Hongjiao Wang

Lilin Wu

Bai Xue

Fang Wang

Zhongkuan Luo

Xianghua Zhang

Laurent Calvez

Ping Fan
email: fanping@szu.edu.cn

Bo Fan

Improving Cycling Stability of the Lithium Anode by a Spin-Coated High

Keywords: lithium anode, Li3PS4, spin-coated, artificial SEI layer, sulfide solid electrolyte, cycle stability, lithium-metal batteries

INTRODUCTION

Lithium (Li) has received widespread attention because of its high theoretical capacity of 3860 mAh g -1 and low electrochemical potential (-3.04 V vs standard hydrogen electrode). [START_REF] Chen | Long Cycle Life Lithium Metal Batteries Enabled with Upright Lithium Anode[END_REF][START_REF] Fan | Protection of Lithium Anodes by Fibrous Silica Nanospheres[END_REF][START_REF] Gu | Toward High-Performance Li Metal Anode via Difunctional Protecting Layer[END_REF][START_REF] Hou | Facile Construction of a Hybrid Artificial Protective Layer for Stable Lithium Metal Anode[END_REF] However, there are still some problems that hinder the application of Li anode. The unstable solid electrolyte interphase (SEI) formed on the lithium-metal electrode is one of the important issues. [START_REF] Lee | Long-Lasting Solid Electrolyte Interphase for Stable Li-Metal Batteries[END_REF][START_REF] Palacin | Why do Batteries Fail?[END_REF] Owing to the extremely low redox potential, the Li electrode inevitably reacts with the electrolyte to form the heterogeneous SEI. Usually, the SEI is fragile and suffers a vicious cycle of formation, rupture and reformation. This will not only lead to the formation of dead Li and electrode powdering, but also aggravates the uneven deposition of Li which induces dendrite growth. [START_REF] Wang | Confronting the Challenges in Lithium Anodes for Lithium Metal Batteries[END_REF][START_REF] Wang | Constructing Nitrided Interfaces for Stabilizing Li Metal Electrodes in Liquid Electrolytes[END_REF][START_REF] Lin | Reviving the Lithium Metal Anode for High-energy Batteries[END_REF] Unstable SEI will cause a sharp drop in battery performance. Then, electrolyte additives are widely used to stabilize the SEI. The electrolyte additives change the composition of the SEI by participating in the formation reaction of the SEI.

It is revealed that the additives such as lithium bisfluorosulfonimide (LiFSI) [START_REF] Miao | Novel Dualsalts Electrolyte Solution for Dendrite-free Lithium-metal Based Rechargeable Batteries with High Cycle Reversibility[END_REF] , lithium hexafluorophosphate (LiPF6) [START_REF] Zheng | Electrolyte Additive Enabled Fast Charging and Stable Cycling Lithium Metal Batteries[END_REF] , lithium nitrate (LiNO3) [START_REF] Zhang | Highly Stable Lithium Metal Batteries Enabled by Regulating the Solvation of Lithium Ions in Nonaqueous Electrolytes[END_REF] , and polysulfide [START_REF] Cheng | Sulfurized Solid Electrolyte Interphases with a Rapid Li + Diffusion on Dendrite-free Li Metal Anodes[END_REF] can increase the inorganic contents such as LiF, Li3N and Li2S in the SEI, improving its ionic conductivity and toughness. This promotes the stable cycling of the Li electrode.

The SEI modified by the electrolyte additives still exhibits a multi-phase heterogeneous mosaic structure, which harms the deposition uniformity of Li and the toughness against the stress caused by the volume change of the Li anode. [START_REF] Liu | An Artificial Solid Electrolyte Interphase with High Li-ion Conductivity, Mechanical Strength, and Flexibility for Stable Lithium Metal Anodes[END_REF][START_REF] Liu | Lithiophilic Ag/Li Composite Anodes via a Spontaneous Reaction for Li Nucleation with a Reduced Barrier[END_REF][START_REF] Yan | Self-Formed Protection Layer on a 3D Lithium Metal Anode for Ultrastable Lithium-Sulfur Batteries[END_REF][START_REF] Zhang | Dendrite-Free Li-Metal Anode Enabled by Dendritic Structure[END_REF][START_REF] Luo | Interfacial Challenges Towards Stable Li Metal Anode[END_REF] Moreover, with the continuous consumption of the additives, the protection ability of the modified SEI is gradually weakened during the long-term cycling.

Recently, the artificial SEIs are constructed on the surface of the Li anode before battery assembling, whose microstructure can be better controlled in comparison with the additive-modified SEI. [START_REF] Hu | An Artificial Hybrid Interphase for an Ultrahigh-rate and Practical Lithium Metal Anode[END_REF][START_REF] Zhang | Composite Lithium Protective Layer Formed In Situ for Stable Lithium Metal Batteries[END_REF] Li et al. [START_REF] Li | An Artificial Solid Electrolyte Interphase Layer for Stable Lithium Metal Anodes[END_REF] reported that an artificial SEI layer of Li3PO4 could be formed by the reaction of polyphosphoric acid with lithium, which suppressed parasitic reactions between lithium anode and electrolyte. More attention has been appealed to the Li3PS4-based SEI layers, viewing that the high ionic conductivity of the SEI layer improves the lithium deposition/plating homogeneity. Pang et al. [START_REF] Pang | An In Vivo Formed Solid Electrolyte Surface Layer Enables Stable Plating of Li Metal[END_REF] used Li2S6 and P2S5 as additives which in-vivo reacted with Li anode to form Li3PS4 SEI layer.

The strategy improved the cycle performance of lithium symmetrical cells and inhibited the formation of lithium dendrites. The thickness and composition of the in-vivo formed Li3PS4 SEI layer were somehow difficult to control due to the complex reaction condition within the battery, which resulted in a relatively large hysteresis voltage of lithium symmetrical cells during long-term cycling. Liang et al. [START_REF] Liang | In Situ Li3PS4 Solid-State Electrolyte Protection Layers for Superior Long-Life and High-Rate Lithium-Metal Anodes[END_REF] reacted P4S16 and lithium to prepare Li3PS4-protected Li anodes. Benefitting from the good controllability of the reaction out of the batteries, the thickness of the protective layer could be controlled to nanoscale so that excellent cycling performance in lithium symmetrical cells and Li-S batteries were obtained. These studies show that high phase purity and nanoscale thickness are essential to achieve good Li-protection ability of the Li3PS4based SEI layer.

Since the complexity of the reaction between lithium and the phosphor-sulfur compounds, the thickness and phase purity of these ex-situ Li3PS4 SEI layers are still difficult to control. [START_REF] Li | P4S10 Modified Lithium Anode for Enhanced Performance of Lithium-sulfur Batteries[END_REF] Here, a new strategy is proposed to prepare a nanoscale Li3PS4 artificial SEI layer with good phase purity for lithium anode. This strategy includes the synthesis of a lithium-compatible Li3PS4 sol and a following spin-coating of the sol to

EXPERIMENTAL SECTION

Preparation of Li3PS4 sol

The chemicals used were sulfur powder (99.5%, Alfa Aesar), tetrahydrofuran (≥99.5%, Aladdin), LiEt3BH (1 M, Innochem), Li2S (99.9%, Alfa Aesar), P2S5 (99%, Aladdin), poly(1,1-difluoroethylene) (Macklin), N-methyl-2-pyrrolidone (99.9%, Aladdin), 1.0 M LiN(CF3SO2)2 (LiTFSI) in 1,3-dioxolane/dimethyl ether (DOL/DME, 50-50 vol.%) with 1.0 wt.% LiNO3 and 1.0 M LiPF6 in ethylene carbonate/diethyl carbonate (EC/DEC, 50-50 vol.%) were used as electrolytes, which were purchased from Do Do Chem. All the chemicals were used as is, without further purification.

Dual-precursor liquid phase synthesis method was applied to prepare Li3PS4 sol.

First, 4 mmol sulfur powder was added into 8 mL of tetrahydrofuran (THF) solution containing LiEt3BH. After stirring for 3 h, the Li2S sol was obtained. The sol was further purified by centrifugation and then redispersed in THF. Second, a certain amount of Li2S and P2S5 (molar ratio 1:1) was added into 3 mL of THF, and stirred for 40 min to obtain a Li2S•P2S5 solution. Finally, the Li2S•P2S5 solution was added to the Li2S sol to get the Li3PS4 sol, and the molar ratio of Li2S and P2S5 was 3:1. The concentration of the Li3PS4 sol was controlled to be 0.1, 0.08, 0.06, 0.04 and 0.02 M, respectively, by adjusting the concentration of the two precursors. All the operations were conducted in an argon-filled glovebox (O2 < 0.5 ppm, H2O < 0.5 ppm).

Preparation of Li3PS4 artificial SEI layer

The Li3PS4 artificial SEI layer was prepared by spin-coating. First, 100 μL of the Li3PS4 sol was dropped on the Li sheet. Then the Li sheet spun at 4000 rpm to spread the sol. After drying at room temperature for 2 min, a thin layer of Li3PS4 formed on the Li sheet. This operation could be repeated several times to achieve the desired thickness. The Li3PS4 sols with different concentrations (0.1, 0.08, 0.06, 0.04 and 0.02 M) were used for spin-coating, so that the quality of the Li3PS4 artificial SEI layer can be optimized. The obtained Li3PS4-protected lithium electrode is denoted as Li/LPS-0.1, Li/LPS-0.08, Li/LPS-0.06, Li/LPS-0.04 and Li/LPS-0.02 respectively. The spincoating was conducted in an argon-filled glovebox (O2 < 0.5 ppm, H2O < 0.5 ppm).

