XRS, particularly in its elastic form, has become an indispensable tool for studying energy storage materials due to its ability to probe structures at multiple length scales without altering the sample. Elastic scattering, where X-ray photons interact coherently with the material, provides essential information on atomic and molecular arrangements. This is especially critical in energy storage materials, where the arrangement of atoms and larger structural features can significantly influence properties like ion mobility, electrochemical stability, and overall battery performance.

Wide-angle X-ray scattering (WAXS) plays a key role in analyzing the atomic structure of materials. By focusing on scattering angles greater than 5°, WAXS provides insights into atomic positions, lattice parameters, and the degree of crystallinity. This information is essential for understanding the behavior of key battery components, such as the cathode and anode, which rely on well-ordered crystal structures for efficient ion intercalation and electronic conductivity. WAXS can also be used to monitor phase transitions in battery materials during operation, offering valuable insights into degradation mechanisms and material stability over time.

On the other hand, small-angle X-ray scattering (SAXS) investigates larger-scale structures by analyzing scattering at angles close to 0°. SAXS is particularly effective for studying nanoscale features, such as particle size, pore distribution, and morphology, which can strongly influence the performance of energy storage materials. For instance, in lithium-ion batteries (LIBs), the nanoscale morphology of electrode materials can affect ion transport and cycling stability. SAXS can reveal changes in these features during battery operation, enabling researchers to optimize material design for improved performance.

The combination of WAXS and SAXS allows for a multi-scale understanding of energy storage materials, from atomic-level structures to mesoscale features. This comprehensive approach is critical for developing materials with enhanced electrochemical properties. For example, a well-ordered crystal structure at the atomic level, combined with an optimized particle size and porosity at the nanoscale, can lead to improved ion diffusion and increased energy storage capacity. Thus, XRS techniques provide valuable tools for guiding the design and optimization of next-generation energy storage materials.

X-ray diffraction (XRD) is a subset of XRS that occurs when X-rays pass through a crystalline material and undergo elastic and coherent scattering (Fig. 1)[7]. Thus, XRD and WAXS are complementary techniques with distinct objectives, widely used to investigate material structures. XRD is a cornerstone of crystallography, providing precise information on the periodic atomic arrangements within crystalline materials. In contrast, WAXS extends its scope beyond crystallinity, enabling the simultaneous examination of both ordered and disordered regions, making it particularly valuable for studying complex structural heterogeneity [8].

The fundamental difference between XRD and WAXS lies in their focus. XRD primarily analyzes Bragg peaks, delivering detailed information about crystal structures, such as lattice parameters and atomic arrangements. On the other hand, WAXS captures scattering signals from both crystalline and amorphous phases, offering insights into both short-range and long-range structural order. This dual capability makes WAXS especially suitable for analyzing materials with diverse structural domains. A key distinction between the two techniques is the range of scattering angles they cover. XRD typically operates within narrow angles, concentrating on lattice plane-specific reflections. Conversely, WAXS spans a broader angular range, reaching higher q-values, which allows the investigation of smaller structural features such as molecular packing and nanostructures. In terms of applications, XRD excels in characterizing crystalline materials, offering precise lattice parameter determination, phase identification, and single-crystal orientation analysis. It is particularly effective in systems dominated by crystallinity. WAXS, by contrast, is indispensable for studying semi-crystalline, amorphous, or nanostructured materials. Its ability to address both ordered and disordered regions has made it a key tool in fields like polymer science, where the interplay between these domains is critical. Finally, sample requirements also differentiate the techniques. XRD typically demands highly crystalline samples for accurate measurements, while WAXS accommodates a broader range of sample types, including those with mixed or poorly ordered phases. This flexibility enhances WAXS’s applicability to real-world systems, such as biological samples, composites, and polymers.

One of the most important applications of XRD, particularly in the form of wide-angle X-ray diffraction (WAXD), is in the characterization of sub-nanometer-scale structures. By analyzing the diffraction patterns generated by WAXD, researchers can gain detailed insights into the atomic arrangement and structural integrity of energy storage materials, which are vital for their performance.

The interpretation of WAXD patterns relies on analyzing the position, intensity, and width of the diffraction peaks. The position of the peaks, defined by the diffraction angle (2θ), corresponds to specific crystallographic planes within the material, as governed by Bragg’s Law. This relationship between X-ray wavelength, diffraction angle, and the interplanar spacing allows for the identification of the material’s crystal structure and lattice parameters.

The intensity of diffraction peaks provides further information regarding the preferred orientation of the crystal. If certain crystallographic planes exhibit much higher peak intensities than others, it suggests that those planes are preferentially aligned in a particular direction. For instance, a pronounced peak at the (hkl) plane suggests that the material has a preferred orientation along that plane, which may influence its mechanical and physical properties.

