Renewal of Lead Battery Electrodes Using a Plasma—Chemical Processing Method

Abstract

The increasing use of renewable energy sources increases the need for electricity storage systems. In this work, the possibility of renewing worn-out battery Pb electrodes by applying Ar and O₂ gas plasma in a magnetron vacuum system was investigated. As shown by the SEM results of the surface of Pb electrodes, after exposure to O₂ plasma, the surface of Pb electrodes consists of irregularly shaped microstructures. Analyzing the XRD results revealed that the spectrum peaks of the Pb₄O₃SO₄ compound are more pronounced in the case of the electrodes of the worn battery than those electrodes that were exposed to Ar and O₂ gas plasma. It was observed that the process of renewing Pb electrodes in Ar and O₂ plasma has a positive effect. This effect is also confirmed by the results of EDS studies, as the chemical compounds that inhibit the charge/discharge process are reduced, and the percentage of pure lead increases from 68.2% (in the case of worn electrodes) to 81–82% (when the worn electrodes were processed using Ar and O₂ plasma).

1. Introduction

Recently, the rapidly developing industry of both renewable energy sources (such as solar power plants, etc.) and mobile devices (household battery tools, telephones, backup power sources, etc.) has been paying increasing attention to electrical energy storage devices of various types and sizes. Batteries store DC electricity using chemical reaction processes. Depending on the type of battery, the process of storing and releasing electricity is reversible or irreversible [1,2,3,4]. There are two types of batteries: primary and secondary. Lead-type batteries are called secondary batteries because they are rechargeable. Lead-type batteries can be divided into starter and industrial batteries. Starter batteries are designed to provide high power impulse requiring systems, such as, e.g., vehicle starter systems, electric cars, backup power systems, etc. Industrial batteries are intended for long-term use and storage of electricity, e.g., for backup power supply of special-purpose buildings in the event of malfunctions of the external network, etc. [5,6,7,8]. Pb-type batteries are produced in large quantities in the world, reaching 95 million units per year or more. In addition, the production scale of this type of battery is growing by about 2% every year. The chemical reaction during the charging of a lead-type battery: 2PbSO₄ + 2H₂O → PbO₂ + Pb + H₂SO₄, while the chemical reaction during the discharge process is: PbO₂ + Pb + H₂SO₄ → 2PbSO₄ + 2H₂O [9,10,11,12,13].

The electrodes of modern lead-type batteries are manufactured using a grid-shaped concept in order to save the materials and achieve the lowest possible mass of the device and to create the lowest possible amount of recyclable waste. During the charge/discharge cycle, the chemical reactions occur simultaneously with the corrosion of the Pb electrodes, which gradually wear and lose their original state and functionality. This process affects the number of life cycles and electrical capacity of Pb batteries. In order to reduce the corrosion of Pb electrodes, an alloy of lead and antimony is often used in the production process of this type of battery [14,15,16,17]. In order to extend the service life of lead batteries and reduce the corrosion of Pb grid-shaped electrodes, it is very important to monitor the electrolyte level in the batteries, which should be sufficient. This factor is especially important in the operation of Pb-type liquid electrolyte batteries. During long-term operation of batteries, their service life depends not only on the corrosion process of Pb electrodes, but also on the formation of lead sulfate crystals on the surface of the electrodes. Lead sulfate crystals hinder the battery charging process [18,19,20,21].

Lead-type batteries are characterized by the number of charge–discharge cycles of about 500–750, after which they wear out and become unusable, so there is a need to safely dispose them or recycle into the secondary raw materials. The main danger when operating the batteries is the possible release of lead particles and electrolyte into the environment. Lead is a sufficiently heavy element whose density is about 11.3 times higher than water, and lead waste also has the property of accumulating in soil, animal organisms, etc. [1,22,23,24]. Other dangerous products that are left in used batteries are glass fiber in AGM-type batteries and silicon dioxide in GEL-type batteries. Almost all battery parts can be recycled. The main steps in recycling of Pb-type batteries are: collecting batteries and transporting them to a recycling facility, separating battery components, melting and refining lead components, washing plastic components, then crushing or melting them, cleaning and treating the sulfuric acid electrolyte, and waste treatment and disposal. Currently, about 98% of Pb batteries are recycled by utilizing their products or using them as secondary raw materials in circular economy processes [2,3,25,26,27]. As was already mentioned, the Pb charging processes are stopped by the formation of various compounds on the surface of the electrodes. In order to recover pure Pb during battery recycling, a deduction process is carried out, during which pure lead is obtained as a secondary raw material for battery recycling. Later, the recovered Pb is melted down and used in the production of future batteries. Despite the high recycling efficiency of Pb batteries, this process has a number of disadvantages: melting consumes a lot of energy, which is obtained by burning fossil fuel, which emits a considerable amount of CO₂ (1 kg of melted lead accounts for 0.55 kg of CO₂). In order to improve the process of recycling and secondary use of Pb batteries, new methods are being sought [28]. One such method that partially helps to solve the problem of secondary use of this type of batteries is the regeneration of Pb electrodes [1,29].

