Preparation of lithium silicon nitrides and their lithium ion conductivity

Six single phases were prepared by reactions of Li₃N and Si₃N₄ at 1075 K and 1475 K in nitrogen stream. Chemical analysis showed that they were ideally formulated as LiSi₂N₃, Li₂SiN₃, Li₁₈Si₃N₁₀, Li₂₁Si₃N₁₁ and Li₈SiN₄. Their X-ray diffraction patterns could be indexed with the following lattice parameters except phase II, Li₂SiN₂: phase I, LiSi₂N₃; orthorhombic, a=9.198(3), b=5.307(2), c=4.779(2); phase III, Li₅SiN₃; cubic, a=4.7240(3); phase IV, Li₁₈Si₃N₁₀; tetragonal, =14.168(4), c=14.353(8); phase V, Li₂₁Si₃N₁₁; tetragonal, a=9.470(3), c=9.530(8); phase VI, Li₈SiN₄; tetragonal a=10.217(2), c=9.536(3) Å. The unit cell dimensions of phases III to VI were related to each other. All phases are lithium ion conductors. A new phase, Li₈SiN₄, has the highest lithium ion conductivity (5×10^-2 Sm^-1 at 400K) and the lowest activation energy (46kJ/mol) among the present products.