ACID STRATIFICATION MITIGATION, ELECTROLYTES, DEVICES, AND METHODS RELATED THERETO

Abstract

Methods of reducing acid stratification with an acid-soluble and acid-stable polymer with a high molecular weight are disclosed herein. Electrolytes and separators for an energy storage device are disclosed herein. The separator includes a coating containing an acid-soluble and acid-stable polymer with a high molecular weight. The electrolyte includes sulfuric acid and an acid-soluble and acid-stable polymer with a high molecular weight. Methods of making the separators disclosed herein and methods of making batteries are also disclosed herein.

Claims

1. A method of reducing acid stratification in a battery, the method comprising: introducing an acid-soluble and acid-stable polymer with a high molecular weight into an electrolyte of a battery, to thereby increase the viscosity of the electrolyte.

2. The method of claim 1, further comprising introducing enough of the acid-soluble and acid-stable polymer with a high molecular weight to increase the viscosity of the electrolyte by at least 25%, by about 1.25 times to about 15 times, by about 1.25 times to about 10 times, or by about 1.5 times to about 7 times.

3. The method of claim 1, in which the acid-soluble and acid-stable polymer comprises about 0.05 wt % to about 5 wt %, about 0.05 wt % to about 1 wt. %, about 0.05 wt % to less than 1 wt. %, or about 0.5 wt % to about 2.0 wt % of the electrolyte.

4. The method of claim 1, further comprising adding the acid-soluble and acid-stable polymer with a high molecular weight directly into the electrolyte before or after formation of the battery and dissolving at least a portion the acid-soluble and acid-stable polymer with a high molecular weight into the electrolyte.

5-26. (canceled)

27. A battery electrolyte comprising sulfuric acid and an acid-soluble and acid-stable polymer with a high molecular weight.

28. The battery electrolyte of claim 27, in which the high molecular weight is greater than 500,000 g/mol.

29. The battery electrolyte of claim 27, in which the high molecular weight is 500,000 g/mol to 30 million g/mol, 1 million g/mol to 30 million g/mol, 1 million g/mol to 10 million g/mol, and 1 million g/mol to 7 million g/mol.

30. The battery electrolyte of claim 27, in which the acid-soluble and acid-stable polymer comprises polyacrylamide, polyvinyl pyrrolidone, polyarylate, polyacrylate, polymethacrylate, polyethylene oxide, copolymers including one or more of the foregoing, or mixtures of any of the foregoing, or in which the acid-soluble and acid-stable polymer comprises polyacrylamide, polyvinyl pyrrolidone, copolymers including one or more of the foregoing, or mixtures of any of the foregoing.

31. The battery electrolyte of claim 27, further comprising up to 10 wt % phosphoric acid.

32. The battery electrolyte of claim 27, in which the electrolyte further comprises additives.

33. The battery electrolyte of claim 32, in which the additives comprise antimony suppression agents, additives for reducing water loss, deep discharge supporting agents, cycle life enhancers, or combinations thereof.

34. The battery electrolyte of claim 27, in which the acid-soluble and acid-stable polymer comprises 0.05 wt % to about 5 wt %, about 0.05 wt % to about 1 wt. %, about 0.05 wt % to less than 1 wt. %, or about 0.5 wt % to about 2.0 wt % of the electrolyte.

35. A battery comprising: a positive electrode; a negative electrode; a battery separator; and an electrolyte comprising sulfuric acid and an acid-soluble and acid-stable polymer with a high molecular weight.

36. The battery of claim 35, in which the battery separator comprises a microporous silica-filled polyethylene web, a phenol-resorcinol-formaldehyde web, microporous polyether sulfone web, a glass mat, an absorbent glass mat (AGM), or a combination thereof.

37. The battery of claim 35, in which the positive electrode and the negative electrode are dry-charge electrodes.

38. A method of making a battery, the method comprising introducing the electrolyte of claim 27 into an incomplete battery.

