Structure, Size, and Morphology Control of Nanocrystalline Lithium

Jan 20, 2011 - Kirsten M. Ш. Jensen,† Mogens Christensen,† Christoffer Tyrsted,† Martin Bremholm,†,‡ and Bo B. Iversen*,†. †Center for ...
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DOI: 10.1021/cg101271d

Structure, Size, and Morphology Control of Nanocrystalline Lithium Cobalt Oxide

2011, Vol. 11 753–758

Kirsten M. Ø. Jensen,† Mogens Christensen,† Christoffer Tyrsted,† Martin Bremholm,†,‡ and Bo B. Iversen*,† †

Center for Materials Crystallography, Department of Chemistry and iNANO, Aarhus University, DK-8000 A˚rhus C, Denmark, and ‡Department of Chemistry, Princeton University, Princeton, New Jersey 05844, United States Received September 28, 2010; Revised Manuscript Received December 19, 2010

ABSTRACT: Hydrothermal synthesis of nanocrystalline LiCoO2 from CoOOH and LiOH is studied by in situ powder X-ray diffraction. The study confirms that transformation from CoOOH to LiCoO2 takes place through a dissolution-recrystallization process, where the dissolution step is rate determining. Therefore, a high solubility of CoOOH is needed to ensure a fast reaction, and this can be achieved by increasing the temperature or the pH value. The growth rates and the final sizes of the LiCoO2 nanoparticles are observed to be highly dependent on the synthesis temperature. The nanoparticles are disk shaped, and the in situ data show that the degree of anisotropy in the morphology is temperature dependent. Thus, not only particle size but also morphology is controllable. The in situ data also show that at low temperature and low LiOH concentration, the Fd3m polymorph of LiCoO2 is first crystallized, but sustained reaction leads to a transformation to the R3m polymorph. At other conditions the R3m polymorph forms directly within the time resolution (10 s). Finally, the unit cell is observed to change significantly with particle size. However, at low LiOH concentrations peculiar anisotropic unit cell changes are observed, which cannot be explained by particle size effects or Li defects.

*To whom correspondence should be addressed. E-mail: þ45 89423969 [email protected].

To obtain control over the nanoparticle characteristics, it is important to understand the chemical reactions governing the formation of LiCoO2. In a defining study, Okubo et al. established a size controlled synthesis of LiCoO2 nanoparticles using hydrothermal methods, and based on ex situ data, they proposed that the material is formed through a dissolution-recrystallization process.6 In ex situ studies the details of the formation reaction are inferred from the observed products. It is clearly of interest if one could obtain a confirmation (or rejection) of the dissolution-recrystallization proposal based on data measured while the reaction takes place, that is, based on in situ data. Here, we report on in situ time-resolved synchrotron powder X-ray diffraction (SR-PXRD) studies of hydrothermal synthesis of LiCoO2 from solid CoOOH suspended in LiOH. The in situ study follows the formation reaction of LiCoO2, while it takes place and the data directly provide information not only on the formation mechanism, but also on the evolution of particle size and morphology. Because of the excellent time resolution hundreds of data points become available e.g. for correlations between particle sizes and unit cell sizes, which are often drawn based on a few points. In situ X-ray studies have become increasingly applied in the study of hydrothermal and solvothermal reactions within the past two decades.18-25 In recent years, we have done extensive studies of the formation and growth of nanoparticles such as Fe3O4,26 ZrO2,27 CexZrx-1O2,28 AlOOH,29 and Bi2Te330 using different custom designed reactors.31,32 A unique feature of our reactors is their ability to withstand very high pressures and high temperatures, and this allows studies of reactions both in sub- and supercritical water conditions. It should be noted that the present experimental setup does not allow representative samples to be collected at specific times for later ex situ microscopy characterization.

