High-Entropy Doping Boosts Ion/Electronic Transport of Na4 Fe3 (PO4 )2 (P2 O7 )/C Cathode for Superior Performance Sodium-Ion Batteries.

Abstract

Fe-based mixed phosphate cathodes for Na-ion batteries usually possess weak rate capacity and cycle stability challenges resulting from sluggish diffusion kinetics and poor conductivity under the relatively low preparation temperature. Here, the excellent sodium storage capability of this system is obtained by introducing the high-entropy doping to enhance the electronic and ionic conductivity. As designed high-entropy doping Na4 Fe2.85 (Ni,Co,Mn,Cu,Mg)0.03 (PO4 )2 P2 O7 (NFPP-HE) cathode can release 122 mAh g-1 at 0.1 C, even 85 mAh g-1 at ultrahigh rate of 50 C, and keep a high retention of 82.3% after 1500 cycles at 10 C. Besides, the cathode also exhibits outstanding fast charge capacity in terms of the cyclability and capacity with 105 mAh g-1 at 5 C/1 C, corresponding 94.3% retention after 500 cycles. The combination of in situ X-ray diffraction, density functional theory, conductive-atomic force microscopy, and galvanostatic intermittent titration technique tests reveal that the reversible structure evolution with optimized Na+ migration path and energy barrier boost the Na+ kinetics and improve the interfacial electronic transfer, thus improving performance.