Advancing sodium-ion batteries for sustainable energy storage

Sodium-ion batteries (SIBs) have been proposed as a low-cost, environmentally friendly option for lithium-ion batteries but in reality are plagued by low energy density, slow ion diffusion, and integrity loss following repetitive cycling. Phase-engineered nanostructured materials have been proven capable of simultaneously optimizing sodium ion diffusion pathways and structure integrity in SIBs. Phase engineering of nanostructured materials as documented in this review, represents a promising strategy for addressing the disadvantages by crafting crystallographic phases atomically. We describe how phase engineering of anodes, cathodes, electrolytes, and separators enhances sodium-ion diffusion, structural stability, and electrochemical activity, thereby extending cycle life and safety. Recent advancements, including defect/vacancy engineering of layered oxides, phase-stabilized polyanionic networks, and hybrid anode composites, demonstrate the capability of phase engineering to overcome inherent SIB bottlenecks. The review also emphasizes scalability issues, long-term stability, and cost-effectiveness and presents future directions such as in situ phase transformation, interfacial phase tuning, and machine learning based phase prediction. By bridging fundamental principles with component level applications, phase engineering is a critical tool to enable high performance, sustainable, and market relevant sodium-ion energy storage systems.