A Strategy for Synthesis of Nanosheets Consisting ... - ACS Publications

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A Strategy for Synthesis of Nanosheets Consisting of Alternating Spinel Li4Ti5O12 and Rutile TiO2 Lamellas for High-Rate Anodes of Lithium-Ion Batteries Libo Wu,† Xuning Leng,† Yan Liu,‡ Sufeng Wei,§ Chunlin Li,† Guoyong Wang,*,† Jianshe Lian,† Qing Jiang,† Anmin Nie,∥ and Tong-Yi Zhang∥ †

Key Laboratory of Automobile Materials, Department of Materials Science and Engineering, Jilin University, Changchun 130025, P.R. China ‡ Key Laboratory of Bionic Engineering (Ministry of Education) and State Key Laboratory of Automotive Simulation and Control, Jilin University, Changchun 130022, P.R. China § Key Laboratory of Advanced Structural Materials, Changchun University of Technology, Changchun 130012, P.R. China ∥ Shanghai University Materials Genome Institute and Shanghai Materials Genome Institute, Shanghai University, Shanghai 200444, P.R. China S Supporting Information *

ABSTRACT: Ultrathin dual phase nanosheets consisting of alternating spinel Li4Ti5O12 (LTO) and rutile TiO2 (RT) lamellas are synthesized through a facile and scalable hydrothermal method, and the formation mechanism is explored. The thickness of constituent lamellas can be controlled exactly by adjusting the mole ratio of Li:Ti in the original reactants. Alternating insertion of the RT lamellas significantly improves the electrochemical performance of LTO nanosheets, especially at high charge/discharge rates. As anodes in lithium-ion batteries (LIBs), the dual phase nanosheet electrode with the optimized phase ratio can deliver stable discharge capacities of 178.5, 154.9, 148.4, 142.3, 138.2, and 131.4 mA h g−1 at current densities of 1, 10, 20, 30, 40, and 50 C, respectively. Meanwhile, they inherit the excellent cyclic stability of pure spinel LTO and exhibit a capacity retention of 93.1% even after 500 cycles at 50 C. Our results indicate that the alternating nanoscaled lamella structure is a good alternative to facilitate the transfer of both the Li ions and electrons into the spinel LTO, giving rise to an excellent cyclability and fast rate performance. Therefore, the newly prepared carbon-free LTO-RT nanosheets with high safety provide a new opportunity to develop high-power anodes for LIBs. KEYWORDS: lithium titanate, rutile titanium oxide, dual phase, hydrothermal method, lithium-ion batteries

1. INTRODUCTION With growing concerns over fossil fuel consumption and global warming, considerable efforts have been devoted to the development of pure electric vehicles (EVs)/hybrid electric vehicles (HEVs) with low emissions.1,2 As energy-storage devices, lithium-ion batteries (LIBs), which generally have high energy density and long cycling performance, are particularly adapted for applications in HEVs/EVs where a long cruising range is needed after each recharge in a short time.3,4 However, conventional graphite anodes cannot meet the demands of HEVs/EVs because they have low Li-intercalation potential voltage (almost 0.1 V vs Li+/Li) and a low Li-ion diffusion rate. Thus, lithium metal dendrite can easily grow in the conventional graphite anodes, especially during the high rate discharging process, which usually causes severe safety issues.5,6 Spinel-type lithium titanate (Li4Ti5O12, LTO) has been considered as a candidate with great promise compared with conventional graphite anodes.7 It possesses a long flat charge/discharge plateau at a potential voltage of 1.55 V vs Li+/Li, which guarantees a large capacity in © XXXX American Chemical Society

the voltage window of 1−2.5 V, well above the potential to form a solid-electrolyte interphase (SEI) layer (usually occurring below 1.0 V vs Li+/Li) and dendritic lithium, thus showing high safety.8,9 In addition to being environmentally friendly and relatively inexpensive, LTO also exhibits excellent Li-ion insertion/extraction reversibility with nearly zero strain change during the Li-ion insertion/extraction process and possesses outstanding thermal stability, ensuring a long cycling lifetime.10,11 Further, LTO has a low Li-ion diffusion coefficient (ca. 10−9 to 10−13 cm2·s−1) and low electrical conductivity (ca. 10−13 S·cm−1), both of which are vital kinetic barriers greatly limiting the rate performance.12 The rate of Li-ion diffusion and electron transfer should be simultaneously improved because the slower of the two determines the entire rate performance of LTO. Recently, numerous researches have been conducted on coating,13,14 Received: November 22, 2016 Accepted: January 12, 2017

