Photon Factory Activity Report 2014 #32 (2015) B

NW2A/2014G542

In-Situ 2-Dimentional XAFS Imaging of Lithium Nickel Oxide Cathodein Operating Lithium Ion Battery
Taro Uenoyama, Ryota Miyahara, Kaito Tsukagoe, Hirona Yamagishi, Misaki Katayama,
and Yasuhiro Inada*
Graduate School of Life Sciences, Ritsumeikan University,
1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan
1 Introduction
The improvement of the lithium ion battery (LIB)
performance is an urgent mission for the modern society.
In addition to the capacity increase, the enhancement of therate property is also important to apply the LIB for a
motive power source of vehicles. For the safe utilization of
the large current, an inhomogeneity of the electrodereaction during the charge/discharge process becomes aserious problem because of the locally produced highcurrent density at the reacting area. Such inhomogeneous
progress of the electrode reaction is also a disadvantage forthe effective use of the active species leading to a decline
in the battery capacity.
The 2-dimentional (2D) XAFS imaging technique is a
powerful tool to observe the progress of redox reactionsoccurred in the electrode sheet because the XANES
spectrum is sensitive to the valence state of the active metal
center [1]. The spatially resolved detection using a 2D
detector to monitor the transmitted X-ray can reveal theinhomogeneity of the valence state, which is directlyrelated to the local progress of the electrode reaction. In
this study, the 2D XAFS imaging has applied to thecharge/discharge processes of the lithium nickel oxide(LNO, LiNiO2) cathode of LIB. The spatial distribution
will be compared with that of the lithium iron phosphate(LFP, LiFePO4) cathode reported previously [2].
2 Experiment
The cathode sheet composed of LNO, the carbon
additive, and the binder polymer was prepared, and the LIB
cell was complied using the cathode sheet, the separator
sheet, the Li anode, and the 1 M LiPF6 electrolyte solution.
All parts were sealed in a gas-barrier laminated bag. The
LIB capacity was 86 mA h g–1, and wascharged/discharged with the rate of 0.2 C. The XAFS
imaging measurement was performed during thecharge/discharge process.
The XAFS imaging was carried out at NW2A of PFAR (KEK). The Si(111) double-crystal monochromator
was used without any focusing optics. This higher-orderharmonics was removed by the Rh-coated double-mirrorsystem. The incident X-ray intensity was monitored by anionization chamber, and a CMOS image sensor (ORCAFlash 2.8) coupled with the Gd2O2S:Tb phosphor was usedas the transmitted X-ray detector. The measurement area
was 4 x 3 mm. The XANES measurement was carried out
at the Ni K edge, and the total measurement time for one
XAFS image was 10 min.

Fig. 1: Chemical state map of LNO and LFP cathode at 0,50, and 100 % state during the charge process and at
50 % state during the discharge process. The blue and red
pixel denotes the oxidized (uncharged) and reduced(charged) state of the active material, respectively. The
image size is 0.24 x 0.18 mm for LNO and 2.8 x 2.1 mm
for LFP.
3 Results and Discussion
The measured chemical state maps of the LNO cathodewere shown in Fig. 1 at the 0, 50, and 100 % state during
the charge process. The E0 value of a XANES spectrum
observed at each detector element was used to judge thechemical state of LNO between the oxidized (uncharged,LiNiO2) and reduced (charged, Li1–xNiO2) states. In Fig. 1,
the blue and red pixel represents LiNiO2 and Li1–xNiO2,respectively. Similar to the reported LFP cathode [2], theinhomogeneous reaction distribution was observed also inthe case of the LNO cathode. The size of theinhomogeneous reacting spots of LNO was much smaller
than that of LFP.
The most impressive difference between LNO and LFP
is appeared when the images are compered between thecharge and discharge process. The reverse pattern isobserved in the maps of LFP, suggesting that the electronic
conductance due to the carbon network controls spatiallyinhomogeneous reaction. In contrast, the maps of LNO
shows very similar pattern. This indicates the existence ofthe gateway for the lithium-ion insertion/desertion on theLNO cathode sheet, maybe caused by the accumulation oforganic materials formed by the decomposition of organic
solvent.
References[1] M. Katayama et al., J. Synchrotron Rad., 19, 717 (2012).
[2] M. Katayama et al., J. Power Sourc., 269, 994 (2014).
* yinada@fc.ritsumei.ac.jp