When you want to make a structure stronger, put a girder across it. It’s a simple principle that every civil engineer knows well. But a team at Warwick Manufacturing Group has found that it applies just as well on very small scales as in megastructures. Melanie Loveridge and colleagues are studying methods for improving lithium-ion batteries, and have found that minute girders could provide an answer to a problem that has been plaguing the field.
Ever since their first introduction in the early 1990s, the anode of lithium batteries has been made of graphite. It has long been apparent that silicon would be a better material, as it can hold ten times more charge per gramme than carbon. But the mechanics of lithium ion batteries, where lithium ions are absorbed into the anode, create problems.
When silicon is lithiated, it expands. But it is an inelastic material, and repeated expansion and contraction — as happens during charge-discharge cycles — can lead to cracking and crumbling, which makes the capacity of the battery fade over time. Graphene has been tried as a reinforcing material for nanostructured silicon, but this has led to other problems.
Loveridge’s team is looking at a material known as FLG (few-layer graphene). As the name implies, this is composed of a few connected layers of single-atom-thick graphene sheets, which can be manipulated together.
In a paper in Nature Scientific Reports, the WMG team describes how FLG can improve the performance of anodes containing micron-sized particles of silicon. The team started with a mixture of 60 per cent micro-silicon, 16 per cent FLG, 14 per cent sodium/polyacrylic acid and 10 per cent carbon additives, and put these anodes through 100 charge-discharge cycles.
“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 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," Loveridge said.
Moreover, she added, the graphene girders keep the silicon particles apart. In their absence, the particles tend to ‘weld’ together, restricting lithium diffusion through the anode and reducing the surface area available for lithiation.
“The presence of FLG in the mixture tested by the WMG University of Warwick led researchers to hypothesise that this phenomenon is highly effective in mitigating electrochemical silicon fusion,” Loveridge stated.
The team is now working on scaling up their graphene girders discovery to produce pouch cells based on their reinforced anodes, as part of a two-year graphene flagship project along with Varta Micro-innovations, Cambridge University, CIC, Lithops and IIT (Italian Institute of Technology).