| Silicon is one of the most promising anode materials for next-generation lithium-ion batteries.It has the advantages of high theoretical capacity,low discharge potential,good environmental compatibility,and high abundance.However,the large volume expansion and unstable solid electrolyte interface(SEI)during lithiation hinder the commercial application of silicon anodes.Silicon nanostructures and composites have been used to solve the above-mentioned problems and achieved better electrochemical performance,but there are still some fundamental understandings which need to be clarified,such as the lithiation behavior of silicon nanotubes,the influences of amorphous carbon coating on silicon anodes,and the influences of vertical graphene coating on silicon anodes.Accordingly,the following studies are carried out in this dissertation:Firstly,silicon nanotube arrays with different sizes are prepared.The effects of crystal phase,crystal orientation,wall thickness,and inner radius on lithiation behavior of silicon nanotubes are investigated.The experimental and simulation results show that the radial expansion of amorphous silicon nanotubes is isotropic during lithiation,and the critical outer diameter to avoid fracture is larger than 2 ?m;whilst the expansion and fracture of crystalline silicon nanotubes are anisotropic.In particular,the expansion towards the inner hole is much smaller than that towards the outside.For <111> silicon nanotubes,the fracture ratio and maximum tensile hoop stress are positively correlated with wall thickness and inner radius,and the optimal thickness–outer radius ratio is 2/3.At this optimal value,the critical outer diameter of the crystalline silicon nanotubes to avoid fracture can reach 700 nm.Secondly,a silicon nanocone–amorphous carbon(SNC-C)composite structure with clear geometry and pure components is prepared.The unique effects of amorphous carbon coating on SEI formation,structural stability,and electrochemical performance of silicon nanocone electrode are investigated.The experimental results show that the SEI layer on the SNC-C electrode is only about 10 nm after 50 cycles,and the nanocone structure is almost the same as the original electrode;whilst the silicon nanocone(SNC)electrode is covered with a thick SEI layer after 50 cycles,and the structure of the electrode is damaged and disordered.The elemental analysis on the surface of the electrode is consistent with the morphological analysis.Furthermore,electrochemical test shows that the SNC-C electrode achieves longer cycle life(>725 cycles),higher Coulombic efficiency(>99%),better rate capability,and lower electrode polarization.Finally,a silicon nanocone–vertical graphene(SNC-G)composite structure is prepared.The vertical graphene coating is innovatively patterned;thus both the coated area and the uncoated area can be directly compared side by side on one electrode.The effects of the three-dimensional vertical graphene coating on the SEI formation and structural stability of the SNC electrode are more convincingly demonstrated.The experimental results show that the SNC-G area retains a thin SEI layer and stable nanocone structure after 100 cycles,whilst the structure of the SNC area is gradually damaged and covered with a thick SEI layer.Furthermore,electrochemical test shows that the SNC-G electrode achieves longer cycle life(1715 cycles),higher Coulombic efficiency(98.2%),lower electrode polarization,and better rate capability.In summary,the effects of inner space and surface coating on the structural evolution and electrochemical performance of silicon anodes are studied,which will help to deepen the understanding of silicon-based anode materials and further improve the performance of silicon anodes.Furthermore,micro/nano-fabrication techniques such as fabrication of nanotube arrays,etching of nanocones,and patterning of coating layers would also provide new ideas and future directions for the research of other battery electrode materials. |
PDF Full Text Request