Layered Na₂Mn₃O₇ as a 3.1 V Insertion Material for Li-Ion Batteries

Abstract

Exploring Mn-based battery insertion materials, layered Na2Mn3O7 was synthesized via single pot solid-state method. Rietveld analysis confirmed the formation of triclinic structured Na2Mn3O7 with P-1 symmetry. With no further optimization, the as-synthesized Na2Mn3O7 was found to be an efficient host for Li+ (de)intercalation. Na2Mn3O7 delivered a discharge capacity of ∼160 mA h g–1 by altering the redox couple Mn(IV)/Mn(III) with a nominal voltage of 3.12 V (vs Li/Li+). Interestingly, the first discharge of pristine Na2Mn3O7 led to the formation of LixNa2Mn3O7 (2 < x < 3) having completely different phase similar to the LiCoO2 structure. Lithiation triggered phase transformation from triclinic (P-1) to trigonal (R-3m) structure. This electrochemically reformed structure imparts stability to [Mn3O7]−2∞ building layers, preventing irreversible capacity loss. Underlying a single-phase (solid-solution) redox mechanism, Na2Mn3O7 acts as a robust cathode material for Li-ion batteries.