Stabilizing Li10SnP2S12/Li Interface via an in Situ Formed SolidElectrolyte Interphase LayerBizhu Zheng,† Jianping Zhu,† Hongchun Wang,‡ Min Feng,† Ediga Umeshbabu,† Yixiao Li,†

Qi-Hui Wu,§ and Yong Yang*,†,‡

†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 ishindered by the poor stability against Li metal. Herein, toaddress that issue, a simple strategy is proposed anddemonstrated for the first time, i.e., in situ modification ofthe interface between Li metal and Li10SnP2S12 (LSPS) bypretreatment with specific ionic liquid and salts. X-rayphotoelectron spectroscopy and electrochemical impedancespectroscopy results reveal that a stable solid electrolyteinterphase (SEI) layer instead of a mixed conducting layer isformed on Li metal by adding 1.5 M lithium bis-(trifluoromethanesulfonyl)imide (LiTFSI)/N-propyl-N-meth-yl pyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr13TFSI) ionic liquid, where ionic liquid not only acts as a wettingagent but also improves the stability at the Li/LSPS interface. This stable SEI layer can prevent LSPS from directly contactingthe Li metal and further decomposition, and the Li/LSPS/Li symmetric cell with 1.5 M LiTFSI/Pyr13TFSI attains a stable cyclelife of over 1000 h with both the charge and discharge voltages reaching about 50 mV at 0.038 mA cm−2. Furthermore, theeffects of different Li salts on the interfacial modification is also compared and investigated. It is shown that lithiumbis(fluorosulfonyl) imide (LiFSI) salt causes the enrichment of LiF in the SEI layer and results in a higher resistance of the cellupon 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 portableelectronic products and electric vehicles. All-solid-state lithiumbatteries (ASLBs) are considered as a promising way to realizethe lithium batteries with high energy density and safety.1

Because inorganic solid-state electrolytes (SSEs) have apotential to prevent Li dendrite growth for their highmechanical strength, making it possible to enable Li metalanode (with an extremely high theoretical specific capacity of3860 mAh g−1 and the lowest negative electrochemicalpotential of −3.04 V vs standard hydrogen electrode) inlithium batteries. The better safety of ASLBs also benefits fromthe noninflammability and absence of leakage and vaporizationof SSEs. Inorganic lithium-conducting solid-state electrolytescan be mainly divided into sulfides and oxide ceramics, and thesulfide SSEs usually exhibit a higher ionic conductivity than thelatter.1−3 For example, Li10GeP2S12 shows an extremely highionic conductivity of 14 mS cm−1, even comparable to that ofconventional liquid electrolytes.2 However, the practical power

density for ASLBs with sulfide SSEs is still far from satisfactoryon the account of high interfacial resistance at the electrode/solid electrolyte interface.4

Regarding the Li metal/sulfide solid electrolyte interface, thereasons for high resistance mainly include two aspects. First,most sulfide solid electrolytes show poor stability at a lowpotential and the undesired decomposition products will leadto large interfacial resistance.5−7 Despite the extremely high Li+

conductivity, both experiment and theoretical calculationresults show that the instability of Li10MP2S12 (M = Ge, Si,Sn) materials against Li metal causes a spontaneous formationof an interphase layer consisting of Li2S, Li3P, and Li−M alloyat the Li metal/Li10MP2S12 interface.

6,8 In contrast to Li2S andLi3P, Li−M alloy allows the simultaneous transport of Li+ andelectrons, and thus a mixed ionic and electronic conductinglayer is formed at the interface, and not able to passivate

Received: May 28, 2018Accepted: July 10, 2018Published: July 10, 2018

Research Article

www.acsami.orgCite This: ACS Appl. Mater. Interfaces 2018, 10, 25473−25482

© 2018 American Chemical Society 25473 DOI: 10.1021/acsami.8b08860ACS Appl. Mater. Interfaces 2018, 10, 25473−25482

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Li10MP2S12 against further Li reduction.9 Many solid electro-

lytes comprising multivalent metal cations are prone to formsuch a mixed conducting layer in contact with Li metal andimpedes the application of Li metal in those solid electrolytesystems.10,11 Moreover, the volume changes during Lideposition and dissolution can also deteriorate the interfacebetween Li metal and solid electrolyte due to high Young’smodulus of inorganic solid electrolytes.12

To reduce the interfacial resistance between Li and solidelectrolytes, several effective strategies have been reported inthe literature. (i) Alloy anode acts as an alternative to pure Limetal. For example, Li−In alloy with the potential of 0.62 V vsLi/Li+ is suitable for sulfide solid electrolyte system.13−15 (ii)Double-layer SSE configuration can provide a stable interfacebetween electrodes and SSE layers.16−18 Nevertheless, thesetwo methods often sacrifice the energy density of batteries. (iii)A buffer layer interposed between Li metal and solidelectrolyte can suppress side reaction and promote intimatecontact.19−21 It turns out that to suppress the side reaction atthe Li metal/solid electrolyte interface, an electron-insulatingbut ion-conducting interface layer is critical. However, theinterfacial modification strategies shown in literature areusually highly costly and quite complicated. Thus, it isnecessary to develop simple and efficient methods to modifyand control the solid/solid interfaces conveniently in all solid-state batteries.Ionic liquids (ILs) have attracted great interest in lithium

