In Situ Atomic Force Microscopy (AFM) Study of Oxygen Reduction

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In situ Atomic Force Microscopy (AFM) Study of Oxygen Reduction Reaction on a Gold Electrode Surface in a Dimethyl Sulfoxide (DMSO) Based Electrolyte Solution Can Liu, and Shen Ye J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08718 • Publication Date (Web): 10 Oct 2016 Downloaded from http://pubs.acs.org on October 15, 2016

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The Journal of Physical Chemistry

In situ Atomic Force Microscopy (AFM) Study of Oxygen Reduction Reaction on a Gold Electrode Surface in a Dimethyl Sulfoxide (DMSO) Based Electrolyte Solution Can Liu, and Shen Ye* Institute for Catalysis, Hokkaido University, Sapporo 001-0021, Japan.

ABSTRACT: In the present study, the morphological changes on a gold electrode during the oxygen reduction (ORR) and oxygen evolution reaction (OER) processes in a dimethyl sulfoxide (DMSO) based electrolyte solution were investigated using an electrochemical atomic force microscope (EC-AFM) with the help of vibrational spectroscopy measurements. The growth of the ORR products on the electrode surface, which was mainly assigned to lithium peroxide (Li2O2), was directly confirmed by the EC-AFM. It was found that the water concentration in the solution significantly affects the morphology of the ORR products. The growth of anisotropic Li2O2 particles on the

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gold electrode surface has been confirmed to be an electrochemical process. No evidence was found to support the disproportionation growth mechanism. These ORR products were fully decomposed at a potential as high as 4.4 V (vs. Li+/Li) in the subsequent OER process, more positive than that determined by a surface enhanced Raman spectroscopy (SERS) measurement. Combining with infrared (IR) absorption spectroscopy and SERS measurements, we propose that the oxidation decomposition of the Li2O2 deposits first occurs at its interface with the gold electrode surface, while that of the remaining particles take place at a higher overpotential. On the other hand, the ORR deposits could be fully decomposed at a potential as low as 3.6 V when tetrathiafulvalene (TTF) was included in the solution. We confirmed by EC-AFM that the electrochemically generated TTF+ can mediate the decomposition of the Li2O2 at a lower potential through a homogeneous oxidation mechanism.

Introduction The non-aqueous lithium-oxygen (Li-O2) battery has attracted significant research interest in recent years due to its highest theoretical specific energy among the “beyond Li-ion” batteries.1-3 However, the development of the Li-O2 batteries seriously suffers from several technological challenges, including a high charge overpotential, poor rechargeability and low cyclability, especially for its cathode reaction.2-3 Generally, the main electrochemical reactions on the cathode in the Li-O2 battery have been proposed


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as shown below due to the oxygen reduction and oxygen evolution reactions (ORR/OER):4 (1) O2 + e– ⇄ O2–

E0 = 2.65V

Li+ + O2– ⇄ LiO2 (2) O2 + 2Li+ + 2e– ⇄ Li2O2

E0 = 2.96 V

It was found that the potential for the OER, especially that for Li2O2 (Eq. 2) is much higher than its equilibrium potential. This has been attributed to the poor conductivity of Li2O2 and the side reactions, such as the oxidation of the organic solvents and carbon electrode during the ORR/OER. Overall, the ORR/OER mechanisms on the cathode are still not fully understood at a molecular level, which significantly hinders the development and practical applications of the Li-O2 battery. As an aprotic polar solvent, dimethyl sulfoxide (DMSO) shows an excellent stability to superoxide (LiO2), which is one of the ORR products (Eq. 1) with an extremely high oxidation ability.5 Thus, DMSO has been regarded as a promising solvent candidate although it still has disadvantages of a higher activity to Li metal and relatively high evaporation rate at room temperature. Bruce and co-workers reported a 95% capacity retention after 100 discharge/charge cycles in a DMSO-based electrolyte solution on a gold electrode.6 Recently, based on in-situ surface enhanced Raman spectroscopy (SERS) and UV-Vis spectroscopy studies on a gold electrode in a DMSO-based electrolyte solution, we found that the ORR yield for LiO2 was higher than that of Li2O2 under the same conditions. For example, the partial yields for LiO2 (Li2O2) during the potential sweep at 10 and 2 mV/s were found as high as 73% (27%) and 86% (14%), respectively,



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assuming only two products are generated.7 SERS measurements showed that the Li2O2 on the gold electrode surface disappeared around 4 V (vs. Li+/Li) while most of the LiO2 still remained in the solution at the end of the OER.7 On the other hand, electrochemical quartz crystal microbalance (EQCM) measurements reported sluggish decomposition of the ORR deposits on the gold electrode surface.8-9 The differential electrochemical mass spectrometry (DEMS) found that the ratio between oxygen evolved and oxygen consumed is not unity in the DMSO-based solution.2,

