Phase Transformation Processes in the Active Material of Lead-acid Batteries

Abstract

The good performance of a lead-acid battery (LAB) is defined by the good practice in the production. During this entire process, PbO and other additives will be mixed at set conditions in the massing procedure. Consequently, an active material mainly composed of unreacted PbO, lead sulfate crystals, and amorphous species will be obtained. Later, the same mass will be pasted on the grids and the curing step will be performed. In this way, the previous pasted mass will be modified and a new hard porous structure will be formed in the active material. Furthermore, this structure will be bounded to the grid through a corrosion layer. Thus, the formed plate will be conducted to the following soaking and formation procedures. In these manufacturing steps, thanks to the major role of H2SO4, the active non-conductive material will be transformed into an electrically conductive element. Therefore, the prior compounds (PbO and lead sulfate crystals) will be converted to new phases: Pb or oxidized to PbO2 on the negative and positive plate, respectively. Because of the importance of the previous phase transformations, new advanced designs are focused on the internal structure of the active material to improve the LAB performance.

1. Introduction

Lead-acid batteries (LABs) are supported by a large and well-organized network of suppliers and manufacturers. Additionally, in terms of market, this type of device is recognized as the leader for automotive batteries and the second most important for industrial batteries. Nowadays, the systems known as advanced LABs are commonly used for commercial and domestic purposes. On the other hand, distinct technologies are found in LABs, such as the flooded or the valve-regulated sealed (VRLA) ones. Furthermore, two types of grating are manufactured either the flat pasted one or the tubular one. Unlike a standard battery, the negative plate of an advanced battery is modified in several ways. Thus, the plate can be composed of the active material and a supercapacitor (active carbon covering) or directly composed of a single supercapacitor. In addition to the supercapacitor, the carbon compounds are also used as negative plate additives in small contents. Nevertheless, the composition of the positive plate for either standard or advanced LABs is practically the same. Another interesting point about the LABs is their remarkable sustainability. Because 60% of world production is formed by batteries build through recycled lead [1].

The battery performance is very influenced by the several phase changes in the active material. These changes are mainly caused and controlled by the manufacturing steps. Therefore, the LAB manufacturing process is so important.

In the first production step, the PbO powder is used as a precursor compound and it is massed with other additives. As a note, the PbO specie can be found as a mixture of two polymorphic structures (α PbO and β PbO). Therefore, when the massing procedure is finished, new compounds are formed (lead sulfate crystals and amorphous species). As soon as the grids are pasted, the curing step is carried out. As a result, the hard porous structure of the mass is built. Once the plates are cured, they are formed. Nevertheless, there is a previous procedure where the cured plates are soaked in H₂SO₄ for a long period of time. Thus, the internal pores of the active material are filled with acid and several transformations into the active material are found. Finally, in the last step, the formation is developed to obtain electrically conductive plates. Hence, the negative plate is mainly composed of Pb and the positive plate of a mixture of α and β PbO₂, being the β phase more stable in acidic media.

After this entire process, the LAB can be used for a specific application. Therefore, during the battery lifetime, different reactions are performed by charge and discharge processes. The main ones are defined by the following systems: PbO₂/ PbSO₄ (positive plate) and Pb/PbSO₄ (negative plate). Nevertheless, these systems are followed by another secondary process. Some of them are the sulfation, the water consumption, and the self-discharge. Furthermore, in some cases, a battery failure can be originated from these secondary reactions [2].

Lastly, as mentioned previously, a good battery operation is influenced by the internal structure of the LAB plates. For this reason, in the last few years, different investigations have been developed to enhance the battery performance through studies focused on modifications of the active material [3, 4, 5, 6].

2. Lead oxide as the precursor compound

The active material spent to build the LAB plates is essentially composed of PbO. This lead compound is obtained by thermal oxidation of metallic lead in previous manufacturing steps. The resulting product is not entirely PbO since a portion of metallic lead cannot be oxidized. In this way, the percentage of PbO content is usually around 80–20% for free Pb. In addition, PbO is composed of two polymorphic structures in nature: the red tetragonal PbO or the α PbO and the yellow orthorhombic PbO or the β PbO. Moreover, PbO is basically made up of α PbO and a little β PbO content (around 5%). Additionally, the massing procedure is affected by the prior polymorphic compounds. Since the composition of the active material will be influenced by them. In this way, the formation of tribasic lead sulfate (3PbO·PbSO₄·H₂O/3BS) is mostly caused by the α PbO. However, the collecting of tetrabasic lead sulfate (4PbO·PbSO₄/4BS) is obtained by the β PbO. Nevertheless, the best 4BS production conditions were experimentally demonstrated as 80% α PbO + 20% β PbO [2].

Comparing the relevance of the 3BS and 4BS compounds, the 3BS one is more used in the LAB industry. Because its nature characteristics (particle size, structure, and agglomerates) are ideal for the production of both negative and positive plates. In addition, because of previous treatments, the 4BS preparation is more complicated and expensive than the 3BS one. For example, the 4BS formation is commonly produced by the 3BS curing above 80°C for 48–72 h [2].

