Study On The Design Of High Specific Capacity Iron-based/Silicon-based Anode Materials And Their Lithium Storage Performance And Mechanism

Under the background of "double carbon",our country has been vigorously expediting the development of renewable energy in China,which promotes the rapid growth of electric vehicles,smart grids and energy storage power stations,resulting in higher requirements for energy density,working life,safety and cost of lithium-ion batteries(LIBs).Electrode materials are the key factor to determine the performance of LIBs.However,commercially used graphitic anode in LIBs can not meet the requirements of high specific energy density,so it is urgent to develop new highperformance anodes to replace graphite.Among all candidate anodes,the Fe-based anodes with conversion-type reaction mechanisms and the Si-based anodes with alloytype reaction mechanisms have much higher specific capacity and relatively safer working potential than those of graphite anodes.Moreover,their high natural abundance and low cost make them considered as promising anodes for the next-generation LIBs,attracting widespread attention of researchers.Unfortunately,the main challenge of Febased and Si-based anodes is their large volume variation during lithiation/delithiation processes,which leads to the structural destruction of electrode materials and the formation of unstable SEI film.Furthermore,they also exhibit inferior electronic conductivity.These disadvantages lead to poor cycle life and rate performance of the batteries,hindering their practical application.Therefore,it is imperative to develop efficient optimization and modification strategies to solve the above key problems of Febased and Si-based anodes,improving their electrochemical performances and realizing their practical application in the next-generation high-performance LIBs.In this dissertation,we develop several optimization strategies including nanostructure engineering,nanocomposite construction and binder modification to improve the overall electrochemical performance of iron phosphide and silicon anodes.A variety of spectroscopy and electron microscopy characterizations,combined with computational simulations,have been emloyed to reveal the effects of microstructure,composition,interface and other factors on the electrochemical properties.The electrochemical reaction mechanism of electrode materials during charge/discharge process have been systematically studied.These research results provide some new ways and insights for the design and synthesis of high-performance anode materials for nextgeneration LIBs.The main conclusions are summarized as follows:(1)Guided by density functional theory calculations,amorphous FeP nanoparticles were encapsulated in ultra-thin 3D cross-linked phosphorus-doped porous carbon nanosheets(denoted as FeP@CNs)by a facile approach.The 3D cross-linked phosphorus-doped porous carbon skeleton can not only provide good electron/ion transport channel and abundant active sites,but also can prevent the aggregation of FeP nanoparticles.Moreover,it can also effectively buffer the volume expansion of active materials and maintain the stability of electrode structure.Therefore,the obtained FeP@CNs composite electrodes show outstanding rate capability and long-term cycling stability,delivering a high capacity of 403 mA h g-1 at 16 A g-1 and 563 mA h g-1 at 4 A g1 over 2500 cycles with capacity retention as high as 98%.(2)The three-dimensional network composites of carbon-coated and graphenecoated Si nanoparticles(Si@Carbon@Graphene)were prepared by sol-gel method and electrostatic adsorption method assisted by cationic surfactant.Combined with the heat treatment of electrodes,the inactive Cu3Si buffer phase was directly formed inside the Si nanoparticles at high temperature,generating the Cu3Si-Si@Carbon@Graphene(Cu3SiSCG)composite materials.The external carbon-based coating and the internal Cu3Si phase can buffer the volume expansion and release the stress during the reaction process of Si,thus guaranteeing the structural integrity and the formation of a stable SEI film on the surface of electrodes.Benefitting from the synergistic effect of carbon-based coating and Cu3Si phase,the composite electrodes exhibit excellent electrochemical performance.(3)A facile thermal condensation reaction was applied to construct a robust carboxymethyl cellulose/phytic acid(CMC/PA)cross-linked binder network used for Si anode by employing the phytic acid(PA)with abundant hydroxyl functional groups as a cross-linking agent,combined with the conventional CMC binder.Different from CMC binder,the cross-linked CMC/PA binder has higher modulus and more contact points,which can better buffer the volume expansion of Si and form stronger hydrogen bonding interaction with Si,helping to maintain the structural stability of the electrode and promote the formation of a stable SEI film during charge/discharge process.Thus,the asprepared Si-CMC/PA electrodes present excellent electrochemical performance.