Ultrafast sodium storage in anatase TiO2 nanoparticles embedded on carbon nanotubes

Nano Energy (2015) 16, 218–226

Available online at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/nanoenergy

COMMUNICATION

Ultrafast sodium storage in anatase TiO2 nanoparticles embedded on carbon nanotubes Jang-Yeon Hwanga,1, Seung-Taek Myungb,1, Joo-Hyeong Leea, Ali Abouimranec, Ilias Belharouakc,n, Yang-Kook Suna,nn a

Department of Energy Engineering, Hanyang University, Seoul 133-791, South Korea Department of Nano Engineering, Sejong University, Seoul 143-747, South Korea c Qatar Environment and Energy Research Institute, Qatar Foundation, P.O. Box 5825, Doha, Qatar b

Received 22 April 2015; received in revised form 15 June 2015; accepted 17 June 2015 Available online 27 June 2015

KEYWORDS Anatase TiO2; Carbon nanotubes; Nanocrystalline; Anode; Sodium batteries

Abstract

The main disadvantage of using transition metal oxides for Na + -ion batteries is the sluggish kinetics of insertion of Na + ions into the structure. Here, we introduce nanosized anatase TiO2 that is partially doped with fluorine (TiO2 δFδ) to form electro-conducting trivalent Ti3+ as an ultrafast Na + insertion material for use as an anode for sodium-ion batteries. In addition, the F-doped TiO2 δFδ is modified by electro-conducting carbon nanotubes (CNTs) to further enhance the electric conductivity. The composite F-doped TiO2 embedded in CNTs is produced in a one-pot hydrothermal reaction. X-ray diffraction and microscopic studies revealed that nanocrystalline anatase-type TiO2 δFδ particles, in which fluorine is present with TiO2 particles, are loaded on the CNTs. This yields a high electric conductivity of approximately 5.8 S cm 1. The first discharge capacity of the F-doped TiO2 embedded in CNTs is approximately 250 mA h (g-oxide) 1, and is retained at 97% after 100 cycles. As expected, a high-rate performance was achieved even at the 100 C discharging rate (25 A g 1) where the composite material demonstrated a capacity of 118 mA h g 1 under the 0.1 C-rate charge condition. The present work also highlights a significant improvement in the insertion and extraction of Na + ions when the material was charged and discharged under the same rate of 35 C (8.75 A g 1), delivering approximately 90 mA h (g-oxide) 1. & 2015 Elsevier Ltd. All rights reserved.

n

Corresponding author. Tel.: +974 4454 6820. Corresponding author. Tel.: +82 2 2220 0524; fax: +82 2 2298 5416. E-mail addresses: [email protected] (I. Belharouak), [email protected] (Y.-K. Sun). 1 These authors contributed equally to this work. nn

http://dx.doi.org/10.1016/j.nanoen.2015.06.017 2211-2855/& 2015 Elsevier Ltd. All rights reserved.

Ultrafast sodium storage in anatase TiO2 nanoparticles

Introduction Since sodium is substantially less expensive and more abundant than lithium, sodium-ion batteries (SIBs) are considered a desirable alternative to lithium-ion batteries [1–6], which have to date become the most suitable technology for applications ranging from consumer electronics to vehicles and energy storage devices (ESS). Increased demand for mid- to large-scale batteries for ESS and electric vehicles (EVs) may, however, lead to serious problems such as the shortage of lithium resources in the near future. In addition, high cost of raw materials such as nickel and cobalt is another obstacle to the mass production of large format batteries. Because of the low cost and abundance of sodium, room-temperature SIBs that use inexpensive sodium-based transition metal oxides (Fe and Mn) as cathodes, and Na + -ions insertion, conversion, or alloy materials as anodes are potential alternatives, especially for stationary energy storage. As for anode side, conversion [7–11] and alloy electrode materials [12–15] are attractive because of their high capacity when reacting with sodium ions, even at high rates. However, the deposition of metallic Na on the anode surface is an issue, which, in turn, jeopardizes the safety of the battery. Hard carbon allows Na + insertion with a voltage profile that is the lowest (near zero V) versus Na/Na + among all existing anode materials, and with a reasonable capacity of 300 mA h g 1 and good cycling (200 cycles) under low rate conditions [16–18]. This is advantageous because hard carbon contributes to the increases of the energy density of sodium-ion cells. However, the intrinsic physical properties and random structure of hard carbon significantly reduce its capacity at the high current density [19]. Recently, several metal oxides have been introduced as anodes for SIBs, such as anatase TiO2 [20–24], TiO2(B) [25– 27], Li4Ti5O12 [28,29], P2-Na0.66Li0.22Ti0.78O2 [30], Na2Ti3O7 [31,32], and Na2C8H4O4 [33]. Note that most of the above insertion materials have Ti4 + /3 + as the active redox couple. Drawing from works dedicated to titanium dioxide (TiO2) as a stable, nontoxic and inexpensive anode for LIBs, recent reports also suggested the use of the material for SIBs. Of significance, amorphous nanotube TiO2 [34], mesoporous TiO2 [35], nanoparticle anatase TiO2 [36] and nanorod anatase TiO2 [37] have been investigated. An interesting feature of these materials is that they are made of nanoarchitectures that enable them achieve reasonable capacity and cyclability and excellent rate capability. A carbon coating was found to be advantageous to further improve the TiO2 performance, as demonstrated in our previous reports [37,38]. Despite the beneficial effect of the nanostructure and surface modification, the insertion of Na + into Na-free compounds was predicted to be slower than Li as computed by Ong et al. [39]. Experimentally, we verified this hypothesis for TiO2 nanorods [37] and Li4Ti5O12 [40], even though these materials have exhibited the best performances to date. The significant drop in capacity in the case of sodium reflects the difficulty of enabling highrate insertion anodes for SIBs. Beyond the approaches of material design with nanostructure and modification, TiO2 can be enabled for high-rate operations through the partial reduction of Ti4 + to Ti3 + .

