Title of Invention: POLYMER COMPOSITE MEMBRANE, PREPARATION METHOD FOR SAME, AND LITHIUM-ION BATTERY INCLUDING SAME Abstract: The disclosure relates to the field of lithium-ion batteries, and discloses a polymer composite membrane and a method for preparing same. The disclosure further includes a lithium-ion battery for which the foregoing polymer composite membrane is used. The polymer composite membrane includes a porous base membrane and a heat-resistant fiber layer covering at least one side surface of the porous base membrane, where materials of the heat-resistant fiber layer contain a first polymeric material and a second polymeric material. Detailed Description: CROSS-REFERENCE TO RELATED APPLICATIONS This application is a National Stage Entry of PCT Application No. PCT/CN2017/097405, filed on Aug. 14, 2017, which claims priority to and benefits of Chinese Patent Application No. 201610750354.7, filed with the State Intellectual Property Office of P. R. China on Aug. 29, 2016. The entire content of the above-referenced applications are incorporated herein by reference. FIELD The disclosure relates to the field of lithium-ion batteries, and specifically, to a polymer composite membrane and a preparation method for same. The disclosure further includes a lithium-ion battery in which the foregoing polymer composite membrane is used. BACKGROUND A lithium-ion battery is mainly formed by a positive/negative electrode material, an electrolyte, a membrane, and a battery case packaging material. The membrane is an important component of the lithium-ion battery, and is configured to play a role of separating positive and negative electrodes, to prevent the battery from being internally short-circuited; and allowing ions of the electrolyte to pass freely, to complete an electrochemical charge/discharge process. The performance of the membrane determines the interface structure, the internal resistance, and the like of the battery, and directly affects characteristics of the battery such as the rate performance, the cycle performance, and the safety performance (the high-temperature resistance performance). A membrane whose performance is excellent plays an important role of improving the comprehensive performance of a battery, and is referred to as a “third electrode” of the battery in the industry. Conventional membrane preparation methods mainly include two main types: a “melt-spinning and cold-stretching method” and a “thermally induced phase separation method”, where the preparation principle of the “melt-spinning and cold-stretching method” is: crystallizing a polymer melt such as polyolefin in a high stress field, forming a structure having lamellar crystals arranged in parallel and perpendicular to an extrusion direction, and then obtaining a hard-elastic material through heat treatment. After a hard-elastic polymer membrane is stretched, the lamellar crystals are separated from each other, and a large quantity of microfibers appear. Therefore, a large quantity of microporous structures are formed, and then a microporous membrane is manufactured through heat setting. The “thermally induced phase separation method” is a method for preparing a microporous membrane developed in recent years. In the method, a high polymer such as polyolefin and some small molecule compounds having high boiling points are used to form a homogeneous solution at a relatively high temperature (which is usually higher than the melting temperature Tm of the polymer), and solid-liquid or liquid-liquid phase separation occurs when the temperature is reduced. In this way, an additive phase is contained in a polymer-rich phase, and a polymer phase is contained in an additive-rich phase. After the stretching, if low molecular polymers are removed, microporous membrane materials in communication with each other may be made. However, for a membrane prepared by using the “melt-spinning and cold-stretching method”, although the costs are relatively low and the mechanical strength (toughness and strength) is relatively good, the high-temperature resistance stability (thermal shrinkage) is poor; and for a membrane prepared by using the “thermally induced phase separation method”, although the high-temperature resistance stability is improved, the costs are relatively high, and the mechanical strength of the material is deteriorated (hardened), which limits development of the material in the battery field. The research and development personnel are always seeking a manner that can give consideration to both the mechanical strength and the high-temperature resistance stability of the membrane, so as to adapt to rapid development of the battery industry. SUMMARY An objective of the disclosure is to provide a polymer composite membrane, a method for preparing same, and a lithium-ion battery including same, so as to balance mechanical strength and high-temperature thermal stability of the polymer composite membrane. To achieve the foregoing objective, according to a first aspect of the disclosure, a polymer composite membrane is provided, where the polymer composite membrane includes a porous base membrane and a heat-resistant fiber layer covering at least one side surface of the porous base membrane, where materials of the heat-resistant fiber layer contain a first polymeric material and a second polymeric material; the first polymeric material is a heat-resistant polymeric material whose melting point is above 180° C.; and the melting point of the second polymeric material is lower than that of the first polymeric material, and a liquid absorption rate of the second polymeric material in an electrolyte at 25° C. is above 40% and has an error of ±5%. According to a second aspect of the disclosure, a method for preparing a polymer composite membrane is provided, where the preparation method includes the following steps: S1: providing a porous base membrane; and S2: preparing a spinning solution containing a first polymeric material and a second polymeric material, and forming a heat-resistant fiber layer on at least one side surface of the porous base membrane through electrostatic spinning by using the spinning solution, where the first polymeric material is a heat-resistant polymeric material whose melting point is above 180° C.; and the melting point of the second polymeric material is lower than that of the first polymeric material, and a liquid absorption rate of the second polymeric material in an electrolyte at 25° C. is above 40% and has an error of ±5%. According to a third aspect of the disclosure, a lithium-ion battery is provided, where the lithium-ion battery includes a positive electrode, a negative electrode, and a battery