Tailoring the Electrochemical Properties of Nickel-based Mixed Phosphate Cathodes for Sodium-Ion Batteries

Mixed phosphate cathode materials have garnered significant research interest due to their outstanding structural properties, featuring open frameworks with 3D Na channels. These materials offer high theoretical capacity and stable three-dimensional structures that exhibit minimal volume changes during charging and discharging, ensuring good cycling stability and safety. However, their insufficient conductivity impedes their electrochemical performance and rate capability, hence limiting their widespread application. Ion substitution and carbon coating are considered as an effective strategy for improving the conductivity of these materials. This study addresses this limitation by modifying nickel (Ni)-based mixed phosphate cathode materials through iron (Fe) substitution. This approach combines the advantages of Fe-based cathodes, including lower cost, greater abundance, improved thermal stability, and reduced environmental impact, with nickel's higher energy density and voltage potential. A simple solid-state method has been used to synthesize Fe-substituted Na4Ni3-xFex(PO4)2P2O7 (x = 1,2), and thorough structural and electrochemical analyses were conducted for materials’ characterization. XRD analysis confirmed the phase-pure synthesis of the materials, while TGA analysis revealed their outstanding thermal stability. Electrochemical characterization demonstrated enhanced initial specific charge/discharge capacity of Fe-substituted carbon coated Na4Ni3(PO4)2P2O7 compounds compared to pristine Na4Ni3(PO4)2P2O7. Henceforth, this study presents a promising approach for developing high-performance sodium-ion batteries (SIBs) using lower-cost, highly stable mixed phosphate cathode materials.