219 Indeed, a scenario of mix-valence states can help in significantly improving the electronic conductivity of TiO2 [41–44]. The simplest way to achieve this is through the synthesis of oxygen-deficient TiO2 δ, in which trivalent Ti is partially produced. However, this does not readily occur in the anatase TiO2 structure because the tolerable deficiency level is in a very narrow oxygen stoichiometric range (TiO2 δ, δo0.001) [45], above which anatase TiO2 undergoes a spontaneous phase transformation to other types of titanium oxide such as the Magnéli form (TiO2 δ, δ = 0.001– 0.1). In an attempt to overcome the insulating character of anatase TiO2 (10 8 S cm 1), we synthesized F-doped anatase nanoparticles in which fluorine replaces oxygen and allows the partial formation of trivalent Ti. The resulting compound was embedded in carbon nanotubes (CNTs) to further improve the electric conductivity to 107 S cm 1 (Scheme 1). This synergetic approach yielded an ultrafast storage anode for non-aqueous sodium batteries.

Experimental Materials synthesis HNO3-treated multi-wall CNTs (0.2 g, CM-100, Hanwha Chemical Industry) were dispersed by ultrasonication in anhydrous ethanol (Merck) (Solution 1). Appropriate amounts of high-purity TiCl4 (Sigma-Aldrich) and TiF4 (Sigma Aldrich) were added drop-wise to distilled water in an ice bath. Urea was then added to mixture with continuous stirring. Thereafter, ammonium sulfate (Junsei Chemical) was added to the resulting solution which was then stirred for 2–4 h (Solution 2). Solution 2 was added to Solution 1 and stirred for 1 h. The mixture was transferred into a Teflon-lined autoclave and a hydrothermal process was carried out at a temperature of 115 1C for 5 h. After cooling the autoclave to room temperature, the resulting precipitate was filtered and washed several times with deionized water and ethanol. It was then dried in a vacuum oven at 60 1C for 24 h and calcined at 400 1C for 5 h in an Argon gas atmosphere. Three materials were the subject of the present study: (1) TiO2 baseline, (2) TiO2 doped with fluorine (TiO2-F), and (3) TiO2 doped with fluorine and supported on carbon nanotubes (TiO2-F/ CNTs).

Scheme 1 Schematic of F-doped TiO2 embedded on CNTs.

220

Electrochemical measurement Electrochemical testing was performed in a R2032 coin-type cell with Na metal (Alfa Aesar, USA) as the anode. The electrodes were fabricated by blending the composite TiO2 powders (80 wt%), carbon black (10 wt%), and polyacrylic acid (10 wt%) in N-methylpyrrolidinone. The resulting slurry was applied to a copper foil and dried at 110 1C for 12 h in a vacuum oven, and then disks were punched out of the foil. The loading amount of the electroactive material in the electrode was 2.0 mg cm 2. The electrolyte solution was 0.5 M NaPF6 in a (98:2 v/v) mixture of propylene carbonate (Tokyo Chemical Industry) and fluoroethylene carbonate. All cells were prepared inside an Argon-filled glove box. The cell was cycled between 0.01 and 2.0 V versus Na + /Na. In this study, the 1 C rate was equivalent to the current density of 250 mA h g 1.

