Oxygen Partial Pressure as a Kinetic Descriptor for Single‐Crystal Morphology in Mid‐Nickel Layered Cathodes

Establishing the single-crystal morphology essential for durable mid-nickel layered cathodes requires high-temperature calcination, in which strongly coupled densification and grain-growth kinetics constrain scalable processing. Here, we combine controlled-atmosphere calcination experiments with phase-field modeling to resolve how oxygen partial pressure regulates the competing kinetics of densification and grain growth. We find that diffusional transport governs early densification, whereas grain-boundary mobility dominates grain growth. This indicates that processing conditions must be carefully optimized to balance accelerated grain growth with the efficient removal of trapped pores. Increased oxygen partial pressure raises the activation barrier for grain-boundary migration, suppressing excessive grain growth while maintaining microstructural uniformity, consistent with experimental observations. Based on these findings, we propose an air-oxygen sequential calcination process that dynamically modulates grain-boundary mobility during the dwell stage, overcoming the trade-off inherent to single-atmosphere calcination and enabling time-efficient single-crystal formation with enhanced cycling stability. The optimized air-oxygen sequence achieves a capacity retention of 83.04% after 100 cycles at 45°C, compared to approximately 77% under single-atmosphere conditions. These results establish oxygen partial pressure as an accessible process variable and quantitative kinetic descriptor for predictive single-crystal cathode synthesis.