Study On Synthesis And Modification Of Lithium-rich Manganese-based Cathode Material

Lithium-rich manganese-based cathode materials（LMCM）has high reversible specific capacity（>200m Ah/g）.They are promising to become a new generation of high-performance lithium-ion battery cathode materials.The materials have received extensive attentions from researchers.However,they have some defects,such as high initial irreversible capacity,poor rate performance,and serious capacity decay after many cycles,which hinder their application in practice.In this paper,the Lithium-rich manganese-based cathode materials,Li_1.2Mn_0.54Ni_0.13Co_0.13O₂,were the research object.The materials were synthesized by the co-precipitation method,and were modified by Gd-doping and F-doping respectively.In this paper,lithium-rich manganese-based cathode materials,Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ were prepared by oxalate co-precipitation.In order to improve the electrochemical performance of the materials,the preparation conditions were optimized.The materials had the best electrochemical performance when the calcination temperature was 900℃,the calcination time was 15h,and the amount of over-lithium was 5%.The first discharge specific capacity was 223.1m Ah/g,the coulombic efficiency is 68.3%,the discharge specific capacity after 30 cycles is 202.1m Ah/g,and the capacity retention rate is 90.4%.The discharge specific capacities at 1C and 2C rates were 131.7m Ah/g and 102.3m Ah/g,respectively.In this paper,the Gd-doped lithium-rich manganese-based materials Li_1.2Mn_0.54-xGd_xNi_0.13Co_0.13O₂（x=0,0.01,0.02,0.05,0.08）was synthesized by the co-precipitation method.The crystal structure,surface morphology,and electrochemical performance of the materials were studied,and the influences of Gd-doping on the material performance was analyzed.The XRD analysis results show that materials doped by a small amount of Gd still maintained the original layered crystal structure,and reducing ion mixing and widens the diffusion channel of Li⁺.The SEM characterization found that the microscopic morphology of the Gd-doped materials presented spherical secondary particles with smooth surfaces,and the particle size was between 400 and 500 nm.Although Gd-doping reduced the first discharge capacity of the material,it increases the coulombic efficiency of the material by 2.9%.The Gd doping stabilized the crystal structure of the materials,and the higher binding energy of the Gd-O bond restricted the oxygen loss,the ion mixing the dissolution of Mn⁴⁺by the electrolyte,and the reversible specific capacity decay during cycle.The discharge specific capacity of the materials LMCM-Gd2（x=0.02）after 30 cycles was 211.4m Ah/g,and the capacity retention rate was 96.3%.The capacity retention rate of the materials LMCM-Gd2 increased by 5.9%.Gd doping reduced the impedance of the materials effectively.The reversible discharge specific capacity of the materials at high current was improved.Compared with pure phase materials,the discharge specific capacities of LMCM-Gd2 at 1C and2C increased by 38m Ah/g and 42.2m Ah/g respectively.In this paper,the F-doped lithium-rich manganese-based materials Li_1.2Mn_0.54-xF_xNi_0.13Co_0.13O₂（x=0,0.01,0.02,0.05,0.08）were synthesized by co-precipitation method,and the effect of F doping was analyzed.XRD analysis showed that materials doped by a small amount of F would not destroy the original crystal structure,and the doping reduces the mixing of ions in the material,increases the interlayer spacing of the crystal,and widens the diffusion channel of Li⁺.SEM characterization found that the microscopic morphology of the Gd-doped materials presented spherical secondary particles with smooth surfaces,and the particle size was between400 and 500 nm.The first discharge specific capacity of LMCM-F2（x=0.02）and LMCM-F5（x=0.05）are 224.4m Ah/g,208.7m Ah/g,and the coulombic efficiencies are 70.2%and 73.7%,respectively.The lower valence F replaced part of the oxygen in the crystal lattice and reduced the activation of the Li₂Mn O₃ component.The initial discharge capacity of the material had decreased,but the coulombic efficiencies of LMCM-F2 and LMCM-F5materials had increased by 1.9%and 5.4%,respectively.The crystal lattice was doped with F ions to form Li-F bonds and F-Mn bonds which had larger binding energy,and prevents Mn⁴⁺from being dissolved by the electrolyte during the cycle,and the crystal structure was more stable.F-doping optimized the rate performance of the F-doped materials.Compared with pure phase materials,the discharge specific capacity of the LMCM-F5materials at 1C and 2C increased by 16.5m Ah/g and 28.1m Ah/g,respectively.