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Solid Electrolyte Interphase (SEI) at TiO2 Electrodes in Li-ion Batteries. Defining Apparent and Effective SEI Based on Evidence from XPS and SECM Edgar Ventosa, Edyta Madej, Giorgia Zampardi, Bastian Mei, Philipp Weide, Hendrik Antoni, Fabio La Mantia, Martin Muhler, and Wolfgang Schuhmann ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13306 • Publication Date (Web): 30 Dec 2016 Downloaded from http://pubs.acs.org on December 31, 2016

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ACS Applied Materials & Interfaces

Solid Electrolyte Interphase (SEI) at TiO2 Electrodes in Li-ion Batteries. Defining Apparent and Effective SEI Based on Evidence from XPS and SECM Edgar Ventosa,[1]* Edyta Madej,[1] Giorgia Zampardi,[1,2] Bastian Mei,[1,3,4] Philipp Weide,[3] Hendrik Antoni,[3] Fabio La Mantia,[2,5] Martin Muhler,[3] Wolfgang Schuhmann,[1]*

[1]

Analytical Chemistry – Center for Electrochemical Sciences (CES) , Ruhr-Universität Bochum, Universitätsstr. 150, D-44780 Bochum, Germany.

[2]

Semiconductor and Energy Conversion – Center for Electrochemical Sciences (CES), RuhrUniversität Bochum, Universitätsstr. 150, D-44780 Bochum, Germany [3]

Laboratory of Industrial Chemistry, Ruhr-Universität Bochum, Universitätsstr. 150, D-44780 Bochum, Germany

[4]

Photocatalytic Synthesis Group, MESA+ Institute for Nanotechnology, Faculty of Science and Technology, University of Twente, Meander 229, P.O. Box 217, 7500 AE Enschede, The Netherlands [5]

Energiespeicher- und Energiewandlersysteme, Universität Bremen, Wiener Str. 12, D-28359 Bremen, Germany Email: [email protected] and [email protected]

ACS Paragon Plus Environment

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ABSTRACT The high (de-)lithiation potential of TiO2 (ca. 1.7 V vs. Li/Li+ in 1M Li+) decreases the voltage and, thus, the energy density of a corresponding Li-ion battery. On the other hand it offers several advantages such as the (de-)lithiation potential far from lithium deposition or absence of a solid electrolyte interphase (SEI). The latter is currently under controversial debate as several studies reported the presence of a SEI when operating TiO2 electrodes at potentials above 1.0 V vs. Li/Li+. We investigate the formation of a SEI at anatase TiO2 electrodes by means of X-ray photoemission spectroscopy (XPS) and scanning electrochemical microscopy (SECM). The investigations were performed in different potential ranges, namely, during storage (without external polarization), between 3.0 – 2.0 V and 3.0 – 1.0 V vs. Li/Li+, respectively. No SEI is formed when a completely dried and residues-free TiO2 electrode is cycled between 3.0 – 2.0 V vs. Li/Li+. A SEI is detected by XPS in case of samples stored for 6 weeks or cycled between 3.0 – 1.0 V vs. Li/Li+. Using SECM it is verified that this SEI does not possess the electrically insulating character as expected for a “classic” SEI. Therefore, we propose the term apparent SEI for TiO2 electrodes to differentiate it from the protecting and effective SEI formed at graphite electrodes.

KEYWORDS Li-ion batteries, solid electrolyte interphase, X-ray photoemission spectroscopy, scanning electrochemical microscopy, titanium dioxide


