Molybdenum-Doped Titanium Dioxide and Its Superior Lithium

Article pubs.acs.org/JPCC

Molybdenum-Doped Titanium Dioxide and Its Superior Lithium Storage Performance Jingjing Zhang, Tao Huang,* Lijuan Zhang, and Aishui Yu* Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, Lab of Advanced Materials, Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, Shanghai 200438, China ABSTRACT: Molybdenum-doped titanium dioxide obtained using a facile hydrothermal process delivers a high reversible capacity of 408 mA h g−1 at 60 mA g−1 after 200 cycles, which is more than twice of that of undoped TiO2. In addition, this material also exhibits a better rate capability. Structural and surface analyses indicate that doping with Mo6+ has significant effects on the transformation of brookite to anatase and on the electronic structure, thereby improving the electrical conductivity. Remarkably, Mo6+ doping also induces a substantial decrease in particle size and lattice distortion, which greatly ameliorates the lithium diffusion and enhances the interfacial lithium storage. In combination with the extra lithium storage capacity through the conversion reaction of Mo6+ with Li+, the electrochemical performance of molybdenumdoped titanium dioxide dramatically improves.

electrical conductivity.12−16 Second, the reaction of dopants with Li ions improves the theoretical capacity of the electrodes.17,18 Third, the morphological and structural properties are further improved when host ions are partially substituted.19,20 Fehse et al. and Anh et al. reported that Nb5+ or V5+ doping significantly influenced the electronic structure and the particle size of TiO2, and thus the electrical conductivity was improved and the lithium diffusion promoted due to the much shorter diffusion length.12,14 Wang et al. mentioned that the reaction of the Sn dopants with Li ions improved the specific capacity of the Sn4+-doped TiO2 electrodes.18 It is worth mentioning that Mo6+ doping in this context not only improves the electrical conductivity and the theoretical capacity of the electrodes and induces a substantial decrease in particle size, which is the key to facilitating lithium diffusion due to the short, nanoscopic, through-particle diffusion path, but also dramatically augments lithium mass transport within the crystalline lattice because of the resulting lattice distortion.10,21,22 In addition to the benefit associated with a short path length, interfacial lithium storage can play an important role beyond the conventional bulk intercalation of lithium into the TiO2 lattice as the particle size decreases because excess Li accommodated at the interface of the nanosized particles leads to a higher energy density.23−26 Doping with Mo also restrains the agglomeration of TiO2 nanoparticles, leading to the formation of uniform particles with many mesopores. The high porosity and large surface area can further improve the interfacial lithium storage. Additionally, the mesopores play a significant role in shortening the lithium

1. INTRODUCTION Over the past decade, substantial efforts have been focused on developing advanced lithium-ion batteries (LIBs) with high storage capacities and power densities for applications in highperformance portable devices and hybrid electric vehicles (HEVs). One of the major challenges is to develop novel electrode materials with fast charge/discharge rates, high capacities, and long cycle lives. TiO2 is considered to be one of the most promising anode materials for LIBs because of its structural characteristics, low cost, high abundance, and environmental benignity.1−5 In addition, TiO2 possesses the advantages of a small volume expansion ratio (3%) upon the insertion/extraction of Li ions and good cyclic stability.6 In general, the insertion/extraction of Li ions in TiO2 occurs according to the following reaction x Li+ + TiO2 + x e− = LixTiO2

(0 ≤ x ≤ 1)

where x is the amount of lithium inserted into TiO2, which depends upon the properties of the materials used. For anatase, the maximum value of x is generally 0.5 for the fully reversible reaction, which is accompanied by a phase transformation from tetragonal TiO2 to orthorhombic Li0.5TiO2, corresponding to a capacity of 168 mA h g−1.7 However, low capacity, poor electrical conductivity (∼10−12 Ω−1 cm−1),8,9 and slow lithium diffusion (∼10−17 cm2 s−1)10 limit the widespread application of TiO2 in LIBs. Doping TiO2 with metallic ions that possess attractive electrochemical properties appears to be a promising method for improving the electrochemical performance of TiO2. First, substituting Ti4+ with an aliovalent dopant introduces additional charge carriers, which improve the low bulk conductivity of TiO2.11,12 Vanadium (V5+), niobium (Nb5+), and tantalum (Ta5+) have previously been introduced to TiO2 to improve its © 2014 American Chemical Society

