Abstract Understanding the local electronic and atomic structure of materials is central to modern physics, chemistry, materials science, and nanotechnology. X-ray absorption spectroscopy (XAS) provides element-specific insight into these local environments through core-level X-ray absorption processes, making it an irreplaceable technique for studying complex and heterogeneous materials, particularly at the nanoscale. Despite its broad applicability, XAS measurements and data interpretation often remain technically complex and strongly dependent on synchrotron facilities, underscoring the need for clear, accessible, and standardized guidance, particularly for new and occasional users. In this work, we present a set of foundational yet practical protocols for performing XAS measurements and interpretation, covering experimental principles, beamline setup, sample preparation, energy and mode selection, as well as data processing and analysis. We also introduce common tools, methods, and typical practices alongside training modules. These protocols aim to facilitate researchers across disciplines to perform XAS measurements, navigate existing resources, and interpret results for their specific research objectives with greater accuracy, efficiency, and reproducibility.

The development of material acceleration platforms in battery research requires integrating complementary techniques and correlating heterogeneous experimental datasets. Here, this challenge is tackled in a large-scale multimodal program involving fifteen laboratories and facilities across Europe. Coordinated multi-site experiments are performed on state-of-the-art graphite / LiNiO2 Li-ion full cells to address two archetypal scientific questions: is the electrolyte composition impacting electrode properties, and how do electrode materials evolve when cells are cycled to their end-of-life? A fully standardized and centralized workflow is demonstrated, from sample production and delivery, to metadata and data handling, generating seventy-five concatenated datasets shared among all partners. Their integrated analysis shows that scientific conclusions depend critically on both the observable chosen to describe electrode properties, and the measurement technique employed. Individual experiments provide detailed information into specific aspects, such as crystal structures, redox activity, surface processes, morphology, etc., but can also function as binary diagnostic tool. Two-dimensional observable-technique patterns are introduced, in which each pixel encodes a yes, no or uncertain answer to a given scientific question. These patterns serve as multi-property metaviews, e.g. visual genotypes, enabling to classify material behavior and technique suitability according to predefined user demand and criteria, highlighting the interdependencies between measurement choices, extracted parameters and scientific interpretation. This multimodal workflow establishes a proof-of-concept for correlative analysis and underscores challenges toward fully integrated, automated and holistic approaches in energy material science.

Operando synchrotron X-ray techniques have become essential tools for investigating rechargeable batteries as they provide real-time insights into electrochemical processes. However, the high brilliance of synchrotron radiation can alter the electrochemical mechanisms within the battery, thereby compromising the reliability and reproducibility of operando measurements. In this study, we introduce a novel methodology that directly correlates the local X-ray dose with the Ni4+/Ni3+ redox activity in a LiNiO2 positive electrode. Full-field transmission X-ray absorption spectroscopy imaging (FFI-XAS) is employed to probe the charge-compensation mechanism during lithium extraction at the micrometre scale, using two beam configurations with different focal distances (far-focus and near-focus). While the spatially averaged XAS spectra exhibit sluggish reaction, regions exposed to lower dose rates exhibit the expected electrochemical evolution. This contrast enables the identification of a dose threshold for reliable operando measurements. This approach establishes a practical dose limit and provides spatially resolved insight into beam-induced effects, offering both a diagnostic framework and a pathway toward more reliable operando experiments.

In the search for sustainable cathode materials for Na-ion batteries, Mn-based oxides have emerged as attractive candidates owing to their earth abundance and structural versatility. In this context, a comprehensive investigation of the electrochemical behavior of the sodium-rich Mn2+ oxide Na10Mn4O9 toward sodium extraction/insertion is presented. The structural framework of this phase, prepared through a simple solid-state synthesis at 450 degrees C, is built on MnO4 and MnO3 polyhedra. Seven sodium ions per unit formula can be extracted during the first charge, and XAS analysis reveals the main transformation into a layered NaxMnO2Na2O composite. Upon further cycling, once this phase is formed, the material delivers a reversible capacity of 110 mAh g(-1) at an average potential of 2.24 V vs Na+/Na.

