  Nickel-rich layered oxides have been widely used as positive electrode
materials for high-energy-density lithium-ion batteries, but their degradation
has severely affected cell performance, in particular at a high voltage and
temperature. However, the underlying degradation mechanisms have not been well
understood due to the complexity and lack of predictive models.Here we present
a model at the particle level to describe the structural degradation caused by
phase transition in terms of loss of active material (LAM), loss of lithium
inventory (LLI), and resistance increase. The particle degradation model is
then incorporated into a cell-level P2D model to explore the effects of LAM and
LLI on capacity fade in cyclic ageing tests. It is predicted that the loss of
cyclable lithium (trapped in the degraded shell) leads to a shift in the
stoichiometry range of the negative electrode but does not directly contribute
to the capacity loss, and that the loss of positive electrode active materials
dominates the fade of usable cell capacity in discharge. The available capacity
at a given current rate is further decreased by the additional resistance of
the degraded shell layer. The change pattern of the state-of-charge curve
provides information of more dimensions than the conventional capacity-fade
curve, beneficial to the diagnosis of degradation modes. The model has been
implemented into PyBaMM and made available as open source codes.
