FREESTANDING, DIMENSIONALLY STABLE MICROPOROUS WEBS

Abstract

A thin, freestanding, microporous polyolefin web with good heat resistance and dimensional stability includes an inorganic surface layer. A first preferred embodiment is a microporous polyolefin base membrane in which colloidal inorganic particles are present in its bulk structure. Each of second and third preferred embodiments is a thin, freestanding microporous polyolefin web that has an inorganic surface layer containing no organic hydrogen bonding component for the inorganic particles. The inorganic surface layer of the second embodiment is achieved by hydrogen bonding with use of an inorganic acid, and the inorganic surface layer of the third embodiment is achieved by one or both of hydrogen bonding and chemical reaction of the surface groups on the inorganic particles.

Claims

1-30. (canceled)

31. A freestanding polyolefin web, comprising: a microporous polyolefin membrane having a surface and a bulk structure; and an aqueous dispersion-formed porous inorganic surface layer comprising inorganic particles and an inorganic acid, wherein the porous inorganic surface layer covers at least a portion of the surface of the polyolefin membrane, and wherein the polyolefin web exhibits in-plane dimensional stability above the melting point of the polyolefin membrane.

32. The polyolefin web of claim 31, wherein the inorganic particles are independently selected from a group of metal oxides including silica, alumina, titania, zirconia, and combinations thereof.

33. The polyolefin web of claim 31, wherein the inorganic particles comprise fumed inorganic particles or aggregates of inorganic primary particles.

34. The polyolefin web of claim 33, wherein the inorganic surface layer further includes other inorganic particles consisting of colloidal particles and boehmite.

35. The polyolefin web of claim 31, wherein the fumed inorganic particles have a mean aggregate size of about 100 nm to about 300 nm.

36. The polyolefin web of claim 35, wherein the fumed inorganic particles have a specific surface area of about 50 m.sup.2/g to about 225 m.sup.2/g.

37. The polyolefin web of claim 31, wherein the microporous polyolefin membrane comprises a polyolefin matrix and colloidal inorganic particles distributed therein.

38. The polyolefin web of claim 31, in which the polyolefin web exhibits pore collapse in the polyolefin membrane and less than 5% shrinkage in either of its in-plane axes at 200° C.

39. The polyolefin web of claim 31, wherein the inorganic surface layer contains an organic hydrogen bonding component

40. The polyolefin web of claim 39, wherein the organic hydrogen bonding component comprises polyvinyl alcohol (PVOH), polyvinylpyrrolidone (PVP), carboxymethyl cellulose (CMC), polyacrylic acid, polyethylene oxide, or mixtures thereof.

41. The polyolefin web of claim 39, wherein the inorganic surface layer contains less than or equal to 5 wt. % of the organic hydrogen bonding component.

42. The polyolefin web of claim 31, wherein the inorganic acid comprises boric acid.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 is a photograph of a Mayer rod used in coating an inorganic surface layer on a polyolefin membrane in accordance with the methodology disclosed.

[0026] FIGS. 2A and 2B present scanning electron micrographs (SEMs) showing the openings in the surface of a polymer membrane at 20,000× and 40,000× magnifications, respectively.

[0027] FIGS. 3 and 4 are cross-sectional schematic diagrams showing, respectively, a polyolefin membrane and a colloid-modified polyolefin membrane coated with microporous inorganic surface layers.

[0028] FIG. 5 presents upper and lower rows of SEM images showing with three different magnifications the surface and bulk structure, respectively, of a polyolefin membrane in which colloidal particles penetrated the surface of the membrane and into its bulk structure.

[0029] FIG. 6 is a diagram of a stainless steel fixture used for electrical resistance measurement of a battery separator.

[0030] FIG. 7 is a graph of measured data from which electrical resistance (ER) was determined for an embodiment of a battery separator.

[0031] FIG. 8 shows surface and machine direction (MD) fracture SEM images of a silica-coated separator made as described in Example 9.

[0032] FIG. 9 shows surface and machine direction (MD) fracture SEM images of a coated separator made as described in Example 16.

[0033] FIG. 10 is a bar graph showing the results of peel strength tests performed on two inorganic surface layer-coated separators subjected to various heat treatment conditions.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0034] The base membrane used is comprised of a polyolefin matrix. The polyolefin most preferably used is an ultrahigh molecular weight polyethylene (UHMWPE) having an intrinsic viscosity of at least 10 deciliter/gram, and preferably in the range from 18-22 deciliters/gram. It is desirable to blend the UHMWPE with other polyolefins such as HDPE or linear low density polyethylene (LLDPE) to impact the shutdown properties of the membrane. Membranes can also be manufactured from other polyolefins or their blends, such as, for example, ethylene-propylene copolymers, polypropylene, and polymethyl pentene.

[0035] The plasticizer employed is a nonevaporative solvent for the polymer and is preferably a liquid at room temperature. The plasticizer has little or no solvating effect on the polymer at room temperature; it performs its solvating action at temperatures at or above the softening temperature of the polymer. For UHMWPE, the solvating temperature would be above about 160° C., and preferably in the range of between about 180° C. and about 240° C. It is preferred to use a processing oil, such as a paraffinic oil, naphthenic oil, aromatic oil, or a mixture of two or more such oils. Examples of suitable processing oils include: oils sold by Shell Oil Company, such as Gravex™ 942; oils sold by Calumet Lubricants, such as Hydrocal™ 800; and oils sold by Nynas Inc., such as HR Tufflo® 750.