3. Massing procedure

After the production of positive and negative active masses, the paste can be composed of unreacted PbO, lead sulfate crystals, and amorphous species, such as hydrated lead oxides, lead carbonates, and free lead. In the case of lead sulfate crystals, several species can be found: PbSO₄, monobasic lead sulfate (PbO·PbSO₄/1BS), 3BS, and 4BS [2, 7].

During the massing, the paste composition is modified by several experimental parameters, such as the pH, the temperature, and H₂SO₄/PbO ratio.

On the one hand, early in the procedure (pH from 1.0 to 8.0), the H₂SO₄ is consumed, and the formation of PbSO₄ and 1BS is noticeable Eqs. (1, 2). However, a small amount of 3BS is also formed Eq. (3). Thus, with the progress of the procedure, the pH is increased (pH > 9.5). Under these new conditions, the formation of 3BS is carried out as the main reaction, while the solubility of PbO is greatly decreased. As a consequence, the formation of 3BS crystals is only found on the surface of the PbO particles and thus, part of the PbO is isolated. Consequently, a percentage of PbO is not reacted, although the H₂ SO₄ content was stoichiometrically correct. Hence, according to their stability, distinct lead sulfate crystals are found as the predominant compound at three clear pH regions: 3BS at pH > 9.5, 1BS at 8.5 < pH < 9.5 and PbSO₄ at pH > 8.5 [2, 8].

$$\mathit{PbO}+H_2{\mathit{SO}}_4\leftrightarrow {\mathit{\text{PbSO}}}_4+H_{2}O$$E1

$$2\mathit{PbO}+H_2{\mathit{SO}}_4\leftrightarrow \mathit{PbO}·{\mathit{\text{PbSO}}}_4+H_{2}O$$E2

$$4\mathit{PbO}+H_2{\mathit{SO}}_4\leftrightarrow 3\mathit{PbO}·{\mathit{\text{PbSO}}}_4·H_{2}O$$E3

On the other hand, two different boundary conditions are established regarding the temperature. If the mass temperature is higher than 75°C, most of the mass is composed of 4BS crystals along with α and β PbO Eqs. (4, 5). Nevertheless, the composition is affected when the temperature is below 60°C since the mass is mainly composed of 3BS, α PbO, and β PbO crystals [2, 9].

$$5\mathit{PbO}+H_2{\mathit{SO}}_4\leftrightarrow 4\mathit{PbO}·{\mathit{\text{PbSO}}}_4+H_{2}O$$E4

$$2\left(3\mathit{PbO}·{\mathit{\text{PbSO}}}_4·H_{2}O\right)+\alpha \mathit{PbO}+\beta \mathit{PbO}\to 2\left(4\mathit{PbO}·{\mathit{\text{PbSO}}}_4\right)+2H_{2}O$$E5

Finally, as it was previously mentioned, the composition of the active mass is influenced by the H₂SO₄/PbO ratio. Even the size of the 3BS crystals is affected by this6factor since it is reduced due to the increment of the ratio. So at H₂SO₄ / PbO ratio < 4%, the 3BS formation is developed and partially inhibited by a parallel mechanism caused by the significant alkaline conditions. Thus, the mix of PbO species (α PbO + β PbO) is hydrated Eq. (6) and consequently, the β PbO level is increased Eq. (7). Hence, the content of the 3BS precursor (α PbO) is reduced. At a ratio of 4% < H₂SO₄ / PbO < 8%, the formation of 3BS is less inhibited, but the unreacted PbO content remains significant. Furthermore, the hydrated secondary species are affected by the higher H₂SO₄ concentration. Thus, new amorphous species are formed Eq. (8). On the other hand, when the ratio is increased (8% < H₂SO₄/ PbO ratio < 10%), the paste is mainly composed of 3BS crystals, although there are others, such as 1BS and β PbO. However, if the PbO is previously mixed with water, the presence of β PbO is blocked. Therefore, under these conditions, the formation of 1BS is achieved, and the pH influence on β PbO is confirmed. Even so, at H₂SO₄/PbO ratio of around 5%, the formation of 4BS crystals is favored, and the 3BS levels are reduced [2, 8].

$$\mathit{PbO}+H_{2}O\to \mathit{Pb}{\left(\mathit{OH}\right)}_2$$E6

$$\mathit{Pb}{\left(\mathit{OH}\right)}_2\to \beta \mathit{PbO}+H_{2}O$$E7

$$2\mathit{Pb}{\left(\mathit{OH}\right)}_2+H_2{\mathit{SO}}_4\to \mathit{Pb}{\left(\mathit{OH}\right)}_2·{\mathit{\text{PbSO}}}_4+2H_{2}O$$E8

As explained at the beginning of this section, there are other amorphous species in the paste, such as hydroxides, hydrated lead sulfates, and carbonates. The presence of these compounds (15% of the paste content) is considered important because the lead sulfate crystals can be bonded to each other through them. Hydroxides and hydrated lead sulfates are generated by the hydration of PbO and 3BS compounds. In the case of PbO, part of its content is previously generated by a portion of free Pb (around 2.5%) in the early stages of massing Eq. (9). In this way, when the temperature is increased during the procedure and 3BS formation is carried out, the hydration of PbO and newly formed 3BS crystals Eqs. (6, 10) is performed. Furthermore, if the massing time is longed and the presence of CO₂ is increased, lead hydrocarbonate compounds will be also formed Eq. (11) [2].

