Restriction of voltage decay by limiting low-voltage reduction in Li-rich oxide materials.

ABSTRACT

Li-rich layered oxides are recognized as promising candidates for next-generation Li-ion batteries owing to the high capacity of >250 mAh g-1, but the severe voltage fade has prevented their commercialization. It is widely known that high-voltage charge processes result in layered-to-spinel structural evolution and voltage fade in Li-rich layered oxides. This work emphasizes that limiting the low-voltage reduction can maintain the structure and voltage stability of Li-rich layered oxides after the 4.6 V high-voltage charge processes. A strategy of limiting the low-voltage (<2.8 V) reduction by cycling at 4.6-2.8 V was performed in traditional Li1.2Ni0.13Mn0.54Co0.13O2 and high-Ni Li1.2Ni0.222Mn0.504Co0.074O2. After 300 cycles, traditional Li1.2Ni0.13Mn0.54Co0.13O2 and high-Ni Li1.2Ni0.222Mn0.504Co0.074O2 cycling at 4.6-2 V showed midpoint discharge voltages of 2.83 V and 2.97 V with high voltage fade rates of 2.25 mV/cycle and 2.24 mV/cycle, respectively. While the two materials cycling at 4.6-2.8 V can maintain discharge midpoint voltages of 3.34 V and 3.49 V, with low voltage decay rates of 0.692 mV/cycle and 0.632 mV/cycle, respectively. To better understand the voltage performance, their electric structures were calculated by density functional theory. Physical characterizations were also used to analyze their differences in structural evolution. The results suggested that limiting low-voltage reduction in Li-rich layered oxides is highly necessary for maintaining their structure and voltage stability.