Understanding solid electrolyte interphase formation in hydroborate-based all-solid-state batteries

Hydroborate solid electrolytes are attracting increasing attention as alternatives to argyrodite electrolytes for all-solid-state lithium and sodium batteries. In this work, we first summarize recent progress in mixed-anion closo-hydroborate and closo-hydrocarborate electrolytes and derive criteria to select anion compositions that balance ionic conductivity, electrochemical stability, and interface compatibility. Building on a diffusion-limited interphase growth model, we quantitatively compare the interface resistance growth rate of Li₃(CB₁₁H₁₂)₂(CB₉H₁₀) and Li₆PS₅Cl in contact with lithium metal, lithiated silicon, and delithiated NMC811 by monitoring impedance as a function of time using electrochemical impedance spectroscopy. Despite its lower reductive stability, the hydroborate exhibits substantially slower interface resistance buildup in contact with lithium metal than the argyrodite, emphasizing that the transport properties of the solid electrolyte interphase, rather than the bulk electrolyte stability alone, govern long-term interfacial degradation. In contact with lithiated silicon, both electrolytes show markedly slower resistance growth, with the hydroborate showing higher stability than the argyrodite. In contact with delithiated NMC811, neither electrolyte shows any resistance growth at an open-circuit potential of 3.8 V vs. Li/Li⁺. At 4.1 V vs. Li/Li⁺, the hydroborate electrolyte shows resistance growth due to electrolyte oxidation, despite its higher oxidative stability compared to the argyrodite, which shows no resistance growth at this potential. The cell resistance is dominated by the interface resistance at the cathode, highlighting the importance of protective coatings to prevent electrolyte oxidation. Our findings highlight closo-hydroborates and closo-hydrocarborates as promising electrolytes for high-energy solid-state lithium and sodium batteries and underline the critical importance of tailoring interphase composition and transport to unlock their full potential.