Separators suitable for use in lithium-ion batteries

Abstract

The present invention provides a separator suitable for use in a lithium ion battery, comprising a core paper containing 30-70% by weight of aramid short cut fiber, 10-45% by weight of PET, and 5-40% by weight of a binder, the core paper having a density of 5-30 g / m 2 and a thickness of 5-30 microns, at least one side of the core paper is provided with a coating layer, said coating layer comprising refractory particles and a coating binder, and the separator has a surface pore size such that at least 90% of the surface pores have a pore size of up to 0.5 microns on the side of the paper provided with the coating layer. The separator according to the invention combines a low thickness, good dimensional stability, and good thermal stability and thermal conductivity with good barrier properties. The manufacture and use of the separator are also claimed.

Description

[Technical field]

[0001] The present invention relates to separator paper suitable for use in lithium ion batteries.

[0002] Lithium-ion batteries, also known as Li-ion batteries, are used in many applications where high energy density, high voltage, and rechargeability are desirable. They are especially found in many mobile devices.

[0003] The main elements of a Li-ion battery are the positive electrode, the negative electrode, a separator, and an electrolyte. The separator is placed between the positive and negative electrodes. Its main function is to physically separate the positive electrode from the negative electrode to prevent short circuits within the battery while allowing lithium ions to pass through the electrolyte-containing separator.

[0004] Commercially available Li-ion cells use polyolefins, such as polyethylene or polypropylene, as the separator.

[0005] US Patent No. 9,431,643 describes a separator for a lithium-ion battery, which includes a substrate film and a coating on the substrate film. The substrate film is preferably selected from at least one of a polyethylene film, a polypropylene film, a polypropylene / polyethylene / polypropylene composite film, an aramid film, and a polyimide film.

[0006] EP 2955773 describes a nonwoven base for lithium-ion secondary battery separators that is composed primarily (i.e., at least 70% by weight) of PET, i.e., crystallized PET fibers and a PET binder.

[0007] US Patent Publication No. 2012251890 describes a membrane that includes a flat, flexible substrate having a plurality of openings and has a porous inorganic coating located on and within the substrate. The substrate includes aramid fibers and fibers that have a melting point lower than the decomposition point of the polyaramid fibers.

[0008] While polyolefin-based separators exhibit sufficient properties, it has been found that there is room for improvement. In particular, there is a need in the art for separators that combine low thickness, good dimensional stability, and good thermal stability and thermal conductivity with good barrier properties.

[0009] The present invention provides a separator that meets these requirements.

[0010] The present invention provides a separator suitable for use in a lithium ion battery, comprising a core paper containing 30-70% by weight of aramid short cut fiber, 10-45% by weight of PET, and 5-40% by weight of a binder, the core paper having a density of 5-30 g / m 2 and a thickness of 5 to 30 microns, a coating layer is provided on at least one surface of the core paper, the coating layer comprising refractory particles and a coating binder, and the separator has a surface pore size on the surface of the paper provided with the coating layer such that at least 90% of the surface pores have a pore size of up to 0.5 microns.

[0011] It has been found that by combining different elements of the claims it is ensured that a separator, sometimes called separator paper, having good properties is obtained, as will be discussed in more detail below.

[0012] The present invention is illustrated by, but is not limited to, the following figures. [Brief description of the drawings]

[0013] [Figure 1a]2 is a SEM photograph of the surface of the separator of the present invention. [Figure 1b] FIG. 1b is a close-up of the box in FIG. 1a, showing surface pores as black "holes."

[0014] An important feature of the separator paper of the present invention is that it has a surface pore size such that at the side of the paper provided with the coating layer, at least 90% of the surface pores have a pore size of up to 0.5 microns. If this requirement is not met, the separator will have insufficient barrier properties. Since it is intended to limit the number of pores with a pore size of more than 0.5 microns, it will be apparent that the pore size is the maximum diameter of the pores in question.

[0015] As will be clear to those skilled in the art, surface pore size can be determined by microscopic examination of the surface, for example, using a scanning electron microscope (SEM) with appropriate image analysis software. Using standard software, it is well within the scope of a skilled artisan to select the contrast and magnification that allows the measurement of pore size. One possibility would be to manually draw a line along the longest axis of the pore and let the software determine the length of the applied line. Depending on the equipment used, more sophisticated approaches may also be possible.

[0016] It is preferred that at least 95% of the surface pores of the separator on the side provided with the coating layer have a pore size of up to 0.5 microns.It is preferred that at least 80% of the surface pores of the separator on the side provided with the coating layer have a pore size of up to 0.4 microns, more preferably up to 0.3 microns.

[0017] In one embodiment, the number average pore size of the surface pores of the separator on the side provided with the coating layer is at most 0.3 microns, particularly at most 0.2 microns, more particularly at most 0.10 microns.

[0018] It has been found that certain combinations of core paper and coating layers make it possible to obtain a separator paper having the requisite surface pore size combined with further desirable properties as described herein.

[0019] Core paper: 5~30g / m 2 and a thickness of 5 to 30 microns. The core paper is 5 g / m 2 If the core paper has a basis weight of less than 30 g / m or a thickness of less than 5 microns, the separator paper will be difficult to process in the roll-to-roll manufacturing process. Furthermore, its barrier properties to prevent short circuits may be insufficient. On the other hand, if the core paper has a basis weight of less than 30 g / m 2 If the thickness is greater than 7 g / m2, or if the thickness is greater than 30 microns, the paper becomes too thick and heavy to be commercially attractive. To balance these properties, the core paper should have a thickness of at least 7 g / m2 2 It may also be preferred that the core paper has a basis weight of up to 25 g / m 2 , especially up to 20 g / m 2 , more particularly up to 15 g / m 2 , in some embodiments up to 10 g / m 2 It may be preferred to have a basis weight of

[0020] With regard to thickness, for the reasons given above for basis weight, it is preferred that the core paper has a thickness of at least 10 microns, and it may be preferred that the core paper has a thickness of up to 25 microns, particularly up to 20 microns and in some embodiments up to 15 microns.

