Development of High-Performance Iron-Based Phosphate Cathodes toward Practical Na-Ion Batteries

Iron-based phosphate cathode of Na₄Fe₃(PO₄)₂(P₂O₇) has been regarded as a low-cost and structurally stable cathode material for Na-ion batteries (NIBs). However, their practical application is greatly hindered by the insufficient electrochemical performance and limited energy density. Here, we report a new iron-based phosphate cathode of Na_4.5Fe_3.5(PO₄)_2.5(P₂O₇) with the intergrown heterostructure of the maricite-type NaFePO₄ and orthorhombic Na₄Fe₃(PO₄)₂(P₂O₇) phases at a mole ratio of 0.5:1. Benefited from the increased composition ratio and the spontaneous activation of the maricite-type NaFePO₄ phase, the as-prepared Na_4.5Fe_3.5(PO₄)_2.5(P₂O₇) composites deliver a reversible capacity over 130 mA h g^-1 and energy density close to 400 W h kg^-1, which is far beyond that of the single-phase Na₄Fe₃(PO₄)₂(P₂O₇) cathode (∼120 mA h g^-1 and ∼350 W h kg^-1). Moreover, the kg-level products from the scale-up synthesis demonstrate a stable cycling performance over 2000 times at 3 C in pouch cells. We believe that our findings could show the way forward the practical application of the iron-based phosphate cathodes for NIBs.