Dynamic Behavior at the Interface between Lithium Cobalt Oxide and

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Dynamic Behavior at the Interface between Lithium Cobalt Oxide and an Organic Electrolyte Monitored by Neutron Reflectivity Measurements Taketoshi Minato,*,† Hiroyuki Kawaura,† Masaaki Hirayama,§ Sou Taminato,§ Kota Suzuki,§ Norifumi L. Yamada,¶ Hidetaka Sugaya,† Kentaro Yamamoto,‡ Koji Nakanishi,† Yuki Orikasa,‡ Hajime Tanida,† Ryoji Kanno,§ Hajime Arai,† Yoshiharu Uchimoto,‡ and Zempachi Ogumi† †

Office of Society-Academia Collaboration for Innovation, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan ¶ Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), 203-1 Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1106, Japan ‡ Graduate School of Human and Environmental Studies, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan §

S Supporting Information *

ABSTRACT: Clarification of the interaction between the electrode and the electrolyte is crucial for further improvement of the performance of lithium-ion batteries. We have investigated the structural change at the interface between the surface of a 104oriented epitaxial thin film of LiCoO2 (LiCoO2(104)), which is one of the stable surfaces of LiCoO2, and an electrolyte prepared using a carbonate solvent (1 M LiClO4 in ethylene carbonate and dimethyl carbonate) by in situ neutron reflectivity measurements. Owing to the decomposition of the organic solvent, a new interface layer was formed after contact of LiCoO2(104) with the electrolyte. The composition and thickness of the interface layer changed during Li+ extraction/insertion. During Li+ extraction, the thickness of the interface layer increased and the addition of an inorganic species is suggested. The thickness of the interface layer decreased during Li+ insertion. We discuss the relationship between battery performance and the dynamic behavior at the interface.

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the electrolyte/LiCoO2 interface, reduction of Co3+ ions after contact with the organic electrolyte was observed.6,8,14 During Li+ extraction/insertion reactions, the structure of the LiCoO2 surface was irreversibly changed.6 Also, at the electrolyte/ LiCoO2 interface, it has been reported that an interface layer is formed due to decomposition of the electrolyte.15,16 On LiCoO2 particles, the thickness of the interface layer was estimated to be 5.3 nm by photoelectron spectroscopy.17 Changes in the interface layer during the Li+ extraction/ insertion process due to the decomposition of the organic solvent have been monitored by in situ polarization modulation Fourier transform infrared (PM-FTIR) spectroscopy.18 Changes in the structure (thickness and roughness) and composition of the interface layer at the electrolyte/LiCoO2 interface have not previously been analyzed in situ.

INTRODUCTION Lithium-ion batteries (LIBs) are commonly used in portable electronic devices owing to their high storage capacity and relatively low weight.1,2 Efforts to improve the performance of the LIBs used in these devices have increased significantly in recent decades. More recently, the potential use of LIBs in electric and hybrid vehicles has attracted considerable attention. To improve the performance of LIBs, understanding of the electrode/electrolyte interface is needed because it has a significant impact on the battery performance.3−12 The behavior of the geometric and electronic structure of the electrode/electrolyte interface has been reported during the Li+ extraction/insertion process.3−12 However, much is still unknown about the behavior at the electrode/electrolyte interface. LiCoO2 is one of the most widely used positive electrode materials in LIBs because of the reversible nature of Li+ extraction/insertion from LiCoO2 to Li0.5CoO2 with a high electrochemical potential of up to 4.2 V (vs Li+/Li).13 The electrolyte/LiCoO2 interface has been widely investigated. At © XXXX American Chemical Society

Received: March 10, 2016 Revised: August 2, 2016

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DOI: 10.1021/acs.jpcc.6b02523 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C In situ neutron reflectometry (NR) is a powerful method for determining the composition, thickness, and roughness of the electrolyte/electrode interface.19−24 By using in situ NR, we previously monitored changes in the liquid phase at the electrolyte/electrode interface in LIBs.19 Because neutrons have high sensitivity for light elements and high penetrating power through a substrate, NR is capable of detecting changes in the interface layer due to decomposition of the electrolyte at the electrolyte/electrode interface. Dura et al. showed that in situ NR can be used to monitor the interface layer on Cu.20 Recently, NR has been applied for the analysis of the e l e c t r o l y t e / e l e c t r od e i n t e r f a c e o n Si 2 1 , 2 3 , 2 4 a n d LiMn1.5Ni0.5O422 electrodes. However, the analysis of changes in the interface layer on LiCoO2 electrodes, which are the most widely used electrodes for LIBs, by NR has not previously been reported. In this work, changes in the structure and composition of the interface layer at the electrolyte/LiCoO2 interface were investigated by in situ NR. To monitor changes in the interface layer on LiCoO2, we used the surface of a 104-oriented epitaxial film of LiCoO2 (LiCoO2(104)), because this surface has high flatness, which is very important for the detection of changes in the interface layer by NR. Also, LiCoO2(104) (Figure 1) is one