membrane located between the positive electrode and the negative electrode, and the battery membrane is the polymer composite membrane described in the disclosure. Application of the polymer composite membrane, the method for preparing same, and the lithium-ion battery including same of the disclosure has the following beneficial effects: (1) The heat-resistant polymeric material is used as the first polymeric material, to facilitate improvement in the high-temperature resistance performance (transverse/longitudinal thermal shrinkage percentage) of the polymer composite membrane, so that thermal shrinkage of the polymer composite membrane at a high temperature (180° C.) is relatively small, to further facilitate avoidance of contact between the positive and negative electrodes caused by shrinkage of the polymer composite membrane because the battery is heated (for example, caused by tiny short-circuit) cause, thereby ensuring the high-temperature resistance safety performance of the battery. (2) The second polymeric material whose liquid absorption rate in an electrolyte at 25° C. is above 40% is used, and a characteristic in which the second polymeric material can perform liquid absorption and swelling in the electrolyte and is partially gelatinized is used, so that the gelatinized second polymeric material has a particular viscosity, to further facilitate enhancement in compatibility between the heat-resistant fiber layer and the ceramic layer, and/or between the heat-resistant fiber layer and an outer layer structure (the bonding layer or the positive and negative electrodes) and enhancement in a binding force between the heat-resistant fiber layer and the ceramic layer, and/or between the heat-resistant fiber layer and an outer layer structure (the bonding layer or the positive and negative electrodes), and facilitate improvement in the mechanical performance of the prepared polymer composite membrane at a high temperature. (3) Both the first polymeric material and the second polymeric material are used, and a characteristic in which the first polymeric material has a relatively high melting point (above 180° C.) and can keep a relatively good strength at a high temperature is used, so that the first polymeric material becomes a backbone of an entire spinning fiber network structure; and the characteristic in which the second polymeric material (whose liquid absorption rate in an electrolyte at 25° C. is above 40%) can perform liquid absorption and swelling in the electrolyte and is partially gelatinized is used, so that the gelatinized second polymeric material has a particular viscosity, and then is attached to the backbone of the spinning fiber network structure formed by the first polymeric material, to play a particular role of enhancing the backbone of the spinning fiber network structure, and then improve the mechanical strength (the transverse stretching strength, the longitudinal stretching strength, and the needling strength) of the heat-resistant fiber layer and the polymer composite membrane. (4) The heat-resistant fiber layer is formed by using the first polymeric material and the second polymeric material, and this heat-resistant fiber layer has a spinning fiber network structure, so that a substance layer (relatively thin) generated when the second polymeric material is gelatinized is attached to a surface of the first polymeric material, and therefore does not hinder migration of lithium ions, to facilitate maintenance in the ion conductivity of the polymer composite membrane, and then improve the performance of the battery such as the cycle life to a particular extent. Other features and advantages of the disclosure are described in detail in the subsequent specific implementations. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawing is used to further understand the disclosure and constitute a part of the specification, and is used to explain the disclosure together with the following specific implementations, but does not constitute a limitation on the disclosure. In the accompanying drawing: FIG. 1 shows a SEM picture of a polymer composite membrane F1 obtained according to Embodiment 1 of the disclosure, where a magnification factor is 2000. DETAILED DESCRIPTION Specific implementations of the disclosure are described in detail below. It should be understood that, the specific implementations described herein are used to only describe and explain the disclosure, but are not used to limit the disclosure. Endpoints of all ranges and all values disclosed in the disclosure are not limited to the precise ranges or values, and these ranges or values should be understood as including values close to these ranges or values. For value ranges, endpoint values of the ranges, an endpoint value of each range and an independent point value, and independent point values may be combined with each other to obtain one or more new value ranges, and these value ranges should be considered as being specifically disclosed herein. The disclosure provides a polymer composite membrane, where the polymer composite membrane includes a porous base membrane and a heat-resistant fiber layer covering at least one side surface of the porous base membrane, where materials of the heat-resistant fiber layer contain a first polymeric material and a second polymeric material; the first polymeric material is a heat-resistant polymeric material whose melting point is above 180° C.; and the melting point of the second polymeric material is lower than that of the first polymeric material, and a liquid absorption rate of the second polymeric material in an electrolyte at 25° C. is above 40% and is optionally 40% to 100% and has an error of ±5%. In the disclosure, the error being ±5% herein means an error existing when the liquid absorption rate of the second polymeric material is measured. In the disclosure, a method for measuring a “liquid absorption rate” includes: dissolving a to-be-measured material in a corresponding solvent, pouring to form a sample having a designated size (for example, a wafer whose diameter is 17 mm), drying, then weighing the sample of a mass m1 in a glove box (at 25° C.) full of argon, then immersing the sample in an electrolyte (the electrolyte contains a lithium salt LiPF6 (lithium hexafluorophosphate) and an organic solvent system, the content of the lithium salt is 1 mol/L, and the organic solvent system contains 32.5 wt % of EC (ethylene carbonate), 32.5 wt % of EMC (ethyl methyl carbonate), 32.5 wt % of DMC (dimethyl carbonate), and 2.5 wt % of VC (vinylene carbonate) by using 100 wt % of a total weight of the organic solvent system as a reference) for 24 h, then taking