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1. INTRODUCTIONS The large cell voltage (above 3 V) provided by Li-ion batteries (LIBs) contributes to their high energy density.1 However, the high cell voltage requires using solvents with a large electrochemical stability window. For decades, mixtures of linear and cyclic alkyl carbonates have been, and still are, used as solvents for LIBs. Despite their large electrochemical stability window, carbonate-based solutions decompose at graphite negative electrodes due to the very cathodic operating conditions (ca. 0.1 V vs. Li/Li+ in 1M Li+). Fortunately, the products of the cathodic decomposition precipitate on the surface of the electrode forming an electrically insulating film that is usually called solid electrolyte interphase (SEI). This SEI prevents the electrolyte solution from further decomposition, which allows operating at potentials outside the stability window of the electrolyte solution. The SEI plays a key role on the one hand in determining the life cycle of LIBs by avoiding the continuous consumption of electrolyte solution and on the other hand it determines the safe operation of the LIB by preventing contact between very reactive lithiated graphite and the electrolyte solution.2-4 The cathodic decomposition of the most commonly used carbonate-based solutions has been reported to occur at 1.0 – 0.8 V vs. Li/Li+ (1M Li+) on carbonaceous materials.5-7 Higher safety and prolonged durability are achieved at expense of energy density, by deploying negative electrode materials which operate at more anodic potentials (> 0.8 V vs. Li/Li+ in 1M Li+). This becomes especially important when operating above 60 °C, due to the thermal dissolution of SEI.8,9 Titanate materials such as Li4Ti5O12 or TiO2 were proposed as high-power negative electrode materials since their (de-)lithiation potential of 1.5 – 1.8 V vs. Li/Li+ (1M Li+) avoids the electrochemical decomposition of carbonate-based solutions and offers a large voltage gap to lithium deposition which prevents electroplating of metallic lithium at high charge-rates.10-12


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Titanates may be considered as SEI-free negative electrode materials due to their comparatively high (de-)lithiation potentials. On the other hand, there are studies reporting that electrolyte solution reacts with Li4Ti5O12 (LTO) electrodes, even spontaneously, leading to the evolution of gases (so-called gassing).13-15 The electrolyte decomposition and gassing prevent full scale implementation of LTO for high-power and high-safety LIBs. Consequently, great efforts have recently been devoted to the understanding of SEI formation and gassing process at LTO electrodes. Meanwhile, the understanding of electrolyte decomposition at TiO2 electrodes did not receive much attention despite the excellent performances delivered by this material for LIBs.1620

Gassing process at TiO2 electrodes is yet to be properly addressed, and whether the formation

of a SEI occurs is still under debate. We suppose that the definition of the SEI is the origin of the debate. It is commonly agreed that the electronically insulating character of the SEI is its most important property since it is preventing continuous decomposition of the electrolyte solution. Therefore, despite the fact that the SEI refers, strictly speaking, to an electronically insulating solid film formed at the electrode/electrolyte interface, it has been frequently used in literature to generally indicate any solid film deposited during battery operation. The term cathode electrolyte interphase (CEI) is sometimes used to refer to a film formed at the positive electrode since compositions and properties of SEI and CEI differ significantly.21-23 In order to allow for a more clear differentiation between possible types of deposited solid films on electrode surfaces, we propose to distinguish two types of SEI: the effective SEI and the apparent SEI referring to electronically insulating and conducting films, respectively. In this contribution, we investigate the formation of a SEI on TiO2 electrodes to determine the nature of the formed film. The formation of a SEI of


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any type was examined by XPS measurements, whereas the electronic properties of the formed SEI were determined by means of feedback-mode SECM.

