Graphite Battery

Electrochimica Acta 256 (2017) 307–315

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Research Paper

Tetrafluoroterephthalonitrile: A Novel Electrolyte Additive for High-Voltage Lithium Cobalt Oxide/Graphite Battery Yanlin Liua , Kang Wanga , Yilong Lina , Yunmin Zhua , Wenqiang Tua , Mengqing Xua,b , Xiang Liua,b,* , Bin Lia,c , Weishan Lia,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), Key Laboratory of ETESPG (GHEI), and Innovative Platform for ITBMD (Guangzhou Municipality), South China Normal University, Guangzhou 510006, China c School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, China b

A R T I C L E I N F O

Article history: Received 31 July 2017 Received in revised form 14 September 2017 Accepted 9 October 2017 Available online 10 October 2017 Keywords: Lithium cobalt oxide Graphite Lithium-ion battery Cyclic stability Tetrafluoroterephthalonitrile

A B S T R A C T

A novel electrolyte additive, tetrafluoroterephthalonitrile (TFTPN), is proposed to improve the cyclic stability of lithium cobalt oxide (LiCoO2)/graphite lithium-ion full cells up to 4.4 V. Electrochemical measurements indicate that TFTPN can be reduced on graphite electrode and oxidized on LiCoO2 electrode preferentially compared to the baseline electrolyte, 1.0 M LiPF6 in EC/DEC/EMC (1/1/1, in weight), and thus improves the cyclic stability of graphite/Li and LiCoO2/Li half cells, respectively. Further charge/discharge tests demonstrate that the cyclic stability of LiCoO2/graphite full cell can be significantly improved by TFTPN. A high capacity retention of 91% is achieved for the full cell using 0.5% TFTPN-containing electrolyte after cycling at 0.5C between 3.0 and 4.4 V for 300 cycles, compared to the 79% for that using the baseline electrolyte. This effect is attributed to the simultaneously formed protective interphase films on graphite and LiCoO2 by TFTPN due to its preferential reduction or oxidation. The resulting interphase films are verified by physical characterizations and theoretical calculations. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction Lithium-ion batteries (LIBs) have been successfully used in portable electronic devices because of its high energy density and long cycling life compared to other secondary batteries [1–5]. In order to meet the demands of advanced applications such as in electric vehicles, further improvement in energy density of LIBs is required [6,7]. Enhancing the operating voltage of individual cells is considered as one of the ways to improve the energy density of LIBs [8,9]. Among various cathode materials that have been used commercially, layered lithium cobalt oxide (LiCoO2) is most attractive, because additional capacity can be attained by raising end off charge voltage [10]. Unfortunately, the charging to over 4.2 V (vs. Li/Li+), the end off charge voltage of current LiCoO2/ graphite battery, is always accompanied by electrolyte oxidation decomposition and cobalt dissolution from LiCoO2 [11–15], leading to the deteriorated cyclic performance. The dissolved cobalt might

* Corresponding authors at: School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China. E-mail addresses: [email protected] (X. Liu), [email protected] (W. Li). https://doi.org/10.1016/j.electacta.2017.10.059 0013-4686/© 2017 Elsevier Ltd. All rights reserved.

destroy the solid electrolyte interphase (SEI) film on graphite of the battery [15]. Many efforts have been devoted to solve this issue via surfacemodifying LiCoO2 with organic and inorganic materials, such as polymerized C60 [16], polypyrrole [17], Al2O3 [18], MgO [19], ZrO2 [20], and Li3PO4 [21], or doping LiCoO2 with other elements [22,23]. The cyclic stability of LiCoO2 can be improved by these approaches but the improvement is unsatisfactory. Additionally, surface-coating and element-doping usually involves complicated synthetic processes and high manufacturing cost. Graphite is currently the most used anode for state-of-the-art commercial LIBs due to its low cost, relatively high capacity and good cyclic stability [24,25]. The layered structure of graphite anode may be exfoliated by the co-intercalation of electrolyte solvents, which will be accentuated by dissolved transition metal cations from cathode [26,27]. Therefore, electrolyte additives are necessary to form a robust SEI film to protect the graphite from exfoliation. So far, various electrolyte additives such as vinylene carbonate (VC) [28], vinyl ethylene carbonate (VEC) [29], prop-1ene-1,3-sultone (PES) [30], tetrachloroethylene (TCE) [31] and 4fluorophenyl acetate (4-FPA) [32] have been proposed for the formation of a protective SEI film on graphite anode. Among these

