Defects in Hydrothermally Synthesized LiFePO4 and LiFe1-xMnxPO4

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Defects in Hydrothermally Synthesized LiFePO4 and LiFe1‑xMnxPO4 Cathode Materials Kirsten M. Ø. Jensen,† Mogens Christensen,† Haraldur P. Gunnlaugsson,‡ Nina Lock,†,§ Espen D. Bøjesen,† Thomas Proffen,∥ and Bo B. Iversen*,† †

Center for Materials Crystallography, Department of Chemistry and iNANO, Aarhus University, DK-8000 Aarhus C, Denmark Department of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark § Institut für Anorganische Chemie, Georg-August-Universität Göttingen, D-37077 Göttingen, Germany ∥ Neutron Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6475, United States

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‡

S Supporting Information *

ABSTRACT: The crystal structure and defect chemistry of hydrothermally synthesized LiFe1‑xMnxPO4 (x = 0, 0.25, and 0.50) particles have been characterized by simultaneous neutron and X-ray Rietveld refinement as well as X-ray and neutron pair distribution function (PDF) analysis, crystallinity determination, Mössbauer spectroscopy, ion coupled plasma (ICP) studies, and scanning electron microscopy (SEM). The very detailed structural refinements show that fast hydrothermal synthesis causes partial Feoccupancy and vacancies on the Li (M1) site, while the Fe (M2) site is always fully occupied by iron. Thus, the defect is not merely a Li/ Fe antisite defect, and excessive amounts of Fe are the origin of the disorder in the structure. Neutron and X-ray total scattering with PDF analysis show that after fast hydrothermal synthesis, the crystalline, defective LixFeyPO4 coexists with amorphous Li/ Fe-PO4 structures having just short-range order. Iron excess is only seen in the crystalline part of the particles, and as the crystallinity of the samples increases with longer synthesis time, the crystalline Fe/Li ratio approaches 1. The present data thus suggest that when crystalline particles initially form, Fe is included faster in the structure from the amorphous precursor than Li, causing the defects in the structure. Only when all Li have been incorporated into the crystal structure and 100% crystallinity is achieved, fully ordered, defect free samples can be obtained. The Fe occupancy on the M1 site is therefore directly linked to the crystallinity of the sample. In LiFe1‑xMnxPO4 samples, the transition metal defect on the M1 site is only Fe and not Mn. Furthermore, the presence of Mn locks in the defects, and thus the Fe disorder is not suppressed with extended synthesis time. KEYWORDS: LiFePO4, defects, powder diffraction, total scattering

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INTRODUCTION Ever since Goodenough et al. first suggested the use of LiFePO4 as a cathode material for Li ion batteries in 1997,1 the compound has received immense interest. LiFePO4 is cheap and nontoxic, and it shows good electrochemical properties such as high energy density, good cyclability, and high stability.2−5 As shown in Figure 1a, it crystallizes in the olivine structure and has an orthorhombic unit cell (space group Pnma), where edge-sharing LiO6 octahedra (M1 site, violet) form chains along b, while corner-sharing FeO6 octahedra (M2 site, red) form a zigzag pattern in the b/c-plane. During charge and discharge the 1D Li-ion diffusion takes place along the bdirection as illustrated in Figure 1b.6−8 During delithiation, the iron is oxidized to yield FePO4 through a two phase reaction for bulk material9−11 and as a single phase reaction for small nanoparticles.12 LiFePO4 is already used in commercial Li-ion batteries, but much effort still goes into developing new synthesis methods to reduce the cost of the batteries.13,14 During the past decades, hydrothermal synthesis has proven itself as a cheap, environ© 2013 American Chemical Society

mentally benign, and easily scalable way of producing inorganic materials. In 2001 Whittingham et al.15−17 were the first to hydrothermally prepare LiFePO4, and many studies have since been published for both LiFePO4 and LiFe1‑xMnxPO4.18−25 However, when synthesized hydrothermally at low temperatures, the material shows disappointing electro-chemical properties due to defects in the crystal structure. This is believed to be due to the transition metal partly occupying the Li M1 sites and thereby blocking the Li-ion diffusion pathway.16 The exact nature of the defect has been discussed in the literature using several approaches. Islam et al. have shown by theoretical calculations that Fe/Li interchange is the defect of lowest energy,7,26 and the Fe/Li antisite defect has also been studied experimentally using both diffraction and microscopy methods.27,28 Further theoretical studies of antisite defects have also been done.29−32 However, other studies Received: March 13, 2013 Revised: May 1, 2013 Published: May 2, 2013 2282