batteries22−28 for their superior properties, including high ionicconductivity, high chemical stabilities, and especially non-inflammability, high decomposition temperature, and lessvolatility compared to traditional organic electrolyte; but thepoor wettability of the ILs with the separator restricts the ratecharacteristic of the batteries.29 As reported in previousliterature, ionic liquids were already used as wetting agentsto reduce the interfacial impedance22−25 or to preparecomposite polymer solid electrolytes26−28,30 in solid-statebatteries. Liu et al.22 and Oh et al.23 reported that theintroduction of ionic liquid to the cathode composite couldchange the contact mode and provide an ionic conductingnetwork so that it could ensure good contact between solidparticles. In another study, Guo et al.28 preparedLi1+xAlxGe2−x(PO4)3 (LAGP)−poly(vinylidene fluoride-co-hexafluoropropylene)-based solid polymer composite electro-lyte. They speculate that the improved cycle performance ofthe cell may be due to the solid electrolyte interphase (SEI)layer formed by the decomposition of IL. Because the polymerin the composite electrolyte can cover the surface of LAGPparticles and promote the intimate contact between Li andelectrolyte, the effects of ionic liquid are not clear if excludingthe influence of polymer electrolytes on the improvedelectrochemical performance of the cell. So far, there is noreport to further explore the effects of ionic liquid in solid-statebatteries, including but not limited to the wetting agent.Therefore, the role of ionic liquid in solid-state batteries needsfurther investigation with clear evidence.In this work, we propose and demonstrate clearly a novel

and simple approach by utilizing an extremely small amount ofionic liquid to improve the stability at the Li10SnP2S12 (LSPS)solid electrolyte/Li metal interface and reveal the roles of ionicliquid on the interfacial properties in solid-state batteries.Herein, the Pyr13TFSI ionic liquid was employed to modify theinterface between Li metal and LSPS for its advantagesincluding high ionic conductivity, high viscosity, and high

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

2. MATERIALS AND METHODS2.1. Preparation of Materials. 2.1.1. Preparation of 1.5 M

LiTFSI/Pyr13TFSI and 1.5 M LiFSI/Pyr13TFSI Ionic Liquid. N-Propyl-N-methyl pyrrolidinium bis(trifluoromethanesulfonyl) imide(Pyr13TFSI) (Shanghai CHENGJIE Corporation, China) (named asIL 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 wasprepared by dissolving a stoichiometric amount of LiTFSI (providedby Zhuhai Smoothway Electronic Materials Co., Ltd, China) or LiFSIsalt (purchased from NIPPON SHOKUBAI company, Japan) intodried Pyr13TFSI ionic liquid.

2.1.2. Preparation of C/LSPS/C Symmetric Cells. One hundredfifty milligrams of Li10SnP2S12 (NEI Corporation) electrolyte powderwas cold-pressed into a pellet (Φ = 10 mm, 380 MPa), followed bypressing carbon foil as blocking electrodes to both sides of the LSPSpellet. C/LSPS/C symmetric cells were assembled to measure theionic conductivity of the LSPS pellet.

2.1.3. Preparation of Li/Li Symmetric Cells. Li/LSPS/Lisymmetric cells were assembled by attaching Li metal foils to thetwo sides of the LSPS pellet and pressing them by hand in custom-made stainless steel molds. Quasi-solid-state symmetric cells Li/LSPS/Li with ionic liquid were assembled via the same method and10 mg of pure Pyr13TFSI ionic liquid (or 1.5 M LiTFSI/IL, or 1.5 MLiFSI/IL) was spread on both sides of the pellet between Li metal andLSPS pellet. For comparison, the liquid Li/Li symmetric cells wereassembled 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 thebatteries were assembled in a dry glovebox filled with argon gas.

2.1.4. Preparation of Quasi-Solid-State Batteries. The cathodecomposite was prepared using the same method as described in theliterature.22 LiFePO4 (LFPO, BTR Corporation, China) was groundwith acetylene black in a ratio of 75:15 (wt %, LFPO: acetylene black)thoroughly using a mortar. Then, the composite was mixed with 1.5M 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 sideof the LSPS pellet and attached Li metal. The cells without ionicliquid at the anode side were assembled for comparison. The massloading of the LiFePO4 cathode is 2.5−3 mg cm−2. All the batterieswere assembled in a dry glovebox filled with argon gas.

2.2. Electrochemical Measurements. Galvanostatic charge anddischarge measurements of the batteries were conducted on a LANDCT-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) measurementswere carried out by using a Versa STAT MV Multichannelpotentiostat/galvanostat (Princeton Applied Research) from 1 Hzto 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-raydiffractometer using a Cu Kα radiation (λ = 1.5406 Å), and Mylarfilm was used to seal the sample to avoid reactions between the solidelectrolyte and moist air. Rietveld refinement was performed by usingthe General Structure Analysis System (GSAS) software package to

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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 Rietveldrefinement 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.

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get the lattice parameters of the commercial LSPS powder samples.33

Scanning electron microscopy (SEM, Hitachi S-4800) and energy-dispersive spectrometer (EDS) were utilized to characterize themorphology and elemental distribution of the materials. Li metal forSEM and EDS characterization was washed copiously with diethylether and dried under vacuum at RT before the experiment. X-rayphotoelectron spectroscopy (XPS) was carried out on a PHI 5000Versa Probe III spectrometer (ULVAC-PHI, Japan) and etchingexperiments were performed on the sample surface using argon ionbeam gun operating at 25.1 W. The binding energy scale wascalibrated from the hydrocarbon contamination using the C 1s peak at284.8 eV. For SEM and XPS characterizations, an airtight specimenholder is used to avoid moisture and air contamination during sampletransfer.

