Synthesis And Electrochemical Performance Of High Capacity Electrode Materials For Lithium Ion Batteries

Lithium ion batteries（LIBs）,as the efficient energy storage devices,are widely used in portable electronics and even high-power systems such as electric vehicles/hybrid vehicles and green energy storage,which demands the improvement of energy density and validity of LIBs.The electrode materials are the most critical parts for the electrochemical performance of LIBs.Therefore,it is very important to develop high capacity,long cycle-life as well as low cost electrode materials.Among many candidates,lithium iron silicate（Li₂FeSiO₄）is regarded as an ideal cathode material for the next generation LIBs because of its high theoretical capacity(332 mAh g^-1),high thermal stability through strong Si-O bonding,low cost and environmental benignity.Unfortunately,the inherent low electronic conductivity and sluggish lithium ion diffusivity limit the electrochemical activity of Li₂FeSiO₄ electrode.Another important problem with Li₂FeSiO₄ is the difficulty in controlling synthetic parameters and the appearance of Fe3+-based impurities in the final product.As for anode materials,zinc sulfide（ZnS）has received great attention due to its advantageous material properties,such as high theoretical specific capacity(963 mAh g^-1),abundant natural resources,low cost,as well as environmental friendliness.While it also suffers from low electronic conductivity and huge volume variation during lithiation and delithiation processes,which can cause the low capacity and the serious capacity decay.All the problems limit the practical application of Li₂FeSiO₄ and ZnS electrodes for LIBs.In order to solve the problem of impurity in Li₂FeSiO₄,the influence of the calcination temperature and oxygen partial pressure on the oxidation state of iron ions in a compound was analyzed from the viewpoint of thermodynamics.Besides,different kinds of carbon coated electrode materials were synthesized by different methods.The formation mechanisms of composites with various morphologies were studied.The influence of particle size,carbon coating and carbon content on electrochemical performance was investigated.Li₂FeSiO₄/C composite was synthesized by a co-precipitation method followed by calcination using Fe3+ salt as iron source and polyethylene glycol（PEG 200）as surfactant and carbon source.Based on the thermodynamic calculation,the temperature and oxygen partial pressure where FeO is stable were obtained,and the suitable synthesis parameters for pure phase Li₂FeSiO₄ were proposed.The addition of PEG200 was beneficial to preventing the aggregation of the precipitates during co-precipitation process as well as forming carbon layer on the surface of the particle,thus can improve the electrical conductivity of the material.The Li₂FeSiO₄/C electrode delivers a reversible capacity of 190 mAh g^-1 at 0.1 C,corresponding to the extraction of 1.37 Li+ from lattice.It also demonstrates a good long-term cycling stability,with 90%capacity retention being achieved at 0.5 C over 400 cycles.This work proposes a low cost,simple and fast fabrication method for Li2FeSi04/C cathode material,which is suitable for large-scale production.Core-shell structured nano-ZnS-C composite（100¹50 nm）was designed and obtained by a two-step synthesis process,including a chitosan-assistant hydrothermal synthesis of nano-ZnS grains,followed by a carbon coating process,which is done by chemical vapor deposition（CVD）.Chitosan plays a key role in the synthesis of highly dispersed ZnS nanoparticles.The thermal deposition of acetylene（C2H2）can form a carbon coating layer on the individual nanoparticle during the CVD process.Such core-shell structure with small and uniform particle size allows shortening the pathways of Li+ diffusion,as well as facilitating the reversible Li+ diffusion kinetics.The thin,outer carbon layer can improve the electrical conductivity of the ZnS nanocomposite,thereby ensuring excellent cycling performance.After 600 cycles,the nano-ZnS-C material delivers the reversible capacity of 530 mAhg^-1 and 506 mAhg^-1 at current density of 0.1 A g^-1 and 0.5 A g^-1,respectively.Even at high current density of 5 A g^-1 the reversible capacity of 363 mAhg^-1 is retained.ZnS/C composite was prepared successfully by a simple solvothermal method followed by an annealing process.With the help of polyvinylpyrrolidone（PVP）and glucose,ZnS/C spherical particle has a uniform size distribution and exhibits hierarchical-type structure.The obtained ZnS/C nanocomposite consists of primary ZnS nanocrystals（～10 nm）that are encapsulated by the in situ formed carbon matrix.The nano-sized ZnS particle provides short diffusion length for lithium ion transport,and the amorphous carbon endows good electrical conductivity of the material,as well as accommodates the big volume change during repeated charge/discharge processes.At the current density of 0.5 A g^-1,the ZnS/C electrode exhibits an initial reversible capacity of 813 mAh g^-1.After 900 cycles,the capacity of 653 mAh g^-1 is delivered,which is 81%of the initial one.When the current density increases to 4 A g^-1,the specific capacity still remains at around 747 mAh g^-1,which is 77%of the theoretical capacity.This implies the excellent cycling stability and rate capability of the ZnS/C electrode.ZnS@C nanospheres with uniform size distribution and good dispersion were synthesized by a low temperature liquid phase process.A single ZnS nanosphere consists of primary nanocrystals with an average diameter of 10 nm,forming a porous architecture.The carbon layer covers the surface of the nanosphere,exhibiting a perfect core-shell structure.The pores between the nanocrystals afford void space for volume change of ZnS,thus can prevent the cracking of active particles during cycling.The excellent electronic conducting feature of carbon layer provides a uniform charge distribution on the particle surface during charge/discharge process,which promotes the maximum utilization of active material for electrochemical reactions.At a current density of 0.1 A g^-1,ZnS@C electrode exhibits the reversible capacity of 637 mAh g^-1.After 700 cycles at a current density of 0.5 A g^-1,the constructed electrode shows capacity of 670 mAh g^-1.Even at 4 A g^-1,a high reversible capacity of 374 mAh g^-1 can be still achieved.This method realizes the construction of hierarchical ZnS nanospheres and the controllable coating of uniform carbon layer,which provides a new strategy for the preparation of core-shell structured metal sulfide-based composites.