Unlocking the Potential of Sodium-Based Batteries: The Role of Buffered Melts

Sodium-based batteries hold promise for energy storage due to their abundance and low cost compared to lithium. However, one of the biggest challenges has been the high operating temperatures required for their effective function. If the temperature could be reduced below the melting point of sodium (98°C), the safety and efficiency of these systems could be significantly enhanced. This reduction would eliminate the necessity for expensive and brittle separators that are only effective above 175°C.

One major limitation of traditional molten salts used in sodium cells is the lack of sodium ions within the electrolyte, preventing effective operation. Although claims exist about the electrochemical utility of conventional melts, sodium reduction occurs at a more negative potential compared to the reduction of other cations, such as ethyl-methyl-imidazolium (EMI). This means that sodium cannot be effectively plated from the melts before the EMI cation is reduced, thereby limiting the practicality of these systems.

Researchers Melton et al. have addressed some of these challenges by introducing alkali metal halides, like LiCl or NaCl, into an acidic melt. This addition stabilizes the melt at the neutral point, facilitating improved sodium and lithium plating processes. The halide-buffered melts have shown to be effective in enhancing battery performance while maintaining low interference levels, particularly for lithium and sodium cycling.

To further improve sodium plating efficiency, additional modifications have been made, such as the introduction of protons and other additives. These adjustments help to suppress the undesirable reduction of the EMI cation. By incorporating compounds like hydrochloric acid (HCl), researchers have been able to shift the reduction potential of the EMI cation to a more negative value, allowing for improved sodium deposition.

Significant advancements have been observed in plating efficiencies on electrodes like tungsten and platinum, achieving rates as high as 89%. However, it has been noted that maintaining a specific partial pressure of HCl is crucial to ensure that sodium plating remains effective. This delicate balance highlights the intricate nature of developing sodium-based batteries, where both chemical composition and environmental conditions play vital roles in performance.

By continuing to explore the chemistry of buffered melts and their interactions, researchers are paving the way for safer, more efficient sodium-based battery technologies. With ongoing innovation, the potential for these batteries to compete with established lithium-ion technology is becoming increasingly viable.