39. The method of claim 38, in which the electrolyte is introduced prior to electrode formation.

40. The method of claim 38, in which the electrolyte is introduced as replacement electrolyte after electrode formation.

41. The method of claim 38, in which the electrolyte is introduced into a dry charge battery.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1 depicts sulfuric acid density as a function of concentration at 20° C. (1).

[0020] FIG. 2 illustrates convective flows of the electrolyte during charge and discharge.

[0021] FIG. 3 depicts a relationship between diffusion coefficient and concentration of sulfuric acid at various temperatures (2).

[0022] FIG. 4A depicts predicted and measured convection velocity gradients across the electrolyte space between the positive and negative electrodes at half-height of the electrodes during charge for 15 minutes (5).

[0023] FIG. 4B depicts predicted and measured convection velocity gradients across the electrolyte space between the positive and negative electrodes at half-height of the electrodes during charge for 30 minutes (5).

[0024] FIG. 4C depicts predicted and measured convection velocity gradients across the electrolyte space between the positive and negative electrodes at half-height of the electrodes during charge for 60 minutes (5).

[0025] FIG. 5 depicts a comparison of predicted and measured vertical concentration profiles in the middle of the electrolyte space during charge at 9.4 ma/cm.sup.2 at charge times of 15, 30, and 60 minutes (5).

[0026] FIG. 6 depicts a mechanism of the redistribution of charge during open circuit in a battery with acid stratification (2).

[0027] FIG. 7 illustrates the results of water porosity experiments for examples 2, 6, and comparative example 1.

[0028] FIG. 8 illustrates the results of electrical resistance experiments for examples 2, 6, and comparative example 1.

[0029] FIG. 9 illustrates cumulative pore volume as determined by mercury porosimetry for examples 2, 6, and comparative example 1.

[0030] FIG. 10 illustrates pore size distribution as determined by mercury porosimetry for examples 2, 6, and comparative example 1.

[0031] FIG. 11 illustrates surface SEMs for example 2 (top panel at 2 microns; bottom panel at 100 nm).

[0032] FIG. 12 illustrates surface SEMs for example 6 (top panel at 2 microns; bottom panel at 100 nm).

[0033] FIG. 13 illustrates surface SEMs for an uncoated control separator (top panel at 2 microns; bottom panel at 100 nm).

[0034] FIG. 14 depicts the results of viscosity measurements for particular concentrations of two different grades of an exemplary acid-soluble and acid-stable polymer dissolved in acid.

[0035] FIG. 15 depicts the results of certain acid stratification experiments at different concentrations of acid-soluble and acid-stable polymers dissolved in acid, as compared to control.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0036] As discussed above, it has been discovered that introducing an acid-soluble and acid-stable polymer with a high molecular weight into the electrolyte of a battery can be used to reduce acid stratification during use of the battery. Preferably, enough of the acid-soluble and acid-stable polymer with a high molecular weight is dissolved in the electrolyte to increase the viscosity of the electrolyte by at least 25%, by about 1.25 times to about 15 times, by about 1.25 times to about 10 times, or by about 1.5 times to about 7 times. For example, this preferred viscosity increase can be achieved if the acid-soluble and acid-stable polymer constitutes about 0.05 wt % to about 5 wt %, about 0.05 wt % to about 1 wt. %, about 0.05 wt % to less than 1 wt. %, or about 0.5 wt % to about 2.0 wt % of the electrolyte. Multiple ways of introducing the acid-soluble and acid-stable polymer to the electrolyte are disclosed herein.

[0037] In a first preferred embodiment, battery separators having a separator incorporating an acid-soluble and acid-stable polymer with a high molecular weight can be used and the acid-soluble and acid-stable polymer with a high molecular weight allowed to dissolve into the electrolyte.

[0038] The separator will typically have first and second opposite major surfaces. The acid-soluble and acid-stable polymer can be coated on one or both of the first and second opposite major surfaces. The coating can be continuous or discontinuous on one or both of the first and second opposite major surfaces. Examples of discontinuous coatings include separators patterned with dots, stripes, or other patterns. The coating can penetrate completely or partially into the bulk structure of one or more components of the separator. Alternatively, the coating may be only on the surface of one or more components of the separator.

