J Solid State Electrochem DOI 10.1007/s10008-016-3243-2

ORIGINAL PAPER

Lead sulfate used as the positive active material of lead acid batteries Ke Zhang 1 & Wei Liu 1 & Beibei Ma 1 & Mohammed Adnan Mezaal 1 & Guanghua Li 1 & Rui Zhang 1 & Lixu Lei 1

Received: 5 January 2016 / Revised: 27 April 2016 / Accepted: 8 May 2016 # Springer-Verlag Berlin Heidelberg 2016

Abstract Lead sulfate is produced when a lead acid battery discharges, and it is also known that big PbSO4 crystals are less active than the smaller ones because they dissolve slower, thus result in failure of the battery. However, little is known if chemically prepared PbSO4 can be used as active material of lead acid batteries. Here, we report the preparation of PbSO4 by facile chemical precipitation of aqueous lead acetate with sodium sulfate and its utilization as the positive active material. The results show that the PbSO4 alone is not good enough for the purpose, but its mixtures with Pb3O4 are as excellent as the industrial leady oxide. For example, the mixtures containing 5, 10, 20, and 30 wt.% of Pb3O4 discharge 78.2, 92.9, 88.0, and 91.5 mAh g−1 at a current density of 100 mA g−1, respectively. Also, the one with 10 % Pb3O4 remains 93 % capacity in 150, 100 % DOD cycles. Keywords Lead sulfate . Red lead . Positive active material . Lead acid battery

Introduction Due to the growing energy and environment requirements, the world is focusing on sustainable and renewable power sources [1]. Recently, the electric vehicles and hybrid electric vehicles bring out more challenges for battery technology innovation [2–5]. Although much attention is attracted to Li-ion battery,

* Lixu Lei [email protected] 1

School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China

lead acid batteries are still the dominant rechargeable batteries in the world owing to their high performance to cost ratio, mature manufacturing technology, reliability, and safety [6]. However, lead acid batteries have a defect, which produces toxic lead and may form environmental hazard while in producing and recycling processes. Traditionally, the material used in manufacture of lead acid battery is leady oxide made from metallic lead via either ball milling or Barton pot technique, which typically contains 20 ~ 30 wt.% of metallic lead and 70 ~ 80 wt.% of lead monoxide [7]. Although the hazards can be prevented by deploying fine mechanicals and workers under strict measures, the large particles and metallic lead in this leady oxide make the curing time exhausting and the utilization of the active materials very low, which normally discharges only about 70 mAh g−1 at 100 mA g−1. Consequently, it is urgent to seek cleaner, safer, and especially, more active materials instead of the leady oxide [8]. For the recycling processes, both pyrometallurgical and hydrometallurgical methods have been proposed; however, only the pyrometallurgical methods are in industrial use and facing criticism from the public and governments due to the possible lead dust and SO2 emissions. Given to the environmental concerns, hydrometallurgical methods have attracted scientific interest recently. Typical industrial technology comprises desulfation and reduction of lead compounds, or leaching with acids and electro-winning, and their purpose is to recover metallic lead [9, 10]. However, there is one question here, why should we obtain lead metal rather than prepare PbO, or even PbSO4 from the spent batteries, as re-oxidation of the metallic lead is necessary to produce leady oxide at high temperature, exhausting more energy? Answering this question leads to novel routes for recycling the lead acid batteries, which produce lead compounds.

