Potassium Decahydrido- closo-Decaborane Urea Complex as a Potential Solid-State Electrolyte for Potassium Metal Batteries

All-solid-state potassium metal batteries have been considered promising candidates for large-scale energy storage because of abundance and wide availability of K resources, elimination of flammable liquid organic electrolytes, and incorporation of high-capacity K metal anode. However, unideal K-ion conductivities of most reported K-ion solid electrolytes have restricted the development of these batteries. Herein, a novel K₂B₁₀H₁₀·CO(NH₂)₂ complex is reported, forming by incorporating urea into K₂B₁₀H₁₀, to achieve an enhanced K-ion conductivity. The crystal structure of K₂B₁₀H₁₀·CO(NH₂)₂ was determined as a monoclinic lattice with the space group of C2/c (No. 15). K₂B₁₀H₁₀·CO(NH₂)₂ delivers an ionic conductivity of 2.7 × 10^-8 S cm^-1 at 25 °C, and reaching 1.3 × 10^-4 S cm^-1 at 80 °C, which is about 4 orders of magnitude higher than that of K₂B₁₀H₁₀. One possible reason is the anion expansion in size due to the presence of dihydrogen bonds in K₂B₁₀H₁₀·CO(NH₂)₂, resulting in an increase in the K-H bond distance and the electrostatic potential, thereby enhancing the mobility of K⁺. The K-ion conductivity is also higher than those of most hydridoborate-based K-ion conductors reported. Besides, K₂B₁₀H₁₀·CO(NH₂)₂ reveals a wide electrochemical stability window and remarkable interface compatibility with K metal electrodes, suggesting a promising electrolyte for all-solid-state K metal batteries.