Search

EP-4178023-B1 - SEPARATOR FOR LITHIUM SECONDARY BATTERY AND METHOD FOR MANUFACTURING THE SAME

EP4178023B1EP 4178023 B1EP4178023 B1EP 4178023B1EP-4178023-B1

Inventors

  • MUN, SUNG CIK
  • LEE, JOO-SUNG
  • HAN, SUNG-JAE

Dates

Publication Date
20260506
Application Date
20211111

Claims (14)

  1. A separator for a lithium secondary battery comprising a porous polyolefin substrate, wherein the polyolefin contained in the separator shows a tan(δ) of 0.3 or less as determined by the following Formula 1 under the following condition, and wherein the polyolefin contained in the separator shows an 'a' value of 0.03-0.25 as determined by the following Formula 2 under the following condition: tan δ = loss modulus G " B / storage modulus G ′ A a = d log G ′ / d log angular frequency wherein tan(δ) is determined under the condition of a temperature of 230°C and an angular frequency of 0.1 rad/s, wherein Formula 2 means a gradient of frequency-storage modulus curve, the horizontal axis of which is the angular frequency of the polyolefin contained in the separator, converted into a log scale, and the vertical axis of which is storage modulus G' of the polyolefin contained in the separator, converted into a log scale, wherein 'a' value is determined under the condition of a temperature of 230°C and an angular frequency of 0.1 rad/s.
  2. The separator for a lithium secondary battery according to claim 1, wherein tan(δ) of Formula 1 is 0.1-0.3.
  3. The separator for a lithium secondary battery according to claim 1, wherein 'a' value of Formula 2 is 0.04-0.23.
  4. The separator for a lithium secondary battery according to claim 1, wherein the porous polyolefin substrate comprises a plurality of fibrils and pores formed by the fibrils entangled with one another, and polyolefin chains forming the fibrils are crosslinked directly with one another.
  5. The separator for a lithium secondary battery according to claim 4, wherein the surfaces of fibrils are crosslinked.
  6. The separator for a lithium secondary battery according to claim 1, wherein the storage modulus is 1.0x10 5 to 1.0x10 7 Pa determined at a temperature of 230°C and a frequency of 0.1 rad/s using a rheological property analyzer.
  7. The separator for a lithium secondary battery according to claim 1, wherein the loss modulus is 1.0x10 6 Pa or less determined at a temperature of 230°C and a frequency of 0.1 rad/s using a rheological property analyzer.
  8. The separator for a lithium secondary battery according to claim 1, which has a melt-down temperature of 160°C or higher determined through thermomechanical analysis (TMA) as described in the chapter "determination of meltdown temperature" of the description.
  9. The separator for a lithium secondary battery according to claim 1, which further comprises an inorganic composite porous layer disposed on at least one surface of the porous polyolefin substrate, and containing an inorganic filler and a binder polymer.
  10. The separator for a lithium secondary battery according to claim 1, which further comprises: an inorganic composite porous layer disposed on at least one surface of the porous polyolefin substrate, and containing an inorganic filler and a first binder polymer; and a porous adhesive layer disposed on the inorganic composite porous layer, and containing a second binder polymer.
  11. A method for manufacturing the separator for a lithium secondary battery as defined in claim 1, comprising the steps of: (S1) preparing a non-crosslinked porous polyolefin substrate; (S2) applying a Type 2 photoinitiator composition comprising a Type 2 photoinitiator to the non-crosslinked polyolefin porous substrate; and (S3) irradiating UV rays to the porous polyolefin substrate applied with the Type 2 photoinitiator composition.
  12. The method for manufacturing the separator for a lithium secondary battery according to claim 11, wherein the Type 2 photoinitiator comprises isopropyl thioxanthone (ITX), an isopropyl thioxanthone derivative, thioxanthone (TX), benzophenone (BPO), a benzophenone derivative, 4-hydroxybenzophenone, or two or more of them.
  13. The method for manufacturing the separator for a lithium secondary battery according to claim 11, wherein the Type 2 photoinitiator composition in step (S2) further comprises a Type 1 photoinitiator.
  14. A lithium secondary battery comprising a positive electrode, a negative electrode and a separator interposed between the positive electrode and the negative electrode, wherein the separator is the separator for a lithium secondary battery as defined in any one of claims 1 to 10.