Material Characterizations

The morphology of the samples was characterized by a field-emission scanning electron microscope (SEM, SU8010, Hitachi Inc., Japan). An energy-dispersive X-ray microanalyzer (EDS) integrated with the scanning electron microscope was used to characterize the composition of the samples. The phase structure of the samples was characterized by X-ray diffraction (XRD, D8 Advance, Bruker AXS GmbH, Germany).

Raman spectra were recorded by a Raman spectrometer (in Via, Renishaw Inc., UK) with a 532-nm diode-pumped solid-state laser as the excitation source. The samples for XRD and Raman spectroscopy were sealed by a polyimide film to protect them from air humidity. The interfacial chemistry of the Li electrodes was analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific Inc., US) with Al Kα-radiation.

Electrochemical Measurements

Lithium symmetrical cells were assembled with Li/LPS or bare Li as the electrodes.

A Celgard 2300 membrane was used as the separator. 1 M LiN(CF3SO2)2 (LiTFSI) in 1,3-dioxolane/dimethyl ether (DOL/DME, 50-50 vol.%) with 1 wt.% LiNO3 additive was used as the electrolyte. The components were packaged into CR2032 coin-type cells. Lithium plating/stripping cycles were monitored by a battery tester (CT2001, Wuhan Land Electronics Co. Itd., China). The impedance spectra of the cells were recorded by an electrochemical workstation (CHI660E, CH Instruments Ins., China).

The CR2032 coin-type Li-S batteries were assembled using S@C electrode as the cathode, Li3PS4-protected Li or bare Li as the anode, Celgard 2300 membrane as the separator and 1 M LiTFSI dissolved in DOL/DME (50-50 vol.%) with 1 wt.% LiNO3 as the electrolyte. The S@C electrode was prepared as follows: First, the S@C composite was prepared by vacuum-heating the premixed sulfur and ketjen black (7:3) at 155℃ for 12 h. Then a slurry of S@C, poly(1,1-difluoroethylene), and acetylene black with a mass ratio of 8:1:1 was mixed in N-methyl-2-pyrrolidone (NMP). The slurry was blade-coated onto an aluminum foil. After drying in an oven at 60°C for 12 h, the aluminum foil was cut into Ø13-mm discs. The sulfur loading of the S@C electrode was 0.9-1.2 mg cm -2 . Galvanostatic charge/discharge cycles of the Li-S batteries were conducted with a voltage range of 1.6-2.7 V.

The CR2032 coin-type Li-LFP batteries were assembled using LFP electrode as the cathode, Li3PS4-protected Li or bare Li as the anode, Celgard 2300 membrane as the separator and 1.0 M LiPF6 in ethylene carbonate/diethyl carbonate (EC/DEC, 50-Technology Co. Ltd, China) electrode was used in this study. The active material loading was 11.5 mg cm -2 . The voltage cutoffs of the batteries were 2.10 and 4.20 V.

RESULTS AND DISCUSSION

Characterization of Li3PS4 artificial SEI layer

Traditional liquid-phase synthesis method by adding a stoichiometric ratio of Li2S and P2S5 in THF can only produce Li3PS4 suspension with micron-sized particles. [START_REF] Liu | Anomalous High Ionic Conductivity of Nanoporous Beta-Li3PS4[END_REF] This Li3PS4 suspension cannot form continuous film by spin-coating owing to the large particles size. Therefore, a novel synthesis route of nano-sized Li3PS4 has been developed here. The preparation of the Li3PS4 artificial SEI layer is described in Schematic 1. This work uses the reaction of LiEt3BH and S to prepare a THF sol containing Li2S nanoparticles (Figure S1a). Then, a THF solution containing P2S5•Li2S is prepared, and it is added dropwise to the Li2S sol to trigger the reaction 2Li2S+P2S5•Li2S = 2Li3PS4. Benefiting from the nanoscale particle size of Li2S, the reaction completes within 1 h, resulting in a transparent sol containing Li3PS4 nanoparticles (Figures S1b,c). Finally, the sol was spin-coated onto the Li sheet and dried at room temperature to form the artificial SEI layer.

The phase structure of the as-prepared high-purity Li3PS4 is identified by XRD (Figure 1a). After drying the sol at 60°C, the diffraction peaks attributed to Li3PS4•3THF can be clearly seen. [START_REF] Liu | Anomalous High Ionic Conductivity of Nanoporous Beta-Li3PS4[END_REF] When the sample is heat-treated at 140°C, the signal of Li3PS4•3THF disappears and that of β-Li3PS4 appears. This indicates that the asprepared Li3PS4 in the sol is solvated by THF, while the de-solvation can be achieved by 140°C heat treatment. The conclusion is further supported by the Raman results in Figures 1b,c. A single peak at about 423 cm -1 dominates the low-wavenumber region of the Raman spectra, which is assigned to PS4 3-. [START_REF] Fan | Influence of precipitate/supernatant ratio during liquid-phase synthesis of solid electrolyte Li7P3S11[END_REF] The slight shift of the peak to 421 cm -1 is observed in the 60°C-dried sample, associated with the solvation effect of THF.

The absence of the signals of P2S6 2-(380 cm -1 ) and P2S7 4-(410 cm -1 ) excludes the impurity such as Li2P2S6 and Li4P2S7. [START_REF] Pang | An In Vivo Formed Solid Electrolyte Surface Layer Enables Stable Plating of Li Metal[END_REF][START_REF] Hayashi | Preparation and Ionic Conductivity of Li7P3S11-z Glass-ceramic Electrolytes[END_REF] In the high-wavenumber region, C-H stretching signal from THF (2900-3000 cm -1 ) is observed in the 60°C-dried sample, which disappears after 140°C heat treatment, confirming the de-solvation of THF at 140°C. At low magnification, the top-view SEM image of Li3PS4-protected Li shows a flat surface without observable graininess (Figure 1d). Comparing the smooth surface morphology of the Li3PS4-protected Li (Figure 1e) with the textured surface morphology of bare Li (Figure 1g) at high magnification, it can be further confirmed that the spin-coated Li3PS4 homogeneously covers the surface of Li, forming a dense and smooth layer. The thickness of the protective layer is measured from its cross-sectional SEM image in Figure 1f, and it turned out to be about 650 nm. Then, in Figure 1h, the elemental analysis of the Li3PS4 film is carried out. As shown by the EDS spectrum in Figure S2, evident signals of P and S are observed, which come from the Li3PS4 artificial SEI layer. The O element may come from THF (Figure 1i). S1 reveal that the atomic ratio of P and S elements is about 1:3.4, closing to the stoichiometric ratio of Li3PS4. The homogeneous distribution of S and P on the lithium surface can be seen in the EDS mapping (Figures 1j,k), further confirming that the lithium surface is evenly covered by the Li3PS4 artificial SEI layer.

Quantitative analysis results listed in Table

Lithium symmetrical cell test

The Li symmetrical cells are assembled with the bare Li electrode and the Li/LPS-0.06M electrode. Their lithium plating/stripping performance is compared. Here we focus the discussion on the LPS-0.06M electrode. While the Li3PS4 layer prepared with other concentrations of the precursor sol exhibit considerable Li protective ability (Figures S3-S10), the optimized Li/LPS-0.06M electrode has the most compact protective layer and shows the best cycling stability in the symmetrical cells.

The as-assembled cells firstly stand for 24 h, during which the evolution of their electrochemical impedance spectra (EIS) is monitored (Figures 2a,b). The initial interfacial resistance of the Li/LPS-0.06M symmetrical cell is 25 Ω, slightly greater than 13 Ω of the bare Li symmetrical cell. This additional resistance comes from the Li3PS4 layer. With the standing time increasing, the interfacial resistance of the bare Li symmetrical cell increases significantly. This reflects that the SEI layer grows continuously owing to the parasitic reactions between the Li electrode and the electrolyte. [START_REF] Wood | Dendrites and Pits: Untangling the Complex Behavior of Lithium Metal Anodes through Operando Video Microscopy[END_REF][START_REF] Chen | Dead Lithium: Mass Transport Effects on Voltage, Capacity, and Failure of Lithium Metal Anodes[END_REF] In contrast, the interfacial resistance of the Li/LPS-0.06M symmetrical cell increases slowly, and even decreases in 15-24 h. The stable interfacial resistance benefits from the suppression of the side reactions by the Li3PS4 protective layer.

The lithium plating/stripping voltage profiles of the symmetrical cells at a current density of 1 mA cm -2 and a capacity of 1 mAh cm -2 are presented in Figure 2c. The initial voltage of the Li/LPS-0.06M symmetrical cell is -0.062 V, which is smaller than -0.080 V of the bare Li symmetrical cell. This is due to the larger interfacial resistance of the bare Li electrode. Once the cycling starts, the hysteresis voltage of both cells decreases due to the activation of lithium-ion-conduction in the SEI layers. [START_REF] Gunnarsdottir | Noninvasive In Situ NMR Study of "Dead Lithium" Formation and Lithium Corrosion in Full-Cell Lithium Metal Batteries[END_REF] In Figure 2c, the bare Li symmetrical battery stably cycles for 200 h and then its hysteresis voltage increases rapidly. In contrast, the stable cycling of the Li/LPS-0.06M symmetrical cell greatly extends to more than 800 h. The lithium plating/stripping at a high current density of 3 mA cm -2 is also tested. The voltage of the Li/LPS-0.06M symmetrical cell has been demonstrated to operate stably for 350 hours. In contrast, the voltage of the bare Li symmetric cell increased rapidly after 200 h (Figure S11).

