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 Na 3 MnTi(PO 4 ) 3 with the NASICON structure was investigated and shown to have a 0.5 eV separation of the localized-electron Mn 3+ /Mn 2+ and Mn 4+ /Mn 3+ redox energies. Used as a cathode in a sodium-ion half-cell, the redox couples of Mn 3+ /Mn 2+ and Mn 4+ /Mn 3+ exhibit two voltage plateaus located at about 3.6 and 4.1 V vs Na + /Na, respectively. Na 3 MnTi(PO 4 ) 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.