Fabricating High-Performance Lamellar β″-Al₂O₃ Solid Electrolytes via an MnO₂-Mediated Redox Reaction Strategy

The development of a high-performance β″-Al₂O₃ solid electrolyte (BASE) is pivotal for advancing sodium-based energy storage systems, yet the intrinsic mechanism of mineralizers regulating its phase evolution and microstructure remains elusive. This work clarifies for the first time that MnO₂ undergoes multistep redox reactions during heat treatment, accompanied by the release of reactive oxygen species and a reduction in the activation energy required for β-phase formation. Meanwhile, the interfacial electron transfer originating from the MnO₂-mediated redox reaction suppresses the formation of impurity α-Al₂O₃, thereby promoting the formation of a high-purity β″-phase. Specifically, MnO₂ induces preferential crystal adsorption along the (001) crystal plane, restrains crystal growth along the c-axis direction, and facilitates two-dimensional expansion along the ab plane. This results in the formation of parallel-stacked layered structures (diameter: 1-4 μm, thickness: 0.2-0.8 μm) and the construction of continuous Na⁺ migration channels. The prepared electrolyte exhibits an ionic conductivity of 3.6 × 10^-3 S/cm at 300 °C, which is 1 order of magnitude higher than that of the electrolyte prepared by the traditional MnO₂-free method.