A lithium battery and a preparation method thereof

Abstract

The application discloses a lithium battery and a preparation method thereof. The lithium battery comprises a positive electrode sheet, a negative electrode sheet, a diaphragm and a thermal safety element. The thermal safety element comprises a base layer, which is electrically connected to the positive electrode sheet or the negative electrode sheet; an impedance layer, which is arranged on at least one side of the base layer, is insulated from ions and has electrical resistance; and an insulation layer, which is arranged on the outside of the base layer and the impedance layer and covers at least the impedance layer, is insulated from electrons and ions at normal temperature and has a heat resistance threshold. When the battery temperature is lower than the heat resistance threshold, the insulation layer remains insulated, and the thermal safety element does not participate in the electrochemical reaction; when the temperature reaches or exceeds the heat resistance threshold, the insulation layer fails, the impedance layer directly contacts the positive and negative electrode sheets through the failure area to form an internal short circuit, and energy is released gently. The application can actively trigger a gentle internal short circuit at a pre-warning temperature lower than the oxygen release temperature of the positive electrode without affecting the normal working performance of the battery, and effectively improves the safety of high-nickel ternary lithium batteries.

Description

Technical Field

[0001] This invention relates to the field of battery safety technology, and in particular to a lithium battery and its preparation method. Background Technology

[0002] High-nickel ternary materials are widely used as positive electrode active materials in power batteries for electric vehicles due to their high specific capacity and good rate performance. However, high-nickel ternary materials have poor structural thermal stability, and their structure collapses and releases oxygen at around 150°C. The released oxygen reacts with the lithium storage anode in a redox reaction, generating a large amount of heat, causing the separator to collapse and triggering a strong internal short circuit, ultimately leading to complete thermal runaway of the battery.

[0003] To address the aforementioned safety issues, CN112820934B discloses a self-protection structure positioned between the positive and negative electrode structures. This self-protection structure deforms when the battery temperature reaches a first preset temperature threshold, causing the positive and negative electrode structures to conduct, thereby inducing a mild internal short circuit within the battery and halting thermal runaway before reaching the positive electrode oxygen release temperature. However, this self-protection structure must simultaneously possess a certain ionic conductivity and a low melting point in its operational state, limiting its material selection to fusible polymers. The ionic conductivity of fusible polymers is far lower than that of conventional electrolytes, which severely impacts the battery's charge-discharge performance and cycle life when used as an electrolyte. Therefore, existing technologies cannot effectively solve the thermal runaway problem of high-nickel ternary batteries without affecting their normal operating performance. Summary of the Invention

[0004] To address the problem that the self-protection structure in existing technologies affects the normal operation of batteries due to its reliance on fusible polymers, this invention proposes a lithium battery and its preparation method.

[0005] The specific technical solution is as follows: A lithium battery includes a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, and further includes:

[0006] Thermal safety element, the thermal safety element comprising:

[0007] The base layer is electrically connected to the positive electrode or the negative electrode.

[0008] An impedance layer is disposed on at least one side of the base layer, the impedance layer being ion-insulating and having resistance;

[0009] An insulating layer is disposed outside the base layer and the impedance layer and at least covers the impedance layer. The insulating layer is insulating against electrons and ions at room temperature and has a heat resistance threshold.

[0010] Specifically, when the battery temperature is below the heat resistance threshold, the insulating layer remains in an insulating state, and the thermal safety element does not participate in the electrochemical reaction of the battery; when the battery temperature reaches or exceeds the heat resistance threshold, the insulating layer loses its insulating function, and the impedance layer directly contacts the positive electrode and the negative electrode through the failure area of ​​the insulating layer to form an electrical connection, thereby forming an internal short circuit inside the battery.

[0011] Furthermore, the thermal safety element is disposed between adjacent positive and negative electrode plates, the insulating layer is in direct contact with one of the positive and negative electrode plates, and the base layer is electrically connected to the other of the positive and negative electrode plates.

