Renewable energy sources such as solar, wind, and tide energy have been implemented to decrease air pollution due to common fossil fuel-generated electricity [1]. However, those systems are intermittent; creating the need for an energy storage system (ESS) that stores over-generated energy for later use and effectively matches the power fluctuation generated because of the sporadic demand throughout the day [2]. A possible solution to this problem is to couple renewable sources with rechargeable batteries. The most widespread electrochemical battery in the market is Lithium-ion, owing to its high energy density and lifetime and capability to resist frequent changes in charging-discharging rates [3]. Nevertheless, the current battery industry already requires 50% of the world's available lithium [4]. Foremost, lithium-ion battery is composed of critical metals such as cobalt, nickel, and manganese. The anticipated growing demand for these metals will lead to their scarcity [5]. Therefore, this study aims to develop strategy to enable a sodium-ion battery based on soluble seawater sodium and address the electrochemical and engineering problems.

Seawater batteries have an open cathode compartment that can utilizes Na⁺ infinite source in the ocean as the active material [6]. There are three main components in this open structure seawater battery design. First is the non-aqueous liquid electrolyte facilitating the sodium ions transfer and deposition on the anode compartment [7-8]. Subsequently, the solid-state electrolyte (SSE) enables the flow of sodium ions from the sweater cathode to the anode which is typically copper current collector [9]. Lastly, a current collector that provides reaction sites for cathode reactions that could be made of carbon-based materials, such as carbon paper, carbon felt, or carbon cloth [10].

The Solid-state electrolyte is the component that requires the most attention. It must have high ionic conductivity to increase sodium-ions transfers and maintain good mechanical and physical properties as it represents the interface between cathode and anode, preventing the water from penetrating the anode compartment and short-circuiting the cell.

To increase its ionic conductivity, it is necessary to reduce its thickness as much as possible. Through the palletization and sintering process, a ceramic SSE was fabricated with a thickness of ~ 250 µm and ionic conductivity of 0.62 mS/cm. Subsequently, symmetric cells (Na||SSE||Cu) were assembled to further test the pellet's performance. Cells that were tested under continuous charge/discharge cycling for 360 cycles showed stable charge capacity and high Coulombic efficiency (> 95%). Performance of full cells using seawater at the cathode was also demonstrated. Addressing various issues such as water permeation through the SSE, electrode corrosion, Na deactivation in the anode, and catalytic activity of the carbon cathodes are also investigated.

Figure 1. Charge/discharge profile of a symmetric Na||SSE||Cu cell at a current density of 0.10 mA/cm².

References:

[1] Hussain, Akhtar, et al. “Emerging Renewable and Sustainable Energy Technologies: State of the Art.” Renewable and Sustainable Energy Reviews, Pergamon, 8 Jan. 2017,

[2] CAISO, 2016. Fast Facts: What the Duck Curve Tells Us about Managing a Green Grid.

https://www.caiso.com/Documents/FlexibleResourcesHelpRenewables_FastFacts.pdf

[3] Schmuch, R., Wagner, R., Hörpel, G. et al. Performance and cost of materials for lithium-based rechargeable automotive batteries. Nat Energy 3, 267–278, 2018.

[4] Vaalma, C., Buchholz, D., Weil, M. et al. A cost and resource analysis of sodium-ion batteries. Nat Rev Mater 3, 18013 (2018).

[5] Prior, Timothy, et al. “Sustainable Governance of Scarce Metals: The Case of Lithium.” Science of The Total Environment, Elsevier, 12 June 2013,

[6] Hwang, S. M., Park, J.-S., Kim, Y., Go, W., Han, J., Kim, Y., Kim, Y. “Rechargeable Seawater Batteries—From Concept to Applications” Adv. Mater. 2019, 31, 1804936.

[7] S. Lee, I. Y. Cho, D. Kim, N. K. Park, J. Park, Y. Kim, S. J. Kang, Y. Kim, S. Y. Hong, “Redox-Active Functional Electrolyte for High-Performance Seawater Batteries” ChemSusChem 2020, 13, 2220.

[8] Kim, Y., Kim, G.-T., Jeong, S., Dou, X., Geng, C., Kim, Y., & Passerini, S. (2018, April 26). Large-scale stationary energy storage: Seawater batteries with high rate and reversible performance. Energy Storage Materials.

[9] Wang, Yumei, et al. “Development of Solid-State Electrolytes for Sodium-Ion Battery–A Short Review.” Nano Materials Science, Elsevier, 21 Mar. 2019,

[10] Park, Jehee, et al. “Hybridization of Cathode Electrochemistry in a Rechargeable Seawater Battery: Toward Performance Enhancement.” Journal of Power Sources, Elsevier, 18 Dec. 2019.