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 K 2 B 10 H 10 ·CO(NH 2 ) 2 complex is reported, forming by incorporating urea into K 2 B 10 H 10, to achieve an enhanced K-ion conductivity. The crystal structure of K 2 B 10 H 10 ·CO(NH 2 ) 2 was determined as a monoclinic lattice with the space group of C 2/ c (No. 15). K 2 B 10 H 10 ·CO(NH 2 ) 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 K 2 B 10 H 10 . One possible reason is the anion expansion in size due to the presence of dihydrogen bonds in K 2 B 10 H 10 ·CO(NH 2 ) 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, K 2 B 10 H 10 ·CO(NH 2 ) 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.