Article Cite This: J. Phys. Chem. C 2018, 122, 18795−18801
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Morphology and Electrochemical Response of LiFePO4 Nanoparticles Tuned by Adjusting the Thermal Decomposition Synthesis Pablo S. Martinez,†,‡ Enio Lima, Jr.,† Fabricio Ruiz,§ Javier Curiale,†,‡ and M. Sergio Moreno*,† †
J. Phys. Chem. C 2018.122:18795-18801. Downloaded from pubs.acs.org by UNIV FRANKFURT on 08/25/18. For personal use only.
Instituto de Nanociencia y Nanotecnología CNEA-CONICET, Centro Atómico Bariloche, Av. Bustillo 9500, 8400 San Carlos de Bariloche, Argentina ‡ Instituto Balseiro, Universidad Nacional de Cuyo, CNEA, Av. Bustillo 9500, 8400 San Carlos de Bariloche, Argentina § Centro Atómico Bariloche, Av. Bustillo 9500, 8400 San Carlos de Bariloche, Argentina ABSTRACT: Development of nanostructured electrodes for application in Li-ion batteries implies the control of particle morphology and size distribution. Using the high-temperature thermal decomposition synthesis for different temperatures and changing the reactant concentration, we obtained highly crystalline LiFePO4 nanoparticles with different morphologies and mean size. Our results show that an increase of the temperature produces an increase in crystallinity and size of the particles, whereas an increment in reactant concentration generates a slightly increase of the particle size without changes in morphology. The electrochemical capacity of annealed samples is enhanced for those synthesized at higher temperatures, indicating that changes induced by changing the synthesis temperature, as for example differences in the morphology of the particles, are important.
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particle size.12 In a previous work, we have used this method to produce highly crystalline LiCuxFe1−xPO4 nanoparticles with a mean size in the range of 30−40 nm and different amounts of Cu (x = 0, 0.001 and 0.042).13 Jiang et al.11 also used a similar decomposition method with different synthesis conditions to obtain faceted LiFePO4 nanoparticles with a mean size of 80 nm. This kind of method seems very attractive for the production of LiFePO4 nanoparticles, although a study about the effect of the reaction variables results necessary. Here, we compare the morphology and electrochemical response of lithium iron phosphate (LiFePO4) nanoparticles obtained by high-temperature thermal decomposition of organometallic precursors at two different temperatures. We also explore the effect of the concentration of the precursors in the morphology of the system while maintaining the temperature constant. We have studied as-made and annealed samples by X-ray diffraction (XRD), electron diffraction, and transmission electron microscopy (TEM). The Fourier transform infrared (FTIR) technique was used to analyze the organic layer in the as-made sample. Electrical transport as a function of the thermal history of the nanoparticles was studied by impedance spectroscopy (IS). Charge/discharge cycles technique was used for electrochemical capacity determination
INTRODUCTION The use of nanostructured materials in electrodes of lithium ion batteries has become fundamental because of their advantages to overcome typical problems associated with the larger scale of bulk materials used in batteries. Some of these advantages include the decrease of the diffusion distance for lithium ions, and consequently reducing hysteresis effects, enhancement of the electron transport, and a large surface area that improves the contact with electrolyte solution.1,2 Taking into account these advantages several methods have been explored for the production of nanoparticles, some of the most commonly used methods are the solid state,3 hydro/ solvothermal4 and sol−gel5 among others. Since the discovery of lithium iron phosphate (LiFePO4) as cathode material,6 it became one of the most attractive materials for lithium ion batteries because of properties such as a reasonable high capacity (170 mA h g−1) and flat voltage profile, it is environmental friendly, and has lower cost than other cathodes materials.7 However, LiFePO4 has as principal drawback the low electrical and ionic conductivity. Many solutions to these disadvantages have been proposed, for example, carbon coating,8 cation doping,9 particle size reduction,10 improved crystallinity,11 and so forth. Following the strategy of reducing the particle size to improve the LiFePO4 performance, the high-temperature decomposition of organometallic precursors is a widely used method to produce metal-oxide nanoparticles with a tunable © 2018 American Chemical Society
