Crosslinked polyolefin separator for lithium ion circulating batteries and method of making same

Abstract

The invention relates to a cross-linked polyolefin separator for a lithium ion circulating battery and a method of making the same. A battery circulating lithium ions includes a negative electrode, a positive electrode, and a separator sandwiched between opposing major facing surfaces of the negative electrode and the positive electrode. The separator has an open microporous structure and contains a polyolefin having a cross-linked structure. The separator has a thermal shrinkage of less than or equal to about 5% in a direction parallel to opposing major facing surfaces of the negative and positive electrodes when the separator is heated at a temperature of greater than or equal to about 145 degrees Celsius for about 1 hour. The separator may be fabricated from a precursor film having an open cellular structure and comprising linear or branched polyolefin molecules. The precursor film may be exposed to a source of free radicals such that covalent bonds are formed between the polyolefin molecules and a crosslinked polyolefin having a relatively high molecular weight.

Description

Technical Field

[0001] The present disclosure relates to separators for cycling lithium-ion batteries, and more particularly to separators having improved thermal shrinkage resistance and improved wettability. Background Art

[0002] The information provided in this section is intended to generally introduce the background of the present disclosure. To the extent that work currently named as inventors is described in this section, and aspects of the specification that may not have been otherwise identified as prior art at the time of filing, are not admitted, either explicitly or implicitly, as prior art to the present disclosure.

[0003] A battery pack for circulating lithium ions generally includes a negative electrode and a positive electrode separated from each other by a separator, and an ion-conductive electrolyte that permeates the separator, which provides a medium for lithium ion conduction between the negative electrode and the positive electrode during discharge and charge of the battery pack. The separator is configured to physically separate and electrically isolate the negative electrode and the positive electrode from each other while allowing lithium ions to pass through. Commercial separators are generally made of microporous polyolefin materials such as polypropylene (PP) and / or polyethylene (PE), which exhibit a combination of good mechanical strength and chemical stability. However, such polyolefin materials may shrink if exposed to high temperatures (e.g., temperatures greater than or equal to about 130°C), and due to their non-polar nature, may exhibit poor wettability and absorption of electrolytes containing polar organic solvents. Summary of the invention

[0004] According to one or more embodiments of the present disclosure, a battery pack for circulating lithium ions includes a negative electrode, a positive electrode, and a separator. The negative electrode and the positive electrode are spaced apart from each other and have relative main facing surfaces. The separator has an open microporous structure and is sandwiched between the relative main facing surfaces of the negative electrode and the positive electrode. The separator comprises a polyolefin having a cross-linked structure, and when the separator is heated at a temperature greater than or equal to about 145 degrees Celsius for about 1 hour, the separator has a thermal shrinkage of less than or equal to about 5% in a direction parallel to the relative main facing surfaces of the negative electrode and the positive electrode.

[0005] In embodiments, the separator may have a thermal shrinkage of less than or equal to about 10% in a direction parallel to opposing major facing surfaces of the negative electrode and the positive electrode when the separator is heated at a temperature greater than or equal to about 200 degrees Celsius for about 1 hour.

[0006] The polyolefin may include polyethylene, polypropylene, or a combination thereof.

[0007] The polyolefin may have a cross-linking degree of greater than or equal to about 10% and less than or equal to about 80%.

[0008] In aspects, the polyolefin can have a degree of crosslinking of greater than or equal to about 60% and less than or equal to about 70%.

[0009] The separator may further include a ceramic material.

[0010] The spacer may have a thickness greater than or equal to about 5 micrometers and less than or equal to about 500 micrometers.

[0011] The battery may further include a non-aqueous polar aprotic organic solvent that permeates the open microporous structure of the separator.

[0012] According to one or more embodiments of the present disclosure, a method for manufacturing a battery pack for circulating lithium ions includes exposing a precursor film to a free radical source. The precursor film has an open microporous structure and contains linear or branched polyolefin molecules. The precursor film is exposed to the free radical source so that covalent bonds are formed between the polyolefin molecules and a separator containing a cross-linked polyolefin is formed, and the cross-linked polyolefin has a relatively high molecular weight compared to the linear or branched polyolefin molecules in the precursor film. The separator is sandwiched between the opposite main facing surfaces of the negative electrode and the positive electrode and is infiltrated with an electrolyte.

[0013] The precursor film may include polyethylene, polypropylene, or a combination thereof.

[0014] Exposing the precursor film to the free radical source may include irradiating the precursor film with an electron beam, a plasma, ionizing radiation, non-ionizing radiation, or a combination thereof.

[0015] Exposing the precursor film to the free radical source may include irradiating the precursor film with an electron beam or gamma radiation. In this case, the precursor film may be exposed to the free radical source in the presence of water.

[0016] Exposing the precursor film to the free radical source may include applying a chemical cross-linking agent to the precursor film. The chemical cross-linking agent may include a peroxide, a benzophenone, or a combination thereof.

