LITHIUM-BASED BATTERY SEPARATOR AND METHOD FOR MAKING THE SAME

Abstract

Lithium-based battery separator comprising: a porous polymer membrane having opposing surfaces; a porous carbon coating formed on one of the opposing surfaces of the porous polymer membrane; and polycations incorporated into the porous carbon coating, the porous polymer membrane, or both the porous carbon coating and the porous polymer membrane, wherein the polycations are incorporated into the porous polymer membrane and have a gradient distribution increasing in the direction of the other of the opposing surfaces of the porous polymer membrane, or the polycations are incorporated into the porous carbon coating and have a gradient distribution increasing in the direction of one of the opposing surfaces of the porous polymer membrane.

Description

CROSS REFERENCE TO RELATED APPLICATIONSThis application claims priority to U.S. Provisional Patent Application Serial No. 61 / 941,054 (U.S. Patent No. 2015 / 0 236 324 A1), filed February 18, 2014, which is incorporated herein by reference in its entirety.BACKGROUNDSecondary or rechargeable lithium-sulfur batteries or lithium-ion batteries are often used in many stationary and portable devices, for example those encountered in the consumer electronics, automotive and aeronautical industries. The lithium class of batteries has gained popularity for various reasons, including relatively high energy density, general non-occurrence of memory effect when making comparisons with other types of rechargeable batteries, relatively low internal resistance, and low self-discharge rate when not in use. The ability of lithium batteries to undergo repeated energy cycling over their useful life makes them an attractive and reliable energy source.US 2012 / 0 171 573 A1 discloses battery separators which can consist of a porous polymeric material and are coated with ionic particles. These particles consist of a metal oxide core and an anionic or cationic polyelectrolyte applied to the core.CN 102 185 158 A discloses lithium-sulfur batteries in which an adsorption layer, which can contain porous carbon, is located on a separator membrane made of a polymer material. In the battery, this adsorption layer is positioned between the positive electrode and the separator membrane.SUMMARYA lithium-based battery separator includes a porous polymer membrane having opposing surfaces. On one of the opposite surfaces of the porous polymer membrane, a porous carbon coating is formed. Polycations are incorporated into the porous carbon coating, the porous polymer membrane or both the porous carbon coating and the porous polymer membrane, the polycations being incorporated into the porous polymer membrane and having a gradient distribution increasing in the direction of the other of the opposing surfaces of the porous polymer membrane, or the polycations being incorporated into the porous carbon coating and having a gradient distribution increasing in the direction of the one of the opposing surfaces of the porous polymer membrane.Examples of the lithium-based battery separator disclosed herein may be included in a separator for a lithium ion battery or a lithium-sulfur battery.BRIEF DESCRIPTION OF THE DRAWINGSFeatures and advantages of examples of the present disclosure will become apparent from the following detailed description and drawings, in which like reference numerals correspond to similar, though perhaps non-identical, components. For the sake of brevity, reference numerals or features having a previously described function may or may not be described in connection with other drawings in which they occur. FIG. 1 is a schematic perspective view of an example of a lithium-sulfur battery showing a charge and discharge state, the battery including an example of the separator according to the present disclosure. FIG. 2 is a schematic perspective view of another example of the lithium-sulfur battery showing a charge and discharge state, the battery including another example of the separator according to the present disclosure. FIG. 3 is a schematic perspective view of an example of a lithium ion battery showing a discharge state, the battery including an example of the separator according to the present disclosure. FIG. 4 is a schematic perspective view of another example of a lithium ion battery showing a discharge state, the battery including another example of the separator according to the present disclosure. FIG. 5 is a graph depicting capacity (mAh / g s, left Y-axis labeled "C") vs. cycle number (X-axis labeled "#") and coulombic efficiency (%, right Y-axis labeled "%") vs. cycle number (X-axis labeled "#") for an example lithium-sulfur cell with an example of the lithium-sulfur battery separator disclosed herein for a first comparative lithium-sulfur cell with a first comparative battery separator and for a second comparative lithium-sulfur cell with a second comparative battery separator. FIG. 6 is a graph depicting the voltage profiles (voltage (V) vs. capacitance (C)) for the example lithium-sulfur cell, the first comparative lithium-sulfur cell, and the second comparative lithium-sulfur cell in the second cycle. FIG. 7A is a backscattered electron image of the second comparative battery separator; and FIG. 7B is a gray-scale representation of the element map of fluorine (F) for the backscattered electron image of FIG. 7A.DETAILED DESCRIPTIONLithium-sulfur and lithium ion batteries generally operate by reversibly flowing lithium ions between a negative electrode (sometimes called anode) and a sulfur-based or lithium-based positive electrode (sometimes called cathode). The negative and positive electrodes are located on opposite sides of a porous polymer separator impregnated with an electrolyte solution suitable for conducting the lithium ions. Each of the electrodes is connected to respective current collectors connected by an interruptible external circuit that allows an electric current to flow between the negative and positive electrodes.For a lithium-sulfur battery, the useful life may be limited by the relatively poor conductivity of sulfur and by the migration, diffusion, or shuttle of lithium polysulfide intermediates (LiS x, where x 2<x<8) from the sulfur-based positive electrode during the battery discharge process, through the porous polymer separator, to the negative electrode. The lithium polysulfide intermediates produced at the sulfur-based positive electrode are soluble in the electrolyte and can migrate to the negative electrode where they react with the negative electrode in a parasitic fashion to produce lower order lithium polysulfide intermediates. These lower order lithium polysulfide intermediates diffuse back to the cathode and regenerate the higher forms of lithium polysulfide intermediates. As a result, a shuttle effect occurs. This effect results in reduced sulfur utilization, self-discharge, poor cyclability, and reduced Coulombic efficiency of the battery. Even a small amount of lithium polysulfide intermediates form an insoluble molecule, for example, dilithium sulfide (Li 2 S), which can permanently bind to the negative electrode. This results in parasitic loss of active lithium at the negative electrode, preventing reversible electrode operation and reducing the