Earth‐Abundant Na‐Mg‐Fe‐Mn‐O Cathode with Reversible Hybrid Anionic and Cationic Redox

Abstract Na‐ion batteries (NIBs) are promising for grid‐scale energy storage applications. However, the lack of Co, Ni‐free cathode materials has made them less cost‐effective. In this work, Mg 2+ is successfully utilized to activate the oxygen redox reaction in earth‐abundant Fe/Mn‐based layered cathodes to achieve reversible hybrid anionic and cationic redox capacities. A high first charge capacity of ≈210 mAh g −1 with balanced charge–discharge efficiency is achieved without O‐loss, showing a promising energy cost of $2.02 kWh −1 . Full cell against hard carbon anode without pre‐sodiation shows energy density exceeding ≈280 Wh kg −1 with a decent capacity retention of 85.6% after 100 cycles. A comprehensive analysis of the charge compensation mechanisms and structural evolution is conducted. Voltage and capacity loss resulting from partially reversible Fe 3+ migration to the Na layer is confirmed, shedding light on further improvements for low‐cost NIB cathodes in application scenarios.