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 <i>D</i> 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