Article

Chemical Imaging of Cathode−Electrolyte Interphase (CEI) in Sulfide Solid-State Batteries

Surface Analysis Spotlight: XPS

	by Cesar Saucedo Staff Scientist

Key Takeaways

Interfacial degradation: Sulfide electrolyte decomposition occurs primarily at the NMC electrolyte interface, while the bulk electrolyte remains largely intact. This indicates that degradation is localized and driven by interfacial interactions rather than uniform processes.

Oxidation reactions: High voltage alone is insufficient to degrade the sulfide electrolyte, proximity to the NMC cathode is required due to its strong oxidizing nature and possible oxygen release, which can convert S^2- into S⁰ and sulfur oxides.

Accurate characterization of interfacial degradation processes is essential for understanding performance and stability in sulfide‑based batteries. While bulk‑averaged measurements provide valuable compositional insight, they lack the spatial and chemical‑state resolution needed to resolve degradation phenomena occurring at the cathode–electrolyte interface (CEI), a region that is critical to understand for optimized battery performance.

In this work, published in ACS Energy Letters (2026), a team from the University of Houston and Nissan ARC led by Lihong Zhao employs X-ray photoelectron spectroscopy (XPS) and scanning transmission X-ray microscopy (STXM) to map the chemistry across the NMC–sulfide interface in a solid-state cell.¹ The main objective of this work is to directly map, with chemical‑state sensitivity, the evolution and spatial distribution of degradation chemistry at the NMC–argyrodite interface, enabling nanoscale view CEI formation and growth.

When the cell was charged to high voltage, the STXM data shows significant oxidation of the sulfide electrolyte at the boundary with the cathode particles. In these zones, the sulfur L-edge spectra shifted to higher energy, revealing the presence of oxidized sulfur species, specifically elemental sulfur (S⁰) and sulfur oxoanions (sulfites/sulfates). Meanwhile, the interior of electrolyte particles, including those in contact with conductive carbon but not touching NMC, showed little change, retaining the signature of unoxidized S²⁻. This suggests that high potential alone isn’t enough to break down the sulfide; the proximity of the NMC cathode is crucial to trigger the side reactions. The scientists propose that during charging, the cathode’s oxidizing power and possibly oxygen release from the charged NMC surface drives nearby electrolyte anions to oxidize.

Figure 1: XPS spectra of NMC-argyrodite interphase

XPS offers a complementary, quantitative view of the chemistry with a higher degree of surface sensitivity. XPS analysis was performed on the cycled cathodes using a PHI VersaProbe III instrument. Figure 1 is from the paper showing the XPS spectra of NMC-argyrodite interphase.

The XPS results agreed with the microscopic maps: after 100 cycles at high voltage, the electrolyte’s sulfur 2p peaks showed a decrease in the fraction of S²⁻ and a corresponding increase in S⁰ content. A small amount of sulfate species was detected but remained minor. Similarly, XPS detected a growth of phosphorus–sulfur oxide signals (P₂Sₓ and POₓ species) in the interfacial region with cycling. This suggested that sulfide electrolyte near the cathode is being partially oxidized to elemental sulfur and complex phosphates, consuming part of the electrolyte in the process. The absence of those changes in the control experiment further support that NMC is indispensable in catalyzing the rich interphase chemistry observed.

Overall, this study provides an understanding of the cathode–electrolyte interphase in sulfide solid‑state batteries, an area of high relevance for next‑generation energy storage. By demonstrating that electrolyte decomposition is localized at the cathode–electrolyte contact, while the bulk electrolyte remains largely intact, the work resolves a long‑standing question in the field. These findings show that CEI formation arises from the reactive synergy between high‑voltage cathodes and sulfide electrolytes, rather than from independent electrolyte breakdown driven solely by high electrochemical potential.

1. Lihong Zhao, Qiuyi Yuan, Wen Ren, Toshihiro Asada, Chaoshan Wu, Takanori Itoh, Suusaku Ogiu, Takashi Sanada, Shohei Yamashita, Daisuke Shibata, and Yan Yao. ACS Energy Letters 2026 11 (3), 2668-2676. DOI: 10.1021/acsenergylett.5c03034