The WMG team says it found an effective approach to replacing graphite in the anodes of lithium-ion batteries using silicon, by reinforcing the anode's structure with graphene girders, which may result in this life extension, as well as increasing the battery capacity.
Silicon is said to be ideal because it’s an abundantly available element with ten times the gravimetric energy density of graphite. However, the material has several challenges that limit its commercial exploitation. Over time, silicon particles can electrochemically agglomerate in ways that impede further charge-discharge efficiency, due to its volume expansion upon lithiation. Silicon is also not intrinsically elastic enough to cope with the strain of lithiation when it is repeatedly charged. This can lead to cracking, pulverisation and rapid physical degradation of the anode's composite microstructure, contributing to issues such as capacity fade.
According to WMG, its research may offer a solution. Led by Dr Melanie Loveridge, the researchers claim to have discovered and tested a new anode mixture of silicon and a form of chemically modified graphene which could create viable silicon anode lithium-ion batteries.
This approach could be practically manufactured on an industrial scale and without the need to resort to nano sizing of silicon and its associated problems.
The team says it has managed to separate and manipulate a few connected layers of graphene, resulting in a material they refer to as few-layer graphene (FLG).
This study has apparently found that FLG can improve the performance of larger micron-sized silicon particles when used in an anode, causing power to increase and battery life to extend significantly.
The team claim to have created anodes that are a mixture of 60% micro silicon particles, 16% FLG, 14% Sodium/Polyacrylic acid, and 10% carbon additives. They then examined the performance (and the changes in structure of the material) over a 100 charge-discharge cycles.
Loveridge said: "The flakes of FLG were mixed throughout the anode and acted like a set of strong, but relatively elastic, girders. These flakes of FLG increased the resilience and elasticity 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 very effective at preserving the degree of separation between the silicon particles. Each battery charge cycle increases 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 tested by the WMG University of Warwick led researchers to hypothesize that this phenomenon is highly effective in mitigating electrochemical silicon fusion. This has been supported by systematic investigations"
The team are working to develop this technology further, with a view to advance in pre-industrial production of silicon/graphene composites and their subsequent processing into lithium-ion batteries for high-energy and high-power applications.