The details of the voltage profiles of the two symmetrical cells at 1 mAh cm -2 are compared in Figures 2d-f Alleviating concentration polarization not only stabilizes the plating/stripping voltage, but also suppresses the pulverization of lithium electrodes and the growth of dendrites. As shown by the morphology of the lithium electrodes after 500 h cycling, the bare Li electrode shows the porous and dendritic structure (Figures 3a,b), and the pulverized layer is as thick as 122 μm (Figure 3c). In contrast, the surface of the cycled Li/LPS-0.06M electrode is smooth (Figures 3d,e). The cross-sectional SEM image shows that the deposited lithium layer is constructed by micron-sized lithium grains and the thickness is only 26 μm (Figure 3f). These morphological characteristics are consistent with the voltage curves in Figure 2c, that is, the dendritic structure intensifies the parasitic reaction between lithium and the electrolyte, resulting in poor morphological stability of the lithium electrode and large fluctuation of the cell voltage, while the dense structure of the LPS film promotes the homogenous plating/stripping of lithium to achieve a stable voltage platform.

To further reveal the mechanism beneath the cycling stability of the Li/LPS-0.06M electrode, the impedance of the symmetrical cell after 10 cycles is measured (Figure S12 and Table S2). The resistance of the Li3PS4 artificial SEI layer is revealed by the high-frequency semicircle and turns to be 0.8 Ω, which is much smaller than the resistance 2.9 Ω of the spontaneously formed SEI layer on the bare Li, showing the good ion-conduction ability of the artificial SEI layer. [START_REF] Ma | Enhanced Cycle Performance of a Li-S Battery Based on a Protected Lithium Anode[END_REF] It has been demonstrated previously that the ether-solvated Li3PS4 has high ionic conductivity when contacted with ether-based liquid electrolytes. [START_REF] Fan | Will Sulfide Electrolytes be Suitable Candidates for Constructing a Stable Solid/Liquid Electrolyte Interface?[END_REF] Moreover, compared with the complex composition and microstructure of the spontaneous SEI, the microstructure of the Li3PS4 artificial SEI layer can be effectively controlled by the spin-coating process, forming a stable lithium-ion transport pathway. The good ion-conduction ability of the SEI layer favors homogeneous Li + flux within it, which is an important factor for the stable plating/stripping of the Li/LPS-0.06M electrode.

Besides the good ion-conduction ability which homogenizes the Li + flux, the Li3PS4 artificial layer effectively suppresses the parasitic reactions between the lithium electrode and the electrolyte, which is studied by XPS of the bare Li and Li/LPS-0.06M electrodes after 500 h cycling. For the Li/LPS-0.06M electrode, the S 2p3/2 peak is near 161.4 eV is assigned as PS4 3-(Figure 4a), which corresponds to the P 2p3/2 peak at 132.9 eV in the P 2p spectrum (Figure 4b). The signals of PS4 3-are absent in the S 2p and P 2p spectra of the bare Li electrode. [START_REF] Wenzel | Interphase Formation and Degradation of Charge Transfer Kinetics Between a Lithium Metal Anode and Highly Crystalline Li7P3S11 Solid Electrolyte[END_REF] It can be concluded that Li3PS4 still exists on the Li/LPS-0.06M electrode after 500 h cycling. The S 2p3/2 peaks at 169.0 eV and 167.1 eV are attributed to -SO2CF3 and -SO2CF2 + , from LiTFSI and its reduction products. [START_REF] Pang | An In Vivo Formed Solid Electrolyte Surface Layer Enables Stable Plating of Li Metal[END_REF][START_REF] Li | P4S10 Modified Lithium Anode for Enhanced Performance of Lithium-sulfur Batteries[END_REF][START_REF] Lu | Pre-modified Li3PS4 Based Interphase for Lithium Anode Towards High-performance Li-S Battery[END_REF][START_REF] Aurbach | On the Surface Chemical Aspects of Very High Energy Density, Rechargeable Li-Sulfur Batteries[END_REF] Compared with the Li/LPS-0.06M electrode, the proportion of -SO2CF2 + to -SO2CF3 has increased for the bare Li electrode, indicating that LiTFSI has been deeply reduced and decomposed. The over-reduced LiTFSI on the Li anode leads to high resistance and an unstable interface. [START_REF] Miao | Novel Dualsalts Electrolyte Solution for Dendrite-free Lithium-metal Based Rechargeable Batteries with High Cycle Reversibility[END_REF][START_REF] Jeong | A Biopolymer-based Functional Separator for Stable Li Metal Batteries with an Additivefree Commercial Electrolyte[END_REF][START_REF] Jin | Robust Cycling of Ultrathin Li Metal Enabled by Nitrate-Preplanted Li Powder Composite[END_REF][START_REF] Kang | Lithium Metal Anode with Lithium Borate Layer for Enhanced Cycling Stability of Lithium Metal Batteries[END_REF] In contrast, the Li3PS4 layer can alleviate the over-reduction of LiTFSI and promote the cycle stability of the Li/LPS-0.06M symmetrical cells.

For the Li 1s spectra in Figure 4c, the signals of Li-F (56.1 eV), Li-O (55.6 eV), Li-S (54.8 eV), ROCO2Li (54.6 eV) and Li-N (54.2 eV) are detected. [START_REF] Hu | An Artificial Hybrid Interphase for an Ultrahigh-rate and Practical Lithium Metal Anode[END_REF][START_REF] Fu | A Salt-in-Metal Anode: Stabilizing the Solid Electrolyte Interphase to Enable Prolonged Battery Cycling[END_REF] The Li-F and Li-N peaks of the Li/LPS-0.06M electrode are lower than that of the bare Li electrode.

Correspondingly, the N 1s and F 1s spectra of the Li/LPS-0.06M electrode also show weak intensity. It indicates that the reaction of Li with LiTFSI and LiNO3 is suppressed by the Li3PS4 layer. As shown in Figure 4d, the N 1s peaks attributed to TFSI -(399.0 eV) and Li3N (397.2 eV) exist in both electrodes. The formation of Li3N is due to the decomposition of LiNO3 and TFSI -. [START_REF] Aurbach | On the Surface Chemical Aspects of Very High Energy Density, Rechargeable Li-Sulfur Batteries[END_REF][START_REF] Xiong | Properties of Surface Film on Lithium Anode with LiNO3 as Lithium Salt in Electrolyte Solution for Lithium-sulfur Batteries[END_REF][START_REF] Tang | Lithium Silicide Surface Enrichment: A Solution to Lithium Metal Battery[END_REF] The F 1s spectra (Figure 4e) are predominated by the peaks attributed to -CF2 (687.5 eV) and LiF (684.8 eV). [START_REF] Jie | High-performance PVDF-HFP Based Gel Polymer Electrolyte with a Safe Solvent in Li Metal Polymer Battery[END_REF] The higher relative intensity of the Li3N and LiF peak can be observed for the bare Li electrode, indicating that the lithium salt is reduced more deeply. From Figures 4c-e Second, the robust artificial SEI layer provides enough mechanical strength to reduce the contact as well as the parasitic reactions between Li anode and the organic liquid electrolytes, so that a highly resistive pulverized lithium layer can keep thin. To simultaneously achieve the two aspects, the artificial layer should have a proper composition with high ion conductivity, dense microstructure to keep the robust and optimized thickness to balance the ion-conduction ability and the mechanical strength.

Benefitting from the high phase purity and well-controlled nanoscale thickness of the spin-coated Li3PS4 layer, the lithium symmetrical cell of this work has a longer cycle life under high current densities than those with the Li3PS4 layers generated by the direct reaction on the lithium surface (Table S3). [START_REF] Liang | In Situ Li3PS4 Solid-State Electrolyte Protection Layers for Superior Long-Life and High-Rate Lithium-Metal Anodes[END_REF][START_REF] Li | P4S10 Modified Lithium Anode for Enhanced Performance of Lithium-sulfur Batteries[END_REF][START_REF] Lu | Pre-modified Li3PS4 Based Interphase for Lithium Anode Towards High-performance Li-S Battery[END_REF] The thickness and phase structure of the Li3PS4 SEI generated by the reaction between lithium and phosphorsulfur compounds are difficult to control, limiting the cycling performance of the lithium electrode at a current density higher than 1 mA cm -2 . For example, the SEI formed by reacting P4S10 with lithium makes the lithium symmetrical cell cycles for 270 h at 1 mA cm -2 . [START_REF] Li | P4S10 Modified Lithium Anode for Enhanced Performance of Lithium-sulfur Batteries[END_REF] In this study, the spin-coating route of the sulfide artificial SEI layer provides an effective way to control the thickness and phase purity, and consequently, the Li/LPS-0.06M symmetrical cell can stably cycle at 1 mA cm -2 for 800 h with a voltage hysteresis of less than 25 mV. The protected lithium electrode can even cycle for 300 h at 3 mA cm -2 , which is competitive with the high-rate performance of the lithium electrodes stabilized by other strategies, such as lithiophilic coating and 3D-structured electrode. [START_REF] Yang | Lithium Dendrite Inhibition via 3D Porous Lithium Metal Anode Accompanied by Inherent SEI Layer[END_REF][START_REF] Xia | Stabilized Lithium Metal Anode by an Efficient Coating for High-performance Li-S Batteries[END_REF]

Li-S and Li-LFP battery test

To test the practical application, the Li/LPS-0.06M electrode and bare Li were evaluated in Li-S full batteries. When the battery is operated at a rate of 0.2 C, the initial capacity of the Li/LPS-0.06M electrode is 1270 mAh g -1 , while the initial capacity of the bare lithium electrode is only 1090 mAh g -1 (Figure S13a). In Figure 5a, when the current density is increased to 1 C, the battery with the Li/LPS-0.06M anode has an initial discharge capacity of 910 mAh g -1 , which is 1.2 times higher than that of 750 mAh g -1 with the bare Li anode. Remarkably, from the 240 th cycle to the 800 th cycle.

