Controlling Size, Crystallinity, and Electrochemical Performance of

Dec 5, 2013 - The effect of crystallite size and crystallinity on the electrochemical performance of LTO NPs was investigated in half-cells, and excel...
22 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/cm

Controlling Size, Crystallinity, and Electrochemical Performance of Li4Ti5O12 Nanocrystals Yanbin Shen, Jakob R. Eltzholtz, and Bo B. Iversen* Centre for Materials Crystallography, Department of Chemistry and iNANO, Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark S Supporting Information *

ABSTRACT: Synthesis of nanosized lithium ion battery electrode materials with high purity, controllable crystallinity, and uniform sizes is important for high power applications, but it is challenging using conventional methods. Here, we report on a rapid new synthesis approach, which potentially allows for large scale preparation of nanoparticles (NPs) of the promising zero-expansion anodic material, Li4Ti5O12 (LTO). A pulsed flow supercritical reactor is used for one-step synthesis, and by adjusting the synthesis temperature and pulse frequency LTO NPs with tunable crystallite size from ∼2 to ∼20 nm and controllable crystallinity (50−100%) can be synthesized in a matter of minutes. The effect of crystallite size and crystallinity on the electrochemical performance of LTO NPs was investigated in half-cells, and excellent properties are observed even for as-prepared NPs used without post-synthesis treatment. The LTO NPs with small crystallite size and high crystallinity show the best high-rate ability and long-term cyclability probably due to a combination of small charge transfer and Li+ diffusion impedance. KEYWORDS: Li-ion batteries, anode materials, lithium titanate, nanoparticles, supercritical synthesis, crystallinity, crystallite size

■

INTRODUCTION Lithium ion batteries have been successfully commercialized and widely applied in portable electronic devices in the last two decades,1−4 and are now considered to be a promising choice for hybrid electrical vehicles and energy storage devices.5−7 To meet the future challenges of energy storage, increasing the energy and power density by nanosizing the electrode material is an effective approach.8−10 Reducing the crystallite size of the electrode materials to nanometer scale can reduce the solid state diffusion distance for Li+. This can remarkably improve the overall ionic conductance of a battery since the diffusion of Li+ in the solid state is the rate limiting step. Nanosizing can also decrease the blocking point defects of the particles, which will increase the conductivity of the electrode materials. In addition, nanosizing will increase the contact areas between the electrode and electrolyte, leading to higher charge−discharge rates.11,12 However, there are also some disadvantages for the application of nanomaterials. The inferior packing of nanoparticles will lead to lower volumetric energy densities, and nanosizing also results in high surface areas and hence an increase in the undesirable reactions between electrode and electrolyte.8 Furthermore, the synthesis of NPs is typically more complicated, and significant challenges remain with respect to developing simple and scalable synthesis methods providing the required control over the NPs characteristics. Sol−gel synthesis, spray pyrolysis, and hydrothermal synthesis are common methods for preparing nanosized battery materials due to their relatively low synthesis temperature.13−18 © 2013 American Chemical Society

The drawback of these methods is that they usually require long reaction time (6−12 h) and high temperature (>600 °C) post-treatment to enhance the crystallinity.11,16 Hydrothermal synthesis under supercritical conditions is another well-known method for preparing crystalline NPs with uniform size distribution.14,19−21 Supercritical fluids have properties that lie between those of gases and liquids, which facilitate rapid uniform mixing of reactants and extremely high heating rates. In general, classical batch reactors are good for preparing highly crystalline particles and large single crystals,22 whereas continuous flow reactors are useful for rapid production of nanocrystals.14,19,23−27 Continuous flow systems provide synthesis control through high heating rates and quick quenching of the product. However, the residence time and heating rate in a continuous flow reactor is highly dependent on its geometry, and it is difficult to prepare highly crystalline NPs for lithium ion batteries.19 As an example, Laumann et al. produced LTO NPs using continuous flow hydrothermal synthesis, but the poor crystallinity of the particles necessitated post-calcination at 600 °C for 6 h, which also resulted in significant crystallite growth.19 In the case of the widely used cathode material LiFePO4, Jensen et al. showed that a synthesis time in excess of 5 min is necessary to avoid Li−Fe antisite defects, and this is difficult to achieve in normal continuous flow reactors.28 Since Received: July 15, 2013 Revised: November 18, 2013 Published: December 5, 2013 5023