$$2\mathit{Pb}+O_2\to 2\mathit{PbO}$$E9

$$3\mathit{PbO}·{\mathit{\text{PbSO}}}_4·H_{2}O+2H_{2}O\to 3\mathit{Pb}{\left(\mathit{OH}\right)}_2·{\mathit{\text{PbSO}}}_4$$E10

$$10\mathit{Pb}{\left(\mathit{OH}\right)}_2+6{\mathit{CO}}_2\to \mathit{PbO}·6{\mathit{\text{PbCO}}}_3·3\mathit{Pb}{\left(\mathit{OH}\right)}_2+7H_{2}O$$E11

4. Curing plate

Once the massing of the active material is finished and the subsequent grid pasting is performed, the new plates can be cured. The curing procedure is carried out in chambers and is usually divided into two stages. In the first one, which is properly considered as the curing step, the initial moisture of the active material must be exceeded by the set relative humidity (RH). After hours of curing, the later drying step is performed. In this case, the RH must be reduced below the moisture of the active material. In addition, the temperature can be adjusted to different values during the curing process. Plates rich in 3BS crystals will be obtained if the temperature is limited to 60°C. Thus, crystals up to 2 μm in length will be obtained and they will be interconnected, building agglomerates. Even so, if the aim is focused on the formation of 4BS crystals, the temperature will be increased above 80°C. Regardless of whether the active material has been previously produced with 3BS or 4BS compounds. Therefore, crystals up to 20 μm in length and 5 μm in diameter with a good interconnection between themselves will be found [2, 10].

For this reason, according to the above explanation, the 3BS crystals of pasted plate can be modified to 4BS crystals by a curing procedure. This mechanism is firstly initiated by the reaction of the unreacted α PbO particles to the β PbO polymorphic ones Eq. (12). And meanwhile, the 3BS crystals and unreacted PbO particles are hydrated Eqs. (6, 13). Later, a new formed hydrated compound of 3BS and PbO is observed. As a result, a complex hydrated structure of 4BS crystal is generated Eq. (14). Finally, 4BS crystal is obtained by dehydration of the previous complex structure Eq. (15) [2, 11, 12].

$$\alpha \mathit{PbO}\leftrightarrow \beta \mathit{PbO}$$E12

$$3\mathit{PbO}·{\mathit{\text{PbSO}}}_4·H_{2}O+a{H}_{2}O\to 3\mathit{Pb}{\left(\mathit{OH}\right)}_2·{\mathit{\text{PbSO}}}_4·{\mathit{bH}}_{2}O$$E13

$$\begin{array}{l}\left[3\mathit{Pb}{\left(\mathit{OH}\right)}_2·{\mathit{\text{PbSO}}}_4·{\mathit{bH}}_{2}O+\mathit{Pb}{\left(\mathit{OH}\right)}_2+{\mathit{cH}}_{2}O\right]\\ \to \left[4\mathit{Pb}{\left(\mathit{OH}\right)}_2·{\mathit{\text{PbSO}}}_4·{\mathit{dH}}_{2}O+{\mathit{aH}}_{2}O\right]\end{array}$$E14

$$\left[4\mathit{Pb}{\left(\mathit{OH}\right)}_2·{\mathit{\text{PbSO}}}_4·{\mathit{dH}}_{2}O+H_{2}O\right]\to 4\mathit{PbO}·{\mathit{\text{PbSO}}}_4·{\mathit{eH}}_{2}O+{\mathit{bH}}_{2}O$$E15

On the one hand, a hard porous internal mass (or “skeleton”) is built up in the active material after the curing procedure. The mechanism is generated by the evaporation of the water portion. As a consequence, an interconnection between the different crystals and the unreacted PbO particles is generated. As it is showed in the Figure 1, the water portion indicated by the orange section is evaporated during the curing process. Thus, a hard porous mass structure is represented as a result. Furthermore, as long as the moisture of the active material is fixed at around 9%, the amorphous compounds solved and lead sulfate crystals will be recrystallized, causing a growth of their sizes. Lastly, while all these changes are carried out, the remaining free Pb content is oxidized to PbO Eq. (9). On the other hand, the performance of a positive will be influenced by the curing procedure. Since the basis of its structure is generated after this manufacturing step. Instead, the negative plate will not be affected at all because a new internal structure is generated in the following formation step.

Figure 1.

Changes caused in the plate by the curing procedure.

After hours of the first curing stage, the mass structure is mechanically weak. For this reason, the active material moisture will be reduced to 0.2% in the subsequent drying stage. In this way, part of the water content in the capillaries and between the particles is evaporated in the beginning. Consequently, internal pores are formed and the active material is shrunk. Later, while the drying step is extended, the residual water content is finally evaporated. Furthermore, the hydroxide species solved in this residual water content is precipitated and bound to the dried particles. Thus, a hard porous mass structure into the active material is achieved [2].

