Understanding and Controlling the Surface Chemistry of LiFeSO

Article pubs.acs.org/cm

Understanding and Controlling the Surface Chemistry of LiFeSO4F for an Enhanced Cathode Functionality Adam Sobkowiak,*,† Matthew R. Roberts,† Reza Younesi,† Tore Ericsson,† Lennart Hag̈ gström,† Cheuk-Wai Tai,‡ Anna M. Andersson,§ Kristina Edström,† Torbjörn Gustafsson,† and Fredrik Björefors*,† †

Department of Chemistry − Ångström Laboratory, Uppsala University, Box 538, SE-751 21 Uppsala, Sweden Department of Materials and Environmental Chemistry − Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden § ABB Corporate Research, SE-721 78 Västerås, Sweden ‡

S Supporting Information *

ABSTRACT: The tavorite polymorph of LiFeSO4F has recently attracted a lot of interest as a cathode material for lithium ion batteries stimulated by its competitive specific capacity, high potential for the Fe2+/Fe3+ redox couple, and lowtemperature synthesis. However, the synthesis routes explored to date have resulted in notably varied electrochemical performance. This inconsistency is difficult to understand given the excellent purity, crystallinity, and similar morphologies achieved via all known methods. In this work, we examine the role of the interfacial chemistry on the electrochemical functionality of LiFeSO4F. We demonstrate that particularly poor electrochemical performance may be obtained for pristine materials synthesized in tetraethylene glycol (TEG), which represents one of the most economically viable production methods. By careful surface characterization, we show that this restricted performance can be largely attributed to residual traces of TEG remaining on the surface of pristine materials, inhibiting the electrochemical reactions. Moreover, we show that optimized cycling performance of LiFeSO4F can be achieved by removing the unwanted residues and applying a conducting polymer coating, which increases the electronic contact area between the electrode components and creates a highly percolating network for efficient electron transport throughout the composite material. This coating is produced using a simple and scalable method designed to intrinsically favor the functionality of the final product. KEYWORDS: battery, fluorosulfate, polymer coating, powder X-ray diffraction, Mössbauer spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy

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INTRODUCTION Since their commercialization in the 1990s, rechargeable lithium ion (Li-ion) batteries have found wide usage within portable electronic devices due to their high gravimetric and volumetric energy densities. Today, they are also considered as promising candidates for large-scale energy storage within future sustainable energy systems, such as wind and solar power, as well as in electric vehicles. These outlooks have not only driven, but also significantly intensified, the development of Li-ion batteries over the past few years, where research on new electrode materials has been a major focus. The interest in iron-based cathode materials containing polyatomic anions was raised with the discovery of olivine LiFePO4,1 which showed reasonable electrochemical performance with improved stability and safety as compared to conventional metal oxide electrodes (such as LiCoO2). The utilization of abundant iron as the transition metal redox couple (Fe2+/Fe3+) also provides compounds with low price and toxicity, which are equally important factors for commercial viability. This has triggered an increase in activity to identify © XXXX American Chemical Society

new iron-based polyatomic anion compounds with similar or improved electrochemical properties, and has led to the discovery of materials such as the NASICON structure phosphate2 (Li3Fe2(PO4)3), phosphate fluoride3 (Li2FePO4F, also known as “fluorophosphate”), silicate4 (Li2FeSiO4), and borate5 (LiFeBO3). One of the most recently discovered materials within this class of compounds is the lithium iron sulfate fluoride,6 LiFeSO4F (commonly named “lithium iron fluorosulfate” in the literature). This material has attracted much attention within the lithium battery community because it demonstrates an increase in the potential of the Fe2+/Fe3+ redox couple when compared to other previously studied ironbased compounds, for example, LiFePO4. The higher potential for LiFeSO4F can be generally explained in terms of a lower covalent nature of the transition metal−ligand bonds, known as the inductive effect,7−9 due to the higher electronegativity of Received: April 2, 2013 Revised: June 22, 2013

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dx.doi.org/10.1021/cm401063s | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

