KR20210015552A - Stack and folding-type electrode assembly and Secondary battery device comprising the same - Google Patents

Abstract

Disclosed is a stack-folding-type electrode assembly having a structure in which a plurality of unit cells having a positive electrode, a negative electrode, and a separator for a unit cell interposed between the positive electrode and the negative electrode, are overlapped with each other, and a separator for continuous folding is interposed in each of the overlapping portions. The separator for a unit cell comprises a first cross-linked polyolefin separator having siloxane cross-linking, and the separator for folding comprises a second cross-linked polyolefin separator having siloxane cross-linking. The melt-down temperatures of the first and second cross-linked polyolefin separators are all 170°C or greater, the shut-down temperature of the first cross-linked polyolefin separator is lower than the shut-down temperature of the second cross-linked polyolefin separator, and the mechanical strength of the second cross-linked polyolefin separator is greater than the mechanical strength of the first cross-linked polyolefin separator. A secondary battery having the stack-folding type electrode assembly according to the present invention has improved thermal safety and improved mechanical safety against external impact.

Description

Stack and folding-type electrode assembly and secondary battery device comprising the same

The present invention relates to a stack-folding electrode assembly and a secondary battery having the same, and more particularly, to a stack-folding electrode assembly having a crosslinked polyolefin separator having siloxane crosslinking and a secondary battery having the same.

With the development of technology and the increase in demand for mobile devices, the demand for secondary batteries is also rapidly increasing. Among them, lithium secondary batteries with high energy density and operating voltage and excellent storage and lifespan characteristics are used for various mobile devices as well as various electronic products. It is widely used as an energy source.

Secondary batteries are roughly classified into cylindrical batteries, prismatic batteries, and pouch-type batteries according to external and internal structural features, and among them, prismatic batteries and pouch-type batteries that can be stacked with a high degree of integration and have a small width compared to their length are particularly noted. Receiving.

Electrode assemblies having a positive/separator/cathode structure constituting a secondary battery are largely classified into a jelly-roll type (winding type) and a stack type (stack type) according to their structure. The jelly-roll type electrode assembly is coated with an electrode active material, etc. on a metal foil used as a current collector, dried and pressed, cut into a band of a desired width and length, and separated from the negative electrode and the positive electrode using a separator, and then spirally It is manufactured by winding it. The jelly-roll type electrode assembly is suitable for a cylindrical battery, but when applied to a prismatic or pouch type battery, there are disadvantages such as a peeling problem of an electrode active material and low space utilization. On the other hand, the stacked electrode assembly is a structure in which a plurality of positive and negative unit cells are sequentially stacked, and has the advantage of being easy to obtain a square shape, but the manufacturing process is complicated and the electrode is pushed when an impact is applied, causing a short circuit. There is a drawback that is caused.

In order to solve this problem, as an electrode assembly in the form of a mixture of the jelly-roll type and the stack type, unit cells having a certain unit size, for example, full cells of an anode/separator/cathode structure or an anode (cathode) A stack-folding electrode assembly having a structure in which bicells of /separator/cathode (anode)/separator/anode (cathode) structure are folded and overlapped using a long-length continuous folding separator has been developed.

1 schematically shows an exemplary structure of such a stack-folding electrode assembly.

Referring to FIG. 1, as a unit cell, C-type bi-cells (10, 13, 14) having a cathode/unit cell separator/anode/unit cell separator/cathode structure and anode/unit cell separator/cathode/unit cell separator/ A-type bi-cells 11 and 12 having an anode structure are alternately overlapped, and a folding separator 20 is interposed in each overlapping portion. A unit cell separator 30 is interposed between the anode and the cathode constituting the bi-cell which is a unit cell. The folding separator 20 has a unit length that can wrap a bicell, and is bent inward for each unit length, starting from the bicell 10 at the center, and continuing to the bicell 14 at the outermost side. It is interposed in the overlapping part of the bi-cell in a surrounding structure. The distal end of the folding separator 20 is heat-sealed or an adhesive tape 25 is applied to finish.

On the other hand, a polyolefin separator has been used as the separator 30 for a unit cell and the separator 20 for folding of such a stack-folding electrode assembly. However, polyethylene or polypropylene/polyethylene/polypropylene separators that have been commercially used have a problem that heat resistance is not sufficient (see Korean Patent Publication No. 10-0610261).

A first object of the present invention is to provide a stack-folding electrode assembly having improved thermal safety and a secondary battery having the same.

A second object of the present invention is to provide a stack-folding type electrode assembly having excellent durability against external impact while improving thermal safety and a secondary battery having the same.

In order to solve the above technical problem, according to the first embodiment of the present invention,

A stack-folding type electrode assembly having a structure in which a plurality of unit cells having a positive electrode, a negative electrode, and a unit cell separator interposed between the positive and negative electrodes are overlapped, and a continuous folding separator is interposed at each overlapping portion,

The separator for the unit cell includes a first crosslinked polyolefin separator having a siloxane crosslink, and the folding separator includes a second crosslinked polyolefin separator having a siloxane crosslink,

The meltdown temperature of the first and second crosslinked polyolefin separators are both 170°C or higher,

The shutdown temperature of the first crosslinked polyolefin separator is lower than the shutdown temperature of the second crosslinked polyolefin separator,

There is provided a stack-folding type electrode assembly, characterized in that the mechanical strength of the second crosslinked polyolefin separator is greater than that of the first crosslinked polyolefin separator.

According to the second embodiment of the present invention, the shutdown temperature of the first crosslinked polyolefin separator may be 2 to 15°C lower than the shutdown temperature of the second crosslinked polyolefin separator, and particularly 2 to 12°C.

According to the third embodiment of the present invention, in any one or more embodiments of the first or second embodiments, the mechanical strength of the second crosslinked polyolefin separator is 500 to greater than the mechanical strength of the first crosslinked polyolefin separator. It can be 2500 kgf/cm ² large, especially 800 to 1500 kgf/cm ² large.