copy (XAS). The XAS measurements were conducted using the BL28XU beamline located at the SPring-8 facility in Hyogo, Japan. The XAS measurements were conducted via X-ray absorption near-edge spectroscopy (XANES) in the Co K-edge region. The X-ray irradiation was applied from the [010] direction of LiCoO2. The incident angle of the X-ray irradiation was 2.2°, which meant that fluorescence occurred from the bulk of the LiCoO2(104) film.6 The X-ray fluorescence was detected by a solid-state detector (21-element SSD). The orientation of the film was determined by X-ray diffraction (XRD). The XRD measurements were performed using an ATX-G (Rigaku) diffractometer with Cu Kα radiation. For electrochemical measurements, 1 M LiClO4 in ethylene carbonate (EC)/dimethyl carbonate (DMC) (vol. 1:2) was used as the electrolyte. Electrochemical Li+ extraction/insertion was performed using a 2032-type coin cell containing a Li anode by means of a potentiogalvanostat (Toyo System). The charge−discharge characteristics were examined at a current density of 152 mA g−1. The dQ/dV curve was obtained from the differential of the capacity−voltage curve. In situ NR measurements were performed using the SOFIA horizontal-type reflectometer at the BL16 beamline, Materials and Life Science Experimental Facility, Japan Proton Accelerator Research Complex (J-PARC), Ibaraki, Japan.29,30 A custom three-electrode cell was used in the NR measurements.19,31 The neutron reflectivity was measured as a function of momentum transfer perpendicular to the substrate, Qz: Q z = (4π sin θ )/λ

where θ is the incident angle and λ is the wavelength of the neutrons. We used deuterated EC and DMC electrolytes containing 1 M LiClO4 to prevent background signals arising from incoherent scattering by 1H. Analysis of the NR data was performed using Parratt3232 and Motofit33 software. To model changes in the surface of the LiCoO2 layer due to reaction with the electrolyte,6,8,14−16 the LiCoO2 layer at open-circuit voltage, 4.2 V, and 3.3 V (vs Li+/Li) was separated into two layers (LiCoO2-1 and LiCoO2-2) for analyzing NR profiles. For the as-prepared sample in air (Table 1(i)), the thickness, scattering length density (SLD), and roughness of the LiCoO2, SrRuO3, and Nb−SrTiO3 layers were determined by fitting. In the analysis after immersion in the electrolyte (Table 1(ii)), after Li+ extraction at 4.2 V (vs Li+/Li) (Table 1(iii)), and after Li+ insertion at 3.3 V (vs Li+/Li) (Table 1(iv)), all parameters (SLD, thickness, and roughness) of the layers were determined by fitting, in which the thickness of the SrRuO3 layer and roughness of the SrRuO3 and Nb−SrTiO3 layers were fixed at the values in air.

Figure 1. Surface structure of LiCoO2(104) sliced from the bulk structure (red ball, O; purple ball, Li; blue octahedron, O-coordinated Co).

of the most stable surfaces25,26 and is expected to be an electrochemically active surface for Li+ extraction/insertion. Therefore, a thorough understanding of the structure of the electrolyte/LiCoO2(104) interface during Li+ extraction/ insertion is important for improving the performance of LIBs. We successfully observed a change in the interface layer at the interfaces. On the basis of our results, we discuss the relationship between LIB performance and the behavior of the interface layer at electrolyte/LiCoO2(104) interfaces.

