Li10SnP2S12 composite polymer electrolyte

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Poly(ethylene oxide)/Li10SnP2S12 composite polymer electrolyte enable high performance all-solid-state lithium sulfur battery Xue Li, Donghao Wang, Hongchun Wang, Hefeng Yan, Zhengliang Gong, and Yong Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05212 • Publication Date (Web): 03 Jun 2019 Downloaded from http://pubs.acs.org on June 4, 2019

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ACS Applied Materials & Interfaces

Poly(ethylene

oxide)/Li10SnP2S12

Composite

Polymer

Electrolyte Enable High Performance All-Solid-State Lithium Sulfur Battery

Xue Li,a Donghao Wang,a Hongchun Wang,a Hefeng Yan,a Zhengliang Gong,a,* Yong Yanga,b

aCollege bState

of Energy, Xiamen University, Xiamen 361005, People’s Republic of China

Key Laboratory for Physical Chemistry of Solid Surface, Department of Chemistry,

College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People’s Republic of China ∗

Corresponding author. E-mail: [email protected], Tel: +86-529-2880703.

1

ACS Paragon Plus Environment

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Abstract Composite polymer electrolyte membranes are fabricated by incorporation of Li10SnP2S12 into polyethylene oxide matrix using a solution casting method. The incorporation of Li10SnP2S12 plays a positive role on Li-ionic conductivity, mechanical property and interfacial stability of the composite electrolyte, thus significantly enhances the

electrochemical

performance

of

solid

state

Li-S

battery.

The

optimal

PEO-1%Li10SnP2S12 electrolyte presents a maximum ionic conductivity of 1.69×10−4 S cm−1 at 50 °C and highest mechanical strength. The possible mechanism for the enhanced electrochemical performance and mechanical property is analyzed. The uniform distribution of Li10SnP2S12 in PEO matrix inhibits crystallization and weakens the interactions among the PEO chains. The PEO-1%Li10SnP2S12 electrolyte exhibits lower interfacial resistance and higher interfacial stability with lithium anode than pure PEO/LiTFSI electrolyte. The Li-S cell comprising PEO-1%Li10SnP2S12 electrolyte exhibits outstanding electrochemical performance with high discharge capacity (ca. 1000 mAh g-1), high coulombic efficiency and good cycling stability at 60 oC. Most importantly, PEO-1%Li10SnP2S12 based cell still possesses attractive performance with high specific capacity (ca. 800 mAh g-1) and good cycling stability even at 50 oC, where the PEO/LiTFSI based cell can’t be successfully discharged due to the low ionic conductivity and high interfacial resistance of PEO/LiTFSI electrolyte. Keywords: Li-S batteries; Solid polymer electrolyte; Polyethylene oxide; Sulfide lithium ionic conductor; Interfacial stability 2


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Introduction Lithium-sulfur (Li-S) batteries have attracted extensive attention as promising alternatives to the current lithium ion batteries (LIBs) due to their high theoretical energy densities and low cost.1-3 Despite the aforementioned advantages, the development of high performance Li-S batteries has been impeded by the electrical insulating nature and the poor ionic conductivity of elemental sulfur and its discharge product Li2S.4-7 To overcome these drawbacks, numerous research works have been conducted. The electronic conductivity of the sulfur cathode has been enhanced by incorporating sulfur with conductive matrix (such as carbon, polymers and inorganic compounds).8-13 The poor ionic conductivity of 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-S battery, as the electrochemical reactions at the sulfur electrode mainly take place at the “liquid–solid” interface of the dissolved lithium polysulfides and the surface

of

conducting

matrix.14-18

Despite

the

significant

improvements

in

electrochemical performance, the practical application of conventional Li-S batteries is still obstructed by several critical issues, such as lithium ploysulfides shuttle effect, long-term stability of lithium metal anode with organic liquid electrolytes and the safety concerns related to the lithium anode and liquid electrolyte.19-21 All-solid-state lithium batteries are considered as one of the most promising techniques to address the safety challenges of lithium ion batteries.22-26 Especially, poly(ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) have been extensively investigated due to their superior 3



