Defect and Surface Area Control in Hydrothermally Synthesized LiMn0

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Crystal Growth & Design

Defect and Surface Area Control in Hydrothermally Synthesized LiMn0.8Fe0.2PO4 Using a Phosphate Based Structure Directing Agent Jürgen Schoiber†, Gerold Tippelt†, Günther J. Redhammer†, Chihiro Yada‡, Oleksandr Dolotko§, Raphael J. F. Berger†*, Nicola Hüsing† †

Materials Science and Physics, Paris Lodron University Salzburg, 5020 Salzburg, Austria

‡

Battery Research Division, Higashifuji Technical Center, Toyota Motor Corporation, Susono, Shizuoka 410-1193, Japan §

Maier-Leibnitz Zentrum MLZ Forschungsreaktor München FRM-II, Lichtenbergstr. 1, 85747 Garching, Germany

ABSTRACT: As confirmed by ion coupled plasma-mass spectrometry and powder X-ray diffraction, stoichiometrically pure samples of olivine type LiFe0.2Mn0.8PO4 in the presence of a phosphoric acid ester based structure directing agent have been prepared. A Rietveld analysis of X-ray and n0 diffraction data suggests that the solids are largely free of defect occupation of Fe at the lithium sites. This is confimed by 57Fe-Mößbauer spectroscopic investigations. The use of the structure directing surfactant results in significantly higher specific surface areas (SSA) and smaller particle sizes as compared to samples prepared without using a surfactant. In using NO3- salt educts or different surfactants a further increase in SSA but at a cost of lower stoichiometric lithium contents can be achieved.

1. INTRODCUTION Olivine type lithium transition metal phosphates LiMPO4 (M = Fe, Mn, Co, Ni) are amongst the most promising candidates for cathode materials in next-generation rechargeable lithium-ion batteries. For this reason they have been in the focus of intense research activities in the past one and a half decades.1─4 One of the most important development goals is to improve the charge/discharge capacity by a proper choice of the composition and the structure of the battery materials. The theoretical maximum capacity of LiFePO4 as a cathode material is 170 mAh/g.5 Values closely approaching this upper limit have been reported for materials which have been prepared via solid state reaction or with hydrothermal synthesis methods.4 In contrast partially manganese substituted materials of type LiFe1-xMnxPO4 prepared in hydrothermal reactions often show disappointing electrochemical properties significantly below this value.6 This is often attributed to lattice defects, like Fe/Li interchange (which is often also called an ‘anti-site defect’) or an surplus of M2+-ions (M = Me, Fe) at lithium sites (which are also called M1 sites in the olivine structure, as compared to the positions of the transition metal which are called M2 sites) (see figure 1). Such defects presumably block Li ions in the diffusion pathway.7 Theoretical considerations and model calculations suggest that the most favourable defect occurring in LiMPO4 (M= Fe, Mn, Co, Ni) olivines is the Li+/M2+ interchange (anti−site defect), with an occurrence lower than 2%.8 Hereby is the actual amount expected to largely depend on synthesis conditions especially the reaction temperature. Systems with M = Fe or Co, are expected to be especially prone to Li+/M2+ interchange defects.9 Whereas the influence of experimental conditions on structural defects for the end members LiMnPO4 and LiFePO4 of the solid solution series LiMn1-xFexPO4 have already been investigated in detail8-12 much less is documented for mixed stoichiometries such as LiMn1-xFexPO4.13 In a very recent work Jensen et al. have investigated the structural nature of these lattice defects in LiFe1-xMnxPO4 materials.6 From combined X-ray diffraction (XRD), neutron-diffraction experiments supported by inductively coupled plasma optical emission spectrometry (ICP-OES) measurements, Mößbauer spectroscopy and scanning electron microscopy (SEM), they found a direct correlation between the frequency of a defect occupation of Fe2+-Ions at the M1 site and the degree of crystallinity of the prepared materials. This was attributed to the kinetics of Li+, Fe2+ and Mn2+ incorporation (slow, fast and very fast) into the solids and the kinetics of the formation of crystalline products from amorphous precursors and intermediates. They could show for LiFePO4 that the occurrence of structural defects is decreasing with increasing reaction time. Moreover they have found that the presence of Mn2+ ions in LiMn1-xFexPO4 results in structural defects which are not curable by

