Evolution of Solid Electrolyte Interface (SEI) on TiO2 electrodes in

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Evolution of Solid Electrolyte Interface (SEI) on TiO2 electrodes in Aqueous Li ion Battery Studied Using Scanning Electrochemical Microscopy Dongqing Liu, Qipeng Yu, Shuai Liu, Kun Qian, Shuwei Wang, Wei Sun, Xiao-Qing Yang, Feiyu Kang, and Baohua Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01412 • Publication Date (Web): 01 May 2019 Downloaded from http://pubs.acs.org on May 1, 2019

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The Journal of Physical Chemistry

Evolution of Solid Electrolyte Interface (SEI) on TiO2 electrodes in Aqueous Li ion Battery Studied Using Scanning Electrochemical Microscopy Dongqing Liua,b, Qipeng Yua,c, Shuai Liua,c, Kun Qiana,c,d, Shuwei Wanga,c, Wei Sunb, Xiao-Qing Yange, Feiyu Kanga,c,d, Baohua Li*a,f

a

Engineering Laboratory for the Next Generation Power and Energy Storage Batteries, Graduate School at Shenzhen, Tsinghua University, Shenzhen, 518055, China b

Sunwoda Electronic Co., Ltd., Baoan District, Shenzhen 518055, China

c

Laboratory of Advanced Materials, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China d

Shenzhen Environmental Science and New Energy Technology Engineering Laboratory, Tsinghua-Berkeley Shenzhen Institute, Shenzhen 518055, China e Chemistry f

Division, Brookhaven National Laboratory, Upton, New York 11973, USA

Shenzhen Geim Graphene Center, Shenzhen, 518055, China

Corresponding Author. E-mail: [email protected]

Abstract: Scanning electrochemical microscopy (SECM) was applied for in situ visualization of solid electrolyte interface (SEI) evolution on TiO2 anode in concentrated aqueous electrolyte during cycling. In non-aqueous electrolytes, the SEI is an electronic insulative layer composed of organic and inorganic components covering the electrode surface. However, in concentrated aqueous electrolyte, the SEI is mostly composed of inorganic compounds formed by electrolyte decomposition, such as LiF, Li2CO3, which are scatteredly distributed over the TiO2 surface. In addition, the Ti3+ and Li residuals accumulated through multiple cycling lead to an increase in the

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overall electronic conductivity of the TiO2 anode. During the resting period after cycling, an inverse process is observed with partial dissolution of the formed SEI and the diffusion of Li residuals back into the electrolyte.

1. Introduction Aqueous lithium-ion batteries (ALIBs) are attracting more and more attention due to their intrinsic non-flammable nature 1. However, the narrow electrochemical window (1.23V) limits their practical application 2. Recently, it was reported that by using highly concentrated aqueous electrolyte, one could expand the electrochemical stability window to ~3.0V, making many Li-ion chemistries possible to achieve energy densities higher than 100Whkg-1 3-5. In aqueous lithium-ion batteries, the formation of an interface on the anode surface can be manipulated and the structure of Li+-solvation sheath plays an important role in the stabilization during high voltage operation

6-8.

Borrowing the

solid electrolyte interphase (SEI) concept from non-aqueous electrolytes, the SEI terminology is used here for aqueous electrolyte. The SEI is formed by the preferentially decomposition of salt anions on the anode surface before hydrogen evolution, and mainly consists of LiTFSI salt

9-11.

LiF in the case of using

To date, information about the SEI formation and its properties has been studied

using various in situ/ex situ techniques, including transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), soft X-ray absorption spectroscopy (SXAS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), differential electrochemical mass spectrometry (DEMS), atomic force microscopy (AFM) and pressure measurements 6-8, 10-15. Scanning electrochemical microscopy (SECM) is an advanced technique that has been used for determining the local interfacial kinetics on a variety of electrodes

16.

In recent years, SECM has

demonstrated its unique and powerful capabilities in investigating the electrochemical processes, especially the formation and evolution of the SEI in LIB

17.

For example, the feedback mode for

SECM (FB-SECM) works by monitoring the modulations in the reaction rate of the redox mediator at the substrate. It can provide laterally resolved information about the electrochemical reactivity of the electrode surface, which is directly related to the electronic character of the SEI


18.

