In-situ crosslinked single ion gel polymer electrolyte with superior performances for lithium metal batteries

Journal Pre-proofs In–situ Crosslinked Single Ion Gel Polymer Electrolyte with Superior Performances for Lithium Metal Batteries Xiang Guan, Qingping Wu, Xiaowan Zhang, Xuhong Guo, Chilin Li, Jun Xu PII: DOI: Reference:

S1385-8947(19)32345-9 https://doi.org/10.1016/j.cej.2019.122935 CEJ 122935

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

5 July 2019 17 September 2019 21 September 2019

Please cite this article as: X. Guan, Q. Wu, X. Zhang, X. Guo, C. Li, J. Xu, In–situ Crosslinked Single Ion Gel Polymer Electrolyte with Superior Performances for Lithium Metal Batteries, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.122935

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In–situ Crosslinked Single Ion Gel Polymer Electrolyte with Superior Performances for Lithium Metal Batteries

Xiang Guan, Qingping Wu, Xiaowan Zhang, Xuhong Guo, Chilin Li and Jun Xu*

X. Guan, Q. P. Wu, X. W. Zhang, Prof. X. H. Guo, Prof. J. Xu State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China. Email: [email protected]

Q. P. Wu, Prof. C. L. Li State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 585 He Shuo Road, Shanghai 201899, China.

Abstract: Conventional dual-ion conductive gel polymer electrolytes (GPEs) attracted tremendous interest for their combined advantages of liquid and solid electrolytes. However, their cycling performance usually degrades due to the concentration polarization of anions. Single lithium ion gel polymer electrolytes (SLIGPEs), which have anions covalently bonded to the polymer or inorganic

backbone, can suppress the motion of anions by the interactions between the pedant anions and mobile ions. Especially, Li+ deposition in SLIGPEs can be regulated uniformly to avoid lithium dendrite growth. Herein, novel 3D-structured SLIGPEs were

prepared

by

in-situ

crosslinking

of

lithium

(4-styrenesulfonyl)

(trifluoromethanesulfonyl) imide (LiSTFSI) with polyethylene glycol dimethacrylate (PEGDMA), soaked with liquid electrolyte. The obtained SLIGPEs not only promote the absorption of liquid electrolyte and accelerate the transport of Li+, but also show a high limitation for the migration of anions, proved by high ionic conductivity (2.74×10-5 S·cm-1 at room temperature) and high lithium ion transference number (0.622) of SLIGPE-3.5, respectively. The long cycle life (cycling for over 600 h under 0.05 mA·cm-2 and 0.1 mA·cm-2) of symmetric lithium batteries with SLIGPEs as both separator and electrolyte exhibits good interfacial stability between SLIGPEs and electrodes. Meanwhile, LiFePO4/SLIGPE-2/Li shows a specific discharge capacity of 132.1 mAh·g-1 at 0.1 C, 80.0% of the capacity retention ratio and 99.0% of the coulombic efficiency after 150 cycles at room temperature. Moreover, confirmed by SEM, lithium dendrite growth is significantly suppressed during repeated Li plating/stripping cycles. Density functional theory calculations (DFT) confirm the mutual interactions between STFSI- pendants and ions in electrolytes and that between PEO segments and ions. We believe that these in-situ SLIGPEs with superior performances have great potential for the application in lithium metal batteries.

Keywords: lithium metal battery; single lithium ion; gel polymer electrolyte; in situ crosslinking; density functional theory calculations.

1. Introduction Liquid electrolytes used in lithium ion battery are playing irreplaceable roles in electrochemical energy storage for their high ionic conductivities (10-3~10-2 S·cm−1 at room temperature) and good surface contact with electrodes in the latest decades.[1,2] However, safety issues always exist due to the use of combustible liquid organic electrolyte, especially in the industrial application of electrical vehicles. Although efforts have been made to improve the safety of lithium batteries, including utilization of novel electrolytes[3-5], electrolyte additives[6,7] or anodes[8-14], the risks of leakage, internal short circuit, overheating and even combustion for liquid electrolytes are hardly avoided in real-world application. As an alternative, solid electrolytes have attracted much attention in recent years, which can avoid leakage and dendrite growth to alleviate the safety issues. However, the ionic conductivity of solid electrolytes at room temperature is comparably low (10-8~10-6 S·cm−1), which is out of the requirements of lithium batteries.[15-18] Moreover, solid electrolytes provide poor contact interfaces with electrodes, resulting in deteriorated cycling performance. Gel polymer electrolyte (GPE) is a polymer network, which can absorb liquid electrolytes in its microstructure. It provides promising solutions for the safety problems of lithium metal battery, because of its strong cohesion to solid electrode and high dispersive conductivity to liquid electrolyte. Moreover, GPEs can render the energy

storage devices with flexible shapes, which are candidate materials for the emerging portable and wearable electronics.[19–21] The elasticity of GPEs is also inclined to brook the volume change caused by electrode materials and lithium dendrite growth during lithium plating/stripping processes.[22-25] Traditional dual-ion conductor GPEs, in which both cations and anions are mobile similarly to those in liquid electrolyte solutions, are formed by the dissolution of electrolytic salts in polymers, like polyacrylonitrile (PAN),[26,27] poly(vinylidene fluoride) (PVDF),[28-31] poly(ethylene oxide) (PEO) and poly(methyl methacrylate) (PMMA).[32,33] They have attracted much attention owing to their great chemical stability and decent wettability, whereas their further applications have been limited because of several disadvantages, such as low thermal and electrochemical stability, poor cycling and rate performances.[34] To overcome the shortcomings, various polymers, salts and solvent were combined through solution casting,[35,36] in-situ polymerization,[37,38] extraction-activation,[39] or phase inversion method.[40–44] However, owing to the concentration polarization, these dual GPEs still suffer from eclipse of battery performance, including the increase of internal impedances, voltage losses, and undesirable reactions occurred in the electrolytes. The lithium ion transference number (tLi+) of dual GPEs is generally lower than 0.5, since the transport of Li+ is inhibited by the segmental motion of the polymer host. To reduce concentration polarization and increase the tLi+, the migration of anions inside of the GPEs should be suppressed. Thus, single lithium ion GPEs (SLIGPEs) attract further attention for its relative high conductivity and high lithium ion transference number