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EXPERIMENTAL SECTION The epitaxial LiCoO2(104) film was grown by a pulsed laser deposition (PLD) technique.27 The substrate was Nb-doped (0.2 wt %) SrTiO3(100) (Nb−SrTiO3(100)) (15 mm × 15 mm × 2 mm). Prior to deposition of LiCoO2, SrRuO3 was grown on the Nb−SrTiO3(100) surface by PLD to be used as a buffer layer.28 The thickness of each film was determined by cross-sectional measurements using transmission electron microscopy (TEM). The TEM measurements were performed on a JEM-ARM200F (JEOL) microscope with an electron acceleration energy of 200 kV. No impurity in the thin film was observed by electron energy loss spectroscopy equipped with a TEM apparatus. The electronic structure of Co ions in the prepared films was investigated by X-ray absorption spectros-

RESULTS

The orientation of the prepared film was determined by XRD measurements. Parts a and b of Figure 2 show out-of-plane XRD patterns obtained from the prepared film. In the angle range (2θ = 15−100°) of the obtained XRD patterns, three peaks at 22.8, 46.5, and 72.6° are observed. These peaks are assigned to SrTiO3 100, 200, and 300 and SrRuO3 100, 200, and 300 planes. This shows that SrRuO3 is epitaxially grown on the SrTiO3(100) substrate and oriented to (100). In addition, a peak is observed at 46.2°. This peak is assigned to LiCoO2 104. No other peaks are observed. This shows that the prepared LiCoO2 film is oriented to (104). From a cross-sectional TEM image (Figure 2c), the thickness of each layer was determined B

DOI: 10.1021/acs.jpcc.6b02523 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Table 1. Thickness, Scattering Length Density (SLD), and Roughness Determined by NR Analysis from (i) the AsPrepared Sample, (ii) the Sample after Immersion in Electrolyte (1 M LiClO4 in Deuterated EC:DMC (vol. 1:2)), (iii) the Sample after Li+ Extraction at 4.2 V (vs. Li+/Li), and (iv) the Sample after Li+ Insertion at 3.3 V (vs. Li+/Li) of LiCoO2(104) Layer/SrRuO3(100) Layer/Nb−SrTiO3(100) layer layer

SLD/nm−2

thickness/nm

roughness/nm

(i) surface layer LiCoO2 SrRuO3 Nb−SrTiO3

0.9 24.2 19.9 −

electrolyte interface layer LiCoO2-1 LiCoO2-2 SrRuO3 Nb−SrTiO3

− 30.6 4.5 16.3 19.9 −

electrolyte interface layer LiCoO2-1 LiCoO2-2 SrRuO3 Nb−SrTiO3

− 48.8 1.6 19.8 19.9 −

electrolyte interface layer LiCoO2-1 LiCoO2-2 SrRuO3 Nb−SrTiO3

− 35.6 5.8 15.1 19.9 −

1.48 3.68 4.86 3.56

× × × ×

10−4 10−4 10−4 10−4

0.8 1.5 2.0 1.6

5.54 8.51 3.05 3.68 4.86 3.56

× × × × × ×

10−4 10−5 10−4 10−4 10−4 10−4

− 25.9 5.9 7.3 2.0 1.6

5.54 3.33 4.09 3.93 4.86 3.56

× × × × × ×

10−4 10−4 10−4 10−4 10−4 10−4

− 9.3 6.1 4.1 2.0 1.6

5.54 2.17 3.50 3.83 4.86 3.56

× × × × × ×

10−4 10−4 10−4 10−4 10−4 10−4

− 24.3 10.9 5.0 2.0 1.6

(ii)

(iii)

(iv)

to be 17.7 and 28.0 nm for the SrRuO3 and LiCoO2 layers, respectively. The 104 orientation of the LiCoO2 layer was confirmed by electron diffraction of the LiCoO2 layer (Figure 2d). Hereafter, the SrRuO3 and LiCoO2 layers are denoted as the SrRuO3(100) and LiCoO2(104) layers, respectively. The oxidation state of Co in the LiCoO2(104) layer was also investigated by XAS measurements. The red curve in Figure 2e shows the Co K-edge XANES spectrum of the LiCoO4(104) layer. The obtained spectrum corresponds with the spectrum obtained from a standard sample of LiCoO2 powder (SigmaAldrich Co., LLC) (green curve in Figure 2e), which shows that the Co ions in the LiCoO2(104) layer were in the form of Co3+. The electrochemical behavior of the LiCoO2(104) layer was investigated by measurements of dQ/dV. Figure 3 shows the resulting curve of dQ/dV using an electrolyte solution of 1 M LiClO4 in EC:DMC. The anodic and cathodic current peaks correspond to a first-order phase transition between the two hexagonal phases of LiCoO2. These are observed at 3.91 V in the first cycle (red curve in Figure 3). The peak voltages are well matched with the anodic and cathodic peaks of reported results for polycrystalline or composite LiCoO2 electrodes.6−13,34,35 These data confirm that the LiCoO2(104) layer electrode is an appropriate model electrode. Compared with previous reports of thin-film LiCoO2 electrodes,6−12,34 the decrease in the capacity of the LiCoO2(104) electrode was not

