ORIGINAL PAPER

Ethylene sulfate as film formation additive to improvethe compatibility of graphite electrode for lithium-ion battery

Xuecheng Li & Zhoulan Yin & Xinhai Li & Chao Wang

Received: 12 September 2013 /Revised: 10 November 2013 /Accepted: 24 November 2013# Springer-Verlag Berlin Heidelberg 2013

Abstract Ethylene sulfate (DTD) is investigated as a novelfilm formation electrolyte additive for graphite anode ma-terial in lithium-ion battery. The CV results reveal thatDTD is reduced prior to ethylene carbonate (EC) at theinterface between graphite and electrolyte, while it cannotprevent the sustained reduction of propylene carbonate(PC) when the amount of DTD is lesser than 3 wt% inthe PC-based electrolyte. XPS analyses demonstrate thatthe reduction products of DTD, Li2SO3, and ROSO2Li areformed at the surface of graphite in the EC-based electro-lyte, which is beneficial to lower the interfacial resistanceas suggested by the EIS results. In addition, SEM imagesshow a smoother and homogeneous surface film at thesurface of graphite when DTD is incorporated into theelectrolyte. Consequently, the Li/graphite half cells cycledin EC-based electrolyte containing DTD exhibit higherspecific capacity and improved cycling capability than thatwithout DTD.

Keywords Lithium-ion battery . Ethylene sulfate . Filmformation additive . Electrolyte

Introduction

In the past few decades, lithium-ion batteries (LIBs) havedeveloped as a promising power source for electric vehicles(EVs) and plug-in hybrid electric vehicles (HEVs) because oftheir high energy density and long life cycle [1, 2]. Commer-cial anode electrode material is mainly natural graphite inrechargeable LIBs [3]. It is well known that each kind ofgraphite electrode has its most suitable electrolyte. However,there still exist many problems about the commercial electro-lytes which can match the graphite electrode with high energydensity and high power density.

During the first cycle of lithium-ion batteries, solvent de-composition reaction at the surface of the electrode will resultin formation of a passivating surface film, which is commonlynamed as the solid electrolyte interface (SEI). More impor-tantly, the properties of SEI film directly determine the wholeperformance of the battery [4]. A small amount of film-forming additive added into the electrolyte will improve theperformance of the battery because of occurrence of reductivedecomposition preferentially, which form SEI at the surface ofthe graphite electrode [5–8]. The commercial anode materialsfor lithium-ion battery include natural graphite, artificialgraphite (AG), mesocarbon microbeads (MCMB), and soon. However, there have been few studies about the modifiedAG of high density capacity because of the bad wetabilitybetween electrolyte and graphite. In this study, a newAG fromShanghai Shanshan Super Energy Composite Materials Co.,Ltd. was chosen as the anode electrode material, which hashigh compaction density and high-specific capacity comparedwith ordinary graphite, but the corresponding electrolyte doesnot exist yet in the market.

Reports about the compatibility between electrolyte andanode electrode material of graphite can be found elsewherenow [9, 10]. Xu et al [11] considered that the compatibilitybetween electrolyte and graphite is essentially based on the

X. Li : Z. Yin (*) :X. LiCollege of Chemistry and Chemical Engineering, Central SouthUniversity, Changsha, Hunan 410083, Chinae-mail: lixuecheng0125@163.com

Z. Yine-mail: yzllxh@gmail.com

C. WangJiangxi Youli New Material Co., Ltd, Pingxiang,Jiangxi 337000, China

X. LiSchool of metallurgy and Enviroment, Central South University,Changsha, Hunan 410083, China

IonicsDOI 10.1007/s11581-013-1036-5

reduction reaction of electrolyte at the surface of anode-active-material particles to generate effective SEI film. In order tofind good electrolyte matching with graphite, it is necessary tosearch for functional additives which can form effective SEIfilm. Sulfide additives such as SO2 [12], CS2 [13], and sulfinyladditives [14–16] have been studied for many years. Thestudy of Yu [17] suggests that the performance of theLiCoO2/graphite battery can be significantly improved when0.3 wt% ES is added into 1 mol/L LiPF6/EC+DEC+DMC(1:2:2, v:v:v). In our researching process of effective film-forming additive, we found that ethylene sulfate (DTD) andethylene sulfite (ES) have similar structure (Fig. 1), whichsuggests that DTD may be a promising additive. In addition,the LUMO energy (0.11 eV) of DTD is lower than ethylenecarbonate (EC) (0.95 eV) according to our quantum chemicalcalculation results, which means it is easier to be reducedat the graphite anode. However, it was not reported so farto our best knowledge. To understand the compatibilitybetween the electrolyte containing DTD and graphite,DTD has been studied as an electrolyte additive for AGhalf battery with 1 mol L−1 LiPF6/EC+DMC+EMC(1:1:1, w:w:w) electrolyte.

