TiO₂–Fe₂O₃ nanocomposites as high-capacity negative electrode materials for rechargeable sodium-ion batteries

Abstract

In this study, we report TiO₂–Fe₂O₃ nanocomposites as high-capacity negative electrode materials for rechargeable sodium-ion batteries with an ionic liquid electrolyte. The TiO₂–Fe₂O₃ electrode in the Na[FSA]-[C₃C₁pyrr][FSA] (FSA = bis(fluorosulfonyl)amide; C₃C₁pyrr = N-methyl-N-propylpyrrolidinium) ionic liquid electrolyte at 363 K delivers a high reversible capacity exceeding 360 mA h g⁻¹ at a current density of 10 mA g⁻¹, exhibiting good rate capability and cycling performance.

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Introduction

The development and utilization of renewable energy, such as solar energy and wind energy, will play a key role in meeting future energy demand and protecting the environment. Energy storage, especially battery storage, is one of the key technologies in developing renewable energy. Rechargeable sodium-ion batteries (SIBs), as a replacement for lithium-ion batteries, are attracting much attention as large-scale power storage devices for applications in electric vehicles and stationary energy storage, because sodium is a more abundant natural resource than lithium. The major challenge for the practical application of SIBs is the development of new active materials with large capacities, long cycle lives, and excellent rate capabilities. Although numerous materials have been reported as negative electrodes for SIBs, none of them display sufficiently high capacity, good cycling performance, adequate charge–discharge potential, good rate capability, natural abundance, etc. Alloy materials^1–4 exhibit high reversible capacities; however, they also suffer from low cycling performance due to the large volume changes that occur during charge–discharge cycles. Carbon materials,^5–9e.g., hard carbon,^8,9 were also investigated as negative electrode materials, showing reversible capacities of 200–300 mA h g⁻¹. Nevertheless, carbon materials always exhibit low volumetric performance because of their relatively low density. Moreover, hard carbon negative electrodes also tend to show a potential plateau below 0.1 V vs. Na/Na⁺, raising safety issues because of metallic sodium plating and dendrite formation. Some sodium titanium oxides such as Na₂Ti₃O₇ (ref. 10–12) and NaTiO₂ (ref. 13) were also investigated as alternative negative electrode materials, showing reversible capacities below 200 mA h g⁻¹ and poor rate capabilities.

Iron oxide, Fe₂O₃, is considered a promising negative electrode candidate for SIBs because of its abundance, low cost, high theoretical capacity (1007 mA h g⁻¹), and non-toxicity. Valvo et al.¹⁴ reported a nanostructured Fe₂O₃ negative electrode with a reversible capacity of about 420 mA h g⁻¹ at 40 mA g⁻¹. Jian et al.¹⁵ reported Fe₂O₃ nanocrystals anchored onto graphene nanosheets as a negative electrode material for SIBs, with a reversible capacity of 535 mA h g⁻¹ at 100 mA g⁻¹. Jiang et al.¹⁶ investigated a porous Fe₂O₃ thin film negative electrode, obtaining a reversible capacity of 550 mA h g⁻¹ at 50 mA g⁻¹. Despite exhibiting high reversible capacities, Fe₂O₃ electrodes usually suffer from a large volume change during Na-ion insertion/extraction, which leads to the rapid agglomeration of metal oxide particles and pulverization of electrode materials, finally causing capacity loss and deteriorated cycling performance.^15,17

Recently, titanium dioxides, including amorphous TiO₂,¹⁸ TiO₂ (B),¹⁹ TiO₂ (H),²⁰ anatase TiO₂,^21–24 and Nb-doped rutile TiO₂,²⁵ have been investigated as negative electrode materials for SIBs, with anatase TiO₂ showing the highest reversible capacity. Xu et al.²¹ investigated nanocrystalline anatase TiO₂ and reported a reversible discharge capacity of about 150 mA h g⁻¹. Kim et al.²² reported that an anatase TiO₂ nanorod electrode exhibited a discharge capacity of 193 mA h g⁻¹ in the first charge–discharge cycle and showed good cycling performance. However, the obtained reversible capacity was still lower than the theoretical capacity of 335 mA h g⁻¹. In a previous study, we used the Na[FSA]-[C₃C₁pyrr][FSA] (FSA = bis(fluorosulfonyl)amide; C₃C₁pyrr = N-methyl-N-propylpyrrolidinium) ionic liquid as an electrolyte to investigate the use of carbon-coated nanoscale anatase TiO₂ in SIBs. This TiO₂ negative electrode showed a relatively high reversible capacity of 275 mA h g⁻¹ and good cycling performance at 363 K.²⁶ However, the reversible capacity still needs to be further increased for practical applications.

