Preparation of microencapsulated NH4H2PO4 solution and flame suppression of lithium battery thermal runaway

This study addresses the bottleneck issue of insufficient functionality of traditional extinguishing agents in the fire prevention and control of lithium batteries in new energy vehicles. Specifically, water-based extinguishing agents can trigger chain reactions upon contacting the electrolyte due to exothermic reactions; dry powder extinguishing agents fail to cool quickly enough due to low specific heat capacity; and foam extinguishing agents are prone to damage of their coverage layer under jet impact. To tackle these challenges, an innovative sodium alginate-gelatin/ammonium dihydrogen phosphate (SA-GE/NH4H2PO4) core-shell microcapsule system has been designed and constructed. A saturated ammonium dihydrogen phosphate solution, known for its significant chain reaction suppression capability, serves as the core. Utilizing the electrostatic complexation mechanism of gelatin (GE) and sodium alginate (SA), an efficient microencapsulation method is achieved through coacervation. Key process parameters were optimized, resulting in the following conditions: coacervation temperature of 40 °C, pH 3.8, shell material concentration of 8 wt%, and core-shell mass ratio of 4:1. Under these parameters, the microcapsules exhibited well-defined morphology, good monodispersity, and an encapsulation efficiency of 26.9 ± 0.6 %. Thermal analysis revealed that the SA-GE shell increased the liquid core's release initiation temperature from 75 °C to 125.3 ± 2.5 °C. Accelerated aging tests (40 °C/75 % RH, 30 days) confirmed a mass retention rate of 94.2 ± 0.8 %. Fire extinguishing efficacy evaluation demonstrated that in a ternary lithium battery standard thermal runaway model, the microcapsule extinguishing response time was 9 ± 0.5 s, which is a reduction of 25 % compared to the saturated ammonium dihydrogen phosphate solution (12 ± 1.2 s) and 30.8 % compared to perfluorohexane (13 ± 1.0 s). This system also achieved a 25.1 % reduction in peak surface temperature of the battery (control group: 19.1 % and 13.0 %) and completely eliminated the phenomenon of re-ignition. These results validate that the microcapsule system provides an innovative solution for lithium battery fire prevention and control through a ''physical barrier-thermal phase change-chemical suppression'' triple synergistic mechanism.