The capacity of the battery with the Li/LPS-0.06M anode first increases, reaching its maximal capacity at the 576 th cycle, and then decreases. By comparing the discharge curves at the 320 th cycle and 640 th cycle in Figure 5c, it can be observed that the latter has a longer and flatter second discharge plateau, indicating that the increased capacity is mainly attributed to the more thorough conversion of the low-order polysulfides to Li2S/Li2S2. According to relevant literature reports, [START_REF] Cheng | Ultra-high rate Li-S batteries based on a novel conductive Ni2P yolk-shell material as the host for the S cathode[END_REF][START_REF] Xu | An Extremely High Rate Li-S Battery with Hybrid Electrolyte[END_REF] this phenomenon of abnormal capacity increase can be explained by the activation of the S@C cathode during highrate cycling. During the activation of the S@C cathode, the capacity will increase to a certain extent. After the S@C cathode is fully activated, the capacity will normally decay. The anormal trend of cycling performance results in a very low-capacity loss at the 800 th cycle (551 mAh g -1 ) in comparison with that at the 240 th cycle (564 mAh g -1 ).

The average capacity decay rate from 240 th to 800 th cycle is only 0.004%/cycle. In contrast, the capacity of the Li-S battery with the bare Li anode begins decaying rapidly after the 267 th cycle, and only 346 mAh g -1 are left after the 800 th cycle. Additionally, the second discharge plateau of the battery with the Li/LPS-0.06M anode well keeps at 2.0 V during the long-term cycling (Figure 5b), while the one with the bare Li anode decreases from 2.0 V to 1.96 V after 640 cycles (Figure 5c). Figures S13b andc show the charge-discharge profiles of the Li-S batteries with the bare Li anode and Li/LPS-0.06M anode, at a 0.2 C rate. Similarly, the battery with the Li/LPS-0.06M anode exhibits smaller potential polarization and higher specific capacity than those of the battery with the bare Li anode, at a low current density (0.2 C). During standing or discharging of the Li-S cells, the long-chain polysulfide formed in the cathode diffuses to the surface of the Li anode and is reduced to short-chain polysulfide. Then the shortchain polysulfide moves back to the cathode and is oxidized to long-chain polysulfide. [START_REF] Li | P4S10 Modified Lithium Anode for Enhanced Performance of Lithium-sulfur Batteries[END_REF] Such reactions continue to proceed in the batteries, reducing the full utilization of active materials and accelerating the capacity decay. As mentioned above, the Li3PS4 artificial SEI layer suppresses the parasitic reactions which include the reactions between the polysulfides and the lithium anode. Therefore, the Li-S batteries with the Li3PS4protected Li anode have better capacity retention than those with the bare Li anode.

Besides the capacity retention, the Li3PS4 artificial SEI layer can also improve the rate performance of the Li-S batteries. As shown in Figure 5d, when operating at a 2 C rate, the battery capacity with a bare Li anode drastically decreases to 175 mAh g -1 , while the battery capacity with a Li/LPS-0.06M anode remains at 682 mAh g -1 .

Comparing the 2 C galvanostatic charge-discharge curves of the two batteries, it can be seen that the one with the bare Li anode shows a much larger overpotential than the one with the Li/LPS-0.06M anode (Figure 5e). A major factor that limits the high-rate performance of the battery with liquid electrolytes is the concentration polarization. [START_REF] Bai | Transition of Lithium Growth Mechanisms in Liquid Electrolytes[END_REF] Increasing the Li + transfer number by substituting liquid electrolyte with solid electrolyte has been demonstrated to be an effective strategy to alleviate the concentration polarization. [START_REF] Kato | High-power All-solid-state Batteries Using Sulfide Superionic Conductors[END_REF] In this case, the Li3PS4 artificial SEI layer increases the local Li + transfer number near the Li anode, where the concentration polarization is the most severe. This helps to lower the overpotential of the batteries when cycling at a high current density.

The Li3PS4-protected Li anode is also applied in Li-LFP batteries, to demonstrate the adaptability of this artificial SEI layer for the ester-based electrolytes. The cycling performance of the Li-LFP battery with the Li/LPS-0.06M anode and bare Li anode at 1 C is shown in Figures 6a-c. It can be seen that the capacity of the Li/LPS-0.06M/LFP battery decreased by about 36% during the first 145 cycles, while the capacity of the bare Li/LFP battery decreased by as high as 60%. Moreover, a soft short circuit occurs in the bare Li/LFP battery after 100 cycles, leading to a sharp drop in the coulombic efficiency. In contrast, the coulombic efficiency of the Li/LPS-0.06M/LFP battery can stabilize at about 98% over 145 cycles. The morphology of the protected Li anode and the bare Li anode after 145 cycles at 1 C are characterized by SEM (Figure S14). It can be clearly seen that the surface of the cycled bare Li anode presents a dendritic appearance, indicating that the corrosion reaction and powdering are serious. Generally, the formation of dendritic Li causes excessive volume expansion during the lithium deposition process, increases the charge transfer resistance, and leads to low lithium utilization and failure of the battery. [START_REF] Liu | Lithiophilic Ag/Li Composite Anodes via a Spontaneous Reaction for Li Nucleation with a Reduced Barrier[END_REF][START_REF] Yang | Suppressed Dendrite Formation Realized by Selective Li Deposition in All-solid-state Lithium Batteries[END_REF] On the contrary, the surface of the Li3PS4protected Li anode is still smooth after 145 cycles, corresponding with the stable coulombic efficiency of the battery. As well known, different from the ether-based electrolyte, the ester-based electrolyte (1.0 M LiPF6 in EC/DEC, 50-50 vol.%) used for Li/LPS batteries cannot spontaneously form stable SEI with the Li anode, so the corrosion of the Li anode is more serious. [START_REF] Qiu | Stable Lithium Metal Anode Enabled by Lithium Metal Partial Alloying[END_REF] The results show that the Li3PS4 artificial SEI layer can effectively suppress the formation of an unstable SEI layer between the Li anode and the ester-based electrolyte, so that the Li/LPS-0.06M/LFP batteries can operate stably.

CONCLUSIONS

In this work, 650-nm-thick high-purity Li3PS4 film is directly spin-coated onto the lithium electrode with a lithium-compatible Li3PS4 sol, to serve as an artificial SEI layer.

The modification of the lithium electrodes by Li3PS4 limits the parasitic reactions between Li and the organic electrolyte, makes the Li-ion flux uniform, and stabilizes the SEI layer to inhibit Li dendritic growth. As a result, the lithium symmetrical cells with Li3PS4-modified lithium electrodes can be cycled stably for 800 h at 1 mA cm -2 .

Using the Li3PS4-modified lithium anode, the Li-S full battery exhibits an initial capacity of 910 mAh g -1 , and a low-capacity decay rate of 0.004% per cycle from 240 th to 800 th cycles at 1 C. In addition, the Li3PS4 layer significantly enhances the stability of the lithium anode in ester-based electrolytes, so the Li-LiFePO4 battery with an active material loading of 11.5 mg cm -2 show stable operation over 145 cycles at 1 C.

These results demonstrate that with the spin-coating route developed in this work, the structure of the sulfide-electrolyte-based artificial SEI layer can be well controlled, so that the spin-coated SEI layer effectively improves the stability of the lithium electrodes.

ASSOCIATED CONTENT *sı Supporting Information

  form a 650-nm thick Li3PS4 layer on the Li electrode. With the protection of the Li3PS4 layer, the lithium symmetrical cells can stably cycle for 800 h under 1 mA cm -2 with a low hysteresis voltage of about 25 mV, showing great advantages over the cells with the bare lithium electrodes, whose stable cycling lasts only 200 h. The improved electrochemical performance of the Li3PS4-protected Li electrode is also demonstrated in Li-S batteries and Li-LiFePO4 (LFP) batteries. At 1 C rate, the Li-S battery with the protected Li anode shows a quite low average capacity decay rate of 0.004%/cycle from 240 th cycle to 800 th cycle. For the Li-LFP battery, the Li3PS4 artificial SEI layer guarantees stable operation of the batteries over 145 cycles, while the battery with the bare Li anode fails in 100 cycles.

Schematic 1 .

 1 Synthesis route of the Li3PS4 sol and the preparation of the Li3PS4 artificial SEI layer on Li by a spin-coating process.

Figure 1 .

 1 Figure 1. (a) XRD patterns of Li3PS4 prepared at different drying temperatures. (b, c) Raman spectra

  . During the first 20 h, both cells exhibit voltage fluctuation. This voltage fluctuation is associated with the concentration polarization of the liquid electrolyte near the lithium electrodes. 31 With the plating/stripping cycling continues, the voltage fluctuation of the Li/LPS-0.06M symmetrical cell gradually attenuates, while the voltage fluctuation of the bare Li symmetrical cell is still obvious, as shown by the profile during 200-220 h. Finally, the voltage of the Li/LPS-0.06M symmetrical cell keeps very stable at 480-500 h, while the hysteresis voltage of the bare Li symmetrical cell dramatically increases and becomes unstable. Compared with the fragile spontaneous SEI layer, the Li3PS4 artificial SEI layer has better toughness and more stable morphology so that the transfer number of Li + within the SEI layer can be effectively augmented to close unity. Consequently, concentration polarization can be alleviated by the artificial SEI layer.

  , we can conclude that compared with bare Li electrodes, Li/LPS-0.06M electrodes can alleviate the decomposition of LiTFSI and change the component content of the SEI layer.The Li3PS4 artificial SEI layer protects the lithium electrode from two aspects (Schematic 2). First, the good lithium-ion conductivity and high lithium transfer number promote the homogenous plating and stripping of lithium, which alleviates the local concentration of Li-ion flux and suppresses the formation of lithium dendrites.

