Effect of an Electrolyte Additive of Vinylene Carbonate on the

Graduate School of Human and Environmental Studies, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan. § Hitachi Research Laboratory, Hitachi, Ltd., ...
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Effect of an Electrolyte Additive of Vinylene Carbonate on the Electronic Structure at the Surface of a Lithium Cobalt Oxide Electrode under Battery Operating Conditions Daiko Takamatsu,†,∥ Yuki Orikasa,‡ Shinichiro Mori,‡ Takayuki Nakatsutsumi,‡ Kentaro Yamamoto,‡ Yukinori Koyama,*,† Taketoshi Minato,† Tatsumi Hirano,§ Hajime Tanida,† Hajime Arai,† Yoshiharu Uchimoto,‡ and Zempachi Ogumi† †

Office of Society-Academia Collaboration for Innovation, Kyoto University, Uji-shi, Kyoto 611-0011, Japan Graduate School of Human and Environmental Studies, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan § Hitachi Research Laboratory, Hitachi, Ltd., Hitachi-shi, Ibaraki 319-1292, Japan ‡

ABSTRACT: Lifetimes of lithium-ion batteries are often affected by deterioration of positive electrodes. It is well-known that the deterioration of the positive electrodes can be reduced by using electrolyte additives; however, the mechanism underlying this cyclability improvement needs to be clarified. In this study, we investigate electronic structure at the electrode/electrolyte interface using in situ total-reflection fluorescence X-ray absorption spectroscopy to elucidate the mechanism underlying the cyclability improvement of a LiCoO2 electrode upon addition of vinylene carbonate (VC) to the electrolyte. The results indicate that the reduction of cobalt ions at the surface of the LiCoO2 electrode, which occurs upon soaking in the electrolyte in the absence of VC, is suppressed by the presence of the VC additive. The VC additive also suppresses irreversible change in the electronic structure of the cobalt ions at the LiCoO2 surface during successive charge/discharge processes. The effects of the VC additive can be attributed to the formation of a layer of decomposed VC molecules at the LiCoO2/electrolyte interface, which plays an important role in the suppression of the irreversibility at the LiCoO2 surface during the charge/discharge processes.

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INTRODUCTION

The phenomena observed at electrode/electrolyte interface in a composite electrode composed of electrode active material, electric conductive additive, and binder are not easily interpreted because the composite electrode contains rough and inhomogeneous surface. The scale of an interfacial region is speculated to be in the nanometric order, close to the Debye length.22 For the analysis of the dynamic behavior at the electrode/electrolyte interface, a well-defined flat surface is essential and thus the use of a thin film is suitable. Among the few methods that can be used to analyze the dynamic behavior at the interface of a flat thin-film electrode at a nanometric-scale spatial resolution, in situ total-reflection fluorescence X-ray absorption spectroscopy (TRF-XAS) has risen to prominence as a potent technique to analyze the electronic structure at the electrode/electrolyte interface under real battery operating conditions.23−27 XAS provides information concerning the electronic structure of the target species, which is advantageous for the analysis of the electrode surface. In situ TRF-XAS has revealed that surface coating on a LiCoO2 electrode by ZrO224 and MgO25,26 suppresses the parasitic reactions between the electrode and electrolyte, thereby enhancing the electrode reversibility. With the fact that electrolyte additives affect the

Lithium-ion batteries (LIBs) are used in a variety of applications, such as power sources for mobile devices and electric vehicles.1 Improvements of the durability, energy density, and safety of LIBs are required to extend their range of application. It has been recognized that the lifetimes of LIBs are primarily limited by parasitic reactions that occur at the electrode/electrolyte interface,2 and the importance of deterioration at the positive-electrode/electrolyte interface, in particular, has been demonstrated in recent years.3−5 Electrolyte additives such as vinylene carbonate (VC) are known to be effective for the cyclability improvement at both the positiveelectrode6−15 and the negative-electrode sides.16−21 It is suggested that the VC addition to the electrolyte changes the film formed at the electrode/electrolyte interface upon decomposition of the electrolyte and the film arising from the VC additive inhibits the parasitic reactions at the interface.8,9 The nature of the surface film formed on the positive electrode by the VC decomposition has been analyzed using ex situ methods such as X-ray photoelectron spectroscopy (XPS).8,9 As the electrode/electrolyte interface can be destroyed on disassembling the cell, in situ analysis is necessary to elucidate the dynamic behavior of the electrode surface at the vicinity of the electrolyte during the charge/discharge processes and also how the behavior is affected by the electrolyte additives. © 2015 American Chemical Society