According to the fourth embodiment of the present invention, in any one or more of the first to third embodiments, the shutdown temperature of the first crosslinked polyolefin separator may be 123 to 134 °C, in particular 125 to 133 °C. Can be

According to the fifth embodiment of the present invention, in any one or more of the first to fourth embodiments, the mechanical strength of the second crosslinked polyolefin separator may be 1500 kgf/cm ^{2 or} more, and in particular 2000 kgf/cm ^It can be ^{2 or} more.

According to a sixth embodiment of the present invention, in any one or more embodiments of the first to fifth embodiments, the first crosslinked polyolefin separator is modified high-density polyethylene, low-density polyethylene, and substituted with alkenes having 5 to 8 carbon atoms, and A mixture of at least one low-crystalline polyethylene and high-density polyethylene selected from the group consisting of linear low-density polyethylene may be formed of a cross-linked polyolefin cross-linked with siloxane.

According to the seventh embodiment of the present invention, in any one or more of the first to sixth embodiments, the second crosslinked polyolefin separator is a crosslinked crosslinked siloxane of high density polyethylene or a mixture of high density polyethylene and ultrahigh molecular weight polyethylene. It may be made of polyolefin.

According to the eighth embodiment of the present invention, in any one or more embodiments of the first to seventh embodiments, the meltdown temperature of the second crosslinked polyolefin separator is 3 to 70 than the meltdown temperature of the first crosslinked polyolefin separator. ℃ can be high.

The present invention provides a secondary battery including a stack-folding electrode assembly, which is one or more embodiments of the first to eighth embodiments. Such a secondary battery may be a lithium secondary battery.

The electrode assembly of the present invention is a stack-folding type, and the disadvantages of the jelly-roll and stacked electrode assembly are complemented.

In addition, in the stack-folding electrode assembly of the present invention, since the separator for the unit cell and the separator for folding both have a separator made of crosslinked polyolefin having a siloxane crosslink having a high meltdown temperature, commercially used polyethylene or polypropylene/polyethylene/ It is more excellent in heat resistance than a polypropylene separator.

In addition, by designing the shutdown temperature of the first crosslinked polyolefin separator provided in the unit cell separator to be lower than the shutdown temperature of the second crosslinked polyolefin separator provided in the folding separator, the current flow in the unit cell is lower when the secondary battery is overheated. The thermal stability of the secondary battery is improved by blocking it at temperature first.

Meanwhile, the mechanical strength of the second crosslinked polyolefin separator is designed to be higher than that of the second crosslinked polyolefin separator, so that the mechanical safety of the secondary battery against external impact is improved.

Such an electrode assembly can be usefully applied to a secondary battery such as a lithium secondary battery, particularly a lithium secondary battery having a high-capacity cell or a nickel-rich cathode material.

1 is a schematic diagram of an exemplary structure of a stack-folding type electrode assembly.
FIG. 2 is a schematic diagram illustrating an arrangement combination of unit cells in a manufacturing process of the stack-folding electrode assembly of FIG. 1.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to this, terms or words used in the specification and claims should not be construed as being limited to their usual or dictionary meanings, and the inventors appropriately explain the concept of terms in order to explain their own invention in the best way. Based on the principle that it can be defined, it should be interpreted as a meaning and concept consistent with the technical idea of the present invention. Accordingly, the embodiments described in the present specification and the configurations shown in the drawings are only the most preferred embodiment of the present invention, and do not represent all the technical spirit of the present invention, and thus various alternatives that can be substituted for them at the time of application It should be understood that there may be equivalents and variations. In addition, it should be understood that the same reference numerals denote the same elements in the specification.

In the first embodiment of the present invention, a plurality of unit cells having a positive electrode, a negative electrode, and a unit cell separator interposed between the positive and negative electrodes are overlapped, and a continuous folding separator is interposed in each overlapping portion. A stack-folding electrode assembly, wherein the unit cell separator includes a first crosslinked polyolefin separator having a siloxane crosslink, the folding separator includes a second crosslinked polyolefin separator having a siloxane crosslink, and the first and The meltdown temperature of the second crosslinked polyolefin separator is both 170°C or higher, the shutdown temperature of the first crosslinked polyolefin separator is lower than the shutdown temperature of the second crosslinked polyolefin separator, and the mechanical strength of the second crosslinked polyolefin separator Is greater than the mechanical strength of the first crosslinked polyolefin separator.

Here, the crosslinked polyolefin separator having a siloxane crosslink refers to a separator made of a material in which the polyolefin base resin is crosslinked with each other with a crosslinking group including a siloxane bond (-S-O-Si-).

Here, the meltdown temperature of the separator is defined as follows. After collecting the separator sample, insert a 10mm long sample into the TMA equipment (Thermomechanical Analysis, TA Instrument, Q400), and apply a tension of 10mN in the machine direction (MD) of the separator. Start and expose to 5° C./min). As the temperature increases, the length of the sample is changed, and the temperature at which the sample breaks due to the rapid increase in length is measured. In addition, the temperature at which the sample breaks is measured under the condition that the same tension is applied in the transverse direction (TD) of the separator. Among the temperatures at which the sample to which tension is applied in the MD and TD directions is cut, the higher temperature is defined as the melt down temperature of the sample.

Here, the shutdown temperature of the separator is defined as follows. While measuring the air permeability of the separator according to JIS P8117, the separator sample is exposed to increasing temperature conditions (starting at 30° C. and 5° C./min). The shutdown temperature of a separator is defined as the temperature at which the air permeability of the separator (Gurley value) exceeds 100,000 seconds/100cc for the first time. Air permeability can be measured using an air permeability meter (Asahi Seiko, EGO-IT).

Here, the mechanical strength of the separator is defined as follows. Using a UTM measuring instrument (manufacturer: Instron: model name: 3345 UTM), hold both ends of the separator specimen (grip length: 10 cm, width 1.5 cm) at a speed of 50 mm/min and pull it in the MD direction to endure the specimen until it is broken. The intensity measured as the maximum value of is measured three times and the average value is obtained. Subsequently, the intensity in the TD direction is measured three times in the same manner, and the average value is obtained. The value obtained by the average of the obtained strength in the MD direction and the strength in the TD direction is defined as the mechanical strength of the separator.

