Importance of Crystallographic Sites on Sodium-Ion Extraction from NASICON-Structured Cathodes for Sodium-Ion Batteries.

The V/V (3.4 V) redox couple has been well-documented in cathode material NaV(PO) for sodium-ion batteries. Recently, partial cation substitution at the vanadium site of NaV(PO) has been actively explored to access the V/V redox couple to achieve high energy density. However, the V/V redox couple in partially substituted NaV(PO) has a voltage far below its theoretical voltage in NaV(PO), and the access of the V/V redox reaction is very limited. In this work, we compare the extraction/insertion behavior of sodium ions from/into two isostructural compounds of NaVGa(PO) and NaVAl(PO), found that, by DFT calculations, the lower potential of the V/V redox couple in NaVM(PO) (M = Ga or Al) than that in NaV(PO) is because of the extraction/insertion of sodium ions through the V/V redox reaction at different crystallographic sites, that is, sodium ions extracting from the Na(2) site in NaVM(PO) while from the Na(1) site in NaV(PO), and further evidenced that the full access of the V/V redox reaction is restrained by the excessive diffusion activation energy in NaVM(PO).