Lithium secondary battery including a separator membrane

Abstract

The present application relates to a separator for a lithium secondary battery and a lithium secondary battery including the same.

Description

Technical Field

[0001] This application claims the benefit of the filing date of Korean Patent Application No. 10-2022-0012968, filed with the Korean Intellectual Property Office on January 28, 2022, and all of its contents are incorporated herein by reference.

[0002] This application relates to a lithium secondary battery including a separator.

Background Art

[0003] Due to the rapid increase in the use of fossil fuels, the demand for alternative and clean energy has been increasing. As part of this, the field most actively studied is power generation and energy storage using electrochemical reactions.

[0004] Currently, a typical example of an electrochemical device that uses such electrochemical energy is a secondary battery, and its usage area is increasingly expanding.

[0005] As the technology development and demand for mobile devices increase, the demand for secondary batteries as an energy source has been rapidly increasing. Among such secondary batteries, lithium secondary batteries having a high energy density, voltage, long cycle life, and low self-discharge rate have been commercialized and widely used. In addition, research on methods for manufacturing high-density electrodes with a higher energy density per unit volume as electrodes for such high-capacity lithium secondary batteries has been actively proceeding.

[0006] Generally, a secondary battery is composed of a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode includes a negative electrode active material that inserts and desorbs lithium ions emitted from the positive electrode, and silicon-based particles having a large discharge capacity can be used as the negative electrode active material.

[0007] Particularly in recent years, in response to the demand for high-density energy batteries, as negative electrode active materials, Si / C or SiO having a capacity more than 10 times larger than that of graphite-based materials xWhile research is actively underway on methods to increase capacity by using silicon-based compounds such as those mentioned above, silicon-based compounds, although offering higher capacity compared to conventionally used graphite, have the problem of rapidly expanding in volume during the charging process, disrupting the conductive path and degrading battery performance.

[0008] Furthermore, while silicon-based anodes undergo a carbonization process, more than 50% of the anode binder contained in the silicon-based anode disappears during this process, resulting in a problem where the surface becomes extremely rough. In other words, the carbonization process of the silicon-based anode makes the surface of the silicon-based anode extremely rough, which can cause micro-shorts to occur as the silicon penetrates the separation membrane and comes into contact with the positive electrode.

[0009] Therefore, in order to resolve the problems that arise when using silicon-based compounds as negative electrode active materials, various solutions are being researched, such as adding a coating layer to the surface of the separation membrane or changing the material of the separation membrane. However, no suitable solution has yet been found.

[0010] Therefore, research is needed on separation membranes that can withstand the pressure and roughness when the separation membrane is compressed by expansion during the charging and discharging of silicon-based negative electrodes with rough surfaces. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] Japanese Patent Publication No. 2009-080971 [Overview of the Initiative] [Problems that the invention aims to solve]

[0012] When the separator is compressed due to the expansion during charge and discharge of a silicon-based negative electrode having a rough surface, as a result of research on a separator that can withstand such pressure and roughness, it has been confirmed by research that when adjusting the physical properties of the separator itself, the above problems can be solved. Accordingly, the present application relates to a lithium secondary battery including a separator under specific conditions.

Means for Solving the Problems

[0013] One embodiment of the present specification is a lithium secondary battery including a silicon-based negative electrode; a positive electrode; a separator positioned between the negative electrode and the positive electrode; and an electrolyte, wherein the silicon-based negative electrode includes a negative electrode current collector layer; a negative electrode active material layer including a negative electrode active material layer composition provided on one or both surfaces of the negative electrode current collector layer, the negative electrode active material layer composition includes at least one selected from the group consisting of a silicon-based active material, a negative electrode conductive material, and a negative electrode binder, the silicon-based active material includes at least one selected from the group consisting of SiO x (x = 0) and SiO x (0 < x < 2), and based on 100 parts by weight of the silicon-based active material, the SiO x (x = 0) includes 70 parts by weight or more, the separator satisfies a thickness of 1 μm or more and 15 μm or less, has a compression resistance defined by the following formula 1 of 1% or more and 14.5% or less, and has a breakdown voltage of 400 V or more, and provides a lithium secondary battery.

[0014] [Formula 1] (T1 - T2) / T1 × 100 In the above formula 1, T1 means the initial thickness in a state where three of the separators are laminated, T2 means the thickness after laminating three of the separators and pressing them at 2000 kgf and 70 °C for 10 seconds with a hot press (QM-940A Grade manufactured by Q-mesys), The breakdown voltage means the voltage at the point of dielectric breakdown when the cell including the separator is boosted from 0 V to 6000 V at a rate of 100 V / s with a Hipot tester (AC / DC / IR Tester) of Chroma.

Advantages of the Invention

[0015] A lithium secondary battery according to one embodiment of the present invention is characterized by using a silicon-based negative electrode, which satisfies all three parameters: a thickness of 1 μm to 15 μm so that the separation membrane can maintain a rigid shape even when the rough negative electrode surface due to volume expansion during charging and discharging of the silicon-based negative electrode comes into contact with the separation membrane; a compressive resistance defined by formula 1 of 1% to 14.5%; and a dielectric breakdown voltage of 400 V or more.

[0016] In other words, the lithium secondary battery of this application has the characteristic of being able to solve existing problems that occur in lithium secondary batteries using silicon-based negative electrodes by setting three parameters, defined as the thickness range of the separator membrane, the dielectric breakdown voltage, and the compressibility, within appropriate ranges. [Brief explanation of the drawing]

[0017] [Figure 1] This figure shows a stacked structure of a lithium secondary battery according to one embodiment of the present application. [Modes for carrying out the invention]

[0018] Before describing the present invention, let us first define some terms.

