Chemical and Electrochemical Lithiation of LiVOPO₄ Cathodes for Lithium-Ion Batteries

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 phase changes accompanying the second Li+ insertion into alpha-LiVOPO4 and beta-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 infrared spectroscopy measurements indicate that a composition of Li2VOPO4 can 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 a significant amount of lithium can be inserted into alpha-LiVOPO4 before a phase change occurs. In contrast, lithiation of beta-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.