Apart from the prior internal mechanisms, there is another one in which the internal active material can be adhered to the grid (Figure 1). This mechanism is the formation of the corrosion layer on the grid surface. During the curing process, the layer is formed and its thickness is increased as well. As it is reflected in the Figure 1, a new “red” corrosion layer is formed between the grid and the active material. The mechanism is based on ion diffusion through oxygen vacancies. In this way, oxygen vacancies are formed with the oxidation of Pb at the interface between the grid and the active material Eq. (16). The electrons and oxygen vacancies then are moved from the formed corrosion layer to the interface between the layer and the active material. So, they are reacted by H₂O and O₂, obtaining lead hydroxides Eqs. (17–20). In addition, during the first curing stage, the decrease of water content must be compensated by a high RH value (more than 40%) for a good development in the mechanism [2].

$$\mathit{Pb}\to {\mathit{PbO}}_n^{2+}+2e^{-}$$E16

$${\mathit{PbO}}_n^{2+}+2e^{-}+H_{2}O\to \mathit{PbO}+H_2$$E17

$${\mathit{PbO}}_n^{2+}+2e^{-}+\frac{1}{2}{O}_2\to \mathit{PbO}$$E18

$$\mathit{PbO}+H_{2}O\to \mathit{Pb}{\left(\mathit{OH}\right)}_2$$E19

$${\mathit{PbO}}_n^{2+}+2e^{-}+\frac{1}{2}{O}_2+H_{2}O\to \mathit{Pb}{\left(\mathit{OH}\right)}_2$$E20

Talking more about the structure of the corrosion layer, two structural levels are found. The most internal one is basically constituted by PbO particles. These ones are previously obtained due to the oxidation of free Pb. The second level is obtained as a result of the hydration of PbO to Pb(OH)₂. Thus, the second level constituted by hydrated PbO particles can be bonded with the partially hydrated 3BS and 4BS crystals. Therefore, the connection is formed by the surfaces of the hydrated compounds. In the case of 3BS crystals, the lead sulfate compound is partially incorporated into the second level of the corrosion layer. Instead, the 4BS crystals are simply bonded to the layer [2].

5. Soaking plate

The first step of the formation is the soaking procedure. In this new stage, the cured plates are stored in H₂SO₄. In this way, the active material will be sulfated and hydrated.

The active material composition is considered as an important fact in the soaking procedure. Since the following mechanisms will be quite influenced by the content of either 3BS or 4BS crystals. For this reason, the procedure can be more accelerated with a 3BS plate. In addition, the soaking is influenced by the acid density. Therefore, at low-density values (1.06 g/mL), the structure of the positive active material (PAM) and negative active material (NAM) will be not significantly affected during the procedure. Even no distinctions between the soaking of 3BS or 4BS plates will be found. On the other hand, at higher values of sulfuric acid density (1.20 g/mL), the NAM and PAM structures will be modified during soaking. And the process on 3BS plates will be more significant [2].

Therefore, at low H₂SO₄ density values, the soaking procedure will be limited by the acid diffusion. Furthermore, the active material structure (especially the positive one) obtained after the subsequent formation will be defined by the pre-soaking procedures. Nevertheless, if the H₂SO₄ density is increased, the soaking will be limited by the active material reactivity. Additionally, the structure of the subsequently formed active material will be fixed by the soaking procedure [2, 11].

The soaking is carried out by a direct contact between the plate surface and the H₂SO₄Eqs. (3, 21, 22), provided that the active material is composed of 3BS crystals and PbO particles. Thus, a PbSO₄ layer is formed on the plate surface. Additionally, part of this layer will be resolved and then recrystallized, generating larger PbSO₄ crystals. Later, when the soaking is prolonged, the plate porous structure will be penetrated by the acid. In this way, the 3BS crystals and PbO particles located in the pores are attacked by H₂SO₄, forming water, PbSO_4, and 1BS crystals Eqs. (3, 21, 22). As a consequence, the acidic solution will be modified to neutral one inside the plate. So, as soon as the new pores zones are reached by this solution, the formation of PbO, 1BS, and 3BS hydrated is executed Eqs. (6, 23). Then, the new hydrated species are sulfated by the acid flow Eq. (24). And even part of these sulfated species can be dehydrated Eq. (25) or sulfated again Eq. (26). Therefore, some areas of the inner of the plate will be composed of crystals and amorphous compounds. Furthermore, while the active material is penetrated by the acid, the growth of the PbSO₄ layer is extended to the plate surface. As a result, the sulfation process is slowed down because the penetration of H₂SO₄ is impeded. Since the size of the newly formed PbSO₄ crystals is increased on the plate surface. Finally, after more than 4 h of soaking, most of the 3BS crystals and PbO particles will be hydrated in the deepest zone of the plate [2].