dx.doi.org/10.1021/cm4008393 | Chem. Mater. 2013, 25, 2282−2290

Chemistry of Materials

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atomic numbers, while Li only scatters weakly. In the case of neutrons, Li and Mn both have negative scattering lengths, while Fe has a positive scattering length. By combining neutron and X-ray data in a simultaneous analysis, we are able to get complete information on Fe, Mn, and Li as well as P and O positions and occupancies. Furthermore, by use of PDF analysis, we are able to get atomic structural information from the noncrystallized material in the sample. The present study thus provides a detailed analysis and understanding of the structure of hydrothermally synthesized LiFe1‑xMnxPO4 particles, and based on the results we propose an explanation for the formation of defects in LiFePO4 during hydrothermal synthesis. Our studies show that the presence of Fe disorder is directly related to partial sample crystallinity, which will strongly affect the electrochemical properties.

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EXPERIMENTAL METHODS

Synthesis. All samples were synthesized hydrothermally in 14 mL steel autoclaves with Teflon linings. For the LiFePO4 synthesis, 2 mL of 1.9 M H3PO4 (Riedel de Haen) was mixed with 2 mL of 1.9 M FeSO4 (Sigma-Aldrich, >99%).15 Then 4 mL of 2.85 M 7LiOH (Sigma-Aldrich, >98%, 97% 7Li) was added, forming a thick green precursor gel. 7Li97%OH was used because of the lower neutron absorption cross section of 7Li. The LiFe1‑xMnxPO4 (x = 0.25 and 0.50) samples were prepared in the same way, by replacing the appropriate amount of FeSO4 with MnSO4 (Sigma-Aldrich, >98%) of the same concentration.23 All syntheses were performed at 170 °C. In order to obtain different defect concentrations the reaction time was varied with synthesis durations of 40 min, 2 h, and 7 h. For all syntheses, 0.08 g of ascorbic acid (Sigma-Aldrich, >99%) was added to act as a reducing agent to avoid the formation of iron(III)-compounds. The pressure was autogenously generated in the autoclaves, which were 50% filled. In summary, the following samples were prepared: 1 LiFePO4 (170 °C, 40 min.), 2 LiFePO4 (170 °C, 2 h), 3 LiFePO4 (170 °C, 7 h), 4 LiFe0.75Mn0.25PO4 (170 °C, 40 min.), 5 LiFe0.75Mn0.25PO4 (170 °C, 7 h), 6 LiFe0.50Mn0.50PO4 (170 °C, 40 min.), 7 LiFe0.50Mn0.50PO4 (170 °C, 7 h). For each sample, four identical syntheses (using identical autoclaves in the same position in the oven) were performed to obtain enough material for neutron experiments. X-ray and Neutron Scattering Experiments. High resolution powder X-ray diffraction (PXRD) data for Rietveld refinement were measured at beamline BL44B2 at Spring-8, Japan using a large Debye− Scherrer camera.36 The X-ray wavelength was determined to be 0.49995(2) Å by Rietveld refinement of a CeO2 standard (a = 5.411102 Å). The samples were loaded into 0.2 mm glass capillaries, and the measurements were done at room temperature. For the LiFePO4 samples, X-ray data for crystallinity determination were measured by mixing a small amount of LiFePO4 with 100% crystalline diamond powder. X-ray total scattering data were obtained at beamline 11-ID-B, Advanced Photon Source, USA, using an X-ray wavelength of 0.212(1) Å and a Perkin-Elmer amorphous silicon detector. The samples were loaded in 1.0 mm kapton capillaries, and the measurements were done at room temperature. Neutron powder diffraction and total scattering data were measured at room temperature at the time-of-flight diffractometer NPDF, Los Alamos National Laboratory, USA. The samples of ca. 2 g were loaded into vanadium cans (6 mm in diameter). Background corrections were determined by measurements on the empty sample chamber and the empty vanadium sample can. The detector efficiency was normalized by measuring data on a vanadium rod. Each data collection took 8 h. Rietveld Refinement. The X-ray and neutron diffraction data were analyzed by simultaneous Rietveld refinement in GSAS37 using the EXPgui interface.38 For the X-ray synchrotron data, the 2θ range from 3° to 65° (i.e., qmax = 13.5 Å−1) was included in the refinement. The background was modeled by interpolation between points with refinable intensity values. The scale factor and the zero point were refined as well as the peak shape, which was described by the