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advantages, such as good flame-resistance, no-leakage, flexible geometry and low cost in design, as well as light weight.27-30 It provides an easily achievable approach to surpass the energy density of current Li-ion batteries by using SPEs with membranes thickness below 100 μm.31 Normally, SPEs membranes are fabricated by dissolving lithium salt into PEO matrix. Among the lithium salts, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is the most commonly used conducting salt for PEO-based SPEs. However, all-solid-state Li-S batteries with PEO/LiTFSI based SPEs can’t be cycled normally with severe overcharge when using non-modified element sulfur based cathode, owing to the dissolution of polysulfides into PEO electrolyte and the poor interfacial stability of PEO/LiTFSI electrolyte against lithium metal, which can’t withstand the corrosion of polysulfide species.32,33 To improve the stability of Li/electrolyte interface, extensive studies have been devoted to electrolyte modifications, such as using lithium salt (e.g. lithium bis(fluorosulfonyl)imide

(LiFSI),

lithium

(trifluoromethanesulfonyl)

(n-nonafluorobutanesulfonyl)imide (LiTNFSI)) with good compatibility with lithium metal, and dispersing inorganic ceramic fillers (e.g. Al2O3, TiO2, SiO2, and LiAlO2, etc.) into PEO matrix.34-36 However, LiFSI and LiTNFSI are much more expensive than LiTFSI, which increase the cost of batteries. Also, the inorganic ceramic fillers have higher density than that of PEO, which decrease 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 and Li garnet etc.)37-40 and sulfides (eg. Li3PS4 and 4


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Li10GeP2S12)41-44, have been widely used as fillers to fabricate the composite polymer electrolytes. Compared with inactive fillers (e.g. Al2O3 and SiO2), Li+ conductors can contribute to the ionic conductivity of the SPEs, which significantly promote the ionic conductivity, interfacial stability and mechanical property of the SPEs.41,42 Among them, sulfides have lower density (~2.0 g cm-3) than oxides (>4.0 g cm-3) and high ionic conductivity (~ 10-3 S cm-1).43,45 As fillers, they can substantially improve the electrochemical performance of SPEs without severe sacrifice on energy density.46 Moreover, sulfide based ionic conductors can be dissolved in organic solvents; particularly, Li10SnP2S12 is highly soluble in acetonitrile (ACN), which is beneficial for fabricating

uniformly

dispersed

nano-filler

incorporated

composite

electrolyte

membranes by a solution casting process. With these considerations, herein the PEO-Li10SnP2S12 inorganic-polymer composite membranes are investigated as polymer electrolytes for all-solid-state Li-S batteries for the first time. The composite electrolytes with various Li10SnP2S12 content are prepared through incorporating of LSPS as filler into PEO matrix by a simple solution casting method. The effects of LSPS on ionic conductivity and physico-chemical properties of PEO-LSPS SPEs are investigated. Our results demonstrate that PEO-LSPS SPEs exhibit enhanced ionic conductivity and improved interfacial stability. Therefore, the all-solid-state Li-S batteries with the SPEs show superior electrochemical performance, with high capacity, good capacity retention and high coulombic efficiency.

5



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Experimental Preparation of solid-state composite polymer electrolytes (SPEs) The pure PEO and PEO-LSPS composite electrolyte membranes were fabricated via a conventional solution-casting method. PEO (Mw = 600000, 99.9%, Aldrich) was dried at 60 oC for 24 h and LiTFSI (99.9%, Aldrich) was dried at 100 oC for 24 h in vacuum before use. The procedures are sensitive for water and oxygen, so all experiments must be carried out in the Ar-filled glove box with H2O and O2 contents below 1 ppm. PEO and LiTFSI

were

dissolved

into

acetonitrile

(ACN)

with

the

molar

ratio

of

-CH2-CH2O-(EO)/Li+ (EO/Li+ = 20). Then the Li10SnP2S12 (LSPS, NEI Corporation) powder with different mass ratio (1% or 3%) was added to the previous solution respectively. After stirring at room temperature for 24 hours, the resulting solution was casted onto a Teflon plate, and then dried with 4Å molecular sieves for 48 h. Finally, the SPEs membranes with thickness ~70 μm were obtained. The uniform thin films were peeled off and punched into 10 mm and 19 mm diameter membranes for further measurements. Sulfur electrodes preparation Elemental sulfur and acetylene black (AB) were dried at 100 oC for 24 h before use. Composite sulfur cathodes consist of S, AB and LiTFSI/PEO electrolyte with the weight ration of 40:15:45. To prepare the cathode laminates, the mixture of S, AB and and LiTFSI/PEO electrolyte were added into acetonitrile, and ball milled at 500 rpm for 3 h to form a slurry. Then the mixed slurry was casted onto nickel foam and dried at 50 oC 6


