A secondary battery based on a gradient-cured polymer electrolyte and a method of manufacturing the same

Abstract

A secondary battery based on a gradient-cured polymer electrolyte and its preparation method are disclosed, relating to the field of electrochemical energy device technology. The secondary battery includes a bare cell, a battery casing, and a polymer electrolyte. The bare cell is installed inside the battery casing, and the polymer electrolyte fills the space between the bare cell and the battery casing. The polymer electrolyte includes a first electrolyte layer, a gradient interface layer, and a second electrolyte layer with increasing crosslinking density from the inside out. The first electrolyte layer is impregnated inside the bare cell, and the second electrolyte layer fills the space between the gradient interface layer and the inner wall of the battery casing. The battery prepared by this invention simultaneously possesses high interfacial ion transport efficiency and bulk mechanical suppression strength, exhibiting high ionic conductivity, high rate performance, and long cycle life. Furthermore, it effectively suppresses cell deformation, significantly improving safety.

Description

Technical Field

[0001] This application relates to the field of electrochemical energy device technology, and in particular to a secondary battery based on gradient-cured polymer electrolyte and its preparation method. Background Technology

[0002] Polymer electrolyte batteries are a type of battery that uses solid or gel-state polymers as electrolytes. This is an important development direction following traditional liquid electrolyte batteries, and it can fundamentally solve safety issues and break through the energy density bottleneck.

[0003] Existing polymer electrolyte batteries mainly consist of homogeneous solid or gel-state polymer electrolytes, typically formed through a single-stage impregnation and in-situ polymerization reaction. For example, patent application CN118507850A proposes a method for preparing a gel polymer solid electrolyte battery through in-situ polymerization. The gel monomer is polyethylene glycol diacrylate. A precursor solution is injected into the battery casing, and the battery is placed in an oven for in-situ polymerization to obtain the gel polymer solid electrolyte battery. This in-situ polymerization results in a tight contact between the electrolyte and the electrodes, effectively reducing the impedance between the electrolyte material and the positive and negative electrode interfaces, and exhibiting high ionic conductivity. However, the electrode material, especially the negative electrode, undergoes repeated volume changes during cycling, generating micro-stress. Long-term accumulation not only damages the electrode interface but also leads to macroscopic deformation of the entire cell (such as bulging), affecting the overall lifespan and safety of the battery pack. Therefore, it is necessary to develop a polymer electrolyte battery that achieves a balance between interfacial ion transport efficiency and bulk mechanical inhibition strength. Summary of the Invention

[0004] The purpose of this invention is to overcome the defects and shortcomings of the prior art and provide a secondary battery based on gradient-cured polymer electrolyte, which has both high interfacial ion transport efficiency and bulk mechanical suppression strength, high ionic conductivity, high rate performance and long cycle life, and can effectively suppress cell deformation and greatly improve safety.

[0005] Another objective of this invention is to provide a method for preparing a secondary battery based on a gradient-cured polymer electrolyte.

[0006] The above-mentioned objective of this invention is achieved through the following technical solution: This invention protects a secondary battery based on a gradient-cured polymer electrolyte, comprising a bare cell, a battery casing, and a polymer electrolyte. The bare cell is installed inside the battery casing, and the polymer electrolyte fills the interior of the bare cell and the space between the bare cell and the battery casing. The polymer electrolyte includes a first electrolyte layer, a gradient interface layer, and a second electrolyte layer with increasing crosslink density from the inside to the outside of the bare cell. The polymer matrix of the polymer electrolyte is a (meth)acrylate polymer. The first electrolyte layer is located inside and on the surface of the bare cell, and the average molecular weight (Mc) between the crosslinking points of the first electrolyte layer is ≥3000; The gradient interface layer is a composite layer formed by the interface between the first electrolyte layer and the second electrolyte layer. The second electrolyte layer fills the space between the gradient interface layer and the inner wall of the battery casing, and the average molecular weight between the crosslinking points of the first electrolyte layer is ≤2500.

[0007] In the secondary battery of the present invention, the bare cell uses a first electrolyte layer with a low cross-linking density, which is beneficial for wetting the electrode. The low viscosity of the monomer ensures that it fully penetrates the electrode pores and forms an excellent ion transport channel. At the same time, the low modulus can better adapt to the volume change of the electrode during charging and discharging, buffer the interfacial stress, and form a stable SEI / CEI film. Its low polymer content reduces the hindrance to ion migration.

[0008] Secondly, the gradient interface layer is an interpenetrating network structure combining the first electrolyte layer and the second electrolyte layer. The average molecular weight between its cross-linking points is between the first electrolyte layer and the second electrolyte layer, thereby forming a gradient transition of mechanical properties between the flexible inner layer and the rigid outer layer, effectively dispersing and absorbing stress.

