2017 Volume 85 Issue 7 Pages 403-408
Charge and discharge properties of a graphite/SiOx composite electrode were studied over a wide range of charge/discharge rates (1/20 to 5 C) for use in automotive applications. The graphite/SiOx (90/10 by weight) composite electrode gave a high reversible capacity (453 mAh·g−1), and showed a good capacity retention at a low rate of 1/20 C. However, the capacity decreased significantly on cycling at a high rate of 2 C. From the analysis of the charging and discharging processes, it was found that the charging reaction occurs predominantly at SiOx particles initially at higher potentials and then proceeds at graphite particles at lower potentials to be fully charged. This tendency was also supported by a dependence of the activation energy of the charge transfer reaction on the state of charge (SOC) estimated by ac impedance analysis. Because the composite electrode contains only 10% SiOx, the current was excessively concentrated to the SiOx particles at the initial state when charged at high rates. This caused crack formation in SiOx particles, and the resulting contact loss between particles was considered as the reason for the observed poor cycleability at 2 C.