Research Of All Solid-State Batteries Based On Li₁₀SnP₂S₁₂ Sulfide Solid Electrolyte

In all-solid-state batteries,Li10SnP2S12(LSPS)with high ionic conductivity(4 mS cm-1),soft nature and moderate price is a promising solid electrolyte for the commercial application.However,the instability of LSPS and LSPS/electrodes interfaces would cause poor cycle performance issues in the LSPS-based all-solid-state batteries,which have not been well understood.Herein,we address and unravel the decomposition products of LSPS and their Li+transfer characteristics,especially on the surface of LSPS/electrodes by using solid-state nuclear magnetic resonance(ss NMR)spectroscopy coupled with X-ray photoelectron spectroscopy(XPS).The results reveal that the high mechanical energy during ball-milling process leads to the decomposition of LSPS into Li4SnS4 and Li3PS4.During charge/discharge cycling,specific capacity fading of batteries originates from the formation of new interfacial layer at LSPS/Acetylene black cathode and LSPS/Li metal anode interfaces.Furthermore,our results demonstrate that the rough and porous morphology of the interface formed after cycling,rather than the decomposition products,is the critical factor which results in the increases of the interfacial resistance at LSPS/Li interface and serious formation of Li dendrite.Our results highlight the significant roles of(electro)chemical and interfacial stability of sulfide solid electrolyte in the development of all-solid-state batteries.Towards the poor stability against Li metal,a simple strategy is proposed and demonstrated for the first time,i.e.,in situ modification of the interface between Li metal and Li10SnP2S12(LSPS)by pretreatment with specific ionic liquid and salts.Xray photoelectron spectroscopy and electrochemical impedance spectroscopy results reveal that a stable solid electrolyte interphase layer instead of a mixed conducting layer is formed on Li metal by adding 1.5 M lithium bis-(trifluoromethanesulfonyl)imide(LiTFSI)/N-propyl-N-methyl pyrrolidinium bis(trifluoromethanesulfonyl)imide(Pyr13TFSI)ionic liquid,where ionic liquid not only acts as a wetting agent but also improves the stability at the Li/LSPS interface.This stable interface layer can prevent LSPS from directly contacting the Li metal and further decomposition,and the Li/LSPS/Li symmetric cell with 1.5 M LiTFSI/Pyr13TFSI attains a stable cycle life of over 1000 h with both the charge and discharge voltages reaching about 50 mV at 0.038 mA cm-2.Furthermore,the effects of different Li salts on the interfacial modification is also compared and investigated.It is shown that lithium bis(fluorosulfonyl)imide(LiFSI)salt causes the enrichment of LiF in the SEI layer and results in a higher resistance of the cell upon a long cycling life.The implementation of LiNi0.8Co0.1Mn0.1O2(NCM811)Ni-rich cathode material in all solid-state batteries is regarded to effectively improve the energy density and reduce the cost of batteries.Herein,uncoated single crystal NCM811 was compared to conventional polycrystalline NCM811 in NCM811/Li10SnP2S12(LSPS)/Li4Ti5O12(LTO)all solid-state batteries.Regardless of the upper cutoff voltage,the single crystal NCM811 batteries all show dramatically higher capacity and superior cycle stability than polycrystalline NCM811.The poor electrochemical performance of polycrystalline NCM811 originates from the structural instability of polycrystalline particles,induced by the space voids and microcracks inside polycrystalline particle during electrode pressing process and extremely obvious volume change during cycling.In stark contrast,the single crystal NCM811,even without any surface modification,presents dramatically high discharge capacity of 173 mAh g-1 at 0.1 C and outstanding cycle retention of 89.5%after 50 cycles and 61.5%after 100 cycles.Our study emphasizes the good structural stability of single crystal NCM811 materials during electrode pressing process and cycling,giving their importance in the application of all solid-state batteries.It needs to mention that,for the long cycle performance,the capacity degradation of both NCM811 fits with the common sense for the inevitable side reactions between uncoated NCM811 and sulfide solid electrolyte.Therefore,surface modification on cathode material is still imperative to further improve the cycle performance of SNCM811/LSPS/LTO.Herein,several coating methods have been adopted to modify the S-NCM811 material.It reveals that coating 1 wt%LiNbO3 on S-NMC811 greatly improves the capacity and cycle retention of S-NCM811 all solid-state batteries.The discharge capacity and the cycle retention at 0.3C has been enhanced from 140 mAh g-1 to 153 mAhg-1 and 67.9%to 79.7%after 100 cycles.