Unraveling the Interplay between Degradation Mechanisms during Battery Cycling Conditions: Influence on SEI Growth
Authors: del Olmo Diego , Elzaurdi Maria , Baraldi Giorgio , Echeverria Maria , Ayerbe Elixabete 
Published: February 2024

Although the solid electrolyte interphase (SEI) is integral for the operation and stability of Li-ion batteries, its continuous growth over the battery life leads to in loss of cyclable material and overall degradation which results in capacity face. Therefore, a proper understanding of the mechanisms and factors involved in the SEI evolution during battery operation is integral for both reliable ageing models, allowing proper prognosis of the state of health (SoH), and improvement of battery lifetime.

Generally, studies of SEI growth have focused on calendar ageing experiments, in which the battery is left undisturbed for long periods of time and periodic check-up cycles are performed to estimate the capacity loss over time. Despite significant advances in the understanding of SEI formation with this kind of experiments, the specific limiting mechanism for SEI growth is still under discussion, with electron or Li-interstitial diffusion through the SEI being the main candidates in recent studies. Calendar ageing provides a suitable framework to study SEI growth in almost isolation from other degradation mechanisms but neglects the synergistic effects that can occur between them in real operation conditions, in which the SEI growth will be influenced by other degradation processes. For example, it has been reported that lithium plating can favor SEI formation and that new SEI will growth in exposed active material through cracks in the particles or in the existing SEI. Although initial efforts towards accounting for these synergies are already present in the literature, they have been applied to sparse sets of data and particularized for single chemistries.

In this work we focus on the study SEI growth during cell operation and its interplay with other degradation mechanisms in the cell in different regimes to partially isolate degradation processes (e.g., varying cut-off voltages to favor lithium plating). To this end, we carry out electrochemical cycling tests with LNO/Gr and NMC811/Gr full coin cells and also assemble 3-electrode cells to differentiate between positive and negative electrode contributions. Moreover, the individual contributions from the different degradation mechanisms are assessed by combining EIS measurements, incremental capacity analysis and morphological characterization.

The results of the characterization are then used to parametrize the implementation of degradation models (SEI, particle cracking, lithium plating, loss of active material...), with both semi-empirical and physics-based approaches, within a Doyle-Fuller-Newman pseudo-2D framework. The results of the model are then compared with the full coin cells experiments and used to evaluate the degradation synergies and provide best-practice approaches to balance battery performance and lifetime. Moreover, the flexibility of the model with degradation is tested using larger publicly available and literature datasets for different chemistries to assess its suitability in cases for which the complete model parameterization is not available (as it is often the case in commercial cells).