r 2011 American Chemical Society

Published on Web 01/20/2011

Introduction Lithium cobalt oxide is one of the most widely used materials for Li-ion battery cathodes because of its high capacity, high voltage and excellent cyclability, and for this reason, LiCoO2 has been studied extensively for the past two decades.1-3 Lithium-ion batteries are already widely used in portable electronics, but for applications, for example, in hybrid and electric vehicles, a very high charge/discharge rate is required, and here the slow diffusion of Li-ions in the LiCoO2 structure is problematic. The use of nanoelectrodes is a way to reduce this problem, as the small particle size shortens the diffusion pathway for Liþ.4,5 However, nanosizing the LiCoO2 particles can influence other properties, such as the electrochemical capacity, and extreme size reduction is therefore not always favorable.6,7 For this reason, size control of LiCoO2 is crucial during synthesis. Furthermore, the morphology of the particles can also be expected to play a role in the battery properties of the compound. The diffusion in the LiCoO2 structure takes place in the Li-ion layers, parallel to the ab plane. Optimally, the particles should therefore be shortest along the ab direction (to achieve a short Liþ diffusion pathway) and have an extended length along the c-direction (to have more layers). So far, morphology control has not been described in the literature. Many different synthesis methods have been used to produce nanocrystalline LiCoO2 including sol-gel methods,8,9 coprecipitation methods,10 as well as both batch and flow hydrothermal methods.11-17 Hydrothermal synthesis using a flow reactor is particularly attractive, since this is a fast, cheap and efficient way to produce small particles of high crystallinity.

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Figure 1. (Left) Experimental setup. (Right) Contour plots of the time-resolved synchrotron PXRD data for the experiment carried out at 160 C with Li/Co = 1.3.

Experimental Section Experimental Setup. The in situ X-ray measurements were carried out at beamline I711, MAXII, MAX-lab, Sweden, during two different beam time periods, and a schematic view of the setup used is shown in Figure 1. The experimental setup has been described in detail elsewhere.32 The reactor is a sapphire capillary with inner and outer diameters of 0.7 and 1.6 mm, respectively. The capillary is pressurized with water, while the heating is done by a jet of hot air. The small volume of the capillary ensures fast heating rates, which makes the synthesis conditions mimic the conditions in a hydrothermal flow reactor. The X-ray wavelength was 0.998 A˚ during the first beam time period and 0.997 A˚ during the second. A beam size of 400*400 μm2 was used, and the MAR165 CCD detector was placed ∼9.5 cm from the capillary during the first beam time and ∼9.0 cm during the second. The readout time in all experiments was about 6 s, and with an exposure time of 4 s the time resolution was ∼10 s. Since no internal standard was used, the absolute values of the unit cell parameters are not available, and all conclusions are based on relative changes in the refined values. Precursor Preparation. The precursor consisted of CoOOH suspended in aqueous LiOH. The CoOOH particles were synthesized prior to the synchrotron experiments using the method reported by Okubo et al.6 For this synthesis, aqueous Co(NO3)2 (100 mL, 0.2 M) was slowly added to a solution of NaOH (100 mL, 5.0 M). The resulting pink suspension of Co(OH)2 was diluted to 2 L, and stirred for 24 h in air, during which the color changed to dark brown. The suspension was centrifuged and the product was washed twice with water. The particles were dried at 80 C for 3 h, resulting in crystalline CoOOH. The precursor suspension was prepared by adding 0.919 g CoOOH to a syringe containing 10 mL of LiOH solution. Two different LiOH solutions with concentrations of 0.533 and 4 M, respectively, were used in order to obtain Li/Co molar ratios of 1.3 and 10. A small magnet was introduced in the syringe, and the precursor suspension was injected to the reactor under constant stirring. The magnetic stirring of the suspension in the syringe during injection ensures that the Li/Co ratio in the capillary is the same as in the syringe. After injection of the precursor, the system was pressurized. When the pressure was stable at 250 bar, the heat gun was turned on simultaneous with initiation of the X-ray exposure. Experiments were carried out at three different temperatures, 160, 200, and 250 C, and in all cases, the pressure was fixed at 250 bar. Overall, five different experiments were carried out. During the first beam time period data were collected with Li/Co = 1.3 at 160 and 250 C, and with Li/Co = 10 at 200 C. During the second beam time period data were collected with Li/Co = 10 at 160 and 250 C. Data Treatment. The wavelength and detector distance were determined by calibration with a NIST LaB6 sample using the program Fit2D.33 The raw data frames were integrated in Fit2D, and subsequently analyzed by Rietveld refinement using the FullProf suite.34 Sequential refinements allowed extraction of the phase composition, unit cell, particle size and shape. The refinements employed the 2θ-range from 8 to 38. The background was modeled using linear interpolation with 28 parameters. The refinements were done based on the structure of HT-LiCoO2 in the spacegroup R3m, though modeling using the LT-LiCoO2 structure in the Fd3m space group gave comparable R-values. Structural information on the two phases (unit cell parameters, atomic positions and thermal parameters) was

