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

The V4+/V3+ (3.4 V) redox couple has been well-documented in cathode material Na3V2(PO4)(3) for sodium-ion batteries. Recently, partial cation substitution at the vanadium site of Na3V2(PO4)(3) has been actively explored to access the V5+/V4+ redox couple to achieve high energy density. However, the V5+/V4+ redox couple in partially substituted Na3V2(PO4)(3) has a voltage far below its theoretical voltage in Na3V2(PO4)(3), and the access of the V5+/V4+ redox reaction is very limited. In this work, we compare the extraction/insertion behavior of sodium ions from/into two isostructural compounds of Na3VGa(PO4)(3) and Na3VAl(PO4)(3), found that, by DFT calculations, the lower potential of the V5+/V4+ redox couple in Na3VM(PO4)(3) (M = Ga or Al) than that in Na3V2(PO4)(3) is because of the extraction/insertion of sodium ions through the V5+/V4+ redox reaction at different crystallographic sites, that is, sodium ions extracting from the Na(2) site in Na3VM(PO4) 3 while from the Na(1) site in Na3V2(PO4)(3), and further evidenced that the full access of the V5+/V4+ redox reaction is restrained by the excessive diffusion activation energy in Na3VM(PO4)(3).

Automated summary

The study compares the extraction/insertion behavior of sodium ions in two isostructural compounds and finds that the voltage of the V5+/V4+ redox couple in partially substituted Na3V2(PO4)3 is lower due to excessive diffusion activation energy.