Co-doping Lithium Manganate Spinel Cathode Materials

Spinel LiMn₂O₄ has become one of the most promising cathode materials of Li-ion batteries because of its advantages such as source abundance, low price, easy preparation and environmental friendliness. However, LiMn₂O₄ undergoes rapid capacity fade at high charge and discharge voltage due to the Jahn-Teller effect and Mn²⁺ dissolution, which becomes one of the biggest obstacle for the commercialization of the spinel LiMn₂O₄ and also a research hot topic at present. In the present work, well-crystallized LiMn₂O₄ was prepared by a high temperature solid phase reaction using Li₂CO₃ and EMD as raw materials and modification on LiMn₂O₄ by co-doping of two kinds of metal ions was also carried out on the basis of the research at home and abroad.It was found that well-crystallized LiMn₂O₄ prepared by two-stage temperature elevation shows a capacity of 120 mAh g^-1, and the Li and Ni modified spinel, Li_1.05Mn_1.95O₄ and LiNi_0.05Mn_1.95O₄, show better cycle performances compared to LiMn₂O₄, but their voltage plateau and polarization during charge and discharge were not very good.Considering that little research works of modifying the spinel by rare earth (RE) have even been done and co-doping with two metals could be of interest, the present work was focused on the experimental investigations of the effect of Li/RE and Li/Ni co-doping on the structure and electrochemical performances. It was found that obvious improvement in performance, especially the high temperature performance at rapid charge and discharge rate of 0.5C, was observed for the Li/RE co-doped LiMn₂O₄ compared to the un-doped one. Li_1.01Nd_0.01Mn_1.98O₄ shows a first discharge capacity of 111 mAh g^-1 at 55℃ with a 97% retention rate after 50 cycles at 0.5C. It was indicated by the structural analyses that the cell parameters were increased by rare earth doping, which expands the tunnels for Li-ions diffusion and also stabilizes the LiMn₂O₄ frame. The reason for the improvement in electrochemical performances of LiMn₂O₄ was explained by the crystal field theory.However, there is a large difference in radii between the rare earth and Mn ions. Thus, impurity can easily formed during doping. Based on the above research, attempt was made to use Li/Ni co-doping with mole ratio of 0.05. It was found by electrochemical tests that Li_1.05Ni_0.05Mn_1.9O₄ spinel shows improved electrochemical cycling performances with a little lower first discharge capacity compared to the un-doped one. Li_1.05Ni_0.05Mn_1.95O₄ shows a first discharge capacity of 101.6 mAh g^-1 at 55℃ with a 93% retention rate after 50 cycles at 0.5C.Ex-situ XRD was performed to study the structural changes of Li1.05Nio.05Mri1.9O4 during Li de-intercalation process. It was found that Li/Ni co-doping could reduce changes of the cell parameter effectively during the first stage (0.4 < x <1) and obviously during the second stage (0.1