Iron-based polyanionic cathodes for sustainable sodium-ion batteries

Sodium-ion batteries (SIBs) have emerged as a compelling alternative to lithium-ion batteries, driven by the abundance of raw materials and lower costs. Iron-based polyanionic compounds, particularly Na_2+xFe_1+x(PO₄)_xP₂O₇ (NFPP), stand out as promising cathode materials due to their structural stability, high operating voltage, and superior cycling performance. This review offers a comprehensive overview of recent advances in NFPP cathodes, addressing their crystal structure, electrochemical mechanisms, synthesis techniques, and performance-enhancing modifications. Key challenges—including low electronic conductivity, impurity phase formation, and constrained energy density—are critically examined. To mitigate these issues, strategic approaches such as phase optimization, carbon coating, doping, and heterostructure design are systematically evaluated for their efficacy in improving conductivity, stability, and energy output. Furthermore, the barriers to scaling NFPP production, such as synthesis scalability and cost-efficient processing, are discussed in the context of commercialization. Finally, future research priorities are proposed, emphasizing advanced nanostructures, novel doping elements, and sustainable synthesis routes to accelerate the development of high-performance NFPP cathodes. These efforts aim to pave the way for practical, economically viable, and environmentally sustainable SIB technologies.