| In order to strengthen the construction of ecological civilization,it is imperative to develop and utilize new energy.Lithium ion batteries(LIBs)have received wide attention in recent years as a new type of high performance energy storage device.With the development of social economy,the electronic devices of new generation have put forward higher requirements for LIBs.Since the specific capacities of the cathode materials are much lower than that of the anode materials,and their proportions in the quality and cost of the batteries are as high as～40%,the cathode materials are generally considered to be the major limiting factors for high performance LIBs.The ternary layered oxides(LiNi1-x-yCoxMnyO2 or xLi2MnO3·(1-x)LiMO2,M=Ni,Co,Mn,etc.)are currently the most popular and highly promising cathode materials due to their excellent comprehensive properties.However,they still face problems such as structural deterioration,capacity loss,and surface side reactions.From the view of practical applications,considering the energy density,stability and safety,the rational design of the compositions and structures of the ternary materials still faces enormous challenges.In this thesis,the reasonable structural design modifications of the ternary layered oxide cathode materials were carried out,and the related reaction mechanisms and structure-activity relationships were analyzed and verified,so as to explore the design idea of a novel class of cathode materials for LIBs.The main work includes:1.The effect of niobium(Nb)doping in one-dimensional nanostructured LiNii/3Co1/3Mn1/3O2 cathode has been systematically investigated.It can be confirmed that Nb doping can effectively diminish cation disorder,improve the structural stability,and can weaken the electrochemical polarization and increase the Li+ ion diffusion coefficient.Furthermore,owing to the valence compensation,Mn4+ ions are partially reduced to Mn3+ions,which can further improve the conductivity and cycling stability.Therefore,superior electrochemical performances are observed for the Nb-doped LiNi1/3Coi1/3Mn1/3O2 cells,which achieve a discharge capacity of 200.4 mAh·g-1 at 0.1 C with a coulombic efficiency of 92.3%.Moreover,a discharge capacity of 118.7 mAh g’1 at 5 C with 83.3%capacity retention after 200 cycles is also achieved.2.Nb-doped LiNi0.4Co0.2Mn0.4O2(Nb-NCM)nanobelts have been successfully fabricated for the first time through a facile electrospinning method by the delicate control of PAN pyrolysis and Nb-NCM formation(including nucleation and subsequent growth processes).The experimental results show that the formation of nanobelts undergoes a morphology evolution from nanofibres to nanotubes,and finally to the nanobelts composed of subunit nanoparticles.At the same time,the Nb-NCM nanobelts exhibit superior electrochemical performances.The Nb-NCM nanobelts can deliver a high discharge capacity of 148.9 mAh·g-1 at 1 C after 100 cycles.At 2 C and 5 C rates,after 200 cycles,the specific capacities of 127.5 and 109.6 mAh g-1 were released,respectively.3.Free-standing Lil.2Mn0.54Ni0.13Co0.13O2/MWCNTs framework electrodes have been successfully fabricated by vacuum filtration for high energy density LIBs.The free-standing electrodes exhibit excellent electrochemical performances by constructing a superior three-dimensional conductive network.Specifically,they achieve an initial discharge capacity of 318.2 mAh·g-1 at 0.1 C with a coulombic efficiency of 91.1%.After 200 cycles at 1 C,they can still deliver 209.2 mAh g-1,corresponding to a capacity retention of 95.9%.Furthermore,compared with conventional electrodes,the free-standing electrodes can yield up to 47%reduction in the total weight and the quality of the active materials can be increased by 2 times in the case of a certain pole piece area. |
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