Figure 2 .

 2 Figure 2. Evolution of the electrochemical impedance spectroscopy of (a) the bare Li symmetrical

Figure 3 .

 3 Figure 3. Morphology of the symmetrical cells with the bare Li and Li/LPS-0.06M electrodes after

Figure 4 .

 4 Figure 4. (a) S 2p, (b) P 2p, (c) Li 1s, (d) N 1s and (e) F 1s XPS spectra and the fitting results of the

Figure 5 .Figure 6 .

 56 Figure 5. Electrochemical performance of the Li/S batteries with the Li/LPS-0.06M and bare Li

Additional

  Figures S1-S14: SEM images of Li2S nanoparticles, Li3PS4 nanoparticles and photograph of as-prepared Li3PS4 sol; EDS spectrum of the Li/LPS-0.06M electrode; morphologies and electrochemical performance of Li/LPS-0.1M, Li/LPS-0.08M, Li/LPS-0.04M and Li/LPS-0.02M symmetric cells; EIS test of the bare Li and Li/LPS-0.06M symmetric cell after 10 cycles; electrochemical performance and SEM of the Li/LPS-0.06M and the bare Li anodes; additional Tables S1-S3: elemental analysis of the Li/LPS-0.06M electrode; fitting the EIS test; comparison of the Li symmetrical batteries with sulfurized SEI and other SEI strategies.

ACKNOWLEDGMENTS

The work is financially supported by the National Natural Science Foundation of China (51702216), the Natural Science Foundation of Guangdong Province (2021A1515011725/ 2020A1515011430), the Stable Support Plan for Shenzhen Higher Education Institutions (20200811211215001) and the Shenzhen Science and Technology Foundation (JCYJ20210324095808023).

Author Contributions

Hongjiao Wang: Conceptualization, Methodology, Writing-Original Draft.

Lilin Wu and Bai Xue: Validation, Investigation and Data curation.

Fang Wang, Zhongkuan Luo, Xianghua Zhang and Laurent Calvez:

Notes

There are no conflicts of interest to declare.

By spin-coating of a Li 3 PS 4 sol, a Li 3 PS 4 artificial SEI layer is prepared on the lithium electrode, which uniformizes the Li + flux and isolates the parasite reaction. The modified electrode cycles for 800 h at 1 mA cm -2 .


Lilin Wu

Bai Xue

Hongjiao Wang

Zhenghua Su

Zhongkuan Luo

Fang Wang

Xianghua Zhang

Laurent Calvez
email: laurent.calvez@univ-rennes1.fr.

Bo Fan

Self-densified ultrathin solid electrolyte membrane fabricated from monodispersed sulfide electrolyte nanoparticles

Keywords: nanoparticles, sulfides, electrolyte, ionic conductivity, membranes, all-solid-state batteries

Reducing particle size of sulfide electrolytes to nanoscale is essential for preparing ultrathin sulfide electrolyte membrane of all-solid-state batteries. However, there lacks effective way to synthesize sulfide electrolyte nanoparticles with good morphology uniformity. In this work, a dual precursor sol route is developed to speed up the synthesis of lithium thiophosphate from tens of hours by conventional liquidphase routes to < 50 min. The promoted reaction kinetics inhibit the overgrowth of the product particles so that monodispersed lithium thiophosphate nanoparticles with sizes of 44±6 nm are prepared.

Membranes fabricated from the nanoparticles achieve a thickness of 0.5-1.0 μm, which are reduced by one order of magnitude in comparison with those of reported sulfide electrolyte membranes. Moreover, the monodispersity makes the nanoparticles closely packed in the green membrane, so that the membrane can be self-densified without applying pressure, while 100-MPa-level pressure is indispensable for conventional membrane fabrication technologies. The ultrathin LPS membrane is also successfully employed in all-solid-state LiFePO4 thin film batteries. This work not only provides a heuristic strategy to synthesis sulfide electrolyte nanoparticles, but also develops a pressing-free fabrication route of sulfide electrolyte membranes, which is eagerly demanded for high-energy-density all-solid-state batteries.

Introduction

All-solid-state batteries have been considered as one of the candidates of next generation batteries because of their reliable safety and great potential of improving power density [START_REF] Ding | Solid-state lithium-sulfur batteries: Advances, challenges and perspectives[END_REF][START_REF] Schnell | All-solid-state lithium-ion and lithium metal batteries -paving the way to large-scale production[END_REF] . To achieve the comparable energy density with conventional lithiumion secondary batteries, all-solid-state batteries eagerly require solid electrolyte membranes with thicknesses < 30 μm. [START_REF] Li | Rodriguez-Martinez Estimation of energy density of Li-S batteries with liquid and solid electrolytes[END_REF] Viewing the high ionic conductivity and the good mechanical properties of sulfide electrolytes, [START_REF] Kato | High-power allsolid-state batteries using sulfide superionic conductors[END_REF][START_REF] Zhang | High Capacity, Superior Cyclic Performances in All-Solid-State Lithium-Ion Batteries Based on 78Li2S-22P2S5 Glass-Ceramic Electrolytes Prepared via Simple Heat Treatment[END_REF] great efforts have been made to fabricate sulfide electrolyte membranes, including slurry process [START_REF] Lee | High-energy long-cycling all-solid-state lithium metal batteries enabled by silver-carbon composite anodes[END_REF][START_REF] Nam | Bendable and Thin Sulfide Solid Electrolyte Film: A New Electrolyte Opportunity for Free-Standing and Stackable High-Energy All-Solid-State Lithium-Ion Batteries[END_REF] and dry-film process [START_REF] Zhang | Flexible Sulfide Electrolyte Thin Membrane with Ultrahigh Ionic Conductivity for All-Solid-State Lithium Batteries[END_REF] , which typically using binder [START_REF] Liu | Ultra-thin free-standing sulfide solid electrolyte film for cell-level high energy density all-solid-state lithium batteries[END_REF] or scaffold [START_REF] Zhu | A Self-Limited Free-Standing Sulfide Electrolyte Thin Film for All-Solid-State Lithium Metal Batteries[END_REF][START_REF] Xu | Cathode-Supported All-Solid-State Lithium-Sulfur Batteries with High Cell-Level Energy Density[END_REF] to help shaping the membrane.

Because the membranes are made from micron-sized sulfide electrolyte particles, [START_REF] Kim | Formation of the high lithium ion conducting phase from mechanically milled amorphous Li2S-P2S5 system[END_REF][START_REF] Minami | Lithium ion conductivity of the Li2S-P2S5 glass-based electrolytes prepared by the melt quenching method[END_REF] the thickness of the membranes is limited to tens of microns. [START_REF] Zhu | A Self-Limited Free-Standing Sulfide Electrolyte Thin Film for All-Solid-State Lithium Metal Batteries[END_REF] Moreover, the irregular shape makes the particles cannot be closely packed. Therefore, a cold/hot-pressing process with a high pressure of 150-500 MPa is indispensable to densify the green membrane, which increases the process complexity for mass production. Recently, nano-sheets of sulfide electrolyte with lateral sizes of 1 μm and thicknesses of 80 nm are prepared by a newborn liquid-phase synthesis route. [START_REF] Hood | Fabrication of Sub-Micrometer-Thick Solid Electrolyte Membranes of β-Li3PS4 via Tiled Assembly of Nanoscale, Plate-Like Building Blocks[END_REF][START_REF] Wang | Fabrication of ultrathin solid electrolyte membranes of β-Li3PS4 nanoflakes by evaporation-induced selfassembly for all-solid-state batteries[END_REF] Benefiting of the reduced particle size, submicron sulfide electrolyte membranes are prepared from the nanosheets. To further improving the film-forming ability of the sulfide electrolyte particles, the morphology uniformity is another important factor, but has not been studied.

Rapid reaction kinetics, which favor burst nucleation and inhibit Ostwald ripening, are critical for synthesizing monodispersed nanoparticles. [START_REF] Sugimoto | Monodispersed Particles (Second Edition)[END_REF] Morphology uniformity of sulfide electrolyte particles at nanoscale should be achieved by identifying and accelerating the kinetics-limiting synthesis step. Here, the synthesis of Li3PS4 is studied, because it is a solid intermediate for most liquid-phase synthesized sulfide electrolytes, which controls the morphology of the final products. [START_REF] Wang | Mechanism of Formation of Li7P3S11 Solid Electrolytes through Liquid Phase Synthesis[END_REF][START_REF] Phuc | Fast synthesis of Li2S-P2S5-LiI solid electrolyte precursors[END_REF] The reaction pathway for the formation of Li3PS4 includes two steps (Figure 1a). First, Li2S and P2S5 rapidly react with a 1:1 molar ratio to form Li2S•P2S5 solution. Second, the excess Li2S attacks the P-S-P bond in Li2S•P2S5 and Li3PS4 precursor (solvated by solvent molecular) precipitates. [START_REF] Calpa | Formation Mechanism of Thiophosphate Anions in the Liquid-Phase Synthesis of Sulfide Solid Electrolytes Using Polar Aprotic Solvents[END_REF] Owing to the insolubility of Li2S and Li3PS4, the second step, Li2S+ Li2S•P2S5  Li3PS4, is a sluggish solid-liquid reaction. Granular commercial Li2S and P2S5 are usually used for liquid-phase synthesis. This even retards the second step, leading to the overgrowth and irregular shape of the products. Consequently, the Li3PS4 precursor appearing in conventional liquid-phase synthesis is always a suspension (Fig. 1b).