useful life of the lithium-sulfur battery.In addition, it has been found that lithium ion batteries are impaired by dissolution of transition metal cations from the positive electrode, leading to accelerated capacity fading and thus to a loss of battery durability. The transition metal cations migrate from the positive electrode to the negative electrode of the battery, resulting in their "poisoning". In one example, a graphite electrode is metabolized by Mn 2+-, Mn 3+- or Mn 4+- cations, which are soluble from spinel Li x Mn 2 O 4 of the positive electrode. For example, the Mn 2+- cations can migrate through the battery electrolyte and deposit on the graphite electrode. When the Mn 2+- cations are deposited on the graphite, they become Mn metal. It has been shown that a relatively small amount (e.g., 90 ppm) of Mn atoms can poison the graphite electrode and prevent reversible electrode operation, thereby reducing the useful life of the battery. The deleterious effect of the Mn deposited on the negative electrode is significantly enhanced during battery exposure to temperatures above ambient temperature (>40 °C), regardless of whether exposure occurs only during storage (i.e., simply standing at open circuit voltage in a certain state of charge) or during battery operation (i.e., during charging, during discharging, or during charge / discharge cycling).The shuttle of lithium polysulfide intermediates to the negative electrode in the lithium-sulfur battery or the poisoning of the lithium ion battery by transition metals that separate from the positive electrode can be reduced or prevented by using the battery separator disclosed herein. The examples of the separator include a porous polymer membrane, a porous carbon coating, and a polycation present in the coating and / or in the membrane (wherein the presence in the membrane depends, at least in part, on the polycation molecular weight), wherein the polycations are incorporated into the porous polymer membrane and have a gradient distribution increasing in the direction of the other of the opposing surfaces of the porous polymer membrane, or the polycations are incorporated into the porous carbon coating and have a gradient distribution increasing in the direction of the one of the opposing surfaces of the porous polymer membrane. The porous carbon coating can mitigate or prevent the shuttle of lithium polysulfide intermediates in the lithium-sulfur battery or the migration of transition metal cations in the lithium-ion battery by a variety of mechanisms. As an example, the porous carbon coating may absorb the lithium polysulfide intermediates. As another example, a soft acid-soft base interaction can occur between the polycation (i.e., the soft acid) and the lithium polysulfide intermediates (i.e., the soft base). As yet another example, the polycations may fill at least some of the pores of the membrane and / or the coating, thereby further contributing to mitigation or prevention of lithium polysulfide intermediates shut off in the lithium-sulfur battery. In yet another example, the presence of the polycation present in the coating and / or in the membrane may repel the transition metal cation(s) and prevent them from migrating through to the negative electrode. The reduction or elimination of lithium polysulfide intermediate migration in the lithium-sulfur battery results in higher sulfur utilization and enhanced lithium-sulfur battery cyclability and overall performance. Similarly, the reduction or elimination of transition metal cation migration in the lithium ion battery results in higher graphite, silicon, or other anode material utilization and enhanced lithium ion battery cyclability and overall performance.Referring now to FIGS. 1 and 2, two examples of the lithium-sulfur battery 30, 30' are shown. The illustrated examples of the lithium-sulfur battery 30, 30' in FIGS. 1 and 2, respectively, include different examples of the lithium-sulfur battery separator 10, 10' disclosed herein.Separators 10, 10' disclosed herein include porous polymer membrane 12 having two opposing surfaces 13, 15, porous carbon coating 14 formed on one of opposing surfaces 13, and polycation(s) 16 present / present in membrane 12 (as shown in Figure 1) or present / present in coating 14 (as shown in Figure 2). It should be understood that in yet another example, although not shown, the polycation(s) 16 may (may) be present in both the membrane 12 and the coating 14.The porous polymer membrane 12 may be formed of, for example, a polyolefin. The polyolefin may be a homopolymer (derived from a single monomer component) or a heteropolymer (derived from more than one monomer component) and may be either linear or branched. When a heteropolymer derived from two monomer components is used, the polyolefin may assume any copolymer chain arrangement including that of a block copolymer or a random copolymer. The same applies when the polyolefin is a heteropolymer derived from more than two monomer constituents. As examples, the polyethylene (PE), polypropylene (PP), a mixture of PE and PP, or multilayer structured porous films of PE and / or PP may be used. Commercially available porous polymer membranes 12 include single layer polypropylene membranes, for example, CELGARD 2400 and CELGARD 2500 from Celgard, LLC (Charlotte, NC). It is appreciated that the porous polymer membrane 12 may be coated or treated or uncoated or untreated. For example, the porous polymer membrane may or may not be coated or may not comprise surfactant treatment thereon.In other examples, the porous polymer membrane 12 may be formed of another polymer selected from polyethylene terephthalate (PET), polyvinylidene fluoride (PVdF), polyamides (nylon), polyurethanes, polycarbonates, polyesters, polyetheretherketones (PEEK), polyethersulfones (PES), polyimides (PI), polyamideimides, polyethers, polyoxymethylene (e.g., acetal), polybutylene terephthalate, polyethylene naphthenate, polybutene, polyolefin copolymers, acrylonitrile-butadiene-styrene copolymers (ABS), polystyrene copolymers, polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), polysiloxane polymers (e.g., polydimethylsiloxane (PDMS)), polybenzimidazole (PBI), Polybenzoxazole (PBO), polyphenylenes (e.g., PARMAXTM (Mississippi Polymer Technologies, Inc., Bay Saint Louis, Mississippi)), polyarylene ether ketones, polyperfluorocyclobutanes, polytetrafluoroethylene (PTFE), vinylidene fluoride copolymers and terpolymers, polyvinylidene chloride, polyvinyl fluoride, liquid crystalline polymers (e.g., VECTRANTM (Hoechst AG, Germany), and ZENITE® (DuPont, Wilmington, DE)), polyaramides, polyphenylene oxide, and / or combinations thereof. It is believed that another example of a liquid crystalline polymer that can be used for the porous polymer membrane is poly(p-hydroxybenzoic acid). In yet another example, the porous polymer membrane 12 may be selected from a combination of the polyolefin (e.g., PE and / or PP) and one or more polymers listed above.The porous polymer membrane 12 may be a single layer or a multilayer (e.g., two-layer, three-layer, etc.) laminate prepared by a dry or wet process. A single layer of the polyolefin and / or other listed polymer may form, for example, the entirety of the porous polymer membrane 12. As another