In the fifth of Michael McDonagh’s series we learn how creating the active material can lead to some of the very best and very worst battery performances.
This article critically examines the materials and methods used to make the active material which provide the electron reservoir required for the battery function. It is this stage in the production process where the element of individualism becomes more pronounced. It is where companies with their own unique experience and personal histories, seek to avert previous disasters or to differentiate themselves from the competition with better performance, lower cost or unique advantages. Similar ingredients, similar equipment, but different chefs, all companies will attempt to convince the purchaser that they have the best recipes to fulfill their requirements.
It is this stage in the production process which either ensures the integrity and performance of the product or results in a physical and chemical structure which creates a plethora of irreversible problems. The problems which can occur are low capacity, low cold crank performance, high internal resistance, low end of charge voltages, low charge acceptance and the much discussed premature capacity loss (antimony free effect) The margin for error which distinguishes between a well performing saleable battery and a lump of scrap lead and plastic is surprisingly small. In this article, the processes of paste mixing, belt pasting and curing will be examined.
The processes examined are: paste mixing hopper trowel roller pasting and flash drying. Curing and formation processes, which have a direct dependency on these processes, and themselves have the potential to create disastrous results if not correctly executed, will be dutifully examined in subsequent articles.
The effect of variables in materials, processes and equipment functions are analysed with a view to their effect on subsequent processes and the final product. Current market conditions are placing increasing pressure on prices of batteries from manufacturers. This has resulted in manufacturers reducing the lead materials in their batteries to absolute minimum quantities with tight controls on production tolerances. If the process parameters are not well controlled during these pasting processes, the variation in performances this can produce may mean the difference between a product fit for purpose and a very expensive string of warranty claims.
The control of these processes, the consequences of variations and the streamlining of operating parameters and ingredients to particular applications are discussed below.
Figure 1 shows the processes schematically from mixing the paste ingredients to the finished battery plate prior to the curing process. There are critical aspects of the process to control in order to ensure that each of the stages of each sub process provides a sub product of the correct chemical and physical structure, suitable for the next stage of the pasting process. The mixing stage requires precise control of weights of ingredients to ensure that the correct density of paste and balance of expanders is achieved for the battery application. The rate of acid addition and control of temperature in the paste mixer will affect the structure of the crystal morphology, which will affect the mechanical properties of the paste for the grid pasting operation. It is also a critical factor in the determination of the cycle life and high rate discharge capability of the battery. In fact a difference of 10ºC as a maximum temperature during the mixing process can mean the difference between a reliable battery with long life and an early warranty failure.
The paste application using a trowel roller in a hopper requires several conditions, the correct moisture content, correct temperature and correct physical structure to provide the mechanical strength for the trowel rolling process. The moisture content must be sufficient to enable material flow and to retain high enough water content at the end of flash drying to ensure that the curing process is not inhibited. The temperature at which the paste is dropped from the mixing vessel into the pasting hopper is a critical factor in determining this.
The next process of flash drying has the primary function of drying the surface of the plates to prevent their sticking when stacked in groups. However, it also plays a critical part in the determination of the moisture content of the plates and the structure of the active material. It can also cause shrinkage of the active material away from the grid causing lack of contact with high internal resistance. The temperature and residence time in the dryer will affect the plate moisture content, this must be controlled between limits to ensure the curing process is successful. Likewise, plates which are placed on racks with spaces must not be left out of the curing chambers for long periods to prevent their drying out.
This process brings together the materials which form the basis of the active mass. The purpose is to create a paste with the correct mechanical, as well as chemical, properties to enable its application to the conductive part of the electrode, i.e. the grid. In order to do this there is a paste mixer to mix the ingredients, weigh batch hopper for the oxide, and measuring containers for the acid and water additions. This process is usually automated with manual quality checks at the end of the process. Considerable heat is generated during the mixing process which necessitates heat removal by air convection and/ or water cooling of the reaction vessel. Usually mixes are in one tonne batches including water, acid and additives. The order of additions is as follows: the dry lead oxide is weighed and then dispensed into the paste mixer, the additives are blended into the dry mix either individually or as a pre weighed package, this is followed by the metered water addition and allowed to mix for several minutes to ensure uniform dispersion. Finally the acid is added slowly to ensure localised action is avoided and the temperature rise from the heat of the reaction is not excessive. Figure 2 shows a typical control panel with automated control systems for the paste mixing process.
The most common method of mixing paste utilises weigh batch hoppers for the oxide and acid additions, manual weigh scales or pre mixed packages for the additives, with flow meters to measure the water additions. The oxide, water and acid mixture provides the right consistency and chemistry for the pasting and curing process which provides the basis for the electrochemical function of the battery. The additives are a blend of carbon, barium sulfate and organic wood derivatives, which in combination, improve the life and performance of the battery. Different applications require different blends of additives. This depends upon whether cycle life, power or energy density, have priority. The effect of these additives and the reasons for their choice are discussed separately.
