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A new phosphate-formate precursor method for the preparation of carbon coated nano-crystalline LiFePO4
Journal of Alloys and Compounds 476 (2009) 950–957
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A new phosphate-formate precursor method for the preparation of carbon coated nano-crystalline LiFePO4 V. Koleva ∗ , E. Zhecheva, R. Stoyanova Institute of General and Inorganic Chemistry, Acad. G. Bonchev St, bl. 11, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
a r t i c l e
i n f o
Article history: Received 31 July 2008 Received in revised form 18 September 2008 Accepted 28 September 2008 Available online 14 November 2008 Keywords: Energy storage materials Chemical synthesis X-ray diffraction Scanning electron microscopy SEM Thermal analysis
a b s t r a c t Mixed phosphate-formate precursors, LiFePO4 Hx (HCOO)x ·yH2 O (x ∼ 1, 1 < y < 1.3 and x < y), were prepared by freeze-drying of solutions containing Li+ , Fe2+ , PO4 3− and HCOO− in a ratio of 1:1:1:2. Fe(HCOO)2 ·2H2 O was used as an iron source. The concentration of the freeze-dried solutions allows affecting the morphology of phosphate-formate precursors. Powder XRD, IR spectroscopy, DTA, BET measurements, SEM and XPS analyses were used for the characterization of the precursors and LiFePO4 . IR spectroscopic study of the precursors shows that the deprotonated phosphate and formate ions are coordinated around the metal ions. The thermal heating of the lithium-iron phosphate-formate precursors at 350 ◦ C for a short period yields stoichiometric defectless LiFePO4 having a high-specific surface area of about 35 m2 /g, a mesoporous structure and containing up to 1.5 mass% carbon. By variation of the solutions concentration subjected to freeze-drying, the morphology of LiFePO4 and the carbon content are changed. The morphology of target LiFePO4 comprises micrometric aggregates, which are composed of nanometric particles with close particle size distribution in the range of 60–140 nm. The nanometric particles are interconnected in a way to form mesoporous network. This method is suitable for the preparation of LiFePO4 as cathode material for lithium-ion batteries since it allows affecting both the particle dimensions and carbon content. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Lithium iron phosphates, LiFePO4 , with olivine-type structure have been considered as promising electrode materials for highpower lithium-ion batteries due to high cycling stability, excellent safety characteristics and low materials cost [1–3]. The state-ofthe-art research is mainly devoted to the improvement of rate capability of LiFePO4 electrodes by engineering of nano-sized phosphates [4–8], by coating with carbon [6,9–12] and by doping with alien cations [13–15]. Soft chemistry techniques have been shown to be effective for the preparation of nanometric LiFePO4 . Two varieties of these techniques can be outlined. The first one uses reflux or co-precipitation methods with an aim to prepare pure LiFePO4 with particle dimensions varying between 40 and 140 nm [16,17]. The second one comprises sol–gel techniques based on metal-organic precursors leading to the formation of carbon containing LiFePO4 [8,18–20]. The characteristic feature of these methods is the selection of organic components (such as citric acid, l-ascorbic acid, glycolic acid, lauric acid, PEG, etc.) which can serve as carbon sources, from one hand, and can ensure the complexation of metal ions in the
precursors, from the other. Sol–gel techniques are also suitable to obtain crystalline LiFePO4 doped with alien ions such as Mg, Zr, Ti [21,22]. Although many studies have been devoted on the preparation of nano-sized LiFePO4 or carbon containing LiFePO4 , little works is done on the development of methods that are able to control both particle dimensions and carbon content. In this study, we report a new and simple method for the preparation of LiFePO4 that allows affecting both the particle dimensions and the carbon content. This method is based on the formation of homogeneous lithium-iron phosphate-formate precursors by freeze-drying of aqueous solutions containing Li+ , Fe2+ , PO4 3− and HCOO− . Thermal heating of the lithium-iron phosphate-formate precursors yields at temperature above 300 ◦ C nanometric LiFePO4 containing up to 2 mass% carbon. By variation of the solution concentration, it is possible to change the morphology of the precursors and the target LiFePO4 , as well as the carbon content. Powder XRD, IR spectroscopy, BET measurements, SEM and X-ray photoelectron spectroscopy (XPS) analyses have been used for the characterization of LiFePO4 . 2. Experimental 2.1. Preparation of lithium-iron phosphate-formate precursors
∗ Corresponding author. Tel.: +359 2 9793725; fax: +359 2 870 50 24. E-mail address: [email protected] (V. Koleva). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.09.144
The starting reagents are Fe(HCOO)2 ·2H2 O, Li2 CO3 and H3 PO4 . All reagents used were of “p.a.” quality (Merck). As an iron source we used crystalline Fe(HCOO)2 ·2H2 O
V. Koleva et al. / Journal of Alloys and Compounds 476 (2009) 950–957
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since it displays: (i) a moderate stability at storage (with respect to both oxidation to Fe3+ and dehydration), which ensures an exact stoichiometry in the final product; (ii) a relatively lower temperature of decomposition; (iii) appearance of reducing gaseous products (H2 and CO) during the decomposition. Fe(HCOO)2 ·2H2 O was prepared as described in Ref. [23]. The transparent phosphate-formate solution of lithium and iron was obtained by mixing two solutions: A and B. Solution A containing Li+ and PO4 3− in a molar ratio of 1:1 was prepared by dissolution of Li2 CO3 in an aqueous solution of phosphoric acid. Crystalline Fe(HCOO)2 ·2H2 O was dissolved in water at pH ≈ 3.5 (solution B). The pH value of both solutions before mixing was adjusted to 2 with addition of a small quantity of HCOOH (1:1). The ratio between the components in the resulting solution was Li+ :Fe2+ :PO4 3− :HCOO− = 1:1:1:2, the concentration being varied between 0.05 and 0.2 M (with respect to metal ions). The solution thus prepared was cooled down to room temperature, then frozen instantly with liquid nitrogen and dried in vacuum (20–30 mbars) at −20 ◦ C with an Alpha-Crist Freeze-Dryer. For the sake of simplicity, the precursors obtained from solutions with different concentrations will be further on denoted as Pr–S005, Pr–S01 and Pr–S02 obtained from 0.05, 0.1 and 0.2 M solutions, respectively. After drying, the solid precursors were decomposed under flowing argon at 350 ◦ C for 2 h. The solid residues were further annealed at 400, 500 and 600 ◦ C for 5–15 h in an argon atmosphere. The argon gas was of 99.999% purity. The LiFePO4 samples obtained from the three different precursors are denoted as LiFePO4 –S005, LiFePO4 –S01 and LiFePO4 –S02, respectively. In addition, Fe(H2 PO4 )2 ·2H2 O was also prepared by dissolving a powder iron in 70% H3 PO4 as described in Ref. [24]. 2.2. Methods for samples characterization The total iron content in the samples was determined complexometrically at pH 1.5 using sulfosalicylic acid as an indicator after preliminary oxidation of Fe2+ with hydrogen peroxide. The lithium content was determined by atomic absorption analysis. The purity of the LiFePO4 powders with respect to impurities of Fe3+ was determined by oxidation–reduction titration using standard K2 Cr2 O7 and diphenylamine as indicator [25]. Elemental analysis (C and H) was performed using Elementar Analysensysteme GmbH (VarioEL analyser). X-ray structural analysis was made by a Bruker Advance 8 diffractometer with Cu K␣ radiation. Step-scan recordings for structure refinement by the Rietveld method were carried out using 0.03◦ 2 steps of 12 s duration. The computer program FULLPROF was used in the calculations. The crystallite size of LiFePO4 was calculated by the Scherrer equation from the line width of the (2 0 0) reflection peak: D200 = /((ˇ2 − ˇ2 o )1/2 cos 200 ) where (Cu K␣) = 0.15405 nm, ˇ is the peak width at the half height corrected with instrumental broadening and hkl is the Bragg angle. The line width was determined by profile analysis using a WinPlotr programme. The IR spectra were recorded on a Fourier transform Nicolet Avatar-320 instrument using KBr pellets (resolution < 2 cm−1 ). The thermal analysis (simultaneously obtained DTA-, TG- and DTG-curves) of the precursors was carried out by a “Stanton Redcroft” apparatus in the temperature range up to 650 ◦ C under an argon stream (1 l/h, 99.9%), a heating rate of 5 ◦ C/min and sample mass of 10 mg. SEM images of the precursors and LiFePO4 powders coated with gold were obtained by Philips SEM 515 and by JEOL (JSM-5510) scanning electron microscopes. The porous texture of the samples was examined by low-temperature (77.4 K) nitrogen adsorption in a conventional volume-measuring adsorption apparatus. The chemical state and atomic concentrations of elements in the surface layer of the samples were determined using XPS. XPS studies were performed in a VG ESCALAB MkII electron spectrometer under a base pressure of 1 × 10−8 Pa. The photoelectron spectra were excited using un-monochromatized Al K␣ radiation (h = 1486.6 eV) with a total instrumental resolution of ∼1 eV. The C 1s line of adventitious carbon at 285.0 eV was used as internal standard to calibrate the binding energies.