3. RESULTS AND DISCUSSION

3.1. Characterization of Li10SnP2S12 Material. It can beseen in Figure 1a that the particle size of the LSPS powder isnonuniform and shows a clear aggregation. The thickness andthe diameter of the LSPS pellet are 1 mm and 1 cm (Figure1b,c), respectively. The XRD measurement and Rietveldrefinement are used to identify the phase purity andcrystallinity of Li10SnP2S12 material, respectively, as shown inFigure 1d. The powder XRD pattern indicates that Li10SnP2S12is contaminated with Li2SnS3 impurity phase at 34°, which isconsistent with the result of Ilyas Tarhouchi et al.34 Therefore,two phases were used for the refinement based on Li10SnP2S12structure (proposed by Bron et al.35) and Li2SnS3 structure.And, the impurity Li2SnS3 accounts for 3.8% weight fraction inthe electrolytes we used. Brant et al.36 showed that Li2SnS3 has

a low ionic conductivity of 1.5 × 10−5 S cm−1 at 25 °C andgood stability in air under ambient conditions. Consequently,the existence of Li2SnS3 should decrease the total conductivityof the LSPS material, but improve the stability of LSPS in air.Our results display that Li10SnP2S12 shows a tetragonal crystalstructure (space group P42/nmc (137), a = b = 8.745 Å, c =12.783 Å), close to those proposed in the literature. EISanalysis was performed to obtain the ionic conductivity ofLSPS. As shown in Figure 1e, the C/LSPS/C symmetric cellshows one semicircle (corresponding to the grain boundary)and a spike (corresponding to electrode contributions), andthe intercept is related to the bulk conductivity of 1.3 × 10−3 Scm−1. The total ionic conductivity is 2.2 × 10−4 S cm−1.

3.2. Interfacial Modification at Li Metal/LSPS Electro-lyte Interface with Ionic Liquid. To suppress the sidereaction between Li metal and LSPS solid electrolyte, anextremely small amount of 1.5 M LiTFSI/IL ionic liquid wasadded into the Li/LSPS/Li symmetric cells for interfacialmodification, and such a high concentrated ionic liquid withhigh viscosity can avoid the leakage problem. Therefore, theLi/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. Theoverpotential of the Li/LSPS/Li symmetric cell increasesgradually with time (Figure 2a), revealing a continuousreaction between Li and LSPS. The symmetric cell with 1.5M LiTFSI/IL, in contrast, exhibits a stable cycling life of over1000 h and the overpotential of Li+ platting/stripping remainssmaller than 50 mV at a current density of 0.038 mA cm−2,

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 differentstorage 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.

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indicating that a stable interface might be formed between Liand LSPS. Although, the overpotential of the cell increases at ahigher current density of 0.115 mA cm−2, the cell stillmaintains stable cycling for over 350 h (Figure 2b). Moreover,the interfacial resistance of symmetric cell (Figure 2d) isdecreased from 1960 to 250 Ω with the addition of 1.5 MLiTFSI/IL on account of the fact that ionic liquid provides anionic conducting network and changes the solid−solid contactinto the solid−liquid contact.Figure S2a,b denotes the morphology of the LSPS pellet

with and without ionic liquid. Significant voids are observed onthe 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 willmake it very difficult to form a stable and intimate contact withLi metal anode in all solid-state batteries and thus can lead tohigh interfacial resistance. As shown in Figure S2b, the voids ofthe LSPS pellet are filled with ionic liquid. The EDS results(Figure S2c−f) reveal that the ionic liquid can cover thesurface of the electrolyte particles uniformly and exhibits agood interfacial wettability. Furthermore, the high viscosity ofthe ionic liquid can make sure it attaches itself to the surface ofthe LSPS pellet and prevents liquid leakage.Time-resolved impedance spectrum measurements were

performed on the symmetric cells with or without 1.5 MLiTFSI/IL ionic liquid to confirm whether the ionic liquid cansuppress the chemical reaction between Li metal and LSPS ornot (Figure 3). The Li/LSPS/Li symmetric cells with andwithout ionic liquid were stored in air (but it cannot react withair or moisture because of the airtight molds) under roomtemperature and EIS measurement was conducted at different

days of storage. It is evident that both cells exhibit an apparentoverall increase in cell resistance with increase in the storagetime. However, the Li/LSPS/Li cell (Figure 3a) shows adramatic overall increase in resistance and exhibits a muchhigher interfacial resistance than the Li/LSPS/Li cell with 1.5M LiTFSI/IL (Figure 3c) after a long time aging. After storage,both symmetric cells were carried out with galvanostaticcycling experiment. The Li/LSPS/Li cell (Figure 3b) showsnoticeably high polarization potential, whereas the cell with 1.5M ionic liquid (Figure 3d) maintains a stable cycling. Thisdifference is associated with the unstable interface between Limetal and LSPS pellet. Chemical side reactions happen at theLi metal/LSPS electrolyte interface, and the interface layerformed after storage with high resistance deteriorates theelectrochemical performance of the batteries. In contrast, themodest increase in the resistance of the Li/LSPS/Li cell with1.5 M LiTFSI/IL means that a stable surface layer rather than agrowing and unstable interface layer may be formed at theinterface with the addition of LiTFSI/IL ionic liquid additiveand suppresses further side reaction during cycling.

