Li interface via an in-situ formed solid

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Stabilizing Li10SnP2S12/Li Interface via an in Situ Formed Solid Electrolyte Interphase Layer Bizhu Zheng,† Jianping Zhu,† Hongchun Wang,‡ Min Feng,† Ediga Umeshbabu,† Yixiao Li,† Qi-Hui Wu,§ and Yong Yang*,†,‡ †

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Collaborative Innovation Center of Chemistry for Energy Materials, State Key Laboratory for Physical Chemistry of Solid Surface, College of Chemistry and Chemical Engineering and ‡College of Energy, Xiamen University, Xiamen 361005, China § Department of Materials Chemistry, School of Chemical Engineering and Materials Science, Quanzhou Normal University, Quanzhou 362000, China S Supporting Information *

ABSTRACT: Despite the extremely high ionic conductivity, the commercialization of Li10GeP2S12-type materials is hindered by the poor stability against Li metal. Herein, to address that issue, a simple strategy is proposed and demonstrated for the first time, i.e., in situ modification of the interface between Li metal and Li10SnP2S12 (LSPS) by pretreatment with specific ionic liquid and salts. X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy results reveal that a stable solid electrolyte interphase (SEI) layer instead of a mixed conducting layer is formed on Li metal by adding 1.5 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)/N-propyl-N-methyl pyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr13TFSI) ionic liquid, where ionic liquid not only acts as a wetting agent but also improves the stability at the Li/LSPS interface. This stable SEI layer can prevent LSPS from directly contacting the Li metal and further decomposition, and the Li/LSPS/Li symmetric cell with 1.5 M LiTFSI/Pyr13TFSI attains a stable cycle life of over 1000 h with both the charge and discharge voltages reaching about 50 mV at 0.038 mA cm−2. Furthermore, the effects of different Li salts on the interfacial modification is also compared and investigated. It is shown that lithium bis(fluorosulfonyl) imide (LiFSI) salt causes the enrichment of LiF in the SEI layer and results in a higher resistance of the cell upon a long cycling life. KEYWORDS: sulfide solid electrolyte, Li metal, symmetric batteries, ionic liquid, solid electrolyte interphase layer, Li10SnP2S12

1. INTRODUCTION Over the past several decades, lithium-ion batteries (LIBs) have become an essential energy storage device in portable electronic products and electric vehicles. All-solid-state lithium batteries (ASLBs) are considered as a promising way to realize the lithium batteries with high energy density and safety.1 Because inorganic solid-state electrolytes (SSEs) have a potential to prevent Li dendrite growth for their high mechanical strength, making it possible to enable Li metal anode (with an extremely high theoretical specific capacity of 3860 mAh g−1 and the lowest negative electrochemical potential of −3.04 V vs standard hydrogen electrode) in lithium batteries. The better safety of ASLBs also benefits from the noninflammability and absence of leakage and vaporization of SSEs. Inorganic lithium-conducting solid-state electrolytes can be mainly divided into sulfides and oxide ceramics, and the sulfide SSEs usually exhibit a higher ionic conductivity than the latter.1−3 For example, Li10GeP2S12 shows an extremely high ionic conductivity of 14 mS cm−1, even comparable to that of conventional liquid electrolytes.2 However, the practical power © XXXX American Chemical Society

density for ASLBs with sulfide SSEs is still far from satisfactory on the account of high interfacial resistance at the electrode/ solid electrolyte interface.4 Regarding the Li metal/sulfide solid electrolyte interface, the reasons for high resistance mainly include two aspects. First, most sulfide solid electrolytes show poor stability at a low potential and the undesired decomposition products will lead to large interfacial resistance.5−7 Despite the extremely high Li+ conductivity, both experiment and theoretical calculation results show that the instability of Li10MP2S12 (M = Ge, Si, Sn) materials against Li metal causes a spontaneous formation of an interphase layer consisting of Li2S, Li3P, and Li−M alloy at the Li metal/Li10MP2S12 interface.6,8 In contrast to Li2S and Li3P, Li−M alloy allows the simultaneous transport of Li+ and electrons, and thus a mixed ionic and electronic conducting layer is formed at the interface, and not able to passivate Received: May 28, 2018 Accepted: July 10, 2018 Published: July 10, 2018 A