The regeneration of Pb electrodes is carried out using the reverse charge desulfation process, and, when at a constant voltage of 2.67 V, up to 80% of the previous battery capacity can be obtained. Another way is when the pulsed voltage mode is used [4,30]. It is reported in the literature that, when using the pulse mode, it was possible to restore up to 90% of the previous battery capacity [1,3,31]. However, the regeneration methods discussed have several major drawbacks: (1) the duration of the battery regeneration process, which can reach up to several days; (2) although the battery capacity can be restored up to 90% of its initial value by means of regeneration, the number of additional life cycles is sufficiently small (up to 10% of the initial number of cycles) [32,33,34,35]. Therefore, the life of an old worn-out battery can only be extended by 10% of its former life.

When solving the problem of disposal and secondary use of batteries, it would be useful to try the process of Pb electrodes renewal (e.g., using plasma surface treatment and renewal technologies), during which the worn-out electrodes would be restored to their original state without changing their shape and could be used in the production of new batteries. Applying such a method would avoid the melting of lead, which requires a lot of energy. The use of plasma treatment for the renewal of Pb electrodes surface is a completely new technology not discussed in the scientific literature.

The aim of this scientific work is to apply plasma chemical processing to the process of Pb electrodes renewal, during which electrode surfaces are processed in a reactive gas plasma in order to renew them to their original state.

2. The Experimental Setup and Methodology

Deep-cycle (multi-cycle) Pb-type batteries with liquid electrolytes were used for the research. A new battery and old battery after 500 life-cycles were used. After such a number of cycles, the Pb-type battery was considered worn out. In order to carry out the research, the old worn-out battery and a new battery were disassembled. In this way, the lead electrodes needed for further technological processing and research were removed from the batteries. A magnetron vacuum system was used to renew the lead electrodes. Renewable lead electrodes were placed on the surface of the magnetron. Ar and O₂ gases were used to affect the surface. Some worn electrodes were renewed using Ar gas plasma, and others were renewed using O₂ plasma. If the gas was inert, it was intended to realize the removal of unwanted compounds on the surface of the Pb electrode during bombardment, while the O₂ gas plasma was used to realize the etching of the surface of the Pb electrodes. Before the cleaning, the vacuum chamber was evacuated to a pressure of 10⁻² Pa. Next, argon or oxygen gas is introduced so that the pressure in the chamber becomes 10 Pa, the cleaning voltage is 700 V, and the exposure time is 15 min.

A Hitachi S-3400 N scanning electron microscope (Hitachi, Tokyo, Japan) was used to analyze the surface of the affected Pb electrodes. The elemental composition of the surface was evaluated by energy dispersive X-ray spectroscopy (EDS) using a Bruker Quad 5040 spectrometer (Hamburg, Germany). The crystal structure of the electrodes is analyzed by X-ray diffraction (XRD) (DRON-UM1, with standard Bragg-Brentan focusing geometry) in the range 10–100° using CuKα (λ = 0.154059 nm) radiation. Measurements were performed using a D8 Discover X-ray diffractometer (Bruker ASX GmbH, Billerica, MA, USA) in a peeking (grazing) incidence configuration in the range 20–55° at an incidence angle of 0.2. The specific electrical conductivity was measured using the four-probe method, with a 100 μm electric current flowing across the sample surface at 10 V.

3. The Results and Discussion

The surface morphology of the battery’s lead electrodes was analyzed using a scanning electron microscope. The studies of the surface morphology of Pb electrodes are presented in Figure 1. In Figure 1a, we can see the surface morphology of the new (unused) Pb electrode of the battery. As can be seen in this case, the surface is uneven, composed of irregularly shaped microstructures. In this case, the dimensions of the microstructures vary in a wide range of 10–50 µm. Figure 1b shows the surface morphology of the Pb electrode when the battery was charged/discharged for 500 cycles, which is considered completely worn out. In this case, the visible surface is more uniform than the electrodes of a new battery. This is because chemical reactions occur on the surface of the electrodes during the charging/discharging process. At the same time, the process of surface etching takes place, during which, part of the microstructures are destroyed, or their geometric dimensions are reduced. In the picture Figure 1c, we see the surface of the Pb electrodes of a worn battery that has been exposed to an Ar gas plasma [1,3]. As SEM analysis shows, the surface morphology in this case is similar to Figure 1b. This is because the Ar gas is inert, and chemical etching does not take place when the surface is exposed to it (the surface is only bombarded with Ar ions and molecules), so it is unlikely that new microstructures will form. However, the Ar gas exposure process can be useful to renew the structure of Pb electrodes in batteries. Figure 1d shows the surface of a Pb electrode after being etched in O₂ plasma. In this case, the visible surface is uneven and consists of microstructures of fine structure and irregular shape. Since, in this case, the surface etching with reactive gas takes place, some microstructures are destroyed, but, at the same time, new microstructures are formed on the surface of the Pb electrode.