$$3\mathit{PbO}·{\mathit{\text{PbSO}}}_4·H_{2}O+H_2{\mathit{SO}}_4\leftrightarrow 2\left(\mathit{PbO}·{\mathit{\text{PbSO}}}_4\right)+2H_{2}O$$E21

$$\mathit{PbO}·{\mathit{\text{PbSO}}}_4+H_2{\mathit{SO}}_4\leftrightarrow 2{\mathit{\text{PbSO}}}_4+H_{2}O$$E22

$$3\mathit{PbO}·{\mathit{\text{PbSO}}}_4·H_{2}O+3H_{2}O\to 3\mathit{Pb}{\left(\mathit{OH}\right)}_2·{\mathit{\text{PbSO}}}_4·H_{2}O$$E23

$$3\mathit{Pb}{\left(\mathit{OH}\right)}_2·{\mathit{\text{PbSO}}}_4·H_{2}O+H_2{\mathit{SO}}_4\to 2\left(\mathit{Pb}{\left(\mathit{OH}\right)}_2·{\mathit{\text{PbSO}}}_4\right)+3H_{2}O$$E24

$$\mathit{Pb}{\left(\mathit{OH}\right)}_2·{\mathit{\text{PbSO}}}_4\to \mathit{PbO}·{\mathit{\text{PbSO}}}_4+H_{2}O$$E25

$$\mathit{Pb}{\left(\mathit{OH}\right)}_2·{\mathit{\text{PbSO}}}_4+H_2{\mathit{SO}}_4\to 2{\mathit{\text{PbSO}}}_4+2H_{2}O$$E26

The soaking process will be initiated in the same way if the plate is basically conformed by 4BS crystals. Thus, a PbSO₄ layer is formed on the plate surface due to the direct contact between the 4BS crystals and the H₂SO₄Eq. (27). Nevertheless, as the 4BS crystal size is remarkable, only its surface layer will be completely transformed into PbSO₄. In addition, the superficial PbSO₄ crystals will be resolved and later recrystallized, generating larger crystals. When the soaking is prolonged, the active area in which the plate can be penetrated by the acid is reduced. So, the acid penetration into the plate will be more and more impeded. Hence, the pH value of the acid solution located in the pores will be increased. Consequently, the formation of 1BS and 3BS crystals will be carried out. Therefore, the inside of the plate will be composed of PbSO₄ and other lead sulfate crystals [2, 11].

$$4\mathit{PbO}·{\mathit{\text{PbSO}}}_4+4H_2{\mathit{SO}}_4\leftrightarrow 5{\mathit{\text{PbSO}}}_4+4H_{2}O$$E27

During the soaking procedure, the corrosion grid layer is reached by the acid flow and consequently, PbSO₄ crystals are formed. Even so, the inner of the grid will not be affected due to the acid will be isolated by the PbSO₄ layer recently generated. Additionally, as the acid flow is diluted into the active material pores, the interface between the active material and the corrosion layer will be hydrated as well. This mechanism is similar to plates rich in 3BS or 4BS crystals.

6. Formation of the active material

In this new manufacturing step, the active non-conductive material of the plates is transformed into an electrically conductive element. In this way, the initial compounds such as PbO particles, PbSO₄ and basic lead sulfate crystals are reduced to Pb or oxidized to PbO₂ on the negative and positive plate respectively. So, electromotive forces can be generated by the LAB with formed plates mechanically connected.

6.1 Positive plate

With the first current flow, PbO₂ is formed in the grid because it is the only electronic conductor element on the plate. Then, the formation of the active material is developed through two well-defined stages. In the first one, the α PbO₂ crystal structure is mainly obtained in the inner of the plate. Because the PbO particles and the basic lead sulfate crystals are oxidized Eqs. (28–30). In the case of 3BS crystals, the process is begun with its hydration Eq. (31). Later, the oxidation state of lead is increased Eq. (32) and followed by the formation of α PbO₂Eq. (33). In a parallel, the formation of PbSO₄Eq. (34) is detected from the previous hydration Eq. (31). Thus, the internal pores of the plates are also sulfated [13].

$$\mathit{PbO}+H_{2}O\to {\mathit{PbO}}_2+2H^{+}+2e^{-}$$E28

$$\mathit{PbO}·{\mathit{\text{PbSO}}}_4+3H_{2}O\to 2{\mathit{PbO}}_2+{\mathit{SO}}_4^{2-}+6H^{+}+4e^{-}$$E29

$$3\mathit{PbO}·{\mathit{\text{PbSO}}}_4·H_{2}O+4H_{2}O\to 4{\mathit{PbO}}_2+{\mathit{SO}}_4^{2-}+10H^{+}+8e^{-}$$E30

$$3\mathit{PbO}·{\mathit{\text{PbSO}}}_4·H_{2}O+2H_{2}O\to 4{\mathit{Pb}}^{2+}+{\mathit{SO}}_4^{2-}+6{\mathit{OH}}^{-}$$E31

$${\mathit{Pb}}^{2+}\to {\mathit{Pb}}^{4+}+2e^{-}$$E32

$${\mathit{Pb}}^{4+}+2H_{2}O\to {\mathit{PbO}}_2+4H^{+}$$E33

$${\mathit{Pb}}^{2+}+{\mathit{SO}}_4^{2-}\to {\mathit{\text{PbSO}}}_4$$E34

Even so, this first stage is not only located in the core of the plate because other processes are found near the plate surface. In these reactions, the PbO and 3BS compounds are sulfated Eqs. (21, 22), obtaining1BS and PbSO₄ crystals as products. Because of these separate mechanisms, different structures are found in the formed PAM.

