HK1177774A - Method for increasing the strength and solvent resistance of polyimide nanowebs - Google Patents

Description

Method for increasing strength and solvent resistance of polyimide nanowebs

Technical Field

The present invention relates to the use of nanoweb polyimide separators in lithium (Li) and lithium ion (Li-ion) batteries, as well as other electrochemical cells.

Background

An important practical aspect of modern energy storage devices is the ever increasing energy and power density. Security has been found to be a major concern. Lithium ion batteries, which are currently in widespread commercial use, are among the highest energy density batteries in common use and require a multi-stage safety device, including an external fuse and a temperature sensor, which shuts down the battery in the event of overheating before a short circuit, which can occur due to a mechanical failure of the battery separator. Lithium ion (Li-ion) batteries can explode and catch fire in the event of a short circuit due to mechanical or thermal failure of the separator. Lithium ion secondary batteries present specific durability challenges after multiple charge and discharge cycles. Commercially available lithium ion batteries typically employ microporous polypropylene as the battery separator. The microporous polypropylene begins to shrink at 120 ℃, limiting the method of cell manufacture, the temperature at which the cell is used, and the power available from the cell.

Wound electrochemical cells, common in the market place, impose severe mechanical stress on the device separator during manufacture and use. Those stresses can lead to manufacturing defects and device failure. Mechanical stresses may include, for example, the strong tensile and compressive forces of the separator during manufacture, which stresses are used to create tight windings. After fabrication is complete, the device layers remain under compressive and tensile stresses. Furthermore, the manufacturing apparatus is subject to shock and impact stresses during use.

The need to select improved separators for lithium ion batteries and other high energy density electrochemical devices is complex. Suitable separators combine good electrochemical properties, such as high electrochemical stability, charge-discharge-recharge hysteresis, first cycle irreversible capacity loss, and the like, with good mechanical aspects, such as strength, toughness, and thermal stability.

Investigations have been conducted on known high performance polymers for use as battery separators. One such polymer is polyimide.

The Handbook of Batteries(edited by David Lindon and Thomas Reddy, McGraw-Hill, (3 rd edition), 2002) describes the loss of first cycle discharge capacity as an important criterion for secondary batteries (P.35.19). It is also noted that nonwoven separators have generally been found to exhibit insufficient strength for use in lithium and lithium ion batteries. (P.35.29). For this reason, polyethylene microporous films with low melting points tend to be used as separators in lithium batteries and lithium ion batteries. However, polyethylene microporous films are not suitable for high temperatures in terms of thermal properties, which are occasionally associated with rapid discharge end uses or end uses in high temperature environments.

Adv.mat.doi of Huang et al: 10.1002/adma.200501806 discloses the preparation of polyimide nanofiber mats: the polyamic acid was electrospun and then imidized into a polymer represented by the following structure.

The mat thus prepared was then heated to 430 ℃ and held for 30 minutes, resulting in an increase in strength. No mention is made of battery separators.

U.S. published patent application 2005/0067732 to Kim et al discloses a method of making a polymeric nanoweb by electrospinning of a polymer solution comprising a polyimide solution. No mention is made of battery separators.

JP2004-308031A to Honda et al discloses the preparation of polyimide nanowebs by electrospinning a polyamic acid solution followed by imidization. Use as a battery separator is disclosed.

JP2005-19026A to Nishibori et al discloses the use of polyimide nanowebs with sulfone functional groups in the polymer chain as separators for lithium metal batteries. Polyimide is described as being soluble in organic solvents and nanowebs are made by electrospinning a polyimide solution. The actual battery is not illustrated. The nanofiber web is disclosed as being heated to about 200 ℃.

WO2008/018656 to Jo et al discloses the use of polyimide nanowebs as battery separators in lithium and lithium ion batteries.

EP 2,037,029 discloses the use of polyimide nanowebs as battery separators in lithium and lithium ion batteries.

However, there remains a need for lithium and lithium ion batteries prepared from materials that combine good electrochemical properties, such as high electrochemical stability, charge-discharge-recharge hysteresis, first cycle irreversible capacity loss, and the like, with good mechanical aspects, such as strength, toughness, and thermal stability.

Summary of The Invention

In one aspect, the present disclosure provides a multilayer article comprising a first electrode material, a second electrode material, a porous separator disposed between and in contact with the first and second electrode materials, wherein the porous separator comprises a nanoweb that comprises a plurality of nanofibers, wherein the nanofibers consist essentially of a fully aromatic polyimide.

In another aspect, the present disclosure provides an electrochemical cell comprising a housing having an electrolyte disposed within the housing and a multilayer article at least partially immersed in the electrolyte; the multilayer article includes a first metallic current collector, a first electrode material in electrically conductive contact with the first metallic current collector, a second electrode material in ionically conductive contact with the first electrode material, a porous separator disposed between and in contact with the first electrode material and the second electrode material; and a second metallic current collector in electrically conductive contact with the second electrode material, wherein the porous separator comprises a nanoweb that comprises a plurality of nanofibers, wherein the nanofibers consist essentially of a fully aromatic polyimide.

In another aspect, the present invention provides a method of making a nanoweb: assembling nanofibers to form a nanofiber web, wherein the nanofibers comprise polyamic acid; imidizing the polyamic acid nanofibers at a selected temperature to provide a nanoweb comprising polyimide nanofibers; and subjecting the nanoweb to a temperature of at least 50 ℃ above the selected temperature for a period of time in the range of from about 5 seconds to about 20 minutes.

Brief Description of Drawings

Fig. 1 is a schematic representation of one embodiment of a multilayer article herein.

Fig. 2 is a schematic representation of another embodiment of a multilayer article herein.

Fig. 3a and 3b are schematic illustrations of additional embodiments of multilayer articles of the present invention.

Fig. 4 is a schematic illustration of the assembly process of one embodiment of the multilayer article herein.

Fig. 5 is a schematic representation of a spiral wound embodiment of the multilayer article herein.

Figure 6 is a schematic diagram of a suitable electroblowing apparatus.

Figure 7 is a schematic view of an alternative suitable electroblowing apparatus.

FIG. 8 is a graphical depiction of a comparison between imidization and tensile strength versus temperature for samples prepared in example 5.

FIG. 9 is a graphical depiction of a comparison between tensile strength and crystallinity index versus temperature for the samples prepared in example 5.

FIG. 10 is a bar graph showing the solvent exposure effect on the tensile strength of the sample in example 13 versus the effect on comparative example CNW-B.

Fig. 11 is a bar graph showing the relationship between the breaking strength and the crystallinity index of the sample in example 14.

Detailed Description

For the purposes of the present invention, the abbreviations and nomenclature shown in table 1 consistent with practices in the polyimide art will be employed:

it will be understood that other dianhydrides and diamines not listed in table 1 are also suitable for use in the present invention, provided that the suitable dianhydrides and diamines meet the limitations described below.

A fundamental property of practical separators for lithium ion batteries is that they maintain a high degree of mechanical integrity under conditions of use. If there is insufficient mechanical integrity within the separator, the battery will not only cease to operate, but will actually short circuit and explode. Polyimides are well known to be strong and chemically inert in a wide variety of environments. However, many polyimides exhibit undesirably high solvent uptake when subjected to common electrolyte solvents with a concomitant decrease in strength and toughness. In extreme cases, the polymer will dissolve completely. The battery separator is naturally insulating and therefore non-conductive.

