Elucidating Influence of Mg- and Cu-Doping on Electrochemical Properties of O3-Nax [Fe,Mn]O2 for Na-Ion Batteries

Abstract

Although O3-NaFe_1/2 Mn_1/2 O₂ delivers a large capacity of over 150 mAh g^-1 in an aprotic Na cell, its moist-air stability and cycle stability are unsatisfactory for practical use. Slightly Na-deficient O3-Na_5/6 Fe_1/2 Mn_1/2 O₂ (O3-Na_5/6 FeMn) and O3-Na_5/6 Fe_1/3 Mn_1/2 Me_1/6 O₂ (Me = Mg or Cu, O3-FeMnMe) are newly synthesized. The Cu and Mg doping provides higher moist-air stability. O3-Na_5/6 FeMn, O3-FeMnCu, and O3-FeMnMg deliver first discharge capacities of 193, 176, and 196 mAh g^-1 , respectively. Despite partial replacement of Fe with redox inactive Mg, oxide ions in O3-FeMnMg participate in the redox reaction more apparently than O3-Na_5/6 FeMn. X-ray diffraction studies unveil the formation of a P-O intergrowth phase during charging up to >4.0 V.

Keywords: anion redox; layered oxides; positive electrode materials; sodium intercalation; sodium-ion batteries.