[0021] The core paper contains 30-70% by weight of aramid short cut fiber, 10-45% by weight of PET, and 5-40% by weight of binder.

[0022] In the context of this specification, aramid refers to aromatic polyamides, which are condensation polymers of aromatic diamines and aromatic dicarboxylic acid halides. Aramids can exist in meta and para forms, both of which can be used in the present invention. It is believed that it is preferable to use aramids in which at least 85% of the bonds between aromatic moieties are para-aramid bonds. Typical members of this group include poly(paraphenylene terephthalamide), poly(4,4'-benzanilide terephthalamide), poly(paraphenylene-4,4'-biphenylenedicarboxylic acid amide), and poly(paraphenylene-2,6-naphthalenedicarboxylic acid amide), copoly(paraphenylene / 4,4'-dioxydiphenylene terephthalamide), or copoly(paraphenylene / 3,4'-dioxydiphenylene terephthalamide). The use of aramids in which at least 90% of the bonds between the aromatic moieties, more particularly at least 95%, are para-aramid bonds, is considered to be preferred. The use of poly(paraphenylene terephthalamide), also called PPTA, is particularly preferred. This applies to all aramid components present in the paper according to the invention, unless otherwise specified. More particularly, to ensure sufficient dimensional stability, In order to achieve this, it may be preferred that the aramid in the core paper is composed of at least 10% by weight of para-aramid. This can be achieved, for example, by using a meta-aramid short cut in combination with a para-aramid fibrid, a para-aramid short cut in combination with a meta-aramid fibrid, a mixture of meta-aramid short cut and para-aramid short cut in combination with a meta-aramid fibrid, a para-aramid fibrid or a combination thereof, or in other combinations. In one embodiment, the aramid in the core paper is composed of at least 20% by weight, in some embodiments at least 40% by weight, in some embodiments at least 60% by weight, or at least 80% by weight, or at least 90% by weight of para-aramid. In one embodiment, all the aramid components in the core paper are para-aramid components.

[0023] Aramid short cut fibers, also known as aramid flock, are known in the art. They are generally obtained by cutting aramid fibers to the desired length, generally in the range of 0.5 to 25 mm. In a preferred embodiment, the average length is at least 2 mm, in particular at least 3 mm. In some embodiments, it may be at least 4 mm. The average length of the short cuts is preferably at most 15 mm, in one embodiment at most 10 mm.

[0024] Short cuts generally have a linear density in the range of 0.05 to 5 dtex. It has been found that short cuts with a linear density of less than 0.05 dtex and long lengths are difficult to process. Short cuts with a linear density of more than 5 dtex may result in paper with less attractive properties. For processing reasons, it may be preferred that the short cut has a linear density of at least 0.3 dtex. On the other hand, it has been found that the use of short cuts with lower linear densities results in separators with improved properties. It may therefore be preferred that the aramid short cut fibers have a linear density of at most 1.1 dtex, in particular at most 0.9 dtex, more in particular at most 0.7 dtex, in some embodiments at most 0.55 dtex.

[0025] The shortcut preferably has a diameter of at most 10 microns, in particular at most 8 microns.

[0026] The short cut may be a para-aramid short cut, a meta-aramid short cut, or a mixture of meta-aramid and para-aramid short cuts. The use of the para-aramid short cut is considered to be preferred.

[0027] The core paper contains 30-70% by weight of aramid short cut fibers. If the amount of short cut is less than 30% by weight, the strength and dimensional stability of the paper will be insufficient. If the amount of short cut is more than 70% by weight, there will be insufficient room for other components of the paper required to obtain the desired properties. It may be preferred that the core paper contains more than 30% by weight of short cut, in particular at least 32% by weight of short cut, more in particular at least 35% by weight of short cut, or at least 40% by weight of short cut, or at least 45% by weight of short cut, and / or up to 60% by weight of short cut, in particular up to 55% by weight of short cut.

[0028] The core paper of the separator of the present invention contains 5 to 40% by weight, preferably 10 to 35% by weight, of a binder.

[0029] The binder is intended to help hold the paper together. Generally, the binder is a polymer. It can be fibrous in nature, but non-fibrous binders, also called resinous or polymeric binders, can also be used. Temperature stability is an important feature since the separator can reach high temperatures. It is therefore considered preferable to use a heat-resistant polymer, such as a polymer that does not degrade under the conditions that prevail during the use of the battery. Degradation includes physical as well as chemical degradation. Examples of suitable heat-resistant polymers are polyaramids, polyarylates, polyimides, polybenzoxazoles, polyethersulfones, polyurethanes, polyacrylics, aromatic polyethers, and heat-resistant crosslinked polyesters. Thermosetting systems based on, for example, phenolic resins, melamines, and epoxies can also be used.

[0030] In one embodiment, the binder is selected from one or more of aramid fibrids, aramid pulp, jet-spun aramid fibrids, jet-spun aramid pulp, and combinations thereof. Within this group, the use of aramid fibrids, particularly para-aramid fibrids, may be preferred.

[0031] In another embodiment, the binder is a heat-resistant polymer, in particular a heat-resistant polymer selected from the group of polyimides, polybenzoxazoles, polyethersulfones, polyurethanes, polyacrylics, aromatic polyethers, heat-resistant crosslinked polyesters, phenolic resins, melamines, and epoxies. It may be preferred that the binder is a heat-resistant crosslinked polyester, for example a heat-resistant crosslinked polyester with hexamethoxymethylmelamine as a crosslinking agent.

[0032] In the context of this specification, the term aramid fibrids refers to small, non-granular, non-fibrous, non-rigid film-like particles. Film-like fibrid particles have two of their three dimensions on the order of microns and one dimension less than 1 micron. In one embodiment, the fibrids used in the present invention have an average length in the range of 0.2 to 2 mm, an average width in the range of 10 to 500 microns, and an average thickness in the range of 0.001 to 1 micron. In one embodiment, the aramid fibrids contain less than 40%, preferably less than 30%, fine particles, which are defined as particles having a length-weighted length (LL) of less than 250 microns.

