Louis G. Hector

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.

Density functional theory (DFT) is used to reveal that the polycrystalline Young's modulus of graphite triples as it is lithiated to . This behavior is captured in a linear relationship between and lithium concentration suitable for continuum-scale models aimed at predicting diffusion-induced deformation in battery electrode materials. Alternatively, Poisson's ratio is concentration-independent. Charge-transfer analyses suggest simultaneous weakening of carbon–carbon bonds within graphite basal planes and strengthening of interlayer bonding during lithiation. The variation in bond strength is shown to be responsible for the differences between elasticity tensor components, , of lithium–graphite intercalation (Li-GIC) phases. Strain accumulation during Li intercalation and deintercalation is examined with a core–shell model of a Li-GIC particle assuming two coexisting phases. The requisite force equilibrium uses different Young's moduli computed with DFT. Lithium-poor phases develop tensile strains, whereas Li-rich phases develop compressive strains. Results from the core–shell model suggest that elastic strain should be defined relative to the newest phase that forms during lithiation of graphite, and Li concentration-dependent mechanical properties should be considered in continuum level models.

This paper aims to help fill a gap in the literature on Li-ion battery electrode materials due to the absence of measured elastic constants needed for diffusion induced stress models. By examining results from new first principles density functional theory (DFT) calculations of LiCoO2, LiMn2O4, (and their delithiated hosts, CoO2 and MnO2), LixAl alloys, and data from the extant literature on LiFePO4 (and FePO4), LiTi2O4 (and Li2Ti2O4), LixSi, LixSn and lihtium graphite-interaction-compounds, a compelling picture emerges on the dependency of the elastic properties on Li concentration. Specifically, three distinct categories of behavior are found: (a) the averaged Young's moduli change very minimally upon lithiation of the spinel and olivine structures; (b) lithiation induced stiffening is observed only when new and stronger bonds between the Li ions and the host materials are formed in layered compounds; and (c) for alloy-forming electrode materials, such as Si, β-Sn and Al, the averaged Young's moduli of lithiated compounds follow the linear rule of mixtures. The tendency of ductile or brittle behavior electrode materials is investigated with the Pugh criterion, and a ductile to brittle transition was found to occur during lithiation of Al and β-Sn, but not in Si.