Received: June 27, 2014 Revised: October 12, 2014 Published: October 14, 2014 25300

dx.doi.org/10.1021/jp506401q | J. Phys. Chem. C 2014, 118, 25300−25309

The Journal of Physical Chemistry C

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battery test system (Land CT2001A, Wuhan Jinnuo Electronic Co. Ltd.) at a constant current density of 60 mA g−1 in the potential range from 0.01 to 3.0 V. EIS measurements were measured for the fresh cells at open potential and for post cycling cells with an ac amplitude of 5 mV over the frequency range from 105 to 10−2 Hz (Zahner_IM6e).

diffusion length in the solid phase, as long as the electrolyte can penetrate the pores, thus enabling fast transport toward the interior of the solid. In this study, Mo6+-doped TiO2 nanoparticles were produced via an extremely simple and cost-effective hydrothermal method. The effect of Mo 6+ doping was thoroughly investigated. The Mo6+-doped TiO2 nanoparticles obtained using this method exhibit a high capacity, excellent cycling performance, and remarkable rate capability.

3. RESULTS AND DISCUSSION 3.1. Morphologies and Structures of the Obtained Materials. The phase compositions of the prepared samples determined from the EDX analyses and the unit cell parameters calculated by the Rietveld method are presented in Table 1. In

2. EXPERIMENTAL SECTION 2.1. Preparation of Mo6+-Doped TiO2 Nanoparticles (MTO). All reagents were purchased from the Shanghai Sinopharm Chemical Reagent Co., Ltd., were analytically pure, and were used as-received without further purification. The amount of Mo was optimized to obtain the best lithium storage performance. In a typical synthesis,27 an aqueous solution containing a defined amount of MoCl5 and TiCl4 precursors was prepared. The sols were then transferred to a 50 mL stainless-steel autoclave covered with Teflon for hydrothermal treatment (200 °C, 2 h). After the hydrothermal treatment, the autoclave was quickly cooled to room temperature. The products were obtained by filtering, were sequentially washed several times with water and ethanol, and named MTO-1, MTO-2, MTO-3, MTO-4, and MTO-5. Similarly, undoped TiO2 was prepared using the same method but without adding MoCl5. 2.2. Characterization and Electrochemical Measurements. The crystalline structure and morphology were characterized using X-ray diffraction (XRD, Bruker D8 Advance, Cu Ka radiation, λ = 1.5406 Å), field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800), transmission electron microscopy (TEM, JEM -2100F), and selected area electron diffraction (SAED, TEM, JEM-2100F) measurements. The lattice parameters for the materials were refined using the Rietveld method (Total Pattern Analysis Solutions). Sample MTO-4 was further studied using in situ high-temperature X-ray diffraction (XRD, Bruker D8 Discovery, Cu Kα radiation, λ = 1.5406 Å), and the diffractometer was equipped with a high-temperature appendix (PAR-1200N). The measurements started at room temperature and finished at 800 °C, using steps of 5 °C. The proportion of Mo to Ti was determined by energy-dispersive X-ray spectroscopy (EDX, Hitachi S-4800), and elemental mapping (EM) was conducted using the same instrument. The specific surface area and pore size distribution of the sample were determined using the multipoint Brunauer−Emmett−Teller (BET) method and the Barrett−Joyner−Halenda (BJH) model, respectively (QUDRASORB SI, Quantachrome Instruments U.S.). X-ray photoelectron spectroscopy (XPS) experiments were conducted on a RBD upgraded PHI-5000C ESCA system (PerkinElmer) with Al Kα radiation (hυ = 1486.6 eV). The binding energies were calibrated using the containment carbon (C 1s = 284.6 eV). Electrochemical tests were performed using a CR2016-type coin cell. An assembled coin cell was composed of lithium as the counter electrode and the working electrode consisting of 80% active material, 10% super P carbon black, and 10% polyvinylidene fluoride (PVDF) made on copper foil. Cointype half-cells were assembled in an argon-filled glovebox (Mikarouna, Superstar 1220/750/900) with a 1 M LiPF6 solution in ethylene carbonate/diethyl carbonate (EC:DEC = 1:1, v/v) as the electrolyte and Celgard 2300 as the separator. The galvanostatic charge−discharge tests were performed on a

