Photoelectron Spectroscopic Evidence for Overlapping Redox

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Photoelectron Spectroscopic Evidence for Overlapping Redox Reactions for SnO Electrodes in Lithium-Ion Batteries 2

Solveig Boehme, Bertrand Philippe, Kristina Edstrom, and Leif Nyholm J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01529 • Publication Date (Web): 20 Feb 2017 Downloaded from http://pubs.acs.org on February 22, 2017

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The Journal of Physical Chemistry

Photoelectron Spectroscopic Evidence for Overlapping Redox Reactions for SnO2 Electrodes in Lithium-Ion Batteries Solveig Böhme,

†

Bertrand Philippe,

‡

Kristina Edström,

†

and Leif Nyholm

∗,†

†Department of Chemistry - Ångström, Uppsala University, 751 21 Uppsala, Sweden ‡Department of Physics and Astronomy, Uppsala University, 751 21 Uppsala, Sweden E-mail: [email protected]

Phone: 0046 18 471 3742. Fax: 0046 18 513548

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ACS Paragon Plus Environment


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Abstract In-house and synchrotron-based photoelectron spectroscopy (XPS and HAXPES) evidence is presented for an overlap between the conversion and alloying reaction during the cycling of SnO2 electrodes in lithium-ion batteries (LIBs). This overlap resulted in an incomplete initial reduction of the SnO2 as well as the inability to regenerate the reduced SnO2 on the subsequent oxidative scan. The XPS and HAXPES results clearly show that the SnO2 conversion reaction overlaps with the formation of the lithium tin alloy and that the conversion reaction gives rise to the formation of a passivating Sn layer on the SnO2 particles. The latter layer renders the conversion reaction incomplete and enables lithium tin alloy to form on the surface of the particles still containing a core of SnO2 . The results also show that the reoxidation of the lithium tin alloy is incomplete when the formation of tin oxide starts. It is proposed that the rates of the electrochemical reactions and hence the capacity of SnO2 based electrodes are limited by the lithium mass transport rate through the formed layers of the reduction and oxidations products. In addition, it is shown that a solid electrolyte interphase (SEI) layer is continuously formed at potentials lower than about 1.2 V Li+ /Li during the rst scan and that a part of the SEI dissolves on the subsequent oxidative scan. While the SEI was found to contain both organic and inorganic species, the latter were mainly located at the SEI surface while the inorganic species were found deeper within the SEI. The results also indicate that the SEI dissolution process predominantly involves the organic SEI components.

1. Introduction

SnO2 has been investigated for a long time as an anode materials in lithium-ion batteries (LIBs) because of its high energy densities (theoretical capacity: 1491 mAh/g) and its ability to provide high power densities. 14 However, there is still a lack of understanding regarding the fundamental electrochemistry of tin oxide. The material displays both the conversion reaction to Sn and the alloying reaction between Sn and Li (see reactions schemes 1 and 2 2


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below). The electrochemical reaction taking place at 1.2 V vs. Li+ /Li has been described as the conversion of SnO2 to Sn and Li2 O in reaction scheme 1 ( SnO2 : E0 = 1.566 V). 5 At potentials below 0.5 V vs. Li+ /Li the alloying reaction in reaction scheme 2, which results in big volume changes, can be observed. 3,5,6

4Li+ + 4e− + SnO2 * ) 2Li2 O + Sn

(1)

xLi+ + 4e− + Sn * ) Lix Sn

(2)

In two recent studies, in-house X-ray photoelectron spectroscopy (XPS) has been used to study SnO2 electrodes exposed to cyclic voltammetric (CV) cycling in LIBs. 7,8 The results of these experiments indicated that the cycling behavior was limited by the lithium mass transport rates due to the formation of passivating tin oxide layers and that the conversion reaction was partially reversible in the absence of the alloy forming reaction. 7,8 These studies also suggested that it was very dicult to separate the reduction peaks due to the conversion and the alloying reactions as these were found to overlap even at rather low scan rates (i.e., 0.1 mV/s). This made evaluations of the conversion and alloying charges practically impossible. 7,8 While other groups likewise have found a certain overlap between the conversion and alloying reaction during CV experiments, the implications of such an overlap on the electrochemical behavior of SnO2 electrodes has not yet been properly evaluated. 9,10 One consequence of the overlap between the conversion and alloy forming reactions could then be that the SnO2 conversion may not be completed prior to the onset of the alloy forming reaction. This has been proposed to give rise to SnO2 particles coated with surface layers of Sn and Lix Sn. 8 While the latter hypothesis should be possible to test using, e.g., photoelectron spectroscopic investigations of cycled SnO2 electrodes we are not aware of any such reports. There is therefore a need for such spectroscopic data as a complement to the electrochemical results discussed above. Similar studies should also be performed to investigate

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if the dealloying and conversion reactions likewise overlap during the oxidation cycle. These types of studies would most likely also benet from the use of synchrotron based hard X-ray photoelectron spectroscopy (HAXPES) as this technique can provide more bulk information than in-house XPS. 11 By using dierent excitation energies it would thus be possible to straightforwardly obtain important information about the locations of dierent species in the case that layers of, e.g., Lix Sn and Sn are indeed formed on the SnO2 particles. Studies of the SnO2 conversion and alloy forming reactions are, unfortunately, complicated by the fact that a solid electrolyte interphase (SEI) layer is formed predominantly during the rst reduction cycle. The SEI is formed as the electrolyte is electrochemically unstable at the low potentials, i.e., potentials below about 1.0 V vs. Li+ /Li, employed with

SnO2 electrodes. 1214 There are, however, several recent reports stating that the SEI could start to form already at potentials between 1.5 V and 1.2 V vs. Li+ /Li. 15,16 For tin based electrodes, the SEI formation has also been reported to take place during several cycles as a result of dierent formation and reorganization processes and evidence for a partial dissolution of the SEI layer during the oxidation scan has also been presented. 14,1719 As the SEI process can be assumed to aect the electrochemical performance of SnO2 electrodes it is therefore important to also study the SEI formation process on SnO2 electrodes in more detail. The main aim of the present work is to provide photoelectron spectroscopic evidence regarding the proposed overlap between the conversion and alloy forming redox reactions for SnO2 electrodes in LIBs. In this process, the surfaces of electrodes cycled to dierent potentials during the rst voltammetric cycle have been studied with dierent photoelectron spectroscopic (PES) methods, e.g., in-house XPS as well as synchrotron based HAXPES. This was done to obtain information regarding the composition of the cycled SnO2 electrodes employing dierent analysis depths. It is demonstrated that these results provide new important insights about the overlapping conversion and alloying reactions as well as the composition and thickness of the SEI layer formed on the SnO2 electrodes.

