The rechargeable batteries you have on your smartphone right now is primarily thanks to the concept of reversible electrochemical reactions; using the battery pushes the reaction in one direction, while recharging it pushes it back the other way. The batteries that may be sitting behind the screen you’re reading this on is likely a variant of the lithium ion battery; in it, the lithium ions in the lithium-based cathode travel through the electrolyte towards the carbon-based graphite anode, while electrons travel through the external circuit, making it available for use for your smartphone’s circuitry. Naturally, this process goes backwards every time you plug your phone into its charging cable. Of course, this battery technology has its restrictions and limitations; engineers are thus always chasing after an alternative to the ubiquitous battery tech, perhaps with higher charge capacities or cycle life (or how many times a battery can be discharged, then charged). This new study, published in Nature, showcases a potential alternative to lithium-ion batteries called alkali metal-chlorine batteries, but the authors made use of a peculiar participant: chlorine.
Researchers from Stanford University found a way to make use of chlorine ions in its battery experiments. In it, they designated their cathodes to be alkali-chlorine compounds, like sodium chloride (NaCl, table salt) or lithium chloride (LiCl2). Chloride ions, however, are notorious for their reactivity; so despite their ubiquity in other ionic compounds, it’s not usually considered when thinking of battery mechanism alternatives. After all, it’d be hard to sell a battery with extremely reactive contents, as they may pose a danger to consumers and the technology they power. What the Stanford-based researchers did to circumvent this, however, is in the anode they used; instead of the usual graphite, the team made use of carbon nanospheres as anode material.
The surface feature of the nanospheres gave it porosity, which conveniently housed some of the chloride ions during discharge; as the reactive chloride ions are now locked in place within the pores of the “nanospherical” carbon structure, they are rendered unable to react with much else in the system until the reaction is reversed, and the chlorine ions revert back into their chloride salt forms. According to study co-author Guanzhou Zhu: “The chlorine molecule is being trapped and protected in the tiny pores of the carbon nanospheres when the battery is charged. […] Then, when the battery needs to be drained or discharged, we can discharge the battery and convert chlorine to make NaCl – table salt – and repeat this process over many cycles.”
This not only made the use of chloride ions in batteries feasible—it made the batteries slightly rechargeable, as well as gave the batteries high amounts of energy density, or the amount of charge it can hold per gram of material. The experiments yielded up to 1,200 mAh/g (milliampere-hours per gram)—six times that of normal lithium-ion batteries. The novel batteries can be charged and discharged up to 200 times before failing; lithium-ion batteries, on the other hand, can reach up to 1000 cycles. Study co-author Hongjie Dai likens the rechargeable battery to “a rocking chair,” as it tips back and forth whenever you add or consume energy from it; he refers to what they developed as “a high-rocking rocking chair,” given its penchant for potentially large amounts of energy density.
Much work must be done, however, to allocate this technology anywhere near commercial use. The research team also posits that the chlorine-based battery tech, should it be deployed, is better suited for applications where devices are infrequently charged, like satellites, remote sensors, or hearing aids. It is also not directly translatable to battery designs used for smartphones; the new technology must then be reconfigured in such a way as to allow usage for these commonplace devices. Nevertheless, it promises a new avenue for researchers to do follow-up experimentation, or to find even better battery tech.
Bibliography
- Lavars, N. (2021, August 29). Experimental chlorine battery holds 6 times more charge than lithium-ion. New Atlas. Retrieved September 14, 2021, from https://newatlas.com/energy/stabilized-chlorine-battery-6-times-charge/
- Lumen Learning. (n.d.). Other Rechargeable Batteries. Lumen Learning. Retrieved September 14, 2021, from https://courses.lumenlearning.com/introchem/chapter/other-rechargeable-batteries/
- Zhu, G., Tian, X., Tai, H.-C., Li, Y.-Y., Li, J., Sun, H., Liang, P., Angell, M., Huang, C.-L., Ku, C.-S., Hung, W.-H., Jiang, S.-K., Meng, Y., Chen, H., Lin, M.-C., Hwang, B.-J., & Dai, H. (2021). Rechargeable na/cl2 and li/cl2 batteries. Nature, 596(7873), 525–530. https://doi.org/10.1038/s41586-021-03757-z
