Electrolyte Interface of Fe2O3 Composite

Article pubs.acs.org/cm

Investigation of the Electrode/Electrolyte Interface of Fe2O3 Composite Electrodes: Li vs Na Batteries Bertrand Philippe,*,† Mario Valvo,‡ Fredrik Lindgren,‡ Håkan Rensmo,† and Kristina Edström‡ †

Department of Physics and Astronomy, Uppsala University, P.O. Box 516, SE-75121, Uppsala, Sweden Department of ChemistryÅngström Laboratory, Uppsala University, Box 538, SE-75121 Uppsala, Sweden

‡

ABSTRACT: We have investigated the properties of the electrode/electrolyte interfaces of composite electrodes based on nanostructured iron oxide cycled in Li- and Na-half cells containing analogous electrolytes (i.e., LiClO4 or NaClO4 in ethylene carbonate:diethyl carbonate (EC:DEC)). A meticulous nondestructive step-by-step analysis of the first discharge/charge cycle has been conducted via soft X-ray photoelectron spectroscopy using synchrotron radiation. In this way, different depths were probed by varying the photon energy (hν) for both electrochemical systems. The results of this thorough study clearly highlight the differences and the similarities of their respective solid electrolyte interface (SEI) layers in terms of formation, composition, structure, or thickness, as well as their conversion mechanisms. We specifically point out that the SEI coverage is more pronounced, and a homogeneous distribution rich in inorganic species exists in the case of Na, compared to the organic/inorganic layered structure observed for the Li system. The SEI formation gradually occurs during the first discharge in both Li- and Na-half cells. For Na, a predeposit layer is formed directly by simple contact of the electrode with the electrolyte. Despite using similar electrolytes, the nature of the cation (Li+ or Na+) has significant impact on the overall composition/structure of the resulting SEI. counterpart: Na-ion batteries.6 Sodium is often seen as an almost-infinite resource, since it is much easier to extract worldwide and it can be easily recycled in less-expensive ways. Many investigations have been focused so far on finding electrode materials and electrolytes that can be used in upcoming Na-ion batteries, mainly exploiting the basis of previous knowledge derived from the Li-ion battery field. However, to date, only a few works have been dedicated to the study of the solid electrolyte interface (SEI) formed on negative electrodes in Na-ion batteries.7,8 It is well-established that the stability of the SEI layer is crucial for maintaining good performances in a Li-ion battery.9,10 Moreover, the studies of this layer have given valuable information to improve the entire battery assembly and operation, e.g., electrolyte formulation (i.e., solvents, salts, use of additives), suitable voltage window for cycling, etc.11 It has been shown that electrolytes are also unstable at low potential in Na-ion cells leading to a SEI formation;12 however, few studies have been focused on this specific point. Therefore, the investigation of the SEI on Na-ion batteries is an important point that should not be neglected, especially if a deeper understanding of the electrochemical mechanisms is the key toward major advancements in this field. The SEI layer results from the degradation (and/or the reduction) of the electrolyte compounds, which forms, in this

1. INTRODUCTION Energy storage technologies, such as rechargeable batteries, have been playing an important role for the last three decades. In particular, Li-ion batteries have emerged as one of the main rechargeable electrochemical devices, which currently dominate the market of power sources for portable electronics. Since the early commercialization of Li-ion cells by Sony in 1991,1 many electrodes materials have been investigated with the aim to develop low-cost, safe, and efficient Li-ion batteries.2−5 Nonetheless, Li-ion battery technology can be considered to still be in its infancy, especially when the target set by the new markets of energy requires massive stationary electrical storage and advanced batteries for zero-emission transportation. Both applications clearly require a very large-scale production of Liion batteries with unmatched features, in terms of energy and power densities. Unavoidably, this perspective raises crucial questions about the abundance of the raw materials, e.g., lithium reserves: where, for how long, and at what cost? Mostly found in South America, lithium and its extraction processes imply costs that might sensibly increase if massive production of large-scale battery would be required. In addition, Li recycling is also another important and costly issue, which cannot be neglected in such global context. Alternative battery chemistries are currently the subject of intense investigation to possibly provide different routes toward large-scale (stationary) electrical storage. Among viable approaches, an appealing opportunity consists in replacing lithium with sodium in order to develop an analogous © 2014 American Chemical Society

Received: June 12, 2014 Revised: August 18, 2014 Published: August 19, 2014 5028

dx.doi.org/10.1021/cm5021367 | Chem. Mater. 2014, 26, 5028−5041

Chemistry of Materials

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SEI layer upon lithiation and sodiation in Fe2O3, because this will not only enable highlighting the similarities and differences for these chemistries, but it will also expand the overall limited knowledge on the surface processes occurring in negative electrodes in Na-ion batteries. The SEI layer formed on a Fe2O3 electrode in a Li-ion battery has been recently reported by Tian et al.30 This study was performed on a planar iron oxide thin film prepared by thermal oxidation, using a LiClO4−propylene carbonate (PC) electrolyte for related Li-half cells and conducting a surface characterization via depth-profiling analyses (XPS and ToFSIMS). The SEI appeared to be stable in composition upon cycling (e.g., mostly Li2CO3 with a minor amount of alkyl carbonates (ROCO2Li); however, its thickness evolved upon the first discharge/charge cycle. Conversion and deconversion processes were followed, and their depth profiling indicated that unconverted and/or partially converted material remained in the inner part of the electrodes. Consequently, the deconversion appeared to be only partially reversible. Herein, composite electrodes embedding nanostructured Fe2O3, carbon black, and a carboxymethyl cellulose (CMC) binder have been used following a previous study,28 which closely compares Li- and Na-half cells containing such active formulations. The objective of the present work is to study the interfacial mechanisms occurring during the first electrochemical cycle, with a special attention to the first discharge. The discussion will be focused mainly on the different features in Li- and Na-half cells regarding the formation and the composition of the SEI. A similar electrolyte has been used in both cases to conveniently highlight these differences. Furthermore, some information will be provided on the conversion mechanism, when access to the buried elements was possible (i.e., the SEI covering was thin enough to look through it). It is important to observe that no surface etching or any other destructive method have been employed in this investigation to achieve an extremely accurate surface-sensitive characterization of the exposed electrodes after their electrochemical cycling. A methodology based on photoelectron spectroscopy (PES) or X-ray photoelectron spectroscopy (XPS) has been used here. Classical XPS is one of the few techniques that can give detailed chemical information on surface layers (i.e., a few nanometers thick). In this work, a nondestructive depth-sensitive analysis has been carried out by changing the photon energy. In this way, the electrodes were studied with soft X-ray PES (photon energy hν = 100−1200 eV) using synchrotron radiation. The low photon energy available at the synchrotron facility allowed probing of the outermost electrode surface, i.e., ∼2−10 nm, a perfect depth range for SEI investigation. A schematic illustration of the depth dependency as a function of the photon energy can be found in a previous paper.17

