Surface coating engineering of titanate anodes based on tannic acid-formaldehyde polymer chelating bismuth ions for advanced sodium-ion capacitors

As a battery-type anode material for sodium ion capacitors (SICs), titanate (H₂Ti₂O₅·H₂O, HTO) exhibits good rate capability due to its layered structure, easy to insert Na⁺ ions and low potential during sodium-ion storage. However, the structure is unstable due to the lattice distortion resulting from the irreversible embedment of Na⁺ in the process of sodium storage. So there is a significant mismatch between the dynamic reaction of the HTO anode and the capacitive cathode. Surface coating engineering is a useful strategy for stabilizing the HTO structure, which is critical for improving the kinetic response. In this work, a surface coating technique is designed to enhance the surface of HTO nanoarrays on titanium foil by using the oligomers of tannic acid formaldehyde polymer (TAF) chelated Bi³⁺ ions (Bi-TAF). As a binder-free anode, HTO coated with Bi-TAF (HTO@Bi-TAF) exhibits more excellent capacity (335.2 mA h g^-1, 0.1 A g^-1), rate capability (212.3 mA h g^-1, 2.0 A g^-1), and cycle stability (97 % capacity maintenance following 2000 cycles at 1.0 A g^-1) than HTO and HTO coated with TAF (HTO@TAF). At the sweep rate of 1.0 mV s^-1, the kinetic investigation reveals that the capacitance contribution of HTO@Bi-TAF is 86 %. The SICs exhibit a significant energy/power density (89.4 Wh kg^-1/250 W kg^-1). This work shows that the Bi-TAF polymer coating has a dual effect of rate capability improvement and structural protection on the prepared HTO. This results in a reasonable and effective surface coating strategy that provides outstanding rate capability and extended cycle performance of titanium-based anode materials for SICs.