Image courtesy of Google images, creative commons.
Every day around the world, modern luxuries are plugged in, charged, drained, and the cycle begins anew. Critical to this energy dependence is the lithium-ion battery—the electrochemical backbone behind cell phones, laptops, electric cars, and most other battery-powered devices. In the US alone, electronics consume some two thousand metric tons of lithium annually, of which over fifty percent arrives as imports from other countries such as China, Chile, and Australia.
Yiren Zhong, a postdoctoral associate in Yale’s Department of Chemistry, understands the need to find a substitute for lithium-ion batteries. “We all know that the lithium is a very very limited resource, not only in the Earth’s crust but also in the oceans, in the lakes,” Zhong said. Resource scarcity and related environmental concerns have inspired chemists—including Zhong—to look for candidates no further than a row down in the periodic table. One promising candidate is sodium, an alkali metal like lithium. Sodium and lithium have many similar qualities due to periodic trends, with the notable difference that sodium is larger in atomic size and has less electric potential overall. However, sodium is also far more abundant naturally on Earth. Would this periodic similarity make sodium a prime candidate for battery production? This is what Zhong set out to study with his research, published in August of 2021 in the Journal of the American Chemical Society.
Despite it’s natural abundance, sodium has a long way to go before replacing lithium as a primary battery component. There are pros and cons for sodium metal as an electrode; for one, sodium has good reversibility compared to lithium, however, it cannot be charged or discharged very quickly. Reversibility for a battery is the ability to return the electrochemical reaction back to its original reactants, meaning batteries with good reversibility can be recharged and reused. Sodium’s reversibility in batteries is promising, however intrinsic elemental properties stand in the way of sodium’s potential. Zhong’s research group investigated these properties through rigorous experiments testing sodium batteries at varying power levels, then examining the electrode’s physical and chemical structure after both charging and discharging. The research team performed the experiments with currents at high levels closer to those of lithium batteries and far lower currents for comparison. When the sodium electrode was discharged at the higher currents it performed with only zero to sixty percent Coulombic efficiency—the efficiency with which a battery outputs usable electrons (electricity) that it produces— and an interesting physical reaction indicated a key elemental difference between lithium and sodium. When built through charging, sodium metal electrodes naturally form in dendritic structures, which are long, thin columns of metal that form porous, microscopic forests on the surface of the electrode. However, when charged at high current densities these dendritic structures are formed with non-metallic impurities that reduce the reversibility of the battery overall. When discharged at high current densities, these impure porous surfaces allowed fast- moving current to react unevenly—especially at the base of the electrode—causing electrode erosion and eventual electrical disconnection. These results indicate that the low performance of sodium batteries stems from theirits elemental characteristics, namely their its larger atomic size. Electrodes made from sodium metal have more spread- out dendritic structures due to larger atomic size, which creates the porous surface that allows for erosion of the electrode foundation layer at high currents, like waves washing away the base of a sand castle. However, Zhong’s findings also suggest a favorable future for sodium. At low power levels, the sodium battery did not decay and performed favorably with Coulombic efficiencies as high as 99.5 percent. At these low current densities, sodium batteries may demonstrate commercial usefulness in technologies like short- range transportation tools.
Having observed sodium’s intrinsic characteristic limiting its potential in batteries, Zhong’s group laid the foundation for future sodium battery technology. One of his newest ideas involves the electrode shape itself. “My current thinking is trying to use a three- dimensional electrode,” Zhong said. He theorizes that a three- dimensional electrode may reduce local current density across a larger surface area, which subsequently improves electrochemical reaction in the battery. As our society’s energy dependence grows each year, more environmentally friendly batteries become a necessity rather than a goal. “The number one reason for that is because we need to develop a battery future. . . by the year 2050, I would envision that sodium would be one of the major components in the battery market,” Zhong said. Sodium metal has the potential to help build a sustainable battery future, and thanks to the continued work of ingenuitive innovative chemists, that future is in reach.
Jaskula, B. W. (2021, January). Lithium. US Geological Service. Retrieved September 2021.
Zhong, Y. (2021, September 16). Personal interview [Zoom interview].
Zhong, Y., Shi, Q., Zhu, C., Zhang, Y., Li, M., Francisco, J. S., & Wang, H. (2021). Mechanistic
insights into fast charging and discharging of the sodium metal battery anode: A comparison with lithium. Journal of the American Chemical Society, 143(34), 13929–13936. https://doi.org/10.1021/jacs.1c06794