Figure 1. a) LiFePO4 structure where violet octahedra show LiO6, red octahedral show FeO6, and blue dots are the P atoms. b) Illustration of the 1D Li channels along b. The red octahedra are FeO6 and the blue tetrahedra are PO4.

report defects due to nonstoichiometry with Li deficiency resulting in Fe on the M1 sites.33,34 Masquelier et al. have studied the structure of LiFePO4 nanoparticles synthesized by precipitation, and they observed Fe on the M1 site and Fe deficiency on the M2 site.12 We have previously investigated the hydrothermal synthesis of LiFe1‑xMnxPO4 by in situ X-ray diffraction35 and observed that when LiFe1‑xMnxPO4 particles are initially formed, a large amount of Fe is present at the Li site. The structure orders with increasing synthesis time and higher temperature. However, in situ studies are inherently a compromise between data quality and time resolution, and our in situ data were therefore not suited for a detailed investigation of the true nature of the defect chemistry. Here we present an ex situ study, where we have combined neutron and X-ray diffraction data for both simultaneous Rietveld refinements and PDF (pair distribution function) analysis. The structural analysis is complemented with ICP elemental analysis, Mössbauer spectroscopy, SEM images, and crystallinity determination. Only by a combination of all these techniques, we are able to get a full picture of the defect chemistry of the hydrothermally synthesized samples. Especially the simultaneous refinement of X-ray and neutron data is essential to understand the crystal structure and defects in the compounds. For X-rays, Mn and Fe are both strongly scattering but practically indistinguishable due to the similar 2283



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Figure 2. Simultaneous Rietveld refinement of the data collected on sample 3, i.e. LiFePO4 particles obtained after 7 h synthesis at 170 °C. a-d) Neutron banks. e) X-ray data. The black lines show the data, the red the pattern calculated from the model, and the blue line the difference between the two. qdamp value was obtained from refinement of a LaB6 standard. For the neutron data, qdamp and qbroad were provided for the instrument. ICP Analysis. The elemental composition of the samples was analyzed by ICP analysis using a Spectro Arcos ICP spectrometer. Prior to the analysis, the samples were dissolved in an aqueous HNO3 solution. Scanning Electron Microscopy. Scanning electron microscopy (SEM) images were recorded using a Nova 600 Nano SEM from FEI. A Low Vacuum Detector was used due to the insulating nature of the particles, and a water atmosphere (61 Pa) was applied. Mö ssbauer Spectroscopy. Mössbauer spectra were obtained for the LiFePO4 samples at room temperature in transmission geometry using a 57Co:Rh source of ∼5−10 mCi mounted on a conventional drive system. Velocities and isomer shifts are given relative to the center of the spectrum of α-Fe. The spectra were analyzed with one component due to Fe3+ and one component due to Fe2+ which was simulated using a quadrupole splitting distribution based on linear segments in the distribution function.44 The coupling between the quadrupole splitting and isomer-shift was assumed to be the same in all cases.

Thompson-Cox-Hasting pseudo-Voigt function using U, V, W, X, and Y. Corrections for X-ray absorption were done by GSAS. For the neutron data, measurements from all 4 detector banks at the NPDF instrument were included in the fit, covering q-space to 30 Å−1. For all banks, the background was described by GSAS model 4 (sum of exponential functions) where 4 parameters were refined. Furthermore, for each bank, a scale factor was refined along with the zero point, and the profile parameters were refined in profile function 4 in GSAS, which is a convolution of back-to-back exponential functions and a Pseudo-Voigt function. Corrections for neutron absorption were done by GSAS. The X-ray and neutron data were weighted equally in the refinement as the counting statistics are similar. No correlation between refined parameters larger than 80% were observed. The structure was described in the Pnma space group, and unit cell, atomic positions, and isotropic Debye−Waller factors were refined for each site. An average of the neutron scattering length of 6Li and 7Li was used in accordance with their abundance in the samples. Fe was allowed on the Li site and Li on the Fe site independently without constraints. For the Mn containing samples two models were refined: 1) both Mn and Fe on the M1 defect site and 2) only Fe on the M1 defect site. For the crystallinity determination, the weight fractions of crystalline LiFePO4 and diamond in the mixed samples were found by Rietveld refinement using the FullProf program package.39 The crystallinity of the samples was then obtained from the amounts of sample according to Crystallinity =

