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Functional Nanostructured Materials (including low-D carbon)

The Critical Role of the Crystallite Size in Nanostructured Li4Ti5O12 Anodes for Lithium Ion Batteries Junpei Yue, Felix M. Badaczewski, Pascal Voepel, Thomas Leichtweiß, Doreen Mollenhauer, Wolfgang G. Zeier, and Bernd M. Smarsly ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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ACS Applied Materials & Interfaces

The Critical Role of the Crystallite Size in Nanostructured Li4Ti5O12 Anodes for Lithium Ion Batteries Junpei Yue†, Felix M. Badaczewski†, Pascal Voepel†, Thomas Leichtweiß§, Doreen Mollenhauer†,§, Wolfgang G. Zeier†,§ and Bernd M. Smarsly†,§,* † §

Institute of Physical Chemistry, Justus-Liebig-University Giessen, Heinrich-Buff-Ring 17, 35392 Giessen, Germany Center for Materials Research (LaMa), Heinrich-Buff-Ring 17, 35392 Giessen, Germany

KEYWORDS: Li4Ti5O12; Nanoparticles; Mesoporous Fiber; Mesoporous Film; Size Effects; Interfacial Charge Storage; Pair Distribution Function

ABSTRACT: Lithium titanate Li4Ti5O12 (LTO) is regarded as a promising alternative to carbon-based anodes in lithium-ion batteries. Despite its stable structural framework, LTO exhibits disadvantages such as the sluggish lithium-ion diffusion and poor electronic conductivity. In order to modify the performance of LTO as an anode material, nanosizing constitutes a promising approach, and the impact is studied here by systematical experimental approach. Phase-pure polycrystalline LTO nanoparticles (NPs) with high crystallinity and crystallite sizes ranging from 4 to 12 nm are prepared by an optimized solvothermal protocol and characterized by several state-of-the-art technologies including HRTEM, XRD, PDF (pair distribution function) analysis, Raman spectroscopy and XPS. Through a wide array of electrochemical analyses, including charge/discharge profiles, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), a crystallite size of approx. 7 nm is identified as optimum particle size. Such NPs exhibit as good reversible capacity as the ones with larger crystallite sizes, but a more pronounced interfacial charge storage. By decreasing the crystallite size to about 4 nm the interfacial charge storage increases remarkably, however resulting in a loss of reversible capacity. An in-depth structural characterization using the PDF obtained from synchrotron XRD data indicates an enrichment in Ti for NPs with the small crystallite sizes, and this Ti-rich structure enables a higher Li storage. The electrochemical characterization confirms this result and furthermore points to a plausible reason why a higher Li-storage in very small nanoparticles (4 nm) results in a loss in the reversible capacity.

Introduction Spinel Li4Ti5O12 (LTO) is regarded as a promising alternative to carbon-based anodes for lithium-ion batteries (LIBs) applied in hybrid electric vehicles (HEVs) and sustainable energy storage devices.1-3 The high and flat lithium-ion intercalation potential (1.55 V vs. Li+/Li) avoids the decomposition of carbonate-based electrolytes and the formation of solid electrolyte interphases (SEI).4-6 Li4Ti5O12 crystallizes in a spinel structure (Fd3തm), in which Li+ entirely occupies tetrahedral 8a sites, while the the octahedral 16d sites are occupied by one-sixth Li+ and five-sixth Ti4+. Thereby, the Li+ ions at 16d sites are distributed as homogenously as possible to minimize strain in the material.7 The oxygen ions on 32 (e) sites form a cubic close-packed structure. Intriguingly, during lithiation, the Li4Ti5O12 spinel structure undergoes a phase change to a rock salt structure, in which lithium ions at 8a sites move towards octahedrally coordinated 16c sites that are unoccupied in case of spinel Li4Ti5O12. Overall, three Li+ ions can be accommodated: (Li3)8a[Li1Ti5]16dO1232e + 3 Li+ + 3 e− [Li]616c[Li1Ti5]16dO1232e.

This structural rearrangement is accompanied by a minor volume change of only ~0.2%.4,8 In case of graphite anodes, the formation of SEI layers and the strong volume expansion during lithium-uptake turn out to be safety risks and deteriorate the lifetime of LIBs.9-11 On this account, LTObased anodes offer advantageous stability and safety properties compared to graphite anodes. The gassing issue is regarded as a big challenge for LTO anode and attributed to the moisture residue in the cell according to the report in literature, therefore a totally dry electrode can exclude this effect.12,13 Furthermore, the sluggish lithium-ion diffusion in LTO and the inherently poor electronic conductivity limit the rate performance as anode material. Numerous strategies have been employed to overcome these drawbacks in the past decade, including doping with other elements,14-16 combining with conductive compounds17-21 or designing nanostructured materials22-26. In this regard, Passerini et al. reported LTO nanoparticles (NPs) possessing a size of 20 to 30 nm which were prepared by flame spray pyrolysis.26 The NPs revealed a high specific capacity of 115 mAh·g−1 at 10C (1C means that the charge/discharge current density is 175 mA·g−1).26 Mesoporous LTO prepared by soft templating showed a similar rate capability (105 mAh·g−1 at 10C).18,25 Besides