DOI: 10.1021/acsami.8b08860 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Li10MP2S12 against further Li reduction.9 Many solid electrolytes comprising multivalent metal cations are prone to form such a mixed conducting layer in contact with Li metal and impedes the application of Li metal in those solid electrolyte systems.10,11 Moreover, the volume changes during Li deposition and dissolution can also deteriorate the interface between Li metal and solid electrolyte due to high Young’s modulus of inorganic solid electrolytes.12 To reduce the interfacial resistance between Li and solid electrolytes, several effective strategies have been reported in the literature. (i) Alloy anode acts as an alternative to pure Li metal. For example, Li−In alloy with the potential of 0.62 V vs Li/Li+ is suitable for sulfide solid electrolyte system.13−15 (ii) Double-layer SSE configuration can provide a stable interface between electrodes and SSE layers.16−18 Nevertheless, these two methods often sacrifice the energy density of batteries. (iii) A buffer layer interposed between Li metal and solid electrolyte can suppress side reaction and promote intimate contact.19−21 It turns out that to suppress the side reaction at the Li metal/solid electrolyte interface, an electron-insulating but ion-conducting interface layer is critical. However, the interfacial modification strategies shown in literature are usually highly costly and quite complicated. Thus, it is necessary to develop simple and efficient methods to modify and control the solid/solid interfaces conveniently in all solidstate batteries. Ionic liquids (ILs) have attracted great interest in lithium batteries22−28 for their superior properties, including high ionic conductivity, high chemical stabilities, and especially noninflammability, high decomposition temperature, and less volatility compared to traditional organic electrolyte; but the poor wettability of the ILs with the separator restricts the rate characteristic of the batteries.29 As reported in previous literature, ionic liquids were already used as wetting agents to reduce the interfacial impedance22−25 or to prepare composite polymer solid electrolytes26−28,30 in solid-state batteries. Liu et al.22 and Oh et al.23 reported that the introduction of ionic liquid to the cathode composite could change the contact mode and provide an ionic conducting network so that it could ensure good contact between solid particles. In another study, Guo et al. 28 prepared Li1+xAlxGe2−x(PO4)3 (LAGP)−poly(vinylidene fluoride-cohexafluoropropylene)-based solid polymer composite electrolyte. They speculate that the improved cycle performance of the cell may be due to the solid electrolyte interphase (SEI) layer formed by the decomposition of IL. Because the polymer in the composite electrolyte can cover the surface of LAGP particles and promote the intimate contact between Li and electrolyte, the effects of ionic liquid are not clear if excluding the influence of polymer electrolytes on the improved electrochemical performance of the cell. So far, there is no report to further explore the effects of ionic liquid in solid-state batteries, including but not limited to the wetting agent. Therefore, the role of ionic liquid in solid-state batteries needs further investigation with clear evidence. In this work, we propose and demonstrate clearly a novel and simple approach by utilizing an extremely small amount of ionic liquid to improve the stability at the Li10SnP2S12 (LSPS) solid electrolyte/Li metal interface and reveal the roles of ionic liquid on the interfacial properties in solid-state batteries. Herein, the Pyr13TFSI ionic liquid was employed to modify the interface between Li metal and LSPS for its advantages including high ionic conductivity, high viscosity, and high

stability.31,32 Furthermore, the effects of different Li salts on the interfacial modification are also explored. The results demonstrate that 1.5 M LiTFSI/Pyr13TFSI ionic liquid can effectively improve the long cycle performance of Li/LSPS/Li symmetric cells. In this solid−liquid hybrid electrolyte system, ionic liquids can not only be used as a wetting agent but also improve the stability of the interface between Li metal and sulfide solid electrolyte for the in situ formed SEI layer on Li metal by decomposition of an ionic liquid. It facilitates the application of ILs with poor wettability and sulfide solid electrolyte with instability against Li metal in the practical use in lithium batteries.

