Low-Cost Iron/Manganese-Based Polyanionic Cathodes for Sustainable Na-Ion Batteries

Sustainable Na-ion batteries (NIBs) have attracted widespread attention in transportation vehicles and large-scale energy storage systems due to abundant sodium sources, high safety records, and favorable all-climate performance. However, their overall performance and cost are largely governed by cathode materials, which determine practical application and commercial market prospects. Iron- and/or manganese-based polyanionic cathodes not only reduce cost but also provide high working voltage and desirable cycling stability, benefiting from strong inductive effects and robust structural covalency of polyanionic groups, making them attractive for sustainable batteries. This review summarizes material design concepts, research progress, technological bottlenecks, and development directions for various types of phosphate, pyrophosphate, and sulfate cathodes. Besides, we focus on clarifying the structure–function relationships by comprehensively comparing synthesis methods, crystal structures, electrochemical behavior, and reaction mechanisms of Fe/Mn-based polyanionic cathodes for NIBs. This review provides critical guidance for the future advancement of practical NIBs.