dx.doi.org/10.1021/cm402366y | Chem. Mater. 2013, 25, 5023−5030

Chemistry of Materials

Article

structure.31,39 However, coarse LTO is characterized by poor electronic conductivity and dull ionic diffusion that limits its application in high power batteries. To improve the performance of LTO for high power applications, nanosizing would be effective. In the present study, we have for the first time synthesized LTO NPs with tunable size and tunable crystallinity via a rapid, facile one-step reaction using pulsed flow supercritical synthesis. Subsequently, we have systematically investigated the effect of crystallite size and crystallinity on the electrochemical performance of the LTO nanocrystals, and LTO anode material with excellent properties can be obtained even without post-synthesis treatment.

the crystallinity of the NPs is critical in Li-ion battery applications, it is clearly desirable to develop a supercritical flow reactor with precise control of residence time, temperature and pressure. Recently, we have developed a pulsed flow reactor, which allows for rapid preparation of NPs in a continuous supercritical flow mode with full control of reaction pressure, temperature, and time, as schematically shown in Figure 1a. It is composed

■

EXPERIMENTAL SECTION

Synthesis. The LTO NPs were synthesized by reacting Li/Ti precursor with deionized (DI) water in the pulsed flow reactor at high temperature and pressure for specific residence times. The Li/Ti precursor was prepared by stoichiometric (with 5% lithium excess) mixing of lithium ethoxide and titanium tetraisopropoxide (TTIP, purity ≥ 97 wt %, Sigma-Aldrich) under stirring conditions for 10 min. The lithium ethoxide with a concentration of 0.25 M was prepared by dissolving lithium chips (purity ≥99 wt %, MTI Corporation) in absolute ethanol. In order to fine-tune the crystallite size and crystallinity, a series of LTO NPs were synthesized by varying the synthesis temperature from 350 to 500 °C, and the residence time in the reactor from 30 to 300 s. The pressure for all syntheses was kept fixed at 250 bar, and the ratio of the pumping volumes of DI water to Li/Ti precursor was kept at two. For a typical synthesis, the pumping volume, temperature, residence time, and pressure were programmed prior to the synthesis. The Li/Ti precursor and DI water were simultaneously pumped in a pulsed fashion into the preheated reactor chamber. The reaction was initiated immediately when the precursor met the DI water, and the mixture remained in the hot zone at the chosen temperature and pressure within the preset residence time before an additional isopropanol pulse pushed the products into the cooling zone and eventually through the outlet valve. The as-prepared products were washed with isopropanol and DI water, centrifuged and dried at 80 °C overnight. The scalability of the synthesis was also tested by carrying out synthesis with different reactor tube diameters of ϕ1 mm and ϕ4 mm. The large tube diameter allows synthesis of large amounts of material as e.g. needed for electrochemical characterization. In general, the large tube diameter results in NPs which are ∼2 nm smaller than for the smaller tube diameter when identical reactor settings are used. Nanoparticle Characterization. Powder X-ray diffraction (PXRD) was employed to characterize the crystal structure and estimate volume averaged crystallite sizes. The diffraction patterns were collected in a parallel beam geometry using a Rigaku Smart-Lab diffractometer equipped with Cu Kα radiation. The rotating anode generator was operated at 40 kV, 180 mA, and diffraction patterns were measured in steps of 0.015° over the 2θ range of 10−70°. The data were used for Rietveld refinement in program FullProf.32 Details of the refinements as well as the crystallinity and size determination from PXRD are given in the Supporting Information (SI). The specific surface areas of the pristine LTO NPs were measured by nitrogen adsorption isotherms using a Nova 2200e surface area analyzer (Quantachrome). Prior to the analysis, the samples were degassed under vacuum for 5 h at 100 °C. The data were analyzed using the Brunauer−Emmett−Teller (BET) method.33 The morphology of the as-prepared samples was characterized using a scanning electron microscope (SEM, NOVA600, FEI). A Philips CM20 transmission electron microscope (TEM) was utilized to analyze the particle size and shape. Electrochemical Characterization. The electrochemical properties of the samples were studied using a CR2032 type half-cell. The cathode electrode material was prepared by mixing 65 wt % asprepared LTO, 20 wt % acetylene black, and 15 wt % polyvinylidene difluoride (PVdF) binder in N-methyl-2-pyrrolidone (NMP). The

