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

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

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 NaFe2(PO4)(SO4)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 Al3+-substituted NaFe2(PO4)(SO4)2 (NFAPS) cathode in this study, synthesised by a straightforward solid-state ball-milling method. Herein, Al3+ 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, Al3+ 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, NaFe1.93Al0.07(PO4)(SO4)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 NaFe2(PO4)(SO4)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 Al3+ doping in NFAPS. Such multivalent cation-doped NASICONs can be adapted to economically design next-generation high-energy–density NIB.