(tLi+). The limitation of anions’ motion in electrolytes will not affect the performance of battery since that the anions are not involved in electrode reactions.[45] Various strategies were tried to graft the anionic group of lithium salt onto the polymer backbone or add trapping agents into the electrolytes to limit the movement of anions.[46] In current methods commonly used, SLIGPEs are prepared by means of mixing polymers with adhesives.[46] However, the obtained membranes are often relatively heterogeneous with poor mechanical properties, which result in the reduction of ionic conductivity. To acquire homogenous membrane with both high lithium ion transference number and mechanical properties, in-situ crosslinking approach is a reasonable and feasible choice.[37] Herein, we propose an in-situ crosslinking strategy for synthesizing homogenous SLIGPEs with high lithium ion transference number. The –C6H4-SO2-N--SO2-CF3 (STFSI-) anions are introduced as the pendants of GPEs, which can enhance the ionic conductivity of Li+ by suppressing the formation of ion-pairing between counter ions and lithium cations. Besides, polyethylene glycol-dimethacrylate (PEGDMA), as the crosslinking agent, not only improves the mechanical properties of GPEs, but introduces PEO segments into the branches which benefit the transport of Li+.[47] Without any membrane-forming addictives, homogenous membranes were prepared by this in-situ crosslinking reaction. Membranes with various ratios of PEGDMA/LiSTFSI were prepared and investigated. Their chemical structures are characterized by Fourier Transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). Thermal properties and glass transition temperature are

investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Ionic conductivity is tested by Electrochemical Impedance Spectroscopy (EIS) in SS/SLIGPEs/SS battery, and lithium ion transference numbers are measured by EIS and chronoamperometry in Li/SLIGPEs/Li battery. Symmetric lithium battery assembled with the SLIGPEs, in which the molar ratio of PEGDMA/LiSTFSI is 1:2, exhibits high charge-discharge stability (over 600 h under 0.05 and 0.1 mA·cm-2, with 0.16 V and 0.24 V of over-potential, respectively). In LiFePO4/SLIGPE-2/Li batteries, a high specific discharge capacity of 132.1 mAh·g-1 at 0.1 C after 150 cycles at room temperature is also found, due to the lithium dendrites being significantly suppressed on lithium anode surface, which is observed by scanning electron microscope (SEM). Eventually, the interactions between the SLIGPEs and mobile ions are studied by density functional theory calculations (DFT). Therefore, SLIPGEs reported in the present work are promising in the practical application in lithium metal batteries.

2. Experimental section 2.1 materials 4-styrenesulfonic acid sodium salt (9dingchem, 99%), oxalyl chloride (Adamas, 98%), N,N-dimethylformamide (GreaGent, 99.8%), acetonitrile (J&K, 99.9%), trimethylamine (GreaGent, 99.8%), trifluoromethanesulfonamide (9dingchem, 98%), 4-dimethylaminopyridine (JG-Chemical, 99%), dichloromethane (GreaGent, 99.8%), sodium bicarbonate (Macklin, 99.8%), hydrochloric acid (Greagent, 36-38%), diethyl

ether (Greagent, 99.5%), potassium carbonate (Greagent, 99%), lithium perchlorate (Alfa, 99%), DMSO (J&K, 99.7%), LiTFSI (Aldrich, 99.95%) were used as received. PEGDMA (Aldrich) was dried by freeze drying at 0.008 mbar at room temperature, followed by addition of anhydrous 4 Ao molecule sieves (dried at 500 °C in muffle furnace). PEO (Mw = 2,000,000 g·mol-1) (Aladdin) was dried at 50 °C for 24h under vacuum before using. Azodiisobutyronitrile (AIBN; Adamas) was recrystallized twice from methanol before using. Liquid electrolyte (1.0 M LiTFSI, 0.1M LiNO3 in DME:DOL=1:1 Vol%) was purchased from Duoduo Chem.

2.2 Synthesis of single lithium ion gel polymer electrolytes (SLIGPEs) An improved method was adopted to synthesize the KSTFSI monomer.[48] 4.0 mL of oxalyl chloride (46.6 mmol) and DMF (0.1546 ml, 2 mmol) were added in 55 mL of dry acetonitrile in sequence, and then stirred for 6 hours under N2 protection. After stirring, the Vilsmeier-Haack solution was obtained and its color became yellow. Then 8 g of 4-styrenesulfonic acid sodium salt was added slowly to the solution at 0 °C under N2 protection. The acquired mixture was stirred for another 36 hours under N2 protection. NaCl precipitate was removed by filtration, then the brown solution was cooled to 0 °C. In this process, 16.2 mL of triethylamine (116.2 mmol), 5.78 g of trifluoromethylsulfonamide (38.8 mmol) and DMAP (1.58 g) were added in 45 mL of dry acetonitrile in succession. The solution was stirred for 2 h to dissolve all

