Harvesting energy from natural resources is of significant interest because of their abundance and sustainability. Seawater is the most abundant natural resource on earth, covering two‐thirds of the surface. The rechargeable seawater battery is a new energy storage platform that enables interconversion of electrical energy and chemical energy by tapping into seawater as an infinite medium. Here, an overview of the research and development activities of seawater batteries toward practical applications is presented. Seawater batteries consist of anode and cathode compartments that are separated by a Na‐ion conducting membrane, which allows only Na+ ion transport between the two electrodes. The roles and drawbacks of the three key components, as well as the development concept and operation principles of the batteries on the basis of previous reports are covered. Moreover, the prototype manufacturing lines for mass production and automation, and potential applications, particularly in marine environments are introduced. Highlighting the importance of engineering the cell components, as well as optimizing the system level for a particular application and thereby successful market entry, the key issues to be resolved are discussed, so that the seawater battery can emerge as a promising alternative to existing rechargeable batteries.

Metallic lithium (Li) and sodium (Na) anodes have received great attention as ideal anodes to meet the needs for high energy density batteries due to their highest theoretical capacities. Although many approaches have successfully improved the performances of Li or Na metal anodes, many of these methods are difficult to scale up and thus cannot be applied in the production of batteries in practice. In this work, we introduce nanocrevasses in a carbon fiber scaffold which can facilitate the penetration of molten alkali metal into a carbon scaffold by enhancing its wettability for Li/Na metal. The resulting alkali metal/carbon composites exhibit stable long-term cycling over hundreds of cycles. The facile synthetic method is enabled for scalable production using recycled metal waste. Thus, the addition of nanocrevasses to carbon fiber as a scaffold for alkali metals can generate environmentally friendly and cost-effective composites for practical electrode applications.

New concepts or chemistry is an urgent requirement for rechargeable batteries to achieve a low-cost, user-friendly nature with adequate energy densities and high levels of safety.

Spinel-structured transition metal oxides are promising non-precious-metal electrocatalysts for oxygen electrocatalysis in rechargeable metal-air batteries. We applied porous cobalt manganese oxide (CMO) nanocubes as the cathode electrocatalyst in rechargeable seawater batteries, which are a hybrid-type Na-air battery with an open-structured cathode and a seawater catholyte. The porous CMO nanocubes were synthesized by the pyrolysis of a Prussian blue analogue, Mn[Co(CN)]·nHO, during air-annealing, which generated numerous pores between the final spinel-type CMO nanoparticles. The porous CMO electrocatalyst improved the redox reactions, such as the oxygen evolution/reduction reactions, at the cathode in the seawater batteries. The battery that used CMO displayed a voltage gap of ∼0.53 V, relatively small compared to that of the batteries employing commercial Pt/C (∼0.64 V) and Ir/C (∼0.73 V) nanoparticles and without any catalyst (∼1.05 V) at the initial cycle. This improved performance was due to the large surface area (catalytically active sites) and the high oxidation states of the randomly distributed Co and Mn cations in the CMO. Using a hard carbon anode, the Na-metal-free seawater battery exhibited a good cycle performance with an average discharge voltage of ∼2.7 V and a discharge capacity of ∼190 mAh g during 100 cycles (energy efficiencies of 74-79%).