There have been many studies which have examined the processes used and the products resulting from the mixing of acid, oxide and water, into a lead‑acid battery plate paste. There are differences between the positive and negative plate mix but the fundamental chemistry is the same.
When the acid first contacts the wet PbO, the most likely reaction is to form normal lead sulfate (PbSO4). This then reacts with PbO to form basic lead sulfates.
At temperatures below about 60ºC, tribasic lead sulfate (TRB) is stable. At higher temperatures, (above 70ºC), tetrabasic lead sulfate has the higher stability.
4PbO + H2SO4 = PbSO43PbOH2O (Tri basic lead sulfate or 3BS )
PbSO43PbOH2O + PbO = PbSO44PbO + H2O
(Tetra basic lead sulfate or 4BS )
Other factors which influence the phase composition are the ratios of acid to lead oxide and the process times.
According to LABD:
Below 60ºC and with an H2SO4/lead oxide ratio up to 12%.
The paste contains 3PbO.PbSO4.H2O (3BS) + tetragonal PbO + orthorhombic PbO + Pb.
The Maximum content of 3BS is obtained at 10% H2SO4/LO.
Above 70ºC , at H2SO4/LO ratio up to 7%.
This will produce a paste which contains 4PbO.PbSO4 (4BS) + tetragonal PbO + orthorhombic PbO + Pb. Maximum content of 4BS is obtained at 6.5% H2SO4/LO.
At the beginning of the mixing process, 3BS and orthorhombic PbO are formed first.
Then 3BS + tetragonal PbO + orthorhombic PbO react to form 4BS.
H2SO4/LO ratio between 7% and 12%.
The paste contains 3BS + 1BS + tetragonal PbO + orthorhombic PbO.
4BS nucleation is the slower process which starts at the higher temperature and can be inhibited by the presence of surface active additives such as expanders.
4BS and orthorhombic PbO are not formed at all.
With time of stirring 3BS crystals grow up to 2-4 µm in size and 4BS ones reach sizes of up to 20-50 µm.
These consequences are important in determining not only the performance characteristics of the battery but also the integrity of the subsequent processes.
Effect of process parameters on battery performance and characteristics
As discussed in article 5 the proportions of 3BS and 4BS crystals in the paste mix will influence the properties of the battery. 3BS tends to produce a finer structure with higher cold crank performance but structurally less robust than a larger grained 4 BS structure. The curing and formation process also affect the proportions which are formed, however, if the pasting process variables and material ratios are well controlled, the subsequent paste formed will enable the desired ratios to be more easily achieved.
Generally, cyclic applications require a robust structure capable of repeated deep discharges as in traction, semi traction and many valve regulated or standby power applications. For this a higher proportion of 4BS in the structure is desirable, however, this is at the expense of initial capacity. For this reason, red lead is sometimes incorporated into the positive plate in order to reduce the formation energy and improve initial capacities. In addition, the positive paste needs to perform a different role to the negative inasmuch as it has to resist the paste shedding due to the volume changes associated with cyclic applications.
In contrast automotive batteries have a shallow cycle requirement but with the ability to provide high rate discharges (cold cranking). In this, a more porous structure with higher surface area is required. The production of 3 BS with a smaller crystal size is to be promoted, this is encouraged by lower mixing temperatures.
These different requirements help to select the paste densities, additives and process variables which should be employed to produce the optimum performance for each battery design and market sector.
For industrial battery pastes a temperature limit of 60ºC is not critical. It is in fact desirable for industrial-battery positive plates to contain tetrabasic lead sulfate (4BS), which begins to form at temperatures above 70ºC. The presence of TTB crystals provides sites for the further growth of TTB during the curing process. TTB is not formed in the negative-paste mix due to the inhibiting effect of the expander.
The chemical nature of the lead sulfate in the paste is important since it affects the facility with which the plate can be formed, its electrochemical performance and its durability under discharge/charge cycling. TRB is the preferred compound for engine-starting battery plates. It is readily converted to PbO2 during formation and confers high initial performance.
TTB is considerably more difficult to form, particularly in sulfuric acid solutions of concentrations greater than 15%, but it contributes to the formation of a strong and stable morphology in the crystal structure of the plate. In battery types where TTB is preferred, it is more common to produce this during the curing process than during paste mixing.
Leady oxide made by the Barton process contains up to 10% orthorhombic (β) PbO. It has lower stability than tetragonal (α) modification and is partially converted to the tetragonal phase during mixing. The finished paste, at the point of completion of pasting, will contain the following compounds:
βPbO (Barton oxide only) Pb(OH)2
The use of additives or components which enhance rather than create the electrochemical actions, which provide the energy and power associated with lead‑acid batteries is now standard practice in lead‑acid battery manufacture. There are four principal addititions to the paste formulations used in lead‑acid batteries, these are:
Long chain polymer or glass fibres (floc) used principally in the positive plate to bind the particles together and prevent shedding. These are also used in many automotive negative formulations
Barium Sulfate which helps to reduce capacity loss and provide better cycle life in automotive as well as traction batteries.
