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Page 24 of 30 Guo et al. Microstructures 2023;3:2023038 https://dx.doi.org/10.20517/microstructures.2023.30

To investigate the growth of sodium dendrites on the surface of sodium electrodes, it is common to use
ex-situ digital photos or SEM images to compare the changes in the surface of the negative electrode before
and after reactions. For example, Kim et al. effectively alleviated the problem of sodium dendrite formation
[131]
by designing a 1M NaBH /ether-based (glyme) electrolytes (NaBH /DEGDME and NaBH /TEGDME) .
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For control experiments, 1.0 M NaClO in EC/PC + 5 wt% fluoroethylene carbonate (FEC) and 1M NaOTf
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in glyme electrolytes (denoted as NaClO EC/PC + 5 wt% FEC and NaOTf/TEGDME) as reference
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electrolytes. As shown in Figure 15. NaClO in carbonate solvents containing + 5 wt% FEC was rapidly
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decomposed upon reaction with sodium to form a solid-electrolyte interphase layer. This layer exhibited
inhomogeneity, which facilitated dendrite growth. Therefore, porous and dendritic surfaces of sodium
metal could be observed through SEM images and digital photos. [Figure 15A-C]. Although the sodium
surface cycled by NaOTf/TEGDME is smoother and less porous than that cycled by NaClO EC/PC + 5 wt%
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FEC electrolytes, a wrinkled surface with uneven local thickness was obtained [Figure 15D-F]. By contrast, it
is observed that NaBH -based (glyme) electrolytes had a non-dendritic dense surface morphology, which
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proved that NaBH /glyme can be used as an effective strategy for ‘solid-electrolyte interphase layer
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reconstruction’ on sodium metals [Figure 15G-I].
PRACTICAL APPLICATIONS
The SMABs have promising applications in various fields, such as environmental detection, channel
measuring, military operations, etc. [132,133] . The discharging capacity of the package of SMABs can be
controlled from small-scale (< 1 kWh) to medium-scale (1-10 kWh), even to large-scale (> 1 MWh) by
connecting each unit pack together. For practical applications, for instance, the small-scale SMABs can be
used in light buoys and detectors. Light buoys are floating devices that can be used for marking navigational
channels, indicating hazards, or serving as reference points for marine activities. These buoys are designed
to be easily deployed and controlled. They are often connected to a range of devices, such as lights,
reflectors, and sensors, to provide visual and navigational information to vessels. The seawater battery in the
light buoy can be charged by solar cells in the sunlight, thus providing more electricity to the LED lights.

The applications of medium-scale SMABs are aimed at providing electricity for various small autonomous
machines, such as maritime exploration robots and drones. They are typically equipped with various
sensors and cameras to monitor and search for targets in seawater, such as drifting vessels or people who
have fallen overboard. These robots and drones can keep long-duration for the undersea operations using
the seawater batteries. The high energy density and long lifespan of seawater batteries are being considered
as ideal energy sources for maritime search and rescue robots or drones. By retrofitting and modifying
existing robots or drones and integrating seawater batteries into their design, their endurance and
operational efficiency can be enhanced, thereby better supporting the maritime search and rescue tusks.
Moreover, there is great potential for the large-scale SMABs with energy capacities exceeding 1 MWh.
These energy storage systems play a crucial role in providing electricity, stabilizing the grid, and integrating
renewable energy sources into the power system. They can be utilized in various equipment, such as power
plants, utility-scale renewable energy projects, industrial facilities, and large commercial buildings.

CONCLUSION AND OUTLOOK
SMABs directly utilize seawater as the electrolyte to achieve high energy density, long-term stability, and
distributed, in-situ power supply systems, which possess broad application prospects. However, the
application of OER/ORR catalysts as air cathodes is different from traditional alkaline metal-air batteries
-
due to the influence of high content of Cl in seawater on the electrocatalytic processes. To improve the
intrinsic OER/ORR electrocatalytic activity and stability, various electrocatalyst materials have been
developed, such as noble metal-based, non-noble metal-based, and metal-free electrocatalysts. Some of these