Investigation of Alkali-Ion (Li, Na, and K) Intercalation in KxVPO4 F(x ~ 0) Cathode

This work compares the intercalation of K, Na, and Li in KxVPO4F (x~0). The KxVPO4F (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 KxVPO4F (x~0) framework. While the material shows high rate capability for all the alkali ions, Li migration in KxVPO4F (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.