[0017] Exposing the precursor film to the free radical source may further include irradiating the precursor film with non-ionizing radiation.

[0018] The precursor film may be exposed to the free radical source in an inert gas environment or a negative pressure environment.

[0019] The cross-linked polyolefin may have a cross-linking degree of greater than or equal to about 10% and less than or equal to about 80%.

[0020] In aspects, the cross-linked polyolefin may have a degree of cross-linking of greater than or equal to about 60% and less than or equal to about 70%.

[0021] The precursor film may further comprise a ceramic material.

[0022] The precursor film may have a thickness greater than or equal to about 5 micrometers and less than or equal to about 500 micrometers.

[0023] The present invention relates to the following scheme:

[0024] Scheme 1. A battery pack for circulating lithium ions, the battery pack comprising:

[0025] Negative electrode;

[0026] a positive electrode spaced apart from the negative electrode, the negative electrode and the positive electrode having opposing major facing surfaces; and

[0027] A separator having an open microporous structure and sandwiched between the relative main facing surfaces of the negative electrode and the positive electrode, the separator comprising a polyolefin having a cross-linked structure, and when the separator is heated at a temperature greater than or equal to about 145 degrees Celsius for about 1 hour, the separator has a thermal shrinkage of less than or equal to about 5% in a direction parallel to the relative main facing surfaces of the negative electrode and the positive electrode.

[0028] Option 2. A battery pack according to Option 1, wherein when the separator is heated at a temperature greater than or equal to about 200 degrees Celsius for about 1 hour, the separator has a thermal shrinkage of less than or equal to about 10% in a direction parallel to the relative main facing surfaces of the negative electrode and the positive electrode.

[0029] Option 3. The battery pack according to Option 1, wherein the polyolefin comprises polyethylene, polypropylene or a combination thereof.

[0030] Embodiment 4. The battery pack according to embodiment 1, wherein the polyolefin has a cross-linking degree greater than or equal to about 10% and less than or equal to about 80%.

[0031] Embodiment 5. The battery pack according to embodiment 1, wherein the polyolefin has a cross-linking degree of greater than or equal to about 60% and less than or equal to about 70%.

[0032] Option 6. The battery pack according to Option 1, wherein the separator further comprises a ceramic material.

[0033] Option 7. The battery pack according to Option 1, wherein the separator has a thickness greater than or equal to about 5 microns and less than or equal to about 500 microns.

[0034] Solution 8. The battery pack according to Solution 1, further comprising:

[0035] A non-aqueous polar aprotic organic solvent penetrates the open microporous structure of the separator.

[0036] Scheme 9. A method for manufacturing a cycling lithium ion battery, the method comprising:

[0037] exposing a precursor film having an open microporous structure and comprising linear or branched polyolefin molecules to a free radical source so that covalent bonds are formed between the polyolefin molecules and a separator comprising a cross-linked polyolefin having a relatively high molecular weight compared to the linear or branched polyolefin molecules in the precursor film is formed;

[0038] sandwiching the separator between opposing major facing surfaces of the negative electrode and the positive electrode; and

[0039] The separator is impregnated with an electrolyte.

[0040] Option 10. The method according to Option 9, wherein the precursor film comprises polyethylene, polypropylene or a combination thereof.

[0041] Option 11. The method according to Option 9, wherein exposing the precursor film to the free radical source includes irradiating the precursor film with an electron beam, plasma, ionizing radiation, non-ionizing radiation, or a combination thereof.

[0042] Option 12. The method according to Option 9, wherein exposing the precursor film to the free radical source comprises irradiating the precursor film with an electron beam or gamma radiation, and wherein the precursor film is exposed to the free radical source in the presence of water.

[0043] Option 13. The method of Option 9, wherein exposing the precursor film to the free radical source comprises applying a chemical cross-linking agent to the precursor film.

[0044] Embodiment 14. The method according to embodiment 13, wherein the chemical cross-linking agent comprises a peroxide, a benzophenone or a combination thereof.

[0045] Embodiment 15. The method according to Embodiment 13, wherein exposing the precursor film to the free radical source further comprises irradiating the precursor film with non-ionizing radiation.

[0046] Option 16. The method according to Option 9, wherein the precursor film is exposed to the free radical source in an inert gas environment or a negative pressure environment.

[0047] Embodiment 17. The method according to Embodiment 9, wherein the cross-linked polyolefin has a degree of cross-linking greater than or equal to about 10% and less than or equal to about 80%.

[0048] Embodiment 18. The method according to Embodiment 9, wherein the cross-linked polyolefin has a degree of cross-linking greater than or equal to about 60% and less than or equal to about 70%.

[0049] Option 19. A method according to Option 9, wherein the precursor film further comprises a ceramic material.

[0050] Option 20. A method according to Option 9, wherein the precursor film has a thickness greater than or equal to about 5 microns and less than or equal to about 500 microns.

