The doping effect on the electrochemical performance of LiNi0.305Mn0.33Co0.33M0.025O2 (M=Al, Y, Cr) for lithium-ion batteries

LiNi_0.305Mn_0.33Co_0.33M_0.025O₂ (M=Al, Y, Cr) materials were synthesized via the sol-gel method. The structure and electrochemical properties were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and charge/discharge tests. The materials were maintained the α-NaFeO₂ type layered structure and the discharge capacity was also increased by doping. The Y- and Al- doping slightly decrease the initial discharge capacity of LiNi_0.33Mn_0.33Co_0.33O₂ while the Cr-doping can increase the initial discharge capacity. The cycling performances are improved after doping. When cycled at 1.0C, about 99%, 98.6% and 97.1% of their initial capacities can be retained after 40 cycles for Y-, Al- and Cr-doped materials, respectively. Their rate capabilities are also better than that of the un-doped one. EIS measurement shows that the Y-doped electrode has the lowest resistance impedance value during cycling.