Table 1. EDX Results (Molar Ratio) and Structural Analysis (Tetragonal Unit Cell Constants a and c) for the Obtained Materials sample

MTO-1

MTO-2

MTO-3

MTO-4

a (Å) c (Å) Mo/Ti

3.807 9.495 0.0431

3.814 9.473 0.0953

3.818 9.451 0.2336

3.821 9.439 0.4158

addition, the crystal structures investigated with the XRD method are shown in Figure 1a. When no MoCl5 was added to the starting solution, well-distinguished peaks corresponding to anatase (204), (220), (215), and brookite (121) were observed, which confirms that the as-prepared TiO2 nanoparticles are a mixture of two crystal phases: anatase and brookite. However, because anisotropic peak broadening occurs in nanomaterials as

Figure 1. XRD patterns of (a) undoped TiO2, MTO-1, MTO-2, MTO-3, MTO-4, and MTO-5 and of (b) undoped TiO2 with a prolonged reaction time of 12 h. 25301



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The high-temperature XRD patterns of MTO-4 are shown in Figure 3. Only anatase (JCPDS 21-1272) and rutile (JCPDS

a result of crystallite shapes, defects, and microstrain, some peaks were overlapped and unresolved, such as the (101) peak of anatase, which was overlapped by the (120) and (111) peaks of brookite (JCPDS 21-1272 and 29-1360). As the content of MoCl5 during the synthesis increased, brookite began to transform into anatase, resulting in an increase in the intensity of the anatase diffraction peaks in MTO-1, and brookite completely transformed to anatase in MTO-2. Notably, the phase separation in TiO2 was retained under the preparation conditions, even with a prolonged reaction time (200 °C, 12 h, Figure 1b). Hence, it can be concluded that doping TiO2 with Mo6+ promotes the transformation of brookite to anatase. Furthermore, note that all of the diffraction peaks were shifting toward lower angles, indicating a gradual expansion of the crystal lattice of TiO2 when Mo6+ is incorporated due to the larger ionic radius of Mo6+ (0.062 nm) compared with that of Ti4+ (0.0605 nm).28,29 The XRD patterns of the obtained materials were further analyzed using Rietveld refinements (Table 1). It has been found that doping with Mo6+ induces an increase in the a lattice constant and a decrease in the c lattice constant of the TiO2 unit cell, which confirms the possibility of incorporating metal ions as a substituent in the TiO2 lattice.30,31 For example, the refinement results of undoped TiO2, MTO-1, and MTO-2 are presented in Figure 2. All of the samples fit

Figure 3. High-temperature XRD patterns of MTO-4.

77-0446) were observed during heating, and no diffraction lines corresponding to Mo-containing phases or other phases, with the exception of the Al1.92Cr0.08O3 sample holder, were observed, further confirming that the sample was a homogeneous single-phase solid solution rather than a composite of crystalline or amorphous metal oxides. TEM and FE-SEM were employed to investigate the microstructure and crystallinity of the different materials obtained. Figures 4a−e show TEM micrographs of undoped TiO2 and MTO with different Mo/Ti molar ratios. It can be observed that doping TiO2 with Mo6+ leads to pronounced changes in the particle sizes and surface morphologies of the samples. Increasing the Mo content not only reduces the average grain size (from 10 nm for undoped TiO2 to 3 nm for MTO-4) but also restrains the agglomeration of TiO2, leading to the formation of uniform particles. This phenomenon arises from the nucleation effect, which also suggests a more rapid formation of the desired phase.6,11 This doping effect has a substantial impact on the electrochemical properties of doped materials, which implies improved lithium diffusion and higher surface lithium storage. Figure 4f shows the SEM image of MTO-4, which consists of well-distributed particles with many mesopores. The distribution of Mo, Ti, and O in MTO-4 was also determined through elemental mapping (EM) at low magnification. From the Mo, Ti, and O element maps shown in Figure 5, the three elements are uniformly distributed in the material without phase separation. It is concluded that the Mo dopant is uniformly doped throughout the entire TiO2 lattice. Nitrogen adsorption isotherms were measured to determine the

