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Lithium Ion Intercalation into Graphite in SO-Based Inorganic Electrolyte Towards High-Rate Capable and Safe Lithium Ion Batteries Ayoung Kim, Hojae Jung, Juhye Song, Hyun Jong Kim, Goojin Jeong, and Hansu Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20025 • Publication Date (Web): 08 Feb 2019 Downloaded from http://pubs.acs.org on February 8, 2019

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ACS Applied Materials & Interfaces

Lithium Ion Intercalation into Graphite in SO2Based Inorganic Electrolyte Towards High-Rate Capable and Safe Lithium Ion Batteries

Ayoung Kim1, Hojae Jung1, Juhye Song1, Hyun Jong Kim1, Goojin Jeong2,* and Hansu Kim1,*

1 Department 2 Advanced

of Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea Batteries Research Center, Korea Electronics Technology Institute, Seongnam 463-816, Republic of Korea

* Corresponding author: [email protected]; [email protected]

Keywords: lithium ion battery, inorganic electrolyte, high rate performance, graphite, non-flammable

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Abstract

Herein we have identified that lithium ions in an SO2-based inorganic electrolyte reversibly intercalate and de-intercalate into/out of graphite electrode using ex situ Xray diffraction and various electrochemical methods. X-ray photoelectron spectroscopy shows that the solid electrolyte interphase on the graphite electrode is mainly composed of inorganic compounds, such as LiCl and lithium sulfur-oxy compounds. Graphite electrode in SO2-based inorganic electrolyte has stable capacity retention up to 100 cycles and outstanding rate capability performance. This can be attributed to low interfacial impedance and the high ionic-conductivity of the SO2-based inorganic electrolyte, which are superior to those of conventional organic electrolytes. Considering the remarkable rate capability and intrinsically non-flammable properties of the electrolyte, use of graphite and an SO2 electrolyte will likely facilitate the development of advanced lithium ion batteries.


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Introduction

Lithium ion batteries (LIBs) have been used widely as power sources for portable electronic devices and regarded as promising power sources for nextgeneration electric vehicles and large-scale energy storage system applications. To meet the demands of these emerging applications, the safety, power density and energy density of Li batteries need to be improved 1-3. In particular, safety concern is that the flammable organic electrolytes used in LIBs can cause the LIBs to fire and explode under extreme operating conditions4-5. To ensure the safety of LIBs, new electrolyte systems such as ionic liquid and solid electrolytes have been investigated6-11. Solid electrolyte can potentially avoid the safety problems that are associated with electrolyte leakage and flammability6-7. Furthermore, the high mechanical strength of solid state electrolytes can suppress dendritic growth of lithium metal anode7-9. However, the two major drawbacks of solid state electrolytes are poor power density due to limited ionic conductivity and rapid capacity fading of batteries due to poor contact between the electrode and electrolyte interface8,

10.

Ionic liquids have attracted considerable

attention because of their low vapor pressure, non-flammability, high thermal stability, and electrochemical stability11-13. However, the high cost, high viscosity, and low ionic conductivity (about 10 mS cm-1) of these ionic liquids are challenges that still need to be overcome11-12. Both electrolytes have limited ionic conductivity in common, and therefore they could not fully satisfy the requirements for electric vehicles6-8, 13. The Li-SO2 rechargeable battery system was introduced about 30 years ago, and the employed SO2-based inorganic liquid electrolyte (LiAlCl4·xSO2) showed nonflammability, high ionic conductivity (about 80 mS cm-1 at room temperature (Figure S1)), and a wide range of working temperatures14-17. Considering these outstanding