[0012] Furthermore, the impedance layer is not ionicly conductive, and the insulating layer is completely insulated from electrons and ions when below the heat resistance threshold, so that the thermal safety element is electrically insulated from the electrochemical reaction circuit of the battery during normal charging and discharging.

[0013] Furthermore, the insulating layer is an electronic insulating material layer that loses its insulating properties in a temperature range of 100°C to 130°C.

[0014] Furthermore, the insulating layer is a porous polyethylene membrane with a heat resistance threshold of 110°C.

[0015] Furthermore, the impedance layer comprises a polymer matrix and an electronically conductive filler uniformly dispersed in the polymer matrix, and the impedance layer provides a resistance of 15 mΩ to 5000 mΩ.

[0016] Furthermore, the polymer matrix is ​​electrochemically stable within the battery operating voltage window, and the conductive filler is a carbon material.

[0017] Furthermore, the base layer is a metal foil; when the base layer is electrically connected to the positive electrode, the metal foil is an aluminum foil; when the base layer is electrically connected to the negative electrode, the metal foil is a copper foil.

[0018] A method for preparing a lithium battery, applicable to the aforementioned lithium battery, comprising:

[0019] The polymer matrix and conductive filler are dispersed in a solvent to form a coating solution;

[0020] The coating liquid is applied to at least one side surface of the base layer and dried to form an impedance layer, thus obtaining a base layer-impedance layer unit.

[0021] The base-impedance layer unit is covered with an insulating layer to form a thermal safety element;

[0022] The positive electrode, negative electrode, separator, and thermal safety element are stacked to form a bare battery.

[0023] The bare battery is packaged, tabs are welded, electrolyte is injected, and formation is performed to obtain a lithium battery.

[0024] Furthermore, when the base layer is electrically connected to the negative electrode sheet, the stacking sequence includes, in sequence: negative electrode sheet, separator, positive electrode sheet, thermal safety element, positive electrode sheet, separator, negative electrode sheet, separator, positive electrode sheet, separator, negative electrode sheet; and the positive electrode sheet adjacent to the thermal safety element has no positive electrode active material on the side closest to the thermal safety element.

[0025] The above technical solution has the following advantages or technical effects:

[0026] 1. This invention, through a thermal safety element composed of a base layer, an impedance layer, and an insulating layer, achieves active triggering of a mild internal short circuit at a warning temperature below the positive electrode oxygen release temperature, effectively improving the safety of high-nickel ternary lithium batteries without affecting their normal operating performance.

[0027] 2. By controlling the resistance value of the impedance layer within the range of 15-5000 mΩ and ensuring that the impedance layer has no ionic conductivity and the insulating layer is completely insulated at room temperature, the present invention enables the thermal safety element and the battery electrochemical reaction circuit to coexist independently, thus avoiding the negative impact of fusible polymer electrolytes on battery performance in the prior art.

[0028] 3. By setting the heat resistance threshold of the insulating layer to 100-130℃, this invention ensures that the internal short circuit is triggered in time before the positive electrode releases oxygen, thus fundamentally blocking the thermal runaway chain reaction. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the structure of the lithium battery when the thermal safety element and the positive electrode tab of the present invention are turned on;

[0030] Figure 2 This is a schematic diagram of the structure of the lithium battery when the thermal safety element and the negative electrode tab of the present invention are turned on. Detailed Implementation

[0031] To make the technical solution of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0032] A lithium battery includes a positive electrode 110, a negative electrode 120, and a separator 300 disposed between the positive electrode 110 and the negative electrode 120. The following methods for preparing the positive electrode 110 and the negative electrode 120 are applicable to all embodiments and comparative examples.

[0033] Preparation of positive electrode 110:

[0034] The binder, polyvinylidene fluoride, was prepared as a 7 wt% N-methylpyrrolidone adhesive solution. A conductive agent (carbon nanotubes) and a positive electrode active material (…) were then added sequentially to the adhesive solution. The mixture is thoroughly stirred to form a uniformly dispersed adhesive solution. The mass ratio of the binder, conductive agent, and positive electrode active material is 2:2:96. This adhesive solution is coated onto the surface of the current collector, dried to remove the solvent, and a positive electrode layer is formed. The current collector with the positive electrode layer is then rolled to obtain the positive electrode sheet 110.

