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A novel imidazole-based electrolyte additive for improved electrochemical performance of high voltage nickel-rich cathode coupled with graphite anode lithium ion battery
Journal of Power Sources 332 (2016) 312e321
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A novel imidazole-based electrolyte additive for improved electrochemical performance of high voltage nickel-rich cathode coupled with graphite anode lithium ion battery Haibo Rong a, Mengqing Xu a, b, *, Yunmin Zhu a, Boyuan Xie a, Haibin Lin a, Youhao Liao a, b, Lidan Xing a, b, Weishan Li a, b, ** a
School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China Engineering Research Center of MTEES (Ministry of Education), Research Center of BMET (Guangdong Province), Engineering Lab. of OFMHEB (Guangdong Province), Key Lab. of ETESPG (GHEI), and Innovative Platform for ITBMD (Guangzhou Municipality), South China Normal University, Guangzhou 510006, China
b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
SDM is investigated as an electrolyte additive for graphite/LiNi0.5Co0.2Mn0.3O2 cell. The cycling performance is enhanced with appropriate amount of SDM addition. The surface layer derived from SDM on the cathode is more stable and robust. Dissolution of Mn, Co, and Ni can be reduced by adding SDM.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 July 2016 Received in revised form 23 August 2016 Accepted 3 September 2016
1,10 -sulfonyldiimidazole (SDM) has been investigated as a novel carbonate-based electrolyte additive for high voltage nickel-rich cathode chemistry, graphite/LiNi0.5Co0.2Mn0.3O2 cells. Upon cycling at high voltage for 50 cycles, graphite/LiNi0.5Co0.2Mn0.3O2 cells with SDM containing electrolyte have superior cycling performance than the cells with baseline electrolyte, specifically, 96.9% and 73.1% capacity retention, respectively. Moreover, cells with 0.25 wt. % SDM have lower impedance and better elevated temperature storage performance as well. The functional mechanism of electrolyte containing SDM on improved cycling performance is elucidated with ex-situ analytical techniques, including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and fourier transform infrared spectroscopy (FTIR), etc. The surface analysis result reveals that SDM has been involved into the surface film forming on the LiNi0.5Co0.2Mn0.3O2 cathode and graphite anode as well, which can simultaneously provide protection for both cathode and anode upon cycling to high voltage, leading to enhanced cyclability of the high voltage (4.5 V vs. Li/Liþ) graphite/LiNi0.5Co0.2Mn0.3O2 cells with the presence of SDM. © 2016 Elsevier B.V. All rights reserved.
Keywords: Lithium ion battery 1,10 -sulfonydiimidazole Electrolyte additive LiNi0.5Co0.2Mn0.3O2 High voltage Interface
* Corresponding author. School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China. ** Corresponding author. School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China. E-mail addresses: [email protected] (M. Xu), [email protected] (W. Li). http://dx.doi.org/10.1016/j.jpowsour.2016.09.016 0378-7753/© 2016 Elsevier B.V. All rights reserved.
1. Introduction Lithium ion batteries (LIBs) have been used widely as dominant power source for portable electronic devices and also have
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attracted growing interest for the application in electric vehicles and hybrid electric vehicles [1e5]. In order to meet the demands of these applications, further improvement in energy density of LIBs are required [6e8]. The energy density of LIBs can be improved by enhancing the specific capacity and the operating voltage of individual cells [9]. Among various cathode materials, layer-structured Ni-rich cathodes (Ni content 0.5), such as LiNi0.5Co0.2Mn0.3O2, LiNi0.6Co0.2Mn0.2O2 and LiNi0.8Co0.1Mn0.1O2 have been considered as prospective cathode candidates for high energy density LIBs [10e13]. Extra capacity could be attained by charging the cells to high voltage (4.5 V vs. Li/Liþ) with the use of Ni-rich cathodes [14e17]. Unfortunately, high voltage is always accompanied by serious oxidative decomposition of the traditional LiPF6/carbonate electrolyte [18e21], and these decomposed products lead to continuous growth of an undesirable surface film on the cathode, which is impeditive and erratic, resulting in bad cycling performance and poor coulombic efficiency [22]. It has been reported in the literatures that transition metal cations tend to dissolve from the Ni-rich cathode and subsequently move toward the graphite anode during cycling and aging process. This can destroy the formed solid electrolyte interface (SEI) on the graphite surface, resulting in additional lithium loss and capacity fading upon cycling [16,23e25]. Surface modification of Ni-rich cathodes with Al2O3, AlF3, SiP2O7 and LiAlO2 have been verified as an effective approach for improving the cycling performance of Ni-rich cathodes at high voltage [26e30]. But the surface modification usually involves complicated synthetic processes and high manufacturing cost [31]. Graphite is the preferred anode material and has been widely used for state-of-the-art LIBs due to its excellent advantages: relatively high capacity, low cost and environmental friendliness [32e36]. However, the layered structure of graphite anode is vulnerable and may be damaged when co-intercalation of electrolyte solvent occurres [37]. Therefore, a robust and protective SEI should be built up on the graphite anode. It has been reported that electrolyte additives such as vinylene carbonate (VC), fluoroethylene carbonate (FEC), 4-fluorophenyl acetate (4-FPA), propane sultone (PS) and 1-fluoropropane-2-one (FA) are able to form a SEI film on the graphite anode [36,38e45]. The additive-derived SEI film protects the graphite and reduces further decomposition of the electrolyte. The concept by forming stable SEI via incorporation of additives to inhibit electrolyte reduction has been expanded to cathode surface film forming as well [46]. According to some recent studies, cathode film forming additives, including methylene methyl disulfonate (MMDS), tris (trimethylsilyl) borate (TMSB) and tris (trimethylsilyl) phosphate (TMSP), have been used to improve the cycling performance of the cells with Ni-rich cathode upon cycling at high voltage. These film forming additives tend to incorporate into cathode surface film forming process, resulting in a robust surface film on Ni-rich cathodes [47e50]. Several electrolyte additives reported recently are able to simultaneously form protective surface film on cathode and anode, and this unique functionality demonstrates more convenient and effective approach to improve the electrochemical performance of LIB. Li et al. reported that the cycling performance of the graphite/ LiMn2O4 cell at elevated temperature can be significantly improved with the use of prop-1-ene-1,3-sulfone (PES) [37], which can simultaneously form protective film on the graphite electrode and the LiMn2O4 electrode. Xu et al. reported lithium difluoro (oxalato) borate (LiDFOB) was considered as an effective additive for graphite/LiFePO4 cell with the formation of a oxalate-borate derived film on both anode and cathode, as well [51]. In this study, we report a novel electrolyte additive, 1,10 - sulfonyldiimidazole (SDM), to improve the electrochemical performance
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of graphite/LiNi0.5Co0.2Mn0.3O2 cell upon cycling at 4.5 V vs. Li/Liþ. SDM possesses unique functionality by offering simultaneous protection to the graphite anode and LiNi0.5Co0.2Mn0.3O2 cathode upon cycling at high voltage, 4.5 V vs. Li/Liþ. Electrochemical methods combination with ex-situ surface analyses have been used to elucidate the effects of SDM containing electrolyte on the improved performance of graphite/LiNi0.5Co0.2Mn0.3O2 cell upon cycling at 4.5 V vs. Li/Liþ. 2. Experimental section 1.0 M lithium hexafluorophosphate (LiPF6) dissolved in a solution of 3:5:2 ethylene carbonate (EC)/ethyl methyl carbonate (EMC)/diethyl carbonate (DEC) (wt.%) (baseline electrolyte) was obtained from Dongguan Kaixin Battery Materials Co., Ltd., China, and used without further purification. 1,10 - sulfonyldiimidazole (SDM, assay 99.5%) was purchased from Fujian Chuangxin Technology Co., Ltd., China, and used without further purification. Electrolytes with SDM contain 0.25 or 0.5% by weight. Molecular structure of SDM is shown in Scheme 1. The composite LiNi0.5Co0.2Mn0.3O2 cathodes obtained from Foshan Advanced Electronics Energy Co., Ltd., China, containing 97.5% active material, super P (1.0%) and PVDF (1.5%). The composite graphite anodes obtained from Foshan Advanced Electronics Energy Co., Ltd., China, consisting of 95.5% graphite, super P (1.0%), CMC (1.5%) and SBR (2.0%). Graphite/LiNi0.5Co0.2Mn0.3O2 2025-type coin cells with Celgard 2400 as separator were assembled in an argon glove box for electrochemical performance measurements. The electrochemical charge/discharge behavior of Li/LiNi0.5Co0.2Mn0.3O2 cells and graphite/LiNi0.5Co0.2Mn0.3O2 cells were explored on a Land cell test system (Land CT2001A, China) according to the following procedure: cells cycled at 25 C were first charged to 4.5 V at an constant current (0.2 C for full cells and 0.5 C for half cells) followed by a constant voltage charge (V ¼ 4.5 V) until the current decreased to 10% of the applied charging current, and then the cells were discharged to 2.75 V with the same constant current. The Li/graphite cells were charged and discharged between 0.01 and 2.5 V at 0.2 C. Cyclic voltammetry was performed on Solartron-1480 (England) in the potential range of 0.01e2.5 V at a scanning rate of 0.1 mV s1. The electrochemical impedance spectroscopy (EIS) were carried out under discharged state, with the amplitude of 5 mV, and the frequency ranging from 105 Hz to 0.01 Hz by using a PGSTAT-30 electrochemical station (Autolab Metrohm, Netherlands). Self-discharge measurements of the graphite/LiNi0.5Co0.2Mn0.3O2 cells were performed according to the following protocols: cells were charged to 4.5 V with a constant current of 0.2 C, followed by a constant voltage (V ¼ 4.5 V) charge with a cut-off current of 10% of applied charging current, and then the cells were stored at open circuit voltage for 25 days at elevated temperature (55 C). The cycled graphite/LiNi0.5Co0.2Mn0.3O2 cells were disassembled in a glove box filled with high purity argon. Both the cycled graphite anode and LiNi0.5Co0.2Mn0.3O2 cathode were rinsed with anhydrous dimethyl carbonate (DMC) three times to remove the residual EC
Scheme 1. Molecular structure of SDM.
