The search for more sustainable and environmentally-friendly energy sources has led to a renewed interest in metal-free lithium-ion batteries. These batteries have the potential to provide high levels of energy at a lower cost than their metal-based counterparts. However, one of the challenges facing researchers is finding a suitable cathode material. Croconic acid, a small organic molecule, has shown promise as a metal-free cathode material. In a recent study, a joint research team from Tohoku University and UCLA showed that croconic acid can enable lithium-ion batteries to operate at voltages as high as 4 V. This is a significant advancement toward metal-free, high-voltage lithium-ion batteries. The researchers believe that croconic acid has the potential to revolutionize battery technology, making it more sustainable and affordable. With further development, these batteries could play an important role in the transition to a low-carbon economy.
Organic batteries have several advantages over their conventional lithium-ion counterparts. For one, they exploit naturally abundant elements such as carbon, hydrogen, nitrogen, and oxygen, rather than rare-earth materials like cobalt and lithium. In addition, organic batteries have greater theoretical capacities than conventional lithium-ion batteries because their use of organic materials renders them lightweight. Most reported organic batteries to date, however, possess a relatively low (1-3V) working voltage. Increasing organic batteries' voltage will lead to higher energy density batteries—batteries that can store more energy per unit of weight. This would be a major breakthrough for portable electronics, electric vehicles, and other applications where weight and space are constrained. Researchers are currently exploring a variety of strategies for boosting the voltage of organic batteries, and it is only a matter of time until a high-voltage organic battery hits the market.
Itaru Honma, a professor of chemistry at Tohoku University's Institute of Multidisciplinary Research for Advanced Materials, Hiroaki Kobayashi, an assistant professor of chemistry at Tohoku University, and Yuto Katsuyama, a graduate student at UCLA, found that croconic acid, when used as a lithium-ion battery cathode material, maintains a strong working voltage of around 4 V. This is significant because most cathode materials lose their charge quickly as the voltage drops below 4 V. The team's findings were published in the journal Nature Communications. In addition to its high working voltage, croconic acid is also inexpensive and environmentally friendly. The research team is now working on further improving the performance of this new battery material.
Croconic acid is a five-carbon molecule bonded to oxygen in a pentagonal form. Its high theoretical capacity of 638.6 mAh/g makes it an attractive option for use in lithium-ion batteries, as it can store more energy than the conventional cathode material, LiCoO2 (140 mAh/g). In addition, croconic acid is less likely to undergo the reversible oxidation-reduction reactions that are responsible for many of the problems associated with lithium-ion batteries, such as capacity fade and thermal runaway. As a result, croconic acid could potentially be used to create safer, more stable lithium-ion batteries with a longer lifespan. "We investigated the electrochemical behaviour of croconic acid in the high-voltage range above 3 V using theoretical calculations and electrochemical experiments," said Kobayashi. "We discovered that croconic acid stores lithium ions at roughly 4 V, giving a very high theoretical energy density of 1949 Wh/kg, which is larger than most inorganic and organic lithium-ion batteries."
While the theoretical capacity of croconic acid was not achieved in this study, the researchers are optimistic that it can be enhanced through the development of stable electrolytes at high voltage and chemical modifications to the molecules. Most electrolytes cannot stand for such a strong working voltage of croconic acid, making the development of new electrolytes essential. Additionally, the structures of small organic molecules, including croconic acid, can be easily modified. Appropriate structural modification can stabilize the molecule, leading to greater capacity and reversibility. Through these efforts, it is hoped that the potential of croconic acid will be fully realized in future studies.