A square channel vanadium phosphite framework as a high voltage cathode for Li- and Na-ion batteries

Abstract

A vanadium phosphite compound, LiV(HPO₃)₂, has been synthesized using low-melting flux composed of phosphorous acid and lithium hydroxide. The crystal structure of the compound as refined using the Rietveld method matched with the previously reported unit cell parameters (space group: I2d, a = b = 10.5987(4), c = 9.9843(5) Å). It has a 3-dimensional open-framework structure with intersecting channels. Eight-membered ring square channels can be viewed along the c-axis and 6-membered ring channels along the a- and b-axis. The Li-ions are located in the six-membered ring channels exactly at the opening window to the eight-membered ring channels. The magnetic property measurements confirmed the +3 oxidation state of vanadium. The as-synthesized V-phosphite exhibits facile electrochemical properties with an insertion voltage of 4.05 V vs. Li⁺/Li and 3.7 V vs. Na⁺/Na in Li- and Na-ion batteries, respectively. When cycled between 4.5 and 1.2 V, the compound shows an insertion of second lithium at ∼2 V. The galvanostatic charge–discharge profiles indicate that LiV(HPO₃)₂ can be cycled between V⁴⁺ and V²⁺, where a 4 V slightly sloppy plateau and a sloping profile after 2 V represent a solid-solution type mechanism during reductions for V⁴⁺/V³⁺ and V³⁺/V²⁺ redox couples, respectively, whereas during charging there is evidence of a biphasic reaction. The compound has been further characterized by TGA, IR, and diffuse reflectance spectroscopy.