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

Although O3‐NaFe 1/2 Mn 1/2 O 2 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 O 2 (O3‐Na 5/6 FeMn) and O3‐Na 5/6 Fe 1/3 Mn 1/2 Me 1/6 O 2 (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.