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RESULTS AND DISCUSSION Crystal Structure of LiFePO4 − Dependence on Synthesis Time. Figure 2 shows examples of the Rietveld fits to both X-ray and neutron data. As seen from the difference curves, good fits with RF values around 5% were obtained for all the data sets. Generally, the quality of the fit decreased slightly with shorter synthesis time indicating some structural disorder not included in the model. The refined parameters and R-values for all samples can be found in the Supporting Information. The Rietveld refinements reveal that the unit cell size is dependent on the synthesis time. This is seen in Figure 3a, where the changes in the unit cell parameters are plotted relative to the final values (7 h data) as function of synthesis time. As reported earlier16,34,35 the unit cell size decreases along a and c with synthesis time, while b increases slightly. The change in cell volume is believed to be related to the defects in the sample, which is apparent from the occupancy of Fe on the M1 site shown as the black line in Figure 3b. The refinements show that short syntheses times produce disordered samples with a high concentration of Fe on the Li site, and this agrees

m%Crystalline LiFePO4 mdiamond · 100% m%diamond mLiFePO4

PDF Analysis. PDF analysis was done for the LiFePO4 and LiFe0.50Mn0.50PO4 samples. Neutron PDFs were calculated from the TOF data using PDFgetN.40 All 4 detector banks were included to obtain the total scattering function S(q) which was Fourier transformed using qmax = 25 Å−1. X-ray PDFs were obtained in PDFgetX241 using qmax = 23 Å−1. Both the neutron and X-ray PDFs were modeled individually in PDFgui.42 The structure was described as for the Rietveld refinements, however with the difference that the metal occupancies were not refined but kept fixed at the values obtained from the Rietveld refinements. The pair distribution r-ranges from 1−5 Å and 5−20 Å were used in the analysis. Apart from the structural parameters, the scale factor and the quadratic dynamic correlation factor were refined.43 For the X-ray measurements, the 2284



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The macroscopic strain in the crystal results in broadening of the peak profile with a tan(θ) dependency and based on the instrument corrected profile parameters, the strain S can thus be calculated (see the Supporting Information).37 This is plotted as a function of synthesis time in Figure 3c. The plot suggests that the strain arises from the differences in unit cell sizes due to the Fe occupancies on the M1 site. Formation of LiFePO4 from Amorphous Particles: Relation between Crystallinity and Defects. Previously, we have shown by means of in situ PXRD studies that LiFePO4 forms from a precursor gel consisting of small particles of iron and lithium phosphates, which are only slightly crystalline and where the precise structure depends on the local pH value in the gel.35 From the present diffraction data, there are no clear observations of any remaining crystalline precursor gel and good Rietveld fits are obtained for all data sets. However, from the crystallinity measurements of the LiFePO4 samples, it is clear that a considerable fraction of the samples synthesized for 40 min and 2 h is not crystalline LiFePO4 as shown in Figure 4a

Figure 3. a) Change in unit cell parameter for LiFePO4 relative to the value obtained after 7 h. b) Refined concentration of Fe (black) and vacancies (red) on the M(1) Li site (red). c) Microstrain plotted as function of Fe occupancy on M1 (Li) sites. The error bars on all refined parameters are smaller than the size of the symbols.

well with the unit cell changes. Due to the slight size difference of Li+ and Fe2+ (Shannon ionic radii of 76 and 78 pm,45 respectively) the presence of Fe2+ will expand the unit cell slightly along a and c, while it is almost unaffected along b, where there is more space for the Fe2+ ion in the Li diffusion channels. The Rietveld refinements show that after short syntheses, large amounts of Fe are present on the M1 site, while there is no Li on the M2 site. For all samples M2 is fully occupied with Fe. The results thereby confirm earlier studies stating that the crystallographic disorder present in hydrothermally synthesized LiFePO4 samples is not an antisite defect but rather Fe excess.33,34 We suggest that this deviation from ideal LiFePO4 stoichiometry is caused by reaction kinetics and is directly linked to the crystallinity of the samples, as will be discussed further below. The simultaneous refinement of X-ray and neutron data allow refinement of the full site occupancy of Li and Fe on the two metal sites, and this also provides a determination of the metal site vacancy concentration; calculated as occ(vacancyM1) = 1 − occ(LiM1) − occ(FeM1). This is plotted as the red curve in Figure 3b. The vacancy and Fe concentration on M1 sites follow the same trend, and fewer vacancies are seen with longer synthesis time. However, the vacancy concentration is always slightly higher than the Fe occupancy and for charge balance conservation; substitution of a Li+ ion with Fe2+ requires formation of only one single Li-site vacancy. The ‘additional’ M1 vacancies in the refined model are believed to be due to the presence of Fe3+on the M2 (Fe) sites, as is corroborated by the Mössbauer measurements discussed below. In powder diffraction, the Bragg peak width is affected by the finite extent of coherently diffracting domains in the crystal.