In this work, the kinetics-limiting reaction Li2S+ Li2S•P2S5  Li3PS4 is significantly accelerated by replacing the commercial powder of Li2S and P2S5 with Li2S sol and Li2S•P2S5 solution respectively, so that the overgrowth of the products is inhibited. Consequently, nanoparticles of lithium thiophosphate (LPS) are obtained for the first time, which possess excellent morphology uniformity and greatly reduced particle size (44±6 nm). Benefiting from the good film-forming ability of the monodispersed nanoparticles, uniform and compact submicron (0.5-1.0 μm) solid electrolyte membranes with resistances of 0.8 Ω cm 2 are prepared by simply spin coating. Remarkably, self-densification of the green membrane occurs without applying pressures. This work not only provides a heuristic strategy to synthesis nanoparticles of LPS and its derivatives, but also develops a pressing-free fabrication route for sulfide electrolyte membranes, which is practical for mass production of solid electrolyte membranes for high-energy-density all-solid-state batteries.

Experimental Section

Synthesis of materials

Li3PS4 precursor sol was synthesized by a dual precursor method. First, sulfur powder (99.5%, Alfa Aesar) and LiEt3BH solution in tetrahydrofuran (THF, 1 M, Innochem) were used as the starting materials to prepare Li2S sol with a concentration of 1M. The as-prepared Li2S was centrifugated and re-dispersed in THF (≥99.5%, Aladdin). Then, 322-mg of a mixture of commercial Li2S (99.9%, Alfa Aesar) and P2S5 (99.9%, Aladdin) with a molar ratio of 1:1 was dispersed in 15 mL acetonitrile (ACN, ≥99.5%, Aladdin). A light yellow Li2S•P2S5 solution was obtained after 30-min stirring.

Finally, the Li2S sol and Li2S•P2S5 solution were mixed to obtain a colorless and transparent Li3PS4 precursor sol, in which totally the molar ratio of Li2S and P2S5 was 3:1. The precursor sol was dried at 60℃ and subsequently heat-treated at 250℃ under argon for 1h to obtain the LPS solid electrolytes. For comparison, β-Li3PS4 was also prepared by conventional suspension method. Commercial powders of Li2S and P2S5 at a molar ratio of 3:1 were mixed in ACN. The mixture was stirred for 3 days. The resultant precursor suspension was dried and heat-treated in the same way as above.

The nano-Li2S was also applied in conventional route, that is, mixed with the P2S5 powder in a mixed solvent (THF: ACN=2:1) to prepare the LPS solid electrolyte.

Considering the high price of the LiEt3BH solution, an alternative route by replacing the LiEt3BH solution with a Li-naphthalene (Naph) solution was also studied.

The Li2S sol could also be prepared by mixing Li-Naph solution and sulfur powder.

First, naphthalene and lithium chips were dispersed in 1,2-dimethoxyethane (DME) to prepare 0.5 M Li-Naph solution, into which the sulfur powder was then added and stirred for 3 h to synthesize Li2S nanoparticles. Then, the as-prepared Li2S nanoparticles were centrifugated and re-dispersed in THF. The obtained Li2S sol was used to react with the Li2S•P2S5 solution so that the Li3PS4 precursor sol could be prepared as described above. Considering the sensitivity of the materials to moisture and oxygen, all the operations were conducted in an argon-filled glovebox (H2O<0.5 ppm, O2<0.5 ppm).

Characterization of materials

The X-ray diffraction patterns of the powder samples were collected on a Bruker D8 Advance X-ray diffractometer with a Cu Kα radiation source. Diffraction data was collected in the 2𝜃 range from 10° to 60°. Raman spectroscopy was performed using a Raman spectrometer (inVia, Renishaw Inc., UK) equipped with a green laser (532 nm).

The laser power was 5% and a 50X objective was used. Samples for XRD and Raman measurements were prepared in glovebox on glass slides and sealed with polyimide film. All the XRD patterns show two broad peaks around 12° and 20°, which were attributed to the polyimide film. Morphology of the samples was observed by a fieldemission scanning electron microscope (Zeiss Supra 55, Carl Zeiss GmbH, Germany).

Samples were transferred from the glovebox to SEM through a vacuum sample holder.

The ionic conductivity of materials was examined by AC impedance spectroscopy using an impedance analyzer (Solartron1260A, Solartron Analytical Inc., UK) in the frequency range of 1.0 MHz-0.1 Hz. The amplitude of the AC voltage was 15 mV. For the impedance measurement, the solid electrolyte powder (about 200 mg) was cold-pressed into disks (φ 15 mm) under 360 MPa pressure. Gold film was then deposited by a low vacuum coater (EM ACE200, Leica Microsystems, Germany) on the both sides of the disks as the inert electrodes.

Fabrication of LPS solid electrolyte membranes

Submicron LPS membranes were fabricated using Li3PS4 precursor sol via spin coating technique. Glass sheets covered with gold film were used as the substrates for spin coating. The glass substrates were ultrasonically cleaned in detergent (1% in water), anhydrous ethanol and deionized water in sequence, and dried with N2 flow. Then gold film was deposited on the glass substate with a low vacuum coater (EM ACE200, Leica Microsystems, Germany). The concentration of the LPS sol was adjusted to be 0.13 M for spin coating. The spin coating was conducted at 4000 rpm. The free solvent rapidly evaporated, resulting in a transparent membrane. The membrane was then transferred onto a hotplate preset at 250°C to remove the solvated organic molecules. The thickness of the membrane could be increased by repeating the above operations. Finally, the membranes were sintered at 250°C under vacuum for 1 h. The spin coating was operated in a glovebox.

Assembling of LiFePO4 thin film battery

The LiFePO4 cathode was prepared by mixing LiFePO4 powder, KB, PVDF binder, PEO, and bistrifluoromethanesulfonimide lithium salt (LiTFSI) with a weight ratio of 6:1:1:1:1 in N-methylpyrrolidone (NMP). The resulting slurry was casted on a carboncoated aluminum foil with a doctor blade and dried at 80°C for 24 h under vacuum. The mass loading of the active material was~0.5 mg cm -2 .

A LPS membrane with a thickness of 860 nm was spin-coated on an Au/glass substrate, with a corner of the Au film exposed as a bottom electrode. To attach the cathode to the LPS membrane, the surface of the cathode was covered with a sticky layer of polyethylene oxide (PEO) electrolyte by spin-coating. The PEO electrolyte was prepared by dissolving 0.3 g PEO and LiTFSI (EO:Li + =18:1) in 10 mL ACN. Then, the LiFePO4 cathode was pressed on the LPS membrane at 80°C. The freshly assembled LiFePO4 battery was conducted to electroplate lithium on Au film at 60°C at a current of 25 μA cm -2 with a capacity of 37.5 μAh cm -2 , and the obtained Li-Au alloy was used as anode of the LiFePO4 thin film battery.

Results and discussion

Monodispersed sulfide electrolyte nanoparticles

Inspired by the two-step reaction mechanism of the conventional liquid-phase synthesis of LPS, we design a dual precursor route to improve the morphology control of the products by promoting the reaction kinetics. As shown in Fig. 1c. The route starts with the preparation of two precursors, a sol containing nano-Li2S and a solution containing Li2S•P2S5. Here the Li2S sol is synthesized by the reaction between S and LiEt3BH. The two precursors replace commercial powders of Li2S and P2S5 respectively. Then they are mixed to react. The large specific area of Li2S nanoparticles and the wettability of Li2S•P2S5 solution significantly promote the reaction kinetics so that overgrowth of the product is greatly inhibited. Finally, solvated Li3PS4 nanoparticles are obtained. The transparent LPS precursor sol shows obvious Tyndall effect (a typical scattering phenomenon of sol) under the illumination of a green laser The nanoscale size distribution of the LPS particles prepared by dual precursor route is unambiguously verified by SEM. The nano-Li2S is of good size uniformity and the size is 18±3 nm (Fig. 1e), corresponding to the broadening of XRD and Raman signals (Fig. S1). These monodispersed nano-Li2S particles not only provide large amounts of solid/liquid interface to accelerate the react with Li2S•P2S5, but also create uniform additional driving force for the reaction by their spherical surfaces with monodispersed curvatures. Consequently, nanoparticles of LPS precursor with average sizes of 44 nm and a small standard deviation of 6 nm are prepared (Fig. 1f). It is worth noting that these LPS particles show a uniform spherical morphology, different from those prepared by suspension route, which show prismatic shape with large size of ~10 μm or irregular nanosheets with lateral sizes of ~1 μm. [START_REF] Wang | Fabrication of ultrathin solid electrolyte membranes of β-Li3PS4 nanoflakes by evaporation-induced selfassembly for all-solid-state batteries[END_REF][START_REF] Liu | Anomalous high ionic conductivity of nanoporous β-Li3PS4[END_REF] LPS precursor containing solvated solvent molecules. XRD results identify that its principal phase is Li3PS4•ACN (Fig. S2a). After heat treatment at 250°C, the solvent is removed (Fig. S2b and Fig. S3). The obtained sulfide electrolyte contains β-Li3PS4 and thio-LISICON Ⅱ, and is free of poorly conductive impurities such as Li2S and Li4P2S6 (Fig. 1g). β-Li3PS4 and thio-LISICON Ⅱ are room-temperature metastable phase with high ionic conductivities, which are initially discovered in Li2S-P2S5 glass-ceramics with Li2S content higher than 75 mol.%. [START_REF] Hayashi | Formation of superionic crystals from mechanically milled Li2S-P2S5 glasses[END_REF][START_REF] Mizuno | High lithium ion conducting glass-ceramics in the system Li2S-P2S5 Solid State Ionics[END_REF] The room-temperature conductivity of the obtained sulfide electrolyte is 3.87×10 -5 S cm -1 (Fig. S4). This value is somehow lower than that prepared by conventional suspension routes, [START_REF] Liu | Anomalous high ionic conductivity of nanoporous β-Li3PS4[END_REF][START_REF] Teragawa | Liquid-phase synthesis of a Li3PS4 solid electrolyte using N-methylformamide for all-solid-state lithium batteries[END_REF] which may be caused by the residual by-products such as Et3B. Kaup et al. proposed that, when Li3PS4 is prepared by liquid phase method, to completely remove the residual solvent and decomposition products, the annealing temperature should be above 400°C, but the nanostructure could be destroyed. [START_REF] Kaup | Impact of the Li substructure on the diffusion pathways in alpha and beta Li3PS4: an in situ high temperature neutron diffraction study[END_REF] Thus, we try a moderately higher heat-treatment temperature of 300°C to obtain purer Li3PS4, and achieve higher conductivity of 1.69×10 -4 S cm -1 (Fig. S5), which is comparable to that prepared by conventional liquid-phase synthesis.