way, a passivation layer on the surface of the negative electrode in a Li-ion battery.13,14 In this framework, the first questions that would naturally arise are: (i) Does the knowledge acquired on the SEI layer for the Li-ion systems apply to analogous Naion batteries? (ii) Can it be directly transferred to the Na-ion technology? (iii) If similar materials and electrolytes are to be used, can we expect to simply get the Na+-related species, instead of Li+ ones, or could different phenomena be expected? The SEI is a complex system and most of the studies on Liion batteries suggest that this layer form on different types of negative electrodes (i.e., graphite,15,16 Li-alloys (Si,17−19 Sn), conversion materials (CoO20) or intermetallics (AlSb21,22 MnSn223)), as the electrolyte is thermodynamically instable below 0.8 V vs Li+/Li. The SEI formed in all above-mentioned examples exhibits many similarities when an analogous electrolyte is used, and, in all the cases, it appears to be dominated by the presence of degradation products of the solvents. In this scenario, a first approach would be to focus on a particular type of negative electrodes and study their behavior in both Li- and Na-half cells, where similar nonaqueous liquid electrolytes with alkyl carbonates (ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), etc.) can be used as solvents. Under such conditions, could analogous species, similar formation processes and species distribution within their respective SEI be expected for such different chemistries? In addition, the same negative electrodes are exposed to a potential that is less reductive in a sodium system than in an analogous lithium cell, since the standard redox potential of Li+/Li and Na+/Na are −3.04 V and −2.71 V vs SHE, respectively. Consequently, can a less-pronounced SEI be expected in a Na cell versus an analogous Li battery, when it comes to the overall coverage of the electrode? In this respect, it is now well-known that the SEI formation is voltagedependent and that it occurs at low potential during the entire discharge process for Li half cells.24 Clearly, all these questions and issues must be addressed. The lack of studies on the SEI layer for Na-ion batteries most probably comes from the fact that, so far, only a few efficient negative electrode materials have been proposed as possible candidates. Among these studies, the SEI formed on a hard carbon electrode in a full cell device has been analyzed by Komaba et al.7 and more recently by Ponrouch et al.8 Moreover, the electrochemical performance for the insertion compound Na2Ti3O725 has been shown to be severely limited by surface phenomena related to the SEI instability.26 In the present study, a typical conversion material (i.e., iron oxide) has been investigated as another model example. This compound is interesting because of its ability to reversibly react with both Li+ and Na+ ions27,28 undergoing a characteristic conversion reaction at low voltage, vs Li+/Li and Na+/Na, respectively. This electrochemical mechanism enables a higher electrical storage in this compound, when compared to other insertion hosts.29 Iron oxide is also abundant, inexpensive, and nontoxic, thus potentially fitting the opportunity of developing large-scale batteries with contained costs and a limited environmental footprint, provided that other technical issues can be fixed (e.g., poor energy efficiency per cycle, limited initial reversibility). In particular, insights for the surface phenomena occurring in a Na+-based electrolyte are totally lacking so far for this compound and, more in general, for negative electrodes based on conversion-type materials. Therefore, it is important to study and compare the formation of the

2. EXPERIMENTAL DETAILS 2.1. Synthesis of the Active Material and Electrode Preparation. Nanostructured iron oxide with a porous morphology was synthesized via the pyrolysis, under vacuum, of anhydrous iron acetate (Aldrich), according to a preparation earlier reported.28 The electrode coatings consisted of a mixture of nanostructured Fe2O3, NaCMC (Aldrich, Mw ≈ 700 000, D.S. 0.90), and carbon black (CB) (Super P, Timcal Graphite & Carbon) having a Fe2O3/CMC/CB weight ratio of 65:15:20. The resulting paste, which was obtained after 1 h of ball milling, was casted on a copper foil by a dedicated coating equipment (KR-K Control Coater) and dried overnight in a convection oven at 80 °C. Coated disks with a diameter of 10 mm 5029



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were optimized by a weighted least-squares fitting method using CasaXPS software, 70% Gaussian and 30% Lorentzian line shapes, if not otherwise stated. The spectra were energy-calibrated by the hydrocarbon C 1s peak set to 285.0 eV after curvefitting. The time period between the electrochemical cycling and the PES measurements was approximately the same for all the cells presented in this work (5−6 days). Before PES characterization, the potential of each cell was checked to ascertain that no short circuit or major selfdischarge had occurred during this time lapse. Then, each electrode was carefully separated from the rest of the battery components in an argon-filled glovebox and washed thoroughly with dimethyl carbonate (DMC, Novolyte Technologies) in three successive baths in order to remove any remaining electrolyte species on the surface of each sample. For each bath, the electrode was put into 2 mL of DMC of ultralow water content in a clean and dry aluminum container, maintaining a mild manual agitation for 1 min. Then each electrode was removed from the container and put into the next one. After the third bath, the specimens were quickly dried and mounted on a sample holder for the PES analysis. The samples were transferred from the glovebox to the spectrometer via a stainless steel transfer system directly connected to the load-lock chamber of the end-station.34 In this way, it was possible to move the samples in a completely inert gas atmosphere, avoiding moisture/air exposure and thus possible surface contamination.

