Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Separator – retainer or spacer insulating structure
Reexamination Certificate
2002-02-14
2004-03-09
Tsang-Foster, Susy (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Separator, retainer or spacer insulating structure
Reexamination Certificate
active
06703161
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to novel porous separators for electric lead-acid storage batteries. According to another aspect the invention relates to lead-acid storage batteries comprising such a novel separator.
BACKGROUND OF THE INVENTION
Basically, battery separators serve as electronic insulators and ionic conductors, i.e. they prevent the direct electronic contact of electrodes of opposite polarity while enabling the ionic current between them. To meet these two functions, separators are usually porous insulators with pores as small as possible to prevent electronic short circuits by dendrites or plate particles and with a porosity as high as possible to minimize the internal battery resistance. In lead-acid batteries, the separator also determines the proper plate spacing and thereby defines the amount of electrolyte which participates in the cell reaction. The separator has to be stable over the life time of the battery, i.e. to withstand the highly aggressive electrolyte and oxidative environment.
Beyond these basically passive functions, separators in lead-acid batteries can also actively affect the battery performance in many ways. In valve regulated lead-acid (VRLA) batteries they additionally determine properties like oxygen transfer, electrolyte distribution and plate expansion. Due to their outstanding influence on the performance of VRLA batteries the separator is even referred to as the “third electrode” or “fourth active material” (Nelson B., Batteries International, April 2000, 51-60).
VRLA stands for valve-regulated lead-acid batteries which are also called sealed or recombinant batteries. In VRLA batteries oxygen, which is generated during charging at the positive electrode, is reduced at the negative electrode. Thus the battery can be charged and even be overcharged without water consumption and is therefore theoretically maintenance-free. The formation of hydrogen at the negative electrode is suppressed, for instance by using larger negative than positive plates in order to generate oxygen at the positive plate before the negative plate is fully charged.
For VRLA batteries two technologies are predominant, i.e. batteries with an absorptive glassmat (AGM) and gel batteries. In batteries with AGM, the absorptive glassmat immobilizes the electrolyte and simultaneously functions as a separator. In gel batteries, the acid is immobilized by means of fumed silica and an additional separator is required to fix the plate distance and to prevent electronic shorts. Compared to AGM batteries, the manufacturing cost of gel batteries is considered to be higher and their specific power is lower due to a higher internal resistance.
In AGM batteries the electrolyte is completely absorbed by the glass mat. AGM separators have a very high porosity in excess of 90%. This high porosity together with a good wettability is reflected in a very high acid absorption and low electrical resistance in the battery, the acid saturation of AGM separators is usually in a range of 85 to 95%. This increases the effective electrical resistance versus fully saturated separators but creates open channels of relatively large pores that enable a very efficient oxygen transfer from the positive to the negative plate. The average pore size of AGM separators is usually within the range of 3 to 15 &mgr;m with an anisotropic distribution, i.e. pore sizes of about 0.5 to 5 &mgr;m in the x-y-plane of the separator which is the plane parallel to the electrode plates and pore sizes of about 10 to 25 &mgr;m in the z-direction perpendicular to the electrodes. A potential drawback of the high oxygen transfer rate is the so-called thermal runaway, caused by the self-propelling exothermic consumption of oxygen at the negative plate and a premature capacity loss by undercharging of the negative plate.
Due to the relatively large pores and the good wettability, the wicking rate (speed of acid pick-up) of AGM is fairly high which facilitates the filling process of batteries. On the other hand a drawback of the large pores of AGM is the risk of internal short circuits caused by dendrite growth through the separator especially at low electrode plate distance and especially in cycling applications.
It was suggested to include thin microporous sheets as part of the separator system in order to control dendrite formation and oxygen transfer to the negative plate. An example of such a microporous separator is the DuraGard™ separator introduced by ENTEK International LLC (Weighall M. J.; ALABC Project R/S-001, October 2000). DuraGard™ has an average pore size of 0.014 &mgr;m and a membrane thickness of 0.10 mm (Fraser-Bell G., New developments in Polyethylene separators, Presentation at the 7
th
European Lead Battery Conference, September 19-22, 2000, Dublin, Ireland).
However, if the separator has a very small pore size, it will act as a barrier to oxygen transport, and the gas will rise to the top of the plates and go over the top or around the sides of the barrier layer of the separator. This means that only the top and edges of the negative plate will participate in the oxygen reduction reaction. This is not a desirable situation as the oxidation of the pure lead in the negative plates is a highly exothermic reaction, resulting in a build up of heat in a very localised area. This results in increased danger of premature water loss and deactivation of the negative plates. It was therefore suggested to use a separator with a larger average pore size, for example a microporous PVC separator having a mean pore size of 5 &mgr;m and a thickness of 0.57 mm, sandwiched between two layers of AGM with a thickness of 0.52 mm at 10 kPa (Weighall M. J., see above; Lambert U., A study of the effects of compressive forces applied onto the plate stack on cyclability of AGM VRLA batteries, 5
th
ALABC Members and Contractors' Conference Proceedings, Nice, France, Mar. 28-31, 2000). Although this separator configuration might provide for acceptable oxygen transfer and improved resistance to dendrite growth when compared to AGM separators, the pore size is still within the range of the particle size of the active material of the electrodes and thus the risk of metal particle deposition and subsequent shorting is still existing.
U.S. Pat. No. 3,862,861 describes sealed lead-acid batteries which preferably comprise separators made from fiber glass material. The edges of the negative plate are not covered by free electrolyte, i.e. the lead sponge is directly exposed to the oxygen which is said to readily react with the lead.
U.S. Pat. No. 6,124,059 describes microporous separators for sealed lead-acid batteries which essentially consist of a homogeneous mixture of a thermoplastic polymer, filler and optionally a plasticizer. The separators contain at least 20 percent by volume of pyrogenic silica and permit diffusion of the oxygen to the negative electrode. Oxygen diffusion presumable takes place through large pores which are not filled with acid.
SUMMARY OF THE INVENTION
The present invention relates to a battery separator for a lead-acid battery comprising at least one first fibrous layer, at least one second fibrous layer, and at least one microporous polymer layer which is sandwiched between two fibrous layers, wherein said microporous polymer layer has an average pore size of less than 1 &mgr;m, and wherein said at least one first fibrous layer has a thickness of at least 0.6 mm.
It is the object of the invention to provide a battery separator for a lead-acid battery with improved resistance to dendrite formation without impairing the reduction of oxygen at the negative electrode.
It is a further object of the invention to provide a battery separator which can be produced in a cost effective manner.
It is still a further object of the invention to provide an improved valve-regulated lead-acid battery with high cycling performance.
REFERENCES:
patent: 3862861 (1975-01-01), McClelland et al.
patent: 4137377 (1979-01-01), McClelland
patent: 4448862 (1984-05-01), Schulte et al.
patent: 4908282 (1990-03-01),
Daramic, Inc.
Nields & Lemack
Tsang-Foster Susy
LandOfFree
Multilayer separator for lead-acid batteries does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Multilayer separator for lead-acid batteries, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Multilayer separator for lead-acid batteries will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3238183