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

Iron-based phosphate cathode of Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) 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 4.5 Fe 3.5 (PO 4 ) 2.5 (P 2 O 7 ) with the intergrown heterostructure of the maricite-type NaFePO 4 and orthorhombic Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) phases at a mole ratio of 0.5:1. Benefited from the increased composition ratio and the spontaneous activation of the maricite-type NaFePO 4 phase, the as-prepared Na 4.5 Fe 3.5 (PO 4 ) 2.5 (P 2 O 7 ) 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 Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) 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.