Synthesis And Electrochemical Performance Of Si-based Anode Material For Li-ion Batteries

Silicon (Si) has been regarded as the promising anode material for next generation of rechargeable lithium ion batteries due to its highest theoretical gravimetric capacity (4200 mAh g-1), low discharging potential and relative abundance on the earth. However, practical application of Si anode materials is seriously hampered by the huge volume variation (over 300%) during repeated lithiation/delithiation process, which lead to cracking and pulverization of anode structure, and thus loss of electrical contact. That is harmful for columobic efficiency, cycling stability and reversible capacity. Generally, fabricating nanosized Si and adding second phase into Si anode are demonstrated as efficient approach to improve the electrochemical performance. On one hand, CVD, magnesiothermic reduction of SiO2, organic solution reduction of SiCl4, and etching bulk Si were widely utilized to prepare nanostructured Si anode materials. On the other hand, designing SiM (M= graphene, graphite, carbon and metal) composite is beneficial for better Li-storage performance. Tn this paper, we developed a series of synthetic route to fabricate high-performance Si based anode materials. we developed a molten salt system to synthesize nanosized Si through metallothermic reduction of SiCl4 and silicon oxides in molten salt, and metathesis reaction between Mg2Si and ZnCl2; Si/graphene and Si@C/graphene is prepared via polymer assisted approach; prepatarion of Si/Cu/C and Si/Ge is realized by using Mg2Si as reductant and Si resources; fabrication of Si/graphite and SiOx/graphite is carried out by simple mechanical ball-milling; deriving Si@C composite from Si-rich biomass such as bamboo leaves and rice husks is realized. The specific content is as follows:1. Preparation of nanosized Si in molten salt systemCrystalline Si nanoparticles are prepared by reduction of SiCl4 with metallic magnesium in the molten salt of AlCl3 at 200 ℃ in an autoclave (2Mg+SiCl4+ 4AICl3â†'2MgAl2Cl8+Si). AlCl4 not only acts as molten salt, but also participates in the reaction. As anode for rechargeable lithium ion batteries, the as-prepared Si delivers the reversible capacity of 3083 mAh g-1 at 1.2 A g-1 after 50 cycles, and 1180 mAh g-1 at 3 A g-1 over 500 cycles. This work is published in Angew. Chem. Inter. Ed., and highlighted by Nature Materials.Preparing crystalline Si nanoparticles is carried out through the reduction of micro-sized high silicon zeolite by metallic Al (or Mg) in molten AlCl3,4Al+3SiO2 +2AICl3â†'3Si+6AlOCl。The by-product AlOCl is easily removed by washing in diluted HCl solution. The reaction can be initiated at 200 ℃, and the yield is about 40%. As the reaction temperqature increases to 250 ℃, the yield can reach to be about 75%. When the prepared Si was used as anode for Li-ion batteries, a reversible capacity of 2663 mAh g-1 at 0.5 A g-1 after 50 cycles, and 870 mAh g-1 at 3 A g-1 after 1000 cycles can be obtained. Similarly, this synthetic strategy is employed to synthesize Si nanoparticles starting from various abundant raw materials including SiO2 powder, kieselguhr, fiberglass, and even the natural mineral of albite. This work is published in Energy Environ. Sci..Silicon (Si) nanoparticles have been prepared by a "metathesis" reaction of magnesium silicide (Mg2Si) and zinc chloride (ZnCl2) in an autoclave at 300 ℃ (Mg2Si+2ZnCl2â†'2MgCl2+2Zn+Si. The alloying reaction between Si and Zn can not be ignited at 300 ℃. The as-prepared Si nanoparticles exhibit a reversible capacity of 795 mA h g-1 at a current density of 3.6 A g-1 over 250 cycles. This work is published in Chem. Commun..Preparation of porous Si@C nano-composite from Si-rich biomass such as bamboo leaves and rice husks is realized through baking the precursor at 400 ℃ in air atmosphere for controllable pyrolysis, and followed by thermal reduction in molten AlCl3 at 200 ℃. In this process, both Si and C components in those natural