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SO42− and F− as compared to PO43−. However, the redox potential is also affected by additional factors related to the crystal structure. Such effects have been observed for various iron-based phosphate compounds10 and are also considered to be the main contributor to the potential difference between the two known polymorphs of pure LiFeSO4F,6,11−13 isostructural to the minerals tavorite and triplite, operating at 3.6 and 3.9 V, respectively. With its three-dimensional channel framework, facilitating fast Li-ion transport, the tavorite polymorph is particularly interesting from an electrochemical point of view, and has the potential to be utilized in high rate applications. The synthesis of tavorite LiFeSO4F, which is an intermediate meta stable phase to the energetically favored triplite polymorph,14 relies on a kinetically controlled topotactic reaction,15 where the crystal water within the structurally similar FeSO4·H2O precursor is exchanged for LiF. This reaction has been demonstrated using different reaction media, such as ionic liquid,6 polymers,16 and glycol.17 Low-temperature solid-state preparations have also been successful, but at the expense of reaction kinetics.18 Reviewing the literature on LiFeSO4F leads one to conclude that, in general, materials synthesized employing the ionothermal preparation route (using EMI-TFSI) generate the best electrochemical performance, utilizing approximately 80−85% of the 151 mAh/g theoretical capacity. The EMI-TFSI has been suggested to enhance the electrochemical performance of related compounds by leaving a thin residual coating on the material particles after synthesis, facilitating the ionic conductivity through the electrode.19 Conversely, we see that the electrochemical performance of pristine LiFeSO4F synthesized using tetraethylene glycol (TEG) as reaction media may be significantly limited, despite being equally phase pure. While the poor electrochemical performance of materials prepared in TEG is a substantial drawback, the use of this solvent as a reaction media should still be pursued as it presents a far more economical and scalable synthesis route when compared to that presented by expensive ionic liquids. Therefore, understanding the limitations of the materials synthesized in TEG, and developing strategies to negate these drawbacks, is extremely interesting. This is driven by a fundamental need both to explain the differences in performance for phase pure materials synthesized in different solvents6,16,17 and to develop a commercially practical and scalable synthesis of high performing LiFeSO4F. In materials with a particle size in the micrometer range (as generally produced for tavorite LiFeSO4F) and poor electronic conductivity (which is known to be low in tavorite LiFeSO4F6), electronically conducting surface coatings have been shown to be an effective way of enhancing material performance. Typically, this involves a carbon coating produced using a high-temperature pyrolysis of organic materials.20 Such methods are, however, inappropriate for LiFeSO4F, which is unstable at the required temperatures.6 An alternative coating strategy is to use electronically conducting polymers, which can be fabricated using a facile low-temperature synthesis. Coatings of this kind have been shown to improve the electrochemical performance of several different materials,21−26 increasing the practical specific capacities and improving the cycle life and rate capabilities. In this study, we highlight the importance of controlling the surface chemistry when preparing high performing LiFeSO4F using economical and scalable methods. This is carried out by synthesizing phase pure tavorite LiFeSO4F using TEG as

reaction media, and then introducing a conducting polymer coating (poly-3,4-ethylenedioxythiophene (PEDOT)) to the pristine material, enhancing its electrochemical performance. The coating method utilized22 relies on the intrinsic oxidation power of Li(1−x)FeSO4F, which can oxidize the EDOT monomer and form the PEDOT coating directly on the surface of the LiFeSO4F particle, as shown schematically in Figure 1. By thorough surface characterizations at different

Figure 1. Schematic illustration explaining the two-step procedure of coating LiFeSO4F with a layer of PEDOT. The propagation of the polymerization reaction (step 2) intrinsically favors the functionality of the final product as Li-ions are required to enter Li(1−x)FeSO4F and penetrate the continuously growing PEDOT layer, establishing transportation paths.

stages of the coating procedure, and by coupling these results to the electrochemical behavior, we gain a better understanding of the mechanisms that are limiting the performance of LiFeSO4F prepared in TEG.

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EXPERIMENTAL SECTION

The title compound (LiFeSO4F) was prepared by a low-temperature solvothermal approach17 using a pressurized reaction vessel. First, a FeSO4·H2O precursor was prepared by dehydration of commercially available FeSO4·7H2O (Sigma-Aldrich, >99%) at 100 °C for 3 h, under constant flow of N2. The FeSO4·H2O precursor was then mixed with a slight excess (1.15:1) of LiF (Alfa Aesar, >99%), summing to a total batch mass of ∼1 g. This batch was then ball-milled for 1 h with acetone, with no sealing from the laboratory atmosphere. The powder mixture was then dispersed in 30 mL of TEG by stirring for 30 min under ambient conditions. The dispersion was then heated in a 45 mL Teflon-lined steel autoclave (Parr Instruments) at 220 °C for 3 days. The product, having an ivory white color, was then collected by centrifugation, washed with acetone, dried at room temperature under N2 flow, and finally stored in an Ar-filled glovebox. For clarity, this sample will be labeled “LiFeSO4F” from here on. B