Description

TECHNICAL FIELD The present disclosure relates to a separator for a lithium secondary battery and a method for manufacturing the same. Particularly, the present disclosure relates to a separator for a lithium secondary battery which maintains elasticity even at high temperature and a method for manufacturing the same. The present application claims priority to Korean Patent Application No. 10-2020-0150262 filed on November 11, 2020 in the Republic of Korea. BACKGROUND ART Recently, energy storage technology has been given an increasing attention. Efforts into research and development for electrochemical devices have been actualized more and more, as the application of energy storage technology has been extended to energy for cellular phones, camcorders and notebook PC and even to energy for electric vehicles. In this context, electrochemical devices have been most spotlighted. Among such electrochemical devices, development of rechargeable secondary batteries has been focused. Among the commercially available secondary batteries, lithium secondary batteries developed in the early 1990's have been spotlighted, since they have a higher operating voltage and significantly higher energy density as compared to conventional batteries, such as Ni-MH, Ni-Cd and sulfuric acid-lead batteries using an aqueous electrolyte. Such a lithium secondary battery includes a positive electrode, a negative electrode, an electrolyte and a separator. Particularly, it is required for the separator to have insulation property for separating and electrically insulating the positive electrode and the negative electrode from each other and high ion conductivity for increasing lithium ion permeability based on high porosity. Meanwhile, there is a need for remarkable improvement of the safety and cost of lithium secondary batteries in order to apply the lithium secondary batteries to electric vehicles (EV). In the case of a typical polyolefin separator, a polyethylene (PE) separator, it has a low melting point (Tm) and may cause ignition and explosion, when a battery is used abnormally, and the battery temperature may be increased to the melting point of polyethylene or higher to generate a meltdown phenomenon, resulting in ignition and explosion. As a method for reinforcing the safety of a separator, there has been an attempt to use a PE/PP/PE trilayer separator by blending polypropylene (PP) having a relatively higher melting point as compared to polyethylene, instead of a polyethylene monolayer separator. Such a PE/PP/PE trilayer separator is advantageous in that it shows an increased meltdown temperature as compared to the polyethylene monolayer separator, but shows a limitation in that it requires a more complicated manufacturing process as compared to the wet monolayer polyethylene separator. In addition, as another method for reinforcing the safety of a separator, there has been an attempt to form crosslinked bonds in polyethylene fibrils by using a crosslinking agent. However, the method shows low processing efficiency due to the production of byproducts and causes formation of undesired foreign materials in a separator. EP 4 040 589 A1 describes a crosslinked separator for a lithium secondary battery comprising a polyolefin porous substrate including a plurality of fibrils and pores formed by the fibrils entangled with one another, wherein polyolefin chains forming the fibrils are crosslinked directly with one another. US 2020/0335755 A1 describes a separator for an electricity storage device comprising a silane-modified polyolefin. DISCLOSURE Technical Problem The present disclosure is designed to solve the problems of the related art, and therefore the present disclosure is directed to providing a separator maintaining elasticity even at high temperature and an electrochemical device including the same. In addition, when the separator maintains elasticity at high temperature, it ensures resistance against external force in the state of high-temperature exposure, and thus can ensure the safety of an electrochemical device. Technical Solution In one aspect of the present disclosure, there is provided a separator for a lithium secondary battery according to any one of the following embodiments. According to the first embodiment, there is provided a separator for a lithium secondary battery including a porous polyolefin substrate, wherein the polyolefin contained in the separator shows a tan(δ) of 0.3 or less as determined by the following Formula 1 under the following condition, and wherein the polyolefin contained in the separator shows an 'a' value of 0.03-0.25 as determined by the following Formula 2 under the following condition: tanδ=lossmodulusG"B/storagemodulusG′A a=dlogG′/dlogangularfrequency (wherein tan(δ) is determined under the condition of a temperature of 230°C and an angular frequency of 0.1 rad/s)(wherein Formula 2 means a gradient of frequency-storage modulus curve, the horizontal axis of which is the angular fre