In situ synthesis of amorphous titanium dioxide supported RuO₂ as a carbon-free cathode for non-aqueous Li–O₂ batteries

Abstract

Amorphous TiO₂ supported crystalline RuO₂ (a-TiO₂/c-RuO₂ hybrid) was prepared as a carbon-free cathode by in situ sol–gel synthesis, and its electrochemical performance in non-aqueous Li–O₂ batteries was investigated for the first time. The a-TiO₂/c-RuO₂ hybrid enhanced battery performance, and this enhancement was attributed to the crystallinity of the TiO₂. It was found that amorphous TiO₂ is more electrochemically active toward the discharge/charge processes than crystalline TiO₂. The a-TiO₂/c-RuO₂ hybrid exhibited good reversibility as well as cyclic stability, making it a promising carbon-free cathode material for non-aqueous Li–O₂ batteries.

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To meet the ever-growing demand for high energy density rechargeable batteries in various applications, Li–O₂ batteries have attracted much attention and have been intensively studied as an effective energy storage system, because they possess an extremely high energy density.¹ However, in practical use Li–O₂ batteries also have limitations, such as poor cycle life, low round-trip efficiency, electrolyte degradation and decomposition of the carbon cathode.² Among these shortcomings, the decomposition of the carbon cathode is responsible for poor battery performance because it results in the production of lithium carbonates (Li₂CO₃). Since the fundamental energy storage mechanisms in Li–O₂ batteries are the formation and decomposition of lithium peroxide (Li₂O₂)³ upon discharge and charging, respectively, the formation of Li₂CO₃ is undesirable.⁴ Moreover, deposited Li₂CO₃ covers and blocks the available cathode surface and does not decompose, which further deteriorates battery performance, including the development of large overpotential.⁵ As a result, it has become necessary to develop alternative carbon-free cathode materials for Li–O₂ batteries.

Titanium dioxide (TiO₂) nanoparticles have been intensively investigated in many industrial and scientific areas due to their high chemical and physical stability, low-cost, non-toxicity and a straightforward synthesis route.⁶ In particular, TiO₂ nanoparticles with diverse polymorphs, e.g., amorphous, anatase, brookite and rutile forms, have emerged in applications for energy storage devices, exhibiting excellent electrochemical activities.⁷

In this study, amorphous TiO₂ was chosen as a catalyst support and was combined with ruthenium oxide (RuO₂) as a catalyst in recognition of its great catalytic activity toward both discharge and charge.⁸ Although TiO₂ has been widely applied in many research fields including energy storage systems, employing TiO₂ as a catalyst support in Li–O₂ batteries has been rarely reported since the first report from Zhao et al. that Pt modified anatase TiO₂ nanotube arrays.⁹ Only few reports have been published on TiO₂ as a carbon-free cathode including a TiO₂(B) nanofiber@porous RuO₂ composite from Guo et al.¹⁰ Different from previous reports on TiO₂, which are crystalline in phase, the prepared TiO₂ is amorphous in phase. To the best of our knowledge, amorphous TiO₂ has not yet been reported as a catalyst support for Li–O₂ batteries.

Herein, we report a simple in situ sol–gel synthesis of amorphous TiO₂ supported crystalline RuO₂ as a carbon-free cathode material, and its electrochemical performance in Li–O₂ batteries is evaluated for the first time. The in situ synthesis is a convenient method for preparing TiO₂/RuO₂ hybrids owing to the simple and facile synthesis route. Moreover, the sol–gel process, which is one of the most commonly used methods for the preparation of metal oxide nanoparticles, provides chemical homogeneity at relatively low temperature.¹¹ The experimental details are shown in the ESI.†

