Enhancing layered structural stability via mg doping at the Mn site to suppress irreversible oxygen redox activity in Li1.2Mn0.4–XMgXCo0.4O2 cathode materials

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The object of the study is Mg-doped Li- and Mn-rich layered oxide cathode materials with the composition Li1.2Mn0.4–xMgxCo0.4O2, where x = 0, 0.02, 0.04, and 0.06. The materials were synthesized by solid-state reaction. Mg doping at the transition-metal/Mn site was carried out to improve the stability of the layered structure and suppress irreversible oxygen-redox activity during Li2MnO3 activation. All samples contained R-3m LiMO2-like and C2/m Li2MnO3-like phases, with the Li2MnO3-like phase fraction increasing from 46.014 wt. % in the undoped sample to 52.464 wt. % at x = 0.06, while the I(003)/I(104) ratio increased from 1.66 to 1.92 at x = 0.04. The undoped sample showed the highest initial charge capacity of 171.46 mAh g⁻1 and discharge capacity of 131.4 mAh g⁻1. The Mg-doped sample with x = 0.02 showed the highest discharge capacity among the doped samples, at 112.58 mAh g⁻1, and also exhibited the best Li⁺ diffusion, 1.60 × 10⁻15 cm2 s⁻1, whereas the sample with x = 0.06 showed a decrease to 4.85 × 10⁻16 cm2 s⁻1. Overall, the results show that Mg doping improved the ordering of the layered structure and suppressed irreversible oxygen activity during Li2MnO3 activation at 4.50–4.55 V. However, this suppression of the oxygen-redox process also caused the Mg-doped samples to have lower capacity than the undoped sample. The excessive Mg addition can reduce capacity and hinder Li⁺ transport. In this study, the composition x = 0.02 showed the best balance among capacity, structural stability, and Li⁺ diffusion. These results can serve as a reference for optimizing Mg content in Li- and Mn-rich layered oxide cathodes for Li-ion batteries operating at high voltage, where structural stability, oxygen-redox control, capacity, and Li⁺ transport need to be balanced

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