Figure 1. (a) Schematic drawing of the synthesis system: (1) Pulsed pumps for reactant and solvents. (2) Heating block. (3) Manometer to monitor pressure. (4) Spiral cooler for quenching the product as it leaves the reactor. (5) Proportional relief valve for pressure adjustment. (6) Product collector. (b) The temperature inside the reactor was recorded as a function of the pulse length during different synthesis run.

of three pumps, a reactor, and a cooling system, and the operation is entirely automated using computer control. A detailed technical description of the reactor has been reported elsewhere.29 Figure 1b demonstrates the evolution of the temperatures in the reactor during synthesis, and indeed very rapid heating to the set-point temperature is achieved. Thus, the pulsed flow reactor eliminates the problem of insufficient mixing, which is typically encountered in continuous flow mode. Apart from the strong control over reaction parameters, a novel aspect of the pulsed flow reactor is that the same reactor can be used both for laboratory synthesis of NPs and for in situ X-ray scattering studies of NPs formation and growth.30 In the past decades there has been a huge growth in in situ studies of hydrothermal reactions, but is it often difficult to transfer the in situ information obtained in specialized miniature reactors to actual laboratory synthesis conditions.23−25 As a proof of concept for the pulsed supercritical synthesis method, we have targeted LTO due to its strong potential as anode material in high power density applications, although recent studies show that LTO will react with alkylcarbonate solvents, generate gases and result in swelling problems of the lithium ion battery.38 LTO exhibits excellent cyclability due to its limited volume change upon lithium insertion/removal, and also shows good rate ability resulting from its 3D spinel 5024

dx.doi.org/10.1021/cm402366y | Chem. Mater. 2013, 25, 5023−5030

Chemistry of Materials

Article

resulting electrode mixture thus obtained was coated on Al foil and dried overnight under vacuum at 100 °C. The electrodes were punched in the form of disks with a diameter of 12 mm, and the loading of the electrode was ∼5 mg/cm2. Metallic lithium foil was used as anode. A solution of 1 M LiPF6 dissolved in EC/DMC/DEC (1:1:1 in volume) was used as electrolyte, and porous polypropylene membrane was used as the separator. The capacity was measured by discharging and charging the Li/Li4Ti5O12 half-cell between 1.0 and 3.0 V at 0.1C (17.5 mAh·g−1) and 1C (175 mAh·g−1). The rate ability was measured by discharging and charging the half-cell between 1.0 and 3.0 V at various C rates. Electrochemical impedance spectroscopy (EIS) analysis of the electrodes was carried out using a CHI660B impedance analyzer in half-cells with metallic lithium as the counter and reference electrodes. The impedance spectra were measured under open circuit conditions using a 5 mV amplitude signal over a frequency range from 100 kHz to 0.01 Hz on the cell. Prior to the EIS measurement, each half-cell was charged to 2.0 V and rested for 5 h to establish electrochemical equilibrium.

