LiAlCl4·3SO2 as a high conductive, non-flammable and inorganic non-aqueous liquid electrolyte for lithium ion batteries

Electrochimica Acta 286 (2018) 77e85

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LiAlCl4$3SO2 as a high conductive, non-flammable and inorganic nonaqueous liquid electrolyte for lithium ion batteries Tiantian Gao a, Bo Wang a, b, **, Lei Wang a, Guijing Liu a, Fei Wang a, Hao Luo a, Dianlong Wang a, * a

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, 150001, Harbin, China School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, China

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 April 2018 Received in revised form 4 July 2018 Accepted 8 August 2018 Available online 11 August 2018

Safety problems of lithium ion batteries (LIBs), such as fire and explosion caused by flammable commercial organic electrolyte (OE) during abuse-testing conditions, hinder the further application of LIBs. To address these issues, an inorganic non-aqueous liquid electrolyte-LiAlCl4$3SO2 (IE) was synthesized and investigated for LIBs. This study demonstrated that the IE possessed preeminent comprehensive properties, such as non-flammable property, sufficient ionic conductivity (23.77 mS cm
1 at room temperature) and high lithium ion transport number (tLiþ ¼0.47). Therefore, compared with OE, LiFePO4 (LFP) cathode in IE exhibited outstanding rate capacity and stable cycle performance at room temperature. At room temperature the discharge capacity of LFP in IE could still achieved ~80 mAh$g
1 at 10 C, while the reversible capacities of LFP in OE is almost 0 mAh$g
1. In addition, at 0  C the reversible capacities of LFP in IE is much higher than the LFP in OE, and graphite electrode with IE also shows better rate capabilities than graphite electrode with OE. All these results demonstrate that the presented IE could be served as a good electrolyte candidate for LIBs. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Lithium ion batteries Non-aqueous liquid electrolyte Non-flammable High ionic conductivity Low temperature

1. Introduction Currently, lithium ion batteries (LIBs) have been considered as an attractive power source for a wide variety of applications, such as electronic devices, electric vehicles and energy storage systems, due to their high energy density, environment friendliness and long cycle life [1]. However, conventional carbonate-based electrolyte are highly volatile and flammable [2,3]. Once the cell is ignited caused by internal/external short, the combustible electrolytes readily participate in additional combustion reactions, because they can act as a fuel [3]. It implies that the flame retardancy of conventional electrolytes should be improved for the further applications of LIBs. Various approaches have been investigated to improve the safety of electrolyte for LIBs: (i) Add flame retardant to commercial organic electrolyte (OE); (ii) Use ionic liquids (ILs) as

* Corresponding author. ** Corresponding author. MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, 150001 Harbin, China. E-mail addresses: [email protected] (B. Wang), wangdianlonghit@ 163.com (D. Wang). https://doi.org/10.1016/j.electacta.2018.08.033 0013-4686/© 2018 Elsevier Ltd. All rights reserved.

the solvent of the electrolyte; (iii) Application of solid-state electrolytes. Introduction of flame retardant into the electrolyte is an economic and efficient strategy to improve the safety characteristics of LIBs in the battery industry. Research based on flame retardant has focused on organic phosphorus compounds, such as trimethyl phosphate, triphenyl phosphate, dimethyl methylphosphonate and triethyl phosphate, to improve the thermal stability of electrolytes [4]. However, these additives have some disadvantages, such as environmental pollution, toxicity and poor electrochemical compatibility with electrode materials [2,5,6]. In addition, ILs have also been investigated as one promising alternative electrolyte to replace OE, because the strong ionic bonding nature of ILs allows them to be nonflammable as well as nonvolatile. In addition, wide electrochemical windows of the ILs seem to be well compatible with high-voltage electrode materials, which should be useful for high energy density application of the cells [7,8]. However, the high viscosity of ILs, resulting in low ionic conductivity at low or at room temperature, is a fatal drawback which hinders their commercial application [3,9]. Solid-state electrolytes can also overcome the safety problem of LIBs, because they do not contain flammable liquid chemicals [10,11]. However, solid-state electrolytes usually

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have low ionic conductivity [11], and the electrolyte/electrolyte interface in solid-state electrolyte is unstable during the process of charging/discharging [12,13]. In order to fundamentally prevent the combustion of the electrolyte without fading of the electrochemical performance of LIBs, a non-flammable, inorganic non-aqueous liquid electrolyteLiAlCl4,3SO2 has been studied by our group. SO2 complexesLiAlCl4,3SO2 (IE) can be formed by the reaction between LiCl and AlCl3 in SO2 environment [14]. The IE exhibits various promising performances, including high ionic conductivity (close to 0.1 S cm
1 at room temperature) [15,16], non-flammability [16], high concentration of lithium cation [17] and unusually high transference number of the lithium cation [17]. Kinds of batteries based on IE have been studied, such as lithium-SO2 rechargeable batteries [16,18e20], lithium-CuCl2 rechargeable batteries [21] and lithium ion batteries used LiCoO2 as cathode material [22,23]. The excellent electrochemical performance of these batteries depends mainly on the promising properties of IE. However, as far as we know, the electrochemical performance of LiFePO4 (LFP) in IE has not been reported before. Being a candidate for lithium ion batteries cathode material, olivine structured LFP has various advantages, such as high theoretical capacity (170 mAh g
1), high electrochemical potential, good thermal stability, low cost, environmental friendliness, and nontoxicity. LFP is considered as an ideal cathode material for energy storage devices with both high energy and power densities [24]. Therefore, in this paper, electrochemical performance of LFP with the IE was systematically studied by comparing with the performance of OE. IE owns higher ionic conductivity and higher lithium ion transport number, which reduces the internal resistance of the battery and eliminates the concentration gradients within the electrode at a high current density, and thus, enhances the specific capacity and improves the rate performance of the LFP cathode even at low temperature. The compatibility of graphite anode with IE and the solid electrolyte interface (SEI) film formed in IE on the surface of graphite anode have also been studied.

