Investigation of metastable Na₂FeSiO₄ as a cathode material for Na-ion secondary battery

Li₂FeSiO₄ has been widely investigated as a promising cathode material due to its high theoretical capacity of 330 mAh g^-1 via two electron reactions. However, its Na-equivalent was not reported yet even it could considerably reduce the potential product costs due to the abundance of sodium resources in nature. In this study, sodium iron silicate with an empirical formula unit, Na₂FeSiO₄, was synthesized via solvothermal synthesis with two unavoidable impurities, Na₂SiO₃ and Fe₃O₄, for the first time. Galvanostatic charge/discharge test vs. Na/Na⁺ showed an initial discharge capacity of 126 mAh g^-1 at 1/40 C in a voltage range of 1.0-4.1 V (1C = 138 mA g^-1). However, during the initial charge, sodium iron silicate underwent irreversible crystal lattice breakdown and became amorphous.