Na₃MnZr(PO₄)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 Na₃MnZr(PO₄)₃ 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 Mn⁴⁺/Mn³⁺ and Mn³⁺/Mn²⁺ redox couples are reversibly accessed in Na₃MnZr(PO₄)₃, providing high discharge voltage plateaus at 4.0 and 3.5 V, respectively. A high discharge capacity of 105 mAh g^-1 was obtained from Na₃MnZr(PO₄)₃ with a small variation of lattice parameters and a small volume change on extraction of two Na⁺ ions per formula unit. Moreover, Na₃MnZr(PO₄)₃ 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 Na₃MnZr(PO₄)₃ where Mn and Zr are disordered within the structure. We further show that the cooperative Jahn-Teller distortion of Mn³⁺ is suppressed in the cathode and that Na₂MnZr(PO₄)₃ is a stable phase.