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.