Exploring Interactions between Electrodes in Li[Ni x Mn y Co 1-xy ]O 2 /Graphite Cells through Electrode/Electrolyte Interfaces Analysis
Abstract
Interactions occurring between positive and negative electrodes during constant current-constant voltage cycling in Li[NixMnyCo1-xy]O2/graphite pouch cells at various upper cutoff voltages was investigated. Below 4.3 V, good capacity retention, high/stable coulombic efficiency (CE) and low gas production were observed. At 4.5 V, however, extensive capacity loss, dramatic/unstable CE and large gas generation were observed due to extensive electrolyte degradation at the NMC electrodes. XPS experiments highlighted that solvents (-CO containing species) and salt (mostly LiF) degradation products including gas and NMC dissolution products are produced at the NMC electrodes then migrate to the graphite electrodes surface where they are either reduced or simply deposited. Although these phenomena were greatly accelerated at high voltage, the dominant failure mechanism was the formation of a Li-ion insulator/electron conductor surface layer (maybe rock salt) due to irreversible structural change of the NMC particles surface. At low voltage, the failure mechanism was explained by accumulation of SEI at the graphite electrode surface that hinder the ionic transport through the electrode porosity. Overall, both failure mechanisms are driven by oxidative parasitic reactions at the positive electrode and interaction with the negative electrode. Passivation of the positive electrode appears therefore crucial to promote long lifetime of Li-ion cells.