Na-ion capacitor using sodium pre-doped hard carbon and activated carbon

We assembled a sodium-ion capacitor (Na-IC) by combining sodium pre-doped hard carbon (HC) as the negative- and activated carbon (AC) as the positive-electrode. The electrochemical properties were compared with two lithium-ion capacitors (Li-ICs) in which the negative electrodes were prepared with Li pre-doped HC and mesocarbon microbeads (MCMB). The positive and negative electrodes were prepared using the established doctor blade method. The negative electrodes were galvanostatically pre-doped with Na or Li to 80% of the full capacity of carbons. The potential of the negative electrodes after pre-doping was around 0.0 V vs. Na/Na+ or Li/Li+, which resulted in the higher output potential difference of the Na-IC and Li-ICs than that of the conventional electrochemical double-layer capacitors (EDLCs) because AC positive electrode works in the same principle both in the ion capacitors and in the EDLC. The state-of-charge of the negative electrode varied 80 ± 10% during the electrochemical charging and discharging. The capacity of the cell was evaluated using galvanostatic charge–discharge measurement. At the discharge current density of 10 mA cm−2, the Na-IC maintained 70% of the capacity that obtained at the current density of 0.5 mA cm−2, which was comparable to the Li-ICs. At 50 mA cm−2, the capacities of the Li-IC(MCMB) and the Na-IC dropped to 20% whereas the Li-IC(HC) retained 30% of the capacity observed at 0.5 mA cm−2. The capacities of the Na-IC and Li-ICs decreased by 9% and 3%, respectively, after 1000 cycles of charging and discharging.