Li-ion battery is used in fields such as transportation, electronics, and grid-scale energy storage. However, some challenges remain in achieving higher energy density, enhanced safety, and lower cost for large-scale energy storage and electric vehicles applications. To address these issues, a comprehensive understanding on the structural evolution of electrode materials, the stability of the electrolyte, and the reaction mechanism at the electrode interface is required. Nevertheless, conventional characterizations are limited by brightness, spectral range, and spatial/temporal coherence of light source, thus being unable to resolve the reaction details and structure/micro-structure evolution during the charge/discharge. Synchrotron radiation is an electromagnetic radiation emitted tangentially by relativistic electrons deflecting in a magnetic field. Compared with conventional light sources, synchrotron radiation shows high brightness, highly collimated, wide spectral range, etc.. These superior characters make it a powerful tool for battery study. For instance, synchrotron radiation XRD offers a superior lattice resolution as small as 0.10% or even 0.01%, thereby showing a subtle lattice collapse after lithium de-intercalation. Moreover, synchrotron radiation extended X-ray absorption fine structure (EXAFS) can determine the chemical valence of a selected element and the coordination structure of the element. Furthermore, the EXAFS can be combined with microscopy/imaging to directly provide the spatial distribution of oxidation state and coordination environments of a selected element within electrode materials. This review represents some challenges of Li-ion battery that can be strategically addressed by synchrotron radiation characterizations. In addition, the related progress in this field through a few case studies are described.

Synchrotron Radiation X-ray Diffraction: Compared with conventional X-ray diffraction, synchrotron radiation XRD can accurately capture the subtle lattice changes, phase transformation, and structure/micro-structure evolution during the charge/discharge. For instance, in polycrystalline and single-crystalline NCM523 materials, synchrotron radiation XRD reveals the~0.2% sudden changes in lattice constants and clarifies the significant differences in the phase transformation paths between different crystalline states. Synchrotron radiation XRD can also track the reversible structure evolution from trigonal to cubic to tetragonal during the charge/discharge of the sodium-ion battery cathode material Na_2-xFe[Fe(CN)₆ · nH₂O, and confirm its excellent structural reversibility and cycling stability.

Synchrotron Radiation Extended X-ray Absorption Fine Structure: The EXAFS can provide the coordination information around a specific element, which is crucial for in-depth research on the structure changes of electrode materials. For instance, regarding the Jahn-Teller distortion of Ni in high-nickel layered cathode materials LiNi_0.9Mn_0.05Co_0.05O₂, the dynamic evolutions of Ni-O coordination environment during the battery cycling can be in-situ monitored. The results demonstrate that the modified coating layer can effectively buffer the Jahn-Teller distortion of Ni³⁺, thereby significantly reducing the lattice distortion and enhancing the structural stability of the high-nickel layered materials. For the issues of Mn dissolution and electrolyte decomposition in the LiNi_0.5Mn_1.5O₄ cathode, the EXAFS characterization confirms that Fe³⁺ in the additive can dynamically and uniformly dope the LNMO cathode, therefore significantly reducing the lattice damage caused by Mn dissolution and improving the cycling stability of the electrode material.

Synchrotron Radiation X-ray Imaging: Synchrotron radiation X-ray computed tomography (XCT) is a high-resolution, non-destructive, three-dimensional characterization technique. Combined with the wavelength tunability of synchrotron radiation, the XCT can dynamically image the internal structural evolution (such as crack propagation, phase distribution) inside the electrode materials and valence and coordination environment distribution through combination with the EXAFS. For instance, in a Li-rich NMC cathode grain, the XCT reveals the gradient distribution of Mn valence and its evolution during the charging/discharging, providing a key basis for understanding the reaction kinetics.

Synchrotron Radiation Infrared Spectroscopy: Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), as an interface-sensitive technique, can be effectively used to characterize the composition and structure evolution inside the SEI between the electrode and electrolyte. However, as the main vibrational modes of Li salts are located in the low-energy region while conventional Globar gives only a weak output in the corresponding mid-infrared range, high bright synchrotron radiation infrared source is expected to play an important role in the study of SEI. For instance, there is a significant difference between the mid-infrared spectra of LiF thin film obtained from ATR-FTIR and scanning infrared near-field spectroscopy (which should be a surface sensitive technique), indicating that the distribution and coordination environment inside the film is nonuniform. As the gradient is closely related to the reaction/diffusion dynamics, a comprehensive characterization of the distribution of various components eventually bring us a deeper understanding of the formation and dissolution of the Li inorganics in the SEI film.