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Hydrogenated Anatase TiO2 as Lithiumion Battery Anode: Size-Reactivity Correlation Jing Zheng, Lei Liu, Guangbin Ji, Qifan Yang, Lirong Zheng, and Jing Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05993 • Publication Date (Web): 19 Jul 2016 Downloaded from http://pubs.acs.org on July 20, 2016

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ACS Applied Materials & Interfaces

Hydrogenated Anatase TiO2 as Lithium-ion Battery Anode: Size-Reactivity Correlation

Jing Zheng,a Lei Liu, a Guangbin Ji, a,* Qifan Yang, a Lirong Zheng,b and Jing Zhangb a

College of Materials Science and Technology, Nanjing University of Aeronautics

and Astronautics, Nanjing, 210016, P. R. China b

Institute of High Energy Physics, Chinese Academy of Sciences, Beijing,100049, P.

R. China

* Corresponding authors: E-mail: [email protected] (G.B. Ji)

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ABSTRACT An improved hydrogenation strategy for controllable synthesis of oxygen-deficient anatase TiO2 (H-TiO2) is performed via adjusting the particle size of starting rectangular anatase TiO2 nanosheets from 90 to 30 nm. The morphology and structure characterizations obviously demonstrate that the starting materials of TiO2 nanosheets are transformed into nanoparticles with distinct size reduction; meanwhile the concentration of oxygen vacancy is gradually increased with the decreasing particle size of starting TiO2. As a result, the Li-storage performance of H-TiO2 is not only much better than that of the pure TiO2 but also elevated stage by stage with the decreasing particle size of starting TiO2; especially the H-TiO2 with highest concentration of oxygen vacancy from smallest TiO2 nanosheets shows the best Li-storage performance with a stable discharge capacity 266 mAh g-1 after 100 cycles at 1 C. Such excellent performance should be attributed to the joint action from oxygen vacancy and size effect, which promises significant enhancement of high electronic conductivity without weaken Li+ diffusion via hydrogenation strategy.

KEYWORDS:

anatase

TiO2 nanosheets,

controllable

oxygen

vacancy,

hydrogenation process, size-reactivity correlation, lithium-ion battery, anode materials

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INTRODUCTION Owing to the increasing concerns on the environmental issues and energy crisis, lithium-ion batteries (LIBs) have been considered as one of the widespread energy storage devices recently. As a versatile functional material, titanium dioxide (TiO2) has a wide range of applications, such as solar cells, photocatalytic water splitting, gas sensing, and so on.1-3 Due to its excellent advantages of low cost, nontoxicity, environmentally benign, thermally and chemically stability, TiO2 has been developed to be a promising anode material for LIBs.4 Even though the low volume expansion during

the

intercalation/deintercalation

could

lead

to

a

stable

and

safe

charge/discharge process for TiO2 anode material, it is much difficult to reach a satisfactory electrochemical performance with high and stable capacity, which is most limited by the intrinsic properties of poor electronic conductivity and Li+ ion diffusion for TiO2. The poor Li+ mobility in TiO2 has been improved via employing unique nanostructures to reduce the length of Li+ diffusion pathway.5, 6 On the other hand, the electronic conductivity for TiO2 could be obviously enhanced by coating a conductive layer (e.g. polymers and carbon) on its surface or by doping foreign atoms into its crystal lattice to modulate its energy band structures.7,

8

Recently hydrogenation

strategy has been developed as an novel and effective route to fabricate high-performance TiO2 anode materials with a plenty of oxygen vacancy accompanied with Ti3+ (Ti3+/oxygen vacancy).9-11 Experimental results and theory calculations show that the introduced Ti3+/oxygen vacancy can act as electronic 3