3.3. Mechanistic Study on the Improved Stability atLi/LSPS Interface with Ionic Liquid. To figure out why theionic liquid can improve the cycling performance of the cells,XPS was utilized to analyze the change in the surface of theLSPS pellet and Li metal. The results presented in Figure 4a,bdisplay that the peaks at 163 and 162 eV in the S 2p spectracan be assigned to the bridging sulfur atoms (P−S−P groups)and terminal sulfur atoms (PS groups), respectively.37 Whencompared to the pristine LSPS pellet, the surface of the LSPSpellet reacts with Li metal during cycling and generates Li2S asthe undesirable product observed at 161 eV, consistent with

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) C1s 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.)

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the result in the literature.6 Chemical instability of the LSPSsulfide solid electrolyte against Li metal induces the growth ofthe interface layer with increase in time and then causes anoticeable increase in impedance. For the LSPS pellet with 1.5M LiTFSI/IL ionic liquid after cycling (Figures 4c and S4), thepeaks in the S 2p spectra are related to the ionic liquid, and noobvious peaks of the side reactions products and LSPSmaterials are observed on the LSPS electrolyte for the coverageof an ionic liquid.To exclude the influence of coverage of an ionic liquid and

get more structural information of the decomposed com-pounds underneath, XPS etching experiments were conductedon the surface of Li metal with and without 1.5 M LiTFSI/ILionic liquid after cycling. The C 1s and F 1s spectra in Figure4d indicate that when compared to the Li metal without ionicliquid, the presence of −CF3 and LiF on the surface of Li metalwith 1.5 M LiTFSI/IL ionic liquid after cycling is associatedwith the decomposition products of TFSI− anion, respectively.As reported in the literature,38 LiF is the dominatedcomponent of the SEI layer formed on the surface of Limetal in LiTFSI/Pyr1xTFSI (x = 3, 4) ionic liquid. Moreover,the EDS result (Figure S3) confirms the uniform distributionof F element on the surface of Li metal, further demonstratingthe existence of a SEI layer on Li metal. A SEI layer comprisingof TFSI− anion reduction products is already formed on thesurface of Li metal and hence suppresses the side reactions atthe Li/LSPS interface. Such an observation supports the resultsof the electrochemical cycling behavior (Figure 2a) andNyquist plots (Figure 3a,c) of symmetric cells. In addition,XPS experiment under the same conditions was also conductedon Li metal with 1.5 M LiTFSI/IL without cycling. The resultsin Figure S5 indicate that the SEI layer is formed on the surfaceof Li metal even without cycling, but the SEI layer is muchthinner than that formed after cycling. Therefore, the thin SEIformed after Li metal comes in contact with the ionic liquidcan prevent the direct contact between Li metal and LSPSbefore cycling, which also accounts for the results in Figure3c,d.For the purpose of exploring whether this hybrid solid−

liquid electrolyte system can show better electrochemicalperformance than ionic liquid system or not, we compared theLi+ stripping/plating curves of the Li/LSPS/Li quasi-solid-statesymmetric cell with 1.5 M LiTFSI/IL and Li/Li liquidsymmetric cell with 1.5 M LiTFSI/IL. The results in FigureS6 show that during each alternating periods of charge and

discharge, steady Li+ stripping/plating plateau appears rapidlyin the quasi-solid-state symmetric cell Li/LSPS/Li with 1.5 MLiTFSI/IL. By comparison, the curve of the liquid symmetriccell is serrated and the polarization voltage is slightly higherthan that of the quasi-solid-state symmetric cell. The electricaldouble layer (EDL) of ionic liquid-electrode is complex andconsists of multilayers of ions (anion + cation).39 We speculatethat the special serrated curve in the liquid Li/Li symmetriccell may be ascribed to the charge redistribution andreorientational movement of Pyr13

+ cation at the EDL.40 Asfor the quasi-solid-state symmetric cell Li/LSPS/Li with 1.5 MLiTFSI/IL, LSPS solid electrolyte in contact with lithiummetal destroys the EDL and provides channel to Li iontransport directly for its high ionic conductivity, accounting forthe flat and rectangular Li+ stripping/plating curves. Besides,considering the improved stability of LSPS against Li metal byadding ionic liquid and the good wettability of ionic liquid inthe LSPS electrolyte, a combination of LSPS with ionic liquidis a good way to achieve a whole greater than the sum of itsparts.

3.4. Electrochemical Performance of LiFePO4/LSPS/LiBatteries with and without Interfacial Modification atthe Anode Side. Based on this interfacial modificationmethod, quasi-solid-state batteries with commercial LiFePO4as the cathode, LSPS as the electrolyte, and Li metal as theanode were assembled. Figure 5 shows the electrochemicalperformance of the batteries assembled with and without 1.5M LiTFSI/IL at the anode side. It is clearly shown that the cellwith 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 capacityof 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 liquidshows a discharge capacity retention of 84.7% (Figure 5b) anda steady Coulombic efficiency after 30 cycles, whereas thecapacity of the cell without ionic liquid decays rapidly within10 cycles. Because the instability between Li metal and LSPSpellet deteriorates, so does the cycle performance of thebatteries. Therefore, a small amount of 1.5 M LiTFSI/IL at theanode side can significantly improve the electrochemicalperformance of the LiFePO4/LSPS/Li batteries. And, thelong cycle performance of the cell needs further improvement

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 of0.1C (14 mA g−1) under room temperature. (a) The initial charge and discharge curves and (b) cycle performance.