[0009] Furthermore, the second electrolyte layer, located on the inner wall of the casing, has a high cross-linking density, providing a safety barrier. It possesses excellent mechanical strength and thermal stability, effectively suppressing sodium dendrite penetration and preventing internal short circuits. Simultaneously, as a robust barrier, it can prevent or delay the spread of flame and oxygen in the early stages of thermal runaway, enhancing the battery's intrinsic safety. Its high-modulus outer layer acts like a "skeleton," resisting macroscopic deformation caused by accumulated internal stress during long-term storage and cycling.

[0010] In some embodiments, the average molecular weight between the crosslinking points of the first electrolyte layer is 4000-5500.

[0011] In some embodiments, the average molecular weight between the crosslinking points of the second electrolyte layer is 800-2000.

[0012] In some embodiments, the first electrolyte layer and the gradient interface layer are formed by in-situ polymerization of the first electrolyte precursor. The first electrolyte precursor comprises: 1-15 wt% of a first polymerizable monomer, 0.1-1 wt% of a first polymerization initiator, and the balance being a first electrolyte solution; The acrylate group in the first polymerizable monomer has a functionality of 1-2.

[0013] In some embodiments, the mass content of the first polymerizable monomer that can achieve the purpose of the present invention is any one or a range between two of 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, and 15%.

[0014] In some embodiments, the first polymerizable monomer is selected from at least one of polyethylene glycol diacrylate (PEGDA), polyethylene glycol methacrylate (PEGMA), methoxy polyethylene glycol methacrylate (mPEGA), methoxy polyethylene glycol methacrylate (mPEGMA), tridecylfluorooctyl methacrylate, and hexafluorobutyl acrylate.

[0015] In some embodiments, the first polymerization initiator is a photoinitiator and / or a thermal initiator; Preferably, the photoinitiator is selected from at least one of acylphosphine oxide, α-hydroxy ketone (Irgacure 184), and acylphosphine oxide (Irgacure TPO) initiators.

[0016] Specifically, the acylphosphine oxide is selected from phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide (Omnirad 819), the α-hydroxy ketone initiator is selected from Irgacure 184, and the acylphosphine oxide initiator is selected from Irgacure TPO.

[0017] In some embodiments, the second electrolyte layer is formed by in-situ polymerization of a second electrolyte precursor; the second electrolyte precursor includes: 12-40 wt% of a second polymerizable monomer, 0-16 wt% of a functional monomer, 0.1-2 wt% of a second polymerization initiator, and the balance being a second electrolyte solution. The functionality of the acrylate groups in the second polymerizable monomer is ≥3; The functional monomer contains at least one acrylate group.

[0018] In some embodiments, the mass content of the second polymerizable monomer that enables the present invention to be achieved is any one or a range between two of the following: 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, and 40%.

[0019] In some embodiments, the second polymerizable monomer is selected from at least one of pentaerythritol tetraacrylate (PETA), dipentaerythritol penta / hexaacrylate (DPEPA), ethoxylated pentaerythritol tetraacrylate (PPTTA), trimethylolpropane triacrylate (TMPTA), and ethoxylated trimethylolpropane triacrylate (TMP(EO)TA).

[0020] In some embodiments, the functional monomer is selected from polyether acrylate oligomers, polyurethane acrylate oligomers, fluorinated polyurethane acrylate oligomers, and dicationic ionic liquid monomers. Specifically, the functional monomer is selected from polyethylene glycol diacrylate (PEGDA), aromatic polyurethane acrylate, 1H,1H,2H,2H-perfluorooctyl acrylate, fluorinated polyurethane acrylate, and 1-acryloyloxyethyl-3-methylimidazolium bis(fluorosulfonyl)imide (AcOEMImFSI).

[0021] In some embodiments, the second polymerization initiator is a thermal initiator; preferably, the thermal initiator is selected from organic peroxides and / or azo compounds; more preferably, the thermal initiator is selected from at least one of benzoyl peroxide (BPO), tert-butyl peroxide (TBPB), di(4-tert-butylcyclohexyl) peroxydicarbonate, 2,2'-azobisisobutyronitrile (AIBN), 1,1'-azobis(cyclohexane-1-carboxynitrile) (V-40), and dimethyl azobisisobutyronitrile (AIBME).

[0022] In some embodiments, the solute in the second electrolyte solution is at least one of NaPF6, LiPF6, NaClO4, and NaFSI at a concentration of 0.8-1.2M, and the solvent is ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), or at least one of the above-mentioned fluorinated derivatives. Preferably, it is any one of EC / PC, EC / DEC, EC / PC / DEC, or TFEP / FEC.

