Rechargeable Seawater Batteries: From Electrochemistry to Energy Storage

Rechargeable seawater batteries are emerging as sustainable electrochemical energy‐storage systems that directly exploit natural seawater as an abundant electrolyte and ion reservoir. Beyond conventional seawater‐activated primary batteries and half‐seawater rechargeable systems, fourth‐generation seawater batteries represent a distinct concept in which both electrodes operate directly in natural seawater without nonaqueous anolytes or NASICON‐type ceramic separators. This review clarifies the conceptual boundary between seawater‐based batteries, where seawater mainly serves as a solvent, catholyte, or supporting medium, and genuine rechargeable seawater batteries, where intrinsic seawater ions participate in charge storage and charge compensation. We discuss the fundamental electrochemistry of seawater electrolytes, including ion transport, oxygen/chlorine side reactions, and seawater–electrode interfacial chemistry. Recent advances in cathode materials, including cation‐intercalation hosts and oxygen electrocatalysts, are summarized together with emerging anode materials for single‐ion storage and multi‐ion costorage. Key challenges involving chloride‐induced corrosion, competitive ion storage, interfacial instability, and marine deployment are critically assessed. Future directions are proposed to guide the development of efficient, durable, and scalable rechargeable seawater batteries for marine energy storage.