Patent Application
20110305927
The field of the invention is energy storage devices, and more particularly bipolar lead acid batteries (BLAB) and valve regulated bipolar lead acid batteries (VR-BLAB).
The general concept of bipolar lead acid batteries is well known for several decades, and the first operable batteries made from single lead sheets were reported by Peter Kapitsa in the 1930's. Despite their apparent simplicity, bipolar thin film batteries provide numerous significant advantages. For example, as the internal path length is relatively short and as the electrode area relatively large, internal resistance is typically very low, resulting in rapid charge and discharge cycles at minimal heat generation. It is these and other advantages that make bipolar lead acid batteries attractive for hybrid vehicles and regenerative braking systems on automobiles and locomotives. Moreover, due to their bipolar configuration, the weight is reduced and production is simplified.
However, several drawbacks have so far prevented widespread use of bipolar lead acid batteries. Among other things, lead is a fairly poor construction material as it creeps under load (i.e., a sheet of lead will slump under its own weight unless attached to a stronger support such as steel), and extra material is often needed to support the lead resulting in an increased weight. Still further, creeping of lead typically leads to surface cracking and formation of crevices, which will in most cases accelerate corrosion (stress corrosion).
To reduce the overall weight of a bipolar electrode, a non-conductive carrier material can be used to which the active electrode material can then be applied as described, for example, in EP 0 607 620 where a plastic honeycomb structure was filled with lead, or in EP 0 848 442 where two opposing and electrically connected webbings were arranged on either side of a non-conductive plastic plate. Similarly, as taught in U.S. Pat. No. 5,126,218, electrically conductive plugs comprising sub-stoichiometric titanium dioxide materials were used to provide a non-conductive light-weight carrier with conductive pathways connecting both sides of the carrier, while U.S. Pat. No. 3,819,412 teaches use of lead clamps for the same purpose. Alternatively, it was described to directly incorporate conductive materials into an otherwise non-conductive polymeric plate to so form a bipolar electrode as described in GB 2 371 402. While such electrode configurations advantageously increase the potential capacity/weight ratio, several drawbacks nevertheless remain.
For example, anodic corrosion of lead is a common failure mode for conventional lead acid batteries and well known to the person of ordinary skill in the art. Examination often reveals fractures of the supporting grids along stress corrosion cracks. When the lead grid fractures, active material is typically dislodged from the grid and accumulates in the mud space at the bottom of the cells, eventually forming a bridge that causes short circuits in the cells. To overcome such problems, large industrial lead electrodes (e.g., as those used in commercial electrosynthesis) often use steel plates inserted and welded to the lead sheets to eliminate creep. Such electrodes advantageously increase lifetime of the electrochemical device in strong sulfuric acid and often delay or even prevent stress corrosion. However, such configurations are typically not desirable for bipolar lead acid batteries due to the substantial weight and dimensional requirements.
Furthermore, one of the major factors contributing to failure of lead acid batteries with bipolar electrodes is the migration of electrolyte through a seal around the edges of the bipolar electrode (bipole), which is typically driven by the Marangoni effect. This sealing problem is especially persistent on the positive side of the bipole which has turned out to be virtually impossible to seal in a reliable and permanent fashion. In such failure event, the electrolyte creates a conductive bridge between the positive and negative sides of the bipole, and numerous attempts have been undertaken to more tightly seal the bipole. However, due to many factors, and especially thermal expansion, the Marangoni effect, and the relatively aggressive environment in lead acid batteries, such attempts have not yielded satisfactory results. Still further, the positive electrode material tends to shed over time and accumulate in the space below the electrode, ultimately leading to short circuits and battery malfunction.
Therefore, there is still an unmet need to provide improved battery configurations and methods to improve life time and cycle characteristics in lead acid batteries, and especially bipolar lead acid batteries.
