Diaphragm, electrochemical device and electric equipment

Abstract

The invention provides a diaphragm, an electrochemical device and electric equipment. The diaphragm comprises a base film, the thickness of the base film is H, and the cross section of the base film in the thickness direction comprises a first area within H / 4 away from the surface of the base film and a second area above H / 3 away from the surface of the base film; based on the molar weight of the element C in the first region, the total molar percentage content of the element N and the element O in the first region is c1; based on the molar weight of the element C in the second area, the total molar percentage content of the element N and the element O in the second area is c2; and c2 / c1 is greater than or equal to 1.05. According to the diaphragm disclosed by the invention, the second region on the inner side of the base membrane is controlled to have relatively high electrolyte wettability relative to the first region on the outer side, so that the lithium separation safety of the electrochemical device can be remarkably improved.

Description

Technical Field

[0001] This application belongs to the field of electrochemistry technology, and particularly relates to a separator, an electrochemical device, and an electrical equipment. Background Art

[0002] With the wide application of lithium-ion batteries in the fields of consumer electronics and new energy, the requirements for their safety are becoming increasingly strict. As one of the key components of lithium-ion batteries, the separator has a crucial impact on the safety performance of the battery. Summary of the Invention

[0003] The inventors of this application have found through research that traditional separators have many drawbacks. For example, the wettability of the electrolyte in them is poor. On the one hand, it affects the injection efficiency of the electrolyte, thereby reducing the production efficiency of the battery; on the other hand, it will directly increase the internal resistance of the battery, affecting the fast charge and discharge performance and cycle life of the battery. In addition, since it is difficult for electrolyte molecules to penetrate into the polymer molecular chains of the separator for diffusion, during the charge and discharge process, once there is a lack of electrolyte, it is difficult to be replenished in time, resulting in the risk of lithium plating, thus affecting the safety of the battery. In view of this, the purpose of this application is to provide a separator, an electrochemical device, and an electrical equipment to improve the wettability of the electrolyte in the separator to meet the requirements of battery production efficiency, electrochemical performance, and safety performance.

[0004] The first aspect of the present application provides a separator, including a base film; the thickness of the base film is H, and the cross-section of the base film along its thickness direction includes a first region within H / 4 from the surface of the base film and a second region above H / 3 from the surface of the base film; based on the molar amount of C element in the first region, the total molar percentage content of N element and O element in the first region is c1; based on the molar amount of C element in the second region, the total molar percentage content of N element and O element in the second region is c2; satisfying: c2 / c1≥1.05. By regulating that the second region on the inner side of the separator base film contains more electrolyte-affinitive groups than the first region on the outer side (reflected in c2 being relatively higher than c1), the second region on the inner side of the base film has higher electrolyte wettability than the first region on the outer side, thereby significantly improving the lithium plating safety of lithium-ion batteries. For example, during high-temperature cycling, the gas generated by side reactions in lithium-ion batteries easily causes local electrolyte loss in the separator base film. For the separator of the present application, since the second region on the inner side of the base film contains more electrolyte-affinitive groups, the diffusion rate of the electrolyte inside the base film is relatively fast. This enables the electrolyte to preferentially fill the pores inside the base film when pores are generated due to gas production causing local electrolyte loss in the separator base film, reducing the situation where the gas pores inside the base film cannot be filled by the electrolyte due to the preferential filling of the electrolyte on both sides. In this way, during subsequent charging, the lithium plating phenomenon caused by the failure of the local pores inside the base film to be replenished by the electrolyte can be effectively reduced, thereby greatly improving the lithium plating safety of lithium-ion batteries.

[0005] In some embodiments, 1.1≤c2 / c1≤10. In this way, the replenishment of the electrolyte inside the electrolyte-deficient part of the base film can be further promoted, and the lithium plating safety of the lithium-ion battery can be improved. Further, in some embodiments, 1.3≤c2 / c1≤5.

[0006] In some embodiments, 3.4%≤c1≤15%. Further, in some embodiments, 4.6%≤c1≤15%. By regulating the value of c1 within the above range, the wettability of the electrolyte in the first region on the outer side of the base film can be improved, the replenishment of the electrolyte on the surface layer of the electrolyte-deficient part of the base film can be promoted, the lithium plating risk caused by the failure to replenish the electrolyte on the surface layer of the base film in time can be reduced, and the lithium plating safety of the lithium-ion battery can be improved.

[0007] In some embodiments, 4.3%≤c2≤20%. Further, in some embodiments, 6.1%≤c2≤20%. By regulating the value of c2 within the above range, the wettability of the electrolyte in the second region on the inner side of the base film can be improved, the replenishment of the electrolyte inside the electrolyte-deficient part of the base film can be promoted, the lithium plating risk caused by the failure to replenish the local pores inside the base film by the electrolyte can be further reduced, and the lithium plating safety of the lithium-ion battery can be improved.

[0008] In some embodiments, the wetting length of the electrolyte on the base film is L, satisfying L≥10 mm; wherein, the electrolyte is an organic solvent dissolved with lithium hexafluorophosphate, and the composition of the organic solvent is ethylene carbonate, propylene carbonate and diethyl carbonate with a mass ratio of 1:1:2. Based on the mass of the electrolyte, the mass percentage of lithium hexafluorophosphate is 12.5%. By regulating the wetting length of the electrolyte on the base film L≥10 mm, the present application can further improve the wetting rate of the electrolyte in the second region inside the base film, thereby promoting the replenishment of the electrolyte inside the electrolyte-deficient part of the base film and further improving the lithium deposition safety of the lithium-ion battery.

[0009] In some embodiments, the contact angle θ of the electrolyte on the surface of the base film satisfies θ≤10°; the electrolyte is an organic solvent dissolved with lithium hexafluorophosphate, and the composition of the organic solvent is ethylene carbonate, propylene carbonate and diethyl carbonate with a mass ratio of 1:1:2. Based on the mass of the electrolyte, the mass percentage of lithium hexafluorophosphate is 12.5%. By regulating the contact angle θ of the electrolyte on the surface of the base film to be θ≤10°, the present application intuitively shows that the electrolyte has extremely excellent wettability to the surface layer of the base film, thereby further promoting the replenishment of the electrolyte on the surface layer of the electrolyte-deficient part of the base film and improving the lithium deposition safety of the lithium-ion battery.

