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, Mg2+ 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 Fe3+ migration to the Na layer is confirmed, shedding light on further improvements for low-cost NIB cathodes in application scenarios.