3. Results and discussions 3.1. Chemical and structural characterization of the Li-Fe phosphate-formate precursors Freeze-drying of the solution containing Li+ , Fe2+ , PO4 3− and HCOO− in the ratio 1:1:1:2 yields amorphous pale-green powders. Irrespective of the concentration of the freeze-dried solutions, the chemical analysis shows that in the solid precursors the ratio between the lithium, iron and phosphate ions remains the same, while the concentration of the formate ion tends to 1, i.e., Li+ :Fe2+ :PO4 3− :HCOO− = 1:1:1:1. This means that about 1 mol of formic acid is sublimated during the freeze-drying process. The coordination of the phosphate and formate ions in the freeze-dried solids is studied by IR spectroscopy. Fig. 1 compares the IR spectra of the freeze-dried solids with those of Fe(HCOO)2 ·2H2 O
Fig. 1. IR spectra of Fe(HCOO)2 ·2H2 O, Fe(H2 PO4 )2 ·2H2 O, precursors Pr–S005, Pr–S01, Pr–S02 and of precursor Pr–S02 heated at 350 ◦ C.
and Fe(H2 PO4 )2 ·2H2 O. Three types of characteristic vibrations due to the formate and phosphate ions and the water molecules can be outlined. The IR spectra of freeze-dried solids display bands, which are typical for formate ion vibrations [26]: 1600–1580 cm−1 (as (CO)); 1395–1360 cm−1 (ıCH and s (CO)); 760–700 cm−1 (ıs (OCO)). For the sake of comparison, the formate ion vibrations in Fe(HCOO)2 ·2H2 O and LiHCOO·H2 O appear at: 1580 and 1590 cm−1 for as (CO); 1395 and 1383 cm−1 for ıCH ; 1372/1358 cm−1 and 1371 cm−1 for s (CO); 756/693 cm−1 and 796 cm−1 for ıs (OCO), respectively [27]. This comparison shows clearly that the positions of formate ion vibrations in the freeze-dried solids and in both pure iron and lithium salts are close. It is worth to mention that the characteristic bands of protonated formate (C O at about 1700 cm−1 , C–O at 1260 cm−1 and OH at 975 cm−1 [28]) are not observed in the IR spectra of precursors. This means that the formate ion in the precursors is deprotonated and coordinated around the metal ions. The second type of characteristic vibrations is associated with the phosphate groups. The stretching P–O and bending OPO vibrations of the phosphate groups appear in the ranges of 920–990 cm−1 (1 ), 950–1160 cm−1 (3 ), 460–320 cm−1 (2 ), and 650–470 cm−1 (4 ). In addition to the internal PO4 vibrations, other vibrations involving OH motions are the characteristic of the protonated phosphate ions (H2 PO4 − or HPO4 2− ), namely: both out-of-plane ıOH and in-plane OH bending P–O–H vibrations, which appear at 1230–1260 and 800–900 cm−1 , respectively [29,30]. While the IR vibrations due to the protonated phosphate groups are clearly
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Fig. 2. SEM images of precursors Pr–S005 (A), Pr–S02 (B) and of corresponding LiFePO4 –S005 (C) and LiFePO4 –S02 (D) annealed at 600 ◦ C.
resolved in the spectrum of Fe(H2 PO4 )2 ·2H2 O (1262 cm−1 and the duplet at 820/790 cm−1 , Fig. 1), the freeze-dried solids show the characteristic vibrations for deprotonated phosphate ions only (3 at 1015–1070 cm−1 and 4 at 560 cm−1 ). Further on, the observed upshift of the asymmetric stretching 3 vibration compared to that of the free phosphate ion (1017 cm−1 ) evidences for the coordination of PO4 3− to the metal ions in the freeze-dried compositions. The third spectral feature in the IR spectra is the water molecule vibrations. A broad featureless band at 3400 cm−1 with a shoulder around 3200 cm−1 dominates in the IR spectra of the precursors. As a comparison, for Fe(H2 PO4 )2 ·2H2 O and Fe(HCOO)2 ·2H2 O where the water molecules are coordinated to the Fe2+ ions, the OH stretching vibrations appear in the range of 3150–3370 cm−1 (Fig. 1). For LiHCOO·H2 O, the OH stretching vibrations of the water molecule bonded to Li+ are at 3398 and 3109 cm−1 [27]. Hence, the broad band observed for Li-Fe phosphate-formate precursors could be due to the superposition of the OH stretching vibrations of the water molecules bonded to both metal ions. The weak band around 900 cm−1 could be tentatively assigned to the water libration mode [31]. Summarizing, it appears that during the freeze-drying process solids with compositions LiFePO4 Hx (HCOO)x ·yH2 O (x ∼ 1; 1 < y <1.3 and x < y) are obtained, where the metal ions are bonded to the deprotonated phosphate and formate ions. The charge neutrality is achieved by insertion of one proton (most probably H3 O+ ) in the precursor framework. It is worth mentioning that the appearance of H3 O+ within the crystal structure of different compounds is well documented, for example H3 OM(SO4 )2 (M3+ Al, Ga, In, Ti, and Fe) [32], (H3 O)UO2 PO4 ·3H2 O [33], etc. In compounds containing different protonic species like H3 O+ , H5 O2 + , etc., the stretching OH vibrations appear in the range of 3500–2500 cm−1 and the bending ı(H3 O+ ) vibration—in the range of 1700–1750 cm−1 [34,35]. Therefore, in the spectra of the precursors the shoulder observed at 1720 cm−1 could be related with the ı(H3 O+ ) vibration and the OH bands of H3 O+ could be overlapped with the vibrations of H2 O molecules as it is reported in Ref. [35]. While the concentration of the freeze-dried solution does not influence the compositions and structure of precursors, the pre-
cursor’s morphology is affected. Fig. 2 shows the SEM images of the precursors obtained from the diluted and concentrated solutions. While the precursor Pr–S005 obtained from the diluted solution comprises fine flake-like aggregates (Fig. 2(A)), the thick platelike aggregates gives rise to the morphology of Pr–S02 obtained from the concentrated solution (Fig. 2(B)). The transformation of fine aggregates into thicker ones by increasing the concentration of freeze-dried solutions has also been established for the preparation of layered LiCo/NiO2 cathode materials by freeze-drying of lithium–cobalt citrate solutions [36]. 3.2. Structural characterization of LiFePO4 Homogeneous Li-Fe phosphate-formate precursors are unstable and, above 50 ◦ C, they are decomposed (Fig. 3). Decomposition process is developed from 50 to 350 ◦ C with a total weight loss of 28%. Based on the numerous data on thermal decomposition of metal formates [37,38] the first broad endothermic effect between 50 and
Fig. 3. TG (solid line), DTA (dot line) and DTG (dash line) curves for the precursor Pr–S01.
