Multivalent cation substitution boosted sodium-ion storage in NASICON-type iron-phospho-sulphate cathodes

• A cost-effective multi-valent Al-ion substituted Fe-rich NASICON cathode synthesised via a facile solid-state ball milling. • Al-doping enhanced the spin-states of Fe lowering the band gap and activation energy for charge transport. • Experimental and first principle calculations reveal the charge storage mechanism in the optimised cathode. • Impressive energy density of 372 Wh kg −1 , rate tolerance, and cycling stability make such cathodes promising . Despite advancements in NASICON cathodes, their widespread use in sodium-ion batteries (NIBs) remains limited due to low energy density, durability issues, and the use of scarce transition metals like vanadium. While the NASICON-type NaFe 2 (PO 4 )(SO 4 ) 2 cathode shows potential in addressing these challenges, it encounters issues with electron transport and Na + diffusion. To overcome these hurdles, we introduce a novel Al 3+ -substituted NaFe 2 (PO 4 )(SO 4 ) 2 (NFAPS) cathode in this study, synthesised by a straightforward solid-state ball-milling method. Herein, Al 3+ is strategically incorporated at the Fe site, and MWCNT is added in situ during NFAPS synthesis. The doping reduces the band gap, improves charge mobility, and maintains structural integrity during the Na + insertion and extraction processes. Further, Al 3+ enhances the spin state of Fe by attenuating the energy gap of undoped NFAPS cathodes, resulting in improved electrochemical performance, as evidenced by temperature-dependent magnetization susceptibility (M−T) and electron paramagnetic resonance (EPR) measurements. The optimized cathode, NaFe 1.93 Al 0.07 (PO 4 )(SO 4 ) 2 (NFAPS07) delivered a high specific discharge capacity of 124 mAh/g at C/20 (1C = 127 mAh/g), impressive rate capability (93.49 mAh/g at C/5 and 78.85 mAh/g at C/2) and good cycle life even at higher current rates. Ex-situ XRD analysis of NFAPS electrodes at various (de)sodiation voltages shows negligible volume expansion with minimal structural distortion. Further, NFAPS07 exhibits the highest reported energy density of 372 Wh kg −1 among all NASICON-based NaFe 2 (PO 4 )(SO 4 ) 2 cathode. Both experimental and first-principles studies confirm that enhanced charge migration, electrical conductivity, and lower activation barrier stem from synergistic effects of optimised Al 3+ doping in NFAPS. Such multivalent cation-doped NASICONs can be adapted to economically design next-generation high-energy–density NIB.