Boosting the Lithium-Ion Conductivity in Li₇TaP₄ by Aliovalent Li versus Ta Substitution by Three Orders of Magnitude

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 Li_9.5Ta_0.5P₄. Li₇TaP₄ and Li_9.5Ta_0.5P₄ 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.