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Defect chemistry of phospho-olivine nanoparticles synthesized by a microwave-assisted solvothermal process
Journal of Solid State Chemistry 205 (2013) 197–204
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Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Defect chemistry of phospho-olivine nanoparticles synthesized by a microwave-assisted solvothermal process Craig A. Bridges a,n, Katharine L. Harrison c, Raymond R. Unocic b, Juan-Carlos Idrobo b, M. Parans Paranthaman a, Arumugam Manthiram c a
Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA c Electrochemical Energy Laboratory and Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX 78712, USA b
art ic l e i nf o
a b s t r a c t
Article history: Received 18 May 2013 Received in revised form 11 July 2013 Accepted 12 July 2013 Available online 20 July 2013
Nanocrystalline LiFePO4 powders synthesized by a microwave-assisted solvothermal (MW-ST) process have been structurally characterized with a combination of high resolution powder neutron diffraction, synchrotron X-ray diffraction, and aberration-corrected HAADF STEM imaging. A significant level of defects has been found in the samples prepared at 255 and 275 1C. These temperatures are significantly higher than what has previously been suggested to be the maximum temperature for defect formation in LiFePO4, so the presence of defects is likely related to the rapid MW-ST synthesis involving a short reaction time (∼5 min). A defect model has been tentatively proposed, though it has been shown that powder diffraction data alone cannot conclusively determine the precise defect distribution in LiFePO4 samples. The model is consistent with other literature reports on nanopowders synthesized at low temperatures, in which the unit cell volume is significantly reduced relative to defect-free, micron-sized LiFePO4 powders. Published by Elsevier Inc.
Keywords: Lithium-ion batteries LiFePO4 cathode Antisite disorder Neutron diffraction
1. Introduction Due to the critical importance of particle size and morphology on the electrochemical performance of battery electrode materials, there has been a great deal of interest in finding optimum approaches to electrode material preparation. In the case of LiFePO4 (LFP) [1], synthesis approaches that provide highly crystalline nanoparticles are essential to obtain high-rate capability with cyclability [2–4]. Surface modification and coating can further improve the electrochemical properties [2–5]. Of particular importance to the electrochemical performance is the formation of antisite defects on the lithium site, which if present are expected to inhibit diffusion of Li+ ions through channels in the [010] direction. A number of different native point defects may occur in LiFePO4, depending upon the synthesis conditions employed. The possible defects include the presence of Fe on the Li site (FeLi), lithium on the Fe site (LiFe), and vacancies on the Li (VLi) and Fe (VFe) sites. Furthermore, these defects have been predicted to occur in one stable charge state, corresponding to FeLi+, LiFe , VLi and VFe2 [6]. These charged defects may form complexes containing pairs of these defects, such as the FeLi+–LiFe antisite
n
Corresponding author. Fax: +1 865 574 4961. E-mail address: [email protected] (C.A. Bridges).
0022-4596/$ - see front matter Published by Elsevier Inc. http://dx.doi.org/10.1016/j.jssc.2013.07.011
defect pair, or the combination of antisite defect and vacancy represented as FeLi+–VLi . Several computational investigations have predicted that the FeLi–LiFe antisite defect pair is energetically favored with respect to other possible defects [7,8], and complimentary investigations have suggested that the type of defect that is energetically favorable may depend upon the synthesis conditions [6,9]. Research has also focused on the effect of aliovalent or non-aliovalent doping on the electrochemical properties and defect formation in LiFePO4 [8–16]. A number of publications have been reported to experimentally characterize the distribution of Li and Fe cations in Olivine LiFePO4, both during synthesis and during electrochemical cycling in a Li-ion battery. Briefly, the results suggest that for micron-sized samples prepared at elevated temperatures (typically ∼700 1C), few defects are observed for stoichiometric samples, but that for samples prepared under Li-deficient conditions it is possible to observe the formation of FeLi+–VLi defects [17,18]. This supports related computational investigations on the influence of synthesis environment given above [6]. Several STEM investigations, supported by DFT-based computational investigations, suggest that FeLi+ [10,19], FeLi+–VLi [20], and FeLi+–LiFe [21] defects may cluster even at low defect concentrations. In the case of FeLi+– LiFe , it has been reported that LiFe has been observed in a cluster of the neutral defect complex [21]. The observation of LiFe defects have generally not been reported due to the difficulty of observing
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intensity changes in a column of Fe cations with substitution of a small fraction of Li cations, but due to the clustering of the neutral defect complex, it may be possible to overcome this limitation. The type of defects present in LiFePO4 may be influenced by synthesis under lower temperatures, potentially non-equilibrium conditions that are typically used to prepare nanoparticle samples. In situ diffraction studies of hydrothermal synthesis suggest that antisite defects may disappear in minutes at temperatures above ∼270 1C (with some dependence upon the starting precursors used), but that defects may persist for longer times at lower temperatures [22]. Hydrothermal synthesis has previously been shown to promote antisite defect formation at temperatures below ∼200 1C [23]. Furthermore, single phase samples with unusually low Li:Fe ratios have been reported for very low-temperature synthesis of nanoparticles based upon a co-precipitation process [24]. The formation of nanoparticles is particularly important as it can enhance Li+ diffusion in LiFePO4 for Li-ion battery applications when the formation of detrimental FeLi+ defects can be avoided [25,26]. Interestingly, particle size may also influence the lithiation/ delithiation mechanism, and therefore the distribution of lithium within the structure. Nanoparticles below a certain size limit may demonstrate single phase cycling behavior (solid solution behavior) [24,27] whereas larger particles typically exhibit phase segregation into Li-rich and Li-poor compositions [28–30]. Phase segregation during cycling is suggested to be promoted by the presence of edge-sharing FeO6 octahedra and PO4 tetrahedra, which limit the structural flexibility to accommodate solid solution behavior. However, this picture may be further modified under non-equilibrium cycling conditions, resulting in a mixture of solid solution and phase segregation behavior regardless of particle size [27,31]. Clearly the synthesis conditions, the resulting particle size, and the eventual battery cycling conditions may all influence how lithium is distributed within the olivine structure. The microwave-assisted solvothermal (MW-ST) approach has been shown to produce highly crystalline nanorods of up to 1 μm length at temperatures ranging from ∼230 to 300 1C [3,32,33]. Moreover, reaction times as short as 5–15 min can be used to obtain single phase powders. Previous hydrothermal experiments have suggested that LFP produced at temperatures above ∼180 1C should not contain a significant concentration of antisite defects [23,34]. For the preparation of MW-ST LFP nanorods, experiments have shown that for synthesis temperatures below 250 1C under solvothermal conditions, the samples do not crystallize completely. Therefore, we were interested to understand whether antisite defects might be stabilized under solvothermal processes at higher temperatures than 180 1C. To investigate this possibility, we present here a detailed structural analysis of particles synthesized by the MW-ST method, combining high quality, high resolution powder X-ray diffraction, neutron diffraction, and electron diffraction data. This work shows that antisite defects are present for samples synthesized at 255 and 275 1C, and that post-annealing at 700 1C is shown to reduce these defects in agreement with the minimal defect concentrations observed in higher temperature synthetic approaches with stoichiometric Li:Fe molar ratios.
