Preparation And Electrochemical Performance Of Si-based Anode Material For High Performance Lithium-ion Batteries

Currently,silicon has been considered as the most promising and prospective anode material for the next generation lithium ion battery because of its theoretical capacity(3572 mAh g^-1),environmental friendliness and low cost.However,the enormous volume effect of silicon and low intrinsic conductivity leads to the initial low coulomb efficiency,high irreversible capacity as well as poor cycle stability,which cause the bad cycling performance,seriously limiting the commercial application of silicon-based materials.Targeted at the above challenges,we design and further prepare silicon nanoparticles and silicon oxide materials with different microstructure,and then combines them with synergistic metal oxides and conductive carbon matrix,which can solve effectively the problems of Si-based as anode materials and enable outstanding electrochemical performances.The innovative achievements in this paper are listed as follows:（1）The hollow mesoporous silicon nanobox dually encapsulated by SnO₂/C（HM-Si/SnO₂/C）were prepared by Fe₂O₃ template,sol-gel method and in-situ polymerization method,and the effects of the double coating layers on the structure,and features of nanocomposite are systematically investigated.The results show that the composite with a special microstructure,and the interior cavity could afford sufficient buffer space to absorb huge volume expansion during（de）lithiation processes and the thin porous Si shell with appropriate proportion could significantly shorten the electronical and ionic diffusion pathways between electrolytes and Li⁺and the SnO₂layer and amorphous carbon shell on the HM-Si nanobox could attain a high volumetric density to offset the defects of the hollow nanostructure and can provide sufficient conducting channels to Li⁺transportation and further make sure the cycling stability of anode material.Electrochemical tests show that HM-Si/SnO₂/C composites have high reversible capacity and excellent rate performance.The first discharge specific capacity of the HM-Si/SnO₂/C samples is about 2685 mAh g^-11 with coulomb efficiency remained74.3%at 0.2 A g^-1 within the voltage scope of 0.01-1.5 V.The specific capacity is still1535 mAh g^-1 with the coulomb efficiency retained 82%200 cycles later.Besides,the Si/Sn@N-C nanohybrids still offer a reversible discharge capacity of 858 mAh g^-1 at 6A g^-1.（2）The hollow mesoporous Si-Sn nanospheres encapsulated by N-doped carbon matrix have been successfully achieved via a simple one-step magnesiothermic reduction procedure and sol-gel approach.The composites are characterized by X-ray diffraction（XRD）,field emission scanning electron microscopy（FE-SEM）,transmission electron microscopy（TEM）,X-ray photoelectron spectroscopy（XPS）and constant current charge-discharge tests.The results show that the Si-Sn@N-C composites own hollow mesoporous sphere morphology,which can shorten the transport path of the Li⁺and electron and offer adequate buffer space to assimilate enormous volume expansion of the silicon material.The introduction of Sn element possesses a step-by-step alloying process due to the different potential plateau of Si and Sn element,which provide a buffer for the volume change of each other.Additionly,employing heteroatom doping in the carbon matrix further enhances the electrical conductivity in a way of offering sufficient conducting passageways to fast charge transfer and Li⁺diffusion.Electrochemical tests show that the Si/Sn@N-C composite has high reversible capacity and excellent rate performance.The first discharge specific capacity of the Si-Sn@N-C sampless is 2171 mAh g^-1 with coulomb efficiency remained 77.8%under 0.5 A g^-1 within the voltage scope of 0.01-2 V,and the specific capacity is still 1236 mAh g^-1 with average coulomb efficiency retained as high as 78%after 200 cycles.Furthermore,the Si/Sn@N-C nanohybrids still offer a reversible discharge capacity of 613 mAh g^-11 at 8 A g^-1.（3）Currently,silicon suboxide（SiO_x,0<x<2）anodes are inclined to achieve smaller volume expansion compared to silicon element,which results from the in-situ generation of inert irreversible during the initial lithiation process,serving as a cushion matrix to relief remarkably the volume effect of Si-based materials.We adopt a valid strategy to synthesis the SiOx nanosphere double-wrapped by MoS2 and CNTs as anode material.The lithium intercalation mechanism and the influences of coatings are probed detailly.Electrochemical tests show that the SiO_x/MoS₂-CNTs composite own high reversible capacity and excellent rate performance.The specific capacity is still 798mAh g^-1 with average coulomb efficiency retained as high as 81.4%after 300 cycles.And the HS-SiO_x sample has a discharge specific capacity of 206 mAh g^-1 150 cycles later under the same test conditions.The synergistic effect of the high electroconductibility of the CNTs and the existence of sheet-structure MoS₂ and further can offer more exposed active sites and further promote the electronic conductivity,which still offer a reversible discharge capacity of 413 mAh g^-1 at 4 Ag^-1...