Abstract text (incl. figure legends and references)

Soft X-Ray Emission Spectrometer (SXES), which can be installed on SEM and EPMA (Electron probe microanalyzer), is a useful method for chemical state analysis by detecting the energy state of valence electrons. In particular, since it is possible to detect low-energy characteristic X-rays such as lithium (Li) K-line and analyze the chemical state, it is expected as a method for chemical state analysis of Lithium Ion secondary Battery (LIB) materials in a charged state. Field Emission-SEM equipped with SXES enables us to obtain chemical state information in the vicinity of the bulk top surface by using low-kV analysis. (1) However, it is known that the irradiation of an electron beam causes a change in the chemical state of the beam-sensitive materials such as lithium compounds during the observation. (2) This study shows the improvement of chemical state analysis for beam-sensitive materials using the SEM-SXES system with a cooling stage.

The Schottky FE-SEM, JSM-IT800(JEOL), combined with the SXES, SS-94000SXES (JEOL), and cooling stage C1003 (Gatan, Inc.), was used to examine Lithium Sulfide (Li2S) reagent for the solid electrolyte of LIB. To prevent deterioration of the samples due to atmospheric exposure, these samples were transferred to the SEM-SXES system using a transfer vessel for air isolation. The changes in the chemical state of Li2S were confirmed by time-resolved SXES analysis. (2)

SXES point analysis for Li2S at room temperature (RT), was performed at an accelerating voltage of 3 kV and a probe current of 20 nA (Fig. 1 and Fig. 2). When Li2S is continuously irradiated with an electron beam for 450 seconds, formation of　deposition on the sample surface can be confirmed in the backscattered electron (BSE) image (Fig.1). And the spectrum of the precipitated Li and S are detected in the SXES analysis result at an acquisition time of 450 seconds (Fig.2 left). By contrast, in the time-resolved SXES analysis performed every 15 seconds from 0 to 450 seconds, the Li spectrum shape changes in response to the irradiation time (Fig.2 right). This demonstrates that Li is metallized as the dose of the electron beam increases. Figure 3 shows the result of SXES point analysis with -100℃ cooling at 3kV and 20 nA prove current. In the BSE image (Fig.3 left), there are no change in contrast. It can be seen that no precipitation has occurred on the surface of Li2S after performing SXES analysis with an acquisition time of 450 seconds. Even in the SXES spectrum (Fig. 3 right), which was time-resolved every 15 seconds from 0 to 450 seconds, no spectral change with the passage of time was confirmed. Comparing the SXES results obtained at room temperature and at -100 ° C, it can be seen that the Li K spectrum derived from metallic lithium was not detected and the Li K spectrum derived from Li2S was obtained by cooling at -100 ° C (Fig. 4).

This result lead to the possibility of a new technique for SEM-SXES analysis using specimen cooling system for materials whose chemical state changes due to electron beam irradiation.In this presentation, more examples of specimen cooling application for SEM-SXES analysis of materials whose chemical state changes due to electron beam irradiation, such as polymers and negative electrodes of solid-state batteries, will also be introduced.

References:

(1) Yamamoto. Y et al, Microscopy and Microanalysis 24(S1):1062-1063(2018)

(2) Yamamoto. Y et al, Microscopy and Microanalysis 26(S2):68-70(2020)