propane and Fluoroethylene Carbonate as Cosolvents for Nonaqueous

Mar 25, 2014 - propane (HFE) and fluoroethylene carbonate (FEC) as cosolvents is ... reduce the surface tension of the electrolyte, while FEC increase...
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Compatibility of Graphite with 1,3-(1,1,2,2Tetrafluoroethoxy)propane and Fluoroethylene Carbonate as Cosolvents for Nonaqueous Electrolyte in Lithium-Ion Batteries Guochun Yan, Xinhai Li,* Zhixing Wang, Huajun Guo, and Jiexi Wang School of Metallurgy and Environment, Central South University, Changsha 410083, People’s Republic of China ABSTRACT: The compatibility of graphite with 1,3-(1,1,2,2-tetrafluoroethoxy)propane (HFE) and fluoroethylene carbonate (FEC) as cosolvents is investigated with Li/graphite cells. Having lower surface tension, HFE can act as a “surfactant” to reduce the surface tension of the electrolyte, while FEC increases it. Charge−discharge tests validate that the Li/graphite cells show a superior cycling performance and rate ability in 1 M (M = mol L−1) LiPF6-ethylene carbonate (EC)/ethyl methyl carbonate (EMC)/HFE (mixing ratio: 1/1/1 in weight). Cyclic voltammetry, electrochemical impedance spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy results reveal the mechanism that HFE is reduced with EC together in an EC-based electrolyte, forming a compact surface film at the graphite surface with lower interface resistance compared with FEC. In addition, we proposed that HFE will produce fluoroalkyl lithium compounds (LiOCF2CF2CF2CF2OLi) through radical termination reaction in the reduction process.

1. INTRODUCTION Graphite-based anode materials have been widely used in commercial lithium-ion batteries (LIBs). In the past few decades, many researchers focused on the interface properties between graphite and electrolyte, such as the poor compatibility of graphite with propylene carbonate (PC) due to the cointercalation of PC1,2 and highly reversibly intercalation and deintercalation of Li+ in EC-based electrolytes.3 In this stage, Peled and Besenhard et al. proposed some models to explain the mechanism of solid electrolyte interface (SEI) formation, which is considered a vital factor affecting the electrochemical performance of graphite electrodes.4−6 It is commonly accepted that the electrolyte components, including additives, lithium salts, and solvents, have a great impact on the stability and interface resistance of the SEI film. On the one hand, a series of film-forming additives such as vinylene carbonate (VC),7,8 ethylene sulfite (ES),9 and FEC10 were introduced into the electrolyte successively. It was confirmed as an effective way to improve the cyclability and to reduce the interfacial impedance of LIBs. Generally, the additives will be reduced prior to the decomposition of solvents because the lowest unoccupied molecular orbital (LUMO) energies of additives are lower than the solvents. Then, the constitution of the SEI film at the surface of graphite is altered with the participation of these additives, enhancing the stability of the SEI film. On the other hand, some lithium salts, such as lithium bis(oxalato)borate (LiBOB), 11 lithium difluoro(oxalato)broate (LiDFOB),12 and lithium bis(fluorosulfonyl)imide (LiFSI),13 exhibit a superior compatibility with graphite. The decomposition products of these lithium salts can form a stable SEI film at the surface of graphite with a very low interfacial impedance. In addition, the solvents of nonaqueous © 2014 American Chemical Society

electrolytes also affect the electrochemical properties of graphite considerably. For example, 1,2-dimethoxyethane (DME) is an attractive electrolyte solvent because of the large donor number, high chemical stability, and low viscosity. Moreover, the lithium salt can dissolve in DME solutions easily as the strong coordination of Li ions with the lone pair of O atoms in DME.13,14 However, the graphite anode material does not work reversibly in DME-based electrolytes.14 Recently, substituting H atoms in ethers and esters by F atoms was confirmed an effective strategy to lower their highest occupied molecular orbital (HOMO) energies and LUMO energies. Lower HOMO energies lead to strong tolerance to oxidation. Thereby, it can be used as cosolvents in high voltage electrolyte to support the normal operation of high voltage cathode material.15,16 Meanwhile, fluorinated carbonates and ethers were investigated as film-forming additives for graphite because of the higher reduction potential compared with traditional carbonate solvents.10,17 However, there is no published research to report and compare the effect of FEC and HFE as cosolvents on graphite to the best of our knowledge. It is meaningful to study the compatibility of graphite with fluorocarbonates and fluoroethers. In the present study, we introduced a new fluoroether, HFE, as a solvent for EC-based electrolytes. In addition, we compared the interfacial properties between graphite and electrolyte using HFE and FEC as cosolvents in Li/graphite cells. Received: December 5, 2013 Revised: March 15, 2014 Published: March 25, 2014 6586