[0051] Further areas of applicability of the present disclosure will become apparent from the detailed description, claims and drawings.The detailed description and specific examples are intended for illustration only and are not intended to limit the scope of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] The present disclosure will be more fully understood from the detailed description and accompanying drawings, in which:

[0053] Figure 1 is a schematic perspective view of a motor vehicle powered by a battery pack including a plurality of battery modules.

[0054] Figure 2 yes Figure 1 Schematic cross-sectional view of a portion of one of the battery modules of FIG. 1 , the battery module including a plurality of cycling lithium-ion electrochemical cells or batteries.

[0055] Figure 3 is a schematic cross-sectional view of a battery stack for cycling lithium ions, the battery stack including a positive electrode, a negative electrode, a porous separator, and an electrolyte that permeates the positive and negative electrodes and the porous separator.

[0056] Among the drawings, reference numerals may be repeated to identify similar and / or identical elements. DETAILED DESCRIPTION

[0057] The disclosed method can be used to form a separator for cycling lithium ion batteries having improved resistance to thermal shrinkage and improved electrolyte wettability.The separator can be formed by crosslinking a polyolefin-containing film to form a highly crosslinked polyolefin structure.

[0058] Figure 1 A motor vehicle 2 is depicted that is powered by an electric motor 4 that draws power from a battery pack 6 that includes one or more battery modules 8. The battery modules 8 may be electrically coupled together in a series and / or parallel arrangement to meet the desired capacity and power requirements of the electric motor 4. The vehicle 2 may be a pure electric vehicle and may be powered entirely by the electric motor 4, or the vehicle 2 may be a hybrid vehicle and may be powered by the electric motor 4 and an internal combustion engine (not shown).

[0059] like Figure 2As shown in , each battery module 8 includes one or more electrochemical cells or batteries 10 that circulate lithium ions. In practice, the batteries 10 in the battery module 8 are typically assembled as stacked layers, which include a negative electrode layer 12, a negative electrode current collector 13, a positive electrode layer 14, a positive electrode current collector 15 and a separator layer 16. Each battery 10 is defined by a negative electrode layer 12 and a positive electrode layer 14 separated from each other by a separator layer 16. In practice, the separator layer 16 can be permeated with an electrolyte that provides a medium for lithium ion conduction between the negative electrode layer 12 and the positive electrode layer 14, or the separator layer 16 itself can serve as an electrolyte. The negative electrode layer 12 is disposed on the negative electrode current collector 13 and is electrically connected to the negative electrode current collector 13, while the positive electrode layer 14 is disposed on the positive electrode current collector 15 and is electrically connected to the positive electrode current collector 15. As shown Figure 2 As shown in , for efficiency, these layers can be stacked so that some of the negative electrode collectors 13 and some of the positive electrode collectors 15 are double-sided and include negative electrode layers 12 or positive electrode layers 14 on both sides thereof, respectively. In this arrangement, adjacent negative electrode layers 12 and positive electrode layers 14 share a single negative electrode collector 13 or positive electrode collector 15, respectively.

[0060] Figure 3 An electrochemical cell or battery 20 of circulating lithium ions is depicted. The battery 20 can generate an electrical current during discharge that can be used to power a load device (e.g., an electric motor 4), and can be recharged by connection to a power source. Figure 1 and 2 1 , in aspect, the battery pack 20 may be used to supply power to the motor 4 of the motor vehicle 2. Additionally or alternatively, the battery pack 20 may be used in other transportation applications (e.g., motorcycles, boats, tractors, buses, motorbikes, mobile homes, campers, tanks, and aircraft), and may be used to power stationary and / or portable electronic devices, components, and devices used in a wide variety of other industries and applications, including industrial, residential and commercial buildings, consumer products, industrial equipment and machinery, agricultural or farm equipment, and heavy machinery (as non-limiting examples).

[0061] The battery 20 includes a negative electrode 22, a positive electrode 24, a separator 26, and an electrolyte 28, which provides a medium for lithium ion conduction between the negative electrode 22 and the positive electrode 24. The negative electrode 22 has a major facing surface 38, and the positive electrode 24 has a major facing surface 40, which faces the negative electrode 22 opposite the major facing surface 38 of the negative electrode 22. The negative electrode 22 is disposed on a major surface of a negative electrode current collector 30, and the positive electrode 24 is disposed on a major surface of a positive electrode current collector 32. In practice, the negative electrode current collector 30 and the positive electrode current collector 32 are electrically coupled to a power source or load 34 (e.g., a motor 4) via an external circuit 36. The negative electrode 22 and the positive electrode 24 are configured so that an electrochemical potential difference is established between the negative electrode 22 and the positive electrode 24 when the battery 20 is at least partially charged. During discharge of the battery 20, the electrochemical potential established between the negative electrode 22 and the positive electrode 24 drives spontaneous reduction and oxidation (redox) reactions within the battery 20 and releases lithium ions and electrons at the negative electrode 22. The released lithium ions travel from the negative electrode 22 through the separator 26 and the electrolyte 28 to the positive electrode 24, while the electrons travel from the negative electrode 22 to the positive electrode 24 via the external circuit 36, which generates an electric current. After the negative electrode 22 has been partially or completely depleted of lithium, the battery 20 can be charged by connecting the negative electrode 22 and the positive electrode 24 to a power source 34, which drives non-spontaneous redox reactions within the battery 20 and releases lithium ions and electrons from the positive electrode 24. Repeated discharge and charging of the battery 20 may be referred to herein as a "cycle", with a complete charge event followed by a complete discharge event being considered a complete cycle.