[0033] Meta-aramid fibrids can be obtained by shear precipitation of a polymer solution in a coagulation liquid, as known, for example, from US Patent 2,999,788. Fibrids of fully aromatic polyamides (aramids) are also known, for example, from US Patent 3,756,908, which discloses a method for preparing poly(metaphenylene isophthalamide) (MPIA) fibrids. Para-aramid fibrids can be obtained by high shear processes, as described, for example, in WO 2005 / 059247, and are also called jet-spun fibrids.

[0034] Para-aramid fibrids, meta-aramid fibrids, or combinations thereof may be used. It is preferred that the aramid fibrid is a para-aramid fibrid, in particular a para-aramid fibrid having a Shopper-Riegler (SR) value of 50 to 90, preferably 70 to 85. Preferably, the fibrid is a 10 m 2 / g, more preferably 0.5 to 10m 2 / g, most preferably 1 to 4m 2 / g specific surface area (SSA).

[0035] In one embodiment, fibrids are used with an LL0.25 of at least 0.3 mm, particularly at least 0.5 mm, more particularly at least 0.7 mm. In one embodiment, the LL0.25 is at most 2 mm, more particularly at most 1.5 mm, even more particularly at most 1.2 mm. The LL0.25 represents the length-weighted length of the fibrid particles, and particles having a length less than 0.25 mm are not considered.

[0036] The binder may also include aramid pulp or aramid jet spun pulp.

[0037] Aramid pulp is well known in the art. Aramid pulp may be derived from aramid fibers that have been chopped to lengths of, for example, 0.5-6 mm, and then subjected to a fibrillation step that pulls the fibers apart to form fibrils, whether or not they are attached to thicker stems. This type of pulp may be characterized by, for example, a length of 0.5-6 mm and a Schopper-Riegler of 15-85. In some embodiments, the pulp may be cut into pieces of 4-20 mm thick. 2 / g of surface area.

[0038] In the present context, the term pulp also encompasses "pulps" that mainly contain fibrils, i.e. fibrillated parts, and little or no fiber stalks. This pulp, sometimes called aramid fibrils, can be obtained by direct spinning from the solution, for example as described in WO 2004 / 099476. In one embodiment, the pulp has a structural irregularity, expressed as a difference in CSF (Canadian Standard Freeness) of the never-dried pulp and the dried pulp, of at least 100, preferably at least 150. In one embodiment, the pulp has a Canadian Standard Freeness (CSF) value of less than 300 ml in the wet phase and less than 7 m after drying. 2 Fibrils are used that have a specific surface area (SSA) of less than 1.2 mm / g and in particular a weight-weighted length (WL0.25) for particles with a length of more than 250 microns of less than 1.2 mm, more preferably less than 1.0 mm. Suitable fibrils and methods for their preparation are described, for example, in WO 2005 / 059211.

[0039] The core paper contains 5-40% by weight of binder. If the amount of binder is too small, the strength of the core paper will be insufficient. It is therefore preferred that the amount of binder is at least 10% by weight, in particular at least 15% by weight. On the other hand, if the amount of binder is too large, the permeability of lithium ions may be insufficient. It may be preferred that the amount of binder is at most 35% by weight, in particular at most 30% by weight, in some embodiments at most 25% by weight.

[0040] The core paper contains 10 to 45% by weight of PET.

[0041] PET is known in various forms such as particles, flakes, fibrids, and fibers. Surprisingly, when PET is incorporated in the form of fibers into the core paper in the papermaking process, the formation of the paper, expressed in terms of pinhole reduction, is improved. This is therefore a preferred embodiment of the present invention. It is therefore preferred that the core paper is manufactured using 10-45% by weight of PET fibers. It is also preferred that all the PET in the core paper is provided in the form of fibers.

[0042] PET (polyethylene terephthalate) fibres are known in the art, for example from US Pat. No. 6,411,497. In one embodiment, the PET fibres used in the core paper according to the invention generally have a length in the range of 0.5 to 25 mm. It may be preferred that the fibres have a length of at most 15 mm, in particular at most 10 mm, more in particular at most 6 mm, since fibres that are too long may not provide good paper properties. It may be preferred that the fibres have a length of at least 2 mm, in particular at least 3 mm, since fibres that are too short may not provide sufficient performance. PET fibres generally have a linear density in the range of 0.01 to 0.20 dtex. Fibres with a linear density of less than 0.01 dtex are difficult to obtain and therefore unattractive from an economic point of view. If the linear density is too high, the advantages associated with the presence of the fibres are not obtained and the pore size of the paper becomes too large. It may be preferred that the PET fibres have a linear density of at most 0.15 dtex, in particular at most 0.1 dtex. It may also be preferred that the PET fibres have a linear density of at least 0.01 dtex, especially at least 0.03 dtex.

[0043] PET is present in the core paper in an amount of 10-45% by weight. Too little PET will not provide the benefits associated with its presence. On the other hand, too much PET may affect the dimensional stability of the separator at higher temperatures, i.e. the risk of shrinkage at higher temperatures increases. It may be preferred that the amount of PET is at least 15% by weight, in particular at least 20% by weight, more in particular at least 30% by weight. On the other hand, it may be preferred that the amount of PET in the core paper is up to 40% by weight, in particular up to 35% by weight.

[0044] Optionally, the core paper may contain 0.1-10 wt.% polyamidoamine epichlorohydrin (PAE), which has been found to increase the strength of the paper. Polyamidoamine epichlorohydrin (PAE) polymers are known in the art and do not require further explanation. PAE resins are commercially available, inter alia, from Solenis under the trade name Kymene. PAE resins generally have an average molecular weight of at least 10,000 g / mol, for example in the range of 50,000 to 2,000,000 g / mol. It may be preferred that the amount of PAE resin present in the paper is at most 8 wt.%, in particular at most 5 wt.%, in some embodiments at most 4.5 wt.%, at most 4.0 wt.%, or at most 3 wt.%. An amount of at most 2 wt.% may also be preferred. On the other hand, it may be preferred that the amount of PAE is at least 0.3 wt.%, in particular at least 0.5 wt.%. When used, the PAE can be added to the paper at any stage in the papermaking process, for example to a suspension of one or more starting materials or mixtures thereof, or to the finished paper. It is also possible to contact the finished paper with a PAE solution.