10

These

results imply the complexity of the ORR/OER processes on the cathode surface. In fact, the growth and decomposition mechanisms of the Li2O2 on the electrode surface during the ORR/OER are still far from clear.11-12 Therefore, a direct observation of the morphology of the cathode surface is deemed necessary to reveal the details of the reaction processes in the Li-O2 batteries. Zakharchenko et al.13 reported that the ORR products grow as film comprising thin Li2O2 plates on the Au electrode after the ORR in the DMSO-based electrolyte solution as observed by scanning electronic microscopy (SEM). Marchini et al. examined the formation of discrete Li2O2 particles on either a highly oriented pyrolytic graphite (HOPG) surface or an Au-(111) surface after the ORR in the DMSO-LiPF6 solution using ex situ atomic force microscope (AFM) observations.14-15 In the subsequent OER, they found that these particles were not fully decomposed at a potential more negative than 4.15 V (vs. Li+/Li). On the other hand, it was found that the ORR products on a porous carbon surface had a toroidal or spherical shape.16-18 The toroid particles are expected to be Li2O2 thin plates with the large (001) facets and a typical thickness of 10 nm.19-21 It has


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been proposed by several groups that the appearance of the Li2O2 toroid is a result of a disproportionation reaction of LiO211-12, 22, i.e. a chemical reaction such as 2Li+ (sol) + 2O2─ (sol) ↔ Li2O2 (s) + O2 (g), although substantial evidence on the existence of the disproportionation reaction has never been fully demonstrated. Since the morphology of the ORR products closely correlates with the ORR/OER performance of the Li-O2 battery20-21,

23-24,

it is extremely important to address the

relationship between the morphology and the growth/decomposition mechanism of Li2O2 on the electrode surface during the ORR/OER under in situ conditions. For such a purpose, AFM is one of the best choices, which can be carried out in an electrolyte solution under electrochemical control. Wen et al. previously reported an electrochemical AFM (EC-AFM) observation during the ORR/OER on an HOPG electrode surface in a tetraglyme-based electrolyte solution and found that the Li2O2 is formed in a layer growth mode.25 Due to the different solubility and stability of Li2O2 and LiO2, which are typical products and intermediate for the ORR (Eqs. 1 and 2), in tetraglyme and DMSO,7, 12 the growth and decomposition mechanism of the ORR products on the electrode surface could be very different. Furthermore, as discussed in our previous studies, one should also consider the difference in the reaction and diffusion conditions between the porous electrodes and the flat electrodes due to presence of the three-phase interface.26 In this study, EC-AFM is employed to study the morphological changes in the ORR deposits on a gold electrode surface during the ORR/OER in a DMSO-LiClO4 solution with the help of vibrational spectroscopy. The morphological changes can be mainly



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attributed to the dynamic ORR/OER processes, shedding some light on their reversibility and efficiency. We found that L2O2 is electrodeposited following a particle growth mode, while the particles tend to develop into layered plates as more water molecules are included in the solution. The complete decomposition of all the ORR products seems to occur at a higher overpotential than that expected from the SERS measurements. On the contrary, the decomposition of the Li2O2 occurs in the more negative potential region with the help of TTF. These puzzling mechanisms of the ORR/OER will be discussed based on these results.

Experimental methods A home-made AFM cell with a three-electrode assembly was used in a commerciallyavailable AFM (Agilent 5500) in the tapping mode under electrochemical potential control. An Au thin-film of 80-nm thickness with a root-mean-squared (RMS) roughness of 0.7 nm was used as the working electrode in the study. Li wires were employed as the counter and reference electrodes, and a 0.1 M LiClO4 dissolved in DMSO saturated with O2 was used as the electrolyte solution. All potentials were referred to Li+/Li. The AFM image scanning was recorded using a silicon cantilever (OMCL-AC240TS-C2, spring constant of 2 N/m, resonance frequency of 70 kHz, Olympus). The resonance frequency at 18 kHz was chosen when the cantilever was immersed into the electrolyte. Typically, each image was acquired at a resolution of 256 lines in 260 s. Electrochemical controls were obtained carried out using a potentiostat (PS-07, TOHO Technical


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Research). Details for the sample preparation and AFM cell setup are given in the Supporting Information. All chemicals were purchased from Kanto Chemical Co., Inc. (Tokyo, Japan) unless otherwise mentioned. DMSO was dried in freshly activated molecular sieves (types 3 Å and 4 Å) for several days before use. The concentration of water in the DMSO was 6 ppm, measured by Karl Fischer titration (MKC-710, Kyoto Electronics). The electrolyte solutions were freshly prepared in a glove box (Labmaster MB10-C, MBraun) filled with Ar. The water content of the electrolyte was found to increase to ca. 30 ppm after the O2 purging. The AFM cell was assembled in advanced in the glove box. Careful attention was paid when transferring the AFM cell to a standard sealed chamber (Agilent), continuously purged with dry air (dew point < -70 °C). After the cell was loaded onto the AFM microscope, the scanner was demounted for introducing the electrolyte solution and then immediately remounted. Due to the short exposure to the atmosphere, the water content in the cell typically increased by 200~300 ppm or even higher (by adding water). The specific water concentration of the electrolyte was determined at the end of each EC-AFM observation. The Fourier transform infrared spectroscopy (FTIR) measurements were carried out by an FTIR spectrometer (Perkin-Elmer) with an attenuated total reflectance (ATR) accessory in the glove box. The in-situ SERS measurement was also employed to track the ORR/OER process. Details have been described elsewhere.7



