Chromium-Modified Li4Ti5O12 with a Synergistic Effect of Bulk Doping

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Chromium-Modified Li4Ti5O12 with a Synergistic Effect of Bulk Doping, Surface Coating, and Size Reducing Hailin Zou, Xin Liang, Xuyong Feng, and Hongfa Xiang* School of Materials Science and Engineering, Hefei University of Technology, Anhui Hefei 230009, P.R. China S Supporting Information *

ABSTRACT: Bulk doping, surface coating, and size reducing are three strategies for improving the electrochemical properties of Li4Ti5O12 (LTO). In this work, chromium (Cr)-modified LTO with a synergistic effect of bulk doping, surface coating, and size reducing is synthesized by a facile sol−gel method. X-ray diffraction (XRD) and Raman analysis prove that Cr dopes into the LTO bulk lattice, which effectively inhibits the generation of TiO2 impurities. Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) verifies the surface coating of Li2CrO4 on the LTO surface, which decreases impedance of the LTO electrode. More importantly, the size of LTO particles can be significantly reduced from submicroscale to nanoscale as a result of the protection of the Li2CrO4 surface layer and the suppression from Cr atoms on the long-range order in the LTO lattice. As anode material, Li4‑xCr3xTi5−2xO12 (x = 0.1) delivers a reversible capacity of 141 mAh g−1 at 10 °C, and over 155 mAh g−1 at 1 °C after 1000 cycles. Therefore, the Cr-modified Li4Ti5O12 prepared via a sol−gel method has potential for applications in highpower, long-life lithium-ion batteries. KEYWORDS: lithium titanate, chromium doping, sol−gel method, lithium-ion batteries, Li2CrO4 either Li+ ion or electronic conduction can enhance the fast charging−discharging ability of LTO. Commonly used methods include reducing the particle size, incorporating a conductive component, and doping alien ions. Reducing the particle size not only can shorten the path lengths for Li+ transport but can also offer a large contact area between the electrode and the electrolyte,11−13 which facilitates the Li+ ion conduction in an LTO electrode.14 In particular, nanostructured LTO with various morphologies, such as nanowires,15 nanosheets,16 and nanotubes,17 has been investigated and usually exhibits enhanced rate capability due to the short path length for Li+ transport. However, either intrinsic or extrinsic electronic conductivity of LTO is not improved greatly, and both the electrode/electrolyte side reactions and the preparation methods are complex in this strategy.18 Comparatively, incorporating a conductive component can enhance the electronic conductivity of the LTO particles and in between and also avoid serious electrode/electrolyte side reactions. The conductive components for LTO usually include amorphous carbon,19 graphene,20 carbon nanotubes,21 and metals and some oxides.22,23 However, the surface coating strategy is unable to affect the properties of the LTO bulk, e.g., electronic conductivity and Li+ diffusion coefficient in the LTO lattice. Substitution of Ti4+, Li+, or O2− with alien ions can

1. INTRODUCTION Energy storage has become a worldwide topic over the past decade as a result of energy insufficiency and environmental pollution caused by vast fossil-fuel consumption. An energy storage system (ESS) is necessary to assimilate variable solar and wind energy for the smart grid. Lithium ion batteries (LIBs) have received increasing research interest as one of the most popular EESs, due to its competitive energy density, long lifetime, and high energy efficiency.1−5 However, state-of-theart graphite-anode-based LIBs are handicapped by poor cycling stability, low rate capability, and serious safety concerns. One of the reasons for the safety concerns is that lithium dendrite formed on the graphite surface at high current densities can induce an internal short circuit of a LIB, which can further trigger thermal runaway of the battery or even a fire/explosion.6 As an attractive candidate for graphite, spinel-type lithium titanium oxide (Li4Ti5O12, denoted as LTO) has been intensively investigated because of its advantages to lifetime and safety characteristics.7−9 To be widely applied in the ESSs, a fast charging− discharging (corresponding to Li+ intercalation and deintercalation, respectively) ability of LTO is needed. During transformations in Li-ion chemistry, charge transfer consists of Li+ ion and electron conduction. Li+ ion conduction in an LTO electrode is determined by the Li+ ion diffusion coefficient (DLi+) and the particle size (L) of LTO, while electronic conduction is determined mainly by electronic conductivity in and between LTO particles.10 Usually the improvements on © 2016 American Chemical Society

Received: June 25, 2016 Accepted: August 1, 2016 Published: August 1, 2016 21407

DOI: 10.1021/acsami.6b07742 ACS Appl. Mater. Interfaces 2016, 8, 21407−21416

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ACS Applied Materials & Interfaces

Figure 1. XRD patterns of as-prepared Li4−xCr3xTi5−2xO12 (0 ≤ x ≤ 0.5) (a, b). Li4Ti5O12 prepared at various temperatures (c) and the Raman spectra of Li4−xCr3xTi5−2xO12 (0 ≤ x ≤ 0.5) (d).

