Na 3 MnZr(PO 4 ) 3 : A High-Voltage Cathode for Sodium Batteries

Sodium batteries have been regarded as promising candidates for large-scale energy storage application, provided cathode hosts with high energy density and long cycle life can be found. Herein, we report NASICON-structured Na MnZr(PO ) as a cathode for sodium batteries that exhibits an electrochemical performance superior to those of other manganese phosphate cathodes reported in the literature. Both the Mn /Mn and Mn /Mn redox couples are reversibly accessed in Na MnZr(PO ) , providing high discharge voltage plateaus at 4.0 and 3.5 V, respectively. A high discharge capacity of 105 mAh g was obtained from Na MnZr(PO ) with a small variation of lattice parameters and a small volume change on extraction of two Na ions per formula unit. Moreover, Na MnZr(PO ) exhibits an excellent cycling stability, retaining 91% of the initial capacity after 500 charge/discharge cycles at 0.5 C rate. On the basis of structural analysis and density functional theory calculations, we have proposed a detailed desodiation pathway from Na MnZr(PO ) where Mn and Zr are disordered within the structure. We further show that the cooperative Jahn-Teller distortion of Mn is suppressed in the cathode and that Na MnZr(PO ) is a stable phase.