2. MATERIALS AND METHODS 2.1. Preparation of Materials. 2.1.1. Preparation of 1.5 M LiTFSI/Pyr13TFSI and 1.5 M LiFSI/Pyr13TFSI Ionic Liquid. N-Propyl-Nmethyl pyrrolidinium bis(trifluoromethanesulfonyl) imide (Pyr13TFSI) (Shanghai CHENGJIE Corporation, China) (named as IL for short hereafter) was dried by using molecular sieves before use. And, 1.5 M LiTFSI/IL or 1.5 M LiFSI/IL with high viscosity was prepared by dissolving a stoichiometric amount of LiTFSI (provided by Zhuhai Smoothway Electronic Materials Co., Ltd, China) or LiFSI salt (purchased from NIPPON SHOKUBAI company, Japan) into dried Pyr13TFSI ionic liquid. 2.1.2. Preparation of C/LSPS/C Symmetric Cells. One hundred fifty milligrams of Li10SnP2S12 (NEI Corporation) electrolyte powder was cold-pressed into a pellet (Φ = 10 mm, 380 MPa), followed by pressing carbon foil as blocking electrodes to both sides of the LSPS pellet. C/LSPS/C symmetric cells were assembled to measure the ionic conductivity of the LSPS pellet. 2.1.3. Preparation of Li/Li Symmetric Cells. Li/LSPS/Li symmetric cells were assembled by attaching Li metal foils to the two sides of the LSPS pellet and pressing them by hand in custommade stainless steel molds. Quasi-solid-state symmetric cells Li/ LSPS/Li with ionic liquid were assembled via the same method and 10 mg of pure Pyr13TFSI ionic liquid (or 1.5 M LiTFSI/IL, or 1.5 M LiFSI/IL) was spread on both sides of the pellet between Li metal and LSPS pellet. For comparison, the liquid Li/Li symmetric cells were assembled by using ionic liquid (1.5 M LiTFSI/IL or 1.5 M LiFSI/IL) as an electrolyte and fiberglass as a separator in the coin cell. All the batteries were assembled in a dry glovebox filled with argon gas. 2.1.4. Preparation of Quasi-Solid-State Batteries. The cathode composite was prepared using the same method as described in the literature.22 LiFePO4 (LFPO, BTR Corporation, China) was ground with acetylene black in a ratio of 75:15 (wt %, LFPO: acetylene black) thoroughly using a mortar. Then, the composite was mixed with 1.5 M LiTFSI/IL ionic liquid in the ratio of 40:60 (wt %, composite: IL). The as-prepared slurry was coated on one side of the LSPS pellet, followed by 10 mg of 1.5 M LiTFSI/IL ionic liquid on the other side of the LSPS pellet and attached Li metal. The cells without ionic liquid at the anode side were assembled for comparison. The mass loading of the LiFePO4 cathode is 2.5−3 mg cm−2. All the batteries were assembled in a dry glovebox filled with argon gas. 2.2. Electrochemical Measurements. Galvanostatic charge and discharge measurements of the batteries were conducted on a LAND CT-2001A (Wuhan, China) battery test system at room temperature (RT). The ionic conductivities of 1.5 M LiTFSI/IL and 1.5 M LiFSI/ IL were obtained by using a DDS-307A conductivity meter at RT. The electrochemical impedance spectroscopy (EIS) measurements were carried out by using a Versa STAT MV Multichannel potentiostat/galvanostat (Princeton Applied Research) from 1 Hz to 1 MHz with an amplitude of 10 mV at RT. 2.3. Characterization of Materials. X-ray powder diffraction (XRD) analyses were performed by a Rigaku Ultima IV powder X-ray diffractometer using a Cu Kα radiation (λ = 1.5406 Å), and Mylar film was used to seal the sample to avoid reactions between the solid electrolyte and moist air. Rietveld refinement was performed by using the General Structure Analysis System (GSAS) software package to B


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Figure 1. (a) SEM images of LSPS solid electrolyte powder and (b) the diameter and (c) thickness of LSPS pellet. (d) The two-phase Rietveld refinement results of Li10SnP2S12 with X-ray powder diffraction data and GSAS software. (e) Nyquist plot of C/LSPS/C cell at room temperature. The inset was the equivalent circuit model for the symmetric cell.

