Hierarchical Structure Design And Surface Modification Of Li_1.2mn_0.54ni_0.13co_0.13o₂ Cathode Materials

Owing to the advantages of high specific capacity and low cost,Li-rich Mn-based materials are considered to be the ideal cathodes for next-generation lithium-ion batteries.However,the practical application of Li-rich Mn-based cathodes is hindered by its poor rate performance,cycle instability,and severe voltage fade.To address the above problems,hierarchical structure design and surface modification are taken in this thesis to optimize the electrochemical performance of Li1.2Mn0.54Ni0.13Co0.13O2 cathodes.Combined with multiple physical characterization and electrochemical testing,this thesis aims to study the relationship among morphology,crystal orientation,Li⁺diffusion kinetics at the cathode-electrolyte interface,surface lattice oxygen stability,local structure,and the electrochemical performance.The main research contents of this thesis are listed as follows:Yolk-shell-like hierarchical structured Li1.2Mn0.54Ni0.13Co0.13O2 cathodes is synthesized by one-step solvothermal method and the subsequent high temperature sintering process,the formation mechanism of the yolk-shell-like structure is elaborated,and the influence of the yolk-shell-like structure on the Li⁺transport dynamics and structural stability is explored.The research shows that compared with the solid hierarchical structure,the larger specific surface area and pore volume of the yolk-shell-like hierarchical structure can provide more electrochemical active sites and shortened Li⁺diffusion paths,and the internal cavity is beneficial to alleviate the accumulation of internal stress during the rapid charge and discharge process,thereby improving the rate performance and cycle stability of the cathodes at the same time.The yolk-shell-like structured cathodes exhibit a high specific capacity of 156.5 and122.7 m Ah g^-1 at 5.0 and 10.0 C(1.0 C=200 m A g^-1),respectively,and a superior capacity retention rate of 85.2%after cycling at 10.0 C for 1000 cycles.Hierarchical structured Li1.2Mn0.54Ni0.13Co0.13O2 cathodes with various morphologies are synthesized by the solvent-controlled solvothermal method and the subsequent high temperature sintering process.The influence of solvent on morphologies of hierarchical structure is probed,the formation mechanism of the hierarchical structure is demonstrated,and the relationship among morphologies and orientation of the hierarchical structure,Li⁺intercalation/deintercalation kinetics,and mechanical properties is investigated.The research shows that the morphologies of hierarchical structure is closely related to the component of the solvent,when fixing the volume ratio of N,N-dimethylformamide and deionized water as 1:1,the oriented two-dimensional nanosheets self-assembled three-dimensional hierarchical structured Li-rich Mn-based cathodes own the best rate performance and cycle stability,including the high specific capacities of 141.7 and 117.6 m Ah g^-1 at 10.0 and 20.0 C,respectively,and the excellent capacity retention rate of 75.0%after cycling at the high current density of 20.0 C for 1200 cycles.Excellent electrochemical performance of nanosheet self-assembled hierarchical structure is attributed to the synergistic effect of two-dimensional nanosheets,micron-scale frameworks,and exposed active{010}crystal planes in promoting Li⁺diffusion and maintaining structural stability during rapid Li⁺transmission.The fast ion conductor lithium tungstate coating layer is used for the surface modification of Li1.2Mn0.54Ni0.13Co0.13O2 cathodes,the mechanism for the improvement of electrochemical performance in lithium tungstate coated cathodes is investigated.The research shows that the fast ion conductor lithium tungstate coating can enhance the Li⁺transport at the cathodes-electrolyte interface and increase the binding of the surface lattice oxygen through the strong W-O bond,reducing the phase transition from layered to spinel-like phase that triggered by oxygen loss,thereby significantly improving the rate performance,cycle and voltage stability of Li-rich Mn-based cathodes.The lithium tungstate coated Li-rich Mn-based cathodes demonstrate the high discharge specific capacities of 166.5 and 136.7 m Ah g^-1at 5.0 and 10.0 C,respectively,and exhibit superior capacity retention rate of 90.4%and 81.1%after cycling at 5.0 C for 200 and 300 cycles,respectively.In addition,the voltage fade is also obviously reduced upon cycling at 1.0 C.The spinel structured lithium cobalt oxide coating layer is used for the surface modification of Li1.2Mn0.54Ni0.13Co0.13O2 cathodes,combining with synchrotron X-ray diffraction patterns,the pair distribution function analysis,synchrotron X-ray absorption near edge structure spectra,and density functional theory calculations,the local structure of the modified Li-rich Mn-based cathodes is studied,the modulation of surface ligand configuration by cubic spinel structured lithium cobalt oxide which is lattice compatible with the layered structure is clarified,and the relationship among the surface ligand configuration,relative band position,and anionic redox reversibility is explored.The research shows that the cubic spinel structured lithium cobalt oxide changes the local bonding environment of the Li2Mn O3 phase,triggers the two-band redox process,thus affecting the reversibility of anionic redox.The electrochemical results show that the voltage fade of the modified cathode after cycling at 0.5 C for 140 cycles is only half of the unmodified material.In addition,the high capacity retention rate of 85.9%after cycling at 1.0 C for300 cycles is obtained in the modified cathodes.The excellent capacity and voltage retention rate of the modified material are attributed to the reasonable modulation of the local bonding environment that couples the relative band position,improving the reversibility of anion redox.