Defective alluaudite-type sodium iron sulfate, Na2.6Fe1.7(SO4)3 (NFS), represents a highly critical non-stoichiometric phase within the Na(2+2x)Fe(2-x)(SO4)3 family (x=~0.3). While stoichiometric Na2Fe2(SO4)3 theoretically delivers an exceptional operating potential (3.8 V vs. Na+/Na) driven by the strong inductive effect of the polyanionic [SO4]^{2-} groups, synthesis limitations typically necessitate working with these iron-deficient/sodium-rich configurations. Managing and engineering these native defects is central to unlocking their viability for low-cost, high-voltage sodium-ion battery (SIB) cathodes.

The composition Na2.6Fe1.7(SO4)3 is inherently a thermo-deficient phase. Because sulfate frameworks cannot withstand high calcination temperatures without decomposing (>400°C), they are synthesized via low-temperature sol-gel, spray drying, or aqueous precipitation. This creates distinct crystalline defects:

(1) Iron Vacancies and Multi-Site Na Distortions: To maintain charge neutrality when Fe^{2+} is deficient (1.7 instead of 2.0), additional Na+ ions populate the framework. This forces excess sodium into interstitial sites or directly substitutes onto Fe structural positions. (2) Fe Migration and Phase Irreversibility: During high-voltage extraction (desodiation), the structural instability of these heavily defected sites allows Fe^{3+} ions to migrate into adjacent vacant sodium pathways. This blocks the 1D/3D percolation channels, causing sluggish Na+$ transport kinetics. (3) Lattice Heterogeneity and Splitting: The native strain induced by the Na/Fe disorder causes highly localized polyhedral distortions. Under high-rate cycling, this structural mismatch results in catastrophic polyhedral fracture, micro-cracking of the cathode particles, and massive electrochemical decay.

Part Number	CSIBCDATNFS (C-SIB-C-DATNFS)
Chemical Formula	Na2.6Fe1.7(SO4)3
Chemical Composition	Na: 13.31 wt%, Fe: 21.25 wt%, S: 20.77 wt%, C: 2.27 wt%
Particle Size Distribution	D10 = 1.65 um D50 = 5.07 um D90 = 11.62 um
Tap Density	0.94 g/cm3
Compaction Density	2.15 g/cm3
Specific Area	7.29 m2/g
XRD
First Discharging Capacity	~95 mAh/g (0.1 C, 2.0-4.5 V vs. Na+/Na)   Electrolyte: 1 M NaClO4 in EC:PC=1:1 with 5% FEC
First Columbic Efficiency	94.0%
Package Grade	100-500 g/bottle

Notes: (1) Please store the defective NFS powder in a dry area (glovebox is preferred); (2) The battery powder is highly recommended to be dried at 80-100°C in a vacuum oven for 6-12 h before use.

References: 

1. J. Wang, et al. NaO6 Octahedron-Engineered Sodium Iron Sulfate Cathodes for High-Rate and Sustainable Sodium-Ion Batteries, Angew Chem Int Ed, DOI: 10.1002/anie.8606538.
2. S. Zhou, et al. Expanding the Fe–Fe distance for superior electrochemical performance of Na2.6Fe1.7(SO4)3 cathodes for sodium-ion batteries, J. Mater. Chem. A, 2025,13, 42038-42047