components under the protection of N2, then cooled to 0 °C. The 4-styrene sulfonyl chloride synthesized in the last step was added slowly to above solution and stirred vigorously for 22 h. All Solvents were removed by vacuum evaporation. The obtained brown solid was dissolved in 70 mL of dichloromethane, then washed with 40 mL of 4% NaHCO3 aqueous solution for twice and 40 mL of 1 M HCl solution for one time. KSTFSI was obtained by neutralized with excess K2CO3 (2.68 g) in water. The resulting suspension was stirred for 2 h, separated by centrifugation and dried for 8 h at 60 °C. Finally, the acquired light yellow solid was purified by recrystallization in water. LiSTFSI was prepared by a metathesis reaction through ion exchange between KSTFSI and LiClO4 in CH3CN with nearly quantitative yield in a glove box (H2O and O2<0.1 ppm).[49] 2 g KSTFSI was dissolved in 80 ml of CH3CN. Insoluble substance was separated by centrifugation. Afterwards, certain moles of LiClO4 dissolved in CH3CN were dropwise added into equal moles of KSTFSI solution. After stirred overnight, the precipitate of KClO4 was filtered with 0.45 μm pore-sized PTFE membrane filter. Solvent was removed by vacuum evaporation and white solid was obtained. Then the solid was dissolved in CH3CN again and residue precipitate was filtered by a 0.22 μm pore-sized PTFE membrane filter. After the removal of solvent by vacuum evaporation, the purified solid was transferred to vacuum oven to remove residue CH3CN at 50 °C for 8 h. LiSTFSI with light white solid appearance was obtained and stored in dryer.

PEGDMA and LiSTFSI, with various molar ratios of 1:1, 1:2, 1:3.5, were added into a certain volume of dry DMSO solvent, fully dissolved by using supersonic treated for 1 h. Afterwards, a certain mass of AIBN (0.5~1 wt%) was added to the above solution and stirred for 10 min. Before pouring the solution into PTFE plate, oxygen was removed by bubbling with N2 for several minutes at 0 °C. The plate was transferred into drying oven for 15 h at 60 °C and kept horizontal in N2. Polymer membrane was obtained after the crosslinking reaction. Solvent inside the membrane was removed by dried at 75 °C overnight. Then the membrane was washed with diethyl ether for three times, kept in vacuum oven at 95 °C for 24 h to remove water and residue solvent. Single lithium ion membranes with various molar ratios of PEGDMA/LiSTFSI (1:1, 1:2, 1:3.5) are abbreviated as SLIM-1, SLIM-2, SLIM-3.5. After being punched into circular pieces (d=19 mm), the membrane was transferred into the glove box. SLIGPEs were obtained by soaking the membrane in the liquid electrolyte (1.0 M LiTFSI, 0.1M LiNO3 in DME:DOL=1:1 Vol%) for about 3 min. Corresponding SLIMs soaked with liquid electrolyte are abbreviated as SLIGPE-1, SLIGPE-2, SLIGPE-3.5. Pure PEGDMA membrane (SLIM-0) and gel electrolyte (SLIGPE-0) were synthesized similar to the above synthesis procedures.

2.3 Preparation of cathode LiFePO4 was mixed with carbon black and PEO with the mass ratio of 60:20:20 and thoroughly grounded into a mortar. Then the mixture was dispersed in CH3CN and stirred overnight.[50] The slurry was spread on the aluminum foil to produce

electrode films with loading of active materials around 0.5-1 mg·cm-2. The prepared cathode slices were dried at 90 °C for 12 h under vacuum and punched into circular pieces (d=9 mm) before use.

2.4 Characterization 1H

NMR spectra was employed to confirm the chemical structures of the

synthesized KSTFSI (Potassium 4-styrenesulfonyl (trifluoromethylsulfonyl) imide) and LiSTFSI by using a Brukeravance 500 spectrometer (500 MHz) with DMSO-d6 as solvent. The functional groups in SLIMs were studied by FTIR spectroscopy using Nicolet 7800 Fourier transform infrared spectrometer with ATR as tableting. Before measurements, samples were dried at 70 °C under vacuum for 24 h and then stored in a glove box for 2 h. Images of the surface and cross-section of SLIMs were observed by using NOVA NanoSEM 450 scanning electron microscopy (SEM) with an accelerating voltage of 5 kV. The crystallization of SLIMs were characterized by X-ray diffraction (XRD) using a Bruker D8 X-ray diffractometer (40 kV, 100 mA) with a copper target and Ni filter. The angle of 2θ is ranged from 10° to 75° with an increment of 0.02°. Thermogravimetric analysis (TGA) was used to evaluate the thermal stability of SLIMs by a TA SDT Q600 analyzer. Samples are heated from 25 °C to 800 °C with the heating rate of 10 °C min-1 in N2 flow.

The glass transition temperature (Tg) of SLIMs was studied by TA Q2000 differential scanning calorimetry (DSC). The mechanical properties of SLIMs were measured by the stress–strain test using Hengyi HY-0580 universal testing machine with a 100 N loading cell at the rate of tensile is 50 mm/min. The width and length of the sample is 4 mm and 5 cm, respectively. The contact angles between the SLIMs and water were measured by using a contact angle measuring system (JC2000D, CHINA POWEREACH) at room temperature. The absorbed rate of liquid electrolyte (n) by SLIMs was calculated according to the Equation (1):

n

Wt  W0 100% W0

(1)

in which W0 and Wt are the weights of the membranes before and after the absorption of organic liquid electrolyte, respectively. The whole process was done in a glove box. Temperature-resolved dynamic mechanical analysis (DMA) of SLIGPEs was conducted using a Q800 dynamic mechanical analyzer (TA Instruments) with 1 Hz of frequency at 10 °C·min-1 ramp.

2.5 Computational method All calculations were conducted by density functional theory calculations (DFT) at the B3LYP/6-31g++(d) level in Gaussian 09 package. A large molecule consisting of two EO segments was constructed to represent the PEGDMA, and a large anion

consisting of styrene and trifluorosulfimide represents STFSI- pendants. The binding energy (Eb) is defined as the energy shown in Equation (2):

Eb  E1  E2  Etotal

(2)

in which E1 is energy of mobile ions in SLIGPEs, E2 is the energy of PEGDMA or STFSI- pendants and Etotal is the energy of combining system of PEGDMA or STFSIpendants and mobile ions in SLIGPEs. Basis set superposition error was considered in the calculation of binding energy.

