Structural Design Of Carbon Matrix Composite Anode Materials And Their Energy Storage Mechanism

New energy storage systems represented by lithium/sodium/potassium ion batteries have been widely used in electric and vehicles aerospace.However,with the increasing demand for energy,the current commercial energy storage battery is gradually difficult to meet the market demand.Exploiting the lithium/sodium/potassium ion batteries materials with high capacity and long cycle life is,of great significance to energy storage fields.Iron-based and tin-based anode materials are appealing anodes materials because of their high theoretical capacity.These materials,however,suffer from severe problems of poor intrinsic conductivity and huge volumetric variations upon lithiation/delithiation,sodiation/desodiation and potassiation/depotassiation,resulting in low specific capacities and poor cycling stabilities.The design and optimization of iron-based/carbon composites and tin-based/carbon composites can effectively promote the electronic/ions transport and alleviate the volume expansion of electrode,thus improving the capacity and cycle stability.This thesis aims to exploiture the lithium/sodium/potassium ion batteries materials with high performance by construction of metal-organic framework（MOF）derived carbon/transition metal nanocomposite structures,transition metal phosphide hierarchical nanostructures,and metal/metal oxide heteronanostructures.Specifically,by these approaches,the problems of rapid capacity decay caused by volume changes during the charging and discharging process can be well resolved.The electrical conductivity and stability of the iron-based and tin-based compound anode materials are obviously improved,and their energy storage mechanisms are studied furtherly at the same time.The main research contents and innovations of the thesis are listed below:（1）Preparation of MOF derived carbon-coated metallic iron nanoparticles composites（MOF-Fe@C）and the mechanisms of lithium ion storage.The spindle-shaped MOF-Fe@C was prepared by the calcination of an Fe based metal-organic framework（Fe₃-MOF）,involving carbonization of Fe₃-MOF under N₂ flow and reduction of Fe₂O₃ under H₂/N₂ flow.Depending on the unique structural features of MOF materials,the obtained MOF-Fe@C consists of interconnected carbon networks integrated with carbon-coated small-sized Fe nanoparticles,significantly improving the electrical conductivity and stability.When used in the LIBs,the MOF-Fe@C exhibits high stable reversible capacity,excellent rate capability and cycling stability.Density functional theory calculations indicate that although Fe is unalloyable with Li,its empty 3d orbitals can accept electrons from Li,making the Fe nanoparticles capable for lithium ion storage.The high performance of MOF-Fe@C arises from the high activity of the surface Fe atoms towards the adsorption of Li.Density functional theory calculation results show that the lithiation of the transition metal compounds produces the transition metal nanograins,which can further store the Li ions through the adsorption of the Li ions at their surfaces,thus endow the transition metal compounds exhibit extra capacities beyond their theoretical limits.These results provide a significative reference to develop the transition metal-based anode materials with excellent electrochemical performance.（2）Kirkendall effect-induced formation of MOF derived carbon-coated hollow iron phosphide nanoparticles composites（MOF-H-FeP@C）and the mechanisms of lithium/sodium/potassium ions storage.In order to improve the lithium storage capacity of transition metal based anode materials and extend them to the application of sodium ion batteries and potassium ion batteries,the spindle-like MOF-H-FeP@C composite with hierarchical nanostructures comprising MOF derived carbon-coated hollow FeP nanoparticles and the interconnected carbon network was synthesized through high temperature phosphorization of MOF-Fe@C.The formation of the hollow FeP nanoparticles is a result of the Kirkendall effect,in which the fast solid-state outward diffusion of the Fe atoms occurs after the phosphorization of the surface Fe atoms on the Fe nanoparticles,facilitating the formation of the FeP nanoparticles with a hollow structure.When evaluated as an anode for alkali metal ions