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and LiPF6 salt precipitated on the surface, followed by drying overnight under vacuum at 25 C before ex-situ characterizations. The surface morphologies of graphite anode and LiNi0.5Co0.2Mn0.3O2 cathode were observed with scanning electron microscopy (SEM) (JSM-6510, Japan). Fourier transiform infrared spectroscopy (FTIR) was carried out on a Bruker Tensor 27 (Germany) with an attenuated total reflectance (ATR) accessory to identify organic functional groups on the surface of both anode and cathode. X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD, England) with Al Kɑ line as an X-ray source. Calibration of XPS peak was made by recording XPS spectra for reference compounds, which are present on the electrode surface: LiF, PVDF and LixPOyFz. The universal contamination of CeH bond at 284.6 eV was used as a reference for the final adjustment of the energy scale in the spectra. Elemental concentration was calculated based on the equation: P Cx¼(Ix/Sx)/( Ix =Sx ), where Ix is the relative intensity of the element, and Sx is the sensitivity value of the element. 3. Results and discussion According to previous studies, the cathode film forming additives should be easier to be oxidized than the electrolyte solvents [48]. Therefore, Linear sweep voltammetry (LSV) was conducted to evaluate the oxidation potential of SDM on a Pt electrode. As presented in Fig. S1, it can be observed that the electrolyte with SDM oxidizes from 3.8 V (vs. Li/Liþ), while the baseline electrolyte decomposition begins 4.5 V, indicating SDM has lower oxidation potential than carbonate solvents of the electrolyte. In addition, the oxidative current of the electrolyte with SDM is significantly lower than the baseline electrolyte from 4.5 to 5.5 V. This result implies the oxidation product of SDM can passivate the Pt electrode and suppress severe oxidative decomposition of the electrolyte at high voltage. Fig. 1a shows the cycling performance of the graphite/LiNi0.5Co0.2Mn0.3O2 cells with baseline electrolyte and electrolyte with added SDM at room temperature. The initial discharge capacity of the cells with added SDM additive is slightly lower than that with baseline electrolyte, which may be related to the irreversible
decomposition of SDM. Evidence for support this assumption will be discussed below. Surprisingly, a significant improvement in the cycling performance for the cells with SDM can be observed upon cycling at high voltage. The capacity retention for the cells with 0.25 and 0.5 wt. % SDM is 96.9% and 87.8%, respectively; while the cell with baseline electrolyte after 50 cycles at high voltage is only 73.1%, suggesting over 20% improvement by incorporation of 0.25 wt. % SDM after 50 cycles. In addition, the initial Coulombic efficiency (CE) of the cell with baseline electrolyte is 72.75%, while is 70.86% and 63.35% for the electrolytes with 0.25 and 0.5 wt.% SDM, respectively (Fig. 1b). The lower initial CE maybe related to the irreversible decomposition of SDM. Interestingly, it is noteworthy that the CE for the cells with 0.25 (99.62%) or 0.5 wt. % (99.5%) is slightly higher than that cycled with baseline electrolyte (98.92%) after 50 cycles at high voltage. As is well known, a high CE of greater than 99.5%, which is vital for practical applications of LIBs. Based on the above eletrochemical data, the 0.25 wt.% concentration of SDM in the electrolyte yields better capacity retention and CE. Hence, the following study is based on the data set with 0.25 wt. % SDM. With an effort to obtain a better understanding of the benefits of SDM for the graphite/LiNi0.5Co0.2Mn0.3O2 cells, control experiments have been done by adding SDM to the electrolyte of Li/graphite cell and Li/LiNi0.5Co0.2Mn0.3O2 cell, respectively. Li/LiNi0.5Co0.2Mn0.3O2 cells with and without SDM suffered capacity fading after 80 cycles at high voltage, but the Li/LiNi0.5Co0.2Mn0.3O2 cell containing SDM shows better cycling stability than that with baseline electrolyte, as seen in Fig. 1c. Additionally, a Li/LiNi0.5Co0.2Mn0.3O2 cell with added SDM has a lower initial discharge capacity than that without SDM. The lower discharge capacity is most likely due to the SDM additive undergoing the surface filming process and consuming the active Liþ source on the surface of LiNi0.5Co0.2Mn0.3O2 cathode at high voltage, and the irreversible decomposition of SDM is consistent with the results of the LSV test as discussed above. Consequently, the improvement of Li/LiNi0.5Co0.2Mn0.3O2 cell can be ascribed to surface film incorporated with SDM on the LiNi0.5Co0.2Mn0.3O2 cathode, which can suppress continuous oxidative decomposition of electrolyte upon cycling at high voltage. Investigation of the
Fig. 1. Cycling performance of (a) and the Coulombic efficiency (b) of graphite/LiNi0.5Co0.2Mn0.3O2 cells, (c) Li/LiNi0.5Co0.2Mn0.3O2 cells and (d) Li/graphite cells with SDM containing electrolyte and baseline electrolyte.
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effect of SDM on the graphite anode was also carried out. It can be seen from Fig. 1d that the cycling performance of the Li/graphite cell can be improved with added SDM, suggesting that addition SDM is beneficial to the graphite anode as well. Analysis of the results of using SDM in LiNi0.5Co0.2Mn0.3O2 cathode and graphite anode as discussed above yields the conclusion that use SDM can simultaneously protect the cathode and anode thus resulting in improvement of the graphite/LiNi0.5Co0.2Mn0.3O2 cell. To gain more insight into the effect of added SDM on the graphite anode, cyclic voltammogram of this system is conducted. Fig. 2a and b compare the initial CVs of graphite anodes in the 1 M LiPF6 EC:EMC:DEC ¼ 3:5:2 electrolytes with and without SDM. The anodic processes are mainly related to Liþ deintercalation from graphite; while the cathodic processes are indicative of the Liþ intercalation into graphite and the reduction of the electrolyte species, and the formation of SEI film [38]. Apparently, the initial decomposition of the electrolyte with added SDM occurs earlier than that of baseline electrolyte, from 1.3 to 1.5 V (vs. Li/Liþ) (Fig. 2b), indicating the preferred reduction of SDM over the solvents. The reduction peak that appears at around 0.5 V corresponds to reductive decomposition of electrolyte, which is weaker for the electrolyte with added SDM than the baseline electrolyte, suggesting that an SEI film originating from SDM mitigates the decomposition of the electrolyte. To further confirm the preferential reductive behavior of SDM on the graphite anode, CVs for graphite anodes in the PC-based electrolyte with and without SDM were also obtained, as shown in Fig. 2c and d. In the PC-based electrolyte without SDM additive (Fig. 2c), a large reductive current at 0.5 V can be clearly observed. This is attributed to the cointercalation of PC into the graphite anode, indicating nonprotective SEI formed on graphite [43]. Unlike the electrolyte without SDM, a reduction peak can be seen from 1.3 to 1.5 V (Fig. 2d), very consistent with the reduced peak observed in ECbased electrolyte system, which suggests SDM has incorporated into the SEI film forming process on the graphite and the reduction peak at 0.5 V is much weaker for the graphite with SDM than that without additive. These observations imply that co-intercalation of PC can be effectively mitigated by the formation of a steady and
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protective SDM derived SEI on graphite anodes. As a result, the improved cycling performance of Li/graphite half cell can be ascribed to the robust and protective SEI film on the graphite via the preferred reduction of SDM. This in turn alleviates the decomposition of electrolyte and provides the protection for the graphite anode. Fig. 3a shows the voltage vs. discharge capacity profiles of graphite/LiNi0.5Co0.2Mn0.3O2 cells with baseline electrolyte. The discharge capacity and the discharge potential plateau of the graphite/LiNi0.5Co0.2Mn0.3O2 cell with baseline electrolyte are significantly decreased upon cycling at high voltage. This observation is likely due to the following factors. First, the severe oxidative decomposition of electrolyte on the LiNi0.5Co0.2Mn0.3O2 cathode at high voltage exaggerates the irreversible capacity and polarization. Second, breakdown of the SEI on the graphite anode resulting from the irreversible reduction of electrolyte solvents and the deposition of transition metal ions originating from the cathode during the cycling process caused extra Liþ loss and larger polarization of the full cell. However, graphite/LiNi0.5Co0.2Mn0.3O2 cells with added SDM electrolyte showed less capacity loss and a negligible decrease of the potential plateau (Fig. 3b). This result indicates the use of SDM in a graphite/LiNi0.5Co0.2Mn0.3O2 cell can dramatically reduce the capacity loss and polarization of the full cell upon cycling at high voltage. The electrochemical impedance spectroscopy (EIS) of graphite/ LiNi0.5Co0.2Mn0.3O2 cells with baseline electrolyte and added SDM after 50 cycles at high voltage are depicted in Fig. 3c. The spectra contain two main features: a semicircle from high to medium frequency, which can be ascribed to the impedance of surface film formed on the surface of the electrodes; another semicircle occurring at medium to low frequency is attributed to the chargetransfer processes. Fig. 3c clearly shows that both the surface film impedance and charge-transfer impedance are smaller for the cell with SDM than that with baseline electrolyte after cycling at high voltage. The reduced impedance of the graphite/LiNi0.5Co0.2Mn0.3O2 cell is most likely a result of the protective surface layers formed on the graphite anode and LiNi0.5Co0.2Mn0.3O2 cathode originated from SDM. SDM can reduce the decomposition
Fig. 2. First cycle CV curves of graphite anodes in EC- based electrolytes with (b) and without (a) SDM and PC- based electrolytes with (d) and without SDM (c), respectively.