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In 2009, R.V. Kumar’s group published their results on producing fine PbO powders from model compounds of lead acid battery by treating PbSO4, PbO2, and PbO with citric acid/sodium citrate [11, 12]; at about the same time, we independently patented a novel method to reproduce lead acid batteries from the spent ones based on physically separated electrodes. Later, both methods have been advanced to the real spent batteries. For examples, in 2012 and 2013, Zhu et al. reported a process to prepare PbO by desulfation, leaching, and combustion-calcinations at low temperature from lead paste [13, 14]; we have also published our results on methanothermal reduction of the spent positive materials to produce PbO, which can be directly used in new battery production and discharges about 120 mAh g−1 at 50 mA g−1 [15, 16]. All those process saves more energy, consumes less chemicals, and releases much less environmental hazards. It is known that a few basic lead sulfates, such as PbO· PbSO4 (1BS), 3PbO·PbSO4 (3BS), and 4PbO·PbSO4 (4BS) form during curing process in traditional technique, researches on those basic lead sulfates have been done [17–21]. During our research progressing, one new question arises: it is generally known that PbSO4 is formed in both electrodes of lead acid batteries when they discharge, which means that PbSO4 is the active material of the lead acid batteries, and PbSO4 can be easily prepared from a variety of lead compounds, including materials from spent lead acid batteries, why do we not use lead sulfate as active materials directly? We have reported the first example that PbSO4 was used as the negative active materials [22]. Our research showed that PbSO4 negative electrode could discharge a capacity of 103 mAh g−1 at the current density of 120 mA g−1, which was higher than the electrode made from leady oxide under the same condition, also its cycle life of 100 % DOD (depth of discharge) reached 550 cycles. Although there have been two reports concerning PbSO4 as the positive active material of lead acid batteries, some important information have not been disclosed. In 2002, Yan et al. [8] used PbSO4, prepared from the time-exhausting reaction of leady oxide and sulfuric acid, as positive material in lead acid batteries, together with two undisclosed additives, the specific energy by weight of 12V10Ah battery at 5- and 2-h rate reached 37.19 and 35.47 Wh kg−1, the cycle life of 55 % DOD attained 450 times. In 2012, Foudia et al. [23] prepared PbSO4 by the precipitation of lead nitrate with sodium sulfate and formed the plates in electrolyte of different pHs, which showed that average crystallite size, phase composition, and positive active material capacity were affected by pH, and the maximum discharge capacity was 92 mAh g−1 for the formation at pH > 12. However, we know that PbSO4 is not stable at such pH and will convert into Pb(OH)2, which makes the result questionable. To make things simpler, we attempted to directly use chemically synthesized PbSO4 as the positive active material, but we had soon found that the electrochemical

properties of pure lead sulfate are very poor. Consequently, we need to find out possible ways to improve the electrochemical performance of PbSO4. Red lead, that is, Pb3O4, a well-known industrial product, has a wide range of applications [24]. It was also used by early battery manufacturers but was later replaced by leady oxide [25]. Recently, the use of red lead has again drawn attention of the lead-acid battery manufacturers due to its ability to promote plate formation and deep-cycle performance [26–28]. Based on this, we investigated the mixtures of Pb3O4 and PbSO4 to see the effect on their electrochemical performances.

Experimental Preparation of lead sulfate 0.01 mol of Na2SO4 in 100 ml of distilled water at 70 °C was poured into the solution of 0.01 mol of Pb(CH3COO)2·3H2O in 100 ml of distilled water at 70 °C in water bath with stirring. The formed mixture was kept at 70 °C for another 30 min, and then the solid was filtered out, washed several times with deionized water, and dried overnight at 80 °C. Electrode preparation Herein, to make sure the thicknesses of the electrode are the same, the total mass of the mixture is the same (0.6 g). Accordingly, Pb3O4 (99.95 %, Aladdin Chemical Reagent Co. Ltd.) of 0, 5, 10, 20, and 30 wt.% of the total mass were added to lead sulfate, respectively. Hereafter, the mixtures will be denoted as A, B, C, D, and E sequentially. Each of the mixtures was placed in an agate mortar, and then 0.0006 g of acetylene black, 0.0018 g of polypropylene fibers together with some deionized water was slowly added. The obtained paste was put evenly onto a Pb-Ca alloy grid with dimensions of 10 × 8 × 2 mm3, which was then pressed to get the positive plates. The plates were then immersed in sulfuric acid of 8 wt.% for 5 s; after that, they were put into an oven, maintaining at the temperature of 100 °C for 5 min, and then put into an oven at 75 °C with relative humidity >95 % for 24 h; lastly, the plates were dried at 70 °C for 48 h. The actual mass of the active material was the difference of the mass of dried plate and that of the blank grid. Characterization Powder X-ray diffraction patterns of samples were measured on Bruker D8 Discover instrument operating at 40 KV and 20 mA, by using Cu K radiation (λ = 0.15406 nm). Scanning electron micrographs (SEM) of samples were carried out on a Hitachi S-4800 microscope.

