Poly(ethylene oxide)−Li10SnP2S12 Composite Polymer ElectrolyteEnables High-Performance All-Solid-State Lithium Sulfur BatteryXue Li,† Donghao Wang,† Hongchun Wang,† Hefeng Yan,† Zhengliang Gong,*,† and Yong Yang†,‡

†College of Energy and ‡State Key Laboratory for Physical Chemistry of Solid Surface, Department of Chemistry, College ofChemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People’s Republic of China

*S Supporting Information

ABSTRACT: Composite polymer electrolyte membranes arefabricated by the incorporation of Li10SnP2S12 into thepoly(ethylene oxide) (PEO) matrix using a solution-castingmethod. The incorporation of Li10SnP2S12 plays a positive roleon Li-ionic conductivity, mechanical property, and interfacialstability of the composite electrolyte and thus significantlyenhances the electrochemical performance of the solid-stateLi−S battery. The optimal PEO−1%Li10SnP2S12 electrolytepresents a maximum ionic conductivity of 1.69 × 10−4 S cm−1

at 50 °C and the highest mechanical strength. The possiblemechanism for the enhanced electrochemical performance andmechanical property is analyzed. The uniform distribution ofLi10SnP2S12 in the PEO matrix inhibits crystallization andweakens the interactions among the PEO chains. The PEO−1%Li10SnP2S12 electrolyte exhibits lower interfacial resistance and higher interfacial stability with the lithium anode than thepure PEO/LiTFSI electrolyte. The Li−S cell comprising the PEO−1%Li10SnP2S12 electrolyte exhibits outstandingelectrochemical performance with a high discharge capacity (ca. 1000 mA h g−1), high Coulombic efficiency, and goodcycling stability at 60 °C. Most importantly, the PEO−1%Li10SnP2S12-based cell possesses attractive performance with a highspecific capacity (ca. 800 mA h g−1) and good cycling stability even at 50 °C, whereas the PEO/LiTFSI-based cell cannot besuccessfully discharged because of the low ionic conductivity and high interfacial resistance of the PEO/LiTFSI electrolyte.

KEYWORDS: Li−S batteries, solid polymer electrolyte, polyethylene oxide, sulfide lithium ionic conductor, interfacial stability

■ INTRODUCTION

Lithium−sulfur (Li−S) batteries have attracted extensiveattention as promising alternatives to the current lithium-ionbatteries (LIBs) because of their high theoretical energydensities and low cost.1−3 Despite the aforementionedadvantages, the development of high performance Li−Sbatteries has been impeded by the electrical insulating natureand poor ionic conductivity of elemental sulfur and itsdischarge product Li2S.

4−7 To overcome these drawbacks,numerous research works have been conducted. The electronicconductivity of the sulfur cathode has been enhanced byincorporating sulfur with a conductive matrix (such as carbon,polymers, and inorganic compounds).8−13 The poor ionicconductivity of the sulfur cathode is addressed by using ether-based electrolytes with good solubility for lithium polysulfide,which is favorable for the circulation of a conventional Li−Sbattery, as the electrochemical reactions at the sulfur electrodemainly take place at the “liquid−solid” interface of thedissolved lithium polysulfides and on the surface of theconducting matrix.14−18 Despite the significant improvementsin the electrochemical performance, the practical application ofconventional Li−S batteries is still obstructed by several criticalissues, such as the lithium polysulfide shuttle effect, long-term

stability of the lithium metal anode with organic liquidelectrolytes, and safety concerns related to the lithium anodeand the liquid electrolyte.19−21 All-solid-state lithium batteriesare considered to be most promising to address the safetychallenges of LIBs.22−26 Especially, poly(ethylene oxide)(PEO)-based solid polymer electrolytes (SPEs) have beenextensively investigated because of their superior advantages,such as good flame-resistance, no leakage, flexible geometry,and low cost in design as well as their light weight.27−30 Itprovides an easily achievable approach to surpass the energydensity of current LIBs by using SPEs with membranethickness below 100 μm.31

Normally, SPE membranes are fabricated by dissolving thelithium salt into the PEO matrix. Among the lithium salts,lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is themost commonly used conducting salt for PEO-based SPEs.However, all-solid-state Li−S batteries with PEO/LiTFSI-based SPEs cannot be cycled normally with severe overchargewhen using a nonmodified element sulfur-based cathode,