Figure 2. (a) Li+ stripping/plating curves of Li/LSPS/Li symmetric cells with and without 1.5 M LiTFSI/IL at a current density of 0.038 mA cm−2 and the cycle performance of Li/LSPS/Li symmetric cell with 1.5 M LiTFSI/IL (b) at 0.115 mA cm−2 and (c) at different current densities. (d) Nyquist plots measured with Li/LSPS/Li symmetric cells with and without 1.5 M LiTFSI/IL at room temperature. C


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Figure 3. Nyquist plots measured from symmetric cells (a) Li/LSPS/Li without ionic liquid and (c) Li/LSPS/Li with 1.5 M LiTFSI/IL at different storage times. Cycling behavior of the symmetric cells recorded after storage: (b) Li/LSPS/Li and (d) Li/LSPS/Li with 1.5 M LiTFSI/IL.

a low ionic conductivity of 1.5 × 10−5 S cm−1 at 25 °C and good stability in air under ambient conditions. Consequently, the existence of Li2SnS3 should decrease the total conductivity of the LSPS material, but improve the stability of LSPS in air. Our results display that Li10SnP2S12 shows a tetragonal crystal structure (space group P42/nmc (137), a = b = 8.745 Å, c = 12.783 Å), close to those proposed in the literature. EIS analysis was performed to obtain the ionic conductivity of LSPS. As shown in Figure 1e, the C/LSPS/C symmetric cell shows one semicircle (corresponding to the grain boundary) and a spike (corresponding to electrode contributions), and the intercept is related to the bulk conductivity of 1.3 × 10−3 S cm−1. The total ionic conductivity is 2.2 × 10−4 S cm−1. 3.2. Interfacial Modification at Li Metal/LSPS Electrolyte Interface with Ionic Liquid. To suppress the side reaction between Li metal and LSPS solid electrolyte, an extremely small amount of 1.5 M LiTFSI/IL ionic liquid was added into the Li/LSPS/Li symmetric cells for interfacial modification, and such a high concentrated ionic liquid with high viscosity can avoid the leakage problem. Therefore, the Li/LSPS/Li symmetric cells with and without 1.5 M LiTFSI/ IL ionic liquid were assembled as shown in the schematic (Figure S1) and charged/discharged at constant current. The overpotential of the Li/LSPS/Li symmetric cell increases gradually with time (Figure 2a), revealing a continuous reaction between Li and LSPS. The symmetric cell with 1.5 M LiTFSI/IL, in contrast, exhibits a stable cycling life of over 1000 h and the overpotential of Li+ platting/stripping remains smaller than 50 mV at a current density of 0.038 mA cm−2,

get the lattice parameters of the commercial LSPS powder samples.33 Scanning electron microscopy (SEM, Hitachi S-4800) and energydispersive spectrometer (EDS) were utilized to characterize the morphology and elemental distribution of the materials. Li metal for SEM and EDS characterization was washed copiously with diethyl ether and dried under vacuum at RT before the experiment. X-ray photoelectron spectroscopy (XPS) was carried out on a PHI 5000 Versa Probe III spectrometer (ULVAC-PHI, Japan) and etching experiments were performed on the sample surface using argon ion beam gun operating at 25.1 W. The binding energy scale was calibrated from the hydrocarbon contamination using the C 1s peak at 284.8 eV. For SEM and XPS characterizations, an airtight specimen holder is used to avoid moisture and air contamination during sample transfer.

3. RESULTS AND DISCUSSION 3.1. Characterization of Li10SnP2S12 Material. It can be seen in Figure 1a that the particle size of the LSPS powder is nonuniform and shows a clear aggregation. The thickness and the diameter of the LSPS pellet are 1 mm and 1 cm (Figure 1b,c), respectively. The XRD measurement and Rietveld refinement are used to identify the phase purity and crystallinity of Li10SnP2S12 material, respectively, as shown in Figure 1d. The powder XRD pattern indicates that Li10SnP2S12 is contaminated with Li2SnS3 impurity phase at 34°, which is consistent with the result of Ilyas Tarhouchi et al.34 Therefore, two phases were used for the refinement based on Li10SnP2S12 structure (proposed by Bron et al.35) and Li2SnS3 structure. And, the impurity Li2SnS3 accounts for 3.8% weight fraction in the electrolytes we used. Brant et al.36 showed that Li2SnS3 has D


