Structural Engineering and Functionalization of Carbon-Based Anodes for Sodium-Ion Batteries: From Biomass to Composites

Sodium-ion batteries (SIBs) are becoming more popular as a sustainable and affordable alternative to lithium-ion batteries for electric energy storage. One of the key challenges of SIB development lies in the cell components, including anode material capable of reversible hosting of Na⁺ ions. Carbon-based materials are still the best choice for this purpose because they can be modified easily and produced in larger quantities, while accommodating a large amount of stored sodium. This review provides an overview of hard carbon (HC)- and reduced graphene oxide (rGO)-based anode materials, from precursors made from biomass and polymers to structurally engineered graphene derivatives and carbon–transition metal composites. This review focuses on how the synthesis protocols, carbon structure properties, porosity, surface functionalization, and introduction of the inorganic components affect the sodium storage mechanism and performance. The review provides insights into rational material design strategies and underlines key challenges in the pursuit of scalable, high-efficiency SIB anodes.