Here we investigated the electrochemical performance of molybdenum nitride films as an efficient electrode for asymmetric micro-supercapacitors. Molybdenum nitride films were successfully deposited and optimized by reactive magnetron sputtering. On the one hand, the tuning of several deposition parameters (pressure, gas flow rates) allows obtaining molybdenum nitride electrode with high porosity and high electrical conductivity. On the other hand, Operando X-ray diffraction, operando Raman spectroscopy and operando X-ray absorption spectroscopy are combined to unveil the charge storage process in 1 M KOH aqueous electrolyte. These measurements clearly reveal the role of molybdenum oxide species in the pseudocapacitive mechanism at the oxide/electrolyte interface. High volumetric capacitance up to 624 F & sdot;cm-3 with excellent capacitance retention of 95 % over 20 000 cycles was achieved in 1 M KOH.

Representative NiO2 layers composed of edge-sharing NiO6 octahedra and a Li interlayer from the LiNiO2 crystal structure, highlighting a local NiO6 unit. A simplified band diagram illustrates the negative charge transfer from O 2p to Ni 3d states.

In AgNbO3 perovskite structure, electrochemical activation is speculated during the first lithiation cycle enabling the material to reversibly store Li+ by the contributions of both Ag and Nb cation. However, the origin of electrochemically induced structural activation and understanding of cations involvement in complex Li+ storage mechanism is still elusive. Herein, operando synchrotron X-ray absorption spectroscopy (XAS) was applied to clarify this mechanism under different cycling conditions. Ag K-edge XAS measurements during first lithiation revealed a gradual Ag+ to Ag0 reduction starting at a relatively high potential of 1.0 V vs Li+/Li, thus creating vacancies in the lattice for Li+ insertion and inducing a crystalline-to-amorphous structural transition. Below 0.3 V vs Li+/Li, metallic Ag forms multiple intermetallic Li-Ag alloys, resulting in lithium-rich Li9Ag at the end of lithiation. Simultaneously, Nb K-edge XAS measurements indicate an irreversible Nb5+ to Nb3+ reduction with formation of metastable phases during first lithiation. Upon extended cycling at high current densities, intermediate phases sustain reversible Li+ storage through Nb-redox activity and Li-Ag (de)alloying reactions, facilitating fast charging capability. This study will help in designing new conversion-alloying type negative electrodes for fast-charging batteries.

The interest in Ni-rich layered oxide positive electrode materials has been increasing due to its wide applicability particularly in electric vehicles as high capacity and high energy density electrode materials. However, the Ni-O bond array which builds the overall framework and plays a critical role in the charge compensation mechanism of the material requires deeper understanding. This work presents a correlative approach elucidating the role of the local highly covalent Ni-O bonds in LiNiO2 (LNO) model material. Pristine and electrochemically obtained LNO positive electrodes are analyzed using ex situ X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) to compare the average and local structural evolution upon Li+ ion de-intercalation. Insights from Ni K-edge X-ray absorption near-edge structure (XANES) and non-resonant Ni Kbeta X-ray emission spectroscopy (XES) spectra are combined to track the electronic environment of Ni. X-ray Raman scattering (XRS) spectra at the Ni L2,3-edges and O K-edge provide direct bulk electronic information with regards to the interplay between Ni 3d and O 2p states. The overall findings imply that O plays a significant role in the charge compensation process, contributing to the substantial negative charge transfer from the O 2p orbitals, because of the covalency in the Ni-O bonds inside the NiO2 framework within the edge-sharing NiO6 octahedra. The utilization of complementary X-ray spectroscopy techniques clarifies the intricate electronic environment of LNO, which is helpful in understanding Ni-rich positive electrode materials and offering new insights into their covalent nature.