2. EXPERIMENTAL SECTION Anatase TiO2 nanoparticles with a specific surface area of ca. 300 m2 g–1 provided by Sachtleben Chemie were annealed at 450 °C in synthetic air for 1 h. The BET specific surface area of the annealed sample was 130 m2 g-1. Electrode slurries were prepared with 76:15:9 wt% composition of active material, C65 carbon black (Timcal) and polyvinylidene difluoride (PVdF) binder (Solef S5130, Solvay) dispersed in N-methyl pyrrolidone (NMP) (Sigma-Aldrich) and mixed thoroughly for 30 min at 4000 rpm using an ultra-turrax disperser (Ika). The slurry was then deposited onto a copper current collector (15 µm) using the “doctor blade” technique (200 µm film thickness) and dried at 60 °C for 2 h. 12 mm disk electrodes were punched out with a commercially available hole punch (Hoffmann) and dried overnight at 110 °C in a vacuum oven (Büchi) resulting in an active material loading of ca. 2 mg cm–2. Cells were assembled in an argon-filled glove box (O2 < 2 ppm and H2O < 1 ppm). Metallic lithium (Sigma-Aldrich, d = 0.38 mm) was used as counter and reference electrodes, Whatman GF/D glass fibre filters as separator and 1 M LiPF6 in ethylene carbonate (EC):diethyl carbonate (DEC), 1:1 wt% (LP40, Merck) as electrolyte. Pure dimethyl carbonate (DMC) from Sigma-Aldrich was employed to remove electrolyte salt from the electrodes that were disassembled inside the glovebox after experiments. Galvanostatic cycling (GC) experiments were carried out using a Bio-Logic VMP-3 potentiostat (Bio-Logic). Charge/discharge rates of 0.5 C (assuming the theoretical capacity of TiO2 as 330 mA g–1) were applied during cell cycling within potential windows of 3.0 V – 2.0 V


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or 3.0 V – 1.0 V, respectively. All potentials are reported versus Li/Li+ in 1 M LiPF6 and all electrochemical experiments were performed with a 3-electrode cell setup using standard Swagelok® T-cells. For the evaluation of spontaneous formation of a SEI, the cells were assembled without lithium counter-electrode to avoid possible self-discharge. By this, the cell consisted of the TiO2 paste electrode and the separator soaked with electrolyte. Consequently, the presence of any formed SEI film would result from the spontaneous reaction between the carbonate-based electrolyte solution and anatase TiO2. SECM measurements were carried out inside an Ar-filled glovebox using a set-up as described elsewhere.24,25 A platinum ultra-microelectrode (UME) with 10 µm diameter was employed as SECM tip. Ferrocene (Fc; 98% Sigma Aldrich) with a concentration of 10 mM was used as free-diffusing redox species in solution. The supporting electrolyte was 1 M lithium perchlorate (battery grade, dry, 99.99% Sigma Aldrich). All electrolytes were based on ethylene carbonate (EC) (anhydrous 99%, Sigma Aldrich) and propylene carbonate (PC) (anhydrous 99.7%, Sigma Aldrich), 1:1 weight ratio. All potential values are referred to a reference electrode composed of metallic lithium immersed in 1 M LiClO4 dissolved in EC:PC solution and separated from the main body of the cell by a ceramic frit. The SECM tip was positioned at a distance of 7 µm above the sample surface for operando SECM measurements. XPS measurements were carried out in an UHV set-up equipped with a Gammadata-Scienta SES 2002 analyser. The base pressure in the measurement chamber was 5 × 10−10 mbar. Monochromatic Al Kα (1486.6 eV; 14.5 kV; 30 mA) was used as incident radiation and a pass energy of 200 eV was chosen resulting in an energy resolution better than 0.6 eV. Charging


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effects were compensated using a flood gun, and binding energies were calibrated by positioning the main C 1s peak at 284.5 eV which originates from carbon black.

3. RESULTS AND DISCUSSION. 3.1 Spontaneous formation of a SEI on TiO2. The evolution of gas (gassing) has been reported to occur spontaneously at Li4Ti5O12 (LTO) electrodes.13-15 Although the gassing mechanism at LTO electrodes is not fully elucidated, several possible mechanisms have been proposed,26-28 and several strategies have been developed to overcome this issue.15,29-31 However, only Liu et al. investigated the spontaneous evolution of gas when anatase TiO2 electrodes were soaked with carbonate based electrolyte solutions.32 The volume of the evolved gas was dependent on the type of electrolyte solution, the storage time and temperature. The presence of ROLi and ROCOOLi species was confirmed by infrared spectroscopy and X-ray photoemission spectroscopy (XPS). Although the authors did not use the term SEI, this study suggests that the formation of a SEI occurs spontaneously at anatase TiO2 electrode. The question remains as to whether this film is electrically passivating or not, i.e. if an effective or apparent SEI was formed. The spontaneous formation of a SEI on carbon electrodes (the same composite electrodes however without TiO2) was first evaluated in order to elucidate whether the spontaneous reaction occurs at TiO2 or at any other element of the electrode. XPS measurements on carbon electrodes stored in 1 M LiPF6 in ethylene carbonate/diethyl carbonate (EC:DEC, 1:1 wt/wt) for 6 weeks at 25 °C (Figure S1 and S2) led to two important conclusions. On the one hand, organic species from the SEI were not detected in the C 1s region suggesting that a spontaneous reaction does not take place in the absence of TiO2. It should be noted that an organic layer spontaneously


