Anodic Dissolution of the Aluminum Current Collector in Lithium-ion Cells with LiFSI, LiPF6, and LiBF4

Anodic dissolution (often referred to as corrosion) of the positive electrode aluminum current collector can be a limiting factor for the performance, lifetime, and safety of lithium-ion cells. The superior performance of lithium bis(fluorosulfonyl)imide (LiFSI) salt, especially in high-temperature lithium-ion cells, motivates this study of the Al current collector corrosion in electrolytes with and without LiFSI. With pure lithium hexafluorophosphate (LiPF6) and lithium tetrafluoroborate (LiBF4) or LiFSI blends of these salts, only minor Al corrosion currents were found in cyclic voltammetry (CV), open-circuit voltage storage, cycling experiments, and ultra-high precision coulometry; however severe corrosion was found with pure LiFSI. Importantly, there is a discrepancy between the results of CV experiments and actual NMC532/graphite pouch cell cycling in terms of the magnitudes and onset voltages of Al corrosion. For the latter, severe anodic dissolution was only observed when using LiFSI above 4.2 V, but for the former severe corrosion currents were already observed at 3.8 V vs Li+/Li when using bare Al foil in CV experiments. This discrepancy is likely due to proton scavenging by H+/Li+ exchange and/or solvent-mediated hydrogenation of layered oxide positive electrode materials - two mechanisms that are absent in cells with bare Al electrodes.