greatly improve the Li+ ion diffusion coefficient in LTO particles by adjusting the paths for Li+ ion transport.24,25 Although inequivalent ion doping can enhance the electronic conductivity of LTO bulk by reducing Ti4+ to more conductive Ti3+, the electronic conduction between LTO particles still needs be improved. Additionally, the introduction of alien ions into the lattice usually leads to reduced crystallinity.26 Therefore, simultaneous improvements on both Li+ ion and electronic conduction could significantly enhance the fast charging−discharging ability of LTO.27−29 Ion doping is recognized to reduce the structural order of the lattice. However, Song et al. reported that charge redistribution increased the structural order of the Cr-doped LTO lattice.30 The main reason is that Cr doping can lead to a straightened Ti−O−Ti bond and an expanded oxygen sublattice by stretching the Ti−O bond. This work triggered studies on Cr-doped LTO again,10,31−33 even though the relative materials had been reported 10 years ago.34,35 Feng et al. reported that LiCrTiO4 (Li4−x/3Ti5−2x/3CrxO12, x = 3) prepared via an acrylic acid polymerization method contained lithium chromium oxide (LiCrxOy, y > 1/2 + 3x/2) as a modifier for the conductivity of the electrode.31 However, the Li+ diffusion coefficient in LiCrTiO4 would be lower than that in pristine LTO, because of the shrunken lattice parameters after Cr doping.35 Additionally, the theoretical capacity of LiCrTiO4 (157 mAh g−1) is lower than that of Li4Ti5O12 (175 mAh g−1). A simple surface modification with an aqueous CrO3 solution was also proved to greatly improve the rate capability of LTO.32 Thus, both Cr doping and surface modification have been proved to effectively enhance the fast charging−discharging ability of LTO, respectively. But to the best of our knowledge, combination of Cr doping and surface modification via a facile method,

especially with the synergistic effect on size reducing, was seldom reported to improve the electrochemical performance of LTO. In this work, we prepare Cr-modified LTO anode materials with a synergistic effect of bulk doping, surface coating, and size reducing via a facile sol−gel route. Cr doping in the LTO bulk, lithium chromium oxide (Li2CrO4) coating on the surface of LTO, and the size reducing of LTO particles can synergistically improve the Li+ ion and electronic conduction of LTO anode materials. Cr-modified LTO is predetermined as Li4−xCr3xTi5−2xO12 (0 ≤ x ≤ 0.5). The Li4−xCr3xTi5−2xO12 (x = 0.1) with the best synergistic effect exhibits superior rate capability (over 141 mAh g−1 at 10 °C) and excellent cycling stability along with a high capacity of over 155 mAh g−1 after 1000 cycles at 1 C.

2. EXPERIMENTAL SECTION Materials Preparation. Li4−xCr3xTi5−2xO12 (0 ≤ x ≤ 0.5) was synthesized by a facile sol−gel method. Lithium acetate (Li(CH3COO)·2H2O), tetrabutyl titanate (Ti(OC4H9)4, TBT), and chromium nitrate (Cr(NO3)3·9H2O) were dissolved in 20 mL of dehydrated ethanol containing 2.5 mL of acetic acid to form a solution. The Li, Cr, and Ti sources were predetermined at a molar ratio of 1.05 × (4 − x):3x:(5 − 2x), and they all were purchased from Sinopharm Chemical Reagent Co., Ltd. After 2 mL of deionized water was dropwise added into the solution above under stirring at 60 °C for 2− 3 h, a transparent sol was obtained. Afterward, a gel was formed by drying the sol at 80 °C for 10 h. The final products Li4−xCr3xTi5−2xO12 (0 ≤ x ≤ 0.5) were obtained after heat treatment at 400 °C for 5 h and at 700 °C for 6 h in air. The colors of Li4−xCr3xTi5−2xO12 (0 ≤ x ≤ 0.5) change from white to olive with increasing chromium content (Figure S1a). For comparison, the Li4−xCr3xTi5−2xO12 (0 ≤ x ≤ 0.5) samples were washed with deionized water to remove soluble surface compositions. 21408


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Figure 2. Ti 2p XPS spectra of as-prepared Li4−xCr3xTi5−2xO12: (a) x = 0.0 and (b) x = 0.1. First, 1.0 g of product was dispersed in 10 mL of deionized water. After 4 h stirring, the precipitate was obtained by the use of a Beckman Coulter J2-MC centrifuge at 8000 rpm. Then the final precipitate was dried at 100 °C for 10 h. After washing, the colors of the samples were slightly faded, and the colors of the filtered solution darkened gradually with the increase of x (Figure S1). Physiochemical Characterization. The crystal structures of these Li4−xCr3xTi5−2xO12 (0 ≤ x ≤ 0.5) powders (washed before and after) were determined by X-ray diffraction (XRD) on a diffractometer (Dmax 2500 V) with a Cu target. The diffraction patterns were recorded at room temperature in a 2θ range of 10−80°. Raman spectra of the samples were obtained on a Micro-Raman spectrometer (LABRAM-HR confocal laser) using an argon laser with a wavelength of 514.5 nm. The morphology and particle size of Li4−xCr3xTi5−2xO12 (0 ≤ x ≤ 0.5) samples were measured on a JEOL JSM-6390LA scanning electron microscope (SEM) and an FEI Titan 80−300 transmission electron microscope (TEM). The chemical states of various elements at the surface were analyzed on a PerkinElmer PHI5000C multifunctional X-ray photoelectron spectrophotometer (XPS). Electrochemical Measurements. The electrochemical properties of Li4−xCr3xTi5−2xO12 (0 ≤ x ≤ 0.5) were measured in the Li∥LTO coin cells (CR2032-type) using various Li4−xCr3xTi5−2xO12 electrodes. Each Li4−xCr3xTi5−2xO12 electrode was composed of an aluminum foil as the current collector and a coated mixture of 80 wt % Li4−xCr3xTi5−2xO12, 10 wt % carbon black, and 10 wt % polyvinylidene fluoride (PVDF) binder. For coin cell assembly, the Li4−xCr3xTi5−2xO12 electrode was designed as discs with a diameter of 14 mm, and the Li4−xCr3xTi5−2xO12 mass loading was controlled to about 3 mg. The specific capacity is based on the mass of the Li4−xCr3xTi5−2xO12. In all the Li∥LTO cells, the separator was Celgard 2400 polypropylene microporous membrane and the electrolyte was 1 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1, w/w). All the cells were assembled in an argon-filled MBraun glovebox. The electrochemical properties of Li2CrO4 and LiCrTiO4 were measured similarly by substituting Li4−xCr3xTi5−2xO12 with Li2CrO4 or LiCrTiO4 in the process of the electrode preparation and cell assembly. Charge−discharge tests were performed at a range of 1.0−2.5 V on a multichannel battery cycler (Arbin BT2000). All charge−discharge rates were denoted by using the C-rate (1 C = 175 mA g−1). Electrochemical impedance spectroscopy (EIS) of the coin cells was collected on a CHI 604 electrochemical workstation (Shanghai Chenhua Corp.) in a frequency range from 105 Hz to 0.1 Hz. Before the EIS measurements, the Li∥LTO cells were charged to 2.5 V after two formation cycles at 0.1 C.

