Regular Papers
Sodium-Based Dual Carbon Batteries with Graphene-Like Graphite: Achieving High Reversible Capacity and Stable Cycling
2025 Volume 93 Issue 5 Pages 057002
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2025 Volume 93 Issue 5 Pages 057002
Sodium-based dual carbon batteries (Na-DCBs) are promising next-generation secondary batteries with low environmental impact and minimal resource risk, as they can be constructed without lithium ions, transition metal oxides in the cathode, and copper current collectors in the anode. Our previously reported carbon material, named graphene-like graphite (GLG), exhibits a higher reversible capacity than graphite when used as a cathode active material in lithium-based dual carbon batteries. Additionally, it shows comparable performance to hard carbon as an anode material for sodium-ion batteries. Therefore, in this study, we fabricated a Na-DCB using GLG as both electrodes and evaluated its performance in full-cell configuration. Precycling of the anode facilitated the formation of a stable solid electrolyte interphase (SEI), enabling highly reversible charge–discharge cycles in the full-cell configuration. When the upper cutoff voltage was set to 4.5 V, the maximum capacity reached 139 mAh g−1 based on the mass of the cathode active material. This value largely exceeds previously reported capacities of DCB full cells with graphite cathodes. These results clearly demonstrated the feasibility of constructing high-capacity Na-DCBs using GLG as active materials.
Dual carbon batteries (DCBs) are next-generation secondary batteries in which the cathode reaction of lithium-ion batteries (LIBs) is replaced by anion intercalation/deintercalation into/from layered carbon materials.1–5 Because DCBs do not contain transition metal oxides as active materials, they offer lower environmental impact and strategic advantages in terms of elemental resource utilization. Additionally, unlike transition metal oxides, carbon cathodes do not pose risks such as thermal runaway caused by oxygen release, making DCBs inherently safer than LIBs.
Thus far, graphite has been the most commonly used cathode material for DCBs, with various anions such as PF6−,1–9 BF4−,10 ClO4−,11–14 bis(trifluoromethanesulfonyl)amide (TFSA−),13–21 and bis(fluorosulfonyl)amide (FSA−)20–22 being reported to reversibly intercalate into its interlayers. However, graphite cathodes exhibit high anion intercalation potentials, making the electrolyte solution prone to oxidative decomposition, which significantly limits the choice of the electrolyte solutions. Furthermore, even when charged to high potentials exceeding 5.0 V vs. Li+/Li, the maximum reported capacity remains around 110 mAh g−1, which is relatively low compared to conventional LIB cathode materials.2–22 These limitations highlight the need for carbon materials that enable intercalation at moderate potentials and provide higher capacities.
To address this issue, we have previously proposed graphene-like graphite (GLG) as a promising cathode material for DCBs.23–29 GLG is synthesized through the thermal treatment of graphite oxide (GO), resulting in a carbon material with an interlayer spacing comparable to or slightly larger than that of graphite.30–34 Importantly, GLG retains several weight percent of oxygen atoms in the form of some oxygen-containing functional groups.30,31 These functional groups alter the overall electronic state of GLG, facilitating electron withdrawal from the material and enabling anion intercalation to commence at lower potentials than in graphite.26 As a result, GLG achieves higher capacities at lower potentials. For example, in the case of FSA− anion intercalation, GLG has demonstrated a capacity of 150 mAh g−1 when charged to 4.8 V vs. Li+/Li.24 Additionally, recent studies have reported that oxygen content and interlayer distance of GLGs influences the kinetics of the anion intercalation/deintercalation process.27
In our previous work, we successfully fabricated a DCB with full-cell configuration using GLG as both cathode and anode and lithium-based electrolyte solution, demonstrating excellent electrochemical performance.35 These findings indicated that high-performance DCBs could be realized without relying on transition metal oxides. However, the system still depended on lithium and complete elimination of rare metals was not yet achieved.
