Physics-based, reduced order degradation model of lithium-ion batteries

Abstract

A physics-based, reduced order framework is developed to calculate the charge capacity loss contributions from spatially homogeneous and heterogeneous degradation mechanisms, chemomechanical cycling and initial capacity recovery. The formulation goes well beyond prevalent coulomb-counting models and is tuned solely based on experimentally measurable parameters, although it neglects any buildup of internal resistance. The model is compared against the largest data set available to date for commercial LiFePO4-graphite cells and shows less than 10% error for 92% of the cells. Results suggest that in most cells the charge capacity increases through the first ∼50 cycles, beyond which homogeneous SEI growth dominates capacity loss up to ∼500 cycles. Above ∼500 cycles and at high current densities, the model attributes capacity loss primarily to heterogeneous SEI growth proceeding at microstructurally favored locations, further assisted by chemomechanical failure of the graphite anode particles towards the end of cell life. The developed model sets the stage for on-the-fly capacity loss calculations in hybrid and electric vehicles, especially at low currents and constant voltage holds and could be extended to capture the deleterious effects of high current densities in fast-charging scenarios by including resistive losses.