The Influence of Formation Protocol on Heterogeneous Degradation of Lithium-ion Pouch Cells Revealed by Spatially Resolved Synchrotron X-ray Diffraction

<jats:p>The formation cycle strongly influences lithium-ion battery lifetime and performance but remains a costly manufacturing bottleneck, while its spatially resolved structural consequences remain poorly understood. Here, operando spatially resolved synchrotron X-ray diffraction reveals how formation C-rate and voltage-hold protocols govern lithiation heterogeneity and degradation in NMC811||graphite pouch cells. Cells formed at C/20, 1C, C/10, and using a modified voltage-hold protocol are compared after formation, with the same C/20 and 1C cells re-examined after 250 cycles. Slow C/20 formation produces spatially uniform graphite and NMC811 lattice responses that are largely retained during cycling. By contrast, 1C formation creates pronounced heterogeneity, including locally inactive regions and correlated lattice variations across opposing electrode areas. These features intensify with ageing and coincide with accelerated capacity loss, demonstrating that sub-optimal formation seeds persistent degradation pathways. Crucially, both the C/10 and voltage-hold protocols produce substantially more homogeneous responses than 1C, showing that uniform lithiation does not require slow C/20 formation. Although X-ray diffraction does not directly probe the solid electrolyte interphase, it reveals how formation-dependent interfacial processes manifest in electrode-scale lattice evolution. These findings establish spatially resolved diffraction as a powerful validation tool and show that accelerated formation can preserve electrode homogeneity without sacrificing long-term performance.</jats:p>