Received: November 14, 2014 Revised: April 15, 2015 Published: April 16, 2015 9791

DOI: 10.1021/jp511405g J. Phys. Chem. C 2015, 119, 9791−9797

The Journal of Physical Chemistry C

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interfacial phenomena,14,21 TRF-XAS is anticipated to yield information that is beneficial to understanding the effects of such additives. In this study, we have investigated how the addition of VC to the electrolyte suppresses the deterioration of a LiCoO2 electrode using the in situ TRF-XAS technique. LiCoO2 thin films have been prepared via pulsed laser deposition (PLD) to serve as model electrodes for the observation of interfacial phenomena at the nanometric-scale spatial resolution.23−29 The effect of the VC additive on the electrochemical properties of LiCoO2 has been examined using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements. Observations of the electronic structure of the cobalt ions at the electrode/electrolyte interface demonstrate that the surface of the LiCoO2 electrode is stabilized by the presence of VC, thereby enhancing the electrode reversibility during repeated charge/discharge processes.

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RESULTS AND DISCUSSION

Figure 1a and Figure 1b present the CVs of the LiCoO2 thinfilm electrodes using the VC-free and VC-added electrolytes, respectively. The same peak positions were observed in the CVs for both electrolytes. The conspicuous sharp peak at ∼3.9 V (versus Li/Li+) corresponds to a first-order phase transition between the two hexagonal phases of LiCoO2.30 The small

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EXPERIMENTAL SECTION Preparation of LiCoO2 Thin Films. Polycrystalline LiCoO2 thin films were prepared on mechanically polished polycrystalline platinum substrates via PLD at a substrate temperature of 873 K with an oxygen partial pressure of 0.01 Pa for 30 min. The PLD target contained 15 wt % excess Li2O to compensate for lithium loss during deposition. A Nd:YAG 4HG laser (266 nm, 200 mW) was used in the deposition process with a repetition frequency of 10 Hz. The prepared LiCoO2 thin films had thicknesses of ∼50 nm and the surface roughness values (Ra) of 2.0 nm, as reported in our previous studies.23−29 Electrochemical Measurements. For electrochemical measurements, a three-electrode electrochemical cell was assembled, in which the LiCoO2 thin film was used as the working electrode and lithium metal foil as the reference and counter electrodes. 1 mol dm−3 LiClO4 dissolved in a 1:1 volumetric mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) was used as a “VC-free electrolyte”. The VCfree electrolyte was mixed with 1 vol % VC to yield a “VCadded electrolyte”. The amount of VC in the VC-added electrolyte is enough to cover the surface of the flat thin-film electrode. CV measurements were conducted in an argon-filled glovebox with an HZ-5000 potentiostat (Hokuto Denko Corp., Japan). EIS measurements were performed at frequencies ranging from 105 to 10−2 Hz with an amplitude of 10 mV using a Solartron 1255B frequency response analyzer coupled with a Solartron 1287 potentiostat. In Situ TRF-XAS Measurements. A homemade electrochemical cell23 was used to perform the TRF-XAS measurements. The cell was assembled in a glovebox under an argon atmosphere, and helium gas was flowed into the cell during the TRF-XAS measurements. The TRF-XAS measurements of the Co K edge region were performed using a solid-state detector at beamlines BL01B1, BL28XU, and BL37XU in SPring-8 (Hyogo, Japan). The in situ TRF-XAS measurements were conducted under the total-reflection condition with an X-ray incidence angle of 0.2°. The estimated penetration depth of the incident X-ray was approximately 3.0 nm from the LiCoO2 surface.23 The X-ray absorption near edge structure (XANES) obtained under the total-reflection and non-total-reflection (with an X-ray incidence angle of 2.2°) conditions is hereafter described as “surface XANES” and “bulk XANES”, respectively. The obtained TRF-XAS spectra were calibrated using the Co K edge of cobalt foil (7708.9 eV).