Characterization The synthesized materials were characterized by powder Xray diffraction (Rint-2000, Rigaku) using Cu-Kα radiation. Particle morphologies of the precursor and as-synthesized powders were observed by scanning electron microscopy (JSM 6400, JEOL) and transmission electron microscopy (JEOL 2010). The DC electrical conductivity was measured by a direct volt-ampere method (CMT-SR1000, AIT Co.), in which disk samples were contacted by a four-point probe. Nitrogen sorption measurements were performed using a Quantachrom Autosorb-1 apparatus after the sample had been degassed at 200 1C for 4 h. The fluorine doping level was determined by X-ray photoelectron spectroscopy (ESCALAB 220-I, VG).

Results and discussion Characterizations of TiO2-F embedded on CNTs Hydrothermal reaction of the solution mixture resulted in formation of anatase TiO2 powders with different colors: white (without TiF4 and CNTs), pale yellow (without CNTs) and black (with CNTs and TiF4) (Figure S1). The resulting products exhibit low crystallinity in XRD and the resulting selected-area diffraction results since highly crystalline anatase TiO2 usually shows clear and sharp peak splitting between the (103), (004), and (112) peaks [46]. Since the starting TiF4 is soluble in water, fluorine reacted during the formation of anatase TiO2 which results in a dark yellow colored TiO2-F (Figure S1b). Embedding TiO2-F in CNTs yielded a black colored composite (Figure S1c). The precipitation of nanoparticles on CNTs is evident from microscopic images (Figure 1a and b). The primary particles are, however, agglomerated, and the estimated particle size is less than 10 nm (Figure 1c). The electron energy loss spectroscopy (EELS) image also indicates that the produced anatase TiO2 nanoparticles are attached to the CNTs (Figure 1d). Note that white dots corresponding to fluorine appear within the agglomerates indicating the incorporation

J.-Y. Hwang et al. of F into TiO2, presumably through the replacement of oxygen. In addition, TiO2-F and TiO2-F/CNTs exhibit slight increase of the lattice parameters a and c (Table S1). On the basis of ionic radii considerations (Ti3 + : 0.67 Å, Ti4 + : 0.605 Å [47]), a slight increase in the lattice parameters can be associated with the partial formation of Ti3 + in the fluorine doped titania. HR-TEM images reveal well-resolved lattice fringes (Figure 1e). The cell parameter a value calculated from XRD is consistent with the distance measured between the atomic planes shown in the fast Fourier Transform (FFT) image (Figure 1f). Nitrogen adsorption–desorption isotherm exhibits a typeIV isotherm with a hysteresis loop between the H1 and H2 types that is related to capillary condensation behavior for the produced powders (Figure 2a). This is further evidenced from the pore size distribution, which appears to be similar for all samples (Figure 2b and Table S2). The resulting specific surface area and total pore volume vary greatly when CNTs are used during embedding (Table S2). It is likely that the nanopores facilitate electrolyte penetration deep into electrodes. As a result, this would assist to improve the electrochemical performance. Also, the small primary nanoparticles (below 10 nm) obviously shorten the diffusion length of Na ions. We assumed that the slightly larger crystalline lattice observed for the F-doped materials is attributed to the partial formation of trivalent Ti3 + (Table S1). Therefore, we analyzed TiO2 and TiO2-F/CNTs by XPS for further confirmation (Figure 3). Since the particle sizes of the produced TiO2 are below 10 nm, Ar + ion etching was performed for 10 s to remove air contaminants on the surface of the powders. The normalized intensity spectra indicate the presence of tetravalent Ti4 + at 458.9 eV for both TiO2 and TiO2-F/CNTs (Figure 3a). Moreover, the binding energy associated with Ti3 + for TiO2-F/CNTs is in the range of 458–455 eV, although the integrated area is much smaller compared to that of Ti4 + . The ratio of Ti3 + to Ti4 + was calculated to be 2.7:97.3 for the outer surface, which yielded a composition of TiO1.987. Regarding the binding energy for F (Figure 3b), there is an obvious appearance of a fluorine binding energy for TiO2-F/CNTs, in comparison with the F-free TiO2. It is, therefore, anticipated that the F anion partially replaced the O2 anion in the TiO2 matrix, resulting in the formation of stoichiometric TiO1.987F0.013 obtained from XPS, TEM, and TG results, in which Ti3 + is partially produced for charge compensation. As reported by Szot et al. [45], the amount of oxygen deficiencies is highly critical for retaining the crystal structure of TiO2 anatase. Otherwise, the anatase TiO2 structure undergoes phase transformation to the Magnéli form (TiO2 δ, δ= 0.001–0.1), which does not represent an anatase phase, but instead represents titanium oxide. It is more likely that fluorine would have replaced oxygen in the anatase TiO2. In the present study, such a phase transformation was not observed, but there was an obvious color change from white to dark yellow, which was attributed to the partial formation of trivalent Ti3 + in the anatase matrix. Therefore, the above results confirm that the doping with fluorine occurred while retaining the anatase crystal structure, as clearly evidenced by (1) the presence of fluorine in the EELS image, (2) the slight increase in the crystalline lattice (Table S1 and FFT image), and (3) the appearance of Ti3 + and F in XPS.

