Preparation And Modification Of Lithium-rich Layered Cathode Material For High Performance Lithium-ion Batteries

Lithium-rich layered cathode material xLi₂MnO₃·（1-x）LiMO₂（M=Ni,Co,Mn,etc.）,as one of the most promising cathode materials for future generations of high performance lithium-ion batteries（LIBs）due to their extremely high reversible capacities（>250 mAh/g）and high operating voltages（>4.6 V vs.Li/Li+）,has attracted enormous interests during recent years.Despite their excellent capacities and high energy densities,several fundamental challenges for lithium-rich layered cathode material still need to be settled before large-scale commercial application,such as large initial irreversible capacity loss,substandard rate capacity and insufficient capacity retention.To overcome above technical obstacles,some strategies such as surface modification,micro/nano structure design,morphology and size optimization were adopted to enhance the electrochemistry performances of lithium-rich layered cathode material in this dissertation.And the primary research tasks are as follow:（1）The spherical Li_1.5Ni_0.25Mn_0.75O₂+δ cathode material was firstly prepared via co-precipitation method.Subsequently,the LiTi₂（PO₄）₃（LTP）coating layer was uniformly coated on the surface of lithium-rich cathode material baded on hydrothermal strategy.The influences of coating layer with various amounts on the structure,morphology,and electrochemical performance were studied in detail.The result indicates that the LTP was homogeneously coated in the form of nanoparticles on the surface of Li_1.5Ni_0.25Mn_0.75O₂+δ material and does not affect the layered structure.Moreover,LTP coating layer can not only improve the initial irreversible capacity,but also enhance the cyclic stability and rate capacity of lithium-rich layered cathode material.The sample modified with 3 wt% LTP shows the best performance among all samples,it displays an initial reversible capacity of 232.8 mAh/g at 0.5 C with a high capacity retention of 95.9% after 100 cycles and exhibits a discharge capacity of 103.2 mAh/g even at a high rate of 10 C.Besides,the LTP coating layer is amorphous with electrochemical activity,which can efficiently restrict the formation of the SEI layer and reduce the charge transfer resistance during long-term cycling process as well as facilitate the transportation of Li+ and electron.（2）A new-type 0.5Li₂MnO₃·0.5LiNi_1/3Co_1/3Mn_1/3O₂ layered cathode material with porous fusiform micro/nano structure has been successfully synthesized through self-assembly on the basis of a facile and mild co-precipitation approach.The microsized matrixes assemblied with interconnected nanosized primary particles can maintain the structure and guarantee excellent cycle performance of lithium-rich layered cathode material.Meanwhile,the porous structures can not only enlarge the surface area of the as-prepared material,but also increase the reaction sites between the electrode material and the electrolyte as well as shorten the diffusion distance of Li+,thus markedly improve the rate capacity of lithium-rich layered cathode material.The galvanostatic charge/discharge testing demonstrates that the as-synthesized material delivers a high initial discharge capacity of 294.8 mAh/g at 0.1 C in a potential range of 2.0-4.6 V and retains a discharge capacity of 220.6 mAh/g after 200 cycles with high capacity retention of 87.1%.Benefitting from the special structural features,it also exhibits excellent rate capacity with remarkable specific capacity of 139.5 mAh/g at a high rate of 10 C.（3）In order to profoundly enhance the electrochemical properties of lithium-rich layered cathode material,a series of micro/nano structured lithium-rich layered cathode materials Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ with controllable morphology and size was designed and successfully prepared.By rationally regulating and controlling the volume ratio of ethylene glycol in hydroalcoholic solution,the morphology and size of the as-prepared materials can be tailored from rod-like to olive-like,and further evolved into shuttle-like with the assistance of surfactant.The results showed that the electrochemical performances of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ cathode materials were highly dependent on their morphology and size.Compared to rod-like and shuttle-like cathode materials,olive-like cathode material with regular shape and suitable size exhibits much better electrochemical performances in terms of initial reversible capacity（297.0 mAh/g）and cycle performance（95.4% capacity retention after 100 cycles at 0.5 C）as well as rate capacity（142.8 mAh/g at 10 C）.The excellent electrochemical properties of the as-prepared materials could be related to the synergistic effect of well-regulated morphology and appropriate size as well as their micro/nano structure.