| Silicon(Si),as a popular anode material for lithium ion batteries,attracts lots of researchers'attention because it possesses the advantages such as ultrahigh theoretical capacity(3579 m A h g-1)and moderate discharge potential(under 0.5 V vs.Li+/Li).However,the intrinsic deficiencies such as relatively low electronic conductivity(?10-3 S·cm-1)and large volume variation beyond 300%during lithiation process confine its practical application.In recent years,Si-C composite nanofiber film structural anode material has been widely studied.This structure combines the high capacity of Si nanoparticles with the good electrical conductivity of CNFs,and acts as a free-standing anode for LIBs without binder and current collector.Nevertheless,there are still some problems with this structure.On the one hand,during the electrochemical reaction,the contact between the fibers is weakened,even the active material is separated from the electrode because of the constant expansion and shrinkage,causing the decrease of capacity and low rate performance.On the other hand,the Si NPs embedded in the CNF are covered with a thick layer of amorphous C,which slows down the reaction rate between lithium ions and Si NPs,resulting in a low rate capacity.Based on the abovementioned problems,our work aims at improving cyclic stability and rate performance of the anode materials of LIBs.Using electrospinning technique,we fabricated the Si/CNFs structure for LIBs anode material and by means of electrospinning technology,and enhanced its electrochemical performance by the following measures.1.Si/CNFs is wrapped by copper foam,which has high electronic conductivity and is barrier-free for the migration of lithium ions.It can not only improve the electrical conductivity of the Si/CNFs electrode,but also effectively limit the separation of fibers and ensure the structural-stability of the electrode.The results showed that the specific capacity of Si/CNFs-Cu electrode reached 813.9 m Ah g-1 after 80 cycles at the current density of 0.5 A g-1,and the cycling retention rate is 2.74 times as much as that of Si/CNFs electrode.At the same time,the rate capacity under 5.0 A g-1 is increased from 183.3 m Ah g-1 to 565.2 m Ah g-1.2.Cross-linked structural Si/CNFs can be obtained directly by pressing the electrospun nanofibers film.When the pressure condition of 1 MPa is selected,a good cross-linking effect can be formed between the nanofibers.The electrolyte infiltration into the sample can be avoided due to very few pores are shown on the surface of the fiber film,and the electronic conductivity can be improved along the direction of thickness of the film,and an optimal electrochemical lithium storage performance can be obtained.After 300 cycles at the current density of 0.5 A g-1,the specific capacity of the anode material is 926 m Ah g-1.At 5 A g-1,the multiplier capacity reached 709.7m Ah g-1.3.Si/PCNFs is obtained by one-step pore-forming using polyethylene glycol.By measuring the diffusion coefficient of lithium ion,we have proved that the introduction of pore-forming agent can effectively promote the migration of lithium ion and improve the rate performance of the electrode.By controlling the mass ratio of pore-forming agent,we optimized the pore structure of the Si/PCNFs.Finally,the sample named Si/PCNFs-0.4 had a high first-charge/discharge coulombic efficiency(ICE),excellent cyclic stability,and the rate capacity is increased to 1033.4 m Ah g-1 under the current density of 5.0 A g-1. |
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