Water-soluble ammonium polyphosphate synchronously enables mechanically robust and flame-retardant cellulose composite separator for high safety lithium batteries

Water-insoluble ammonium polyphosphate (APP) is widely used to impart flame retardancy to combustible materials, while the use of water-soluble APP is limited due to its high water-solubility. Taking the nonaqueous strength of electrolyte for lithium batteries, in this work, a composite separator (BLA) is fabricated by purposefully introducing a highly water-soluble APP into a bacterial cellulose (BC) aqueous dispersion together with a mixed aqueous solution of lignosulfonate and polyamide-epichlorohydrin (LPC), followed by freeze drying and pressing treatments. The incorporation of LPC improves the tensile strength of BC, and on this basis, besides endowing the separator with flame retardant function, the utilization of water-soluble APP synchronously and significantly enhances the separator's tensile strength, which is not achievable with the water-insoluble APP. Good compatibility between BC, LPC and APP and their multiple interactions including electrostatic attraction, hydrogen bonding and covalent crosslinking contribute to the mechanical improvement. Meanwhile, the BLA separator has a micro-nano porous structure, high porosity, and electrolyte uptake thanks to the ideal mix of these components and the freeze drying method, which also gives the cell outstanding cycling stability with a capacity retention of 97% after 150 cycles at 1 C. Therefore, the as-prepared BLA membrane may be a promising separator choice for lithium batteries when considering safety and battery performance.