Dopant-induced electronic design of redox-active elements in LiMn₂O₄ spinel structures

Despite the importance of the electronic-level design of inorganic cathode materials for high-performance secondary batteries, studies attempting to clarify the correlation between the electronic structure and performance are relatively scarce compared to the broad range of inorganic cathode materials developed to date. This study highlights that the symmetricity/asymmetricity of e_g/t_2g orbitals in redox-active elements would be a core factor to determine the degree of the Jahn-Teller distortion of LiM_0.125Mn_1.875O₄ (M = Mn, Co, Cr, Cu, Fe, and Ni) spinel-type cathode materials during the discharging process. The presence of redox-active Mn³⁺ ions accompanied by a significant collapse in the symmetry of orbital e_g during the discharging process is highlighted as the main reason for poor structural durability and electrochemical performance of the cathode material. This limitation can be most effectively overcome by removing Mn³⁺ ions by adding Ni²⁺ as a carefully selected dopant. Further investigation reveals that the electrochemical impact of the introduced dopants strongly relies on the change in the symmetricity/asymmetricity of their e_g/t_2g orbital configurations during the discharging process and the resultant energy benefit/penalty.