Fe₂O₃ exhibits high capacity, while anatase TiO₂ shows good cycling performance. If the advantages of these materials are combined, a promising negative electrode material can well be expected. In this work, the electrochemical performance of TiO₂–Fe₂O₃ nanocomposites is examined in sodium half-cells using the Na[FSA]-[C₃C₁pyrr][FSA] ionic liquid as an electrolyte. The Na[FSA]-[C₃C₁pyrr][FSA] ionic liquid can be used in a wide temperature range and exhibits higher ionic conductivity at higher temperature.²⁷ In order to obtain a high capacity, a temperature of 363 K was adopted. The TiO₂–Fe₂O₃ negative electrodes show high reversible capacities above 360 mA h g⁻¹ at 10 mA g⁻¹ in the first cycle and exhibit good cycling performance at 363 K. Thus, TiO₂–Fe₂O₃ negative electrodes are promising electrodes for application in stationary sodium batteries. However, the TiO₂–Fe₂O₃ negative electrodes also present high irreversible capacities in the first cycle, which will be one of the main future challenges.

Results and discussion

The TiO₂–Fe₂O₃ nanocomposites were synthesized by mixing and heating commercially available anatase TiO₂ nanopowders and Fe(NO₃)₃. Nanocomposites with 10, 20, 30, and 40 wt% Fe₂O₃ were denoted as TiO₂–Fe₂O₃-10, TiO₂–Fe₂O₃-20, TiO₂–Fe₂O₃-30, and TiO₂–Fe₂O₃-40, respectively. Fig. 1 shows the X-ray diffraction (XRD) patterns of the TiO₂–Fe₂O₃ nanocomposites with different Fe₂O₃ contents, with anatase TiO₂ being the main crystalline phase in all nanocomposites. The diffraction peaks of Fe₂O₃ in TiO₂–Fe₂O₃-40 are very weak, implying that the synthesized Fe₂O₃ in the nanocomposites has poor crystallinity.

Fig. 1 XRD patterns of TiO₂–Fe₂O₃ nanocomposites with different Fe₂O₃ contents.

The field emission scanning electron microscopy (FE-SEM) image of TiO₂–Fe₂O₃-20 nanocomposites shows the presence of two types of particles (Fig. 2a) with sizes of 50–100 and less than 20 nm. The anatase TiO₂ nanopowders have particle sizes of 50–100 nm (Fig. S1a†), and the synthesized Fe₂O₃ nanopowders under the same conditions have a particle size of less than 50 nm (Fig. S1b†). Thus, it is concluded that the larger particles are TiO₂ and the smaller particles consist of amorphous Fe₂O₃. Fig. 2b presents the energy-dispersive X-ray (EDX) spectrum of TiO₂–Fe₂O₃-20 nanocomposites, showing the presence of Ti, Fe, and O elements with atomic molar fractions of 26.5%, 6.2%, and 67.3%, respectively, which coincide well with the theoretical composition of TiO₂–Fe₂O₃-20. This means that the synthesized powders include TiO₂ and Fe₂O₃ phases, with the other nanocomposites (Fe₂O₃-10, TiO₂–Fe₂O₃-30, and TiO₂–Fe₂O₃-40) showing similar results.

Fig. 2 (a) SEM image and (b) EDX spectrum of TiO₂–Fe₂O₃-20 nanocomposites.

