Si Anode for next gen LIB

Review

Silicon-based materials as high capacity anodes for next generation lithium ion batteries

Bo Liang a ,*,Yanping Liu a ,Yunhua Xu b

a

Department of Material Forming and Control Engineering,School of Automobile and Mechanic Engineering,Changsha University of Science &Technology,Changsha 410076,PR China b

Department of Chemical and Biomolecular Engineering,University of Maryland,College Park,MD 20742,USA

h i g h l i g h t s

The paper reviewed the merits of silicon-based anodes in lithium ion batteries. The mechanisms were discussed from in-situ TEM and ?rst-principles simulation. The challenges faced in silicon-based anodes were analyzed and forecast.

a r t i c l e i n f o

Article history:

Received 2October 2013Received in revised form 30April 2014

Accepted 21May 2014

Available online 29May 2014Keywords:

Lithium-ion batteries Silicon Anode Composite

Nano-structure

a b s t r a c t

Silicon (Si)-based materials have the highest capacity among the investigated anode materials and have been recognized as one of the most promising materials for lithium-ion batteries.However,it is still a signi ?cant challenge to obtain good performance for practical applications due to the huge volume change during the electrochemical process.To date,the most successful strategy is to introduce other components into Si to form composite or alloy materials.In this review,the recent progress in Si-based materials utilized in lithium-ion batteries is reviewed in terms of composite systems,nano-structure designs,material synthesis methods,and electrochemical performances.The merits and disadvantages of different Si-based materials,the understanding of the mechanisms behind the performance enhancement as well as the challenges faced in Si anodes are also discussed.We are trying to present a full scope of the Si-based materials,and help understand and design future structures of Si anodes in lithium-ion batteries.

?2014Elsevier B.V.All rights reserved.

1.Introduction

Among all investigated anode materials,silicon (Si)has a theoretical capacity of 3590mAh g à1(almost ten times higher than that of graphite)based on the fully alloyed form of Li 15Si 4at room temperature (at high temperature Li 22Si 4can be reached,giving a capacity of 4200mAh g à1),placing it on top of all other anode materials [1e 3].In addition,Si anodes show moderate working potential (~0.5V vs.Li/Li t)[4],which,in contrast to graphite an-odes (~0.05V vs.Li),avoid the safety concern of lithium deposition upon cell overcharge as well as avert the energy penalty of battery cells assembled with the Li 4Ti 5O 12anodes (1.5V vs.Li/Li t)[5].Fig.1showed a schematic of a lithium battery with a Si anode and a

lithium metal oxide (LiM x O y )intercalation cathode [3].However,the large number of lithium ion insertion/extraction results in huge volume change (~370%assuming ?nal alloy of Li 15Si 4),which leads to structural pulverization and electrical disconnection between the active materials and the current collector,and ?nally fast ca-pacity fading [2].To circumvent these problems,tremendous ef-forts have been attempted,including methods from physical to chemical,from inorganic to organic.Among them the most suc-cessful strategy is to incorporate other components into Si mate-rials,such as polymers,carbon,and metals.Signi ?cant progress and promising results have been achieved based on these composites.The material designs and performance results will be summarized and discussed in the following sections.

Failure of the Si anodes is mainly caused by the large volume expansion/contraction during lithium insertion/extraction because the intermetallics of Li/Si have much greater molar volume than the nanostructure Si phase,as shown in Fig.2[6].Large stresses

*Corresponding author.

E-mail addresses:liangbo26@https://www.wendangku.net/doc/058497296.html, ,lbscut@https://www.wendangku.net/doc/058497296.html, (B.

Liang).Contents lists available at ScienceDirect

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Journal of Power Sources 267(2014)469e 490

induced by the repetitive volume expansion and contraction can cause cracking and pulverization of the Si anodes,which leads to loss of electrical contact and eventual capacity fading [5,7e 9]as shown in Fig.3.

The behavior of lithiation-induced deformation and stress has been extensively studied in recent years.On the basis of theoretical calculation,several groups have investigated the lithium-insertion-induced swelling and stress and proposed or predicted the results in mechanical failure.For instance,Christensen and Newman calculated swelling and stress [10,11],Sastry and co-workers simulated the stress generation during lithiation under galvano-static control [12],Cheng and coworkers calculated the strain en-ergy under both potentiostatic and galvanostatic operations in spherical particles [13,14].Lithiation-induced stress in Si has also been calculated [15,16].Several recent papers have reported lithiation-induced fracture by applying electrochemical mechanics [17e 19].

The large volume changes also cause signi ?cant challenges on the morphology and integrity of the entire electrodes.The drastic electrode morphology change can in ?uence the integrity of the electrodes and further result in the capacity fading.The electrode

cracking and peeling-off have been observed by several research groups.Kim et al.investigated the variations in volume and density as a function of Li content [20].They found that when the Li content was suf ?ciently high,alloying between Li and silicon was ener-getically favorable as evidenced by the negative mixing enthalpy;the alloy was most stable around 70atom%Li in the crystalline phase and 70±5atom%Li in the amorphous phase.Many other researchers also studied the volume expansion and received similar results [21e 23].The previous studies have shown that the large deformation of Si electrodes during electrochemical process could be alleviated by plastic ?ow [24].The electrode morphology and integrity are especially relevant to the geometries of the nano-structured electrodes.Motivated by this phenomenon,Zhao et al.elucidated the plastic deformation in lithiated Si under uniaxial tension [23].The microscopic mechanism of large plastic defor-mation is attributed to continuous lithium-assisted breaking and reforming of Si e Si bonds and the creation of nano-pores.

Solid-electrolyte interphase (SEI)?lm that resulted from the decomposition of electrolytes is another important factor in the battery performance,which leads to irreversible capacity loss.The formation of this passivating SEI ?lm on the Si surface has been con ?rmed by high resolution transmission electron microscopy (HRTEM),Fourier Transform infrared spectroscopy (FTIR),and X-ray photoelectron spectroscopy (XPS)[25e 27].The SEI stability at the interface between Si and the liquid electrolyte is a critical factor in cycle life and coulombic ef ?ciency.For example,Arie et al.investigated the electrochemical performance of phosphorus-and boron-doped nanostructure Si thin-?lm anodes [28].They found that the doped Si showed much better cycling stability and higher capacity than the un-doped Si ?lms,which is attributed to the formation of stable SEI layer on the surface of the doped

electrodes.

Fig.1.Schematic of a lithium battery containing a Si anode and lithium metal oxide cathode during (a)charging and (b)discharging [3]

.

Fig.2.Structure demolition of Si anode during lithium insertion [6]

.

Fig.3.Electrochemical performance and CE of solid-state nano-Si composite anodes cycled at a rate of C/20under compressive pressures of 3(blue),150(red)and 230(black)MPa [9].(For interpretation of the references to color in this ?gure legend,the reader is referred to the web version of this article.)

B.Liang et al./Journal of Power Sources 267(2014)469e 490

470

On the contrary,the un-doped Si electrode showed high electrode polarization and poor cycling performance because of an unstable surface layer formed on the electrode surface.

In addition,the large volume change cracks the SEI?lms and makes Si expose to electrolytes,and thus keep the SEI?lms growing.As a result,active Si particles would be isolated by the electric-insulated SEI?lms and lose electrochemical activity,i.e. capacity fading.

2.Nanostructured silicon anodes

Here four generation nanostructures,namely hollow nano-spheres,nanotubes,nanowire arrays,and porous structures was gone through.These nanostructures can effectively withstand the stress induced by heterogeneous changes in the volume of Si an-odes without fracturing or improving the electrochemical proper-ties of Si electrodes.

The action mechanism in hollow nanospheres is actually the same as that in nanotubes.During lithiation,the thin layers of the active materials thicken or expand inwards without structural fracture.During delithiation,the thickness of the lithiated layers and the morphology of the entire nanostructures can almost recover.

The electrochemical characteristics of hollow Si tubes have been shown to exhibit good performances[29e31].Song et al.[29] developed a group IVA based nanotube heterostructure array, consisting of a high-capacity Si inner layer and a highly conductive Ge outer layer,to yield both favorable mechanics and kinetics in battery applications.This type of Si/Ge double-layered nanotube array electrode exhibits stable capacity retention(85%after50 cycles)and doubled capacity at a3C rate.These results stem from reduced maximum hoop strain in the nanotubes,supported by theoretical mechanics modeling,and lowered activation energy barrier for Li diffusion.Park et al.fabricated novel Si nanotubes by reductive decomposition of a Si precursor in the alumina template and etching[31].The use of Si nanotubes increased the surface area accessible to the electrolyte,allowing lithium ions to intercalate from both the interior and the exterior of the nanotubes.The nanotube electrodes have ultrahigh reversible charge capacities of 3200mAh gà1with capacity retention of89%after200cycles at a rate of1C in practical Li-ion cells with improved rate stability.After that,Song et al.reported a Si nanotube,consisting of arrays of sealed,tubular geometries capable of accommodating large volume changes associated with lithiation[30].High initial coulombic ef-?ciencies(~85%)and stable capacity retention(80%after50cycles) were obtained.Wen et al.described a promising strategy for large-scale fabrication of Si nanotubes prepared directly from Silica nanotubes[33].The Si nanotubes showed signi?cantly improved rate capability and long-term cycling performance compared with commercial Si meshes,a capacity of~1000mAh gà1was retained after90cycles at a rate of0.5C.For hollow nanospheres,Cui and co-workers synthesized interconnected hollow Si nanospheres with an inner radius of about175nm and a layer thickness of about 25nm,exhibiting a high initial discharge capacity of2725mAh gà1 when used as anode materials in LIBs[34].The discharge capacity only degraded by8%per100cycles for700total cycles.The shell of the hollow spheres can also consist of irregularly arranged subunits (such as nanosheets,nanorods,and nanotubes)instead of one in-tegral layer.

Si nanowires(SiNWs)have recently attracted intense attention for application as lithium-ion battery anodes[26,35e51]for the following attributes:(1)SiNWs can be directly connected to the current collector without additional binders or conducting addi-tives;(2)SiNWs can offer direct one-dimensional electronic path-ways for rapid charge transport;(3)SiNWs can provide rapid charge/discharge rates due to high surface-to-volume ratio;and(4) SiNWs with small diameters can accommodate large volume changes stemming from the alloying reaction and can prevent fragmentation.

Cui and coworkers designed high performance Si nanostructure anodes to ful?ll the requirements of electric vehicles.In2008,they reported a novel anodes made of vapor e liquid e solid(VLS)SiNWs, which was able to accommodate large strain caused by lithium ion insertion and extraction,as schematically shown in Fig.4(a),(b) [35].The SiNWs anode achieved a charging capacity of 4277mAh gà1during the?rst charging operation and maintained a discharge capacity close to75%of the maximum value during subsequent cycling(Fig.4(c)).In another interesting work,Kumta and co-workers fabricated crystalline e amorphous core e shell SiNW-based anodes[35].The core e shell SiNW anodes exhibited a high charge storage capacity three times that of carbon with90% capacity retention over100cycles,and an excellent electrochemical performance.

After the pioneering work on SiNWs,numerous studies have been focused on nanowire-based electrodes to further improve the electrochemical performance of the SiNWs-based electrodes.Cui and coworkers also observed the SiNWs before and after lithiation and discovered their impacts on volume expansion[42,47,52e55]. In order to meet the signi?cant quantities needed for commercial LIB applications,Aaron et al.demonstrated the creation of a SiNW nonwoven fabric and showed the speci?c capacity of800mAh gà1 for a standalone anode material without the need of additional conductive?llers(activated carbon)or polymeric binders[48].

Several other approaches have been used to grow SiNWs.For example,Yang and coworker synthesized SiNWs on stainless steel foil by the Cu-catalyzed chemical vapor deposition(CVD)

method,

Fig.4.Testing of Si nanowires as the battery anode.(a)Concept schematic of Si nanowire electrode assembled on the current collector;(b)scanning electron microscopy(SEM) image of Si nanowires that comprise the device anode;(c)capacity versus cycle number for various electrode con?gurations.The Si nanowires show stable capacity (~3500mAh gà1)without any degradation with increase in the number of cycles[35].

B.Liang et al./Journal of Power Sources267(2014)469e490471

showing a high coulombic ef?ciency of89%at?rst cycle with a high

speci?c capacity of over2000mAh gà1in several dozens of cycles

[56].They con?rmed that the introduction of vinylene carbonate

(VC)additives into electrolytes greatly improved the cycle perfor-

mance and rate capability of the SiNWs and showed a slow capacity

degradation of0.25%per cycle during100cycles test.The choices of

catalyst affect the irreversible capacity loss for the anode.Emma

et al.designed Sn catalyzed SiNWs using a high boiling point

solvent-vapor-growth(SVG)system as a low cost approach to

realize high density NW growth[57].The SiNWs remained excel-

lent capacities of1078mAh gà1after50charge/discharge cycles.

Concurrently,Peng and coworkers demonstrated aligned metal-

catalyzed electroless etching(MCEE)SiNWs as scalable anode

materials for lithium-ion batteries[37].In contrast to the VLS

SiNWs[1],the MCEE SiNWs inherit the electrical characteristics of

the original Si and thus no additional doping is needed.Moreover,

the rough surfaces make MCEE SiNWs more promising for lithium-

ion battery anode applications.The MCEE SiNWs showed a

discharge capacity of nearly0.5mAh cmà2and a longer cycling

stability than bulk Si.The surface roughness of MCEE SiNWs

increased after cycling due to the large volume change.Chakrapani

et al.evaluated the performance of SiNW electrodes in ionic liquid

(IL)and organic electrolyte[58],which showed good performance

with charge and discharge capacities of2014and1836mAh gà1.