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by modification at the cathode side for the instability of LSPSat a high potential.41

3.5. Effect of Lithium Salt on the Improved Electro-chemical Performance of Batteries. Additionally, toinvestigate the effect of different lithium salts on the improvedinterfacial stability at the Li metal/LSPS solid electrolyteinterface, we also assembled the Li/LSPS/Li symmetric cellwith pure Pyr13TFSI or 1.5 M LiFSI/Pyr13TFSI ionic liquid forcomparison (symmetric cells with pure Pyr13TFSI, 1.5 MLiTFSI/Pyr13TFSI, or 1.5 M LiFSI/Pyr13TFSI are labeled asCell-pure, Cell-1.5 M LiTFSI, and Cell-1.5 M LiFSI,respectively). As shown in Figure 6, Cell-pure exhibits a stable

cycle initially, but the polarization of symmetric cell shows arapid increase after cycling for several hundred hours, withboth charge and discharge voltages reaching over 0.5 V,indicating the unstable interface between Li metal and solidelectrolyte. However, for Cell-1.5 M LiFSI, the potentialincreases gradually with time, and the resistance increasesnoticeably during cycling compared to that of Cell-1.5 MLiTFSI (Figures S9 and S10). This indicates the continuousgrowth and the highly resistive nature of the SEI layer formedin the presence of LiFSI. It is interesting to find that the ionic

conductivities of 1.5 M LiTFSI/IL and 1.5 M LiFSI/IL are0.543 and 0.778 mS cm−1, respectively. The higher ionicconductivity of 1.5 M LiFSI/IL corresponds to its lowerviscosity. Unlike the results in quasi-solid-state symmetric cells,the liquid Li/Li symmetric cells with 1.5 M LiTFSI/IL or 1.5M LiFSI/IL shows a stable Li+ stripping/plating performance(Figure S8). These differences in Li+ stripping/plating curvesbetween the quasi-solid-state system and the liquid system maybe due to the amount of ionic liquid and thus influence theresistance 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 differentetching time under the same conditions are compared inFigure 7. The F 1s spectra after 60 and 120 s etching timereveal that the amount of LiF in Cell-pure is far less than thatin Cell-1.5 M LiTFSI. Moreover, the intensity of the peakcorresponding to LiF in Cell-pure symmetric cell decreaseswith the etching time. The results suggest that the SEI layerformed in Cell-1.5 M LiTFSI symmetric cells is thicker andmore stable. Because the Li metal anode undergoes largevolume changes during lithiation and delithiation, the SEI layeris under dynamic strain. A thin SEI layer is easy to crack duringlong cycling, whereas a stable SEI layer can maintain a goodinterface between Li metal and LSPS solid electrolyte. Hence,the addition of LiTFSI salt promotes the formation of a stableSEI layer, facilitating the long-term cycling stability of Cell-1.5M LiTFSI. However, Figure 7c presents that the SEI layerformed in the presence of LiFSI salt is richer in LiF than thatformed in the LiTFSI-containing or pure Pyr13TFSI ionicliquid. And, the peak of −CF3 corresponds to thedecomposition of TFSI− anion from Pyr13TFSI. It is knownthat LiFSI salt provides a more donatable fluorine than LiTFSIsalt and thus the number of LiF generated per LiFSI moleculeis higher than that generated per LiTFSI molecule.42 It isworth noting that excessive LiF will cause an increase in theresistance due to its low ionic conductivity43 and ultimatelydeteriorate the cycle performance of the symmetric cell.Because Li metal shows intrinsically high chemical reactivityand low thermal stability, it is hard to characterize themorphology of the SEI layer without damaging by conven-tional transmission electron microscopy (TEM).44,45 Herein,we deduce a possible schematic illustration of the interfacialmodification mechanism based on the results of the electro-

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 MLiFSI/IL, and (red) Li/LSPS/Li with pure Pyr13TFSI at the currentdensity of 0.038 mA cm−2 under room temperature.

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 MLiTFSI/Pyr13TFSI)/Li, and (c) Li/(LSPS + 1.5 M LiFSI/Pyr13TFSI)/Li symmetric cells after cycling for 140 h.

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chemical performance and XPS analysis results as shown inFigure 8.

4. CONCLUSIONSIn conclusion, a novel and simple approach to stabilize theinterface between Li metal and Li10SnP2S12 sulfide solidelectrolyte is successfully demonstrated in this work. The cycleperformance of the Li/LSPS/Li symmetric cells at roomtemperature is enhanced greatly by the addition of anextremely small amount of 1.5 M LiTFSI/Pyr13TFSI ionicliquid. Furthermore, the LFPO/LSPS/Li quasi-solid-statebatteries with 1.5 M LiTFSI/Pyr13TFSI ionic liquid at theanode side shows a much higher initial discharge capacity andretention after cycling compared to batteries without ionicliquid. Therefore, the significantly enhanced interfacial stabilitybetween Li metal and LSPS sulfide solid electrolyte is achievedby in situ forming a SEI layer on Li metal as a passivation layer.In addition, when compared to the pure Pyr13TFSI ionicliquid, LiTFSI salt in Pyr13TFSI ionic liquid can form a morestable SEI layer, enabling stable Li deposition/dissolution.However, the presence of LiFSI salt leads to the enrichment ofLiF in the SEI layer and thus deteriorates the cycleperformance of the symmetric cell. Our new strategy providesa new concept of overcoming the inherent shortcomings of ILsand sulfide solid electrolyte, such as poor wettability of ILsagainst separator and the instability of sulfide solid electrolyteagainst Li metal, enabling the application of ILs and sulfidesolid electrolyte in the practical use in lithium batteries.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.8b08860.