The PXRD patterns of representative LTO samples prepared at different conditions are shown in Figure 2b. The LTO NPs synthesized at different temperatures and residence times were named in the format of temperature-residence time, (i.e., 425 °C−30s refers to a sample synthesized at 425 °C for 30 s). For comparison, a sample synthesized at 425 °C for 60 s was annealed at 600 °C for 5 h (referred to as 425 °C−60s−C). All as-prepared LTO NPs have broad peaks, and all the main diffraction peaks can be indexed as spinel LTO. Anatase TiO2 and Li2TiO3 impurities were detected by XRD, especially for samples with residence times exceeding 120 s, and this can be associated with the prolonged mixing time of the Li−Ti precursor and DI water at room temperature between the pulses. For future studies, a new configuration of the mixing pipe for our setup is being designed to ensure separation of precursor solutions between pulses and hence to avoid impurities in the product. The amount of crystalline anatase TiO2 was obtained by Rietveld refinement analysis, and the values are lower than 1 wt % for all the samples (425 °C−30s and 475 °C−30s, too small to be quantified; 425 °C−60s, 0.13 wt %; 425 °C−120s, 0.42 wt %; 425 °C−210s, 0.61 wt %; 425 °C−60s−C, 0.265 wt %). To determine whether any amorphous impurities, which cannot be detected by XRD, are present in the as-prepared sample, PXRD data were collected on the annealed LTO (425 °C−60s-C, olive curve, Figure 2b). If there is amorphous impurity in the as-prepared sample, then the amorphous impurity will crystallize during the annealing and then be detected by XRD. No additional peaks were detected after annealing and the original impurity peaks from TiO2, and Li2TiO3 also did not get more intense compared with the pristine sample (425 °C−60s, red curve, Figure 2b), suggesting that the as-prepared LTO NPs contain no amorphous impurities. The full width at half-maximum (fwhm) of the peaks show a clear trend of getting narrower with increasing residence time and temperature reflecting the increase of the crystallite size (see below). Volume Averaged Crystallite Size. The volume averaged crystallite size of the as-prepared LTO NPs was estimated from the PXRD patterns. The calculation was performed by analyzing the most intense diffraction peak (111) of the LTO patterns using the Scherer equation.35 Details of the crystallite size calculation are given in the SI. The effect of synthesis temperature and residence time on the crystallite size is shown in Figure 3a. It is clear that LTO NPs with crystallite size ranging from ∼2 nm to ∼20 nm can be obtained by varying the synthesis conditions. The volume averaged crystallite size of the LTO NPs increases from ∼2.5 to ∼12 nm with increasing temperature from 350 to 500 °C at a constant residence time of 30 s. The impact of temperature on the crystallite size is minute at low temperature, where only an ∼1 nm increase of the crystallite size is observed from 350 to 425 °C. However, the crystallite size increases dramatically at elevated temperature (>425 °C). Hence, we conclude that the crystallization process of LTO NPs preferably takes place at temperature higher than 425 °C. Similar trends in the crystallite growth are observed for prolonged residence times, but an accelerated increase in size with increasing residence time is observed at high temperature (Figure 3a). Even though LTO NPs with similar crystallite size can be obtained using either a relative high temperature and a short time, or a lower temperature and prolonged residence time, there are significant differences in the crystallinity between such particles.

■

RESULTS AND DISCUSSION Crystal Structure. Figure 2a shows a typical PXRD pattern from an as-prepared sample synthesized at 450 °C for 120 s.

Figure 2. (a) Representative PXRD pattern and Rietveld refinement of an as-prepared sample (450 °C, 120 s). The black dots are experimental points, the red line is the calculated pattern and the green line is the difference curve. The blue tick bars indicate the Bragg reflections of Li4Ti5O12 in space group Fd3̅m. (b) PXRD patterns of representative samples synthesized with different residence times and temperatures. The name on the patterns reflects the synthesis temperature and residence time of the corresponding sample. The top line shows the pattern of a 425 °C−60 s sample after sintering for 5 h at 600 °C.

Rietveld refinement analysis confirmed that the sample can be identified as Li4Ti5O12 with space group Fd3̅m. The refined lattice constant is 8.3648(2) Å, which is slightly smaller than the value of 8.367(2) Å reported by Colbow et al.,34 but it is much larger than that reported in a single crystal study (8.352(4) Å) by Kataoka et al.31 Details of the Rietveld refinement are given in the SI. 5025

dx.doi.org/10.1021/cm402366y | Chem. Mater. 2013, 25, 5023−5030

Chemistry of Materials

Article

volume averaged crystallite size of ∼2.4 nm, the unit cell size is ∼8.386 Å, which is much larger than that reported in a single crystal study (8.352(4) Å).31 Unit cell expansion in the nanoregime has been noted in many other nanoparticle systems and it has often be explained by surface effects.22 Nanoparticle Morphology. The microstructure of the asprepared LTO samples was further investigated by SEM and TEM. Figure 5 shows representative images of the samples prepared at three different conditions. Figure 5a shows a SEM image from a sample synthesized at 450 °C for 30 s (450 °C− 30s), and it is seen that the as-prepared LTO sample is composed of homogeneous nanosized particles. The morphology of the 450 °C−30s sample was further observed by TEM as shown in Figure 5b, in which two selected areas have been enlarged in the insets. Even though the limited image resolution and the agglomeration make it challenging to determine the exact shape and size of the individual particles, it is seen that the particles are cube shaped with a size of ∼6.5 nm. This is consistent with the average size values obtained from PXRD. Figure 5c and d shows the TEM images obtained from a sample prepared at higher temperature (475 °C) for the same residence time (30 s) and a sample prepared at the same temperature (450 °C) but with longer synthesis time (120 s). Similar to the results from PXRD, the TEM images show a particle growth with increasing temperature and residence time. The TEM particle sizes of the 475 °C−30s and 450 °C−120s samples are ∼10 and ∼12 nm, respectively, which are also in agreement with the volume averaged values obtained from PXRD. Formation Mechanism. As shown in Figures 2 and 3, the LTO particles form rapidly, and the particles size and crystallinity grow with increasing temperature and reactor residence time. The details of the formation mechanism are currently being investigated using in situ total X-ray scattering methods. However, according to the present results, we propose that the formation of LTO in the pulse flow reactor can be described in two steps, as shown in Figure 6a. First, Li/ Ti gel with a crystalline LTO core form immediately when the Li/Ti precursor mix with the DI water in the hot reactor (Figure 6a-2), then the Li/Ti gel layer develop into crystalline LTO as the residence time increases (Figure 6a-3). As shown in Figure 6b, the impact of temperature on the crystallite size is small at low temperature, but the crystallite size increases dramatically at elevated temperature (>425 °C). In contrast the residence time is crucial for obtaining high crystallinity. Thus, even when heating to 500 °C, a residence time of 120 s is needed to obtain a nearly 100% crystallinity. This explains why highly crystalline LTO NPs cannot be obtained using conventional flow reactors, which normally do not allow broad residence time control.

