A Fluorinated Ether Electrolyte Enabled High PerformancePrelithiated Graphite/Sulfur BatteriesShuru Chen, Zhaoxin Yu, Mikhail L. Gordin, Ran Yi, Jiangxuan Song, and Donghai Wang*

Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, UnitedStates

*S Supporting Information

ABSTRACT: Lithium/sulfur (Li/S) batteries have attracted greatattention as a promising energy storage technology, but so far theirpractical applications are greatly hindered by issues of polysulfideshuttling and unstable lithium/electrolyte interface. To addressthese issues, a feasible strategy is to construct a rechargeableprelithiated graphite/sulfur batteries. In this work, a fluorinatedether of bis(2,2,2-trifluoroethyl) ether (BTFE) was reported toblend with 1,3-dioxolane (DOL) for making a multifunctionalelectrolyte of 1.0 M LiTFSI DOL/BTFE (1:1, v/v) to enable highperformance prelithiated graphite/S batteries. First, the electrolytesignificantly reduces polysulfide solubility to suppress the deleterious polysulfide shuttling and thus improves capacity retentionof sulfur cathodes. Second, thanks to the low viscosity and good wettability, the fluorinated electrolyte dramatically enhances thereaction kinetics and sulfur utilization of high-areal-loading sulfur cathodes. More importantly, this electrolyte forms a stablesolid−electrolyte interphase (SEI) layer on graphite surface and thus enables remarkable cyclability of graphite anodes. Bycoupling prelithiated graphite anodes with sulfur cathodes with high areal capacity of ∼3 mAh cm−2, we demonstrate prelithiatedgraphite/sulfur batteries that show high sulfur-specific capacity of ∼1000 mAh g−1 and an excellent capacity retention of >65%after 450 cycles at C/10.

KEYWORDS: batteries, fluorinated electrolytes, lithium/sulfur, prelithiated, graphite

1. INTRODUCTION

The rechargeable lithium/sulfur (Li/S) battery is a promisinghigh-energy-density storage technology, as the elemental sulfurcathode can deliver a theoretical capacity of 1675 mAh g−1 andan energy density of ∼2600 Wh kg−1 upon complete reductionby lithium to form Li2S.

1,2 The attempts to develop Li/Sbatteries began in the 1960s,3 but with limited progress in thefollowing decades due to the insulating nature of sulfur andLi2S and the dissolution and shuttle effect of lithium polysulfideintermediates in liquid electrolytes.4−8 Significant efforts hadbeen made in the past decade by the development ofnanoporous or functionalized conductive host/sulfur compo-sites, as they provide higher overall cathode conductivity, bettertrapping of soluble polysulfides, and greater resistance toelectrode pulverization and thus can improve the utilization andcyclability of sulfur cathodes.9−21 In order to achieve highpractical energy density of Li/S batteries, more recent effortsare focusing on rational design of high-areal-loading sulfurcathodes or functional electrochemically active electrolytes thatcan boost cell capacity.22−24 It was pointed out that withincreasing sulfur loading/areal capacity the cycle life of Li/Sbatteries becomes much worse due to the exaggerated issues ofpolysulfide shuttling and instability of lithium/electrolyteinterfaces that can lead to fast electrolyte depletion andanode degradation.24−28 The issues related to the electrolyte

and lithium metal anodes have been recognized as majorlimiting factors for practical applications of Li/S batteries.The key to address the above issues is to prevent the anode

from continuous side reactions with both polysulfides andelectrolyte solvents. Highly concentrated electrolytes or solvateionic liquids where solvent molecules are all stronglycoordinated with Li+ ion have proven to be quite effective forboth suppression of polysulfide dissolution and shuttling, andformation of stable lithium deposition/stripping reaction, thusfacilitating good cyclability of Li/S batteries.29−32 However,without polysulfide dissolution in those electrolytes, thereaction kinetics of the sulfur cathodes could be very sluggishowing to the poor ionic conductivity of sulfur species. Inaddition, those electrolytes usually have high viscosity, low ionicconductivity, and high liquidus temperature, all of which mightseverely limit the battery performance for high-sulfur-loadingelectrodes at high current density and/or low temperature. Thereal promise of this concept thus remains to be verified.Researchers have also examined approaches of protecting

lithium anode by either ex situ coated layer or by an in situformed solid electrolyte interphase (SEI) layer from thedecomposition of electrolyte additives, such as LiNO3, P2S5,

Received: September 2, 2016Accepted: February 3, 2017Published: February 3, 2017

Research Article

www.acsami.org

© 2017 American Chemical Society 6959 DOI: 10.1021/acsami.6b11008ACS Appl. Mater. Interfaces 2017, 9, 6959−6966

or fluorinated ethers.33−37 Suppressed polysulfide shuttling andself-discharge were reported by using those approaches;however, the role of protection layers on lithium anode iscomplicated, and their long-term stability/effectiveness in Li/Sbatteries is still elusive due to the presence of polysulfides inelectrolyte and the infinite volume change of lithium anodeassociated with repeated lithium deposition and stripping.Electrolyte depletion, lithium dendritic deposition, anddegradation may therefore remain huge problems for develop-ment of safe and long-cycling Li/S batteries.Another strategy to address those issues is to use alternative

anodes, such as graphite,38−41 hard carbon,42 tin, andsilicon,43,44 which can be directly paired with Li2S cathodesor be prelithiated to pair with sulfur cathodes. Among them,graphite seems promising despite its lower capacity (372 mAhg−1), as it has been successfully adopted in commercial Li-ionbatteries for decades thanks to its limited volume change andstable SEI layer. However, electrolytes based on ethers such asDOL and 1,2-dimethoxyethane (DME), which are so far mostcommonly used for Li/S batteries, have long been known asincompatible with graphite anodes, owing to the exfoliation ofgraphite by solvent co-intercalation.45 It is thus pivotal todevelop alternative electrolytes that work with graphite whilefacilitating reversible cycling of high capacity sulfur cathodes. Inthis regard, some highly concentrated electrolytes or solvateionic liquids were adopted to enable prelithiated graphite/sulfurbatteries.38−41 However, the disadvantages of those electrolytesas mentioned above still apply, making it difficult to apply tosulfur cathodes with areal sulfur loadings of up to 2 mg cm−2 inthe previous report.In this paper, we report a multifunctional fluorinated ether-

