First, the role of the grid:
The grid is the bone of the plate, also known as the lattice, which has two main functions. On the one hand it is the carrier of the active substance, on the other hand it acts to conduct the current and to distribute the current evenly.
The electrode active material of lead storage battery is generally only blended with sulfuric acid solution, without compression molding, sintering, bonding, etc.: the structure is relatively loose, and it is not suitable for itself, and it is not easy to form. Therefore, a carrier is needed to make the activity The substance can undergo processes such as formation, drying, assembly, etc. without dispersing and falling off, and it is necessary to maintain a certain capacity after many times of charge and discharge. This is the first role of the grid.
When charging, it is transferred from the external power source to the active material; when discharging, the current generated by the reaction of the active material is transmitted to the external circuit, and the grid acts as such. However, if the current distribution is not uniform, that is, the current of some parts of the active material is larger than that of other parts, then the volume changes due to and should be larger, and these parts are larger than others, resulting in The active substance expands or even falls off. Therefore, the grid is made of square lattices with horizontal and vertical ribs, diamond lattices or circular spaces. There should be proper spacing between the ribs. If the interval is too wide, the air will be unevenly distributed. If the interval is too narrow, the weight of the entire grid will increase. In order to facilitate the connection of the active materials, the ribs each take a staggered form (Fig. 1), and the cross-sectional area thereof is usually elliptical, rhombic, triangular, and the like.
In recent years, for the positive electrode of the power battery, a tubular plate is used. At this time, the bearing effect of the active material is mainly carried by the glass fiber tube, and the effect of the grid as a carrier is correspondingly reduced, according to the lead-bismuth alloy grid. Some people believe that the enthalpy that dissolves into the positive active material may play a beneficial role. Therefore, the role of the grid will also change with the development of the lead battery manufacturing process, not static.
Second, lead-bismuth alloy and its casting
Pure lead is not suitable for grid materials because:
(1) Pure lead is too soft, the strength is very poor, and it is easy to deform when subjected to force, so it cannot withstand the strong stress in the operation of coating, assembly and other processes.
(2) Due to the poor strength, it is not possible to carry an excessively large active material weight unless the grid is made thick.
(3) It is difficult to cast into a grid of complicated shapes.
(4) In long-term use, when the surface corrosion products are formed, under the stress of volume change, when the grid is made of pure lead, the ribs will elongate and even warp.
Therefore, lead alloys, mainly lead-bismuth alloys, are used instead of pure lead to make the grid.
(a) lead-bismuth alloy and its properties
Table 1 lists some properties of lead-bismuth alloys, and compared with pure lead, it can be seen that lead-bismuth alloys have the following advantages:
(1) High mechanical strength; tensile strength and hardness are much higher than pure lead, and tensile strength and hardness are all aspects of mechanical strength. Tensile strength is the force required to pull an object apart. It is expressed by the force used for the unit cross-sectional area (ie, the number of kilograms per square centimeter). The greater the tensile strength, the stronger the strength of the material resisting breaking. Hardness is to press a steel ball under a certain pressure to the material, and use the indentation area to remove the load force to determine the hardness of the material. Therefore, Brinell hardness represents an intensity of this material that resists the intrusion of foreign matter into the body. The high content of niobium and the increase in tensile strength and hardness are also advantageous for casting grids, especially for thin and complex grids.
Elongation at break is the percentage of elongation of the object when it is broken. The elongation is the largest at 5% Sb. When the amount of bismuth is higher, it is reduced. The decrease of elongation is beneficial to prevent the deformation of the grid after multiple charge and discharge. .
(2) The casting property is good; the fluidity during melting is better than that of pure lead. One reason for the good fluidity is that the melting point of lead-bismuth alloy is lower than that of pure lead. We know that solids have no fluidity and become liquid to have fluidity. The more the liquid temperature is higher than the melting point, the greater the fluidity is generally. From Table 1, the melting point of pure lead is 327 ° C, while the melting point of a 9% Sb alloy is only 265 ° C. Therefore, the liquid Pb at 400 ° C is 73 ° C higher than the melting point, while the 9% Sb liquid alloy at 400 ° C is higher than the melting point. At 135 ° C, it is almost twice as large as pure lead. Obviously, the fluidity of the alloy is larger than that of pure lead, and the fluidity is good, so it is easy to fill the inner cavity of the casting when casting.
(3) The coefficient of linear expansion of lead-bismuth alloy is smaller than that of pure lead. The linear expansion system is the ratio of the elongation at 1 ° C when the temperature of the object rises to the length at 0 ° C. For example, the linear expansion of the lead from Table 1 is 0.0002292, that is, a one-meter-long lead strip is raised. At 1 ° C, it becomes 1.0000292 meters long. Although this number is small, when the temperature rises and falls, the elongation of lead is reduced. There is another type of expansion system called body expansion, which is the fraction of the volume of the volume when the temperature of the object rises by 1 ° C. Elongation increases in one direction, and expansion increases in all three directions so the coefficient of bulk expansion is approximately three times the coefficient of linear expansion.