The articles of the present invention comprise polyimide nanoweb separators that exhibit desirably high strength and toughness, desirably low solvent absorption, and desirably high strength retention after solvent exposure, as compared to polyimide nanoweb separators of the art. Polyimide nanoweb separators also exhibit desirable electrochemical stability and performance. Polyimide nanoweb separators suitable for the practice of the invention comprise a plurality of nanofibers wherein the nanofibers consist essentially of a fully aromatic polyimide. Polyimide separators suitable for use in the present invention exhibit less than 20% electrolyte uptake by weight after 1300 hours of exposure. Polyimide nanoweb separators of the art include non-fully aromatic nanofibers according to the definition below, and have been found to exhibit more than twice the solvent uptake with greater performance degradation.

The invention also provides electrochemical cells comprising the inventive articles, i.e., the polyimide nanoweb separators herein, as a separator between a first electrode material and a second electrode material.

The present invention provides a method for enhancing the properties of polyimide nanoweb separators suitable for use in the present invention by subjecting the polyimide nanoweb to a temperature at least 50 ℃ above its imidization temperature for a period of time from 5 seconds to 20 minutes. The resulting nanoweb was stronger and had less solvent absorption than the same nanoweb prior to treatment.

For the purposes of the present invention, the definition of ISO 9092 by the term "nonwoven" article will be utilized: manufactured sheets, webs or batts of "oriented or random oriented fibers bonded together by friction, and/or cohesion and/or adhesion, excluding paper and products that are woven, knitted, tufted, stitch-bonded incorporating binder yarns or filaments, or felted by wet milling, whether or not additionally needled. The fibers may be of natural or manufactured origin. They may be staple or continuous filaments or formed in situ. The term "nanoweb" as used herein represents a subset of nonwoven articles in which the fibers are designated as "nanofibers" characterized by a cross-sectional diameter of less than 1 micron. As used herein, a nanoweb defines a relatively flat, flexible and porous planar structure and is formed by laying down one or more continuous filaments.

As used herein, the term "nanofiber" refers to a fiber having a number average diameter of less than 1000nm, even less than 800nm, even between about 50nm and 500nm, and even between about 100nm and 400 nm. For nanofibers having a non-circular cross-section, the term "diameter" as used herein refers to the largest cross-sectional dimension. The nanofibers employed in the present invention consist essentially of one or more fully aromatic polyimides. For example, the nanofibers employed in the present invention can be made from greater than 80 weight percent of one or more fully aromatic polyimides, greater than 90 weight percent of one or more fully aromatic polyimides, greater than 95 weight percent of one or more fully aromatic polyimides, greater than 99 weight percent of one or more fully aromatic polyimides, greater than 99.9 weight percent of one or more fully aromatic polyimides, or 100 weight percent of one or more fully aromatic polyimides.

The nanoweb can be processed by a method selected from the group consisting of: electro-blowing, electro-spinning, and melt-blowing. The nanofiber webs used in the specific embodiments listed below have been prepared by electroblowing. Electroblowing of polymer solutions to form nanofiber webs is described in considerable detail in op.

As used herein, the term "fully aromatic polyimide" specifically refers to a polyimide that is at least 90% imidized and wherein at least 95% of the bonds between adjacent benzene rings in the polymer backbone are affected by covalent bonds or ether bonds. Up to 25%, preferably up to 20%, most preferably up to 10% of the bonds may be affected by aliphatic carbon, sulfide, sulfone, phosphide or phosphine functional groups or a combination thereof. Up to 5% of the aromatic rings making up the polymer backbone may have ring substitutions of aliphatic carbons, sulfides, sulfones, phosphides, or phosphines. By 90% imidization is meant that 90% of the amic acid functionality in the polyamic acid precursor has been converted to imide. Preferably, the fully aromatic polyimides suitable for use in the present invention are 100% imidized and preferably do not contain aliphatic carbons, sulfides, sulfones, phosphides, or phosphines.

Polyimide nanowebs suitable for use in the invention are prepared by imidization of a polyamic acid nanoweb, wherein the polyamic acid is a condensation polymer prepared by the reaction of one or more aromatic dianhydrides with one or more aromatic diamines. Suitable aromatic dianhydrides include, but are not limited to, pyromellitic dianhydride (PMDA), biphenyl tetracarboxylic dianhydride (BPDA), and mixtures thereof. Suitable diamines include, but are not limited to, diaminodiphenyl ether (ODA), 1, 3-bis (4-aminophenoxy) benzene (RODA), and mixtures thereof. Preferred dianhydrides include pyromellitic dianhydride, biphenyltetracarboxylic dianhydride, and mixtures thereof. Preferred diamines include diaminodiphenyl ether, 1, 3-bis (4-aminophenoxy) benzene and mixtures thereof. PMDA and ODA are most preferred.

In the polyamic acid nanoweb imidization process herein, a polyamic acid is first prepared in solution; typical solvents are Dimethylacetamide (DMAC) or Dimethylformamide (DMF). In one method suitable for the practice of the present invention, the polyamic acid solution is formed into a nanoweb by an electroblowing process, as described in detail below. In an alternative process suitable for the practice of the present invention, the polyamic acid solution is formed into a nanoweb by electrospinning as described by Huang et al, in op. In either case, it is necessary that the nanoweb be formed from a polyamic acid solution, and the resulting nanoweb subsequently be subjected to imidization. Unlike solvent soluble polyimides employed in the art in nanoweb separators of electrochemical cells in the art, the fully aromatic polyimides employed in the present invention are highly insoluble. One skilled in the art can choose to electro-blow or electro-spin the polyimide solution or polyamic acid solution followed by imidization. The practitioner of the present invention must first form a nanoweb from polyamic acid and then imidize the nanoweb so formed.

Imidization of the polyamic acid nanoweb thus formed can be conveniently carried out as follows: first subjecting the nanoweb to solvent extraction in a vacuum oven with nitrogen purge at a temperature of about 100 ℃; after extraction, the nanoweb is then heated to a temperature of 300 to 350 ℃ for about 10 minutes or less, preferably 5 minutes or less, more preferably 30 seconds or less, to fully imidize the nanoweb. Imidization according to the process herein results in at least 90%, preferably 100% imidization. In most cases, analytical methods showed that 100% imidization was rarely achieved, even after long periods of imidization. In fact, complete imidization is achieved when the slope of the imidization percentage versus time curve is zero.

In one embodiment, the polyimide nanoweb consists essentially of polyimide nanofibers formed from pyromellitic dianhydride (PMDA) and Oxydianiline (ODA) having monomer units represented by the structure,

polyimides are commonly referred to by the names of the condensation reactants that form the monomer units. This convention will be followed herein. Thus, a polyimide consisting essentially of the monomer units represented by structure I is designated PMDA/OD.

Although the invention is not so limited, it is believed that the polymerization process can also affect the polyimide behavior in the electrolyte solution. The stoichiometric configuration of the excess dianhydride is allowed to result in a polyimide with amine end groups. These amine end groups have active hydrogens capable of reacting with the electrolyte solution. Those active hydrogens are deactivated by adjusting the stoichiometry to have a slight excess of dianhydride or by capping the diamine with a mono anhydride (e.g., phthalic anhydride), thereby reducing the reaction with the electrolyte solution.