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Fig. 3. Discharge curves of graphite/LiNi0.5Co0.2Mn0.3O2 cells at different cycles with (b) and without (a) SDM, electrochemical impedance spectra after 50 cycles for graphite/ LiNi0.5Co0.2Mn0.3O2 cells with and without SDM (c), self-discharge curves of graphite/LiNi0.5Co0.2Mn0.3O2 cells cycled with and without SDM at elevated temperature (d).
of the bulk electrolyte deposited on the anode and cathode upon cycling at high voltage. Elevated temperature self-discharge measurement of the graphite/LiNi0.5Co0.2Mn0.3O2 cells with and without SDM has been conducted and shown in Fig. 3d. Significant difference was observed for the charged cell storing with baseline and SDM-containing electrolyte. The voltage of the charged cell in the baseline electrolyte declined very slowly during the first 17 days, and then dropped from 4.3 to 0.7 V at a sudden. As respect to the SDM added electrolyte, the voltage declined gradually during
the entire process, and no sudden drop of voltage was observed. This observation indicates that the charged graphite/LiNi0.5Co0.2Mn0.3O2 cell in baseline electrolyte underwent more severe self-discharge during storage; while this scenario did not occur via the presence of SDM added electrolyte. Deep insight understanding of this self-discharge process could predict that more electrolyte suffered oxidative degradation in the baseline electrolyte on the charged LiNi0.5Co0.2Mn0.3O2 particle surface and lost electrons during this irreversible electrochemical side-reactions. And the free
Fig. 4. SEM images of fresh LiNi0.5Co0.2Mn0.3O2 electrode (a), and cycled LiNi0.5Co0.2Mn0.3O2 electrodes with (c) and without SDM (b) extracted from graphite/LiNi0.5Co0.2Mn0.3O2 cells.
H. Rong et al. / Journal of Power Sources 332 (2016) 312e321 Table 1 Element concentrations of fresh and cycled LiNi0.5Co0.2Mn0.3O2 electrodes with and without SDM containing electrolyte.
Fresh Baseline SDM
C(%)
O(%)
F(%)
P(%)
Ni(%)
Co(%)
Mn(%)
N(%)
S(%)
58 48.8 55.4
13.8 22.94 11.17
22.86 22.9 26.72
0.46 4.51 1.02
2.63 0.2 2.22
0.63 0.06 0.46
1.61 0.78 1.12
1.52
0.38
electrons flow into the charged/high valance particle surface, leading to sudden drop of the cell voltage as we observed for cell in baseline electrolyte. Self-discharge measurements indicate the SDM derived films on the surface layers of the anode and cathode can effectively suppress the self-discharge. SEM images of fresh cathode and cathodes cycled with either baseline electrolyte or SDM containing electrolyte are presented in Fig. 4. The spherical particles of LiNi0.5Co0.2Mn0.3O2 can be identified clearly on the cathode before cycling, as depicted in Fig. 4a. After 50 cycles at high voltage, it is obvious that some of the LiNi0.5Co0.2Mn0.3O2 particles cycled with baseline electrolyte suffered serious destruction (Fig. 4b), which might be ascribed to the
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transition metal dissolution in the bulk material during cycling at high voltage [49]. However, the structure of the cathode particles is maintained well in presence of electrolyte with added SDM (Fig. 4c). This suggests a surface film incorporated with SDM can protect the cathode upon cycling at high voltage. There are no distinct differences between the surface films for the cycled cathodes with and without SDM as measured by SEM. X-ray photoelectron spectroscopy (XPS) was employed to further understand the surface and structure of the cycled cathodes. This method is useful for investigating surface films because the method analyzes about the top 5 nm of the surface, has good sensitivity for the elements of interest and provides elemental concentration data [52]. XPS spectra of fresh cathode and cathodes extracted from the full cells with and without SDM were acquired, and the corresponding elemental concentrations are listed in Table 1. Analysis of the cathode surfaces reveals significant difference in the surface film species. The concentrations of C, Ni, Co and Mn are decreased for both cycled cathodes with and without SDM; while the concentrations of P and F are increased compared to the fresh cathode electrode, suggesting formation of a surface film composed of electrolyte decomposition products covering the metal oxide and
Fig. 5. XPS spectra of the fresh LiNi0.5Co0.2Mn0.3O2 electrode, and the cycled LiNi0.5Co0.2Mn0.3O2 electrodes with and without SDM extracted from graphite/LiNi0.5Co0.2Mn0.3O2 cells.
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Fig. 6. SEM images of the fresh graphite electrode (a), and cycled graphite electrodes with (c) and without SDM (b) extracted from graphite/LiNi0.5Co0.2Mn0.3O2 cells.
binder upon cycling at high voltage. Elements S and N were detected on the surface film of the cathode cycled in electrolyte with SDM added as well, and these unique species can be ascribed to the added SDM. Apparently the concentrations of Ni, Co and Mn, of the cathode cycled with SDM, are much higher than of the with baseline electrolyte, indicating a thicker cathode surface film for the baseline electrolyte. Fig. 5 contains the XPS spectra of LiNi0.5Co0.2Mn0.3O2 cathodes from the cycled full cells, with a comparison of the fresh cathode. The fresh cathode contains peaks characteristic of PVDF binder at 285.7 and 290.6 eV and conductive carbon at 284.8 eV in the C 1s spectrum. Related peak characteristic of PVDF binder can be observed in the F 1s spectrum at 687.6 eV. In the fresh O 1s spectrum, the peak at 529.5 eV is due to the metal oxide, and the peak at 531.7 eV is assigned to the Li2CO3, which is frequently present on fresh cathode [31]. Analysis of the cathodes cycled at high voltage reveals new species in the C 1s, O 1s, F 1s and P 2p spectra, which could be identified as electrolyte decomposition products on the surface of the cathodes. New peaks characteristic of CeO and C]O are present at 286 eV and 288 eV in the C 1s spectrum and 533.5 eV and 532.5 eV in the O 1s spectra for the cycled cathodes with and without SDM additive, CeO and C]O species are consistent with the presence of lithium alkyl carbonates and polycarbonates [36]. In addition, the intensity of the metal oxide peak (529.5 eV) in the O 1s spectrum is stronger for the cathode cycled with SDM containing electrolyte than cathode cycled with baseline electrolyte, indicating a thinner surface film formed on the cathode with the use of SDM after cycling, or less transition metals dissolution occurring during cycling at high voltage, 4.5 V vs. Li/Liþ. Along with the peak corresponding to PVDF at 687.8 eV, new peak at 684.8 eV, characteristic of LiF, appeared in the F 1s spectra. It is evident that the intensity of the LiF peak is weaker for the cathode with SDM than that with baseline electrolyte. It is well known that LiF is one of the decomposition products of LiPF6 and the interfacial impedance of the cell could increase with the deposition of LiF on the cathode surface [53]. In the P 2p spectrum, peaks at 134.3 eV and 136.5 eV are attributed to LixPFyOz and LixPFz, which can be ascribed to LiPF6 decomposition on the cathode [54]. The cathode cycled in the SDM containing electrolyte shows weaker intensities of peaks belonging
to LiF, LixPFyOz and LixPFz, indicating that there is less decomposition products of LiPF6 on the surface of the cathode with electrolyte containing SDM. In the S 2p and N 1s spectra, peaks around 168e169 eV are assigned to Li2SO3 or ROSO2Li [37], and peaks at 398.5 eV and 401 eV can be attributed to pyridinic N and pyrrolic N, respectively [55]. The results obtained from the S 2p and N 1s indicate that SDM has been incorporated into the surface film on the cathode during cycling at high voltage. The spectra of transition metals (Ni, Co and Mn) also have been obtained. The intensities of Ni, Co and Mn are stronger for the cathode cycled with SDM containing electrolyte than that with baseline electrolyte. This further confirms a thicker film generated on the cathode without additive caused by the oxidative decomposition of the electrolyte or transition metal dissolution occurring after cycling at high voltage, or less transition metals dissolution occurring during cycling at high voltage, 4.5 V vs. Li/Liþ. In summary, the results of XPS analysis reveal that a thinner surface film originated from the addition of SDM covers the LiNi0.5Co0.2Mn0.3O2 cathode, and less transition metals dissolution occurring during cycling at high voltage, 4.5 V vs. Li/Liþ. The interphase plays a role in suppressing the continuous decomposition of the LiPF6 and the electrolyte, and transition metal dissolution as well, while cycling at high voltage. The FTIR spectra of the LiNi0.5Co0.2Mn0.3O2 cathodes with and without added SDM after cycling at high voltage are shown in Fig. S2. The IR spectra are dominated by characteristics of PVDF binder at 1400, 1271, 1170, 1070 and 877 cm1 for both cathodes. As opposed to the cathode cycled in the baseline electrolyte, an additional peak can be observed at 770 cm1 for the cathode cycled with SDM containing electrolyte. This peak is characteristic of CeSeO species. Presence of this peak confirms the SDM has been
Table 2 Element concentrations of fresh and cycled graphite electrodes with and without SDM containing electrolyte.
Fresh Baseline SDM
C(%)
O(%)
F(%)
P(%)
83.7 48.11 52.41
15.41 32.35 35.69
5.47 13.97 9.74
2.3 3.35
Ni(%)
Co(%)
Mn(%)
N(%)
S(%)
1.21 0.65
0.46 0.19
0.55 0.19
0.7
0.43
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Fig. 7. XPS spectra of the fresh graphite electrode, and the cycled graphite electrodes with and without SDM extracted from graphite/LiNi0.5Co0.2Mn0.3O2 cells.
incorporated into the surface film on the cathode after cycling at high voltage. This observation is also consistent with the results of XPS results of the cathodes as discussed above. SEM images of the graphite anode before and after cycling at high voltage can be found in Fig. 6. In Fig. 6a, the graphite particles can be observed clearly on the anode electrode before cycling. However, the surface of the graphite anode after cycling with either electrolyte results in an amorphous surface film (Fig. 6b). The graphite particles cycled with baseline electrolyte appear damaged or covered by a thick SEI film. This can be ascribed to the electrolyte decomposition or precipitation of transition metal migrated from cathode side due to the dissolution in the electrolyte. With the addition of the SDM into to the bulk electrolyte, the graphite particles can still be identified and are observed with a thin surface film (Fig. 6c), suggesting the use of SDM can protect the graphite anode and reduce electrolyte decomposition during cycling.