example, however, multiple separate layers of similar or different polyolefins and / or polymers may be assembled into the porous polymer membrane 12. In one example, a separate layer of one or more of the polymers may be coated onto a separate layer of the polyolefin to form the porous polymer membrane 12. Further, the polyolefin (and / or other polymer) layer and any other optional polymer layers may be included in the porous polymer membrane 12 as a fibrous layer to aid in providing the porous polymer membrane 12 with suitable structural and porosity characteristics. Still other suitable porous polymeric membranes 12 include those having a ceramic layer affixed thereto (positioned between the remainder of the membrane 12 and the porous carbon coating 14) and those having ceramic filler in the polymeric matrix (i.e., an organic-inorganic composite matrix).The porous carbon coating 14 is formed on a surface 13 of the porous polymer membrane 12. As shown in both Figures 1 and 2, this surface 13 is the surface 13 or 15 which faces (or will face) the positive electrode when incorporated into the lithium-sulfur battery 30, 30'.The polycation(s) 16 penetrates (penetrate) into at least some of the pores of the porous polymer membrane 12 and / or the porous carbon coating 14. As schematically illustrated in FIGS. 1 and 2, the polycation(s) 16 has a gradient distribution across the membrane 12 (FIG. 1 ) and / or the coating 14 (FIG. 2 ). By "gradient distribution" is meant that the concentration of the polycation(s) 16 is highest in a region of the porous polymer membrane 12 and / or the porous carbon coating 14 facing or to be facing the negative electrode 20 and decreases through the porous polymer membrane 12 and / or the porous carbon coating 14 in the direction of a region facing or to be facing the positive electrode 18. In the example shown in FIG. 1, the gradient distribution (100→0) decreases moving from the surface 15 to the surface 13. Accordingly, in the example shown in FIG. 2, the gradient distribution (100→0) decreases moving from the surface 13 to an outer surface 17 of the porous carbon coating 14. When the polycation(s) 16 is (are) present in both the porous polymer membrane 12 and the porous carbon coating 14, the gradient distribution decreases moving from the surface 15 of the porous polymer membrane 12 to the outer surface 17 of the porous carbon coating 14.The gradient distribution of the polycation(s) 16 may be formed during the formation of the porous carbon coating 14 on the porous polymer membrane 12. This method will now be described.Both the gradient distribution of the polycation(s) 16 and the porous carbon coating 14 may be formed using a slurry. The slurry comprises porous carbon particles and a polycation solution.The porous carbon particles used in the slurry may be formed by exposing a carbon precursor to a predetermined temperature in an inert atmosphere (e.g., argon gas, nitrogen gas, etc.). This process carbonizes the carbon precursor to form the porous carbon particles having a pore volume in the range of about 5 cm 3 / g to about 6 cm 3 / g. In one example, porous carbon having a pore volume of about 5.68 cm 3 / g is obtained using an organometallic framework-5 (MOF-5) as the carbon precursor and about 1000° C. as the predetermined temperature. As other examples, the carbon precursor may be Al-PCP (Al(OH)(1,4-naphthalenedicarboxylate) or ZIF-8 (Zn(2-methylimidazolate)2). Generally, the carbonization temperature may range from about 800° C. to about 1200° C.The porous carbon particles used in the slurry may also be commercially available carbons. Examples include activated carbon, for example AX-21 or XE-2, and mesoporous carbon, for example, cmk-3 or cmk-8.The polycation solution used in the slurry may comprise an organic solvent and the polycation(s) 16. In one example, the polycation solution comprises N-methyl-2-pyrrolidone (NMP) as the organic solvent and polyethyleneimine (trifluoromethanesulfonyl) imide (PEI-TFSI) as the polycation 16. Further examples of the organic solvent comprise dimethylformamide (DMF), methanol, etc. However, it is understood that the organic solvent that is selected may depend on the solubility of the selected polycation. Further examples of the polycation 16 include poly(diallyldimethylammonium chloride) in which chlorine (Cl) is replaced by TFSI or poly(acrylamide-co-diallyldimethylammonium chloride) (AMAC) in which chlorine (Cl) is replaced by TFSI.The polycation solution comprising NMP and PEI-TFSI can be formed by first titrating an aqueous polyethyleneimine (PEI) solution to pH 7 using a bis-(trifluoromethanesulfonyl)imide (HTFSI) methanol solution. This forms the PEI-TFSI product in water and methanol. The reaction of PEI and HTFSI is shown below: wherein TFSI x- is a bis(trifluoromethylsulfonyl)imide anion, x is in the range of 100 to 100,000, and n is in the range of 100 to 100,000. It is understood that the value of x depends on the number average molecular weight of the polyethylene imine used.Water and methanol can be removed from the solution using a vacuum or evaporator. The liquid removal process leaves the PEI-TFSI product which can be dissolved in the desired organic solvent. Generally, the amount of PEI-TFSI product in organic solvent yields a polycation solution having about 20 wt% of polycation 16.To form the slurry, the porous carbon particles may be added to the polycation solution, or the polycation solution may be added to the porous carbon particles. In one example, the ratio of carbon to polycation solution in the slurry is in the range of about 5% to about 50%.The slurry may then be deposited on the porous polymer membrane 12 using any suitable technique. As examples, the slurry may be poured onto the surface 13 of the porous polymer membrane 12, or may be spread on the surface 13 of the porous polymer membrane 12, or may be applied to the surface 13 of the porous polymer membrane 12 using a slot die coater.The deposited slurry may then be subjected to a drying process. The drying process forms i) the porous carbon coating 14 on / on the surface 13 and ii) the gradient distribution of the polycation(s) 16. Thus, the gradient distribution formed may depend, at least in part, on the drying rate. Generally, drying occurs at a temperature ranging from about 25° C. to about 65° C. for a time ranging from about 12 hours to about 48 hours. In one example, the drying process is performed at about 25° C. for about 24 hours. For example, the higher the temperature used during the drying process, the smaller the concentration gradient of the polycation(s) 16 from the porous carbon coating 14 to the polymer membrane 12.Whether the polycation(s) 16 penetrates (penetrates) the porous polymer membrane 12, the porous carbon coating, or both, 12 and 14, depends, at least in part, on the molecular size of the polycation(s) 16 and the size of the pores of the porous polymer membrane 12.If the pores of the porous polymer membrane 12 are larger than the molecular size of the polycation(s) 16, the polycations 16 may penetrate into the membrane 12 during drying as shown in Figure 1. If the drying process is conducted at a sufficiently low temperature and / or for a sufficiently long time and the polycations 16 are smaller than