Carbon black or graphite originally added as a conductor to assist in the charging process of deeply discharged batteries. It now is recognised as an expander which helps the negative plate to maintain its porosity and cranking performance during the batteries lifetime. Also of increasing significance is the effect of carbon on the charge acceptance of the negative plate. Enhanced flooded batteries used in stop‑start applications utilise high levels of carbon in the negative paste mix.
Lignosulphonates, their principal chemical function is to inhibit the formation of tetrabasic lead‑sulfate during both paste mixing and the curing process. They also help to promote a smaller crystal size both in the paste mixing and initial formation process, which increases the surface area of the plates which increases the cold crank ability. This makes these additives of particular importance in the automotive industry. For traction batteries they improve the end of charge voltages and help to reduce water loss through gassing.
Organic expanders of ligno-sulphonates, BaSO4 and active carbon are added to the negative paste. It has been established that although BaSO4 is isomorphic with PbSO4 it has no effect on the phase composition of the paste. It has also been shown that lignosulphonates suppress the formation of 4PbO.PbSO4 and orthorhombic PbO phases. Formation of 4BS is suppressed due to the suppression of orthorhombic PbO formation.
The recipes for mixing pastes are normally determined by the application. The density and use of additives for the negative are the main differences. Table 2 gives a summary.
Industrial batteries, as defined here, covers those designed for motive and standby power use, it includes valve-regulated types. These batteries are designed to provide higher cycle‑life in operating conditions that will include repetitive deep cycling and in the case of stand-by power applications, long periods of float charging. Under these conditions, plate active material can soften and shed from the plate, to reduce this, the paste has a higher density to give it more strength.
If density is too high, it is acceptable to add water to the mix while it is still in the paste mixer. Water is never added after the paste mix has been discharged into the cone feeder or the pasting hopper. Water added at this stage does not mix properly into the paste, resulting in inconsistent pasting machine performance and variations in pasted plates. If the density is too low, oxide cannot be effectively added to the paste mix at any stage. The oxide never becomes properly incorporated into the paste. The paste formula is adjusted to add more initial water during paste preparation until the density is in the desired range.
Paste density and physical characteristics
Paste density is determined by the ratio liquid (H2SO4 solution+H2O) to lead oxide. The total pore volume of the paste (dried) depends on paste density while the average pore radius and the consistency are determined by the phase composition of the paste. The effects of acid/oxide ratio (A/O), peak mixing temperature (PT) and peak curing temperature (CT), on the surface area are given in figure 2. From this it can be seen that the surface area is reduced with increasing acid/oxide ratio. This may be attributed to the increase in the formation of lead sulfates with increased acid quantities. Lead sulfates have a lower density than PbO and therefore occupy a high volume which reduces the pore size with a proportional reduction in the surface area.
The physical characteristics determined by the crystal structure are also important in determining the rheological characteristics of the paste. The shear strength and plasticity are important in ensuring that the trowel rolling process which applies the paste to the grid is able to function. Without the characteristic long needles of lead sulfate produced during the addition of the acid, there would be insufficient shear strength within the paste to be pushed from the hopper, into the grid by the rotating pasting bars. The rate of addition of acid and the mix time are critical factors as well as the maximum temperature reached during the process.
Apart from the surface area and another critical parameter of the paste density is the pH of the aqueous media in the pores of the paste matrix. The chemistry of these interstitial areas should promote a good connection between the grid and the active material. In the case of lead calcium grids it is believed that under certain conditions, a passivation layer may be formed between the grid and active material structure. This layer inhibits recharge of the battery by providing a high resistance or less conducting barrier between the AM and the conducting grid. This leads to the so‑called premature capacity loss (PCL) effect which has been blamed for many early battery failures made with lead calcium grids.
The higher density paste will have a lower acid content and consequently higher pH in the pores of the active material. The formation of tribasic or tetrabasic lead sulfate crystals at the grid/active material interface requires corrosion of the grid surface during curing. For optimum attachment of the active material, the grid surface must be alkaline to permit solubility of the lead ions in the water of the paste.
Leady oxide from Barton Pot or Ball Mill, different water and acid absorption, Barton Pot has lower water and acid absorption, also, the surface area is different which will influence the relative quantities of ingredients used and the method of mixing. Table 4 compares the two most common forms of lead sub oxide used in flat plate manufacture, Barton Pot and Ball Mill products.