frequency range from 0.01 Hz to 1 MHz using CHI 660E electrochemical workstation (Shanghai Chenhua, China). The test system was “stainless steel (SS) jelectrolytejstainless steel (SS)” blocking type experimental cell. The experimental cells with OE and IE were recorded as “SSjOEjSS” and “SSjIEjSS”, respectively. The ionic conductivity was calculated from Eq. (1)

s ¼ d=ðRb $SÞ

(1)

Where d is the thickness of the separator (thickness of celgard2500 (polypropylene) is 25 mm, and thickness of glass fiber separator is 680 mm), S is the contact area between electrolyte and stainless steel disk. The resistance Rb of the bulk OE and IE at different temperature was obtained by electrochemical impedance spectroscopy and was retrieved from the intercept of the straight line on the Z0 axis.

2.2.2. Lithium ion transference numbers The lithium ion transference numbers (tLiþ ) was obtained by chronoamperometry using CHI 660E electrochemical working station. The test system was “lithium metal/electrolyte/lithium metal” blocking type experimental cell and the step potential was 10 mV. Electrochemical impedance spectra (EIS) were measured 10 mHz~106 Hz with a perturbation amplitude of 5 mV. The tLiþ was calculated from Eq. (2)

tLiþ ¼

Is ðV
I0 R0 Þ I0 ðV
Is Rs Þ

(2)

Where I0 and Is are the initial and steady-state current, respectively. V presents the applied voltage, R0 is the initial resistance, and Rs is the steady-state resistance.

2.3. Electrochemical measurement 2. Experimental 2.1. Preparation of IE and the details of OE The IE was prepared as follows: LiCl (99.99%, aladdin) was vacuums-dried at 120  C for 24 h, while anhydrous AlCl3 (99.999%, Alfa Aesar) was used without any purification. At room temperature, SO2 gas (99.99%, Tongda Gas Distribution Co., Ltd, Harbin, China) was passed into the mixture of LiCl and AlCl3 in a homemade glass vessel. The molar ration of LiCl to AlCl3 was 1.1:1 to avoid the presence of free AlCl3, which is known to be corrosive against alkali metals, such as lithium metal. As soon as SO2 gas was in contact with the mixture, it became a liquid of transparent light yellow color. The SO2 gas was blown until the reaction was completed, then the home-made glass vessel with IE was transferred into the Ar filled glove box, avoiding the contact with air. The excess LiCl was removed by filtration to obtain pure IE. The concentration of SO2 was determined by weighting the electrolyte vessel before and after blowing the SO2 gas. OE is the conventional commercial electrolyte (Shanshan Battery Materials Co., Ltd., Dongguan, China) with typical composition, that is 1.0 M LiPF6 in propylene carbonate (PC)/ethylene carbonate (EC)/diethyl carbonate (DEC) solvent (1: 1: 1 in volume). 2.2. Characterization of IE 2.2.1. Ionic conductivity Ionic conductivity at different temperature (253, 263, 273, 283, 293, 303 and 313 K) was tested by an AC impedance analysis over a

2025-size coin cell was manufactured by the conventional method for electrochemical measurement. The LFP electrodes were prepared by a doctor-blading and the mass ratio of LFP: carbon black: PVDF was 8: 1: 1. The commercial graphite electrodes were provided by ZhuHai COSLIGHT Battery Co., Ltd., China. The cell composed of LFP cathode, OE and lithium metal anode is recorded as LijOEjLFP cell. Similarly, LijIEjLFP cell, LijOEjgraphite cell and LijIEjgraphite cell were also manufactured for electrochemical measurement. Specially, the separator matched with OE is PP/PE/PP (celgard 2500), and the separator matched with IE is glass microfiber filters (GF/D, Whatman). Galvanostatic charge-discharge were tested on Neware Battery Test System (Shenzhen Neware, China). Galvanostatic chargedischarge cycles of LijOEjgraphite cell and LijIEjgraphite cell were tested between 0.001 V and 2 V at various rates. Galvanostatic charge-discharge cycles of LijOEjLFP cell and LijIEjLFP cell were tested between 3.2 V and 3.8 V at various rates. In particular, during the charging process, there is a constant-voltage charging process for 5 min after each galvanostatic charging process. Cyclic voltammograms (CV) curves for LijOEjLFP cell and LijIEjLFP cell were recorded on a CHI660E electrochemical workstation (Shanghai Chenhua, China) in a potential range of 3.0e3.8 V (vs. Liþ/Li) at a scan rate of 0.1 mV/s. The EIS measurement of LijOEjLFP cell and LijIEjLFP cell were performed over a frequency range of 100 KHz to 10 mHz with an applied amplitude of 5 mV on an electrochemical workstation (CHI 660E). The parameters of the equivalent circuit were calculated and analyzed by computer simulations using the ZView software.