Received: April 25, 2018 Revised: August 1, 2018 Published: August 3, 2018 18795
DOI: 10.1021/acs.jpcc.8b03908 J. Phys. Chem. C 2018, 122, 18795−18801
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The Journal of Physical Chemistry C
The TEM experiments were carried out using a Tecnai F20 G2 operated at 200 kV and room temperature. The TEM specimens were prepared by diluting the nanoparticles in chloroform and dropping it on a copper grid with an ultrathin carbon film. Sample crystallinity was analyzed by inspection of the fast Fourier transform (FFT) of single particles using the standard tools implemented in DigitalMicrograph software. Kinetic TGA−DTA was performed in a Shimadzu DTG60H, in a temperature range from room temperature to 1073 K using heating rates of 1, 2, 5, 7, and 10 K min−1. Transport measurements were performed on the sample L653 with four different histories: as-made (coated with organic layer), fat-free (sample as-made without the organic layer), as-made annealed at 673 K for 2 h, and as-made annealed at 973 K for 4 h. For the annealing treatment, we used a heating rate of 5 K min−1 in an Ar/H2 (10%) atmosphere. IS was measured at room temperature with a precision impedance analyzer Agilent 4294A for the frequency range of 40 Hz to 110 MHz. The equipment worked without bias voltage and with an oscillator amplitude of 1 V. The impedance, defined as Z = Z′ + jZ″, was normalized by its geometrical factor. Samples for IS measurements consisted of cylindrical pellets of 7 mm diameter and typical thickness of 200 μm, obtained by uniaxially pressing the nanoparticles with 8 tons in a hydraulic press. On each face of the pellets, electrical contacts were made using silver paste (4929N from DuPont). The thermal treatment of samples coated with organic layers produces a graphitic surface which improves the conductivity. For this reason, the electrochemical measurements were performed in the annealed sample at 973 K because of its higher conductivity. The resultant powders were mixed with conductive carbon black and polyvinylidene fluoride binder in a ratio 80:10:10, respectively, in N-methyl-2-pyrrolidone solvent. A three-electrode system cell (Swagelok type) was used for these measurements. Aluminium foil was used as current collector for the cathode, and lithium foils were employed as anode and reference electrode. The electrolyte was 1 M LiPF6 in ethylene carbonate/dimethyl carbonate solution (1:1 in volume). The cell was assembled in an argonfilled glovebox and tested electrochemically in cycles of charge/discharge in a voltage range of 4.2−2.5 V at 0.1 C. Also, cyclic voltammetry (CV) at 0.1 mV s−1 scan rate between 4.2 and 2.5 V was performed. All electrochemical measurements were carried out in a Gamry Interface 1000 potentiostat.
of the annealed samples. The activation energy of the chemical reaction also was obtained from kinetic measurements of the thermal decomposition of the precursors by thermogravimetric and differential thermal analyses (TGA−DTA).
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EXPERIMENTAL SECTION Lithium acetylacetonate (Li(acac)), iron(III) acetylacetonate ( F e ( a c a c ) 3 ) , a n d a m m on i um p h os p h a te d i b a si c ((NH4)2HPO4) were used as precursors. The solvents used were oleic acid (C18H34O2) and oleylamine (C18H37N). The precursors, in a molar ratio of 1:1:1, were put together with the high-temperature solvents into a three-neck flask connected to a condenser and under nitrogen atmosphere. In Figure 1, the
Figure 1. Temperature profile for sample synthesized at 653 K. Dehydration of system and reflux of solvent are indicated with arrows over their respective temperature plateaus. The nucleation, growth, and crystallization time windows are also indicated.