based electrolyte of 1.0 M LiTFSI DOL/BTFE (1:1, v/v) toenable cycling prelithiated graphite/sulfur batteries with highperformance. This electrolyte significantly reduces polysulfidesolubility to suppress the deleterious polysulfide shuttling;

meanwhile, it dramatically enhances the reaction kinetics andsulfur utilization for high-areal-loading sulfur cathodes (up to 7mg S cm−2), making it suitable for achieving both good cyclelife and high practical energy density of sulfur cathodes.Moreover, a stable SEI on graphite surface was found with thisfluorinated electrolyte, enabling reversible lithium intercalationof graphite anodes in prelithiated graphite/sulfur batteries toachieve enhanced safety, lower cost, and even better cycling life.By using prelithiated graphite anodes with capacity of ∼3 mAhcm−2 and cathodes with sulfur loading of ∼3 mg cm−2, wedemonstrate prelithiated graphite/sulfur batteries that showhigh sulfur-specific capacity of ∼1000 mAh g−1 and an excellentcapacity retention of >65% after 450 cycles at C/10.

2. RESULTS AND DISCUSSION

2.1. Improved Performance of High-Loading SulfurCathodes with DOL/BTFE Electrolyte. We first comparedthe performance of sulfur cathodes with conventional electro-lyte of 0.4 M LiNO3 + 1.0 M LiTFSI DOL/DME (1:1, v/v)and the fluorinated electrolyte of 1.0 M LiTFSI DOL/BTFE(1:1, v/v) in Li/S cells. To demonstrate the advantages of thefluorinated electrolyte in enhancing the cyclability of sulfurcathodes, commercial Ketjenblack carbon was chosen toproduce carbon/sulfur composite cathode containing 70 wt %of sulfur following the widely reported melted-diffusionmethod.11,12

The discharge−charge profiles of sulfur cathodes with arealsulfur loading of ∼2 mg cm−2 in conventional and the DOL/BTFE-based electrolytes are shown in Figures 1a and 1b,respectively. Both cells show typically two discharge plateaus,one at 2.2−2.4 V and the other at 2.1−2.0 V, suggesting thesame discharge mechanism of sulfur cathodes by a “two step”reduction to form soluble lithium polysulfides and thenultimately to less soluble Li2S2 and Li2S. However, someinteresting differences in the discharge−charge profiles were

Figure 1. (a, b) Discharge−charge profiles and (c) cycling performance of Li/S half-cells in conventional DOL/DME and DOL/BTFE electrolytesat C/10 with sulfur loading ∼2 mg cm−2. (d) Photos demonstrating (A) the dissolution of 0.25 M Li2S8 in the DME/DOL electrolyte and (B) therelative insolubility of Li2S8 in the DOL/BTFE electrolyte by stirring stoichiometric amounts of Li2S and sulfur in the electrolytes for 24 h.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.6b11008ACS Appl. Mater. Interfaces 2017, 9, 6959−6966

6960

also observed between these two electrolyte systems. First,another discharge plateau <1.8 V was found only in the firstseveral cycles with conventional electrolyte, which does notexist without the addition of LiNO3 additive (Figure S1) andthus can be ascribed to the reduction of LiNO3 additive on thecathode.46,47 Second, compared to the cell with conventionalelectrolyte (Figure 1a), the cell with the DOL/BTFE-basedelectrolyte showed a greater voltage drop in the slope regionbetween the two discharge plateaus. This indicates slowerreaction kinetics for the reduction of Li2S8 to Li2S6 in the sloperegion but similar or even better kinetics for followingelectrochemical reactions in the second plateau with theDOL/BTFE-based electrolyte. Third, despite similar initialdischarge capacity of approximately 1200 mAh g−1, the Li/Scell cycling in conventional electrolyte encounters fast capacityloss at both discharge plateaus, indicating the irreversible loss ofactive materials due to dissolution of polysulfide andirreversible deposition of Li2S. On the contrary, the cell inthe DOL/BTFE electrolyte shows much more stabledischarge−charge profiles during cycling.The cycling performances of sulfur cathodes with areal sulfur

loading of 2 mg cm−2 in conventional and DOL/BTFEelectrolytes are compared in Figure 2c. In conventionalelectrolyte, the discharge capacity drops very fast from >1200to ∼550 mA h−1 in first 50 cycles, despite its high averageColumbic efficiency (CE) close to 100%. The high CE isascribed to the LiNO3 additive, which is known to suppress thepolysulfide shuttling even when significant amount ofpolysulfides dissolves in the conventional electrolyte (Figure1d, photo A).34,35 Nevertheless, LiNO3 seems not to improvethe cycle performance of sulfur cathode at all. In fact,decomposition of LiNO3 on the cathode surface under adischarge voltage of 1.8 V has been reported to negativelyimpacting sulfur cathode performances.47 This may be one ofthe reasons for the fast capacity fading observed in theconventional electrolyte. It was also reported that LiNO3 ishighly oxidizing, and it will gradually decompose on lithium