In one aspect, the present disclosure provides a multilayer article comprising a first electrode material, a second electrode material, a porous separator disposed between and in contact with the first and second electrode materials, wherein the porous separator comprises a nanoweb that comprises a plurality of nanofibers, wherein the nanofibers consist essentially of a fully aromatic polyimide. In one embodiment, the first and second electrode materials are different, and multiple layers thereof are used in the battery. In an alternative embodiment, the first and second electrode materials are the same and the multilayer article thereof is used in a capacitor, especially of the type known as an "electric double layer capacitor".

In one embodiment, the first electrode material, separator and second electrode material can be in adhesive contact with each other in the form of a laminate. In one embodiment, the electrode material is mixed with a polymer and other additives to form a paste that is adhesively applied to opposing surfaces of the nanoweb separator. Pressure and/or heat can be applied to form the adhesive laminate.

In one embodiment in which the multilayer article of the present invention is used in a lithium ion battery, the negative electrode material comprises an intercalation material for lithium ions, such as carbon, preferably graphite, coke, lithium titanate, lithium tin alloy, silicon, carbon silicon composite, or mixtures thereof; and the positive electrode material includes lithium cobalt oxide, lithium iron phosphate, lithium nickel oxide, lithium manganese phosphate, lithium cobalt phosphate, MNC (LiMn (1/3) Co (1/3) Ni (1/3) O₂)、NCA(Li(Ni_1-y-zCo_yAl_z)O₂) Lithium manganese oxide, or mixtures thereof.

In one embodiment, the multilayer article further comprises at least one metallic current collector in adherent contact with at least one of the first or second electrode materials. Preferably, the multilayer article further comprises a metallic current collector in adhering contact with each electrode material.

In another aspect, the present disclosure provides an electrochemical cell comprising a housing having an electrolyte disposed within the housing and a multilayer article at least partially immersed in the electrolyte; the multilayer article comprises a first metallic current collector, a first electrode material in electrically conductive contact with the first metallic current collector, a second electrode material in ionically conductive contact with the first electrode material, a porous separator disposed between and in contact with the first electrode material and the second electrode material; and a second metallic current collector in electrically conductive contact with the second electrode material, wherein the porous separator comprises a nanoweb that comprises a plurality of nanofibers, wherein the nanofibers consist essentially of a fully aromatic polyimide. The ion-conducting component and the material transmit ions, and the conducting component and the material transmit electrons.

In one embodiment of the electrochemical cell herein, the first and second electrode materials are different, and the electrochemical cell herein is a cell, preferably a lithium ion cell. In an alternative embodiment of the electrochemical cell herein, the first and second electrode materials are the same, and the electrochemical cell herein is a capacitor, preferably an electric double layer capacitor. When it is stated herein that the electrode materials are the same, it is meant that they comprise the same chemical composition. However, they may differ in certain structural compositions such as granularity.

In another embodiment of the multilayer article of the present invention, at least one electrode material is coated onto a non-porous metal sheet used as a current collector. In a preferred embodiment, both electrode materials are so coated. In a cell embodiment of the electrochemical cells herein, the metal current collectors comprise different metals. In the capacitor embodiments of the electrochemical cells herein, the metal current collectors comprise the same metal. The metallic current collector suitable for use in the present invention is preferably a metal foil.

FIG. 1 depicts one embodiment of an article of the present invention. Referring to fig. 1, the inventive articles described herein comprise a porous nanoweb separator 1 disposed between a negative electrode 2 and a positive electrode 3, the separator consisting essentially of polyimide nanofibers consisting essentially of a fully aromatic polyimide, each electrode being deposited on a non-porous conductive metal foil 4a and 4b, respectively. In one embodiment, the negative electrode 2 comprises carbon, preferably graphite, and the metal foil 4a is a copper foil. In another embodiment, the positive electrode 3 is lithium cobalt oxide, lithium iron phosphate, or lithium manganese oxide, and the metal foil 4b is an aluminum foil.

In one embodiment, a multilayer article comprises:

a first layer comprising a first metallic current collector;

a second layer comprising a first electrode material in adherent contact with the first metallic current collector;

a third layer comprising a porous separator in adhering contact with the first electrode material;

a fourth layer comprising a second electrode material in adhering contact with the porous separator;

and

a fifth layer comprising a second metallic current collector in adherent contact with the second electrode material.

In one embodiment, the first layer is copper foil and the second layer is carbon, preferably graphite. In another embodiment, the third layer is a nanoweb consisting essentially of PMDA/ODA nanofibers. In another embodiment, the fourth layer is lithium cobalt oxide and the fifth layer is aluminum foil. In one embodiment, the first layer is copper foil, the second layer is carbon, preferably graphite, the third layer is a nanoweb consisting essentially of PMDA/ODA nanofibers, the fourth layer is lithium cobalt oxide and the fifth layer is aluminum foil.

In another embodiment, the foil is coated on both sides with positive or negative electrode active material. This makes it easy to form stacks of arbitrary size and voltage, as depicted in fig. 2, by alternately layering the double-sided foil with separators. The stack so depicted comprises a plurality of interconnected multilayer articles of the present invention as depicted in fig. 1. Referring to fig. 2, a plurality of porous polyimide nanoweb separators 1 are stacked with alternating layers of negative electrodes 2 and positive electrodes 3. In one embodiment, the negative electrode material 2 is carbon, preferably graphite, deposited on both sides of a copper foil 4a, and the positive electrode material 3 is lithium cobalt oxide deposited on both sides of an aluminum foil 4 b.

An alternative embodiment of the article of the present invention is shown in fig. 3 a. Referring to fig. 3a, the article of the invention comprises a porous nanoweb separator 1 suitable for use in the invention, consisting essentially of nanofibers of a fully aromatic polyimide, disposed between a negative electrode 2 and a positive electrode 3, each electrode being deposited directly on opposite sides of the nanoweb. The electrode material is deposited onto the nanoweb, for example, by methods well known in the art, including paste extrusion, printing. In one embodiment, the negative electrode comprises carbon, preferably graphite. In another embodiment, the positive electrode comprises lithium cobalt oxide, lithium iron phosphate, or lithium manganese oxide, preferably lithium cobalt oxide.

Another embodiment of the configuration of fig. 3a is depicted in fig. 3b, wherein a layer of metal foil 4 is added to the structure of fig. 3a as shown. In a preferred embodiment, the multilayer structure of fig. 3b is subjected to lamination to provide intimate surface-to-surface contact and adhesion between the layers.

Referring again to fig. 2, the electrochemical cell of the present invention is formed when the layered stack shown in fig. 2 is enclosed within a liquid-tight housing 5, which may be a metal "can" containing a liquid electrolyte 6. In another embodiment, the liquid electrolyte includes an organic solvent and a lithium salt dissolved in the organic solvent. In another embodiment, the lithium salt is LiPF₆、LiBF₄Or LiClO₄. In another embodiment, the organic solvent comprises one or more alkyl carbonates. In another embodiment, the one or more alkyl carbonates include a mixture of ethylene carbonate and dimethyl carbonate. The optimum ranges for salt and solvent concentrations may vary depending on the particular materials employed and the intended use conditions; for example, according to the expected operating temperature. In one embodiment, the solvent is 70 parts by volume ethylene carbonate and 30 parts by volume dimethyl carbonate and the salt is LiPF₆. Alternatively, the electrolyte salt may comprise lithium hexafluoroarsenate, lithium bistrifluoromethylsulphonylimide, lithium dioxalate borate, lithium difluorooxalate borate, or a Li + salt of a polyfluorinated cluster anion, or a combination of these.

