Copper-induced lattice distortion in Na4Fe3(PO4)2(P2O7) cathode enabling high power density Na-ion batteries with good cycling stability

Abstract

Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) has attracted attention due to its high theoretical capacity, low cost, and good structure stability for Na-ion batteries. However, its practical application is limited by the low intrinsic electronic conductivity and sluggish ion diffusion kinetics. To tackle those problems, lattice strain engineering by heteroatom doping is applied to tune the local molecular structure and optimize the electrochemical properties of electrode materials. Copper cation (+2) with a smaller ionic radius was chosen to dope at Fe-site in NFPP, and the doping amount was also optimized, illustrating the optimized sample of Na₄Fe_2.7Cu_0.3(PO₄)₂P₂O₇ (NFPP-0.3Cu) delivered a high capacity of 119.01 mAh g^-1 at 1 C (1 C = 129 mA g^-1) and a high capacity retention of 82.76 % after 3000 cycles at 20 C. Notably, full cells with NFPP-0.3 Cu as cathode and hard carbon as anode delivered a high energy density of 230 Wh kg^-1 and a power density of 2280 W kg^-1. The exceptional electrochemical performances are attributed to the modulated electronic structure and abundant lattice defects by Cu-doping. Furthermore, in-situ X-ray diffraction technique and theoretical calculation have jointly proved that the lattice distortions originated from Cu-doping have reduced the band gap of the NFPP-0.3Cu and altered the coordination environment of Fe, shortening the Fe-O and Cu-O bond lengths, significantly enhancing the intrinsic ionic conductivity and the diffusion kinetics of Na⁺. This work provides new point of views on lattice strain engineering and reaction kinetics of cathode materials in promoting high energy and power density sodium-ion batteries.