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charge carrier to significantly improve the electronic conductivity; while too high Ti3+/oxygen vacancy concentration will weaken the Li+ diffusion because of depression of free Li+ concentration owing to association with excess electrons.12, 13 Therefore, developing an improved hydrogenation strategy that can enhance the electronic conductivity and Li+ diffusion simultaneously is much desired for high-performance H-TiO2 anode materials. Considering the nanoscale insertion/extraction effects in nanostructured materials14 as well as reaction dynamics during hydrogenation process, the particle size of starting TiO2 could probably play an important role on the electronic conductivity and Li+ diffusion speed via influencing the introduction of Ti3+/oxygen vacancy and Li+ diffusion path, respectively. In this work, as demonstrated in Scheme 1, we present an improved hydrogenation strategy that can modulate the concentration of Ti3+/oxygen vacancy in H-TiO2 via controlling the particle size of starting TiO2 in the range of 30 to 90 nm. Systematic characterizations demonstrated that oxygen vacancy with a higher concentration can be easily introduced into smaller-sized start material, promising a strengthened electronic conductivity; meanwhile, due to the size reduction derived shortened Li ion transport path, the hindering effect for Li ion diffusivity from oxygen vacancy is well neutralized. As a result, the synergistic effect from oxygen vacancy and size is well established. Such size-reactivity correlation suggests that tailoring particle size of starting TiO2 is an effective way for hydrogenation strategy that can boost the electronic conductivity and Li+ diffusion

4


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speed simultaneously, leading to a further enhanced Li storage performance especially for hydrogenated TiO2 from the smallest sized material.

Scheme 1. Schematic illustration of the important role of size effect for simultaneous regulation of the electron conductivity and Li+ diffusivity via improved hydrogenation strategy.

EXPERIMENTAL SECTION Synthesis of Size-Controllable Anatase TiO2 Nanosheets. A modified hydrothermal route was employed to synthesize size-controlled TiO2 nanosheets via altering the reaction temperature and the volume of hydrofluoric acid.15 In a typical experiment, 25 mL of Ti(OBu)4 (Sinopharm Chemical Reagent Co., P. R. China) and 2 mL of HF (35.35 wt.% Nanjing Chemical Reagent Co., P. R. China) were mixed in a 100 mL dried Teflon-lined autoclave for 5 min and then maintained at 180oC for 24 h. After being cooled to room temperature, the white TiO2 nanosheets with average side length of 30nm were obtained. In addition, other hydrothermal reaction of 25 mL Ti(OBu)4 5



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with 3 mL HF or 4 mL HF both at 200 oC for 24 h was used to obtain TiO2 nanosheets with average side length of 60 nm and 90 nm, respectively. The three sizes of pure TiO2 samples are obtained. Synthesis of H-TiO2 Nanoparticles from TiO2 Nanosheets with Different Size. In a typical experiment, the pre-synthesized white TiO2 nanosheets were put into an electric tube furnace under a flow of pure Ar (purity 99.999%, 100 sccm) for 30 min to remove the air in tube furnace. After that, the sample was heated to 500oC at a ramping rate of 5oC/min and maintained for 0.5 h under a flow of pure hydrogen (99.999%) with a flow rate of 50 sccm. This as-obtained H-TiO2 sample is denoted as H-Xnm (X= the size of original TiO2 nanoshhets, i.e., H-30 nm, H-60 nm, H-90 nm). As contrast, pure TiO2 samples without oxygen vacancy (denoted as Pure-Xnm, i.e., Pure-30 nm, Pure-60 nm, Pure-90 nm) were also prepared by heat treatment of pre-synthesized three sizes TiO2 nanosheets at the same temperature in air. Structure Characterization. The anatase TiO2 nanosheets before and after hydrogenation treatment were extensively characterized by power X-ray Diffraction analysis (XRD, Bruker D8 Advance X-ray diffractometer using a Cu Ka X-ray source), UV-Vis diffuse reflectance spectra (Shimadzu UV 3600), Raman spectroscopy thermal dispersive spectrometer (JY HR800, a laser with an excitation wavelength of 532 nm at laser power of 10 mW), Fourier Transform Infrared Spectroscopy (FT-IR, Perkin-Elmer spectrum 100 using KBr disks), Field Emission Scanning Electron Microscopy (FESEM, HITACHI S4800), High Resolution Transmission