[0045] The core paper used in the present invention can be manufactured by methods known in the art. In one embodiment, a suspension, generally an aqueous suspension, containing at least some of the components, particularly solid fiber components, is prepared and the suspension is applied onto a porous screen so as to lay a web of randomly interwoven material on the screen. The liquid medium is removed from the web, for example by pressing and / or applying a vacuum, followed by drying to produce the paper. In one embodiment, all the components of the paper are provided in the suspension. In another embodiment, some components are provided in the suspension and further components are applied onto the paper. For example, a polymeric binder can be applied by spraying or coating onto the web of randomly interwoven material prepared from the fiber components, similar to the processes described in, for example, US Pat. No. 5,389,716 or US Pat. No. 6,838,401.

[0046] Optionally, the dried paper is subjected to a calendering step, which is known in the art and generally involves passing the paper through a pair of rolls, optionally at elevated temperatures.

[0047] In one embodiment, the manufacture of paper includes a heat treatment, the paper being subjected to a temperature of at least 175°C, in particular at least 200°C, more in particular at least 240°C, even more in particular at least 260°C. Such heat treatment has been found to result in improved paper properties, in particular one or more of increased breaking force (BF), increased breaking strength (BT) and increased elongation at break (EaB). Without wishing to be bound by theory, it is believed that the effect may be related to softening or melting of the PET, since the effect is more pronounced in the case of paper containing PET. The maximum temperature may include values ​​of up to 400°C. Above this temperature, degradation of the polymer may occur. Generally, temperatures of up to 350°C, in particular up to 320°C, more in particular up to 300°C will be sufficient. The heat treatment does not require a long time. Generally, a heat treatment of up to 2 hours will be sufficient. Depending on the equipment used, times in the range of 1 second to 1 hour, in particular 1 second to 30 minutes, more in particular 1 second to 10 minutes, even more in particular 1 second to 5 minutes, or even 1 second to 30 seconds may be mentioned, provided that the paper reaches the appropriate temperature.

[0048] The heat treatment can be carried out using various methods, for example in an oven, using a hot plate or heated rolls, by infrared radiation, or using hot air, batchwise or continuously. A combination of methods can also be applied.

[0049] When polymeric binders are used, especially crosslinkable or curable polymeric binders, it is possible that a partial curing step is carried out immediately after papermaking and full curing is carried out in a heat treatment / calendering step, similar to that described, for example, in U.S. Pat. No. 6,838,401.

[0050] In the separator according to the present invention, a coating layer is provided on at least one surface of the core paper. The coating layer includes refractory particles and a coating binder. After application to the paper, the refractory particles together with the coating binder become part of the paper structure, and the particles with the coating binder are partially or entirely embedded in the paper structure. This can be seen by viewing a cross section of the paper under a microscope.

[0051] The coating layer includes a coating binder and refractory particles. The refractory particles are thermally conductive but electrically insulating. The coating layer is intended to reduce the surface pore size of the separator. Furthermore, the presence of the thermally conductive particles ensures a good dissipation of heat above the separator, resulting in a lower heating rate and risk of overheating in a short time. The presence of the thermally conductive particles may also contribute to the improved stability of Li-ion batteries, since they may trap HF formed in the battery during use.

[0052] The coating layer is applied to at least one surface of the core paper, and it is preferable to provide a coating layer on both surfaces of the core paper.

[0053] The refractory particles are inert under the conditions prevailing in the battery. They are generally selected from inorganic oxide particles, inorganic nitride particles, and inorganic carbide particles. Suitable inorganic oxide materials include aluminum oxide (e.g., gamma alumina), aluminum oxyhydroxide (e.g., boehmite), zirconia, silica, titania, magnesia. Suitable inorganic nitrides may include boron nitride. Suitable inorganic carbides may include silicon carbide. Alumina is considered preferred because it has suitable heat conducting properties while at the same time being economically attractive. Combinations of various types of particles may also be used.

[0054] The refractory particles generally have an average particle size (d50 in the context of this application) in the range of 100-1000 nm, preferably between 200-600 nm. If the particles are too large, the desired surface pore size will not be obtained. If the particles are too small, they may interfere with other properties of the separator.

[0055] The coating layer may be applied to the paper as a composition comprising a mixture of the coating binder, the refractory particles, generally one or more solvents, and optionally one or more additives.

[0056] The coating binder is intended to hold the particles together and bind them to the core paper. The coating binder is polymeric in nature, and suitable binders include, for example, polyvinylidene fluoride (PVDF), polyurethane, polyethylene oxide (PEO), polypropylene oxide (PPO), polyacrylonitrile (PAN), polyacrylamide, polymethyl acrylate, polymethyl methacrylate, polyvinyl acetate, polyvinylpyrrolidone, polytetraethylene glycol diacrylate, cellulose nanofibers or microfibers, copoly(paraphenylene / 4,4'-dioxydiphenylene terephthalamide), copoly(paraphenylene / 3,4'-dioxydiphenylene terephthalamide), metaphenylene isophthalamide (MPIA available under the trade name Teijin Conex), copolymers of the above, and combinations thereof.

[0057] The separator generally comprises 0.5 to 20% by weight, in particular 1 to 10% by weight, more in particular 2.5 to 8% by weight, of a binder derived from the coating.

[0058] From a chemical standpoint, the paper binder and the coating binder may be the same or different.