[0010] In some embodiments, the air permeability T of the base film satisfies: 50 s / 100 cc≤T≤150 s / 100 cc.

[0011] By controlling the air permeability T of the base film within the above range, the present application can ensure good diffusion and transmission of the electrolyte inside the base film. When local electrolyte deficiency is caused by gas generation during high-temperature cycling, the moderate air permeability, combined with the rapid electrolyte diffusion characteristics of the second region inside the base film, enables the electrolyte to replenish the pores more smoothly, further improving the lithium deposition safety of the lithium-ion battery. Further, in some embodiments, 80 s / 100 cc≤T≤110 s / 100 cc.

[0012] In some embodiments, the infrared spectrum of the base film is at 2200 cm -1 to 2280 cm -1 In the wavenumber range, and at least one absorption peak exists in the wavenumber range of 1700 cm -1 to 1780 cm -1 When the infrared spectrum of the base film has an absorption peak in a specific wavenumber range in the present application, this indicates that specific functional groups exist in the base film. For example, the absorption peak in the wavenumber range of 2200 cm -1 to 2280 cm -1 corresponds to the presence of a cyano group, while the absorption peak in the wavenumber range of 1700 cm -1 to 1780 cm -1The absorption peaks in the wavenumber range are related to ester groups. These electrolyteophilic groups can enhance the affinity between the base film and the electrolyte, further accelerating the diffusion rate of the electrolyte within the base film, thereby more efficiently replenishing the pores formed due to gas generation and improving the lithium plating safety of the lithium-ion battery.

[0013] In some embodiments, the base film comprises a polyolefin grafted with acrylonitrile and / or acrylate monomers, and the polyolefin comprises at least one of polyethylene, polypropylene, and ethylene-propylene copolymer.

[0014] In some embodiments, the base film comprises a first surface layer, an intermediate layer, and a second surface layer. The intermediate layer is located between the first surface layer and the second surface layer. The first surface layer, the intermediate layer, and the second surface layer each independently comprise a first polymer, and the first polymer comprises at least one of acrylonitrile polymers, vinyl acetate polymers, maleic anhydride grafted polymers, acrylate polymers, acrylate salts polymers, polyethers, polyamides, and polyimides; based on the mass of the intermediate layer, the mass percentage of the first polymer in the intermediate layer is m1; based on the mass of the first surface layer, the mass percentage of the first polymer in the first surface layer is s1; based on the mass of the second surface layer, the mass percentage of the first polymer in the second surface layer is s2; and it satisfies: m1 > s1, m1 > s2.

[0015] In this application, by regulating the content of the first polymer in the first surface layer, the intermediate layer, and the second surface layer, since the mass percentage of the electrolyteophilic first polymer in the intermediate layer is higher, the diffusion of the electrolyte inside the base film is faster. When local electrolyte deficiency in the base film is caused by gas generation during high-temperature cycling, it can promote the electrolyte to preferentially replenish the pores inside the base film, reducing the situation where the gas pores inside the base film cannot be filled with electrolyte due to the preferential filling of the electrolyte on both sides, thereby improving the lithium plating safety of the lithium-ion battery.

[0016] In some embodiments, m1 / s1 ≥ 1.1. In some embodiments, m1 / s2 ≥ 1.1. By regulating m1 / s1 and / or m1 / s2 within the above range in this application, when local electrolyte deficiency in the base film is caused by gas generation during high-temperature cycling, it can promote the electrolyte to preferentially replenish the pores inside the base film, thereby improving the lithium plating safety of the lithium-ion battery.

[0017] In some embodiments, m1 ≥ 10%. Thus, it can improve the diffusion rate of the electrolyte inside the base film. When local electrolyte deficiency in the base film is caused by gas generation during high-temperature cycling, it can promote the electrolyte to quickly replenish the pores inside the base film, thereby improving the lithium plating safety of the lithium-ion battery.

[0018] In some embodiments, s1 ≥ 5%. In some embodiments, s2 ≥ 5%. By regulating s1 and / or s2 within the above range, when high-temperature cyclic gas generation causes local electrolyte loss in the base film, it can promote the rapid replenishment of the electrolyte into the pores on the surface layer of the base film, reduce the risk of lithium plating caused by the failure to timely replenish the electrolyte on the surface layer of the base film, and thus improve the lithium plating safety of the lithium-ion battery.

[0019] In some embodiments, the first surface layer, the intermediate layer, and the second surface layer each independently comprise a polyolefin, and the polyolefin comprises at least one of polyethylene, polypropylene, and ethylene-propylene copolymer.

[0020] In a second aspect, the present application provides an electrochemical device, comprising a positive electrode sheet, a negative electrode sheet, and a separator located between the positive electrode sheet and the negative electrode sheet, wherein the separator comprises the separator provided in any one of the embodiments of the first aspect. Due to the use of the separator provided in the first aspect, the electrochemical device of the present application can have significantly improved lithium plating safety.

[0021] In a third aspect, the present application provides an electrical device, comprising the electrochemical device provided in the second aspect. Description of the Drawings

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present application, and those skilled in the art can also obtain other drawings based on these drawings.

[0023] Figure 1 SEM photograph of the base film of Comparative Example 1;

[0024] Figure 2 SEM photograph of the base film of Example 1;

[0025] Figure 3 Infrared absorption spectra of the base films of Example 1 and Comparative Example 1;

[0026] Figure 4 In the figure, (a) is the lithium plating degree test photograph of Example 1; (b) is the lithium plating degree test photograph of Comparative Example 1. Detailed Embodiments

[0027] In order to make the objectives, technical solutions, and advantages of the present application more clear, the following further details the present application in conjunction with specific embodiments. It should be understood that the specific embodiments described are only used to illustrate the present application, rather than to limit the present application.