Figure 2. Rietveld refinements of (a) undoped TiO2, (b) MTO-1, and (c) MTO-2.

well with their respective XRD parameters, confirming the phase transition of brookite to anatase and the formation of Mo6+-doped TiO2 nanoparticles mentioned above. Additionally, the values of the Rwp factor for the three samples were 3.309, 3.027, and 3.318%, respectively, indicating that the fittings were accurate. However, as indicated by the arrowheads in Figure 1a, a trace impurity was formed at higher Mo content (MTO-5). Therefore, this sample is not included in the following discussion, and further work is in progress to understand this. 25302



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Figure 4. TEM micrographs of (a) undoped TiO2, (b) MTO-1, (c) MTO-2, (d) MTO-3, and (e) MTO-4 and SEM micrograph of (f) MTO-4.

Figure 5. Elemental mapping of Ti/Mo/O for MTO-4.

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pore structure and the Brunauer−Emmett−Teller (BET) surface area of MTO-4 (Figure 6). As shown, the

adsorption/desorption isotherm is of type IV with a distinct hysteresis loop observed in the range of 0.5−1.0 P/P0, indicative of a mesoporous material (Figure 6a). Furthermore, the BET surface area is as high as 210 m2 g−1. Figure 6b shows the PSD curve using the BJH method, and this curve indicates that the pore size of MTO-4 is approximately 7 nm. The presence of mesopores in the final product can be reasonably attributed to the spacing between the nanoparticles. Figures 7a and c present HRTEM images of undoped TiO2 and MTO-4, and the corresponding selected area diffraction patterns (SAED) are shown in Figures 7b and d. The HRTEM image of undoped TiO2 reveals that the lattice plane distances are 0.353 and 0.292 nm, which are well matched with the (101) and (121) planes of anatase and brookite, respectively. Additionally, this result is in good agreement with the SAED pattern (Figure 7b), confirming the presence of both anatase and brookite phases. The crystallinity of MTO-4 is clearly apparent from the well-defined fringe at 0.362 nm (Figure 7c). Due to the replacement with Mo cations in the nanoparticles and resulting lattice expansion, the measured d101 is slightly larger than that of anatase. Furthermore, the absence of the (121) diffraction ring in the SAED pattern (Figure 7d) indicates that the presence of Mo6+ dopants in the TiO2 nanostructure has a significant effect on the transformation of brookite to anatase and that the obtained Mo6+-doped TiO2 nanoparticles are a single-phase solid solution with the anatase TiO2 structure. To identify the states of the Mo and Ti species, the undoped TiO2 and MTO-4 samples were analyzed using XPS (Figure 8). The two peaks in the Ti 2p spectrum (Figure 8b; 459.3 and 465 eV) of MTO-4 show a small positive shift of 0.7 eV compared to those of undoped TiO2 (Figure 8e),32 which suggests a certain electron drain from the Ti4+ in the oxide matrix due to the presence of Mo6+. Consistent with this result, the Mo 3d spectrum (Figure 8c; 234.8 and 231.6 eV) of the same doped sample exhibits a negative shift of 1.05 eV compared with the

Figure 6. (a) N2 adsorption/desorption isotherm and (b) pore size distribution of MTO-4.