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electrolyte additives, phenyl or fluorine-containing compounds can form more robust SEI films that are composed of stable phenyl or fluorinated polymers [32–35]. However, HF might be formed during the SEI formation from fluorine-containing additives, which is detrimental to the cathode. Recently, the concept of forming stable SEI film via electrolyte additives has been expanded to form cathode interphase film for the structural stability improvement of the cathodes. Many electrolyte additives including fumaronitrile (FN) [14], 4-(trifluoromethyl) benzonitrile (4-TB) [36], tris (trimethylsilyl) borate (TMSB) [37], and tris (trimethylsilyl) phosphate (TMSP) [38], have been proposed to construct protective interphase films of various high voltage cathodes. Interestingly, some of these electrolyte additives are able to form protective interphase films simultaneously on cathode and anode [39–42]. This feature is beneficial for reducing the amount of the electrolyte additives used in LIBs. Extra additives in the electrolyte might deteriorate the physical and chemical properties of the electrolyte such as ionic conductivity and interfacial compatibility with electrodes and membranes. Most importantly, the resulting cathode interphase films can prevent transition metal ion dissolution that is detrimental to anode. Compared to the massive number of the electrolyte additives that can form interphase films on cathode or anode individually, however, few are available for cathode or anode simultaneously, especially under high end off charge voltage [43– 48]. In this work, a novel electrolyte additive, tetrafluoroterephthalonitrile (TFTPN), is proposed to improve the cyclic stability of LiCoO2/graphite battery under an end off charge voltage of 4.4 V in a baseline electrolyte of 1.0 M LiPF6 in EC/DEC/EMC (1/1/1, in weight). This proposal is based on the molecular structure of TFTPN that contains one phenyl, four
F and two
CN groups in a simple molecule structure. As mentioned above, phenyl and
F groups are helpful for constructing a robust interphase film on graphite anode. On the other hand,
CN group is beneficial for the formation of a protective interphase film on LiCoO2 cathode under high voltage [36]. With this special molecular structure, TFTPN is expected to be effective for forming stable interphase films simultaneously on graphite anode and LiCoO2 cathode of high voltage LIBs. This selection of electrolyte additives has never been reported before. Electrochemical measurements and physical characterizations demonstrate that TFTPN can be preferentially oxidized and reduced compared to the baseline electrolyte, forming interphase films simultaneously on LiCoO2 cathode and graphite anode, leading to the significantly improved cyclic stability of LiCoO2/graphite battery. 2. Experimental 2.1. Preparation LiCoO2 electrode was prepared by coating a mixture of 97.5 wt. % LiCoO2 (Ningbo Jinhe New materials Co., Ltd.), 1.0 wt. % super-p and 1.5 wt. % poly (vinylidene difluoride) (PVDF) onto aluminum current collector (thickness: 14 mm). Graphite electrode was prepared by coating a mixture of 95.5 wt. % graphite (Shanghai Shanshan Tech Co., Ltd.), 1.0 wt. % super-p, 1.5 wt. % carboxymethyl cellulose (CMC) and 2.0 wt. % styrene butadiene rubber (SBR) onto copper current collector (thickness: 8 mm). The average mass loadings of cathode and anode electrodes were 37 mg cm
2 and 21 mg cm
2, respectively. An electrolyte (Shenzhen CAPCHEM Technology Co. Ltd., China), 1.0 M hexafluorophosphate (LiPF6) in the mixed solvent of ethylene carbonate (EC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) with a ratio of 1:1:1 in weight, was used as baseline electrolyte. TFTPN (>98.0%) was purchased from TCI (Tokyo Chemical Industry Co., Ltd. Japan). The