[0028] The inventors of the present application have found through research that traditional diaphragms have many drawbacks. For example, the wettability of the electrolyte in them is poor. On the one hand, it affects the electrolyte injection efficiency, thereby reducing the battery production efficiency; on the other hand, it will directly lead to an increase in the battery internal resistance, affecting the fast charge and discharge performance and cycle life of the battery. In addition, since it is difficult for electrolyte molecules to penetrate into the space between the polymer chains of the diaphragm for diffusion, during the charge and discharge process, once there is a lack of electrolyte in a certain part, it is difficult to be replenished in time, resulting in the risk of lithium plating, thus affecting the safety of the battery. In view of this, the purpose of the present application is to provide a diaphragm, an electrochemical device, and an electrical device using the same to improve the wettability of the electrolyte in the diaphragm to meet the requirements of battery production efficiency, electrochemical performance, and safety performance. The diaphragm of the present application will be described in detail below.

[0029] <Separator>

[0030] In the first aspect of the present application, a separator is provided, including a base film; the thickness of the base film is H, and the cross-section of the base film along its thickness direction includes a first region within H / 4 from the surface of the base film and a second region more than H / 3 from the surface of the base film; based on the molar amount of C element in the first region, the total molar percentage content of N element and O element in the first region is c1; based on the molar amount of C element in the second region, the total molar percentage content of N element and O element in the second region is c2; and it satisfies: c2 / c1 ≥ 1.05.

[0031] In the present application, by regulating that the second region on the inner side of the separator base film contains more electrolyte-philic groups than the first region on the outer side (reflected in c2 being relatively higher than c1), the second region on the inner side of the base film has higher electrolyte wettability than the first region on the outer side, thereby significantly improving the lithium plating safety of lithium-ion batteries. For example, during the high-temperature cycle, the gas generated by side reactions in lithium-ion batteries easily causes local electrolyte deficiency in the separator base film. For the separator of the present application, since the second region on the inner side of the base film contains more electrolyte-philic groups, the diffusion rate of the electrolyte inside the base film is relatively fast. This enables the electrolyte to preferentially fill the pores inside the base film when pores are generated due to gas production, reducing the situation where the gas pores inside the base film cannot be filled with electrolyte because the electrolyte preferentially fills both sides. In this way, during the subsequent charging process, the lithium plating phenomenon caused by the lack of electrolyte backfilling in the local pores inside the base film can be effectively reduced, thus greatly improving the lithium plating safety of lithium-ion batteries.

[0032] Exemplarily, the value of c2 / c1 can be 1.05, 1.1, 1.2, 1.3, 1.5, 1.6, 1.8, 2.0, 3.0, 4.0, 5.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0, 13.0, 14.0, 15.0, or a range composed of any two of the above numerical values.

[0033] In some embodiments, 1.1 ≤ c2 / c1 ≤ 10. Thus, it is possible to further promote the replenishment of the electrolyte inside the electrolyte-deficient part of the base film and improve the lithium plating safety of the lithium-ion battery. Further, in some embodiments, 1.3 ≤ c2 / c1 ≤ 5.

[0034] In some embodiments, 3.4% ≤ c1 ≤ 15%. Exemplarily, the value of c1 can be 3.4%, 3.7%, 3.9%, 4.1%, 4.3%, 4.5%, 4.6%, 5.5%, 7.5%, 8.5%, 10.0%, 12.5%, 13.5%, 15.0%, or a range composed of any two of the above numerical values. Further, in some embodiments, 4.6% ≤ c1 ≤ 15%. By regulating the value of c1 within the above range, it is possible to improve the wettability of the electrolyte in the first region outside the base film, promote the replenishment of the electrolyte on the surface layer of the electrolyte-deficient part of the base film, reduce the lithium plating risk caused by the failure to replenish the electrolyte in time on the surface layer of the base film, and improve the lithium plating safety of the lithium-ion battery.

[0035] In some embodiments, 4.3% ≤ c2 ≤ 20%. Exemplarily, the value of c2 can be 4.3%, 4.5%, 5.5%, 6.1%, 7.5%, 10.0%, 11.0%, 12.0%, 14.0%, 16.0%, 17.0%, 18.0%, 20.0%, or a range composed of any two of the above numerical values. Further, in some embodiments, 6.1% ≤ c2 ≤ 20%. By regulating the value of c2 within the above range, it is possible to improve the wettability of the electrolyte in the second region inside the base film, promote the replenishment of the electrolyte inside the electrolyte-deficient part of the base film, further reduce the lithium plating risk caused by the failure to replenish the electrolyte in some local pores inside the base film, and improve the lithium plating safety of the lithium-ion battery.

[0036] In some embodiments, the wetting length of the electrolyte on the base film is L, satisfying L ≥ 10 mm; wherein, the electrolyte is an organic solvent dissolved with lithium hexafluorophosphate, and the composition of the organic solvent is ethylene carbonate, propylene carbonate, and diethyl carbonate with a mass ratio of 1:1:2, and the mass percentage content of lithium hexafluorophosphate based on the mass of the electrolyte is 12.5%.

[0037] Exemplarily, L can be 10 mm, 11 mm, 12 mm, 13 mm, 15 mm, 17 mm, 18 mm, 20 mm, or a range composed of any two of the above numerical values.

[0038] By controlling the wetting length L of the electrolyte on the base film such that L≥10 mm, the present application can further improve the wetting rate of the electrolyte in the second region inside the base film, thereby promoting the replenishment of the electrolyte inside the electrolyte-deficient part of the base film and further enhancing the lithium deposition safety of the lithium-ion battery.

[0039] In some embodiments, the contact angle θ of the electrolyte on the surface of the base film is θ≤10°, the electrolyte is an organic solvent dissolved with lithium hexafluorophosphate, the composition of the organic solvent is ethylene carbonate, propylene carbonate and diethyl carbonate with a mass ratio of 1:1:2, and based on the mass of the electrolyte, the mass percentage content of lithium hexafluorophosphate is 12.5%.

[0040] Exemplarily, θ is 10°, 9°, 8°, 7°, 6°, 5°, 4°, 2°, 1°, 0 or a range composed of any two of the above values.

[0041] By controlling the contact angle θ of the electrolyte on the surface of the base film such that θ≤10°, the present application intuitively shows that the electrolyte has extremely excellent wettability to the surface layer of the base film, thereby further promoting the replenishment of the electrolyte on the surface layer of the electrolyte-deficient part of the base film and enhancing the lithium deposition safety of the lithium-ion battery.