Viewing the importance of the kinetics of the reaction Li2S+ Li2S•P2S5  Li3PS4 for morphology control during liquid-phase synthesis, how fast this reaction is promoted by dual precursor route is examined. It is done by monitoring the phase evolution of the solid electrolytes with different reaction time. The reaction time is recorded since the Li2S•P2S5 solution and Li2S sol are mixed, until the obtained dual precursor sol is dried completely under an argon flow. To avoid the acceleration of the reaction by heating, the drying is conducted at room temperature. Thus, the minimal reaction time, which is achieved by directly starting the drying after the raw material mixing, is 50 min. The solid electrolytes with reaction duration of 50 min and 2 h 20 min present identical XRD patterns (Fig. 2a), both containing β-Li3PS4 and a small amount of thio-LISICON Ⅱ, while the signals of the raw material Li2S completely disappear. Correspondingly, except the Raman band attributing to the symmetric stretching of P-S bonds in PS4 3-(420 cm -1 ), 25 which constitutes β-Li3PS4 and thio-LISICON Ⅱ, [START_REF] Mizuno | High lithium ion conducting glass-ceramics in the system Li2S-P2S5 Solid State Ionics[END_REF] signals belonging to other structure units are hardly detected for all reaction durations (Fig. 2b). It is therefore concluded that the reaction of dual precursor route is completed within 50 min. For comparison, LPS suspension is prepared with conventional liquid-phase synthesis, by directly mixing commercial Li2S and P2S5 in ACN. As shown by the XRD results, the signals of Li2S weaken and the signals of β-Li3PS4 intensify after a long-time reaction (Fig. 2c). However, Li2S is still observed after 72-h reaction, indicating that the reaction is not completed.

Remarkably, the promoted reaction kinetics not only provide better morphology control of the products, but also eliminate the impurities. During conventional suspension route, the unreacted Li2S•P2S5 turns into Li4P2S6 after heat treatment, [START_REF] Wang | Mechanism of Formation of Li7P3S11 Solid Electrolytes through Liquid Phase Synthesis[END_REF] as shown by the X-ray diffraction peaks and characteristic Raman bands (Fig. 2c-d). The obvious band at 389 cm -1 attributes to the stretching of P-S bonds in PS3 terminal modes of P2S6 4-. [START_REF] Machida | Preparation of amorphous 75L2S•P2S3•(25-x)P2S5 (mol%) solid electrolytes by a high-energy ballmilling process and their application for an all-solid-state lithium battery[END_REF] The weak peak at 310 cm -1 can be attributed to the stretching of P-P and the bending of S-P-S in P2S6 2-, and the peak between 460 cm -1 and 480 cm -1 is the signal of the stretching of P-S bonds in P2S6 2-. [START_REF] Delnick | Investigation of Complex Intermediates in Solvent-Mediated Synthesis of Thiophosphate Solid-State Electrolytes[END_REF] The LPS is also synthesized by mixing the P2S5 powder with the nano-Li2S in a mixed solvent (THF: ACN=2:1). Similar reaction time of 65 min with the dual precursor route is chosen, and 1 h heat treatment at 250℃ under argon is subsequently applied. As shown by the XRD results (Fig. S6), besides β-Li3PS4 and thio-LISICON Ⅱ, the signal of Li2S can still be observed in the product. These results confirm that, preparing the Li2S•P2S5 solution in advance is very crucial to promote the reaction homogeneity and eliminate the residual raw materials. The thorough reaction between Li2S and Li2S•P2S5 by dual precursor route eliminates these poorly conductive impurities (Fig. 2a-b), benefiting the ionic conductivity of the solid electrolyte.

Using the reaction between LiEt3BH and S to prepare nano-Li2S greatly increases the cost of the dual precursor sol route. Fortunately, there are many other inexpensive synthetic methods of nano-Li2S. [START_REF] Jiang | Ultrasmall Li2S-Carbon Nanotube Nanocomposites for High-Rate All-Solid-State Lithium-Sulfur Batteries[END_REF][START_REF] Xue | 3D Printed Li-S Batteries with In Situ Decorated Li2S/C Cathode: Interface Engineering Induced Loading-Insensitivity for Scaled Areal Performance[END_REF][START_REF] Li | Facile Synthesis of Lithium Sulfide Nanocrystals for Use in Advanced Rechargeable Batteries[END_REF] As an example, here it is demonstrated that by using the nano-Li2S prepared by an inexpensive way, that is, mixing sulfur powder and Li-Naph solution, [START_REF] Duchardt | Flow-Oriented Synthesis of Li2S and Li3PS4•3THF: Opening Up a Completely Solvent-Based Solid Electrolyte Value Chain[END_REF] the LPS nanoparticles can also be successfully synthesized by the dual precursor sol method (Fig. S7). Similarly, after heat treatment at 250°C, sulfide electrolyte containing β-Li3PS4 and thio-LISICON Ⅱ is obtained (Fig. S8), and its ionic conductivity at room temperature is 2.8×10 -5 S cm -1 (Fig. S9). This demonstrates the high flexibility of the dual precursor sol route for the synthesis of LPS nanoparticles, which benefits the modification of the synthesis route for satisfying the requirements for specific applications.

Self-densified submicron sulfide electrolyte membrane

Compared with the conventional synthesis technologies of sulfide electrolytes, the novel dual precursor route significantly reduces the particle size to nanoscale and improves the morphology uniformity. These features endow the nanoparticles with distinguished film-forming ability (Fig. 3a). First, the nanoparticles provide large amounts of highly active surface. Combined with the high Li diffusivity of sulfide electrolytes which facilitates low-temperature sintering, [START_REF] Fan | Ionic conductive GeS2-Ga2S3-Li2S-LiI glass powders prepared by mechanical synthesis[END_REF] the nanoparticles exhibit excellent sinterability. Second, the good morphology uniformity makes the nanoparticles closely packed, reduces the porosity of the green membranes (Fig. 1f).

Consequently, the ultrathin solid electrolyte membrane can be fabricated by spincoating of the precursor sol. Even though a high sintering temperature of 300℃ greatly improves the ionic conductivity of LPS, it causes serious thermal stress in the membrane. Therefore, the green membrane is sintered at lower temperature of 250°C.

Despite the conductivity of the 250°C-sintered LPS is lower, it is sufficient for fast ion conduction in a submicron solid electrolyte membrane as shown below. It is worth noting that no pressing is needed here, while it is an indispensable step to densify sulfide electrolyte membranes by other reported technologies. Fig. 3b shows the digital picture and SEM image of the LPS membrane prepared from the sulfide electrolyte nanoparticles. The picture shows a transparent LPS membrane on gold film. In the SEM image, it can be seen that the surface of LPS membrane is dense and uniform, without any micropores and cracks. There is an obvious sintering phenomenon among LPS particles in comparison with the green membrane (Fig. 1f). The cross-sectional SEM image (Fig. 3c) shows that the thickness of the LPS membrane is about 550 nm. This also shows the compact structure over the whole membrane. To characterize the ion conduction ability of the LPS membrane, the membrane is fabricated on gold-film-coated glass substrates by spin coating. After sintering, gold disks are further deposited on the membrane as top electrodes for the impedance spectroscopy measurement. Fig. 3d is the Nyquist plot of the LPS film. Owing to the high-frequency limit of the measurement system, the impedance spectroscopy does not show a complete semicircle corresponding to the resistance of the film, and presents only an oblique line attributed to the polarization near the electrodes. It can be seen that the intersection of the oblique line and the Z' axis is 0.8 Ω cm 2 . It represents the specific resistance of the LPS film which is much smaller than that of the thick membranes fabricated by cold pressing. [START_REF] Ito | A synthesis of crystalline Li7P3S11 solid electrolyte from 1,2-dimethoxyethane solvent[END_REF][START_REF] Phuc | Preparation of Li3PS4 solid electrolyte using ethyl acetate as synthetic medium[END_REF] Remarkably, the ionic conductivity of the membrane is calculated to be 6.7× 10 -5 S cm -1 . This value is even higher than 3.87× 10 -5 S cm -1 , which is measured from the impedance spectra of the cold-pressed LPS pellet. It implies that the self-densification of the LPS nanoparticles by low temperature sintering is more effective than that by cold-pressing, a conventional densifying approach for sulfide electrolytes. [START_REF] Sakuda | Sulfide Solid Electrolyte with Favorable Mechanical Property for All-Solid-State Lithium Battery[END_REF]

