Lithium-ion batteries (LIB) are widely used in various mobile electronic devices. Concerns over global warming and climate change have recently increased the demand for LIBs for electric vehicles and smoothing solar photovoltaic power. Silicon (Si) has been studied as an active material due to its high theoretical capacity — 3578 mAh/g, which is about ten times higher than graphite (372 mAh/g).
Recently, a team of researchers from Osaka University used silicon nanopowder in the form of flakes wrapped in ultrathin graphite sheets (GS) to make high-capacity LIB electrodes in terms of area and current density.
Silicon chips, which are considered industrial waste, are typically produced at a rate of 100,000 tons per year globally. They come from silica, which is processed into silicon ingots at temperatures of 1000–1800 °C. Water-based coolants and wire saws with fixed abrasive grains can help use silicon chips as an anode-active material with high capacity at a reduced cost.
Nanocarbon materials were applied to silicon electrodes to improve electrical conductivity and cyclability. Many strategies have already been demonstrated for working with large volume changes of silicon electrodes, but they are often associated with high costs. However, silicon-based electrodes do not meet all the performance requirements, such as cost reduction, material and process sustainability, and a circular economy.
"In this study, the Si/graphite sheet composite made from Si chips and expanded graphite utilizes reduced costs and thermal budgets. The Si nanopowder disperses and wraps between GS made from expanded graphite," explains the first author, Jaeyoung Choi. "GS bridges form through cracks and suppress cracking and delamination of Si. Agglomerated GS wraps the Si/GS composites and acts as stable scaffolds that protect electrolyte pathways and provide buffer spaces for the volume change of Si."
The Si/GS composite structure and delamination constraints improve cyclicity to 901 cycles at 1200 mAh/g. The delamination capability area and current density of Si/GS electrodes linearly increase to 4 mAh/cm² and 5 mA/cm², respectively, under mass loading over more than 75 cycles, while thick C-coated Si electrodes made from C₂H₄ are not competitive.
"High-capacity silicon anode batteries with high current density have the potential to be used in electric vehicles. This potential, combined with the increasing formation of silicon chips as industrial waste, will allow our work to contribute to reducing greenhouse gas emissions and achieving the SDGs," says corresponding author Taketoshi Matsumoto.