Phase Purification and Crystallology Engineering Enable Fast-Charging Na5Fe2.9Ni0.1(PO4)2(P2O7) Cathode for Wide-Temperature Sodium-Ion Batteries

Abstract

Na4Fe3(PO4)2(P2O7) (NFPP) holds considerable promise among sodium-ion batteries (SIBs) cathodes benefit from non-toxicity and structural stability. However, the concomitant inactive NaFePO4 impurities (NFP) and low intrinsic electronic conductivity hinder the practical application, especially for fast-charging and wide-temperature utilization. Herein, a novel NFP absent phase-purified Na5Fe2.9Ni0.1(PO4)2(P2O7) (N5FNPP) with high electronic conductivity and low Na+ diffusion barrier is successfully developed through Na enrichment and Ni introducing. The incorporated strategy aiming at hybrid orbital diversification (Ni─O) and spin-state transition (Fe 3d) is predicted by DFT calculation to narrow the band gap, as well as the structural framework stabilizing and Na+ diffusion accelerating, induced by lattice tuning and electronic delocalization. Benefiting from the original coupling designs, the N5FNPP cathode exhibits an excellent discharging capacity (112.9 mAh g⁻¹ at 0.1 C), and outstanding fast-charging performance (87.1 mAh g⁻¹ at 10 C, 95% for 2000 cycles). Particularly, it exhibits excellent applicability in a wide temperature range of −30–60 °C. This work sheds light on the comprehensive consideration of electron tuning, lattice adjustment, and phase purification on advanced polyanion phosphate cathode construction, as well as provides a guideline for fabricating other advanced cathodes and fast-charging, wide-temperature SIBs.