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The PEDOT coating of the title compound was performed in a two-step process.22 In the first step, LiFeSO4F was chemically delithiated by using the oxidation agent NO2BF4 (Figure 1, step 1), according to the following equation: LiFeSO4 F + x NO2 BF4 → Li(1 − x)FeSO4 F + x LiBF4 + x NO2 ↑

as the peak separation in the symmetrical (no texture assumed) doublet. Fourier transform infrared spectroscopy (FTIR) was used to probe chemical differences between the samples. The measurements were carried out in the attenuated total reflection (ATR) mode using a PerkinElmer Spectrum One FT-IR spectrometer. The spectra were obtained by addition of 50 scans swept between 550 and 4000 cm−1 with a resolution of 4 cm−1. Thorough surface characterization of the samples was performed by X-ray photoelectron spectroscopy (XPS) using a commercial PHI 5500 spectrometer with monochromatic Al Kα radiation (1487 eV) and an electron emission angle of 45°. All spectra were energy calibrated by using the hydrocarbon peak positioned at the binding energy of 285.0 eV. Samples were prepared by carefully applying a thin layer of powder on the surface of aluminum substrates. Complete coverage of the Al substrate was confirmed as no spectroscopic contribution from the substrate was observed. To reduce the effect of differential charging, an electron gun was used during the measurements of the uncoated samples. The spectra were intensity normalized to unity. Scanning electron microscopy (SEM) measurements were performed using a Zeiss LEO1550 field emission microscope. Images were recorded at 15 kV with an in-lens secondary electron detector. A transmission electron microscopy (TEM) study was carried out in a field emission microscope (JEOL JEM-2100F) equipped with ultra high-resolution pole-piece (Cs = 0.5) operated at 200 kV. The samples were dispersed in acetone and dropped onto a TEM copper grid with holey carbon supporting films. The images were recorded using a CCD camera (Gatan Ultrascan 1000). Thermal gravimetric analysis (TGA) was used to estimate the gravimetric amount of polymer in the PEDOT-LiFeSO4F composite sample. The analysis was carried out between 25 and 700 °C under a constant flow of dry air with a heating rate of 10 °C/min using a thermogravimetric analyzer from Mettler Toledo, model TGA/ SDTA851e. The sample size was ∼10 mg, loaded in an aluminia crucible. The polymer content in the composite material was quantified by subtracting the thermogravimetric trace of the asprepared LiFeSO4F from the PEDOT-LiFeSO4F trace, where the difference is attributed to decomposition of the polymer by combustion in air. Electrochemical characterization was carried out in traditional Swagelok cells, equipped with aluminum and nickel current collectors at the positive and negative electrode, respectively. A spring mechanism was used in the cell to maintain a stack pressure. The cells were typically loaded with 8−10 mg of electrode powder, consisting of noncoated material (LiFeSO4F or Li(1−x)FeSO4F) or polymer composite (PEDOT-LiFeSO4F) mixed with 15 wt % of carbon black (Super P, TIMCAL Graphite & Carbon). All samples were cycled against metallic lithium as a combined reference and counter electrode, using a commercial electrolyte (Merck) consisting of 1 M LiPF6 dissolved in equal volumetric amounts of ethylene carbonate (EC) and diethylene carbonate (DEC). Two glass microfiber filter sheets (Whatman, GE Healthcare) were used as separator. The cells were cycled in galvanostatic mode in the potential window of 2.5−4.2 V.

(1)

Here, “Li(1−x)FeSO4F” refers to a degree of delithiation of x Li-ions per formula unit of the title compound, and this notation will be used from here on for simplicity when referring to the partially delithiated sample (because in the literature LiFeSO4F is described as a two-phase insertion compound, a more correct notation would be “(1− x)LiFeSO4F(x)FeSO4F”). In the second step, the intrinsic oxidation power of Li(1−x)FeSO4F during chemical reinsertion of Li-ions was utilized as a driving force to initiate the polymerization reaction of the EDOT monomers, forming the final composite material (labeled “PEDOT-LiFeSO4F”) (Figure 1, step 2). The reaction process can be informally described by the following simplified reaction scheme (a more detailed reaction is given in the Supporting Information):

Li(1 − x)FeSO4 F + LiTFSI + EDOT → PEDOT‐LiFeSO4 F

(2)