Figure 7. HRTEM micrographs of (a) undoped TiO2 and (c) MTO-4 and the corresponding selected area diffraction patterns (b) and (d). 25304



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Figure 8. (a) XPS spectrum of MTO-4; (b) the Ti 2p3 line, (c) the Mo 3d5 line, (d) the O 1s line of MTO-4; and (e) the Ti 2p3 line and (f) the O 1s line of undoped TiO2.

attributed to the surface hydroxyl group Ti−OH and chemisorbed H2O molecules.36−38 However, for MTO-4 (Figure 8d), the binding energy of the lattice oxygen shifts to 530.08 eV due to the difference in the electronegativities of the two metal elements.39 This observation further confirms the formation of a Mo−O−Ti linkage in the Mo6+-doped TiO2 nanoparticles. The remaining two peaks of MTO-4 at 531.79 and 533.27 eV are attributed to the same chemical constituents as those assigned for the undoped TiO2. 3.2. Electrochemical Performance. The electrochemical properties of undoped TiO2, MTO-2, MTO-3, and MTO-4 as anode materials for LIBs were investigated. As discussed above, there was an impurity in MTO-5; thus, this sample is not considered with respect to the electrochemical performance. For comparison, only the cycling performance is provided. The charge/discharge curves of the as-prepared samples for the first cycle obtained at a current density of 60 mA g−1 in the voltage window of 0.01−3.0 V (vs Li/Li+) are shown in Figure 9. In

standard Mo 3d spectrum listed in Handbook of X-ray Photoelectron Spectroscopy. Indeed, this type of charge transfer occurring between two cationic species has been well investigated using XPS, in which the metallic ions are mixed at an atomic level, forming a heterogeneous linkage of MA−O− MB (where Mi represents different metallic species).33−35 Depending on the cation charge and the electronegativity of the element, the electron cloud tends to transfer from one type of cation to the other through their oxygen bridge in the linkage structure. In the present case, electron transfer is expected to occur from Ti4+ to Mo6+ because of their difference in electronegativity (Mo = 2.16 vs Ti = 1.54). Therefore, the above complementary binding energy shifts confirm that Mo6+ ions are incorporated into TiO2, which is in good agreement with our above findings. Furthermore, the O 1s spectrum of undoped TiO2 can be divided into three peaks (Figure 8f). The peak at 529.78 eV is assigned to lattice oxygen in this oxide, whereas the two peaks at 531.72 and 533.25 eV can be 25305



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charge capacities are 210, 246, 318, and 461 mA h g−1, respectively, implying that the first discharge and charge capacities are considerably improved through the doping with Mo6+. The initial capacity loss is caused by (i) irreversible intercalation of Li ions into TiO2, (ii) irreversible conversion of Mo6+ by Li ions, and (iii) irreversible formation of the SEI on the electrode surface. More importantly, the voltage profiles of the doped samples display a smaller polarization (Vch − Vdis) than that of the undoped sample due to the considerably improved electrical conductivity. Figure 10 presents the dQ/dV curves of the first discharge/ charge profiles of the undoped TiO2, MTO-2, MTO-3, and MTO-4 electrodes. For undoped TiO2, the peaks at around 1.75 and 1.51 V are characteristic for the insertion of Li ions into anatase and brookite, respectively. The peak at around 2.0 V is attributed to the extraction of Li ions from TiO2. The peak at around 0.77 V is associated with decomposition of the electrolyte and the formation of the solid electrolyte interphase (SEI) layer. However, the lithiation process in the Mo6+-doped samples appears to be considerably different. For MTO-4, the reduction and oxidation peaks of TiO2 are weaker due to fewer sites in TiO2 for the insertion of Li ions. Furthermore, there are an additional two peaks: the first peak at around 0.42 V is attributed to the conversion reaction of the Mo6+ dopants with Li+,41−43 and the second peak at around 0.13 V could be related to the lithium storage occurring at the nanoparticle surfaces/ interfaces.24,26 These two processes are primarily responsible for the extra reversible capacity of the nanoparticles. Similar reduction and oxidation peaks were observed at almost the same positions in the differential capacity curves for MTO-2

Figure 9. First discharge/charge profiles of the undoped TiO2, MTO2, MTO-3, and MTO-4 electrodes at 60 mA g−1 in the voltage range of 0.01−3.0 V.