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2. Experimental Section

2.1. Electrode preparation Slurry electrodes were prepared using SnO2 particles with a size of 35 - 55 nm (US Research Nanomaterials, Inc.). The slurry was composed of 85 wt% SnO2 , 10 wt% Carbon Black (CB) (Imerys) as well as 5 wt% binder (i.e., a 3:1 mixture of CMC (carboxymethyl cellulose, Aldrich) and SBR (styrene butadiene rubber, Tagray)). After suspending the slurry mixture in 5 ml of water, the slurry was ball-milled for two hours and the suspension was then cast on a copper foil and dried at room temperature (22 o C ) for 24 hours. Circular electrodes (d = 20 mm) with a lm thickness of about 20 µm were then cut out. Prior to electrochemical testing, the electrodes were dried again for 12 hours at 90 o C in a vacuum furnace (Büchi) placed in an Ar-lled glove box (M-Braun).

2.2. Cyclic voltammetry (CV) The electrochemical behavior was studied with CV using two-electrode pouch cells containing a Li foil as the counter electrode and a glass ber separator (Whatman, 70 µm). The cells were assembled in a sealed, Ar-lled glove box (M-Braun) to prevent side-reactions caused by water and oxygen. The electrolyte consisted of 0.5 ml 1 M LiP F6 dissolved in a mixture of 1:1 ethylene carbonate (EC) and diethyl carbonate (DEC), i.e., LP40 (BASF). The CV experiments, which were carried out with a BioLogic SA VMP2 instrument using the ECLab software, included an initial cathodic scan from the open circuit voltage (OCV) at a scan rate of 1 mV/s, if not stated otherwise.

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2.3. Characterization of cycling products 2.3.1. Photoelectron spectroscopy (PES)

PES was employed to investigate the surfaces of electrodes cycled to 1.2; 0.4; 0.05; 1.0 and 2.5 V vs. Li+ /Li. In addition, PES measurements were also performed on pristine electrodes, i.e., slurry electrodes that were cast and dried as described above, but not included in a battery. All cells were dismantled in an Ar-lled glove box (M-Braun) and the electrodes were rinsed with DMC (dimethylcarbonate) (Aldrich) and transferred to the spectrometer with a special built transfer system. 20 During the rinsing, DMC was used instead of EC/DEC to minimize the dissolution of the SEI layer. 21 In-house XPS spectra were obtained with a PHI 5500 Multi-Technique system (Perkin Elmer) using a monochromatic Al Kα X-ray source hν = 1487eV (yielding an analysis depth of about 10 nm 22 ) and a pass energy of 23.5 V. The pressure in the analysis chamber was about 3 · 10−9 bar. The pressure in the analysis chamber was about 3 · 10−9 bar. The HAXPES measurements were carried out at the Bessy II synchrotron facility (HIKE end station 23 , KMC-1 beamline 24 , Helmholtzzentrum Berlin, Germany) employing two xed excitation energies, i.e., hν = 2005eV (yielding an analysis depth of about 14 nm 22 ) and hν = 6015eV (yielding to an analysis depth of about 40 nm 22 ). Calibration of the binding energy scale was carried out based on the hydrocarbon C1s peak at 285.0 eV and the peaks were analyzed using a non-linear Shirley-type background. 25 The Casa XPS software was used to curve t the obtained spectra. Fingerprint spectra were recorded at the beginning and at the end of each measurement to ensure that there was no X-ray irradiation damage to the samples. 2.3.2. Scanning electron microscopy (SEM)

The electrodes used in the cycling product characterization were cycled and subsequently treated as described in the description of the XPS characterization. The SEM micrographs were taken using a SEM LEO 1550 instrument from Zeiss using a beam voltage of 2.5 kV

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and an aperture width of 10 µm.

3. Results and Discussion

As mentioned above, this PES work was carried out to investigate if there is an overlap between the SnO2 conversion and alloying reactions by analyzing spectra obtained for electrodes cycled to dierent potentials. Cyclic voltammetry was used during the cycling as this technique allows the potentials to be controlled while the current is measured, providing better control of the reactions taking place. The shapes of the CVs also provide important information regarding the factors limiting the current at the dierent potentials.

3.1. Cyclic voltammetry (CV) In Figure 1 the CV between 0.05 V and 2.5 V vs. Li+ /Li of the rst cycle is shown. The red circles mark the potentials at which batteries were stopped to take out the electrodes and measure PES spectra. Charge/Oxidation

5 1.0 V

2.5 V

0

Current [mA]

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1.2 V

Discharge/Reduction

-5 0.4 V

-10 -15 0.0

0.05 V

0.5

1.0

1.5

2.0

2.5

+

Potential [V vs. Li /Li]

Figure 1: Cyclic voltammogram of the rst cycle for SnO2 cycled vs. Li between 0.05 V and 2.5 V vs. Li+ /Li at a scan rate or 1 mV/s. Red circles mark at which potentials PES spectra were obtained.

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In Figure 1, which depicts the rst cycle CV recorded between 0.05 V and 2.5 V vs.

Li+ /Li using a scan rate of 1 mV/s, the red circles denote the potentials at which the cycling was stopped prior to the recording of the PES spectra. On the cathodic scan (discharge), a broad reduction peak can be seen between 1.2 and 0.4 V vs. Li+ /Li followed by an increasing reduction current down to 0.05 V vs. Li+ /Li. The reduction current between 1.2 V and 0.4 V vs. Li+ /Li is typically ascribed to the reduction (i.e., conversion) of SnO2 yielding

Sn and Li2 O, while the current at lower potentials is attributed to the formation of the Lix Sn alloy. As is seen in the voltammogram, the conversion and alloy formation reactions appear to overlap, in good agreement with our previous ndings. 8 During the oxidative scan (charge) several oxidation peaks can be observed, one between 0.5 and 1.0 V vs. Li+ /Li which generally is ascribed to the dealloying reaction (i.e., Lix Sn * ) xLi+ + Sn + xe− ). The two broad, overlapping peaks observed between 1.0 and 2.2 V vs. Li+ /Li have, on the other hand, been ascribed to the oxidation of Sn to SnO and SnO2 , respectively. 8 In this case, it can also be seen that the rst and the subsequent two overlapping peaks overlap to some extent, indicating that the oxidation of the Lix Sn alloy may not be complete at the onset of the SnO formation. Analogous overlaps between the dierent reduction and oxidation peaks could likewise be seen when using a scan rate of 0.1 mV/s (rather than 1 mV/s), see Figure S5 in the Supporting Information. While the lower scan rate gave rise to sharper peaks and smaller overpotentials, an overlap between the conversion and the alloying reaction could still be observed both on the reductive and oxidative scans. The electrochemical data consequently indicate that there are signicant overlaps between the dierent reduction and oxidations reactions during the cycling of the SnO2 electrodes in LIBs. This suggests that the SnO2 reduction may not be complete during the reduction step and that the reduced electrode hence may contain a mixture of SnO2 , Sn and Lix Sn. To investigate this further, the cycled electrodes were analyzed using photoelectron spectroscopy as is described in the next section.