out the sample, drying the liquid on the surface of the sample by using filter paper (not performing pressing processing), weighing the sample of a mass m2, and then calculating a corresponding liquid absorption rate according to a calculation formula: “liquid absorption rate=(m2−m1)/m1×100%”. In a process of measuring a liquid absorption rate, different operators may have different operation manners for the step of “drying the liquid on the surface of the sample by using the filter paper”. This affect may a measurement result, and in the disclosure, an allowable measurement error is ±5%. According to the polymer composite membrane of the disclosure, in an implementation, a liquid absorption rate of the first polymeric material in an electrolyte at 25° C. is below 5% and has an error of ±5%. By controlling the liquid absorption rate of the first polymeric material, it is favorable to better keep the backbone of the spinning fiber network structure formed by the first polymeric material at a high temperature, and optimize heat-resistant stability (heat-resistant safety) of the prepared polymer composite membrane. According to the polymer composite membrane of the disclosure, in an implementation, the glass transition temperature of the first polymeric material is above 100° C. By selecting the first polymeric material whose glass transition temperature is above 100° C., it is favorable for the prepared polymer composite membrane to keep a relatively high strength in a temperature increasing process (from a room temperature to 100° C.), so that the second polymeric material that can perform liquid absorption and swelling in the electrolyte and is partially gelatinized can be better bonded onto the backbone of the spinning fiber network structure formed by the first polymeric material, to further optimize heat-resistant stability of the prepared polymer composite membrane. According to the polymer composite membrane of the disclosure, in an implementation, optionally, the first polymeric material includes but is not limited to one or more of polyetherimide (PEI), poly(ether ether ketone) (PEEK), polyether sulfone (PES), polyamide-imide (PAI), polyamide acid (PAA), and polyvinylpyrrolidone (PVP). The poly(ether ether ketone) (PEEK) includes copoly(ether ether ketone) (CoPEEK) and modified poly(ether ether ketone), provided that the melting point of the poly(ether ether ketone) satisfies the foregoing requirement. Specifically, the first polymeric material that may be used includes but is not limited to one or more of polyetherimide ultem 1000 (which has a glass transition temperature of 215° C. and a liquid absorption rate of 0.1% in an electrolyte at 25° C.) commercially available from SABIC Innovative Plastics (Shanghai) Co., Ltd., a polyvinylpyrrolidone product (which has a glass transition temperature of 110° C. to 130° C. and a liquid absorption rate of 1% in an electrolyte at 25° C.) with a trade mark K90 commercially available from Hangzhou Sunflower Technology Development Co., Ltd., and a poly(ether ether ketone) product (which has a glass transition temperature of 143° C. and a liquid absorption rate of 0.5% in an electrolyte at 25° C.) with a trade mark ketaspire commercially available from Solvay Corporation. According to the polymer composite membrane of the disclosure, in an implementation, the melting point of the second polymeric material is 100° C. to 150° C.; and the glass transition temperature of the second polymeric material is below 25° C. By selecting the second polymeric material whose glass transition temperature is relatively low, at a normal use temperature (a room temperature to 40° C.) of a battery, the second polymeric material is already softened, and is bonded onto the backbone of the spinning fiber network structure formed by the first polymeric material, to further optimize heat-resistant stability of the prepared polymer composite membrane. In an implementation, optionally, the second polymeric material includes but is not limited to one or more of modified polyvinylidene fluoride (PVDF), polyacrylate, polyphenyl ethylene, and polyethylene oxide (PEO). It should be noted that, in the disclosure, the foregoing types of raw materials (modified polyvinylidene fluoride, polyacrylate, polyphenyl ethylene, and polyethylene oxide) may be used as the second polymeric material, but a precondition is that the used raw material needs to satisfy the requirement of the disclosure for the liquid absorption rate of the second polymeric material. Using polyvinylidene fluoride as an example, the liquid absorption rate of unmodified polyvinylidene fluoride is usually 10% to 20%, and does not conform to the use requirement of the disclosure, and modified polyvinylidene fluoride such as polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) that is modified so that the liquid absorption rate satisfies the foregoing requirement is selected in the disclosure; and using polyacrylate as an example, polyacrylate whose liquid absorption rate satisfies the foregoing requirement includes but is not limited to polymethyl acrylate, polymethyl methacrylate, and polyethylene acrylate. Specifically, the second polymeric material that may be used includes but is not limited to one or more of a PVDF-HFP product (which has a glass transition temperature of −55° C. to −40° C. and a liquid absorption rate of 45% to 60% in an electrolyte at 25° C.) with a trade mark LBG commercially available from Arkema Corporation, a polyethylene oxide product (which has a glass transition temperature of −65° C. and a liquid absorption rate of 1000% in an electrolyte at 25° C., where Mw=600000) commercially available from Aladdin Corporation, and a polymethyl methacrylate product (which has a liquid absorption rate of 55% in an electrolyte at 25° C.) commercially available from Aladdin Corporation. Optionally, a liquid absorption rate of the second polymeric material in an electrolyte at 25° C. is 40% to 100% and has an error of ±5%. According to the polymer composite membrane of the disclosure, selection for a material in the heat-resistant fiber layer is not limited. Based on use of both the first polymeric material and the second polymeric material, other raw materials such as nanometer ceramic particles may be further properly added with reference to a regular material use requirement in the field. Certainly, in the disclosure, the materials of the heat-resistant fiber layer may be formed by a mixture of the first polymeric material and the second polymeric material. By using the mixture of the first polymeric material and the second polymeric material to prepare the heat-resistant fiber layer of the polymer