Electrochemical Deposition and Dissolution of Aluminum in NaAlCl4 Melts Influence of MnCl2 and Sulfide Addition

Effects of the additives MnC12, sulfide, and their combined influence on a luminum deposition and dissolution in NaA1C14 saturated with NaC1 have been studied by polarization measurements, galvanostatic deposition, and current reversal chronopotentiometry (CRC). The solubility of MnC12 was found to be 0.086 _+ 0.006 m/o in the melt at 175~ Aluminum-manganese alloys can be deposited in NaA1C14 saturated with both NaC1 and MnC12, resulting in a slight increase in cathodic overpotentials. The codeposition of the binary alloys at current densities below 4 mAJcm 2 gave rise to formation of deposits so compact that their specific average volumes are of the theoretical value. The content of manganese in the alloy deposits was found to be between 8-11 a/o. It was found that the manganese content in the deposits did not depend on the current efficiency. The presence of sulfide in NaA1C]4 results in the formation of more spongy deposits at low current densities, and thus destroys the beneficial effects of MnC12 when both additives are present, but prevents formation of dendritic deposits at high current densities. Coulombic ratios of dissolution to deposition were found by CRC measurements to be affected by the nature of the substrate. Aluminum electrodeposition from A1C13-based molten salts has been the subject of many investigators (1). In most cases the electrolysis has been performed in the following ranges: temperature 150~176 current density 20-50 mA/cm ~, and concentration of AIC13 56-64 mole percent (m/o). These melts have a high vapor pressure which may lead to a high evaporation of A1C13. At preferable concentrations, close to 50 m/o A1C13, with A1CI~ bound as the stable complex A1CI(, smooth a luminum layers could be deposited only in the current density range 2-10 mAJcm 2. At higher current densities a luminum was deposited in the form of dendrites or very porous powders. Surface-active components such as tetraalkylammonium halides (2-5) and urea (5) have been suggested as electroplating aiding * Electrochemical Society Active Member. agents. In the case of rechargeable batteries, the oxidationreduction conditions are more demanding, and slow decomposition can make organic additives unfavorable (6). The effects of many inorganic additives have also been investigated. Among them, BaC12 (7) was found to have little effect, whereas PbC12 (3, 8), SnC12 (3, 9, 10), MnC12 (3, I1-13), NaBr, and NaI (3, 14) were found to have some beneficial effects on a luminum deposition. Iron chloride as an impurity in the chemicals, however, might make deposits even worse (15). In our preliminary investigations (16), the effects of some inorganic additives on the morphology of a luminum were widely investigated. There were: GaCI~, InCl,, BiC13, HgC12, CdC12, PbC12, SnC12, and MnC]2. Sulfide is of interest because it is present in the electrolyte of a newly developed battery having transition metal sulfides (e.g., Downloaded 16 Oct 2009 to 192.38.67.112. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp J. Electrochem. Sac., Vol . 137, No. 9, S e p t e m b e r 1990 9 The Electrochemical Society, Inc. 2795 Ni2S3) as cathode (17). MnCI=, among others, was found to be the most favorable additive. In the present paper, the influences of manganese chloride, sulfide, and their combination on a luminum deposition and dissolution are presented.