Alternatively, the electrolyte solvent may comprise propylene carbonate, an ester, ether or trimethylsilyl derivative of ethylene glycol or poly (ethylene glycol), or a combination of these. In addition, the electrolyte may contain a variety of known additives to enhance the performance or stability of lithium ion batteries as reviewed by k.xu in chem.rev., 104, 4303(2004) and s.s.zhang in j.power Sources, 162, 1379 (2006).

For layered stacks, the stack depicted in fig. 2 can be replaced with the multilayer article depicted in fig. 1. Means are also present but not shown to be used to connect the respective negative and positive terminals of the battery to an external electrical load or charging device. When the individual cells in a stack are electrically connected to each other in series (positive to negative), then the output voltage from the stack is equal to the combined voltage of each cell. When the individual cells making up the stack are electrically connected in parallel, then the output voltage from the stack is equal to the voltage of one cell. It will be clear to a person of ordinary skill in the power art when a series arrangement is appropriate and when a parallel arrangement is appropriate.

Lithium ion batteries are available in a variety of forms including cylindrical, prismatic, pouch, wound, and laminated. Lithium ion batteries find use in a variety of different applications (e.g., consumer electronics, electric tools, and hybrid vehicles). The lithium ion battery is manufactured in a manner similar to other batteries such as NiCd and NiMH, but is more sensitive due to the reactivity of the materials used in the lithium ion battery.

The positive and negative electrodes of a lithium ion battery suitable for use in one embodiment of the invention are similar in form to each other and are prepared by similar methods on similar or identical equipment. In one embodiment, the active material is coated on both sides of a metal foil, preferably aluminum or copper foil, which acts as a current collector, conducting current into and out of the cell. In one embodiment, the negative electrode is prepared by coating graphitic carbon on copper foil. In one embodiment, by reacting lithium metal oxide (e.g., LiCoO)₂) Coating on aluminum foil to prepare the anode. In another embodiment, the foil thus coated is wound on a large reel and dried at a temperature in the range of 100-150 ℃ before it is brought into a drying room for battery manufacture.

Referring to fig. 4a, for each electrode, an active material 41 is mixed with a binder solution 42 and a conductive filler 43 such as acetylene black. The mixture thus formed is fed through a fine conditioner 44 into a mixing tank 45 where the mixture is stirred until it assumes a uniform appearance. Suitable binders include, but are not limited to, poly (vinylidene fluoride) homopolymers and copolymers, styrene butadiene rubber, polytetrafluoroethylene, and polyimide. The slurry so formed is then gravity fed or pressure fed to a pump 46 which pumps the slurry through a filter 47 and thence to a coating head 48. The coating head deposits a controlled amount of slurry onto the surface of a moving metal foil 49 fed by a feed roll 410. The foil thus coated is transported through an oven 412 set at 100-. The knife edge 413 arranged at the oven entrance is positioned an adjustable distance above the foil; the thickness of the electrode formed is thus controlled by adjusting the gap between the blade and the foil. In the oven, the solvent is typically volatilized by a solvent recovery unit 414, and the thus dried electrode is then conveyed to a take-up roll 415.

The electrode thickness achieved after drying is typically in the range of 50-150 microns. If it is desired to produce a coating on both sides of the foil, the foil coated on such one side is returned to the coater, but only the uncoated side is set to accept the slurry deposition. After coating, the electrode thus formed is then calendered and optionally cut into narrow strips for different size cells. Any burrs on the edges of the foil strips cause internal short circuits in the cells, and the cutting machine must be precisely manufactured and maintained.

In one embodiment of the electrochemical cell of the invention, the electrode assembly thereof is a spirally wound structure for use in a cylindrical cell. A structure suitable for use in a spirally wound electrode assembly is shown in fig. 5. In an alternative embodiment, the electrode assembly is a stacked structure similar to that of fig. 2 suitable for prismatic cells. Prismatic batteries can also be prepared in wound form. In the case of prismatic batteries, the wound battery is pressed to form a rectangular structure, which is then pushed inside a rectangular case.

To form a cylindrical implementation of the lithium ion battery of the present invention, the electrode assembly is first wound into a spiral structure as depicted in fig. 5. Tabs are then applied to the electrode edges to connect the electrodes to their corresponding terminals. In the case of high power cells, it is desirable to employ multiple tabs welded along the edges of the electrode strip to carry high currents. The tabs are then welded to the can and the spirally wound electrode assembly is inserted into the cylindrical housing. The housing is then sealed and left open for electrolyte to be injected into the housing. The cell is then filled with electrolyte and then sealed. The electrolyte is typically a mixture of a salt (LiPF6) and a carbonate-based solvent.

The electrode assembly is preferably carried out in a "dry room" as the electrolyte reacts with water. The water can cause LiPF₆Hydrolysis forms HF, which degrades the electrode and adversely affects battery performance.

After the cell is assembled, it is formed (conditioned) by undergoing at least one precisely controlled charge-discharge cycle to activate the working substance. For most lithium ion chemistries, this involves creating an SEI (solid electrolyte interface) layer on the negative (carbon) electrode. Which is a passivation layer that substantially prevents further reaction of the lithium carbon with the electrode.

In one embodiment, the multilayer article of the present invention comprises a nanoweb that is a reinforced nanoweb as described below. In another embodiment, the electrochemical cell herein comprises a nanoweb separator that is a reinforced nanoweb as described below. In one embodiment, the reinforced nanoweb is characterized by a crystallinity index of at least 0.2. In one embodiment, the reinforced nanoweb is a reinforced nanoweb consisting essentially of PMDA/ODA nanofibers having a crystallinity index of at least 0.2.

In another aspect of the present invention, a method for making a nanoweb wherein the nanoweb comprises a plurality of nanofibers wherein the nanofibers consist essentially of a fully aromatic polyimide is provided. The nanofiber web produced may be a reinforced nanofiber web, by which is meant a nanofiber web having higher strength, lower electrolyte solvent uptake, and reduced electrolyte solvent induced loss of physical properties. In a preferred embodiment, the nanofiber webs reinforced herein are characterized by a crystallinity index of at least 0.2. The term "crystallinity index" as used herein is defined below. The reinforced nanoweb so prepared provides an enhanced level of safety when used as a separator as described above in a lithium ion battery. As discussed above, high strength and toughness are key features of lithium ion batteries. The maintenance of those characteristics is also important in use-i.e. in the presence of the electrolyte solvent. While the nanoweb separator of the invention provides excellent strength, toughness and its retention upon solvent exposure, the enhanced nanoweb separator of the invention provides further improvements to the nanoweb consisting essentially of fully aromatic polyimide nanofibers. Although not limiting to the invention, it is believed that the enhanced properties observed are due, at least in part, to the increased crystallinity provided by the enhanced nanofiber webs herein.

