Synthesis And Electrochemical Properties Of Li-rich Manganese-based Nanofibers Cathode Materials

In recent years,the lithium rich manganese based cathode materials have become a research hotspot of next generation lithium ion batteries because of their high specific capacity,high energy density,environmental friendliness and low cost.However,the transformation of the materials structure during the charging and discharging progress cause many problems,such as the irreversible capacity loss in the first cycle,the voltage drops during cycling and the poor rate performance.In this paper,we use lithium-rich manganese-based layered oxide Li_1.2Mn_0.54Ni_0.13Co_0.13O₂as the research object,and use the electrospinning method to prepare a one-dimensional hollow nanofiber lithium-rich manganese-based material.What's more,we have studied the surface coating modification of the material and deeply analyzed the effects of micromorphology,crystal structure and surface modification on the electrochemical properties of the materials.The following results have been achieved:?1?One-dimensional lithium-rich manganese-based nanofiber cathode materials were prepared by electrospinning process.The effects of different calcination temperatures and different holding times on the microstructure and crystal structure of the materials were investigated.The experimental results show that the material obtained under the conditions of calcination temperature of 800°C and holding time of 7 h is hollow nano-tubular.The particle size distribution of the material is uniform and the crystallization quality of the material is good.For the electrochemical performance test,the 1D lithium-rich manganese-based nanotube materials have a specific discharge capacity of 272.69 mAh g^-1 in the first cycle,and the first coulombic efficiency is 75.3%.After 100 cycles at a 1 C,the specific discharge capacity of the material is 124 mAh g^-1,and the capacity retention rate is 85%.The excellent electrochemical performance is attributed to the large contact area between the nanotube material and the electrolyte.Moreover,one-dimensional structures have shorter lithium ion transport and diffusion paths.?2?The Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ nanotubes were coated with an alumina film.After impregnating the aluminum nitrate solution and the two-step calcination,a spinel layer appears between the Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode and the alumina coating.The effects of different coatings on the morphology of the materials were discussed.The coating of alumina and the appearance of spinel bands did not change the overall structure and morphology of the materials.The surface-modified sample has a significant improvement in electrochemical performance.Firstly,the presence of the spinel phase increases the capacity of the materials in the first cycle.The discharge specific capacity of the sample in the first cycle is 282.04 mAh g^-1 and the initial coulomb efficiency is 85.2%.Secondly,the surface-modified material exhibits excellent cyclic performance,which is attributed to the Al₂O₃ coating prevents the direct contact between electrode and electrolyte,and reduce the side reaction at the material interface.The discharge capacity of the S-LMNCO-4 sample is 205 mAh g^-1after 90 cycles,and the capacity retention rate is 97.6%.What's more,since the spinel phase can provide a three-dimensional lithium ion diffusion channel and accelerate the lithium ion transmission rate,the rate performance of the S-LMNCO-4 sample has improved,and the specific capacity at 5 C high current density is 107.3 mAh g^-1.?3?The polymer conductive polymer polypyrrole was uniformly coated on the surface of the Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ nanotube by gas phase oxidation polymerization,and the coating amount of the polypyrrole was controlled by changing the polymerization time.The effects of different polypyrrole coatings on the structures,morphologies and electrochemical properties of the materials were investigated.The coated composite material has an excellent cyclic stability.After100 cycles,the discharge specific capacity of the LMNCO sample without PPy coating decreased from 201.5 mAh g^-1 to 157.5 mAh g^-1,and the capacity retention rate was 78.2%.The specific discharge capacity of the P-LMNCO-1 sample with a coating time of 2 min was 209.3 mAh g^-1 after 120 cycles,and the capacity retention rate was 99%.The discharge specific capacity of the P-LMNCO-1 sample at 5 C was159.8 mAh g^-1.