Kagomé lattices as cathode: Effect of particle size and fluoride substitution on electrochemical lithium insertion in sodium- and ammonium Jarosites

Highly crystalline sodium and ammonium Jarosites, NaFe₃(SO₄)₂(OH)₆ and NH₄Fe₃(SO₄)₂(OH)₆, have been synthesized employing hydrothermal synthesis routes. The structures consist of anionic layers of vertex-sharing FeO₆ octahedra and SO₄ tetrahedral units with interlayer space occupied by Na and ammonium ions, respectively. The corner-sharing FeO₆ octahedral units form six and three rings similar to hexagonal tungsten bronze sheets also known as kagomé lattice. These sodium and ammonium Jarosites are thermally stable up to 400 °C and undergo facile electrochemical lithium insertion through the reduction of Fe³⁺ to Fe²⁺. Galvanostatic charge–discharge indicates that up to 2.25 and 2 lithium ions per formula unit can be inserted at an average voltage of 2.49 and 2.26 V to the sodium and the ammonium Jarosites, respectively, under slow discharge rate of C/50. The cycle-life and experimental achievable capacity show strong dependence on particle sizes and synthesis conditions. A small amount of fluoride substitution improves both achievable capacity and average voltage. - Graphical abstract: Discharge capacity of jarosite phases as a function of particle size and fluoride substitution. - Highlights: • Synthesis of natro- and ammonium Jarosites. • Jarosites as cathodes for lithium ion batteries. • Li-ion electrochemistry of Jarosites. • Mössbauer spectroscopy of Jarosites.