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

The first-cycle irreversibility of Li1.048(Ni1/3Co1/3Mn1/3)0.952O2 (LiMO2) 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/LiMO2 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 Li2MO2-like phase on the oxide particle surface due to very sluggish lithium diffusion in Li1−ΔMO2 with Δ → 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 Li2MO2-like phase on the particle surface, followed by slow lithium diffusion in the layered structure. Experiments conducted in Li4Ti5O12/LiMO2 lithium-ion cells confirmed that the physical loss of lithium (via surface film formation or parasitic electrochemical reactions, etc.) from LiMO2 was negligible up to an oxide voltage of 4.3 V vs. Li+/Li.