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Results and Discussion 3.1 In situ AFM observation for ORR Figure 1a shows a cyclic voltammogram (CV, 5 mV/s) for the ORR process on a gold electrode in 0.1 M LiClO4-DMSO saturated by O2. The water concentration in the solution was 875 ppm. Two cathodic peaks were observed at 2.54 V and 2.43 V, which can be attributed to the electrochemical reduction of O2 to LiO2 and Li2O2, respectively.7, 12

The morphological changes on the gold electrode surface were recorded by in situ AFM observations during the potential sweep of the ORR in Figure 1b (from 3.09 V → 2.10 V → 2.41 V) and Figure 1c (2.41 V → 2.77 V, then switched to OCP, ca. 2.8 V) in a


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scanning area of 1×1 µm2. When the potential is high, a flat surface with many small islands is observed (lower part, Figure 1b). This should be attributed to the clean Au substrate surface where many Au domains with dimensions of 30~80 nm are uniformly distributed. The average roughness of the gold electrode was ca. 0.7 nm. No clear change in the morphology is observed as the cathodic current starts to flow from 2.7 V (Figure 1a). As the potential approaches 2.55 V, i.e., the first cathodic peak, some tiny particles start to deposit on the surface and swiftly grow into anisotropic particles in the shape of rods from 2.44 V, i.e., the second cathodic peak potential. The morphological changes become much slower as the potential decreases to 2.3 V, while a small cathodic current continues to flow (Figure 1a). Ultimately, the gold electrode surface is covered by a uniform film of anisotropic particles (Figure 1c). This in-situ AFM observation validates that the deposition of Li2O2 takes place in the potential region of the second cathodic peak in the CV (Figure 1a). These particles deposited should be attributed to Li2O2, which are insoluble in DMSO and precipitated on the gold electrode surface. Based on our previous studies, in addition to the formation of Li2O2 on the gold electrode surface, a major part of the cathodic current was used to produce LiO2, which is fully soluble in DMSO and could not be imaged by the AFM observation. Growth of a two-dimensional film for Li2O2 was not found on the gold electrode surface. The present features are quite different from that by Wen et al.25, where they reported rapid growth of Li2O2 nanoplates with dimensions of several hundreds of nanometers on HOPG in a TEGDME electrolyte solution.



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Figure 1d shows two cross-sectional profiles marked in Figure 1b (dashed trace, L1) and 1c (dashed trace, L2). The height of the deposited particles is estimated to be 10~15 nm. The lateral view of these particles exhibits an elongated shape with the long size in the range of 30~60 nm and the short size of 10~20 nm, forming the shape of rods. Figures 1e and 1f exhibit the subsequent AFM images observed in a wide area (2×2 µm2) at OCP (2.8 V). The morphology of the ORR deposits is uniform, which excludes the probe scraping effect and confirms the accuracy of the in-situ imaging results. The morphology does not change during repeated imaging at OCP, meaning that no deposition occurs at OCP even if a large amount of LiO2 is already produced in the DMSO solution during the ORR. The present observation is a good agreement with previous SERS and UV-Vis observations7, suggesting that Li2O2 grows on the gold electrode surface through an electrodeposition process instead of disproportionation reaction by LiO2 in the solution.12

3.2 AFM observation for ORR/OER in high water concentration The in situ AFM observation is useful to track the morphological changes on the electrode surface under potential control. However, the AFM tip may also have some influence on the reaction due to the close distance to the electrode surface (see Figure S2 in Supporting Information). To avoid the tip-induced influence on the electrochemical reaction, in most of AFM measurements, we avoid the direct in situ observation. Instead, we lifted up the AFM tip by 600 µm during the ORR or OER process and then re-approached the tip to the electrode surface after the reaction to image the electrode


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without potential control (i.e. at the OCP) unless specially noted. Although a shift of scanning area occurs during every re-approaching process, it was found that these AFM images are independent on the surface position and thus can represent the morphologies on the electrode surface under each reaction condition. By using this method, the morphological changes on a gold electrode surface (Figure 2) were evaluated in a LiClO4-DMSO solution containing a similar amount of water as in Figure 1. Two cathodic peaks are also observed (black trace, Figure 2a), being attributed to the formation of LiO2 and Li2O2. Figure 2b shows an AFM image before the ORR, showing a clean Au substrate surface. After the potential was swept to 2.1 V,