[0036] The polymer/oil mixture is extruded through a sheet die or annular die, and then it is biaxially oriented to form a thin, oil-filled sheet. Any solvent that is compatible with the oil can be used for the extraction step, provided it has a boiling point that makes it practical to separate the solvent from the plasticizer by distillation. Such solvents include 1,1,2 trichloroethylene; perchloroethylene; 1,2-dichloroethane; 1,1,1-trichloroethane; 1,1,2-trichloroethane; methylene chloride; chloroform; 1,1,2-trichloro-1,2,2-trifluoroethane; isopropyl alcohol; diethyl ether; acetone; hexane; heptane; and toluene. In some cases, it is desirable to select the processing oil such that any residual oil in the polyolefin membrane after extraction is electrochemically inactive.

[0037] FIGS. 2A and 2B show scanning electron micrographs (SEMs) of an embodiment of a 16 μm polymer membrane at 20,000× and 40,000× magnification, respectively. FIGS. 2A and 2B show that the openings in the surface of the polymer membrane are typically less than 250 nm in diameter, though many are smaller.

[0038] The coating formulation used in the first preferred embodiment is composed of inorganic particles dispersed in an aqueous-based dispersion in which greater than 50% water is counted in the liquid phase. The inorganic particles are typically charge stabilized and stay suspended in the alcohol/water mixture. An organic hydrogen bonding component, such as low molecular weight, water-soluble polymer, is also present. It is desirable to choose a polymer with numerous hydrogen bonding sites to minimize its concentration, yet achieve a robust, microporous inorganic surface layer that does not easily shed inorganic particles. Polyvinyl alcohol is a preferred organic hydrogen bonding component such that fewer than 5 parts of PVOH can be used with 95 parts or more of the inorganic particles. This organic hydrogen bonding component imparts high peel strength and good in-plane dimensional stability to the coated membrane, while being suitable for coating application from an aqueous-based dispersion.

[0039] FIG. 3 shows a cross-sectional schematic of a polyolefin membrane coated with microporous inorganic surface layers. FIG. 4 shows the cross-sectional schematic of FIG. 3 but with a colloid-modified polyolefin membrane.

[0040] In addition to controlling the amount of organic hydrogen bonding component and inorganic particles in the coating formulation, applicants believe it is important to control the particle size distribution of the inorganic particles. Furthermore, the coating formulation was carefully applied to the polyolefin base membrane to control the thickness of the resultant inorganic surface layer.

[0041] Examples 1 and 2 demonstrate that the colloidal particles penetrate through the surface and into the bulk structure of the polyolefin membrane.

Example 1

[0042] A 16 μm thick, microporous ultrahigh molecular weight polyethylene-containing separator, Entek® KLP (Entek Membranes LLC, Oregon) (see FIGS. 2A and 2B) was used as a polyolefin membrane for coating. The 16 μm Entek® KLP membrane, before coating, is referred to herein as the control. The membrane was dipped through a 275 part isopropyl alcohol:1000 part water solution containing colloidal silica (LUDOX; Sigma-Aldrich Co. LLC) at the following concentrations: 5, 10, and 20 wt. %. Two #00 Mayer rods were used (one on each surface of the membrane) to remove the wet surface layer, and the membrane was then dried with a series of air knives and transported through a vertical oven set at 120° C.

[0043] The samples were examined by scanning electron microscopy and energy dispersive x-ray analysis to show that colloidal silica particles penetrated the membrane surface and were present in the bulk structure, as shown with three different magnifications in the SEMs arranged in the bottom row (MD fracture) of FIG. 5.

Example 2

[0044] The thermal shrinkage values of the colloidal-modified separators in Example 1 were compared with the 16 μm Entek® KLP control. Three 100 mm×100 mm samples were cut from each separator type. The sample groups were held together with a small binder clip fixed in a corner. The samples were then suspended in an oven at 200° C. for 30 minutes. After closure of the oven, it was evacuated and then backfilled with argon for this test. Upon removal, the samples were cooled to room temperature and then measured to determine their shrinkage in the machine direction (MD) and the transverse direction (TD). The results in Table 1 show that there was a substantial reduction in transverse direction shrinkage as the separators were exposed to higher concentrations of colloidal silica.

TABLE-US-00001 TABLE 1 200° C. shrinkage results 200° C. shrinkage Sample MD % TD % 16 μm Entek KLP control 78.7 69.2 5% Ludox 71.1 58.7 10% Ludox 60.0 57.8 20% Ludox 65.6 49.5

[0045] Examples 3-17 relate to inorganic surface layer coating formulations in accordance with a first preferred embodiment.

Example 3

[0046] A 16 μm thick, microporous ultrahigh molecular weight polyethylene-containing separator, Entek® KLP (Entek Membranes LLC, Oregon) was coated with an aqueous-based dispersion that contained the following:

TABLE-US-00002 25 g Polyvinyl alcohol (87-89% hydrolyzed; MW = 13-23K; Aldrich) 610 g Distilled water 275 g Isopropanol 59 g LUDOX HS-40 (40 wt. % colloidal silica; Sigma-Aldrich Co. LLC) 1484 g CAB-O-SPERSE 1030 K (30 wt. % fumed silica; Cabot Corporation).

[0047] The coating dispersion contained 20% solids with a 90:5:5 1030K:LUDOX:PVOH mass ratio. The CAB-O-SPERSE 1030K is an aqueous dispersion of fumed silica with a mean aggregate size of 150 nm and a surface area of 90 m.sup.2/g. Two #7 Mayer rods were used (one on each surface of the membrane) in the dip coating operation (residence time about 7 seconds), and the wetted separator was dried as described in Example 1. Shrinkage values of the coated separator in the machine direction (MD) and the transverse direction (TD) were determined, as described in Example 2. The separator had a final thickness of 20.2 μm, a basis weight increase of 3.9 g/m.sup.2, a thermal shrinkage of 3.1% in the MD and 2.7% in the TD, and a Gurley value of 483 seconds. A Gurley value is a measure of air permeability determined with use of a Gurley® densometer Model 4340, which measures the time in seconds (s) for 100 cc of air to pass through a 6.45 cm.sup.2 membrane at an applied pressure of 1215 Pa.

