Durable flame-retardant cotton fabrics with tannic acid complexed by various metal ions

•Various tannin-metal ions assembly coatings were developed for cotton fabrics.•Different flame-retardant effects of the coatings on cotton fabrics were revealed•Catalytic carbonization process of different metal ions on tannin was tracked.•Flame-retardant mechanism of different tannin-metal ions complexes was clarified.