Boosting Reversibility of Mn‐Based Tunnel‐Structured Cathode Materials for Sodium‐Ion Batteries by Magnesium Substitution

Electrochemical irreversibility and sluggish mobility of Na⁺ in the cathode materials result in poor cycle stability and rate capability for sodium-ion batteries. Herein, a new strategy of introducing Mg ions into the hinging sites of Mn-based tunnel-structured cathode material is designed. Highly reversible electrochemical reaction and phase transition in this cathode are realized. The resulted Na_0.44Mn_0.95Mg_0.05O₂ with Mg²⁺ in the hinging Mn-O₅ square pyramidal exhibits promising cycle stability and rate capability. At a current density of 2 C, 67% of the initial discharge capacity is retained after 800 cycles (70% at 20 C), much improved than the undoped Na_0.44MnO₂. The improvement is attribute to the enhanced Na⁺ diffusion kinetics and the lowered desodiation energy after Mg doping. Highly reversible charge compensation and structure evolution are proved by synchrotron-based X-ray techniques. Differential charge density and atom population analysis of the average electron number of Mn indicate that Na_0.44Mn_0.95Mg_0.05O₂ is more electron-abundant in Mn 3d orbits near the Fermi level than that in Na_0.44MnO₂, leading to higher redox participation of Mn ions.