composite membrane of the disclosure, the effect that the disclosure intends to achieve can be achieved relatively well. According to the polymer composite membrane of the disclosure, a weight ratio of the first polymeric material to the second polymeric material in the heat-resistant fiber layer may be random, provided that based on using the particular first polymeric material as a main raw material, the objective of the disclosure can be achieved to a particular extent by adding some second polymeric material. However, in comprehensive consideration of temperature resistant stability, strength, and toughness of the entire polymer composite membrane, in the disclosure, optionally, a weight ratio of the first polymeric material to the second polymeric material in the heat-resistant fiber layer is (0.5 to 10):1, is optionally (1 to 5):1, and is optionally (1 to 3):1. According to the polymer composite membrane of the disclosure, in an implementation, in the heat-resistant fiber layer, the first polymeric material is polyetherimide, and the second polymeric material is polyvinylidene fluoride-hexafluoropropylene; and the materials of the heat-resistant fiber layer are a mixture of polyetherimide and polyvinylidene fluoride-hexafluoropropylene. Polyetherimide is characterized in that polyetherimide can keep a relatively good strength at a high temperature, so that polyetherimide becomes the backbone of the entire spinning fiber network structure, and polyvinylidene fluoride-hexafluoropropylene can perform liquid absorption and swelling in an electrolyte, and has a gelatinization characteristic and therefore has bonding performance to some extent, and then can be quite well attached to the backbone of the spinning fiber network structure formed by polyetherimide, thereby playing a role of enhancing on the backbone of the spinning fiber network structure, and further improving the mechanical strength of a first heat-resistant fiber layer and the polymer composite membrane. According to the polymer composite membrane of the disclosure, in an implementation, in the foregoing polymer composite membrane, the porosity of the heat-resistant fiber layer is above 70%, and optionally the porosity of the heat-resistant fiber layer is 70% to 95%, for example, 75% to 95%. The porosity of the heat-resistant fiber layer is high, so that the ion conductivity of the polymer composite membrane may be effectively ensured. In the disclosure, a manner of measuring the porosity of the heat-resistant fiber layer is: preparing a heat-resistant fiber layer sample of a particular size, weighing, then immersing the heat-resistant fiber layer sample in isobutanol, measuring the weight of the sample after adsorption and balancing, and then calculating the porosity of the heat-resistant fiber layer by using a formula: Mass ⁢ ⁢ after ⁢ ⁢ adsorption - Mass ⁢ ⁢ before ⁢ ⁢ adsorption ρ i ⁢ s ⁢ o ⁢ b ⁢ utanol ⁢ Sample ⁢ ⁢ volume × 1 ⁢ 0 ⁢ 0 ⁢ % . According to the polymer composite membrane of the disclosure, in an implementation, the surface density of the heat-resistant fiber layer is 0.2 g/m2 to 15 g/m2, and is optionally 3 g/m2 to 6 g/m2. The surface density of the heat-resistant fiber layer is the mass of a substance applied onto a base material membrane on a unit area. In the disclosure, when the surface density of the heat-resistant fiber layer falls within the foregoing range, the conductivity may be effectively ensured without affecting migration of lithium ions, and better bonding performance is provided, to facilitate improvement in the safety performance of the battery. According to the polymer composite membrane of the disclosure, the thickness of the heat-resistant fiber layer and the diameter of fiber in the heat-resistant fiber layer are not specially limited, and in an implementation, the single-sided thickness of the heat-resistant fiber layer is 0.5 μm to 30 μm, and is optionally 1 μm to 20 μm; and in an implementation, the diameter of the fiber is 100 nm to 2000 nm. In the disclosure, when the thickness of the heat-resistant fiber layer falls within the foregoing range, the positive and negative electrodes and the membrane may be effectively bonded, thereby improving the cycle performance of the battery. According to the polymer composite membrane of the disclosure, the heat-resistant fiber layer may be formed on one side surface of the porous base membrane, or formed on two side surfaces of the porous base membrane. In an implementation, the heat-resistant fiber layer is formed on each of two side surfaces of the porous base membrane. According to the polymer composite membrane of the disclosure, the porous base membrane may be a polymer base membrane, or may be a ceramic membrane, where a polyolefin membrane generally used for a lithium-ion battery may be used as the polymer base membrane, for example, includes but is not limited to a polypropylene (PP) membrane, a polyethylene (PE) membrane, and a PE/PP/PE three-layered membrane; and the ceramic membrane is the same as a regular ceramic membrane in the field, and includes a polymer base membrane (which is the same as the foregoing polymer base membrane) and a ceramic layer disposed on at least one side surface of the polymer base membrane. According to the polymer composite membrane of the disclosure, in an implementation, the porous base membrane is a ceramic membrane, where no special requirement is imposed on a ceramic layer in the ceramic membrane, and a regularly used ceramic layer in the field may be selected. However, the inventor of the disclosure finds through in-depth research that, in the prior art, a ceramic layer of a ceramic membrane is usually intentionally or unintentionally fabricated to have low density and high porosity. In this way, the air permeability of the ceramic membrane may be greatly increased, but it is difficult for such a ceramic membrane to be high-temperature resistant, and obvious thermal shrinkage usually occurs in the ceramic membrane above 160° C. to affect the safety performance of the battery. Moreover, it is mentioned in CN105355825A that the surface density of a ceramic layer may be controlled to be between 0.2 mg/cm2 and 1.8 mg/cm2 to improve the pressure resistant performance and the ion permeation performance of the ceramic layer, but a thickness factor is not eliminated for the surface density, that is, the surface density of the ceramic layer may be increased because the ceramic layer is thicken but not because the ceramic layer is accumulated more densely. Such an increase in the surface density may improve the safety by improving the thermal resistance of the ceramic membrane, but the high-temperature resistance