To better understand the surface chemistry on the graphite anode after cycling to high voltage, XPS was employed to investigate the components of the graphite surface. Table 2 contains the elemental concentrations of the fresh graphite, and graphite anodes extracted from graphite/LiNi0.5Co0.2Mn0.3O2 cells after cycling at high voltage with baseline electrolyte and SDM containing electrolyte. After cycling at high voltage with both electrolytes, the concentration of C is distinctly decreased, while the concentrations of O and F are increased relative to the fresh graphite anode, suggesting SEI films were formed on the cycled graphite anodes, in accord with SEM observations. Besides the cathode, N and S are observed on the anode cycled with SDM containing electrolyte, indicating SDM is involved in the formation of surface film on the graphite anode as well, resulting from the preferential reduction of SDM. Most importantly, the concentrations of Ni, Co and Mn are much lower for the anode cycled with SDM than that with baseline
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electrolyte, indicating less transition metal deposition on the graphite electrode with the use of SDM. The XPS elemental spectra are shown in Fig. 7. In the C 1s spectra, the peak located at 284.8 eV is assigned to graphite, and the peaks appearing at 286 eV and 288 eV correspond to CeO and C]O species. Apparently, the intensity of graphite on the electrode cycled with SDM containing electrolyte is much stronger than that of the baseline electrolyte, suggesting less decomposition products on the anode in the presence of SDM. The O 1s spectrum at 531.7 eV and 533.5 eV corresponds to Li2CO3 and lithium alkyl carbonates, respectively. The peak attributed to Li2CO3 is stronger for the anode cycled with SDM. The intensity of lithium alkyl carbonates is stronger for the anode cycled with baseline electrolyte than that with SDM containing electrolyte, which can be ascribed to the more decomposition of electrolyte on the anode in baseline electrolyte during cycling. In addition, peaks of N 1s (398.5 eV, 401 eV) and S 2p (167.2 eV, 169 eV) can be observed on the anode extracted from the cell cycled with SDM containing electrolyte, confirming that SDM is involved in the anode surface film forming process. The F 1s spectrum of the cycled graphite anodes present two peaks at 684.8 eV and 687.0 eV, corresponding to LiF and LixPOyFz, respectively. As it has been discussed above, LiF, and LixPOyFz are the decomposition products of LiPF6. Particularly, the presence of LiF on the surface of the active electrodes would increase the interfacial impedance of the cell, leading to poor cycling performance. The intensities of the LiF and LixPOyFz are slightly stronger for the anode cycled with baseline electrolyte than that with SDM containing electrolyte, suggesting SDM derived films on the graphite anode can reduce the decomposition of the electrolyte. Finally, transition metal (Ni, Co and Mn) ions appear on the surface of cycled anodes, suggesting soluble transition metal ions reach the graphite through the separator and are reduced on the anodes. The intensities of Ni, Co and Mn on the anode with SDM containing electrolyte are weaker than those for the anode without SDM, which indicates that the SDM derived film inhibits the deposition of transition metals on anode surfaces. According to previous reports, deposition of transition metal ions on the graphite anode surface likely destabilizes the anode SEI leading to additional electrolyte reduction and related capacity loss [49]. The XPS results of the graphite anodes reveal that the protective surface film resulting from SDM can reduce the decomposition of the electrolyte and reduce deposition of transition metal ions. The FTIR spectra of graphite anodes extracted from graphite/ LiNi0.5Co0.2Mn0.3O2 cells with added SDM and baseline electrolyte after 50 cycles at high voltage are depicted in Fig. S3. Anodes with either electrolyte are similar and are dominated by CMC and SBR [56,57], which have characteristic absorptions at 1636, 1241, 1060, 966 and 840 cm1. However, for the graphite anode cycled with SDM containing electrolyte, an additional peak around 690e730 cm1 and 1010 cm1, is indicative of OeSeO species. This further confirms SDM has been involved in the formation of surface films on the graphite anode. 4. Conclusion In this work, 1,10 -sulfonyldiimidazole (SDM) is firstly proposed as a novel electrolyte additive for improving the electrochemical performance of graphite/LiNi0.5Co0.2Mn0.3O2 cell upon cycling at high voltage (4.5 V, vs. Li/Liþ). SDM is simultaneously sacrificially reduced and oxidized on graphite and LiNi0.5Co0.2Mn0.3O2 cathode, respectively, resulting in forming protective surface layers on both anode and cathode simultaneously. The desired forming surface layers lead to highly stable interface of the electrolyte/electrodes, which can effectively suppress the electrolyte decomposition parasitic reactions and transition metal dissolution from bulk
cathode material upon cycling at high voltage. Ex-situ analysis of LiNi0.5Co0.2Mn0.3O2 and graphite electrodes is conducted via a combination of SEM, XPS and FTIR to further support that the highly stable interfaces derived from SDM is the dominant contributor to the enhanced cycling performance of the graphite/ LiNi0.5Co0.2Mn0.3O2 cell. Acknowledgements The authors thank the National Natural Science Foundation of China (Grant No. 21573081), the Natural Science Foundation of Guangdong Province (Grant No. 2014A030313444), and the key project of Science and Technology of Guangdong Province (Grant No. 2014B010123002) for financial support. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.09.016. References [1] K. Xu, Nonaqueous liquid electrolytes for lithium-based rechargeable batteries, Chem. Rev. 104 (2004) 4303e4418. [2] J.-M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359e367. [3] J.B. Goodenough, K.-S. Park, The Li-ion rechargeable battery: a perspective, J. Am. Chem. Soc. 135 (2013) 1167e1176. [4] V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Challenges in the development of advanced Li-ion batteries: a review, Energy Environ. Sci. 4 (2011) 3243e3262. [5] Z.H. Ma, C.W. Sun, Y.H. Lyu, Y.S. Wang, Y.S. Kim, L.Q. Chen, A new oxyfluorinated titanium phosphate anode for a high-energy lithium-ion battery, ACS Appl. Mater. Interfaces 7 (2015) 1270e1274. [6] M. Hu, X.L. Pang, Z. Zhou, Recent progress in high-voltage lithium ion batteries, J. Power Sources 237 (2013) 229e242. [7] L.W. Su, Y. Jing, Z. Zhou, Li ion battery materials with core-shell nanostructures, Nanoscale 3 (2011) 3967e3983. [8] D.H. Long, M.-G. Jeong, Y.-S. Lee, W.C. Choi, J.K. Lee, I.-H. Oh, H.-G. Jung, Coating lithium titanate with nitrogen-doped carbon by simple refluxing for high-power lithium-ion batteries, ACS Appl. Mater. Interfaces 7 (2015) 10250e10257. [9] J. Li, L.E. Downie, L. Ma, W. Qiu, J.R. Dahn, Study of the failure mechanisms of LiNi0. 8Mn0.1Co0.1O2 cathode material for lithium ion batteries, J. Electrochem. Soc. 162 (2015) A1401eA1408. [10] P. He, H. Yu, D. Li, H. Zhou, Layered lithium transition metal oxide cathodes towards high energy lithium-ion batteries, J. Mater. Chem. 22 (2012) 3680e3695. [11] J. Shu, R. Ma, L. Shao, M. Shui, K. Wu, M. Lao, D. Wang, N. Long, Y. Ren, In-situ X-ray diffraction study on the structural evolutions of LiNi0.5Co0.3Mn0.2O2 in different working potential windows, J. Power Sources 245 (2014) 7e18. [12] S.H. Ju, I.-S. Kang, Y.-S. Lee, W.-K. Shin, S. Kim, K. Shin, D.-W. Kim, Improvement of the cycling performance of LiNi0.6Co0.2Mn0.2O2 cathode active materials by a dual-conductive polymer coating, ACS Appl. Mater. Interfaces 6 (2014) 2546e2552. [13] J. Cho, T.-J. Kim, J. Kim, M. Noh, B. Park, Synthesis, thermal, and electrochemical properties of AlPO4-coated LiNi0.8Co0.1Mn0.1O2 cathode materials for a Li-ion cell, J. Electrochem. Soc. 151 (2004) A1899eA1904. [14] Y.-M. Lee, K.-M. Nam, E.-H. Hwang, Y.-G. Kwon, D.-H. Kang, S.-S. Kim, S.W. Song, Interfacial origin of performance improvement and fade for 4.6 V LiNi0.5Co0.2Mn0.3O2 battery cathodes, J. Phys. Chem. C 118 (2014) 10631e10639. [15] M. Noh, J. Cho, Optimized synthetic conditions of LiNi0.5Co0.2Mn0.3O2 cathode materials for high rate lithium batteries via co-precipitation method, J. Electrochem. Soc. 160 (2013) A105eA111. [16] X. Xiong, Z. Wang, H. Guo, Q. Zhang, X. Li, Enhanced electrochemical properties of lithium-reactive V2O5 coated on the LiNi0.8Co0.1Mn0.1O2 cathode material for lithium ion batteries at 60 C, J. Mater. Chem. A 1(2013) 1284e1288. [17] K. Chen, Y. Shen, J. Jiang, Y. Zhang, Y. Lin, C.-W. Nan, High capacity and rate performance of LiNi0.5Co0.2Mn0.3O2 composite cathode for bulk-type all-solidstate lithium battery, J. Mater. Chem. A 2 (2014) 13332e13337. [18] M. Xu, L. Zhou, Y. Dong, U. Tottempudi, J. Demeaux, A. Gursuch, B.L. Lucht, Improved performance of high voltage graphite/LiNi0.5Mn1.5O4 batteries with added lithium tetramethyl borate batteries and energy storage, ECS Electrochem. Lett. 4 (2015) A83eA86. [19] H. Rong, M. Xu, L. Xing, W. Li, Enhanced cyclability of LiNi0.5Mn1.5O4 cathode in carbonate based electrolyte with incorporation of tris (trimethylsilyl)
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A novel imidazole-based electrolyte additive for improved electrochemical performance of high voltage nickel-rich cathode coupled with graphite anode lithium ion battery Haibo Rong a, Mengqing Xu a, b, *, Yunmin Zhu a, Boyuan Xie a, Haibin Lin a, Youhao Liao a, b, Lidan Xing a, b, Weishan Li a, b, ** a
School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China Engineering Research Center of MTEES (Ministry of Education), Research Center of BMET (Guangdong Province), Engineering Lab. of OFMHEB (Guangdong Province), Key Lab. of ETESPG (GHEI), and Innovative Platform for ITBMD (Guangzhou Municipality), South China Normal University, Guangzhou 510006, China
b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
SDM is investigated as an electrolyte additive for graphite/LiNi0.5Co0.2Mn0.3O2 cell. The cycling performance is enhanced with appropriate amount of SDM addition. The surface layer derived from SDM on the cathode is more stable and robust. Dissolution of Mn, Co, and Ni can be reduced by adding SDM.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 July 2016 Received in revised form 23 August 2016 Accepted 3 September 2016
1,10 -sulfonyldiimidazole (SDM) has been investigated as a novel carbonate-based electrolyte additive for high voltage nickel-rich cathode chemistry, graphite/LiNi0.5Co0.2Mn0.3O2 cells. Upon cycling at high voltage for 50 cycles, graphite/LiNi0.5Co0.2Mn0.3O2 cells with SDM containing electrolyte have superior cycling performance than the cells with baseline electrolyte, specifically, 96.9% and 73.1% capacity retention, respectively. Moreover, cells with 0.25 wt. % SDM have lower impedance and better elevated temperature storage performance as well. The functional mechanism of electrolyte containing SDM on improved cycling performance is elucidated with ex-situ analytical techniques, including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and fourier transform infrared spectroscopy (FTIR), etc. The surface analysis result reveals that SDM has been involved into the surface film forming on the LiNi0.5Co0.2Mn0.3O2 cathode and graphite anode as well, which can simultaneously provide protection for both cathode and anode upon cycling to high voltage, leading to enhanced cyclability of the high voltage (4.5 V vs. Li/Liþ) graphite/LiNi0.5Co0.2Mn0.3O2 cells with the presence of SDM. © 2016 Elsevier B.V. All rights reserved.
