Corrosion of Lithium in Alkaline Solution

Abstract

The corrosion of Li was studied in 3-5M LiOH at 25~ and in 4.5M LiOI-I at 4~176The Arrhenius relationship is obeyed and the apparent activation energy for the Li corrosion reaction is found to be 15.5 kcal mole -1. An empirical relationship is derived from which the corrosion rate vf can be obtained at any molar LiOH. At 25~ ,f(kA m -2) ----0.31 ). An equation is also given which allows calculation of the Li corrosion rate when the metal is anodically polarized. The kinetics of the He evolution reaction (e.r.) on Li were determined, the transfer coefficient ~ has a value of 0.14. The rate-controlling process of the Li dissolution reaction is the highly polarized I-I2 evolution at cathodic sites. Based on the ~ value obtained, the enthalpy of activation for the He e.r. on Li at OCV is calculated to be 22.8 kcal mole -1. It is proposed that the protective film on Li has a duplex structure with a thin oxide/ hydroxide layer adjacent to the metal and an outer porous hydrated layer through which the reaction products and reactants can pass freely.The rapid dissolution of Li in alkaline aqueous solutions can be utilized electrochemically to produce high rate battery systems (1). This is possible because the corrosion (parasitic) reaction in 3-5M LiOH can be virtually eliminated if the metal is anodically polarized by about 300 mV. It has been found that the rate of the Li dissolution reaction Lt + H~O-> LiOH -5 Yz H2[i]is essentially constant at constant temperature from OCV to the above-mentioned polarization level. Unlike conventional batteries, in Li-H20 cells the current efficiency is governed by the ratio of two competing reactions, namely, the anodic dissolution reaction and the parasitic reaction (2, 3). The current, efficiency increases with an increase in the anodic dissolution rate (i.e., the current density), and if decreases at elevated temperatures where the corrosion reaction is stimulated. Since current efficiency is critical to cell performance, a fundamental understanding of the factors which influence the parasitic reaction is important.No prior investigations of the corrosion of Li immersed in aqueous electrolytes have been reported. Papers on the electrochemistry of the system have been concerned with anodic passivation and mathematical modeling. This corrosion study of Li in flowing electrolyte under conditions identical to battery operation was undertaken to gain knowledge of three aspects of the Li-H20 reaction.(i) Information on the reaction at various LiOH concentrations and temperatures provides data for the determination of the reaction rate constant. This then allows calculation of corrosion rates in practical battery systems.(it) To characterize the processes which occur across the oxide/hydroxide film in a Li-H20 cell, knowledge of the H20 activity at the active Li surface is necessary.The assumption of the previous mathematical modeling paper (2) was that the exchange current density for the parasitic hydrogen evolution reaction is proportional to the 0.5 power of water activity at the a...

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