The structure of the electrode assembly according to the first embodiment of the present invention will be described as follows.

The electrode assembly is a stack-folding type electrode having a structure in which a plurality of unit cells having a positive electrode, a negative electrode, and a separator for a unit cell interposed between the positive and negative electrodes are overlapped, and a continuous folding separator is interposed at each overlapping portion. If it is an assembly, it is not particularly limited, and various forms may be included. For example, a stack-folding electrode assembly in which a plurality of stacked unit cells are wound with a long-cut folding separator, a Z-type stack-electrode assembly that folds in a zigzag direction when the stacked unit cells are wound with a folding separator sheet, etc. All of these can be included. The stack-folding electrode assembly compensates for the disadvantages of the jelly-roll and stacked electrode assembly.

1 schematically shows an exemplary structure of such a stack-folding electrode assembly.

Referring to FIG. 1, as a unit cell, C-type bi-cells (10, 13, 14) having a cathode/unit cell separator/anode/unit cell separator/cathode structure and anode/unit cell separator/cathode/unit cell separator/ A-type bi-cells 11 and 12 having an anode structure are alternately overlapped, and a folding separator 20 is interposed in each overlapping portion. A unit cell separator 30 is interposed between the anode and the cathode constituting the bi-cell which is a unit cell. The folding separator 20 has a unit length that can wrap a bicell, and is bent inward for each unit length, starting from the bicell 10 at the center, and continuing to the bicell 14 at the outermost side. It is interposed in the overlapping part of the bi-cell in a surrounding structure. The distal end of the folding separator 20 is heat-sealed or an adhesive tape 25 is applied to finish.

Next, a separator provided in the electrode assembly according to the first embodiment of the present invention will be described.

In the stack-folding electrode assembly according to the first embodiment of the present invention, the unit cell separator 30 includes a first crosslinked polyolefin separator having a siloxane crosslink, and the folding separator 20 has a siloxane crosslink. It has a 2nd crosslinked polyolefin separator which has it. That is, the separator 30 for the unit cell and the separator 20 for folding are all in common in that they are made of a crosslinked polyolefin having a siloxane crosslink having a high meltdown temperature, but their physical properties are different.

That is, the first and second crosslinked polyolefin separators are prepared using a crosslinked polyolefin resin having a siloxane crosslink, and the meltdown temperature is both 170°C or higher. For this reason, heat resistance is superior to those of commercially used non-crosslinked polyethylene or polypropylene/polyethylene/polypropylene separators. On the other hand, the shutdown temperature of the first crosslinked polyolefin separator is lower than the shutdown temperature of the second crosslinked polyolefin separator, but the shutdown temperature of the first crosslinked polyolefin separator is 2 to 15 than the shutdown temperature of the second crosslinked polyolefin separator. It can be as low as °C, especially 2 to 12 °C low. Accordingly, when the secondary battery is overheated, the current flow in the unit cell is first blocked at a lower temperature, thereby suppressing an increase in the battery temperature, thereby improving the thermal stability of the secondary battery. At this time, the shutdown temperature of the first crosslinked polyolefin separator may be 123 to 134 °C, and in particular, 125 to 133 °C.

The meltdown temperature of the second crosslinked polyolefin separator may be 3 to 70°C higher than the meltdown temperature of the first crosslinked polyolefin separator. In this case, the unit cell shuts down and the current flow is first blocked, so that the rate of temperature rise of the battery decreases, and even if the first cross-linked polyolefin separator melts down, the second cross-linked polyolefin separator does not melt down or is delayed, causing the battery to explode. And the ignition phenomenon can be improved.

Further, the second crosslinked polyolefin separator is designed to have a mechanical strength greater than that of the first crosslinked polyolefin separator. That is, the mechanical strength of the second crosslinked polyolefin separator is designed to be higher than that of the second crosslinked polyolefin separator, thereby improving the mechanical safety of the secondary battery against external impact. The mechanical strength of the second crosslinked polyolefin separator may be 500 to 2500 kgf/cm ² greater than the mechanical strength of the first crosslinked polyolefin separator, and particularly 800 to 1500 kgf/cm ² . At this time, when the mechanical strength of the second crosslinked polyolefin separator is 1500 kgf/cm ² or more, particularly 2000 kgf/cm ² or more, safety against external impact is further enhanced.

The first crosslinked polyolefin separator having the above-described physical properties having a siloxane crosslinking is at least one low crystal selected from the group consisting of modified high-density polyethylene, low-density polyethylene, and linear low-density polyethylene substituted with alkenes having 5 to 8 carbon atoms as shown in the following formula (1). A mixture of high-density polyethylene and high-density polyethylene (HDPE) may be made of crosslinked polyolefin crosslinked with siloxane.

In Formula 1, a is an alkene having 5 to 8 carbon atoms, and may include 2.5 to 10.0 carbon atoms based on 1,000 carbon atoms in the polyolefin main chain. Depending on the amount of substitution, the polyethylene and shutdown temperature can be lowered, and as the crystallinity decreases, the mechanical strength is relatively lowered. The mixing amount of low crystalline polyethylene including modified polyethylene may be 20 to 100 parts by weight based on 100 parts by weight of high density polyethylene.

In addition, the second crosslinked polyolefin separator having the above-described physical properties having a siloxane crosslinking may be formed of a crosslinked polyolefin in which high density polyethylene or a mixture of high density polyethylene and ultra high molecular weight polyethylene (UHMWPE) is crosslinked with siloxane. Here, ultra-high molecular weight polyethylene refers to polyethylene having a weight average molecular weight of 1 million or more, as is commercially known. The mechanical strength can be increased if high-density polyethylene with high crystallinity is used alone or mixed with ultra-high molecular weight polyethylene as the base resin. Meanwhile, the shutdown temperature becomes relatively high. When the ultra-high molecular weight polyethylene is mixed and used, the mixing amount of the ultra-high molecular weight polyethylene may be 5 to 100 parts by weight based on 100 parts by weight of the high-density polyethylene.