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many particles in the shape of rods were formed on the Au electrode surface (Figure 2c), which are measured as 50~80 nm in length and 20~40 nm in height, while the thickness is always below 20 nm. Such a morphology of anisotropic particles is similar to that observed by in situ observation (Figure 1c). After the ORR observation, the electrode potential was swept positive to 4.0 V at 5 mV/s (red trace, Figure 2a). Three small anodic peaks appeared around 3.4 V, 3.6 V and 3.9 V. The anodic charge seems to be much lower than the cathodic charge (Figure 2a), implying that part of the species formed in the ORR is not fully oxidized in the subsequent OER. According to our previous in-situ spectroscopic studies, the peaks around 3.4 V and 3.6 V can be attributed to the partial oxidation of the Li2O2 on the electrode surface and the solubilized LiO2 in the adjacent solution, respectively (see discussions below).7 The exact origin for the peak around 3.9 V is still unclear. Figure 2d shows an AFM image after being oxidized to 4.0 V. The morphology changes in comparison to that after the ORR (Figure 2c), indicating that part of the particles are decomposed after the potential sweep to 4.0 V, while many of the particles still remain on the surface. Instead of homogeneous removal of the Li2O2 particles on the electrode surface, the oxidative decomposition seems to take place as domains, implying that reactions may occur from the defect sites and laterally grow with the OER process. The potential was then further swept to 4.4 V and a broad anodic peak was observed at 4.2 V (blue trace, Figure 2a). After the potential was swept to OCP, AFM imaging was recorded as Figure 2e. Most of the particles were decomposed, leaving a few residuals


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on the electrode surface. Through a repeated sweep to 4.4 V (pink trace, Figure 2a), all the residuals were completely removed, revealing a clean surface of Au electrode again (Figure 3f). The present observation indicates that only part of the ORR deposits could be oxidized at 4.0 V. This is contradicted by our previous SERS observation under the similar conditions, suggesting that most of the Li2O2 on a gold electrode surface were oxidized at 3.8 V.7 A more positive potential (4.4 V) is necessary to fully decompose the ORR products on the surface. The present potential dependences are generally in agreement with previous ex situ AFM and EQCM observations.8, 15

3.3 AFM observation for ORR/OER in low water concentration As already mentioned in the Experimental section, due to the limitation of the present AFM system, the water concentration in the solution becomes much higher than the asprepared solution. To avoid this problem, the ORR products were first prepared in the glove box where we were able to carry out electrochemistry in the AFM cell filled with DMSO solution having a lower water concentration (ca. 30 ppm). After the ORR (black trace, Figure 3a), the AFM cell filled with the fresh electrolyte solution was transferred to the AFM chamber for the AFM characterization (Figures 3b~3f). Figure 3b shows an AFM image of a gold electrode surface at OCP after an ORR in the glove box (black trace, Figure 3a). The morphology of the Au electrode surface dramatically changes after the ORR. Many particles with a lateral dimension of 10~20 nm cover the entire electrode surface, which are very different from the anisotropic particles observed in Figures 1c and 2c.



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The morphology of the electrode surface was further investigated after the potential was swept to a positive potential region (red trace, Figure 3a). As the potential was swept to 4.0 V, a small anodic peak was observed at 3.4 V with a shoulder around 3.6 V. The anodic current was only contributed by the oxidation of Li2O2 since the soluble ORR products (mainly LiO2) were removed by solution replacement after the ORR in the glove box. The anodic peak charge is much lower than that of the cathodic peak charge. Figure 3c shows an AFM image after the potential was swept to 4.0 V (red trace, Figure 3a). Many particles remained on the electrode surface. It is hard to distinguish the difference in the AFM image before and after OER (Figures 3b and 3c), indicating that many ORR products are still on the gold electrode surface after the potential sweep


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to 4.0 V. On the other hand, it should be mentioned that no difference in the AFM image was observed by the solution replacement. As already mentioned, the sluggish OER process is different from the expectation based on our previous SERS observations.7 As we swept the potential to 4.4 V, a sharp oxidation peak appeared around 4.3 V (blue trace, Figure 3a). The AFM image after the potential sweep shows that the morphology remarkably deteriorated, leaving some aggregates of large particles (Figure 3d). One may see many small domains with a lower height appear after the potential sweep, implying that the decomposition occurs on the edge or at defect sites and the lower domains grow with the reaction. With a repeated sweep to 4.4 V (pink trace, Figure 3a), the AFM image (Figure 3e) shows that the residuals are fully removed, giving a flat surface, identical to that of the gold substrate (Figure 2b). Thus, in the present DMSO solution with a lower water concentration, although the Li2O2 deposits show quite a different morphology, complete decomposition of the ORR products on the electrode surface requires a positive potential as high as 4.4 V. The further confirmed potential dependences were generally in agreement with previous ex situ AFM and EQCM observations.8, 15 There have been many discussions about the specific ORR mechanism, i.e. whether Li2O2 is formed by a direct electrochemical process (O2 + 2Li+ + 2e– → Li2O2, or LiO2 + Li+ + 2e– → Li2O2) or by a solution-mediated disproportionation (2Li+(sol) + 2O2-(sol) ↔ Li2O2(s) + O2(g)).2, 11-12 According to the present EC-AFM study, no disproportionation evidence is observed on the gold electrode surface (see Figures 1~3). In other words, the growth of anisotropic Li2O2 particles on the gold electrode surface is an electrochemical



