実験報告書様式（一般利用課題・成果公開利用）

MLF Experimental Report

提出日 Date of Report
2016/4/27

課題番号 Project No.

装置責任者 Name of responsible person

2015A0110

Norifumi Yamada

実験課題名 Title of experiment

装置名 Name of Instrument/（BL No.）

In-situ analysis on electrode/electrolyte interfaces in Li-ion batteries by

SOFIA/BL16

neutron reflectometry

実施日 Date of Experiment

実験責任者名 Name of principal investigator

2016/2/27-3/1

Hiroyuki Kawaura
所属 Affiliation
Toyota Central R&D Labs., Inc.
試料、実験方法、利用の結果得られた主なデータ、考察、結論等を、記述して下さい。（適宜、図表添付のこと）
Please report your samples, experimental method and results, discussion and conclusions. Please add figures andtables for better explanation.
1. 試料 Name of sample(s) and chemical formula, or compositions including physical form.

Figure 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 with an area of 30×30 mm was used as the working electrode. Aftercleaning, a 20 nm titanium adhesion layer and a 70 nm carbon layer were deposited on the flat silicon wafer using amagnetron sputtering instrument. The carbon electrode used in these experiments consisted of an amorphous carbon
film. 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 in a 1:1 volumetric mixture of ethylene carbonate and diethyl carbonate.

Figure 1. Schematic illustration of the electrochemical cell and cross-section of the model battery system
for in situ NR measurements.
12. 実験方法及び結果 （実験がうまくいかなかった場合、その理由を記述してください。）
Experimental method and results.

If you failed to conduct experiment as planned, please describe reasons.

The electrochemical cell was mounted on the sample stage of the neutron reflectometer and neutrons wereintroduced from the substrate side for in situ neutron reflectivity (NR) measurement at 200 kW. Charge anddischarge tests were carried out using the two-electrode cells. The electrochemical reaction at the carbon
electrode was controlled potentiostatically with an electrochemical analyzer with a scan rate of 0.2 mV/s in thepotential E range from the open circuit voltage (OCV; ca. 3.1 V) to 0.05 V (vs. Li/Li+) during the NR
measurement. The NR data was acquired every 10 minutes with an incident angle θ of 0.4°, whereby thereflectivity profile as a function of the momentum transfer normal to the basal plane Qz; Qz = (4πsinθ)/λ, can beobtained by taking into account wavelength of neutrons λ from 0.09 to 0.33 nm-1.
Figure 2 shows a voltammogram of the carbon electrode during the in situ NR experiment. First, the current I,
was weak from OCV (3.1 V) to 1.5 V, i.e., no characteristic reaction occurred. Next, I began to decrease around
1.5 V, reached a local minimum at 1.2 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 thepresence of a broad reduction peak in the E range between 0.6 and 1.5 V with a peak around 1.2 V.
2. 実験方法及び結果（つづき） Experimental method and results (continued)
A large reductive I was observed at potentials lower than 0.6 V in each solution, which may be assigned partly tolithium intercalation, because the presence of the corresponding oxidation peak assigned to lithium
deintercalation at around 1.5 V. This reductive current is presumably due to decomposition of the electrolyte
and/or organic solvent, which contributes to SEI formation. I decreased again with an increase in the dI/dE slope
below 0.6 V, which mainly indicates the intercalation of Li ions. However, the charge consumed for the reductive
current was much larger than that for the oxidation peak in each solution, and hence, a substantial fraction of thereduction current at potentials < 0.6 V was consumed by irreversible processes such as direct decomposition ofsolvent molecules on the carbon surface.

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Figure 3 shows reflectivity (R) profiles as a function of E during the charge and discharge reaction, in which
the period of fringes and the critical value of Qz at the total reflection in the profiles indicate the thickness, d, andthe scattering length density, ρ, of the interfacial layers, respectively. In contrast to the lack of change from OCV(=3.1 V) to 1.5 V in the profile, even with a decrease in E, the change in the period and critical value began at Efrom 1.5 V to 0.6 V and was more pronounced at E from 0.6 V to 0.05 V during charge reaction. The change ofR in the period and critical value were observed at E from 0.05 V to 2.5 V, and the change of R may be small at
E from 2.5 V to 3.1 V during discharge reaction. 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 Parrattformalism. Figure 4a shows typical NR profiles and their fitting curves at different potentials, OCV (3.1 V),
charge 0.05 V, and 1cycled 3.1 V (vs. Li/Li+). Figure 4b shows the evolution of the scattering length density
profiles as a function of potential, whereby the change in the carbon electrode and the SEI on the electrochemical
reaction at the interface is observed. Figure 5 shows the dependence of ρC, ρI, dC, and dI obtained by theleast-squares fitting, where the subscripts C and I indicate the carbon electrode and interfacial layer, respectively.

3The fitting result indicated the presence of an interfacial layer, even at the OCV, with dI = 20.6 ± 0.7 nm andρI = (2.88 ± 0.08) × 10-4 nm-2. Based on an ex situ X-ray photoelectron spectroscopy analysis of the present
sample, a hydroxyl layer was detected on the surface of the carbon electrode after immersion in the electrolyte
and without charging. This implies that there is an adsorption layer formed upon contact with the 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 thereaction is decomposition of the electrolyte and/or the organic solvent. According to the trend of dI, the SEI
thickened with the charge reaction up to 29.6 ± 4.3 nm at E = 0.05 V and thinned with discharge reaction down
to 20.1 ± 0.7 nm at E = 3.1 V. On the other hand, ρI decreased with charge reaction down to (2.16 ± 0.04) × 10-4
nm-2 at E = 0.05 V and increased up to (2.95 ± 0.05) × 10-4 nm-2 at E = 3.1 V after discharged reaction.
Next, we focused on the intercalation of Li+ ions into the electrode during the charge reaction; dC was 67.2 ±0.2 nm at OCV, increased monotonically up to 87.6 ± 0.3 nm at E = 0.05 V during charge reaction, anddecreased 81.6 ± 0.3 nm at E = 3.1 V after discharge reaction, whereas ρC was (6.46 ± 0.02)×10-4 nm-2 at OCV,
decreased monotonically down to (4.15 ± 0.01) × 10-4 nm-2 at E = 0.05 V during charge reaction, and increased
up to (5.14 ± 0.01) × 10-4 nm-2 at E = 3.1 V after discharge reaction. The change in dC is consistent with thevolumetric change induced by Li+ intercalation/deintercalation into carbon, because the c-axis length of
Li+-intercalated/deintercalated carbon is larger than that of carbon.

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