Vanadium fluoride phosphates are highly intriguing due to their diverse crystal structures, which significantly influence their electrochemical properties, making them favorable candidates for Li-ion and post Li-ion battery applications. In this study, high-energy resolution fluorescence-detected X-ray absorption near-edge structure (HERFD-XANES) spectroscopy and X-ray emission spectroscopy (XES) are combined to describe and understand the distinctive features of Tavorite-type LiVPO4F and KTP-type KVPO4F, along with their deintercalated homeotypic phases. The HERFD-XANES spectra, featuring the 1s2p resonant inelastic X-ray scattering (RIXS) process at the pre-edge region, provide detailed information on the local structure, while a thorough interpretation of the XES signals, including Core-to-Core (CtC) K alpha and CtC K beta, offers complementary information about local structures and oxidation states, respectively. Furthermore, the occupied and unoccupied electronic states close to the Fermi level are described by combining the information from the XES Valence-to-Core (VtC) K beta 2,5 emission line and the HERFD-XANES pre-edge, respectively. The thorough understanding of these spectral signatures is made possible by the application of ab initio calculations. Overall, this comprehensive analysis of both local geometric and electronic structures provides valuable insights into these materials with distinct crystal structures which are applied as alkali ion positive electrodes.

A new positive electrode material, Na2.85Mn0.4V1.6(PO4)(2)F2.4O0.6, is synthesized via a topochemical reaction in an ionic liquid medium, starting with a tailored precursor Mn-0.2(VO)(0.8)PO42H(2)O. Its structural and chemical characterization was conducted using a comprehensive set of techniques including X-ray diffraction, X-ray absorption, and electron paramagnetic resonance spectroscopies, as well as inductively coupled plasma optical emission spectroscopy and electron probe microanalysis. These analyses not only allowed to determine the composition and structure but also shed light on the synthesis reaction mechanism. The resulting active material exhibits promising electrochemical performance, delivering a high capacity of 108 mA h/g at a rate of C/20 with an average potential of 3.75 V vs Na+/Na. Even at a higher rate of 1C, a specific capacity of 90 mA h/g is maintained and an excellent capacity retention of 94% is demonstrated after 200 cycles at C/5. In addition, XAS analysis conducted on materials recovered at different states of charge reveals the redox activity of both manganese and vanadium centers. More generally, this work showcases the feasibility of synthesizing stable Na-deficient polyanionic phases within the NazMnxV2-x(PO4)(2)F3-yOy (0 <= x, y <= 2, and z <= 3.6) material family.

The dynamic formation of chemical species composing the solid electrolyte interphase (SEI) layer at the surface of a carbonaceous electrode in a carbonate-based liquid electrolyte was observed in real-time using operando near-ambient pressure XPS (NAP-XPS). The potential of the glassy carbon electrode vs. metallic lithium was controlled during the XPS experiment. By following the binding energy shifts as a function of applied potential, we could identify the main SEI species and observe their deposition on the electrode surface during the formation of the SEI. These results demonstrate that NAP-XPS is a powerful tool to investigate the SEI formation and stability in Li- and post-Li-ion batteries, paving the way for future studies on the effect of electrolyte additives and solvent mixtures on battery performance.

The intricate relationship between local atomic arrangements and electronic states significantly influences the electrochemical properties of Li-ion battery cathode materials. Despite decades of investigation, a consensus regarding the local atomic and electronic structure of LiNiO2 remains elusive. This ambiguity stems from the potential distortion of Ni sites, either via Jahn-Teller (JT) distortion or bond disproportionation (BD), complicating the understanding of the charge compensation mechanism involving Ni and O. This study compares the structures of LiNiO2 and NaNiO2, a JT system, using an innovative approach that integrates bulk spectroscopy techniques on standardized interoperable samples for enhanced reliability. While X-r and theoretical calculations fail to differentiate between the proposed scenarios, Raman spectroscopy highlights local structural distinctions between monoclinic NaNiO2 and rhombohedral LiNiO2. HAXPES confirms various formal oxidation states for Ni, supported by RIXS data indicating 3d8 states, emphasizing negative charge transfer from Ni and some bond disproportionation in LiNiO2. Regarding charge compensation, XRS and RIXS suggest oxygen hole involvement in redox activity, whereas Raman spectroscopy does not detect molecular oxygen. This comprehensive spectroscopic analysis highlights the importance of correlative characterization workflows in elucidating complex structural-electrochemical relationships.

