Mechanism of the Processes of Formation of Lead-Acid Battery Positive Plates

Abstract

The processes involved in the formation of the positive lead-acid battery plate in H2SO4 with sp gr 1.15 and 1.05 and in 0.7M Na2SO4 were studied by x-ray diffraction, wet chemical analysis, and microscopic observations. It was found that formation takes place in two stages. During the first one, H2SO4 and H20 penetrate from the bulk of the solution into the plate. As a result of chemical and electrochemical reactions tet-PbO and basic sulfates of lead are converted to PbSO4 and a-PbO2. These compounds form zones which advance into the paste. Part of the hydrogen ions evolved during the reaction of formation of PbO2 migrate from the plate in order for the latter to remain electroneutral. During the second period H20 is consumed in the plate. Lead sulfate is oxidized to ~-PbO2. Sulfuric acid originates and diffuses in a direction opposed to that in the first period. Taking into account specific conditions of the chemical and electrochemical reactions in porous electrodes a mechanism is suggested for the formation processes of the positive plate. Ion diffusion and migration are considered to be the rate-limiting steps of the processes. The direction of advance of the PbO2 and PbSO4 zones is explained in terms of this mechanism as well as the a/fl-Pb02 ratio during formation in the three solutions. The parameters which control the phase composition of the active material of the lead-acid battery positive plate are listed.Formation is one stage of the technological process of lead-acid battery production. By mixing oxidized lead powder with sulfuric acid a paste is obtained which consists of lead monoxide and basic sulfates of lead. After pasting on a grid and curing, the paste is subjected to formation. During formation lead monoxide and the basic sulfates are converted by electrochemical reactions to active material, i.e. to lead at the negative plate and to lead dioxide at the positive one. Formation takes place in a sulfuric acid solution. Lead monoxide and basic sulfates of lead are unstable in sulfuric acid, and therefore the electrochemical reactions are accompanied by chemical sulfation processes. All this imparts a great complexity to the formation process. Until now attention has been mainly focused on the influence of paste, electrolyte and current density, of temperature, etc. on the processes at the positive plate (1-15). This interest may be explained by the fact that formed plates contain both the acidic ~ and the alkaline a polymorph of PbO2. On the other hand, the capacity and the life of the positive plate is determined by the ratio between the polymorphs in it. With the increase in the content of the beta polymorph in the active mass, the capacity increases but the life of the plate decreases.Attempts at elucidating the formation processes of the positive plate were mainly concerned with determination of the mechanism of formation of a-PbO2. Two schools of thought are to be found in the literature. According to the first one, the solution in the pores is locally alkalinized which leads to the forma...

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“…Pavlov and collaborators in their papers [4,7,8] have shown that the formation of the positive plates in which changes were made in the phase and in the chemical composition of the mature paste in them, occurs in two stages.…”

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