[0042] In some embodiments, the air permeability T of the base film satisfies: 50 s / 100 cc≤T≤150 s / 100 cc.

[0043] Exemplarily, the value of T can be 50 s / 100 cc, 60 s / 100 cc, 70 s / 100 cc, 80 s / 100 cc, 90 s / 100 cc, 100 s / 100 cc, 110 s / 100 cc, 130 s / 100 cc, 150 s / 100 cc or a range composed of any two of the above values. Further, in some embodiments, 80 s / 100 cc≤T≤110 s / 100 cc.

[0044] By controlling the air permeability T of the base film within the above range, the present application can ensure good diffusion and transmission of the electrolyte inside the base film. When local electrolyte deficiency is caused by gas production during high-temperature cycling, the moderate air permeability, combined with the fast electrolyte diffusion characteristics in the second region inside the base film, enables the electrolyte to replenish the pores more smoothly, further enhancing the lithium deposition safety of the lithium-ion battery.

[0045] In some embodiments, the infrared spectrum of the base film is in the wavenumber range of 2200 cm -1 to 2280 cm -1 and in the wavenumber range of 1700 cm -1 to 1780 cm -1There is an absorption peak in at least one of the wavenumber ranges. In the present application, when the infrared spectrum of the base film has an absorption peak in a specific wavenumber range, this indicates the presence of specific functional groups in the base film. For example, the absorption peak in the wavenumber range of 2200 cm -1 to 2280 cm -1 corresponds to the presence of cyano groups, and the absorption peak in the wavenumber range of 1700 cm -1 to 1780 cm -1 is related to ester groups. These electrolyte-philic groups can enhance the affinity between the base film and the electrolyte, further accelerating the diffusion rate of the electrolyte within the base film, thereby more efficiently replenishing the pores formed due to gas generation and improving the lithium plating safety of the lithium-ion battery.

[0046] In some embodiments, the base film comprises a polyolefin grafted with acrylonitrile and / or acrylate monomers, and the polyolefin comprises at least one of polyethylene, polypropylene, and ethylene-propylene copolymer.

[0047] In some embodiments, the base film comprises a first surface layer, an intermediate layer, and a second surface layer. The intermediate layer is located between the first surface layer and the second surface layer. The first surface layer, the intermediate layer, and the second surface layer each independently comprise a first polymer, and the first polymer comprises at least one of acrylonitrile polymers, vinyl acetate polymers, maleic anhydride grafted polymers, acrylate polymers, acrylate salts polymers, polyethers, polyamides, and polyimides; based on the mass of the intermediate layer, the mass percentage of the first polymer in the intermediate layer is m1; based on the mass of the first surface layer, the mass percentage of the first polymer in the first surface layer is s1; based on the mass of the second surface layer, the mass percentage of the first polymer in the second surface layer is s2; and the following is satisfied: m1 > s1, m1 > s2.

[0048] In the present application, by regulating the content of the first polymer in the first surface layer, the intermediate layer, and the second surface layer, since the mass percentage of the electrolyte-philic first polymer in the intermediate layer is higher, the diffusion of the electrolyte within the base film is faster. When the local electrolyte in the base film is depleted due to gas generation during high-temperature cycling, it can promote the preferential replenishment of the pores inside the base film by the electrolyte, reducing the situation where the gas pores inside the base film cannot be filled with the electrolyte due to the preferential filling of the electrolyte on both sides, thereby improving the lithium plating safety of the lithium-ion battery.

[0049] It can be understood that in the present application, the above-mentioned first surface layer and second surface layer include the aforementioned first region; the above-mentioned intermediate layer includes the aforementioned second region.

[0050] In some embodiments, m1 / s1 ≥ 1.1. In some embodiments, m1 / s2 ≥ 1.1. By adjusting m1 / s1 and / or m1 / s2 within the above ranges, when high-temperature cycling gas generation causes local electrolyte depletion in the base film, it can promote the preferential replenishment of the electrolyte into the pores inside the base film, thereby improving the lithium plating safety of the lithium-ion battery.

[0051] Exemplarily, the value of m1 / s1 can be 1.1, 1.2, 1.6, 1.8, 2, 4, 6, 8, 10 or a range composed of any two of the above numerical values. Exemplarily, the value of m1 / s2 can be 1.1, 1.2, 1.6, 1.8, 2, 4, 6, 8, 10 or a range composed of any two of the above numerical values.

[0052] In some embodiments, m1 ≥ 10%. Thus, it can improve the diffusion rate of the electrolyte inside the base film. When high-temperature cycling gas generation causes local electrolyte depletion in the base film, it can promote the rapid replenishment of the electrolyte into the pores inside the base film, thereby improving the lithium plating safety of the lithium-ion battery. Exemplarily, the value of m1 can be 10%, 20%, 40%, 60%, 80%, 90% or a range composed of any two of the above numerical values.

[0053] In some embodiments, s1 ≥ 5%. In some embodiments, s2 ≥ 5%. By adjusting s1 and / or s2 within the above ranges, when high-temperature cycling gas generation causes local electrolyte depletion in the base film, it can promote the rapid replenishment of the electrolyte into the pores on the surface layer of the base film, reduce the lithium plating risk caused by the failure to replenish the electrolyte on the surface layer of the base film in time, thereby improving the lithium plating safety of the lithium-ion battery.

[0054] Exemplarily, the value of s1 can be 5%, 10%, 20%, 40%, 60%, 80%, 90% or a range composed of any two of the above numerical values. Exemplarily, the value of s2 can be 5%, 10%, 20%, 40%, 60%, 80%, 90% or a range composed of any two of the above numerical values.

[0055] In some embodiments, the first surface layer, the intermediate layer, and the second surface layer each independently comprise a polyolefin, and the polyolefin includes at least one of polyethylene, polypropylene, and ethylene-propylene copolymer.

[0056] The base film in the separator of the present application can be prepared by the following method.