Therefore, in this study, we focus on sodium-based dual-ion batteries (Na-DCBs) as an alternative to lithium-based ones. The advantage of Na-DCB is not only that it does not use lithium, but also that it can replace copper, which is used for the anode current collector in lithium systems, with aluminum. This is because sodium ions do not alloy with aluminum, which could be a significant advantage for practical application.36,37 This allows all cell components, including the current collectors, to be composed of abundant light elements with minimal resource constraints. As a result, Na-DCBs can reduce raw material costs and also eliminate the need for recycling rare metals, thereby achieving low-cost and environmentally friendly energy storage throughout the product lifecycle. As for anode active material, we utilized GLG as well in this study. Although GLG is a layered carbon material structurally similar to graphite, it allows for the reversible intercalation and deintercalation of sodium ions, making it a promising anode material for sodium-ion batteries.38–40
Based on the above, Na-DCBs utilizing GLG as both cathode and anode materials hold great promise as next-generation secondary batteries. However, GLG cathodes have not yet been investigated in sodium-based electrolyte systems, and it remains unclear whether performance comparable to that observed in lithium-based systems can be achieved. Therefore, in this study, we first evaluated the charge–discharge characteristics of GLG cathodes in sodium-based electrolytes using half-cell configurations. Furthermore, as an initial step toward the development of full cells, we constructed a cell incorporating GLG as both electrodes and a sodium reference electrode. In addition to monitoring the overall cell voltage during cycling, the electrode potentials of both electrodes relative to the sodium reference electrode were tracked to evaluate their individual electrochemical behaviors in the full-cell configuration.
Graphite oxide (GO) was synthesized using the modified Brodie’s method. The detailed synthesis conditions are described in our previous publications.29–33 The obtained GO powder was thermally treated at 700 °C under vacuum, yielding graphene-like graphite (GLG). The specific conditions for thermal treatment are also reported in our previous studies.29–33 Additionally, the X-ray diffraction (XRD) pattern of the synthesized GLG matched previously reported data, confirming that the material exhibited the same crystallinity and interlayer spacing as in our previous reports.31
For the cathode composite, GLG, acetylene black (AB), and polytetrafluoroethylene (PTFE) were mixed in weight ratio of 90 : 5 : 5 using a mortar and pestle. The mixture was then molded into a thin sheet by uniaxial pressing. For the anode composite, GLG, AB, carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed in weight ratio of 90 : 5 : 3 : 2 using water as the solvent. The resulting slurry was coated onto a Cu foil, dried, and hollowed out before use. Note that copper foil was used as a current collector in this study because the same method was used to fabricate the composite electrode as in the lithium system. In the future, we plan to establish a method for making composite electrodes using aluminum foil for further study toward practical application.
Electrochemical measurements were conducted using these electrode cells. First, the cathode performance of GLG was evaluated with three-electrode half-cell using either sodium-based or lithium-based electrolyte solutions. The working electrode was the GLG cathode composite and the electrolyte was 1 mol dm−3 NaPF6 or LiPF6/ethylene carbonate (EC) + dimethyl carbonate (DMC) (1 : 1 volume ratio, purchased from Kishida Chemical). Sodium metal or lithium metal was used as both counter and reference electrodes. The cutoff potentials were set to 1.7–4.5 V vs. Na+/Na for sodium-based system and 2.0–4.8 V vs. Li+/Li for lithium-based system. The current density was set to 6.75 mA g−1 for both systems.
Next, a three-electrode cell was constructed with GLG composite electrodes as cathodes and anodes and sodium metal as a reference electrode in a full-cell configuration. The cell was assembled using SB10 (EC Frontier Co., Ltd.), with the GLG cathode and anode facing each other, and the sodium metal reference electrode introduced around them. A glass filter (Whatman GF/A) was used as a separator. The weight ratio of active materials was cathode : anode = 1 : 0.77. In our previous studies, it was reported that GLG cathodes and anodes prepared under the same synthesis conditions exhibited capacities of approximately 140 mAh g−1 and 230 mAh g−1, respectively.23,38 This results in a capacity ratio of cathode : anode = 1 : 1.27. However, since the capacity of GLG cathodes can vary depending on the charge-discharge conditions, this ratio should not be considered a strict value. The same sodium-containing electrolyte solution as in the half-cell tests was used. Before charge-discharge cycles in the full-cell configuration, precycling was performed only on the anode. For precycling, the assembled cell was first connected as a two-electrode system, where the GLG anode served as the working electrode and sodium metal as the counter electrode. A constant current charge-discharge cycle was conducted with HJ-SD8 system (Hokuto Denko Corp.), applying a current density of 30 mA g−1 (based on anode active material weight) within a voltage range of 0.05–2.0 V.
After the precycling process, the cell was rewired to function as the three-electrode full cell, with GLG as both cathode and anode, and sodium metal as the reference electrode. Charge–discharge measurements were conducted using an HZ-Pro S12 system (Hokuto Denko Corp.). The cutoff voltage was controlled based on the voltage between cathode and anode, with a lower limit of 0.1 V and an upper limit increased from 3.9 V to 4.5 V by 0.1 V every three cycles. The current density was set to 10 mA g−1 based on the cathode active material weight.
