Development of a New Mixed-Polyanion Cathode with Superior Electrochemical Performances for Na-Ion Batteries

We report on Na4[Mn1.5Co1.5](PO4)2(P2O7) as a high-energy-density cathode material for sodium-ion batteries. Detailed structural information for Na4[Mn1.5Co1.5](PO4)2(P2O7) is obtained from Rietveld refinement of X-ray diffraction data. We confirm that 2.7 mol Na+ ions per formula unit (116 mA h g–1) can be (de)intercalated from the host Na4[Mn1.5Co1.5](PO4)2(P2O7) structure using first-principles calculation and various electrochemical tests. Moreover, Na4[Mn1.5Co1.5](PO4)2(P2O7) exhibits that 73% of the capacity at C/10 is retained even at 10 C (84 mA h g–1). Notably, Na4[Mn1.5Co1.5](PO4)2(P2O7) delivers a high capacity retention of ∼75% compared with the initial capacity for 300 cycles with a high Coulombic efficiency of more than 99%. The redox reaction mechanism of Na4[Mn1.5Co1.5](PO4)2(P2O7) during charge/discharge is also confirmed through combined studies using first-principles calculation and operando X-ray absorption near-edge structure analyses.