The

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FB-SECM mode has been applied for in situ detection of SEI formation on TiO2 in non-aqueous lithium ion batteries

19-20,

spontaneous spatiotemporal changes of the SEI on graphite21, the

electrically insulating character of the SEI on Si electrode

18,

and etc. To date, not much work has

been reported on using SECM to investigate the SEI formation on anode surface in highly concentrated aqueous electrolyte systems. In this study, the SECM operation in the feedback mode and alternating current SECM (AC-SECM) mode were applied for in situ visualization of the SEI formation on TiO2 substrate during the first cycle and multiple cycles, as well as for study of the stability of the electrochemically formed SEI. This investigation will provide comprehensive understanding of the SEI in highly concentrated aqueous electrolyte, providing valuable guidance for the development of battery material and ALIBs technology.

2. Experimental procedures Electrode material: Carbon-coated anatase TiO2 (99.99%, Sigma-Aldrich) was prepared by carbothermal reduction at sugar/TiO2 mass at 600°C for 2 hours in Ar atmosphere. Electrodes were fabricated by dissolving active material C-TiO2, super P, and polyvinylidene difluoride (PVDF) at a weight ratio of 8:1:1 in N-methyl pyrrolidone (NMP). The slurry was then deposited onto a glass carbon current collector (4 mm diameter) and dried at 60°C for 8h. Electrolyte preparation: The aqueous electrolyte "Water-in-Bisalt" (WIBS) was prepared by dissolving 21 m lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) and 7 m Lithium trifluoromethanesulfonate (LiOTf) (m-molality: mol-salt in kg-solvent) in de-ionized H2O, containing 4 mM potassium ferricyanide (K3[Fe(CN)6]) (99.5% Sigma-Aldrich). Electrochemical measurements: SECM measurements were performed using Bio-Logic M470. The experiment was controlled and analyzed by using M470 and 3DlsoPlot software. The main body of the four-electrode open cell was made of poly tetra fluoroethylene(PTFE). The PTFE cell was filled with the prepared electrolyte, and then one of the working electrodes (TiO2 substrate) was placed at the bottom, with the other working electrode (25 μm Pt microelectrode) sealed in glass as the SECM tip; Ag/AgCl was used as reference electrode, and Pt served as a counter electrode. The electrode



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surface tilt was corrected prior to all measurements. The approach curves were recorded with a step size of 3 μm in the z-axis and 1 μm when the tip current or impedance change was larger than 115%. An area scan was performed after the tip was positioned near the surface. Images with an area of 144 μm×144 μm and 144 points were taken using a wide step size of 12 μm along the x-axis and height step size of 12μm along the y-axis. Data were recorded by moving the tip to (x, y) = (0,-72) initially, sweeping along the x-axis at a step size of 12 μm for the 1st line scan and then moving back to (x,y)=(0, -60) for the 2nd line scan and so on. Materials Characterization: XRD experiments were performed to analyze the crystal structure of (C-)TiO2 with Bruker D8 ADCANCE using Cu Ka radiation in the 2θ range of 15-75° with a step time of 1.16s and 0.0194 steps. The surface morphologies and structure of the C-TiO2 electrode were characterized by SEM (Hitachi, SU8010) and TEM (FEI Tecnai G2 F30). The TiO2 electrode and electrolyte structure was also studied via a confocal micro-Raman system (LabRAM HR800 spectrometer, Horiba) with a 532.05 nm argon-ion laser. Surface chemical composition analysis was conducted by XPS with a PHI 5000 VersaPrboe-II spectrometer using monochromatic Al K(alpha) source under ultrahigh vacuum (10-9 Torr) conditions.

