Sodium Extraction from NASICON-Structured Na3MnTi(PO4)3 through Mn(III)/Mn(II) and Mn(IV)/Mn(III) Redox Couples

Sodium extraction behavior in the rhombohedral Na3MnTi(PO4)3 with the NASICON structure was investigated and shown to have a 0.5 eV separation of the localized-electron Mn3+/Mn2+ and Mn4+/Mn3+ redox energies. Used as a cathode in a sodium-ion half-cell, the redox couples of Mn3+/Mn2+ and Mn4+/Mn3+ exhibit two voltage plateaus located at about 3.6 and 4.1 V vs Na+/Na, respectively. Na3MnTi(PO4)3 also exhibits a small volume change after the extraction of two sodium ions, and the NASICON structure is well-maintained after repeated extraction and insertion of sodium ions.