However,the electrode showed capacity fade of20e30%both in IL

and organic electrolyte after50cycles.

The good capacity retention of thin-?lm anodes was attributed

to the strong adhesion of active material to the conductive support.

Thin-?lm anodes can be prepared by magnetron sputtering

[59e61]or physical vapor deposition[62e64].The performance of

thin-?lm anodes depends strongly on the deposition rate,deposi-

tion temperature,substrate-surface roughness,?lm thickness and

post annealing treatment[65e67].Takamura et al.reported that an

amorphous Si thin?lm deposited50nm thick on a Ni foil exhibited

a high speci?c capacity of over2000mAh gà1and superior cycla-

bility of over1000cycles at a charge rate of12C[62,68].However,

the cycle life was dramatically reduced when the?lm thickness was

increased(200cycles for0.5e1m m?lms and50cycles for1.8m m ?lms)[67].The poor performance of thicker?lms(>1m m) compared to thinner?lms was due to the increased Li diffusion

length,higher electrical resistance and larger internal stress of Li

insertion/extraction[69,70].

Later,Pure Si thin?akes(Si Leaf Powder?(Si-LP))of different

thicknesses were prepared and used as negative electrode mate-

rials[71].High reversible capacity(2200e2500mAh gà1)and good

capacity retention were obtained for50e200nm Si-LPs.For the

thinner?akes,agglomeration and large cracks were con?rmed on

the composite electrodes,but no pulverization of the?akes was

observed.These data suggested that Li atoms diffused easily within

the thinner Si-LPs and the uniformity of Li distribution suppressed

the localized physical stress that caused by alloying and de-

alloying.

Park et al.fabricated amorphous Si thin-?lms with a thickness of

1.5m m and3m m by pulsed laser deposition,and found that the capacity of the former was superior to that of the latter[72].Both ?lms showed good cyclic performances without any abrupt fading in capacity up to70cycles.They attributed the good cyclic perfor-mance to strong adhesion behavior of the Si thin?lm and to the high density of the thin?lms.Paul et al.synthesized nano-structured Si thin?lms with varied oxygen content by evaporating Si in a water ambient using reactive ballistic deposition at glancing angles[73].The incorporated oxygen concentration was controlled by varying the partial pressure of water during the deposition.The combination of homogeneous oxygen incorporation during the synthesis of the?lms and surface oxidation by low-temperature annealing in air provides the best electrode stability.A high ca-pacity(2200mAh gà1)was realized with virtually no capacity fade for the?rst120cycles and slight capacity fade(~0.15%per cycle)in cycles150e300.Baggetto et al.[74]investigated the interaction between Si thin?lm electrodes and various electrolytes(liquid electrolyte:LiClO4in propylene carbonate(PC),LiPF6in ethylene carbonate(EC)/diethyl carbonate(DEC);solid electrolyte:lithium phosphorus oxynitride(LiPON),Li3PO4).For liquid electrolytes,it was found that the SEI formation depended on the type of elec-trolyte used.For solid electrolytes,the deposition of a LiPON elec-trolyte layer rendered the activation of the interface a more complicated process compared to that of pure Li3PO4.An interlayer material due to partial intermixing was observed after depositing LiPON onto Si.Recently,some studies on patterned Si thin-?lm revealed interesting results and unique scienti?c insights [75e77].Xiao et al.prepared patterned Si thin?lms on a Cu sub-strate by electron beam evaporation using three kinds of mesh masks[75].Those Si patterns below7e10m m remained adhered to the Cu substrate during cycling,leading to much better cycling stability and capacity retention than did patterns of other sizes.

The Fig. 5.(a)SEM image of the as-prepared and fully lithiated Si honeycomb.(b) Morphological change of the Si honeycomb structure during delithiation[76].

B.Liang et al./Journal of Power Sources267(2014)469e490 472

gaps between Si patterns allow stress relaxation and support better cycling stability relative to a continuous?lm.Baggetto et al.fabri-cated a honeycomb-structured Si?lm on planar TiN-covered Si substrate by photoetching[76].After lithiation,the honeycombs became highly curved(Fig.5(a)).The thickness,length and height of the curved wall all increased compared with the starting mate-rial.Fig.5(b)illustrated the morphological changes during deli-thiation.Interestingly,the highly curved structure gradually converted back to its original con?guration,and the hexagonal honeycombs were recovered?nally,exhibiting only slight defor-mation.A patterned Si electrode to alleviate the cracking and reduce the side reactions was also reported by He and coworkers [77].

Although a quite number of Si nanostructures have been re-ported as anodes in lithium-ion batteries,nanostructure Si mate-rials show fast capacity fading,which hinders the practical applications.Besides nanostructure Si,another structural design for solving the problem related to the volume change is the con-struction of composites,which consists of Si active materials and other components.The composites act as a buffer to alleviate the stress and accommodate the volume change led by lithium inser-tion/extraction.By using composites/alloys,the electrode integrity and the electronic contact between the active particles and conductive phase can be maintained[4,78].A variety of composites with active/inactive materials and nanostructures have been designed and reported in the latest few years.

3.Silicon/carbon composites

3.1.Silicon/amorphous carbon composites

Carbon coatings on Si electrodes have been reported to modify the SEI morphology and exhibited smooth surfaces with limited pore structure[79,80],compared to the visibly cracked and porous morphology seen in nanostructure Si electrodes[81].Recently, nanocomposites with a nanocrystalline Si powder coated with a thin layer of amorphous carbon has led to moderate success of sustaining reversible capacities on the order of700mAh gà1at ~0.25C in the electrochemical potential window of0.02e1.2V [82,83].Carbon-coated Si particles have also been shown to achieve capacities as high as1000mAh gà1when charging and discharging at a constant current of~0.3mA mgà1[84].Wilson et al.?rstly obtained nanodispersed Si in carbon using CVD and received a reversible capacity of roughly500mAh gà1[34].Similarly,Ng et al. synthesized a spheroidal carbon-coated Si nanocomposite anode, and an initial lithiation capacity of2600mAh gà1and a delithiation capacity of1857mAh gà1for the active materials(44%Sit56%C) was obtained by using a nonrestricted cycling procedure[85].

Some studies were reported that nano-sized Si particles in composites tend to aggregate during charge/discharge cycles,and this size increase results in poor Li insertion/extraction kinetics [86,87].Carbon-coated Si particles with an average size of10nm had a capacity over3000mAh gà1with an ef?ciency of98%[87].In order to prevent aggregation during the cycling,Kwon et al.re-ported the synthesis of Si quantum dots coated with amorphous carbon[88].The new structures gained a?rst charge capacity of 1257mAh gà1with a coulombic ef?ciency of71%.The uniform distribution of Si quantum dots along with the carbon coating prevented aggregation during the cycling.

Huang et al.prepared coin cells using carbon-coated MCEE SiNWs as composite anodes[38].Compared to simple SiNWs anode,the carbon-coated SiNWs composite anode exhibited an excellent?rst discharge capacity of3344mAh gà1with a coulombic ef?ciency of84%and signi?cantly enhanced cycling performance. TEM analysis showed most of the SiNWs remained continuous after 40cycles with a few SiNWs breaking into particles due to non-uniform volume changes among nanowires.The performance improvement is ascribed to the carbon coating that facilitates electronic contact and conduction.

A hierarchical bottom-up self-assembly technique consisting of dendritic carbon structures coated with Si nanoparticles(SiNPs) has recently attracted attention[89].A reversible charge(dein-tercalation)capacity of about1950mAh gà1was reached at a rate of 0.5C.The speci?c capacity of the SiNPs alone was estimated to be 3670mAh gà1,which is close to the theoretical value.The dendrite structure of carbon helped to provide ef?cient electron conduction by acting like a conducting backbone.It also provided the appro-priate porosity required for the SiNPs to undergo volumetric expansion.Carbon inverse-opals coated with amorphous Si was synthesized via templating with ordered colloidal spheres and subsequent Si deposition.Its capacity was above2100mAh gà1 after145cycles,whereas the capacity of a Si inverse-opal coated with amorphous carbon(Si/C inverse-opal)was completely lost by the11th cycle[90].

Hu et al.reported a Si@SiO x/C nanocomposite in which pre-formed SiNPs were coated with carbon by hydrothermal carbon-ization of glucose[91].The nanocomposite showed remarkably improved lithium-storage performance in terms of high reversible lithium-storage capacity(1100mAh gà1)and high rate capability. However,the electrode was much less stable without the additive. Considering the construction of Si-based composites with hollow structures is another effective approach to improve the electro-chemical performance,Cho and coworkers synthesized three-dimensional porous Si with an excellent capacity of 2800mAh gà1at a rate of1C[92].Park et al.reported SiNPs trapped in an ordered mesoporous carbon composite by a one-step self-assembly with solvent evaporation using a triblock copolymer Pluronic F127and a resorcinol-formaldehyde polymer as the tem-plating agent and carbon precursor respectively[93].Such a one-pot synthesis of Si/ordered mesoporous carbon nanocomposite is suitable for large-scale synthesis.The composite showed a high reversible capacity above700mAh gà1during50cycles at2A gà1.

Deng et al.reported a tubular con?guration made from naturally rolled-up C/Si/C trilayer nanomembranes,which exhibits a highly reversible capacity of approximately2000mAh gà1at50mA gà1, and approximately100%capacity retention at500mA gà1after300 cycles,as shown in Fig.6[94].The naturally strain-released nano-membranes could enhance the capability to prevent stress cracking during the electrochemical cycles,and the tubular structures could facilitate fast ion diffusion and electron transport.The sandwich-structured C/Si/C composites,with moderate kinetic properties toward Lition and electron transport,are of the highest quality. The excellent cycling performance is related to the thin-?lm effect combined with carbon coating,which play a structural buffering role in minimizing the mechanical stress induced by the volume change of Si.The energy reduction in C/Si/C trilayer nano-membranes after rolling up into multi-winding microtubes results in a signi?cantly reduced intrinsic strain,which can improve ca-pacity and cycling performance.This synthetic process could be compatible with existing industrial sputtering deposition processes as well as roll-to-roll thin-?lm fabrication technology.

3.2.Silicon/graphene composites

Graphene is an attractive option to improve the performance of the battery by enhancing the overall charge transportation mech-anism[95].The synthesis of Si/graphene composite is presented by a sonochemical method and then magnesiothermic reduction process[96].Silica particles were?rstly synthesized and deposited on the surface of graphene oxide(SiO2e GO)by ultrasonic waves,

B.Liang et al./Journal of Power Sources267(2014)469e490473

subsequent low-temperature magnesiothermic reduction trans-formed SiO 2to SiNPs in-situ on graphene sheets.With the opti-mized ratio of 1:1,SiNPs on graphene sheets was obtained with the average particle size of 30nm.The resultant Si e graphene with 78wt%Si inside delivered a reversible capacity of 1100mAh g à1,with very little fading when the charge rates change from 100mA g à1to 2000mA g à1and then back to 100mA g à1.Similarly,a facile CVD method to prepare Si/carbon/graphite microspheres (Si/C/GM)composite anode by growing Si/C microrods on the surface of commercial GMs was reported [97].The Si/C/GM com-posites with an urchin-like morphology are composed of Si parti-cles,amorphous carbon,and graphite.The composite displayed the best anode properties with a speci ?c capacity of 562mAh g à1at a current density of 50mA g à1,much higher than that of GMs (361mAh g à1),and a good cycling performance (a reversible ca-pacity of 590.5mAh g à1after 50cycles).The improved electro-chemical performance is attributed to the incorporation of Si,together with the formation of a Si/C microrod network,which connects the GMs and buffers the volume change of Si during lithium ion insertion/extraction.Very recently,Ren et al.introduced a chemical vapor deposition process to deposit crystalline Si par-ticles onto graphene sheets by using a liquid chlorosilane as Si source [98].The Si/graphene composite exhibits high utilization of Si in charge/discharge processes.The capacity retention of 90%after 500full cycles and an average coulombic ef ?ciency in excess of 99.5%are achieved in half cells.Zhou et al.presented a double protection strategy by fabricating graphene/carbon-coated SiNP hybrids to improve the electrochemical performance of Si in Li storage [99].The graphene/carbon-coated SiNP hybrids exhibited outstanding reversible capacity of 902mAh g à1after 100cycles at 300mA g à1.This work suggested a strategy to improve the elec-trochemical performance of Li-ion batteries by using graphene as supporting sheets for loading of active materials and carbon as the covering layers.

A unique graphene bonded and encapsulated Si anode using a one-step scalable aerosol spray method was researched (Fig.7

)

Fig.6.(a)Fabrication of naturally rolled-up C/Si/C multilayer microtubes.(b)A cross-section of a single tube that prepared by focused ion beam (FIB)cutting,where tightly wrapped windings are visible;cycling performance showing the charge/discharge capacities of the as-prepared multilayer microtubes at current densities of approximately 50mA g à1;(c)and 500mA g à1;(d)from 0.05V to 1.5V versus Li/Li t,the capacity was calculated based on the total mass of the C/Si/C microtubes [94]

.

Fig.7.Schematic procedure for the synthesis of graphene bonded and encapsulated nano-Si composite [100].

