YolkâShell TiO2@C Nanocomposite as High-Performance Anode

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Yolk-Shell TiO2@C Nanocomposite as HighPerformance Anode Material for Sodium-Ion Batteries Shen Qiu, Lifen Xiao, Xinping Ai, Hanxi Yang, and Yuliang Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12001 • Publication Date (Web): 13 Dec 2016 Downloaded from http://pubs.acs.org on December 15, 2016

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ACS Applied Materials & Interfaces

Yolk-Shell TiO2@C Nanocomposite as HighPerformance Anode Material for Sodium-Ion Batteries Shen Qiu,† Lifen Xiao,‡ Xinping Ai, † Hanxi Yang† and Yuliang Cao *†

†

College of Chemistry and Molecular Sciences, Hubei Key Lab. of Electrochemical Power

Sources, Wuhan University, Wuhan 430072, China. ‡

College of Chemistry, Central China Normal University, Wuhan 430079, P. R. China

* Corresponding author E-mail: [email protected];

Abstract: Yolk-shell TiO2@C nanocomposites have been synthesized successfully through a simple self-catalyzing solvothermal method. The structural and morphological characterizations reveal that TiO2@C nanocomposite has a yolk-shell microsphere morphology with diameters of 1-2 µm, and both yolk and shell are composed of TiO2 nanoparticles (~10 nm). The as-prepared yolk-shell TiO2@C composites exhibit superior sodium storage properties, with a specific capacity of 210 mAh g-1, an outstanding cycle life of 85% capacity retention of 2000 cycles and extraordinary rate performance at 40 C rate. All the results indicate that the yolk-shell TiO2@C

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nanocomposite can be suggested as a promising anode material for high-performance sodium-ion batteries. Keywords: TiO2@C; Yolk-shell nanostructure; Self-catalyzed reaction; Solvothermal synthesis; Na-ion batteries

1. Introduction Over the past two decades Li-ion batteries (LIBs) have been the most significant applications for electric vehicles and portable electronic devices. However, the scarcity and rising cost of lithium resources have created challenges in using lithium-ion batteries in large-scale energy storage applications.

1, 2

Recently, sodium-ion batteries (SIBs) have begun to attract more

attention because of the abundant resources, low cost and environment friendliness of sodium. 3, 4 However a large challenges for the application of SIBs mainly comes from a larger radius of Na ions (1.02Å) compared to Li ions (0.76Å), resulting in sluggish diffusion kinetics and structural collapse during sodium ion insertion/extraction into the electrode material.

5, 6

Therefore,

extensive efforts have explored appropriate electrode materials for SIBs. Until now, many host materials have been successfully studied as Na-storage cathodes with usable capacity and cycle stability, such as layered oxides, 7, 8 phosphates, 9, 10 hexacyanoferrate

11, 12

and so on. However,

the development of anode materials is confined to only a few types, such as carbonaceous materials, 13-15 alloys

16, 17

and alloy compounds

18, 19

and so on. Among these anode materials,

hard carbon anodes demonstrate good sodium storage abilities with reversible capacities of 200~300 mAh g-1, however, their applications are limited because they have a low sodium insertion potential, which can also lead to possible safety hazards. 20-23 Na alloy anodes have received a wid range of interests because of their high reversible capacity. 16, 24 However, the


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sodium storage reactions involing alloying and conversion processes are inevitably accompanied by drastic structural expansion of the electrode materials, which eventually leads to the pulverization of the materials and fast capacity decay. Consequently, it is a key factor to develop novel anode materials which possess improved cycle stability and rate capability for the practical application of SIBs. Recently, titanium dioxide (TiO2) has emerged as a promising candidate for SIB anode materials because of its high structural stability, nontoxicity and low cost. 25-36 Additionally, it is known that insertion and extraction of Na ions in TiO2 can be easily realized because of the tunnel structure formed by the stacked TiO6 octahedra, which offers suitable sites and pathways to accommodate sodium insertion/extraction in the crystal structure. 27 The redox of Ti4+/Ti3+ is expected to obtain high capacity during sodiation and desodiation process, which resembles the mechanism involved in Li-ion batteries.

