First-cycle irreversibility of layered Li–Ni–Co–Mn oxide cathode in Li-ion batteries

The first-cycle irreversibility of Li{sub 1.048}(Ni{sub 1/3}Co{sub 1/3}Mn{sub 1/3}){sub 0.952}O{sub 2} (LiMO{sub 2}) cathode material in lithium and lithium-ion cells has been studied using galvanostatic cycling and in situ synchrotron X-ray diffraction. The so-called 'lost capacity' of a Li/LiMO{sub 2} cell observed during initial cycle in conventional voltage ranges (e.g., 3.0-4.3 V) could be completely recovered by discharging the cell to low voltages (<2 V). During the deep discharge, the lithium cell exhibited an additional voltage plateau, which is believed to result from the formation of Li{sub 2}MO{sub 2}-like phase on the oxide particle surface due to very sluggish lithium diffusion in Li{sub 1-{Delta}}MO{sub 2} with {Delta} {yields} 0 (i.e., near the end of discharge). Voltage relaxation curve and in situ X-ray diffraction patterns, measured during relaxation of the lithium cell after deep discharge to obtain 100% cycle efficiency, suggested that the oxide cathode returned to its original state after the following two-step relaxation processes: relatively quick disappearance of the Li{sub 2}MO{sub 2}-like phase on the particle surface, followed by slow lithium diffusion in the layered structure. Experiments conducted in Li{sub 4}Ti{sub 5}O{sub 12}/LiMO{sub 2} lithium-ion cells confirmed that the physical loss of lithium (via surface film formation or parasiticmore » electrochemical reactions, etc.) from LiMO{sub 2} was negligible up to an oxide voltage of 4.3 V vs. Li{sup +}/Li.« less