Investigating the Nucleation of Lithium Deposits in Polycrystalline Solid Electrolytes

Abstract

All-solid-state batteries (ASSB) are candidates for the next-generation of battery electric vehicles. They potentially enable the use of lithium metal as an anode material thereby highly improving the energy density of the battery. For traditional liquid electrolytes this has not been feasible to date due to the formation of dendritic lithium structures during battery charge. It is generally believed that the mechanical properties of the solid electrolyte stop dendrite growth and several studies using LiPON in a thin film configuration as solid electrolyte demonstrate stable cycling even for elevated current densities of up to 10 mA/cm² [1,2]. However, recently short circuits after only a few cycles under moderate current densities were reported for other polycrystalline electrolyte materials, such as LLZO [3]. Moreover, it was found that lithium does not only deposit at the anode, but also nucleates inside the bulk electrolyte [4], facilitating dendrite growth even for stable solid electrolyte –anode interfaces. The cause for these dendrites might be the higher electronic conductivity of LLZO compared to LIPON, however, this has not yet been resolved conclusively. In our contribution we explore different mechanisms for these internal lithium deposits using models and theories on the continuum scale [5,6]. In our studies we focus on the effect of grain boundaries and structural defects within the solid electrolyte. These structural features are obstacles for the flux of lithium ions and electrons and thereby affect the transient potential landscape during the charging and discharging of the ASSB. In combination with insights on the spatial distribution of lithium ions at the grain boundaries our simulations will provide indications for nucleation centers and, additionally, will allow us to analyze the stability of lithium clusters during discharge. The aim of our work is to get an understanding of the influence of material properties, manufacturing processes, and cycling conditions on the nucleation of local lithium deposits and their effect on dendrite growth. This information will give important directions for the development of future ASSBs.