[0039] In a variation of the first preferred embodiment, the acid-soluble and acid-stable polymer with a high molecular weight can be incorporated into the bulk structure of the separator, as opposed to applied as a coating, and thereby dissolve into the electrolyte from within the bulk structure.

[0040] In another variation of the first preferred embodiment, any substrate, such as a scrim, a battery pasting paper, or other dissolvable or non-dissolvable battery component could incorporate the acid-soluble and acid-stable polymer with a high molecular weight. For example, the battery pasting paper could include the acid-soluble and acid-stable polymer with a high molecular weight and then the pasting paper allowed to dissolve and release the polymer into the electrolyte solution during manufacture of the battery.

[0041] In a second preferred embodiment, the acid-soluble and acid-stable polymer with a high molecular weight can be added directly to the battery electrolyte. The electrolyte can include sulfuric acid and also phosphoric acid, such as up to 10 wt %.

[0042] The concentration of the acid-soluble and acid-stable polymer in the electrolyte (either directly added or as result of dissolving from the separator or other substrate) can be determined based on factors such as cost, viscosity, and electrolyte conductivity. FIG. 14 depicts the results of kinematic viscosity (kinematic viscosity, v=μ/ρ, is the measure of a fluid's resistance to flow when no external force, other than gravity, is acting on it) measurements for different concentrations of two high molecular weight PVP grades (K90 and K120) dissolved in 1.28 specific gravity sulfuric acid. As can be seen from FIG. 14, the kinematic viscosity increases exponentially with increasing polymer concentration. It may be desirable for the acid-soluble and acid-stable polymer concentration to be 5 wt % or less, such as, for example, about 0.05 wt % to about 5 wt %, about 0.1 wt % to about 1 wt. %, about 0.1 wt % to less than 1 wt. %, or about 0.5 wt % to about 2.0 wt %.

[0043] Table 1 depicts changes in sulfuric acid viscosity with addition of different amounts of PVP, as calculated from FIG. 14.

TABLE-US-00001 TABLE 1 Kinematic Viscosity (cSt) Change (X) wt % PVP K90 in 1.28 S.G Acid 0.00 1.96 0.05 2.46 1.26 0.10 2.61 1.33 0.50 4.10 2.09 1.00 7.22 3.68 wt % PVP K120 in 1.28 S.G Acid 0.00 1.96 0.05 2.70 1.38 0.10 2.90 1.48 0.50 5.13 2.62 1.00 10.47 5.34

[0044] Electrolyte including the acid-soluble and acid-stable polymer with a high molecular weight can be added to an incomplete battery (or the polymer added to the electrolyte after it is in the battery), before or after electrode formation. In the case of after electrode formation, the electrolyte would be replacement electrolyte. Additionally, the electrolyte including the acid-soluble and acid-stable polymer with a high molecular weight can be added to a dry charge battery.

[0045] As used herein, “an acid-soluble polymer” refers to a polymer sufficiently soluble in acid to provide at least 0.05 wt % of polymer in the battery electrolyte, across the full range of acid concentrations during charge and discharge of the battery (e.g., sulfuric acid with a specific gravity of 1.1 or less to 1.35 or more). As used herein, “an acid-stable polymer” refers to a polymer capable of being dissolved in 100 mL of 1.21 s.g. sulfuric acid, placed into a sealed 125 mL vessel (such as a Nalgene 4 oz. polypropylene wide mouth bottle), and heated at 70° C. in an oven for one week, without noticeable color change of the solution. Thus, an “an acid-stable and acid-soluble polymer” refers to a polymer that satisfies both of the above criteria.

