Monitoring LixFeSO4F (x = 1, 0.5, 0) Phase Distributions in Operando

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Monitoring LixFeSO4F (x = 1, 0.5, 0) Phase Distributions in Operando To Determine Reaction Homogeneity in Porous Battery Electrodes Andreas Blidberg,† Torbjörn Gustafsson,† Carl Tengstedt,‡ Fredrik Björefors,† and William R. Brant*,† †

Department of Chemistry, Ångström Laboratory, Uppsala University, Box 538, SE-75121, Uppsala, Sweden Scania CV AB, SE-15187, Södertälje, Sweden

‡

S Supporting Information *

ABSTRACT: Increasing the energy and power density simultaneously remains a major challenge for improving electrochemical energy storage devices such as Li-ion batteries. Understanding the underlying processes in operating electrodes is decisive to improve their performance. Here, an extension of an in operando X-ray diffraction technique is presented, wherein monitoring the degree of coexistence between crystalline phases in multiphase systems is used to investigate reaction homogeneity in Li-ion batteries. Thereby, a less complicated experimental setup using commercially available laboratory equipment could be employed. By making use of the intrinsic structural properties of tavorite type LiFeSO4F, a promising cathode material for Li-ion batteries, new insights into its nonequilibrium behavior are gained. Differences in the reaction mechanism upon charge and discharge are shown; the influence of adequate electronic wiring for the cycling stability is demonstrated, and the effect of solid state transport on rate performance is highlighted. The methodology is an alternative and complementary approach to the expensive and demanding techniques commonly employed for time-resolved studies of structural changes in operating battery electrodes. The multiphase behavior of LiFeSO4F is commonly observed for other insertion type electrode materials, making the methodology transferable to other new energy storage materials. By expanding the possibilities for investigating complex processes in operating batteries to a larger community, faster progress in both electrode development and fundamental material research can be realized.

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INTRODUCTION Since their successful commercialization in the 1990s, lithium ion batteries have become a ubiquitous power source for portable electronic devices. More recently, they have been rapidly adapted to hybrid electric vehicles and electric vehicles and as a means of storing excess energy produced from intermittent renewable energy resources.1 Within these areas, improving the energy and power densities has consistently been a key objective of research activities.2 Optimizing the performance of an existing material through understanding the origins of the overpotentials required to drive the electrochemical reaction is a commercially relevant approach to this issue. Generally speaking, once the sources of polarization are minimized, a more homogeneous reaction distribution can be achieved improving the energy efficiency and rate performance of the battery.3−7 To date, the majority of reaction distribution studies have used synchrotron-based techniques for investigating LiFePO4 due to its high commercial and practical relevance, together with the strong dependence of its structural behavior and electrochemical performance on the microstructure and electrode formulation.8,9 Research in this area provides an excellent basis for understanding how other phase separating electrode materials could behave during cycling and, here, is extended to tavorite © 2017 American Chemical Society

type LiFeSO4F using less complicated and commercially available instrumentation. The tavorite type LiFeSO4F polymorph belongs to a family of compounds which are isostructural with the mineral tavorite (LiFePO4OH).10 It is investigated as a potential alternative to LiFePO4 due to its open crystal framework, suggested to provide fast solid state Li-ion transport,10,11 and attractive electrochemical performance when coated with a conductive polymer.12 On charging LiFeSO4F, there is a subtle inflection in the voltage profile corresponding to the intermediate compound Li0.5FeSO4F.13 Interestingly, the intermediate phase also formed during discharge despite no observed inflection in the voltage profile. The asymmetric voltage profile, multiple phases present during electrochemical cycling with minimal potential difference, and its promising performance12 make LiFeSO4F an interesting compound for studying structural changes and reaction homogeneity in operating electrodes. There are four general factors which can influence the degree of reaction homogeneity and distribution of phases in porous Received: March 21, 2017 Revised: August 8, 2017 Published: August 9, 2017 7159

DOI: 10.1021/acs.chemmater.7b01019 Chem. Mater. 2017, 29, 7159−7169

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Chemistry of Materials

available equipment to monitor complex electrochemical processes in operando from a standard unmodified pouch cell is demonstrated. The insights into the reaction dynamics in battery electrodes are essential for achieving improved battery performance, and the methodology presented herein is easily transferable to other chemistries.

