Tin anodes may offer a thermal stability advantage over steel carbon in sodium-ion batteries, new research suggests. The study also shows that electrolyte choice significantly affects how tin behaves as temperatures rise, providing new information for the development of high-energy tin-based batteries.
Professor Lin Ma from the University of North Carolina.
Researchers from the University of North Carolina, the University of California San Diego, Argonne National Laboratory, and Northwestern University, in collaboration with US sodium-ion battery company Peak Energy, compared the behavior of tin, carbon steel, and mixtures of the two under elevated temperatures.
Tin is less reactive than carbon steel
The researchers used acceleration rate calorimetry, a technique that measures when a material begins to generate its own heat as the temperature rises.
This allows researchers to compare the thermal behavior of the charged electrode materials and their interactions with the surrounding electrolyte.
In experiments, fully saturated tin showed greater thermal stability than carbon steel. Tin and carbon steel mixtures showed intermediate behaviour, with thermal stability increasing with increasing tin proportion.
One factor the researchers identified is the difference in surface area between materials. Solid carbon has a much larger surface area than the tin powder used in the study, providing a greater interface between the electrode material and the electrolyte. Thus, surface area may contribute to the difference in thermal interaction observed between the two materials.
Tin is also being studied as a way to increase the energy density of a sodium ion battery. Fully saturated tin can theoretically store much more sodium per unit volume than solid carbon, with research continuing into designs of electrodes and electrolyte systems that support redundant sodium storage.
Electrolyte makes a big difference
The study also showed that the thermal behavior of tin strongly depends on the liquid electrolyte surrounding it.
The researchers compared propylene carbonate, known as PC, with TEGDME, a member of the ether-based solvent family.
The tin started generating heat earlier and reacted more strongly in the PC. In TEGDME, the material remained stable at higher temperatures and showed lower overall reactivity.
This builds on previous research demonstrating the compatibility of Glim-based electrolytes with tin anodes. The latest study shows that, besides its electrochemical performance, Glim chemistry can also influence the thermal behavior of soda-treated tin.
The researchers investigated why the two solvents produce different behavior. When the tin containing sodium was heated, the sodium began to come out of the tin-sodium alloy. In the PC system, this process occurred more easily and was accompanied by greater reaction with the electrolyte and formation of tin oxide.
TEGDME suppressed these reactions and largely preserved the tin in its metallic form. Computer modeling supported the experimental results, showing that PC reduces the energy required for sodium to exit the tin-sodium alloy, increasing the likelihood of subsequent reactions and heat generation.
The study compared selected carbonate- and ether-based systems. Electrolyte salt, tin particle size, electrode design, and operating temperature are among additional factors that can influence thermal behavior and provide areas for further investigation.
Towards practical tin-based cells
Michael Chuck is a doctoral student at the University of North Carolina.
The results show that tin can combine high sodium storage capacity with favorable thermal behavior at the electrode material level.
Electrolyte development will be an important part of translating these findings into whole cells. Glim-based electrolytes show strong compatibility with tin at the anode, while electrolyte compositions must also provide the required stability at the anode. Therefore, research into systems that operate effectively across both electrodes is ongoing.
Overall cell behavior is also affected by cathode chemistry and cell size and design. The current research provides a comparison between tin and solid carbon at the electrode material level, complementing broader studies of complete sodium ion cells.
The results identify another property of tin that is relevant to sodium-ion batteries. Besides its greater ability to store sodium, tin showed less thermal reactivity than carbon steel under the conditions tested, while the choice of electrolyte provided an additional means of influencing its thermal stability.
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