exhibit sharp diffraction peaks which suggest good crystallinity. All the diffraction peaks can be identified to be the Fd3m space group according to JCPDS card no. 49-0207, while some impurity of rutile TiO2 comes out in pristine LTO (x = 0). To confirm Cr doping in the LTO lattice, the strongest peak at around 18° is picked out and zoomed in, as shown in Figure 1b. The diffraction angle (2θ) increases after more Cr doping (18.44° for pristine LTO, 18.49° for Cr0.3-LTO, and 18.55° for Cr0.5-LTO). On the basis of Bragg’s equation, the higher diffraction angle means a smaller lattice parameter, e.g., 8.345 Å for pristine LTO, 8.332 Å for Cr0.3-LTO, and 8.311 Å for Cr0.5LTO. This is in good agreement with Sun’s results.35 In a unit of Li4−xCr3xTi5−2xO12, 3x Cr3+ ions (R = 0.062 nm) replace x Li+ (R = 0.076 nm) and 2x Ti4+ ions (R = 0.061 nm) by the following formula: 3Cr3+ = Li+ + 2Ti4+. Because (0.062−0.076) + 2 × (0.062−0.061) = −0.012 < 0, it is reasonable that the lattice parameters of Cr0.3-LTO and Cr0.5-LTO were smaller than that of pristine LTO. A big shrinkage in lattice parameter is not beneficial for Li+ transport. That is the reason for control of x at 0 ≤ x ≤ 0.5 in this study. For Cr0.1-LTO, a lattice parameter (8.346 Å) comparable with that of pristine LTO suggests that a small amount of Cr doping (i.e., Cr0.1-LTO) does not significantly change the lattice parameter or hinder the Li+ diffusion. In Figure 1c, the rutile TiO2 impurities can be significantly inhibited when the sintering temperature was increased from 700 °C to 800 °C. It is easy to detect the TiO2 impurities in Li4Ti5O12 prepared by a sol−gel method,36 and optimization of the sintering temperature can inhibit the generation of impurities. Considering that no impurity was observed in the Cr-modified LTO samples prepared at 700 °C (Figure 1a), it is evident that Cr modification can reduce the sintering temperature and also improve the purity of the final LTO product. Raman spectroscopy can provide atomic-scale structural information and show the state of metal oxide.37 Figure 1d shows the Raman spectra of as-prepared Li4−xCr3xTi5−2xO12 (0 ≤ x ≤ 0.5). The five Raman bands of A1g + Eg + 3F2g are predicted by factor group analysis of spinel LTO.38 However, at least nine Raman bands at 751, 672, 510, 427, 400, 346, 264, 231, and 101 cm−1 are observed in pristine LTO (x = 0), which is consistent with previous reports.39 Three of these Raman bands, 672, 427, and 231 cm−1 are assigned as A1g, Eg, and F2g modes, corresponding to a symmetric stretching vibration (νsym) of Ti−O bonds in the TiO6 octahedra, an asymmetric stretching vibration (νasym) of Li−O bonds in the LiO4 tetrahedra, and a bending vibration (δ) of O−Ti−O bonds, respectively.37,40 As shown by the red guide lines in Figure 1d,

3. RESULTS AND DISCUSSION The X-ray diffraction patterns of the as-prepared Li4−xCr3xTi5−2xO12 (x = 0, 0.1, 0.3, and 0.5) samples are shown in Figure 1a. These Li4−xCr3xTi5−2xO12 (x = 0, 0.1, 0.3 and 0.5) samples were also marked with pristine LTO, Cr0.1LTO, Cr0.3-LTO, and Cr0.5-LTO, respectively. All the samples 21409


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Figure 3. SEM images of as-prepared Li4−xCr3xTi5−2xO12 with (a) x = 0.0, (b) x = 0.1, (c) x = 0.3, and (d) x = 0.5.