The TiO₂/RuO₂ hybrids were synthesized by an in situ sol–gel method followed by annealing at relatively low temperature. Although it is not easy to demonstrate the exact formation mechanisms of TiO₂/RuO₂ hybrids, hydrolysis and polycondensation are assumed to be the main steps of the reactions.¹² Based on the hydrolysis of titanium oxysulfate and ruthenium chloride in aqueous solution, the resultant precursors are regarded as a composite of titanium hydroxide and ruthenium hydroxide. This in situ preparation may create close interfacial contact between the titanium and ruthenium, which facilitates the uniform distribution of the TiO₂ and RuO₂.¹³ The TiO₂/RuO₂ precursors were annealed at 300 and 600 °C for crystallization and phase transformation from hydroxide to oxide, and the resulting crystal structures of the TiO₂/RuO₂ hybrids are shown in Fig. 1a. When annealed at a given temperature both samples exhibited X-ray diffraction (XRD) patterns corresponding to the RuO₂ (JCPDS#01-071-4825). On the other hand, there was a difference in the diffraction patterns associated with TiO₂: the sample annealed at 600 °C exhibited a mixture of anatase (JCPDS#01-075-2546) and rutile (JCPDS#99-000-3236) phases of TiO₂, whereas the sample annealed at 300 °C exhibited the amorphous phase. Since XRD is not able to reveal the presence of amorphous material, it is essential to confirm the presence of TiO₂. In this regard, Raman spectroscopy was conducted on the a-TiO₂/c-RuO₂ hybrid, and the resultant spectra in Fig. 1b demonstrated the formation of TiO₂.¹⁴ Therefore, it can be concluded that TiO₂/RuO₂ hybrids with different crystalline structure were achieved by changing the annealing temperature. A composite of amorphous TiO₂ with crystalline RuO₂ (a-TiO₂/c-RuO₂) was obtained with the 300 °C-annealing, while crystalline TiO₂ with crystalline RuO₂ (c-TiO₂/c-RuO₂) was obtained with the 600 °C-annealing.

Fig. 1 (a) XRD patterns of TiO₂/RuO₂ hybrids based on the annealing temperature (b) Raman spectroscopy of a-TiO₂/c-RuO₂ hybrid (T: TiO₂, R: RuO₂).

The microstructural characteristics of a-TiO₂/c-RuO₂ and c-TiO₂/c-RuO₂ hybrids were studied using transmission electron microscopy (TEM). In the sol–gel approach, the relative reaction rates of hydrolysis and polycondensation greatly affect the size and morphology of TiO₂/RuO₂.¹⁵ As shown in Fig. 2, both a-TiO₂/c-RuO₂ and c-TiO₂/c-RuO₂ hybrids exhibit semi-spherical particles with diameters under 15 nm, indicating the formation of nanostructured materials, and this signifies that there is no significant morphological deviation or particle growth between the a-TiO₂/c-RuO₂ and c-TiO₂/c-RuO₂ hybrids. Therefore, the effect of annealing temperature on the particle size and morphology are negligible in the TiO₂/RuO₂ hybrid. For further investigation, TEM images were analyzed using energy dispersive X-ray spectroscopy (EDS) mapping, shown in Fig. 2c, to examine the elemental distribution of particles. From the results, it was observed that the elements including Ti, Ru and O, which originated from the TiO₂ and RuO₂, were uniformly distributed over the microstructure.

Fig. 2 TEM images of (a) a-TiO₂/c-RuO₂ (b) c-TiO₂/c-RuO₂ hybrids (c) TEM-EDS mapping images of a-TiO₂/c-RuO₂ hybrid.

The electrochemical performances of a-TiO₂/c-RuO₂ and c-TiO₂/c-RuO₂ hybrids as air electrodes in Li–O₂ batteries were investigated, and the results are shown in Fig. 3. The profiles of the galvanostatic discharge/charge were obtained based on the mass of TiO₂ and were compared at a current density of 100 mA g⁻¹. The a-TiO₂/c-RuO₂ hybrid showed high electrochemical activity toward ORR/OER in the given condition, whereas that of c-TiO₂/c-RuO₂ hybrid showed poor electrochemical activity. The capacity of the a-TiO₂/c-RuO₂ hybrid was 1200 mA h g⁻¹, while the c-TiO₂/c-RuO₂ hybrid was not even effectively discharged in the given range, showing a capacity of only 71 mA h g⁻¹. Since the important difference between a-TiO₂/c-RuO₂ and c-TiO₂/c-RuO₂ hybrids was their respective crystallinity, it is appropriate to assume that crystallinity is the key factor affecting their electrochemical properties in Li–O₂ batteries.

Fig. 3 Voltages profiles of TiO₂/RuO₂ hybrids.