2. Experimental 2.1. Synthesis LiFePO4 was synthesized by a microwave-assisted solvothermal technique that has been described in more detail elsewhere [32]. Stoichiometric amounts of lithium hydroxide (Fisher), iron acetate (STREM), and phosphoric acid (Fisher) were stirred in tetraethylene glycol (Alfa Aesar) such that the solutions were 0.18 M in Li
and P. The solutions were sealed in high pressure quartz vessels and placed on a turntable in an Anton Paar Synthos 3000 microwave reactor. A constant power of 600 W was applied until the temperature reached 255 and 275 1C in two respective reactions. These temperatures were then held for 5 min. Additionally, the sample synthesized at 275 1C was then post-annealed in a flowing 5% H2–95% Ar atmosphere for 6 h at 700 1C with heating and cooling rates of 10 1C/min. In order to obtain samples of sufficient size for neutron diffraction experiments, multiple syntheses were performed under identical conditions using the same starting solutions, and the resulting powders obtained were combined. 2.2. Powder X-ray diffraction Powder X-ray diffraction (XRD) data were collected at beamline 11-BM at the APS at 300 K, with a wavelength of 0.412437 Å and a step size of 0.0011. The instrument has a resolution of ΔQ/Q¼1.7 10-4, corresponding to a peak FWHM at the measurement energy of ∼0.0051 2θ. Powder samples were contained within 0.8 mm Kapton capillary tubes, and for better powder averaging the samples were spun at 60 Hz during data collection. Data were analyzed over a range from 31 to 401 2θ. 2.3. Powder neutron diffraction Powder neutron diffraction (PND) data were collected at 300 K on the POWGEN beamline at the Spallation Neutron Source (SNS). Isotopically enriched lithium (7Li) was used to remove uncertainties in the correct bound coherent neutron scattering length ( 2.22 fm for 7Li) that can exist with natural lithium. The samples were contained in 8 mm vanadium cans. Powder X-ray diffraction data and neutron diffraction data were analyzed for unit cell constants, phase fractions, and structural information with Rietveld refinement [35] with GSAS/EXPGUI [36,37] and Fullprof/ WinPLOTR [38,39]. 2.4. Scanning Transmission Electron Microscopy Atomic-resolution, Z-contrast, high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) experiments were acquired with an aberration-corrected STEM Nion UltraSTEM 100 instrument [49] equipped with a cold-field emission electron source and a corrector of third- and fifth-order aberrations. The electron microscope was operated at 100 kV accelerating voltage, the convergence semiangle of the incident probe was set to 30 mrad, and the HAADF images were collected from 86 to 200 mrad half-angle range. 3. Results Combined Rietveld refinement of the powder neutron diffraction data and X-ray diffraction data confirm that regardless of reaction temperature, the powders can be indexed with the Pnma olivine structure (Fig. 1 and S4; Tables 1 and 2). The samples are largely free of impurities, though the sample prepared at 255 1C apparently contains a very small fraction of spinel Fe3O4 (∼0.22 wt %) and Goethite FeOOH (∼0.16 wt%) impurities, on the basis of comparison with the PDF database for a small number of weak peaks (Fig. 1). While the presence of Fe-rich impurities may imply a Fe deficiency in the 255 1C sample, there is uncertainty due to the fact that the washing step in the synthesis may remove phosphates or Li-containing impurities preferentially to these Fe-rich impurities. Also, the iron precursor is highly hygroscopic, which implies difficulty in adding exactly stoichiometric amounts
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Fig. 1. Comparison of refinements of the Bank 5 TOF powder neutron diffraction data (left) and synchrotron powder X-ray diffraction data (right) for the 255 1C sample (a–b), the 275 1C sample (c–d), and the 700 1C sample (e–f). There are weak (□0.5 wt%) impurities of Fe3O4 and FeOOH in the 255 1C LiFePO4 sample, as indicated in (a) and (b) by the second and third set of reflection markers, respectively.
of iron precursor to the reaction solution. ICP and TGA are used to determine the water content, but these are only accurate to ∼1–3%, so the small fraction of iron impurities can easily be explained by slight inaccuracies in the weight of the iron precursor due to limitations in water content analysis. Other potential impurity phases that have been previously reported in other LiFePO4 syntheses, such as Li3PO4, Fe3(PO4)2(OH)2, Fe3(PO4)2 or Li3Fe2(PO4)3 were not observed [17,33]. The trend in lattice parameters given in Table 1 indicates an overall increase in volume with increasing sample preparation temperature. The volume increases from 290.282(3) Å3 at 255 1C, to 290.845(2) Å3 at 275 1C, and to 290.902(1) Å3 at 700 1C. The atomic displacement parameters (Biso) have similar magnitudes in the range of ∼0.5– 0.85 Å2 with the exception of the lithium site, which is ∼1.6–2.1 Å2.
The larger Biso for the Li site has been previously observed in structural refinements of LiFePO4 [23], and could represent a slight static or dynamic off-center distortion from the (0,0,0) site position (Li site). The olivine structure (Fig. S1) consists of a hexagonal close-packed array of oxide ions, in which half of the octahedral sites are occupied by M cations, and one-eighth of the tetrahedral sites are occupied by phosphorous ions. There are three distinct oxygen sites, one phosphorous site, and two metal sites (M1 and M2, corresponding to Li and Fe sites in the defect-free structure) [40]. To obtain the best fit for a combined refinement of neutron diffraction and X-ray diffraction data, it was necessary to refine all positional parameters, occupancies of the lithium and iron sites, weight fractions, and all atomic displacement parameters with lithium, iron, and oxygen sites refined anisotropically. Further
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Table 1 Refinement results for LiFePO4 samples with structural parameters obtained at 300 K, showing a comparison between refinements at different temperatures with Model C. Preparation method Sample
MW-ST (nano-sized) 255 1C
MW-ST (nano-sized) 275 1C
MW-ST (nano-sized) 700 1C
Lattice parametersa
a (Å) b (Å) c (Å) V (Å3)
10.31626(4) 5.99528(2) 4.69341(1) 290.282(3)
10.32826(5) 5.99997(2) 4.69337(2) 290.845(2)
10.32763(2) 6.00433(1) 4.69118(1) 290.902(1)
Fractional coordinates
Li2/Fe2 x Li2/Fe2 z Px Pz O1 x O1 z O2 x O2 z O3 x O3 y O3 z Li/Fe Li2/Fe2 P O O2 O3 LFP1 Fe3O4 FeOOH
0.28187(5) 0.97537(13) 0.09504(10) 0.41788(20) 0.09734(24) 0.74076(47) 0.45607(25) 0.20774(46) 0.16604(19) 0.04627(26) 0.28348(28) 1.64(13) 0.68 0.73(2) 0.58 0.58 0.58 99.6% 0.21% 0.16%
0.28172(7) 0.97580(17) 0.09524(14) 0.41736(27) 0.09711(28) 0.74022(59) 0.45567(30) 0.20928(59) 0.16584(23) 0.04682(33) 0.28346(36) 2.09 0.85 0.86(3) 0.69 0.69 0.69 100%
0.28204(5) 0.97522(12) 0.09480(9) 0.41802(19) 0.09690(20) 0.74226(43) 0.45637(21) 0.20743(43) 0.16575(16) 0.04679(23) 0.28480(27) 1.73 0.51 0.43(2) 0.53 0.53 0.53 100%
ADP (Biso) (Å2)
Phase fractions
a All crystallography data are for the combined refinement of neutron and X-ray powder diffraction data at 300 K; where the ADP is shown in italics, this indicates an anisotropic refinement was used and the equivalent isotropic thermal parameter is listed here (full description is given in the supplemental); the phase fraction is given as a weight percent.
Table 2 Refinement results for LiFePO4 samples, showing a comparison between refinements with no antisite disorder and different models of antisite disorder. Single phase 255 1C
Single phase 275 1C
Single phase 700 1C
χ2 (global) χ2 (XRD) wRp (XRD) Rp (XRD)
2.17 1.91 8.81 6.71
1.81 1.27 9.15 6.92
2.07 1.28 8.25 6.21
χ2 (global) χ2 (XRD) wRp (XRD) Rp (XRD)
1.6(1)% 1.6(1)% 1:1 2.11 1.89 8.75 6.60
2.4(1)% 2.4(1)% 1:1 1.76 1.23 9.02 6.77
0.3(1)% 0.3(1)% 1:1 2.08 1.27 8.22 6.17
χ2 (global) χ2 (XRD) wRp (XRD) Rp (XRD)
3.8(2)% 0.8(1)% 1.03:0.97 2.10 1.87 8.70 6.59
3.0(2)% 2.2(2)% 1.00:0.99 1.76 1.23 9.02 6.78
1.0(2)% 0.9(2)% 1.00:0.99 2.08 1.27 8.22 6.18
χ2 (global) χ2 (XRD) wRp (XRD) Rp (XRD)
3.6(1)% 0.8(2)% 0.4(14)% 0.99:0.97 2.09 1.87 8.71 6.59
3.0(2)% 2.2(2)% 0% 0.99:0.99 1.75 1.23 9.02 6.78
1.0(2)% 0.8(2)% 0% 0.99:0.99 2.06 1.27 8.21 6.18
Sample Model A: No Antisites Statistical fita
Model B: Constrained antisites LiFeb FeLi Li:Fe ratio Statistical fit
Model C: Independent antisites LiFe FeLi Li:Fe ratio Statistical fit
Model D: Antisites/Vacancies VFe FeLi VLi Li:Fe ratio Statistical fit
a All crystallography data are for the combined refinement of neutron and X-ray powder diffraction data at 300 K; full statistics for the XRD data are shown for comparison between the models of antisite defect formation. For Models A–C, there are no vacancies as the sites are constrained to be fully occupied. b FeLi refers to an antisite defect involving Fe on the Li site, LiFe refers to Li on the Fe site, and VLi and VFe correspond to Li/Fe vacancies; for the “constrained antisite” refinement the FeLi+–LiFe defect pair is assumed.