Figure 3. (a) Particle growth as a function of the synthesis temperature at various residence times. (b) Crystallinity as a function of the residence time at various synthesis temperatures.

Crystallinity. The crystallinities of the as-prepared LTO samples synthesized at various conditions were determined by calculating the phase fraction of crystalline LTO relative to an internal standard of CaF2, which is assumed to be 100% crystalline. Details of the calculation can be found in the SI. The results are plotted as a function of residence time at two different temperatures (450 and 500 °C) in Figure 3b. It is clear that the crystallinity of the particles improve with increasing temperature when the residence time is short (30 and 60 s). However, at residence times above 120 s the two temperatures shows no difference in the crystallinity and highly crystalline particles are obtained. Even at 500 °C, a residence time of 120 s is necessary to obtain ∼98% crystallinity. This result illustrates the importance of residence time control during the synthesis and hence the advantage of the pulsed flow reactor. Unit Cell Volume. The Rietveld refinements results also reveal that the unit cell size of the as-prepared LTO samples depends on the crystallite size (Figure 4). The data points have been fitted to an exponential function and the result is shown as a red line. Clearly, the unit cell expands with decreasing crystallite size, and the volume increase is especially significant when the crystallite size is below 7 nm. For the particle with

■

ELECTROCHEMICAL PERFORMANCE Four different as-prepared LTO samples with different crystallite size and crystallinity were selected for the electrochemical performance evaluation. The LTO NPs were named in the format of crystallite size−crystallinity (i.e., 4 nm−69% refers to samples with a crystallite size of ∼4 nm and 69% crystallinity). Figure 7a shows the first discharge/charge C−V curve at 0.1C (17.5 mAh·g−1) of the as-prepared ∼4 and ∼18 nm particles. The first discharge capacities for ∼18 and ∼4 nm samples are 232 and 260 mAh·g−1, respectively. These capacities are much higher than the theoretically estimated

Figure 4. Crystallite size dependence of the unit cell parameters of nanocrystalline Li4Ti5O12. The line is an exponential fit. 5026

dx.doi.org/10.1021/cm402366y | Chem. Mater. 2013, 25, 5023−5030

Chemistry of Materials

Article

Figure 6. (a) Schematic of the LTO formation mechanism in the pulsed flow supercritical reactor: (1) The reactor is ready with isopropanol at preset temperature and pressure. (2) Li/Ti precursor and DI water are pumped into the reactor and particles form with a crystalline LTO core and a Li/Ti surface layer. (3) The Li/Ti gel develops into highly crystalline LTO as residence time increases. (4) The product is pumped into the spiral cooler at the end of preset residence time. (b) Schematic illustrating the effect of the residence time and temperature on the LTO particle size and crystallinity. The synthesis temperature plays an important role for the particle size, while the residence time is vital for the crystallinity.