25, 26

But, it is worth noting that the low intrinsic

electronic conductivity of TiO2 results in relatively poor reversible capacity and rate capability. Thus, it is very important to increase the diffusion kinetics of Na+ and intrinsic electronic conductivity in TiO2 to achieve improved capacity and rate performance. A common and effiective strategy to improve diffusion kinetics is to construct nanostructural materials. As known, nanostructural materials can immensely enhance the electrochemical capacity due to the short diffusion paths and large electrochemical reaction interface.

28

For

instance, Xu et al. synthesized crystalline anatase TiO2, which showed a stable charge capacity of ~150 mAh g-1 in 100th cycle. 29 Rajh et al. synthesized amorphous TiO2, which reached the reversible capacity of ~150 mAh g-1 over 15 cycles.

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On the other hand, a feasible approach

effective strategy to increase the electronic conductivity is carbon coating, which not only provides a good conductive medium, but also improves the structural stability because it can


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prevent the agglomeration of nanoparticles during sodiation and desodiation processes. Liu et al. reported that TiO2@GO delivered reversible capacity of 186 mAh g-1 after 100 cycles. 31 Chen et al. investigated that Na-ion intercalation pseudocapacitance in graphene-coupled TiO2 exhibited a specific capacity of 265 mAh g-1 at 50 mA g-1. Even at 12000 mA g-1, the electrode can also deliver a high capacity of more than 90 mAh g-1.32 Hwang et al. prepared TiO2 nanoparticles embedded on carbon nanotubes with initial capacity of about 250 mAh g-1 and capacity retention of 97% over 100 cycles. 33 Moreover, unique morphology of anode materials is also suggested to be in favour of the improvement of electrochemical Na storage properties for TiO2 anodes, e.g. TiO2 nanocubes,

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TiO2 mesocages

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and TiO2 nanofibers.

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Therefore, it is evident that

nanosized-TiO2 particles by carbon coating increase the electrochemical performance of TiO2 and the unique morphology plays a key role for the stability of the electrode structure. In order to obtain high performance TiO2 anodes, it is essential to develop a feasible strategy to prepare the carbon-coated TiO2 nanoparticles with unique morphology. Porous and hollow yolk-shell nanostructures possess many advantages in ionic storage, like large contact surface between electrolyte and electrode, and short distance for ions diffusion, all of which benefit the electrochemical reaction kinetics in the electrode. Furthermore, the yolkshell nanoarchitecture helps to alleviate the structural strain, leading to a stable cycling performance. Conventionally, hollow structures with carbon frameworks are achieved by a hydrothermal process involving metal salts and carbon precursors. However, it is hard to controll homogeneous distribution of nanosized TiO2 in the carbon matrix under hydrothermal conditions because titanium salts can be easily hydrolyzed, resulting in aggregation of the particles. Therefore, it is important to control and minimize the hydrolysis process. In our previous work, a self-catalyzed solvothermal method was introduced by using furfural in ethanol to control the


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generation of H2O through ethanolysis and polycondensation reaction of furfural. Herein, a facile self-catalyzed solvothermal method is used to synthesize yolk-shell TiO2@C microspheres with TiO2 nanoparticles (~10 nm) uniformly coated by furfural pyrolytic carbon. In this process, furfural is not just a template for the microsphere structure, but also a hydrolysis catalyst for titanium ions to form controllable H+, which in return catalyzes the condensation of furfural. The yolk-shell TiO2@C microspheres were fully characterized, and then tested for their sodium storage performance in reversible capacity, cycling stability and rate performance. The findings of this work will be useful for the further development of novel and improved anode materials for SIBs.

2. Experimental Section 2.1．．Material Synthesis Firstly, 2 g furfural and 7.5 g TiCl4 were dissolved in 80 ml ethano and vigorously stirred for 2 hour to obtain uniform solution. Subsequently, the mixture solution was then moved into a Teflon-lined stainless steel autoclave (100 mL) and heated up 160 ºC for 4 h, which was named as TiO2@C-4h. And then, the resulting black product was isolated via centrifugation and dried in vacuum oven at 80 ºC for 12 h. The TiO2@C composites were then put into a tube furnace at 550 º

C for 4 h in argon atmosphere. For the sake of investigating the reaction mechanism, the

preparation process of other TiO2@C composites was the same just at different solvothermal reaction time such as 0.1 h, 0.5 h, 1 h, 8 h and 16 h, which were named as [email protected], [email protected], TiO2@C-2h, TiO2@C-4h. TiO2@C-8h and TiO2@C-16h respectively. For the preparation of the bare TiO2, the TiO2@C-4h composite was heat-treated in air at 450 ºC for 3 h to burn up the coated carbon, which was denotes as bare TiO2. 2.2 Structural characterizations


