2FeSO4F, Formed during

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Identification of an Intermediate Phase, Li1/2FeSO4F, Formed During Electrochemical Cycling of Tavorite LiFeSO4F Adam Sobkowiak, Matthew Roberts, Lennart Häggström, Tore Ericsson, Anna Mikaela Andersson, Kristina Edstrom, Torbjorn Gustafsson, and Fredrik Göran Björefors Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm502104q • Publication Date (Web): 11 Jul 2014 Downloaded from http://pubs.acs.org on July 16, 2014

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Chemistry of Materials

Identification of an Intermediate Phase, Li1/2FeSO4F, Formed During Electrochemical Cycling of Tavorite LiFeSO4F Adam Sobkowiak*,†, Matthew R. Roberts†, Lennart Häggström†, Tore Ericsson†, Anna M. Anderssonϟ, Kristina Edström†, Torbjörn Gustafsson†, Fredrik Björefors*,† †

Department of Chemistry – Ångström Laboratory, Uppsala University, Box 538, SE-751 21 Uppsala, Sweden

ϟ

ABB Corporate Research, SE-721 78 Västerås, Sweden

KEYWORDS: lithium-ion battery, fluorosulfate, intermediate phase, x-ray diffraction, Mössbauer spectroscopy

ABSTRACT: Many compounds adopting the tavorite-type crystal structure have attracted considerable attention as cathode materials for lithium ion batteries due to the favorable structural characteristics, facilitating promising electrochemical performances. Recent reports have highlighted the complex mechanism of lithium insertion/extraction in some of these compounds, such as the stabilization of intermediate phases in the LiFeSO4OH and LiVPO4F systems. In the case of tavorite LiFeSO4F, reported density functional theory (DFT) calculations have suggested the possibility of a similar behavior, but thus far, no experimental verification of such a process has, to the best of our knowledge, been successfully demonstrated. In this work, we investigate the structural evolution of LiFeSO4F upon extraction/insertion of lithium ions from/into the host framework. By thorough ex-situ characterizations of chemically and electrochemically prepared LixFeSO4F-samples (0≤x≤1), we demonstrate the stabilization of an intermediate phase, Li1/2FeSO4F, for which one possible structural model is proposed. However, results indicating charge ordering on the iron-sites, suggesting the formation of a super structure with a larger unit cell, are also highlighted. Moreover, the degree of formation of Li1/2FeSO4F is shown to be highly dependent on the rate of lithium extraction as a result of an exceptionally small potential separation (~15 mV during charging) of the two subsequently occurring biphasic processes, LiFeSO4F/Li1/2FeSO4F and Li1/2FeSO4F/FeSO4F. Finally, the intermediate phase is shown to be formed both on charge and discharge during battery cycling, even though an apparent asymmetrical electrochemical trace suggests the contrary.

INTRODUCTION The recently raised interest in large-scale exploitation of renewable energy sources, such as wind and solar power, and in expanding the usage of electric vehicles for a more sustainable future, has intensified the development of suitable integrated energy storage devices. For such applications, lithium ion (Li-ion) batteries represent one of the most promising technologies, and they have already been in operation successfully for decades in portable electronics. To meet the increasing demands on energy and power density, cycling life time, cost, and green chemistry in these systems, research and development of new and improved battery materials is constantly ongoing. Among cathode materials, the layer structured LiCoO2 and its Ni and Mn-substituted derivatives have been dominating the commercial market for many years. However, the discovery of olivine LiFePO41 triggered a considerable interest within academia and industry towards cathode materials based on polyanionic frameworks. This type of materials embodies rich chemistries with large tuning possibilities of the structural, and in turn electrochemical, properties based on the chosen transition metal redox

couple, Mn+/M(n+1)+, and the polyatomic anion (XO4n-) that make the building blocks of the Li-hosting framework. Their inherent limitations, such as low ionic and electronic conductivities, can today be overcome by nano-scaling and surface decoration techniques. In the wake of the successes for LiFePO4, many promising alternative cathode materials have been identified, such as silicates2–7 (Li2MSiO4), borates8 (LiMBO3), NASICON-structure phosphates9,10 (LixM2(PO4)3), pyrophosphates11–14 15–18 (Li2MP2O7), phosphate fluorides (LixMPO4F, commonly called “fluorophosphates”), and sulfate fluorides19–24 (LiMSO4F, commonly called “fluorosulfates”). The two latter systems, incorporating fluoride anions in addition to the polyatomic moieties, have generated considerable attention recently by demonstrating promising electrochemical performances with record-high insertion potentials for the Fe2+/Fe3+ (3.6 and 3.9 V vs. Li/Li+ in LiFeSO4F prepared in the tavorite and triplite structure, respectively19,22,23) and the V3+/V4+ (4.25 V vs. Li/Li+ in tavorite LiVPO4F16) redox couples. In general, the high operation voltages are explained by the inductive effect25–28 where the large electronegativety of the PO4-/SO4-/F--ions gives rise to a high degree of ionicity of the M-(O,F) bonds, resulting in an increased oxidation energy barrier for the

