Realizing the Capacity Benefit of Anionic Redox in Stoichiometric Sodium Manganese Oxides

Sodium-ion batteries offer a cost-effective and sustainable alternative to lithium-ion batteries, yet their lower energy density remains a challenge. Anionic redox in layered oxide cathodes has the potential to enhance capacity beyond the limits of transition-metal redox. This potential is maximized in full-stoichiometric sodium compounds with 3d transition metals, a goal hindered by synthesis challenges. The successful synthesis of O3-NaLi_1/3Mn_2/3O₂ marked a breakthrough, offering high capacity, moisture stability, and no voltage decay. Yet, issues such as irreversibility, voltage hysteresis, and slow kinetics remain. In this concept, we review the state of the art of this promising O3 phase with a brief account of other anionic-redox-based Na compounds. We then discuss strategies and prospects to address these challenges, including tuning chemical compositions, O/P multiphase structures, and superstructure ordering.