The present invention is directed to various BLAB configurations and methods that overcome numerous disadvantages of heretofore known BLABs. More specifically, the BLABs presented herein comprise a compression resistant separator that retains the electrolyte in a gelled form, which not only allows for substantial compression of the cell stack (thus eliminating shedding of positive active materials), but also allows for operation of the BLAB without problems associated with electrolyte migration (even where the bipole fails to have any seal to protect against solvent migration). Still further, as the electrodes in preferred BLABs are configured as quasi-bipolar electrodes, problems otherwise associated with pinhole defects in the electrode are avoided and power-to-weight ratio is substantially increased.
In one exemplary aspect of the inventive subject matter, a bipolar lead acid battery includes a first and a second bipolar electrode that are separated by a compression resistant separator that further includes an electrolyte in a gelled form. Most preferably, the separator comprises pyrogenic silica and an inert filler material, and/or the electrolyte is gelled to a degree sufficient to allow operation of the battery without sealing of a cell formed by the bipolar electrodes. In still further preferred aspects, the cell comprises a void space between the bipolar electrodes and a thermally conductive material is disposed in at least part of the void space to help dissipate heat from within the electrode stack. Moreover, it is generally preferred that a one-way valve (e.g., duckbill valve) is coupled to the cell to thereby allow venting of a gas from the cell.
Additionally, it is preferred that at least one of the electrodes in contemplated bipolar batteries is a quasi-bipolar electrode. For example, suitable quasi-bipolar electrodes comprise a non-conductive carrier with openings formed in the carrier, wherein a conductive material is disposed in at least some of the openings, and wherein thin lead foils are laminated to both surfaces of the electrode.
Therefore, in another exemplary aspect of the inventive subject matter, a bipolar lead acid battery includes a quasi-bipolar electrode having a non-conductive carrier with a plurality of openings formed between a first and a second surface of the carrier. A conductive material is disposed in the plurality of openings, and a first and a second lead foil are coupled to the first and second surfaces, respectively. Most preferably, a layer of positive active material is coupled to the first foil, and a layer of negative active material coupled to the second foil, wherein the layer of negative active material may further comprise a compression resistant spacer structure. It is still further contemplated that in such batteries a first and a second compression resistant separator are coupled to the layer of positive active material and the layer of negative active material, respectively, wherein first and second compression resistant separators comprise the electrolyte in a gelled form.
While not limiting to the inventive subject matter, it is typically preferred that the non-conductive carrier is manufactured from a synthetic polymer and/or a ceramic material, and that the conductive material comprises lead. Similarly, it is generally preferred that the spacer structure is made from a synthetic polymer and/or a ceramic. With respect to the compression resistant separators, it is typically preferred that the separator material includes pyrogenic silica and an inert filler material. Furthermore, and where a second quasi-bipolar electrode is coupled to the quasi-bipolar electrode to form a cell, the battery may also include a one-way valve to allow venting of a gas from the cell, and/or a thermally conductive material disposed in at least part of a void space formed in the cell.
Therefore, and viewed from another perspective, the inventors contemplate a method of reducing migration of an electrolyte in a battery in which a compression resistant separator that comprises an electrolyte in a gelled form is placed between a positive active material of a first bipolar electrode and a negative active material of a second bipolar electrode. The first and second bipolar electrodes and the separator are then aligned to form a cell of the battery, and mechanical pressure of at least 10 kPa (most preferably between 20 kPa to 150 kPa) is applied to the first and second electrodes.
As before, it is generally preferred that the bipolar electrode is configured as a quasi-bipolar electrode, and particularly such that the bipolar electrode comprises a carrier having a first and a second surface, and a first and a second lead foil coupled to the first and second surfaces, respectively. It is still further preferred that the compression resistant separator comprises pyrogenic silica and an inert filler material, and that the negative active material further comprises a compression resistant spacer structure. Additionally, the cell may have a void space between the first and second bipolar electrodes, which is preferably filled with a thermally conductive material. Furthermore, and where desired, a one-way valve may be coupled to the cell to thereby allow venting of gas from the cell.
Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention.