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under vacuum for 12 h. Nickel foam was used as current collector, due to Al is susceptible to corrosion in Li-S batteries with LiTFSI based electrolytes.47 The areal loading amount of the sulfur on the electrodes is about 0.5 mg cm-2. Characterization and instruments The X-ray diffraction (XRD) patterns of samples were recorded using a Rigaku Ultima IV X-ray Diffractometer (Rigaku Corporation, Japan) equipped with Cu Ka radiation (λ = 1.54178 Å) operated at 40 kV and 30 mA with a scanning rate of 5° per min. The electrochemical tests were conducted by using Autolab PGSTA302 electrochemical workstation (Eco Chemie, Netherland). Scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) analysis were performed on a S-4800 (HITACHI, Japan) microscope, operating at 20 kV. Differential scanning calorimetry (DSC) analysis of the polymer electrolytes was performed with a thermogravimetric differential scanning calorimetry simultaneous analyzer (NETZSCH STA449F5, NETZSCH, Germany) with a heating rate of 10 oC min-1. The electrolyte samples (10-20 mg) were hermetically sealed in a gold pan in a glove box. Mechanical properties of the polymer electrolyte membranes were measured using a universal testing machine (UTM-4000, SUNS, Shenzhen) with a stretching speed of 1.66 mm s-1. The membranes for stress-strain measurements are about 1 cm in width, 2 cm in length and 70 µm in thickness. The ionic conductivity of the polymer electrolytes was measured by electrochemical 7



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impedance spectroscopy (EIS) in the frequency range of 100000 Hz to 1 Hz with a signal amplitude of 10 mV, at temperatures in the range from 30 oC to 90 oC. The cells for measurement were assembled by sandwiching the electrolyte membranes between two stainless steel blocking electrodes of diameter 10 mm. For ionic conductivity measurements under each temperature, the cell was allowed to equilibrate for 2 h before EIS test. The ionic conductivity was calculated using Eq. (1):



L RS

(1)

where σ is the ion conductivity (S cm-1), L is the electrolyte membrane thickness (cm), S is the area of membrane (cm2), and R is the bulk resistance of membrane (Ω), respectively. The lithium-ion transference number (𝑡𝐿𝑖 + ) of the polymer electrolytes was measured at 60 oC by a combination measurement of EIS and DC polarization using a Li|SPE|Li symmetric cell, as proposed by Watanabe and Bruce.48-50 A DC bias (0.01 V) was applied to polarize the Li|SPE|Li cell, and the initial current I0 and steady-state current Is flowing through the cell was recorded. EIS measurements were performed before and after the DC polarization, in the frequency range from 100000 to 0.01 Hz with a signal amplitude of 10 mV. The initial electrolyte bulk resistance Ri and Li/electrolyte interfacial resistance R10, and final resistance Rf and R1s were obtained from impedance spectra. And the lithium-ion transference number 𝑡𝐿𝑖 + was calculated by using Eq (2): 𝑡𝐿𝑖 + =

𝐼𝑠𝑅𝑓[𝛥𝑉 - 𝐼 0𝑅0𝑙 ] 𝐼0𝑅𝑖[𝛥𝑉 - 𝐼𝑠𝑅𝑠𝑙]

(2)

The electrochemical stability window of the polymer electrolytes was investigated by 8


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linear sweep voltammetry (LSV) using stainless steel as working electrode and lithium metal as counter electrode. All LSV experiments were performed between open circuit voltage (OCV) and 6 V (vs. Li+/Li) with a sweep rate of 0.5 mV s-1 at 50 °C. The electrochemical stability of Li/SPEs interface was investigated by galvanostatic cycling of the Li|SPEs|Li symmetric 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 2025 type coin cell with the prepared electrode as cathode, lithium metal as anode and the polymer electrolyte membranes as both electrolyte and separator in an argon-filled glove box. Galvanostatic charge/discharge tests were carried out between 1.6 and 2.8 V at 0.1 C (1 C = 1675 mAh g-1) using a Land battery test system (Wuhan, China).