[0059] The coating binder is dissolved or dispersed in a solvent. The nature of the solvent depends on the nature of the polymer binder to allow proper dissolution or dispersion. Water or organic solvents can be used. Depending on the nature and properties of the solvent, the solvent can be removed from the system by evaporation (volatile solvents such as water, ethanol, etc.) or through other means, for example, by adding a non-solvent to coagulate the polymer and removing the solvent by washing. Solvent removal can be accelerated by application of heat and / or reduced pressure.

[0060] Examples of suitable organic solvents include alcohols, such as ethanol, propanol, isopropyl alcohol, ketones, such as acetone, methyl ethyl ketone, acetate, etc. Further suitable solvents include polar aprotic solvents, such as DMAc, DMSO, DMF, and NMP. NMP may be considered preferred.

[0061] The coatings may be applied using methods known in the art, for example, using a roller, doctor blade, slot die, or through other suitable methods.

[0062] In another embodiment, the coating layer may be provided by spatial atomic layer deposition of oxide particles, followed by application of a coating binder, typically using a solvent.

[0063] The separator of the present invention comprises a core paper having a coating layer on at least one side thereof. As shown above, after application of the coating layer, the refractory particles become part of the paper structure, with the particles being partially or wholly embedded in the paper structure. This can be seen when viewing a cross section of the paper under a microscope.

[0064] Generally, the separator contains 10 to 60% by weight, particularly 20 to 55% by weight, and more particularly 35 to 50% by weight of refractory particles.

[0065] Separators according to the present invention are discussed further below.

[0066] For % surface porosity, see above.

[0067] The separator of the present invention generally has a thickness of 5 to 35 g / m 2 The separator has a basis weight of 5 g / m 2 On the other hand, if the separator has a basis weight of 35 g / m or less, its barrier properties to prevent short circuits may be insufficient. 2 If the thickness is greater than 6 g / m, the separator will be too thick and too heavy to be commercially attractive. To balance these properties, the separator should be at least 6 g / m 2 It may be preferable for the separator to have a basis weight of up to 25 g / m 2 , especially up to 20 g / m 2 , more particularly up to 15 g / m 2 , in some embodiments up to 10 g / m 2 It may be preferred to have a basis weight of

[0068] The separator of the present invention generally has a thickness of 5 to 35 microns. If the separator is too thin, its barrier properties may be insufficient. On the other hand, if the separator is too thick, it becomes unattractive for commercial operation. It may be preferred that the separator has a thickness of at least 7 microns. It may also be preferred that the separator has a thickness of at most 25 microns, in particular at most 20 microns, and in some embodiments at most 15 microns. In one embodiment, the separator of the present invention has a porosity of 40 to 70%. If the porosity is too low, the resistivity of the separator to conduct Li ions becomes too low to allow rapid charging and discharging. If the porosity of the separator is too high, other properties may be insufficient. The porosity of the separator according to the present invention is preferably at least 45%, more preferably at least 50%. It may also be preferred that the separator has a porosity of at most 65%, more preferably at most 60%, and in some embodiments at most 55%.

[0069] An important feature is the ionic resistivity of the separator, which should be as good as possible. An important value characterizing the ionic mobility of a certain separator-electrolyte combination is the so-called McMullin number, which is defined as the ratio of the resistivity of the separator saturated with electrolyte to that of the pure electrolyte and is therefore always greater than 1. The McMullin number for the separator according to the invention is preferably at most 10, in particular at most 8, more preferably at most 6. It is particularly preferred that the McMullin number is at most 5, more particularly at most 4.

[0070] A particularly useful parameter for characterizing the conductivity of a separator is CPV=ε% / (t*p*MM) [In the formula, ε%=(1-AW / (r*t))*100%, porosity (%), r=density(g / cm 3 ) AW = areal density (g / m 2 ) p = surface pore size (μm) t = thickness (μm) MM=McMullin number] is a conductivity performance value defined as:

[0071] The CPV combines several relevant parameters for the separator. It is preferred that the CPV be at least 0.5, particularly at least 5, more particularly at least 10, and in some embodiments at least 20.

[0072] The present invention also relates to a battery cell comprising the separator of the present invention. The battery cell according to the present invention comprises a lithium-containing cathode and an anode connected through a lithium-containing electrolyte, the anode and the cathode being separated from each other through a separator as described herein. The present invention also relates to a battery comprising one or more battery cells as discussed above, and a module comprising one or more batteries as described above.

[0073] It will be apparent to one skilled in the art that the various preferred embodiments described herein can be combined unless they are mutually exclusive.

[0074] The present invention is illustrated by, but is not limited to, the following examples.

[0075] Working Example overview The following materials were used in the examples: Para-aramid short cut fibre with a length of 6mm and a linear density of 0.55dtex Para-aramid fibrid 8016 from Teijin Aramid The Netherlands with a Shopper-Riegler (SR) value of more than 75°, preferably more than 80° PET fiber: TA04PN from Teijin Frontier Japan with a length of 3 mm, a linear density of 0.06 dtex, and a diameter of approximately 3 μm Para-aramid pulp: 1094 from Teijin Aramid, The Netherlands, with SR55-60° · Para-aramid jet spun pulp 8077 from Teijin Aramid The Netherlands with CSF50-100.

[0076] The core paper was manufactured through a conventional wet-laying process, where the various components were mixed in an aqueous suspension, followed by a wet-laying paper process through an inclined wire machine. The paper was calendered to the desired thickness using a steel roller at 120°C. If a heat treatment was performed, this is indicated separately.

[0077] Areal weight (Aw) may be determined according to ASTM D646. Thickness and tensile properties may be determined according to ISO 534 and ISO 1924-3, respectively.

[0078] The MacMullin number can be determined at 25 °C by following the main procedure of the RHD application note (https: / / www.rhd-instruments.de / download / appnotes / application_note_macmullin_no.pdf) utilizing a Metrohm Autolab PGSTAT204 with FRA32 module, using 0.5 M LiClO4 in PC / EC / DME (22:8:70 wt%) as electrolyte. Data acquisition and analysis can be performed by Nova2.1 and RelaxIS software, respectively. To avoid the effect of contact resistance, the MacMullin number can be determined from the slope of the resistivity as a function of saturation and number of stacked separators.

