Passivation behaviour of aluminium current collector in ionic liquid alkyl carbonate (hybrid) electrolytes

Sowmiya Theivaprakasam,Maria Forsyth,Patrick Howlett,Gaetan Girard,Douglas MacFarlane,Sagar Mitra

doi:10.1038/s41529-018-0033-6

Abstract

The compatibility of current collectors with the electrolyte plays a major role in the overall performance of lithium batteries, critical to obtain high storage capacity as well as excellent capacity retention. In lithium-ion batteries, in particular with cathodes that operate at high voltage such as lithium nickel cobalt manganese oxide, the cathodic current collector is aluminium and it is subjected to high oxidation potentials (>4 V vs. Li/Li+). As a result, the composition of the electrolyte needs to be carefully designed in order to stabilise the battery performance as well as to protect the current collectors against corrosion. This study examines the role of a hybrid electrolyte composed of an ionic liquid (N-methyl-N-propyl pyrrolidinium bis(trifluoromethanesulfonyl)imide or N-methyl-N-propyl pyrrolidinium bis(fluorosulfonyl)imide) and a conventional electrolyte mixture (LiPF6 salt and alkyl carbonate solvents) with correlation to their electrochemical behaviour and corrosion inhibition efficiency. The hybrid electrolyte was tested against battery grade aluminium current collectors electrochemically in a three-electrode cell configuration and the treated aluminium surface was characterised by SEM/EDXS, optical profilometry, FTIR, and XPS analysis. Based on the experimental results, the hybrid electrolytes allow an effective and improved passivation of aluminium and lower the extent of aluminium dissolution in comparison with the conventional lithium battery electrolytes and the neat ionic liquids at high anodic potentials (4.7 V vs. Li/Li+). The mechanism of passivation behaviour is also further investigated. These observations provide a potential direction for developing improved hybrid electrolytes, based on ionic liquids, for higher energy density devices.

Highlights

In a lithium-ion battery, the flow of lithium ions between the electrodes happens along with the electron transfer in the external circuit
The main difference seen in the scanning electron microscopy (SEM) analysis of the surface exposed to the hybrid electrolyte (HE 1) is the presence of a smoother deposit when the ionic liquid with the [TFSI] anion is mixed with LP40, Fig. 3e (HE 1 sample)
The dissolved aluminium ions react with the electrolyte mixture to form aluminium salts, the nature of which depends on the salt and solvents in the electrolyte system

Summary

INTRODUCTION

In a lithium-ion battery, the flow of lithium ions between the electrodes happens along with the electron transfer in the external circuit. The main difference seen in the SEM analysis of the surface exposed to the hybrid electrolyte (HE 1) is the presence of a smoother deposit when the ionic liquid with the [TFSI] anion is mixed with LP40, Fig. 3e (HE 1 sample). There are significant differences between the samples with the surface films formed from neat ILs or the HE 1 electrolyte containing mainly –SO2F and AlF3, after a small amount of etching whereas a more steady variation in composition was exhibited by the HE 2 sample, suggesting that the Al film may be thicker or significant amounts of other species may be present on the Al surface in this latter case. The hybrid electrolyte based on the bis(trifluoromethanesulfonyl)imide [TFSI] anion performs better than both the neat [TFSI] ionic liquid anodising process, according to the N 1s region spectra, and this is in agreement with FTIR analysis described previously. Electrolyte and bis(fluorosulfonyl)imide [FSI] hybrid electrolyte in contact with aluminium at high anodic potentials (4.7 V vs. Li/Li+)

DISCUSSION

METHODS