A research team has identified a previously unknown degradation mechanism that occurs during the use of lithium-ion batteries. Their findings are published in Advanced Energy Materials.

The team includes researcher Seungyun Jeon and Dr. Gukhyun Lim, led by Professor Jihyun Hong from the Department of Battery Engineering at POSTECH (Pohang University of Science and Technology), in collaboration with Professor Jongsoon Kim’s group at Sungkyunkwan University.

Lithium-ion batteries, which are essential for electric vehicles, typically use nickel-manganese-cobalt (NMC) ternary cathodes. To reduce costs, recent industry trends have favored increasing the nickel content while minimizing the use of expensive cobalt. However, higher nickel content tends to shorten the overall cycle life of the battery.

Until now, battery performance degradation was primarily attributed to overcharging. However, this explanation failed to account for degradation occurring under seemingly stable voltage conditions. The research team focused on the discharging process—the actual operation of the battery—to solve this mystery.

They discovered that when a battery is used for extended periods without recharging, a phenomenon known as the quasi-conversion reaction occurs on the cathode surface. During this reaction, oxygen escapes from the surface and combines with lithium to form lithium oxide (Li₂O) during discharge, particularly around 3.0V. This compound further reacts with the electrolyte, generating gas and accelerating battery degradation.

The quasi-conversion reaction was found to be more severe in high nickel cathodes. The research team confirmed that when batteries are used until most of their capacity is depleted, the effects of the degradation process, including battery swelling, become increasingly pronounced.

Importantly, the study also revealed a simple yet effective solution. The research team significantly extended cycle life by optimizing battery usage and avoiding full discharge. In experiments with high-nickel batteries (containing over 90% nickel), those discharged deep enough to trigger the quasi-conversion reaction retained only 3.8% of their capacity after 250 cycles, whereas batteries with controlled usage maintained 73.4% of their capacity even after 300 cycles.

Prof. Jihyun Hong, who led the research, stated, “The impact of discharge—the actual process of using a battery—has been largely overlooked until now. This research presents an important direction for developing longer-lasting batteries.”