temperature profile used for the samples obtained at 653 K is shown. In this figure, the dehydration of the system at 373 K, the reflux of solvent at 653 K, and different synthesis steps are indicated, specifically the nucleation, growth, and crystallization of particles. After that the heat source was removed and the solution was allowed to cool down to room temperature. After the reaction, ethanol was added to the solution in a ratio of 8:1 (ethanol/sample) and this mixture was centrifuged to precipitate the particles. The precipitated powder was washed with acetone in an ultrasound bath for 10 min. As expected from the synthesis procedure, the samples were coated with a monolayer of oleic acid/oleylamine, making them easily dispersible in organic solvents because of their strongly hydrophobic character. According this procedure, three samples were synthesized: L653, synthesized in a reflux condition at 653 K during 120 min; L653d, at the same time and temperature but with a double relation of [solvents/precursors] (dilute sample), and L573 synthesized at 573 K during 120 min, with the concentration of precursors similar to the sample L653. For one sample obtained at 653 K the organic layer was removed by acetone and ultrasound bath (10 min) at 313 K. This sample was labeled as washed sample (fat-free). Powder X-ray diffractograms were obtained in a PANalytical Empyrean diffractometer, using Cu Kα radiation (λ = 0.154 nm). The FTIR spectra were acquired using a PerkinElmer “spectrum two” equipment with a universal ATR optical system.
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RESULTS AND DISCUSSION In Figure 2, the TGA−DTA for 7 K min−1 heating rate (similar to the heating rate used for the syntheses of the three samples) is shown. In the TGA curve, there are two zones where an important mass loss can be identified. On the one hand, the mass loss around 600 K is related to the solvent evaporation, and on the other hand, the loss in the 400−550 K range corresponds, according to our DTA−TGA analysis (Figure 2) and other authors,14,15 to the precursor decompositions. For this temperature range, the DTA curve does not show peaks associated with an exo/endothermic process further than the corresponding to mass loss. The inset in Figure 2 corresponds to the dTGA/dT versus T curve for this temperature range in which a two-peak structure is found. These peaks are related to the precursor decompositions, where peak 1 belong to Fe(acac)3 and peak 2 to Li(acac). The temperature of each 18796
DOI: 10.1021/acs.jpcc.8b03908 J. Phys. Chem. C 2018, 122, 18795−18801
Article
The Journal of Physical Chemistry C
Figure 2. TGA−DTA curves of L653 sample obtained at 7 K min−1 heating rate. Inset: dTGA/dT vs T (open circles) and fitting (solid line) using curves 1 and 2 (dot).
peak (Tpeak) is estimated by fitting the curve with two Gaussian functions (1 and 2 in the inset) where Tpeak corresponds to the maximum reaction rate. From these results we fixed the minor temperature for the study of synthesis as the highest temperature decomposition of the precursors, near 573 K, and the maximum temperature, 623 K, corresponding to the solvent reflux temperature. We also calculated the activation energy (Ea) of the reaction from the variation of Tpeak with the heating rate (β) based on the Kissinger method.16 Specifically, Ea is calculated from the slope of plotting ln(β/Tpeak2) versus 1/Tpeak, according to eq 1
Figure 4. X-ray diffractograms of as-made and annealed (labeled with an R) samples, obtained at 573 (a), 653 (b), and 653 K, diluted sample (c).
