Department of Electrochemistry of Lead-Acid Batteries

The studies of smooth lead electrode polarization in sulfuric acid solution show that a new electrode system Pb/PbO/PbSO₄ is formed within the potential region -0.400 to +O.950 V vs. a Hg/Hg₂SO₄ reference electrode. It is assumed that its formation is due to the building up of a semipermeable PbSO₄ membrane which hinders the transport of SO₄^2- ions in the pores of the PbSO₄ layer and leads to alkalization of the solution there. This Pb/PbO/PbSO₄ electrode has semiconductive and photoelectrochemical properties. A semiconductive mechanism of PbO oxidation through nonstoichiometric PbO_n (1 < n < 2) to PbO₂ is proposed. PbO_nhas p-type conductivity and its electric resistance is several orders of magnitude lower than that of PbO. When the coefficient n exceeds 1.4, the crystal lattice defects reach a concentration, which leads to transformation of this lattice into alpha-PbO₂.

The corrosion processes on anodic polarization of the lead electrode at potentials higher than 0.950 V are investigated, as well as the influence of Sb, As, Ag, Sn, Cu, Tl additives on these processes. Optimum compositions of the lead alloys are proposed for various battery applications.

The processes occurring during lead-acid battery paste preparation are studied. The phase composition and the structure of the pastes are determined in dependence of the amount of sulfuric acid used and the temperature and time of paste mixing. It is established that at mixing temperatures of up to 60^oC the pastes are composed mainly of 3PbO.PbSO₄.H₂O that crystallizes into small crystals 3-4 microns in size. This fine crystalline structure ensures high initial capacity of the active mass, but its cycle life is not very long. When the paste is prepared at temperatures higher than 80^oC, tetrabasic sulfate is formed comprising large crystals (25-30 microns). The active mass obtained from this paste has lower initial capacity, but considerably longer cycle life.

The curing processes of the paste and the formation of the active masses are examined. It is established that the negative plate formation proceeds in two stages. At the first one, the lead oxide and the basic lead sulfates are reduced to Pb and PbSO₄ is obtained. The lead forms a continuous skeleton of lead crystals. At the second stage, PbSO₄ is reduced to small lead crystals, which are deposited on the lead skeleton. During charge and discharge the small lead crystals take part in these processes. Their morphology and size depend on the type of expander used.

It is found that the positive active mass formation also takes place in two stages. The lead oxide and basic lead sulfate are oxidized and form microporos agglomerates of small PbO₂ particles. These particles consist of crystal, amorphous and hydrated zones. The agglomerates build a macroporos skeleton, which supports the active mass and serves as an electro-conductor. The positive and negative active masses "memorize" the technology of their preparation. The carrier of this memory - the "gene", is the skeleton structure of the active mass.

The processes at the grid/corrosion layer/active mass interfaces are studied. The following steps of grid corrosion are determined Pb ® PbO ® PbO_{n ®} PbO₂. When, after formation, the positive plates are dried at a temperature higher than 85^oC, a continuous PbO layer is built. This layer has very high ohmic resistance and causes plate thermopassivation. If the grid contains Sn, its oxidation generates p-type conductivity of the corrosion layer and plate passivation is eliminated (Sn-free effect).

The phenomena that occur on battery charge and discharge at the interface positive plate grid/active mass, the smallest cross-section, through which the electric current passes, are investigated. The behaviour of this interface is determined by the technological parameters, the alloying additives used and the plate design. Based on the results of these investigations, a new positive plate design and the corresponding manufacturing technology are developed. Experimental batteries are produced using the new plate design and manufacturing technology.

A new concept is proposed for the structure of the positive active mass (PAM) according to which PAM and the corrosion layer (CL) are gel-crystal systems. It is established that the electronic conductivity of the gel zones depends on the gel density and the presence of foreign ions (dopants) of the type Sb, Sn, Bi, As, which are readily hydrated. Thus Sb, Sn and Bi improve the electronic conductivity of gel zones, while As decreases it.

