Communication—Improving Corrosion Resistance of Lead-Alloy Positive Grid of Lead-Acid Battery by an Electrochemical Prepassivation Interphase

All the chemicals and materials used in experiments, including the Pb-Ca-Sn-Al alloy (1.43% Sn, 0.06% Ca, 0.01% Al) and 1.28 g cm⁻³ H₂SO₄ solution, were provided by Herou Power Supply Technology Co. Ltd. The Pb-alloy grid was cut into small samples (35 mm × 10 mm × 2 mm) and polished with a series of SiC papers (mesh size: 500, 800, 1000, 1200, 1500) sequentially. The samples were cleaned with alcohol and deionized water alternately and then dried in air. The samples were sealed in epoxy resin leaving a working area of 0.1 cm² and used as the working electrode. A 2 cm² Pb plate acted as the counter electrode and a Hg/Hg₂SO₄ electrode acted as the reference electrode. Prior to each test, the electrochemical system was cathodic polarized at −1.2 V for 20 min to remove the oxides on the working electrodes. Subsequently, the electrochemical prepassivation was performed by fixing the working electrode potential first at 1.6 V for 5 s for the generation of nucleation sites, and then at 1.4 V for 5 s for the growth of passivation layer. Cyclic voltammetry was adopted to test the growth rate of passivation layer, by which the corrosion resistance is evaluated.

Electrochemical experiments were carried out on a CHI760d workstation (Shanghai Chenhua Instrument Co.) at room temperature (25 °C). X-ray photoelectron diffraction (XPS, Thermo Scientific ESCALAB Xi+, Thermo Fisher) and Raman spectrum (InVia Raman Microscope, Renishaw Leica) were adopted to characterize the components of prepassivation film. Scanning electron microscope (SEM, GeminiSEM 500, Zeiss) was used to analyze the surface morphology of the prepassivation film and the cross-sectional structure of corrosion layer, which was prepared by focused ion beam processing system (FIB, Orion Nanofab, Carl Zeiss).

The passivation curve of Pb-alloy electrode was obtained by linear sweep voltammetry with a slow scan rate of 5 mV s⁻¹ (Fig. 1a). In the AB segment, the surface of Pb-alloy was electrochemically activated and the current was increased with the applied potential. Since the product is the insulate PbSO₄, after achieving a maximum at the defined passivation potential, the anodic current kept decreasing to a minimum and then presented a passivation potential range from −0.8 V to 1.2 V (i.e., the CD segment). Increasing the potential continually, the passivation layer formed in the passivation potential range would be broken, indicating a new surface reaction from insulate PbSO₄ through PbO to conductive PbO₂ and, simultaneously, the oxygen evolution reaction catalyzed by PbO₂ as defined the transpassivation (the DE segment). ^14–16

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Passivation curve provides abundant information on the anti-corrosion parameters, such as the corrosion potential, corrosion current, passivation potential, transpassivation potential, etc, which are relevant to the battery performances. Indeed, the corrosion resistance is determined by the components and compactness of the passivation layer on the Pb-alloy surface. ^17,18 Based on these considerations, we optimized a protocol to produce a compact passivation interphase film on Pb-alloy: Firstly, a cathodic potential was fixed at −1.2 V for 20 min to remove the oxides on the working electrodes; Secondly, an anodic potential was fixed at 1.6 V in the transpassivation range for 5 s to generate the oxygen-containing nucleation sites; Thirdly, the anodic potential was immediately stepped to 1.4 V for 5 s for the formation of prepassive layer. The whole prepassivation processes were finished in 10 s. The insert in Fig. 1b shows the chronopotentiometric curve obtained by the pre-passivation protocol. Indeed, an obvious current stage is observed when the potential is stepped to the transpassivation range (i.e., 1.6 V), indicating a complex surface electrode process from PbSO₄ to PbO_x (x = 1 or 2) mixtures. When the potential is stepped back to 1.4 V, the current falls down indicating the slower growth rate of the prepassivation interphase. This protocol is supported by previous reports, e.g., the potential should be biased higher than the transpassivation potential in order to produce PbO or PbO·PbSO₄, ¹⁵ and the PbO·PbSO₄ interphase will become denser and the grains will become smaller under higher bias potential. ¹⁶

