Electrochemical Reaction of <scp> LiCoO ₂ </scp> Cathode With Optimized <scp> LiBH ₄ </scp> – <scp>MgO</scp> Electrolyte in All‐Solid‐State Lithium‐Ion Batteries

ABSTRACT In this study, MgO was incorporated into the extensively studied solid electrolyte material, lithium borohydride (LiBH 4 ), to enhance its poor ionic conductivity at ambient temperature. The addition of MgO was proved to significantly improve its ionic conductivity by approximately four orders of magnitude at 30°C compared to the low‐temperature phase (less than 115°C) of pristine LiBH 4 . Based on this electrolyte, all‐solid‐state batteries employing LiCoO 2 as the cathode and MgO‐modified LiBH 4 as the electrolyte were successfully fabricated and operated in the low‐temperature range. Moreover, the initial charging process exhibited anomalous electrochemical behavior, delivering a remarkably high specific capacity of 285.2 mAh/g with an unconventional charge plateau at 1.6 V, which deviates substantially from the typical electrochemical characteristics of LiCoO 2 . To understand the charging mechanism from thermochemical and electrochemical views, a series of mechanistic characterizations was performed on the battery. Thermogravimetric analysis revealed a small amount of hydrogen evolution (≤ 0.2 wt%) at phase transition temperatures, while solid‐state NMR spectroscopy confirmed the formation of BO bonds, providing evidence for redox reactions involving LiBH 4 . However, comparative electrochemical experiments and X‐ray diffraction (XRD) analysis excluded the influence of the thermal decomposition of LiBH 4 during the charging process. The charging mechanism that controlled the electrochemical behavior of this system was clarified in a detailed discussion.