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 KCBH ⋅ 2CHN, synthesized by grinding and heating the mixture of potassium decahydrido-monocarba-closo-decaborate (KCBH) and imidazole (CHN) 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 Pna2 by FOX software. The diffusion properties for K in the crystal structure were calculated using the climbing image nudged elastic band (CI-NEB) method. KCBH ⋅ 2CHN exhibited a high ionic conductivity of 1.3×10 S cm at 30 °C, four orders of magnitude higher than that of KCBH. This ionic conductivity is also the highest value of hydridoborate-based K conductors reported. Moreover, KCBH ⋅ 2CHN 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 KCBH ⋅ 2CHN a promising K-ion solid-state electrolyte.