properties, our group has revisited the rechargeable SO2-based inorganic liquid electrolyte battery system using various materials18, alkali metal changed to Na19-21, and various electrolyte configurations22 for high energy density combined with high safety than currently used LIB systems. As explained above, we expect that the nonflammability, high ionic conductivity (about 80 mS cm-1 at room temperature), and wide range of working temperatures of SO2-based inorganic liquid electrolyte will improve LIB performance and safety. Li intercalation reactions in LiFePO4 and LiCoO2 cathode materials using SO2-based inorganic liquid electrolyte have been studied previously in LIBs16, 23-24. Dreher et al.23 reported reversible lithium ion intercalation and de-intercalation reaction of the LixCoO2 cathode using an SO2-based inorganic liquid electrolyte and overcharging properties through charging up to 4.5 V (vs. Li/Li+) to regenerate the electrolyte. Park et al.16 also demonstrated that more detailed investigations of LixCoO2/Li cells in LiAlCl4·3SO2 electrolyte. They found that cell degradation occurred due to undesirable gaseous Cl2 evolution reaction through electrochemical oxidation of the electrolyte on the carbon surface. This degradation was reduced at low temperatures, where Cl2 formation was suppressed. However, operating at low temperatures to reduce side reactions is not practically useful, so other approaches are needed to further improve battery performance. Recently, Grundish et al.24 reported the feasibility of LiFePO4/Li cells with SO2-based inorganic liquid electrolyte. They reported a capacity of 140 mAh g-1 during the first cycle at room temperature and 40 mAh g-1 at a low (-20C) temperature. However, there are few reports of the application of SO2-based inorganic liquid electrolyte to graphite anode material. This is because of the possibility of decomposition of SO2-based liquid electrolyte on the graphite surface. Electrochemical reduction of LiAlCl4·3SO2 electrolyte occurs under 3.1 V (vs. Li/Li+) on the carbon surface, while the 4 ACS Paragon Plus Environment

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electrochemical oxidation of AlCl4- produces Cl2 gas above 4.0 V (vs. Li/Li+)18, 25-26 (Figure S2). As a result, previous studies have focused on cathode materials that have a working potential between 3.1 < V < 4.0 (vs. Li/Li+). Park et al. added LiPF6 to the electrolyte to improve battery capacity retention by stabilizing the lithium anode16. Recently, Gao et al.27 reported that lithium ions can be intercalated into graphite, which suggests that graphite may be suitable as an anode for LIBs when using LiAlCl4·3SO2. However, they reported only the electrochemical performance of graphite in LiAlCl4·3SO2 electrolyte. It is essential to know not only the exact reaction mechanism of lithium insertion into graphite, but also key factors that can affect the anodic performance of the graphite electrode in LiAlCl4·3SO2 electrolyte. Furthermore, the solid electrolyte interface (SEI) that formed on the interface between the graphite and LiAlCl4·3SO2 electrolyte should be clearly investigated because the SEI is one of the most important factors that determines the electrochemical performance of graphite anodes in LIBs such as long-term cycle performance, rate capability, and initial Coulombic efficiency. Here, we investigated the reaction mechanism of graphite anodes using LiAlCl4·3SO2 inorganic liquid electrolyte to assess the potential of SO2based inorganic liquid electrolyte systems in LIBs.



Results and Discussion

Figure 1 shows the electrochemical performance of a graphite electrode in an SO2-based inorganic liquid electrolyte. A schematic illustration (Figure 1a) shows the Li/graphite half-cell configuration in SO2-based inorganic liquid electrolyte. This LiAlCl4·3SO2 electrolyte system is non-flammable because of the intrinsic properties of the electrolyte (Figure 1b). Figure 1c shows the voltage profile of the graphite anode for the first cycle over the voltage range of 0.005 - 2.0 V (vs. Li/Li+). In the SO2-based inorganic liquid electrolyte system, the first charge and discharge capacities of the graphite were 419.3 and 356.9 mAh g-1, respectively, with an initial cycling efficiency of about 85% at a current density of 20 mAg-1. As shown in Figure 1, multiple voltage plateaus were observed in the potential range of 0.05 – 0.25 V (vs. Li/Li+), due to lithium ion intercalation/deintercalation into graphite28-29. In the organic electrolyte system, the first charge and discharge capacities of the same graphite electrode were 372.3 and 345.9 mAh g-1, respectively, with an initial cycling efficiency of about 93% at a current density of 20 mA g-1 (Figure S3). This cycling efficiency is greater than that of the SO2-based inorganic liquid electrolyte system. Irreversible electrochemical reduction of SO2 on the graphite at 2.7 V (vs. Li/Li+)30 is likely one of the main reasons for the low initial Coulombic efficiency of the graphite electrode in the SO2-based inorganic liquid electrolyte system. It should be noted that the amount of electrochemical reduction of the LiAlCl4·3SO2 electrolyte is much smaller than that on various high surface area carbon materials as reported in our previous study18. This is probably because the graphite used in this study has a much lower BET surface areas ranging from 2 to 10 m2g-1 (Figure S4) compared to the high surface area carbon materials used in our previous study (200 - 2000 m2g-1). We also clarified the


