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. Graphical This study presents the chelation chemistry between ferrous ions and sodium humate with neighboring carboxylate groups enabling the aqueous synthesis of impurity-free sulfate cathode materials of Na 2 Fe 2 (SO 4 ) 3 (NFS) and Na 2 0.5 Fe 2 (SO 4 ) 2.5 (PO 4 ) 0.5 (NFSP), as well as their composites with graphene (i.e., NFS/4%G, NFSP/4%G), for sustainable sodium ion batteries.