Review: Insights on Hard Carbon Materials for Sodium‐Ion Batteries (SIBs): Synthesis – Properties – Performance Relationships

Abstract Sodium‐ion batteries (SIBs) have attracted a significant amount of interest in the past decade as a credible alternative to the lithium‐ion batteries (LIBs) widely used today. The abundance of sodium, along with the potential utilization of electrode materials without critical elements in their composition, led to the intensification of research on SIBs. Hard carbon (HC), is identified as the most suitable negative electrode for SIBs. It can be obtained by pyrolysis of eco‐friendly and renewable precursors, such as biomasses, biopolymers or synthetic polymers. Distinct HC properties can be obtained by tuning the precursors and the synthesis conditions, with a direct impact on the performance of SIBs. In this work, an in‐depth overview of how the synthesis parameters of HC affect their properties (porosity, structure, morphology, surface chemistry, and defects) is provided. Several synthesis‐property relationships are established based on a database created using extensive literature data. Moreover, HC properties are correlated with the electrochemical performance (initial Coulombic efficiency (iCE) and reversible capacity) vs. Na, in half‐cells. The Na‐ion storage mechanisms and solid electrolyte interphase (SEI) formation are discussed along with the HC performance in full‐cell devices, as well as the SIB prototypes and a short history of SIBs enterprises.