Received: March 24, 2019Accepted: June 3, 2019Published: June 3, 2019

Research Article

www.acsami.orgCite This: ACS Appl. Mater. Interfaces 2019, 11, 22745−22753

© 2019 American Chemical Society 22745 DOI: 10.1021/acsami.9b05212ACS Appl. Mater. Interfaces 2019, 11, 22745−22753

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owing to the dissolution of polysulfides into the PEOelectrolyte and the poor interfacial stability of the PEO/LiTFSI electrolyte against the lithium metal, which cannotwithstand the corrosion of polysulfide species.32,33 To improvethe stability of the Li/electrolyte interface, extensive studieshave been devoted to electrolyte modifications, such as usinglithium salt [e.g., lithium bis(fluorosulfonyl)imide (LiFSI) andlithium(trifluoromethanesulfonyl)(n-nonafluorobutane-sulfonyl)imide (LiTNFSI)] with good compatibility with thelithium metal and dispersing inorganic ceramic fillers (e.g.,Al2O3, TiO2, SiO2, LiAlO2, etc.) into the PEO matrix.34−36

However, LiFSI and LiTNFSI are much more expensive thanLiTFSI, which would increase the cost of batteries. Also, theinorganic ceramic fillers have higher density than PEO, whichdecrease the energy density of batteries.Recently, inorganic Li+ conductors, such as oxides [e.g.,

Li1.3Al0.3Ti1.7(PO4)3, Li0.33La0.557TiO3, Li1.4Al0.4Ge1.7(PO4)3, Ligarnet etc.]37−40 and sulfides (e.g . , Li3PS4 andLi10GeP2S12),

41−44 have been widely used as fillers to fabricatethe composite polymer electrolytes. Compared with inactivefillers (e.g., Al2O3 and SiO2), Li

+ conductors can contribute tothe ionic conductivity, interfacial stability, and mechanicalproperty of the SPEs.41,42 Among them, sulfides have lowerdensity (∼2.0 g cm−3) than oxides (>4.0 g cm−3) and highionic conductivity (∼10−3 S cm−1).43,45 As fillers, they cansubstantially improve the electrochemical performance of SPEswithout severe sacrifice on the energy density.46 Moreover,sulfide-based ionic conductors can be dissolved in organicsolvents; particularly, Li10SnP2S12 is soluble in acetonitrile(ACN), which is beneficial for fabricating uniformly dispersednanofiller-incorporated composite electrolyte membranes by asolution-casting process.With these considerations, herein, the PEO−Li10SnP2S12

(LSPS) inorganic polymer composite membranes are inves-tigated as polymer electrolytes for all-solid-state Li−S batteriesfor the first time. The composite electrolytes with variousLi10SnP2S12 contents are prepared through incorporating LSPSas a filler into the PEO matrix by a simple solution-castingmethod. The effects of LSPS on the ionic conductivity andphysicochemical properties of PEO−LSPS SPEs are inves-tigated. Our results demonstrate that PEO−LSPS SPEs exhibitan enhanced ionic conductivity and improved interfacialstability. Therefore, the all-solid-state Li−S batteries with theSPEs show superior electrochemical performance, with highcapacity, good capacity retention, and high Coulombicefficiency.

■ EXPERIMENTAL SECTIONPreparation of Solid-State Composite Polymer Electrolytes.

The pure PEO and PEO−LSPS composite electrolyte membraneswere fabricated via a conventional solution-casting method. PEO (Mw= 600 000, 99.9%, Aldrich) was dried at 60 °C for 24 h, and LiTFSI(99.9%, Aldrich) was dried at 100 °C for 24 h in vacuum before use.The procedures are sensitive to water and oxygen; so all experimentsmust be carried out in the Ar-filled glovebox with H2O and O2contents below 1 ppm. PEO and LiTFSI were dissolved into ACNwith the molar ratio of −CH2−CH2O−(EO)/Li+ (EO/Li+ = 20).Then, the LSPS (NEI Corporation) powder with different mass ratios(1 or 3%) was added to the previous solution. After stirring at roomtemperature for 24 h, the resulting solution was casted onto a Teflonplate and then dried with 4 Å molecular sieves for 48 h. Finally, theSPE membranes with a thickness ∼70 μm were obtained. Theuniform thin films were peeled off and punched into 10 and 19 mmdiameter membranes for further measurements.