B.Liang et al./Journal of Power Sources 267(2014)469e 490

474

[100].The functionalized Si sealed by graphene delivered a speci ?c charge capacity of 2250mAh g à1at 0.1C and retained 85%of its initial capacity even after 120charge/discharge cycles.The open-ended graphene shell with defects allowed fast electrochemical lithiation/delithiation,good electronic conductivity for SiNPs and prevented them from aggregating during charge/discharge cycles.The space inside the graphene shell accompanied by its strong mechanical strength can effectively accommodate the volume expansion of Si upon lithiation.The homogeneous distribution of monolayer SiNPs were ?rmly anchored to graphene oxides (GOs)by covalent immobilization [34](Fig.8).The super high surface area of graphenes produced a high stable cycle performance when the hybrid was used as anode material for lithium ion batteries.The capacity of the Si/graphene hybrid anode at the 50th cycle still retained 1203mAh g à1,which was 92.7%of the 1st cycle.

Lee et al.fabricated well-dispersed SiNPs supported by a 3D network of graphene sheets [101].Intimate contact between nanoparticles and graphene sheets was essential for improved electrochemical performance.The Si/graphene composites exhibi-ted high lithium ion storage capacity and cycling stability (>1500mAh g à1after 200cycles).Luo et al.developed a one-step aerosol assisted capillary assembly technique to create

crumpled

Fig.8.(a)Scheme of fabricating Si-GO hybrid and (b)Si e graphene anode [34]

.

Fig.9.Electrochemical performance of the capsules (Si@crumpled graphene).(a)Coulombic ef ?ciency;(b)charge/discharge cycling test of the composite capsules in comparison to the unwrapped SiNPs at a constant current density of 1A g à1;(c)SEM image of the capsules after 250cycles showed that SiNPs were still encapsulated in the crumpled graphene;(d)galvanostatic charge/discharge pro ?les of the composite capsules electrode at various current densities ranging from 0.2to 4A g à1[102].

B.Liang et al./Journal of Power Sources 267(2014)469e 490475

capsules of graphene-wrapped SiNPs [102].As shown in Fig.9,the composite delivered about 1200mAh g à1at a low current density of 200mA g à1.When the current density was increased 20times from 0.2to 4A g à1,a capacity of 600mAh g à1still remained.

Wang et al.reported that graphene nanosheets (GNSs)signi ?-cantly improved the lithium storage capacity of porous single crystalline SiNWs [45].Graphene acts as a conductive additive and the nanosheets cover larger areas of the nanowires,providing greater areas for charge transfer.The interleaved networks of GNSs should produce pathways for transport of electrons and lithium ions,improving both the electrical conductivity as well as the lithium ion diffusion rate of the electrode.The Si/grapheme com-posites showed an initial charge capacity of 2347mAh g à1with capacity retention of 87%after https://www.wendangku.net/doc/058497296.html,yer-by-layer alternation of GNSs coated Si nanocomposites with good cyclability and high capacities were also prepared by Sun et al.[103].Zhu et al.prepared a well-de ?ned core/shell self-assembly structure of SiNWs with spatially de ?ned wrap of grapheme [104].The obtained GNS@SiNWs delivered a reversible capacity of 1648mAh g à1with initial coulombic ef ?ciency as high as 80%.

Wang and coworkers recently published a seminal paper to introduce a novel kind of self-supporting binder-free Si-based anode via the encapsulation of SiNWs with dual adaptable apparels (overlapped graphene sheaths and reduced graphene oxide (RGO)overcoats)(Fig.10(a))[105].Within the novel architecture,the overlapped G sheets acted as adaptable but sealed sheaths,which could transform synergistically with the volume change of embedded SiNWs,prevent the direct contact of Si with the elec-trolyte,avoid the pore formation in Si,and thus secure the integrity of SiNWs during repeated cycling.Moreover,the RGO overcoats,as a mechanically robust and ?exible matrix,accommodated the volume change of embedded SiNW@G nanocables,thus maintained the structural and electrical integrity of the electrode.They achieved a high reversible speci ?c capacity of 1600mAh g à1at 2.1A g à1,80%capacity retention after 100cycles,and superior rate capability (500mAh g à1at 8.4A g à1)on the basis of the total electrode weight (Fig.10(b)).The unique shape and structure of these nanocomposite not only allowed faster transport of the Li ions through the elec-trolyte and the electrode but also guaranteed faster intercalation reactions of the Li ions,thus resulting in a large speci ?c capacity even when operated at high charge e discharge currents.

However,most of the Si/graphene composites resulted from freeze-drying [106],sonication followed by vacuum-?ltration [101]or mechanical blending [107]did not exhibit acceptable perfor-mances,especially in https://www.wendangku.net/doc/058497296.html,posite anodes of SiNPs and RGO sheets with highly dispersed SiNPs were synthesized to investigate the performance-related improvements from particle

dispersion [108].In spite of all the aforementioned advances in the ?eld of composite anodes,the key issue of a weak structural interface between carbon and Si was not properly addressed.Si and carbon exhibited different volume changes associated with inter-calation and de-intercalation of lithium ions.This in turn makes the composite vulnerable to rapid delamination,particularly at higher charge/discharge rates due to signi ?cant mismatch of the induced strains in Si and carbon.

3.3.Silicon/carbon ?ber or nanotube composites

Among 1D carbonaceous material,CNTs are the most-popular additive for improving the electrochemical properties of Si-based materials.A homogeneous dispersion of SiNPs along thin CNTs al-lows optimizing of the electrochemical ef ?ciency of Si [109].The speci ?c hierarchical hybrid nanostructure which deposited 10nm SiNPs on thin CNTs of 5nm in diameter delivered high reversible Li storage capacity of 3000mAh g à1at 1.3C.Li et al.prepared Si/carbon nanotube (CNT)/C composite for lithium-ion battery anode materials [110].The carbon matrix accommodated the volume change of SiNPs and provided continuous pathways for ef ?cient charge transport along the ?ber https://www.wendangku.net/doc/058497296.html,Ts could improve the electronic conductivity and electrochemical performance of the composite anodes.Evanoff et al.reported a route to produce the large-scale fabrication of CNT fabric coated with Si for usage as an electrode for multifunctional Li-ion batteries [32].The composite Si-CNT fabric showed ultimate tensile strengths (UTS)greater than 90MPa after electrochemical cycling and the electrochemical per-formance of the electrodes demonstrated stability for more than 150cycles.The same group explored ultra-thick electrodes composed of Si-coated carbon nanotube (CNT)arrays [111].The electrode had average values of dealloying capacities of

3300mAh g Si à1and 2000mAh g Si à1

at C/5and C/2,respectively,and demonstrated good stability for over 250cycles.Park and co-workers proposed the fabrication of self-supported multi-walled carbon nanotube (MWCNT)-embedded SiNP electrodes through CVD,with acetylene gas and a facile spin coating process using commercial Si nanopowders [112].The void spaces between the MWCNTs were densely ?lled with a considerable amount of SiNPs with a diameter of ~50nm.The MWCNT-embedded SiNP electrodes showed high speci ?c capacity and superior capacity retention (2900and 1510mAh g à1after 10and 100cycles,respectively,at a current density of 840mA g à1)as well as outstanding rate capa-bility.These high electrochemical performances were ascribed to the roles of MWCNTs in terms of providing an ef ?cient electron-transport path and alleviating volume change of SiNPs during Li-alloying/de-alloying

process.

Fig.10.(a)Schematic of the fabrication (upper panel)and adapting (lower panel)of SiNW@graphene@RGO.The fabrication process mainly includes (I)CVD growth of overlapped graphene sheets on as-synthesized SiNWs to form SiNW@graphene nanocables,and (II)vacuum ?ltration of an aqueous SiNW@graphene-GO dispersion followed by thermal reduction.The resulting SiNW@graphene@RGO can transform between an expanded state and a contracted state during lithiation/delithiation cycles,thus enabling the stabilization of the Si material;(b)comparison of capacity retention of different electrodes.All electrodes were cycled at the charge/discharge rate of 210mA g à1for the ?rst three cycles and then 840mA g à1for the subsequent cycles;(c)capacity and Columbic ef ?ciency of the SiNW@graphene@RGO cycled at the designated rate (210mA g à1for the initial cycle and then 2.1A g à1)for 100cycles [105].

B.Liang et al./Journal of Power Sources 267(2014)469e 490

476

Three-dimensional (3D)nano-architectures have attracted tremendous attention for applications in Li-ion batteries due to their unique structural advantages.Studies have further revealed that on anchoring SiNPs to carbon nano ?bers (CNF),a signi ?cant speci ?c capacity improvement achieved [113].The carbonization process anchored the SiNPs induced a strong interaction between Si and carbon through thick amorphous Si oxide layer.The anode delivered a speci ?c capacity of 2500mAh g à1at a current density of 500mA g à1and showed good capacity retention over 50cycles.The polymer-like elasticity of the ?brous carbon matrix could further relieve the induced stress by accommodating Si volume expansion on Li insertion.Klankowski and coworkers reported a high-performance hybrid lithium-ion anode material using coaxially coated Si shells on vertically aligned carbon nano ?ber (VACNF)cores (Fig.11)[114].The unique “cup-stacking ”graphitic micro-structure made VACNFs a good lithium-ion intercalation medium and,more importantly,a robust bush-like conductive core to effectively connect high-capacity Si shells for lithium-ion battery.The vertical core e shell nanowires remained well separated from each other even after coating with bulk quantities of Si (equivalent to 1.5m m thick solid ?lms).This open 3D nanostructured archi-tecture allowed the Si shells to freely expand/contract in the radial direction during lithium-ion insertion/extraction.High-performance Li storage with a mass-speci ?c capacity of 3000e 3650mAh g à1,which was comparable to the maximum value of amorphous Si,was obtained even at the 1C rate.About 89%of the capacity was retained after 100charge e discharge cycles at the 1C rate.The electrode material became even more stable after long cycling.High capacity near the theoretical limit was attained in over 120charge/discharge cycles,showing the invariant lithium-ion storage capacity as the charge/discharge rate is increased by 20times from 0.1C to 2C.Similar Si e C core e shell composites as lithium ion battery anodes have been reported by Cui and co-workers and other researchers as well [112,115e 117].Table 1summarizes the most recent literature data on Si e C based elec-trodes for lithium-ion battery devices.4.Si/polymer composites

4.1.Si/conducting polymer composites

To improve the cyclability of the Si-based anode,other compo-nents have been introduced to act as a buffer to accommodate the large volume change upon cycling.Conducting polymers (CPs)possess many advantages that make them suitable materials for lithium-ion battery,such as good conductivity,?exible mechanical property,and easy structure modi ?cation.The components of CPs not only function as a structural and physical buffer to minimize the mechanical stress,thus keep the particles together in powder-based anodes and make them adhere to the current collector,but also provide a conductive matrix to ensure a good kinetics.

Among the electronically-conducting polymers,polypyrrole (PPy)was the most used polymers as a positive electrode additive material in lithium ion batteries because it is easily doped with cations and anions to produce high conductivity and good stability in air [118,119].Guo et al.reported a novel Si/PPy anode using high-energy mechanical milling [118].It was found that dense agglom-erates (0.3e 3.0m m)of Si particles were connected by PPy.The Si/PPy anode with a 1:1weight ratio of Si to PPy exhibited an initial discharge capacity of up to 1800mAh g à1and retained over 90%of the initial capacity after 10cycles.The improved performance was attributed to the buffering and binding ability of the conductive PPy matrix.Better performance was obtained for anodes derived from PPy-coated Si particles prepared by in-situ polymerization [120]in which Si particles were coated with PPy via in-situ polymerizing pyrrole with FeCl 3(Fig.12).A uniform thin layer of PPy (average thickness,about 2nm)was formed on the surface of Si particles.The Li/PPy-coated Si electrode exhibited improved discharge ca-pacities.Unfortunately,only short cycle life was reported on these composite anodes.The result indicated that PPy coating on the Si nanoparticles was not sustained during cycles,but may contribute in preventing the binding among the particles from loosing and/or preventing cracked Si fragments from being extracted into the electrolyte.

PPy-coated Si nano ?ber networks were prepared using a facile two-step approach:at ?rst PPy nano ?bers were synthesized by electropolymerzation and then Si was deposited on the PPy ?bers via a CVD procedure,as shown in Fig.13(a).[121].With this well-designed con ?guration,the PPy nano ?bers conveniently maintain the structural integrity of the composite ?bers and facilitate charge delivery and gathering,while the porosity of the electrode can adequately buffer the Si swelling during the lithiation process.The reversible capacity of the electrode remained as high as 2800mAh g à1after 100cycles,corresponding to 91%of the ?rst cycle (Fig.13(b)).Good rate capability was also observed on the composite ?bers.The well-preserved morphology of reticular nano ?bers after repeated lithium insertion and extraction demonstrated the robust ability of the PPy nano ?bers.

In order to increase electronic conductivity,Ag was introduced into the Si/PPy composites by chemical polymerization of pyrrole and reduction of AgNO 3[122].Ag not only increased the conduc-tivity but also provided strong mechanical property of the com-posites.For the optimal amount of Ag in the Si/PPy composites,the cycle stability and rate capability of the Si-PPy-Ag composites were greatly enhanced in comparison with the bare SiNPs.A high ca-pacity of more than 823mAh g à1was retained after 100cycles,showing its promising application as anode materials for lithium-ion batteries.

The effects of different microstructures of PPy,nano-wires and micro-particles,on the electrochemical performance were inves-tigated [123].In comparison with the bare Si particles,better per-formance was observed for the Si/PPy nano-wires that

contain

Fig.11.Schematic illustration of the reversible structural changes of the coaxially coated Si on vertically aligned carbon nano ?bers (VACNFs)in (a)extracted (discharged)and (b)inserted (charged)states during a half-cell test for Li-ion batteries [114].