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The morphology of TiO2@C composites were investigated by scanning electron microscopy (SEM, ULTRA/PLUS, ZEISS) and transmission electron microscopy (TEM, JEOL, JEM-2010FEF). A X-ray diffraction using Cu-Kɑ radiation (XRD, Shimadzu XRD-6000) was used to characterize the crystalline structures of the yolk-shell TiO2@C composites. The composition analysis of the TiO2@C composites and bare TiO2 were performed with TG measurement (Diamond TG/DTA300). The surface area and porosity analyzer was determined by N2 adsorption-desorption measurments (ASAP-2020 HD88).

2.3 Electrochemical Measurements The TiO2@C anode was prepared as below: we first mixed 80 wt% active material, 10 wt% Super P and 10 wt% Polyacrylic acid (PAA, 25 wt%) together and then made them into a slurry. The slurry was coated onto a copper foil and then dried in 60 ºC oven under vacuum for 24 h. The loading of the whole material is about 2.3 mg cm-2. The charge and discharge performances of the TiO2@C anode were examined using 2016 coin-type cells. The TiO2@C anode was used as a working electrode. A Na sheet was used as counter and reference electrode. The electrolyte was 1 M NaClO4 dissolved in the mixture of ethylene carbonate (EC) and diethylcarbonate (DEC) (EC/DEC=1:1, by volume) with 2 wt% fluoroethylene carbonate (FEC) as additive. A Celgard 2400 microporous membrane was used as the separator. The Na sheets were home-made by rolling sodium lumps into plate and cutting into circulated disks. We assembled the testing cells in a glove box in which water/oxygen content is lower than 1 ppm and tested these cells at room temperature. The galvanostatic charge-discharge test was carried out on a LAND cycler (Wuhan Kingnuo Electronic Co., China). The discharge/charge capacities were calculated on basis of the pure TiO2 mass in TiO2@C composites by taking out the capacity contribution of the


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pyrolyzed carbon from furfural microsphere and super P (Figure S4). Cyclic votammetric tests were carried out with the 2016 coin-type cells at a scan rate of 0.1 mV s-1 on a CHI 660c electrochemical work station (ChenHua Instruments Co., China).

3. Results and Discussion

Figure 1. TEM images of the samples after a solvothermal reaction at 160 ºC for (a) 0.1 h; (b) 0.5 h; (c) 2 h; (d) 4 h; (e) 8 h; (f) 16 h.

In order to observe the structure and morphology of the materials, the evolution in morphology of TiO2@C was investigated by controlling the heating time under solvothermal


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conditions, as shown in Figure 1. It can be seen clearly the solid microspheres with a glossy surface and uniform dimensions were formed when the solvothermal reaction time was 0.1 h (Figure 1a). When the solvothermal reaction time increased to 0.5 h, the aggregation of the [email protected] microspheres became more intensive and their morphologies had no visible change (Figure 1b) compared to the sample at 0.1 h. However, when the solvothermal reaction was allowed to proceed further for one hour, the interior of the TiO2@C-2h microspheres slowly became empty and the yolk-shell structure gradually formed, which has rough edges and vague shapes due to the loose coalescence of nanoparticles (Figure 1c). Further increasing the reaction time to 4 h, the yolk-shell structure became more obvious (Figure 1d). However, when the reaction time was extended to 8 h and 16 h (Figure 1e and 1f), the morphology of the products had no visible change compared to the sample at 4 h (Figure 1d), indicating that the yolk-shell structure tends to stabilize when the solvothermal reaction time reaches 4 h. From the above observation, the morphology of the TiO2@C composites depend on the solvothermal time. However, it is still not clear how the composition of the TiO2@C nanocomposites depends on the duration of the solvothermal reaction. To understand the composition of the TiO2@C composites under different solvothermal times, thermal analysis was performed in air atmosphere (Figure S1). The decomposition of carbon occurred from 300 to 550 ºC in term of the weight loss. As shown in Figure S1a, the [email protected] sample demonstrated 99.5% mass loss, indicating the absence of TiO2 and only presence of carbon from the polymerization of furfural. When the solvothermal reaction time was increased to 0.5 h, 2 h and 4 h, the content of TiO2 is calculated as 40%, 58% and 69% (Figure S1b, c and d), respectively. The results suggest that the kinetics of furfural polymerization was faster than that of Ti4+ hydrolysis under solvothermal conditions, possibly