Lithium ion batteries incorporating the nanoweb separators of the invention are superior to those of the art in terms of durability with respect to thermal stress and mechanical impact. Lithium ion batteries incorporating the reinforced nanoweb separators of the invention are further improved.

The reinforced nanoweb separator of the invention is prepared as follows: a nanoweb consisting essentially of fully aromatic polyimide nanofibers is heated to a temperature in the annealing range and exhibits enhanced crystallinity, strength and reduced solvent uptake. The extent of annealing is highly dependent on the material composition. The annealing range was 400-500 ℃ for PMDA/ODA. For BPDA/RODA, it is about 200 ℃; BPDA/RODA decomposes if heated to 400 ℃. Generally, in the methods herein, the annealing regime begins at least 50 ℃ above its imidization temperature. For the purposes of the present invention, given that the imidization temperature of the polyamic acid nanoweb is a temperature below 500 ℃, which is the point at which the% weight loss/C ° drops to below 1.0, preferably below 0.5, in thermogravimetric analysis with a heating rate of 50C °/min, with accuracies of ± 0.005% weight% and ± 0.05 ℃. According to the process herein, the fully aromatic polyimide nanoweb is subjected to heating in the annealing range for a period of time of 5 seconds to 20 minutes, preferably 5 seconds to 10 minutes.

In one embodiment, a PMDA/ODA amic acid nanoweb prepared by solution polycondensation followed by electroblowing of the nanoweb is first heated to about 100 ℃ in a vacuum oven with a nitrogen sweep to remove residual solvent. After removal of the solvent, the nanoweb is heated, preferably under an inert atmosphere such as argon and nitrogen, to a temperature in the range of 300-350 ℃ for a period of time of less than 15 minutes, preferably less than 10 minutes, more preferably less than 5 minutes, most preferably less than 30 seconds, until at least 90% of the amine functionality has been converted (imidized) to imide functionality, preferably until 100% of the amine functionality has been imidized. The so imidized nanoweb is then heated to a temperature in the range of 400-500 ℃, preferably 400-450 ℃ for a period of 5 seconds to 20 minutes until a crystallinity index of 0.2 is obtained.

In another aspect, the present invention provides an Electrochemical Double Layer Capacitor (EDLC). Electrochemical double layer capacitors are energy storage devices with a capacitance that can be as high as several farads. Charge storage in double layer electrochemical capacitors is a surface phenomenon that occurs at the interface between an electrode (typically carbon) and an electrolyte. In the double layer capacitor, the separator absorbs and retains the electrolyte, thereby maintaining close contact between the electrolyte and the electrodes. The separator functions to electrically insulate the positive electrode from the negative electrode to facilitate transport of ions in the electrolyte during charging and discharging. Electrochemical double layer capacitors are typically made using a cylindrical wound design in which two carbon electrodes and a separator are wound together, thus requiring the use of a separator of high strength to avoid short circuits between the two electrodes.

Examples

Test method

Crystallinity index method

The parameter "crystallinity index" as used herein refers to the relative crystallinity parameter as determined by wide angle X-ray diffraction (WAXD). X-ray diffraction data were collected using a PANalytical X' Pert MPD with copper radiation equipped with parabolic X-ray mirrors and parallel plate collimators. Samples for transmission spectra were prepared by stacking the films to a total thickness of about 0.7 mm. Data was collected over a 2 theta range of 3-45 degrees with 0.1 degree steps. The count time for each data point is a minimum of 10 seconds with the sample rotating about the transmission axis at a rate of 0.1 revolutions per second.

The WAXD scan so generated consists of three factors: 1) a background signal; 2) scattering from ordered but amorphous regions; 3) scattering from crystalline regions. A polynomial background is fitted to the baseline of the diffraction data. The background function was chosen to be a third order polynomial of the 2 theta diffraction angle variation. The background subtracted data was then fitted with a series of gaussian peaks least squares, which represent ordered amorphous or crystalline components. Guided by experience with numerous samples of the same composition but with distinct degrees of crystallinity, it was determined which peaks represent crystalline regions. The ratio of the integral under the thus selected crystallization peak to the integral under the entire scanning curve with background subtracted is the crystallinity index.

The peaks shown in Table 2 are obtained for PMDA-ODA polyimide.

The absolute crystal content of the sample remains unknown, since it is determined which peaks are sharp enough that they should be considered as a fraction of the crystalline phase, with slight assertions. However, the crystallinity index determined in this manner allows us to compare the relative crystallinity of two polymers of the same polymer type.

TABLE 2

WAXD (2 theta angle)

11.496

15.059

16.828

22.309

Measurement of degree of imidization (DOI)

Measuring the infrared spectrum of a given sample and calculating to 1375cm^-1Absorbance of imide C-N at 1500cm^-1The ratio of absorbance of C-H substituted at the para position. This ratio is taken as the degree of imidization (DOI). It has been found that the PMDA/ODA polyamic acid nanofiber web that has been subjected to imidization conditions for a time sufficient to exceed the time considered necessary to achieve maximum imidization exhibits a DOI of about 0.57. In contrast, the PMDA/ODA film sample had a DOI of 0.65. This difference can be attributed to sample effects such as the orientation of the nanofibers not present in the film.

For the purposes of the present invention, by taking 1375/1500cm of 0.57^-1The peak ratios represent calculated DOI for 100% imidized PMDA/ODA nanowebs. To determine the% imidization for a given sample, the 1375/1500 peak ratio was calculated as a percentage of 0.57.

The polyimide nanowebs herein were analyzed by ATR-IR using the DuraSamplIR (ASI Applied systems) accessory on Nicolet Magna 560FTIR (ThermoFisher scientific). 4000-600cm-1 spectra were collected and corrected for the ATR effect (depth of penetration versus frequency).

Solvent uptake

A 1 square centimeter sample was placed horizontally on top of a piece of corrugated aluminum foil in a sealed 20mL scintillation vial containing 1.5mL of an 70/30(v/v) mixture of ethyl methyl carbonate and ethylene carbonate. The sample is thus suspended above the liquid mixture, allowing only contact with the solvent vapor. At the recorded time points, the samples were removed from the vials, quickly weighed on a microbalance, and placed back into the sealed vials.

Viscosity measurement

The solution viscosity was measured using a Brookfield Engineering HADV-II + programmable viscometer equipped with RV/HA/HB-5 or-6 spindle and calibrated using NIST traceable silicon fluid. The rotor was immersed in the room temperature polymer solution until the liquid level reached the indentation on the rotor. The motor is started with the rotational speed set to produce 10-20% of the nominal torque. For 40-70 poise solutions, 10-20rpm HAs been found to be suitable for RV/HA/HB-5 rotors, and 20rpm for HA/HB-6 rotors.

Fiber sizing

The nanofiber web diameter was measured using the following method:

1. taking one or more SEM (scanning Electron microscope) images of the surface of the nanoweb at a magnification comprising 20-60 measurable fibers.

2. Three locations were selected on each image, which appear by visual observation to represent the average appearance of the nanoweb.

3. The fiber diameters of 60-180 fibers were measured using image analysis software and the mean of the selected regions was calculated.