  Lead-bismuth alloy shrinks when cooled, about 1/3 less than pure lead. The degree of shrinkage during cooling is small, which is advantageous for the grid to maintain the contour of the cavity of the mold.
Therefore, lead-bismuth alloys basically meet the requirements of the manufacturing of the grid in terms of surface mechanical properties.
  However, the use of lead-bismuth alloys also has disadvantages such as:
(1) The resistance of the alloy is larger than that of pure lead.
(2) Corrosion resistance is not as good as pure lead, and positive plate grid corrosion is the main reason for lead-acid batteries. Therefore, this is the main disadvantage of making grids with lead-bismuth alloys.
(3) 锑 will dissolve from the positive grid, causing the negative/self-discharge.
Compared with its shortcomings, the main disadvantage of lead-bismuth alloy is that it is the second one. However, in order to further improve the performance of lead-acid batteries, its shortcomings cannot be ignored, especially the study on the corrosion of lead-bismuth alloys. The essence of corrosion can better grasp it to prevent it.
(b) Precasting of lead-bismuth alloy
The lead-bismuth alloy used in the lead battery grid has a germanium content of 5-12%. Due to the corrosion problem of lead-bismuth alloys, the current trend is to reduce the amount of Sb-containing. Generally, the thinner grid has a higher amount of Sb, about 9-10%. For example: a grid below 2 mm with a Sb content of 9%. Thicker grids contain a lower amount of Sb. For example: a 3.1 mm grid with a Sb content of 6.5%. Lead parts contain a low amount of antimony, about 3-6%.
(1) Lead and antimony are usually made into lead-bismuth alloy in advance. One method is to make a sorghum alloy containing about 20% Sb and add pure lead to the desired niobium content when making the grid. Another method is to directly form the desired niobium-containing alloy, which can be used after melting. An example of the process for preparing the alloy is shown in Figure 2. In the melting step, usually a part of the total amount of lead is first added, the temperature of the alloy crucible is controlled at 350-400 ° C, and then the crucible is added, and the temperature is raised to 500-550 ° C to melt the crucible. Finally, the remaining lead is added to the crucible. To keep stirring, it is evenly mixed.
The special melting method of adding Pb, adding Sb and adding Pb is because the melting point of Sb is high (631 ° C), the melting point of Pb 低 is low (327 ° C), and the complete melting temperature of the alloy is lower (1-11). %Sb is 320-256 °C, see Table 1). Add one-fifth to one-half of Pb, its melting point is 327 ° C, and it can be completely melted by heating to 350-400 Pb ° C (you can also leave a part of the alloy of the previous pot as a primer, the melting temperature will be Lower). At this time, Sb is added and the remaining Pb is finally melted into an alloy, and the temperature is maintained at 400-450 ° C.
The specific gravity of lead is 11.3, the specific gravity of bismuth is 6.7, and the weight of lead is light and it is evenly stirred. Stirring should be continued when adding Sb. After adding Pb, stir for about 15 minutes to ensure that it is evenly mixed.
The amount of Sb contained in the alloy is measured by the "freezing point method", and the measurement method and principle will be described later.
Since the alloy is also melted during the manufacture of the grid, the main problem with the precast alloy is to shorten the melting time. Reduce the melting temperature to save fuel and reduce burn-off ※, and try to make the composition even.
(2) The article number for the purity of lead Pb and Sb raw materials for the preparation of alloys is shown in Table 3. However, it has been found from long-term production practices and scientific experiments that alloys made of crude lead (low grade lead) or secondary lead (recycled from waste plates) have poor corrosion resistance compared to alloys made of pure lead. . Impurities such as arsenic, sulfur, tin, silver, copper, etc., are themselves used as additives for lead-bismuth alloys to improve the corrosion resistance of lead-bismuth alloys (sometimes even improve mechanical properties such as tinning to improve flow), Therefore, through practice and scientific analysis, the use of crude lead or secondary lead to manufacture grid alloys is completely feasible, and excessive requirements for the purity of raw materials are not necessary.
3. Casting of the grid
In the grid casting process, mechanized or semi-mechanized casting has been widely adopted, and the joint engine is gradually being promoted.
(a) The temperature at which the alloy is melted one by one.
(b) Insulation of molds - control of mold temperature
(C) When casting the grid, the causes of the main defects and their prevention measures are summarized:
The above table is only a general situation. In the production practice, different types of batteries have different requirements for the plates, and specific problems must be analyzed.