Figure 4. TEM images of (a) Cr0.1-LTO and (b) Cr0.5-LTO. HRTEM images and SAED patterns of Cr0.1-LTO (c, d). EFTEM images of (e) Cr0.1LTO and (f) Cr0.5-LTO.

the x dependencies of the A1g, Eg, and F2g modes differ. The wave numbers of the A1g (672 cm−1) and the Eg (427 cm−1)

modes decrease with increasing x, whereas the wavenumber of the F2g mode (231 cm−1) increases with increasing x. As for 21410


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Figure 5. Illustration of the formation mechanism for Cr-modified Li4Ti5O12.

with a small size of 100−200 nm. Basically, the Cr modification is proved to suppress the growth of the LTO particles prepared via the sol−gel method, and the reduced particle size means a short distance for both Li+ ion and electron conduction. Herein our results are obviously different from those of Lin’s report10 with similar large particle sizes (about 500 nm) of Li4−xCr3xTi5−2xO12 (0 ≤ x ≤ 1.0) samples prepared by a solid-state route. According to the previous literature,30 alien atoms introduced into the host lattice can result in smaller particle sizes by retarding the long-range order of the lattice. Thermogravimetry (TG) and differential scanning calorimetry (DSC) results of the Li4−xCr3xTi5−2xO12 (0 ≤ x ≤ 0.5) precursors (Figure S2 and Table S1) indicate that chromium nitrate can promote the oxidation−decomposition reaction of TBT to get small-sized and porous LTO materials with large specific surface area.46 With further observations on the morphologies of Cr0.1-LTO and Cr0.5-LTO (Figure 4a,b), Cr0.1-LTO has a size of 100 nm with clear edges, but Cr0.5-LTO particles are not homogeneous with a smaller size of 50 nm. It is well-known that the nanosized particles are prone to agglomerate into big particles at high temperatures, which results in a wide size distribution of Cr0.5LTO particles. The HRTEM images of Cr0.1-LTO are shown in Figure 4c,d. Figure 4c reveals that Cr0.1-LTO is highly crystalline with a spinel structure. However, Li2CrO4 with a particle size of 10 nm is detected on the surface of the Cr0.1LTO particle (the red dotted line in Figure 4d). The lattice fringe of ca. 0.215 nm is consistent with the interplanar distance of (502) planes of the Li2CrO4 crystal, and the SAED pattern in the inset of Figure 4d also confirms the existence of hexagonal Li2CrO4. For the purpose of studying the segregation state of Cr, the energy-filtered (EF) TEM images of Cr0.1-LTO and Cr0.5-LTO are shown in Figure 4e,f. The distribution of Cr is uniform with those of Ti and O in Cr0.1-LTO (Figure 4e). However, as for Cr0.5-LTO (Figure 4f), the Cr content in the parts of the red circle is relatively higher than that in other places, which is attributed to the existence of Li2CrO4 coated on the particle surface. The existence of Li2CrO4 is also proved by XPS analysis (Figure S3) and Raman spectra (Figure S4), besides the TEM images and the Raman spectra in Figure 1d. To illustrate the effects of Cr modification, a possible mechanism is proposed in Figure 5. First, a glaucous dried gel was formed after the solvents were removed from the wet gel consisting of Li, Ti, and Cr sources. A molecular level mixing of Li, Ti, and Cr sources was achieved by the sol−gel method. After heat treatment at 400 °C under air atmosphere, the dried gel was decomposed into a uniform and porous mixture of Li− Cr−Ti oxides. The uniform composition can help reduce the sintering temperature and enhance the suppression of heterogeneous atoms on the long-range order. The porous structure is conducive to the permeation of O2 in the

Cr0.1-LTO, nine Raman bands are evident with negligible shifts compared to those of pristine LTO. However, there are only five obvious Raman bands at 233, 384, 585, and 659 cm−1 for Cr0.3-LTO and Cr0.5-LTO. The peak at 585 cm−1 is assigned to the stretching vibration in the CrO6 octahedra,31 which intensifies with the increase of x. But the strongest Raman band in Cr0.5-LTO is the A1g mode at 659 cm−1, which is different from that of LTO (the strongest Raman band is the F2g mode at 231 cm−1). This indicates a drastic change in the local structure around the TiO6 octahedra with Cr doping in Cr0.3-LTO and Cr0.5-LTO. In addition, another minor peak exists at 841 cm−1 in Cr0.5-LTO (inset of Figure 1d), which is ascribed to the stretching vibration of “Cr6+O”31 because of the formation of Cr6+ from the oxidation of Cr3+ at high temperature.41 To confirm the Cr doping in Cr0.1-LTO, the XPS results of the Ti 2p spectra for pristine LTO and Cr0.1-LTO samples are compared in Figure 2. Two peaks at 464.9 and 459.5 eV (Figure 2a) correspond to the Ti 2p1/2 and Ti 2p3/2 binding energies of Ti4+, respectively.42 The binding energies in Cr0.1LTO are 465.1 (Ti 2p1/2) and 459.6 eV (Ti 2p3/2), which are approximately 0.2 and 0.1 eV higher than those in pristine LTO, respectively. This result can be explained by an enhancement in the electron cloud density of Ti and further implies that Cr does dope into the LTO lattice. Additionally, the Ti3+ peak at 457.8 eV in Figure 2b can be calculated with a content of 35.86% that is much higher than in pristine LTO, which suggests that partial Ti4+ is reduced to Ti3+. According to ́ 43 the Cr3+ ions (R = 0.062 nm) the results reported by Martin, can dope into the octahedral 16d Li/Ti sites of Li4Ti5O12 because the Cr3+ ion radius is much smaller than the Li+ ion radius (R = 0.076 nm) but comparative to the Ti4+ ion radius (R = 0.061 nm). It is possible for more Cr3+ ions than that proposed to occupy the 16d Li sites, which results in the formation of Ti3+ to balance the charge. Therefore, it is proposed that despite the main style of 3Cr3+ = Li+ + 2Ti4+, partial Cr doping is as follows: 3Cr3+ = (1 + x)Li+ + (2 − x)Ti4+ + xTi3+. It was reported that the existence of Ti3+ can efficiently enhance the electronic conductivity of the LTO bulk by increasing the electron−hole concentration.44,45 Herein the existence of Ti3+ detected by XPS proves that Cr3+ doped into the lattice of LTO, which induced more Ti3+ to balance the charge. The morphologies and particle sizes of Li4−xCr3xTi5−2xO12 (0 ≤ x ≤ 0.5) were observed by SEM, as shown in Figure 3. The pristine LTO particles (Figure 3a) reveal a wide size distribution from 100 to 500 nm with an average of about 300 nm. However, Cr0.1-LTO (Figure 3b) and Cr0.3-LTO (Figure 3c) exhibit much smaller particle sizes, especially 50− 100 nm with centralized size distribution from Cr0.1-LTO. In Cr0.5-LTO, there are some large particles of 300−500 nm along 21411