crystal structure of LiFePO4. All the samples synthesized at 653 K present a predominant amount of the LiFePO4 phase with other minority phases, as is detailed in the following: for L653 sample, (Figure 4b), one peak of Li3PO4 phase is found while for the annealed sample small peaks of FeP and Fe2P2O7 phases are also present. In the case of the as-made diluted sample, peaks of Li3PO4 and FeLi5O4 phases are observed. The corresponding annealed sample presents small peaks of FeP and Li4P2O7 impurity phases, as indicated in (Figure 4c). The reductive environment of the synthesis may generate some of these impurities. For instance, Fe2P2O7 is generated in LiFePO4 deficient in lithium (LixFePO4 with x = 0.7− 0.95).17 The formation of FeP is produced by the reduction of Fe2P2O7.18 The reaction of the carbon with LiFePO4 results in the oxidation to CO or CO2, and the reduction of nearby atoms of Fe and P generates Fe 2P or Fe 3P.19 The formation of an insulating phase, Li4P2O7 in LiFePO4 is attributed to the excessive formation of Fe 2P.20 As it was mentioned before, the synthesized particles are coated with an organic layer which will be converted, through an annealing, into the carbon layer necessary to improve the electrochemical performance of this material. The presence of this organic layer was detected through FTIR technique, as it is shown in Figure 5 for the diluted sample. The washed (fatfree) and annealed sample spectra are also included for comparison. From this figure, it is possible to see an important difference between curves in the region corresponding to CH2 peaks. The as-made sample shows peaks with big intensity, whereas in the case of the washed sample the peaks almost disappear because most of the organic layer was eliminated with the washing. The annealed sample, however, does not show these peaks because the organic layer was converted into a carbonaceous layer. A similar behavior is found for carbonoxygenated functional groups. The as-made sample exhibit remarkable peaks that in the washed sample are diminished and in the annealed sample are not detectable.
ij β yz i y zz = lnijjj RA yzzz − jjj Ea zzz lnjjjj z j jj E zz jj RT zzz j Tpeak 2 zz k a{ (1) k peak { k { where A is the pre-exponential factor and R is the gas constant. The linear fitting of temperature peaks 1 and 2 for all heating rates, according to eq 1, is presented in Figure 3. For peak 1 the slope is −19.5 K and Ea1 is 161.85 kJ mol−1. On the other hand, for peak 2 the slope of the fitting curve is −14.5 K and the Ea2 is 117.86 kJ mol−1. Figure 4 shows the XRD patterns of the as-made and annealed (labeled R) samples. The diffractogram of L573 (Figure 4a), does not show any reflections, similar to the diffractogram of an amorphous material. For the annealed sample, however, the reflections correspond only to the olivine
Figure 3. Plot of ln(β/Tpeak2) vs 1000/Tpeak, for the temperature peaks obtained from (dTGA/dT) vs temperature curves, for different heating rates (β = 1, 2, 5, 7, and 10 K min−1). 18797
DOI: 10.1021/acs.jpcc.8b03908 J. Phys. Chem. C 2018, 122, 18795−18801
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The Journal of Physical Chemistry C
graphitic-like carbon layer surrounding the particles with a thickness of ca. 3 nm. The sample L653 obtained at 653 K (Figure 6c) consists of particles with rectangular and hexagonal morphologies with a mean size of 89 nm. When this sample is annealed (Figure 6d), the mean size increases to 185 nm and exhibit a disordered graphitic carbon layer over its surface, as is possible to see in the inset. In Figure 6e, the diluted sample synthesized at 653 K is shown. The morphology of the particles is similar to the L653 sample although the mean size of 57 nm is slightly smaller. This result is consistent with the fact that less concentrated solution produces a smaller particle size. Interestingly for the corresponding annealed sample (Figure 6f), a bimodal size distribution is found with mean particle sizes of 48.5 and 161 nm, showing that a fraction of the initial particle size is preserved. In addition, it is possible to see a disordered graphitic carbon layer surrounding the particles. Figure 7 presents the Bode plot (electrical impedance) for the nanoparticles synthesized at 653 K: as-made, fat-free (sample without the organic layer), annealed at 673 K for 2 h and annealed at 973 K for 4 h. As it can be observed, there are major changes in agreement with the sample history: the higher impedance is observed for the fat-free sample, with impedance response similar to the expected for the LiFePO4 system;13 it is followed by the as-made nanoparticles, where the organic layer plays some role in the impedance values as well as in the cut-off frequency; in the sequence, the sample annealed at 673 K (2 h) evidences an effect of the annealing on the impedance of the sample, probably caused by changes in the organic layer. Finally, the sample annealed at 973 K (4 h), where the particles are also coated with a carbon layer, present a strong reduction in the impedance.