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2. EXPERIMENTAL AND THEORETICAL METHODS

3. RESULTS AND DISCUSSION As low LUMO energy leads to a high reduction potential, it was reported as useful data to predict the decomposition sequence of the solvents at the graphite electrode.19,20 The calculation results of HOMO and LUMO energies are listed in Table 1.

2.1. Density Functional Theory (DFT) Calculations. Full geometry optimizations of these solvents molecular were performed at the B3LYP/6-31G (d) level before conducting energy calculations. The calculations of HOMO and LUMO energies were carried out using DFT calculations with the B3LYP/6-311G (3df, 2p) basis set.18 2.2. Preparation of Electrolytes. Jiangxi Youli New materials Co., Ltd. (China) kindly provided Li battery grade EC, EMC, FEC, and HFE. All electrolytes containing 1 M LiPF6 (Morita Chemical Industries (Zhangjiagang) Co., Ltd., China) were prepared in a glovebox under Ar atmosphere, and the water content of the electrolytes was less than 10 ppm. The water content of electrolytes was measured by Karl Fischer titration (Metrohm, 831 KF Coulometer, Switzerland). The conductivity and viscosity of the electrolytes were tested by the conductivity meter (INESA, DDS-307, China) and viscometer (Brookfield, DVI-prime+LVDV-IPCP, American), respectively. In addition, the surface tension of the electrolytes was measured by a tensiometer (KRÚ SS, K100, Germany). 2.3. Electrochemical Tests. The artificial graphite (FSN-4, Shanghai Shanshan Technology Co., Ltd., China) electrode was prepared by mixing 90 wt % graphite materials, 3 wt % Super P carbon, 7 wt % polyvinylidene fluoride (PVDF), coating on Cu foil. The mass loading of the graphite electrode was 1.30 mg/ cm2. Electrochemical charge−discharge tests were performed with 2025 coin-type cells utilizing a Neware battery circler between 2.0 and 0.01 V (vs Li/Li+) at room temperature. All potentials in this article were referenced to the Li/Li+ redox couple. The cells were discharged and charged at 0.1 C (1 C = 350 mA g−1) three cycles previously to ensure the SEI film formation was finished. For the rate performance tests, the electrodes were discharged at 0.1 C and charged at various current densities from 0.1 to 5 C. Cyclic voltammetry (CV) experiments were carried out with an Electrochemical Workstation (Chenhua, CHI604E, China) at a sweep rate of 0.1 mV s−1. The electrochemical impedance spectroscopy (EIS) of the cells at 100% depth of discharged (DOD) after 1 cycle was performed over a frequency range from 100 kHz to 0.01 Hz. 2.4. XPS, Raman, and TEM Sample Preparation. To analyze the graphite surface of the cycled electrode, the cells were disassembled in a glovebox filled with Ar gas. The anode was rinsed by high purity dimethyl carbonate to remove the electrolyte residue, followed by drying in a vacuum box at 40 °C for 6 h. Raman spectra of the graphite electrodes were obtained from a WiTec Alpha 300 system with a 632.8 nm wavelength laser light. The surface morphology of graphite was observed by a transmission electron microscope (Tecnai, G12, 200 kV). X-ray photoelectron spectroscopy (XPS, PHI 5600, Perkin-Elmer) was employed to identify the element composition at the graphite surface. Before the TEM, XPS, and Raman tests, the cycled electrodes were cut into three pieces in a glovebox. To conduct TEM measurements, the cycled graphite material was scraped from the electrode and dispersed in high purified ethanol absolute by ultrasound. Then, the graphite was transferred onto the microgrid through a pipet and was ready to observe in the vacuum chamber. For the XPS and Raman tests, the cut electrodes were measured directly in the corresponding instruments.