According to the present invention a bipolar lead acid battery, and most preferably a valve regulated BLAB, is constructed in a way that solves the heretofore known problems of bipole leakage, stress corrosion, and relatively high weight in a simple and elegant manner.
In especially preferred methods and devices, the separator of the batteries comprises a material that gels the electrolyte and so prevents leakage around the bipole. Most preferably, such separators are configured to withstand compression to still further improve operational parameters of the battery. Where desired, contemplated batteries will also include a quasi-bipole electrode in which a (typically non-conductive) light-weight plate-shaped material has a plurality of windows. The windows are then filled with lead and the plate is laminated between two thin lead films to so obtain a composite structure that can serve as a basis for the bipole construction.
While not limiting the inventive subject matter, it is further especially preferred that the negative active material (NAM) is conductively coupled to the bipole and includes a grid or otherwise porous structure such that the grid or structure retains the NAM in contact with the bipole while preventing the NAM to be compressed. Preferably, such grid (e.g., skeletal structure) will advantageously have the same height as the NAM thickness at fully charged state. Therefore, the bipole can be compressed at both sides to a desirable pressure without negatively affecting the electrode performance. Void space filling and sealing of the bipole is then implemented using thermally conductive materials, and most preferably using an adhesive material to so form a path for heat dissipation. Moreover, it is generally preferred that such batteries further include a unidirectional valve that allows for independent venting of different quantities of gas from different cells in the battery. In such case, it is especially preferred that the unidirectional valves is a duckbill valve and vents into a common space from which the vented gases my then be released via one or more controlled valves.
For example,
Each of the bipolar electrodes is preferably assembled with a frame to allow stacking of the electrodes together with the separators. In especially preferred aspects, the so formed cells will have void spaces that would normally have to be sealed to avoid leakage of the electrolyte from the cell, and migration of the electrolyte. However, as the electrolyte is retained in the separator in a gelled form, leakage is entirely avoided and the battery can be operated without sealing the cells. Moreover, as the separator is compression resistant, significant force can be exerted onto the terminal electrodes to so compress the cell stack and avoid positive electrode material shedding. Where desirable, at least some of the void spaces in the cells are then filed with a thermally conductive material 260 to facilitate heat transfer from the inside of the battery to the outside. Moreover, it is generally preferred that each cell is provided with a one-way valve 252 to allow for venting of gas (predominantly H2) into a common space above the cells, which can then be vented via a common valve 250 to transfer the gas to a location outside the battery.
Consequently, it should be appreciated that a bipolar (and most preferably a quasi-bipolar) lead acid battery can be produced in which a first and a second (quasi-) bipolar electrode are separated by a compression resistant separator in which an electrolyte is retained in a gelled form. Most preferably, such batteries will therefore include a quasi-bipolar electrode that is formed from a non-conductive carrier having a plurality of openings between a first and a second surface of the carrier, and wherein a conductive material is disposed in the openings. While not limiting to the inventive subject matter, a first and a second lead foil coupled to the first and second surfaces, respectively, and a layer of positive active material is coupled to the first foil, while a layer of negative active material is coupled to the second foil. It is still further generally preferred that the layer of negative active material also include a compression resistant spacer structure. Typically, contemplated batteries will have a first and second compression resistant separator coupled to the layer of positive active material and the layer of negative active material, respectively, wherein first and second compression resistant separators comprises the electrolyte in a gelled form.
The term “compression resistant separator” as used herein refers to a separator that can withstand mechanical compression of at least 30 kPa in a battery stack without loss of thickness or with a loss in thickness that is equal or less than 10%. Most typically, however, preferred compression resistant separators will withstand pressures of at least 50 kPa, and even more typically at least 100 kPa in a battery stack with a loss in thickness that is equal or less than 10%, more preferably equal or less than 5%, and most preferably equal or less than 3%. Consequently, preferred separators will comprise ceramic or polymeric materials suitable to withstand such pressures.
