A widely used remedy to slow down the degradation of the electrochemical performances of lithium-ion batteries is to apply coating materials on cathode surfaces.^[1] Such coatings can serve as physical barriers to inhibit electrode-electrolyte side reactions, and they can also possess an additional functionality such as scavenging the detrimental acids (e.g., the hydrofluoric acid, HF) in electrolytes. Following our previous thermodynamics high-throughput search for functional cathode coatings,^[2] we recently further built on our framework for understanding the battery performance improvement associated with coatings by studying and predicting lithium transport through a number of various crystalline and amorphous coating materials using density functional theory (DFT) calculations.

In the current work, we first investigated the Li diffusion in top candidates of crystalline metal oxides from our thermodynamics screening. With the help of MINT (software developed by our group), we found unique Li interstitial sites and corresponding migration pathway in these crystalline metal oxides. After that, DFT based Nudge Elastic Band (NEB) method was applied to calculate the migration barrier and corresponding Li diffusivity for each material was obtained by Kinetic Monte Carlo (KMC) simulation. Since many reports in the literature show coatings are likely to be in an amorphous form, we further studied the Li transport in a variety amorphous oxides and fluorides by methods that combine first principles density functional theory calculations and statistical mechanics.

In above diffusivity calculations, only dilute Li concentrations is taken into account, since at high Li concentration, phase transitions are likely to occur. Knowing that the Li diffusivity can be improved at high Li concentration, we extended our kinetical framework to some Li-contained metal oxides. All these metal oxides, fluorides and Li-contained metal oxides are top coating candidates examined from previous thermodynamic calculations. After all this kinetical calculation of Li transport, a ranking list is predicted and some materials are suggested as good coating materials in terms of Li diffusivity, which is one of the key factors influencing the rate performance of Li ion battery.

This work is one part of a framework for understanding the battery performance improvement associated with coatings and should aid in the future discovery of functional coating materials.

References

[1] Z. Chen, Y. Qin, K. Amine, Y.-K. Sun, J. Mater. Chem. 2010, 20, 7606.

[2] M. Aykol, S. Kirklin, C. Wolverton, Adv. Energy Mater. 2014, 4, 1400690

Acknowledgement

This computational research work was supported by The DOW Chemical Company.