Figure 2. (a) 15−100° and (b) 45−47.5° regions of out-of-plane XRD patterns, (c) cross-sectional TEM image, and (d) electron diffraction pattern obtained from the LiCoO2 layer. (e) Co fluorescence K-edge XANES spectrum (red curve) of the prepared LiCoO2(104)/ SrRuO3(100)/Nb−SrTiO3(100). The Co transmission K-edge XANES spectrum of LiCoO2 powder (green curve) is also shown as a reference in (e).

significant with successive cycles of Li+ extraction/insertion (inset of Figure 3). Structural changes in the electrolyte/LiCoO2(104) layer interface were monitored by NR measurements. The blue dots and red curve in Figure 4a(i) show the NR profile of the LiCoO2(104) layer/SrRuO3(100) layer/Nb−SrTiO3(100) layer in air and the result of fitting, respectively. The obtained SLD profile in air is shown as the red curve in Figure 4b(i) and the obtained parameters are shown in Table 1(i). The analyzed SLD values of Nb−SrTiO3(100) and the SrRuO3(100) layer are 3.56 and 4.86 × 10−4 nm−2, respectively. The SLD values are well matched with the values calculated from the bulk density and composition (3.52 and 5.07 × 10−4 nm−2 for Nb− SrTiO3 and SrRuO3, respectively). A good fit in NR analysis was obtained by the addition of the surface layer on the LiCoO2 layer (analyzed SLD is 3.68 × 10−4 nm−2). The analyzed SLD and thickness of the surface layer are 1.48 × 10−4 nm−2 and 0.9 C

DOI: 10.1021/acs.jpcc.6b02523 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 4a(ii) shows the NR profile of the LiCoO2(104) layer/SrRuO3(100) layer/SrTiO3(100) layer after contact with the electrolyte, and the obtained fitting parameters are shown in Table 1(ii). A good fit was obtained by the separation of the LiCoO2(104) layer into two layers (LiCoO2-1 and LiCoO2-2; analyzed SLD = 3.05 and 3.68 × 10−4 nm−2). This suggests that the surface of the LiCoO2(104) layer is slightly different from the bulk due to reaction with the electrolyte. Also, a new interface layer with an analyzed SLD of 8.51 × 10−5 nm−2 was observed between the electrolyte and the LiCoO2(104) layer.37 The thickness of the interface layer is 30.6 nm, which is much greater than that of the surface layer observed on the LiCoO2(104) layer before contact with the electrolyte. The roughness of the interface layer is 25.9 nm; this is much larger than the roughnesses of the surface layer and LiCoO2 before contact with the electrolyte. The high roughness would be due to reaction of the LiCoO2 surface with the electrolyte. To determine the composition of the interface layer, we theoretically calculated SLD values of possible species, including poly(ethylene oxide) (PEO), Li2CO3, LiOD, LiCl, LiOCO2CD3, and (LiCO2CD2)2, which are reported to be formed by the decomposition of EC and DMC in a Li+containing electrolyte on LiCoO2.16,18,38−40 The calculated SLD values of each species are shown in Table S1. The material that has an SLD value closest to that of the interface layer is PEO (7.51 × 10−5 nm−2 for C30−C3000), whereas the SLD values of other candidate species (3.48, 3.88, 2.26, 4.74, and 4.45 × 10−4 nm−2 for Li2CO3, LiOD, LiCl, LiOCO2CD3, and (LiCO2CD2)2, respectively) are obviously larger than that of the interface layer. The electrolyte has the largest SLD (5.54 × 10−4 nm−2). Therefore, we deduced that the interface layer is mainly composed of organic species, typically PEO due to the decomposition of the organic solvent. The composition and thickness of the interface layer showed a change after Li+ extraction from LiCoO2. Parts a(iii) and b(iii) of Figure 4 and Table 1(iii) show the NR profile, SLD profile, and fitting parameters of LiCoO2(104) after a Li+ extraction reaction at 4.2 V (vs Li+/Li), respectively. The analyzed SLD values of the LiCoO2-1 and LiCoO2-2 layers increased to 4.09 and 3.93 × 10−4 nm−2, respectively. The increase in the SLD was caused by the extraction of Li+ ions, which have a negative SLD value (the SLD of Li metal is −0.88 × 10−4 nm−2), from LiCoO2.41 The analyzed SLD value of the interface layer increased to 3.33 × 10−4 nm−2 and its thickness increased to 48.8 nm. The determination of the composition of the interface layer by the comparison of species that are reported to be formed on the electrode surface by the decomposition of the organic solvent is very difficult, because there are many species that have similar SLD values. A change in the SLD means a change in the composition of the interface layer. A possible interpretation for the analyzed SLD values is the formation of an inorganic species in the interface layer. Li2CO3 (SLD = 3.48 × 10−4 nm−2), LiCl (SLD = 2.26 × 10−4 nm−2), LiOCO2CD3 (SLD = 4.74 × 10−4 nm−2), and (LiCO2CD2)2 (SLD = 4.45 × 10−4 nm−2) are species that may possibly have contributed to the increase in the SLD of the interface layer. Another possible interpretation of the increase in the SLD on Li+ extraction is the insertion of the electrolyte, which has a high SLD, into the interface layer. If the interface layer exhibits high porosity, insertion of the electrolyte may occur. The thickness of the interface layer decreased to 35.6 nm on Li+ insertion at 3.3 V (Figure 4, a(iv) and b(iv), and Table