[0079] Example 1: Effect of the presence of PET Papers containing various amounts of para-aramid pulp, aramid short cut, jet spun pulp, jet spun fibrids, and PET fibers were prepared. It will be clear that the presence of pinholes needs to be avoided as much as possible. The appearance of pinholes in low basis weight papers is a measure of poor uniformity of formation. There are many approaches to reduce the number of pinholes, including the utilization of additives such as hydroxyethylated starch, polyvinyl alcohol, and carboxymethyl cellulose. However, these materials have a negative effect on battery performance.

[0080] Surprisingly, it was found that the low strength PET fibers significantly improved paper formation, as evidenced by the pinhole count.

[0081] Herein, pinholes are determined using the following method: To determine the number of pinholes, an Epson Perfection V720 Pro scanner was used in combination with the image analysis software Fiji-ImageJ. A paper sheet is placed on the glass of the optical scanner and covered with a black background paper. Images are taken with the following settings: image type: 16 bit, gray, optimal scan mode, resolution: 1200 dpi, format: width 200-height 200 mm. Images were further processed by image analysis software with the following settings: set scale and calibrate: 9440 pixels, distance 200, pixel aspect ratio 11, length unit mm, global on. This results in 47.2 pixels / mm. For measurement parameter analysis, the following parameters were selected: area, standard deviation, centroid, area ratio, ellipse approximation, Feret diameter, decimal places: 3. After inversion and selecting a circular area with a diameter of approximately 200 mm, the outer area was cleared and the threshold was set between 235 and 255 in the remaining area. The particle analysis module was selected with size:0-infinity, circularity0.00-1.00, show:Outlines and activated with Display results, Clear results, and Summarize.

[0082] The results are presented in the table below.

[0083] [Table 1]

[0084] A comparison of Comparative 1 and Sample 1 shows that replacing a portion of the aramid short cut with PET fiber results in better paper formation as represented by a significant reduction in the number of pinholes as determined by the previous method.

[0085] This phenomenon is not limited to the paper compositions listed above: it can be seen that papers containing 15-20% by weight of low strength PET show better formation than papers without PET of otherwise comparable composition.

[0086] Example 2: Effect of heat treatment Various experiments were carried out to investigate the effect of heat treatment on paper.

[0087] Effect on paper without PET A paper was prepared containing 20% ​​by weight of jet spun pulp and 80% by weight of short cut fibers. The paper was sent to a heat treatment in an oven at 250° C. for 30 minutes.

[0088] The results are as follows: [Table 2]

[0089] As can be seen from the table, heat treatment results in a slight increase in breaking force (BF) and breaking strength (BT). (Note that the variation in AW is due to irregularities in the paper composition).

[0090] Effect on paper with PET A paper was prepared containing 20% ​​by weight of jet spun pulp, 50% by weight of short cut fiber, and 30% by weight of PET fiber. The paper was subjected to heat treatment in an oven at 200° C. or 250° C. for 30 minutes. The results are as follows: [Table 3]

[0091] As can be seen from the table, heat treatment, especially at 250° C., results in a significant increase in breaking force (BF), breaking strength (BT), and elongation at break (EaB).

[0092] Effect of heating temperature A paper was prepared containing 15% by weight of jet-spun pulp, 55% by weight of short cut fibers, and 30% by weight of PET fibers. The paper was subjected to heat treatment in an oven at 250° C. or 270° C. for 30 minutes. The results are as follows: [Table 4]

[0093] As can be seen from the table, heat treatment at 250°C, and even more significantly at 270°C, results in significant increases in breaking force (BF), breaking strength (BT) and elongation at break (EaB).

[0094] Effect of heating temperature on further paper composition A paper was prepared containing 15% by weight of jet spun pulp, 60% by weight of short cut fiber, and 25% by weight of PET fiber. The paper was subjected to heat treatment in an oven at 270° C. for 30 minutes. The results are as follows: [Table 5]

[0095] As can be seen, even for this composition, heat treatment at 270° C. leads to a significant increase in breaking force (BF), breaking strength (BT), and elongation at break (EaB).

[0096] Heat treatment under dynamic conditions Paper was prepared containing 15 wt% jet-spun pulp, 55 wt% short cut fibers, and 30 wt% PET fibers, and was sent for heat treatment under dynamic conditions by passing it over a hot plate on a PTFE foil at different temperatures at 0.5 m / min with a residence time of 3 min.

[0097] The results are as follows: [Table 6]

[0098] The same paper was also subjected to heating with an IR heater, with the following results: [Table 7]

[0099] The same paper was also heat treated in a laminator with double belt transport. The results are as follows: [Table 8]

[0100] The same paper was also heat treated in a hot roll at 290°C at different speeds. The results are as follows: [Table 9]

[0101] Example 3: Coating of paper through a continuous process A paper was prepared having the following composition by weight: 30% jet spun fibrids, 26% para-aramid pulp, 32% aramid short cut, and 12% PET fibers. The paper was prepared as described above.

[0102] The papers were coated in a continuous operation using various coating methods. The aqueous coating was a liquid containing Al2O3 and polyvinylpyrrolidone (molecular weight 1.3 Mg / mol) in a mass ratio of 95:5.

[0103] The coated paper was dried in an oven with three drying sections set at 85°C, 85°C, and 95°C respectively.

[0104] The coated papers were tested to determine various properties, the results of which are presented below.

[0105] [Table 10]

[0106] In all samples, the surface pore size of the paper side on which the coating layer was applied was such that at least 95% of the surface pores had a pore size of up to 0.5 microns and at least 80% of the surface pores had a pore size of up to 0.3 microns.

[0107] As can be seen from the table, all the different coating methods result in papers with good properties.

