| Recently,with the rapid growth of energy storage requirements in the fields of mobile devices,electric vehicles and smart grids,lithium-ion batteries have been unable to meet their requirements for energy density,rate capability,working life and cost of use.Electrode materials play a key role in the performance of lithium-ion batteries.There is an urgent need to research and develop new high-performance anode and cathode materials to enable lithium-ion batteries to play an important role in the future revolution of new energy and transportation.The theoretical specific capacity of the metal oxide anode is two to three times that of the commercial graphite anode.Its low cost and wide source make it very promising for the next generation of lithium-ion batteries.However,its intrinsic low electronic conductivity,large volume changes during charging and discharging,and reactivity with the electrolyte result in poor rate performance,low cycle life,making it difficult to commercialize.In addition,the conventional layered LiMO2(M=Ni,Co,Mn),spinel structure LiMn2O4 and olivine structure LiFePO4 cathode materials have a specific capacity limited to 120-200 mA h g-1,which is unable to meet the increased demand for energy density and power density of lithium-ion batteries.Therefore,Li-rich manganese-based cathode materials(Li1+xM1-xO2(x≥0.1,M=Ni,Co,and Mn))have drawn much attention for their high specific capacity of 250-300 mA h g-1 and high energy density over 1000 W h kg-1.However,the low initial Coulombic efficiency,significant capacity fading and poor rate capability of Li-rich manganese-based cathode materials greatly restrict their commercial applications.Therefore,it is urgent to develop efficient optimization methods to improve the cycle stability and rate performance of transition metal oxide anode and Li-rich manganese-based cathode materials,giving full play to their high specific capacity,so that they can be applied to the next generation high-performance lithium-ion batteriesIn this dissertation,we develop several optimization methods including micro-nanostructure design,nanocomposite and atomic defect construction to improve the overall electrochemical performance of transition metal oxide anode and Li-rich manganese-based cathode materials.A variety of characterization methods and computational simulations have been used to study the effects of microstructure,elemental composition,atomic defects and other factors on the electrochemical performance.The lithium insertion and removal mechanism of electrode materials during charge and discharge have been systematically studied to fully understand their electrochemical reactions.These research results provide a new way to design and synthesize high-performance anode and cathode materials for the next generation of high-performance lithium ion batteries.The main conclusions are summarized as follows:(1)Amorphous ZnSnO3 double-shell and yolk-shell hollow microcubes were synthesized by calcination of their corresponding ZnSn(OH)6 precursors pre-prepared through a facile chemical solution method in argon.When used as the anode materials,amorphous ZnSnO3 double-shell hollow microcubes(D-ZnSnO3)reveal better electrochemical properties than ZnSnO3 yolk-shell counterparts(Y-ZnSnO3).The amorphous feature and unique box-in-box hollow architecture of D-ZnSnO3 play a key role in their excellent electrochemical properties.(2)A cationic surfactant induced self-assembly method is developed to construct 3D Fe3O4@reduction graphene oxide(H-Fe3O4@RGO)hybrid architecture in which hierarchical Fe3O4 nano-flowers(H-Fe3O4)are intimately encapsulated by 3D graphene network.When tested as the anode material in lithium-ion batteries,a high reversible capacity of 2270 mA h g-1 after 460 cycles is achieved under a current density of 0.5 A g-1.More impressively,even tested at a large current density of 10 A g-1,a decent reversible capacity of 490 mA h g-1 can be retained,which is still higher than the theoretical capacity of traditional graphite anode,demonstrating the remarkable lithium storage properties.The reasons for the excellent electrochemical performance of H-Fe3O4@RGO electrode have been discussed in detail.(3)By calcination of the facilely prepared cuboid cobalt oxalate in air,Co3O4 nanorods-orientated assembled porous microrods with tunable internal pore structures have been successfully synthesized in this work.The elaborately designed C03O4 microrods possess bicontinuous mesopores and one dimensional macropores.Moreover,the density functional theory calculations indicate that the oxygen vacancies existed in porous Co3O4 microrods would advance to form local in-plane electric fields around oxygen vacancy sites.When applied into lithium ion batteries as anode materials,such hierarchical bicontinuous porous feature and local in-plane electric fields endow Co3O4 microrods with enhanced ionic and electronie conductivity as well as outstanding structural stability.The morphology and electrochemical impedance spectroscopy investigations of electrode materials after cycling are carried out in detail.The result highlights the importance of the skillful regulation of atomic defects,morphology,microstructures and pores structure of anode materials on the improvement of their intrinsic ionic and electronic conductivity and thus lithium storage properties.(4)Unique double-shell LRLO(Li-rich layered oxides)hollow microspheres with sandwich-like carbon@spinel@layered@spinel@carbon shells(LRLO-500@S@C)were successfully synthesized via a facile template-free method,followed by carbothermal reduction treatment.The fabricated LRLO-500@S@C cathode delivers a high initial charge capacity of 312.5 mA hg-1 with a large initial Coulombic efficiency of 89.7%.After cycling 200 times,large and stable discharge capacities of 228.3 mA h g-1 and 196.1 mA h g-1 can be obtained at 1.0C and 5.0C,respectively.The impressive electrochemical performances of LRLO-500@S@C cathode material can be attributed to its multiscale coordinated design based on hierarchical double-shell hollow construction,the special layered@spinel@carbon heterostructured shells and the introduced oxygen vacancies,which benefit to shorten Li-ion diffusion paths,strengthen structural stability and reduce side reactions. |
