Na ₂ Fe ₃ (SO ₄ ) ₄ : A Zero‐Strain Sustainable Positive Electrode Material for Na‐Ion Batteries

Abstract

Abstract 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 (Na 3 V 2 (PO 4 ) 2 F 3 and Na x Cu 1‐y‐z Fe y Mn z O 2 ) 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, Na 2 Fe 3 (SO 4 ) 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) Å 3 . 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.