Facile Synthesis of Potassium Decahydrido‐Monocarba‐closo‐Decaborate Imidazole Complex Electrolyte for All‐Solid‐State Potassium Metal Batteries

All-solid-state potassium metal batteries have caught increasing interest owing to their abundance, cost-effectiveness, and high energy/power density. However, their development is generally constrained by the lack of suitable solid-state electrolytes. Herein, we report a new complex KCB9H10·2C3H4N2, synthesized by grinding and heating the mixture of potassium decahydrido-monocarba-closo-decaborate (KCB9H10) and imidazole (C3H4N2) under mild conditions, to achieve the K-ion superionic solid-state electrolyte. The crystal structure was revealed as an orthorhombic lattice with the space group of Pna21 by FOX software. The diffusion properties for K+ in the crystal structure were calculated using the climbing image nudged elastic band (CI-NEB) method. KCB9H10·2C3H4N2 exhibited a high ionic conductivity of 1.3 × 10-4 S cm-1 at 30 °C, four orders of magnitude higher than that of KCB9H10. This ionic conductivity is also the highest value of hydridoborate-based K+ conductors reported. Moreover, KCB9H10·2C3H4N2 demonstrated a K+ transference number of 0.96, an electrochemical stability window of 1.2 to 3.2 V vs. K/K+, and good stability against the K metal coated by a layer of potassium imidazolate (KIm). These great performances make KCB9H10·2C3H4N2 a promising K-ion solid-state electrolyte.