Figure 1. Cyclic volammograms of LiCoO2 thin-film electrodes with the VC-free and VC-added electrolytes. Cyclic voltammograms were obtained with (a) the VC-free electrolyte and (b) the VC-added electrolyte, at a potential sweep rate of 0.1 mV/s between 3.2 and 4.2 V (versus Li/Li+). (c) Changes in capacities calculated from the peak areas of the cyclic voltammograms as a function of cycle number: bluefilled circles, charge capacity with the VC-free electrolyte; blue open circles, discharge capacity with the VC-free electrolyte; red-filled squares, charge capacity with the VC-added electrolyte; red open squares, discharge capacity with the VC-added electrolyte. 9792

DOI: 10.1021/jp511405g J. Phys. Chem. C 2015, 119, 9791−9797

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The Journal of Physical Chemistry C peaks at 4.06 and 4.18 V (versus Li/Li+) are ascribed to phase transitions between the hexagonal and monoclinic phases of LiCoO2.30 As the cycle number increased, the peak intensity appearing at 3.9 V in the CV with the VC-free electrolyte decreased significantly from the blue curve to the green one shown in Figure 1a. Figure 1c shows capacities calculated from the peak areas of the CVs as a function of the cycle number in the VC-free and VC-added electrolytes. The discharge capacity obtained with the VC-free electrolyte notably decreased from 0.94 to 0.50 μA h after 20 cycles. By contrast, the capacity obtained with the VC-added electrolyte was more stable in subsequent cycles than that with the VC-free electrolyte: the discharge capacity with the VC-added electrolyte decreased only from 1.05 to 0.96 μA h after 20 cycles. The stabilization effect of the VC additive on the cyclic performance of the LiCoO2 thin-film electrode is similar to that observed in previous studies using LiCoO2 composite electrodes.6,7,13,14 Figure 2a and Figure 2b show the EIS spectra obtained at 4.0 V (versus Li/Li+) using the VC-free and VC-added electrolytes, respectively. The change in the interfacial resistance on subsequent charge/discharge cycles is illustrated in Figure 2c. The interfacial resistance with the VC-free electrolyte was 1.3 kΩ at the first cycle and significantly increased to 5.7 kΩ after 10 cycles. By contrast, the interfacial resistance with the VCadded electrolyte was lower and more stable than that for the VC-free electrolyte: it was 0.7 kΩ at the first cycle and 1.3 kΩ after 10 cycles. The EIS results demonstrate that the addition of VC to the electrolyte strongly suppresses the initial interfacial resistance as well as the increase in the interfacial resistance during subsequent charge/discharge cycles. To investigate the effects of the VC additive on the electronic structure at the LiCoO2 surface under real battery operating conditions, in situ TRF-XAS measurements were conducted for the LiCoO2 thin-film electrode with the VC-added electrolyte. Figure 3a and Figure 3b respectively show the normalized Co K edge XANES at the surface and in the bulk of the LiCoO2 thinfilm electrode, which are obtained before and after soaking into the electrolyte. Both the surface and bulk XANES spectra were unchanged upon soaking into the VC-added electrolyte, indicating that the oxidation state of the cobalt ions at the surface was unchanged. These results contrast sharply with the surface XANES spectra with the VC-free electrolyte, in which an energy shift of the spectra toward the lower energy was observed upon soaking into the electrolyte.23 Figure 4 shows the surface (parts a, b) and bulk (parts c, d) Co K edge XANES spectra of the LiCoO2 thin-film electrode with the VC-added electrolyte obtained under potential-controlled conditions. Both the surface and bulk XANES spectra shifted continuously toward higher (oxidation) and lower (reduction) energy levels as electrochemical charging and discharging proceeded, respectively. The energy shift of the XANES spectra is quantified by the absorption energy at a normalized intensity of 0.5 (E0.5) to deduce the oxidation state of cobalt species. Figure 5 shows the E0.5 values of the Co K edge surface XANES spectra with the VC-added electrolyte during the electrochemical processes and those with the VC-free electrolyte for comparison.23 The VC-added electrolyte shows nearly constant E0.5 values of the LiCoO2 surface before (7717.0 eV) and after (7717.1 eV) the electrolyte soaking, as already described. During charging, the E0.5 value of the LiCoO2 surface with the VC-added electrolyte shifted to a higher-energy region (from 7717.1 eV after the soaking to 7718.5 eV at 4.2 V), indicating the oxidation of the cobalt ions at the LiCoO2 surface. During

