The ongoing development of solid electrolytes (SEs) for Li ion batteries has pushed their room temperature ionic conductivities to 1 mS/cm or beyond, bringing the realization of all-solid-state batteries within reach. All-solid-state batteries do not only promise improved safety compared to conventional batteries utilizing flammable liquid electrolytes. Moreover, it is currently investigated if the usage of SEs can sufficiently suppress dendrite growth, which might facilitate Li metal anodes and largely increase the energy density of the battery compared to conventional graphite anodes.

However, there are several issues that still need to be solved. One of these issues is the resistance due to the interface between SE and Li metal anode (SE|Li interface) that keeps rising with increasing charge-discharge cycles. One explanation for this observation is the formation of Li pores during the discharge process: The presence of pores leads to less interface area between SE and Li metal, leading to increased local current densities with high overpotentials. In this context, several studies have argued that one reason for the formation of pores is the low diffusion coefficient of Li atoms in the anode, unable to refill created vacancies at the interface towards the solid electrolyte fast enough.

In this contribution we will put the aforementioned argument to a test. To this end, we present results obtained from kinetic Monte Carlo simulations, in which we have modeled the Li metal anode connected to a virtual SE during the discharge process. By varying the externally applied current density and the interaction strength between SE and Li metal, we show two things: First, the diffusive species of interest for this process are not Li metal atoms, but rather Li vacancies. Second, we find that the SE|Li interface thermodynamics plays a much more crucial role in the kinetics of pore formation than the blamed diffusion coefficients in the metal. Therefore, we conclude that a careful preparation of SE|Li interfaces, potentially by means surface treatments or coatings of the solid electrolyte, are necessary to enable high power densities for all-solid-state batteries employing Li metal anodes. Furthermore, these insights are transferable to Na system and highlight the importance of interface engineering in all-solid-state batteries.