Cyclic voltammetric experiments were performed to further demonstrate the necessity that the prepassivation potential should be biased to the transpassivation range in order to form the PbO/PbSO₄ interphase. Figure 1c shows the cyclic voltammograms obtained with the same initial potential at −1.45 V and different inverse potentials. From Curve Ⅰ to Curve Ⅴ little change can be observed when the inverse scan potential lies in the passivation potential range. Until the reverse scan potential reach to 1.4 V, i.e., in the beginning of transpassivation potential range, a slight current peak can be observed corresponding to the electroreduction of PbO to Pb at −0.85 V (Curve Ⅵ). When the inverse potential arrives at 1.6 V, a pair of sharp current peaks at ∼1.2 V are observed obviously which corresponding to the PbO/PbO2 conversion; Meanwhile, a dramatically increased cathodic current peak is observed for the electroreduction of PbO to Pb at −0.85 V (Curve Ⅶ). All the results are in harmonious accordance with those of chronopotentiometric, elucidating the reasonability of this protocol for the electrochemical formation of the PbO/PbSO₄ prepassivation interphase on the Pb-alloy surface.

X-ray photoelectron spectroscopy (XPS) was adopted to characterize the components of passivation interphase. Figures 2a–2c presented the S_2p, Pb_4f, and O_1s spectra. Comparing the characteristic peak binding energies of Pb_4f-7/2 at 136.3, 138.6 and 139.1 eV with the characteristic peak binding energies of O_1s at 529.6, 531.4 and 532.4 eV, on the basis of standard binding energy data of lead compounds, ^11,19,20 it can be concluded that PbO and PbSO₄ are the main components of the passivation interphase. In addition, the appearance of metallic Pb peak at 136.3 eV indicates the prepassivation interphase is a very thin film, and the O_1s peak at 532.4 eV indicates the adsorbed H₂O molecules on the thin prepassivation film.

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Raman spectroscopy further verified the XPS results. Figure 2d illustrates the Raman spectra of passivation interphase formed on the surface of Pb-alloy electrode. The Raman peaks at 444 cm⁻¹ and 611 cm⁻¹ correspond to the S-O bond bending vibration, and the one at 981 cm⁻¹ represents the S-O bond stretching vibration. The most intense Raman peak at 137 cm⁻¹ is associated with the Pb-O bond. Based on the literature, ^21–23 the peaks at 444 cm⁻¹, 611 cm⁻¹, and 981 cm⁻¹ are ascribed to PbSO₄, and the peak at 137 cm⁻¹ belongs to PbO. The results also elucidated that the prepassivation interphase film was successfully formed on the surface of Pb-alloy electrode, and the main components were PbO and PbSO₄.

Figures 3a and 3b show the SEM images of the Pb-alloy electrode surface without and with the electrochemical prepassivation. It is observed that the passivation interphase is formed with homogeneously distributed and compact nanoparticles, exhibiting a smoother and tighter surface with a larger specific surface area. Furthermore, in order to elucidate the dynamic evolution of the electrochemical prepassivation interphase, cyclic voltammetry was adopted to mimic the charging/discharging cycles of LAB in the experimental potential range between 0.6 V and 1.6 V (Figs. 4a and 4b). The cross-section SEM morphologies of the interphase were characterized shown in Fig. 3c (without prepassivation) and Fig. 3d (with prepassivation). It is obvious that, with electrochemical prepassivation processes, the interphase is much denser. From Fig. 4c, the integrate charge of the PbO₂ electroreduction obtained on the pre-passivation Pb-alloy electrode increases more slowly with the scan cycles than that on the un-passivation one, which indicates a slower growth rate of the interphase and an improved corrosion resistance of the pre-passivation Pb-alloy grids.

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Furthermore, because the PbO/PbSO₄ prepassivation interphase has the same components as the positive lead paste, it should have an excellent compatibility to both the positive Pb-alloy grid and lead paste. The unique compatibility can facilitate the subsequent electrochemical forming processes, avoid the crevice corrosion of the positive Pb-alloy grids, and play a crucial role in improving the corrosion resistance of the pre-passivation Pb-alloy grid.

The corrosion of positive grids is the fatal factor causing the failure of LAB. To this, we propose a feasible strategy to form a nanostructured PbO/PbSO₄ interphase by electrochemical prepassivation. The results show that, comparing to the un-passivation Pb-alloy grid, during the mimic charging/discharging cycles by cyclic voltammetry, the dynamic interphase on the pre-passivation Pb-alloy grid is much denser with a slower growth rate and, thus, presents an improved corrosion resistance.

Financial support of the National Natural Science Foundation of China (22202166, 21827802, 22132003 and 22021001), Fundamental Research Funds for the Central Universities (20720230076) and the 111 Project (B08027, B17027) is appreciated. We thank Tan Kah Kee Innovation Laboratory for providing characterization services. The authors declare no competing financial interests.