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Figure 4. S 2p XPS spectra of (a) pristine LSPS pellet, (b) LSPS pellet after cycling, and (c) LSPS pellet with 1.5 M LiTFSI/IL after cycling. (d) C 1s and F 1s XPS spectra of different Li metals after being etched for 60 s. (Sample 1: Li metal after cycling; Sample 2: Li metal with 1.5 M LiTFSI/ IL after cycling.)

indicating that a stable interface might be formed between Li and LSPS. Although, the overpotential of the cell increases at a higher current density of 0.115 mA cm−2, the cell still maintains stable cycling for over 350 h (Figure 2b). Moreover, the interfacial resistance of symmetric cell (Figure 2d) is decreased from 1960 to 250 Ω with the addition of 1.5 M LiTFSI/IL on account of the fact that ionic liquid provides an ionic conducting network and changes the solid−solid contact into the solid−liquid contact. Figure S2a,b denotes the morphology of the LSPS pellet with and without ionic liquid. Significant voids are observed on the surface of the pristine solid electrolyte pellet (Figure S2a). Because a pore-less monolith is hard to form via clod-pressing, the inevitable rough surface of the solid electrolyte pellet will make it very difficult to form a stable and intimate contact with Li metal anode in all solid-state batteries and thus can lead to high interfacial resistance. As shown in Figure S2b, the voids of the LSPS pellet are filled with ionic liquid. The EDS results (Figure S2c−f) reveal that the ionic liquid can cover the surface of the electrolyte particles uniformly and exhibits a good interfacial wettability. Furthermore, the high viscosity of the ionic liquid can make sure it attaches itself to the surface of the LSPS pellet and prevents liquid leakage. Time-resolved impedance spectrum measurements were performed on the symmetric cells with or without 1.5 M LiTFSI/IL ionic liquid to confirm whether the ionic liquid can suppress the chemical reaction between Li metal and LSPS or not (Figure 3). The Li/LSPS/Li symmetric cells with and without ionic liquid were stored in air (but it cannot react with air or moisture because of the airtight molds) under room temperature and EIS measurement was conducted at different

days of storage. It is evident that both cells exhibit an apparent overall increase in cell resistance with increase in the storage time. However, the Li/LSPS/Li cell (Figure 3a) shows a dramatic overall increase in resistance and exhibits a much higher interfacial resistance than the Li/LSPS/Li cell with 1.5 M LiTFSI/IL (Figure 3c) after a long time aging. After storage, both symmetric cells were carried out with galvanostatic cycling experiment. The Li/LSPS/Li cell (Figure 3b) shows noticeably high polarization potential, whereas the cell with 1.5 M ionic liquid (Figure 3d) maintains a stable cycling. This difference is associated with the unstable interface between Li metal and LSPS pellet. Chemical side reactions happen at the Li metal/LSPS electrolyte interface, and the interface layer formed after storage with high resistance deteriorates the electrochemical performance of the batteries. In contrast, the modest increase in the resistance of the Li/LSPS/Li cell with 1.5 M LiTFSI/IL means that a stable surface layer rather than a growing and unstable interface layer may be formed at the interface with the addition of LiTFSI/IL ionic liquid additive and suppresses further side reaction during cycling. 3.3. Mechanistic Study on the Improved Stability at Li/LSPS Interface with Ionic Liquid. To figure out why the ionic liquid can improve the cycling performance of the cells, XPS was utilized to analyze the change in the surface of the LSPS pellet and Li metal. The results presented in Figure 4a,b display that the peaks at 163 and 162 eV in the S 2p spectra can be assigned to the bridging sulfur atoms (P−S−P groups) and terminal sulfur atoms (PS groups), respectively.37 When compared to the pristine LSPS pellet, the surface of the LSPS pellet reacts with Li metal during cycling and generates Li2S as the undesirable product observed at 161 eV, consistent with E


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Figure 5. Electrochemical performance of LFPO/LSPS/Li batteries with and without 1.5 M LiTFSI/IL at the anode side at the current density of 0.1C (14 mA g−1) under room temperature. (a) The initial charge and discharge curves and (b) cycle performance.