Schematic of (quasi) solid-state symmetric cell, SEMimages and EDS elemental mapping results of thesurface of pristine LSPS pellet with and without ionicliquid and Li metal with ionic liquid, Sn 3d XPS spectraof LSPS pellet with ionic liquid after cycling and F 1sXPS spectra of Li metal with ionic liquid without or aftercycling, galvanostatic charge and discharge performanceof Li/LSPS/Li symmetric cell with ionic liquid andliquid Li−Li symmetric cell, Coulombic efficiency andNyquist plots of batteries of LFPO/LSPS/Li batterieswith and without 1.5 M LiTFSI/IL at the anode side and

Nyquist plots of symmetric cells Li/LSPS/Li with

different Li salts after cycling (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: yyang@xmu.edu.cn. Tel/Fax: +86 592 2185753.ORCIDBizhu Zheng: 0000-0002-2744-394XAuthor ContributionsThe manuscript was written through contributions of all theauthors. All the authors have given approval to the final versionof the manuscript.FundingNational Key Research and Development Program of China,National Natural Science Foundation of China.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors thank Yuanjun Shao for helpful discussion. Thiswork was financially supported by National Key Research andDevelopment Program of China (grant no. 2016YFB0901502and 2018YFB0905400) and National Natural ScienceFoundation of China (grant nos. 21233004, 21473148, and21621091).

■ REFERENCES(1) Bachman, J. C.; Muy, S.; Grimaud, A.; Chang, H. H.; Pour, N.;Lux, S. F.; Paschos, O.; Maglia, F.; Lupart, S.; Lamp, P.; Giordano, L.;Shao-Horn, Y. Inorganic Solid-State Electrolytes for LithiumBatteries: Mechanisms and Properties Governing Ion Conduction.Chem. Rev. 2016, 116, 140−162.(2) Kamaya, N.; Homma, K.; Yamakawa, Y.; Hirayama, M.; Kanno,R.; Yonemura, M.; Kamiyama, T.; Kato, Y.; Hama, S.; Kawamoto, K.;Mitsui, A. A Lithium Superionic Conductor. Nat. Mater. 2011, 10,682−686.(3) Kato, Y.; Hori, S.; Saito, T.; Suzuki, K.; Hirayama, M.; Mitsui, A.;Yonemura, M.; Iba, H.; Kanno, R. High-Power All-Solid-StateBatteries using Sulfide Superionic Conductors. Nat. Energy 2016, 1,No. 16030.(4) Takada, K.; Ohta, N.; Tateyama, Y. Recent Progress inInterfacial Nanoarchitectonics in Solid-State Batteries. J. Inorg.Organomet. Polym. 2014, 25, 205−213.(5) Jung, Y. S.; Oh, D. Y.; Nam, Y. J.; Park, K. H. Issues andChallenges for Bulk-Type ALl-Solid-State Rechargeable Lithium

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

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.8b08860ACS Appl. Mater. Interfaces 2018, 10, 25473−25482