Example 4

[0048] Separator electrical resistance (ER) was measured in a glove box using a fixture with stainless steel electrodes, lithium-ion electrolyte (1M LiPF.sub.6 in 1:1 Ethylene Carbonate:Ethyl Methyl Carbonate (EMC)), and an impedance analyzer (Gamry PC4 750) operating over a frequency range of 100 kHz to 1 kHz. FIG. 6 is a diagram of the stainless steel fixture used for electrical resistance measurement. The real component of the measured impedance at 100 kHz was plotted for 1, 2, and 3 layers of separator. FIG. 7 is a graph of the measured data from which electrical resistance (ER) was determined. The slope of the linear fit of measured resistance to the number of separator layers was used as the electrical resistance of the separator.

[0049] The areal resistance, electrical resistivity, and MacMullin Number measurements were made for the separator samples described in Examples 1 and 3. A comparison to the 16 μm Entek® KLP base membrane is shown in Table 2.

TABLE-US-00003 TABLE 2 Electrical resistivity, areal resistance, and MacMullin number data Test material description: Coated Entek Series Average Areal MacMullin (Test electrolyte: Thickness Resistance Resistivity Number 1.0M LiPF6, Units 1:1 EC:EMC) mm Q-cm.sup.2 Q-cm dim'less J161X831, 16 μm 0.0181 3.13 1730 13.0 KLP, base membrane J161X833, 16 μm 0.0181 2.85 1572 11.8 KLP, base membrane CDL130225.001, 0.0178 3.16 1776 13.4 5% LUDOX CDL130226.004, 0.0189 3.88 2051 15.4 10% LUDOX CDL130204.001, 0.0197 5.61 2848 21.4 20% LUDOX CDL130227.006, 0.0208 3.31 1595 12.0 90/5/5 1030K/ LUDOX/PVOH

Example 5

[0050] A 16 μm thick, microporous ultrahigh molecular weight polyethylene-containing separator, Entek® GLP (Entek Membranes LLC, Oregon) was coated with an aqueous-based dispersion that contained the following:

TABLE-US-00004 14.5 g Polyvinyl alcohol (PVOH, 87-89% hydrolyzed; MW = 13-23K; Aldrich) 1000 g Distilled water 275 g Isopropanol 1172 g CAB-O-SPERSE PG 008 (40 wt. % alumina; Cabot Corporation).

[0051] The coating dispersion contained 19.6 wt. % solids with a 97/3 alumina/polyvinyl alcohol (PVOH) mass ratio. The CAB-O-SPERSE PG 008 is an aqueous dispersion of fumed alumina with a mean aggregate size of 130 nm and a surface area of 81 m.sup.2/g.

[0052] The separator was dip-coated through a bath containing the aqueous-based dispersion, and the thickness of the wet layer was controlled on each side with a #14 Mayer rod. The wetted separator was then dried with a series of air knives and transported through a vertical oven set at 80° C. and wound on a plastic core, prior to testing. The separator had a final thickness of 20.0 μm and a Gurley value of 464 seconds. The basis weight increased 5 g/m.sup.2 after the coating and drying operations.

[0053] The thermal shrinkage of the coated separator was determined. Three 100 mm×100 mm samples were cut from the separator. The samples were then suspended in an oven at 200° C. for 30 minutes. After closure of the oven, it was evacuated and then backfilled with argon for this test. Upon removal, the samples were cooled to room temperature and then measured to determine their shrinkage in the machine direction (MD) and the transverse direction (TD). Results showed average shrinkage values of 3.4% in the MD and 2.2% in the TD.

Example 6

[0054] A 12 μm thick, microporous ultrahigh molecular weight polyethylene-containing separator, Entek® GLP (Entek Membranes LLC, Oregon) was coated with an aqueous-based dispersion that contained the following:

TABLE-US-00005 7.14 g Polyvinyl alcohol (PVOH; 87-89% hydrolyzed; MW = 13-23K; Aldrich) 1000 g Distilled water 275 g Isopropanol 1172 g CAB-O-SPERSE PG 008 (40 wt. % alumina; Cabot Corporation).

[0055] The coating dispersion contained 19.4 wt. % solids with a 98.5/1.5 alumina/polyvinyl alcohol (PVOH) mass ratio. After dip coating the separator through a bath containing the aqueous-based dispersion, two Mayer rods (#9, #12, or #14) were used to control the wet layer thickness on each side. The wetted separator was then dried with a series of air knives and transported through a vertical oven set at 80° C. and wound on a plastic core, prior to testing.

[0056] Cut samples were then suspended in an oven at 200° C. for 30 minutes. Upon cooling, sample shrinkage in the machine direction (MD) and the transverse direction (TD) was determined, as described in Example 5. Table 3 shows the separator coating pickup, high temperature thermal stability, and Gurley values for the coated separators using various Mayer rods. The results illustrate that the coating thickness could be controlled while maintaining excellent high temperature thermal stability and low Gurley values. Additionally, increasing the thickness of the inorganic surface layer did not negatively affect the Gurley values of the separators using this formulation.

TABLE-US-00006 TABLE 3 Coated separator characteristics Wt. Thickness 200° C. pickup pickup shrinkage Gurley Composition Rod # (g/m.sup.2) (μm) MD % TD % (sec/100 ml) 98.5/1.5 Alumina/ #09/09 4.72 3.0 2.9 2.9 410 PVOH 98.5/1.5 Alumina/ #12/12 7.56 5.4 2.2 2.2 389 PVOH 98.5/1.5 Alumina/ #14/14 8.33 6.0 2.7 2.7 378 PVOH

Example 7

[0057] A 16 μm thick, microporous ultrahigh molecular weight polyethylene-containing separator, Entek® KLP (Entek Membranes LLC, Oregon) was coated with an aqueous-based dispersion containing the following:

TABLE-US-00007 62 g Polyvinylpyrrolidone (LUVETEC K115, 10% solution in water; MW = 2.2 million; BASF) 1242 g Distilled water 258 g Isopropanol 500 g CAB-O-SPERSE PG 008 (40 wt. % alumina; Cabot Corporation).