[0057] Method 1: Immerse the polyolefin porous membrane in the grafting solution. Control the composition of the solvent in the grafting solution such that the mass ratio of water to acetone ranges from 1:(1 - 4), and control the concentration of the grafting monomer (relative to the mass of the solvent) to be 10% - 30%. For every 1 L of the solvent, the concentration of the photosensitizer is 0.01 - 0.1 mol / L. Irradiate with ultraviolet light for 5 - 15 min to conduct graft polymerization. After completion, take out, wash, and dry to obtain the required base membrane. The principle of Method 1 is that by controlling the relative content of water and acetone within the above range, the grafting monomer can better swell in the framework of the polyolefin porous membrane, enabling rapid and efficient graft polymerization inside the polyolefin porous membrane. On the surface layer of the polyolefin porous membrane, due to the relatively high concentration of water, it is easy to terminate the graft polymerization, thereby reducing the rate of graft polymerization on the surface layer of the polyolefin porous membrane, and further achieving a higher grafting concentration in the middle part of the polyolefin porous membrane. And controlling the concentration of the grafting monomer ≤ 30% can avoid excessive polymerization precipitation on the surface of the porous membrane during photoinitiation, which may lead to the enrichment of graft polymerization on the surface layer of the polyolefin porous membrane and cause pore blockage, affecting the lithium ion penetration rate of the base membrane.

[0058] Method 2: Blend the first polymer that is electrolyte-philic with polyolefin and a pore-forming agent (mineral oil), adjust the proportion of the first polymer in the intermediate layer and the surface layer, and prepare a laminated film by melt multi-layer co-extrusion. After immersing in an extraction liquid (such as dichloromethane, n-hexane, etc.) to remove the pore-forming agent, stretch it to prepare a base membrane with the first polymer rich in electrolyte-philic in the intermediate layer.

[0059] The separator of the present application may further include a heat-resistant layer on the surface of the base membrane. The heat-resistant layer includes inorganic particles and a binder. In some embodiments, the inorganic particles include at least one of alumina, silica, magnesia, titania, hafnium dioxide, tin dioxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. In some embodiments, the binder in the heat-resistant layer includes at least one of polyvinylidene fluoride, copolymer of vinylidene fluoride - hexafluoropropylene, polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinyl pyrrolidone, polyethylene ether, polytetrafluoroethylene, or polyhexafluoropropylene.

[0060] The separator of the present application may further include an adhesive layer on the surface of the base membrane and / or on the surface of the heat-resistant layer. In some embodiments, the binder in the adhesive layer may include at least one of polyvinylidene fluoride, copolymer of vinylidene fluoride - hexafluoropropylene, polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinyl pyrrolidone, polyethylene ether, polytetrafluoroethylene, or polyhexafluoropropylene.

[0061] In the second aspect of the present application, an electrochemical device is provided, which includes a positive electrode sheet, a negative electrode sheet, and a separator located between the positive electrode sheet and the negative electrode sheet. The separator includes the separator provided in any embodiment of the first aspect. Due to the use of the separator provided in the first aspect, the electrochemical device of the present application can have significantly improved lithium plating safety.

[0062] The electrochemical device of the present application includes, but is not limited to, a lithium-ion battery or a sodium-ion battery.

[0063] <Positive electrode sheet>

[0064] The present application has no particular limitation on the positive electrode sheet, as long as the purpose of the present application can be achieved. For example, the positive electrode sheet includes a positive electrode current collector and a positive electrode material layer provided on at least one surface of the positive electrode current collector. The present application has no particular limitation on the positive electrode current collector, as long as the purpose of the present application can be achieved. For example, the positive electrode current collector may include a metal foil, such as an aluminum foil. In the present application, the positive electrode material layer includes a positive electrode active material. The present application has no particular limitation on the type of the positive electrode active material, and any positive electrode active material known in the art can be used as long as the purpose of the present application can be achieved. For example, the positive electrode active material may include at least one of lithium cobaltate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminate, lithium iron phosphate, lithium manganate, or lithium manganese iron phosphate, etc.

[0065] The positive electrode material layer of the present application may further include a positive electrode conductive agent and a positive electrode binder. The present application has no particular limitation on the positive electrode conductive agent and the positive electrode binder, as long as the purpose of the present application can be achieved. For example, the positive electrode conductive agent may include, but is not limited to, carbon materials, metals, etc. The carbon materials may include at least one of conductive carbon black, carbon nanotubes, carbon fibers, or graphene, etc. The metal may include metal powder or metal fiber. The positive electrode binder may include at least one of polyacrylic acid, polyacrylate, acrylate polymer, polyimide, polyvinyl alcohol, carboxymethyl cellulose or its salt, or polyvinylidene fluoride.

[0066] <Negative electrode sheet>

[0067] The present application places no particular restrictions on the negative electrode sheet, as long as the object of the present application can be achieved. For example, the negative electrode sheet includes a negative electrode current collector and a negative electrode material layer provided on at least one surface of the negative electrode current collector. The present application places no particular restrictions on the negative electrode current collector, as long as the object of the present application can be achieved. For example, the negative electrode current collector may include a metal foil, such as a copper foil. In the present application, the negative electrode material layer includes a negative electrode active material. The present application places no particular restrictions on the type of the negative electrode active material, and any negative electrode active material known in the art can be used, as long as the object of the present application can be achieved. For example, the negative electrode active material may include at least one of natural graphite, artificial graphite, mesocarbon microbeads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon material, silicon-oxygen material, Li alloy, or metallic lithium, etc.

[0068] The negative electrode material layer of the present application may further include a negative electrode binder. The present application places no particular restrictions on the negative electrode binder, as long as the object of the present application can be achieved. For example, the negative electrode binder may include, but is not limited to, at least one of styrene-butadiene rubber, polyacrylic acid, polyacrylate, acrylate polymer, carboxymethyl cellulose or its salt, polyimide, or polyvinyl alcohol.

[0069] The negative electrode material layer of the present application may further include a negative electrode conductive agent. The present application places no particular restrictions on the type of the negative electrode conductive agent, as long as the object of the present application can be achieved. For example, the negative electrode conductive agent may include, but is not limited to, carbon materials, metals, etc. The carbon materials may include at least one of conductive carbon black, carbon nanotubes, carbon fibers, or graphene, etc. The metals may include metal powders or metal fibers.