Polymer preparation

Poly (amic acid) solution 1(PAA-1)

43.13 pounds of PMDA (DuPont Mitsubishi Gas Ltd.) was mixed with 40.48 pounds of 4, 4ODA (Wakayama Seika) and 1.30 pounds of phthalic anhydride (Aldrich Chemical) in 353 pounds of DMF in a 55 gallon Ross Versa Mixer stainless Steel tank. They were mixed and reacted at room temperature by first adding ODA to DMF, then PMDA and finally phthalic anhydride while stirring for 26 hours to yield polyamic acid. The polyamic acid was then filtered using a 25 micron 7 "metal leaf filter and placed in a 55 gallon drum freezer. The resulting polyamic acid had a weight average molecular weight of 133, 097 daltons, as measured by GPC, and had a room temperature solution viscosity of 60 poise.

Poly (amic acid) solution 2(PAA-2)

33.99kg of PMDA (DuPont Mitsubishi Gas Ltd.) was mixed with 32.19kg of 4, 4ODA (Wakayama Seika) and 1.43kg of phthalic anhydride (Aldrich Chemical) in 215.51kg of DMF (DuPont) in a 100 gallon stirred stainless steel reactor. They were mixed and reacted at room temperature while stirring for 30 hours by first adding ODA to DMF, then PMDA and finally phthalic anhydride to yield polyamic acid. The resulting polyamic acid had a room-temperature solution viscosity of 58 poise.

Poly (amic acid) solution 3(PAA-3)

53.87g of PMDA (Aldrich chemical) was mixed with 50.46g of 4, 4ODA (Aldrich chemical) and 1.49g of phthalic anhydride (Baker ACS) in a laboratory stirred 1L glass kettle into 417.31g of DMAC (Chromosolv plus) to obtain a polyamic acid having a stoichiometry of 98% and 20% solids by weight. They were mixed and reacted at room temperature while stirring for 18 hours by first adding ODA to DMAC, followed by PMDA and finally phthalic anhydride to yield polyamic acid. The resulting polyamic acid had a room-temperature solution viscosity of 88 poise.

Poly (amic acid) solution 4(PAA-4)

24.02g of BPDA (Aldrich chemical) were mixed with 24.35g of 4, 4RODA (1, 3-phenylene dioxydianiline) (Aldrich chemical) and 0.49g of phthalic anhydride (Baker ACS) in a laboratory stirred 1L glass kettle into 193.5g of DMF (Chromosolv plus). They were mixed and reacted while stirring at room temperature for 18 hours to form polyamic acid by first adding RODA to DMF, then adding BPDA, and finally adding phthalic anhydride. The resulting polyamic acid had a room-temperature solution viscosity of 60 poise.

Poly (amic acid) solution 5-12(PAA-5-PAA-12)

A plurality of additional poly (amic acid) solutions were prepared from the various reactants using the equipment and procedures described for poly (amic acid) solutions 3 and 4. The reactants, their molar ratios, and percent solids for each polyamic acid solution so prepared are shown in table 3.

Comparative Polymer

Commercially available solvent soluble polyimides were used for the preparation of the comparative examples below. These are P84 and P84HT available from HP Polymers. P84 is a co-condensate of 2, 4TDI/2, 6TDI/MDI with BTDA. P84HT is a co-condensate of 2, 4TDI/2, 6TDI with BTDA/PMDA.

Production of nanofiber webs

Device

Electrospinning is a well known technique described, for example, in Encyclopedia of Polymer science and Technology, DOI 10.1002/0471440264.pst 554. The field of electrospinning finds that spinning becomes unstable and the fiber begins to "wave" around. Due to the swinging action, the fiber diameter is reduced to a desired range. The productivity of electrospinning is extremely low, typically with individual spinning points in the mL/h range. So-called electroblowing techniques have been proposed in an attempt to compensate for yield problems in electrospinning fine fibers. In the electroblowing process, a turbulent air stream is directed at the fibers as they are spun, forming a "cloud" of polymer fibers that are blown down onto a target surface and electrostatically attracted. The combination of blowing and electrostatic forces greatly increases system yield. The electroblowing process is described in detail in U.S. published patent application 2005/0067732.

The nanoweb was prepared from the poly (amic acid) solution prepared above by an electroblowing process. Figure 6 depicts one embodiment of a suitable electroblowing apparatus. In the method, the PAA solution is pumped from the drum through a 25 micron screen (not shown) into a stirred tank (not shown). The PAA solution from the reservoir was filled into a gear pump (not shown) using pressurized air (not shown). The gear pump was then fed to a spinning beam 102 having 76 spinning nozzles arranged in a 1m wide spinneret, with the nozzles spaced 1cm apart. A dc voltage differential is applied between the spinneret and the grounded collector 110. Compressed air from an air compressor, used as process gas, is fed through a heater (not shown) into the spinneret and discharged from the spinneret via a tuyere slot 106 disposed on the blade side of the spinneret containing the spinning nozzles. Each spinning nozzle is characterized by a diameter d of 0.25mm and a length of 2.5 mm. A blower 112 is connected to a perforated plenum 114 with a length of tubing to create a vacuum under a 1m wide steel mesh conveyor 110 driven by rollers 116 and supported by 3 other rotating rollers 115. The fibers 103 are both air blown and electrically attracted to form a web 105 on the surface of the steel mesh collector conveyor 110. The web passes through a hot air dryer 107 and is wound up on a winder 106.

A second electroblowing apparatus is depicted in figure 7. The polymer solution was manually loaded into a 250mL Hoke cylinder 200 using a syringe (not shown). A pressurized nitrogen gas source 201 was used to deliver the solution to a 10cm wide spinneret 102 having 3 nozzles, each having a diameter of 0.38mm and a length of 3.8mm, arranged at 1cm intervals, centered in the spinneret. Heated compressed air 108 is fed into the spinneret and discharged through slots 106. The fibers 103 are both air blown and attracted by the dc voltage potential to the sheet metal collector 202, which remains grounded to the battery operated winder 203. A roll of scrim 204 was mounted at the end of the sheet metal collector. Heated pressurized air 205 is also blown into the apparatus containing the entire spinning apparatusIn a housing 207. Suction fan 206 is used to maintain atmospheric pressure within the hood and remove any evaporated solvent. The common numbered components in fig. 6 and 7 are both identical.

Production of nanofiber webs

Nanofiber web #1(NW-1)

Referring to fig. 6, 50kg of PAA-1 was loaded onto the apparatus and discharged from the spinning nozzle 104 at an outlet pressure of 3.5 bar and a temperature of 39 ℃. The process gas was exhausted at 69 ℃ at a velocity of 5,042m/min from slot 106, which has a gap dimension "a" of 0.7 mm. The product is collected as a nanoweb on an electrically grounded collection belt 110. The distance of the nozzle from the collector was 30 cm. The applied potential difference was 85 kV.

The nanoweb 105 was passed through a hot air dryer 107 at 180 ℃ for 3 minutes. The so dried nanoweb is then wound into a roll. The polyamic acid nanoweb so prepared was then unwound and subsequently imidized and rewound by heating to a temperature of about 325C for 11/2 minutes in a Glenro medium wave infrared oven. The web was then unwound and calendered between a stainless steel calender roll and a package calender roll at 1800 pounds per linear inch on a BF Perkins calender and then rewound.

