Interface characterization of cathode electrode in lithium-ion battery
by operand neutron reflectometry
Kyoto University Tatsumi Hirano
1. Introduction
Lithium-ion batteries (LIBs) are used in consumer mobile electronics and are expected to be
widely applied in products, such as energy-storage systems and electric vehicles, that demand highcurrent/power, high capacity, long-life, improved safety, and low cost. The high current/power
operation of LIBs requires knowledge of lithium-ion transport properties in the cathode material orat the electrode/electrolyte interface during the battery operation. Operand neutron or X-ray
reflectometry is enable us to analyze the density profile in the cathode material and the interface
width at the electrode/electrolyte interface. Previous studies using the reflectometry were based on
in-situ cells in which the electrolyte was fully filled [1, 2]. Since the surface of the thin film cathodewas fully soaked in the electrolyte, it was difficult to form the solid electrolyte interface (SEI) on thethin film. Therefore, we newly designed an in-situ aluminum-laminated cell, which is composed of
the cathode thin film, a separator filled by the electrolyte and Li metal.
Previous studies have reported results of the neutron and X-ray reflectometry measured atseveral state of charge (SOC) based on the in-situ cells filled by the electrolyte [1-3]. In this report,
we successively measured the neutron reflectivity of the newly designed in-situ cell during thebattery operation because of MLF operation in 2017B from 150 kW to 400 kW to clarify lithium-ion
transport properties in the LiCoO2 cathode and at the electrode/electrolyte interface during thebattery operation.

2. Experiment
The cathode film was LiCoO2/Pt/Cr on the Si substrate. The newly designed in-situ cell
laminated by aluminum was composed of the LiCoO2 thin film as cathode, lithium metal as anodeand separator filled by the electrolyte of 1.0 mol dm-3 LiClO4 in a 1:1 volumetric mixture of ethylene
carbonate (EC) and diethyl carbonate (DEC). The in-situ cell was operated with cyclic voltammetry(CV) mode (0.1 mV/s) from OCV (Open Circuit Voltage) to 4.2 V. The neutron reflectivities were
measured at BL17 in MLF during the battery operation. Neutron beams were incident on thesubstrate side of the film and reflect at the LiCoO2 film/electrolyte interface. Three neutronreflectivities are measured for ~ 6 hours with θ/2θ = 0.3/0.6, 0.9/1.8 and 2.0/4.0 degree at OCV, 4.2 V.
Moreover, successive neutron reflectivities were measured with θ/2θ = 0.3/0.6 from OCV to 4.2 Vand time-sliced every 20 minute. We analyzed the measured neutron reflectivities using Motofit
analysis code [4].
3. Results
Figure 1(a) shows the measured and calculated neutron reflectivities at OCV and 4.2 V. The OCV(4.2 V) corresponded to SOC of 0 % (100 %) before (after) the battery operation. Figure 1(b) shows
the scattering length density (SLD) profiles after the analysis of the neutron reflectivities. The SLDand interface width of LiCoO2 increased from OCV to 4.2 V because of the lithium de-intercalation1

from the cathode (LiCoO2 ) to the anode (Li metal) due to the minus value of the lithium SLD. The
time-sliced reflectivities from OCV to 4.2 V are shown in Fig. 2(a) and were different each other.
Figure 2(b) shows the CV curve (red line) and the SLD of LiCoO2 (blue marked line) vs. voltage. The
CV curve shows the 3.65 V unknown peak and 3.9 V peak due to the lithium de-intercalation. The
SLD of LiCoO2 was increased over 3.65 V, i.e. the analysis of the lithium-ion transport at theelectrode/electrolyte interface during the battery operation by the operand neutron reflectometry
based on the newly designed in-situ cell has been realized.
References[1] M. Hirayama et al., J. Power Sources 168 (2007) 493-500.
[2] M. Yonemura et al., J. Phys.: Conf. Ser. 502 (2014) 012054.
[3] M. Hirayama, et al. Electrochemistry, 78, 413-415 (2010).
[4] http://motofit.sourceforge.net/wiki/index.php/Main_Page.

Scattering Length Density /10 A-6345 6 7 85LiCoO2

43Cr
SiO2
2Si
Electrolyte

10200.1
Scattering vector, q /AOCV
4.2V

200

400

600

800

1000-1Distance from Interface /AFig. 1 (a) Measured and calculated neutron reflectivities at OCV and 4.2 V, (b)scattering length density (SLD) profiles of OCV and 4.2 V.
(a)

(b)

-330x105.04.8Current /mA

4.620

4.4
4.2

10

4.0
3.80
2.5

3.6
3.0

3.5

4.0

4.5

Voltage /VFig. 2 (a) Measured reflectivities from OCV to 4.2 V, (b) cyclic voltammetry curve(red line) and SLD of LiCoO2 (blue marked line) vs. voltage.
2-6 -22(b)Scattering Length Density of LiCoO2 /10 AReflectivity-4Pt

6-6 -2OCV
4.2VFitting-2-8 5 6 7 8
0.01

7(a)0