Keywords: Lithium ion battery 1,10 -sulfonydiimidazole Electrolyte additive LiNi0.5Co0.2Mn0.3O2 High voltage Interface
* Corresponding author. School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China. ** Corresponding author. School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China. E-mail addresses: [email protected] (M. Xu), [email protected] (W. Li). http://dx.doi.org/10.1016/j.jpowsour.2016.09.016 0378-7753/© 2016 Elsevier B.V. All rights reserved.
1. Introduction Lithium ion batteries (LIBs) have been used widely as dominant power source for portable electronic devices and also have
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attracted growing interest for the application in electric vehicles and hybrid electric vehicles [1e5]. In order to meet the demands of these applications, further improvement in energy density of LIBs are required [6e8]. The energy density of LIBs can be improved by enhancing the specific capacity and the operating voltage of individual cells [9]. Among various cathode materials, layer-structured Ni-rich cathodes (Ni content 0.5), such as LiNi0.5Co0.2Mn0.3O2, LiNi0.6Co0.2Mn0.2O2 and LiNi0.8Co0.1Mn0.1O2 have been considered as prospective cathode candidates for high energy density LIBs [10e13]. Extra capacity could be attained by charging the cells to high voltage (4.5 V vs. Li/Liþ) with the use of Ni-rich cathodes [14e17]. Unfortunately, high voltage is always accompanied by serious oxidative decomposition of the traditional LiPF6/carbonate electrolyte [18e21], and these decomposed products lead to continuous growth of an undesirable surface film on the cathode, which is impeditive and erratic, resulting in bad cycling performance and poor coulombic efficiency [22]. It has been reported in the literatures that transition metal cations tend to dissolve from the Ni-rich cathode and subsequently move toward the graphite anode during cycling and aging process. This can destroy the formed solid electrolyte interface (SEI) on the graphite surface, resulting in additional lithium loss and capacity fading upon cycling [16,23e25]. Surface modification of Ni-rich cathodes with Al2O3, AlF3, SiP2O7 and LiAlO2 have been verified as an effective approach for improving the cycling performance of Ni-rich cathodes at high voltage [26e30]. But the surface modification usually involves complicated synthetic processes and high manufacturing cost [31]. Graphite is the preferred anode material and has been widely used for state-of-the-art LIBs due to its excellent advantages: relatively high capacity, low cost and environmental friendliness [32e36]. However, the layered structure of graphite anode is vulnerable and may be damaged when co-intercalation of electrolyte solvent occurres [37]. Therefore, a robust and protective SEI should be built up on the graphite anode. It has been reported that electrolyte additives such as vinylene carbonate (VC), fluoroethylene carbonate (FEC), 4-fluorophenyl acetate (4-FPA), propane sultone (PS) and 1-fluoropropane-2-one (FA) are able to form a SEI film on the graphite anode [36,38e45]. The additive-derived SEI film protects the graphite and reduces further decomposition of the electrolyte. The concept by forming stable SEI via incorporation of additives to inhibit electrolyte reduction has been expanded to cathode surface film forming as well [46]. According to some recent studies, cathode film forming additives, including methylene methyl disulfonate (MMDS), tris (trimethylsilyl) borate (TMSB) and tris (trimethylsilyl) phosphate (TMSP), have been used to improve the cycling performance of the cells with Ni-rich cathode upon cycling at high voltage. These film forming additives tend to incorporate into cathode surface film forming process, resulting in a robust surface film on Ni-rich cathodes [47e50]. Several electrolyte additives reported recently are able to simultaneously form protective surface film on cathode and anode, and this unique functionality demonstrates more convenient and effective approach to improve the electrochemical performance of LIB. Li et al. reported that the cycling performance of the graphite/ LiMn2O4 cell at elevated temperature can be significantly improved with the use of prop-1-ene-1,3-sulfone (PES) [37], which can simultaneously form protective film on the graphite electrode and the LiMn2O4 electrode. Xu et al. reported lithium difluoro (oxalato) borate (LiDFOB) was considered as an effective additive for graphite/LiFePO4 cell with the formation of a oxalate-borate derived film on both anode and cathode, as well [51]. In this study, we report a novel electrolyte additive, 1,10 - sulfonyldiimidazole (SDM), to improve the electrochemical performance
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of graphite/LiNi0.5Co0.2Mn0.3O2 cell upon cycling at 4.5 V vs. Li/Liþ. SDM possesses unique functionality by offering simultaneous protection to the graphite anode and LiNi0.5Co0.2Mn0.3O2 cathode upon cycling at high voltage, 4.5 V vs. Li/Liþ. Electrochemical methods combination with ex-situ surface analyses have been used to elucidate the effects of SDM containing electrolyte on the improved performance of graphite/LiNi0.5Co0.2Mn0.3O2 cell upon cycling at 4.5 V vs. Li/Liþ. 2. Experimental section 1.0 M lithium hexafluorophosphate (LiPF6) dissolved in a solution of 3:5:2 ethylene carbonate (EC)/ethyl methyl carbonate (EMC)/diethyl carbonate (DEC) (wt.%) (baseline electrolyte) was obtained from Dongguan Kaixin Battery Materials Co., Ltd., China, and used without further purification. 1,10 - sulfonyldiimidazole (SDM, assay 99.5%) was purchased from Fujian Chuangxin Technology Co., Ltd., China, and used without further purification. Electrolytes with SDM contain 0.25 or 0.5% by weight. Molecular structure of SDM is shown in Scheme 1. The composite LiNi0.5Co0.2Mn0.3O2 cathodes obtained from Foshan Advanced Electronics Energy Co., Ltd., China, containing 97.5% active material, super P (1.0%) and PVDF (1.5%). The composite graphite anodes obtained from Foshan Advanced Electronics Energy Co., Ltd., China, consisting of 95.5% graphite, super P (1.0%), CMC (1.5%) and SBR (2.0%). Graphite/LiNi0.5Co0.2Mn0.3O2 2025-type coin cells with Celgard 2400 as separator were assembled in an argon glove box for electrochemical performance measurements. The electrochemical charge/discharge behavior of Li/LiNi0.5Co0.2Mn0.3O2 cells and graphite/LiNi0.5Co0.2Mn0.3O2 cells were explored on a Land cell test system (Land CT2001A, China) according to the following procedure: cells cycled at 25 C were first charged to 4.5 V at an constant current (0.2 C for full cells and 0.5 C for half cells) followed by a constant voltage charge (V ¼ 4.5 V) until the current decreased to 10% of the applied charging current, and then the cells were discharged to 2.75 V with the same constant current. The Li/graphite cells were charged and discharged between 0.01 and 2.5 V at 0.2 C. Cyclic voltammetry was performed on Solartron-1480 (England) in the potential range of 0.01e2.5 V at a scanning rate of 0.1 mV s1. The electrochemical impedance spectroscopy (EIS) were carried out under discharged state, with the amplitude of 5 mV, and the frequency ranging from 105 Hz to 0.01 Hz by using a PGSTAT-30 electrochemical station (Autolab Metrohm, Netherlands). Self-discharge measurements of the graphite/LiNi0.5Co0.2Mn0.3O2 cells were performed according to the following protocols: cells were charged to 4.5 V with a constant current of 0.2 C, followed by a constant voltage (V ¼ 4.5 V) charge with a cut-off current of 10% of applied charging current, and then the cells were stored at open circuit voltage for 25 days at elevated temperature (55 C). The cycled graphite/LiNi0.5Co0.2Mn0.3O2 cells were disassembled in a glove box filled with high purity argon. Both the cycled graphite anode and LiNi0.5Co0.2Mn0.3O2 cathode were rinsed with anhydrous dimethyl carbonate (DMC) three times to remove the residual EC
Scheme 1. Molecular structure of SDM.