[0058] The coating dispersion contained 10 wt. % solids with a 97/3 alumina/polyvinylpyrrolidone (PVP) mass ratio. Two #14 Mayer rods were used (one on each surface of the membrane) to control the wet layer thickness; and the separator was then dried with a series of air knives, transported through a vertical oven set at 80° C., and wound on a plastic core, prior to testing. Shrinkage values of the coated separator in the machine direction (MD) and the transverse direction (TD) were determined, as described in Example 5. Table 4 shows the separator coating pickup, high temperature thermal stability, and Gurley values for the coated separator. Results showed excellent high temperature thermal stability and low Gurley values can be obtained for separators with inorganic surface layers containing PVP as the organic hydrogen bonding component.

TABLE-US-00008 TABLE 4 Coated separator characteristics Wt. Thickness 200° C. pickup pickup shrinkage Gurley Composition Rod # (g/m.sup.2) (μm) MD % TD % (sec/100 ml) 97/3 Alumina/ #14/14 4.66 3.6 3.9 3.4 402 PVP

Example 8

[0059] A 16 μm thick, microporous ultrahigh molecular weight polyethylene-containing separator, Entek® KLP (Entek Membranes LLC, Oregon) was coated with an aqueous-based dispersion that contained the following:

TABLE-US-00009 62 g Polyvinylpyrrolidone (LUVETEC K115, 10% solution in water; MW = 2.2 million; BASF) 942 g Distilled water 258 g Isopropanol 800 g AERODISP W 925 (25 wt. % alumina; Evonik Corporation).

[0060] The coating dispersion contained 10 wt. % solids with a 97/3 alumina/polyvinylpyrrolidone (PVP) mass ratio. The AERODISP W 925 is an aqueous dispersion of fumed alumina with a mean aggregate size of 100 nm and a surface area of 81 m.sup.2/g. Two #14 Mayer rods were used (one on each surface of the membrane) in the dip coating operation, and the coated separator was dried as described in Example 5. Shrinkage values at high temperatures were determined by suspending in an oven at 200° C. for 30 minutes and then measuring the change in machine and transverse dimensions upon cooling (see Example 5). Characteristics of the coated separator are described in Table 5.

TABLE-US-00010 TABLE 5 Coated separator characteristics Wt. Thickness 200° C. pickup pickup shrinkage Gurley Composition Rod # (g/m.sup.2) (μm) MD % TD % (sec/100 ml) 97/3 Alumina/ #14/14 4.14 4.8 5.5 3.9 362 PVP

[0061] A 12 μm thick, microporous ultrahigh molecular weight polyethylene-containing separator, Entek® GLP (Entek Membranes LLC, Oregon) was coated with an aqueous-based dispersion containing:

TABLE-US-00011 60 g Polyvinylpyrrolidone (LUVETEC K115, 10% solution in water; MW = 2.2 million; BASF) 1058 g Distilled water 247 g Isopropanol 35 g Colloidal silica (LUDOX; Sigma-Aldrich Co. LLC) 600 g Fumed silica dispersion (CAB-O-SPERSE 1030 K; 30 wt. % solids; Cabot Corporation).

[0062] The coating dispersion contained 10 wt. % solids with a 90/7/3 fumed silica/colloidal silica/polyvinylpyrrolidone (PVP) mass ratio. Two Mayer rods were used (one on each surface of the membrane) to control the wet layer thickness, and dried as described in Example 5. The separator had a final thickness of 19.8 μm, a weight pickup of 2.9 g/m.sup.2, and a Gurley value of 560 seconds. Surface and MD fracture SEM images of the coatings are shown in FIG. 8.

Example 10

[0063] A 12 μm thick, microporous ultrahigh molecular weight polyethylene-containing separator, Entek® GLP (Entek Membranes LLC, Oregon) was coated with aqueous-based dispersions containing the following:

TABLE-US-00012 14.5 g Polyvinyl alcohol (Kuraray; Mowil 4-88, 88% hydrolyzed) 1275 g Distilled water 1172 g Cabosperse PG008 (40 wt. % alumina; Cabot Corporation),
with varying concentrations of surfactant (Dow Q2-5211; 0 wt. %, 0.01 wt. %, 0.1 wt. %, and 0.2 wt. %).

[0064] The coating dispersion contained 19.6 wt. % solids with a 97/3 alumina/polyvinyl alcohol mass ratio. Two #09 Mayer rods were used (one on each surface of the membrane) to control the wet layer thickness; and the separator was then dried with a series of air knives, transported through a vertical oven set at 80° C., and wound on a plastic core, prior to testing. Shrinkage of the coated separator in the machine direction (MD) and the transverse direction (TD) was determined, as described in Example 5. Table 8 presents the coating pickup, high temperature thermal stability, and Gurley values for the coated separator. The data show that the inorganic surface layer exhibits excellent high temperature thermal stability, irrespective of whether a surfactant or isopropanol is present in the coating formulation.

TABLE-US-00013 TABLE 8 Coated separator characteristics Wt. Thickness 200° C. pickup pickup shrinkage Gurley Composition Rod # (g/m.sup.2) (μm) MD % TD % (sec/100 ml) 97/3 Alumina/ #09/09 5.2 3.8 2.4 2.1 315 PVOH, no IPA, no surfactant 97/3 Alumina/ #09/09 5.5 3.7 2.6 1.9 344 PVOH, 0.02% surfactant 97/3 Alumina/ #09/09 5.6 3.6 2.6 2.6 393 PVOH, 0.1% surfactant 97/3 Alumina/ #09/09 6.3 4.3 2.9 2.7 359 PVOH, 0.2% surfactant

Example 11

[0065] A 12 μm thick, microporous ultrahigh molecular weight polyethylene-containing separator, Entek® GLP (Entek Membranes LLC, Oregon) was coated with aqueous-based dispersions containing the following:

TABLE-US-00014 14.5 g Polyvinyl alcohol (Kuraray; Mowil 4-88, 88% hydrolyzed) 1172 g Cabosperse PG008 (40 wt. % alumina; Cabot Corporation).

