Accessing Cation and Anion Redox in a 1-D Iron-Chalcogenide, Na₃Fe₂S4-XSex (x = 0, 2, & 4), in Na-Ion Batteries

The utilization of anionic redox activity is gaining significant interest in chalcogen based alkali-ion batteries. In this study, we are reporting reversible Na-ion (de)intercalation in one-dimensional structure of Na 3 Fe 2 S 4-x Se x (x = 0, 2, and 4). The crystal structure of Na 3 Fe 0.5 2+ Fe 0.5 3+ Q 4 (Q = S and Se) can be visualized as a mixed valent iron, Fe 2+ /Fe 3+ , forming infinite edge-shared chain of Fe Q 4 tetrahedra with Na-ion residing in the interchain voids. While cycling the cathodes within a potential range of 1.5 – 2.5 V vs. Na/Na+, we could reversibly (de)intercalate one mole of Na from the structure. Mössbauer spectroscopy reveals a complete Fe 2+ to Fe 3+ oxidation in all the charged samples. Substitution of less electronegative Se 2– in the structure systematically lowers the Fe 2+/3+ redox couple during cycling. After 100 cycles at 1C rate, the cathode retains 83 % of its initial reversible capacity. While cycling the same cathode at a higher voltage range of 1.5 – 3.0 V, we observed a flat plateau profile at ~2.6 V with capacity reaching 255 mAh/g for Na 3 Fe 2 S 4 cathode. This high capacity can be directly attributed to the anion oxidation for the charge compensation process during ~3.0 Na-ion deintercalation as supported by XPS analysis. The flat plateau profile further decreases from 2.6 to 2.5 V with the substitution of Se 2– . The GITT results show two distinct kinetic behavior for the cation (fast) and anion (sluggish) redox. The decoupling of cation and anion redox in this system can enable us to understand and optimize the utilization of both redox chemistries to achieve a high energy density cathode.