Nanofiber web #2(NW-2)

50kg of PAA-2 was filled into the apparatus depicted in FIG. 6. The solution was electroblown according to the method described in the preparation of NW-1, except that the solution was discharged from the spinning nozzle 104 at a temperature of 37 ℃. The process gas was fed at 5,833m/min and 72 ℃ from slot 106, which had a gap dimension "a" of 0.6mm, to form a nanoweb of polyamic acid fibers. The nanoweb was then unwound manually and cut with a manual hob cutter into hand-fed sheets approximately 12 "long and 10" wide. The hand sheets were then calendered on a BFPerkins calender at 1800 lb/linear inch between a rigid steel roll and a package roll at room temperature.

Nanofiber web #3(NW-3)

60cc of PAA-3 was manually filled into the apparatus depicted in FIG. 7. The nozzle 104 was spaced 35.6cm from the flat panel collector 202 and the potential difference applied between the nozzle and the belt was 110 kV. The solution outlet pressure was 32psig and the process gas velocity was 4480m/min at a temperature of 22 ℃. A secondary air source 205 was heated to 102 ℃ and blown into the fiber spinning chamber 207 at a flow rate of 9 cubic feet per minute. The nanoweb structure was laid on an aluminum foil scrim.

Nanofiber web #4-13(NW 4-13)

PAA-4-PAA-12 was used for the preparation of nanowebs using the equipment and procedures employed in the preparation of NW-3. The specific ingredients and conditions are summarized in table 4.

Comparative nanoweb A-D (CNW-A-D)

The nanoweb can also be made from the non-fully aromatic comparative polymers P84 and P84 HT. These are named CNW 1-3.

12.5 grams of P84HT polyimide 200 mesh powder (HP Polymer Inc.) was dried overnight in a vacuum oven at 90 ℃ and then dissolved in 50mL of DMF at room temperature to give a 25 wt.% solids polyimide solution having a solution viscosity of 43 poise. This solution was named S-1.

12.5 g of P84 powder were similarly dried and subsequently dissolved in 50mL of DMF at room temperature, again resulting in a 25 wt.% solids polyimide solution having a viscosity of 43 poise and designated S-2.

12.5 g of P84 were similarly dried and subsequently dissolved in 50mL of DMAC at room temperature, again giving a 25% by weight solids polyimide solution of unrecorded viscosity, designated S-3.

Each of the polyimide solutions so prepared was then manually loaded into the electroblowing apparatus depicted in FIG. E-2 using a syringe using the conditions and methods described for preparation of NW-3. The nanoweb structure was laid on a polyester scrim. Other parameters for each polymer solution are summarized in table 5.

TABLE 5

The resulting web was peeled off the scrim and hand cut into hand-fed sheets approximately 12 "long and 4" wide. The hand sheets were then dried in a convection oven and allowed to cool, followed by calendering on a BF Perkins calender between steel calender rolls and a package calender roll at 1500 pounds per linear inch. One of the CNW-B sheets was then heated again in a convection oven to a temperature of 400 ℃ for two minutes. See table 6 for post-processing setup.

TABLE 6

Examples 1-4 and comparative example A

After the entire roll of NW-1 imidization and calendering was complete, the infrared oven was heated from 325 ℃ to 450 ℃ at a rate of approximately 8 ℃/min. The calendered web was unwound and placed back in the oven at the beginning of the heating, thereby obtaining samples annealed at different temperatures calculated from the initial temperature of 325 ℃ and the time spent when the samples were removed. These samples are shown in table 7. The crystallinity index (c.i.), imidization Degree (DOI), and% solvent uptake were determined for each sample using the methods described above. The results are shown in Table 7.

Example 4 was 37 microns thick with a basis weight of 17.3 grams per square meter; 67.5% porosity. Porosity is determined by dividing the sample weight by the sample size to provide the apparent density. This density was compared to the known density of polyimide of 1.43 gm/cc. Therefore, the porosity (%) was 100 × (1-apparent density/1.43). The tensile strength was 21.8MPa and the tensile modulus was 979.7MPa (according to ISO 9073-3). The average fiber diameter was determined to be 531 nanometers by viewing a scanning electron microscope image and estimating the diameter of 100 fibers using image analysis software.

Three coin cells were made with each of the NW-1 samples so annealed.

Coin cells (model CR 2032) were assembled from commercial components (Pred Materials International, inc., New York, NY 10165) with the following components: the negative electrode is graphite on copper foil, 60 + -3 μm thick (Japan piezoics Co. Ltd., distributed by Pred Materials International), 1.5 + -0.1 mAh/cm²Like a disk of 5/8 "diameter. Positive electrode is LiCoO on aluminum foil₂60 + -3 μm thick (Japan Piconics Co. Ltd., by Pred Materials International), 1.5 + -0.1 mAh/cm²Like a disk of 9/16 "diameter. The electrolyte solution was 1.OM LiPF in a 2: 1 by weight ethylene carbonate/ethyl methyl carbonate (Ferro) mixture₆Stored and dispensed in an argon glove box.

Porous polyolefin (Celgard LLC, Charlotte, NC28273) like 11/16 "diameter disks was used as a comparative example to represent the current commercial separator.

The cell hardware was dried overnight at 90 ℃ under reduced pressure and transferred to an argon-filled glove box for storage, filling and assembly. The cell was assembled with two layers of separator interposed between the negative and positive electrodes, filled with electrolyte solution, sealed by means of crimped polyolefin gaskets and removed from the glove box.

The cell test was performed at room temperature by means of a Maccor series 4000 cell tester (Maccor, inc. tulsa, OK 74107). Each cell was first subjected to six formation cycles at 0.25mA between 2.7V and 4.2V with a 10 minute dwell period between each half cycle, followed by 250 charge at 1.OmA and discharge at 2.5mA with a 10 minute dwell period between each half cycle. The charge and discharge capacity (in mAh) per cycle was recorded. The results are shown in Table 8.

Example 5

After preparation of NW-2, nanoweb samples of dried and calendered but still imidized PAA nanofibers were subsequently heated as follows: placing the sample in a linerThe metal trays of the films and then the trays with the samples thereon were placed in a laboratory convection oven preheated to a temperature range of 200 ℃ to 475 ℃ for 2 minutes. The sample heated at 475C for 2 minutes had an average fiber diameter of 707nm and a porosity of 50.5%. The samples that were not otherwise heated had an average fiber diameter of 775nm and a porosity of 50.8%.

The imidization Degree (DOI), crystallinity index (c.i.) and tensile strength (ISO9073-3) were also measured. The results are shown in table 9 and graphically in fig. 8 and 9.

These experiments show that although imidization appears to have been completed by heating at 300 ℃ for 2 minutes, crystallinity and fracture strength steadily increase with annealing temperature from 300 ℃ to 450 ℃.

Examples 6 to 11 and comparative examples AA-EE

Nanowebs #3, 5, 7, 8, 9, 10, and 11 were dried in an air convection oven at 200 ℃ for 2 minutes. They were then subjected to calendering, imidization and annealing according to the conditions shown in table 10.

Watch 10

The so-processed nanowebs and CNW-B and CNW-D were incorporated into coin cells following the methods employed in examples 1-4 and using the equipment employed in examples 1-4; the first formation cycle irreversible capacity loss was determined. These too areCommercially available separators were compared. The results are shown in Table 11. Table 11 also shows the% solvent uptake after 1300 hours exposure to mixed vapors of ethylene carbonate and dimethyl carbonate.

