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

摘要

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, Li8SiP4 and Li2SiP2, 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) x 10(-6) Scm(-1) at 0 degrees C to 1.2(2)x10(-4) Scm(-1) at 75 degrees C (Li8SiP4) and from 6.1(7)x10(-8) Scm(-1) at 0 degrees C to 6(1)x10(-6) Scm(-1) at 75 degrees C (Li2SiP2), as determined by impedance measurements. Temperature-dependent solid-state Li-7 NMR spectroscopy revealed low activation energies of about 36 kJ mol(-1) for Li8SiP4 and about 47 kJ mol(-1) for Li2SiP2. Both compounds were structurally characterized by X-ray diffraction analysis ( single crystal and powder methods) and by Li-7, Si-29, and P-31 MAS NMR spectroscopy. Both phases consist of tetrahedral SiP4 anions and Li counterions. Li8SiP4 contains isolated SiP4 units surrounded by Li atoms, while Li2SiP2 comprises a three-dimensional network based on corner-sharing SiP4 tetrahedra, with the Li ions located in cavities and channels.