This in turn will affect the properties of the resultant paste and the operation of the belt pasting and flash drying processes. Barton Pot has lower absorbtion values than Ball Mill oxide, the reasons were discussed in the previous article. This means that the shear strength and plasticity of the Barton Pot oxide will be optimised at lower water and acid values. In turn it is more difficult to achieve the required water content for curing after the flash drying stage. However, if possible, it would be useful to study compare the distribution of the water within the Barton Pot and Ball Mill pastes at similar moisture contents.
The physical properties are affected by the particle shape, the void size and hence water and acid absorption. The effect of water additions on plasticity of the oxide in suspension has been modelled in order to predict its behaviour during the processes and to try to optimise the water content. This modelling ignores the crystal structures of the sulfates produced from acid oxide reactions and assumes void spaces resulting from spherical particles. And as such it is best described as a load of balls (in colloidal suspensions).
The physical form of the paste after mixing of the ingredients can best be described as needle like lead sulfate crystals in association with leady oxide particles with an aqueous layer. The water content is important in providing plasticity (both lubrication and surface tension effects) and increasing the density of the paste due to the voids between the particles and lead sulfate crystals being filled as the water content increases.
However, it is not simply the plasticity which determines the mechanical properties of the paste, figure 4 shows an electron microscope picture of a needle‑like structure of a plate surface before curing. It is this needle like structure which contributes to the required mechanical properties to enable the paste to be transported through the hopper and onto the grid, but still maintain sufficient pliability to enable its relatively precise application to the grid.
At the end of the paste mixing cycle it is normal to conduct two quality checks, one is the cube weight or density and the other is a penetrometer reading. Another less‑well documented check is to hold the paste in one hand and squeeze. This should give a characteristic crunching feel due to the needle like structure of the sulfate crystals. Basically it is an indication that the process has been successful in producing a structure suitable for the remaining processes.
Paste hopper and flash drying
After the paste mixing process, the paste is discharged into the pasting cone and pasting hopper, once it has cooled sufficiently, usually to around 40ºC or less. The paste is then pushed into the conducting frame or grid structure, using rotating trowel bars which constantly squeeze the paste through a narrow opening at the base of the hopper. Underneath the hopper, a travelling flat fabric belt transports the grids from a stack under the hopper which contains adjustable counter rotating rollers which determine the thickness of the active material. Provided that the density and the moisture content of the paste are within tolerance, then the weight of active material can be controlled by the setting of the roller height. Figure 5 shows the pasting belt, grid feed and hopper with rollers attached. At this stage it is normal to have a weight check to ensure that the correct amount of active material is obtained to meet the capacity and performance requirements of the battery. Normal tolerances are within ± 2%.
It is important not to dispense the paste too hot as it quickly loses moisture through evaporation. Loss of moisture will result in lower plasticity making it more difficult to apply to the grid, even stopping the process by jamming the grids under the hopper. This same effect can occur if the paste has insufficient shear strength to hold its structure whilst being forced into the moving grid on the pasting belt. This is often the case when water is added to the hopper during pasting to compensate for excessive water loss in the hopper, or paste which is too dry being dispensed from the paste mixer. Also, excess moisture in the paste can result in the paste sticking to the belt and being pulled away from the grid as it is transported onto the mesh belt of the flash dryer.
Water addition to the pasting hopper is bad practice for two reasons: firstly, the structure of the paste and distribution of water within the matrix is a key part of the curing and AM/grid cementation process, secondly, the water content is not homogeneously dispersed within the hopper batch. This will give batches of plates which have inferior properties and which will almost certainly result in warranty claims.
The wet plates are next transported onto a metal mesh belt, which carries them through the flash dryer before being stacked in racks or on pallets depending on the method of curing. The flash drying process has three principal functions: the surface drying of the plates to allow contact without sticking, the controlling of the moisture content to be within the limits required for the seasoning reaction to occur and to provide heat which helps to kick start the curing process.
It is important to have sufficient aqueous media in alkaline condition to ensure proper dispersion of fine particles and dissolution of lead sulfates. This allows the precipitation and distribution of cementing particles within the voids during curing. The final structure and strength of the active material/grid bond before formation is dependent upon getting all the process parameters right.
The active material should at the end of the flash drying process contain moisture contents of 7–9.5% H2O for Barton Pot oxide and 8‑10.5% H2O for Ball Mill oxide.
The process of paste production should provide a plate with an active material structure of basic sulfate crystals, lead oxide free lead and moisture, see figure 4. The distribution of these materials and their concentration is critical to ensuring that the processes of curing and formation provide a product which achieves the maximum performance from the minimum material content. Provided that appropriate checks are made at each stage and that the parameters of material selection, balance and process controls are suitable for the product application, the plates produced will provide the foundation for the subsequent curing and formation processes. It is this control of the active material production, which forms the basis of producing well performing batteries, with minimal life and performance failures.