Facile Synthesis of Inorganic Li ₂ B ₁₂ H ₁₂ /LiI Solid Electrolytes for High‐Voltage All‐Solid‐State Lithium Batteries

Abstract Hydroborate‐based solid electrolytes (SEs), distinguished by their eco‐friendliness and non‐flammability have emerged as a research hotspot in energy storage research. Despite these advantages, their widespread adoption is constrained by insufficient ionic conductivity and poor high‐voltage compatibility. To overcome this challenge, a novel hydroborate SEs, 400‐0.6Li ₂ B ₁₂ H ₁₂ ‐0.4LiI (denoted as 400‐0.6B ₁₂ ‐0.4I), is engineered through a facile synthesis strategy combining high‐temperature processing (400 °C, 2 h, in Ar) and LiI doping. 400‐0.6B ₁₂ ‐0.4I exhibits an outstanding ionic conductivity up to 0.75 mS cm ⁻¹ at 25 °C, which is the highest level among the reported inorganic Li ₂ B ₁₂ H ₁₂ ‐based SEs so far. It also improves the cycling stability against lithium metal anodes, limits dendrite growth with a high critical current density of 5.0 mA cm ⁻² , which enables the Li symmetrical cells to cycle over 1000 h at 25 °C with only 10 mV of overpotential. Moreover, it cycles well in cells using lithium anodes and various high voltage (>4 V vs Li ⁺ /Li) cathodes materials such as LiCoO ₂ , NCM811, and LiMn ₂ O ₄ . The proposed 400‐0.6B ₁₂ ‐0.4I SEs offer valuable guidance for designing next‐generation lithium‐ion solid electrolytes with superior ionic conductivity, enhanced interfacial stability, and exceptional electrode compatibility.