anode during cycling, causing gassing issue in Li/S pouchcells.48 In this context, development of reliable electrolyteswithout oxidizing additives is highly demanded for Li/Sbatteries. By using the DOL/BTFE electrolyte withoutLiNO3 additive, even though the average CE is found to be90−95% due to the minor dissolution (Figure 1d, photo B) andshuttling of polysulfides, the capacity retention of sulfurcathode is remarkably improved: the discharge capacitymaintains at ∼1160 mAh g−1 in the first 10 cycles, followedby very slow degradation in latter cycles. It remains above 1000and 800 mA h g−1 after 50 and 100 cycles, respectively. Such anexcellent capacity retention is believed to resulting from the lessdissolution loss and less passivation of the cathode surface byLi2S in the DOL/BTFE electrolyte. The results indicate thatthe DOL/BTFE electrolyte is promising for improving thecycle life of sulfur cathodes in Li/S batteries.High areal sulfur loading (>2 mg cm−2) is still necessary for

further reducing cell dead weight from current collectors inorder to realize high practical energy density of Li/S batteries.Therefore, we also tested sulfur cathodes with high sulfurloading of ∼3.5 and ∼7 mg cm−2. Compared to the discharge−charge curves with sulfur loading of ∼2 mg cm−2 shown inFigures 1a and 1b, the cells with the increased sulfur loadingsshow remarkably enlarged polarization and decreased specificcapacities in the conventional electrolyte, while only slightdecrease of discharge potential and slight reduction in capacityretentions were observed in the DOL/BTFE electrolyte (Figure2). The huge polarization exists in second discharge plateau andLiNO3 reduction plateau at ∼1.8 V disappears with conven-tional electrolyte, indicative of the sluggishness of diffusion andelectrochemical reduction of the dissolved polysulfides as wellas LiNO3 additive toward conductive surface within the high-areal-loading sulfur cathodes. Previous reports showed thatfluorinated ethers have both low viscosity and poorcoordination with Li+ ion, which can thus function as“lubricant” to enable use of highly viscous electrolytesystems.40,49 BTFE is thus believed to play a key role in the

Figure 2. (a, b) Discharge−charge profiles, (c) cycling performance and Coulombic efficiency, and (d) areal discharge capacity of Li/S half-cells atC/10 in conventional DOL/DME and DOL/BTFE electrolytes with S loading of ∼3.5 and ∼7 mg cm−2.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.6b11008ACS Appl. Mater. Interfaces 2017, 9, 6959−6966

6961

DOL/BTFE electrolyte as a “lubricant”, which improves thewettability of cathodes with electrolyte and facilitates ions andmass transportation in the electrode, leading to significantimprovement in the reaction kinetics and utilization of high-areal-loading sulfur cathodes. As evidence, cells with DOL-based electrolyte without BTFE were also tested, whichsimilarly show low sulfur utilization and poor cyclingperformance as that of cells with conventional electrolyte(Figure S2).

Figures 2c and 2d compare the cycling performance of thehigh-areal-loading sulfur cathodes in both conventional andDOL/BTFE electrolytes. The electrodes with sulfur loading of∼3.5 and ∼7 mg cm−2 can deliver a high discharge capacity of1150−1200 mAh g−1 at C/20 initially in DOL/BTFEelectrolyte (Figure 2c), corresponding to an areal capacityloading of ∼4 and 8 mAh cm−2 (Figure 2d), which arecomparable to that of cathode electrodes in commercial high-energy Li-ion batteries. The capacity remains ∼800−850 mAh

Figure 3. (a) Discharge−charge profiles of Li/graphite half-cell with conventional electrolyte at C/10. (b) Discharge−charge profiles, (c) cyclingperformance, and (d) Coulombic efficiency of Li/graphite half-cell with the fluorinated DOL/BTFE electrolyte at C/10.

Figure 4. (a−c) SEM images of graphite electrodes: (a) fresh, (b) postcycled with DOL/BTFE electrolyte, and (c) postcycled with conventionalDOL/DME electrolyte. (d) EDS and (e) XRD spectra of the corresponding graphite electrodes shown in (a), (b), and (c). All the postcycledelectrodes were collected at delithiated states from Li/graphite half-cells.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.6b11008ACS Appl. Mater. Interfaces 2017, 9, 6959−6966

6962

g−1 corresponding to ∼3.0 and 5.5 mAh cm−2 at C/10 after 50cycles. Such a high sulfur specific/areal capacity and stablecycling at such a mass loading level have rarely been reportedfor sulfur cathodes.2.2. Compatibility of DOL/BTFE Electrolyte with

Graphite Anode. It has been reported that when the arealcapacity of sulfur cathodes reaches a certain level (e.g., largethan 3 mAh cm−2), the cycling stability of lithium anodebecomes a dominant killing factor for lithium batteries due toits continuous evolution into the porous structure andaccumulation of cell resistance resulting from growth of SEIlayers.27,50 Such morphological changes on lithium anodes inhigh-areal-loading cells cycled with DOL/BTFE electrolytewere observed (Figure S3), indicating the exaggeratedinstability of lithium/electrolyte interface. Therefore, despitethe significant improvement owing to the reduced polysulfidesolubility and enhanced reaction kinetics with DOL/BTFEelectrolyte, Li/S cells with increasing areal sulfur loadings of 2,3.5, and 7 mg cm−2 still show increased capacity fading of∼0.3%, 0.5%, and ∼0.7% per cycle within 50 cycles,respectively. More severe capacity degradation can be expectedif cells are tested for longer cycles under practical environmentssuch as limited lithium sources and electrolyte amount. Thismotivates us to replace lithium with graphite anode that iscapable of forming a stable SEI and thus to study the newelectrolyte.To test the compatibility of the electrolytes with graphite

anodes, coin cells with lithium metal negative electrodes andgraphite positive electrodes with areal loading of ∼9 mg cm−2

were fabricated. Figure 3 shows galvanostatic discharge−chargeprofiles and cycling performance of graphite electrodes in bothconventional DOL/DME and DOL/BTFE electrolytes. Themeasurements were carried out at a current rate of C/10. Inline with previous reports,38 graphite electrodes show very poorcyclability with conventional DOL/DME electrolyte, delivering

