Interface properties are key for the performance of battery systems. A variety of experimental methods are applied in order to study interface properties, but often their execution and interpretation are difficult. At this stage atomistic simulations can provide valuable details of the processes occurring at the interface.
As a first step, total energy calculations based on Density Functional Theory (DFT) are routinely applied to investigate the thermodynamics of the systems of interest. From these calculations, the stability of materials can be judged by, e.g., decomposition reactions, if potential reaction products are considered.
In a second, more elaborated step, explicit interface models can be constructed and deliver additional information about the interface and the formation of interphases. In order to overcome the static nature of standard DFT relaxations, even ab-inito molecular dynamics (AIMD) can be applied.
However, the construction of interface models can be non-trivial as DFT suffers from size limitations. Either only a limited number of surface orientations are feasible or one or both involved materials need to be deformed in order to simultaneously fit the simulation cell and the periodic boundary conditions.
Here, we present a third possibility on how to evaluate interface stabilities: our approach relies on the analysis of the thermodynamics of point defects of involved species. To underline our approach, we show examples of instable LiPON/Li and Li4P2S6/Li interfaces and stable Li4P2S6/a‑Li4P2S7 interfaces. We are confident that our methodology helps to screen new material combinations for battery applications and allows estimating their interface stability. In addition, the presented approach is not limited to only battery systems but can be employed wherever interfaces are key to the performance of the system of interest.
Journal of Power Sources 354 (2017) 124-133
Chemistry of Materials 29 (18) (2017) 7675-7685
Solid State Ionics 319 (2018) 53-60