3. Results and discussion 3.1 FB-SECM and AC-SECM Here, carbon-coated TiO2 (denoted as C-TiO2) was used as the working electrode; the detailed characterization of the C-TiO2 is summarized in Figure S1 (XRD, Raman and SEM). A mixed salt system consisting of 21 m LiTFSI and 7 m LiOTf, abbreviated as "water-in-bisalt"(WIBS), was used as the electrolyte. The molecular structure characterization by Raman spectroscopy is shown in Figure S2. The intercalation potential for Li ions into anatase C-TiO2 occurs at approximately 1.9 V vs. Li/Li+ in WIBS electrolyte (Figure S3). The free-diffusion redox mediator is potassium ferricyanide (K3[Fe(CN)6]) dissolved in the WIBS electrolyte, showing a redox potential of 3.6 V vs. Li/Li+ at the Pt tip of SECM (Figure 1a). The Fe3+ ions are generated at the Pt tip at an applied potential of 3.8 vs. Li/Li+. During the process of the tip approaching the surface of the substrate, the


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Fe3+ ions are reduced back to Fe2+ with an increase in the reaction rate observed and, hence, an increased tip current if the substrate is conductive, referred to as positive feedback. Above an insulating substrate, the regeneration rate for Fe2+ is reduced and reflected as negative feedback (Figure 1b). The tip current IT is always normalized by the mass transport limit current IBulk in the bulk of the electrolyte solution as: iT=IT/IBulk. A typical SECM approach curve to the C-TiO2 substrate surface is shown in Figure 1c with the normalized tip current iT versus normalized distance d/r. The Fe3+ generated at the tip at an applied potential of 3.8 V vs. Li/Li+ is reduced back to Fe2+ upon reaching the TiO2 substrate surface. The positive feedback is reflected with an increased tip current, demonstrating that the added Super P and C-TiO2 cover a considerable fraction of the electrode. In the AC-SECM mode, a high-frequency voltage is applied between the SECM tip and counter electrode. Thus, the AC impedance can be described by an equivalent circuit consisting of a series combination of solution resistance and double-layer capacitance. At high frequencies, the capacitances behave as short and the current is limited by the solution resistance Rts as the tip approaches the conductive substrate (Figure 1d). As the tip approaches an insulating surface, all the ionic current flows through the solution, and the solution resistance Rt becomes dominant (Figure 1e)

22-23.

The decrease (increase) of the real impedance |Z| as the AC-SECM tip approaches a

conductive (insulative) substrate follows a similar dependence to that found for the tip current with SECM feedback mode containing the redox mediator. AC-SECM offers an advantage over FB-SECM because the measurements can be performed in a solution free of any redox mediator, thereby avoiding undesirable reactions between the redox mediator with the electrode material 24. In Figure 1f, the decrease in the real impedance |Z| with distance towards the surface agrees with the tip current response from the SECM feedback in Figure 1c.



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Figure 1. a Cyclic voltammograms between 3.3V and 3.9V vs. Li/Li+ at a scan rate of 2 mV/s at the SECM tip positioned far away from the sample surface before experiment. The electrolyte is 21 m LiTFSI/7 m LiOTf containing 4 mM potassium ferricyanide (K3[Fe(CN)6]) as redox mediator. b SECM feedback approach curve to conductive substrate and insulative substrate. The SEI modulates the reduction rate of Fe3+ at the electrode interface. c Approach curve of the SECM tip towards TiO2 electrode. (d is the distance from the surface, r is the radius of the tip). d/e Equivalent circuit modeling the solution between the tip of the probe and substrate over conductive and insulative substrate. f AC impedance approach curve of SECM tip towards TiO2 electrode. The electrolyte is 21 m LiTFSI/7 m LiOTf without any redox mediator.

3.2 FB-SECM curves and area scan in one cycle To evaluate the electronic property of the TiO2 substrate within one cycle, in situ FB-SECM was employed. The tip was positioned above the TiO2 electrode substrate surface and kept at a constant distance of ~20μm (z-value) and a constant potential of 3.8 V vs. Li/Li+, corresponding to the diffusion-limited oxidation current for Fe2+. The feedback current at the tip (green line in Figure 2a) was recorded while a cyclic voltammogram was performed at the TiO2 electrode (black line in Figure 2a) in the potential range of 2.8 V to 1.7 V vs. Li/Li+ at a scan rate of 2mV/s (Figure 2a).