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3.2. Characterization by photoelectron spectroscopy (PES) To identify the formed reduction and oxidations products, PES experiments were carried out on the SnO2 electrodes after stopping the scan at the potentials indicated in Figure 1. The measurements were carried out using both in-house XPS and HAXPES to obtain information for dierent analysis depths as this should facilitate the detection of any compositional gradients in the cycled electrodes. 11 The chosen potentials were situated either before or after the reduction and oxidation peaks, i.e., the scan was stopped at 1.2; 0.4 and 0.05 vs.

Li+ /Li on the rst reduction scan as well as at 1.0 and 2.5 V vs. Li+ /Li on the rst oxidation scan. For comparison, pristine electrodes that had not been assembled into batteries were also analyzed. 3.2.1. Conversion and alloying

In Figures 2a and b, the Sn3d5/2 peaks are displayed for the three dierent excitations energies and the dierent reduction and oxidation potentials, respectively. In the gures, the analysis depth is increasing from left to right, showing the in-house XPS spectra (1487 eV) to the left and the HAXPES spectra obtained at the synchrotron source Bessy II in the middle (2005 eV) and to the right (6015 eV). The spectra feature three dierent components which can be assigned to SnO2 at about 486.6 eV (before cycling) and at 486.3 eV (after cycling), Sn at about 484.8 eV and Lix Sn at about 483.4 eV. 26,27 It can also be concluded that the electrode stopped at 0.05 V vs. Li/ Li+ contained a Lix Sn rich surface layer on top of a Sn rich layer and that the electrode still contained unreacted SnO2 . As the results clearly show that both Sn and Lix Sn were present after stopping at 0.4 V, the results conrm the presence of a signicant overlap between the conversion and alloy forming reactions on the rst reductive scan. A shift in the Sn3d5/2 peaks towards lower binding energies can also be observed, especially for the Sn and Lix Sn peaks, when scanning from 0.4 V to 0.05 V vs.

Li+ /Li. In this potential range, the SEI can be assumed to grow thicker based to the O1s, C1s and F 1s spectra (see below). The shift towards lower binding energies has therefore 9



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been ascribed to the formation of an electric potential gradient at the interface between the electrode and the SEI. 28 In connection with Figure 2b it should be mentioned that it is generally dicult to determine which tin oxide, i.e., SnO or SnO2 , is present in the oxidized electrodes as the dierence between the binding energies for these merely should be about 0.7 eV. 27 The latter value should be compared to the SnO2 peak width of about 3 eV found for the pristine sample and the electrode cycled to 1.2 V vs. Li+ /Li. Based on our previous ndings, a mixture of both tin oxides is most likely formed when the oxidation of the Sn starts. 8 The peak for such a mixture would consequently be located at an intermediate binding energy and should also be somewhat broader than the SnO2 peak for the pristine sample. The larger tin oxide peak width and the slight shift towards lower binding energies hence support the hypothesis that the electrode cycled to 2.5 V vs. Li+ /Li contained a mixture of SnO2 and SnO. The two broad oxidation peaks seen above 1.0 V vs. Li+ /Li seen in Figure 1 can then be explained by the formation of SnO and SnO2 as previously suggested. 8 As a result of this diculty in dierentiating between SnO and SnO2 , the formed tin oxide peaks have been denoted SnOx in the Sn3d5/2 and O1s spectra. As is seen in Figure 2a, the pristine samples as well as the electrodes cycled only to 1.2 V vs. Li+ /Li, i.e., to a potential above the SnO2 reduction potential, only exhibited a SnO2 peak for all analysis depths. This is in good agreement with the electrochemical ndings as the CV peak between 1.2 V and 0.4 V is normally assigned to the reduction (i.e., conversion) of SnO2 . 5,6,12,29 The XPS spectra obtained at 0.4 V vs. Li+ /Li, on the other hand, feature both Sn and Lix Sn peaks and it is also evident that the intensity for the Sn peak increased with increasing analysis depth. As already indicated above, this observation and the shape of the voltammogram in Figure 1 demonstrate that the very broad reduction peak between 1.2 V and 0.4 V vs. Li+ /Li is due to an overlap between the conversion and alloying reactions. When continuing the scan to 0.05 V vs. Li+ /Li, Sn is still visible, especially for the larger analysis depths indicating that the alloy is formed as the deposited lithium is diusing into

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a

hv = 1487 eV

hv = 2005 eV

hv = 6015 eV

pristine SnO 2

SnO2

SnO 2

1.2 V Discharge/Reduction

0.4 V

0.05 V Li x Sn

Sn

LixSn

Sn

Li x Sn Sn

490

488

486 484 482 Binding ener gy [eV] Binding energy [eV]

480

490

488

486 484 482 Bindingenergy energy[eV] [eV] Binding

480

490

488

486 484 482 Binding Bindingenergy energy[eV] [eV]

480

Increasing analysis depth

b

hv = 1487 eV

hv = 2005 eV

hv = 6015 eV

L i2

2.5 V SnOx

SnO x

SnOx

Charge Charge/Oxidation

Sn

1.0 V LixSn

Li x Sn

Sn

Sn

LixSn

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490

488

486 484 482 Binding gy[eV] [eV] Binding ener energy

480

490

488

486

484

482

480

490

Binding energy energy [eV] Binding [eV]

488

486 484 482 Binding Bindingenergy energy[eV] [eV]

480

Increasing analysis depth Figure 2: XPS spectra of the Sn3d5/2 peak at dierent potentials a) Reduction: 1.2 V, 0.4 V and 0.05 V vs. Li+ /Li as well as b) Oxidation: 1.0 V and 2.5 V vs. Li+ /Li including dierent analysis depths.