an initial discharge capacity of ∼450 mAh g−1 followed by ahuge capacity loss to a low charge capacity of only ∼50 mAhg−1 in the subsequent charge process (Figure 3a).Such a poor reversibility of graphite anode in conventional

electrolyte is due to exfoliation of graphite by cointercalation ofDOL/DME solvents that occurs at potential of 0.9−0.2 Vduring initial discharge. The solvent cointercalation can beconfirmed by a significant increase in oxygen peak from theenergy dispersive X-ray spectroscopy (EDS) of the cycledgraphite electrode (Figure 4d). In addition, compared to thepristine graphite (Figure 4a), a large amount of cracks on thecycled graphite particles was observed from the SEM images(Figure 4c), which explains the disappearance of the sharpXRD peak at 26.4° (Figure 4e) corresponding to the (002)planes of the graphite. In sharp contrast, with the DOL/BTFEelectrolyte, graphite delivers an initial discharge and chargecapacity of ∼400 and 350 mAh g−1 (Figure 3a), respectively,resulting in a high initial CE of ∼88% (Figure 3d). The firstdischarge profile exhibits a short plateau at 0.6−0.4 V thatdisappears in the subsequent discharge processes, which can beascribed to the decomposition of organic solvents (DOL andBTFE) and the formation of a stable SEI layer. In thesubsequent cycles, multistep lithium intercalation/deintercalac-tion curves are observed within 0.01−0.3 V, with stable specificcapacity of ∼340 mAh g−1 (Figure 3c) and high CE of >99%(Figure 3d), indicating the structural stability and highelectrochemical reversibility of graphite in the DOL/BTFEelectrolyte. As evidence, similar morphology of cycled graphiteanode as the pristine one without any surface cracks orexfoliation was observed from SEM images (Figure 4a,b). Inaddition, the sharp XRD peak at 26.4° maintains well beforeand after cycling in DOL/BTFE electrolyte (Figure 4e), furtherconfirming its structural integrity of graphite anode in this newelectrolyte.

Figure 5. (a, c) Discharge−charge profiles and (b, d) cycling performance of prelithiated graphite/sulfur full cells at (a, b) C/10 and (c, d) differentC rates in the DOL/BTFE electrolyte.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.6b11008ACS Appl. Mater. Interfaces 2017, 9, 6959−6966

6963

These results clearly suggest that the co-intercalation ofsolvents was greatly suppressed by replacing DME with BTFEas a cosolvent, and desolvation of Li+ ions effectively proceededat the graphite/electrolyte interface. This is believed to resultfrom the nonsolvation of Li+ ion with BTFE and the capabilityof BTFE to facilitate a stable SEI layer on graphite electrode,e.g., by reacting with lithiated graphite to form a conjugatedCC bond and LiF,36,37,51 before cointercalation of DOLoccurs.49,52 The mechanism of desolvation of Li+ ion at theinterface of graphite/electrolyte and the suppression ofcointercalation of ethers by using another fluorinated ether(HFE) were also confirmed in a previous report by Watanabeet al.41 The increase in oxygen and fluorine peaks from the EDSof graphite electrodes cycled in DOL/BTFE electrolyte (Figure4d) provides evidence that BTFE plays a key role in theformation of a better SEI layer. The DOL/BTFE electrolytethus enables high capacity and excellent cyclability of graphite,making it a promising candidate as anode for replacing lithiummetal in Li/S batteries to improve both their cycle life andsafety.2.3. High-Performance Prelithiated Graphite/Sulfur

Batteries Enabled by DOL/BTFE Electrolyte. The remark-able cyclability offered by both the sulfur cathode and graphiteelectrode in lithium half-cells with DOL/BTFE electrolyte mayafford their use in prelithiated graphite/sulfur batteries, eventhough the stability of SEI layer on graphite remainsquestionable when it is exposed to highly reactive polysulfidesin electrolyte. To verify this setup, prelithiated graphite/sulfurcells were constructed by using electrochemically prelithiatedgraphite electrodes with capacity of ∼3.0 mAh cm−2 and sulfurcathodes with an areal sulfur loading of ∼3 mg cm−2 in the 1.0M LiTFSI DOL/BTFE (1:1, v/v) electrolyte. The arealcapacity ratio of negative to positive electrodes (N/P ratio) isabout 1.0−1.2, close to that in commercial Li-ion batteries, byassuming an initial discharge capacity of 1150 mAh g−1 for thesulfur cathodes.Figure 5 demonstrates promising electrochemical perform-

ance of the prelithiated graphite/sulfur cells in terms ofreversible capacity, Coulombic efficiency, stability, and ratecapability. The discharge−charge profiles at C/10 (Figure 5a)show typically two discharge plateaus over cycling, similar tothose of the Li/S cells while at slightly lower voltage of 2.2 and1.9 V due to the higher potential to “extract” Li+ ion fromgraphite. The full cell exhibits a highest second dischargecapacity of ∼950 mAh g−1 (3.1 mAh cm−2) based on the massof sulfur after the first cycle activation and shows improvedaverage CE of >95% without LiNO3 additive and remarkablecycle stability with discharge capacity greater than 600 mA hg−1 after 450 cycles (Figure 5b). The capacity fading rate at C/10 drops significantly to 0.082% per cycle for the prelithiatedgraphite/sulfur full cell, about an order of magnitude lower thanthat of Li/S cells at similar testing conditions (Figure 2b). Thissuggests that the loss of lithium and sulfur by irreversiblydeposits of Li2S2/Li2S at the anode, which plays a key role inthe capacity fading of Li/S batteries, becomes a minor factor inprelithiated graphite/sulfur cells. Moreover, such a long cyclelife and high capacity retention of the prelithiated graphite/sulfur cells were achieved with a low N/P ratio and high arealcapacity, suggesting that the SEI layer on graphite anode can bestable enough to endure the attack from polysulfides and tosuppress the continuous decomposition of electrolyte. There-fore, the growth of SEI layer, the increase in cell innerresistance, and fast depletion of electrolyte and lithium source

