Crystal Chemistry of Electrochemically and Chemically Lithiated Layered α_I-LiVOPO₄

LiVOPO 4 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 VOPO 4 and Li 2 VOPO 4 . Among the three known forms of LiVOPO 4 (α, β, and α I ), the α I -LiVOPO 4 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 -LiVOPO 4 is metastable and difficult to prepare by conventional approaches. We present here a facile synthesis of highly crystalline α I -LiVOPO 4 and α I -LiVOPO 4 /rGO nanocomposite by a microwave-assisted solvothermal method and its electrochemical/chemical lithiation. The LiVOPO 4 /rGO cathodes exhibit a high reversible capacity of 225 mAh g –1, indicating the insertion of more than one lithium into VOPO 4 . Both electrochemical and chemical lithiation imply a solid-solution reaction mechanism on inserting the second lithium into α I -LiVOPO 4, 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 -Li 2 VOPO 4 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 -Li 2 VOPO 4 retains the tetragonal P 4/ nmm symmetry of the parent α I -LiVOPO 4 structure, where the second lithium ions are located in the lithium layers rather than in the VOPO 4 layers.