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The normalized feedback current detected at the tip increases upon cathodic polarization of the TiO2 electrode and decreases with an electrode anodic scan to 2.8 V vs. Li/Li+. Note that in Figure 2a, the positive and negative feedback correspond to yellow and green colors, respectively. The normalized feedback current is mainly affected by three factors: (1) The cathodic/anodic polarization of the substrate can induce a change in the driving force for the reduction of Fe3+ at the substrate surface 25. (2) The formation of a conductive or insulative SEI on the substrate surface influences the regeneration rate of Fe2+ 21. (3) The (de)intercalation of Li+ affects the transfer number of Fe2+/Fe3+ and the effect of the electric field on the normalized feedback current

26.

In Figure 2a, the

normalized feedback current at the tip increases upon cathodic polarization of the TiO2 electrode. As mentioned above, cathodic polarization of the substrate with respect to the formal potential of Fe2+/Fe3+ will increase the driving force for local reduction of Fe3+ and increased feedback current. When Li+-ion insertion takes place as the substrate potential is changed from 2.3 V to 1.7 V vs Li/Li+, the increase in the feedback current is more pronounced. In addition to the cathodic polarization effect, the conductivity of the substrate electrode material is substantially enhanced, which is ascribed to the reduction of Ti4+ to Ti3+ with Li insertion

27-28.

Moreover, the intercalation

leads to a depletion of Li+ in the vicinity of the electrode, leading to an increase in the transference number of Fe2+/Fe3+ and a predominant effect of the electric field on the normalized feedback current. It has been suggested that this effect is influenced by the scan rate, and that the SEI formation process can be precisely monitored at low scan rates

25.

When the applied potential is

scanned anodically from 1.7V to 2.8V, the feedback current is decreased again. In addition to the anodic polarization and transfer number effects, it indicates that the conductivity of the substrate is not decreased within one cycle. This is quite different with our expectation based on the electronic insulating properties of the SEI film. If the surface is covered by an insulating SEI film, the overall feedback current should decrease rather than increase. Herein, we propose that this could be caused by the structure of the SEI film formed in the WIBS electrolyte. In the organic electrolyte system, an insulating organic/inorganic hybrid SEI film can be formed because of the decomposition and deposition of organic composition and lithium salt. However, in the WIBS electrolyte, the inorganic products of electrolyte decomposition couldn’t form an interconnected insulative SEI film, like that found to occur in the organic electrolyte. Therefore, the overall feedback current is generated from the SEI-covered area and the TiO2 substrate without the SEI film. At the delithiation state in Figure



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2a, the TiO2 substrate with certain amount of lithium residues exhibits increased conductivity and the SEI composed of salt decomposition products is insulating. When the conductivity of the TiO2 exposed areas increases and offsets the conductivity decline caused by the SEI-covered area, the overall feedback current of the substrate can be expected to increase. To gain spatially resolved information about the electronic property of the interface, in situ SECM areal scan operation in the feedback mode were taken to visualize the local electrochemical activity of an144×144 μm2 area. Firstly, the TiO2 substrate is scanned from the open-circuit potential (OCV) to the preset potential of 2.2 V, and then rested until a stable current density is reached. The corresponding area scan images for an identical region with different applied potentials are shown in Figure 2b. Figure S4 provides more scanned images at other intermediate potentials. Generally, the features shown in the scan area at pristine state are ascribed to topography variations of the paste electrode. Because the dimensions of the TiO2 particles are at scale of ~50nm (Figure S2), which are too small to be resolved by SECM (25μm tip diameter). The area scan images are probing 144 points with different distances (z) over the interface, which results in reliable observation of the changes in the reaction rate and electronic character detection. The area feedback currents increase with cathodic sweeping from OCV to 1.7V vs. Li/Li+, and then decrease when scanned anodically to 2.4V vs. Li/Li+. The change in the average area current (Iavg.) with substrate potential is shown in Figure 2c, which is in agreement with the normalized tip current (Figure 2a). This demonstrates that the point behavior presented in Figure 2a is representative of the general behavior observed for most of the surface area. However, point feedback current cannot reflect the heterogeneity of the interface. Provided that the electrode surface is distributed homogeneously, the difference percentage for IT(x, y) with the average area current Iavg. should remain constant for an identical region. However, the region around (x, y)=(12, 12) (denoted as A1) shows values that are slightly higher than Iavg. initially but exhibits much higher IT(12,12) values through the lithiation-delithiation process. The region around (12, 130) (denoted as A2) presents an example of the opposite trend. A relatively large IT (12, 130) initially decreased to a value far lower than Iavg. To evaluate the interface heterogenicity and its evolution, the feedback current difference with the average area current, δ= | IT(x,y) - Iavg.| / Iavg., is defined to quantify the change. The current difference δ for regions A1 and A2 at different applied potentials are shown in Figure 2d and e. These features reflect the spatial heterogeneity of the SEI