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details of the refined parameters are given for one model of antisite defects (Model C, described below) in Table 1, and are representative of the results for different antisite defect models. The variation in peak full-width at half maximum (FWHM) versus diffraction angle for the 255 and 275 1C samples did not follow a simple polynomial function. Clear anisotropy in FWHM versus angle is shown in Fig. 2 (and over a wider 2θ range in Fig. S5), in which the peaks with larger h indices were markedly broader than the other reflections. This indicates that the crystalline domain size is significantly smaller along the a axis. To better quantify the anisotropy in domain size, orthorhombic spherical harmonic anisotropic size broadening was introduced into the Rietveld refinement in Fullprof. The size broadening parameters indicate a much more
Fig. 2. Comparison of peak FWHM versus diffraction angle derived from synchrotron powder X-ray diffraction data for the 255 1C sample (filled triangle), the 275 1C sample (open circle), and the 700 1C sample (filled square). Indexed X-ray diffraction data for the 255 1C sample are given below. The sharper reflections are highlighted with a yellow background in both plots. The broader reflections are those with a larger h index, due to the shorter coherent domain size along the a axis. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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anisotropic peak broadening for the 255 and 275 1C samples than for the 700 1C sample (Figs. 3 and S3). Interestingly, the refined crystallite domain sizes for the lower-temperature (non-annealed) samples are essentially identical within the error of the fit despite the slightly smaller FWHM values derived from the refinement. The smaller FWHM values for the 275 1C sample suggests that the average domain sizes are slightly larger than for the 255 1C sample, but are similar enough that they appear the same within the error of the Rietveld-based size determination. Due to the presence of competing models for refinement of LiFePO4 antisite disorder in the literature, a number of different antisite defect models have been examined here and are compared in Tables 2 and S1. The purpose of this approach is to examine the possibility that the MW-ST synthetic method could produce a different defect distribution than has previously been reported in the literature, and further to test the limits of high quality diffraction data in determining the distribution of defects in LiFePO4. For the Rietveld refinements, we consider that the PO4 sublattice is fully occupied (i.e., there are no V PO4 ) to serve as a reference for occupancy refinement of the Li and Fe sites. The antisite defect models used in the Rietveld refinements, and summarized in Table 2, are: (A) no antisite defects; (B) constrained antisite defect formation in which equal amounts of LiFe and FeLi are present (i.e., FeLi+–LiFe expected for a Li-rich environment), such that the overall formula of “LiFePO4” is maintained; (C) a model in which the level of LiFe and FeLi defects is refined independently on each site, and the sites are constrained to be fully occupied; (D) same as Model C, but with vacancies possible on the Li and Fe sites, and no LiFe defects. As a non-exhaustive list, examples of antisite defect Model B are found in the work of Chung et al. [21,41] and Islam et al. [8], and discussion of defect models related to Models C/D has been examined by Lee et al. [20] and Hamelet et al. [42]. A small but significant improvement in the refinement over Model A (global χ2 of 2.17) is observed upon the addition of antisite defects in Model B, with the result of global χ2 of 2.11, and for the XRD data a χ2 of 1.89, wRp of 8.75, and Rp of 6.60. For the Model B refinements of the lower-temperature samples, the level of antisite defects are ∼1–2%, and clearly higher than that of the 700 1C sample, which has a very low level of defects (∼0.3%). For model C, the independent refinement of antisite defects suggests that for the lower-temperature samples, there could be a greater fraction of LiFe antisite defects than FeLi defects. For the 255 1C sample, this produces a composition of [Li0.992(12)Fe0.008(2)][Fe0.962(2)Li0.038(2)] PO4. This composition should produce an imbalance in the ratio of Li:Fe:P, and the relative increase in Li+ content on the fully occupied sites should suggest a partial oxidation of iron to form Fe3+. For Model D, the MW-ST 255 1C sample refinement was initially carried out with LiFe defects for a refined composition of
Fig. 3. Average coherent domain size of LiFePO4 increases and becomes less anisotropic with a higher-temperature post-annealing. The refined maximum c axis domain sizes are ∼142 nm, ∼120 nm, and ∼164 nm for the 255, 275, and 700 1C samples, respectively. Average apparent domain sizes are 70(30) nm, 72(23) nm, and 126(20) nm for the 255, 275, and 700 1C samples, respectively.
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[Li0.988(12)Fe0.008(2)□0.004(14)][Fe0.962(2)Li0.038(2)]PO4, with a global χ2 of 2.10, and for the XRD data a χ2 of 1.87, wRp of 8.70, and Rp of 6.59. Replacement of the LiFe antisite defects with vacancies (VFe) gives a composition [Li0.988(12)Fe0.008(2)□0.004(14)][Fe0.964(2)□0.036(2)] PO4 with the same goodness-of-fit. In the case of the refinement with VFe defects, the refined ratio of Li:Fe is close to 1:1, as expected from samples produced by the MW-ST synthesis [3,32,33]. It is immediately apparent when comparing the goodness-of-fit indicators for different antisite defect models that the fits are of essentially identical quality for the three different models B, C, and D, indicating that despite the combination of high quality X-ray and neutron diffraction data, the refinement cannot clearly distinguish between vacancies and lithium defects on the Fe site. It should be noted that a significant asymmetry was evident in the more intense peaks of the high resolution synchrotron X-ray diffraction data for the 255 1C sample, which was not immediately evident in the TOF neutron diffraction data. This asymmetry could not be fully fitted by the asymmetry functions available in Fullprof, which suggests that a small compositional inhomogeneity may exist in this sample. To improve the fit to the data and examine this possibility, refinements were made using two LiFePO4 phases, with the results shown in Fig. S2 and Table S1. The additional parameters provided by the second phase improved the fit, with a decrease in the global χ2 from 2.10 to 1.94. The improvement was far more prominent for the fit to the high resolution X-ray data, and the goodness of fit parameters are, therefore, presented for these data in addition to the global χ2. The structural parameters resulting from the single phase refinement are found to generally be an average of the parameters obtained for the two phase refinement, as may be expected given that the similar refined weight fraction of the two phases (∼50:50 ratio of the two phases). The two-phase refinement produces cell volumes of 290.05 Å3 and 290.58 Å3, as compared to 290.28 Å3 for the single-phase refinement, and both phases show evidence of antisite defects in the refinement. The results suggest that for the lower-temperature synthesis, there may be a small compositional gradient within the powders that relates to the quantity and distribution of defects on the Li and Fe sites. This could in part relate to the slight temperature gradient that exists within or between the vessels, or that at lower temperatures, it is more difficult to obtain precisely reproducible conditions for different batches of powders. Interestingly, the refined occupancies for larger volume cell
suggest a greater level of defects on the Fe site and fewer defects on the Li site. Direct imaging of antisite defects in LiFePO4 can be visualized using atomic-resolution Z-contrast, high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM). To observe FeLi antisite defects, the aberration-corrected Nion UltraSTEM (operating at 100 kV) was used to acquire HAADF STEM images along the b-axis of the orthorhombic crystal structure of LiFePO4, as the [010] crystallographic orientation allows for the direct imaging and projection of Li columns [10]. The FeLi antisite defect can directly be interpreted from atomic-resolution HAADF STEM images when there is an increased contrast as a result of the higher atomic number Fe atoms being positioned onto the Li sites. Fig. 4 shows a raw and unprocessed HAADF STEM image of LiFePO4 that was imaged along the [010] direction. To aid in the clarification of the atomic structure of LiFePO4, atoms corresponding to Fe (orange), P (gray), and Li (blue) are overlaid onto a subset of this image. Although there are many Li columns that appear to be free of antisite defects, there are several locations (marked by white arrows) that show a discernible increase in contrast within several Li columns, which is attributed to antisite cation ordering where Fe atoms occupy Li sites. While FeLi antisite defects can readily be observed with this imaging method, it was not possible to determine for these samples if LiFe antisite defects occurred because the minimal contrast from Li was not discernible when compared to the higher atomic number Fe and/or P that occupy their respective lattice positions.
4. Discussion For low pO2 conditions (such as found in the MW-ST synthesis), it is expected that for a Li-poor environment the neutral defect complex FeLi+–VLi should be favored, while the FeLi+–LiFe defect pair should occur in a Li-rich synthesis environment [6]. For these defects, to maintain electrical neutrality and a 2+ iron oxidation state, the FeLi+–VLi defect can be written (Li1 2xFexVFe(x))FePO4, and the FeLi+–LiFe defect can be written (Li1 xFex)(Fe1 xLix)PO4. Fe3+ cations are not expected to be stable under reducing conditions at higher temperatures in the LiFePO4 olivine structure. The limit of solubility of Fe on the Li site has been reported to be x≈0.06 for the defect complex (Li1 2xFexVFe(x))FePO4, with Fe3(PO4)2 impurity forming for higher Li deficiency [17]. In the
Fig. 4. Atomically-resolved HAADF-STEM image of the 255 1C LiFePO4 sample oriented along the b-axis [010] crystallographic direction. Right: direct imaging of FeLi defect sites. One example of this antisite defect is indicated with an arrow.