previous works, the anatase anode was always discharged to 1.0 V. In this study, a voltage range of 0.01−3.0 V (vs Li/Li+) was used for the conversion reaction of the Mo6+ dopants with Li+. The doped samples show plateaus in their respective curves at ∼1.85 and ∼2.05 V for discharging and charging, respectively, which is consistent with the presence of anatase as the electroactive material.40 Additionally, note that the initial discharge capacities of undoped TiO2, MTO-2, MTO-3, and MTO-4 are 467, 549, 646, and 797 mA h g−1, whereas the

Figure 10. dQ/dV differential curves of the first discharge/charge profiles of the (a) undoped TiO2, (b) MTO-2, (c) MTO-3, and (d) MTO-4 electrodes at 60 mA g−1 in the voltage range of 0.01−3.0 V. 25306



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high rate of 1200 mA g−1, the specific capacity is still above 148 mA h g−1, and nearly 91% of the charge capacity could be recovered (369 mA h g−1) when the current density is reduced back to 60 mA g−1 during 90 cycles, which is much higher than that of undoped TiO2 when cycling at the current from 60 mA g−1 (200 mA h g−1) ramped to 1200 mA g−1 (51 mA h g−1). In conclusion, these results demonstrate that doping TiO2 with Mo6+ is able to not only increase the charge/discharge capacities but also impart the materials with remarkable rate capabilities. As discussed above, doping TiO2 nanoparticles with aliovalent ions modifies its electrical conduction properties. To verify this hypothesis, electrochemical impedance spectroscopy (EIS) measurements were performed to compare the conductivities of undoped TiO 2 and Mo 6+ -doped TiO 2 electrodes (Figure 12). An intermediate-frequency semicircle

and MTO-3 but with lower heights, suggesting that more Li ions are involved in the insertion/desertion reactions in MTO4. Figure 11a shows the cycling performances of the obtained materials at 60 mA g−1 in the voltage window of 0.01−3.0 V (vs

Figure 11. (a) Cycling performances of the undoped TiO2, MTO-2, MTO-3, MTO-4, and MTO-5 electrodes at 60 mA g−1 in the voltage range of 0.01−3.0 V (open-discharge, filled-charge) and (b) rate capabilities (60−1200 mA g−1) of the undoped TiO2, MTO-2, MTO3, and MTO-4 electrodes.

Li/Li+). Among the obtained materials, MTO-4 exhibits the highest capacity and best reversibility. After 200 cycles, the charge capacity is as high as 408 mA h g−1, which is more than two times of that of undoped TiO2 (156 mA h g−1). Notably, the performance of MTO-4 is superior to those reported to date for TiO244−48 and for doped TiO2.14,49,50 Additionally, MTO-2, MTO-3, and MTO-4 retained their charge capacities of 195, 273, and 171 mA h g−1 after 200 cycles, respectively. Therefore, a higher Mo content and smaller particle size result in more Li ions participating in the conversion and surface reactions, leading to an increase in capacity with increasing Mo6+ content. However, as the amount of Mo6+ is increased to the level of that in MTO-5, capacity fading occurs, which could be explained by the fact that an impurity is formed with higher Mo6+ contents. The rate capabilities of undoped TiO2, MTO-2, MTO-3, and MTO-4 from 60 to 1200 mA g−1 for five cycles at each current rate are presented in Figure 11b. Among the obtained materials, MTO-4 exhibits the best rate capacity. No obvious fading of the charge capacity is observed during the first five cycles when testing at various rates. At a lower current density of 60 mA g−1, MTO-4 exhibits a charge capacity of 407 mA h g−1. Even at a

Figure 12. AC impedance (Nyquist plots) of the undoped TiO2, MTO-2, MTO-3, and MTO-4 electrodes: (a) before cycling and (b) after cycling (30th) at 60 mA g−1.

and a low-frequency tail are observed in Figure 12a. The intermediate-frequency semicircle is related to charge transfer resistance (Rct), whereas the low-frequency tail is associated with the Li ion diffusion process in the solid phase of the electrode. After 30 cycles, an additional high-frequency semicircle, attributed to the formation of the SEI film upon cycling, is observed, as shown in Figure 12b. Before charging, it is observed that the diameter of the semicircle for MTO-2 is shorter than that for undoped TiO2, implying a smaller Rct in the former. Moreover, Rct is further decreased for MTO-3 and MTO-4 with more Mo6+ doping. After 30 cycles, the Rct values of both samples dramatically decrease as a result of electrochemical activation.51−53 According to the Krö ger−Vink 25307



