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Hydrogenated Na₂Ti₃O₇ Epitaxially Grown on Flexible N-Doped Carbon Sponge for Potassium-Ion Batteries

Li, Peihao, Wang, Wei, Gong, Sheng, Lv, Fan, Huang, Hanxin, Luo, Mingchuan, Yang, Yong, Yang, Chao, Zhou, Jinhui, Qian, Chang, Wang, Bin, Wang, Qian, Guo, Shaojun
ACS applied materials & interfaces 2018 v.10 no.44 pp. 37974-37980
X-ray photoelectron spectroscopy, anodes, batteries, carbon, electrochemistry, hydrogenation, nanowires
With its inherent zig-zag layered structure and open framework, Na₂Ti₃O₇ (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 Na₂Ti₃O₇ (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 KTiₓOy-based materials. Our work highlights the importance of hydrogenation treatment and N-doped CS in enhancing the electrochemical property for KIBs.