Si Swarf Wrapped by Graphite Sheets for Li-Ion Battery Electrodes with Improved Overvoltage and Cyclability

Composites of flake-shaped Si nanopowder from swarf treated as an industrial waste and ultrathin graphite sheets (GSs) (Si:C = 5:1 wt) are used in Li-ion battery electrodes. Si nanopowder is dispersed and wrapped between GSs fabricated from expanded graphite. The delithiation capacity of the Si/GS composite electrode during 300 cycles is 1.69 ∼ 0.83 mAh cm−2 (0.5 C), while that of the electrode with C-coated Si nanopowder (Si:C = 10:1 wt) fabricated in C2H4 is 1.55 ∼ 0.72 mAh cm−2. The series resistances (Rs) for the Si/GS electrode are a half and two-thirds of those for the C-coated Si electrode at the 6th and 300th cycles, respectively. The charge transfer resistance (Rct) for the Si/GS electrode is two-thirds of that for the C-coated Si electrode at the 300th cycle. GS bridges are formed across cracks, and suppress cracking and peeling-off of Si. Agglomerated GSs wrap Si/GS composites, and work as stable frameworks that secure electrolyte paths and buffer spaces for Si volume change. In the C-coated Si electrodes, Si frameworks fuse after the 300th cycle, leading to low delithiation capacities. The delithiation capacity of 4 mAh cm−2 for more than 75 cycles is achieved by the Si/GS electrode at the current density of 5 mA cm−2 with delithiation limitation at 1200 mAh g−1.