Preparation And Modification Of Lithium Rich Manganese Based Cathode Materials For Lithium Ion Battery

Li-rich layered oxides,as a kind of cutting-edge cathode material,are supposed to be candidate for the next-generation high energy density electrode materials due to their capacity more than 250 mAh g^-1 and high operating voltage up to 4.8 V.However,this class of materials are still far from commercial applications on account of the drawbacks:low Coulombic efficiency?CE?in the initial cyclic process due to the release of oxygen from its lattice and subsequent structural rearrangement;dreadf ul rate performance attributed to the limited electronic conductivity of tetravalent manganese ion and thick solid electrolyte interface?SEI?layer formed on the material surface;inadequate cycling stability at high cut-off voltage related to the side reaction between active material and electrolyte;gradual plateau decay during long cycling due to the irreversible phase transformation from layer to spinel.In this thesis,Li-rich materials are prepared by co-precipitation method.Various strategies are adopted to modify surface and bulk structures of Li-rich materials.Physical characterizations and electrochemical measurements are used to study the relationship between material structure and electrochemical properties.The precursors of Mn_4/6Ni_1/6Co_1/6CO₃ are prepared by co-precipitation method,and the influences of preparation process on morphology and structure of precursors are investigated.The procedure and technological parameters of solid-state sintering are studied.The amount of lithium in Li-rich material is optimized and the effects of lithium content on electrochemical properties are studied.A water treatment and a microwave treatment are adopted to modify the surface structure of Li-rich material.Structure and composition analyses show that partial Li₂MnO₃ are pre-activated after treatment.Pre-activation conditions and their effects on material structure and electrochemical properties are further studied.It is found that harsh chemical conditions are not necessary to realize pre-activation and stable solvents show less undesirable impact on the structural stability.By studying the activation process of water treated material,it is found that pre-activation can be regarded as a specific form of gradient activation,which can play a great role on the improvement of electrochemical performance.Pre-activation also contributes to optimize the structure evolution of Li-rich material in cyclic process.After pre-activation,the properties of Li-rich material,such as initial Coulombic efficiency,rate capability,and circulatory stability are significantly improved.An approach is reported to fabricate ultrathin nano-coating layer on the surface of Li-rich material,by using solid-state reaction process after the precursor was coated with magnesium pyrophosphate.Pyrophosphate as a complexing agent,can control the reaction rate effectively.Therefore,the magnesium pyrophosphate can be slowly generated on the cathode material surface and forms a uniform protective layer.Structure and chemical state analyses indicate that magnesium and phosphorus are incorporated into the crystal structure which induces the larger lattice spacing and lower cation mixing.After coating,the resultant Li-rich material displays extremely high Coulombic efficiency of 91.8%and discharge capacity of 288.4 mAh g^-1,showing prominent cycling stability of 89.2%after 200 cycles.Furthermore,this strategy also suppresses the attenuation of voltage and potential drop is only 0.56 mV per cycle from 25th to 200th cycle.The excellent electrochemical performance can be ascribed to the combined merits of surface protection and bulk doping.A simple co-precipitation method with dual sedimentating agent is adopted to realize phosphor doping in both surface and bulk.The detailed process of phosphor doping is presented by theoretical analysis and experimental verification.The result s show that concentration of pyrophosphate in precursor goes through a process of starting from scratch,gradually increasing and finally stabilizing.XRD Rietveld refinement method,synchrotron radiation X-ray imaging,and X-ray absorption fine-structure are used to investigate the effects of phosphor doping on structure and phase distribution of Li-rich material.The results indicate that phosphor doping not only increases lattice spacing and lowers cation mixing,but also has significant effect on the uniformity of phase distribution and chemical states.The doped sample displays high discharge capacity of 295 mAh g^-1 with initial Coulombic efficiency of 90.5%at 0.1 C,showing high rate performance of 247 mAh g^-1 at 1 C and superior capacity retention of 73%after 500 cycles.Moreover,this doping strategy also inhibits the critical voltage decay of Li-rich materials during cycling.The prolonged structural evolution analysis demonstrates that phosphor doping can play a stabilizing role in Li-rich materials to restrain the transformation from layer to spinel.In order to overcome the shortcomings of complex modification methods and single performance improvement,several optimization schemes are proposed to prepare high performance Li-rich manganese-based cathode materials.Firstly,methods and solvents for pre-activation are expanded,and some new understanding on the improvement of electrochemical performance of Li-rich materials are provided.Secondly,it provides a new idea for coating modification,and solves the problem that coating treatment is difficult to stabilize inner crystal structure of Li-rich material.Thirdly,the application conditions of phosphorus doping have been extended,and the effects of phosphorus doping,especially on structure and phase distribution,are recognized.