Research On Modification Of High Voltage Li-rich Manganese-based Cathode For Li-ion Batteries

Li-rich manganese-based materials exhibit high-capacity and high-voltage properties as cathodes for lithium-ion batteries.Although the redox of anionic oxygen at high voltage can increase the upper limit of the specific capacity,it often causes problems such as voltage decay and cycling instability,which limit the practical application of Li-rich manganese-based materials.In this thesis,starting from the correlation between the material structure and electrolyte composition of lithium-rich manganese-based cathode,aiming to achieve long-cycle stability and alleviate voltage attenuation at high voltage.The electrochemical properties and related modification mechanisms were discussed by various technical methods such as XRD,SEM,TEM,XPS,GITT,DEMS and EIS.The main research contents and results are as follows:1.Lithium-rich manganese-based cathode materials with oxygen vacancies,surface spinel phases,and element doping were obtained by surface manipulation.The integration of oxygen vacancies,surface spinel phases and element doping was achieved in one step by a mild surface pre-activation treatment.Lithium-rich manganese-based cathode material(abbreviated as VO-LR)with oxygen vacancy and surface spinel phase,and the lithium-rich manganese-based cathode material(abbreviated as VS-LR)with oxygen vacancy,surface spinel phase and doping were obtained,respectively.The synergy of surface oxygen vacancies suppresses the irreversible release of lattice oxygen,so the initial coulombic efficiency of VO-LR0.25 material(the mass ratio of Li-rich manganese-based material to NH4HCO3 is 10:2.5)is increased from 72.1%to 81.2%,and the initial coulombic efficiency of VS-LR0.25(the mass ratio of Li-rich manganese-based material to(NH4)2SO4 is 10:2.5)material is increased to 82.7%.In addition,the capacity retention was greatly improved,After 200 cycles at 1 C,the capacity retention of VO-LR0.25 material increased from 63.5%to 89.5%,and that of VS-LR0.25 material increased to 95.1%.2.The degradation of bulk phase structure related to voltage decay is improved by bulk doping.In-situ integration of spinel phase was realized by lithiation and calcination of sulfide precursor,and layered-spinel heterostructure was simultaneously formed without changing the redox state of other transition metals.The spinel phase possesses a close-packed hexagonal oxygen array similar to Li-rich manganese-based materials,which can serve as a robust framework,thereby reducing the irreversible release of lattice oxygen and mitigating structural damage.The three-dimensional lithium ion diffusion channels of in-situ formed spinel phase also greatly facilitate the transport of lithium ions.Therefore,the material modified by sulfur doping exhibits excellent cycling stability,the capacity retention can still maintain 100%after 250 cycles at 1 C,and the capacity retention is as high as 89.7%after a longer cycle to 500 cycles.The modified material not only greatly inhibits the phase transition from layered to low-voltage spinel phase or even disordered rock-salt phase,but also has a more stable high-potential reactive pair activity,which makes the voltage retention of the material increase from 83.7%to 92.9%after 250 cycles at 1 C,effectively improves the voltage attenuation of materials.And compared with the initial Li-rich manganesebased cathode,the modified cathode also exhibits better thermal stability.3.A cathode-electrolyte interfacial layer(CEI)with active oxygen scavenging function on the electrode surface was constructed.For the first time,β-carotene with antioxidant properties was used as scavenging molecule to scavenge active oxygen species generated during the first charging of high-voltage Li-rich manganese-based cathode materials.The control of active oxygen species effectively alleviates the decomposition of carbonate electrolytes under high voltage,and the introduction of βcarotene additives can be adjusted in situ to generate a functional cathode-electrolyte interface(CEI),which is double-layer structure with external organic components and internal inorganic components.Moreover,the β-carotene-containing electrolyte exhibits better thermal stability.Benefited from these,the modified Li-rich manganesebased cathode has good structural stability and exhibits excellent long-life cycling stability,with a capacity retention of 93.4%after 200 cycles at 1 C,which was greatly improved compared to the 62.5%capacity retention of the reference electrolyte.