2.6 Electrochemical analysis The ionic conductivities were measured by electrochemical impedance spectroscopy (EIS). Blocking-type batteries were assembled, in which the SLIGPEs were sandwiched between two stainless steel (SS) electrodes. Impedance data were collected by the electrochemical working station (CHI660E Chenhua, Shanghai) in the frequency range of 0.1 Hz-1 MHz with 5 mV of potential amplitude between 20 and 90 °C. The ionic conductivity was calculated according to the Equation (3):

  L / ( A  R)

(3)

in which σ is the ionic conductivity, R is the bulk resistance, L is the thickness of the SLIGPEs, and A is the area of contact area of the stainless steel electrode and SLIGPEs. Linear sweep voltammograms were drawn by using the electrochemical working station. A two-electrode cell was assembled for this test, in which lithium foil is the counter and reference electrode, and stainless steel is the working electrode. Each cell were tested between 2.5 and 5.5 V (vs. Li/Li+) at the scan rate of 1 mV·s-1.

The lithium ion transference numbers (tLi+) were measured according to the chronoamperometry. The SLIGPEs are sandwiched between two lithium metal electrodes with an applied voltage of 10 mV. tLi+ was calculated by the Equation (4):

t Li  

I SS V  I 0 R0  I 0 V  I SS RSS

(4)

in which I0 and Iss are the initial and steady-state current, respectively. V is the applied voltage. R0 and Rss are the initial and steady-state resistance, respectively. EIS data were collected before and after the potentiostatic polarization at room temperature, ranging from 0.1 Hz to 1 MHz with a 5 mV amplitude. The dynamically interfacial stability between GPEs and Li metal at room temperature was investigated by the polarization test of Li/SLIGPEs/Li symmetric battery under different current densities for over 600 h. 2032 coin-type batteries were assembled to measure the performance of LiFePO4 cathode, with lithium metal foil as the both counter and reference electrode. The SLIGPEs were used as both the separator and electrolyte. Cycling performance was studied by the Land battery tester (CT2001A). The voltage is between 2.0 and 3.8 V and current density varies from 0.1 C to 0.4 C with LiFePO4 as cathode. 1 C is defined as 170.0 mAh·g-1 for all C rate measurements. All batteries were assembled in an Ar-filled glove box with oxygen content less than 0.1 ppm and H2O content less than 0.1 ppm.

3. Results and discussion SLIMs are obtained by an in-situ crosslinking approach (Scheme 1). The original precursor, consisting of SLI monomer (LiSTFSI) and crosslinker, is a flowable light

yellow liquid. After crosslinking reaction, the acquired polymer exhibits as a solid membrane (Scheme 1a). SLI monomer (LiSTFSI) is synthesized according to the reported method (Scheme 1b).[48] STFSI- groups are introduced as the pendants of polymer backbones, since the diffusion of anion can be limited by the repulsive columbic interactions between the anion and STFSI- pendants (Table S1).[48] The chemical structure of monomer has been characterized by the 1H NMR, consistent with the literature (Fig. S1).[51] (399.76 MHz; DMSO-d6; TMS; ppm): 5.37 (d, J = 11.2 Hz, 1H), 5.98 (d, J = 17.6 Hz, 1H), 6.78 (q, J = 9.6 Hz, 1H), 7.57 (d, J = 8.4 Hz, 2×1H), 7.71 (d, J = 8.4 Hz, 2×1H). Except as a crosslinker, PEGDMA can also enhance both the flexibility and mobility of Li+ of SLIGPEs for its PEO segment. For a typical in-situ crosslinking process of SLIMs (Scheme 1c), LiSTFSI and PEGDMA are crosslinked by free radical polymerization with AIBN as an initiator in a mould. SLIGPEs are finally obtained by immersing the SLIMs into liquid electrolytes (1.0M LiTFSI, 0.1M LiNO3 in DME:DOL=1:1 Vol%). SLIM-0 and SLIGPE-0 are also prepared via similar method as references.

Scheme. 1 (a) The images of the precursor solution and SLIMs after in-situ crosslinking, with the corresponding change of structures. (b) Synthsis procedures and the molecular structure of LiSTFSI. (c) Preparation process of SLIMs by in-situ crosslinking reaction.

The chemical structures of SLIM-0, SLIM-1, SLIM-2 and SLIM-3.5 are characterized by FTIR spectra (Fig. 1a). The absorption of ester C=O for the PEGDMA crosslinker is at 1714.0 cm-1.[47] After crosslinked, the absorption of ester

C=O in SLIM-0 is shifted to 1727.3 cm-1, due to the vanishment of adjacent C=C bonds in SLIM-0.[52,53] The absorptions of ester C=O in SLIM-1, SLIM-2 and SLIM-3.5 are at 1721.2, 1719.7 and 1717.1 cm-1, respectively. Compared to that of SLIM-0, the shift decreases with the increase of STFSI- pendant content. Absorptions at 1325.0, 1321.8 and 1323.1 cm-1 are attributed to the asymmetric stretching vibrations of S=O in sulfonimies (SO2-N-SO2), and absorptions at 1296.6, 1296.7 and 1296.7 cm-1 are caused by the stretching vibration of CF3 groups, for SLIM-1, SLIM-2, SLIM-3.5 respectively.[48,54] From SLIM-1 to SLIM-3.5, the increase of relative intensity of C=O to –CF3 indicates the enhanced content of STFSI- pendants in SLIMs. Absorptions from 1200 to 1000 cm-1 are caused by stretching vibration of CF and CF2.[48,55] The presence of these characteristic absorptions of functional groups in membranes identifies the success of in-situ crosslinking between PEGDMA and LiSTFSI. Since organic components in SLIMs completely decompose above 800 °C, the mass of residue measured by TGA (0%, 16.1%, 19.52% and 21.77% for SLIM-0, SLIM-1, SLIM-2 and SLIM-3.5, respectively) shows the contents of the lithium salts originated from STFSI- pendants after decomposition (Fig. 1b). The temperature of initial decomposition for SLIMs is around 300 °C, indicating that these membranes possess much higher thermal stability than the conventional liquid electrolytes. The residual mass of KSTFSI (before ion exchanging) after thermal decomposition is exact 44% of the original mass (Fig. S2a). In addition, the XRD spectrum of this residue in Fig. S2b matches well with the standard KCl peaks. Moreover, it is