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Mn+/M(n+1)+ redox couple. Moreover, many of the LiMXO4F compounds adopt the tavorite-type crystal structure (named after the mineral LiFePO4OH29) which has been shown to facilitate high ionic conductivities due to the spaceous framework with three-dimensional cationtransportation channels30,31. This negates the necessity of nano-scaling the material to achieve a functional electrochemical performance. However, further enhancements can be achieved by applying a surface confined electronically conducting layer, as we previously demonstrated for LiFeSO4F32. It is well known that the insertion and extraction of Li+ in materials adopting the tavorite structure proceeds mainly by two-phase reactions, typically seen as domains of fairly constant potential in the electrochemical trace of galvanostatically cycled battery cells19,32–36, as predicted by Gibbs’ phase rule. For compounds of LiMXO4F-type, such a biphasic reaction can be described by the following general reaction scheme: LiMXO4F ⇌ MXO4F + Li+ + e- (1) However, recently both LiVPO4F and LiFeSO4OH in the tavorite structure were shown to undergo two subsequent biphasic processes during the extraction of one Li+ per formula unit due to the formation of distinct intermediate phases, Li2/3VPO4F33,34 and Li1/2FeSO4OH36, respectively. In the case of tavorite-type LiFeSO4F, reported density functional theory (DFT) calculations highlight the possibility of a similar behavior by forming a hypothetical intermediate phase Li1/2FeSO4F, but the authors stress that the stabilization of it is negligible and that there is yet no clear experimental evidence of its existence36. Thus, LiFeSO4F has so far been considered as a classic two-phase material undergoing one biphasic reaction (between LiFeSO4F and FeSO4F) upon extraction and insertion of Li+ 19,36–38. During the course of our previous work on surface decoration of tavorite LiFeSO4F with an electronically conducting polymer32, we noticed certain peculiarities in the analyses of chemically oxidized and reduced samples, including unidentifiable Bragg reflections in the X-ray diffraction patterns and preferentially oxidized/reduced iron-sites in the Mössbauer spectra. Thus in this work, to investigate the cause of the previously observed features, we are taking a deeper look into the structural evolution of the tavorite LiFeSO4F framework as a function of Li+composition, LixFeSO4F (0≤x≤1). This is carried out by thorough ex-situ characterizations using X-ray diffraction and Mössbauer spectroscopy on a series of samples prepared both by chemical and electrochemical extraction (and re-insertion) of Li+ from pristine LiFeSO4F. Based on this experimental data, we point out the existence of a distinct intermediate phase, Li1/2FeSO4F, never before experimentally identified.

