Early capacity loss of lead-acid batteries (PCL-3) - irreversible sulfation of the negative electrode

The research shows that: At different discharge rates, the distribution of PbSO4 generated by the negative electrode is different. At low discharge rates (< 0.5C20), PbSO4 crystals are uniformly distributed inside the plate, and the crystal particles are relatively coarse, while at high discharge rates (> 4C20), the crystal particles of PbSO4 crystals are small and dense on the surface of the plate. According to Ostwald ripening mechanism, small lead sulfate crystals tend to be transformed into coarse lead sulfate crystals by recrystallization under the action of specific surface energy. This coarse lead sulfate crystal is difficult to charge and transform because of its low solubility, forming "irreversible sulfation". The effect of irreversible sulfation: the life under low current deep discharge condition, the life under high current and high power discharge condition, and the life under long-term undercharge seriously restrict the application condition and service life of lead-acid batteries.
Irreversible sulfation is further aggravated by the shrinkage of the specific surface of the sponge lead during the cycle
As an active substance, the negative spongiform lead will shrink continuously under the action of specific surface energy during repeated charge and discharge, which is an irreversible process. The aperture of the plate will be larger than that of the surface shrinkage, which is more conducive to the formation of coarser lead sulfate crystals, leading to the intensification of the irreversible process. The solution to the sulfation of the negative electrode is to use surfactants (lignin, humic acid) to inhibit the surface area shrinkage of the active substance. Lead sulfate particles were refined by barium sulfate crystal nucleus. Add carbon black, graphite, etc. to increase electrical conductivity, "called anti-expansion agent."
Surfactants - lignin, humic acid, etc
Principle: Using the surface adsorption of lignin, the specific surface area of sponge lead is increased when PbSO4 is reduced to sponge lead. Defects: In an acidic environment, hydrolysis will occur, oxidation will occur when oxygen recombination, and hydrogen hydrolysis will occur when charging, resulting in no durability of the lignin's role, and it begins to fail at about 200 cycles. The higher the temperature, the faster the decomposition rate.

The complex mechanism of carbon material in the negative electrode

Due to the complex structure of carbon materials, the mechanism of action of carbon materials in the negative electrode is also very complex.
The action mechanism of carbon materials in the negative electrode is summarized into physical and chemical processes:
Physical processes - electrical conductivity, double layer capacitance, surface area effect (utilization) maintain specific surface area during charge and discharge.
Chemical process - Carbon materials can catalyze the conversion of Pb2+ to Pb (electrocatalysis).
The negative electrode is easy to sulfate and the positive electrode is rarely sulfated, because the volume of the negative sponge lead changes greatly during the conversion of lead sulfate, which provides a favorable space for the growth of lead sulfate crystals, and carbon materials can fill the void to generate steric hindrance.
In the process of charging, the electrochemically active carbon material has an electrocatalytic effect on the reduction of PbSO4 in the negative electrode, and the charging voltage is reduced by about 200~300mV. Further research found that the Pb2+ reduction crystallization process occurred on the surface of carbon material and lead surface at the same time, making the carbon material and sponge lead connected into a whole, the current on the surface of the carbon material can reduce the current density of the plate, reduce polarization, and promote the reduction of lead sulfate, which is called the "parallel mechanism" during charging.

Carbon black

Effects: (1) It is generally believed that carbon black conduction can promote the conversion of lead sulfate; (2) Adsorption of balance beam; (3) The Japanese energy storage battery company increased the amount of carbon black by 10 times of the conventional amount, and found that it had very good high-rate partial charge state performance; (4) Pavlov's study found that carbon black could change the skeleton structure of sponge lead, and too much carbon black would be embedded in sponge lead but reduce the conductivity of sponge lead skeleton. Defects: (1) Excessive dosage will leak from the plate, resulting in micro short circuit; (2) Excessive dosage destroys the skeleton structure of the sponge lead, resulting in the negative electrode slime. (3) Excessive hydrogen evolution is serious. Activated carbon function: (1) Activated carbon has a high specific surface area, a relatively high double electric layer capacitance, can form an asymmetric supercapacitor with positive lead dioxide, high magnification performance; (2) Pavlov research shows that during the charging process, lead dendrites will grow on the surface of activated carbon and form a finishing skeleton structure with sponge lead, which is conducive to the charging and discharging of double-layer capacitors. (3) Our study found that the growth morphology of lead dendrites is different with different activated carbon structures, and the crystallinity of graphitic microcrystals that constitute activated carbon and the regularity of surface defects are higher, with high crystallinity, good electrical conductivity and regularity, which is more conducive to the formation of lamellar dendrites higher than the surface, which is conducive to the reversibility of the electrode cycle. Defects: (1) Activated carbon is an internal pore structure with a high specific surface and high hydrogen evolution active point, so it is not easy to adjust the hydrogen evolution potential; (2) Lead deposition will block the hole, and the capacitance of the double electric layer will gradually decay with the progress of the cycle; (3) The porous structure has strong adsorbability and will carry out irreversible adsorption of lignin in the electrode.

graphite

Effects: (1) J. Settelein studied the crystallization of lead dendrites on the surface of expanded graphite and spherical graphite, and found that expanded graphite was more favorable to the growth of lead dendrites; (2) Karel Micka believes that graphite has a resistance effect in the negative electrode, which can inhibit the growth of lead sulfate crystals; (3) We studied the lead dendrite growth of spherical graphite and natural flake graphite, and found that natural flake graphite is more conducive to the formation of lamellar dendrites with good dispersion, while the dendrites on the surface of spherical graphite form a coating structure around the surface of spherical graphite, which is not conducive to the improvement of the surface area of sponge lead. Defects: (1) lower than the surface, no capacitance effect; (2) The particle is thick, the density is high, the dosage is large, and the surface area effect is not obvious.

Carbon nanotubes (Research focus)

Functions: (1) Carbon nanotubes are two-dimensional materials with high conductivity and long conductive path, which is conducive to improving the conductivity of the electrode. (2) Studies have shown that the addition of carbon nanotubes to the negative electrode can improve the charge acceptance, and at the same time, it is more conducive to the formation of fine particles of lead sulfate crystals during discharge.
Defects: (1) difficulty in dispersion; (2) The price is relatively expensive.

Graphene (Research hotspot)

Functions: (1) two-dimensional material with excellent electrical conductivity;
(2) Excellent electrical conductivity, induce lead dendrite growth radiation range is wider;
(3) Reversible adsorption of lignin was formed by two-dimensional planar structure;
(4) The steric effect on lead sulfate is more significant;
(5) More obvious than surface area effect.
Defects: (1) hydrogen evolution overpotential needs to be further inhibited; (2) Manufacturing costs need to be further reduced.