Upcycling Iron Sulfate Waste into Low-Cost and High-Stability Na 2 Fe(SO 4 ) 2 Cathode Materials for Sodium-Ion Batteries

Titanium dioxide is extensively utilized as a pigment and is typically produced via the sulfuric acid method. However, this process generates substantial amounts of iron sulfate waste. So far, there exists a significant challenge to recycle iron sulfate waste due to a lack of an economic and reliable separation method. Herein, we propose an efficient upcycling method that converts iron sulfate waste to low-cost sodium iron sulfate Na 2 Fe(SO 4 ) 2 cathode materials for sodium-ion batteries (SIBs). The obtained Na 2 Fe(SO 4 ) 2 cathode demonstrates superior long-term cycling stability with a capacity retention of 81% after 5000 cycles at 5 C, much higher than the 59% of Na 2 Fe(SO 4 ) 2 cathodes prepared using iron sulfate chemical. Meanwhile, it is demonstrated that less crystal water in iron sulfate waste can increase the specific surface area of Na 2 Fe(SO 4 ) 2 materials, thereby enhancing the electrochemical reaction kinetics, such as the Na ions diffusion coefficient and capacitive contribution for high stability of Na 2 Fe(SO 4 ) 2 cathodes. This strategy offers new insight into upcycling of iron sulfate waste and provides novel ideas for low-cost and high-performance Na 2 Fe(SO 4 ) 2 cathode materials preparation.