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improving the rate capability, it is assumed that nanosizing exerts a significant influence on thermodynamic parameters, such as surface energy, strain and furthermore the surface electrochemical potential of lithium.27 Recent studies have shown that the interfacial charge storage amounts for a significant contribution in the overall charge storage, if the crystallite size reaches the nano-scale dimension. The intercalated interfacial charge storage is found in many metal oxide NPs such as anatase TiO2, Nb2O5 and MoO3, all of which are discussed with respect to their behavior as pseudocapacitors, possessing relatively high energy and power density.28-30 In case of LTO, Ganapathy et al. showed by density functional theory calculations (DFT) that the interfacial charge storage which can exceed the bulk capacity, originated from the occupation of lithium on 8a sites additional to the fully occupied 16c positions.31 Furthermore, the interfacial charge storage was determined to be dependent on particle size and exposed facets.31,32 Electrochemical data of this group on LTO NPs with sizes of 12 and 31 nm revealed that 12 nm particles exhibited higher interfacial charge storage compared to 31 nm particles.32 However, quite in contrast to anatase TiO2, Nb2O5 and MoO3, the high interfacial charge storage in the smaller LTO particles resulted in a low reversible capacity because of surface restructuring. Kavan et al. studied LTO films of different crystallite sizes and indicated that nanoparticles with a size of 20 nm revealed a larger specific capacity as well as a higher stability than LTO particles with smaller sizes.33 Hence, those studies indicated an optimized particle size of 20 nm, below which LTO exhibited a decreased reversible capacity due to the high interfacial charge storage. However, the value of the optimal crystallite size is related to three major quantities, namely the interfacial charge storage, the reversible capacity and the rate capability, and is still not fully resolved. As a recent study reported, LTO films revealed high reversible capacities and cycling stabilities, whereas the interfacial charge storage was reported to be negligible in spite of the crystallite size of approx. 11 nm.23 This finding implies that the crystallite size of LTO should be smaller than 10 nm in order to study the interfacial charge storage. In order to resolve the optimal particle size of nano-sized LTO and to elucidate the interfacial charge storage, one of the challenges constitutes the synthesis of LTO NPs with crystallite sizes smaller than 10 nm. Several groups prepared LTO NPs with different sizes and investigated their electrochemical performance. Iversen et al. reported LTO NPs with sizes ranging from 2 to 20 nm synthesized by a pulseflow supercritical reaction. However, these NPs possessing sizes less than 10 nm consisted of a large amount of amorphous metal oxides and impurities such as anatase, aggravating the usage for fundamental studies addressing the interfacial charge storage of LTO NPs.24 Pinna et al. prepared LTO NPs with crystallite sizes of a few nanometers based on the benzyl alcohol route, while these NPs contained varying amounts of TiO2 anatase.34 By optimizing the reaction route of Pinna et al., in the present study we aimed at the synthesis of phase-pure highlycrystalline LTO NPs with sizes below 10 nm (4 and 7 nm). Furthermore, we demonstrate that the dispersibility of the NPs can allow for the synthesis of mesoporous LTO films and fibers. The NPs were thoroughly investigated by several

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techniques such as high-resolution Transmission electron microscope (HRTEM), X-ray diffraction (XRD) with Rietveld refinement, Pair distribution function (PDF) analysis using synchrotron radiation, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR) and Thermogravimetric analysis-mass spectroscopy (TG-MS), in particular with respect to organic moieties being attached to the surface owing to the synthesis. The influence of the crystallite size on the electrochemical performance was carried out on the annealed samples with crystallite sizes ranging from 4 to 12 nm and a commercial LTO with a crystallite size of about 60 nm. The lithium storage in these NPs was studied by galvanostatic charge/discharge, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Experimental methods Materials. Lithium metals (99.9%), Titanium (IV) isopropoxide (97%), Titanium (IV) ethoxide (97%) were purchased from Sigma Aldrich. Titanium (IV) n-butoxide (98%) and 2-methoxyethanol were obtained from Fluka. Benzyl alcohol (99%) was obtained from Grüssing. Poly(isobutylene)-b-poly(ethylene oxide) copolymer, H[C(CH3)2CH2]50C6H4(OCH2CH2)45OH, referred to as PIB50b-PEO45 was obtained from BASF SE and used as structuredirecting agent in this work. Polyethylene oxide with average Mv of 1000000 g/mol was purchased from Alfa Aesar. Preparation of LTO NPs. The preparation of LTO NPs is based on the same protocol published before.35 In detail, 20 mg metallic lithium were dissolved into 25 ml benzyl alcohol at 70 °C for 2 hours under the flow of argon and then stoichiometric Ti(O-iso-Pr)4 was added. After one hour stirring, the homogeneous solution was transferred into a Teflon sealed autoclave. The autoclave was put into an oven at 230 °C for three days. After cooling down, the samples were collected and washed by acetone for three times and then dried at room temperature. In order to study the effects of temperature, precursors of titanium and concentration on the crystalline structure, these parameters changed, see Table S1. The NPs obtained at 220 and 230 °C were named as LTO-220 and LTO-230, respectively. Furthermore, this reaction completed in a microwave reactor (Anton Paar) with a reaction time of 2 hours and a reaction temperature of 270 °C. Assembly of LTO NPs. In order to remove the surface components and obtain LTO NPs with different crystallite sizes, the LTO-220 sample was annealed at 400 °C for one hour and LTO-230 was annealed at 400, 500 and 600 °C for one hour, respectively. The samples annealed at 500 and 600 °C were named LTO-500 and LTO-600, while the samples annealed at 400 °C kept their pristine names since their crystallite sizes were not changed. These samples were applied to evaluate the impact of crystallite sizes on electrochemical properties. Mesoporous LTO nanofibers were prepared by electrospinning and the detailed experimental parameters were the following: 40 mg PEO with Mv = 1000000 g/mol were added and totally dissolved into a 1 mL LTO-water dispersion with 30 mg LTO NPs by constant stirring. The electrospinning was carried out at room temperature and humidity with applying a voltage of 1 kV cm−1 and a pump rate of 0.3 mL


