Which LiVOPO4 Phase Works Best for Two Li Cycling?

Abstract

Vanadyl phosphates show promise as cathode materials in Li-ion batteries due to the ability to hold multiple Li-ions per redox center. Specifically, the V5+/V4+ and V4+/V3+ couples allow for the material to hold up to two Li-ions per vanadium. The great structural diversity of this family, producing seven VOPO4 and three LiVOPO4 polymorphs, raises a question on which of them would work the best as a Li-ion battery cathode. The electrochemical performance of some of the polymorphs was reported in the literature, but the comparison is complicated by differences is materials morphologies, electrode preparation methods, current densities, and voltage windows used. Here we use LiVOPO4-2H2O as the precursor to synthesize all three LiVOPO4 phases: α1 (layered, tetragonal), β (orthorhombic), and ε (triclinic), and compare their electrochemical performances under the same conditions. The transformation between phases and their relative stabilities are studied using x-ray diffraction and differential scanning calorimetry. The role of oxygen partial pressure in phase stability is discussed based on the experimental data and first-principle calculations results. The electrochemical performance of each phase is evaluated using galvanostatic discharge-charge between 1.5 to 4.5 V at various current rates. The lithium diffusion coefficients are calculated using cyclic voltammetry and galvanostatic intermittent titration technique. These tests reveal that all the three phases can work as multi-electron cathodes; the Li transport details specific to each phase are discussed in conjunction with the structures. This research is supported as part of the NorthEast Center for Chemical Energy Storage (NECCES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0012583.