[0035] Preparation of negative electrode 120:

[0036] Sodium carboxymethyl cellulose (CMC) binder was prepared as a 1.3 wt% aqueous adhesive solution. Conductive agent (carbon fiber), binder (polyacrylic acid), negative electrode active material (silicon-carbon negative electrode), negative electrode active material (graphite), and binder (styrene-butadiene rubber latex) were added sequentially to the adhesive solution, and the mixture was thoroughly stirred to form a uniformly dispersed adhesive solution. The mass ratio of sodium carboxymethyl cellulose, polyacrylic acid, carbon fiber, silicon-carbon negative electrode, graphite, and styrene-butadiene rubber latex was 1:1:1:19.4:77.6. The adhesive solution was coated onto the surface of the current collector, dried to remove the solvent, and a negative electrode layer was formed. The current collector with the negative electrode layer was rolled to obtain a negative electrode sheet 120.

[0037] The diaphragm 300 used in this specific embodiment is a ceramic diaphragm 300, the substrate of which is a porous polyethylene membrane with a ceramic layer coated on the surface.

[0038] It also includes a thermal safety element 200, which includes a base layer 230, an impedance layer 220, and an insulating layer 210.

[0039] The base layer 230 is a metal foil. When the base layer 230 needs to be electrically connected to the positive electrode 110, aluminum foil is selected for the base layer 230; when the base layer 230 needs to be electrically connected to the negative electrode 120, copper foil is selected for the base layer 230.

[0040] An impedance layer 220 is disposed on at least one side of the base layer 230. The impedance layer 220 is ion-insulating and has resistance. The impedance layer 220 includes a polymer matrix and an electronically conductive filler uniformly dispersed in the polymer matrix. The polymer matrix is ​​selected from one or more of polyacrylonitrile, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polytetrafluoroethylene, polybutylene terephthalate, polysulfone, and polyimide. The conductive filler is selected from one or more of graphite, carbon nanotubes, graphene, carbon black, and carbon fiber. The polymer matrix is ​​electrochemically stable within the battery operating voltage window. The conductive filler is a carbon material.

[0041] The impedance layer 220 is prepared by dispersing a polymer matrix and conductive filler in a solvent at a certain mass ratio to form a uniform coating liquid. The coating liquid is then applied to at least one surface of the base layer 230, and the solvent is removed by drying to form the impedance layer 220. Subsequently, the base layer 230 with the impedance layer 220 is rolled to obtain a base layer 230-impedance layer 220 unit. The resistance value provided by the impedance layer 220 can be controlled by adjusting the mass ratio of the polymer matrix to the conductive filler.

[0042] An insulating layer 210 is disposed outside the base layer 230 and the impedance layer 220 and at least covers the impedance layer 220. The insulating layer 210 is electrically and ion-insulating at room temperature and has a heat resistance threshold. When the battery temperature is below the heat resistance threshold, the insulating layer 210 remains insulating; when the battery temperature reaches or exceeds the heat resistance threshold, the insulating layer 210 loses its insulating function. The insulating layer 210 is an electronic insulating material layer that loses its insulating properties in a temperature range of 100°C to 130°C. Specifically, the insulating layer 210 can be a polyethylene porous membrane, a polypropylene porous membrane, or a multilayer composition thereof.

[0043] The above-mentioned base layer 230-impedance layer 220 unit is covered with an insulating layer 210 to form a complete thermal safety element 200. The following embodiments are used to illustrate the specific structure of the lithium battery of the present invention.

[0044] Example 1

[0045] A lithium battery includes a positive electrode 110, a negative electrode 120, a separator 300, and a thermal safety element 200.