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DOI: 10.1021/acsami.6b15021 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Nucleation and growth procedures of dual phase LTO-RT nanosheets and the crystal structure of the Li4Ti5O12 unit cell. In the unit cell, blue tetrahedra represent lithium, and green octahedra represent disordered lithium and titanium.

doping,15−17 morphological optimization, and nanostructuring18,19 to enhance the rate properties of LTO. Nanostructuring accompanied by addition of an electronically conductive coating appears to be an effective method because nanostructuring can shorten Li-ion diffusion distance, while coating can enhance the electronic conductivity. To avoid safety problems with carbonaceous materials, the coating technique is preferred to be carbonfree for achieving high tap density, outstanding surface stability, and excellent rate properties. Recently, epitaxial growth nanolayers of well-crystallized rutile-TiO2 (RT) were coated on the edges of LTO nanosheets.20 Benefiting from the unique structure, the LTO nanosheets showed superior rate capability as anode electrodes for LIBs. In this paper, dual phase nanosheets consisting of alternating spinel LTO and RT lamella sequences were synthesized through a facile and low-cost hydrothermal method. The lamella thickness was exactly controlled by tuning the mole ratio of Li:Ti in the original reactants. Alternating insertion of the RT lamellas endows the LTO nanosheet with an excellent rate capability. The discharge capacity of a dual phase nanosheet electrode with an optimized phase ratio is as high as 178.5 and 131.9 mA h g−1 at 1 and 50 C, respectively. After the nanosheet was cycled at 50 C for 500 cycles, the capacity retention still remained 93.1%, suggesting the decay rate per cycle is only 0.014%.

2. EXPERIMENTAL SECTION

(TG-DSC, SDT 2960) measurement of the precursor was performed from 30 to 750 °C in flowing air. The morphology and microstructure were characterized by field emission scanning electron microscopy (FESEM, ZEISS SUPRA 40, operating at 15 kV) and transmission electron microscopy (TEM, JEOL 2100F, operating at 200 kV). The Brunauer−Emmett−Teller (BET) surface area was characterized by N2 adsorption−desorption measurements using a Micromeritics ASAP 2020 analyzer. 2.3. Electrochemical Characterization. Electrochemical characterizations were tested in CR 2025-type coin cells with pure Li foils acting as counter electrodes. Typically, a mixture of obtained materials, super-P (SP), and polyvinylidene fluoride (PVDF) with a weight ratio of 8:1:1 was homogeneously dissolved in a certain amount of N-methyl-2-pyrrolidinone (NMP). The slurry was pasted on a Cu foil current collector followed by vacuum drying at 110 °C for 12 h. Then, the dried foil was punched into 12 mm diameter circular strips, and these strips were assembled in an argon-filled glovebox. The typical active material mass loading of each electrode was approximately 2 mg/cm2. The electrolyte is a mixture of 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) with a weight ratio of 1:1, and the separator is a type of polypropylene film (Celgard 2400). Galvanostatic charge/discharge performance was examined by a LAND CT2001A multichannel battery tester between 1 and 2.5 V (vs Li+/Li) at various rates of 1, 10, 20, 30, 40, and 50 C (1 C = 175 mA g−1). Cyclic voltammogram (CV) and electrochemical impedance spectroscopy (EIS) tests were examined by an IVIUMSTAT electrochemical workstation. The specific capacity of each electrode was calculated based on the total weight of active materials, including LTO and RT.