the result in the literature.6 Chemical instability of the LSPS sulfide solid electrolyte against Li metal induces the growth of the interface layer with increase in time and then causes a noticeable increase in impedance. For the LSPS pellet with 1.5 M LiTFSI/IL ionic liquid after cycling (Figures 4c and S4), the peaks in the S 2p spectra are related to the ionic liquid, and no obvious peaks of the side reactions products and LSPS materials are observed on the LSPS electrolyte for the coverage of an ionic liquid. To exclude the influence of coverage of an ionic liquid and get more structural information of the decomposed compounds underneath, XPS etching experiments were conducted on the surface of Li metal with and without 1.5 M LiTFSI/IL ionic liquid after cycling. The C 1s and F 1s spectra in Figure 4d indicate that when compared to the Li metal without ionic liquid, the presence of −CF3 and LiF on the surface of Li metal with 1.5 M LiTFSI/IL ionic liquid after cycling is associated with the decomposition products of TFSI− anion, respectively. As reported in the literature,38 LiF is the dominated component of the SEI layer formed on the surface of Li metal in LiTFSI/Pyr1xTFSI (x = 3, 4) ionic liquid. Moreover, the EDS result (Figure S3) confirms the uniform distribution of F element on the surface of Li metal, further demonstrating the existence of a SEI layer on Li metal. A SEI layer comprising of TFSI− anion reduction products is already formed on the surface of Li metal and hence suppresses the side reactions at the Li/LSPS interface. Such an observation supports the results of the electrochemical cycling behavior (Figure 2a) and Nyquist plots (Figure 3a,c) of symmetric cells. In addition, XPS experiment under the same conditions was also conducted on Li metal with 1.5 M LiTFSI/IL without cycling. The results in Figure S5 indicate that the SEI layer is formed on the surface of Li metal even without cycling, but the SEI layer is much thinner than that formed after cycling. Therefore, the thin SEI formed after Li metal comes in contact with the ionic liquid can prevent the direct contact between Li metal and LSPS before cycling, which also accounts for the results in Figure 3c,d. For the purpose of exploring whether this hybrid solid− liquid electrolyte system can show better electrochemical performance than ionic liquid system or not, we compared the Li+ stripping/plating curves of the Li/LSPS/Li quasi-solid-state symmetric cell with 1.5 M LiTFSI/IL and Li/Li liquid symmetric cell with 1.5 M LiTFSI/IL. The results in Figure S6 show that during each alternating periods of charge and

discharge, steady Li+ stripping/plating plateau appears rapidly in the quasi-solid-state symmetric cell Li/LSPS/Li with 1.5 M LiTFSI/IL. By comparison, the curve of the liquid symmetric cell is serrated and the polarization voltage is slightly higher than that of the quasi-solid-state symmetric cell. The electrical double layer (EDL) of ionic liquid-electrode is complex and consists of multilayers of ions (anion + cation).39 We speculate that the special serrated curve in the liquid Li/Li symmetric cell may be ascribed to the charge redistribution and reorientational movement of Pyr13+ cation at the EDL.40 As for the quasi-solid-state symmetric cell Li/LSPS/Li with 1.5 M LiTFSI/IL, LSPS solid electrolyte in contact with lithium metal destroys the EDL and provides channel to Li ion transport directly for its high ionic conductivity, accounting for the flat and rectangular Li+ stripping/plating curves. Besides, considering the improved stability of LSPS against Li metal by adding ionic liquid and the good wettability of ionic liquid in the LSPS electrolyte, a combination of LSPS with ionic liquid is a good way to achieve a whole greater than the sum of its parts. 3.4. Electrochemical Performance of LiFePO4/LSPS/Li Batteries with and without Interfacial Modification at the Anode Side. Based on this interfacial modification method, quasi-solid-state batteries with commercial LiFePO4 as the cathode, LSPS as the electrolyte, and Li metal as the anode were assembled. Figure 5 shows the electrochemical performance of the batteries assembled with and without 1.5 M LiTFSI/IL at the anode side. It is clearly shown that the cell with ionic liquid exhibits smaller polarization (Figure 5a), better cycle performance (Figure 5b), and smaller resistance (Figure S7b) when compared to the cell without modification. The cell with ionic liquid delivers an initial discharge capacity of 144 mAh g−1 at 0.1C with the voltage window range of 2.5− 3.9 V (Figure 5a) and the initial Coulombic efficiency of 93.9% (Figure S7a), higher than those of the cell without ionic liquid (103 mAh g−1, 83.6%). Furthermore, the cell with ionic liquid shows a discharge capacity retention of 84.7% (Figure 5b) and a steady Coulombic efficiency after 30 cycles, whereas the capacity of the cell without ionic liquid decays rapidly within 10 cycles. Because the instability between Li metal and LSPS pellet deteriorates, so does the cycle performance of the batteries. Therefore, a small amount of 1.5 M LiTFSI/IL at the anode side can significantly improve the electrochemical performance of the LiFePO4/LSPS/Li batteries. And, the long cycle performance of the cell needs further improvement F


