Tweet

English version 14 January, 2021 on EurekAlert!

A new method to analyze chemical status of lithium was developed by using a synchrotron-based scanning transmission soft X-ray microscope (STXM). A key of the method is installation of a newly designed X-ray lens, a low-pass filtering zone plate, to the STXM to improve quality of a monochromatic X-ray. 2-dimensional chemical state of a test electrode of Li-ion battery was successfully analyzed with spatial resolution of 72 nm.

-----------------------------------------

Lithium-ion batteries (LIB) are widely used for daily products in our life, such as hybrid cars, cell phone, etc. but their charge/discharge process is not fully understood yet. To understand the process, behaviors of lithium ion, distribution and chemical composition and state, should be revealed. A research group in Institute for Molecular Science noticed on a scanning transmission X-ray microscope (STXM, shown in Fig. 1) as a powerful technique to perform X-ray absorption spectroscopy (XAS) with high spatial resolution. By using absorption edge of a specific element, 2-dimensional chemical state of a sample can be obtained. To analyze lithium by the STXM, the Li K absorption edge (55 eV) in low energy region makes it difficult to measure the XAS due to lack of a proper optical element and tremendous higher-order harmonics from a monochromator which contaminate XAS. Therefore, a low-pass filtering zone plate (LPFZP), a focusing optical element of the STXM, was developed to overcome these issues. The LPFZP uses silicon of 200 nm thick as a substrate of the zone plate and the substrate works as a low-pass filter above 100 eV by using Si L_2,3 edges. The hybrid optics of the LPFZP can suppress the higher-harmonics without installing an additional optical component into the STXM. As a result, the STXM with the LPFZP suppresses higher-order harmonics down to 0.1% of an original intensity and enables to measure XAS spectra of the Li K-edge. Then the spatial resolution was estimated as 72 nm.

A thin section sample of a test electrode of the LIB was analyzed. The sample is made of Li₂CO₃ by a focusing ion-beam process. The STXM image at 70 eV and the Li K-edge XAS spectra are shown in Fig. 2(a) and 2(b), respectively. The XAS spectra are successfully obtained from the regions indicated by circles in Fig. 2(a).

To understand the behavior of lithium in the LIB is necessary to improve its performance. Then, the STXM with the LPFZP will be helpful to analyze that with high spatial resolution.
 

Fig. 1. Schematic of an optical system of a scanning transmission soft X-ray microscope (STXM).
 

Fig. 2. (a) STXM image of a lithium carbonate sample with 70 eV soft X-ray and (b) Lithium K-edge X-ray absorption spectra at areas 1-4 indicated by circles on Panel (a)

Information of the paper

Authors: Takuji Ohigashi, Hayato Yuzawa and Nobuhiro Kosugi

Journal Name: Review of Scientific Instruments

Journal Title: “A low-pass filtering Fresnel zone plate for soft x-ray microscopic analysis down to the lithium K-edge region”

DOI: 10.1063/5.0020956

Financial Supports

JSPS Grant-in-Aid for Scientific Research (JP16K05022, JP16H03877, JP17H03013)
 

Contact Person

Takuji Ohigashi
TEL/FAX: +81-564-55-7203
E-mail: ohigashi_at_ims.ac.jp
*Please replace the "_at_" with @