Figure 2. Electrochemical impedance spectra for LiCoO2 thin-film electrodes with (a) the VC-free electrolyte and (b) the VC-added electrolyte measured at 4.0 V (versus Li/Li+). (c) Changes in the interfacial resistance as a function of cycle number with the VC-free electrolyte (blue circles) and the VC-added electrolyte (red squares). The interfacial resistance is obtained by fitting the electrochemical impedance spectra shown in (a) and (b) to the equivalent circuit shown in the inset of (c). Rele, Rint, and CPE are electrolyte resistance, interfacial resistance, and constant phase element, respectively.

discharging, the E0.5 value shifted to a lower-energy region (from 7718.5 eV at 4.2 V to 7718.0 eV at 3.8 V), indicating the reduction of the cobalt ions at the LiCoO2 surface. The difference between the E0.5 values at 3.8 V before and after the charge/discharge cycle is much smaller (0.5 eV) with the VC9793

DOI: 10.1021/jp511405g J. Phys. Chem. C 2015, 119, 9791−9797

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The Journal of Physical Chemistry C

Figure 5. Shifts of the Co K edge surface XANES spectra measured by absorption energy at a normalized intensity of 0.5 (E0.5) during subsequent charge/discharge processes of LiCoO2 thin-film electrodes with the VC-free (blue squares) and VC-added (red circles) electrolytes. R3.8 V denotes a potential-controlled condition at 3.8 V (versus Li/Li+) during the discharge process. The data with the VCfree electrolyte are from ref 23.

discharge cycle is attributed to the reduction of the cobalt ions at the LiCoO2 surface upon soaking into the electrolyte, which causes the formation of an irreversible layer at the surface.23 This irreversible layer results in capacity attenuation and the increase in the interfacial resistance. With the VC-added electrolyte, the reduction of the cobalt ions at the LiCoO2 surface by the electrolyte soaking is significantly suppressed, and the irreversible change in the oxidation state of the cobalt ions at the LiCoO2 surface during subsequent charge/discharge cycles is reduced. The correlation between the electrochemical behavior (Figures 1 and 2) and the electronic structure at the LiCoO2 surface (Figures 3−5) can be explained as follows. The electrochemical potential of an electron in the electrode differs from that in the electrolyte, as schematically illustrated in Figure 6a and Figure 6c for the VC-free and VC-added

Figure 3. Co K edge (a) surface and (b) bulk XANES spectra of a LiCoO2 thin-film electrode with the VC-added electrolyte before (blue line) and after (red line) electrolyte soaking obtained by in situ TRFXAS. The insets are the enlarged views of the changes in the absorption edge.

added electrolyte than that with the VC-free electrolyte (1.5 eV). This clearly demonstrates that the reversibility of the electronic structure of the cobalt ions at the LiCoO2 surface during charge/discharge cycle is improved by the VC additive. The irreversible change in the E0.5 value during the charge/

Figure 4. Co K edge (a, b) surface and (c, d) bulk XANES spectra of a LiCoO2 thin-film electrode with the VC-added electrolyte during (a, c) the charge and (b, d) the discharge processes obtained by in situ TRF-XAS. The insets are the enlarged views of the changes in the absorption edge. R3.8 V denotes a potential-controlled condition at 3.8 V (versus Li/Li+) during the discharge process. 9794

DOI: 10.1021/jp511405g J. Phys. Chem. C 2015, 119, 9791−9797

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Figure 6. Schematic illustration of the electronic structure at the LiCoO2/electrolyte interface for (a, b) the VC-free and (c, d) the VC-added electrolytes. ϕE and ϕL are electrochemical potentials of an electron in the electrode and the electrolyte, respectively. The blue and green pentagons represent EC and VC molecules, respectively. Other species in the electrolytes, such as Li+, anions, and DEC, are omitted for simplicity.

with less reduction of the cobalt ions at the LiCoO2 surface than the case with the VC-free electrolyte. The layer of the decomposed VC molecules physically separates the LiCoO2 surface and the electrolyte solution. This can suppress the further oxidation of the solvent molecules and the associated reduction of the cobalt ions, namely, the formation of the irreversible layer at the LiCoO2 surface (Figure 6d). The smaller interfacial resistance at the first cycle with the VC-added electrolyte (0.7 kΩ) compared with the VC-free electrolyte (1.3 kΩ) can be ascribed to the suppression of the irreversible layer formation at the LiCoO2 surface. The physical separation of the LiCoO2 surface and the electrolyte solution for the VC-added electrolyte and the resultant suppression of the irreversible layer formation are also effective during the electrochemical processes, leading to the mitigation of the capacity decrease and the reduction in the interfacial resistance increase during subsequent charge/discharge cycles.

