Patent Application
20110318629
The present invention relates to a separator for a flooded or wet cell lead-acid electrochemical battery.
A typical flooded lead-acid battery includes positive and negative plates separated by porous separators and immersed in an electrolyte. Positive and negative active materials are manufactured as pastes that are coated on the positive and negative electrode grids, respectively, forming positive and negative plates. The electrode grids, while primarily constructed of lead, are often alloyed with antimony, calcium, or tin to improve their mechanical characteristics. Antimony is generally a preferred alloying material for deep discharge batteries. The positive and negative active material pastes generally comprise lead oxide (PbO or lead (II) oxide). The electrolyte typically includes an aqueous acid solution, most commonly sulfuric acid (H2SO4). Once the battery is assembled, the battery undergoes a formation step in which a charge is applied to the battery in order to convert the lead oxide of the positive plates to lead dioxide (PbO2 or lead (IV) oxide) and the lead oxide of the negative plates to lead (Pb).
After the formation step, a battery may be repeatedly discharged and charged in operation. During battery discharge, the positive and negative active materials react with the sulfuric acid of the electrolyte to form lead (II) sulfate (PbSO4). By the reaction of the sulfuric acid with the positive and negative active materials, a portion of the sulfuric acid of the electrolyte is consumed. However, under normal conditions, sulfuric acid returns to the electrolyte upon battery charging. The reaction of the positive and negative active materials with the sulfuric acid of the electrolyte during discharge may be represented by the following formulae.
Reaction at the negative electrode:
Pb(s)+SO42−(aq)PbSO4(s)+2e−
Reaction at the positive electrode:
PbO2(s)+SO42−(aq)+4H++2e−PbSO4(s)+2H2O(l)
As shown by these formulae, during discharge, electrical energy is generated, making the flooded lead-acid battery a suitable power source for many applications. For example, flooded lead-acid batteries may be used as power sources for electric vehicles such as forklifts, golf cars, electric cars, and hybrid cars. Flooded lead-acid batteries are also used for emergency or standby power supplies, or to store power generated by photovoltaic systems.
During operation of a flooded lead-acid battery using an electrode grid alloyed with antimony, antimony may leach or migrate out of the electrode grid. Once the antimony deposits on the surface of negative electrode, it will change potential of negative electrode and cause battery to be overcharged easily during application. This will undesirably shorten battery life. Rubber is known to be an effective barrier for preventing or delaying the antimony from leaching from the positive electrode to the negative electrode. Accordingly, some separators for flooded lead acid batteries include a glass mat (i.e., a glass fiber mat) against the positive electrode and a porous rubber sheet between the glass mat and the negative electrode. However, when immersed in the acidic electrolyte of a flooded lead-acid battery, a rubber separator sheet may oxidize and crack. When a rubber separator cracks, lead dendrites may grow from the negative to the positive electrode, thus causing the battery to short circuit. Accordingly, some have proposed using thicker rubber sheets for lead-acid batteries. However, this increases the cost of the separators, increases the internal resistance, and additionally, does not prevent the rubber separator sheet from oxidizing and splitting.
Due to the expense of rubber, some manufacturers have abandoned the use of rubber altogether, instead, preferring to use a polymer separator for flooded lead-acid batteries. A polymer separator is much sturdier than a rubber separator, and thus does not tend to split when used in a flooded-lead acid battery. Such a separator may prevent the short circuits caused by lead dendrite growth, but does not prevent antimony migration. Thus, batteries using only a polymer separator have shortened battery life.
Alternatively, some have attempted to make and use a mixed rubber and polymer separator. Such separators generally include a porous polymer matrix filled with rubber. It was believed that these mixed separators would have improved strength and prevent antimony from transferring to negative electrode. While such separators are more sturdy than rubber alone, and additionally may prevent some antimony leaching, they allow more antimony transfer than a rubber separator alone. Accordingly, due to antimony leaching, flooded lead-acid batteries using a mixed rubber polymer separator have a reduced battery life.
An embodiment of the present invention is directed to a separator for a flooded deep discharge lead-acid battery. The separator includes a first layer made of a rubber material, a rubber layer, and a second layer made of a polymer material, a polymer layer.
In embodiments of the present invention, the rubber material may be natural rubber and the polymer material may be polyethylene, polyvinyl chloride, or polyester.
In embodiments of the present invention, the rubber layer may have a backweb having a first side and a second side, the first side being flat and the second side having a plurality of ridges extending therefrom. In embodiments of the present invention, the separator may further include a glass mat. The glass mat may be adjacent to the plurality of ridges and the polymer layer may be adjacent to the first side.