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because there is no H2O in initial solvothermal system, so the polymerization of furfural occurs first. Furthermore, the water originating from the polycondensation of furfural promotes the hydrolysis of Ti4+ to form TiO2 nanoparticles, so that the TiO2 contents increase gradually as the the solvothermal reaction proceeds. However, as shown in Figure S1e and Figure S1f, the content of TiO2 in the TiO2@C-8h and TiO2@C-16h samples are 68% and 67% respectively, which are very close to that of the TiO2@C-4h, suggesting that the Ti4+ has been consumed completely and the content and morphology of TiO2 would not change significantly after 4 h of solvothermal reaction.

Figure 2. The XRD patterns of the TiO2 samples after a solvothermal reaction at 160 ºC for 0.1h, 0.5h, 2 h, 4 h, 8 h, 16 h (calcined at 550 ºC in Ar) and the bare TiO2

Figure 2 shows the X-ray powder diffraction (XRD) patterns of the samples after solvothermal reactions at 160 ºC for 0.1 h, 0.5 h, 2 h, 4 h, 8 h and 16 h. In accordance with the TG result (Figure S1a), a broad diffraction peak around 25º can be attributed to amorphous carbon because there is scarcely any TiO2 in the [email protected]. When the solvothermal reaction time is 0.5 h, the [email protected] still exhibits an amorphous structure from the XRD pattern


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(Figure 2) though 40% TiO2 has been present in the [email protected] (Figure S1b), demonstrating that the growing TiO2 grains are very tiny or amorphous. When the solvothermal reaction time reaches 2 h, the TiO2@C-2h shows a anatase structure (JCPDS no. 21-1272) in term of the whole of the observed diffraction peaks. The obvious diffraction peaks indicate great crystallinity of these samples. In addition, the XRD peaks became sharper and the width of the half peaks become narrower as prolonging the solvothermal reaction times, suggesting that the structure of TiO2 grains transformed from amorphous to anatase phase and the TiO2 grain size become larger. Based on Scherer formula, the mean crystallite size of the TiO2 particles in the TiO2@C microspheres at the solvothermal reaction of 4, 8 and 16 h were calculated to be ~10, ~16 and ~20 nm, respectively. Interestingly, the XRD patterns shows that the bare TiO2 sample exhibits sharper diffraction peaks and narrower peak widths at half-height compared with the TiO2@C samples, indicating a larger TiO2 grain size.

Figure 3. Schematic illustration of the preparation process for the yolk-shell TiO2@C mirospheres.


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On basis of the experimental observtions mentioned above, it is evident that the facile solvothermal process using TiCl4 and furfural as starting materials can successfully prepare yolkshell TiO2@C microspheres. The plausible formation mechanism of this structure is schematically presented in Figure 3. Firstly, due to the absence of water in the solvothermal system, the hydrolysis reaction of TiCl4 cannot take place, while the furfural monomers can crosslink and bond with each other to form oligomers under solvothermal conditions. Furfural oligomer further undergo polycondensation to form a hydrophobic polymer, which then precipitates from ethanol to produce carbon microspheres, which is due to the carbonization process of the furfural. At this time, there still exists a lot of –OH and –COOH groups on the chain of the polycondensates. Thus, Ti4+ can easily adsorb onto the oligomers and bond with these hydrophilic groups, adhering inside the carbon matrix. Ti4+ then gets hydrolyzed by the H2O generated from the polymerization of furfural to produce TiO2 and H+. In turn, the produced H+ catalyzes the polycondensation of furfural to generate H2O again. The possible self-catalyzed mechanism has been discussed in detail in our previous paper.

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Accompanied by the self-

catalyzed reaction, the TiO2 grains coated by poly-furfural are gradually formed and tightly accumulated inside the microsphere, because the generation of H2O mainly takes place in the interior of the microsphere when furfural polycondensates. Time-dependent experiments showed that the TiO2@C solid microspheres are formed after the reaction time of 0.5 h (Figure 1b) while a hollow structure is observed with a longer reaction (Figure 1c). It is proposed that the evolution of the yolk-shell TiO2@C undergoes the formation of poor crystalline solid microspheres and the subsequent transformation of these solid microspheres to yolk-shell ones on basis of the Ostwald ripening mechanism.

