A pEek on the lab

Beamlines and experimental endstation used for reference-free X-ray spectrometry experiments in the PTB laboratory at the synchrotron radiation facility BESSY II in Berlin.

X-ray spectrometry (XRS) based on radiometrically calibrated instruments, which ensures the physical traceability of quantification to the SI units, is a unique feature of PTB. For X-ray spectrometry, various beamlines are available at PTB’s laboratory at BESSY II in the spectral ranges of soft and hard X-rays (78 eV to 10.5 keV), as well as the "BAMline" for photon energies up to 60 keV [1–4].

With X-ray spectrometry, surfaces, solids, liquids, nanolayers and nanostructures can be characterized regarding their physical and chemical properties – such as chemical composition, elemental depth profiles, layer thicknesses, and species and coordination fractions.

Based on radiometry with synchrotron radiation [5], PTB uses calibrated detectors for X-rays to develop and apply X-ray-spectrometric measurement procedures and quantification algorithms for a characterization of materials that is physically traceable to the SI units [6]. 

The methodology developed by PTB, i.e. reference-free X-ray fluorescence analysis (XRF), is in the focus of the constant instrumental and scientific development of X-ray spectrometry for application in the aforementioned subject areas [7]. Especially the extension of reference-free XRF for the geometry of grazing incidence (GIXRF) and the utilization of the information depth (which can be tuned via the angle of incidence) and of the increased detection sensitivity for nanostructured materials are some of the goals of these activities. X-ray absorption spectrometry was further developed for the depth-sensitive identification and quantification of chemical valence states in nanostructured materials, and the information depth, which depends on the angle of incidence and on the photon energy, was investigated. 

Besides XRF, synchrotron radiation is used in the PTB laboratory at BESSY II also for research and development in the field of high-resolution X-ray emission spectrometry (XES). With an in house built Von Hamos spectrometer measurements are possible in the photon energy range from 2.4 to 18 keV. Activities in the field of high-resolution XES have been focused on investigating the electronic structure of molecules performing resonant inelastic X-Ray scattering (RIXS) measurements [8], and on the determination of relevant atomic fundamental parameters (FPs) such as transition probabilities, chemical shifts, fluorescence yield, line energies, and line width. [9,10] 

For a traceable quantification with small uncertainties, the FPs have to be known with good accuracy.

[1] F. Senf, U. Flechsig, F. Eggenstein, W. Gudat, R. Klein, H. Rabus, G. Ulm: J. Synchrotron Rad. 5, 780 (1998).

[2] M. Krumrey, G. Ulm: Nucl. Instrum. Meth. A 467–468, 1175 (2001)

[3] W. Görner, M. P. Hentschel, B. R. Müller, H. Riesemeier, M. Krumrey, G. Ulm, W. Diete, U. Klein, R. Frahm: Nucl. Instrum. Meth. A 467–468, 703 (2001)

[4] M. Richter, G. Ulm: in this publication on p. 3

[5] R. Klein, R. Fliegauf, S. Kroth, W. Paustian, M. Richter, R. Thornagel: in this publication on p. 16

[6] F. Scholze, M. Procop: X-ray Spectrom. 30, 69 (2001) 

[7] B. Beckhof: J. Anal. At. Spectrom. 23, 845 (2008)

[8] K. Bzheumikhova, J. Vinson, R. Unterumsberger, Y. Kayser, T. Jach, B. Beckhoff: Phys. Rev. B 106, 125133 (2022)

[9] Y. Kayser, P. Hönicke, M. Wansleben, A. Wählisch, B. Beckhof: X-Ray Spectrom., 22083 (2022)

[10] M. Wansleben, Y. Kayser, P. Hönicke, I. Holfelder, A. Wählisch, R. Unterumsberger, B. Beckhoff: Metrologia 56, 065007 (2019)

XRF is a widely used method for the non-destructive characterization of materials for determination of the elemental composition, layer thicknesses, mass distributions and minimum contaminations [12]. Quantification is normally traced to certified calibration standards and reference materials with the most similar matrix elemental composition (chemical traceability) possible. The decisive advantage of XRF without reference samples – as developed by PTB – is that it is absolutely independent of suitable calibration standards and reference materials, as these are often not available or would have to be developed and certified with great effort [11]. Especially when characterizing new materials with very short and dynamic development cycles, reference-free XRF can be used very flexibly and without any limitation due to a lack of calibration standards. 