EXPERIMENTAL SECTION

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LiFeSO4F was synthesized using a low temperature solvothermal approach39. First, a FeSO4·H2O precursor was prepared by partial dehydration of commercially available FeSO4·7H2O (Sigma-Aldrich, >99%) at 100 °C under a constant flow of N2 for 3 h. The FeSO4·H2O precursor was then mixed with a slight excess (1.15:1 molar ratio) of LiF (Alfa Aesar, >99%) by ball-milling the powders in acetone for 1 h. This precursor mixture, summing to a total batch mass of approximately 1 g, was then dispersed in 30 mL of tetraethylene glycol (TEG) by stirring for 30 min. The dispersion was heated in a 45 mL Teflon-lined steel autoclave (Parr Instruments) using a temperature program optimized for obtaining a product of high purity and atomic order40. The program consists of a slow initial temperature ramp (RT-200 °C at 1.5 °C/min and 200-220 °C at 0.07 °C/min), followed by a temperature dwell at 220 °C for 50 h, and a final ramp step to 230 °C (0.17 °C/min) before allowing for a slow cooling. The resulting product, having an ivory white color, was collected by centrifugation, washed with acetone (BDH, 99.5% min.), dried at room temperature under N2 flow, and was finally stored under an Ar atmosphere. Chemical oxidations (Li+-extractions), to prepare a series of LixFeSO4F-samples within the composition range 0≤x≤1, were performed by mixing ~100 mg of pristine LiFeSO4F together with desired stoichiometric amounts of the oxidation agent NO2BF4 (Alfa Aesar, 96%) in ~15 mL of anhydrous acetonitrile (Sigma-Aldrich, 99.8%). The solutions were stirred for 12 h under an Ar atmosphere to allow for the oxidation reaction to complete. The samples were then washed with acetonitrile and acetone, recovered by centrifugation, and analyzed with X-ray diffraction (XRD) and Mössbauer spectroscopy (MS), as described below. Electrochemical cycling of LiFeSO4F was perform on a surface modified sample – coated with ~10 wt % of an electronically conducting layer of poly-3,4ethylenedioxythiophene (PEDOT) - in order to enhance its battery performance. The details regarding the preparation of this LiFeSO4F-PEDOT composite material and information about how the coating affects the electrochemical performance are presented elsewhere32. The electrochemical cycling of the LiFeSO4F-PEDOT composite was carried out in Swagelok-type cells equipped with a spring mechanism in order to maintain a stack pressure. The cells were typically loaded with 15-25 mg of LiFeSO4FPEDOT composite powder (consisting of 90 wt % of active material) mixed with 15 wt % of carbon black (Super P, TIMCAL Graphite & Carbon). Metallic lithium was used as a combined reference and counter electrode, and two stacked glass microfiber filter sheets (Whatman, GE Healthcare) were employed as separator. Aluminium and nickel current collectors were used at the positive and negative electrode, respectively. A commercial electrolyte (Merck) was employed, consisting of 1 M LiPF6 dissolved in equal volumetric amounts of ethylene carbonate (EC) and diethylene carbonate (DEC). All cells were cycled at room temperature (~21° C) in galvanostatic mode. The cells were also subjected to a preconditioning cycle at the


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rate C/10 (extraction/insertion of one Li+ per formula unit of active material in 10 hours) to minimize overpotentials in the electrochemical trace related to the polymer coating32. After completion of the electrochemical experiments, the cells were disassembled and the cathode powders were retrieved for ex-situ X-ray diffraction and Mössbauer spectroscopy characterizations. XRD patterns were collected using a Bruker D8 diffractometer equipped with a Cu X-ray tube (λ1 = 1.54056 Å, λ2 = 1.54439 Å) and a Lynxeye linear detector with fluorescence suppression. Crystal structure refinements were performed using the Rietveld method41 implemented in the FullProf42 software. Mössbauer spectra were collected in transmission mode using a 57CoRh source of constant acceleration, covering a velocity span of ± 5 mm/s. The absorbers were prepared by mixing ~30 mg of active material (LixFeSO4F, 0≤x≤1) with a suitable amount of an inert filler (boron nitride), which was then spread evenly over a 13 mm in diameter absorber disc. Due to the moisture sensitivity of the active material, the absorbers were kept under a constant flow of N2 during the measurements. The spectra were Lorentzian line least-square fitted43 using the Recoil software. The center shift, CS, being the sum of the true isomer shift and the second order Doppler shift, is given relative to metallic iron (α-Fe) at room temperature. The magnitude of the quadrupole splitting, QS, is given as the peak separation in the doublet, and the line width, W, is the experimental FWHM of the spectral peaks.

RESULTS AND DISCUSSION The first part of the Results and Discussion addresses characterization of the as-prepared LiFeSO4F. This is followed by XRD and MS characterizations of a series of samples, LixFeSO4F (0≤x≤1), prepared by chemical oxidation of the pristine LiFeSO4F, with the aim to study the structural evolution upon delithiation. Finally, ex-situ analyses are performed on electrochemically prepared samples, focusing on lithium compositions of specific interest. The X-ray diffraction pattern (Figure 1) of the asprepared LiFeSO4F powder confirmed that the desired tavorite phase was obtained via the solvothermal synthesis (described in the Experimental section), with expected weak reflections from LiF which was used in excess in order to ensure a complete reaction. No other crystalline impurities were observed. The LiFeSO4F-structure was refined within the space group P-1 (triclinic system), using a structural model with one Li-site, (Fe(2))1a(Fe(1))1b{S2i[O2i]4}F2iLi2i, as suggested from reported neutron diffraction studies44. The refinement resulted in unit cell parameters of a = 5.1754(1) Å, b = 5.4896(2) Å, c = 7.2216(2) Å, α = 106.514(3)°, β = 107.191(3)°, γ = 97.843(3)°, giving a cell volume of V = 182.395(9) Å3, in good agreement with previous reports19,39 (more refinement parameters are given in Table S1, Supporting Information).