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Electrochemical performance test All the formation and the cycling test were carried out with an NEWARE Cycler (BTS-5V3A Shenzhen Electronics Co., Ltd., China). The positive plate was assembled with a negative plate cut from a commercial plate (the area is about 3 times of the positive plate) which is separated by absorptive glass mat (AGM). They were inserted into a case and the electrolyte was a H2SO4 solution with relative density of 1.23 g ml−1 of 12 ml. The cell was sealed and put into a water bath with the temperature maintaining at 32 ± 2 °C. After the plate had been soaked for 2 h, the formation was started. The formation was divided into three steps. Firstly, the electrode was charged at a current density of 18.75 mA g−1 to 187 mAh g−1; then it is charged at 31.25 mA g−1 for another 250 mAh g−1; and finally, it was charged at 10 mA g−1 for another 187 mAh g−1. Since the negative electrode had a much larger capacity than the positive electrode, the cell was positive electrode limited. After the battery formation, the cell was discharged at a current density of 5 mA g−1 until the voltage fell to 1.75 V. Then, it was used for cyclic test. The electrode was charged at 50 mA g−1 until the cell voltage reached to 2.45 V, then it was charged at 25 mA g−1 until the charged charges was 110 % of the first discharge capacity and then discharged at a current density of 100 mA g−1. The cyclic voltammetry (CV) tests were carried out on the Corrtest model CS350 electrochemical working station (WUHAN CORRTEST Instrument Co. Ltd. China). The CV curves were determined with a three-electrode system. A double platinum foil electrode was used as the counter electrode, and the Hg/Hg2SO4/K2SO4 (sat.) was employed as the reference electrode; sulfuric acid (36 wt.%) was used as electrolyte. The measurement was carried out at a potential scan rate of 20 mV s−1 from 0 to 1.8 V.

Fig. 1 X-ray diffraction patterns of the prepared PbSO4

Results and discussion

which will transform into α-PbO2 and β-PbO2 in the process of formation; the ratio of the two kinds of PbO2 influences the electrochemical performance of the positive plate. We are wondering how the mixture of PbSO4 and Pb3O4 changes. Figure 3 gives XRD patterns of the five samples after curing; it is obvious that no basic lead sulfate is identified, and they are still mixtures of PbSO4 and Pb3O4. Figure 4 gives the XRD patterns of the five mixtures after formation. It is seen that sole lead sulfate (sample A) does not change (JCPDS NO. 82–1854); it means PbSO4 is hard to be oxidized; those mixtures with red lead can be oxidized and most of PbSO4 converts into β-Pb0.986O2 (JCPDS NO. 75– 2417), and their compositions after formation are shown in Table 1, which shows that the content of PbO2 is similar and independent on the initial composition. This is in accordance with the well-known fact that Pb3O4 or PbO2 can accelerate the formation of the positive plates, because conductive βPbO2 can help the construction of conductive network in the electrode. The red lead can transform into β-PbO2 very quickly during soaking because of the reaction (1) [28, 30]:

Characterization of samples

Pb3 O4 þ 2 H2 SO4 →β−PbO2 þ 2 PbSO4 þ 2 H2 O

The XRD pattern of the prepared PbSO4 is shown in Fig. 1. All diffraction peaks can be indexed to a primitive orthorhombic cell of PbSO4 (JCPDS NO. 36–1461), and the parameters are a = 6.95 Å, b = 8.47 Å, c = 5.39 Å. Consequently, phase pure PbSO4 was obtained. Figure 2 is the SEM image of as-prepared PbSO4, which shows that the PbSO4 contain irregular flakes over 5 μm wide and 1 μm thick. Unlike those in our previous report [22], we did not obtain the nanocrosses possibly due to longer reaction time or longer aging time. As we know, in the process of curing, the positive electrode material will form tribasic and tetrabasic lead sulfate [29],

