Full Activation of Mn⁴⁺/Mn³⁺ Redox in Na₄MnCr(PO₄)₃ as a High-Voltage and High-Rate Cathode Material for Sodium-Ion Batteries

Developing high-voltage cathode materials is critical for sodium-ion batteries to boost energy density. NASICON (Na super-ionic conductor)-structured Na_xMnM(PO₄)₃ materials (M represents transition metal) have drawn increasing attention due to their features of robust crystal framework, low cost, as well as high voltage based on Mn⁴⁺/Mn³⁺ and Mn³⁺/Mn²⁺ redox couples. However, full activation of Mn⁴⁺/Mn³⁺ redox couple within NASICON framework is still a great challenge. Herein, a novel NASICON-type Na₄MnCr(PO₄)₃ material with highly reversible Mn⁴⁺/Mn³⁺ redox reaction is discovered. It proceeds a two-step reaction with voltage platforms centered at 4.15 and 3.52 V versus Na⁺/Na, delivering a capacity of 108.4 mA h g⁻¹. The Na₄MnCr(PO₄)₃ cathode also exhibits long durability over 500 cycles and impressive rate capability up to 10 C. The galvanostatic intermittent titration technique (GITT) test shows fast Na diffusivity which is further verified by density functional theory calculations. The high electrochemical activity derives from the 3D robust framework structure, fast kinetics, and pseudocapacitive contribution. The sodium storage mechanism of the Na₄MnCr(PO₄)₃ cathode is deeply studied by ex situ X-ray diffraction (XRD) and ex situ X-ray photoelectron spectroscopy (XPS), revealing that both solid-solution and two-phase reactions are involved in the Na⁺ ions extraction/insertion process.