X-ray absorption spectroscopy is a structural characterization technique based on the photoelectric effect. An x-ray beam is used to excite an electron from an inner shell of an atom in the sample, creating a core hole. The filling of the core hole by an electron from a higher shell is accompanied by the emission of an x-ray photon. Both x-ray absorption and x-ray fluorescence can be used for detection. In the former mode, the x-ray source, sample and detector are arranged in a 180 degree geometry, in the latter in a 45 degree geometry. For light elements like sulfur and first-row transition metals usually K-edge absorption is used with element specific absorption energies of 2.47 keV for sulfur and 5 to 10 keV for first-row transition metals. For heavy atoms, the L-edge is more commonly used because of the high K-edge energy. For example, the platinum K-edge has an energy of 78.4 keV while the L-edge energy is 13.9 keV.
Usually a synchrotron serves as x-ray source because of the high intensity and wide continuous spectral range.
X-ray absorption spectroscopy can be subdivided into x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) spec-troscopy. A XANES spectrum is measured close to the absorption edge and contains information on the amount (height of edge jump) and the oxidation state (exact edge position) of the respective atom. EXAFS is based on the inelastic scattering of the photo electron with neighboring atoms. From the interference pattern, measured at energies up to several hundered eV’s above the absorption edge, it is possible to de-termine the distance and amount of neighboring atoms and deduce the local geometry of the scattering atom. In contrast to diffraction techniques, no long range order is required for EXAFS analysis.
X-ray absorption spectroscopy is a common tool in battery research and has already been applied under in situ/operando conditions. Operando XANES is especially popular for the investigation of the complex discharge mechanism in lithium-sulfur cells, as it can differentiate between terminal and internal sulfur atoms in polysul-fides and thereby determine the average polysulfide chain length (e.g., Li2Sx with 1 ≤ x ≤ 8).[129;130;131;132] In most studies, an averaged signal from both the sulfur cathode and the polysulfide containing electrolyte in the separator is measured. In contrast, a special operando XAS cell design developed by our group allows the se-lective measurement in either the anode, cathode or separator.[131] The concept of this operando XAS cell, which operates in fluorescence mode, is shown in Figure 2.5a.
One side of the flat, rectangular electrodes and the separator (10 x 10 mm) is pressed against the front plate of the cell which contains a 2 mm tapered hole serving as the x-ray window. The window is sealed with an aluminized Kapton® foil (100 nm Al-film on 8 µm Kapton®) which is an air and water permeation barrier. For the sulfur K-edge at 2.47 keV the 8 µm aluminized Kapton® foil has a transmission of about 50%, for the iron K-edge at 7.11 keV a transmission of over 80% is reached even if a second (non-aluminized) 25 µm thick Kapton® foil is additionally added for enhanced mechanical stability of the window. The penetration depth into the re-spective electrodes is about 50 µm at 2.47 keV and about 500 µm at 7.11 keV. It is important to obtain a homogeneous electrochemical reaction over the entire electrode area because only the uppermost part of the electrodes and the separator is measured (50 to 500 µm penetration depth versus 10 mm electrode side length). The cell 1 is mounted in a 45 angle with respect to the incoming x-ray beam 2 and the detector 3 as shown in Figure 2.5b. The size of the incoming x-ray beam has to be adjusted according to the required spatial resolution. Figure 2.5 c shows the top view of the x-ray window (without Kapton® foil) with the aluminum current collector cube 4 , the cathode 5 (120 µm thickness), two compressed gass fiber separators 6 (about 350 µm thickness), the anode 7 (120 µm thickness) and the copper current collector cube 8 . The dashed red line represents the size (100 x 1000 µm) and orientation of the incoming x-ray beam used for the spatially resolved XAS experiments. In most beamlines, the beam can be focused in one direction and is cut by slits in the other.
In contrast to focusing, cutting with slits causes a significant loss of the incident beam intensity. The advantage of slits is a very well defined and sharp beam width while focusing yields a rather broad peak-like intensity distribution. Therefore the beam is cut in horizontal direction and focused in the vertical direction.
In the beginning of a spatially resolved XAS experiment, detailed mapping has to be carried out to determine the exact position of the electrodes. Note, that the aluminized Kapton® is not permeable for visible light and a standard optical fluorescence screen is therefore not helpful. Mapping is based on line scans (Figure 2.5d) of the total flu-orescence intensity using a certain, fixed x-ray energy. The cell sample stage is moved in µm-steps in horizontal or vertical direction while the beam position and energy are kept constant. In order to reduce the time of line scans, the total fluorescence is measured without spectral resolution. As a consequence, a ”copper line scan” (see red line in Figure 2.5 d), measured with a fixed x-ray energy slightly above the copper edge, is also sensitive to other elements with a slightly lower K-edge energy (e.g. Mn,
Fe, Co). The copper line scan is carried out to determine the position of the cur-rent collector/anode interface. A line scan of a metal contained in the cathode active material is carried out to determine the position of the cathode/separator interface.
In combination with the known thickness of anode, cathode and separator a detailed map of the cell interior is obtained and measuring positions for the operando XAS experiment can be defined. Figure 2.5d shows the successful mapping of a preaged LTO/NMC cell. The steep decline of the red curve (copper line scan) at x = 3.5 mm corresponds to the current collector/LTO interface. The maximum of the green line (manganese line scan) in a similar x-position corresponds to manganese deposited in the LTO electrode. The steep increase of the green curve at x = 3.7 mm corresponds
Figure 2.5 a) General concept of operando XAS cell with spatial resolution (Image adjusted from open access article [131]); b) Fluorescence set-up with 45 degree angle between the cell (1), the incoming x-ray beam (2) and the detector (3); c) Microscope image of an LTO/NMC cell without the aluminized Kapton® foil window with size and orientation of x-ray beam (red dashed line) and detached LTO particle (red circle); d) Mapping line scans for determination of the electrode and separator positions.
to the separator/NMC electrode interface. Two factors have to be considered for the correct interpretation of the line scans: i) the cell is mounted in a 45 degree angle, so moving the sample stage by a distance d results in an effective displacement of cos(d) and ii) the fluorescence intensity measured with a beam width in x-direction of ∆xin position p gives an averaged value for p±∆x/2.
The general electrochemical performance of the XAS cell design is very good. The potential curve shown in Figure 1.4 (Section 1.2, page 8) was measured in the XAS cell and looks identical to a potential profile in a standard cell design. The XAS cell can be cycled for up to one week with a similar capacity retention as a standard cell design. A major problem are cycling conditions which cause heavy gassing; the resulting pressure increase first causes bending out and then rupture of the Kapton®window. Therefore, experiments with harsh cycling conditions like high cut-off potentials are currently limited to a few hours.
A major challenge for the study of the transition metal deposition at the anode (Sec-tion 3.2) is the presence of small detached cathode active material particles. The amount of transition metal deposited in the anode electrode is typically on the order of 1 mmol/Lelectrode, where Lelectrode refers to the volume of the anode electrode (in liter) including graphite active material and electrode porosity. In comparison, the transition metal amount in a solid NMC particle is about 50 mol/LNMC (ρN M C = 4.72 g cm-3) which is four to five orders of magnitude more than the expected concen-tration increase in the anode. As a consequence, even a µm-sized particle of cathode active material which may have gotten detached from the cathode during the cell as-sembly can dominate the transition metal XAS spectra measured in the anode position if it comes to rest on the anode close to the x-ray window. In Figure 2.5c, the red cir-cle shows a rather big detached LTO particir-cle. To avoid this problem, adhesion of the cathode coating has to be essentially perfect and electrode cutting and cell assembly has to be carried out very carefully.