insertion type electrodes. They are (a) electronic wiring through the electrode,14−16 (b) Li-ion transport within the porous electrode,6,14,17,18 (c) nucleation kinetics,14,17,19 and (d) solid state transport within the active material.20 Li-ion transport into the porous electrode and electronic conduction limitations create an inhomogeneous reaction profile within the electrode; a “reaction front” between the current collector and the bulk electrolyte phase.6,14,17,18,21 Both are controlled through several parameters such as the electrode thickness,18 electronic contacts,15,22,23 electrode porosity,24 and formulation.25 Inhomogeneity can also be induced in regions of the electrode which are not as effectively connected (ionically or electronically), such as agglomerates of particles.7,17 Solid state transport limitations, on the other hand, can produce a moving phase boundary within single grains.17,19,26,27 The severity of the reaction inhomogeneity arising from the above four factors is heavily dependent on the current rate applied to the cell. The effects of Li-ion transport in the electrolyte and electronic wiring are more pronounced at higher currents, leading to inhomogeneous phase distributions through the whole electrode.7,14,16,21 On the other hand, nucleation barriers in individual grains are expected at low cycling rates,28 and therefore, higher currents might also induce a more concurrent reaction throughout the whole electrode as more particles are activated due to the larger overpotential generated.5,26 Increasing the current further can lead to solid state transport limitation that also influences the phase distributions within individual particles.5,19 As a result of the complicated influence of the factors a−d, there is a heavy emphasis in the literature on techniques which provide local information (e.g., X-ray microscopy5,7,19,29 and transmission electron microscopy17). Investigation of electrode behavior with these methods is often performed ex situ (i.e., after disassembling the battery) or through the use of specialized cells with windows to allow transmission of the Xray or electron beam. There is a risk in using nonstandard cell designs as the presence of a window may alter the electrochemical behavior.30 Other studies have employed diffraction methods which provide average information from the whole electrode to detect nonequilibrium phases26,31,32 and energy dispersive21 or microbeam33 diffraction to study local effects. In parallel with these synchrotron-based studies, there has been a significant improvement of studying electrodes under operating conditions (in operando) on laboratory diffractometers34−36 compared to the early reports.37 The opportunity to carry out sophisticated analysis of operating electrodes with commercially available equipment is desirable, as it increases its accessibility to a wider research community. However, the complex processes occurring in insertion type electrodes are still challenging to investigate with such equipment. Herein, we extend in operando XRD further by showing that reaction homogeneity can be tracked on a commercially available laboratory diffractometer if more than two crystalline phases are present in the electrode. Thanks to the three phases formed during charging and discharging for LixFeSO4F (x = 0, 0.5, 1) an uncomplicated experimental set up could be employed. The width and asymmetry of the evolving phase distribution provides information on the degree of inhomogeneity in the electrode reaction. The effect of the electronic wiring, mass loading, the ionic transport in the electrolyte, and solid state mass transport on the reaction homogeneity is identified. Thereby, the capability of using commercially

