Operando Measurement of Solid Electrolyte Interphase Formation at

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Operando Measurement of Solid Electrolyte Interphase Formation at Working Electrode of Li-ion Battery by Time-slicing Neutron Reflectometry Hiroyuki Kawaura, Masashi Harada, Yasuhito Kondo, Hiroki Kondo, Yoshitake Suganuma, Naoko Takahashi, Jun Sugiyama, Yoshiki Seno, and Norifumi L Yamada ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01170 • Publication Date (Web): 31 Mar 2016 Downloaded from http://pubs.acs.org on April 5, 2016

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ACS Applied Materials & Interfaces

Operando Measurement of Solid Electrolyte Interphase Formation at Working Electrode of Li-ion Battery by Time-slicing Neutron Reflectometry Formatted: Font color: Auto †

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Hiroyuki Kawaura* , Masashi Harada , Yasuhito Kondo , Hiroki Kondo , Yoshitake Suganuma†, Naoko Takahashi†, Jun Sugiyama†, Yoshiki Seno†, and Norifumi L. Yamada‡ †

Toyota Central Research & Development Laboratories, Inc.

41-1 Yokomichi, Nagakute, Aichi 480-1192, JAPAN Phone: +81-561-71-7564 Fax: +81-561-63-6136 ‡

Institute of Materials Structure Science, High Energy Accelerator Research Organization

203-1 Shirakata, Tokai-Mura, Naka-gun, Ibaraki 319-1106, JAPAN Phone: +81-29-284-4274 Fax: +81-29-284-4899

Keywords: Thin-film electrode, Solid electrolyte interphase, Lithium-ion battery, in situ neutron reflectometry, Operando measurement, Charge reaction, Lithium intercalation

Abstract: We report the first operando measurement of solid electrolyte interphase (SEI) formation at an electrode using in situ neutron reflectometry. The results revealed the growth of the SEI and intercalation of ions during the charge reaction. Furthermore, we propose a way of evaluating the charge used for the SEI formation.

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The development of rechargeable lithium-ion batteries (LIBs) that provide high power density and durability is important to meet demands for high-power applications such as electric vehicles, hybrid vehicles, and aerospace applications. To improve the safety and durability of LIBs to accommodate such special demands, a clear understanding of what occurs inside LIBs during the charge/discharge reactions is required. The durability of LIBs originates largely in side reactions that occur at the electrode/electrolyte interface, especially those at the negative electrode. Electrochemical intercalation and deintercalation reactions of Li+ ions occur at the interface between an electrode and the electrolyte in LIBs. Other reactions, such as decomposition of the electrolyte solvent, charge transfer from Li+ ions to the surface of an electrode, and the formation of a surface layer also occur simultaneously in the interfacial region. As a result, a solid electrolyte interphase (SEI) layer is formed at the boundary between an electrode and the electrolyte, particularly at the negative electrode.1-9 Li+ ions diffuse across the SEI to reach the electrode; therefore, a clear understanding of the nature of the SEI is required for further improvement of the present LIBs and for the development of advanced next-generation batteries. An ideal SEI should exhibit high ionic conductivity for Li+ ions but low electric conductivity to suppress further decomposition of the electrolyte and enhance the Li+ intercalation/deintercalation reactions at the electrodes. The durability of present LIBs, in which graphite is typically used as a negative electrode material, is strongly correlated with the thickness and composition of the SEI.6-9 Therefore, numerous studies have been conducted to investigate the nature of the SEI using surface-sensitive techniques such as X-ray photoelectron spectroscopy (XPS),3,5,7-13 time-of-flight secondary ion mass spectrometry,8,10,11 and Fourier transform infrared spectroscopy.3,5,8,9,11-13 However, these techniques cannot be applied to observe an active electrode in contact with the electrolyte due to the experimental limitations; specifically, what can be observed with these techniques is an “empty shell” of the SEI in the absence of the electrolyte. Therefore, these ex situ techniques are insufficient to investigate the structural properties of the SEI, such as thickness and density, especially for the SEI composition/decomposition process during cyclic charge/discharge reactions. As such, an in situ technique is required to observe SEI formation and the intercalation of Li ions during battery operation to elucidate the charging reaction at the electrode. In this study, neutron reflectometry (NR) was employed for direct observation of the electrode/electrolyte interface of a LIB because neutrons can penetrate through the electrode and provide interference that reflects the interfacial structure.14-16 The depth dependence of the neutron scattering length density ρ, can be evaluated with this method on the scale from 2