can be greatly alleviated, leading to significant improvement inthe cycling stability of the prelithiated graphite/sulfur cells.The excellent electrochemical reversibility of the sulfur

cathode and graphite anode and fast ion transport through thelow-viscosity 1.0 M LiTFSI DOL/BTFE electrolyte also enablethe prelithiated graphite/sulfur cells to deliver high capacitieseven at high rates. The good rate capability is illustrated inFigure 5c,d. The discharge capacity was close to 1000 mAh g−1

for the initial cycle at C/20, which faded gradually withincreasing rate followed by a higher polarization in thedischarge plateaus to ∼400 mAh g−1 at 1 C after 25 cycles.When the rate was gradually reset to C/20 after more than 65cycles, the capacity recovered to ∼800 mAh g−1 and remainedhigher than 700 mAh g−1 after further cycled at C/5 for up to100 cycles. The cyclability of prelithiated graphite/sulfur cells atelevated rates was also investigated by charging and dischargingit at C/2 and then C/5, with remaining discharge capacity of>400 mAh g−1 after totally 1000 cycles (Figure S4).Note that although the 1.0 M LiTFSI DOL/BTFE

electrolyte diminishes polysulfide solubility and forms a stableSEI on graphite anode, some amount of dissolved polysulfidemay still be able to penetrate through the SEI and be reducedby prelithiated graphite anode to form shorter chainpolysulfides, leading to a minor shuttle effect which can beevidenced from the average CE of ∼95%. The polysulfideshuttling can be further suppressed and CE can be improved toclose to 100% by increasing the concentration of LiTFSI to 2 Mto further reduce polysulfides dissolution and diffusion.However, no significant difference in the capacity retentionwas found between the prelithiated graphite/sulfur cells using 1and 2 M LiTFSI (Figure S5). This suggests that the loss oflithium and sulfur from irreversible deposition of Li2S2/Li2S atthe graphite anode may become negligible in the 1 M LiTFSIDOL/BTFE electrolyte, regardless of its slight shuttle effectand less than 100% of CE. Indeed, only very small amount ofsulfur species was observed on graphite anodes cycled in theprelithiated graphite/sulfur cell (see Figure S6 for SEM andEDS characterizations). The slow capacity fading in theprelithiated graphite/sulfur cells may mainly come from thesulfur cathode side, such as gradual structural deterioration andLi2S agglomeration due to the dissolution and redeposition ofsulfur species, and the resultant large volume changes.Therefore, advanced sulfur cathodes with high sulfur contentsand high loadings, enhanced polysulfide absorption, and betterstructural integrity could be further employed to optimizecycling performance and energy density of the prelithiatedgraphite/sulfur cells.

3. CONCLUSIONIn conclusion, by developing a fluorinated ether-basedelectrolyte of 1 M LiTFSI DOL/BTFE (1:1, v/v), wesuccessfully demonstrated long cycle life of prelithiatedgraphite/sulfur batteries with cell configurations close topractical applications (e.g., limited lithium source and highareal loading) using graphite as anodes. The exceptionalperformance of the batteries is mainly ascribed to multiplefunctions of the new fluorinated electrolyte which include (i)the electrolyte significantly reduces polysulfide solubility tosuppress the deleterious polysulfide shuttling and thus improvescapacity retention of sulfur cathodes, (ii) the electrolytedramatically enhances the reaction kinetics and utilization ofhigh-S-loading cathodes owing to its low viscosity and goodwettability, and (iii) this fluorinated electrolyte enables

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.6b11008ACS Appl. Mater. Interfaces 2017, 9, 6959−6966

6964

reversible cycling of graphite anodes by forming a stable SEIlayer on graphite surface. These advantages offered by the newfluorinated electrolyte enable a new prelithiated graphite/sulfurbattery to achieve excellent cycling stability. These promisingresults could bring prelithiated graphite/sulfur technique a bigstep forward toward practical applications.

4. EXPERIMENTAL PROCEDURES4.1. Material Preparation and Electrochemical Testing. The

electrolytes were made by dissolving 1.0 M lithium bis(trifluoro-methanesulfonyl)imide (LiTFSI) in mixtures of 1,2-dimethoxyethane(DME, BASF) or bis(2,2,2-trifluoroethyl) ether (BTFE, Sigma) with1,3-dioxolane (DOL, BASF) (v/v, 1:1). To improve the cyclingefficiency, 0.4 M LiNO3 was added to the DOL/DME electrolyte. Theconductivity of the 1 M LiTFSI DOL/BTFE (1:1, v/v) electrolyte is3.6 mS cm−1, which is lower than that of the DOL/DME electrolyte(11.2 mS cm−1). The viscosity of the DOL/BTFE and DOL/DMEbased electrolyte is 1.5 and 2.2 cP, respectively.Sulfur cathode materials were made by thermal treatment of 70 wt

