Investigation of Alkali‐Ion (Li, Na, and K) Intercalation in K_xVPO₄F (x ∼ 0) Cathode

This work compares the intercalation of K, Na, and Li in K<sub>x</sub>VPO<sub>4</sub>F (x~0). The K<sub>x</sub>VPO<sub>4</sub>F (x~0) cathode delivers reversible capacities of ≈90-100 mAh g<sup>-1</sup> 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<sub>x</sub>VPO<sub>4</sub>F (x~0) framework. While the material shows high rate capability for all the alkali ions, Li migration in K<sub>x</sub>VPO<sub>4</sub>F (x~0) is more difficult than with Na and K. Here, 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.