Na₂Fe₃(SO₄)₄: A Zero-Strain Sustainable Positive Electrode Material for Na-Ion Batteries

One of the challenges in the energy transition is minimization of the battery cost for energy grid storage. Na-ion batteries are a promising alternative to Li-based analogues thanks to low cost, abundance of constituents, and an already set-up industry. However, already commercialized positive electrode materials for Na-ion batteries (Na₃V₂(PO₄)₂F₃ and Na_xCu_1-y-zFe_yMn_zO₂) contain critical raw elements such as V, Cu, and Mn, boosting the research towards abundant Fe-based materials. In this work, we report a novel sodium iron sulfate phase, Na₂Fe₃(SO₄)₄ (NFS), synthesized by ball milling, as a positive electrode material. This new compound crystallizes in the orthorhombic Pbca space group with cell parameters a = 9.682(1) Å, b = 8.739(1) Å, c = 29.300(4) Å, and V = 2479.0(5) ų. Tests of thick NFS positive electrodes (18 mg cm^-2) in Na-half coin cells delivered 62 mAh g^-1 at C/30 (corresponding to the exchange of 2 e-) or 69% of the theoretical capacity (90 mAh g^-1), with a good capacity retention of 47 mAh g^-1 at a high discharge rate 2C. Operando X-ray diffraction (XRD) showed minimal (<1%) changes in lattice parameters during cycling. Dominantly octahedra coordinated iron atoms are oxidized/reduced, as confirmed by operando synchrotron Mössbauer spectroscopy.