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manganese, etc. The specific composition and ratio of the simulated seawater electrolyte can be adjusted and
optimized according to the specific research purposes and experimental requirements. In addition, the
organic electrolytes containing NaCl were also being developed for their application in R-SMABs. This is
because the organic electrolyte possesses good ion conductivity and fewer side reactions for charging and
discharging. Commonly used organic electrolytes include ethylene glycol dimethyl ether (EGDME),
acrylonitrile (AN), and propylene glycol (PG), which have good solubility and high ion conductivity.

Current collector
Although there is no solid electrode in this battery system, a current collector is still needed. It facilitates the
transport of electrons released from the anode part during the deintercalation/dealloying process.
Moreover, the current collector carries cathodic reactions (OER and ORR), which are necessary for related
charge transport. Compared to the typical organic batteries with solid cathodes, current collectors usually
[74]
require special properties, such as stability in saltwater .

In the cathode chamber, the role of the current collector is to support the active materials and collect and
transfer the electrons to the external circuit. In order to effectively promote the sluggish oxygen reaction
kinetics, the current collector on the cathode should possess the advantages of high electronic conductivity,
good electrochemical and mechanical stability, and large specific surface area.

In the anode chamber, the current collector should remain stable and work well with the anode materials
(such as Mg, Al, and their alloys) and the electrolytes in the batteries. During working, the current collector
should ensure the electrons transfer stably from electrolytes to the anode active materials. The commonly
used current collectors are generally porous metal foams, such as nickel foam, copper foil, stainless steel, etc.
Recently, newly developed carbon-based collectors are commonly used in seawater batteries, which could
not only offer catalytic sites for the ORR and OER processes but also ensure an efficient charge transferring
rate [74,75] .

Separator membrane
Sodium-ion-conducting membranes are widely used as separators to separate the anode and cathode
compartments in R-SMABs [Figure 3B] [33,76] . Such membrane materials are commonly used as solid
electrolytes in solid-state batteries. The stability of the membrane in different types of liquid electrolytes
should be stable to ensure the stability of the entire battery in both organic and aqueous solutions, especially
for some SMABs that should work under high current density and voltage conditions. Therefore, the
separator membrane should remain stable in a relatively wide electrochemical potential window to keep the
battery working for long-term utilization. In addition, the solid electrolyte as the separator for seawater
batteries also needs to have high sodium ion conductivity, good mechanical properties, and ultra-low
porosity to avoid electrolyte penetration.

Criteria for evaluating the seawater metal-air battery performance
The SMABs should be assembled into a package before testing, and they generally contain three parts: an
organic anode chamber, a seawater cathode chamber, and a separator. These three parts were sealed to
avoid the electrolyte linkage. The distance between an anode, cathode, and separator should be kept
constant to evaluate the performance of different SMABs. To evaluate the battery performance, the open
circuit voltage, discharging capacity, power density, and other indexes were systematically tested. Typically,
the capacity is one of the most important performance indexes to measure seawater battery performance,
which describes the charge storage capacity of the SMAB (the amount of charge Q that a battery can
provide or store) [Figure 3C], usually expressed as ampere-hour (Ah) and normalized to mass (Ah g ) or
−1
volume (Ah cm ). The value of battery capacity can be influenced by the discharge rate, discharge current,
−3