Electron

Microscopy

(HRTEM,

JEOL

6


JEM

2100F),

X-ray

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Photoelectron Spectrometry (XPS, Thermo ESCALAB 250), Electron Paramagnetic Resonance (EPR, Bruker EMX-10/12). XAFS Measurement and Analysis. Ti K-edge X-ray absorption spectra of TiO2 were conducted at the beamline 1W1B (XAFS station) of the Beijing Synchrotron Radiation Facility (BSRF). The storage ring was operated at 2.5 GeV with an electric current between 160 and 250 mA within about 8 h. During the experiments, a Ti foil and a Si (111) double-crystal monochromatic was used for energy calibration and to reduce the harmonic content of the monochrome beam, respectively. To compensate for the diminishing amplitude due to the decay of photoelectron wave, the back-subtracted Extended X-ray Absorption Fine Structure spectroscopy (EXAFS) function was converted into k space and weighted by k3. The Fourier transforming of the k3-weighted EXAFS data was performed in the range of k 2-10.5 Å-1 using a Hanning window function to get the radial structure function (RSF) of Ti K-edge. Reduction and analysis of all X-ray Absorption Fine Structure spectroscopy (XAFS) data were performed by curve-fitting analysis using the ARTEMIS module implemented in the IFEFFIT package.16 Final fits of the n samples were made in Artemis in R-range 0.9–2.1 Å. The peaks shown in the Fourier transform was related to “shells” of surrounding back-scattering atoms with number of atoms in the shell (N), the absorber-scatter distance (R), and a Debye-Waller factor (σ2).

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Electrochemical Measurement. The two-electrode lithium-ion half cells were assembled in an Ar-filled glove box. To prepare working electrode, the prepared TiO2 nanosheets, conductive carbon black, and polyvinylidene difluoride (PVDF) at a weight ratio of 75:15:10 were mixed in appropriate amount of N-methyl pyrrolidinone (NMP, solvent) to obtain homogeneous yogurt-like slurry. And then the slurry was coated onto a thin Cu-foil, drying at 80oC for 12 h under vacuum condition. Electrode discs with a diameter of 12 mm were punched from the Cu-foil which is used as current collectors. The typical loading of active material was approximately 1.0 mg/cm2. Coin cell tests were done with CR2032 coin-type cells, which were fabricated in the above glove box using a couple of pure lithium metal foils as the reference electrode and a polypropylene microporous film as the separator. The electrolyte was 1 M LiPF6 with a volume ratio of 1:1 (w/w) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The charge-discharge performances of the assembled cells were tested at a voltage range of 1.0-3.0 V vs. Li/Li+ with LAND CT-2001A instrument (Wuhan, P. R. China) at room temperature (26±0.5 oC). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed on a CHI 660D electrochemical workstation (CH Instrument, Shanghai, P. R. China).

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RESULTS AND DISCUSSION

Fig. 1 The optical photographs of the pure TiO2 (pure-30 nm) and different particle size after hydrogenation reduction (H-30 nm, H-60 nm, H-90 nm).

XRD patterns of the pure TiO2 and H-TiO2 samples in Fig. 2a demonstrate the same phase with pure anatase TiO2 (Pure-30 nm, JCPDS 21-1272, space group: I41/amd), without other phases such as rutile or brookite detected. Furthermore, the peak intensity of H-TiO2 is weakened with the particle size of starting TiO2 from 90 to 30 nm, indicating a significant size effect on nanocrystalline structure for the H-TiO2. More importantly, the colour of the anatase TiO2 is changed from white (Pure-30 nm) to grey (H-90 nm), blue (H-60 nm) and then black (H-30 nm) (Fig. 1) after hydrogenation treatment, which is widely believed to the introduction of oxygen vacancy and/or surface disorder.17 To reveal the structure changes from hydrogenation treatment, UV/Vis reflectance spectra, Raman scattering spectroscopy and FT-IR spectra were carried out on pure and hydrogenated samples. Interestingly, the significant colour change of pure TiO2 during hydrogenation process suggests the introduction of oxygen vacancy in the H-TiO2 samples. The UV/Vis reflectance 9



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spectra in Fig. 2b obviously demonstrate that the pure and hydrogenated TiO2 samples have similar UV region absorption (200-400 nm) but possess an significant difference in visible light region (400-800 nm), which is probably due to the existence of local Ti3+ centres (band at 620 nm) and oxygen vacancy (at 1060, 441, and 486 nm) in H-TiO2.18 The absorption in visible light region is gradually increased with the particle size of starting TiO2 from 90 to 30 nm, indicating that the concentration of Ti3+/oxygen vacancy is increased stage by stage with reduced size of starting TiO2.

