MATERIALS DESIGN OF CRYSTALLINE LITHIUM IONIC CONDUCTORS WITH FRAMEWORK STRUCTURE

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Lithium superionic conductors exhibit high lithium-ionic diffusion in one of their ionic sublattice - the mobile ion sublattice - at temperature well below melting points. It is of great interest to understand their fast-ionic transport in "solid' materials, which is widely recognized, although still relatively rare, phenomenon. They are also of technological importance for future applications as solid electrolyte for all-solid lithium battery and may solve the safety problems of the rechargeable lithium ion battery using non-aqueous liquid electrolytes.Ceramic crystalline electrolytes have advantages over liquid, polymer, gel or even glass electrolytes for their chemical and electrochemical stability. Many attempts to synthesize new ceramic lithium superionic conductors have been made ; the highest conductivity of 10^<-3> S cm^<-1> was previously reported for H-doped Li_3N.However, the low decomposition potential of 0.445V restricts its application as a lithium solid electrolyte. The new thio-LISICON (LIthium SuperIonic CONductor) found in the Li_2S-GeS_2, Li_2S-GeS_2-ZnS, and Li_2S-GeS_2-Ga_2S_3 systems have structures related to the gamma-Li_3PO_4-type. Six new materials were found (Li_2GeS_3, Li_4GeS_4, Li_2ZnGeS_4, Li_<4-x>Zn_xGeS_4, Li_5GaS_4, and Li_<4+x+delta>(Ge_<1-delta'-x>GaS_x)SS_4), and LiS_<4+x+delta>(GeS_3GaS_x)S_4 showed the highest conductivity of 6.5x10^<-5> S cm^<-1> at room temperature. Our results on electrochemical measurements indicated that LiS_<4+x+delta>(Ge_<1-delta'-x>Ga_x)S_4 is stable up to 5V vs. Li.