Protect Lithium Anode and Improve Cycle Performance for Li–S Batteries LiODFBI添加剂 有电解液文献

An E?ective Approach To Protect Lithium Anode and Improve Cycle Performance for Li?S Batteries

Feng Wu,?,?,∥Ji Qian,?,∥Renjie Chen,*,?,?Jun Lu,*,§Li Li,?,?Huiming Wu,§Junzheng Chen,?

Teng Zhao,?Yusheng Ye,?and Khalil Amine*,§

?Beijing Key Laboratory of Environmental Science and Engineering,School of Chemical Engineering and Environment,Beijing Institute of Technology,Beijing100081,P.R.China

?National Development Center of High Technology Green Materials,Beijing100081,P.R.China

§Chemical Sciences and Engineering Division,Argonne National Laboratory,9700South Cass Avenue,Lemont,Illinois60440, United States

*Supporting Information

LiODFB,passivation layer,electrochemistry

INTRODUCTION

Rechargeable batteries with high energy density and long cycle life are in urgent demand for electric vehicles and other energy storage systems.1The Li?S battery is a promising electro-chemical system because of its high theoretical speci?c capacity (1672mAh/g)and energy density(2600Wh/kg).In addition, the active material,sulfur,is low cost,highly abundant,and nontoxic,which makes the Li?S battery even more attractive. However,despite much development e?ort in the past years, Li?S batteries have not yet reached commercialization because of several technical bottlenecks.2?4One of the biggest challenges is the insulating nature of sulfur and its reduction products(Li2S and Li2S2),which result in low electronic conductivity within sulfur electrodes.Cathode materials that incorporate sulfur into a di?erent carbon matrix,5?11or conductive polymers,12?14were developed to improve the electrical conductivity and accommodate the electrode volume expansion during cell operation.Another challenge is the solubility of the long-chain polysul?de ions generated during the charge and discharge of the cell,which gives rise to a “shuttle”mechanism.This shuttle e?ect not only decreases utilization of the active material,but also markedly reduces the Coulombic e?ciency of the cell.15To overcome this problem, the cathode materials have been tuned to better encapsulate sulfur and thereby suppress the dissolution of polysul?des into the electrolyte;however,such cathode structures only provide limited success in solving the polysul?de dissolution problem.

Various researchers have investigated optimization of the electrolyte as a di?erent avenue to tackle this problem.In particular,a pseudosolid-state electrolyte(the solvent-in-salt electrolyte16with an ultrahigh salt concentration)and an all-solid-state electrolyte17have been developed for Li?S cells, which demonstrate improved Coulombic e?ciency and cycle stability because of the dramatic decrease in the solubility of the lithium polysul?des.However,the challenge facing solid-state electrolytes is the low ionic conductivity at room temperature, which has a signi?cant impact on the rate performance of the cell.In addition,these electrolytes are usually very costly and di?cult to fabricate at the current stage;therefore,organic liquid remains the electrolyte of choice for Li?S batteries. Nonetheless,as mentioned earlier,the shuttle e?ect from the polysul?de dissolution that occurs in pure organic liquid electrolytes is problematic.For that reason,electrolyte additives are commonly used to alleviate the shuttle problem.For instance,LiNO3was widely used as the additive or cosalt in the electrolyte for Li?S cells because it helps to

protect the lithium electrode by forming a surface protective?lm.18?20This?lm can e?ectively suppress the redox shuttle reactions,and thus improves the cycle performance and Coulombic e?ciency of the Li?S cell.18However,LiNO3can be reduced on the Received:July4,2014

Accepted:August6,2014

Published:August6,2014

cathode at potentials lower than 1.6V,and the formed byproducts severely a?ect the reversibility of the sulfur cathode. The narrower voltage window of a cell containing LiNO3in the electrolyte causes a loss of capacity.20,21A strong oxidant in the presence of acidity,LiNO3will potentially increase the safety issues of the cell during long-term cycling if it is not consumed completely.Therefore,researchers are seeking alternative electrolyte additives that can reduce the shuttle e?ect in the Li?S cell.22,23

Lithium oxalyldi?uoroborate(LiODFB)salt,which has a chemical structure that comprises the half molecular moieties of LiBOB and LiBF4,24has been reported to improve the electrochemical performance of Li-ion batteries.Since LiODFB exploits advantages of both LiBOB and LiBF4,it possesses thermal stability,optimized ionic conductivity over a wide temperature range,and the ability to passivate Al at high potential.24?26In this work,we studied the e?ects of LiODFB as an electrolyte additive on the electrochemical performance of Li?S cells for the?rst time.The results demonstrate that the addition of an appropriate amount of LiODFB to the electrolyte promotes the formation of a surface passivation layer on the lithium electrode,which signi?cantly suppresses the parasitic reactions between polysul?des and the lithium electrode and o?ers the promise of high Coulombic e?ciency and a long cycle life for the Li?S battery.

