Probing the Limiting Mechanism of Sodium-Ion Extraction in the Na5V(PO4)2F2 Cathode

Abstract

Layered Na5V(PO4)2F2 has been recently identified as a cathode material for sodium-ion batteries due to its high voltage and excellent cycling stability. However, the capacity loss is a main challenge for the application of Na5V(PO4)2F2. Here, by using first-principles calculations and AIMD simulations based on density functional theory, we explored the structural evolution, structural stability, electronic properties, and sodium-ion diffusion of Na5V(PO4)2F2 during the desodiation process. After the removal of one sodium ion, the migration barrier of the sodium ion is 0.57 eV, which is significantly increased compared to Na5V(PO4)2F2. When two sodium ions are removed, the Na3V(PO4)2F2 will undergo a lattice deformation and decompose into VOPO4, NaF, and NaPO3 under standard conditions. The decreasing diffusivity will affect the rate capacity, while the decomposition of the structure during the deep sodium extraction may cause the irreversible capacity of the Na5V(PO4)2F2. In addition, density of states and spin moments analysis demonstrated that V participated in the redox reaction.