Comparison of Pb–Sm–Sn and Pb–Ca–Sn alloys for the positive grids in a lead acid battery

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Abstract

The anodic behavior of Pb–0.73 wt.% Sm–0.63 wt.% Sn and Pb–0.088 wt.% Ca–0.63 wt.% Sn alloys in sulfuric acid solution has been studied using linear sweep voltammetry, impedance–time curves and cyclic voltammetry. The experimental results show that the samarium in the Pb–Sm–Sn alloy can inhibit the growth of the anodic corrosion layer (PbO2) formed on the alloy. Moreover, the rate of the oxygen evolution at the Pb–Sm–Sn alloy electrode is lower than that at the Pb–Ca–Sn alloy electrode. 2V-200Ah VRLA batteries were separately manufactured using the Pb–Sm–Sn and Pb–Ca–Sn positive grids. The results of the test show that the capacity loss of the battery with the Pb–Sm–Sn positive grids is significantly less than that of the battery with the Pb–Ca–Sn positive grids.

Introduction

So far, in the manufacture of most valve regulated lead acid (VRLA) batteries, low-antimony and lead–calcium alloys are still widely used as the materials for positive grids [1], [2], [3], [4]. Sb added to Pb alloy can effectively improve the charge–discharge performance of lead-acid batteries, but it can also decrease the overpotential of the hydrogen evolution on lead alloys owing to the transference of the antimony during charging, which can lead to early excessive water loss and increase the self-discharge for the VRLA battery [5], [6]. Compared with Sb, Ca added to Pb alloys can decrease the water loss of the batteries, which is helpful for batteries to have a rather good maintenance-free performance. However, corrosion at the grain boundaries occurs between the crystal grains in the Pb–Ca alloy, and a high-resistivity oxide corrosion layer forms on the Pb–Ca alloy during charging, which can give rise to a bad influence on the deep charge–discharge cycles of the batteries [7], [8].

Owing to the mentioned detrimental influences of Pb–Sb and Pb–Ca alloys, in the last several decades there have been numerous research papers on lead and lead alloys to choose a suitable additive of alloys and to improve their performance [4], [5], [9], [10], [11], [12], e.g. Sn, Bi, Sr, Cd, and Ag.

It has been demonstrated that Sn as an additive to the Pb–Ca alloy can effectively improve the Pb–Ca alloy performance [13]. Hence, the Pb–Ca–Sn alloy has been widely used for the grid materials of the VRLA battery [9]. Although the addition of Sn can reduce the resistivity of the anodic lead oxide films to some extent, the growth of the high resistivity layer on the Pb–Ca alloy cannot be essentially eliminated owing to the presence of calcium in the alloy. Thus, the performance of Pb–Ca–Sn alloy is still not satisfactory [14].

In order to overcome the detrimental influence of Pb–Sb and Pb–Ca alloys, we attempted to research and develop some new lead alloys instead of the lead alloys containing Sb or Ca. In previous work we discovered that some rare-earth elements, such as Ce added to Pb–Ca–Sn alloy can inhibit the corrosion of the alloy at the deep discharge potential of 0.9 V (vs. Hg/Hg2SO4) and decrease the resistance of the anodic Pb(II) film significantly [15]. As Sm and Ce have similar properties, we studied the Pb–Sm–Sn alloy as the positive grid material and compared it with the Pb–Ca–Sn alloy in this work.

Section snippets

Electrochemical measurements

A proprietary Pb–0.73 wt.% Sm–0.63 wt.% Sn alloy [16] (Pb–Sm–Sn) rod and a conventional Pb–0.088 wt.% Ca–0.63 wt.% Sn alloy (Pb–Ca–Sn) rod were used as working electrodes. The alloys were manufactured by the Shanghai Powerson Power Supply Co. with lead (99.99%), samarium (99.9%), calcium (99.9%) and tin (99.9%), and mixed using a vacuum fusion method [16]. The rods were sealed with epoxy resin in the lower part of an L-shaped glass tube, so that a cross-sectional area of 0.2–0.3 cm2 was exposed

Linear sweep voltammetry (LSV)

Fig. 1 shows the voltammgrams of the anodic films formed on the Pb–Sm–Sn and Pb–Ca–Sn alloys at the floating charge potential of 1.28 V for 1 h. In Fig. 1, peak a (1.07 V) corresponds to the reduction of β-PbO2 to PbSO4 [17]. Peak b (1.03 V) is caused by the oxidation of a small amount of Pb(II) to α-PbO2 [18].

Table 1 lists the reduction charges of peak a (Qa) of the two alloy electrodes in Fig. 1. It can be seen from Table 1 that the Qa of the Pb–Sm–Sn (29.3 mC cm−2) is smaller than that of

Conclusions

Samarium as the substitute for the calcium in the Pb–Ca–Sn alloy can inhibit the growth of the anodic corrosion layer (PbO2) and reduce the impedance of the anodic corrosion layer (PbO2) formed on the lead alloy. Moreover, the rate of the oxygen evolution at the Pb–Sm–Sn alloy is also lower than that at the Pb–Ca–Sn alloy.

The capacity loss of the battery with the Pb–Sm–Sn positive grids is significantly less than that of the battery with the Pb–Ca–Sn positive grids and the life of the battery

Acknowledgements

This work was supported by the National Natural Science Foundation of China, Project Nos. 20173013 and 29873013. The authors thank the Shanghai Powerson Power Supply Co. Ltd. performing the accelerating life test of the 2V-200Ah VRLA batteries.

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