Reviewed by Lexie CornerApr 1 2025
Under the direction of Professor Jihyun Hong from the Department of Battery Engineering at POSTECH (Pohang University of Science and Technology), a research team consisting of Researcher Seungyun Jeon and Dr. Gukhyun Lim has identified a previously unknown degradation mechanism in lithium-ion batteries during use. The team collaborated with Professor Jongsoon Kim’s group at Sungkyunkwan University.
Discharge voltage-Dependent Structural Degradation and Gas Evolution. (a) Irreversible structural changes were observed on the surface after 150 charge/discharge cycles within the voltage range in which the quasi-conversion reaction occurs. (b) The original layered structure was preserved even after 150 cycles within a more stable voltage range. (c) When high nickel pouch-type cells were cycled more than 250 times at 1.9 V (where the quasi-conversion reaction occurs) and 3.15 V (where it does not), capacity retention improved significantly solely by adjusting the discharge cutoff voltage. (d) Increasing the discharge voltage effectively suppressed oxygen loss and subsequent side reactions at the surface, resulting in a significant reduction in the evolution of gases (CO, CO₂, CH₄, and C₂H₄). Image Credit: Pohang University of Science and Technology
NMC ternary cathodes are widely used in lithium-ion batteries, which are essential for electric vehicles. Recent industry trends have favored increasing the nickel content while reducing the use of cobalt to lower costs. However, higher nickel concentrations typically lead to shorter battery cycle life.
Previously, overcharging was believed to be the primary cause of battery performance degradation. However, this view does not account for degradation that occurs under seemingly stable voltage conditions. To address this, the research team focused on the discharging process, which occurs during the actual operation of the battery.
They found that when a battery is used for extended periods without recharging, a process called the “quasi-conversion reaction” occurs on the cathode surface. During discharge, oxygen is released from the surface and reacts with lithium to form lithium oxide (Li₂O) at approximately 3.0V. This molecule then reacts with the electrolyte, producing gas and accelerating battery degradation.
The quasi-conversion reaction was found to be more severe in high-nickel cathodes. The researchers confirmed that when batteries are discharged to near-empty, the effects of this degradation process, such as battery swelling, become more pronounced.
The study identified a simple but effective solution. By managing battery usage and avoiding full discharge, the researchers were able to significantly extend the battery’s cycle life.
In experiments with high-nickel batteries (containing more than 90 % nickel), those that were deeply discharged to trigger the quasi-conversion process retained only 3.8 % of their capacity after 250 cycles, while batteries with controlled usage maintained 73.4 % of their capacity after 300 cycles
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.
Jihyun Hong, Professor, Department of Battery Engineering, Pohang University of Science and Technology
This study was funded by the Korea Institute for Advancement of Technology (KIAT) through the Ministry of Trade, Industry, and Energy (MOTIE) (HRD Program for Industrial Innovation). It was also supported by the Korea Planning and Evaluation Institute of Industrial Technology (KEIT).
Journal Reference:
Jeon, S. et. al. (2025) Reduction-Induced Oxygen Loss: the Hidden Surface Reconstruction Mechanism of Layered Oxide Cathodes in Lithium-Ion Batteries. Advanced Energy Materials. doi.org/10.1002/aenm.202404193