Paradoxical role of structural degradation of nickel-rich layered oxides in capacity retention upon storage of lithium-ion batteries

Batteries experience a mixture of active cycling and long idle storage during their lifetime. While the cycling-induced degradation mechanisms and corresponding mitigation strategies have been extensively investigated, the distinct impacts of storage in the absence of cycling have been largely overlooked and unexplored. Battery performance also degrades over time with a peculiar dependence on the state-of-charge (SoC) of batteries at rest, for instance, retaining higher capacity during storage at SoC100 than at SoC70. In this study, nondestructive operando X-ray diffraction (XRD) coupled with gas analysis reveals the complex interplay of structural degradation of active materials, interfacial side reactions, and their impact on full-cell aging during idle storage. Capacity fading during SoC70 storage predominantly resulted from the electrode slippage and Li inventory loss within a full-cell, with minor structural degradation of Ni-rich layered oxide cathodes. SoC100 storage caused more detrimental structural degradation of Ni-rich cathodes and side reactions. Paradoxically, severe side reactions suppressed Li inventory loss, electrode slippage, and full-cell capacity fading during SoC100 storage. In addition to conventional degradation mechanisms such as Li/Ni cation mixing, surface reconstruction layer formation, and the appearance of fatigued phases, SoC100-stored cathodes exhibited an unexpected contraction of interlayer spacing during cycling after high-temperature storage, indicating unusual impacts of storage. Based on the capacity fading mechanisms revealed in this study, mitigation strategies for storage-induced aging are demonstrated. Our work provides insights into battery manufacturing and management to improve calendar lifetime.