Controllable Preparation And Electrochemical Performance Of Li-rich Manganese-based Cathode Materials And Sodium Ion Battery Layer Materials

Li-rich manganese-based layered cathode material is expected to become the next major lithium-ion battery cathode material with its excellent discharge specific capacity(>250 mA h g-1)and high voltage platform,therefore its special structure also leads to a series of problems such as voltage decay and capacity fading limiting the development of Li-rich materials.In this thesis,the synthesis and modification of Li-rich cathode materials were studied.A method for improving the cycle stability of Li-rich materials was proposed and analyzed.Compared with lithium-ion batteries,sodium-ion batteries have the advantages of low cost and wide resources,but the capacity of batteries is not as good as that of lithium-ion batteries.In this paper,the synthesis process and phase transition reaction of sodium ion battery layered cathode material were studied.The main contents are as following:(1)This paper prepared the precursors of Li-rich manganese-based cathode though hydrothermal reaction,after that synthesized the Li-rich cathode by high temperature solid-phase reaction.By setting up a control experiment,the effects of calcination temperature and amount of surfactant on material synthesis were investigated.And we compared the difference of two calcination methods between the two-stage method and the two-step method.(2)On this basis,we designed a method of doping the spinel phase to synthesize a lithium-rich material with a layered-spinel heterostructure.On the effect of spinel phase,the material presents excellent electrochemical perfaormance,exhibiting a discharge capacity of 253 mA h g-1 at 0.2 C and a high energy density of 852 W h kg-1 at 0.5 C.Moreover,the discharge capacity was changed from 186 mA h g-1 to 170 mA h g-1 at 2 C,the capacity retention was 91.5%.It was found to be excellent after being assembled into a full-cell test with graphite.The excellent performance is due to the unique advantages of the spinel phase and the synergistic effect of the layer phase and the spinel phase.(2)The P2 phase layered cathode material of Na-ion battery was synthesized successfully.And the calcination temperature and sodium content were explored by experimental process.It is found that the calcination temperature has a certain influence on the crystallinity and morphology of the material.When the calcination temperature is 900?,the performance of the material is the most stable.Increasing the amount of sodium(sodium salt)could affect the morphology of the material without significant phase change,and more sodium salts will affect the electrochemical properties of the material.After studying the reaction phase change,the mechanism of charge and discharge of sodium ion battery has been deeply investigated.