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properties rather than topography difference. A1 could be a region filled with active TiO2 without SEI coverage, and the increase in the mediator regeneration rate with (de) lithiation could be induced by the existence of Ti3+ and Li residuals. In contrast, A2 could be a region covered by insulative SEI materials with certain passivation effect to reduce the regeneration rate of mediator. In summary, a discontinuous interface film with passivation effect was formed with certain active region exposed to the electrolyte.



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Figure 2. a Cyclic voltammogram at the anatase TiO2 electrode at a scan rate of 2 mV/s. The substrate current (black) and corresponding feedback current at the SECM tip (green) are plot vs. time. The electrolyte is 21 m LiTFSI/7 m LiOTf and 4 mM (K3[Fe(CN)6]/20 mM KCl. b SECM feedback images of an identical region at different SOC: OCV, 2.2V, 1.7V, 2.4V. (The image of much more potential points are shown in Figure S4) c Average feedback currents (Iavg.) of image at different SOC (lithiation: OCV, 2.2.V, 2.0V, 1.8V, 1.7V; delithiation: 1.9V, 2.2V, 2.4V). d/e Difference percentage of the feedback currents at point A1(12,12) and point A2(12, 130) with the average areal feedback current Iavg. during lithiation-delithiation.

3.3 FB-SECM and AC-SECM feedback area scan with cycling Previous publication indicate that SEI formation is a gradual process that needs at least tens of cycles in the highly concentrated electrolyte

9-11.

FE-SECM images are used to study the film formation

process on TiO2 substrate over an identical area after 3, 10, 30 and 60 cycles (Figure 3a). It can be seen that the overall feedback current first increases before becoming relatively stable with cycling (Figure 3b), which demonstrates that the conductivity of the substrate, with the SEI-covered and active materials exposed areas, increases with the number of cycles. As mentioned previously, the conductivity of the TiO2 substrate increases with lithium insertion and the transformation of Ti4+ to Ti3+. Although the FB-SECM images were all obtained at the delithiation states, the TiO2 particles may still contain some residual lithium, more specifically, the lithium ions are not totally extracted upon charging. With the formation of an insulating SEI, the electrochemical polarization will increase, and more residual lithium will be generated. When the increase of TiO2 conductivity offsets the passivation effect caused by the SEI coverage, an increase of overall feedback current of the substrate is observed as shown in Figure 3b. From this point, the rapid increase in overall feedback current indicates the SEI formation processes and that the SEI film is well formed in the WIBS electrolyte after approximately 20 cycles. In addition, one can expect the surface conductivity to be very inhomogeneous with the formation of the discontinues SEI film. This is well demonstrated by the surface roughness of the feedback current density shown in Figure 3b, with more details found from the roughness change. At the OCV, the surface roughness can reflect the topography variation with different working distances, as mentioned in Figure 2b. With cycling, the surface roughness reflects a combination of the effects of topography variation and lateral heterogeneity in the


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regeneration kinetics due to interface evolution. The feedback surface roughness initially increases with cycling before decreasing slightly after 10 cycles. This demonstrates that the difference in the regenerate kinetics generated by topography vibration is gradually reduced by the interface evolution, indicating increased regeneration rates at various positions, and especially those positions with an initially larger working distance d. A much detailed lateral heterogeneity of the regenerate kinetics with the defined parameter δ is schematically illustrated in Figure 3c and d. The region identification could roughly be determined through the change of δ during cycling. For region i and ii, the feedback currents are initially lower than average due to topography reasons. The feedback currents increase to values above average during cycling especially for region ii. These regions could be filled with active TiO2 materials without SEI coverage. There are also regions filled with active materials initially, but gradually covered with insulative SEI products as demonstrated by region iii. For region iv, the feedback currents remain low during cycling and it could be a region of inactive material such as binder. In summary, the substrate conductivity increased over cycles without interconnected insulative SEI formed; the insulative SEI products with passivation effect are distributed scatteredly over the interface. This will be further verified by morphology, chemical, and structure characterization using TEM, XPS and DFT calculation.