The first crosslinked polyolefin separator having a siloxane crosslink and the second crosslinked polyolefin separator having a siloxane crosslink are introduced and mixed with a polyolefin, a diluent, an alkoxy group-containing vinylsilane, and an initiator into an extruder to prepare a silane-grafted polyolefin composition. Reactive extrusion; Molding and stretching the reaction-extruded silane-grafted polyolefin composition into a sheet form; Extracting a diluent from the stretched sheet to prepare a porous membrane; And crosslinking the porous membrane in the presence of moisture, but is not limited thereto.

First, a polyolefin, a diluent, an alkoxy group-containing vinylsilane, and an initiator are introduced into an extruder and mixed to react and extrude the silane-grafted polyolefin composition.

As the polyolefin, base resins of the first crosslinked polyolefin separator having a siloxane crosslink and the second crosslinked polyolefin separator having a siloxane crosslink may be used, but are not limited thereto. Among these polyolefin base resins, the weight average molecular weight of high-density polyethylene, low-density polyethylene, and linear low-density polyethylene may be 200,000 or more, more preferably 220,000 to 800,000, more preferably 250,000 to 500,000, and even more preferably 300,000 to 450,000.

As the diluent, liquid or solid paraffin, wax, soybean oil, etc., which are generally used in the manufacture of wet separators, may be used. As a diluent, a diluent capable of liquid-liquid phase separation with polyolefins can also be used, examples of which include phthalic acid esters such as dibutyl phthalate, dihexyl phthalate, and dioctyl phthalate. (phthalic acid ester); Aromatic ethers such as diphenyl ether and benzyl ether; Fatty acids having 10 to 20 carbon atoms such as palmitic acid, stearic acid, oleic acid, linoleic acid, and linolenic acid; Fatty acid alcohols having 10 to 20 carbon atoms, such as palmitic alcohol, stearic alcohol, and oleic alcohol; Of fatty acid groups such as palmitic acid mono-, di-, or tryster, stearic acid mono-, di-, or tryster, oleic acid mono-, di-, or tryster, linoleic acid mono-, di-, or tryster. Saturated and unsaturated fatty acids having 4 to 26 carbon atoms, or one or two or more fatty acids in which the double bond of the unsaturated fatty acid is substituted with epoxy is ester-bonded with an alcohol having 1 to 8 hydroxy groups and 1 to 10 carbon atoms. Fatty acid esters; The diluent may be used as a mixture containing two or more of the above-described components. The weight ratio of polyolefin to diluent may range from 50:50 to 20:80, preferably from 40:60 to 30:70.

As the alkoxy group-containing vinylsilane, trimethoxyvinylsilane, triethoxyvinylsilane, triacetoxyvinylsilane, or the like may be used alone or as a mixture of two or more. Such an alkoxy group-containing vinylsilane is grafted onto a polyolefin by a vinyl group, and a crosslinking reaction proceeds by the alkoxy group to crosslink the polyolefin. The content of the alkoxy group-containing vinylsilane may be 0.1 to 10 parts by weight, preferably 0.1 to 5 parts by weight, and more preferably 0.5 to 2 parts by weight, based on 100 parts by weight of the total content of the polyolefin and the diluent.

As the initiator, if an initiator capable of generating radicals is applicable, for example, benzoyl peroxide, acetyl peroxide, dilauryl peroxide, di-ter-butyl peroxide, dicumyl peroxide, cumyl peroxide, hydrogen peroxide. Oxide, potassium persulfate, and the like, but are not limited thereto. The content of the initiator may be 0.2 to 100 parts by weight, preferably 1 to 50 parts by weight, more preferably 2 to 10 parts by weight, based on 100 parts by weight of the alkoxy group-containing vinylsilane.

The silane grafted polyethylene composition may further include a crosslinking catalyst in the presence of moisture, that is, a crosslinking catalyst that promotes crosslinking, if necessary.In addition, the composition may include an oxidation stabilizer, a UV stabilizer, an antistatic agent, and a nucleating agent if necessary. agent) and general additives for improving specific functions may be further added.

Next, the reaction-extruded silane-grafted polyolefin composition is molded and stretched into a sheet form. For this process, a conventional single screw extruder, twin screw extruder, and stretching device can be used. Extrusion conditions, stretching conditions, and heat setting conditions are not different from the range of ordinary separation membrane processing conditions. Accordingly, the step of forming and stretching into a sheet form may include cooling the silane-grafted polyolefin composition extruded into a sheet form to form a sheet; And forming a stretched sheet by biaxially stretching the resultant formed into a sheet in a longitudinal direction and a transverse direction. That is, the silane-grafted polyolefin composition is extruded using an extruder equipped with T-Dice, etc., and then a cooled extrudate may be formed using a general casting method or calendering method using water cooling or air cooling. Thereafter, a sheet is formed by stretching using the cooled extrudate. According to an embodiment of the present invention, such stretching may be performed by sequential or simultaneous stretching of a roll method or a tenter method. The draw ratio may be 3 times or more and 5 to 10 times, respectively, in the longitudinal and transverse directions, and the total draw ratio may be 20 to 80 times. In this case, the stretching temperature can be adjusted according to the melting point of the polyolefin used and the concentration and type of the diluent.

Thereafter, the diluent is extracted from the obtained stretched sheet to prepare a porous membrane. Specifically, the diluent is extracted using an organic solvent from the porous membrane, and then dried. The organic solvent that can be used at this time is not particularly limited, and the resin is extruded.

Any solvent that can extract the used diluent can be used. For example, as the organic solvent, methyl ethyl ketone, methylene chloride, which has high extraction efficiency and quick drying,

Hexane and the like are suitable. As the extraction method, all general solvent extraction methods, such as an immersion method, a solvent spray method, and an ultrasonic method, may be used individually or in combination. In addition, the extraction time varies depending on the thickness of the porous film to be produced, but in the case of a porous film having a thickness of 10 to 30 μm, 2 to 4 minutes is appropriate.