[0062] The separator 26 is configured to physically separate and electrically isolate the negative electrode 22 and the positive electrode 24 from each other while allowing lithium ions to pass through. The separator 26 has an open microporous structure including a plurality of open pores that define a plurality of channels extending from a first side 46 thereof to an opposite second side 48. The separator 26 is sandwiched between the opposite major facing surfaces 38, 40 of the negative electrode 22 and the positive electrode 24. The separator 26 is configured to resist thermal shrinkage in a direction 42 parallel to a plane 44 defined between and extending parallel to the major facing surfaces 38, 40 of the negative electrode 22 and the positive electrode 24, which can help prevent short circuits in the battery pack 20. The separator 26 may have a thickness extending in a thickness direction perpendicular to the major facing surfaces 38, 40 of the negative electrode 22 and the positive electrode 24 of greater than or equal to about 5 microns (μm), optionally greater than or equal to about 10 μm, or optionally greater than or equal to about 20 μm and less than or equal to about 500 μm, optionally less than or equal to about 200 μm, or optionally less than or equal to about 50 μm.

[0063] The separator 26 comprises a polyolefin or a poly(olefin). The polyolefin may include a homopolymer or copolymer of two or more olefin monomers, or a copolymer of one or more olefin monomers and one or more different monomers. Examples of polyolefins include polyethylene (PE), polypropylene (PP), polymethylpentene (PMP), polybutene-1 (PB-1), ethylene-octene copolymers, olefin block copolymers, propylene-butane copolymers, polystyrene (PS) and combinations thereof. In an embodiment, the separator 26 may include a laminate of polyolefins, such as a laminate of PE and PP. Prior to infiltration with the electrolyte 28, the polyolefin having a highly cross-linked structure may account for greater than or equal to about 80% by weight of the separator 26, optionally greater than or equal to about 90%, optionally greater than or equal to about 95%, or optionally greater than or equal to about 98%.

[0064] The polyolefin in the separator 26 has a highly cross-linked structure, which means that the polymer chains of the polyolefin are chemically connected to each other through multiple covalent bonds called bridges. The cross-linking of the polymer chains of the polyolefin increases the molecular weight of the polyolefin. The polymer with a cross-linked structure is different from the polymer with a linear structure, in which the polymer chains are bound together by relatively weak van der Waals forces or hydrogen bonds. The polymer with a cross-linked structure is different from the polymer with a branched structure, in which short chains extend from the polymer main chain (or main chain) but do not form a bridge.

[0065] The highly cross-linked structure of the polyolefin provides excellent heat shrinkage resistance and improved wettability to the separator 26 compared to a microporous polyolefin membrane without any cross-linking or with a relatively low degree of cross-linking. The polyolefin in the separator 26 may have a degree of cross-linking (also referred to as a cross-linking density) of greater than or equal to about 10%, optionally greater than or equal to about 20%, optionally greater than or equal to about 30%, optionally greater than or equal to about 40%, optionally greater than or equal to about 50%, optionally greater than or equal to about 60%, or optionally greater than or equal to about 70% and less than or equal to about 80%.

[0066] The relatively high degree of crosslinking in the polyolefin ensures that the polyolefin undergoes a relatively small amount of thermal shrinkage when heated at a temperature of about 130 degrees Celsius (° C.) or greater. The thermal shrinkage of separator 26 can be calculated according to formula (1):

[0067] Thermal shrinkage (%) = (SA0-SA) / SA0×100%, (1)

[0068] Wherein SA0 is the surface area of ​​the separator 26 before being heated, and SA is the surface area of ​​the separator 26 after being heated. The surface area (SA0 and SA) of the separator 26 is the area of ​​the main surface of the separator 26 extending in the direction 42 substantially parallel to the main facing surfaces 38, 40 of the negative electrode 22 and the positive electrode 24, perpendicular to the thickness direction of the separator 26. For example, the surface area (SA0 and SA) of the separator 26 can be defined by the area of ​​the first side 46 or the second side 48 of the separator 26.

[0069] In an embodiment, when the separator 26 is heated at a temperature of about 145 degrees Celsius or greater for about 1 hour, the separator has a thermal shrinkage of less than or equal to about 10%, or optionally less than or equal to 5%, in a direction 42 parallel to the relative major facing surfaces 38, 40 of the negative and positive electrodes 22, 24. In an embodiment, when the separator 26 is heated at a temperature of about 200 degrees Celsius or greater for about 1 hour, the separator may have a thermal shrinkage of less than or equal to about 20%, or optionally less than or equal to 10%, in a direction 42 parallel to the relative major facing surfaces 38, 40 of the negative and positive electrodes 22, 24.

