Electrochemical Behavior of Lithium in Alkaline Aqueous Electrolytes. II. Point Defect Model

Abstract

Experimental studies on lithium in concentrated lithium hydroxide aqueous solutions have confirmed the formation of a passive film on the surface that controls the rate of the dissolution process. 1,2 The prevailing consensus is that the layer is an oxide-hydroxide film. However, it has been shown in Part I of this series 3 that, under opencircuit conditions, lithium hydride and lithium hydroxide are stable phases, whereas lithium oxide cannot form even as a metastable phase. Accordingly, the passive film formed under open-circuit conditions is of a bilayer lithium hydride/lithium hydroxide structure.We analyze experimental data for lithium dissolution in terms of a theoretical model postulating a lithium hydride-hydroxide bilayer structure for the film. The model incorporates charge-transfer phenomena, hydrogen evolution, barrier-layer formation and dissolution, and metal dissolution, and allows for a change in the porosity of the outer layer as a function of the electrolyte composition and applied voltage. The model succeeds in explaining a wide panorama of experimental data with minimal variation of the model parameters. Indeed, the variation of only one parameter, related to the porosity of the outer layer, is sufficient to account for all the observed trends in the total current density and in the anodic and cathodic partial current densities with respect to the independent experimental variables (voltage and electrolyte and additive concentrations).Theory Thermodynamic calculations on the Li/H 2 O system indicate that at the open-circuit potential (OCP) of the system (about Ϫ2.8 V SHE 1,4-6 ), the phase in contact with lithium is lithium hydride. 3 The existence of a porous LiOH layer on the metal surface can be inferred from the limited solubility of this compound in water (and especially in the concentrated KOH solution used in this study), and from the dissolution/precipitation mechanism that was postulated by Littauer and co-workers, 1,2 who performed much of the early work on the electrochemistry of the Li/H 2 O system. We postulated that under open-circuit conditions a hydride barrier layer forms next to the metal, whereas the outer layer consists of LiOH.

Effect of magnesium on the electrochemical behavior of lithium anode in LiOH aqueous solution used for lithium–water battery

“…This research is consistent with previous research, Ref. [23], in which additives to the solution had a proportional effect on the rate of Li-dissolution and the rate of H 2 evolution with the current efficiency effectively unchanged. This research is also consistent with previous research, Ref.…”

“…11a). Cation vacancies are the predominant charge carriers in LiH film [23,24]. Consequently, the current efficiency is not markedly affected by magnesium addition (Fig.…”

“…The cathodic and anodic partial current densities were computed as −2FN H 2 and I T + 2FN H 2 . The coulombic efficiency of the system (Á) was calculated from the following equation [23,24],Á =…”

“…The model and the interfacial reactions within the passive film on lithium are shown in Fig. 13 [10,23,24]. All the results above suggest that addition of magnesium to lithium changed the property of the bilayer film on the lithium surface.…”

“…Interfacial reactions leading to generation and annihilation of point defects within the passive film on lithium[10,23,24].…”