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conductivities of 1.5 M LiTFSI/IL and 1.5 M LiFSI/IL are 0.543 and 0.778 mS cm−1, respectively. The higher ionic conductivity of 1.5 M LiFSI/IL corresponds to its lower viscosity. Unlike the results in quasi-solid-state symmetric cells, the liquid Li/Li symmetric cells with 1.5 M LiTFSI/IL or 1.5 M LiFSI/IL shows a stable Li+ stripping/plating performance (Figure S8). These differences in Li+ stripping/plating curves between the quasi-solid-state system and the liquid system may be due to the amount of ionic liquid and thus influence the resistance of the SEI layer. The XPS results for Li metal in Cell-pure, Cell-1.5 M LiFSI, and Cell-1.5 M LiTFSI (after cycling for 140 h) with different etching time under the same conditions are compared in Figure 7. The F 1s spectra after 60 and 120 s etching time reveal that the amount of LiF in Cell-pure is far less than that in Cell-1.5 M LiTFSI. Moreover, the intensity of the peak corresponding to LiF in Cell-pure symmetric cell decreases with the etching time. The results suggest that the SEI layer formed in Cell-1.5 M LiTFSI symmetric cells is thicker and more stable. Because the Li metal anode undergoes large volume changes during lithiation and delithiation, the SEI layer is under dynamic strain. A thin SEI layer is easy to crack during long cycling, whereas a stable SEI layer can maintain a good interface between Li metal and LSPS solid electrolyte. Hence, the addition of LiTFSI salt promotes the formation of a stable SEI layer, facilitating the long-term cycling stability of Cell-1.5 M LiTFSI. However, Figure 7c presents that the SEI layer formed in the presence of LiFSI salt is richer in LiF than that formed in the LiTFSI-containing or pure Pyr13TFSI ionic liquid. And, the peak of −CF 3 corresponds to the decomposition of TFSI− anion from Pyr13TFSI. It is known that LiFSI salt provides a more donatable fluorine than LiTFSI salt and thus the number of LiF generated per LiFSI molecule is higher than that generated per LiTFSI molecule.42 It is worth noting that excessive LiF will cause an increase in the resistance due to its low ionic conductivity43 and ultimately deteriorate the cycle performance of the symmetric cell. Because Li metal shows intrinsically high chemical reactivity and low thermal stability, it is hard to characterize the morphology of the SEI layer without damaging by conventional transmission electron microscopy (TEM).44,45 Herein, we deduce a possible schematic illustration of the interfacial modification mechanism based on the results of the electro-

by modification at the cathode side for the instability of LSPS at a high potential.41 3.5. Effect of Lithium Salt on the Improved Electrochemical Performance of Batteries. Additionally, to investigate the effect of different lithium salts on the improved interfacial stability at the Li metal/LSPS solid electrolyte interface, we also assembled the Li/LSPS/Li symmetric cell with pure Pyr13TFSI or 1.5 M LiFSI/Pyr13TFSI ionic liquid for comparison (symmetric cells with pure Pyr13TFSI, 1.5 M LiTFSI/Pyr13TFSI, or 1.5 M LiFSI/Pyr13TFSI are labeled as Cell-pure, Cell-1.5 M LiTFSI, and Cell-1.5 M LiFSI, respectively). As shown in Figure 6, Cell-pure exhibits a stable