[0070] It is noteworthy that the degree of crosslinking in the polyolefin can be controlled or adjusted so that the polyolefin retains the ability to perform a thermal shutdown function. During thermal shutdown of the battery pack 20, the polyolefin in the separator 26 softens, which causes the pores of the separator 26 to close and block the lithium ion transmission between the negative electrode 22 and the positive electrode 24, effectively preventing the operation of the battery pack 20. In other words, the degree of crosslinking in the polyolefin can be controlled or adjusted so that the polyolefin does not become a thermosetting polymer, which does not soften when heated. In an embodiment of a polyolefin including polyethylene, when the separator 26 is heated at a temperature greater than or equal to about 130°C and less than or equal to about 145°C, the thermal shutdown of the battery pack 20 can be initiated by the polyolefin.

[0071] In an embodiment, the separator 26 may include a ceramic material. In an embodiment, the ceramic material may be in the form of a coating disposed on the first side 46 and / or the second side 48 of the separator 26. In an embodiment, the ceramic material may be throughout the separator 26, between the first side 46 and the second side 48 of the separator 26. In an embodiment, the ceramic material may be substantially uniform throughout the separator 26. Examples of ceramic materials include aluminum oxide (Al2O3) and / or silicon dioxide (SiO2).

[0072] The electrolyte 28 is ionically conductive and provides a lithium ion conductive medium for passing through the separator 26 (between the negative electrode 22 and the positive electrode 24). In practice, the electrolyte 28 is introduced into the battery pack 20 so that the electrolyte 28 penetrates the open micropores of the separator 26. The electrolyte 28 comprises an organic solvent and a lithium salt in the organic solvent. The organic solvent is a non-aqueous polar aprotic organic solvent. Non-limiting examples of non-aqueous polar aprotic organic solvents include cyclic carbonates (e.g., ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC)); linear carbonates (e.g., dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC)); aliphatic carboxylic acid esters (e.g., methyl formate, methyl acetate, methyl propionate); lactones (e.g., γ-butyrolactone, γ-valerolactone and / or δ-valerolactone); nitriles (e.g., succinonitrile); , glutaronitrile and / or adiponitrile); sulfone (e.g., tetramethylene sulfone, ethyl methyl sulfone, vinyl sulfone, phenyl sulfone, 4-fluorophenyl sulfone, benzyl sulfone and / or cyclopentane sulfone); aliphatic ether (e.g., triethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, 1,3-dimethoxypropane, 1,2-dimethoxyethane, 1,2-diethoxyethane and / or ethoxymethoxyethane); cyclic ether (e.g., 1,4-dioxane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane); phosphate (e.g., triethyl phosphate and / or trimethyl phosphate); and combinations thereof. The lithium salt is soluble in an organic solvent and provides a path for lithium ions to pass through the electrolyte 28. The lithium salt may include an inorganic lithium salt, an organic lithium salt, or a combination thereof. Examples of lithium salts include lithium hexafluorophosphate (LiPF6), lithium difluorophosphate (LiPO2F2), lithium perchlorate (LiClO4), lithium tetrachloroaluminate (LiAlCl4), lithium iodide (LiI), lithium bromide (LiBr), lithium thiocyanate (LiSCN), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(trifluoromethane)sulfonyl imide (LiN(CF3SO2)2), lithium bis(fluorosulfonyl)imide (LiN(FSO2)2)(LiSFI), lithium tetraphenylborate (LiB(C6H5)4), lithium bis(oxalato)borate (LiB(C2O4)2)(LiBOB), lithium difluoro(oxalato)borate (LiBF2(C2O4))(LiDFOB), and combinations thereof.

[0073] The negative electrode 22 is configured to store and release lithium ions to facilitate charging and discharging of the battery 20, respectively. The negative electrode 22 includes an electrochemically active (electroactive) material that can store and release lithium ions by undergoing a reversible redox reaction with lithium during charging and discharging of the battery 20. Examples of electroactive negative electrode materials include lithium, lithium-based materials, lithium alloys (e.g., alloys of lithium and silicon, aluminum, indium, tin, or a combination thereof), carbon-based materials (e.g., graphite, activated carbon, carbon black, hard carbon, soft carbon, and / or graphene), silicon, silicon-based materials (e.g., silicon oxide, alloys of silicon and tin, iron, aluminum, cobalt, or a combination thereof and / or a composite of silicon and / or silicon oxide and carbon), tin oxide, aluminum, indium, zinc, germanium, silicon oxide, lithium silicon oxide, lithium silicide, titanium oxide, lithium titanate, and a combination thereof. In aspects, the negative electrode 22 may further include a polymer binder and an optional conductive material.

