Aqueous Lithium Hydroxide Chemistry Based on Hierarchically Assembled Hydrogenated Borophene/Cobalt-Nickel Compounds for Rechargeable High-Performance Supercapattery

Lithium hydroxide (LiOH), recognized for its chemical stability, presents a promising discharge product for safe and efficient electrochemical energy storage. While water-driven protonation is crucial for facilitating LiOH chemistry, the reaction mechanism of LiOH in aqueous electrolytes remains unclear. Here, a hierarchically assembled hydrogenated borophene/cobalt-nickel compound (HB/CoNiC) is synthesized using electrochemical-electrophoretic deposition to investigate aqueous LiOH chemistry. The electrophoretic deposition of reducible HB results in the formation of undercoordinated oxygens near partially reduced CoNi alloy nanoparticles. In bi-salt-in-water electrolytes, HB/CoNiC demonstrated a significantly enhanced specific capacitance of 1102 F g⁻¹ at a current density of 1 A g⁻¹, which is a 250% improvement compared to CoNiOOH, due to the pseudocapacitive contributions from lithiation and protonation reactions. Furthermore, HB/CoNiC-based supercapatteries exhibited a remarkably high energy density of 112.5 Wh kg⁻¹ at a power density of 40 kW kg⁻¹ and a capacitance retention of 95.7% over 10 000 cycles at a current density of 20 A g⁻¹, outperforming state-of-the-art aqueous energy storage devices. Electrochemical measurements, ex-situ characterizations, and computational calculations demonstrate that the oxygen nonbonding states in undercoordinated oxygen atoms act as crucial redox centers in lithiation and protonation processes, facilitating efficient and reversible LiOH chemistry in aqueous electrolytes.