X-ray photoelectron spectroscopy (XPS) is an indispensable tool for probing the surface chemistry of energy materials, particularly in battery electrodes, solid electrolytes, and catalytic systems. Its ability to provide quantitative elemental composition and chemical state information with high surface sensitivity makes it uniquely suited for studying interfacial phenomena, degradation mechanisms, and reaction pathways critical to energy storage and conversion technologies.

In lithium-ion batteries, XPS plays a pivotal role in characterizing electrode-electrolyte interfaces, particularly the solid-electrolyte interphase (SEI). The SEI forms due to electrolyte decomposition during initial cycling and significantly influences battery performance and longevity. XPS identifies the chemical composition of SEI layers, typically composed of inorganic species like LiF, Li2CO3, and Li2O alongside organic compounds such as polycarbonates and alkoxides. By analyzing core-level shifts in binding energies, XPS distinguishes between these components and tracks their evolution during cycling. For instance, the Li 1s peak around 55-56 eV indicates Li2O, while peaks near 56.5-57.5 eV correspond to Li2CO3. Similarly, the F 1s peak at 685 eV signifies LiF, a common SEI component.

Degradation mechanisms in battery electrodes are also elucidated through XPS. Transition metal dissolution in cathodes, such as Mn in LiMn2O4 or Ni in NMC materials, can be detected via changes in the Mn 2p or Ni 2p spectra. Oxidation state shifts in these metals, such as Mn2+ to Mn4+, are discernible through peak deconvolution. In silicon or graphite anodes, XPS reveals the formation of irreversible Li-containing compounds and electrolyte decomposition products that contribute to capacity fade. The C 1s spectrum, for example, differentiates between conductive carbon (284.5 eV), C-O bonds (286-287 eV), and carbonate species (289-290 eV), providing insights into parasitic reactions.

Solid-state batteries benefit from XPS analysis of interfacial stability between solid electrolytes and electrodes. Sulfide-based electrolytes like Li6PS5Cl or oxide-based ones such as LLZO often react with electrode materials, forming resistive interphases. XPS identifies these reaction products, such as Li2S or P2Sx species in sulfide systems, which degrade ionic conductivity. The S 2p spectrum is particularly informative, with peaks at 161-162 eV corresponding to sulfide ions (S2-) and higher binding energies indicating oxidized sulfur species. Similarly, oxide electrolytes exhibit shifts in O 1s spectra due to lattice oxygen (529-530 eV) versus surface hydroxides or carbonates (531-533 eV).

Catalytic energy materials, including those for fuel cells and water splitting, rely on XPS to monitor active site oxidation states and surface adsorbates. In oxygen reduction catalysts like Pt-Co alloys, XPS tracks the Co 2p and Pt 4f states to assess electronic structure modifications that enhance activity. For transition metal oxides used in oxygen evolution reactions (OER), such as NiFeOx, the Ni 2p and Fe 2p spectra reveal mixed oxidation states (e.g., Ni2+/Ni3+ and Fe3+/Fe4+) critical for catalytic performance. The O 1s region further distinguishes lattice oxygen (O2-) from hydroxyl (OH-) and adsorbed water, providing clues about reaction mechanisms.

Despite its strengths, XPS faces challenges in analyzing air-sensitive energy materials. Many battery and catalytic materials react with moisture or oxygen, necessitating inert handling. Glovebox-integrated XPS systems address this by enabling sample transfer without air exposure. These systems maintain ultra-high vacuum conditions while allowing electrodes or catalysts to be transferred directly from synthesis or electrochemical testing environments. For example, studies on lithium metal anodes require such setups to prevent artifact formation from ambient reactions.

Another challenge is spectral interpretation due to overlapping peaks and complex backgrounds. Advanced fitting techniques, including Shirley or Tougaard background subtraction and peak deconvolution using reference spectra, are essential. Charge compensation in insulating samples, such as solid electrolytes or oxide catalysts, can also distort spectra, requiring low-energy electron flood guns or metallic coatings to stabilize the surface potential.

Recent advancements in operando XPS enable real-time monitoring of electrochemical processes. By integrating electrochemical cells with XPS systems, researchers observe dynamic changes in oxidation states and interfacial chemistry during cycling or catalysis. For instance, operando studies of sulfur cathodes in Li-S batteries capture polysulfide evolution through S 2p spectra, revealing intermediate species that contribute to shuttle effects.

In summary, XPS provides unparalleled insights into the surface and interfacial chemistry of energy materials. Its ability to quantify elemental composition, oxidation states, and degradation products is critical for optimizing battery electrodes, solid electrolytes, and catalytic systems. While challenges like air sensitivity and spectral complexity persist, advancements in glovebox integration and operando techniques continue to expand its capabilities in energy research.