precursors are recovered as active materials. The obtained crystallized Si nano-particles are homogeneously embedded in the pyrolyzed porous carbon matrix. As anode for Li-ion batteries, the Si@C nano-composite delivers a reversible capacity of 1117 mAh g-1 at 0.4 A g-1 after 250 cycles, and about 600 mAh g-1 at 2.0 A g-1 even after 3700 cycles. When the anode is coupled with LiCoO2 cathode, a full cell exhibits a capacity of 1048 mAh g-1 at 150 mA g-1.2. Polymer assisted synthesis of Si and graphene compositeSi/reduced graphene oxide-based composite (Si@RGO) with 3D framework is constructed by the typical cross-linking reaction between polyacrylamide and graphene oxides. The prepared Si@RGO composite with the unique structure is beneficial for overcoming the drawbacks of Si anode materials such as huge volume change and low conductivity, which exhibits high reversible capacity of 1610 mAh g-1 at a current density of 1.2 A g-1 after 200 cycles, good rate capability, and cycling stability with negligible capacity degradation over 200 cycles. More importantly, the present strategy is very easy to carry out under facile condition, and could also be extended to other fascinating anode and cathode materials for next generation of lithium ion batteries. This work is published in J Mater. Chem. A.Fabricating Si@carbon/reduced graphene oxides composite assisted by polyaniline (PANI) is developed. Here, PANI not only is serveed as "glue" to combine Si nanoparticles with graphene oxides through electrostatic attraction, but also can be pyrolyzed as carbon layer coated on Si particles during subsequent annealing treatment. The assembled composite delivers high reversible capacity of 1121 mAh g-1 at a current density of 0.9 A g-1 over 230 cycles with improved initial coulombic efficiency of 81.1%. The enhanced electrochemical performance of Si@C/RGO can be attributed to the dual protection of carbon layer and graphene sheets, which are synergistically capable of overcoming the drawbacks of these inner Si particles such as huge volume change and low conductivity and providing protective and conductive matrix to buffer the volume variation, prevent the Si particles from aggregating, enhance the conductivity, and stabilize the solid electrolyte interface membrane during cycling. This work is published in ACS applied Mater. Interface.3. Preparation of Si based composite using active Zintl phase Mg2SiSi/Ge nanocomposite composed of interconnected Si and Ge nanoparticles is prepared through one-step solid-state metathesis reaction between Mg2Si and GeO2, Mg2Si+GeO2â†'2MgO+Ge+Si. The electrochemical lithiation process is investigated by ex situ XRD analysis, showing a step by step lithiation approach of Si and Ge component. As an anode, the Si/Ge electrode exhibits reversible capacity of 2404.7 mAh g-1 at 0.5 A g-1 over 60 cycles and long-term cycling stability with a capacity of 1260 mAh g-1 over 500 cycles even at 5 A g-1. This work is published in J Mater. Chem. A.Micron-sized bulk Si is chemically converted into nano-sized Si/Cu/C ternary composite. The Si particles, Cu crystals, and amorphous carbon are generated synchronously and mixed uniformly,4Mg2 Si+Cu(CH3COO)2.H2Oâ†'8MgO+ 4Si+Cu+2C+4H2. As an anode, the Si/Cu/C exhibits a capacity of 1560 mAh g1 after 80 cycles at 0.5 mA g-1, long-term cycling stability with a capacity of 757 mAh g-1 at 2 A g-1 after 600 cycles, and fine rate capability. This work is published in J Mater. Chem. A.In addition, Ge-Sn, Si-Sn and Si-C composite are also prepared via the similar approach.4. Si/graphite and SiOx/graphite composite prepared by ball-milling approachWe prepared Si/graphite and SiO/graphite composite via simple mechanical ball-milling approach. The specific capacity of these sample could be tuned by variating the weight ratio of Si (SiOx) and graphite. It is demonstrated that the the graphite content plays a key role to improve the cycling stability of the composite. Noteworthy, the developed approach is so simple to scale up.