Deciphering Sodium Storage in Hard Carbon Anodes: A Review on the Interplay of Microstructure, Interfacial Engineering, and Electrochemical Performance

Hard carbon (HC) is the most commercially viable anode for sodium-ion batteries (SIBs). Yet, its full potential remains unrealized due to persistent ambiguities in its sodium storage mechanism and the resulting variability in electrochemical performance. While numerous reviews have catalogued the progress in HC materials, a critical gap remains in systematically correlating the vast parameter space of precursor selection and synthesis strategies with the nuanced microstructural features that ultimately govern performance. This review addresses this need by providing a holistic, mechanism-centric analysis. Herein, we critically deconstruct the prevailing sodium storage models, from early “insertion-adsorption” concepts to the more comprehensive “adsorption-intercalation-filling” paradigm. The core novelty of this work lies in its in-depth exploration of how engineered defects, heteroatom doping, pore architecture, and graphitization degree directly influence the contributions of the sloping and plateau regions of the voltage profile. Furthermore, we elucidate the pivotal role of the electrode–electrolyte interphase, systematically linking electrolyte formulation to the chemical composition and stability of the solid electrolyte interphase, a critical factor for achieving high initial Coulombic efficiency and long-term cyclability. Bridging fundamental insights with practical application, this review synthesizes the current challenges. It highlights emerging trends, including scalable synthesis pathways and the transformative potential of advanced characterization, computational modeling, and machine learning to accelerate the rational design of next-generation HC anodes. Ultimately, this review provides a foundational guide and a forward-looking perspective, aiming to bridge the gap between laboratory-scale discovery and the industrial realization of high-performance, cost-effective SIBs.