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the electrode still containing the Sn formed in the conversion reaction. Note that the PES results also indicate that SnO2 was still present at a potential of 0.05 vs. Li+ /Li, most likely due to the fact that the reduction of the SnO2 (SnO2 + 4e− + 4Li+ * ) Sn + 2Li2 O) should become limited by the Li+ diusion rate though the growing Sn layer. 8 Analogously the formation of the Lix Sn takes place from the electrode surface and inwards, generating an electrode with an Lix Sn surface on top of a layer of tin and a core still containing unreacted

SnO2 . This explains the larger relative Lix Sn peak area seen with the smallest analysis depth (in-house XPS, 1487 eV) as compared to for the larger depths (HAXPES, 2005 eV and 6015 eV). The alloying hence starts before the conversion of the SnO2 is complete. As Lix Sn formed on the electrode surface, the diusion of Li+ into the electrode stops as the electrode now is at a potential where Li+ is reduced to Li. The only way to reduce the remaining SnO2 in the electrode is then via the following redox reaction 4Li + SnO2 * ) Sn + 2Li2 O taking place as a result of Li diusing further into the electrode. This explains why it is very dicult to reach the full theoretical capacity of SnO2 on the rst cycle with electrodes composed of particles with sizes larger than 10 nm at conventional cycling rates. 3032 It should, however, also be pointed out that the full SnO2 capacity may still not be obtained even with particle sizes less than 10 nm if the electrode is made too thick. Theoretical capacity is therefore most likely to be reached for electrodes composed of monolayers of small SnO2 nanoparticles immobilized on a conducting substrate. Such electrodes are, unfortunately, of less practical interest as their capacities tend to be very low as a result of the low mass loadings. In Figure 2a, small amounts of SnO2 are still visible even at a potential of 0.05 V vs.

Li+ /Li for all analysis depths. The reason for this could be that the particles gradually crack due to the volume expansion associated with the alloying reaction. In this process new surfaces with SnO2 could be exposed and become possible to detect with PES. A small amount of SnO2 could also be detected for the electrode stopped at 0.4 V vs. Li+ /Li and analyzed using the (1487 eV) in-house XPS instrument, even though no such SnO2 peaks could be seen for the other electrodes analyzed with the higher excitation energies. Since

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the remaining SnO2 should be easier to detect using a higher excitation energy we believe that this eect merely stemmed from random dierences in the degree of the cracking of the electrodes cycled to 0.4 V vs. Li+ /Li. Given the overlap between the conversion and alloying reactions, the tendency to form cracks would already be present at 0.4 V vs. Li+ /Li and the inuence of the eect should naturally increase as the potential is decreased further. This would explain the appearance of the 0.05 V SnO2 peaks for all the analysis depths. Based on the oxidation results in Figure 2b it is immediately evident that the dealloying reaction was incomplete at 1.0 V vs. Li+ /Li as a large Lix Sn peak still could be seen for all analysis depths. This may be somewhat surprising as the oxidation peak seen between about 0.3 V and 1.0 V vs. Li+ /Li generally is ascribed to the dealloying reaction ( Lix Sn * )

xLi+ + xe− + Sn). 5,6,33 A comparison of the HAXPES spectra also shows that less Lix Sn was present at 0.05 V vs. Li+ /Li than at 1.0 V vs. Li+ /Li (i.e., for the 2005 eV and 6015 eV excitation energies). The latter can, however, be explained by the fact that the electrochemical results (see Figure 1) demonstrate that the deposition of the alloy took place up to a potential of 0.4 V vs. Li+ /Li on the oxidative scan. This is further supported by the 0.05 V vs. Li+ /Li Sn3d5/2 PES spectra in Figure 2 in which a more dominant Lix Sn peak can be seen for the shallowest analysis depth. These observations hence suggest that a

Lix Sn gradient is formed during the alloying reaction with a higher Lix Sn concentration at the surface and more Sn remaining underneath at 0.05 V vs. Li+ /Li. During the anodic scan from 0.05 to 1.0 V vs. Li+ /Li, lithium is therefore diusing further into the electrode which gives rise to a thicker Lix Sn layer. The fact that PES experiments were carried out some time after the electrochemical experiments would also favor the detection of the Lix Sn since the oxidation of the Lix Sn alloy results in the formation of a layer of Sn at the surface of the electrode into which lithium (from the alloy present below) would diuse to regenerate a surface layer of the alloy (note that the analysis depths were merely about 10, 14 and 40 nm, respectively). To obtain a complete oxidation of the Lix Sn alloy all the deposited lithium present must hence have time to diuse to the electrode surface during the time domain of

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the experiment. In Figure 2b, a small tin oxide peak can also be seen, at least for the two lowest analysis depths, indicating the presence of an overlap between the dealloying and the oxide formation reactions also on the anodic scan. As the SnOx peak was most clearly seen for the smallest analysis depth it is immediately evident that the SnOx layer was formed on the surface of the electrode, i.e., that the SnOx layer was formed on top of the Lix Sn still present in the electrode. The latter would most likely make it more dicult to reach a full oxidation of the Lix Sn (see below). Note also that there must be an intermediate Sn layer between the

Lix Sn and the SnOx layers as the latter would be reduced to Sn in contact with elemental lithium. This means that the oxidation of the Lix Sn (i.e., the lithium in this alloy) requires that the lithium diuses towards the electrode surface where it can react with the SnOx . This also means that a complete regeneration of SnO2 requires that all elemental lithium present in the electrode is rst oxidized. It is consequently also reasonable to assume that mainly SnO rather than SnO2 will be present below the surface of the electrode as long as there is lithium diusing towards the electrode surface. The spectra recorded for the electrodes scanned to 2.5 V vs. Li+ /Li featured only SnOx and Sn peaks. Since Sn could be detected in the oxidized electrodes the reconversion of Sn to

SnO2 must also have been incomplete. Note also that the data indicate that a mixture of SnO and SnO2 rather than only SnO2 was formed during the oxidative scan. This is, however, not surprising given the lithium diusion eect discussed above and as the formation of a SnO layer on Sn (and the formation of a SnO2 layer on SnO) would generate a growing passivating layer through which Li+ would need to diuse (e.g., 2Sn + Li2 O * ) 2SnO + 2Li+ + 2e− ) to maintain the electroneutrality within the electrode. This mass transport controlled oxidation is most likely one of the reasons why the conversion reaction often has been reported to be irreversible. Further evidence for the overlapping redox reactions can be obtained from the O1s spectra seen in Figures 4a and b which likewise were recorded for the reduced and oxidized

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electrodes employing the three dierent analysis depths. In the spectra there are basically four dierent peaks which can be ascribed to C = O species, i.e., carbonates, e.g., Li2 CO3 and polycarbonates, as well as esters ( −CO2 −) at about 531.5 eV; organic C − O species, e.g., ethers (C − O − C ) at around 532.5 eV; tin oxide ( SnOx ) and/or lithium alkoxides (C − OLi) at about 530 eV and Li2 O, seen at about 528 eV for the lowest potentials. 20,22 The pristine electrodes as well as the electrodes cycled to 1.2 V vs. Li+ /Li all exhibit SnO2 peaks at about 530 eV and this peak grows stronger with increasing analysis depth indicating that the other peaks stemmed from oxygen containing species present on the surface of the electrode. Peaks with lower intensities, which still can be attributed to SnO2 , are also observed for the electrodes cycled to 0.4 V and 0.05 V vs. Li+ /Li, supporting the incomplete

