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

The first-cycle irreversibility of Li(1.048)(Ni(1/3)CO(1/3)Mn(1/3))(0.952)O(2) (LiMO(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(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(2)MO(2)-like phase on the oxide particle surface due to very sluggish lithium diffusion in Li(1-Delta)MO(2) with Delta -> O (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(2)MO(2)-like phase on the particle surface, followed by slow lithium diffusion in the layered structure. Experiments conducted in Li(4)Ti(5)O(12)/LiMO(2) lithium-ion cells confirmed that the physical loss of lithium (via surface film formation or parasitic electrochemical reactions, etc.) from LiMO(2) was negligible up to an oxide voltage of 4.3 V vs. Li(+)/Li. (C) 2008 Elsevier Ltd. All rights reserved.