University of Warwick (Coventry, United Kingdom) researchers developed an approach for using silicon to replace graphite in the anodes of lithium-ion batteries by reinforcing the anodes’ structure with graphene girders. This could more than double the life of rechargeable batteries by extending the electrode’s lifespan and also raising capacity.
According to the researchers, manufacturers have long wanted to replace graphite with silicon, as it is abundantly available and has 10 times the gravimetric energy density. Silicon, however, has undesirable performance characteristics. For example, due to their volume, silicon particles can electrochemically agglomerate in ways that impede charging efficiency. Silicon also is not intrinsically elastic enough to cope with strain when it is repeatedly charged, leading to cracking, pulverization, and rapid physical degradation of the anode’s composite microstructure.
To address this, the researchers tested an anode mixture of silicon and a form of chemically modified graphene, which they say could create viable silicon anodes for lithium-ion batteries. Such an approach could be manufactured on an industrial scale without the need for nanosizing the silicon.
Using nanosized silicon particles is problematic, they say, because the process increases the amount of reactive surface available—leading to more lithium being deposited on the silicon during the first charge cycle. This forms a solid-electrolyte interphase barrier between the silicon and electrolyte, thus reducing the lithium inventory and the battery’s useful life.
For their route, the researchers explain that graphene comprises one layer of graphite—an allotrope of carbon. By separating and manipulating connected layers of graphene, they form a few-layer graphene (FLG) material. According to the researchers, their study shows that FLG can boost the performance of larger micron-sized silicon particles when used as an anode, thus extending the life of lithium-ion batteries.
In their study, researchers used anodes that were a mixture of 60% microsilicon particles, 16% FLG, 14% sodium polyacrylic (C3H3NaO2)n acid, and 10% carbon additives, and examined their performance as well as changes in the material’s 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,” explains Melanie Loveridge, senior research fellow at the University of Warwick. “These flakes increased the resilience and elasticity of the material, greatly reducing the damage caused by the physical expansion of the silicon. 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.”
Further studies and research are planned in the months ahead.