The second stage is achieved when the potential is increased to about 200 mV and the formation is carried out for at least 6 h. Then, the required oxidation potential of PbSO₄ is reached, and therefore, the formation of the other PbO₂ crystal structure (β PbO₂) is developed Eq. (35) [13].

$${\mathit{\text{PbSO}}}_4+2H_{2}O\to {\mathit{PbO}}_2+{\mathit{SO}}_4^{2-}+4H^{+}+2e^{-}$$E35

Normally, the distribution of the two PbO₂ phases is well established. On the one hand, the α phase is located inside the plate, and the β phase is usually found in outer areas. Furthermore, as it was demonstrated in the prior mechanisms, the flow of H₂O, H⁺ and ${\mathrm{SO}}_4^{2-}$ ions are crucial for the positive plate formation. Since the pH of the active material is compensated by the crossover of ions and water molecules between the active material and the electrolyte [13].

In contrast, the formation mechanisms are altered if the positive plate is rich in 4BS crystals. When the period of formation is longer than 15 h, a higher percentage to 70% is not reached in the formed plate. This low formation rate is caused by the complexity of the mechanism. Firstly, the external surface of PbSO₄ crystals is oxidized to PbO₂Eq. (35), generating H₂SO₄ as a product. Thus, the internal 4BS crystals are partially reacted by the newly formed acid Eq. (27). So, an innermost surface of 4BS crystal will be composed of a 4BS + PbSO₄ layer. Thus, this new layer will be coated by PbO₂. Lastly, the inner of the 4BS crystal will be oxidized to PbO₂ provided that the active material is penetrated by H₂O. And the H⁺ and${\mathit{SO}}_4^{2-}$ ions are driven out to the electrolyte. Nevertheless, the mechanism can be impeded due to the low ion transport rate. Therefore, the formation process is further impeded when the plate is composed mainly of 4BS crystals [13].

Lastly, the corrosion layer formed on the grid surface is oxidized during the forming process. There are two oxidation ways according to the dissolution of the external PbSO₄ layer. The mechanism without dissolution is started by the partial oxidation of PbSO₄ crystals Eq. (35). Thus, the surface of the corrosion layer is composed of PbSO₄ + PbO₂. Nevertheless, this layer will be completely oxidized during the battery cycling life. In the case of a dissolution mechanism, the corrosion layer is first oxidized to PbO₂Eq. (35). Later, the PbSO₄ crystals are solved and the Pb²⁺ ions are oxidized near the new PbO₂ layer, forming a thicker layer. Lastly, the solved PbSO₄ crystals will be involved by a PbO₂ layer [2].

6.2 Negative plate

The formation of the negative plate starts on the grid. Afterward, the process is conducted on the plate surface. Firstly, the entire surface is reacted and then the process is aimed at the inner of the active material.

According to previous studies, the formation can be divided into two stages. In the first one, PbO particles and lead sulfate crystals are reduced to Pb Eqs. (36–38). But PbO and 3BS compounds are partially reduced because the rest is attacked by the H₂SO₄Eqs. (21, 22). Therefore, in a plate constituted by 3BS crystals, the mechanism is carried out by the reaction between lead sulfate crystals and H₂O Eq. (39). Then, the oxidation state of the obtained Pb²⁺ ions are transformed and reduced to Pb Eq. (40). Additionally, as parallel reactions, other products of 3BS hydration are generated (H₂O Eq. (41) and PbSO₄ crystals Eq. (34). In this way, the pH of the system is changed to near neutral [13].

$$\mathit{PbO}+2e^{-}+2H^{+}\to \mathit{Pb}+H_{2}O$$E36

$$3\mathit{PbO}·{\mathit{\text{PbSO}}}_4·H_{2}O+8H^{+}+8e^{-}\to 4\mathit{Pb}+H_2{\mathit{SO}}_4+4H_{2}O$$E37

$$\mathit{PbO}·{\mathit{\text{PbSO}}}_4+4H^{+}+4e^{-}\to 2\mathit{Pb}+H_2{\mathit{SO}}_4+H_{2}O$$E38

$$3\mathit{PbO}·{\mathit{\text{PbSO}}}_4·H_{2}O+2H_{2}O\to 4{\mathit{Pb}}^{2+}+{\mathit{SO}}_4^{2-}+6{\mathit{OH}}^{-}$$E39

$${\mathit{Pb}}^{2+}+2e^{-}\to \mathit{Pb}$$E40

$${\mathit{OH}}^{-}+H^{+}\to H_{2}O$$E41

After more than 6 h of formation process, when the inner of the active material is not completely reduced to Pb, the second stage is performed. In this new stage, the reduction of PbSO₄ crystals to Pb Eq. (42) is located in the outer area of the plate. Thus, when the first stage is ended, the PbSO₄ crystal reduction is aimed at the core of the plate. Thus, as soon as the second stage is finished, the pH value decreased because H₂SO₄ is obtained as a product [13].

$${\mathit{\text{PbSO}}}_4+2H^{+}+2e^{-}\to \mathit{Pb}+H_2{\mathit{SO}}_4$$E42

Finally, the rate of the negative plate formation is controlled by the solved ion diffusion. Because during the first stage, the reaction layer is overloaded by negative solved ions (OH$-$ and ${\mathit{SO}}_4^{2-}$). In this way, as the electronic neutrality must be achieved, an ionic crossing is induced between the active material and the electrolyte. So, the plate is penetrated by H⁺ ions from the electrolyte and the newly formed ${\mathit{SO}}_4^{2-}$ ions are expelled from the same plate [13].