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nanometers to hundreds of nanometers. ρ is related to the elemental composition and their density at the interface; therefore, NR can be used to detect not only the SEI formation, but also the intercalation of Li+ ions in situ. NR was used to study SEI formation on a thin-film Cu electrode and a LiMn1.5Ni0.5O4 film electrode as a function of the potential,14,15 and the surface structural changes in epitaxial LiFePO4 during an electrochemical process.16 However, these studies were conducted under a constant potential, so only the static structures of the SEIs were observed. Thus, an operando study is strongly required to evaluate the electrochemical reaction at an electrode and understand the mechanism for the formation of the SEI. Here, we report the first direct observation of the SEI growth process and the intercalation of Li+ ions at an electrode during the charge reaction using time-slicing NR with high-flux pulsed neutrons generated by a megawatt-class proton accelerator. Fig. 1 shows a schematic of the electrochemical cell used for the present work. A carbon/titanium multilayer thin film on a 3 mm thick silicon substrate (Shin-Etsu Chemical Co., Ltd.) with an area of 30×30 mm was used as the working electrode. Immediately after cleaning, a 20 nm titanium adhesion layer and a 70 nm carbon layer were deposited on the flat silicon wafer using a magnetron sputtering instrument (Toshima Manufacturing Co., Ltd.). We confirmed the carbon electrode used in these experiments has an amorphous structure

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using Raman spectroscopy and ex situ NR in air (Figs. S1 and S3a in Supporting Information). The composition, uniformity, and roughness of the multilayered thin film were confirmed by bright-field scanning transformation electron microscopy (BF-STEM) (Fig. S2 in Supporting Information). The substrate was set in an electrochemical cell with a lithium foil, counter/reference electrode. The electrodes were separated by a microporous polypropylene membrane (separator) soaked with an electrolyte consisting of 1 mol/dm3 LiPF6 + 1 wt% vinylene carbonate dissolved in a 1:1 volumetric mixture of ethylene carbonate and diethyl carbonate. The electrochemical cell was mounted on the sample stage of the neutron reflectometer (SOFIA, J-PARC17,18) and neutrons were introduced from the substrate side for in situ NR measurement. The electrochemical reaction at the carbon electrode was controlled potentiostatically with an electrochemical analyzer (BAS Inc., ALS Model 660A) with a scan rate of 1.0 mV/s in the potential range from 2.9 to 2.0 V (vs. Li/Li+) and at 0.2 mV/s from 2.0 to 0.05 V during the NR measurement. The NR data was acquired every 5 minutes with an incident angle θ of 0.4°, whereby the reflectivity profile as a function of the momentum transfer normal to the basal plane Qz; Qz = (4πsinθ)/λ, can be obtained by taking into account wavelength of neutrons λ from 0.2 to 0.88 nm. 3




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Fig. 2 shows a voltammogram of the carbon electrode during the in situ NR experiment, in which three processes are identified with decreasing E. First, the current I, was weak from the open circuit voltage (OCV; ca. 2.9 V) to 1.5 V, i.e., no characteristic reaction occurred (phase I). The electrochemical reaction at the carbon electrode was controlled potentiostatically with


an electrochemical analyzer with a scan rate of 1.0 mV/s in the potential range from 2.9 to 2.0 V (vs. Li/Li+) and at 0.2 mV/s from 2.0 to 0.05 V during the NR measurement. So, the small hump at 2 V in Fig. 2 is most likely a contribution from the electric current change due to scan late switching. Next, I began to decrease around 1.5 V, reached a local minimum at 1.0 V, and then slightly increased until 0.6 V. The negative value of I indicates that the Li+ intercalation reaction is a reductive reaction for carbon, so that the I(E) curve shows the presence of a broad reduction peak in the E range between 0.6 and 1.5 V with a peak around 1.0 V. This reductive current is presumably due to decomposition of the electrolyte and/or organic solvent, which contributes to SEI formation (phase II).4,11 Finally, I decreased again with an increase in the dI/dE slope below 0.6 V, which mainly indicates the intercalation of Li ions (phase III). Fig. 3 shows reflectivity profiles as a function of E during the charge reaction, in which the period of fringes and the critical value of Qz at the total reflection in the profiles indicate the thickness, d, and the scattering length density, ρ, of the interfacial layers, respectively. In contrast to the lack of change in the profile during phase I, even with a decrease in E, the change in the period and critical value began in phase II and was more pronounced in phase III. To convert the change in reciprocal space to that in real space, the R profile at each E was analyzed to obtain a depth profile of ρ based on a least-squares fitting with the Parratt formalism (detailed fitting procedures and Fig. S3 in Supporting Information).19,20 Fig. 4 shows the dependence of ρC, ρI, dC, and dI obtained by the least-squares fitting, where the subscripts C and I indicate the carbon electrode and interfacial layer, respectively. In air,


dC and ρC were evaluated to be 69.2±0.3 nm and (6.45±0.05)×10-4 nm-2, respectively (Fig. S3a). These values were quite consistent with those obtained at OCV: dC = 69.1±0.3 nm and

