Study On Preparation And Lithium Storage Properties Of Coated Si-based Anode Materials

Silicon（Si）,with an high theoretical specific capacity(about 4200 m Ah g^-1),abundant reserves and good stability,is considered as one of the highly promising anode materials for lithium-ion batteries.However,the huge volume expansion during charge/discharge process and poor electrical conductivity of Si greatly limits its development and practical applications.The combinations of Si with graphite,amorphous carbon or other carbon materials are effective ways to improve the electrochemical performance of Si anode.The carbon matrix can improve the conductivity and buffer the volume expansion of Si-based anode.However,the traditional carbon coating is insufficient to accommodate the volume change during the long cycling process,resulting in the coating breaking and losing its function.Based on the way of traditional carbon coating,the Ti C coating with certain structural strength and carbon coating with hierarchical porous structure are used in this thesis to improve the stability of Si-based anode during the high rate and long cycling process.The main contents of this study are as follows:（1）Using graphite-coated Si（Si@G）powders as raw material,the Si@G@Ti C（SGT）composite powders with inner flexible and outer rigid double-layer coating were prepared by in-situ synthesis of titanium carbide（Ti C）layer on the surface of Si@G by a molten-salt method.Molten salt medium can promote the dissolution and migration of Ti,contributing to the formation of a uniformly Ti C coating layer.The graphite matrix can serve as a good conductive network and prevent the agglomeration of Si nanoparticles,and buffer the volume change of Si during charge/discharge process.The outer Ti C layer can further limit the volume expansion of Si,and form stable solid electrolyte interphase（SEI）films.When the content of Ti powder was 10%of graphite,the as-prepared SGT-10%electrode showed the best electrochemical performance with a reversible specific capacity of 600 m Ah g^-1 at 0.2 A g^-1 after 300 cycles and a capacity retention rate of 85%.（2）The porous carbon coated Si-based composite powders（Si@p-C）were prepared by using nano-Si powders as Si source,phenol formaldehyde resin as carbon source and nano Zn O particle as pore former.The large introduction of graphite would sacrifice the high specific capacity characteristics of Si materials,which is not conducive to the promotion of the mass or volume energy density of the battery.The porous carbon matrix can not only provide more buffer space for the volume expansion of Si and alleviate the huge stress generated during the lithiation/delithiation process,but also be able to shorten the diffusion path of lithium ions.The as-prepared Si@p-C electrode exhibited a reversible capacity of 1604 m Ah g^-1 after 50 cycles at 0.2 A g^-1with a capacity retention rate of 87.8%.Even at a current density of 1.0 A g^-1,it still had a high specific capacity of 956 m Ah g^-1 after 100 cycles,showing excellent cycle stability and rate capability.（3）The self-supporting electrodes of porous carbon fibers loaded with Si nanoparticles(Si_0.5/p-CF)and Si@p-C nanoparticles(（Si@p-C）_0.5/p-CF)respectively were prepared by electrospinning using polyacrylonitrile（PAN）as carbon source and nano Zn O as pore former,and the electrochemical performances as anode material for lithium-ion batteries were investigated.The three-dimensional carbon fiber framework formed by PAN carbonization as a support for self-supporting electrodes not only exhibits better flexibility and electrolyte penetration ability,but also has a good electron transport ability.Compared with Si_0.5/p-CF self-supporting electrode,the multi-layer carbon coatings of（Si@p-C）_0.5/p-CF electrode can more effectively prevent the pulverization of Si nanoparticles,and it exhibit a high discharge specific capacity of 940 m Ah g^-1 at 0.2 A g^-1 after 400 cycles,showing a good electrochemical performance.