LiMn2O4 is a very important cathode material in the field of plug-in
hybrids. However, it has limited large-scale practical application
because of its severe electrode polarization and rapid capacity decay.
By means of synergistic enhancement of double-cation (Na+, Mg2+) doping and three-dimensional graphene (3DG) coating,
the Li0.94Na0.06Mg0.08Mn1.92O4/3DG composite cathode material was successfully synthesized
via hydrothermal synthesis, high-temperature solid-phase sintering,
and freeze-drying. Electrochemical test results display that the initial
discharge specific capacity of the Li0.94Na0.06Mg0.08Mn1.92O4/3DG material reaches
146 mAh/g (137 mAh/g for LiMn2O4) at 0.5C, and
the specific capacity remains at 130 mAh/g after 100 cycles (99 mAh/g
for LiMn2O4). At 10C, the initial discharge
specific capacity of the Li0.94Na0.06Mg0.08Mn1.92O4/3DG material is 90 mAh/g
(69 mAh/g for Li0.94Na0.06Mg0.08Mn1.92O4 and 38 mAh/g for LiMn2O4), which should be resulted from the synergistic enhancement of double-cation
(Na+, Mg2+) doping and 3DG coating. Na+, Mg2+ codoping effectively expands the Li+ diffusion path and the diffusion coefficient of Li0.94Na0.06Mg0.08Mn1.92O4/3DG,
and inhibits the dissolution of Mn3+. 3DG coating effectively
improves the conductivity and reduces electrode polarization during
the charge and discharge course. In addition, 3DG coating also plays
an important role in inhibiting the dissolution of Mn3+.