An Ultrathin Paraffin@SiO ₂ Nanocapsule Coating Enabled a Composite Separator for Thermal Shutdown in Lithium-Ion Batteries

Thermal runaway represents a fundamental safety barrier for high-energy-density lithium-ion batteries, hindering their widespread application. Consequently, there is a critical need for intelligent safety strategies that can deactivate batteries during thermal runaway events without compromising their electrochemical performance. This study synthesized core–shell paraffin@SiO2 nanocapsules (100–200 nm) via interfacial polycondensation, utilizing paraffin (Pn) with a phase change temperature of 85 °C as the thermal-responsive core. The synthesized nanocapsules were then uniformly coated onto a commercial PP separator to form a composite separator, denoted as PP/Pn@SiO2 (PPS), which featured an ultrathin, continuous functional layer approximately 5 μm thick. The hydrophilic SiO2 shell markedly improves both the electrolyte wettability and interfacial compatibility of the PPS separator, as evidenced by a low contact angle of 19.8° and a reduced interfacial impedance of 28.63 Ω. This optimized interface ensures stable electrochemical performance at room temperature, yielding a high-capacity retention of 99.18% over 100 cycles. Upon heating to 90 °C, the paraffin core in the Pn@SiO2 nanocapsules undergoes a solid–liquid phase change, causing it to melt, expand, and diffuse through the mesoporous silica shell. The molten Pn subsequently permeates and seals both the internanocapsule gaps and the micropores on the separator surface, thereby blocking the transport pathways of lithium ion. This integrated mechanism initiates an autonomous battery shutdown during the incipient stages of thermal runaway. Consequently, the engineered PPS composite separator provides an effective solution to mitigate thermal runaway risks in lithium-ion batteries while preserving their electrochemical performance.