Table 1. Chemical Structure and HOMO/LUMO Energies of EC, FEC, HFE, and EMC

FEC owns the lowest LUMO energies among these five solvents, indicating that it will be reduced at the graphite surface first. Thereby, FEC will decompose prior to the EC since the LUMO energies is lower than that of EC, which illustrates that the passivation layer derived from the decomposition of FEC may alter the constitution of the SEI film. Nevertheless, the LUMO energy of HFE is slightly less than that of EC, which indicates that HFE may be reduced before EC reduction. Meanwhile, it may decompose with EC together as the LUMO energies are so close to each other. In addition, EMC possesses the highest LUMO energies, suggesting that it is the last one to be decomposed in the cathodic process. Furthermore, the HOMO energies of FEC and HFE are lower than conventional carbonate solvents, which is consistent with the results of previous studies.16,21 To investigate the compatibility of HFE and FEC as cosolvents with graphite, we formulated three electrolytes as shown in Table 2. Basic physicochemical properties of the Table 2. Physicochemical Properties of the Electrolytes at 25 °C mark EE EEF EEH

composition (wt %) 1 M LiPF6 in EC/EMC (3/7) 1 M LiPF6 in EC/EMC/FEC (1/1/1) 1 M LiPF6 in EC/EMC/HFE (1/1/1)

conductivity (ms cm−1)

viscosity (cP)

8.82 8.36

2.88 4.64

5.41

5.32

electrolytes are also presented in Table 2. The conductivity of electrolytes decreases obviously with the addition of HFE, while it drops a little using FEC as a cosolvent. The conductivity results from the amount of charges in the electrolyte, and a higher dielectric constant of the solvent means that the lithium salt dissolves in the solvent easily to produce more charges.6,20 Therefore, a high dielectric constant leads to a large conductivity. In addition, the viscosity will also affect the conductivity according to the Einstein equation (correlation between ionic mobility and diffusivity) and the Stokes−Einstein 6587

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distinctions in terms of the peak voltage and peak current, which relate to the SEI film forming process. More specifically, EC starts to be reduced around 1.5 V, and FEC decomposes at about 1.8 V. As for the EEH electrolyte sample, HFE is initially reduced at about 1.5 V. The results indicate that HFE and EC may be reduced together. In addition, there is a cathodic peak positioned at about 0.7 V in both the EE and the EEH sample, which is due to the reduction of EMC. In contrast, that peak disappears in the EEF electrolyte sample, demonstrating that the SEI film originating from the decomposition of FEC can prevent the reduction of EMC. These CV results are in accordance with the reduction sequence revealed by the LUMO energies. Moreover, the peak current of the peak around 1.4 V in EEF sample is almost double that of EE and EEH as can be seen from Figure 2a−c. As a result, the SEI film formed with participation of FEC should be thicker than that of HFE. The anodic peak in three electrolyte samples are similar to each other, and the peak at 0.25 V can be assigned to the delithiation process of graphite. As for the second cycle, the cathodic peaks related to the reduction of solvents do not appear except for the peak corresponding to the lithiation process. To investigate the effect of the SEI film originating from FEC and HFE on the interface impedance, EIS experiments of Li/ graphite cells cycled in EEF and EEH electrolyte after 1 cycle were carried out. As can be seen apparently from Figure 2d, the interfacial impedance of the cell in EEF electrolyte (91.84 Ω) is larger than that in the EEH electrolyte (66.35 Ω). It reveals that the enhanced cycling performance of graphite in EEH sample is due to the favorable film formation of HFE at the electrode surface. To explore further the compatibility of graphite with FEC and HFE as cosolvents for electrolytes, Figure 3 shows the cycling performance and rate capability of the brother cells presented in Figure 1. After 50 cycles at 0.1 C, the cell with the EEH electrolyte can still maintain 99.25% of its initial charge capacity, while the cell with the EEF sample has 95.59%. When it comes to the following 100 cycles at 2 C rate, the capacity gap enlarges considerably between the cells with the EEH and EEF samples. More specifically, the cell cycling in EEH electrolyte delivers a capacity of 327.4 mAh g−1, whereas it is only 282.7 mAh g−1 for the cell cycled in the EEF electrolyte. Those experimental data from Figure 3a strongly demonstrate that HFE enhances not only the cycling performance but also the rate capability compared with FEC as a cosolvent in a nonaqueous electrolyte for LIBs. Figure 3b shows the rate capability difference of the cells with EEH and EEF samples more clearly as the cell cycling in the EEH sample displays a higher capacity than that in the EEF sample at various rates. Such results are a bit beyond our comprehension because the conductivity of the EEF sample is higher than that of the EEH sample as listed in Table 2. With the curiosity to trace the distinction of the physiochemical properties of FEC and HFE, we measured the surface tension of FEC, HFE, and electrolyte samples. As shown in Table 3, the surface tension of HFE is much lower than that of FEC. When HFE is incorporated into the electrolyte as a cosolvent, HFE can act as a “surfactant” to reduce the surface tension. Consequently, the surface tension of EEH (27.94 mN m−1) is less than that of EEF (37.15 mN m−1). As a valid fact, lower surface tension means that the electrolyte can wet the graphite electrode better. In addition, the EIS results reveal that the SEI film with participation of