Fig. 2 SEM images of the prepared PbSO4

ð1Þ

J Solid State Electrochem Table 1 The composition of the active material after formation (wt.%)

Sample

PbSO4

PbO2

A

100

B C

6.9 6.2

93.1 93.8

D E

5.8 5.2

94.2 94.8

The composition was determined according to the XRD patterns shown in Fig. 4 with the software jade

confirmed the four samples formed nanorods in different dimensions, and the length of sample C is smaller than others.

Fig. 3 XRD patterns of the five samples with different contents of red lead after curing: (A) 0 %, (B) 5 %, (C) 10 %, (D) 20 %, and (E) 30 %

To observe the morphology of the five samples after formation, the active materials were peeled from the electrode for SEM observations. Figure 5 shows that sample A, the pure lead sulfate electrode, is very different from the others; the particle surface is very smooth. The change in morphology compared with the original may be related to the grinding during the paste making. All the other four samples, B, C, D, and E, form tiny rod aggregates, but the sizes of rods are very different: sample C have the best-developed and biggest crystals among the four, while the sample D the worst and cloudy. The β-PbO2 particles present dimensions of 50 nm in diameter and 100–200 nm in length after formation in Fig. 5, which are closely interconnected and form aggregates. Figure 6 gives TEM images of the four samples with different portion of red lead after formation; we

Fig. 4 XRD patterns of the five samples with different proportion of red lead after formation: (A) 0 %, (B) 5 %, (C) 10 %, (D) 20 %, and (E) 30 %

Fig. 5 SEM images of the five samples with different proportion of red lead after formation: (A) 0 %, (B) 5 %, (C) 10 %, (D) 20 %, and (E) 30 %

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Figure 8 shows the discharge curves at a current density of 100 mA g−1 during the 25th cycle. The discharge capacity of the samples B through E are 78.2, 92.9, 88.0, and 91.5 mAh g−1, respectively. We have to count all the Pb atoms in the mixture of PbSO4 and Pb3O4, which form PbSO4 when the electrode is discharged and PbO2 when it is charged. Thus, the theoretical discharge capacity C in mAh g−1 of the mixture containing x% Pb3O4 and (1-x)% PbSO4 can be calculated from the formula (2):   3x 1−x ð2Þ þ C ¼ 53600 M Pb3 O4 M PbSO4

Fig. 6 TEM images of the four samples with different proportion of red lead after formation: (B) 5 %, (C) 10 %, (D) 20 %, and (E) 30 %

Electrochemical performance According to the XRD patterns in Fig. 4, SEM images in Fig. 5, and TEM images in Fig. 6, we know that red lead is helpful for PbSO4 formation. Figure 7 shows cyclic performance of the five samples, during which the discharge current density was 100 mA g−1. Taking the test error of experiments into account, the discharge capacity of samples C, D, and E has the very close discharge capacity, which are stablized about 90 mAh g −1 and sample B is stablized about 78 mAh g−1; they are much higher than the pure lead sulfate sample A, which is only about 10 mAh g−1 after 50 cycles. Therefore, the electrochemical performance of PbSO4 can be significantly promoted by mixing with Pb3O4.

Fig. 7 Discharge capacity of the five samples with different proportion of red lead versus cycle number

where M is the molar weight of the substance indicated in the subscript. Consequently, the theoretical capacity of the four samples containing 5, 10, 20, and 30 wt.% of Pb3O4 are 179.6, 182.5, 188.3, and 194.1 mAh g−1, respectively. If the measured discharge capacities after 25th cycle are divided by those theoretical ones, the utilization of the four samples can be calculated, which are 43.5, 50.9, 46.7, and 47.1 %, respectively. Therefore, the sample C with 10 % of red lead behaves the best, which will be chosen for further studies. The variation of the discharge capacity along with the current density is shown in Fig. 9. As expected, the discharge capacity decreases as the current density increases. It can be seen that curves of samples C and E overlap almost completely and that of sample D is a bit lower, and sample B is the worst. At the lowest current density 5 mA g−1, all the four samples discharge a similar capacity, around 130 mAh g−1, and the sample B behaves the best. All those data indicate that Pb3O4 in the initial mixture starts efficient formation of the electrode, which makes the electrode behave better, especially the performance at high current density. Voltammetry tests were conducted in the potential range from 1.8 to 0 V (vs. Hg/Hg2SO4 electrode in saturated

