A Class of Sodium Transition-Metal Sulfide Cathodes With Anion Redox

Sodium-ion batteries (SIBs) are entering commercial relevance as a sustainable and low-cost alternative to lithium-ion batteries. Improving the energy density of SIBs is critical to enable their widespread adoption. Here, a new class of cathode materials Na6MS4 (M = Co, Mn, Fe, and Zn) that exhibit high charge-storage capacity is reported. Using Na6CoS4 as a prototypical example, a six-electron conversion reaction dominated by anion redox is observed, confirmed through various electrochemical and spectroscopic techniques. After the initial cycle, Na6CoS4 delivers a high capacity of 392 mA h g-1 with a long lifespan of over 500 cycles. The reaction involves, initially, the transformation of crystalline Na6CoS4 to a nearly amorphous structure consisting of mainly CoS and sulfur nanoparticles, which then reversibly cycles between nearly amorphous a-CoS/S and a-Na6CoS4. Such anion-redox-driven conversion-type cathodes hold the potential to enable energy-dense, stable SIBs. A new class of Na6MS4 (M = Mn, Fe, Co, and Zn) with anion redox is demonstrated as cathodes for sodium-ion batteries. As a model system, Na6CoS4 is found during the initial charge process to transform from a highly crystalline to an amorphous structure, mainly consisting of nanosized CoS and sulfur particles. Afterward, the conversion is highly reversible between amorphous CoS/S and Na6CoS4. image