[0066] The coating dispersion contained 40.7 wt. % solids with a 97/3 alumina/polyvinyl alcohol mass ratio. Two Mayer rods were used (one on each surface of the membrane) to control the wet layer thickness; and the separator was then dried with a series of air knives, transported through a vertical oven set at 80° C., and wound on a plastic core, prior to testing. Shrinkage of the coated separator in the machine direction (MD) and the transverse direction (TO) was determined, as described in Example 5. Table 9 shows the coating pickup, high temperature thermal stability, and Gurley values for the coated separator. This example illustrates that PVOH can be directly dissolved into the aqueous-based dispersion to obtain an inorganic surface layer with high temperature thermal stability.

TABLE-US-00015 TABLE 9 Coated separator characteristics Wt. Thickness 200° C. pickup pickup shrinkage Gurley Composition Rod # (g/m.sup.2) (μm) MD % TD % (sec/100 ml) 97/3 Alumina/ #07/07 8.04 5.3 2.4 2.1 370 PVOH 97/3 Alumina/ #09/09 13.84 10.6 2.4 1.4 379 PVOH

Example 12

[0067] A 32 μm thick microporous ultrahigh molecular weight polyethylene-containing separator composed of two individual 16 μm thick membrane layers,

TABLE-US-00016 Entek ® HPIP (Entek Membranes LLC, Oregon) was coated with an aqueous-based dispersion containing the following: 14.5 g Polyvinyl alcohol (Kuraray; Mowil 4-88, 88% hydrolyzed) 1000 g Distilled water 275 g Isopropanol 1172 g Cabosperse PG008 (40 wt. % alumina; Cabot Corporation).

[0068] The coating dispersion contained 20 wt. % solids with a 97/3 alumina/polyvinyl alcohol mass ratio. Two Mayer rods were used (one on each side of the membrane) to control the wet layer thickness; and the separator was then dried with a series of air knives and transported through a vertical oven set at 80° C., as described in Example 5. The separator was then split into its individual layers, leaving one side uncoated and one side coated for each layer. Each layer was wound onto a plastic core prior to testing. Table 10 shows the coating pickup and Gurley values for the coated separators. This example illustrates an extremely efficient method of manufacturing a separator with an inorganic surface layer on only one side of the polyolefin membrane.

TABLE-US-00017 TABLE 10 Coated separator characteristics Wt. Thickness pickup pickup Gurley Composition Rod # (g/m.sup.2) (μm) (sec/100 ml) 97/3 Alumina/ #14 4.4 3.2 209 PVOH, Side 1 97/3 Alumina/ #14 4.3 3.5 216 PVOH, Side 2

Example 13

[0069] A 16 μm thick, microporous polyethylene-based separator prepared using a dry process (Foresight Separator, Foresight Energy Technologies Co. Ltd) was coated with an aqueous-based dispersion containing the following:

TABLE-US-00018 100 g Selvol 21-205 Polyvinyl alcohol aqueous solution (21 wt. %; 88% hydrolyzed; Sekisui) 205 g Distilled water 1697.5 g Cabosperse PG008 (40 wt. % alumina; Cabot Corporation).

[0070] The coating dispersion contained 35 wt. % solids with a 97/3 alumina/PVOH mass ratio. After dip coating the separator into a bath containing the alumina dispersion; two Mayer rods (#5, #7, or #10) were used (one on each surface of the membrane) to control the wet layer thickness, and the separator was then dried with a series of air knives, transported through a vertical oven set at 80° C., and wound on a plastic core, prior to testing. Shrinkage of the coated separator in the machine direction (MD) and the transverse direction (TO) was determined, as described in Example 5. Table 11 shows the coating pickup, high temperature thermal stability, and Gurley values for the coated separator.

TABLE-US-00019 TABLE 11 Coated separator characteristics Wt. Thickness 200° C. pickup pickup shrinkage Gurley Composition Rod# (g/m.sup.2) (μm) MD % TD % (sec/100 ml) 97/3 alumina/ #05/05 2.24 2.2 8.2 1.1 267 PVOH 97/3 alumina/ #07/07 4.55 4.2 7.2 1.1 340 PVOH 97/3 alumina/ #10/10 9.67 8.9 2.7 1.6 347 PVOH

Example 14

[0071] A 16 μm thick, microporous ultrahigh molecular weight polyethylene-based separator, Entek® KLP (Entek Membranes LLC, Oregon) was coated with an aqueous-based dispersion containing the following:

TABLE-US-00020 7.1 g Selvol 21-205 Polyvinyl alcohol aqueous solution (21 wt. %; 88% hydrolyzed Sekisui) 60 g Distilled water 70 g Boehmite (AlO—OH) [5 wt. % in water; see J. Appl. Chem. Biotechnol. 1973, 23, 803-09 for preparation] 112.5 g Cabosperse PG008 (40 wt. % alumina; Cabot Corporation).

[0072] The coating dispersion contained 20 wt. % solids with a 90/7/3 alumina/boehmite/PVOH mass ratio. The separator was dip-coated into a bath containing the aqueous-based dispersion. The coated polyolefin membrane was then dried in an oven set to 80° C. for 30 minutes prior to testing. Shrinkage of the coated separator in the machine direction (MD) and the transverse direction (TD) was determined, as described in Example 5. Table 12 shows the coating pickup, high temperature thermal stability, and Gurley values for the coated separator.