B.Liang et al./Journal of Power Sources 267(2014)469e 490477

10wt%PPy,while no improvement for Si/PPy micro-particles was observed.The possible reason is that the vast network of PPy nano-wires matrix can accommodate the huge volume change during the alloying/dealloying reactions.

Polyaniline(PANi),another conducting polymer,was also used in Si composite anodes.Cai et al.synthesized Si/PANi composites by chemically polymerizing aniline on Si nanoparticles[124].The initial delithiation capacity was1940mAh gà1at100mA gà1and maintained a capacity of1870mAh gà1after25cycles with a ca-pacity fading rate of0.3%per cycle.PANi/Si composite materials prepared by dispersing Si-NPs in PANi have been used as the electrode material for supercapacitors[125].The PANi/Si composite showed high power(220W gà1)and energy-storage(30Wh kgà1) capabilities as well as good device stability during1000charging/ discharging cycles.However,the composites showed limited rate capability,especially at high charge and discharge current densities,indicating that the conductivity of the composites should be further improved.

Very recently,Cui group reported a well-connected three dimensional network structure of a SiNPs/PANi composite.SiNPs were conformally coated by the conducting polymer using in-situ polymerizing with SiNPs dispersed in hydrogel.This hierarchical hydrogel framework has multiple advantages:continuous electri-cally conductive PANi network,binding between Si and polymer, and porous space for volume expansion of Si particles.With these features,excellent performance was demonstrated:a cycle life of 5000cycles with over90%capacity retention at current density of 6.0A gà1.

Poly(3,4-ethylenedioxythiophene)(PEDOT),one of the most successful conducting polymers,has received great attention for applications in material sciences.PEDOT has high electrical con-ductivity in the p-doped state,fast electrochemical switching,and

Table1

Electrochemical performance of Si e C composite anodes for lithium-ion batteries.

Anode Speci?c

capacity

(mAh gà1)

Cycling stability Synthesis methods References

Carbon-coated Si10001000mAh gà1after57cycles Fluidized-bed type of chemical

vapor deposition

[84] Carbon-coated Si26001489mAh gà1after20cycles Spray-pyrolysis technique[85]

Si/a-C1257Capacity retention after30cycles was84%Annealing at700or900 C[88]

Si/dendritic C367050wt%of Si1950mAh gà1at C/20,

volumetric capacity was determined to

be1270mAh cmà3at C/20Hierarchical bottom-up assembly

technique

[89]

Si/C21002100mAh gà1after145cycles Templating[90] Si@SiO x/C1100A current density of150mA gà1Coated carbon by hydrothermal

carbonization on glucose

[91]

Si/C1275Reversible capacity of700mAh gà1

during50cycles at2A gà1One-step self-assembly with

solvent evaporation

[93]

C/Si/C?lm2000Approximately100%capacity retention

at500mAh gà1after300cycles Naturally roll-up C/Si/C trilayer

nanomembranes

[94]

C/SiNWS3344At a rate of150mA gà1between2and0.02V metal catalytic etching of Si wafers

and pyrolyzing of carbon aerogel

[38]

SiNW/GNSs23472041mAh gà1at the20th cycle,87%capacity

retention after20cycles Liquid-phase graphite exfoliation

method and an electroless HF/AgNO3

etching process

[45]

Si/graphene1100A reversible capacity of1300mAh gà1,little

fading from100to2000mA gà1back to

100mA gà1Combination of sonochemistry and

Mg-assisted reduction

[96]

Si/C/GMS562A reversible capacity of590.5mAh gà1

after50cycles

Chemical vapor deposition[97] Si/graphene sheets1365553mAh gà1retained after500cycles.Chemical vapor deposition[98]

Si/graphene902Remained902mAh gà1after100cycles at

300mA gà1Wrapped Si nanoparticles between

graphene and amorphous carbon

[99]

APS e Si e graphene225076%of initial capacity after120charge/discharge

cycles at1C

Scalable aerosol spray method[100]

Si/graphene15001500mAh gà1after200cycles3D network of grapheme sheets[101] Si/graphene1200940mAh gà1after250cycles One-step capillary-driven assembly

route in aerosol droplets.

[102]

SiNW/G/RGO1600500mAh g-1at8.4A gà1,80%capacity

retention after100cycles Chemical vapor deposition and

thermal reduction

[105]

Si/CNT/C1410837mAh gà1at the30th cycle,the57.6%

capacity retention with300mA gà1at the

50cycle

Electrospinning and carbonization[110]

a-Si/C3086Capacity retention displaying~0.03%fade

in capacity up to50cycles and~0.2%after

50cycles

Radio frequency magnetron sputtering[227]

Si/CNT2000~1000mAh gà1at the100th cycle Two-step liquid injection chemical

vapor deposition proces

[228] Si/CNF40503890mAh gà1after100cycles A unique dual plasma deposition

technique

[229]

Si/PCNF2643~1104mAh gà1under0.5A gà1after

100cycles,with a capacity retention

of69.1%A single-nozzle electrospinning

technique and subsequent calcination

and HF etching process

[230]

Si/C-PDA26911601mAh gà1at the50th cycle,72.6%

after50cycles Conventional electrospinning and

multiple depositions and annealing

[231]

Si?lm/CNT20831711mAh gà1after50cycles,with82%

capacity retention Chemical vapor deposition of a-Si on

CNT?lms or by CNT-SiNP compositing technique

[232]

B.Liang et al./Journal of Power Sources267(2014)469e490 478

good thermal and chemical stability.But it is soluble in common solvents,which makes it dif ?cult to process in current fabrication technologies.Poly(styrenesulfonate)(PSS)is usually used as a charge-balancing dopant and dispersing agent during polymeri-zation to form a stable aqueous suspension of PEDOT:PSS.PEDOT/PEDOT:PSS is attractive for use in lithium-ion batteries due to its polymer ?exibility and high electric conductivity.It has been studied as cathode materials or additives of cathode materials in lithium-ion batteries [126e 128].Additionally,previous studies show that the heteroatom sulfur contained in each repeat unit of both PEDOT and PSS may improve the electrochemical perfor-mance when used in anode materials [129e 132].

Zhang and co-workers reported a nano-Si/carbon composite with S-doped carbon matrix,which was prepared from an in-situ chemical polymerization of EDOT with nano-Si particles in a PSS aqueous solution and subsequent carbonization of Si/PEDOT:PSS [133].A good cycling stability was obtained with capacity retention of 768mAh g à1and a columbic ef ?ciency of 99.2%after 80cycles.

Zhang and co-workers prepared Si/S-doped C composites by a magnesiothermic reduction reaction of mesoporous SiO 2,subse-quently coated with sulfur-containing polymer (PEDOT)and post-carbonization [134].The Si/S-doped C composite materials

exhibited a remarkably improved lithium storage performance,capacity retention,and rate capability.The Si/S-doped C composite displayed a ?rst discharge capacity of 1947mAh g à1with ?rst coulombic ef ?ciency of 76.1%and cycling performance with a ca-pacity of 539mAh g à1at the 100th cycle,which are much better than those of bare Si electrode.

Yao et al.coated the conductive polymer of PEDOT on Si nano-wires (SiNWs)[135].The PEDOT-coated Si-NWs demonstrated improved cycling stability,increasing the capacity retention after 100charge/discharge cycles from 30%to 80%over bare SiNWs.The improvement in cycling stability was attributed to the conductive coating that maintained the mechanical integrity of the cycled Si material and preserved the electrical connections between SiNWs.4.2.Effects of polymer binders on Si anodes

Polymer binders are components used in preparation of elec-trodes to keep the integrity of the electrodes.Previous studies show that binders play a critical role in cell performance.A number of polymers have been investigated as binders in lithium-ion batteries.

Polyvinylidene ?uoride (PVDF),one conventional binder,ex-hibits a good ductility and has been examined for Si anodes [136e 139].But it was not a good binder agent.The Si anodes using PVDF always suffered fast capacity fading [136,140].In contrast,respectable Li-ion battery performance was obtained with poly (acrylic acid)(PAA),sodium carboxymethyl cellulose (CMC),and sodium alginate.They all have carboxylic functional groups and are soluble in water with environment-friendly characteristic.CMC,a bio-derived polymer,was used as a promising binder for Si elec-trodes [141e 145].Superior cycle stability,high capacity,and high energy and power density was demonstrated on a Si-NPs/CMC scaffold structure.At 1.5C,the CMC scaffold-nano-Si electrode could provide discharge capacity of 1800mAh g à1(75%of the ca-pacity at 0.05C)[145].

Pure PAA,possessing certain mechanical properties comparable to those of CMC but containing a higher concentration of carboxylic functional groups,offer superior performance as a binder for Si anodes [146].Komaba and coworkers reported PAA and its polymer salts [sodium polyacrylate (PANa)]led to graphite-Si electrodes with good reversibility [139,147e 150].The polyacrylates improved the binding ability (adhesion strength)of the composite electrode.The thin coating layer of polyacrylates uniformly covered the active materials and suppressed electrolyte decomposition as well as self-discharges.

Cross-linked PAA can be modulated by the addition of the pol-ycarbodiimide (PCD)as a cross-linker [151].The initial reversible capacity of Si/graphite composite electrodes was increased

with

Fig.12.Schematic representation for the preparation of PPy-coated Si particles [120]

.

Fig.13.(a)Schematic illustration of the synthesis procedure of 3D reticular PPy-Si core e shell nanocables.The PPy backbone network (red)is synthesized by electropolymerization,and Si (green)is deposited by CVD as the lithium storage host;(b)discharge/charge capacity and coulombic ef ?ciency versus cycle number for the PPy-Si nanostructured electrode [121].(For interpretation of the references to color in this ?gure legend,the reader is referred to the web version of this article.)

B.Liang et al./Journal of Power Sources 267(2014)469e 490479

suppressed electrolyte decomposition by the use of cross-linked PAA.The electrode adhesion strength onto current collector was increased by the cross-linking of PAA with PCD.The Si-composite electrode with optimal amount of 1wt%PCD cross-linker delivers more than 1000mAh g à1of reversible capacity with improved capacity retention.

Inspired by Bionics applications,Murase et al.examined and compared three different natural products polysaccharides (amylose,amylopectin,and glycogen)as binders for Si-based electrodes [152].Yushin et al.showed that mixing SiNPs with alginate,a natural polysaccharide extracted from brown algae,could yield stable Si nanoparticle anodes [136].The ideal binder for Si should have a weak binder e electrolyte interaction,easy access to the Si and the ability for building stable SEI ?lm.Alginate appears to combine all of the above advantages.Therefore,it signi ?cantly improved the electrochemical performance of the Si anode.At a current density of 4200mA g à1,the reversible capacity is in the range of 1700e 2000mAh g à1.Coulombic ef ?ciency approaching 99.9%was observed during cycling of the alginate-based Si anode,thus the alginate binder might also be helpful in building a stable passivating SEI layer.

Choi and coworkers reported mussel-inspired binders for high-performance SiNPs anodes [153].In the case of the catechol-conjugated binders,the robust contacts promote ef ?cient elec-trical conducting pathways within the electrodes.Taking note of the rigidity of polymer backbone in retaining the capacity of the Si electrodes during cycling,they conjugated adhesive catechol functional groups to well-known poly(acrylic acid)(PAA)and alginate backbones with high Young 's moduli (Fig.14).The Coulombic ef ?ciencies after 400cycles were 99.1%for Si e Alg e C.In addition,Si e Alg e C also showed good rate capability.Si e PAA e C showed a capacity increase (220mAh g à1)over the entire cycle range compared to that of Si e PAA.

Contrasting other polymer binders,a tailored electronic struc-ture of the new polymer enables lithium doping under the battery environment.The polymer thus maintains both

electric

Fig.14.(a)Structural formula of Alg e C and PAA e C alongside a simpli ?ed structure of a conjugated polymer binder;the black solid line represents the polymer backbone with carboxylic acid functional groups attached and red circles represent catechol moieties conjugated to the backbone;(b)a graphical illustration of the SiNP anode structure [153].(For interpretation of the references to color in this ?gure legend,the reader is referred to the web version of this

article.)

Fig.15.Schematics of the technical approaches to address volume change issue in battery materials.(a)Traditional approaches use acetylene black (AB)as the conductive additive and PVDF polymer as mechanical binder;(b)conductive polymer with dual functionality,as a conductor and binder,could keep both electrical and mechanical integrity of the electrode during the battery cycles;(c)the molecular structure of the PF-type conductive polymers,with two key function groups in PFFOMB,carbonyl and methylbenzoic ester,for tailoring the conduction band and for improving the mechanical binding force,respectively [154].

B.Liang et al./Journal of Power Sources 267(2014)469e 490

480

conductivity and mechanical integrity during the battery operation, holding the promise of dual functionality of both binder and conductive additive.Liu et al.developed a new conductive binder to address the volume change problems in high capacity through a combination of advanced materials synthesis,spectroscopic anal-ysis and theoretical simulations,as shown in Fig.15[154].The in-tegrated experimental and theoretical results showed that the developed polymer features improved electronic conductivity and robust mechanical binding forces,which simultaneously main-tained electrical connectivity and accommodated the Si volume https://www.wendangku.net/doc/058497296.html,posite electrodes based on Si particles and poly(9,9-dioctyl?uorene-co-?uorenone-co-methylbenzoic acid(PFFOMB) binder,without any conductive additive,exhibited high capacity, long-term cycling,low over potential between charge and discharge,and good rate performance.The same group developed a multifunctionality binder polymer to maintain high electronic conductivity,mechanical adhesion,ductility,and electrolyte uptake [155].These critical properties were achieved by designing poly-mers with proper functional groups PEFM,as shown in Fig.16. Cycling of Si particles with a full capacity of3750mAh gà1was demonstrated from the multifunctionality conductive polymer binder.Table2summarized recent reports on Si/polymer based electrodes for lithium-ion battery devices.