electrolytes containing 0.2 wt. %, 0.5 wt. % and 1.0 wt. % TFTPN were prepared by adding TFTPN into the baseline electrolyte in a highly pure argon-filled glove box (MBraun, Germany). The ionic conductivity of the prepared electrolytes was measured on 865 conductivity module of Metrohm (Netherlands). The contact angles of the prepared electrolytes on PE membrane (Entek) were observed on the Contact Angle Measurement Instrument (JC2000D3P, POWEREACH, China). The self-extinguishing time was measured by the time from igniting a nickel foam soaking up electrolyte to the flame extinguishing. 2025-type LiCoO2/Li and graphite/Li half cells and 454261-type (1520 mAh, based on LiCoO2) pouch cells were fabricated in the glove box, with PE membrane as separator. 2.2. Electrochemical measurements Charge/discharge tests for half cells were carried out on Land test system (Wuhan, China). LiCoO2/Li half cells were charged/ discharged with 0.2C (1C = 170 mAh g
1) in the voltage range of 2.75-4.40 V (vs. Li/Li+) and graphite/Li half cells were charged/ discharged with 0.5C (1C = 372 mAh g
1) in the voltage range of 0.01-2.5 V (vs. Li/Li+). Electrochemical impedance spectroscopy was performed on Autolab PGSTAT302 (Netherlands) with frequencies ranging from 100 kHz to 0.01 Hz with a potential amplitude of 5 mV at discharge state for LiCoO2/Li half cells before and after three cycles at 0.1C in the voltage range of 2.75-4.40 V (vs. Li/Li+). Cyclic voltammetry was performed on a CHI test system with a scan rate of 0.2 mV s
1 in the voltage range of 2.0-5.0 V (vs. Li/Li+) for LiCoO2/Li half cells and 0.01-3.0 V (vs. Li/Li+) for graphite/ Li half cells. The cycling performance of the LiCoO2/graphite pouch cells were evaluated on an Arbin Instrument with a constant charge/discharge current of 0.5C (1C = 1520 mAh) over the voltage range of 3.0-4.4 V at room temperature (25  C). 2.3. Physical characterizations The cycled pouch cells were disassembled in the glove-box mentioned above, and the cycled LiCoO2 and graphite electrodes were rinsed with dimethyl carbonate (DMC) for three times to remove residual electrolyte, followed by vacuum drying overnight at room temperature. The crystal structure of cycled cathode was characterized by X-ray diffraction diffractometer (SHIMADZU XRD-6100) with Cu Ka radiation in the 2u range of 10–80 at an interval of 0.02 and a scanning rate of 2 min
1. The XRD patterns were refined by using MDI Jade 6 software. Surface morphology of the cycled electrodes was observed by scanning electron microscopy (SEM, ZEISS Ultra 55) and transmission electron microscopy (TEM JEM-2100HR). For TEM analysis, the powders from cycled electrodes were dispersed ultrasonically in DMC for 30 min, cast on a copper TEM grid and dried under an infrared lamp. X-ray photoelectron spectroscopy was performed on Axis Ultra DLD system (England) using Al Ka radiation (hn=1486.6 eV) under ultrahigh vacuum. The obtained spectra were fitted by XPSPeak 4.1, with graphite peak at 284.3 eV as a reference for the final adjustment of the energy scales.

2.4. Theoretical calculations Gaussian 09 package was used to calculate the energy of solvents and TFTPN before and after accepting or losing electron. The density functional theory (DFT) with B3LYP method together with 6-311+G(d) level basis set (C, H, N, F) was used to optimize the molecular structures. Polarized continuum model (PCM) was applied to investigate the effect of bulk solvent (dielectric constant is 20.5).

