A first-principles study on stabilizing disordered LiNi_0.5Mn_1.5O₄ cathode material by doping

High-voltage LiNi_0.5Mn_1.5O₄ (LNMO) is a promising cathode material for high energy-density Li-ion batteries. The LNMO phase exhibits either ordered or disordered structure; meanwhile, the Mn ions possess 3+ and 4+ mixed valence states. The main challenges of LNMO cathode material for real applications are that the ordered LNMO phase possesses poor rate capability and that the Mn^3+/4+ redox reaction leads to Mn dissolution. Doping has been a straightforward approach to address these issues, by enhancing the stability of the disordered LNMO phase and reducing the Mn³⁺ fraction in LNMO. However, experimental trials involved different setups and uncertainties, which usually cannot be directly compared, and thus an investigation of doping influence on disordered LNMO based on fair comparison is required. Herein, we screened and proposed the best dopants that can improve LNMO performance, and we clarified the corresponding mechanism based on first-principles calculations and thermodynamics. The stable doping systems and preferred doping sites were obtained through phase stability evaluation. Mechanism of disordered LNMO stabilization was obtained through analyzing the relation between the change of transition metal valence states after doping and the ability of dopants to stabilize disordered LNMO. It was found that not only extra-electron dopants stabilize disordered LNMO, as most of the studies reported, but extra-hole dopants could also stabilize disordered LNMO. Furthermore, considering the Mn^3+/4+ redox reaction, extra-hole dopants, including Mg, Na, and Zn, were suggested to be the dopants that could improve LNMO performance with simultaneously increasing the fraction of disordered phase and avoiding Mn^3+/4+ redox reaction.