SIMS Surface Analysis for Battery Cathode-Electrolyte Interfaces: Depth Profiling of SEI Composition

Chemical Origin of the Interphase

At the battery cathode-electrolyte interface, the SEI/CEI develops through a series of electrolyte decomposition reactions that progress during cycling. The key pathways responsible for generating the interphase include the:

• Oxidation of electrolyte solvents- producing organic fragments and polymeric species that contribute to the outer regions of the interphase.
• Decomposition of lithium salts- yielding lithium fluoride, fluorophosphates, and other phosphate-containing species that accumulate within the inorganic-rich domains.
• Reactions between electrolyte components and the cathode surface- generating metal oxides and metal fluorides that form closest to the cathode.

The interplay of these reactions generates a layered, chemically diverse interphase, representing the cathode-specific form of the SEI, whose composition evolves with voltage, temperature, and cycling history during battery operation.

Layered Nature of the Interphase

Driven by competing reaction pathways, the SEI that forms at the battery cathode-electrolyte interface develops with a layered structure. It is not uniform; instead its layered arrangement typically places organic fragments in the outermost region, salt-derived inorganic species within the the intermediate domain, and metal-containing compounds closest to the cathode surface. As this structure varies over only a few nanometres, effective characterisation requires surface sensitivity and controlled depth profiling, capabilities that can be achieved using SIMS surface analysis in battery research.

SIMS surface analysis can characterise the SEI at the battery cathode-electrolyte interface with high chemical and spatial resolution. This technique operates by directing a focused primary ion beam onto the sample’s surface, sputtering away material and releasing secondary ions for mass analysis. The resulting spectra reveal elemental species and molecular fragments that reflect the composition and evolution of the interphase during battery operation.

SIMS surface analysis is particularly valuable for studying the battery cathode-electrolyte interface because it:

• Detects a broad range of molecular fragments and elemental species, capturing the chemical diversity of SEI components
• Offers high sensitivity, enabling the identification of both dominant and trace decomposition products
• Analyses the upper most nanometres of the surface, precisely where SEI formation and early-stage modifications occur in battery systems.

Chemical and Spatial Insight

Another capability of SIMS surface analysis is that it can generate chemical images that show how SEI species are distributed across the battery cathode-electrolyte interface. These lateral variations may indicate local reaction environments, degradation pathways, or structural heterogeneity within the electrode. By combining depth-resolved chemistry with spatial mapping, SIMS surface analysis delivers a detailed view on how the SEI forms, evolves, and influences cathode behaviour throughout battery cycling.

SIMS surface analysis can look beyond the SEI surface and examine how its chemistry changes with depth at battery cathode-electrolyte interfaces. The information ascertained from depth profiling can help clarify how different SEI components are distributed through the interphase.

How Depth Profiling Works

Depth profiling with SIMS surface analysis involves gradually sputtering the SEI and measuring the secondary ions released as each layer is removed. Tracking changes in ion intensities as a function of sputter time can produce a depth-resolved profile that reveals how chemical species are arranged throughout the interface.

What Depth Profiling Reveals

Applying depth profiling to battery cathode-electrolyte interfaces with SIMS surface analysis can show the composition of the SEI changes as a function of depth. SIMS surface analysis helps distinguish which species appear only at the surface, which are present throughout the interphase, and which accumulate close to the cathode. It also highlights how these distributions shift with battery cycling, electrolyte formulation, or operating conditions.

The capabilities of depth profiling can be used to reveal:

• Where different SEI components reside within the interphase
• How the balance or organic and inorganic species changes with depth
• Whether certain products accumulate preferentially near the cathode
• How the internal structure of the SEI evolves during cycling.

Such insights show not only how the SEI is arranged, but how its internal chemistry changes in response to real battery operating environments.

Interpreting Depth-Resolved Data

Depth-resolved measurements from SIMS surface analysis can be shaped through factors such as variable sputter rates and ion yields within the SEI. Considering them helps ensure depth-dependent trends are interpreted reliably and that any artefacts affecting the apparent interphase structure are recognised.