The electrochemical performance of the TiO₂–Fe₂O₃ nanocomposites was evaluated at 363 K in half-cells using Na metal as the counter electrode. Fig. 3a shows the charge–discharge curves of the TiO₂–Fe₂O₃-20 electrode at a current rate of 10 mA (g-TiO₂–Fe₂O₃)⁻¹ in a voltage range of 0.01–2.5 V. In the first charge–discharge cycle, the TiO₂–Fe₂O₃-20 electrode shows a large charge capacity of 743 mA h (g-TiO₂–Fe₂O₃)⁻¹ and exhibits two distinct voltage plateaus between 1.7 and 1.5 V and 0.4 and 0.3 V. The first voltage plateau (1.7 to 1.5 V) is attributable to side reactions like electrolyte reduction and formation of a solid-electrolyte interphase (SEI),²⁶ while the second one (0.4 to 0.3 V) possibly corresponds to the insertion of Na into TiO₂–Fe₂O₃ and further decomposition of the electrolyte.²⁶ The first discharge capacity of the TiO₂–Fe₂O₃-20 electrode is 386 mA h g⁻¹, which is much higher than that of TiO₂ electrodes reported in the literature.^21–24,26 In subsequent cycles, the voltage plateaus disappear, and a stable cycling behavior with a reversible capacity of approximately 360 mA h g⁻¹ is observed.

Fig. 3 (a) Charge–discharge curves of the TiO₂–Fe₂O₃-20 electrode, (b) the first charge–discharge curves of the TiO₂–Fe₂O₃ electrodes with different Fe₂O₃ contents, and (c) charge–discharge curves of the TiO₂ and Fe₂O₃ electrodes at a current rate of 10 mA g⁻¹.

Fig. 3b shows the first charge–discharge curves of the TiO₂–Fe₂O₃ electrodes with different Fe₂O₃ contents at a current rate of 10 mA g⁻¹. All TiO₂–Fe₂O₃ electrodes exhibit similar charge–discharge behavior. With the increase of the Fe₂O₃ content, however, the TiO₂–Fe₂O₃ electrodes show higher discharge capacities of 367, 386, 420, and 459 mA h g⁻¹ for TiO₂–Fe₂O₃-10, TiO₂–Fe₂O₃-20, TiO₂–Fe₂O₃-30, and TiO₂–Fe₂O₃-40, respectively. The theoretical capacities are 402, 469, 537 and 604 mA h g⁻¹ for TiO₂–Fe₂O₃-10, TiO₂–Fe₂O₃-20, TiO₂–Fe₂O₃-30 and TiO₂–Fe₂O₃-40, respectively. Although the obtained discharge capacities are lower than the theoretical capacities, the discharge capacity of the TiO₂–Fe₂O₃ electrodes increases almost linearly with increasing Fe₂O₃ content, indicating that these nanocomposites are promising high-capacity negative electrode materials for SIBs. Fig. 3c shows the first charge–discharge curves of the TiO₂ and Fe₂O₃ electrodes at a current rate of 10 mA g⁻¹. The discharge capacities are 308 and 672 mA h g⁻¹ for the TiO₂ and Fe₂O₃ electrodes, respectively. Since the high capacity results from the high reversible capacity of Fe₂O₃,^14–16 increasing the Fe₂O₃ content can improve the reversible capacity of TiO₂–Fe₂O₃ electrodes.

According to the literature, Fe₂O₃ reversibly reacts with Na ions to form Fe metal and Na₂O during the charge–discharge process.^14–16 The formation of Fe metal could improve the conductivity and resistance of the TiO₂–Fe₂O₃ electrodes, which were investigated by electrochemical impedance spectroscopic analysis. Fig. S2† shows the Nyquist plots of the TiO₂–Fe₂O₃-30 electrode at different discharge states. As shown in Fig. 3, a small amount of charge–discharge reaction occurs for the TiO₂–Fe₂O₃ electrodes even at a cut-off voltage of 2.5 V. The semicircle at intermediate frequencies is related to the charge-transfer process at the electrode/electrolyte interface. Although the semicircle may be due to the charge-transfer process, SEI, and counter electrode reaction, the charge-transfer process should be the major factor because the impedances from the SEI^28,29 and counter electrode (Fig. S3†) are comparatively small. The charge-transfer resistance increases with discharging the electrode from 0.5 V to 2.5 V. Fig. 4 shows Nyquist plots of the TiO₂–Fe₂O₃ electrodes with variable Fe₂O₃ content after five charge–discharge cycles. With increasing Fe₂O₃ content, the charge-transfer resistance obviously decreases. Thus, the electrochemical performance of the TiO₂–Fe₂O₃ electrodes is improved by increasing the Fe₂O₃ content.

