Structure and kinetics of formation and decomposition of corrosion layers formed on lithium compounds exposed to atmospheric gases

Abstract

A thorough review of the literature on the structure, growth kinetics and decomposition of (corrosion) layers formed upon the reaction of LiH and other lithium compounds with atmospheric gases, particularly water, suggests that simple models, consistent with thermodynamics, similar to those postulated to explain corrosion in other materials, can effectively explain all of the data. For example, data from a host of techniques, including calorimetry, microbalance, X-ray photoelectron spectroscopy, secondary ion mass spectrometry and Rutherford back-scattering, indicate that on LiH the corrosion layer formed upon water exposure grows at a nearly constant rate and consists of two layers, a very thin layer of Li2O sandwiched between LiOH and the original LiH ('tri-layer' model). These form during the reaction of water at two subsurface interfaces: LiH/Li2O and Li2O/LiOH. These reactions result in the release of hydrogen (2LiH+H2O→Li2O+2H2), and the growth of the LiOH layer (Li2O+H2O→2LiOH). Alternative models found in the literature, which include features such as the existence of LiOH/LiH interfaces, are shown to be inconsistent with thermodynamics. It is also shown that confusion regarding the mechanism of decomposition of LiOH upon heating results from a failure to account for the fact that the decomposition process of LiOH 'on top of' LiH is fundamentally different from that of pure LiOH. In fact, thermodynamics clearly predicts that even at room temperature the trilayer model is not stable, and should eventually decompose to two layers, Li2O on top of LiH. Thus, the layer of LiOH formed on top of LiH, produced for example for surface studies, is in fact composed of both LiOH and Li2O on top of LiH (i.e. the 'tri-layer' structure) and this structure will gradually decompose, releasing hydrogen, even at room temperature. This process will lead eventually to the simpler structure Li2O/LiH. In contrast, pure LiOH is stable except at temperatures above 400 K even in the driest environments. Finally, recent data show a 'barrier layer' will form during exposure of LiH to a gas containing both CO2 and water. Once fully formed (∼1 μm thickness), this barrier effectively prevents the attack of LiH by water.