US researchers have given new insights into single-crystal cathode degradation that can lead to longer-lasting, safer batteries.
Researchers have solved a major battery mystery that has led to capacity degradation, shortened lifespan, and, in some cases, fire. New research from Argonne National Laboratory and the UChicago Pritzker School of Molecular Engineering uncovered some of the root causes – and ways to mitigate – the nanoscopic strains that can lead to cracking in an increasingly popular battery type for electric vehicles and other technologies.
“Electrification of society needs everyone’s contribution,” said one of the corresponding authors, Khalil Amine, Argonne Distinguished Fellow and Joint Professor at UChicago. “If people don’t trust batteries to be safe and long-lasting, they won’t choose to use them.”Major degradation mechanism identifiedBecause of the long-standing cracking issues in lithium-ion batteries that use polycrystalline Ni-rich materials in their cathodes, researchers over the last few years have turned toward single-crystal Ni-rich layered oxides . But they have not always performed as well as, or even better than, the older model, according to researchers.The new work revealed the underlying issue: assumptions drawn from polycrystalline cathodes were being incorrectly applied to single-crystal materials. “When people try to transition to single-crystal cathodes, they have been following similar design principles as the polycrystal ones,” said first author Jing Wang, now a postdoctoral researcher working with UChicago and Argonne. “Our work identifies that the major degradation mechanism of the single-crystal particles is different from the polycrystal ones, which leads to the different composition requirements.”Study unravelled the distinct nanoscopic strain evolutionPublished in Nature Nanotechnology, the study unravelled the distinct nanoscopic strain evolution in SC-NMC during battery operation, challenging the conventional composition-driven strategies and mechanical degradation indicators used for PC-NMC. Through particle-level chemomechanical analysis, researchers reveal a decoupling between mechanical stability and lattice volume change in SC-NMC, identifying that structural instability in SC materials is primarily driven by multidimensional lattice distortions induced by kinetics-driven reaction heterogeneity and progressively deactivating chemical phases.Using this mechanical failure mode, researchers redefined the roles of cobalt and manganese in maintaining mechanical stability. Unlike cobalt’s detrimental role in PC-NMC, researchers find that cobalt is critical for enhancing the longevity of SC-NMC by mitigating localized strain along the extended diffusion pathway, whereas manganese exacerbates mechanical degradation.The study not only challenged conventional design but also the materials used, redefining the roles of cobalt and manganese in battery’s mechanical failure. “Not only are new design strategies needed, but different materials will also be required to help single-crystal cathode batteries reach their full potential,” said Meng, who is also the director of the Energy Storage Research Alliance based at Argonne.“By better understanding how different types of cathode materials degrade, we can help design a suite of high-functioning cathode materials for the world’s energy needs.”As a polycrystal cathode battery charges and discharges, the tiny, stacked primary particles swell and shrink. This repeated expansion and contraction can widen the grain boundaries separating the polycrystals, similar to how repeated freezing and thawing can create potholes in city streets, according to a press release.“Typically, it will suffer about five to 10% volume expansion or shrinkage,” Wang said. “Once an expansion or shrinkage exceeds the elastic limits, it will lead to particle cracking.”Degradation in single-crystal NMC cathodesIf the cracks widen too much, electrolyte can get in, leading to unwanted side reactions and oxygen release that raise safety concerns, including the risk of thermal runaway. But, barring those dramatic circumstances, a more day-to-day effect is capacity degradation – the batteries fade over time, becoming increasingly incapable of delivering the same charge they did when new.Researchers demonstrated that a distinct mechanical failure mode predominantly governs degradation in single-crystal NMC cathodes.“By identifying this previously underappreciated mechanism, this work establishes a direct link between material composition and degradation pathways, providing deeper insight into the origins of performance decay in these materials,” said another corresponding author, Tongchao Liu, a chemist at Argonne.
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