Optionally, it may further include heat setting between the step of extracting the diluent from the sheet to prepare a porous membrane, and crosslinking the porous membrane. That is, the dried porous membrane may undergo a heat setting step when it is necessary to reduce the residual stress and reduce the high-temperature shrinkage of the final separator to 5% or less in the longitudinal direction and the transverse direction, as in the case of a battery separator. Heat setting is to fix the porous membrane and apply heat to forcefully hold the porous membrane to shrink to remove residual stress. A high heat setting temperature is advantageous for lowering the shrinkage rate, but if it is too high, the microporous formed by partially melting the porous membrane may become clogged and the transmittance may decrease. The heat setting time should be relatively short when the heat setting temperature is high, and can be made relatively long when the heat setting temperature is low. Preferably, about 5 seconds to 1 minute is appropriate.

Next, the porous membrane prepared by extracting the diluent is subjected to a crosslinking step in the presence of moisture. Such crosslinking may be carried out in a constant temperature and humidity chamber at a temperature of 50 to 100°C, more preferably 60 to 90°C, and a humidity of 50 to 100%, more preferably 70 to 100%, or immersed in hot or boiling water for several hours. Alternatively, it can proceed over several days. In order to promote the crosslinking reaction, a crosslinking catalyst may be used. As such a crosslinking catalyst, in general, a carboxylate, an organic base, an inorganic acid, and an organic acid of a metal such as tin, zinc, iron, lead, and cobalt may be used. Specifically, dibutyl tin dilaurate, dibutyl tin diacetate, stannous acetate, stannous caprylic acid, zinc naphthenate, zinc caprylate, cobalt naphthenate, ethylamine, dibutyl amine, hexyl There may be inorganic acids such as amine, pyridine, sulfuric acid, hydrochloric acid, and organic acids such as toluene sulfonic acid, acetic acid, stearic acid, and maleic acid. As a method of using the crosslinking catalyst, a method of adding a crosslinking catalyst at the time of preparing a silane grafted polyethylene solution, a method of applying a solution or dispersion of a crosslinking catalyst to a porous membrane, and the like.

The prepared first crosslinked polyolefin separator having siloxane crosslinking is interposed between the positive electrode and the negative electrode as a unit cell separator to prepare a unit cell such as a full cell or a bicell.

y≤0.7 임)으로 표현되는 리튬 니켈계 산화물; Li_{1 + z}Ni_{1 / 3}Co_{1 / 3}Mn_{1 / 3}O₂, Li_1+zNi_0.4Mn_0.4Co_0.2O₂ 등과 같이 Li_{1 + z}Ni_bMn_cCo_{1 - (b+c+d)}M_dO_(2-e)A_e (여기서, -0.5

z≤0.5, 0.1≤b≤0.8, 0.1≤c≤0.8, 0≤d≤0.2, 0≤e≤0.2, b+c+d<1 임, M = Al, Mg, Cr, Ti, Si 또는 Y 이고, A = F, P 또는 Cl 임)으로 표현되는 리튬 니켈 코발트 망간복합산화물; 화학식 Li_{1 + x}M_{1 - y}M'_yPO_{4 - z}X_z(여기서, M = 전이금속, 바람직하게는 Fe, Mn, Co 또는 Ni 이고, M' = Al, Mg 또는 Ti 이고, X = F, S 또는 N 이며, -0.5

x≤+0.5, 0≤y≤0.5, 0≤z≤0.1 임)로 표현되는 올리빈계 리튬금속 포스페이트 등을 들 수 있고, 특히 니켈이 다량 함유된 (Ni rich) 양극 활물질이 사용될 수 있지만, 이들만으로 한정되는 것은 아니다. The positive electrode may be prepared by coating a slurry prepared by mixing a positive electrode mixture with a solvent such as N-methyl pyrrolidone (NMP) on a negative electrode current collector, followed by drying and rolling. The positive electrode mixture may optionally contain a conductive material, a binder, a filler, etc. in addition to the positive electrode active material. The positive electrode current collector is not particularly limited as long as it has conductivity without causing a chemical change in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or carbon on the surface of aluminum or stainless steel, A surface treated with nickel, titanium, silver, or the like may be used. Preferably, the positive electrode current collector may contain 99.0 to 99.9% of aluminum, and the balance may be made of inevitable impurities. The inevitable impurities may include at least one element selected from the group consisting of C, Si, Fe, Cu, Mn, Mg, Ti, and Zn. The cathode active material is a material capable of causing an electrochemical reaction, as a lithium transition metal oxide, containing two or more transition metals, for example, lithium cobalt oxide (LiCoO ₂ ), lithium nickel substituted with one or more transition metals Layered compounds such as oxide (LiNiO ₂ ); Lithium manganese oxide substituted with one or more transition metals; Formula LiNi _{1 - y} M _y O ₂ (where, M = Co, Mn, Al, Cu, Fe, Mg, B, Cr, Zn or Ga and contains one or more of the above elements, 0.01

a lithium nickel-based oxide represented by y≤0.7; _{Li 1 + z Ni 1/3} Co 1/3 Mn 1/3 O 2, Li 1 + z Ni 0.4 Mn 0.4 Co 0.2 O 2 , such as Li _{1 + z} Ni _b Mn _c Co _{1 - (b + c + d )} M _d O _(2-e) A _e (where -0.5

z≤0.5, 0.1≤b≤0.8, 0.1≤c≤0.8, 0≤d≤0.2, 0≤e≤0.2, b+c+d<1, M = Al, Mg, Cr, Ti, Si or Y And A = F, P or Cl) lithium nickel cobalt manganese composite oxide; Formula Li _{1 + x} M _{1 - y M'y} PO _{4 - z} X _z (where M = transition metal, preferably Fe, Mn, Co or Ni, M'= Al, Mg or Ti, X = F, S or N, and -0.5

x≤+0.5, 0≤y≤0.5, 0≤z≤0.1) olivine-based lithium metal phosphate and the like, and in particular, a positive electrode active material containing a large amount of nickel may be used, but these It is not limited to only.

Conductive materials, binders, fillers, and the like may also use conventional components used in manufacturing the positive electrode.

