Synthesis,Electrochemical Performance And Mechanisms Of Silicon And Ferric Oxide-based Composite Anode Materials

Si and Fe₂O₃ have been considerd to be one of the most prosimising anode materials for high-performance LIBs and attracted significant attention because of their high theoretical lithium storage capacity and good safety.However,the practical application of Si and Fe₂O₃ anodes is hindered due to their low conductivity,large volume expansion,quickly fading capacity,and poor rate capability.To address these issues,based on the review of the research development of Si-based and iron oxide-based anode materials,a new strategy was proposed to improve electrochemical properties of Si-based and iron oxide-based anode materials by introducing metal hydride into the preparation process in this work.Micron-sized Si-based and iron oxide-based composites were successfully synesized by a facile heating and ball milling process.The structural characteristics and electrochemical performances of the prepared composite materials were studied systematically,and mechanisms related to the improved performances were elucidated.Inspired by the nanocrystalline 'dispersion strengthening' mechanism of amorphous materials,we successfully fabricated a novel micrometre-sized silicon composite with polycrystalline Si particles embedded in amorphous SiOC matrix containing 3-10 nm SiC and Li2SiO3 nanocrystals via a simple and scalable method of the room-temperature mechanical milling of prelithiated Si microparticles in CO₂ atmosphere.The dispersed SiC and Li2SiO3 nanocrystals in the amorphous SiOC remarkably increased the toughness and hardness of the protective matrix with a dispersion strengthening effect.This strengthened buffer matrix effectively supressed the cracking,fracture,and pulverization of Si microparticle caused by the volume change during lithiation/delithiation,and consequently alleviated the degradation of anodes upon cycling.As a result,a high discharge/charge specific capacity?1824/1268 mA·h g-1 at 100 mA g-1?,a long-term cyclability?957 mA·h g-1 after 400 cycles?,a good rate performance?895 mA·h g-1 at 1000 mAg-1?and a high volumetric capacity?1268 mA h cm-3?are achieved in the microparticle silicon composite anodes.To improve the available capacities of microparticle silicon composite anodes,the degree of surface prelithiation for Si microparticles was reduced by reacing with less LiH.And then,a multicomponent amorphous layer composed of SiOX,C,SiC and Li2SiO3 was successfully coated on the surface of Si microparticles by ball milling the partially prelithiated microparticle Si in a CO₂ atmosphere.The coating level stronglydepended on the milling duration.The presence of SiC remarkably enhanced the mechanical properties of the SiOx/C coating matrix as the elastic modulus and the hardness values were increased,which effectively enhanced the pulverization resistance of the microparticle silicon composite in the lithiation/delithiation process.Moreover,the existence of Li2SiO3 insured high Li-ion conductivity of the coating layer.Meanwhile,the in situ formed multicomponent amorphous coating layer also stabilized the SEI film of the electrode surface upon cycling.These processes worked together to allow the amorphous multicomponent layer coated Si microparticle to offer high reversible capacity?1924 mA-h g-1?,high Coulombic efficiency?78%?,and better cycling stability?its reversible capacity remains at 1439 mA h g-1 after 100 cycles?.On the basis of above studies,the prelithiated Si was also mechanically milled under O₂ atmosphere to prepare Si microparticles coated by amorphous SiOx buffer species-containing Li2SiO3 nanocrystals.Then,these Si microparticles were encapsulated by the electrostatic self-assembly of graphene oxide sheets and the subsequent high-temperature reduction.This gave rise to the formation of a graphene sheets enveloped microparticle Si composite.The graphene layer effectively improved the electronic conductivity of Si composite.Despite the pulverization process of the inner Si particles,the pulverized Si particles were well encapsulated with the graphene layers;this kept Si particles high electrochemical activity within the carbon network and built stable SEI films on the surface to avoid continuous lithium consumption.As a result,the graphene sheets-enveloped microparticle Si composite exhibited significantly improved electrochemical performance with high reversible capacities?1450 mA-h g-1 at current density of 100 mA g-1?,good cycling stability?89%capacity retention after 100 cycles?,and high rate capability?937 mA-h g-1 at 5 A g-1?.Moreover,an amorphous Li2CO3-coated nanocrystalline a-Fe₂O₃ hierarchical structure was synthesized for the first time using a facile one-step mechanochemical process at room temperature,taking advantage of the concurrence of repeated fracture-cold welding of materials' particles and a gas-solid redox reaction.The amorphous Li2CO3 coating layer restrained the fracture and exfoliation of the active particles as a strain buffer,prevented Fe nanocrystalline agglomeration,stabilized the formed SEI layer and also promoted the rapid transmission of Li+ during cycling.Therefore,the amorphous Li2CO3-coated Fe₂O₃ nanocrystallines delivered superior lithium storage performance,with high reversible capacity?985 mA-h g-1?,remarkable cycling stability?99%capacity retention after 400 cycles?,and better rate performance?537 mA-h g-1 at current density of 3 A g-1?.In addition,the coating technique developed here demonstrated general applicability for other transition metal oxides,such as NiO and CoO,to improve their lithium storage performances.To increase the initial Coulombic efficiency and the overall electrochemical properties of iron oxide,a coralloid porous LiFeO₂/Fe composite was fabricated by chemical prelithiating Fe₂O₃ using LiH upon heating.An high initial Coulombic efficiency of?87.2%was achieved because of high reversibility of the prelithiated porous LiFeO₂/Fe composite.In addition,The high porosity facilitated the effective contact with electrolyte and the rapid Li-ion transport in the 3D coralloid architecture.Meanwhile,numerous void spaces involved in porous structure were beneficial to maintain the integrity of the electrode and buffer the volumetric effect upon cycling.Moreover,the existence of Fe reduced the global volume expansion,offered a good structural stability of the porous LiFeO₂/Fe composite as an inert phase,and also improved the electrical conductivity.As a result,a high specific capacity?976 mAh g-1 at 100 mA g-1?,a good cyclability?992 mA·h g-1 after 200 cycles?,and a good rate performance?621 mA·h g-1 at 2000 mA g-1?were achieved for the porous LiFeO₂/Fe composite.