[0070] <Electrolyte>

[0071] In this application, the electrochemical device further includes an electrolyte, which includes a lithium salt and a non-aqueous solvent. There is no particular limitation on the lithium salt in this application, as long as the object of this application can be achieved. For example, the lithium salt may include, but is not limited to, LiPF6. There is no particular limitation on the content of the lithium salt in the electrolyte in this application, as long as the object of this application can be achieved. There is no particular limitation on the non-aqueous solvent in this application, as long as the object of this application can be achieved. For example, the non-aqueous solvent may include, but is not limited to, at least one of carbonate compounds or carboxylate compounds. The above carbonate compounds may include, but are not limited to, at least one of linear carbonate compounds or cyclic carbonate compounds. The above linear carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, or methyl ethyl carbonate. The above cyclic carbonates may include, but are not limited to, at least one of ethylene carbonate, propylene carbonate (PC), or butylene carbonate. The above carboxylate compounds may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolide, valerolactone, or caprolactone.

[0072] In the third aspect of this application, an electrical device is provided, which includes the electrochemical device provided in the second aspect of this application. There is no particular limitation on the electrical device of this application, and it can be any electrical device known in the prior art. In some embodiments, the electrical device may include, but is not limited to, mobile phones, laptop computers, drones, Bluetooth headsets, smart watches, electric vehicles, etc.

[0073] The technical solutions of this application will be described in more detail below with specific examples and comparative examples.

[0074] Example 1

[0075] 1. <Preparation of the separator>:

[0076] Step 1. Preparation of the base film: Select a polyethylene-polypropylene blend porous film with an average thickness of 4.5 μm, a porosity of 41%, and a Gurley value of 79 s / 100 cc for air permeability; in a mixed solvent with a mass ratio of water to acetone of 1:3, for every 1 L of the above mixed solvent, add 0.06 mol of the photosensitizer benzophenone (BP); relative to the mass of the mixed solvent, add acrylonitrile monomer with a mass fraction of 20% to prepare a grafting solution. Immerse the polyethylene-polypropylene blend porous film in the above grafting solution and irradiate it with ultraviolet light for 7 min to obtain the required base film. Before grafting, the areal density of the polyethylene-polypropylene blend porous film is 11.5 g / m 2 ; after grafting, the areal density of the obtained base film is 14.6 g / m 2 ; the acrylonitrile grafting rate is (14.6 - 11.5) / 14.6 = 21%.

[0077] Step 2: After mixing alumina and the binder polyvinylidene fluoride in a mass ratio of 85:15, add deionized water for dispersion to obtain the heat-resistant layer slurry; add the binder polyvinylidene fluoride to deionized water and stir to obtain the binder layer slurry with a solid content of 12%.

[0078] Step 3: After coating the heat-resistant layer slurry on one side surface of the base film, dry it. Among them, the thickness of the heat-resistant layer is 2 μm.

[0079] Step 4: Uniformly coat the binder layer slurry on the surface of the heat-resistant layer and the other side surface of the base film. The coating surface density of the single-sided binder layer is 1 g / m 2 , dry to obtain the required separator.

[0080] 2. <Preparation of the positive electrode sheet>

[0081] Add the positive electrode active material lithium cobaltate, the conductive agent Super P, and the binder polyvinylidene fluoride in a weight ratio of 96:2.5:1.5 to N-methylpyrrolidone (NMP), stir evenly to obtain the positive electrode slurry; uniformly coat the positive electrode slurry on one side surface of the positive electrode current collector aluminum foil, and repeat the above steps on the other side surface of the aluminum foil after drying to obtain a positive electrode sheet with a positive electrode material layer coated on both sides, and then obtain the positive electrode sheet after cold pressing and cutting.

[0082] 3. <Preparation of the negative electrode sheet>

[0083] Add the negative electrode active material artificial graphite, sodium carboxymethylcellulose, and styrene-butadiene rubber in a weight ratio of 96:2:2 to deionized water, stir evenly to obtain the negative electrode slurry; uniformly coat the negative electrode slurry on one side surface of the negative electrode current collector copper foil, and repeat the above steps on the other side surface of the copper foil after drying to obtain a negative electrode sheet with a negative electrode material layer coated on both sides, and then obtain the negative electrode sheet after cold pressing and cutting.

[0084] 4. <Assembly of the lithium-ion battery>

[0085] Stack the positive electrode sheet, the separator, and the negative electrode sheet in sequence, so that the separator is located between the positive and negative electrode sheets to form an electrode assembly with a laminated structure, where the heat-resistant layer side of the separator faces the positive electrode sheet. After welding the electrode tabs, place them in the outer packaging aluminum-plastic film, inject the electrolyte, and obtain the lithium-ion battery after vacuum packaging, standing, and formation.

[0086] Examples 2 to 5

[0087] The difference from Example 1 lies in that in the <Preparation of diaphragm> step, the mass of acrylonitrile monomer relative to the mixed solvent in the grafting solution is adjusted to be 10%, 15%, 25%, and 30% respectively, so as to realize the regulation of the grafting amount in the middle part of the base film. The rest is the same as Example 1.

[0088] Examples 6 to 8

[0089] The difference from Example 1 lies in that in the <Preparation of diaphragm> step, the composition of the mixed solvent is adjusted to: the mass ratio of water to acetone is 1:1, 1:2, and 1:4 respectively. The rest is the same as Example 1.

[0090] Examples 9 to 13

[0091] The difference from Example 1 lies in that in the <Preparation of diaphragm> step, ethyl acrylate monomer is used to replace acrylonitrile monomer, and the addition amount of ethyl acrylate monomer relative to the mass of the mixed solvent is 10%, 15%, 20%, 25%, and 30% respectively. The rest is the same as Example 1.

[0092] Example 14

[0093] The difference from Example 1 lies in that the base film in this example includes a first surface layer, an intermediate layer, and a second surface layer, and the intermediate layer is located between the first surface layer and the second surface layer.