electrolytes, respectively. The difference in the electrochemical potential between the electrode and electrolyte is compensated at the interface through the formation of a space-charge layer on the electrode side and an electrical double layer on the electrolyte side.31 The low electrochemical potential in the LiCoO2 electrode results in the reduction of the cobalt ions at the LiCoO2 surface and the associated oxidation of the electrolyte solvent molecules upon soaking into the VC-free electrolyte (Figure 6b), and this leads to the formation of an irreversible layer at the LiCoO2 surface.23,27 During subsequent charge/discharge cycles, (i) the reduction of the cobalt ions, (ii) the oxidation of the solvent molecules, and (iii) the formation of the irreversible layer proceed, resulting in the further decrease in capacity and increase in interfacial resistance (Figures 1 and 2). By contrast, stable behavior of the electronic structure at the LiCoO2 surface upon soaking and improved reversibility during subsequent charge/discharge cycles was observed with the VC-added electrolyte (Figures 3−5). Yu et al. reported preferential adsorption of the EC molecules on the LiCoO2 surface compared to the DEC and dimethyl carbonate (DMC) molecules in mixed solutions of EC, DEC, and DMC by sum frequency generation (SFG) vibrational spectroscopy.32 The molecular proportion on the surface largely differs from that in the solution bulk: >90 mol % EC at the surface versus 43−65 mol % in the bulk. They suggested that the preferential adsorption of EC is related to its high dielectric constant (89.8) in comparison with that of DEC (2.8) and DMC (3.1). The higher dielectric constant of VC (127)33 is expected to yield a high proportion of the VC molecules on the LiCoO2 surface, and formation of decomposed VC molecules, likely as a polymer of VC, at the LiCoO2/electrolyte interface has been reported.8,9,21 This is because the VC molecules are more easily oxidized than the EC molecules34,35 and, by contrast to the EC molecules, the oxidized VC molecules initiate polymerization8,9,21 without further oxidation from LiCoO2. Therefore, soaking into the VC-added electrolyte forms a layer of the decomposed VC molecules at the LiCoO2/electrolyte interface

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CONCLUSION The effects of the VC additive to the electrolyte on the electronic structure of the cobalt ions at the surface of the LiCoO2 thin-film electrode under real LIB operating conditions were investigated using in situ TRF-XAS. The addition of VC to the electrolyte suppressed the reduction of the cobalt ions upon electrolyte soaking. The reversibility of the electronic structure at the LiCoO2 surface during subsequent charge/ discharge cycles was significantly improved by the addition of VC. The effects of the VC additive can be attributed to the formation of the decomposed VC layer at the LiCoO2/ electrolyte interface, which plays an important role in mitigating the degradation of the capacity and the increase in the interfacial resistance of LiCoO2 during subsequent charge/ discharge cycles.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-774-38-4966. 9795

DOI: 10.1021/jp511405g J. Phys. Chem. C 2015, 119, 9791−9797

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The Journal of Physical Chemistry C Present Address

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D.T.: Hitachi Research Laboratory, Hitachi, Ltd., Hitachi-shi, Ibaraki 319-1292, Japan. Author Contributions

D.T., Y.O., Y.K., H.A., and Y.U. designed the experiments. D.T., S.M., and T.N. performed the electrochemical measurements on the LiCoO2 thin-film electrodes. D.T., Y.O., S.M., T.N., T.H., and H.T. performed the in situ TRF-XAS experiments. The manuscript was written by D.T., Y.O., K.Y., Y.K., T.M., 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 Batteries (RISING Project)” of the New Energy and Industrial Technology Development Organization (NEDO), Japan. The synchrotron radiation experiments were performed at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposals 2010B1027, 2011A1011, 2011B1019, 2011B1023, 2011B1033, 2011B1037, 2012A1028, 2012A1029, 2012A1030, 2012A7600, and 2012B7600). We thank Dr. Kiyofumi Nitta, Dr. Masugu Sato (JASRI), Koki Shimada, Tomoya Kawaguchi, Dr. Hisao Yamashige, and Dr. Haruno Murayama (Kyoto University) for their supports in the synchrotron radiation experiments.

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DOI: 10.1021/jp511405g J. Phys. Chem. C 2015, 119, 9791−9797

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DOI: 10.1021/jp511405g J. Phys. Chem. C 2015, 119, 9791−9797