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process. Therefore, there is no correlation between the morphology feature of the Li2O2 and the ORR mechanism (whether surface growth or solution disproportionation). It is rather doubtful to directly assign the formation of discrete Li2O2 plates or toroids to the solution-mediated disproportionation.11-12 One needs to carefully consider the influence from the three-phase interface and mass diffusion conditions on the surface of the electrode, especially that of the porous carbon electrode, where the reaction mechanism could be different from a flat electrode surface due to the different interfacial conditions. 3.4 Morphology of the ORR products in DMSO with different amounts of water As shown in Figure 1~3, the morphologies for the ORR products on the gold electrode surface change with the water concentration in the DMSO electrolyte solution. Thus, a systematic investigation on the morphology of the ORR products was carried out in the DMSO-based solutions containing different amounts of water (33~1269 ppm, Figure 4). The CVs (5mV/s) were recorded in each solution by scanning the potential from the initial OCP (ca. 3.1 V) to 2.1 V and then back to 2.8 V (Figure 4a). With the increasing water contents in solution, the cathodic current becomes broader and two peaks can be distinguished at 2.54 V and 2.47 V (Figure 4a). AFM images of the gold electrode surface after the ORR were taken in a DMSO solution containing water of 33 ppm (Figure 4b), 298 ppm (Figure 4c), 533 ppm (Figure 4d), 813 ppm (Figure 4e) and 1269 ppm (Figure 4f). When the water concentration is low, the ORR products are uniformly deposited on the gold electrode surface as small particles of 10~20 nm (Figure 4b). As the water content increases to 298 ppm, the


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diameter of the particles increases to 10~30 nm (Figure 4c). In the DMSO electrolyte solutions containing water of 500~800 ppm, the particles grow even larger, with one dimension preferably developed, leading to plate-like feature (Figure 4d-4e). As the water concentration increases to 1269 ppm, large aggregations composed of particles in the shape of plates are produced (Figure 4f). On the other hand, we did not find a big difference in the decomposition process of the ORR deposits on the electrode surface (i.e. OER) in the DMSO solution with the different water concentrations. All of them require a very high overpotential to fully decompose the ORR products on the gold electrode surface. In spite of many preceding studies, the mechanism of the water effect is still not well understood due to both the intrinsic complexity and the difficulty of concentration control in practical batteries.11, 18, 27-32 It has been found that trace amounts of electrolyte



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additives, such as water in the organic solvents, could enhance the formation of Li2O2 toroids.11,

27

Aetukuri

et

al.11

proposed

that

the

solution-mediated

Li2O2

disproportionation mechanism plays an important role. However, evidence for the disproportionation reaction has not been obtained in any of the present AFM observations. Different explanations have also been proposed by other groups.27-28 No experimental evidence has been obtained in the present study to support these assumptions. More experimental and theoretical efforts are necessary to understand the dependence of the water concentration on the ORR/OER behaviors on the cathode surface.

3.5 Vibrational Spectroscopy Characterizations The potential dependence for the decomposition of the ORR products on the electrode surface evaluated by the AFM observations present different potential dependences obtained from previous SERS measurements. The SERS signals for Li2O2 formed during the ORR disappeared when the potential became approximately 3.7 V, while the AFM observations (Figure 1~3) demonstrated that part of the ORR deposits remained on the gold electrode surface even after a potential was swept to 4.4 V. To understand the discrepancy from the two different measurements, several complementary experiments were carried out using IR and Raman spectroscopies under similar condition. The Au electrodes used for vibrational spectroscopic characterization were prepared by a sputtering method, which produces strong SERS effects. It is assumed that these gold electrodes show similar ORR/OER behaviors


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except for the different surface morphologies and surface area. The galvanostatic ORR/OER cycle at a current density of 20 µA/cm2 was used. Figure 5a shows the ATR-FTIR spectra (400 ~ 840 cm─1) of the Au electrodes after the ORR (@2.7 V, -20 µAh/cm2) and OER (@3.5 V, +1 µAh/cm2; @4.0 V, +5 µAh/cm2 and @4.5 V, +20 µAh/cm2). The IR spectra of several standard materials as possible ORR products are also given in the spectra for comparison. After the ORR, a strong IR absorption peak is observed at 570 cm─1 (black trace, Figure 5a), which could be attributed to the Li-O stretching mode of Li2O2. Only Li2O2 can be identified as a major ORR product on the gold electrode surface by the ex situ IR observation. On the other hand, after the OER to 3.5 V, the IR peak of Li2O2 at 570 cm─1 decreased by only 20% (red trace, Figure 5a). Almost no change for the IR peak of Li2O2 was observed at 4.0 V (blue trace, Figure 5a). This indicates that a large amount of Li2O2 still remained on the gold electrode surface even at 4.0 V. As the potential is polarized to 4.5 V, this IR peak



