Investigation of Alkali-Ion (Li, Na, and K) Intercalation in KxVPO4F (x ∼ 0) Cathode

This work compares the intercalation of K, Na, and Li in KxVPO4F (x similar to 0). The KxVPO4F (x similar to 0) cathode delivers reversible capacities of approximate to 90-100 mAh g(-1) in K, Na, and Li cells, at an average voltage of approximate to 4.33 V for K, approximate to 3.98 V for Na, and approximate to 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 KxVPO4F (x similar to 0) framework. While the material shows high rate capability for all the alkali ions, Li migration in KxVPO4F (x similar to 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.