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At the beginning of Ostwald ripening process, Ti4+ bonds


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with hydrophilic groups (–OH and –COOH) on the furfural oligomer and then gets hydrolyzed by the H2O generated from the polymerization to produce TiO2. In this case, the produced TiO2 grains adhere to the surface of the furfural oligomer by weak forces so that the less-crystalline TiO2 grains can easily dissolve. Then, recrystallization occurs on the surface of spheres according to the Ostwald ripening mechanism. However, during the further reaction, the produced TiO2 grains are difficult to dissolve because they are coated by poly-furfural at this time. The Ostwald ripening mechanism cannot proceed easily so the morphology of the products do not change further as the reaction time is prolonged. Thus, the final product would be yolkshell microspheres.

Figure 4. The charge-discharge profiles of the TiO2@C after a solvothermally reaction at 160 ºC for (a) 2 h; (b) 4 h; (c) 8 h; (d) 16 h at a current density of 0.1C (1C=200 mA g-1).

In order to evaluate the electrochemical performance of the yolk-shell TiO2@C microspheres for sodium ion storage, galvanostatic charge-discharge tests were tested to determine the correlation between the electrochemical performance and morphology, composition and


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crystallinity of the yolk-shell TiO2@C microspheres after a solvothermal reactions at 160 ºC for 2 h, 4 h, 8 h and 16 h. For comparison, the charge-discharge profiles of the yolk-shell TiO2@C electrodes during the first ten cycles at 0.1C (1C=200 mA g-1) are shown in Figure 4. All four electrodes display similar shapes of the charge-discharge curves. Besides the TiO2@C-2h electrode (~140 mAh g-1) in Figure 4a, the other three electrodes (TiO2@C-4h, 8h and 16h) also exhibit similar charging capacity of about 210 mAh g-1 in Figure 4b, c and d (the capacity is calculated on the basis of pure TiO2 mass in the TiO2@C composites by deducing the capacity of the pyrolyzed carbon and super P. If there are no special considerations, the reversible capacity in this work can be assigned to the capacity based on pure TiO2 mass). 17, 37 The results indicate that the TiO2 content is low (58%) in the TiO2@C-2h electrode (Figure S1c), the electrochemical capacity of TiO2 is also very low compared to those of the TiO2@C-4h, 6h and 8h electrodes with high content (~69%) of TiO2 (Figure S1 d, e and f). This phenomenon is possibly due to low crystallinity of TiO2 in the TiO2@C-2h microspheres. Additionally, the initial coulombic efficiency (~25%) of the TiO2@C-2h electrode is also lower than those (~41%) of the other three electrodes, which is due to the large irreversible capacity contributed by a large number of the pyrolyzed carbon in the TiO2@C-2h electrode. Therefore, considering the tradeoff between the electrochemical performance, TiO2 content and solvothermal time, the yolk-shell TiO2@C-4h microsphere is considered to be optimal choice for high-performance anode in sodium-ion batteries. Therefore, more physicochemical and electrochemical characteristics of the yolk-shell TiO2@C-4h microsphere are described in the following sections.


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Figure 5. (a and b) SEM images; (c) TEM image; (d) high resolution TEM image of the hollow core-shell TiO2@C-4h mirospheres.

To further verify the morphology and particle characteristics of the as-synthesized yolk-shell TiO2@C-4h microspheres, the detailed SEM and TEM analyses were performed, and the results are in Figure 5. As shown in the Figure 5a, the as-prepared yolk-shell TiO2@C-4h microspheres demonstrate a uniformly spherical shape with diameters ranging from 1 to 2 µm. The sample consists of a lot of nanosized particles so that the exterior of the microspheres appears very rough. In addition, the yolk-shell structure could also be clearly observed from some damaged microspheres. Figure 5b reveals that the internal structure of microspheres observed from one of the damaged microspheres is an integrated sphere constructed from many tiny nanoparticles like its exterior. This yolk-shell structure was further analyzed by TEM (Figure 5c), which clearly reveals that the diameter of the yolk and the thickness of the shell are ~0.5 µm and ~0.1 µm, respectively. In fact, the yolk looks like a hollow sphere. The high resolution TEM image (Figure.