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3. Results and Discussion 3.1. Effect of TFTPN on electrochemical performances of graphite and LiCoO2 The effect of TFTPN on the electrochemical performance of graphite anode was evaluated on graphite/Li half cells using baseline and TFTPN-containing electrolytes. The obtained results are presented in Fig. 1. From the voltammogram at the first cycle shown in Fig. 1a, the currents for the reduction of electrolyte and the intercalation of lithium ion can be observed for the cell using baseline electrolyte, represented by the peaks at 1.52, 1.04 and 0.7 V (vs. Li/Li+), and the quickly increasing current at the potential close to 0 V (vs. Li/Li+) [30,49]. The electrolyte decomposition results in a solid electrolyte interphase film that can protect the layered structure of graphite from exfoliation to some extent. With this interphase film, the lithium ion can be extracted reversibly, as indicated by the oxidation current of voltammogram in the first cycle (Fig. 1a) and the subsequent cycle (Fig. 1b). However, the interphase film formed from the electrolyte decomposition is unstable and cannot provide graphite anode with good cyclic stability. This is why an interphase film formation additive is needed for graphite-based LIBs. For the formation of effectively protective interphase film on graphite, the electrolyte additive should be reduced preferentially compared to the baseline electrolyte [1,30]. Comparatively, two additional reduction peaks at 1.97 and 1.75 V (vs. Li/Li+) appear for the cell when TFTPN is applied (Fig. 1a), indicating that TFTPN can be reduced at the potential higher than that for the reduction of the baseline electrolyte. In addition, larger oxidation current is recorded for the cell using TFTPN-containing electrolyte than that using baseline electrolyte, suggesting that a more protective interphase film has been formed from TFTPN than baseline electrolyte. This feature provides graphite anode with

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improved cyclic stability, as shown in Fig. 1c, which was obtained at 0.1C for initial two cycles and 0.5C for the subsequent cycles. It can be found from Fig. 1c that the cell using baseline electrolyte suffers a fast capacity decay: from 285 mAh g
1 (initial capacity at 0.5C) to 183 mAh g
1, with a capacity retention of 64% after 120 cycles. Such a low capacity retention should be related to the unstable interphase film formed from the reduction decomposition of the baseline electrolyte, which cannot protect graphite effectively and leaves the electrolyte decomposition proceeding in the subsequent cycling. Due to the obvious electrolyte decomposition, the graphite electrode has a low coulombic efficiency of 65% at the first cycle. When TFTPN is applied, the initial coulombic efficiency is increased to 83%, 89% and 77% for the graphite electrodes in the electrolytes containing 0.2 wt. %, 0.5 wt. % and 1.0 wt. % TFTPN, respectively. Apparently, the preferential reduction of TFTPN yields a protective interphase film that inhibits the electrolyte decomposition. Consequently, the cyclic stability of graphite electrode is improved, with the capacity retention of 78%, 89% and 90% after 120 cycles for the electrolytes containing 0.2 wt. %, 0.5 wt. % and 1.0 wt. % TFTPN, respectively. The electrolyte decomposition can be indicated clearly by the long voltage plateaus at about 0.5 V (vs. Li/ Li+) for the first discharge curve of the graphite electrode in the baseline electrolyte, which disappears for the electrode in the TFTPN-containing electrolyte, as shown in Fig. 1d. It should be noted that, as the TFTPN concentration increases, the coulombic efficiency is increased and then reduced. This phenomenon can be explained by the increased amount of the reduced TFTPN when its concentration is increased. The reduction of more TFTPN might cause thicker interphase film that increases the interfacial impedance. In fact, the initial capacity of graphite electrode decreases although the capacity retention increases with increasing the TFTPN concentration, as shown in Fig. 1c. Comparatively, the graphite electrode with 0.5 wt. % TFTPN delivers a high

Fig. 1. Cyclic voltammograms of graphite electrodes in the electrolytes with and without 0.5 wt. % TFTPN at the first cycle (a) and the second cycle (b) at a scan rate of 0.2 mV s
1; cyclic stability of the graphite electrodes in the electrolytes containing various TFTPN concentrations at 0.5C for the initial two cycles and at 0.1C for the subsequent cycles between 0.01 and 2.5 V (vs. Li/Li+) (c); and initial charge-discharge curves of graphite electrodes in the electrolytes with and without TFTPN (d).