TABLE-US-00021 TABLE 12 Coated separator characteristics Wt. Thickness 200° C. Coating pickup pickup shrinkage Gurley) Composition (g/m.sup.2) (μm) MD % TD % (sec/100 ml) 90/7/3 alumina/ 11.5 8.6 2.8 1.6 345 boehmite/PVOH

Example 15

[0073] A 12 μm thick, microporous ultrahigh molecular weight polyethylene-containing separator, Entek® GLP (Entek Membranes LLC, Oregon) was coated with an aqueous-based dispersion that contained the following:

TABLE-US-00022 53.4 g Pentaerythritol (Aldrich) 1116 g Distilled water 295 g Isopropanol 1200 g CAB-O-SPERSE PG 008 (40 wt. % alumina; Cabot Corporation).

[0074] The coating dispersion contained 20 wt. % solids with a 90/10 alumina/pentaerythritol mass ratio. Two #09 Mayer rods (one on each surface of the membrane) were used to control the wet layer thickness; and the separator was then dried with a series of air knives, transported through a vertical oven set at 80° C., and wound on a plastic core, prior to testing. Shrinkage values of the coated separator in the machine direction (MD) and the transverse direction (TD) were determined, as described in Example 5. Table 13 shows the separator coating pickup, high temperature thermal stability, and Gurley values for the coated separator.

TABLE-US-00023 TABLE 13 Coated separator characteristics WL Thickness 200° C. pickup pickup shrinkage Gurley Composition Rod# (g/m.sup.2) (μm) MD % TD % (sec/100 ml) 90/10 Alumina/ #09/09 5.67 3.0 4.5 3.1 406 Pentaerythritol

Example 16

[0075] A 12 μm thick, microporous ultrahigh molecular weight polyethylene-containing separator, Entek® GLP (Entek Membranes LLC, Oregon) was coated with aqueous-based dispersions that contained 20 wt. % solids with 90/10 and 80/20 alumina/sucrose mass ratios. Compositions of each of the aqueous-based dispersions prepared are described in Table 14.

TABLE-US-00024 TABLE 14 Dispersion Compositions Isopropyl PG008 Description Sucrose DI water Alcohol Dispersion of coating (g) (Aldrich) (g) (g) (g) Cabot) 90/10 mass ratio 20 420 110 450 Alumina/Sucrose 80/20 mass ratio 40 450 110 400 Alumina/Sucrose

[0076] Two #09 Mayer rods were used (one on each surface of the membrane) to control the wet layer thickness; and the separator was then dried with a series of air knives, transported through a vertical oven set at 80° C., and wound on a plastic core, prior to testing. Shrinkage of the coated separator in the machine direction (MD) and the transverse direction (TD) was determined, as described in Example 5. Table 15 shows the coating pickup, high temperature thermal stability, and Gurley values for the coated separator. Surface and MD fracture SEM images of a 90/10 alumina/sucrose coating mass ratio is shown in FIG. 9. This example illustrates that small molecules with high hydrogen bonding abilities can be incorporated into the inorganic surface layer to yield high temperature thermal stability of the coated separator.

TABLE-US-00025 TABLE 15 Coated separator characteristics Wt. 200° C. pickup shrinkage Gurley Composition Rod # (g/m.sup.2) MD % TD % (sec/100 ml) 90/10 alumina/ #09/09 5.5 2.4 1.2 401 sucrose 80/20 alumina/ #09/09 5.0 23.6 26.2 437 sucrose

Example 17

[0077] A 12 μm thick, microporous ultrahigh molecular weight polyethylene-containing separator, Entek® GLP (Entek Membranes LLC, Oregon) was coated with aqueous-based dispersions that contained 20 wt. % solids with 90/10, 80/20, and 70/30 alumina/maltitol mass ratios. Compositions of each of the coating dispersions prepared are described in Table 16.

TABLE-US-00026 TABLE 16 Dispersion Compositions Isopropyl PG008 Description Maltitol DI water Alcohol Dispersion of coating (g) (Aldrich) (g) (g) (g) Cabot) 90/10 mass ratio 20 420 110 450 Alumina/Maltitol 80/20 mass ratio 40 450 110 400 Alumina/Maltitol 70/30 mass ratio 60 480 110 350 Alumina/Maltitol

[0078] Two #09 Mayer rods were used (one on each surface of the membrane) to control the wet layer thickness; and the separator was then dried with a series of air knives, transported through a vertical oven set at 80° C., and wound on a plastic core, prior to testing. Shrinkage of the coated separator in the machine direction (MD) and the transverse direction (TD) was determined, as described in Example 5. Table 17 shows the coating pickup, high temperature thermal stability, and Gurley values for the coated separator. This example further shows that small molecules with high hydrogen bonding abilities can be incorporated into the inorganic surface layer to yield high temperature thermal stability of the coated separator.

TABLE-US-00027 TABLE 17 Coated separator characteristics Wt. 200° C. pickup shrinkage Gurley Composition Rod # (g/m.sup.2) MD % TD % (sec/100 ml) 90/10 alumina/ #09/09 5.08 2.6 3.1 389 maltitol 80/20 alumina/ #09/09 5.63 8.8 22.9 393 maltitol 70/30 alumina/ #09/09 4.88 27.2 39.3 644 maltitol

[0079] Example 18 related to an inorganic surface layer coating formulation achieved by hydrogen bonding with use of an inorganic acid, in accordance with a second preferred embodiment.

Example 18

[0080] A 16 μm thick, microporous ultrahigh molecular weight polyethylene-containing separator, Entek® KLP (Entek Membranes LLC, Oregon) was coated with aqueous-based dispersions containing the following:

TABLE-US-00028 10 g Boric acid (Aldrich) 405 g Distilled water 110 g Isopropanol 475 g Cabosperse PG008 (40 wt. % alumina; Cabot Corporation).