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150 ◦ C is due to the dehydration, while the second strong endothermic peak at 240 ◦ C followed by the broad complex exothermic effect between 300 and 400 ◦ C corresponds to the decomposition of the formate ions. The decomposition process is completed at 350 ◦ C. This is confirmed by the IR spectrum of the precursor Pr–S02 heated at 350 ◦ C for 20 min (Fig. 1), where no IR bands due to the formate ions vibrations are observed. It should be mentioned that the observed broadening of the exothermic effect at a temperature higher than 350 ◦ C accompanied by a small weight loss can be explained by partial oxidation of Fe2+ as a result of oxygen contaminant in a small amount in the argon used. The formation of the olivine-type phase LiFePO4 starts at 350 ◦ C immediately after the release of the formic acid (Fig. 1). The IR spectrum of the sample obtained at 350 ◦ C for 10 h coincides with the IR spectrum typical of the olivine-type phase LiFePO4 [39]. The shoulder at 945 cm−1 and the band at 970 cm−1 are assigned to the symmetric stretching P–O vibration of the PO4 3− group, while the rest three bands at 1060, 1098 and 1138 cm−1 are due to the asymmetric stretching P–O vibrations. The asymmetric bending OPO mode splits into four bands at 550, 578, 636 and 648 cm−1 . The bands at 500 and 470 cm−1 originate essentially from the Li+ translations. It is important that the IR bands profile of the sample obtained at 350 ◦ C for 10 h is identical with the IR bands profiles of samples annealed at 400, 500 and 600 ◦ C (Figs. 1 and 4). From the IR spectroscopic data it is evident that the olivine-type phase LiFePO4 is already formed at 350 ◦ C and remains stable during the annealing between 400 and 600 ◦ C. Irrespective of the concentration of the freeze-dried solution, well-crystalline powders of LiFePO4 without any XRD detectable impurities are obtained in the temperature range of 400–600 ◦ C (Fig. 5). The data from chemical analysis show some impurity of Fe3+ ions with concentration less than 1.5%. The diffraction peaks have been indexed to a single-phase having an ordered olivinetype structure, S.G. Pnma [40]. The unit-cell dimensions determined from the Rietveld refinement of the XRD patterns are given in Table 1. On the first glance, the lattice parameters and unit-cell volumes are not sensitive towards both the annealing temperature and the concentration of freeze-dried solutions. For LiFePO4 annealed between 500 and 600 ◦ C, the lattice parameters coincide well with those reported in literature for LiFePO4 obtained at temperature higher than 600 ◦ C [7,18,41]. This means that the freeze-drying method allows formation of “defectless” LiFePO4 phases even at lower temperatures. In addition, close inspection of Table 1 reveals a slight expansion of the unit-cell volume with increasing the annealing temperature from 400 to 500 ◦ C. Slightly lower unit-cell parameters have been established by Kim et al. [16] and Masquelier and co-workers [17] when carbon-free LiFePO4 with nanometric particle dimensions (about 40 and 140 nm, respectively) are obtained: 289.8 Å3 [16] and 288.7 Å3 [17] as compared to 289.8–290.0 Å3 for our samples of LiFePO4 annealed at 400 ◦ C. The physical meaning of the contracted unit-cell volumes observed for nano-sized LiFePO4 is not clear yet [17]. Another parameter derived from XRD patterns is the Scherrer crystallites size. Fig. 6(A) shows the dependence of the Scher-
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Fig. 4. IR spectra of LiFePO4 annealed at 400, 500 and 600 ◦ C.
rer crystallites size (D200 ) on the annealing temperature as well as on the concentration of the solutions subjected to freezedrying. At 400 ◦ C, crystallites with sizes of about 27–28 nm are formed irrespective of the solution concentration. By increasing the annealing temperature, there is a crystallite growth especially for LiFePO4 originating from Pr–S02. It is noticeable that, for LiFePO4 –S005 originating from the diluted solution, the crystallite growth between 400 and 600 ◦ C is restricted. The observed trends in the crystallite growth are further supported by the dependence of the specific surface areas on the annealing temperature (Fig. 6(B)). At 400 ◦ C, the LiFePO4 samples display a maximumspecific surface area of about 34–35 m2 /g which is not sensitive towards the solutions concentration. After increasing the annealing temperature, a stronger reduction in the specific surface area is observed for LiFePO4 –S02 obtained from the concentrated solution (from 35 to 13 m2 /g), while the specific surface areas of the samples LiFePO4 –S005 obtained from the most diluted solution are slightly decreased (from 35 to 32 m2 /g) (Fig. 6(B)). The effect of the concentration of the freeze-dried solution on the particle dimensions is illustrated in Fig. 7. So, nano-sized isometric particles are seen in the powders prepared from the concentrated freeze-dried solutions (Fig. 7(B and C)), while (at the same magnification) individual particles cannot be distinguished for the
Table 1 Lattice parameters, unit-cell volumes and specific surface areas of LiFePO4 prepared from different freeze-dried precursors and annealing temperatures. Pr–S005
a (Å) b (Å) c (Å) V (Å3 ) SBET (m2 /g) a
Pr–S01
Pr–S02
400a
500a
600a
400a
500a
600a
400a
500a
600a
10.3116(7) 5.9963(4) 4.6905(3) 290.02(3) 34
10.3187(5) 6.0017(6) 4.6940(6) 290.69(3) 32
10.3243(3) 6.0041(2) 4.6908(1) 290.78(2) 32
10.3195(6) 5.9937(3) 4.6909(3) 289.86(4) 34
10.3201(6) 6.0010(5) 4.6937(4) 290.68(3) 30
10.3228(4) 6.0047(4) 4.6918(2) 290.83(2) 18
10.3089(5) 5.9939(3) 4.6908(2) 289.86(2) 35
10.3203(4) 5.9995(2) 4.6920(2) 290.51(4) 25
10.3257(3) 6.0042(2) 4.6902(1) 290.79(2) 13
Temperature (◦ C)
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Fig. 5. Rietveld refinement of the XRD patterns of LiFePO4 –S02 annealed at 400 and 600 ◦ C.
