Synergistic Flame Retardancy of Microcapsules Based on Ammonium Polyphosphate and Aluminum Hydroxide for Lithium-Ion Batteries

[Image: see text] Flame retardants have important theoretical research and applied value for lithium-ion battery safety. Microcapsule flame retardants based on ammonium polyphosphate (APP) and aluminum hydroxide (ATH) were synthesized for application in lithium-ion batteries. First, the ATH–APP was prepared by coating a layer of ATH on the surface of the core APP. Then, the ATH–APP was encapsulated by poly(urea-formaldehyde) (PUF) to obtain en-ATH–APP. The structure and flame-retardant property of en-ATH–APP, the influence of en-ATH–APP on the thermal stability of the electrode, and the electrochemical performance of the battery were studied. The results of Fourier transform infrared and scanning electron microscope experiments indicated that APP was coated with ATH and PUF in turn. The results of differential scanning calorimetry and the fire extinguishing test for electrodes manifested that en-ATH–APP had better flame-retardant property than APP because of the synergistic effect between APP and ATH. Moreover, the flame-retardant efficiency of en-ATH–APP was comparable to that of ATH–APP, indicating that the presence of PUF had almost no effect on the flame-retardant property. The results of electrochemical experiments indicated that en-ATH–APP had the best electrochemical compatibility for the battery compared with APP and ATH–APP. The research lights the way to improve inherent safety of lithium-ion batteries by adding en-ATH–APP to the cathode.