Ultrafast sodium storage in anatase TiO2 nanoparticles

221

Figure 1 Physicochemical properties of TiO2-F embedded on CNTs. (a) SEM image, (b,c) TEM image, (d) EELS image, (e) HR-TEM image and (f) FFT image of TiO2-F embedded on CNTs.

Electrochemical performance The TiO2 electrode delivers a discharge capacity of approximately 160 mA h g 1 at the 0.1 C-rate, as typically reported in the literature [38]. The discharge capacity increases to 195 mA h g 1 for the F-doped TiO2, presumably because of the presence of conducting Ti3 + in the oxide matrix. The delivered capacity is maximized to 250 mA h (g-oxide) 1 for the F-doped TiO2 embedded on CNTs. This synergetic effect

of the presence of Ti3 + in the oxide matrix and electroconducting CNTs is likely to significantly assist to improve the electrochemical performance. However, Na + insertion into the CNTs seems to be difficult in the present experiment because of the large ionic size of Na + (1.02 Å). Provided that the insertion progresses, gradual capacity fade is predicted to occur during the first 10 cycles, as seen for C/Fe3O4 composites [48], implying that the electrochemical reaction is solely related to F-doped TiO2 supported

222

J.-Y. Hwang et al.

Figure 2 (a) Isotherm profiles and surface areas and (b) pore size distributions of TiO2-F embedded on CNTs.

Normalized intensity / A.U.

TiO2-F on CNTs

4+

Ti : 458.9 eV 3+ Ti : 456 eV

Pristine TiO2

462

460

458

456

454

452

Binding energy / eV

684.1 eV 688.8 eV Intensity

by conducting CNTs. It is known that TiO2 suffers from a large irreversible capacity during the first charge in a nonaqueous sodium electrolyte [37]. Therefore, the presodiation treatment via direct contact with Na metal was effective to minimize side reactions such as reductive decomposition of electrolyte and formation of a solidelectrolyte interface (SEI). As a result, the coulombic efficiency between charge and discharge capacity can approach 100%. Our products also exhibited a low coulombic efficiency (36.3%) at the 0.1 C rate during the first cycle (Figure 4a). Hence, we adopted the pre-sodiation treatment which is typically used in previous reports [37]. Under this conditions, our electrode showed the discharge capacity of 241 mA h g 1 with high coulombic efficiency (96%) at the 0.1 C rate during the first cycle. With or without presodiation, the F-doped TiO2 embedded on CNTs electrode displayed the excellent cycle retention of 97% of the first discharge capacity after 100 cycles (Figure 4b, Figure S2, 3). The rate test highlights the excellent electrode performance of F-doped TiO2 embedded on CNTs compared to TiO2 and F-doped TiO2. Prior to discharge, all cells were subjected to charge at a constant current of 25 mA g 1 (0.1 C-rate) (Figure 4c). TiO2 delivered approximately 75 mA h g 1 at 2.5 A g 1 (10 C-rate). The measured capacities at the same current density are 165 mA h g 1 for TiO2-F and 210 mA h g 1 for TiO2-F/CNTs. Moreover, the cell remained highly active even at the 100 C-rate (25 A g 1), showing 118 mAh g 1 capacity, which is the highest value reported to date. The effect of F doping and CNTs was more evident when the same currents were applied for both the charge and discharge (Figure 4d). While TiO2 only delivered 42 mA h g 1 capacity at the 10 C-rate because of the sluggish Na + kinetics, as suggested by Ong et al. in the case of the sodium-free anodes [40], the capacity reached 80 mA h g 1 for the F-doped TiO2, and markedly increased to 150 mA h g 1 for TiO2-F/CNTs. At the 35 C-rate for both the charge and discharge, the F-doped TiO2 embedded on CNTs still delivered 90 mA h g 1, which means that introducing F and CNTs has been effective in overcoming the poor sodium intercalation ability in TiO2. In addition, the cycling test measured by application of a 5 C-rate (1.25 mA g 1) for both charge and discharge clearly demonstrates excellent capacity retention during 200 cycles for the F-doped TiO2

