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.