SnO2 reduction hypothesis. It should, however, be noted that the latter peaks also at least partly could originate from lithium alkoxides formed in the SEI formation process. 34,35 The fact that the intensity of the peaks around 530 eV increased when the analysis depth was increased, nevertheless, indicate that the peaks did stem from SnO2 (or possibly SnO). This implies that some SnO2 (or SnO) did not undergo conversion to Sn and Li2 O in excellent agreement with the Sn3d5/2 spectra discussed above. In Figure 3a it can also be seen that a Li2 O peak was seen for the electrodes cycled to 0.4 and 0.05 V vs. Li+ /Li. This can be explained either based on the conversion reaction (SnO2 + 4Li+ + 4e− * ) Sn + 2Li2 O) or the formation of an SEI layer containing Li2 O. The latter is, however, less likely as the Li2 O peak is most clearly seen for the larger analysis depths. It can also be seen that the relative intensities for the Li2 O peaks were lower for the electrodes cycled to 0.05 rather than to 0.4 V vs. Li+ /Li. This can readily be explained by the formation of the Lix Sn as the Li+ present in the Li2 O in the vicinity of the Sn would be reduced to elemental lithium ( xLi+ + xe− + Sn * ) Lix Sn) as soon as the formation of the alloy becomes possible. This implies that the Lix Sn formation rate becomes controlled by the Li+ diusion rate and that the process to maintain the electroneutrality within the electrode also may involve diusion of O2− towards the electrode surface, a process which

15



a

hv = 1487 eV

hv = 2005 eV

hv = 6015 eV

pristine SnO 2

C-O

SnO 2

C-O

SnO 2


0.4 V

Li 2 O

536

ROLi/ SnO x

526

C=O

534 532 530 528 Binding ener [eV] gy[eV] [eV] Binding Binding energy energy

Li 2 O

ROLi/ SnO x

Li 2 O

C=O

C=O

536

ROLi/SnO x

0.05 V

C-O

534 532 530 528 Bindi ngenergy ener gy[eV] [eV] Binding

526

536

534 532 530 528 Bindi ngenergy ener gy [eV] Binding [eV]

526


hv = 1487 eV

hv = 2005 eV

536

528 530 532 534 Binding energy [eV] energy[eV] Binding

526

536

Li 2 O

526

C=O

532 530 528 Binding energy [eV]

Li 2 O

C-O

Li 2 O

534

ROLi/ SnO x

C-O 536

C=O

1.0 V

ROLi/SnO x

x

C-O

ROLi/SnO

2.5 V

hv = 6015 eV

Charge/Oxidation

b

C=O

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 46

534 532 530 528 Bindingenergy energy[eV] [eV] Binding

526

Increasing analysis depth Figure 3: XPS spectra of the O1s peak at dierent potentials a) Reduction: 1.2 V, 0.4 V and 0.05 V vs. Li+ /Li as well as b) Oxidation: 1.0 V and 2.5 V vs. Li+ /Li including dierent analysis depths.

16


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will further decrease the likelihood of a complete regeneration of the SnO2 on the subsequent anodic scan. As is seen in Figure 3b, SnOx peak were found for the electrodes scanned up to 1.0 and 2.5 V. The relative intensity of these peaks, which generally was highest for the largest analysis depth, increased when the electrode was scanned from 0.05 to 1.0 V vs. Li+ /Li indicating that the tin oxide formation was active already at 1.0 V vs. Li+ /Li. These results are hence in in good agreement with the results presented in Figure 2 concerning the overlap between the dealloying and the oxide forming reactions on the anodic scan. For the electrodes cycled to 2.5 V vs. Li+ /Li, no Li2 O peaks could be seen indicating that the Li2 O had reacted with

Sn to yield a layer of tin oxide on the surface of the electrodes. A comparison of the 0.05 and 1.0 V spectra further shows that the relative intensities of the Li2 O and SnOx peaks were higher for the latter potential when using the largest analysis depth. This eect was most likely caused by a decrease in the C = O peak intensity which suggests that there was a decrease in the thickness of the latter layer when the potential was increased from 0.05 to 1.0 V vs. Li+ /Li (see below). Based on the electrochemical and PES results it is immediately evident that there is a signicant overlap between the electrochemical reactions taking place on the rst reduction and oxidation cycle for SnO2 electrodes. It is therefore unrealistic to assign each voltammetric peak to a single specic reaction, particularly as the electrochemical reactions result in the formation of layers on the electrode protecting the unreacted material present below. It is hence reasonable to assume that the rates of the electrochemical reactions are limited by the diusion rate of Li+ through the formed Sn and Li2 O and SnOx layers, as well as the diusion rate of Li in Sn. 3.2.2. SEI formation

As discussed in the introduction, the reduction of SnO2 is always accompanied by the formation of an SEI layer on the electrode surface. The formation of this layer can, however, 17



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be expected to be very similar on dierent electrodes since the reduction merely involves the electrolyte. As the electrode merely acts as a reducing agent, dierent results for dierent electrodes can only be expected if the electrode can act as a catalyst in the SEI formation process and then only as long as the rate of the SEI formation process is limited by the electron transfer process. Since the SEI layer generally serves as a passivating layer and as the SEI formation takes place over a wide potential region, the SEI formation process is most likely diusion controlled under conventional experimental conditions. This means that the composition of the SEI layer should be practically independent of the electrode material used. The O1s results obtained for the present SnO2 electrodes (see Figure 3) indicate that the general composition of the SEI layer agrees well with those previously described for other types of electrodes cycled in similar electrolytes. The C − O peaks seen in the O1s spectra thus originate mainly from the C − O − C groups in the CMC binder as well as ether and alcohol containing surface species visible at about 532.5 eV. The fact that some C = O could be observed also for the pristine samples can be explained by the −CO2 − groups in the CMC binder. Upon scanning to 1.2 V vs. Li+ /Li the C = O peaks grow somewhat followed by a signicant increase in the peak intensity when the potential is decreased from 1.2 to 0.4 or 0.05 V vs. Li+ /Li. This eect can be ascribed to the formation of carbonate ( CO3 ) species as a part of the SEI formation process. These results consequently indicate that the main SEI process takes place at a potential lower than 1.2 V. Figures 4a and b display the corresponding C1s spectra. The main peaks can in this case be ascribed to Carbon Black (about 283 eV), organic C − H species (285 eV), organic

C − O species, e.g., mainly ethers, C − O − C (approximately 286.5 eV), −CO2 − species, i.e., esters and polyethers (about 289 eV) and CO3 species, carbonates, i.e., Li2 CO3 and organic carbonates (about 290 eV). 20,22 Normally, graphitic carbons like Carbon Black give rise to a shake-up satellite peak at about 290 eV but this peak is most likely masked by the contributions from the CMC/SBR binder, and later the SEI components. 36,37