equation (correlation between diffusivity and viscosity). Moreover, the viscosity of EEH is just a little larger than that of EEF. Hence, the change of viscosity will not influence the conductivity so much. In contrast, the dielectric constant of FEC (78.4) is much larger than that of HFE because ether has a low dielectric constant based on the universal law.6,22 It is the main reason that the conductivity of EEH drops a lot. Initial discharge (lithiation) and charge (delithiation) curves at 0.1 C and cycling performance of Li/graphite cells in various electrolytes are presented in Figure 1. As shown in the inset of

Figure 1. Initial discharge−charge curves (a) and cycling performance (b) of Li/graphite cells in various electrolytes, where the inset in panel (a) shows sectional details with an enlarged scale.

Figure 1a, the following trend is observed in decreasing decomposition potential: EEF (1.57 V) > EEH (1.21 V) > EE (1.15 V). In addition, the initial Coulombic efficiency is improved from 89.89% (EE) to 92.58% (EEH) when HFE was used as a cosolvent. Although the incorporation of FEC also increases the initial Coulombic efficiency (90.37%), it is inferior to HFE. Moreover, the cycling performance of the cells in electrolyte with HFE and FEC as solvents is superior to that without HFE and FEC as can be seen obviously from Figure 1b. Specifically, the cells cycled in EEH sample deliver the best cycling performance with a capacity retention of 98.34% after 100 cycles, indicating the graphite electrodes cycled in the EEH sample may own a better SEI film than that in the EEF sample. The excellent cycle ability of the graphite anode material commonly correlates with the outstanding SEI film at the surface of the electrode. For the sake of understanding the film forming process, CV experiments of the cells with EE, EEF, EEH electrolytes were performed as shown in Figure 2. In the cathodic process at the first cycle, the cathodic peak below 0.15 V is ascribed to the intercalation of Li ions into the graphite layer. All electrolyte samples show cathodic peaks around 1.4 V with fine 6588

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Figure 2. Cyclic voltammograms of Li/graphite cells in the electrolyte sample of EE (a), EEF (b), and EEH (c) at a sweep rate of 0.1 mV s−1, where insets show the details with an enlarged scale; Nyquist plots of Li/graphite cells after 1 cycle (d), where the inset shows the equivalent circuit model.