This study elucidates the role of Fe3+ incorporation in birnessite and underscores the significance of the alkaline site's nature in the interlayer spacing for achieving optimized energy storage performance.

Sodium layered oxides NaxMO2 (x <= 1 and M = transition metal) are of great interest for sodium-ion batteries due to their high energy density and cost-effectiveness. However, these materials, whether they are stoichiometric (Na/M approximate to 1 as in O3 NaMO2) or not (Na/M approximate to 0.7 as in P3/P2 NaxMO2), have certain disadvantages, namely sensitivity to humidity or inadequate capacity, respectively. Herein, we propose an intermediate composition Na0.85Ni0.38Zn0.04Mn0.48Ti0.1O2 that we succeed to stabilize in either O3 or a nanoscale mixture of O3-P3 or O3-P2 phases as proven by X-ray diffraction and transmission electron microscopy, through complex synthesis approaches including quenching, slow cooling and annealing in different atmospheres (Ar, air, O-2 etc). We rationalize the stabilization of different phases and microstructure as a function of synthesis conditions and show how it influences the electrochemical performance. Through this study we identified a single phase O3 Na0.85Ni0.38Zn0.04Mn0.48Ti0.1O2 synthesized at 1000 degrees C in air, which exhibits a high capacity of similar to 170 mAh/g and good moisture stability. Furthermore, thanks to the synthesis-structure- electrochemical performance relationship identified here, we believe that this study will provide a reliable basis for optimizing the synthesis for best performing sodium layered oxides for commercialization.

The intimate correlation between the local atomic arrangement and electronic states in Li-ion battery cathode materials plays a crucial role in determining their electrochemical properties, including capacity, cycling stability, and rate capability. Despite almost 30 years of research efforts on high performance cathodes based on Ni rich layered oxides, there is still no consensus on LiNiO2 local atomic and electronic structure. Ni sites could be either Jahn-Teller distorted or bond disproportionated and the role of Ni and oxygen in the charge compensation mechanism remains unclear. In this study, we compare the local and electronic structure of LiNiO2 and NaNiO2, a long-range Jahn-Teller system, using a novel approach which aims at correlating the results from bulk spectroscopy techniques, particularly under operando conditions, obtained on standard samples to ensure sample interoperability and enhance the reliability and robustness of our results. Despite being a site-selective and local technique, XAS is unable to discriminate between the proposed scenarios, as confirmed also by theoretical calculations. On the contrary, Raman spectroscopy show local structural differences between monoclinic distorted NaNiO2 and rhombohedral LiNiO2. Additionally, HAXPES confirms the presence of multiple formal oxidation states for Ni, and RIXS data provides evidence of 3d8 states, confirming the negative charge transfer character of Ni and some degree of bond disproportionation in LiNiO2. Regarding the charge compensation mechanism, XRS and RIXS support the participation of oxygen holes in the redox activity, while Raman spectroscopy does not detect molecular oxygen. By combing several high-fidelity spectroscopy datasets, this study shows the value of correlative characterization workflows to provide insights into complex structural-electrochemical relationships.