TiO2-F on CNT TiO

690

688

686

684

682

2

680

Binding energy / eV Figure 3 X-ray photoelectron spectroscopy (XPS) spectra. (a) Ti 2p3/2 and (b) F 1s of Pristine TiO2 and TiO2-F embedded on CNTs particles.

Voltage / V

Ultrafast sodium storage in anatase TiO2 nanoparticles

223

3.0

TiO2

2.5

TiO2-F

2.0

TiO2-F on CNT

1.5 1.0 0.5 0.0 0

100 200 300 400 500 600 700 800 -1

Specific capacity / mAh g

TiO 2

2.0

2.0 0.1C 0.2C 0.5C 1C 3C 5C 10C

1.5 1.0 0.5

TiO 2 0.1C 0.2C 0.5C 1C 5C 10C

1.5 1.0 0.5 0.0

0.0

TiO 2-F Voltage / V

Voltage / V

TiO 2-F 2.0 0.1C 1C 10C 30C 50C 70C 100C

1.5 1.0 0.5

0.1C 0.2C 0.5C 1C 5C 10C 15C 20C 25C 30C 35C

2.0 1.5 1.0 0.5 0.0

0.0

TiO 2-F on CNT

TiO 2-F on CNT

2.0

2.0

1.5

1.5

1.0

1.0

0.5

0.5 0.0

0.0 0

50

100

150

200

250

300

-1

Specific capacity / mAh g

0

50

100

150

200

250

300

-1

Specific capacity / mAh g

Figure 4 (a) First charge and discharge curves with a constant current of 25 mA g 1 (0.1 C-rate), (b) cycling performance for 100 cycles with a constant current of 100 mA g 1 (0.5 C-rate), rate capability results of TiO2, TiO2-F and TiO2-F on CNTs electrodes: (c) various discharge current densities from 0.1 C-rate (25 mA g 1) to 100 C-rate (25 A g 1) at constant charge current of 25 mA g 1 and (d) same charge and discharge current density from 0.1 C-rate (25 mA g 1) to 35 C-rate (8.75 A g 1). All cells are tested in voltage range of 0.01–2.0 V at 30 1C.

supported by conducting CNTs (Figure S4). The measured electric conductivity was 6  10 7 S cm 1 for TiO2, and was slightly increased through F doping to 2  10 6 S cm 1 for the F-doped TiO2 because of the partial formation of Ti3 + . When the F-doped TiO2 is supported on CNTs, the conductivity drastically rose to 5.8 S cm 1. From these data, we concluded that the extraordinarily high rate capability of the TiO2-F/CNTs composite is due to the enhancement of the electronic conductivity owing to the presence of Ti3 + within the titania particles supported by CNTs. In terms of electrode performances, doping TiO2 with fluorine and

embedding it on CNTs are more effective than carbon coating [37,38] or graphene-related compounds [49].

In-situ XRD study Ex-situ XRD is a simple method to observe the crystal structure resulting from desodiation. However, the reactivity of sodium in the ambient atmosphere caused rapid oxidation of the charged electrodes even after protection with a Mylar film. For this reason, an in-situ XRD measurement was carried out for the F-

40

* Cu Foil (204)

(105)

*

Intensity (arb. unit)

*

(211)

J.-Y. Hwang et al.

(200)

224

1 cylce 2.0 V end

1 cylce 0.01V end

(1) a stable structure that reversibly host Na + ions at high rates, (2) the partial formation of Ti3 + that improves the electronic conductivity, (3) and more importantly, the creation of electric conducting pathways through CNTs. We suggest that the disclosed synthesis method be applicable to other non-sodium based oxides as a way for enabling high power sodium ion batteries.