% of elemental sulfur and 30 wt % of commercial Ketjenblack EC-600JD (KB) mixtures at 159 °C in sealed vials for 10 h, denoted asKB-S70. The KB-S70 composite was mixed with carbon nanofiber andpolyvinylpyrrolidone (PVP, Mw = 360 000) in a mass ratio of 70:20:10and stirred into a slurry with water for 4 h, then blade cast ontocarbon-coated aluminum foil with different thickness, and vacuum-dried overnight at 55 °C. Graphite electrode was prepared by mixing92.5 wt % of MesoCarbon MicroBeads (MCMB), 3.5 wt % of Super Ccarbon, 1.5 wt % of carboxymethyl cellulose (CMC), and 2.5 wt % ofstyrene−butadiene rubber (SBR) in water solution, then cast ontocopper foil, and vacuum-dried overnight at 100 °C. After drying, theelectrodes were pressed at 15 MPa and punched into round pieceswith diameter of 12 mm for sulfur cathode and 13 mm for graphiteelectrode. The thickness is ∼80 μm for both graphite anode and sulfurcathode with areal sulfur loading of ∼3 mg cm−2. Graphite wasprelithiated by discharge it to 0.01 V in coin cells with DOL/BTFEelectrolyte. CR2016 coin cells were assembled using lithium foil orprelithiated graphite as anode in an Ar-filled glovebox with H2O andO2 level below 0.1 ppm. Cycling performance was carried out on anArbin BT2000 or Land CT2001A battery tester from 1.6 to 2.6 V and0.01 to 1 V for sulfur and graphite half-cells, respectively, and 1.4−2.8V for full cells. 1 C rate is defined as 1680 mA g−1 of sulfur or 350 mAg−1 of graphite in half-cells. For the full cells, 1 C rate is defined as1680 mA g−1 of sulfur, and the specific capacity is calculated based onthe mass of sulfur.4.2. Characterizations. XRD was done on Rigaku Miniflex II

using an airtight specimen holder with beryllium windows purchasedfrom Rigaku. Co. SEM was carried out on a FEI Nova NanoSEM 630FESEM. The postcycled electrodes were collected in glovebox, washedwith DOL, vacuum-dried, and sealed in holders before they were takenout for characterizations.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.6b11008.

SEM images of cycled lithium anode and graphiteanodes, more discharge−charge profiles and cyclingperformance of Li/S and prelithiated graphite/sulfurcells (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: dwang@psu.edu (D.W.).

ORCIDDonghai Wang: 0000-0001-7261-8510

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the Department of Energy, Officeof Energy Efficiency and Renewable Energy (EERE), underAwards DE-EE0005475 and DE- EE0007795.

■ REFERENCES(1) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J. M.Li-O2 and Li-S Batteries with High Energy Storage. Nat. Mater. 2011,11, 19−29.(2) Manthiram, A.; Fu, Y.; Chung, S.-H.; Zu, C.; Su, Y.-S.Rechargeable Lithium-Sulfur Batteries. Chem. Rev. 2014, 114,11751−11787.(3) Bhaskara, R. M. L. Organic Electrolyte Cells. US Patent 3413154,1968.(4) Yamin, H.; Peled, E. Electrochemistry of a Nonaqueous Lithium/Sulfur cell. J. Power Sources 1983, 9, 281−287.(5) Shim, J.; Striebel, K. A.; Cairns, E. J. The Lithium/SulfurRechargeable Cell - Effects of Electrode Composition and Solvent onCell Performance. J. Electrochem. Soc. 2002, 149, A1321−A1325.(6) Cheon, S. E.; Ko, K. S.; Cho, J. H.; Kim, S. W.; Chin, E. Y.; Kim,H. T. Rechargeable Lithium Sulfur Battery - II. Rate Capability andCycle Characteristics. J. Electrochem. Soc. 2003, 150, A800−A805.(7) Cheon, S. E.; Ko, K. S.; Cho, J. H.; Kim, S. W.; Chin, E. Y.; Kim,H. T. Rechargeable lithium sulfur battery - I. Structural Change ofSulfur Cathode during Discharge and Charge. J. Electrochem. Soc. 2003,150, A796−A799.(8) Akridge, J. R.; Mikhaylik, Y. V.; White, N. Li/S FundamentalChemistry and Application to High-Performance RechargeableBatteries. Solid State Ionics 2004, 175, 243−245.(9) Xin, S.; Gu, L.; Zhao, N. H.; Yin, Y. X.; Zhou, L. J.; Guo, Y. G.;Wan, L. J. Smaller Sulfur Molecules Promise Better Lithium-SulfurBatteries. J. Am. Chem. Soc. 2012, 134, 18510−18513.(10) Zhang, B.; Qin, X.; Li, G. R.; Gao, X. P. Enhancement of LongStability of Sulfur Cathode by Encapsulating Sulfur into Micropores ofCarbon Spheres. Energy Environ. Sci. 2010, 3, 1531−1537.(11) Ji, X. L.; Lee, K. T.; Nazar, L. F. A Highly OrderedNanostructured Carbon-Sulphur Cathode for Lithium-Sulphur Bat-teries. Nat. Mater. 2009, 8, 500−506.(12) Chen, S. R.; Zhai, Y. P.; Xu, G. L.; Jiang, Y. X.; Zhao, D. Y.; Li, J.T.; Huang, L.; Sun, S. G. Ordered Mesoporous Carbon/SulfurNanocomposite of High Performances as Cathode for Lithium-SulfurBattery. Electrochim. Acta 2011, 56, 9549−9555.(13) Jayaprakash, N.; Shen, J.; Moganty, S. S.; Corona, A.; Archer, L.A. Porous Hollow Carbon@Sulfur Composites for High-PowerLithium-Sulfur Batteries. Angew. Chem., Int. Ed. 2011, 50, 5904−5908.(14) Wang, Y.-X.; Huang, L.; Sun, L.-C.; Xie, S.-Y.; Xu, G.-L.; Chen,S.-R.; Xu, Y.-F.; Li, J.-T.; Chou, S.-L.; Dou, S.-X. Facile Synthesis of aInterleaved Expanded Graphite-Embedded Sulphur Nanocomposite asCathode of Li-S Batteries with Excellent Lithium Storage Performance.J. Mater. Chem. 2012, 22, 4744−4750.(15) Song, J.; Yu, Z.; Gordin, M. L.; Wang, D. Advanced SulfurCathode Enabled by Highly Crumpled Nitrogen-Doped GrapheneSheets for High-Energy-Density Lithium-Sulfur Batteries. Nano Lett.2016, 16, 864−870.(16) Song, J.; Gordin, M. L.; Xu, T.; Chen, S.; Yu, Z.; Sohn, H.; Lu,J.; Ren, Y.; Duan, Y.; Wang, D. Lithium Polysulfide Chemisorption onElectroactive Sites of Nitrogen-Doped Carbon Composites For High-Performance Lithium-Sulfur Battery Cathodes. Angew. Chem., Int. Ed.2015, 54, 4325−4329.(17) Yang, Y.; Zheng, G. Y.; Cui, Y. Nanostructured Sulfur Cathodes.Chem. Soc. Rev. 2013, 42, 3018−3032.(18) Wei Seh, Z.; Li, W.; Cha, J. J.; Zheng, G.; Yang, Y.; McDowell,M. T.; Hsu, P.-C.; Cui, Y. Sulphur-TiO2 Yolk-Shell Nanoarchitecturewith Internal Void Space for Long-Cycle Lithium-sulphur batteries.Nat. Commun. 2013, 4, 1331.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.6b11008ACS Appl. Mater. Interfaces 2017, 9, 6959−6966

