The Study Of Structural Design And Electrochemical Performance Of High Performance Anode Materials For Lithium Ion Batteries

Lithium ion batteries(LIBs)have become the dominant power supply in portable electronic devices due to their long lifespan and high energy density.However,commercial graphite anode used currently has been reached its theoretical limit,and has not yet met the demand of the emerging applications,such as electric vehicles and stationary energy storage.Thus,many new anode materials with high specific capacity or high safety have been extensively investigated.This paper selects Si(alloy-type lithiation mechanism),Co3O4,Fe3O4(conversion reaction-type lithiation mechanism),and Li metal as the candidates of high specific capacity anode material,and Li3VO4(insertion-type lithiation mechanism)as the candidate of high safety anode material.Based on the structural design,we try to solve the problems in the development of high performance LIB anode materials.And assisted with the novelty of preparation method,we attempt to establish a universal preparation methodology.Our work mainly focuses on the following 4 parts in details:(1)The preparation of high capacity Si based anodes by the electrophoretic deposition(EPD)method;(2)The preparation of high capacity transition metal oxide anodes by the EPD method;(3)Carbon fiber 3D conductive host coating on glass fiber filter for Li metal anodes;(4)The preparation of high safety Li3VO4 anodes by the spray drying method.(1)The preparation of high capacity Si based anodes by the EPD method.A binder-free Si based electrode for LIBs is fabricated in an organic solvent through one-step EPD.The nano-sized Si and Acetylene black(AB)particles are bonded tightly together to form a homogeneous co-deposited film with 3-D porous structure through the EPD process.The 3-D porous structure provides buffer spaces to alleviate the mechanical stress due to volume changes of Si during cycling,and improves lithium-ion conductivity by shortening ion diffusion length and better ion conducting pathway.The electrode prepared with 5 seconds deposition duration shows the best cycling performance among electrodes fabricated by EPD method,and thus it was selected to be compared with the silicon electrode prepared by the conventional method.Our results demonstrate that the Si nanoparticle electrode prepared through EPD exhibits smaller cycling capacity decay rate,and better rate capability than the electrode prepared by the conventional method.A simple yet versatile binder-free EPD method has been developed to fabricate carbon-coated Si/rGO(Si@C/rGO)nanocomposite electrode as a high performance anode for lithium ion battery application.In this specific nanostructure,high specific capacity Si NPs are uniformly coated by the carbon layer and embedded in graphene sheets to form an integrated,robust and conductive framework.Our electrochemical studies show that this nano-structured Si@C/rGO electrode exhibits a very high reversible specific capability(1165 mAh g-1 at 0.1 A g-1 3 times of that of graphite),and the excellent cycling stability(capacity retention of 96.8%at 1A g-1,and 95.4%at 2 Ag-1 after 100 cycles).(2)The preparation of high capacity transition metal oxide anodes by the EPD method.A new electrophoretic deposition Co3O4/graphene(EPD C03O4/G)hybrid electrode is developed to improve the electrochemical performance.Through EPD,Co3O4 nanocubes can be homogeneously embedded between graphene sheets to form a sandwich-like structure.Owing to the excellent flexibility of graphene and a large number of voids formed in this sandwich-like structure,the structural integrity and unobstructed conductive network can be maintained during cycling.Moreover,the electrode kinetics has been demonstrated to be a fast surface-controlled lithium storage process.As a result,the Co3O4/G hybrid electrode exhibits high specific capacity and excellent electrochemical cycling performance.The Co3O4/G hybrid electrode was also further studied by in-situ electrochemical XRD to understand the relationship of its structure and performance:?the observed LixCo3O4 indicates an intermediate of possible small volume change in the first-discharging;? The theoretical capacity achievement of the Co3O4 in hybrid electrode was evidenced;? The correlation between the electrochemical performance and the structural evolution of the Co3O4/G hybrid electrode was discussed detailedly.We have developed a facile electrophoretic deposition route to fabricate the Fe3O4/CNTs(carbon nanotubes)/rGO(reduced graphene oxide)composite electrode,simultaneously achieving the material synthesis and electrode assembling.Even without binders,the adhesion and mechanical firmness of the electrode is strong enough to be used for LIB anode.In this specific structure,Fe3O4 nanoparticles(NPs)interconnected by CNTs are sandwiched by rGO layers to form a robust network with good conductivity.The resultant Fe3O4/CNTs/rGO composite electrode exhibits much improved electrochemical performance(high reversible capacity of 540 mAh g-1 at a very high current density of 10 A g-1,and remarkable capacity of 1080 mAh g-1 can be maintained after 450 cycles at 1 A g-1)compared with commercial Fe3O4 NPs electrode.(3)Carbon fiber 3D conductive host coating on glass fiber filter for Li metal anodes.We report a rational design of vapor grown carbon fiber(VGCF)3D conducting host coating on glass fiber filter for its use as a lithium metal anode to inhibit dendrite growth and enhance cycling stability.The high coulombic efficiency of 91.1%and stable cycling can be maintained after 100 cycles(965 h)at 0.5 mA cm-2 for 2.5 mAh cm-2 in the carbonate electrolyte.(4)The preparation of high safety Li3VO4 anodes by the spray drying method.Mesoporous Li3VO4/C hollow spheres have been prepared by a facile drying method and subsequent heat treatment process.The unique structure of the composite offers a synergistic effect to facilitate the transport of Li+ ions and electrons and afford an anode with superior rate capability and cyclic stability.Li3VO4/C/rGO(HC-LVO/C/G)ternary composite with honeycomb-like structure is successfully prepared through a simple spray drying method with polystyrene(PS)microspheres as soft template-In this characteristic structure,carbon-coated Li3VO4 nanoparticles are well wrapped in rGO sheets and uniformly distributed within the honeycomb-like micrometer-sized clusters.Double coating layers of amorphous carbon and rGO can avoid the direct exposure of Li3VO4 nanoparticles to the electrolyte and enhance the electronic conductivity.Meanwhile,the honeycomb-like structure can shorten the diffusion paths of Li+ ions-and favors the relaxation of the-strin/stress during cycling.The resultant HC-LVO/C/G composite exhibits significantly improved high-rate performance and long cycle-life(the high reversible capacity of 312 mAh g-1 can be maintained after 1000 cycles at 10 C)compared with the contrastive Li3VO4/C composite synthesized by a typical solid-state reaction method.