Full Activation of Mn4+ /Mn3+ Redox in Na4 MnCr(PO4 )3 as a High-Voltage and High-Rate Cathode Material for Sodium-Ion Batteries.

Abstract

Developing high-voltage cathode materials is critical for sodium-ion batteries to boost energy density. NASICON (Na super-ionic conductor)-structured Nax MnM(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 Na4 MnCr(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 Na4 MnCr(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 Na4 MnCr(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.