A Novel NASICON-Type Na₄ MnCr(PO₄ )₃ Demonstrating the Energy Density Record of Phosphate Cathodes for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have attracted incremental attention as a promising candidate for grid-scale energy-storage applications. To meet practical requirements, searching for new cathode materials with high energy density is of great importance. Herein, a novel Na superionic conductor (NASICON)-type Na₄ MnCr(PO₄ )₃ is developed as a high-energy cathode for SIBs. The Na₄ MnCr(PO₄ )₃ nanoparticles homogeneously embedded in a carbon matrix can present an extraordinary reversible capacity of 160.5 mA h g^-1 with three-electron reaction at ≈3.53 V during the Na⁺ extraction/insertion process, realizing an unprecedentedly high energy density of 566.5 Wh kg^-1 in the phosphate cathodes for SIBs. It is intriguing to reveal the underlying mechanism of the unique Mn²⁺ /Mn³⁺ , Mn³⁺ /Mn⁴⁺ , and Cr³⁺ /Cr⁴⁺ redox couples via X-ray absorption near-edge structure spectroscopy. The whole electrochemical reaction undergoes highly reversible single-phase and biphasic transitions with a moderate volume change of 7.7% through in situ X-ray diffraction and ex situ high-energy synchrotron X-ray diffraction. Combining density functional theory (DFT) calculations with the galvanostatic intermittent titration technique, the superior performance is ascribed to the low ionic-migration energy barrier and desirable Na-ion diffusion kinetics. The present work can offer a new insight into the design of multielectron-reaction cathode materials for SIBs.