[0074] The positive electrode 24 is configured to store and release lithium ions during discharge and charge of the battery 20. The positive electrode 24 comprises an electroactive material, a polymer binder, and an optional conductive material. The electroactive material of the positive electrode 24 can store and release lithium ions by undergoing a reversible redox reaction with lithium at a higher electrochemical potential than the electrochemically active material of the negative electrode 22, so that there is an electrochemical potential difference between the negative electrode 22 and the positive electrode 24. The electroactive material of the positive electrode 24 may include a material capable of lithium insertion and deinsertion or a material capable of conversion reaction with lithium. In aspects, where the electroactive material of the positive electrode 24 includes an intercalation host material that can reversibly insert or embed lithium ions, the electroactive material of the positive electrode 24 may include a lithium transition metal oxide.

[0075] The negative electrode current collector 30 and the positive electrode current collector 32 are conductive and provide electrical connections between the external circuit 36 ​​and the negative electrode 22 and the positive electrode 24, respectively. In one aspect, the negative electrode current collector 30 and the positive electrode current collector 32 can be made of metal and can be in the form of a non-porous metal foil, a perforated metal foil, a porous metal mesh, or a combination thereof. The negative electrode current collector 30 can be made of copper, nickel, or alloys thereof, stainless steel, or other suitable conductive materials. The positive electrode current collector 32 can be made of aluminum (Al) or other suitable conductive materials.

[0076] method

[0077] The separator 26 can be manufactured by subjecting a precursor film having an open microporous structure and comprising a polyolefin to a crosslinking treatment. The polyolefin in the precursor film comprises a plurality of linear or branched polyolefin molecules. In some embodiments, the precursor film may further comprise a ceramic material, such as Al2O3 and / or SiO2, which may be formed as a coating on the surface of the precursor film and / or substantially uniformly throughout the precursor film.

[0078] The precursor film can be formed by molding the polyolefin-containing precursor into the shape of a film, for example, by extrusion, casting, blowing, or a combination thereof, and then optionally subjecting the formed film to a uniaxial or biaxial stretching process. The precursor film can be obtained from commercial battery separator manufacturers, such as Asahi Kasei Corporation, Celgard, ENTEK (e.g., ENTEK12EPH), LG Chem Ltd., SEMCORP Global, SK Innovations LTD., Sojo Electric Co., Toray, UBE Corporation, and Sumitomo Chemical Co. Ltd.

[0079] Crosslinking of the precursor film can be performed by exposing the precursor film to a free radical source, so that covalent bonds are formed between the polyolefin molecules in the precursor film and a highly crosslinked polyolefin having a relatively high molecular weight compared to the molecular weight of the linear or branched polyolefin molecules present in the precursor film is produced. The precursor film can be exposed to the free radical source, for example, by irradiation, application of a chemical crosslinking agent and / or by heat treatment.

[0080] In embodiments, the precursor film can be crosslinked by irradiating the precursor film with an electron beam, plasma, ionizing radiation, non-ionizing radiation, and combinations thereof. Examples of ionizing radiation include X-rays, gamma radiation, high energy ultraviolet light, and combinations thereof. Examples of non-ionizing radiation include radio waves, microwaves, visible light, infrared light, ultraviolet light, and combinations thereof. In some embodiments, the precursor film can be crosslinked by irradiating the precursor film in the presence of water.

[0081] Chemical crosslinking agents can contribute to the generation of free radicals and / or act as a free radical source during the crosslinking process. In embodiments where the precursor film is crosslinked by irradiation with non-ionizing radiation and / or by thermal treatment, it may be necessary to use a chemical crosslinking agent. Examples of chemical crosslinking agents include inorganic peroxides, organic peroxides, azo compounds, amines, amides, silanes, epoxy resins, isocyanates, and combinations thereof. More specific examples of chemical crosslinking agents include diacyl peroxides, hydroperoxides, peresters, peroxyesters, peroxycarbonates, dialkyl peroxides, perketals, ketone peroxides, peroxyketals, cyclic peroxides, diaryl ketones (e.g., benzophenone), peroxycarbonates, dicumyl peroxides, benzoyl peroxides, dibenzoyl peroxides, dilauryl peroxides, methyl ether ketone peroxides, and combinations thereof.

[0082] In the embodiment of using chemical crosslinking agent during crosslinking, chemical crosslinking agent can be applied to precursor film, so that chemical crosslinking agent is deposited on the surface of precursor film substantially uniformly, including its open micropores. In aspect, chemical crosslinking agent can be applied to precursor film by preparing solution containing chemical crosslinking agent in polar solvent, and then infiltrating or impregnating precursor film with the solution, so that the solution fills its open micropores. Polar solvent can include aqueous solvent (for example, water) or organic solvent. Examples of organic solvents include acetic acid, formic acid, alcohol (ethanol, methanol and / or isopropanol), ethyl acetate, tetrahydrofuran (THF), dichloromethane, acetone, acetonitrile (ACN), dimethylformamide (DMF) and combination thereof. Precursor film can be infiltrated or impregnated with solution containing chemical crosslinking agent, for example, by ultrasonic spraying, dip coating or flow coating.