[0094] The preparation of the base film in this example is as follows: the mixtures of the intermediate layer, the first surface layer, and the second surface layer are respectively prepared. Among them, the composition of the skeleton resin in the mixture of the intermediate layer is polyethylene and ethylene-vinyl acetate copolymer (the mass percentage content of vinyl acetate is about 15%) with a mass ratio of 1:4, and the mass ratio of the skeleton resin to the pore-forming agent mineral oil is 1:7; the composition of the skeleton resin in the mixtures of the first surface layer and the second surface layer is the above-mentioned polyethylene and ethylene-vinyl acetate copolymer with a mass ratio of 1:2, and the mass ratio of the skeleton resin to the pore-forming agent mineral oil is 1:7. After forming a laminated film by melt multi-layer co-extrusion and extracting the pore-forming agent with an extraction solution, the required base film is obtained by stretching. Among them, the average thickness of the first surface layer and the second surface layer is 1.5 μm, and the average thickness of the intermediate layer is 3 μm. The rest is the same as Example 1.

[0095] Examples 15 to 16

[0096] The difference from Example 14 lies in that in the composition of the skeleton resin in the mixture of the intermediate layer, the mass ratio of polyethylene and ethylene-vinyl acetate copolymer is 1:8 and 1:14 respectively. In the composition of the skeleton resin in the mixture of the surface layer, the mass ratio of polyethylene and ethylene-vinyl acetate copolymer is 1:5 and 1:8 respectively. The rest is the same as Example 14.

[0097] Example 17

[0098] The difference from Example 16 is that ethylene-ethyl acrylate copolymer (the mass percentage content of ethyl acrylate is about 16%) is used to replace ethylene-vinyl acetate copolymer. The rest is the same as Example 16.

[0099] Example 18

[0100] The difference from Example 16 is that ethylene-methyl acrylate copolymer (the mass percentage content of methyl acrylate is about 20%) is used to replace ethylene-vinyl acetate copolymer, and the rest is the same as Example 16.

[0101] Example 19

[0102] The difference from Example 16 is that ethylene-butyl acrylate copolymer (the mass percentage content of butyl acrylate is about 35%) is used to replace ethylene-vinyl acetate copolymer, and in the composition of the skeleton resin in the mixture of the surface layer, the mass ratio of polyethylene to ethylene-butyl acrylate copolymer is 1:3 respectively. The rest is the same as Example 16.

[0103] Comparative Example 1

[0104] The difference from Example 1 is that an ungrafted polyethylene-polypropylene blend porous membrane is used as the base membrane. The rest is the same as Example 1.

[0105] Comparative Example 2

[0106] The difference from Example 1 is that in the <preparation of the diaphragm> step, the mass ratio of water to acetone is adjusted to 1:9 as the mixed solvent, and the rest is the same as Example 1.

[0107] Comparative Example 3

[0108] The difference from Example 14 is that in the preparation of the base membrane, the compositions of the intermediate layer and the surface layer of the base membrane are exchanged.

[0109] Testing method

[0110] Testing of the concentrations of N and O: Use an ion polishing instrument CP to prepare the cross-section of the base membrane, observe the cross-section of the base membrane under a scanning electron microscope, select the regions within 1 / 4 of its thickness H from the surface of the base membrane and the regions more than 1 / 3 of its thickness H from both surfaces of the base membrane respectively, conduct EDX testing, select 3 places in each region for testing, and take the average value as the molar concentrations c1 and c2 of N element and O element relative to C element in the first region and the second region.

[0111] Infiltration test: At 25°C, a 10-cm long and 8-mm wide base film sample is horizontally straightened along the length direction of the base film sample and placed on the first and second sample platforms with a 7-cm interval; along the length direction of the base film sample, the width of the first sample platform is 1 cm; the length of the first end of the base film sample exceeding the first sample platform is 2 cm; fix the separator sample and the second sample platform; at a first reference line 2 cm away from the first end, use a 1-cm wide first Teflon tape to bond the surface of the base film sample facing away from the first sample platform and the first surfaces of the first sample platforms on both sides in the width direction of the base film sample, where the first side edge in the width direction of the first Teflon tape is aligned with the first reference line, and the second side edge in the width direction of the first Teflon tape is farther from the first end relative to the first side edge (i.e., the width of the first Teflon tape coincides with the width of the first sample platform); the tail section from the first end of the base film sample to the first reference line extends vertically in the horizontal direction and extends into the electrolyte pool. When the electrolyte is added to the electrolyte pool until the liquid level is 1.5 cm higher than the first end, start timing. At 30 min, record the length exceeding the second side edge of the first Teflon tape infiltrated by the electrolyte as the infiltration length L of the base film; where the composition of the electrolyte is an organic solvent dissolved with lithium hexafluorophosphate, and the composition of the organic solvent is ethylene carbonate, propylene carbonate, and diethyl carbonate with a mass ratio of 1:1:2. Based on the mass of the electrolyte, the mass percentage content of lithium hexafluorophosphate is 12.5%.

[0112] Electrolyte contact angle test: Use a contact angle measuring instrument for testing. Drop an appropriate amount of electrolyte on the base film, take a microscope photo, and then measure the contact angle θ between the base film and the electrolyte.

[0113] Among them, the following method is used to prepare the electrolyte: Under an argon atmosphere with a water content less than 10 ppm, mix ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) evenly according to a mass ratio of 1:1:2, and add the lithium salt lithium hexafluorophosphate and mix evenly to obtain the electrolyte. Among them, based on the mass of the electrolyte, the mass percentage content of lithium hexafluorophosphate is 12.5%.

[0114] Air permeability T test: Use a Gurley air permeability instrument to test the air permeability of the base film, where the air column volume of the air permeability instrument is 100 cm 3 , the test area of the base film is 6.45 cm 2 , during the test, keep the base film flat, repeat the test 3 times, and take the average value as the final air permeability value.

[0115] Test for the presence or absence of infrared characteristic absorption peaks: Use an infrared spectrometer to perform an infrared test on the base film sample to determine whether there are infrared characteristic absorption peaks at 2200 cm -1 -2280 cm-1 Range (corresponding to the -CN absorption peak), 1700 cm -1 -1780 cm -1 Whether there is a characteristic absorption peak in at least one of the ranges (corresponding to the ester group C=O absorption peak).