5.Si-based alloys/composites

Si-germanium(Ge)alloys have been extensively studied for application in the semiconductor industry.Recently,Ge was stud-ied as energy storage materials due to its high electronic conduc-tivity,ionic diffusivity and capacity by several groups[156e160].It has been found that Ge acts as a buffer against the volume change of the Si and contributes to the elevating lithium ion diffusion[161]. Si0.41Ge0.34Mo0.25nanocomposites with high energy density(1st charge,1193mAh gà1),long cycle ability(~870mAh gà1over100 cycles),and high initial coulombic ef?ciency(~96%)were prepared by Hwang et https://www.wendangku.net/doc/058497296.html,ing a magnetron sputtering method[162].Song developed a Si/Ge double-layered electrode based on nanotube heterostructure array,consisting of a high-capacity Si inner layer and a highly conductive Ge outer layer,to yield both favorable mechanics and kinetics in lithium ion battery applications[29].The electrode exhibited improved electrochemical performances over the analogous homogeneous Si system,such as higher capacity retention after50cycles at a3C rate.These results stem from reduced maximum hoop strain in the nanotubes supported by theoretical mechanics modeling,and lowered activation energy barrier for Li diffusion.Hashimoto et al.prepared a Li4.4Si(1àx)Ge x composite using ball milling process,which was applied as an active material in all-solid-state batteries[163].Wang et al.inves-tigated Si(1àx)Ge x that was sputtered onto a copper nanowire array and found that capacity retention varied with its composition,with

P E F M

Fig.16.The molecular structure of the functional groups incorporated in polymer

binder,P:poly?uorene with octyl side chains;E:?uorene with triethyleneoxide

monomethyl ether side chains;F:?uorenone;M:methyl benzoate ester[155].

Table2

Electrochemical performance of Si/polymer composite anodes for lithium-ion batteries.

Anode Speci?c

capacity

(mAh gà1)

Cycling stability Synthesis methods Reference

Si/PPy1800Speci?c capacity90%of the

initial capacity after10cycles

High-energy mechanical milling[118]

Si/PPy e568mAh gà1capacity of the

50%Si/50%PPy composite

electrode after10cycles.

Chemical polymerization[119]

Si/PPy25901000mAh gà1discharge

capacity in the10th cycle

In-situ polymerization[120]

Si/PPy3076~91%reversible capacity

remaining after100cycles

In-situ polymerization[121]

Si/PPy/Ag1816823mAh gà1after100cycles

0.2mA cmà2

Chemical polymerization method[122]

Si/PANi19401807mAh gà1after25cycles

with a capacity fading rate of

0.3%per cycle

Chemical polymerization process.[124]

Si/PEDOT:PSS936768mAh gà1after80cycles,

with an initial capacity fade of

0.48%per cycle

In-situ chemical polymerization[133]

Si/PEDOT:PSS1947539mAh gà1after100cycles,a

reversible capacity of450mAh gà1

at a current rate of6000mA gà1.Magnesiothermic reduction of mesoporous

SiO2,coating with S-containing polymer(PEDOT),

and post-carbonization

[134]

Si/Alginate30401200mAh gà1Si after1300cycles A high-modulus natural polysaccharide

extracted from brown algae.

[136] Si/PCD1000Capacity retention75%after30cycles Cross-linked poly(acrylic acid)(PAA)with

polycarbodiimide(PCD)utilized as a binder

[151]

Si/PFFOMB20501400mAh gà1(2100mAh gà1for Si)

after650cycles Dispersing Si nanoparticle in the conductive

polymer chlorobenzene solution

[154]

Si/PEFM3650The PEFM binder effectively maintained

stable capacity over the50-cycles 100%utilization of Si material embedded in

the PEFM binder

[155]

SiNW/TMV1cys33431656mAh gà1at4C,0.20%loss per

cycle at1C Metal coatings on patterned3D TMV1cys

templates used as a substrate

[180]

B.Liang et al./Journal of Power Sources267(2014)469e490481

the most optimal composition of Si0.6Ge0.4[164].In contrast with these deposition techniques,Mullins et al.synthesized nano-structured amorphous thin?lms by glancing angle deposition (GLAD)[165].The electronic conductivity increases while the spe-ci?c capacity decreases as germanium content increases.Addi-tionally,the high-rate performance of the material increased substantially with increasing germanium content.However,the high cost and low abundance of germanium are hurdles to its generalization[166].Very recently,Geaney et al.grew Si/Ge axial heterostructure nanowires with high yield using a versatile wet chemical approach[167].Heterostructure nanowires growth was achieved using the vapor zone of a high boiling point solvent as a reaction medium with an evaporated tin layer as the catalyst.The precise tuning of the heterostructure nanowires interface would allow for exact determination and control of transport properties at the atomic level and greatly enhance their application in electronic devices.

Sakaguchi et al.[168]synthesized TiO2/Si composites by sol e gel method and evaluated the electrochemical properties of the thick-?lm electrodes using the composites.The remarkably improved performance was obtained for the composite electrodes of TiO2/Si (43/57wt%):the discharge capacity of710mAh gà1could be ach-ieved at the900th cycle.

Kim and coworkers synthesized nanocomposites of Si and tita-nium nitride(TiN)by using high-energy mechanical milling[169]. Si particles were homogeneously distributed inside the TiN matrix. The electrochemical activities of the obtained lithium ions nano-composites were investigated,in which TiN provided structural stability.Zhou et al.synthesized a unique heteronanostructure that consists of two-dimensional TiSi2nanonets(NNs)and particulate Si coating(Fig.17(a))[170].The high conductivity and structural integrity of TiSi2nanonets core were proved as great merits to permit reproducible lithium ions insertion and extraction into and from the Si coating.Nanoscale Si particles are attached to the NNs as the medium to react with lithium ions.This heteronanostructure was tested as the anode material for lithium ions storage.At a charge/discharge rate of8400mA gà1,the electrode delivered speci?c capacities of over1000mAh gà1and showed only an average of0.1%capacity fade per cycle between the20th and the 100th cycles(Fig.17(b)).The good performance,high capacity,long capacity life,and fast charge/discharge rate made it one of the best anode materials that have been reported.The remarkable perfor-mance was attributed to its capability to preserve the crystalline of the TiSi2core during the charge/discharge process.It was also found that there was a large capacity fade in the?rst10cycles.The possible reason is that TiSi2backbone alloyed with Li ions at a potential of60mV,which could destruct the backbone structure and result in the fast capacity loss.The cycling stability can be improved by applying a narrower charge/discharge voltage win-dow to exclude the TiSi2reaction.Mesoporous TiO2has also been tested as a Li battery anode at high rates of10C and20C.However, the capacities were relatively low(~140mAh gà1and~100mAh gà1 at the respective C-rates[171]).In addition,the preparation process of the above composite materials was too complex and unfavorable for its commercial application.

Park et al.reported a simple synthesis of Ti x Si y coated SiNPs via a silicothermic reduction process,in which Si acted as the reducing agent and titanium oxide as a source material of Ti[172].The synthetic approach of Ti x Si y-coated SiNPs is brie?y described in Fig.18(a).The Ti x Si y-coated Si electrodes showed signi?cantly improved high thermal stability compared to bare Si electrodes. When a cell was heated above a certain temperature,exothermic reactions between the electrodes and the electrolyte took place and led to an increase of the internal cell temperature.If the generated heat is greater than the energy that can be dissipated,the cell temperature will increase rapidly,accelerating chemical reactions, eventually resulting in the thermal runaway of batteries.As depicted in Fig.18(b),the Ti x Si y coating layer can effectively miti-gate various exothermic reactions caused by the thermal decom-position reactions of an electrolyte with lithiated Si at elevated temperatures.Thus,the titanium silicide enhanced the electrical conductivity of SiNPs and provided a highly stable solid electrolyte interface layer during the cycling.As a result,high reversible ca-pacity(1470mAh gà1)and high rate capability(1150mAh gà1at 20C rate)were demonstrated(Fig.18(c),(d)).

Ohara et al.reported a Si?lm deposited on a Nickel(Ni)sub-strate[64].The cycle life was signi?cantly improved because Ni develops a passivating layer that acted as an excellent binding agent between the substrate and the Si?lm,which is essentially due to the strong af?nity of Si to oxygen.These cells showed ca-pacities of as high as1700e2200mAh gà1for750cycles at a2C charge/discharge https://www.wendangku.net/doc/058497296.html,ter,nanocrystalline Si-embedded Ni e Ti composite anode materials were synthesized using a two-stage high-energy mechanical milling(HEMM)method by Soo et al. [173].Cells with10h milled powders showed relatively low ca-pacity fading,which was attributed to the nanocomposite

structure

Fig.17.(a)Schematic of the Si/TiSi2heteronanostructure.SiNPs are deposited on highly conductive TiSi2nanonets to act as the active component for Li storage;(b)a low-mag TEM picture manifests the particulate nature of the Si coating on TiSi2nanonets;(c)charge capacity and coulombic ef?ciency of the Si/TiSi2heteronanostructure with8400mA gà1 charge/discharge rate tested between0.15and3.00V[169].

B.Liang et al./Journal of Power Sources267(2014)469e490

482

that comprised of Si nanocrystals embedded in the amorphous Ni e Ti matrix phase.

Sakaguchi et al.[174]coated Si particles with Ni,Ni e Sn,and Ni e P by using the electroless deposition (ELD)technique,and fabricated thick-?lm electrodes by the gas deposition (GD)method using the coated-Si particles.The electrode of Ni-coated Si had a discharge capacity of 580mAh g à1at the 1000th cycle.The elec-trode of Si coated with Ni 3P with a lower coating amount exhibited higher initial capacity and excellent cycling performance with a

capacity of 750mAh g à1at the 1000th cycle.The excellent per-formance in the case of the Ni 3P coating can be attributed to the smaller amount of coating,its high elastic modulus,and the mod-erate reactivity of Ni 3P with Li.

Zhang et al.reported the fabrication and characterization of a nanoscale Ni metal scaffold supported bicontinuous 3D Si anode [175].This structure provided both conductivity pathway and me-chanical support,which maintained a good electrical connectivity and stable structure during cycling.The initial capacity of

the

Fig.18.(a)Schematic illustration preparing Ti x Si y layer on the surface of Si particles;(b)schematic illustration showing the thermal decomposition reactions of an electrolyte with lithiated Si at elevated temperatures;(c)cycling performances of both electrodes are obtained at 0.1C (?rst cycle)and 0.2C (from second cycle);(d)rate capabilities of Ti x Si y -coated Si (solid circle)and bare Si (solid square)were obtained at 0.2C e 20C rates [172].

B.Liang et al./Journal of Power Sources 267(2014)469e 490483

bicontinuous Si anode was3568(Si basis)and1450mAh gà1 (including the metal framework)at0.05C.85%of the capacity remained after100cycles at0.3C.Although the3D porous scaffold could eliminate the usage of the current collector foil and decrease the total mass of inactive parts,Ni is a relatively dense metal and additional research on lightweight electrochemically stable conductive3D scaffolds may further advance this concept.

There is a growing interest in harnessing the viruses for use in energy-storage device[176,177].The experiment revealed that the virus is electro-chemically inactive and stable.Different from pre-viously reports on the synthesis of nanomaterials using biological templates where the active materials were mixed with binding and conductive agents and cast on current collectors,Wang et al. directly fabricated a Ni-core/Si-shell nanowire array electrode us-ing a virus template[178].They developed Tobacco mosaic virus (TMV)as a novel bioinorganic template that can be easily patterned on metal substrates to form nanoscaled3D structures[179].In this work,SiNW anodes were constructed by electrochemically depositing Si on a3D current collector of self-assembled and Ni-coated TMV1cy array[180,181].The virus-assembled nano-struc-tures produced a13e80fold increase in reactive surface area.The nano-structured3D Si anodes provided high capacities (3300mAh gà1),good charge e discharge cycling stability(0.20% loss per cycle at1C),and excellent rate capabilities(46.4%at4C) [180].They also constructed Si anodes using an electrochemical deposition method on the TMV1cys/Ni nanowire current collectors. The electrochemical deposited TMV1cys/Ni/Si structure provided high capacity(2300mAh gà1)as well as improved stability (>1200mAh gà1at173cycles)with a high columbic ef?ciency (99.5%).

Copper(Cu)coating on Si has also been reported to enhance the electrical conductivity of Si anodes,which is due to the formation of a thin layer of Cu3Si alloy at the interface of Cu and Si[182]. Increased coulombic ef?ciency and cycling stability have been demonstrated on the Cu-coated Si anodes compared to carbon-coated Si electrodes[183,184].Joyce et al.investigated the in?u-ence of copper coating as both a binder and conductive additive on the electrochemical performance of Si electrodes[185].They found that copper coating layer formed a metallic net on Si and identi?ed the existence and the electrochemically activity of the interfacial layer of silicides.A unique,scalable chemical approach for syn-thesizing copper-coated amorphous Si particles(Cu-coated a-Si:H) through a polyol reduction method was reported by Stevenson et al.[186].The Cu-coated a-Si:H particles exhibited signi?cantly enhanced lithium storage capacity over pristine a-Si:H particles in about sevenfold.They attributed the enhancement to the improved charge transfer kinetics by the copper coating.High charge storage capacity and improved cycling stability was observed.