2.1. Synthesis of Dual Phase LTO-RT Nanosheets. Dual phase LTO-RT nanosheets were synthesized through a simple one-pot hydrothermal process. Initially, 1.7 mL (5 mmol) of tetrabutyl titanate (Ti(OC4H9)4 CP, Sinopharm Chemical Reagent Co., LTD) was added dropwise into 20 mL of anhydrous ethanol (C2H5OH AR, Sinopharm Chemical Reagent Co., LTD) with continuous stirring at room temperature. Meanwhile, 0.1678 g (4.0 mmol) of lithium hydroxide monohydrate (LiOH·H2O AR, Tianjin Guangfu Technology Development Co., LTD) was dissolved in 20 mL of deionized water (H2O 18.2 MΩ, Millipore Direct-Q System) under magnetic stirring for around 10 min control the Li/Ti molar ratio in the reactant more accurately, for it has a higher solubility in deionized water than anhydrous ethanol. Then, a white suspension appeared immediately once the LiOH·H2O aqueous solution was slowly dropped into the Ti(OC4H9)4 ethanol solution. After intense stirring for 30 min, the suspension was poured into a 50 mL Teflon-lined stainless steel autoclave and placed in an oven at 180 °C for 24 h. The precipitate was collected by centrifugation and washed with deionized water and ethanol and then dried in an oven at 80 °C for 8 h. The precursor was ground to powder, followed by calcination at 600 °C for 6 h in horizontal tube furnace in air. Other samples with different phase fractions were also synthesized with a similar process, except the Li:Ti mole ratio was different. For simplicity, the samples with the different Li:Ti mole ratio of 4.5:5, 4.2:5, 4.1:5, and 4.0:5 were named LTO-RT-0, LTO-RT-1, LTO-RT-2, and LTO-RT-3, respectively. 2.2. Structural Characterization. The phase and crystal structure data of these as-synthesized samples were collected by powder X-ray diffraction (XRD, Rigaku D/max 2500 pc, Cu Kα radiation: λ= 1.5406 Å). Thermal gravimetric and differential scanning calorimetry

3. RESULTS AND DISCUSSION The synthetic process of the dual phase nanosheets is schematically illustrated in Figure 1. The precursor can nucleate immediately as LiOH·H2O aqueous solution is dropped into Ti(OC4H9)4 ethanol solution. The nucleates go through ionexchange with solution and grow into nanosheets during hydrothermal treatment.21,22 After calcination, the precursor nanosheets transit to pure LTO or dual phase LTO-RT nanosheets depending on the Li:Ti mole ratio. All of the precursors harvested from hydrothermal treatment have the same crystal structure, as investigated by XRD (Figure S1a, Supporting Information), which can be identified as orthorhombic Li1.81H0.19Ti2O5·2H2O (JCPDS card no. 47-0123).23 TG-DSC tests (Figure S1b, Supporting Information) were carried out on the LTO-RT-0 precursor to deduce possible chemical reactions and phase transitions. Two endothermic peaks in the DSC curve corresponding to two weight-loss regions in the TG curve were observed. The first endothermic peak at 100 °C should relate to the volatilization of surface adsorbed substances such as water, which induces 5% weight loss in the first region of TG. The second endothermic peak starting at about 200 °C is also accompanied by a dramatic decline in the TG curve, which indicates the crystal water desorption of the compound.24 Following the second endothermic peak, an intensive exothermic peak appeared immediately. There is no independent change in B


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The details of the morphology and structure for LTO-RT-2 were well-investigated by TEM. Figure 4a shows a typical low magnification TEM image of the LTO-RT-2 samples, indicating a nonuniform contrast distribution in individual nanosheets. An enlarged TEM image of a single sheet is shown in Figure 4b. The sheet contains lots of parallel lamellas, as indicated by the alternating diffraction contrast. In addition, the lamella thickness of each sample was also measured, and the size distribution of LTO-RT-2 was plotted in the inset of Figure 4b. Such phenomenon was also clearly observed in the TEM images of LTO-RT-1 and LTO-RT-3 (Figures S3a and b Supporting Information). The mean thicknesses of the lamellas were 7.0, 7.5, and 13.5 nm for LTO-RT-1, LTO-RT-2, and LTO-RT-3, respectively, which agree well with the statistical results from the FESEM images. The diffraction contrast difference, in fact, originates from alternating phase distribution in the sheet, which was further confirmed by the HRTEM analysis (Figure 4c). The HRTEM image and corresponding selected area fast Fourier transform patterns in Figure 4c confirm a RT lamella lying on the top of a LTO lamella. Two white lines were used to denote the (110) facet of RT and (100) facet of LTO. They are almost parallel, which indicates that the (110) facet of RT prefer to grow on the (100) facet of LTO. The oxygen atoms located on the (110) facet of RT and (100) facet of LTO are both shown in Figures S4 a and b (Supporting Information), respectively. After the two images were compared, it was found that the oxygen atoms on the two crystallographic facets were almost coherent. Such a growth manner would cause less lattice distortion on the interface, and the interfacial energy should be quite low. A solid phase transforming into two other solid phases is a general phenomenon in metallurgy, which is named the eutectoid reaction. The microstructure of the sheet after calcination consists of alternating ultrathin lamellas of LTO and RT, which shows a feature of the eutectoid reaction. In fact, the crystal structure of rutile TiO2 is also very similar to spinel Li4Ti5O12 in terms of oxygen ions, as shown by the insets in Figure S4. Oxygen ions in both phases occupy the face-centered cubic sites, denoted as 32e sites in some references (see unit cell of LTO in Figure 1).28,29 All titanium ions occupy the octahedral sites (16d sites), taking a half of all octahedral sites in the face-centered cubic O2−. But, for LTO, only part of lithium ions occupy the 16d sites, which may cause the crystal structure to be disordered to some extent. The other part of lithium ions in LTO occupy only the tetrahedral sites (8a sites), taking one-eighth of all tetrahedral sites in the face-centered cubic O2−, where it is empty in RT.30 If lithium ions can partially occupy the 8a sites and 16d sites in RT, RT will transit to LTO. That is why Li4Ti5O12 can be made by the solid-state reaction between rutile TiO2 and Li2CO3/LiOH at a very high calcination temperature.31−33 The hydrothermal reaction and high-temperature calcination always cause the loss of lithium. Although lithium is enough or excess in all solutions to generate LTO, pure LTO only formed in the LTO-RT-0, in which there was a 12.5% lithium excess. In the parent phase where lithium is less, the generation of LTO and RT phases requires the redistribution of the lithium by diffusion. Figure 1 schematically illustrates microstructural changes accompanying the reaction. The directions of lithium diffusion are indicated by the curved arrows. Lithium atoms have to totally spread out from the RT regions to the LTO lamellas as the newly formed phase lamellas extend from one side of the sheet to the unreacted parent phase region. The alternating lamella structure forms