[0046] Regardless of delivery method, the molecular weight of the polymer is preferably greater than 500,000 g/mol, such as 500,000 g/mol to 30 million g/mol, 1 million g/mol to 30 million g/mol, 1 million g/mol to 10 million g/mol, and 1 million g/mol to 7 million g/mol, to impart sufficient viscosity to the electrolyte at a low concentration of the polymer. Preferred polymers include polyacrylamides, polyvinyl pyrrolidones (PVP), copolymers including either or both, and mixtures of the foregoing. Non-limiting examples of other possible polymers include polyarylates, polyacrylates (such as, polyhydroxyethyl acrylate (pHEA) and polyhydroxypropyl acrylate (pHPA)), polymethacrylates (such as, polyhydroxyethyl methacrylate (pHEMA), polyglycidyl methacrylate (pGMA), and polyhydroxypropyl methacrylate (pHPMA)), polyethylene oxides, copolymers including one or more of the foregoing, or mixtures of any of the foregoing.

[0047] In the first preferred embodiment, the separator may also include additives (such as in admixture with the acid-soluble and acid-stable polymer with a high molecular weight). In the second preferred embodiment, the battery electrolyte may also include other additives. Exemplary separator additives include rubber latex, antimony suppression agents, additives for reducing water loss (e.g., gassing prevention), wettability-enhancing agents, and inorganic materials. Exemplary electrolyte additives include antimony suppression agents, additives for reducing water loss (e.g., gassing prevention), deep discharge supporting agents, such as sodium sulfate, and cycle life enhancers, such as phosphoric acid. Some of the additives may have multiple functions and/or may be applicable to both preferred embodiments.

[0048] Antimony suppression agents include many organic molecules that mask the presence of antimony deposited on the negative electrode due to corrosion of the positive electrode grid. These substances inhibit hydrogen evolution from the antimonial sites and include substituted benzaldehydes, such as vanillin, salicylaldehyde, anisaldehyde, veratraldehyde, and p-propoxyacetophenone.

[0049] Additives for reducing water loss also include many surfactants that act as hydrogen-evolution inhibitors. Examples of hydrogen-evolution inhibitors include non-ionic surfactants such as polyoxyethyleneglycol octophenyl ether (Triton X-100), fatty alcohol ethoxylates, and ethylene-propylene oxide block copolymers.

[0050] Wettability-enhancing agents can include surface active molecules, such as sodium dodecylbenzene sulfonate or sodium dihexyl sulfosuccinate.

[0051] The inorganic material can include an inorganic oxide, carbonate, or hydroxide, such as, for example, alumina, silica, zirconia, titania, mica, boehmite, magnesium hydroxide, calcium carbonate, and mixtures thereof. The inorganic material can be in particulate form (e.g., colloidal silica and fumed silica) or powder form (e.g., phenolics).

[0052] In either preferred embodiment, the separator can include a microporous silica-filled polyethylene web, such as manufactured by ENTEK. In another embodiment, the separator can include any web, such as a phenol-resorcinol-formaldehyde web (e.g., Darak) or a polyether sulfone web. In yet another embodiment, the separator can include a fiber mat (woven or nonwoven), a synthetic pulp separator (such as with or without inorganic filler), a glass mat (including absorbent glass mat (AGM)), or combinations thereof. The foregoing possible separators can also be combined with each other and/or other substrates, such as a microporous silica-filled polyethylene web combined with a glass mat or scrim.

[0053] In the first preferred embodiment, when the separator is a composite, the entire composite can be coated with the acid-soluble and acid-stable polymer. Alternatively, only one component of the composite can be coated with the acid-soluble and acid-stable polymer. For example, in a composite containing a microporous silica-filled polyethylene web and a glass mat, only the glass mat may be coated, only the polyethylene web may be coated, or both components may be coated. One or more components of the composite can be coated prior to assembly as the composite or coated post-composite assembly.

[0054] The term “coating” as used herein does not limit the manner of applying the coating to one or more components of the separator. The coating can be applied by dipping the separator in a bath of the polymer, spraying the polymer on the separator, gravure roll coating, reverse roll coating, slot die coating, knife edge coating, or combinations thereof. The coating can also be applied during formation of one or more components of the separator. For example, the acid-soluble and acid-stable polymer can function as a binder for a component of the separator. Thus, by way of non-limiting example, the acid-soluble and acid-stable polymer may be incorporated into a glass mat as a polymer binder, such as during manufacture of the glass mat via a paper making process.

