Study On The Structural Tune And Electrochemical Performance Of Lithium-Rich Manganese-Based Cathode Materials For Lithium-ion Batteries

Lithium-rich manganese-based cathode materials(LRMOs)offer joint reversible cationic and anionic redox processes at a high voltage,thus promising the high specific capacity and average discharge voltage for the next-generation lithium-ion batteries(LIBs).In despite of furnishing high capacity,the unique reaction mechanism,especially the indisciplinable oxygen redox,triggers some issues.For instance,the anion redox in the LRMOs is coupled with irreversible loss of lattice oxygen and cation migration on continuous cycling,which can induce irreversible structural transformation from layered to deficient spinel.The phase transformation has proved to be a major factor of the continuous voltage and capacity fading.Moreover,the anion redox in the LRMOs exhibits sluggish charge-transfer and diffusion kinetics,which was disadvantageous for fast charge/discharge cycling.In brief,the severe voltage decay,successive capacity fading and inferior rate capability cause severe restrictions for the commercialization of LRMOs.In this dissertation,various modification methods have been presented to enhance-the electrochemical properties of LRMOs.Furhtermore,The element distribution,crystal structure,morphology,phase structure,element valence,local environment around lattice oxygen and anionic/cationic redox activity in these materials have been carefully investigated,and the relationship of these features with electrochemical properties has been evaluated.The main works are as follows:A facile La3+doping strategy for the preparation of LRMOs has been successfully put forward.The La3+can be introduced into Mn(4g)site via the solvothermal method and subsequent high-temperature calcination technique,generating a structural stabilization effect on the materials.The voltage fading can be mitigated and the Li+diffusion velocity can be enhanced by La3+substitution for partial Mn4+.It's worth noting that the Li1.2Mn0.52Ni0.13Co0.13La0.02O2 materials display optimal electrochemical performance among all La3+ doped samples.Mg2+doped LRMOs have been successfully synthesized via a co-precipitation process and subsequent high-temperature solid state method.The Coulomb repulsion between Mn4+and Mg2+is strong,since the introduction of inert Mg2+in Li+(4h)site can suppress the migration of TM ions into Li sites and enhance the structural stability of material.Moreover,the Mg2+ can play a role of pillar to stabilize the structural stability of the generated phase.Thus,the phase transformation from layered to spinel-like structure and the deterioration of the generated spinel-like phase can be effectively mitigated.Furthermore,the Mg2+ doping can tune the size of primary grain to reduce the diffusion path of Li+,thus building a stable and high-efficiency path for Li+intercalation/deintercalation.The "core-gradient shell" structural LRMOs have been successfully prepared via a co-precipitation process and subsequent high-temperature calcination technique.In the concentration-gradient shell,the Ni content decreases gradually towards the outer surface of the particle,the concentration of Mg shows an inverse distribution,and the Mn content remains nearly constant.The existence of the Mg concentration-gradient shell can protect the host material from electrolyte erosion and facilitate Li+diffusion,thus enhancing the electrochemical properties.The full concentration-gradient structural LRMOs with core-multilayer shells have been rationally designed and successfully prepared through layer-by-layer self-assembly deposit with a co-precipitation process.The microspheres are composed of an inner core and hierarchically multilayer concentric circle shells with porous structure.The void space between layers show the buffering action for volume change during charge/discharge process and serve as a reservoir for electrolyte,which allows much easier penetration of the electrolyte into the inside of the microspheres.Moreover,the Ni content decreases gradually from the inner core to the outer surface of the particle,the concentration of Co shows an inverse distribution,and the Mn content remains nearly constant.The low concentration of Ni in the outer layer of materials can reduce side reactions between highly reactive Ni4+and the electrolyte when charging to high voltage,which can keep the electrode/electrolyte interface stable during the Li+insertion/extraction process,and the high concentration of Co in the outer layer can enhance the conductivity of the materials.Accordingly,the full concentration-gradient structural LRMOs with core-multilayer shells can exhibit excellent electrochemical performances.The LRMO microspheres self-assembled with nano scaled grains and radial nanoplates have been successfully synthesized based on an evolutionary co-precipitate method.The unique hierarchical architecture that combines the advantages of hierarchical porous structure and electrochemically active planes,generates both stable matrix and efficient Li+diffusion channel.Noticeably,the hierarchically structural LRMOs with exposed active planes for LIBs can reciprocate preeminent electrochemical performance.The spinel/layered heterostructured LRMO nanowires self-assembled from nano scaled primary grains have been synthesized by a solvothermal method and subsequent high-temperature calcination technique.The unique architecture,which combines the advantages of 1D porous structures and build-in 3D spinel tunnels,generates efficient Li+diffusion channels and good structure stability.Particularly,the spinel/layered heterostructure can mitigate the phase transformation from layered structure to deficient spinel structure and structural degeneration,thus consolidating the stability of the host structure during cycling.The Sb3+doped LRMO nano fibers have been successfully synthesized by electrospinning method and subsequent high-temperature calcination technique.The 1D porous micro/nano morphology is in favor of Li+diffusion,and the Sb3+doping can expand the layered phase lattice and further improve the Li+diffusion coefficient.Moreover,The Sb3+can act as another charge compensation donator during Li+extraction,which can stabilize the lattice O2-,thereby suppressing the O2 gas evolution and the degradation of layered structure.Accordingly,the prepared materials can reciprocate preeminent electrochemical performance for LIBs.The Te6+doped LRMO microspheres with heterogeneous protective layer have been successfully synthesized based on a combination of co-precipitation process and wet coating method.Abundant nonbonding On-(n<2)species can be obtained in the bulk lattice when charged to high voltage due to the less directional Te-O bond,which can significantly improve anionic redox activity during cycling process.Meanwhile,the Te6+ dopant in the bulk can expand the interslab spacing and induce partial Ni3+ ions to reduce into Ni2+ions,thus maximizing TM ions redox capacity and promoting Li+ ions transport within the whole particles.The surface Mg3(PO4)2 component with stable P=O bond can withstand the erosion of acidic species.Moreover,the redox activity of surface lattice oxygen can be greatly subdued by introducing the Li/TM cations disordered structure in the subsurface,which prevents the layered-to-spinel transformation.As a result,the unique architecture can significantly boost the electrochemical performance.