Comparative Studies on Graphite and Hard Carbon Negative Electrodes for Alkali Metal-Ion Batteries Using FSA-Based Ionic Liquids

Abstract

1. Introduction Most of the non-renewable energy resources such as coal and petroleum being used all over the world, are causing serious problems of climate change. Thus, the need of the hour is to reduce the usage of these resources and increase that of renewable ones, which is realized by bringing energy storage devices into picture. Batteries are the potential candidates of energy storage devices. The development of lithium-ion battery (LIB) revolutionized the electronic industry and has impacted the vehicle industry [1]. Although LIBs are predominant choices for many applications in day to day life, the depletion of cobalt and lithium resources is a matter of growing concern. In order to mitigate the concern, we need to find alternatives for LIBs. One of the promising ways to fulfill such demands is to analyze the practical use of other alkali metal-ion batteries, such as Na-ion battery (NIB) and K-ion battery (KIB) [2, 3].In the present paper, we have conducted comparative studies on graphite (Gr) and hard carbon (HC) negative electrodes for alkali metal-ion batteries, i.e., LIBs, NiBs and KIBs, using FSA-based ionic liquids (FSA: bis(fluorosulfonyl)amide). These carbon materials are the most common intercalation-type electrode materials. They are also cheap, easy to prepare, and environmental benign. Moreover, the organic solvent-based electrolytes that are primarily used for batteries are inherently very volatile, flammable and unsafe to use, whereas the ionic liquid electrolytes used in the present study are non-volatile and safe to use. We have focused on the FSA-based ionic liquids because they possess high electrochemical and thermal stability, which is advantageous for the rechargeable batteries operating at higher temperatures as well [4–6]. 2. Experimental The working electrodes (Gr and HC) coated onto a copper foil and a separator were vacuum impregnated with the electrolyte at 333 K overnight before assembling the cells. The mass loading of active materials for all the electrodes was around 3.8–4.2 mg cm−2. The electrolytes used were M[FSA] (M = Li, Na, K)–[C3C1pyrr][FSA] (C3C1pyrr: N-methyl-N-propylpyrrolidinium) ionic liquids. Molar fraction of M[FSA], x(M[FSA]), was 0.20 in all cases of LIB, NIB and KIB. A glass fiber filter (Whatman GF/A, 260 μm t ) was used as the separator, and metallic lithium, sodium and potassium were used as the counter electrodes for LIB, NIB and KIB, respectively. CR2032-type coin cells were fabricated in an argon filled glovebox and the operating temperature was set at 298–333 K. 3. Results and discussion The charge–discharge tests were performed at current rates from 0.1C to 2C (1C = 372 mA g−1) for graphite (Gr) electrodes and 20 mA g−1 to 200 mA g−1 for hard carbon (HC) electrodes.As shown in Fig. 1a, for Li/Gr cells at 333 K, we obtained a discharging capacity of 337 mAh (g-Gr)−1 at 0.2C (74.4 mA g−1) current rate. The cell exhibited good cycleability, retaining a discharging capacity of 348 mAh (g-Gr)‒1 and 99.9% coulombic efficiency even after 4 cycles. For the Na/Gr cell (Fig. 1c), a discharging capacity of only 3 mAh (g-Gr)−1 was obtained, being consistent with the well-known fact that graphite does not form intercalation compounds with sodium.For the Li/HC cell (Fig. 1b), the initial charging and discharging capacities were 447 and 361 mAh (g-HC)−1, respectively, at 20 mA (g-HC)−1 current rate. The coulombic efficiency was 80% for the first cycle. A discharging capacity of 330 mAh (g-HC)−1 was obtained after 5 cycles at 20 mA (g-HC)−1. For the Na/HC cell, as shown in Fig. 1d, the initial charging and discharging capacities were 305 and 263 mAh (g-HC)−1, respectively, at 20 mA (g-HC)−1. The coulombic efficiency was 86% for the first cycle and more than 99% for the rest of the cycles.In the presentation, more detailed results including the ones of KIB will be given. Acknowledgement A part of this study was supported by JSPS KAKENHI grant (JP18K14320). Graphite powder was supplied from SEC Carbon Ltd.