TG curve corresponding to this peak. It is possible that some phase transition took place as the crystal water was desorbed. The heat flux returned to zero at 450 °C as indicated by the DSC curve, while the TG curve remained constant. The chemical reaction and phase transition completed, as the temperature was above 450 °C. It has been reported that the Li4Ti5O12 nanosheets synthesized below 500 °C are unstable, and those synthesized above 700 °C are too thick for the transportation of Li ions.20 Thus, the calcination temperature was set at 600 °C in this experiment. The crystal structures of all samples after calcination were characterized by XRD, as shown in Figure 2. Only LTO-RT-0

Figure 2. XRD patterns of the LTO-RT-0, LTO-RT-1, LTO-RT-2, and LTO-RT-3 nanosheets and the standard PDF cards of spinel type Li4Ti5O12 and rutile TiO2.

has a pure phase structure, which was identified as LTO (JCPDS card no. 49-0207).25 Besides LTO, RT phase (JCPDS card no. 21-1276) was also detected in the other three samples.26 The intensity of diffraction peaks of one crystalline phase in a multiphase mixture depends on the weight fraction of the corresponding phase in the mixture.27 Thus, the weight fraction of RT can be calculated by measuring the intensities of (110) peak of RT and (111) peak of LTO. Accordingly, the weight fraction of the RT phase was measured to be around 9.6, 14.4, and 26.5% for LTO-RT-1, LTO-RT-2, and LTO-RT-3, respectively. Obviously, the weight fraction of RT rises as the molar ratio of Li:Ti declines. Lattice parameters refined according to the Rietveld method are listed in Table S1 (Supporting Information). It can be seen that the addition of RT has a negligible impact on the intrinsic crystal structure of LTO, which might be beneficial for maintaining the outstanding electrochemical stability. Increasing the weight fraction of RT phase also caused a change in the sample morphology. As shown in Figure S2 (Supporting Information), all of the precursors have a similar morphology, which is comprised of smooth rectangle sheets with dimensions of about 300 × 500 nm and a thickness of around 30 nm. However, after calcination, only LTO-RT-0 remained smooth, as shown in Figures 3a and b. The other samples fragmented into a series of sheets to some extent, as shown in Figures 3c−h. In fact, the sheets of LTO-RT-1, LTO-RT-2, and LTO-RT-3 transited to a sequence of nanorods, the length of which was comparable to the width of the sheet, but the rod width was different. The rod width of each sample was carefully measured, and the size distribution was plotted in each inset of the FESEM image. According to the statistics, the mean rod widths were 6.7, 8.5, and 12.1 nm for LTO-RT-1, LTO-RT-2, and LTO-RT-3, respectively. C


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Figure 3. FESEM images of (a and b) LTO-RT-0, (c and d) LTO-RT-1, (e and f) LTO-RT-2, and (g and h) LTO-RT-3 nanosheets; the insets in panels c, e, and g are the lamella thickness distributions of each sample summarized from FESEM images.