To investigate the effect of cations in electrolyte, the intercalation/deintercalation behaviors of PF6− anions into/from GLG were examined by charge-discharge measurements using lithium-based and sodium-based electrolyte solutions. The results are shown in Fig. 1. The electrode potentials were converted to the standard hydrogen electrode (SHE) using reference values of Na+/Na = −2.714 V vs. SHE and Li+/Li = −3.045 V vs. SHE.41 During the initial charge, a larger overpotential was observed in the sodium-based electrolyte. However, at the second cycle, the charge curves were nearly identical for both electrolytes. The discharge curves also exhibited excellent agreement above 0.5 V, though the sodium-based system showed slightly lower capacity in the lower voltage region. This difference is probably attributed to variations in the ionic conductivity of the electrolytes. Nonetheless, the initial discharge capacities were comparable, with 95.6 mAh g−1 in the lithium-based system and 88.9 mAh g−1 in the sodium-based system, confirming that PF6− anion intercalation/deintercalation proceeded reversibly in the sodium-based system as well. It should be noted that in our previous study using a higher salt concentration of 3 mol dm−3 LiPF6, a reversible capacity of approximately 137 mAh g−1 was achieved at the same upper cutoff potential.23 Although the capacity obtained with the 1 mol dm−3 electrolyte solution in this study was lower than that in the previous case, this can be partly attributed to the reduced rate capability in low-concentration electrolyte solutions, as we recently reported.42 However, as shown later, the cathode capacity increased significantly with cycling in full-cell configuration, indicating that the use of this electrolyte solution did not result in a disadvantage in terms of capacity.
Charge-discharge curves of GLG cathode in 1 mol dm−3 NaPF6/EC+DMC and 1 mol dm−3 LiPF6/EC+DMC.
Next, charge-discharge performance was evaluated in full-cell configuration using GLG as both cathode and anode. Figure 2 shows the precycling results for the GLG anode, where the charge capacity was 630 mAh g−1 and the discharge capacity was 270 mAh g−1. Compared to our previous study on a GLG anode for sodium-ion batteries,38 the irreversible capacity was somewhat larger, and this might be caused by difference in cell configuration. However, the shape of the curves and reversible capacity remained consistent with the report. In our previous study on lithium-based DCBs, it was found that oxidative decomposition products formed at the cathode migrated to the anode, and the products were reduced again, leading to continuous side reactions, which is called cross-talk reaction.35 At the same time, we demonstrated that precycling the anode formed a stable solid electrolyte interphase (SEI), which effectively suppressed the cross-talk reactions.35 The irreversible capacity in the current precycling process suggests that a similar SEI was formed, which would contribute to the suppression of cross-talk reactions. It should be noted that, from a practical manufacturing standpoint, precycling of the anode is not desirable. Therefore, we are currently conducting investigations to identify conditions under which charge-discharge can be carried out without precycling, and the results will be presented in the future.
Charge-discharge curves of GLG anode for precycling.
After the precycling, charge-discharge performance in the full-cell configuration was evaluated. The charge-discharge curves are shown in Fig. 3. In this series of measurements, the cell voltage between the cathode and anode, indicated by the bold solid lines, was used as the cutoff criterion, and the upper cutoff voltage was increased by 0.1 V every three cycles. Meanwhile, by employing a sodium reference electrode, the potentials of both the cathode and anode relative to Na+/Na were simultaneously monitored. These potentials are represented by the dotted and thin solid lines, respectively. Hereafter, the term “voltage” will be used to refer to the voltage between the positive and negative electrodes, and the term “potential” will be used to refer to the potential of the positive and negative electrodes vs. Na+/Na. The charge-discharge cycles were found to be fully reversible across all cycles. We also conducted a control experiment without precycling under the same conditions. In that case, the cathode and anode potentials plateaued, indicating the occurrence of cross-talk reactions. In contrast, when precycling was performed (Figs. 2 and 3), the potential profiles of both electrodes closely resembled those observed in half-cell and precycling tests, confirming that charge-discharge reactions proceeded without cross-talk reactions. Furthermore, as the upper cutoff voltage was increased every three cycles, the reversible capacity increased.
Charge-discharge curves in the full-cell configuration with sodium metal reference electrode. The bold, dotted, and thin lines represent the cell voltage (GLG cathode vs. GLG anode), cathode potential (GLG cathode vs. Na+/Na), and anode potential (GLG anode vs. Na+/Na), respectively. The upper cutoff voltage was increased by 0.1 V every three cycles, namely 3.9 V (a), 4.0 V (b), 4.1 V (c), 4.2 V (d), 4.3 V (e), 4.4 V (f), and 4.5 V (g). The capacity is divided by cathode weight.