The distribution of elements on the surface of Pb electrodes was studied using the EDS method. Elemental composition studies are presented in Figure 2. The investigations of the elemental composition of the Pb electrode of the new battery are presented in Figure 2a. As we can see in this case, the predominant elements are Pb, Si, C, O, and Al. Observing only the color difference in the picture, it is difficult to assess which element has the largest amount. The elements Al and C are likely to appear during the electrode manufacturing process, while S and O are likely to be a component of the battery electrolyte, with small amounts deposited on the surface of the electrodes. Figure 2b shows the elemental composition of the surface when the battery is considered worn out. As can be seen, the predominant elements are the same as in the case of a new battery. It can also be noted that Ca, Fe, Na, and Mg elements are visible in the case of a worn-out battery, but as further EDS analysis showed, their amount is negligible, probably due to measurement error.

The percentage composition of elements on the surface of Pb electrodes was measured using the EDS method. The results of the percentage elemental composition studies are presented in

Table 1. Elemental composition of lead electrodes obtained by EDS.

Element	Pb (New) [wt.%]	Pb (Old) [wt.%]	Pb (Ar) [wt.%]	Pb (O2) [wt.%]
C	5	4	3.2	3.5
O2	11	21	10.7	11.3
Pb	80	68.2	82	81
Al	1.4	0.9	0.8	1.4
Si		1.4	1	1.2
Na		0.7	0.5	0.8
S	2.6	2.2
Ca		1.4	1.8	0.8
Mg		0.2

.

Analyzing the results of the EDS measurements, it can be observed that, in the case of a new battery, the largest content of the electrode material is lead—about 80%, while the remaining elements are oxygen, carbon, and sulfur, the percentages of which are 11%, 5% and 2.6%, respectively. In the case of a worn out battery, and the percentage of Pb is about 68.2%, while the percentage of oxygen is about 21%. This is because, during the charge/discharge process, part of the lead is lost during the ongoing chemical reactions, and various compounds are formed that further delay the charging process. As it can be seen, in the case of Pb electrodes exposed to Ar and O₂ gas plasma, the lead content is similar to that of a new battery and reaches about 81–82%. Meanwhile, in both of these cases, a reduced amount of oxygen can be observed, which varies from 10.7 to 11.3%. As we can see, the sulfur is not detected at all after the exposure of Pb electrodes in Ar and O₂ gas plasma. The increase in lead and the disappearance of sulfur in this case can be explained by the fact that the formed surface sulfur compounds (including lead) are eroded, and the surface of the electrode is renewed.

The XRD method was used to identify the chemical compounds with a crystalline structure on the surface of Pb. The XRD studies are presented in Figure 3. Analysis of the the XRD spectrograms showed that the new electrode is mainly composed of lead, and small peaks of Pb₄O₃SO₄ are also noticeable. This compound is likely formed when the electrolyte reacts with the surface of the Pb electrode. Meanwhile, the data on the orientation of lead crystals can also be seen in Figure 3. In the case of the worn-out battery and electrodes exposed to Ar and O₂ gas plasma, it was possible to identify Pb₂(SO₄)O, Pb₂SO₅, and Pb₂SO₅ compounds in XRD X-ray images. These compounds are likely formed during the charge/discharge cycles when the Pb surface interacts with the liquid electrolyte. Additionally, analyzing the XRD results, it became clear that the peaks of the spectrum of the Pb₄O₃SO₄ compound are more pronounced in the case of worn-out battery electrodes than in the electrodes that were exposed to Ar or O₂ gas plasma. Therefore, it can be said (this is also confirmed by EDS studies) that the amount of various sulfur and lead compounds on the surface of affected Pb electrodes is reduced, thus the electrodes are renewed, and the amount of pure lead in them increases.