confirmed that the residues of SLIMs after thermal decomposition are LiF (from monomer) and KCl (from LiClO4 and KSTFSI during ion exchange process), and the mass fractions are 20.7% and 79.3% proven by XRD method, respectively (Fig. S2b). Thus, the STFSI- pendant contents in SLIM-1, SLIM-2 and SLIM-3.5 are 46.99%, 56.97% and 63.52%, respectively.[47] (Detailed calculation is shown in supporting information) These results are roughly consistent with the feeding mass ratio (39.10%, 56.22%, 65.83% for SLIM-1, SLIM-2 and SLIM-3.5) and the deviation comes from the inevitable errors of the experimental procedure.

Fig. 1. (a) FTIR spectra of SLIM-0, SLIM-1, SLIM-2 and SLIM-3.5. (b) TGA of SLIM-0, SLIM-1, SLIM-2 and SLIM-3.5. (10 °C·min-1 of heating rate and under N2 protection). (c) DSC

curves of SLIM-0, SLIM-1, SLIM-2 and SLIM-3.5 (10 °C·min-1 of heating rate and under N2 flow). Inserts are zoomed in part for determination of glass transition temperature. (d) Contact angle between water and Celgard 2400 or SLIMs within 40 s. (e) Images for contact angle between water and Celgard 2400 or SLIM-2 at 0 s and 40 s.

Thermal behaviors of SLIMs have been investigated by means of DSC (Fig. 1c). Tg of SLIM-0 is -40.09 °C. Tg of SLIM-1, SLIM-2 and SLIM-3.5 are -18.61, -13.39 and -13.01 °C, which increase with the increase of STFSI- pendant contents in the membranes. In addition, no exothermic peak is found during the cooling process, indicating that all SLIMs are in amorphous state.[47] Besides, XRD results (Fig. S3) and no observed melting endothermic peaks within the tested DSC temperature range suggest the absence of crystallization in the SLIMs. Notably, the amorphous structure of the crosslinked polymer membranes would facilitate the transport of lithium ions. The surface and cross-section of SLIM-0 and SLIM-2 membranes have been observed by SEM. Both of them appear homogenous and flat surface (Fig. S4), which is in favor of the interfacial contact between GPEs and electrodes. From the cross-section images (the inset of Fig. S4), no pores are observed inside of the membranes. The thicknesses of both membranes are around 200~300 μm. Both membranes are transparent and exhibit highly flexible characteristics (Fig. S5). The continuous and homogenous structures of SLIMs with excellent mechanical property (Fig. S6) are essential to obtain uniform GPEs. The wetability of the SLIMs has been evaluated by contact angle test between SLIMs and water droplets.[56] As shown in Fig. 1d, the contact angle between Celgard

2400 separator and water keeps stable over 40 s. After 40 s, the contact angle is 112.3°, which is higher than 90°, indicating hydrophobic surface of the commercial separator (Fig. 1e). Interestingly, the contact angles between SLIMs and water are all below 90° after 40 s, suggesting the hydrophilic characteristic of SLIMs (Fig. 1e). Besides, the water contact angles for SLIMs decrease over time. Meanwhile, with the increase of ratio of STFSI- pendants, the contact angles between SLIMs and water apparently decrease. The contact angle between SLIM-0 and water is 87.8° while the contact angle obviously decreases to 53.8° for SLIM-3.5. It can be concluded that STFSI- pendants are able to intensively enhance the wettability of membranes and improve electrolyte uptake (Fig. S7), thus increase the ionic conductivity of SLIGPEs.[17] The mechanical strength of GPEs usually dramatically decreases and exhibits unsteadiness after soaked with liquid electrolytes. However, from 10 to 90 °C, the storage modulus of SLIGPEs almost remain the same (Fig. S8), indicating their high thermal stability. Even so, optical photos of SLIGPEs exhibit that SLIGPE-1 is easy to crack into pieces (Fig. S9a) and SLIGPE-3.5 is inclined to wrinkle and form fracture (Fig. S9c). Therefore, both of them can not be used to assemble batteries. SLIGPE-2 can be applied to assemble batteries because it is hardly damaged (Fig. S9b). For this reason, the ionic conductivities and lithium transference numbers are measured for all SLIGPEs, but the electrochemical window, cycle life and rate performance are only investigated by the SS/SLIGPE-2/Li, Li/SLIGPE-2/Li and LiFePO4/SLIGPE-2/Li battery with SLIGPE-0 as references.

