Spectroscopic techniques provide critical insights into the atomic-scale properties of sulfide solid electrolytes, which are essential for understanding their ionic conductivity, structural stability, and degradation mechanisms. Among the most powerful methods are nuclear magnetic resonance (NMR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and extended X-ray absorption fine structure (EXAFS). These techniques reveal details about local coordination environments, ion mobility, and chemical changes during cycling, enabling the rational design of high-performance solid-state batteries.

Nuclear magnetic resonance spectroscopy is particularly valuable for probing lithium or sodium ion dynamics in sulfide electrolytes. NMR measures the interaction between nuclear spins and their local environments, providing information about ion hopping rates and activation energies. For lithium-containing sulfides such as Li7P3S11 or Li10GeP2S12, lithium-7 NMR can distinguish between different crystallographic sites and quantify ion mobility through spin-lattice relaxation (T1) and spin-spin relaxation (T2) measurements. The temperature dependence of relaxation rates reveals ion transport mechanisms, with typical activation energies ranging from 0.2 to 0.4 eV for high-conductivity phases. NMR also detects amorphous domains in partially crystallized electrolytes, where broader peaks indicate disordered lithium environments. For sodium analogs like Na3PS4, sodium-23 NMR similarly resolves mobile versus immobile species, aiding in the optimization of composition and processing conditions.

X-ray photoelectron spectroscopy offers direct chemical state analysis of sulfur, phosphorus, and transition metal elements in sulfide electrolytes. The binding energy shifts in core-level spectra reflect changes in oxidation states and local bonding configurations. For instance, the S 2p spectrum often shows doublet peaks corresponding to bridging (S-S) and terminal (S-M, where M is P, Ge, or Sn) sulfur environments. The relative intensities of these peaks indicate the degree of polymerization in thiophosphate-based systems. XPS also identifies decomposition products such as Li2S or polysulfides that form at electrode-electrolyte interfaces. Depth profiling with argon sputtering can further map chemical gradients near surfaces, revealing reactions with moisture or electrode materials. In argyrodite-type electrolytes like Li6PS5Cl, XPS confirms halogen incorporation and its influence on interfacial stability.

Extended X-ray absorption fine structure spectroscopy provides atomic-scale structural details beyond the range of diffraction techniques. EXAFS analyzes the oscillatory fine structure above an element's absorption edge, yielding precise interatomic distances and coordination numbers. For sulfide electrolytes, sulfur K-edge or phosphorus K-edge measurements reveal the local environment around these anions. In crystalline Li7P3S11, EXAFS resolves distinct P-S and Li-S distances, while in glass-ceramic variants, it quantifies the degree of disorder. Transition metal K-edge EXAFS is equally important for doped systems, such as Sn-substituted Li10SnP2S12, where it confirms the substitution site and its impact on lattice parameters. The technique also detects metastable intermediates during synthesis, such as the progressive formation of PS4 tetrahedra in mechanochemically prepared samples.

Combining these techniques provides a comprehensive understanding of structure-property relationships in sulfide electrolytes. NMR captures dynamic processes, XPS tracks chemical changes, and EXAFS refines local atomic arrangements. For example, in Li6PS5Br, NMR shows enhanced ion mobility with bromine doping, XPS verifies bromide incorporation without sulfur oxidation, and EXAFS confirms minimal distortion of the PS4 tetrahedra. Such multimodal analysis is crucial for optimizing ionic conductivity, which often exceeds 10 mS/cm in the best-performing sulfides.

Decomposition pathways are another critical area where spectroscopy excels. During cycling, sulfide electrolytes may react with lithium metal anodes or oxide cathodes, forming interphases that impede ion transport. NMR detects the emergence of new lithium environments, such as Li2S or Li3P, while XPS identifies redox products like polysulfides or reduced phosphorus species. EXAFS tracks the evolution of coordination environments, such as the breakdown of PS4 units into shorter P-S bonds. These insights guide interface engineering strategies, such as applying protective coatings or adjusting stack pressure to mitigate degradation.

Operando spectroscopic studies further enhance understanding by capturing real-time changes during battery operation. High-temperature NMR probes ion mobility under operating conditions, while time-resolved XPS or EXAFS tracks phase evolution during charge-discharge cycles. Such experiments reveal transient intermediates and kinetic barriers that influence performance. For instance, operando XPS has demonstrated the progressive sulfidation of lithium metal interfaces, while EXAFS has shown reversible structural changes in sulfide hosts during ion extraction.

Despite their strengths, each technique has limitations. NMR requires isotopes with favorable nuclear properties, XPS is surface-sensitive and may require ultra-high vacuum conditions, and EXAFS demands synchrotron radiation for high-quality data. Careful sample preparation is essential to avoid artifacts, such as air exposure for moisture-sensitive sulfides. Data interpretation often relies on complementary simulations, such as density functional theory calculations to correlate spectral features with atomic models.

Recent advances continue to expand spectroscopic capabilities. High-field NMR improves resolution for quadrupolar nuclei like lithium-7, while ambient-pressure XPS enables studies under more realistic conditions. Quick-EXAFS setups at synchrotrons allow faster time resolution for kinetic studies. These developments promise deeper insights into sulfide electrolyte behavior, from synthesis to cycling and aging.

In summary, NMR, XPS, and EXAFS are indispensable tools for characterizing sulfide solid electrolytes. They reveal the atomic-scale details governing ion transport, structural stability, and interfacial reactions, providing a foundation for advancing solid-state battery technology. By correlating spectroscopic data with electrochemical performance, researchers can design more conductive, stable, and compatible electrolytes for next-generation energy storage systems.