Peak width offers insight into the crystallite size and potential defects within the material. Narrower peaks generally indicate larger crystallite sizes, while broader peaks suggest smaller crystals, often due to the presence of nano-sized particles. The Scherrer equation provides a method for estimating crystallite size from the width of the diffraction peaks. Nanomaterials, for instance, tend to show broader peaks compared to materials with large, well-formed crystals, which exhibit sharp peaks.

Moreover, WAXD can be employed for phase identification and phase transitions, as different phases of a material display unique diffraction patterns. Additionally, any unexpected peaks may signal the presence of impurities or the coexistence of multiple phases within the material. Variations in temperature or external pressure can also induce phase transitions, which are detectable by shifts in the WAXD pattern.

Fig. 2a–c show the results from the XRD analysis and Rietveld refinements, revealing that the unit cell dimensions of the Ni-based layered materials decrease with increasing nickel content [9]. Specifically, the lattice parameters follow the trend: NCM523 (a_hex. = 2.86973 Å, c_hex. = 14.23785 Å) > NCM622 (a_hex. = 2.86924 Å, c_hex. = 14.21737 Å) > NCM721 (a_hex. = 2.86796 Å, c_hex. = 14.1959 Å). This reduction in the c-lattice parameter suggests a corresponding decrease in the height of the Li-ion channel (T_LiO6), potentially hindering Li-ion mobility (Fig. 2d). However, the precise determination of Li-ion channel size requires detailed knowledge of the transition metal and oxygen ion positions within the lattice. In these materials, Li-ions, transition metals (Ni, Co, Mn), and oxygen occupy specific Wyckoff positions in the rhombohedral R3¯m space group, with the z-coordinate of oxygen ions (z_ox.) playing a crucial role in defining the height of the Li-ion channels. Highresolution XRD data, such as those obtained from synchrotron radiation, are necessary to detect subtle differences in these atomic positions and ensure accurate structural modeling.

The height of the transition metal (T_TMO6) and Li-ion layers (T_LiO6) can be derived from the crystal lattice using the following equations: [9–12]

Calculations of the T_LiO6, based on the Wyckoff position of oxygen ions, show an opposite trend for the c_hex.- lattice parameters. As the Ni content increases, the T_LiO6 increases: NCM523 (2.540 Å) < NCM622 (2.585 Å) < NCM721 (2.598 Å). This indicates that lattice parameters alone are insufficient to fully describe the Li-ion channel environment, and careful consideration of oxygen ion positioning is essential.

In addition to structural parameters, cation disorder plays a critical role in Li-ion transport. The presence of Ni ions in Li-ion sites, due to their similar ionic size between Li^⁺ (0.76 Å) and Ni²⁺ (0.69 Å), decreases with increasing Ni content: NCM523 (4.71%) > NCM622 (3.29%) > NCM721 (1.81%). Ni ions occupying Li sites disrupt the Li-ion migration pathways, impeding Li-ion movement during battery cycling. As the proportion of Ni ions in the Li layer decreases, the environment becomes more conducive to Li-ion migration, enhancing the material’s electrochemical performance.

The contour plots for in situ XRD patterns (Fig. 2e) reveal a clear phase transition from the H1 phase to the H2 phase during the charging process. In the NCM523 cathode, a distinct two-phase reaction is observed, with the intensity of the H1 phase decreasing and the H2 phase increasing as charging progresses. However, in NCM622 and NCM721, where Ni content is higher, the transition appears more continuous, resembling a pseudo-one phase reaction. This smoother transition suggests that higher Ni content leads to a more uniform phase evolution, reducing structural stress during charging.

The nature of these phase transitions is intrinsically linked to the ordering of Li-vacancies within the crystal structure [13–17]. When the Li-vacancy ordering is similar between the H1 and H2 phases, as seen in NCM721, the transition occurs more smoothly, with fewer structural distortions. In contrast, NCM523, with more pronounced differences between the H1 and H2 lattice parameters, experiences greater strain during the phase transition. This observation is supported by the differences in cation disorder and Li-ion channel size, as shown in Fig. 2d. As Ni content increases, the degree of cation disorder decreases, and the Li-ion channel size increases, both of which favor easier Li-ion migration. This suggests that materials with higher Ni content are more conducive to efficient electrochemical cycling due to their more stable phase transition behavior and enhanced Li-ion mobility. Accordingly, NCM721 demonstrates the best rate performance, achieving a capacity retention of 65.8%, as calculated from the discharge capacity ratio between C/3 and 3C. The capacity retention values follow the trend: NCM523 (55.1%) < NCM622 (63.9%) < NCM721 (65.8%) (Fig. 2e).