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longer reaction times. This was explained by a blocking of M2 sites with Mn2+ ions for the reoccupation with Fe-ions from M1 sites. This work is a continuation of our efforts to develop new synthetic strategies for high surface area LiMnPO4 materials using structure directing agents.14 We report about the preparation of almost Li+/M2+ (M = Mn, Fe) interchange defect free LiFe1-xMnxPO4 materials with high specific surface areas using a phosphate based structure directing agent and without restriction to high crystallinity. The structural state of the prepared materials was analyzed by Rietveld refinements based on X-ray and neutron diffraction data as well as 57Fe-Mößbauer spectroscopy and ICP-OES measurements.

Figure 1. Crystal structure of LiMPO4 (M = Mn, Fe) a) defect free and anti-site defect (arrow) and b) M2+-excess on M1-site; view along c-axes. 2. EXPERIMENTAL METHODS Synthesis. All chemicals were purchased from Sigma-Aldrich in purities > 99%, and used without further purification. Water was deionized, degassed in vacuo at RT and saturated with Ar prior to usage. (1) To 130 mL of H2O 8.6 g CH3COOLi∙2H2O was added and stirred until complete dissolving. Then 3.23 g H3PO4 (85 %) and 5.5 g Mn(CH3COO)2∙4H2O was added under precipitation of a white solid. To this suspension 1.01 g FeC2O4∙2H2O and 0.55 g ascorbic acid were added and the suspension was stirred for 5 min and transferred to a 200 mL stainless steel autoclave with a teflon inlet. The autoclave was closed and the reaction mixture was vigorously stirred with a teflon-covered magnetic stirring bar and it was heated to 200 °C upon autogenously generated pressure in the autoclave. After 20 h of reaction time the sample was allowed to cool to RT. Solid precipitates were separated from solution by centrifugation and decantation. The precipitate was washed twice with a mixture of 40 mL tetrahydrofuran (THF, VWR Company, technical grade) and 40 mL of H2O and dried for 1 h at 60 °C in air to yield 3.2 g of 1 (72.6 %.in relation to Mn). (2) was prepared in analogy to 1 except, that 13.13 g of Triton H-66® were added prior to Li acetate and the amount of phosphoric acid was reduced to 1.01 g. 3.4 g of 2 was yielded (77.2 %.in relation to Mn). X-ray and Neutron Scattering Experiments. Powder X-ray diffraction (PXRD) measurements (5° ≤ 2Θ ≤ 80°, continuous scan) were carried out in reflection-mode on a BRUKER D8 DaVinci Design diffractometer using CuKα radiation, fixed 0.3° divergence slit, primary and secondary side 0.04 rad Soller slits, anti-scatter slit 4.0° and the Lynxeye detector (opening angle 2.93°). Neutron diffraction experiments were performed at the Heinz Maier-Leibnitz Forschungsneutronenquelle (FRM II) Garching/München, Germany. Powder diffraction data were acquired in constant wavelength mode using the high resolution powder diffractometer SPODI.15 The sample was contained in a 14 mm diameter vanadium can at 300 K using Ge551 monochromatized neutron radiation (λ = 1.548203 Å). Experiments were performed in a 2Θ range 3° ≤ 2Θ ≤ 152°, step width 0.04°. Rietveld Refinement. The data refinement was done using the FULLPROF16-suite of programs. The pseudo-Voigt function was chosen to model peak shape, all atoms were refined using isotropic atomic displacement parameters. Starting values for the refinement of PXRD data were taken from the literature.19 ICP-OES Analysis. The elemental composition was analyzed by ICP-OES analysis using Ultima2 from Horiba Jobin Yvon. The samples were dissolved by acidic fusion with aqua regia in the microwave oven Multiwave 3000 from Anton Paar. All stoichiometric quantities in this work are given relative to the total content in phosphate [(PO4)x with x = 1.0]. Scanning Electron Microscopy. Scanning electron microscopy (SEM) images were taken using a ZEISS FE-SEM ULTRAPLUS from ZEISS and using its in-lens secondary electron detector and an accelerating voltage of 10kV. Mößbauer Spectroscopy. Transmission 57Fe Mößbauer spectra were collected at room temperature using a Mößbauer apparatus (HALDER electronics, Germany) in horizontal arrangement (57Co/Rh single line thin source, constant acceleration mode, symmetric triangular velocity shape, multi-channel analyser with 1024 channels, velocity scale calibrated to α-iron). For Mößbauer absorber preparation samples were carefully ground under ethanol, filled into Curings (inner diameter 10 mm and covered with a high-purity Al-foil on one side) and mixed with epoxy resin to fix the sample. The folded spectra were analyzed using classical full static Hamiltonian site analysis (using Lorentzian shaped doublets).


