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

In recent years,with the development of society and people demand higher,high specific capacity and high energy density of rich lithium manganese base cathode material to become the next generation of lithium ion battery cathode material attention hot spot,but rich lithium manganese base cathode material is still has some defects,such as spontaneous Mn³⁺redox reaction,the electrolyte to the cathode material,phase transformation of structure in charging and discharging process.In this paper,cobalt-free rich lithium manganese base Li_1.2Mn_0.6Ni_0.2O₂ nanofibers were prepared by electrostatic spinning method,and the materials were modified by doping,coating and other methods in order to inhibit the phase transition and side reactions of the materials and improve the electrochemical properties of the materials.The results are as follows:（1）The preparation and electrochemical properties of cobalt-free Li_1.2Mn_0.6Ni_0.2O₂nanofibers by electrostatic spinning were studied,and the effects of calcination parameters such as calcination temperature and holding time on the microstructure and electrochemical properties of the materials were discussed.It was found that the electrochemical properties of the materials were the best when the calcination temperature was 850℃and the holding time was 8 h.The capacity retention rate of the materials was 78.3%after 100 cycles at 0.5 C.At the same time,the sample under the optimal parameters is hollow nanofiber,the diameter of the nanofiber is about 180nm,and its single particle diameter is about 50 nm.The solid structure of the porous hollow nanofiber can shorten the transmission path of Li⁺,which is conducive to the improvement of the electrochemical performance of the material.（2）The preparation and electrochemical properties of Cobalt-free Li_1.2Mn_0.6Ni_0.2O₂ nanofibers doped with Al and B elements were studied.It was found that Al doping of substitutive Mn did not change the layered crystal structure and fiber morphology of cobalt-free rich Li Mn-based cathode material,but Al doping could significantly improve the cyclic stability of the material,and the capacity retention rate of LMNO-A2 sample could reach 95.6%after 100 cycles at 0.5 C.The capacity retention rate of undoped Li_1.2Mn_0.6Ni_0.2O₂ nanofibers is only 78.3%.The layered crystal structure and fiber morphology of the cobalt-free rich lithium manganese base cathode material were not significantly changed by a small amount of B doping.However,the cycling stability and rate performance of the nanofibers were improved by B doping.After 100 cycles of charging and discharging at 0.5 C,the discharge specific capacity of LMNO-B1 sample was 125.3 m Ah/g.The specific capacity can still be maintained at 71.5 m Ah/g under 2 C.（3）The preparation and electrochemical properties of Al₂O₃ coated Li_1.2Mn_0.6Ni_0.2O₂ nanofibers were studied.Firstly,Li_1.2Mn_0.6Ni_0.2O₂ nanofibers were prepared by electrostatic spinning method,and then Al₂O₃-coated Li_1.2Mn_0.6Ni_0.2O₂nanofibers（LMNO@A）were prepared by impregnating aluminum nitrate solution nine water and two-step calcination.It is found that Al₂O₃ coating can significantly improve the electrochemical performance of Li_1.2Mn_0.6Ni_0.2O₂ nanofibers.The specific capacity of LMNO@A material is 225.1 m Ah/g at 0.1c and 181.1 m Ah/g at 0.5 C.After 100cycles of continuous charging and discharging,the discharge specific capacity still retains 152.9 m Ah/g,and the retention rate is 84.9%.