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discharge capacity with a good capacity retention. Therefore, 0.5 wt. % TFTPN was selected for further consideration. Fig. 2 presents the effect of TFTPN on the electrochemical performance of LiCoO2 electrode, evaluated in LiCoO2/Li half cells. In the baseline electrolyte (Fig. 2a), the initial oxidation current appears at 3.88 V (vs. Li/Li+). This oxidation current is attributed to the extraction of lithium ion from LiCoO2, which is accompanied by the oxidation of Co2+ to Co3+ and Co4+, corresponding to the peak voltages at 4.2 and 4.6 V (vs. Li/Li+), respectively [13]. This voltammetric behavior can also be observed for the LiCoO2 electrode in the TFTPN-containing electrolyte, as shown in Fig. 2a. Differently, the initial oxidation current appears at 3.85 V (vs. Li/Li+) for the electrode in the TFTPN-containing electrolyte, lower than that in the baseline electrolyte (3.88 V), suggesting that the oxidation of TFTPN takes place on LiCoO2 before lithium extraction. This preferential oxidation of TFTPN might help build a protective interphase film on LiCoO2, which inhibits the oxidation decomposition of the baseline electrolyte [37]. The improved cyclic stability of LiCoO2 electrode by using TFTPN, as shown in Fig. 2b, confirms the contribution of TFTPN. When it is cycled at 0.2C between 2.75 and 4.4 V (vs. Li/Li+) in the baseline electrolyte, LiCoO2 electrode delivers an initial capacity of 135 mAh g
1 but retains a capacity of only 65 mAh g
1 with a capacity retention of only 48% after 30 cycles. Apparently, LiCoO2 exhibits poor cyclic stability under 4.4 V (vs. Li/Li+). This drawback comes from the electrolyte decomposition and the subsequent destruction of LiCoO2. The initial coulombic efficiency of LiCoO2 electrode in baseline electrolyte is only 90%, indicative of the electrolyte decomposition. When TFTPN is applied, however, LiCoO2 delivers an initial capacity of 168 mAh g
1 with a capacity retention of 88% after 30 cycles. Additionally, the initial coulombic efficiency of the LiCoO2 electrode is improved to 92%. It should be noted from Fig. 2b that LiCoO2 delivers a lower initial capacity in the baseline electrolyte than in the TFTPN-

containing electrolyte. This difference can be explained by the increased interfacial impedance due to the oxidation decomposition of the baseline electrolyte under charging at a high voltage (4.4 V). The increased interfacial impedance can be indicated by the pressed semicircles in Fig. 2c and d. As shown in Fig. 2c, both fresh electrodes show large interfacial impedance, suggesting that the electrode has poor wettability with electrolytes. After three cycles, the interfacial impedance is reduced to a great extent, but the electrode in the baseline electrolyte shows its interfacial impedance far larger than that in TFTPN-containing electrolyte (Fig. 2d). Under high voltage, the oxidation decomposition of the baseline electrolyte takes place seriously, which increases the interfacial impedance and reduces the capacity delivery of LiCoO2. This effect can be mitigated by applying TFTPN. Interestingly, the initial impedance in the baseline electrolyte is slightly larger than in the TFTPN-containing electrolyte (Fig. 2c), suggesting that the baseline electrolyte has poor wettability with electrode materials than the TFTPN-containing electrolyte. The improved wettability of electrode materials can be confirmed by contact angle experiments. As shown in Fig. S1, the contact angle becomes smaller when TFTPN is added in the baseline electrolyte. Since TFTPN contains fluorine, it should possess flame retardance ability, which can be confirmed by the decreased self-extinguishing time when TFTPN is used, as shown in Table S1 and Fig. S2. Like other electrolyte additives, TFTPN will reduce the ionic conductivity of the baseline electrolyte, as shown in Table S2 and Fig. S3. Fortunately, the ionic conductivity is reduced insignificantly when TFTPN is used in a concentration of as low as 0.5% (Fig. 3S). These features make it suitable for TFTPN as an electrolyte additive to be used in practice. 3.2. Effect of TFTPN on cyclic stability of LiCoO2/graphite full cell With the contribution of TFTPN to the simultaneously improved cyclic stability of graphite anode and LiCoO2 cathodes, the cyclic

Fig. 2. Voltammograms of LiCoO2 electrodes in the electrolytes with and without 0.5 wt. % TFTPN at the first cycle with scan rate of 0.2 mV s
1 (a) and corresponding cyclic stability at 0.1C for the initial three cycles and at 0.2C for the subsequent cycles between 2.75 V and 4.40 V (vs. Li/Li+) (b); electrochemical impedance spectra of LiCoO2 electrode in baseline and 0.5% TFTPN-containing electrolyte before (c) and after (d) three cycles at 0.1C between 2.75-4.4 V.

