近来尖晶石相LiNi_0.5Mn_1.5O₄被认为是一种有前景的二次锂离子电池正极材料. 但是其相对较差的循环性能和倍率性能限制了LiNi_0.5Mn_1.5O₄的大规模应用. 金属掺杂被认为是一种提高其电化学性能的有效方法. 然而, 还急需深层次地理解掺杂对材料结构和电化学性质的影响. 采用第一性原理方法, 系统地研究了金属掺杂的LiM_0.125Ni_0.375Mn_1.5O₄ (M为Cr, Fe和Co)电极体系的结构与电子性质. 计算结果显示, 少量的过渡金属M取代LiNi_0.5Mn_1.5O₄晶格中的Ni, 能够有效抑制材料在电化学脱嵌锂过程中的体积变化(从锂化相到脱锂相, 体积变化率约为4%, 而未掺杂的情况为4.7%), 提高材料循环性能. 体系态密度表明金属掺杂能够减小体系的带隙, 进而提高材料的电子传导. 另外, 通过Li离子的扩散计算, 我们发现与未掺杂的LiNi_0.5Mn_1.5O₄相比, Co掺杂使得Li在材料中两条不同扩散路径的扩散能垒分别降低了约90 meV和140 meV, 表明Co掺杂有利于Li在材料中的快速扩散.

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.