Study On Synthesis And Electrochemical Properties Of MnO-LiF Composite Cathode Material

MnO-LiF composite is a new type of cathode material based on conversion reaction,and its research is of great significance to the expansion of cathode materials for lithium ion batteries.In this paper,the preparation,coating modification and doping modification of MnO-LiF composites were systematically studied.The MnO-LiF-KB composites were prepared by spray drying-sintering method.The structure,morphology and electrochemical properties of the samples under different conditions were investigated.The optimal preparation conditions were obtained as follows:the sintering temperature was 550?,the sintering time was 20min,and the molar ratio of manganese oxide and lithium fluoride was 1:2.XRD and SEM results show that the MnO-LiF-KB composite prepared under the optimum conditions has high purity and no impurity phase.N₂ adsorption-desorption test showed that mesoporous structure was also found in the material,with specific surface area of 408.18 m~2/g and pore size of 22.6nm.The initial discharge capacity is 165.8m Ah/g at the current of 0.05C and the voltage range of1.5?4.8V,which is reduced to 137.2m Ah/g after 50 cycles.The rate test results show that when the discharge rate gradually increases from 0.05C to 0.1C,0.2C and 0.5C,the corresponding discharge specific capacities of the composites at various rates are 168.7,136.4,103.6 and 70.4m Ah/g,and the capacity decays rapidly.When the discharge rate returned to0.05C,the recovery rate of specific capacity is 77.4%.The AC impedance test results show that the main reason for the poor electrochemical performance of the material is the increase of the charge transfer resistance,which needs further exploration and modification.In order to improve the electrochemical performance of the materials,the materials prepared under the optimal conditions were coated with Mo S₂ to prepare MnO-LiF-KB/Mo S₂composites.XRD and SEM results showed that the structure of the material did not change after coating Mo S₂.When the current is 0.05C and the voltage range is 1.5?4.8V,the initial discharge capacity of the material is 199.9m Ah/g,which is reduced to 170.9m Ah/g after 50cycles,and the capacity retention rate is 85.5%.The rate test results show that when the discharge rate gradually increases from 0.05C to 0.1C,0.2C and 0.5C,the corresponding discharge specific capacities of the composites at various rates are 199.9,148.9,117.1 and80.5m Ah/g,which are higher than those of the uncoated materials.When the discharge rate returned to 0.05C,the specific capacity retention rate is 52.8%.From the results of AC impedance and cyclic voltammetry,it can be seen that the charge transfer resistance of the material coated with 2wt%Mo S₂ decreases significantly,and the lithium ion diffusion coefficient increases significantly,which indicates that the coated Mo S₂ can effectively improve the electrochemical performance of the material.In addition,this paper also explored the effects of different FeO doping levels on the structure,morphology and electrochemical properties of Mn_1-xFe_xO-LiF-KB composites.The results show that the optimum doping conditions are as follows:sintering temperature is650?,sintering time is 5h.XRD results showed that the Mn_1-xFe_xO-LiF-KB composite prepared under the optimal conditions had high purity and no impurity phase,and MnO and FeO formed a solid solution.The first discharge capacity is reduced due to activation,and the cycle performance is improved significantly.When the current is 0.05C and the voltage range is 1.5?4.8V,the initial discharge capacity of the material is 112.9m Ah/g,which increases to173.6m Ah/g after 50 cycles.The discharge rate gradually increased from 0.05C to 0.1C,0.2C,0.5C and 1C,and the corresponding discharge capacities of the composites were 107.1,101,84.8,62.8,27.9 and 112.5m Ah/g.When the discharge rate returned to 0.05C,the specific capacity retention rate is 105%.Further cycling tests show that,after 50 cycles the cycle performance of FeO doped materials is significantly improved,and the discharge capacity is stable at about 150 m Ah/g.It can be seen from the results of AC impedance and cyclic voltammetry that the charge transfer resistance of the material doped with FeO decreased significantly,and the diffusion coefficient of lithium ion increased significantly,indicating that although the doping FeO prolonged the activation period,it effectively improved the cyclic performance of the material.