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EXPERIMENTAL SECTION

Material Synthesis. Tavorite type LiFeSO4F was synthesized with a solvothermal method38 under the same conditions as Sobkowiak et al.13 The precursors were FeSO4·H2O (prepared by dehydration of FeSO4·7H2O, ≥99.0% Sigma-Aldrich) and LiF (99.85%, Alfa-Aesar, used in 10 mol % excess) and the reaction medium tetraethylene glycol (99%, Aldrich) in a Teflon lined steel autoclave (Parr Instruments). LiFeSO4F was coated with poly(3,4-ethylenedioxythiophene)-bis(trifluoromethane)-sulfonimide (PEDOT-TFSI), via a route developed for LiFePO4,39 and adopted for LiFeSO4F.12 The same synthesis conditions were applied in the present study. The amount of PEDOT coating was determined to be 13 wt % by thermogravimetric analysis performed on a TA Instruments Q500. The weight loss of the LiFeSO4F−PEDOT composite was compared to that of a thoroughly washed uncoated reference sample. With the capacities 151 mAh g−1 for LiFeSO4F and 38 mAh g−1 for PEDOTTFSI, the theoretical capacity of the LiFeSO4F−PEDOT composite was estimated to be 136 mAh g−1. Li0.5FeSO4F and FeSO4F reference samples were obtained electrochemically by slowly charging Swagelok battery cells to 3.59 and 4.1 V with respect to Li+/Li, respectively, followed by a potentiostatic step until the current reached 0.7 mA g−1. The batteries were loaded with LiFeSO4F−PEDOT composite mixed with carbon black but no binder. The powders were washed with dimethylcarbonate (Selectilyte, BASF) and dried under vacuum. Electrochemical Evaluation. For electrochemical evaluation, the LiFeSO4F−PEDOT composite was mixed with carbon black (Super C65, Timcal) and poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP, Kynar FLEX 2801, Arkema) in n-methyl-2-pyrrolidone (NMP, 99.9%, WVR) in a planetary ball mill for 45 min. The weight ratio of LiFeSO4F/PEDOT-TFSI/carbon black/PVdF-HFP was set to 70/10.5/8/11.5. The slurry was cast onto a 22 μm thick carbon coated Al-foil, dried at 70 °C in air, punched out to circular electrodes (d = 10 mm), dried, pressed with 10 kN for 30 s, and again dried at 120 °C under vacuum for 12 h. The electrode thickness was between 20 and 100 μm, and the porosity was ca. 60% (estimated by comparing the actual coating thickness with a theoretical completely dense coating (Section SI)). Pouch cells were assembled using a 9 μm thick aluminum foil coated with a 100 μm poly(ethylene) layer on the inside and a 12 μm thick poly(ethylene terephthalate) on the outside as pouch material, an aluminum tab as the positive current collector, a nickel tab attached to a ca. 80 μm thick Li-foil as counter/reference electrode, and 1 M LiPF6 in ethylene carbonate and diethyl carbonate in the volume ratio of 1/1 (LP40, BASF) as electrolyte, soaked into a 12 μm thick polyethylene separator (Solupor, Lydall Performance Materials). The battery assembly was performed in an argon filled glovebox. Galvanostatic cycling and cyclic voltammetry were carried out on VMP2 or MPEG2 equipment (Bio-Logic). All cells were precycled on a BT-2043 (Arbin) or a BTS600 (Digatron) prior to in operando experiments. For in operando X-ray diffraction (XRD) at different C-rates (the reciprocal of the time required to charge the cell to its theoretical capacity at a certain current), an SP-200 (Bio-Logic) was used. Material Characterization and in Operando Diffraction. Scanning electron microscopy (SEM) images were collected on a Zeiss Sigma Series SEM, with an acceleration voltage of 5 kV. Cross sections were obtained by gently tearing an electrode in two pieces. The samples were coated with a ca. 10 nm layer of platinum by physical vapor deposition using a Quorum G150T ES sputter (Quorum Technologies) to obtain high quality images. Mössbauer spectroscopy was carried out in transmission mode using a 57CoRh source at constant acceleration. Absorber discs (d = 13 mm) were prepared by mixing typically 20 mg of LiFeSO4F with an inert boron 7160


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Chemistry of Materials nitride filler and kept under a constant nitrogen flow during measurements. The spectra were Lorentzian line least-squares fitted using the Recoil software. The center shift is given relative to metallic iron (α-Fe) at room temperature. High resolution synchrotron XRD patterns were collected on the powder diffraction beamline, 10-BM-1, at the Australian Synchrotron with a MYTHEN-II microstrip detector.40 Using LaB6 (NIST standard 660b), the wavelength was determined to be 0.688417(1) Å. In operando XRD measurements were carried out in transmission mode through an unmodified pouch cell (Figure 1) on a Stoe & Cie GmbH Stadi X-ray powder