[0023] In some embodiments, the solute in the second electrolyte solution is at least one of NaPF6, LiPF6, NaClO4, and NaFSI at a concentration of 0.8-1.2M, and the solvent is ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), or at least one of the above-mentioned fluorinated derivatives. Preferably, it is any one of EC / PC, EC / DEC, EC / PC / DEC, or TFEP / FEC.

[0024] This invention protects a method for preparing a secondary battery based on a gradient-cured polymer electrolyte, comprising the following steps: S1, First electrolyte injection: The bare cell is placed inside the battery casing, and a first electrolyte precursor is injected into the battery casing. The first electrolyte precursor includes: 1-15 wt% of a first polymerizable monomer, 0.1-1 wt% of a first polymerization initiator, and the remainder is a first electrolyte solution. S2, First stage polymerization reaction: The first electrolyte precursor is induced to undergo a polymerization reaction by external stimulation, and a gradient-cured first electrolyte layer is formed inside and on the surface of the bare cell respectively; S3, Second electrolyte injection: Inject a second electrolyte precursor into the battery casing after step S2. The mass ratio of the second electrolyte precursor to the first electrolyte precursor is (1-3):(7-9). The second electrolyte precursor includes: 12-40wt% of a second polymerizable monomer, 0-16wt% of a functional monomer, 0.1-2wt% of a second polymerization initiator, and the remainder is a second electrolyte solution. S4, Second stage polymerization reaction: The second electrolyte precursor is induced to undergo polymerization reaction by external stimulation to form a second polymer electrolyte. After sealing and formation treatment, the secondary battery based on gradient solidification polymer electrolyte is obtained.

[0025] In the preparation method of the present invention, a first electrolyte precursor and a second electrolyte precursor are sequentially injected for stepwise wetting and controlled polymerization reaction; the precursors are polymerized in situ from the inside to the outside inside the battery cell to form a three-dimensional network polymer electrolyte with mechanical property gradient.

[0026] Preferably, the mass ratio of the second electrolyte precursor to the first electrolyte precursor is (1.8-2.2):(7.8-8.2).

[0027] In some embodiments, the first-stage polymerization reaction in step S2 is a photoinduced reaction, and the reaction conditions are: 365nm UV light irradiation for 15-30 minutes. In some embodiments, the second-stage polymerization reaction in step S4 is a thermally induced reaction, and the reaction conditions are: heating and curing at 70-95°C for 4-8 hours.

[0028] In some embodiments, the positive electrode material in the bare battery cell is a nickel-cobalt-manganese ternary metal oxide, lithium cobalt oxide, layered oxide sodium electrode positive electrode material, sodium iron pyrophosphate (NFPP), Prussian blue / white, sodium iron sulfate, lithium iron phosphate, lithium manganese oxide, and the negative electrode material is graphite, hard carbon, soft carbon, mesophase carbon microspheres, silicon carbide, or graphene.

[0029] Compared with the prior art, the beneficial effects of the present invention are: The present invention discloses a secondary battery based on gradient-cured polymer electrolyte, which uses a first electrolyte layer with increasing cross-linking density from the inside to the outside, a gradient interface layer and a second electrolyte layer to form a three-dimensional network polymer electrolyte with a mechanical property gradient. It can simultaneously have high interfacial ion transport efficiency and bulk mechanical suppression strength, and has high ionic conductivity, high rate performance and long cycle life. It can also effectively suppress cell deformation and significantly improve safety. Detailed Implementation

[0030] The present invention will be further described below with reference to specific embodiments, but the embodiments do not limit the present invention in any way. Unless otherwise stated, the raw materials and reagents used in the embodiments of the present invention are conventionally purchased raw materials and reagents.

[0031] Example 1 A secondary battery based on gradient-cured polymer electrolyte includes a bare cell, a battery casing, and a polymer electrolyte. The bare cell is installed inside the battery casing, and the space between the bare cell and the battery casing is filled with the polymer electrolyte. The polymer electrolyte comprises a first electrolyte layer, a gradient interface layer, and a second electrolyte layer, wherein the crosslinking density of the polymer matrix increases gradually from the inside to the outside. The polymer matrix of the first electrolyte layer is formed by in-situ polymerization of a first polymerizable monomer, which is selected from methoxy polyethylene glycol acrylate (mPEGA, Mn=480); The polymer matrix of the second electrolyte layer is formed by in-situ polymerization of a polymerizable monomer, wherein the second polymerizable monomer is selected from trimethylolpropane triacrylate (TMPTA).

