Ageing variability in single-crystal NMC811 under mission-profi le cycling: stochastic and diffusion-controlled insights

Abstract

Nominally identical nickel-rich NMC811 cathodes do not age in the same way. Even under identical cycling conditions, capacity trajectories diverge over time. Three cells from a fork-lift mission-profile dataset were analyzed. Results were compared with a larger fast-charging dataset containing 46 commercial lithium-ion cells. Capacity fade was evaluated round by round, and variability was quantified using the coefficient of variation (CV%) and an early-cycle deviation metric. Cycling caused variability in both datasets to rise from less than 1% CV to more than 3%. A diffusion-limited square-root ageing model was used to represent transport-controlled ageing. The model produces smoother capacity trends and a modest reduction in variability, but cell-to-cell differences persist. Early-stage heterogeneity remains linked to long-term divergence. Al₂O₃ coatings can promote diffusion-limited behaviour, while the model used here represents this regime conceptually rather than a specific coating system. The results point to a two-stage ageing pattern, with early divergence followed by partial moderation under transport influence.