Amorphous materials belonging to the family of Lithium Phosphorus Oxynitrides are increasingly popular solid electrolytes for thin-film Li-ion batteries. Despite the ever-growing usage in devices, the relationship between the structure of these materials and their properties has not been clarified, yet. Theoretical work offers an invaluable insight into the atomistic properties of solids, but the rationalization of these properties must rely on valid structural models. The simulation of glassy structures represents a main challenge from a computational point of view, and is further complicated by their non-trivial composition. In this contribution, a new approach to the ab-initio simulation of amorphous structures of virtually any desired composition is described. This strategy consists of the use of an evolutionary algorithm followed by simulated annealing in the framework of Density Functional Theory. A realistic composition has been suggested by experiments recently conducted by academic partners. While investigating the defect thermodynamics of LiPON, evidence has been found of the instability of the interface between the electrolyte and metallic lithium. The occurrence of side reactions at the interface between LiPON and lithium has been recently observed and quantified experimentally, and represents a crucial step in the formation of the harmful solid-electrolyte interphase (SEI) which is responsible for the degradation of the performance of Li-ion batteries. Following up on this result, the structural and electronic properties of the interface between LiPON and lithium have been investigated with a special focus on its reactivity. This work does not only describe a novel approach to the simulation of a more realistic electrolyte, but also provides unprecedented insights, supported by experimental results, into its stability and reactivity under operational conditions.