Recent Studies on Lithium Solid Electrolytes (LiI)_x(LiPO₃)_1-x for Secondary Battery

Recently, there has been increasing demand on finding new solid electrolytes based on glassy material, due to their potential applications for solid state rechargeable battery. Superionic conducting glasses have many advantageous compared with the crystalline, or composite electrolytes, such as easy in preparation, low melting point, no grain broundary, wide composition range and easy to be formed as thin film. Among superionic glasses, the family of silver phosphate glasses (AgI) _x (AgPO ₃ ) _{1- x} and lithium phosphate glasses (LiI) _x (LiPO ₃ ) _{1- x} , are the most interested materials to be investigated. Many experiments have been performed to investigate the properties of those materials. This paper will review the recent results of the lithium glasses (LiI) _x (LiPO ₃ ) _{1- x} , such as the preparation, appearance, structure, electrical, and thermal properties. The x-ray data shows the precipitate occured at x =0.4, indicating the solubility limit of (LiI) _x (LiPO ₃ ) _{1- x} was reached at this composition. The microstructure data observed by SEM showed the different area between the smooth surface for x =0.3 and appearance of cluster at x =0.4. Comparison between glassy LiPO ₃ and (LiI) _0.3 (LiPO ₃ ) _{1- x} as the result of Raman spectra analysis, suggested a new feature at around 1000 cm ^-1 , indicating the depolymerization of the non bridging oxygen, due to the presence of LiI component. The room temperature conductivities of (LiI) _x (LiPO ₃ ) _{1- x} with x =0.3 and 0.4 are 2.7 ×10 ^-6 and 8.8 ×10 ^-7 S/cm, respectively, which are 3 orders of magnitudes higher than the undoped glass LiPO ₃ . In general, the silver glass has better ionic conductivity than the lithium glass; but in application the lithium battery has better performance and produces higher voltage.