and shrinkage resistance performance of the ceramic membrane is not ideal, and the increase in the thickness unfavorably affects the capacity of the battery. The inventor of the disclosure further finds through in-depth research that, when the surface density p of the ceramic layer of the ceramic membrane at a unit thickness (1 μm) is controlled to be 1.8 mg/cm2<ρ≤2.7 mg/cm2, the corresponding ceramic membrane has quite excellent high-temperature resistance and thermal-shrinkage resistance performance. Based on this, in the disclosure, optionally the ceramic layer contains ceramic particles and a binder, and the surface density p of the ceramic layer at a unit thickness (1 μm) satisfies 1.8 mg/cm2<ρ≤2.7 mg/cm2, optionally satisfies 1.85 mg/cm2≤ρ≤2.65 mg/cm2, and optionally satisfies 1.9 mg/cm2≤ρ≤2.6 mg/cm2. In the method for preparing a ceramic membrane provided in the disclosure, optimized dispersion of ceramic particles is implemented by controlling the use amount of each component in the ceramic layer slurry, the number-average molecular weight of the dispersant, and the rotational speed for forming the ceramic layer slurry, thereby controlling the density of accumulation between the ceramic particles to control the surface density of the ceramic layer at a unit thickness (1 μm) to be 1.8 mg/cm2<ρ≤2.7 mg/cm2. In this way, the high-temperature resistance and thermal-shrinkage resistance performance of the ceramic membrane can be improved on the basis of basically not reducing the air permeability, so that the heat-resistant temperature of the ceramic membrane is above 160° C., that is, the thermal stability of the ceramic membrane is improved without increasing the thickness of the ceramic layer, so as not to affect the energy density of the battery. According to the polymer composite membrane of the disclosure, in an implementation, in the ceramic layer, relative to the ceramic particles of 100 parts by weight, the content of the binder is 2 to 8 parts by weight, and is optionally 4 to 6 parts by weight. When the content of each substance in the ceramic layer is controlled to be within the foregoing optional range, the obtained ceramic membrane is enabled to have better high-temperature resistance and thermal-shrinkage resistance performance and air permeability. According to the polymer composite membrane of the disclosure, the type of the ceramic particles may be regularly selected in the field, for example, may be selected from one or more of Al2O3, SiO2, BaSO4, BaO, TiO2, CuO, MgO, Mg(OH)2, LiAlO2, ZrO2, CNT, BN, SiC, Si3N4, WC, BC, AlN, Fe2O3, BaTiO3, MoS2, α-V2O5, PbTiO3, TiB2, CaSiO3, molecular sieve, clay, boehmite, and kaolin. Moreover, an average particle size of the ceramic particle is optionally 200 nm to 800 nm, and is optionally 300 nm to 600 nm, thereby not only facilitating avoidance of coagulation of the slurry used for the ceramic layer, but also facilitating improvement in the air permeability of the ceramic membrane. According to the polymer composite membrane of the disclosure, the type of the binder in ceramic layer is not particularly limited, and the binder may be various existing substance that can be used to improve the strength of the ceramic membrane, for example, may be at least one of polyacrylate (an optional weight-average molecular weight Mw is 1×104 g/mol to 1×106 g/mol), a copolymer of polyvinylidene fluoride and hexafluoropropylene (an optional weight-average molecular weight Mw is 1×104 g/mol to 1×106 g/mol), a copolymer of polyvinylidene fluoride and trichloro ethylene (an optional weight-average molecular weight Mw is 1×104 g/mol to 1×106 g/mol), polyacrylonitrile (an optional weight-average molecular weight Mw is 1×104 g/mol to 1×106 g/mol), polyvinyl pyrrolidone (an optional weight-average molecular weight Mw is 1×105 g/mol to 1×106 g/mol), polyimide (an optional weight-average molecular weight Mw is 1×104 g/mol to 1×106 g/mol), polyvinyl alcohol (an optional weight-average molecular weight Mw is 1×103 g/mol to 1×105 g/mol), and the like, is optionally polyacrylate, and is optionally polyacrylate whose glass transition temperature satisfies −40° C. to 0° C. The polyacrylate whose glass transition temperature satisfies −40° C. to 0° C. may be specifically at least one of a homopolymer, a copolymer, and the like of methylmethacylate, ethylmethacrylate, butylmethacrylate, and hexylmethacrylate. When the polyacrylate whose glass transition temperature satisfies −40° C. to 0° C. is used as the binder, the manufacturing performance of the ceramic membrane can be improved without affecting the bonding strength of the ceramic membrane, to have a better industrial application prospect. Moreover, optionally, a crosslinked monomer such as methyl methacrylate and/or methylolacrylamide is introduced into the foregoing polyacrylate binder, and the content of the crosslinked monomer is optionally controlled to be within 8 wt %, and is optionally controlled to be 3 wt % to 5 wt %. In this way, the polyacrylate binder may be mildly crosslinked, thereby improving the water resistance of the ceramic membrane and increasing the bonding strength of the ceramic layer. According to the polymer composite membrane of the disclosure, in an implementation, in the ceramic layer, relative to the ceramic particles of 100 parts by weight, a binder of 2 to 8 parts by weight, a dispersant of 0.3 to 1 part by weight, a thickener of 0.5 to 1.8 parts by weight, and a surface treating agent of 0 to 1.5 parts by weight are further included, and the number-average molecular weight of the dispersant is below 50000; optionally, in the ceramic layer, relative to the ceramic particles of 100 parts by weight, the use amount of the binder is 4 to 6 parts by weight, the use amount of the dispersant is 0.4 to 0.8 part by weight, the use amount of the thickener is 0.7 to 1.5 parts by weight, and the use amount of the surface treating agent is 0 to 1.5 parts by weight, and the number-average molecular weight of the dispersant is 5000 g/mol to 20000 g/mol. According to the polymer composite membrane of the disclosure, the type of the dispersant in the ceramic layer is not particularly limited, and the dispersant may be various existing substances that are helpful to dispersion of each substance in the ceramic layer slurry and whose number-average molecular weight is below 50000, is optionally at least one of polyacrylate, polyglycol ether, silicates, phosphates, and guar gum, and is optionally at least one of polyacrylate, aliphatic polyglycol ether, and phosphates. The polyacrylate may be, for example, at least one of potassium polyacrylate, sodium polyacrylate, lithium polyacrylate, and the like. The