Operando-Defined Amorphous Energy Materials for Electrochemical Energy Systems: From Structural Definition to Descriptor-Guided Design

Amorphous materials offer distinctive opportunities for electrochemical energy conversion and storage because their local coordination, structural flexibility and metastability can promote catalysis, ion transport and charge storage. However, amorphousness is often inferred only from broad diffraction features or missing lattice fringes, which cannot distinguish genuine amorphous phases from poorly crystalline, defect-rich, surface-amorphized or reconstructed structures. This review establishes an operando-defined framework for understanding and designing amorphous energy materials. We first clarify their structural hierarchy and identify short-range order, medium-range connectivity, interfacial heterogeneity and reconstruction propensity as key descriptors. We then examine how synthesis controls disorder and how multimodal characterization, operando measurements and experiment-constrained modeling can resolve local and working-state structures. Representative applications in electrocatalysis, batteries, supercapacitors, solid electrolytes and solid oxide electrochemical devices are discussed through structure-performance relationships. Finally, we propose descriptor-guided interface engineering and physics-informed data-driven strategies for rational design. This framework moves the field beyond the empirical "XRD-amorphous" label toward quantitative control of dynamically evolving functional structures.