Preparation And Modification Of Manganese-based Oxide Positive Electrode Material

Lithium-ion battery has become an indispensable power source in the field of energy storage due to its many advantages,such as high working voltage,large specific energy,long cycle life and small self-discharge,which have attracted much attention.Currently,cathode materials are one of the limitations for further development of lithium-ion batteries.Known cathode materials for commercial batteries are mainly Li Fe PO₄,LiCoO₂,LiMn₂O₄ and ternary nickel-cobalt-manganese materials.These materials have their own shortcomings,so it is necessary to develop new cathode materials with better comprehensive performance.Among them,manganese-based materials have the advantages of abundant resources,low price,friendly environment and strong safety.Lithium-rich materials have the characteristics of high specific capacity and high energy density.In this thesis,the preparation and modification of common lithium-rich manganese-based materials with O3 structure and manganese-based oxide cathode materials with O2 structure are studied.The modified cathode material with high capacity,good cycling and rate performance was prepared by means of coating and doping.The main research contents and achievements are as follows:In the first part of the work,nanoparticles Li-rich manganese-based cathode material coated with zirconium-based compounds（ZrO₂,Li₂ZrO₃）were successfully synthesized.First by sol gel and heat treatment method to get the layered structure with nanoscale LRMO(Li_1.2Mn_0.54Co_0.13Ni_0.13O₂),on the basis of using lithium hydroxide zirconium and n-butyl alcohol wet chemical method and heat treatment to achieve the zirconium-based compounds coating.The results show that the modified materials exhibit better electrochemical properties,especially the specific capacity and multiplier properties are significantly improved.This part of work mainly explores the effects of different heat treatment temperatures on the crystal structure of the coating layer and the electrochemical properties of the whole material.When the heat treatment temperature is 400℃,the product obtained by calcination of lithium hydroxide and n-butanol zirconium is the mixture of cubic structure ZrO₂ and monoclinic structure Li₂CO₃.The specific discharge capacity of the modified material obtained under this heat treatment condition is 272 m Ah g^-1 at 0.1 C.When the heat treatment temperature is600℃,the zirconium-based compound coating layer is composed of tetramonal phase ZrO₂and Li₂ZrO₃,and the specific discharge capacity reaches 296 m Ah g^-1 and 120 m Ah g^-1 at 1 C and 5 C,respectively.The zirconium-based compounds consist of monoclinic structure ZrO₂and Li₆Zr₂O₇ at 800℃,and the specific discharge capacity of the material is 283 m Ah g^-1 at0.1 C.In the second part,solvothermal method and ion exchange method were used to prepare manganese-based lithium-ion cathode material(Li_0.67Ni_0.33Mn_0.67O₂,O2-LNMO)with micron-scale secondary spherical O2 structure.Titanium-doped manganese-based manganese oxide cathode material(Li_0.67Ni_0.33Mn_0.67-xTi_xO₂,O2-LNMT)was prepared by solvothermal method after Ni-Mn carbonate precursor was mixed with sodium carbonate and titanium dioxide by grinding and then calcined at high temperature.X-ray energy dispersive analysis（EDS）showed that titanium was uniformly distributed in the main body of manganese-based cathode material.The electrochemical analysis results show that when the molar ratio of titanium is 0.02,the specific discharge capacity of the modified material at 0.1 C is 184.2m Ah g^-1.When the molar ratio of titanium is 0.05,the specific discharge capacity of the modified material is 178.3 m Ah g^-1 at 0.1 C.When the molar ratio of titanium is 0.08,the cycle and rate properties are the best.At 0.1 C,the specific discharge capacity of the material is 203.7 m Ah g^-1.After 100 cycles at 1 C,the capacity retention rate of the material is 64.6%.In the third part,the co-precipitation method and ion exchange method were used to obtain the micron-sized nubbly O2 structure manganese-based oxide cathode material(LiNi_0.33Mn_0.67O₂,O2-LNM).In the process of forming hydroxide precursor,aluminum nitrate is added to realize aluminum doping of manganese-based oxide cathode material.The hydroxide precursor is mixed with sodium carbonate and lithium carbonate.Increase lithium content,improve battery specific capacity.Through the crystal structure analysis,the material is layered with O2 structure,and the aluminum element is successfully doped into the material in a uniform distribution state.Electrochemical tests show that Al-doped samples have better cyclic stability,because Al³⁺partially replaces Mn³⁺,which can slow down the Jahn-Teller effect of Mn³⁺and inhibit the disorderly migration of transition metal ions.When the molar ratio of aluminum is 0.02,the electrochemical performance of the modified material is the best.The specific initial discharge capacity of the material is up to 196.1 m Ah g^-1 at0.1C.After 100 cycles at 1 C current,the capacity retention of the modified material reached87.0%,which was higher than that of the original material（70.5%）.The specific discharge capacity of the modified material at 5 C is still 75 m Ah g^-1.