Sulfur Electrodes Preparation. Elemental sulfur and acetyleneblack (AB) were dried at 100 °C for 24 h before use. Compositesulfur cathodes consist of S, AB, and LiTFSI/PEO electrolyte with theweight ratio of 40:15:45. To prepare the cathode laminates, themixture of S, AB, and LiTFSI/PEO electrolyte was added into ACNand ball milled at 500 rpm for 3 h to form a slurry. Then, the mixedslurry was casted onto a nickel foam and dried at 50 °C under vacuumfor 12 h. The nickel foam was used as the current collector because Alis susceptible to corrosion in Li−S batteries with LiTFSI-basedelectrolytes.47 The areal loading amount of sulfur on the electrodes isabout 0.5 mg cm−2.

Characterization and Instruments. The X-ray diffraction(XRD) patterns of samples were recorded using a Rigaku UltimaIV X-ray diffractometer (Rigaku Corporation, Japan) equipped withCu Kα radiation (λ = 1.54178 Å) operated at 40 kV and 30 mA with ascanning rate of 5° per min. The electrochemical tests were conductedby using Autolab PGSTA302 electrochemical workstation (EcoChemie, The Netherlands).

Scanning electron microscopy (SEM) and energy-dispersivespectrometer (EDS) analysis were performed on an S-4800 (Hitachi,Japan) microscope operating at 20 kV.

Differential scanning calorimetry (DSC) analysis of the polymerelectrolytes was performed with a thermogravimetric DSC simulta-neous analyzer (NETZSCH STA449F5, NETZSCH, Germany) witha heating rate of 10 °C min−1. The electrolyte samples (10−20 mg)were hermetically sealed in a gold pan in a glovebox.

Mechanical properties of the polymer electrolyte membranes weremeasured using a universal testing machine (UTM-4000, SUNS,Shenzhen) with a stretching speed of 1.66 mm s−1. The membranesfor stress−strain measurements were about 1 cm in width, 2 cm inlength, and 70 μm in thickness.

The ionic conductivity of the polymer electrolytes was measured byelectrochemical impedance spectroscopy (EIS) in the frequency rangeof 100 000 to 1 Hz with a signal amplitude of 10 mV at temperaturesranging from 30 to 90 °C. The cells for measurement were assembledby sandwiching the electrolyte membranes between two stainless steelblocking electrodes of diameter 10 mm. For ionic conductivitymeasurements under each temperature, the cell was allowed toequilibrate for 2 h before the EIS test. The ionic conductivity wascalculated using eq 1

LRS

σ =(1)

where σ is the ion conductivity (S cm−1), L is the electrolytemembrane thickness (cm), S is the area of the membrane (cm2), andR is the bulk resistance of the membrane (Ω).

The lithium-ion transference number (tLi+) of the polymerelectrolytes was measured at 60 °C by a combination measurementof EIS and dc polarization using a Li|SPE|Li symmetric cell, asproposed by Watanabe and Bruce.48−50 A dc bias (0.01 V) wasapplied to polarize the Li|SPE|Li cell, and the initial current I0 andsteady-state current Is flowing through the cell were recorded. EISmeasurements were performed before and after the dc polarization inthe frequency range from 100 000 to 0.01 Hz with a signal amplitudeof 10 mV. The initial electrolyte bulk resistance Ri and the Li/electrolyte interfacial resistance R1

0 and the final resistance Rf and R1s

were obtained from the impedance spectra. The lithium-iontransference number tLi+ was calculated by using eq 2

tI R V I RI R V I Rs

Lis f 0 l

0

0 i ls=

[Δ − ][Δ − ]

+

(2)

The electrochemical stability window of the polymer electrolyteswas investigated by linear sweep voltammetry (LSV) using stainlesssteel as the working electrode and lithium metal as the counterelectrode. All LSV experiments were performed between the open-circuit voltage and 6 V (vs Li+/Li) with a sweep rate of 0.5 mV s−1 at50 °C.

The electrochemical stability of the Li/SPEs interface wasinvestigated by galvanostatic cycling of the Li|SPEs|Li symmetric

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cell at 0.1 mA cm−2 with the half-cycle duration of 0.5 h at 50 and 60°C.Solid state Li−S cells were assembled using a 2025-type coin cell

with the prepared electrode as the cathode, the lithium metal as theanode, and the polymer electrolyte membranes as both the electrolyteand the separator in an argon-filled glovebox. Galvanostatic charge−discharge tests were carried out between 1.6 and 2.8 V at 0.1 C (1 C =1675 mA h g−1) using a Land battery test system (Wuhan, China).