18


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Page 19 of 46

a

hv = 1487 eV

hv = 2005 eV

hv = 6015 eV

pristine


0.4 V

CB

CB

C-H

-CO 2 -

CO

0.05 V

292

290 288 286 284 Bindi ng ener gy [eV]

282

280

294

292

Binding energy [eV]

CB

294

CO

Binding energy [eV]

C-H

294 292 290 288 286 284 282 280 Bindi ng ener gy [eV]

CO3

CO

-CO 2 -

CO3

C-H

-CO 2 CO3

290 288 286 284 Binding energy [eV]

282

280

Binding energy [eV]


b

hv = 1487 eV

hv = 2005 eV

hv = 6015 eV

2.5 V

294

Binding energy [eV]

292

290

C-H

-CO 2 - /CO 3

C-H

CO3

-CO 2 -

C-H

294 292 290 288 286 284 282 280 Binding energy [eV]

Charge/Oxidation

CB

CO

CB 294 292 290 288 286 284 282 280 Bindi ng ener gy [eV] Binding energy [eV]

CB

CO

CO

1.0 V -CO 2 CO3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


288

286

284

282

280

Binding [eV] Bindingenergy energy [eV]

Increasing analysis depth Figure 4: XPS spectra of the C1s peak at dierent potentials a) Reduction: 1.2 V, 0.4 V and 0.05 V vs. Li+ /Li as well as b) Oxidation: 1.0 V and 2.5 V vs. Li+ /Li including dierent analysis depths.

19



in-house XPS (1487 eV) HAXPES (2005 eV)

60

CB relative peak areas [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

50 40 30 20 10 0

OCV pristine

A 1.2

B 0.4

C 0.05

D 1.0

E 2.5

Potential [V vs. Li+/Li]

Figure 5: Evolution of the relative areas of the CB peak measured in the C1s spectra during the initial CV cycle for both in-house XPS data (1487 eV) and synchrotron based HAXPES data (2005 eV). The most abundant carbon species in the bulk is the CB, making it the dominant peak in the spectra for the pristine samples. For the pristine electrode, peaks due to C − H , C − O and −CO2 − species can likewise be observed due to the CMC ( C − H , C − O and −CO2 − species) and SBR binders ( C −H species, C = C species) as well as carbon containing surface contamination species. The evolution of the CB relative peak areas for the in-house (1487 eV) and HAXPES (2005 eV) data is visualized in Figure 5 while the relative peak areas of all C1s peaks for the excitation energies 1487 eV and 2005 eV are summarized in the tables included in the Supporting Information. The spectra recorded for the electrodes scanned to 1.2 V vs. Li+ /Li demonstrate that the intensity of the CB peak was lower than for the pristine electrode whereas the intensity of the C − H peak was higher. This indicates that SEI components were indeed formed already at a potential of 1.2 V vs. Li+ /Li. 15,16 Although a small cathodic currents can be seen in this potential region in Figure 1 it is unlikely that this current was dominated by the SEI formation process as the formation of an SEI layer with an assumed thickness of 20 nm only should give rise to a current of the order of 0.1 mA. 7 Carbonates (CO3 ) and organic C −H species are usually major components in the SEI

20


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layers formed on other anode materials, i.e., graphite or silicon, which is why their presence in the SEI layer was anticipated. 20,22,34 The present results show that the relative peak area of the CB peaks (see Figure 5) decreased further between 1.2 V and 0.4 V vs. Li+ /Li while the contributions from the C − H and CO3 peaks increased. This indicates that the main part of the SEI is formed in the latter potential region. When continuing the reductive scan to 0.05 V vs. Li+ /Li, the C − H and CO3 peaks can be seen to grow even more while the CB peak continues to decrease in size (see Figure 5). These observations are in accordance with infrared and Raman data obtained with SnO electrodes. 38,39 In these cases, there was an initial SEI formation at about 1.2 V vs. Li+ /Li followed by a continuous increase in the SEI thickness and CO3 species also could be seen in the SEI layer at potentials below 0.7 V vs. Li+ /Li. 38,39 It is therefore reasonable to assume that the SEI formation process is a continuous process at potentials lower than 1.2 V vs. Li+ /Li and that the growth of the SEI layer is time rather than potential controlled at potentials lower than about 1.2 V vs. Li+ /Li. As the CB peak still could be seen with excitation energy of 6015 eV at 0.05 V vs. Li+ /Li, it is likely that the SEI was thinner than about 10 nm. Other researchers have reported a maximum SEI layer thickness of about 20 nm under similar experimental conditions. 22 In this case, the thickness of the SEI layer should be given by the diusion rate of the solvent through the SEI layer and the solubility of the SEI species in the SEI layer. The thickness of the SEI layer would therefore be expected to decrease once the potential is higher than about 1.2 V vs. Li+ /Li. As is seen in Figure 5, the CB relative peak intensity was in fact found to increase at the expense of the other

C1s components during the oxidative scan to 2.5 V vs. Li+ /Li. This suggests that a part of the SEI was lost during the anodic scan, most likely as a result of the dissolution of some of the SEI components, in good agreement with previous reports. 12,14,19,40 Further information regarding the loss of some SEI components can be obtained from the F 1s spectra displayed in Figure 6. In these, three peaks due to P − F containing species (at about 687 eV), C − F containing species (at about 689 eV) and LiF (at about 684 eV)

21



a

hv = 1487 eV

hv = 2005 eV

hv = 6015 eV

pristine

1.2 V Discharge/Reduction Discharge

0.4 V

0.05 V

682

692

690 688 686 684 Bindi ng ener gy[eV] [eV] Binding energy

682

692

LiF

690 688 686 684 Bindi ng ener gy [eV] Binding energy [eV]

P-F

C-F

LiF

692

LiF

P-F

P-F

C-F

690 688 686 684 Binding [eV] Binding energy energy [eV]

682


b

hv = 1487 eV

hv = 2005 eV

hv = 6015 eV

690 688 686 684 Bindi ng ener gy [eV] [eV] Binding energy

682

692

690

LiF

692

P-F

682

688 686 684 Binding [eV] Binding energy energy [eV]

Charge

LiF P-F

690 688 686 684 Binding energy[eV] [eV] Binding energy

C-F

LiF

692

P-F

1.0 V

Charge/Oxidation Charge

2.5 V

C-F

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 46

682

Increasing analysis depth Figure 6: XPS spectra of the F 1s peak at dierent potentials a) Reduction: 1.2 V, 0.4 V and 0.05 V vs. Li+ /Li as well as b) Oxidation: 1.0 V and 2.5 V vs. Li+ /Li including dierent analysis depths.