[0019] In this specification, when a part "includes" a component, this means, unless otherwise stated, that it may include other components rather than excluding them.

[0020] In this specification, "p~q" means the range "p or greater and q or less".

[0021] In this specification, "specific surface area" is measured by the BET method, specifically calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77K) using BELSORP-mini II from BEL Japan. In other words, in this application, BET specific surface area may mean the specific surface area measured by the above measurement method.

[0022] In this specification, "Dn" refers to the particle size distribution, and means the particle size at the n% point of the cumulative particle number distribution by particle size. That is, D50 is the particle size (central particle size, average particle size) at the 50% point of the cumulative particle number distribution by particle size, D90 is the particle size at the 90% point of the cumulative particle number distribution by particle size, and D10 is the particle size at the 10% point of the cumulative particle number distribution by particle size. Alternatively, the particle size distribution may be measured using the laser diffraction method. Specifically, the powder to be measured is dispersed in a dispersion medium, then introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac S3500), and the difference in diffraction patterns due to particle size as the particles pass through the laser beam is measured to calculate the particle size distribution.

[0023] In this specification, when a polymer contains a monomer as a monomer unit, it means that the monomer participates in the polymerization reaction and is included as a repeating unit within the polymer. In this specification, when a polymer is said to contain a monomer, this is interpreted as meaning that the polymer contains monomers as monomer units.

[0024] In this specification, unless otherwise specified, the term "polymer" is understood to be used in a broad sense, including copolymers.

[0025] In this specification, the weight-average molecular weight (Mw) and number-average molecular weight (Mn) are the molecular weights in terms of polystyrene measured by gel permeation chromatography (GPC) using monodisperse polystyrene polymers (standard samples) of various degrees of polymerization commercially available for molecular weight measurement as the standard substances. In this specification, the molecular weight means the weight-average molecular weight unless otherwise specified.

[0026] Hereinafter, for those with ordinary knowledge in the technical field to which the present invention pertains to easily implement the present invention, a detailed description will be given with reference to the drawings. However, the present invention can be implemented in various different forms and is not limited to the following description.

[0027] One embodiment of this specification is a lithium secondary battery including a silicon-based negative electrode; a positive electrode; a separator located between the negative electrode and the positive electrode; and an electrolyte, wherein the silicon-based negative electrode includes a negative electrode current collector layer; a negative electrode active material layer including a negative electrode active material layer composition provided on one or both surfaces of the negative electrode current collector layer, the negative electrode active material layer composition includes at least one selected from the group consisting of a silicon-based active material, a negative electrode conductive material, and a negative electrode binder, the silicon-based active material includes at least one selected from the group consisting of SiO x (x = 0) and SiO x (0 < x < 2), and based on 100 parts by weight of the silicon-based active material, the SiO x (x = 0) contains 70 parts by weight or more, the separator satisfies a thickness of 1 μm or more and 15 μm or less, has a compressibility defined by the following formula 1 of 1% or more and 14.5% or less, and has a breakdown voltage of 400 V or more, and provides a lithium secondary battery.

[0028] [Formula 1] (T1 - T2) / T1 × 100 In the above formula 1, T1 means the initial thickness in the state where three of the separators are laminated, T2 means the thickness after laminating three of the separation membranes and then pressing them at 2000 kgf and 70°C for 10 seconds using a hot press (QM-940A Grade manufactured by Q-mesys). The dielectric breakdown voltage means the voltage at the point of dielectric breakdown when the voltage of a monocell including the separation membrane is increased from 0 V to 6000 V at a rate of 100 V / s using a Chroma Hipot tester (AC / DC / IR Tester).

[0029] In the case of the separation membrane according to one embodiment of the present invention, it satisfies a thickness of 1 μm to 15 μm, a compressive resistance defined by Formula 1 of 1% or more and 14.5% or less, and a dielectric breakdown voltage of 400 V or more. By satisfying all three parameters, the main feature is that the separation membrane can maintain a firm form even when a rough negative electrode surface contacts the separation membrane during charging and discharging of the silicon-based negative electrode due to volume expansion.

[0030] FIG. 1 is a diagram showing a laminated structure of a lithium secondary battery according to one embodiment of the present application. Specifically, a negative electrode 100 for a lithium secondary battery including a negative electrode active material layer 20 can be confirmed on one surface of a negative electrode current collector layer 10, and a positive electrode 200 for a lithium secondary battery including a positive electrode active material layer 40 can be confirmed on one surface of a positive electrode current collector layer 50. It shows that the negative electrode 100 for a lithium secondary battery and the positive electrode 200 for a lithium secondary battery are laminated with a separation membrane 30 interposed therebetween.

[0031] Hereinafter, a silicon-based negative electrode, which is one of the characteristic parts of the present invention, will be described.

[0032] In one embodiment of the present application, the silicon-based negative electrode includes a negative electrode current collector layer; and a negative electrode active material layer provided on one or both surfaces of the negative electrode current collector layer, and the negative electrode active material layer composition includes at least one selected from the group consisting of a silicon-based active material; a negative electrode conductive material; and a negative electrode binder.

[0033] In one embodiment of the present application, the silicon-based active material is SiO x (x = 0) and SiO xincluding at least one selected from the group consisting of (0 < x < 2), based on 100 parts by weight of the silicon-based active material, the SiO x provides a lithium secondary battery containing 70 parts by weight or more of (x = 0).

[0034] In another embodiment, based on 100 parts by weight of the silicon-based active material, the SiO x (x = 0) may be contained in 70 parts by weight or more, preferably 80 parts by weight or more, more preferably 90 parts by weight or more, and may be contained in 100 parts by weight or less, preferably 99 parts by weight or less, more preferably 95 parts by weight or less.