[0083] Cross-linking of the precursor film may be performed in an inert gas atmosphere (eg, argon and / or nitrogen) and / or in a negative pressure environment (eg, vacuum).

[0084] In embodiments, crosslinking of the precursor film may be accomplished by infiltrating the precursor film with an isopropyl alcohol solution containing 5% by weight benzophenone and then irradiating the precursor film with ultraviolet light having a wavelength of 290 nanometers (nm) to 320 nm (UV-B light) for about 5 minutes.

[0085] The foregoing description is merely exemplary and is absolutely not intended to limit the present disclosure, its application or use. The broad teachings of the present disclosure can be implemented in various forms. Therefore, although the present disclosure includes specific examples, the true scope of the present disclosure should not be limited thereto, because after studying the drawings, the specification and the following claims, other modifications will become apparent. It should be understood that one or more steps in the method may be implemented in different orders (or simultaneously) without changing the principles of the present disclosure. In addition, although each embodiment is described above as having certain features, any one or more features described with respect to any embodiment of the present disclosure may be implemented in any other embodiment and / or combined with the features of any other embodiment, even if the combination is not explicitly described. In other words, the embodiments are not mutually exclusive, and the mutual replacement of one or more embodiments is still within the scope of the present disclosure.

[0086] Various terms are used to describe the spatial and functional relationships between elements (e.g., between modules, circuit elements, semiconductor layers, etc.), including "connected," "engaged," "coupled," "adjacent," "next to," "on top of," "on," "under," and "disposed." Unless explicitly described as "directly," when describing the relationship between a first element and a second element in the above disclosure, the relationship can be a direct relationship in which there are no other intervening elements between the first element and the second element, or an indirect relationship in which there are one or more intervening elements (spatially or functionally) between the first element and the second element. The phrase "at least one of A, B, and C" used herein should be interpreted as meaning a logical (A or B or C) using a non-exclusive logical OR, and should not be interpreted as meaning "at least one of A, at least one of B, and at least one of C." As used herein, the term "and / or" includes a combination of one or more associated listed items.

[0087] The terms used herein are intended to be limited only for the purpose of describing exemplary embodiments. Unless the context clearly indicates otherwise, the singular forms "one" and "the" used herein are intended to also include plural forms. The terms "comprising", "including" and "having" are inclusive, and therefore specify the presence of specified features, elements, compositions, steps, integers, operations and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or their combinations. Although the open terms "comprising", "including" and "having" should be understood as non-limiting terms for describing and requesting the various embodiments described herein, in some aspects, these terms are understood as more restrictive and constraining terms, such as "consisting of..." or "consisting essentially of..." Therefore, for any given embodiment of a composition, material, component, element, ingredient, feature, integer, operation and / or method step, the present disclosure also clearly includes an embodiment consisting of or essentially consisting of these enumerated compositions, materials, components, elements, ingredients, features, integers, operations and / or method steps. In the case of “consisting of,” the alternative embodiment does not include any additional compositions, materials, components, elements, ingredients, features, integers, operations and / or method steps, while in the case of “consisting essentially of,” such an embodiment does not include any additional compositions, materials, components, elements, ingredients, features, integers, operations and / or method steps that substantially affect the basic and novel characteristics, but may include any compositions, materials, components, elements, ingredients, features, integers, operations and / or method steps that do not substantially affect the basic and novel characteristics in the embodiment.

[0088] Any method step, method and operation described herein should not be interpreted as necessarily requiring them to be implemented in the order discussed or illustrated, unless explicitly specified as the implementation order.It is also to be understood that, unless otherwise specified, additional or alternative steps can be used.Although the terms first, second, third, etc. may be used in this article to describe various steps, elements, components, regions, layers and / or sections, unless otherwise specified, these steps, elements, components, regions, layers and / or sections should not be limited by these terms.These terms are only used to distinguish a step, element, component, region, layer or section from another step, element, component, region, layer or section.Unless the context clearly indicates, terms such as "first", "second" and other ordinal terms do not imply order or sequence when used in this article.Therefore, the first step, element, component, region, layer or section discussed below can be referred to as the second step, element, component, region, layer or section without departing from the teaching of exemplary embodiments.

