Elevating Energy Density for Sodium-Ion Batteries through Multielectron Reactions.

It remains a great challenge to explore desirable cathodes for sodium-ion batteries to satisfy the ever-increasing demand for large-scale energy storage systems. In this Letter, we report a NASICON-structured Na4MnCr(PO4)3 cathode with high specific capacity and operation potential. The reversible access of the Mn2+/Mn3+ (3.75/3.4 V), Mn3+/Mn4+ (4.25/4.1 V), and Cr3+/Cr4+ (4.4/4.3 V vs Na/Na+) redox couples in a Na4MnCr(PO4)3 cathode endows a distinct three-electron redox reaction during the insertion/extraction process. The highly stable NASICON structure with a small volume variation upon cycling ensures long-time cycling stability (73.3% capacity retention after 500 cycles within the potential region of 2.5-4.6 V). The impedance analysis and interface characterization indicate that the evolution of a cathode electrolyte interphase at high potential is correlated with the capacity fading, while the robustness of the NASICON framework is redemonstrated.