powder prepared from the diluted solution (Fig. 7(A)). This will be discussed further on. For LiFePO4 obtained from the two concentrated solutions, the particle size distributions determined from SEM images are compared in Fig. 8. It is seen that, for LiFePO4 –S01 more than 3/4 from the particles are within the 60–120 nm range and the mean particle size is 90 ± 15 nm. A little bit larger are the particles of LiFePO4 –S02, where about 3/4 from the particles are distributed between 80 and 140 nm and the mean particle size is 110 ± 12 nm. It is important that SEM particle dimensions coincide with particles sizes determined from the specific surface areas—90 and 130 nm for the samples LiFePO4 –S01 and LiFePO4 –S02, respectively. In addition, the SEM particle dimensions are slightly larger as compared to crystallites sizes determined from XRD line broadening (Table 1). This indicates a formation of well-crystalline nano-sized LiFePO4 from freeze-dried Li-Fe phosphate-formate precursors. The release of the formic acid is accompanied by the deposition of a small amount of in situ formed carbon. It is important that the carbon content depends on the concentration of the freeze-dried solution. At 400 ◦ C, the carbon content is 2.66, 0.64 and 0.53 mass% for the samples LiFePO4 –S005, LiFePO4 –S01 and LiFePO4 –S02, respectively. By increasing the annealing temperature, the carbon content decreases and at 600 ◦ C it is 1.1, 0.43 and ≤0.3 mass% for LiFePO4 –S005, LiFePO4 –S01 and LiFePO4 –S02, respectively. From these data it appears that LiFePO4 samples prepared from the diluted solution have the highest carbon content irrespective of the annealing temperature. It is worth noting that a carbon amount of about 1.45 mass% has been shown to improve significantly the rate capability of LiFePO4 electrode materials [42]. The carbon deposition on LiFePO4 samples was checked by XPS analysis. The C 1s spectra (Fig. 9) display a broad band centred
Fig. 6. Dependence of the crystallite size D200 (A) and of specific surface areas (B) of LiFePO4 prepared from the different precursors on the annealing temperature.
at 285.0 eV with intensities decreasing as the concentration of the freeze-dried solution increases. For the samples LiFePO4 –S005, LiFePO4 –S01 and LiFePO4 –S02 annealed at 600 ◦ C, the surface carbon content is 23.3, 11.1 and 7.4 at.%, respectively. The decrease in the surface carbon content (determined from XPS analysis) is in agreement with the observed trend in changes of the total carbon content determined from the chemical analysis. On the other hand, the ratio of the surface carbon amount to the total carbon amount is higher than 10:1 for all samples studied, which indicates that the carbon is deposed mainly on the particles surface of LiFePO4 . To check this suggestion, Fig. 10 compares the dependences of the total and surface carbon amounts on the specific surface areas of LiFePO4 prepared at 600 ◦ C. There is a good correlation “total carbon amount – surface carbon amount – specific surface area”. The deposition of higher amount of carbon for LiFePO4 –S005 obtained from the diluted solutions is a fact that could explain the restricted particle growth at temperatures higher than 400 ◦ C. It appears that the higher is the carbon amount, the more restricted is the crystallite and particle growth (Figs. 2 and 7). Also, the carbon deposition gives rise to the morphology of target LiFePO4 . The morphology of all samples studied comprises the plate-like aggregates with micrometric dimensions. However, the surface of aggregates seems to be flatter for LiFePO4 –S005 obtained from the diluted solution (Fig. 2(C)). This is, most probably, a consequence of the higher amount of carbon deposed during formic
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Fig. 8. Particle size distributions of LiFePO4 –S01 and LiFePO4 –S02 annealed at 600 ◦ C.
sensitive towards the concentration of freeze-dried solutions and the temperature of annealing. However, the porous structure would be favourable for the application of LiFePO4 as cathode materials [8,11].
Fig. 7. SEM images of LiFePO4 –S005 (A), LiFePO4 –S01(B) and LiFePO4 –S02 (C) annealed at 600 ◦ C.
acid release. In addition, the higher amount of carbon make difficulties in the observation, at higher magnification, of individual primary particles for LiFePO4 –S005 obtained from diluted solutions (Fig. 7(A)). The formation of micrometric aggregates from nano-sized particles determines the complex porous structure of LiFePO4 . Adsorption–desorption isotherms of LiFePO4 prepared at 400 and 600 ◦ C (not presented here) are of IV type, which is characteristic for the presence of large amount of meso- and micropores. The pore size distribution obtained by the method of Pierce [43] (Fig. 11) reveals that the pores are continuously distributed within the mesopore region of RP up to 25–30 nm with average pore sizes between 3.3 and 4 nm. It looks like that porous structure is not
Fig. 9. XPS spectra of LiFePO4 –S005 (A), LiFePO4 –S01 (B) and LiFePO4 –S02 (C) annealed at 600 ◦ C.
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from the diluted solution display better electrochemical performance. Further works are in progress. 4. Conclusion
Fig. 10. Carbon content of LiFePO4 annealed at 600 ◦ C in dependence on the specific surface areas of samples prepared from the freeze-dried solutions with concentrations 0.05, 0.1 and 0.2 M: Curve 1, total carbon content from the chemical analysis; Curve 2, surface carbon content form XPS analysis.
The freeze-drying method is successfully used for the preparation of cathode materials based on layered and spinel oxides [36,44,45]. Concerning LiFePO4 electrodes, the application of freeze-drying technique has recently been reported by Rojo and co-workers [46] using ferrous acetate (FeC4 H6 O4 ) as iron source and citric acid as chelating agent and carbon source. In our variety of the freeze-drying method, we have combined the stability of iron (II) formate salt with the control of the solution concentration as an instrument to design the lithium-iron phosphate-formate precursors. As a result, we are able to change both particle dimensions and carbon content. The freeze-drying method is suitable for the preparation of LiFePO4 as cathode materials for lithium ion batteries. The preliminary electrochemical data show that LiFePO4 obtained at 500 ◦ C
Freeze-drying of solutions containing Li+ , Fe2+ , PO4 3− and HCOO− in a ratio of 1:1:1:2 yields LiFePO4 Hx (HCOO)x ·yH2 O compositions (x ∼ 1, 1 < y < 1.3 and x < y), where deprotonated phosphate and formate ions are coordinated around metal ions. During the freeze-drying process, 1 mol of formic acid is sublimated. By variation of the concentration of the freeze-dried solutions, the morphology of the phosphate-formate precursors is changed from fine flake-like to thicker plate-like aggregates. The formation of the olivine-type phase LiFePO4 starts at 350 ◦ C immediately after the release of the formic acid. Further annealing between 400 and 600 ◦ C yields defectless well-crystalline LiFePO4 powders containing less than 1.5% Fe3+ impurities. The morphology of all samples studied comprises the plate-like aggregates, which are composed of isometric primary particles with close particle size distribution in the nanometric range from 60 to 140 nm. The release of the formic acid is accompanied by the deposition of carbon, whose amount decreases with increasing the concentration of the freeze-dried solutions. The higher content of deposed carbon limits the particle growth as the annealing temperature increases from 400 to 600 ◦ C. As a result, the particle dimensions of LiFePO4 annealed at 600 ◦ C become sensitive towards the concentration of the freeze-dried solution. The ability of the freeze-drying method for precursor design provides an opportunity to rationalize the synthesis of LiFePO4 (in respect of particle dimensions and carbon content) as cathode materials for lithium-ion batteries. By using additional organic chelating agents it is possible to vary smoothly the amount of carbon content keeping the particle dimensions in nanometric scale. Acknowledgment Authors are grateful for the financial support from the National Science Fund of Bulgaria (Ch 1701/2007). References
Fig. 11. Pore size distribution curves of LiFePO4 prepared from the different precursors and annealing temperatures.