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completely disappeared (pink trace, Figure 5a), implying no Li2O2 on the gold electrode surface at this condition. We also compared the in situ SERS spectra of the Au electrode surface obtained under the same conditions as that foe the ATR-IR observations (Figure 5b). A strong Raman peak at 788 cm─1 was observed after the ORR (black trace, Figure 5b). This peak can be assigned to the O-O bond of Li2O2. This is in agreement with the IR observation confirming that Li2O2 was definitely formed on the Au electrode surface during the ORR. It should be mentioned that the Raman peaks can be only observed on the specially sputtered Au electrode with a SERS effect. No Raman signal can be observed on the flat gold electrode used in the AFM measurement. As the potential is polarized to 3.5 V in the OER, the SERS peak at 788 cm─1 decreased by approximately 70% (red trace, Figure 5b), while the absorbance of Li-O bond in the IR spectrum declined by only 20% (red trace, Figure 5a). As the potential is increased to 4.0 V, the SERS peak at 788 cm─1 further dropped to only 5% of the original intensity (blue trace, Figure 5b). As already mentioned, the same sample showed almost the same IR band intensity for Li-O stretching mode of Li2O2 (blue trace, Figure 5a) as that at 3.5 V (red trace, Figure 5a). The SERS peak becomes very weak at 4.0 V and is almost impossible to see any other change with a further OER process although only one-fourth of the OER capacity is completed at the potential. The present results suggest that the SERS signal does not reflect the same information as that obtained by the IR observations. Generally, based on the interaction between the dipole moment and external fields, the IR absorption can measure the coverage of the target species on a metal surface. On


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the other hand, however, the enhancement factor for the SERS significantly depends on the shape and size of the metal nanoparticles on the surface and can dramatically decrease with the distance from the SERS active surface. This is the important reason why SERS can selectively observe the surface species only on the surface. SERS has an extremely high sensitivity, but quantitative analysis of the SERS signals may become very complicated when the interaction and spatial distance between the SERS substrate and the detection species changes during the measurement. All the results demonstrate the limitation of the SERS spectroscopy in characterizing the deposits on the electrode surface during electrochemical reactions. Since the SERS signal significantly depends on the electromagnetic field enhancement caused by surface plasmon resonance, which is nonhomogeneous and dramatically decays in the space away from the substrate surface, the Raman intensity is not a linear function of the film thickness of the deposits. The sensitivity may largely decline as the species get farer away from the substrate surface. The SERS results reveal the variation in the deposits closely adjacent to the electrode surface, while the IR spectra give information on the bulk deposits. In fact, we have found that the SERS peak intensity for Li2O2 on the gold electrode surface becomes saturated even if the ORR capacity is still low. This may be attributed to the thickness dependence of the SERS response.7,33 Detailed characterizations on the relationship between the SERS intensity and coverage of Li2O2 are still in progress. Based on the EC-AFM and vibrational spectroscopic studies, we propose that the deposition of the ORR products and their decomposition take place in different ways.



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During ORR, the Li2O2 particles gradually nucleate on the electrode surface and grow larger in both the lateral dimensions and thickness dimension until the electrode surface is covered by thick particles and no electrons could be transferred to the O2 molecules to sustain the ORR process to the end of the ORR. Such a process agrees with the EC-AFM observations (Figures 1-3) which revealed the particle growth model, and is in good agreement with both the IR and SERS results. Wang et al. recently reported that the ORR occurs at the buried interface between Li2O2 and electrode during the sudden death stage of ORR34 while we are unable to get direct evidence to support it in the present study. The SERS intensity dramatically decreases even when only a very small amount of OER capacity is used.7,33 This rapid decrease in SERS intensity does not reflect the decrease of Li2O2 estimated from the polarization curve and IR measurement. This result is considered as a result of decomposition from the bottom layer of the Li2O2 on the Au substrate surface. Even if small number of Li2O2 particles is oxidatively decomposed, the SERS effect significantly decreases due to the separation of the Li2O2 particle and SERS-active Au substrate surface. One expects that decomposition of the Li2O2 initiates at the interface of the Au|Li2O2 as soon as the potential becomes more positive than 3.1 V during the OER process. The remaining Li2O2 particles are electronically isolated from the electrode surface, thus oxidative decomposition can only occur from the defect sites and thus a higher overpotential is necessary. Although AFM is impossible to directly confirm the buried structure at the interface between the


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Li2O2 and Au electrode, all of the AFM features become easy to understand based on the conclusion from the vibrational spectroscopic results. Zhong et al. investigated the ORR/OER on a supported multiwall carbon nanotube (MWCNT) by in situ transmission electron microscope (TEM) using an all-solid setup in vacuum and reported that the OER preferentially took place at the Li2O2/MWCNT interface, indicating the electronic conductivity to be the limiting factor during charging.