ρC = (6.46±0.02)×10-4 nm-2. In addition, the interfacial layer with dI = 17.0±1.7 nm and ρI = (2.50±0.20)×10-4 nm-2 was required to obtain a better fitting result even at the OCV (Fig. S3b). According to the literature, there are two possible explanations for this layer, a carbonate/hydroxide solvation layer14,15 or an electrolyte double layer.16 In addition, the thicknesses of the interfacial layers are different each other. This difference suggests that the physical property and formation mechanism of the interfacial layer are strongly dependent on 4



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systems even at OCV. In our case, an ex situ XPS analysis detected a hydroxyl layer on the surface of the carbon electrode after immersion in the electrolyte and without charging (Fig. S4a). This implies that there is an adsorption layer formed upon contact with the carbon


electrolyte before SEI formation. Furthermore, the values of dI and ρI changed with a decrease in E, which indicates an SEI starts to form at the interface due to a decomposition reaction. The I(E) curve shown in Fig. 2 indicates the reaction is decomposition of the electrolyte and/or the organic solvent. According to the trend of dI, the SEI gradually thickened with the charge reaction during phases I and II, and the rate of thickening was enhanced during phase III. On the other hand,

ρI remained almost constant during phases I and II, while it increased during phase III up to (3.26±0.72)×10-4 nm-2 at E = 0.05 V. ρ indicates the composition of the SEI; therefore, these results imply a change of the product by the charge reaction. The SEI formed in this system is reported to consist of the following components: polyvinylene carbonate (ρ = 2.84×10−4 nm−2), Li2CO3 (3.48×10−4 nm−2), lithium alkyl carbonate (2.75×10−4 nm−2), polyethylene oxide (PEO; 8.22×10−5 nm−2), LiPF6 (2.21×10−4 nm−2), and LiF (2.30×10−4 nm−2),11,12 which are consistent with the XPS analysis results after charging (Fig. S2b). A scattering length density higher than 3.26×10-4 nm-2 makes Li2CO3 a strong candidate product that contributes to the increase in ρI during phase III. Next, we focused on the intercalation of Li+ ions into the electrode during the charge reaction. As shown before, dC and ρC at OCV were 69.1±0.3 nm and (6.46±0.02)×10-4 nm-2,


respectively. During the charging reaction, dC remained almost constant during phases I and II,


and then increased monotonically up to 84.1±0.8 nm at E = 0.05 V during phase III, whereas

ρC remained constant during phase I, and decreased monotonically down to (4.50±0.27)×10-4 nm-2 at E = 0.05 V during phases II and III. The change in dC indicates the volumetric change induced by Li+ intercalation into carbon, and the decrease in ρ C also indicates the intercalation because Li has a negative coherent scattering length. In the case of graphite, the electrode shows a 12% expansion of c-axis length (LixC6 vs C) during the reversible chemistry between Li and LixC6.21 This expansion rate is insufficient to explain the 21% expansion in dC. On the other hand, the reversible Li storage capacity of a single-wall carbon nanotube increases twice in comparison with that of graphite.22 The composition of the amorphous carbon electrode after lithiation at 0.05 V is estimated to be Li:C = 1:4.8 according to the change in ρ C from OCV. This indicates that the electrode made of amorphous carbon contains more Li ions than that of graphite, such as the carbon nanotube, 5




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which is presumably the reason why the expansion ratio of the carbon electrode is larger than that of graphite. It should be noted here that the number of Li ions intercalated into the carbon electrode with one-electron transfer reaction during the charge reaction can be evaluated based on the changes in dC and ρC. Thus, the total charge can be separated into the charge used for SEI formation and that for Li+ intercalation. The solid line in Fig. 5 represents the total charge Ctotal, used for the electrochemical reaction during the operando measurement, which was evaluated using the following formula:

Ctotal = ocv / ,

(1)

where Eocv is the potential at OCV. The white circles in Fig. 5 represent the charge of Li ions intercalated into the electrode CLi, which is given by: CLi = ∙

∙ ocv ∙ ocv Li

.

