Suppressed Voltage Decay of Li-Rich Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ Electrodes through Delayed Spinel-Phase Formation for Lithium-Ion Batteries

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 Li1.2Ni0.13Mn0.54Co0.13O2 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 Li1.2Ni0.13Mn0.54Co0.13O2 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.