Synthesis and Electrochemistry of Na2.5(Fe1−yMny)1.75(SO4)3 Solid Solutions for Na‐Ion Batteries

Abstract Na 2+2 x Fe 2− x (SO 4 ) 3 was recently reported as a promising cathode material for Na‐ion batteries with high voltage generation (3.8 V vs. Na/Na + ), high energy density, and extreme high‐rate operation, making it competitive to lithium battery cathodes. Following our identification of the narrow off‐stoichiometric region in Na 2+2 x Fe 2− x (SO 4 ) 3 solid solutions ( x around 0.25), we report here the effect of Mn substitution in Na 2.5 (Fe 1− y Mn y ) 1.75 (SO 4 ) 3 ( y =0, 0.25, 0.5, 0.75, and 1.0) solid solutions when increasing the voltage of the Fe 3+ /Fe 2+ redox couple, but with a simple capacity decrease, owing to the entire Mn 2+ inactivity.