Structural Models and Electrochemical Evaluation. Structural models for the tavorite LixFeSO4F (x = 1, 0.5, 0) were refined to highly resolved ex situ synchrotron XRD patterns (Figure 2). Starting models were based on previously reported structures for LiFeSO4F.13 For clarity, only part of the angular range is shown here, with additional details in Figure S2 and Table S1−S5. The degree of oxidation was determined by 57 Fe Mössbauer spectroscopy for all samples. The pristine sample (Figure 2a,b) showed high purity with no contamination from phases containing Fe3+, and only trace amounts of LiF that was used in excess during the synthesis were detected. An improved structure model compared to one previously reported13 was achieved for the intermediate Li0.5FeSO4F (Figure 2c,d). 9 wt % of the end phase FeSO4F was present in the sample according to the Rietveld refinement, and a similar degree of oxidation was verified by Mössbauer spectroscopy. An almost complete conversion from LiFeSO4F to FeSO4F was found for the most oxidized sample (Figure 2e,f). Two Fe3+ doublets were used in the fitting procedure for the Mössbauer spectrum of FeSO4F, despite the satisfying refinement of a structural model to the XRD data using only one Fe-site. Likely, the electrochemical delithiation process resulted in a local variation in Fe−O bond distances with slightly different quadrupole splittings, while on average only one Fe-site sufficed to describe the XRD data. Further details regarding Rietveld refinements and Mössbauer hyperfine parameters are available in Sections II and III. These well-defined structural models were used during refinements of multiphase models to the in operando XRD data, where the evolution of each phase fraction was the focus. After satisfying structural models had been obtained for the three LixFeSO4F phases, the electrochemical performance was thoroughly characterized (Figure 3). LiFeSO4F coated with a conductive polymer was used in all cases, since it significantly improves the electrochemical performance of tavorite type LiFeSO4F12 by reducing a kinetic barrier for the lithium insertion and extraction.43 The amount of conductive additive was slightly increased compared to the previous study43 in order to achieve sufficient electronic wiring while maintaining good ionic transport in the electrolyte within the porous matrix. With an increased amount of conductive additive and a carbon coated current collector, sufficient electronic pathways were obtained even with high electrode porosities (≈60%). The characteristic step in the charge profile, indicative of the intermediate phase Li0.5FeSO4F,13 was observed for such electrodes (Figure 3a). The corresponding differential capacity plot is included in Figure S6. Cyclic voltammetry (CV) data collected at different scan rates (Figure 3b) showed a typical diffusion limited behavior with a linear relation between the peak current (ip) and the square root of the scan rate (ν0.5, Figure 3c).44 The corresponding C-rates for the peak currents were in the range of C/7.5 to 1.3 C (based on 136 mAh g−1 for the capacity of the LiFeSO4F−PEDOT composite.43 The increase in peak current was accompanied by a linear shift in the peak potential (Figure S7a). The corresponding area specific resistance was lower with higher mass loadings, which is expected for electrodes with ideal electronic and ionic networks.28,45 With good electronic and ionic paths, the electrochemically active area increases with increased active material loading, approximately corresponding to increased electrode thickness. The similar behavior for different electrode thicknesses implies that the diffusion limitation observed in Figure 3c was caused by solid state transport. The capacity

Figure 1. Schematic of a standard pouch cell showing the path of the X-ray beam passing through the different cell components. The entire thickness of the cell was ca. 0.5 mm, and the X-ray transmittance ranged from 19% to 44%.

diffractometer equipped with a Ge monochromator (single wavelength Cu Kα1). The beam transmittance with this particular setup was calculated to be 19% for thick (100 μm) electrodes and 44% for thin (20 μm) electrodes. A Mythen 1K Si strip detector was operated in stationary mode, positioned to cover a 19° 2θ range with an angular resolution of 0.015° 2θ. The instrumental setup is shown in Figure S1. Structural refinements were performed with the Rietveld method41 in the FullProf software.42 For the in operando data, atomic positions were fixed, and the cell parameters were only refined for the fully lithiated LiFeSO4F phase at the beginning and end of charge. The dynamic phase distributions were estimated by refining the scale factors for the different LixFeSO4F phases (x = 1, 0.5, 0). Profile parameters were refined from patterns with the strongest intensity of the respective phases.

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RESULTS AND DISCUSSION In order to ensure high accuracy in the information obtained from in operando experiments and its interpretation, high quality models for the different crystal structures were obtained. The following section covers a detailed structural characterization of the three tavorite type LixFeSO4F structures (x = 1, 0.5, 0) using synchrotron X-ray diffraction and Mössbauer spectroscopy. In addition, thorough electrochemical characterization resulted in falsifiable hypotheses that were later tested with in operando XRD, where the nonequilibrium phase transformations were tracked under different conditions. 7161


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Figure 2. Synchrotron X-ray diffraction and 57Fe Mössbauer data of (a, b) LiFeSO4F, (c, d) Li0.5FeSO4F, and (e, f) FeSO4F. The results from Rietveld refinements are shown, and the entire data set for each phase is provided in the Supporting Information.