The negative electrode is manufactured by, for example, applying a negative electrode mixture containing a negative electrode active material on a negative electrode current collector and then drying the negative electrode mixture, if necessary, such as a conductive material, a binder, a filler, etc. Ingredients may be included.

Examples of the negative electrode active material include carbon and graphite materials such as natural graphite, artificial graphite, expanded graphite, carbon fiber, non-graphitizable carbon, carbon black, carbon nanotubes, fullerene, and activated carbon; Metals such as Al, Si, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, Pd, Pt, Ti, etc., which can be alloyed with lithium, and compounds containing these elements; Composites of metals and compounds thereof and carbon and graphite materials; Lithium-containing nitrides, and the like. Among them, a carbon-based active material, a silicon-based active material, a tin-based active material, or a silicon-carbon-based active material is more preferable, and these may be used alone or in combination of two or more.

The negative electrode current collector may be generally manufactured to a thickness of 6 to 20 μm. Preferably, the thickness of the negative electrode current collector may be 8 to 15 μm. Such a negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes to the battery, for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel. Surface treatment with carbon, nickel, titanium, silver or the like, aluminum-cadmium alloy, etc. may be used. In addition, like the positive electrode current collector, it is possible to enhance the bonding strength of the negative electrode active material by forming fine irregularities on the surface thereof, and may be used in various forms such as films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics.

A stack-folding electrode assembly was manufactured using the unit cells thus prepared and a second crosslinked polyolefin separator having a siloxane crosslink as a folding separator.

FIG. 2 is a schematic diagram illustrating an arrangement combination of unit cells in a manufacturing process of the stack-folding electrode assembly of FIG. 1. Such a stack-folding type electrode assembly, for example, by arranging bicells 10, 11, 12, 13, 14 on a folding separator sheet 20 of a long length, and one end of the folding separator sheet 20 It is produced by winding up sequentially starting at (21).

At this time, looking at the arrangement combination of the bicells 10, 11, 12, 13, and 14 as unit cells, the first bicell 10 and the second bicell 11 have a width corresponding to at least one bicell Since it is located at a distance separated by an interval, the outer surface of the first bi-cell 10 is completely coated with the folding separator sheet 20 during the winding process, and then the lower electrode of the first bi-cell 10 (cathode, -) It comes into contact with the top electrode (positive electrode, +) of the second bi-cell 11. In the bicells 12, 13, and 14 after the second bi-cell 11, since the application length of the folding separator sheet 20 increases in the sequential lamination process by winding, the gap between them in the winding direction is increased. They are arranged to increase sequentially. In addition, the bicells 10, 11, 12, 13, 14 should be configured such that the anode and the cathode face each other at the stacked interface when winding. In the first bicell 10, the top electrode is a bipolar electrode. Cell, the second bi-cell 11 and the third bi-cell 12 are bi-cells whose top electrodes are positive, and the fourth bi-cell 13 and the fifth bi-cell 14 have a negative top electrode It is composed of phosphorus bicell. That is, the bi-cell is mounted in an alternate arrangement in two units.

The stack-folding type electrode assembly thus prepared can be applied to secondary batteries such as lithium secondary batteries. For example, a secondary battery may be manufactured by welding a positive electrode and a negative electrode lead to the electrode tab portion of the fabricated electrode assembly, packing the welded cell with an aluminum pouch, and injecting an electrolyte solution, but is not limited thereto. These secondary batteries can be used not only for battery cells used as power sources for small devices, but also for medium and large batteries including a number of battery cells used as power sources for medium and large devices that require high temperature safety, long cycle characteristics, and high rate characteristics. It can be preferably used as a unit cell in the module. Preferred examples of medium and large-sized devices include a power tool that is powered by an omniscient motor; Electric vehicles including electric vehicles (EV), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and the like; Electric two-wheeled vehicles including electric bicycles (E-bikes) and electric scooters (E-scooters); And an electric golf cart.

Hereinafter, examples will be described in detail to illustrate the present invention in detail. However, the embodiments according to the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments of the present invention are provided to more completely describe the present invention to those of ordinary skill in the art.

Example One

Siloxane Crosslinking Having the first crosslinked polyolefin Separator ( Unit cell )of Produce

First, 15 parts by weight of high-density polyethylene with a weight average molecular weight of 300,000 (LG Chemical XL1800), 15 parts by weight of a low crystalline modified high-density polyethylene with a weight average molecular weight of 500,000 (Daehan Emulsification pilot product VH030H), liquid paraffin oil (at 40 ℃) Kinematic viscosity of: 40 cSt) 70 parts by weight of trimethoxyvinylsilane, 0.5 parts by weight based on the total of the high-density polyethylene, the modified high-density polyethylene and the liquid paraffin oil, and the initiator based on 100 parts by weight of trimethoxyvinylsilane 2, 1 part by weight of 5-dimethyl-2,5-di-(tert-butylperoxy)hexane (2,5-dimethyl-2,5-di(tert-butylperoxy)hexane) was added to and kneaded into a twin screw extruder, Reaction was extruded.

The reaction-extruded silane-grafted polyethylene composition was formed into a sheet form through a T-die and a cooling casting roll, and then biaxially stretched with a tenter-type sequential stretching machine of TD stretching after MD stretching. The MD draw ratio and the TD draw ratio were 6 and 5.5 times, respectively. The stretching temperature was 105°C in MD and 115°C in TD.

From the stretched sheet, liquid paraffin oil, which is a diluent, was extracted with methylene chloride and heat-set at 125° C. starting at 1.6 times the draw ratio and decreasing to 1.4 times to prepare a porous membrane. The porous membrane was allowed to stand at 85° C. and 85% relative humidity for 48 hours to proceed with a crosslinking reaction, thereby preparing a first crosslinked polyolefin separator having a siloxane crosslink. The thickness of the obtained first crosslinked polyethylene separator was 9 µm, and the meltdown temperature, shutdown temperature, and mechanical strength were measured and shown in Table 1 below.