capacity (175 mAh·g−1) under the assumption of insertion of three Li+ ions (from Li4Ti5O12 to Li7Ti5O12). This implies that the LTO can host more than three Li+ ions, and the increased storage capacity of the smaller particles has been suggested to reside mainly near the surface.12,36 This also explains the increasing capacity with decreasing particle size. Even though the nanosized particles have very high capacity at the first discharge, they show quite high irreversible capacity loss on the first few cycles (see the SI). These irreversible capacity losses possibly are due organic residues remaining in the LTO sample as shown in the SI (Figure S6) by FT-IR and TGA measurements. These organic residues can be removed by heating the powder at 400 °C. The organic residues probably can be reduced at voltage lower than 1.5 V on the LTO surface in the first few cycles, similar to the formation of solid− electrolyte interfaces (SEI), and this contributes to part of the high irreversible capacity loss. Like SEI, the reduced products of these organic residues could also behave like a protective layer for the LTO particle and this is beneficial in subsequent cycles. Another potential origin of the high irreversible capacity loss is

Figure 5. Representative SEM and TEM images of as-prepared samples: (a, b) Tset = 450 °C, p = 250 bar, residence time = 30 s, (c) Tset = 475 °C, p = 250 bar, residence time = 30 s, and (d) Tset = 450 °C, p = 250 bar, residence time = 120 s sample. 5027

dx.doi.org/10.1021/cm402366y | Chem. Mater. 2013, 25, 5023−5030

Chemistry of Materials

Article

Figure 7. (a) C−V curve of the 1st discharge−charge (0.1C) of asprepared nanoparticles. (b) C−V curve of the fifth discharge (1C) of various as prepared nanoparticles.

Figure 8. (a) Comparison of the discharge rate ability of particles with different size. (b) EIS Nyquist plots of the electrodes with different crystalline size (measured at ∼1.90 V).

reaction between LTO nanocrystals and alkylcarbonate solvent, which is the main cause of the “gassing” problem of LTO. Moreover, the as-prepared nanocrystals possess a curved discharge plateau at ∼1.55 V (more obviously when compared to microsized LTO in the SI), which have been suggested to be related to the strain and interface energy of nanocrystals.37 Figure 7b further compares the 1C discharge capacities of the NPs with different crystallite sizes. This figure reveals a correlation between the discharge capacity and crystallite size, and the discharge capacity is increasing with decreasing crystallite size. To evaluate the rate ability dependence of particle size, the half-cells were discharged at various C-rates (1C, 2C, 4C, 8C, 16C) before 1C charge−discharge cyclability measurement. As shown in Figure 8a, the cell with 9 nm−99% particles (∼56.3% @16C) shows slightly better ability of high current discharge than the cells with 6 nm−80% particles (∼55.3% @16C), and it is much better than the 18 nm−98% particles (∼23.6% @16C). The fact that the 9 nm−99% particles have better rate ability than the 6 nm−80% and 18 nm−98% particles was further illustrated by the discharge rate map measured after 200 charge/discharge cycles at 1C (see the SI). The rate ability of the as-prepared 9 nm−99% particles is also better when compared to a commercial microsized sample. EIS analysis suggests that this is probably due to the smaller charge transfer and lithium diffusion impedance of the as-prepared NPs (see the SI). To understand the correlation between the rate ability and the crystallite size, EIS measurements were conducted on the half-cells before the cyclability measurements. As shown in Figure 8b, the EIS Nyquist plot is composed of a semicircle and an oblique line. The intercept of the plot on the Z-real axis reflects the impedance (Rb) of electron conduction among particles and Li+ conduction in the electrolyte, while the semicircle in the high−middle frequency range corresponds to the charge-transfer impedance (Rct) at the solid/electrolyte

interface. The oblique line at low frequencies refers to lithium ion diffusion impedance (Zw) in the solids. A smaller intercept on the Z-real axis and a smaller diameter of the semicircle represent smaller conductivity impedance and charge transfer impedance. According to the Nyquist plot at Figure 8b, all the three samples show similar Rb, that is, similar impedance of electron conductivity and Li+ conductivity in the electrolyte. However, the 6 nm−80% sample has the smallest charge transfer impedance, while the 18 nm−98% has the biggest. This result fits well with the specific surface area of these three samples (SBET of the 6 nm−80%, 9 nm−99%, and 18 nm− 98% particles are 224, 200, and 100 m2/g, respectively). The reason that the 6 nm−80% sample possesses smaller charge transfer impedance but poor rate ability compared to the 9 nm−99% sample could be due to its low crystallinity (∼80%), which implies that there is an amorphous layer around the crystalline core. This amorphous layer is composed of disordered atoms, possibly leading to higher impedance for the lithium ion to diffuse into the particle core. Note that there is a second depressed semicircle (marked with black dotted circle) in the plots for the 9 nm−99% and 18 nm−98% samples. This may be because too much amorphous acetylene black additive was added to the electrode, since this could give extra impedance for the lithium diffusion on the particle surface. The cyclability of Li/LTO half-cells with three different samples is shown in Figure 9a. The data points show some scatter but nevertheless clear trends. The scatter possibly is due to temperature fluctuations in the laboratory, which did not have temperature control. The 9 nm-99% particles possess better cyclability than the 6 nm−80% and 18 nm−98% particles. There is almost no capacity fading on the 9 nm−99% sample, while there is about 28% capacity fading on the 18 nm−98% particles after 550 cycles at 1C. To understand the cyclability behavior, the Coulombic efficiency of the half-cells is plotted as a function of cycle number in Figure 9b. Even though 5028