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A new phosphate-formate precursor method for the preparation of carbon coated nano-crystalline LiFePO4 V. Koleva ∗ , E. Zhecheva, R. Stoyanova Institute of General and Inorganic Chemistry, Acad. G. Bonchev St, bl. 11, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
a r t i c l e
i n f o
Article history: Received 31 July 2008 Received in revised form 18 September 2008 Accepted 28 September 2008 Available online 14 November 2008 Keywords: Energy storage materials Chemical synthesis X-ray diffraction Scanning electron microscopy SEM Thermal analysis
a b s t r a c t Mixed phosphate-formate precursors, LiFePO4 Hx (HCOO)x ·yH2 O (x ∼ 1, 1 < y < 1.3 and x < y), were prepared by freeze-drying of solutions containing Li+ , Fe2+ , PO4 3− and HCOO− in a ratio of 1:1:1:2. Fe(HCOO)2 ·2H2 O was used as an iron source. The concentration of the freeze-dried solutions allows affecting the morphology of phosphate-formate precursors. Powder XRD, IR spectroscopy, DTA, BET measurements, SEM and XPS analyses were used for the characterization of the precursors and LiFePO4 . IR spectroscopic study of the precursors shows that the deprotonated phosphate and formate ions are coordinated around the metal ions. The thermal heating of the lithium-iron phosphate-formate precursors at 350 ◦ C for a short period yields stoichiometric defectless LiFePO4 having a high-specific surface area of about 35 m2 /g, a mesoporous structure and containing up to 1.5 mass% carbon. By variation of the solutions concentration subjected to freeze-drying, the morphology of LiFePO4 and the carbon content are changed. The morphology of target LiFePO4 comprises micrometric aggregates, which are composed of nanometric particles with close particle size distribution in the range of 60–140 nm. The nanometric particles are interconnected in a way to form mesoporous network. This method is suitable for the preparation of LiFePO4 as cathode material for lithium-ion batteries since it allows affecting both the particle dimensions and carbon content. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Lithium iron phosphates, LiFePO4 , with olivine-type structure have been considered as promising electrode materials for highpower lithium-ion batteries due to high cycling stability, excellent safety characteristics and low materials cost [1–3]. The state-ofthe-art research is mainly devoted to the improvement of rate capability of LiFePO4 electrodes by engineering of nano-sized phosphates [4–8], by coating with carbon [6,9–12] and by doping with alien cations [13–15]. Soft chemistry techniques have been shown to be effective for the preparation of nanometric LiFePO4 . Two varieties of these techniques can be outlined. The first one uses reflux or co-precipitation methods with an aim to prepare pure LiFePO4 with particle dimensions varying between 40 and 140 nm [16,17]. The second one comprises sol–gel techniques based on metal-organic precursors leading to the formation of carbon containing LiFePO4 [8,18–20]. The characteristic feature of these methods is the selection of organic components (such as citric acid, l-ascorbic acid, glycolic acid, lauric acid, PEG, etc.) which can serve as carbon sources, from one hand, and can ensure the complexation of metal ions in the
precursors, from the other. Sol–gel techniques are also suitable to obtain crystalline LiFePO4 doped with alien ions such as Mg, Zr, Ti [21,22]. Although many studies have been devoted on the preparation of nano-sized LiFePO4 or carbon containing LiFePO4 , little works is done on the development of methods that are able to control both particle dimensions and carbon content. In this study, we report a new and simple method for the preparation of LiFePO4 that allows affecting both the particle dimensions and the carbon content. This method is based on the formation of homogeneous lithium-iron phosphate-formate precursors by freeze-drying of aqueous solutions containing Li+ , Fe2+ , PO4 3− and HCOO− . Thermal heating of the lithium-iron phosphate-formate precursors yields at temperature above 300 ◦ C nanometric LiFePO4 containing up to 2 mass% carbon. By variation of the solution concentration, it is possible to change the morphology of the precursors and the target LiFePO4 , as well as the carbon content. Powder XRD, IR spectroscopy, BET measurements, SEM and X-ray photoelectron spectroscopy (XPS) analyses have been used for the characterization of LiFePO4 . 2. Experimental 2.1. Preparation of lithium-iron phosphate-formate precursors
∗ Corresponding author. Tel.: +359 2 9793725; fax: +359 2 870 50 24. E-mail address: [email protected] (V. Koleva). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.09.144
The starting reagents are Fe(HCOO)2 ·2H2 O, Li2 CO3 and H3 PO4 . All reagents used were of “p.a.” quality (Merck). As an iron source we used crystalline Fe(HCOO)2 ·2H2 O
V. Koleva et al. / Journal of Alloys and Compounds 476 (2009) 950–957
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since it displays: (i) a moderate stability at storage (with respect to both oxidation to Fe3+ and dehydration), which ensures an exact stoichiometry in the final product; (ii) a relatively lower temperature of decomposition; (iii) appearance of reducing gaseous products (H2 and CO) during the decomposition. Fe(HCOO)2 ·2H2 O was prepared as described in Ref. [23]. The transparent phosphate-formate solution of lithium and iron was obtained by mixing two solutions: A and B. Solution A containing Li+ and PO4 3− in a molar ratio of 1:1 was prepared by dissolution of Li2 CO3 in an aqueous solution of phosphoric acid. Crystalline Fe(HCOO)2 ·2H2 O was dissolved in water at pH ≈ 3.5 (solution B). The pH value of both solutions before mixing was adjusted to 2 with addition of a small quantity of HCOOH (1:1). The ratio between the components in the resulting solution was Li+ :Fe2+ :PO4 3− :HCOO− = 1:1:1:2, the concentration being varied between 0.05 and 0.2 M (with respect to metal ions). The solution thus prepared was cooled down to room temperature, then frozen instantly with liquid nitrogen and dried in vacuum (20–30 mbars) at −20 ◦ C with an Alpha-Crist Freeze-Dryer. For the sake of simplicity, the precursors obtained from solutions with different concentrations will be further on denoted as Pr–S005, Pr–S01 and Pr–S02 obtained from 0.05, 0.1 and 0.2 M solutions, respectively. After drying, the solid precursors were decomposed under flowing argon at 350 ◦ C for 2 h. The solid residues were further annealed at 400, 500 and 600 ◦ C for 5–15 h in an argon atmosphere. The argon gas was of 99.999% purity. The LiFePO4 samples obtained from the three different precursors are denoted as LiFePO4 –S005, LiFePO4 –S01 and LiFePO4 –S02, respectively. In addition, Fe(H2 PO4 )2 ·2H2 O was also prepared by dissolving a powder iron in 70% H3 PO4 as described in Ref. [24]. 2.2. Methods for samples characterization The total iron content in the samples was determined complexometrically at pH 1.5 using sulfosalicylic acid as an indicator after preliminary oxidation of Fe2+ with hydrogen peroxide. The lithium content was determined by atomic absorption analysis. The purity of the LiFePO4 powders with respect to impurities of Fe3+ was determined by oxidation–reduction titration using standard K2 Cr2 O7 and diphenylamine as indicator [25]. Elemental analysis (C and H) was performed using Elementar Analysensysteme GmbH (VarioEL analyser). X-ray structural analysis was made by a Bruker Advance 8 diffractometer with Cu K␣ radiation. Step-scan recordings for structure refinement by the Rietveld method were carried out using 0.03◦ 2 steps of 12 s duration. The computer program FULLPROF was used in the calculations. The crystallite size of LiFePO4 was calculated by the Scherrer equation from the line width of the (2 0 0) reflection peak: D200 = /((ˇ2 − ˇ2 o )1/2 cos 200 ) where (Cu K␣) = 0.15405 nm, ˇ is the peak width at the half height corrected with instrumental broadening and hkl is the Bragg angle. The line width was determined by profile analysis using a WinPlotr programme. The IR spectra were recorded on a Fourier transform Nicolet Avatar-320 instrument using KBr pellets (resolution < 2 cm−1 ). The thermal analysis (simultaneously obtained DTA-, TG- and DTG-curves) of the precursors was carried out by a “Stanton Redcroft” apparatus in the temperature range up to 650 ◦ C under an argon stream (1 l/h, 99.9%), a heating rate of 5 ◦ C/min and sample mass of 10 mg. SEM images of the precursors and LiFePO4 powders coated with gold were obtained by Philips SEM 515 and by JEOL (JSM-5510) scanning electron microscopes. The porous texture of the samples was examined by low-temperature (77.4 K) nitrogen adsorption in a conventional volume-measuring adsorption apparatus. The chemical state and atomic concentrations of elements in the surface layer of the samples were determined using XPS. XPS studies were performed in a VG ESCALAB MkII electron spectrometer under a base pressure of 1 × 10−8 Pa. The photoelectron spectra were excited using un-monochromatized Al K␣ radiation (h = 1486.6 eV) with a total instrumental resolution of ∼1 eV. The C 1s line of adventitious carbon at 285.0 eV was used as internal standard to calibrate the binding energies.