The ionic conductivity of GPEs is one of essential performances for the lithium metal batteries. Electrochemical impedance spectra (EIS) of a SS/SLIGPEs/SS symmetrical batteries at different temperatures are performed to study the ionic conductivity of SLIGPEs. As is shown in Fig. S10, the bulk resistance (Rb, the resistance of SLIGPEs) decreases with the increase of temperature (20-90 °C). The temperature dependency upon ionic conductivity for SLIGPEs is investigated in the range of 20 °C to 90 °C with 10 °C as increment (Fig. 2a), showing the increase of ionic conductivity over temperature. The ionic conductivities of SLIGPE-0, 1, 2 and 3.5 under room temperature are 9.78×10-6, 1.71×10-5, 1.94×10-5 and 2.74×10-5 S·cm-1, respectively. With the increase of STFSI- pendant contents in the GPEs, the ionic conductivity of SLIGPEs also increases. Apparently, the introducing of STFSIpendants in polymer network improves the permeability of the SLIMs, resulting in higher uptake of liquid electrolyte (Fig. S7). It is noted that the ionic conductivity of SLIGPE-3.5 increases from 2.74×10-5 S·cm-1 under room temperature to 5.14×10-4 S·cm-1 at 90 °C (Fig. 2a), which is higher than that of previous literature.[51] The approximately linear relationship between logσ and 1000/T indicates that no structure has been changed in SLIGPEs over the broad temperature range. Moreover, the Rb of SLIGPE-2 increases from the initial 472 Ω to 551 Ω after 13 days, and then remains constant from 13 days to 33 days, due to the slight aging of SLIGPEs (Fig. S11). For conventional liquid electrolyte, low transference number often causes the high concentration of mobile anions in the electrolyte and subsequently leads to the polarization and unavoidable side reactions of electrodes.[57] In this work, the

introducing of STFSI- pendants in the crosslinking polymer network can remarkably enhance the lithium ion transference number (tLi+) and inhibit the polarization. From Fig. 2b, calculated by Equation 4, the transference number of SLIGPE-3.5 is 0.622, and that of SLIGPE-0, SLIGPE-1 and SLIGPE-2 are 0.429, 0.523 and 0.595, respectively (Fig. S12). Apparently, tLi+ increases with the increase of STFSIpendants in SLIGPEs. With the help of STFSI- pendants, SLIGPEs can restrain the movement of anions in the electrolyte and reduce the polarization. Although the tLi+ of SLIGPE-2 is slightly lower than that of SLIGPE-3.5, SLIGPE-2 exhibits superior mechanical property (Fig. S8). Therefore, SLIGPE-2 is chosen to be further studied.

Fig. 2. (a) The temperature dependency upon ionic conductivity of SLIGPEs. (b) The chronoamperometry profile of a symmetric Li/SLIGPE-3.5/Li battery at the polarization potential of 10 mV, and the EIS before and after the polarization (insert). (c) EIS of a Li/SLIGPE-2/Li

symmetric battery for different aging time at room temperature. The insert is the enlarged region of Rb. (d) Linear sweep voltammetry profile of SLIGPEs at 1 mV/s at room temperature, using stainless steel as the working electrode, Li metal as the counter and reference electrode. (e) Voltage profiles of Li/SLIGPE-2/Li and Li/SLIGPE-0/Li batteries at various current densities under the fixed capacity of 7.85×10-2 mA·h at room temperature. The inserts are the selected regions of voltage profiles. (f) Voltage profiles of Li/SLIGPE-2/Li and Li/SLIGPE-0/Li batteries at current density of 0.05 mA·cm-2 under the capacity of 7.85×10-2 mA·h at room temperature. The inserts are enlarged regions of the voltage profiles.

To evaluate the interfacial compatibility between SLIGPEs and Li metal, EIS of Li/SLIGPE-2/Li symmetric battery upon aging time has been measured. As shown in Fig. 2c, the Rb (the resistance of GPE) increases from initial 425 Ω to 510 Ω after 13 days, in accordance with the results found in the SS/SLIGPE-2/SS symmetric battery (Fig. S11). From 13 to 33 days, no obvious change of resistance suggests that the ionic conductivity of SLIGPE-2 at room temperature is constant over long time period. Furthermore, Ri (interfacial resistance) also reaches constant after 13 days, indicating the formation of uniform interfacial resistance layer on the surface of Li electrodes. All these results indicate that homogenous SLIGPEs (Fig. S4) are stable enough to endure aging. SLIGPEs and Li electrodes also possesses superior interfacial contact. The electrochemical window of SLIGPE-0 and SLIGPE-2 has been investigated by linear sweep voltammetry (LSV) scan. As is shown in Fig. 2d, the SLIGPE-2 and SLIGPE-0 keep stable up to 4.7 V vs. Li+/Li from the anodic scan part, slightly higher than liquid electrolyte, which meets the voltage requirement for the lithium metal

battery with LiFePO4 as the cathode. The dynamical interfacial stability between SLIGPEs and Li metal anode at room temperature has been studied by polarization test of lithium symmetric battery. As shown in Fig. 2e, the over-potential of SLIGPE-0 is much higher than that of SLIGPE-2 at various current densities and fixed capacity of 7.85×10-2 mAh. It indicates that the STFSI- pendants in SLIGPE-2 is able to reduce the polarization of electrolyte. With the increase of current density, the over-potential of SLIGPE-2 slightly increases, while that of SLIGPE-0 increases intensively. It indicates that STFSI- pendants are capable of limiting anions movement in electric field to reduce the increase of over-potential. The insert figure in Fig. 2e displays the specific voltage profiles of batteries with SLIGPE-2 cycled from 5th to 10th, 25th to 30th and 45th to 50th cycles. The emergence of flat plateaus suggests the existence of smooth Li plating/stripping processes, due to the decent ionic conductivity of the SLIGPE-2 at the elevated current density. In contrast, SLIGPE-0 shows a rough process. To identify the dynamically interfacial stability between the interface of Li metal anode and SLIGPEs, the long-time polarization of the symmetric battery at current density of 0.05 mA·cm-2 at room temperature has been investigated (Fig. 2f). It is found that the initial over-potentials are about 0.16 V and 0.52 V for the plating and stripping of the lithium metal in the batteries, respectively. In the presence of SLIGPE-2, low voltage polarization without short circuit after cycled for over 600 h is realized. For SLIGPE-0, the over-potential increases over time, indicating that the cycling reversibility and stability for lithium plating/stripping of battery in the

