Material design of new lithium ionic conductor, thio-LISICON, in the Li2S?P2S5 system

A new lithium ionic conductor of the thio-LISICON (LIthium SuperIonic CONductor) family was found in the binary Li2S–P2S5 system; the new solid solution with the composition range 0.0≤x≤0.27 in Li3+5xP1−xS4 was synthesized at 700 °C and characterized by X-ray diffraction measurements. Its electrical and electrochemical properties were studied by ac impedance and cyclic voltammetry measurements, respectively. The solid solution member at x=0.065 in Li3+5xP1−xS4 showed the highest conductivity value of 1.5×10−4 S cm−1 at 27 °C with negligible electronic conductivity and the activation energy of 22 kJ mol−1 which is characteristic of high ionic conduction state. The extra lithium ions in Li3PS4 created by partial substitution of P5+ for Li+ led to the large increase in ionic conductivity. In the solid solution range examined, the minimum conductivity was obtained for the compositions, Li3PS4 (x=0.0 in Li3+5xP1−xS4) and Li4P0.8S4 (x=0.2 in Li3+5xP1−xS4); this conductivity behavior is similar to other thio-LISICON family with the general formula, LixM1−yMy′S4 (M=Si, Ge, and M′=P, Al, Zn, Ga, Sb). Conduction mechanism and the material design concepts are discussed based on the conduction behavior and the structure considerations.