Suppressed Voltage Decay of Li-Rich Li1.2Ni0.13Mn0.54Co0.13O2 Electrodes through Delayed Spinel-Phase Formation for Lithium-Ion Batteries

Abstract

Despite possessing a high gravimetric capacity above 230 mAh g^–1, Li-rich NMC oxides suffer from the bottleneck of capacity fading and a decrease in the discharge voltage upon cycling. Therefore, suppressing the discharge voltage decay is a major concern for employing these cathodes in Li-ion cells. To understand the structural change during initial cycles, the ex-situ X-ray diffraction investigation of Li-rich NMC cathodes at different charged states (4.0, 4.4, and 4.6 V) after completing one cycle in the potential domain of 2.0–4.7 V is conducted, which reveals the generation of a spinel phase only when polarized to above 4.4 V. Hence, Li-rich Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ cathodes herein are investigated across three different voltage ranges: 2.0–4.6 V, 2.7–4.6 V, and 2.7–4.4 V versus Li, after being activated first by polarization up to 4.7 V, to assess the suitable operational voltage range for their stable cycling behavior. When being cycled at C/5 rate in the voltage domains of 2.0–4.6, 2.7–4.6, and 2.7–4.4 V, the gravimetric charge storage of Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ cathodes is around 230, 226, and 208 mAh g^–1, respectively. Interestingly, the decay in discharge voltage has been suppressed from 0.31 to 0.1 V, and the retention of capacity is improved from 76.5 to 82.4% upon 100 cycles by controlling the voltage window of cycling from 2.0–4.6 V to 2.7–4.4 V vs. Li. Thus, this study reveals the essentiality of controlled cycling of Li-rich NMC oxides for better cycling performance in LIBs without using surface coating and doping of foreign elements.