Modeling Degradation to Predict Capacity and Cell Level Reversible and Irreversible Expansion throughout Life across Various Cycling Conditions
Authors: Siegel Jason , Pannala Sravan , Wan Zhiwen , Stefanopoulou Anna , Jones Matthew , Garrick Taylor R. 
Published: December 2024

In this presentation, we will explore the coupling between active material volume change, mechanics, transport, and various degradation modes for lithium-ion battery pouch cells. An experimental aging study was designed to probe the impact of temperature, SOC cycling window, and preload pressure. Over one hundred pouch cells were manufactured and cycle-aged in custom build fixtures which enable the application of a nearly constant spring-loaded surface pressure while continuously measuring the cell thickness during charging and discharging.
A Single Particle Model with electrolyte dynamics (SPMe) implemented in the PyBaMM (Python Battery Mathematical Modelling) framework was augmented with particle cracking, SEI growth, and Lithium plating degradation mechanisms [1]. This work extends the model with the theory from [2] to address the impact of preloading and external boundary conditions on porosity. We then considered the mechanical impacts of SEI growth and accumulation of dead lithium [3] on the electrode volume change equation. Since the cells were built on the UM Battery Lab pilot-scale manufacturing line the geometric dimensions, electrode loadings, material properties, and half-cell potentials were readily accessible to populate the physics-based model. The remaining kinetic model parameters were tuned to match the beginning of life performance on a modified HPPC profile. Finally, the rates of various aging mechanisms were tuned to match the individual cell capacity and thickness growth at each reference performance test in this unique and rich experimental dataset.
The reversible thickness change (during each charge-discharge cycle) is directly captured by the predicted changes in the positive and negative electrode stoichiometry range based on the model's predicted Loss of Active Material (LAMn,p). The predicted electrode capacity retention and remaining cyclable lithium feed to the model calculation of reversible active material volumetric expansion. In addition, the irreversible thickness growth (across many cycles) is parameterized for different aging mechanisms with a linear relationship on SEI growth, quadratic on the Lithium plating, and linear on the particle fracture. The model is trained with data from slow and fast charge and discharge rates at a full SOC cycling window and its accuracy is evaluated at asymmetric charge-discharge and partial SOC cycling window.
By matching both the cell voltage and thickness change over life with a single parameter set we can achieve higher confidence in the model parameters. This is a key step in understanding the impact of degradation mechanisms on the irreversible changes in electrode thickness. The expected benefit of the proposed simulation tool will aid in the design process by allowing researchers to rapidly assess the implications of cell design choices (for example N/P ratio, electrode, separator and fixture compressibility ratio, and external pressure and dimensions) on performance over life and to understand the implications for packaging and potential performance tradeoffs reducing the need for physical testing.

Sravan Pannala et al 2024 J. Electrochem. Soc. 171 010532

Taylor R. Garrick et al 2017 J. Electrochem. Soc. 164 E3592

Shanshan Xu et al 2019 J. Electrochem. Soc. 166 A3456