the pores of the membrane 12, all of the polycation (all of the polycations) 16 will migrate from the formed porous carbon coating 14 into the porous polymer membrane 12. However, the drying process can be regulated so that polycation migration stops before all of the polycation(s) 16 has been moved into the porous polymer membrane 12. In these cases, the polycation gradient distribution may extend through at least a portion of the porous polymer membrane 12 and at least a portion of the porous carbon coating 14. In one example, the molecular size of the polycation(s) 16may be regulated by selecting a specific molecular weight of the polycation(s) 16. Smaller molecular weight polycations 16 can travel through a variety of membrane pore sizes and allow the polycation(s) 16 to penetrate (penetrate) into the membrane 12 during drying as shown in Figure 1.When the pores of the porous polymer membrane 12 are smaller than the molecular size of the polycation(s) 16, the polycations 16 cannot penetrate the membrane 12 during drying. In these cases, the polycation(s) 16 migrates toward the surface 13 of the porous polymer membrane 12 in the porous carbon coating 14, resulting in the gradient distribution formed in the porous carbon coating 14, as shown in FIG. 2. In one example, the gradient shown in FIG. 2 may be achieved by selecting a high molecular weight polycation(s) 16, thereby preventing the polycation(s) 16 from penetrating the membrane 12 during drying.When the polycation(s) 16 is(are) located at or near the interface between the surface 13 of the porous polymer membrane 12 and the porous carbon coating 14, the polycation(s) 16 may serve as a binder for the porous carbon coating 14.As shown in both FIGS. 1 and 2, the separators 10, 10' are located between the positive electrode 18 and the negative electrode 20 so that the porous carbon coating 14 faces the positive electrode 18. The separators 10, 10' each act as an electrical insulator (preventing the occurrence of a short circuit), mechanical support and barrier to prevent physical contact between the two electrodes 18, 20. Separators 10, 10' also assure passage of lithium ions (identified by Li +) through an electrolyte filling their pores. As discussed above, however, separators 10, 10' also block the passage of polysulfide intermediates by absorption in porous carbon coating 14, a soft acid-soft base interaction between polycation 16 and polysulfide intermediates and / or polycation(s) 16 that fill at least some of the pores of membrane 12 and / or coating 14.The positive electrode 18 of the lithium-sulfur battery 30, 30' may be formed of any sulfur-based active material that sufficiently undergoes lithium intercalation and deintercalation while functioning as a positive terminal of the battery 30, 30'. Examples of sulfur-based active materials include S 8, Li 2 S 8, Li 2 S 6, Li 2 S 4, Li 2 S 3, Li 2 S2, and Li2S. Another example of the sulfur-based active material includes a sulfur-carbon composite material. In one example, the weight ratio of S to C in the sulfur-carbon composite is in the range of 1:9 to 8:1.The positive electrode 16' may also comprise a polymeric binder material to structurally hold the sulfur-based active material together. The polymer binder material may be made of at least one of polyvinylidene fluoride (PVdF), an ethylene propylene diene monomer (EPDM) rubber, carboxymethylcellulose (CMC), styrene butadiene rubber (SBR), styrene butadiene rubber carboxymethylcellulose (SBR CMC), polyacrylic acid (PAA), cross-linked polyacrylic acid-polyethyleneimine, polyimide, polyvinyl alcohol (PVA), polyimide, poly(acrylamide-co-diallyldimethylammonium chloride), polyethylene oxide (PEO), or sodium alginate, or other water soluble binders. Moreover, the positive electrode 18 may include a conductive carbon material. In one example, the conductive carbon material is a high specific surface area carbon, for example, acetylene black (i.e., carbon black). Other examples of suitable conductive fillers that may be used alone or in combination with carbon black include graphene, graphite, carbon nanotubes, and / or carbon nanofibers. A specific example of a combination of conductive fillers is carbon black and carbon nanofibers.The positive electrode 18 may include about 40 wt % to about 90 wt % (i.e., 90 wt %) of the sulfur-based active material. The positive electrode 18 may include from 0% to about 30% by weight of the conductive filler. Additionally, the positive electrode 18 may comprise from 0% to about 20% by weight of the polymer binder. In one example, the positive electrode 18 comprises about 85 wt % of the sulfur-based active material, about 10 wt % of the conductive carbon material, and about 5 wt % of the polymer binder material.The negative electrode 20 of the lithium-sulfur battery 30, 30' may comprise any lithium host material 44 that can sufficiently undergo lithium plating and stripping while acting as the negative terminal of the lithium-sulfur battery 30, 30'. Examples of the lithium host negative electrode material (i.e., the active material) 44 include graphite or an amorphous carbon having a low specific surface area. Graphite is widely used as a lithium host material because it has characteristics of reversible lithium intercalation and deintercalation, is relatively non-reactive, and can store lithium in amounts that produce a relatively high energy density. Commercial forms of graphite that can be used to make the negative electrode 20 are available from, for example, Timcal Graphite & Carbon (Bodio, Switzerland), Lonza Group (Basle, Switzerland) or Superior Graphite (Chicago, IL). Other materials may also be used to form the lithium host material 44 of the negative electrode 20, for example, lithium titanate, silicon or silicon-carbon composites, tin oxide, or lithiated silicon (e.g., LiSi x). In FIGS. 1 and 2, porous silicon is shown as the lithium host material.The negative electrode 20 may also include a polymer binder material 48 to structurally hold the lithium host material together. Examples of binders include polyvinylidene fluoride (PVdF), an ethylene-propylene-diene monomer (EPDM) rubber, sodium alginate, styrene-butadiene rubber (SBR), styrene-butadiene rubber carboxymethylcellulose (SBR-CMC), polyacrylic acid (PAA), cross-linked polyacrylic acid-polyethyleneimine, polyvinyl alcohol (PVA), polyimide, poly(acrylamide-co-diallyldimethylammonium chloride), polyethylene oxide (PEO), or carboxymethylcellulose (CMC). These negative electrode materials may also be mixed with a high specific surface area carbon, for example, acetylene black (i.e., carbon black) or other conductive filler 46. Further examples of suitable conductive fillers 46, which may be used alone or in combination with carbon black, include graphene, graphite, carbon nanotubes, and / or carbon nanofibers. A specific example of a combination of conductive fillers is carbon black and carbon nanofibers. The conductive filler 46 may be used to ensure electron conduction between the lithium negative electrode host material 44 and, for example, a negative-side current collector 20 a.As shown in FIGS. 1 and 2, the lithium-sulfur battery 30, 30' may also include a positive-side current collector 18a and the aforementioned negative-side current