6965

(19) Yang, C.-P.; Yin, Y.-X.; Ye, H.; Jiang, K.-C.; Zhang, J.; Guo, Y.-G. Insight into the Effect of Boron Doping on Sulfur/Carbon Cathodein Lithium-Sulfur Batteries. ACS Appl. Mater. Interfaces 2014, 6, 8789−8795.(20) Wang, X.; Li, G.; Li, J.; Zhang, Y.; Wook, A.; Yu, A.; Chen, Z.Structural and Chemical Synergistic Encapsulation of PolysulfidesEnables Ultralong-Life Lithium-Sulfur Batteries. Energy Environ. Sci.2016, 9, 2533−2538.(21) Ahn, W.; Seo, M. H.; Jun, Y. S.; Lee, D. U.; Hassan, F. M.;Wang, X.; Yu, A.; Chen, Z. Sulfur Nanogranular Film-Coated Three-Dimensional Graphene Sponge-Based High Power Lithium SulfurBattery. ACS Appl. Mater. Interfaces 2016, 8, 1984−1991.(22) Zhou, G.; Li, L.; Ma, C.; Wang, S.; Shi, Y.; Koratkar, N.; Ren,W.; Li, F.; Cheng, H.-M. A Graphene Foam Electrode with HighSulfur Loading for Flexible and High Energy Li-S Batteries. NanoEnergy 2015, 11, 356−365.(23) Qie, L.; Manthiram, A. High-Energy-Density Lithium−SulfurBatteries Based on Blade-Cast Pure Sulfur Electrodes. ACS EnergyLetters 2016, 1, 46−51.(24) Lv, D.; Zheng, J.; Li, Q.; Xie, X.; Ferrara, S.; Nie, Z.; Mehdi, L.B.; Browning, N. D.; Zhang, J.-G.; Graff, G. L.; Liu, J.; Xiao, J. HighEnergy Density Lithium-Sulfur Batteries: Challenges of Thick SulfurCathodes. Adv. Energy Mater. 2015, 5, 1402290.(25) Zhang, S. S. Liquid Electrolyte Lithium/Sulfur Battery:Fundamental Chemistry, Problems, and Solutions. J. Power Sources2013, 231, 153−162.(26) Barchasz, C.; Lepretre, J. C.; Alloin, F.; Patoux, S. New Insightsinto the Limiting Parameters of the Li/S Rechargeable Cell. J. PowerSources 2012, 199, 322−330.(27) Lu, D.; Shao, Y.; Lozano, T.; Bennett, W. D.; Graff, G. L.;Polzin, B.; Zhang, J. G.; Engelhard, M. H.; Saenz, N. T.; Henderson,W. A.; Bhattacharya, P.; Liu, J.; Xiao, J. Failure Mechanism for Fast-Charged Lithium Metal Batteries with Liquid Electrolytes. Adv. EnergyMater. 2015, 5, 1400993.(28) Ma, Y.; Zhang, H.; Wu, B.; Wang, M.; Li, X.; Zhang, H. LithiumSulfur Primary Battery with Super High Energy Density: Based on theCauliflower-like Structured C/S Cathode. Sci. Rep. 2015, 5, 14949.(29) Shin, E. S.; Kim, K.; Oh, S. H.; Cho, W. I. PolysulfideDissolution Control: the Common Ion Effect. Chem. Commun. 2013,49, 2004−2006.(30) Suo, L. M.; Hu, Y. S.; Li, H.; Armand, M.; Chen, L. Q. A NewClass of Solvent-in-Salt Electrolyte for High-Energy RechargeableMetallic Lithium Batteries. Nat. Commun. 2013, 4, 1481.(31) Yamada, Y.; Yamada, A. Review - Superconcentrated Electro-lytes for Lithium Batteries. J. Electrochem. Soc. 2015, 162, A2406−A2423.(32) Dokko, K.; Tachikawa, N.; Yamauchi, K.; Tsuchiya, M.;Yamazaki, A.; Takashima, E.; Park, J.-W.; Ueno, K.; Seki, S.; Serizawa,N.; Watanabe, M. Solvate Ionic Liquid Electrolyte for Li-S Batteries. J.Electrochem. Soc. 2013, 160, A1304−A1310.(33) Lin, Z.; Liu, Z. C.; Fu, W. J.; Dudney, N. J.; Liang, C. D.Phosphorous Pentasulfide as a Novel Additive for High-PerformanceLithium-Sulfur Batteries. Adv. Funct. Mater. 2013, 23, 1064−1069.(34) Liang, X.; Wen, Z. Y.; Liu, Y.; Wu, M. F.; Jin, J.; Zhang, H.; Wu,X. W. Improved cycling performances of lithium sulfur batteries withLiNO3-modified electrolyte. J. Power Sources 2011, 196, 9839−9843.(35) Zhang, S. S. Role of LiNO3 in Rechargeable Lithium/SulfurBattery. Electrochim. Acta 2012, 70, 344−348.(36) Azimi, N.; Xue, Z.; Rago, N. D.; Takoudis, C.; Gordin, M. L.;Song, J.; Wang, D.; Zhang, Z. Fluorinated Electrolytes for Li-S Battery:Suppressing the Self-Discharge with an Electrolyte ContainingFluoroether Solvent. J. Electrochem. Soc. 2015, 162, A64−A68.(37) Gordin, M. L.; Dai, F.; Chen, S.; Xu, T.; Song, J.; Tang, D.;Azimi, N.; Zhang, Z.; Wang, D. Bis(2,2,2-trifluoroethyl) Ether as anElectrolyte Co-solvent for Mitigating Self-Discharge in Lithium-SulfurBatteries. ACS Appl. Mater. Interfaces 2014, 6, 8006−8010.(38) Lv, D.; Yan, P.; Shao, Y.; Li, Q.; Ferrara, S.; Pan, H.; Graff, G. L.;Polzin, B.; Wang, C.; Zhang, J.-G.; Liu, J.; Xiao, J. High Performance

