Tailoring The Composition And Microstructure Of Transition Metal Oxides For Efficient Lithium Storage

To address the ever-increasing energy demand and effectively utilize renewable energy,electrochemical energy storage systems have become a top priority for green and sustainable development.Lithium-ion batteries（LIBs）have been widely used in portable electronics,electric vehicles,and stationary power stations.LIBs’s performance depends largely on the inherent characteristics of electrode materials.The current commercialized electrode materials have almost approached their limits in terms of energy density,service life,and safety.Therefore,it is urgent to develop electrode materials with lower cost,higher energy density,and better stability to achieve large-scale applications.In this thesis,transition metal oxides（TMOs）were chosen as the object for LIBs,which has the advantages of low price and high theoretical energy density.According to the reaction mechanisms,TMOs can be divided into conversion-type and intercalation/deintercalation-type materials.The former has serious volume expansion and poor structural stability during cycling,while the latter has relatively low theoretical specific capacity.In this thesis,the electrochemical performance of the TMOs-based electrodes was optimized by adjusting the composition and designing the micro/nano structure.The main research results and conclusions are as follows:In Chapter 3,the work tries to integrate metal oxides with three-dimensional（3D）porous carbon to effectively improve the capacity and stability of LIBs.This 3D porous carbon networks（3DPCN）reinforced composite electrode was developed by integrating biomass（i.e.,agaric）-derived 3DPCN and Prussian blue analogue（PBAs）-generated carbon-coated metal oxides（Co Fe O_x@C）.To have an integrated structure,PBA precursors were directly deposited and grown on 3DPCN（PBA/3DPCN）,and then PBA/3DPCN was subjected to low-temperature oxidation in air and thermal treatment in Ar to yield Co Fe O_x@C/C.The interconnected Co Fe O_x@C/C electrode,as an advanced LIBs’anode,delivers excellent cycle capacity and stability with typical specific capacity of 1043.4?m A?h?g^-1 at 0.2 A?g^-1/483.2?m A?h?g^-1 at as high as 5.0 A?g^-1after 120 cycles.Such superior electrochemical performance originates from the interconnected structure of 3DPCN,the finely regulated composition of nano-sized Co Fe O_x@C,and the tight contact and synergistic effect between 3DPCN and Co Fe O_x@C.3DPCN not only provides a void space for buffering the volume change of Co Fe O_x@C,but it also acts as a framework for improving the conductivity of the entire electrodes,whilst Co Fe O_x@C contributes the majority of specific capacity.As a typical oxide among TMOs,vanadium pentoxide（V₂O₅）has shown great promise as an anode for LIBs,owing to its high theoretical energy density,relatively low cost,and toxicity.However,the practical application of V₂O₅ is greatly restricted by its poor rate capability and long-term cycle instability.Therefore,the second work of the thesis（Chapter 4）takes V₂O₅ as the research object,and tries to improve the electrochemical performance of the V₂O₅ electrode and investigate the electrode reaction kinetics.First,hollow porous V₂O₅ microspheres（HPVOM）were fabricated by a facile poly（vinylpyrrolidone）and ethylene glycol-assisted soft-template solvothermal method.The spherical precursor microspheres are converted into hollow microspheres after the annealing process.The assembled HPVOM electrode exhibits high capacity and good rate capability,with discharge capacities of 615.7 m Ah g^–1 at200 m A g^–1 after 300 cycles and 407.9 m Ah g^–1 at 1000 m A g^–1 after 700 cycles in the voltage range of 3-0.01 V vs.Li/Li⁺.Furthermore,we systematically recorded the electrochemical impedance spectra（EIS）at different potentials to reveal the electrode kinetics.With the help of an equivalent circuit model（ECM）,the respective changes of resistances during charge and discharge were analyzed.We noticed that the charge-transfer and bulk resistances varied regularly with the charge/discharge state.The preparation method of hollow microspheres,as well as dynamic observations and clarifications of kinetics via EIS with ECM,may be broadly applied to the development of efficient electrodes for LIBs.