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h−1. The as-prepared fibers were heated up to 500 °C with a ramp of 10 °C min−1 and a holding time of 1 h. The mesoporous LTO film was prepared by evaporationinduced self-assembly under the assistance of structuredirecting agents. The homogenous dip-coating solution was prepared by the following procedure: a solution containing 45 mg PIB50-b-PEO45 in mixed solvents of 0.5 mL ethanol and 0.5 mL 2-methoxylethanol was added drop by drop into 1 mL of an LTO-water dispersion with a concentration of 6 wt%. The parameters for dip-coating were a withdrawing speed of 5 mm s−1 and a humidity of 75 RH%. The as-made film was dried at 120 °C for 3 h and the porous structure in the film was stabilized at 300 °C for another 6 h at the ramp of 0.5 °C min−1. PIB50-b-PEO45 was removed by heating up to 500 °C at a ramp of 10 °C min−1.

(3:7) as electrolytes in an Ar-filled glovebox (PO2 and PH2O both less than 0.5 ppm). The measurements of galvanostatic profiles were carried out on a Maccor multichannel battery cycler with the cutoff voltages of 1.0 and 2.5 V vs. Li+/Li. The cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) for each sample were collected on the same cell in an electrochemical workstation (Bio-Logic VMP 300). EIS were recorded one hour after each discharge-charge process (rate:1C) with a voltage amplitude of 20 mV and in the frequency range from 1 MHz to 100 mHz. CVs were recorded with cutoff voltages of 1.0 and 2.5 V vs. Li+/Li and scanning rates of 2.0, 1.0, 0.5, 0.2 and 0.1 mV/s.

Characterization Methods. Bright-field transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images were obtained on a CM30-ST microscope from Philips and on a MERLIN instrument from Carl Zeiss, respectively. Powder X-ray diffraction (XRD) measurements were carried out on an X’Pert PRO diffractometer from PANalytical instruments. The pair distribution function (PDF) was determined from XRD data acquired at room temperature using synchrotron radiation at the Diamond Synchrotron (UK) beamline I-15. High-energy monochromatic X-rays (71.52 keV, bent Laue monochromator) and a PerkinElmer 1621 EN area detector were used. The samples were measured in spinning borosilicate glass capillaries with a diameter of 1 mm (Hilgenberg). A CeO2 standard was used for calibration and empty capillaries were measured for background correction. The raw data was processed using DAWN software. The PDF analysis was conducted using PDFgetX3 and PDFgui. Different PDF functions were created by varying the upper scattering vector limit (Qmax) for the Fourier transformation to distinguish between real peaks and artificial Fourier features. A Qmax value of 22 Å-1 was used for the final computation. The obtained curves were fitted in the range from 1.5 Å to 20.0 Å. A Li4Ti5O12 cubic spinel type structure (ICSD: 160655) with Li/Ti octahedral sites was used for fitting. Raman spectra were collected on a SENTERRA dispersive Raman microscope from Bruker Optics equipped with an objective from Olympus (MPlan N 100×) and a Nd:YAG laser (λ = 532 nm, P = 2 mW). X-ray photoelectron spectroscopy (XPS) spectra were acquired on a VersaProbe II Scanning ESCA Microprobe from Physical Electronics, a monochromatic Al Kα X-ray source. The C 1s signal from adventitious hydrocarbon at 285.0 eV was used as energy reference to correct for charging. Nitrogen physisorption measurements were carried out at 77 K using an Autosorb-6-MP automated gas adsorption station from Quantachrome Corporation. Dynamic light scattering (DLS) measurements are carried out on 1 wt% LTO dispersion in water.

Electrochemical Measurements. The working electrode was prepared by coating a slurry composed of 5 wt% PVDF, 15 wt% carbon black (super-p Termical) and 80 wt% LTO on copper foils. The coin cells were assembled with lithium foil as a counter electrode, Whatman GF/D 17 mm glass microfiber membrane as a separator and 1M LiPF6 in EC:DEC

Figure 1. TEM images and particle size distribution determined by DLS of LTO-220 and LTO-230 NPs. (a), (b) and (c) show LTO-220 NPs. (d), (e) and (f): LTO-230 NPs. (g) Dynamic Light Scattering (DLS) measurement of LTO-220. (h) DLS measurement of LTO-230, indicating an average particle size of 30 nm. (i) 2 wt% LTO-230 dispersion in water.