retention was satisfactory during the CV measurements (Figure S7b), but an increased polarization and decreased accessible capacity were observed during extended galvanostatic cycling (Figure 3d,e). The source of the decreased performance is likely loss of electronic contacts within the electrode, caused by the volume change of the active material particles (10% for tavorite type LiFeSO4F10) during lithium insertion and extraction.21,22,43 Overall, on the basis of the well characterized materials described above, the following hypotheses can be formed: (1) Fresh electrodes have adequate electronic and ionic networks, (2) the electrochemical reaction proceeds via an intermediate phase in two consecutive biphasic reactions, and (3) the electrochemical reaction is limited by solid state transport at rates spanning over at least C/10 to 1C for this specific particle morphology. In the following sections, these statements are systematically tested with simultaneous electrochemical and XRD measurements by evaluating the effect of electrode thickness, the increased electrode resistance over cycling, and cycling at different rates. Prior to this, the capability for carrying

out such in operando XRD measurements on a laboratory diffractometer is discussed in the following section. Capabilities for in Operando XRD on a Laboratory Diffractometer. Figure 4 shows representative in operando data when measuring on operating LiFeSO4F electrodes in pouch cells (also referred to as “coffee bag cells”37). The thin polymer coated Al-foil in the pouch material enabled a commercially relevant setup, even with Cu Kα1 radiation. Collecting high quality diffraction patterns without the need for a dedicated X-ray window is vital for investigating the reaction homogeneity produced under realistic cycling conditions.30 Further, the electronic contact between the current collector and the cast composite is of utmost importance,15,23 making it undesirable to evaluate the electrode performance of composites directly coated onto, e.g., a Be window. Application of a highly efficient Si strip detector operating in stationary mode enabled fast measurements over a fairly wide angular range. With 10 mg cm−2 mass loading (based on the mass of the entire composite), a collection time of 20 min, and a cycling rate of C/50, high quality data was retrieved (Figure 4a,b). It 7162


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Figure 3. Electrochemical characterization of tavorite LiFeSO4F. (a) A representative galvanostatic cycling profile at slow rate, (b) cyclic voltammetry data collected with different scan rates for a thin electrode (2 mg cm−2), (c) the peak current as a function of the square root of the scan rate, (d) galvanostatic cycling curves at C/5 (4 mg cm−2), and (e) the corresponding capacity retention.

was possible to reduce the mass loading to 3 mg cm−2, shorten the collection time to 4 min, and increase the cycling rate to C/ 5 (Figure 4c,d). Although the signal-to-noise ratio is decreased, similar general trends are observed in both cases. The advent and disappearance of the intermediate Li0.5FeSO4F phase and the end FeSO4F phase are clearly seen by new peaks in the diffractograms that appear on charge and disappear on discharge. Representative Rietveld refinements for the first pattern in these data sets are shown in Figure 4b,d. The data retrieved with the fast collection times and low mass loading in Figure 4c is considered the limit of what can be achieved for this particular experimental setup. For higher mass loadings (commercially more relevant25), the collection time could be decreased to 3 min, resulting in a resolution of 60 patterns per cycle at C/2. The corresponding resolution in terms of phase fractions was a change of about 5 wt % between each pattern. Thereby, in operando studies could be carried out while

changing the cycling rate from C/50 to C/2 (ca. 3−68 mA g−1). Phase fractions could be extracted by Rietveld refinements from all the collected patterns, and the evolution of phase distributions is discussed in the next section. For the fully lithiated phase, a limited solid solution region spanned over 1 < x < 0.87 in LixFeSO4F, in line with in situ XRD data collected at open circuit conditions.10 The slight gradual shift in the (1 1 0) reflection for this phase is highlighted with a dashed line in Figure 4a. The unit cell parameters could be refined during the beginning and end of the electrochemical cycling, where the cell parameter b was decreased and the ac plane slightly expanded (Figure S3). The gradual change in the unit cell parameters is an example of what can be obtained from in operando studies using commercially available laboratory equipment. For the intermediate phase Li0.5FeSO4F and the end phase FeSO4F, no shift in the peak positions could be observed within the instrumental resolution. This is indicated in Figure 4a by the 7163


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Figure 4. Typical in operando XRD data collected at (a) C/50 with 20 min collection times and 10 mg cm−2 material loading and (c) C/5 with 4 min collection times and a material loading of 3 mg cm−2. The color scale shows the intensity after background subtraction. (b, d) Rietveld refinement of the first pattern in each data set. Signals from the plastic in the pouch material and from the separators were excluded during the refinements (gray circles).

