Research On Silicon-based Composite Anode Materials For Lithium-ion Batteries

Silicon, owing to its high theoretical specific capacity (4200mAh-1) andappropriate discharge voltage (ca.0.4V), is regarded as the most promisingcandidate for the anode material in lithium ion batteries, in place of graphite.However, the drastic volume changes during cycling cause silicon particlecracks and breakdown of the electrical conducting network. In addition,silicon has a low electronic conductivity. These hinder the application ofsilicon as an anode. Highly porous structure and carbon coating can improveobviously the electrochemical performance of silicon based anode materials.But the complicated preparation technologies of this porous silicon may causehigh cost and be difficult for large scale production.Aiming at the above problems, silicon based anode materials with highelectrochemical performances were developed based on choosing simplepreparation process and cheap raw materials. Meanwhile, suitable binder wasselected for different active materials. Porous silicon/carbon composite wasprepared using simple preparation method and cheap silicon source; Theeffect of different aqueous binders on electrochemical performance of SiO with and without carbon coating were studied as well; The SiOx/Si/Ccomposite was designed by partially reducing SiO to nano-Si. The mainfinding and results were summarized as follows:1. Porous silicon/carbon composite was successfully prepared via amechanochemical ball milling using lithium silicon alloy (Li13Si4) and silicontetrachloride (SiCl4) as raw materials. Different carbon coating routes andcarbon contents were applied and electrochemical behaviors of the resultingsamples were compared and discussed. Meanwhile, specific surface area ofporous silicon/carbon composite could be adjusted by controlling the particlesize distribution of Li13Si4raw material. Electrochemical performance of thecomposite with a specific surface area of73.9-100.9m2g-1and a carbon layerof ca.6nm exhibited excellent electrochemical performance. After200cyclesthe reversible capacity of the composite was still982mAh g-1with excellentcapacity retention of86%versus the3rd cycle. Moreover, it also showedfavorable rate performance. Even increasing rate to10C, the reversiblecapacity was still502.5mAh g-1. The superior electrochemical performancewas attributed to the porous structure, which could accommodate the volumeexpansion, and excellent electronic conductivity of thin carbon layer.2. Macro/meso-structured porous silicon particles were synthesized fromnano-SiO2as both template and silicon precursor via a combination of spray drying and magnesiothermic reduction procedure. Followed nano-layercarbon coating via chemical vapor deposition, porous silicon/carboncomposite was obtained. This material presented a stable capacity of ca.1780.4mAh g-1for100cycles at0.5C. Furthermore, it maintained～1100mAh g-1reversible capacity even at5C. The special structure of compositewith macro/meso pores was benefit to accommodate the volume changes ofsilicon and uniformly thin carbon layer maintained the conducting net-workof the electrode.3. The influence of environmentally friendly aqueous binders and carboncoating on the electrochemical performance of SiO powder anodes had beeninvestigated. The SiO anode with polyacrylic acid (PAA), styrene butadienerubber/sodium carboxymethyl cellulose (SCMC) or sodium alginate (Alg)binder exhibited improved cycling stability. The possible reason for it wasthat their amorphous structure and high adhesion strength related to ester-likebonds stabilized the electrode structure. The cycling performance was furtherenhanced by carbon coating on the surface of the SiO. This was because thatcarbon coating improved the electronic conductivity of SiO and alsoreinforced the mechanical stability of the active phase. The SiO/C electrodewith SCMC binder holded the reversible capacity up to900mAh g-1after100cycles. 4. In order to increase the reversible capacity of SiO, the sub-oxide/silicon(m-SiOx/Si) composite was prepared via a partial reduction reaction betweenSiO and Mg by mechanochemical ball milling. This material was composedof Si-suboxide (SiOx) and embedded silicon nano-crystallite. The particleswere further covered by a uniform carbon layer on the surface via chemicalvapor depositon process. The m-SiOx/Si/C composite provided reversiblecapacity of1145.4mAh g-1after100cycles and the capacity retention was92%versus the6th cycle (compared at the same current rate). The highreversible capacity and favorable cycling performance were benefit touniform particle size, the uniform distribution of the Si nanocrystallites in theSiOxmatrix and uniformly thin carbon layer. In contrast, pre-milled SiOwithout adding Mg presented a reversible capacity of only ca.900mAh g-1under the same carbon coating condition.In summary, by means of designing porous structure, modifying carbonphase, embedding in-active oxide phase and matching suitable binder, thevolume change of silicon during cycling was effectively buffered and theelectronic conductivity was enhanced. Thus, the electrochemicalperformances of silicon based anode materials were improved largely.