phase sheets probably benefits from the fragmentation of the sheets. The lithium storage performances of the LTO-based nanosheets were investigated by assembling them into LTO/ Li half cells. Figure 5a displays representative CV profile of the LTO-RT-2 electrode at a scan rate of 0.1 mV s−1 in the voltage range of 1−2.5 V versus Li+/Li. It is quite similar to that of the LTO-RT-0 electrode, as shown in Figure S6a

because lithium atoms need to diffuse only minimal distances with this configuration. The specific surface areas of LTO-RT-0 and LTO-RT-2 were examined with a BET measurement, and the corresponding N2 adsorption−desorption isotherms are presented in Figures S5a and b (Supporting Information). The BET specific surface area of LTO-RT-2 (32.71 m2 g−1) is higher than that of LTO-RT-0 (22.74 m2 g−1). The high specific surface area of the dual D


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Figure 4. (a) TEM image of LTO-RT-2 nanosheets; (b) TEM image of one slice of LTO-RT-2 sheet, where the inset is the lamella thickness distribution measured from the TEM image; (c) HRTEM image and corresponding fast Fourier transform image of the RT lamella and the LTO lamella, showing clearly that the LTO and RT phases are with the [0−11] and [−110] zone axes, respectively.

Figure 5. (a) CV curves of LTO-RT-2 nanosheets at 0.1 mV S−1 in 1−2.5 V and (b) the galvanostatic charge−discharge curves of the LTO-RT-2 electrode at 1 C in 1−2.5 V.

to Li-ion deintercalation from LTO,34,35 can be clearly identified from the profile CV curves. Remarkably, except for the first cycle, the subsequent cycles shown in the figure overlap

(Supporting Information). Two obvious redox peaks, a cathodic peak at around 1.46 V assigned to Li-ion intercalation into LTO and an anodic peak at approximately 1.66 V assigned E


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Figure 6. Cycle performance of LTO-RT-0, LTO-RT-1, LTO-RT-2, and LTO-RT-3 nanosheets at (a) 1 C and (b) 10 C for 100 cycles. (c) Cycle performance of LTO-RT-2 at 50 C for 500 cycles.

Figure 7. (a) Rate performance of four electrodes from 1 to 50 C and (b) comparison of the capacities at different rates for the LTO-RT-2 electrode with other LTO-based high-rate electrodes.

current density, all the dual phase samples show cycle stability better than that of the pure LTO sample, and the LTO-RT-2 electrode also exhibits the best cycle stability. Figure 6c shows the cycling performance of 500 cycles for the LTO-RT-2 electrode at 50 C. The capacity was 131.9 mA h g−1 initially and remained at 122.8 mA h g−1 after 500 cycles, suggesting the decay rate per cycle is only 0.014% in the prolonged charging/ discharging process. The corresponding Coulombic efficiency is close to 100%, which is of significant importance to the implementation and commercialization. The rate performances of all the four electrodes at different current rates of 1, 10, 20, 30, 40, and 50 C are displayed in Figure 7a, and the corresponding charge/discharge curves can be referred to in Figure S8 (Supporting Information). At 1 C, all dual phase electrodes exhibit a specific capacity loss during the initial cycle, and then a reversible capacity is obtained. It is interesting that the reversible capacities of all the electrodes are comparable at such a low current density. The phenomenon is also observed in Figure 6a. All dual phase electrodes show a better rate ability than the LTO-RT-0 electrode. Remarkably the LTO-RT-2 electrode shows the best rate performance among all the four samples, delivering stable discharge capacities

very well, suggesting an excellent reversibility of dual phase LTO-RT nanosheets. Figure 5b exhibits the galvanostatic charge−discharge curves (first, second, and fifth) of the LTORT-2 electrode at a current density of 1 C in the potential window of 1−2.5 V. The LTO-RT-2 electrode suffers a capacity loss in the following cycles compared to the initial capacity, which is consistent with the CV curves. With the exception of this, they are similar to that of the LTO-RT-0 electrode (Figure S6b, Supporting Information). All of the charge− discharge curves in both samples show a voltage plateau around 1.55 V, corresponding to the lithiation/delithiation reaction for LTO. Notably, the charge−discharge voltage platform of the LTO-RT-2 electrode is narrower than that of the LTO-RT-0 electrode, revealing less polarization in the LTO-RT-2 electrode. The cyclic performances of all the four electrodes are compared at the current density of 1, 10 (Figure 6a and b), 30, and 50 C (Figure S7a and b, Supporting Information). The LTO-RT-2 electrode delivers the highest discharge capacities at each current density. Compared to the initial capacity, the specific capacity decay of LTO-RT-1/2/3/0 after 100 cycles is 2.7/2.2/5.3/8.7% at 1 C, 5.1/2.4/2.3/12% at 10 C, 4.5/2.8/ 2.9/6.4% at 30 C, and 3.7/3.4/3/3.8% at 50 C. Clearly, at each F