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Results and Discussion The LTO NPs studied here are prepared by a solvothermal reaction using the benzyl alcohol following the approach proposed by Pinna et al.34 In essence, the synthesis is based on the condensation to form M−O−M clusters through the elimination of benzyl ether, a C−C cleavage reaction and the Meerwein-Ponndorf-Verley reduction reaction.34,36-38 According to the analysis of the liquid products, the main residues are benzyl alcohol, toluene and benzaldehyde, indicating the key step is the Meerwein-Ponndorf-Verley reduction reaction (Illustration in Figure S1 and S2). The raw product yield reaches a value of at least 90 wt% after three-days autoclave reaction. Furthermore, this reaction can be completed with a similar yield at 270 °C for 2 hours under the assistance of microwave irradiation (Figure S3a). As suggested by Wood et al., the microwave-assisted preparation is more economic than a conventional autoclave reaction.39 The NPs used and analyzed below are all obtained from an autoclave reaction. The freshly-prepared NPs are slightly yellow, which can be ascribed to organic moieties on the surface. The LTO NPs can be dispersed in water with a concentration of more than 2 wt% (Figure 1i), which makes these NPs suitable as building blocks for generating porous structures (fibers and thin films) by using templating strategies. Representative TEM images of LTO NPs (Figure 1) indicate that with increasing temperature, the average particle size grows from around 4 to 7 nm. Thus, the particle size can be tuned by the reaction temperature to a certain extent. Compared with the particle sizes obtained from DLS measurements, the sizes observed in TEM are much smaller, which indicates that the LTO NPs aggregate to form small clusters in water. HRTEM images of LTO NPs exhibit crystalline lattices with a spacing of 0.48 nm corresponding to the (111) plane of Li4Ti5O12 spinel. The three diffraction rings in selected area electron diffraction can be ascribed to the (222), (422) and (442) lattice planes of spinel LTO according to JCPDS card 00-049-0207, proving that these NPs are crystalline and possess the spinel LTO structure. The microstructure of LTO NPs XRD patterns of LTO (Figure 2a and Figure S3) present the characteristics of spinel lithium titanate (Li4Ti5O12, LTO) without detectable impurity phases. Compared with the results using the protocol from Pinna and Wood, a lower concentration of inorganic precursors plays an important role in avoiding the formation of anatase TiO2.34,39 The XRD patterns are analyzed with a Pawley fit using the Full Prof Software40 for LTO-220 and LTO-230, respectively, indicating a reasonable fitting in the light of within the very small crystallite size and corresponding broad Bragg reflections. From the Pawley fit the average crystallite sizes of LTO-220 and LTO-230 (the number denote the reaction temperature) were determined as 4.6 (± 0.9) nm and 6.7 (± 0.6) nm, in good agreement with the corresponding TEM analysis. The crystallite sizes of LTO nanocrystals remain unchanged when the reaction temperature is further increased to 240 and 250 °C (Table S1). LTO-250 nanocrystals are found to aggregate into mesocrystals with a size of several micrometers (see Figure S4). As shown by XPS and FT-IR (Figure S6), benzoate is

Figure 2. The microstructure of LTO NPs. (a) XRD pattern of LTO-230 with Pawley refinement. The yellow crosses are the fitted data, the black solid curve is the experimental data and the dark yellow curve is the difference between them. (b) Long-range order PDF analysis of LTO-220 and LTO-230. (c) Raman spectra of LTO-220 (black) and LTO-230 (red).

produced on the surface of the NPs at higher temperature, which can hinder the further growth of NPs as reported for other oxides prepared via benzyl alcohol, such as nanocrystalline ZrO2.38 Other reaction parameters such as


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ACS Applied Materials & Interfaces lengths) when compared to the average structure model. Indeed, if a local Ti enrichment is present without a full stoichiometric disorder the local bond lengths are expected to change, resulting in a local structural distortion. In summary, the structural investigations using PDF show that the Ti octahedra of the LTO-220 sample are smaller and more distorted compared to LTO-230, suggesting more Ti enrichment in the smaller crystallites with their larger surface areas.

Figure 3. PDF analysis of LTO-220 and LTO-230. The first six atomic distances are indicated. The red, blue and green balls represent the atoms located at 32e (O), 16d (Ti) and 8a (Li) positions, respectively. The fit curves of the G(r) functions (black circles) are shown in red.

titanium precursors and concentrations possess little effects on the crystallite size of NPs (Table S1 and Figure S3). In order to investigate the local atomic environment within the LTO NPs, the pair correlation functions G(r) of the LTO220 and LTO-230 samples were analyzed. Both nanocrystalline materials have a very well-defined local atomic arrangement. The G(r) functions of both materials thus confirm a high crystallinity and the absence of impurities. DSC data of LTO-230 (Figure S5c) indicate only one thermal exchange process for the combustion of organic groups at around 400 °C, which excludes the crystallization of the amorphous phase and implies high crystallinity as well. The distribution G(r) of the LTO-230 sample exhibits sharper and more intense signals at higher r-distances due to bigger crystallite sizes and higher long-range order compared to LTO-220 (see Figure 2b). Despite fitting the local correlated motion factor,41 the fit does not cover the distances at 3 Å and 6 Å very well (Figure 3). A possible explanation is a nonstoichiometric composition across the particles: As these distances correspond to the Ti-Ti distance of the first and second coordination sphere (see Figure 3), these data suggest an enrichment of Ti within the particle. The latter phenomenon has recently been seen in Ba0.5K0.5TiO3.42 A local enrichment of Ti atoms will lead to a higher intensity of the M-M peaks compared to an average occupation of the Ti atoms. Note that in the ideal LTO structure both, Li and Ti occupy octahedral positions. In addition to the additional intensities at 3 Å and 6 Å, the Ti octahedra are more distorted (shorter Ti-O bond