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work of Julien et al. [17], the samples were prepared at 725 1C with an excess or deficiency of lithium (Li3PO4 starting material); with the result that under lithium deficient conditions, the unit cell volume was reported to increase with an increase in FeLi and VFe defects, while lithium excess was found to produce the Li3PO4 impurity plus stoichiometric LiFePO4. Interestingly, samples prepared below 200 1C by a hydrothermal synthesis demonstrate a similar increase in volume with decreasing reaction temperature, with the lattice parameters reported to rise above the nominal value of ∼291.9 Å3 for micron-sized stoichiometric powders prepared at high temperatures [23]. This implies a similar type of defect formation for these micron-sized powders prepared under very different synthetic conditions of temperature and precursor stoichiometry. Given the 1:1 mol ratio of Li:Fe in the precursors, and the rapid synthesis that may create non-equilibrium conditions and stabilize unique defect distributions, it is not clear from theoretical considerations alone what defect distribution should occur with MWST synthesis. The lattice parameters for the MW-ST produced samples indicate a decrease in volume with increasing defects. This is in agreement with previous reports of samples prepared by the MW-ST method, which have consistently shown smaller lattice parameters and cell volumes than LiFePO4 prepared by conventional methods [3,32–33]. Whereas micron-sized particles prepared by conventional heating (i.e., ∼650–700 1C) typically exhibit lattice parameters such as a ¼10.332 Å, b¼6.010 Å, c ¼4.692 Å, V¼ 291.4 Å3, powders prepared through the MW-ST route at 255 1C have parameters a ¼10.31626(4) Å, b¼5.99528(2) Å, c ¼ 4.69341(1) Å, V ¼290.282(3) Å3. Interestingly, this compares closely with nanoparticles prepared by a precipitation route at a temperature slightly above 100 1C with a Li:Fe ratio of 1:1, resulting in lattice parameters of a ¼10.3140(2) Å, b¼ 5.9948(3) Å, c ¼4.6945(1) Å, V ¼290.263(6) Å3 [42]. For the 275 1C MW-ST samples, the lattice parameters are a¼ 10.32826(5) Å, b¼5.99997 (2) Å, c¼ 4.69337(1) Å, V ¼ 290.845(3) Å3, indicating that although the volume increases with increasing sample preparation temperature, it is still significantly lower than that of the conventionally prepared micron-sized powders. The clear differences in the trend in cell volume for the micron-sized versus nano-sized particles containing antisite defects suggests that there may be a particle size dependence on the type of defects formed in LiFePO4 [43–47]. Nanoparticles prepared under non-equilibrium conditions may indeed present defect distributions that differ from those observed under high-temperature preparation conditions. For example, “single-phase” behavior during battery cycling of small (∼40 nm) nanoparticles [24] with an unusually low Li:Fe ratio has been suggested. A further example is the formation of a high-pressure polymorph of LiFePO4 at low temperatures and pressures in nanocrystalline form [48]. For the MW-ST powders prepared at 255 or 275 1C, the powder diffraction data and HAADF STEM images demonstrate the presence of antisite defects. The ∼1–2% level of defects refined for Model B is higher than might be expected on the basis of temperature, when compared with previous reports of hydrothermal- or co-precipitation-based synthesis of nanoparticles. It has been reported that for temperatures above 200 1C and for stoichiometric molar ratios of the precursors, only very few antisite defects should be observed [17,23]. In the model of Hamelet et al. [42], where the Li:Fe ratio is maintained at 1:1 and defects include FeLi+, FeLi2+, LiFe , VLi and VFe2 , the 40 nm particles prepared at a temperature near 100 1C reportedly demonstrated a refined composition of [Li0.93(5)Fe0.02(2)VLi0.05(7)][Fe0.95(9)Li0.04(2)VFe0.01(11)] PO4. This composition agrees within error of the compositions refined in Model C [Li0.992(12)Fe0.008(2)][Fe0.962(2)Li0.038(2)]PO4 and Model D [Li0.988(12)Fe0.008(2)□0.004(14)][Fe0.964(2)□0.036(2)]PO4 for the 255 1C sample. The stoichiometry of Model C requires a Fe2+:Fe3+
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ratio of ∼0.97:0.03, while for Model D a Fe2+:Fe3+ ratio of ∼0.93:0.07 is calculated. The similarity of the refined compositions with the results of Hamelet et al. [42] (within error) is notable, and in addition the MW-ST samples also typically show Li:Fe ratios of 1:1, and the lattice parameters are similar for the 255 1C sample with that of Hamelet et al. [42]. Given the errors present in the refinement of Hamelet et al. [42], we expect that their data are similarly insensitive to the level of lithium on the Fe site as presented by Models C and D in this study. While it has been demonstrated that antisite defects are present in the MW-ST samples, the high quality, high resolution diffraction data presented here suggest that diffraction data alone cannot be used to conclusively determine the type of antisite defects present. While the STEM images presented here can provide definitive evidence for the presence and local distribution of FeLi type defects, and while it was recently suggested that STEM images can be used to image LiFe defects [21], these data cannot provide a conclusive picture of the defect distribution in the bulk of a powder. In fact, for both STEM images and powder diffraction data, the FeLi defects produce a far greater change in the scattering power for the Li site than the same level of LiFe defects on the Fe site, which produces the greatest difficulty in determining a precise model of the defects. In the study of Hamelet et al. [42], Mössbauer data were used to show that oxidized Fe3+ cations, interpreted to arise from the bulk olivine structure and not from impurities, are present and are likely distributed over the Li and Fe sites (of the defect-free LiFePO4 structure). Their highly defective model was therefore justified on the basis of Mössbauer data, combined with ICP data suggesting a ∼1:1 ratio of Li:Fe. The similarity of the refinement results (occupancy and lattice parameters) for the MW-ST 255 1C sample with the ∼40 nm sized particles of Hamelet et al. may indicate that Model C or D is more appropriate for the MW-ST samples. Using the expected 1:1 ratio of Li:Fe, we may tentatively propose that Model D is the most appropriate for our samples, with a significant fraction of VFe defects rather than LiFe defects, and the partial oxidation of iron to Fe3+. It is important to note that while the LiFePO4 samples prepared by the MW-ST method consistently show Li:Fe ratios of ∼1:1, the errors on this measurement are in the range of ∼2–3%, which is significant for the level of defects examined here. The partial oxidation of Fe2+ to Fe3+ has been previously observed by XPS in samples prepared via MW-ST synthesis at temperatures between 260 1C and 280 1C [11]. This was interpreted to involve surface oxidation, though the present work suggests that partial oxidation in the bulk may occur in the presence of defects. The proposed model does not correspond exactly with either of the neutral defect complexes FeLi+–VLi or FeLi+–LiFe , indicating that under non-equilibrium synthesis conditions, other defect distributions are possible. It is likely that the defects are formed due to the more rapid nature of the microwave synthesis during the initial stages of nucleation and growth, and it is possible that if the synthesis time were significantly extended at 255 or 275 1C, these defects could be greatly reduced, as is found for the hightemperature annealed samples. Future work will consider how defects may be stabilized in other non-aqueous environments, such as during ionothermal synthesis, and whether methods such as resonant scattering could provide the additional information needed to more conclusively define a particular defect model.
5. Conclusions High resolution powder diffraction data and HAADF STEM images have been used to demonstrate that antisite defects are present in samples prepared at moderate temperatures (∼255–275 1C) by microwave-solvothermal synthesis. The antisite defects have been
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formed at a significantly higher preparation temperature than expected on the basis of previous reports suggesting that essentially defect free samples are formed above 200 1C. The ability to stabilize these defects at higher temperatures may be related to the fact that the MW-ST synthesis of crystalline particles is achieved over a period of minutes. The shape of crystalline domains within the nanopowders has been quantitatively analyzed with Rietveld refinement, which shows a significant increase in domain size for the sample fired at a higher temperature, and similar domain sizes and shapes for both lower temperature samples. Despite it being a common practice in the literature, diffraction data alone could not conclusively determine the precise distribution of defects over the Li and Fe sites. A defect model involving FeLi+, FeLi2+, VLi and VFe2 defects is tentatively proposed on the basis of Rietveld refinement results, compositional constraints on the ratio of Li:Fe, and comparison with the literature. Acknowledgments This work was supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division, at the University of Texas at Austin (under award number DESC0005397) and at Oak Ridge National Laboratory (ORNL). We acknowledge Ashfia Huq and Jason Hodges for assistance with collection of powder neutron diffraction data at the Spallation Neutron Source (SNS). Research at ORNL's SNS, Center of Nanophase Materials Sciences (CNMS) and Shared Research Equipment (ShaRE) User Facility Programs were sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. One of the authors (KH) thanks the National Science Foundation for the award of a Graduate Research Fellowship. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jssc.2013.07.011.
