Understanding the influence of Mg doping for the stabilization of capacity and higher discharge voltage of Li- and Mn-rich cathodes for Li-ion batteries

Abstract

Although Li- and Mn-rich layered cathodes exhibit high specific capacity, the cathode materials of the general formula Li_1+x[Ni_yMn_zCo_w]O₂ (x + y + z + w = 1) suffer from capacity fading and discharge-voltage decay during prolonged cycling, due to the layered-to-spinel transformation upon cycling to potentials higher than 4.5 V vs. Li. In this paper, we study the effect of Mg doping (by partial replacement of Mn ions) on the electrochemical performance of Li- and Mn-rich cathodes in terms of specific capacity, capacity retention and discharge voltage upon cycling. Mg-doped Li- and Mn-rich Li_1.2Ni_0.16Mn_0.54Mg_0.02Co_0.08O₂ and Li_1.2Ni_0.16Mn_0.51Mg_0.05Co_0.08O₂ cathode materials were synthesized by a self-combustion reaction (SCR), and their electrochemical performance in Li-ion batteries was tested. The replacement of a small amount of Mn ions by Mg ions in these materials results in a decrease in their specific capacity. The doping of a small amount of Mg (x = 0.02) resulted only in the stabilization of the capacity, whereas a greater amount (x = 0.05) resulted in improved capacity retention and discharge voltage upon cycling. Li_1.2Ni_0.16Mn_0.51Mg_0.05Co_0.08O₂ exhibits a low specific capacity of about 160 mA h g⁻¹, which increases and then stabilizes at about 230 mA h g⁻¹, and finally decreases to 210 mA h g⁻¹ during 100 cycles. The substitution of Mg for Mn (x = 0.05) results in a higher discharge voltage than the other two cathode materials examined in this study. Structural analysis of the cycled electrodes suggests that Mg suppresses the activation of Li₂MnO₃ during the initial cycling, and hence, partially prevents layered-to-spinel transformation, resulting in a better electrochemical performance of the Mg-doped cathode material as compared to the undoped material.