■RESULTS AND DISCUSSION

To investigate the e?ect of the LiODFB additive on the electrochemical performance of the Li?S cell,we selected a multiwalled carbon nanotube(MWCNT)with a sulfur composite as the cathode active material.The method for preparation of this cathode material can be found else-where.11,14The sulfur content in the cathode was con?rmed (Figure S1of the Supporting Information)by thermogravi-metric analysis to be67wt%.The pure electrolyte used was1.0 mol/L bis(tri?uoromethane)sulfonimide lithium(LiTFSI)in dimethoxyethane(DME)and1,3-dioxolane(DOL)(see the Experimental section).

Figure1a shows the speci?c capacities and Coulombic e?ciencies as functions of cycle numbers for Li?S cells with di?erent amounts of LiODFB added to the electrolyte(i.e.,0, 1%,2%,4%,and10%,respectively).Clearly,the Li?S cell with no LiODFB shows poor electrochemical performance.The capacity undergoes a fast decay during the?rst5cycles,and the capacity retention of this cell is only50%after50cycles. Meanwhile,the Coulombic e?ciency of this cell drops sharply from97%to below70%in the?rst2cycles.This poor electrochemical performance is believed to be associated with the severe shuttle phenomenon.After di?erent amounts of LiODFB were added to the electrolyte,both the capacity retention and Coulombic e?ciency improved,as illustrated in Figure1,panel(a).Interestingly,the cell with2%LiODFB additive exhibited the best electrochemical performance.For this cell,the initial capacity(based on the mass of sulfur loading)was1146.4mAh/g,and the capacity retention after50 cycles was about70%.Moreover,the Coulombic e?ciency improved dramatically and remained at97%during the cycle test.

Figure1,panel b shows the charge and discharge curves of the Li?S cell with2%LiODFB additive.The discharge curves comprises two typical plateaus(2.3V and2.1V vs Li+/Li), which correspond to the reduction of sulfur to soluble polysul?des and the further reduction of polysul?des to Li2S2/Li2S,respectively.The multistep electrochemical reac-tions of elemental sulfur with lithium ions in these cells are further demonstrated by the cyclic voltammetry results in Figure S2of the Supporting Information.

It is noteworthy that the cell performance decreases when the amount of LiODFB additive increases,as is evident by the low Coulombic e?ciency of the cell with10%LiODFB,which is similar to that of the cell with no LiODFB(Figure1a).This result suggests that a fairly thick layer is formed on the lithium electrode for the cell with more than2%LiODFB added,which likely blocks further ion transportation and thus leads to a low Coulombic e?ciency.In other words,the adjustment of the amount of LiODFB added to the electrolyte is critical to tune the solid?electrolyte interphase(SEI)that forms on the lithium electrode.

The above results demonstrate that the addition of the appropriate amount of LiODFB into the electrolyte improves the electrochemical performance of the Li?S cell.We attribute this improvement to the formation of a protective layer on the lithium electrode to suppress the shuttle e?ect.The scanning electron microscopy(SEM)images of the cross-section of the lithium electrodes after the cycle test in the cell containing2% LiODFB clearly show the formation of a dense passivation layer on the anode(Figures2a,b).

To study the passivation layer on lithium electrodes,an AC impedance analysis of the cells after50cycles was performed over a frequency range of100mHz to1MHz.As shown in Figure2,panel c,the spectra with the LiODFB added displays two semicircles,while that with no LiODFB added only has

Figure1.(a)Capacity versus cycle number for Li?S cells with di?erent amounts of LiODFB added.(b)Discharge and charge curves of the cell with2%LiODFB additive on di?erent cycles.All of the electrochemical measurements were performed at a current of100 mA/g-sulfur.

