Effect of Mg substitution on structural stabilization of Na _x MnO ₂ cathode for Na‐ion batteries

As the Li‐ion batteries market encounters numerous obstacles, such as not sufficient production to the demands, there is a need to develop post‐lithium technologies, such as Na‐ion batteries. Na 0.67 MnO 2 cathode material provides high capacity; however, it suffers from phase transitions during the (de)intercalation process. Herein, it is presented that magnesium substitution in Na 0.67 Mg 0.2 Mn 0.8 O 2 stabilizes the crystal structure during cycling, which was confirmed by operando X‐ray diffraction measurements as well as the long‐term cycling experiment. It also enhances the electronic conductivity from 9 × 10 −6 to 7 × 10 −5 S cm −1 and the ionic one from 3 × 10 −6 to 4 × 10 −5 S cm −1 . X‐ray absorption spectroscopy O K‐edge spectra are in line with the enormous exceeding of the theoretical capacity of Na/Na + /Na x Mg 0.2 Mn 0.8 O 2 cell (nonreversible discharge capacity of 310 with 155 mAh g −1 of theoretical capacity). Electronic structure calculations performed by the Korringa–Kohn–Rostoker method in the framework of local spin‐density approximation + U in ordered Na 2/3 Mg 1/3 Mn 2/3 O 2 approximant support its semiconducting properties as well as confirm that substitution of Mn with Mg markedly enhances the role of oxygen in constituting electronic states around the Fermi level and its role in the electrochemical process leading to the improved capacity.