[0081] The coating dispersion contained 20 wt. % solids with a 95/5 alumina/boric acid mass ratio. Two Mayer rods were used (one on each surface of the membrane) to control the wet layer thickness; and the separator was then dried with a series of air knives, transported through a vertical oven set at 80° C., and wound on a plastic core, prior to testing. Shrinkage of the coated separator in the machine direction (MD) and the transverse direction (TD) was determined, as described in Example 5. Table 18 shows the coating pickup, high temperature thermal stability, and Gurley values for the coated separators. This example illustrates that an inorganic acid can be incorporated to provide excellent high temperature thermal stability of the coated separators. Additionally, this example illustrates the importance of inorganic surface layer coating pickup on the thermal shrinkage properties of the coated separators.

TABLE-US-00029 TABLE 18 Coated separator characteristics Wt. Thickness 200° C. pickup pickup shrinkage Gurley Composition Rod# (g/m.sup.2) (μm) MD % TD % (sec/100 ml) 95/5 alumina/ #07/07 3.59 2.8 48.8 32.7 326 boric acid 95/5 alumina/ #09/09 4.63 3.1 11.7 9.8 382 boric acid 95/5 alumina/ #12/12 6.14 4.6 3.5 1.6 357 boric acid

[0082] Examples 19 and 20 relate to inorganic surface layer coating formulations achieved by one or both of hydrogen bonding and chemical reaction of the surface groups on the inorganic particles.

Example 19

[0083] A 12 μm thick, microporous ultrahigh molecular weight polyethylene-containing separator, Entek® GLP (Entek Membranes LLC, Oregon) was coated with an aqueous-based dispersion that contained the following:

TABLE-US-00030 1000 g Distilled water 275 g Isopropanol 1172 g CAB-O-SPERSE PG 008 (40 wt. % alumina; Cabot Corporation).

[0084] The coating dispersion contained 19.2 wt. % solids and only alumina particles. This coating dispersion is analogous to that of Example 5, with the exception that the resultant inorganic surface layer contains no organic hydrogen bonding component. Two #09 Mayer rods were used (one on each surface of the membrane) to control the wet layer thickness; and the separator was then dried with a series of air knives, transported through a vertical oven set at 80° C., and wound on a plastic core, prior to testing. Shrinkage values of the coated separator in the machine direction (MD) and the transverse direction (TD) were determined, as described in Example 5. Table 19 shows the separator coating pickup, high temperature thermal stability, and Gurley values for the coated separator.

TABLE-US-00031 TABLE 19 Coated separator characteristics Wt. Thickness 200° C. pickup pickup shrinkage Gurley Composition Rod # (g/m.sup.2) (μm) MD % TD % (sec/100 ml) Alumina #09/09 5.44 3.5 4.2 2.1 343

Example 20

[0085] A 16 μm thick, microporous ultrahigh molecular weight polyethylene-based separator, Entek® KLP (Entek Membranes LLC, Oregon) was coated with an aqueous-based dispersion containing the following:

TABLE-US-00032 100 g Boehmite (AlO—OH) [5 wt. % in water; see J. Appl. Chem. Biotechnol. 1973, 23, 803-09 for preparation] 20 g Isopropanol 100 g Cabosperse PG008 (40 wt. % alumina; Cabot Corporation).

[0086] The coating dispersion contained 20.5 wt. % solids with a 89/11 alumina/boehmite mass ratio. The separator was dip-coated into a bath containing the aqueous-based dispersion. The coated polyolefin membrane was then dried in an oven set to 80° C. for 30 minutes prior to testing. Shrinkage of the coated separator in the machine direction (MD) and the transverse direction (TD) was determined, as described in Example 5. Table 20 shows the coating pickup, high temperature thermal stability, and Gurley values for the coated separator.

TABLE-US-00033 TABLE 20 Coated separator characteristics Wt. Thickness 200° C. Coating pickup pickup shrinkage Gurley Composition (g/m.sup.2) (μm) MD % TD % (sec/100 ml) 89/11 alumina/ 19.8 15.6 5.3 5.3 387 boehmite

[0087] Applicants believe that the inorganic surface layers containing no organic hydrogen bonding component bond to the separator as described below. The porous particles in the inorganic surface layer are characterized by open chainlike morphology to form a virtual network at the surface of the polyolefin membrane. The particles of the inorganic surface layer are held together by particle-to-particle contacts that include mechanical interlocking and hydrogen bonding. Preferred metal oxide particles include fumed alumina, silica, titania, and zirconia. The inorganic surface layer is thought to be held to the separator by mechanical interlocking to its surface pores.

[0088] The following example demonstrates the effect of heat treatment on the adhesive strength of coated separators.

Example 21

[0089] Two different separators were used to study the effect of heat treatment on the inorganic surface layer adhesive strength to the polyolefin membrane. In the first case, the inorganic surface layer contained only alumina particles (see Example 19). In the second case, the inorganic surface layer was prepared from an aqueous-based coating dispersion having a 95/5 alumina/boric acid mass ratio (see Example 18).

[0090] To study the effect of heat treatment on coating adhesion strength, three different conditions were employed: [0091] 1) a control condition in which no heat treatment was performed; [0092] 2) a heat treatment using calender rolls (Innovative Machine Corp.), with a gap set to 20 μm, a roll temperature of 125° C., and a roller speed of 1 ft/minute (30.5 cm/minute); and [0093] 3) an oven heat treatment at 125° C. for 4 hours in vacuum.