(i.e., to easily fit in the sample holder for the synchrotron measurements) were cut by a precision perforator (Hohsen). The electrodes were dried at 120 °C for 12 h in a vacuum oven (Büchi) placed in an Ar-filled glovebox (O2 < 1 ppm, H2O < 1 ppm) before cell preparation. 2.2. Cell Assembly and Electrochemical Measurements. The electrodes were assembled in polymer-laminated aluminum pouch cells (i.e., “coffee bags”) in an Ar-filled glovebox (M-Braun) having oxygen and moisture levels below 1 ppm. Different types of batteries were prepared with the same coated disks as working electrodes: (i) Li-half cells having a Li foil as the combined reference and counter electrode and (ii) Ns-half cells, where metallic Na played an analogous role. Different salts were dissolved in EC:DEC (2:1) to obtain 1 M LiClO4 electrolyte for the Li half cells and 1 M NaClO4 electrolytic solution for the Na half cells, respectively. A thin separator (Solupore) was imbued with the respective electrolytes and placed between the electrodes in the corresponding cells. Cyclic voltammetry (CV) was performed via a VMP2 equipment (Bio-Logic) by applying a scan rate of 0.05 mV s−1 in the voltage range between 0.05 and 2.80 V vs Li+/Li or vs Na+/Na for both half cells. Each cell was discharged and charged to a predefined potential during its reduction/oxidation scan and stopped at that particular voltage. Selected states of charge for both Li- and Na-half cells were as follows: the open circuit voltage (OCV) corresponding to an uncycled battery, the upper and the lower cutoff limits (i.e., 2.80 V upon oxidation and 0.05 V upon reduction), and 1.40 V during the reductive part of the scan. Moreover, 0.90 V vs Li+/Li and 0.40 V vs Na+/Na were specifically chosen for the respective reductive half cycles for these batteries. The upper and the lower cutoff voltages will later be referred to as “end of charge” (EOC) and “end of discharge” (EOD), respectively. After cycling, each cell was disconnected and its metal contacts were protected by an adhesive tape to avoid any shortcircuiting during transportation. 2.3. Soft X-ray Photoelectron Spectroscopy. Photoelectron spectroscopy (PES) measurements were carried out at the Beamline I41131 at the Swedish National Synchrotron Facility MaxIV Laboratory in Lund, Sweden. This spectroscopy can be referred to as soft-X-ray PES, as the end-station is provided with a usable photon energy range from 50 eV to 1500 eV. The photon energies were selected using a modified Zeiss SX-700 monochromator, and the photoelectron kinetic energies (K.E.) were measured using a Scienta R4000 WAL analyzer. The electron take-off angle was 70°, and its direction was collinear with the e-vector of the incident photon beam. The pressure in the analysis chamber was ∼10−7 mbar. The samples were sufficiently conductive, so no charge neutralizer was applied during the PES measurements. The depth sensitivity in the PES measurements depends on the inelastic mean free path (IMFP) of the photoelectrons, which is related to their kinetic energy (see, for example, ref 17). Therefore, changing the photon energy will modify the depth sensitivity. Furthermore, the IMFP is slightly dependent on the materials of interest, and values for polyethylene were used in this work32 as a rough approximation for the IMFP of the SEI. The depth sensitivity values reported in the present work were defined as three times the IMFP of the photoelectron, since 95% of the PES signal in a homogeneous material comes from a layer with this thickness. Moreover, the decrease in the signal related to carbon black and/or Fe core level peaks, together with the IMFP for the SEI, can be used for a rough estimation of the SEI coverage. The measurements were carried out in such a way that the same depth sensitivity was obtained for all recorded spectra; thereby, the same photoelectron kinetic energy (Ek) was set for all probed elements. In particular, two depths were investigated corresponding to distinct kinetic energies of 130 and 590 eV (for C 1s, a third one was used, Ek = 840 eV). This approach was possible by tuning the photon energy (hν) for each probed element (hν = Eb + Ek). Thereafter, merely the photon energy used for the different spectra acquisitions will be mentioned. Overview spectra and core peaks were measured with a pass energy (Ep) of 200 eV. The core peaks were analyzed using a nonlinear Shirley-type background.33 The peak positions and areas

3. RESULTS AND DISCUSSION 3.1. Electrochemical Analysis. Figure 1 shows the cyclic voltammograms of the first discharge/charge cycle of a composite nano-Fe2O3 electrode tested in a Li-half cell (Figure 1a) and in a Na-based one (Figure 1b), both between 0.05 and 2.80 V vs X+/X with X = Li and Na. We can note that 0.05 V vs Na+/Na would correspond to ∼0.38 V vs Li+/Li. These measurements were intentionally performed at a low scan rate (0.05 mVs−1) to facilitate tracking of the various electrochemical reactions, even in the presence of sluggish kinetics, thus leaving enough time for the characteristics processes to occur. The voltammograms obtained in this study are similar to those presented in an earlier work.28 However, the lower scan rate applied here (0.05 mV s−1 instead of 0.1 mV s−1) allows better resolution of the electrochemical features, e.g., moredefined peaks, especially upon reduction. An exhaustive analysis of the materials and their associated reactions has already been performed;28,30 therefore, in this section, only the main characteristics will be briefly mentioned. The first half cycle of discharge (Li+ uptake) for the Li-half cell (Figure 1a) exhibits three main reductive features (inflection or peak). The first one at 1.55 V vs Li+/Li is associated with the formation of the insertion compound LixFe2O3 (0 < x < 2). The second one, at ∼0.90 V vs Li+/Li, corresponds to the early stage of iron oxide reduction coupled with structural changes, while the major peak at ∼0.50 V is a combination of the conversion reaction of Fe2O3 with the process of electrolyte degradation (i.e., the SEI layer formation). During the following half cycle of charge (Li+ removal), two oxidation peaks can be observed at ∼1.60 V and ∼1.80 V, which are attributed to the sequential oxidations Fe0 → Fe2+ and Fe2+ → Fe3+, respectively. The overall electrochemical reaction is given in eq 1: Fe2O3 + 6Li ↔ 2Fe + 3Li 2O

(1)

The voltammogram of the Na-half cell shown in Figure 1b is different from the one presented for the Li-half cell analogue. During the half cycle of discharge (Na+ incorporation), two features are clearly visible: first, an inflection at ∼0.9−1.0 V vs 5030



