New research from WMG at the University of Warwick has discovered an effective approach to replacing graphite in the anodes of lithium-ion batteries using silicon by reinforcing the anode’s structure with graphene girders. This development could double the life of rechargeable lithium-ion batteries and increase the capacity delivered by those batteries.

This is a cross section of the Silicon and FLG together in an anode. Source: University of Warwick WMGThis is a cross section of the Silicon and FLG together in an anode. Source: University of Warwick WMG

Graphite has been the default choice of active material for anodes in lithium-ion batteries since their original launch by Sony. But finding a way to replace graphite with silicon has been long sought after because it is a plentifully available element that has ten times the gravimetric energy density of graphite.

But silicon has a few other performance issues that continue to limit the commercial exploitation. Due to the volume expansion that lithiation silicon particles can electrochemically agglomerate in ways that stop further charge-discharge efficiently over time. Silicon is also not elastic enough to cope with the strain of lithiation when it is charged repeatedly. This leads to cracking, pulverization and rapid physical degradation of the anode’s composite microstructure. This contributes significantly to the capacity’s fade and degradation events that occur on the counter electrode, like having to charge your phone longer over the time you own it.

There are a few approaches that have attempted to overcome the issues. The use of nano-sized/structured silicon particles with micron-sized graphene didn’t work — using nano-sized silicon particles dramatically increases the amount of reactive surface available. This leads to more lithium being deposited on the silicon during the first charge cycle. This will form a solid-electrolyte interphase barrier between the silicon and the electrolyte which reduces the lithium inventory and battery’s lifetime. The layer continues to grow on the silicon, which means the lithium loss is continuous. Other methods have attempted to incorporate other materials but they are impractical.

But new research, led by Dr. Melanie Loveridge in WMG at the University of Warwick, has discovered a new anode mixture of silicon and a form of chemically modified graphene, which could resolve these issues. This could also create viable silicon anode lithium-ion batteries. This type of approach could be practically manufactured on an industrial scale without the need to have nanosizing of silicon.

Graphene is a single atom thick layer of graphite. But it is possible to separate and manipulate a few connected layers of graphene, which results in a material called few-layer graphene (FLG). Other research tested FLG with nano-sized silicon but the new study has found that FLG can improve the performance of micron-sized silicon particles when used in an anode. This mixture could significantly improve the life of lithium-ion batteries and increased power capability.

"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 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," Loveridge said. "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 paper on this research was published in Scientific Reports.

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