■ RESULTS AND DISCUSSIONThe uniformity of LSPS distribution in the PEO matrixstrongly affects the ionic conductivity and electrochemicalperformance of the composite polymer electrolytes. Themicrostructure and morphology of the as-obtained electrolytemembranes are characterized by SEM. Figure 1 shows the

photos and SEM images of the composite polymer electrolytes.It can be seen that all electrolyte membranes are flexible andfree-standing. The PEO electrolyte is semitransparent; theincorporation of LSPS slightly decreases the transparency ofthe composite electrolyte membranes. For the LSPS-free PEOelectrolyte membrane, a loose and porous surface morphologycan be observed from the SEM image. The incorporation of1% LSPS to the PEO electrolyte resulted in a compact andsmooth morphology, whereas for the PEO−3%LSPS electro-lyte membrane, obvious agglomeration of LSPS particles canbe observed. The uniform distribution of LSPS in the PEO−1%LSPS membrane is further verified by the cross-sectionSEM and EDS elemental mapping of the electrolytemembranes (Figure S1). For all membranes, C, O, and F arehomogeneously distributed throughout the membrane. S is

also homogeneously distributed in the PEO−1%LSPSmembrane, whereas obvious S agglomeration is observed forthe PEO−3%LSPS membrane.Figure 2 shows the XRD patterns of PEO, LiTFSI, LSPS,

and electrolyte membranes with different LSPS contents. The

XRD pattern of PEO exhibits two distinct sharp peaks ataround 19.2° and 23.6° representing (120) and (112) planes,respectively, consistent with the crystalline property of PEO.51

The XRD patterns of the LiTFSI salt and LSPS also exhibitsharp peaks because of their crystalline nature. The intensitiesof the major reflections from PEO dramatically decrease withthe addition of the LiTFSI salt, indicating a decrease in thedegree of crystallinity of the PEO backbone. This can beascribed to the demolition effect of the LiTFSI salt on theordered arrangement of the PEO chains.52 No diffraction peakscorresponding to the LiTFSI salt appeared in the pattern of thePEO/LiTFSI membrane, suggesting the complete dissolutionof the LiTFSI salt into the PEO matrix. This verified theformation of the polymer−salt complex between the LiTFSIsalt and PEO.53,54 After the incorporation of LSPS, the peaksof PEO slightly shift toward higher diffraction angle, indicatingthe interaction between LSPS and the PEO matrix.55 No peakscorresponding to LSPS appeared in the patterns of SPEmembranes, which is due to the low content of LSPS and lowcrystalline of LSPS precipitated from ACN.The mechanical properties of SPE membranes are important

for the practical development of all-solid-state batteries. TheSPE membranes should withstand high stress and strain duringbattery assembling and cycling. Mechanical studies wereperformed to investigate the effects of LSPS addition on SPEmembranes. The mechanical properties of the SPE membraneswere measured and are shown in Figure 2b. The tensilestrength of the PEO−1%LSPS composite electrolyte wasimproved from 0.56 to 0.79 MPa with high elongation-at-breakat 1230% after the incorporation of 1% LSPS. The enhancedmechanical strength of the PEO−1%LSPS composite mem-brane can be ascribed to the structural changes induced byLSPS. The improved mechanical properties of the PEO−1%LSPS membrane can suppress lithium dendrite growth andthus decrease the possibility of short circuit and realize safelithium metal batteries. In contrast to the PEO−1%LSPSmembrane, PEO−3%LSPS exhibits a decreased tensilestrength of 0.31 MPa with an elongation-at-break at 1037%because of the obvious agglomeration of LSPS particles.It is well known that the crystallinity of the polymer matrix

plays an important role in the ionic conductivity of PEO-basedsolid electrolytes. The ionic conductivity can be enhanced by

Figure 1. Photos of the (a) PEO/LiTFSI membrane, (b) PEO−1%LSPS membrane, and (c) PEOI−3%LSPS membrane. SEM images ofthe surface morphology of the (d) PEO/LiTFSI membrane, (e)PEO−1%LSPS membrane, and (f) PEO−3%LSPS membrane.

Figure 2. (a) XRD patterns of PEO, LiTFSI, LSPS, PEO/LiTFSImembrane, PEO−1%LSPS membrane, and PEO−3%LSPS mem-brane and (b) tensile strengths of different polymer electrolytemembranes.