Figure 1. X-ray diffraction and Rietveld refinement of asprepared LiFeSO4F. The red circles represent the experimental data, the black line shows the calculated fit, and the blue line shows the difference between observed and calculated data. The green bars show the allowed Bragg positions for LiFeSO4F and LiF. The inset shows the corresponding Mössbauer spectrum.

The as-prepared LiFeSO4F was further characterized by Mössbauer spectroscopy (MS). The spectrum (Figure 1, inset (a larger graph is provided in Figure S1a, Supporting Information)) shows two sharp and well resolved doublets with hyperfine parameters (Table S2a, Supporting Information) corresponding to Fe2+ in a high spin coordination, as expected from the two distinct crystallographic iron sites, Fe(1) and Fe(2), within the tavorite-type LiFeSO4F19,45. Here, the outer and inner doublets are ascribed to the Fe(1)-site (Wyckoff position 1b (0, 0, ½)) and to the Fe(2)-site (Wyckoff position 1a (0, 0, 0)), respectively, consistent with previously suggested assignments by us40 and others45. The lack of additional signals from ironcontaining species and the relatively narrow line widths, W, of the spectral peaks with a slight asymmetry of the inner doublet (W-/W+ = 1.10) indicate a sample of high purity and high atomic order, as has been thoroughly discussed elsewhere40. To screen the structural evolution of tavorite LiFeSO4F upon Li+-removal, a series of chemically oxidized samples, LixFeSO4F, with evenly distributed compositions in the range 0≤x≤1 were prepared from the pristine material (as described in the Experimental section). The XRD patterns of these samples are shown in Figure 2b-f, and the given compositions were determined by MS, presented in Figure S1b-f and Table S2b-f, Supporting Information. The diffraction patterns of the fully lithiated (Figure 2a) and fully delithiated (Figure 2f) samples show the characteristic reflections that are expected from these extensively characterized and well known phases19,39,44, with no traces of crystalline impurities. Looking at the diffraction patterns for the entire series, going from high to low Licontent (x=1 to x=0 in LixFeSO4F), it is evident that the



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Figure 2. X-ray diffraction patterns of samples prepared by chemical oxidation of as-prepared LiFeSO4F.

peaks of the LiFeSO4F-phase are decreasing in intensity at the expense of growing FeSO4F-reflections, and only minor shifts of the peak positions are observed. This is in good agreement with previously reported results where it was concluded that the insertion/extraction of Li-ions in LiFeSO4F is occurring mainly through a single biphasic reaction mechanism accompanied by a certain degree of solid solution behavior19,38. However, in addition, we notice that the XRD patterns of the samples with compositions 0.27≤x≤0.74 (Figure 2b-e) also show the presence of new Bragg reflections (the visually most evident and well resolved peak is seen in the 23-23.3° region) that cannot be assigned to either the LiFeSO4F or the FeSO4F endmember, indicating the existence of an additional phase. Interestingly, one can observe similar peaks in a previous work, but these were not identified as a separate phase19. The relative intensity of these new reflections appears to be highest for the samples Li0.44FeSO4F (Figure 2d) and Li0.56FeSO4F (Figure 2c), indicating that the unidentified phase is stabilized at compositions close to Li0.5FeSO4F (i.e. when half the initial Li+-content has been removed from the pristine LiFeSO4F-structure). At a first glance, it appears surprising to have three phases in co-existence in some of the samples (Li0.44FeSO4F; Li0.56FeSO4F, and Li0.74FeSO4F), as it is contradictory to Gibbs’ phase rule. However, considering the aggressive oxidation process that is driven by NO2BF4, having a redox potential of ~5.1 V vs. Li/Li+, it is reasonable to expect an inhomogeneous delithiation of the sample resulting in non-equilibrium situations, where Gibbs’ phase rule does not apply. Figure 3. Voltage-composition plots of galvanostatically cycled LiFeSO4F, presented using a) a full scaled y-axis and b) rescaled y-axis for a more detailed view of the nuances of the electrochemical trace.