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formed on carbon was detected by synchrotron-based XPS providing higher surface sensitivity.33 On the other hand, traces of separator were detected in the samples even after careful washing of the electrode with pure dimethyl carbonate (DMC) inside the Ar-filled glovebox. These traces of separator contain oxides (SiO2, Na2O and K2O) which hinder the analysis of the SEI in the O 1s region. Therefore, the C 1s region of XPS spectra was chosen for the analysis of the SEI since this region was not affected by the impurities making it more reliable. In addition, a carbon electrode was electrochemically cycled between 3.0 – 1.0 V vs. Li/Li+ (1 M Li+) and organic species from SEI were not found in the XPS measurement. Anatase TiO2 electrodes were stored in 1 M LiPF6 in EC:DEC (1:1 wt/wt) for 6 weeks at either 25 °C or 60 °C. The cells were disassembled inside an Ar-filled glovebox, the electrodes were washed with pure DMC and characterized by X-ray photoemission spectroscopy (XPS) and scanning electrochemical microscopy (SECM). Three samples are compared; one sample which had no contact with the electrolyte solution (TiO2(as prep)), one sample which was soaked at 25 °C (TiO2(25°C)) and one sample which was soaked at 60 °C (TiO2(60°C)). Figure 1 shows the high resolution C 1s XPS spectra of the three samples. The peak assignments were made using literature data.7,21,23,34 Five main signals, corresponding to C-C bond from the carbon additive (~284.8 eV), F-C-F bond (~291 eV) from the binder, C-O-C, O-C-O (or –C=O) and O-C=O bonds (~286, 287 and 289 eV, respective) from various organic species, appear in the C 1s region. The C 1s signal of TiO2(as prep) consists mainly of C-C carbon which comes from the carbon additive in the electrode (15 wt% carbon black). The surface groups of carbon black in TiO2(as prep) are responsible for a weak signal observed in C-O region. Interestingly, a substantial increase in intensity in the C-O region was observed for TiO2(25°C). This increase of the C-O like carbon signal indicates the formation of an organic layer on the TiO2 surface.


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Evidently, a SEI is spontaneously formed at the surface of the TiO2 electrode after 6 weeks at 25 °C, which is consistent with the results recently reported by Liu et at.32 The carbon signal in the C-O region did not increase when the sample was stored at 60 °C. Considering the fact that the dissolution of SEI has been reported to occur at moderated temperatures (> 50 °C),8,9 the absence of an organic layer is presumably not due to the suppression of its formation but its enhanced dissolution at elevated temperatures. The O 1s region in the XPS spectra of samples stored in electrolyte was dominated by the traces of separator (Figure S5). The deconvolution of the spectra considering the presence of separator confirmed the formation of an organic film. The F 1s region of the samples revealed the absence of LiF in the organic film (Figure S6), which is a major difference with respect to classic SEI. It should be noted that the samples were exposed to air for few seconds while loading them into the XPS system. A TiO2 electrode was cycled between 3.0 V and 0.1 V vs. Li/Li+ (1M Li+) to ensure the formation of SEI on the surface. The sample was split into two pieces. One piece was exposed to air for few second while the other piece was transported from the Ar-filled glovebox into the XPS system using an Ar-filled vessel avoiding air-contact. As shown in Figure S3 and S4, the intensity of the some organic species slightly changed when air-contact was avoided. However, the presence of an organic film can still be confirmed for samples momentarily exposed to air. Figure S3 and S4 confirm that the transportation of the samples in the Ar-fill vessel is not necessary to determine the presence of the film as long as a quantitative estimation of the species is not intended.