Result and discussion The uniformity of LSPS distribution in the PEO matrix strongly affects the ionic conductivity and electrochemical performance of the composite polymer electrolytes. The microstructure and morphology of the as obtained electrolyte membranes are characterized by SEM. Fig. 1 shows the photos and SEM images of the composite polymer electrolytes. It can be seen that all the electrolyte membranes are flexible and free-standing. The PEO electrolyte is semitransparent, the incorporation of LSPS slightly decrease the transparency of the composite electrolyte membranes. For the LSPS-free PEO electrolyte membrane, a loose and porous surface morphology can be observed from 9



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SEM image. The incorporation of 1% LSPS to the PEO electrolyte resulted in compact and smooth morphology. While for PEO-3%LSPS electrolyte membrane, obvious agglomeration of LSPS particles can be observed. The uniform distribution of LSPS in PEO-1%LSPS membrane is further verified by the cross-section SEM and EDS elemental mapping of the electrolyte membranes (Fig. S1). For all the membranes, C, O and F are homogeneously distributed throughout the membrane. S is also homogeneously distributed in the PEO-1%LSPS membrane. While obvious S agglomeration is observed for the PEO-3%LSPS membrane.

Fig. 1 The photos of (a) PEO/LiTFSI membrane, (b) PEO-1%LSPS membrane and (c) PEOI-3%LSPS membrane, and SEM images of the surface morphology of (d) PEO/LiTFSI membrane, (e) PEO-1%LSPS membrane and (f) PEO-3% LSPS membrane. 10


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Fig. 2 (a) XRD patterns of PEO, LiTFSI, LSPS, PEO/LiTFSI membrane, PEO-1%LSPS membrane and PEO-3%LSPS membrane, (b) Tensile strengths of different polymer electrolyte membranes.

Fig. 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 at around 19.2° and 23.6°, representing (120) and (112) planes, respectively, consistent with the crystalline property of PEO.51 The XRD patterns of LiTFSI salt and LSPS also exhibits sharp peaks due to their crystalline nature. The intensities of the major reflections from PEO dramatically decrease with the addition of LiTFSI salt, indicating a decrease in the degree of crystallinity of the PEO backbone. This can be ascribed to demolition effect of LiTFSI salt on the ordered arrangement of the PEO chains.52 No diffraction peaks corresponding to LiTFSI salt appeared in the pattern of PEO/LiTFSI membrane, suggesting the complete dissolution of LiTFSI salt into PEO matrix. This verified the formation of polymer-salt complex between LiTFSI salt and PEO.53,54 After 11



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the incorporation of LSPS, the peaks of PEO slightly shift toward higher diffraction angle, indicating the interaction between LSPS and PEO matrix.55 No peaks corresponding to LSPS appeared in the patterns SPE membranes, which is due to the low content of LSPS and the low crystalline of LSPS precipitated from ACN. The mechanical properties of SPE membranes are important for the practical development of all-solid-state batteries. The SPE membranes should withstand the high stress and strain during battery assembling and cycling. Mechanical studies were performed to investigate the effects of LSPS addition in SPE membranes. The mechanical properties of the SPE membranes were measured and shown in Fig. 2b. The tensile strength of PEO-1%LSPS composite electrolyte was improved from 0.56 MPa to 0.79 MPa with high elongation-at-break at 1230% after the incorporation of 1% LSPS. The enhanced mechanical strength of the PEO-1%LSPS composite membrane can be ascribed to the structural changes induced by LSPS. The improved mechanical properties of PEO-1%LSPS membrane can suppress lithium dendrite growth, thus decrease the possibility of short circuit and realize safe lithium metal batteries. In contrast to PEO-1%LSPS membrane, PEO-3% LSPS exhibits a decreased tensile strength of 0.31 MPa with an elongation-at-beak at 1037% due to the obvious agglomeration LSPS particles.