Jensen et al. taken from powder diffraction file (PDF) number 01-072-2280 (CoOOH) and 01-072-8323 (LiCoO2). The scale factors, the unit cell parameters and the peak profile parameters were refined. Because of the limited q-range, the atomic positions and thermal parameters were held fixed. Representative fits to the data can be found in the Supporting Information. The peak broadening was used for particle size determination, and the Thompson-Cox-Hasting formulation of the pseudo-Voigt function was applied to describe the peak profiles.35 Before analysis of the sample broadening, the total peak broadening was corrected for the instrumental contribution, which was determined by peak shape analysis of a diffraction pattern measured on the NIST LaB6 sample. Sample broadening can arise from particle size, lattice strain and stacking faults, but due to the small size of the particles, the peak broadening was assumed to stem purely from the particle size, and other contributions were neglected. The sample broadening can then be described by the Scherrer formula,36 β = (K 3 λ)/(ÆDæ 3 cos(θ)), where ÆDæ is the volume weighted crystallite domain size, which here is interpreted as the particle size. For anisotropic nanoparticles, the particle shape needs to be included in the model, as the peak broadening in this case is hkl-dependent. The particle shape can be modeled using the spherical harmonic functions, and the peak broadening in different crystallographic directions can be described by,34,37 βðΘ, ΦÞ ¼

K 3λ K 3λ X ¼ A Ynml ðΘ, ΦÞ ÆDðΘ, ΦÞæ 3 cosðθÞ cosðθÞ nml 3

where Anml are the refinable coefficients (i.e., the peak shape parameters) and Ynml are the spherical harmonic functions. The LiCoO2 and CoOOH structures both belong to the 6/mmm Laue class, and three functions allowed by symmetry, Y00, Y20, and Y40, were used. In the present case the particle size in different directions can therefore be calculated by: ÆDðΘ, ΦÞæ ¼

1 A00 Y00 ðΘ, ΦÞ þ A20 Y20 ðΘ, ΦÞ þ A40 Y40 ðΘ, ΦÞ

Further details are given in the Supporting Information. For all the diffraction patterns, the scale factors for both phases were refined. The peak shape and the unit cell parameters were only refined for one phase at a time. Thus, when more than ∼50% of LiCoO2 was present, the peak shape and the unit cell parameters for LiCoO2 were refined, whereas only the scale factor for CoOOH was refined. This constraint was necessary because the two structures, CoOOH and LiCoO2, have quite similar powder diffraction patterns and therefore many overlapping peaks within the observed q-range.

Results and Discussion Formation Reaction. To understand the reaction mechanism, we investigated the effect of temperature and Li/Co ratio on the formation rate. The time-resolved SR-PXRD data obtained from the experiment done at 160 C with Li/ Co=1.3 are shown in Figure 1. Initially, diffraction peaks from the crystalline precursor CoOOH are present, but after approximately 20 min, the reaction starts, and peaks from LiCoO2 emerge. Figure 2 shows the weight fractions of LiCoO2 as function of time for two different Li/Co ratios at T = 160 C. The LiOH concentration clearly has a huge impact on the reaction rate. For the low LiOH concentration, the reaction takes more than 90 min, while the transformation is complete in less than two minutes when using the high LiOH concentration. The mechanism of the reaction can be studied by following the size of the CoOOH particles. Figure 3 shows the particle size in the c-direction, ÆDcæ, along with the weight percentage of CoOOH and LiCoO2 as function of time. Initially, the CoOOH particle size increases, but then it starts decreasing simultaneous with the onset of the transformation to LiCoO2.

