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Direct Calculation of Li-Ion Transport in the Solid Electrolyte Interphase

Shi, S, P Lu, Z Liu, Y Qi, LG Hector Jr., H Li, and SJ Harris
Journal of the American Chemical Society 134, no. 37: 15476–15487.

The mechanism of Li⁺ transport through the solid electrolyte interphase (SEI), a passivating film on electrode surfaces, has never been clearly elucidated despite its overwhelming importance to Li-ion battery operation and lifetime. The present paper develops a multiscale theoretical methodology to reveal the mechanism of Li⁺ transport in a SEI film. The methodology incorporates the boundary conditions of the first direct diffusion measurements on a model SEI consisting of porous (outer) organic and dense (inner) inorganic layers (similar to typical SEI films).

New experimental evidence confirms that the inner layer in the ∼20 nm thick model SEI is primarily crystalline Li₂CO₃. Using density functional theory, we first determined that the dominant diffusion carrier in Li₂CO₃ below the voltage range of SEI formation is excess interstitial Li⁺. This diffuses via a knock-off mechanism to maintain higher O-coordination, rather than direct-hopping through empty spaces in the Li₂CO₃ lattice.

Mesoscale diffusion equations were then formulated upon a new two-layer/two-mechanism model: pore diffusion in the outer layer and knock-off diffusion in the inner layer. This diffusion model predicted the unusual isotope ratio ⁶Li⁺/⁷Li⁺ profile measured by TOF-SIMS, which increases from the SEI/electrolyte surface and peaks at a depth of 5 nm, and then gradually decreases within the dense layer. With no fitting parameters, our approach is applicable to model general transport properties, such as ionic conductivity, for SEI films on the surface of other electrodes, from the atomic scale to the mesoscale, as well as aging phenomenon.