presence of SLIGPE-2 is superior to that in the presence of SLIGPE-0. Long cycle life and low over-potential are possible, since STFSI- pendants can suppress the high partial current density by regulating the movement of lithium ion and electric field, and thus inhibit the growth of lithium dendrites and dead lithium. The small fluctuation of potential is caused by the change of surrounding temperature. For SLIGPE-2, the over-potential decreases with the increase of cycling time,[58] due to the improvement of interface between SLIGPE-2 and lithium electrode during the plating and stripping process. To study the cycling performance of SLIGPEs at higher current density, the polarization tests of the symmetric batteries with SLIGPE-2 and SLIGPE-0 as electrolytes at current density of 0.1 mA·cm-2 are also studied (Fig. S13a). In the presence of both SLIGPEs, the initial over-potential is about 0.24 V and 1.2 V during the plating and stripping process of lithium metal, respectively. With the progression of cycle time, the over-potential of battery in the presence of SLIGPE-2 decreases and reaches steady at 0.16 V. Moreover, the over-potential of battery with SLIGPE-0 declines intensively due to short circuit at this current density, indicating that SLIGPE-0 can not suppress the irregular deposition of lithium metal and avoid the dendrite growth. The polarization test at higher current density (0.2 mA·cm-2) in the presence of SLIGPE-2 has also been performed (Fig. S13b). The over-potential is steady even after cycling over 160 h without any short circuits. The over-potential of this battery is about 0.2 V. Hence, high ionic conductivity, good interfacial compatibility to lithium metal electrode and high lithium transference number of SLIGPE-2 is able to limit the

growth of lithium dendrites, thus enhancing low over-potential and cycle life of lithium metal batteries. LiFePO4, as a well-developed commercial cathode, is chosen to explore the feasibility of SLIGPEs well. With SLIGPE-2 and SLIGPE-0 as both electrolyte and separator, LiFePO4/SLIGPEs/Li battery has been assembled to evaluate the cycling stability and rate performances. The initial charge-discharge curves of batteries in the presence of SLIGPE-2 or SLIGPE-0 at 0.1 C have been collected at room temperature (Fig. 3a). The over-potentials of both SLIGPEs are similar to each other, but the battery in the presence of SLIGPE-2 exhibits the high initial discharge capacity of 132.1 mAh·g-1 and the coulombic efficiency over 95%, which are much higher than those in the presence of SLIGPE-0. Meanwhile, the specific capacity shows almost unchanged with the capacity retention ratio over 90% after 21 cycles (Fig. S14), verified by no shift of peaks in the CV after 21 cycles (Fig. 3b). The rate performance of batteries with both SLIGPEs as electrolyte at the current densities from 0.1 C to 0.4 C has been investigated (Fig. 3c). The battery with SLIGPE-2 as electrolyte exhibits discharge capacities of 135.4, 111.2, 96.3 and 54.1 mAh·g-1 at 0.1 C, 0.2 C, 0.3 C and 0.4 C, respectively. In contrast, the battery with SLIGPE-0 as electrolyte exhibits lower discharge capacities (101.5, 64.8, 42.7 and 23.9 mAh·g-1 at 0.1 C, 0.2 C, 0.3 C and 0.4 C). The rate performance of batteries in the presence of SLIGPE-2 is superior to that in the presence of SLIGPE-0, due to the former possessing higher lithium ion transference number and ionic conductivity. Particularly, the discharge capacities in

the presence of SLIGPE-2 (137.7 mAh·g-1) and SLIGPE-0 (81.1 mAh·g-1) are recovered when the current density returns to 0.1 C after rate cycles. Moreover, the cycling stabilities of batteries in the presence of both SLIGPEs are studied at the rate of 0.1 C at room temperature (Fig. 3d). The reversible capacity in the presence of SLIGPE-2 is 133.3 mAh·g-1 and reaches plateau at 101.2 mAh·g-1 with over 95% of coulombic efficiency. However, the reversible capacity in the presence of SLIGPE-0 is only 111.3 mAh·g-1 and become steady at 70.4 mAh·g-1. In the presence of GPE with STFSI- pendants, the degradation of battery performances is alleviated through the regulation of electric field at a high current density. Besides, no variation has been found for the over-potentials beyond 100 cycles for the battery with SLIGPE-2 (Fig. 3e). Interestingly, the over-potential of the battery in the presence of SLIGPE-0 increases from 0.1 to 0.3 V after 100 cycles (Fig. 3f), possibly due to formation of dead lithium and lithium dendrites by uneven deposition of lithium. For this reason, morphology

of

lithium

anode

surface

in

LiFePO4/SLIGPE-0/Li

and

LiFePO4/SLIGPE-2/Li batteries after 150 cycles at 0.1 C has been observed by SEM (Fig. S15). It is found that Li anode surface in LiFePO4/SLIGPE-0/Li battery is coarser than that in LiFePO4/SLIGPE-2/Li battery because of the growth of lithium dendrites. The interface between SLIGPE-0 and electrodes during cycles is unstable for the same reason observed by the SEM images (Fig. 3g). In contrast, in the presence of SLIGPE-2, no lithium dendrites are observed even after 150 cycles at 0.1 C (Fig. 3h). Binding energy has been calculated to confirm the interaction intensity between STFSI- pendants or PEGDMA and ions in SLIGPEs by DFT calculation (Fig.

