Polymorphism and fast Potassium-Ion Conduction in the T5 Supertetrahedral Phosphidosilicate KSi2P3.

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 the well-studied sodium-based systems, potassium-based systems offer the advan-tage of low cost and a large electrochemical window but are hardly explored. Here we report on the synthesis and crystal structure of K-ion conducting T5 KSi 2 P 3 , which was inspired by recent discoveries of fast ion conductors in the family of alkali phosphidosilicates. KSi 2 P 3 is composed of SiP 4 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 can be as high as 2.6·10 -4 S/cm at 25 °C with an average activation energy of 0.20 eV. This is remarkable high for a potassium ion conductor at room temperature, and marks KSi 2 P 3 as the first non-oxide solid potassium ion conductor.