aliphatic polyglycol ether may be, for example, polyethylene glycol tert-octylphenyl ether and/or polyethylene glycol monooleyl ether. The phosphates may be, for example, sodium tripolyphosphate and/or sodium hexametaphosphate. According to the polymer composite membrane of the disclosure, the type of the thickener in the ceramic layer is not particularly limited, and the thickener is optionally at least one of polyacrylate, a polyacrylate copolymer, polyvinylpyrrolidone, a cellulose derivative, and polyacrylamide, and is optionally at least one of polyacrylate, a polyacrylate copolymer, and a cellulose derivative. The polyacrylate may be, for example, at least one of potassium polyacrylate, sodium polyacrylate, lithium polyacrylate, and the like. The polyacrylate copolymer may be, for example, at least one of a copolymer of acrylic acid and styrene, a copolymer of acrylic acid and ethyl acrylate, a copolymer of acrylic acid and ethylene, and the like. The cellulose derivative may be, for example, at least one of sodium carboxymethylcellulose, potassium carboxymethylcellulose, hydroxyethyl cellulose, and the like. Moreover, the viscosity of the thickener in an aqueous solution of 1 wt % is 1500 mPa·s to 7000 mPa·s. In this way, the thickener not only can be quite well dispersed in the ceramic layer slurry, but also can be favorable to applying, and more favorable to improvement in the surface density. Moreover, each of the dispersant and the thickener may be polyacrylate, but, the number-average molecular weight of polyacrylate used as the thickener is far greater than the molecular weight of polyacrylate used as the dispersant, the number-average molecular weight of the polyacrylate used as the thickener is usually 300000 to 1500000, and the number-average molecular weight of the polyacrylate used as the dispersant is below 50000. According to the polymer composite membrane of the disclosure, the type of the surface treating agent in the ceramic layer is not particularly limited, and the surface treating agent is optionally 3-glycidyloxypropyltrimethoxysilane and/or 3-glycidyloxypropyltriethoxysilane. In this way, interaction between the ceramic particles and the binder can be further improved, to enhance the strength of the ceramic membrane. According to the polymer composite membrane of the disclosure, in an implementation, the thickness of the polymer base membrane in the ceramic membrane is 5 μm to 30 μm, and is optionally 6 μm to 25 μm. Moreover, the single-sided thickness of the ceramic layer is optionally 1 μm to 5 μm, and is optionally 2 μm to 3.5 μm, so as to be more favorable to improvement in the high-temperature resistance and thermal-shrinkage resistance performance of the ceramic membrane and improvement in the air permeability. According to the polymer composite membrane of the disclosure, in an implementation, the ceramic layer in the ceramic membrane may be formed on one side surface of the polymer base membrane, or formed on two side surfaces of the polymer base membrane. In an implementation, the ceramic layer is formed on each of two side surfaces of the polymer base membrane. According to the polymer composite membrane of the disclosure, in an implementation, the polymer composite membrane further includes a bonding layer, where the bonding layer is formed on at least an outermost layer of one side surface of the polymer composite membrane, and optionally the bonding layer is formed on outermost layers of two side surfaces of the polymer composite membrane; and formation of the bonding layer can improve the viscosity between the polymer composite membrane and the positive and negative electrodes, increase the disposition stability of the polymer composite membrane, and improve the safety performance of the battery. Optionally, in the disclosure, the bonding layer contains an acrylate crosslinked polymer and a styrene-acrylate crosslinked copolymer and/or a vinylidene fluoride-hexafluoropropylene copolymer, what means, the bonding layer contains an acrylate crosslinked polymer and a styrene-acrylate crosslinked copolymer, or the bonding layer contains an acrylate crosslinked polymer and a vinylidene fluoride-hexafluoropropylene copolymer, or the bonding layer contains an acrylate crosslinked polymer, a styrene-acrylate crosslinked copolymer and a vinylidene fluoride-hexafluoropropylene copolymer, and the porosity of the bonding layer is 40% to 65%. When the ceramic membrane further includes the foregoing particular bonding layer, the ceramic membrane not only has good high-temperature resistance and thermal-shrinkage resistance performance, but also has higher bonding strength and ion conductivity. That “the first bonding layer contains an acrylate crosslinked polymer and a styrene-acrylate crosslinked copolymer and/or a vinylidene fluoride-hexafluoropropylene copolymer” means that the bonding layer contains an acrylate crosslinked polymer and a styrene-acrylate crosslinked copolymer and does not contain a vinylidene fluoride-hexafluoropropylene copolymer, or contains an acrylate crosslinked polymer and a vinylidene fluoride-hexafluoropropylene copolymer and does not contain a styrene-acrylate crosslinked copolymer, or contains an acrylate crosslinked polymer, a styrene-acrylate crosslinked copolymer, and a vinylidene fluoride-hexafluoropropylene copolymer. Moreover, “containing a self-crosslinking pure acrylic emulsion and a self-crosslinking styrene acrylic emulsion and/or a copolymer emulsion of vinylidene fluoride and hexafluoropropylene” may also be similarly explained. According to the polymer composite membrane of the disclosure, the acrylate crosslinked polymer is a polymer obtained by performing cross-linking polymerization on reaction-type acrylate monomers. The crosslinking degree of the acrylate crosslinked polymer may be 2% to 30%, and is optionally 5% to 20%. In the disclosure, the crosslinking degree is the percentage of the weight of the crosslinked polymer to the total weight of the polymer. Moreover, the glass transition temperature of the acrylate crosslinked polymer is optionally −20° C. to 60° C., and is optionally −12° C. to 54° C. According to an implementation of the disclosure, the acrylate crosslinked polymer is a mixture of a first acrylate crosslinked polymer and a second acrylate crosslinked polymer and/or a third acrylate crosslinked polymer, or a second acrylate crosslinked polymer, or a third acrylate crosslinked polymer, where the first acrylate crosslinked polymer contains a polymethyl methacrylate chain segment of 70 to 80 wt %, a polyethylene acrylate chain segment of 2 to 10 wt %, a polybutyl acrylate chain segment of 10 to 20 wt %, and