Spinel LiNi_0.5Mn_1.5O₄ is recently considered as a promising cathode material for rechargeable Li-ion batteries, yet its large-scale application is limited due to relatively poor cycling and rate performance. Metal doping is expected to be an effective approach to improve the electrochemical performance of spinel LiNi_0.5Mn_1.5O₄. However, deeper understanding into doping effects on structural and electrochemical properties of LiNi_0.5Mn_1.5O₄ electrode materials is still ambiguous. In this work, systematic first-principles studies based on the density functional theory (DFT) have been carried out to investigate electronic and structural properties of LiM_0.125Ni_0.375Mn_1.5O₄ (where M=Cr, Fe, and Co) cathode. All computations were carried out on the basis of projector augmented wave (PAW) approach as implemented in VASP. The exchange and correlation potential was treated with the generalized gradient approximation (GGA) of Perdew and Wang (PW91). In order to take into account the strong on-site Coulomb interaction (U) presented in the localized d electrons of transition metals, the GGA-U framework was used for evaluating the exchange-correlation energy. Within this framework, the effective single parameters U_eff of 3.5, 4, 5, 5.62 and 5.96 eV were used for Cr, Fe, Mn, Co and Ni, respectively. The electron wave functions were expanded by a high cutoff of 500 eV and the total energy was converged to 10^-5 eV. The following electronic states are treated as valence electrons: Li, 2s¹2p⁰; O, 2s²2p⁴; Cr, 3d⁵4s¹; Mn, 3d⁶4s¹; Fe, 3d⁷4s¹; Co, 3d⁸4s¹; Ni, 3d⁹4s¹; Regarding the accurate calculations of total energy and electronic structure, the tetrahedron method with Blöch correction was adopted for structural relaxation and density of state (DOS) analysis. The cell parameters, volume cells, and positions of all the atoms in the primitive cell were fully relaxed until the residual Hellmann-Feynman force on each atom was less than 10^-2 eV/Â. It is found that doping a small quantity of metal M atoms into the Ni site results in a decrease in the volume variation during the lithiation/delithiation cycle (ca. 4% from lithiated phase to delithiated phase, whereas 4.7% for the undoped case). Electronic calculations suggested that transition metal doping (Cr-, Fe-, and Co-doping) would effectively improve the electronic conductivity of systems. To evaluate effects of dopants on lithium mobility, we calculated the activation energies for lithium diffusion in M-doped LiNi_0.5Mn_1.5O₄ cathode. Our calculations indicate that doping with Co can potentially reduce lithium diffusion barrier as compared to that of pristine LiNi_0.5Mn_1.5O₄ spinel.

Key words: first-principles, spinel LiNi_0.5Mn_1.5O₄, doping, volume variation, Li diffusion barriers