Fig. 2 (a) X-ray diffraction patterns, (b) UV/Vis diffuse reflectance spectra, (c) Raman spectra, and (d) FTIR spectra for the pure TiO2 (Pure-30 nm) and H-TiO2 samples. 10


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Such disordered structures form Ti3+/oxygen vacancy in H-TiO2 can be unambiguously detected by Raman scattering spectroscopy. As shown in Fig. 2c, six characteristic Raman modes of the anatase-phase TiO2 were observed at 141 cm-1 (Eg),197 cm-1 (Eg), 399 cm-1 (B1g), 514 cm-1 (A1g+B1g) and 639 cm-1 (Eg). It can be easily found that the signal intensity of these characteristic peaks is much weakened for H-TiO2. This change on Raman vibration modes is caused by non-stoichiometric property probably from oxygen-deficient structures in H-TiO2. Furthermore, the peak intensity is gradually weakened with reduced particle size of starting TiO2 from 90 to 30 nm. This result indicates that the decreased particle size of starting TiO2 can significantly enhance the non-stoichiometric property (Ti3+/oxygen vacancy) of H-TiO2,19 well accordant with the results of UV/Vis reflectance spectra. In addition, the FT-IR spectra in Fig. 2d present gradual blue-shift and broaden vibration peak of Ti-O-Ti at 500 to 1000 cm-1 for hydrogenated TiO2 with decreasing the particle size of starting TiO2 when compared to pure TiO2, which can be also ascribed to nonstoichiometric property caused by the presence of oxygen vacancies on the crystal lattice produced during hydrogenation treatment.20

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Fig. 3 TEM (row a) and HRTEM (row b) images for pure TiO2 (a1 and b1 for Pure-30 nm; a2 and b2 for Pure-60 nm; a3 and b3 for Pure-90 nm); TEM (row c) and HRTEM (row d) images H-TiO2 (c1 and d1 for H-30 nm; c2 and d2 for H-60 nm; c3 and d3 for H-90 nm). The inset of the corresponding row a and c images are the nanoparticles size distribution images.

The morphology of white pure TiO2 and hydrogenated TiO2 were systematically characterized by TEM and HRTEM. The TEM images of pure TiO2 in Fig. 3a1-a3 demonstrate that all the pure TiO2 samples have uniform structure of rectangular nanosheets with an average side length of about 30 nm, 60 nm and 90 nm, 12


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respectively (inset of Fig. 3a1-a3). By contrast, as shown in Fig. 3c1-c3, after hydrogenation treatment, these well-defined rectangular nanosheets were significantly transferred into some round or oval nanoparticles with a smaller average size of around 15, 21 and 61 nm, respectively, (inset of Fig. 3c1-c3). It can be speculated that the decreasing average size from regular nanosheets to anomalous nanoparticles is probably resulted from the destruction and reconstruction of these unstable nanosheets under hydrogenation treatment. Furthermore, the pure TiO2 samples only present a well-resolved lattice fringe of (101) plane of anatase TiO2 with a lattice distance of 0.357 nm (Fig. 3b1-b3) while a new lattice fringe with a lattice distance of 0.334 nm is observed in H-TiO2 (Fig. 3d1-d3), corresponding to the (004) planes of Ti6O11. This obvious difference also suggests the breaking of stoichiometry in TiO2 after hydrogenation treatment, generating some oxygen vacancies or crystal defects into H-TiO2. Particularly necessary to point out that the main phase of these hydrogenated TiO2 samples is still the TiO2 not the Ti6O11, in accordance with the XRD result.

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Fig. 4 (a) (top) The original XPS spectra of Ti 2p for Pure-30 nm and H-30 nm together (up) with their difference spectrum (“H-30 nm”minus“Pure-30 nm”) (bottom); High-resolution XPS spectra of Ti 2p (b) and O 1s (c) for the Pure-30 nm and H-TiO2, respectively; (d) The relative concentration of both Ti3+ and oxygen vacancy for hydrogenation TiO2; (e) Low-temperature EPR for the pure TiO2 and H-TiO2.

