Sodium Humate Chelating Ferrous Ions in the Aqueous Synthesis of High‐Purity Sulfate Cathode Materials for Sustainable Sodium Ion Storage

Na 2 Fe 2 (SO 4 ) 3 (NFS) is a promising, low‐cost, high‐voltage cathode material with elemental abundance for sustainable sodium ion batteries. However, the high‐performance application of NFS is haunted by the low conductivity and the FeSO 4 impurity. Herein, sodium humate (SH), which acts as a Fe 2+ chelating agent, is applied in a facile aqueous synthesis strategy to obtain high‐purity NFS. Control samples prepared with nonchelating sodium carboxylic acid salts, such as monosodium glutamate, sodium acetate, or sodium gluconate, are accompanied by significant FeSO 4 ·H 2 O impurities. SH also avoids the FeSO 4 ·H 2 O impurity in the synthesis of Na 2.5 Fe 2 (SO 4 ) 2.5 (PO 4 ) 0.5 (NFSP). The chelation between SH and Fe 2+ has been demonstrated through visual experiments, as well as by X‐ray diffraction, Fourier transform infrared spectroscopy, and X‐ray photoelectron spectroscopy characterizations. Furthermore, the integration of NFS or NFSP with graphene enhances the capacity and rate capability. For instance, the initial discharge capacity of NFS/4%G doubles that of NFS (84.77 vs. 40.11 mAh g −1 ) at 0.1 C. This study paves the way for the development of pure‐phase polyanion cathode materials through chelation chemistry.