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

Abstract

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, Li 8 SiP 4 and Li 2 SiP 2 , 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 (Li 8 SiP 4 ) and from 6.1(7)×10 −8 Scm −1 at 0 °C to 6(1)×10 −6 Scm −1 at 75 °C (Li 2 SiP 2 ), as determined by impedance measurements. Temperature‐dependent solid‐state 7 Li NMR spectroscopy revealed low activation energies of about 36 kJ mol −1 for Li 8 SiP 4 and about 47 kJ mol −1 for Li 2 SiP 2 . 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 SiP 4 anions and Li counterions. Li 8 SiP 4 contains isolated SiP 4 units surrounded by Li atoms, while Li 2 SiP 2 comprises a three‐dimensional network based on corner‐sharing SiP 4 tetrahedra, with the Li ions located in cavities and channels.