Post 80% Cycling of Li-Ion Cells, an Update on a Seven Year Cycling Study of Diverse Lithium-Ion Battery Chemistries
Authors: Upadhya Sahana , Wittman Reed M , Rich Chaz , Fresquez Armando , Chalamala Babu , Mahvi Allison , Wagner Michael J , Preger Yuliya 
Published: December 2024

Li-ion batteries are increasingly being used in various sectors, including consumer electronics, electric vehicles (EVs), and grid energy storage, as critical components in modern infrastructure. Convention for the EV market dictated that 80% of the initial capacity of the battery represented the end of life and is therefore commonly marked as the end point of battery aging studies. To exploit the potential applications of a given cell, an understanding of degradation trends beyond 80% capacity is needed and requires the collection of data at various standard use conditions.

This work details a multi-year cycling study of commercial LiFePO4 (LFP), LiNixCoyAl1−x−yO2 (NCA), and LiNixMnyCo1−x−yO2 (NMC) 18650 cells, varying the discharge rate (C-rate), state of charge (SOC) ranges, and environment temperature. This work builds on a previously published analysis performed up to 80% capacity, now focusing on post 80% analysis[1]. The capacity degradation of each chemistry, under varying conditions, were compared. Cycling at varying discharge C-rates (0.5C, 1C, 2C, 3C), SOC ranges (0-100%, 20-80%, and 40-60%) and temperatures (15, 25, and 35 °C) affected each chemistry to different extents. Therefore, the relative importance of each parameter in cyclic aging is different for different chemistries.

Overall, we find that each cell chemistry can conduct significant cycling post 80%, in some cases to a capacity retention of 40%, with minimal loss of round-trip efficiency (RTE). We find that SOC range is the most consistent factor in cell capacity fade rate for all chemistries considered. Temperature had varied impacts on capacity fade depending on cell chemistry, for example NCA cells show no clear trend before 80% capacity, but post 80%, increased temperatures caused an increase in the rate of capacity fade. C-rate of discharge has a more prominent impact on capacity as cells age. LFP cells are the only ones that showed a clear trend of the highest and lowest C-rates causing the most rapid fade before 80%. Post 80%, the NMC and NCA cells start to show a similar trend and the LFP trend becomes more pronounced.

Most batteries do not display a distinct knee point, instead exhibiting a more linear degradation with gradual increase in slope. When they do experience knee points, there is no consistent trend across chemistries and conditions that appear to cause knees. Other metrics such as RTE, total discharged energy, and internal resistance (IR) are also being considered to identify trends. This study represents the broadest public report of post 80% capacity cycling across multiple chemistries and will be valuable in lifetime prediction modelling for efficient battery usage.

References:

1Preger et al. “Degradation of Commercial Lithium-Ion Cells as a Function of Chemistry and Cycling Conditions” J. Electrochem. Soc.,
2020
, 167, 120532.

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