Lithium transport through Lithium-ion battery cathode coatings

TL;DR

This study uses computational methods to analyze lithium ion transport in various insulator coatings for cathodes, revealing that amorphous forms of Al2O3 and AlF3 are more promising for practical battery coatings due to higher diffusivity and suitable resistivities.

Contribution

It provides a detailed theoretical investigation of lithium transport and resistivity in crystalline and amorphous insulator coatings, guiding the design of effective cathode surface coatings.

Findings

Crystalline insulators generally have high lithium migration barriers (>0.9 eV).

Amorphous Al2O3 and AlF3 exhibit higher lithium diffusivity and practical resistivities.

Oriented α-quartz SiO2 shows low migration barrier (0.276 eV), but crystalline forms are less practical.

Abstract

The surface coating of cathodes using insulator films has proven to be a promising method for high-voltage cathode stabilization in Li-ion batteries. However, there is still substantial uncertainty about how these films function, specifically with regard to important coating design principles such as lithium solubility and transport through the films. This study uses Density Functional Theory to examine the diffusivity of interstitial lithium in crystalline {\alpha}-$AlF_3$, {\alpha}-$Al_2O_3$, m-$ZrO_2$, c-MgO, and {\alpha}-quartz $SiO_2$, which provide benchmark cases for further understanding of insulator coatings in general. In addition, we propose an Ohmic electrolyte model to predict resistivities and overpotential contributions under battery operating conditions. For the crystalline materials considered we predict that Li+ diffuses quite slowly, with a migration barrier larger than 0.9 eV in all crystalline materials except {\alpha}-quartz $SiO_2$, which is predicted to have a migration barrier of 0.276 eV along <001>. These results suggest that the stable crystalline forms of these insulator materials, except for oriented {\alpha}-quartz $SiO_2$, are not practical for conformal cathode coatings. Amorphous $Al_2O_3$ and $AlF_3$ have higher Li+ diffusivities than their crystalline counterparts. Our predicted amorphous $Al_2O_3$ resistivity (1789 M{\Omega}m) is near the top of the range of fitted resistivities extracted from previous experiments on nominal $Al_2O_3$ coatings (7.8 to 913 M{\Omega}m) while our predicted amorphous $AlF_3$ resistivity (114 M{\Omega}m) is close to the middle of the range. These comparisons support our framework for modeling and understanding the impact on overpotential of conformal coatings in terms of their fundamental thermodynamic and kinetic properties, and support that these materials can provide practical conformal coatings in their amorphous form.