the present study. In contrast, if activation of single active material grains is the dominating source of polarization, the reaction takes place over the entire thickness of the electrode (given that it consists of homogeneously dispersed particles).5,14 In this case, no phase overlap is expected between the end members for a multiphase material (as presented in Figure 5c by the absence of FeSO4F). That is, the reaction would start around activated sites, e.g., around defects or from the smallest particles,19,46,47 and proceed until an additional increase in potential is required before the nucleation of the next phase is initiated. If solid state transport is dominating the reaction, the entire thickness of the electrode is active.5 However, phase overlap would be observed due to local inhomogeneity within the particles. The electrochemical response would be similar to a situation where Li-ions cannot be supplied sufficiently fast from the electrolyte. The two cases can be separated by changing the electrode thickness, since a thinner electrode would alleviate liquid but not solid state transport at a given C-rate.28 In the next section, the hypotheses previously formulated after the electrochemical evaluation are evaluated on the basis of the above predictions. Effect of Cell Resistance on the Electrochemical Reaction Profile. To test if fresh electrodes had adequate electronic and ionic networks, the effect of the cell resistance at near-equilibrium rate was evaluated (i.e., using a small net current corresponding to C/50, Figure 6). The phase fractions

invariant Bragg position of the overlapping (1 1 0) and (−1−1 2) reflections for the intermediate phase (x = 0.5) and the (−2 0 2) reflection for the end phase (x = 0; see also Figure S4). Also, at faster rates, there was no indication of solid solution regions for Li0.5FeSO4F and FeSO4F within the experimental resolution (Figure S5). After establishing the absence of solid solution regions for lithiation degrees of x < 0.87 in LixFeSO4F, the evolution of the three LixFeSO4F phase fractions under nonequilibrium conditions was used to directly monitor reaction homogeneity throughout operating electrodes. Figure 5 shows the expected phase distributions when the electrochemical reaction is limited by (a) electronic wiring, (b) ionic transport in the electrolyte, (c) nucleation kinetics, or (d) solid state transport (as discussed in the introduction). Depending on which of these mechanisms is controlling the reaction, the phase overlap observed from a diffraction experiment will vary. If either electron or Li-ion transport to the active material grains is limiting the electrochemical performance, a reaction front is expected to form throughout the electrode.18,28 In other words, substantial overlap between all three LixFeSO4F phases would be observed for in operando diffraction experiments in the presence of such reaction fronts. While the two effects cannot be directly differentiated, control experiments can be designed to distinguish their contributions. For example, the electronic conductivity in the electrode can be varied, which is utilized in 7164


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Figure 5. Factors that can limit insertion reactions in porous electrodes, here qualitatively shown for LiFeSO4F during charge where the reaction is mainly limited by (a) electronic wiring, (b) Liion transport in the electrolyte, (c) reaction kinetics related to nucleation phenomena, and (d) solid state Li-ion transport.

are plotted as a function of the time normalized to the total charging time, since no statistically significant difference was observed in the accessible capacities between the cells (Figure S8). The corresponding galvanostatic cycling profiles are also shown. The evolution of phase fractions was almost identical for electrodes with mass loadings of 3 and 10 mg cm−2 (Figure 6a). The similar reaction profiles for the two mass loadings (approximately corresponding to the electrode thickness for a given porosity) indicate that the electronic wiring and the liquid state Li-ion transport were satisfactory, in line with the CV results discussed earlier. The higher resistance of the thin electrode is counterbalanced by the lower current at a given Crate, and the voltage difference between charge and discharge was 46 mV in both cases (evaluated by the maximum of the derivative dQ/dE; see Table S7). The comparable behavior for the two mass loadings means that no reaction front was present, supporting the hypothesis that neither electronic nor ionic wiring was limiting the electrode performance. Indeed, the SEM images in Figure 7 show that particles ranging between 0.35 and 3.55 μm in size are well connected and dispersed throughout the cross section of the electrode. Consequently, minimal coexistence of LiFeSO4F and FeSO4F was observed, and the reaction proceeded via Li0.5FeSO4F in two consecutive biphasic reactions upon charge. More than 80% of the theoretical capacity was utilized showing that the effect of solid state transport was not pronounced during the main part of the cycling, as expected at near-equilibrium rates. By eliminating these factors, nucleation appears to be the

Figure 6. Evolution of phase fractions estimated by in operando XRD for LiFeSO4F cycled at C/50, with the cycling time normalized to the total charging time. (a) Comparison between electrodes with high and low mass loading, (b) the phase fractions for an electrode with increased resistance, and (c) the corresponding voltage profiles.

