Lithium Ion Mobility in Lithium Phosphidosilicates: Crystal Structure, ⁷Li, ²⁹Si, and ³¹P MAS NMR Spectroscopy, and Impedance Spectroscopy of Li₈SiP₄ and Li₂SiP₂

Abstract

The need to improve electrodes and Li-ion conducting materials for rechargeable all-solid-state batteries has drawn enhanced attention to the investigation of lithium-rich compounds. The study of the ternary system Li-Si-P revealed a series of new compounds, two of which, Li₈ SiP₄ and Li₂ SiP₂ , are presented. Both phases represent members of a new family of Li ion conductors that display Li ion conductivity in the range from 1.15(7)×10^-6 Scm^-1 at 0 °C to 1.2(2)×10^-4 Scm^-1 at 75 °C (Li₈ SiP₄ ) and from 6.1(7)×10^-8 Scm^-1 at 0 °C to 6(1)×10^-6 Scm^-1 at 75 °C (Li₂ SiP₂ ), as determined by impedance measurements. Temperature-dependent solid-state ⁷ Li NMR spectroscopy revealed low activation energies of about 36 kJ mol^-1 for Li₈ SiP₄ and about 47 kJ mol^-1 for Li₂ SiP₂ . Both compounds were structurally characterized by X-ray diffraction analysis (single crystal and powder methods) and by ⁷ Li, ²⁹ Si, and ³¹ P MAS NMR spectroscopy. Both phases consist of tetrahedral SiP₄ anions and Li counterions. Li₈ SiP₄ contains isolated SiP₄ units surrounded by Li atoms, while Li₂ SiP₂ comprises a three-dimensional network based on corner-sharing SiP₄ tetrahedra, with the Li ions located in cavities and channels.