Experimental

Ethylene carbonate (EC), ethyl methyl carbonate (EMC),dimethyl carbonate (DMC), and propylene carbonate (PC)were provided by Jiangxi Youli New Materials Co. Ltd.,China. Lithium hexafluorophosphate (LiPF6) was purchasedfrom Morita Chemical Industries (Zhangjiagang) Co., Ltd.,China. The electrolytes of 1 mol L−1 LiPF6/EC+DMC+EMC(1:1:1, w:w:w) were used as the blank electrolyte (BE). DTDwas obtained from Fujian Chuangxin Science and TechnologyDevelops Co., Ltd., China, and it was added with the amountof 0 wt%, 1 wt%, 2 wt%, and 3 wt% into BE in an argon-filledglove box (H2O<10 ppm, O2<1 ppm, Mikrouna). All thesechemicals were used without further purification.

AG was obtained from Shanghai Shanshan Super EnergyComposite Materials Co., Ltd., China. The graphite electrode

was prepared by 90 wt% AG, 3 wt% acetylene black, and7 wt% polyvinylidene fluoride (PVDF), coating onto copperfoil current collector and dried at 60 °C for 8 h under vacuumcondition. Coin-type cells assembled with graphite electrodeas working electrode and lithium foil counter electrode wereassembled in an argon-filled glove box.

The structure of AG was determined by X-ray diffraction(XRD, CuKα radiation, Rint-2000, Rigaku).

The electrochemical cycling performance of the half cellswas tested on Neware battery test system (Shenzhen, China)in the potential range of 0.01–2 V (vs. Li/Li+) at a constantcurrent mode of 0.1 C (1 C=350 mAh g−1). Cyclic voltamm-etry (CV) measurements were performed on electrochemicalworkstation (CHI604E, Chenhua, Shanghai) in the potentialrange of 0–2 V at a scanning rate of 0.1 mV s−1 At the initialand the last cycle, electrochemical impedance spectroscopy(EIS) of the half batteries was observed immediately withfrequency ranging from 0.1 Hz to 1 MHz.

In order to analyze the microstructure and chemical compo-sitions of the graphite electrodes after charge–discharge cyclingmeasurements, the batteries were disassembled in an argon-filled glove box. The graphite electrodes were carefully takenout and rinsed with anhydrous DMC to remove the residualelectrolyte and then dried in vacuum at room temperature toevaporate the DMC solvent. The morphology of the graphiteelectrodes was observed by scanning electron microscopy(SEM, Sirion200). The surface compositions were determinedby a transmission electron microscope (Tecnai G12, 200 kV).

Results and discussion

XRD analysis

Figure 2 shows the XRD pattern of graphite material. It can befound that the 2H (100) and 2H (101) peaks are clearly

Fig. 1 Structural molecular formula of DTD and ES Fig. 2 The XRD spectrum of graphite

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exhibited at 42.5° and 44.7°. The 3R (101) and 3R (012) peaksdo not appear. It suggests that the graphite exists as pure 2Hphase, which has low irreversible capacity loss in the chargingand discharging process of lithium-ion batteries. Disma et al.[18] considered that graphite with 3R phase has some defectwhich can cause to irreversible capacity loss easily. The sharppeak (002) of graphite at 2θ=26.5° indicates that the graphitehas a good graphitization degree structure. The spacing oflayers is 0.3371 nm, which is close to the interlayer distanceof ideal graphite (0.3354 nm). The results illustrate that thegraphite owns good crystalline.

Charge–discharge experiments

Figure 3 presents the initial voltage profiles of Li/graphite halfcells in 1 mol L−1 LiPF6/PC+DMC+EMC (1:1:1, w:w:w)with the amount of DTD (0 wt%, 1 wt%, 2 wt%, and 3 wt%).For the electrolyte with 0 wt%, 1 wt%, and 2 wt% DTD, thecointercalation of PC and the solvated lithium ions into thegraphite anode were observed at the voltage plateau around0.75 V. Apparently, the unsolvated lithium ions cannot beproperly intercalated into the graphite in PC-based electro-lyte because the voltage potential could not reach 0.2 V.But for the electrolyte with 3 wt% DTD, the potentialdeclined in the range of 1.25–0.2 V and a long plateauabout 0.2–0 V can be identified, corresponding to thereduction of DTD and the intercalation of lithium ionsinto the graphite, respectively.