4. Solidification process and principle of lead-bismuth alloy
The manufacturing process of the grid is essentially a cooling and solidification process of a lead-bismuth alloy. The principle will be explained below, which has certain parameter significance for the solidification point of the general alloy.
(a) "Coagulation point method" measures the amount of the alloy to the step-cooling curve.
(i) "Coagulation point method" is to use a handle-type iron tongs with a volume of about 200 ml to hold the alloy solution and insert a thermometer (0-500 ° C).
(ii) Step cooling curve It can be seen that the "freezing point" method actually observes the temperature change curve when the alloy liquid is gradually cooled. This cooling curve, referred to as the non-cold curve, has a shape that is related to the composition of the alloy.
(b) Lead-bismuth alloy phase diagram
The step cold curve does not clearly show the relationship of concentration, which is its disadvantage. This disadvantage is overcome if the temperature at which the solid is precipitated on the step cooling curve is plotted against the concentration of the alloy liquid. This figure is called a binary alloy phase diagram, as shown in Figure 8.
(C) Equilibrium cooling crystallization process of lead-bismuth alloy
The above is all about the extremely slow cooling crystallization process, that is, the equilibrium cooling crystallization process.
(d) Unbalanced cooling crystallization process of lead-bismuth alloy.
In fact, the above-mentioned equilibrium crystallization is rarely achieved, and in most cases, the cooling cannot be very slow and is an unbalanced crystallization process. Balance is relative, temporary conditional, and imbalance is absolute, permanent, and unconditional. But only by mastering the situation of balanced crystallization can we better understand the process of imbalance.
When discussing the properties of the crystalline polymer, it is mentioned that the fine bodies are in contact with each other and hinder each other, and function as a crosslink, which increases the strength and hardness. Therefore, the precipitated eutectic dense mixture also increases the mechanical strength of the Pb-Sb alloy. As shown in Table 1, both the tensile strength and the hardness increase as the amount of Sb increases (the amount of Sb is more than that of eutectic precipitation), and there is a maximum at the eutectic composition. From Fig. 16, it can also be seen that the composition of α-Pb precipitated at a low eutectic point at 252 ° C is point A, containing 3.5% Sb, in equilibrium crystallization, and this α-Pb phase composition is cooled to room temperature. It should be changed along the AS ́S line. The amount of Sb is reduced to 0.44% at 100 °C. In the case of unbalanced crystallization, the temperature drops rapidly, the diffusion in the phase is slow, and the Sb is too late to diffuse out, so the α-Pb phase remains 3.5. The %Sb is composed and cooled to room temperature (as shown in the AK line of Figure 16). In this way, it is an unstable phase, Sb is slowly diffused out, and Sb is precipitated. When the time is enough, many fine Sb grains are precipitated, which further increases the strength and hardness. This phenomenon is called age hardening. Because it is caused by precipitation of crystallites, it is also called precipitation hardening (or precipitation hardening). After the grid is manufactured, it takes a few days to apply the paste, just because of this.

But after being placed for too long, there is a problem of so-called "brittleness". This is because the precipitation is at the crystal interface (also called the intergranular interlayer), and the precipitation is much more, so that the alloy is relatively fragile in the direction of the grain boundary and is easily brittle. When the Pb-Sb alloy grid is placed for 1-3 months, it does not matter, but especially a small amount of impurities such as silver is added to precipitate on the grain boundary, so that the grid is relatively brittle. Therefore, after the pole plate is manufactured, it should not be placed for too long.
The greater the degree of subcooling, the faster the cooling, and the nucleation rate and grain growth rate are all related to it. When the degree of undercooling is not large, both speeds increase with the increase of the coldness, but the growth rate of the nucleus grows faster because the nucleus is formed "out of thin air", and the growth of the formed nucleus is It precipitates on the surface of the crystal nucleus, and has sharp edges and defects on the surface, and precipitation is much easier. When the degree of undercooling is large, both speeds are reduced by a minimum point, and the growth rate is reduced to more. This is because after the solid is precipitated, it is diffused to continue the precipitation. The degree of supercooling is too large, the temperature is too low, and the diffusion is too Slow, so the speed drops.
Although the degree of subcooling is related to the size of precipitated grains. When the degree of subcooling is 2, crystallization occurs, the nucleation rate is small, and the growth rate is large. Therefore, the crystal nuclei which are not much formed are rapidly grown to form relatively coarse and uniform crystal grains, and when the degree of subcooling is 3 The nucleation rate and growth rate of the nucleus are relatively large, and uniform and fine crystal grains can be obtained. When the degree of subcooling is 1, the speeds of both are small, coarse grains are formed, and sometimes shrinkage cavities are formed. Porosity, even cracks, and due to very slow grain growth, impurities have time to accumulate, some in the grain boundary, causing grain boundary corrosion or pitting.