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X-ray photoelectron spectroscopy (XPS) was then employed to further investigate the surface binding properties of H-TiO2 samples. As demonstrated in Fig. 4a (top), the Ti 2p XPS spectrum of pure TiO2 (Pure-30 nm) shows typical Ti 2p3/2 and 2p1/2 peaks centred at 459.77 eV and 465.47 eV, respectively, while a rather different feature can be found in the Ti 2p XPS spectrum of the H-TiO2 (H-30 nm), which can be attributed to the different chemical environment of Ti 2p.21 The similar phenomenon

of obvious

binding energy shift is also found in other H-TiO2 samples (Fig. S1). To observe such differences, a difference Ti 2p XPS spectrum is obtained by subtracting the normalized Ti 2p spectra of H-30 nm with Pure-30 nm sample (“H-30 nm”minus“Pure-30 nm”). As shown in Fig. 4a (bottom), there are two extra peaks (two red arrows shown) centred at 458.2 and 463.6 eV, corresponding to the characteristic Ti 2p3/2 and Ti 2p1/2 peaks of Ti3+.22 These results manifest that Ti3+ is successfully doped into H-TiO2 probably via taking the oxygen atom neighbouring to Ti4+ away from TiO2 crystal lattice under the reduction atmosphere. To uncover the relationship between Ti3+/oxygen vacancy and particle size of starting TiO2, the high-resolution XPS spectra of Ti 2p and O 1s of pure TiO2 and H-TiO2 are well deconvoluted. As shown in Fig. 4b, compared to pure TiO2, the Ti 2p spectra of all three H-TiO2 samples show a negative shift in binding energy, that is, the smaller the particle size of starting TiO2 is, the more the binding energy shift (Table S1). Same phenomenon is also observed in O 1s 15



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XPS spectrum (Fig. 4c),23 suggesting that the surface bonding environments of H-TiO2 are changed step by step with reducing particle size of starting TiO2. Furthermore, the deconvolution results demonstrate that obvious Ti3+ and oxygen vacancy (Ov) are introduced into H-TiO2 during the hydrogenation process. Importantly, the relative concentration of both Ti3+ and Ov is increased stage by stage with decreasing the particle size of starting TiO2 from 90 to 30 nm, owing to growing hydrogenation reaction extent caused by the decreasing size (Fig. 4d and Table S2). These results demonstrate that the hydrogenation effect on Ti3+/oxygen vacancy can be significantly enhanced through improving the reaction dynamic by reducing the particle size of starting TiO2. Considering that the XPS measurement is limited on the surface of the tested materials, low temperature electron paramagnetic resonance (EPR) was then employed here to reveal the Ti3+/oxygen vacancy properties in the bulk of TiO2. The EPR spectra in Fig. 4e show that no apparent signal can be detected from the pure TiO2 while the as-synthesized H-TiO2 samples gave rise to a significant peak at g= 2.0010, corresponding to typical signal from oxygen vacancy. This result indicate that the bulk of H-TiO2 have obvious oxygen vacancies which are probably located not only on the surface but also in the bulk of H-TiO2 nanoparticles. Moreover, the signal intensity of oxygen vacancy increases along with the decrease of the particle size of starting TiO2, indicating an increased relative concentration of oxygen vacancy in H-TiO2 (including the surface and bulk) with reduced particle size of starting TiO2, owing to the 16


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different hydrogenation reaction extent. This overall change mode is well accordant with the surface measured by XPS, suggesting a similar distribution of Ti3+/oxygen vacancy on the surface and in the bulk of H-TiO2.