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relationship between the discharge capacity and physical properties of graphite such as BET surface area, pore volume. We could not find any relationship between the surface area, pore volume and the reversible capacity, as shown in Figure S5. Differential capacity plots (dQ/dV, DCP) of the graphite electrode more clearly revealed the electrochemical reactions of the graphite electrodes with the SO2-based inorganic liquid electrolyte. Figure 1d shows the DCP of the graphite electrode with the potential range between 0.05 – 2 V (vs. Li/Li+) for the first cycle. Three distinctive peaks at around 0.20, 0.11, 0.07 V (vs. Li/Li+) were observed in the DCP of the graphite electrode during lithium intercalation, and these values match well with the differential capacity plots peaks of graphite electrodes reported in earlier studies28,

31-32.

A cyclic

voltammogram (CV) of the graphite electrode in SO2-based inorganic liquid electrolyte revealed findings similar to those in the DCP; lithium intercalation into the graphite within the range of 0.05-0.25 V (vs. Li/Li+) (Figure S6). Despite concerns about the electrochemical reduction of the LiAlCl4·3SO2 electrolyte at 2.7 V (vs. Li/Li+) on the graphite surface, the main electrochemical reaction was lithium intercalation into the graphite anode rather than electrolyte decomposition. Figure 1e shows the cycle performance of graphite for 100 cycles under constant current/constant voltage (CC/CV) mode at a current of 20 mA g-1. The graphite anode exhibited highly stable cycle performance and maintained about 98.1% of its initial capacity even after 100 cycles with a Coulombic efficiency of more than 98.5%, which is comparable to cycle performance of graphite electrode in the organic electrolyte as compared in Figure 1e.



Figure 1. (a) Schematic illustration of the cell configuration with graphite using LiAlCl4·3SO2 electrolyte. (b) Photosnapshots of a flammability test of the LiAlCl4·3SO2 inorganic electrolyte and 1M LiPF6 dissolved in EC : EMC = 1:2 with 2 wt% FEC as the organic electrolyte. (c) First cycle voltage profiles, (d) differential capacity plots, and (e) the cycle performance of the graphite electrode in LiAlCl4·3SO2 electrolyte (red color) and organic electrolyte (green color).


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Figure 2 shows ex situ XRD patterns of the graphite electrode for the first cycle. Upon charging, XRD peaks corresponding to the LiC12 phase observed at 25.3° appeared at 0.07 V (vs. Li/Li+) with disappearance of the graphite peaks, indicating phase transformation to form the second stage compound. When the electrode was fully charged up to 0.005 V (vs. Li/Li+), the LiC12 peaks disappeared and LiC6 peaks, the first stage compound, emerged. During subsequent discharge, reversal phase transitions were observed and finally a sharp (002) graphite peak at 2θ = 26.6° was observed again. Based on these results, we concluded that graphite also exhibited typical electrochemistry with Li in the SO2-based electrolyte. In our previous study18, we showed that the electrochemical reduction of LiAlCl4·3SO2 electrolyte was highly dependent on the structural properties of the carbon materials used in the cathode. Figure S7 shows ex situ XRD patterns of the graphite electrode after the first cycle; a very small amount of LiCl phase was formed as a reaction product of the SO2-based inorganic electrolyte. This result combined with the magnified CV in Figure S6b showing an initial irreversible reduction peak between 2.7 V and 0.9 V (vs. Li/Li+) demonstrates that electrochemical reduction of LiAlCl4·3SO2 generated reduction products, such as LiCl, on the graphite surface. These CV peaks were observed in the same potential range during the second CV cycle with the exception of lithium intercalation into the graphite. This result implies that a protective passivation film formed at the graphite surface during the first cycle. In other words, a passivation layer formed irreversibly during the initial charge process, and then lithium intercalation into graphite occurred subsequently. This passivation layer that originated from the irreversible electrochemical reduction of LiAlCl4·3SO2 electrolyte would suppress continuous electrolyte decomposition and allow migration of lithium ion across the interface, similar to the so-called solid electrolyte interface of organic electrolytes30, 339 ACS Paragon Plus Environment


34.