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3. RESULTS AND DISCUSSION The hydrothermal synthesis we have applied is derived from a previously reported one.14 In attempting to investigate the formation of the solid we have tried to simplify the composition of the reaction mixture. Initially we have replaced hydrazine by ascorbic acid as a reducing agent, since it potentially can undergo an exothermal comproportionation reaction with nitrates to form N2 gas. Later we have replaced all NO3- salts by acetates and in the synthetic procedure we have tried also to exclude all other sources of oxygen and oxidizing agents to prevent Fe(II) being oxidized to Fe(III). As a source of Fe(II) we have used iron(II) oxalate, which is stable against oxidation in air. We also took special care in degassing all solvents and the autoclave was flooded with argon before the preparation. We have noticed that use of even degassed ethanol for the final washing procedure always led to a darkening of the solid product, which we could assign in the course of our investigations to the formation of Fe(III) species. We have found in ICP-OES analysis of crystalline products that stoichiometric excesses of 8:1 in Li vs. Mn/Fe are not required and the ratio can be lowered without gaining stoichiometric lithium deficiencies in the product down to 3:1 as it is also reported in the literature.6 Instead of using glass autoclaves we choose to use stainless steel autoclaves with Teflon inlets since for our synthetic procedure it was required (see below) to heat to 200°C, which generates autogenous pressures for which the glass autoclaves are not suitable anymore. An increase of the reaction temperature from 150 to 200 °C for hydrothermally synthesized LiMPO4 (M = Mn, Fe) should decrease structural defects.7,17 In the following we will discuss the properties of representatives of the two material types 1 and 2 based on the analytical investigations we have performed. In addition to this developed synthesis we want to mention, that LiFePO4 can also be synthesized by using the same structure directing agent (SDA), Triton H-66® as here and previously reported.18 Our initial intention in using a SDA as an additive in the preparation was to achieve higher surface areas of the products. Since we are dealing with Mn and Fe ions which show good complexation with phosphates we have chosen this SDA. However organic phosphoric ester functions are labile towards ester cleavage reactions, especially under harsh conditions, so we have attempted to compensate for additional PO43- contents in the reaction mixtures of type 2 by addition of an equivalent amount of H3PO4 in the preparation of type 1 materials. Both samples crystallize in space group Pnma. PXRD patterns revealed phase pure sample material, subsequent Rietveld refinement (see Figure 2) yielded very good fits [Rf= 5.9 % (1), 3.4 % (2)]. Further data from the PXRD refinement are summarized in table S1 and S2 in the supporting informations. Samples of type 2, show some broadening of the Bragg-peaks, which we assign to the smaller average particle size. This is in agreement with the results from the SEM investigation of the materials (see Figure 3).



