Research On Synthesis And Modification Of Layered LiMnO₂ Cathode Materials For Lithium-ion Batteries

Based on the research and development in detail on lithium-ion battery and its cathode material,layered LiMnO₂ was selected as the research object,and its preparation,composite and doped were also studied in this thesis.o-LiMnO₂ and LiMnO₂/Li₂MnO₃ cathode material was successfully synthesized by the reaction of LiOH and MnO₂ directly,using glucose as carbon source under the reductive atmosphere by one step solid phase method.o-LiMnO₂ cathode material was synthesized by solid-state and hydrothermal method,respectively, using Mn₂O₃,decomposed from MnO₂ at 750℃,as the manganese source.Besides, Mg and Mo were doped in LiMnO₂ materials.Compared to the product using Mn₂O₃ as manganese source,the progress of the one using glucose as carbon source can be performed without high-temperature pretreatment of MnO₂.Then the synthesis process was greatly simplified,and the synthesis cycle was shortened.The phase and micro structure of product were characterized by X-ray diffraction (XRD),scanning electron microscopy(SEM),and transmission electron microscopy(TEM),while the electrochemical performance was investigated by the charge-discharge testing and AC impedance test.Non-situ XRD was used in analysis of the phase change of LiMnO₂/Li₂MnO₃ composites during charging and discharging process.The amount of C in LiMnO₂ material and Li/Mn ratio in composite materials was determined by elemental analyzer and plasma-atomic emission spectrometer,respectively.On the base of a great deal of experimental data,the impacts of the ratio of raw materials,reaction temperature and such conditions on the performance of the product were obtained.The results showed that:the optimum conditions of synthesis of o-LiMnO₂ samples were as followed:the Li:Mn:C mole ratio was 5:4:2;calcinations under 750℃for 12h.Samples have good electrochemical properties,after 15 cycles,the discharge capacity was 122 mAh·g^-1.At the same time,0.23Li₂MnO₃·0.77LiMnO₂ showed a 4.1V and 2.5V charge-discharge platform,different from 3 V and 4V platform of o-LiMnO₂.Change to MnO₂ of part of product existed in charging and discharging process,such changes had a great impact on the electrochemical properties,leading to a discharge efficiency of greater than 100%;after 30 cycle,the discharge capacity of composite materials reached the 218 mAh·g^-1,the charge-discharge performance of o-LiMnO₂ was markedly improved.The performance of o-LiMnO₂ made by hydrothermal method is superior to that by solid-phase method using Mn₂O₃ as manganese source;through doping study of part of as-prepared LiMnO₂,results showed:while Mg-doped content of molar percentage was 10%,after 20 cycles,discharge capacity maintained more than 86.27 percent.The battery cycle performance of Mg-doped samples was markedly improved;while that of Mo-doped material was not as good as Mg-doped,but the same molar percentage of doping is also effective in improve the cycling performance.At the same time,LiMnO₂/VO₂(B)of the electrochemical properties of composite materials also studied.