[0035] In one embodiment of the present application, the silicon-based active material may particularly use pure silicon (Si) as the silicon-based active material. Using pure silicon (Si) as the silicon-based active material means that, as described above, when based on 100 parts by weight of the total silicon-based active material, pure Si particles (SiO x (x = 0)) may be included within the above range.

[0036] In the case of silicon-based active materials, compared with the conventionally used graphite-based active materials, the capacity is significantly higher, and attempts to apply it have been increasing. However, due to the high volume expansion rate during the charge and discharge process, it has been limited to cases such as mixing a small amount with graphite-based active materials.

[0037] Also, in the case of the active material contained in the silicon-based negative electrode, when compared with the carbon-based negative electrode, the surface is very rough, so (In particular, the surface roughness (Ra) of the negative electrode active material layer containing the silicon-based negative electrode active material, which is in contact with the separation membrane, is between 3 μm and 10 μm.) there was a problem of micro short occurring by contacting the positive electrode through the separator. However, in the case of the lithium secondary battery according to the present application, even when using a negative electrode with a very rough surface as described above, by adjusting the physical properties of the separator described later, it has the characteristic that micro short can be prevented while having very excellent capacity characteristics and energy density.

[0038] On the other hand, the average particle size (D50) of the silicon-based active material in the present invention is 5 μm to 10 μm, specifically 5.5 μm to 8 μm, and more specifically 6 μm to 7 μm. When the average particle size falls within the above range, the specific surface area of ​​the particles falls within an appropriate range, and the viscosity of the negative electrode slurry is formed within an appropriate range. This allows for smooth dispersion of the particles constituting the negative electrode slurry. Furthermore, when the size of the silicon-based active material is greater than or equal to the lower limit of the above range, the composite material consisting of the conductive material and the negative electrode binder in the negative electrode slurry provides excellent contact area between the silicon particles and the conductive material, increasing the likelihood of a sustained conductive network and increasing the capacity retention rate. On the other hand, when the average particle size satisfies the above range, excessively large silicon particles are excluded, and the surface of the negative electrode is formed smoothly, thereby preventing the phenomenon of non-uniform current density during charging and discharging.

[0039] In one embodiment of this application, the silicon-based active material generally has a characteristic BET specific surface area. The BET specific surface area of ​​the silicon-based active material is preferably 0.01 m². 2 / g~150.0m 2 / g, more comfortably 0.1m 2 / g~100.0m 2 / g, particularly preferably 0.2m 2 / g~80.0m 2 / g, most preferably 0.2m 2 / g~18.0m 2 The value is / g. The BET specific surface area is measured according to DIN 66131 (using nitrogen).

[0040] In one embodiment of this application, the silicon-based active material may exist, for example, in a crystalline or amorphous form, and is preferably not porous. The silicon particles are preferably spherical or fragmentary particles. Alternatively, the silicon particles may also have a fibrous structure or exist in the form of a silicon-containing film or coating, but these are less preferred.

[0041] In one embodiment of this application, the silicon-based active material may be 60 parts by weight or more based on 100 parts by weight of the negative electrode active material layer composition.

[0042] In another embodiment, the silicon-based active material may be present in amounts of 60 parts by weight or more, preferably 65 parts by weight or more, more preferably 70 parts by weight or more, based on 100 parts by weight of the negative electrode active material layer composition, and may be present in amounts of 95 parts by weight or less, preferably 90 parts by weight or less, more preferably 80 parts by weight or less.

[0043] The negative electrode active material layer composition according to this application, even when using a silicon-based active material with remarkably high capacity within the aforementioned range, utilizes specific conductive materials and binders that suppress the rate of volume expansion during the charge and discharge process, thereby preventing a decrease in negative electrode performance and exhibiting excellent output characteristics during charging and discharging.

[0044] In one embodiment of this application, the silicon-based active material may have a non-spherical shape, and its circularity is, for example, 0.9 or less, for example 0.7 to 0.9, for example 0.8 to 0.9, for example 0.85 to 0.9.

[0045] In this application, the circularity is determined by the following formula A-1, where A is the area and P is the boundary line.

[0046] [Formula A-1] 4πA / P 2

[0047] Traditionally, graphite-based compounds were commonly used as the negative electrode active material. However, in recent years, with the increasing demand for high-capacity batteries, there has been a growing trend to mix in silicon-based compounds to increase capacity. However, silicon-based compounds have a limitation: their volume expands rapidly during the charge / discharge process, damaging the conductive paths formed within the negative electrode active material layer and actually degrading the battery's performance.

[0048] Accordingly, in one embodiment of this application, the negative electrode conductive material may include at least one selected from the group consisting of point conductive material; linear conductive material; and planar conductive material.

[0049] In one embodiment of this application, the point-shaped conductive material can be used to improve the conductivity of the negative electrode, is conductive without inducing a chemical change, and means a spherical or point-shaped conductive material. Specifically, the point-shaped conductive material may be at least one selected from the group consisting of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, conductive fibers, fluorocarbon, aluminum powder, nickel powder, zinc oxide, potassium titanate, titanium oxide, and polyphenylene derivatives, and may preferably include carbon black in that it embodies high conductivity and has excellent dispersibility.

[0050] In one embodiment of this application, the point conductive material has a BET specific surface area of ​​40 m². 2 / g or more 70m 2 / g or less, preferably 45m 2 / g or more 65m 2 / g or less, more preferably 50m 2 / g or more 60m 2 It may be less than / g.

[0051] In one embodiment of this application, the point-shaped conductive material may satisfy a functional group content (volatile matter) of 0.01% or more and 0.05% or less, preferably 0.01% or more and 0.04% or less, and more preferably 0.01% or more and 0.03% or less.