[0055] When the separator includes a microporous silica-filled polyethylene web, and the acid-soluble and acid-stable polymer with a high molecular weight is applied as a coating, the coating preferably decreases the average pore size of the separator of the coated region by at least 5% (including about 5% to about 50% and about 10% to about 40%), prior to exposure of the separator to an electrolyte (which would result in dissolving of at least a portion of the coating). FIGS. 11 and 12 depict surface scanning electron microscope (SEM) images of the coated separators of Examples 2 and 6, respectively. FIG. 13 depicts surface SEM images of an uncoated separator of the same type as Comparative Example 1. As can be seen, the acid-soluble and acid-stable polymer with a high molecular weight seems to largely fill the surface pores of the separator. FIGS. 9 and 10 plot the pore size distributions for the coated separators of Examples 2 and 6 and the uncoated separator of Comparative Example 1. As can be seen, the acid-soluble and acid-stable polymer with a high molecular weight is present in sufficient amount to reduce the average pore size of the coated separator. Or stated another way, the coat weight of the coating is sufficient to reduce the average pore size of the coated separator. Table 2 depicts the reduction in average pore size, as determined by mercury porosimetry, for Comparative Example 1 and Examples 2 and 6.

TABLE-US-00002 TABLE 2 Comparative Example 1 Sample (control) Example 2 Example 6 Median Pore Diameter μm 0.124 0.100 0.087 (Volume) = Change in average % 19 30 Pore Diameter

[0056] Either of the preferred embodiments can be used in an energy storage device, such as a battery. Both of the preferred embodiments can be used together. Alternatively, in the first preferred embodiment, the electrolyte can be a conventional electrolyte, such as an electrolyte that includes sulfuric acid and up to 10 wt % phosphoric acid. Likewise, the second preferred embodiment can be used with a variety of battery separators, instead of with the coated separators of the first preferred embodiment.

[0057] Electrolytes and porous battery separators that reduce acid stratification have a number of benefits. Other benefits of the electrolytes and coated separators disclosed herein will be apparent to those skilled in the art.

Example 1

[0058] 1.5 wt. % Polyacrylamide (PAM) solution was prepared by mixing 15 g of PAM (Mw˜5-6 Million g/mol) in 985 g of deionized water using a high shear mixer. A polyethylene/silica (PE/SiO.sub.2) separator (ENTEK 161-0.9-0.15 GE_LR) (161 mm×127 mm) was coated by dipping it into the 1.5 wt % PAM solution, withdrawing it and hanging it in a convection oven at 80° C. to fully dry. The coated separator was then soaked in 1.28 s.g. sulfuric acid and its electrical resistance determined to be 57.9 mohm-cm.sup.2.

Example 2

[0059] 2.0 wt. % Polyacrylamide (PAM) solution was prepared by mixing 20 g of PAM (Mw˜5-6 Million g/mol) in 980 g of deionized water using a high shear mixer. A polyethylene/silica (PE/SiO.sub.2) separator (ENTEK 161-0.9-0.15 GE_LR) (161 mm×127 mm) was coated by dipping it into the 2.0 wt % PAM solution, withdrawing it and hanging it in a convection at 80° C. to fully dry. The coated separator was then soaked in 1.28 s.g. sulfuric acid and its electrical resistance determined to be 61.3 mohm-cm.sup.2 (see FIG. 8). Water porosity was determined and is illustrated in FIG. 7. Mercury porosimetry was also determined. Cumulative pore volume is illustrated in FIG. 9. Pore size distribution is illustrated in FIG. 10. Surface SEM images are shown in FIG. 11, with the lower panel at increased magnification.

Example 3

[0060] 3.0 wt. % Polyvinyl pyrrolidone (PVP K90) solution was prepared by mixing 30 g of PVP K90 (Mw˜1.5 Million g/mol) in 970 g of deionized water using a high shear mixer. A polyethylene/silica (PE/SiO.sub.2) separator (ENTEK 161-0.9-0.15 GE_LR) (161 mm×127 mm) was coated by dipping it into the 3.0 wt % PVP K90 solution, withdrawing it and hanging it in a convection oven at 80° C. to fully dry. The coated separator was then soaked in 1.28 s.g. sulfuric acid and its electrical resistance determined to be 71.7 mohm-cm.sup.2.

