Research On Preparation,Prelithiation And Cycling Stability Of Silicon Carbon Anode Materials With Core-shell Structure

Silicon-based materials have become one of the most promising candidates to replace the current graphite anode,due to their high specific capacity,suitable discharge platform(?0.4 V vs.Li/Li⁺),abundant reserves,low price,and environmental friendliness.However,the main problems,such as the poor conductivity of silicon-based materials,large volume changes during charge and discharge,and low initial Coulombic efficiency,have led to the rapid capacity decay of silicon-based materials.Since these problems of silicon-based materials have not been well handled,the silicon content in current commercial silicon-carbon anodes is generally less than 20%,which limits the improvement of battery energy density.In this work,the reasonable york-shell structure design and prelithiation of silicon-based materials were studied.The preparation process,physical properties,electrochemical performance,evolution of the in-situ lithiation morphology,and the in-situ electrochemical behavior of the synthesized york-shell structured silicon-based composite materials were also studied and analyzed.In addition,a strategy of surface passivation after prelithiation was proposed to solve the problem of instability of prelithiated electrodes in the air,and the mechanism of passivation was investigated by X-ray photoelectron spectroscopy.Through the york-shell structure design and prelithiation,the cycle stability and the initial Coulomb efficiency of silicon-based materials have been successfully improved,which provides a technical approach and theoretical basis for the large-scale practical application of silicon-based materials.The york-shell structured silicon-based composite materials were synthesized through the method of controlling the growth condition of phenolic resin on nano silicon surface,and selective dissolution of internal phenolic resins with low polymerization degree by acetone.This method can flexibly adjust the structural parameters of the york-shell silicon-carbon composite material by adjusting the ratio of the reactants,including the thickness of the carbon shell,the size of the reserved void,and the number of silicon particles coated in a single carbon shell,so as to facilitate its best electrochemical performance.The optimized york-shell structured silicon-carbon composite had a reversible specific capacity of 1993 m Ah g^-1 at a current density of 0.1 A g^-1,corresponding to an initial Coulombic efficiency of65.3%.It demonstrated a rate capacity of 799 m Ah g^-1 at a large current density of10 A g^-1,and a specific capacity of 1018 m Ah g^-1 with the capacity retention of73.5%at a current density of 2.0 A g^-1 after 1000 cycles.In addition,because this method does not depend on the morphology and synthesis process of electrochemically active materials,it is also applicable to the preparation of other york-shell structure energy storage materials,such as transition metal oxides,transition metal phosphides and sulfides,tin-based materials and so on.A low-cost magnesiothermic reduction method was used to prepare hollow and porous carbon-coated silicon spheres(Hp-Si@C).The introduction of Na Cl heat absorber during the magnesiothermic reduction successfully suppressed the formation of electrochemically inert silicon carbide by-products.The hollow and porous structure design promoted that Hp-Si@C anode has excellent electrochemical performance than that of nano silicon anode.Hp-Si@C presented a reversible specific capacity of 2699 m Ah g^-1 with an initial Coulombic efficiency of71.6%at a current density of 0.1 A g^-1,a rate capacity of 865 m Ah g^-1 at a current density of 10 A g^-1,and a specific capacity of 1228 m Ah g^-1 with the capacity retention of 69%at a current density of 2 A g^-1 after 1000 cycles.Electrochemical behavior analysis,such as in situ differential capacitance curves,electrochemical impedance spectroscopy,and galvanostatic intermittent titration technique,further indicated that Hp-Si@C had better electrochemical reaction kinetics than nano silicon.Considering that SiO_x has a smaller volume expansion during charge and discharge process and is cheaper than Si,a self-transformation method was used to prepare a carbon-coated hollow SiO_x sphere(H-SiO_x@C)with an adjustable shell thickness.H-SiO_x@C with a shell thickness of 35 nm exhibited the best electrochemical performance.It presented a reversible specific capacity of 1475m Ah g^-1 with an initial Coulombic efficiency of 72.2%at a current density of 0.1 A g^-1,a rate capacity of 755 m Ah g^-1 at a current density of 10.0 A g^-1,a reversible capacity of 1126 m Ah g^-1 with capacity retention of 91.2%at a current density of1.0 A g^-1 after 1000 cycles,and a reversible capacity of 876 m Ah g^-1 with capacity retention of 80.1%at a current density of 2.0 A g^-1 after 1000 cycles.In order to improve the initial Coulombic efficiency of silicon-based materials,a air-stable pre-lithiated H-SiO_x@C(ASP-H-SiO_x@C)anode was prepared by electrochemical prelithiation and subsequent thermal passivation.Through XPS characterization to explore the passivation mechanism,it was found that the realization of passivation was attributed to the heating treatment to promote the formation of a air-stable Li₂O layer on the surface of the prelithified H-SiO_x@C,which protected the internally highly active of Li_xSi.This ASP-H-SiO_x@C anode exhibited excellent electrochemical performance after being exposed to air(relative humidity 10%?20%)for 48 hours.It exhibited a reversible specific capacity of1438 m Ah g^-1 with an initial Coulombic efficiency of 99.2%at a current density of0.1 A g^-1,a rate capacity of 753 m Ah g^-1 at a current density of 10.0 A g^-1,and a reversible capacity of 1059 m Ah g^-1 with capacity retention of 89.8%at a current density of 1.0 A g^-1 after 1000 cycles.