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Fig. 3. Cyclic stability of LiCoO2/graphite full cells using the electrolytes with and without 0.5 wt. % TFTPN at 0.5C in the voltage range of 3.0-4.4 V.

stability of LiCoO2/graphite full cell should be also improved by applying TFTPN. Fig. 3 presents the cyclic stability of LiCoO2/ graphite full cells using baseline and 0.5 wt. % TFTPN-containing electrolytes at 0.5C in the voltage range of 3.0-4.4 V. It can be seen from Fig. 3 that the cell using baseline electrolyte suffers a distinct capacity decaying: from an initial discharge capacity of 1584 mAh to 1258 mAh after 300 cycles, with a capacity retention of only 79%. When 0.5% TFTPN was added into the baseline electrolyte, the cyclic stability of the cell is significantly improved: the initial discharge capacity is 1575 mAh and retains 1429 mAh with a capacity retention of 91% after 300 cycles. To confirm further the contribution of TFTPN, the LiCoO2 and graphite electrodes taken from the cycled full cells were performed with characterizations of SEM, TEM, XRD and XPS. Fig. 4 shows the SEM and TEM images of the LiCoO2 electrodes after 300 cycles in LiCoO2/graphite full cells using the electrolytes with and without TFTPN. As shown in Fig. 4a, the layered structure of LiCoO2 collapses (as indicated by the red arrows) for the electrode cycled in the baseline electrolyte, but maintains integrity for the electrode cycled in the TFTPN-containing electrolyte. These observations

Fig. 5. XRD patterns of fresh and the cycled LiCoO2 for 300 cycles in the voltage range of 3.0-4.4 V in the electrolytes without and with 0.5 wt. % TFTPN.

clearly indicate that LiCoO2 suffers structural destruction when it is performed with cycling under high voltage and this destruction can be avoided by applying TFTPN as an electrolyte additive. Under high voltage, electrolyte decomposes oxidatively on LiCoO2 seriously, yielding HF and polymers [14,50]. The resulting HF attacks LiCoO2, leading to its structural destruction. The polymers deposit on the electrodes, as indicated by the arrows in Fig. 4c, which cannot yield any protection for LiCoO2. Differently, as shown in Fig. 4d, a protective interphase film is formed on LiCoO2 due to the preferential oxidation of TFTPN, which protects LiCoO2 from destruction and suppresses the subsequent decomposition of the baseline electrolyte. XRD was used to identify the crystal structure of the LiCoO2 cycled in the baseline and TFTPN-containing electrolytes with a comparison of the pristine LiCoO2. The obtained results are presented in Fig. 5. Compared with the pristine sample, the LiCoO2 cycled in the baseline electrolyte shows a significant change of XRD

Fig. 4. SEM and TEM images of LiCoO2 electrodes after 300 cycles in LiCoO2/graphite full cells using the electrolytes without (a and c) and with 0.5 wt. % TFTPN (b and d).

