Polymorphism and Fast Potassium-Ion Conduction in the T5 Supertetrahedral Phosphidosilicate KSi₂ P₃

The all-solid-state battery (ASSB) is a promising candidate for electrochemical energy storage. In view of the limited availability of lithium, however, alternative systems based on earth-abundant and inexpensive elements are urgently sought. Besides well-studied sodium compounds, potassium-based systems offer the advantage of low cost and a large electrochemical window, but are hardly explored. Here we report the synthesis and crystal structure of K-ion conducting T5 KSi₂ P₃ inspired by recent discoveries of fast ion conductors in alkaline phosphidosilicates. KSi₂ P₃ is composed of SiP₄ tetrahedra forming interpenetrating networks of large T5 supertetrahedra. The compound passes through a reconstructive phase transition from the known T3 to the new tetragonal T5 polymorph at 1020 °C with enantiotropic displacive phase transitions upon cooling at about 155 °C and 80 °C. The potassium ions are located in large channels between the T5 supertetrahedral networks and show facile movement through the structure. The bulk ionic conductivity is up to 2.6×10^-4  S cm^-1 at 25 °C with an average activation energy of 0.20 eV. This is remarkably high for a potassium ion conductor at room temperature, and marks KSi₂ P₃ as the first non-oxide solid potassium ion conductor.