The Preparation Of Hydrogel-derived Three-dimensional Porous Silicon-based Materials And Their Lithium Storage Performance

Silicon-based matreials are expected to become the next anodic candidates to replace graphite anodes for lithium ion batteries(LIBs),due to its superior theoretical specific capacity(4200 mA h g-1)and abundant sources.However,silicon has a serious volume change during the process of lithiation/delithiation and its own poor electronic conductivity,which lead to rapid deterioration of battery capacity and inferior rate performance.Therefore,extensive researchers have focused on how to solve the problem of volumetric expansion of silicon and obtain highly reversible Si-based anodes.In this paper,we obtained a novel nano-porous silicon-carbon composite by a simple hydrogel-derived method,and jointly improved the electrochemical performance of the silicon-based matreials through the compounding of different types of carbon media.The main studies are as follows:(1)We synthesized graphene oxide hydrogel(GO-gel)by electrostatic attraction and hydrogen bonding between graphene oxide(GO)and chitosan(Cs),which immobilized the silicon nanoparticles(Si NPs)in situ.After the subsequent freeze drying and simple heat treatment process,the three-dimensional(3D)Si@G nanoporous composites were prepared.Compared with Si NPs anode,Si@G composites exhibited good cycle performance and rate performance.For example,the Si@G(1:2)(mass ratio)composite retained high reversible capacity of 497.7 mA h g-1 after 200 cycles at a current density of 0.5 A g-1.(2)In order to further improve the reversible capacity of Si@G composites,we in-situ introduced carbon nanotubes(CNT)on the basis of the first part of experiments to obtain Si-CNT@G 3D nano-porous composites.Compared with Si@G composites,the introduction of CNTs makes the graphene sheets form a firmly interconnected conductive carbon network,which greatly improves the charge transport capability,making Si-CNTs @G composites have better electrochemical performance.For example,the Si-CNT@G(1:0.2:2)(mass ratio)composite was able to deliver a high capacity of 673.7 mA h g-1 after 200 cycles at a current density of 0.5 A g-1,the capacity retention rate reaches up to 97%,which is much higher than the corresponding Si@G(1:2)composite.(3)Finally,a Si@C/G nano-porous composite was prepared by a double-network hydrogel(DN-gels)derived method,that is,the DN-gels with polyvinyl alcohol(PVA)and graphene oxide(GO)intercalated to encapsulate the Si NPs as a precursor(Si@PVA/GO).After subsequent freeze-drying and simple heat treatment process,the Si@C/G nano-porous composite can be obtained.The unique dual carbon network structure effectively buffers the volume change of silicon and greatly improves the conductivity of the composite,ensuring that the Si@C/G composite shows excellent lithium storage performance.For example,at a current density of 0.5 A g-1,the Si@C/G nanoporous composite still retain a high reversible capacity of 830.5 mAh g-1 after 100 cycles.Moreover,while the current density increases to 2 A g-1,the Si@C/G electrode retained a high reversible capacity of 565.9 mAh g-1 after 200 cycles.