+ Furthermore, silver ions are observed to segregate at LLZO grain boundaries (Figure ), potentially enhancing the grain boundary fracture toughness. The surface toughening will prevent cracks opening resulting from lithium intrusion, and therefore improve the stability of Li electroplating. Although how the nanoscale surface Ag + modification manifested to the microscale fracture behavior still requires further investigation, our results demonstrate that annealing ultra-thin Ag coating (3 nm) mitigating crack opening.