The use of graphite as an anode material is behind many of the limitations to today’s lithium-ion batteries. Though a reliable material, graphite’s comparatively low theoretical capacity, and tendency to lose this capacity at high current densities, are two factors preventing major improvements to the capacity and performance of commercial lithium-ion batteries.
Silicon is among a few materials vying for researchers' attention as a new anode material, and with around ten times the theoretical capacity of graphite, it certainly has plenty of potential. During battery cycling, however, the silicon tends to expand and subsequently fall apart, leading to rapid performance loss.
Researchers have discovered various solutions to this, including making the silicon porous to begin with, or working with a ‘scaffold’ structure of carbon nanotubes. At this early stage in the research, however, few have focused on the cost or potential scalability of the processes investigated.
Scientists at Mid Sweden University looked to address this, investigating a new process for silicon anode production where nanometer-sized particles of silicon are coated onto flakes of graphite using an ‘aerogel’ process. With this process, they produced an anode with 455 millamp-hours per gram (mAh/g-1) specific capacity and 97% coulombic efficiency at a current density of 100 milliamps per gram. Details of the process and characterization of this anode are published in Scientific Reports, in the paper Silicon-Nanographite Aerogel-Based Anodes for High Performance Lithium Ion Batteries.
The anode relies on graphite as an additive to boost the conductivity and connect the tiny silicon nanoparticles. At the first cycle, the anode reached a capacity of 1050 mAh/g-1, close to silicon’s theoretical limit. This quickly decreased to 57% of the initial value after 30 cycles, and 52% after 100 cycles. The researchers noted that this represents a major improvement in comparison with results from pristine, milled or heat-treated silicon. At the first cycle, the anodes' specific energy was measured at 787 watt-hours per kilogram (Wh/kg), falling to 341.25 Wh/kG after 200 cycles.
While other approaches have shown stronger results in capacity retention, the researchers here note that these often rely on complex or expensive processes not well suited to large-scale production. “Si nanoparticles were grown on nanographite flakes by aerogel fabrication route from Si powder and nanographite using polyvinyl alcohol (PVA) by a simple, cost-efficient, and scalable method, which does not require expensive equipments for the synthesis,” stated the researchers. “In this study, we show that electrodes prepared based on this structure show high specific capacity and cycling stability, thus being a potentially cost-effective method for Si-based anodes.”
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