Visualizing Na Metal Plating and Solid Electrolyte Failure in Seawater Batteries

Among the various types of batteries, the rechargeable seawater battery (SWB) has emerged as a promising alternative to lithium-ion batteries due to its limitless capacity, cost-effectiveness, and thermal stability, all facilitated by the abundant seawater cathode. To take advantage of the extremely high cathode capacity, SWBs consider using sodium (Na) metal as an anode material, which offers superior specific capacity and the lowest potential level. However, metal plating at high rates can lead to the formation of metal dendrites, resulting in the failure of the ceramic NASICON solid electrolyte and eventually causing a short circuit in the SWB.<br/>In previous research on seawater batteries, several strategies have been proposed to suppress metal dendrites. Firstly, using liquid electrolytes facilitates contact between the metal and solid electrolyte. This prevents the localized increase in current, which is common with poor solid/solid contacts, thereby effectively inhibiting the growth of metal dendrites.<br/>Secondly, employing anolyte (NaBP) which has an intrinsic redox potential is another approach. This anolyte participates directly in the redox reactions, suppressing side reactions and SEI layer formation, which in turn prevents localized metal growth. Additionally, the high electronic conductivity of NaBP allows the dissolution of dead metal separated from the electrode during the discharge process, improving the Coulombic efficiency and reversibility of the metal anode.<br/>Although these strategies are effective in suppressing metal dendrites, the problem of solid electrolytes breaking due to metal dendrites still occurs. To further suppress this issue, it is necessary to understand the metal dendrite features in seawater batteries. Particularly on the anode side of seawater batteries, multiple interfaces exist, and the appropriate strategy may vary depending on the interface causing the problem.<br/>In this study, the metal dendrites of seawater batteries (SWB) are primarily investigated using X-ray computed tomography (XCT) visualization, which can directly reveal the failure state. The results show that metal plating occurs at the NASICON/liquid electrolyte interface, and high-current plating leads to uneven metal accumulation. These plating phenomena are discussed in the context of the charge transfer pathway in SWBs, particularly focusing on the role of the electro-conductive liquid anolyte. The principles of mechanical failure in NASICON solid electrolytes are also described, referencing prior research on all-solid-state batteries. Consequently, we suggest a potential solution to prevent dendrite failure at high current densities, which primarily targets the release of metal accumulation on the NASICON surface.