Study The Synthesis Of Doped Lithium Manganese Oxide And The Reaction-Diffusion Process Of Lithium Ion

Lithium manganese oxide(LiMn₂O₄)has been considered as a promising cathode material for lithium ion batteries,since it is high cell voltage,high safety,low cost and low toxicity.Despite such merits,LiMn₂O₄ still has difficulty in practical applications,owing to its severe capacity depletion.To overcome this problem,many approaches have been carried out.In this work,pure and doped LiMn₂O₄ were synthesized by a solid-state reaction.The effects of the synthesis conditions on their microstructure and electrochemical performance of spinel lithium manganese oxide were investigated by X-ray diffraction(XRD),scanning electron microscopy(SEM)and charge-discharge experiments.Electrochemical impedance spectroscopy(EIS)was used to investigate the effects of the substitution on the lithium extraction in the LiBOB based electrolyte.The optimal synthesis conditions of pure LiMn₂O₄ were found to be a preheating temperature 600℃,preheating time 6h,calcination temperature 750℃,calcination time 30h, annealing temperature 600℃and annealing time 6h.The material synthesized using the optimal conditions provided an initial discharge capacity of 138mAh·g^-1,136mAh·g^-1and capacity retention of 86.3%during 50 cycles at 25℃.But its capacity retention was 66.2%at elevated temperature(55℃).The results revealed that the samples substituted by Al³⁺,Li⁺and F^-had a lower fade rate and a lower capacity than pure LiMn₂O₄.But the Al³⁺,Li⁺ and F^-co-substituted sample had better cycle performance and higher initial capacity.Among these doped materials, Li_1.03Al_0.03Mn_1.94O_3.94F_0.06had the best cycle performance.Its initial capacity was 127 mAh·g^-1and its capacity retention was 91.3%after 50 cycles at 55℃.The EIS tests showed that the Al³⁺,Li⁺ and F^-co-substitution improved the stability of the surface film,reduced its thickness and the resistance of charge transfer,but it had little effect on the solid diffusion coefficient of Li⁺.From the analysis of EIS,the lithium extraction processes of pure and doped LiMn₂O₄ was a the reaction-diffusion process of Li⁺,which includes the Li⁺ surface film diffusion processes,the charge transfer processes and Li⁺ solid diffusion in the materials host.The average activation energy of the film diffusion,the charge transfer and the solid diffusion in LiMn₂O₄ was 30.04 kJ·mol^-1,4.71 kJ·mol^-1and 52.74 kJ·mol^-1,respectively,and that in Li_1.03Al_0.03Mn_1.94O_3.94F_0.06was calculated to be 17.51 kJ·mol^-1,7.21 kJ·mol^-1and 52.46 kJ·mol^-1.Compared to the values of the average activation energy of three processes,Li⁺ solid diffusion in the host was supposed to be the rate-determining-step of the electrode process.