Layered Structure Modulation And Performance Study Of Li Rich And Mn Based Cathode Materials

The demand for lithium ion battery is increasing day by day.It has become a hot research topic to find positive materials for lithium ion battery with high capacity,good rate performance and stable circulation.Because of its advantages of high specific capacity,high safety,green and pollution-free,Li-rich and Mn based materials have become the most promising research objects.However,in the process of charging and discharging,the problems such as lattice oxygen precipitation and poor self-rate performance seriously hinder the commercial application of li-mn-rich materials.In this paper,Li_1.2Mn_0.6Ni_0.2O₂（LMNO）with high safety and theoretical specific capacity is taken as the research object,and the properties of LMNO material are studied from the molecular/atomic microscopic point of view through first principles.The structure model of Li-rich Mn-based materials was established and optimized by CASTEP module in MS（Material Studio）software,and the energy band,state density,crystal cell parameters,charge density and other information were calculated after doping,coating and co-modification of different elements.Under the guidance of the calculated results,the precursors with uniform morphology and high crystallization degree were prepared by co-precipitation reaction,and the LMNO materials with high reversible capacity and stable cycling performance in the voltage range of 2-4.8 V were obtained by optimizing sintering process,collective phase doping and surface regulation.The main research contents of this paper are as follows:（1）Li-rich and Mn based precursor was prepared by co-precipitation reaction,and the preparation process of LMNO precursor was optimized by adjusting p H value and reaction time.On this basis,LMNO material was prepared by optimizing the ratio of lithium source to precursor and sintering temperature.The prepared LMNO material exhibits a high reversible specific capacity（～270 m Ah/g at 0.1c current density）.（2）The crystal models of LMNO and LMNO@Ce were built,and the information of energy band,state density and charge density were calculated based on DFT.The results show that the density of states near the Fermi level of LMNO@Ce material is significantly stronger,the number of local electrons near the conduction band is increased,and the charge density near Mn and O atoms becomes more denser,indicating that the conductivity of the doped modified material is significantly improved.Under the guidance of the calculation results,Ce doped LMNO material was prepared by solid state method.The charge transfer impedance of LMNO sample was～300Ω,and that of LMNO@Ce sample was～100Ω,which was the same as the simulation results.Through the electrical performance analysis,it was found that the stability of LMNO@Ce cycle was greatly improved（the capacity retention rate of 200 cycles of 1C cycle was 75%）.（3）On the basis of the previous doping research,the modification strategy of collective phase doping and subsurface spinel reconstruction is proposed.A three-layer LMNO@Ce-Li Mn₂O₄-Li Ce O₂（LMNO@Ce-LCO）heterostructure was constructed on the（003）crystal plane of LMNO.The changes of energy band,state density,conductivity and differential charge density were analyzed by geometric optimization and single point energy calculation.Guided by the simulation results,in this chapter,Ce（NO₃）₃·6H₂O was uniformly coated on the surface of LMNO@Ce by wet coating,and LMNO@Ce-LCO material was prepared.The charge transfer impedance of the sample was further reduced（～100Ω）,which was consistent with the simulation results.TEM analysis showed that the prepared materials had a Li Ce O₂（LCO）layer of 2～3 nm and a spinel structure of 4～5 nm on the sub-surface.Further electrical properties analysis shows that the capacity retention rate of the co-modified material is about 86%at the current density of 1C,which is much higher than that of the original material.