Fast Diffusivity of PF6– Anions in Graphitic Carbon for a Dual-Carbon Rechargeable Battery with Superior Rate Property

The diffusivity of PF6– intercalated into graphitic carbon is investigated using the galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS). The chemical diffusion constant D of PF6– is estimated by the GITT and EIS to be around 10–12 cm2/s. The diffusivity of PF6– in graphitic carbon is comparable to or slightly higher than that of Li+ in oxide cathode materials such as LiFePO4 or LiCoO2 for Li ion batteries. The activation energy for PF6– diffusion is also estimated using EIS and found to be around 0.366 eV. The diffusion route of PF6– in graphitic carbon is probed using density functional theory (DFT), and diffusion is found to be slightly easier along the ⟨100⟩ direction than along the ⟨110⟩ direction. The estimated activation energy for PF6– diffusion obtained by DFT calculation was about 0.23 eV, which is lower than that estimated using EIS. However, both the experimentally measured and theoretically calculated low activation barrier values confirm the fast diffusivity of PF6– along the ⟨100⟩ family of directions in graphitic carbon, which is an interesting phenomenon in light of its large ionic size. This result also suggests that a superior rate property can reasonably be achieved in dual-carbon battery applications.