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Figure 1. XPS spectra for the C 1s regions of TiO2(as prep), TiO2(25°C) and TiO2(60°C) before being electrochemically cycled. Areas in red, blue, magenta, green and orange corresponds to C-C, C-O-C, O-C-O or –C=O, O-C=O and F-C-F, respectively. Note that O 1s and F 1s regions of these samples are shown in supporting information. The surface reactivity of the three samples was investigated by means of feedback mode SECM, which was previously used to study properties of the SEI in Li-ion batteries.24,25,35-37 The measurements were carried out in the presence of ferrocene as free-diffusing redox mediator. The SECM tip, a 10 µm diameter Pt disk electrode, was polarized at a constant potential of 3.6 V vs. Li/Li+ (1M Li+) When approaching to a non-reactive surface, the current at the microelectrode decreases because the diffusion of ferrocene is hindered by the non-reactive surface (negative feedback). On the contrary, the current at the microelectrode increases when approaching to a reactive and conductive surface due to the regeneration of ferrocene (positive


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feedback).38,39 Figure 2a shows the simulated approach curves to flat samples with indefinitely high reaction rate of the mediator at the sample (positive feedback) or reaction rate equals 0 (negative feedback). The positive feedback in the approach curves of the three samples, TiO2(as prep), TiO2(25°C) and TiO2(60°C), revealed that the surface of the electrode remained electrochemically reactive after storage for 6 weeks at 25 °C and 60 °C (Figure 2b).

Figure 2. (a) Simulated approach curves to flat samples (i) with indefinitely high reaction rate of the mediator at the sample (positive feedback) and (ii) reaction rate equals to 0 (negative feedback) for a RG value of 10. SECM approach curves experimentally obtained for (b) TiO2(as prep), TiO2(25°C) and TiO2(60°C) and (c) TiO2(25°C) and TiO2(60°C) after being cycled in the potential range of 3.0 – 0.5 V vs. Li/Li+ (1M Li+). IT/Ibulk (Y-axes) and d/r (X- axes) are the


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current at the microelectrode (tip) normalized by the current recorded far from the electrode (bulk of the electrolyte) and the distance to the electrode normalized by the radius of the microelectrode, respectively. It should be noted that the non-ideal behavior in the approach curve of TiO2(60°C) was probably due to topographic effects. TiO2(25°C) and TiO2(60°C) were electrochemically cycled between 3.0 – 0.5 V vs. Li/Li+ (1M Li+) in an attempt to form a “classic” effective SEI (cyclic voltammograms in Figure S7)The approach curves in both cycled samples (Figure 2c) showed negative feedback indicating that an electrically insulating character of the films is achieved by polarizing the electrode down to 0.5 V vs. Li/Li+ (1M Li+). This drastic change from apparent to effective SEI as detected by SECM cannot be seen by means of XPS (Figure 3). The comparison between the XPS results of an apparent (Figure 1) and effective SEI (Figure 3) does not allow for an unambiguous assignment of any specific species to the insulating character of the film. Although the origin of the spontaneous reaction remains unknown requiring future efforts to be devoted to the understanding of the mechanism of its formation, our results demonstrate that the properties of this spontaneously formed organic film drastically differ from those of a classic SEI.


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Figure 3. XPS spectra for the C 1s of TiO2(as prep), and TiO2(25°C) and TiO2(60°C) after being electrochemically cycled between 3.0 – 0.5 V vs. Li/Li+ (1M Li+). Areas in red, blue, magenta, green and orange correspond to C-C, C-O-C, O-C-O and –C=O, O-C=O and F-C-F, respectively. Note that O 1s and F 1s regions of these samples are shown in supporting information.