Figure 4. a) Crystallinity of the LiFePO4 samples as a function of synthesis. The uncertainties have been estimated to be 5%. b) Ratio between the scale factors from the low r-range PDFs and the high rrange PDFs. The neutron results are shown in black, while the X-ray results are in red. c) Fe/Li ratio from Rietveld analysis (black) and ICP (red) as a function of synthesis time. In b and c, the error bars are smaller than the symbols.

(see the SI for details). Although the absolute crystallinity values are associated with rather large uncertainties arising from the sample preparation and data analysis, it is clearly seen that the mass fraction of crystalline LiFePO4 increases with increasing synthesis time. The atomic structure of the amorphous material can be characterized by PDF analysis. The PDF is based on total scattering data, and it provides therefore information on both 2285



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Figure 5. Modeling of the neutron and X-ray PDF data in the range from 5−20 Å.

Figure 6. SEM images of LiFePO4 particles synthesized at 170 °C for 40 min (A), 2 h (B), and 7 h (C).

plotted in the Supporting Information. Here, it is seen that the region from 1 to 5 Å can in fact be fitted well with the crystalline LiFePO4 structural model. This shows that the atomic structure of the amorphous nanoclusters is very closely related to the bulk phase. The ratio between the scale factors obtained for the 1−5 Å and 5−20 Å fits are plotted in Figure 4b, which clearly shows that the shorter the synthesis time, the larger the amount of amorphous ‘LiFePO4’ with only shortrange order. It should be noted that an additional origin of the poor fit at low r for the defective phase could be local structural disorder induced by the presence of Li at the Fe sites, leading to a split metal site. When using our small-box structural models, this is not observed. However, to fully characterize whether the cation disorder changes the local structure of the metal sites, large box

the amorphous and crystalline parts of the particles. Neutron and X-ray PDF fits for the three LiFePO4 samples are shown in Figure 5. The data are fitted in the r-range from 5 to 20 Å, and in this region the crystallographic model fits well with the experimental PDFs. However, when extending this model to r values below 5 Å, a large fraction of the peak intensities in the experimental PDF is not well described, showing that amorphous lithium and iron phosphate nanoclusters with only local order coexist with the crystalline LiFePO4. The peak seen at 1.5 Å originates from the P−O bond, while the range from 2 to 2.7 Å covers the first Li−O and Fe−O distances. The first metal−P distance is seen around 3.2 Å, and this peak is particularly poorly described in the X-ray data from the shorter syntheses time. To quantify the difference in intensity in the different r-regions, fits based on the LiFePO4 structure were made from both 1−5 Å and 5−20 Å. The low r-region fits are 2286



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have previously been suggested to increase the performance of the cathode material,30 there is no doubt that the presence of Fe in the Li channels blocking the diffusion pathway will reduce the electrochemical capacity significantly. However, the main reason for the low capacity of hydrothermally synthesized samples might very well be the presence of a large amorphous fraction. The lithium and iron phosphate nanoclusters do not contribute to the electrochemical capacity or the electronic or ionic conductivity of the samples. Furthermore, as the amorphous clusters seem to reside on the surface of the crystalline particles, their presence could hinder the diffusion of Li+ into the crystalline particles. Only by ensuring 100% crystallinity, high quality materials can thus be obtained. This can be done by high synthesis temperatures, longer synthesis times, and/or postsynthesis high temperature treatment. Low crystallinity is generally a problem for samples synthesized at low temperatures and affects the properties of almost all functional materials. Although PXRD is a standard characterization technique, simple crystallinity measurements using a crystalline standard are rarely done. By measuring this along with characterization of the crystal structure, size, and morphology, many of the physical properties of the synthesized materials can be explained. Mössbauer Analysis of LiFePO4 Samples. Figure 8 shows the Mössbauer spectra obtained from the LiFePO4 samples. The overall shape of the spectra is due to a dominating Fe2+ component, but close inspection shows two distinct features in the samples compared with a single phase Fe2+ compound. First, there is a feature at v ∼ 0.85 mm/s, most clearly seen in the sample from the shortest synthesis. Most