To understand the effect of the content of DTD additive onthe cycling performance, the first charge–discharge profiles ofgraphite anode in the EC-based electrolytes containing vari-ous contents of DTD (0 wt%, 1 wt%, 2 wt%, and 3 wt%) wereshown in Fig. 4. The charge and discharge platform of halfcells using 1 mol L−1 LiPF6/EC+DMC+EMC (1:1:1, w:w:w)

with various contents of DTD as electrolyte are similar to thatwithout additive. It can be seen from the initial discharge(lithium intercalation) curve that the discharge process appearsin two platforms, the first platform is at about 1.25 V, and thecurve drops slowly from 1.25 V to 0.2 V. The second is a longvoltage platform ranging from 0.2 V to 0.01 V. The firstplatform corresponds to the reduction reaction of DTD. Thesecond one corresponds to lithium intercalation process [19].It is found that the cell using 1 wt% DTD exhibits the highestcharge–discharge capacity. For the electrolyte without DTD,the charge capacity in the first cycle was just 335.9 mAh g−1.For the electrolytes with 1 wt% DTD and 2 wt% DTD, thecapacities were 351.8 mAh g−1 and 340.0 mAh g−1, respec-tively. The reason for the improved initial charge capacity maybe that DTD can preferentially occupy the active sites atthe surface of the graphite and protect more effectively thegraphite electrode from the consumption of electrolyte.While for the electrolyte with 3 wt% DTD, the capacitydecreased. It can be attributed to the thicker SEI, increasinginitial capacity loss and the interface resistance of the cell.Therefore, the optimal content of DTD is 1 wt% in EC-based electrolyte.

Figure 5 shows the results of the cycle performance. Thecapacity retentions of the batteries with 0 wt% DTD, 1 wt%DTD, 2 wt% DTD, and 3 wt% DTD after 50 cycles are85.3 %, 105.3 %, 98.4 %, and 96.0 %, respectively. Thisindicates that the cycling performance of the battery inelectrolyte with 1 wt% DTD is the best. Its capacityincreased instead of decreasing. This suggests that theDTD may affect the formation of SEI to protect thegraphite anode. A more detailed analysis of the effect of1 wt% DTD on electrochemical performance will be furtherstudied in the following work.

Figure 6 shows the cyclic voltammograms of cells at a scanrate of 0.1 mV s−1. In the first cycle, the initial discharge curve

Fig. 3 Initial discharge and charge curves of Li/graphite half cells in1 mol L−1 LiPF6/PC+DMC+EMC (1:1:1, w:w:w) with various contentsof DTD

Fig. 4 Initial charge–discharge curve of Li/graphite half cells in1 mol L−1 LiPF6/EC+DMC+EMC (1:1:1, w:w:w) with various contentsof DTD

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of the battery with 1 wt% DTD, 2 wt% DTD, and 3 wt% DTD(Fig. 6b–d) displays reduction peak at about 1.25 V, but it donot appear at the same potential for the electrolyte withoutadditive (Fig. 6a). This peak indicates that DTD was reducedat the voltage of 1.25 Vat the surface of graphite electrode and

forms a passivation layer in the first cycle. The formed SEIfilm was considered to be thin according to the weak intensityand small area of the peak, which was consistent with thedepletion of Li+ attributes to the formation of SEI film. In thesecond and third lap scans, the sharp peaks of intercalation oflithium ions with graphite at 0–0.2 V area of the electrolytecontaining DTD increase and display good reversibility. Itsuggests that the addition of DTD reduces the polarization.The results show that the formation of SEI film in the firstcycle prevents the further reduction of solvent in the electro-lyte, and DTD as an additive can improve the compatibility ofelectrolyte and the graphite electrode.

AC impedance measurement

The EIS of the cells are shown in Fig. 7. The cell containingDTD shows greater impedance value compared to that with-out additive in the first cycle, whereas it displays small im-pedance value after 50 cycles in Fig. 7d. This suggests that theaddition of DTD increases the initial resistance and decreasesthe film resistance after circulation. The result is consistentwith the battery with DTD that has a higher charge capacity in

Fig. 6 Initial three cyclic voltammograms of cells using a BE, b BE+1 wt% DTD, c BE+2 wt% DTD, d BE+3 wt% DTD

Fig. 5 Cycle performance of Li/graphite half cells using a BE, b BE+1 wt% DTD, c BE+2 wt% DTD, d BE+3 wt% DTD

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the first cycle (Fig. 4). After 50 cycles, the first semicircleattributes to surface film resistance of the graphite with DTDshows smaller value compared with that without DTD. It maybe ascribed that the interfacial film derived from DTD reactswith the active sites of the surface of the graphite, which isformed in the cycling stage.