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Figure 2. X-ray diffraction of both samples [synthesized without SDA ( type 1) and with SDA (type 2)]. ICP-OES analysis show for samples of type 1 a slightly off-stoichiometric composition of Li0.91Mn0.82Fe0.22PO4 while samples of type 2 show compositions which are within the error limits of the method matching the theoretical values of Li1.00Mn0.80Fe0.20PO4. Charge balancing is given for both sample types. We conclude that an excess of M2+ (M = Mn, Fe) for type 1 is possibly present.


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Figure 3. SEM images of both samples (synthesized without SDA ( type 1) and with SDA (type 2)). In order to better characterize the distribution of Li, Fe and Mn on the crystallographic positions M1 and M2 neutron diffraction measurements (NDM) have been performed (see Figure 4). The Rietveld refinement on the diffraction data of both samples yielded RBragg values of ~ 2.5 %. %. In first refinement cycles for cationic distribution in sample 1, M1 was assumed to be occupied by Li+ exclusively, while M2 was refined as (Fe2+ + Mn2+). This – however - resulted in some deficiencies in the (0 1 1) and (1 2 0) Bragg peaks in 1. Thus a partial occupation of the M1 site with either (Li+ + Fe2+) or (Li+ + Mn2+) was tested: While for the case of Mn a negative occupation was obtained. A partial occupation of Fe2+ on the M1 site improved the fit, yielding a partial occupation of 1.6 % Fe on M1. From this, we conclude that Mn2+ is located exclusively on the M2 site. For sample 2 this mismatch in the (0 1 1) and (1 2 0) Bragg peaks could not be detected having Li exclusively on M1 and (Fe + Mn) on M2. Tests with either Mn or Fe besides Li on M1 yielded negative occupancies in sample 2. After doing refinements on the neutron diffraction data, simultaneous refinements of both, PXRD and NDM were performed for both type 1 and type 2 samples. They yielding very similar results to the neutron data with only the Fe content in the type 1 sample being somewhat higher. Results of this refinement are given in Table S1 for comparison. Besides the refined site occupation, also the lattice parameters of the two samples slightly differ, especially along the a─ and c─ axis which are slightly elongated in samples of type 2 (Table 1). This is also in agreement with previously published results.6



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Figure 4. Neutron diffraction of both samples (synthesized without SDA ( type 1) and with SDA (type 2)). Concluding, for samples of type 1 with stoichiometry Li0.91 Mn0.82Fe0.22PO4 only the model assumption of Fe on the Lisite led to a good Rietveld refinement and to a non-zero partial occupation of Fe on M1-site of 1.6%. Whereas for the type 2 LiMn0.8Fe0.2PO4 compound no partial occupation with Fe-ions could be refined at the M1-site. Furthermore, in both samples no Mn could be refined at the M1-site. Table 1: Lattice parameters of LiMn0.8Fe0.2PO4 of type 1 and 2 as determined from the neutron powder diffraction experiments. Ty pe

a [Å]

1

10.44242(14)

2

10.43175(12)

b [Å]

6.08711(8) 6.08515(7)

c [Å]

V [Å3]

Fe on M1 site (%)

4.74477(7)

301.598( 7)

1.6

4.73602(6)

300.636( 6)

0

According to reference [6], a decreasing proportion of structural defects is observed in products prepared with prolonged reaction times. However, in our experiments increasing the reaction time even to 20 hours and the reaction temperature to 200 °C did not yield entirely defect free materials when no structure directing agents was used in the synthesis. With Mößbauer spectroscopy information about the oxidation state of Fe cations as well as the crystallographic site occupation and its relative stoichiometric amount can be obtained. Therefore 57Fe Mößbauer spectroscopy was used routinely to characterize the valence state and Fe-cationic distribution on M1 and M2-site. Two representative spectra for type 1 and 2 are shown in figure 5.