Figure 3. dQ/dV curve of a LiCoO2(104) thin-film electrode in the first cycle. The inset shows the change in discharge capacity in 50 cycles recorded from a LiCoO2(104) thin-film electrode.

Figure 4. (a) Neutron reflectivity and (b) analyzed SLD determined for (i) as-prepared sample, (ii) sample after immersion in electrolyte (1 M LiClO4 in deuterated EC:DMC (vol. 1:2)), (iii) sample after Li+ extraction at 4.2 V (vs Li+/Li), and (iv) sample after Li+ insertion at 3.3 V (vs Li+/Li) of LiCoO2(104) layer/SrRuO3(100) layer/Nb− SrTiO3(100) layer. The blue dots and red curves in (a) correspond to experimental and fitted data, respectively. The neutron reflectivity in (ii), (iii), and (iv) and SLD values in (i), (ii), and (iii) are shifted for offsets.

nm. The surface layer would comprise Li2CO3 and LiOH formed by reaction with air.27,36 The analyzed thicknesses of the SrRuO3(100) and LiCoO2(104) layers are 19.9 and 24.2 nm, respectively, which matches the thicknesses determined by TEM. This shows that our NR measurements and analysis were reliable. D

DOI: 10.1021/acs.jpcc.6b02523 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C 1(iv)). The SLD value of the interface layer after Li+ insertion (2.17 × 10−4 nm−2) did not show a significant change. This suggests that the composition and density of the interface layer did not change, but the layer simply dissolved during the Li+ insertion process.

LiCoO2(110). The characteristic electronic structure of LiCoO2(104) would be one of the main reasons for the formation of the thick interface layer on LiCoO2(104). During Li+ extraction, an increase in the thickness of the interface layer was observed. This result indicates that the electrolyte decomposed further on the surface of the interface layer. Also, the addition of an inorganic species to the interface layer is suggested. This indicates that Li+ extracted from LiCoO2 is partially trapped in the interface layer. The reaction of the electrolyte with Co3+ formed by the oxidation of Co2+ on LiCoO2(104) during Li+ extraction is a possible cause of the growth in the interface layer. If the increase in the SLD of the interface layer is caused by insertion of the electrolyte into the interface layer, the interface layer formed in the Li+ extraction process would exhibit high porosity. The decomposition rate of the electrolyte during Li+ extraction may be high and therefore the interface layer would have a large amount of voids. In the Li+ insertion process, a decrease in the thickness of the interface layer was observed. Similar behavior during Li+ insertion on LixCoO2 was observed using X-ray photoelectron spectroscopy and in situ PM-FTIR spectroscopy.15,18 This was probably caused by stripping of the interface layer by the insertion and diffusion of Li+ in the interface layer. It is expected that the binding of the inside of the interface layer is probably not strong; therefore, the interface layer is partially stripped during the Li+ insertion process. The growth of the thick interface layer on LiCoO2(104) would contribute to the stability of the discharge capacity (Figure 3, inset). In previous reports, we proposed that a spacecharged layer that formed on the LiCoO2 surface affected the stability of the LiCoO2 surface, which led to a decrease in the discharge capacity during cycling.8,9,11,12 The thick interface layer on LiCoO2(104) would suppress the formation of a space-charged layer on the LiCoO2 surface; therefore, the discharge capacity of LiCoO2(104) is stable during cycling.