To study the structural evolution of LiFeSO4F under well defined conditions closer to equilibrium, and investigate the implications on the redox behavior, the material was cycled in electrochemical cells of Swagelok type where the rate of oxidation was controlled


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Chemistry of Materials fined plateau with low polarization and with sharp charge and discharge end points, and the practical specific capacity obtained was 135 mAh/g (corresponding to the extraction of 0.9 Li+ per formula unit of LiFeSO4F). However, when rescaling the y-axis for a more detailed look into the nuances of the redox behavior, two distinct plateaus centered around ~3.585 and ~3.600 V and separated by an inflection point can clearly be distinguished on charging (Figure 3b). This suggests the presence of two subsequent biphasic processes, and the position of the inflection point suggests that the new phase formed has a distinct composition of Li0.5FeSO4F. It should be noted that similar electrochemical behavior has been observed previously21,36, but the two plateaus have been interpreted as a biphasic region followed by a solid solution domain21.

Figure 4. The upper plot (a) shows the ex-situ XRD pattern and Rietveld refinement of the electrochemically prepared sample of global composition Li0.5FeSO4F. The inset shows the corresponding electrochemical trace where the red dot represents the position where the battery was stopped and disassembled. The lower plot (b) shows the corresponding ex-situ Mössbauer spectrum.

galvanostatically. To optimize the battery performance, maximizing the practical capacity and minimizing the polarization during charge and discharge, the as-prepared LiFeSO4F powder was coated with the electronically conducting polymer poly-3,4-ethylenedioxythiophene (PEDOT). A detailed description of the coating procedure and its effects on the electrochemical performance is presented elsewhere32. For the sake of completeness, an XRD analysis of the LiFeSO4F-PEDOT composite material was conducted showing that the desired tavorite structure had been preserved after the coating process (Figure S2, Supporting Information). A complete charge and discharge cycle (recorded after initial conditioning cycling) of the PEDOT coated LiFeSO4F (Figure 3a), using a slow rate of C/100 (extraction/insertion of 1 Li+ per formula unit of LiFeSO4F in 100 hours), show a flat and well de-

To obtain a better insight of the structural aspects of the material at this global composition of Li0.5FeSO4F, another Swagelok cell was cycled at C/100 and stopped at the inflection point (Figure 4a, inset), whereupon it was disassembled to retrieve the electrode powder for ex-situ XRD and MS characterization. The collected Mössbauer spectrum, shown in Figure 4b, confirms an overall sample composition (within the uncertainty of the measurement) of Li0.5FeSO4F, as expected from the charge passed through the cell (the corresponding Mössbauer hyperfine parameters are presented in Table S3, Supporting Information). The diffraction pattern of this sample (Figure 4a) is significantly dominated (~90% contribution to the pattern as suggested by the multi-phase refinement) by reflections that cannot be attributed to either of the endmembers, LiFeSO4F or FeSO4F, which confirms the formation of a completely new phase. Here, we have indexed these reflections in a triclinic cell, space group P-1 (in good agreement with reported DFT calculations36), with unit cell parameters a = 5.1427(3) Å, b = 5.2972(4) Å, c = 7.3234(3) Å, α = 108.830(6)°, β = 109.394(6)°, γ = 94.274(5)°, giving a cell volume of V = 174.382(18) Å3 (more refinement parameters are given in Table S4, Supporting Information). For clarity, this intermediate phase will hereafter be denoted Li1/2FeSO4F. Looking closer into the details of the proposed structural model of Li1/2FeSO4F, it clearly exhibits strong similarities to LiFeSO4F and FeSO4F. Just as the end-members (Figure 5a and 5c), the intermediate phase consists of FeO4F2 octahedra linked together through F-vertices, creating zigzag-like chains along the c-axis (Figure 5b). All oxygen atoms are covalently bound to sulfur, forming SO4 tetrahedra which are bridging the Fe-chains. While the overall structural framework is similar for all three phases, the differences lie in the distinct distortions and rotations of the polyhedra, as well as the subsequent changes of the unit cell parameters (Table 1). In the first biphasic region, when going from LiFeSO4F to Li1/2FeSO4F upon removal of o.5 Li+ per formula unit, one can notice a significant expansion of the c-axis accompanied by contractions of the a and b-axes (as well as changes of the unit cell angles), effectively resulting in an overall decrease of the unit cell volume by ~4.4%. The c-axis expansion is due to an elongation of the Fe-F bonds within the



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Figure 5. Illustrations of the crystal structure of LiFeSO4F (a and d), Li1/2FeSO4F (b and e), and FeSO4F (c and f). Note that FeSO4F is illustrated in the triclinic space group P-1 for a more direct comparison of all three phases. Table 1. Comparison of the unit cell parameters for the three phases formed during electrochemical cycling of tavorite LiFeSO4F. LiFeSO4F