dominating component controlling the reaction, as reported for LiFePO4 and LiNi0.5Mn1.5O4 cathodes at slow cycling rates.5,14,19,46−48 To investigate how the dynamic phase distribution was influenced by increased electrode resistance, an in operando experiment was carried out on a cycled electrode (Figure 6b). The polarization was observed to have increased similar to the later cycles in Figure 3d, and the average voltage difference between the charge and discharge was 131 mV, instead of 46 mV observed for the fresh electrodes at C/50 (Figure 6c). Here, the end phase FeSO4F was observed earlier during charge, and the maximum amount of the intermediate Li0.5FeSO4F phase was decreased. These observations are expected when a reaction front is present during electrochemical cycling of the electrode. Its origin is likely degradation of the electronic wiring due to the relatively large volume change in LiFeSO4F upon lithium extraction (≈10%),10 which is a common fading mechanism for Li-ion battery cathodes.21,22,43 The almost identical evolution of LiFeSO4F to Li0.5FesO4F and finally FeSO4F upon charge for both thin and thick fresh electrodes and the low cycling rate imply that increased resistance from loss of electrolyte Li-ion pathways in 7165


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Figure 7. SEM images of (a) pristine LiFeSO4F and (b) a cross section of a thick electrode (13.9 mg cm−2). 0.35−3.5 μm sized particles were evenly distributed throughout the electrode.

overlap between the three LixFeSO4F phases could be used to detect the accompanying reaction front within the electrode while measuring the average phase distribution across the entire electrode. Evolution of Phase Fractions at Faster Rates. After the effect of cell resistance on the reaction homogeneity had been evaluated at a slow rate (C/50), the influence of solid state transport at rates equal to or faster than C/5 was tested. The evolution of phase fractions was investigated during cycling at C/5 to C/2 (corresponding to 27−68 mA g−1, Figure 8). The maximum amount of the FeSO4F end phase was continuously decreased when the cycling rate increased. The voltage difference between charge and discharge increased with the applied current to values similar to the cycled electrode at C/ 50, and the increase in polarization had a linear relation with the applied current (Figure S7). Despite the linear response in the overpotential when increasing the current, the overlap between the end phases was not as severe as in Figure 6b. In addition, no substantial difference was observed when increasing the mass loadings (again approximately corresponding to the electrode thickness). The similar behavior between the thin and thick electrodes implies that no pronounced reaction front was developed, despite the faster cycling rates, and the reaction was taking place evenly over the thickness of the electrode. However, at corresponding rates, the reaction was found to be controlled by mass transfer in the CV experiments (Figure 3b,c.) These results, together with less FeSO4F end phase observed for increased rates and the similar behavior for thin and thick electrodes, imply that the electrochemical reaction was indeed under solid state transport control between C/5 and C/2. Another interesting detail in Figure 8 is that the evolution of phase fractions is more symmetric during charge and discharge as compared to the near-equilibrium (C/50) cycling in Figure 6a. As previously discussed, the entire thickness of the electrode appeared to be reacting in both cases but with two different controlling mechanisms. During slow cycling, the reaction appears nucleation limited, whereas at faster rates, the reaction was under solid state mass transfer control. On the basis of the more symmetric phase distribution during faster cycling and higher currents being observed during discharge than charge with CV (Figure 3b,c), the lithium insertion reaction appears to be faster than the lithium extraction from LiFeSO4F. The origin of the faster discharge behavior could be a combination of a reduced nucleation barrier as previously discussed, combined

the composite is unlikely. Furthermore, the results show that reaction fronts can be detected with commercially available laboratory equipment. The intermediate phase was present in all cases shown in Figure 6, independent of electrode thickness or resistance. Therefore, its formation appears to be an intrinsic process in the chemically reversible transition from LiFeSO4F to FeSO4F. Still, the phase transformation in Figure 6a is asymmetric between charge and discharge. Furthermore, as previously reported13 and shown in Figure 3a, a step in the voltage profile is only observed during charge. These results imply that the electrochemical reaction proceeds differently upon lithium extraction and insertion, and the overpotential needed to drive the reaction is different on charge and discharge. The maximum amount of intermediate Li0.5FeSO4F was smaller on discharge, and it started to react further to LiFeSO4F before all of the FeSO4F was consumed. The asymmetric phase transformation could be due to the small amounts of start phase (1 < x < 0.87) and intermediate phase (x = 0.5) still present at the end of charge even at the slow rate of C/50. The incomplete reaction is likely due to the fairly large particle sizes of some grains (Figure 7a), with accompanying solid state transport limitations at the end of charge. Possibly, the domains of these phases could act as seed crystal at the beginning of discharge and thereby remove the nucleation barrier to form the start phase (1 < x < 0.87). On the other hand, a fairly wide particle size distribution was observed in Figure 7a, with grain sizes ranging between 0.35 and 3.5 μm. It appears unlikely that the start phase (with 1 < x < 0.87) was still present in the smallest grains, and these parts of the electrode would be subject to a nucleation barrier in order to form the fully lithiated phase again during discharge. An alternative interpretation of the data would then be that the energy barrier to form the fully lithiated phase on discharge is smaller than that to form the fully delithiated phase on charge. Neither of these interpretations can be falsified solely on the basis of in-house in operando XRD where an average of the entire electrode is measured and would require other techniques such as synchrotron-based X-ray microscopy to evaluate the evolution of phases in single LiFeSO4F grains.19 In summary of this section, nucleation phenomena appeared to be controlling the electrochemical reaction for fresh electrodes at slow rates. Upon cycling, the electrode resistance increased and the phase distribution became inhomogeneous, caused by degradation of the electronic network. The degree of 7166


