| Résumé | Conventions established for battery cycling were originally from the domain of chemists and material scientists interested in characterizing the battery materials. Involvement in studies with large energy storage batteries for residential energy systems outfitted with renewables demonstrated that such scenarios were very different compared to cycling regimes used for battery characterization. Transient, irregular power production and storage coupled with stochastic electrical demand result in battery usage where frequent switching between charge and discharge modes occurs. Further, energy storage control systems favor maintaining the battery in an elevated state of charge. In view of these different circumstances, experiments to characterize battery performance for a more generalized and comprehensive operating environment were developed which explored varying amplitudes of battery cycles, as well isolating a full range of instantaneous depth of discharge levels with very small amplitude cycles. To investigate the operations on a mechanistic level, numerical simulations with a pseudo-2D electrochemical model were run to discover what is occurring inside the electrodes under these conditions. It was found that localized degradation effects vary significantly across the battery capacity range, and that the net extent of degradation increased with the duration of uninterrupted charging or discharging. Analysis was performed to find a good correlation between the measured cell degradation with time-dependent transport of electrolyte phase lithium ions in the anode as a driver for SEI formation. These findings have some bearing on claims being made about pulse charging regimes under development aimed at allowing fast charging with minimal cell capacity loss penalties. |
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