Na3V2-xFex(PO4)2O2F: An advanced cathode material with ultra-high stability for superior sodium storage

Abstract

Searching a suitable electrode material is one of the key issues of Sodium-ion batteries (SIBs) for the large-scale applications. Sodium-rich Na3V2(PO4)O2F (NVPOF) with a 3D tunnel structure is considered to be the promising cathode material for SIBs owning to the advantage of its high operating voltage, andstable structure, and good thermal stability. Nevertheless, the inferior reaction kinetics and unsatisfied sodium storage behavior resulting from its low electron conductivity hinder its further practical application. Here, Na3V2-xFex(PO4)2O2F microcuboids were synthesized via iron partially replacing V atom sites of Na3V2(PO4)2O2F by a simple hydrothermal method. Benefiting from the intrinsically improved electronic conductivity and enhanced charge transfer kinetics, Na3V1.85Fe0.15(PO4)2O2F (NVFPOF) with 15 wt% doping concentration shows an initial capacity of 137.2 mAh g−1 at 1C, which is about 31% higher than that of NVPOF. In addition, a full cell based on NVFPOF cathode and Hard carbon anode showed 82.3% capacity retention even after 100 cycles at 0.4C. Moreover, iron atoms can be used as the pillar of the host structure to buffer deformation and effectively alleviate the volume change (less than 1.05%) upon cycling, showing 86.5% capacity retention at 20C after 1000 cycles. The results provide an effective and simple way to construct advanced cathodes for SIBs.