(327g) A Reduced-Order Physics-Based Calendar Life Model for Li-Ion Cells
In order to estimate the state of health of Li-ion cells under storage condition, a control-oriented physics-based calendar life model has been developed. The Solid Electrolyte Interphase (SEI) formation on the anode particles due to a parasitic reaction between the solvent and Li+, is the main degradation mechanism responsible for the cell capacity fade. The reaction rate of the SEI reaction is controlled by diffusion of solvent through SEI and reaction kinetics on the surface of particles. A simple algebraic expression for time dependent SEI thickness and in turn cell capacity fade can be obtained by combining SEI mass balance, linear solvent diffusion and a Tafel kinetic for electrolyte decomposition1-2. The only adjustable parameters of this simple model are the reaction rate constant and the solvent diffusivity which vary with temperature by Arrhenius relation. While this model is able to predict the capacity fade of cells stored at different temperatures and state of charges (SOCs) for long durations, it might fail to estimate the resistance growth particularly for Li-ion cells with high-voltage cathode materials (e.g. Nickel Cobalt Aluminum oxide (NCA), Nickel Manganese Cobalt oxide (NMC)). An additional side reaction is assumed to occur in the cathode side due to the electrolyte instability at high potentials, causing presumably gas generation3. Whereas the cathode side reaction does not consume any Li+ and as a result has no impact on the cell capacity, it increases the cell resistance especially at high temperatures and SOCs. The new calendar life model successfully predicted 8-month storage data including the cell capacity and resistance at four temperatures and three SOCs.
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