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Figure 8. (a) Plots of ΔE of LTO-RT-0 and LTO-RT-2 versus C rates and (b) Nyquist plots over the frequency range from 100 kHz to 0.01 Hz and the used equivalent circuit, where the square dots are experimental data and the solid curves denote the fitting curves; Rs, Rct, CPE, and Zw represent the ohmic resistance, charge-transfer impedance, double layer capacitance, and the Warburg impedance, respectively.

of 178.5, 154.9, 148.4, 142.3, 138.2, and 131.4 mA h g−1 at current densities of 1, 10, 20, 30, 40, and 50 C, respectively. Even when the current density was enlarged by 50 times, 75.3% of the capacity was retained. Importantly, as the current density returns to 1 C after 60 cycles, the capacity recovers to 178 mA h g−1 again, being also comparable to the one obtained after 100 discharge/charge cycles at 1 C, as shown in Figure 6a. Accordingly, the LTO-RT-2 electrode is stable enough for such high current density loading. As a contrast, the pure LTO electrode displays a relatively lower reversible capacity at each current density compared with the LTO-RT-2 electrodes, and the capacity gaps between them become larger as the current density increases. All of the results indicate the introduction of RT phase is significantly beneficial for improving the kinetics of LTO nanosheets toward fast lithium insertion/extraction. The rate capacities, especially at high rates of the LTO-RT-2 electrode, are superior to those of the electrodes recently reported for nanostructured LTO,36−39 Cr-doped LTO,40 carboncoated LTO,41 LTO/carbon textiles or graphene,42,43 dual phase LTO-TiO2 nanowires,44 RT nanocoating LTO,20 and carbon coated dual phase LTO-TiO245 (Figure 7b). The polarization of ΔE versus rate plots of the LTO-RT-0 and LTO-RT-2 electrodes are shown in Figure 8a. The values of ΔE are defined as the differences between the potentials of charge and discharge plateaus. The nearly linear relationship exhibited in both plots with relation to the current rates shows an ohmic performance of the electrode.46 The ohmic resistance (R) of the LTO-RT-2 electrode evaluated from the slope of the linear fitting line is 82 Ω, which is smaller than that (94 Ω) of the LTO-RT-0 electrode. EIS measurements were performed at frequencies from 100 kHz to 0.01 Hz on the four LTO-based electrodes at open-circuit state before cycling. Figure 8b shows the Nyquist plots over the frequency range from 100 kHz to 0.01 Hz and the used equivalent circuit, where the square dots are experimental data and the solid curves denote the fitting curves; Rs, Rct, CPE, and Zw represent the ohmic resistance, charge-transfer impedance, double layer capacitance, and the Warburg impedance, respectively. The interception on the Z′-axis at the high-frequency end denotes the electrolyte resistance (Rs), which represents the ohmic resistance of the electrolyte. The size of the depressed semicircle that encompasses the mediumfrequency region is a symbol of charge-transfer resistance (Rct). The inclined line in the low-frequency range represents the Warburg impedance (Zw), which reflects the diffusion of Li ions into the electrodes.47,48 The fitted parameters listed in Table S2