22


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can be seen. 34,35 Based on the P1s HAXPES spectra included in the Supporting Information (see Figure S2), it is likely that the P − F species included P F6− or decomposition products thereof. The C − F species, which only could be seen with the smaller analysis depths (and therefore were likely to be situated on the electrode surface) could, on the other hand, have been formed during decomposition reactions involving P F6− and the EC/DEC solvent whereas LiF is known to be formed during the decomposition of P F6− . 34,35,41 For the electrodes cycled to 1.2 V vs. Li+ /Li, P − F species, LiF and a small amount of C − F species could be observed. This indicates that the SEI formation started already at this potential in good agreement with the C1s results (see Figures 4 and 6) and previous ndings. 15,16 In-house XPS spectra ( hν = 1487 eV) for a SnO2 electrode that had been stored in batteries at open circuit voltage (OCV) for four hours (see Figures S1a to d in the Supporting Information), however, indicate that LiF and P − F species were present on the uncycled electrode. One explanation for this could be that the electrolyte decomposed upon contact with the Li foil and that some of the decomposition products then were transported to the SnO2 electrode. Another, more likely, possibility is that there was a decomposition of

P F6− due to reactions involving traces of water or other impurities in the solvents (i.e., EC and DEC). 14,41,42 The C − F peak could only be observed for rather small analysis depths and its intensity was found to increase during the scan from 1.2 V to 0.05 V vs. Li+ /Li only to decrease again during the oxidation scan. This peak could in fact not be seen in the F 1s spectra recorded after cycling to 2.5 V vs. Li+ /Li. It is therefore reasonable to assume that there is a partial dissolution of the SEI layer in the potential region where the SEI forming reactions do not take place. It can also be seen that the relative intensity of the LiF peak increased in comparison to those for the P − F and C − F peaks when the larger analysis depths were used and that the relative LiF peak area increased in comparison to that for the P − F peak when scanning from 1.0 V to 2.5 V vs. Li+ /Li. This shows that the P − F and C − F species were located in the outer parts of the SEI whereas the LiF was located deeper inside the SEI layer. This is in good agreement with previous ndings for, e.g., silicon

23



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Page 24 of 46

and graphite electrodes. 43,44 Based on these results it is hence reasonable to assume that the less polar, organic species (e.g., C − F and P − F compounds) present in the outer parts of the SEI dissolve to a larger extent in the EC/DEC based electrolyte than the more polar, inorganic compounds (e.g., LiF ) present in the bulk of the SEI in accordance with previous ndings. 45,46

3.3. Scanning electron microscopy (SEM) b) 2.5 V (after 1 cycle)

a) pristine electrode

2 μm

2 μm

Figure 7: SEM images of a) a pristine electrode that had not been built into a battery and b) an electrode after one complete CV cycle stopped in the oxidized state at 2.5 V vs. Li+ /Li. Figure 7a and b, show SEM images of a pristine electrode and an electrode that had been stopped at 2.5 V vs. Li+ /Li on the rst cycle. As the SnO2 particles easily can be seen in Figure 7a it can be concluded that an SEI layer was absent on the pristine electrode. On the cycled electrode, a poorly conducting layer was, on the other hand, present which made it more dicult to observe the SnO2 particles (see Figure 7b). The SEM images obtained for the other potentials (i.e., 1.2 V; 0.4 V; 0.05 V and 1.0 V vs. Li+ /Li), which are shown in the Supplementary Information (Figure S4), further support the hypothesis that an SEI layer was present at a potential of 1.2 V vs. Li+ /Li. As the SEM image for an electrode stored for four hours at OCV (see the Supplementary data, Figure S3) was very similar to the image for the pristine electrode it is reasonable to conclude that there was no proper SEI formation prior to the electrochemical cycling, even though typical SEI components still 24


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could be seen in the OCV F 1s spectra.

4. Conclusion

The present electrochemical and photoelectron spectroscopic results, which were obtained by using dierent excitation energies, clearly indicate the presence of a signicant overlap between the electrochemical reactions taking place upon the cycling of SnO2 electrodes in LIBs. The results clearly show that it is unrealistic to ascribe the dierent peaks in the voltammograms to single specic reactions. On the rst reduction cycle, the overlap between the conversion and alloy formation reactions resulted in the formation of electrodes containing a surface layer composed of Lix Sn on top of a layer of Sn and a core of unreacted SnO2 . The Sn3d5/2 PES spectra for electrodes cycled to 0.4 V vs. Li+ /Li on the rst reductive scan hence featured peaks due to both Lix Sn and Sn indicating that the reduction of the

SnO2 was incomplete at the onset of the Lix Sn formation. A peak due to SnO2 was also seen for the electrodes cycled to 0.05 V vs. Li+ /Li. The incomplete SnO2 reduction can be explained by the formation of a protecting layer of Sn on top of the remaining tin oxide as the reduction then becomes limited by the diusion rate of the Li+ through the growing layer of Sn and Li2 O generated by the conversion reaction. At the onset of the Lix Sn alloy formation, the reduction of the SnO2 becomes even more dicult since the lithium ions then are reduced to elemental lithium which reacts with Sn to yield the Lix Sn alloy. At this point, the SnO2 can only be reduced via a reaction between the SnO2 and elemental lithium a process which, however, requires that the lithium atoms diuse through the Lix Sn and Sn layers. An analogous redox reaction overlap was also found between the dealloying and tin oxide forming reactions on the rst anodic scan. Large Lix Sn peaks and small SnO2 peaks could hence be observed in the Sn3d5/2 spectra for electrodes cycled to 1.0 V vs. Li+ /Li (i.e., to a potential where the dealloying reaction generally is assumed to be complete). The

25



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regeneration of SnO2 was found to be incomplete as a result of the formation of a passivating

SnOx layer which protected the remaining Sn. The SnO2 regeneration rate is then limited by the diusion rate of Li+ through the growing SnOx layer. A further complication is that the formed SnOx can undergo reduction in contact with elemental lithium diusing towards the electrode surface as the oxidation of the Lix Sn is incomplete at the onset of the oxidation generating SnO. These ndings indicate that the capacity losses seen during the cycling of

SnO2 electrodes can be explained by the incomplete oxidation of the Lix Sn alloy and the problems associated with the attainment of a complete regeneration of the SnO2 . The main SEI formation on the SnO2 electrodes was found to take place at potentials lower than 1.2 V vs. Li+ /Li even though some SEI species could be detected on the SnO2 electrodes already after the assembly of the battery. The C −F and P −F species were found to be present at the SEI surface while the more polar inorganic components, e.g., LiF , were situated in the inner part of the SEI layer. At potentials lower than about 1.2 V vs. Li+ /Li, the SEI formation process is most likely driven by the diusion of the electrolyte though the existing SEI layer. The signicant dissolution of the C − F and P − F species found at the potentials above about 1.2 V vs. Li+ /Li can be explained by the fact that less polar SEI components can be expected to be dissolved to a higher extents in the organic solvent based electrolytes than more polar, inorganic components. These ndings hence indicate that the steady state thickness of the SEI layer is determined by the balance between the SEI formation and dissolution rates.

