Boosting the Lithium-Ion Conductivity in Li

Lithium-ion conductors are one of the key features of all-solid-state lithium-ion batteries. To modify their properties and enable their implementation in high-performance devices, an understanding of the relationship between the crystal structure and the transport properties of the mobile species is important. Lithium phosphidotetrelates and -trielates are classes of lithium-ion conductors reaching ionic conductivities of up to 4.5 × 10-3 cm-1 at room temperature for ω-Li9GaP4. Here, we present the new lithium phosphidotantalate Li7TaP4, and the aliovalent substitution of Ta by Li atoms, which leads to a partial filling of octahedral voids in the structure of Li7TaP4. As a result, the lithium-ion conductivity of Li7TaP4 (1.3 × 10-7 S cm-1) increases by 3 orders of magnitude to 3.7 × 10-4 S cm-1 in Li9.5Ta0.5P4. Li7TaP4 and Li9.5Ta0.5P4 crystallizing in the cubic space groups Pa3̅ and Fm3̅m, respectively, show a close structural relationship. The structure-property relationship is highlighted and compared with the isotypic tetrel element analogues.