Fig. 5 The comparison of Fourier transform magnitudes of the EXAFS function of (a) Pure-30 nm and H-30 nm, and (b) different H-TiO2 samples (H-30 nm, H-60 nm, H-90 nm), respectively. In order to better investigate the local structure around Ti atoms, Fig. S2 shows the Ti K-edge EXAFS spectra of the well-crystallized pure TiO2 and H-TiO2. the quite similar EXAFS curves between the H-TiO2 samples and pure TiO2 indicates the H-TiO2 still remain their anatase structure without any phase transformation, in keeping with the aforesaid XRD and TEM results. Fig. 5 demonstrates the corresponding Fourier transformation (FT) of the χ (k) function in R-space for the as-prepared pure TiO2 and H-TiO2, respectively, which reveals the structural information on each shell at different bond lengths from the core Ti atom. The Fourier transform of the EXAFS spectra display different coordination shell peaks. The positions, areas and widths of these peaks are related to the interatomic distances (R), coordination number (N) and

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Debye–Waller factor (σ2), respectively. The peak at about 1.5 Å and 2.5 Å corresponds to the Ti-O coordination of the first shell and second shell, separately. Obviously, the first peak intensity of H-30 nm is weakened after hydrogenation treatment. It also can be found that the first peak intensity of H-TiO2 is slightly decreased with the decreasing particle size. In addition, the distinct amplitude reduction appear among these samples, which is probably caused by the decrease of Ti-O coordination number (Table S3) resulting from the shrink of particle size.24

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Fig. 6 Electrochemical tests of Pure-30 nm and H-TiO2 samples. Cyclic voltammograms curves (0.2 mV/s, 1.0-3.0V) of pure-30 nm (a) and H-90 nm (b); (c) galvanostatic charge/discharge voltage profiles at 0.5 C between 1 and 3 V. (data taken from the 10th cycle); (d) Cycling performance comparisons up to 100 cycles at 1 C; (e) Specific discharge capacity at various C-rates of 0.2, 0.5, 1, 2, 5, 10, 20 then down to 0.2, 1 C; (f) EIS for pure TiO2 and H-TiO2 samples with frequency range and amplitude of the two samples are set at 100 kHz-10 mHz and 5 mV after the 5th CV cycles, respectively. To evaluate the electrochemical Li-storage performance of pure TiO2 and H-TiO2, electrochemical measurements were conducted in a half-cell configuration battery (Li/TiO2).Obviously, a pair of distinct cathodic/anodic peak was located at around 1.70 and 2.01 V (Fig. 6b and Fig. S3) for hydrogenation samples, which were associated with the lithium insertion/extraction in the anatase lattice, respectively. While there is a dramatic differences between pure and hydrogenated samples: a minor oxidation/ reduction peaks appears on the CV curves for Pure-30 nm (Fig. 6a), demonstrating worse reversibility during Li+ intercalation/de-intercalation process when compared with these hydrogenation samples. Moreover, Fig. 6c presents the 10th cycle galvanostatic discharge/charge curves for pure TiO2 and H-TiO2 at 1 C. The insertion/extraction plateau of H-30 nm is different from Pure-30, H-60 nm and H-90 nm. The voltage plateaus for the Pure-30, H-60 nm and H-90 nm are observed at about 1.75 V (insertion process) and 1.92 V (extraction process) while 1.76 V and 1.90 V for H-30 nm, respectively. As a result, H-30 nm sample show a narrower gap between the charge and discharge than Pure-30, H-60 nm or H-90 nm. Such phenomenon indicates that Li+ ions are much easier to insert into the inside of H-30 19



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nm than other three samples. This advantage should be attributed to the small size of H-30 nm sample (15.1 nm for H-30 nm vs. 21.4 nm for H-60 nm, and 61.2 nm for H-90 nm) that significantly shortens the path for Li+ diffusion, and the high electronic conductivity from the high-level doping of oxygen vacancy/Ti3+ which improves the electron transfer during the charge/discharge process. Notably, the H-30 nm sample possesses a significantly higher discharge specific capacity of 248 mAh g-1 than that of Pure-30 nm (187 mAh g-1) at 100th, the same situation also appeared with the pure-60 nm and pure-90 nm in Fig. S4a. Even though H-90 nm has a lower discharge specific capacity than that of pure-30 nm, it can be easily found that the pure-30 nm shows a specific capacity of only about 83 mAh g-1 after 100 cycles, a worse cycle performance than any of H-TiO2 samples, as shown in Fig. 6d. More importantly, the discharge capacities of H-TiO2 samples at 100th cycle present an increasing trend gradually with reducing the particle size of starting TiO2 from 90 to 30 nm. Especially for H-30 nm, it can maintain a high discharge capacity of 226 mAh g-1 even after 100 cycles and 200 mAh g-1 after 200 cycles, but only 50 mAh g-1 for Pure-30 nm at 200th (in Fig. S5), indicating a better stability after hydrogenation process. It can be easily found that the H-30 nm possesses better cycling performance when compared with some published works, such as the anatase TiO2 nanoparticles 17 (150 mAh/g after 100 cycles at 1 C), traditional hydrogenated blue rutile

