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

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Abstract
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.
Funding Information
  • U.S. Department of Energy
  • Basic Energy Sciences
  • Division of Materials Sciences and Engineering (DE‐SC0005397)

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