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

TLDR

Li⁺ transport through the SEI film remains poorly understood, yet it is critical for Li‑ion battery performance and longevity. This study develops a multiscale theoretical approach to uncover Li⁺ transport mechanisms in SEI films. The approach integrates experimental boundary conditions, DFT calculations revealing excess interstitial Li⁺ as the dominant carrier diffusing via a knock‑off mechanism, and mesoscale two‑layer diffusion equations linking pore diffusion in the outer layer to knock‑off diffusion in the inner layer. Experiments confirm the inner SEI layer is crystalline Li₂CO₃, and the model accurately predicts the measured ⁶Li⁺/⁷Li⁺ isotope profile and can forecast ionic conductivity and aging without fitting parameters.

Abstract

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 Li(2)CO(3). Using density functional theory, we first determined that the dominant diffusion carrier in Li(2)CO(3) 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 Li(2)CO(3) 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 (6)Li(+)/(7)Li(+) 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.