Article

Cryogenic XPS Reveals the Pristine SEI in Lithium Batteries

Surface Analysis Spotlight: XPS

	by Sanchita Chakrabarty Technical Sales

Key Takeaways

• Cryogenic XPS preserves the pristine SEI
• Room-temperature XPS can misrepresent SEI composition
• Cryo-XPS enables robust performance correlations 
• Cryo-XPS reveals SEI heterogeneity and minimizes beam damage Ultra-thin film (< 10 nm) metrology with greater accuracy

Accurate characterization of the solid–electrolyte interphase (SEI) is critical for understanding and optimizing lithium battery performance. While X‑ray photoelectron spectroscopy (XPS) is widely used for SEI analysis due to its chemical specificity and nanoscale depth sensitivity, conventional room‑temperature XPS (RT‑XPS) alters SEI composition through reactions and volatilization under ultra‑high vacuum (UHV) conditions, leading to misrepresentation of the native interface.

This work, performed at Stanford University and published in Nature, demonstrates that cryogenic XPS (cryo‑XPS)—in which samples are plunge‑frozen and analyzed at approximately −110 °C—preserves the pristine SEI by halting chemical reactions and retaining volatile species. Using a PHI VersaProbe III and 4, the researchers investigated time‑dependent compositional changes, formation of reaction products, UHV exposure effects, and X‑ray beam damage by comparing cryo XPS and RT-XPS measurements.

Time‑resolved measurements, seen in Figure 1, show that SEI composition remains stable under cryogenic conditions, whereas RT‑XPS induces irreversible evolution, including increased LiF formation and loss of other components. Comparison of cryo‑XPS and RT‑XPS for LHCE SEI show that room‑temperature analysis induces chemical changes , leading to overestimation of LiF and loss of Li₂O due to reactions and volatilization during heating and UHV exposure. Cryo‑XPS preserves the native SEI composition, revealing Li₂O and performance‑relevant trends that RT‑XPS fails to capture.

Fig.1: Time-dependent SEI preservation and evolution. High resolution spectra for F 1s, O 1s, and N 1s under cryo and cryo heated to RT conditions.  Times shown are at 0 min and at 60 min.

Cryo‑XPS reveals a thicker (>4.2 nm), more chemically heterogeneous SEI, consistent with cryo‑TEM results, while RT‑XPS shows SEI thinning and exposure of metallic lithium due to volatilization and reactions. Importantly, cryo‑XPS detects species such as Li₂O that disappear during RT analysis despite their low volatility, indicating reaction‑driven changes rather than physical loss.

Preservation of the native SEI enables robust, performance‑relevant correlations between interphase chemistry and battery metrics. Salt‑ and additive‑derived compositional ratios measured by cryo‑XPS show strong correlations with coulombic efficiency (Spearman ρ ≈ 0.9), whereas RT‑XPS yields weaker or misleading trends due to SEI evolution during analysis. X‑ray beam effects were found to be minimal compared to reaction and UHV‑driven changes.

Overall, this study establishes cryo‑XPS as a necessary tool for reliable SEI characterization, revealing true interfacial chemistry and heterogeneity that are obscured by conventional RT‑XPS. These insights enable more accurate interpretation of structure–performance relationships and provide a foundation for rational design of advanced lithium metal batteries.