[0032] A method for preparing a secondary battery based on a gradient-cured polymer electrolyte includes the following steps: S1. Preparation of bare cells: The positive electrode, negative electrode, and separator are made into bare cells through winding or stacking processes and placed into the battery casing; the positive electrode is NFPP and the negative electrode is hard ink; S2. First injection: Inject a first electrolyte precursor into the shell, the first electrolyte precursor comprising: 1M NaPF6 EC / PC (v / v, 1:1) liquid electrolyte, 4wt% first polymerizable monomer and 0.5wt% first polymerization initiator; The first polymerizable monomer is selected from methoxy polyethylene glycol acrylate (mPEGA, Mn=480), and the first polymerization initiator is selected from photoinitiator phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide (Omnirad819).

[0033] S3, First stage polymerization reaction: Irradiate with 365nm UV light for 20min to cause the first electrolyte precursor to undergo polymerization reaction and form an ion transport framework; S4, Second electrolyte injection: Inject a second electrolyte precursor into the battery casing after step S2. The mass ratio of the second electrolyte precursor to the first electrolyte precursor is 2:8. The second electrolyte precursor contains: 1M NaPF6 EC / PC (v / v, 1:1) liquid electrolyte, 30wt% of a second polymerizable monomer, and 1wt% of a second polymerization initiator. The second polymerizable monomer is selected from trimethylolpropane triacrylate (TMPTA), and the second polymerization initiator is selected from the thermal initiator 2,2'-azobisisobutyronitrile (AIBN).

[0034] S5. Second stage polymerization reaction: Heating and curing at 75°C for 6 hours induces the polymerization reaction of the second electrolyte precursor and promotes the unreacted monomers in S3 to continue to react, ultimately forming the three-dimensional gradient polymer electrolyte structure inside the battery. S6. Sealing and Formation: The battery is sealed and a standardized formation process is performed.

[0035] Example 2 A secondary battery based on a gradient-cured polymer electrolyte differs from Example 1 in that: in the preparation method of this example, the second electrolyte precursor comprises: 1M NaPF6 EC / PC (v / v, 1:1) liquid electrolyte, 25wt% of a second polymerizable monomer, and 0.5wt% of a second polymerization initiator; The second polymerizable monomer is selected from trimethylolpropane triacrylate (TMPTA), and the second polymerization initiator is selected from the thermal initiator dimethyl azobisisobutyrate (AIBME).

[0036] The conditions for the second stage polymerization reaction are: heating and curing at 80°C for 5 hours.

[0037] Example 3 A secondary battery based on gradient-cured polymer electrolyte differs from Example 1 in that: in the preparation method of this example, the second electrolyte precursor comprises: 1M NaPF6 EC / PC (v / v, 1:1) liquid electrolyte, 30wt% of a second polymerizable monomer, and 1wt% of a second polymerization initiator; The second polymerizable monomer is selected from trimethylolpropane triacrylate (TMPTA), and the second polymerization initiator is selected from the thermal initiator dimethyl azobisisobutyrate (AIBME).

[0038] The conditions for the second stage polymerization reaction are: heating and curing at 80°C for 5 hours.

[0039] Example 4 A secondary battery based on gradient-cured polymer electrolyte differs from Example 1 in that: in the preparation method of this example, the second electrolyte precursor comprises: 1M NaPF6 EC / PC (v / v, 1:1) liquid electrolyte, 25wt% of a second polymerizable monomer, 5wt% of a functional monomer, and 1wt% of a second polymerization initiator; The second polymerizable monomer is selected from ethoxylated trimethylolpropane triacrylate (TMP(EO)TA), the functional monomer is selected from 1H,1H,2H,2H-perfluorooctyl acrylate (THFA), and the second polymerization initiator is selected from thermal initiator 1,1'-azobis(cyclohexane-1-carboxynitrile) (V-40, high-temperature initiator).

[0040] The conditions for the second stage polymerization reaction are: heating and curing at 90°C for 4 hours.

[0041] Example 5 A secondary battery based on gradient-cured polymer electrolyte differs from Example 1 in that: in the preparation method of this example, the second electrolyte precursor comprises: 1M NaFS EC / PC (v / v, 1:1) liquid electrolyte, 15wt% of a second polymerizable monomer, 15wt% of a functional monomer, and 1.2wt% of a second polymerization initiator; The second polymerizable monomer is selected from pentaerythritol tetraacrylate (PETA), the functional monomer is selected from polyethylene glycol diacrylate (PEGDA-400), and the second polymerization initiator is selected from the thermal initiator dimethyl azobisisobutyrate (AIBME).

[0042] The conditions for the second stage polymerization reaction are: heating and curing at 75°C for 7 hours.

[0043] Example 6 A secondary battery based on gradient-cured polymer electrolyte differs from Example 1 in that: in the preparation method of this example, the first electrolyte precursor contains: 1M NaFSI EC / DMC (v / v, 1:1) liquid electrolyte, 8wt% of the first polymerizable monomer and 0.8wt% of the first polymerization initiator; Wherein, the first polymerizable monomer is selected from 1-acryloyloxyethyl-3-methylimidazolium bis(fluorosulfonyl)imide (AcOEMImFSI, ionic liquid monomer), and the first polymerization initiator is selected from photoinitiators acylphosphine oxides (IrgacureTPO).