Figure 5. FTIR spectra of L653d for as-made, washed (fat-free) and annealed samples. Characteristic peaks of functional groups related with oleic acid (solvent), CH2, CO, COO− and −CO, are shown, and peaks corresponding to PO43− are also indicated.
In Figure 6, TEM images and particle size distribution of all samples are shown. In L573 sample (Figure 6a), all the nanoparticles have a circular shape with a mean size of 8 nm. The electron-diffraction pattern clearly shows reflections indicating that the nanoparticles are crystalline. In the XRD diffractogram, no reflections were observed. This could be attributed to the small size of the particles and the organic medium they are immersed making impossible to detect any reflection in the XRD pattern. The corresponding annealed sample is shown in Figure 6b. It is possible to see that particles have grown, reaching a mean size of 33 nm as can be seen in the size distribution histogram, and also they form agglomerates. In the inset we can observe the presence of a
Figure 6. TEM images of as-made (left column) and annealed at 973 K samples (right column), with inset of size distribution histograms of particles and high-resolution TEM with its corresponding FFT. (a) L573, (b) L573R, (c) L653, (d) L653R, (e) L653d, and (f) L653dR. 18798
DOI: 10.1021/acs.jpcc.8b03908 J. Phys. Chem. C 2018, 122, 18795−18801
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disordered graphitic change of the organic/carbon layer on the surface of nanoparticles. The sample annealed at 973 K (4 h) presents promising impedance values for the charge/discharge measurements. The annealed samples at 973 K were electrochemically tested in galvanostatic charge/discharge cycles at C/10 rate (Figure 9). These results show a medium capacity for all
Figure 7. Bode plot for the as-made, fat-free (without the organic layer), and annealed samples at 673 K for 2 h and at 973 K for 4 h.
In Figure 8, we present the Nyquist impedance plot for the same samples. This plot also evidences changes in the ac
Figure 9. Cycling performance of annealed LiFePO4 samples at 0.1 C.
samples. For the samples L653R and L653dR, similar values and behavior through the cycles are found, ca. 80 mA h g−1 at 15th cycle (with initial discharge capacity of 100 mA h g−1), whereas a capacity of about 50 mA h g−1 at 15th cycle is measured for L573R. The electrochemical results in Figure 9 show a medium capacity and a quasireversible system for all samples. Our current understanding is based on the following major ways. The first one is related to the phases present: impurities contribute to a reduced electrochemical response, an inhomogeneous carbon coating on the particles might contribute to a reduced electrical charge transfer. The second one is related to the grain morphology because we have a variety of particle habit and a minor fraction of them are at the optimal orientation for Li diffusion. In addition, our particle size is too small for an optimal electrochemical response (to get the maximum theoretical capacity). In fact in Figure 9, the results obtained on samples prepared at different temperatures support this analysis, in good agreement with similar trends observed on samples prepared in a similar synthesis route11,21,22 and slightly lower than larger particles produced by different synthesis route with organic coating.23,24 In Figure 10, the CV measurements of the annealed samples are presented. The CV curve shows that these samples have a couple of anodic and cathodic peaks, corresponding to the reaction based on the redox couple of Fe2+/Fe3+ during lithium ion extraction and intercalation process in olivine LiFePO4. The absence of other redox peaks in this voltage region indicates that the samples mainly consist of single-phase LiFePO4.25,26 Table 1 summarizes the main electrochemical parameters obtained from experimental results for all samples: the anodic peak area, the corresponding capacity taking into account the scan rate, the peak potentials and the difference among those potentials (ΔEpeak). The capacity values are in agreement with the values shown in Figure 9. The peak potentials are consistent with the values reported by others authors for the redox couple Fe2+/Fe3+.25,27,28 The measured ΔEpeak is higher than ΔEpeak = 0.058 V expected for a reversible system for the three samples, evidencing kinetic limitations,
Figure 8. Nyquist impedance plot for the as-made, the fat-free (without the organic layer), and annealed samples at 673 K for 2 h and at 973 K for 4 h.