collector 20a disposed in contact with the positive electrode 18 and the negative electrode 20, respectively, to move free electrons to an external circuit 22 and collect free electrons from an external circuit 22. The positive-side current collector 18 amay be formed of aluminum or any other suitable electrically conductive material known to those skilled in the art. The negative-side current collector 20 amay be formed of copper or any other suitable electrically conductive material known to those skilled in the art.The positive electrode 18, the negative electrode 20 and the separator 10, 10' are each impregnated with an electrolyte solution. Any suitable electrolyte solution capable of conducting lithium ions between the negative electrode 20 and the positive electrode 18 may be used in the lithium-sulfur battery 30, 30'. In one example, the non-aqueous electrolyte solution may be an ether-based electrolyte stabilized with lithium nitrite. Other non-aqueous liquid electrolyte solutions may comprise a lithium salt dissolved in an organic solvent or a mixture of organic solvents. Examples of lithium salts that can be dissolved in the ether to form the non-aqueous liquid electrolyte solution include LiClO 4, LiAlCl 4, LiI, LiBr, LiB(C 2 O 4)2( LiBO), LiF 2( C 2 O 4) ( LiODFB), LiSC, LiF 4, LiB(C 6 H 5)4, LiAsF 6, LiCF 3 SO 3, LiN(FSO 2)2( LIFSI), LiN(CF 3 SO 2)2( LITFSI), LiPF e, LiPF 4( C 2 O 4) ( LiFOP), LiNO3and mixtures thereof. The ether-based solvents may consist of cyclic ethers, for example 1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, and chain structure ethers, for example 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxymethane, tetraethylene glycol dimethyl ether (PEGDME), and mixtures thereof.The lithium-sulfur battery 30, 30' also includes an interruptible external circuit 22 that connects the positive electrode 18 and the negative electrode 20. The lithium-sulfur battery 30, 30' may also support a load device 24 operatively connected to the external circuit 22. The load device 24 receives a supply of electrical energy from the electrical current passing through the external circuit 22 as the lithium-sulfur battery 30, 30 a discharges. While the load device 24 may be any number of known electrically powered devices, few specific examples of a power-consuming load device include an electric motor for a hybrid vehicle or a fully electric vehicle, a laptop computer, a cell phone, and a cordless power tool. However, the load device 24 may also be a power generating apparatus that charges the lithium-sulfur battery 30, 30' for the purpose of energy storage. For example, the tendency of windmills and solar cells to variably and / or intermittently generate electricity often leads to the need to store excess energy for later use.The lithium-sulfur battery 30, 30' may include a wide range of other components, which, although not shown herein, are known to those skilled in the art. For example, for performance or other purposes, the lithium-sulfur battery 30, 30' may include a housing, gaskets, terminals, tabs, and any other desirable components or materials that may be located between or around the positive electrode 18 and the negative electrode 20. Moreover, the size and shape of the lithium-sulfur battery 30, 30' as well as the design and chemical structure of its major components may vary depending on the particular application for which it is designed. Battery powered vehicles and portable consumer electronics devices are, for example, two cases where the lithium-sulfur battery 30, 30' will most likely be developed with different size, capacity, and power output specifications. The lithium-sulfur battery 30, 30' may also be connected in series and / or parallel with other similar lithium-sulfur batteries 30, 30' to produce a greater voltage output and current (if arranged in parallel) or a higher voltage (if arranged in series) if the load device 24 requires this.The lithium-sulfur battery 30, 30' may generate a usable electric current during battery discharge (shown in FIGS. 1 and 2 by reference numeral 26). During discharge, the chemical processes in the battery 30, 30' include lithium (Li +)- dissolution from the surface of the negative electrode 20, and incorporation of the lithium cations into alkali metal polysulfide salts (i.e., Li 2 S n, for example, Li 2 S 8, Li 2 S 6, Li 2 S 4, Li 2 S2, and Li2S) into the positive electrode 18. Thus, polysulfides are formed within the positive electrode 18 in sequence (sulfur is reduced), while the battery 30, 30' is discharging. The chemical potential difference between the positive electrode 18 and the negative electrode 20 (in the range of about 1.5 to 3.0 volts, depending on the exact chemical structure of the electrodes 18, 20) directs electrons produced by dissolution of lithium at the negative electrode 20 through the outer circuit 22 toward the positive electrode 18.The lithium-sulfur battery 30, 30' may be charged or re-energized at any time by connecting an external power source to the lithium-sulfur battery 30, 30' to reverse the electrochemical reactions that occur during battery discharge. During charging (shown in FIGS. 1 and 2 by reference numeral 28), lithium plating occurs at the negative electrode 20 and sulfur formation occurs in the positive electrode 18. The connection of an external power source to the lithium-sulfur battery 30, 30' forces the otherwise non-spontaneous oxidation of lithium at the positive electrode 18 to produce electrons and lithium ions. The electrons flowing back to the negative electrode 20 through the external circuit 22 and the lithium ions (Li +), carried by the electrolyte through the separator 10, 10' back to the negative electrode 20, coalesce at the negative electrode 20 and refill them with lithium for consumption during the next battery discharge cycle. The external power source that can be used to charge the lithium-sulfur battery 30, 30' may vary depending on the size, design, and particular end use of the lithium-sulfur battery 30, 30'. Some suitable external power sources include a battery charger plugged into an AC outlet and an automotive alternator.In FIGS. 3 and 4, the lithium ion battery 40, 40' includes the negative electrode 20, the negative-side current collector 20 a, a positive electrode 18', the positive-side current collector 18 a, and the separator 10, 10' positioned between the negative electrode 20 and the positive electrode 18'. It is understood that the separator 10 shown in FIG. 3 and the separator 10' shown in FIG. 4 may be the same type of porous separators 10, 10' described with reference to FIGS. 1 and 2, respectively. In addition, in the lithium ion battery 40, 40', the negative current collector 20a and the positive current collector 18a described herein for the lithium-sulfur battery 30, 30' may be used.As shown in both FIGS. 3 and 4, the separators 10, 10' are positioned between the positive electrode 18' and the negative electrode 20 such that the porous carbon coating 14 faces the positive electrode 18'. The separators 10, 10' each act as an electrical insulator (prevent the occurrence of a short circuit), a mechanical support, and a barrier to prevent physical contact between the two electrodes 18', 20. Separators 10, 10' also assure passage