SAXS is particularly well-suited for characterizing the structural features of energy storage materials at the nano- to micro-scale. By analyzing the scattering of X-rays at very small angles, typically close to 0°, SAXS provides valuable information on features such as particle size, pore distribution, and morphology. In the context of LIBs, nanoscale features such as particle size and pore distribution within electrode materials significantly influence the electrochemical performance [18–20]. The ability to maintain efficient ion transport during charging and discharging cycles is essential for maximizing battery capacity, rate capability, and cycling stability. SAXS, by focusing on small scattering angles, can capture key structural changes in these materials as they undergo electrochemical processes.

For instance, in LIBs, the morphology of electrode materials plays a key role in determining the ease with which Li-ions diffuse through the active material. A highly porous structure with well-distributed particles can enhance the movement of ions, thereby improving the overall efficiency of the battery. Conversely, poorly distributed or irregularly shaped particles can hinder ion transport, leading to performance degradation. SAXS allows researchers to analyze these features by providing detailed information on particle size distribution and pore architecture within the electrode material.

Furthermore, SAXS can reveal how these nanoscale features evolve during the operation of a battery. Structural changes, such as particle agglomeration, pore collapse, or alterations in morphology, may occur during repeated charging and discharging cycles. These changes can negatively affect ion transport, reduce capacity, and shorten the battery's lifespan. By employing SAXS, researchers can track these changes in real-time, gaining valuable insights into the mechanisms behind performance degradation. Such insights can then be applied to refine material design, optimizing particle size, pore structure, and overall morphology to enhance the long-term stability and performance of LIBs.

The ability of SAXS to probe these structural factors at multiple scales also makes it a valuable complement to other characterization techniques like WAXS, which focus on atomic and sub-nanometer scale structures. Together, these techniques provide a comprehensive understanding of both the atomic-level and nanoscale features that govern the electrochemical performance of energy storage materials. SAXS, in particular, fills the critical gap by offering detailed information about mesoscale structures, which are often challenging to capture with other methods.

Fig. 3a depicts the real-time observation of mesoscopic structural changes in ordered mesoporous energy storage materials throughout the lithiation and delithiation cycles, employing the synchrotron-based Small-Angle X-ray Scattering (SAXS) technique [18]. This technique focuses on scattering angles of 2θ ranging from 0° to 1.2° at a wavelength of 1.54 Å, allowing for direct monitoring of the volumetric alterations in mesoporous materials as they engage with Li-ions. This capability to track structural modifications in situ is crucial for understanding the underlying mechanisms governing electrochemical reactions within these materials. As Li-ions infiltrate the mesopores during lithiation, they induce changes in both pore volume and structure, subsequently affecting the electrochemical characteristics of the material. In contrast, during delithiation, the extraction of Li-ions prompts further structural transformations. The comprehensive data acquired regarding pore dynamics and volume variations throughout these processes provides valuable insights into the Li-ion storage mechanisms inherent to mesoporous structures. By investigating these dynamic changes, researchers can pinpoint critical factors that influence the performance of nano-engineered energy storage materials.

Fig. 3b–d demonstrate how the in situ SAXS technique can be utilized to evaluate the dynamic volume changes in the mesopore structures of meso-Co_xSn_y materials during the processes of lithiation and delithiation[21]. The data reveals that the relative scattering intensity for each material remains relatively unchanged until a discharge potential of 0.20 V is reached, at which point the peak position begins to shift toward lower angles. This behavior indicates that the mesostructures maintain their integrity while the overall cell volume expands. However, as the potential falls below 0.20 V, both the peak intensities and positions undergo significant alterations, signaling a loss of mesostructural periodicity associated with the alloying reaction that forms a Li-Sn phase. Upon delithiation, the return of peak intensities and positions to their initial values suggests that the ordered mesostructures largely persist despite the alloying interactions with Li-ions. This resilience is notable, indicating that these materials are capable of effectively accommodating the volumetric fluctuations that occur during lithiation and delithiation while preserving their structural integrity.

The impact of Co ions on the behavior of the mesostructures is further explored through the analysis of mesoscopic lattice parameters obtained from the (211) d-spacing (Fig. 3e–g). For example, meso-Co_0.5Sn_0.5 shows a significant decrease in peak intensity (~19%) alongside an increase in the (211) d-spacing (~14% volume change) after lithiation. In contrast, meso-Co_0.3Sn_0.7 exhibits a smaller reduction in peak intensity (~13%) but a more pronounced increase in the (211) d-spacing (~41% volume change). Meanwhile, meso-Co_0.1Sn_0.7 experiences a substantial drop in peak intensity (~52%) and a moderate rise in the (211) d-spacing (~30% volume change)[22]. These observations imply that the ratios of Co and Sn are critical determinants of the electrochemical properties and structural robustness of the mesoporous materials.

