K x VPO4f (x~0): A Host Structure for Versatile Alkali Ion Intercalation

Abstract

Alkali-ion intercalation compounds are commonly used cathode materials for rechargeable batteries, including Li-, Na-, and K-ion batteries. In our previous works, we developed KVPO4F as a high voltage cathode material for K-ion batteries.[1, 2] The KVPO4F framework has large void spaces that can accommodate facile K ion intercalation. In this present work, we investigated KVPO4F as a host structure for versatile alkali ion (Li, Na, and K) intercalation after fully removing the K ions.[3] Interestingly, our work finds that the voltage for Na insertion is even higher than that for Li insertion in K x VPO4F (x~0) host structure, in contrast to the common belief that Li intercalation voltage is always higher than Na intercalation. The lower Li intercalation voltage is likely attributed to unstable Li site in a large cavity in KxVPO4F (x~0), making less stable discharged product upon Li insertion vs. Na insertion. In addition, we discovered that Li intercalation is much more sluggish than Na and K intercalation in KxVPO4F (x~0). Since Li ion is too small compared to the cavity in the KxVPO4F (x~0) structure, Li ions are under-coordinated in the transition state. Therefore, Li ion migration barrier becomes much higher than Na and K migration. This finding indicates that large cavity size (or channel size) is not always good for fast alkali ion migration and we need to finely tune the cavity size suitable for each intercalating ion species.