Acknowledgement

This work was funded by the Swedish Foundation for Strategic Research (SSF) with additional support from StandUp and the Ångström Advanced Battery Center. The authors thank Maria Hahlin and Julia Maibach for their help and support regarding the HAXPES experiments carried out at the Bessy II synchrotron facility. The Helmholtzzentrum Berlin

26


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is acknowledged for providing beam time and technical support during the HAXPES measurements.

Supporting Information Available

The following les are available free of charge. The Supporting Information (SI) includes in-house XPS spectra obtained for a SnO2 electrode stored in a battery for four hours at open circuit voltage (OCV) as well as tables with the relative peak areas for all the observed C1s peaks obtained with excitation energies of 1487 and 2005 eV. The SI further includes P1s HAXPES spectra measured with an excitation energy of 6015 eV as well as additional SEM images for the electrodes for the potentials marked in Figure 1 as well as under open circuit conditions. Finally, the SI features a rst cycle voltammogram recorded with at a scan rate of 0.1 mV/s.

References

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Page 32 of 46

Page 33 of 46

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Graphical TOC Entry

A model for the overlapping between alloying and conversion reactions of

SnO2

in LIBs during reduction as well as oxidation based on observations

made by hard X-ray photoelectron spectroscopy.

33



Current [mA]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Page 34 of 46

Charge/Oxidation

5 1.0 V

2.5 V

0 1.2 V

Discharge/Reduction

-5 0.4 V

-10 -15 0.0

0.05 V

0.5

1.0

1.5

2.0

Potential [V vs. Li+/Li] ACS Paragon Plus Environment

2.5

a

Page 35 of 46

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


hv = 1487 eV

hv = 2005 eV

hv = 6015 eV

pristine SnO 2

SnO2

SnO 2


0.4 V

0.05 V Li x Sn

Sn

LixSn

Sn

Li x Sn Sn

ACS Paragon Plus Environment 490

488


480

490

488

486 484 482 Bindingenergy energy[eV] [eV] Binding

480

490

488

486 484 482 Binding energy [eV] Binding energy [eV]

480


Page 36 of 46

Increasing analysis depth 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

b

hv = 1487 eV

hv = 2005 eV

hv = 6015 eV

L i2

2.5 V SnOx

SnO x

SnOx

Charge Charge/Oxidation

Sn

1.0 V LixSn

Li x Sn

Sn

Sn

LixSn

490

488


480

490

488

486

484

482

480

Binding energy energy [eV] Binding [eV]

ACS Paragon Plus Environment Increasing analysis depth

490

488

486 484 482 Binding energy [eV] Binding energy [eV]

480

a

Page 37 of 46

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


hv = 1487 eV

hv = 2005 eV

hv = 6015 eV

pristine SnO 2

C-O

SnO 2

C-O

SnO 2


0.4 V

Li 2 O

C=O ROLi/ SnO x

Li 2 O

ROLi/ SnO x

C-O

Li 2 O

C=O

C=O

ROLi/SnO x

0.05 V


534 532 530 528 Binding ener [eV] gy[eV] [eV] Binding Binding energy energy

526

536


526

536


526


Page 38 of 46


hv = 1487 eV

hv = 2005 eV

526

536

528 530 532 534 Binding energy [eV] energy[eV] Binding

526


536

Li 2 O

532 530 528 Binding energy [eV]

C=O

C-O

ROLi/ SnO x

534

Li 2 O

C=O

C-O 536

C=O

1.0 V

ROLi/SnO x

x

C-O

ROLi/SnO

2.5 V

hv = 6015 eV

Charge/Oxidation

b

Li 2 O

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

534 532 530 528 Bindingenergy energy[eV] [eV] Binding

526

a

Page 39 of 46


hv = 1487 eV

hv = 2005 eV

hv = 6015 eV

pristine 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


0.4 V

CB

CB

C-H

-CO 2 -

CO

0.05 V

290 288 286 284 Bindi ng ener gy [eV]

Binding energy [eV]

282

280

294

292

290 288 286 284 Binding energy [eV]

Binding energy [eV]

CB

292

C-H


294

CO

CO3

CO

Binding energy [eV]

-CO 2 -

CO3

C-H

-CO 2 CO3

294 292 290 288 286 284 282 280 Bindi ng ener gy [eV]

282

280


Page 40 of 46


b

hv = 1487 eV

hv = 2005 eV

hv = 6015 eV

2.5 V

Binding energy [eV]


294

292

290

C-H

-CO 2 - /CO 3

C-H

-CO 2 -

CO3

294 292 290 288 286 284 282 280 Binding energy [eV]

Charge/Oxidation

CB

CO

CB

C-H

-CO 2 CO3

294 292 290 288 286 284 282 280 Bindi ng ener gy [eV] Binding energy [eV]

CB

CO

CO

1.0 V

288

286

284

Binding [eV] Bindingenergy energy [eV]

282

280

Page 41 of 46

in-house XPS (1487 eV) HAXPES (2005 eV)

60

CB relative peak areas [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


50 40 30 20 10 0

OCV pristine

A 1.2

B 0.4

C 0.05

D 1.0

+ Potential [V vs. Li /Li] ACS Paragon Plus Environment

E 2.5

a 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


hv = 1487 eV

hv = 2005 eV

hv = 6015 eV

Page 42 of 46

pristine

1.2 V Discharge/Reduction Discharge

0.4 V

0.05 V LiF

P-F

LiF

C-F

LiF

P-F

P-F

C-F



682

692


682

692

690 688 686 684 Binding energy [eV] Binding energy [eV]

682

Page 43 of 46



b

hv = 1487 eV

hv = 2005 eV

hv = 6015 eV

LiF P-F

692

690 688 686 684 Bindi ng ener gy [eV] [eV] Binding energy

682


692

690

LiF

682

P-F

690 688 686 684 Binding energy[eV] [eV] Binding energy

C-F

LiF

C-F 692

P-F

1.0 V

Charge/Oxidation Charge

2.5 V

688 686 684 Binding [eV] Binding energy energy [eV]

682

a) pristine electrode

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42



Page 44 of 46

2 μm

Page 45 of 46

b) 2.5 V (after 1 cycle)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42



2 μm


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


Page 46 of 46

Photoelectron Spectroscopic Evidence for Overlapping Redox

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