35

Recently, Kushima et al. employed a liquid-cell in situ TEM to track the

ORR/OER on the Au cathode surface in a DMSO-based electrolyte solution and found the ORR occurred at the interface with the electrolyte, while the OER took place at the gold electrode surface and proposed that the Li-ion diffusivity/electronic conductivity is the limiting factor in ORR/OER, respectively.36 Gittleson et al. reported the similar conclusion on the OER based on a SERS observation on an Au-Ni foam electrode with a DMSO-based electrolyte solution.37 Our results are in good agreement with these previous studies, but the conclusions are confirmed from different angles. The extremely high OER overpotential should be associated with the specific decomposition mechanism of Li2O2 in the OER process.

3.6 Promoting OER Process by a Redox Mediator TTF A high charge overpotential during the OER process has been widely recognized as a challenge for Li-O2 batteries. As also confirmed in the present study, oxidative decomposition of the ORR products on the gold electrode requires a very high overpotential. As already discussed, one of the reasons for the high OER overpotential



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is attributed to the decomposition starting from the interface with the substrate, making the remaining Li2O2 particles to be hardly decomposed. Recently, redox mediators soluble in the electrolyte solution have been applied to oxidize the Li2O2 using homogeneous chemical reactions.38-39 Previously, we have evaluated the ORR/OER on gold and porous carbon electrods in a DMSO-based electrolyte solution containing tetrathiafulvalene (TTF) as a redox mediator by in situ UV-Vis absorption spectroscopy and SERS measurements.33 We found that the electrochemically generated TTF+ was mainly consumed by the oxidative decomposition of LiO2 in solution, but not Li2O2, on the gold electrode surface. On a porous carbon electrode, however, the TTF+ did help oxidize the Li2O2. However, as already mentioned, SERS measurement may be hard to evaluate the coverage of Li2O2 on gold electrode due to the OER mechanism. More examination is necessary. We employed EC-AFM to investigate the OER process in the solution containing TTF. Figure 6a compares the CV in a DMSO-based solution with (black trace) and without TTF (red trace) containing 75 ppm water. Both CVs show a similar shape in the potential region more negative than 3.0 V, implying no influence of the TTF on the ORR. When TTF is included, a pair of redox peaks corresponding to TTF/TTF+ appeared around 3.65 V, while the anodic current profile in the potential region below 3.5 V is the same as that free of TTF. The CVs are similar to those already proposed and suggest that the TTF can mediate the electrochemical reaction of Li2O2 at its reversible redox potential.


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Figure 6b shows an AFM image recorded after a potential sweep to 2.1 V in solution with TTF. As similar AFM image was observed in Figure 3. These particles are attributed to the Li2O2 deposited on the gold electrode surface. No influence of TTF was observed for the ORR process. In order to dynamically track the decomposition process during the OER, a galvanostatic OER at 16 µA/cm2 was performed and the potential changes versus the capacity is shown in Figure S3. The electrode potential quickly increases to 3.6 V and remains almost constant during the OER process. Figures 6c (tip scan from top to bottom) and 6d (tip scan from bottom to top) show the in situ AFM images during the galvanostatic OER process. Both the potential and charge ratio of the OER to ORR are indicated near the images. Generally, little change on the morphology



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of the deposits during the potential region below 3.6 V is observed during the galvanostatic OER. As the potential reaches the plateau around 3.6 V, some protruding particles on the top surface disappear first and the particle film suddenly disappears at 61% of the OER capacity (see bottom part in Figures 6c and 6d). Although the current still flows, no change is observed on the gold electrode surface (Figure 6d). This indicates that the electrochemically generated TTF+ at 3.6 V helps the decomposition of Li2O2 on the gold electrode surface. Although the first oxidation of the Li2O2 from the interface with the gold substrate may be the same, the subsequent decomposition process mediated by TTF seems to be quite different from that without TTF. No defect is observed during the reaction on the gold electrode surface (Figure 6). The Li2O2 particles are fully removed in a uniform way, implying that the uniform oxidative dissolution of the Li2O2 particles by the soluble TTF+ in solution directly control the reaction. Due to the homogeneous oxidation of LiO2 in solution, it is reasonable to have a current flow even if the Li2O2 particles are removed from the surface.7, 33 The present result is consistent with the previous EQCM study on the Au electrode in the DMSObased electrolyte solution containing TTF, which demonstrated a better reversibility in mass variation during cycling.8 Hence, it is concluded that the electrochemically-deposited Li2O2 particles requiring an oxidation potential higher than 4.0 V could be decomposed below 3.63 V with the addition of TTF. Our previous conclusion on the role of TTF on the gold electrode may be not suitable due to the large decrease is the SERS sensitivities as the bottom layer of Li2O2 on the gold electrode substrate is first oxidized.