Example 4

[0061] 5.0 wt. % Polyvinyl pyrrolidone (PVP K90) solution was prepared by mixing 50 g of PVP K90 (Mw˜1.5 Million g/mol) in 950 g of deionized water using a high shear mixer. A polyethylene/silica (PE/SiO.sub.2) separator (ENTEK 161-0.9-0.15 GE_LR) (161 mm×127 mm) was coated by dipping it into the 5.0 wt % PVP K90 solution, withdrawing it and hanging it in a convection at 80° C. to fully dry. The coated separator was then soaked in 1.28 s.g. sulfuric acid and its electrical resistance determined to be 74.4 mohm-cm.sup.2.

Example 5

[0062] 2.5 wt. % Polyvinyl pyrrolidone (PVP K120) solution was prepared by mixing 25 g of PVP K120 (Mw˜3.0 Million g/mol) in 975 g of deionized water using a high shear mixer. A polyethylene/silica (PE/SiO.sub.2) separator (ENTEK 161-0.9-0.15 GE_LR) (161 mm×127 mm) was coated by dipping it into the 2.5 wt % PVP K120 solution, withdrawing it and hanging it in a convection at 80° C. to fully dry. The coated separator was then soaked in 1.28 s.g. sulfuric acid and its electrical resistance determined to be 65.5 mohm-cm.sup.2.

Example 6

[0063] 2.0 wt. % Polyvinyl pyrrolidone (PVP K120) solution was prepared by mixing 20 g of PVP K120 (Mw˜3.0 Million g/mol) in 980 g of deionized water using a high shear mixer. A polyethylene/silica (PE/SiO.sub.2) separator (ENTEK 161-0.9-0.15 GE_LR) (161 mm×127 mm) was coated by dipping it into the 2.0 wt % PVP K120 solution, withdrawing it and hanging it in a convection oven at 80° C. to fully dry. The coated separator was then soaked in 1.28 s.g. sulfuric acid and its electrical resistance determined to be 62.3 mohm-cm.sup.2 (see FIG. 8). Water porosity was determined and is illustrated in FIG. 7. Mercury porosimetry was also determined. Cumulative pore volume is illustrated in FIG. 9. Pore size distribution is illustrated in FIG. 10. Surface SEM images are shown in FIG. 12, with the lower panel at increased magnification.

Example 7

[0064] 5.0 wt. % Polyvinyl pyrrolidone (PVP K90) solution was prepared by mixing 50 g of PVP K90 (Mw˜1.5 Million g/mol) in 950 g of deionized water using a high shear mixer. A cut out piece (162 mm×254 mm) of Evalith™ B10 glass fiber nonwoven (˜330 microns thick) (Johns Manville, Denver, Colo.) was coated by dipping it into the 5.0 wt % PVP K90 solution, withdrawing it, and hanging it in a convection oven at 80° C. to fully dry. The coated glass fiber nonwoven was then used to build to an acid stratification cell test as discussed in example 8 below.

Comparative Example 1

[0065] A polyethylene/silica (PE/SiO.sub.2) separator (ENTEK 161-0.9-0.15 GE_LR) (161 mm×127 mm), without additional treatment, was soaked in 1.28 s.g. sulfuric acid and its electrical resistance determined to be 55 mohm-cm.sup.2 (see FIG. 8). Water porosity was determined and is illustrated in FIG. 7. Mercury porosimetry was also determined. Cumulative pore volume is illustrated in FIG. 9. Pore size distribution is illustrated in FIG. 10.