4), which is listed in Table S1. It shows that the coulomb repulsion between STFSIpendant and TFSI- (-5.54 eV) or NO3- (-0.84 eV) is stronger than that between PEGDMA and TFSI- (-3.92 eV) or NO3- (-0.69 eV), indicating that STFSI- pendants are in favor of limiting the motion of anions and suppressing the concentration polarization. Moreover, the binding energy between STFSI- pendant and lithium ion (6.72 eV) is much higher than that between PEGDMA and lithium ion (1.80 eV), which can enhance the Li+ conductivity by facilitating the dissociation of Li+ and suppress the growth of lithium dendrite by uniformly deposition of lithium metal on anode surface (Fig. S15). Especially, the contributions of STFSI- pendants in these processes are much higher than those of PEO segments in PEGDMA. These calculation results are consistent with the electrochemical measurements and well explain the cycling performance of LiFePO4/SLIGPE-2/Li battery superior to that of LiFePO4/SLIGPE-0/Li battery. In addition, the cycling stability of batteries in the presence of both SLIGPEs under 60 °C has been investigated (Fig. S16). The initial discharge capacities in the presence of SLIGPE-2 and SLIGPE-0 are increased to 144.4 and 121.6 mAh·g-1, respectively. However, their cyclability is not stable, because high temperature accelerates molecular dynamics, causing rapid growth of lithium dendrites and consumption or thermal instability of liquid electrolyte.[59, 60]

Fig. 3. (a) Initial charge and discharge curves at 0.1 C with LiFePO4 as cathode in the presence of SLIGPE-0 and SLIGPE-2 at room temperature. (b) The CV curves of LiFePO4/SLIGPE-2/Li battery at 1st and 21st cycles at room temperature. (c) Rate performances of LiFePO4/SLIGPEs/Li

battery at room temperature. (d) Cycling performances of LiFePO4/SLIGPEs/Li battery at 0.1 C at room temperature. (e) Selected charge-discharge curves in the presence of SLIGPE-2 at various cycles. (f) Selected charge-discharge curves in the presence of SLIGPE-0 battery at various cycles. (g) Top view of SEM image for Li anode surface in LiFePO4/SLIGPE-0/Li battery after 150 cycles at 0.1 C at room temperature. (h) Top view of SEM image for Li anode surface in LiFePO4/SLIGPE-2/Li battery after 150 cycles at 0.1 C at room temperature.

Fig. 4. Sketch of molecule and ions in the SLIGPEs. DFT calculation showing the interaction between STFSI- pendants and (a) Li+, (b) TFSI-, (c) NO3-, and the interaction between PEGDMA and (d) Li+, (e) TFSI-, (f) NO3-. (grey, silver, red, blue, green, yellow and purple balls represent carbon atoms, hydrogen atoms, oxygen atoms, nitrogen atoms, fluorine atoms, sulphur atoms and lithium ions, respectively).

Although the ionic conductivity and tLi+ of SLIGPEs synthesized in our work are not the most prominent compared with previous works in Table S2, it exhibits outstanding specific capacity (133 mAh·g-1) and cyclability of 101.2 mAh·g-1 over 150 cycles. And SLIGPEs obtained by in-situ crosslinking show more homogenous

status than solution casting or any other method. Therefore, relatively high transference number, limitation of anion motion, reduction of lithium dendrites and good interfacial contact between SLIGPE-2 and electrodes, can obviously enhance the reversible capacity, rate performance and cycling stability of batteries. Besides, the promoted compatibility of SLIGPEs with LiFePO4 cathode also benefits its practical application in lithium metal battery at ambient temperature.

4. Conclusions In the present work, SLIMs with 3D network structure were fabricated by in-situ crosslinking of LiSTFSI and PEGDMA. After absorbing liquid electrolytes, the obtained SLIGPEs exhibit robust framework, good thermal stability, high ionic conductivity (2.74×10-5 S·cm-1 at room temperature), high lithium ion transference number (0.622) and broad electrochemical window (4.7 V). Low voltage polarization is found with the symmetric lithium battery cycling for over 600 h without short circuit at the current density of 0.05 mA·cm-2 and 0.1 mA·cm-2, indicating the superior interfacial compatibility between SLIGPE-2 and lithium metal electrodes. Notably, LiFePO4/SLIGPE-2/Li exhibits specific discharge capacity (132.1 mAh·g-1) at 0.1 C, capacity retention ratio (80.0% after 150 cycles), coulombic efficiency (99.0%) and superior rate performance after 150 cycles at room temperature, higher than those of LiFePO4/SLIGPE-0/Li, indicating the excellent electrochemical performance of GPEs with STFSI- pendants and superior compatibility with LiFePO4 cathode materials. Verified by electrochemical analyses and DFT calculations, STFSI-

pendants and PEO segments in PEGDMA are able to enhance the Li+ conductivity by facilitating the dissociation of Li+ and suppress the growth of lithium dendrite through uniformly deposition of lithium metal on anode surface confirmed by SEM. Moreover, the contribution of STFSI- pendants is much higher than that of PEO segments in PEGDMA. In conclusion, batteries with SLIGPEs exhibit high ionic conductivity, uniform lithium metal deposition, superior rate performance and excellent cycling stability, which endow the SLIGPEs with great potential for the application in lithium metal batteries.

Acknowledgements This

work

was

supported

by

National

Key

R&D

Program

of

China

(2016YFB0901600), National Natural Science Foundation of China (21476143, 51003028, 51772313, U1830113 and 51802334) and PetroChina Innovation Foundation (2016D-5007-0211).

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Graphical Abstract

Single lithium ion gel polymer electrolytes with STFSI- pendants have high tLi+ and superior battery performance.

SLIMs are prepared by in-situ polymerization of LiSTFSI and PEGDMA. Motions of anions are suppressed by the interactions between the STFSI- and ions. Li+ deposition in SLIGPEs is regulated uniformly to avoid lithium dendrite growth. SLIGPEs show high tLi+ and interfacial stability with electrodes. LiFePO4/SLIGPEs/Li battery exhibits superior cycle stability and rate performance.