In embodiments of the present invention, the polymer layer may be provided as an envelope adapted to contain and surround an electrode.
In embodiments of the present invention, the polymer layer may include a backweb having a first side and a second side, the first side being flat and the second side having a plurality of ridges extending therefrom. A glass mat may be adjacent to the plurality of ridges of the polymer layer and the rubber layer may be adjacent to the first side.
In embodiments of the present invention, the rubber layer may include foamed rubber. The rubber layer may be adjacent to the plurality of ridges of the polymer layer.
The accompanying drawings, together with the specification, illustrate various aspects and embodiments of the invention:
According to one embodiment of the invention, a separator for a flooded lead-acid battery includes a first layer and a second layer. The first layer may be made of a rubber material.
The second layer may be made of a polymer material. The rubber layer may prevent or reduce antimony transfer, while the polymer layer may prevent short circuits caused by lead dendrite growth.
In one embodiment, as shown schematically in
Suitable polymers for the polymer layer include polyethylene, polyvinyl chloride, polypropylene, copolymers of ethylene and propylene, phenol formaldehyde (PF) resin, polyester, copolymers of styrene and butadiene, copolymers of a nitrile and butadiene, and cellulose based polymers. The polymer layer should be sufficiently porous to allow the electrolyte to be able to transfer through the layer to the negative electrode. In an exemplary embodiment, polyethylene is used for the polymer layer. Suitable rubbers for the rubber layer include natural rubber, synthetic rubber (isoprene), and ethylene propylene diene monomer (EPDM) rubber. As is known in the art, the rubber layer may be partially cross-linked by an electron beam unit. Using this or a similar treatment, a porous rubber layer may be formed. The rubber layer should be sufficiently porous to allow the electrolyte to be able to transfer through the layer to the negative electrode. In an exemplary embodiment, the rubber layer includes natural rubber.
As shown in
Most rubber layer backwebs have a thickness of 0.013 to 0.017 inches. While a rubber layer 140 having a thinner backweb 142 may be more prone to cracking and splitting in the acidic electrolyte, a cracked or split rubber layer 140 still substantially prevents or reduces antimony transfer. Accordingly, as a polymer layer 150 physically prevents lead transfer in separators of the present invention, a thinner rubber layer 140 backweb 142 may be used. For instance, a rubber layer 140 having a backweb 142 of 0.008 to 0.012 inches may be used. The polymer layer 150 may have a thickness of less than 0.010 inches. However, any suitable thicknesses may be used.
The present invention will now be described with reference to the following examples. These examples are provided for illustrative purposes only, and are not intended to limit the scope of the present invention.
Positive and negative electrodes for lead-acid batteries were formed according to customary practices. Separators according to an embodiment of the present invention were formed and placed between each positive and negative plates in a cell. The separators used in Example 1 included a rubber sheet having a backweb and ribs, a flat porous polyethylene sheet, and a glass mat. The separators were assembled with the rubber sheet in the middle, the ribs facing the positive electrode.
Cells were formed as in Example 1 except that a traditional lead-acid separator was used. The traditional lead-acid separators used in Comparative Example 1 included a rubber sheet having a backweb and ribs and a glass mat. The separators were assembled with the glass mat adjacent to the positive electrode and the flat side of the rubber sheet backweb adjacent to the negative electrode.
For the tests, the cells were repeatedly discharged and charged using standard procedures as established by Battery Council International. The corrected capacity and end of charge voltage of Example 1 and Comparative Example 1 were measured after each cycle. As expected, there were no substantial changes to the capacity or end of charge voltage in Example 1. In other words, battery performed was not negatively impacted by the inclusion of an additional layer. However, the use of an additional membrane in a battery, i.e., the use of both a rubber layer and a polymer layer, increases the resistance of a battery. While this may negatively affect a battery if used for high current applications, the increase in resistance generally does not affect the performance of the battery. It is expected that Example 1 will have a significantly higher cycle life than Comparative Example 1. In other words, as the rubber layer of Comparative Example 1 oxidizes and cracks, a short circuit will occur as lead migrates from the negative electrode to the positive electrode. However, in Example 1, even if the rubber layer oxidizes and cracks, the polymer layer should prevent lead migration, and thus should prevent a short circuit.
While the present invention has been illustrated and described with reference to certain exemplary embodiments, those of ordinary skill in the art would appreciate that various modifications and changes can be made to the described embodiments without departing from the spirit and scope of the present invention, as defined in the following claims.