Acknowledgments Start 45

50 55 2 θ (λ=1.54 )

60

Figure 5 In-situ XRD patterns of F-doped TiO2 embedded on CNTs electrode during 1st cycle with a constant current of 25 mA g 1 (0.1 C-rate) at 30 1C.

doped TiO2 embedded on CNTs (Fig. 5). As observed in our previous work [37], there was no change in the XRD patterns down to 0.8 V (Figure S5). This voltage region is mainly associated with the reductive decomposition of the electrolyte and formation of the SEI layer. Thereafter, a progressive shift of the diffraction peaks toward the lower angles was noticed from 0.8 V to 0.01 V, and a similar tendency was observed by Wu et al. [22] and in our prior report [37]. This observation indicates that the reaction is not conversion of TiO2 to Ti metal, which would have shown a diffraction peak at 40.171 (2θ) along with an amorphous state in the XRD patterns. From the expansion in the lattice, the related reaction could be attributed to the electrochemical reduction of Ti4 + /3 + , TiO2NaxTiO2 + Na + +e , allowing Na + insertion into the host structure of anatase TiO2. The diffraction peaks return to their original Bragg peak positions because of the contraction of the lattice on charging, suggesting the extraction of Na + from the sodiated TiO2: NaxTiO2 +Na + +e -TiO2. In consideration of the theoretical capacity of TiO2 (ca. 330 mA h g 1), the delivered capacity on charging is approximately 250 mA h (g-oxide) 1. Hence, the insertion/extraction of Na + into/out of the anatase TiO2 prevails based on Ti4 + /3 + redox couple because the reaction does not exceed the theoretical capacity. The reaction is sufficiently stable, such that the capacity retention reaches approximately 97% for 100 cycles (Figure 4b).

Conclusion The present work represents a significant improvement in the uptake and release of Na + ions in titania modified by an innovative process that yielded nanosize fluorinated TiO2 supported on carbon nanotubes. At the 35 C-rate during the charge and discharge, the F-doped TiO2 embedded on CNTs delivered a capacity of 90 mA h g 1, which clearly demonstrates that introducing F and CNTs has been very effective in improving the kinetics of the intercalation of Na + ions in TiO2. The driving force for the unprecedented performances in terms of great capacity retention and excellent rate capability is the combination of several intrinsic properties of the fluorine doped TiO2 embedded on CNTs including:

This work was supported by the Global Frontier R&D Program (2013M3A6B1078875) on Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT and Future Planning and by a Human Resources Development program (No. 20124010203310) of a Korea Institute of Energy Technology Evaluation and Planning (KETEP) Grant funded by the Korea Government Ministry of Trade, Industry and Energy. The authors thank Qatar Foundation for supporting this work.

Appendix A.

Supplementary information

Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2015.06.017.

References [1] V. Palomares, P. Serras, I. Villaluenga, K.B. Hueso, J. Carretero-González, T. Rojo, Energy Environ. Sci. 5 (2012) 5884–5901. [2] M.D. Slater, D. Kim, E. Lee, C.S. Johnson, Adv. Funct. Mater. 23 (2013) 947–958. [3] S.W. Kim, D.H. Seo, X. Ma, G. Ceder, K.S. Kang, Adv. Energy Mater. 2 (2012) 710–721. [4] B.L. Ellis, L.F. Nazar, Curr. Opin. Solid State Mater. 16 (2012) 168–177. [5] D.A. Stevensa, J.R. Dahn, J. Electrochem. Soc. 147 (2000) 1271–1273. [6] V.L. Chevrier, G. Ceder, J. Electrochem. Soc. 158 (2011) A1011–A1014. [7] K.M. Shaju, F. Jiao, A. Debart, P.G. Bruce, Phys. Chem. Chem. Phys. 9 (2007) 1837–1842. [8] R. Dedryvere, S. Laruelle, S. Grugeon, P. Poizot, D. Gonbeau, J.M. Tarascon, Chem. Mater. 16 (2004) 1056–1061. [9] Q. Su, D. Xie, J. Zhang, G. Du, B. Xu, ACS Nano 7 (2013) 9115–9121. [10] M. Sina, K.W. Nam, D. Su, N. Pereira, X.Q. Yang, G. G. Amatucci, F. Cosandey, J. Mater. Chem. A 1 (2013) 11629–11640. [11] X. Wang, X.L. Wu, Y.G. Guo, Y. Zhong, X. Cao, Y. Ma, J. Yao, Adv. Funct. Mater. 20 (2010) 1680–1686. [12] I.A. Courtney, J.R. Dahn, J. Electrochem. Soc 144 (1997) 2045–2052. [13] C.M. Park, J.H. Kim, H.S. Kim, H.J. Sohn, Chem. Soc. Rev. 39 (2010) 3115–3141. [14] P. Limthongkul, Y.I. Jang, N.J. Dudney, Y.M. Chiang, Acta Mater. 51 (2003) 1103–1113. [15] Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa, T. Miyasaka, Science 276 (1997) 1395–1397. [16] K. Gotoh, T. Ishikawa, S. Shimadzu, N. Yabuuchi, S. Komaba, K. Takeda, A. Goto, K. Deguchi, S. Ohki, K. Hashi, T. Shimizu, H. Ishida, J. Power Sources 225 (2013) 137–140.

