New method for determining empirical force fields of lithium battery materials

Solid-state electrolyte (SSE) materials used in rechargeable batteries, fuel cells, sensors and other applications rely on fast ion transport through the crystalline solid. Computational methods have been used to screen candidate materials to see which have high ion conductivities needed for these applications.

One widely used computational approach is molecular dynamics (MD), which requires accurate descriptions of inter-atomic interactions. Ab-initio MD (AIMD) can produce these, but AIMD is limited to small systems and short simulation times.

Another approach is to use empirical force fields (FFs). Kobayashi et al. propose an automated procedure for producing empirical FFs for SSE materials. The method uses distribution function data obtained from AIMD simulations. The FF parameters are then optimized using an algorithm known as a Cuckoo search, based on the way a cuckoo bird finds nests for its eggs.

The investigators determined an empirical FF for LiZr₂(PO₄)₃, an oxide commonly used in lithium batteries. From MD simulations with the calculated FF, they confirmed that only Li ions migrate through the crystalline solid, but other ions do not. They determined an activation energy for Li-ion diffusion of 0.39 eV, in excellent agreement with experiment.

Additionally, the group looked at fluoride containing solids that have been considered as replacement candidates for Li-ion batteries. In these, the fluoride ion carries charges between the cathode and anode. Calculations with materials doped with strontium showed that the FF correctly reproduces the effects of strain created by Sr ions on the F-ion migration.

“The present approach enables us to create a FF for any novel SSE material,” said author Ryo Kobayashi. This method should enhance the speed at which new SSE materials can be screened.

Source: “High-throughput production of force-fields for solid-state electrolyte materials,” by Ryo Kobayashi, Yasuhiro Miyaji, Koki Nakano, and Masanobu Nakayama, APL Materials (2020) The article can be accessed at https://doi.org/10.1063/5.0015373 .