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Figure 6. Electrochemical performance: (a) Voltage profiles of as-prepared Li4−xCr3xTi5−2xO12 (0 ≤ x ≤ 0.5) at C/10 rate. (b) Incremental capacity plot of the first cycle for as-prepared Li4−xCr3xTi5−2xO12 (0 ≤ x ≤ 0.5). (c) Voltage profiles of the washed Li4−xCr3xTi5−2xO12 (0 ≤ x ≤ 0.5) at C/10 rate. (d) Voltage profile of Li2CrO4 in the first cycle process at a low current density (0.06 mA cm−2) from 1.0 to 2.5 V.

subsequent calcination at 700 °C in air, which is very important for the formation of highly pure Li4−xCr3xTi5−2xO12 phase compared with a small amount of Li2CrO4 phase. Here the migration distance of each ion in the sintering process was reduced greatly because of the molecular level mixing, compared with that in a solid-state reaction. Additionally, Cr3+ also doped into the lattice of Li4−xCr3xTi5−2xO12, which induced more Ti3+ to balance the charge. Cr doping can effectively enhance the electronic conduction of LTO as well as the ionic conductivity.34,35 Li2CrO4 surface coating can lead to Li2+xCrO4 after lithium insertion, which improves both Li+ ionic and electronic conductivities of LTO. Owing to the protection of the surface modification layer and the suppression of heterogeneous atoms on the long-range order, growth and agglomeration of LTO particles can be effectively retarded and thus the particle size can be evidently reduced. Herein via a facile sol−gel route, Cr modification on LTO exhibits a synergistic effect on bulk doping, surface coating, and size reducing. Afterward, the synergistic effect of Cr modification is expected to endow LTO with significantly enhanced electrochemical performance. Galvanostatic charge (Li+ insertion)−discharge (Li+ extraction) tests were carried out to evaluate the electrochemical performance of the Li4−xCr3xTi5−2xO12 (0 ≤ x ≤ 0.5). As shown in Figure 6a, the reversible capacity at 0.1 C, respectively, reaches 155, 170, 171, and 154 mAh g−1 for pristine LTO, Cr0.1-LTO, Cr0.3-LTO, and Cr0.5-LTO, respectively. Obviously, for Cr0.1-LTO and Cr0.3-LTO, the capacities are significantly improved compared to the pristine sample. This is mainly due to the increased crystallinity and the reduced content of rutile TiO2 impurities. However, the capacity of Cr0.5-LTO is the lowest. For Li4−xCr3xTi5−2xO12, compared with pristine LTO,

each Li+ ion and two Ti4+ ions are replaced by three Cr3+ ions. Consequently, the molecular weight of the doped LTO increases with increasing content of Cr3+ so that its theoretical capacity decreases. On the other hand, the first charge curves for the as-prepared Li4−xCr3xTi5−2xO12 contain two plateaus at about 2.1 and 1.55 V (Figure 6a). As shown in Figure 6b, most capacity is in the region around 1.55 V, corresponding to the two-phase transformation between Li4Ti5O12 and Li7Ti5O12.47 The insert in Figure 6b shows that the initial oxidation and reduction peaks shift to low potential with the increase of Cr content, which suggests that the Cr doping results in a lower operating potential, in agreement with a previous report.10 In addition, there is an irreversible peak upon initial reduction around 2.1 V vs Li/Li+, which disappears in the subsequent cycles. The intensity of this peak increases and is gradually divided into two small peaks (1.98 and 2.13 V) with increasing x in Li4−xCr3xTi5−2xO12. This peak corresponding to the plateau of 2.1 V in Figure 6a is assigned to the lithium insertion into Li2CrO4 on the surface of Cr-modified LTO particles. To confirm this point, the as-synthesized Cr-modified LTO samples were washed with water to remove Li2CrO4 that has good solubility in water. For the corresponding LTO samples after washing, as shown in Figure 6c, the plateau at about 2.1 V disappears and the reversible capacity is significantly reduced for Cr0.1-LTO and Cr0.3-LTO. These results suggest that Li2CrO4 has important influence on the electrochemical performance of Cr-modified LTO. There is no obvious change for pristine LTO after washing, which indicates that the washing in water did not change the bulk structure of the LTO. Furthermore, the charge−discharge curves of pure Li2CrO4 as active material are shown in Figure 6d (details for the synthesis and electrochemical measurements of Li2CrO4 can be found in 21412


