Synthesis And Modification Of The Lithium Manganese Oxide For Lithium-ion Batteries

As a high efficiency energy storage technology,lithium-ion batteries can convert,store and transport clean renewable energy such as wind,solar,geothermal and tidal energy.Meanwhile,it can also solve the increasingly serious global energy shortage problem and avoid the environmental issues.Therefore,developing lithium-ion battery with high energy density and power density is of great significance for its further application.Featuring a high working potential（4 V vs.Li⁺/Li）,high theoretical specific capacity(148 mAh g^-1),low cost,environmental friendliness,and three-dimensional lithium ion diffusion channel,lithium manganese oxide（LiMn₂O₄）is a very attractive cathode material,but it suffers fast capacity deterioration especially at elevated temperatures,which limits its practical application.In this paper,the strategies of optimizing the synthesis process,ion doping and surface coating were developed to improve the electrochemical performance of LiMn₂O₄.The specific research contents are summarized as follows:（1）The research of sintering process for LiMn₂O₄.By using a two-stage sintering process,that is,a pre-sintering at 600°C followed by a high temperature of 700°C for 10 h,the as-obtained LiMn₂O₄ sample with good crystallinity and uniform particle size delivers a discharge capacity of 125.2 mAh g^-1 at 0.5 C in the potential range of 3.0-4.3 V in the first cycle,and keeps a capacity retention of 84%after 200 cycles at room temperature,which is superior to that delivered by LiMn₂O₄ prepared by single-stage sintering for ten hours.（2）The research of Nb and Li co-doping on the properties of LiMn₂O₄.By preparing different ratios of Nb and Li co-doped Li_1+xMn_2-yNb_yO₄（x=0.05、0.10、0.15;y=0,0.01,0.02,0.03,0.04）,it is found that the particle size and surface of LiMn₂O₄ sample increases and smoothens with the increase of Nb doping amount,respectively,leading to the smaller contact area with electrolyte and further suppressing the dissolution of manganese.When the Nb and Li doping amounts are 0.03 and 0.05,respectively,Li_1.05Mn_1.97Nb_0.03O₄ sample shows the best electrochemical performance,realizing capacity retentions of 94.9%at 25℃and 84%at 55℃after 100 cycles at 0.5C(1C=148 mA g^-1),respectively,which is significantly higher than that of undoped LiMn₂O₄.At the same time,Li_1.05Mn_1.97Nb_0.03O₄ also exhibits excellent rate performance,showing a discharge capacity of 90 mAh g^-1 at 5C and 50 mAh g^-1 at 10C.（3）The research of surfactant assisted Al₂O₃ coated LiMn₂O₄.Due to the poor coating effect of LiMn₂O₄ by conventional liquid phase coating method,this work successfully prepared Al₂O₃ uniformly coated LiMn₂O₄ composite（LiMn₂O₄@Al₂O₃（PVP））by utilizing the adsorption of metal ions from the surface chemical bond of polyvinylpyrrolidone（PVP）.Compared with LiMn₂O₄ and LiMn₂O₄@Al₂O₃,the as-obtained LiMn₂O₄@Al₂O₃（PVP）can keep capacity retentions of 93.8%after 200 cycles at 25℃and 87.4%after 100 cycles at 55℃at 0.5 C,achieving great improvement in cycle performance.