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2.4. Surface characterizations Graphite electrodes were removed from cycled LijIEjgraphite cell in an Ar filled glove box. All graphite electrodes were washed by SOCl2 for three times to remove the residual electrolyte. The graphite electrodes were dried until the solvent volatilized, and all the processes were taken in the Ar-filled glove box. The micromorphologies of the cycled graphite electrodes were characterized by a scanning electron microscopy (SEM; Hitachi SU8000, Japan). Surface chemistry of the cycled graphite electrode was probed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, America). During transferring process before characterizations, all samples were protected in sealing container with Ar to avoid contact with air.

3. Results and discussion Fig. S1 shows the preparation process of IE. The LiCl and AlCl3 are solid powder at room temperature, however, the mixture of solids became a liquid of transparent light yellow color when they were reacted with SO2 gas. This indicates that the chemical environment of LiCl and AlCl3 changed and formed a new inorganic complex. Rüdiger Mews had pointed out that alkali metal chlorides can react with AlCl3 in SO2 environment with solvate formation according to [25]. SO2

MCl þ AlCl3 ƒƒƒƒƒ!Mþ ½AlCl4 
$nSO2 ;

M ¼ Li; Na; K; NH4

The flame retardancy of the electrolyte is important for the safety of LIBs. The combustibility of OE with celgard-2500 (polypropylene) separator and IE with glass fiber separator are shown in Fig. 1a and b, respectively. A polypropylene separator soaked with OE burned immediately after exposure to an open flame (Fig. 1a). However, it clearly can be seen that the glass fiber separator soaked with IE exhibited good flame retardancy: separator soaked with IE did not burn even after contacted with an open flame about few seconds (Fig. 1b). The corresponding video is also provided. This is mainly because, unlike OE, IE is mainly composed of inorganic non-

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flammable materials: the solvent is SO2, and the conductive salt is LiAlCl4. This non-flammable property of IE could significantly relieve the safety problems of LIBs. Supplementary video related to this article can be found at https://doi:10.1016/j.electacta.2018.08.033. Ionic conductivity is an important factor for liquid electrolyte. Electrolyte with a high ionic conductivity would endow the batteries with a better power performance at a high charging/discharging current density [26]. To evaluate the ion conductivity of OE and IE, the alternating current impedance was carried out on the SSjOEjSS and SSjIEjSS cell over the temperature ranging from 253 to 313 K, as shown in Fig. S2. Usually, a typical impedance plots consist of a high frequency semicircle followed by a sloping line in the low frequency, which correspond to the bulk/grain boundary and the electrode resistance, respectively [27,28]. In the present study, the plot shows the disappearance of the semicircular portion indicating that the current carriers are ions and hence the total conductivity is the result of ion conduction [27,28]. The resistance of the bulk OE and IE at different temperature has been retrieved from the intercept of the straight line on the Z’ axis and has been listed in Table S1. And the ionic conductivity of OE and IE has been calculated based on Eq. (1). Fig. 2a depicted the ionic conductivity of IE and OE over the temperature ranging from 253 K to 313 K. Obviously, the ionic conductivity of IE is much higher than that of OE throughout the whole testing temperature range. The obtained ionic conductivity of IE is 23.77 mS cm
1 at ambient temperature (293 K), which is about 39 times as that of OE (0.607 mS cm
1). These data presents that the motive rate of lithium ion in the IE is higher than those in OE. The high ion conductivity of IE may be due to the high concentration of lithium cations in the IE. tLiþ is also an important factor for liquid electrolyte, because a large tLiþ would reduce electrode polarization and suppress undesirable side reactions on the electrodes [26]. Therefore it is necessary to increase tLiþ of electrolyte to gain favorable performance for LIBs. tLiþ was calculated using the Eq. (2). As shown in Fig. 2b, in Eq. (2), Is and I0 are calculated as 0.123 and 0.143 mA, while R0 and Rs are calculated as 61.14 and 60.52 U (insert in Fig. 2b). By using these data, Eq. (2) gives a tLiþ of 0.42. According to the previous literature, tLiþ of the

Fig. 1. Digital photos for flammability tests of (a) OE and (b) IE. The electrolyte-socked separators were forced to be contact with an open flame.

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Fig. 2. (a) Conductivity of IE and OE at various temperatures. (b) Polarization curves obtained by chronoamperometry with a step potential of 10 mV for the “lithium metal/ electrolyte/lithium metal” symmetrical cell at room temperature. (Inset: Nyquist plots of the symmetrical cell before and after polarization).

commercial liquid electrolyte used in LIBs (LiPF6/EC/DMC) is only 0.27 [27,29]. The results indicate that IE possible contribute to a considerable rate performance of LIBs. The electrochemical performance of the IE and OE was evaluated by using LFP as the cathode and lithium metal as the counter and reference electrode. From the CV (Fig. 3a), it can be seen that for the LFP cathode in IE, the oxidation and reduction peaks appear at around 3.53 and 3.34 V (vs. Liþ/Li), respectively. The potential interval between two peaks is 0.19 V. However, in the case of the LFP cathode in OE, the oxidation and reduction peaks appear at around 3.57 and 3.31 V (vs. Liþ/Li), respectively. The potential difference of oxidation and reduction peaks is 0.26 V, larger than that of LFP in IE. The sharper peak shape, smaller peak potential separation and larger peak currents all indicate that the higher electrochemical

