Short communicationInfluence of Mg2+, Al3+, Co2+, Sn2+ and Sb3+ on the electrical performance of doped β-lead dioxide
Introduction
The positive lead dioxide active material (PAM) of lead acid batteries is formed by electrochemical oxidation of basic lead sulfates and lead oxide. The capacity of the positive plate depends mainly on the ratio between the two forms of lead dioxide α and β-PbO2.
Pavlov [1] showed that lead dioxide active mass is a gel-crystal system with proton and electron conductivity. Mohanov et al. [2] found hydrated structures in the anodic layer formed on lead electrodes in H2SO4 solution. At a given potential, Pb4+ ions are formed on the electrode surface. These are unstable in water solutions and form Pb(OH)4. The Pb(OH)4 is dehydrated partially or completely giving PbO(OH)2 and PbO2. The electrode surface is covered by a layer of PbO2, PbO(OH)2 and Pb(OH)4, which layer has gel-like properties.
According to Pavlov et al. [3] during discharge of the positive battery plate, the reduction of PbO2 and PbO(OH)2 to PbSO4 proceeds in two stages. The first is electrochemical and occurs in the bulk of the agglomerates and particles and gives Pb(OH)2. During the second stage, PbSO4 formation takes place through a chemical reaction between Pb(OH)2 and H2SO4.
In our previous work [4], using an all solid state system exempt of H2SO4, the kinetic tests and coulometric data show that the process of PAM reduction includes two electrochemical stages (one electron is consumed during each stage) taking place in the gel zones according to a proton-electron mechanism or a double-injection process. The role of structural water in the reactivity of the positive active mass has been emphasized by different authors [5], [6], [7], [8], [9], [10], [11], [12], [13]. They concluded that the heat treatment of PAM reduces the discharge capacity and decreases considerably the proton diffusion coefficient.
The effects of metal ions addition in the electrolyte and in the grid alloys on the battery electrical performance have been investigated by several authors [14], [15], [16], [17], [18], [19], [20], [21]. The objective of the present paper is to determine the impact of the following ions (Mg2+, Al3+, Co2+, Sn2+ and Sb3+) on the properties of the active material of positive battery plates.
Section snippets
Materials
The paste was prepared in a mixer at 35 °C from leady oxide (72% PbO). The XRD pattern of the starting material showed the presence of tetragonal PbO and Pb. Leady oxide (LO) was mixed with water and 1.40 s.g. H2SO4 in a ratio equal to 5% H2SO4/LO. β (ND: non doped sample) and doped β lead dioxide were prepared by electroformation of cured battery plates (with grids cast from a Pb-5 wt.% Sb alloy) in acidic solution according to the method described by Voss and Freundlich [22]. The dopants were
Samples characterizations
Table 1 summarizes the obtained results for the chemical composition of the PAM samples formed in solutions with different dopants. The data show that traces of Mg2+and Al3+ are present in the active mass, whereas important quantities of Sn2+, Co2+and Sb3+are found. This affects the chemical composition of the positive mass. The amount of PbO2 decreases, whereas that of PbO increases. In addition, small quantities of unconverted PbSO4 is found in the majority of the samples.
Fig. 1 shows the XRD
Conclusions
The experimental data obtained in this study are in general agreement that the incorporation of metallic ions in the gel zones of the positive active material influences its physico-chemical and electrochemical properties. The experimental findings showed that small quantities of magnesium and aluminium are introduced in PAM. In contrast, tin, antimony and cobalt were doped in β-PbO2. The maximum admissible level of the metal ion depends on the nature of the dopant itself. The different ions
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