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reducing the crystallinity of PEO. The phase transitionbehavior of the composite polymer electrolytes was analyzedusing DSC. Figure 3a shows the DSC thermograms of

electrolyte membranes with different LSPS contents. Theendothermic peaks presented are assigned to the melting of theSPEs, which indicates the PEO crystalline phase. Thecrystallinity (χc) of the electrolyte membranes has beencalculated using eq 3 by taking into account PEO as theperfect crystal46

HH f

100%cm

PEO PEO

χ =Δ

Δ×

(3)

where ΔHm is the melting enthalpy of SPEs, ΔHPEO (196.4 Jg−1) is the melting enthalpy of 100% crystalline PEO, and f PEOis equal to the PEO mass fraction in composite electrolytes.The results including Tm, ΔHm, and χc obtained from DSCthermograms are listed in Table 1.

Table 1 shows that the incorporation of LSPS into the PEOmatrix decreases the value of Tm and χc of the polymerelectrolytes. The composite electrolyte PEO−1%LSPS exhibitsthe lowest Tm (65 °C) and χc (27.5%). The DSC resultssuggest that the incorporation of LSPS can effectively weakenthe interaction among the PEO chains and inhibit polymercrystallization in the SPEs. This may increase the amount offree volume and enhance segmental dynamics; thus, improvedionic conductivity of the SPEs could be expected.Figure 3b shows the temperature dependence of the ionic

conductivity of SPEs with different LSPS contents. The ionicconductivity of all electrolytes show an inflection point nearthe Tm of PEO, at around 50−60 °C, corresponding to thephase transition of PEO from the crystalline to amorphousphase when it is heated to the melting temperature at around60 °C. As expected, the incorporation of LSPS significantly

increases the ionic conductivity of the composite electrolytesover the entire temperature range investigated, especiallybelow 50 °C. The composite electrolyte PEO−1%LSPSexhibits the highest ionic conductivity. It reaches 1.69 ×10−4 S cm−1 at 50 °C and 6.62 × 10−6 S cm−1 at 30 °C, whichis significantly higher than that of the PEO/LiTFSI electrolyte(3.79 × 10−5 S cm−1 at 50 °C and 2.27 × 10−6 S cm−1) at 30°C. The ionic conductivity of the PEO−3%LSPS membrane islower than that of the PEO−1%LSPS membrane but higherthan that of the PEO/LiTFSI membrane. The decreased ionicconductivity of the PEO−3%LSPS membrane can be ascribedto the agglomeration of LSPS particles as observed by SEM.For PEO-based composite polymer electrolytes consisting ofinorganic fillers (e.g., TiO2 and Li6.4La3Zr1.4Ta0.6O12), it hasbeen proved that nanosized fillers are more effective inimproving the ionic conductivity than micronsized fillersbecause of their large specific surface areas.56,57 Variation ofionic conductivity in the SPEs is consistent with the DSCresults, where the PEO−1%LSPS electrolyte shows the lowestTm and χc.Besides the ionic conductivity, lithium-ion transference

number (tLi+) is also very important for the practicalapplication of polymer electrolytes, since the ionic chargecarriers are lithium ions in lithium batteries. Figure S2 showsthe chronoamperometric curves and impedance spectrameasured for both PEO/LiTFSI and PEO−1%LSPS electro-lytes at 60 °C. The resulting data and calculated tLi+ using eq 2are summarized in Table 2. It shows that the incorporation ofLSPS into the PEO electrolyte increases the value of tLi+ from0.25 of the PEO/LiTFSI electrolyte to 0.38 of the PEO−1%LSPS electrolyte. The enhanced tLi+ can be explained with theLewis-acidic effect58 and specific surface chemistry.58,59

Moreover, the sulfhydryl groups in the LSPS surface arefavorable to the bonding between TFSI− and Li10SnP2S12,which can enhance the mobility of Li+.60

The electrochemical stability of the polymer electrolytes iscritical for practical battery applications. To determine theoxidation potential of the SPEs, LSV measurements wereperformed on a stainless steel electrode. As shown in FigureS3, both PEO/LiTFSI and PEO−1%LSPS electrolytes exhibithigh anodic stability, with the oxidation potential up to 5.0 V.The incorporation of LSPS has no obvious effects on theanodic stability of PEO electrolytes. It is worth noting that theelectrochemical stability window of the solid electrolytemeasured using the Li/electrolyte/inert metal semiblockingelectrode configuration is normally relatively higher than thevalue obtained from the bulk-type solid-state Li batteriesbecause of the limited contact area between the solidelectrolyte and the inert metal.61 The stability of the Li/electrolyte interface plays a critical role in the operation oflithium metal batteries because the formation of lithiumdendrite can result in a low Coulombic efficiency, poor cyclingstability, and severe security concerns. The impact of LSPSincorporation on the interfacial resistance and stabilitybetween the polymer electrolytes and the lithium metalanode are evaluated by EIS and galvanostatical cycling of Li|

Figure 3. (a) DSC traces and (b) temperature dependence of theionic conductivity of the three polymer electrolytes.