reactivity and lower ohmic resistance of the LFP cathode can be achieved in IE [30], suggesting that a better electrochemical performance of the LFP cathode can be achieved in IE. Fig. 3b exhibits the charge/discharge profiles of the LijIEjLFP cell and LijOEjLFP cell at rate of 0.5 C at room temperature. The discharge capacity obtained in LijIEjLFP was ~154 mAh g
1, which was a little higher than the discharge capacity obtained in the LijOEjLFP. More importantly, the polarization between the charge and discharge plateaus of LijIEjLFP cell (58 mV) was smaller than the polarization obtained in LijOEjLFP cell (85 mV). It was indicated that IE cell exhibited extremely low polarization and voltage hysteresis due to its large tLiþ value, which suppressed the polarization effectively. This result is consistent with the larger peak separation from CV. Fig. 3c described the rate capability of LijIEjLFP cell and LijOEjLFP cell at

Fig. 3. Electrochemical performance of the LijIEjLFP cell and LijOEjLFP cell. (a) CV curves at the scan rate of 0.1 mV/s; (b) Typical charge-discharge curves at 0.5 C between 3.2 and 3.8 V; (c) Rate capabilities for LijIEjLFP cell and LijOEjLFP at C rates of 0.5 C, 1 C, 5 C, 10 C and 0.5 C; (d) Cycle performance of LijIEjLFP at 5 C. All of the above electrochemical properties were tested at room temperature.

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room temperature. It can be clearly seen that the cell with IE possessed higher capacities and better rate performance than the cell with IE. In the case of LijIEjLFP cell, the reversible capacities of the LFP cathode are 153, 139 and 108 mAh g
1 at 0.5 C, 1 C and 5 C, respectively. At the rate of 10 C, it can still have a discharge capacity of ~80 mAh g
1. In terms of LijOEjLFP cell, the reversible capacities of the cathode are 152, 139 and 59 mAh g
1 at 0.5 C, 1 C and 5 C, respectively. The reversible capacity of LijOEjLFP cell is almost 0 mAh g
1 at the rate of 10 C, much smaller than that based on the LijIEjLFP cell. This better rate behavior for the LijIEjLFP cell is mainly because of the higher lithium ion transference numbers of IE. A higher tLiþ can eliminate the concentration gradients within the electrode and ensure the redox reactions to take place even at a high current density [31]. The cycle performance is crucially important for lithium ion batteries in practical application. Fig. 3d described the cycle performance of LijIEjLFP cell at 5 C at room temperature. After 100 cycles, the discharge capacity of LijIEjLFP cell was still 113 mAh g
1. There is no evident capacity fading during the 100 cycles and the capacity retention of LijIEjLFP cell is 93.7%. The corresponding columbic efficiency remains virtually constant at about 99% for 100 cycles, indicating the relative stable cycle performance of the LijIEjLFP cell. The EIS profiles of LijIEjLFP and LijOEjLFP cell after 5th and 10th cycles at 0.5 C are shown in Fig. 4a. And the EIS profiles were fitted by the equivalent circuit given in Fig. 4b, where RU (U), corresponding to the high-frequency intercepts with the real axis, is related to the total internal ohmic resistance from electrolyte, electrodes, separator and all the internal connections. RSEI (U), corresponding the high frequency region of the semicircle in the Nyquist plots, is ascribed to lithium ion migration through the SEI

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on the electrode surface. Rct is related to the charge transfer resistance (the medium frequency region of the semicircle in the Nyquist plots) [32,33]. According to the fitting values, as listed in Fig. 4c, these figures clearly show that the RSEI of LijIEjLFP cell is much lower than that observed in OE. After five charge-discharge cycles at the rate of 0.5 C, RSEI of LijIEjLFP cell is 27.69 U, while RSEI of LijOEjLFP cell is 499.3 U, implying that high electronic conductivity at the electrode/electrolyte interface can be obtained in IE. After 10th cycles, similar results were observed. The cell with IE exhibits evidently lower RSEI than that with OE (19.02 U for IE, while 569.1 U for OE). Consequently, it demonstrates that SEI film formed in the IE is more stable and conductive, rendering a high electronic conductivity at the electrode/electrolyte interface. In addition, a lower RU and Rct can also be obtained in the LijIEjLFP cell, as listed in Fig. 4c, implying that lower total internal ohmic resistance and charge transfer resistance can also be achieved in the LijIEjLFP cell. The lower RSEI, RU and Rct of LijIEjLFP cell lead to the small and stable over-potential during charge-discharge cycles, which is in accordance with the above-mentioned results. The charge-discharge performance at low temperature is also important for the practical application of LIBs. The rate and cycle performance of the LijIEjLFP cell and LijOEjLFP cell were tested at 0  C. The obtained results are presented in Fig. 5. As shown in Fig. 5a and b, the charge-discharge curves were tested at rates from 0.5 C to 5 C at temperature of 0  C. It can be obviously seen that the LijIEjLFP cell displays a higher specific capacity and more satisfactory charge-discharge potential plateau than LijOEjLFP cell at 0  C. The corresponding specific capacities of the LijOEjLFP cell reduce dramatically. Besides, the electrode polarization of the LijOEjLFP cell increases dramatically at the operation temperature of 0  C,

Fig. 4. (a) EIS of LijIEjLFP cell and LijOEjLFP cell at frequency ranging from 105 to 10
3 Hz amplitude of 10 mV after different cycles at 0.5 C. (b) The simplified equivalent circuit of LijIEjLFP cell and LijOEjLFP cell. (c) Simulated impedance parameters of LijLFP cells with different electrolyte.