[0044] The conditions for the first stage of polymerization reaction were: irradiation with 365nm UV light for 15 minutes.

[0045] The second electrolyte precursor comprises: 1M NaFSI EC / DMC (v / v, 1:1) liquid electrolyte, 25wt% of a second polymerizable monomer, and 1wt% of a second polymerization initiator; The second polymerizable monomer is selected from ethoxylated trimethylolpropane triacrylate (TMP(EO)TA), and the second polymerization initiator is selected from thermal initiator 1,1'-azobis(cyclohexane-1-carboxynitrile) (V-40).

[0046] The conditions for the second stage polymerization reaction are: heating and curing at 85°C for 5 hours.

[0047] Example 7 A secondary battery based on gradient-cured polymer electrolyte differs from Example 1 in that: in the preparation method of this example, the first electrolyte precursor contains: 1M NaPF6 TFEP / FEC (v / v, 7:3) liquid electrolyte, 12wt% first polymerizable monomer and 0.5wt% first polymerization initiator; The first polymerizable monomer is selected from ethyl methacrylate (EPMA), and the first polymerization initiator is selected from α-hydroxy ketone photoinitiators (Irgacure 184).

[0048] The conditions for the first stage of polymerization reaction were: irradiation with 365nm UV light for 25 minutes.

[0049] The second electrolyte precursor comprises: a 1M NaPF6 TFEP / FEC (v / v, 7:3) liquid electrolyte, 10wt% of a second polymerizable monomer, 15wt% of a functional monomer, and 1.5wt% of a second polymerization initiator; The second polymerizable monomer is selected from dipentaerythritol pentaacrylate (DPEPA), the functional monomer is selected from fluorinated polyurethane diacrylate (F-PUA), and the second polymerization initiator is selected from the thermal initiator 2,2'-azobisisobutyronitrile (AIBN).

[0050] The conditions for the second stage polymerization reaction were: heating at 82°C for 6 hours.

[0051] Example 8 A secondary battery based on gradient-cured polymer electrolyte differs from Example 1 in that: in the preparation method of this example, the first electrolyte precursor includes: 1M NaClO4 EC / PC (v / v, 1:1) liquid electrolyte, 10wt% first polymerizable monomer, 1wt% first polymerization initiator and 2wt% co-initiator; The first polymerizable monomer is selected from polyethylene glycol diacrylate (PEGDA-200), the first polymerization initiator is selected from photoinitiator benzophenone, and the co-initiator is selected from methyl diethanolamine.

[0052] The conditions for the first stage of polymerization reaction were: irradiation with 365nm UV light for 10 minutes.

[0053] The second electrolyte precursor comprises: 1M NaClO4 in EC / PC (v / v, 1:1) liquid electrolyte, 35wt% of a second polymerizable monomer, and 2.0wt% of a second polymerization initiator; The second polymerizable monomer is selected from trimethylolpropane triacrylate (TMPTA), and the second polymerization initiator is selected from the thermal initiator benzoyl peroxide (BPO).

[0054] The conditions for the second stage polymerization reaction were: heating at 70°C for 8 hours.

[0055] Example 9 A secondary battery based on a gradient-cured polymer electrolyte differs from Example 1 in that: in the preparation method of this example, the second electrolyte precursor comprises: 1M NaPF6 EC / PC (v / v, 1:1) liquid electrolyte, 20wt% of a second polymerizable monomer, and 1.0wt% of a second polymerization initiator; The second polymerizable monomer is selected from pentaerythritol tetraacrylate (PETA), and the second polymerization initiator is selected from azo thermal initiator 2,2'-azobisisobutyronitrile (AIBN).

[0056] The conditions for the second stage polymerization reaction were: heating at 78°C for 8 hours.

[0057] Example 10 A secondary battery based on gradient-cured polymer electrolyte differs from Example 1 in that: in the preparation method of this example, the first electrolyte precursor contains: 1M NaFSI FEC / DEC (v / v, 3:7) liquid electrolyte, 10wt% first polymerizable monomer, and 0.5wt% first polymerization initiator; The first polymerizable monomer is selected from acryloyloxypropyl-terminated polydimethylsiloxane (PDMS-MA) and monofunctional polyether acrylate (mPEGA) in a mass ratio of 1:1, and the first polymerization initiator is selected from photoinitiator Omnirad 819.

[0058] The conditions for the first stage of polymerization reaction were: irradiation with 365nm UV light for 10 minutes.

