context: Electrochemical impedance spectroscopy (EIS) curves are presented in Fig. S8.† A larger slope, which is associated with the Warburg impedance (Ws), was observed in the low-frequency region of the curve of the Fe-intercalated ML Ti3C2Tx electrode. Such an occurrence is related to the migration of electrolyte ions within the electrode. It is inferred that the faster kinetics of EMIm+ were derived from the increased space caused by Fe pre-intercalation. Ex situ Ti 2p and Fe 2p XPS spectra are presented in Fig. S9;† no obvious shifts were observed. This finding indicates that the charge storage mechanism of the Fe pre-intercalated ML Ti3C2Tx anode is based on the intercalation/de-intercalation behavior of EMIm+ instead of a redox process. Therefore, the larger real interlayer space contributed to the enhanced electrochemical performance. The scheme in Fig. 5d shows the charge storage mechanism of this DIB. Upon charging, EMIm+ cations were inserted into Fe pre-intercalated ML Ti3C2Tx, whereas PF6− anions were intercalated into the graphite cathode. Charge–discharge profiles of the full DIB are presented in Fig. S10†. The DIB provided an energy density of 76 W h kg−1 at a power density of 360 W kg−1. The cycling stabilities and capacity retentions of the electrodes were examined via performing GCD tests at the same current density. After 50 cycles, the DIB retained 94% of its initial capacity (Fig. 5e), indicating its improved stability compared with a DIB using the ML Ti3C2Tx anode (71%). Ragone plots are presented in the inset of Fig. 5e. The full DIB displayed a good rate capacity, with energy densities of 76, 70, and 64 W h kg−1 at power densities of 360, 900, and 1800 W kg−1, respectively. However, when the power density was higher than 3600 W kg−1, the energy density was reduced. The dual-ion intercalation mechanism of this DIB provides a new route for multi-ion storage.
answer: graphite 