Li1/2FeSO4F

FeSO4F

space group

P -1

P -1

C 2/c

Z

2

2

4

a (Å)

5.1754(1)

5.1427(3)

7.303(1)

b (Å)

5.4896(2)

5.2972(4)

7.068(1)

c (Å)

7.2216(2)

7.3234(3)

7.304(1)

α (°)

106.514(3)

108.830(6)

90(-)

β (°)

107.191(3)

109.394(6)

119.741(2)

γ (°)

97.843(3)

94.274(5)

90(-)

V (Å3)

182.395(9)

174.38(2)

327.36(9)

V/Z (Å3)

91.2(1)

87.2(1)

81.8(1)

octahedra, while the a and b-axes contraction is a result of shortened Fe-O bonds. However, the dihedral angle between adjacent FeO4F2 octahedra (O(4)-Fe(2)-Fe(1)-O(2), seen along the c-axis) is effectively preserved when going from pristine tavorite LiFeSO4F (Figure 5d) to Li1/2FeSO4F (Figure 5e), changing from 28.1 to 27.8°, respectively. For the second biphasic process, going from Li1/2FeSO4F to FeSO4F upon removal of the remaining Li+, the a and baxes are further contracted while the c-axis remains fairly constant (note that this is true only if FeSO4F is repre-

sented in the P-1 space group, shown in Table S5, Supporting Information). Together with the changes of the unit cell angles, this result in a further contraction of the unit cell (V/Z) by ~6.1% (in total, the difference in volume between LiFeSO4F and FeSO4F accounts for ~10.3%, in good agreement with previous reports19). The FeSO4F phase presents more symmetric FeO4F2 octahedra that become crystallographically indistinguishable as this second biphasic phase transition leads to an increase in symmetry from P -1 to C 2/c. Moreover, the dihedral angle between adjacent octahedra in FeSO4F (Figure 5f) is significantly reduced compared to the situation in LiFeSO4F (Figure 5d) and Li1/2FeSO4F (Figure 5e). While the overall structural features of the intermediate phase and the evolution of the unit cell parameters in the LiFeSO4F/Li1/2FeSO4F/FeSO4F-system seem reasonable and realistic, it is important to point out details in the results suggesting that the chosen structural representation of Li1/2FeSO4F may not be the most optimal. Firstly, the Mössbauer spectrum of the Li0.5FeSO4F-sample (Figure 4b) shows preferentially oxidized Fe2+-sites, suggesting a charge order in the Li1/2FeSO4F-phase. Interestingly, from the total oxidation degree of 50%, the individual contribution from the crystallographic Fe(1) and Fe(2)sites is ~2/3 and ~1/3, respectively. Surprisingly, this is in contrast to previously reported in-operando MS measurements on LiFeSO4F where no preferential oxidation was observed45. Such a distribution of charges over the two Fe-sites, with a close relation to a ratio of 1/3, could be an indication of the formation of a super structure, where at least a three times larger unit cell should be used for a structural description of Li1/2FeSO4F with distinct ordering of Fe2+ and Fe3+. Such a hypothesis is further


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Figure 6. Ex-situ XRD patterns (a) of electrochemically prepared LixFeSO4F-samples with global compositions x=0.22, x=0.49, and x=0.74, (b) their corresponding electrochemical traces (the red dots represent the position where the cells were stopped and disassembled), and the corresponding Mössbauer spectra for (c) the Li0.74FeSO4F and (d) the Li0.22FeSO4F samples. The contribution from the different phases to the Mössbauer signals, as determined from the corresponding XRD data, are denoted A, B, and C for LiFeSO4F, Li1/2FeSO4F, and FeSO4F, respectively.

