Preparation Of Iron-Based Oxygen(Sulfur)Compound/Carbon(Nitrogen)Composite And Study On Lithium/Sodium Storage Performance

With rapid development of electric vehicles,grid energy storage and portable electronic products,it is urgent to develop new-generation lithium/sodium ion battery electrode materials with high specific capacity and long cyc le life.Fe₃O₄ and FeS₂have the specific lithium/sodium ion storage capacities of 924 m Ah g^-1 and 894 m Ah g^-1,respectively,which are much higher than those of the layered metal oxide cathodes and graphite anodes applied in commercial lithium-ion batteries,and therefore have a broad application prospect.However,the electr ical conductivities of Fe₃O₄ and FeS₂ are not high.The huge volumetric expansion during charge/discharge cycles results in a rapid decline in cyclic performance.Furthormore,FeS₂ is also suffered from the serious polysulfide shuttle effect,which further degrades its cyclic performance.To address these issues,in this thesis,the following investigations have been conducted:(1)Three-layer structured Fe₃O₄/carbon hollow spheres were prepared through self-assembly during spray-pyrolys process,which are composed of a nitrogen-doped carbon outer layer,a large-particle Fe₃O₄/carbon intermediate layer,and a small-sized Fe₃O₄/carbon inner layer.Meso/microporous network is formed in the Fe₃O₄/carbon hollow spheres,which provides abundant channels for rapid Li⁺gransport.In addition,the electrical conductivity reaches(9.90±0.14)×10^-2 S cm^-1.The unique three-layer structure and the central hole endow the Fe₃O₄/carbon hollow with excellent Li⁺storage performance.The reversible specific capacity remains 383 m Ah g^-1 after 1000charge/discharge cycles at a current density of 10 A g^-1 and the capacity fading rate is only 0.019%.(2)Aiming at the serious polysulfide shuttle effect existing in the Li⁺/Na⁺storage process of the FeS₂ cathode material,the three-layer structured Fe₃O₄/carbon hollow spheres are employed as precursors to prepare a three-layer structured FeS₂/carbon hollow spheres through solid-state vulcanization.A strategy of"in-situ SEI film engineering"has been proposed with purpose to effectively inhibit the shuttle effect and improve the cyclic stability of FeS₂ cathode materials,which is to form an in-situ SEI film on the sphere surface using a small current density at a low potential window.Transmission electron microscopy analysis evidences a layer of granular SEI film with a thickness of 20-30 nm on the surface of the FeS₂/carbon hollow spheres that experienced the"in-situ SEI film engineering"treatment.Ele ctrochemical analysis confirms that this SEI film is very effective in inhibiting the polysulfides shuttle,and consequently significantly improve the Li⁺/Na⁺storage performance of FeS₂.In lithium-ion batteries,2000 stable cycles have been achieved at a current density of 2A g^-1,and the capacity fading rate is 0.012%.In sodium-ion battery,a reversible specific capacity of 331 m Ah g^-1 after 200 cycles has been obtained at 0.5 A g^-1current density,and the capacity fading rate is 0.045%.(3)Graphite-like biomass carbon/FeS₂ composite material with multi-stage pore structure was prepared by liquid phase permeation,carbonization and solid phase vulcanization reaction using citron peel with ferric nitrate as starting materials.FeS₂particles with high crystallinity and sizes of ca.100 nm are embeded in the graphitic carbon matrix.As an anode material for lithium/sodium-ion batteries,the graphite-like biomass carbon/FeS₂ composite materials exhibit excellent Li⁺/Na⁺storage performance.In lithium-ion batteries,a reversible specific capacity of 522m Ah g^-1 is achieved at a current density of 20 A g^-1(?22.5 C).In a sodium-ion battery,a reversible specific capacity of 502 m Ah g^-1 is maintained after 100 charge/discharge cycles at a current density of 0.2 A g^-1.