Development of High-Performance Lithium-Ion Capacitor Using Reduced Nitrogen-Doped Graphene as Capacitor-Type Cathode Material Exhibiting Anomaly Anion Storage Mechanism
Authors: Ishikawa Jumpei , Soeda Kazunari , Okuda Daisuke , Ishikawa Masashi 
Published: December 2024

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 LiPF6 / 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 LiPF6 / 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 intercalation mechanism undergoes expansion and shrinking of the interlayer distance of carbon as anions are inserted and removed, whereas the N-rGO cathode goes through expanding the interlayer distance of carbon due to anion insertion at the initial charging, and then the expanded interlayer distance is maintained during subsequent charge-discharge cycles. Morphology difference between these materials may have altered the anion storage mechanism. The microstructure within the interlayer was investigated using XANES. Details will be explained in this poster presentation. Constant current charge-discharge tests were conducted to investigate the performance of LIC with this N-rGO as a capacitor-type cathode. With the previous LIC, we pre-doped Li in the graphite anode and constructed a full cell. The specific capacity was almost the same as the N-rGO/Li half cell, but the capacity retention was poor and the capacity was significantly reduced after 10 cycles. We speculated that this was due to a plateau in the charge curve even after the initial charge, and that the morphological change due to sufficient anion insertion had not occurred. Therefore, in addition to pre-doping Li into the graphite anode, we pre-cycled the N-rGO cathode as a single cycle prior to constructing full cell. As a result, the plateau in the charge process almost disappeared, and the specific capacity and high capacity-retention were almost the same as those of the N-rGO/Li half-cell, and long-term cycling was achieved.
Reference
[1] J. Ishikawa et al., The 64th Battery Symposium Japan, 1F01 (2023).
[2] M. Zhang et al., Energy Stor. Mater., 16 (2019) 65-84.