25

(179.8 mAh/g after 100 cycle at

0.05 C), and nanoporous anatase TiO2 mesocrystals 26 (151.9 mAh/g after 60 cycles at 0.5 C). Such significant improvement on the electrochemical properties can be attributed to the strong synergistic effect of oxygen vacancy and size effect for both 20


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enhancement of the electronic conductivity and Li+ diffusion, leading to a further enhanced Li storage performance for the hydrogenated small-sized TiO2 (H-30 nm). Fig. 6e shows the rate capability of pure TiO2 and H-TiO2 samples with current densities varying from 0.2 C up to 20 C then back to 1 C. Undoubtedly, the H-TiO2 samples demonstrate higher rate capacity than pure TiO2 at most tested rates (Fig. S4b), probably because the strong synergistic effect from oxygen vacancy and size effect that can promise fast transport of both electron and Li ion in hydrogenated TiO2. Moreover, the rate performance at high rates is improved significantly for the H-TiO2 samples. Especially for H-30nm, it has a discharge capacity of about 289 mAh g-1 at 0.2 C vs. 326 mAh g-1 for Pure-30 nm, owing to there is a formation of uneven and Mosaic electrode surface with an inhomogeneous Li-ion deposition/dissolution from an original layer (Li2O-LiOH), resulting in an irreversible capacity occurred during the early cycles. However, because of the exhaustion of original layer and the formation of uniform SEI layer, the H-30 nm has a higher discharge capacity than Pure-30 nm at large current density (like 1 C, 2 C, 5 C or more) and can be still retained at as high as 105 mAh g-1 at 20 C (vs. 30 mAh g-1 of Pure-30 nm). Most importantly, after such rigorous rate performance tests the H-TiO2 samples can still maintain better stability than that of pure TiO2. Particularly, even after 60 cycles with successively increasing current densities (from 0.2 C to 20 C), H-30 nm can still retain a discharge capacity of 170 mAh g-1 at 0.2 C, almost three times of that of pure-30 nm (at 62 mAh g-1). These results present an invaluable merit for the H-TiO2 anode materials that possess good stability during long cycles and excellent tolerance 21



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to high-rate charge/discharge. Especially, we speculate that it is probably the unstable condition when the major shift of uninterrupted current from 20 C to 0.2 C without any standing time, which makes the capacity of H-60 nm is higher than H-30 nm temporarily when back to 0.2 C. Moreover, the TEM images (Fig.S6) reveal that the H-30 nm can keep the nanostructure and size well without obvious decrease after long cycle, whereas the pure-30 nm shows an obvious structure changes from rectangular to spherical morphology with an increased size. That's probably the reason why hydrogenated TiO2 (especially for H-30 nm) has a high and stable capacity. The electrochemical impedance spectroscopy (EIS) is employed to investigate the ionic and electronic resistance behaviour in the battery enclosure. As Fig. 6f demonstrated, the Nyquist plots of pure TiO2 and H-TiO2 are composed of semicircle in the medium frequency region and an inclined line at low frequencies, corresponding to the charge-transfer resistance and the lithium diffusion process within the electrodes, respectively.27 To our knowledge, several published articles have reported that during hydrogenation process the oxygen atoms on the surface of TiO2 nanoparticles are easier to be removed and an oxygen-deficient TiO2-x shell is generated.25, 28 Correspondingly, only the H-30nm has two semicircles in the EIS curve, which probably because the electron in the oxygen-deficient TiO2-x shell transport fast that crystal pure TiO2. The simulated equivalent circuits as well as results are shown in Fig. S7 and Table S4. It can be easily found that the charge-transfer resistance (Rct) for H-30 nm is about 130.0 Ω, much lower than that of pure-30 nm (about 609.2 Ω), suggesting that the Ti3+/oxygen vacancy in H-TiO2 22