(Supporting Information) indicate that the Rct values are 160.9, 97.09, 75.86, and 100.4 Ω, respectively for LTO-RT-0, LTORT-1, LTO-RT-2, and LTO-RT-3 samples. The Rct values of the dual phase samples are much smaller than that of the pure-LTO sample, indicating that the introduction of TiO2 is favorable to improve the electronic conductivity of the nanosheets, enabling much easier charge transfer at the electrode/electrolyte interface.49,50 Meanwhile, LTO-RT-2 has the smallest Rct among all the samples, implying the highest fast charge transfer rate among the four types of samples. The lithium-ion diffusion coefficient (DLi) values obtained from Figure S9 (Supporting Information) are presented in Table S2 (Supporting Information). It clearly shows that all of the dual phase electrodes have lithium-ion diffusion coefficients higher than that of the pure phase electrode, which implies superior electrochemical properties for LIBs. The plots in Figure 8a and the EIS results in Figure 8b both indicate that the lamella structure can improve the kinetics of the nanosheets so effectively that all the dual phase electrodes have higher rate capability, as shown in Figure 7a. The kinetics of the LIB electrode is related to Li-ion transfer and conductivity. The ultrathin sheet structure is very beneficial for Li-ion transfer because Li-ions have a short diffusion distance as they penetrate normally into the sheet. Moreover, the ultrahigh interfacial area between the sheet and electrolyte can also provide even more reaction sites, which is also necessary for improving the kinetics of the electrode. Therefore, for the pure LTO electrode, the ultrathin sheet structure could have a rate capability higher than that of the bulk one.10 In this work, the LTO nanosheets were further divided into even smaller lamellas by the nanoscale RT lamellas, signifying a shorter diffusion distance. Bulk RT has a lithium diffusion coefficient and electrical conductivity higher than those of LTO.20,51 The alternating RT lamellas can also act as expressways for lithium-ion and charge transfer, endowing the dual phase nanosheets much better rate performances. Theoretical calculation and experimental research both observed higher conductivity in nanostructured RT with smaller characteristic size.25,51,52 Our results also show that RT is beneficial for the electrochemical performance of the dual phase nanosheets electrode. It is crucial to obtain an optical balance between the lamellas’ size and the RT phase weight fraction in the nanosheets to achieve a good electrochemical performance. Thus, the LTO-RT-2 electrode exhibits the best rate capability. Contrary to an ambipolar (or concerted) Li+/electron polaron-hopping conductivity of bulk titania,53−55 the charge redistributes in the nanostructured RT under external potential. Li+ and electrons G


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(2) Jung, H.-G.; Myung, S.-T.; Yoon, C. S.; Son, S.-B.; Oh, K. H.; Amine, K.; Scrosati, B.; Sun, Y.-K. Microscale Spherical Carbon-coated Li4Ti5O12 as Ultra High Power Anode Material for Lithium Batteries. Energy Environ. Sci. 2011, 4 (4), 1345−1351. (3) Zhang, Q.; Uchaker, E.; Candelaria, S. L.; Cao, G. Nanomaterials for Energy Conversion and Storage. Chem. Soc. Rev. 2013, 42 (7), 3127−3171. (4) Bruce, P. G.; Scrosati, B.; Tarascon, J. M. Nanomaterials for Rechargeable Lithium Batteries. Angew. Chem., Int. Ed. 2008, 47 (16), 2930−2946. (5) 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 (14), 9162−9169. (6) Pawlitzek, F.; Pampel, J.; Schmuck, M.; Althues, H.; Schumm, B.; Kaskel, S. High-power Lithium ion Batteries Based on Preorganized Necklace Type Li4Ti5O12/VACNT Nano-composites. J. Power Sources 2016, 325, 1−6. (7) Maloney, R. P.; Kim, H. J.; Sakamoto, J. S. Lithium Titanate Aerogel for Advanced Lithium-ion batteries. ACS Appl. Mater. Interfaces 2012, 4 (5), 2318−2321. (8) Wang, J.; Zhou, Y.; Hu, Y.; O’Hayre, R.; Shao, Z. Facile Synthesis of Nanocrystalline TiO2 Mesoporous Microspheres for Lithium-Ion Batteries. J. Phys. Chem. C 2011, 115 (5), 2529−2536. (9) Sha, Y.; Zhao, B.; Ran, R.; Cai, R.; Shao, Z. Synthesis of Wellcrystallized Li4Ti5O12 Nanoplates for Lithium-ion Batteries with Outstanding Rate Capability and Cycling Stability. J. Mater. Chem. A 2013, 1 (42), 13233−13243. (10) 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 (11), 5750−5777. (11) Hao, X.; Bartlett, B. M. Li4Ti5O12 Nanocrystals Synthesized by Carbon Templating from Solution Precursors Yield High Performance Thin Film Li-Ion Battery Electrodes. Adv. Energy Mater. 2013, 3 (6), 753−761. (12) Zhu, G.-N.; Wang, Y.-G.; Xia, Y.-Y. Ti-based Compounds as Anode Materials for Li-ion Batteries. Energy Environ. Sci. 2012, 5 (5), 6652−6667. (13) Cheng, L.; Yan, J.; Zhu, G.-N.; Luo, J.-Y.; Wang, C.-X.; Xia, Y.-Y. General Synthesis of Carbon-coated Nanostructure Li4Ti5O12 as a High Rate Electrode Material for Li-ion Intercalation. J. Mater. Chem. 2010, 20 (3), 595−602. (14) Jung, H.-G.; Kim, J.; Scrosati, B.; Sun, Y.-K. Micron-sized, Carbon-coated Li4Ti5O12 as High Power Anode Material for Advanced Lithium Batteries. J. Power Sources 2011, 196 (18), 7763−7766. (15) Cai, R.; Jiang, S.; Yu, X.; Zhao, B.; Wang, H.; Shao, Z. A Novel Method to Enhance Rate Performance of an Al-doped Li4Ti5O12 Electrode by Post-synthesis Treatment in Liquid Formaldehyde at Room Temperature. J. Mater. Chem. 2012, 22 (16), 8013−8021. (16) Bai, Y.-J.; Gong, C.; Qi, Y.-X.; Lun, N.; Feng, J. Excellent Longterm Cycling Stability of La-doped Li4Ti5O12 Anode Material at High Current Rates. J. Mater. Chem. 2012, 22 (36), 19054−19060. (17) Lin, C.; Ding, B.; Xin, Y.; Cheng, F.; Lai, M. O.; Lu, L.; Zhou, H. Advanced Electrochemical Performance of Li4Ti5O12-based Materials for Lithium-ion Battery: Synergistic Effect of Doping and Compositing. J. Power Sources 2014, 248, 1034−1041. (18) Lee, S. C.; Lee, S. M.; Lee, J. W.; Lee, J. B.; Lee, S. M.; Han, S. S.; Lee, H. C.; Kim, H. J. Spinel Li4Ti5O12 Nanotubes for Energy Storage Materials. J. Phys. Chem. C 2009, 113 (42), 18420−18423. (19) Tang, Y. F.; Yang, L.; Qiu, Z.; Huang, J. S. Preparation and Electrochemical Lithium Storage of Flower-like Spinel Li4Ti5O12 Consisting of Nanosheets. Electrochem. Commun. 2008, 10 (10), 1513−1516. (20) Wang, Y. Q.; Gu, L.; Guo, Y. G.; Li, H.; He, X. Q.; Tsukimoto, S.; Ikuhara, Y.; Wan, L. J. Rutile-TiO2 Nanocoating for a High-rate Li4Ti5O12 Anode of a Lithium-ion Battery. J. Am. Chem. Soc. 2012, 134 (18), 7874−7879.