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Fig. 5. Electrochemical performance of the LijIEjLFP cell and LijOEjLFP cell at 0  C. Charge-discharge curves at rates from 0.5 C to 5 C for (a) LijIEjLFP cell and (b) LijOEjLFP cell. (c) Rate capabilities at C rates of 0.5 C, 1 C, 3 C, 5 C and 0.5 C. (d) Cycle performance at rate of 2 C at 0  C.

which may be due to the sluggishness of lithium diffusion in OE with low temperature during charge and discharge process. The rate capabilities in Fig. 5c shows that the LijIEjLFP cell exhibits discharge capacities of 131, 121, 96 and 78 mAh g
1 at 0.5 C, 1 C, 3 C and 5 C at 0  C, while the capacities reduce to 83 and 50 mAh,g
1 for LijOEjLFP cell at 0.5 C and 1 C at 0  C, and the reversible capacity of LijOEjLFP cell is almost 0 mAh g
1 at the rate of 3 C and 5 C at 0  C, much smaller than that based on the LijIEjLFP cell. As shown in Fig. 5d, LijIEjLFP cell also presents stable cycling performance at 2 C at 0  C. The specific capacity of LijIEjLFP cell increases from 110.6 to 115 mAh g
1 after 100 cycles, maintaining 104% of the first cycle capacity at 0  C. The slight increment may be due to the activation of electrolyte and active materials after several charge-discharge

cycles. By contrast, the specific capacity of LijOEjLFP cell has dropped sharply at 2 C at 0  C. After 20th cycle, the cell has a specific capacity of only 9 mAh g
1; and after 100th cycle, the specific capacity of LijOEjLFP cell is almost 0. At low temperature the better charge-discharge performance for the LijIEjLFP cell is mainly due to the higher lithium ions conductivity of IE even at low temperature. In addition, a higher tLiþ can eliminate the sluggishness of lithium diffusion at low temperature during charge and discharge process and ensure the redox reactions to take place even at low temperature. The superior rate performance and cycling stability of LijIEjLFP cell at 0  C demonstrates that the higher lithium ions conductivity and tLiþ of IE can improve the electrochemical property of LFP material at low temperatures.

Fig. 6. Electrochemical performance of the LijIEjgraphite cell and LijOEjgraphite cell. (a) typical charge-discharge curves at 0.5 C between 0.001 and 2 V at room temperature; (b) rate capabilities for LijIEjgraphite cell and LijOEjgraphite cell at C rates of 0.2 C, 0.5 C, 1 C, 2 C and 3 C.

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Fig. 7. SEM images of the (a) fresh and (b) cycled graphite electrodes after two cycles at 0.2 C in IE. (c) ~ (f) Element mapping images for the graphite electrodes after two cycles at 0.2 C in IE.

The electrochemical performance of graphite electrode in IE and OE has also been studied. The obtained results are presented in Fig. 6. Fig. 6a exhibits the charge/discharge profiles of LijIEjgraphite cell and LijOEjgraphite cell at rate of 0.5 C at room temperature. Similar charge/discharge profiles were observed in IE and OE. And a little higher specific capacity and lower polarization between the charge and discharge plateaus can be obtained in LijIEjgraphite cell. The better electrochemical performance obtained in LijIEjgraphite cell attributed to the higher ionic conductivity and higher tLiþ of IE. Fig. 6 b shows the rate performance of the LijIEjgraphite cell and LijOEjgraphite cell. It can be seen that the cells with IE delivers

approximate 344 mAh g
1 at 0.2 C, 335 mAh g
1 at 0.5 C, and 305 mAh g
1 at 1 C, higher than the cell with OE (335 mAh g
1 at 0.2 C, 291 mAh g
1 at 0.5 C, and 185 mAh g
1 at 1 C). In addition, by using IE to improve the rate performance become more dominate when the discharge rate is increased. For example, at 2 C and 3 C, LijIEjgraphite cell deliver 250 and 208 mAh g
1, respectively; while only 63 and 20 mAh g
1 can be obtained in LijOEjgraphite cell. The enhanced rate performance should be ascribed to the improvement of electrochemical kinetics of the IE with higher tLiþ as we aforementioned. SEM images of the fresh and cycled graphite electrode (after two

Fig. 8. (a) Li 1s, (b) Cl 2p, (c) O 1s, (d) S 2p XPS spectra for graphite anode after 2 cycles at the rate of 0.2 C in IE.

