| ABSTRACT:The development of different cathode material, especially Mn-based solid solution material is reviewed in detail. The thesis presents high-temperature solid state synthesis routes and performance of Li2Mn03, Mn-Ni system0.3Li2MnO3·O.7LiMno.5Ni0.5O2and its Co/Al doping, Mn-Ni-Co system0.3Li2MnO3·0.7LiNi1/3Mn1/3Co1/3O2materials. The structure model of two phase co-exist is proposed initially based on Rietveld fitting method for structure parameter and phase constitution.Li2MnO3was prepared using electrolytic manganese dioxide and lithium carbonate as raw materials via high-temperature solid state synthesis routes. The different synthesis temperature and different charge-discharge voltage range are studied. Experimental results show that the sample synthesized at800℃presents well crystallization. The initial discharge capacity at20mA g-1(0.1C) rate is46.9mAh g-1(2.0～4.8V), while keep rising up to169.5mAh g-1before the sixteen cycles times then a sharp decline. Sample after acid treated presents higher solid electrolyte interface resistance (RSEI) and lower charge transfer process resistance (Rct). Cycle times for capacity ascending, columbic efficiency and discharge capacity and are effectively improved. However, the destruction of surface and structure occurred thus cycle performance get worse.Li1+x(Mn0.65Ni0.35)1-x02in Mn-Ni system with different Li-excess was synthesized using hydroxide prepared from co-precipitation methods as precursors. Co and Al doping was initially used for modification. The sample of x=0.09behaves well crystallization and electrochemical properties with initial efficiency of74%and discharge capacity of217.2mAh g-1(0.1C,2.0～4.8V). The highest capacity can reach240.2mAh g-1between2.0～4.8V; while the value is147.9mAh g-1between3.0～4.3V. With the increase of Li-excess, initial efficiency and discharge capacity down correspondingly.The initial discharge capacity and columbic efficiency in the cyclical process of Co-doped sample are improved, while rate performance becomes better after Al-doing. The SEI impedance decreased drastically. On the other hand, anodic peak potential for Co-doped sample is lower than that of Al-doped and the reduction peak is higher, which presents smaller polarization.The Li2MnO3-LiNi1/3Co1/3Mn1/3O2solid solution Li1+x(Mn0.533Ni0.233Co0.233)1-xO2material in Mn-Ni-Co system was also initially studied comparatively. The sample of x=0.09presents the best initial coulombic efficiency and other electrochemical properties. The discharge capacity is close to260mAh g-1at20mA-g-1rate (0.1C,2.0～4.8V) and186.9mAh g-1at200mA g-1(1C). The capacity ratio of1C/0.1C is73.4%.Through Rietveld fitting method and quantum chemistry calculation, structure parameter and phase constitution analysis for the Mn-Ni and Mn-Ni-Co system solution are performed to XRD data. The structure model of two phases co-exists for Li-rich Mn-based solid solution materials are proposed initially. When the fitting process is carried out based on rock salt structure C2/m of Li2MnO3single phase model, the shape factor and other parameters are splitting. On the other side, while the fitting process is based on layered Rim structure of LiMn0.5Nio.5O2or LiNi1/3Co1/3Mn1/3O2, the XRD pattern fits well except20～25°ranges. However, when the two phases co-exist model of Li2MnO3-LiMn0.5Ni0.5O2or Li2MnO3-LiNi1/3Co1/3Mn1/3O2was adopted, all the patterns fit well and the phase content is in accordance with the theoretical value. Through crystal structure building, total energy calculation, band structure and density of states analysis, the structural stability of single structure was confirmed well than solid solution. Two phases co-exists model for the different Li-rich Mn-based materials is proper. The Rietveld fitting and quantum chemistry calculation method are innovately and instructively for anylasis or deducation of solid-solution systems. |
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