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Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Defect chemistry of phospho-olivine nanoparticles synthesized by a microwave-assisted solvothermal process Craig A. Bridges a,n, Katharine L. Harrison c, Raymond R. Unocic b, Juan-Carlos Idrobo b, M. Parans Paranthaman a, Arumugam Manthiram c a
Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA c Electrochemical Energy Laboratory and Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX 78712, USA b
art ic l e i nf o
a b s t r a c t
Article history: Received 18 May 2013 Received in revised form 11 July 2013 Accepted 12 July 2013 Available online 20 July 2013
Nanocrystalline LiFePO4 powders synthesized by a microwave-assisted solvothermal (MW-ST) process have been structurally characterized with a combination of high resolution powder neutron diffraction, synchrotron X-ray diffraction, and aberration-corrected HAADF STEM imaging. A significant level of defects has been found in the samples prepared at 255 and 275 1C. These temperatures are significantly higher than what has previously been suggested to be the maximum temperature for defect formation in LiFePO4, so the presence of defects is likely related to the rapid MW-ST synthesis involving a short reaction time (∼5 min). A defect model has been tentatively proposed, though it has been shown that powder diffraction data alone cannot conclusively determine the precise defect distribution in LiFePO4 samples. The model is consistent with other literature reports on nanopowders synthesized at low temperatures, in which the unit cell volume is significantly reduced relative to defect-free, micron-sized LiFePO4 powders. Published by Elsevier Inc.
Keywords: Lithium-ion batteries LiFePO4 cathode Antisite disorder Neutron diffraction
1. Introduction Due to the critical importance of particle size and morphology on the electrochemical performance of battery electrode materials, there has been a great deal of interest in finding optimum approaches to electrode material preparation. In the case of LiFePO4 (LFP) [1], synthesis approaches that provide highly crystalline nanoparticles are essential to obtain high-rate capability with cyclability [2–4]. Surface modification and coating can further improve the electrochemical properties [2–5]. Of particular importance to the electrochemical performance is the formation of antisite defects on the lithium site, which if present are expected to inhibit diffusion of Li+ ions through channels in the [010] direction. A number of different native point defects may occur in LiFePO4, depending upon the synthesis conditions employed. The possible defects include the presence of Fe on the Li site (FeLi), lithium on the Fe site (LiFe), and vacancies on the Li (VLi) and Fe (VFe) sites. Furthermore, these defects have been predicted to occur in one stable charge state, corresponding to FeLi+, LiFe , VLi and VFe2 [6]. These charged defects may form complexes containing pairs of these defects, such as the FeLi+–LiFe antisite
n
Corresponding author. Fax: +1 865 574 4961. E-mail address: [email protected] (C.A. Bridges).
0022-4596/$ - see front matter Published by Elsevier Inc. http://dx.doi.org/10.1016/j.jssc.2013.07.011
defect pair, or the combination of antisite defect and vacancy represented as FeLi+–VLi . Several computational investigations have predicted that the FeLi–LiFe antisite defect pair is energetically favored with respect to other possible defects [7,8], and complimentary investigations have suggested that the type of defect that is energetically favorable may depend upon the synthesis conditions [6,9]. Research has also focused on the effect of aliovalent or non-aliovalent doping on the electrochemical properties and defect formation in LiFePO4 [8–16]. A number of publications have been reported to experimentally characterize the distribution of Li and Fe cations in Olivine LiFePO4, both during synthesis and during electrochemical cycling in a Li-ion battery. Briefly, the results suggest that for micron-sized samples prepared at elevated temperatures (typically ∼700 1C), few defects are observed for stoichiometric samples, but that for samples prepared under Li-deficient conditions it is possible to observe the formation of FeLi+–VLi defects [17,18]. This supports related computational investigations on the influence of synthesis environment given above [6]. Several STEM investigations, supported by DFT-based computational investigations, suggest that FeLi+ [10,19], FeLi+–VLi [20], and FeLi+–LiFe [21] defects may cluster even at low defect concentrations. In the case of FeLi+– LiFe , it has been reported that LiFe has been observed in a cluster of the neutral defect complex [21]. The observation of LiFe defects have generally not been reported due to the difficulty of observing
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intensity changes in a column of Fe cations with substitution of a small fraction of Li cations, but due to the clustering of the neutral defect complex, it may be possible to overcome this limitation. The type of defects present in LiFePO4 may be influenced by synthesis under lower temperatures, potentially non-equilibrium conditions that are typically used to prepare nanoparticle samples. In situ diffraction studies of hydrothermal synthesis suggest that antisite defects may disappear in minutes at temperatures above ∼270 1C (with some dependence upon the starting precursors used), but that defects may persist for longer times at lower temperatures [22]. Hydrothermal synthesis has previously been shown to promote antisite defect formation at temperatures below ∼200 1C [23]. Furthermore, single phase samples with unusually low Li:Fe ratios have been reported for very low-temperature synthesis of nanoparticles based upon a co-precipitation process [24]. The formation of nanoparticles is particularly important as it can enhance Li+ diffusion in LiFePO4 for Li-ion battery applications when the formation of detrimental FeLi+ defects can be avoided [25,26]. Interestingly, particle size may also influence the lithiation/ delithiation mechanism, and therefore the distribution of lithium within the structure. Nanoparticles below a certain size limit may demonstrate single phase cycling behavior (solid solution behavior) [24,27] whereas larger particles typically exhibit phase segregation into Li-rich and Li-poor compositions [28–30]. Phase segregation during cycling is suggested to be promoted by the presence of edge-sharing FeO6 octahedra and PO4 tetrahedra, which limit the structural flexibility to accommodate solid solution behavior. However, this picture may be further modified under non-equilibrium cycling conditions, resulting in a mixture of solid solution and phase segregation behavior regardless of particle size [27,31]. Clearly the synthesis conditions, the resulting particle size, and the eventual battery cycling conditions may all influence how lithium is distributed within the olivine structure. The microwave-assisted solvothermal (MW-ST) approach has been shown to produce highly crystalline nanorods of up to 1 μm length at temperatures ranging from ∼230 to 300 1C [3,32,33]. Moreover, reaction times as short as 5–15 min can be used to obtain single phase powders. Previous hydrothermal experiments have suggested that LFP produced at temperatures above ∼180 1C should not contain a significant concentration of antisite defects [23,34]. For the preparation of MW-ST LFP nanorods, experiments have shown that for synthesis temperatures below 250 1C under solvothermal conditions, the samples do not crystallize completely. Therefore, we were interested to understand whether antisite defects might be stabilized under solvothermal processes at higher temperatures than 180 1C. To investigate this possibility, we present here a detailed structural analysis of particles synthesized by the MW-ST method, combining high quality, high resolution powder X-ray diffraction, neutron diffraction, and electron diffraction data. This work shows that antisite defects are present for samples synthesized at 255 and 275 1C, and that post-annealing at 700 1C is shown to reduce these defects in agreement with the minimal defect concentrations observed in higher temperature synthetic approaches with stoichiometric Li:Fe molar ratios.