TABLE 11

Example 12, comparative examples FF and JJ

One 8.5 x 10 inch NW-1 sample and two 8.5 x 10 inch CNW-B samples were cut. One of the CNW-B samples was annealed at 400 ℃ for 2 minutes; and the NW-1 sample was annealed at 450 ℃ for 2 minutes. From each of the nanofiber webs thus prepared, a specimen of 5mm × 60mm was cut and its breaking strength was measured. The breaking strength was determined by clamping the specimen to a spring balance on a test stand and then manually stretching the other end of the specimen until it broke. The breaking strength was calculated by dividing the breaking load by the cross-sectional area.

Another four 5mm x 60mm samples from each nanoweb were immersed in a solution of ethyl methyl carbonate and ethylene carbonate (70/30, v/v) at room temperature for a week seven days, after which they were rinsed extensively with deionized water and dried in a nitrogen purged vacuum oven at 100 ℃ for 16 hours. The breaking strength was measured again. The results are summarized in Table 12. Each data point represents the average of four determinations.

TABLE 12

Example 13 and comparative example KK

The 8.5 '. times.10' CNW-B and NW-2 sheets were annealed at 300 ℃ for 2 minutes. The imidization degree of the NW-2 sample thus treated was determined to be 96.5%. The thus heated sample was cut into 5X 60mm strips. The breaking strength of the four strips of each sample was determined using the spring balance method described above. Four additional strips of each sample were immersed in a solution of ethyl methyl carbonate (TCI) and ethylene carbonate (Sigma Aldrich) (70/30, v/v) in a sealed scintillation vial for a period of 48 hours, after which they were rinsed extensively with deionized water and then treated with N₂The drying was carried out at 105 ℃ for 2 hours in a vacuum furnace with a purge. The breaking strength of the thus solvent-treated specimen was subsequently determined. The results are summarized in table 13 and fig. 10. Each data point represents the average of four determinations. Table 13 shows the breaking load, thickness and basis weight before and after solvent exposure for the two nanowebs.

Watch 13

Example 14

Four 8.5 "× 10" BPDA/RODA sample sheets were heated in an air convection oven at 200 ℃ for 2 minutes and at 220 ℃ for 30 minutes, and then calendered at 1500pli between a package calender roll and a metal calender roll. After calendering, the samples were then annealed in an air convection oven at a temperature range of 300-. The crystallinity index of each sample was determined according to the method above. The break strength of the 6 x 0.5cm strip was determined by using a hand balance to stretch until break and recording the force required to break each sample. Fig. 11 and table 14 show the correlation between the breaking strength and the crystallinity in these samples.

Example 15

Preparation of double electric layer capacitor

NW-3, which had been imidized at 350 ℃ for 2 minutes and then annealed at 450 ℃ for 2 minutes, exhibited the characteristics shown in table 15.

Watch 15

Coin battery assembly

The housing, end caps, gaskets, wave springs and gasket disks for a 2032-type coin cell (Hohsen corp., Osaka Japan via Pred Materials, New York, USA) were stored in a glove box (Vacuum Atmosphere Company, Hawthorne, CA) operated with an argon Atmosphere. Two 0.625 inch diameter commercial grade carbon (PTFE-based activated carbon) coated aluminum foil electrodes were punched from their plates. The electrode disk thus prepared was dried in a vacuum oven (Neytech, Model Number 94-94-400) at 90 ℃ for 18 hours. Two 0.75 inch diameter disks were punched out of 8in 10in NW-3 samples, followed by 9 in a Neytech vacuum ovenDrying at 0 deg.C for 18 hr. Electrolyte solution (A)1M tetraethylammonium tetrafluoroborate in acetonitrile) from Honeywell (Morristown, NJ).

The coin cell was then assembled inside a glove box. The PP washer is pushed into the top end cap. A first carbon electrode disk was placed in the coin cell housing and four drops of electrolyte were added using a plastic pipette. Two layers of NW-3 disks were then placed on top of the wet electrode, followed by a second carbon electrode. Four drops of electrolyte were added to the second electrode. The shim disk was placed on the second carbon electrode, followed by the wave spring and the end cap with washer. The coin cell thus assembled was crimped using an automatic coin cell crimper (Hohsen Corporation, Model No HSACC-D2032). Excess electrolyte outside the coin cell was wiped off and the cell was removed from the glove box for further conditioning and electrochemical testing.

Battery testing

The 2032 coin cell electric double layer capacitor was tested by cycling it between 1.0V and 2.5V for 5 cycles at a current of 10 mA. All cycling tests (charging at a constant current of 10mA followed by discharging at a constant current of 10mA, rest steps at 15 minute intervals) were performed using a Maccor 32 channel circulator (model 4000). The charge and discharge capacitances for cycles 4 and 5 are shown in table 16.

TABLE 16

Claims (14)

1. A method of making a nanoweb, the method comprising: (a) assembling nanofibers to form a nanofiber web, wherein the nanofibers comprise polyamic acid; (b) imidizing the polyamic acid nanofibers at a selected temperature to provide a nanoweb comprising polyimide nanofibers; and (c) subjecting the nanoweb to a temperature of at least 50 ℃ above the selected temperature for a period of time in the range of from about 1 to about 20 minutes.

2. The method of claim 1, wherein the nanoweb comprises a reinforced nanoweb.

3. The method of claim 1, further comprising layering a first electrode material, the nanoweb, and a second electrode material in sequence to produce a multilayer article therefrom.

4. The method of claim 1, wherein the polyimide comprises a fully aromatic polyimide.

5. The method of claim 1 wherein the fully aromatic polyimide comprises PMDA/ODA.

6. The method of claim 3, wherein in the multilayer article the first electrode material, the nanoweb, and the second electrode material are placed in adhering contact with each other in the form of a laminate.

7. The method of claim 3 or claim 6, further comprising placing at least one metallic current collector in adhering contact with at least one of the first or second electrode materials.

8. The method of claim 3, further comprising sequentially layering

A first layer comprising a first metallic current collector;

a second layer comprising the first electrode material in adherent contact with the first metallic current collector;

a third layer comprising the nanoweb in adherent contact with the first electrode material;

a fourth layer comprising the second electrode material in adhering contact with the reinforced nanoweb;

and

a fifth layer comprising a second metallic current collector in adherent contact with the second electrode material.

9. The method of claim 8, wherein in the multilayer article, the first metallic current collector is a copper foil; the first electrode material is graphite; the wholly aromatic polyimide is PMDA/ODA, and the second electrode material is lithium cobalt oxide; and the second metallic current collector is an aluminum foil.

10. The method of claim 3 or claim 6, wherein the first and second electrode materials are the same in the multilayer article.

11. The method of claim 3 or claim 6, wherein the first and second electrode materials are different in the multilayer article.

12. The method of claim 3, wherein the first and second electrode materials are the same in the multilayer article.

13. The method of claim 3, wherein the first and second electrode materials are different in the multilayer article.

14. The method of claim 3, wherein in the multilayer article the first and second metallic current collectors are aluminum foils, the first and second electrode materials are carbon, and the fully aromatic polyimide is PMDA/ODA.