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Figure 3. SECM feedback images of an identical region with cycles after a 3C, 10C, 30C and 60C. b The change of average current and surface roughness of the SECM feedback images with cycles. c Schematic illustration of different regions in the feedback scanned area and d the change of δ in the four regions with cycling.

In accordance with the FB-SECM measurement, an AC-SECM area scan is also used to determine the interface evolution with cycles (Figure 4). Similar to the SECM feedback images, topography variation and lateral heterogeneity of reaction kinetics induced different impedance for an identical area, as shown in Figure 4a. The average interface impedance decreases and surface roughness increase to relatively stable values with cycling (Figure 4b). This is in agreement with the trend determined by SECM feedback images (Figure 3). To evaluate the interface evaluation more preciously, the topography variation of the scanned area at pristine state should be ruled out. It’s assumed that the TiO2 substrate at pristine state is uniformly distributed and the difference in the measured real impedance is only caused by the topography difference. Thus, ΔReal(Z) = |Real(Z)(x,y)-Real(Z)(x,y)pristine| could be used to reflect the change in impedance induced solely by interface evolution. Figure S5 shows the areal scan images with ΔReal(Z) values for the identical


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region after 3 cycles and 60 cycles, respectively. In Figure 4c, the increase of average areal ΔReal(Z) with cycling, corresponding to the change of overall impedance (Figure 4b), indicates the increase of substrate conductivity as discussed above. The impedance difference δΔZ is also applied for the interface heterogeneity evaluation and region identification as shown in Figure 4d and e. Similar as the result of FB-SECM area scan, the cycled interface is covered by disconnected insulative SEI products (region i and ii) with certain active area exposed (region iii and iv). The SECM area scan results are correlated to the electrochemical performance of the C-TiO2 substrate. Figure 4f shows the evolution of the discharge capacity and coulombic efficiency (CE) for TiO2 cycled in WIBS electrolyte. The discharge capacity severely decays in the initial ten cycles before stabilizing afterwards. CE is very low initially and increased to a steady value above 99% above after tens of cycles. The CE, demonstrating the irreversible Li consumption, is usually related to SEI formation. However, in the present case, electrolyte decomposition only takes part of the irreversible Li consumption. The other part is mainly contributed by the residual lithium in the TiO2 electrode material, which can be clearly seen from the increased substrate conductivity during cycling (Figure 3/4 b). Similar to SEI formation, the residual Li in the electrode increases to relatively stable during cycling, corresponding to the change in the CE. Residual Li in the active material will inevitably lead to capacity decay over cycles. The SEI evolution is also evaluated by electrochemical impedance spectroscopy (EIS) measurement at different cycles (Figure 4g). The impedance in the high frequency range Rsei increases with cycling, demonstrating the gradual SEI formation process. This seems to contradict the previous interface conductivity measurement (Figure 4b). Actually, the conductivity measured by (AC-)SECM reflect the electronic conductivity of the substrate. More specifically, the insulative SEI components are not substantial enough to counteract the increase in substrate conductivity. This is demonstrated by the intercept of the impedance spectra with the real axis at higher frequency in the Nyquist plot, as fitted by the equivalent element Ro. In Figure 4g, the intercept resistance values decrease with cycles, indicating increased substrate conductivity. On the other hand, the high-frequency semicircle attributed to the Li diffusion through the SEI interface 29-30, is influenced by the random growing SEI components, which are obstacles for Li diffusion. So, the AC-SECM substrate impedance and EIS measurements are more in agreement.



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Figure 4. a AC-SECM feedback images of an identical region with cycles after 3C, 10C, 30C and 60C. b The change of average real impedance and surface roughness of the SECM feedback images with cycles. c The change of average areal ΔReal (Z) of the AC-SECM images with cycles. d Schematic illustration of different regions in scanned area and e the change of δΔZ in the four regions with cycling. f The discharge capacity and coulombic efficiency as a function of cycle number. g Nyquist plots of C-TiO2 at pristine and after cycles, and the inset show the equivalent circuit.

