(Invited) Anion-Intercalated Cathode Concept for Fast and Stable Cycling of LIC and Batteries
Authors: Ishikawa Masashi , Ishikawa Jumpei , Yoshie Masaru , Okuda Daisuke 
Published: December 2024

Behavior of Graphene-like Graphite Cathode in Ionic Liquid Electrolytes

Research and development of energy storage devices with higher energy densities than those of ordinary supercapacitors is underway. In the field of rechargeable batteries, a rechargeable battery that utilizes anion intercalation between graphite layers for charge compensation is being considered as one of the post-lithium ion batteries1). Anion intercalation is the insertion and desorption of anions between the layers of cathode materials during charging and discharging. The wider the interlayer of the cathode material, the greater the amount of anion insertion, leading to higher battery capacity. Generally, graphite is used as the cathode material for secondary batteries that utilize anion insertion. However, graphite has narrow interlayer spaces, making it impossible to increase intercalation capacity. Therefore, the use of graphene-like graphite (GLG), which has been reported to have wider interlayer space than graphite, is expected to provide high-speed operating performance and high capacity equivalent to that of ordinary capacitors2). In a previous study by the presenter, a two-electrode cell was constructed with GLG as the working electrode, Li metal as the counter electrode, and 1.0M LiFSI / Pyr13FSI ionic liquid electrolyte as the electrolyte, and a high discharge capacity of approximately 200mAh g-1 was demonstrated3).

In this study, we attempted to examine and develop a LIC using GLG as a cathode material. Since both the cathode and anode are carbon materials, the LIC is lighter, safer, and more environmentally friendly than conventional LIBs. In addition, the operating voltage per cell is high due to the large redox potential difference. Therefore, the energy density of the LIC can be expected to be comparable to that of a conventional LIB using transition metal oxides for the cathode. However, the reversible capacity of graphite as a cathode material depends on the type of anion and is lower than that of conventional LIB cathodes with transition metal oxides4). In addition, the high redox potential may cause undesirable side reactions such as oxidative decomposition of the electrolyte, so an electrolyte with oxidation resistance is required. Therefore, we focused on GLG as a cathode material to replace graphite and ionic liquid electrolyte as an oxidation-resistant electrolyte. LICs were fabricated using GLG as the cathode, graphite as the anode, and LiFSI/Pyr13FSI ionic liquid electrolyte as the electrolyte. The results showed that the initial discharge capacity and electrolytic efficiency in the first cycle were lower than those of the GLG cathode-Li half cell. Therefore, we analyzed the structure of each electrode and conducted charge-discharge tests under various conditions to investigate the improvement of discharge capacity and initial coulombic efficiency.

Development of LICs using Nitrogen-Doped Graphene

Conventional dual carbon energy storage devices use graphite with narrow interlayer space as the cathode active material, and the capacity was limited by the reduced utilization of the graphene layer due to the insertion of anions5) . To solve this problem, reduced nitrogen-doped graphene (N-rGO), in which nitrogen is doped into graphene with wider interlayer space than graphite, was applied as cathode active material in LIC and dual carbon batteries. The results showed that N-rGO exhibited higher specific capacitance than graphite. In general, a plateau region originating from the step structure of graphite was observed in the charge-discharge curve of anion insertion-desorption reaction using graphite as the cathode active material6), but no plateau region originating from the step structure was observed in the charge-discharge curve using N-rGO as the cathode active material. This result suggests that the anion insertion/desorption reaction of N-rGO has different reactivity between cathode active materials.

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