sei-layer-analysis

name	sei-layer-analysis
description	Domain knowledge for solid electrolyte interphase (SEI) and battery electrode interface analysis. Covers SEI layer thickness ranges, lithium plating, battery material SLDs, and electrochemical cell conventions. Use when the sample involves batteries, electrochemistry, SEI, lithium plating, anodes, or cathodes.
metadata	{"author":"aure","version":"1.0"}

SEI Layer Properties

The solid electrolyte interphase (SEI) forms between the electrolyte and the electrode surface in battery systems.

• Typical thickness: 50–200 Å
• SLD range: 1.0–4.0 × 10⁻⁶ Å⁻² (composition-dependent)
• Thickness bounds for fitting: 5–500 Å
• Position: between the electrolyte (ambient) and the electrode surface

When adding an SEI layer:

• Set initial thickness to 100 Å (midpoint of typical range)
• Use wide SLD bounds (0.5–5.0) since SEI composition varies widely
• NEVER add with thickness < 5 Å — such layers cannot be resolved

Lithium Plating

Lithium metal plating appears between the SEI and the electrode (e.g., copper).

• Typical thickness: 10–100 Å
• SLD: −0.9 × 10⁻⁶ Å⁻² (pure Li)
• SLD range for fitting: −1.5 to 0.5 × 10⁻⁶ Å⁻²
• Thickness bounds: 5–300 Å

Battery Material SLDs (×10⁻⁶ Å⁻²)

Material	SLD
Lithium metal	-0.9
LiF	2.25
Li₂CO₃	2.0
Li₂O	1.4
LiPF₆ (in solution)	~2.5

Electrochemical Cell Conventions

• If the description says the native oxide "reduces away", do NOT include an oxide layer; replace it with the hypothesized SEI/plating layer(s).
• Layer ordering for a typical battery electrode (substrate to ambient): substrate → electrode metal → (plating) → SEI → electrolyte (ambient)
• Back-reflection geometry is common for in-situ electrochemical cells (neutrons through the substrate).

Electrolyte Solvents

Common battery electrolyte solvents and their SLDs:

Solvent	SLD (H)	SLD (D)
EC (ethylene carbonate)	~1.0	~5.5
DMC (dimethyl carbonate)	~0.5	~5.0
THF	0.18	6.35 (d8-THF)

Refinement Strategy — Battery Interfaces

When refining battery/SEI models:

• SEI thickness uncertainty: SEI layers are inherently diffuse. If the fitted SEI thickness is very different from the 50–200 Å typical range AND χ² is high, first try widening the SLD bounds (0.5–5.0) rather than changing thickness bounds. SEI composition (and therefore SLD) varies more than its thickness.
• Lithium plating detection: If residual fringes suggest a thin (~10–50 Å) unmodeled layer between the electrode and SEI, consider adding a Li plating layer (SLD ≈ -0.9). Only add if the BIC supports it.
• Oxide reduction: If the sample description says the native oxide "reduces away" but an oxide layer is still in the model, remove it and replace with SEI/plating layers as described.
• Electrode roughness: Battery electrodes can have high roughness (10–30 Å). If the electrode roughness is pinned at its upper bound, widen the roughness range before adding interface layers.
• Electrolyte SLD: Verify the fitted electrolyte (ambient) SLD matches the stated solvent. Battery electrolytes are often deuterated for contrast — check for isotope confusion.