Table 1. Data of DSC Measurement Results of the ThreePolymer Electrolytes

SPEs Tm/°C ΔHm/J g−1 χc/%

PEO/LiTFSI 67 66.17 44.6PEO−1%LSPS 65 50.47 27.5PEO−3%LSPS 66 56.44 39.5

Table 2. Data of Li-Ion Transference Number of PEO/LiTFSI and PEO−1%LSPS Electrolytes Measured at 60 °C

SPEs I0/μA Is/μA Ri/Ω Rf/Ω R10/Ω R1

s/Ω ΔV/mV tLi+

PEO/LiTFSI 121.8 61.12 22.94 21.43 47.14 32.87 10 0.25PEO−1%LSPS 117.5 59.03 17.22 16.56 32.78 36.85 10 0.38

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SPEs|Li symmetric cells. As shown in Figure 4, the Nyquistplots consist of a semicircle at high frequencies and a straight

line at low frequencies. The semicircle is related to the Li/electrolyte interfacial resistance Ri and capacitance Ci, and thestraight line is ascribed to Warburg impedance due to thediffusion process. Compared with PEO/LiTFSI, the PEO−1%LSPS electrolyte shows obvious lower interfacial resistance.The value of Ri decreases from 129.6 to 52.24 Ω at 50 °C andfrom 47.14 to 27.41 Ω at 60 °C. As for the PEO−3%LSPSelectrolyte, the values of Ri are only slightly lower than thosefor the PEO/LiTFSI electrolyte but much larger than those forthe PEO−1%LSPS electrolyte. This can be attributed to theagglomeration of LSPS particles as observed by SEM.Furthermore, compared with PEO/LiTFSI, the evolutions of

voltage of the symmetric cell with the PEO−1%LSPSelectrolyte is more stable with a lower overpotential of 25mV at 60 °C (Figure 5). PEO−1%LSPS-based lithium

symmetric cell exhibits very stable evolutions of voltagewithout obvious erratic values or Li infiltration even over600 h. However, the PEO/LiTFSI-based one shows higheroverpotential of 90 mV and fails due to internal short circuitafter being cycled for less than 450 h. The enhanced interfacialproperties of the PEO−1%LSPS electrolyte with the lithiumanode can be attributed to its higher ionic conductivity andbetter mechanical properties.The surface morphology of the lithium anodes after cycling

in lithium symmetric cells were analyzed by SEM and areshown in Figure S4. For the PEO/LiTFSI electrolyte,nonuniform Li deposition accompanied with obvious dendritesis observed. In contrast, uniform Li deposition with a smoothsurface is observed for the lithium anode with the PEO−1%LSPS electrolyte. This testifies that the PEO−1%LSPSelectrolyte can suppress the lithium dendrite growth, thusfavorable for more uniform and dense lithium deposition,which results in enhanced cycling performance of the Li|Li

symmetric cell. The lower Ri and higher interfacial stability ofthe PEO−1%LSPS electrolyte with the lithium anode arefavorable for achieving high-performance all-solid-state bat-teries.To verify the feasibility of using the PEO−1%LSPS

electrolyte membrane for all-solid-state batteries, the electro-chemical performance of Li−S cells utilizing PEO/LiTFSI andPEO−1%LSPS electrolyte membranes have been evaluated at60 and 50 °C. Figure 6 shows the charge−discharge profiles

and cycling performance of Li−S cells with the two electrolytescycled at 60 °C. The cells with both electrolytes exhibit typicalpolymer Li−S battery discharge curves with two dischargeplateaus, one short plateau at ∼2.4 V and one flat and longplateau at ∼2.05 V. The first discharge plateau at 2.40 V isascribed to the reduction of elemental sulfur (S8) to high-orderpolysulfides (Li2S8, Li2S6, and Li2S4), and the second plateau at2.05 V is attributed to the further reduction of high-orderpolysulfides to Li2S2 or Li2S.

