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.