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Figure 2. Weight percentage of LiCoO2 as function of time at a temperature of 160 C. Squares show the results from Li/Co = 1.3 and circles Li/Co = 10. The lines are guides to the eye. For clarity, only every third data point is shown after t = 2 min.

Figure 3. (Upper plot) The weight percent of LiCoO2 and CoOOH as function of time. (Lower plot) The size of the CoOOH particles in the crystallographic c-direction. For clarity, only every third data point is shown. The data were obtained with Li/Co = 1.3 and T = 160 C.

The in situ experiment thereby confirms that the reaction is a dissolution-recrystallization process as suggested by Okubo et al. based on ex situ data,6 and the data reject the earlier suggested solid state transformation.15 The first step in the reaction, i.e. the dissolution of the CoOOH particles, controls the rate of the reaction. A high solubility of CoOOH is therefore necessary for the reaction to occur quickly. The solubility of CoOOH increases with pH, and consequently, the dissolution is faster for the experiments done with high LiOH concentration. This is directly seen in the large difference in reaction rate between the experiments done with low and high LiOH concentration, Figure 2. Another way of increasing the solubility of the CoOOH particles is to increase the temperature, as shown in Figure 4, where the weight percentage of LiCoO2 is plotted as function of time for three different temperatures. The time before the reaction starts is highly temperature dependent, whereas the rate of the reaction is almost constant. Particle Growth. The particle growth curves are shown in Figure 5. The LiCoO2 particles start growing immediately after the nucleation, but after a short time, the growth almost stops. The growth rate and the stabilized particle size are highly temperature dependent. The particles are disk shaped, i.e. elongated in the a- and b-direction compared with the

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Figure 4. Weight percentage of LiCoO2 as function of time for experiments done at 160 (triangles), 200 (crosses) and 250 C (circles) with Li/Co = 10. The lines are guides to the eye.

c-direction. This is consistent with earlier ex-situ work done on the hydrothermal synthesis of LiCoO2 particles.6,17 Figure 5 clearly illustrates that the thickness to diameter ratio of the disk shaped particles is temperature dependent. The in situ data clearly show that the particle size can be precisely controlled by synthesis time and temperature. This was also documented in earlier ex situ studies.6 However, in addition the in situ data also show that the particle morphology, i.e. the aspect ratio of the anisotropic disk shaped particles, is temperature dependent, and therefore controllable. Size control during synthesis is important for the applications of the materials since the cathode properties are highly size dependent. Presumably, the morphology control is also of very large interest when considering the properties of the particles because the diffusion of Liþ happens in the ab plane of the structure. The morphology of the disk shaped particles that form during hydrothermal synthesis is not favorable, but if the particles can be elongated in the c-direction better performance is expected, as this increases the number of layers for Liþ diffusion. The in situ data suggest that elevating the synthesis temperature makes the Li diffusion easier in the produced material. Ongoing efforts concentrate on reproducing the in situ results in larger scale in a flow reactor and subsequently carry out electrochemical characterization of the materials. To gain further understanding of the particle growth, analysis of the growth curves was undertaken using kinetic models. The growth of the nanoparticles often proceeds by several mechanisms depending on the specific system and the local concentration of precursor in the solvent. Semiquantitative information about the mechanisms can be extracted by applying a generalized N-exponential growth model, D(t) = D0 þ k(t - t0)N, where D is the size of the particle and t is time. D0 is the particle size at time t0, and k is a constant dependent on the probed microenvironment.38 The value of N provides information about the growth mechanism. If the surface reaction between the precursor and growing particles is fast, then the growth is usually diffusion limited. In this case, the Lifshitz-Slyozov-Wagner (LSW) theory states that the volume of the particles increases linearly with time, and consequently N should be close to 1/3. If the surface reaction kinetics is the limiting factor, then the surface of the particles increases linearly with time and N is close to 1/2.39 The fitted parameters obtained from the growth curves are listed in Table 1, and the fits are shown along with the data in Figure 5. For the highest