One important parameter in XRS is the full width at half maximum (FWHM) of XRD peaks, particularly in the wide-angle region. According to Scherrer’s equation [22], the FWHM provides insights into the crystallite size, thereby allowing researchers to assess the structural integrity and potential performance of materials in various applications. In addition to this, SAXS has emerged as a valuable technique for studying mesostructured materials. The broadening of SAXS peaks serves as an effective means to evaluate the size of coherently scattering domains, reflecting the degree of disorder present in the mesostructures[23,24]. This multifaceted approach to analyzing crystallite and domain sizes provides a comprehensive understanding of the material properties that influence their electrochemical behavior.

Fig. 3h presents the FWHM plots for the meso-Co_xSn_y materials as a function of potential during the lithiation and delithiation cycles. Notably, the FWHM values remain relatively stable until the lithiation potential reaches 0.2 V, after which a sharp increase is observed as the potential decreases below this threshold. For instance, the FWHMs for the meso-Co_0.5Sn_0.5 and meso-Co_0.3Sn_0.7 electrodes show changes from 0.071° to 0.078° (indicating a domain size decrease of approximately 10 nm) and from 0.070° to 0.088° (resulting in a 24 nm decrease in domain size), respectively, after complete lithiation. In contrast, the meso-Co_0.1Sn_0.9 electrode experiences a substantial variation in FWHM, increasing from 0.069° to 0.12°, which corresponds to a reduction in domain size from 120 nm to 73 nm—more than double the peak broadening observed in the other materials.

This variation in FWHM suggests that the degree of disorder within the mesoporous structures is closely linked to the amount of electrochemically active Sn present in the meso-Co_xSn_y compositions. The increase in disorder may account for the corresponding decrease in SAXS peak intensities observed during lithiation. Furthermore, all intermetallic electrode materials display narrowing of the SAXS peaks following full delithiation, indicating a return towards a more ordered structure. However, it is noteworthy that the FWHM for the mesoCo_0.1Sn_0.9 electrode remains slightly larger than that of the Li-free mesostructured material (99 nm domain size), suggesting that some disordered characteristics persist even after the lithiation–delithiation process.

The discharge capacities measured for the meso-Co_xSn_y materials underscore the significant role of Co content, with values of 822, 1321, and 1493 mAh g^–1 for meso-Co_0.5Sn_0.5, meso-Co_0.3Sn_0.7, and meso-Co_0.1Sn_0.7, respectively. The superior performance of meso-Co_0.3Sn_0.7 indicates that a Co-to-Sn ratio of 3:7 achieves an optimal balance, enhancing both structural stability and electrochemical capacity. The cycling performance of this material reinforces the idea that Co functions as both a chemical and physical buffer, effectively mitigating the adverse effects of volume changes during electrochemical cycling (Fig. 3i).

XAS is a versatile and powerful tool for probing the local structure and electronic properties of materials [25,26]. By examining how X-rays are absorbed by atoms, XAS can provide valuable insights into the chemical valence states, local atomic geometries, and electronic configurations surrounding specific elements in a material[27–34]. The absorption process occurs when X-ray photons transfer their energy to electrons within an atom, causing the electrons to transition to higher energy levels according to the Fermi golden rule.

XAS spectra are typically divided into two key regions: X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS)[35]. Each region offers unique insights into the material’s characteristics. XANES provides information on the oxidation state and local geometric arrangement of atoms, while EXAFS reveals details about the short-range order and local atomic environment. These features make XAS an invaluable technique for studying complex materials, including energy storage systems, catalysts, and electronic devices.

The XANES region of the XAS spectrum is particularly informative for understanding oxidation states and local geometries. The pre-edge and main edge features in this region arise due to electron transitions between orbitals, which follow selection rules such as Δℓ = ± 1, making dipole transitions (e.g., from 1s to 4p) more probable than quadrupole transitions (e.g., from 1s to 3d) which are typically forbidden by the dipole selection rule. However, under certain conditions, low-probability quadrupole transitions can occur, especially when the centrosymmetry of the absorbing atom is distorted.

For instance, in materials with tetrahedral coordination, the 3d and 4p orbitals mix, leading to an enhancement in pre-edge intensity due to the increased probability of forbidden transitions like 1s to 3d. This change in intensity can be used as a probe to assess the local geometry of the absorbing atom. Similarly, the main-edge transition, typically involving the 1s to 4p transition, provides valuable information about the oxidation state of the absorber. As the oxidation state increases, the binding energy of core electrons also increases, requiring higher photon energies to induce transitions. This trend is governed by the electrostatic model, where atoms with higher positive charges require more energetic X-rays for electron excitation.

