Exploring Factors Limiting Three-Na<sup>+</sup> Extraction from Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub>

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Abstract

<p>NASICON-type Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> is a promising cathode material for Na-ion batteries. Although it is well known that two Na<sup>+</sup> can be extracted from Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> by charging the cathode material, an electrochemical three-Na<sup>+</sup> extraction has not been reported yet, to the best of our knowledge. In this work, we studied factors that limit the three-Na<sup>+</sup> extraction from Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub>. In DFT calculations, the voltage of the third-Na<sup>+</sup> extraction is predicted to be more than 4.5 V vs. Na<sup>+</sup>/Na<sup>0</sup>, which is above the potential windows of the conventional organic electrolytes. Our study of Na<sub>3</sub>V<sub>1.5</sub>Al<sub>0.5</sub>(PO<sub>4</sub>)<sub>3</sub> 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<sup>+</sup> extraction from the Na1 site is predicted to be 753 meV for NaV<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub>. Ab-initio molecular dynamics simulations also suggest that the Na ions which remain in NaV<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> are kinetically locked up in Na1 sites. Our results indicate that the three-Na<sup>+</sup> extraction is limited due to the high voltage and the large activation energy. We also compare Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> with Li<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub>, in which the three-Li<sup>+</sup> extraction has been reported.</p>
Original languageEnglish
Pages (from-to)457-462
Number of pages6
JournalElectrochemistry
Volume88
Issue number5
DOIs
Publication statusPublished - 2020

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