one.As has been intensively studied,27?30the semicircle at high frequency is associated with the passivation surface ?lm,which can be characterized by a parallel combination between the resistance (R sei )and capacitance (C sei )of the SEI on the surface of the electrode,while the semicircle at a medium frequency is related to charge-transfer processes and can be described on the basis of the parallel combination between the charge-transfer resistance (R ct )and the double-layer capacitance (C dl ).Speci ?cally,while no main electrochemical reactions take place,the R ct is so high that its semicircle becomes invisible.28Roughly speaking,the passivation ?lm resistance (R sei )increases with the LiODFB weight percent,but the R sei is lowest with 2%LiODFB added.According to the results in other papers,18,22LiTFSI ’s contribution to the passivation ?lm is negligible,and with no LiODFB,the main component of the SEI is Li 2S,which is a poor lithium-ion conductor,but in the case of LiODFB added,it participates in the formation of a passivation layer that contributes to Li-ion conduction.An increase in the amount of LiODFB added beyond 2wt %may thicken the passivation ?lm,delivering a larger R sei .We thus believe that the low R sei is a major factor behind the much improved electrochemical performance of the Li ?S cell containing 2%LiODFB.We used SEM to investigate the surface morphology of the lithium electrodes after 10cycles with various amounts of LiODFB added.As seen in Figure S3b of the Supporting Information,the surface of the lithium electrode with no LiODFB became rough and loose after 10cycles compared to the smooth and compact surface of the pristine lithium electrode (Figure S3a,Supporting Information).As previously reported,31,32the rough lithium surface has a higher speci ?c surface area,which is detrimental to the performance of Li ?S cells and may lead to some other safety problems such as cell swelling and thermal instability.Fortunately,adding LiODFB into the electrolyte for Li ?S cells can help alleviate the problem.As shown in Figure S3(Supporting Information),a smoother and denser surface morphology on the anode appears when the LiODFB additive was used.Furthermore,the roughness of the surface morphology decreases when the LiODFB weight percent increases.When we replaced the LiTFSI with LiODFB (1mol/L)as the lithium electrolyte salt,the lithium electrode exhibited a highly uniform and compact surface morphology (Figure S4,Supporting Information).In this way,the reactions of the lithium anode and electrolyte solvent or the dissolved polysul ?des can be e ?ectively controlled,which results in capacity fading and thermal instability.Figure 3,panels a and b show the surface morphology of the lithium electrode after 50cycles with no and 2%LiODFB added into the electrolyte,respectively.As shown,the surface morphology of the lithium electrode with no LiODFB became looser and rougher,while that with 2%LiODFB was relatively smooth,except for some cracks (as indicated by the rectangles in Figure 3b).Once a crack appears before cycling or on the initial cycle,the lithium in this area will exhibit relatively higher reactivity with the electrolyte,which leads to growth of the crack.This process is similar to the mechanism of dendrite formation in Li-ion batteries,and the rough surface morphology that occurs is harmful for batteries,as described previously.As con ?rmed in other papers,16,18,22the improved cycling e ?ciency in the modi ?ed electrolyte system is related to the SEI formed on the surface of the lithium electrode.To determine the chemical composition of the passivation ?lm,we performed energy dispersive X-ray (EDX)spectroscopy and X-ray photoelectron spectroscopy (XPS).The EDX results of the lithium electrode surface after 50cycles with no and 2%LiODFB added are presented in Figure 3,panels c and d and in Table S1of the Supporting Information.The ?uorine elemental map (Figure 3c)proves that the surface is uniformly covered by the passivation ?lm;however,some concentrated areas appear in the sulfur elemental map (marked by a white rectangle in Figure 3d)and correspond to the cracks in Figure 3,panel b.This ?nding demonstrates that a greater part of Li 2S 2/Li 2S was precipitated in these areas,which con ?rmes the higher reactivity of the lithium in these areas.The EDX results listed in Table S1(Supporting Information)show the maps of several elements (O,C,F,and S)on the lithium electrode surface.(Since oxygen and carbon might arise from the organic compounds in the electrolyte or the atmosphere within the glovebox used during cell assembly and operation,they will be ignored here.)The data in Table S1of the

Supporting

Figure 2.Schematic con ?guration of the Li ?S cell (on the right side).Scanning electron micrographs of the cross-section of the lithium electrode with 2%LiODFB after 50cycles at (a)low and (b)high magni ?cations.(c)The alternating current (AC)impedance measurements of the lithium electrode with 2%LiODFB added into the electrolyte.The inset shows the AC impedance of the cells with di ?erent amounts of LiODFB added.All of the lithium electrodes measured were after 50cycles.

Information indicate that,after the addition of 2%LiODFB,the molar ratio of ?uorine to sulfur on the surface of the lithium electrode increased from 2.17to 6.93.This ?nding suggests that the ?uorine-containing substance,namely LiODFB,is involved

in the formation of the passivation layer,which can inhibit the

reaction between lithium and the electrolyte and thus reduces

the amount of Li

2S 2/Li 2S deposited on the surface of the

lithium https://www.wendangku.net/doc/b716642538.html,plementary information on the lithium electrode

surfaces with 2%LiODFB was provided by the XPS analysis

(Figures 3e,f and Figure S5of the Supporting Information).

