Investigations On Composite With High Capacity For Anode Of Lithium-ion Battery

Si-based and Tin-based anodes are recognized as potential candidates to take place of commercial graphite anode due to their high theoretical capacity,low charge/discharge plateau,environmental friendly and crustal reserve.However,the unstable solid state electrolyte(SEI)caused by the sever volume effect during the lithiation/delithiation process leads to poor cycling performance which largely hinders their practical application.Recently,Si-based composites and Tin-based composites,especially the Si/C and Tin/C composites,are believed to be one of effective ways to overcome the above issue.Therefore,Si-based and Tin-based composites electrodes with high capacity and long cycling life will be discussed in this thesis.Firstly,a well dispersed nitrogen doped graphene wrapped Si nanoparticles(Nano-Si@NG)composite was synthesized through chemical reduction and calcination process to improve the structural stability and conductivity of the electrode.The pyridinic-to pyrrolic nitrogen ratio in NG is not only controllable but also there is an optimal ratio for the enhancement of battery performance.DFT calculation was used to get better understanding of the effect of different nitrogen doping motifs on the transportation of lithium-ion.The as-obtained Nano-Si@NG electrode delivered a reversible capacity of 551.5 mA h g^-1 after 200 cycles at a current density of 2 A g^-1,which implies great enhancement of cycling stability on Si-based anode.Secondly,graphene wrapped phosphorus doped Si(P-Si/G)was prepared by chemical reduction method.The insertion energy of lithium ion into Si crystal will be increased due to introducing of phosphorus atoms into Si crystal structure,thus the degree of lithiation is limited and the volume expansion is decreased to some extent.The cycling performance of P-Si/G electrode is further improved by utilizing poly-glutamate(PGA)as binder with self-healing capability.The mechanism of self-healing capability of PGA is demonstrated.The P-Si/G electrode with PGA as binder can deliver a reversible capacity of 1002 mA h g^-1 after 150 cycles at a current density of 1 A g^-1.Thirdly,the volume effect of Si nanoparticles was further improved by constructing 3D Si/C composites.Porous Si consisting of Si nanoparticles/C composite was synthesized by magnesium thermal method and in-situ polymerization process.The pore structure serves as volume expansion buffer which can significantly improve the structural stability of the electrode.The other 3D Si/C composite is achieved by embedding Si nanoparticles into the 3D conductive network consisting of1D S doped nanofibers and 2D graphene.This consecutive conductive network can improve the electrical contact between the active material and avoid the side reaction with electrolyte.Both of these two 3D Si/C composite contribute to superior electrochemical performance of Si electrode.Lastly,SnO₂ nanoparticles was synthesized by hydrothermal method and was utilized as the precursor to prepare Li Sn₂(PO₄)₃ by solid-state method.The calcination temperature and time as two key factors that affect the formation of Li Sn₂(PO₄)₃crystal are discussed.By combing with amorphous carbon,the electrochemical property of Li Sn₂(PO₄)₃/C is further improved.The as obtained Li Sn₂(PO₄)₃/C electrode retain 381.5 mA g^-1 after 50 cycles at 100 mA g^-1,displaying better cycling performance than bare SnO₂.The electrochemical performance of SnO₂ can be improved by incorporating with activated graphene nanoplates(a-GNPs).With a-GNPs as support,the SnO₂/a-GNPs composite is achieved by optimizing the p H value during the wet chemical process and the loading of SnO₂.The well dispersed SnO₂/a-GNPs electrode delivered a reversible capacity of 609.77 mA g^-1 after 100cycles at mA g^-1.