High Energy Density Polyanion Electrode Material: LiVPO4O1–xFx (x ≈ 0.25) with Tavorite Structure

Abstract

Polyanion compounds as an electrode material in lithium-ion battery are of great interest due to superior structural and thermal stability, but they have relatively low energy density. Here, we report high energy density electrode material, LiVPO4O1-xFx (x ≈ 0.25) with tavorite structure that is first synthesized by single-step solid-state reaction and that can use more than 1.6 electrons during charging/discharging process. The doping of F instead of O into LiVPO4O was confirmed with a structural solid-solution measured by neutron powder diffraction and with a mixed valence state of V measured by X-ray absorption near edge spectroscopy. The resulting material has distinct and beneficial electrochemical properties compared to both LiVPO4F and LiVPO4O. LiVPO4O0.75F0.25 has higher average operating voltage and better electrochemical activity than LiVPO4O and shows higher operating voltage (∼2.4 V) than LiVPO4F (∼1.8 V). As a result, LiVPO4O0.75F0.25 can achieve high specific capacity of 260 mA h/g in the voltage range from 2.0-4.8 V and thereby can deliver 820 W h/kg, which is relatively high energy density achieved in known polyanion compounds. © 2017 American Chemical Society.