The X-ray absorption process starting from the excitation of the core-level electrons makes the XAS technique highly element specific [
14]. Therefore we can observe the behaviors of various elements to fulfill different experimental purposes. For example, C and O K-edge XAS can be used to investigate the interactions between the aforementioned conductive carbon based materials with S [
11]. Additionally, C and O K-edge XAS can be used to investigate the degradation mechanism of the cathode side of the Li/S cells [
10], one example has also been provided above. Besides these, some 3d transition metal based additives have been added to the cathode to improve the cell performances [
27,
28,
29], and metal L-edge XAS can provide directly insight to the additive’s electronic and structural evolutions and probe the interactions among the components in the cathode materials. More importantly, S K-edge XAS can be applied to investigate S species, the major element in Li/S cells, the evolution during the sample preparation and the cell cycling process [
21,
30,
31]. This specific information enables us get a clear and comprehensive understanding of the Li/S cell system. The C and O K-edge XAS features originating from transitions from 1s to 2p orbitals appear at energies from 280 to 320 eV and 525 to 560 eV, respectively; 3d-transition metal L-edge XAS features arising from transition from 2p to 3d show up between 350 and 1100 eV (Ca to Zn); and the S K-edge XAS peaks arising from transition from S 1s to S 3p mainly occur between 2465 and 2520 eV. The energy regions of the C and O K-edges and 3d transition metal L-edges are in the soft X-ray region, which require a vacuum system for measurement; while the energy region of the S K-edge is in the tender X-ray region which can be performed in either vacuum or ambient pressure. To get detailed information about the S speciation in the cell and the interactions of additives and the S species during the charge/discharge process,
in-situ/
in-operando studies are highly desirable. Several
in-situ/
in-operando setups have been designed, but mostly are model cells [
21,
30]. We have used a modified coin cell, which can provide accurate information of the nature of the electrochemical processes. A 1.0 mm × 0.5 mm rectangular hole was punched with a laser beam on the stainless steel coin cell shell. This small opening was sealed with Al coated Mylar film using epoxy. The scheme is illustrated in
Figure 3a. The 2.6 μm thickness aluminized Mylar film can provide more than 90% transmission at the S K-edge, and seals the cell well. Furthermore, thicker Mylar film, such as 10–20 μm thickness Mylar film, can also be applied with good transmission of the X-ray beam, as shown in the
Figure 3c. The small opening can eliminate the cell structure differences induced changes of the S speciation during the electrochemical process. The modified cell works well for tender or hard X-ray spectroscopy, but it is not vacuum-compatible. Therefore, this kind of cell cannot be used for soft X-ray absorption spectroscopy (sXAS). In previous studies, sXAS has been applied to study the LiFePO
4 based Li-ion cell with solid electrolyte [
32]. However, solid electrolytes have many limitations and are not good candidates for widespread application [
33]. To address these issues, a vacuum-compatible cell was developed, illustrated in
Figure 3b and described in detail in previous papers [
14,
34]. Basically, the cell body was built of PEEK (polyetheretherketone) material; a vacuum-sealed O-ring, a copper ring, a Si
3N
4 membrane window and a metal lead were used to seal the cell. A gold-coated Si
3N
4 membrane was attached to the copper ring and further connected to the Cu base, acting as the working electrode. Two holes at the cell bottom accommodate Al wires going through and acting as reference and counter electrodes. The holes were sealed well with epoxy. The 100 nm thickness of Si
3N
4 allows more than 95% transmission for X-rays at the S K-edge energy and lower but still good transmission for the energy regions of C and O K-edge and 3d transition metal L-edge. It has been well established that this design can be applied for the three-electrode electrochemical reactions. However, the applications of this cell have been highly limited into several aspects, including the catalyst reactions and some early starting of Mg-ion batteries studies. Therefore, it is highly worthwhile to check the possibility of adapting this design to the Li/S cell system by calculating the attenuation of the soft X-ray through different materials with different thicknesses. Thereafter, as described above and shown in the
Figure 3d, we provided evidences that this cell design is suitable for the Li/S cell system and enable us to extend the interests to the soft X-ray regions. Moreover, this vacuum cell can also be applied to study most cathode materials used in Li-ion batteries. In conclusion, this work provided the basic perspectives on the application of the cell design on the Li/S cell system and enable us revisit and explore more details in this topic.
Figure 3.
The scheme of (
a) a modified coin cell and (
b) a three-electrode electro-chemical cell (figure adapted from Reference [
14]); and X-ray transmission at (
c) soft X-ray energy region and (
d) tender X-ray energy region through different materials.