Preparation And First Principle Calculations Of The LiMn₂O₄ As The Cathode For Lithium-ion Battery

Spinel LiMn2O4 was found to be one of the most challenging cathode materials for Li-ion batteries, which was attributed to its abundant Mn resource, eco-friendliness, high voltage, good thermal safety, etc. However, However, the poor stability and the rapidly capacity loss during the process of charge and discharge, hindered its practical applications seriously. It was mainly due to the Jahn-Teller distortion of the structure and the dissolution of Mn3+ in the acid electrolytes. It was evidenced that nanoporous LiMn2O4 constructed with nanometer-size primary particles, can sustain high rate capability and good cycling performance due to the larger surface to volume ratio, shorter diffusion path length and better structure stability.In this thesis, nanoporous LiMn2O4 have been prepared by sol-gel and hydrothermal method, respectively. The carbon spheres prepared from hydrothermal method were used as the template.The mesoporous structure is found in the LiMn2O4 prepared by sol-gel method. The surface area is 11.5 m2/g. The pores diameter are in the range of 0.6–30 nm. The initial discharge capacities are 129, 124, 121, 108, 85 and 53 mAhg-1 at 0.2, 0.5, 1, 2, 5, and 10 C, respectively. At 0.5 C, the coulomb efficiency is over 99% and the capacity retention rate is about 88%. The CV analysis suggests that the diffusion coefficient is between 9.83 ×10-9 and 4.70×10-8 cm2s-1. After the charge and discharge cycle, LiMn2O4 still keeps the cubic spinel.The mesoporous structures are also found in the LiMn2O4 prepared by hydrothermal method, which shows the surface area of 13.175 m2g-1. The valence state of Mn element in LiMn2O4 are +3 and +4, respectively. The results show that the initial discharge capacities are 135, 126, 118, 106, 89 mAhg-1 cycled at 0.2, 0.5, 1, 2, 5 C, respectively. At 0.5 C, the coulomb efficiency is over 98% and the capacity retention rate is about 88%. The diffusion coefficient is from 4.50×10-8 to 7.86×10-8 cm2s-1 and the resistances of RS, RSEI, Rct are 6.34, 50.51 and 9.43 ?, respectively. After the charge and discharge cycle, LiMn2O4 still keeps the cubic spinel structure but locally deformed.In addition, the structure and electronic properties of the spinel LiMn2O4 are studied by first principle calculations using GGA and GGA+U method, respectively. The results show that the spinel LiMn2O4 is metallic for the GGA method, which is different from the experimental result. Three configurations, FM, AFM-I and AFM-II of cubic LiMn2O4 are calculated from GGA+U method, respectively. The results show that LiMn2O4 with FM configuration is a metallic metastable compound. In AFM configurations, the results show that LiMn2O4 with the AFM-Ⅱis more stable. In AFM-Ⅱ, LiMn2O4 is cubic structure with the lattice parameter of 8.513 ? and is semiconducting compound with the bandgap of 0.33 eV.