The seawater battery is a rechargeable electrochemical energy storage system that uses sodium ions dissolved in natural seawater as the active catholyte, separated from the negative electrode by a sodium super-ionic conductor (NASICON) solid ceramic electrolyte membrane. The concept was originally proposed and developed primarily by researchers at POSTECH (South Korea) from around 2014 onwards, and interest has grown significantly for marine, subsea, and coastal energy storage applications.
Overview
In a seawater battery, the positive side of the cell is open to the surrounding seawater, which acts as both the electrolyte and the source of sodium ions for the cathode reaction. The NASICON ceramic membrane selectively transports Na⁺ ions between the seawater side and the internal (negative) side of the cell, while blocking water, chloride ions, and other seawater species. The negative electrode (anode during discharge) is typically hard carbon (for Na⁺ intercalation) or sodium metal.
Electrochemistry
During discharge:
Positive (seawater side): 2Na⁺ + 2e⁻ → 2Na (or Na⁺ insertion into a carbon cathode framework; or O₂ reduction in open-cell designs)
Negative (internal side): Na → Na⁺ + e⁻ (sodium metal) or CₙNa → C + Na⁺ + e⁻ (hard carbon)
The NASICON membrane (typical composition Na₁₊ₓZr₂SiₓP₃₋ₓO₁₂, 0 ≤ x ≤ 3) provides selective Na⁺ conductivity of ~10⁻³ S cm⁻¹ at room temperature while maintaining electrochemical stability in the harsh seawater environment. The open-circuit voltage is approximately 3.0–3.5 V.
Key Performance Characteristics
- Open-circuit voltage: ~3.0–3.5 V
- Specific capacity (first cycle): ~120 mAh g⁻¹ (cathode active material)
- Coulombic efficiency: >91% after initial cycles
- NASICON conductivity: ~10⁻³ S cm⁻¹ at 25°C
- Technology readiness level: TRL 3–4 (laboratory demonstration)
Advantages
- Effectively unlimited catholyte: seawater provides an inexhaustible supply of Na⁺ and water, eliminating the need for a catholyte tank
- Low-cost electrolyte: no catholyte chemicals to procure, store, or manage
- Sodium is the sixth most abundant element on Earth; no critical mineral supply chain for the active ion
- Potential for multifunctional operation: seawater batteries have been proposed for simultaneous energy storage, desalination, and resource extraction (e.g., lithium, magnesium from seawater)
- Environmentally benign: no toxic liquid electrolytes; seawater is the working medium
Limitations
- NASICON membrane durability: long-term stability in biofouling seawater environments is a critical unresolved challenge
- Sodium metal anode: dendrite formation, reactivity with moisture, and handling safety
- Low ionic conductivity of NASICON limits power density and charge/discharge rates
- Membrane fabrication cost and reproducibility at scale remain engineering challenges
- Biofouling of the open seawater electrode degrades performance over time
- Early-stage technology: no commercial systems; predominantly laboratory-scale studies
Applications
Targeted applications include subsea and marine energy storage (AUVs, underwater infrastructure, offshore renewables), coastal stationary storage where seawater access is direct, and multifunctional systems combining energy storage with water desalination. The technology is not suited to terrestrial or mobile applications due to its dependence on seawater access.
References
- Kim, Y. et al. “Na ion-Conducting Ceramic as Solid Electrolyte for Rechargeable Seawater Batteries.” Electrochimica Acta, 2016. doi:10.1016/j.electacta.2016.01.193
- Jang, M. et al. “Development and Implementation of Sodium-Ion Conductive NASICON Ceramic Electrolytes for Seawater-Based Electrochemical Devices.” International Journal of Energy Research, 2026. doi:10.1155/er/5428960
- Kim, D. et al. “Seawater to resource technologies with NASICON solid electrolyte: a review.” Frontiers in Batteries and Electrochemistry, 2023. doi:10.3389/fbael.2023.1301806
- Lee, M.H. et al. “Seawater batteries for energy storage, desalination and carbon sequestration.” Nature Reviews Clean Technology, 2026. doi:10.1038/s44359-026-00158-1
- Kim, S.H. et al. “Assessing the Efficacy of Seawater Batteries Using NASICON Solid Electrolyte.” Applied Sciences, 2025. doi:10.3390/app15073469