Structure and Electrochemistry of NaxFexMn1-xO₂(1.0 ≤ x ≤ 0.5) for Na-Ion Battery Positive Electrodes

The primary objective of research activities in Na-ion battery development is the implementation of inexpensive, sustainable and environmentally friendly battery materials. Here we report an investigation of Na x Fe x Mn 1-x O 2 electrode materials which contain elements commonly found in the earth's crust, have very low toxicity and cost, and can be prepared in open air atmosphere using simple procedures. Samples with x > 0.65 were found to have the same structure as O3 α-NaFeO 2 under selected synthesis conditions. Samples with x < 0.65 did not have the P2 or O3 structures, but retained a layered structure in which staging likely occurs. Samples having x = 0.5 were found to cycle reversibly at a capacity over 170 mAh/g (corresponding to a theoretical energy density of approximately 460 Wh/kg) with 85% capacity retention after 20 cycles. When the capacity is restricted to 135 mAh/g (340 Wh/kg), capacity retention approaches 95% after 20 cycles.