Crystal Chemistry of Electrochemically and Chemically Lithiated Layered α_I-LiVOPO₄

LiVOPO₄ is an attractive cathode for lithium-ion batteries with a high operating voltage and the potential to achieve the reversible insertion of two lithium ions between VOPO₄ and Li₂VOPO₄. Among the three known forms of LiVOPO₄ (α, β, and α_I), the α_I-LiVOPO₄ has a layered structure that could promote better ionic mobility and reversibility than others. However, a comprehensive study of its lithiated product is not available as α_I-LiVOPO₄ is metastable and difficult to prepare by conventional approaches. We present here a facile synthesis of highly crystalline α_I-LiVOPO₄ and α_I-LiVOPO₄/rGO nanocomposite by a microwave-assisted solvothermal method and its electrochemical/chemical lithiation. The LiVOPO₄/rGO cathodes exhibit a high reversible capacity of 225 mAh g^-1, indicating the insertion of more than one lithium into VOPO₄. Both electrochemical and chemical lithiation imply a solid-solution reaction mechanism on inserting the second lithium into α_I-LiVOPO₄, but a two-phase reaction feature could also occur under certain conditions such as insufficient time for equilibration of Li⁺ diffusion in the structure. The fully lithiated new α_I-Li₂VOPO₄ phase was characterized by combined Rietveld refinement of neutron diffraction and X-ray diffraction data and by bond-valence sum maps. The results suggest that α_I-Li₂VOPO₄ retains the tetragonal P4/nmm symmetry of the parent α_I-LiVOPO₄ structure, where the second lithium ions are located in the lithium layers rather than in the VOPO₄ layers.