Natrium superionic conductor (NASICON)-type phosphates have aroused a great interest as cathode materials for sodium-ion batteries (SIBs) by virtue of their stable 3-dimensional frameworks, flexible molecular formula tunability, and superior ionic conductivity. Nevertheless, the intrinsic low electronic conductivity and relatively low theoretical specific capacity place obstacles in their way toward achieving higher electrochemical performance. In addition, only 2-electron reactions in most NASICON cathodes and poor reversibility of high-voltage redox couples severely limit their energy density. To address the above tough issues, an in-depth understanding of transition metal selection, elements ratio optimization, and Na-storage mechanism is of paramount importance. Here, this mini review summarizes the latest progresses on the NASICON-type phosphate cathodes for SIBs from the perspective of redox couple modulation. NASICON cathodes featuring high operating voltage and multielectron reactions are discussed in detail. Finally, the remaining challenges and personal outlooks based on redox couple regulation are put forward, shedding light on the designing rules for high-energy and long-durability NASICON-type phosphate cathodes for SIBs in the future.

The development of high-energy and long-lifespan NASICON-type cathode materials for sodium-ion batteries has always been a research hotspot but a daunting challenge. Although Na₄MnCr(PO₄)₃ has emerged as one of the most promising high-energy-density cathode materials owing to its three-electron reactions, it still suffers from serious structural distortion upon repetitive charge/discharge processes caused by the Jahn-Teller active Mn³⁺. Herein, the selective substitution of Cr by Zr in Na₄MnCr(PO₄)₃ was explored to enhance the structural stability, due to the pinning effect of Zr ions and the ≈2.9-electron reactions, as-prepared Na_3.9MnCr_0.9Zr_0.1(PO₄)₃/C delivers a high capacity retention of 85.94% over 500 cycles at 5 C and an ultrahigh capacity of 156.4 mAh g⁻¹ at 0.1 C, enabling the stable energy output as high as 555.2 Wh kg⁻¹. Moreover, during the whole charge/discharge process, a small volume change of only 6.7% was verified by in situ X-ray diffraction, and the reversible reactions of Cr³⁺/Cr⁴⁺, Mn³⁺/Mn⁴⁺, and Mn²⁺/Mn³⁺ redox couples were identified via ex situ X-ray photoelectron spectroscopy analyses. Galvanostatic intermittent titration technique tests and density functional theory calculations further demonstrated the fast reaction kinetics of the Na_3.9MnCr_0.9Zr_0.1(PO₄)₃/C electrode. This work offers new opportunities for designing high-energy and high-stability NASICON cathodes by ion doping.