Raman spectra provide further evidence for a pure spinel LTO, as this technique can identify even minute amounts of impurity phases. The results for both materials are shown in Figure 2c. Five first-order Raman-modes given by 1×A1g, 1×Eg and 3×F2g modes of the cubic Li4Ti5O12 spinel structure according to group theory can be clearly observed.43-45 In detail, the five typical vibration bands located at 230 (F2g), 271 (F2g), 337 (F2g), 427 (Eg), 677 (A1g) and 750 cm−1, can be assigned to the bending vibrations of O-Ti-O bonds (230 cm−1), stretching vibrations of the Li-O bonds in LiO6 (337 cm−1) and LiO4 (427 cm−1) polyhedral and vibrations of Ti-O bonds in TiO6 octahedral (677 and 750 cm−1), respectively.43-45 Raman bands of impurities such as anatase TiO2 are not observable. The other noticeable Raman bands at 618 and 1000 cm−1 (marked by “+”) are attributed to a phenyl ring, commonly found on metal oxide particles prepared by the benzyl-alcohol route.36 The signal at 1080 cm−1 (marked by “*”) is attributed to lithium carbonate, which is formed in trace amounts upon the exposure of nanocrystalline LTO to air.46 Overall, single-phase nanocrystalline spinel LTO NPs can be obtained by adjusting the reaction conditions, thus advancing the original work of Pinna et al. To investigate the elemental composition and chemical states of the LTO nanocrystals, X-ray photoelectron spectroscopy (XPS) was carried out. XPS survey spectra and scans of the Ti 2p and C 1s core levels are shown in Figure 4 and S5. The atomic ratios between Li and Ti are found to be 0.82 and 1.05 for LTO-230 and LTO-220, respectively, and the high lithium amounts may be caused by lithium carbonate on the surface. From the ICP-MS analysis of LTO-230, this ratio is found to be 0.78, which is reasonable agreement with XPS analysis and indicates the system has a small enrichment in Ti, supporting the results from PDF analysis. Due to spinorbit splitting the Ti 2p signal is composed of a doublet peak at binding energies of 464.5 eV and 458.8 eV , which is in good agreement with reported values for Ti4+ in spinel LTO.23,25 No Ti with reduced valence is found. The C 1s core level spectrum can be deconvoluted into five peaks located at 285.0, 286.4, 289.0, 290.2 and 291.7 eV corresponding to carbon species with different chemical bonding/oxidation states, which can supply detailed information about the organic moieties on the surface. The signal at the lowest binding energy can be attributed to adventitious hydrocarbons overlapping with the C−C bonds of the aromatic ring of benzoate (see below). The peak at 286.4 eV can be assigned to C−O−C emerging from residue alkoxide groups, the amount in LTO-220 being larger than that in LTO-230. The residue alkoxide groups can be hydrolyzed, leading to a negative charging of the NPs (zeta potential of −40 mV) and the resulting electrostatic repulsion explains the good dispersibility in water. The signal at 289.0 eV is due to O−C=O bonds of benzoate, which is further supported by FT-



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IR data (Figure S6).47,48 It is reasonable that the amount of benzoate observed for LTO-220 is lower than in case of LTO-230, because the generation of benzoate is facilitated at higher temperature.47 The peak at 290.2 eV is attributable to carbonate (as typically found on the surface of LTO) and the signal at 291.7 eV most likely stems from C−F bonds resulting from the F-containing polymer (Teflon) within the reactor. 25,46-48 This peak overlaps with a very small π−π* shake up satellite from the aromatic structure of benzoate. Assembly of LTO nanocrystalsIn order to eliminate the organic moieties on the surface and to obtain LTO with bigger crystallite sizes, the LTO-230 NPs were annealed for 1 hour at 400 °C, 500 °C (LTO-500) and 600 °C (LTO600), respectively, while LTO-220 NPs were heated at 400°C for 1 hour. The XRD patterns (Figure S7) prove that these samples present pure spinel structure. The crystallite size is calculated from Rietveld refinement and shown in Table 1. The SEM images of the annealed samples (Figure 5a and 5b) indicate a porous micron-sized spherical nature. As reported by Amine et al., the morphology of micronsized spheres combined with an internal nanoporous structure is beneficial for both, energy density and power density.3

Figure 4. The surface component analysis of LTO NPs. (a) XPS detail spectra of the Ti 2p core level. (b) XPS detail spectra of the C 1s core level. The bubbles represent experimental data and the solid red curve is the envelope from peak fitting.

