Intergranular Cracking as a Major Cause of Long-Term Capacity Fading of Layered Cathodes.

Capacity fading has limited commercial layered Li-ion battery electrodes to <70% of their theoretical capacity. Higher capacities can be achieved initially by charging to higher voltages, however, these gains are eroded by a faster fade in capacity. Increasing lifetimes and reversible capacity are contingent on identifying the origin of this capacity fade to inform electrode design and synthesis. We used operando X-ray diffraction to observe how the lithiation-delithiation reactions within a LiNi0.8Co0.15Al0.05O2 (NCA) electrode change after capacity fade following months of slow charge-discharge. The changes in the reactions that underpin energy storage after long-term cycling directly correlate to the capacity loss; heterogeneous reaction kinetics observed during extended cycles quantitatively account for the capacity loss. This reaction heterogeneity is ultimately attributed to intergranular fracturing that degrades the connectivity of subsurface grains within the polycrystalline NCA aggregate.