3. Results and discussions 3.1. Chemical and structural characterization of the Li-Fe phosphate-formate precursors Freeze-drying of the solution containing Li+ , Fe2+ , PO4 3− and HCOO− in the ratio 1:1:1:2 yields amorphous pale-green powders. Irrespective of the concentration of the freeze-dried solutions, the chemical analysis shows that in the solid precursors the ratio between the lithium, iron and phosphate ions remains the same, while the concentration of the formate ion tends to 1, i.e., Li+ :Fe2+ :PO4 3− :HCOO− = 1:1:1:1. This means that about 1 mol of formic acid is sublimated during the freeze-drying process. The coordination of the phosphate and formate ions in the freeze-dried solids is studied by IR spectroscopy. Fig. 1 compares the IR spectra of the freeze-dried solids with those of Fe(HCOO)2 ·2H2 O
Fig. 1. IR spectra of Fe(HCOO)2 ·2H2 O, Fe(H2 PO4 )2 ·2H2 O, precursors Pr–S005, Pr–S01, Pr–S02 and of precursor Pr–S02 heated at 350 ◦ C.
and Fe(H2 PO4 )2 ·2H2 O. Three types of characteristic vibrations due to the formate and phosphate ions and the water molecules can be outlined. The IR spectra of freeze-dried solids display bands, which are typical for formate ion vibrations [26]: 1600–1580 cm−1 (as (CO)); 1395–1360 cm−1 (ıCH and s (CO)); 760–700 cm−1 (ıs (OCO)). For the sake of comparison, the formate ion vibrations in Fe(HCOO)2 ·2H2 O and LiHCOO·H2 O appear at: 1580 and 1590 cm−1 for as (CO); 1395 and 1383 cm−1 for ıCH ; 1372/1358 cm−1 and 1371 cm−1 for s (CO); 756/693 cm−1 and 796 cm−1 for ıs (OCO), respectively [27]. This comparison shows clearly that the positions of formate ion vibrations in the freeze-dried solids and in both pure iron and lithium salts are close. It is worth to mention that the characteristic bands of protonated formate (C O at about 1700 cm−1 , C–O at 1260 cm−1 and OH at 975 cm−1 [28]) are not observed in the IR spectra of precursors. This means that the formate ion in the precursors is deprotonated and coordinated around the metal ions. The second type of characteristic vibrations is associated with the phosphate groups. The stretching P–O and bending OPO vibrations of the phosphate groups appear in the ranges of 920–990 cm−1 (1 ), 950–1160 cm−1 (3 ), 460–320 cm−1 (2 ), and 650–470 cm−1 (4 ). In addition to the internal PO4 vibrations, other vibrations involving OH motions are the characteristic of the protonated phosphate ions (H2 PO4 − or HPO4 2− ), namely: both out-of-plane ıOH and in-plane OH bending P–O–H vibrations, which appear at 1230–1260 and 800–900 cm−1 , respectively [29,30]. While the IR vibrations due to the protonated phosphate groups are clearly
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Fig. 2. SEM images of precursors Pr–S005 (A), Pr–S02 (B) and of corresponding LiFePO4 –S005 (C) and LiFePO4 –S02 (D) annealed at 600 ◦ C.
resolved in the spectrum of Fe(H2 PO4 )2 ·2H2 O (1262 cm−1 and the duplet at 820/790 cm−1 , Fig. 1), the freeze-dried solids show the characteristic vibrations for deprotonated phosphate ions only (3 at 1015–1070 cm−1 and 4 at 560 cm−1 ). Further on, the observed upshift of the asymmetric stretching 3 vibration compared to that of the free phosphate ion (1017 cm−1 ) evidences for the coordination of PO4 3− to the metal ions in the freeze-dried compositions. The third spectral feature in the IR spectra is the water molecule vibrations. A broad featureless band at 3400 cm−1 with a shoulder around 3200 cm−1 dominates in the IR spectra of the precursors. As a comparison, for Fe(H2 PO4 )2 ·2H2 O and Fe(HCOO)2 ·2H2 O where the water molecules are coordinated to the Fe2+ ions, the OH stretching vibrations appear in the range of 3150–3370 cm−1 (Fig. 1). For LiHCOO·H2 O, the OH stretching vibrations of the water molecule bonded to Li+ are at 3398 and 3109 cm−1 [27]. Hence, the broad band observed for Li-Fe phosphate-formate precursors could be due to the superposition of the OH stretching vibrations of the water molecules bonded to both metal ions. The weak band around 900 cm−1 could be tentatively assigned to the water libration mode [31]. Summarizing, it appears that during the freeze-drying process solids with compositions LiFePO4 Hx (HCOO)x ·yH2 O (x ∼ 1; 1 < y <1.3 and x < y) are obtained, where the metal ions are bonded to the deprotonated phosphate and formate ions. The charge neutrality is achieved by insertion of one proton (most probably H3 O+ ) in the precursor framework. It is worth mentioning that the appearance of H3 O+ within the crystal structure of different compounds is well documented, for example H3 OM(SO4 )2 (M3+ Al, Ga, In, Ti, and Fe) [32], (H3 O)UO2 PO4 ·3H2 O [33], etc. In compounds containing different protonic species like H3 O+ , H5 O2 + , etc., the stretching OH vibrations appear in the range of 3500–2500 cm−1 and the bending ı(H3 O+ ) vibration—in the range of 1700–1750 cm−1 [34,35]. Therefore, in the spectra of the precursors the shoulder observed at 1720 cm−1 could be related with the ı(H3 O+ ) vibration and the OH bands of H3 O+ could be overlapped with the vibrations of H2 O molecules as it is reported in Ref. [35]. While the concentration of the freeze-dried solution does not influence the compositions and structure of precursors, the pre-
cursor’s morphology is affected. Fig. 2 shows the SEM images of the precursors obtained from the diluted and concentrated solutions. While the precursor Pr–S005 obtained from the diluted solution comprises fine flake-like aggregates (Fig. 2(A)), the thick platelike aggregates gives rise to the morphology of Pr–S02 obtained from the concentrated solution (Fig. 2(B)). The transformation of fine aggregates into thicker ones by increasing the concentration of freeze-dried solutions has also been established for the preparation of layered LiCo/NiO2 cathode materials by freeze-drying of lithium–cobalt citrate solutions [36]. 3.2. Structural characterization of LiFePO4 Homogeneous Li-Fe phosphate-formate precursors are unstable and, above 50 ◦ C, they are decomposed (Fig. 3). Decomposition process is developed from 50 to 350 ◦ C with a total weight loss of 28%. Based on the numerous data on thermal decomposition of metal formates [37,38] the first broad endothermic effect between 50 and
Fig. 3. TG (solid line), DTA (dot line) and DTG (dash line) curves for the precursor Pr–S01.