However, it was not clear whether the amorphous TiO₂ provides a higher electrochemical activity than that of crystalline TiO₂, because it was reported in our previous study that RuO₂ crystallinity can also influence battery performance, based on its different catalytic activity.¹⁶ To determine the effect of TiO₂ crystallinity on battery performance, it is necessary to minimize the effect of the crystallinity of the RuO₂ catalyst in the TiO₂/RuO₂ hybrid. For that purpose, we employed the ex situ method, which allowed the amorphous TiO₂ and crystalline RuO₂ to be synthesized separately, and then mixed together in an agate mortar. The a-TiO₂/c-RuO₂ hybrid compound prepared by the ex situ method was fabricated into cathode material for Li–O₂ batteries, and the results were compared. As can be seen, the a-TiO₂/c-RuO₂ hybrid prepared by the ex situ method showed a capacity of 470 mA h g⁻¹ with an overpotential of 1.4 V, which are much better values than those of the c-TiO₂/c-RuO₂ hybrid. Therefore, it was demonstrated that amorphous TiO₂ is more electrochemically active as a catalyst support than crystalline TiO₂.

In order to provide insights into the reversibility and stability of the a-TiO₂/c-RuO₂ hybrid for Li–O₂ battery cathodes, the surface morphology at discharge/charge states and cycle life were examined. The FE-SEM image of a-TiO₂/c-RuO₂ hybrid taken after discharge demonstrates that the discharge products are deposited on the surface of the cathode, which clogs the oxygen penetration through the cathode (Fig. 4b). This blocked surface is recovered after subsequent charging, as presented in Fig. 4c, which shows that the deposited discharge products have completely disappeared, and the morphology of the surface is identical to pristine a-TiO₂/c-RuO₂ hybrid. To capture the details of discharge products, high-resolution X-ray diffraction (HR-XRD) and X-ray photoelectron spectroscopy (XPS) were applied and the results are shown in Fig. 4d and S1.† The HR-XRD result exhibited the peak corresponding to Li₂O₂ appeared after the first discharge. In Fig. S1,† the Li 1s spectrum obtained after discharge exhibited the Li₂O₂ relevant binding energy peak, which utterly disappeared after recharge.¹⁷ This strongly supports the result from FE-SEM that Li₂O₂ was produced as a main discharge product and was decomposed after recharge. Therefore, the reversibility of charge/discharge reaction was verified.

Fig. 4 FE-SEM images of a-TiO₂/c-RuO₂ hybrid: (a) pristine (b) after first discharge (c) after recharge, (d) HR-XRD pattern after first discharge (e) terminal voltages and energy efficiency as a function of cycle numbers.

For further investigation of reversibility and stability, the cycle life of a-TiO₂/c-RuO₂ hybrid at limited capacity was measured, and the results are provided in Fig. 4e. The cell was cycled at a DOD of 300 mA h g⁻¹ at a current density of 100 mA g⁻¹. The cell was observed to maintain capacity without fading up to 132 cycles with no significant decrease in discharge terminal voltage, or increase in charge terminal voltage. The energy efficiency of the a-TiO₂/c-RuO₂ hybrid in accordance with cycle life indicated that energy efficiency was more than 82% after 132 cycles. Based on this account, it is clear that as a cathode material the a-TiO₂/c-RuO₂ hybrid facilitates reversible and rechargeable Li–O₂ battery performance.

In summary, a a-TiO₂/c-RuO₂ hybrid was prepared by the in situ sol–gel route and its electrochemical performance as a new carbon-free cathode material for non-aqueous Li–O₂ batteries was evaluated for the first time. The a-TiO₂/c-RuO₂ hybrid achieved an overpotential of 1.0 V at a specific capacity of 1200 mA h g⁻¹, and these values were much better performances than those of the c-TiO₂/c-RuO₂ hybrid. The results strongly suggest that the electrochemical activity of TiO₂ as a catalyst support differs based on whether it is crystalline, and amorphous TiO₂ is more electrochemically active toward discharge/charge processes than crystalline TiO₂. The reversibility and stability of the a-TiO₂/c-RuO₂ hybrid was demonstrated, confirming its applicability as an alternative to carbon material for non-aqueous Li–O₂ batteries.

Acknowledgements

This research was supported by the Government-Funded General Research & Development Program by the Ministry of Trade, Industry and Energy, Republic of Korea.

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Footnote

† Electronic supplementary information (ESI) available: Details of the synthesis. See DOI: 10.1039/c6ra16645h

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