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5d) displays the lattice distance of ~3.5Å assigned to the (101) plane of anatase TiO2 (JCPDS no. 21-1272). It can be also seen that the TiO2 nanoparticles are uniformly surrounded by amorphous carbon originated from the carbonization of furfural. Due to the presence of the carbon coating, the aggregation of TiO2 nanoparticles during heat treatment can be effectively restricted , so the microspheres exhibit a final size of ~10 nm.

Figure 6. (a) N2 adsorption-desorption isotherms curve; (b) pore size distribution of the yolkshell TiO2@C-4h mirospheres.

The specific surface area and porous structure of the yolk-shell TiO2@C-4h microspheres were determined by nitrogen adsorption-desorption experiments, as displayed in Figure 6a. The N2 adsorption-desorption isotherms revealed a distinct hysteresis loop, which can be considered a type IV isotherm with H3 hysteresis loop. The adsorption isotherm displays a sharp knee at medium pressure and a hysteresis loop at higher pressures, which is characteristic of a mesoporous structure. The Brunauer-Emmett-Teller (BET) surface area and pore volume of the yolk-shell TiO2@C-4h microspheres are 144 m2 g-1 and 0.16 cm3 g-1, respectively. To further characterize the porosity structure of the material, the Barrett-Joyner-Halenda (BJH) method was carried out to analyze the pore size distribution. As displayed in Figure 6b, the material demonstrates a mesoporous structure with a narrow size distribution of 12 nm. The mesoporous


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structure and large surface area provide a high interfacial reaction areas and electrolyte penetration, thus enabling a great improvement of the kinetic performance of the TiO2 electrodes.

Figure 7. Electrochemical performance of the TiO2@C-4h electrode: (a) CV curves at a scan rate of 0.1 mV s-1; (b) Rate performance at different rates from 0.1C to 40C (1C=200 mA g-1); (c) Long-term cycling performance at a charge-discharge current density of 1C (the cell has been firstly cycled at 0.1 C for 5 cycles, and then cycled at 1C for 2000 cycles).

To evaluate the electrochemical Na-intercalation properties of the TiO2@C-4h electrode, half cell has been assemblyed using TiO2 as cathode and Na metal as anode. Figure 7a displays the typcal cyclic voltammetry (CV) curves of the TiO2@C-4h electrode in the range of 0-3 V. The irreversible reduction peaks at approximately 2.0 and 0.5 V are assigned to the electrolyte decomposition and the formation of a solid electrolyte interface (SEI) film. However, the reduction peak at ~0.01V becomes more distinguishable in the following fo cycles, and is most


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likely arisen from the capacity contribution of the conductive carbon as demonstrated by the oxidation peak approaching to this potential, probably related to the reversible sodium extraction, Furthermore, from the second cycle onwards, reduction and oxidation reactions occurred at ~0.67 and ~0.83 V, respectively, as a result of the sodiation and desodiation of TiO2. Therefore, it is conjectured that the large irreversible capacity probably originated from electrolyte decomposition and formation of SEI film during the first cycle and disappeared from subsequent cycles, which can further explain the low initial coulombic efficiency. The rate capacity of the yolk-shell TiO2@C-4h electrode was measured over the range of 0.1 to 40 C rate, with 5 cycles at each rate (Figure 7b). The reversible capacities are found to be 210, 188, 174, 158, 136, 121, 111, 100, 94, 90, 82 and 75 mAh g-1 at current densities of 0.1C, 0.2C, 0.5C, 1C, 2C, 4C, 8C, 12C, 20C, 25C, 30C and 35C, respectively. Even at the high rate of 40C (8000 mA g-1), the capacity was still capable of reaching 70 mAh g-1. When the current rate went back to 1C, the reversible capacity recovered its original value (158 mAh g-1), demonstrating superior electrochemical reversibility of the yolk-shell TiO2@C-4h electrode for the Na storage reaction. The high rate capability of the TiO2@C-4h electrode clearly indicates that the unique yolk-shell structure offers a robust and porous framework, large electrode/electrolyte contact area, and smaller Na+ ion diffusion distance, all of which alleviate the electrochemical polarization in the electrode. Besides its excellent rate performance, the superior cycling stability of the yolk-shell TiO2@C-4h electrode at a moderate current density of 1C (1C=200 mA g-1) as shown in Figure 7c. After 2000 cycles, the capacity slightly decreased from 160 mAh g-1 to 136 mAh g-1, demonstrating very high capacity retention of 85%. The cycling coulombic efficiency of the TiO2@C electrode remained steady at 99.5%, demonstrating superior reversibility for the Na storage reaction. In addition, a comparison of the electrochemical performance of the TiO2


