Hard Carbon for Sodium-Ion Batteries: From Fundamental Research to Practical Applications.

Sodium-ion batteries (SIBs) have emerged as a promising technology for large-scale energy storage due to their unique performance characteristics and raw material accessibility. Among various anode materials, hard carbon (HC) stands out due to its high Na storage capacity, structural stability, and intrinsic safety. However, the structural complexity and heterogeneity of HC present ongoing challenges in understanding its structural models and Na storage mechanisms, impeding the rational design and performance optimization of HC-based anodes. This review provides a systematic overview of recent advances in HC for SIBs, beginning with an in-depth examination of representative structural models and the underlying structure-property relationships. The review critically analyzes Na storage mechanisms and bridges these insights with a diverse array of modification strategies-including precursor design, structural tailoring, and surface/interface optimization. Special emphasis is placed on improving initial Coulombic efficiency, rate capability, and long-term cycling stability. Furthermore, practical challenges related to full-cell integration are discussed, such as pre-sodiation techniques and electrolyte/interface engineering to enhance the real-world applicability of HC. By integrating fundamental understanding with forward-looking design strategies, this review provides a valuable reference for the development of high-performance, cost-effective SIB systems, and to inspire future research directions in sodium-ion energy storage.