batteries,the MOF-H-FeP@C can exhibit excellent electrochemical performance for lithium/sodium/potassium ions storage.Its superior electrochemical performance mainly arises from the unique hierarchical nanostructures of the MOF-H-FeP@C.The porous structure of MOF-H-FeP@C and the hollow structure of the FeP nanoparticles allow for an easy inward and outward diffusion of lithium/sodium/potassium ions,facilitating the fast reaction kinetics of charge and discharge.The interconnected carbon network and the carbon shell of the FeP nanoparticles can improve the electric conductivity of the MOF-H-FeP@C and allow for the ready transfer of charge between carbon and FeP during the charge/discharge processes.Additionally,the hierarchical nanostructures of the MOF-H-FeP@C can relieve the volume expansion and structural damage of the anode material during the intercalation process of Li/Na/K,endowing the MOF-H-FeP@C with structural stability during charging/discharging.The hierarchical nanostructure provides a new idea for the design and preparation of the high-performance anode materials for lithium/sodium/potassium ion batteries.（3）Preparation of carbon-coated and carbon nanotubes supported Antimony doped Tin oxide（ATO）hetero-nanoparticles composites（C@Sb-ATO/MWCNTs）and the mechanisms of lithium/sodium/potassium ions storage.To further improve the electrochemical performance of the materials that are adaptable simultaneously as the anode for lithium/sodium/potassium ions batteries and lower the cost,the Sb-ATO heteronanostructures/carbon composite was synthesized,using cheap and conductive carbon nanotubes as the carrier,based on SnO₂ with high theoretical specific capacity.The C@Sb-ATO/MWCNTs was synthesized by in-situ thermal metal exclusion.That is,the formation of the Sb NPs was attributed to the in-situ partial metal exclusion of Sb from ATO during the thermal annealing of the carbon-coated and carbon nanotubes supported ATO composites（PPY@ATO/MWCNTs）.Depending on the specific structure features of the C@Sb-ATO/MWCNTs,which integrate the advantages of nanostructuring,elemental doping,heterostructuring,and carbon integration,the C@Sb-ATO/MWCNTs shows improved performance for Li⁺/Na⁺/K⁺storage.The heteronanostructures of Sb-ATO hetero-nanoparticles can facilitate their better accessibility to the electrolyte,shorten the diffusion path length of Li⁺/Na⁺/K⁺ions in the Sb-ATO hetero-nanoparticles,and reduce the total internal strain of the Sb-ATO hetero-nanoparticles upon lithiation/delithiation,sodiation/desodiation and potassiation/depotassiation.MWCNTs and carbon coating can improve the electric conductivity and cycling stabilities of the C@Sb-ATO/MWCNTs.Density functional theory calculations indicate that the Sb doping can increase the electric conductivity of the ATO and promote the diffusion and storage of Li⁺/Na⁺/K⁺ions in the ATO.Additionally,the electrical conductivity of the Sb-ATO hetero-nanoparticles can be increased by the Sb-ATO heterostructuring,thus improve the lithiation/delithiation,sodiation/desodiation and potassiation/depotassiation kinetics of the C@Sb-ATO/MWCNTs.The results represent here indicate that the strategy in combination of nanostructuring,elemental doping,heterostructuring,and carbon integration is a new design approach to improve the battery performance of tin-based compound anode materials.In conclusion,the electrochemical performance of LIBs,SIBs,and PIBs can be well improved by construction of MOF derived carbon/transition metal nanocomposite structures,transition metal phosphide hierarchical nanostructures,and metal/metal oxide heteronanostructures.These approaches can effectively resolve the problems of rapid capacity decay associated with LIBs,SIBs,and PIBs through alleviating the huge volume change during charging and discharging,enhancing the conductivity and structural stability of the electrode materials,and accelerating electrode reaction kinetics,thereby improving the battery performance of the iron-based and tin-based compound anode materials.The results of this thesis are therefore of great significance since it provides new approaches to improve the battery performance of iron-based and tin-based compound anode materials and is of great significance for promoting commercial applications of lithium/sodium/potassium ion batteries.