Graphite has long been the default choice of material for anodes, but researchers dream of replacing graphite with silicon, which has ten times the gravimetric energy density. Unfortunately, silicon has several issues that limit its commercial use – it expands during lithiation, causing particles to agglomerate in ways that impede efficiency, and it is not elastic enough to cope with the strain of lithiation over repeated charge/discharge cycles.
Researchers at the University of Warwick have addressed these problems by reinforcing the anode’s structure with graphene girders. They believe the new approach could double battery life by extending the operating lifetime of the electrode, and also increase capacity.
In Electrochemical Evaluation and Phase-related Impedance Studies on Silicon-Few Layer Graphene (FLG) Composite Electrode Systems, published in Nature Scientific Reports, Dr. Melanie Loveridge and colleagues explain how they tested a new anode mixture of silicon and a form of chemically modified graphene.
Graphene is a one-atom-thick layer of graphite. However, it also possible to separate and manipulate a few connected layers of graphene, creating a material called few-layer graphene (FLG). Previous research has tested the use of FLG with nano-sized silicon, but the new study found that FLG can also dramatically improve the performance of larger micron-sized silicon particles when used in an anode.
The researchers created anodes that were a mixture of 60% micro-silicon particles, 16% FLG, 14% sodium/polyacrylic acid, and 10% carbon additives, then examined the performance and changes in structure over 100 charge/discharge cycles.
“The flakes of FLG were mixed throughout the anode and acted like a set of strong, but relatively elastic, girders,” said Loveridge. “These flakes of FLG increased the resilience and tensile properties of the material, greatly reducing the damage caused by the physical expansion of the silicon during lithiation. The graphene enhances the long-range electrical conductivity of the anode and maintains a low resistance in a structurally stable composite.”
“More importantly, these FLG flakes can also prove effective at preserving the degree of separation between the silicon particles, increasing the chance that silicon particles become electrochemically welded to each other. This increased agglomeration increasingly reduces and restricts the electrolyte access to all the particles in the battery and impedes effective diffusion of lithium ions, which of course degrades the battery’s life and power output. The presence of FLG in the mixture led researchers to hypothesize that this phenomenon is highly effective in mitigating electrochemical silicon fusion. This has been supported by systematic investigations.”
Source: University of Warwick