Example 8—Acid Stratification Cell Test

[0066] A rectangular test cell was fabricated with approximate internal dimensions of 17 cm wide, 20 cm high and ¾ cm thick made from polycarbonate. Electrodes were harvested from a commercially available Deka YB16-B motorcycle battery. Each cell contained a positive and negative electrode which ⅛″ 99% lead wire was used to add contacts. Pieces of ENTEK 161-0.9-0.15 GE_LR separator, 24.5 cm and 12.25 cm long, were prepared as described in examples 1, 3, 4, and 5 and comparative example 1. In separate experiments, each 24.5 cm piece was used to envelope the negative electrode. Then the positive electrode was positioned between the envelope and the respective 12.25 cm piece of separator. In a further separate experiment, pieces of B10 glass fiber nonwoven, 24.5 cm and 12.25 cm long, were prepared as described in example 7. An uncoated ENTEK 162-0.80-0.25 GE_LR separator was used to envelope the negative electrode. The 24.5 cm piece was wrapped around the outside ribs of the uncoated separator. Then the positive electrode was positioned between the envelope and the 12.25 cm piece of coated B10 glass fiber nonwoven. The ribbed side of an uncoated piece of ENTEK 162-0.80-0.25 GE_LR separator was placed adjacent the coated nonwoven 12.25 cm piece and sandwiched between the positive electrode and one of the test cell walls. In each experiment, the cell was then filled with 170 mL of 1.21 s.g. sulfuric acid, charged at 0.3 A to 2.6 V, then discharged at 0.5 A to 1.8 V. The cell was cycled 5 times between 2.6 V and 1.8 V at constant current. After the cycling was completed, the acid density was measured using an Anton Paar DMA35 density meter with a 16.5 cm sample tube. The density was measured at 1 cm below the surface of the acid, then at 11.5 cm below the surface of the acid. Care was taken to prevent acid mixing either by movement of the cell, sample tube or air bubbles. The difference between the top and bottom density measurements is shown in Table 3. The cells with separators treated with high molecular weight polymers showed much lower differences in acid gravity between the top and the bottom of the cell, indicating a decrease in acid stratification compared to the untreated separator.

TABLE-US-00003 TABLE 3 Acid Density Example Sample Difference Compar. Control, 161-0.9-0.25 GE_LR, 170 mL 0.0519 1.21 s.g., no treatment 1 PAM (5-6 mil g/mol MW) Coated, 161- 0.0094 0.9-0.25 GE_LR, 170 mL 1.21 s.g. 3 K90 PVP Coated, 161-0.9-0.25 GE_LR, 0.0062 170 mL 1.21 s.g. 4 K90 PVP Coated, 161-0.9-0.25 GE_LR, 0.0035 170 mL 1.21 s.g. 5 K120 PVP Coated, 161-0.9-0.25 GE_LR, 0.0032 170 mL 1.21 s.g. 6 K90 PVP Coated, Evalith ™ B10 glass 0.0115 fiber nonwoven, 170 mL 1.21 s.g.

Example 9

[0067] A sulfuric acid solution was prepared with a final density of 1.21 s.g. 3.63 g of K120 polyvinyl pyrrolidone (PVP) (Mw˜3.0 Million g/mol) was added to 600 mL of 1.21 s.g. acid and mixed at room temperature until all the polymer was dissolved.

[0068] A rectangular test cell was fabricated with approximate internal dimensions of 17 cm wide, 20 cm high and ¾ cm thick made from polycarbonate. Electrodes were harvested from a commercially available Deka YB16-B motorcycle battery. Each cell contained a positive and negative electrode which ⅛″ 99% lead wire was used to add contacts. A 24.5 cm and a 12.25 cm piece of ENTEK 161-0.9-0.15 GE_LR separator was used in the test cell. The 24.5 cm piece was used to envelope the negative electrode. Then the positive electrode was positioned between the envelope and the 12.25 cm piece of separator. The cell was then filled with 170 mL of the sulfuric acid solution containing PVP. The cell was then charged at 0.3 A to 2.6 V then discharged at 0.5 A to 1.8 V. The cell was cycled 5 times between 2.6 V and 1.8 V at constant current. After the cycling was completed the acid density was measured using an Anton Paar DMA35 density meter with a 16.5 cm sample tube. The density was measured at 1 cm below the surface of the acid, then at 11.5 cm below the surface of the acid. Care was taken to prevent acid mixing either by movement of the cell, sample tube or air bubbles. The difference between the top and bottom density measurements is shown in Table 4. The cell with acid containing 0.5% PVP showed a much lower difference in acid gravity between the top and the bottom of the cell (as compared to comparative example 1), indicating a decrease in acid stratification. The results are depicted in FIG. 15.