HFE has a lower interface resistance. Therefore, the cells with EEH electrolyte display excellent cycling performance and rate capability with a relatively low conductivity of 5.41 ms cm−1. Nevertheless, with that experimental evidence we still cannot explain the essence of compatibility of graphite with the EEH electrolyte completely. Raman spectroscopy has been confirmed an effective detection means to characterize the structure of graphitic materials as it can provide valuable information about the defects and stacking of the graphene layers. The remarkable features in the Raman spectra of graphite are the so-called D band (about 1360 cm−1) and the G band (around 1580 cm−1), and the integrated intensity ratio R (R = ID/IG) is widely used to evaluate the content of disordered carbon.23,24 The increased value of R is often associated with the decrease of the degree of graphitization. Figure 4 shows the Raman spectra of fresh graphite electrode and cycled graphite electrode with EEF and EEH electrolyte after 1 cycle and 110 cycles. First, the fresh graphite material represents a high degree

Figure 3. Cycling performance (a) and rate capability at various current densities (b) of Li/graphite cells with EEF and EEH electrolyte samples.

Table 3. Surface Tension of FEC, HFE, and Electrolyte Samples at 25 °C sample

FEC

HFE

EE

EEF

EEH

surface tension (mN m−1)

46.16

24.65

31.04

37.15

27.94

Figure 4. Raman spectra obtained from the fresh and the cycled graphite electrode with EEF and EEH electrolytes after 1 cycle and 110 cycles. 6589

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Figure 5. TEM images of fresh graphite (a); cycled graphite in EEF (b) and EEH (c) after 1 cycle; and cycled graphite in EEF (d) and EEH (e) after 110 cycles.

of graphitization with a low R value (0.1989). The R values of the cycled graphite surface with EEF and EEH electrolytes all increase apparently compared with the fresh graphite, which implies that the degradation of the graphite surface occurred in the intercalation of Li+ into graphite. In addition, the value of R for the graphite cycled in EEH electrolyte (0.5464) is lower than that in EEF electrolyte (0.6368) at first cycle, suggesting there is less disordered carbon at the graphite surface. However, the R values are almost the same between the EEF (R = 0.7261) and EEH (R = 0.7288) samples after 110 cycles, while we noticed that the peak at 600 cm−1 is more pronounced in the EEH electrolyte. Moreover, the peak around 600 cm−1 can be ascribed to the amorphous carbon film at the graphite surface, and it appears in hydrogen-free carbon films.25 It may relieve the damage to the graphite because of the amorphous film. On the basis of above results, we can conclude that the SEI film formed in the electrolyte of EEH is more effective in terms of protecting the surface structure of graphite. For a further investigation of the graphite surface, ex situ TEM images of fresh electrode (Figure 5a), cycled graphite electrode in EEF (Figure 5b,d), and EEH (Figure 5c,e) electrolyte were obtained. As expected, the fresh graphite shows a smooth surface, while the cycled graphite in both EEF and EEH electrolytes displays a rough surface. In addition, there are some holes and large crystal grains at the edge of the cycled graphite in EEF sample, which leads to a loose structure at the graphite surface from Figure 5b,d. It has a large amount of activate sites at the graphite surface, causing the decomposition of solvents. In contrast, the graphite surface cycled in EEH sample contains more relatively small crystal grains and shows a compact surface. As can be seen more clearly in Figure 5d, it

seems that some crystal grows at the surface of graphite cycled in EEF electrolyte after 110 cycles. In comparison, the graphite cycled in EEH electrolyte remains a flat surface as shown in Figure 5e. The stable surface film derived from HFE will avoid the sustaining reduction of solvents and the structure degradation of graphite surface, improving the electrochemical properties of the graphite electrode. Meanwhile, the surface composition analysis of the fresh and cycled graphite was conducted by XPS. Figure 6a displays the atomic ratio of O, Li, C, and F at the surface of fresh and cycled graphite without sputtering by Ar ions. It can be seen clearly that the concentration changed obviously after cycling. For graphite cycled in EEF and EEH electrolytes, the constitution is similar except that there is a little more C content at the surface of EEH-110th sample. After sputtering 3 min by Ar ions, the C content decreased by almost by half, and the F content increased as shown in Figure 6b. In addition, the F content at the surface of the graphite cycled in EEF electrolyte is higher than that in the EEH electrolyte. Those results revealed that more carbon compounds cover the outer layer of SEI film and the fluoro compounds distribute in the inner layer of SEI film in both EEF and EEH samples. Moreover, more C and less F elements exist in the SEI film of graphite cycled in the EEH electrolyte. For identifying the source of specific elements at the graphite surface, high resolution XPS patterns of F1s were obtained as displayed in Figure 7. The F content at the surface of graphite without sputtering in EEF is almost the same with that in the EEH sample from Figure 7a,b, which is consistent with the surface composition analysis in Figure 6. The peak intensities of F1s at 683.2 eV (corresponding to LiF)26,27 of the graphite in 6590

