High Energy Density Polyanion Electrode Material: LiVPO₄O_1–xF_x (x ≈ 0.25) with Tavorite Structure

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, LiVPO₄O_1–xF_x (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. LiVPO₄O_0.75F_0.25 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, LiVPO₄O_0.75F_0.25 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.