Novel method to extend lifecycle of Li-ion batteries

Researchers from Pohang University of Science and Technology (POSTECH) have identified a degradation mechanism that occurs during the use of lithium-ion batteries. Lithium-ion batteries typically used nickel-manganese-cobalt (NMC) ternary cathodes. To reduce costs, recent industry trends have favoured increasing the nickel content while minimising the use of expensive cobalt. However, a higher nickel content can shorten the overall lifecycle of the battery.

Until now, battery performance degradation was primarily attributed to overcharging. However, this fails to account for degradation occurring under seemingly stable voltage conditions. The researchers focused on the discharging process — the actual operation of the battery — to address this issue.

They found 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 3.0 V. 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 researchers 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.

The study also found a simple and effective solution; the researchers were able to extend the battery’s lifecycle by optimising 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.

Professor Jihyun Hong, who led the research, said the research findings present an important direction for the development of longer-lasting batteries. “The impact of discharge — the actual process of using a battery — has been largely overlooked until now,” Hong said.

The research findings have been published in Advanced Energy Materials.

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