Transition-metal Compounds As Lithium-ion Battery Anodes

Nowadays,fast depletion of fossil fuel leads to severe environmental pollution and energy crisis.To address these problems,it is urgent to find efficient,eco-friendly,low-cost and sustainable energy.Among the existing energy storage techniques,lithium-ion batteries（LIBs）have been considered undoubtedly as the main power source for devices,including portable electronics,electric vehicles,and so on.But for the LIBs,the graphite-based anodes and lithium-phosphate-based cathodes are reaching their limits of energy density and power performance.To satisfy requirements of next generation lithium-ion batteries,the development of high-capacity anode materials is needed for successful implementation of advanced lithium-ion batteries.Conversion-type transition-metal compounds such as transition-metal oxides and transition-metal sulfides,with high theoretical specific capacities,have been widely attracted.But their drawbacks such as low electrical conductivity and severe volume expansion have hindered seriously their development.This thesis tries to solve the issues and provides three methods to prepare transition-metal compounds.Based on the severe volume expansion of Co₃O₄ during process of lithium-ion inserted,we designed two-dimensional（2D）porous Co₃O₄ nanosheets via one step.The 2D structure can greatly shorten the distance of Li⁺diffusion and expose relatively high area surface to improve the contact interface between electrolyte and electrode,improving reaction efficiency.In addition,the hierarchical porous structure can efficiently alleviate volume strain during lithium-ion insertion and extraction,and is advantageous for the electrolyte penetration.In this work,a simple,eco-friendly and low-cost method was adopted to synthesize 2D porous Co₃O₄nanosheets.This Co₃O₄ nanosheets show excellent electrochemical performance as lithium-ion battery anode,and a high reversible capacity of about 850 mAh g^-1 was reached after 100 cycles with the coulombic efficiency of about 99%at a current density of 300 mA g^-1.To explore the 2D material in lithium-ion batteries anode,we used leaf-shaped zeolitic imidazolate framework-L（ZIF-L）as precursor to prepare metal sulfides/carbon matrix composite.In fact,single metal ZIF-L after sulfidation cannot obtain leaf-shaped metal sulfides/carbon matrix.When the distribution of Zn²⁺and Co²⁺was changed in bimetallic ZIF-L,the morphology will be influenced greatly after treatment.Finally,we found bimetallic ZIF-L with Co²⁺/Zn²⁺=2can maintain the leaf-shaped morphology after sulfidation treatment.L-Co₂Zn₁-S as lithium-ion battery anode displays great cycling performance of specific capacity of 527 mAh g^-1 at a current density of 0.5 A g^-1 after 200 cycles with coulombic efficiency of about 99%.In addition,L-Co₂Zn₁-S also shows excellent rate capability.This superior electrochemical performance can be ascribed to 2D carbon matrix and synergistic effect of bimetallic sulfides.This work not only casts a new light on the design of bimetallic sulfides but also provides a manufacturing method of2D carbon matrix.To solve the low electrical conductivity of sulfides,3D carbon network connecting Co₉S₈ nanoparticles was prepared.Nanocellulose was used as precursor to construct 3D network structure where ZIF-67 in-situ grew and then formed 3D Co₉S₈/C network after sulfidation treatment.This structure takes advantage of 3D carbon network as electron transport channels to connect Co₉S₈ nanoparticles and effectively improve the reaction efficiency.In addition,this structure can maintain the Co₉S₈ fixed in carbon network rather than separation so that can experience long cycles during electrochemical reaction.This 3D Co₉S₈/C network shows excellent cycling performance as lithium-ion battery anode,reaching 700 mAh g^-1 at current density of 500 mA g^-1 after 150 cycles.It also displays great rate capability(774,728,646,578,486 mAh g^-1at current densities of 100,200,500,1000,2000 mA g^-1,respectively).In this thesis,2D Co₃O₄ porous nanosheets,leaf-shaped bimetal sulfide/carbon matrix and 3D Co₉S₈/C network were prepared,respectively.These transition-metal oxides and sulfides were synthesized in simple,low-toxic and eco-friendly methods.These materials show excellent electrochemical performance as lithium-ion anode,and we also hope these methods and ideas can cast a new light on the preparation of transition-metal compounds.