modeling (e.g., by the Reverse Monte Carlo technique) is needed. The Fe/Li ratios obtained from the crystallographic analysis as well as from ICP are plotted in Figure 4c. The stoichiometry of the crystalline LiFePO4 changes as a function of synthesis time, and for all samples the Fe/Li ratio is higher than 1. The ICP results show similar trends, although here the ratio between Fe and Li is much closer to that of stoichiometric LiFePO4. The diffraction results express the stoichiometry of the crystalline part of the sample, while the ICP analysis gives the composition of the entire sample, including the amorphous part. The discrepancy between the results from the two techniques thus indicates that there are differences in the way that Li and Fe are incorporated into the crystal structure during the synthesis. Thus, Fe is included relatively faster into both the M1 and M2 sites of the crystal structure than Li, leading to the Fe excess. SEM images of the three LiFePO4 samples are shown in Figure 6. All the samples are very heterogeneous and consist of particles of several different sizes and shapes. After 40 min at 170 °C, rhombic shaped particles coexist with smaller, spherical-like particles. The smallest particles are most likely amorphous, while the rhombic particles are crystalline. After 2 h, the rhombic particles have grown bigger, and the size distribution has broadened. The largest particles are more irregular but still keep the basic rhombic shape. Again, smaller particles without the rhombic morphology are also observed. After 7 h of synthesis, some of the particles have grown significantly to more than 10 μm, Much smaller rhombic and some irregular particles are also observed. By combining the results from Rietveld analysis, PDF analysis, ICP, and SEM, a picture of the formation mechanism emerges, as illustrated in Figure 7. The amorphous structures

Figure 7. Formation mechanism for LiFePO4.

present after short synthesis surround the crystalline, defective particles, and more of the lithium phosphates may be found in solution. The amorphous particles then act as ion donors to the growing crystallites. The Fe-PO4 parts are incorporated in the crystal structure faster than Li, leading to defects in the structure. However, as the synthesis proceeds, more LiFePO4 crystallize, and the remaining amorphous lithium phosphate is included in the crystalline particles. As this happens, the structure orders, and Fe is moved from the M1 site to the M2 site. Thus, only when full crystallinity is obtained, defect free LiFePO4 can be achieved. The presence of defects is thus directly related to the crystallinity of the particles. With our novel understanding of defects and crystallinity of hydrothermally synthesized LiFePO4, a new interpretation of the poor electrochemical properties of samples synthesized using this method can be given. First, we have confirmed that the defects present are not antisite defects but in fact excess Fe and vacancies occupying the Li sites. Although Li vacancies

Figure 8. a-c) Room-temperature Mössbauer spectra of the three LiFePO4 samples. The experimental data are shown as black dots, while the solid lines shows the fitting components from Fe2+ (green), Fe3+ (blue), and their sum (red). d) Quadrupole splitting distribution determined for the LiFePO4 sample synthesized for 40 min. 2287



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Table 1. Hyperfine Parameters and Spectral Areas Found from Simultaneous Analysis of the Mössbauer Spectraa FeIIb,c

FeIIId

sample

⟨δ⟩ (mm/s)

⟨ΔEQ⟩ (mm/s)

area (%)

δ (mm/s)

ΔEQ (mm/s)

Γ (mm/s)

area (%)

LiFePO4, 40 min LiFePO4, 2 h LiFePO4, 7 h

1.24(4) 1.23(4) 1.23(4)

2.73(5) 2.78(5) 2.85(4)

88(1) 93(2) 98(1)

0.30(4)

1.13(7)

0.57(9)

12(1) 7(1) 2(1)

a

The table lists the values of (average) isomer-shift (δ), quadrupole splitting (ΔEQ), and FWHM line width (Γ). bThe coupling between isomer shift and quadrupole splitting was found as dδ/dΔEQ = −0.08(1). cThe same parameters of FeIII were used for all samples. dIn all cases, the peak of the quadrupole splitting distribution was at 2.92(4) mm/s.