The morphology of the graphite electrode in LiAlCl4·3SO2 electrolyte was

investigated by scanning electron microscopy (SEM) after fully charged state (Figure S8).

Figure 2. Ex situ XRD patterns of the graphite electrode before and during the first cycle.

Because the electrolyte reduction products of the SO2-based inorganic liquid electrolyte used in this work would be different from the those generated by organic electrolyte decomposition, we characterized the composition of the passivation layer. X-ray photoelectron spectroscopy (XPS) spectra were collected to identify the chemical composition of the LiAlCl4·3SO2 electrolyte reduction products on the graphite electrode. Figure 3 shows the Li 1s, Cl 2p, O 1s, S 2p XPS spectra of the graphite electrode taken after the first charge and after the first cycle, as well as those of the pristine graphite electrode. The Li 1s XPS spectra of the fully charged state showed 10 ACS Paragon Plus Environment

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three peaks at binding energies of 57.1 eV, 56.2 eV and 55.5 eV corresponding to LiC635, Li-Cl36, and a lithium oxide and lithium sulfur-oxy compound37, respectively (Figure 3a). After the first cycle, the peaks at 56.2 eV and 55.5 eV remained unchanged, indicating that LiC6 formed after full lithiation of the graphite electrode. LiCl, lithium oxide, and lithium sulfur-oxy compounds such as Li2S2O4 and Li2SO4 that formed on the surface of the graphite electrode would come from LiAlCl4·3SO2 electrolyte reduction reactions. The Cl 2p, O 1s, and S 2p spectra in Figure 3b-d showed the same tendency to Li 1s XPS spectra38-40. Note that the C 1s XPS spectra (Figure S9) corresponding to the graphite disappeared after fully charged and appeared again after fully discharged, indicating Li ion intercalation into graphite, which is in excellent agreement with Figure 2. XPS analysis revealed that the SEI that formed on the graphite in LiAlCl4·3SO2 electrolyte was mainly composed of the inorganic reduction products of the SO2-based inorganic electrolyte such as LiCl, Li2S, lithium oxide, and lithium sulfur-oxy compounds, which are completely different from the main components of the SEI formed on the graphite electrode in conventional organic electrolyte, which include organic compounds such as ROCO2Li, CH3OLi, (CH2OCO2Li)2, lithium carbonate, and inorganic compounds such as Li2O, Li2CO3, and LiF33,

40-41.

This

difference suggests that the SEI that formed on the graphite in the SO2-based inorganic liquid electrolyte would have different physical and transport properties from the SEI that formed in the organic electrolyte on the graphite electrode. Hence, different compositions of the passivation layer, mainly inorganic components, would result in different electrochemical performance in terms of lithium ion transportation and electrochemical kinetics for the intercalation and de-intercalation of lithium ions.



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Figure 3. (a) Li 1s, (b) Cl 2p, (c) O 1s, and (d) S 2p XPS spectra collected from various state of the SEI layer on the graphite electrodes in LiAlCl4·3SO2 electrolyte (pristine, fully charged state, and fully discharged state).

To characterize the electrochemical properties of the SEI that formed on the graphite in the SO2-based inorganic electrolyte, we employed electrochemical impedance spectroscopy (Figures 4a-b). We focused on the electrolyte resistance (Rs) and electrode interfacial polarization resistance, including electrolyte transport resistance, in the SEI (Rf) and charge transfer resistance (Rct) for lithium ion intercalation into the graphite. Figure 4c is the equivalent circuit used to analyze electrochemical reactions of the graphite electrode in LiAlCl4·3SO2 electrolyte

42-43.

Figure 4a shows a Nyquist plot of the OCV state graphite electrode in each electrolyte. Intrinsic electrolyte resistance (Rs) and the diameter of the semicircle in the OCV state, which represents the charge transfer resistance (Rct), were much smaller for graphite in the SO2-based inorganic electrolyte than graphite in the organic electrolyte, indicating that lithium ion intercalation into graphite in the SO2-based inorganic electrolyte had lower resistance than graphite in the organic electrolyte. Figure 4b


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shows a Nyquist plot of the fully charged state graphite electrode in each electrolyte. The additional semicircle in the high frequency region indicates the resistance of the interfacial layer (Rf). More specific fitting results for the electrochemical impedance spectroscopy measurements are summarized in supporting information Table 1. The organic electrolyte provided a higher Rf and Rct than the SO2-based inorganic electrolyte, implying that the inorganic electrolyte was able to generate a more efficient SEI layer that could facilitate lithium ion transportation through the surface film of graphite and charge transfer in electrochemical reactions than the conventionally used organic electrolyte. This finding combined with the XPS results indicated that inorganic components produced a compact film that adhered tightly to the graphite anode than the SEI from the organic electrolyte and showed improved electrochemical performance, consistent with previous reports33, 40, 44.