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notation,54 the partial substitution of Ti4+ ions with Mo6+ ions creates conduction band electrons, interstitial oxygen defects, oxygen vacancies, or titanium vacancies to compensate for the imbalance in the charge caused by doping,55 thereby significantly enhancing the electrical conductivity of the TiO2 material, enabling much easier charge transfer, and consequently, decreasing the overall internal resistance of the battery. Furthermore, we investigated the lithium diffusion behavior by analyzing the low-frequency Warburg contribution in the Nyquist EIS spectra. The lithium diffusion coefficient is calculated by the following formula21,22 2 1 ⎡⎛ Vm ⎞ dE ⎤ DLi = ⎢⎜ ⎟ ⎥ 2 ⎢⎣⎝ FAσw ⎠ dx ⎥⎦

where the values of DLi for undoped TiO2, MTO-2, MTO-3, and MTO-4 are ca. 4.20 × 10−17, 1.34 × 10−16, 2.76 × 10−15, and 3.73 × 10−15 cm2 s−1, respectively. In general, the diffusion coefficients tend to decrease with decreasing particle size for Liinsertion hosts.56,57 However, this trend is not observed in our materials, which further illustrates the effect of Mo6+ doping. In other words, the lithium mass transport within the crystalline lattice is not characterized by particle size alone; other effects, such as lattice distortion in this context and the appearance of oxygen vacancies, are at play.10,21,22 Now, another question arises: can MTO retain its structural stability during cycling due to the conversion and surface reactions? To answer this question, post cycling electrode materials were investigated using ex situ XRD measurements, as shown in Figure 13. Well-distinguished peaks of anatase are observed in the same positions for both the pristine MTO-4 electrode and the MTO-4 electrode after the 200th charge, and no impurity is observed, demonstrating that the structural changes accompanying the insertion/extraction of Li in the MTO nanoparticles are completely reversible. The retention of the primitive anatase framework even after 200 cycles fully demonstrates the high structural stability of the currently developed MTO anode materials, which leads to the outstanding cycling performance mentioned above.

Figure 13. (a) Ex situ XRD patterns of the pristine MTO-4 electrode and the electrode after the 200th charge. * corresponds to diffraction peaks from the Cu current collector under the anode electrode film and (b) magnified image of part a.

which greatly improves the theoretical capacity of the electrode. The stable anatase structure also contributes to the improved cycling performance.

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AUTHOR INFORMATION

Corresponding Authors

4. CONCLUSIONS We showed that the Mo dopant is homogeneously incorporated in the TiO2 lattice by substituting Ti. A well-developed crystal structure with a gradual expansion in the crystal lattice upon the introduction of Mo6+ is confirmed by XRD and HRTEM. XPS analysis reveals the presence of Mo−O−Ti linkages, which is consistent with the observed lattice expansion. The results from the in situ high-temperature XRD analysis further confirm the formation of a homogeneous single-phase solid solution without any crystalline or amorphous Mo-containing phases. The TEM and BET results both show that the particle size of TiO2 is substantially reduced, leading to a mesoporous structure with a high specific surface area. When used as an anode material, the charge/discharge capacities, cycling performance, and rate capability are significantly improved by Mo6+ doping. We primarily attribute these improvements to three observed Mo-doping effects: first, the global increase in the electrical conductivity, which is related to the change in electronic structure; second, the decrease in crystal size and lattice distortion, which are beneficial for Li+ diffusion and interfacial lithium storage; and third, the conversion reaction of Mo6+ dopants with Li ions,

*Tel.: +86-21-51630320. Fax: +86-21-51630320. E-mail: [email protected] *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the National Key Basic Research Program of China (973 Program, 2013CB934103), the National Natural Science Foundation (No. 21173054), and the Science & Technology Commission of Shanghai Municipality (No. 08DZ2270500), China.

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REFERENCES

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