Electrochemical and thermal properties of α-NaFeO₂ cathode for Na-ion batteries

α-NaFeO₂ is promising as minor-metal free cathode materials for low cost sodium-ion batteries. It has a flat voltage plateau at 3.3 V vs. Na metal and a stable reversible capacity of 85 mAh g^-1. Fe³⁺/Fe⁴⁺ redox reaction on charge/discharge cycle was confirmed by ⁵⁷Fe Mössbauer spectrometer. The thermal stability of NaFeO₂ cathodes with/without 1 mol dm^-3 NaClO ₄/EC-DMC electrolyte was investigated by DSC measurements. The fully-charged Na_0.58FeO₂ powder decomposed thermally at a temperature higher than 300°C, with Fe₂O₃ as a possible product. On the other hand, the mixture of Na_0.58FeO ₂ powder and electrolyte showed exothermic heat in a temperature range of 220-300°C. However, NaFeO₂ showed better thermal stability in the electrolyte than LiCoO₂ counterparts in Li ion battery systems, including less heat generation and higher exothermic onset temperature. This indicated that the Na-ion batteries might have comparable thermal stability with Li-ion batteries.