Figure 4. Electrochemical impedance spectra of the graphite electrode in LiAlCl4·3SO2 electrolyte (green colored dots) and organic electrolyte (purple colored dots) (a) at OCP, (b) after the first lithiation, and (c) the equivalent circuit used for the analysis.

We anticipated that the graphite electrode in the SO2-based inorganic liquid electrolyte would show higher rate capability than that of a graphite electrode in organic electrolyte, mainly due to the higher ionic conductivity of the inorganic electrolyte and its lower electrochemical impedance than that of a typical LIB organic electrolyte. Figure 5 compares the rate capability of the graphite with two different types of the electrolytes. The graphite electrode was tested by increasing the discharge C rate every 5 cycles from a 0.1 C-rate to a 10 C-rate and then returned to 1 C-rate. Figure 5a shows the charge-discharge voltage profiles of the graphite electrode in LiAlCl4·3SO2 electrolyte at various C-rates. The gradual increase of the discharge capacity during the initial few cycles might be attributed to the electrochemical reduction of LiAlCl4·3SO2 electrolyte to form SEI layer on the surface of graphite anode. At 0.5 C and 10 C rate, the graphite electrode delivered almost the same discharge capacity of about 350.7 and 350.8 mAh g-1, respectively. Voltage hysteresis between charge and discharge in the SO2-based inorganic electrolyte was much lower than that observed in the organic electrolyte (Figure 5c). The discharge capacity of the graphite electrode in LiAlCl4·3SO2 electrolyte at various C-rates is compared in Figure 5b; discharge capacity did not decrease even with an increase in current density. However, the obtained discharge capacity of the graphite electrode in the organic electrolyte was about 363, 362, 354, 335, 211 mAh g-1 at 0.2, 0.5, 1, 2 and 5 C-rate, respectively (Figure


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5d). As shown in the comparison of rate capability tests, graphite in the SO2-based inorganic electrolyte exhibited remarkable capacity retention even at a high current density. We attributed the outstanding rate capability of graphite in the SO2-based inorganic electrolyte to the low interfacial impedance and high lithium ion conductivity of the SO2-based inorganic electrolyte.

Figure 5. Rate capability test of the graphite electrodes. (a) Voltage profiles in LiAlCl4·3SO2 electrolyte at current densities of 0.1, 0.5, 1, 5, and 10 C (1 C = 372 mA g-1). (b) Rate capabilities in LiAlCl4·3SO2 electrolyte at current densities of 0.1, 0.2, 0.5, 1.0, 2.0, 5.0, and 10.0 C (1 C = 372 mA g-1). (c) Voltage profiles in organic electrolyte at current densities of 0.1, 0.5, 1, and 5C (1 C = 372 mA g-1). (d) Rate capabilities in organic electrolyte at current densities of 0.1, 0.2, 0.5, 1.0, 2.0, and 5.0 C (1 C = 372 mA g-1).



Conclusions

Herein we demonstrated that lithium ions in SO2-based inorganic liquid electrolyte could be reversibly intercalated and de-intercalated into/out of the graphite electrode using ex situ XRD analaysis. We found that the initial Coulombic efficiency of graphite electrode is closely related to the BET surface area of the graphite. We also found that the SEI formed on the grpahite anode in LiAlCl4·3SO2 is completely different from the SEI formed in the conventioally used organic electrolytes in lithiumion batteries using ex situ XPS analays as well as electrochemical impedance spectroscopy. Graphite in LiAlCl4·3SO2 electrolyte showed stable cycle performance and outstanding rate capability compared to graphite in conventional organic electrolyte. Such remarkable electrochemical performance could be attributed to highly efficient SEI layer with relatively low impedance mainly consisting of inorganic compounds as well as the high ionic-conductivity of the LiAlCl4·3SO2 electrolyte. We suggest that LIBs comprising a graphite anode with non-flammable and highly conductive SO2-based inorganic electrolyte, may facilitate the development of nextgeneration LIB systems with high rate capability that are safe to use.