supported by the significantly distorted polyhedra, in particular the SO4-tetrahedron, observed in the refined structural model for Li1/2FeSO4F. This could be a result of averaging the geometries of several distinct polyhedra situated in the unit cell of a hypothetical super structure through the use of this smaller unit cell. However, we have not yet spotted any clear evidence of a superstructure from the collected XRD patterns. Nevertheless, these findings strongly encourage further studies to fully understand the structural nature of the intermediate phase. Even though Li1/2FeSO4F is significantly dominating the contribution to the XRD pattern shown in Figure 4a, it should be noted that the pattern also contains traces of the end-members (contributing with ~5% each). This highlights the difficulty of stabilizing solely the intermediate phase in a powder sample, even by electrochemical

cycling at the slow rate of C/100. To obtain an insight into the role of the cycling rate on the stabilization of Li1/2FeSO4F, another cell was cycled and stopped at the inflection point during charge, but using the significantly higher cycling rate C/20. The ex-situ XRD pattern and Mössbauer spectrum of this sample (Figure S3, with corresponding hyperfine parameters in Table S6, Supporting Information) show a high resemblance with the results from the sample cycled at C/100. However, at the higher cycling rate the amount of Li1/2FeSO4F formed is slightly lower (~80% contribution to the pattern as determined by a multiphase refinement). Thus, the direct comparison of the XRD results from electrochemical cycling at C/100 and C/20, together with the chemical oxidations discussed above, leads one to conclude that a faster, more aggressive, delithiation reaction results in formation of less amount of the intermediate phase, and more of the



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end-members, LiFeSO4F and FeSO4F. Such an observation seems reasonable considering that; (i) one can expect a faster galvanostatic charge (or discharge) to induce a larger distribution of local electrochemical potentials over the whole electrode as a result of variations in the electronic wiring between the electrode particles (giving rise to different iR-drops) and (ii) one can expect to have local variations in Li+-accessibility in the electrode due to different particle sizes of the active material. Both these effects are expected to result in local variations of the sample composition that is deviating from the global composition (in this case Li0.5FeSO4F). Considering the exceptionally low energetical separation of up to ~15 meV between the three, seemingly thermodynamically stable phases of this system, such a composition distribution in the powder is expected to trigger crystallization of all of them. Furthermore, the equilibration process of the system, reducing the FeSO4F phase by LiFeSO4F to form only Li1/2FeSO4F, is expected to be slow given the small potential driving force. Finally, to confirm the existence of two subsequent biphasic domains during electrochemical extraction of Li+, another two cells were cycled at a rate of C/100 and stopped during charging (Figure 6b, also presented with rescaled y-axis in Figure S4, Supporting Information), targeting nominal compositions of LixFeSO4F x=0.25 and x=0.75, respectively. After cell disassembly, ex-situ MS and XRD measurements were carried out on the retrieved electrode powders and compared to the results of the sample with the global composition Li0.5FeSO4F, with known predominance of the intermediate phase. The Mössbauer spectra (Figure 6c-d, with corresponding hyperfine parameters in Table S7, Supporting Information) indeed confirmed compositions close to those desired, namely Li0.22FeSO4F and Li0.74FeSO4F. Looking at the corresponding XRD analysis of these samples (Figure 6a), the pattern of the sample with a composition of Li0.74FeSO4F clearly shows a mixture of Bragg reflections from the LiFeSO4F and Li1/2FeSO4F phases, and no contribution from FeSO4F can be observed. Moreover, the sample with a composition of Li0.22FeSO4F shows a mixture of contributions from the Li1/2FeSO4F and FeSO4F phases, with no presence of LiFeSO4F. These results, combined with the observation of the two voltage plateaus upon electrochemical cycling (Figure 3b), strongly suggest the occurrence of two subsequent biphasic processes in the LiFeSO4F-system, which can be summarized by the following equations: LiFeSO4F ⇌ Li1/2FeSO4F + 0.5Li+ + 0.5e-

(2)

Li1/2FeSO4F ⇌ FeSO4F + 0.5Li+ + 0.5e-

(3)

Additionally, both these biphasic processes are accompanied by certain degrees of solid solution reactions, as indicated by small shifts of the diffraction peaks for all three phases during multi-phase refinements of XRD data. It should also be mentioned that the direct two-

