Force-balance criteria for droplet transport in laser cutting of LiFePO₄ cathodes in a multi-sensor framework

Droplet and spatter formation during laser processing is governed by a balance between capillary retention and vapor-driven transport forces, a framework well established for metallic melts but rarely tested on materials with markedly different wetting and surface-tension behavior. This is an open and industrially relevant question for high-throughput laser cutting of composite electrodes, which is widely adopted in lithium-ion battery manufacturing. Specifically, LiFePO₄ cathodes are affected by resolidified droplet defects along the cut edge and material ejection, which can compromise electrode integrity and safety. Despite their relevance, the mechanisms governing their formation and their real-time detectability remain poorly understood. This work combines high-speed imaging and multi-sensor photodiode monitoring to test the force-balance droplet-transport criteria and to identify optical signatures for real-time process discrimination during continuous-wave laser cutting of LiFePO₄ cathodes. Three distinct regimes are identified, driven by the balance between capillary, recoil, drag and gravity forces. Changing laser power and cutting speed shifts the process from a melting-dominated to a vapor-driven regime, each associated with characteristic defects. Results show that defects mainly originate from melting of the active material and that, under vapor-driven conditions, droplet size is reduced while material ejection becomes more widespread and intense. Photodiode monitoring provides evidence of a correlation between vaporization regime and optical signature of the process, enabling reliable discrimination between complete and incomplete cuts. This work elucidates defect generation mechanism and supports real-time process monitoring for high-quality battery electrode manufacturing.