2. Experimental 2.1. Synthesis LiFePO4 was synthesized by a microwave-assisted solvothermal technique that has been described in more detail elsewhere [32]. Stoichiometric amounts of lithium hydroxide (Fisher), iron acetate (STREM), and phosphoric acid (Fisher) were stirred in tetraethylene glycol (Alfa Aesar) such that the solutions were 0.18 M in Li
and P. The solutions were sealed in high pressure quartz vessels and placed on a turntable in an Anton Paar Synthos 3000 microwave reactor. A constant power of 600 W was applied until the temperature reached 255 and 275 1C in two respective reactions. These temperatures were then held for 5 min. Additionally, the sample synthesized at 275 1C was then post-annealed in a flowing 5% H2–95% Ar atmosphere for 6 h at 700 1C with heating and cooling rates of 10 1C/min. In order to obtain samples of sufficient size for neutron diffraction experiments, multiple syntheses were performed under identical conditions using the same starting solutions, and the resulting powders obtained were combined. 2.2. Powder X-ray diffraction Powder X-ray diffraction (XRD) data were collected at beamline 11-BM at the APS at 300 K, with a wavelength of 0.412437 Å and a step size of 0.0011. The instrument has a resolution of ΔQ/Q¼1.7 10-4, corresponding to a peak FWHM at the measurement energy of ∼0.0051 2θ. Powder samples were contained within 0.8 mm Kapton capillary tubes, and for better powder averaging the samples were spun at 60 Hz during data collection. Data were analyzed over a range from 31 to 401 2θ. 2.3. Powder neutron diffraction Powder neutron diffraction (PND) data were collected at 300 K on the POWGEN beamline at the Spallation Neutron Source (SNS). Isotopically enriched lithium (7Li) was used to remove uncertainties in the correct bound coherent neutron scattering length ( 2.22 fm for 7Li) that can exist with natural lithium. The samples were contained in 8 mm vanadium cans. Powder X-ray diffraction data and neutron diffraction data were analyzed for unit cell constants, phase fractions, and structural information with Rietveld refinement [35] with GSAS/EXPGUI [36,37] and Fullprof/ WinPLOTR [38,39]. 2.4. Scanning Transmission Electron Microscopy Atomic-resolution, Z-contrast, high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) experiments were acquired with an aberration-corrected STEM Nion UltraSTEM 100 instrument [49] equipped with a cold-field emission electron source and a corrector of third- and fifth-order aberrations. The electron microscope was operated at 100 kV accelerating voltage, the convergence semiangle of the incident probe was set to 30 mrad, and the HAADF images were collected from 86 to 200 mrad half-angle range. 3. Results Combined Rietveld refinement of the powder neutron diffraction data and X-ray diffraction data confirm that regardless of reaction temperature, the powders can be indexed with the Pnma olivine structure (Fig. 1 and S4; Tables 1 and 2). The samples are largely free of impurities, though the sample prepared at 255 1C apparently contains a very small fraction of spinel Fe3O4 (∼0.22 wt %) and Goethite FeOOH (∼0.16 wt%) impurities, on the basis of comparison with the PDF database for a small number of weak peaks (Fig. 1). While the presence of Fe-rich impurities may imply a Fe deficiency in the 255 1C sample, there is uncertainty due to the fact that the washing step in the synthesis may remove phosphates or Li-containing impurities preferentially to these Fe-rich impurities. Also, the iron precursor is highly hygroscopic, which implies difficulty in adding exactly stoichiometric amounts
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Fig. 1. Comparison of refinements of the Bank 5 TOF powder neutron diffraction data (left) and synchrotron powder X-ray diffraction data (right) for the 255 1C sample (a–b), the 275 1C sample (c–d), and the 700 1C sample (e–f). There are weak (□0.5 wt%) impurities of Fe3O4 and FeOOH in the 255 1C LiFePO4 sample, as indicated in (a) and (b) by the second and third set of reflection markers, respectively.
of iron precursor to the reaction solution. ICP and TGA are used to determine the water content, but these are only accurate to ∼1–3%, so the small fraction of iron impurities can easily be explained by slight inaccuracies in the weight of the iron precursor due to limitations in water content analysis. Other potential impurity phases that have been previously reported in other LiFePO4 syntheses, such as Li3PO4, Fe3(PO4)2(OH)2, Fe3(PO4)2 or Li3Fe2(PO4)3 were not observed [17,33]. The trend in lattice parameters given in Table 1 indicates an overall increase in volume with increasing sample preparation temperature. The volume increases from 290.282(3) Å3 at 255 1C, to 290.845(2) Å3 at 275 1C, and to 290.902(1) Å3 at 700 1C. The atomic displacement parameters (Biso) have similar magnitudes in the range of ∼0.5– 0.85 Å2 with the exception of the lithium site, which is ∼1.6–2.1 Å2.
The larger Biso for the Li site has been previously observed in structural refinements of LiFePO4 [23], and could represent a slight static or dynamic off-center distortion from the (0,0,0) site position (Li site). The olivine structure (Fig. S1) consists of a hexagonal close-packed array of oxide ions, in which half of the octahedral sites are occupied by M cations, and one-eighth of the tetrahedral sites are occupied by phosphorous ions. There are three distinct oxygen sites, one phosphorous site, and two metal sites (M1 and M2, corresponding to Li and Fe sites in the defect-free structure) [40]. To obtain the best fit for a combined refinement of neutron diffraction and X-ray diffraction data, it was necessary to refine all positional parameters, occupancies of the lithium and iron sites, weight fractions, and all atomic displacement parameters with lithium, iron, and oxygen sites refined anisotropically. Further
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Table 1 Refinement results for LiFePO4 samples with structural parameters obtained at 300 K, showing a comparison between refinements at different temperatures with Model C. Preparation method Sample
MW-ST (nano-sized) 255 1C
MW-ST (nano-sized) 275 1C
MW-ST (nano-sized) 700 1C
Lattice parametersa
a (Å) b (Å) c (Å) V (Å3)
10.31626(4) 5.99528(2) 4.69341(1) 290.282(3)
10.32826(5) 5.99997(2) 4.69337(2) 290.845(2)
10.32763(2) 6.00433(1) 4.69118(1) 290.902(1)
Fractional coordinates
Li2/Fe2 x Li2/Fe2 z Px Pz O1 x O1 z O2 x O2 z O3 x O3 y O3 z Li/Fe Li2/Fe2 P O O2 O3 LFP1 Fe3O4 FeOOH
0.28187(5) 0.97537(13) 0.09504(10) 0.41788(20) 0.09734(24) 0.74076(47) 0.45607(25) 0.20774(46) 0.16604(19) 0.04627(26) 0.28348(28) 1.64(13) 0.68 0.73(2) 0.58 0.58 0.58 99.6% 0.21% 0.16%
0.28172(7) 0.97580(17) 0.09524(14) 0.41736(27) 0.09711(28) 0.74022(59) 0.45567(30) 0.20928(59) 0.16584(23) 0.04682(33) 0.28346(36) 2.09 0.85 0.86(3) 0.69 0.69 0.69 100%
0.28204(5) 0.97522(12) 0.09480(9) 0.41802(19) 0.09690(20) 0.74226(43) 0.45637(21) 0.20743(43) 0.16575(16) 0.04679(23) 0.28480(27) 1.73 0.51 0.43(2) 0.53 0.53 0.53 100%
ADP (Biso) (Å2)
Phase fractions
a All crystallography data are for the combined refinement of neutron and X-ray powder diffraction data at 300 K; where the ADP is shown in italics, this indicates an anisotropic refinement was used and the equivalent isotropic thermal parameter is listed here (full description is given in the supplemental); the phase fraction is given as a weight percent.
Table 2 Refinement results for LiFePO4 samples, showing a comparison between refinements with no antisite disorder and different models of antisite disorder. Single phase 255 1C
Single phase 275 1C
Single phase 700 1C
χ2 (global) χ2 (XRD) wRp (XRD) Rp (XRD)
2.17 1.91 8.81 6.71
1.81 1.27 9.15 6.92
2.07 1.28 8.25 6.21
χ2 (global) χ2 (XRD) wRp (XRD) Rp (XRD)
1.6(1)% 1.6(1)% 1:1 2.11 1.89 8.75 6.60
2.4(1)% 2.4(1)% 1:1 1.76 1.23 9.02 6.77
0.3(1)% 0.3(1)% 1:1 2.08 1.27 8.22 6.17
χ2 (global) χ2 (XRD) wRp (XRD) Rp (XRD)
3.8(2)% 0.8(1)% 1.03:0.97 2.10 1.87 8.70 6.59
3.0(2)% 2.2(2)% 1.00:0.99 1.76 1.23 9.02 6.78
1.0(2)% 0.9(2)% 1.00:0.99 2.08 1.27 8.22 6.18
χ2 (global) χ2 (XRD) wRp (XRD) Rp (XRD)
3.6(1)% 0.8(2)% 0.4(14)% 0.99:0.97 2.09 1.87 8.71 6.59
3.0(2)% 2.2(2)% 0% 0.99:0.99 1.75 1.23 9.02 6.78
1.0(2)% 0.8(2)% 0% 0.99:0.99 2.06 1.27 8.21 6.18
Sample Model A: No Antisites Statistical fita
Model B: Constrained antisites LiFeb FeLi Li:Fe ratio Statistical fit
Model C: Independent antisites LiFe FeLi Li:Fe ratio Statistical fit
Model D: Antisites/Vacancies VFe FeLi VLi Li:Fe ratio Statistical fit
a All crystallography data are for the combined refinement of neutron and X-ray powder diffraction data at 300 K; full statistics for the XRD data are shown for comparison between the models of antisite defect formation. For Models A–C, there are no vacancies as the sites are constrained to be fully occupied. b FeLi refers to an antisite defect involving Fe on the Li site, LiFe refers to Li on the Fe site, and VLi and VFe correspond to Li/Fe vacancies; for the “constrained antisite” refinement the FeLi+–LiFe defect pair is assumed.