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150 ◦ C is due to the dehydration, while the second strong endothermic peak at 240 ◦ C followed by the broad complex exothermic effect between 300 and 400 ◦ C corresponds to the decomposition of the formate ions. The decomposition process is completed at 350 ◦ C. This is confirmed by the IR spectrum of the precursor Pr–S02 heated at 350 ◦ C for 20 min (Fig. 1), where no IR bands due to the formate ions vibrations are observed. It should be mentioned that the observed broadening of the exothermic effect at a temperature higher than 350 ◦ C accompanied by a small weight loss can be explained by partial oxidation of Fe2+ as a result of oxygen contaminant in a small amount in the argon used. The formation of the olivine-type phase LiFePO4 starts at 350 ◦ C immediately after the release of the formic acid (Fig. 1). The IR spectrum of the sample obtained at 350 ◦ C for 10 h coincides with the IR spectrum typical of the olivine-type phase LiFePO4 [39]. The shoulder at 945 cm−1 and the band at 970 cm−1 are assigned to the symmetric stretching P–O vibration of the PO4 3− group, while the rest three bands at 1060, 1098 and 1138 cm−1 are due to the asymmetric stretching P–O vibrations. The asymmetric bending OPO mode splits into four bands at 550, 578, 636 and 648 cm−1 . The bands at 500 and 470 cm−1 originate essentially from the Li+ translations. It is important that the IR bands profile of the sample obtained at 350 ◦ C for 10 h is identical with the IR bands profiles of samples annealed at 400, 500 and 600 ◦ C (Figs. 1 and 4). From the IR spectroscopic data it is evident that the olivine-type phase LiFePO4 is already formed at 350 ◦ C and remains stable during the annealing between 400 and 600 ◦ C. Irrespective of the concentration of the freeze-dried solution, well-crystalline powders of LiFePO4 without any XRD detectable impurities are obtained in the temperature range of 400–600 ◦ C (Fig. 5). The data from chemical analysis show some impurity of Fe3+ ions with concentration less than 1.5%. The diffraction peaks have been indexed to a single-phase having an ordered olivinetype structure, S.G. Pnma [40]. The unit-cell dimensions determined from the Rietveld refinement of the XRD patterns are given in Table 1. On the first glance, the lattice parameters and unit-cell volumes are not sensitive towards both the annealing temperature and the concentration of freeze-dried solutions. For LiFePO4 annealed between 500 and 600 ◦ C, the lattice parameters coincide well with those reported in literature for LiFePO4 obtained at temperature higher than 600 ◦ C [7,18,41]. This means that the freeze-drying method allows formation of “defectless” LiFePO4 phases even at lower temperatures. In addition, close inspection of Table 1 reveals a slight expansion of the unit-cell volume with increasing the annealing temperature from 400 to 500 ◦ C. Slightly lower unit-cell parameters have been established by Kim et al. [16] and Masquelier and co-workers [17] when carbon-free LiFePO4 with nanometric particle dimensions (about 40 and 140 nm, respectively) are obtained: 289.8 Å3 [16] and 288.7 Å3 [17] as compared to 289.8–290.0 Å3 for our samples of LiFePO4 annealed at 400 ◦ C. The physical meaning of the contracted unit-cell volumes observed for nano-sized LiFePO4 is not clear yet [17]. Another parameter derived from XRD patterns is the Scherrer crystallites size. Fig. 6(A) shows the dependence of the Scher-
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Fig. 4. IR spectra of LiFePO4 annealed at 400, 500 and 600 ◦ C.
rer crystallites size (D200 ) on the annealing temperature as well as on the concentration of the solutions subjected to freezedrying. At 400 ◦ C, crystallites with sizes of about 27–28 nm are formed irrespective of the solution concentration. By increasing the annealing temperature, there is a crystallite growth especially for LiFePO4 originating from Pr–S02. It is noticeable that, for LiFePO4 –S005 originating from the diluted solution, the crystallite growth between 400 and 600 ◦ C is restricted. The observed trends in the crystallite growth are further supported by the dependence of the specific surface areas on the annealing temperature (Fig. 6(B)). At 400 ◦ C, the LiFePO4 samples display a maximumspecific surface area of about 34–35 m2 /g which is not sensitive towards the solutions concentration. After increasing the annealing temperature, a stronger reduction in the specific surface area is observed for LiFePO4 –S02 obtained from the concentrated solution (from 35 to 13 m2 /g), while the specific surface areas of the samples LiFePO4 –S005 obtained from the most diluted solution are slightly decreased (from 35 to 32 m2 /g) (Fig. 6(B)). The effect of the concentration of the freeze-dried solution on the particle dimensions is illustrated in Fig. 7. So, nano-sized isometric particles are seen in the powders prepared from the concentrated freeze-dried solutions (Fig. 7(B and C)), while (at the same magnification) individual particles cannot be distinguished for the
Table 1 Lattice parameters, unit-cell volumes and specific surface areas of LiFePO4 prepared from different freeze-dried precursors and annealing temperatures. Pr–S005
a (Å) b (Å) c (Å) V (Å3 ) SBET (m2 /g) a
Pr–S01
Pr–S02
400a
500a
600a
400a
500a
600a
400a
500a
600a
10.3116(7) 5.9963(4) 4.6905(3) 290.02(3) 34
10.3187(5) 6.0017(6) 4.6940(6) 290.69(3) 32
10.3243(3) 6.0041(2) 4.6908(1) 290.78(2) 32
10.3195(6) 5.9937(3) 4.6909(3) 289.86(4) 34
10.3201(6) 6.0010(5) 4.6937(4) 290.68(3) 30
10.3228(4) 6.0047(4) 4.6918(2) 290.83(2) 18
10.3089(5) 5.9939(3) 4.6908(2) 289.86(2) 35
10.3203(4) 5.9995(2) 4.6920(2) 290.51(4) 25
10.3257(3) 6.0042(2) 4.6902(1) 290.79(2) 13
Temperature (◦ C)
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Fig. 5. Rietveld refinement of the XRD patterns of LiFePO4 –S02 annealed at 400 and 600 ◦ C.
