Li₁₀GeP₂S_12-xSe_x solid solution as a new Li⁺ solid-state electrolyte

A new type electrolyte Li₁₀GeP₂S_12-xSe_x (LGPSS, 0 < x ≤ 7) was designed and synthesized by solid-state reaction method. The cell parameters a and c of Li₁₀GeP₂S_12-xSe_x (0 ≤ x ≤ 7) increase from 8.7295(8) to 8.995(2) Å and 12.675(1) to 13.102(3)Å with x increasing from 0 to 7, respectively. S2 site is mainly occupied by Se, while the secondly occupied is S3 site. Li1 is occupied by Li completely and embedded in framework, while other Li sites are all partially occupied and involved in conducting. The occupancy of Li2, Li3, Li4 site are 0.71, 0.70 and 0.47, respectively, when x = 4. Li2 has lowest energy, and Li2 site are preferred to be the destination of Li⁺ migration pathway. Substitution of S is proved to be selective and there is a priority for Se to bind with Ge. The maximum conductivity 4.98 mS/cm is obtained by Li₁₀GeP₂S₈Se₄ at 25 °C, which is about three times higher than that of Li₁₀GeP₂S₁₂ (LGPS). The activation energy (E_a) and electrochemical window of Li₁₀GeP₂S₈Se₄ are 0.23 eV and 0–3 V. Li⁺ migration mechanism is calculated using bond valency theory, the migration of Li ⁺ presents 1D pathway has maximum barrier of 0.233 eV and 3D pathway barrier is 0.392 eV Li₁₀GeP₂S₈Se₄ has favorable stability that there is no breakdown occurring under 0.72 mA/cm² current density.