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electrodes in previous works is also presented in Table S1. It can be seen from Table S1 that the electrochemical performance of the as-prepared yolk-shell TiO2@C is superior compared to the materials in previous works in terms of the overall performance in capacity, rate capability and cycling stability. The enhanced electrochemical performance of the yolk-shell TiO2@C can be attributed to the unique yolk-shell structure which effectively buffers the structural strain and volume changes concerned with the processes of repeated Na+ insertion/extraction, as well as the carbon coating to improve the electronic contact. To illustrate the impact of carbon coating on the electrochemical performance, we also tested the electrochemical performance of the bare TiO2 sample, in which the carbon was removed from the yolk-shell TiO2@C-4h nanocomposites through additional calcination in air. Although the bare TiO2 electrode also retained the yolkshell structure (Figure S2), it exhibited poor specific capacity, especially under high rate (Figure 7b). Under high rates (>2C), its specific capacities decreased rapidly and approach zero value finally. In addition, it not only delivers a much lower initial reversible capacity of 32 mAh g-1 at a current rate of 1C, but also exhibited much poorer cyclic stability with capacity retention of only 50% over 10 cycles (Figure S3), demonstrating inferior capacity retention. There are two main reasons for this phenomenon. One reason is that the agglomeration of TiO2 grains which occurs during calcination in air upon removal of the carbon coating outside the TiO2 grains, which is confirmed by XRD analysis (Figure 2). The other reason is that the bare TiO2 without carbon coating has very low electrical conductivity. Therefore, the results further demonstrate that the drastic improvement of electrochemical properties for the yolk-shell TiO2@C microspheres originates from its uniform carbon coating and unique morphology that effectively alleviate the growth of TiO2 during calcination and enhance the electron transport of the electrode.


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Conclusions In summary, a facile synthesis of the yolk-shell TiO2@C microspheres by a self-catalyzed solvothermal approach has been successfully reached in this work. Due to the interaction between Ti4+ and furfural, the amount, size and structure of the TiO2 nanoparticles were effectively controlled during the solvothermal reaction. Furthermore, aggregation of the TiO2 nanoparticles was effectively prevented by carbon coating during subsequent carbon-thermal reduction reaction. The TiO2@C electrode demonstrates a high initial capacity of 210 mAh g-1 at 0.1C (1C=200 mA g-1), impressive cycling performance of 85% capacity retention over 2000 cycles at a middle rate capability at 1C, and excellent rate capability (the reversible capacity of 70 mA g-1 at 40C). The enhanced electrochemical properties is mainly attributed to the small crystallite size, superior electrical contact of TiO2 nanoparticles with carbon coating, and unique yolk-shell morphology that increases the electrical conductivity and accommodates the structural strain during Na-ion intercalation and deintercalation. As a result of the structural advantages,

the

obtained

yolk-shell

TiO2@C

microspheres

display

excellent

electrochemical properties as potential anode materials for SIBs. In addition, the synthesis method for the yolk-shell TiO2@C microspheres via a self-catalyzed solvothermal approach may be adopted to fabricate other hollow core-shell spheres for various fields like energy, biomaterial and catalysis.

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Supporting Information Available. Additonal details and figures as mentioned in the text. This material is available free of charge via the Internet at http:// pubs.acs.org.

Author Information Corresponding Authors *E-mail: [email protected];

Acknowledgements We thank financial support by the National Key Research Program of China (No. 2016YFB0901501), National Science Foundation of China (No. 21673165, 21333007, and 21273090), and Hubei National Funds for Distinguished Young Scholars (2014CFA038).

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Yolk-shell TiO2@C nanocomposites are synthesized successfully via a facile self-catalyzing solvothermal method, which exhibit superior sodium storage properties, demonstrating a reversible capacity of 210 mAh g-1, an excellent cycling stability of 85 % capacity retention over 2000 cycle and high rate capability, capably serving as a promising anode material for high-performance sodium-ion batteries. 274x173mm (144 x 144 DPI)


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YolkâShell TiO2@C Nanocomposite as High-Performance Anode

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YolkâShell TiO2@C Nanocomposite as High-Performance Anode