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Figure 7. Electrochemical performance: Rate performance of Li4−xCr3xTi5−2xO12 (0 ≤ x ≤ 0.5) as-prepared (a) and after being washed (b) at different charge−discharge rates. (c) Rate performance of the samples as-prepared and after being washed (x = 0.1) in comparison with literature data for Li3.917Ti4.834Cr0.25O12 (ref 30), Li4Ti5O12 (ref 48) and LiCrTiO4 (ref 10). (d) Cycle performance of the as-prepared Li4−xCr3xTi5−2xO12 (0 ≤ x ≤ 0.5) at 1 C.

Figure 8. CV curves of as-prepared Li4−xCr3xTi5−2xO12 at various sweep rates: (a) x = 0.0, (b) x = 0.1. (c) The relationship between the peak current (Ip) and the square root of scan rate (v1/2) cathodic process of as-prepared Li4−xCr3xTi5−2xO12 (0 ≤ x ≤ 0.5). (d) The Nyquist plots of the asprepared x = 0.0 sample and the as-prepared and postwashed x = 0.1 samples.

these plateaus during the charge process means that Li2CrO4 is not an active material for Li storage and does not contribute an evident reversible capacity by itself. It is noted that such a 2.1 V

Supporting Information). During charging from 2.5 to 1.0 V, it displays two plateaus at 2.3 and 2.0 V, which are close to 2.1 V observed in Li4−xCr3xTi5−2xO12. The irreversible character of 21413


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kinetics during charging, to obtain the outstanding charge and discharge performance. Additionally, there is a small cathodic peak at low potential, and the linear relationship between its peak current and scan rate (insert in Figure 8b) indicates a pseudocapacitive effect as previously reported.50 Similar results can be found in Cr0.3-LTO (Figure S7a). The pseudocapacitive effect corresponds to the distinct surface lithium storage in Cr0.1-LTO and Cr0.3-LTO because of their small particle sizes and large specific surface areas.51 The small cathodic peak was not observed in pristine LTO (Figure 8a) and Cr0.5-LTO (Figure S7b) with relatively large particle sizes. According to the previous literature,52 the smallest Ipa/Ipc values for Cr0.1LTO at different scan rates suggest the best reversibility for Li insertion/extraction (Table S2). The relationship between the peak current Ip [mA] and the square root of scan rate v [mV s−1] in Figure 8c can be used to calculate the diffusion coefficient D [cm2 s−1] of lithium ions in LTO. On the basis of the CV data and the following equation:53

plateau has not been reported in the Cr-doped LTO materials prepared by a solid-state method.43 The same difference also appeared in the different methods for preparation of LiCrTiO4 materials31 (Figure S5), and the 2.1 V plateau related to lithium chromium oxide impurity formed in the liquid phase method is more obvious than that formed in the solid-state method. Similarly, in our facile sol−gel method, more Li2CrO4 is inclined to be formed simultaneously on the surface of the Crdoped LTO particles. Figure 7a,b shows the rate capabilities of the as-prepared Li 4−x Cr 3x Ti 5−2x O 12 (0 ≤ x ≤ 0.5) samples and the corresponding samples after washing. The specific reversible capacity of pristine LTO declines most sharply during the initial three cycles at 0.1 C and from 0.1 to 1 C. The main reason is its poor electronic conductivity, large particle size, and existence of rutile TiO2 impurity. Nevertheless, the reversible capacities of Cr0.1-LTO and Cr0.3-LTO are comparable and maintain high capacity retention as the C-rates are increased. Even at 10 C, Cr0.1-LTO and Cr0.3-LTO still maintain high capacities of 141 and 139 mAh g−1 (83% and 81% of initial charge capacity), which are much higher than the others (25 and 75 mA h g−1 for pristine LTO and Cr0.5-LTO, respectively). After Li2CrO4 is removed by washing with water, the rate capabilities of Cr0.1LTO and Cr0.3-LTO become worse and are no longer close (Figure 7b). Meanwhile, the capacity retentions of Cr0.1-LTO at various current rates are substantially higher than those of the Cr-free LTO sample synthesized by the same method48 and the Cr-doped LTO samples by the solid-state method10,34 (Figure 7c). Therefore, the presence of Li2CrO4 benefits the higher rate capability of Li4−xCr3xTi5−2xO12. Feng et al.32 reported that Li2CrO4 had a high electrical conduction and could result in the formation of Li2+xCrO4 with good Li+ ion conduction during Li insertion. It is worth noting that the rate capabilities of the Crmodified LTO samples are all better compared to pristine LTO with/without washing (Figures 7a,b). Although the Cr3+ doping remarkably increased the electronic conductivity of particles, the LTO lattice parameters gradually decrease with increasing Cr3+ doping level, which inhibits the diffusion of Li ions. For Cr0.1-LTO and Cr0.3-LTO, the electronic conductivity in particles increased significantly, but the lattice constant decreases and thus results in lower ion diffusion for Cr0.3LTO. The Coulombic efficiency of rate capability for the preand postwashed samples always maintains a quite high level of ∼100% after formation cycles (Figure S6a and S6b). As shown in Figure 7d and Figure S6c, Cr0.1-LTO exhibits excellent cycling stability compared to other samples. After 1000 cycles, Cr0.1-LTO still delivers a capacity of 154 mAh g−1 with a retention of 96%. The good cycling performance is far better than that of pristine LTO. For Cr0.3-LTO, good cycling stability is similar to Cr0.1-LTO in the first several hundred cycles, but after 600 cycles a distinct capacity fading appears. This may be because the products of Li2+xCrO4 are not stable during the long cycling time. Cyclic voltammogram (CV) results on the Li4−xCr3xTi5−2xO12 (0 ≤ x ≤ 0.5) anodes are shown in Figure 8. Figure 8a and 8b displays the CV curves of pristine LTO and Cr0.1-LTO at different sweep rates, respectively. A pair of redox peaks at 1.45/1.66 V at 0.2 mV s−1 agree well with Li insertion/ extraction in the LTO,49 and the polarization potential increases with an increase in sweep rate. At a high scan rate of 1.0 mV s−1, the sharp redox peaks in Cr0.1-LTO are still maintained with excellent reversibility. This suggests that the synergistic strategy can significantly improve the electrode

