Study On Preparation?modification And Performance Of Layered-spinel Lithium-rich Cathode Materials

Lithium-rich cathode materials, which are considered as one of the main tendency of lithium ion batteries, have the advantages of high discharge capacity, low cost and environmental benignity. They have been the focus of intense research interest since their first report. Nevertheless, the drawbacks of their cyclic durability problems, low initial coulombic efficiency, poor rate capability and the voltage fading have always been the focus of the researchers, which also limit the practical uses of these cathodes.In an attempt to put forward a promising technical route to improve the performance of lithium-rich cathode materials, the layer-spinel hybrid structure lithium-rich cathode materials have been designed and sythiesized in this dissertation. This dissertation particularly investigated the preparation, structural characters, technological conditions and the electrochemical performance of the layer-spinel hybrid structure lithium-rich cathode materials. And the main works are as follows:1. Homogeneous lithium-rich layered-spinel hybris structure microspheres were prepared by a solvothermal method and subsequent gradient calcinations method. These cathode materials contained the layered Li2MnO3 phase and spinel Li4Mn5O12 phase inside, the Li4Mn5O12 phase can provide a fast Li-ion diffusion rate due to the 3D holes, and the Li2MnO3 phase can provide a high capacity by the layered Li-Mn-Ni-Co-O component. The research results proved that the prepared cathode materials can combine the advantages of these two phases which exhibited a high discharge capacity, an improved cycle stability and rate performance. It can deliver a high initial discharge capacity of 289.6 mAh/g at 0.1 C, and a discharge capacity of 122.8 mAh/g at 10 C between 2.0 and 4.6 V. In addition, the retention of the capacity was still as high as 88 % after 200 cycles.2. Based on the previous studies, the influential factors of structural composition structure were investigated, and electrochemical performances of the lithium-rich cathode materials with different layered-spinel phase ratios were compared in order to determine a composite which can exhibit the best properties. The results showed that the relative amounts of each phase depended on the calcination temperature and calcination time in gradient calcination process:(i) when the temperature stays at 750 oC, the spinel phase will be formed mainly, and it will lead to a formation of layered phase when the temperature stays at 800 oC;(ii) when the calcination time at 750 oC is less than 10 hours, the formation of spinel phase is quite inactive, and the major components is layered phase, but once the calcination time at 750 oC is longer than 10 hours, the quantity of spinel phase in the hybrid structure will increase rapidly; besides, the different time stayed at 800 oC will lead to a transformation from spinel phase to layered phase in different quantities.3. In order to overcome the drawback of the poor electroconductivity for the layered-spinel hybrid composite, the spherical lithium-rich cathode material with a layered-spinel hybrid structure was successfully synthesized and coated by polyaniline(PANI) following the previous work. It was found that the modification of PANI can apparently improve the electrical conductivity of the Li-rich cathode material with no effects on the hybrid structure, and the impedance of the materials during the cyclings was also decreased obviously. In the meanwhile, the discharge capacity of the as-prepared materials at 0.1 C was as high as 302.9 mAh/g, this work also showed a much better rate performance which can keep a discharge capacity of 146.2 mAh/g at 10 C, a high initial columbic efficiency of 83.5 % and a high capacity retention of 92.4 % after 200 cycles at 0.5 C.4. In order to make a further improvement on the elelctrochemcial properties of lithium-rich cathode materials, the solvothermal method used in previous sections was applied to the preparation of other lithium-rich cathode materials. Based on the homogeneous and well-dispersed particles with a diameter of 1 ?m, the approach of Na-doping conined with porous structure was carried out in this section. There was a large ionic radii difference between Na ion(r Na+ = 1.02 ?) and the transition metal ions(Ni2+, Mn4+, Co3+), introducing Na ions into the Li layer can stabilize the bulk lattice and expand the Li slab space, so as to facilitate the lithium ion diffusion and effectively enhance the stability of the layered structure; meanwhile, porous structure can keep the structural integrity via a whole moderate and controllable method, and the porous structure can provide extra active sites for the Li+ storage. Results showed that the prepared cathode materials can deliver a high initial discharge capacity of 305.2 mAh g-1 between 2.0 V and 4.6 V at a rate of 0.1 C, and the discharge capacity at 0.5 C, 5 C, 10 C even reached 252.3 mAh/g?200.8 mAh/g and 177.2 m Ah/g.