[Technical Field] [1] This application claims the benefit of priority to Korean Patent Application No. 2021-0035591 filed on March 19, 2021 and Korean Patent Application No. 2022-0033523 filed on March 17, 2022, the disclosures of which are incorporated herein by reference in theirs entireties. 10 [2] The present invention relates to a separator for a lithium secondary battery with improved heat resistance. More particularly, the present invention relates to a separator for a lithium secondary battery configured such that heat shrink at a high temperature is reduced, 15 whereby heat resistance is improved. 【Background Art】 [3] A lithium secondary battery may be manufactured by receiving an electrode assembly configured such that a 20 separator is interposed between a positive electrode and a negative electrode in a battery case, injecting an electrolytic solution into the battery case, and hermetically sealing the battery case. 3 [4] The separator, which interrupts electrical 5 connection between the positive electrode and the negative electrode in order to secure insulation, may be configured to have a structure in which a coating layer including an inorganic material and a binder is formed on a polyolefin-based substrate made of a porous material 10 such that lithium ions are movable. A polyolefin-based material has low heat resistance, and high-temperature safety and mechanical properties may be improved by addition of the coating layer. [5] In order to increase the strength of the 15 separator and to thin the separator, the separator may be uniaxially or biaxially oriented. When the temperature of the lithium secondary battery increases, the separator may shrink in the oriented direction. As a result, short circuit may occur due to contact between the positive 20 electrode and the negative electrode in the electrode assembly, which may cause ignition and explosion of the lithium secondary battery. [6] When the capacity per volume of the lithium secondary battery increases, the heat generation amount 25 at the time of internal short circuit increases. Various research to solve a safety problem with a high-capacity lithium secondary battery has been conducted. [7] Patent Document 1 discloses a separator 4 manufacturing method of coating one surface of a porous 5 substrate with an aqueous inorganic dispersion solution to form an inorganic layer and forming an electrode adhesion layer made of a binder composition on each of opposite surfaces of the porous substrate having the inorganic layer formed thereon, whereby the force of 10 adhesion of the separator to an electrode is increased. [8] In Patent Document 1, the force of adhesion between the separator and the electrode is increased, whereby it is possible to prevent short circuit between a positive electrode and a negative electrode. However, 15 the resistance of the separator may be increased due to the electrode adhesion layer, and technology capable of inhibiting shrink of the separator is not suggested. [9] Patent Document 2 discloses a composite separator for secondary batteries, the composite separator 20 including a heat resistance layer formed on one surface or opposite surfaces of a porous substrate layer and an adhesive layer formed on the heat resistance layer, wherein inorganic particles are connected and fixed to each other by a binder polymer in the heat resistance 25 layer, and the adhesive layer contains polymer particles. [10] The separator of Patent Document 2 includes the heat resistance layer and the adhesive layer, thereby showing the results in which the strength of adhesion 5 between the separator and an electrode at a high 5 temperature is increased and heat shrinkage is reduced; however, air permeability is increased due to addition of the heat resistance layer, and therefore it is not possible to solve problems in that an electrolytic solution impregnation rate is reduced and resistance is 10 increased. [11] In a separator for a lithium secondary battery, therefore, there is a need for technology capable of reducing heat shrinkage of the separator without provision of an adhesive layer, whereby it is possible to 15 prevent internal short circuit between a positive electrode and a negative electrode, and improving heat resistance of the separator. [12] (Prior Art Documents) [13] (Patent Document 1) Korean Registered Patent 20 Publication No. 1888732 (2018.08.14) [14] (Patent Document 2) Korean Patent Application Publication No. 2016-0109669 (2016.09.21) 【Disclosure】 25 【Technical Problem】 [15] The present invention has been made in view of the above problems, and it is an object of the present invention to provide a separator for a lithium secondary 6 battery configured such that short circuit between a 5 positive electrode and a negative electrode at a high temperature is prevented, whereby safety and heat resistance are improved. 【Technical Solution】 10 [16] A separator for a lithium secondary battery according to the present invention to accomplish the above object includes a separator substrate including a porous polymer resin and a heat resistance layer on at least one side of the separator substrate, the heat 15 resistance layer including a phenolic resin configured to be hardened when heated. [17] An inorganic layer may be present between the separator substrate and the heat resistance layer. [18] The heat resistance layer may include a resol-20 containing phenolic resin. [19] The resol-containing phenolic resin may be present in the heat resistance layer in an amount ranging from 0.5 wt% to 2.0 wt% based on the total solid content of the heat resistance layer. 