In addition to the information provided by XANES, the EXAFS region offers insights into the local atomic structure by analyzing the quantum-mechanical interference of photoelectron waves scattered by neighboring atoms[36]. The interaction between forward-propagating and backscattered photoelectrons leads to constructive or destructive interference patterns, which encode the distances and coordination numbers of atoms surrounding the absorber. Because EXAFS probes short-range ordering, it is particularly useful for studying local environments even in materials lacking long-range crystalline order, such as amorphous solids or disordered nanomaterials.

The ability of synchrotron-based X-ray sources to generate tunable, high-flux beams over a broad energy range further enhances the utility of XAS. By using soft X-rays (typically from tens of electron volts to around 1 keV) and hard X-rays (from 5 keV to tens of keV), researchers can explore the electronic structure and local environments of a wide range of elements, from light to heavy atoms, providing critical insights into both their chemical and physical properties[37].

Thus, synchrotron radiation, with its tunable photon energy, is essential for XAS experiments. By spanning a wide energy range, from soft X-rays to hard X-rays, researchers can explore the local and electronic structures of various elements. Soft X-rays are particularly effective for studying lighter elements, while hard X-rays can penetrate deeper into materials and are ideal for investigating heavier atoms. This broad tunability allows XAS to be applied across diverse fields, including the characterization of catalysts, energy materials, and biological systems, where understanding the local atomic structure is essential for optimizing performance.

Hard XAS (HXAS) is a valuable technique in battery research due to its deep penetration depth, ranging from microns to millimeters. By focusing on the transition metal K-edge, where a 1s electron is excited to unfilled p orbitals, HXAS provides detailed insights into both the electronic and local structure of these materials.

Its ability to probe the transition metal K-edge, coupled with the complementary information provided by XANES and EXAFS, makes it invaluable for studying both electronic structure and local atomic environments. XANES reveals crucial information about the valence states and local symmetry, while EXAFS provides detailed insights into the coordination numbers, bond lengths, and structural ordering of the materials. This combined analysis helps researchers optimize battery materials for improved performance and durability.

The XANES region provides information about the valence state and local site symmetry around the absorbing atom (Fig. 4). The weak 1s to 3d transition, typically forbidden by dipole selection rules, becomes partially allowed due to the mixing of 3d and 4p orbitals, or through direct quadrupole coupling. The intensity of the pre-edge peak is highly sensitive to the local symmetry of the transition metal center. For example, in octahedral coordination, where the centrosymmetric environment is preserved, the intensity of the pre-edge feature is relatively low. In contrast, as the symmetry is distorted, such as in tetrahedral coordination, the pre-edge intensity increases significantly. This relationship makes the pre-edge region a powerful diagnostic tool for assessing the degree of symmetry distortion in a material.

Another important feature of the XAS spectrum is the ligand-to-metal charge transfer (LMCT) transition, specifically the 1s to 4p with LMCT shake-down transition. This transition provides valuable information about the degree of covalency between the metal and ligand atoms, which is an important factor in determining the material's electronic properties. A higher degree of covalency, characterized by more significant mixing between metal 3d and ligand 2p orbitals, leads to a stronger LMCT transition. This feature can be used to assess how strongly bonded the metal is to its surrounding ligands, which has direct implications for material stability and reactivity, particularly in battery electrodes where charge transfer is a critical process.

The direct 1s to 4p transition, another prominent feature in the XAS spectrum, is closely linked to the oxidation state of the transition metal. As the oxidation state of the metal increases, the energy of this transition shifts to higher values. This shift occurs because higher oxidation states result in a greater positive charge on the metal atom, requiring more energy to excite the 1s electron into the higher 4p orbital. This makes the 1s to 4p transition a useful tool for tracking oxidation state changes during electrochemical processes such as charging and discharging batteries.

Beyond the absorption edge, EXAFS provides detailed structural information, including bond lengths, coordination numbers, and disorder factors in the vicinity of the absorbing atom. This is critical for understanding the short- and long-range structural ordering in electrode materials. During battery operation, structural changes such as lattice expansion, contraction, or phase transitions can impact the stability and cycling life of the material. EXAFS helps quantify these changes by revealing how the coordination environment of the transition metal evolves as Li-ions are inserted or extracted. For instance, in layered transition metal oxides, EXAFS can track the subtle distortions in bond lengths and angles that occur during Li intercalation, which may affect the material’s capacity and degradation behavior over multiple charge-discharge cycles.

By combining XANES and EXAFS data, researchers can build a comprehensive picture of both the electronic states and atomic arrangements within the material, enabling more informed decisions about how to improve material design for higher energy density, better stability, and longer battery lifespans.