Vlad et al.demonstrated an operational full cell3.4V lithium-polymer Si nanowire(LIPOSIL)battery which is mechanically ?exible and scalable to large dimensions[187].As shown in Fig.19, an electroless growth protocol was developed to wrap the Si nanowires with a thin porous,electrically interconnected Cu layer. The conformal Cu-wrapped Si nanowires showed an

improved

Fig.19.(a)Conformal Cu coating of high aspect ratio Si nanowires with coaxial morphology through an electroless deposition protocol;(b)snapshots at different height positions evidencing the uniform Cu coating along the Si nanowires;(c)schematic representation of the Cu-wrapped Si nanowires.The Cu shell has a porous,electrically interconnected structure to allow for a faster Litinsertion,volume expansion accommodation and ef?cient current collection;(d)discharge/charge pro?les for the Si-core@Cu-shell composite anodes cycled between1.5and0.02V;(e)capacity retention at a cycling rate of C/20of the Si and Si-core@Cu-shell polymer electrolyte composite anodes;(f)rate capability of the Si and Si-core@Cu-shell polymer electrolyte composite anodes.The theoretical capacity of graphite is highlighted for comparison[187].

B.Liang et al./Journal of Power Sources267(2014)469e490

484

capacity retention and rate capabilities as compared to pristine nanowires when integrated into LIPOSIL architecture.The obtained Si-core@Cu-shell nanowires displayed enhanced electrochemical performances due to improved current collection ef?ciency and Si encapsulation.The porous Cu was used to stabilize the electrodes over extended cycles and provided ef?cient current collection.The ?rst discharge capacity of3900mAh gà1was higher than that of pristine Si nanowires and the composite could sustain a capacity of 2000mAh gà1over extended cycling with little capacity decay.The Cu coating rules out the conductivity limitations and enables lith-iation of the entire Si mass not only at the?rst discharge,but also during the subsequent cycling.

Anode performances of LaSi2/Si composites were studied in ionic liquid electrolytes[188].The LaSi2/Si electrode exhibited much better performance,with reversible capacity at250th cycle and its retention were800mAh gà1and80%.This performance is attributed to a higher stability of cations in the ionic liquid and an easier des-olvation of Li ions and the anions.Kohandehghan et al.synthesized magnesium(Mg)-and magnesium silicide(Mg2Si)-coated SiNWs for Li-ion batteries[189].Both Mg-and Mg2Si-coated materials showed signi?cant improvement in coulombic ef?ciency.The large void space between the nanowire assembly and the substrate produced during cycling makes nanowires lose electrical contact with the substrate,which has been accepted as a main degradation mecha-nism of electrodes.An overview of the characteristics of various Si/ metal based electrode materials is given in Table3.

6.Understanding of the performance improvement in

silicon-based composites

Study of the internal stress/strain distribution and materials failure mechanisms in nano-Si has motivated intense research.The methodologies adopted for reducing the capacity loss in Si anodes and the challenges that remain in using Si-based anodes have been illustrated in the last few sections.A comprehensive understanding of the(de)lithiation mechanisms is among the key issues in designing and fabricating high-performance Si anodes[190,191]. Impressive results have been obtained by using in-situ Trans-mission Electron Microscopy(TEM)and?rst-principles simulation.

6.1.In-situ TEM

Understanding the microscopic mechanisms of electrochemical reaction and material degradation is crucial for the design of high-performance Si anodes for lithium ion batteries(LIBs).A novel nanobattery assembly and testing platform inside a TEM has been designed,which allows a direct study of the structural evolution of individual nanowire or nanoparticle electrodes with near-atomic resolution in real time.The in-situ TEM technique allows for bet-ter understanding of volume changes in SiNWs electrodes[192]. Very recently,Huang et al.developed a nanoscale open cell elec-trochemical device[193],allowing real-time observation of the charging/discharging behavior of individual SiNW electrodes.They demonstrated ultrafast and full electrochemical lithiation of indi-vidual carbon coated SiNWs by direct real-time observation using in-situ TEM in ionic liquid electrolytes(Fig.20).The SiNWs didn't fracture despite the ultrahigh lithiation rates and~300%volume expansion.Such anisotropic expansion was consistent with Cui's ?nding[194],and was attributed to the interfacial processes of accommodating large volumetric strains at the lithiation reaction front that depend sensitively on the crystallographic orientation. The same group studied the morphological changes in SiNWs after electrochemical cycling,and observed interesting stepwise surface roughening and nanopore evolution[195].The study also helped explain capacity loss in Si anodes in general.In another work,the effect of metallic coatings on Si expansion was studied[55].

McDowell et al.studied the lithiation kinetics in c-SiNPs with different initial diameters using in-situ TEM[196].They measured the

Table3

Electrochemical performance of Si/metal anodes for lithium-ion batteries.

Anode Speci?c

capacity

(mAh gà1)

Cycling stability Synthesis methods Reference

Si/Ge1544.6Capacity retentioning85%

after50cycles and doubled

capacity at3C rate Template-assisted synthesis method

based on chemical vapor deposition process

[29]

Si/Ni1700e22001500mAh gà1after700

cycles at2C rate

Vacuum evaporation method[64]

Multi-layered Si/Ge1981Retention rate(49.3%)100

cycles

Magnetron sputtering[161]

Si0.41Ge0.34Mo0.251193~870mAh gà1over100

cycles

Magnetron sputtering method[162]

Cu e Si0.6GE0.439541506mAh gà1after75cycles,

with the retention of76.9%

Sputtered Si(1àx)Ge x onto copper nanowires[164]

Si/TiSi2nanonets1990937mAh gà1at the100th

cycle,>99%capacity retention

per cycle over100cycles

Chemical vapor deposition[170]

Si/Ti x Si y14701430mAh gà1at90cycles,

capacity retention87.8%at

20C

Silicothermic reduction[172]

Si/Ni35682660mAh gà1after100cycles,~85%

capacity remaining after100cycles at

0.3C

Chemical vapor deposition[175]

Ni/Si1130580mAh gà1at the1000th cycle Electroless deposition followed by gas deposition[174] Ni3P/Si1590790mAh gà1at the1000th cycle Electroless deposition followed by gas deposition[174]

SiNW/TMV1cys/Ni23001200mAh gà1after173cycles,0.25%

capacity decreased per cycle Electrodeposition on a virus-structured current

collector

[178]

SiNW/Cu39002000mAh gà1over extended cycling

with little capacity decay

Roll up nanowire battery from silicon chips[187]

TiO2/Si2000710mAh gà1at the900th cycle,1870

mAh g(Si)à1at a current rate of4.8C

Sol e gel method[168]

LaSi2/Si3300mAh g(Si)à1800mAh gà1at250th cycle,80%retention.Mechanical alloying method[188]

B.Liang et al./Journal of Power Sources267(2014)469e490485

positions of the reaction front during lithiation.This work inspired another group to combine experimental and theoretical study of the lithiation kinetics in individual SiNWs,as shown in Fig.21[197].The results provided the quantitative data of lithiation kinetics in SiNWs and revealed the self-limiting behavior of lithiation,which is attrib-uted to the stress-retardation effect.The same group studied the physical and chemical transformations during the Li e Si reactions [198].The results showed that amorphous Si has more favorable ki-netics and fracture behavior when reacting with Li than does crys-talline Si,making it advantageous to use in battery electrodes.

Lithiation of crystalline SiNWs leads to highly anisotropic mor-phologies.This has been interpreted as due to anisotropy in equi-librium interface energies,but this interpretation does not capture the dynamic,nonequilibrium nature of the lithiation process.The volume expansion during initial lithiation of c-Si is highly aniso-tropic [36,191,194,199]with the <110>orientation growing at a much faster rate than other low-index orientations,which will cause stress concentration and fracture in certain directions.Ex-periments also indicated that the lithiation reaction front (RF)is atomically sharp (~1nm)[36,200]and progresses linearly with time [36].Although the lithiation behavior of Si has been under-stood,lithium ion transport behavior across a network of Si and carbon is still unclear.Wang and coworkers probed the lithiation behavior of SiNPs attached to and embedded in a carbon nano ?ber using in-situ TEM,and found that aggregated SiNPs show contact ?attening upon initial lithiation,which is characteristically analo-gous to the classic sintering of powder particles by a neck-growth mechanism [201].

6.2.First-principles simulation

Analysis of the delithiation and relithiation processes provided insights into the underlying physics of the lithiation-delithiation

process,thus providing ?rm conceptual foundations for future design of improved Si anodes for Li ion battery applications.First-principles calculations based on density-functional theory (DFT)is widely used in investigating the electronic structure and study-ing a wide range of different atomic systems,including molecules,surfaces,nano-structures and bulk materials.The mechanism of lithium insertion and the interaction between Li and Si electrode must be quantitatively understood at the atomic level by ?rst-principles simulation.

Chevrier et al.developed the protocol to simulate the molecular dynamics of Si lithiation at room temperature while avoiding the lengthy diffusion dynamics [21,202].However,Chan et al.tested the insertion of Li into different interstitial sites and found that in only approximately 20%of all cases,the site farthest away from existing atoms is the most energetically favorable site after relaxation [203].Tritsaris et al.found that both the energy barriers for diffusion and the topology of the atomic structure control the ion diffusion.They also established that not all of the diffusion pathways participate equally in mediating the ?ow of Li atoms in the material,even if all energy barriers were assumed to be equal [204].

Experimental studies have provided strong evidence for the formation of various stable Li e Si crystalline phases during high-temperature lithiation [36,197,205].The equilibrium titration curves of the Li e Sn and Li e Si systems are shown superimposed in Fig.22[205].It can be seen that the phase Li 2.6Sn is stable over a range of potential,including that at which the reaction Li tSi ]Li 1.71Si takes place,approximately 0.338V vs.Li.as shown in Fig.22.Many simulations have been proposed to study the structures of these crystalline phases [206e 208].In contrast to high-temperature lithiation,room-temperature Si lithiation frequently leads to amorphous lithium silicide (a-Li x Si),which is a particularly important phase transition and structural change [209].Using ab initio schemes,the amorphization process and structural

evolution

Fig.20.(a)Schematic illustration of the electrochemical device;(b)simulated Li and stress distributions in a [111]-oriented Si nanowire;(c)morphology evolution of the Si nanowire during lithiation in the solid cell;(d)anisotropic swelling and crack formation of Si nanowire during lithiation [36].

B.Liang et al./Journal of Power Sources 267(2014)469e 490

486

of Li x Si compounds can be studied,and many works have focused on this issue [20,210].

Using ?rst-principles calculations,several groups investigated the characteristics of Li insertion into c-Si [203],but the reaction mechanism involving the progression of the sharp RF was not considered in these studies [211e 213].In an attempt to rationalize the experimentally observed behavior,two different groups [203,214]have recently studied the energetics of equilibrium structures at the <110>,<111>,and <100>interfaces between c-Si and amorphous Si (a-Si),showing that the ?rst of these interfaces was signi ?cantly more stable than the other two.However,since lithiation of Si is a nonequilibrium process,thermodynamically stable states are not expected to form during lithiation [196,215,216].Accordingly,arguments based on relative energies of equilibrium structures cannot capture the essence of the observed behavior;instead,kinetic rates and the dynamic evolution of the structure are crucial for understanding the lithiation process.Cubuk et al.provided a comprehensive explanation of experi-mentally observed morphological changes identifying reaction

paths and associated structural transformations for Li insertion into the Si <110>and <111>surfaces and calculated the relevant energy barriers from density functional theory methods [217].Then they performed kinetic Monte Carlo simulations for nanowires with surfaces of different orientations,which resembled the experi-mentally observed pro ?les and the relative reaction front rates to a remarkable degree.To provide insight into the pertinent kinetic processes,Pharr et al.presented an experimental study quantifying the kinetics of the initial lithiation of crystalline Si [218].The ex-periments identi ?ed the existence of a moving phase boundary for <100>,<110>,and <111>orientations,indicating that short range processes at the a-Li h Si/c-Si interface signi ?cantly contribute to the kinetics of the lithiation process,as shown in Fig.23.

Besides employing DFT to examine Li incorporation in nano-structure Si [21,42,203,207,219]and grapheme [220e 222],the lithiation behavior of Si/graphene composites was also examined [223].Chou et al.demonstrated charge transfer from Li to both Si and C (in graphene):the excess electrons on graphene create an electric ?eld,which attracts Li cations while repels Si anions and thus results in a distinct alternative Li e Si layering structure near graphene.The facile interfacial Li ions diffusion contributed to high performance anodes with fast charge/discharge rates.However,the presence of graphene tends to have no signi ?cant impact on the structural evolution of Si during lithiation,as Li atoms are mostly incorporated in the Si matrix rather than at the Si/graphene interface.

Nuclear Magnetic Resonance (NMR)and Auger Electron Spec-troscopy (AES)have also been used successfully to observe different Li environments in Si [142,224e 226]at many stages of the (de)lithiation process.

7.Summary and perspective

This review has illustrated Si-based anodes in lithium batteries.Si-based anodes have considerable potential for improvement in charge capacity,coulombic ef ?ciency and capacity retention over conventional graphite-based anodes.However,signi ?cant chal-lenges need to be overcome before Si anodes can be utilized in practical lithium batteries.The challenges include the control of

Si-

Fig.21.In-situ lithiation of SiNWs.Modeling of stress generation and self-limiting lithiation in a SiNW [197]

.