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Conclusions In summary, the dynamic process of Li2O2 growth/ decomposition during the ORR/OER on a gold electrode in a DMSO-based electrolyte solution has been investigated by EC-AFM with the help of vibrational spectroscopy. Discrete Li2O2 nanoparticles are deposited by the ORR process, whose shape is significantly influenced by the water concentration. The irreversibility of the ORR/OER in the DMSO-based electrolyte solution was demonstrated by the EC-AFM observations. The decomposition of the ORR deposits requires a potential of 4~4.5 V, regardless of the water concentration in the solution. Based on the IR and SERS analysis, it was found that oxidation of the Li2O2 initiates at the Au|Li2O2 interface at a low potential, leaving the remaining Li2O2 particles hard to be oxidized. The decomposition potential of Li2O2 is dramatically reduced to 3.6 V by including a redox mediator of TTF in the solution. The AFM observation suggests that electrochemically generated TTF+ is involved in the oxidative removal of Li2O2 particles on the gold electrode surface through a homogenous chemical reaction mechanism. No evidence was found in the present ECAFM study to support the solution-mediated disproportionation process of LiO2 for the formation of anisotropic Li2O2 particles on the gold electrode surface.

AUTHOR INFORMATION Corresponding Author



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* E-mail: [email protected]; Tel: +81-117069126. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT The authors thank Dr. Xu Shi for his technical assistance with the deposition of gold film electrodes. This work was supported by the Advanced Low Carbon Technology Research and Development Program (ALCA), specially promoted research for innovative next generation batteries (SPRING) from the Japan Science and Technology Agency (JST).

SUPPORTING INFORMATION Supporting Information Available: detailed experimental method section and supporting Figures S1-S3 (the schematic of EC-AFM cell, the probe effect for the in-situ OER imaging, the galvanostatic OER curve with TTF in the electrolyte solution). This material is available free of charge via the internet at http://pubs.acs.org.


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Table of Contents



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Figure 1. (a) CV curve for ORR process acquired during in-situ AFM imaging in the Li-O2 cell at 5 mV/s. The potential was swept negatively from initial OCP (3.1 V) to 2.1 V. (b) Upward and (c) downward in-situ AFM images during negative potential sweeping corresponding to the CV in (a). The cross-sectional profiles of the same position before and after deposition (L1 and L2) were displayed in (c). (e) Upward and (f) downward AFM images in the area of 2×2 μm2 at OCP (2.8 V) after ORR is finished. The electrolyte solution contained 875 ppm of water. Scale bar = 200 nm.

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Figure 2. (a) CVs in a Li-O2 cell at 5 mV/s. The potential was firstly swept negatively from initial OCP (3.1 V) to 2.1 V (black line), then positively from the OCP (2.9 V) to 4.0 V (red line), and further increased to 4.4 V (blue line). The pink dash line represents direct oxidation from the initial OCP to 4.4 V on a fresh Au electrode. The water content in the solution is 1000 ppm. AFM images obtained (b) at initial OCP of 3.1 V, after (c) being reduced to 2.1 V, (d) being oxidized to 4.0 V, (e) further to 4.4 V and (f) finally to 4.4 V again. Scale bar = 300 nm.



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Figure 3. (a) CVs in a Li-O2 cell at 5 mV/s. The cell was firstly placed in the glove box for cathodic potential scanning, i.e. from initial OCP to 2.1 V (black line), with the water content of 33 ppm. Then the cell was transferred to the AFM box for image scanning and anodic potential scanning, i.e. from OCP to 4 V (red line) and subsequently from OCP to 4.4 V (blue line). AFM images obtained after (b) being reduced to 2.1 V, then oxidized (c) to 4.0 V, (d) to 4.4 V, and (e) to 4.4 V again. Scale bar =300 nm.


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Figure 4. (a) CV curves from initial OCP (ca. 3.1 V) to 2.1 V at 5 mV/s, performed in ECAFM cell with the electrolytes containing different water content. The AFM images of the ORR products corresponding to water content of (b) 33 ppm, (c) 298 ppm, (d) 533 ppm, (e) 813 ppm, and (f) 1269 ppm in the solutions, respectively. Scale bar = 300 nm.



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Figure 5. (a) Ex-situ ATR-FTIR spectra and (b) in-situ SERS spectra of the Au film electrodes deposited on glass substrates being galvanostatically discharged to 2.6 V (black line), recharged to 3.5 V (red line), to 4.0 V (blue line) and to 4.5 V (pink line) at 20 μA/cm2 in O2-saturated 0.1 M LiClO4-DMSO electrolyte, with a water content of 76 ppm. Reference FTIR spectra of standard Li2O2, Li2CO3, and LiOH are also plotted.


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Figure 6. (a) CVs performed in EC-AFM cell with and without 10 mM TTF in O2saturated DMSO electrolytes containing 0.1 M LiClO4, swept at 50 mV/s. (b) The resulted AFM image after CV reduction from OCP 3.2 V to 2.1 V at 5 mV/s with 10 mM TTF in the solution. (c-d) The successive dynamic AFM image scanned downward (c) and upward (d) simultaneously during the galvanostatic OER at 16 μA/cm2 to the same capacity of ORR. The green arrows and lines mark the specific potential and the charge ratio of OER to ORR. Scale bar = 200 nm.


In Situ Atomic Force Microscopy (AFM) Study of Oxygen Reduction

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