Focusing on the cathode potential profiles shown as dotted lines in Fig. 3, when the upper cutoff voltage between cathode and anode was set above 4.2 V (Figs. 3d–3g), the final potential of the cathode was approximately 4.5 V vs. Na+/Na, almost unchanged for all the conditions. On the other hand, the overall capacity increased with cycling. In our previous study on lithium-based systems, we reported a similar trend of capacity increase with increasing cycle number, where capacity in the lower voltage region gradually increased when the cut-off voltage was held constant over multiple cycles.23 This behavior was attributed to the gradual expansion of the interlayer spacing in GLG.23 In the current study, increasing the cycle number led to an increase in capacity, enhancing the overall reversible capacity. As mentioned earlier, the maximum cathode potential remained nearly unchanged even when the upper cutoff voltage was increased. Therefore, the observed capacity increase is attributed not to changes in the upper potential limit, but rather to the increase in cycle number. Examining the anode potential profiles (shown as thin lines), we observed that the anode potential gradually decreased as the upper voltage limit increased, which in turn led to an increase in the reversible capacity of the anode. This behavior contrasts with that of the cathode. As a result, the charge–discharge depths of the cathode and anode became more balanced. Finally, in Fig. 3g, where the cutoff voltage was set to 4.5 V, the anode potential reached ca. 0 V vs. Na+/Na at the end of charging, achieving an optimal balance between the charge depths of both electrodes. At this condition, the maximum reversible capacity reached 139 mAh g−1, comparable to the values reported in our lithium-based half-cell study. By converting the capacity based on the total mass of active materials in both the cathode and anode, the resulting reversible capacity is 78 mAh g−1. Recently, in the rapidly advancing field of sodium-ion battery research, reversible capacities of approximately 100 mAh g−1 per total active material mass have been reported.43 Although the present result did not reach this benchmark, it is noteworthy that such performance has been achieved at this early stage of Na-DCB research. These findings suggest that there is substantial potential for improvement through further optimization of cell fabrication and charge-discharge conditions. In addition, increasing the cutoff voltage led to a slight increase in irreversible capacity. This is presumably due to side reactions such as electrolyte solution decomposition. Therefore, exploration of electrolyte solutions will also be an important subject for future study.
From the above results, it is concluded that even DCBs using sodium ion as the cation can achieve high capacity and stable cycle performance in the full-cell configuration. To date, numerous studies have reported dual carbon batteries using graphite cathodes. However, to the best of our knowledge, no previous study on DCB full cells with various carrier ions has reported such a high capacity. This highlights the strong potential of GLG as an active material for DCB.44–49 Particularly in sodium-based systems, graphite cannot be used as an anode material, making it difficult to develop DCB using the same material for both electrodes. The ability of GLG for both electrodes in sodium-based system makes it highly unique, and we believe it is a promising material that can greatly expand the potential of Na-DCB, a next-generation secondary battery with low environmental impact and resource risk.
In this study, we developed a sodium-based dual carbon battery (Na-DCB) using GLG as both cathode and anode, aiming for a new secondary battery with low environmental impact and resource risk, and evaluated its performance in full-cell configuration. Initial investigations in the cathode half-cell demonstrated that the sodium-based electrolyte solution could achieve a capacity comparable to that of the lithium-based system. The results in full-cell configuration showed that precycling facilitated the formation of an SEI layer on the anode surface, effectively suppressing the cross-talk reaction and enabling reversible charge-discharge cycling. The maximum capacity reached 139 mAh g−1 based on the cathode weight, significantly surpassing the values reported for DCBs using graphite cathodes. These findings highlight the excellent potential of GLG as both cathode and anode material for Na-DCB and suggest a promising pathway toward the practical application of this next-generation secondary battery.
This work was partially supported by Grant-in-Aid for Transformative Research Areas (JP21H05237).
Junichi Inamoto: Conceptualization (Lead), Data curation (Supporting), Formal analysis (Lead), Investigation (Supporting), Methodology (Equal), Project administration (Lead), Writing – original draft (Lead)
Shinsuke Nakano: Data curation (Lead), Investigation (Lead), Methodology (Equal)
Akane Inoo: Supervision (Supporting), Writing – original draft (Supporting)
Yoshiaki Matsuo: Conceptualization (Supporting), Funding acquisition (Lead), Supervision (Lead), Writing – original draft (Supporting)
The authors declare no conflict of interest in the manuscript.
Japan Society for the Promotion of Science: JP21H05237
J. Inamoto, A. Inoo, and Y. Matsuo: ECSJ Active Members
