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Binson Babu1 Sanjay Ullattil2 Ranjith Prasannachandran1 Jithesh Kavil2 Pradeepan Periyat2 M.M. Shaijumon1

1, Indian Institute of Science Education and Research, Thiruvananthapuram, , India
2, University of Calicut, Calicut, , India

With the on-going thrust on sodium chemistry, owing to the huge abundance of sodium over lithium resources and similar electrochemical properties with that of lithium, sodium ion batteries (NIBs) are being projected as alternatives to lithium ion batteries (LIBs). Several intercalation materials have been studied as electrodes for NIB, most of which showed poor rate capabilities, owing to the huge size of sodium ions compared to lithium ions.[1] Inorder to overcome these conventional power limitations, the concept of hybrid ion capacitors, which combines the advantages of both faradaic and non-faradaic processes, seems to be a promising approach. The cathode provides high power density, being a double layer capacitive electrode, while the anode imparts higher energy density.[2,3,4] Thus a hybrid capacitor can achieve both the power and energy densities with very long cycle life. However, developing efficient sodium storage materials for the development of high performance Na-ion capacitor systems still remains a challenge. Herein, for the first time, we study the sodium-ion intercalation pseudocapacitance behavior of black TiO2 nanotubes for their application as efficient anode material for Na-ion hybrid capacitor. Black TiO2 nanotubes with a flower-like morphology, obtained through hydrothermal route, is characterized by several techniques such as X-ray photoelectron spectroscopy (XPS), electron energy loss spectroscopy (EELS), UV-absorbance, FTIR, Raman spectroscopy, etc. The presence of Ti3+ during TiO2 reduction by foreign donor atoms or the oxygen deficiencies or defects created in the nanostructures, enhanced the conductivity due to the colossal bandgap narrowing (~1.51 eV). Black TiO2 electrode exhibited excellent electrochemical properties revealing Na-ion intercalation pseudocapacitive behavior when cycled against Na-metal, with 57% of capacitive storage at 1.0 mV s-1. Further, we carried out detailed studies on diffusion coefficient of Na-ions inside the black TiO2 by using Galvanostatic Intermittent Titration Technique (GITT) and Electrochemical Impedance Spectroscopy (EIS) methods.[5] A hybrid Na-ion capacitor is fabricated with black-TiO2 as anode and activated carbon as cathode, and the device showed a high energy density of ~ 68 Wh kg-1 and a high power density of ~12.5 kW kg-1 with excellent cycling stability up to 10,000 cycles with ~ 80% capacitve retention. Thus the study reveals black TiO2 as a promising candidate for sodium ion hybrid capacitors.

References
[1] M. D. Slater et al., Adv. Funct. Mater. 23, (2013), 947–958.
[2] K.Naoi et al., Energy Environ.Sci. 5, (2012), 9363-9373.
[3] B. Babu, P. G. Lashmi and M. M. Shaijumon, Electrochim. Acta 211, (2016), 289-296.
[4] Binson Babu and M. M. Shaijumon, J. Power Sources 353 (2017) 85-94
[5] M.V. Reddy et al., Electrochim. Acta, 128 (2014) 198-202.

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