Polyaniline Intercalation and Encapsulation of Titanate as Anodes for High-Performance Sodium-Ion Capacitors

As an intercalation-type anode material for sodium storage, layered Na 2 Ti 2 O 5 exhibits good rate capability and operates at a low potential. However, its intrinsically low electrical conductivity and Na-ion diffusion coefficient, together with lattice distortion caused by irreversible Na-ion insertion, lead to sluggish diffusion kinetics that are difficult to match those of the capacitive cathode in sodium-ion capacitors (SICs). By means of an acid-base reaction, aniline monomers are inserted into the Ti-O layers of H 2 Ti 2 O 5 (HTO) and subsequently polymerized to form polyaniline (PANI) by air or FeCl 3 oxidation. Simultaneously, a PANI layer coats the HTO surface through electrostatic or chemical adsorption. After low-temperature heat treatment, PANI-intercalated and encapsulated HTO is obtained. Intriguingly, the polymerization route, air oxidation or FeCl 3 oxidation, dictated the PANI content both in the interlayer and on the external surface, thereby modulating sodium storage performance. Compared with pristine HTO, the interlayer spacing of PANI-modified HTO by air oxidation expands from 0.775 to 0.853 nm, and the band-gap energy decreases from 3.41 to 3.34 eV. Consequently, this sample delivers a high specific capacity of 389.3 mAh g −1 at 0.1 A g −1 and excellent rate capability (158.1 mAh g −1, 10 A g −1 ). SICs assembled with the PANI-modified HTO anode achieve an energy density of 58.4 Wh kg −1 at a high power density of 3000 W kg −1 and retain 80.9% of their initial capacity after 5000 cycles at 1 A g −1 . This dual-modification strategy based on conductive polymers offers an effective pathway for developing high-performance titanate anodes in SICs.