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cycles at 0.2 C in IE) are shown in Fig. 7a and b, respectively. The smooth surface of the fresh graphite electrode can be observed clearly in Fig. 7a. After cycling, a thin and uniform film can be found on the surface of cycled graphite electrode (Fig. 7b), which can be attributed to the SEI film formation. In order to understand the interfacial stability of the graphite electrode in IE, EDS was applied. The results of element mapping analyses are given in Fig. 7cef. As can be seen clearly from Fig. 7cef, various elements (C, S, O and Cl) are evenly distributed on the surface of the graphite electrode, which further indicate that a uniform and dense SEI film is formed on the surface of the graphite electrode. For a closer investigation of the formation of the SEI film, the surface of cycled graphite anode was characterized by XPS to characterize the chemical structure of the SEI film on the cycled graphite anode. Fig. 8aed shows the Li 1s, Cl 2p, O 1s and S 2p XPS along with computer-fitted curves of the spectrum for the graphite anode surface after two charge-discharge cycles at the rate of 0.2 C in IE. The Li 1s spectrum (Fig. 8a) for graphite anode consisted of two peaks. The first one at 55.3 eV was associated with Li2CO3 [34,35]. The next one at 56.1 eV was attributed to LiCl [36]. The Cl 2p spectra (Fig. 8b) show the Cl 2p3/2-2p1/2 spin-orbit components (Cl 2p3/2 at 198.6 eV) with an energy separation of 1.7 eV, in good agreement with the values observed for elemental chlorine and some of its compounds [37e39]. The peak corresponding to CleC covalent bonds was close to 200 eV [40]. The O 1s spectrum (Fig. 8c) of the cycled graphite anode showed three peaks. Peak centered around 532 eV, which can be assigned to CeO bonds within Li2CO3 (graphite anode surface impurity). Peaks at 532.6 eV indicates the SeO bonds (sulfone and sulfite) [41]. While the O 1s peak was close to 533 eV corresponding to water oxygen [42,43]. Fig. 8d shows a high resolution S 2p XPS spectrum of the graphite anode surface. It appears that at least three different S oxidation states are present. According to reference data, the binding energy at 167.2 eV was assigned to S2O2
3 [44], while those at 168.4 eV and 169.2 eV were 2
assigned to SO2
3 and SO4 , respectively [45,46]. Based on the XPS results, the composition of the SEI films for graphite anode surface in IE may include LiCl, Li2S2O3, Li2SO4 and Li2SO3, except for graphite surface impurity (Li2CO3 and H2O) produced by the contacting with the air.

4. Conclusion In summary, an inorganic non-aqueous liquid electrolyteLiAlCl4,3SO2 (IE) was prepared for lithium ion batteries (LIBs) which LiFePO4 is used as cathode materials. Compared with the commercial organic electrolyte (OE), the IE displays a lot of outstanding characteristics. First of all, the IE shows non-flammable property, suggesting good safety performance for LIBs. Secondly, the ionic conductivity of the IE over the temperature ranging from 253 to 313 K is far higher than that of OE (ionic conductivity of IE is 23.77 mS,cm
1 at 293 K, while the conductivity of OE is only 0.607 mS,cm
1), and it also presents higher transference number of lithium ions (0.47). Based on these excellent physical and chemical properties, LIBs with IE exhibits better electrochemical performance. When IE was tested as electrolyte, a LiFePO4 cathode displays excellent rate performance and stable cyclic stability. Furthermore, when the LIBs with IE was tested at 0  C, a LiFePO4 cathode still displays superior electrochemical properties including high discharge capacity, excellent rate performance and stable cycling. A graphite electrode with IE also shows better rate capabilities than graphite electrode with OE. These results show that IE is attractive for LIBs requiring good electrochemical performance and high security.

