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

<p>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 Na₃V_1.5Al_0.5(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.</p>