[0089] Throughout this disclosure, numerical values ​​represent approximate measurements or range limits, and include slight deviations from given values ​​and embodiments that have approximately the listed values ​​as well as embodiments that have exactly the listed values. In addition to the examples provided at the end of the specific embodiments, all numerical values ​​of parameters (such as amounts or conditions) in this specification should be understood to be modified by the term "about" in all cases, regardless of whether "about" actually appears before the numerical value. The numerical values ​​of the parameters in the attached claims should be understood to be modified by the term "about" only when the term "about" appears before the numerical value. "Approximately" means that the specified numerical value allows a certain slight imprecision (close to the accuracy of the value; approximately or reasonably close to the value; almost). If the imprecision provided by "approximately" is not understood in this ordinary sense in the art, "approximately" used herein at least refers to changes that may be caused by ordinary methods of measuring and using such parameters. For example, "approximately" may include changes of less than or equal to 5%, optionally less than or equal to 4%, optionally less than or equal to 3%, optionally less than or equal to 2%, optionally less than or equal to 1%, optionally less than or equal to 0.5%, and in some aspects, optionally less than or equal to 0.1%. Additionally, disclosure of ranges includes disclosure of all values ​​within the entire range and further subdivided ranges, including endpoints and subranges given within those ranges.

[0090] As used herein, unless otherwise indicated, the terms "composition" and "material" are used interchangeably to refer generally to materials that contain at least a preferred chemical component, element, or compound, but may also contain additional elements, compounds, or substances, including trace impurities. An "X-based" composition or material generally refers to a composition or material in which "X" is the single largest component of the composition or material on a weight percent (%) basis. This can include compositions or materials having greater than 50% by weight of X, and can also include compositions or materials having less than 50% by weight of X, as long as X is the single largest component of the composition or material based on its total weight. When a composition or material is referred to as being "substantially free" of a substance, the composition or material may contain less than 5%, optionally less than 3%, optionally less than 1%, or optionally less than 0.1% by weight of the substance.

[0091] As used herein, the term "metal" may refer to a pure elemental metal, or to an alloy of an elemental metal and one or more other metals or nonmetallic elements, referred to as "alloying" elements. Alloying elements may be selected to impart certain desirable properties to the alloy that are not exhibited by the base metal element.

Claims

1. A battery pack for circulating lithium ions, the battery pack comprising: Negative electrode; a positive electrode spaced apart from the negative electrode, the negative electrode and the positive electrode having opposing major facing surfaces; A separator having an open microporous structure and sandwiched between the opposite main facing surfaces of the negative electrode and the positive electrode, the separator comprising a polyolefin having a cross-linked structure, the polyolefin comprising polyethylene, polypropylene or a combination thereof, the polyolefin having a cross-linking degree of greater than or equal to about 10% and less than or equal to about 80%, and when the separator is heated at a temperature of greater than or equal to about 145 degrees Celsius for about 1 hour, the separator has a thermal shrinkage of less than or equal to about 5% in a direction parallel to the opposite main facing surfaces of the negative electrode and the positive electrode; and A non-aqueous polar aprotic organic solvent penetrates the open microporous structure of the separator.

2. The battery pack according to claim 1, wherein: When the separator is heated at a temperature greater than or equal to about 200 degrees Celsius for about 1 hour, the separator has a thermal shrinkage of less than or equal to about 10% in a direction parallel to opposing major facing surfaces of the negative electrode and the positive electrode. 3 . The battery pack according to claim 1 , wherein the polyolefin has a cross-linking degree of greater than or equal to about 60% and less than or equal to about 70%.

4. The battery of claim 1, wherein the separator has a thickness greater than or equal to about 5 microns and less than or equal to about 500 microns, and optionally wherein the separator further comprises a ceramic material.

5. A method of manufacturing a cycling lithium ion battery, the method comprising: exposing a precursor film having an open microporous structure and comprising linear or branched polyolefin molecules to a free radical source so that covalent bonds are formed between the polyolefin molecules and a separator comprising a cross-linked polyolefin having a relatively high molecular weight compared to the linear or branched polyolefin molecules in the precursor film, the precursor film comprising polyethylene, polypropylene, or a combination thereof, the cross-linked polyolefin having a cross-linking degree of greater than or equal to about 10% and less than or equal to about 80% is formed; sandwiching the separator between opposing major facing surfaces of the negative electrode and the positive electrode; and impregnating the separator with an electrolyte, Wherein exposing the precursor film to the free radical source comprises irradiating the precursor film with an electron beam, a plasma, an ionizing radiation, a non-ionizing radiation, or a combination thereof.

6. The method of claim 5, wherein exposing the precursor film to the free radical source comprises irradiating the precursor film with an electron beam or gamma radiation, and wherein the precursor film is exposed to the free radical source in the presence of water.

7. The method of claim 5, wherein exposing the precursor film to the free radical source comprises irradiating the precursor film with non-ionizing radiation, wherein exposing the precursor film to the free radical source further comprises applying a chemical crosslinking agent to the precursor film, and wherein the chemical crosslinking agent comprises a peroxide, a benzophenone, or a combination thereof.

8. The method of claim 5, wherein the precursor film is exposed to the free radical source in an inert gas environment or a negative pressure environment.

9. The method of claim 5, wherein the cross-linked polyolefin has a degree of cross-linking of greater than or equal to about 60% and less than or equal to about 70%.

10. The method of claim 5, wherein the precursor film has a thickness greater than or equal to about 5 microns and less than or equal to about 500 microns, and optionally wherein the precursor film further comprises a ceramic material.

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