In this article, Dr Mike McDonagh explains the chemical and physical processes involved in forming lead dioxide and pure lead in lead-acid battery plates, detailing how initial compounds are electrochemically converted into active materials.
The lead acid battery, and indeed, all rechargeable conversion electrochemical batteries, operate on the same principle. By design, they have two electrodes in chemical states that have different energy levels and an electrolyte to enable the reactions required to release or absorb electrons to generate power.
We have to create an energy imbalance between the two electrodes by forming two different chemical species for the positive and the negative plate, i.e. lead dioxide, PbO2 , for the positive and pure lead, Pb, for the negative. The formation process is so called because it is the first time the material has been converted into electrodes.
The chemical forms should also allow free movement of electrons, when connected across a resistor, to enable the reactions that will balance out the energy differences. So far, in previous articles we have discussed the processing conditions which will create an active mass which will maximise the lead acid battery’s performance. It is the formation process which will now turn these complex lead compounds into electrochemical machines, capable of storing and providing electrical energy.
Before formation, both plates have similar chemistry, mainly lead oxides and sulphates, so when immersed in dilute sulphuric acid, there is almost no voltage and no energy difference to create an electron flow when the plates are connected. Once a current is applied, the polarity is chosen to ensure that the electron flow is in the right direction to pump electrons from the positive into the negative plate.
Since most of the lead compounds in the active mass are the divalent form, the removal of two additional electrons by application of an electric current results in formation of tetravalent lead. Two oxygen molecules provide the required number of electrons, whilst the two electrons transferred to the negative plate fill the electron vacancies in the divalent lead ions of the active mass, i.e. PbO2 is formed in the positive and Pb at the negative. Eq 1.
The general overall reaction can be simplified to the classic charge-discharge reaction of the double sulphate theory: Eq 2.
Sulphuric acid is a by-product of the formation reaction. The concentration of the dilute acid therefore increases as the reaction proceeds. This is important as the kinetics and efficiency of the reaction are reduced by increasing concentration. Fig 1 shows a concentration gradient that changes with time during the formation process. The less steep the gradient, the higher the concentration of SO4– ions in the electrolyte. As the curve becomes less steep, the ion diffusion from the plate into the bulk solution slows down as the energy needed to eject the ions from the AM particles’ surfaces increases. This higher energy requirement raises the electrical resistance and therefore the cell voltage as the reactions progresses.
The theoretical quantity of electrical energy required for formation can be simply calculated by using the Faraday (96,500 coulombs) which will liberate one gram equivalent. One coulomb is one amp for one second. To obtain this in ampere hours, divide by 3,600 which is 26.8 ampere hours. The gram equivalent also takes into account the valency of the material. Whilst lead is multi valent, most of the lead acid battery reactions are divalent. Because the coulombic value is based on a single electron, the two-electron conversion of lead means that the equivalent weight is halved. I.e. 103.5g of lead is liberated by 96,500 coulombs. We can write the electrochemical equivalent of lead (g/Ah) as: Eq 3.
The dry cured active material is not a single chemical. Whilst there is a plate soaking phase in sulphuric acid before the electrical charging process, it is unlikely that the cured AM will be fully converted to lead sulphate. To calculate the theoretical energy of conversion to lead dioxide and pure lead, it is possible to either calculate the lead content of the chemical species or use the molecular weights of the plate chemicals to calculate the theoretical Ah/Kg needed for their conversion.
Table 1 gives these values for commonly found lead compounds in most lead acid battery dried and cured plates. However, it has to be stressed that these are theoretical values. There is a plethora of factors which ensures that the theoretical value lies very far from the practical one. These include:
- Parasitic reactions:
These are triggered by the rise in voltage as the conversion process advances.
- Increase in resistance due to a rise in the electrolyte sulphate content from the reaction:
This is especially true of container formation where there is limited cell volume.
- Original chemistry of the plates’ active material. The use of additives and different lead oxides such as Pb3O4 can greatly influence the energy needed to create PbO2.
- Heat generation due to I2R effect as resistance increases.
- Increasing reaction resistance due to the longer diffusion paths of the reacting chemical species as conversion of the particles in the AM progresses.
- Temperature of the electrodes and electrolyte. Higher temperatures increase the reaction rate but decrease the overvoltage temperatures for oxygen and hydrogen evolution.
The positive and negative plates have different conversion efficiencies with the negative energy requirement lying closer to the theoretical value than that of the positive. In general, however, the process of forming the positive and negative electrodes can be regarded as having common, distinct stages. These stages are defined by the chemical, physical and electrical changes which occur within the active masses during the formation period. These stages can be identified as follows:
- Formation of a bonded interface between the grid and active material. For the positive this will be a corrosion layer and for the negative it will be a growth of metallic lead from the grid into the interconnected active mass.
- In the positive, another layer forms on the corrosion layer. This is what Pavlov calls the Active Mass Collecting Layer (AMCL). For the negative plate, this stage is characterised by a rapidly growing zone of lead sulphate and pure lead which extends initially from the grid to the plate surface then grows inwards to the rest of the active mass. This forms the interconnecting lead structure which provides the current path for active material conversion.
- Conversion of the cured paste material into the positive and negative active masses. This is the part of the process in which the capacity of the battery is determined. It is also the period in which the voltage of the battery begins to rise more rapidly.
- Electrochemical decomposition of water. Once the battery voltage has risen above the threshold for evolution of oxygen and hydrogen, water is electrolysed and is lost as gas bubbles nucleating on the electrodes.
The conversion of the cured paste material into the negative active masses as a function of percentage completion, is represented by Fig 2.
Likewise, the changes in the positive AM as a function of percentage formation are given in Fig 3.
