SiO and Silicon Graphite Composite Anode Materials for Lithium-ion Batteries
Date
2023-08-30
Authors
Jiang, Hairui
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Lithium-ion batteries have been widely used in portable digital devices and electric vehicles. Silicon based anode materials including SiO and silicon graphite composites have attracted much attention due to their high volumetric capacity compared to conventionally used graphite. However, huge volume changes of Si-based materials can result in particle fracture and thick solid electrolyte interphase formation, leading to battery failure. One strategy of overcoming this is to utilize special binders that can maintain connection among electrode components and protect the active from side reactions. Another strategy is to design composite materials that include a Si component that is isolated from contacting the electrolyte. This thesis explores both these strategies.
Description
Poly(acrylic acid) is known to be one of the best binders for Si-based anodes. A systematic investigation was undertaken on the effect of sodium poly(acrylic acid) binder molecular weight on silicon monoxide cycling performance. It was found that cycling performance was poor for low molecular weight binder, however this was primarily related to poor slurry rheology. It was found that if propylene glycol is used instead of water as a slurry solvent, the propylene glycol can take over the role of viscosity modifier from the binder. When propylene glycol is used as a solvent for slurries low MW NaPAA binders (e.g. 1.5 k NaPAA), the cycling outperformance of the resulting SiO electrodes outperforms conventionally made SiO electrodes with high molecular weight binder (e.g. 250 k NaPAA). These results show that binder molecular weight does only affect the cycling performance of Si-alloy based electrodes because of its role as a slurry viscosity modifier. If propylene glycol is used to increase slurry viscosity, then the molecular weight has little effect. This research enables small molecules to act as binders opening a new avenue of binder research.
Silicon-graphite composite anode materials for Li-ion cells were synthesized by embedding nano-Si into voids within natural graphite by mechanofusion followed by carbon coating by chemical vapor deposition. In the resulting structure, voids within the natural graphite were completely filled with silicon nanoparticles encapsulated in carbon. This resulted in the silicon being protected from reaction with the electrolyte. The as-prepared silicon-graphite composites were shown to cycle well in electrodes with no special binders (PVDF) and in conventional electrolytes without any additives (i.e. no fluoroethylene carbonate). Furthermore, nearly all the Si within the voids were fully active. This provides a promising and simple strategy for silicon-graphite composites that can be used as a drop-in replacement for graphite to increase Li-ion cell capacity. The electrochemical data and silicon morphology were discussed and investigated in this thesis.
Keywords
lithium-ion batteries, silicon graphite composites, anode materials, binder