[0046] The base layer 230 of the thermal safety element 200 is aluminum foil. The polymer matrix of the impedance layer 220 is polyacrylonitrile, and the conductive filler is carbon black, with a mass ratio of polyacrylonitrile to carbon black of 90:10. The impedance layer 220 is coated on both sides of the aluminum foil, with a thickness of 20 μm on each side. The resistance of the base layer 230-impedance layer 220 unit is 5 mΩ. The insulating layer 210 is a 16 μm thick porous polyethylene membrane with a heat resistance threshold of 110℃.

[0047] The thermal safety element 200 is disposed between adjacent positive electrode 110 and negative electrode 120. For example... Figure 1 As shown, the base layer 230 of the thermal safety element 200 is electrically connected to the positive electrode tab 111, meaning the thermal safety element 200 is connected in parallel with the positive electrode. The outer side of the insulating layer 210 of the thermal safety element 200 is in direct contact with the negative electrode 120. In the stacking sequence, the thermal safety element 200 is located between two negative electrode sheets 120.

[0048] Example 2

[0049] The difference between this embodiment and Product Embodiment 1 lies in the composition of the impedance layer 220: the mass ratio of polyacrylonitrile to carbon black is 95:5, and the resistance of the base layer 230-impedance layer 220 unit is 50 mΩ. The rest of the structure is the same.

[0050] Example 3

[0051] The difference between this embodiment and Product Embodiment 1 lies in the composition of the impedance layer 220: the mass ratio of polyacrylonitrile to carbon black is 98:2, and the resistance of the base layer 230-impedance layer 220 unit is 500 mΩ. The rest of the structure is the same.

[0052] Example 4

[0053] The difference between this embodiment and Product Embodiment 1 lies in the composition of the impedance layer 220: the mass ratio of polyacrylonitrile to carbon black is 98.5:1.5, and the resistance of the base layer 230-impedance layer 220 unit is 5000 mΩ. The rest of the structure is the same.

[0054] Example 5

[0055] The difference between this embodiment and Embodiment 3 is that the impedance layer 220 is only coated on one side of the base layer 230, and the coating thickness is 40 μm. The resistance of the base layer 230-impedance layer 220 unit is 500 mΩ. The rest of the structure is the same.

[0056] Example 6

[0057] A lithium battery includes a positive electrode 110, a negative electrode 120, a separator 300, and a thermal safety element 200.

[0058] The base layer 230 of the thermal safety element 200 is copper foil. The polymer matrix of the impedance layer 220 is polyvinylidene fluoride (PVDF), and the conductive filler is carbon black. The mass ratio of PVDF to carbon black is 90:10. The impedance layer 220 is coated on both sides of the copper foil, with a thickness of 20 μm on each side. The resistance of the base layer 230-impedance layer 220 unit is 15 mΩ. The insulating layer 210 is a 16 μm thick porous polyethylene membrane with a heat resistance threshold of 110 °C.

[0059] The thermal safety element 200 is disposed between adjacent positive electrode 110 and negative electrode 120. For example... Figure 2 As shown, the base layer 230 of the thermal safety element 200 is electrically connected to the negative electrode tab 121, meaning the thermal safety element 200 is connected in parallel with the negative electrode. The outer side of the insulating layer 210 of the thermal safety element 200 is in direct contact with the positive electrode plate 110. The side of the positive electrode plate 110 adjacent to the thermal safety element 200 has no positive electrode active material.

[0060] Comparative Example 1

[0061] This comparative example provides a lithium battery, which differs from Example 3 in that the battery does not contain a thermal safety element 200.

[0062] Comparative Example 2

[0063] The difference between this comparative example and Example 3 is that the insulating layer 210 is a 17µm polypropylene porous membrane with a heat resistance threshold of 165°C.

[0064] Comparative Example 3

[0065] The difference between this comparative example and Example 3 is that the insulating layer 210 is a 17µm polyethylene oxide film with a heat resistance threshold of 80°C.

[0066] The present invention also proposes a method for preparing lithium batteries applicable to Examples 1 to 6.