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SEI formation and the conversion reaction, is observed in the Li system, whereas only a weak and broad peak, attributed to similar phenomena, can be seen in the reaction with Na+. This circumstance suggests that the properties of the respective SEI layers might be different. Therefore, the SEI formation and its evolution have been investigated through a step-by-step PES analysis of the first CV cycle, which are discussed in detail in the following section and subsections. 3.2. PES Analysis. The first part of this section is devoted to the study and the comparison of the respective SEI layers: their formation and evolution, the determination of their main species, and their surface dependency obtained by variation of the depth sensitivity, as previously explained. It is important to mention that we used the same solvent and perchlorate anion in the electrolytes in the Li- and Na-half cells, i.e., 1 M (X+, ClO4−) in EC:DEC with X = Li or Na. 3.2.1. Study of the SEI Layers. (a). Carbonaceous Species of the SEI: Li-Half Cells. The spectra related to C 1s of the pristine composite electrode are shown in Figure 2a, as a function of the photon energy and, thus, depth sensitivity (hν = 415 and 880 eV). The spectra contain four components; the first peak, with a binding energy at 284.2 eV (black), corresponds to the carbon black (CB). The peak at 285.0 eV (white) is assigned to hydrocarbons (C−C and C−H bonds). Since the pristine electrode materials do not contain any hydrocarbon sources, this peak is due to surface contamination, which is commonly observed during PES measurements. The two peaks at higher binding energies, 286.5 and 288.5 eV (light gray and white) are attributed to the C−O and OC−O (−CO2) carbon environments found in the structure of the CMC binder. When the photon energy increases from 415 eV to 880 eV (i.e., access to a higher probing depth), a relative increase of the intensity of the peak assigned to carbon black can be observed, whereas the (C−C, C−H) peak decreases and the ratio −CO2/ −CO remains stable. These features are related to the surface concentration dependency of the different materials: carbon black is covered by the binder (≤2 nm), together with some hydrocarbon contamination which is expected in samples prepared in a non-UHV environment. The evolution of the C 1s spectra during the first electrochemical cycle in Li-half cells is presented in Figure 2b. The data are sorted horizontally as a function of the photon energy (hν = 415, 880, and 1125 eV) and vertically as a function of the cycling step. The C 1s spectra corresponding to the OCV state of the electrode are very similar to those of the pristine material previously commented, thus suggesting that no crucial modifications occur by simple contact at the electrode/electrolyte interface after 7 days from the initial preparation of the cell. The presence of a weak additional peak at a higher binding energy ∼290.0 eV (dark gray), however, can be observed and assigned to a −CO3 carbon environment (e.g., carbonates). This component results from some species adsorbed at the electrode surface due to the contact with the electrolyte. Besides, the signal associated with the carbon black continues to increase when a higher photon energy (hν = 1125 eV) is applied, as mentioned earlier. During the first half cycle (lithiation from point A to point C in Figure 2b), significant changes of the C 1s spectra are visible, in correspondence of the initial reaction step (point A, 1.4 V vs Li+/Li). In particular, the intensity of the carbon black component (in black) gradually decreases during the discharge;

Figure 1. Cyclic voltammograms of the first cycle for the composite nano-Fe2O3 electrode between 0.05 and 2.80 V, (a) in 1 M LiClO4, EC:DEC (2:1) vs Li metal, and (b) in 1 M NaClO4, EC:DEC (2:1) vs Na metal. The samples analyzed by PES are highlighted by gray (Li half-cell) and blue (Na half-cell) symbols, each one referring to a particular type of battery. OCV (i.e., uncycled) samples are marked by open circles, whereas cycled samples are represented by solid points.

Na+/Na, followed by a peak at ∼0.55 V, while only a single peak was observed (∼0.5 V) when a higher scan rate was applied (i.e., 0.1 mV s−1).28 These differences are likely due to the use of a lower scan rate, which here enables the detection of the slight inflection at ∼1.0 V. The peak at 0.55 V vs Na+/Na corresponds to the conversion of the iron oxide coupled with the SEI formation, while the first inflection can be correlated to the early stage of Fe2O3 reduction and related structural changes. A tiny inflection can also be observed at higher potential (∼1.4 V) and associated with the formation of a NaxFe2O3 phase. During Na+ release, two oxidation features can be detected, at ∼1.0 V and ∼1.4 V vs Na+/Na and similarly attributed to the two-step oxidation Fe0 → Fe2+ and Fe2+ → Fe3+. Also, a third faint feature is seen at ∼2.0 V, which could be due to completion of the oxidation process of some particles having different sizes, because of the previous conversion step. Despite the different overall aspect for the two voltammograms in Figure 1, it can be observed that most features can be attributed to similar electrochemical processes, thus suggesting a global reaction for this system such as that of eq 2: Fe2O3 + 6Na ↔ 2Fe + 3Na 2O

(2)

The noticeable difference observed upon the first half cycle of discharge between Li- and Na-half cells points directly at the different interface phenomena that might occur in these two systems. Indeed, an intense reduction feature, mainly due to the 5031



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Figure 2. (a) C 1s spectra of the Fe2O3/CMC/CB pristine electrode measured as a function of the analysis depth (hν = 415 and 880 eV). (b) Evolution of C 1s spectra of the Fe2O3 composite electrode at different steps of the first discharge/charge when cycled vs Li+/Li. Evolution as a function of the analysis depth (hν = 415, 880, and 1125 eV). Note that the corresponding points are reported in the CV cycle in Figure 1a.

reach a maximum thickness roughly estimated below 6 nm (estimation explained in the Experimental Section). This percentage slightly increases during the following charge (delithiation) suggesting that a partial dissolution occurs when Li+ is removed. Nevertheless, the formation of the SEI remains mainly irreversible. Looking back to Figure 2, it can further be noticed that, at the lowest depth sensitive analysis (hν = 415 eV), no carbon black peak is detected at the EOD and EOC, indicating that no uncovered areas remain on the pristine electrode. Furthermore, it is worth mentioning that, during the discharge, the binding energy of the carbon black peak gradually decreases and reaches 283.0 eV at the end of the discharge. This phenomenon is related to the gradual formation of LixC with x increasing

this behavior is clearly visible at the largest analysis depths (i.e., hν = 880 and 1125 eV). Figure 3a shows the evolution of the relative intensity of the carbon black peak in the C 1s spectra recorded with 880 eV during the first electrochemical cycle in the Li-based system. The signature of the carbon black originates from the electrode materials and not from the SEI, contrary to the other components found in the C 1s spectra. Consequently, the behavior of its intensity gives a rather good indication of the covering by the SEI and its evolution. It can be seen that its percentage declines during the discharge, because of the progressive SEI covering of the electrode. Carbonaceous species are gradually deposited throughout the first lithiation and the coverage gets thicker until the end of the discharge, to 5032



