Development of High-Performance Lithium-Ion Capacitor Using Reduced Nitrogen-Doped Graphene as Capacitor-Type Cathode Material Exhibiting Anomaly Anion Storage Mechanism

1.Introduction Lithium-ion capacitor (LIC) is a capacitor with improved energy density by applying a battery-type active material capable of lithium storage as the anode material. However, a typical cathode of LIC applies a capacitor-type active material that adsorbs anions, limiting its specific capacity as LIC. Therefore, we synthesized and applied reduced nitrogen-doped graphene (N-rGO) as a new cathode active material to achieve high energy density in LIC. The N-rGO cathode exhibits capacitance via an anion intercalation mechanism, a faradaic reaction, and initial charge-discharge characteristics in the N-rGO/Li half-cell showed a high specific capacitance exceeding 160 mAh g -1 in a 2.0 - 4.8 V voltage range and a current density of 150 mA g -1 . We also reported that N-rGO cathode exhibits high rate characteristics due to a unique anion storage mechanism in which the capacity increases proportionally to the voltage after the first charge cycle. [1, 2] This suggests that it is suitable as a new cathode material for LIC. This unconventional anion storage mechanism would be ascribed to a change in the N-rGO morphology due to anion intercalation during the initial charging, but there have been still many unresolved points. In this study, we further analyzed the anion storage mechanism in N-rGO cathode and constructed a LIC device using N-rGO, which exhibits an anomaly anion storage mechanism, and examined the possibility of achieving high capacity. 2.Experimental section N-rGO was synthesized by a hydrothermal reaction of ball-milled graphene oxide (GO) as a N-rGO precursor with urea as a nitrogen source and distilled water as a solvent in an autoclave at 180°C in 12 hours under high pressure conditions. N-rGO and graphite electrodes were prepared by mixing each active material with acetylene black (AB), carboxymethyl cellulose (CMC), and styrene butadiene rubber (SBR) in the weight ratio of 90:5:3:2, respectively. The slurry was coated onto Al and Cu current collector, respectively. Cathode half-cells were assembled with the N-rGO cathode, Li-metal as the counter electrode, and 1.0 mol dm -3 LiPF 6 / EC : DMC = 1 : 1 (by vol.) as the electrolyte to analyze the anion storage mechanism of N-rGO cathode. Full-cells were assembled with the N-rGO cathode, graphite anode, and 1.0 mol dm -3 LiPF 6 / EC : DMC = 1 : 1 (by vol.) as the electrolyte, and its performance as LIC was evaluated. 3.Results and discussion In our previous report, a conventional anion inte