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Figure 5. Mößbauer spectra of both samples. The spectra of type 1 can be fitted with two Lorentzian shaped doublets and represent Fe2+-ions at M1 and M2 site. No Fe3+-species are detected. The spectra of type 2 can be fitted with two Lorentzian shaped doublets, too, but representing Fe2+- and Fe3+-ions only at M2 site. The isomer shift assigned to the Fe2+-ions at the M2 site found here is in good agreement with the value found by Jensen et al. in a previous study.6 We have obtained an isomer shift of 0.42 mm/s and a quadrupole splitting of 0.78 mm/s for the Fe3+-ions, while Jensen et al. found 0.30 mm/s and 1.13 mm/s6, which they have assigned to an amorphous phase containing the Fe3+-ions. In contrast, our data for type 2 materials suggest that the Fe3+-ions are still in a crystalline environment at M2-sites. These sites are apparently less distorted as it was found in related materials, where quadrupole splitting higher than 1 mm/s occurs.20 Assuming an excess of Fe-ions in the Mn-rich phase Li0.91Mn0.82Fe0.22PO4 (type 1) the local environment of the Fe-ions at the M1-site is stronger distorted and lead therefore to a higher isomer shift and quadrupole splitting as in case of the Fe-ions on the M2-site. The results are summarized in Table 2. Combining the results of ICP-OES, Rietveld refinement of NSM and Mößbauer spectroscopy we conclude that for type 1 samples we have an excess of Fe2+-Ions on the M1-sites and therefore structural defects in the (Li0.91Fe0.04) Mn0.82Fe0.18PO4 compound but not any classic anti-site defect. For type 2 samples we could not detect any structural defects at all. Table 2: Mößbauer spectroscopy data of LiMn0.8Fe0.2PO4 of type 1 and 2 Type 1 IS (mm/s)

QS (mm/s)

w+ (mm/s)

Area (%)

Fe2+ at M2

1.33

2.82

0.17

80

Fe2+ at M1

1.35

3.08

0.14

20

-

-

0

3+

Fe at M2

-

Type 2



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Fe2+ at M2

IS (mm/s)

QS (mm/s)

w+ (mm/s)

Area (%)

1.24

2.95

0.14

96

2+

Fe at M1

-

-

-

0

Fe3+ at M2

0.42

0.78

0.21

4

Comparing the mean distances of the M1and M2-site in Li1-xFe1+xPO4 (excess of Fe on M1-site) or (Li0.99Mn0.01)(Mn0.99Li0.01)PO421–24 (exchange of Li and Mn on M1- and M2-site), see table S3, with the coordinating oxygen atoms in octahedral arrangement (Figure 6) it can be seen, that the distances from M1-site to the O1-site slightly increases for both compounds with different structural defects. Contrarily, in Li1-xFe1+xPO4 the M2-site only shows a decrease in the bond length for O3b-sites, the LiMnPO4 compound shows a different behaviour where no excess of M2+-Ions occurs, but anti-site defect is present. Additionally to the decrease of the mean distance between the M1-site and the O1-site the bond length to the O3b-site increases. Furthermore, a dramatic increase of the bond lengths between the M2-site to the O1-, O2- and O3a-site can be seen. The difference can be assigned to the different ionic radii of Li+, Fe2+ and Mn2+ 0.76, 0.78 and 0.83 Å, respectively.25 Comparing the mean distances of the defect free end members of the solid solution series LiMn1-xFexPO4 with x = 022 or 121 with intermediate members like x = 0.2719 an increase between M1- and M2-site to the surrounding oxygen atoms in octahedral arrangement from x = 1 to x = 0 is observed (see table S3). However, the mean distance between M1-site to O3-site decreases. Materials of type 2 also follow this trend (see Table S4). Beside the decrease of the bond length between M1-site and O1-site another decrease between M2-site and O2-site occurs. Furthermore, an increase between M2-site to O1- and O3b-site appears.