Among all positive electrode materials for K-ion batteries (KIBs), KVPO 4 F offers a high theoretical capacity of 131 mAh·g -1 and an average working potential reaching above 4.3 V vs K + /K, resulting in a theoretical energy density of up to 520 Wh·kg -1 . [1] The anionic substitution of fluorine by oxygen in KVPO 4 F 1-x O x (x = 0, 0.25, 0.5, 0.75, 1) induces an increasing working potential by activating the V 3+/4+ and V 4+/5+ redox couples and replacing the VO 4 F 2 “ionic” entity by {V=O}O 5 unit with a highly “covalent” vanadyl-type bond. [2] The accurate prediction of the electrode potential as well as the understanding of the mechanisms involved upon cycling requires the determination of how the coordination environment of the transition metal ion influences the ionicity/covalency of the metal-ligand bonds and thus the electronic structure. Nevertheless, discriminating these ligands remains a challenge due to the limited sensitivity to light elements of common characterization techniques such as X-ray diffraction and extended X-ray absorption fine structure. Despite the valuable information from these techniques, fundamental concerns still need to be addressed which include: i) the implication of the covalent V=O vanadyl bond in the electronic structure; ii) the difference of V cis and V trans , the two different sites for V, in the electronic configuration of the material; and finally, (iii) the impact of F/O ligands on the electrochemical mechanism upon battery cycling. To address this issue, we employed valence-to-core Kβ X-ray emission spectroscopy (VtC XES) combined with ab initio modelling and conducted a systematic investigation of KVPO 4 F 1-x O x : two distinct regions were identified in the spectra, Kβ" and Kβ 2,5 , which are critical to probe the electronic structure close to the Fermi level and to discriminate different ligands coordinated with the vanadium atoms. [3] Our approach allows distinguishing in KVPO 4 F 1-x O x the contributions of V-F, V-O, and V=O bonds, with intensities at the Kβ" region highly correlated to the F - and O 2- anionic composition. Additionally, the evolution of the features at the Kβ 2,5 region is highly associated to the presence of short V=O bonds, strongly influencing the electrode potential of the material. Overall, we present a detailed and reliable approach for understanding the occupied electronic states of the electrode material, proving valuable for a thorough comprehension of the structural and redox mechanisms involved in batteries. Acknowledgement This work was supported by the DESTINY Marie Skłodowska-Curie Actions COFUND PhD Programme (Grant Agreement #945357) co-funded by the European Union's Horizon2020 research and innovation program and the Synchrotron SOLEIL. ANR is also acknowledged for funding the RS2E network through the STORE-EX Labex Project ANR-10-LABX-76-01, and the ANR TROPIC project ANR-CE05-0026. Alistore-ERI network is also acknowledged. References [1] K. Chihara et al, Chem. Commun. 2017, 53, 5208-5211 [2] R. Wernert et al, Chem. Mater. 2022, 34, 4523-4535 [3] E. Gallo and P. Glatzel, Adv. Mater. 2014, 26, 7730-7746

Fast charging is a critical concern for the next generation of electrochemical energy storage devices, driving extensive research on new electrode materials for electrochemical capacitors and micro-supercapacitors. Here we introduce a significant advance in producing thick ruthenium nitride pseudocapacitive films fabricated using a sputter deposition method. These films deliver over 0.8 F cm–2 (~500 F cm–3) with a time constant below 6 s. By utilizing an original electrochemical oxidation process, the volumetric capacitance doubles (1,200 F cm–3) without sacrificing cycling stability. This enables an extended operating potential window up to 0.85 V versus Hg/HgO, resulting in a boost to 3.2 F cm–2 (3,200 F cm–3). Operando X-ray absorption spectroscopy and transmission electron microscopy analyses reveal novel insights into the electrochemical oxidation process. The charge storage mechanism takes advantage of the high electrical conductivity and the morphology of cubic ruthenium nitride and Ru phases in the feather-like core, leading to high electrical conductivity in combination with high capacity. Accordingly, we have developed an analysis that relates capacity to time constant as a means of identifying materials capable of retaining high capacity at high charge/discharge rates. Fast charging is driving extensive research on enhanced electrodes for high-performance electrochemical capacitors and micro-supercapacitors. Thick ruthenium nitride pseudocapacitive films are shown to exhibit enhanced capacitance with a time constant of less than 6 s.