[0059] The second electrolyte precursor comprises: 1M NaFSI FEC / DEC (v / v, 3:7) liquid electrolyte, 28wt% of a second polymerizable monomer, and 1.2wt% of a second polymerization initiator; The second polymerizable monomer is selected from ethoxylated pentaerythritol tetraacrylate (PPTTA), and the second polymerization initiator is selected from the thermal initiator dimethyl azobisisobutyrate (AIBME).

[0060] The conditions for the second stage polymerization reaction are: heating at 80°C for 5 hours.

[0061] Example 11 A secondary battery based on gradient-cured polymer electrolyte, which differs from Example 1 in that: in the preparation method of this example, the positive electrode of the bare cell is NCM613 and the negative electrode is graphite; The first electrolyte precursor comprises: 1M LiPF6 EC / EMC / DEC (1:1:1, vol%) liquid electrolyte, 4wt% first polymerizable monomer, and 0.5wt% first polymerization initiator; The first polymerizable monomer is selected from methoxy polyethylene glycol acrylate (mPEGA, Mn=480), and the first polymerization initiator is selected from photoinitiator Omnirad 819.

[0062] The conditions for the first stage of polymerization reaction were: irradiation with 365nm UV light for 20 minutes.

[0063] The second electrolyte precursor comprises: 1M LiPF6 EC / EMC / DEC (1:1:1, vol%) liquid electrolyte, 30wt% second polymerizable monomer, and 1wt% second polymerization initiator; The second polymerizable monomer is selected from trimethylolpropane triacrylate (TMPTA), and the second polymerization initiator is selected from the thermal initiator 2,2'-azobisisobutyronitrile (AIBN).

[0064] The conditions for the second stage polymerization reaction are: heating and curing at 75°C for 6 hours.

[0065] Comparative Example 1 A method for preparing a gel electrolyte sodium-ion battery includes the following steps: S1. Preparation of bare cells: The positive electrode, negative electrode, and separator are made into bare cells through winding or stacking processes and placed into the battery casing; the positive electrode is NFPP and the negative electrode is hard ink; S2. Liquid injection: Injecting an electrolyte precursor into the housing, the electrolyte precursor comprising: 1M NaPF6 EC / PC (v / v, 1:1) liquid electrolyte, 15wt% polymerizable monomer and 0.5wt% polymerization initiator; The polymerizable monomer is selected from polyethylene glycol diacrylate (PEGDA-400), and the polymerization initiator is selected from azobisisobutyronitrile (AIBN).

[0066] S3, polymerization reaction: heating and curing at 75°C for 6 hours to form a single, homogeneous gel polymer electrolyte network; S4, Sealing and Formation: The battery is sealed and standardized in the formation process.

[0067] Comparative Example 2 A method for preparing a gel electrolyte lithium-ion battery includes the following steps: S1. Preparation of bare cells: The positive electrode, negative electrode, and separator are made into bare cells through winding or stacking processes and placed into the battery casing; the positive electrode is NCM613 and the negative electrode is graphite; S2. Liquid injection: Injecting an electrolyte precursor into the housing, the electrolyte precursor comprising: 1M LiPF6 EC / EMC / DEC (1:1:1, vol%) liquid electrolyte, 15wt% polymerizable monomer and 0.5wt% polymerization initiator; The polymerizable monomer is selected from polyethylene glycol diacrylate (PEGDA-400), and the polymerization initiator is selected from azobisisobutyronitrile (AIBN).

[0068] S3, polymerization reaction: heating and curing at 75°C for 6 hours to form a single, homogeneous gel polymer electrolyte network; S4, Sealing and Formation: The battery is sealed and standardized in the formation process.

[0069] The raw material components of the above embodiments are shown in Table 1: Table 1

[0070] Performance testing The above embodiments and comparative examples were subjected to the following performance tests, and the results are shown in Table 2: 1. Polymer electrolyte crosslinking density The crosslinking density of the polymer electrolyte is expressed as the average molecular weight M between crosslinking points. C (g / mol) represents M as an ideal network formed from neutral polymer chains with tetrafunctional crosslinking points. C The calculation can be performed using the following simplified form of the Flory-Rehner equation:

[0071] The physical meanings of the symbols in the formula are as follows: : Functionality of crosslinking points; vˉ: Specific volume of the polymer (reciprocal of density). V1 ： Molar volume of the solvent n 2r Volume fraction of polymer in the relaxed state (percentage of polymer after crosslinking and before swelling). n 2m : The volume fraction of the polymer in the equilibrium swollen state (the volume fraction of the polymer in the entire gel after swelling equilibrium). x : Flory-Huggins polymer-solvent interaction parameters. Calculation logic: The volume or mass changes of the polymer network in the dry state, the post-preparation relaxed state, and the final equilibrium swollen state in the solvent are obtained through precise experimental measurement. n 2r and n 2m By substituting these macroscopic measurements into the theoretical equations above, the microscopic chain structure parameters can be derived.

