LiβΊ cations (teal) and anions (grey) drift between two electrode
plates through a liquid electrolyte. Ionic conductivity depends on how
many charge carriers there are (salt concentration), how fast they
move (temperature, via an Arrhenius-like activation barrier) and how
much the solvent resists their motion (viscosity). On first contact
with the anode, a solid-electrolyte-interphase (SEI) film nucleates
and grows β too thin and the electrolyte keeps decomposing every
cycle, too thick and it blocks ion transport, so a good electrolyte
additive builds a thin, stable, ion-conducting SEI fast.
Ο = Ξ£α΅’ nα΅’ zα΅’Β² eΒ² Dα΅’ / (kT)
Dα΅’ β exp(βEβ / RT) / Ξ·(solvent)
SEI growth rate β [additive] Β· exp(βthickness / Ξ»)
- Salt concentration β more dissolved salt means more charge carriers, but at high concentration ion pairing starts to lower mobility.
- Temperature β raises ion mobility (faster drift) but also speeds up unwanted side reactions at the electrode surface.
- Solvent viscosity β a thicker solvent slows ion diffusion even at fixed concentration and temperature.
- SEI additive β a sacrificial molecule that reduces first, forming a thin protective film that self-limits its own growth.