Ip = 2.69 × 105An3/2C0D1/2v1/2

(1)

where n is the mole number of electrons during Li intercalation, A [cm2] is the surface area of the LTO anode, and C0 [mol cm−3] is the Li+ concentration in LTO. The Li+ diffusion coefficient in as-prepared Li4−xCr3xTi5−2xO12 (x = 0, 0.1, 0.3, and 0.5) are 3.80 × 10−11, 1.37 × 10−10, 1.09 × 10−10, and 5.07 × 10−11 cm2 s−1, respectively. The diffusion coefficients of Crmodified LTO are higher than those of pristine LTO and decrease with an increase in the content of Cr (decreasing of lattice parameter). These results show that Cr-modified LTO with a synergistic modification has higher Li+ diffusion, which enhances the rate capability of LTO. But excessive Cr modification will severely decrease the lattice parameters and thus lead to poor electrochemical properties. AC impedance spectra were investigated to understand the electrochemical performance of Cr0.1-LTO. Herein pristine LTO and the as-prepared and washed Cr0.1-LTO were cycled three times for the AC impedance measurements. In Figure 8d, Cr-modified LTO samples exhibit a semicircle smaller than that of the pristine sample, suggesting that a small amount of Cr modification can reduce the charge transfer impedance of LTO, especially for as-prepared Cr0.1-LTO. The possible reasons are as follows. First, Cr3+ doping induced more Ti3+ in the body region, which can enhance the electronic conductivity of LTO bulk particles. Second, the presence of some Li2CrO4 on the surface is helpful to improve the electrical conductivity between the particles. More importantly, evident size reducing of LTO particles facilitates the fast Li+ ion diffusion during Li insertion/ extraction. All these factors mentioned above result in the reduced impedance and enhanced electrochemical performance of Cr-modified LTO materials.

4. CONCLUSIONS In this work, a series of Cr-modified spinel Li4Ti5O12 materials were synthesized by a very facile sol−gel method, which can simultaneously realize bulk doping, surface coating, and particle size reducing. First of all, introduction of Cr3+ into the 16d Li sites changes Ti4+ into Ti3+ in the body region so that the electronic conductivity of the LTO material is enhanced. At the same time, lithium chromium (Li2CrO4) with a size of about 10 nm is coated on the LTO surface, which further improves the conductivity of the electrode, as shown by comparing the as21414


