Exploring Factors Limiting Three-Na+ Extraction from Na3V2(PO4)3

NASICON-type Na3V2(PO4)3 is a promising cathode material for Na-ion batteries. Although it is well known that two Na+ can be extracted from Na3V2(PO4)3 by charging the cathode material, an electrochemical three-Na+ extraction has not been reported yet, to the best of our knowledge. In this work, we studied factors that limit the three-Na+ extraction from Na3V2(PO4)3. In DFT calculations, the voltage of the third-Na+ extraction is predicted to be more than 4.5 V vs. Na+/Na0, which is above the potential windows of the conventional organic electrolytes. Our study of Na3V1.5Al0.5(PO4)3 reveals that such a high voltage is required when Na ions are extracted from Na1 sites in the NASICON structure. From NEB calculations, the activation energy of the Na+ extraction from the Na1 site is predicted to be 753 meV for NaV2(PO4)3. Ab-initio molecular dynamics simulations also suggest that the Na ions which remain in NaV2(PO4)3 are kinetically locked up in Na1 sites. Our results indicate that the three-Na+ extraction is limited due to the high voltage and the large activation energy. We also compare Na3V2(PO4)3 with Li3V2(PO4)3, in which the three-Li+ extraction has been reported.