Kinetically facile intercalation of anions into graphite electrodes is essential for the good performance of dual-ion batteries. The intercalation of BF 4 À anions into graphite electrodes is found to be strongly impeded in the electrolyte of 1.25 m LiBF 4 À ethylmethyl carbonate. For this, both the strong attraction between Li + and BF 4 À clusters and the special solvation state of the anion are to blame. Two strategies are employed to tailor the solvation states of ions in the solutions, which prove easy and effective in facilitating anion intercalation into graphite electrodes.

Anion-intercalated graphite is one of the feasible solutions for high-energy devices. Various characteristics of graphite may influence its overall performance. In this work, three homologous graphites with different particle sizes are selected as positive electrodes to accommodate tetrafluoroborate anion from propylene carbonate, based on activated carbon/graphite capacitors. In situ X-ray diffraction, Raman spectroscopy and electrochemical dilatometry are combined organically to identify the hierarchical storage behavior of anions in graphite electrodes. The graphite with minimal size exhibits a prior anion accumulation in the shallow region before notable expansion in the crystal structure. This extra charge storage leads to a higher capacity at the normal current density (0.1 A g −1 ), however, also drags down the anionic de-intercalation process and causes a significant capacity fade at high discharge rate.