[0072] Wherein, A represents the electrolyte of the first electrolyte layer, and its sampling site is on the surface of the bare cell; B represents the electrolyte of the second electrolyte layer, and its sampling site is on the inner wall of the battery casing.

[0073] 2. Room temperature cycling test: The battery was placed in a constant temperature oven (25°C) for 4 hours, then charged at a constant current of 0.5C to 3.25V, then charged at a constant voltage until the current dropped to 0.05C, and then discharged at a constant current of 0.5C to 1.5V. This cycle was repeated, and the initial capacity and discharge capacity after 500 cycles were recorded.

[0074] Capacity retention rate = Discharge capacity over 500 cycles / Initial capacity × 100%.

[0075] 3. Ratio Performance Test: The battery is charged at a constant current of 0.5C to 3.25V in an environment of 25℃, charged at a constant voltage until the current drops to 0.05C, and discharged at a constant current of 5C to 1.5V. This discharge capacity is recorded as the cell's discharge capacity, and the initial capacity is the discharge capacity at 0.2C.

[0076] Capacity retention rate = Discharge capacity / Initial capacity × 100% 4. Needle prick test: The battery was charged to 3.25V at a constant current of 0.5C in an environment of 25℃, with a cutoff current of 0.05C, a descent rate of 25 mm / s, a 3mm steel needle, and held for 30 minutes.

[0077] 5. Full charge storage test: The battery was charged at a constant current of 0.5C to 3.25V in an environment of 25℃, and then charged at a constant voltage until the current dropped to 0.02C. This was recorded as the initial thickness of the cell. The thickness of the cell was then tested every 18 days thereafter.

[0078] Table 2

[0079] As shown in Table 1, the secondary battery based on gradient-cured polymer electrolyte of this invention has an average molecular weight (MC) between crosslinking points in the first electrolyte layer of 4390-5320 g / mol and an average molecular weight (MC) between crosslinking points in the second electrolyte layer of 1009-1880 g / mol, exhibiting a significant increasing crosslinking density distribution. This design achieves both high interfacial ion transport efficiency and strong bulk mechanical inhibition. Results show that its room temperature ionic conductivity is ≥0.9 mS / cm, its capacity retention rate after 1500 cycles at 0.5C is ≥82%, its capacity ratio at 5C / 0.2C rates is ≥50%, and its thickness expansion rate after 18 days at 60℃ is ≤6.2% through needle penetration testing.

[0080] In terms of safety, Example 4 with fluorine-containing flame-retardant shell and Example 7 with full-area flame retardancy have the best safety, with the highest temperature in the needle penetration test being below 85°C and 70°C respectively, and the amount of smoke is extremely small, achieving an intrinsic safety improvement from "passive protection" to "active inhibition".

[0081] In terms of cycle life, the battery prepared by this invention meets the requirement of ≥82% capacity retention after 1500 cycles at 0.5C, with Examples 2 and 10 reaching 92.1% and 93.5% respectively, while the single-component gel electrolyte battery of Comparative Example 1 only has a retention rate of 76.2%.

[0082] This invention can also effectively suppress cell deformation. Full-charge storage deformation data shows that this invention can reduce the thickness expansion rate from 8.7% for homogeneous electrolytes to below 5.5%. In particular, Examples 5 and 9 further reduce it to 2.8% and 2.5%, respectively, proving that the high-strength outer shell layer plays an effective "skeleton" constraint role for the cell.

[0083] Furthermore, Example 6 uses an ionic liquid electrolyte with an ionic conductivity as high as 1.60 mS / cm and a rate performance (5C / 0.2C capacity ratio of 82%), which simultaneously achieves the effects of high safety and high rate performance.

[0084] The above embodiments of the present invention are merely examples for clearly illustrating the present invention and are not intended to limit the implementation of the present invention. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively describe all possible implementations here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A secondary battery based on a gradient-cured polymer electrolyte, comprising a bare cell, a battery casing, and a polymer electrolyte, wherein the bare cell is installed inside the battery casing, and the polymer electrolyte fills the interior of the bare cell and the space between the bare cell and the battery casing, characterized in that, The polymer electrolyte includes a first electrolyte layer, a gradient interface layer, and a second electrolyte layer with increasing crosslink density from the inside to the outside of the bare cell. The polymer matrix of the polymer electrolyte is a (meth)acrylate polymer. The first electrolyte layer is located inside and on the outer surface of the bare cell, and the average molecular weight between the crosslinking points of the first electrolyte layer is ≥3000; The gradient interface layer is a composite layer formed by the interface between the first electrolyte layer and the second electrolyte layer. The second electrolyte layer fills the space between the gradient interface layer and the inner wall of the battery casing, and the average molecular weight between the crosslinking points of the second electrolyte layer is ≤2500.