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and LiPF6 salt precipitated on the surface, followed by drying overnight under vacuum at 25 C before ex-situ characterizations. The surface morphologies of graphite anode and LiNi0.5Co0.2Mn0.3O2 cathode were observed with scanning electron microscopy (SEM) (JSM-6510, Japan). Fourier transiform infrared spectroscopy (FTIR) was carried out on a Bruker Tensor 27 (Germany) with an attenuated total reflectance (ATR) accessory to identify organic functional groups on the surface of both anode and cathode. X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD, England) with Al Kɑ line as an X-ray source. Calibration of XPS peak was made by recording XPS spectra for reference compounds, which are present on the electrode surface: LiF, PVDF and LixPOyFz. The universal contamination of CeH bond at 284.6 eV was used as a reference for the final adjustment of the energy scale in the spectra. Elemental concentration was calculated based on the equation: P Cx¼(Ix/Sx)/( Ix =Sx ), where Ix is the relative intensity of the element, and Sx is the sensitivity value of the element. 3. Results and discussion According to previous studies, the cathode film forming additives should be easier to be oxidized than the electrolyte solvents [48]. Therefore, Linear sweep voltammetry (LSV) was conducted to evaluate the oxidation potential of SDM on a Pt electrode. As presented in Fig. S1, it can be observed that the electrolyte with SDM oxidizes from 3.8 V (vs. Li/Liþ), while the baseline electrolyte decomposition begins 4.5 V, indicating SDM has lower oxidation potential than carbonate solvents of the electrolyte. In addition, the oxidative current of the electrolyte with SDM is significantly lower than the baseline electrolyte from 4.5 to 5.5 V. This result implies the oxidation product of SDM can passivate the Pt electrode and suppress severe oxidative decomposition of the electrolyte at high voltage. Fig. 1a shows the cycling performance of the graphite/LiNi0.5Co0.2Mn0.3O2 cells with baseline electrolyte and electrolyte with added SDM at room temperature. The initial discharge capacity of the cells with added SDM additive is slightly lower than that with baseline electrolyte, which may be related to the irreversible
decomposition of SDM. Evidence for support this assumption will be discussed below. Surprisingly, a significant improvement in the cycling performance for the cells with SDM can be observed upon cycling at high voltage. The capacity retention for the cells with 0.25 and 0.5 wt. % SDM is 96.9% and 87.8%, respectively; while the cell with baseline electrolyte after 50 cycles at high voltage is only 73.1%, suggesting over 20% improvement by incorporation of 0.25 wt. % SDM after 50 cycles. In addition, the initial Coulombic efficiency (CE) of the cell with baseline electrolyte is 72.75%, while is 70.86% and 63.35% for the electrolytes with 0.25 and 0.5 wt.% SDM, respectively (Fig. 1b). The lower initial CE maybe related to the irreversible decomposition of SDM. Interestingly, it is noteworthy that the CE for the cells with 0.25 (99.62%) or 0.5 wt. % (99.5%) is slightly higher than that cycled with baseline electrolyte (98.92%) after 50 cycles at high voltage. As is well known, a high CE of greater than 99.5%, which is vital for practical applications of LIBs. Based on the above eletrochemical data, the 0.25 wt.% concentration of SDM in the electrolyte yields better capacity retention and CE. Hence, the following study is based on the data set with 0.25 wt. % SDM. With an effort to obtain a better understanding of the benefits of SDM for the graphite/LiNi0.5Co0.2Mn0.3O2 cells, control experiments have been done by adding SDM to the electrolyte of Li/graphite cell and Li/LiNi0.5Co0.2Mn0.3O2 cell, respectively. Li/LiNi0.5Co0.2Mn0.3O2 cells with and without SDM suffered capacity fading after 80 cycles at high voltage, but the Li/LiNi0.5Co0.2Mn0.3O2 cell containing SDM shows better cycling stability than that with baseline electrolyte, as seen in Fig. 1c. Additionally, a Li/LiNi0.5Co0.2Mn0.3O2 cell with added SDM has a lower initial discharge capacity than that without SDM. The lower discharge capacity is most likely due to the SDM additive undergoing the surface filming process and consuming the active Liþ source on the surface of LiNi0.5Co0.2Mn0.3O2 cathode at high voltage, and the irreversible decomposition of SDM is consistent with the results of the LSV test as discussed above. Consequently, the improvement of Li/LiNi0.5Co0.2Mn0.3O2 cell can be ascribed to surface film incorporated with SDM on the LiNi0.5Co0.2Mn0.3O2 cathode, which can suppress continuous oxidative decomposition of electrolyte upon cycling at high voltage. Investigation of the
Fig. 1. Cycling performance of (a) and the Coulombic efficiency (b) of graphite/LiNi0.5Co0.2Mn0.3O2 cells, (c) Li/LiNi0.5Co0.2Mn0.3O2 cells and (d) Li/graphite cells with SDM containing electrolyte and baseline electrolyte.
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effect of SDM on the graphite anode was also carried out. It can be seen from Fig. 1d that the cycling performance of the Li/graphite cell can be improved with added SDM, suggesting that addition SDM is beneficial to the graphite anode as well. Analysis of the results of using SDM in LiNi0.5Co0.2Mn0.3O2 cathode and graphite anode as discussed above yields the conclusion that use SDM can simultaneously protect the cathode and anode thus resulting in improvement of the graphite/LiNi0.5Co0.2Mn0.3O2 cell. To gain more insight into the effect of added SDM on the graphite anode, cyclic voltammogram of this system is conducted. Fig. 2a and b compare the initial CVs of graphite anodes in the 1 M LiPF6 EC:EMC:DEC ¼ 3:5:2 electrolytes with and without SDM. The anodic processes are mainly related to Liþ deintercalation from graphite; while the cathodic processes are indicative of the Liþ intercalation into graphite and the reduction of the electrolyte species, and the formation of SEI film [38]. Apparently, the initial decomposition of the electrolyte with added SDM occurs earlier than that of baseline electrolyte, from 1.3 to 1.5 V (vs. Li/Liþ) (Fig. 2b), indicating the preferred reduction of SDM over the solvents. The reduction peak that appears at around 0.5 V corresponds to reductive decomposition of electrolyte, which is weaker for the electrolyte with added SDM than the baseline electrolyte, suggesting that an SEI film originating from SDM mitigates the decomposition of the electrolyte. To further confirm the preferential reductive behavior of SDM on the graphite anode, CVs for graphite anodes in the PC-based electrolyte with and without SDM were also obtained, as shown in Fig. 2c and d. In the PC-based electrolyte without SDM additive (Fig. 2c), a large reductive current at 0.5 V can be clearly observed. This is attributed to the cointercalation of PC into the graphite anode, indicating nonprotective SEI formed on graphite [43]. Unlike the electrolyte without SDM, a reduction peak can be seen from 1.3 to 1.5 V (Fig. 2d), very consistent with the reduced peak observed in ECbased electrolyte system, which suggests SDM has incorporated into the SEI film forming process on the graphite and the reduction peak at 0.5 V is much weaker for the graphite with SDM than that without additive. These observations imply that co-intercalation of PC can be effectively mitigated by the formation of a steady and
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protective SDM derived SEI on graphite anodes. As a result, the improved cycling performance of Li/graphite half cell can be ascribed to the robust and protective SEI film on the graphite via the preferred reduction of SDM. This in turn alleviates the decomposition of electrolyte and provides the protection for the graphite anode. Fig. 3a shows the voltage vs. discharge capacity profiles of graphite/LiNi0.5Co0.2Mn0.3O2 cells with baseline electrolyte. The discharge capacity and the discharge potential plateau of the graphite/LiNi0.5Co0.2Mn0.3O2 cell with baseline electrolyte are significantly decreased upon cycling at high voltage. This observation is likely due to the following factors. First, the severe oxidative decomposition of electrolyte on the LiNi0.5Co0.2Mn0.3O2 cathode at high voltage exaggerates the irreversible capacity and polarization. Second, breakdown of the SEI on the graphite anode resulting from the irreversible reduction of electrolyte solvents and the deposition of transition metal ions originating from the cathode during the cycling process caused extra Liþ loss and larger polarization of the full cell. However, graphite/LiNi0.5Co0.2Mn0.3O2 cells with added SDM electrolyte showed less capacity loss and a negligible decrease of the potential plateau (Fig. 3b). This result indicates the use of SDM in a graphite/LiNi0.5Co0.2Mn0.3O2 cell can dramatically reduce the capacity loss and polarization of the full cell upon cycling at high voltage. The electrochemical impedance spectroscopy (EIS) of graphite/ LiNi0.5Co0.2Mn0.3O2 cells with baseline electrolyte and added SDM after 50 cycles at high voltage are depicted in Fig. 3c. The spectra contain two main features: a semicircle from high to medium frequency, which can be ascribed to the impedance of surface film formed on the surface of the electrodes; another semicircle occurring at medium to low frequency is attributed to the chargetransfer processes. Fig. 3c clearly shows that both the surface film impedance and charge-transfer impedance are smaller for the cell with SDM than that with baseline electrolyte after cycling at high voltage. The reduced impedance of the graphite/LiNi0.5Co0.2Mn0.3O2 cell is most likely a result of the protective surface layers formed on the graphite anode and LiNi0.5Co0.2Mn0.3O2 cathode originated from SDM. SDM can reduce the decomposition
Fig. 2. First cycle CV curves of graphite anodes in EC- based electrolytes with (b) and without (a) SDM and PC- based electrolytes with (d) and without SDM (c), respectively.
