Defect structures in supercapacitor electrodes: non-oxide 2D materials and metal oxides

This review analyzes how crystallographic and interfacial defects govern charge storage in supercapacitor electrodes by shaping electronic structure, charge/ion transport, and redox kinetics. We explicitly decouple chemistry (oxide vs. non-oxide) from dimensionality (2D vs. 3D) to avoid conflation, treating non-oxide 2D materials and metal oxides as orthogonal categories. Across point defects, edge/termination states, and lattice disorder, we show how defect-driven mixed valence, oxygen-vacancy formation, and small-polaron conduction set the density and accessibility of (pseudo)capacitive sites and control the trade-offs between rate capability, stability, and safety. A concise framework connects defect type and distribution to measurable electrochemical responses (capacitance, kinetics, cyclability), supported by multimodal spectroscopy and in situ/operando methods that quantify defect populations under working conditions. Finally, we distill practical design rules-vacancy/termination control, aliovalent doping, and phase/strain engineering-that translate defect chemistry into targeted performance gains, providing a unifying roadmap for defect-engineered supercapacitor electrodes.