a polyacrylic acid chain segment of 2 to 10 wt %, the second acrylate crosslinked polymer contains a polymethyl methacrylate chain segment of 30 to 40 wt %, a polyethylene acrylate chain segment of 2 to 10 wt %, a polybutyl acrylate chain segment of 50 to 60 wt %, and a polyacrylic acid chain segment of 2 to 10 wt %, and the third acrylate crosslinked polymer contains a polymethyl methacrylate chain segment of 50 to 80 wt %, a polyethylene acrylate chain segment of 2 to 10 wt %, a polybutyl acrylate chain segment of 15 to 40 wt %, and a polyacrylic acid chain segment of 2 to 10 wt %; the glass transition temperature of the first acrylate crosslinked polymer is 50° C. to 60° C., the glass transition temperature of the second acrylate crosslinked polymer is −20° C. to −5° C., and the glass transition temperature of the third acrylate crosslinked polymer is 30° C. to 50° C. According to the polymer composite membrane of the disclosure, the styrene-acrylate crosslinked copolymer is a copolymer obtained by copolymerizing a styrene monomer and a reaction-type acrylate monomer. A weight ratio of a styrene structure unit to an acrylate structure unit in the styrene-acrylate crosslinked copolymer may be (0.5 to 2):1, and is optionally (0.67 to 1.5):1. The crosslinking degree of the styrene-acrylate crosslinked copolymer may be 2% to 30%, and is optionally 5% to 20%. Moreover, the glass transition temperature of the styrene-acrylate crosslinked copolymer is optionally −30° C. to 50° C., and is optionally −20° C. to 50° C. According to an implementation of the disclosure, the styrene-acrylate crosslinked copolymer contains a polyphenyl ethylene chain segment of 40 to 50 wt %, a polymethyl methacrylate chain segment of 5 to 15 wt %, a polyethylene acrylate chain segment of 2 to 10 wt %, a polybutyl acrylate chain segment of 30 to 40 wt %, and a polyacrylic acid chain segment of 2 to 10 wt %; and the glass transition temperature of the styrene-acrylate crosslinked copolymer is 15° C. to 30° C. According to the polymer composite membrane of the disclosure, the glass transition temperature of the vinylidene fluoride-hexafluoropropylene copolymer is optionally −65° C. to −40° C., and is optionally −60° C. to −40° C. According to an implementation of the disclosure, the vinylidene fluoride-hexafluoropropylene copolymer contains a polyvinylidene fluoride chain segment of 80 to 98 wt % and a polyhexafluoropropylene chain segment of 2 to 20 wt %, and optionally contains a polyvinylidene fluoride chain segment of 90 to 96 wt % and a polyhexafluoropropylene chain segment of 4 to 10 wt %; and the glass transition temperature of the vinylidene fluoride-hexafluoropropylene copolymer is −60° C. to −40° C. According to the polymer composite membrane of the disclosure, in an implementation, the bonding layer contains the acrylate crosslinked polymer and the styrene-acrylate crosslinked copolymer and does not contain the vinylidene fluoride-hexafluoropropylene copolymer, and a weight ratio of the acrylate crosslinked polymer to the styrene-acrylate crosslinked copolymer is 1:(0.05 to 2), and is optionally 1:(1 to 2); or the bonding layer contains the acrylate crosslinked polymer and the vinylidene fluoride-hexafluoropropylene copolymer and does not contain the styrene-acrylate crosslinked copolymer, and a weight ratio of the acrylate crosslinked polymer to the vinylidene fluoride-hexafluoropropylene copolymer is 1:(0.3 to 25), and is optionally 1:(0.4 to 19); or the bonding layer contains the acrylate crosslinked polymer, the styrene-acrylate crosslinked copolymer, and the vinylidene fluoride-hexafluoropropylene copolymer, and a weight ratio between the acrylate crosslinked polymer, the styrene-acrylate crosslinked copolymer and the vinylidene fluoride-hexafluoropropylene copolymer is 1:(0.01 to 2):(0.3 to 5), and is optionally 1:(0.05 to 1.5):(0.45 to 3). The inventor of the disclosure finds through in-depth research that, when the foregoing several polymers are cooperatively used according to the foregoing particular proportion, it is quite favorable to improvement in the liquid absorption rate and the conductivity of the polymer composite membrane and improvement in the manufacturing performance. According to the polymer composite membrane of the disclosure, in an implementation, the bonding layer contains a first acrylate crosslinked polymer, a second acrylate crosslinked polymer, and the styrene-acrylate crosslinked copolymer and does not contain the vinylidene fluoride-hexafluoropropylene copolymer, and a weight ratio between the first acrylate crosslinked polymer the second acrylate crosslinked polymer and the styrene-acrylate crosslinked copolymer is (5 to 10):1:(10 to 13); or the bonding layer contains the first acrylate crosslinked polymer, the second acrylate crosslinked polymer, and the vinylidene fluoride-hexafluoropropylene copolymer and does not contain the styrene-acrylate crosslinked copolymer, and a weight ratio between the first acrylate crosslinked polymer, the second acrylate crosslinked polymer and the vinylidene fluoride-hexafluoropropylene copolymer is (5 to 15):1:(5 to 12); or the bonding layer contains the second acrylate crosslinked polymer and the vinylidene fluoride-hexafluoropropylene copolymer and does not contain the styrene-acrylate crosslinked copolymer, and a weight ratio of the second acrylate crosslinked polymer to the vinylidene fluoride-hexafluoropropylene copolymer is 1:(5 to 20); or the bonding layer contains the second acrylate crosslinked polymer, the styrene-acrylate crosslinked copolymer, and the vinylidene fluoride-hexafluoropropylene copolymer, and a weight ratio between the second acrylate crosslinked polymer, the styrene-acrylate crosslinked copolymer and the vinylidene fluoride-hexafluoropropylene copolymer is 1:(0.5 to 2):(1 to 5); or the bonding layer contains the third acrylate crosslinked polymer, the styrene-acrylate crosslinked copolymer, and the vinylidene fluoride-hexafluoropropylene copolymer, and a weight ratio between the third acrylate crosslinked polymer, the styrene-acrylate crosslinked copolymer and the vinylidene fluoride-hexafluoropropylene copolymer is 1:(0.5 to 2):(1 to 5); or the bonding layer contains the first acrylate crosslinked polymer, the second acrylate crosslinked polymer, the styrene-acrylate crosslinked copolymer, and the vinylidene fluoride-hexafluoropropylene copolymer, and a weight ratio between the first acrylate crosslinked polymer, the second acrylate crosslinked polymer, the styrene-acrylate crosslinked copolymer and the vinylidene fluoride-hexafluoropropylene copolymer is (10 to 15):1:(0.5 to 2):(5 to 10), where the first acrylate