tend to accumulate at the boundary, which can be even bigger to compensate external potential in larger nanostructured RT than the Debye length.35,51 The charge accumulation may explain the large initial capacity of the dual phase electrodes.

4. CONCLUSIONS In summary, we successfully synthesized ultrathin dual phase nanosheets consisting of alternating spinel Li4Ti5O12 and rutile TiO2 lamellas through a facile and cheap hydrothermal method. Alternating insertion of the RT lamellas into the LTO lamellas significantly improves the electrochemical performance of the LTO nanosheets, especially at a high discharge/charge rate. When the dual phase nanosheets with optimized phase ratios are used as the anode of a half-cell LIB, 75.3% of the capacity can be retained as the current density is enlarged 50× from 1 to 50 C. Our results indicate that the excellent rate performance is attributed to the alternating insertion of RT lamellas, which enhances Li-ion diffusion and electron transfer. The facial synthesis and outstanding rate performance of the alternating LTO-RT nanosheets could be an alternative anode material for next-generation high-rate LIBs.

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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.6b15021. XRD patterns of four precursors, TG-DSC curves of LTO-RT-0 precursor, lattice parameters refined from the XRD patterns, FESEM images of precursors, crystal structures of RT and LTO, BET data, CV curves of LTO-RT-0, cycle performance at 30 and 50 C, galvanostatic charge/discharge curves, and fitted parameters from EIS (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel.: +86 431 85095875; Fax: +86 431 85095876. ORCID

Anmin Nie: 0000-0002-0180-1366 Tong-Yi Zhang: 0000-0002-9646-9668 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 51401083, 51371089, and 51631004), Science and Technology Development Project of Jilin Provence (No. 20150519007JH), and the Program for Innovative Research Team (in Science and Technology) at the University of Jilin Province. A.M.N. and T.Y.Z. are grateful for financial support from the research grants (14DZ2261200 and 15DZ2260300) from the Science and Technology Commission of Shanghai Municipality. A.M.N. acknowledges support by the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning.

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REFERENCES

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