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DISCUSSION During immersion in the electrolyte, the appearance of a thick (30.6 nm) interface layer, which was mainly composed of PEO, was observed on LiCoO2(104). In our previous work, the formation of a thick interface layer during immersion in an electrolyte could not be observed on LiCoO2(003) or LiCoO2(110) by X-ray reflectometry measurements, because the densities of the electrolyte and interface layer were similar.27 By in situ NR measurements, we successfully detected the formation of an interface layer during Li+ extraction/ insertion. We discuss the reaction mechanism of the formation of the thick interface layer on LiCoO2(104). Earlier work using sum frequency generation spectroscopy showed that at the interface of LiCoO2 and an EC/DMC electrolyte, the molar percentage of EC is significantly higher due to the high polarization of EC.44 In our experiments, at the interface of LiCoO2(104) and the electrolyte EC molecules should contact the LiCoO 2 (104) surface. During the contact of the LiCoO2(104) surface and EC, Co ions on the LiCoO2(104) surface were reduced, as has been observed on polycrystalline LiCoO2 thin films.6−12 A PEO layer was formed by oxidative polymerization of EC molecules. A previous density functional theory calculation by Leggesse et al. showed that the oxidation of EC by the ClO4− anion produces an unstable radical species 1 as follows:45 •

[EC−ClO4 −] − e− → OCH−CH 2 + CO2 + HClO4 1

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The reaction of the radical species 1 with EC molecules forms the radical species 2 and 3.

CONCLUSION Structural changes at the interface of LiCoO2(104) and an electrolyte during immersion in the electrolyte and Li+ extraction and insertion were successfully monitored by in situ NR measurements. The appearance of an interface layer resulting from the decomposition of organic solvents was observed after immersion in the electrolyte. During Li+ extraction/insertion, the growth/dissociation of the interface layer was observed. The interface layer contributed to the stability of the discharge capacity during cycling. Analysis of the structural changes in electrolyte/electrode interfaces during battery operation is useful for the development of LIBs.

•

OCH−CH 2 + EC 1 •

→ CO2 + OCH−CH 2−O−CH 2 − CH 2 2

•

OCH−CH 2−O−CH 2−CH 2 + EC 2 •

→ CO2 + OCH−CH 2−O−C2H4−O−CH 2−CH 2 3

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A general formula of the products of additional polymerization reactions is given as 4.

ASSOCIATED CONTENT

S Supporting Information *

•

OCH−CH 2 −[−O−CH 2−CH 2 −]n −O−CH 2−CH 2

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b02523. Calculated values of scattering length density (PDF)

4

The product 4 is mainly composed of a PEO group ([−O− CH2CH2−]n−). Thus, a PEO layer is formed at the electrolyte/ LiCoO2(104) interface. If the energy level of the conduction band minimum on LiCoO2(104) is lower than that on other surfaces, the oxidative decomposition of the electrolyte would be enhanced. First-principles calculations by Qian et al. showed that the band gap of LiCoO2(104) (intermediate-spin (IS) configuration) is smaller than that of LiCoO2(110).26 This suggests that the energy level of the conduction band minimum of LiCoO2(104) (IS configuration) is lower than that of

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AUTHOR INFORMATION

Corresponding Author

*(T.M.) E-mail: [email protected]. Telephone: +81-774-38-4966. Author Contributions

T.M., H.K., M.H., and H.A. designed the experiments. M.H. and K.S. prepared the LiCoO2(104) epitaxial film and E

DOI: 10.1021/acs.jpcc.6b02523 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

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performed the electrochemical measurements. T.M., H.K., M.H., S.T., K.S., and N.L.Y. performed in situ NR measurements. T.M., H.K., H.S., K.Y., K.N., and H.T. performed XAFS measurements. The manuscript was written by T. M., M. H., and H. A. All authors discussed the results and approved the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the “Research and Development Initiative for Scientific Innovation of New Generation Battery (RISING project)” of the New Energy and Industrial Technology Development Organization (NEDO), Japan. The synchrotron radiation experiments were performed with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2013A7600, 2013B7600). The neutron experiments were performed as projects approved by the Japan Proton Accelerator Research Complex (2013B0166).

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DOI: 10.1021/acs.jpcc.6b02523 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.6b02523 J. Phys. Chem. C XXXX, XXX, XXX−XXX