3.4 Chemical and structure information of the SEI To correlate the interface electrochemical kinetics property with its chemical composition and structure information, XPS and TEM were used to provide a more comprehensive picture. TiO2 Electrode material cycled in WIBS electrolyte for more than 60 cycles was used as the experimental


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model. XPS with Ar+-sputtering was used to analyze the SEI components and their depth distribution. From F1, C1s, and O1s, the presence of LiF and Li2CO3 on the surface was revealed, which are typical SEI components (Figure 5a-c). Both LiF and Li2CO3 decrease with Ar+ sputtering. In contrast, M-O of O1s spectra and Ti4+/Ti3+ of Ti2p spectra increase with depth profiling, demonstrating the existence of Ti3+ (Figure 5d). Quantitative analysis for each element and the components are shown in Figure 5e-h. This confirmed LiF to be the dominant ingredient of the SEI composite, which is formed by the reduction of anions or clusters

6, 11.

The intensity of Li2CO3 is

nearly one tenth that of LiF at the surface. The steep decrease in Li2CO3 is accompanied by an increase of the M-O band. The M-O band could be composed of Li2O and TixOy. It has been suggested that the formation of Li2CO3 and Li2O is induced by the reduction of O2 and CO2 dissolved in electrolyte 11. TEM images of C-TiO2 taken before and after cycling are shown in Figure 5i and j. The images of the cycled TiO2 also provide evidence for LiF and Li2CO3/Li2O formation. Numerous isolated nano-crystalline particles are randomly distributed on the substrate. The substrate has a large interplanar distance of ~0.35 nm, which is related to the (101) interplanar of TiO2. A large part of the nano-crystalline particles are formed with a small interplanar space of approximately 0.2 nm, close to the (200) interplanar spacing of LiF. A part of the particles have an interplanar space of 0.272 nm. This is either related to the (002) of Li2CO3 or the (111) interplanar of Li2O 31. Schematic illustration of the interface structure is shown in Figure 5k. The SEI is not an interconnected interface covering the whole electrode surface. Instead, the inorganic SEI components, LiF, Li2CO3 and Li2O, are randomly dispersed on the electrode substrate. The inorganic SEI components are highly resistive and serve as electron barriers

32-33.

DFT calculation was explored to assist the estimation of the

electronic tunneling barrier and SEI thickness(Figure S7). Electrode materials with smaller electronic tunneling barrier values are prone to form a thicker SEI (denoted as ΔEt) 11. In this study, lithiated TiO2 was found to have a smaller ΔEt value, with the SEI components more likely to be formed on its surface.



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Figure 5. X-ray Photoelectron Spectroscopy(XPS) conducted on cycled TiO2 with Ar+ sputtering intervals (~7nm/min) a F 1s spectra b C 1s spectra c O 1s spectra d Ti 2p spectra. e The atomic concentration of various elements with etching depth. The intensity and (relative) atomic concentration of (f) LiF, (g) Li2CO3 and (h) M-O with various etching duration. TEM image of C-TiO2 before (i) and after cycling (j). k Schematic illustration of the interface structure after cycling.

3.5 Stability of the aqueous SEI Wang et al. suggested that in aqueous system, integrated SEI formation is dependent on the duration time and that superconcentrated electrolyte is essential to maintain such an SEI. In their experiment, a high quality SEI was obtained either at 0.1C in a few cycles or a high rate of 1C within 10 cycles


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11.