dx.doi.org/10.1021/cm402366y | Chem. Mater. 2013, 25, 5023−5030

Chemistry of Materials

■

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors would like to thank Andreas Laumann for helpful discussions; Marcel Ceccato and Kim Daasbjerg for help with the EIS measurement; and the Danish National Research Foundation (DNRF93) for funding.

■

(1) Scrosati, B. Nature 1995, 373, 557. (2) Scrosati, B.; Garche, J. J. Power Sources 2010, 195, 2419. (3) Goodenough, J. B.; Park, K.-S. J. Am. Chem. Soc. 2013, 135, 1167. (4) Yoshino, A. Angew. Chem., Int. Ed. 2012, 51, 5798. (5) Braun, P. V.; Cho, J.; Pikul, J. H.; King, W. P.; Zhang, H. Curr. Opin. Solid State Mater. Sci. 2012, 16, 186. (6) Kim, T. H.; Park, J. S.; Chang, S. K.; Choi, S.; Ryu, J. H.; Song, H. K. Adv. Energy Mater 2012, 2, 860. (7) Thackeray, M. M.; Wolverton, C.; Isaacs, E. D. Energy Environ. Sci. 2012, 5, 7854. (8) Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J. M.; Van Schalkwijk, W. Nat. Mater. 2005, 4, 366. (9) Tarascon, J. M.; Armand, M. Nature 2001, 414, 359. (10) Wagemaker, M.; Mulder, F. M. Acc. Chem. Res. 2013, 46, 1206. (11) Wang, D.; Ding, N.; Song, X.; Chen, C. J. Mater. Sci. 2009, 44, 198. (12) Borghols, W. J. H.; Wagemaker, M.; Lafont, U.; Kelder, E. M.; Mulder, F. M. J. Am. Chem. Soc. 2009, 131, 17786. (13) Wu, X.; Gong, Z.; Tan, S.; Yang, Y. J. Power Sources 2012, 220, 122. (14) Nugroho, A.; Kim, S. J.; Chung, K. Y.; Kim. Electrochim. Acta 2012, 78, 623. (15) Prakash, A. S.; Manikandan, P.; Ramesha, K.; Sathiya, M.; Tarascon, J. M.; Shukla, A. K. Chem. Mater. 2010, 22, 2857. (16) Fu, L. J.; Liu, H.; Li, C.; Wu, Y. P.; Rahm, E.; Holze, R.; Wu, H. Q. Prog. Mater. Sci. 2005, 50, 881. (17) Burukhin, A.; Brylev, O.; Hany, P.; Churagulov, B. R. Solid State Ionics 2002, 151, 259. (18) Kobayashi, H.; Shigemura, H.; Tabuchi, M.; Sakaebe, H.; Ado, K.; Kageyama, H.; Hirano, A.; Kanno, R.; Wakita, M.; Morimoto, S.; Nasu, S. J. Electrochem. Soc. 2000, 147, 960. (19) Laumann, A.; Bremholm, M.; Hald, P.; Holzapfel, M.; Fehr, K. T.; Iversen, B. B. J. Electrochem. Soc. 2012, 159, A166. (20) Adschiri, T.; Kanazawa, K.; Arai, K. J. Am. Ceram. Soc. 1992, 75, 1019. (21) Aymonier, C.; Loppinet-Serani, A.; Reveron, H.; Garrabos, Y.; Cansell, F. J. Supercrit. Fluids 2006, 38, 242. (22) Okubo, M.; Hosono, E.; Kim, J.; Enomoto, M.; Kojima, N.; Kudo, T.; Zhou, H.; Honma, I. J. Am. Chem. Soc. 2007, 129, 7444. (23) Hald, P.; Becker, J.; Bremholm, M.; Pedersen, J. S.; Chevallier, J.; Iversen, S. B.; Iversen, B. B. J. Solid State Chem. 2006, 179, 2674. (24) Becker, J.; Hald, P.; Bremholm, M.; Pedersen, J. S.; Chevallier, J.; Iversen, S. B.; Iversen, B. B. ACS Nano 2008, 2, 1058. (25) Bremholm, M.; Felicissimo, M. P.; Iversen, B. B. Angew. Chem., Int. Ed. 2009, 48, 4788. (26) Hong, S.-A.; Nugroho, A.; Kim, S.; Kim, J.; Chung, K.; Cho, B.W.; Kang, J. Res. Chem. Intermed. 2011, 37, 429. (27) Cabanas, A.; Darr, J. A.; Lester, E.; Poliakoff, M. J. Mater. Chem. 2001, 11, 561. (28) Jensen, K. C. M.; Tyrsted, C.; Ivensen, B. B. J. Appl. Crystallogr. 2011, 44, 287. (29) Eltzholtz, J. R.; Iversen, B. B. Rev. Sci. Instrum. 2011, 82.