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Apart from carbon black, also other contributions can be observed in Figure 2. Indeed, the components of some new carbonaceous species gradually overlap the peaks of the pristine electrode (CB and CMC) up to the point C (0.05 V vs Li+/Li). These species exhibit SEI-characteristic C 1s signatures located at 285.0, 286.5, 288.5, and 290.0 eV, which can be assigned to hydrocarbon and carbons bonds to one, two and three oxygen atoms, respectively. These new species result from solvent reduction (i.e., EC and DEC) byproducts, as suggested by previous investigations on the reduction mechanisms of classical carbonate-based solvents and their role in the SEI formation and composition.38−43 The −CO3 carbon environment (290 eV) is due to carbonate species, such as Li2CO3 or alkyl carbonates ROCO2Li, which are commonly found in the SEI of most of the other negative electrodes. The peak at 288.5 eV (−CO2) and 286.5 eV (−CO) cannot be attributed only to the CMC binder, since the ratios (−CO2/carbon black) and (−CO/ carbon black) clearly increase upon the discharge, in agreement with a deposition of new species containing −CO and −CO2 carbon environments. The possible existence of lithium oxalate44 and/or the formation of RCOOLi species were suggested in the literature to explain the −CO2 component in the SEI generated on various materials, such as other conversion materials (e.g., CoO20 or, more recently, CuO45). The C 1s core peak assigned

Figure 3. Evolution of the relative intensity of the carbon black component measured in the C 1s spectra (hν = 880 eV) during the first discharge/charge cycle (i.e., C 1s (carbon black)/C 1s (total)) (a) in the Li system and (b) in the Na system. The corresponding points are shown in the voltammograms in Figure 1.

during the discharge and decreasing during the charge. This evolution was also observed on other negative materials such as graphite,35 Si,17 MnSn2,36 or Ni3Sn4.37

Figure 4. Evolution of C 1s spectra for the composite Fe2O3 electrode cycled vs Na+/Na at different voltage stages during its first discharge/charge. Evolution as a function of the analysis depth (hν = 415, 880, and 1125 eV). The points (I, II, etc.) refer to those indicated in the voltammogram in Figure 1b. 5033



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The −CO3 (dark blue), which is barely observed in the Lirelated OCV electrodes, is indeed significant here. In addition, the peak assigned to the carbon black displays an intensity that is strongly attenuated when compared to the analogous Li system. This fact indicates that a thin layer is readily deposited on the composite electrode after simple contact with the electrolyte. The overall behavior of the SEI during the first electrochemical cycle of the Na-half cells can be again evaluated by looking at the evolution of the carbon black peak (black). The histogram presented in Figure 3b shows the evolution of the relative intensity of the carbon black peak in the C 1s spectra during the first electrochemical cycle for the Na-based system (hν = 880 eV). A gradual decrease of the percentage can be observed upon discharge, whereas no carbon black could be detected at the EOD. However, its contribution can be noticed at the EOC. As mentioned above for the Li system, the SEI gradually grows upon the discharge and a slight dissolution occurs for it during the subsequent charge. Surprisingly, the relative carbon black intensity values regarding the Na system are much lower than those for its Li counterpart, starting from the early OCV stage (see Figure 3a). Two important results can be extracted from this preliminary comparison. First, the deposited layer is more pronounced in the Na-half cells after a full discharge and its thickness can be roughly estimated above 7 nm at the EOD. Second, a predeposited layer covers the composite electrode in the Na system after a simple contact with the electrolyte. From one voltage stage to another in Figure 3b, the variation of the percentage is ∼1%−2% for Na, while it is ∼2%−7% for Li (Figure 3a), showing that the formation of the SEI occurs largely during the first electrochemical discharge in the Li-half cells. By contrast, a substantial surface layer is present for the Na system even before the electrochemical cycling. Still, an additional layer is formed onto the “predeposit” during the initial discharge. This particular point can be put in relation to the CV profiles presented in Figure 1 and, in fact, highlights why no prominent reduction peak was observed for the Na-half cells. It is important to mention that the histograms in Figure 4 do not take into account the effect of the CMC binder. The binder components are, in fact, overlapped by the SEI contributions; thus, it is difficult to take into consideration the mixing and contribution of the electrode covering by the CMC. However, as the pristine electrodes used in the Li- (Figure 4a) and Na(Figure 4b) half cells are taken from the same batch, the binder contribution is very similar for both systems and, consequently, the difference observed can only originate from their distinct SEI features. The other C 1s components presented in Figure 4 are assigned to the carbonaceous species found in the SEI. Previous works on hard carbon by Komaba et al.7 and Ponrouch et al.8 suggest that the species formed on this type of electrodes are similar in both Li-ion and Na-ion batteries, as they result from similar solvent (decomposition). The −CO3 peak can be assigned to Na2CO3 and/or alkylcarbonates ROCO2Na. The −CO2 environment might be attributed to oxalate and/or ester linkage (RCOONa), while the peak at 286.7 eV, due to −CO carbon environments, result from ROCO2Na, PEO (−CH2− CH2−O−)n chains or alkoxides species (RONa). Finally, the peak at 285.0 eV can be assigned to the common hydrocarbon contamination, as well as to other hydrocarbon chains found in

to −CO (286.5 eV) is explained by alkyl carbonates ROCO2Li; however, other species, such as lithium alkoxide (ROLi) and/or PEO (−CH2−CH2−O−)n are often reported.34,41 The main component at 285.0 eV in Figure 2b is attributed to hydrocarbon contamination, as previously mentioned; still, it could also be related to organic species containing hydrocarbon chains (CHx−) possibly found in the earlier discussed species (R−COCO2Li, R−COOLi, and R−COLi) or in polymers. The latter has been proposed by Peled et al.46,47 The C 1s spectra presented by Tian et al.30 displayed many more carbonates species and less hydrocarbon chains than in the present study. Such difference can be explained by the different solvent used here (EC: DEC), while only PC was utilized in their work. Considering the global evolution of the C 1s spectra upon cycling at a given depth sensitive analysis, it can be seen that the related signal gradually evolves from the OCV to the EOD. Nevertheless, the overall shape remains similar until the EOC, apart from very weak fluctuations. This evolution supports the idea that the SEI formation is potential-dependent during the first discharge and that it grows continuously. Moreover, once completely formed (i.e., EOD), the SEI layer is stable at a given depth. In this respect, it is interesting to consider the depth dependency of the carbonaceous species by looking at the C 1s signal evolution, as a function of the depth sensitivity, when the SEI layer is totally formed (i.e., EOD) and after a full cycle (i.e., EOC). It is clear that the peak at 285 eV is predominant at the most surface sensitive analysis (415 eV) and that it decreases as a function of the probing depth. This suggests that the organic species containing the CHx− chains are mainly present in the outer part of the SEI. At the same time, a slight increase of the (−CO3/−CO2) and (−CO3/−CO) ratios between hν = 415 and 880 eV can be noticed, thus indicating that Li2CO3 is likely found at a deeper level (i.e., in the inner part of the SEI layer). The overall shape of the C 1s core peak is more stable between hν = 880 and 1125 eV, suggesting a relatively good homogeneity of the carbonaceous compounds deeper within the SEI. Interestingly, a comparable species distribution has been observed on graphite48 or silicon18 by using a similar nondestructive depth-sensitive analysis approach. These preliminary results regarding the carbonaceous species, as well as the formation and the evolution of the SEI during the first cycle, are similar to those previously reported for other negative electrodes in Li-ion batteries. Indeed, the SEI formation on the composite Fe2O3/CB/CMC electrodes is proven here to be an irreversible process that occurs during the initial electrochemical discharge (i.e., Fe2O3 reduction), although a slight dissolution of this layer is also observed upon subsequent charge (i.e., Fe0 oxidation). The thickness of this SEI does not exceed 6 nm and its composition is slightly heterogeneous, with an outer part richer in species containing hydrocarbon chains, while carbonates are mainly found in proximity of the electrode material. (b). Carbonaceous Species of the SEI: Na-Half Cells. The evolution of the C 1s spectra for the Fe2O3 electrode during the first electrochemical cycle in Na-half cells is presented in Figure 4. The data are sorted in the same way of that reported in Figure 2b. The C 1s spectra associated with the Na-related OCV electrode contain four main peaks at 285.0, 286.7, 288.5, and 289.8 eV, which can be assigned, respectively, to hydrocarbons, −CO, −CO2, and −CO3 carbon environments, as described above. These spectra are clearly different from the signature of the Li-associated OCV electrode (see Figure 2b). 5034