The choice of LiTFSI as the Li-ion source is based on its stability under ambient conditions27 and negligible redox currents at normal operation potentials of Li-ion batteries.28 Moreover, the use of TFSI as counterion for the oxidized state of PEDOT has been shown to result in polymeric materials with a high electronic conductivity.25 Aiming for a final PEDOT-LiFeSO4F composite containing ∼9 wt % of PEDOT, an appropriate composition of Li0.7FeSO4F (x = 0.3) was targeted in the delithiation step to provide enough oxidative reagent for the polymerization step both to completely consume all of the EDOT monomers and to allow for a certain level of p-doping of the formed PEDOT coating, making it electronically conductive. To obtain this nominal composition, 500 mg of as-prepared LiFeSO4F was mixed with 112 mg of NO2BF4 in 15 mL of anhydrous acetonitrile, and stirred for 12 h under Ar-atmosphere. This partially delithiated compound, Li(1−x)FeSO4F (which had a gray appearance), was then washed with acetonitrile and acetone, and finally dried under a flow of N2 at room temperature. For the polymerization reaction step, a stock solution was prepared by dissolving 0.400 g of EDOT and an excess of LiTFSI (3.2 g) (Aldrich) in 15.8 g of methanol. Next, 1.6 g of the stock solution was mixed with 0.350 g of Li(1−x)FeSO4F to form a homogeneous clay-like slurry, which was heated in a Petri dish at 70 °C for 3 h under Ar-atmosphere. The PEDOT-LiFeSO4F powder, which had turned dark blue after solvent evaporation, was washed with acetonitrile, methanol, and acetone, dried at 70 °C for 12 h under vacuum, and finally stored in an Ar-filled glovebox. Sample purity and qualitative composition analysis of the LiFeSO4F, Li(1−x)FeSO4F, and PEDOT-LiFeSO4F samples was performed by powder X-ray diffraction (XRD). The diffractometer, a Bruker D8 with Cu Kα radiation (λ1 = 1.54056 Å, λ2 = 1.54439 Å), was equipped with a Lynxeye linear detector with fluorescence suppression. Crystal structure refinement of the as-prepared LiFeSO4F was performed using the Rietveld method29 implemented in the program FullProf.30 Sample purity and quantitative composition analysis of LiFeSO4F, Li(1−x)FeSO4F, and PEDOT-LiFeSO4F samples were performed using 57 Fe Mössbauer spectroscopy (MS). The measurements were run in the transmission geometry using a spectrometer of constant acceleration type, with 1024 memory cells for storing the unfolded data. The source, 57CoRh, was always held at room temperature. The absorbers were prepared by mixing ∼30 mg of active material with a suitable amount (some tenths of a milligram) of boron nitride, which was then spread evenly over the absorber disc (diameter 13 mm). The amount of material was chosen to give optimal thicknesses, resulting in the optimal signal-to-noise ratio.31 The folded spectra (512 memory cells), covering a velocity span of ±6 mm/s or less, were least-squares fitted with Lorenzian lines using the software Recoil.32 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

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RESULTS AND DISCUSSION The first part of the Results and Discussion addresses the characterization of the as-prepared LiFeSO4F and the analysis of its surface at different stages of the PEDOT coating procedure (as-prepared, partially delithiated, and polymer coated). This is followed by electrochemical characterizations, where the observed trends are coupled to the differences in surface chemistry. Finally, a model is presented describing the proposed mechanism responsible for the observed cycling behaviors. Powder X-ray diffraction (XRD) of the as-prepared LiFeSO4F confirmed that the desired tavorite phase was obtained, C



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with only weak reflections from excess LiF, as expected. The diffraction pattern (Figure 2) was indexed in a triclinic cell,

Figure 3. XRD patterns of (a) as-prepared LiFeSO4 F, (b) Li(1−x)FeSO4F, and (c) PEDOT-LiFeSO4F, showing the change in composition of the title compound at different stages of the PEDOT coating procedure. The black bars show the Cu Kα1 Bragg positions of the lithiated tavorite structure, and the black stars show peaks belonging to the delithiated phase.

Figure 2. XRD pattern and Rietveld refinement of as-prepared LiFeSO4F. Red circles represent experimental data, black solid line shows the calculated fit, blue line shows the difference between observed and calculated data, and green bars show the Bragg positions. The inset shows an illustration of the resulting calculated structure.

doublets with hyperfine parameters (Table S2, Supporting Information) corresponding to a high spin Fe2+ state. These two doublets correspond to the two distinct crystallographic iron sites in tavorite LiFeSO4F, Fe1 and Fe2, and are in good agreement with that previously reported for the material.18 No signature of Fe3+ could be detected, confirming a pure and fully lithiated sample. The partially oxidized sample, Li(1−x)FeSO4F, showed clear evidence that the delithiation had occurred as an expected Fe3+-signal could be observed in the spectrum (Figure 4b), together with significantly affected Fe2+-doublets. The composition was estimated to Li0.74FeSO4F, close to the targeted Li0.70FeSO4F, showing that the chemical delithiation was practically fully efficient. Finally, the spectrum of the PEDOT-LiFeSO4F composite (Figure 4c) revealed a large decrease of the Fe3+/Fe2+ signal ratio, confirming that a partial relithiation was successful, and in turn, indicating a successful propagation of the polymerization reaction. The composition of the title compound after polymer coating was estimated to Li0.91FeSO4F, which is in good agreement with the XRD results, showing that a complete relithiation was not obtained. Additionally, the spectra of Li(1−x)FeSO4F and PEDOTLiFeSO4F showed more disordered Fe2+ environments in comparison to the as-prepared LiFeSO4F, indicating a decrease in crystallinity upon the chemical oxidation and reduction steps. Fourier transform infrared spectroscopy (FTIR) was used to probe the differences between the prepared materials at various stages of the coating procedure. The broad and most intense band seen in all spectra (Figure 5a−c), with maximum intensity at 1100 cm−1, originates from the tetrahedral sulfate anion33 within the title compound, and is, together with the sharp band at 1000 cm−1, a characteristic feature seen in FTIR spectra for pure tavorite LiFeSO4F.34 Hence, these bands stayed practically unchanged in all spectra. However, the spectrum of the asprepared LiFeSO4F (Figure 5a) also contained bands indicating the presence of TEG residues from the synthesis, even though the sample had been washed several times with acetone. The main peaks attributed to the TEG are seen as broad bands with maximum intensities at 3400 and 2875 cm−1,35,36 correspond-