12


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

It is well known that the crystallinity of the polymer matrix plays an important role in the ionic conductivity of PEO-based solid electrolytes. The ionic conductivity can be enhanced by reducing the crystallinity of PEO. The phase transition behavior of the composite polymer electrolytes was analyzed using DSC. Fig. 3a shows the DSC thermograms of electrolyte membranes with different LSPS contents. The endothermic peaks presented are assigned to the melting of the solid polymer electrolytes, which indicates the PEO crystalline phase. The crystallinity (χc) of the electrolyte membranes has been calculated using Eq. (3) by taking into account of PEO perfect crystal46 𝜒𝑐 =

𝛥𝐻𝑚 𝛥𝐻𝑃𝐸𝑂𝑓𝑃𝐸𝑂

× 100%

(3)

Where, ΔHm is the melting enthalpy of SPEs, ΔHPEO (196.4 J g-1) is the melting enthalpy of 100% crystalline PEO, as well as fPEO is equal to the PEO mass fraction in composite electrolytes. The results including Tm, ΔHm, χc, obtained from DSC thermograms are 13



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listed in Table 1.

Table 1 The data of DSC measurement results of the three polymer electrolytes. SPEs

Tm/oC

ΔHm/J g-1

χc/%

PEO/LiTFSI

67

66.17

44.6

PEO-1%LSPS

65

50.47

27.5

PEO-3%LSPS

66

56.44

39.5

It shows that the incorporation of LSPS into PEO matrix decreases the value of Tm and χc of the polymer electrolytes. The composite electrolyte PEO-1%LSPS exhibits the lowest Tm (65 °C) and χc (27.5%). The DSC results suggest that the incorporation of LSPS can effectively weaken the interaction among the PEO chains and inhibit polymer crystallization in the SPEs. This may increase the amount of free volume and enhance segmental dynamics, thus improved ionic conductivity of the SPEs could be expected.

Table 2 The 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

R1S/Ω ∆V/mV

𝑡Li +

PEO/LiTFSI

121.8

61.12

22.94

21.43

47.14

32.87

10

0.25

PEO-1%LSPS

117.5

59.03

17.22

16.56

32.78

36.85

10

0.38 14


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Fig. 3b shows the temperature dependence of the ionic conductivity of SPEs with different LSPS contents. The ionic conductivity of all electrolytes show an inflection point near the Tm of PEO, at around 50–60 °C, corresponding to the phase transition of PEO from crystalline to amorphous phase when it is heated to the melting temperature at around 60 °C. As expected, the incorporation of LSPS significantly increases the ionic conductivity of the composite electrolytes over the entire temperature range investigated, especially below 50 °C. The composite electrolyte PEO-1%LSPS exhibits 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, which is 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 PEO-3%LSPS membrane is lower than that of PEO-1%LSPS membrane, while higher than PEO/LiTFSI membrane. The decreased ionic conductivity of PEO-3%LSPS membrane can be ascribed to the agglomeration of LSPS particles as observed by SEM. For PEO based composite polymer electrolytes consisting of inorganic fillers (e.g. TiO2 and Li6.4La3Zr1.4Ta0.6O12, it has been proved that nano-sized fillers are more effective in improving the ionic conductivity than micron-sized fillers due to their large specific surface areas.56,57 Variation of ionic conductivity in the SPEs is consistent with the DSC results, where PEO-1%LSPS electrolyte shows the lowest Tm and χc. Besides the ionic conductivity, lithium ion transference number (𝑡Li + ) is also very important for the practical application of polymer electrolytes, since the ionic charge carriers are lithium ions in lithium batteries. Fig. S2 shows chronoamperometric curves 15



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and impedance spectra measured for both the PEO/LiTFSI and PEO-1%LSPS electrolytes at 60 °C. The resulted data and calculated tLi+ using Eq. (2) are summarized in Table 2. It shows that the incorporation of LSPS into PEO electrolyte increase the value of 𝑡Li + from 0.25 of PEO/LiTFSI electrolyte to 0.38 of PEO-1%LSPS electrolytes. The enhanced 𝑡Li + can be explained with the Lewis-acidic effect58 and specific surface chemistry.58,59 Moreover, the sulfhydryl groups in LSPS surface are favorable to the bonding between TFSI− and Li10SnP2S12, which can enhance the mobility of Li+ .60

Fig. 4 The electrochemical impedance spectra of Li|electrolytes|Li symmetrical cells at 50 oC (a) and 60 oC (b).