(2)

Here, A is the reaction area (20×20 mm2), lLi is the scattering length of a Li atom (-1.9 fm), and e is the elementary charge (1.6×10-19 C). In addition, there is an extra charge not used for Li intercalation (red area in Fig. 5), which is the charge consumed for the decomposition of the electrolyte and/or organic solvent at the electrode, i.e., the charge that contributes to SEI formation. The trend of the extra charge is consistent with the changes in the thickness and scattering length density of the SEI layer obtained by the operando NR analysis. In conclusion, we have succeeded in the first operando measurement of SEI formation at the working electrode of a LIB using a combination of time-slicing NR and electrochemical analysis. The results obtained clearly show the formation of a SEI layer at the carbon electrode and the intercalation of Li ions into the electrode. This method has the potential to be used to determine the chemical composition of an SEI and to evaluate the absolute amounts of charge used in SEI formation and Li intercalation during an electrochemical reaction. Although a well-defined multilayered thin film was used as a model battery for the measurement, the basic principle of the formation and growth of an SEI is expected to be the same as that for an actual carbon electrode in current LIBs. Therefore, the operando NR technique opens a door to shed light on the “live” composition/decomposition process of SEIs under operation and deepen our understanding of the formation mechanism and its relationship with the performance of LIBs, which will lead to further improvement of the present LIBs and the development of advanced next-generation batteries.

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Associated Content Supporting Information The Supporting Information is available free of charge via the ACS Publications website at DOI:xxxx. Figure S1. Raman spectrum of the carbon thin film of the multilayer fabricated by spattering


process. Figure S2. BF-STEM image of multilayer film sample on a silicon wafer by


spattering process. Figure S3. (a) Neutron reflectivity vs. Qz and fitting results at different


potentials. (b) Evolution of scattering length profiles obtained by fitting of the electrochemical reaction at the model electrode. Figure S4. C 1s, O 1s, and F 1s XPS spectra


+

of the carbon thin-film electrodes (a) after soaking, and (b) after charging at 0.05 V vs. Li/Li .

Author Information Corresponding author: ∗

E-mail: [email protected]

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Acknowledgements The authors are grateful to the staff of the Materials and Life Science Facility (MLF), Japan Proton Accelerator Research Complex (J-PARC), Japan for their help and useful suggestions. In situ NR measurements were performed at the BL16 beamline with the approval of J-PARC (Proposal No. 2013B0247, No. 2014A0294). JS was partially supported by a Kakenhi Grant-in-Aid (No. 23108003) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan.

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References: (1) Peled, E. The Electrochemical Behavior of Alkali and Alkaline Earth Metals in Nonaqueous Battery Systems ̶ The Solid Electrolyte Interphase Model. J. Electrochem. Soc. 1979, 126, 2047−2051. (2) Peled, E.; Golodnitsky,D.; Ardel G. Advanced Model for Solid Electrolyte Interphase Electrodes in Liquid and Polymer Electrolytes. J. Electrochem. Soc. 1997, 144, L208−L210. (3) Aurbach, D.; Markovsky, B.; Weissman, I.; Levi, E.; Ein-Eli, Y. On the Correlation between Surface Chemistry and Performance of Graphite Negative Electrodes for Li Ion Batteries. Electrochim. Acta 1999, 45, 67−86. (4) Jeong, S.-K.; Inaba, M.; Mogi, R.; Iriyama, Y., Abe, T.; Ogumi, Z. Surface Film Formation on a Graphite Negative Electrode in Lithium-Ion Batteries: Atomic Force Microscopy Study on the Effects of Film-Forming Additives in Propylene Carbonate Solutions. Langmuir 2001, 17, 8281−8286. (5) Xu, K. Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries. Chem. Rev. 2004, 104, 4303−4417. (6) Zhang, S.S. A Review on Electrolyte Additives for Lithium-Ion Batteries. J. Power Sources 2006, 162, 1379−1394. (7) Verma, P.; Mairel, P.; Novak, P. A Review of the Features and Analyses of the Solid Electrolyte Interphase in Li-Ion Batteries. Electrochim. Acta 2010, 55, 6332–6341. (8) Xu, K. Electrolytes and Interphases in Li-Ion Batteries and Beyond. Chem. Rev. 2014, 114, 11503−11618. (9) Agubra, V. A.; Fergus, J.W. The Formation and Stability of the Solid Electrolyte Interface on the Graphite Anode. J. Power Sources 2014, 268, 153−162. (10) Peled, E.; Tow, D. B.; Merson, A.; Gladkich, A.; Burstein, L.; Golodnistsky, D. Composition, Depth Profiles and Lateral Distribution of Materials in the SEI Built on HOPG-TOF SIMS and XPS Studies. J. Power Sources 2001, 97−98, 52−57. (11) Ota, H.; Sakata, Y.; Inoue, A.; Yamaguchi, S. Analysis of Vinylene Carbonate Derived SEI Layers on Graphite Anode. J. Electrochem. Soc. 2004, 151, A1659−A1669. (12) Aurbach, D.; Gamolsky, K.; Markovsky, B.; Gofer, Y.; Schmidt, M.; Heider, U. On the Use of Vinylene Carbonate (VC) as an Additive to Electrolyte Solutions for Li-Ion Batteries. Electrochim. Acta 2002, 47, 1423−1439. (13) Tsubouchi, S.; Domi, Y.; Doi, T.; Ochida, M.; Nakagawa, H.; Yamanaka, T.; Abe, T.; Ogumi, Z. Spectroscopic Characterization of Surface Films Formed on Edge Plane Graphite in Ethylene Carbonate-Based Electrolytes Containing Film-Forming Additives. J. 8