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to form LiF.28 It is the reason that the graphite surface in the EEF sample contains more LiF. The raised content of LiF in the SEI film will hinder the transmission of Li ions due to its poor conductivity, increasing the interface impedance. Those results suggest that the SEI film layer originating from the reduction of FEC contain more inorganic compounds (LiF, Li2CO3) and less organic compounds (ROLi) compared with that derived from HFE. On the basis of above experimental results, we speculate that the HFE decomposes as described in Scheme 2 (Schemes 1, 2, and 3 are shown in Figure 8). During the decomposition of HFE, it will produce propylene and radical. As the reduction potential of EC is close to HFE, we reproduced the reduction mechanism of EC as shown in Scheme 1.6,29 The radical originated from EC will react with the radical derived from HFE through radical termination, which is represented in Scheme 3. The decomposition product of HFE (LiOCF2CF2CF2CF2OLi) is stable compared to alkyl carbonate (LiOCOOCH2CH2OOCOLi) derived from EC as it will not react with HF to produce LiF, reducing the amount of LiF in the SEI film. Thus, the interfacial impedance can be lowered, improving the diffusion of Li ions through the SEI film at the graphite surface. We believe that HFE is more appropriate as a cosolvent than FEC in the EC-based nonaqueous electrolyte for the graphite electrode. Furthermore, HFE is a promising cosolvent for high voltage electrolytes because of the low HOMO energy. We expect to acquire valuable experimental data from a more comprehensive perspective through evaluating the HFEcontaining electrolyte in commercialized full cells in further studies.

Figure 6. The element concentration at the surface of fresh and cycled graphite electrodes without sputtering (a) and sputtering for 3 min (b).

4. CONCLUSIONS We have introduced HFE into an EC-based nonaqueous electrolyte as a cosolvent and compared the compatibility of graphite with HFE and FEC. CV and electrochemical tests results demonstrated that HFE is reduced with EC together to form a SEI film at the graphite surface. The SEI film derived from HFE contains more organic compounds and less

the EEF sample are nearly 4 times that in the EEH sample after sputtering 3 min as can be seen from Figure 7c,d, which implies that the SEI film originating from the decomposition of FEC contains more LiF. The decomposition of FEC will produce VC and HF, followed by the reaction of HF with ROLi or Li2O

Figure 7. F1s XPS patterns of the graphite electrode cycled in the EEF electrolyte (a) and (c); EEH electrolyte (b) and (d) after 110 cycles. 6591

