Enhancing the rate capability of Na4Fe3(PO4)2(P2O7) by partial co-substitution of V3+ and Al3+ for Fe2+

Abstract

Sodium iron pyrophosphate (NFPP) is a cost-effective and safe cathode material for sodium-ion batteries (NIBs). However, NFPP suffers from low electron conductivity and sluggish Na+ diffusion kinetics, resulting in a poor rate capability. Here, we show that partial co-substitution of V3+ and Al3+ for Fe2+ synergistically enhances the rate capability of NFPP with improved Na+ storage capacity. The V3+/Al3+ co-substitution narrows the band gap energy of NFPP from 2.99 to 1.53 eV to increase electron conductivity, reduces Na+ diffusion energy barrier from 0.529 eV to 0.398 eV to enhance Na+ diffusivity, leads to the formation of Na vacancies to further facilitate Na+ transport, and improves Na+ storage capacity is because of enhanced electroactivity of the Fe2+/Fe3+ couple. At 0.1, 5 and 10C, the Na+ storage capabilities of the V3+/Al3+ co-substituted NFPP cathode are 120, 85 and 72 mAh g−1, respectively. At 5C, this electrode maintains 81% capacity after 1000 cycles. A full cell fabricated with the co-substituted NFPP as the cathode and a commercial hard carbon (HC) as the anode delivers a Na+ storage capacity of 87 mAh g−1 with 77% capacity retention after 100 cycles at 1C, outperforming a counterpart full cell fabricated with NFPP as the cathode and the anode and electrolyte.