25 [20] MD-direction and TD-direction heat shrinkage of the separator for a lithium secondary battery at 150°C may be 60% or less. [21] The loading amount of the heat resistance layer 7 may be 2.0 g/m2 or less. 5 [22] An inorganic layer may be present on at least one of a first surface of the separator substrate and a second surface of the separator substrate, and the heat resistance layer may be present on an upper surface of the inorganic layer and an upper surface of the 10 separator substrate on which the inorganic layer is not formed. [23] The separator may be an aqueous separator. [24] The present invention provides a cylindrical lithium secondary battery having a structure in which an 15 electrode assembly including the separator for a lithium secondary battery is received in a cylindrical battery case. [25] The present invention provides a battery pack including the lithium secondary battery as a unit cell, 20 wherein the battery pack is used as an energy source of an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, or an energy storage system. [26] In addition, the present invention may provide various combinations of the above solving means. 25 【Advantageous Effects】 [27] As is apparent from the above description, in a separator for a lithium secondary battery according to 8 the present invention, heat shrinkage at a high 5 temperature is remarkably improved, whereby it is possible to prevent short circuit between a positive electrode and a negative electrode due to shrink of the separator when a lithium secondary battery is under a high-temperature environment. 10 [28] In addition, it is possible to maintain the resistance of the separator at a low level even in the state in which a heat resistance layer is formed, whereby it is possible to prevent deterioration in performance of the lithium secondary battery. 15 【Description of Drawings】 [29] FIG. 1 is a structural formula of a resol-based phenolic resin. [30] FIG. 2 is an FE-SEM photograph of the surface of 20 a separator manufactured according to Example 1. [31] FIG. 3 is an FE-SEM photograph of the surface of a separator manufactured according to Example 2. [32] FIG. 4 is an FE-SEM photograph of the surface of a separator manufactured according to Example 3. 25 [33] FIG. 5 is an FE-SEM photograph of the surface of a separator manufactured according to Comparative Example 1. [34] FIG. 6 is a DMA evaluation graph. 9 5 【Best Mode】 [35] Now, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings such that the preferred embodiments of the present invention can be easily 10 implemented by a person having ordinary skill in the art to which the present invention pertains. In describing the principle of operation of the preferred embodiments of the present invention in detail, however, a detailed description of known functions and configurations 15 incorporated herein will be omitted when the same may obscure the subject matter of the present invention. [36] In addition, the same reference numbers will be used throughout the drawings to refer to parts that perform similar functions or operations. In the case in 20 which one part is said to be connected to another part throughout the specification, not only may the one part be directly connected to the other part, but also, the one part may be indirectly connected to the other part via a further part. In addition, that a certain element 25 is included does not mean that other elements are excluded, but means that such elements may be further included unless mentioned otherwise. [37] In addition, a description to embody elements 10 through limitation or addition may be applied to all 5 inventions, unless particularly restricted, and does not limit a specific invention. [38] Also, in the description of the invention and the claims of the present application, singular forms are intended to include plural forms unless mentioned 10 otherwise. [39] Also, in the description of the invention and the claims of the present application, “or” includes “and” unless mentioned otherwise. Therefore, “including A or B” means three cases, namely, the case including A, the case 15 including B, and the case including A and B. [40] A separator for a lithium secondary battery according to the present invention may include a separator substrate configured to have a porous structure, the separator substrate being made of a polymer resin, 20 and a heat resistance layer added to at least one side of the separator substrate, the heat resistance layer including a phenolic resin configured to be hardened when heated. [41] For example, the separator substrate may be at 25 least one selected from the group consisting of high-density polyethylene, low-density polyethylene, linear low-density polyethylene, ultra-high-molecular-weight polyethylene, polypropylene, polyethylene terephthalate, 11 polybutylene terephthalate, polyester, polyacetal, 5 polyamide, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenylene oxide, polyphenylenesulfidro, polyethylene naphthalene, and a mixture thereof. [42] Even during normal use of a lithium secondary 10 battery, such as repetitive charging and discharging, the temperature of a battery cell is