Synthesis and Characterization of the Lithium-Rich Phosphidosilicates Li10Si2P6 and Li3Si3P7.

The lithium phosphidosilicates Li10Si2P6 and Li3Si3P7 are obtained by high-temperature reactions of the elements or including binary Li-P precursors. Li10Si2P6 (P21/n, Z = 2, a = 7.2051(4) Å, b = 6.5808(4) Å, c = 11.6405(7) Å, ß = 90.580(4)°) features edge-sharing SiP4 double tetrahedra forming [Si2P6]10- units with a crystal structure isotypic to Na10Si2P6 and Na10Ge2P6. Li3Si3P7 (P21/m, Z = 2, a = 6.3356(4) Å, b = 7.2198(4) Å, c = 10.6176(6) Å, ß = 102.941(6)°) crystallizes in a new structure type, wherein SiP4 tetrahedra are linked via common vertices and which are further connected by polyphosphide chains to form unique ∞2[Si3P7]3- double layers. The two-dimensional Si-P slabs that are separated by Li atoms can be regarded as three covalently linked atoms layers: a defect α-arsenic type layer of P atoms sandwiched between two defect wurzite-type Si3P4 layers. The single crystal and powder X-ray structure solutions are supported by solid-state 7Li, 29Si, and 31P magic-angle spinning NMR measurements.

Lithium Ion Mobility in Lithium Phosphidosilicates: Crystal Structure, 7 Li, 29 Si, and 31 P†MAS NMR Spectroscopy, and Impedance Spectroscopy of Li8 SiP4 and Li2 SiP2.

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.