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Figure 5. (Left) Particle size along the crystallographic c-direction, ÆDcæ, and a-direction, ÆDaæ, as function of time for experiments done at 160 (circles), 200 (crosses), and 250 C (triangles) using Li/Co = 10. For clarity, only every third data point is shown after t = 2 min. The curves represent the kinetic models fitted to the data. (Right) The aspect ratio, ÆDcæ/ÆDaæ of the LiCoO2 particles as a function of time (using Li/Co = 10) at 160 C (circles), 200 C (crosses), and 250 C (triangles). For clarity, only every third data point is shown after t = 2 min. Table 1. Parameters Obtained by Fitting Kinetic Models to the Growth Curves, D(t) = D0 þ k(t - t0)N, According to the LSW Theory growth direction

T [C]

time interval [min]

N

a c a c

160 160 200 200

1.4-30.0 1.4-30.0 1.0-10.0 1.0-10.0

0.353(7) 0.329(3) 0.39(2) 0.42(1)

temperatures, the growth is too fast to obtain reliable fitting results. For the low-temperature experiment (Tcap = 160 C), the N-parameter is very close to 1/3 for both growth directions, and the growth therefore appears to be limited by diffusion, that is, the amount of precursor around the particle nuclei. However, as the temperature is increased to 200 C, N increases to 0.39 and 0.42 in the a and c directions, respectively. Thus, the growth is no longer purely limited by diffusion, but surface reaction effects start to be important. The difference between the growth mechanisms is related to the dissolutionrecrystallization process. When the temperature is low, the dissolution proceeds slowly, and the amount of precursor for the formation of the LiCoO2 particles is therefore limited. At higher temperatures, the dissolution can proceed much faster, and therefore, the diffusion of precursor no longer limits the formation of the LiCoO2 particles. Structure and Unit Cell Parameters. Stoichiometric LiCoO2 exists in two different polymorphs. The high temperature polymorph, HT-LiCoO2, is a layered structure, described in space group R3m, whereas the low temperature polymorph, LT-LiCoO2 is a spinel-like structure, where the positions of the cobalt ions deviate from the ideal spinel structure. This structure is usually represented in the Fd3m space group, but it is also describable in R3m as a disordered layered structure. For this reason it is difficult to distinguish between HT- and LT-LiCoO2 by Rietveld refinement.40 However, the ratio of the unit cell dimensions, that is, c/a, provides a way of discriminating between the two structures. For the spinel-type structure, this ratio is about 4.9, whereas it is 5.0 for the layered structure.40 In Figure 6 the c/a ratio is plotted versus time. For the experiment done at 160 C with Li/Co = 1.3 the c/a ratio is initially close to 4.9, but then it increases to ∼5.0. This indicates that LT-LiCoO2 forms and subsequently transforms into HT-LiCoO2. This can be understood as a disorder-order

Figure 6. Ratio between c/a as function of time for different synthesis conditions.

transformation; in the initially formed particles, the Liþ and Co3þ ions are disordered, but with time, the ordered layers of Liþ and Co3þ in the HT-structure form. This transition is only observed for the experiment conducted at low temperature with low LiOH concentration. All other experiments have c/a ratios close to 5.0, that is, we do not observe the disordered LT-structure within the time resolution of the experiment. The unit cell parameters change during the growth of the particles. In Figure 7a, the development of the a axis is plotted as function of particle volume. The volume of the particles has been calculated assuming a cylindrical shape, using ÆDcæ (the size along c) as the height, and ÆDaæ (the size along ab) as the diameter. The a axis decreases with increasing particle size. This was also reported by Okubo et al. based on a limited number of data points from ex situ experiments,6 and the effect was explained as stemming from surface effects. However, a completely different trend is observed for the c axis. As seen in Figure 7b, the development of the c-axis length for experiments done at high LiOH concentration agree with the surface theory described by Okubo et al.,6 but the opposite behavior is observed for the experiments done at low LiOH concentration. The latter development in the c axis length may not be related to particle size. Since it is only observed for the particles synthesized using low LiOH concentrations it could be related

