Preparation And Electrochemical Performances Of Spinel Modified Layered Lithium-rich Manganese-based Materials And Their F-doped Materials

Lithium-rich manganese-based layered oxides cathode?LLMOs?have been paid more and more attention since they were firstly reported to date due to their extremely high discharge specific capacity(>300 m Ah·g^-1),low cost,and environmental friendliness.They are also considered to be one of the extremely promising cathode materials for next-generation lithium-ion batteries.However,these cathode oxides also have many defects,such as irreversible deoxygenation reaction during the first cycle,poor rate performance caused by low ion and electronic conductivity,and continuous attenuation of the voltage platform during the long cycle,etc,which hinder their further commercialization application.This thesis focuses on the enhancement of electrochemical performances of the LLMO cathode materials by structure design and anion doping.A novel type of spinel-modified lithium-rich manganese-based materials are firstly prepared via a simple precursor treatment process,and then further modification of the as-prepared cathode materials by anion doping are carried out.Those efforts are aim to provide some practical ideas and solutions for the improvement of electrochemical performances of LLMOs.Detail research contents including the preparation method,the exploration of process conditions,the intrinsic structures and the electrochemical performances of the as-prepared materials are elaborated in dtetail.The main research works and results are as follows:?1?A sreils of lithium-rich manganese-based materials with the chemical compositions of Li_1+xMn_0.54Ni_0.13Co_0.13O₂?x=0.08,0.14,0.20 and 0.26?are prepared from a simple sol-gel method with an annealing process,by adjusting the lithium amounts in the mixed precursors and.X-ray diffraction?XRD?patterns in combination of their refinement results,Raman spectra,high resolution transmission electron microscopy?HRTEM?,selected electron diffraction?SAED?and electrochemical measurements of the as-prepared materials indicate the presence of different amounts of novel cobalt-rich spinel phase?Li Co Mn O₄?within these materials,which increases with the decrease of relative lithium compositions.It is found that an appropriate amount of new spinel phase induced by the control synthesis process?using reasonable lithium usage and annealing treatment?contributes to the improvement of the electrochemical performance of LLO,specifically,the Li_1.14Mn_0.54Ni_0.13Co_0.13O₂(Li_1.14)sample delivers a high initial discharge specific capacity and ICE of 303.0 m A h·g^-1 and 77.0%,respectively at 20 m A·g^-1,good rate capability(?200 m A h·g^-1 at 1.0 C rate;1.0 C=200 m A g^-1)and capacity retention capability?83.7%after 100 cycles at 1.0 C rate?.Therefore,a suitable amount of Li Co Mn O₄ spinel phase can be successfully introduced into the pristine LLMO structure by a simple sol-gel route combined with the synergistic strategy of lithium usage and annealing process,which can improve the electrochemical performance of the sample.?2?Two new materials(Li_1.14-F_0.03 and Li_1.20-F_0.03)with the same doping amount of F are successfully prepared from the above-mentioned Sol-gel route by addition of the Li F in the mixed precursors.It is found that the spinel amoumts in the two samples are both increased.In addition,the Li_1.14-F_0.03 sample exhibits bad electrochemical performance of than that of the Li_1.14 sample,while the Li_1.14-F_0.03 sample displays superior electrochemical performance than that of the Li_1.20 sample.After that,a series of graded F-doped materials(Li_1.20-F_0.03,Li_1.20-F_0.05and Li_1.20-F_0.07)were prepared and further comparsions of their electrochemical performances,to achieve the best F-doping amont for the F-doped LLMOs.It is indicated that the primary particle sizes and the content of spinel phase of these samples increse with the increasing of the F-doping content,based on the structure characterization results.The electrochemical performances analysis suggests that the Li_1.20-F_0.03sample exhibits the highest specific capacity,the best rate and cycle performance among the three samples,with a discharge specific capacity of 198 m Ah·g^-1 at 1.0 C and 84.0%of capacity retention after 100 cycles.