Design And Electrochemical Properties Of Sulfur-based Solid Electrolytes In All-solid-state Batteries

Lithium-ion batteries have already dominated electrical vehicle market and would probably be widely applied in the field of large-scale energy storage in the near future.However,the principal obstacle that hinders further expansions of its application areas and markets lies in the potential safety hazard and the limited energy density of the current commercial batteries comprising organic liquid electrolytes.Therefore,researchers in the battery community are turning their foci into all-solid-state batteries constructed by solid electrolytes,especially inorganic solid electrolytes.Sulfide solid electrolytes have been intensively studied because of their intrinsic merits including high ionic conductivities,suitable electrochemical windows and better mechanical properties.The sulfide solid electrolytes can be roughly divided into three types,i.e.,glassy,glass-ceramic and crystalline according to their crystalline structures.Among them,the crystalline sulfide electrolytes have the highest ionic conductivities.This type can be further categorized into thio-LISICON,LGPS and Argyrodite types based on their crystal structure.This thesis focus on the structural design of crystalline sulfide solid electrolyte and its lithium ion conduction mechanism.The main contents of this thesis are summarized as the following:1.Design and Synthesis of Room Temperature Stable Li-Argyrodite Superionic Conductors via Cation DopingWe report the structural simulation and the experimental evaluation of a new series of Li-argyrodites Li7+xMxP1-xS6(M=Si,Ge).The structural stability of argyrodite-type Li7+xMxP1-xS6(M=Si,Ge)is predicted via the density functional theory method.Meanwhile,the argyrodite-type Li7+xMxP1-xS6(M=Si,Ge)is successfully synthesized by solid state reaction method.With the utility of structure refinements of X-ray diffraction data and Raman spectroscopy,the structure refinements have been conducted which show that the P in 4b site has been successfully substituted by Si and Ge.Si and Ge aliovalent doping in original composition Li7PS6 can make the desired cubic high-temperature(HT)phase of Li-argyrodites stabilized at room temperature and improvesthe ionic conductivities,over 10-3 S cm-1,which is 3 orders higher than that of the original Li7PS6.Moreover,the argyrodite-type Li7+xMxP1-xS6(M=Si,Ge)shows wide electrochemical window and stability with lithium metal.The stabilized Li-argyrodites are suitable solid electrolytes for applications in all-solid-state batteries.2.Ion conductivity enhanced by lithium substitution in halogenated argyroditesInspired by the former work,we firstly substituted P in Li6PS5I by Sn4+to make a solid solution.We found that the ionic conductivity of Li6+xP1-xSnxS5I is two orders of magnitude higher than that of Li6PS5I.We then tried to replace P and Li in the halogen-doped argyrodite with different cations to investigate observed whether the ionic conductivity can be improved.Secondly,based on the previous studies on the substitution of Li+by Li7Ge3PS12,we found that the ionic conductivity of halogen-doped argyrodite were improved by replacing Li+sites with Al3+.The ionic conductivity of Li5.4Al0.2PS5Br reaches 2.4 ×10-3 S cm-1,which was much higher than that of Li6PS5Br(0.9 ×10-3 S cm-1).In addition,TiS2/SE/Li all-solid-state battery assembled with Li5.4Al0.2PS5Br as the electrolyte material also showed good compatibility with the lithium metal.At the same time,this method of increasing vacancies by replacing Li+with cations provides a new way for the design of the solid electrolytes.3.Li4-xSbxSn1-xS4 solid solutions for air-stable solid electrolytesAir-sensitive problems impact the development of sulfur-based solid electrolytes.Li4SnS4 is a thio-LISICON type fast Li-ionic conductor,which has a excellent air stability and relatively low conductivity.Solid solution Li4-xSncSb1-xS4 has been synthesized using conventional method.Structural analysis using Rietveld refinements of X-ray data and Raman spectroscopy real that Sb has occupied the position of Sn randomly and shows a ionic conductivity of 3.5 ×10-4 S cm-1(Li3.8Sb0.2Sn0.8S4),which is 5 times that of LiSnS4 and 3 orders of magnitude higher than that of Li3SbS4,respectively.The new sulfur-based solid electrolyte shows good air stability and reduced H2S release in humid air.Moreover,a LTO/SE/LCO all-solid-state battery is assembled and shows good performance.This work not only enriches the family of air-stable sulfide solid electrolytes but also affords Li3.8Sb0.2Sn0.8S4-type structure as template for the further design of new solid electrolyte.To summarize,this thesis explored the structure and ionic conduction mechanism of argyrodite and thio-LISICON type of sulfide solid electrolytes.New solid solution structures were proposed and the ionic conductivity was improved.Based on this,the effect of crystal structure change on ion conduction was studied.It stands a reference for the further design of sulfide solid electrolyte with improved performance.