As mentioned above, the as-made NPs possess excellent dispersibility in polar solvents such as water, ethanol etc., and thus serve as robust building blocks to construct hierarchical LTO nanostructures as exemplified with mesoporous LTO fibers and films. The LTO fibers were prepared by electrospinning a solution including LTO nanocrystals and appropriate polymers. Well-defined fibers possessing an average diameter of 200 nm can be obtained and the fibers are highly porous. Mesoporous films can be prepared by block copolymer templating using PIB50-b-PEO45. Compared with their preparation based on molecular compounds, the preparation based on nanocrystals allows the removal of structure-directing agents at moderate temperatures.18,22,23,25

Figure 5. SEM images of assembled LTO structures. (a) LTO500. (b) LTO-600. (c) and (d) mesoporous LTO fibers. (e) topview SEM image of mesoporous LTO film. (f) cross-sectional SEM image of a mesoporous LTO film.

The size distribution of micelles formed from diblock polymer PIB50-b-PEO45 in mixed solvents of alcohol and water is much wider than for those formed in pure alcohols. Thus the mesoscopic ordering is less developed compared to the preparation in alcohols, e.g. in comparison to mesoporous Nbdoped TiO2 films.49 The average pore size is approx. 20 nm by accounting 100 pores in SEM images. The thickness of the mesoporous thin film is around 600 nm and can be tuned by the variation of the withdrawal rates.


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ACS Applied Materials & Interfaces of underlying NPs. In fibers, the pore size distribution is much wider, from 6 to 30 nm. Here, it can be deduced that there are two classes of pores: one originating from the NP packing and one from templated polymers (PEOs). The pore volume of LTO fibers accounts to 0.74 cm3 g−1, which is two or three times higher than for non-templated samples and agrees well with recent studies.51 Wessel et al. showed that high porosities and surface areas can be achieved using preformed TiO2 for the generation of corresponding nanofibers, which is based on the interplay between polar and nonpolar constituents in the electrospinning solution.51 The pore volumes are 0.285, 0.441, 0.334 and 0.162 cm3 g−1 for LTO-220, LTO-230, LTO-500 and LTO-600, respectively. Mesoporous LTO thin films were prepared by dip-coating under the assistance of the special diblock copolymer PIB50-bPEO45. The N2 physisorption of a film with a total area of 78 (± 2) cm2 and thickness of 700 (± 100) nm presents nonclosure isotherms, which is commonly observed for thin films because of too little material present. The pore size distribution is evaluated by the same method, indicating the size of from 6 nm to 20 nm. The absolute adsorbed pore volume is 3.14×10-3 cm3 and the porosity of this film is around 57%. The absolute BET surface area of this film is 1.11 m2, corresponding to specific surface area of 203 m2/cm3. Electrochemical performance of LTO nanocrystals

Figure 6. N2 physisorption isotherms (77 K) of LTO nanoparticles. (a) LTO nanostructured powders and fiber. (b) Mesoporous LTO film obtained from nanoparticle assembly. The pore size distributions are all calculated from the NLDFT method based on a spherical/cylindrical pore model, using the adsorption branch.

The nanoscaled porosity of all assembled LTO nanostructures was evaluated by N2 physisorption at 77 K (Figure 6 and Table 1). The BET surface areas for LTO-220, LTO-230, LTO-500, LTO-600 and LTO fibers were 300, 218, 164, 84 and 185 m2 g−1, respectively. In order to calculate the pore size distribution, nonlocal density function theory was applied on the adsorption branch of the isotherm based on sphere/cylinder pore shape (Fig. 5b).50 The average pore size of LTO-220 (3 to 8 nm) is smaller than that for LTO-230, LTO-500 and LTO600 (5 to 15 nm), which coincides well with the different sizes

In order to address the influence of the crystallite size on the electrochemical performance, five different LTO samples were used: LTO-220 after annealing at 400°C for 1 hour with a crystallite size of around 4 nm, LTO-230 after annealing at 400°C for 1 hour with a crystallite size of around 7 nm, LTO500 with a crystallite size of 8 nm, LTO-600 with a crystallite size of 12 nm and commercial LTO with a crystallite size of around 60 nm (Figure S7 and Table 1). The lithium-ion storage capability of these nanostructured LTO materials was evaluated by galvanostatic charge/discharge profiles (Figure 7). The intercalation plateau potentials for LTO nanocrystals is around 1.52 V, which corresponds to the transformation of the spinel Li4Ti5O12 into the rock salt-type Li7Ti5O12. The two-phase region followed by a capacitive slope gets smaller with the decrease in crystallite sizes, which is commonly observed in nanomaterials.22,31,52 The interfacial charge storage mechanism enables the material to store more charge, therefore enhancing the energy density. Here the specific capacities at the first cycle for LTO-220, LTO-230, LTO-500 and LTO-600 are around 230, 223, 199 and 180 mAh·g−1 at C/2, respectively, all being higher than the theoretical value of 175 mAh·g−1.



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Figure 7. The electrochemical performance of nanostructured LTO during the first cycles at C/2. (a) Comparison of galvanostatic discharge and charge profiles of LTO with different crystallite sizes at the first cycle. (b) The Coulombic efficiency of LTO with different crystallite sizes. (c) The first three cycles of LTO-500. (d) Impedance spectra of half cells with LTO as active materials after discharging at 20 cycles. The solid curves are the fitted results and the balls are experimental results. The inset shows the charge-transfer resistance R4 . (e) CV of half cells with LTO as active materials at 0.1 mV s−1. (f) Log-log plot of the anodic peak current i against the scan rate v.