of lithium ions (identified by Li +) through an electrolyte filling its pores. However, as discussed above, separators 10, 10' include positively charged polycation(s) 16 that can (can) repel the positively charged transition metal cations and thus block passage of the transition metal cations through separators 10, 10').In FIGS. 3 and 4, the positive electrode 18' may be formed from any lithium-based active material that sufficiently undergoes lithium insertion and de-insertion while aluminum or other suitable current collector acts as the positive terminal of the lithium ion battery 40, 40'. A common class of known lithium-based active materials 42 suitable for the positive electrode 18 includes lithium transition metal layered oxides. Some specific examples of the lithium-based active materials 42 include spinel lithium manganese oxide (LiMn 2 O 4), lithium cobalt oxide (LiCoO 2), nickel manganese oxide spinel [Li(Ni 0.5 Mn 1.5) O 2], a nickel manganese cobalt layered oxide [Li(Ni x Mn y CO z) O 2 or Li(Ni x Mn y Co z) O 4] or a lithium iron polyanion oxide, for example lithium iron phosphate (LiFePO 4) or lithium iron fluorophosphate (Li 2 FePO4F). Other lithium-based active materials 42 may also be used, for example, LiNi x M 1-x O 2( M consists of any ratio of Al, Co, and / or Mg), aluminum-stabilized lithium manganese oxide spinel (Li x Mn 2-x Al y O 4), lithium vanadium oxide (LiV 2 O 5), Li 2 MSiO4(M consists of any ratio of Co, Fe, and / or Mn), xLi 2 MnO 3-(1- x)LiMO 2( M consists of any ratio of Ni, Mn and / or Co) and any other high efficiency nickel manganese cobalt material. By "any ratio" is meant that any element may be present in any amount. For example, M could be Al with or without Co and / or Mg, or any other combination of the listed elements.The lithium-based positive electrode active material 42 18' may be blended with a polymer binder and a conductive filler (e.g., high specific surface area carbon) (neither of which is shown). Any of the binders previously described for the negative electrode 20 of the lithium-sulfur battery 30, 30' may be used in the positive electrode 18' of the lithium-ion battery 40, 40'. The polymer binder structurally holds the lithium-based active materials and the high-specific-surface-area carbon together. An example of the carbon having a high specific surface area is acetylene black. The high specific surface area carbon ensures electron conduction between the positive-side current collector 18 aand the positive electrode active material particles 18'.The negative electrode 20 of the lithium ion battery 40, 40' may comprise any lithium host material 44 that can sufficiently undergo lithium intercalation and deintercalation while copper or other suitable current collector 18a acts as the negative terminal of the lithium ion battery 40, 40'. Any of the lithium host materials (i.e., active materials) previously described for the negative electrode 20 of the lithium-sulfur battery 30, 30' may be used in the negative electrode 20 of the lithium-ion battery 40, 40'. For example, the lithium host negative electrode material may be graphite or a silicon-based material. In FIGS. 3 and 4, a porous silicon is shown as the lithium host material 33.The lithium-based negative electrode active material 20 may be blended with a polymer binder 48 and a conductive filler 44 (e.g., high specific surface area carbon). Any of the binders described above for the negative electrode 20 of the lithium-sulfur battery 30, 30' may be used in the negative electrode 20 of the lithium-ion battery 40, 40'.Any suitable electrolyte solution capable of conducting lithium ions between the negative electrode 20 and the positive electrode 18 may be used in the lithium ion battery 40, 40'. The positive electrode 18', the negative electrode 20 and the separator 10, 10' are each impregnated with an electrolyte solution. In one example, the electrolyte solution may be a non-aqueous liquid electrolyte solution comprising a lithium salt dissolved in an organic solvent or a mixture of organic solvents. Examples of lithium salts that can be dissolved in an organic solvent to form the non-aqueous liquid electrolyte solution include LiClO 4, LiAlCl 4, LiI, LiBr, LiB(C 2 O 4)2( LiBO), LiF 2( C 2 O 4) ( LiODFB), LiSC, LiF 4, LiB(C 6 H 5)4, LiAsF 6, LiCF 3 SO 3, LiN(FSO 2)2( LiFSI), LiN(CF 3 SO 2)2( LITFSI), LiPF 6, LiPF 4( C 2 O 4) ( LiFOP), LiNO3, and mixtures thereof. Some examples of the organic-based solvent include cyclic carbonates (ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate), linear carbonates (dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate), aliphatic carboxylic esters (methyl formate, methyl acetate, methyl propionate), γ-lactones (γ-butyrolactone, γ-valerolactone), chain structure ethers (1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, tetraglyme), cyclic ethers (tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane), and mixtures thereof.As shown in FIGS. 3 and 4, the lithium ion battery 40, 40' also includes an interruptible external circuit 22 that connects the negative electrode 20 and the positive electrode 18. The lithium ion battery 40, 40' may also supply a load device 24 that may be operatively connected to the external circuit 22. The load device 24 receives a supply of electric energy from the electric current flowing through the external circuit 22 when the lithium ion battery 40, 40' is discharged. Although the load device 22 may be any number of known electrically operated devices, few specific examples of a power-consuming load device 24 include an electric motor for a hybrid automobile or a fully electric vehicle, a laptop computer, a portable telephone, and a cordless power tool. However, the load device 24 can also be an electrical current-generating apparatus which charges the lithium ion battery 40, 40' for the purpose of energy storage. For example, the tendency of windmills and solar cells to variably and / or intermittently generate electricity results in the need to store surplus energy for later use.The lithium ion battery 40, 40' may also include a wide range of other components, which, although not shown herein, are nevertheless known to those skilled in the art. For example, the lithium ion battery 40, 40' may include a housing, gaskets, terminals, tabs, and any other suitable components or materials disposed between or around the negative electrode 20 and the positive electrode 18' for performance or other purposes. Moreover, the size and shape of the lithium ion battery 40, 40' as well as the design and chemical structure of its major components may vary depending on the particular application for which it is designed. Battery-powered automobiles and portable consumer electronics devices are, for example, cases where the lithium ion battery 40, 40' would most likely be developed with varying size, capacity, and power delivery specifications. The lithium ion battery 40, 40' may also be connected in series and / or parallel with other similar lithium ion batteries to produce a higher voltage output and current (when arranged in parallel) or voltage (when arranged in series) when the load device 24 so requires.The lithium ion battery 40, 40' generally operates by reversibly flowing lithium ions between the negative