3.2 Electrochemical formation of SEI at operating potentials above 2.0 V vs. Li/Li+. There is no agreement on the onset potential of the electrochemical SEI formation at TiO2 electrodes. We first studied the formation of a SEI in the potential range of 3.0 – 2.0 V vs. Li/Li+ (1M Li+) as the electrochemical formation of a SEI at TiO2 electrodes was recently reported to occur even above 2.0 V vs. Li/Li+ (1M Li+).40 Commercially available anatase TiO2 was airannealed at 450 °C which was shown to drastically improve the cyclability of corresponding TiO2 electrodes. This was attributed to the dehydration of TiO(OH)2 residues minimizing side


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reactions.41 In addition to the standard drying procedure (vacuum oven at 110 °C overnight), the sample was dried inside the glovebox using a hot plate at 130 °C (TiO2(dried)) to further reduce the amount of water adsorbed on TiO2. No significant changes in the C 1s (Figure 4) nor O 1s (Figure S8) regions of the XPS spectra were observed between TiO2(dried) and TiO2(as prep) indicating that SEI was not formed at TiO2 electrodes when cycled in the potential range of 3.0 – 2.0 V vs. Li/Li+ (1M Li+). Additionally the F 1s region (Figure S8) also remained unchanged. Thus, hydrolysis of LiPF6 can be excluded. For comparison, a sample was stored in water saturated (TiO2(sat)) atmosphere for a few days to increase the amount of water adsorbed at the TiO2 surface, and then cycled in the same potential range as TiO2(dried). A slight increase in C=O and O-C=O signals was observed in the C 1s region of TiO2(sat) which was consistent with the increase in C=O signal in the O 1s region (Figure S8). As expected, the hydrolysis of LiFP6 caused by traces of water in the sample resulted in a substantial increase of the signal intensity in the F 1s region (Figure S8).26,27 Comparison between different TiO2 materials cycled in the potential range of 3.0 – 2.0 V vs. Li/Li+ (1M Li+) shows that the formation of a SEI at TiO2 above 2.0 V vs. Li/Li+ (1M Li+) may be triggered by or originated from side reactions. The presence of traces of water in Li4Ti5O12 electrodes was proposed to trigger the formation of a SEI-like layer by reduction of water and formation of OH– that leads to a nucleophilic attack and opening of cyclic EC.27 Similarly, the drastic improvement in the cyclability of commercial TiO2 by simple air-annealing at 450 °C likely originated from removal of traces of water.41 Obviously, side reactions must be taken into consideration when evaluating the formation of an organic surface layer on a TiO2 surface.


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Figure 4. XPS spectra for the C 1s region of TiO2(as prep), and TiO2(dried) and TiO2(sat) after being cycled within the potential range of 3.0 – 2.0 V vs. Li/Li+ (1M Li+). Areas in red, blue, magenta, green and orange correspond to C-C, C-O-C, O-C-O and –C=O, O-C=O and F-C-F, respectively. Note that O 1s and F 1s regions of these samples are shown in supporting information. The electrochemical reactivity of the electrode surface was investigated by operando SECM in the potential range of 3.0 – 2.0 V vs. Li/Li+ (1M Li+). Figure 5 shows the cyclic voltammogram of a TiO2(dried) sample together with the feedback current simultaneously recorded at the SECM tip positioned 7 µm above the sample surface. The feedback current did not change in the investigated sample potential range. A positive IT/Ibulk value of 1.5 was recorded at the SECM tip while polarizing the TiO2 electrode between 3.0 – 2.0 V vs. Li/Li+ (1M Li+). It was possible to successfully carry out SECM measurements for TiO2(sat). Although several attempts were


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made, the microelectrode fouled during the experiment. Although the nature of the fouling is not clear, it has to be related to products released by TiO2(sat) during electrochemical cycling. Therefore, it was not possible to determine the electrically insulating character of the SEI formed at TiO2(sat) within the potential range of 3.0 – 2.0 V vs. Li/Li+ (1M Li+).