likely, this is due to the right-leg of a quadrupole split Fe3+ component, where the left-leg overlaps with the left-leg of the dominating peak due to Fe2+. Second, there is an asymmetry in the line-shape of the Fe2+ component, suggesting the presence of additional component(s) with lower quadrupole splitting than the dominating component. The results of the modeling are summarized in Table 1. The Fe3+ content is highest for the shortest syntheses, and after 7 h only very small amounts of Fe3+ are observed. The results agree with the trends from Rietveld analysis, which indirectly showed the Fe3+ as ‘additional’ vacancies. However, the Mössbauer results show 12(1)% Fe3+ in the sample synthesized for 40 min, which is much higher than the ∼2% extra vacancies seen. This indicates that a large fraction of the Fe3+ ions could be present in the amorphous part of the sample. As the amount of Fe3+ decreases with synthesis time, the results indicate that Fe3+ does not actually form during the synthesis. Possibly, it comes from Fe2+ in the amorphous phase that gets oxidized during handling of the samples after the synthesis, when the reducing ascorbic acid is no longer present. Masquelier et al. have studied nanoparticles of LiFePO4 after moderate thermal treatments in air,46 and they also observed a large fraction of Fe3+ in their samples but concluded that this was present at the M1 site in the structure. They saw that the unit cell volume decreased with increasing Fe3+ content due to the smaller ionic radii of Fe3+ (63 pm).45 We see the opposite effect (the unit cell is largest for the samples with the highest Fe3+ content), indicating that the content and position of Fe3+in the crystal structure is very dependent on both particle size and treatment method. The hyperfine parameters for Fe2+ in the peak of the quadrupole splitting distribution (δ = 1.22(3) mm/s, ΔEQ = 2.92(4) mm/s) are in a reasonable agreement with parameters obtained on natural LiFePO4 i.e. triphylite.47 The lower average quadrupole splitting for the sample synthesized for 40 min and 2 h (cf. Table 1) are due to the quadrupole splitting distribution as shown in Figure 8d. The Mössbauer results thus indicate that several different Fe2+environments exist, in agreement with the diffraction and total scattering results, where Fe2+ was observed at M2, M1, and in the amorphous Fe-PO4 structure. Bini et al.48 saw a similar asymmetry of the Fe2+ component in samples produced by microwave-assisted hydrothermal synthesis route. They analyzed their spectra in terms of three Fe2+ components and saw a similar negative trend between isomer shift and quadrupole splitting as observed here. Metal Disorder in LiFe1‑xMnxPO4. The refined unit cell parameters for the manganese substituted compounds are plotted in Figure 9. As Mn2+ has a larger ionic radius than Fe2+, substitution of Fe for Mn increases the unit cell volume. Just as for the LiFePO4 samples, the unit cell also increases for short synthesis time due to disorder on the metal sites. The results

Figure 9. Unit cell parameters for all samples as a function of synthesis time. The error bars on the refined parameters are smaller than the symbols.

from refinements using the two different disorder models are shown in Table 2. When both Mn and Fe are allowed on the Li site (model 1), the refinement increases the vacancy concentration significantly (20% for LiFe0.50Mn0.50PO4, 40 min), which does not seem reasonable and does not agree with the ICP results. Model 2, where only Fe is allowed on the M1 site, gives a comparatively better and more physical result. This indicates that in LiFe1‑xMnxPO4 only Fe defects and not Mn defects are present. The Shannon ionic radii of Mn2+ and Fe2+ are 83 pm and 78 pm, respectively, compared with 76 pm for Li+, and thus the manganese ion may be too large to be incorporated on the M1 site.45 The defect concentrations from model 2 are plotted in Figure 10, which includes a comparison with LiFePO4. For short syntheses (40 min) less Fe occupancy is seen on M1 with increasing Mn content. However, after long synthesis times (7 h), where the disorder is almost completely suppressed in 2288



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Table 2. Defect Concentration and Sample Compositions in LiFe1‑xMnxPO4a total Fe and Mn occupancy on M1 (%) sample 4, 5, 6, 7, a

LiFe0.75Mn0.25PO4 LiFe0.75Mn0.25PO4 LiFe0.50Mn0.50PO4 LiFe0.50Mn0.50PO4

(40 min) (7 h) (40 min) (7 h)