Siloxane Crosslinking Having a second crosslinked polyolefin Separator ( For folding ) Of manufacture

First, 10 parts by weight of ultra-high molecular weight polyethylene (VH150U) having a weight average molecular weight of 1,500,000, 20 parts by weight of high-density polyethylene (LG Chemical XL1800) having a weight average molecular weight of 300,000, liquid paraffin oil as a diluent (kinetic viscosity at 40 °C: 40 cSt) ) 70 parts by weight, based on the total of 100 parts by weight of the ultra-high molecular weight polyethylene, high-density polyethylene, and liquid paraffin oil, trimethoxyvinylsilane 0.5 parts by weight based on 100 parts by weight of the trimethoxy vinylsilane 2,5-die 1 part by weight of methyl-2,5-di-(tert-butylperoxy)hexane (2,5-dimethyl-2,5-di(tert-butylperoxy)hexane) was added and kneaded into a twin screw extruder, followed by reaction extrusion. .

The reaction-extruded silane-grafted polyethylene composition was formed into a sheet form through a T-die and a cooling casting roll, and then biaxially stretched with a tenter-type sequential stretching machine of TD stretching after MD stretching. The MD draw ratio and the TD draw ratio were 8.0 and 8.0 times, respectively. The stretching temperature was 118°C in MD and 128°C in TD.

Liquid paraffin oil as a diluent was extracted from the stretched sheet with methylene chloride, and heat-set at 135° C. starting at 1.8 times and decreasing to 1.6 times to prepare a porous membrane. The porous membrane was allowed to stand at 85° C. and 85% relative humidity for 48 hours to proceed with a crosslinking reaction, thereby preparing a second crosslinked polyolefin separator having a siloxane crosslink. The thickness of the obtained second crosslinked polyethylene separator was 9 µm, and the meltdown temperature, shutdown temperature, and mechanical strength were measured and shown in Table 1 below.

Manufacture of anode

Positive electrode active material by adding 92% by weight of lithium cobalt composite oxide as positive electrode active material particles, 4% by weight carbon black as a conductive material, and 4% by weight PVDF as a binder to N-methyl-2 pyrrolidone (NMP) as a solvent. A slurry was prepared. The positive electrode active material slurry was coated on an aluminum (Al) thin film of a positive electrode current collector having a thickness of 20 μm, dried to prepare a positive electrode, and then roll press was performed.

Preparation of cathode

Carbon powder as a negative electrode active material, polyvinylidene fluoride (PVdF) as a binder, and carbon black as a conductive material are set to 96% by weight, 3% by weight, and 1% by weight, respectively, and N-methyl-2 as a solvent. It was added to rolidone (NMP) to prepare a negative active material slurry. The negative electrode active material slurry was coated on a copper (Cu) thin film of a negative electrode current collector having a thickness of 10 µm, and dried to prepare a negative electrode, followed by roll press.

Battery manufacturing

The unit cells were assembled by stacking the positive electrode, the negative electrode, and the first crosslinked separator prepared by the above method, and the unit cells were rolled in a folding manner according to FIG. 2 using a second crosslinked separator to obtain an electrode assembly having the structure of FIG. It was prepared and inserted into a pouch. Then, an electrolyte solution (ethylene carbonate (EC)/ethylmethyl carbonate (EMC) = 1/2 (volume ratio), lithium hexafluorophosphate (LiPF6) 1 mol)) was injected to prepare a lithium secondary battery.

Example 2

When manufacturing the first crosslinked polyolefin separator, the content of high-density polyethylene (LG Chemical XL1800) having a weight average molecular weight of 300,000 is 5 parts by weight, and the content of low crystalline modified high-density polyethylene (Daehan Emulsification pilot product VH030H) having a weight average molecular weight of 500,000 is 25. It was prepared in the same manner as in Example 1, except that it was changed to parts by weight.

Example 3

In the manufacture of the first crosslinked polyolefin separator, the MD draw ratio and the TD draw ratio were changed to 5.5 and 4.0 times, respectively, when biaxially stretched with a tenter-type sequential stretching machine, and when heat setting after extracting the liquid paraffin oil, the draw ratio was reduced from 1.5 times to 1.2 times at 125°C. It was prepared in the same manner as in Example 1, except that it was changed to heat setting.

Example 4

Example 1, except that 30 parts by weight of ultra-high molecular weight polyethylene having a weight average molecular weight of 1,000,000 (Daehan Emulsification VH100U) was used instead of 10 parts by weight of ultra-high molecular weight polyethylene (Daehan Emulsification VH150U) having a weight average molecular weight of 1,500,000 when manufacturing the second crosslinked polyolefin separator. It was prepared in the same way as.

Example 5

Low crystalline high-density polyethylene with a weight average molecular weight of 500,000 when manufacturing the first crosslinked polyolefin separator (Daehan Petrochemical pilot product VH030H) 14 parts by weight of a low-crystalline modified high-density polyethylene having a weight average molecular weight of 500,000 (Hanwha Total Petrochemical pilot product V0620-1) 30 parts by weight of ultra-high molecular weight polyethylene (Daehan Petrochemical VH100U) 30 parts by weight was used instead of 10 parts by weight of ultra-high molecular weight polyethylene (Daehan Emulsion VH150U) having a weight average molecular weight of 1,500,000 when manufacturing the second crosslinked polyolefin separator. Except it was prepared in the same manner as in Example 1.

Comparative example One

It was prepared in the same manner as in Example 1, except that the first and second crosslinked polyolefin separators were prepared without adding trimethoxyvinylsilane and an initiator.

Comparative example 2

The first crosslinked polyolefin separator was prepared in the same manner as in Example 1, except that trimethoxyvinylsilane and an initiator were not added.

Comparative example 3

The second crosslinked polyolefin separator was prepared in the same manner as in Example 1, except that trimethoxyvinylsilane and an initiator were not added.

