Influence of Water and Other Contaminants in Electrolyte Solutions on Lithium Electrodeposition
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ABSTRACT Electrochemistry of a nickel electrode in propylene carbonate [ PC ] containing LiC1O4, LiCF3SO3, LiPF6 was studied through a micro electrode ( 0 =25 /I m ) techniques in the wide potential range between +4.5 and -0.2 V vs. Li/Li*. Common pronounced peaks were observed in the potential range positive to lithium electrodeposition on nickel in all electrolyte solutions examined. Thus, these peaks can be attributed to reactions related to Li+ or commonly contained contaminants such as water and acids. In particular, the peak which appeared at the most negative potential seemed to be underpotential deposition ( UPD ) of lithium. To prove this hypothesis a nickel electrode in highly dried PC ( water content : 3 - 8 ppm ) intentionally contaminated with a small amount of water and CF3COOH was examined via cyclic voltammetry. Changing the content of water and acid ( and its ratio ) in PC resulted in a variety of voltammograms and one of them was identical to the one observed in PC containing lithium electrolytes. These facts preclude the existence of UPD of lithium on nickel in the electrolyte solutions. Instead, the existence of NiOH on nickel and its redox reaction mechanism have been postulated. The mechanism is consistent with the experimental facts : a nickel electrode passivates in PC with a small amount of water, and a small amount of acid, CF3COOH, can prevent passivation. The vicinity of the electrode surface may be exposed to an alkaline atmosphere owing to the reduction product of water. This seems to be the cause of troubles we run into with the electrodes at cathodic potentials INTRODUCTION The contaminants in electrolyte solution do affect the surface chemistry of a lithium electrode [ I ], because all compounds will be reduced around the equilibrium potential of a lithium electrode. Thus, much attention has been paid to the purity of the chemicals used for lithium batteries. Meanwhile the electrochemistry of non-lithium electrodes ( such as nickel, gold etc. ) in lithium electrolyte solutions in the potential range positive to lithium deposition [ 2, 3, 4 ], where the contaminants can be reduced, has been a minor subject compared to that of lithium electrodes in electrolyte solutions. However, it could give us important information about the contaminants in solutions, which has not been obtained from experiments with a lithium electrode. The cyclic voltammograms with a nickel electrode in PC containing 0.1 M LiAsF6 show a few peaks at potentials positive to bulk lithium deposition [ 2 ]. These were attributed to the reduction of water or oxygen-related reactions except for the pair of peaks which appear around 0.4 V( cathodic ) and 0.9 V (anodic). So far, the existence of underpotential deposition ( UPD ) and dissolution has been presupposed for these peaks without proof. One objective of this paper is identification of these peaks on the voltammogram. Further, the role of a trace of water and acids for lithium deposition on nickel has been investigated. EXPERIMENT The electrolytes used for the exp
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