The short corrosion life of the positive grid is an important factor limiting the life of lead-acid batteries, and it is also a common problem faced by the entire industry. At the 2024 SMM 19th Lead-Zinc Conference and Lead-Zinc Technology Innovation Forum - Lead-Acid Battery Technology Forum hosted by SMM, Zhang Zhengdong, general manager of EDV Energy Technology (Jiangsu) Co., Ltd., popularized the three major failure mechanisms of lead-acid batteries, analyzed the causes of positive grid corrosion, and introduced methods to delay positive grid corrosion.
There are three major failure mechanisms of lead-acid batteries
Lead-acid battery is a complex electrochemical system, and its main failure mechanisms are three items: oxidative corrosion of the positive grid, shedding of the positive active material, and irreversible sulfation of the negative plate. Among them, the oxidation and corrosion of the positive grid is an important reason for the failure of lead-acid batteries.
The grid mainly plays the role of skeleton support and electronic conduction in the lead-acid battery, and the positive grid needs to have:
Good corrosion resistance: slow down the grid fracture, increase of interface resistance and decrease of conductivity caused by corrosion;
Good mechanical properties: alleviate grid growth and corrosion creep during charging and discharging of the battery.
Causes of positive grid corrosion
The grid is used as the electronic current collector of the battery, so in the process of charging and discharging the lead-acid battery, the battery temperature is too high, the electrolyte density is too large, and the electrolyte contains corrosive acids or organic salts, which will cause obvious corrosion of the positive grid.
It has been established that a thin layer of less oxidized PbOx (mainly PbO) layer is always formed between the gate and PbO2. When the PbO layer comes into contact with H2SO4 in the electrolyte, PbSO4 is formed and becomes an insulator. Grid corrosion is the main cause of premature failure of lead-acid batteries in the first 10-15 cycles of constant voltage charging.
Ball et al. used optical microscopy to analyze the corrosion layer of the positive grid and found that the infiltration of sulfuric acid caused small cracks in the positive grid, as shown in Figure 1. It is concluded that this crack is common at the junction of the grid boundary and the transverse and longitudinal ribs, and the cause of the crack can be mainly attributed to the change of crystal volume after the conversion of PbO2 to PbSO4, the thermal cycling caused by the change of operating temperature and the gas generated by oxygen evolution corrosion destroy the microstructure of the grid.
A method to delay the corrosion of the positive grid
Improve the grid manufacturing process
Sakai et al. changed the grid manufacturing process and used the powder rolling process to make grid alloys with lead-calcium-tin and lead-tin powder, and through the corrosion resistance test, it was found that compared with the traditional casting process, the recrystallization phenomenon was alleviated, the corrosion resistance of the alloy was improved, and the alloy had no obvious corrosion growth, which helped to inhibit the intergranular corrosion of the alloy.
Naresh et al. used potentiostatic polymerization to deposit Py on the positive grid, and compared it with the conventional grid. Through CV, EIS, SEM, corrosion test, battery charge and discharge test, it was found that when the coating thickness was 200 μm, the specific surface area characteristics of the grid were better, which could promote the grid/active material bonding. After the modification of the positive grid, the coating can slow down the growth of corrosion products, thereby improving the corrosion resistance of the positive grid.
FT-IR analysis confirmed the presence of ppy on the surface of the lead alloy cathode grid after 175 cycles. The presence of PPY after 175 cycles was confirmed by SEM analysis. PPY is found to have amorphous particle characteristics due to continuous cycling and exists with active species after cycling. The FT-IR and SEM images obtained confirm the stability of the ppy, even after 175 cycles.
Ppy-coated lead-alloy batteries showed more PbSO4 and PbO2 conversions, while conventional lead-alloy grid cells had lower PbSO4 conversion to PbO2 conversions. This is the main reason why the capacity of conventional lead-acid batteries decreases slowly during the deep discharge of the battery.
The presence of a PPY coating significantly enhances the corrosion resistance of the grating and inhibits the rate of oxygen release. Compared to conventional lead-acid batteries, the capacity is increased by about 15–20% at low charge-discharge rates.
3.2 Electrolyte additives
3.2.1 Phosphoric acid can significantly reduce the shedding of the cathode active material and prevent the formation of dense lead sulfate on the PbO2 electrode, thereby inhibiting anodic corrosion of the gate material and reducing self-discharge.
Saminathan et al. investigated the effects of electrolyte additives. Studies have shown that the addition of phosphoric acid to the electrolyte can inhibit the formation of PbO2 in the positive grid, increase the oxygen evolution overpotential, and reduce the oxygen precipitation rate.
3.2.2 Sodium hexametaphosphate (SHMP) as an electrolyte additive for lead-acid batteries. It is a non-toxic, colorless polymer that is not genotoxic to bacteria and harms the environment.
By adding SHMP, the current density of the oxidation and reduction peaks decreases, and the C2 peak current decreases further, which means that smaller PbSO4 is formed in the presence of SHMP and the formation of larger PbSO4 particles is inhibited.