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pattern. The peak intensity of (003) diffraction becomes much stronger, while other diffraction peaks become weaker and even disappear. In addition, new diffraction peaks appear at (009), (021), (116) and (202). These differences confirm that the LiCoO2 suffers structural destruction after cycling in the baseline electrolyte under 4.4 V. Differently, the LiCoO2 cycled in TFTPN-containing electrolyte shows its diffraction patterns similar to those of the pristine sample, confirming that the structure integrity of LiCoO2 is maintained due to the application of TFTPN. XPS was conducted to analyze the surface compositions of LiCoO2 electrodes after 300 cycles in the electrolytes with and without TFTPN. As shown in Fig. 6, in the C 1s spectrum, the peak at 284.3 eV is ascribed to the conductive carbon [51], the peaks at 285.7 eV and 290.8 eV correspond to the C-H and C-F of PVDF, respectively [52], and the peaks at 286.5 eV, 288.6 eV and 289.9 eV are assigned to C-O, C¼O and OCO2 groups of ROCO2Li, ROLi and Li2CO3 species, respectively, which result from the electrolyte decomposition [15,53]. It should be noted that three new peaks at 284.6, 286.2 and 287.6 eV in C 1s spectrum, corresponding to C 1s peaks of the aromatic ring C-C bonding, CRN and C-F species, appear on the electrode cycled in the TFTPN-containing electrolyte. Considering this difference together with the peaks at 398.8 and 400.2 eV in the N 1s spectrum and the peak at 687.8 eV in the F 1s spectrum, which do not exist on the electrode in the baseline electrolyte but appear on the electrode in TFTPN-containing electrolyte, we can conclude that the oxidation products of TFTPN are incorporated into the protective interphase phase film on LiCoO2 [53–55]. In the O 1s spectrum, the peaks at 533–534 eV (C-O), 532– 534 eV (C¼O) and 529.8 eV (M-O) are characteristic of lithium alkyl carbonates, polycarbonates and metal oxide, respectively [56]. The weaker intensity of these peaks for the electrode in TFTPNcontaining electrolyte than that in baseline electrolyte, suggests that less electrolyte decomposition products exist on the electrode and LiCoO2 is well protected by the interphase film when TFTPN is used. In the F 1s spectra, the peaks of 687.5 eV, 686.2 eV and 684.5 eV correspond to the PVDF, LixPOyFz and LiF, respectively [55], while in P 2p spectra, there two main peaks at 135.9 eV

(LixPFy) and 133.7 eV (LixPOyFz) [56]. The weaker peak intensity of PVDF for the electrode in baseline electrolyte than in TFTPNcontaining electrolyte suggests that PVDF has been covered by the electrolyte decomposition products. LixPOyFz and LixPFy might come from oxidation decomposition of electrolyte anion and TFTPN and thus no significant difference can be identified between two samples. The significantly weaker intensity of LiF for the electrode cycled in TFTPN-containing electrolyte is indicative of the contribution of TFTPN to the inhibition of the electrolyte decomposition. Fig. 7 presents the SEM and TEM images of the graphite electrodes after 300 cycles in LiCoO2/graphite full cells using the electrolytes with and without TFTPN. The pulverized flake graphite can be observed for the electrode cycled in the baseline electrolyte (Fig. 7a), but the typical flake graphite is retained for the electrode cycled in the TFTPN-containing electrolyte (Fig. 7b), confirming that graphite suffers serious structural destruction when it is cycled in the baseline electrolyte, but it can be well protected when TFTPN is applied. In the baseline electrolyte, the reduction decomposition of the electrolyte takes place on graphite anode freely as the cycling proceeds. This decomposition becomes more seriously when the transition metal ions from cathode deposit on the anode, which cannot yield a protective interphase film on graphite but causes the graphite exfoliation and pulverization (Fig. 7c). Differently, an interphase film of about 10 nm can be clearly identified on the graphite for the electrode cycled in the TFTPN-containing electrolyte (Fig. 7d), confirming that TFTPN provides not only LiCoO2 cathode but also graphite anode with effective protection. The existence of interphase film formed from TFTPN on graphite anode and the protection of LiCoO2 by TFTPN can be confirmed by XPS analyses. Fig. 8 presents the XPS patterns of the graphite electrodes after 300 cycles in the electrolytes with and without TFTPN at the voltage range of 3.0-4.4 V. In the C 1s spectrum, the peak at 284.3 eV is assigned to the graphite [51]. The peaks at 286.2 eV and 287.7 eV are ascribed to the C-O bond in CMC binder, ethers and carbonates, while the peak at 289.5 eV corresponds to the C¼O bond in Li2CO3/ROCO2Li and polycarbonates [57,58]. The

Fig. 6. XPS patterns of the cycled LiCoO2 for 300 cycles in the voltage range of 3.0-4.4 V in the electrolytes with and without 0.5 wt. % TFTPN.

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Fig. 7. SEM and TEM images of graphite electrodes after 300 cycles in LiCoO2/graphite full cells using the electrolytes without (a and c) and with 0.5 wt. % TFTPN (b and d).

