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

Area of Science:

• Materials Science
• Electrochemistry
• Solid-State Chemistry

Background:

• High-performance solid electrolytes are crucial for the advancement of sodium-based energy storage systems.
• The precise mechanism by which mineralizers influence the phase evolution and microstructure of beta double prime alumina (β″-Al2O3) remains poorly understood.
• Achieving a high-purity β″-Al2O3 phase is essential for optimal ionic conductivity.

Purpose of the Study:

• To elucidate the role of manganese dioxide (MnO2) as a mineralizer in the synthesis of β″-Al2O3.
• To clarify the intrinsic mechanism of MnO2 in regulating phase evolution and microstructure.
• To investigate the impact of MnO2 on the formation of β″-Al2O3 and its ionic conductivity.

Main Methods:

• Investigated the heat treatment process of β″-Al2O3 in the presence of MnO2.
• Analyzed the redox reactions of MnO2 and the release of reactive oxygen species.
• Examined the effects of interfacial electron transfer and crystal growth kinetics on phase formation.

Main Results:

• MnO2 undergoes multistep redox reactions, reducing activation energy for β-phase formation and releasing reactive oxygen species.
• MnO2-mediated interfacial electron transfer suppresses α-Al2O3 impurity formation, promoting high-purity β″-Al2O3.
• MnO2 induces preferential crystal growth, leading to layered structures and continuous Na+ migration channels, resulting in a conductivity of 3.6 × 10^-3 S/cm at 300 °C (one order of magnitude higher than MnO2-free methods).

Conclusions:

• MnO2 effectively acts as a mineralizer, significantly enhancing the formation of high-purity β″-Al2O3.
• The redox chemistry and interfacial effects of MnO2 are key to controlling phase evolution and microstructure.
• The optimized β″-Al2O3 electrolyte demonstrates superior ionic conductivity, paving the way for improved sodium-based energy storage.