25480

Batteries using Sulfide Solid Electrolytes. Isr. J. Chem. 2015, 55, 472−485.(6) Ong, S. P.; Mo, Y.; Richards, W. D.; Miara, L.; Lee, H. S.; Ceder,G. Phase Stability, Electrochemical Stability and Ionic Conductivity ofThe Li10±1MP2X12(M = Ge, Si, Sn, Al or P, and X = O, S or Se)Family of Superionic Conductors. Energy Environ. Sci. 2013, 6, 148−156.(7) Han, F.; Zhu, Y.; He, X.; Mo, Y.; Wang, C. ElectrochemicalStability of Li10GeP2S12 and Li7La3Zr2O12 Solid Electrolytes. Adv.Energy Mater. 2016, 6, No. 1501590.(8) Wenzel, S.; Randau, S.; Leichtweiß, T.; Weber, D. A.; Sann, J.;Zeier, W. G.; Janek, J. Direct Observation of the Interfacial Instabilityof the Fast Ionic Conductor Li10GeP2S12at the Lithium Metal Anode.Chem. Mater. 2016, 28, 2400−2407.(9) Wenzel, S.; Leichtweiss, T.; Kruger, D.; Sann, J.; Janek, J.Interphase Formation on Lithium Solid ElectrolytesAn In SituApproach to Study Interfacial Reactions by Photoelectron Spectros-copy. Solid State Ionics 2015, 278, 98−105.(10) Hartmann, P.; Leichtweiss, T.; Busche, M. R.; Schneider, M.;Reich, M.; Sann, J.; Adelhelm, P.; Janek, J. Degradation ofNASICON-Type Materials in Contact with Lithium Metal:Formation of Mixed Conducting Interphases (MCI) on SolidElectrolytes. J. Phys. Chem. C 2013, 117, 21064−21074.(11) Chung, H.; Kang, B. Mechanical and Thermal Failure Inducedby Contact between a Li1.5Al0.5Ge1.5(PO4)3 Solid Electrolyte and LiMetal in an All Solid-State Li Cell. Chem. Mater. 2017, 29, 8611−8619.(12) Zhang, W.; Schroder, D.; Arlt, T.; Manke, I.; Koerver, R.;Pinedo, R.; Weber, D. A.; Sann, J.; Zeier, W. G.; Janek, J.(Electro)chemical Expansion during Cycling: Monitoring thePressure Changes in Operating Solid-State Lithium Batteries. J.Mater. Chem. A 2017, 5, 9929−9936.(13) Kanno, R.; Murayama, M.; Inada, T.; Kobayashi, T.; Sakamoto,K.; Sonoyama, N.; Yamada, A.; Kondo, S. A Self-Assembled BreathingInterface for All-solid-state Ceramic Lithium Batteries. Electrochem.Solid-State Lett. 2004, 7, A455−A458.(14) Ohta, N.; Takada, K.; Sakaguchi, I.; Zhang, L.; Ma, R.; Fukuda,K.; Osada, M.; Sasaki, T. LiNbO3-Coated LiCoO2 as CathodeMaterial for All Solid-State Lithium Secondary Batteries. Electrochem.Commun. 2007, 9, 1486−1490.(15) Sakuma, M.; Suzuki, K.; Hirayama, M.; Kanno, R. Reactions atthe Electrode/Electrolyte Interface of All-Solid-State LithiumBatteries Incorporating Li−M (M = Sn, Si) Alloy Electrodes andSulfide-Based Solid Electrolytes. Solid State Ionics 2016, 285, 101−105.(16) Woo, J. H.; Trevey, J. E.; Cavanagh, A. S.; Choi, Y. S.; Kim, S.C.; George, S. M.; Oh, K. H.; Lee, S. H. Nanoscale InterfaceModification of LiCoO2 by Al2O3 Atomic Layer Deposition for Solid-State Li Batteries. J. Electrochem. Soc. 2012, 159, A1120−A1124.(17) Trevey, J. E.; Jung, Y. S.; Lee, S.-H. High Lithium IonConducting Li2S−GeS2−P2S5 Glass−Ceramic Solid Electrolyte withSulfur Additive for All Solid-State Lithium Secondary Batteries.Electrochim. Acta 2011, 56, 4243−4247.(18) Yao, X.; Huang, N.; Han, F.; Zhang, Q.; Wan, H.; Mwizerwa, J.P.; Wang, C.; Xu, X. High-Performance All-Solid-State Lithium−Sulfur Batteries Enabled by Amorphous Sulfur-Coated ReducedGraphene Oxide Cathodes. Adv. Energy Mater. 2017, 7, No. 1602923.(19) Zhang, Z.; Zhao, Y.; Chen, S.; Xie, D.; Yao, X.; Cui, P.; Xu, X.An Advanced Construction Strategy of All-Solid-State LithiumBatteries with Excellent Interfacial Compatibility and UltralongCycle Life. J. Mater. Chem. A 2017, 5, 16984−16993.(20) Liu, B.; Gong, Y.; Fu, K.; Han, X.; Yao, Y.; Pastel, G.; Yang, C.;Xie, H.; Wachsman, E. D.; Hu, L. Garnet Solid Electrolyte ProtectedLi-Metal Batteries. ACS Appl. Mater. Interfaces 2017, 9, 18809−18815.(21) Zhang, Z.; Chen, S.; Yang, J.; Wang, J.; Yao, L.; Yao, X.; Cui, P.;Xu, X. Interface Re-Engineering of Li10GeP2S12 Electrolyte andLithium Anode for All-Solid-State Lithium Batteries with UltralongCycle Life. ACS Appl. Mater. Interfaces 2018, 10, 2556−2565.