Figure 4 shows the FTIR spectra of Pb electrodes. As can be seen in all cases, the spectra contain characteristic Pb peaks in the wavelength range of 500–1200 cm⁻¹. Peak characteristics of lead are at 600, 650, 1000, and 1250 cm⁻¹. C-H, C=C and O-H groups were also identified, with characteristic peaks at 1450, 1500, and 3600 cm⁻¹ wavelength values, respectively. It is likely that the mentioned groups are characteristic of the compounds formed on the surface of Pb electrodes during chemical reactions with the electrolyte and during charge/discharge processes. As it can be seen from the spectra, the least pronounced peak of the O-H group is for Pb electrodes that were exposed to Ar or O₂ plasma. Such a result can be explained by the fact that surface compounds are eroded or destroyed during Pb surface treatment, which have characteristic O-H groups visible in the spectra. In this way, it can be said that the Pb surface of the battery electrodes is renewed, and compounds that interfere with the charging process are removed from the surface [1,2,3,4,5].

Table 2. The electrical conductivity of Pb electrodes.

Pb (New) [S]	Pb (Old) [S]	Pb (Ar) [S]	Pb (O2) [S]
6.2	6.7	6.7	6.2

shows the electrical conductivity measurements of the Pb electrodes. As the results show, the electrical conductivity of both new Pb electrodes and old and affected in Ar and O₂ plasmas changes slightly. The electrical conductivity of the new Pb electrode and the electrode exposed to O₂ plasma is about 6.2 S. Meanwhile, the electrical conductivity of the worn battery electrode and the electrode exposed to Ar plasma is about 6.7 S.

It can be assumed that the processing or wear of the Pb electrodes has no noticeable effect on their electrical conductivity. Meanwhile, electrical conductivity probably does not have a noticeable effect on the charging process, since the charging of the battery itself does not depend on the electrical conductivity of the electrodes. However, concerning the amount of various chemical compounds formed on the surface of Pb and the thickness of the layer, in later stages, it hinders the movement of charge carriers and the progress of useful chemical reactions.

It is worth noting that the processing of Pb electrodes using Ar and O₂ plasma obtained in a magnetron system has a noticeable effect on the surface of Pb electrodes (during the process, formations are removed from the surface of Pb electrodes, which hinder the movement of electric charge carriers) [35]. Such electrodes can be used in the production of new Pb-type accumulators in accordance with the principles of the circular economy when the waste is not wasted and used as a secondary raw material.

4. Conclusions

Ar and O₂ plasmas were used to renew the Pb electrodes. The process took place in a vacuum magnetron system. Using Ar and O₂ plasmas, the aim was to renew the surface of the worn Pb electrodes of batteries by removing the formed surface compounds that hinder the charge/discharge process. As can be seen from the images obtained by SEM, the surface of the Pb electrode of the new battery is composed of irregularly shaped microstructures of various sizes, the dimensions of which vary in the range of 10–50 µm. Meanwhile, the surfaces of worn battery electrodes and those affected by Ar plasma electrodes are more uniform and less pronounced. XRD studies showed that a Pb₄O₃SO₄ compound is formed on the surface of the Pb electrode of the new battery. From the XRD spectrograms, it can be seen that the peaks of the spectrum of the Pb₄O₃SO₄ compound are more pronounced in the case of the electrodes of the worn battery compared to the electrodes that were exposed to Ar and O₂ gas plasma. In the FTIR spectra of Pb electrodes, characteristic peaks of Pb are observed in the wavelength range of 500–1200 cm⁻¹. The least pronounced peak of the O-H group, with a position at 3600 cm⁻¹, is found in Pb electrodes exposed to Ar and O₂ plasma. The measurements show that the wear of Pb electrodes and exposure to Ar and O₂ plasma have almost no effect on the electrical conductivity, since the electrical conductivity of both new Pb electrodes that are worn and those exposed in plasma ranges from 6.2 to 6.7 S. After the analysis of the research results in general, it can be said that the process of renewal of Pb electrodes in Ar and O₂ plasma has a positive effect. This effect is also confirmed by the results of EDS and XRD studies, as the chemical compounds, inhibiting the charge/discharge process, are reduced, and the percentage of pure lead increases from 68.2% (in the case of worn electrodes without plasma processing) up to 81–82% (when the worn electrodes were processed using Ar and O₂ plasma). Meanwhile, in the compounds of oxygen-containing electrode material, according to EDS studies, the content of old electrodes (untreated with Ar and O₂ plasma) is about 21%, while the content of oxygen in plasma-treated electrodes is 10.7% and 11.3% (treated with Ar and O₂ plasma, respectively), which is approximately the same as in the new electrode—11%. This also confirms the effectiveness of the treatment method for these Pb electrodes, since the composition of the old electrodes becomes similar to the chemical composition of the new electrode.