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(5) Wang, S.; Xia, L.; Yu, L.; Zhang, L.; Wang, H.; Lou, X. W. FreeStanding Nitrogen-Doped Carbon Nanofiber Films: Integrated Electrodes for Sodium-Ion Batteries with Ultralong Cycle Life and Superior Rate Capability. Adv. Energy Mater. 2016, 6, 1502217. (6) Zhao, B. Z.; Deng, X.; Ran, R.; Liu, M. L.; Shao, Z. P. Facile Synthesis of a 3D Nanoarchitectured Li4Ti5O12 Electrode for Ultrafast Energy Storage. Adv. Energy Mater. 2016, 6, 1500924. (7) Bae, S.; Nam, I.; Park, S.; Yoo, Y. G.; Yu, S.; Lee, J. M.; Han, J. W.; Yi, J. Interfacial Adsorption and Redox Coupling of Li4Ti5O12 with Nanographene for High-Rate Lithium Storage. ACS Appl. Mater. Interfaces 2015, 7, 16565−16572. (8) Balogun, M. S.; Zhu, Y.; Qiu, W.; Luo, Y.; Huang, Y.; Liang, C.; Lu, X.; Tong, Y. Chemically Lithiated TiO2 Heterostructured Nanosheet Anode with Excellent Rate Capability and Long Cycle Life for High-Performance Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2015, 7, 25991−26003. (9) Jia, X.; Kan, Y.; Zhu, X.; Ning, G.; Lu, Y.; Wei, F. Building Flexible Li4Ti5O12/CNT Lithium-Ion Battery Anodes with Superior Rate Performance and Ultralong Cycling Stability. Nano Energy 2014, 10, 344−352. (10) Lin, C.; Fan, X.; Xin, Y.; Cheng, F.; Lai, M. O.; Zhou, H.; Lu, L. Li4Ti5O12-Based Anode Materials with Low Working Potentials, High Rate Capabilities and High Cyclability for High-Power Lithium-Ion Batteries: a Synergistic Effect of Doping, Incorporating a Conductive Phase and Reducing the Particle Size. J. Mater. Chem. A 2014, 2, 9982−9993. (11) Zhan, L.; Wang, S.; Ding, L. X.; Li, Z.; Wang, H. Grass-Like Co3O4 Nanowire Arrays Anode with High Rate Capability and Excellent Cycling Stability for Lithium-Ion Batteries. Electrochim. Acta 2014, 135, 35−41. (12) Zhang, L.; Xiang, H.; Li, Z.; Wang, H. Porous Li3V2(PO4)3/C Cathode with Extremely High-Rate Capacity Prepared by a Sol-GelCombustion Method for Fast Charging and Discharging. J. Power Sources 2012, 203, 121−125. (13) Fang, J.; Wang, S.; Li, Z.; Chen, H.; Xia, L.; Ding, L.; Wang, H. Porous Na3V2(PO4)3@C Nanoparticles Enwrapped in Three-Dimensional Graphene for High Performance Sodium-Ion Batteries. J. Mater. Chem. A 2016, 4, 1180−1185. (14) Zhang, Q.; Uchaker, E.; Candelaria, S. L.; Cao, G. Nanomaterials for Energy Conversion and Storage. Chem. Soc. Rev. 2013, 42, 3127−3171. (15) Jo, M. R.; Jung, Y. S.; Kang, Y. M. Tailored Li4Ti5O12 Nanofibers with Outstanding Kinetics for Lithium Rechargeable Batteries. Nanoscale 2012, 4, 6870−6875. (16) Hasegawa, G.; Kanamori, K.; Kiyomura, T.; Kurata, H.; Nakanishi, K.; Abe, T. Hierarchically Porous Li4Ti5O12 Anode Materials for Li- and Na-Ion Batteries: Effects of Nanoarchitectural Design and Temperature Dependence of the Rate Capability. Adv. Energy Mater. 2015, 5, 1400730. (17) Liu, J.; Song, K.; van Aken, P. A.; Maier, J.; Yu, Y. SelfSupported Li4Ti5O12-C Nanotube Arrays as High-Rate and Long-Life Anode Materials for Flexible Li-Ion Batteries. Nano Lett. 2014, 14, 2597−2603. (18) Yi, T. F.; Yang, S. Y.; Xie, Y. Recent Advances of Li4Ti5O12 as a Promising Next Generation Anode Material for High Power LithiumIon Batteries. J. Mater. Chem. A 2015, 3, 5750−5777. (19) Long, D. H.; Jeong, M. G.; Lee, Y. S.; Choi, W.; Lee, J. K.; Oh, I. H.; Jung, H. G. Coating Lithium Titanate with Nitrogen-Doped Carbon by Simple Refluxing for High-Power Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2015, 7, 10250−10257. (20) Ge, H.; Hao, T.; Osgood, H.; Zhang, B.; Chen, L.; Cui, L.; Song, X. M.; Ogoke, O.; Wu, G. Advanced Mesoporous Spinel Li4Ti5O12/ rGO Composites with Increased Surface Lithium Storage Capability for High-Power Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2016, 8, 9162−9169. (21) Ni, H.; Fan, L. Z. Nano-Li4Ti5O12 Anchored on Carbon Nanotubes by Liquid Phase Deposition as Anode Material for High Rate Lithium-Ion Batteries. J. Power Sources 2012, 214, 195−199.

prepared and postwashed Cr-doped LTO samples. Finally, the protection of the Li2CrO4 surface layer and the suppression from Cr atoms on the long-range order in the LTO lattice cooperatively suppress the growth of LTO particles, and thus Cr-modified LTO with small sizes (50 nm for Cr0.1-LTO) is obtained. The collaboratively modified LTO has excellent electrochemical performance. Among those samples, Cr0.1-LTO exhibited the best rate and cycling performance due to the enhanced electronic conductivity from a small amount of Cr3+ doped in the bulk phase, the appropriate Li2CrO4 on the surface, and improved Li diffusion from the reduced particle size. A reversible capacity of 141 mA h g−1 was still reached at 10 C, and over 155 mAh g−1 capacity remained after 1000 cycles at 1 C. We firmly believe that chromium modification is a simple and economic strategy for enhancing the battery performance of LTO anode materials. As a promising extension, this synergistic strategy can be further applied to the electrochemical improvements of other LIB electrode materials, such as LiMn2O4.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b07742. Photos of Li4−xCr3xTi5−2xO12 (0 ≤ x ≤ 0.5) powders asprepared and after being washed and the supernatant fluid of Li4−xCr3xTi5−2xO12 (0 ≤ x ≤ 0.5) samples washed solution; TG and DSC of the precursor samples; Cr 2p XPS spectra of the as-prepared x = 0.1 sample. The Raman spectrum of Li2CrO4. Voltage profiles of the different methods prepared of LiCrTiO4materials. The Coulombic efficiency of rate capability and cycling stability. The CV curves of the as-prepared materials with x = 0.3 and 0.5 at various sweep rates (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Science Foundation of China (grants 51372060 and 51402289). X. Feng acknowledges China Postdoctoral Science Foundation (grant 2015M580531).

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REFERENCES

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Chromium-Modified Li4Ti5O12 with a Synergistic Effect of Bulk Doping

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