Preparation Of Halide Li₃HoCl₆ Solid State Electrolyte And Substitution Modification Study

Compared to conventional organic electrolytes,inorganic solid electrolytes have the advantages of high-temperature resistance and non-flammability.It is possible to assemble lithium metal batteries that match lithium metal directly to achieve maximum energy density.Among them,halide solid electrolytes have attracted great interest due to their wide electrochemical window,stability under dry air,and low interfacial impedance when assembling all-solid-state battery.Li₃Ho Cl₆ solid-state electrolyte has even been successfully applied to Li-Se all-solid-state batteries and overcomes many problems of Se dissolution,high instability,poor interfacial compatibility,and electrolyte-electrode decomposition in Li-Se batteries when using conventional electrolyte.It has good prospects for application in all-solid-state batteries,but it also still has some shortcomings such as strict synthesis conditions and relatively low ionic conductivity.At present,the main methods of preparing halide solid electrolytes are solid phase synthesis（vacuum ball milling,vacuum sintering）,liquid phase synthesis and chemical vapour phase synthesis.The liquid phase synthesis is easy to operate and can be generated in situ on the cathode material to reduce the interfacial resistance.However,only a few halide solid electrolytes have been synthesized by the liquid phase synthesis.The preparation of Li₃Ho Cl₆ solid electrolyte using the liquid phase synthesis has not yet been investigated.In this work,Li₃Ho Cl₆ solid electrolyte was successfully prepared by liquid phase synthesis and further substituted Cl^-and Ho³⁺with Br^-and In³⁺to study the effect of lattice structure on ion transport capacity of Li₃Ho Cl₆.The details are as follows:（1）Li₃Ho Cl₆ solid electrolyte was successfully prepared by liquid phase synthesis,and the effect of sintering temperature on its structure and properties was investigated.And when the sintering temperature is 550°C,the Li ₃Ho Cl₆ solid electrolyte crystallinity and purity are the highest,the ionic conductivity is 1.2×10^-4 S cm^-1.It has a wide electrochemical window and good thermal stability,making it an ideal solid-state electrolyte material.（2）Li₃Ho Cl_6-xBr_x（0≤x≤6）powder materials with different substitution ratios were prepared by substituting the Cl^-with Br^-of large ionic radius.As the amount of Br^-substitution increases,the crystal plane spacing is continuously expanded and its ionic conductivity increases,which shows that the larger crystal face spacing is conducive to the ion migration in the solid electrolyte.When x≥3,the crystal structure of Li₃Ho Cl_6-xBr_x solid electrolyte changes from the initial triangular phase to a monoclinic phase more favorable for ion migration,and its ionic conductivity is further improved.However,the monoclinic phase has narrow electrochemical stability window（1.75-3.11 V vs Li⁺/Li）is not conducive to the assembly of high energy density cells.In contrast,the Li ₃Ho Cl₄Br₂ with the triangular phase solid electrolyte has a relatively wide electrochemical window（1.5-3.5 V vs Li⁺/Li）along with a high ionic conductivity(>10^-3 S cm^-1),which is promising for applications.（3）Li₃Ho_1-xIn_xCl₆（0≤x≤0.4）powder materials with different substitution ratios were prepared by substituting Ho³⁺with In³⁺in the Li₃Ho Cl₆ solid electrolyte.The addition of In³⁺causes the lattice volume to shrink and the structure collapses into an amorphous structure at x=0.3.Later,Li Cl impurities will appear when In³⁺substitution concentration continues to increase.Meanwhile,the ionic conductivity also increases with the increase of In³⁺amount,when x=0.3,the highest ionic conductivity is 2.5×10^-4 S cm^-1.Later,ionic conductivity is reduced due to Li Cl impurities.Besides,In³⁺substitution does not degrade the electrochemical window of the solid-state electrolyte.This makes it possible to directly match the high-voltage cathode to increase the energy density of the all-solid-state battery.