[Image above] Next-generation smartphones may charge even faster if aluminum-ion batteries are successfully commercialized. Credit: Pixabay
Aluminum-ion batteries are of increasing interest as alternatives to lithium-ion batteries. In addition to being safer and cheaper, these batteries have a volumetric energy density that is theoretically four times higher than lithium-ion batteries (~8.04 Ah cm–3, compared to 2.06 Ah cm–3 for lithium). They also charge faster due to their ability to exchange three electrons per ion when recharging, in contrast to the one-to-one exchange of lithium ions.
However, finding an appropriate material for the cathode in aluminum-ion batteries is proving to be a barrier to commercialization. Aluminum ions have a higher charge density than lithium ions, which hinders ionic diffusion in host materials. Thus, there is a limited number of materials that can reversibly intercalate and de-intercalate aluminum ions.
Based on studies to date, various metal-oxides and carbon-based materials show the most potential as cathode materials for aluminum-ion batteries. Yet few studies have explored the potential of composites that contain both metal-oxides and carbon.
In a recent paper, researchers from AGH University of Science and Technology in Poland and the Polish Academy of Sciences’ Jerzy Haber Institute of Catalysis and Surface Chemistry investigated the potential of carbon–tungsten oxide composites as cathode materials.
They first explored the potential of carbon–tungsten oxide composites in a 2019 paper. However, while the results in that paper were promising, “the performance of the constructed cells was not satisfactory and further improvement of these materials is still possible,” they write.
For the recent study, the researchers used sol-gel and chemical vapor deposition processes to obtain samples of tungsten oxide, carbon, and carbon–tungsten oxide composites as potential cathode materials.
- Tungsten oxide: The researchers synthesized tungsten oxide via the sol-gel method, using ammonium metatungstate hydrate as the starting precursor.
- Carbon: The researchers used carbon obtained from either Norit, an activated carbon manufacturer, or via processing of potato starch. In the latter case, they diluted potato starch with water, heated and stirred it to form a gel, then added ethanol to perform solvent exchange. After 10 days of solvent exchange, they dried and calcinated the gel under an argon atmosphere.
- Carbon–tungsten oxide composites: The researchers synthesized carbon–tungsten oxide composites using the same process as for carbon from potato starch. They also obtained a carbon–tungsten oxide sample with a core-shell structure via chemical vapor deposition, with tungsten oxide powder and methane used as the starting precursors.
The researchers analyzed the samples using various spectroscopy and microscopy techniques. Then, they used these potential cathode materials to construct prototype aluminum-ion cells, which also featured an aluminum foil anode and a Celgard membrane separator.
Based on their results, the researchers determined that electrical conductivity of the cathode materials depended on the material’s phase composition. “The lowest conductivity was found in the case of WO3 [tungsten oxide], while the modification with carbon led to the increase of electrical conductivity in all composite materials. This effect was better observable in the case of Norit-based than in the case of Potato Starch-based materials,” they write.
Regarding the prototype cells, the researchers found the single-phase tungsten oxide and Norit samples demonstrated the highest values of open-cell voltage and capacity, which could be explained by accounting for differences in average grain size and the Brunauer–Emmett–Teller (BET) specific surface area.
While the carbon–tungsten oxide composites did not lead to improvements in open-cell voltage and capacity, the researchers did observe improved cyclability of the prototype cells that contained these composites.
“After 30 cycles of work, WO3 CS composite [carbon–tungsten oxide with core-shell structure] capacitance, Cp, was found to be unchanged. On the contrary, in the case of single phase WO3 and Carbon PS [Potato Starch] cathode materials, 62% and 66% loss of Cp were observed, respectively,” they write.
“Thus, the formation of composite cathode materials may be beneficial from the point of view of the capacitance long-term stability,” they conclude.
The paper, published in Electrochimica Acta, is “Carbon tungsten oxide composite cathode materials for aluminum-ion batteries” (DOI: 10.1016/j.electacta.2022.140606).