3.2.2 Sodium hexametaphosphate (SHMP) as an electrolyte additive for lead-acid batteries.
The large PbSO4 particles remaining on the surface of the blank electrode will accumulate on the electrode surface during the cycle, passivating the electrode, and the addition of SHMP can significantly reduce the size of the generated PbSO4 particles, which can be completely reduced.
3.2.2 Ionic liquids, which have the characteristics of good conductivity and low volatility, can increase the electrochemical stability of the electrolyte by adding ionic liquids to the electrolyte.
3.3 Alloy Components
The addition of Ag, Sn, Ba, Li and rare earth elements to the lead-calcium alloy can improve the physical and chemical properties of the positive grid to a certain extent.
3.3.1 The addition of Sn can improve the mechanical and electrochemical properties of the positive grid, inhibit the oxygen evolution reaction and hydrogen evolution reaction, and inhibit the formation of PbO with poor conductivity in the corrosion layer, thereby improving the conductivity of the grid corrosion layer.
Taking Pb-Ca alloy as an example, the grid is coated with tin by electrodeposition by using the H2SO4 corrosion weight loss and time diagram of the four Sn electrodeposition cases. Rasters with three different coating thicknesses are produced within a specified range according to the following deposition times: one-minute, two-minute, and three-minute. Increasing the amount of Sn coating can significantly reduce the weight loss and corrosion rate of the material, thereby improving corrosion resistance.
3.3.2 The addition of Li can significantly improve the corrosion resistance of lead-calcium alloy, and can effectively inhibit the growth of corrosion film and improve the service life of the battery.
The study showed that the addition of lithium to Pb-Ca-Sn led to the formation of a Pb-Ca-Sn-Li alloy, reducing the corrosion rate of the alloy by a factor of 2.65 (from 134.2 mpy to 50.3 mpy).
3.3.3 Adding an appropriate amount of BA to the alloy can inhibit the obsolescence of traditional lead-calcium alloys, keep the mechanical properties at a high and stable level, reduce the growth of the grid area, promote balanced corrosion, and improve the corrosion resistance of the grid.
3.3.3 Ag can improve the corrosion resistance and creep resistance of the positive grid.
Taking the Pb-Ca alloy as the research object, an isolation unit was formed on the surface of the battery grid by isolating tin and silver to the subgrain boundary (magnification 500x).
It turns out that a moderate tin content and a high silver alloy content segregate the subgrain boundaries between grains and dendrites, thus significantly reducing the corrosion rate. However, during the casting process, due to the low freezing point of Ag, this may cause it to solidify first during casting, causing cracks in the alloy. In addition, due to the high corrosion resistance of AG alloy, the corrosion layer generated on the surface of the grid may be insufficient, resulting in poor binding between the grid and the active material, which in turn leads to the detachment of the active material during the cycle test of the battery, resulting in the reduction of battery life.
3.3.4 The addition of rare earth elements to the positive grid alloy can be deposited on the surface of the grain boundary during the solidification process of the alloy, which can inhibit the growth of grains, thereby refining the grains, improving the mechanical properties, corrosion resistance and conductivity of the positive grid and improving the performance of lead-carbon batteries.
The addition of Ce can improve the corrosion resistance of the positive grid alloy, inhibit the growth of Pb(II), improve the porosity of the corrosion film, and increase the conductivity of the corrosion layer. The principle is that the rare earth element Ce can make the particle distribution in the impedance layer more uniform, prevent further corrosion of the alloy, rare earth has a large atomic radius, easy to occupy the vacancy in the alloy, block the diffusion of other elements, refine the grain, can ensure the formation of a dense protective film, can make the intergranular intergranular layer chemically inert, slow down the occurrence of corrosion;
When the addition of La,La is 0.006 wt.% and 0.054 wt.%, it can inhibit the oxygen evolution reaction of the electrode, and inhibit the formation of oxides with poor conductivity, and increase the conductivity of the corrosion layer.
The addition of Sm can inhibit the growth of the corrosive layer and promote the conversion of PbSO4 to PbO2, and the impedance of the corrosive layer is reduced. With the addition of Sm and Yb, the growth of PbO in the corrosion layer can be inhibited, and the conductivity of the corrosion layer can be improved, and the hydrogen and oxygen evolution reactions can be mitigated.
3.4 Grid structure
The structure of the grid reinforcement, the position and number of tabs, and the height-to-width ratio of the grid have a direct impact on the distribution and potential drop of the plate equipotential line.
The size of the area between the equipotential lines reflects the speed of potential change, and the larger the area between adjacent potential lines, the slower the potential change, the lower the internal resistance per unit area, and the more uniform the current distribution.
The multi-tab grid can significantly reduce the potential drop of the plate, reduce the internal resistance, reduce the temperature rise of the battery charge and discharge, and prolong the corrosion life of the grid.
summary
By optimizing the alloy material, optimizing the electrolyte additives and optimizing the production process, the corrosion resistance of the positive grid of the acid battery was improved, and the purpose of improving the life of the lead-acid battery was realized.
On the basis of the research on improving the corrosion resistance of the positive grid alloy, the preparation process of the positive plate was optimized, the corrosion growth law of the positive grid/active material interface was studied, and the kinetic influence of the positive electrode potential on the corrosion rate of the alloy was explored.