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(17) Yamada, N. L.; Torikai, N.; Mitamura, K.; Sagehashi, H.; Sato, S.; Seto, H.; Sugita, T.; Goko, S.; Furusaka, M.; Oda, T.; Hino, M.; Fujiwara, T.; Takahashi, H.; Takahara, A.; Design and performance of horizontal-type neutron reflectometer SOFIA at J-PARC/MLF Eur. Phys. J. Plus 2011, 126, 1−13. (18) Mitamura, K.; Yamada, N.L.; Sagehashi, H.; Torikai, N.; Arita, H.; Terada, M.; Kobayashi, M.; Sato, S.; Seto, H.; Goko, S.; Furusaka, M.; Oda, T.; Hino, M.; Jinnai, H.; Takahara, A.; Novel neutron reflectometer SOFIA at J-PARC/MLF for in-situ soft-interface characterization. Polym. J. 2013, 45, 100−108. (19) Parratt, L.G. Surface Studies of Solids by Total Reflection of X-rays. Phys. Rev. 1954, 95, 359−369. (20) Nelson, A. Co-refinement of Multiple-contrast Neutron/X-ray reflectivity Data using MOTOFIT. J. Appl. Crystallogr. 2006, 39, 273−276. (21) Ohzuku, T.; Iwakoshi, Y.; Sawai, K.; Formation of Lithium-Graphite Intercalation Compounds in Nonaqueous Electrolytes and Their Application as a Negative Electrode for a Lithium Ion (Shuttlecock) Cell. J. Electrochem. Soc. 1993, 140, 2490−2498. (22) Shimoda, H.; Gao, B. ; Tang, X. P.; Kleinhammes, A.; Fleming, L.; Wu, Y.; Zhou, O. Lithium Intercalation into Opened Single-Wall Carbon Nanotubes: Storage Capacity and Electronic Properties. Phys. Rev. Lett. 2002, 88, 015502-1−015502-4.

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Figure captions Fig. 1. Schematic illustration of the electrochemical cell and cross-section of the model battery system for in situ NR measurements. Fig. 2. Voltammogram of the carbon thin film electrode in 1 mol dm-3 LiPF6 + 1 wt% vinylene carbonate dissolved in a 1:1 volumetric mixture of ethylene carbonate and diethyl carbonate.

Fig. 3. NR profiles as a function of E during the charging reaction obtained by operando measurement.

Fig. 4. Evolution of the interfacial structure at the carbon electrode during electrochemical reaction: (a) thickness and (b) scattering length density of the electrode and interfacial layer as a function of E.

Fig. 5. Evaluation of total charge of the electrochemical reaction during operando NR measurement (black line), charge used for Li intercalation into the carbon electrode during reaction (white circles), and extra charge used for decomposition of the electrolyte and/or organic solvent that contributes to SEI formation.

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TOC Li foil Electrolyte SEI layer Carbon film Ti layer Si substrate

Carbon electrode Neutron

SEI Electrolyte

Si

Ti


Operando Measurement of Solid Electrolyte Interphase Formation at

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