powder prepared from the diluted solution (Fig. 7(A)). This will be discussed further on. For LiFePO4 obtained from the two concentrated solutions, the particle size distributions determined from SEM images are compared in Fig. 8. It is seen that, for LiFePO4 –S01 more than 3/4 from the particles are within the 60–120 nm range and the mean particle size is 90 ± 15 nm. A little bit larger are the particles of LiFePO4 –S02, where about 3/4 from the particles are distributed between 80 and 140 nm and the mean particle size is 110 ± 12 nm. It is important that SEM particle dimensions coincide with particles sizes determined from the specific surface areas—90 and 130 nm for the samples LiFePO4 –S01 and LiFePO4 –S02, respectively. In addition, the SEM particle dimensions are slightly larger as compared to crystallites sizes determined from XRD line broadening (Table 1). This indicates a formation of well-crystalline nano-sized LiFePO4 from freeze-dried Li-Fe phosphate-formate precursors. The release of the formic acid is accompanied by the deposition of a small amount of in situ formed carbon. It is important that the carbon content depends on the concentration of the freeze-dried solution. At 400 ◦ C, the carbon content is 2.66, 0.64 and 0.53 mass% for the samples LiFePO4 –S005, LiFePO4 –S01 and LiFePO4 –S02, respectively. By increasing the annealing temperature, the carbon content decreases and at 600 ◦ C it is 1.1, 0.43 and ≤0.3 mass% for LiFePO4 –S005, LiFePO4 –S01 and LiFePO4 –S02, respectively. From these data it appears that LiFePO4 samples prepared from the diluted solution have the highest carbon content irrespective of the annealing temperature. It is worth noting that a carbon amount of about 1.45 mass% has been shown to improve significantly the rate capability of LiFePO4 electrode materials [42]. The carbon deposition on LiFePO4 samples was checked by XPS analysis. The C 1s spectra (Fig. 9) display a broad band centred
Fig. 6. Dependence of the crystallite size D200 (A) and of specific surface areas (B) of LiFePO4 prepared from the different precursors on the annealing temperature.
at 285.0 eV with intensities decreasing as the concentration of the freeze-dried solution increases. For the samples LiFePO4 –S005, LiFePO4 –S01 and LiFePO4 –S02 annealed at 600 ◦ C, the surface carbon content is 23.3, 11.1 and 7.4 at.%, respectively. The decrease in the surface carbon content (determined from XPS analysis) is in agreement with the observed trend in changes of the total carbon content determined from the chemical analysis. On the other hand, the ratio of the surface carbon amount to the total carbon amount is higher than 10:1 for all samples studied, which indicates that the carbon is deposed mainly on the particles surface of LiFePO4 . To check this suggestion, Fig. 10 compares the dependences of the total and surface carbon amounts on the specific surface areas of LiFePO4 prepared at 600 ◦ C. There is a good correlation “total carbon amount – surface carbon amount – specific surface area”. The deposition of higher amount of carbon for LiFePO4 –S005 obtained from the diluted solutions is a fact that could explain the restricted particle growth at temperatures higher than 400 ◦ C. It appears that the higher is the carbon amount, the more restricted is the crystallite and particle growth (Figs. 2 and 7). Also, the carbon deposition gives rise to the morphology of target LiFePO4 . The morphology of all samples studied comprises the plate-like aggregates with micrometric dimensions. However, the surface of aggregates seems to be flatter for LiFePO4 –S005 obtained from the diluted solution (Fig. 2(C)). This is, most probably, a consequence of the higher amount of carbon deposed during formic
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Fig. 8. Particle size distributions of LiFePO4 –S01 and LiFePO4 –S02 annealed at 600 ◦ C.
sensitive towards the concentration of freeze-dried solutions and the temperature of annealing. However, the porous structure would be favourable for the application of LiFePO4 as cathode materials [8,11].
Fig. 7. SEM images of LiFePO4 –S005 (A), LiFePO4 –S01(B) and LiFePO4 –S02 (C) annealed at 600 ◦ C.
acid release. In addition, the higher amount of carbon make difficulties in the observation, at higher magnification, of individual primary particles for LiFePO4 –S005 obtained from diluted solutions (Fig. 7(A)). The formation of micrometric aggregates from nano-sized particles determines the complex porous structure of LiFePO4 . Adsorption–desorption isotherms of LiFePO4 prepared at 400 and 600 ◦ C (not presented here) are of IV type, which is characteristic for the presence of large amount of meso- and micropores. The pore size distribution obtained by the method of Pierce [43] (Fig. 11) reveals that the pores are continuously distributed within the mesopore region of RP up to 25–30 nm with average pore sizes between 3.3 and 4 nm. It looks like that porous structure is not
Fig. 9. XPS spectra of LiFePO4 –S005 (A), LiFePO4 –S01 (B) and LiFePO4 –S02 (C) annealed at 600 ◦ C.
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from the diluted solution display better electrochemical performance. Further works are in progress. 4. Conclusion
Fig. 10. Carbon content of LiFePO4 annealed at 600 ◦ C in dependence on the specific surface areas of samples prepared from the freeze-dried solutions with concentrations 0.05, 0.1 and 0.2 M: Curve 1, total carbon content from the chemical analysis; Curve 2, surface carbon content form XPS analysis.
The freeze-drying method is successfully used for the preparation of cathode materials based on layered and spinel oxides [36,44,45]. Concerning LiFePO4 electrodes, the application of freeze-drying technique has recently been reported by Rojo and co-workers [46] using ferrous acetate (FeC4 H6 O4 ) as iron source and citric acid as chelating agent and carbon source. In our variety of the freeze-drying method, we have combined the stability of iron (II) formate salt with the control of the solution concentration as an instrument to design the lithium-iron phosphate-formate precursors. As a result, we are able to change both particle dimensions and carbon content. The freeze-drying method is suitable for the preparation of LiFePO4 as cathode materials for lithium ion batteries. The preliminary electrochemical data show that LiFePO4 obtained at 500 ◦ C
Freeze-drying of solutions containing Li+ , Fe2+ , PO4 3− and HCOO− in a ratio of 1:1:1:2 yields LiFePO4 Hx (HCOO)x ·yH2 O compositions (x ∼ 1, 1 < y < 1.3 and x < y), where deprotonated phosphate and formate ions are coordinated around metal ions. During the freeze-drying process, 1 mol of formic acid is sublimated. By variation of the concentration of the freeze-dried solutions, the morphology of the phosphate-formate precursors is changed from fine flake-like to thicker plate-like aggregates. The formation of the olivine-type phase LiFePO4 starts at 350 ◦ C immediately after the release of the formic acid. Further annealing between 400 and 600 ◦ C yields defectless well-crystalline LiFePO4 powders containing less than 1.5% Fe3+ impurities. The morphology of all samples studied comprises the plate-like aggregates, which are composed of isometric primary particles with close particle size distribution in the nanometric range from 60 to 140 nm. The release of the formic acid is accompanied by the deposition of carbon, whose amount decreases with increasing the concentration of the freeze-dried solutions. The higher content of deposed carbon limits the particle growth as the annealing temperature increases from 400 to 600 ◦ C. As a result, the particle dimensions of LiFePO4 annealed at 600 ◦ C become sensitive towards the concentration of the freeze-dried solution. The ability of the freeze-drying method for precursor design provides an opportunity to rationalize the synthesis of LiFePO4 (in respect of particle dimensions and carbon content) as cathode materials for lithium-ion batteries. By using additional organic chelating agents it is possible to vary smoothly the amount of carbon content keeping the particle dimensions in nanometric scale. Acknowledgment Authors are grateful for the financial support from the National Science Fund of Bulgaria (Ch 1701/2007). References
Fig. 11. Pore size distribution curves of LiFePO4 prepared from the different precursors and annealing temperatures.
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