space group P1̅, and the lattice parameters were refined to a = 5.1760(4) Å, b = 5.4909(4) Å, c = 7.2214(5) Å, α = 106.511(3)°, β = 107.187(3)°, γ = 97.847(2)° (more refinement details are given in Table S1, Supporting Information). This batch of pristine material was then polymer coated, as described in the Experimental Section. The structural effects on the active material were monitored by collecting diffraction patterns after each step of the coating process. The diffraction pattern of the partially oxidized sample, Li(1−x)FeSO4F (Figure 3b), showed clear evidence of a successful delithiation as it contained new Bragg reflections not corresponding to the fully lithiated phase (Figure 3a) (consistent with those previously reported for this material6). The final diffractogram (Figure 3c), obtained after the last coating step (the polymerization step), confirmed reinsertion of lithium into the structure of Li(1−x)FeSO4F because the reflections of the delithiated phase decreased in intensity, indicating that the polymerization reaction had propagated to form the PEDOT-LiFeSO4F composite material as anticipated. However, the peaks appertaining to the delithiated phase did not disappear entirely, implying that a complete lithium reinsertion was not obtained. Optimization of this process was not the objective of this Article; however, there is room for improvement if the material was to be used in a practical full cell, where the highest possible capacity is desired. Mössbauer spectroscopy (MS) was used for quantitative analysis of the Fe2+ and Fe3+ concentrations in the title compound after each step of the coating procedure, which gave information about the material composition. MS is also a valuable method to probe the sample purity because it not only detects crystalline phases, but can also detect possible amorphous iron species, which would be difficult to detect with XRD. The Mössbauer spectrum of the as-prepared LiFeSO4F (Figure 4a) showed two sets of sharp and resolved D



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Figure 5. FTIR spectra of (a) as-prepared LiFeSO 4 F, (b) Li(1−x)FeSO4F, and (c) PEDOT-LiFeSO4F, showing the difference in surface chemistry between the samples. The features positioned between 1800 and 2300 cm−1 are instrumental noise.

characteristic absorption bands37 at 1513, 1345, and 849 cm−1, not present in the other samples. In addition, X-ray photoelectron spectroscopy (XPS) was used for a thorough surface characterization of the composite material to identify the components present in the polymer layer. Figure 6 shows the deconvoluted F 1s and S 2p XPS spectra from all samples. The two spin−orbit split peaks in the S 2p spectra of the uncoated samples, as-prepared LiFeSO4F and Li(1−x)FeSO4F, showed the presence of one main component with a binding energy of 169.5 eV. This component is assigned to sulfate anions within the title compound, as it is

Figure 4. Mössbauer spectra of (a) as-prepared LiFeSO4F, (b) Li(1−x)FeSO4F, and (c) PEDOT-LiFeSO4F, showing the changes of the Fe2+/Fe3+ ratio (indicated as percentages) at different stages of the PEDOT coating procedure.

ing to stretching modes of O−H and C−H bonds, respectively. Moreover, the less pronounced features in the range of 800− 1600 cm−1 are also in good agreement with the characteristic FTIR spectrum of pure TEG.35,36 However, in the spectrum of Li(1−x)FeSO4F (Figure 5b), the characteristic bands appertaining to the TEG residues could no longer be seen, suggesting that the majority of the solvent impurities were removed from the surface during chemical oxidation and the following washing steps. Finally, the spectrum of the polymer-coated sample (Figure 5c) confirmed the presence of p-doped PEDOT, which could be identified by the appearance of

Figure 6. F 1s and S 2p XPS spectra of as-prepared LiFeSO4F, Li(1−x)FeSO4F, and PEDOT-LiFeSO4F samples. E



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Figure 7. SEM (a−c), low magnification TEM (d−f), and high magnification TEM (g−i) images of as-prepared LiFeSO4F (left column), Li(1−x)FeSO4F (middle column), and PEDOT-LiFeSO4F (right column) samples, respectively.