Fig. 8. XPS patterns of the cycled graphite for 300 cycles in the voltage range of 3.0-4.4 V in the electrolyte without and with 0.5 wt. % TFTPN.

peaks for C-O and C¼O containing species can also be observed at 533–534 eV and 532–533 eV, respectively, in the O 1s spectrum. Comparatively, the intensity of C¼O bond is much stronger for the electrode cycled in the TFTPN-containing electrolyte than in the baseline electrolyte, and three additional C peaks at 284.6 eV, 286.2 eV and 287.6 eV in C 1s spectrum are only observed in the electrode cycled in the TFTPN-containing electrolyte. This difference can be ascribed to the TFTPN, whose reduction products have been incorporated into the interphase film [59]. The stronger F peaks in F 1s spectrum for the electrode cycled in the TFTPNcontaining electrolyte than in the baseline electrolyte, together with the N peak at 398.8-400.6 eV in N 1s spectrum only observed

on the electrode cycled in TFTPN-containing electrolyte, confirm that TFTPN is involved in the formation of interphase film on graphite anode [53,55]. Co 2p spectrum, which is split into two parts (2p3/2 and 2p1/2) due to spin-orbit coupling, is clearly observed on the electrode cycled in the baseline electrolyte but is absent for the electrode cycled in the TFTPN-containing electrolyte. This observation confirms that cobalt has been dissolved from the LiCoO2 cathode and deposited on graphite anode in the cell using the baseline electrolyte and that TFTPN has successfully yielded a protection for LiCoO2 cathode. The transition metal deposited on anode can catalyze the decomposition of electrolyte on the anode [41]. With the contribution of TFTPN that

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simultaneously forms protective interphase films on anode and cathode, LiCoO2/graphite full cell exhibits excellent cyclic stability under high voltage. The effect of TFTPN on the cyclic stability of LiCoO2/graphite full cells can be illustrated in Fig. S4. In the TFTPN-containing electrolyte, TFTPN is oxidized on LiCoO2 cathode and reduced on graphite anode preferentially compared to the baseline electrolyte, forming protective cathode and anode interphase films, respectively. The preferential reduction or oxidation of TFTPN can be confirmed by its low energy after accepting electron or low oxidation potential after losing electron. Fig. S5 presents the electronic affinity (EA) obtained by theoretical calculations, together with the calculated bond lengths. It can be seen from Fig. S5 that the EA of TFTPN is much lower than those of EC, EMC and DEC, whether it is under effect of Li+ or not, indicating that TFTPN is more easily reduced than solvents. The C-F bond becomes longer when TFTPN combines one electron, suggesting that this bond will be broken preferentially when TFTPN is reduced. Fig. S6 presents the calculated oxidation potentials of the solvents and TFTPN. It can be seen from Fig. S6 that TFTPN has lower oxidation potential than solvents, whether it is under effect of anion PF6
or not. With these results, the reduction and oxidation decomposition reactions of TFTPN can be inferred, as shown in Fig. S7. On graphite anode, the resulting radical forms polymer that is the main composition of the anode interphase film, while on the LiCoO2 cathode, the resulting cation combines anion, forming salt that is the main composition of the cathode interphase film. 4. Conclusion TFTPN as an electrolyte additive can significantly improve the cyclic stability of LiCoO2/graphite full cell when the cell is performed with cycling under 4.4 V. This improvement is attributed to the contribution of TFTPN that can build simultaneously protective interphase films on anode and cathode. On the one hand, TFTPN is preferentially oxidized on LiCoO2 cathode compared to the baseline electrolyte, forming a protective cathode interphase film that suppresses the oxidation decomposition of the electrolyte and protects LiCoO2 from structural collapse. On the other hand, TFTPN is preferentially reduced on graphite anode compared to the baseline electrolyte, forming a protective anode interphase film that suppresses the reduction decomposition of the electrolyte and protects graphite anode from structural collapse. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 21573080), the Natural Science Foundation of Guangdong Province (Grant No. 2014A030313424), the key project of Science and Technology in Guangdong Province (Grant Nos. 2016B010114001 and 2017A010106006), and Guangzhou City Project for Cooperation among Industries, Universities and Institutes (Grant No. 201604016011). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.electacta.2017.10.059. References [1] K. Xu, Nonaqueous liquid electrolytes for lithium-based rechargeable batteries, Chem. Rev. 104 (2004) 4303.

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