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Fig. 3. Discharge curves of graphite/LiNi0.5Co0.2Mn0.3O2 cells at different cycles with (b) and without (a) SDM, electrochemical impedance spectra after 50 cycles for graphite/ LiNi0.5Co0.2Mn0.3O2 cells with and without SDM (c), self-discharge curves of graphite/LiNi0.5Co0.2Mn0.3O2 cells cycled with and without SDM at elevated temperature (d).
of the bulk electrolyte deposited on the anode and cathode upon cycling at high voltage. Elevated temperature self-discharge measurement of the graphite/LiNi0.5Co0.2Mn0.3O2 cells with and without SDM has been conducted and shown in Fig. 3d. Significant difference was observed for the charged cell storing with baseline and SDM-containing electrolyte. The voltage of the charged cell in the baseline electrolyte declined very slowly during the first 17 days, and then dropped from 4.3 to 0.7 V at a sudden. As respect to the SDM added electrolyte, the voltage declined gradually during
the entire process, and no sudden drop of voltage was observed. This observation indicates that the charged graphite/LiNi0.5Co0.2Mn0.3O2 cell in baseline electrolyte underwent more severe self-discharge during storage; while this scenario did not occur via the presence of SDM added electrolyte. Deep insight understanding of this self-discharge process could predict that more electrolyte suffered oxidative degradation in the baseline electrolyte on the charged LiNi0.5Co0.2Mn0.3O2 particle surface and lost electrons during this irreversible electrochemical side-reactions. And the free
Fig. 4. SEM images of fresh LiNi0.5Co0.2Mn0.3O2 electrode (a), and cycled LiNi0.5Co0.2Mn0.3O2 electrodes with (c) and without SDM (b) extracted from graphite/LiNi0.5Co0.2Mn0.3O2 cells.
H. Rong et al. / Journal of Power Sources 332 (2016) 312e321 Table 1 Element concentrations of fresh and cycled LiNi0.5Co0.2Mn0.3O2 electrodes with and without SDM containing electrolyte.
Fresh Baseline SDM
C(%)
O(%)
F(%)
P(%)
Ni(%)
Co(%)
Mn(%)
N(%)
S(%)
58 48.8 55.4
13.8 22.94 11.17
22.86 22.9 26.72
0.46 4.51 1.02
2.63 0.2 2.22
0.63 0.06 0.46
1.61 0.78 1.12
1.52
0.38
electrons flow into the charged/high valance particle surface, leading to sudden drop of the cell voltage as we observed for cell in baseline electrolyte. Self-discharge measurements indicate the SDM derived films on the surface layers of the anode and cathode can effectively suppress the self-discharge. SEM images of fresh cathode and cathodes cycled with either baseline electrolyte or SDM containing electrolyte are presented in Fig. 4. The spherical particles of LiNi0.5Co0.2Mn0.3O2 can be identified clearly on the cathode before cycling, as depicted in Fig. 4a. After 50 cycles at high voltage, it is obvious that some of the LiNi0.5Co0.2Mn0.3O2 particles cycled with baseline electrolyte suffered serious destruction (Fig. 4b), which might be ascribed to the
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transition metal dissolution in the bulk material during cycling at high voltage [49]. However, the structure of the cathode particles is maintained well in presence of electrolyte with added SDM (Fig. 4c). This suggests a surface film incorporated with SDM can protect the cathode upon cycling at high voltage. There are no distinct differences between the surface films for the cycled cathodes with and without SDM as measured by SEM. X-ray photoelectron spectroscopy (XPS) was employed to further understand the surface and structure of the cycled cathodes. This method is useful for investigating surface films because the method analyzes about the top 5 nm of the surface, has good sensitivity for the elements of interest and provides elemental concentration data [52]. XPS spectra of fresh cathode and cathodes extracted from the full cells with and without SDM were acquired, and the corresponding elemental concentrations are listed in Table 1. Analysis of the cathode surfaces reveals significant difference in the surface film species. The concentrations of C, Ni, Co and Mn are decreased for both cycled cathodes with and without SDM; while the concentrations of P and F are increased compared to the fresh cathode electrode, suggesting formation of a surface film composed of electrolyte decomposition products covering the metal oxide and
Fig. 5. XPS spectra of the fresh LiNi0.5Co0.2Mn0.3O2 electrode, and the cycled LiNi0.5Co0.2Mn0.3O2 electrodes with and without SDM extracted from graphite/LiNi0.5Co0.2Mn0.3O2 cells.
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Fig. 6. SEM images of the fresh graphite electrode (a), and cycled graphite electrodes with (c) and without SDM (b) extracted from graphite/LiNi0.5Co0.2Mn0.3O2 cells.
binder upon cycling at high voltage. Elements S and N were detected on the surface film of the cathode cycled in electrolyte with SDM added as well, and these unique species can be ascribed to the added SDM. Apparently the concentrations of Ni, Co and Mn, of the cathode cycled with SDM, are much higher than of the with baseline electrolyte, indicating a thicker cathode surface film for the baseline electrolyte. Fig. 5 contains the XPS spectra of LiNi0.5Co0.2Mn0.3O2 cathodes from the cycled full cells, with a comparison of the fresh cathode. The fresh cathode contains peaks characteristic of PVDF binder at 285.7 and 290.6 eV and conductive carbon at 284.8 eV in the C 1s spectrum. Related peak characteristic of PVDF binder can be observed in the F 1s spectrum at 687.6 eV. In the fresh O 1s spectrum, the peak at 529.5 eV is due to the metal oxide, and the peak at 531.7 eV is assigned to the Li2CO3, which is frequently present on fresh cathode [31]. Analysis of the cathodes cycled at high voltage reveals new species in the C 1s, O 1s, F 1s and P 2p spectra, which could be identified as electrolyte decomposition products on the surface of the cathodes. New peaks characteristic of CeO and C]O are present at 286 eV and 288 eV in the C 1s spectrum and 533.5 eV and 532.5 eV in the O 1s spectra for the cycled cathodes with and without SDM additive, CeO and C]O species are consistent with the presence of lithium alkyl carbonates and polycarbonates [36]. In addition, the intensity of the metal oxide peak (529.5 eV) in the O 1s spectrum is stronger for the cathode cycled with SDM containing electrolyte than cathode cycled with baseline electrolyte, indicating a thinner surface film formed on the cathode with the use of SDM after cycling, or less transition metals dissolution occurring during cycling at high voltage, 4.5 V vs. Li/Liþ. Along with the peak corresponding to PVDF at 687.8 eV, new peak at 684.8 eV, characteristic of LiF, appeared in the F 1s spectra. It is evident that the intensity of the LiF peak is weaker for the cathode with SDM than that with baseline electrolyte. It is well known that LiF is one of the decomposition products of LiPF6 and the interfacial impedance of the cell could increase with the deposition of LiF on the cathode surface [53]. In the P 2p spectrum, peaks at 134.3 eV and 136.5 eV are attributed to LixPFyOz and LixPFz, which can be ascribed to LiPF6 decomposition on the cathode [54]. The cathode cycled in the SDM containing electrolyte shows weaker intensities of peaks belonging
to LiF, LixPFyOz and LixPFz, indicating that there is less decomposition products of LiPF6 on the surface of the cathode with electrolyte containing SDM. In the S 2p and N 1s spectra, peaks around 168e169 eV are assigned to Li2SO3 or ROSO2Li [37], and peaks at 398.5 eV and 401 eV can be attributed to pyridinic N and pyrrolic N, respectively [55]. The results obtained from the S 2p and N 1s indicate that SDM has been incorporated into the surface film on the cathode during cycling at high voltage. The spectra of transition metals (Ni, Co and Mn) also have been obtained. The intensities of Ni, Co and Mn are stronger for the cathode cycled with SDM containing electrolyte than that with baseline electrolyte. This further confirms a thicker film generated on the cathode without additive caused by the oxidative decomposition of the electrolyte or transition metal dissolution occurring after cycling at high voltage, or less transition metals dissolution occurring during cycling at high voltage, 4.5 V vs. Li/Liþ. In summary, the results of XPS analysis reveal that a thinner surface film originated from the addition of SDM covers the LiNi0.5Co0.2Mn0.3O2 cathode, and less transition metals dissolution occurring during cycling at high voltage, 4.5 V vs. Li/Liþ. The interphase plays a role in suppressing the continuous decomposition of the LiPF6 and the electrolyte, and transition metal dissolution as well, while cycling at high voltage. The FTIR spectra of the LiNi0.5Co0.2Mn0.3O2 cathodes with and without added SDM after cycling at high voltage are shown in Fig. S2. The IR spectra are dominated by characteristics of PVDF binder at 1400, 1271, 1170, 1070 and 877 cm1 for both cathodes. As opposed to the cathode cycled in the baseline electrolyte, an additional peak can be observed at 770 cm1 for the cathode cycled with SDM containing electrolyte. This peak is characteristic of CeSeO species. Presence of this peak confirms the SDM has been
Table 2 Element concentrations of fresh and cycled graphite electrodes with and without SDM containing electrolyte.
Fresh Baseline SDM
C(%)
O(%)
F(%)
P(%)
83.7 48.11 52.41
15.41 32.35 35.69
5.47 13.97 9.74
2.3 3.35
Ni(%)
Co(%)
Mn(%)
N(%)
S(%)
1.21 0.65
0.46 0.19
0.55 0.19
0.7
0.43
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Fig. 7. XPS spectra of the fresh graphite electrode, and the cycled graphite electrodes with and without SDM extracted from graphite/LiNi0.5Co0.2Mn0.3O2 cells.
incorporated into the surface film on the cathode after cycling at high voltage. This observation is also consistent with the results of XPS results of the cathodes as discussed above. SEM images of the graphite anode before and after cycling at high voltage can be found in Fig. 6. In Fig. 6a, the graphite particles can be observed clearly on the anode electrode before cycling. However, the surface of the graphite anode after cycling with either electrolyte results in an amorphous surface film (Fig. 6b). The graphite particles cycled with baseline electrolyte appear damaged or covered by a thick SEI film. This can be ascribed to the electrolyte decomposition or precipitation of transition metal migrated from cathode side due to the dissolution in the electrolyte. With the addition of the SDM into to the bulk electrolyte, the graphite particles can still be identified and are observed with a thin surface film (Fig. 6c), suggesting the use of SDM can protect the graphite anode and reduce electrolyte decomposition during cycling.
