Preparation Of O₂ Li-Mn-Co-MO Li-rich Manganese-based Cathode Materials And Their Electrochemical Properties

Lithium-rich manganese-based cathode materials have become one of the research hotspots in the field of lithium-ion batteries cathode materials because of their high specific capacity.Among them,O₃-type lithium-rich transition metal oxides have attracted much attention because of their large specific capacity obtained by oxygen redox reaction.However,the redox reaction of anions in the charge-discharge process leads to the migration of transition metal ions,which eventually leads to the transition metal layer to spinel phase.And the voltage and capacity decrease seriously with the change of structure,which hinders its industrial application.In order to overcome its shortcomings,this paper mainly focuses on the preparation and modification of O₂-type lithium-rich manganese materials and the optimization of electrochemical performance.we did a lot of modified methods,then characterized and researched it structure,morphology,charge and diacharge performances,cyclic performances and rate performances of the modified materials by XRD,SEM,TEM,and electrochemical performance testing.The research contents and main results are as follows:Li_1.25Mn_0.75O₂ cathode materials were synthesized by coprecipitation-high temperature sintering-ion exchange method.The effects of p H,sintering temperature and ion-exchange lithium content on the electrochemical properties were studied.The results showed that the p H of the precursor synthesis was 8.0,and then the mixture was calcined in air at 450°C for 4 h and then at 800°C for 8 h to form a black powder,finally the target product was subjected to molten salt ion exchange 4h under the condition of lithium excess of 5%.The synthesized target product has a specific capacity of 208.309 m Ah/g for the first charge and 188.342 m Ah/g for reversible discharge under the current density of 0.1C.After 40 cycles,the specific capacity drops to 107.887 m Ah/g.Li1.25Co_0.20Mn_0.55O2?Li1.25Co_0.25Mn_0.5O2?Li1.25Co_0.30Mn_0.45O2 cathode materials of Li_1.25Mn_0.75-xCo_xO₂ series were prepared to research the effect of cobalt substitution partial manganese on lithium-rich manganese cathode materials.The results show that the first charge-discharge specific capacity of Li1.25Co_0.25Mn_0.50O2 cathode material at a current density of 0.1C is 200.101 m Ah/g,the discharge specific capacity is 213.838 m Ah/g after20 cycles,and the retention rate of visible specific capacity is very high.After 30 cycles,the average voltage of the material decreased just 0.0258V.In order to improve the electrochemical properties,the dooping of vanadium and molybdenum metal elements during the synthesis precursor,the doping of these two metal elements did not change the material structure through the XRD patterns.The first discharge specific capacity of Li_1.25Mn_0.75O₂/V materials with a doping content of 5%vanadium is 290.963 m Ah/g,which is more 102.621 m Ah/g than pristine.After 40 cycles of charge/discharge,the capacity retention rate was 89.8%,while the pristine material only57.3%,which increase of 32.5%.After 30 cycles,the average voltage of the material decreased only 0.0786 V,obviously,the doping of vanadium not only increased the specific capacity of the cathode material,but also s reduced the rate of capacity and voltage decay.The first discharge specific capacity of Li_1.25Mn_0.75O₂/Mo material with a doping content of 6%molybdenum is 242.437 m Ah/g,which is more 54.095m Ah/g than pristine.After 40charge/discharge cycles,the capacity retention was 68.6%,while the pristine retention was only 57.3%,which increase of 32.5%.To improve electrochemical performance through increase the electrical conductivity of cathode materials,the composite was prepared by ball milling with the mass ratio of acetylene black to Li_1.25Mn_0.75O₂ of 12:88.The results showed that the structure of composite materials was not changed.The first discharge specific capacity was 309.222m Ah/g at current density of 0.1C,which was 120.879m Ah/g higher than pristine,a nd the capacity retention rate reached 80.8%after 40 charge-discharge cycles,which was 23.5%higher than that without adding acetylene black.After 30 cycles,the average voltage of the material decreased by only 0.0763V.