Table 1. The influence of LTO parameters (crystallite size, BET surface area--SBET, pore diameter--dpore and pore volume--Vpore) on electrochemical properties (specific capacity at first discharge process at C/2, 1C, 20C and 50C, Coulombic coefficient of the first cycle, R4 at 20 cycles and the slope of the Log(i) vs. Log(v) plots (b value).

LTO-220 LTO-230 LTO-500 LTO-600 LTO-fiber

Crystallite sizes / nm 4 7 8 12 7

Specific capacity / mAh/g C/2 1C 20C 50C 230 144 117 88 220 155 129 70 199 157 123 47 180 159 125 46 198 157 137 110

Coulombic coefficient % 73 75 85 92 84

For all samples, the Coulombic efficiency (Figure 7b) at first cycle is very low, for instance around 75% for LTO-220, which is probably caused by the irreversible Li trapping in the crystalline structure. A first-cycle capacity loss due to electrolyte decomposition and SEI formation can be excluded as this process occurs below 0.9 V (seeing CV in the range of 2.5 V to 0.05 V, Figure S8). Moreover, with the crystallite size increasing to 12 nm, the Coulombic efficiency increases to 92% at the first cycle. This value implies that the irreversible charge storage in the LTO anode is highly dependent on the crystallite sizes. The specific capacities for LTO-220, LTO230, LTO-500 and LTO-600 decrease to 144, 155, 157 and 159 mAh·g−1 after 20 cycles at 1C, indicating a significant decrease in the reversible specific capacity for LTO-220. The discharge profiles at the first three cycles (Figure 7c) indicate that the decrease in capacity is mainly derived from the plateau part (two-phase equilibrium). Wagemaker et al. reported similar phenomena for LTO NPs with crystallite size of 20 nm and recognized the fierce interfacial restructuring after the uptake of a high amount of Li ions as an important reason.32 In combination with the above-shown analysis,

b value

R4 / Ω

SBET / m2/g

dpore / nm

Vpore / cm3/g

0.65 0.60 0.57 0.57 --

5 16 13 27 --

300 218 164 84 185

3~8 5~15 5~15 5~15 6~30

0.28 0.44 0.33 0.16 0.74

below a crystallite size of around 7 nm the high interfacial storage results in a low Coulombic efficiency and a low reversible capacity. According to the local structure description obtained from PDF analysis, there is a more pronounced Ti-richment in LTO-220 than in LTO-230 nanoparticles. The Ti-rich and Li-poor structure in LTO-220 can accommodate more lithium, which increases the Listorage capacity. This detailed structural examination supports the electrochemical results obtained on LTO-220 and LTO230. The structural stability of LTO-220 after accommodating more lithium is still under investigation and a suitable theoretical approach would help understanding this issue. In order to obtain detailed information about the kinetic and thermodynamic processes of lithium insertion into LTO with different crystallite sizes, impedance spectra and cyclic voltammograms were recorded (Figures 7 and S9). The analysis of EIS is based on an equivalent circuit of R1+R2/Q2+R3/Q3+R4/Q4+Q5, as reported in literature, where constant-phase elements Q2, Q3 and Q4 were used to replace an ideal capacitor C with the consideration of the nature of rough


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surface of electrode.53-56 R1 is around 4 Ω for all cells and remains unchanged after 20 cycles, and is assigned to electrolyte resistance. R2/Q2 and R3/Q3 corresponding to frequencies above 16 Hz are superimposed at the first cycle and then dramatically decrease during the following cycles. This can be assigned to the electrode/electrolyte interfacial resistance of the LTO/current collector and Li/electrolyte interfaces.53,54 R4/Q4 at the mid-to-low frequency (below 16 Hz) represents the LTO/electrolyte interface.53-56 At low frequencies, the constant-phase element Q5 instead of W (Warburg impedance) is applied to describe the diffusion process due to the roughness of electrode.53,54 This process can be detected after discharging and disappears after charging, agreeing with previous reports.55,56 In order to study the resistance corresponding to the Li-transfer involved, the main attention was focused to R4 according to equivalent circuits (Figure 7d). For LTO-220, there are two semicircles in this frequency range at the first discharging process, which maybe induced by the interfacial restructuring. Furthermore, it can be observed that R4 for NPs with bigger crystallite sizes becomes stable much faster than the one for smaller crystallite sizes, which explains the cycling performance. Comparing with LTO-230, LTO-500 and LTO-600, the R4 in LTO-220 after 20 cycles is the smallest, which implies the smallest Li-insertion thermodynamic barrier. From CV data, the anodic peak potentials for LTO-230, LTO-500 and LTO-600 are nearly identical - about 1.51 V which is in good agreement with the plateau potential in the discharge profile at C/2. Besides the redox peaks, the pseudocapacitive contribution can be clearly observed (seeing Figure 7e and S8) and LTO-220 presents a bigger pseudocapacitive current than LTO-500 and LTO-600. As reported before, smaller crystallite size induces higher pseudocapacitive currents. With an increase of the scanning rate, the intercalation potential shifts to lower potentials, indicating an electrochemically irreversible process. The interfacial charge storage of the particles can be evaluated by the approach mentioned before49,52: jpeak=νb and Log( jpeak) = b Log(ν), where jpeak is the peak current and ν is the scanning rate. The current obtained from the bulk intercalation reaction is proportional to the square root of the scanning rate (b = 0.5), and the current determined from the capacitive part is proportional to the scanning rate (b = 1). By plotting Log jpeak against Log ν, a linear relationship is found and the slopes are 0.65, 0.60, 0.57 and 0.57 for LTO-220, LTO-230, LTO-500 and LTO-600, respectively. The calculation from cathode process gives the same trend, seeing Figure S11. These results confirm that smaller crystallite sizes correspond to more pronounced interfacial charge storage. Theses nanostructured LTO electrodes present an excellent rate capability as well, compare Figure 8. At the charge/discharge rate less than 20C, the LTO-230, LTO-500 and LTO-600 samples all exhibit an excellent rate capability, e.g. the specific capacity for LTO-600 only decreases from 160 to 144 mAh·g−1 when the discharge/charge rate is increased from 1C to 10C. At further increased rates, e.g. 20C or 50C, LTO-220 and LTO-230 present a higher rate capability than LTO-500 and LTO-600. From the porosity analysis and crystallite size analysis (Table 1), electrolyte diffusion and Li-ion diffusion in NPs can be excluded as the rate-dependent steps, instead the electron transport is the most

