Unraveling the Role of Earth-Abundant Fe in the Suppression of Jahn–Teller Distortion of P′2-Type Na_2/3MnO₂: Experimental and Theoretical Studies

Layered Na_2/3MnO₂ suffers from capacity loss due to Jahn–Teller (J–T) distortion by Mn³⁺ ions. Herein, density functional theory calculations suggest Na_2/3[Fe_xMn_1–x]­O₂ suppresses the J–T effect. The Fe substitution results in a decreased oxygen–metal–oxygen length, leading to decreases in the b and c lattice parameters but an increase in the a lattice constant. As a result, the capacity retention and rate capability are enhanced with an additional redox pair associated with Fe^4+/3+. Finally, the thermal properties are improved, with the Fe substitution delaying the exothermic reaction and reducing exothermic heat.