To better understand the surface chemistry on the graphite anode after cycling to high voltage, XPS was employed to investigate the components of the graphite surface. Table 2 contains the elemental concentrations of the fresh graphite, and graphite anodes extracted from graphite/LiNi0.5Co0.2Mn0.3O2 cells after cycling at high voltage with baseline electrolyte and SDM containing electrolyte. After cycling at high voltage with both electrolytes, the concentration of C is distinctly decreased, while the concentrations of O and F are increased relative to the fresh graphite anode, suggesting SEI films were formed on the cycled graphite anodes, in accord with SEM observations. Besides the cathode, N and S are observed on the anode cycled with SDM containing electrolyte, indicating SDM is involved in the formation of surface film on the graphite anode as well, resulting from the preferential reduction of SDM. Most importantly, the concentrations of Ni, Co and Mn are much lower for the anode cycled with SDM than that with baseline
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electrolyte, indicating less transition metal deposition on the graphite electrode with the use of SDM. The XPS elemental spectra are shown in Fig. 7. In the C 1s spectra, the peak located at 284.8 eV is assigned to graphite, and the peaks appearing at 286 eV and 288 eV correspond to CeO and C]O species. Apparently, the intensity of graphite on the electrode cycled with SDM containing electrolyte is much stronger than that of the baseline electrolyte, suggesting less decomposition products on the anode in the presence of SDM. The O 1s spectrum at 531.7 eV and 533.5 eV corresponds to Li2CO3 and lithium alkyl carbonates, respectively. The peak attributed to Li2CO3 is stronger for the anode cycled with SDM. The intensity of lithium alkyl carbonates is stronger for the anode cycled with baseline electrolyte than that with SDM containing electrolyte, which can be ascribed to the more decomposition of electrolyte on the anode in baseline electrolyte during cycling. In addition, peaks of N 1s (398.5 eV, 401 eV) and S 2p (167.2 eV, 169 eV) can be observed on the anode extracted from the cell cycled with SDM containing electrolyte, confirming that SDM is involved in the anode surface film forming process. The F 1s spectrum of the cycled graphite anodes present two peaks at 684.8 eV and 687.0 eV, corresponding to LiF and LixPOyFz, respectively. As it has been discussed above, LiF, and LixPOyFz are the decomposition products of LiPF6. Particularly, the presence of LiF on the surface of the active electrodes would increase the interfacial impedance of the cell, leading to poor cycling performance. The intensities of the LiF and LixPOyFz are slightly stronger for the anode cycled with baseline electrolyte than that with SDM containing electrolyte, suggesting SDM derived films on the graphite anode can reduce the decomposition of the electrolyte. Finally, transition metal (Ni, Co and Mn) ions appear on the surface of cycled anodes, suggesting soluble transition metal ions reach the graphite through the separator and are reduced on the anodes. The intensities of Ni, Co and Mn on the anode with SDM containing electrolyte are weaker than those for the anode without SDM, which indicates that the SDM derived film inhibits the deposition of transition metals on anode surfaces. According to previous reports, deposition of transition metal ions on the graphite anode surface likely destabilizes the anode SEI leading to additional electrolyte reduction and related capacity loss [49]. The XPS results of the graphite anodes reveal that the protective surface film resulting from SDM can reduce the decomposition of the electrolyte and reduce deposition of transition metal ions. The FTIR spectra of graphite anodes extracted from graphite/ LiNi0.5Co0.2Mn0.3O2 cells with added SDM and baseline electrolyte after 50 cycles at high voltage are depicted in Fig. S3. Anodes with either electrolyte are similar and are dominated by CMC and SBR [56,57], which have characteristic absorptions at 1636, 1241, 1060, 966 and 840 cm1. However, for the graphite anode cycled with SDM containing electrolyte, an additional peak around 690e730 cm1 and 1010 cm1, is indicative of OeSeO species. This further confirms SDM has been involved in the formation of surface films on the graphite anode. 4. Conclusion In this work, 1,10 -sulfonyldiimidazole (SDM) is firstly proposed as a novel electrolyte additive for improving the electrochemical performance of graphite/LiNi0.5Co0.2Mn0.3O2 cell upon cycling at high voltage (4.5 V, vs. Li/Liþ). SDM is simultaneously sacrificially reduced and oxidized on graphite and LiNi0.5Co0.2Mn0.3O2 cathode, respectively, resulting in forming protective surface layers on both anode and cathode simultaneously. The desired forming surface layers lead to highly stable interface of the electrolyte/electrodes, which can effectively suppress the electrolyte decomposition parasitic reactions and transition metal dissolution from bulk
cathode material upon cycling at high voltage. Ex-situ analysis of LiNi0.5Co0.2Mn0.3O2 and graphite electrodes is conducted via a combination of SEM, XPS and FTIR to further support that the highly stable interfaces derived from SDM is the dominant contributor to the enhanced cycling performance of the graphite/ LiNi0.5Co0.2Mn0.3O2 cell. Acknowledgements The authors thank the National Natural Science Foundation of China (Grant No. 21573081), the Natural Science Foundation of Guangdong Province (Grant No. 2014A030313444), and the key project of Science and Technology of Guangdong Province (Grant No. 2014B010123002) for financial support. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.09.016. References [1] K. Xu, Nonaqueous liquid electrolytes for lithium-based rechargeable batteries, Chem. Rev. 104 (2004) 4303e4418. [2] J.-M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359e367. [3] J.B. Goodenough, K.-S. Park, The Li-ion rechargeable battery: a perspective, J. Am. Chem. Soc. 135 (2013) 1167e1176. [4] V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Challenges in the development of advanced Li-ion batteries: a review, Energy Environ. Sci. 4 (2011) 3243e3262. [5] Z.H. Ma, C.W. Sun, Y.H. Lyu, Y.S. Wang, Y.S. Kim, L.Q. Chen, A new oxyfluorinated titanium phosphate anode for a high-energy lithium-ion battery, ACS Appl. Mater. Interfaces 7 (2015) 1270e1274. [6] M. Hu, X.L. Pang, Z. Zhou, Recent progress in high-voltage lithium ion batteries, J. Power Sources 237 (2013) 229e242. [7] L.W. Su, Y. Jing, Z. Zhou, Li ion battery materials with core-shell nanostructures, Nanoscale 3 (2011) 3967e3983. [8] D.H. Long, M.-G. Jeong, Y.-S. Lee, W.C. Choi, J.K. Lee, I.-H. Oh, H.-G. Jung, Coating lithium titanate with nitrogen-doped carbon by simple refluxing for high-power lithium-ion batteries, ACS Appl. Mater. Interfaces 7 (2015) 10250e10257. [9] J. Li, L.E. Downie, L. Ma, W. Qiu, J.R. Dahn, Study of the failure mechanisms of LiNi0. 8Mn0.1Co0.1O2 cathode material for lithium ion batteries, J. Electrochem. Soc. 162 (2015) A1401eA1408. [10] P. He, H. Yu, D. Li, H. Zhou, Layered lithium transition metal oxide cathodes towards high energy lithium-ion batteries, J. Mater. Chem. 22 (2012) 3680e3695. [11] J. Shu, R. Ma, L. Shao, M. Shui, K. Wu, M. Lao, D. Wang, N. Long, Y. Ren, In-situ X-ray diffraction study on the structural evolutions of LiNi0.5Co0.3Mn0.2O2 in different working potential windows, J. Power Sources 245 (2014) 7e18. [12] S.H. Ju, I.-S. Kang, Y.-S. Lee, W.-K. Shin, S. Kim, K. Shin, D.-W. Kim, Improvement of the cycling performance of LiNi0.6Co0.2Mn0.2O2 cathode active materials by a dual-conductive polymer coating, ACS Appl. Mater. Interfaces 6 (2014) 2546e2552. [13] J. Cho, T.-J. Kim, J. Kim, M. Noh, B. Park, Synthesis, thermal, and electrochemical properties of AlPO4-coated LiNi0.8Co0.1Mn0.1O2 cathode materials for a Li-ion cell, J. Electrochem. Soc. 151 (2004) A1899eA1904. [14] Y.-M. Lee, K.-M. Nam, E.-H. Hwang, Y.-G. Kwon, D.-H. Kang, S.-S. Kim, S.W. Song, Interfacial origin of performance improvement and fade for 4.6 V LiNi0.5Co0.2Mn0.3O2 battery cathodes, J. Phys. Chem. C 118 (2014) 10631e10639. [15] M. Noh, J. Cho, Optimized synthetic conditions of LiNi0.5Co0.2Mn0.3O2 cathode materials for high rate lithium batteries via co-precipitation method, J. Electrochem. Soc. 160 (2013) A105eA111. [16] X. Xiong, Z. Wang, H. Guo, Q. Zhang, X. Li, Enhanced electrochemical properties of lithium-reactive V2O5 coated on the LiNi0.8Co0.1Mn0.1O2 cathode material for lithium ion batteries at 60 C, J. Mater. Chem. A 1(2013) 1284e1288. [17] K. Chen, Y. Shen, J. Jiang, Y. Zhang, Y. Lin, C.-W. Nan, High capacity and rate performance of LiNi0.5Co0.2Mn0.3O2 composite cathode for bulk-type all-solidstate lithium battery, J. Mater. Chem. A 2 (2014) 13332e13337. [18] M. Xu, L. Zhou, Y. Dong, U. Tottempudi, J. Demeaux, A. Gursuch, B.L. Lucht, Improved performance of high voltage graphite/LiNi0.5Mn1.5O4 batteries with added lithium tetramethyl borate batteries and energy storage, ECS Electrochem. Lett. 4 (2015) A83eA86. [19] H. Rong, M. Xu, L. Xing, W. Li, Enhanced cyclability of LiNi0.5Mn1.5O4 cathode in carbonate based electrolyte with incorporation of tris (trimethylsilyl)
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