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can effectively boost the electronic conductivity of TiO2 via modulating its energy band structures. Moreover, with the particle size of starting TiO2 decreasing from 90 to 30 nm, an obvious increasing tendency for electronic conductivity of these H-TiO2 samples is observed. These results demonstrate that the electronic conductivity of TiO2 can be controllably enhanced via increasing the concentration of Ti3+/oxygen vacancy to some extent. On the other hand, the Warburg impedance (W) in H-30 nm (0.419 Ω) is slightly reduced when compared to pure-30nm (0.429 Ω), suggesting an enhanced Li+ diffusion process after hydrogenation treatment. Interestingly the W value is increased smoothly with the particle size of starting TiO2 from 90 to 30 nm, indicating that too much Ti3+/oxygen vacancies will weaken the Li+ diffusion, well accordance with previous results.17 Even so, the H-30nm with highest concentration of Ti3+/oxygen vacancy still has a faster Li+ diffusion speed than Pure-30nm, which must be attributed to the shortened Li+ diffusion path via reducing the particle size of starting TiO2. These results vividly demonstrate that the rational regulation of Ti3+/oxygen vacancy and microstructure sizes can significantly enhance the electronic conductivity as well as the Li+ diffusion of the TiO2 electrodes simultaneously, thus facilitating the fast kinetics of electrochemical reactions and leading to long cycles and high-rate performance.29

CONCLUSION In summary, we propose an improved hydrogenation strategy to synthesis high-performance TiO2 anode material via facile size regulation. Systematic 23



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physical and electrochemical measurements demonstrate that reduce-sized TiO2 could be conducive to more oxygen vacancies introduced into H-TiO2 and then result into a better electron conductivity of hydrogenated TiO2. Meanwhile, the reduced size can also shorten the Li+ diffusion path, which can effectively neutralize the negative influence for the Li+ diffusion from oxygen vacancy. As a result, the strong synergistic effect from oxygen vacancy and particle size promises a significant enhancement of high electronic conductivity and fast Li+ diffusion simultaneously by this modified hydrogenation strategy. It is believed that

such

improved

strategy

is

also

a

useful

route

to

synthesize

high-performance other metal oxide-based electrode materials for energy storage and conversion application.

Supporting Information Supplementary Information available: The original XPS curves and binding energy of Ti 2P peaks, the cyclic voltammograms curves, the cycling performance comparisons up to 200 cycles at 1 C, the TEM images after 100 cycles, the equivalent circuits and fitted impedance parameters, the EXAFS fitting parameters and Ti K-edge EXAFS spectra for pure TiO2 and hydrogenation TiO2. The characteristic peak area with its corresponding relative percentage of 2p 3/2 and O 1S for hydrogenation TiO2. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT

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Financial support from the National Natural Science Foundation of China (No. 11575085), the Open Research Fund of Jiangsu Provincial Key Laboratory for Nanotechnology of Nanjing University and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) is gratefully acknowledged.

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Table of Contents

Hydrogenated Anatase TiO2 as Lithium-ion Battery Anode: Size-Reactivity Correlation

Jing Zheng,a Lei Liu, a Guangbin Ji, a,* Qifan Yang, a Lirong Zheng,b and Jing Zhangb

A modified hydrogenation strategy is developed to construct oxygen-deficient anatase TiO2 (H-TiO2) with controllable concentration of Ti3+/oxygen vacancy as well as microstructure size. The Li-storage performances of H-TiO2 show an obvious size-reactivity correlation with the starting particle size. Such size-reactivity correlation should be attributed to the strong synergistic effect between Ti3+/oxygen vacancy and microstructure size, which promise an enhancement of high electronic conductivity and fast Li+ diffusion simultaneously.

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Hydrogenated Anatase TiO2 as Lithium-Ion Battery ... - ACS Publications

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