Moreover, it is particularly preferred that the separators according to the inventive subject matters also have the capability to retain the electrolyte while in contact with the active materials of the battery. Such capability is preferably achieved by retention of the electrolyte in a gelled form, wherein all known gelling agents are deemed suitable for use herein. For example, suitable gelling agents may be organic polymers or inorganic materials. In one particularly preferred aspect of the inventive subject matter, the electrolyte is immobilized in a micro-porous gel forming separator to so prevent conductive bridges between the positive and negative sides of the bipole and thus enables the bipolar battery to have a calendar and cyclic life comparable or better than that of a conventional lead acid battery.
Among other appropriate separators, the inventors have discovered that an AJS (acid jelling separator) (e.g., commercially available from Daramic, LLC) was not only capable of withstanding compressive forces but also capable of arresting migration of the electrolyte beyond the electrode boundary. Such advantage has not been recognized in the field of BLABs. Indeed, the inventors discovered that using such electrolyte immobilization a BLAB can be made that can continuously operate (i.e., over several charge/discharge cycles) without any sealing of the cells in the BLAB. The Daramic AJS is a synthetic micro-porous material filled with 6 to 8 wt % of dry pyrogenic silica. When the AJS is saturated with 1.28 s.g. (specific gravity) electrolyte, its silica component reacts with the latter to form a gel. Thus, it is contemplated that the electrolyte becomes immobilized by hydrogen bonding or Van-der-Waals forces of gel and/or by pores in the separator such that even in air nothing leaks. The limited mobility of the gel electrolyte prevents conductive bridges to occur between the positive and negative sides of the bipole. Further suitable materials are described in U.S. Pat. No. 6,124,059, which is incorporated by reference herein. However, in alternative aspects of the inventive subject matter, it is noted that all combinations of dimensionally stable materials (i.e., materials that can withstand compression at forces of 100 kPa at a loss of thickness of less than 10%, and more preferably less than 5%) with a gelled electrolyte are considered suitable for use herein.
It should be noted that heretofore known monopolar VRLA (valve regulated lead-acid) batteries do not have an issue with electrolyte bridges shunting the active materials (as no bipole electrode is present), and even if the separator in the monopolar configuration would leak, the battery would not be affected. In contrast, bipolar batteries have significant leakage issues that have not been reliably remedied with heretofore known technologies. Thus, immobilization of the electrolyte using AJS provides a unique and effective solution. Viewed from a different perspective, it should be appreciated that the AJS material in bipolar configurations immobilizes both the catholyte and the anolyte by gelling and retention in the micropores.
It should be especially appreciated that a further important advantage of the AJS material is its very limited dimensional yield under the compression force that are typically applied to the bipoles in lead acid batteries, and especially VRLAs. Unlike the ordinarily used AGM (fibrous absorbent glass mat) separators that often yield under compression, the AJS material allows compression the active materials to the desired pressure of 30 to 100 kPa, and even higher.
With respect to the compression of the separator materials it should be noted that it is almost impossible to apply the above noted compression forces during the manufacture of conventional monopolar batteries without destruction to the battery as in most monopolar batteries construction groups of monopoles are connected together with the separators already inserted. The groups of plates are then pushed into the cavity of a plastic box to make consecutive cells. Therefore, to push the electrodes into the box some clearance must be provided or friction would rub out some of the active material. Any pressure to keep the electrodes from shedding is thus generated during cycling as the box is dimensionally stable.
In the case of a BLAB pressure can be applied across all the cells after assembly before cycling. Each electrode plus its separators is assembled one on top of the other and pressed tight, and the packing pressure can be set using an external jig such that no shedding will occur during assembly and during cycling. It should be especially appreciated that shedding is most likely during charge when the active material shrinks (consider the volume occupied by lead sulfate compared to lead dioxide in the anode and lead compared to lead sulfate in the cathode). Therefore, the particular and heretofore unrecognized advantage of contemplated separator materials in BLAB batteries is that the materials do not yield under compression and/or drip electrolyte as the volume is changed due to change in thickness of the active material. Conventional gelling electrolytes fail to afford these advantages.