Fe:Mn:Li − Rietveld crystalline stoichiometry

vacancy concentration on M1 (%)

model 1

model 2

model 1

model 2

model 1

model 2

Fe:Mn:Li − ICP total sample stoichiometry

7.2(1) 3.8(1) 4.0(1) 2.7(1)

6.6(1) 3.6(1) 6.2(1) 4.0(1)

12.8(1) 8.2(1) 20.1(1) 19.6(1)

7.4(1) 4.7(1) 7.8(1) 4.0(1)

0.96:0.38:1 0.85:0.33:1 0.72:0.82:1 0.66:0.74:1

0.90:0.33:1 0:85:0.31:1 0.61:0.62:1 0.55:0.58:1

0.84:0.34:1 0.77:0.29:1 0.56:0.64:1 0.52:0.61:1

In model 1, both Mn and Fe were allowed on the M1 site, whereas for model 2, only Fe was allowed on the M1 site.

particles the defects are suppressed, and thus high quality battery materials are obtained either from extended synthesis time or from higher synthesis temperature. LiFe1‑xMnxPO4 is also disordered on M1, but the refinements show that only Fe occupies the Li site. The presence of Mn in the structure locks the Fe disorder even for long synthesis times, and suppression of the defect formation is challenging for hydrothermal synthesis. The present study underpins the need for thorough structural investigations of cathode materials synthesized by low temperature methods. The disappointing electrochemical properties of hydrothermally synthesized LiFePO4 have so far been explained by the presence of Fe in the Li channels. Although there is no doubt that this will influence the capacity of the resulting cathode, the low crystallinity of defective samples might have an even larger effect.

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Figure 10. Fe occupancy on the M1 site in LiFe1‑xMnxPO4 as a function of synthesis time. The error bars on the refined parameters are smaller than the symbols.

ASSOCIATED CONTENT

* Supporting Information S

Refined parameter values and R-factors for all Rietveld refinements. Calculations of strain from peak widths. Crystallinity calculations. Low r PDF fits of LiFePO4. PDF analysis of LiFe0.50Mn0.50PO4. This material is available free of charge via the Internet at http://pubs.acs.org.

LiFePO4, there are still significant amounts of Fe on the Li site, when Mn is present in the structure. Thus, Mn seems to lock the defective structure, making it challenging to synthesize defect free LiFe1‑xMnxPO4 nanoparticles hydrothermally. This agrees with theoretical calculations.26 When comparing the Rietveld and ICP stoichiometry results, the Fe/Li ratio is higher in the crystalline part of the sample than in the whole sample, just as for pure LiFePO4, again showing that Fe is incorporated faster into the crystal structure than Li. However, the Mn/Li ratios found from the two methods are almost the same, even after short synthesis times. This indicates that Mn is immediately incorporated in the crystal structure when the synthesis is initiated. This agrees well with our previous in situ study showing that Mn substituted samples form faster than LiFePO4.35 The PDF results for LiFe0.5Mn0.5PO4 are similar to those of LiFePO4, and details are given in the Supporting Information. Poor fits to the low rregion are seen, which is believed to be due to the presence of amorphous content. Again, local disorder induced by the split cation site (from both Fe, Mn, and Li) is not apparent in our small-box refinements.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Danish National Research Foundation (Center for Materials Crystallography, DNRF93) and the Danish Research Council for Nature and Universe (Danscatt). The research was performed on the NPDF instrument at the Lujan Center at Los Alamos National Laboratory supported by DOE-Basic Energy Sciences under FWP #2012LANLE389. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The synchrotron radiation experiment at the SPring-8 synchrotron was conducted with the approval of the Japan Synchrotron Radiation Research Institute. The RIKEN-SPring8 Center is thanked for access to the BL44B2 beamline.

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CONCLUSION Simultaneous Rietveld refinement of high quality neutron and X-ray powder diffraction data has been used to obtain unprecedented detailed information about the structure of hydrothermally synthesized LiFe1‑xMnxPO4 particles. Initially in the synthesis, Fe is found to partially occupy the M1 Li site and fully occupy the M2 site, and overall the structure therefore contains excess Fe. The defects arise due to differences in the rate with which Li and Fe are introduced in the crystalline LiFePO4 structure as shown by the fact that LixFeyPO4 coexists with amorphous Li/Fe-PO4 units. For the 100% crystalline

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Defects in Hydrothermally Synthesized LiFePO4 and LiFe1-xMnxPO4

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