[0052] Specifically, by improving the dispersibility of the point-shaped conductive material, even if the content of the point-shaped conductive material is increased from a negative electrode slurry having the same solid content, the viscosity of the negative electrode slurry can be maintained at an appropriate level. This allows for stable processability and improved uniformity of the formed negative electrode.

[0053] In one embodiment of this application, the particle size of the dot-like conductive material is 10 nm to 100 nm, preferably 20 nm to 90 nm, and more preferably 20 nm to 60 nm.

[0054] In one embodiment of this application, the conductive material may include a planar conductive material.

[0055] The aforementioned planar conductive material can improve conductivity by increasing surface contact between silicon particles within the negative electrode, and at the same time suppress the disruption of the conductive path due to volume expansion. It can be described as a plate-type conductive material or a bulk-type conductive material.

[0056] In one embodiment of this application, the planar conductive material may include at least one selected from the group consisting of plate graphite, graphene, graphene oxide, and graphite flakes, and preferably plate graphite.

[0057] In one embodiment of this application, the average particle size (D50) of the planar conductive material is 2 μm to 7 μm, specifically 3 μm to 6 μm, and more specifically 4 μm to 5 μm. When the above range is met, the particle size is sufficient to facilitate dispersion without causing an excessive increase in the viscosity of the negative electrode slurry. Therefore, when dispersion is performed using the same equipment and time, the dispersion effect is excellent.

[0058] In one embodiment of this application, a negative electrode composition is provided in which the planar conductive material has a D10 of 0.5 μm or more and 1.5 μm or less, a D50 of 2.5 μm or more and 3.5 μm or less, and a D90 of 7.0 μm or more and 15.0 μm or less.

[0059] In one embodiment of this application, the planar conductive material may be a high specific surface area planar conductive material with a high BET specific surface area, or a low specific surface area planar conductive material.

[0060] In one embodiment of this application, a high specific surface area planar conductive material or a low specific surface area planar conductive material can be used without limitation as the planar conductive material. However, since the planar conductive material according to this application may be affected to some extent by dispersion in terms of electrode performance, it is particularly preferable to use a low specific surface area planar conductive material that does not cause dispersion problems.

[0061] In one embodiment of this application, the planar conductive material has a BET specific surface area of ​​5 m². 2 It may be more than / g.

[0062] In another embodiment, the planar conductive material has a BET specific surface area of ​​5 m². 2 / g or more 500m 2 / g or less, preferably 5m 2 / g or more 300m 2 / g or less, more preferably 5m 2 / g or more 250m 2 It may be less than / g.

[0063] In another embodiment, the planar conductive material is a high specific surface area planar conductive material with a BET specific surface area of ​​50 m². 2 / g or more 500m 2 / g or less, preferably 80mg 2 / g or more 300m 2 Less than / g, more preferably 100m 2 / g or more 300m 2 The range of / g or less may also be satisfied.

[0064] In another embodiment, the planar conductive material is a low specific surface area planar conductive material with a BET specific surface area of ​​5 m². 2 / g or more 40m 2 / g or less, preferably 5m 2 / g or more 30m 2 / g or less, more preferably 5m 2 / g or more 25m 2 The range of / g or less may also be satisfied.

[0065] Other conductive materials may include linear conductive materials such as carbon nanotubes. The carbon nanotubes may be bundle-type carbon nanotubes. The bundle-type carbon nanotubes may contain multiple carbon nanotube units. Specifically, unless otherwise specified, "bundle type" here refers to a secondary shape in the form of a bundle or rope, in which multiple carbon nanotube units are arranged or intertwined in substantially the same orientation along their longitudinal axes. The carbon nanotube units have a graphite sheet that is cylindrical with a nanoscale diameter and has an sp2 bond structure. In this case, the properties of a conductor or semiconductor can be determined by the angle and structure in which the graphite sheet is wound. Compared to entangled-type carbon nanotubes, the bundle-type carbon nanotubes can be uniformly dispersed during anode manufacturing, smoothly form a conductive network within the anode, and improve the conductivity of the anode.

[0066] In one embodiment of this application, the negative electrode conductive material includes a linear conductive material, and the linear conductive material may be a carbon nanotube.

[0067] In one embodiment of this application, the carbon nanotubes may be SWCNTs and / or MWCNTs. When the linear conductive material is SWCNTs, the length of the SWCNTs may be 0.5 μm to 100 μm, preferably 1 μm to 80 μm.

[0068] In one embodiment of this application, the negative electrode conductive material may be 10 to 40 parts by weight based on 100 parts by weight of the negative electrode active material layer composition.

[0069] In another embodiment, the negative electrode conductive material may be present in an amount of 10 to 40 parts by weight, preferably 10 to 30 parts by weight, and more preferably 10 to 20 parts by weight, based on 100 parts by weight of the negative electrode active material layer composition.

[0070] The negative electrode conductive material of this application has a completely different structure from the conductive material applied to the positive electrode. Specifically, the negative electrode conductive material of this application plays the role of maintaining contact between silicon-based active materials, which undergo very large volume expansion during charging and discharging, while the positive electrode conductive material acts as a buffer during rolling, while also imparting some conductivity. Its structure and role are completely different from the negative electrode conductive material of the present invention.

[0071] Furthermore, the negative electrode conductive material described in this application is applied to silicon-based active materials and has a completely different structure from conductive materials applied to graphite-based active materials. In other words, conductive materials used in electrodes with graphite-based active materials simply have particles that are smaller than the active material, thus improving output characteristics and imparting some conductivity. This is completely different in structure and role from negative electrode conductive materials applied together with silicon-based active materials, as in the present invention.

