Structure, Magnetism, and Electrochemistry of LiMg_1-ZnVO₄ Spinels with 0 ≤ x ≤ 1

Negative electrode materials with lower operating voltages are urgently required to increase the energy density of lithium-ion batteries. In this study, LiMgVO₄ with a Na₂CrO₄-type structure, LiZnVO₄ with a phenacite structure, and their mixture were treated under a high pressure of 12 GPa and a high temperature of 1273 K, and their electrochemical reactivities were examined in a nonaqueous lithium cell. Synchrotron X-ray diffraction (XRD) measurements and Raman spectroscopy revealed that the LiMg_1-ZnVO₄ samples with 0 ≤ x ≤ 1 are in a single phase of the inverse spinel structure that forms a solid solution compound over the whole x range. All of the samples were brown or light black due to the presence of a small amount of V⁴⁺ ions with S = 1/2 and oxygen deficiencies. Since the majority of the vanadium ions are located at the route of the Li⁺ ion conduction pathway, no rechargeable capacity (Q_recha) would be expected. Nevertheless, all LiMg_1-ZnVO₄ samples exhibited a Q_recha value of more than 200 mAh g^-1 with an operating voltage of ∼0.8 V. This operating voltage is ∼1.6 V lower than that of LiV₂O₄ with a normal spinel structure. Furthermore, the x = 0.5 sample demonstrated an extremely stable cycle performance over 1 month. Ex situ XRD measurements clarified that the reversible electrochemical reaction can be attributed to the movement of vanadium ions from the tetrahedral 8a to octahedral 16c sites during the initial discharge reaction. Details regarding the crystal structure, magnetism, and electrochemistry of LiMg_1-ZnVO₄ are presented.