Synthesis And Modification Of Sulfide Solid Electrolyte By Liquid Phase Method

Compared with liquid electrolytes,the inflammability of solid electrolytes can perfectly solve the safety problem of lithium batteries,and the lithium metal negative electrode and high voltage positive electrode can be matched into all-solid-state lithium battery with higher energy density.The development of solid electrolytes is the key.Sulfide solid electrolytes are widely favored because of its excellent ionic conductivity.However,the high energy consumption,uneven reaction and harsh process conditions of the traditional solid phase preparation method,have hindered the industrialization of sulfide solid electrolytes.In recent years,the liquid phase synthesis of sulfide solid electrolytes has the advantages of low energy consumption,uniform reaction and mass production.Li₂S and P₂S₅ were used as raw material,and ACN、DME and ACN-DME were used as solvents,Li₂S-P₂S₅solid electrolytes were prepared by the liquid phase method.The effects of solvent type and heat treatment temperature on the physical structure and lithium ion conductivity of the prepared sulfide solid electrolytes,were investigated.Aimed at Li₇P₃S₁₁from ACN solvent liquid phase synthesis of liquid,effects of Li₄SnS₄amount of covering Li₇P₃S₁₁on the physical and chemical structures,lithium ion conductivity and air stability of the composite solid electrolytes,were investigated.Finally,the effects of Al F₃surface modification of Li₇P₃S₁₁by a liquid phase method on the physicochemical structure,lithium ion conduction properties and electrochemical stability of the synthesized composite solid electrolytes,were investigated.The physical and chemical structures of the synthesized samples were analyzed by XRD,Raman,TG-DSC,BET and SEM characterization,and the lithium ion conductivity,air stability and electrochemical stability of the samples were tested.The main results were as follows:（1）The chemical bonding of DME complexing with the synthetic sulfides is stronger than that of ACN complexing with the synthetic sulfides,and this strong complexation bonding may have a greater impact on the reaction selectivity in the liquid phase synthesis and determine the P₂S₆^4-of choice when DME is used as a solvent.P₂S₇^4-and PS₄^4-were chosen when ACN is used as solvent in the synthesis reaction.Of course,this is mainly caused by the difference in the molecular structures of the two solvents.ACN containing N atoms may has a stronger ability to adsorb sulfur atoms,which may be the essential reason.The metastable Li₇P₃S₁₁with high ionic conductivity was easily prepared in ACN.The glass-ceramic phase Li₇P₃S₁₁formed after heat treatment at 250℃has the highest ionic conductivity,1.17×10^-4S/cm,and ion migration number is 0,therefore it is a very ideal solid electrolyte material.（2）The ionic conductivity of solid electrolyte Li₇P₃S₁₁from ACN liquid phase synthesis varies with the increase of heat treatment temperature from 200℃to 500℃.The increase of ionic conductivity with the increase of temperature before 250℃is due to the increase of crystallinity,and the ions conduction of irregular glass phase changes into the transmission of conductive ions along the crystal phase,and after 250℃,the ionic conductivity decreases with the increase of heat treatment temperature due to the reaction of P₂S₇^4-→P₂S₆^4-+S.Li₄P₂S₇with Li conductive phase was changed into Li₄P₂S₆with Li non-conductive phase,resulting in a gradual decrease in lithium ion conduction performance with the increase of heat treatment temperature.（3）Li₄SnS₄ coating amount is increased from 10%wt to 40%wt,and when the coating amount is increased to 40%,complete encapsulation is achieved.The composite solid electrolyte had good air stability,but the lithium ion conductivity decreases with the increase of the Li₄SnS₄coating amount.Al F₃coating amount is increased from 1%wt to 5%wt,and the coating amount of 3%wt is more suitable as Al F₃is inert material of lithium ion conductive.The ionic conductivity of the composite solid electrolyte decreases with the increase of the coating amount.The coating significantly improves the electrochemical stability of the composite solid electrolyte and enables it to stabilize at high voltage when Al F₃coating amount reaches 3% wt.