Formation of positive plates
There are three basic stages to the positive plate formation process.
- Formation of a conduction layer at the grid/active material interface.
- Growth of a skeletal áPbO2 conducting layer throughout the active mass containing macropores – (Fig 4).
- Formation of the βPbO2 agglomerate structure, providing the bulk of the plate capacity.
According to Pavlov: “The matrix of 4BS crystals is transformed into aggregates of PbO2 particles, which preserve the shape of the initial substance. PbO2 aggregates interconnect to form a skeleton with a structure similar to that of the initial cured paste. The strength of this skeleton depends on the length and thickness of its branches as well as on the strength of their connections. In the case of PAM formed from 4BS pastes, the size of the skeleton branches is predetermined by the size of the 4BS crystals of the paste and by the connections between them formed during the process of curing. Hence, the PAM skeleton formed is stable and batteries with 4BS positive plates have long cycle life.” See Fig 5.
Most likely, the rate-limiting steps of the positive plate formation process are the diffusion of ions into and out of a solid AM particle during conversion. The build-up of acid concentration around the particle as SO4– ions are created will also have a significant impact on the diffusion rate within the electrolyte. From this, it is evident that a high concentration of sulphate ions and sulphuric acid molecules surrounding a partially formed AM particle would inhibit the reactions forming lead dioxide. During the first formation stage, the direction of advance of (PbO2 + PbSO4) zones into the paste particles and the bulk PAM of the plate is determined by the degree of paste sulphation, formation current, temperature and phase composition of cured paste.
The formation of the two allotropic forms of lead dioxide is an important factor in determining the properties of the finished plate. The α and β forms have been shown to have different contributions to the performance of the positive plate. The β modification, formed in the last stage of formation, appears to provide the bulk of the electricity during discharge, whilst the α form seems to provide a rigid interconnecting network of electron conducting material to the grid/AM interface Fig 4. The effect of the α/β ratio on the battery performance will be discussed in a later article which will be describing the optimisation of the processes.
The structure of the forming active mass is influenced by the original structure. In 4BS pastes, there are long needle-like structures which are preserved during formation and which become the α PbO2 conducting network with the PAM structure. Fig 5 is an SEM picture taken from work done by Pavlov in 2001 and is a striking verification of this mechanism.
Formation of negative plates
Again, similar to the positive plate, there are stages during the formation process. In the first stage, after acid soaking, there is an initial high resistance interface between the cured paste mass and the lead alloy conductor. This is mostly due to Pb(OH) and PbO2 crystals creating a high resistance layer between the AM and the grid. As a result, there is a high initial voltage as soon as the current is switched on. This rapidly reduces as the interface is transformed. Once the resistance drops and the interface materials have bonded and partially transformed to Pb, the conversion of dry cured paste to negative active material (NAM) commences. This ensuing process has two distinct stages:
- The first stage is the electrochemical reduction of PbO and basic lead sulphates to lead and sulphuric acid. During this time, a lead skeletal framework is formed. This consists of a network of bonded crystals of irregular shape with PbSO4.
- The SO42- ions, produced from the reduction of the lead sulphate, help to produce the acidic conditions necessary for the deposition of lead rather than lead hydroxide into the structure. In this way, zones are formed on both plate surfaces before spreading into the interior of the plate.
The probable generalised chemical reactions are given below: Eq4.
From this it can be seen that lead, lead sulphate and water are produced, the H+ ions that create the water are provided from the bulk electrolyte.
In the second stage, reduction of PbSO4 to Pb occurs and lead crystals are deposited on the lead skeletal surface in a strongly acidic solution produced from the generation of sulphate ions. It is during this phase that sulphuric acid is produced, the lead sulphate crystals on the surface of the Pb crystal network are reduced to Pb and water electrolysis creates hydrogen gas. From this point in the process, the negative plate contributes to the rise in electrolyte density and the gassing associated with the latter part of the formation cycle.
The lead sulphate produced by the first stage reactions will be reduced to lead which will provide sulphate ions to form sulphuric acid in the bulk electrolyte. Eq5.
The provision of electrons to the negative plate from the formation current reduces the lead ions liberated from the lead sulphate, to form metallic lead. The electrolysis process breaks down water to create hydrogen, which is evolved as a gas on the negative plate. Efficiency and process variables There is a significantly lower conversion efficiency for the positive plate compared with the negative plate.
This is partly due to an increase in voltage which promotes oxygen evolution in the early stages of formation, and partly due to the difference in conductivity of the two active masses.
Lead dioxide in the positive is a semiconductor which will not be as effective as the pure lead matrix of the negative active mass. These factors increase the charge factor required to complete the formation process. A factor is used to multiply the theoretical energy requirement for formation to ensure that maximum conversion in a reasonable time with reasonable energy usage is achieved. This charge factor is also dependent upon process conditions, initial material composition and plate design.
There are several influences on the efficiency of the process and also on the resulting structure and properties of the active masses. These influences affect the performance characteristics of the battery, affecting cycle life as well as capacity and high-rate discharge capabilities. The process variables considered are:
- Formation current density
- Temperature
- Electrolyte concentration
- Total coulombic input
The next article will concentrate on the different formation methods commonly in use for flooded lead acid batteries, notably-single shot container, two shot container, recirculating acid and open tank formation.
The different battery constructions of tubular plate, flat plate, AGM, and GEL will be described in subsequent articles. The technologies of open tank plate, water cooled container and recirculating acid methods will also be described and evaluated.
By using knowledge of the mechanisms described for the production of PAM and NAM in lead acid batteries, it will be shown that the formation process can be modified to control and enhance the battery performance in the chosen market sectors.