effects in the spectra; (i) the peaks are significantly broader for the uncoated samples, implying a higher conductivity of the coated sample,40 (ii) the peaks of the uncoated samples displayed a shoulder at low binding energies in both the F 1s and S 2p spectra, which is a common feature for poorly conducting samples.41 This tail-feature was most clearly seen for the as-prepared LiFeSO4 F, but decreased for the Li(1−x)FeSO4F sample and was practically nonexistent in the case of the PEDOT-LiFeSO4F composite, suggesting an incremental increase in electronic conductivity, in respective order. To obtain an insight into the morphological aspects of the three samples, both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used. The SEM images of the as-prepared LiFeSO4F (Figure 7a) and Li(1−x)FeSO4F (Figure 7b) sample showed sharp edged polyhedral shaped crystallites in the micrometer range. However, the PEDOT-LiFeSO4F composite (Figure 7c) was found to have lost this characteristic shape, and instead, a rough surface could be seen on micrometer range sized particles, indicating a successful polymer coverage. Furthermore, significant agglomeration of particles was observed after the PEDOT coating. It was also notably easier to collect good quality SEM images of the polymer-coated sample, in comparison to the other samples which generated image artifacts resulting from charging effects. This reduced charging effect is a further qualitative indication of significantly enhanced electronic conductivity within the composite material (conducting layers of gold or carbon are often added to poorly conducting SEM samples to obtain similarly improved SEM image quality). The interpretations of the SEM results could be further confirmed by a thorough examination using TEM. Low magnification TEM images of the as-prepared LiFeSO4F (Figure 7d) and Li(1−x)FeSO4F (Figure 7e) sample showed no evidence of surface confined species, while the active

the only expected source of sulfur in these samples. As expected, this sulfate peak was also present in the spectrum of the PEDOT-LiFeSO4F composite powder, appearing at a binding energy of 169 eV. In addition, the S 2p spectrum of the PEDOT-LiFeSO4F composite powder consists of three new peaks, indicating the presence of new surface confined species. The peaks at 164.1 and 165.8 eV are attributed to the S atom in neutral PEDOT and to cationic S+ within p-doped PEDOT,38 respectively, while the peak at 170.6 eV is attributed to the S atoms within the TFSI anion.39 In addition, the presence of TFSI could be further confirmed by the F 1s spectrum. In the case of the as-prepared LiFeSO4F and Li(1−x)FeSO4F samples, the spectra revealed one main peak at 684.9 eV. This peak is attributed to the F− anions within the title compound as it is expected to be the main source of fluorine in these two samples (except for small impurities of LiF which are also expected at this binding energy), and it was thus also found in the spectrum of PEDOT-LiFeSO4F. However, the F 1s spectrum of PEDOTLiFeSO4F showed clear evidence of the presence of TFSI anions, identified by the peak at 688.6 eV.39 Thus, the XPS results confirm the presence of partially p-doped PEDOT on the LiFeSO4F particles, as well as TFSI acting as counterions to the positively charged polymer. This is a favorable situation since, as mentioned earlier, it has been previously shown that the use of TFSI as counterion to the oxidized state of PEDOT results in coatings with high electronic conductivity.25 It is worth mentioning that the relative surface composition of the samples obtained by XPS (not presented here) revealed that the relative amount of Fe decreased from the as-prepared LiFeSO4F and Li(1−x)FeSO4F to the PEDOT-LiFeSO4F sample. Hence, this provides further evidence that a surface layer was formed on the latter. The XPS results also gave a qualitative indication that the electronic conductivity was significantly improved within the PEDOT-LiFeSO4 F composite, as compared to the uncoated samples. This is indicated by two F



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thermogravimetric trace of the reference measurement from the PEDOT-LiFeSO4F trace (Figure 8, inset), was found to be approximately 10 wt %. However, a fraction of this mass can be attributed to the expected attachment of TFSI as counterions to the partially oxidized PEDOT. Electrochemical characterization was performed on samples taken from the different stages of the coating procedure utilizing identical testing conditions (galvanostatic cycling between 2.5 and 4.2 V at a rate of C/10) for a direct comparison of their performance. The as-prepared LiFeSO4F sample (Figure 9a) displayed very high polarization, and, consequently, very low practical capacities were observed. After several cycles, a stable reversible capacity of only 10 mAh/g was achieved (Figure 9d). The Li(1−x)FeSO4F sample (Figure 9b) showed much improved performance with a significantly reduced polarization and better defined two-phase plateau region. An improvement in the specific capacity was also observed with the initial cycle achieving a specific capacity of approximately 80 mAh/g (Figure 9d). Nevertheless, this sample displayed notable capacity fading over the initial 20 cycles. The best performance was observed for the PEDOT-LiFeSO4F composite material (Figure 9c), which displayed the lowest polarization and a stable capacity of approximately 110 mAh/g (Figure 9d). One notable feature observed for this sample was a significant overpotential at the end of the first charging and discharging cycle, which corresponded to a region of approximately 25% of the theoretical capacity. The source of this behavior was not studied in detail, but it is unlikely to be related to a redox activity of the polymer coating, because the amount of PEDOT present in the composite could theoretically only contribute with a maximum of 3−4% to the total capacity (based on the theoretical maximum amount of polymer produced in the polymerization step, in combination with a maximum reversible doping level of about 0.3 electrons per repeating unit, commonly assumed for PEDOT and other closely related conducting polymers42,43). These features are, however, absent from the ensuing cycling where the observed behavior is close to that of an ideal two-phase system, with a flat voltage plateau region and sharp increases and decreases in potential at the end of charge and discharge, respectively. We believe that the significant differences in electrochemical performance seen for the three samples are related to the difference in surface chemistries, as observed by the various characterization methods (FTIR, XPS, SEM, and TEM). The poor cyclability of the as-prepared LiFeSO4F is ascribed to the adsorbed TEG residues, creating a barrier that inhibits efficient electron transfer between active material particles and the conducting matrix (in this case, carbon black) in the cathode, as shown schematically in Figure 10a. This scenario is not very surprising given the insulating nature of the TEG. After the removal of this TEG layer, as in the delithiated sample, a significantly improved electrochemical performance then was seen (a similar improvement was obtained when the TEG layer was removed from the pristine sample by extensive washing (Figures S3 and S4, Supporting Information), confirming that the performance improvement is not related to the delithiation reaction as such, but rather to the removal of TEG). However, still a low practical capacity was observed for the Li(1−x)FeSO4F sample, which most likely is related to the poor intrinsic electronic conductivity of the title compound (in combination with the relatively large particle size) and a poor electronic connectivity between electrode particles (involving active material and carbon black). A poor electronic connectivity of