Ultrafast sodium storage in anatase TiO2 nanoparticles [17] S. Komaba, T. Ishikawa, N. Yabuuchi, W. Murata, A. Ito, Y. Ohsawa, ACS Appl. Mater. Interfaces 3 (2011) 4165–4168. [18] Y. Bai, Z. Wang, C. Wu, R. Xu, F. Wu, Y. Liu, H. Li, Y. Li, J. Lu, K. Amine, ACS Appl. Mater. Interfaces 7 (2015) 5598–5604. [19] V.G. Pol, E. Lee, D. Zhoua, F. Dogana, J.M. Calderon-Morenob, C.S. Johnson, Elcetrochim. Acta 127 (2014) 61–67. [20] Y. Xu, E.M. Lotfabad, H. Wang, B. Farbod, Z. Xu, A. Kohandehghanab, D. Mitlin, Chem. Commun. 49 (2013) 8973–8975. [21] L. Wu, D. Buchholz, D. Bresser, L.G. Chagas, S. Passerini, J. Power Sources 251 (2014) 379–385. [22] L. Wu, D. Bresser, D. Buchholz, G.A. Giffi, C.R. Castro, A. Ochel, S. Passerini, Adv. Energy Mater. 5 (2015) 1401142. [23] J. Lee, Y.M. Chen, Y. Zhu, B.D. Vogt, ACS Appl. Mater. Interfaces 6 (2014) 21011–21018. [24] D. Dambournet, I. Belharouak, K. Amine, Chem. Mater. 22 (2010) 1173–1179. [25] D.H. Lee, J.G. Park, K.J. Choi, H.J. Choi, D.W. Kim, Eur. J. Inorg. Chem. (2008) 878–882. [26] A.G. Dylla, G. Henkelman, K.J. Stevenson, Acc. Chem. Res. 46 (2013) 1104–1112. [27] M.V. Koudriachova, M. Matar, ECS Trans. 16 (2009) 63–68. [28] X. Yu, H. Pan, W. Wan, C. Ma, J. Bai, Q. Meng, S.N. Ehrlich, Y. S. Hu, X.Q. Yang, Nano Lett. 13 (2013) 4721–4727. [29] Y. Ge, H. Jiang, K. Fu, C. Zhang, J. Zhu, C. Chen, Y. Lu, Y. Qiu, X. Zhang, J. Power Sources 272 (2014) 860–865. [30] Y. Wang, X. Yu, S. Xu, J. Bai, R. Xiao, Y.S. Hu, H. Li, X.Q. Yang, L. Chen, X. Huang, Nat. Commun. 4 (2013) 2365. [31] P. Senguttuvan, G. Rousse, V. Seznec, J.M. Tarascon, M. R. Palacín, Chem. Mater. 23 (2011) 4109–4111. [32] A. Rudola, K. Saravanan, C.W. Masona, P. Balaya, J. Mater. Chem. A 1 (2013) 2653–2662. [33] L. Zhao, J. Zhao, Y.S. Hu, H. Li, Z. Zhou, M. Armand, L. Chen, Adv. Energy Mater. 2 (2012) 962–965. [34] H. Xiong, M.D. Slater, M. Balasubramanian, C.S. Johnson, T. Rajh, J. Phys, Chem. Lett. 2 (2011) 2560–2565. [35] Y. Xu, E.M. Lotfabad, H. Wang, B. Farbod, Z. Xu, A. Kohandehghanab, D. Mitlin, Chem. Commun. 49 (2013) 8973–8975. [36] L. Wu, D. Buchholz, D. Bresser, L.G. Chagas, S. Passerini, J. Power Sources 251 (2014) 379–385. [37] K.T. Kim, G. Ali, K.Y. Chung, C.S. Yoon, H. Yshiro, Y.K. Sun, J. Lu, K. Amine, S.T. Myung, Nano Lett. 14 (2014) 416–422. [38] S.M. Oh, J.Y. Hwang, C.S. Yoon, J. Lu, K. Amine, I. Belharouak, Y.K. Sun, ACS Appl. Mater. Interfaces 6 (2014) 11295–11301. [39] S.P. Ong, V.L. Chevrier, G. Hautier, A. Jain, C. Moore, S. Kim, X. Ma., G. Ceder, Energy Environ. Sci. 4 (2011) 3680–3688. [40] K.T. Kima, C.Y. Yua, C.S. Yoon, S.J. Kim, Y.K. Sun, S.T. Myung, Nano Energy 12 (2015) 725–734. [41] B.J. Morgan, G.W. Watson, Phys. Rev. B: Condens. Matter Mater. Phys. 82 (2010) 144119. [42] C.D. Valentin, G. Pacchioni, A. Selloni, J. Phys. Chem. C 133 (2009) 20543–20552. [43] K.S. Han, J.W. Lee, Y.M. Kang, J.Y. Lee, J.K. Kang, Small 4 (2008) 1682–1686. [44] S.T. Myung, M. Kikuchi, C.S. Yoon, H. Yashiro, S.J. Kim, Y. K. Sun, B. Scrosati, Energy Environ. Sci. 6 (2013) 2609–2614. [45] K. Szot, M. Rogala, W. Speier, Z. Klusek, A. Besmehn, R. Waser, Nanotechnology 22 (2011) 254001. [46] D. Reyes-Coronado, G. Rodriguez-Gattorno, M.E. EspinosaPesqueira, C. Cab, R. de Coss, G. Oskam, Nanotechnology 19 (2008) 145605. [47] E.J.W. Whittaker, R. Muntus, Geochim. Cosmochim. Acta 34 (1970) 945–956. [48] S. Hariharan, K. Saravanan, V. Ramar, P. Balaya, Phys. Chem. Chem. Phys. 15 (2013) 2945–2953.

