Can Ion Implantation Enable Solid-State Batteries? An Atomistic Investigation of Radiation Induced Damage in Garnet Solid Electrolytes

Li-metal based batteries with a solid electrolyte are often considered the holy grail of energy storage solutions in that they are higher energy density and safer than their liquid electrolyte counterparts. However, one persistent problem that plagues these batteries is the growth of lithium filaments through the solid electrolytes, which leads to short-circuits and failure early in life. Microstructural characteristics (i.e. cracks and grain-boundaries) drive this failure mode by either orchestrating brittle fracture of the electrolyte or by increasing the local electronic conductivity such that lithium preferentially precipitates at local “hot spots.” <br/>Ion implantation of solid electrolytes may curb both failure mechanisms by inducing compressive stresses which inhibit crack growth and deflect dendrites. It may also increase the wettability of the grain boundaries with respect to lithium, leading to an increased thermodynamic barrier to plating at grain-boundaries. However, the process of ion implantation necessarily leads to radiation induced damage in the material, and the precise nature of the damage as well as its effects on the material properties is ill understood at this early stage in ion-implanted electrolyte development. As ion implantation can be a time-consuming and expensive process, using nano- and mesoscale modeling tools can be a more efficient way to answer key research questions regarding the effects that the implantation process has on solid electrolytes.<br/>To that end, we employ a suite of molecular dynamics (MD) techniques, using Li₇La₃Zr₂O₁₂ (LLZO) as a pedagogical example, to understand (i) the nature of the amorphous damage introduced by ion implantation (ii) the effect of anneal temperature on the crystal structure and resulting Li ion conductivity (iii) the effect of defects on grain-boundary and bulk fracture behavior. We demonstrate that during the irradiation process, oxygen and lithium comprise the majority of interstitial defects while oxygen vacancies are comparatively common. Furthermore, anneal temperature and defect concentration must be simultaneously optimized to maximize Li ion conductivity. Lastly, we derive cohesive zone laws derived for both grain-boundaries and the bulk crystal at different Frenkel pair concentrations, showing that the fracture energy of LLZO is significantly altered in both the bulk and at the grain-boundaries. While, on its face, an increase in fracture energy constitutes a benefit to the long-term reliability of solid electrolytes, an unfortunate byproduct is that the applied compressive stress required to deflect dendrites increases commensurately. In total, our results show that ion implantation may help enable solid electrolytes for Li metal batteries, but that care and attention must be paid in optimizing the balance between the various alterations to material properties.