A new energy conversion and storage system, named ‘seawater battery’, has recently been introduced in electrochemical academia. The recently introduced seawater battery concept is an eco-friendly energy storage system that offers appealing electrochemical performance. Its radically innovative design, compared to conventional lithium-ion batteries, makes use of seawater as an almost infinite sodium reservoir for the positive electrode (catholyte) and, thereby, avoids the use of expensive, scarce, and toxic elements like nickel and cobalt.

Herein, we report a phosphorus/carbon composite anode material which was successfully employed as the anode in seawater batteries, showing good cycling stability with high reversible capacities exceeding 920 mAh g^-1_composite with coulombic efficiencies over 92% for 80 cycles and good rate capabilities. These results show that the seawater battery system would be a new way to overcome the limitations caused by the use of high-capacity alloying reaction-based anodes in existing batteries with a closed system. The full cell system configuration problem caused by low coulombic efficiencies using alloying reaction anode materials were solved through the seawater batteries by supplying unlimited Na-ions. Therefore, these results represent an important step toward practical application of the sodium-based seawater battery technology.