Nickel-rich positive electrodes (e.g., NMC811) are becoming popular thanks to their high energy density and good rate capability. However, their layered structures are prone to phase transitions to disordered spinel and rock-salt structures when a large fraction of lithium diffuses out, leading to capacity fade and electrode degradation. To model this type of degradation, we developed a particle degradation model and incorporated it into the framework of the DFN model within PyBaMM especially for (dis)charging under high C-rates.
The particle degradation model describes lithium diffusion in the active core and the phase transition from the active core to degraded shell. The degraded shell is assumed to hold a fixed content of lithium and serve as a lithium conductor, bearing some resistance and leading to voltage drop across the shell. The phase transition proceeds with oxygen generation at the interface and diffusion in the degraded shell. The moving of the phase boundary towards the particle center defines the degradation in terms of loss of active material and loss of lithium inventory. These two metrics are calculated to study the capacity fade in cyclic aging and storage aging tests.
With the particle degradation model embedded into the DFN model, we can further study the spatial heterogeneity of degradation in the electrode thickness direction. This degradation mechanism can then be integrated into other battery degradation mechanisms (e.g., SEI growth and lithium plating) available in PyBaMM to comprehensively simulate battery aging in all kinds of operating scenarios.