Thanks to the development and validation of the concept of "effective solid angle of detection", reference-free XRF can also be used for beam geometries of grazing incidence [13]. This has allowed the scope of application of this method to be considerably extended. In the area of TXRF, for example, extremely small amounts of substance were determined on various substrate materials [14]. In the transition range between TXRF and conventional 45° geometry, GIXRF allows the composition and thickness of thin layers and multilayer systems to be determined down into the sub-nanometer range [15]. In addition, the adjustable information depth can be used for the quantitative characterization of element depth profiles such as, for example, ultra-shallow implantations or diffusion profiles [16].

[11] B. Beckhof: J. Anal. At. Spectrom. 23, 845 (2008)

[12] B. Beckhof, B. Kanngießer, N. Langhof, R. Wedell, H. Wolf (Eds.): Handbook of Practical X-Ray Fluorescence Analysis, Springer, Berlin, 1st Edition (2006)

[13] B. Beckhof, R. Fliegauf, M. Kolbe, M. Müller, Jan Weser, G. Ulm: Anal. Chem. 79, 7873 (2007)

[14] M. Müller, A. Nutsch, R. Altmann, G. Borionetti, T. Holz, C. Mantler, P. Hönicke, M. Kolbe, B. Beckhof: Sol. State Phenom. 187, 291 (2012)

[15] M. Müller, P. Hönicke, B. Detlefs, C. Fleischmann: Materials 7, 3147 (2014)

[16] P. Hönicke, B. Beckhof, M. Kolbe, D. Giubertoni, J. A. van den Berg, G. Pepponi: Anal. Bioanal. Chem. 396, 2825 (2010)

Our in-house designed measurement chamber addresses several requirements related to the various X-ray spectrometry (XRS) methods as well as reference-free approach. It offers a high flexibility as well as very precise sample movements under ultra-high vacuum (UHV) conditions. It can be operated at two different synchrotron beamlines at BESSY, offering a broad range of available incident photon energies (78 eV - 10.5 keV). With an energy-dispersive X-ray detector perpendicular to the incident beam TXRF, GIXRF and conventional XRF measurements can be performed. The chamber is equipped with a 9-axis manipulator, allowing the movement of the sample stage in a wide range with respect to all degrees of freedom. The attached load-lock with a sample transfer system allows for optimized vacuum conditions inside the chamber and quick sample change. Sample transfer in inert atmosphere is possible.

[17] J. Lubeck, B. Beckhoff, R. Fliegauf, I. Holfelder, P. Hönicke, M. Müller, B. Pollakowski, F. Reinhardt, and J. Weser: Rev. Sci. Instrum. 84, 045106 (2013)

The PTB’s von Hamos spectrometer is based on a modified von Hamos geometry. [18] It is equipped with two full-cylinder crystals to conduct X-ray emission spectroscopy (XES). The spectrometer is characterized by its compact dimensions, its versatility with respect to the number of crystals used in series in the detection path, and the option to perform calibrated XES measurements. The full-cylinder crystals used are based on highly annealed pyrolytic graphite HAPG) with a thickness of 40 µm, which was bent to a radius of curvature of 50 mm. The flexible design of the spectrometer allows for an easy change between measurements with one crystal for maximized efficiency or two crystals for increased spectral resolving power. The spectrometer can be used at different end-stations of synchrotron radiation beamlines or can be laboratory-based. The main application focus of the spectrometer is the determination of X-ray fundamental atomic parameters [19] and chemical speciation [9] in the photon energy range from 2.4 to 18 keV. 

[9] Y. Kayser, P. Hönicke, M. Wansleben, A. Wählisch, B. Beckhof: X-Ray Spectrom., 22083 (2022)

[18] I. Holfelder, M. Wansleben, Y. Kayser, R. Gnewkow, M. Müller, J. Weser, C. Zech, and B. Beckhoff: Rev. Sci. Instrum. 92, 123150 (2021).