Figure 9. (a) Cycle life of samples with various particle sizes after rate map measurement. (b) Coulombic efficiency corresponding to cycles in (a).

the Coulombic efficiency shows similar scatter corresponding to that on the discharge capacity curve in Figure 9a, it is clear that the 9 nm−99% sample shows a higher average value than the other two samples. This could be one of the reasons that the 9 nm−99% sample possesses the best cyclability performance.

■

CONCLUSIONS In this work, we have successfully used a facile one step reaction to prepare LTO NPs with sizes ranging from ∼2 to ∼20 nm and crystallinity ranging from 50% to 100%. The synthesis was performed in a novel pulsed flow supercritical reactor, which provides strong control of reaction parameters such as temperature, pressure, and residence time. Correlation between crystallite size, crystallinity, and synthesis conditions have been obtained based on detailed characterization with PXRD, SEM, TEM and BET. Lattice expansion was observed for nanosized LTO with decreasing crystalline size. Electrochemical performance measurements suggest that ∼9 nm particles with high crystallinity have better rate ability and cyclability than particles with smaller crystallite size and low crystallinity, and larger particles with high crystallinity. Excellent electrochemical properties were obtained for as-synthesized particles without any post treatment procedures. This emphasizes the strong need for synthesis methods that provide accurate control over particle characteristics.

■

REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

Results of Rietveld refinement, methods for crystalline size and crystallinity calculation, additional TEM images, FTIR and TGA measurements, additional discharge rate maps (PDF file). This material is available free of charge via the Internet at http://pubs.acs.org. 5029

dx.doi.org/10.1021/cm402366y | Chem. Mater. 2013, 25, 5023−5030

Chemistry of Materials

Article

(30) Eltzholtz, J. R.; Tyrsted, C.; Jensen, K. M. O.; Bremholm, M.; Christensen, M.; Becker-Christensen, J.; Iversen, B. B. Nanoscale 2013, 5, 2372. (31) Kataoka, K.; Takahashi, Y.; Kijima, N.; Akimoto, J.; Ohshima, K.-i. J. Phys. Chem. Solids 2008, 69, 1454. (32) Rodriguez-Caravajal, J. http://www.ill.eu/sites/fullprof/. (33) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. (34) Colbow, K. M.; Dahn, J. R.; Haering, R. R. J. Power Sources 1989, 26, 397. (35) Scherrer, P. Nachr. Göttinger Ges. 1918, 2, 98. (36) Ganapathy, S.; Wagemaker, M. ACS Nano 2012, 6, 8702. (37) Wagemaker, M.; Mulder, F. M.; Van der Ven, A. Adv. Mater. 2009, 21, 2703. (38) He, Y. B.; Li, B.; Liu, M.; Zhang, C.; Lv, W.; Yang, C.; Li, J.; Du, H.; Zhang, B.; Yang, Q. H.; Kim, J. K.; Kang, F. Sci. Rep. 2012, 2, 913. (39) Haetge, J.; Hartmann, P.; Brezesinski, K.; Janek, J.; Brezesinski, T. Chem. Mater. 2011, 23, 4384.

5030

dx.doi.org/10.1021/cm402366y | Chem. Mater. 2013, 25, 5023−5030