Investigating the first-cycle irreversibility of lithium metal oxide cathodes for Li batteries

摘要

Layered lithium metal oxide cathodes typically exhibit irreversibility during the first cycle in lithium cells when cycled in conventional voltage ranges (e.g., 3–4.3 V vs. Li+/Li). In this work, we have studied the first-cycle irreversibility of lithium cells containing various layered cathode materials using galvanostatic cycling and in situ synchrotron X-ray diffraction. When cycled between 3.0 and 4.3 V vs. Li+/Li, the cells containing LiCoO₂, LiNi_0.8Co_0.15Al_0.05O₂, and Li_1.048(Ni_1/3Co_1/3Mn_1/3)_0.952O₂ as cathodes showed initial coulombic efficiencies of 98.0, 87.0, and 88.6%, respectively, at relatively slow current (8 mA/g). However, the “lost capacity” could be completely recovered by discharging the cells to low voltages (<2 V vs. Li+/Li). During this deep discharge, the same cells exhibited voltage plateaus at 1.17, 1.81, and 1.47 V, respectively, which is believed to be associated with formation of a Li₂MO₂-like phase (M = Ni, Co, Mn) on the oxide particle surface due to very sluggish lithium diffusion in Li_εMO₂ with ε → 1 (i.e., near the end of discharge). The voltage relaxation curve and in situ X-ray diffraction patterns, obtained from a Li/Li_1.048(Ni_1/3Co_1/3Mn_1/3)_0.952O₂ cell, showed that the oxide cathode reversibly returned to its original state [i.e., Li_1.048(Ni_1/3Co_1/3Mn_1/3)_0.952O₂] during relaxation following the deep discharge to achieve 100% cycle efficiency.