An anionic polymerization strategy for improving ionic conductivity in solid hydroborate electrolytes: the anion paddle-wheel effect

Owing to their wide electrochemical stability window and enhanced ionic conductivity, <i>conjuncto</i>-type hydroborates are promising candidates for high-voltage solid-state electrolytes. However, despite experimental evidence of improved performance as electrolytes compared with Na₂B₁₂H₁₂ with a single B₁₂H₁₂ cage, the mechanisms driving these changes remain unexplored systematically. In this study, first-principles calculations were employed to investigate the electronic properties, electrochemical stability, ionic conductivity properties, and mechanical properties of the <i>conjuncto</i>-type hydroborates Na_<i>n</i>+1B_12<i>n</i>H_11<i>n</i>+1 (<i>n</i> = 2, 3, 4). The polymerization strategy of [B₁₂H₁₂]^2- anions can effectively reduce diffusion barriers and strengthen the anion paddle-wheel effect to enhance the ionic conductivity while preserving the electronic insulation and electrochemical stability of <i>conjuncto</i>-type hydroborate electrolytes. Moreover, the polymerized long-chain anions exhibit more flexibility, further improving the interfacial compatibility and mechanical processability. Our work provides insights into the design of advanced hydroborate electrolytes, highlighting the significant influence of anionic polymerization on both ion conductivity and mechanical properties.