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Figure 7. (Left) Relative a-axis value (a - afinal) as function of particle volume. (Right) Relative c-axis value (c - cfinal) as function of particle volume. For clarity only every third data point is shown.

to the Li-content in the structure. However, a lower Li content gives a longer c axis due to repulsion between the oxygenlayers.1 Therefore, the behavior of the c-axis cannot be explained by a nonstoichiometric LiCoO2 phase with Li deficiency. Li-deficiency has been shown to be the defect of the lowest energy,41 but other defects have also been reported in the literature. These include oxygen dislocations and deficiency.3 However, were are not able to determine if these defects are responsible for the structural changes observed, as the data does not allow us to refine the occupancies of Liþ and O2-. The effect is very clear in the data, but it is not possible based on PXRD Rietveld refinements to properly understand its origin. Other characterization methods are needed to fully understand the structural changes taking place. In situ neutron diffraction experiments would potentially make it possible to refine Li occupancy and disorder. Furthermore, in situ pair distribution function studies using both X-rays and neutrons could also provide additional insight into the subtle structural changes, which may be distinct from long-range ordering. Conclusions In situ time-resolved SR-PXRD was applied to study the hydrothermal synthesis of LiCoO2 from CoOOH and LiOH. The data follow the reaction as it occurs, and they confirm that the reaction from the nanocrystalline CoOOH particles proceeds as a dissolution-recrystallization process and not by a solid state transformation. It is furthermore established that the dissolution-step is rate determining. The rate of the reaction can be controlled by adjusting the LiOH concentration or the temperature to increase the solubility of CoOOH. Not only the size of the LiCoO2 nanoparticles, but also the morphology (aspect ratio of disk shaped nanoparticles) can be precisely controlled by adjusting the synthesis time and temperature. This could be a highly important new insight with regard to further improving the performance of Li ion batteries since the Liþ diffusion takes place in the a,b plane. The optimal nanoparticle therefore will have minimal in-plane size to shorten the diffusion path and a maximal size along the c-axis to attain more layers. Kinetic models were applied to the observed particle growth, and they showed diffusion limited growth below 200 C in agreement with the dissolutionrecrystallization mechanism. Above 200 C the reaction

becomes increasingly surface growth limited. Plots of the unit cell c/a ratio reveal that at low temperature and low LiOH concentration the Fd3m polymorph of LiCoO2 is initially formed but sustained reaction leads to a transformation to the R3m polymorph. At other conditions, the R3m polymorph forms immediately. The unit cell a axis is observed to decrease with increasing particle volume, and at higher Li/Co ratios this is also observed for the c-axis. However, at low Li/Co ratio the c-axis increases with increasing particle size leading to highly anisotropic crystal structure changes with particle size. This peculiar effect cannot be explained by Li deficiency in the crystal structure and other types of in situ data (e.g., in situ neutron diffraction or PDF analysis) are required to address this question. Acknowledgment. The work was supported by the Danish Strategic Research Council (CEM), The Danish National Research Foundation (CMC), the Danish Research Council for Nature and Universe (Danscatt) and SCF Technologies A/S. MAX-lab is acknowledged for beamtime, and Yngve Cerenius and Dorthe Haase is thanked for support during beamtime. Jacob Becker-Christensen is thanked for assistance during the experiments. Supporting Information Available: Two dimensional PXRD data, selected one-dimensional PXRD data with corresponding Rietveld refinements results, example of size determination from refined peak shape parameters. This material is available free of charge via the Internet at http://pubs.acs.org.

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