The electrochemical stability of the polymer electrolytes is critical for the practical battery applications. To determine the oxidation potential of the SPEs, LSV measurements were performed on a stainless steel electrode. As shown in Fig. S3, both the PEO/LiTFSI and PEO-1%LSPS electrolytes exhibit high anodic stability, with 16


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oxidation potential up to 5.0 V. The incorporation of LSPS has no obvious effects on the anodic stability of PEO electrolytes. It is worth noting that the electrochemical stability window of solid electrolyte measured using the Li/electrolyte/inert metal semiblocking electrode configuration is normally relatively higher than the value obtained from the bulk-type solid-state Li batteries, due to the limited contact area between solid electrolyte and inert metal.61 The stability of Li/electrolyte interface plays a critical role in the operation of lithium metal batteries, since the formation of lithium dendrite can result in low coulombic efficiency, poor cycling stability and severe security concerns. The impacts of LSPS incorporation on interfacial resistance and stability between polymer electrolytes and lithium metal anode are evaluated by EIS and galvanostatical cycling of Li|SPEs|Li symmetric cells. As shown in Fig. 4, the Nyquist plots consist of a semi-circle at high frequencies and a straight line at low frequencies. The semi-circle is related to Li/electrolyte interfacial resistance Ri and capacitance Ci, and the straight line is ascribed to Warburg impedance due to diffusion process. Compared with PEO/LiTFSI, PEO-1%LSPS electrolyte shows obvious lower interfacial resistance. The value of Ri decrease from 129.6 to 52.24 Ω at 50 oC, and from 47.14 to 27.41 Ω at 60 oC. As for PEO-3%LSPS electrolyte, the values of Ri are only slightly lower than PEO/LiTFSI electrolyte, while much larger than PEO-1%LSPS electrolyte. This can be attributed to the agglomeration of LSPS particles as observed by SEM.

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Fig. 5 Comparison of galvanostatic cycling performance of Li|PEO/LiTFSI|Li and Li|PEO-1%LSPS|Li symmetric cells at a current density of 0.1 mA cm−2 at 60 oC. Insets show the magnified curves from 1 to 9 h and 591 to 600 h.

Furthermore, compared with PEO/LiTFSI, the evolutions of voltage of the symmetric cell with PEO-1%LSPS electrolyte is more stable with a lower overpotential of 25 mV at 60 oC (Fig.5). PEO-1%LSPS based lithium symmetric cell exhibits very stable evolutions of voltage without obvious erratic values or Li infiltration even over 600 h. However, the PEO/LiTFSI based one shows higher overpotential of 90 mV and fails due to intern short circuit after being cycled for less than 450 h. The enhanced interfacial properties of PEO-1%LSPS electrolyte with lithium anode can be attributed to its higher ionic conductivity and better mechanic properties. The surface morphology of the lithium anodes after cycled in lithium symmetric cells 18


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were analyzed by SEM and shown in Fig. S4. For PEO/LiTFSI electrolyte, nonuniform Li deposition accompanied with obvious dendrites is observed. In contrast, uniform Li deposition with a smooth surface is observed for lithium anode with PEO-1%LSPS electrolyte. This testifies that PEO-1%LSPS electrolyte can suppress the lithium dendrite growth, thus favorable for more uniform and dense lithium deposition, which results in enhanced cycling performance of Li|Li symmetric cell. The lower Ri and higher interfacial stability of PEO-1%LSPS electrolyte with lithium anode is favorable for achieving high performance all-solid-state batteries. To verify the feasibility of using PEO-1%LSPS electrolyte membrane for all-solid-state batteries, electrochemical performance of Li-S cells utilizing PEO/LiTFSI and PEO-1%LSPS electrolyte membranes have been evaluated at 60 and 50 °C. Fig. 6 shows the charge/discharge profiles and cycling performance of Li-S cells with the two electrolytes cycled at 60 oC. The cells with both electrolytes exhibit typical polymer Li-S battery discharge curves with two discharge plateaus, one short plateau at ~ 2.4 V and one flat and long plateau at ~ 2.05 V. The first discharge plateau at 2.40 V is ascribed to the reduction of elemental sulfur (S8) to high order polysulfides (Li2S8, Li2S6 and Li2S4), and the second plateau at 2.05 V is attributed to the further reduction of high order polysulfides to Li2S2 or Li2S.62

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Fig. 6 Charge-discharge curves (a, b) and cycling performance (c, d) of the Li-S cells with PEO-1%LSPS electrolyte (a, c) and PEO/LiTFSI electrolytes (b, d) at 60 oC.