According to the XPS data and previous work reported by

other groups,16,18the main components of the SEI layer (2%

LiODFB added)are similar to those of the SEI layer with no

LiODFB,except for much more LiF and a trace of boron

detected.It is worth noting that

the Li ?S cells with a ?uorinated electrolyte show improved performance.33?35Also,HF can be generated through reactions within the electrolyte-

containing LiBF

4in Li-ion batteries and simultaneously will

react with the alkali components in the SEI layer to form LiF 36

and thus improves the morphology of the graphite electrode for lithium deposition.37Our experimental results indicate

that

Figure 3.

SEM images

of the lithium electrodes after 50cycles with (a)no LiODFB and (b)2%LiODFB.(c,d)EDX mapping of the lithium electrode in panel b,which shows the distribution of ?uorine and sulfur.(e,f)XPS spectra (F 1s;S 2p)of the lithium electrode surface with 2%

LiODFB after 50cycles.Scheme 1.Structures of the Lithium Salts and Organic Solvents

Used

Scheme2.Possible Reactions of the?B[ox]Radical with(a)DME and(b)DOL,Where?B[ox]Represents Oxalatoboryl Radical,and the DFT Estimates for Enthalpies of Individual Reaction Steps at the Temperature of Absolute Zero are Given

Scheme3.Proposed Reactions between LiODFB and DOL/DME,Where F2B[ox]?Represents the

Oxalyldi?uoro(oxalate)borate Anion

LiODFB helps stabilize the lithium surface through the reactions between LiODFB and other substances in this system during cycling,but the mechanism is not entirely clear.We thus performed ab initio simulations in the form of the density functional theory (DFT)to further study the mechanism.The oxalyldi ?uoro(oxalate)borate anion (F 2B[ox]?)was reported to reduce at 1.6V,38which corresponds to the cyclic voltammetry results (Figure S2,Supporting Information),and the reduction process is the stepwise elimination of F ?anions,which produces the oxalatoboryl radical (?B[ox]).39The tentative oxalatoboryl radical may react with DME and DOL molecules (structures are shown in Scheme 1),and the DFT calculations of the individual reaction steps at the temperature of absolute zero are listed in Scheme 2.The reactions,which produce several chain hydrocarbons,are favored energetically (enthalpies of the individual reaction steps are marked in red in Scheme 2),and the proposed net reactions of the F 2B[ox]?anion with DME and DOL are given in Scheme 3.The resulting terminal products might account for the benign role of LiODFB in SEI formation,but the process is not fully understood,and research into this mechanism is ongoing.Overall,LiODFB has the ability to facilitate SEI formation on the lithium electrode,which improves cell performance.A simpli ?ed process is shown in Scheme 4.To verify the compatibility of the modi ?ed electrolyte with another sulfur composite cathode,we chose a graphene-based sulfur/MWCNT composite with 70wt %sulfur (GS-MWCNT@S).Figure 4shows the performance of the Li ?S cells with no and 2%LiODFB operated for 100cycles.In the case of 2%LiODFB,the Coulombic e ?ciency increased slowly during cycling and reached a level of 94.6%after 100cycles,which is obviously higher than that of the cell with no LiODFB.In addition,the cell with 2%LiODFB delivered an initial capacity of 1189.5mAh/g.These results imply that the modi ?ed electrolyte with 2%LiODFB could be compatible with other sulfur composite cathodes for the Li ?S cell and delivers a stable electrochemical performance.But it can be observed that the cell capacity drops still exist,so novel cathode materials,with the properties of high conductivity,favorable adsorption properties,and mechanically robust character,need to be developed.

■CONCLUSIONS

In summary,we investigated the e ?ects of LiODFB as an electrolyte additive on the electrochemical properties of Li ?S cells.The Li ?S cells with the LiODFB-added electrolyte exhibited extremely high Coulombic e ?ciency and better cycle performance,and the experimental results demonstrate that the appropriate amount of LiODFB electrolyte additive is 2wt %.The role of LiODFB in Li ?S cells is to promote a LiF-rich passivation layer on the lithium anode surface.The passivation layer not only blocks the polysul ?de shuttle mechanism,thus improving the electrochemical performance,but also stabilizes the lithium surface.In general,LiODFB is a promising electrolyte additive for Li ?S batteries,but the mechanism is still not clear,and a deeper understanding and more detailed knowledge of it are needed.

