Preparation And Application Of Lithium-rich Manganese-based Cathode Materials In High Energy Density Batteries

Lithium-ion(Li-ion)batteries are one of the most important energy storage system for new energy vehicles because of its environmental protection,long cycle life and high energy density.The greater energy density means higher cruising mileage,which is also of positive significance of reducing the power consumption of the battery electric vehicles.At present,the energy density of mainstream single Li-ion battery with energy density of 280 Wh kg^-1 can be relized,and further development of Li-ion battery or new type battery system with energy density over 300 Wh kg^-1 and high safety performance has important scientific significance and application value.Herein,high performance Li-rich Mn-based cathode material(Li_1.2Mn_0.54Ni_0.13Co_0.13O₂)and sulfur(S)composite cathode material(S/g-C3N4-CNTs)are prepared,respectively,which are based on a detailed summary and discussion of the research and development of Li-ion and Li-S batteries in recent years.These two high capacity cathode materials that matched the corresponding chemical system and cell model are used to design and develop new batteries with energy density of 300 Wh kg^-1 and 400 Wh kg^-1,respectively.(1)The precursor of Li-rich Mn-based cathode material is prepared by co-precipitation method.By analyzing the thermodynamic equilibrium relationship of the co-precipitation system and examining the influence of pH,charging speed,reactant concentration,reaction time and other factors on the precursor structure during the precipitation process,the synthesis method is optimized to prepare a spherical precursor with large tap density and uniform particle size.The effects of calcination temperature,precursor morphology,and Li salt adding ways on the structure and electrochemical performance during the calcination process are also investigated.An innovative coprecipitation-gel method has been developed to prepare high-performance Li-rich manganese cathode materials with an octahedral morphology.Compared to that synthesized by the traditional coprecipitation method,the Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ synthesized by the coprecipitation-gel method exhibits higher discharge capacity and Coulombic efficiency,from 73.9%and 251.5 mAh g^-1 for the spherical polycrystal material to 86.2%and 291.4 mAh g^-1.(2)On the basis of detailed XRD,Rietveld refinement,SEM,TEM,EELS,electrochemical kinetic process,etc.It is found that the as-prepared material consists of an octahedral morphology and a new type of core-shell structure with a spinel-layered heterostructure inside,which is the result of overgrowth of the spinel structure with{111} s facets on {001}L facets of the layered structure in a single orientation.The surface area of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ crystals where the spinel phase is located possesses sufficient Li and O vacancies,resulting in the reinsertion of Li into position after the first charge,thereby improving the discharge capacity and charge-discharge efficiency of the material.An anti-core-shell phase transformation was observed for the first time in Li-rich cathode materials.The O loss at pristine surface regions is dynamically replenished from the bulk region,which,accordingly,results in a lower O concentration in the adjoining bulk structure while relatively maintaining the O content in the surface regions.This phase transition can reduce the mechanical failure of materials caused by interface stress,which is associated with the volume change of the crystal upon Li-ion extraction and reinsertion,and maintains the stability of the electrode/electrolyte interface.(3)Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ that prepared by coprecipitation-gel method is used as the cathode material to develop a high energy density 18650 cylindrical cell in which matches a kind of high-capacity SiOx/C as anode material.The density energy of the cylindrical cell reaches 285 Wh kg^-1,which is 96%of the designed target.Similarly,the high-nickel NCM811 material that maches the same SiOx/C anode material is used to develop an 18650 cylindrical cell with an energy density of 250 Wh kg^-1(93%of target value).The electrochemical performances in different voltage ranges and security features of the two types of cells are investigated and compared.The results show that the Li-rich Mn-based 18650 battery has advantages in cycle life,thermal stability,safety performance,and energy density when the upper limit voltage over 4.4 V,while NCM811 battery has better rate performance and higher energy density when the upper limit voltage is less than 4.4 V.According to the results of battery disassembly and SEM characterization,the capacity degradation of both batteries is caused by superposition of multiple reasons that include loss of Li+insertion sites,increase in internal resistance,decomposition of electrolyte at high voltage and mismatch of electrochemical system.In particular,the change of crystal volume,crystal structure,internal stress,etc.that result from by the deep de-and intercalation of Li+at high potential will cause a series of irreversible damage such as cracking,breaking,and shedding of the electrode piece.