transport properties with the sample’s history. On the one hand, as-made and fat-free nanoparticles, despite difference in the impedance values, present a typical capacitive response. On the other hand, the sample with moderate annealing evidences two contributions to the plot, probably indicating that a partial decomposition/transformation of the organic layer with the thermal treatment improve the conductivity of these component with respect to the LiFePO4. Finally, the complete transformation of the organic layer in a disordered graphitic carbon layer with the annealing at 973 K (4 h) leads to a strong reduction of the sample resistivity. According to these results, the conductivity of the system is strongly dependent of the sample’s history, being dominated by the amorphous to 18799
DOI: 10.1021/acs.jpcc.8b03908 J. Phys. Chem. C 2018, 122, 18795−18801
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this synthesis method is obtained from a TGA−DTA analysis. On the basis of transport measurement, we observed that the organic layer in the nanoparticles is converted in a conducting layer. CV shows the typical reduction/oxidation peaks related to redox couple Fe2+/Fe3+, associated with insertion/extraction of lithium ion in LiFePO4 compound. Differences related with synthesis temperature are observed in the electrochemical properties of the particles. Samples synthesized at 653 K show greater capacity than those synthesized at 573 K. However no changes with reactant concentration are observed for samples prepared at 653 K, indicating that changes in the morphology of the particles are important. Figure 10. CV of annealed samples at 1 mV s−1.
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Table 1. Parameters Extracted from CV Measurements
*E-mail:
[email protected]. Phone: +54 2944-445100. Fax: +54 2944 44 5299.
anodic area (V A mg−1) capacity (mA h g−1) anodic peak potential (V) cathodic peak potential (V) ΔEpeak (V)
AUTHOR INFORMATION
Corresponding Author
L573R
L653R
L653dR
0.214 59 3.60 3.27 0.33
0.255 71 3.63 3.21 0.42
0.289 80 3.67 3.20 0.47
ORCID
Pablo S. Martinez: 0000-0001-5529-3574 M. Sergio Moreno: 0000-0001-5815-1029 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are indebted to CONICET for the support and Ph.D fellowship (P.S.M.). Funding by ANPCyT (Argentina) through project PICT2012-1136 is acknowledged.
with L573R sample presenting a slightly reversible behavior than the others. Moreover, in all cases, the intensity of the reduction peak is smaller than that of the oxidation peak, which is a typical behavior of a slow transfer process. In the case of LiFePO4, this difference among the intensities can be generated by the lithium ion insertion/extraction process, a mechanism produced in regions with different phases FePO4 and LiFePO4.6,26 The difference in the capacity between L573R and L653(R and dR) is clearly influenced by the synthesis temperature. From TEM images, it can be seen that the morphology of the as-made nanoparticles is different, although this difference is attenuated by annealing. Nevertheless, the particle sizes are different, being biggest the nanoparticles synthesized at 653 K, and this difference presents no significant changes with annealing. At the same time, it is observed that the increase in the solvent volume in the sample L653dR decreases the average size of the nanoparticles, attenuating the effect of temperature. Comparing the capacity values obtained for L573R and L653(R and dR), we can see that there is a clear influence of the synthesis temperature on this parameter. It is observed that the capacity is nearly the same for samples synthesized at 653 K; therefore, it can be inferred that the effect of particle size would not be significant on the capacity value. The difference observed in capacity among the samples synthesized at different temperatures could be attributed to morphology, even if samples synthesized at 653 K present some impurities, it just seems to improve the electronic conductivity.
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CONCLUSIONS Modifying the synthesis temperature and concentration of reactants, we can control the morphology and size of the samples. An increment in the synthesis temperature improves crystallinity and increases the size of nanoparticles. When the synthesis temperature is diminished, the morphology is different, showing spherical shape with a much smaller size than the original particles. In addition, the activation energy for 18800
DOI: 10.1021/acs.jpcc.8b03908 J. Phys. Chem. C 2018, 122, 18795−18801
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