[0108] Example 4: Coating solution with aramid polymer In a continuous three-roll coating device, each with a gap between 70 and 95 μm and operated at 1 m / min, a coating mixture of 81.9 wt% CaCl2 solution in N-methylpyrrolidone (4.5 wt%), 1.8 wt% copolymerized polyparaphenylene terephthalamide containing 3,4'-oxydiphenylenediamine as the copolymerized diamine component in an amount of 50 mol% based on the moles of the total diamine components from Teijin Aramid, and 16.3 wt% Al2O3 was applied to a paper based on 30 wt% jet-spun fibrids, 26 wt% para-aramid pulp, 32 wt% aramid short cut, and 12 wt% PET fibers, prepared as described above.

[0109] During the coating process, the paper was supported by PET foil (Melinex ST504 from DuPont). The coated paper was passed through several half water baths to wash off the NMP / CaCl2, resulting in the solidification of the polymer. The conductivity at the end of the washing water was up to 9.9 μS / cm, indicating good removal of ions. The paper was peeled off from the supporting foil and dried in an oven containing five different heating zones set at 90°C, 100°C, 110°C, 120°C, and 120°C, respectively.

[0110] The results are provided in the table below.

[0111] [Table 11]

[0112] In all samples, the surface pore size of the paper side on which the coating layer was applied was such that at least 95% of the surface pores had a pore size of up to 0.5 microns and at least 80% of the surface pores had a pore size of up to 0.3 microns.

[0113] As can be seen from the above table, application of this coating results in a separator paper with very good properties.

[0114] Core paper 1 and core paper 2 of Examples 3 and 4 were subjected to heat treatment at 150° C. for 0.5 hours. The shrinkage of the paper and separator was 0.2% and 0.4%, respectively, which was very low, indicating the dimensional stability of the paper and separator.

[0115] Example 5: Alternative Binder - Heat Resistant Polyester In this example, a paper was prepared containing aramid short cut fibers, PET fibers, and a binder based on a polyester dispersion with hexamethoxymethylmelamine as a crosslinker, which will be further referred to as an x-linked binder.

[0116] The paper had the following composition: Paper A: 50 wt% aramid short cut fiber, 20 wt% PET fiber, and 30 wt% x-bonded binder. Paper B: 55 wt% aramid short cut fiber, 20 wt% PET fiber, and 25 wt% x-bonded binder. Paper C: 55 wt% aramid short cut fiber, 30 wt% PET fiber, and 15 wt% x-bonded binder.

[0117] The paper was calendered at 125° C. and heat treated at 290° C. on hot rolls at 10 m / min.

[0118] The coating compositions were prepared by mixing Al2O3 with a solution of 3 wt% copolymerized polyparaphenylene terephthalamide containing 3,4'-oxydiphenylenediamine as the copolymerized diamine component in an amount of 50 mol% based on the moles of the total diamine component dissolved in NMP / CaCl2 (4.5 wt%). Different polymer / Al2O3 ratios were applied. Paper A was coated with the coating composition using a doctor blade (DB) or a Mayer bar (MB). After coating, the coated paper was contacted with NMP / water = 25 / 75 to solidify the polymer. The paper was washed with demineralized water and dried at 95 °C. The properties of the resulting separators are provided in the table below.

[0119] [Table 12]

[0120] Paper B was coated through a similar process and the results are presented in the table below.

[0121] [Table 13]

[0122] Paper C was used to investigate different coating compositions. A first coating composition was prepared by mixing Al2O3 and a solution of 3 wt% copolymerized polyparaphenylene terephthalamide containing 3,4'-oxydiphenylenediamine as copolymerized diamine component in an amount of 50 mol% based on the moles of the total diamine component in NMP / CaCl2 (4.5 wt%). For this first coating, a polymer / Al2O3 ratio of 15 / 85 was applied. This coating was deposited to obtain sample 10. To obtain sample 11, a further coating was prepared containing boehmite instead of alumina. To obtain sample 12, a coating containing PVDF (polyvinylidene difluoride) to improve adhesion with the electrode was used. Sample 13 was prepared using the same coating as sample 10, except that a thicker coating layer was deposited, as evidenced by the higher areal weight.

[0123] Paper C was coated as described in Example 4, except that a slot die coating device in contact mode with a 100 μm aperture was used. The system was operated at 4 m / min.

[0124] [Table 14]

[0125] As can be seen from these tables, especially the CPV, papers with good separator properties were obtained. All samples showed that the surface pore size on the side of the paper provided with the coating layer was such that at least 95% of the surface pores had a pore size of maximum 0.5 microns and at least 80% of the surface pores had a pore size of maximum 0.3 microns.

[0126] Figure 1a shows an SEM photograph of the surface of sample 11. Figure 1b shows an enlarged view of the box visible in Figure 1a. The black "holes" are surface pores. The diameter of the surface pores can be determined from the SEM photograph.

[0127] Example 6: Alternative Binders - Acrylates and PVP Papers were prepared using different binders, namely, an acrylate binder (AkzoNobel Cetabever Blanc 0103, 32 wt% solids) and a polyvinylpyrrolidone binder (PVP K60 Ashland, 46.5 wt% solids). All papers contained 30 wt% aramid short cut, 30 wt% PET fibers, and 40 wt% binder (calculated as solids).

[0128] The paper was prepared as follows: Hand sheets were prepared from PET fibers and aramid short cuts. The sheets were then sprayed with each binder composition and dried between two hot plates at 105°C for at least 6 minutes. The sheets were removed from the support board and weighed. The process was repeated as necessary to achieve the final resin content. The sheets were subjected to a calendering step under the following conditions: PVP-containing sheets: 125°C, acrylate-containing sheets 35°C. Heat treatment was then carried out at 270°C for 3 minutes.

[0129] The final paper properties are presented in the table below.

[0130] [Table 15]

[0131] Coating applied to Paper C, Sample 10, of Example 5, using alumina as inorganic oxide, i.e., the paper was coated using 3 wt. % copolymer in NMP / CaCl2 as solvent. The ratio of copolymer to alumina was 15:85.

