Abstract
Sodium-ion batteries are emerging as a complementary electrochemical energy-storage technology to lithium-ion batteries, particularly for stationary, grid-scale, and other cost-sensitive applications. However, assessing the practical relevance of newly developed sodium-ion materials remains challenging because early-stage electrochemical evaluation is still dominated by Na-metal half-cells. These configurations are indispensable for establishing reversible capacity within defined voltage windows, redox behavior, kinetic characteristics, structural evolution, initial Coulombic efficiency, and electrode-specific interfacial processes, but they do not reproduce the finite sodium inventory, electrode balancing, realistic loading, shared electrolyte environment, or coupled ageing that govern practical full cells. This review develops a translational framework for evaluating sodium-ion battery materials across successive levels of experimental realism. It first defines the appropriate role of half-cell testing and then examines the transition toward realistic composite electrodes and balanced full cells, with particular emphasis on mass loading, areal capacity, electrode density, N/P ratio, sodium-inventory management, electrolyte amount, full-cell energy, Coulombic efficiency, impedance evolution, and reproducibility. Interfacial chemistry is considered in terms of the transition from electrode-specific SEI/CEI formation to chemically coupled full-cell behavior, including interelectrode cross-talk, transition-metal migration, electrolyte-derived species, gas evolution, and their consequences for sodium loss and impedance growth. Operando and in situ characterization are discussed according to an architecture–mechanism–diagnostic framework in which X-ray methods, vibrational spectroscopies, imaging and microscopy, impedance analysis, and multimodal approaches are matched to the degradation processes they are intended to resolve under increasingly representative cell conditions. Finally, a staged validation pathway is proposed, progressing from material-level half-cell characterization to realistic electrode validation, balanced full-cell testing, operando diagnosis, and prototype-level assessment. By distinguishing what each experimental stage can establish and which questions require more representative architectures, this review provides a framework for assessing whether favorable material-level performance can be translated into practical sodium-ion battery operation.