relevant process for ultrafast discharge/charge rates. The huge potential polarization at high current density (observed in Figure S12) is mainly due to the poor electronic conductivity. The LTO fibers present the best rate capability (more than 100 mAh·g−1 at 50C) because of the small fiber (200 nm) and biggest specific pore volume among the samples. For a practical power density evaluation, a half-cell equipped with LTO-230 nanocrystals was tested at 10C for 800 cycles. The results indicate the specific capacity starts at a value of 143 mAh·g−1 and decreases to 129 mAh·g−1 after 800 cycles, corresponding to a capacity decay of only 0.023% per cycle at 10C despite the contribution from partial capacity decay due to the metallic lithium counter electrode. The Coulombic efficiency reaches to 99.9% after 10 cycles. Among all of samples, LTO-230 represents the most promising anode material for LIBs among the LTO samples tested here.

Figure 8. (a) The rate capability of nanostructured LTO. (b) Cycling stability at high C-rate of LTO-230.

Conclusions In this study, LTO nanoparticles were prepared by an optimized solvothermal protocol, possessing high phase purity, high dispersibility in water and tunable crystallite sizes (4, 7, 8, 12 nm). These advantageous properties enable them to be assembled to hierarchical porous structures (films and fibers) and allowed for a systematic studying the size effect on their electrochemical performance.



The results indicated that LTO with smaller sizes presents higher interfacial charge storage and lower reversible capacity at the first several cycles, in good agreement with previous reports.30 In contrast to previous literature suggesting an optimum crystallite size of ca. 20 nm, here a crystallite size of 7 nm is found as critical dimension for LTO with respect to electrochemical Li storage. LTO with a crystallite size of 7 nm exhibits as good reversible capacity as particles with bigger crystallite sizes in spite of possessing interfacial charge storage capacity. Furthermore, Li-storage experiments showed specific capacities of e.g. 130 mAh/g after 800 cycles at 10C for LTO-230 and more than 100 mAh/g at 50C for LTO fibers (with LTO loading of around 3.5 mg/cm2), proving the highrate and long-life anodes for lithium ion batteries. Furthermore, the reason for the interfacial charge storage mechanism in LTO nanoparticles and its electrochemical irreversibility was addressed. The careful examination of the crystalline structure of LTO NPs by PDF analysis indicated that the NPs with a smaller size present a Ti-rich structure, which offers the opportunity for storing more lithium ions compared with ideal large LTO crystals. Whether the interfacial structure of LTO(111), which is determined by Kitta et al. to be an oxygen-poor or oxygen-rich surface structure,57 correlates with the crystallize size is currently being investigated in a separate theoretical study.

ASSOCIATED CONTENT Supporting Information. These materials including GC-MS, FTIR, TGA-MS, XRD with Rietveld Refinement, and CVs and impedance are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Bernd M. Smarsly [email protected]

ACKNOWLEDGMENT The authors thank Hubert Wörner for TGA-MS. B.M.S. and D. M. acknowledges financial support within the LOEWE program of excellence of the Federal State of Hessen (project initiative STORE-E). Dr. Denis Badocco and Prof. Paolo Pastore (Department of Chemical Sciences, University of Padova), are gratefully acknowledged for ICP-MS analyses. J.Y. is grateful for the financial support by the China Scholarship Council (CSC, #2011631008). The synchrotron diffraction was supported by Diamond Light Source (beamtime award EE13560) with beamtime proposal SP13560. We would like to thank the Center of Materials Research (LaMa) at Justus Liebig University Giessen for the support of this project.

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Small Li4Ti5O12(LTO) nanoparticles were repared by special sol-gel methods and studied with respect to the relationship between crystallite size and electrochemical storage of Li.


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Critical Role of the Crystallite Size in ... - ACS Publications

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