It is worth noting that the Li−S cell with PEO-1%LSPS electrolyte exhibits a high initial discharge capacity of 1016 mAh g-1, while the PEO/LiTFSI based cell shows a low initial discharge capacity of 934 mAh g-1. Moreover, the Li−S cell with PEO-1%LSPS electrolyte exhibits good cycling stability and high coulombic efficiency close to 100% for all the cycles. The discharge capacity maintains at 1000 mAh g-1 after 40 cycles. Fig.S5 displays the long term cycling performance of the Li-S cells with PEO-1%LSPS electrolyte at 0.5 C. It shows that the Li−S cell with PEO-1%LSPS electrolyte also possesses good cycling stability at 0.5 C. It exhibits a high reversible capacity of 562 20


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mAh g-1 after the first formation cycle at 0.1 C. The capacities are maintained at around 518 mAh g−1 after 150 cycles with high coulombic efficiency close to 100%. While, the cell with PEO/LiTFSI electrolyte shows severe overcharging with very low coulombic efficiency. The severe overcharging can be ascribed to the side reaction between the dissolved polysulfides and lithium anode due to the relatively poor SEI formed in LiTFSI/PEO electrolyte.32

Fig. 7 Charge-discharge curves (a, b) and cycling performance (c, d) of the Li-S cells with PEO-1%LSPS electrolyte (a, c) and PEO/LiTFSI electrolyte (b, d) at 50 oC. A photograph of the solid state polymer Li-S battery with PEO-1%LSPS lighting a red LED 21



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device is inserted in Fig. 7 (c). More importantly, when cycled at 50 oC (Fig.7), the cell with PEO-1%LSPS electrolyte can work well after the activation of the first several cycles. The discharge capacity increases from 330 to 789 mAh g-1 after 5 cycles and maintains at above 800 mAh g-1 after 50 cycles. On the other hand, the cell with PEO/LiTFSI electrolyte can’t be cycled normally. The initial discharge capacity is very low and slightly increases to 150 mAh g-1 after 35 cycles. The superior electrochemical performance of PEO-1%LSPS electrolyte can be attributed to its higher ionic conductivity, lower interfacial resistance and higher interfacial stability with lithium anode.

Fig. 8 EIS spectra of all-solid-state Li-S cells with PEO/LiTFSI and PEO-1%LSPS polymer electrolytes measured at (a) 50 oC and (b) 60 oC.

In order to understand the superior electrochemical performance of PEO-1%LSPS based cell, EIS of PEO/LiTFSI and PEO-1%LSPS electrolytes based cells were collected 22


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and the results are presented in Fig. 8. The Nyquist plots of all the cells show a depressed semi-circle at high frequencies with a slope tail at low frequencies. The first intercept with the real axis (at high frequency) is related to the ohmic resistance (Ro) of the cell from the electrolyte and electrode. The semicircle is assigned to Li-ion diffusion through the solid electrolyte interface (SEI) layer (Rs) and charge transfer resistance (Rct) in parallel with double-layer capacitance (Cdl), and the low frequency slope tail is related to Li-ion diffusion in the bulk material. It can be clearly seen that the PEO-1%LSPS based cells show much lower SEI and charge transfer resistance. This result confirms the lower interfacial resistance of PEO-1%LSPS electrolyte, thanks to the formation of stable SEI layer. Therefore, the enhanced electrochemical performance of PEO-1%LSPS based cells is closely related to the improved interfacial properties.

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

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Fig. 10 Cross-sectional SEM images (a, d) and EDS mapping images (b, c, e and f) of the Li-S cells after 20 cycles in PEO/LiTFSI (a, b and c) and PEO-1%LSPS (d, e and f) electrolytes.