Li-ion Sulfur Batteries Enabled by Intercalation Chemistry. Chem.Commun. 2015, 51, 13454−13457.(39) Bhargav, A.; Wu, M.; Fu, Y. A Graphite-Polysulfide Full Cellwith DME-Based Electrolyte. J. Electrochem. Soc. 2016, 163, A1543−A1549.(40) Moon, H.; Mandai, T.; Tatara, R.; Ueno, K.; Yamazaki, A.;Yoshida, K.; Seki, S.; Dokko, K.; Watanabe, M. Solvent Activity inElectrolyte Solutions Controls Electrochemical Reactions in Li-Ionand Li-Sulfur Batteries. J. Phys. Chem. C 2015, 119, 3957−3970.(41) Li, Z.; Zhang, S.; Terada, S.; Ma, X.; Ikeda, K.; Kamei, Y.; Zhang,C.; Dokko, K.; Watanabe, M. Promising Cell Configuration for Next-Generation Energy Storage: Li2S/Graphite Battery Enabled by aSolvate Ionic Liquid Electrolyte. ACS Appl. Mater. Interfaces 2016, 8,16053−16062.(42) Bruckner, J.; Thieme, S.; Bottger-Hiller, F.; Bauer, I.;Grossmann, H. T.; Strubel, P.; Althues, H.; Spange, S.; Kaskel, S.Carbon-Based Anodes for Lithium Sulfur Full Cells with High CycleStability. Adv. Funct. Mater. 2014, 24, 1284−1289.(43) Liu, N.; Hu, L.; McDowell, M. T.; Jackson, A.; Cui, Y.Prelithiated Silicon Nanowires as an Anode for Lithium Ion Batteries.ACS Nano 2011, 5, 6487−6493.(44) Hassoun, J.; Scrosati, B. A High-Performance Polymer TinSulfur Lithium Ion Battery. Angew. Chem., Int. Ed. 2010, 49, 2371−2374.(45) Aurbach, D.; Zaban, A.; Ein-Eli, Y.; Weissman, I.; Chusid, O.;Markovsky, B.; Levi, M.; Levi, E.; Schechter, A.; Granot, E. RecentStudies on the Correlation between Surface Chemistry, Morphology,Three-Dimensional Structures and Performance of Li and Li-CIntercalation Anodes in Several Important Electrolyte Systems. J.Power Sources 1997, 68, 91−98.(46) Rosenman, A.; Elazari, R.; Salitra, G.; Markevich, E.; Aurbach,D.; Garsuch, A. The Effect of Interactions and Reduction Products ofLiNO3, the Anti-Shuttle Agent, in Li-S Battery Systems. J. Electrochem.Soc. 2015, 162, A470−A473.(47) Zhang, S. S. Effect of Discharge Cutoff Voltage on Reversibilityof Lithium/Sulfur Batteries with LiNO3-Contained Electrolyte. J.Electrochem. Soc. 2012, 159, A920−A923.(48) Hagen, M.; Hanselmann, D.; Ahlbrecht, K.; Maca, R.; Gerber,D.; Tubke, J. Lithium−Sulfur Cells: The Gap between the State-of-the-Art and the Requirements for High Energy Battery Cells. Adv. EnergyMater. 2015, 5, 1401986.(49) Cuisinier, M.; Cabelguen, P. E.; Adams, B. D.; Garsuch, A.;Balasubramanian, M.; Nazar, L. F. Unique Behaviour of Nonsolventsfor Polysulphides in Lithium-Sulphur Batteries. Energy Environ.Energy Environ. Sci. 2014, 7, 2697−2705.(50) Cao, R.; Xu, W.; Lv, D.; Xiao, J.; Zhang, J.-G. Anodes forRechargeable Lithium-Sulfur Batteries. Adv. Energy Mater. 2015, 5,1402273.(51) Azimi, N.; Weng, W.; Takoudis, C.; Zhang, Z. ImprovedPerformance of Lithium-Sulfur Battery with Fluorinated Electrolyte.Electrochem. Commun. 2013, 37, 96−99.(52) Zu, C.; Azimi, N.; Zhang, Z.; Manthiram, A. Insight intoLithium-Metal Anode in Lithium-Sulfur Batteries with a FluorinatedEther Electrolyte. J. Mater. Chem. A 2015, 3, 14864−14870.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.6b11008ACS Appl. Mater. Interfaces 2017, 9, 6959−6966

6966