Acknowledgement Financial support from the National Natural Science Foundation of China (No. l51604089), the Fundamental Research Funds for the Central Universities (No. HIT.NSRIF.2017024), the China Postdoctoral Science Foundation (No. 2016M601431 and 2018T110308), and the Heilongjiang Province Postdoctoral Science Foundation (No. LBH-Z16056 and LBH-TZ1707) are gratefully acknowledged. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.electacta.2018.08.033. References [1] B. Wang, T. Liu, A. Liu, G. Liu, L. Wang, T. Gao, D. Wang, X.S. Zhao, A hierarchical porous C@LiFePO4/Carbon nanotubes microsphere composite for high-rate lithium-ion batteries: combined experimental and theoretical study, Adv. Energy Mater. 6 (2016), 1600426. [2] R.J. Chen, Y.Y. Zhao, Y.J. Li, Y.S. Ye, Y.J. Li, F. Wu, S. Chen, Vinyltriethoxysilane as an electrolyte additive to improve the safety of lithium-ion batteries, J. Mater. Chem. A 5 (2017) 5142e5147. [3] H.-T. Kim, J. Kang, J. Mun, S.M. Oh, T. Yim, Y.G. Kim, Pyrrolinium-based ionic liquid as a flame retardant for binary electrolytes of lithium ion batteries, ACS Sustain. Chem. Eng. 4 (2015) 497e505. [4] A.M. Haregewoin, A.S. Wotango, B.-J. Hwang, Electrolyte additives for lithium ion battery electrodes: progress and perspectives, Energy Environ. Sci. 9 (2016) 1955e1988. [5] X. Zhu, X. Jiang, X. Ai, H. Yang, Y. Cao, Bis(2,2,2-Trifluoroethyl) ethylphosphonate as novel high-efficient flame retardant additive for safer lithiumion battery, Electrochim. Acta 165 (2015) 67e71. [6] Z. Zeng, X. Jiang, B. Wu, L. Xiao, X. Ai, H. Yang, Y. Cao, Bis(2,2,2-trifluoroethyl) methylphosphonate: an novel flame-retardant additive for safe lithium-ion battery, Electrochim. Acta 129 (2014) 300e304. [7] H.-T. Kim, O.M. Kwon, J. Mun, S.M. Oh, T. Yim, Y.G. Kim, Novel pyrroliniumbased ionic liquids for lithium ion batteries: effect of the cation on physicochemical and electrochemical properties, Electrochim. Acta 240 (2017) 267e276. [8] T. Yong, L. Zhang, J. Wang, Y. Mai, X. Yan, X. Zhao, Novel choline-based ionic liquids as safe electrolytes for high-voltage lithium-ion batteries, J. Power Sources 328 (2016) 397e404. [9] R. Chen, Y. Chen, L. Zhu, Q. Zhu, F. Wu, L. Li, A facile approach of introducing DMS into LiODFBePYR14TFSI electrolyte for lithium ion batteries, J. Mater. Chem. A 3 (2015) 6366e6372. [10] R. Raj, J. Wolfenstine, Current limit diagrams for dendrite formation in solidstate electrolytes for Li-ion batteries, J. Power Sources 343 (2017) 119e126. [11] Z. Xue, D. He, X. Xie, Poly(ethylene oxide)-based electrolytes for lithium ion batteries, J. Mater. Chem. A 3 (2015) 19218e19253. [12] B. Liu, K. Fu, Y. Gong, C. Yang, Y. Yao, Y. Wang, C. Wang, Y. Kuang, G. Pastel, H. Xie, E.D. Wachsman, L. Hu, Rapid thermal annealing of cathode-garnet interface toward high-temperature solid state batteries, Nano Lett. 17 (2017) 4917e4923. ^ne, E.C. Reyes, D. Rentsch, Z. Łodziana, [13] Y. Yan, R.-S. Kühnel, A. Remhof, L. Duche C. Battaglia, A lithium amide-borohydride solid-state electrolyte with lithiumion conductivities comparable to liquid electrolytes, Adv. Energy Mater. 7 (2017), 1700294. [14] R. Mews, E. Lork, P.G. Watson, B. Gortler, Coordination chemistry in and of sulfur dioxide, Coord. Chem. Rev. 197 (2000) 277e320. [15] D.L. Foster, H.C. Kuo, C.R. Schlaikjer, A.N. Dey, New highly conductive inorganic electrolytes-the liquid SO2 solvates of alkali and alkaline earth metal tetrachloroaluminates, J. Electrochem. Soc. 135 (1988) 2682e2686. [16] G. Jeong, H. Kim, J.H. Park, J. Jeon, X. Jin, J. Song, B.R. Kim, M.S. Park, J.M. Kim, Y.J. Kim, Nanotechnology enabled rechargeable Li-SO2 batteries: another approach towards post-lithium-ion battery systems, Energy Environ. Sci. 8 (2015) 3173e3180. [17] R. Hartl, M. Fleischmann, R. Gschwind, M. Winter, H. Gores, A liquid inorganic electrolyte showing an unusually high lithium ion transference number: a concentrated solution of LiAlCl4 in sulfur dioxide, Energies 6 (2013) 4448e4464. [18] A.N. Dey, H.C. Kuo, P. Piliero, M. Kallianidis, Inorganic electrolyte Li-SO2 rechargeable system development of a prototype hermetic C cell and evaluation of its performance and safety characteristics, J. Electrochem. Soc. 135 (1988) 2115e2120. [19] T.H. Yang, S. Kim, Y. Jung, Importance of specific capacity based on the mass of active material in the high energy density Li-SO2 secondary batteries with an inorganic electrolyte, Bull. Kor. Chem. Soc. 37 (2016) 917e922. [20] S.S. Park, Y. Jung, Influence of the electrode materials on the electrochemical performance of room temperature Li-SO2 rechargeable battery, Int. J. Electrochem. Sc. 10 (2015) 7574e7581.

T. Gao et al. / Electrochimica Acta 286 (2018) 77e85 [21] A.N. Dey, W.L. Bowden, H.C. Kuo, M.L. Gopikanth, C. Schlaikjer, D. Foster, Inorganic electrolyte Li/CuCl2 rechargeable cell, J. Electrochem. Soc. 136 (1989) 1618e1621. [22] J. Dreher, B. Haas, G. Hambitzer, Rechargeable LiCoO2 in inorganic electrolyte solution, J. Power Sources 44 (1993) 583e587. [23] C.W. Park, S.M. Oh, Performances of Li_LixCoO2 cells in LiAlCl43SO2 electrolyte, J. Power Sources 68 (1997) 338e343. [24] B. Wang, W.A. Abdulla, D. Wang, X.S. Zhao, A three-dimensional porous LiFePO4 cathode material modified with a nitrogen-doped graphene aerogel for high-power lithium ion batteries, Energy Environ. Sci. 8 (2015) 869e875. €rtler, Coordination chemistry in and of [25] R. Mews, E. Lork, P.G. Watson, B. Go sulfur dioxide, Coord. Chem. Rev. 197 (2000) 277e320. [26] J. Chai, Z. Liu, J. Zhang, J. Sun, Z. Tian, Y. Ji, K. Tang, X. Zhou, G. Cui, A superior polymer electrolyte with rigid cyclic carbonate backbone for rechargeable lithium ion batteries, ACS Appl. Mater. Interfaces 9 (2017) 17897e17905. [27] Y. Zhu, F. Wang, L. Liu, S. Xiao, Z. Chang, Y. Wu, Composite of a nonwoven fabric with poly(vinylidene fluoride) as a gel membrane of high safety for lithium ion battery, Energy Environ. Sci. 6 (2013) 618e624. [28] J. Malathi, M. Kumaravadivel, G.M. Brahmanandhan, M. Hema, R. Baskaran, S. Selvasekarapandian, Structural, thermal and electrical properties of PVAeLiCF3SO3 polymer electrolyte, J. Non-Cryst. Solids 356 (2010) 2277e2281. [29] J. Zhao, J. Zhang, P. Hu, J. Ma, X. Wang, L. Yue, G. Xu, B. Qin, Z. Liu, X. Zhou, G. Cui, A sustainable and rigid-flexible coupling cellulose-supported poly(propylene carbonate) polymer electrolyte towards 5 V high voltage lithium batteries, Electrochim. Acta 188 (2016) 23e30. [30] B. Wang, D. Wang, Q. Wang, T. Liu, C. Guo, X. Zhao, Improvement of the electrochemical performance of carbon-coated LiFePO4 modified with reduced graphene oxide, J. Mater. Chem. A 1 (2013) 135e144. [31] Y.S. Zhu, S.Y. Xiao, M.X. Li, Z. Chang, F.X. Wang, J. Gao, Y.P. Wu, Natural macromolecule based carboxymethyl cellulose as a gel polymer electrolyte with adjustable porosity for lithium ion batteries, J. Power Sources 288 (2015) 368e375. [32] J. Ha, S.K. Park, S.H. Yu, A. Jin, B. Jang, S. Bong, I. Kim, Y.E. Sung, Y. Piao, A chemically activated graphene-encapsulated LiFePO4 composite for highperformance lithium ion batteries, Nanoscale 5 (2013) 8647e8655. [33] B. Wang, A. Liu, W.A. Abdulla, D. Wang, X.S. Zhao, Desired crystal oriented LiFePO4 nanoplatelets in situ anchored on a graphene cross-linked conductive network for fast lithium storage, Nanoscale 7 (2015) 8819e8828. [34] L. Cheng, E.J. Crumlin, W. Chen, R. Qiao, H. Hou, S.F. Lux, V. Zorba, R. Russo, R. Kostecki, Z. Liu, K. Persson, W. Yang, J. Cabana, T. Richardson, G. Chen,

