Fabrication of Si nanopowder from Si swarf and application to high-capacity and low cost Li-ion batteries

Si nanopowder is expected as a high capacity anode active material for Li-ion batteries (LIBs) due to its superior theoretical capacity of ∼3600 mA h/g, which is approximately one order of magnitude higher than that of conventional graphitic materials. We have succeeded in fabrication of Si nanopowder from Si swarf using the cost-effective beads milling method. The Si nanopowder consists of Si nano-crystals having flake-like shape with two largely different sizes, i.e., several hundred nm sizes and sizes less than 15 nm. The mode diameter of Si crystallites fabricated from Si swarf is 6.3 nm. The performance of the nano-Si electrode for Li ion batteries is greatly improved by carbon-coating on Si nanomaterials and addition of fluoroethylene carbonate (FEC) in an electrolyte solution. By addition of 10-15 wt% FEC, a stable and thin (10-20 nm) solid-electrolyte interphase (SEI) layer is formed on the electrode, and the number of cracks on the electrode surfaces after the 100th cycle greatly decreases. Addition of FEC improves the cyclability and decreases the SEI resistance (RSEI) and the charge transfer resistance (Rct). However, excess FEC (25 wt%) suppresses free volume expansion of Si nanopowder due to formation of thick SEI, leading to smooth Si surfaces, and resulting in an increase in Rct. Carbon-coating on Si nanopowder greatly improves the cyclability even with the low C/Si ratio of 0.1. In the case of excess carbon-coating, Si nanopowder agglomerates during carbon-coating, which causes peeling-off of a part of carbon-coated Si nanopowder during lithiation, resulting in a decrease of the capacity. The delithiation capacity higher than 1600 mA h/g after the 100th cycle with the current density of 1800 mA/g is achieved by setting the C/Si ratio at 0.1 and the FEC concentration at 10 wt%.