Construction And Electrochemical Properties Study Of Silicon-based Anode In Lithium Ion Batteries

With the development of portable electronic devices,the lithium-ion batteries with high energy density are urgently needed.Silicon-based anode features a suitable de-lithiation potential and the capacity of 3600-4200 m Ah g^-1(Li₁₅Si₄,Li₂₂Si₅),which can greatly boost the device energy density.However,due to the huge volume change(300%)during cycling,the silicon particles tend to be broken and pulverized,resulting in the rapid capacity attenuation and low coulombic efficiency.The incorporation of silicon and carbon is the most promising solution to the problem of Si-based anodes.Focusing on the structure improvement of composites and the optimization of synthesis strategy,the paper was carried out to construct three kinds of nanosilicon-based/carbon composites with point-line contact,point-surface contact and full-encapsulated contact.The electrochemical properties of prepared composites were systematically studied.The commercial melamine formaldehyde resin was used as the precursor to load carbon nanotubes(CNTs).After carbonization,the porous CNTs/C matrix was obtained.Polyaniline-silicon(PANI-Si)core-shell materials were constructed on the surface of interconnected porous CNTs/C matrix.The silicon nanoparticles(Si NPs)were evenly coated with PANI layers.The point-line contact between Si NPs and CNTs was achieved to ensure electronic conductivity.During the alloying and dealloying process,the volume change of Si NPs was jointly afforded by the outer PANI coating layer and the inner CNTs/C matrix,avoiding the separation of Si NPs from the conductive network.The PANI-Si@CNTs/C composite exhibits the discharge capacity of 727 m Ah g^-1(0.1 A g^-1,100 cycles).The discharge capacity of652 m Ah g^-1 can still be maintained,when the current density was changed from 1.0A g^-1 to 0.1 A g^-1.Two kinds of sandwiched nanosilicon-based/carbon free-standing electrodes with point-surface contact were prepared by the vacuum filtration and freeze casting-drying using bacterial cellulose as binder.Bacterial cellulose nanofibers with a large amount of hydroxyl groups can uniformly bind with Si NPs.The graphene oxide loaded with Si NPs could self-assemble onto the surface of bacterial cellulose loaded with Si NPs.After vacuum filtration and reduction treatment,the free-standing graphene/bacterial cellulose/Si NPs(GN/BC/Si NPs)electrode was obtained.The discharge capacity of 1251 m Ah g^-1 can be delivered after 100 cycles at 0.1 A g^-1.At 6.4 A g^-1,405 m Ah g^-1 discharge capacity can be achieved.The GN/BC/Si NPs electrode can still maintain 1050 m Ah g^-1 discharge capacity after the rate test.BC was oxidized by the TEMPO-Na Cl O-Na Br solution(TOBC).The hydrogen bonding between TOBC and Si-OH functional groups ensures the uniform dispersion of Si NPs.Graphite microsheets(GM)could self-assemble onto TOBC.The solution was frozen and freeze-dried.The corresponding aerogel material was obtained.Due to the support of TOBC,the aerogel structure was intact.After compacting and rolling,the GM/TOBC/Si NPs film was used as an anode.The GM/TOBC/Si NPs free-standing anode delivers the capacity of 639.4 m Ah g^-1(1.0A g^-1,400 cycles).The film battery was assembled with Li Fe PO₄ as cathode and BC membrane as separator.The initial capacity of the assembled film battery was 121.7m Ah g^-1.Furthermore,the film battery can operate well in the bent state(73.4 m Ah g^-1,100 cycles).A series of SiO_x/C hollow microspheres with full-encapsulated contact was synthesized as anode materials for lithium-ion batteries.First,monodispersed hollow polymer microspheres(500-1200 nm)were obtained by one-step chemical polymerization without any template or surfactant.Through the condensation of aldehyde groups and amino groups,different dialdehyde molecules and3-aminopropyltriethoxysilane were successfully polymerized.¹³C and ²⁹Si SS-NMR spectra demonstrate the reaction process.After carbonization,the organic carbon in polymers was in-situ converted to amorphous carbon,resulting in a full-encapsulated contact with SiO_x nanoclusters.The introduction of carbon units is favourable for the charge transfer and meanwhile,comprehensively relaxes the dynamical stress effect of SiO_x nanoclusters.The discharge capacities of the three SiO_x/C hollow microspheres were in the range of 500-900 m Ah g^-1.We developed a new method to synthesize SiO_x/C hollow microspheres and achieve the uniform dispersion of SiO_x and C at the nanocluster scale.The electrodes exhibited good cycle stability and high coulombic efficiency.