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Figure 8. The proposed reduction mechanism of HFE (Scheme 2) and the possible reaction between EC and HFE (Scheme 3). Rechargeable Lithium Batteries. J. Electrochem. Soc. 1998, 145, 3482− 3486. (5) Besenhard, J. O.; Winter, M.; Yang, J.; Biberacher, W. Filming Mechanism of Lithium-Carbon Anodes in Organic and Inorganic Electrolytes. J. Power Sources 1995, 54, 228−231. (6) Xu, K. Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries. Chem. Rev. 2004, 104, 4303−4417. (7) Aurbach, D.; Gamolsky, K.; Markovsky, B.; Gofer, Y.; Schmidt, M.; Heider, U. On the Use of Vinylene Carbonate (VC) as an Additive to Electrolyte Solutions for Li-ion Batteries. Electrochim. Acta 2002, 47, 1423−1439. (8) Ota, H.; Sakata, Y.; Inoue, A.; Yamaguchi, S. Analysis of Vinylene Carbonate Derived SEI Layers on Graphite Anode. J. Electrochem. Soc. 2004, 151, A1659−A1669. (9) Wrodnigg, G. H.; Besenhard, J. O.; Winter, M. Ethylene Sulfite as Electrolyte Additive for Lithium-Ion Cells with Graphitic Anodes. J. Electrochem. Soc. 1999, 146, 470−472. (10) Mcmillan, R.; Slegr, H.; Shu, Z. X.; Wang, W. D. Fluoroethylene Carbonate Electrolyte and Its Use in Lithium Ion Batteries with Graphite Anodes. J. Power Sources 1999, 81−82, 20−26. (11) Xu, K.; Zhang, S. S.; Jow, T. R. Formation of the Graphite/ Electrolyte Interface by Lithium Bis(oxalato)borate. Solid-State Lett. 2003, 6, A117−A120. (12) Chen, Z. H.; Liu, J.; Amine, K. Lithium Difluoro(oxalato)borate as Salt for Lithium-Ion Batteries. Electrochem. Solid-State Lett. 2007, 10, A45−A47. (13) Li, L. F.; Zhou, S. S.; Han, H. B.; Li, H.; Nie, J.; Armand, M.; Zhou, Z. Z.; Huang, X. J. Transport and Electrochemical Properties and Spectral Features of Non-Aqueous Electrolytes Containing LiFSI in Linear Carbonate Solvents. J. Electrochem. Soc. 2011, 158, A74−A82. (14) Abe, T.; Kawabata, N.; Mizutani, Y.; Inaba, M.; Ogumi, Z. Correlation Between Cointercalation of Solvents and Electrochemical Intercalation of Lithium into Graphite in Propylene Carbonate Solution. J. Electrochem. Soc. 2003, 150, A257−A261. (15) Kitagawa, T.; Azuma, K.; Koh, M.; Yamauchi, A.; Kagawa, M.; Sakata, H.; Miyawaki, H.; Nakazono, A.; Arima, H.; Yamagata, M.; et al. Application of Fluorine-Containing Solvents to LiCoO2 Cathode in High Voltage Operation. Electrochemistry 2010, 5, 345−348. (16) Zhang, Z. C.; Hu, L. B.; Wu, H. M.; Weng, W.; Koh, M.; Redfern, P. C.; Curtiss, L. A.; Amine, K. Fluorinated Electrolytes for 5

inorganic compounds at the graphite surface in comparison with FEC, leading to a compact surface with lower interface resistance. Moreover, HFE can act as the role of “surfactant” to reduce the surface tension of the electrolyte. Consequently, the graphite electrode displays an excellent cycling capability and rate performance in EC-based electrolytes employing HFE as a cosolvent.

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AUTHOR INFORMATION

Corresponding Author

*Address: School of Metallurgy and Environment, Central South University, No. 932, Lushan South Road, Yuelu District, Changsha City, Hunan Province, China. Tel: +86-73188836633. Fax: +86-731-88836633. E-mail: [email protected]. cn (current institution email address); [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from the National 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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REFERENCES

(1) Dey, A. N.; Sullivan, B. P. The Electrochemical Decomposition of Propylene Carbonate on Graphite. J. Electrochem. Soc. 1970, 117, 222− 224. (2) Yamada, Y.; Koyama, Y.; Abe, T.; Ogumi, Z. Correlation between Charge-Discharge Behavior of Graphite and Solvation Structure of the Lithium Ion in Propylene Carbonate-Containing Electrolytes. J. Phys. Chem. C 2009, 113, 8948−8953. (3) Fong, R.; Sacken, U. Y.; Dahn, J. R. Studies of Lithium Intercalation into Carbons Using Nonaqueous Electrochemical Cells. J. Electrochem. Soc. 1990, 137, 2009−2013. (4) Peled, E.; Golodnitsky, D.; Menachem, C.; Bar-Tow, D. An Advanced Tool for the Selection of Electrolyte Components for 6592

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp4119106 | J. Phys. Chem. C 2014, 118, 6586−6593