The voltage profile of Li_6–xCoO₄ during Li-ion extraction (Fig. 5a) shows distinct regions that correspond to different state-of-charges. By correlating these regions with Co K-edge XANES spectra (Fig. 5b), we observe two main regions of interest. In Region I (x = 0–2), as Li-ion is extracted, the A transition peak in the XANES spectra shifts to higher energy, and the intensities of the A and B transition peaks decrease (Fig. 5c). This suggests an increase in the oxidation state of Co, approaching Co⁴⁺, along with a reduction in the tetrahedral symmetry of the Co–O bonds. The observed shift in the C transition edge further supports the oxidation of cobalt in this region. These changes indicate that Co undergoes significant electronic structure changes as it donates electrons during delithiation.

In Region II (x = 2–5), the spectral features of the A and B transitions remain largely unchanged, yet a shift in the main edge position of C to lower energy is noted. This is an unusual observation, implying that the average oxidation state of Co decreases, which is unexpected during further delithiation. This suggests the involvement of oxygen in the charge compensation mechanism, likely through an anionic redox reaction. The evolution of oxygen (O₂) from the structure could explain the observed capacity exceeding that expected from the Co redox reactions alone (Co²⁺ to Co⁴⁺). This highlights the contribution of oxygen anions to the overall redox process in Li_6–xCoO₄, which may play a crucial role in enhancing the material’s energy storage capacity.

Structural insights from EXAFS analysis further complement these observations. The increase in the firstshell Co–O peak intensity during Li-ion extraction (Fig. 5d) indicates a transformation in the local environment of cobalt. EXAFS fitting (Fig. 5e) reveals a shift from tetrahedral to octahedral coordination as Li-ion is removed. This structural reorganization, coupled with the appearance of a new peak around 2.5 Å in the FT magnitude spectra, suggests the formation of a new Co–Co structure with a reduced interatomic distance, indicative of phase transitions. Furthermore, the emergence of LiCoO₂-like and Co₃O₄-like phases in the 3–6 Å radial distance range after extensive delithiation (x > 2) reflects the complex phase evolution in Li_6–xCoO₄.

Soft XAS (SXAS) is a powerful technique that utilizes soft X-rays with energies between 100 and 3000 eV. It is highly effective for investigating the electronic structure of light elements such as boron (B), carbon (C), nitrogen (N), oxygen (O), and fluorine (F), as well as the L-edges of first-row transition metals (Fig. 6)[27,40–43]. SXAS stands out for its ability to analyze the surface and near-surface regions of materials, with penetration depths typically ranging from tens to hundreds of nanometers. This makes it ideal for examining surface-specific phenomena that bulk-sensitive methods, like HXAS, cannot access. In particular, SXAS proves essential in battery research, where understanding surface and interfacial behavior is critical for optimizing performance, ensuring cycling stability, and mitigating degradation.

SXAS is especially effective in analyzing the transition metal L-edge spectra, which involves the excitation of 2p electrons to unfilled d orbitals. This transition provides detailed information about the electronic states and bonding characteristics of transition metals, such as nickel (Ni), cobalt (Co), and manganese (Mn), which are commonly used in LIB cathodes. Additionally, the K-edge spectra of light elements, such as oxygen, reveal critical insights into the role of anionic redox activity. The growing interest in oxygen redox chemistry, for example, has made SXAS an indispensable tool for understanding how oxygen contributes to charge compensation during battery operation. These insights are essential for designing materials with enhanced energy density and stability.

The versatility of SXAS is further enhanced by the availability of multiple detection modes, each offering different sensitivities and probing depths. Total electron yield (TEY) mode is highly surface-sensitive, providing information about the electronic structure of materials at depths of approximately 10 nm[44]. This mode is particularly useful for studying surface reactions, such as solid-electrolyte interphase (SEI) formation, which can significantly impact battery performance. Fluorescence yield (FY) mode, on the other hand, provides a deeper probe into the material, detecting signals from depths greater than 100 nm[45–48]. This allows for the study of both surface and near-surface processes, making it ideal for investigating bulk and interfacial regions. Partial electron yield (PEY) mode offers even greater surface sensitivity than TEY, while partial fluorescence yield (PFY) mode restricts the energy range of emitted photons, providing bulk information with high specificity.

SXAS can provide a comprehensive understanding of the electronic structure across different material regions by combining data from these detection modes. For example, TEY mode can be used to monitor surface degradation processes, while FY or PFY modes can offer insights into the behavior of bulk materials. This multidimensional analysis is essential for optimizing the design of battery materials, as both surface and bulk properties are critical to performance. SXAS enables researchers to trace changes in oxidation states, coordination environments, and bonding interactions, allowing for the optimization of energy storage materials.