Figure 6. Li+ stripping/plating curves of the symmetric cells (black) Li/LSPS/Li with 1.5 M LiTFSI/IL, (blue) Li/LSPS/Li with 1.5 M LiFSI/IL, and (red) Li/LSPS/Li with pure Pyr13TFSI at the current density of 0.038 mA cm−2 under room temperature.

cycle initially, but the polarization of symmetric cell shows a rapid increase after cycling for several hundred hours, with both charge and discharge voltages reaching over 0.5 V, indicating the unstable interface between Li metal and solid electrolyte. However, for Cell-1.5 M LiFSI, the potential increases gradually with time, and the resistance increases noticeably during cycling compared to that of Cell-1.5 M LiTFSI (Figures S9 and S10). This indicates the continuous growth and the highly resistive nature of the SEI layer formed in the presence of LiFSI. It is interesting to find that the ionic

Figure 7. F 1s XPS spectra of Li metal at different etching times disassembled from (a) Li/(LSPS + pure Pyr13TFSI)/Li, (b) Li/(LSPS + 1.5 M LiTFSI/Pyr13TFSI)/Li, and (c) Li/(LSPS + 1.5 M LiFSI/Pyr13TFSI)/Li symmetric cells after cycling for 140 h. G


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Figure 8. Schematic illustration of interfacial modification mechanism of different Li salt/Pyr13TFSI.

chemical performance and XPS analysis results as shown in Figure 8.

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4. CONCLUSIONS In conclusion, a novel and simple approach to stabilize the interface between Li metal and Li10SnP2S12 sulfide solid electrolyte is successfully demonstrated in this work. The cycle performance of the Li/LSPS/Li symmetric cells at room temperature is enhanced greatly by the addition of an extremely small amount of 1.5 M LiTFSI/Pyr13TFSI ionic liquid. Furthermore, the LFPO/LSPS/Li quasi-solid-state batteries with 1.5 M LiTFSI/Pyr13TFSI ionic liquid at the anode side shows a much higher initial discharge capacity and retention after cycling compared to batteries without ionic liquid. Therefore, the significantly enhanced interfacial stability between Li metal and LSPS sulfide solid electrolyte is achieved by in situ forming a SEI layer on Li metal as a passivation layer. In addition, when compared to the pure Pyr13TFSI ionic liquid, LiTFSI salt in Pyr13TFSI ionic liquid can form a more stable SEI layer, enabling stable Li deposition/dissolution. However, the presence of LiFSI salt leads to the enrichment of LiF in the SEI layer and thus deteriorates the cycle performance of the symmetric cell. Our new strategy provides a new concept of overcoming the inherent shortcomings of ILs and sulfide solid electrolyte, such as poor wettability of ILs against separator and the instability of sulfide solid electrolyte against Li metal, enabling the application of ILs and sulfide solid electrolyte in the practical use in lithium batteries.

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Nyquist plots of symmetric cells Li/LSPS/Li with different Li salts after cycling (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: +86 592 2185753. ORCID

Bizhu Zheng: 0000-0002-2744-394X Author Contributions

The manuscript was written through contributions of all the authors. All the authors have given approval to the final version of the manuscript. Funding

National Key Research and Development Program of China, National Natural Science Foundation of China. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Yuanjun Shao for helpful discussion. This work was financially supported by National Key Research and Development Program of China (grant no. 2016YFB0901502 and 2018YFB0905400) and National Natural Science Foundation of China (grant nos. 21233004, 21473148, and 21621091).

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REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b08860. Schematic of (quasi) solid-state symmetric cell, SEM images and EDS elemental mapping results of the surface of pristine LSPS pellet with and without ionic liquid and Li metal with ionic liquid, Sn 3d XPS spectra of LSPS pellet with ionic liquid after cycling and F 1s XPS spectra of Li metal with ionic liquid without or after cycling, galvanostatic charge and discharge performance of Li/LSPS/Li symmetric cell with ionic liquid and liquid Li−Li symmetric cell, Coulombic efficiency and Nyquist plots of batteries of LFPO/LSPS/Li batteries with and without 1.5 M LiTFSI/IL at the anode side and H


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Li interface via an in-situ formed solid

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