Spinel Type Of Lithium-ion Battery Cathode Materials Of Limn <sub> 2 </ Sub> The O <sub> 4 </ Sub> High-temperature Performance Failure Mechanism And Its Improvement

Systematic research on the mechanism and solutions of the failure of LiMn2O4 spinel at elevated temperature was performed in this thesis.HF generates from the decomposition of electrolyte at elevated temperature, and part of the capacity loss is induced by Mn dissolution due to the attack of HF to LiMn2O4. Irreversible capacity loss results from the structure change due to Jahn-Teller effect.Four pure LiMn2O4 spinels were prepared by different lithium manganese sources using melt-impregnation method. The lattice parameters of the samples were calculated by self-developed refinement program based on least square method. The result showed that four samples are Li-deficient spinel, and their specific surface area, lattice parameter and electrochemical performance are different to some extent. The samples synthesized by LiOH H2O+EMD is confirmed to be best in view of storage and cycling performance.Cr and anion A were selected as doping element and LiCrxMn2-xO4-yAy composite lithium manganese oxide was prepared for the first time. Lattice parameter for Cr-doping spinel decreases compared with stoichiometric spinel and that of both Cr and A-doping increases compared with Cr-doping spinel. Ion distribution model of LiCrxMn2-xO4-yAy was founded and the theoretical capacity was calculated. The result showed that the theoretical capacity is increased by A-doping. The crystal structure of spinel is stabilized by Cr-doping and the account of dissolved Mn lessened in electrolyte, thus improve the storage performance and cyclability of electrode material. Mn-A bond is stronger than that of Mn-O bond after A-doping, and further stabilize the structure of spinel, thus further better the cyclability of LiMn2O4 spinel. The C-rate performance of LiMn2O4 was improved by Cr-doping. As far as Li/LiCr0.1Mn1.9O4 battery is concerned, the best cyclability of which was achieved in the charge and discharge rate of 1C. The differential chronopotentiongram of charge and discharge showed that the averagecharge voltage increase and the average discharge voltage decreases, and the polarization increases, thus results in the insufficient charge and part of capacity loss.Theory of surface chemistry was applied to the surface modification of the surface of LiMn2O4, and LiCoO2-coated LiMn2O4 and MgO-coated LiMn2O4 were synthesized. SEM showed that an inorganic layer formed on the surface of LiMn2O4 for the two samples. The elevated temperature performance of LiMn2O4 was improved due to the better performance of LiCoO2 at elevated temperature. The initial charge and discharge capacity of MgO-coated LiMn2O4 is smaller due to polarization. And which become higher since the 2nd cycle. And the cyclability is improved.Dynamics research of the electrochemical process showed that elevated temperature speed up the movement of lithium ion in the electrolyte, thus enhance the conductance and reduce the resistance of the electrolyte, thus enhance the working voltage at elevated temperature. Polarization curve of the battery showed that the rate of electrochemical reaction was enhanced by elevated temperature. Diffusion coefficient of Li+ was determined by Chronocoulometry, which is confirmed to be 1.43 X10"12cm2/s and 4.38 X10"12 cm2/s at room temperature and elevated temperature respectively. It showed that Li+ diffuses faster in the lattice of LiMn2O4 at elevated temperature.