Construction of a Green Flame-Retardant Shield for Thermoplastic Polyurethane Composites via Interactive Effects of Nickel–Cobalt–Manganese and Ammonium Polyphosphate: From Waste Batteries to Flame Retardants

The annual volume of battery waste steadily increases, yet its recycling rate stays sluggish. Notably, battery waste comprises various transition metals, which hold a high potential in the flame-retardant field. This study explores a recycling-based strategy to repurpose spent nickel-cobalt-manganese (NCM) battery cathodes as flame-retardant additives in thermoplastic polyurethane (TPU). Unlike conventional metal oxides, the multicomponent transition metal nature of recycled NCM provides a unique catalytic interface with ammonium polyphosphate (APP), influencing char formation and fire behavior. In response, this study puts forward an innovative approach to utilize solid battery waste. Specifically, we have recycled NCM material from the battery cathode and incorporated it into a TPU elastomer, combining with APP. The interactive effect originates from interface-mediated catalytic carbonization between NCM and APP within the TPU matrix, leading to enhanced flame-retardant performance. As revealed, the limiting oxygen index (LOI) of the APP8.75/NCM0.25/TPU composite reaches 26.8 vol %, higher than that of neat TPU (22.5 vol %) and comparable to APP9/TPU (27.3 vol %). The fire property results further illustrate that, as for the APP8/NCM1/TPU composite, the peak heat release rate (PHRR), total heat release (THR), total smoke release (TSR), peak CO release, and peak CO₂ release are reduced by 80.0, 56.7, 54.5, 39.8, and 56.9% respectively. Overall, these findings clearly indicate that the APP/NCM flame-retardant system effectively enhances the fire safety of the composite. This study not only paves the way for the high-value utilization of NCM but also offers novel insights into the development of flame-retardant polymer composites.