Fig. 8 The 25th discharge curves of potential versus capacity

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samples B and D, where the formers are bigger and uniform, the latters are much smaller and cloudy (Please refer Fig. 5). We believe it is the big and uniform PbO2 crystals that make the difference because they conduct electricity better and make the discharge easier and faster. Figure 11 shows cycle stability of the electrode made from sample C, which shows that the discharge capacity maintains at about 93 ~ 100 mAh g−1 in 150 cycles. After the fomation, the electrode is activated in 5 cycles and reaches its best performance. The rise after 50th cycle comes from an interuption of the cycling. This is marvelous if we consider about the large discharge current density, 100 mA g−1, and 100 % DOD cycle here.

Fig. 9 Discharge capacity versus the current density of the four samples. The mass ratios of red lead were (B) 5 %, (C) 10 %, (D) 20 %, and (E) 30 %

Conclusion

K2SO4) at a scan rate of 20 mV s−1 after the 50th cycle, which are presented in Fig. 10. It can be seen that the oxidation of PbSO4 to PbO2 starts at 1.10 V vs. Hg/Hg2SO4, the evolution of oxygen occurs beyond 1.33 V, and the reduction peak of PbO2 to PbSO4 emerges between 0.8 ~ 0.86 V, which is similar to those of Visscher [31]. It is interesting that adding more red lead in lead sulfate in the initial mixture increases the peak current of the reduction or oxidation. This should be related to the fact that Pb3O4 decomposes easily in H2SO4 to form electric conductive PbO2 [30], and more PbO2 can be formed if more Pb3O4 is used. Why the existance of Pb3O4 in the initial mixture of active material is so important? From the data presented above, we know that Pb3O4 has little effect in curing (Please refer to Fig. 3) and it do make great difference in the electrode composition after formation (Please refer to Table 1). Probably, the difference in morphlogy accounts for the difference in Fig. 9: the crystals of PbO2 in samples C and E develop better than

The lead sulfate was prepared by a chemical precipitation of Pb(CH3COO)2 with Na2SO4 in 30 min at 70 °C. The crystals of PbSO4 are irregular flakes dimensioned over micrometers. Five mixtures of Pb3O4 and PbSO4 have been prepared. Although Pb3O4 has little effect on the curing process, it makes the formation easier and the most lead sulfate convert to rod-like β-PbO2 crystal sized 100–200 nm in length and 50 nm in diameter. Electrochemical performances of PbSO4 have been significantly improved by Pb3O4. The discharge capacities are about 78 mAh g−1 for the one with 5 wt.% Pb3O4, 90 mAh g−1 for the ones with 10, 20, and 30 wt.% of Pb3O4. By converting the values according to the Pb atoms in the mixture, we have found that the mixture with 10 wt.% of red lead is the best for both the utilization of Pb and the economy, and during 100 % DOD cycles at 100 mA g−1, 93 % of the initial discharge capacity comes only after 150 cycles. These results show that lead sulfate can be used as positive active material if appropriate amount of red lead is used.

Fig. 10 Cyclic voltammetry curves of the five samples. The mass ratios of red lead were (B) 5 %, (C) 10 %, (D) 20 %, and (E) 30 %

Fig. 11 The cyclic performance of sample C

J Solid State Electrochem Acknowledgments The authors would like to thank the Department of Science and Technology, Jiangsu Province (BY2013073-03), Jiangsu Key Laboratory for Advanced Metallic Materials (BM2007204), Huafu Holding Group, and Analytical Test Fund of Southeast University (201226) for the financial support.

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