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Figure 5. (a) C 1s spectra of the Fe2O3 composite electrode after one CV cycle (EOC) measured with a photon energy of 1125 eV. Comparison between the Li (gray-shaded) and the Na (blue-shaded) systems. The spectra are normalized versus the covered carbon black peak in order to be compared. (b) Relative intensities of the C 1s components detected after the first discharge (EOD) and the first charge (EOC) for the Li and the Na systems at hν = 880 eV. The carbon black component has been excluded in order to compare only the other carbonaceous species found in the SEI.

present study. As we indicate in the case of Li, which is here compared to the work of Tian et al.,30 such a difference is most likely due to their use of PC as the solvent, while EC:DEC has been used in our study. Figure 5a compares the C 1s spectra of the Fe2O3 electrode at the end of a full CV cycle vs Li+/Li (in gray) and vs Na+/Na (in blue). The data show the greatest depth sensitive analysis (hν = 1125 eV) in order to observe the carbon black peak. In this way, these spectra can be normalized versus this component (in black). This figure clearly shows that a more prominent deposited layer is observed in the case of the Na-half cells (i.e., blue area ≫ gray area). As discussed earlier, analogous species can be found in the SEI of the two systems; however, it is clear from this picture that their distribution/ proportion within their respective SEI are different, as indicated by the distinct intensity ratios of the four components (Figure 5b). The histograms in Figure 5b illustrate this difference and show the relative intensity of the four main components (i.e., 285.0, 286.5, 288.5, and 290.0 eV) of the C 1s core peak related to the carbonaceous species of the SEI in both the EOD and the EOC stages for the Li (gray-shaded) and the Na (blueshaded) systems, respectively. First of all, it is worth noticing that, for a given system, the histograms in both the EOD and the EOC are rather similar. However, this is not the case if one closely compares the two systems. The proportion of the species containing a −CO2 carbon environment (oxalates and or ester) is similar, still the Na system exhibits less C−C (−(CH2)x− chains) but more −CO (PEO chains and ROX) and −CO3 (X2CO3 and/or alkyl carbonates ROCO2X with X = Li or Na). This first part of the study devoted to the SEI formation, with a special emphasis to the carbonaceous species, has shown that, in both Na and Li systems, a major part of the SEI results from the solvent degradation. Nevertheless, the nature of the cation

the species detailed earlier (i.e., R−COCO2Na, R−COONa, R−CONa, or a polymer phase (−CH2−)n). It can be observed that the peak at 285.0 eV decreases in intensity at the beginning of the discharge (from I to II in Figure 4) and then it increases during the charge. Despite this feature, which is mainly observed at the outermost surface (hν = 415 eV), only minor fluctuations are detected with an overall, almost-stable signature at a given depth sensitive analysis. Besides, the composition in carbonaceous species is close to that of the initial deposited layer visible in the OCV stage. Considering the depth dependency of these species at the EOD and the EOC, an increase of the −CO3 peak can first be noticed compared to those related to −CO and −CO2, especially between hν = 415 eV and hν = 880 eV. A similar evolution is observed for the Li system and, consequently, Na2CO3 appears to be found preferably in the inner part of the SEI, closer to the electrode. Nevertheless, the other components remain very stable at the three depths investigated, especially the peak at 285 eV compared to the other ones. This last feature is very different from its Li analogue, where the peak at 285 eV was predominant and a surface dependency was clearly observed. In the Na system, the −CO component is predominant in all probed depths. With respect to the Na system, this point suggests that (1) we do not have an outer SEI part rich in organic species (e.g., containing −CHx− chains), and (2) the SEI is much more homogeneous and mostly contains inorganic species, together with a minority of homogeneously dispersed organic components. However, some depth distribution also exists in this system and Na2CO3 is much more present at a deeper level in the SEI. These results are in good agreement with those earlier observed for hardcarbon electrodes7 and based on both XPS and time-of-flight secondary-ion mass spectroscopy (TOF-SIMS) measurements. However, we can note that, in the C 1s spectra presented by Komaba et al.,7 many more carbonates are detected than in the 5035



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(Li+ or Na+) has a significant impact on the stability of the solvents and on their ultimate degradation products. (c). Electrolyte Salts and Their Degradation Products: Li vs Na. As earlier mentioned, LiClO4 and NaClO4 were used as electrolyte salts in the respective Li- and Na-half cells. The degradation of these two salts can be observed through the signature of the Cl 2p core peak. Figure 6 shows the Cl 2p

probed depths. In the case of the Na-half cells (Figure 6a), a significant contribution from the degradation products (i.e., white and light blue components) is present in the OCV electrode surface and it keeps increasing during the CV cycle. Similar to the carbonaceous species of the SEI, the degradation of the salt and the deposition of the resulting products occur mainly during the discharge in the Li system, different from the Na system, where these processes occur before cycling. In both cases, after one cycle, the degradation products of XClO4 (X = Li or Na) are found homogeneously within the SEI layer, as the overall signal is maintained at the two depths (hν = 340 and 800 eV), thus suggesting that no particular distribution of these species exists over the SEI thickness. At this point, it is worth trying to roughly estimate if one system contains more salt and salt degradation products than the other by looking at the intensity ratios of (Cl 2p)/(C 1s), in order to complete the comparison of the separately formed SEI layers in the Li- and Na-based systems. The ratios presented in Table 1 are not corrected by the analyzer transmission Table 1. Ratios between Cl 2p and C 1s Intensities (Cl/C) Measured at the EOD and the EOC for Both Li- and Na-Half Cellsa Relative Intensity Ratio, (Cl 2p/C 1s) [× 10−2] Li system Na system

end of discharge (EOD), 0.05 V

end of charge (EOC), 2.8 V

2.45 4.60

3.87 6.48

a

The data have been extracted from the overview spectra measured with photon energies of 835 eV. Figure 7 illustrates, in a schematic way, how these ratios have been calculated for the respective systems.

