The team at the Warwick Manufacturing Group (WMG) found that replacing graphite in the anodes of lithium-ion batteries using silicon with graphene girders also increased the energy capacity.
Researchers and manufacturers have long sought a way to replace graphite with silicon, as it is easily available and provides ten times the energy density, but has a number of problems. The volume expansion of the silicon particles impede further charge-discharge efficiency over time and the material is is elastic enough to cope with the strain of lithiation when it is repeatedly charged, leading to cracking, pulverisation and a low lifecycle.
Dr Melanie Loveridge from the WMG found that a mixture of silicon and a form of chemically modified graphene which could resolve these issues and create a viable silicon anode in a lithium-ion batteries. This could be practically manufactured on an industrial scale and without the need to use nano-sized silicon particles.
Separating and manipulating a few connected layers of graphene gave the researchers a few-layer graphene (FLG) material. The study has found that FLG can dramatically improve the performance of larger micron-sized silicon particles when used in an anode.
The researchers created anodes that were a mixture of 60% micro silicon particles, 16% FLG, 14% Sodium/Polyacrylic acid, and 10% carbon additives, and then examined the performance (and the changes in 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,” said Dr Loveridge, Senior Research Fellow at the WMG. “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. Each battery charge cycle increases the chance that silicon particles become electrochemically welded to each other,” she said. “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.”
The team has already started using this for a two year project led by Varta Micro-innovations, along with Cambridge University, CIC, Lithops and IIT (Italian Institute of Technology) into production of silicon/graphene composites and processing into lithium-ion batteries for high-energy and high-power applications.