While such compression is desirable for positive active material (PAM, typically made from a combination of lead oxides and basic lead sulfates) to mitigate its shedding, it is detrimental to negative active material (NAM) by reducing its porosity and thickness. To circumvent at least some of the problems associated with NAM compression, the inventors have now incorporated a skeletal structure to which the NAM is coupled and which has contact with the negative electrode surface.
In particularly preferred aspects of the inventive subject matter, the skeletal structure comprises a grid that is made of a glass fiber mesh of the thickness equal to the thickness of the NAM. The negative paste is then filled into the cavities of the mesh even with its surface facing the separator (there is no over-pasting of the grid wires). Such design enables sheltering of the NAM from the compression exerted by the AJS. The AJS, while having a good interface with NAM, is stopped from exerting the force on the latter. Of course, it should be noted that numerous alternative skeletal structures are also suitable, including a perforated plate and other porous and structurally stable materials (typically non-conductive). Most preferably, the skeletal structure is made of a material that is stable in sulfuric acid and has the required mechanical properties (e.g., thermoplastic materials such as ABS, PP, or PC). The skeletal material will typically have the same thickness as the NAM at the 100% state of charge to so act as a buttress between a separator NAM contained in the void space of the skeletal material.
Consequently, and viewed from a different perspective, it is noted that the inventors also contemplate a method of reducing or even entirely eliminating migration of electrolyte in a bipolar lead acid battery by placing between a positive active material of a first bipolar electrode and a negative active material of a second bipolar electrode a compression resistant separator that comprises the electrolyte in a gelled form. Once the first and second bipolar electrodes and the separator are assembled or otherwise aligned to form a cell of the battery, mechanical pressure of at least 10 kPa is applied to the first and second bipolar electrode to so form a component of a BLAB. In most cases, higher pressure (e.g., between 20 kPa to 150 kPa, and even higher) is applied to allow for longer lifetime and reduce positive active material shedding.
It should further be noted that where the pressure is at or above 10 kPa, the negative active material can be protected from undesirable compaction. Most typically, such protection is achieved by including a compression resistant spacer structure such that the negative active material is disposed in void spaces of the spacer structure. There are numerous configurations and materials suitable for such spacer structure, and all of them are deemed suitable for use herein. However, it is especially preferred that the spacer structure is configured as a grid and manufactured from an acid resistant polymeric material. With respect to compression resistance of the spacer structure it is generally preferred that the spacer structure can withstand pressure of at least 100 kPa at a loss in thickness of less than 10%, and more preferably less than 5% (supra).
It is further contemplated that in the batteries according to the inventive subject matter all known bipole electrodes can be used. However, it is generally preferred that the bipole electrode is configured as a quasi-bipole. Such configuration advantageously overcomes performance decay of heretofore known bipolar batteries due to pinhole formation in the bipoles. In one preferred embodiment, two conventional lead foils are laminated to a non-conductive substrate (e.g., made of a thin plastic material) on each side of the substrate. The lead foils are then electrically connected with each other through perforations in a plastic carrier as schematically illustrated in
Remarkably, such quasi-bipoles have a rather uniform flow of current and low Ohmic resistance. Moreover, there is also an advantage of using pure lead foil on the positive side as pure lead has the best resistance to anodic corrosion. Furthermore, the compression of the battery stack does considerably mitigate the corrosion activity on the positive side due to a dense, fissure free protective layer of the lead dioxide formed on the surface of the lead. Among further advantages, the quasi-bipoles are very easy to assemble into a battery stack owing to the plastic substrate compatibility with epoxy.
It should still further be appreciated that as the leakage of electrolyte from the AJS has allowed a dry seal surface, voids around the bipoles can now be filled with thermally conductive materials. For example, suitable materials include industrially available adhesive materials such as epoxy, silicon, or acrylic, which are then primarily used to produce a heat dissipation path rather than for perimeter sealing. Many other temperature stable materials which work between −30 to +70° C. as adhesives are also suitable as fillers. The filler materials provide a good contact and hence conduction path for the heat generated by the inner components of the battery.