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rate was explained by a combination of a reduced nucleation barrier and a faster Li-ion transport in the lithiated phase. Furthermore, it was possible to ascribe the increased overpotential of extensively cycled electrodes to loss of electronic contacts. An increased coexistence between the different LixFeSO4F phases was interpreted as a reaction front starting from the current collector and progressing throughout the electrode. Finally, increasing the cycling rate resulted in less FeSO4F end phase formed, but changing the electrode thickness did not affect the evolution of phases substantially. Thereby, solid and liquid state mass transport control could be distinguished. In a wider perspective, it was shown that advanced in operando studies can be carried out on a laboratory diffractometer by using intrinsic material properties to probe the degree of reaction. The methodology can be extended to other insertion materials that undergo multiple phase transitions, e.g., LiCoPO4, A3V2(PO4)2F3 (A = Li, Na), and Na3/4CoO2. The presence of reaction fronts would also be possible to probe for materials exhibiting solid solution behavior by examining the Bragg reflection broadening. Further, time-resolved monitoring of dynamic phase transformations is relevant also outside the battery field and could be employed for carbon capture chemistry, catalysis, and materials synthesis. Lastly, the results presented here are in good agreement with the extensive characterization of LiFePO4 available in the literature. Thus, these findings appear to largely apply to phase separating insertion materials in general.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01019. Crystal Information for FeSO4F, Li0.5FeSO4F and LiF (CIF) Calculation of electrode porosity, details regarding Rietveld refinements and in operando XRD, Mössbauer hyperfine parameters, and additional electrochemical evaluation (PDF)

Figure 8. (a) The evolution phase fractions retrieved from in operando XRD at C/5, C/3.5, and C/2 as a function of the normalized cycling time and (b) the corresponding voltage profiles as a function of gravimetric capacity. The resolution in terms of phase fractions in (a) corresponded to at most a change of 5 wt % between each pattern.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

with the faster Li-ion transport reported for LixFeSO4F with high values of x.49

ORCID

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Torbjörn Gustafsson: 0000-0003-2737-4670 Fredrik Björefors: 0000-0002-3598-3903 William R. Brant: 0000-0002-8658-8938

CONCLUSIONS Time resolved observations on phase distributions in multiphase insertion materials were made on a commercially available laboratory X-ray diffractometer. By utilizing the latest detectors, it was possible to push the limits of in operando studies of energy storage devices further, even when using a Cu Kα1 X-ray source, improving the accessibility to a wider community. Here, new insights into the behavior of tavorite type LiFeSO4F under operating conditions were gained. The three phases of tavorite type LixFeSO4F (x = 0, 0.5, 1) differ only slightly in their electrode potential, and the degree of coexistence between the phases during battery operation was used to probe the reaction homogeneity throughout the electrode. The results show that the evolution of LixFeSO4F phases is different on charge and discharge. The faster discharge

Funding

The work presented here was realized by support from the Swedish Foundation for Strategic Research (SSF) through the project From road to load − A Swedish lithium battery materials group. Additionally, this work was supported by the Swedish strategic research program StandUp for Energy. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A part of the work was performed on the powder diffraction beamline at the Australian Synchrotron, Victoria, Australia, and the authors acknowledge the help of Justin Kimpton. The 7167


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authors are thankful to Håkan Rundlöf and Henrik Eriksson for technical support and to Tore Ericsson and Lennart Häggström for assistance with Mössbauer spectroscopy.

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Monitoring LixFeSO4F (x = 1, 0.5, 0) Phase Distributions in Operando

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