Fig.22.Equilibrium titration curves of the Li e Sn and Li e Si systems at 415 C [205].

B.Liang et al./Journal of Power Sources 267(2014)469e 490487

based electrodes that exist and operate in a state far away from equilibrium,particularly in charged state,and the accommodation of large volumetric and/or structural changes over many charge/discharge cycles.Nanoscale morphologies have the potential to achieve long cycling lifetimes and good reversibility if stress management and stable passivation layer formation during cycling can be properly addressed.

Using conductive polymer elastomer for build-up of a coated Si anode could assure the long term stability of the anode.In addition,exterior shell can allow free expansion of the anode while retains electrical contact and serves as a barrier from further electrolyte decomposition.Carbon/alloy-Si composites have also shown immense promise.Carbon/alloy can provide a stable SEI and accommodate the stress resulted from the Si volume expansions,while Si can ensure the high capacity due to larger lithium uptake.One dimensional nanostructured materials such as NTs and NWs exhibit a signi ?cant improvement of electrochemical performance due to the short diffusion lengths and better accommodation of large volume changes.

Further research is needed to improve high-rate performance in order to make these batteries feasible for high power applications.Finally,researchers will need to look for ways to extend the cycle life of the electrodes since longevity will be a prime requirement for lithium-ion batteries to enter the commercial market.

Acknowledgments

The authors sincerely appreciate the ?nancial support given by the National Natural Science Foundation of China (50803008),Natural Science Foundation of Hunan (11B001,14JJ4035),Key Lab-oratory for Power Technology of Renewable Energy Sources (2011KFJJ006),the State Key Laboratory of Luminescent Materials and Devices at South China University of Technology (2013-skllmd-08),China Postdoctoral Science Foundation (20100480946,201104508).

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For-next循环

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10、具有三种振荡模式：线形、圆形、8字形，可设定震荡速度、振幅及振荡时间。具有仪器外（outside）振荡功能，在程序运行的过程中，微孔板可以伸出仪器外部振荡，便于实现程序运行中观察振荡效果，而不必中止程序。 11、具有板孔扫描功能：可选孔内圆形或方形区域中的多点扫描检测，适用于贴壁细胞或不均匀样本检测，以减少因样品分布不均匀造成的检测偏差。软件可自动优化调节检测器Z轴高度，以保证检测的灵敏度，减少孔间信号串扰。 12、配备专业仪器自动化控制及数据分析处理软件，软件友好，易学易用。具备线性拟合、动力学、剂量效应等多种常用的数据计算及分析功能，结果可以Excel、文本、网页、图片等多种格式输出。 三、技术服务要求 3.1设备安装调试 在用户指定的地点完成安装调试，并配合用户进行测试验收。 3.2 技术培训及服务 3.2.1完成设备现场安装调试和验收后，在用户所在地免费提供专业培训，就 设备的操作使用和保养维护等内容进行重点培训。 *3.2.2 要能提供原厂AlphaScreen/AlphaLISA检测试剂以及时间分辨荧光检 测试剂，并具有专业技术开发实验室及服务能力，并由应用技术工程师提供蛋 白-蛋白等分子间相互作用实验的现场操作培训以及后期新实验开发培训。 3.3质保期 整机保修1年，保修期自验收签字之日起计算。 3.4维修响应时间 接到维修通知后，2小时内作出响应，24小时内到场排除故障。

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多功能酶标仪SpectraMax i3的标准操作规程Standard Operation of SpectraMax i3 部门Department 签名/日期Signature/Date 起草人：Prepared by 樊小川，Xiaochuan Fan QC 审核人：Reviewed by 黄思佳，Sijia Huang QC 审核人：Reviewed by 褚夫兰，Fulan Chu QA 批准人：Approved by 张伯彦，Boyan Zhang 质量负责人 1 目的 建立多功能酶标仪SpectraMax i3的标准操作程序，规范SpectraMax i3 多功能酶标仪检测时的操作。 2 适用范围

本规程适用于所有对多功能酶标仪SpectraMax i3的操作 3 术语或定义 多功能酶标仪：指功能较强、精度较高的单体台式酶标仪，可检测吸光度（Abs)、荧光强度(FL)、时间分辨荧光（TRF)、化学发光（Lum)等。 4 责任 4.1 仪器负责人负责多功能酶标仪的日常及定期维护，出现故障时负责联系厂家维修，保 证运行正常。 4.2 实验操作人员需严格按照本规程执行，保证仪器正常使用，每次用完后填写仪器使用 记录，且使用后及时清理台面，保持仪器洁净。 5 EHS要求 N/A 6 程序 6.1 仪器安装 仪器与电脑连接完毕并连接电源以后，按仪器背面的按钮可以直接启动仪器，经过几分钟后的仪器自检后就可以开始用于检测。在连有电源的情况下，保持24小时开机，不要罩防尘罩以保持透气，隔1-2月关机重启一次。 6.2 SoftMax Pro软件操作基本步骤 6.2.1 打开软件 点击“SoftMax Pro 6.3”图标，打开SoftMax Pro 软件，出现"Plate Setup Helper"对话框。若无则点击图标。 6.2.2 连接仪器 点击‘Choose a diffecient instrument’，打开‘Instrument Connection’对话框，在“Availible Instruments”菜单中选择仪器连接线与电脑所接的串口号(COM)。 选择所购买的酶标仪型号SpectraMax i3或添加模块卡盒型号。最后点击“OK”键连接。 6.2.3 仪器检测参数设定 点击图标后出现“Settings”对话框，对话框最上方出现Read Mode（读板模式）和Read Type（读板类型）两个选项，读板模式有ABS(光吸收)、FL(荧光强度)、LUM(化学发光)和TRF(时间分辨荧光)，读板类型有终点检测（Endpoint）、动力学（Kinetic）、单孔扫描（Well Scan）和光谱扫描（Spectrum）四种。使用者根据实验

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２）能力目标：培养学生分析问题，解决问题的能力。 ３）情感目标：激发学生学习热情，培养学生学习的积极性。 二、教学过程 1、创设问题情境 师：同学们，请先看这个图形(画5个竖行排列的“*”)，想想看用以前学过的程序设计语言怎样来编写它的程序呢？(本节程序均设置为单击命令按钮cmdstart运行即代码加在private sub cmdstart_click()) 生（稍做思考，然后回答）：使用PRINT语句 PRINT “*” PRINT “*” PRINT “*” PRINT “*” PRINT “*” 师：同学们做得很好，那么，我想画10行，100行，1000行“*”呢？难道就这样顺序写下去吗？这样编写是不是太繁琐了。如果能让计算机去完成这部分重复的内容，而我们只要告诉计算机重复操作的次数就可以了，这个愿望能否实现呢？能!通过我们今天学习的FOR/NEXT循环语句，就可以很容易的实现这个愿望。 [疑问是建构教学的起点。新课伊始，就提出一个真实的问题，力求创设一种教学情境，它可以激起学生的未知欲，有利于建立新的认识结构。] 2、给出程序，并通过流程图加以理解 师出示上题程序代码并通过流程图和卡通图片分析

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光源灯发出的光波经过滤光片或单色器变成一束单色光，进入塑料微孔极中的待测标本.该单色光一部分被标本吸收，另一部分则透过标本照射到光电检测器上，光电检测器将这一待测标本不同而强弱不同的光信号转换成相应的电信号，电信号经前置放大，对数放大，模数转换等信号处理后送入微处理器进行数据处理和计算，Z后由显示器和打印机显示结果。 微处理机还通过控制电路控制机械驱动机构X方向和Y方向的运动来移动微孔板，从而实现自动进样检测过程。而另一些酶标仪则是采用手工移动微孔板进行检测，因此省去了X，Y方向的机械驱动机构和控制电路，从而使仪器更小巧，结构也更简单。 微孔板是一种经事先包理专用于放置待测样本的透明塑料板，板上有多排大小均匀一致的小孔，孔内都包埋着相应的抗原或抗体，微孔板上每个小孔可盛放零点几毫升的溶液。其常见规格有40孔板，55孔板，96孔板等多种，不同的仪器选用不同规格的孔板，对其可进行一孔一孔地检测或一排一排地检测。 酶标仪测定是在特定波长下，检测被测物的吸光值。随着检测方式的发展，拥有多种检测模式的单体台式酶标仪叫做多功能酶标仪，可检测吸光度(Abs)、荧光强度(FI)、时间分辨荧光(TRF)、荧光偏振(FP)、和化学发光(Lum)。 酶标仪从原理上可以分为光栅型酶标仪和滤光片型酶标仪。光栅型酶标仪可以截取光源波长范围内的任意波长，而滤光片型酶标仪则根据选配的滤光片，只能截取特定波长进行检测。 酶标仪的结构 酶标仪所用的单色光既可通过相干滤光片来获得，也可用分光光度计相同的单色器来得到。在使用滤光片作滤波装置时与普通比色计一样，滤光片即可放在微孔板的前面，也可放在微孔板的后面，其效果是相同的。光源灯发出的光经聚光镜，光栏后到达反射镜，经反射镜作90°反射后垂直通过比色溶液，然后再经滤光片送到光电管。 酶标仪可分为单通道和多通道2种类型，单通道又有自动和手动2种之分。自动型的仪器有X，Y方向的机械驱动机构，可将微孔板L的小孔一个个依次送入光束下面测试，手动型则靠手工移动微孔板来进行测量。

For—Next循环语句教学设计(初中信息技术精品)

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1) 了解循环的概念、理解循环结构的基本思想； 2) 掌握for…next语句的基本结构； 3) 理解for…next语句的执行过程； 4) 尝试采用循环结构编写简单的程序，解决实际问题。 2. 过程与方法： 经历分析、实践、讲解、探究、归纳，通过循序渐进、层层深入，逐步深化对循环思想和执行过程的理解。3. 情感、态度与价值观： 1) 通过一个个任务的实战演练，感知使用循环结构解决问题的便捷和优越，培养学生运用循环思想解决实际问题的能力，进一步激发学生学习编程的兴趣。 2) 通过在实际的问题中分析提炼循环结构，从程序设计领域进一步提升学生的信息素养。 四、教学重点、难点: 1) 教学重点：①掌握for…next语句的基本结构；②理解for…next语句的执行过程 2) 教学难点：根据需要采用循环结构解决实际问题，并提炼出for语句的基本结构。 五、教学方法：讲授演示法、对比分析法、任务驱动法、分层教学法等。 六、教学过程： （一）创设情景、激情导入

多功能酶标仪SpectraMax-i3的操作规程

多功能酶标仪SpectraMax-i3的操作规程编号 M-SOP-2-0083 No. 标准操作规程版本号 Standard Operating Procedure 01 Version 多功能酶标仪SpectraMax i3的标准操作规程 多功能酶标仪SpectraMax i3的标准操作规程 Standard Operation of SpectraMax i3 部门签名/日期 Department Signature/Date 起草人: XX， Xiaochuan X Prepared by QC 审核人: XX， Sijia XX Reviewed by QC 审核人: XX， Fulan X Reviewed by QA 批准人: XX， Boyan XX Approved by 质量负责人 颁发部门执行日期质量保证部-QA Issued by Effective Date 替换文件复审日期 Version 00 Replaced For Review Date 分发部门 QA，QC Distributed to 1 of 7 编号 M-SOP-2-0083 No. 标准操作规程版本号 Standard Operating Procedure 01 Version 多功能酶标仪SpectraMax i3的标准操作规程 1 目的 建立多功能酶标仪SpectraMax i3的标准操作程序，规范SpectraMax i3 多功能酶标仪 检测时的操作。 2 适用范围

本规程适用于所有对多功能酶标仪SpectraMax i3的操作 3 术语或定义 多功能酶标仪:指功能较强、精度较高的单体台式酶标仪，可检测吸光度(Abs)、荧 光强度(FL)、时间分辨荧光(TRF)、化学发光(Lum)等。 4 责任 4.1 仪器负责人负责多功能酶标仪的日常及定期维护，出现故障时负责联系厂家维修，保 证运行正常。 4.2 实验操作人员需严格按照本规程执行，保证仪器正常使用，每次用完后填写仪器使用 记录，且使用后及时清理台面，保持仪器洁净。 5 EHS要求 N/A 6 程序 6.1 仪器安装 仪器与电脑连接完毕并连接电源以后，按仪器背面的按钮可以直接启动仪器，经过几分钟后的仪器自检后就可以开始用于检测。在连有电源的情况下，保持24小时开机，不要罩防尘罩以保持透气，隔1-2月关机重启一次。 6.2 SoftMax Pro软件操作基本步骤 6.2.1 打开软件 点击“SoftMax Pro 6.3”图标，打开SoftMax Pro 软件，出现 "Plate Setup Helper"对话框。若无则点击图标。