The O K-edge XAS spectra of Li_1–xNCA cathode material (Fig. 7a) reveal two distinct peaks: one below 534 eV, representing transitions of O 1s electrons to the hybridized TM 3d-O 2p orbitals, and another above 535 eV, corresponding to transitions into the TM 4sp-O 2p hybridized state [49]. The intensity of the peak below 534 eV reflects the hole state of the TM 3d-O 2p hybrid, which complicates the interpretation of oxygen redox activity because the transition metal's 3d band also changes during charging. This makes it challenging to isolate oxygen’s contribution to the redox chemistry based solely on this peak. However, the peak above 535 eV, associated with the TM 4sp-O 2p hybridized state, offers clearer insight into the role of oxygen, as it indicates the presence of holes in the oxygen 2p state, given the absence of electrons in the TM 4sp orbitals.

During the initial charging phase (Region I, x ≤ 0.6 in Li_1–xNCA), the peak intensity for the TM 3d-O 2p hybridized state shows a significant increase, indicating that transition metal ions serve as the primary redox centers. This suggests that the generated capacity in this region is largely driven by the oxidation of transition metals. The TM 4sp-O 2p intensity also increases, but less dramatically, implying limited involvement of oxygen in the redox process during the early stages of charging.

In Region II (x > 0.6), a notable shift occurs. The TM 4sp-O 2p hybridized state intensity exhibits a dramatic increase, indicating the formation of oxygen holes, while the TM 3d-O 2p intensity begins to slow down its upward trend. This shift suggests a transition in the redox mechanism, where oxygen begins to participate more significantly in the charge compensation process. The noticeable increase in oxygen hole states implies that oxygen redox reactions are contributing to the overall capacity. This shift from transition metal-driven redox reactions to oxygen-based redox activity is a crucial factor in extending the charge capacity of Li_1−xNCA beyond the conventional limits of transition metal oxidation. However, this change in the electronic structure at the atomic level also affects the material's crystal structure, as excessive oxygen hole formation can lead to structural instability and degradation, which can impact long-term battery performance.

Scanning transmission X-ray microscopy (STXM) and near-edge X-ray absorption fine structure (NEXAFS) analysis are powerful techniques for visualizing the chemical and structural changes at the nanoscale. As shown in Fig. 7b,c, STXM images of the pristine and cycled NCM523 electrodes reveal significant differences in the chemical and structural state of the material[50]. These images, captured from a cross-section of the electrode particles, show a comprehensive view of the oxidation states of Ni, Co, and Mn, providing insights into the degradation mechanisms during long-term cycling. The extracted L-edge spectra of Ni, Co, and Mn from the integrated regions of the STXM images offer detailed information about the local chemical environment.

The L₃-edge spectra of both the pristine and cycled NCM523 samples for Co show a main peak at ~781 eV and a weaker shoulder at ~783 eV, which are characteristic of Co³⁺ in an octahedral crystal field[52–54]. Notably, the Co oxidation state remains unchanged after cycling, indicating the electrochemical stability of Co ions in NCM523. Similarly, the Mn L₃-edge spectra display a peak at ~644 eV, consistent with Mn4+ in the pristine sample [55], and no significant change in the Mn oxidation state is observed after long-term cycling. This suggests that the Mn ions maintain their oxidation state and play a stabilizing role in the material’s structure.

The most significant changes occur in the Ni L₃-edge spectra, where two main peaks at ~853 eV and ~855 eV correspond to Ni²⁺ and Ni⁴⁺, respectively. In the pristine NCM523 sample, the ratio of these peaks suggests a balanced distribution of Ni²⁺ and Ni⁴⁺, reflecting the initial oxidation state of Ni ions[51,56]. However, after long-term cycling, two distinct regions with different Ni oxidation states are observed in the cycled particles. In the areas near microcracks and grain boundaries (green regions), the Ni²⁺ peak intensity increases, indicating the reduction of Ni ions. This reduction leads to the formation of a NiO-like rock-salt phase, which has been associated with performance degradation in NCM cathodes. The formation of microcracks during cycling likely accelerates this phase transformation by facilitating localized reductions of Ni, particularly at the grain boundaries and cracks, contributing to a loss of capacity and increased impedance. Interestingly, the STXM images reveal that the spatial distribution of different oxidation states is not aligned with the grain boundaries of the primary particles, suggesting that other factors, such as variations in Li-ion concentration, might influence the local chemistry. This highlights the complex nature of chemical and structural heterogeneity in NCM materials during cycling.