2. The secondary battery based on gradient-cured polymer electrolyte according to claim 1, characterized in that, The average molecular weight between the crosslinking points of the first electrolyte layer is 4000-5500; The average molecular weight between the crosslinking points of the second electrolyte layer is 800-2000.

3. The secondary battery based on gradient-cured polymer electrolyte according to claim 1, characterized in that, The first electrolyte layer is formed by in-situ polymerization of the first electrolyte precursor; The first electrolyte precursor comprises: 1-15 wt% of a first polymerizable monomer, 0.1-1 wt% of a first polymerization initiator, and the balance being a first electrolyte solution; The acrylate group in the first polymerizable monomer has a functionality of 1-2.

4. The secondary battery based on gradient-cured polymer electrolyte according to claim 3, characterized in that, The first polymerizable monomer is selected from at least one of polyethylene glycol diacrylate, polyethylene glycol methacrylate, methoxy polyethylene glycol methacrylate, tridecylfluorooctyl methacrylate, and hexafluorobutyl acrylate.

5. The secondary battery based on gradient-cured polymer electrolyte according to claim 3, characterized in that, The first polymerization initiator is a photoinitiator and / or a thermal initiator; the photoinitiator is selected from at least one of acylphosphine oxides, α-hydroxy ketones and acylphosphine oxide initiators; the thermal initiator is selected from organic peroxides and / or azo compounds.

6. The secondary battery based on gradient-cured polymer electrolyte according to claim 1, characterized in that, The second electrolyte layer is formed by in-situ polymerization of the second electrolyte precursor; The second electrolyte precursor comprises: 12-40 wt% of a second polymerizable monomer, 0-16 wt% of a functional monomer, 0.1-2 wt% of a second polymerization initiator, and the balance being a second electrolyte solution; The functionality of the acrylate groups in the second polymerizable monomer is ≥3; The functional monomer contains at least one acrylate group.

7. The secondary battery based on gradient-cured polymer electrolyte according to claim 6, characterized in that, The second polymerizable monomer is selected from pentaerythritol tetraacrylate, dipentaerythritol penta / hexaacrylate, ethoxylated pentaerythritol tetraacrylate, trimethylolpropane triacrylate, and ethoxylated trimethylolpropane triacrylate. The functional monomer is selected from at least one of polyethylene glycol diacrylate, aromatic polyurethane acrylate, 1H,1H,2H,2H-perfluorooctyl acrylate, fluorinated polyurethane acrylate and 1-acryloyloxyethyl-3-methylimidazolium bis(fluorosulfonyl)imide.

8. The secondary battery based on gradient-cured polymer electrolyte according to claim 2 or 6, characterized in that, In the first electrolyte solution or the second electrolyte solution, the solute is at least one of NaPF6, LiPF6, NaClO4 and NaFSI with a concentration of 0.8-1.2M, and the solvent is at least one of ethylene carbonate, propylene carbonate, diethyl carbonate or its fluorinated derivatives.

9. A method for preparing a secondary battery based on a gradient-cured polymer electrolyte as described in any one of claims 1-8, characterized in that, Includes the following steps: S1, First electrolyte injection: The bare cell is placed inside the battery casing, and a first electrolyte precursor is injected into the battery casing. The first electrolyte precursor includes: 1-15 wt% of a first polymerizable monomer, 0.1-1 wt% of a first polymerization initiator, and the remainder is a first electrolyte solution. S2, First stage polymerization reaction: The first electrolyte precursor is induced to undergo a polymerization reaction by external stimulation, and a gradient-cured first electrolyte layer is formed inside and on the surface of the bare cell respectively; S3, Second electrolyte injection: Inject a second electrolyte precursor into the battery casing after step S2. The mass ratio of the second electrolyte precursor to the first electrolyte precursor is (1-3):(7-9). The second electrolyte precursor includes: 12-40wt% of a second polymerizable monomer, 0-16wt% of a functional monomer, 0.1-2wt% of a second polymerization initiator, and the remainder is a second electrolyte solution. S4, Second stage polymerization reaction: The second electrolyte precursor is induced to undergo polymerization reaction by external stimulation to form a second polymer electrolyte. After sealing and formation treatment, the secondary battery based on gradient solidification polymer electrolyte is obtained.

10. The method for preparing a secondary battery based on a gradient-cured polymer electrolyte according to claim 9, characterized in that, The conditions for the first stage polymerization reaction in step S2 are: irradiation with 365nm UV light for 15-30 minutes; The conditions for the second stage polymerization reaction in step S4 are: heating and curing at 70-95℃ for 4-8 hours.