The surface morphology of the lithium anodes after cycled in PEO/LiTFSI and PEO-1%LSPS based Li-S cells were analyzed by SEM and shown in Fig. S6. Compared with the smooth surface of the lithium anode from PEO-1%LSPS electrolyte, corrugated surface texture with an additional layer of material was clearly observed on the surface of lithium anode from PEO/LiTFSI electrolyte. The uneven surface morphology observed for lithium anode from PEO/LiTFSI electrolyte may be attributed to the growth of lithium dendrites and corrosion of lithium metal by shuttled polysulfides. EDS results show that the deposition of sulfur species on lithium anode surface from PEO-1%LSPS electrolyte is lower than that one from PEO/LiTFSI electrolyte. Fig. 9 shows the representative cross-sectional morphologies of Li anode after cycled in PEO/LiTFSI and 24


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PEO-1%LSPS based Li-S cells. For the PEO/LiTFSI based cell, the lithium anode shows an obvious degradation layer around 40 μm thick due to the corrosion of lithium caused by the soluble polysulfides. However, for the PEO-1%LSPS based cell, the lithium anode shows a well-preserved bulk structure with a dense passivation layer (∼10 μm) formed on the surface. Fig. 10 shows the cross-sectional morphologies and EDS elemental mapping of both Li/electrolyte and sulfur cathode/electrolyte interfaces after cycled in PEO/LiTFSI and PEO-1%LSPS electrolytes. For both cells, the electrolyte layers contain significant amounts of S, indicating the dissolution of polysulfides into the polymer electrolyte. For the PEO/LiTFSI based cell, lithium anode is severely corroded by the polysulfides, and the sulfur-containing species penetrate deeply (~40 μm) into bulk lithium. On the contrary, for the PEO-1%LSPS based cell, lithium anode shows a smooth and intact structure and no obvious corrosion or crack is noted. The results above suggest that PEO-1%LSPS electrolyte is favorable for the formation a stable SEI layer on lithium anode and mitigation the corrosion of lithium metal by lithium polysulfides, thus enhance the electrochemical performance of polymer Li-S batteries.

Conclusions PEO/Li10SnP2S12 SPEs were prepared by a simple solution casting method, and investigated for the first time as polymer electrolyte membranes for all-solid-state Li-S batteries. The as-prepared SPEs exhibit enhanced ionic conductivity and mechanical properties compared to the pure PEO/LiTFSI electrolyte. The PEO-1%LSPS electrolyte 25



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shows the highest ionic conductivity (10-4 S cm-1 at 50 oC). DSC analysis indicates that the enhanced Li-ion conductivity can be ascribed to weakening interactions among the PEO chains and inhibiting crystallization by the incorporation of LSPS. Compared with PEO/LiTFSI electrolyte, PEO-1%LSPS electrolyte also shows higher lithium ion transference number, lower interfacial resistance and higher interfacial stability with lithium anode. Li-S batteries with PEO-1%LSPS electrolyte exhibited outstanding electrochemical performance benefits from the enhanced ionic conductivity, mechanical properties and interfacial stability against lithium anode. It delivers a high specific capacity of 1000 mAh g-1 with high coulombic efficiency (close to 100%) and good cycling stability at 60 oC. In contrast, PEO/LiTFSI based cell shows lower discharge capacity and severe overcharging with very low coulombic efficiency, due to the inferior quality of SEI layer formed between Li anode and PEO/LiTFSI electrolyte, which can’t withstand the corrosion of polysulfide species. More importantly, PEO-1%LSPS based cell can work well even at 50 oC, and present high discharge capacity of 800 mAh g-1 as well as good cyclability, while PEO/LiTFSI based cell can’t be cycled normally, and exhibit extremely low discharge capacity.

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ASSOCIATED CONTENT Supporting Information SEM images and EDS mapping images of the cross-section of membrane, Chronoamperometric curves and EIS spectra of the Li|SPEs|Li symmetrical cell, LSV curves of the polymer electrolytes, SEM images of the Li metal surface after 150 cycles of stripping-plating in the Li|SPEs|Li symmetrical cells, Long term cycling performance of the Li-S cells with PEO-1%LSPS electrolyte at 0.5 C, SEM images and EDS data of the Li metal surface after 20 cycles in Li-S cells.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Zhengliang Gong: 0000-0003-4671-4044 Notes There are no conflicts of interest to declare.

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grants No. 21875196, 21761132030 and U1732121), National Key R&D Program of China (Grant No. 2018YFB0905400 and 2016YFB0901500) and the Science and Technology Planning Projects of Fujian Province, China (Grant No. 2019H0003).

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[Graphical abstract]

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Li10SnP2S12 composite polymer electrolyte

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