function; however, the different photoionization cross section for Cl and C have been taken into account and estimated using database values.50 These values can be used in a convenient way to compare their respective surface coverage. The way in which Table 1 has been obtained is illustrated in Figure 7. Table 1 shows that the Cl/C ratio is higher in the Na system than in its Li counterpart by a factor of 2, independent of their state of charge (EOD or EOC). This significant difference

Figure 6. Cl 2p spectra of the Fe2O3/CMC/CB electrode in the OCV stage and after the first CV cycle (EOC) measured as a function of the analysis depth (hν = 340 and 800 eV) for (a) the Li system and (b) its Na analogue.

spectra in the OCV stage and after one CV cycle (EOC), as a function of the photon energy (hν = 340 and 800 eV) for the Li-half cell (Figure 6a) and the Na-half cell (Figure 6b), respectively. A Cl 2p peak is typically composed of two components, due to spin−orbit splitting (2p3/2 and 2p1/2 with ΔE = 1.7 eV). Here, only the binding energy of the 2p3/2 peak will be discussed. The Cl 2p spectra display two main components: the characteristic signature of the electrolyte salt at ∼208.4 eV (i.e., LiClO4 (dark gray shading in Figure 6a) and NaClO4 (dark blue shading in Figure 6b)) and a component at a lower binding energy (∼198.5 eV) assigned to Cl species with an oxidation state of −1, e.g. LiCl (shaded light gray in Figure 6a) and NaCl (highlighted by light blue shading in Figure 6b). A weak component at ∼206.5 eV can also be observed, which is most probably due to XClO349 (oxidation state of +5) with X = Li or Na. When considering the Li-half cells (Figure 6a), the OCV electrode already shows some degradation products. The latter are mainly located at the outermost surface, as the peaks assigned to LiCl and LiClO3 decrease in intensity or disappear going from hν = 340 eV to hν = 800 eV. After a full CV cycle, the intensity of the LiCl peak significantly increases at both

Figure 7. Enlarged view of an overview spectrum for the EOC of a Fe2O3 composite electrode in a Li-half cell, hν = 835 eV. The ratios presented in Table 1 have been obtained by the ratio of the Cl 2p core peak intensities (light gray area) by those associated with the C 1s (dark gray area) after a correction by their relative photoionization cross sections (σ). 5036



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suggests that the SEI covering the electrode in the Na-half cells is not only thicker but also richer in inorganic components resulting from the salt degradation. Furthermore, it can be noticed that this ratio rises from the EOD to the EOC for both the systems. This trend is in direct relation with the partial dissolution of the SEI occurring during the charge observed earlier in this paper and indicates that the dissolution processes mainly affect the carbonaceous species of the SEI. 3.2.2. The Conversion Reaction. So far, the main part of this study has been focused on the investigation of the SEI and the comparison of its properties in Li- and Na-half cells embedding composite Fe2O3/CMC/CB electrodes with similar electrolytes. Nonetheless, valuable information about the conversion mechanisms of iron oxide can be obtained as well, especially if the access to the buried elements composing the electrodes is possible without using a destructive technique (e.g., argon sputtering). The following part of the discussion is aimed at conveying such information and comparing the related findings with the results reported in the literature, which are still controversial and highly debated. (a). O 1s Core Peak. The O 1s spectra can provide information on both the SEI and the conversion/deconversion reactions occurring during the first electrochemical cycle. The spectra of the pristine electrode are presented in Figure 8a, as a function of the depth sensitivity (hν = 665 and 1125 eV). Three components can be detected, the peak with the lowest binding energy (∼530 eV in red) is assigned to Fe2O3. The two components at ∼531.8 and ∼533.0 eV (white area) are attributed to the −CO and −CO2 oxygen environments found in the CMC structure, in good agreement with the C 1s spectra shown in Figure 2a. When the photon energy increases (i.e., access to higher depth sensitivity), the intensity of the Fe2O3 peak rises compared to the signature of the CMC, which confirms that the binder effectively covers the active material and also the conductive additive, as shown earlier. The evolution of the O 1s spectra of the Fe2O3 composite electrode during the first electrochemical cycle is presented in Figure 8b, where a photon energy of 1125 eV has been used. The spectra related to the Li-half cells are framed by a gray line, whereas those associated with Na-half cells are enclosed by a blue frame. Considering first the Li system, it can be noticed that the OCV spectrum is similar to that associated with the pristine electrode, as mentioned earlier very few species are deposited on the electrode at this stage. The overall shape of the main peaks contribution (shown in white in Figure 8) evolves upon cycling due to the gradual formation of the SEI. At the EOD, the two peaks found at 532.0 and 533.5 eV are in good agreement with the presence of carbonates, alkylcarbonates, and others organic species detected on the C 1s spectra and usually found in the SEI of Li-ion devices.51 A third peak, at ∼531.0 eV, appears at the EOD and remains during the charge. The latter can be assigned to LiOH resulting from the presence of traces of water retained in the CMC binder, as suggested by Marino et al.52 This last component is present in the inner part of the SEI (close to the electrode), as its intensity is very weak when the samples were probed with a lower photon energy, i.e., more surface sensitive analysis (hν = 665 eV, not shown). Following the evolution of the Fe2O3 signal from the OCV to 1.4 V vs Li+/Li (point A), it is seen that the peak marked in red is shifted toward a lower binding energy (from 530.0 eV to 529.7 eV). This feature can be correlated to the initial lithiation of the iron oxide and the formation of LixFe2O3, as previously