Example 10

[0069] A rectangular test cell was fabricated with approximate internal dimensions of 17 cm wide, 20 cm high and ¾ cm thick made from polycarbonate. Electrodes were harvested from a commercially available Deka YB16-B motorcycle battery. Each cell contained a positive and negative electrode which ⅛″ 99% lead wire was used to add contacts. A 24.5 cm and a 12.25 cm piece of ENTEK 161-0.9-0.15 GE_LR separator was used in the test cell. The 24.5 cm piece was used to envelope the negative electrode. Then the positive electrode was positioned between the envelope and the 12.25 cm piece of separator. The cell was then filled with 170 mL of 1.21 s.g. sulfuric acid. Polymer was directly added to the top of the cell at 0.25-1 wt % of the acid as follows:

A) 1.0 wt % K120 PVP

B) 0.25 wt % K120 PVP

C) 0.5 wt % PAM

D) 0.1 wt % K90 PVP

E) 1.0 wt % K90 PVP

F) 0.5 wt % PEO

G) 0.5 wt % PAA

[0070] The cell was then charged at 0.3 A to 2.6 V then discharged at 0.5 A to 1.8 V. After 1 cycle the polymer was dissolved in the acid. The cell was cycled 5 times between 2.6 V and 1.8 V at constant current. After the cycling was completed the acid density was measured using an Anton Paar DMA35 density meter with a 16.5 cm sample tube. The density was measured at 1 cm below the surface of the acid, then at 11.5 cm below the surface of the acid. Care was taken to prevent acid mixing either by movement of the cell, sample tube or air bubbles. The difference between the top and bottom density measurements is shown in Table 1. The cell with acid containing dissolved polymer showed a lower difference in acid gravity between the top and the bottom of the cell (as compared to comparative example 2), indicating a decrease in acid stratification. The results for 10A-10D are depicted in FIG. 15.

Comparative Example 2

[0071] A polyethylene/silica (PE/SiO.sub.2) separator (ENTEK 161-0.9-0.15 GE_LR) (161 mm×127 mm), without additional treatment, was soaked in 1.28 s.g. sulfuric acid and its electrical resistance determined to be 55 mohm-cm.sup.2. The acid stratification cell test of example 1 was performed with the PE/SiO.sub.2 separator, but the cell was filled with 170 mL of 1.21 s.g. sulfuric acid, instead of with sulfuric acid having PVP dissolved therein. The difference between the top and bottom density measurements is shown in Table 4.

TABLE-US-00004 TABLE 4 Direct addition of acid-soluble and Example acid-stable polymer # Sample Difference Compar. 2 Control, 161-0.9-0.25 GE_LR, 170 mL 1.21 sg 0.0519  9 161-0.9-0.25 GE_LR, 170 mL 0.5 wt % K120 0.0080 PVP added to the 1.21 sg acid 10A 161-0.9-0.25 GE_LR, 170 mL 1.0 wt % K120 0.0011 PVP added to the top of the cell 10B 161-0.9-0.25 GE_LR, 170 mL 0.25 wt % K120 0.0020 PVP added to the top of the cell 10C 161-0.9-0.25 GE_LR, 170 mL 0.5 wt % PAM 0.0164 added to the top of the cell 10D 161-0.9-0.25 GE_LR, 170 mL 0.1 wt % K90 0.0011 PVP added to the top of the cell 10E 161-0.9-0.25 GE_LR, 170 mL 1.0 wt % K90 0.0046 PVP added to the top of the cell 10F 161-0.9-0.25 GE_LR, 170 mL 0.5 wt % PEO 0.0313 added to the top of the cell 10G 161-0.9-0.25 GE_LR, 170 mL 0.5 wt % PAA 0.0414 added to the top of the cell

[0072] It will be apparent to those having skill in the art that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention.

REFERENCES CITED

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