Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-11-30
2002-10-01
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S228000
Reexamination Certificate
active
06458489
ABSTRACT:
TECHNICAL FIELD
The invention relates to lead-acid batteries having improved performance compared with current lead-acid batteries.
Batteries of this type are particularly intended for the production of high-performance batteries, for electric vehicles for example.
STATE OF THE ART
During the 1970's, research into high performance batteries intended for electrical vehicles confirmed a known dilemma, but which particularly concerns lead-acid batteries: choosing between weight performance and endurance, improvement in one being achieved to the detriment of the other. At times priority was given to energy-to-weight ratio to such an extent that lifetimes fell to scarcely acceptable values. Therefore research was conducted to arrive at a better compromise.
During a 5-hour discharge under constant current, the energy-to-weight ratio of lead-acid batteries intended for electric vehicles ranges from 30 to 40 Wh/kg. On board vehicles, this range of magnitude is reduced by 20%. Therefore, at discharge times of 1 to 2 hours, the lead-acid battery proves to give two times less performance than the nickel-cadmium battery, and three times less than the sodium-sulphur battery.
It would therefore appear that the lead-acid battery ranks the lowest among possible candidates for the market of electric vehicles. Yet, over and above its price, some prospects plead in its favour.
Its performance in terms of charge time should increase with the arrival of specific vehicles that are lighter and more energy-saving. Its lifetime could be increased through intelligent management of its energy. Moreover, the lead-acid battery is probably, among those batteries competing for the new markets, the one which has the highest relative margin for progress.
To improve the performance of lead-acid batteries, it is required to increase the coefficient of use of the active materials of the electrodes. Two pathways of research can be considered to achieve this result. One concerns the collection of charges on the side of the active electrode materials. The other concerns a better distribution of the reagents within the electrodes.
The active material of the electrodes is in fact only used very little, even during a so-called complete discharge, the percentage of converted material being in the region of 25 to 30%.
In respect of charge collection, the electron exchanges between the active material and the outer circuit are ensured by lead alloy conductors in the form of grids or ribs. The size of these collectors is determined by the two following factors: the fact that they form the mechanical support for the electrodes, and the need to resist corrosion phenomena to which they are subject inside the electrodes. In consequence, in current assembly technology, so-called single pole assembly, the weight of the collectors represents between 40 and 50% of the weight of the electrodes for electric vehicle applications.
Regarding the distribution of reagents inside the electrodes, this is limited by the electrode porosities which may be used, as will be seen below.
For over a century, the plates of lead-acid batteries have been made using the Fauré method, the so-called grid and added oxide method. More precisely, a grid in lead alloy is lined with a paste made of lead oxide, sulphuric acid and water. The proportions of these various constituents were empirically determined having regard to the performance of the battery. It soon became apparent that the increase in the quantity of water, that is to say in resulting porosity, increases initial capacity to the detriment of lifetime. Values held to be acceptable for these two parameters limit variations in porosity to a narrow range, in the region of 10%.
Inside an electrode of a lead-acid battery, the quantity of sulphuric acid is much lower than the quantity of active matter likely to be oxidized or reduced. Their ratio is in the order of 15%. To this acid, present on site, additions are made during discharge brought by diffusion from outside the plates, which are more significant the slower the discharge. The coefficients of use of the active matter are highest in the vicinity of the surface of the plates. They may reach 60 to 70%, to be compared with the average value of 25 to 30% for the entire electrode.
Document FR-A-2 438 346 [I] and the publication by J. Alzieu et al., at the Fifth International Electric Vehicle Symposium, Philadelphia, Oct. 2-5, 1978 [2], describe lead-acid batteries with a long lifetime. These batteries have a positive electrode, a negative electrode, an electrolyte formed of sulphuric acid, a set of separator elements arranged between the positive electrode and the negative electrode and means for applying pressure to the whole assembly. It is indicated that with the application of pressure it is possible in particular to increase the lifetime of these lead-acid batteries.
The document J. Electrochem. Soc., vol 130, No. 11, 1983, pages 2144-2149 [3] illustrates a lead-acid battery which uses an active material for the positive electrode having a density of 3.9 g/cm
3
1
, and an electrolyte made of sulphuric acid having a density of 1.28 at 20° C. With the use of such materials it is possible to restrict changes in the structure and physical properties of the active material of the positive electrode, for the purpose of improving its lifetime. In this document, pressure is also applied to the assembly of electrodes, by which means it is also possible to limit electrode structural changes.
The improvement brought by placing the electrodes under stress is of interest, but the energy-to-weight ratio of lead-acid batteries still remains insufficient compared with the performance it is desired to obtain.
Other improvements in lead-acid batteries have been considered by H. Ozgun et al., Journal of Power Sources, 52, 1994, pages 159-171 [4]. These improvements concern variation in the density of the active material of the electrodes. In this latter case, the authors recommend increasing the density of the active material in order to increase the battery's charging/discharging cycle capacity.
P. W. Appel and D. B. Edwards in Journal of Power Sources, 55, 1995, pages 81-85 [5] endeavoured to improve the performance of the lead-acid battery by improving the conductivity of the active material through incorporation of conductor particles, but they did not succeed in finding particles that were able to withstand the particularly corrosive medium of a positive electrode in a lead-acid battery.
Application of pressure to the electrode should bring about an improvement in the coefficient of use of the reagents inside the electrodes by using electrodes in the form of thin plates. It can be understood that a thin plate, that is to say in which every active material element is near a surface delimiting the plate, can have improved performance; the overall coefficient of use should be expected to be 60 to 70%.
With compression it is possible to remedy the special fragile nature of these thin plates. After considering a move in this direction, currently one of the lines of research adopted by the ALABA Advanced Lead Acid Battery Consortium, this approach has been abandoned since unsuspected experimental results have opened up new prospects.
The subject of the present invention is precisely a lead-acid battery of the type described in documents [1] to [3], which has an improved energy-to-weight ratio due to arrangements allowing an increase in the coefficient of use of the active materials of the electrodes by achieving a better distribution of the reagents within the electrodes.
DISCLOSURE OF THE INVENTION
The subject of the present invention is a leadacid battery containing:
a positive electrode containing lead oxide as active material,
a negative electrode containing lead sponge as active material,
an electrolyte formed of a solution of sulphuric acid,
a separator element between the positive electrode and the negative electrode, and
means for applying a stress to the entire assembly p
Alzieu Jean
Robert Jack
Burns Doane Swecker and Mathis LLP
Centre National de la Recherche Scientifique
Ryan Patrick
Tsang-Foster Susy
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