3D Hierarchical Macro-/mesoporous Silicon:Synthesis and Application as Lithium-ion Battery Anodes

Author:Zuo Xiu Xia

Supervisor:zhu jin cheng ya jun


Degree Year:2018





Silicon has been regarded as one of the most promising next generation lithium-ion battery(LIB)anodes to replace commercial graphite anode due to its exceptionalcapacity,properworkingvoltage,reasonablesafety,environmental-friendliness and enormous abundance.However,drastic volume change during lithiation/delithiation processes has caused severe detrimental consequences,such as pulverization,disconnection between electrode and current collector,continuous consumption of lithium ions during the continuous formation-breaking-reformation process of solid electrolyte interface(SEI)layer,leading to very poor cyclic stability.It has greatly hampered the practical application of the silicon anode.Recently,remarkable progress has been made to improve the electrochemical performance by introducing 3D porous structure to accommodate volume change during the cycles.In this thesis,3D continuous macro/-mesoporous silicon and silicon/carbon composites with different structures are synthesized,where the structures and electrochemical performance are systematically studied.(1)3D hierarchical macro-/mesoporous silicon has been successfully synthesized via a magnesiothermic reduction process using 0D 410 nm St(?)ber silica particles as the starting material.Based on a systematic structure characterization,a self-templating mechanism responsible for the formation of the hierarchical porous silicon is proposed.Compared to the commercial nano-sized andmm-sized silicon particles,the macro-/mesoporous silicon exhibits significantly improved cyclic and rate performance.A reversible capacity of 959 mAh g-1 is retained after 300 cycles at0.2 A g-1 with a high mass loading density of 1.4 mg cm-2.At the high current density of 2 A g-1,a reversible capacity of 632 mAh g-1 is achieved.The coexistence of both macropores and mesopores can effectively mitigate the mechanical stress caused by volume change and enhance the electrochemical performance.(2)In order to elucidate the structure-property correlation of the hierarchical macro-/mesoporous silicon,the structures and morphologies of the porous silicon is tuned by systematically varying the magnesiothermic reduction protocols including reduction temperature and length of reduction time.It is found that with either increased reduction temperature or extended reduction time,the pore diameter is gradually decreased and wall thickness is continuously thickened,where the specific surface area and pore volumes are decreased as well.Due to structure change of the porous silicon,the electrochemical performance is modified.Overall,as the reduction time extends and the reduction temperature increases,the initial specific capacities of the porous silicon are increased,while the capacity retention values are decreased.The results suggest that that the porous silicon with larger pores and thinner walls are beneficial for improved cycle stability.Besides,it is found that the extended reduction time effectively improves the rate performance,but the increased reduction temperature does not have a significant impact on the rate performance.Generally,the3D hierarchical macro-/mesoporous silicon exhibits significantly improved cyclic and rate performance than the commercial nano-sized andmm-sized silicon particles,due to existence of the hierarchical macro-/mesoporous structures.(3)Based on the research work presented above,the influence of the size of the silica template on the structure and electrochemical performance of the as-synthesized porous silicon is investigated.Silica nanoparticles with the diameter of 130 nm are employed to generate porous silicon via magnesiothermic reduction at different temperatures.The results confirm that the 130 nm-sized silica can still exercise a self-templating effect in the magnesiothermic reduction process to form 3D continuous macro-/mesoporous structures.However,due to the small size feature of the silica particles,the in situ formed silicon needs to overcome large radius curvature to build macropores during the thermally induced fusion process.As a result,less ordered macropores with a less complete structure feature are observed with low reduction temperatures.With increasing reduction temperature,the specific surface area and pore volume of the porous silicon decrease gradually.Regarding the electrochemical performance,the initial reversible capacity,the initial coulombic efficiency and the rate performance are improved with increased reduction temperature,which may be due to better formed macropores with an improved structure order.(4)The influence of the silica particle size on the structure and properties of the as-synthesized porous silicon is further studied using 720 nm-sized silica particles as the starting material.Different reduction temperatures are applied to the magnesiothermic reduction process to tune the structure and performance of the porous silicon.Because of the large size of the silica particles,it is difficult to form complete macroporous structures under 700°C and 800°C,which results in a slightly improved cyclic performance and comparable rate capability compared with the commercial micro-sized silicon.When the reduction temperature is increased to900°C,the structure of the macropores is improved due to enhanced reduction process between magnesium vapor/melt and silica at the solid-liquid/solid-gas interface.As a result,the cyclic and rate performance are significantly improved compared with the reduction products obtained under low temperature.(5)Si/C nanocomposite with 3D hierarchical macro-/mesoporous silicon encapsulated by carbon matrix is successfully synthesized based on a self-templating mechanism via the magnesiothermic reduction process of the silica/carbon composite.Difunctional methacrylate monomers are used as solvent and carbon source to synthesize the SiO2/C composite.As a result,macroporous structures are retained within carbon matrix after the magnesiothermic reduction.Due to the presence of macro-/mesoporous structures and compositing with carbon matrix,the cyclic stability of the Si/C composite is significantly improved compared to those of the bare porous Si andmm Si/C composite.Good rate performance is also exhibited,which is comparable to that of the bare porous Si and better than that of themm Si/C.