In this study, a C-TiO2 three-electrode cell was cycled at 3C for more than 60 cycles. SECM

feedback images are used to study the stability of the as-formed SEI in WIBS. An identical area of 136×136 μm2 was scanned after 3 h, 4 h, 5 h, and 6 h rest time after SEI formation, as shown in Figure 6a. The average feedback current density of the scanned area decreased with the rest time, which could be due to the decrease in the substrate conductivity (Figure 6b). The regeneration kinetics for the redox mediator within the scanned area change with interface evolution. The active area in the center area shrinks in size and moves to the edge near the x-axis with prolonged resting. The center area could be composed of bare active material that is gradually covered by insulating SEI components with dissolution. The area scan results are analyzed in combination with OCV and EIS spectra for the C-TiO2 electrode, which were measured simultaneously during rest (Figure 6b and c). The OCV increases rapidly in the initial 4 h before increasing more gradually toward a stable value of ~3.36 V vs. Li+/Li. The increase in the OCV could be caused by the release of Li residuals from the substrate active material and the gradual achievement of an equilibrium condition. This process will lead to a decrease in the substrate conductivity, which is in accordance with the change in the average feedback current density. In addition, the change in the substrate conductivity could also be reflected by the change in the EIS-fitted element Ro, moving towards higher values with time (Figure 6c). The high-frequency semicircle, corresponding to the resistance of the SEI (Rsei), decreases with rest time (Figure 6d). This demonstrates that the SEI formed in this experiment condition still suffers from dissolution in concentrated electrolyte. The dissolution process could be partially reflected by the evolution of the SECM area scan images. A schematic illustration of the interface and its evolution is summarized in Figure 6e. The SEI film faces a certain amount of dissolution, as seen from the decrease in the EIS element Rsei and SECM area scan. The electrode rest to equilibrium condition with overall conductivity decrease, manifested by the increase in the EIS element Ro value and decreased average area feedback current density.



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Figure 6. a SECM Feedback Image of an identical region of cycled TiO2 after 3h 4h 5h and 6h. b The change of OCV and average area feedback current density with rest time. c Nyquist plots of the cycled TiO2 with rest time, the intercept of impedance plot with real axis is Ro. d The change of open circuit potential (OCV) and the interface resistance Rsei with rest time. e Schematic illustration of the interface structure and its evolution with rest time.

4. Conclusion In summary, for the first time, FB-SECM and AC-SECM modes have been applied for in situ studies of SEI formation on C-TiO2 electrode as well as for the investigation of the stability of the SEI in WIBS electrolyte for aqueous LIBs. It is found that the TiO2 substrate conductivity increases during


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lithiation before decreasing back again upon (de)lithiation, which is mainly due to Ti3+ and Li residuals. This also results in an increase in the overall conductivity through multiple cycles. Lateral heterogeneity for the regeneration kinetics is also observed from feedback images, which can be divided into SEI-covered region and active material exposed region. By combining the results from SECM with XPS and TEM data, it is found that the TiO2 surface is not covered by a continuous insulative SEI layer but, instead, is covered by scattered distribution of SEI components composed of LiF, Li2CO3 and Li2O formed during cycling. Through DFT calculation, electrolyte decomposition products such as LiF and Li2CO3 are more prone to be formed on the active material with Li residuals. The SEI evolution during the first 10 cycles is correspondingly related to changes in the coulombic efficiency and in good agreement with the EIS measurement. For the resting period after SEI formation, an inverse process was observed. Such SEI dissolution with resting time and the stabilization of the substrate towards the equilibrium condition with decreasing conductivity could be understood from the change in the area scan, substrate OCV and EIS measurement.

Supporting Information Description XRD, Raman, and SEM of the c-TiO2 electrode; Raman spectra of the electrolytes; Cyclic voltammetry of the tip and TiO2 substrate in the WIBS electrolyte; SECM area scan images at different potentials within one cycle; AC-SECM area scan images with ΔReal(Z) values; The real impedance, imaginary impedance, impedance magnitude, AC current magnitude and impedance phase of AC-SECM image; Calculation of the energy bands for the c-TiO2 electrode and electrolyte interface.

Acknowledgement This work was supported by National Nature Science Foundation of China (No. 51872157) Shenzhen Technical Plan Project (No. KQJSCX20160226191136, JCYJ20170412170911187, and JCYJ20170817161753629 ), Guangdong Technical Plan Project (No. 2015TX01N011), and Local



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Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01N111). The work at BNL was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Vehicle Technology Office of the U.S. DOE through the Advanced Battery Materials Research (BMR) Program, including Battery500 Consortium under Contract DE-SC0012704.

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Table of Contents Graphics


Evolution of Solid Electrolyte Interface (SEI) on TiO2 electrodes in

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