Figure 5. Operando feedback mode SECM measurement of TiO2(dried) cycled in the potential range of 3.0 – 2.0 V vs. Li/Li+ (1M Li+). The black line is the cyclic voltammogram of the TiO2 electrode and the red line is the simultaneously recorded feedback current at the SECM tip. Insets (i) and (ii) are schematic representations of positive feedback and negative feedback, respectively. Neither XPS nor SECM detected the formation of a SEI at dried TiO2 electrodes when cycled between 3.0 V – 2.0 V vs. Li/Li+ (1M Li+). Presence of traces of water is suggested to be possibly the source of the formation of surface organic layer at TiO2 electrodes in this potential range. 3.3 Electrochemical formation of SEI at operating potentials above 1.0 V vs. Li/Li+.


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Finally, we studied the formation of a SEI at TiO2 electrodes within the most commonly used potential range of 3.0 – 1.0 V vs. Li/Li+ (1M Li+). Again, TiO2(dried) and TiO2(sat) were characterized by XPS after being electrochemically cycled. In contrast to TiO2(dried) and TiO2(sat) cycled in the potential range of 3.0 – 2.0 V vs. Li/Li+ (1M Li+) (Figure 4 and Figure S8), the formation of an organic layer with a composition closely resembling that of an SEI layer formed at graphite electrodes7,22,23 was observed when cycled between 3.0 – 1.0 V vs. Li/Li+ (Figure 6 and Figure S9). Furthermore, a thicker SEI layer was observed for TiO2(sat) which is in agreement with the results obtained in the potential range of 3.0 – 2.0 V vs. Li/Li+ (1M Li+). The electrochemical formation of an organic surface layer at carefully dried TiO2 electrodes is initiated below 2.0 V vs. Li/Li+ (1M Li+), whereas other side reactions are likely responsible for the presence of an organic layer above 2.0 V vs. Li/Li+ (1M Li+).

Figure 6. XPS spectra of the C 1s region of TiO2(as prep), and TiO2(dried) and TiO2(sat) after being cycled within the potential range of 3.0 – 1.0 V vs. Li/Li+ (1M Li+). Areas in red, blue,


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magenta, green and orange correspond to C-C, C-O-C, O-C-O and –C=O, O-C=O and F-C-F, respectively. Note that O 1s and F 1s regions of these samples are shown in supporting information. The electrochemical reactivity of the electrode surface upon electrochemical cycling in the potential range of 3.0 – 1.0 V vs. Li/Li+ (1M Li+) was evaluated by operando feedback mode SECM performed for TiO2(dried) (Figure 7a). Besides the formation of SEI, there are several phenomena affecting the feedback current. Li+ uptake and release in/from TiO2 induces migration of ions affecting the mass transport of the redox mediator or the increased electric conductivity of TiO2 upon reduction enhances the kinetics of regeneration of redox mediator. However, the formation of a classic SEI results in a unique behavior of the feedback current: a change from positive to negative feedback (from IT/Ibulk > 1 to IT/Ibulk 1) indicates cracking of an existing SEI. These two scenarios are clearly illustrated in Si thin film model electrodes.42 Therefore, the crossing of IT/Ibulk = 1 can be unambiguously attributed to processes related to the SEI film. The feedback current always remained well above unity during the entire cyclic voltammetry, and a similar positive feedback value of ca. 2 was obtained before and after polarizing the electrode in this potential range. SECM measurements showed that the electrochemical reactivity of the electrode surface did not decrease upon electrochemical cycling in the potential range of 3.0 – 1.0 V vs. Li/Li+ (1M Li+). In situ approach curves of the SECM tip towards TiO2(dried) before and after electrochemical cycling confirmed that the surface electrode remained electrochemically reactive (Figure 7b). Either the properties of this film are different from those of a SEI formed at graphite electrodes or the thickness of the film is not sufficient to supress the electrochemical reactivity of the electrode surface. An apparent SEI is


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formed when cycling an anatase TiO2 electrode between 3.0 – 1.0 V vs. Li/Li+ (1M Li+) as revealed by XPS. However, this film does not possess the electrochemically passivating character of an effective SEI. This fact is presumably responsible for the debate on the presence of a SEI at TiO2 electrodes, since besides the XPS results the electrically insulating character of the SEI needs to be evaluated using SECM. Consequently, we propose using the term apparent SEI for TiO2 electrodes when operating above 1.0 V vs. Li/Li+ (1M Li+) to differentiate it from the protecting and effective SEI formed at graphite electrodes below 0.8 V vs. Li/Li+ (1M Li+).


