New research has found that tin anodes may offer a thermal stability advantage over hard carbon in sodium-ion batteries. The study also shows that electrolyte choice significantly influences 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, University of California San Diego, Argonne National Laboratory and Northwestern University, together with US sodium-ion battery company Peak Energy, compared the behaviour of tin, hard carbon and mixtures of the two under elevated temperatures.
Tin is less reactive than hard carbon
The researchers used accelerating rate calorimetry, a technique that measures when a material begins generating its own heat as temperature increases.
This allows researchers to compare the thermal behaviour of charged electrode materials and their interactions with the surrounding electrolyte.
In the experiments, fully sodiated tin showed greater thermal stability than hard carbon. Mixtures of tin and hard carbon showed intermediate behaviour, with thermal stability increasing as the proportion of tin increased.
One factor identified by the researchers is the difference in surface area between the materials. Hard carbon has a substantially larger surface area than the tin powder used in the study, providing more interface between the electrode material and electrolyte. Surface area may therefore contribute to the difference in thermal reactivity observed between the two materials.
Tin is also being investigated as a way of increasing sodium-ion battery energy density. Fully sodiated tin can theoretically store considerably more sodium per unit volume than hard carbon, with research continuing into electrode designs and electrolyte systems that support repeated sodium storage.
Electrolyte makes a major difference
The study also showed that tin’s thermal behaviour depends strongly on the liquid electrolyte surrounding it.
Researchers compared propylene carbonate, commonly known as PC, with TEGDME, a member of the glyme family of ether-based solvents.
Tin began generating heat earlier and reacted more strongly in PC. In TEGDME, the material remained stable to higher temperatures and showed lower overall reactivity.
This builds on previous research demonstrating the compatibility of glyme-based electrolytes with tin anodes. The latest study shows that, alongside their electrochemical performance, glyme chemistry can also influence the thermal behaviour of sodiated tin.
The researchers investigated why the two solvents produced different behaviour. As sodiated tin was heated, sodium began leaving the tin-sodium alloy. In the PC system, this process occurred more readily and was accompanied by greater reaction with the electrolyte and the formation of tin oxide.
TEGDME suppressed these reactions and largely preserved the tin in its metallic form. Computer modelling supported the experimental results, showing that PC lowers the energy required for sodium to leave the tin-sodium alloy, which increases 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 the additional parameters that can influence thermal behaviour and provide areas for further investigation.
Towards practical tin-based cells
Michael Chak PhD student at the University of North Carolina.
The results show that tin can combine high sodium-storage capacity with favourable thermal behaviour at electrode-material level.
Electrolyte development will be an important part of translating these results into complete cells. Glyme-based electrolytes show strong compatibility with tin at the anode, while electrolyte formulations must also provide the required stability at the positive electrode. Research into systems that perform effectively across both electrodes is therefore continuing.
Overall cell behaviour is also influenced by cathode chemistry, cell size and design. The current research provides a comparison of tin and hard carbon at electrode-material level, complementing broader studies of complete sodium-ion cells.
The findings identify another characteristic of tin relevant to sodium-ion batteries. Alongside its high volumetric sodium-storage capacity, tin showed lower thermal reactivity than hard carbon under the conditions tested, while electrolyte selection provided an additional means of influencing its thermal stability.
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