Cause of the detrimental charge voltage rise in lithium-air batteries identified

NIMS has found for the first time that a voltage increase that occurs in lithium-air batteries when they are being charged—a major issue preventing their practical use—is strongly and positively correlated with the degree of crystallinity of lithium peroxide (Li₂O₂), a compound produced during a discharge cycle. This discovery is expected to provide significant insight into resolving this issue.

Because theoretical energy densities of lithium-air batteries are extremely high, they have the potential to be used in a wide variety of technologies, such as drones, IoT devices, electric vehicles and home electricity storage systems. The most serious issue concerning these batteries is the elevated voltage during a charge cycle (i.e., overpotential), which induce side reactions detrimental to their cycle lives. The causes of overpotentials had previously been largely unknown.

To address this issue, this research team investigated the crystallinity of Li₂O₂—a product formed during a discharge cycle—and found that the Li₂O₂ with a more disordered crystalline structure (i.e., a low degree of crystallinity) is more susceptible to decomposition at a lower voltage during a charge cycle. We are the first to report this fact. Two types of discharge reaction pathways leading to the formation of Li₂O₂ have been known: reactions occurring on the surface of the carbon electrode and reactions occurring within the electrolyte (in the form of disproportional reactions). Our research found that the Li₂O₂ formed through pathway is decomposable at 3.5 V or lower voltage during a charge cycle while that formed through pathway is more resistant to decomposition, requiring 4 V or higher voltage. Furthermore, we found that the pathway Li₂O₂ had a more disordered crystalline structure than the pathway Li₂O₂ . These results indicate that the elevated charge voltage is induced by the Li₂O₂ with a highly ordered crystalline structure formed through pathway and that the high voltage can be lowered by suppressing the formation of this type of Li₂O₂.

Based on these results, we will investigate ways to enable lithium-air batteries to primarily produce Li₂O₂ with a low degree of crystallinity, thereby significantly increasing their cycle lives and accelerating research efforts in putting them into practical use at the NIMS-SoftBank Advanced Technologies Development Center.

This research was published in Advanced Science, an open access journal, on August 11, 2020.