[0072] In one embodiment of this application, the plate-shaped conductive material used as the negative electrode conductive material described above has a different structure and role from carbon-based active materials generally used as negative electrode active materials. Specifically, the carbon-based active material used as the negative electrode active material may be artificial graphite or natural graphite, and refers to a material that is processed into a spherical or point-like form for use in order to facilitate the storage and release of lithium ions.

[0073] On the other hand, planar (plate-shaped) conductive materials used as negative electrode conductive materials are substances that have a planar or plate-like form and can be described as plate-shaped graphite. In other words, they are substances included to maintain conductive pathways within the negative electrode active material layer, and do not play a role in lithium storage and release, but rather refer to substances that secure conductive pathways in a planar manner within the negative electrode active material layer.

[0074] In other words, the use of planar (plate-shaped) graphite as a conductive material in this application means that it was used not to store or release lithium after being processed into a planar or plate shape, but rather as a material that secures a conductive path. In this case, the negative electrode active material included together has high capacity characteristics for lithium storage and release, and plays a role in storing and releasing all lithium ions transmitted from the positive electrode.

[0075] On the other hand, in this application, the use of a carbon-based active material as an active material means that it was processed into a point-like or spherical shape and used as a material that plays a role in storing or releasing lithium.

[0076] In other words, in one embodiment of this application, the carbon-based active material, artificial graphite or natural graphite, is in a point-like form, and has a BET specific surface area of ​​0.1 m². 2 / g or more 4.5m 2 The range of less than or equal to / g may also be satisfied. In addition, plate-shaped graphite, which is a planar conductive material, has a planar BET specific surface area of ​​5m². 2 It may be more than / g.

[0077] In one embodiment of this application, the negative electrode binder may contain at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and substances in which the hydrogen atoms thereof are substituted with Li, Na, Ca, etc., or it may contain various copolymers thereof.

[0078] The negative electrode binder according to one embodiment of this application plays a role in holding the active material and conductive material to prevent twisting and structural deformation of the negative electrode structure during the volume expansion and relaxation of the silicon-based active material. Any general binder can be applied as long as it satisfies the above role, and specifically, an aqueous binder can be used, and more specifically, a PAM-based binder may be used.

[0079] In one embodiment of this application, the negative electrode binder may be 30 parts by weight or less, preferably 25 parts by weight or less, more preferably 20 parts by weight or less, based on 100 parts by weight of the negative electrode active material layer composition, and may also be 5 parts by weight or more, or 10 parts by weight or more.

[0080] In one embodiment of this application, the negative electrode current collector layer generally has a thickness of 1 μm to 100 μm. Such a negative electrode current collector layer is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery, and may be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., or aluminum-cadmium alloy. Furthermore, fine irregularities can be formed on the surface to strengthen the bonding force of the negative electrode active material, and it may be used in various forms such as film, sheet, foil, net, porous body, foam, or nonwoven fabric.

[0081] In one embodiment of this application, a lithium secondary battery is provided in which the thickness of the negative electrode current collector layer is 1 μm or more and 100 μm or less, and the thickness of the negative electrode active material layer is 10 μm or more and 100 μm or less.

[0082] In one embodiment of this application, a lithium secondary battery is provided in which the thickness of the negative electrode active material layer is 10 μm or more and 100 μm or less.

[0083] However, the thickness can vary depending on the type and application of the negative electrode used, and is not limited to this.

[0084] In one embodiment of this application, the porosity of the negative electrode active material layer may be in the range of 10% to 60%.

[0085] In another embodiment, the porosity of the negative electrode active material layer may be in the range of 10% to 60%, preferably 20% to 50%, and more preferably 30% to 45%.

[0086] The aforementioned porosity varies depending on the composition and content of the silicon-based active material, conductive material, and binder contained in the negative electrode active material layer. In particular, by including the silicon-based active material and conductive material according to this application in specific compositions and content amounts, the range of the aforementioned porosity is satisfied, and thereby the electrical conductivity and resistance of the electrode are within an appropriate range.

[0087] The separation membrane described in this application satisfies all three of the above conditions, and each of these conditions will be explained below.

[0088] The separation membrane according to one embodiment of this specification satisfies a thickness of 1 μm to 15 μm.

[0089] Another embodiment of the separation membrane may satisfy a thickness of 1 μm to 15 μm, preferably 3 μm to 15 μm, more preferably 6 μm to 15 μm, or 10 μm to 15 μm.

[0090] The thickness of the separation membrane can refer to the thickness of one membrane if one membrane is used, or to the total thickness range including two or more membranes if two or more membranes are used.

[0091] If the thickness of the separation membrane satisfies the aforementioned range, damage to the separation membrane can be minimized even if contact occurs when the silicon-based negative electrode with a rough surface expands. If it is less than the aforementioned range, even if the compressive resistance and dielectric breakdown voltage described later are satisfied, the separation membrane may rupture due to the expansion of the negative electrode. If it exceeds the aforementioned range, lithium ion exchange between the positive and negative electrodes may not occur smoothly, potentially leading to a failure of the lithium secondary battery.

[0092] A separation membrane according to one embodiment of this application may satisfy a compressive resistance range of 1% to 14.5%, as defined by the following formula 1.

[0093] [Formula 1] (T1-T2) / T1×100 In the above formula 1, T1 refers to the initial thickness when the three separation membranes are stacked together. T2 refers to the thickness after stacking three of the aforementioned separation membranes and then hot-pressing them (QM-940A Grade, manufactured by Q-mesys) at 2000 kgf and 70°C for 10 seconds.

[0094] In another embodiment, the compressive resistance defined by Formula 1 may satisfy the ranges of 1% to 14.5%, 5% to 14.5%, 10% to 14.5%, and 10.5% to 14.5%.