Figure 8. (a) O 1s spectra of the Fe2O3/CMC/CB pristine electrode, measured as a function of the analysis depth (hν = 665 and 1125 eV). (b) Evolution of O 1s spectra of the Fe2O3 composite electrode at different voltage stages during the first discharge/charge (hν = 1125 eV). Comparison between the Li (on the left) and Na (on the right) systems. The corresponding points are reported in the CV cycles in Figure 1.

reported by Tian et al.30 Besides, proceeding from A to B, the attribution of the LixFe2O3 species at 529.7 eV is still observed, despite the SEI growth, thus meaning that the amount of LixFe2O3 keeps increasing. A weak peak at a very low binding energy appears at 528.2 eV, where Li2O is expected. This indicates that the conversion reaction in eq 1 had already started at 0.9 V vs Li+/Li. At the EOD, the peak assigned to LixFe2O3 is no longer detected and Li2O has increased in intensity, in agreement with a total consumption of Fe2O3 to Li2O and Fe0 metal at the surface of the active material. At the EOC, the signal of Li2O disappears; however, the recovery of iron oxide cannot be detected, probably due to the overlapping by the irreversible LiOH components presented after a full CV cycle. In the case of the Na-half cell, it can be noticed that the OCV spectrum is clearly different from that of the pristine electrode by following the evolution of the Fe2O3 signature. The ratio between the two peaks related to the −CO and −CO2 environment is different and the intensity of the signature of 5037



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Fe 2 O 3 is weaker, thus confirming the presence of a predeposited layer covering the electrode. The main peaks (∼533.2 and 531.5 eV) remain stable during cycling, displaying similar intensities. It can be also noted that their binding energy is also slightly lower, compared to the peaks in the Li-half cells. This shift is easily explained by the difference in electronegativity between Na and Li, which plays an important role in the chemical shift in photoelectron spectroscopy. The predominant intensity of the CO peak in the C 1s spectra is confirmed by the O 1s spectra and demonstrates that the Na system contains much more ROX species or PEO chains than its Li analogue. These two main peaks are the only ones observed at the lowest depth sensitivity (hν = 665 eV, not shown). A third species at ∼531 eV appears at the beginning of the discharge and remains through the entire cycle. This species can be due to the presence of NaOH, similar to the case for the Li system. The signal of Fe2O3 can also be detected in the OCV electrode in this case. At 1.40 V vs Na+/Na (point I), its signature is shifted, because of the formation of the NaxFe2O3 compound. Finally, the signal of an iron oxide component after a discharge to 0.40 V vs Na+/Na is concealed by the SEI; therefore, no further information can be extracted on the conversion mechanism of Fe2O3 by Na+ ions. (b). Na/Li/Fe Spectra Area. Fe core peaks have been analyzed in this last part of the study, in order to have a complete vision of the surface processes occurring in these Liand Na-half cells. The Fe 3p and Fe 2p spectra of the Fe2O3/CB/CMC pristine electrode are presented in Figure 9a, to the left and to the right, respectively. PES in this context also enables to discern the signature from different iron oxide phases (e.g., Fe2O3, Fe3O4, FexO), as they result in different characteristic spectra.53 The Fe 3p spectrum (hν = 647 eV) in Figure 9a shows a typical single, broad asymmetric peak at 55.5 eV, in good agreement with a pure Fe2O3 phase for the synthesized powders.28 The feature detected at ∼64 eV is the Na 2s core peak due to the sodium present in the CMC binder. Furthermore, the Fe 2p spectrum was also recorded with a photon energy of 1020 eV. The 2p3/2 and 2p1/2 contributions are located at ∼711 eV and ∼725 eV, respectively, and their associated satellites at ∼719 and ∼733 eV can be clearly seen. The energy difference between each core peak and its related satellite is ∼8 eV, again in good agreement with a pure Fe2O3 phase.30,53 The evolution of the Fe 2p core peak upon the electrochemical cycling will not be presented here, because once covered by the SEI, its signal was very weak due to its high binding energy, the Beamline I-411 facility being optimized for lower photon energies. The evolution of the Fe 3p/Na 2s/Li 1s spectra area during the first cycle is presented in Figure 9b. At first glance, it can appear surprising to use the Fe 3p core level, as its binding energy overlaps the Li 1s core peak. Nevertheless, the binding energy of the species containing Li are found in a narrow area centered at ∼55.5 eV, giving an overall symmetric and sharp peak, contrarily to the broad asymmetric Fe 3p peak. In particular, the latter can show a significant shift toward lower binding energies when its oxidation number decreases from Fe3+ to Fe0 (ΔEb ≈ 2.5 eV);54 thus, the results are useful to follow the conversion reaction. At this point it is worth recalling that the spectrum of the Lihalf cell left in a OCV state was very similar to that of the pristine electrode (C 1s and O 1s core peak) and that mainly

Figure 9. (a) Na 2s/Fe 3p and Fe 2p spectra of the Fe2O3/CMC/CB pristine electrode measured with a photon energy of 647 and 1020 eV, respectively. (b) Na 2s, Li 1s, and Fe 3p spectra of the Fe2O3/CMC/ CB electrode at different states of charge during the first discharge/ charge cycle (hν = 647 eV). Comparison between the Li system (left) and its Na analogue (right). The corresponding points are reported in the CV cycle in Figure 1.

lithium salt was observed on its outermost surface (from the Cl 2p core peak). Here, the Fe 3p/Li 1s spectrum associated with the OCV stage is also similar to that of the pristine electrode. The various contributions are easily distinguishable: CMC is shown in white in Figure 9, Fe2O3 is shown in red in Figure 9, and the Li 1s core peak is shown in yellow in Figure 9, explaining the extra asymmetry of the overall shape of this peak. When the battery is discharged to 1.4 V vs Li+/Li (point A), the gradual SEI formation would suggest that the Li 1s core peak should increase in intensity, compared to the iron oxide signature. As a matter of fact, it is observed that the main peak exhibits a sharp top part, while its base is much broader. In addition, the red peak is shifted toward a lower binding energy (55.3 eV), in agreement with the formation of the LixFe2O3 phase observed in the O 1s core peak in Figure 8b. 5038



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NaxFe2O3) was similarly detected at the beginning of the first discharge of the Na-half cell. However, further information about the following reactions was unfortunately not accessible, because of the pronounced SEI covering. 4.2. SEI Layer. The main differences observed during the formation/evolution of the SEI and a close comparison of their respective compositions in Li- or Na-half cells are highlighted below: • A predeposit layer of few nanometers (

Electrolyte Interface of Fe2O3 Composite

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