Understanding the intricate crystal structure of polyanionic positive electrode materials is essential for elucidating the mechanisms involved during cycling and predicting the working potential of the electrode. To achieve this goal, a clear comprehension of how the coordination environment of the transition metal ion influences the ionicity/covalency of the metal-ligand bonds is necessary. Yet, discriminating these ligands poses challenges due to the limited sensitivity to light elements of common characterization techniques such as X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) analysis. To address this issue, we employed valence-tocore K beta X-ray emission spectroscopy combined with ab initio modelling and conducted a systematic investigation using potassium vanadium oxyfluoride phosphate compounds with the general formula KVPO4F1-xOx (x = 0, 0.25, 0.5, 0.75, 1). Our approach allows distinguishing the contributions of V-F, V-O, and V--O bonds at the K beta" region, with intensities highly correlated to the F- and O2- anion composition. Additionally, the evolution of the features at the K beta 2,5 region is highly correlated to the presence of short V--O bonds, strongly influencing the electrode potential of the material. Density of state (DOS) analysis based on ab initio modelling of the end member compounds KVPO4F and KVOPO4 further supports the existence of V-F and V--O bonding through the mixing of F/O p-DOS with V d-DOS. Overall, we present a detailed and reliable approach for understanding the occupied electronic states of the bulk material, proving valuable for a thorough comprehension of the structure of positive electrode materials in batteries.

The emergence of high brilliance synchrotron sources and the availability of more sophisticated infrastructure have opened doors to advanced material characterization, enabling a profound understanding of new processes and mechanisms in battery research. [1] More specifically, non-resonant Valence-to-Core X-ray emission spectroscopy (VtC-XES) and X-ray Raman scattering (XRS) stand out as complementary photon-in/photon-out X-ray spectroscopy techniques. The unique capability of these techniques lies in their access to soft X-ray edges using a hard X-ray beam, providing comprehensive information on both the electronic and local structure, due to their high sensitivity to local structure and coordination. [2,3] In this work, two different families of materials with different compositions and crystal structures are studied using XES and XRS. The first part focuses on the detailed study of oxygen substituted KVPO 4 F 1-x O x (x = 0, 0.25, 0.5, 0.75, 1) using VtC-XES combined to ab initio theoretical calculations. The average working potential increases with increasing oxygen content by activating the V 3+/4+ and V 4+/5+ redox couples and replacing the VO 4 F 2 “ionic” entity by {V=O}O 5 unit with a highly “covalent” vanadyl-type bond. [4] VtC-XES allowed us to distinguish the contributions of V-F, V-O, and V=O bonds in the local environment, with spectral intensities highly correlated to the F - and O 2- anionic composition. In addition, specific spectral evolutions are also associated to the presence of highly covalent V=O bonds, strongly influencing the electrode potential of the material. The second part examines a series of electrochemically obtained Li y NiO 2 layered oxide materials (y = 0.02, 0.25, 0.33, 0.5, 0.67, 1.0). VtC-XES performed at the Ni Kβ emission line allows to study the local structure evolution around Ni, whereas XRS spectroscopy at both the O K-edge and Ni L 2,3 L-edges gives an access to the interplay between Ni and O through the hybridized states of Ni 3 d and O 2 p . [5] This work provides a survey on both the cationic and anionic redox processes, and unveils the underlying phenomena related to the charge compensation processes occurring in this material. Overall, we present a comprehensive study of different polyanionic and layered oxide positive electrode materials using complementary X-ray spectroscopic techniques to probe both the local and electronic structures. We demonstrate that they allow to follow and understand evolution and changes in the local structure in materials of intricate crystal structures. Acknowledgement This work was supported by the DESTINY Marie Skłodowska-Curie Actions COFUND PhD Programme (Grant Agreement #945357) co-funded by the European Union's Horizon2020 research and innovation program and the Synchrotron SOLEIL. ANR is also acknowledged for funding the RS2E network through the STORE-EX Labex Project ANR-10-LABX-76-01. Alistore-ERI network is also acknowledged. References [1] M. Fehse et al, Phys. Chem. Chem. Phys. 2021, 23, 23445-23465 [2] M. Rovezzi et al, Semicond. Sci. Technol. 2014, 29 [3] W. Yang et al, J. Power Sources. 2018, 389, 188–197 [4] R. Wernert et al, Chem. Mater. 2022, 34, 4523-4535 [5] X. Wang et al, Adv. Funct. Mater. 2021, 31, 1–10