A scanning electron microscope image of the novel anode material comprising FLG flakes and micron-sized silicon particles. Image: University of Warwick.
A scanning electron microscope image of the novel anode material comprising FLG flakes and micron-sized silicon particles. Image: University of Warwick.

Researchers in WMG at the University of Warwick, UK, have found an effective approach for replacing graphite in the anodes of lithium-ion batteries with silicon, by reinforcing the silicon anode’s structure with graphene girders. This could more than double the lifetime of rechargeable lithium-ion batteries by greatly extending the operating lifetime of the anode and also increase the capacity delivered by those batteries.

Graphite has been the default choice of material for the anodes in lithium-ion batteries since their original launch by Sony. But researchers and manufacturers have long sought a way to replace graphite with silicon, which is an abundantly available element with 10 times the gravimetric energy density of graphite. Unfortunately, silicon has several performance issues that continue to limit its commercial exploitation.

Due to the volume expansion caused by the intercalation of lithium ions, or lithiation, during charging, silicon particles can electrochemically agglomerate in ways that degrade the battery’s charge-discharge efficiency over time. Silicon is also not intrinsically elastic enough to cope with the strain of lithiation when it is repeatedly charged, leading to cracking, pulverization and rapid physical degradation of the anode’s composite microstructure. This contributes significantly to reduction in the capacity of the battery over time, along with degradation events that occur on the counter electrode – the cathode. This is why mobile phones have to be charged for longer as they age and why they don’t hold their charge for as long.

Scientists have tried numerous ways to overcome these issues, which include using nano-sized and nano-structured silicon particles with micron-sized graphene. Although the nano-sized silicon particles dramatically increase the amount of reactive surface available, they also lead to much more lithium being deposited on the silicon during the first charge cycle. This results in the formation of a solid-electrolyte interphase barrier between the silicon and the electrolyte, greatly reducing the lithium inventory and thus the battery’s useful lifetime. This layer also continues to grow on silicon and so the lithium loss becomes continuous. Other methods for incorporating materials such as graphene at different scales have been deemed impractical for large-scale manufacture.

In new research, a team led by Melanie Loveridge in WMG at the University of Warwick has discovered and tested a new anode material comprising silicon with a form of chemically modified graphene, which could resolve these issues and create viable silicon anode lithium-ion batteries. Such an approach could be practically manufactured on an industrial scale and without the need for nano-sized silicon and its associated problems. Loveridge and her colleagues report their research in a paper in Scientific Reports.

Graphene is a single-atom thick layer of graphite (an allotrope of carbon). However, it is also possible to separate and manipulate a few connected layers of graphene to produce a material known as few-layer graphene (FLG). Previous research had tested the use of FLG with nano-sized silicon, but this new study found that FLG can also dramatically improve the performance of larger micron-sized silicon particles when used in an anode. Loveridge and her team found that this mixture of FLG and micron-sized silicon could significantly extend the lifetime of lithium-ion batteries while also offering increased power capability.

The anodes actually comprised a mixture of 60% micro-silicon particles, 16% FLG, 14% sodium/polyacrylic acid and 10% carbon additives. The researchers examined the performance of these anodes (and changes in the structure of the material) over a 100 charge-discharge cycles.

"The flakes of FLG were mixed throughout the anode and acted like a set of strong, but relatively elastic, girders,” explained 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 very effective at preserving the degree of separation between the silicon particles, [otherwise] the 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. "

This story is adapted from material from the University of Warwick, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.