[0132] [Table 16]

[0133] This example shows that the use of a polymer binder also allows the production of good quality separator paper. In both samples, the surface pore size of the paper side provided with the coating layer was shown to be such that at least 95% of the surface pores had a pore size of up to 0.5 microns and at least 80% of the surface pores had a pore size of up to 0.3 microns.

[0134] Example 7: Water-based coating A paper was prepared containing 55% by weight of aramid short cut fibers, 30% by weight of PET fibers, and 15% by weight of a combined polyester binder system. The paper was calendered at 125°C, heat treated in a hot roll at 285°C at 10 m / min, and finally slit into 100 mm wide paper.

[0135] The following coating compositions were prepared: [Table 17]

[0136] A coating layer having a width of 80 mm was applied onto the above paper by a slot-die contact mode coating process, followed by non-contact drying in two hot air ovens at 80° C. and 120° C., respectively. The properties of the so obtained paper are provided in the table below.

[0137] [Table 18]

[0138] Coating 1 was applied at a speed of 2 m / min and a thickness of 50 microns. Coating 2 was applied at a speed of 1 m / min and a thickness of 100 microns. Coating 3 was applied in the same manner as Coating 2, except that a slightly thicker coating layer was applied, as evidenced by the higher areal weight.

[0139] The CPV values ​​in the table show that separator papers with good properties are obtained. All samples showed that the surface pore size of the paper side provided with the coating layer was such that at least 95% of the surface pores had a pore size of maximum 0.5 microns and at least 80% of the surface pores had a pore size of maximum 0.3 microns.

[0140] Example 8: Effects of three ingredients One example and two comparative examples were prepared to illustrate the effect of the presence of three components, namely, aramid short cut, PET, and binder. The binder was an aramid fibrid binder. The compositions of the various papers are provided in the table below: [Table 19]

[0141] The strength of the paper containing PET and short cut but no binder was too low so that the sheets could not be handled, therefore further properties were not determined.

[0142] The sheets of Example 1 and Comparative Example A were subjected to a calendering step at 125° C. and a heat treatment at 270° C. for 3 minutes, after which further properties were determined. The results are in the table below: [Table 20]

[0143] As can be seen from this table, the paper containing only aramid fibrids and no PET has a much higher number of pinholes at the same PET content. It should be noted that absolute comparisons with the paper of Example 1 cannot be made given the differences in composition, especially the short-cut content.

Claims

1. A separator suitable for use in a lithium ion battery, comprising a core paper comprising 30-70 wt% aramid short cut fiber, 10-45 wt% PET, and 5-40 wt% binder, the core paper having a density of 5-30 g / m ２ and a thickness of 5 to 30 microns, wherein the core paper has a coating layer on at least one surface thereof, the coating layer comprising refractory particles and a coating binder, and the separator has a surface pore size on the surface of the paper having the coating layer such that at least 90% of the surface pores have a pore size of up to 0.5 microns.

2. 2. The separator of claim 1, wherein at least 95% of the surface pores have a pore size of at most 0.5 microns and / or at least 80% of the surface pores have a pore size of at most 0.4 microns, more preferably at most 0.3 microns.

3. The core paper has a thickness of at least 7 g / m ２ and / or a maximum of 25 g / m ２ , especially up to 20 g / m ２ , more particularly up to 15 g / m ２ , in some embodiments up to 10 g / m ２ The separator of claim 1 having a basis weight of

4. 10. The separator of claim 1, wherein the aramid in the core paper comprises at least 20% by weight, in some embodiments at least 40% by weight, and in some embodiments at least 60% by weight, or at least 80% by weight, or at least 90% by weight para-aramid.

5. The separator of claim 1 , wherein the binder is selected from one or more of aramid fibrids, aramid pulp, jet-spun aramid fibrids, jet-spun aramid pulp, and combinations thereof.

6. 2. The separator of claim 1, wherein the amount of PET in the core paper is at least 15 wt.%, particularly at least 20 wt.%, more particularly at least 30 wt.%, and / or at most 40 wt.%, particularly at most 35 wt.%.

7. 2. The separator of claim 1, having a porosity of 40 to 70%, and / or a McMullin number of 10 to 1, particularly 8 to 1, more particularly 6 to 1, even more particularly 5 to 1, and even more particularly 4 to 1, and / or a CPV of at least 4.

8. A core paper suitable for use in the separator of any one of claims 1 to 7, comprising 30 to 70 wt% aramid short cut fibres, 10 to 45 wt% PET and 5 to 40 wt% binder, with a density of 5 to 30 g / m ２ and a thickness of 5 to 30 microns.

9. 10. A method for manufacturing a core paper according to claim 8, comprising the steps of providing a suspension containing at least a portion of the paper components, applying the suspension to a porous screen to form a web, removing the liquid medium from the web, and drying the paper.

10. 10. The method of claim 9, wherein the PET is incorporated into the core paper in the form of fibers, for example PET fibers having a linear density of at most 0.15 dtex, in particular at most 0.1 dtex, and / or at least 0.01 dtex, in particular at least 0.03 dtex.

11. 10. The method of claim 9, wherein the core paper is subjected to a heat treatment at a temperature of at least 175°C, in particular at least 200°C, more in particular at least 240°C, even more in particular at least 260°C, and / or up to 400°C.

12. A method for producing a separator according to any one of claims 1 to 7, comprising the steps of: - 30-70% by weight of aramid short cut fibers, 10-45% by weight of PET, and 5-40% by weight of binder, with a density of 5-30 g / m ２ and a thickness of 5 to 30 microns; providing a coating layer on at least one surface of said core paper by contacting said core paper with a composition comprising refractory particles and a coating binder; A method comprising:

13. 8. A Li-ion battery cell comprising a lithium-containing cathode and an anode connected through a lithium-containing electrolyte, the anode and cathode being separated from each other by the separator according to any one of claims 1 to 7.

14. A battery module comprising at least one Li-ion battery cell according to claim 13.

15. 10. Use of a separator according to any one of claims 1 to 7 in a Li-ion battery.

Images