Hydrogenated Na 2 Ti 3 O 7 Epitaxially Grown on Flexible N-Doped Carbon Sponge for Potassium-Ion Batteries.

With its inherent zig-zag layered structure and open framework, Na ₂ Ti ₃ O ₇ (NTO) is a promising anode material for potassium-ion batteries (KIBs). However, its poor electronic conductivity caused by large band gap (∼3.7 eV) usually leads to low-performance KIBs. In this work, we synthesize the fluff-like hydrogenated Na ₂ Ti ₃ O ₇ (HNTO) nanowires grown on N-doped carbon sponge (CS) as a binder-free and current-collector-free flexible anode for KIBs (denoted as HNTO/CS). High-resolution X-ray photoelectron spectroscopy (XPS) and electron spin-resonance spectroscopy (ESR) confirm the existence of Ti-OHs and O vacancies in HNTO. The first-principles calculation discloses that both Ti-OHs and O vacancies are equivalent to n-type doping because they can shift the Fermi level up to the conduction band, thus leading to a higher electronic conductivity and better performance for KIBs. In addition, the N-doped CS can further reinforce the conductivity and avoid the aggregation of HNTO nanowires during cycling. As a result, the as-made HNTO/CS can deliver a capacity of 107.8 mAh g ^-1 at 100 mA g ^-1 after 20 cycles, and keep the capacity of 90.9% and 82.5% after 200 and 1555 cycles, respectively, much better than the samples without hydrogenation treatment or N-doped CS and reported KTi _x O _y -based materials. Our work highlights the importance of hydrogenation treatment and N-doped CS in enhancing the electrochemical property for KIBs.