Surface Engineering Strategy Using Urea To Improve the Rate Performance of Na₂Ti₃O₇ in Na‐Ion Batteries

Abstract

Na2Ti3O7 (NTO) is considered a promising anode material for Na‐ion batteries due to its layered structure with an open framework and low and safe average operating voltage of 0.3 V vs. Na+/Na. However, its poor electronic conductivity needs to be addressed to make this material attractive for practical applications among other anode choices. Here, we report a safe, controllable and affordable method using urea that significantly improves the rate performance of NTO by producing surface defects such as oxygen vacancies and hydroxyl groups, and the secondary phase Na2Ti6O13. The enhanced electrochemical performance agrees with the higher Na+ ion diffusion coefficient, higher charge carrier density and reduced bandgap observed in these samples, without the need of nanosizing and/or complex synthetic strategies. A comprehensive study using a combination of diffraction, microscopic, spectroscopic and electrochemical techniques supported by computational studies based on DFT calculations, was carried out to understand the effects of this treatment on the surface, chemistry and electronic and charge storage properties of NTO. This study underscores the benefits of using urea as a strategy for enhancing the charge storage properties of NTO and thus, unfolding the potential of this material in practical energy storage applications.

Surface engineering of anode materials for improving sodium-ion storage performance

“…Metal oxides, such as titanium dioxide (TiO 2 ), [59][60][61][62][63] tin dioxide (SnO 2 ), 64,65 vanadium oxide (V 2 O 5 ), 66 and sodium titanate (Na 2 Ti 3 O 7 ) 67,68 are promising anode candidates for NIBs. For example, TiO 2 has merits of excellent cycling stability and low cost.…”

“…Introduction of OVs on the surface of metal oxides can signicantly enhance electronic conductivity and lower the sodiation energy barrier to facilitate Na ion intercalation and migration kinetics, achieving enhanced charge transport kinetics. 59,74 The typical OV fabrication methods include treatment using a reductant (e.g., NaHB 4 or urea) 61,63,67 or in a reductive atmosphere (e.g., H 2 ), 68 and carbothermal reduction. 65 Ji and co-workers employed NaBH 4 as a reductant to treat anatase to introduce surface OVs into TiO 2 .…”

“…Costa et al used urea as the reducing agent to form OVs on the surface of Na 2 Ti 3 O 7 particles. 67 At high temperature, urea decomposes into ammonia which further decomposes to produce reactive H 2 that removes oxygen from the Na 2 Ti 3 O 7 structure, forming surface OV decorated Na 2 Ti 3 O 7. It was found that the concentration of OVs could be controlled by controlling the concentration of urea.…”

Enabling both ultrahigh initial coulombic efficiency and superior stability of Na2Ti3O7anodes by optimizing binders

“…10,11 Thus, a series of designs and optimizations to address these issues are highly desired. To date, there have been various strategies mainly involving: (i) bulk or surface modulation of the crystal structure and reduction of Ti 4+ to Ti 3+ through doping 12,13 or fabricating defects, 14–16 (ii) regulating the morphology and nanostructure engineering, 17,18 (iii) composite fabrication with carbon 19 or Na 2 Ti 6 O 13 , 20 and (iv) surface coating using conducting carbon or inert metal oxide layer. 21…”

“…10,11 Thus, a series of designs and optimizations to address these issues are highly desired. To date, there have been various strategies mainly involving: (i) bulk or surface modulation of the crystal structure and reduction of Ti 4+ to Ti 3+ through doping 12,13 or fabricating defects, [14][15][16] (ii) regulating the morphology and nanostructure engineering, 17,18 (iii) composite fabrication with carbon 19 or Na 2 Ti 6 O 13 , 20 and (iv) surface coating using conducting carbon or inert metal oxide layer. 21 From the above-undertaken efforts, it is not difficult to nd that these modifying methods exclusively concentrate on Na 2 Ti 3 O 7 itself, which has undoubtedly achieved great advancements; nonetheless, the studies on relevant inert components (i.e., the binder and conductive agent), occupying a minor content but playing an irreplaceable role, are rather scarce and generally ignored.…”