Experimental
For different commercial 18650 lithium-ion cells with graphite anode and NMC, NCA, or LFP cathode, we investigated calendar aging at different temperatures, states-of-charge (SoC), and storage periods. In our study, we used a fine SoC resolution with 16 values from 0% to 100% to gain a thorough understanding of the impact of storage SoC on calendar aging. In total, more than 200 cells were examined. To analyze battery aging, we used conventional capacity and impedance measurements but also differential voltage analysis (DVA) and coulometry.
Results
As illustrated in Figure 1a for the NCA cells after a storage period of almost 10 months, the capacity loss from calendar aging is not increasing steadily with higher SoC. In fact, there are two plateau regions in which the capacity fade is rather independent from SoC. A high capacity fade is observed between 65% and 95% SoC and a medium capacity fade is observed between 30% and 55% SoC.
Using DVA [1], a clear correlation between these plateaus and the different intercalation stages of graphite was identified for all cell types examined. For the NCA cells, Figure 1b shows a reconstruction of the full-cell differential voltage spectrum from half-cell data, highlighting the central graphite peak which separates different intercalation stages of the graphite anode. Analyzing differential voltage spectra over time confirmed that the capacity fade resulted predominantly from a loss of lithium inventory, since no or only minor degradation of the electrodes’ active materials was determined.
Coulometry [2,3] enabled to localize loss of lithium inventory at the anode. Furthermore, side reactions causing reversible capacity losses were detected, especially for storage at high SoC.
Conclusions
All in all, there is a more complex dependence of calendar aging on SoC than generally assumed. Our aging study, comprising different cell chemistries, has revealed that lowering the SoC of a lithium-ion battery to reduce calendar aging during storage is not really effective as long as the SoC remains in the plateau region of high capacity fade. To avoid this SoC region, a storage SoC below 55%-70%, depending on the balancing of the graphite anode, is required. To maximize battery life in an electric vehicle, it is beneficial to keep the SoC at 50% or lower when the vehicle is not being used.
References
[1] I. Bloom, A. N. Jansen, D. P. Abraham, J. Knuth, S. A. Jones, V. S. Battaglia, G. L. Henriksen, Journal of Power Sources, 139 (2), 295-303 (2005).
[2] A. J. Smith, J. C. Burns, D. Xiong and J. R. Dahn, Journal of The Electrochemical Society, 158 (10), A1136-A1142 (2011).
[3] R. D. Deshpande, P. Ridgway, Y. Fu, W. Zhang, J. Cai and V. Battaglia, Journal of The Electrochemical Society, 162 (3), A330-A338 (2015).