[0095] In one embodiment of this application, the compressive resistance of formula 1 can mean the ratio of the thickness of the separation membrane after pressure is applied to it under certain conditions to the thickness when no pressure is applied, and low compressive resistance can mean that the separation membrane is compressed less.

[0096] When the compression resistance meets the aforementioned range, the durability of the separation membrane is increased. If it is below the range, the separation membrane cannot flexibly adapt to the rough surface of the negative electrode, resulting in the separation membrane rupturing or developing holes. If it exceeds the range, the separation membrane becomes too rigid, making it difficult for lithium ions to pass through, which may lead to an increase in diffusion resistance.

[0097] Furthermore, separation membranes consisting only of inorganic layers, without polymers like PE or PP, are evaluated as having very low compressibility. In this case, they remain almost uncompressible, and the pores of the separation membrane are maintained in their initial state. As a result, lithium ions pass smoothly during charging and discharging, and the diffusion resistance does not increase further.

[0098] In one embodiment of this application, the dielectric breakdown voltage of the separator film may be 400V or more.

[0099] In another embodiment, the dielectric breakdown voltage of the separation film can satisfy a range of 400V or more, preferably 450V or more, a range of 6000V or less, preferably 5000V or less, and may also satisfy a range of 2000V or less.

[0100] The dielectric breakdown voltage is measured by boosting the voltage of the monocell containing the separator membrane from 0V to 6000V at a rate of 100V / s using Chroma's Hipot tester (AC / DC / IR Tester), and the voltage at which dielectric breakdown occurs. A higher measured voltage indicates that the separator membrane can withstand higher voltages.

[0101] When the dielectric breakdown voltage satisfies the aforementioned range, the durability of the separator membrane is increased. If it is below the range, the separator membrane may rupture or develop holes due to the rough surface of the negative electrode. The measurement of the dielectric breakdown voltage indicates that charge is flowing, and if it falls below the lower limit of the aforementioned range, it can be determined that a short circuit has occurred.

[0102] In other words, the separation membrane according to this application is characterized in that, only when all three of the above conditions are met, it can drive a lithium secondary battery without damage to the separation membrane, even if the volume of the silicon-based negative electrode with a rough surface expands.

[0103] In one embodiment of this application, the compressibility and dielectric breakdown voltage may be satisfied by adjusting the molecular weight and internal pore distribution of the separation membrane.

[0104] Specifically, the larger the molecular weight of the material constituting the separation membrane, the stronger its compressibility and dielectric breakdown voltage. In this case, the separation membrane according to this application satisfies the range of compressibility and dielectric breakdown voltage by adjusting the molecular weight and thickness together to meet the aforementioned requirements.

[0105] Furthermore, by adjusting the pore distribution to satisfy the range of compressibility and dielectric breakdown voltage, the pores of the separation film, which generally have a level of 10 nm or more, can be uniformly distributed, and the range of compressibility and dielectric breakdown voltage can be adjusted to the aforementioned range.

[0106] In other words, the separation membrane according to this application is characterized by satisfying the aforementioned compressibility and dielectric breakdown voltage by adjusting the molecular weight range, internal pore distribution degree, and thickness range to appropriate ranges.

[0107] In one embodiment of this application, the weight-average molecular weight of the separation membrane may include a range of 1,000,000 g / mol or more and 10,000,000 g / mol or less.

[0108] In another embodiment, the weight-average molecular weight of the separation membrane may be in the range of 1,000,000 g / mol or more and 10,000,000 g / mol or less, preferably 1,000,000 g / mol or more and 9,000,000 g / mol or less, and more preferably 1,000,000 g / mol or more and 8,000,000 g / mol or less.

[0109] Furthermore, in one embodiment of this application, the pore size of the separation membrane may satisfy the range of 10 nm to 100 nm, specifically the range of 20 nm to 60 nm, and the compressive resistance and dielectric breakdown voltage can be adjusted by suitably adjusting the distribution degree according to the size of the pores.

[0110] For example, in a separation membrane with 40 nm pores and a separation membrane with 20 nm pores, if the pore distribution of the separation membrane with 20 nm pores is made uniform, the compressive resistance and dielectric breakdown voltage can be increased.

[0111] In one embodiment of this application, the separation membrane is a wet separation membrane; or a dry separation membrane, providing a separation membrane for a lithium secondary battery.

[0112] The wet separation membrane and the dry separation membrane can be distinguished by the differences in the processes themselves.

[0113] Specifically, a wet separation membrane can be obtained by mixing polyethylene and paraffin, extruding the mixture, and then passing the resulting film through methylene chloride (MC) to dissolve the paraffin. In this process, since polyethylene does not dissolve in MC, the wet separation membrane can be manufactured in a way that creates pores wherever the paraffin has dissolved.

[0114] The dry separation membrane can be manufactured by extruding a polypropylene polymer into a film and then stretching it to create gaps between the polymer chains.

[0115] In one embodiment of this application, the separation membrane provides a separation membrane for a lithium secondary battery comprising at least one selected from the group consisting of a porous polymer film; a porous nonwoven fabric; and a ceramic component.

[0116] Specifically, the separation membrane separates the negative and positive electrodes and provides a pathway for lithium ions to move. Any membrane commonly used as a separation membrane in secondary batteries is generally acceptable, but those with low resistance to ion movement in the electrolyte while exhibiting excellent electrolyte moisture absorption capacity are particularly preferred. Specifically, porous polymer films, such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminated structures of two or more layers thereof, may be used. Alternatively, ordinary porous nonwoven fabrics, such as high-melting-point glass fibers or polyethylene terephthalate fibers, may be used. Furthermore, coated separation membranes containing ceramic components or polymeric substances may be used to ensure heat resistance or mechanical strength, and may be selectively used in single-layer or multi-layer structures.