[0116] Lithium plating test: Let the lithium-ion battery stand still at 45°C for 30 min to make the lithium-ion battery reach a constant temperature state. Charge it at a constant current of 4C until the voltage reaches 4.5V, then charge it at a constant voltage until the current reaches 0.05C to make the lithium-ion battery reach a fully charged state. Then discharge it at a constant current of 4C until the voltage reaches 3.0V. Repeat the above steps 100 times. Then let the lithium-ion battery stand still in an environment of 0°C for 30 min to make the lithium-ion battery reach a constant temperature state. Charge it at a constant current of 4C until the voltage reaches 4.5V, then charge it at a constant voltage until the current reaches 0.05C to make the lithium-ion battery reach a fully charged state. Disassemble the lithium-ion battery and observe the severity of lithium plating on the negative electrode surface. Measure the ratio α of the lithium plating area of the negative electrode sheet to the area of the negative electrode active layer. α>20% is defined as severe lithium plating, 10%<α≤20% is defined as moderate lithium plating, 5%<α≤10% is defined as mild lithium plating, and α≤5% is defined as no lithium plating.

[0117] See Table 1 for the experimental test results of Examples 2 to 19 and Comparative Examples 1 to 3.

[0118] Table 1

[0119]

[0120]

[0121] Result analysis: Through the comprehensive comparison of Examples 1 to 19 and Comparative Examples 1 to 3, it can be found that when c2≥1.05c1 is satisfied, the lithium plating safety of the lithium-ion battery is significantly improved. Compared with Comparative Examples 2 to 3, the reason for this improvement may be that during high-temperature cycling, the gas generated by side reactions in the lithium-ion battery is likely to cause the lack of local electrolyte in the separator base film. When the middle second region contains more electrolyte-philic groups, the diffusion rate of the electrolyte inside the base film is relatively fast, which can preferentially replenish the pores formed by gas generation, preventing the electrolyte from first filling the pores on the surface layer of the base film and making it difficult for the gas pores inside the base film to be replenished by the electrolyte, thereby improving the lithium plating phenomenon of the lithium-ion battery during subsequent charging. Further analysis of the data of Examples 1 to 19 shows that when c2≥6.1% and c1≥4.5% are further satisfied, the lithium plating safety of the lithium-ion battery can be further significantly improved. This is because satisfying c2≥6.1% can further increase the diffusion rate of the electrolyte inside the base film, and at the same time satisfying c1≥4.5% can enhance the diffusion rate of the electrolyte from the inside of the base film to the surface layer, so as to timely fill the pores formed by gas generation during high-temperature cycling, thereby effectively improving the lithium plating safety of the battery.

[0122] The above are only the preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, and improvements made within the principles of the present application shall be included in the protection scope of the present application.

Claims

1. A diaphragm, comprising a base film; the base film has a thickness of H, and a cross section of the base film along its thickness direction comprises a first region within H / 4 from the base film surface, and a second region at a distance of more than H / 3 from the base film surface; characterized in that: Based on the molar amount of the C element in the first region, the total molar percentage of the N element and the O element in the first region is c1; Based on the molar amount of the C element in the second region, the total molar percentage of the N element and the O element in the second region is c2; satisfying: c2 / c1≥1.

05.

2. The diaphragm according to claim 1, characterized in that The base film satisfies at least one of the following conditions: (1) 1.1≤c2 / c1≤10; (2)4.6％≤c1≤15％； (3)6.1％≤c2≤20％。 3. The diaphragm according to claim 1, characterized in that The infiltration length of the electrolyte to the base film is L, satisfying L≥10mm; wherein the electrolyte is an organic solvent containing lithium hexafluorophosphate, the composition of the organic solvent is ethylene carbonate, propylene carbonate and diethyl carbonate in a mass ratio of 1:1:2, and based on the mass of the electrolyte, the mass percentage of the lithium hexafluorophosphate is 12.5%.

4. The diaphragm according to claim 1, characterized in that The contact angle θ of the electrolyte on the surface of the base film is ≤10°. The electrolyte is an organic solvent in which lithium hexafluorophosphate is dissolved. The composition of the organic solvent is ethylene carbonate, propylene carbonate and diethyl carbonate in a mass ratio of 1:1:

2. Based on the mass of the electrolyte, the mass percentage of the lithium hexafluorophosphate is 12.5%.

5. The diaphragm according to claim 1, characterized in that At least one of the following conditions is met: (1) The air permeability T of the base film satisfies: 50s / 100cc≤T≤150s / 100cc; (2) The infrared spectrum of the base film is at 2200 cm -1 Up to 2280cm -1 Wave number range, 1700cm -1 Up to 1780cm -1 An absorption peak exists in at least one of the wavenumber ranges; (3) The base film comprises polyolefin grafted with acrylonitrile and / or acrylate monomers, and the polyolefin comprises at least one of polyethylene, polypropylene, and ethylene-propylene copolymer.

6. The diaphragm according to claim 1, characterized in that The base film comprises a first surface layer, an intermediate layer and a second surface layer, wherein the intermediate layer is located between the first surface layer and the second surface layer, and the first surface layer, the intermediate layer and the second surface layer each independently comprise a first polymer, wherein the first polymer comprises at least one of acrylonitrile polymer, vinyl acetate polymer, maleic anhydride grafted polymer, acrylate polymer, acrylate polymer, polyether, polyamide and polyimide; Based on the mass of the intermediate layer, the mass percentage of the first polymer in the intermediate layer is m1; Based on the mass of the first surface layer, the mass percentage of the first polymer in the first surface layer is s1; Based on the mass of the second surface layer, the mass percentage of the first polymer in the second surface layer is s2; Satisfies: m1>s1, m1>s2.

7. The diaphragm according to claim 6, characterized in that At least one of the following conditions is met: (1) m1 / s1 ≥ 1.1; (2) m1 / s2 ≥ 1.1; (3)m1≥10％； (4)s1≥5％； (5)s2≥5％。 8. The diaphragm according to claim 6, characterized in that The first surface layer, the middle layer and the second surface layer each independently include polyolefin, and the polyolefin includes at least one of polyethylene, polypropylene and ethylene-propylene copolymer.

9. An electrochemical device, characterized in that: It comprises a positive electrode sheet, a negative electrode sheet and a separator located between the positive electrode sheet and the negative electrode sheet, wherein the separator comprises the separator according to any one of claims 1 to 8.

10. An electrical device, characterized in that: An electrochemical device comprising the electrochemical device of claim 9.

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