Upcycling Iron Sulfate Waste into Low-Cost and High-Stability Na₂Fe(SO₄)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 Na2Fe(SO4)2 cathode materials for sodium-ion batteries (SIBs). The obtained Na2Fe(SO4)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 Na2Fe(SO4)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 Na2Fe(SO4)2 materials, thereby enhancing the electrochemical reaction kinetics, such as the Na ions diffusion coefficient and capacitive contribution for high stability of Na2Fe(SO4)2 cathodes. This strategy offers new insight into upcycling of iron sulfate waste and provides novel ideas for low-cost and high-performance Na2Fe(SO4)2 cathode materials preparation.