Advances in hard carbon anodes for sodium-ion batteries: understanding defect-pore interactions and interphase evolution, degradation mechanisms, operando/in-situ analysis

Hard carbon is a promising anode material for rechargeable sodium-ion batteries due to its exceptional performance, including high reversible specific capacity, low sodium storage potential, cost-effectiveness, and abundant biomass-derived precursor sources. However, its commercialization is hindered by limited understanding of defect-pore interactions, unstable solid electrolyte interphase (SEI), and irreversible sodium loss, leading to poor initial Coulombic efficiency (ICE). This review explores the relationships between hard carbon microstructure and properties, including biomass-derived synthesis impact, degradation mechanisms, and advanced structural design approaches. Additionally, it discusses strategies to enhance ICE, such as microwave/H2 activation and heterostructure-induced interfacial regulation, as well as in-situ interface engineering for stable SEI layers. Beyond the advancements reviewed, these strategies have practical implications for reducing battery manufacturing costs and promoting sustainability through the utilization of biomass-derived materials. This enables the scalability of sodium-ion systems for grid-level and community-based energy storage. By integrating low-cost, earth-abundant materials, sodium-ion technology aligns with global efforts to combat global warming and achieve sustainable energy infrastructure. As a result, it positions itself as a viable complement to lithium-ion systems in addressing large-scale energy storage and security challenges. Advanced characterization techniques provide insights into SEI and cathode electrolyte interface (CEI) evolution, bridging fundamental mechanisms with scalable synthesis and performance optimization for practical sodium-ion battery applications.