Fe-Substituted Sodium β″-Al₂O₃ as a High-Rate Na-Ion Electrode

The great interest in Na-ion batteries has led to the development of a number of materials based on the analogues of Li-ion battery materials. However, the larger ion radius and the lower operating potential of Na compared to those of Li show the limitation of that approach and underscore the need for new methods of electrode material design. In this paper, the well-known superionic conductor, Na β″-Al₂O₃, is converted into a high-rate Na⁺ negative electrode by substituting Fe for Al and achieving mixed electron–ion conduction; that is, the presence of Fe²⁺/Fe³⁺ in the spinel block of the structure produces electronic conduction, while Na⁺ transport is retained in the conduction plane. The 52 mol % Fe-substituted Na β″-Al₂O₃ exhibited a high ionic conductivity (∼10^–3 S cm^–1), near theoretical capacity values at 1C, and retained ∼70% of theoretical capacity at 20C. Kinetic analysis revealed that a surface-controlled charge-storage mechanism is responsible for the high-rate capability. Although the gravimetric capacity of this converted superionic conductor is limited by the small number of Na-ion storage sites available per transition metal, the transformation of Na β″-Al₂O₃ into a mixed electronic–ionic conducting electrode provides a set of design rules for achieving high-rate redox electrodes from solid-state electrolytes.