[19] M. Wansleben, J. Vinson, I. Holfelder, Y. Kayser, B. Beckhoff, X-Ray Spctrom. 48, 102 (2019)

Quantitative XRF is based on the precise knowledge of the atomic fundamental parameters (FPs) involved, which allows physical modeling of the characteristic X-ray fluorescence radiation emitted by a sample. The FPs consist of mass absorption coefficients, photoionization and scattering cross sections, fluorescence yields, Coster-Kronig factors, and transition probabilities. The different databases available for these fundamental parameters are based on – partly – rather old experimental and theoretical data with estimated uncertainties amounting to up to 50 % [20]. Using radiometrically calibrated instruments of PTB [17], different atomic FPs can be newly determined with traceable and reduced uncertainties [21–23]. For this purpose, dedicated experiments are carried out with – usually – thin, self-supporting foils of the elements to be investigated. For example, the photon-energy-dependent mass attenuation coefficients can be obtained from transmission measurements, whereas determining fluorescence yields or Coster-Kronig factors requires experiments with energy- or wavelength-dispersive X-ray detectors [24].

[17] J. Lubeck, B. Beckhof, R. Fliegauf, I. Holfelder, P. Hönicke, M. Müller, B. Pollakowski, F. Reinhardt, J. Weser: Rev. Sci. Instrum. 84, 045106 (2013)

[20] G. Zschornack: Handbook of X-Ray Data, Springer, Berlin (2007)

[21] P. Hönicke, M. Kolbe, M. Müller, M. Mantler, M. Krämer, B. Beckhof: Phys. Rev. Lett. 113, 163001 (2014)

[22] M. Müller, B. Beckhof, R. Fliegauf, B. Kanngießer: Phys. Rev. A 79, 032503 (2009)

[23] D. Sokaras, A. G. Kochur, M. Müller, M. Kolbe, B. Beckhof, M. Mantler, Ch. Zarkadas, M. Andrianis, A. Lagoyannis, A. G. Karydas: Phys. Rev. A 83, 052511 (2011)

[24] M. Kolbe, P. Hönicke, M. Müller, B. Beckhof: Phys. Rev. A 86, 042512 (2012)

With X-ray spectrometry techniques and the approach of reference sample-free quantification of elemental mass deposition we have a powerful tool to investigate the transition metal dissolution fading process for manganese in lithium-ion batteries with NMC cathodes. [25] Through the quantification we can evaluate the magnitude of deposited transition metals on the anode for aged cells. With near-edge X-ray absorption fine structure analysis (NEXAFS) the electronic structure of specific elements can be studied. 

The combination of the different methods generates a clearer understanding of the chemical processes in the battery and can therefore help to improve the performance by exploring the absolute mass deposition and the electronic structure of elements contained in enhanced material combinations. 

By modified coin cells the operando analysis of the polysulfide formation in LiS batteries allows the correlation between processes upon cycling and the impact on battery capacity loss. [26]

Post mortem studies of anodes after 50 full cycle with elevated cut-off voltage:

The cells show a capacity decrease of 12.5%, while 1.6‰ of the cathodic manganese was found deposited in the anode. The manganese deposited was determined to be in di- and tetra-valent states, studied by near-edge X-ray absorption fine structure analysis (NEXAFS) at the manganese L-edges and K-edges. Due to the application of significantly different excitation energies sensitivity to different sample depths is provided and tetravalent manganese is solely found in the anode bulk. [25]

Operando characterization of both electrodes of LiS coin cells

Operando Near-Edge X-ray Absorption Spectroscopy (NEXAFS) traceable to the SI units were performed during multiple charge–discharge cycles on both electrodes of LiS coin cells which enables an absolute quantification of dissolved polysulfides with no need for calibration samples or reference material. 

During the charging process, polysulfide (PS) movement from the negative to the positive electrode is inhibited. This leads to a steady increase of dissolved polysulfides at the anode side and, therefore, is one of the key points for capacity fading. 

At different state of charge and state of health the appearance of PS at the cathode and anode side to is analyzed to characterize the transport mechanisms of the polysulfide shuttle phenomena and to reveal quantitative information about their evolution. The cell design suppresses the contribution of cathodic sulfur, which is mandatory for reference-free quantification in X-ray spectrometry and allows us to use only slightly modified standard coin cell batteries. [26]

[25] C. Zech, M. Evertz, M. Börner, Y. Kayser, P. Hönicke, M. Winter, S. Nowak, B. Beckhoff: J. Anal. At. Spectrom., 36, 2056 (2021)

[26] C. Zech, P. Hönicke, Y. Kayser, S. Risse, O. Grätz, M. Stamm, B. Beckhoff: J. Mater. Chem. A, 9, 10231 (2021)