electrode 20 and the positive electrode 18'. In the fully charged state, the voltage of the battery 40, 40' is at a maximum (typically in the range of 2.0 to 5.0 V), while the voltage of the battery 40, 40' in the fully discharged state is at a minimum (typically in the range of 0 V to 2.0 V). Importantly, the Fermi energy levels of the active materials in the positive and negative electrodes 18, 20 change during battery operation, and so does the difference between the two, known as battery voltage. The battery voltage decreases during discharge, with the Fermi levels approaching each other. During charging, the reverse process occurs, with the battery voltage increasing as the Fermi levels move away from each other. During battery discharge, the external load device 24 allows an electric current to flow in the external circuit 22 in such a direction that the difference between the Fermi levels (and correspondingly the cell voltage) decreases. The reverse occurs during battery charging: the battery charger forces an electric current flow in the external circuit 22 in such a direction that the difference between the Fermi levels (and correspondingly the cell voltage) increases.At the beginning of a discharge, the negative electrode 20 of the lithium ion battery 40, 40' contains a high concentration of intercalated lithium, while the positive electrode 18' is relatively depleted. If the negative electrode 20 contains a sufficiently higher relative amount of intercalated lithium, the lithium ion battery 40, 40' can generate a usable electrical current through reversible electrochemical reactions that occur when the external circuit 24 is completed connecting the coated electrode 20 and the positive electrode 18'. The development of the closed external circuit under such circumstances causes the extraction of intercalated lithium from the negative electrode 20.The chemical potential difference between the positive electrode 18' and the negative electrode 20 (in the range of about 2.0 volts to about 5.0 volts, depending on the exact chemical composition of the electrodes 20, 18) directs the electrons (e -), produced by the oxidation of intercalated lithium at the negative electrode 20, through the external circuit 22 toward the positive electrode 18'. The lithium ions are simultaneously carried from the electrolyte solution through the porous separator 10, 10' toward the positive electrode 18'. The electrons (e -), which flow through the external circuit 22, and the lithium ions which migrate through the porous separator 10, 10' in the electrolyte solution, eventually coalesce to form intercalated lithium at the positive electrode 18'. The electrical current passing through the external circuit 22 may be utilized and passed directly through the load device 24 until the level of intercalated lithium in the negative electrode 20 falls below a working level or the need for electrical energy ceases.The lithium ion battery 40, 40' may be recharged after partial or complete discharge of its available capacity. To (charge) the lithium ion battery 40, 40' an external battery charger is connected to the positive and negative electrodes 18', 20 to control the reversal of the electrochemical reactions of the battery discharge. During recharging, the electrons (e -) flow back through the external circuit 22 to the negative electrode 10, and the lithium ions are carried by the electrolyte through the porous separator 10, 10' back toward the negative electrode 20. The electrons (e -) and lithium ions are recombined at the negative electrode 20 filling them with intercalated lithium for consumption during the next battery discharge cycle.The external battery charger that can be used to charge the lithium ion battery 40, 40' can vary depending on the size, configuration, and particular end use of the lithium ion battery 40, 40'. Some suitable external battery chargers include a battery charger plugged into an AC wall socket and an automotive alternator.Examples of the battery 30, 30', 40, 40' may be used in a variety of different applications. For example, the batteries 30, 30', 40, 40' may be used in various devices, for example, a battery powered or hybrid vehicle, a laptop computer, a cellular telephone, a cordless power tool, or the like.To further illustrate the present disclosure, an example is provided herein. It is to be understood that this example is provided for illustrative purposes and is not intended to be limiting of the scope of the disclosed example(s).EXAMPLEA separator having a porous carbon coating and a gradient distribution of PEI-TFSI was prepared.A 10 wt% polyethyleneimine aqueous solution was titrated to pH 7 using a 10 wt% bis(trifluoromethanesulfonyl)imide methanol solution. The PEI-TFSI product was obtained after removal of water and methanol by vacuum. The PEI-TFSI product was dissolved in NMP to give a polycation solution containing 20% by weight of PEI-TFSI.Porous carbon having a pore volume of 5.68 cm 3 / g was obtained by heating MOF-5 to 1000°C in argon.A slurry was prepared by mixing the porous carbon with the polycation solution. This solution was cast onto a polypropylene membrane (CELGARD 2500) and dried at room temperature (~25 °C) for about 24 hours. This formed the example separator.Two comparative porous separators were also used. The first comparative porous separator was an unmodified polypropylene separator (CELGARD 2500).The second comparative separator was prepared with a coating of some of the porous carbon and polyvinylidene fluoride described above on a polypropylene separator (CELGARD 2500). The second comparative separator was prepared in particular by mixing 20 mg of the porous carbon with 80 mg of PVDF. 200 mg of NMP was added to prepare a slurry (or magnetic suspension) with magnetic stirring for about 12 hours. The slurry was then poured onto the separator (CELGARD 2500). Finally, the second comparative separator was obtained by drying at 25° C. for about 24 hours.The first comparative example separator ( 1), the second comparative example separator ( 2), and the example separator ( 3) were each evaluated using half cells. In the half cells, the first comparative example separator (1), the second comparative example separator (2) and the example separator (3) were paired with a sulfur positive electrode having a sulfur loading of 1.85 g / cm 2 in 0.6 M LiNO 3 plus 0.4 M LiTFSI in dimethoxyethane:1,3-dioxolane (MDE:DIOXO, 1:1). The galvanostatic cycling performance of the first comparative example separator ( 1), the second comparative example separator ( 2), and the example separator ( 3) was studied by cycling between 1.7 V and 2.75 V at a rate of C / 10 at room temperature for up to 60 cycles.The results for cycling performance and coulombic efficiency are shown in Figure 5. The capacity (mAh / g s) is shown on the left Y axis (marked with "C"), the Coulombic efficiency (%) is shown on the right Y axis (marked with "%"), and the cycle number is shown on the X axis (marked with "#"). As stated above, "1" represents the results for the half cell with the first comparative example separator, "2" represents the results for the half cell with the second comparative example separator, and "3" represents the results for the half cell with the example separator. Both