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

Abstract 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 KCB 9 H 10 ⋅ 2C 3 H 4 N 2 , synthesized by grinding and heating the mixture of potassium decahydrido‐monocarba‐ closo ‐decaborate (KCB 9 H 10 ) and imidazole (C 3 H 4 N 2 ) 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 Pna 2 1 by FOX software. The diffusion properties for K + in the crystal structure were calculated using the climbing image nudged elastic band (CI‐NEB) method. KCB 9 H 10 ⋅ 2C 3 H 4 N 2 exhibited a high ionic conductivity of 1.3×10 −4 S cm −1 at 30 °C, four orders of magnitude higher than that of KCB 9 H 10 . This ionic conductivity is also the highest value of hydridoborate‐based K + conductors reported. Moreover, KCB 9 H 10 ⋅ 2C 3 H 4 N 2 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 KCB 9 H 10 ⋅ 2C 3 H 4 N 2 a promising K‐ion solid‐state electrolyte.