[0117] In other words, the type of separation membrane described in this application is not limited as long as it falls within the range that satisfies the three parameters mentioned above.

[0118] In one embodiment of this application, the separation membrane may consist of a single layer or two or more layers.

[0119] In one embodiment of this application, the positive electrode may include a positive electrode current collector layer and a positive electrode active material layer formed on the positive electrode current collector layer and containing the positive electrode active material.

[0120] In the positive electrode, the positive electrode current collector layer is not particularly limited as long as it is conductive without inducing chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surfaces treated with carbon, nickel, titanium, silver, etc. may be used. The positive electrode current collector layer may also typically have a thickness of 3 μm to 500 μm, and fine irregularities can be formed on the surface of the current collector to enhance the adhesion of the positive electrode active material. For example, it may be used in various forms such as film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0121] The positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material may be a layered compound such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; lithium iron oxide such as LiFe3O4; or a compound with the chemical formula Li 1+c1 Mn 2-c1 Lithium manganese oxides such as O4 (0 ≤ c1 ≤ 0.33), LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, Cu2V2O7; chemical formula LiNi 1-c2 M c2 Ni-site type lithium nickel oxide represented as O2 (where M is at least one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, satisfying 0.01 ≤ c2 ≤ 0.3); chemical formula LiMn 2-c3 M c3 Lithium manganese composite oxides represented by O2 (where M is at least one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, satisfying 0.01 ≤ c3 ≤ 0.1) or Li2Mn3MO8 (where M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn); LiMn2O4 in which part of the Li in the chemical formula is substituted with alkaline earth metal ions, etc., are examples, but are not limited to these. The positive electrode may be Li metal (Li-metal).

[0122] In one embodiment of this application, the positive electrode is LiNi x Co y Mn z O2(x+y+z=1);LiNi a Co b Mn c Al d O2(a+b+c+d=1);LiMn2O4;LiNi 0.5 Mn 1.5 O2 and LiM x Fe y The present invention provides a lithium secondary battery comprising at least one selected from the group consisting of PO4 (M: transition metal, x+y=1).

[0123] The positive electrode active material layer may also include a positive electrode conductive material and a positive electrode binder, along with the positive electrode active material described above.

[0124] In this case, the positive electrode conductive material is used to impart conductivity to the electrode, and can be used without particular limitations as long as it has electronic conductivity in the battery without causing a chemical change. Specific examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. One of these alone or a mixture of two or more may be used.

[0125] Furthermore, the positive electrode binder plays a role in improving the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector layer. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one of these alone or a mixture of two or more may be used.

[0126] Examples of the aforementioned electrolytes include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries.

[0127] Specifically, the electrolyte may contain a non-aqueous organic solvent and a metal salt.

[0128] As the non-aqueous organic solvent, for example, aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate, and ethyl propionate may be used.

[0129] In particular, ethylene carbonate and propylene carbonate, which are cyclic carbonates among the carbonate-based organic solvents, have a high dielectric constant as high-viscosity organic solvents and can be preferably used to dissociate lithium salts well. When such cyclic carbonates are mixed and used with low-viscosity and low-dielectric-constant linear carbonates such as dimethyl carbonate and diethyl carbonate at an appropriate ratio, an electrolytic solution having a high electric conductivity can be produced and can be more preferably used.

[0130] As the metal salt, a lithium salt can be used. The lithium salt is a substance that is easily dissolved in the non-aqueous electrolytic solution. For example, as the anion of the lithium salt, F - , Cl - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , PF6 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - can be used, and one or more selected from the group consisting of them can be used.

[0131] In addition to the electrolyte components, the electrolyte may further contain one or more additives for the purpose of improving battery life characteristics, suppressing battery capacity reduction, and improving battery discharge capacity, such as haloalkylene carbonate compounds like difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexamethyl phosphate triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride.

[0132] One embodiment of the present invention provides a battery module and a battery pack containing the secondary battery as a unit cell. Because the battery module and battery pack include the secondary battery having high capacity, high rate characteristics and cycle characteristics, they can be used as a power source for medium to large devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles and power storage systems. [Examples]

[0133] The following are preferred embodiments to aid in understanding the present invention, but these embodiments are for illustrative purposes only, and it will be apparent to those skilled in the art that various changes and modifications are possible within the scope of this description and the technical concept. It is obvious that such variations and modifications fall within the scope of the appended claims.

[0134] <Manufacturing example> <Manufacturing of negative electrodes> A Si slurry was prepared by mixing inorganic Si (Wacker), polyacrylamide (Aldrich), and conductive material (Super-C, Immerys) in a ratio of 7:2:1 in DI water at a concentration of 34 wt%.

[0135] This slurry was applied to both sides of an 8 μm thick Cu foil (SK Nexilis), and then rolled to produce a negative electrode with a total negative electrode thickness of 53 μm.

[0136] <Manufacturing of positive electrodes> A cathode slurry with a solid content of 75.5% was prepared by mixing NCMA cathode material (LG Chem HN803 Grade) and PVdF binder (Arkema KF9709) in an NMP solvent in a ratio of 97.74:2.26.

[0137] The aforementioned cathode slurry was applied to both sides of a 12 μm thick Al foil (A1100 grade, Sana Aluminium Co., Ltd.), and then rolled to produce a cathode with a final cathode thickness of 142 μm.

[0138] The positive and negative electrodes manufactured as described above were assembled into a monocell in the order of positive electrode / separation membrane / negative electrode / separation membrane together with a separation membrane that satisfies the physical properties shown in Table 1 below.

[0139] [Table 1]

[0140] In Table 1 above, the compressive strength and dielectric breakdown voltage can be measured as follows.