Figure 6. Balls-and-sticks representation of the local environment for the M1- and M2-site. In agreement with what has been previously observed, the addition of the phosphate ester based structure directing agent (SDA) Triton H-66® does not only lead to a decrease the particle size but also a change of the particle morphology can be observed.14,16 Moreover, the addition of the SDA has a positive influence on the formation of structure defect free LiMn0.8Fe0.2PO4. It has been assumed, that the faster incorporation of Mn2+ in the phosphoolivine structure locks the defect structure and does not allow any further reassembling and depressing the disorder.6 In general phosphate esters show larger complexation constants with Mn2+ than with Fe2+-ions.26 So we assume that during the synthesis the coordination of Mn2+ with the SDA is competing with the incorporation of Mn2+ into the bulk material in a dynamic equilibrium. In this way the reaction will be driven into the direction of the thermodynamic product i.e. the defect free olivine structure, and is not as strongly influenced by the kinetic tendency of defect formation. 4. CONCLUSION Stoichiometrically pure samples of LiFe0.2Mn0.8PO4 (x ~ 0.8) using a similar synthetical procedure as has been previously published6, tpye 1 and using additional phosphate ester based structure directing agent14,17 (type 2) have been prepared via hydrothermal synthesis. Rietveld analysis of X-ray and neutron diffraction data suggests that the solids of type 1 show the same defect occupation of Fe at the lithium (M1) sites as has been reported previously for a similar synthetic strategy.6 This interpretation is also confirmed by a close inspection of the 57Fe-Mößbauer spectroscopic data. In contrast samples of type 2 show no signs of Fe occupation at the M1 site, neither in the diffraction data nor in the Mößbauer spectra. In addition to the use of the SDA in the preparation significantly higher


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specific surface areas (SSA, ~ 6 m2/g) and smaller particle sizes (0.5 ─ 0.9 μm) as compared to sample type 1 can be achieved. Therefore, the type 2 LiMn0.8Fe0.2PO4 is a good candidate to investigate its electrochemical behavior as cathode material in lithium ion batteries.

ASSOCIATED CONTENT Supporting Information. Tables on: atom parameters from Rietveld refinement from X-ray- and neuron diffraction measurements for 1 and 2; unit cell refinement parameters from Rietveld refinement from X-ray- and n0-diffraction measurements for 1 and 2, Mean bond length between M1- and M2-site to the surrounding oxygen atoms of LiMn1-xFexPO4, Mean bond length between M1- and M2-site to the surrounding oxygen atoms of type 1 and 2. This material is available free of charge via the internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Email: [email protected].

Author Contributions The manuscript was written through contributions of all authors.

Funding Sources .

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT th

This work is dedicated to Prof. Dr. Hubert Schmidbaur on the occasion of his 80 birthday. We acknowledge Mubera Sulijc (University of Salzburg) for N2 absorption measurements.

ABBREVIATIONS SSA, specific surface area; XRD, X-ray diffraction; ICP-OES, inductively coupled plasma optical emission spectrometry; SEM, scanning electron microscopy; PXRD, Powder X-ray diffraction; SDA, structure directing agent.

REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26)

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For Table of Contents Use Only Defect and Surface Area Control in Hydrothermally Synthesized LiMn0.8Fe0.2PO4 Using a Phosphate Based Structure Directing Agent Jürgen Schoiber, Gerold Tippelt, Günther J. Redhammer, Chihiro Yada, Oleksandr Dolotko, Raphael J. F. Berger*, Nicola Hüsing, TOC graphic, and synopsis.

Stoichiometrically pure samples of olivine type LiFe1-xMnxPO4 have been prepared in the presence of a structure directing agent. Analytic data suggest that the solids are free of defect occupation of Fe at the lithium sites. The use of the structure directing surfactant results in significantly higher specific surface areas and smaller particle sizes as compared to samples prepared without using a surfactant.


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Defect and Surface Area Control in Hydrothermally Synthesized LiMn0

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