62

It is worth noting that the Li−S cell with the PEO−1%LSPSelectrolyte exhibits a high initial discharge capacity of 1016 mAh g−1, whereas the PEO/LiTFSI-based cell shows a low initialdischarge capacity of 934 mA h g−1. Moreover, the Li−S cellwith the PEO−1%LSPS electrolyte exhibits good cyclingstability and high Coulombic efficiency close to 100% for allcycles. The discharge capacity maintains at 1000 mA h g−1

after 40 cycles. Figure S5 displays the long-term cyclingperformance of the Li−S cells with the PEO−1%LSPSelectrolyte at 0.5 C. It shows that the Li−S cell with thePEO−1%LSPS electrolyte also possesses good cycling stabilityat 0.5 C. It exhibits a high reversible capacity of 562 mA h g−1

after the first formation cycle at 0.2 C. The capacities aremaintained at around 518 mA h g−1 after 150 cycles with ahigh Coulombic efficiency close to 100%, whereas the cell withthe PEO/LiTFSI electrolyte shows severe overcharging with avery low Coulombic efficiency. The severe overcharging can beascribed to the side reaction between the dissolved polysulfidesand the lithium anode because of the relatively poor solidelectrolyte interface (SEI) formed in the LiTFSI/PEOelectrolyte.32

More importantly, when cycled at 50 °C (Figure 7), the cellwith the PEO−1%LSPS electrolyte can work well after the

Figure 4. EIS spectra of Li|electrolytes|Li symmetrical cells at 50 (a)and 60 °C (b).

Figure 5. Comparison of galvanostatic cycling performance of Li|PEO/LiTFSI|Li and Li|PEO−1%LSPS|Li symmetric cells at a currentdensity of 0.1 mA cm−2 at 60 °C. Insets show the magnified curvesfrom 1 to 9 and 591 to 600 h.

Figure 6. Charge−discharge curves (a,b) and cycling performance(c,d) of the Li−S cells with the PEO−1%LSPS electrolyte (a,c) andthe PEO/LiTFSI electrolyte (b,d) at 60 °C.

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activation of the first several cycles. The discharge capacityincreases from 330 to 789 mA h g−1 after 5 cycles andmaintains at above 800 mA h g−1 after 50 cycles. On the otherhand, the cell with the PEO/LiTFSI electrolyte cannot becycled normally. The initial discharge capacity is very low andslightly increases to 150 mA h g−1 after 35 cycles. The superiorelectrochemical performance of the PEO−1%LSPS electrolytecan be attributed to its higher ionic conductivity, lowerinterfacial resistance, and higher interfacial stability with thelithium anode.To understand the superior electrochemical performance of

the PEO−1%LSPS-based cell, the EIS spectra of PEO/LiTFSIand PEO−1%LSPS electrolyte-based cells were collected, andthe results are presented in Figure 8. The Nyquist plots of all

cells show a depressed semicircle at high frequencies with aslope tail at low frequencies. The first intercept with the realaxis (at high frequency) is related to the Ohmic resistance (Ro)of the cell from the electrolyte and the electrode. Thesemicircle is assigned to Li-ion diffusion through the SEI layer(Rs) and charge-transfer resistance (Rct) in parallel with thedouble-layer capacitance (Cdl), and the low-frequency slope tailis related to Li-ion diffusion in the bulk material. It can beclearly seen that the PEO−1%LSPS-based cells show muchlower SEI and charge-transfer resistance. This result confirmsthe lower interfacial resistance of the PEO−1%LSPS electro-lyte, thanks to the formation of a stable SEI layer. Therefore,

the enhanced electrochemical performance of PEO−1%LSPS-based cells is closely related to the improved interfacialproperties.The surface morphology of the lithium anodes after cycling

in PEO/LiTFSI and PEO−1%LSPS-based Li−S cells wereanalyzed by SEM and are shown in Figure S6. Compared withthe smooth surface of the lithium anode from the PEO−1%LSPS electrolyte, a corrugated surface texture with anadditional layer of material was clearly observed on the surfaceof the lithium anode from the PEO/LiTFSI electrolyte. Theuneven surface morphology observed for the lithium anodefrom the PEO/LiTFSI electrolyte may be attributed to thegrowth of lithium dendrites and corrosion of lithium metal byshuttled polysulfides. EDS results show that the deposition ofsulfur species on the lithium anode surface from the PEO−1%LSPS electrolyte is lower than that from the PEO/LiTFSIelectrolyte. Figure 9 shows the representative cross-sectional