the example separator and the second comparative example separator (which comprised a carbon / polymer coating) were improved in capacity and Coulombic efficiency as compared to the first comparative example (i.e., the uncoated separator). The capacitance results for the example separator were much better than for both the first and second comparative electrodes. Coulombic efficiency for the example separator was much better than for the first comparative electrode and was comparable to the second comparative electrode. Overall, the example separator had the best performance, which may be due, at least in part, to the soft acid (ammonium) base (polysulfide) interaction.FIG. 6 illustrates the voltage profile of the three half cells in the second cycle. In FIG. 6, the voltage (V) is shown on the Y axis (denoted by "V"), and the specific capacitance (mAh / g s) is shown on the X axis (denoted by "C"). As shown in FIG. 6, the introduction of polycations maintains plateau stress and improved the utilization of the sulfur-based active materials. This results in a high energy density.A reflection electron image of the second comparative example was taken, and this is shown in FIG. 7A. As shown, the second comparative example included the CELGARD 2500 membrane (labeled 12) and the carbon / PVDF coating (labeled 14'). In this comparative example, PVDF is only the source of F, and so the gradient formed in this comparative example can be easily seen by element mapping (FIG. 7B ). In the example separator, the N signal from PEI-TFSI is very weak. As a result, it is difficult to obtain a similar element map of F for the example separator. The example separator is believed to have a similar polycation gradient to the second comparative example, at least in part because of the same solvent (i.e., NMP) and drying conditions used.It is to be understood that the ranges provided herein include the stated range and any value or sub-range within the stated range. For example, a range from 100 to 100,000 should be interpreted as including not only the explicitly stated limits from 100 to 100,000, but also individual values, for example 225, 3,000, 50,050, etc., as well as sub-ranges such as 500 to about 75,000, from 1,100 to 95,000, etc. Moreover, when "about" is used to describe a value, it means that less variance (up to + / -5%) from the stated value is included.References throughout the specification to "1 example", "another example", "an example", and so forth, means that a particular element (e.g., feature, structure, and / or characteristic) described in connection with the example is included in at least one example described herein and may or may not be present in other examples. In addition, it is to be understood that the elements described for any example may be combined in any suitable manner in various examples, unless the context clearly dictates otherwise.In describing and claiming the examples disclosed herein, the singular forms "a" and "the" also include plural referents unless the context clearly dictates otherwise.Although several examples have been described in detail, it will be apparent to those skilled in the art that the disclosed examples may be modified. Therefore, the foregoing description is to be considered non-limiting.

Claims

A lithium-based battery separator comprising: a porous polymer membrane having opposed surfaces; a porous carbon coating formed on one of the opposed surfaces of the porous polymer membrane, and polycations incorporated into the porous carbon coating, into the porous polymer membrane, or into both the porous carbon coating and the porous polymer membrane, wherein the polycations are incorporated into the porous polymer membrane and have a gradient distribution that increases in the direction of the other of the opposed surfaces of the porous polymer membrane, or the polycations are incorporated into the porous carbon coating and have a gradient distribution that increases in the direction of the one of the opposed surfaces of the porous polymer membrane.The lithium-based battery separator of claim 1, wherein the polycations have the following structure: wherein TFSI x- is a bis(trifluoromethylsulfonyl)imide anion, x is in the range of 100 to 100,000, and n is in the range of 100 to 100,000.A method of manufacturing a lithium-based battery separator, the method comprising: casting a slurry onto one of two opposing surfaces of a porous polymer membrane, the slurry comprising porous carbon particles and polycations; and subjecting the cast slurry to a drying process whereby i) a porous carbon coating is formed on the one of the two opposing surfaces of the porous polymer membrane and ii) a gradient distribution of the polycations in the porous carbon coating, in the porous polymer membrane or in both the porous carbon coating and the porous polymer membrane.The method of claim 3, wherein prior to casting the slurry, the method further comprises preparing a polycation solution and adding the porous carbon particles to the polycation solution; and wherein preparing the polycation solution comprises: titrating an aqueous polyethyleneimine solution to a pH of 7, wherein a bis(trifluoromethanesulfonyl)imide-methanol solution is used, thereby forming a polycation product in water and methanol; removing the water and the methanol; and dissolving the polycation product in an organic solvent.The method of claim 4, further comprising preparing the porous carbon particles by exposing a carbon precursor to a predetermined temperature in an inert atmosphere, thereby carbonizing the carbon precursor.The method of claim 5, wherein the carbon precursor is an organometallic framework-5 (MOF-5), and wherein the drying process is performed at about 25°C for about 24 hours.A lithium-based battery comprising: a negative electrode; a positive electrode comprising an active material; a separator positioned between the negative electrode and the positive electrode, the separator comprising: a porous polymer membrane having opposing surfaces, one of the opposing surfaces facing the positive electrode; a porous carbon coating formed on the one of the opposing surfaces and facing the positive electrode, and polycations incorporated into the porous carbon coating, the porous polymer membrane, or both the porous carbon coating and the porous polymer membrane, and an electrolyte solution impregnated through the positive electrode, the negative electrode, and the separator, respectively, wherein the polycations are incorporated into the porous polymer membrane and have a gradient distribution increasing toward the other of the opposing surfaces of the porous polymer membrane, or the polycations are incorporated into the porous carbon coating and have a gradient distribution increasing toward the one of the opposing surfaces of the porous polymer membrane.The lithium-based battery of claim 7, wherein the lithium-based battery is a lithium-sulfur battery, or wherein the lithium-based battery is a lithium-ion battery.The lithium-based battery of claim 7, wherein the polycations have the structure: wherein TFSI x- is a bis(trifluoromethylsulfonyl)imide anion, x is in the range of 100 to 100,000, and n is in the range of 100 to 100,000.

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