[0094] An inorganic surface layer adhesive strength test was performed, in which each coated separator was placed horizontally on a steel plate and magnetic strips were placed on the edges of the separator to secure the separator. A pressure sensitive tape (3M Scotch® Magic™ Tape 810, ¾ inch (1.9 cm) width), was applied to the coated separator. The free end of the tape was secured to a fixture clip, and the tape was peeled at 1800 from the original tape orientation (i.e., 180° peel test configuration) at a speed of 0.1 inch/second (2.54 mm/second) and a distance of 4.5 inches (11.4 cm). A force gauge (Chatillon, DFGS-R-10) with a 10±0.005 lbs. (4 kg±2.7 g) load cell capacity was used to measure the force required to remove the inorganic surface layer from the base polyolefin membrane, and the maximum load was recorded. The test was repeated at least three times for each sample. All testing was performed at room temperature. FIG. 10 is a bar graph showing the results of a peel strength test of the two coated separators that underwent (1) no heat treatment, (2) calender roll heat treatment at 125° C., and (3) oven heat treatment at 125° C. in vacuum.

[0095] Results showed that both coated separators had improved inorganic surface layer adhesive strength after heat treatment. A comparison between heat treatments revealed that, the longer the residence time, the better the adhesion. Coated separators containing boric acid showed much improved adhesive strength after heat treatment compared to coated separators containing only alumina particles. Only small differences in Gurley values were observed before and after heat treatment. For the sample containing 95/5 mass ratio alumina/boric acid, the average Gurley value before oven heat treatment was 324 s compared to 352 s after oven heat treatment. This example illustrates that heat treatment can be used to improve the adhesive strength of the coating with only a minimal decrease in air permeability.

[0096] The following example demonstrates the effect of corona treatment on adhesive strength and wetting.

Example 22

[0097] A 16 μm thick, microporous ultrahigh molecular weight polyethylene-containing separator, Entek® KLP (Entek Membranes LLC, Oregon) was corona treated with an Enercon TL Max™ web surface treater. The corona treatment settings were adjusted to a Watt density of 3.99 Watts/ft.sup.2/min, gap distance of 0.06 inch (1.5 mm), and a speed of 65 ft/min (19.8 m/min). After corona treatment, the surface energy increased from 35 Dynes to 52 Dynes, and the water contact angle decreased from 86° to 56°.

[0098] Entek®16 μm KLP membranes with and without corona treatment were passed through three different aqueous-based coating dispersions: (1) a coating dispersion containing 20 wt. % solids with only alumina particles (Cabosperse PG008), (2) a coating dispersion containing 20 wt. % solids with a 95/5 alumina/boric acid mass ratio, and (3) a coating dispersion containing 20 wt. % solids with a 95/10 alumina/boric acid mass ratio. Compositions for each of the aqueous-based dispersions are described in Table 21

TABLE-US-00034 TABLE 21 Coating Compositions Isopropyl PG008 Description Boric acid DI water Alcohol Dispersion of coating (g) (Aldrich) (g) (g) (g) Alumina coating 0 390 110 500 95/5 Alumina/ 10 405 110 475 Boric Acid 90/10 Alumina/ 20 420 110 450 Boric Acid

[0099] Each of the separators was dip-coated through a bath containing the aqueous-based dispersion, and the thickness of the wet layer was controlled on each side with a #9 Mayer rod. The separator was then dried with a series of air knives, transported through a vertical oven set at 80° C., and wound on a plastic core, prior to testing. Thermal shrinkage of the coated separator in the machine direction (MD) and the transverse direction (TD) was determined, as described in Example 5.

[0100] Table 22 shows the coating weight/thickness pickup, high temperature thermal stability, and Gurley values for the coated separators prepared. A higher weight/thickness pickup was seen when coating onto corona treated separators as compared to when coating onto untreated separators. Additionally, there was a clear improvement in wetting in the separator upon corona treatment. For example, when attempting to coat the aqueous-based dispersion containing a 90/10 alumina/boric acid mass ratio on an untreated separator, the aqueous-based dispersion beaded up, thus resulting in a very uneven coating with poor quality. In contrast, when applying this same aqueous-based dispersion to the corona treated separator, the coating was applied very smoothly, and the quality of the coating was much improved.

TABLE-US-00035 TABLE 22 Coated separator characteristics 200° C. Coating Corona Basis Wt. Thickness Shrinkage Gurley Composition Treatment? g/m.sup.2 μm MD % TD % Sec/100 ml Alumina (PG008) No 13.9 19.9 26.2 17.3 352 Alumina (PG008) Yes 14.6 20.9 16.3 11.4 350 95/5 Alumina/ No 14.2 20.6 17.7 9.8 332 Boric Acid 95/5 Alumina/ Yes 15.6 21.9 3.1 1.6 362 Boric Acid 90/10 Alumina/ No Coating beaded Boric Acid 90/10 Alumina/ Yes 14.5 22.2 3.1 2.6 358 Boric Acid

[0101] The inorganic surface layer adhesive strength was determined using the peel test method described in Example 21. Results are shown in Table 23, illustrating that the inorganic surface layer adhesive strength was significantly improved when the corona treatment was applied. Additionally, formulations with higher concentrations of boric acid resulted in more substantial improvements in the inorganic surface layer adhesive strength. This example illustrates the ability to improve adhesion of the inorganic surface layer and wetting of the coating dispersion when corona treatment is applied to the base polyolefin membrane.

TABLE-US-00036 TABLE 23 Effect of corona treatment on peel strength of coated separators Coating Peel Strength (lbs) Composition No Treatment (SD) Corona Treatment (SD) Alumina 0.025 0.003 0.043 0.008 95/5 Alumina/ 0.023 0.003 0.082 0.012 Boric Acid 90/10 Alumina/ — — 0.182 0.003 Boric Acid

[0102] It will be obvious 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. For example, an inorganic surface layer may be applied as a coating on a portion (e.g., a patterned coating) of the surface or the entire surface of a polyolefin membrane. The scope of the invention should, therefore, be determined only by the following claims.