material in the PEDOT-LiFeSO4F sample (Figure 7f) was clearly surrounded by polymer protuberances, bridging the particles together, as indicated by the black arrows in the figure. It is reasonable to think that such junctions could contribute in forming a percolating network, which would significantly improve the electronic conductivity throughout the electrode. Furthermore, the high magnification image of the PEDOTLiFeSO4F sample (Figure 7i) also revealed that the particles were covered by a continuous and conformal polymer layer of approximately 5−10 nm in thickness (indicated by the white arrows in the image), providing an increased intimate electronic contact area between each particle (both between LiFeSO4FLiFeSO4F and LiFeSO4F-carbon in the final electrode composite). The uncoated samples, LiFeSO4F (Figure 7g) and Li(1−x)FeSO4F (Figure 7h), did not show any evidence of such surface layer. It is noteworthy that the TEG residues found by FTIR on the as-prepared LiFeSO4F could not be detected in the TEM analysis at the given magnifications, indicating that they must have occurred as a very thin film of adsorbed molecules or only in discrete regions of the particles, which are not observed in the samples used for TEM. The gravimetric amount of polymer in the PEDOTLiFeSO4F composite material was estimated by thermal gravimetric analysis (TGA). The thermogravimetric trace of the as-prepared LiFeSO4F reference sample (Figure 8a)

Figure 8. TGA analysis of (a) as-prepared LiFeSO4F and (b) PEDOTLiFeSO4F composite. The inset shows the relative mass difference between the PEDOT-LiFeSO4F composite and the as-prepared LiFeSO4F reference sample.

showed a weight gain starting at 250 °C and continuing to 500 °C, indicating a partial oxidation of the compound. Beyond 500 °C, the material started to decompose, and a total weight loss of more than 15% was obtained at 700 °C, attributed to loss of SO2 and formation of Li2SO4, Fe2O3, FeS, and Fe2(SO4)3.6 The overall characteristics of the TGA results for the LiFeSO4F reference sample are in good agreement with previously reported TGA measurements for this material.6 The thermogravimetric trace of the PEDOT-LiFeSO4F composite material (Figure 8b) showed similar key features as the reference sample, including partial oxidation above 250 °C and decomposition of active material above 500 °C. However, in addition, a significant weight loss could also be observed starting just above 200 °C, ascribed to decomposition of the polymer coating by combustion. The amount of polymer within the composite material, quantified by subtracting the G



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Figure 9. Potential as a function of composition from galvanostatic cycling of (a) as-prepared LiFeSO4F, (b) Li(1−x)FeSO4F, and (c) PEDOTLiFeSO4F. The capacity retention (d) during the first 20 cycles for each material is also shown.

of C/20, where a reversible capacity of 122 mAh/g was observed. The performance of the PEDOT-LiFeSO4F material is that of a classic two-phase insertion compound with a very flat plateau region and very well-defined distinct and sharp endings. This is in contrast to many of the published results on tavorite LiFeSO4F6,16−18 where the cycling curves often are far more sloped in nature, and seldom are such clear end points reported. Also, the composite material showed very promising capacity retention tested at different cycling rates (Figure 11, inset), which indicates that the polymer coating maintains excellent contact to the active material during cycling, accommodating the dynamic volume changes in the electrode without significant degradation. Additionally, the rate performance of the PEDOT-LiFeSO4F composite was tested at rates ranging from C/20 to 2C (Figure 12), with at least three complete cycles at each rate (observed discharge capacities are shown in Figure 12, inset). As expected, the specific discharge capacity decreased with increasing rates, probably primarily due to mass transfer limitations of Li-ions within the LiFeSO4F structure. However, the material exhibited a promising behavior at rates up to 1C, where a capacity of just below 90 mAh/g could be retained (approximately 80% of the capacity at C/10). At a rate of 2C, the discharge capacity dropped to only 50 mAh/g due to the large increase in polarization; however, after the high rate cycling the full slow rate (C/10) capacity of 110 mAh/g could be recovered.

