First-principles study on metal-doped LiNi0.5Mn1.5O4 as a cathode material for rechargeable Li-ion batteries

Abstract

Spinel LiNi0.5Mn1.5O4 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 LiNi0.5Mn1.5O4. However, deeper understanding into doping effects on structural and electrochemical properties of LiNi0.5Mn1.5O4 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 LiM0.125Ni0.375Mn1.5O4 (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 Ueff 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 105 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 102 eV/Å. 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 LiNi0.5Mn1.5O4 cathode. Our calculations indicate that doping with Co can potentially reduce lithium diffusion barrier as compared to that of pristine LiNi0.5Mn1.5O4 spinel. © 2013 Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences.