10 Years of Uninterrupted Shelf Storage: How Long-Term Calendar Aging Affects the Performance of Commercial LFP/C Cells

Many companies are required to keep lithium-ion battery cells in inventory as reserve parts, e.g. for warranty purposes. This can result in long periods of shelf storage, during which cells remain in a resting state at a specific state of charge and ambient temperature for several years without being used. Due to these extended timeframes, data on shelf storage of lithium-ion cells is limited. It is unclear how batteries will perform after such long storage duration, and if they are still able to fulfill specific power requirements for their application.
We aim to bridge this research gap by investigating the influence of 10 years of uninterrupted shelf storage on 100 commercial 26650 LFP/C lithium-ion cells. We compare data collected today, after 10 years of shelf storage at 50% state of charge and 6°C, with measurements taken on the identical cells in 2014 after shipping.
By performing a check-up procedure at 25°C, we quantify the influence of shelf storage on cell capacity and observe that all 100 cells retain between 96% and 98% percent of their original capacity. We identify consistent aging behavior across two production batches, indicating reproducible degradation after shelf storage. The cell-to-cell variations within one production batch after shelf storage remain within a range of what is to be expected of fresh production batches.
Open-circuit voltage measurements at 0.01C and rate tests between 1C and 3C indicate the absence of loss of active anode material and electrolyte salt depletion and thus suggest that aging during shelf storage is mainly attributed to SEI growth. Recent literature shows that SEI growth naturally slows over time during storage, but can accelerate again if interrupted by cyclic measurements. Since our cells remained undisturbed for 10 years, SEI growth likely diminished to a negligible rate, thus potentially explaining the minimal capacity loss observed.