Structure Design And Electrochemical Performance Of Silicon Based Anode Nanomaterials For Lithium Ion Batteries

Si-based materials have received great attention as anode for lithium batteries,due to the high theoretical specific capacity,relatively low discharge potential plateau,natural abundance and environmentally friendly.However,most Si anodes have limited cycle life and poor rate capabilities,which arise from huge volume change during lithiation/delithiation and low inherent electrical and ionic conductivity.In this thesis,in order to improve the cycling stability of Si-based anodes,a series of Si-based materials with ordered mesoporous structure,3D hierarchical structure,hollow heterostructure and sandwich structure were prepared.The above designed structures shorten the transmission path of lithium ions in anode materials,enlarge the contact area between anode materials and electrolyte,and thus promoting the electrode reaction kinetics process.In addition,by introducing amorphous carbon,nitrogen-doped carbon and graphite with good conductivity into the Si-based materials,the electron-transport in the electrodes is accelated and as a result the rate capability of electrode is remarkably enhanced.The synthetic mechanism of the Si-based materials,the relationship between the structure characteristic and the electrochemical properties of the prepared various Si-based materials were systematically investigated.SiOx/C anode material with arrayed mesoporous architecture was synthesized via a template assisted hydrothermal route and a subsequent carbon-coating process.The uniformly arrayed mesoporous can buffer the volume change of SiOx particle during the alloying/dealloying,increase the electrode/electrolyte contact area,and shorten the lithium/electron diffusion pathways.The in-situ carbon coating layer can provide fast electron transport for electrode reaction,and inhibit the volume change of SiOX nanoparticles during cycling.As a result,the electrode exhibits outstanding lithium storage performance with high first reversible capacity of 841 mA h g-1,superior cyclic stability(780 mA h g-1 after 350 cycles,0.02%decay per cycle),good rate capability(540 mA h g-1 at the current of 1.0 A g-1).3D hierarchical SiOx/NC(H-SiOx/NC)composite was synthesized by a combine of self-assembly and N-doped carbon coating process.The PVP acts as the"nucleator" for realizing self-assembly of H-SiO2 nanoparticles.The obtained H-SiOx/NC composite shows 3D hierarchical structure with highly porous SiOx nanoparticles enwrapped with porous N-doped carbon layer.The ultrafine SiO,can reduce the transport length of Li+ions and electron,while the particle size below critical size of Si(20 nm)can suppress the fracture and pulverization of the SiOx during cycling.The N-doped carbon layer can improve the electronic conductivity and alleviate the volume change of H-SiO,nanoparticles.Thanks to the above structural benefits,the H-SiOx/NC composite realizes improved reversible capacity of 842.8 mA h g-1 at 0.1 A g-1,an outstanding capacity retention of 93.6%after 100 cycles,respectable rate capability of 327 mA h g-1 at 2 A g-1.Hollow heterostructure Si/C composite was prepared by magnesiothermic reduction of SiO2 and coating with a layer of amorphous carbon fabricated through CVD process.The hollow structure of Si/C composite provides sufficient space for the volume expansion of Si during cycling.In addition,the thin-wall of the Si/C composite shortened the diffusion path of lithium ion and the cabon coating layer increased the electronic conductivity of material,and eventually improved the electrode reaction kinetics.The prepared Si/C electrode delivered an initial reversible capacity of 1150 mA h g-1 at 2 A g-1,an excellent capacity retention of 90.2%after 500 cycles,corresponding to 0.1 8%decay per cycle.The average coulombic efficiency is as high as 99.69%at 2 A g-1.The Ni/Si-NPs/GC anode material with sandwich structure was succefully synthesized via drop-coating and CVD process.The 3D Ni foam acted as a conducting network providing efficient electrode conduction and improving mass loading of active materials,which enhanced the energy density.The conformal,flexible graphite layer with the electrode that restricts the Si-NPs to fall off from the Ni foam and ensures sufficient electrical conductivity throughout cycling,which improves the cycling stability and rate capability.In addition,the hybrid anode does not require any polymeric binder and conductive additives and has excellent flexibility.All the above critical features make the Ni/Si-NPs/GC electrodes exhibit outstanding electrochemical performance including an initial capacity of 2860 mA h g-1 at 2 A g-1,an excellent capacity retention of 2790 mA h g-1 after 500 cycles,and respectable rate capability of 1500 mA h g-1 at 4 A g-1.