■EXPERIMENTAL SECTION Synthesis.The pure electrolyte used was 1.0mol/L LiTFSI salt (3M)in a solvent of DME and DOL (volume ratio of 1:1,Alfa Aesar).Di ?erent amounts (1%,2%,4%,and 10%,by weight)of LiODFB (Hongyang Chemical,China)were added to the electrolyte.Structures of the lithium salts and organic solvents are illustrated in Scheme 1.

Electrochemical Measurements.A MWCNT/S composite was prepared by a simple melt-di ?usion strategy.Then,cathode slurries were

produced by mixing 70%MWCNT/S composite,20%acetylene black,and 10%polyvinylidene ?uoride binder in N -methyl-2-pyrrolidinone.The mixtures were ball milled for 4h to form

homogeneous slurries.After stirring,each slurry was coated onto aluminum foil using a roll press.The coated electrodes were dried in a vacuum oven at 60°C for 24h.The electrodes were cut into disks with

a diameter of 11mm.Two-electrode coin cells (CR2025)with Li

foil as the anodes were assembled in an argon-?lled glovebox for electrochemical experiments.The cells were discharged and charged

between cuto ?potentials of

1.0and

3.0V using an electrochemical

station (LAND,Wuhan,China)to test their cycle life,where the current was 100mA/g.Cyclic voltammograms were recorded on an

electrochemical workstation (CHI660D,Shanghai Chenhua,China)between 1.0and 3.0V to characterize the redox behavior and kinetic reversibility of the cells.Characterizations and Computation.Thermal gravimetric analysis (TGA)of the cathode material was carried out using a

thermal analyzer (6200EXSTAR)at a heating rate of 10°C/min

under an air atmosphere.The cells were unpacked in the glovebox after cycling,and the lithium electrodes were removed and then thoroughly washed with a large amount of DOL three times.The AC impedance of the lithium electrodes after 50cycles was measured with an impedance analyzer (Zahner Zennium).The AC amplitude was ±5mV,and the applied frequency range was 100mHz to 1MHz.The morphology and composition of the surfaces of the lithium electrode surface were investigated with EDX and SEM (HTACHI S-4800).Also,XPS (ESCALAB 250)was performed using a monochromatized Al K α

source.

Geometry optimizations of the radicals were carried out

with the all-electron density functional program DMol3in Materials Studio 5.5(Accelrys)using the Becke ?Lee ?Yang ?Parr (BLYP)

Scheme 4.Schematic Illustration of the Role of LiODFB in Forming the Passivation Layer

a a Lithium anode (a)before cycling and (b)after cycles with no

LiODFB added.(c)Lithium anode after cycles with LiODFB additive,which promotes the formation of the passivation

layer.Figure https://www.wendangku.net/doc/b716642538.html,parison of the cycle performance between cells with no and 2%LiODFB added,where the cathode active material is GS-MWCNT@S composite.The electrochemical measurements were performed at a current of 0.1C (1C =1675mA/g-sulfur).

maximum atomic displacement.

■ASSOCIATED CONTENT

*Supporting Information

EDX results from the SEM images shown in this work.TGA curve of the MWCNT/S composite.Cyclic votammograms of the Li?S cell.SEM images of the Li electrodes after10cycles. XPS spectra of the Li electrode surface.This material is available free of charge via the Internet at https://www.wendangku.net/doc/b716642538.html,.■AUTHOR INFORMATION

Corresponding Authors

*E-mail:chenrj@https://www.wendangku.net/doc/b716642538.html,.

*E-mail:junlu@https://www.wendangku.net/doc/b716642538.html,.

*E-mail:amine@https://www.wendangku.net/doc/b716642538.html,.

Notes

The authors declare no competing?nancial interest.

∥F.W.and J.Q.contributed equally to this work.

■ACKNOWLEDGMENTS

This work was supported by the National Science Foundation of China(21373028),the National863Program (2011AA11A256),the New Century Educational Talents Plan of Chinese Education Ministry(NCET-12-0050),the Beijing Novel Program(Z121103002512029),and the Ford University Research Program(URP)project.This work was also supported by the U.S.Department of Energy under Contract No.DE-AC0206CH11357from the Vehicle Tech-nologies O?ce,Department of Energy,O?ce of Energy E?ciency and Renewable Energy(EERE).Argonne National Laboratory is operated for the U.S.Department of Energy by UChicago Argonne,LLC,under Contract No.DE-AC02-06CH11357.

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