3D Framework Structure of a New Lithium Thiophosphate, LiTi2(PS4)3, as Lithium Insertion Hosts

Abstract

The M2(PS4)3 (M = transition metal) structure has been investigated as a new open 3D-framework structure for electrode materials for lithium rechargeable batteries. One of these compounds, LiTi2(PS4)3, was synthesized from solid state reaction. From the single crystal X-ray diffraction analysis, it was observed that the LiTi2(PS4)3 crystallizes in the hexagonal space group P6cc (No. 184) with a = 19.8978 (4) Å, c = 11.5198(3) Å, and Z = 8. The resemblance of the powder X-ray diffraction pattern and the lattice parameters obtained from single crystal X-ray diffraction measurement indicates that the crystal structure is isomorphous with NaTi2(PS4)3. This structure is built of TiS6 octahedra linked by edges to PS4 tetrahedra and vice versa to build a 3D framework. Most attractively, very wide tunnels are obtained along the c-axis where lithium ions are mobile. Electrochemical insertion of lithium into LiTi2(PS4)3 was carried out. The initial structure of LiTi2(PS4)3 was changed with initial insertion of lithium, but ∼7 Li per formula unit could be inserted in the modified structure without decomposition of the starting material. However, such a large capacity was not retained on cycling.