[0141] - Compression resistance (%) The thickness (T1) after stacking the three separation membranes shown in Table 1 was measured, and the thickness (T2) after pressing with a hot press (Q-MESYS QM-940A Grade) at 2000 kgf, 70°C, and 10 seconds was measured, and the calculation was performed according to Equation 1 above.

[0142] - Dielectric breakdown voltage (V) The dielectric breakdown voltages in Table 1 were measured using a Chroma Hipot tester (AC / DC / IR Tester) to increase the voltage from 0V to 6000V at a rate of 100V / s while the monocells of Examples 1 to 4, Comparative Example 1, and Comparative Example 2, including the separation film, were measured at the point of dielectric breakdown.

[0143] The monocells of Examples 1 to 4, Comparative Example 1, and Comparative Example 2, manufactured as described above, were observed for short circuits using an AC resistance meter. Specifically, 20 monocells were produced for each example, and it was checked whether or not a short circuit occurred. If a short circuit occurred, it was marked as "Fail," and the results are shown in Table 2 below.

[0144] [Table 2]

[0145] As can be seen from Tables 1 and 2, the lithium secondary battery separation membranes of Examples 1 to 4 satisfy all three parameters: a thickness of 1 μm to 15 μm, a compressive resistance defined by Formula 1 of 1% to 14.5%, and a dielectric breakdown voltage of 400 V or more. Furthermore, it was confirmed through the experiments in Table 2 that the separation membrane maintains a robust shape even when the rough anode surface comes into contact with the separation membrane due to volume expansion during charging and discharging of the silicon-based anode.

[0146] In the case of Comparative Examples 1 and 2, when the compressive resistance falls outside the range specified in this application, the separation membrane becomes excessively rigid, making it difficult for lithium ions to pass through. This leads to a problem of increased diffusion resistance, and it was confirmed that the short-circuit evaluation was inferior to that of the examples.

[0147] Furthermore, in the case of Comparative Example 2, the dielectric breakdown voltage falls outside the range specified in this application (below the lower limit), and in the case of Comparative Example 2, the durability of the separation membrane is inferior, and the separation membrane ruptures or develops holes due to the rough surface of the negative electrode. As a result, it was confirmed that the short-circuit evaluation most frequently resulted in a "Fail" case. [Explanation of Symbols]

[0148] 10 ···Negative electrode current collector layer 20...Negative electrode active material layer 30...Separation membrane 40...Cathode active material layer 50 ···Positive electrode current collector layer 100 ···Negative electrode for lithium secondary batteries 200 ···Positive electrode for lithium secondary batteries

Claims

1. A lithium secondary battery comprising: a silicon-based negative electrode; a positive electrode; a separator membrane located between the negative electrode and the positive electrode; and an electrolyte; The silicon-based negative electrode includes a negative electrode current collector layer; and a negative electrode active material layer comprising a negative electrode active material layer composition provided on one or both sides of the negative electrode current collector layer; The negative electrode active material layer composition comprises a silicon-based active material; a negative electrode conductive material; and a negative electrode binder. The aforementioned silicon-based active material is SiO ｘ (x=0) is included, and based on 100 parts by weight of the silicon-based active material, the SiO ｘ It contains 70 parts by weight or more of (x=0), The separation membrane satisfies a thickness of 10 μm to 15 μm, has a compressive resistance defined by the following formula 1 of 10% to 14.5%, and has a dielectric breakdown voltage of 400 V or more, lithium secondary battery: [Formula 1] (T1-T2) / T1×100 In the above formula 1, T1 represents the initial thickness when the three separation membranes are stacked together. T2 refers to the thickness after stacking three of the separation membranes and then hot-pressing them (QM-940A Grade, manufactured by Q-mesys) at 2000 kgf and 70°C for 10 seconds. The dielectric breakdown voltage refers to the voltage at which dielectric breakdown occurs when the monocell containing the separator membrane is boosted from 0V to 6000V at a rate of 100V / s using Chroma's Hippot tester (AC / DC / IR tester). The monocell is assembled by sequentially assembling the positive electrode, the separator membrane, the negative electrode, and the separator membrane.

2. The lithium secondary battery according to claim 1, wherein the separation membrane comprises at least one selected from the group consisting of a porous polymer film; a porous nonwoven fabric; and a ceramic component.

3. The lithium secondary battery according to claim 1, wherein the separation membrane comprises a single layer or two or more layers.

4. The lithium secondary battery according to claim 1, wherein the pore size of the separation membrane is 10 nm or more and 100 nm or less.

5. A lithium secondary battery according to any one of claims 1 to 4, wherein the silicon-based active material is contained in an amount of 60 parts by weight or more per 100 parts by weight of the negative electrode active material layer composition.

6. The lithium secondary battery according to claim 1, wherein the negative electrode conductive material includes at least one selected from the group consisting of point conductive material; linear conductive material; and planar conductive material.

7. The lithium secondary battery according to claim 1, wherein the surface roughness (Ra) of the surface of the negative electrode active material layer in contact with the separation membrane is 3 μm or more and 10 μm or less.

8. The lithium secondary battery according to claim 1, wherein the thickness of the negative electrode active material layer is 10 μm or more and 100 μm or less.

9. The positive electrode is LiNi ｘ Co ｙ Mn ｚ O ２ (x + y + z = 1); LiNi ａ Co ｂ Mn ｃ Al ｄ O ２ (a + b + c + d = 1); LiMnO ２ O ４ ; LiNi ０．５ Mn １．５ O ２ ; and LiM ｘ Fe ｙ PO ４ (M: transition metal, x + y = 1), the lithium secondary battery according to claim 1, comprising at least one selected from the group consisting of.

Images