7. Battery performance

After the formation process, the battery can be considered as an operative energy storage system. In state of charge, the PAM is composed of two different PbO₂ crystalline structures: the orthorhombic PbO₂ (α PbO₂) and the tetragonal PbO₂ (β PbO₂). And the NAM is composed of Pb. In addition, the role of the electrolyte as reactant is observed as well (Figure 2). Therefore, in the discharge process (see Figure 2), PAM will be reduced to PbSO₄ and water will be formed as a product Eq. (43). And NAM will be oxidized to PbSO_4, and proton ions will be solved into the electrolyte Eq. (44). Thus, a voltage of 2.0 V is provided by both plates Eq. (45) [7].

Figure 2.

A scheme of a LAB discharge process.

$${\mathit{PbO}}_2+H_2{\mathit{SO}}_4+2H^{+}+2e^{-}\leftrightarrow {\mathit{\text{PbSO}}}_4+2H_{2}O$$E43

$$\mathit{Pb}+H_2{\mathit{SO}}_4\leftrightarrow {\mathit{\text{PbSO}}}_4+2H^{+}+2e^{-}$$E44

$${\mathit{PbO}}_2+\mathit{Pb}+2H_2{\mathit{SO}}_4\leftrightarrow 2{\mathit{\text{PbSO}}}_4+2H_{2}O$$E45

During the charging process, PAM will be oxidized to PbO₂Eq. (43) and the NAM will be reduced to Pb Eq. (44). However, the charge current is consumed by other secondary reactions due to their enhanced kinetics. These reactions are located on the surface of the positive and negative plates. Therefore, on the PAM`s surface, the oxygen evolution reaction (OER) is observed Eq. (46), and on the NAM`s surface, the hydrogen evolution reaction (HER) is found Eq. (47). So, the battery will be negatively affected by a high water consumption and an increase of its internal resistance if the HER and OER are not controlled [14]. Besides, part of the O₂ generated on PAM can be driven to the NAM surface and therefore reduced to H₂O Eq. (48), this mechanism is known as “recombination process.”

$$H_{2}O\to \frac{1}{2}{O}_2+2H^{+}+2e^{-}$$E46

$$2H^{+}+2e^{-}\to H_2$$E47

$$\frac{1}{2}{O}_2+2H^{+}+2e^{-}\to H_{2}O$$E48

One of the most common issues during the LAB operation is the plate sulfation. This issue is caused by the irreversibility of the PbSO₄ crystals formed after the discharge process. Focusing on the negative plate, after numerous charge and discharge cycles, some of the PbSO₄ crystals cannot be reduced to Pb. Additionally, the crystal size is increased in each cycle. In this way, a battery failure can be caused due to this mechanism. Therefore, the harmful effects of the sulfation can be reduced by the use of additives. For example, the incorporation of expander additives is experimentally confirmed as a useful strategy. Furthermore, other materials, such as organic expanders, the BaSO_4, and the carbon compounds are also used to prevent sulfation. In the case of carbon additives, different options can be used: the carbon black, the graphite, the graphene, and the activated carbons. Therefore, longer battery cycle life can be achieved through an appropriate content of the above additives into the NAM [15].

In the Figure 3, cured plates of PAM (A) and NAM (B) are showed. In this case, the plates were assembled in a flooded battery for the transportation market. Besides, a commercial battery (C) and its system (D) are showed as example.

Figure 3.

A) Example of a cured PAM. B) Example of a cured NAM. C) Example of a commercial automotive LAB. D) Scheme of the LAB assembly.

8. Premature capacity loss: Corrosion layer growth in the positive plate

When a LAB is overcharged, the main reactions on the plates are moved to a secondary role because most of the PbSO₄ crystals are reacted to Pb (negative plate) or PbO₂ (positive plate). In this way, as it was previously explained in the section 7, the charge current is consumed by other secondary reactions, on the negative plate: the HER and ORR and on the positive plate: the OER. Nevertheless, the grid corrosion of the positive plate is found as another important secondary reaction [16].

Deeping into this process, this mechanism originated when the LAB is charged or overcharged. As it is represented in the Figure 4, the process is started when the PAM and the corrosion layer are penetrated by the oxygen generated in the OER. The sense of the penetration is indicated by the arrows in the Figure 4. Thus, the surface of the grid is attacked by oxygen and oxidized to PbO. Then, the formed PbO may then undergo further oxidation, generating PbO_x species. This new layer is represented by the pink color in the Figure 4, and it is located between the active material and the corrosion layer interface. In addition, the process is favored by two facts: the high anodic potential values reached on the positive plate and its thermodynamic nature. Since the process is spontaneous with the contact between the PAM and the grid. Even so, the kinetic of the process may be reduced by the passive layer formed between the active material and the grid [17].