Fig. 4 Nyquist plots of TiO₂–Fe₂O₃ electrodes with different Fe₂O₃ contents after five charge–discharge cycles.

Rate capability is also an important parameter for describing electrode performance. Fig. 5a shows the charge–discharge curves of the TiO₂–Fe₂O₃-20 electrode at current rates of 10–2000 mA g⁻¹. When the current rate is increased, the shapes of the charge–discharge curves remain unchanged. However, the discharge capacities gradually decrease with an increase in the current rate. The other TiO₂–Fe₂O₃ electrodes also show similar charge–discharge behavior. Fig. 5b compares the rate capabilities of the TiO₂–Fe₂O₃ electrodes with different Fe₂O₃ contents. At current rates below 500 mA g⁻¹, the TiO₂–Fe₂O₃ electrodes with a high Fe₂O₃ content show high discharge capacities. However, when the current rate exceeds 500 mA g⁻¹, the effect of Fe₂O₃ content on the discharge capacity is attenuated due to the poor capability of Fe₂O₃ at high current rates. TiO₂–Fe₂O₃-10, TiO₂–Fe₂O₃-20, TiO₂–Fe₂O₃-30, and TiO₂–Fe₂O₃-40 show discharge capacities of 107, 91, 97, and 93 mA h g⁻¹ at 2000 mA g⁻¹ and 253, 258, 280, and 299 mA h g⁻¹ at 200 mA g⁻¹, respectively. This suggests that the capacity retention at high current rates is impaired, especially for the TiO₂–Fe₂O₃ electrodes with a high Fe₂O₃ content. In addition, as the current rate is decreased from 2000 to 20 mA g⁻¹, the discharge capacity recovers to high values.

Fig. 5 (a) Charge–discharge curves of the TiO₂–Fe₂O₃-20 electrode at current rates of 10–2000 mA g⁻¹; and (b) rate capabilities of TiO₂–Fe₂O₃ electrodes with different Fe₂O₃ contents at current rates of 10–2000 mA g⁻¹.

Fig. 6a shows the cycling performance of the TiO₂–Fe₂O₃ electrodes with different Fe₂O₃ contents at a current rate of 200 mA g⁻¹. For comparison, the cycling performance of the TiO₂ and Fe₂O₃ electrodes at a current rate of 200 mA g⁻¹ is shown in Fig. 6b. For all TiO₂–Fe₂O₃ electrodes, the discharge capacity decreases quickly in the first 50 cycles, remaining almost unchanged in subsequent cycles. The quick decrease of the discharge capacity is likely attributed to the quick capacity degradation of the Fe₂O₃, as shown in Fig. 6b. The TiO₂–Fe₂O₃ electrodes with a high Fe₂O₃ content show high discharge capacities in the first 50 cycles. However, after 100 cycles, the highest discharge capacity is exhibited by the TiO₂–Fe₂O₃-20 electrode. Except for the initial few cycles, the coulombic efficiencies of all TiO₂–Fe₂O₃ electrodes are higher than 99%. After 300 cycles, the discharge capacities are 227, 237, 217, and 199 mA h g⁻¹ for TiO₂–Fe₂O₃-10, TiO₂–Fe₂O₃-20, TiO₂–Fe₂O₃-30, and TiO₂–Fe₂O₃-40, respectively, with the respective capacity retention values being 80%, 72%, 63%, and 60%. This indicates that a high Fe₂O₃ content is not beneficial to the cycling performance of TiO₂–Fe₂O₃ electrodes, despite increasing their initial capacity.