morphologies of the Li anode after cycling in PEO/LiTFSI andPEO−1%LSPS-based Li−S cells. For the PEO/LiTFSI-basedcell, the lithium anode shows an obvious degradation layeraround 40 μm thick because of the corrosion of lithium causedby the soluble polysulfides. However, for the PEO−1%LSPS-based cell, the lithium anode shows a well-preserved bulkstructure with a dense passivation layer (∼10 μm) formed onthe surface. Figure 10 shows the cross-sectional morphologiesand EDS elemental mapping of both Li/electrolyte and sulfurcathode/electrolyte interfaces after cycling in PEO/LiTFSIand PEO−1%LSPS electrolytes. For both cells, the electrolytelayers contain significant amounts of S, indicating thedissolution of polysulfides into the polymer electrolyte. Forthe PEO/LiTFSI-based cell, the lithium anode is severelycorroded by the polysulfides, and the sulfur-containing speciespenetrate deeply (∼40 μm) into bulk lithium. On the contrary,for the PEO−1%LSPS-based cell, the lithium anode shows asmooth and intact structure, and no obvious corrosion or crackis noted. The results above suggest that the PEO−1%LSPSelectrolyte is favorable for the formation of a stable SEI layeron the lithium anode and mitigation of the corrosion of thelithium metal by lithium polysulfides, thus enhancing theelectrochemical performance of polymer Li−S batteries.

■ CONCLUSIONSPEO/Li10SnP2S12 SPEs were prepared by a simple solution-casting method and investigated for the first time as polymerelectrolyte membranes for all-solid-state Li−S batteries. Theas-prepared SPEs exhibit enhanced ionic conductivity andmechanical properties compared to the pure PEO/LiTFSIelectrolyte. The PEO−1%LSPS electrolyte shows the highest

Figure 7. Charge−discharge curves (a,b) and cycling performance(c,d) of the Li−S cells with the PEO−1%LSPS electrolyte (a,c) andthe PEO/LiTFSI electrolyte (b,d) at 50 °C. A photograph of thesolid-state polymer Li−S battery with PEO−1%LSPS lighting a redLED device is inserted in (c).

Figure 8. EIS spectra of all-solid-state Li−S cells with PEO/LiTFSIand PEO−1%LSPS polymer electrolytes measured at (a) 50 and (b)60 °C.

Figure 9. Cross-sectional SEM images of the Li metal surface after 20cycles in Li−S cells with the (a) PEO/LiTFSI electrolyte and (b)PEO−1%LSPS electrolyte.

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ionic conductivity (10−4 S cm−1 at 50 °C). DSC analysisindicates that the enhanced Li-ion conductivity can be ascribedto weakening interactions among the PEO chains andinhibiting crystallization by the incorporation of LSPS.Compared with the PEO/LiTFSI electrolyte, the PEO−1%LSPS electrolyte also shows higher lithium-ion transferencenumber, lower interfacial resistance, and higher interfacialstability with the lithium anode. Li−S batteries with the PEO−1%LSPS electrolyte exhibited outstanding electrochemicalperformance benefits from the enhanced ionic conductivity,mechanical properties, and interfacial stability against thelithium anode. It delivers a high specific capacity of 1000 mA hg−1 with a high Coulombic efficiency (close to 100%) andgood cycling stability at 60 °C. In contrast, the PEO/LiTFSI-based cell shows lower discharge capacity and severeovercharging with a very low Coulombic efficiency becauseof the inferior quality of the SEI layer formed between the Lianode and the PEO/LiTFSI electrolyte, which cannotwithstand the corrosion of polysulfide species. Moreimportantly, the PEO−1%LSPS-based cell can work welleven at 50 °C and presents a high discharge capacity of 800mA h g−1 as well as good cyclability, whereas the PEO/LiTFSI-based cell cannot be cycled normally and exhibit an extremelylow discharge capacity.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.9b05212.

SEM images and EDS mapping images of the cross-section of the membrane, chronoamperometric curvesand EIS spectra of the Li|SPEs|Li symmetrical cell, LSVcurves of the polymer electrolytes, SEM images of the Limetal surface after 150 cycles of stripping−plating in Li|SPEs|Li symmetrical cells, long-term cycling perform-ance of the Li−S cells with the PEO−1%LSPSelectrolyte at 0.5 C, and SEM images and EDS data ofthe Li metal surface after 20 cycles in Li−S cells (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: zlgong@xmu.edu.cn. Phone: +86-529-2880703.ORCIDZhengliang Gong: 0000-0003-4671-4044

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work was supported by the National Natural ScienceFoundation of China (grant nos. 21875196, 21761132030, andU1732121), the National Key R&D Program of China (grantnos. 2018YFB0905400 and 2016YFB0901500), and theScience and Technology Planning Projects of Fujian Province,China (grant no. 2019H0003).

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