Performance of the sulfide electrolyte membrane

The characteristics of typical reported sulfide electrolyte membranes are compared (Fig. 4a and Tab. S1). It can be seen that the membrane prepared by the LPS nanoparticles exhibits a low resistance approaching the state-of-the-art value, while its thickness is reduced by one order of magnitude. More importantly, no pressure is needed to densify the membrane, simplifying the membrane fabrication process. To demonstrate how the ultrathin LPS membrane benefits the energy density of the battery, the energy density of an all-solid-state lithium-sulfur battery using a LPS membrane is estimated following the approach described in Ref. 5. The parameters for the estimation are listed in Tab. S2. As shown in Fig. 4b, when using a 1-μm thick membrane, only a low cathode loading of 1.7 mg cm -2 is required to achieve a practical energy density of 400 W h kg -1 . To achieve this energy density level, the cathode loading should be increased to 5 mg cm -2 when using a 30-μm thick membrane. Increasing cathode loading is accompanied with a series of issues, such as deteriorated electrical conduction in the cathode, mechanical collapse of the composite structure owing to large volumetric fluctuation of active materials and a high current density which induces additional problems at the lithium anode side. [START_REF] Yang | Towards high-performance solid-state Li-S batteries: from fundamental understanding to engineering design[END_REF] A combination of a ultrathin solid electrolyte membrane and a relative low active material loading could be a facile way to achieve practical energy density of all-solid-state batteries. To demonstrate the electrochemical performance of the ultrathin LPS membrane, a +) Au /LPS membrane/ Li (-half-cell is assembled and lithium deposition behavior is tested. The schematic of the cell structure is illustrated in Fig. 4c. First, a LPS membrane with a thickness of 860 nm (Fig. 4d) is spin-coated on an Au/glass substrate, with a corner of the Au film exposed as a bottom electrode. Then, a lithium foil spincoated with a thin layer of PEO electrolyte is pressed on the LPS membrane at 80°C, so that the half-cell is assembled. To prepared the PEO-coated lithium foil, 0.030 g PEO and 0.012 g LiTFSI (EO:Li + =18:1) is firstly dissolved in 10 mL 1,3-dioxolane (DOL), and then 100 μL solution is spin-coated on the lithium foiled and left drying at room temperature to remove the solvent. The lithium deposition test is conducted with a current of 65 μA cm -2 at 60°C. Generally, the lithium deposition includes alloying and lithium dendrite growth stages. [START_REF] Yan | Selective deposition and stable encapsulation of lithium through heterogeneous seeded growth[END_REF][START_REF] Wang | Linking the Defects to the Formation and Growth of Li Dendrite in All-Solid-State Batteries[END_REF] Firstly, Au is slowly lithiated and Li2Au alloy is formed. Then, with the continuous deposition of lithium, Li2Au is transformed into Li3Au. When the lithium in Li-Au alloy reaches saturation, lithium dendrite tends to grow. These processes correspond to three voltage platforms respectively. It can be seen from the voltage profile (Fig. 4c), once lithium deposition begins, the voltage drops sharply. Then, the voltage gradually stabilizes at 0.23 V, which represents that Au is gradually lithiated and Li2Au alloy forms. Then the second platform at 0.10 V(~2000-2500 s) emerges which is associated with the formation of Li3Au alloy. The total deposition capacity is 46.3 μAh cm -2 . The excellent lithium deposition stability benefits from the great densification and uniformity of the LPS membrane. As can be seen from the cross-sectional SEM image in Fig. 4d, the thickness of Au film increases significantly after lithium deposition, from initial 85 nm (Fig. S10) to 320 nm. The EDS mapping and line scanning results (Fig. S11) show that the signal of Au weakens and spreads, which is consistent with the formation of Li-Au alloy during lithium deposition. Additionally, element analysis (Fig. 4d andS11) reveals the diffusion of elements P and S into the deposited metal layer, which is a direct manifestation of the reaction between LPS and Li. Theoretical and experimental studies clarify that the reaction of sulfide electrolytes with lithium metal can be driven by thermodynamic factors and generates Li3P, Li2S and other decomposition products, which constitute solid-electrolyte interphase (SEI) layer. [START_REF] Zhu | Origin of Outstanding Stability in the Lithium Solid Electrolyte Materials: Insights from Thermodynamic Analyses Based on First-Principles Calculations[END_REF][START_REF] Richards | Interface Stability in Solid-State Batteries[END_REF][START_REF] Sang | Evolution at the Solid Electrolyte/Gold Electrode Interface during Lithium Deposition and Stripping[END_REF] It is worth noting that evident migration of P and S into the deposited metal layer occurs within only 2500 s, indicating a rapid reaction occurs concomitantly with the lithium deposition. It implies the intimate contact between the bottom electrode and the spin-coated LPS membrane since good contact promotes interfacial reaction rate. [START_REF] Wenzel | Interphase formation and degradation of charge transfer kinetics between a lithium metal anode and highly crystalline Li7P3S11 solid electrolyte[END_REF] The ultrathin LPS membrane is further applied to all-solid-state LiFePO4 thin film batteries. As shown in Fig. 5, two discharge platforms are observed at 3.24 V and 3.05 V. The equilibrium potential of LiFePO4 is 3.42 V vs. Li/Li + . [START_REF] Kasavajjula | Discharge Model for LiFePO4 Accounting for the Solid Solution Range[END_REF] Considering the fact that the potential of Li3Au and Li2Au are about 0.10 and 0.23 V vs. Li/Li + (Fig. 4c), and considering also the overpotential during discharge of the battery, it can be concluded that the two discharge platforms correspond to the potential platforms of Li3Au and Li2Au, respectively. The initial discharge capacity is 45.9 mAh g -1 . It becomes 16.1 mAh g -1 after 20 cycles. Meanwhile, the coulombic efficiency increases from 84.9% to 93.2%. The low capacity may be ascribed to the consumption of Li by the reaction with LPS, which is supported by the low initial coulombic efficiency. The consumption of Li also hinders the transition from Li2Au to Li3Au, leading to the gradually disappearance of the discharge platform at 3.24 V. This problem is expected to be solved by using Li electrode with an excess amount of Li instead of the Au electrode. This requires a modified LPS precursor sol compatible with lithium, which is under development in our laboratory. Whatever, the operation of the LiFePO4 thin film battery proves that the membrane prepared by LPS nanoparticles is of great potential to develop all-solid-state batteries.

Remarkably, reducing the particle size of sulfide electrolytes into nanoscale not only provides opportunities for the preparation of self-densified ultrathin membranes, but also potentially opens up a new way towards many applications. For example, studies show that reducing particle size of solid electrolytes in composite solid cathode can improve the electrode-electrolyte contact and augment cathode loading. [START_REF] Yang | High-areal-capacity all-solid-state lithium batteries enabled by rational design of fast ion transport channels in vertically-aligned composite polymer electrodes[END_REF][START_REF] Han | High-Performance All-Solid-State Lithium-Sulfur Battery Enabled by a Mixed-Conductive Li2S Nanocomposite[END_REF] Therefore, this new technology of nano-LPS electrolyte is of great potential to be used for nano-composite cathodes. In addition, porous structure can be easily fabricated from the sol, so that the sol-synthesis route can be further developed to prepare nano-porous sulfide solid electrolytes. The porous structure has been demonstrated having excellent anti-lithium-penetration ability for all-solid-state lithium metal batteries. [START_REF] Xu | Three-Dimensional, Solid-State Mixed Electron-Ion Conductive Framework for Lithium Metal Anode[END_REF]

Conclusion

In summary, the sluggish kinetics of a universal reaction, Li2S+Li2S•P2S5  Li3PS4, result in overgrowth and poor morphology controllability of liquid-phase synthesized sulfide electrolytes. By replacing the commercial powder of Li2S and P2S5 with Li2S sol and Li2S•P2S5 solution, this basic reaction is significantly accelerated to less than 50 min. Consequently, monodispersed Li3PS4 nanoparticles with size of 44±6 nm are prepared for the first time, which are then used to construct self-densified ultrathin solid electrolyte membranes. Benefiting from the excellent film-forming ability of the monodispersed nanoparticles, ultrathin solid electrolyte membranes with a thickness of ~550 nm are fabricated by spin coating. The as-prepared membranes can be simply densified after sintering at low temperature of 250°C, without applying any pressure. The membranes possess remarkably small specific resistance of 0.8 Ω cm [START_REF] Schnell | All-solid-state lithium-ion and lithium metal batteries -paving the way to large-scale production[END_REF] and stable lithium deposition performance at 65 μA cm -2 . Besides, the LPS membrane is successfully employed in all-solid-state LiFePO4 batteries. The dual precursor sol method opens new path toward the nano-sized sulfide electrolytes, and provides an effective strategy of preparing ultrathin sulfide solid electrolyte membranes for high energy density all-solid-state batteries.

Figure 1 (

 1 Figure 1 (a) Two-step reaction mechanism of conventional suspension route of LPS. The sluggish

Figure 2

 2 Figure 2 Comparison of the reaction rate by dual precursor route and by conventional suspension route.

Figure 3 (

 3 Figure 3 (a) Schematic of the fabrication of self-densified ultrathin LPS membrane from the precursor

Figure 4 (

 4 Figure 4 (a) Thickness, resistance and densify pressure of reported sulfide electrolyte membranes. Unit

Figure 5 (

 5 Figure 5 (a) The galvanostatic charge/discharge curve and (b) Cycling performance at a current of

Acknowledgements

The work is financially supported by the Natural Science Foundation of Guangdong Province (2021A1515011725), the Stable Support Plan for Shenzhen Higher Education Institutions (20200811211215001) and the Shenzhen Science and Technology Foundation (20210312150925001).

Conflict of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Supporting Information.

Supporting Information is available online.