(22) Liu, L.; Qi, X.; Ma, Q.; Rong, X.; Hu, Y. S.; Zhou, Z.; Li, H.;Huang, X.; Chen, L. Toothpaste-like Electrode: A Novel Approach toOptimize the Interface for Solid-State Sodium-Ion Batteries withUltralong Cycle Life. ACS Appl. Mater. Interfaces 2016, 8, 32631−32636.(23) Oh, D. Y.; Nam, Y. J.; Park, K. H.; Jung, S. H.; Cho, S.-J.; Kim,Y. K.; Lee, Y.-G.; Lee, S.-Y.; Jung, Y. S. Excellent Compatibility ofSolvate Ionic Liquids with Sulfide Solid Electrolytes: TowardFavorable Ionic Contacts in Bulk-Type All-Solid-State Lithium-IonBatteries. Adv. Energy Mater. 2015, 5, No. 1500865.(24) Ito, S.; Unemoto, A.; Ogawa, H.; Tomai, T.; Honma, I.Application of Quasi-Solid-State Silica Nanoparticles−Ionic LiquidComposite Electrolytes to All-Solid-State Lithium Secondary Battery.J. Power Sources 2012, 208, 271−275.(25) Zhang, Z.; Zhang, Q.; Shi, J.; Chu, Y. S.; Yu, X.; Xu, K.; Ge, M.;Yan, H.; Li, W.; Gu, L.; Hu, Y.-S.; Li, H.; Yang, X.-Q.; Chen, L.;Huang, X. A Self-Forming Composite Electrolyte for Solid-StateSodium Battery with Ultralong Cycle Life. Adv. Energy Mater. 2017, 7,No. 1601196.(26) Osada, I.; Hosseini, S. M.; Jeong, S.; Passerini, S. Novel TernaryPolymer Electrolytes Based on Poly(lactic acid) from SustainableSources. ChemElectroChem 2017, 4, 463−467.(27) Wang, Z.; Tan, R.; Wang, H.; Yang, L.; Hu, J.; Chen, H.; Pan,F. A Metal- Organic-Framework-Based Electrolyte with NanowettedInterfaces for High-Energy-Density Solid-State Lithium Battery. Adv.Mater. 2018, 30, No. 1704436.(28) Guo, Q.; Han, Y.; Wang, H.; Xiong, S.; Li, Y.; Liu, S.; Xie, K.New Class of LAGP-Based Solid Polymer Composite Electrolyte forEfficient and Safe Solid-State Lithium Batteries. ACS Appl. Mater.Interfaces 2017, 9, 41837−41844.(29) Watanabe, M.; Thomas, M. L.; Zhang, S.; Ueno, K.; Yasuda, T.;Dokko, K. Application of Ionic Liquids to Energy Storage andConversion Materials and Devices. Chem. Rev. 2017, 117, 7190−7239.(30) Choi, H.; Kim, H. W.; Ki, J.-K.; Lim, Y. J.; Kim, Y.; Ahn, J.-H.Nanocomposite Quasi-Solid-State Electrolyte for High-Safety LithiumBatteries. Nano Res. 2017, 10, 3092−3102.(31) MacFarlane, D. R.; Meakin, P.; Sun, J.; Amini, N.; Forsyth, M.Pyrrolidinium Imides: A New Family of Molten Salts and ConductivePlastic Crystal Phases. J. Phys. Chem. B 1999, 103, 4164−4170.(32) Howlett, P. C.; MacFarlane, D. R.; Hollenkamp, A. F. HighLithium Metal Cycling Efficiency in a Room-Temperature IonicLiquid. Electrochem. Solid-State Lett. 2004, 7, A97−A101.(33) Toby, B. H. EXPGUI, a Graphical User Interface for GSAS. J.Appl. Crystallogr. 2001, 34, 210−213.(34) Tarhouchi, I.; Viallet, V.; Vinatier, P.; Menetrier, M.Electrochemical Characterization of Li10SnP2S12: An Electrolyte or aNegative Electrode for Solid State Li-Ion Batteries? Solid State Ionics2016, 296, 18−25.(35) Bron, P.; Johansson, S.; Zick, K.; Schmedt auf der Gunne, J.;Dehnen, S.; Roling, B. Li10SnP2S12: An Affordable Lithium SuperionicConductor. J. Am. Chem. Soc. 2013, 135, 15694−15697.(36) Brant, J. A.; Massi, D. M.; Holzwarth, N. A. W.; MacNeil, J. H.;Douvalis, A. P.; Bakas, T.; Martin, S. W.; Gross, M. D.; Aitken, J. A.Fast Lithium Ion Conduction in Li2SnS3: Synthesis, PhysicochemicalCharacterization, and Electronic Structure. Chem. Mater. 2014, 27,189−196.(37) Auvergniot, J.; Cassel, A.; Foix, D.; Viallet, V.; Seznec, V.;Dedryvere, R. Redox Activity of Argyrodite Li6PS5Cl Electrolyte inAll-Solid-State Li-Ion Battery: An XPS Study. Solid State Ionics 2017,300, 78−85.(38) Howlett, P. C.; Brack, N.; Hollenkamp, A. F.; Forsyth, M.;MacFarlane, D. R. Characterization of the Lithium Surface in N-Methyl-N-Alkylpyrrolidinium Bis(trifluoromethanesulfonyl)AmideRoom-Temperature Ionic Liquid Electrolytes. J. Electrochem. Soc.2006, 153, A595−A606.(39) Kornyshev, A. A. Double-Layer in Ionic Liquids: ParadigmChange? J. Phys. Chem. B 2007, 111, 5545−5557.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.8b08860ACS Appl. Mater. Interfaces 2018, 10, 25473−25482

25481

(40) Roling, B.; Druschler, M.; Huber, B. Slow and Fast CapacitiveProcess Taking Place at the Ionic Liquid/Electrode Interface. FaradayDiscuss. 2012, 154, 303−311.(41) Richards, W. D.; Miara, L. J.; Wang, Y.; Kim, J. C.; Ceder, G.Interface Stability in Solid-State Batteries. Chem. Mater. 2016, 28,266−273.(42) Suo, L.; Xue, W.; Gobet, M.; Greenbaum, S. G.; Wang, C.;Chen, Y.; Yang, W.; Li, Y.; Li, J. Fluorine-Donating ElectrolytesEnable Highly Reversible 5-V-Class Li Metal Batteries. Proc. Natl.Acad. Sci. U. S. A. 2018, 115, 1156−1161.(43) Pan, J.; Cheng, Y.-T.; Qi, Y. General Method to PredictVoltage-Dependent Ionic Conduction in A Solid Electrolyte Coatingon Electrodes. Phys. Rev. B 2015, 91, No. 134116.(44) Wang, X.; Zhang, M.; Alvarado, J.; Wang, S.; Sina, M.; Lu, B.;Bouwer, J.; Xu, W.; Xiao, J.; Zhang, J. G.; Liu, J.; Meng, Y. S. NewInsights on the Structure of Electrochemically Deposited LithiumMetal and Its Solid Electrolyte Interphases via Cryogenic TEM. NanoLett. 2017, 17, 7606−7612.(45) Li, Y.; Li, Y.; Pei, A.; Yan, K.; Sun, Y.; Wu, C.-L.; Joubert, L.-M.; Chin, R.; Koh, A. L.; Yu, Y.; Perrino, J.; Butz, B.; Chu, S.; Cui, Y.Atomic Structure of Sensitive Battery Materials and InterfacesRevealed by Cryo-Electron Microscopy. Science 2017, 358, 506−510.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.8b08860ACS Appl. Mater. Interfaces 2018, 10, 25473−25482

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