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Figure 7. (a) Operando feedback mode SECM measurement of TiO2(dried) in the potential range of 3.0 – 1.0 V vs. Li/Li+ (1M Li+). The black line is the cyclic voltammogram of the TiO2 electrode and the red line is the simultaneously recorded feedback current at the SECM tip. Insets (i) and (ii) are schematic representations of positive feedback and negative feedback, respectively. (b) In site approach curves to TiO2(dried) after being electrochemically cycled between 3.0 V – 1.0 V vs. Li/Li+ (1M Li+). IT/Ibulk and d/r are the current at the microelectrode (tip) normalized by the current recorded far from the electrode (bulk of the electrolyte) and the distance to the electrode normalized by the radius of the microelectrode, respectively.

4. CONCLUSIONS The formation of a SEI at anatase TiO2 electrodes during storage in contact with the electrolyte (without external polarization) and in potential ranges of 3.0 – 2.0 V and 3.0 – 1.0 V vs. Li/Li+ (1M Li+), was investigated by XPS and SECM. XPS was employed to detect the presence of any surface film, whereas SECM was used to determine the electrochemical reactivity of the electrode surface. Storage of TiO2 electrodes in 1 M LiPF6 in EC:DEC for 6 weeks at 25 °C led to the spontaneous formation of a SEI-like layer. Increasing the storage temperature to 60 °C resulted in a thinner film likely due to its partial dissolution. In both cases, the electrochemical reactivity of the electrode surface was not suppressed by the formation of an apparent SEI. In the potential range of 3.0 – 2.0 V vs. Li/Li+ (1M Li+), no SEI was detected. Expectedly, the electrochemical reactivity of the electrode surface did not change in this potential range. A SEI was formed above 2.0 V vs. Li/Li+ (1M Li+) when the TiO2 electrode was stored in water saturated


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atmosphere and dried improperly, which suggests that the formed SEI observed in other studies above 2.0 V may be originating from side reactions. In the most commonly used potential range of 3.0 – 1.0 V vs. Li/Li+ (1M Li+), an apparent SEI was formed at properly dried TiO2 electrodes. Therefore, apparent SEI is formed at TiO2 electrode by either prolonged storage in carbonate-based electrolyte or operating within the potential range of 3.0 – 1.0 V vs. Li/Li+ (1M Li+). For TiO2 electrodes the formation of a “classic” effective SEI can be excluded.

Supporting Information. General survey XPS spectra of several samples (Figure S1). XPS spectra for the C 1s and O 1s region of several samples (Figure S2). XPS spectra for the O 1s region (Figure S3) and C 1s region (Figure S4) of a sample with and without being exposed to air. XPS spectra for the O 1s region and F 1s region of TiO2(as prep), TiO2(25°C) and TiO2(60°C) (Figure S5). Cyclic voltammetry of several samples, i.e. TiO2(as prep), TiO2(25C) and TiO2(60C), in the range 3.0 – 0.5 V vs. Li/Li+ (Figure S6). XPS spectra for O 1s and F 1s region of several samples, i.e. TiO2(as prep), and TiO2(sat) and TiO2(dried), in different potential ranges, i.e. 3.0 – 2.0 V, 3.0 – 1.0 V and 3.0 – 0.5 V vs. Li/Li+ (Figure S7, S8 and S9, respectively).

Corresponding Author *Prof. Wolfgang Schuhmann, e-mail: [email protected], tel: +49 234 32 26200 *Dr. Edgar Ventosa, e-mail: [email protected], tel: +49 234 32 25474 ACKNOWLEDGMENT Financial support from the DFG (Deutsche Forschungsgemeinschaft) in the framework of the Cluster of Excellence RESOLV (EXC1069) is gratefully acknowledged.


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