For-Next循环语句

课题编制计算机程序解决问题 --For/Next循环语句 课时一课时 课型新授授课人韦开静授课时间2012.3.12 授课班级高一（7）学科信息技术 教材分析 循环结构是程序设计的三种基本结构之一，是程序设计的基础；它的主要应用方向是让计算机重复做大量相同或相似的事情。教材只是通过SIN函数引出了For/Next循环语句，并没有给出它的语法格式，及其语句的具体执行过程。我认为这样会导致一些学生进行简单模仿，而不是真正的掌握和理解。学生只有熟练掌握了For/Next循环语句的格式，理解循环执行过程，才能在实际应用中游刃有余。所以，本节课我们将学习For/Next循环语句。 学情分析 教学对象为高一的学生，对程序的接触不太多，前面的课程只讲了程序中的基本元素，初步了解了流程图的画法，但没有通过实际的编程来上机实践。所以，本节课从简单的实例着手，让学生搞清楚什么情况下要去使用循环结构，怎么样来使用它。 教学目标1、知识技能目标： ①掌握For/Next循环语句的格式 ②理解For/Next循环语句的功能和执行步骤 2、过程方法目标： ①能够分析简单的For/Next循环语句功能，尝试编写简单的For/Next 循环程序 ②培养学生分析问题，解决问题的能力。 3、情感态度目标： 感受用计算机程序解决问题的魅力，激发学生学习程序设计的兴趣。 重点掌握For/Next循环语句的格式与执行步骤 难点运用For/Next循环语句编制简单的计算机程序解决实际问题 教学方式讲授法、任务驱动法、小组协作 教学准备多媒体网络教室、PPT 教学过程 教学环节教师活动学生活动设计意图 复习编制计算机程序解决问题的基本过 程：分析问题→算法设计→编写程序 →调试运行→检测结果 回答问题 唤起学生记忆，为 新课做铺垫

多功能酶标仪常见配置及全参数

Infinite M200 PRO 1．一般参数： 支持的检测模式：光吸收、荧光顶部底部、时间分辨荧光（TRF）、连续发光、瞬时发光、双色发光、BRET、光吸收和荧光波长扫描 1.1 光源：UV高能闪烁氙灯，10^8次闪烁寿命，自带光强监测、校准，免维护无需预热 1.2 光栅杂光率：＜10^-6 1.3 支持板型：6-384孔板，预设常用品牌型号；自动扫描并定义特殊规格板型，NanoQuant微量检测板，4位卧式比色杯，立式比色杯（选配） 1.4 孔内多点读数（光吸收/荧光顶底）：6-384孔板，最多15×15个点（依板型模式可有不同），覆盖全孔，7种点布局 1.5 温度控制：环境温度+5℃至42℃（选配） 1.6 振荡：线性或圆形可选；振幅1-6mm，0.5mm步进；1-1000秒可调 1.7 多标记多模式检测：单个实验中可进行无限制的多波长、多模式检测（光吸收、荧光、发光、波长扫描） 1.8 最快读板速度：96孔板：20秒；384孔板：30秒 1.9 波长扫描速度（FI EX/EM）：150秒；450-550nm，5nm步进，96孔板 1.10 认证：DLReady，Transcreener Red FI 2.光吸收模块 2.1 波长范围：230-1000nm，1nm连续可调 2.2 波长选择：双光栅 2.3 带宽：＜5nm（λ≦315nm），＜9nm（λ＞315nm） 2.4 波长准确性：＜±0.3nm（λ≦315nm），＜±0.5nm（λ＞315nm） 2.5 波长重复性：＜±0.3nm（λ≦315nm），＜±0.5nm（λ＞315nm） 2.6 检测器：紫外硅光电二级管 2.7 检测范围：0-4 OD 2.8 检测分辨率：0.0001 OD 2.9 检测准确性：＜0.5% @260nm 2.10 检测精确性：＜0.2% @260nm 2.11 260/280nm精度：±0.07

酶标仪工作原理及使用方法

酶标仪简介： 酶标仪即酶联免疫检测仪是酶联免疫吸附试验的专用仪器又称微孔板检测器。可简单地分为半自动和全自动2大类，但其工作原理基本上都是一致的，其核心都是一个比色计,即用比色法来进行分析。测定一般要求测试液的最终体积在250μL以下，用一般光电比色计无法完成测试，因此对酶标仪中的光电比色计有特殊要求。 酶标仪原理： 酶标仪实际上就是一台变相光电比色计或分光光度计，其基本工作原理与主要结构和光电比色计基本相同. 图示是一种单通道自动进样的酶标仪工作原理图.光源灯发出的光波经过滤光片或单色器变成一束单色光，进入塑料微孔极中的待测标本.该单色光一部分被标本吸收，另一部分则透过标本照射到光电检测器上，光电检测器将这一待测标本不同而强弱不同的光信号转换成相应的电信号.电信号经前置放大，对数放大，模数转换等信号处理后送入微处理器进行数据处理和计算，最后由显示器和打印机显示结果. 微处理机还通过控制电路控制机械驱动机构X方向和Y方向的运动来移动微孔板，从而实现自动进样检测过程.而另一些酶标仪则是采用手工移动微孔板进行检测，因此省去了X,Y方向的机械驱动机构和控制电路，从而使仪器更小巧，结构也更简单. 微孔板是一种经事先包理专用于放置待测样本的透明塑料板，板上有多排大小均匀一致的小孔，孔内都包埋着相应的抗原或抗体，微孔板上每个小孔可盛放零点几毫升的溶液. 光是电磁波，波长100nm~400nm称为紫外光，400nm~780nm之间的光可被人眼观察到，大于780nm称为红外光。人们之所以能够看到色彩，是因为光照射到物体上被物体反射回来。绿色植物之所以是绿色，是因为植物吸收的大部分为红橙光和蓝紫光，但对绿色不吸收，反射出来，所以植物呈现为绿色。酶标仪测定的原理是在特定波长下，检测被测物的吸光值。 随着检测方式的发展，拥有多种检测模式的单体台式酶标仪叫做多功能酶标仪，可检测吸光度(Abs)、荧光强度(FI)、时间分辨荧光(TRF)、荧光偏振(FP)、和化学发光(Lum)。 酶标仪从原理上可以分为光栅型酶标仪和滤光片型酶标仪。光栅型酶标仪可以截取光源波长范围内的任意波长，而滤光片型酶标仪则根据选配的滤光片，只能截取特定波长进行检测。 检测单位 光通过被检测物，前后的能量差异即是被检测物吸收掉的能量，特定波长下，同一种被检测物的浓度与被吸收的能量成定量关系。

For-next循环

For... Next循环语句 For…next循环简称为For循环。它是一种指定循环次数（事先知道循环次数）的循环程序结构。在这种结构中，使用了一个称为循环变量的特殊变量作为计数器，指定它的初始数值，然后每重复执行一次循环，循环变量就会自动增加或减少一个指定的数值(称为步长值)，直到循环变量的改变达到最终的指定值，循环才停止执行。 1．For …Next语句的语法格式 For <循环变量>=<初值> To <终值> [Step步长] [语句块] [Exit For] Next [循环变量] 功能：用来控制重复执行一组语句。指定循环变量以步长为增量，从初值到终值依次取值，并且对于循环变量的每一个值，把语句块执行一次。 说明： （1）关键字For和Next成对出现，For是循环语句的开始，Next是循环语句的终端，必须出现在For语句的后面。在关键字For和Next之间的语句块叫循环体，它们将被重复执行指定的次数，执行的次数由初值、终值、步长值决定。 （2）初值、终值和步长值都是数值表达式，步长值可以是正数，也可以为负数。如果步长值为1，可以省略不写，即系统默认步长值为1。 （3）<循环变量>为必要参数，是用作循环计数器的数值变量，这个变量不能是数组元素。在循环体内，一般不提倡再给循环变量另外赋值。循环变量从初值开始，逐次按照步长值增加或减少而改变，直到超过终值，这时循环停止执行。这里所说的“超过”有两种含义：即大于或者小于。 （4）<初值>和<终值>也都是必要参数。当初值小于终值时，<步长值>必须是正数；反过来，如果初值大于终值，则步长值必须为负数。 （5）如果循环体中安排了Exit For 语句，当程序执行到该语句时直接跳出循环结构，不再执行循环体中Exit For的后续语句（如果有），而是转到Next后面的其他指令继续执行。 （6）Next语句中的[<循环变量>]可以省略。 2. For... Next语句的执行过程： 进入For...Next循环后，程序按照以下步骤执行： （1）若初值、终值和步长值为表达式，求出它们的值，并保存起来：

for next语句

For/next语句 一、教材分析 For/next语句是《算法与程序设计》中一个重点，也是后面学习面向对象程序设计的一个基础，如何有效教学，跨越这个门槛是我头痛之处，经了解，学生在数学上学习过循环结构，于是，通过整合数学资源实现突破。 二、学情分析 本节课教学对象是高二学生，通过一段时间的学习，学生已经具备了一定的抽象逻辑思维能力，并处于不断发展的阶段；积累了用计算机编程解决现实中的问题的初步经验。在此基础上学习for/next语句，再加上我校学生基础好，学习态度端正，习惯好。学好本节课内容不算什么难事。 三、教学目标 1、知识与技能： （1）、掌握For/Next语句的格式，理解For/Next循环语句的功能和执行步骤 （2）、能够分析简单的For/Next循环语句功能，尝试编写简单的For/Next 循环程序 2、过程与方法 首先，通过绘制同心圆的问题，让学生发现绘制不同半径的同心圆，反复使用circle函数所带来的麻烦，从而引出for/next语句，进而解决代码重复所带来的麻烦。让学生感到欣喜的同时，渴望知识。其次，通过任务设置进一步掌握for/next语句的使用方法，为学习双层循环做准备。最后，通过打印九九乘法表达到掌握双重循环的目的。 3、情感价值观 通过信息技术对其他学科的整合，提高学生学习算法的兴趣，激发学生编程的热情，同时也培养了学生的细心和耐心，加深了对计算机这一工具的认识，也增强了用计算机编程来解决一些无法用人工来计算的问题的信心。 四、教学重点与难点 For/next语句的使用方法和功能以及执行步骤。 根据实际情况，确定for/next语句的循环变量条件和循环体。 五、教学策略 本节课首先采用问题探究方法，引导学生发现问题，进而引出for/next语句；其次通过讲授for语句的使用方法和功能，增强学生对for语句的理解；最后通过实践任务的设置来巩固所学，达到学以致用的效果。 六、教学环境 多媒体机房 七、教学过程

For-Next循环语句--(第1课时)

For-Next循环语句--(第1课时) 【适用教材】广东教育出版社《信息技术》册 【适用年级】初二年级 一、教学内容分析 本节课讲授的是For-Next循环语句，因为之前学生学习过顺序结构，分支结构中的条件语句，对编程有了一定的基础，但是循环语句相对于条件语句来说，语法和语句的工作流程都复杂了，所以在讲述For-Next循环语句时，可以让学生分析程序的具体执行过程，引导学生分析For-Next 循环语句是如何实现程序的循环功能的，加深学生对循环功能的实现方法的理解。 二、教学对象分析 本节课的教学对象是初二学生，因为初二学生的理解能力有限，而这节课涉及的循环语句比较抽象，较难理解，因此在教学中宜比较自然地引入循环语句的功能、格式以及使用方法。并且为了学生更好地理解For-Next语句，尽可能使用程序与流程图结合的方法进行讲解。 三、教学目标 ．初步理解循环结构的定义和作用； ．初步掌握循环语句的一般格式； ．结合For-Next循环语句的执行流程图理解循环结构

程序的执行过程。 四、教学重点以及教学难点 理解及初步掌握For-Next循环语句。 五、教学过程设计 复习程序的顺序结构 前面我们讲过程序的顺序结构，计算机最基本的结构。计算机在执行程序时，按照从上往下的顺序依次执行语句，这样的结构称为顺序结构。 复习程序的分支结构 有时候处理问题时，比如判断一个年份是否闰年，需要根据某个条件进行判断，然后再决定程序的执行过程，这种程序结构称为分支结构。前面我们所学过的If-Then-Else 条件语句就可实现条件的判断。 格式：条件语句的执行过程： IfThen Else EndIf 讲述新 引入： 有时，在解决一些问题时，经常需要重复执行一些操作，

酶标仪的分类方式

酶标仪的分类方式 酶标仪是实验室常用的一种仪器，酶标仪的分类方式一般可以按照滤光 方式的不同和功能的不同进行划分。一。酶标仪基于滤光方式的不同可分为滤光片式的酶标仪和光栅式酶标仪。滤光片式酶标仪采用滤光片来进行波长的选择，酶标仪内置滤光片轮，可选择试验所需的不同波长的滤光片来进行分光，光源发出的全波谱光经过滤光片后，大部分被过滤，只剩下滤光片本身允许的波长通过，这样就可就通过滤光片来获得特定的波长。滤光片轮一般包含4-6 块滤光片，通过选择不同的滤光片可获得不同的波长，但是获得的波长都是固定的，受到一定的限制，不能获得任意所需的波长，而且更换滤光片价格比较昂贵。一般波长固定的滤光片有405，450，490，630NM。光栅式酶标仪是美国分子仪器公司最先发明使用的（Versamax），虽然才出现短短十年，但是发展非常迅猛，现在已经是主流的酶标仪。它采用光栅进行分光，光源发出的全波谱光线经过光栅后，通过光栅上面分布的一系列狭缝的分光，就可以获得任意波长的光，波长连续可调，一般递增量为1nm.光栅式酶标仪使用方便灵活，可以通过软件选择任意波长的光，而且可以进行全波长的扫描，通过全波长扫描可以获得未知样品的吸收峰，从而可以达到检测未知样品的目的，在实验室中很受欢迎，可以进行更多实验的检测。因此目前在国内的普及程度很高。 二。按照功能的划分，酶标仪可以分为光吸收酶标仪，荧光酶标仪，化学发光酶标仪和多功能的酶标仪。光吸收酶标仪是用来进行可见光与紫外光吸光度的检测。特定波长的光通过微孔板中的样品后，光能量被吸收，而被吸收的光能量与样品的浓度呈一定的比例关系，由此可以用来定性和定量的检测。光吸收的检测技术成熟，成本低，操作简单，但是动态范围窄，灵敏度比较低，