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phase transition from LiFeSO4F to FeSO4F is probably also possible, as suggested by DFT calculations36, especially at higher cycling rates and for electrode composites with poor electronic wiring, resulting in larger overpotentials and a less pronounced inflection point in the electrochemical trace. Hence, the improved electronic wiring through the electrode provided by the PEDOT coating used in this work32 should effectively facilitate the formation of Li1/2FeSO4F, and could be an explanation why we spot this intermediate phase more clearly compared to many other studies where no electronically conductive coatings are usually used. Until this point, the conducted experiments in this work have mainly considered the extraction process of Li+ from LiFeSO4F. Focusing now on the insertion process, interestingly, the electrochemical trace from the full cycle obtained at C/100 (Figure 3b) shows no evidence of two plateaus on discharge. This is in contrast to the electrochemical behavior observed for the isostructural LiFeSO4OH-system36, but similar to that of LiVPO4F33,34. For LiFeSO4OH, an intermediate phase, Li0.5FeSO4OH, was indeed identified during the lithiation process, as expected. In the case of LiVPO4F, the lack of a clear inflection point on discharge was attributed to the occurrence of only one biphasic process during electrochemical lithiation, with no stabilization of the intermediate phase, Li0.67VPO4F. Thus, at this point, it was intriguing to investigate the structural behavior of the LiFeSO4F-system during lithiation, so another cell was cycled at C/100 and stopped as close as possible to a targeted composition of Li0.5FeSO4F during discharge (Figure S5a, inset, and Figure S5b, Supporting Information). Just as previously, the cell was disassembled to retrieve the electrode powder for ex-situ characterizations. MS confirmed a sample composition of Li0.5FeSO4F (within the uncertainty of the measurement), and interestingly, a preferential reduction of the iron sites was observed (Figure S5c, with corresponding hyperfine parameters in Table S8, Supporting Information). Surprisingly, the collected XRD pattern of the sample (Figure S5a, Supporting Information) revealed a predominant contribution from Li1/2FeSO4F (60% contribution to the pattern as suggested by a multi-phase refinement), besides the co-existing LiFeSO4F and FeSO4F (contributing with ~20% each), showing a significant stabilization of the intermediate phase despite the lack of clear features in the electrochemical trace. So far, this rather contradictable phenomenon is not well understood, which further points out the need of a thorough insitu XRD study.

CONCLUSIONS In this Article, we have investigated the structural evolution of the tavorite-type LiFeSO4F upon chemical and electrochemical Li+-extraction (and re-insertion). The results show that the delithiation/lithiation occurs through two subsequent biphasic processes due to the stabilization of an intermediate phase, Li1/2FeSO4F, never before experimentally characterized. Surprisingly, even though the Li1/2FeSO4F is shown to form both on charge


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and discharge of a half cell (vs. Li/Li+), the electrochemical trace shows a significant asymmetry with a clear inflection point appearing only during the former process. Due to the overall low energetical separation observed between the phases of this system (LiFeSO4F, Li1/2FeSO4F, and FeSO4F), namely ~15 meV during charging, the degree of formation of the intermediate phase at a global sample composition of Li0.5FeSO4F is shown to be particularly sensitive to the rate of Li+-extraction/insertion. Slower delithiations were shown to maximize the amount of Li1/2FeSO4F formed, which was attributed to lower local composition inhomogeneities within the powder samples/electrodes. However, even at exceptionally slow electrochemical delithiation rates of C/100, the maximum degree of Li1/2FeSO4F formed within a powder sample was ~90%. Finally, the Rietveld refinement of ex-situ powder X-ray diffraction data presents an overall reasonable structural model for Li1/2FeSO4F with realistic evolution of the unit cell parameters between the intermediate and end-member phases. Nevertheless, we also highlight details in the results indicating charge ordering between the two distinct Fe-sites in Li1/2FeSO4F, suggesting that a larger unit cell might be needed for a more accurate structural representation. We hope that the presented results have provided a deeper insight into the Li+-extraction/insertion mechanism in tavorite LiFeSO4F, and that it will trigger further studies to obtain a deeper understanding of the complex phenomena observed.

SUPPORTING INFORMATION Rietveld refinement parameters for LiFeSO4F and Li1/2FeSO4F. Mössbauer spectra, alternative voltagecomposition graphs, and/or hyperfine parameters for the series of chemically oxidized samples of various compositions and the electrochemically prepared samples of composition Li0.22FeSO4F, Li0.5FeSO4F, and Li0.74FeSO4F, stopped during charging at C/100. Ex-situ XRD and MS of electrochemically prepared samples of composition Li0.5FeSO4F, stopped during charging at C/20 and discharging at C/100. XRD pattern of the LiFeSO4F-PEDOT composite material. Comparison of the unit cell parameters between LiFeSO4F, Li1/2FeSO4F, and FeSO4F, all represented in a triclinic crystal structure (space group P-1). This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT The work presented here is undertaken within a joint development project at the HVV (www.highvoltagevalley.se) consortium and financed by the Swedish Governmental Agency for Innovation Systems (Vinnova).

AUTHOR INFORMATION Corresponding Author *E-mail addresses: [email protected] (A.S.), [email protected] (F.B.)

Notes

The authors declare no competing financial interest.

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2FeSO4F, Formed during

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