This work compares the intercalation of K, Na, and Li in K x VPO 4 F (x ∼ 0). The K x VPO 4 F (x ∼ 0) cathode delivers reversible capacities of ≈90-100 mAh g −1 in K, Na, and Li cells, at an average voltage of ≈4.33 V for K, ≈3.98 V for Na, and ≈3.96 V for Li. This is so far the highest average voltage known for a K-intercalation cathode. The lower voltage of Li insertion compared to Na is attributable to undercoordinated Li ions in the K x VPO 4 F (x ∼ 0) framework. While the material shows high rate capability for all the alkali ions, Li migration in K x VPO 4 F (x ∼ 0) is more difficult than with Na and K. This work suggests that a large cavity is not always good for insertion of alkali ions and cathode materials need to be suitably tailored to each intercalating ion species.

NASICON-type Na 3 V 2 (PO 4 ) 3 is a promising cathode material for Na-ion batteries. Although it is well known that two Na + can be extracted from Na 3 V 2 (PO 4 ) 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 Na 3 V 2 (PO 4 ) 3 . In DFT calculations, the voltage of the third-Na + extraction is predicted to be more than 4.5 V vs. Na + /Na 0 , which is above the potential windows of the conventional organic electrolytes. Our study of Na 3 V 1.5 Al 0.5 (PO 4 ) 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 NaV 2 (PO 4 ) 3 . Ab-initio molecular dynamics simulations also suggest that the Na ions which remain in NaV 2 (PO 4 ) 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 Na 3 V 2 (PO 4 ) 3 with Li 3 V 2 (PO 4 ) 3 , in which the three-Li + extraction has been reported.

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