The obtained knowledge of the manufacture and operation processes is used for the development of new technologies for positive and negative plates production, which will ensure high performance of lead-acid batteries.
The Department of Lead-Acid Batteries has a battery testing laboratory outfitted with modern computerized equipment for testing of all types of lead-acid batteries. Battery tests can be performed according to all test standards adopted in the battery practice worldwide, as well as applying test programs developed by the Laboratory to meet specific user requirements. The Department of Lead-Acid Batteries can provide expert opinions about the performance of the batteries. It can also give recommendations related to optimization of the technology of battery manufacture and the solution of technological problem. These abilities are derived out of the results from battery tests, as well as from additional analyses and observations from the type: X-ray diffraction, porometry, electron microscope observations, DSC, TGA, DTA, AAS etc.

The Department of Lead-Acid Batteries has established wide international contacts with many research battery companies worldwide. During the last 10 years, the Department of Lead-Acid Batteries has participated in joint research projects with a number of foreign companies and international organizations as follows: VARTA Batterie AG (Germany), ILZRO (USA), Oerlikon (Switzerland), GNB (USA), Maschinenfabrik EIRICH (Germany), the European Community, Borregaard LygnoTech (Norway), JSB (Japan).

Specialists from the Department of Lead-Acid Batteries have presented lecture courses in many countries all over the world: Australia, Brazil, Canada, China, Finland, India, Japan, Korea, Romania, Taiwan, USA. These lectures were basically focused on the processes, taking place during battery manufacture and the relation of these processes with the performance of the lead-acid batteries.

The Department of Lead-Acid Batteries is the organizer of five international lead-acid battery conferences LABAT'89, LABAT'93, LABAT'96, LABAT'99, and LABAT'02,which are to continue with further issues in the future. These conferences were attended by over 250 delegates - scientists, researchers, and manufacturers of lead-acid batteries and battery equipment, from 30 countries worldwide. More than 60 papers were presented at each of the conferences and major manufacturers and suppliers of batteries and battery equipment exhibited their latest products during the conferences.

Publications

1. D. Pavlov, Processes of formation of divalent lead oxide compounds on anodic oxidation of lead in sulfuric acid, Electrochim. Acta, 13 (1968) 2051.

2. D. Pavlov, N.Iordanov, Growth processes of the anodic crystalline layer on potentiostatic oxidation of lead in sulfuric acid, J. Electrochem. Soc., 117 (1970) 1103.

3. D. Pavlov, G. Papazov, V. Iliev, Mechanism of the processes of formation of lead-acid batteries positive plates, J.Electrochem. Soc., 119 (1972) 8.

4. D. Pavlov, V. Iliev, G. Papazov, E. Bashtavelova, Formation processes of the lead-acid battery negative plate, J. Electrochem. Soc., 121 (1974) 854.

5. D. Pavlov, G. Papazov, Dependence of the properties of the lead-acid battery positive plate paste on the processes occurring during its production, J. Appl. Electrochem., 6 (1976) 339.

6. D. Pavlov, S. Zanova, G. Papazov, Photoelectrochemical properties of the lead electrode during anodic oxidation in sulfuric acid solution, J. Electrochem. Soc., 124 (1977) 1522.

7. D. Pavlov, T. Rogatchev, Dependence of the phase composition of the anodic layer on oxygen evolution and anodic corrosion of lead electrode in lead dioxide potential region, Electrochim. Acta, 23 (1978) 1237.

8. D. Pavlov, S. Ruevski, Thermopassivation of the lead dioxide plate of lead-acid batteries, J. Electrochem. Soc., 126 (1979) 1100.

9. D. Pavlov, V. Iliev, Structure of the active mass of the negative plate of lead-acid batteries, J. Power Sources, 7 (1981) 153.

10. D. Pavlov, E. Bashtavelova, A model of the structure of the positive lead-acid battery active mass, J. Electrochem. Soc., 131, (1984) 1468.