the electrode components is effectively leading to a significant portion of the active material being electronically disconnected from the current collector, and thus unable to undergo redox reactions (as illustrated in Figure 10b). Moreover, the immediate and steady capacity loss observed for this sample indicates that more particles lose the electronic contact within the electrode upon cycling, possibly related to the volume changes of 10.6% between the lithiated and delithiated phase (LiFeSO4F/FeSO4F).44 Finally, when cycling the PEDOTLiFeSO4F composite material, a much more ideal electrochemical performance was observed. This improvement is attributed to the increase in electronic contact area between the active material particles (provided by the intimate and continuous polymer coating), and also between active material and carbon black. The increase in contact area between the electrode components, effectively increasing the amount of contact points, can be thought of as a reduction of the electronic contact resistance, providing a more efficient charge transfer process. The intimate polymer layer also improves the electron distribution in the active material by enabling charge transfer from all around the particle, as illustrated in Figure 10c. In addition, the conducting polymer provides a more effective electronic wiring throughout the electrode, connecting the majority of the active material together and facilitating the redox reactions required to achieve the observed high capacity. This mechanism is similar to that proposed for related cathode materials coated with carbon.45 To emphasize the near ideal performance seen for the PEDOT-LiFeSO4F composite, Figure 11 shows three consecutive cycles (recorded after initial conditioning cycling) at a rate

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CONCLUSION In this contribution, we have demonstrated that controlling the surface chemistry is a very important factor when preparing H



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Figure 11. Galvanostatic cyling of PEDOT-LiFeSO4F composite at a rate of C/20. The inset shows the capacity cycling stability of the composite material at three different rates.

Figure 12. Rate capability test showing discharge curves of the PEDOT-LiFeSO4F composite at different rates. The inset shows the discharge capacity as a function of cycle number at different rates and follows the same color code as in the main plot. Figure 10. Schematic illustration of (a) as-prepared LiFeSO4F, (b) Li(1−x)FeSO4F, and (c) PEDOT-LiFeSO4F, explaining the suggested mechanisms of surface controlled performance improvements by the facilitated electron transport.

contact resistance between the electrode components and creates a highly percolating network for electron transport throughout the electrode. The PEDOT-LiFeSO4F composite material displays redox characteristics of a classic two-phase insertion compound with very well-defined sharp and distinct charge and discharge end points. This is in contrast to many previous reports where typically effects of resistance limitations are observed as sloped charge and discharge curves with significantly muted end points. Furthermore, the composite material shows promising rate capabilities, even though no emphasis was put on optimizing the particle size and morphology of the LiFeSO4F in this Article. The electrochemical and mechanical stability of the polymer coating allows for a sustained stable cycling performance (up to 100 cycles). Finally, besides emphasizing the importance of controlling the surface chemistry, this study demonstrates one way of producing high performing tavorite LiFeSO4F by using simple and cost-effective methods, which also can be easily applied to many other related materials.

high performing tavorite LiFeSO4F. Our results show that the use of the cost-effective TEG as a reaction media leaves residual solvent traces at the surface of LiFeSO4F particles, and that these severely restrict the performance, presumably as a result of their insulating nature. Moreover, we have shown that the electrochemical performance of LiFeSO4F can be significantly enhanced by removing the residual TEG and applying a surface confined conducting PEDOT coating. The coating is produced using a targeted method, which utilizes the oxidation power of the title compound. This negates the need for an external oxidation agent, which could potentially contaminate the final product and interfere with the electrochemistry. Two morphologies of the polymer deposit were observed: (i) a directly adhered, continuous, and conformal layer on the surface of the LiFeSO4F particles, and (ii) particle bridging protuberances, acting to link electrode components together. We propose that this hierarchical structure decreases the I



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ASSOCIATED CONTENT

S Supporting Information *

Detailed polymerization reaction, Rietveld refinement details, Mössbauer hyperfine parameters, FTIR spectrum, and electrochemical behavior of extensively washed pristine LiFeSO4F (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (A.S.), fredrik. [email protected] (F.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to acknowledge Sara Frykstrand for assistance during the TGA measurements, Habtom Desta Asfaw for assistance in using the SEM equipment, and Leif Nyholm and Maria Hahlin for valuable discussions regarding the properties of the conducting polymer and the XPS data, respectively. The Knut and Alice Wallenberg Foundations are acknowledged for an equipment grant for the transmission electron microscopy facilities at Stockholm University. 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).

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Understanding and Controlling the Surface Chemistry of LiFeSO

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