SEI film analysis

The surface morphology of the pristine electrode and thegraphite electrodes without and with 1 wt% DTD after50 cycles is shown in Fig. 8. It can be seen clearly thatthe morphology of graphite in this study is needle; thesurface of graphite before the electrochemical experimentsdoes not appear in layer, as shown in Fig. 8a. Both theelectrolytes without and with 1 wt% DTD are completelycovered with a passivation layer at the electrode surfaceafter 50 cycles, as shown in Fig. 8b and c. The surfacemorphology of the electrode after 50 cycles is significantlydifferent when the electrolyte is with 1 wt% DTD or not.Specifically, the surface film of graphite electrode withoutadditive in the electrolyte after 50 cycles is rough anduneven, while the surface film becomes smoother andmore homogeneous when 1 wt% DTD is added into theelectrolyte. This means that the morphology of SEI film atthe surface of graphite electrodes is improved as DTD isadded. It is mainly because of the relatively low LUMOand the high reduction potential of DTD, which candecompose at the surface of the graphite electrode andchange the composition of SEI film.

The film components of the graphite electrodes were ana-lyzed by XPS as shown in Fig. 9. The C 1 s spectra exhibitslarge difference between graphite electrodes cycled in electro-lyte without and with 1 wt% DTD. For graphite anode cycledin electrolyte without additive, there exist four Gaussian peakswith binding energies of 285.6 eV (C-OH), 289.4 eV (C=O

bond in lithium alkyl carbonates (RCH2OCO2Li) or PVDF),292.0 eV (C=O bond in Li2CO3), and 295.8 eV (C=O),respectively. As for graphite anode cycled in electrolyte with1 wt% DTD, the intensities of C=O bond in lithium alkylcarbonates (RCH2OCO2Li) or Li2CO3 are changed. This indi-cates that DTD changes the component percentage of

Fig. 7 EIS patterns of the cells without and with 1 wt% DTD before(a and b) and after (c and d) 50 cycles

Fig. 8 SEM images of AG surface: a pristine electrode, cycled electrodeafter 50 cycles in b BE, and c BE+1 wt% DTD

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RCH2OCO2Li and Li2CO3 in SEI through participating in theformation of film and improves the stability of SEI.

There are four peaks in theO 1 s spectra of graphite electrodecycled in electrolyte without additive: 531.4 eV (Li2CO3),533.9 eV, 536.1 eV, and 538.6 eV (C=O bond). But these peaksshift and the number of peaks is reduced when DTD is added inthe electrolyte. The possible reason is that the addition of DTDprompts the formation of new compounds, which affect thelocation and the number of C=O peak.

Two absorption peaks of S 2p are observed in Fig. 9 forgraphite electrodes cycled in electrolyte with the addition of1 wt% DTD. From the S 2p XPS pattern, Li2SO3 (169.7 eV)and ROSO2Li (172.3 eV) are detected at the surface of graphitecycled in the electrolyte with 1 wt% DTD. However, those

peaks do not appear in the surface of graphite cycled in theelectrolyte without 1 wt% DTD. It indicates that DTD isinvolved in the formation of SEI film, which is consistent withthe CV results.

Mechanism analysis

Based on these experimental results, the reactions of filmformation process can be speculated as shown in Fig. 10.Above all, the O atoms of DTD get an electron in the processof electrochemical reduction. After that, the five-memberedring breaks off and forms the free radical intermediates. Theradical intermediates are lively, and it can further react withother DTD molecules or other intermediates. At last, it turns

Fig. 9 XPS spectra of graphite electrodes after 50 cycles for C 1 s, O 1 s, and S 2p

Fig. 10 The proposeddecomposition mechanism ofDTD

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out into sulfites, such as Li2SO3, and ROSO2Li, covering atthe surface of graphite electrode.

Conclusions

Compatibility between graphite electrode and DTD-containingelectrolyte is investigated. It is found that DTD can be used asan effective electrolyte additive to improve the cycling perfor-mance of the cell. The electrochemical tests show that the cellwith electrolyte containing 1 wt% DTD exhibits the highestcapacity and the best cycle performance. The reason is that theelectrolyte containing DTD preferentially occupies the activesites of the graphite to form relatively thin and compact SEIfilm where only Li+ can pass through and result in lowerinterfacial impedance. Therefore, DTD would be a promisingfilm formation additive in lithium-ion battery.

Acknowledgments This work was financially supported by theNational Basic Research Program of China (973 Program 2014CB643406)and the Major Special Plan of Science and Technology of Hunan Province,China (No.2011FJ1005).

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