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.