Figure 4.

Real example of a corrosion layer growth and a representation of its mechanism.

Lastly, the battery performance can be adversely affected by the corrosion process if the PbO reaction rate is more favored than the PbO_x one Eq. (49). Consequently, an ohmic resistance layer would be formed causing the polarization of the positive plate, the gradual loss of capacity and the formation of pores and cracks in the grid. This issue was avoided by different studies focused on the alloy composition of the positive grid [2, 13, 16, 17, 18, 19].

$$\mathit{Pb}+\frac{x}{2}{O}_2\to {\mathit{PbO}}_x$$E49

9. Battery improvements: Studies focused on the active material as a new research approach

According to the important role of the active material, different research projects were carried out to improve the LAB performance through enhancements of the internal structure. One research strategy was focused on the synthesis of a more porous and spongier nanostructured PbO. For this purpose, Karami et al. applied ultrasonic waves with fixed frequencies as an experimental synthesis tool [3]. Thus, they obtained PbO after using Pb(NO₃)₂ and NaOH as initial reagents and applying different sonication, dehydration, drying, and filtration procedures. Then, they used the new nanostructured PbO to build negative and positive plates. As a result, better capacity values and higher number of life cycles were showed by the batteries fabricated with the new PbO [3].

The use of a LAB at high-rate partial state of charge (HRPSoC) is increasing every year. However, the performance of the plates at these operation modes can be improved. Since the sulfation process is very aggressive during its performance. Because of this issue, improvements to the internal porous structure of the active material or the use of common battery additives with new structural or chemical modifications have been studied.

On the one hand, Jin et al. studied how the control of the PbO₂ particle size of the positive plate can extend the battery life since the reversibility of PbSO₄ crystals could be improved [4]. In this way, they compared two formed PAMs whose PbO₂ particle sizes were 10 and 100 nm in diameter. After 75 days of discharge, the batteries were recharged for 24 h. During these periods, the phase transformations of the PAM were studied through multiple techniques: scanning electron microscope (SEM), focused ion beam (FIB) combined with transmission electron microscopy (TEM) cross-section, and a thermal test. After the research, a longer lifetime was showed by the 100 nm PAM due to the shedding of its active material was reduced [4]. On the other hand, Yang et al. studied the use of PbO/graphene composites as an additive in NAM. In this way, the aggregation of irreversible PbSO₄ crystals could be reduced and the reversibility of the Pb/PbSO₄ system could be improved. Thus, they synthesized the compound using graphene oxide and Pb(NO₃)₂ as reagents. Lastly, they obtained the final product after sonication and pyrolysis procedures. The results showed that part of the PbO was absorbed by the graphene oxide layers causing improvements in the battery cycle life and a reduction in the HER rate [5].

One of the newest concepts in LAB technology is the ultra battery system. In this type of battery, the negative plate is covered by a carbon layer or the plate itself is compressed to a capacitor electrode. Thus, a higher charge acceptance and longer cycle life will be achieved by this new technology. In keeping with the above information, Thangarasu et al. measured the effect of a Pb nanoparticle impregnated negative carbon plate. Finally, better results were found in the cell capacity, the HER rate and the cycle life through the use of this negative plate covered by a modified carbon layer [6].

10. Conclusions

As demonstrated, the LAB performance is highly dependent on the manufacturing process. Since the structure and composition of the active material are modified during the distinct production steps. These changes can be summarized by two important ones: the formation of the hard porous mass and the corrosion layer in the curing process. And the oxidation and reduction of the active material during the formation procedure. Furthermore, the previous phase transformations are highly influenced by the manufacturing conditions and some critical species. Because the battery could not be lead an electromotive force system without their important role. In the case of the species, the non-reacted PbO particles, the lead sulfate crystals, and the solved ions stand out from the rest. Moreover, these phase changes are not limited to the production process, as many others occur during the battery lifetime. For example, the sulfation of the plates during the discharge process or the secondary reactions occurring in the charging process, such as the water consumption, recombination, and corrosion layer growth. For this reason, recent researches have been aimed to improve the performance of the plates by structural modifications of their active material. In this way, the negative effects caused by sulfation or water consumption can be reduced, and therefore, the battery performance can be improved.

Nomenclature

LAB

Lead-acid battery

VRLA

Valve-regulated lead acid

α PbO

Polymorphic structures (tetragonal)

β PbO

Polymorphic structures (orthorhombic)

α PbO₂

Polymorphic structures (orthorhombic)

β PbO₂

Polymorphic structures (tetragonal)

1BS

Monobasic lead sulfate PbO·PbSO₄

3BS

Tribasic lead sulfate 3PbO·PbSO₄·H₂O

4BS

Tetrabasic lead sulfate 4PbO·PbSO₄

RH

Relative humidity

PAM

Positive active material

NAM

Negative active material

OER

Oxygen evolution reaction

HER

Hydrogen evolution reaction

HRPSoC

High-rate partial state of charge

SEM

Scanning electron microscope

TEM

Transmission electron microscopy

FIB

Focused ion beam