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details of the refined parameters are given for one model of antisite defects (Model C, described below) in Table 1, and are representative of the results for different antisite defect models. The variation in peak full-width at half maximum (FWHM) versus diffraction angle for the 255 and 275 1C samples did not follow a simple polynomial function. Clear anisotropy in FWHM versus angle is shown in Fig. 2 (and over a wider 2θ range in Fig. S5), in which the peaks with larger h indices were markedly broader than the other reflections. This indicates that the crystalline domain size is significantly smaller along the a axis. To better quantify the anisotropy in domain size, orthorhombic spherical harmonic anisotropic size broadening was introduced into the Rietveld refinement in Fullprof. The size broadening parameters indicate a much more
Fig. 2. Comparison of peak FWHM versus diffraction angle derived from synchrotron powder X-ray diffraction data for the 255 1C sample (filled triangle), the 275 1C sample (open circle), and the 700 1C sample (filled square). Indexed X-ray diffraction data for the 255 1C sample are given below. The sharper reflections are highlighted with a yellow background in both plots. The broader reflections are those with a larger h index, due to the shorter coherent domain size along the a axis. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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anisotropic peak broadening for the 255 and 275 1C samples than for the 700 1C sample (Figs. 3 and S3). Interestingly, the refined crystallite domain sizes for the lower-temperature (non-annealed) samples are essentially identical within the error of the fit despite the slightly smaller FWHM values derived from the refinement. The smaller FWHM values for the 275 1C sample suggests that the average domain sizes are slightly larger than for the 255 1C sample, but are similar enough that they appear the same within the error of the Rietveld-based size determination. Due to the presence of competing models for refinement of LiFePO4 antisite disorder in the literature, a number of different antisite defect models have been examined here and are compared in Tables 2 and S1. The purpose of this approach is to examine the possibility that the MW-ST synthetic method could produce a different defect distribution than has previously been reported in the literature, and further to test the limits of high quality diffraction data in determining the distribution of defects in LiFePO4. For the Rietveld refinements, we consider that the PO4 sublattice is fully occupied (i.e., there are no V PO4 ) to serve as a reference for occupancy refinement of the Li and Fe sites. The antisite defect models used in the Rietveld refinements, and summarized in Table 2, are: (A) no antisite defects; (B) constrained antisite defect formation in which equal amounts of LiFe and FeLi are present (i.e., FeLi+–LiFe expected for a Li-rich environment), such that the overall formula of “LiFePO4” is maintained; (C) a model in which the level of LiFe and FeLi defects is refined independently on each site, and the sites are constrained to be fully occupied; (D) same as Model C, but with vacancies possible on the Li and Fe sites, and no LiFe defects. As a non-exhaustive list, examples of antisite defect Model B are found in the work of Chung et al. [21,41] and Islam et al. [8], and discussion of defect models related to Models C/D has been examined by Lee et al. [20] and Hamelet et al. [42]. A small but significant improvement in the refinement over Model A (global χ2 of 2.17) is observed upon the addition of antisite defects in Model B, with the result of global χ2 of 2.11, and for the XRD data a χ2 of 1.89, wRp of 8.75, and Rp of 6.60. For the Model B refinements of the lower-temperature samples, the level of antisite defects are ∼1–2%, and clearly higher than that of the 700 1C sample, which has a very low level of defects (∼0.3%). For model C, the independent refinement of antisite defects suggests that for the lower-temperature samples, there could be a greater fraction of LiFe antisite defects than FeLi defects. For the 255 1C sample, this produces a composition of [Li0.992(12)Fe0.008(2)][Fe0.962(2)Li0.038(2)] PO4. This composition should produce an imbalance in the ratio of Li:Fe:P, and the relative increase in Li+ content on the fully occupied sites should suggest a partial oxidation of iron to form Fe3+. For Model D, the MW-ST 255 1C sample refinement was initially carried out with LiFe defects for a refined composition of
Fig. 3. Average coherent domain size of LiFePO4 increases and becomes less anisotropic with a higher-temperature post-annealing. The refined maximum c axis domain sizes are ∼142 nm, ∼120 nm, and ∼164 nm for the 255, 275, and 700 1C samples, respectively. Average apparent domain sizes are 70(30) nm, 72(23) nm, and 126(20) nm for the 255, 275, and 700 1C samples, respectively.
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[Li0.988(12)Fe0.008(2)□0.004(14)][Fe0.962(2)Li0.038(2)]PO4, with a global χ2 of 2.10, and for the XRD data a χ2 of 1.87, wRp of 8.70, and Rp of 6.59. Replacement of the LiFe antisite defects with vacancies (VFe) gives a composition [Li0.988(12)Fe0.008(2)□0.004(14)][Fe0.964(2)□0.036(2)] PO4 with the same goodness-of-fit. In the case of the refinement with VFe defects, the refined ratio of Li:Fe is close to 1:1, as expected from samples produced by the MW-ST synthesis [3,32,33]. It is immediately apparent when comparing the goodness-of-fit indicators for different antisite defect models that the fits are of essentially identical quality for the three different models B, C, and D, indicating that despite the combination of high quality X-ray and neutron diffraction data, the refinement cannot clearly distinguish between vacancies and lithium defects on the Fe site. It should be noted that a significant asymmetry was evident in the more intense peaks of the high resolution synchrotron X-ray diffraction data for the 255 1C sample, which was not immediately evident in the TOF neutron diffraction data. This asymmetry could not be fully fitted by the asymmetry functions available in Fullprof, which suggests that a small compositional inhomogeneity may exist in this sample. To improve the fit to the data and examine this possibility, refinements were made using two LiFePO4 phases, with the results shown in Fig. S2 and Table S1. The additional parameters provided by the second phase improved the fit, with a decrease in the global χ2 from 2.10 to 1.94. The improvement was far more prominent for the fit to the high resolution X-ray data, and the goodness of fit parameters are, therefore, presented for these data in addition to the global χ2. The structural parameters resulting from the single phase refinement are found to generally be an average of the parameters obtained for the two phase refinement, as may be expected given that the similar refined weight fraction of the two phases (∼50:50 ratio of the two phases). The two-phase refinement produces cell volumes of 290.05 Å3 and 290.58 Å3, as compared to 290.28 Å3 for the single-phase refinement, and both phases show evidence of antisite defects in the refinement. The results suggest that for the lower-temperature synthesis, there may be a small compositional gradient within the powders that relates to the quantity and distribution of defects on the Li and Fe sites. This could in part relate to the slight temperature gradient that exists within or between the vessels, or that at lower temperatures, it is more difficult to obtain precisely reproducible conditions for different batches of powders. Interestingly, the refined occupancies for larger volume cell
suggest a greater level of defects on the Fe site and fewer defects on the Li site. Direct imaging of antisite defects in LiFePO4 can be visualized using atomic-resolution Z-contrast, high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM). To observe FeLi antisite defects, the aberration-corrected Nion UltraSTEM (operating at 100 kV) was used to acquire HAADF STEM images along the b-axis of the orthorhombic crystal structure of LiFePO4, as the [010] crystallographic orientation allows for the direct imaging and projection of Li columns [10]. The FeLi antisite defect can directly be interpreted from atomic-resolution HAADF STEM images when there is an increased contrast as a result of the higher atomic number Fe atoms being positioned onto the Li sites. Fig. 4 shows a raw and unprocessed HAADF STEM image of LiFePO4 that was imaged along the [010] direction. To aid in the clarification of the atomic structure of LiFePO4, atoms corresponding to Fe (orange), P (gray), and Li (blue) are overlaid onto a subset of this image. Although there are many Li columns that appear to be free of antisite defects, there are several locations (marked by white arrows) that show a discernible increase in contrast within several Li columns, which is attributed to antisite cation ordering where Fe atoms occupy Li sites. While FeLi antisite defects can readily be observed with this imaging method, it was not possible to determine for these samples if LiFe antisite defects occurred because the minimal contrast from Li was not discernible when compared to the higher atomic number Fe and/or P that occupy their respective lattice positions.
4. Discussion For low pO2 conditions (such as found in the MW-ST synthesis), it is expected that for a Li-poor environment the neutral defect complex FeLi+–VLi should be favored, while the FeLi+–LiFe defect pair should occur in a Li-rich synthesis environment [6]. For these defects, to maintain electrical neutrality and a 2+ iron oxidation state, the FeLi+–VLi defect can be written (Li1 2xFexVFe(x))FePO4, and the FeLi+–LiFe defect can be written (Li1 xFex)(Fe1 xLix)PO4. Fe3+ cations are not expected to be stable under reducing conditions at higher temperatures in the LiFePO4 olivine structure. The limit of solubility of Fe on the Li site has been reported to be x≈0.06 for the defect complex (Li1 2xFexVFe(x))FePO4, with Fe3(PO4)2 impurity forming for higher Li deficiency [17]. In the
Fig. 4. Atomically-resolved HAADF-STEM image of the 255 1C LiFePO4 sample oriented along the b-axis [010] crystallographic direction. Right: direct imaging of FeLi defect sites. One example of this antisite defect is indicated with an arrow.