225 [49] H.A. Cha, H.M. Jeong, J.K. Kang, J. Mater. Chem. A 2 (2014) 5182–5186. Jang-Yeon Hwang received his B.S. degree from the Department of Chemical Engineering in Hanyang University in 2012. He is presently a Ph.D. candidate in the Department of Energy Engineering at Hanyang University, Korea, under the supervision of Professor Yang-Kook Sun. His research focuses on materials development in the fields of energy conversion and storage, such as cathode, anode and electrolyte materials for Sodium-ion batteries. Joo-Hyeong Lee received his B.S. degree from the Department of Material Science and Engineering in Hanyang University in 2014. He is presently a M.S. candidate in the Department of Energy Engineering at Hanyang University, Korea, under the supervision of Professor Yang-Kook Sun. His research focuses on materials development in the fields of energy conversion and storage, such as cathode for Sodium-ion batteries and Silicon based anode materials for Li-ion batteries. Seung-Taek Myung is an Associate Professor of Nano Engineering at Sejong University, South Korea. He received his Ph.D. degree in Chemical Engineering from Iwate University, Japan, in 2003. His research interests embrace development of electro-active materials and corrosion of current collectors of rechargeable lithium and sodium batteries.

Ali Abouimrane is a Principal Scientist and a Team Leader at Qatar Environment and Energy Research Institute (QEERI), Qatar Foundation, Doha, Qatar. He has over 15 years' experience in lithium batteries, advanced materials and electrochemical energy devices for the development of renewable energy technologies. His research area also includes materials and nanotechnology. He received his Ph.D. in Physical Chemistry in 2000 from University Hassan II, Casablanca, Morocco and a Specialized Graduate Diplomas (D.E.S.S.) in Management in 2007 from HEC, Montreal, Canada. Ilias Belharouak is the Chief Scientist and Energy Storage Group Leader at the Qatar Environment and Energy Research Institute, Qatar Foundation, Doha, Qatar. His works involve high power and high energy lithium ion batteries, sulfur batteries and sodium batteries for consumer, transportation and grid applications. He was recognized with several awards including US. R&D-100 Awards and US. Federal and State Laboratory Awards. Dr. Belharouak holds Ph.D. (1999) and Master's (1996) Degrees in materials science from the Institute for Solid State Chemistry, Bordeaux, France.

226 Yang-Kook Sun received his M.S. degree and Ph.D. degree from the Seoul National University, Korea. In 1996 he was a principal researcher at Samsung Advanced Institute of Technology and contributed to the commercialization of the lithium polymer battery. He has worked at the Hanyang University in Korea as a professor since 2000. His research interests are the synthesis of new electrode materials for lithium ion batteries, Na ion batteries, Li-S batteries, and Li-air batteries. In 2007 and 2011, he was awarded the Energy Technology Division Research Award and Research Award in Battery Division of the Electrochemical Society.

J.-Y. Hwang et al.