Meltdown temperature (℃)
For unit cell/folding Shutdown temperature (℃)
For unit cell/folding Mechanical strength (kgf/cm ² )
(MD/TD) Example 1 180/186 133/137 For unit cell: 1150 (1200 / 1100) For folding: 2100 (2150 / 2050) Example 2 175/181 131.5/137 For unit cell: 1210 (1250 / 1170) For folding: 2100 (2150 / 2050) Example 3 171/184 132/137 For unit cell: 1020 (1100 / 940) For folding: 2100 (2150 / 2050) Example 4 178/185 133/138 For unit cell: 1150 (1200 / 1100) For folding: 2595 (2650 / 2540) Example 5 177/185 130/138 For unit cell: 1200 (1220 / 1180) For folding: 2595 (2650 / 2540) Comparative Example 1 149/151 134/138.5 For unit cell: 1210 (1270 / 1150) For folding: 2150 (2200 / 2100) Comparative Example 2 149/184 134/137 For unit cell: 1210 (1270 / 1150) For folding: 2100 (2150 / 2050) Comparative Example 3 183/149 133/138.5 For unit cell: 1150 (1200 / 1100) For folding: 2100 (2150 / 2050)

<Battery safety evaluation>

Nail penetration test

10 samples of each of the pouch-type batteries of Examples and Comparative Examples were prepared and fully charged at 4.35V, and then the fully charged battery was passed through a 2.5mm diameter nail at a speed of 5m/min to evaluate the stability. When there is no ignition or explosion among the 10 samples, it is judged as a pass, and the number of passed samples/number of samples is shown in Table 2 below.

impact Test

Each sample of 10 pouch-type batteries of Examples and Comparative Examples was fully charged at 4.35V, and a bar having a diameter of 15.8 mm was placed on the fully charged battery, and a weight of 9.1 Kg was dropped to a height of 630 mm to conduct stability evaluation. When there is no ignition or explosion among the 10 samples, it is judged as a pass, and the number of passed samples/number of samples is shown in Table 2 below.

Hot box (Hot Box) test

A hot box test was performed on 10 samples of each of the pouch-type batteries of Examples and Comparative Examples. The hot box test was carried out by maintaining the battery fully charged at 4.35V at a heating rate of 5°C/min, 130°C and 1 hour, and then holding the battery at a rate of 5°C/min, 150°C and 1 hour again, and ignition out of 10 samples B. When there is no explosion, it is judged as a pass, and the number of passed samples/number of samples is shown in Table 2 below.

safety
Item Example Comparative example One 2 3 4 5 One 2 3 Nail penetration test Number of passes/ number of samples 10/10 10/10 10/10 10/10 10/10 4/10 3/10 7/10 Impact test 8/10 9/10 9/10 10/10 9/10 10/10 9/10 10/10 Hot box test 8/10 10/10 10/10 9/10 10/10 4/10 4/10 6/10

Referring to Table 2, it can be seen that the battery having the polyolefin separators of Comparative Examples 1 to 3 in which at least one or more of the separator for the unit cell and the separator for folding does not have siloxane crosslinks has lower safety compared to the Examples.

11, 12: A type bicell 10, 13, 14: C type bicell
20: folding separator 21: one end of the folding separator
25; Adhesive tape
30: separator for unit cell

Claims (13)

A stack-folding type electrode assembly having a structure in which a plurality of unit cells having a positive electrode, a negative electrode, and a unit cell separator interposed between the positive and negative electrodes are overlapped, and a continuous folding separator is interposed at each overlapping portion,
The separator for the unit cell includes a first crosslinked polyolefin separator having a siloxane crosslink, and the folding separator includes a second crosslinked polyolefin separator having a siloxane crosslink,
The meltdown temperature of the first and second crosslinked polyolefin separators are both 170°C or higher,
The shutdown temperature of the first crosslinked polyolefin separator is lower than the shutdown temperature of the second crosslinked polyolefin separator,
A stack-folding type electrode assembly, characterized in that the mechanical strength of the second crosslinked polyolefin separator is greater than that of the first crosslinked polyolefin separator. The method of claim 1,
A stack-folding electrode assembly, wherein a shutdown temperature of the first crosslinked polyolefin separator is 2 to 15°C lower than that of the second crosslinked polyolefin separator. The method of claim 1,
A stack-folding electrode assembly, characterized in that the mechanical strength of the second crosslinked polyolefin separator is 500 to 2500 kgf/cm ² greater than the mechanical strength of the first crosslinked polyolefin separator. The method of claim 1,
A stack-folding electrode assembly, characterized in that the mechanical strength of the second crosslinked polyolefin separator is 800 to 1500 kgf/cm ² greater than the mechanical strength of the first crosslinked polyolefin separator. The method of claim 1,
A stack-folding electrode assembly, characterized in that the shutdown temperature of the first crosslinked polyolefin separator is 123 to 134°C. The method of claim 1,
A stack-folding electrode assembly, characterized in that the shutdown temperature of the first crosslinked polyolefin separator is 125 to 133°C. The method of claim 1,
Stack-folding type electrode assembly, characterized in that the mechanical strength of the second crosslinked polyolefin separator is 1500 kgf / cm ² or more. The method of claim 1,
Stack-folding type electrode assembly, characterized in that the mechanical strength of the second crosslinked polyolefin separator is 2000 kgf / cm ² or more. The method of claim 1,
The first crosslinked polyolefin separator is a siloxane crosslinked crosslinked mixture of at least one low-crystalline polyethylene and high-density polyethylene selected from the group consisting of modified high-density polyethylene, low-density polyethylene, and linear low-density polyethylene substituted with alkenes having 5 to 8 carbon atoms. Stack-folding type electrode assembly, characterized in that made of polyolefin. The method of claim 1,
The second crosslinked polyolefin separator is a stack-folding type electrode assembly, characterized in that it is made of a crosslinked polyolefin in which high density polyethylene or a mixture of high density polyethylene and ultra high molecular weight polyethylene is crosslinked with siloxane. The method of claim 1,
A stack-folding type electrode assembly, characterized in that the meltdown temperature of the second crosslinked polyolefin separator is 3 to 70°C higher than the meltdown temperature of the first crosslinked polyolefin separator. A secondary battery comprising the stack-folding electrode assembly of claim 1. The method of claim 12,
The secondary battery is a secondary battery, characterized in that the lithium secondary battery.

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