Fig. 6 Cycling performance of (a) TiO₂–Fe₂O₃ electrodes with different Fe₂O₃ contents and (b) TiO₂ and Fe₂O₃ electrodes at a current rate of 200 mA g⁻¹.

Conclusions

In summary, the prepared TiO₂–Fe₂O₃ nanocomposites were evaluated as negative electrode materials for rechargeable sodium-ion batteries using the Na[FSA]-[C₃C₁pyrr][FSA] ionic liquid as an electrolyte. These nanocomposites consisted of anatase TiO₂ and amorphous Fe₂O₃ particles with sizes of 50–100 nm and less than 20 nm, respectively. The TiO₂–Fe₂O₃ electrodes with 10, 20, 30 and 40 wt% Fe₂O₃ exhibit high reversible capacities, with discharge capacities of 367, 386, 420, and 459 mA h g⁻¹ at 10 mA g⁻¹, respectively. In addition, these electrodes also show good rate capability and cycling performance, with the Fe₂O₃ content affecting both of these parameters. These results suggest that TiO₂–Fe₂O₃ nanocomposites are attractive high-capacity negative electrode materials for ionic liquid-based sodium-ion secondary batteries.

Experimental

Commercially available anatase TiO₂ nanopowders (Sigma Aldrich) and Fe(NO₃)₃·9H₂O (Wako) were used as raw materials to synthesize TiO₂–Fe₂O₃ nanocomposites. Fe(NO₃)₃·9H₂O was dissolved in ethanol, with the anatase TiO₂ nanopowders subsequently dispersed in this solution by stirring. The mixture was dried at 333 K and heated at 573 K for 3 h to obtain TiO₂–Fe₂O₃ nanocomposites, which were synthesized with 10, 20, 30 and 40 wt% Fe₂O₃. For comparison, Fe₂O₃ nanopowders were also synthesized under the same conditions. The crystal structure and morphology of these nanocomposites were investigated by X-ray diffraction (XRD, Rigaku SmartLab) and field emission scanning electron microscopy (FE-SEM, Hitachi SU8000) with energy-dispersive X-ray spectroscopy (EDX).

Electrochemical characterization was performed using coin-type 2032 cells with sodium foil as the counter electrode. TiO₂–Fe₂O₃ working electrodes were fabricated using a conventional coating method. A slurry consisting of TiO₂–Fe₂O₃ nanocomposites (80 wt%), acetylene black (10 wt%), and polyamide-imide (10 wt%) in N-methyl-2-pyrrolidone (NMP) was prepared and uniformly spread onto Al foil. Polyamide-imide was used in this study because it has higher chemical stability and mechanical strength at high temperatures compared to conventional polyvinylidene fluoride (PVdF). The obtained TiO₂–Fe₂O₃ electrodes were dried in a vacuum at 393 K overnight before being transferred into an Ar-filled glovebox. For comparison, TiO₂ and Fe₂O₃ electrodes were also fabricated under the same conditions. The Na[FSA]-[C₃C₁pyrr][FSA] ionic liquid with a molar ratio of 2 : 8 was used as an electrolyte. A glass fiber filter (Whatman, GF-A, 260 mm) was used as a separator. The TiO₂–Fe₂O₃ electrodes and separators were vacuum-impregnated with Na[FSA]-[C₃C₁pyrr][FSA] before being assembled into coin cells. Charge–discharge tests were conducted at constant current rates of 10–2000 mA g⁻¹ in a voltage range of 0.01–2.5 V at 363 K. Electrochemical impedance measurements of the TiO₂–Fe₂O₃ electrodes after charge–discharge cycles were performed in the frequency range of 200 kHz to 100 mHz using an AC voltage signal of 10 mV.

Acknowledgements

This study was partly supported by the Advanced Low Carbon Technology Research and Development Program (ALCA, No. 3428) of the Japan Science and Technology Agency (JST) and the Elements Strategy Initiative to Form Core Research Center program of the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6se00024j

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