First-Principle Study On The Doped Spinel LiMn₂O₄

As an electrode material with rich resources and low price, high-potential, environmentally friendly, high security, LiMn₂O₄ is the most promising new generation of lithium-ion battery cathode material replaced LiCoO₂. However, its charge-discharge cycle process was mainly due to the poor stability of the structure caused by the severe capacity fading phenomenon of instability and high temperature, which is the most important factor to further restrict its market-oriented commercialization. Over the years, through a variety of preparation methods (such as doping, surface coating, etc.), many researchers tried to change its structure to enhance stability and electrochemical properties. As for reports on the micro-electronic structure calculation of LiMn₂O₄ are relatively rare, this paper attempt study the systems of spinel materials LiMn₂O₄ and its electronic structure, so as to explain doping mechanism and provide meaningful guidance in theory for LiMn₂O₄ cathode material in the process of synthesis and improved properties.Based on the first-principle of the Density Functional Theory (DFT) and ultra-soft pseudo potential plane-wave method, we establish the LiMn₂O₄ doped crystal model by doping Ni and Co and calculate electronic structure of LiMn₂O₄ and the doping system to study their characteristics and changes of electronic structure such as band structures, density of states and so on. The results show that:LiMn₂O₄ is a direct-gap ion-conductor, in which the Li-ion is relatively free. Near the Fermi level of Mn-3d band and O-2p band overlap strongly, so there are strong interactions between Mn and O which played a role in the stability of structure. All doping system with Ni, Co maintain a single spinel structure with the decline in total energy and cell size, and wider width of top valence band. Mn-O bond order is stronger and bond lengths of Mn-O are reducing while net charges of Mn and O are increasing. Mn-O interaction stronged by doping Ni and Co increase the average strength of Mn-O bond so that increase the stability of the structure. It can be effectively restrain changes of structure in the charging and discharging of lithium-ion batteries and also conducive to improve the performance cycle. Density of states of Li iron tend to decrease in doping system, so the bond order of Li-ion and O-ion reduce with bond length increasing. Weakened interaction between Li and O result in the improvement of electrical conductivity performance. Ni separate doping, the Fermi energy rise, while a separate Co-doping, Fermi energy lower; Energy gap of compound-doped system go down evidently, which increase the electrical conductivity properties; Ni has greater effect than Co on the electronic characteristics of the compound-doped system.