Investigating the first-cycle irreversibility of lithium metal oxide cathodes for Li batteries
2008
Kang, Sun-Ho | Yoon, Won-Sub | Nam, Kyung-Wan | Yang, Xiao-Qing | Abraham, Daniel P.
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, LiNi₀.₈Co₀.₁₅Al₀.₀₅O₂, and Li₁.₀₄₈(Ni₁/₃Co₁/₃Mn₁/₃)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 LiεMO₂ 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/Li₁.₀₄₈(Ni₁/₃Co₁/₃Mn₁/₃)₀.₉₅₂O₂ cell, showed that the oxide cathode reversibly returned to its original state [i.e., Li₁.₀₄₈(Ni₁/₃Co₁/₃Mn₁/₃)₀.₉₅₂O₂] during relaxation following the deep discharge to achieve 100% cycle efficiency.
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