NASICON-structured Na₃Fe₂PO₄(SO₄)₂: a potential cathode material for rechargeable sodium-ion batteries

The cost-effective and abundant availability of sodium offers an opportunity for rechargeable Na-ion batteries as an ideal replacement for rechargeable Li-ion batteries. However, the larger size and strong Na⁺-Na⁺ interaction create multidimensional phase instability and transformation problems, especially in layer-structured NaMO₂ (Mn, Co, Fe, and Ni) that inhibit the direct transformation of rechargeable Li-ion battery technology to Na-ion batteries. However, framework structures offer superior structural stability due to the interconnection of polyanions or polyhedra forming cationic octahedra. Sodium superionic conductor (NASICON)-type structures are well known for their superior Na⁺ ion transport and are identified as intercalative hosts as electrodes for rechargeable Na-ion batteries. Here, we report the synthesis of Na₃Fe₂PO₄(SO₄)₂ in a NASICON framework structure and its investigation as a cathode in a Na/Na₃Fe₂PO₄(SO₄)₂ cell working on the Fe³⁺/Fe²⁺ redox couple. The cell provides a single-phase reaction having a capacity approaching 70 mA h g^-1 at 0.1 C after 50 cycles in the voltage range of 2 to 4.2 V, with a columbic efficiency approaching 100%. The large availability of Na and Fe with the stable redox and charge/discharge performance of NASICON-type Na₃Fe₂PO₄(SO₄)₂ make it a possible cathode candidate for next-generation rechargeable sodium-ion batteries.