Effect of Fe in suppressing the discharge voltage decay of high capacity Li-rich cathodes for Li-ion batteries

Fe substituted Li-rich cathodes Li_1.2Mn_0.56Ni_0.16Co_0.04Fe_0.04O₂ and Li_1.2Mn_0.56Ni_0.16Fe_0.08O₂ synthesized by self-combustion reaction (SCR), characterized by XRD, SEM, TEM, and Raman spectroscopy, were studied as cathode materials for Li-ion batteries. The electrochemical performance including specific capacity, average voltage, and rate capability of these two cathode materials are compared with that of Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ in order to understand the effect of Fe. While Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ exhibits a discharge specific capacity of about 270 mAh g⁻¹, the specific capacities are about 254 and 210 mAh g⁻¹ for Li_1.2Mn_0.56Ni_0.16Co_0.04Fe_0.04O₂ and Li_1.2Mn_0.56Ni_0.16Fe_0.08O₂, respectively in the first cycle. The substitution of Co by Fe results in a decrease in the specific capacity of these materials. However, the specific capacities are found to be 190, 200, and 175 mAh g⁻¹ after 80 cycles, thus retaining about 75, 82, and 86 % of the initial capacity (second cycle) for Li_1.2Mn_0.56Ni_0.16Co_0.08O₂, Li_1.2Mn_0.56Ni_0.16Co_0.04Fe_0.04O₂, and Li_1.2Mn_0.56Ni_0.16Fe_0.08O₂, respectively. Thus, the presence of Fe improves the electrochemical cycling stability. The Fe substituted materials exhibited around 100 mAh g⁻¹ at 4C rate, which is close to the value of 110 mAh g⁻¹ obtained for Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ cathodes at the same rate. The substitution of Co by Fe also helps to mitigate the decrease in the average discharge voltage upon cycling. In turn, electrochemical impedance studies show that the charge-transfer resistance of Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ is lower than that of the Fe substituted cathode materials. Also, it seems that the substitution by Fe stabilizes the layered rhombohedral phase of these materials upon cycling.