Modification Strategies for Hard-Carbon-Based Anode Materials in Sodium-Ion Batteries

Sodium-ion batteries offer abundant resources, low cost, and high safety, making them promising for large-scale energy storage. Hard carbon, a key anode material, enables Na+ storage through its disordered structure, enlarged interlayer spacing, and abundant defects and pores. However, these features also cause electrolyte side reactions and sluggish Na+ transport, leading to low initial Coulombic efficiency (ICE), poor rate capability, and limited low-voltage plateau capacity. This paper explores the correlation between hard carbon microstructure and sodium storage, and the mechanisms by which different modifications improve electrochemical performance. Drawing on representative studies published in recent years, this review summarizes the progress in hard-carbon anode modification, with particular emphasis on structural regulation, sodium-storage mechanisms, and electrochemical performance optimization. It also discusses the advantages, limitations, and potential synergies of different modification strategies. The results reveal that the electrochemical performance of hard carbon is influenced by multiple factors like surface structure, carbon layer arrangement, pore characteristics, and the transport process, while a single structural optimization cannot simultaneously meet the requirements of ICE, rate performance, and plateau capacity. The coordinated regulation of multi-scale structures is key to balancing performance. However, current research on hard carbon still faces issues such as insufficient quantitative relationships between structural parameters and sodium storage behavior, unclear matching rules for different scale structures, and a gap between laboratory performance and actual application requirements. Future efforts should integrate structural characterization, theoretical analysis, and engineering evaluation to establish structure-performance relationships and advance hard carbon anodes.