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

The mechanism of Li+ transport through the solid electrolyte interphase (SET), 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 SET consisting of porous (outer) organic and dense (inner) inorganic layers (similar to typical SET films). New experimental evidence confirms that the inner layer in the similar to 20 nm thick model SET is primarily crystalline Li2CO3. Using density functional theory, we first determined that the dominant diffusion carrier in Li2CO3 below the voltage range of SET 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 Li-6(+)/Li-7(+) 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 SET films on the surface of other electrodes, from the atomic scale to the mesoscale, as well as aging phenomenon.