Another example is to observe thermal decomposition reactions by separating the material from the surface and bulk viewpoints respectively. The normalized Ni L-edge spectra of Li_0.33Ni_0.8Co_0.15Al_0.05O₂ (Li_0.33NCA), measured using both PEY and FY modes at various temperatures, provide a detailed view of the thermal behavior of Ni in the material (Fig. 7d,e) [51]. The bulk-sensitive FY mode spectra (Fig. 7e) show no significant shift in the energy position of the Ni L₃-and L₂-edges across the temperature range between 25°C to 300°C, indicating that the bulk of the Li_0.33NCA material remains chemically stable during heating. Thus, no significant thermal degradation is detected in the bulk, suggesting that the material retains its integrity in the core. In contrast, the surface-sensitive PEY mode spectra (Fig. 7d) reveal a noticeable shift in the energy position of the Ni L₃- and L₂-edges to lower values, particularly around 200°C. This shift suggests a reduction in the Ni oxidation state at the surface, leading to the formation of a NiO-like rock-salt phase. The formation of this rock-salt structure is a well-documented phenomenon in Ni-rich cathodes, where surface degradation typically precedes bulk degradation. This surface layer is associated with the loss of active Li-ion and can contribute to increased resistance and reduced battery performance over time. The shift in the PEY spectra provides direct evidence of the onset of thermal degradation at the surface of the material, even while the bulk remains relatively unaffected.

X-ray-based techniques have become indispensable tools in material science, providing a wealth of structural, chemical, and electronic information across different length scales. These techniques allow researchers to investigate crystal structures, particle sizes, chemical states, and bonding environments, which are critical for understanding the behavior and performance of materials, particularly in advanced applications like energy storage, catalysis, and nanotechnology. Two primary categories of X-ray techniques include XRS and XAS, each offering unique insights into the material properties (Fig. 8).

The technique of WAXS (2θ > 5°) focuses on identifying crystal structures by analyzing the position of diffraction peaks. The peak position is directly related to the lattice parameters, allowing researchers to determine the unit cell dimensions and symmetry. The peak profile (line shape) can be analyzed to estimate the crystallite size and microstrain in the material. Broader peaks generally indicate smaller crystallites or higher strain. The peak intensity provides information about the atomic positions, site occupancy, and thermal factors (B), which describe the atomic vibrations. Additionally, the relative intensity of the peaks can be used to calculate phase fractions in multiphase systems.

In the case of SAXS (2θ < 5°), it offers insight into micro- and mesoscopic structures, particularly for nanomaterials. In this low-angle region, SAXS can determine (nano)particle size distribution, particle shapes, porosity, and pore size. These parameters are essential for materials used in applications such as catalysis and energy storage, where surface area and pore architecture play crucial roles. Additionally, it provides information on characteristic distances, which describes the average spacing between particles or structural units within the material.

The technique of HXAS (> 5000 eV) allows researchers to probe deeper into materials, revealing information about electronic structure. XANES, which occurs near the absorption edge, is sensitive to the site symmetry of atoms and their valence states, making it particularly useful for studying oxidation states and electronic configurations. EXAFS, which extends beyond the XANES region, provides information on the coordination number, bond lengths, and bond angles, offering a quantitative understanding of the local structural environment around the absorbing atom. EXAFS can also assess the degree of disorder in the atomic arrangement by analyzing the disordering factor.

In the case of SXAS (150–5000 eV), it is a powerful technique for studying surface or near-surface and bulk phenomena, which are critical in fields like catalysis, energy storage, and thin-film technologies. SXAS can probe the electronic structure, including the energy levels of inner shells, revealing information about the oxidation states and redox processes occurring at the surface. This technique also provides insight into the nature of chemical bonding (covalent or ionic) and the coordination structure of atoms at the surface. Importantly, SXAS offers surface-sensitive analysis of interfacial reactions and structural transformations, which are vital for understanding processes such as interphase formation in batteries.

In summary, synchrotron-based X-ray techniques are transformative tools in the study of energy storage materials, offering unparalleled precision and flexibility for probing structural and electronic properties. These techniques enable in-depth investigation of critical phenomena such as ion diffusion, charge transfer, and phase transitions, providing real-time insights into material behavior under operational conditions. Key methods like XRS and XAS play complementary roles, with XRS focusing on structural aspects (e.g., crystal structure and phase transitions) and XAS uncovering electronic dynamics (e.g., oxidation state and redox processes). The combination of these approaches offers a multidimensional understanding of the interplay between structural and electronic changes, which is essential for optimizing battery performance, safety, and longevity. Thus, the combination of XRS and XAS techniques provides a holistic view of material properties, from crystallographic parameters to electronic structures. These methods are crucial for understanding the fundamental properties of materials, especially in applications where both bulk and surface characteristics play a pivotal role, such as energy storage materials. As material science continues to evolve, the use of X-ray techniques will remain indispensable for developing advanced materials with tailored properties for specific applications. Accordingly, these techniques will continue to drive innovation in energy storage, guiding the development of next-generation batteries. Their ability to analyze complex material systems and uncover mechanisms of degradation and phase evolution positions them as indispensable tools for addressing the challenges in modern energy technologies. This integration of real-time insights with practical applications ensures their central role in the future of battery research and development.