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

Synthesis and Investigation of the Effect of Substitution on the Structure, Physical Properties, and Electrochemical Properties of Anthracenodifuran Derivatives

A series of <i>syn</i>/<i>anti</i> mixtures of anthradifuran (ADF) and substituent compounds were systematically synthesized, and the effect of substitution at the 5,11-positions on the neutral and radical states of ADF was investigated. All compounds were measured and analyzed by absorption and fluorescence spectroscopy, cyclic voltammetry, electrochemical absorption spectroscopy, and DFT calculations. The absorption spectra of 5,11-substituent compounds in their neutral state were red-shifted. In addition, the substituted compounds exhibited increased thermal stability with respect to the parent <b>1a</b> because of elongation of the π-conjugation and an increased steric hindrance effect due to the bulky ethynyl substituent groups. The cyclic voltammograms of all of the compounds exhibited irreversible reduction potentials and irreversible oxidation potentials, except in the case of (trimethylsilyl)­silylethynyl-substituted ADF. When the materials were subjected to oxidation/reduction potentials, the radical cation and anion species were generated. The absorption spectra of the radical-cation species of the compounds exhibited similar characteristics and similar absorption ranges (550–1400 nm), whereas the spectra of the radical anion species were blue-shifted (550–850 nm) compared than that of the parent <b>1a</b>^{<b>•–</b>} (550–1100 nm). The DFT computation results suggested that the radical states of lowest energy transitions occurred primarily from π to π_SOMO or from π_SOMO to π*