Investigation of Alkali‐Ion (Li, Na, and K) Intercalation in K x VPO 4 F ( x ∼ 0) Cathode

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.