Abstract
The theoretical capacity of LiVOPO4 could be increased from 159 to 318 mAh/g with the insertion of a second Li+ ion into the lattice to form Li2VOPO4, significantly enhancing the energy density of lithium-ion batteries. The changes accompanying the second Li+ insertion into -LiVOPO4 and -LiVOPO4 are presented here at various degrees of lithiation, employing both electrochemical and chemical lithiation. Inductively coupled plasma, X-ray absorption spectroscopy, and Fourier transform spectroscopy measurements indicate that a composition of Li2VOPO4 could be realized with an oxidation state of V3+ by the chemical lithiation process. The accompanying structural changes are evidenced by X-ray and neutron powder diffraction. Spectroscopic and diffraction data collected with the chemically lithiated samples as well as diffraction data on the electrochemically lithiated samples reveal that significant amount of lithium can be inserted into -LiVOPO4 before a more dramatic structural change occurs. In contrast, lithiation of -LiVOPO4 is more consistent with the formation of a two-phase mixture throughout most of the lithiation range. The phases observed with the ambient-temperature lithiation processes presented here are significantly different from those reported in the literature.