increased by heat generated in the battery cell. The separator substrate may be manufactured through uniaxial or biaxial orientation, and a separator thus oriented may shrink to 15 the original size thereof when the temperature increases. [43] In the case in which a material that exhibits high heat shrinkage is used, as described above, when the separator shrinks during use of the lithium secondary battery, a positive electrode and a negative 20 electrode come into contact with each other, whereby internal short circuit may occur. [44] Accordingly, in the present invention, the heat resistance layer may be added to at least one side surface of the separator substrate in order to reduce 25 heat shrinkage of the separator. [45] In a concrete example, the heat resistance layer may include a resol-based phenolic resin configured to be hardened at a high temperature. FIG. 1 shows a 12 structural formula of the resol-based phenolic resin. 5 [46] Specifically, the heat resistance layer may include a resol-based phenolic resin configured to be hardened at a high temperature, as follows. In the present invention, as described above, the heat resistance layer including the resol-based phenolic resin 10 located at the outermost side of the separator may be hardened when the temperature of the lithium secondary battery increases, whereby it is possible to prevent shrinkage of the separator, and therefore it is possible to provide a separator with improved heat resistance. 15 [47] [48] In general, when only a separator substrate including a polyolefin-based polymer resin is used as the separator, not only does heat resistance of the 20 separator become a problem but also there is a problem in that the mechanical strength of the separator is low. In order to remedy the above problems, an inorganic layer including an inorganic material and a binder may be provided on one surface or opposite surfaces of the 25 13 separator substrate. 5 [49] In the present invention, the heat resistance layer is added to at least one side of the separator substrate, wherein an inorganic layer may be formed on any one of a first surface and a second surface of the separator substrate, and the heat resistance layer may 10 be added to an upper surface of the inorganic layer and an upper surface of the separator substrate, on which the inorganic layer is not formed. That is, the inorganic layer may be formed on the surface of the separator substrate on which the inorganic layer is 15 formed, which is one of the first surface and the second surface, between the separator substrate and the heat resistance layer. [50] Alternatively, a first inorganic layer and a second inorganic layer may be formed on the first 20 surface and the second surface of the separator substrate, respectively, and the heat resistance layer may be added to each of the first inorganic layer and the second inorganic layer. [51] For example, when an electrode assembly for 25 cylindrical secondary batteries is manufactured, it is possible to increase the capacity and energy density of a battery by using a single-sided coated separator having an inorganic layer formed on only one surface of 14 a separator substrate, since safety securing criteria 5 are relatively low due to the characteristics of the cylindrical secondary batteries. [52] Alternatively, when an electrode assembly for pouch-shaped secondary batteries is manufactured, it is possible to obtain a heat resistance improvement effect 10 by using a double-sided coated separator having inorganic layers formed on opposite surfaces of a separator substrate, since safety securing criteria are relatively high. [53] The inorganic layer may include an inorganic 15 material and a binder. The binder may maintain bonding between inorganic particles and may increase the force of adhesion between an electrode and the separator. [54] The kind of the binder is not particularly restricted as long as the binder does not cause any 20 chemical change in a separator coating layer. For example, the binder may be made of at least one selected from the group consisting of polyolefin, such as polyethylene or polypropylene; a fluorine-containing resin, such as polyvinylidene fluoride or 25 polytetrafluoroethylene; fluorine-containing rubber, such as a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer or an ethylene-tetrafluoroethylene copolymer; a styrene-butadiene 15 copolymer or a hydride thereof; a (meth)acrylic acid 5 ester copolymer, such as a methacrylic acid ester copolymer, an acrylonitrile acrylic acid ester copolymer, or a styrene acrylic acid ester copolymer; rubber, such as ethylene propylene rubber; polyvinyl acetate; a resin having a melting point or a glass transition temperature 10 of 180°C or higher, such as polyphenylene ether, polysulfone, polyethersulfone, polyphenylene sulfide, polyetherimide, polyamide, polyimide, polyamide imide, polyetheramide, polyester, aromatic polyester, or polyetheretherketone; polycarbonate; polyacetal; and a 15 water soluble resin, such as carboxyalkyl cellulose, alkyl cellulose, hydroxyalkyl cellulose, starch, polyvinyl alcohol, sodium alginate, polyethylene glycol, cellulose ester, polyacrylic acid, polyacrylamide, or polymethacrylic acid. 20 [55] The inorganic material constituting the inorganic layer may be at least one selected from the group consisting of BaTiO3, Pb(Zr,Ti)O3 (PZT), Pb1-xLaxZr1-yTiyO3 (PLZT) (0