In yet another aspect of the inventive subject matter, it is noted that in traditional bipolar VRLA (valve regulated lead acid) batteries with thin cells it is often very difficult and costly to provide pressure relieve valves (PRV) for each cell as it is believed to be necessary for the oxygen recombination cycle. It was recently successfully proven by such VRLA batteries as Exide “Optima” that a single PRV can handle multiple cells connected in series and assembled into the common housing. The Optima battery has six wound 2 volts cells of monopolar design that are assembled into a common body with vent channels leading to a PRV. However, the inventors contemplate that such configuration may also be extended to bipolar configurations that may comprise at least 24 cells (or more likely, as many as 100 or more cells).
In an effort to implement regulated venting of a large number of cells, the inventors have used thin-walled small diameter plastic tubes, with each cell receiving a single tube. A small portion of each tube was protruding upward ending into a longitudinal channel of approximately 5×4 mm cross section molded in the battery lid. The length of the channel depended on the quantity of cells in the battery stack. That common channel was then connected to a single PRV. While this design proved to be functional a further improvement was introduced as follows. The top 2 to 4 mm of each tube were collapsed in a press with heated platens to form two flat walls touching each other to so form a duckbill valve. The so modified tube was able to perform as a one-way relieve feature for each of the cells by letting the excess gas out of the cell or cells into the channel while not allowing the gas from the channel to get into the other cells. This simple yet effective modification to the vent tubes has noticeably improved the voltage balance of the cells during charging of the high voltage bipolar battery. The flattened at the end thin wall plastic tubes acting as a one-way gas relieving feature by letting the excess gas out of the cell, while not allowing the gas to get into the other cells.
A VR-BLAB was constructed similar to that depicted in
To test cycling efficiency, two different cycling programs were performed using two VR-BLABs according to the inventive subject matter and a comparative commercially available VRLA, wherein cycling was performed using a moderate profile and an aggressive profile. (a) Moderate cycling program was performed as follows: Charging in two stages, with the 1st Stage at constant current 0.8 A, voltage cutoff 14.4V, and the 2nd stage at constant voltage 14.4V until capacity equals 1.57 Ah. Charging was followed by a 10 min rest period. Discharge was at 1 A for 1.5 h with voltage cutoff at 10.5V, and cycling was stopped at 2.1 Ah (70% rated capacity at C/3). (b) Aggressive cycling program was performed as follows: Charging in two stages, with the 1st stage at constant current 0.9 A, voltage cutoff 14.7V, and the 2nd stage at constant voltage 14.7V, time cutoff 4.5 h, followed by 10 min rest period. Discharge was at 1.7 A until voltage cutoff at 10.5V, and cycling was stopped at 2.45 Ah (70% rated capacity at C/2). The results for two BLABs according to the inventive subject matter are depicted in the tables of
However, it should be noted that the batteries according to the inventive subject matter have a significantly higher energy density. Most typically, contemplated batteries will achieve power densities of at least 35 Wh/kg, more typically at least 38 Wh/kg, and most typically at least 40 Wh/kg. In contrast, current monoblock technology will allow for energy densities of 35 Wh/kg in a best-case scenario.
Thus, specific embodiments and applications of improved bipolar lead acid batteries have been disclosed. It should be apparent, however, to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive subject matter, therefore, is not to be restricted except in the spirit of the appended claims. Moreover, in interpreting both the specification and the claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms “comprises” and “comprising” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced.
This application claims priority to our copending U.S. provisional application with the Ser. No. 61/088,911, which was filed Aug. 14, 2008.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US09/42122 | 4/29/2009 | WO | 00 | 4/11/2011 |
| Number | Date | Country | |
|---|---|---|---|
| 61088911 | Aug 2008 | US |
