In-Situ Diagnostics of Ageing Mechanisms in Li-Ion Batteries Using Entropy Spectroscopy and Incremental Capacity Analysis
Authors: Vie Preben J. S. , Maehlen Jan Petter , Andersen Hanne Flåten , Kirkengen Martin 
Published: February 2020

Knowledge and the ability to predict capacity decay, battery state of health and ageing mechanism will be vital information to enable long, safe and profitable operation of large Li-ion battery systems.
In this paper a 30 Ah high-energy commercial NMC-graphite pouch cell was aged through accelerated cycling at 3 temperatures and 2 selected current rates. The temperatures were 5, 25 and 45 °C, and the current rates were either 1C or 1.5 C for both charge and discharge. The remaining capacity after accelerated cycling at a C/10 discharge at 25 °C is defined as the battery’s state of health (SoH).
Several in-situ techniques were applied to characterize and diagnose the observed ageing, e.g.:

Entropy spectroscopy (ES) is a battery’s entropy profile with respect to state of charge (SoC) [1]. The entropy spectrum is specific to a Li-ion battery’s anode and cathode chemistry. The changes in entropy spectra can be used as indicators of a battery’s state-of-health [1-3] and a diagnostic tool for ageing mechanisms. This method has been extended at Institute for Energy Technology to record entropy spectra for cells up to 50 Ah size.

Incremental capacity analysis (ICA) [4] involves measuring the differentiated of capacity as a function of voltage during charge and discharge, also known as dQ/dV analysis. Dubarry et al claims that ICA on data during ageing can obtain knowledge (qualitative and quantitive) on the degradation mechanisms [4-6].

The entropy spectra for selected cells cycled at the 3 temperatures are presented in the figure. The cells cycled at 5 °C had fast capacity decay, losing more than 20% capacity after less than 20 cycles. A well-known ageing mechanism explaining this is the plating of Lithium metal on the anode electrode during charge at lower temperatures [7, 8]. We observe a shift of the 13 J/Kmol entropy peak for these cells with approx. 20 and 25 % on the SoC axis correlating to the change in SoH. This shift can correspond to the shift of the Li-ion balance between anode and cathode during Lithium plating.
The cells cycled at 45 °C obtained more than 500 normalized cycles before reaching a SoH of 80%. However, at this stage, the SoH was declining fast and reached almost 70% after only 530 normalized cycles. In the graph we observed a different change of the entropy spectra for these cells and the spectrum at 72% SoH has a wider and lower entropy peak at about 9 J/Kmol. This change can possibly be interpreted as changes to the electrode materials during cycling. The spectrum for the cell cycled at 25 °C corresponds more to the change observed for cycling at 5 °C and the smaller shift of the 13 J/Kmol peak correlate to the higher SoH of 85%. This suggests that the possible degradation mechanism for the cycling at 25 °C correlates better to a Li-plating mechanism than the high temperature degradation mechanism occurring at 45 °C.
The observed in-situ ES results were correlated to an ICA study of the C/10 characterization data at 25 °C, and the conclusions were sought verified by ex-situ analysis techniques opening the cycled pouch cells.

References

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Figure 1