11. D. Pavlov, I. Balkanov, P. Rachev, Orthorhombic PbO formation during discharge of lead-acid batteries PbO₂ active mass, J. Electrochem. Soc., 134 (1987) 2390.

12. D. Pavlov, B. Monahov, M. Maja, N. Penazzi, Mechanism of Action of Sn on the Passivation Phenomena in the Lead-Acid Battery Positive Plate (Sn-free effect), J. Electrochem. Soc., 136 (1989) 27.

13. D. Pavlov, The lead-acid battery lead dioxide active mass a gel-crystal system with proton and electron conductivity, J.Electrochem.Soc., 139 (1992) 3075.

14. D. Pavlov, A Theory of the Grid/Positive Active Mass (PAM) Interface and Possible Methods to Improve PAM Utilization and Cycle Life of Lead/Acid Batteries, J. Power Sources, 53 (1995) 9.

15. D. Pavlov, B. Monahov, Mechanism of the elementary electrochemical processes taking place during oxygen evolution on the lead dioxide electrode, J. Electrochem. Soc., 143 (1996) 3616.

16. D. Pavlov, A. Dakhouche, T. Rogachev, Influence of Antimony Ions and PbSO4 Content in the Corrosion Layer on the Properties of the Interface Grid/Active Mass in Positive Lead-Acid Battery Plates, J. Applied Electrochem., 27 (1997) 720.

17. D. Pavlov, Energy Balance of the Closed Oxygen Cycle and Processes Causing Thermal Runaway in Valve Regulated Lead-Acid Batteries, J. Power Sources, 64 (1997) 131.

18. M. Dimitrov, D. Pavlov, Influence of Grid Alloy and Fast Charge on Battery Cycle Life and Structure of the Positive Active Mass of Lead Acid Batteries, J. Power Sources, 93 (2000) 234.

19. D. Pavlov, G. Petkova, M. Dimitrov, M. Shiomi, M. Tsubota, Influence of Fast Charge on the Cycle Life of Positive Lead-Acid Batteries Plates, J. Power Sources, 87 (2000) 39.

20. D. Pavlov, B. O. Myrvold, T. Rogachev, M. Matrakova, A new generation of highly efficient expander products and correlation between their chemical composition and the performance of the lead-acid battery, J. Power Sources, 85 (2000) 79.

21. D. Pavlov, S. Ruevski, V. Naidenov, G. Sheytanov, Influence of Temperature, Current and Number of Cycles on the Efficiency of the Closed Oxygen Cycle in VRLA Batteries, J. Power Sources, 85 (2000) 164.

22. D. Pavlov, S. Ruevski, Semi-suspension technology for preparation of 4PbO-PbSO₄ pastes for lead acid batteries. J.Power Sources, 95 (2001) 191.

23. D. Pavlov, V. Naidenov, S. Ruevski, V. Mircheva and M. Cherneva, New modified AGM separator and its influence on the performance of VRLA batteries, J. Power Sources, 113 (2003) 209-227.

24. A. Kirchev, D. Pavlov and B. Monahov, Gas-diffusion approach to the kinetics of oxygen recombination in lead-acid batteries, J.Power Sources, 113 (2003) 245-254.

25. D. Pavlov, G. Papazov and B. Monahov, Strap grid tubular plate - a new positive plate for lead-acid batteries. Processes of residual sulphation of the positive plate, J.Power Sources, 113 (2003) 255-270.

26. B. Monahov, D. Pavlov, A. Kirchev and S. Vasilev, Influence of pH of the H₂SO₄ solution on the phase composition of the PbO₂ active mass and of the PbO₂ anodic layer formed during cycling of lead electrodes, J. Power Sources, 113 (2003) 281-292.

27. G. Petkova and D. Pavlov, Influence of charge mode on the capacity and cycle life of lead-acid battery negative plates, J.Power Sources, 113 (2003) 355-362.

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