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work of Julien et al. [17], the samples were prepared at 725 1C with an excess or deficiency of lithium (Li3PO4 starting material); with the result that under lithium deficient conditions, the unit cell volume was reported to increase with an increase in FeLi and VFe defects, while lithium excess was found to produce the Li3PO4 impurity plus stoichiometric LiFePO4. Interestingly, samples prepared below 200 1C by a hydrothermal synthesis demonstrate a similar increase in volume with decreasing reaction temperature, with the lattice parameters reported to rise above the nominal value of ∼291.9 Å3 for micron-sized stoichiometric powders prepared at high temperatures [23]. This implies a similar type of defect formation for these micron-sized powders prepared under very different synthetic conditions of temperature and precursor stoichiometry. Given the 1:1 mol ratio of Li:Fe in the precursors, and the rapid synthesis that may create non-equilibrium conditions and stabilize unique defect distributions, it is not clear from theoretical considerations alone what defect distribution should occur with MWST synthesis. The lattice parameters for the MW-ST produced samples indicate a decrease in volume with increasing defects. This is in agreement with previous reports of samples prepared by the MW-ST method, which have consistently shown smaller lattice parameters and cell volumes than LiFePO4 prepared by conventional methods [3,32–33]. Whereas micron-sized particles prepared by conventional heating (i.e., ∼650–700 1C) typically exhibit lattice parameters such as a ¼10.332 Å, b¼6.010 Å, c ¼4.692 Å, V¼ 291.4 Å3, powders prepared through the MW-ST route at 255 1C have parameters a ¼10.31626(4) Å, b¼5.99528(2) Å, c ¼ 4.69341(1) Å, V ¼290.282(3) Å3. Interestingly, this compares closely with nanoparticles prepared by a precipitation route at a temperature slightly above 100 1C with a Li:Fe ratio of 1:1, resulting in lattice parameters of a ¼10.3140(2) Å, b¼ 5.9948(3) Å, c ¼4.6945(1) Å, V ¼290.263(6) Å3 [42]. For the 275 1C MW-ST samples, the lattice parameters are a¼ 10.32826(5) Å, b¼5.99997 (2) Å, c¼ 4.69337(1) Å, V ¼ 290.845(3) Å3, indicating that although the volume increases with increasing sample preparation temperature, it is still significantly lower than that of the conventionally prepared micron-sized powders. The clear differences in the trend in cell volume for the micron-sized versus nano-sized particles containing antisite defects suggests that there may be a particle size dependence on the type of defects formed in LiFePO4 [43–47]. Nanoparticles prepared under non-equilibrium conditions may indeed present defect distributions that differ from those observed under high-temperature preparation conditions. For example, “single-phase” behavior during battery cycling of small (∼40 nm) nanoparticles [24] with an unusually low Li:Fe ratio has been suggested. A further example is the formation of a high-pressure polymorph of LiFePO4 at low temperatures and pressures in nanocrystalline form [48]. For the MW-ST powders prepared at 255 or 275 1C, the powder diffraction data and HAADF STEM images demonstrate the presence of antisite defects. The ∼1–2% level of defects refined for Model B is higher than might be expected on the basis of temperature, when compared with previous reports of hydrothermal- or co-precipitation-based synthesis of nanoparticles. It has been reported that for temperatures above 200 1C and for stoichiometric molar ratios of the precursors, only very few antisite defects should be observed [17,23]. In the model of Hamelet et al. [42], where the Li:Fe ratio is maintained at 1:1 and defects include FeLi+, FeLi2+, LiFe , VLi and VFe2 , the 40 nm particles prepared at a temperature near 100 1C reportedly demonstrated a refined composition of [Li0.93(5)Fe0.02(2)VLi0.05(7)][Fe0.95(9)Li0.04(2)VFe0.01(11)] PO4. This composition agrees within error of the compositions refined in Model C [Li0.992(12)Fe0.008(2)][Fe0.962(2)Li0.038(2)]PO4 and Model D [Li0.988(12)Fe0.008(2)□0.004(14)][Fe0.964(2)□0.036(2)]PO4 for the 255 1C sample. The stoichiometry of Model C requires a Fe2+:Fe3+
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ratio of ∼0.97:0.03, while for Model D a Fe2+:Fe3+ ratio of ∼0.93:0.07 is calculated. The similarity of the refined compositions with the results of Hamelet et al. [42] (within error) is notable, and in addition the MW-ST samples also typically show Li:Fe ratios of 1:1, and the lattice parameters are similar for the 255 1C sample with that of Hamelet et al. [42]. Given the errors present in the refinement of Hamelet et al. [42], we expect that their data are similarly insensitive to the level of lithium on the Fe site as presented by Models C and D in this study. While it has been demonstrated that antisite defects are present in the MW-ST samples, the high quality, high resolution diffraction data presented here suggest that diffraction data alone cannot be used to conclusively determine the type of antisite defects present. While the STEM images presented here can provide definitive evidence for the presence and local distribution of FeLi type defects, and while it was recently suggested that STEM images can be used to image LiFe defects [21], these data cannot provide a conclusive picture of the defect distribution in the bulk of a powder. In fact, for both STEM images and powder diffraction data, the FeLi defects produce a far greater change in the scattering power for the Li site than the same level of LiFe defects on the Fe site, which produces the greatest difficulty in determining a precise model of the defects. In the study of Hamelet et al. [42], Mössbauer data were used to show that oxidized Fe3+ cations, interpreted to arise from the bulk olivine structure and not from impurities, are present and are likely distributed over the Li and Fe sites (of the defect-free LiFePO4 structure). Their highly defective model was therefore justified on the basis of Mössbauer data, combined with ICP data suggesting a ∼1:1 ratio of Li:Fe. The similarity of the refinement results (occupancy and lattice parameters) for the MW-ST 255 1C sample with the ∼40 nm sized particles of Hamelet et al. may indicate that Model C or D is more appropriate for the MW-ST samples. Using the expected 1:1 ratio of Li:Fe, we may tentatively propose that Model D is the most appropriate for our samples, with a significant fraction of VFe defects rather than LiFe defects, and the partial oxidation of iron to Fe3+. It is important to note that while the LiFePO4 samples prepared by the MW-ST method consistently show Li:Fe ratios of ∼1:1, the errors on this measurement are in the range of ∼2–3%, which is significant for the level of defects examined here. The partial oxidation of Fe2+ to Fe3+ has been previously observed by XPS in samples prepared via MW-ST synthesis at temperatures between 260 1C and 280 1C [11]. This was interpreted to involve surface oxidation, though the present work suggests that partial oxidation in the bulk may occur in the presence of defects. The proposed model does not correspond exactly with either of the neutral defect complexes FeLi+–VLi or FeLi+–LiFe , indicating that under non-equilibrium synthesis conditions, other defect distributions are possible. It is likely that the defects are formed due to the more rapid nature of the microwave synthesis during the initial stages of nucleation and growth, and it is possible that if the synthesis time were significantly extended at 255 or 275 1C, these defects could be greatly reduced, as is found for the hightemperature annealed samples. Future work will consider how defects may be stabilized in other non-aqueous environments, such as during ionothermal synthesis, and whether methods such as resonant scattering could provide the additional information needed to more conclusively define a particular defect model.
5. Conclusions High resolution powder diffraction data and HAADF STEM images have been used to demonstrate that antisite defects are present in samples prepared at moderate temperatures (∼255–275 1C) by microwave-solvothermal synthesis. The antisite defects have been
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formed at a significantly higher preparation temperature than expected on the basis of previous reports suggesting that essentially defect free samples are formed above 200 1C. The ability to stabilize these defects at higher temperatures may be related to the fact that the MW-ST synthesis of crystalline particles is achieved over a period of minutes. The shape of crystalline domains within the nanopowders has been quantitatively analyzed with Rietveld refinement, which shows a significant increase in domain size for the sample fired at a higher temperature, and similar domain sizes and shapes for both lower temperature samples. Despite it being a common practice in the literature, diffraction data alone could not conclusively determine the precise distribution of defects over the Li and Fe sites. A defect model involving FeLi+, FeLi2+, VLi and VFe2 defects is tentatively proposed on the basis of Rietveld refinement results, compositional constraints on the ratio of Li:Fe, and comparison with the literature. Acknowledgments This work was supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division, at the University of Texas at Austin (under award number DESC0005397) and at Oak Ridge National Laboratory (ORNL). We acknowledge Ashfia Huq and Jason Hodges for assistance with collection of powder neutron diffraction data at the Spallation Neutron Source (SNS). Research at ORNL's SNS, Center of Nanophase Materials Sciences (CNMS) and Shared Research Equipment (ShaRE) User Facility Programs were sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. One of the authors (KH) thanks the National Science Foundation for the award of a Graduate Research Fellowship. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jssc.2013.07.011.
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