crosslinked polymer contains a polymethyl methacrylate chain segment of 70 to 80 wt %, a polyethylene acrylate chain segment of 2 to 10 wt %, a polybutyl acrylate chain segment of 10 to 20 wt %, and a polyacrylic acid chain segment of 2 to 10 wt %, the second acrylate crosslinked polymer contains a polymethyl methacrylate chain segment of 30 to 40 wt %, a polyethylene acrylate chain segment of 2 to 10 wt %, a polybutyl acrylate chain segment of 50 to 60 wt %, and a polyacrylic acid chain segment of 2 to 10 wt %, and the third acrylate crosslinked polymer contains a polymethyl methacrylate chain segment of 50 to 80 wt %, a polyethylene acrylate chain segment of 2 to 10 wt %, a polybutyl acrylate chain segment of 15 to 40 wt %, and a polyacrylic acid chain segment of 2 to 10 wt %; the styrene-acrylate crosslinked copolymer contains a polyphenyl ethylene chain segment of 40 to 50 wt %, a polymethyl methacrylate chain segment of 5 to 15 wt %, a polyethylene acrylate chain segment of 2 to 10 wt %, a polybutyl acrylate chain segment of 30 to 40 wt %, and a polyacrylic acid chain segment of 2 to 10 wt %; the vinylidene fluoride-hexafluoropropylene copolymer contains a polyvinylidene fluoride chain segment of 80 to 98 wt % and a polyhexafluoropropylene chain segment of 2 to 20 wt %; and the glass transition temperature of the first acrylate crosslinked polymer is 50° C. to 60° C., the glass transition temperature of the second acrylate crosslinked polymer is −20° C. to −5° C., the glass transition temperature of the third acrylate crosslinked polymer is 30° C. to 50° C., the glass transition temperature of the styrene-acrylate crosslinked copolymer is 15° C. to 30° C., and the glass transition temperature of the vinylidene fluoride-hexafluoropropylene copolymer is −60° C. to −40° C. According to the polymer composite membrane of the disclosure, in an implementation, the bonding layer further contains at least one of an acrylonitrile-acrylate copolymer, a vinyl chloride-propylene copolymer, and a butadiene-styrene copolymer. When the bonding layer further contains an acrylonitrile-acrylate copolymer, it is favorable to improvement in the ion conductivity of the polymer composite membrane inside the battery; and when the bonding layer further contains a vinyl chloride-propylene copolymer and/or a butadiene-styrene copolymer, it is favorable to reduction in the liquid absorption rate of the polymer composite membrane, so that the liquid absorption rate cannot go so far as to be excessively high. If the liquid absorption rate is excessively high, the positive electrode and the negative electrode inside the battery are lack of an electrolyte and consequently the performance of the battery is deteriorated. When the bonding layer further contains the acrylonitrile-acrylate copolymer, a weight ratio of the acrylonitrile-acrylate copolymer to the acrylate crosslinked polymer is optionally (0.05 to 2):1, and is optionally (0.08 to 1.85):1. When the bonding layer further contains the vinyl chloride-propylene copolymer, a weight ratio of the vinyl chloride-propylene copolymer to the acrylate crosslinked polymer is optionally (0.15 to 7):1, and is optionally (0.2 to 6):1. When the bonding layer further contains the butadiene-styrene copolymer, a weight ratio of the butadiene-styrene copolymer to the acrylate crosslinked polymer is optionally (0.05 to 2):1, and is optionally (0.08 to 1.85):1. Moreover, the single-sided surface density of the bonding layer is optionally 0.05 mg/cm2 to 0.9 mg/cm2, and is optionally 0.1 mg/cm2 to 0.6 mg/cm2. The single-sided thickness of the bonding layer is optionally 0.1 μm to 1 μm, and is optionally 0.2 μm to 0.6 μm. Moreover, the disclosure further provides a method for preparing a polymer composite membrane, where the preparation method includes the following steps: S1: providing a porous base membrane; and S2: preparing a spinning solution containing a first polymeric material and a second polymeric material, and forming a heat-resistant fiber layer on at least one side surface of the porous base membrane through electrostatic spinning by using the spinning solution, where the first polymeric material is a heat-resistant polymeric material whose melting point is above 180° C.; and the melting point of the second polymeric material is lower than that of the first polymeric material, and a liquid absorption rate of the second polymeric material in an electrolyte at 25° C. is above 40% and has an error of ±5%. Optionally, a liquid absorption rate of the first polymeric material in an electrolyte at 25° C. is below 5% and has an error of ±5%; optionally, the glass transition temperature of the first polymeric material is above 100° C.; optionally, the melting point of the second polymeric material is 100° C. to 150° C.; and optionally, the glass transition temperature of the second polymeric material is below 25° C.; optionally, a liquid absorption rate of the second polymeric material in an electrolyte at 25° C. is 40% to 100% and has an error of ±5%; and optionally, in the spinning polymer, a weight ratio of the first polymeric material to the second polymeric material is (0.5 to 10):1, is optionally (1 to 5):1, and is optionally (1 to 3):1. According to the preparation method of the disclosure, the following method may be further used in step S2: preparing a spinning solution containing a first polymeric material and a second polymeric material, forming a heat-resistant fiber layer through electrostatic spinning on a substrate by using the spinning solution, and then compounding the formed heat-resistant fiber layer onto at least one side surface of the porous base membrane. According to the preparation method of the disclosure, the following steps may be taken in step S2: S201: respectively independently preparing a spinning solution A containing the first polymeric material and a spinning solution B containing the second polymeric material; and S202: performing electrostatic spinning by using the spinning solution A and the spinning solution B. In this case, the formed heat-resistant fiber layer contains both the first polymeric material and the second polymeric material. According to the method for preparing a polymer composite membrane, the following steps may alternatively be taken in step S2: S211: mixing the first polymeric material and the second polymeric material (an optional mixing condition includes: mixing for 2 h to 6 h under a condition of a normal temperature and a rotational speed of 300 to 2000 rpm), to form a mixture; and then preparing a spinning solution containing the mixture; and S212: performing electrostatic
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