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Experimental Section Preparation of the LiAlCl4·3SO2 electrolyte. LiAlCl4·3SO2 inorganic electrolyte was prepared as described in our previous report18. Briefly, LiCl and AlCl3 at a ratio of 1.1 to 1.0 were placed in a glass/Teflon vessel and thereafter SO2 gas was blown through a mixture of LiCl and AlCl3. After the reaction was completed, Li chips were added into the electrolyte vessel to remove AlCl3 or H2O residue. Caution: SO2 gas is a toxic gas and should be handled in an operating fume hood in a well-ventilated room. It is also advisable to wear a respirator, safety goggles and gloves whenever working with the gas. Electrochemical measurements. Graphite (BTR, China), styrene-butadienerubber (SBR, BM400, Zeon, Japan), and sodium carboxyl methylcellulose (CMC, Cellogen, Dai-ichi Kogyo Seiyaku, Japan) were used as anode materials. Graphite electrodes were prepared by coating slurries containing the graphite (96 wt%) as the active material and SBR (2 wt%) and CMC (2 wt%) as water-based mixed binder materials mixed in deionized water on Cu foil substrate. After coating, the electrodes were dried at 120°C for 2 hr and pressed under a pressure of 200 kg cm−2. The loading mass of the electrodes was fixed at about 4.0 mg cm−2. The cell was assembled using graphite as a working electrode, Li foil as a counter electrode, a polyethylene (PE) membrane as the separator and the electrolyte used was LiAlCl4·3SO2 and 1 M LiPF6 dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (1:2, v/v, Panax Etec Co., Ltd.) with 2.0 wt% fluoroethylene carbonate (FEC) in an Ar-filled glove box. CR2032 coin-type cells consisting of electrodes, separator, and each electrolyte were assembled in an Ar-filled glove box for the discharge/charge tests. The assembled cells were aged for 12 hr at room temperature and then electrochemically tested using a TOSCAT battery measurement system. Cells were 17 ACS Paragon Plus Environment


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galvanostatically charged (Li+ insertion) and discharged (Li+ extraction) in the voltage range of 0.005–2.0 V vs. Li/Li+ at room temperature at various current densities. Cyclic voltammetry (CV) was carried out using a potentiostat (VSP-300, BioLogic) in a threeelectrode cell, consisting of glassy carbon as a working electrode, lithium foil as both counter and reference electrode in LiAlCl4·3SO2 electrolyte. CV were performed in the potential range between 0.005 and 2.0 V (vs. Li/Li+) at a sweep rate of 0.1 mV s−1. Characterization. X-ray diffraction (XRD) patterns were obtained using an Empyrean

diffractometer

(PANalytical)

equipped

with

monochromated

Cu

K radiation (= 1.54056 Å). For ex situ analysis, the discharged or charged graphite anodes were covered with polyimide (Kapton) tape as a protective film. After the cell reacted to a certain level, the electrode was carefully disassembled from the cell and then rinsed with SOCl2 or EMC in an Ar-filled glove box to remove residual LiAlCl4·3SO2 or organic electrolyte. The surface chemistry of graphite was investigated by means of X-ray photoelectron spectroscopy (XPS; Thermo Fisher Scientific Co., Inc., Waltham, MA, USA). Electrochemical impedance spectroscopy (EIS) measurements were collected using an impedance analyzing potentiostat (VSP300, BioLogic) with an alternating current amplitude of 10 mV and a frequency range of 5 mHz and 1 MHz for the graphite electrode before cycling and after one cycle of lithiation.


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Conflicts of interest There are no conflicts of interest to declare.

Acknowledgements This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIT) (NRF- 2017R1A2B2012847) and LG Chem, Ltd (Grant No. AP1700081).

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Graphical abstract

The intrinsic merits of the SO2-based inorganic liquid electrolyte, such as, high ionic conductivity and non-flammability, could improve the battery performance of graphite electrode 22 ACS Paragon Plus Environment

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Lithium-Ion Intercalation into Graphite in SO 2 ... - ACS Publications

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