Redox-Targeting Semi-Liquid Electrode with Hard Carbon for High-Energy-Density Seawater Batteries
Authors: Kim Dowan , Lee Wang-Geun 
Published: December 2024

Seawater batteries (SWBs) are emerging as the next generation of energy storage systems owing to their high potential energy densities. However, they suffer from low practical energy densities. Utilizing an open-structured cathode that utilizes the unlimited supply of sodium ions from seawater, SWBs are inherently limited in energy density owing to the capacity of the anode. To maximize the amount of active material in a limited anode space, efforts with highly loaded electrodes present challenges in terms of sufficient reversibility, while liquid electrodes have low capacity, requiring alternative methods. To enhance the energy density of the SWB anode, this study used the “redox targeting” method, which has been used to enhance the energy density in redox flow battery systems. Redox targeting was achieved by combining a hard carbon (HC) active material with a sodium biphenyl (Na-BP) redox mediator in a semi-liquid electrode (SLE) form.
Na-BP has a potential of approximately 0.09 V compared to sodium, while hard carbon has a reaction potential ranging from 0 V to 2 V. During the manufacturing process, the sodium source and electrons of Na-BP are transferred to hard carbon spontaneously. Morphological analysis of the HC in the synthesised SLE was carried out using scanning electron microscopy and energy dispersive spectroscopy elemental mapping, which showed that the sodium is uniformly distributed throughout the HC particles. During manufacturing, sodium and electrons were transferred effectively from Na-BP to HC.
Ex-situ high-resolution transmission electron microscopy was used to confirm the structure of the HC of SLE after the charging and discharging process. The d-spacing between the graphite layers in the HC of SLE as-synthesis is approximately 3.7 Å, which is similar to the d-spacing of the pristine HC. However, the d-spacing of the HC after the SLE is fully charged increases to 4.3 Å. Conversely, the d-spacing of the hard carbon in the fully discharged SLE is reduced, similar to its initial as-synthesized state. In general, HC undergoes an intercalation reaction, causing an increase in d-spacing when sodium is stored between the layers of graphite. Sodium is reversibly intercalated and extruded into the HC of the SLE system.
In a seawater battery half-cell system that uses a sodium ferrocyanide aqueous solution as the cathodic redox material, the electrochemical performance of the SLE during galvanostatic charge-discharge was compared with the theoretical voltage of 3.07 V. When operated at a current density of 10 mA gHC-1 (equivalent to 0.55 mA cm-2), the SLE showed a charging voltage of 3.2 V and a discharging voltage of 2.9 V. The cycling performance of the SLE was evaluated under various states of charge (SOC) at a current density of 20 mA gHC-1. The SLE demonstrated reversibility for 20 cycles with a capacity equivalent to 33.3 mAh cm-2 at an SOC of 100% and 75 cycles with a capacity equivalent to 22.2 mAh cm-2 at an SOC of 66.6%. The Coulombic efficiency was over 92.5%, and the energy efficiency was approximately 80%. Notably, the reversible capacity of 11.1 mAh cm-2 with a SOC of 33.3% was maintained for over 500 cycles, indicating the potential for over 5000 h of operation.
After evaluating the electrochemical performance of the SWB half-cell system (using the sodium ferrocyanide catholyte), the SLE was applied to a full-cell system of the SWB via the oxygen evolution/reduction reaction (OER/ORR), which utilizes dissolved oxygen from natural seawater. At a current density of 10 mA gHC-1, the HCBP-SLE exhibited a charging voltage ranging from 3 V to 3.6 V and a discharging voltage ranging from 3.2 V to 2.5 V. The electrochemical performance was evaluated at a capacity of 33.3 mAh cm-2, which corresponds to an SOC of 100%, and exceeded 98.2% Coulombic efficiency and approximately 80% energy efficiency over 15 cycles. Therefore, it was confirmed that reversible operation with a high areal capacity is feasible even under natural seawater conditions.
A semi-liquid electrode was produced utilizing an HC active material and an Na-BP redox mediator and then applied to seawater batteries. The semi-liquid electrode approach utilized a redox mediator to facilitate charge transfer, allowing energy storage in the active solid material. When applied to a SWB half-cell system, reversible operation for 20 cycles at an areal capacity of 33.3 mAh cm-2 and stable operation for more than 500 cycles (5000 h) at an areal capacity of 11.1 mAh cm-2 were demonstrated. These results can be employed to achieve high energy densities not only in seawater batteries but also in other battery systems, such as redox flow batteries or air batteries.

Figure 1