Critical overview of polyanionic frameworks as positive electrodes for Na-ion batteries

Na-ion batteries (NIBs) are an important technological alternative to Li-ion batteries (LIBs) for developing energy storage systems that are cost-effective and less constrained by geographical supply chains, with similar energy densities. Analogous to LIBs, cathodes play a critical role in determining the energy density of NIBs, and layered transition metal oxide compounds are widely used as NIB cathodes. However, the instability-driven irreversible phase transitions in layered frameworks, especially at large degrees of Na removal, has necessitated the exploration of rigid structural frameworks that are resistant to structural changes with Na exchange. Thus, polyanionic frameworks, which primarily consist of transition metal polyhedra interconnected by PO 4 , SiO 4 , and/or SO 4 units, with or without the presence of fluorine, have been investigated as NIB cathodes. This review provides a critical overview of recent studies using polyanion cathodes, which will be useful in guiding the scientific community towards the development of better NIB cathodes. Graphical abstract