Electrochromic Smart Window: Ion Intercalation

Applied Bias

Voltage V 0.00 V

Material Parameters

Switching time constant τ 6.0 s

Coloration efficiency CE 42 cm²/C

Live Readouts

Equilibrium — clear

Transmittance T

80.0%

Optical density

0.00

Inserted charge q

0.0 mC/cm²

Li⁺ fraction in WO₃

How it works

An electrochromic window sandwiches a tungsten-oxide (WO₃) film and a Li⁺-ion-storage counter-electrode (NiO) around a solid/gel electrolyte, between two transparent ITO conductors. A bias voltage drives Li⁺ ions and electrons across the stack:

WO3 + x Li+ + x e-  ⇌  LixWO3   (colored tungsten bronze)
              coloring (V < 0)  →
              ← bleaching (V > 0)

Ion insertion relaxes toward a voltage-set equilibrium charge with a diffusion-limited time constant τ (set by the slider — larger τ mimics a thicker film / slower Li⁺ diffusivity D, since τ ≈ d²/D):

dq/dt = (q_eq(V) − q) / τ
q_eq(V) = q_max · 1 / (1 + exp(1.8·V))

Each inserted W⁶⁺ → W⁵⁺ polaron adds visible-light absorption proportional to the charge density, via the coloration efficiency CE (cm²/C), giving the standard electrochromic relation:

ΔOD = CE · q            (q in C/cm²)
T    = T_clear · 10^(−ΔOD)

Voltage slider — sets the driving bias; negative pulls Li⁺ into WO₃ (coloring), positive pulls it back out (bleaching).
τ slider — switching speed; a thin film with fast ion diffusion switches in a couple of seconds, a thick film takes tens of seconds — exactly the tradeoff real smart-glass manufacturers (e.g. SageGlass) balance against film durability.
CE slider — how dark the film gets per unit of charge moved; higher CE means deeper tint for the same inserted charge, so less power is needed to fully darken the pane.
The 3D view shows Li⁺ ions (blue spheres) migrating between the NiO reservoir and the WO₃ film through the electrolyte layer, and the WO₃ slab itself darkening as charge accumulates — this is the real mechanism behind smart windows in office towers and the Boeing 787's dimmable cabin windows.

Li⁺ ion WO₃ electrochromic layer NiO counter-electrode

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