Graphite has been the default choice of active material for 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, as it is an abundantly available element with ten times the gravimetric energy density of graphite. Unfortunately, silicon has several other performance issues that continue to limit its commercial exploitation.
However 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 and create viable silicon anode lithium-ion batteries.
She said: “The flakes of graphene were mixed throughout the anode and acted like a set of strong, but relatively elastic, girders. These flakes 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.”
After manipulating a few connected layers of graphene, know as few-layer graphene (FLG), Dr Loveridge found that the substance can also dramatically improve the performance of larger micron-sized silicon particles when used in an anode. So much so that this mixture could significantly extend the life of lithium-ion batteries and also offer increased power capability.
Due to its volume expansion upon lithiation, silicon particles can electrochemically agglomerate in ways that impede further 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, pulverisation and rapid physical degradation of the anode’s composite microstructure. This contributes significantly to capacity fade, along with degradation events that occur on the counter electrode – the cathode.
“These FLG flakes can prove very effective at preserving the degree of separation between the silicon particles. This decreases the chance that silicon particles become electrochemically welded to each other. When they do undergo agglomeration, this 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.”