Investigation of potassium-ion conductivity and bottlenecks in khibinskite, K2ZrSi2O7

Potassium solid-state electrolytes (K-SSEs) offer improved safety and energy density to potassium-ion batteries. Inspired by the high ionic conductivity in zirconium phosphosilicates like Na 3 Zr 2 Si 2 PO 12 , we explore ionic conductivity in khibinskite, a zirconium silicate with chemical formula K 2 ZrSi 2 O 7 . Khibinskite, a rare mineral local to Russia’s Kola Peninsula, is formed of alternating layers of ZrO 6 octahedra and Si 2 O 7 dimers, with potassium cations occupying interstitial sites in the Si 2 O 7 layer. Examination of the khibinskite structure reveals two-dimensional K + conduction planes through the Si 2 O 7 layers, allowing for an activation energy of 0.77 eV. Bond valence site energy (BVSE) analysis shows that K + hops occur through hexagonal bottlenecks. Based on the shape of each bottleneck, we introduce a categorization system for the bottlenecks (Type I, II, and III) and demonstrate with BVSE analysis that Type II bottlenecks are particularly narrow. We then map the distribution of the different bottleneck types within the two-dimensional conduction planes of khibinskite, showing that narrow Type II bottlenecks hinder K + migration in both dimensions of conductivity. Finally, based on this bottleneck analysis, we assess the potential for improved K + diffusion in khibinskite via substitution with larger pyro-anions.