Abstract: Hydroborate/carborate electrolytes represent an emerging and newly rediscovered solid electrolyte used in various all-solid-state batteries (such as lithium-ion batteries and sodium-ion batteries). High ionic conductivity, wide chemical/electrochemical stability, low density, and favorable mechanical properties make hydroborate/carborate electrolytes a promising candidate for solving the difficult challenges faced by the device integration and processing of all-solid-state batteries. It is remarkable that the ionic conductivity of solid electrolytes can be simply adjusted up to 10^?3 S cm^?1, and the optimized ionic conductivity can even reach 10^?2 S cm^?1. Furthermore, hydroborate/carborate electrolytes have been successfully formed and applied to?~?5 V high-voltage solid-state batteries. However, due to certain characteristics of hydroborate/carborate electrolytes, such as anion rotation and phase transition, it is challenging to understand the mechanism of their high ionic conductivity. Therefore, in this review, we summarized the latest research progress on hydroborate/carborate electrolytes, highlighted various mechanisms underlying the conductivity, described emerging characterization techniques and theoretical calculations, and listed general guidelines to unravel the high conductivity of hydroborate/carborate compounds. Novel strategies and suggestions on hydroborate/carborate work are also proposed. Following emerging research trends, we project promising future development toward the realization of hydroborate/carborate electrolytes in practical applications.

Key words: All-solid-state batteries, Ion migration mechanism, Hydroborate/carborate electrolytes