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42] [43]

[44]

[45]

[46]

85

M. Doeff, The origin of high electrolyte-electrode interfacial resistances in lithium cells containing garnet type solid electrolytes, Phys. Chem. Chem. Phys. 16 (2014) 18294e18300. W. Xia, B. Xu, H. Duan, Y. Guo, H. Kang, H. Li, H. Liu, Ionic conductivity and air stability of Al-Doped Li7La3Zr2O12 sintered in alumina and Pt crucibles, ACS Appl. Mater. Interfaces 8 (2016) 5335e5342. R.T. Carlin, J. Fuller, W.K. Kuhn, M.J. Lysaght, P.C. Trulove, Electrochemistry of room-temperature chloroaluminate molten salts at graphitic and nongraphitic electrodes, J. Appl. Electrochem. 26 (1996) 1147e1160. , L.M. Scolaro, G. De Luca, G. Salvato, XPS analysis of L. Silipigni, L. Schiro nanocomposite Li2x
yMn1
xPS3(C13H11N2)y films, Appl. Surf. Sci. 280 (2013) 572e577. Y.V. Larichev, B.L. Moroz, V.I. Bukhtiyarov, Electronic state of ruthenium deposited onto oxide supports: an XPS study taking into account the final state effects, Appl. Surf. Sci. 258 (2011) 1541e1550. re, Redox J. Auvergniot, A. Cassel, D. Foix, V. Viallet, V. Seznec, R. Dedryve activity of argyrodite Li6PS5Cl electrolyte in all-solid-state Li-ion battery: an XPS study, Solid State Ionics 300 (2017) 78e85. E. Stratakis, K. Savva, D. Konios, C. Petridis, E. Kymakis, Improving the efficiency of organic photovoltaics by tuning the work function of graphene oxide hole transporting layers, Nanoscale 6 (2014) 6925e6931. M.I. Nandasiri, L.E. Camacho-Forero, A.M. Schwarz, V. Shutthanandan, S. Thevuthasan, P.B. Balbuena, K.T. Mueller, V. Murugesan, In situ chemical imaging of solid-electrolyte interphase layer evolution in LieS batteries, Chem. Mater. 29 (2017) 4728e4737. J. Janlamool, B. Jongsomjit, Catalytic ethanol dehydration to ethylene over nanocrystalline c- and g-Al2O3 catalysts, J. Oleo Sci. 66 (2017) 1029e1039. € m, D. Brandell, M. Hahlin, Interface layer C. Xu, B. Sun, T. Gustafsson, K. Edstro formation in solid polymer electrolyte lithium batteries: an XPS study, J. Mater. Chem. A 2 (2014) 7256e7264. B. Jung, A. Safan, B. Batchelor, A. Abdel-Wahab, Spectroscopic study of Se(IV) removal from water by reductive precipitation using sulfide, Chemosphere 163 (2016) 351e358. X. Feng, M.K. Song, W.C. Stolte, D. Gardenghi, D. Zhang, X. Sun, J. Zhu, E.J. Cairns, J. Guo, Understanding the degradation mechanism of rechargeable lithium/sulfur cells: a comprehensive study of the sulfur-graphene oxide cathode after discharge-charge cycling, Phys. Chem. Chem. Phys. 16 (2014) 16931e16940. Q. Zhang, Q. Tao, H. He, H. Liu, S. Komarneni, An efficient SO2-adsorbent from calcination of natural magnesite, Ceram. Int. 43 (2017) 12557e12562.