[0067] Example 7

[0068] A 9 wt% N-methylpyrrolidone adhesive solution of polyacrylonitrile was prepared, and carbon black was added at a mass ratio of 90:10. The mixture was stirred until homogeneous to form a coating solution. The coating solution was applied to both sides of an aluminum foil and dried to form an impedance layer 220, resulting in a base layer 230-impedance layer 220 unit. The impedance layer 220 on each side had a thickness of 20 μm and a resistance of 5 mΩ. This unit was then coated with a 16 μm thick porous polyethylene membrane to form a thermal safety element 200.

[0069] The positive electrode 110, negative electrode 120, thermal safety element 200, and ceramic separator 300 are vacuum dried at 85°C for 12 hours. The electrodes are then stacked in the following order: negative electrode 120, separator 300, positive electrode 110, separator 300, negative electrode 120, thermal safety element 200, negative electrode 120, separator 300, positive electrode 110, separator 300, negative electrode 120, forming a bare battery and encapsulating it in an aluminum-plastic film.

[0070] The positive electrode 110 and the thermal safety element 200 are welded to the positive electrode tab 111, and the negative electrode 120 is welded to the negative electrode tab 121. Electrolyte is then injected; the electrolyte solute concentration is 1 mol / L. The solvent is a mixture of ethylene carbonate, methyl ethyl carbonate, and diethyl carbonate in a volume ratio of 1:1:1. After vacuum sealing, it is formed to obtain a lithium battery.

[0071] Example 8

[0072] The difference from Example 1 of the preparation method is that the mass ratio of polyacrylonitrile to carbon black is 95:5, and the unit resistance of the base layer 230-impedance layer 220 is 50 mΩ. The remaining steps are the same.

[0073] Example 9

[0074] The difference from Example 1 of the preparation method is that the mass ratio of polyacrylonitrile to carbon black is 98:2, and the unit resistance of the base layer 230-impedance layer 220 is 500 mΩ. The remaining steps are the same.

[0075] Example 10

[0076] The difference from Example 1 of the preparation method is that the mass ratio of polyacrylonitrile to carbon black is 98.5:1.5, and the unit resistance of the base layer 230-impedance layer 220 is 5000 mΩ. The remaining steps are the same.

[0077] Example 11

[0078] The difference from Example 3 of the preparation method is that the coating liquid is only coated on one side of the aluminum foil, the coating thickness is 40 μm, and the unit resistance of the base layer 230-impedance layer 220 is 500 mΩ. The remaining steps are the same.

[0079] Example 12

[0080] Polyvinylidene fluoride (PVDF) was prepared as a 7 wt% N-methylpyrrolidone (NMP) solution. Carbon black was added, with a PVDF to carbon black mass ratio of 90:10. The mixture was stirred until homogeneous to form a coating solution. The coating solution was applied to both sides of a copper foil and dried to form an impedance layer 220, resulting in a base layer 230-impedance layer 220 unit. The impedance layer 220 on each side had a thickness of 20 μm and a resistance of 15 mΩ. This unit was then encapsulated with a 16 μm thick porous polyethylene membrane to form a thermal safety element 200.

[0081] When preparing the positive electrode 110, the positive electrode active material is not coated on the side of the positive electrode 110 adjacent to the thermal safety element 200.

[0082] The positive electrode 110, negative electrode 120, thermal safety element 200, and ceramic separator 300 are vacuum dried at 85°C for 12 hours. The electrodes are stacked in the following order: negative electrode 120, separator 300, positive electrode 110, thermal safety element 200, positive electrode 110, separator 300, negative electrode 120, separator 300, positive electrode 110, separator 300, negative electrode 120, forming a bare battery and encapsulating it in an aluminum-plastic film.

[0083] The positive electrode 110 is welded to the positive electrode tab 111, and the negative electrode 120 and the thermal safety element 200 are welded to the negative electrode tab 121. Electrolyte is injected (same as in Example 1 of the preparation method), and after vacuum sealing, a lithium battery is obtained.

[0084] The present invention tests the lithium batteries prepared in the above embodiments. The lithium batteries prepared in each embodiment and the comparative example are charged at 25°C with a constant current of 1C to 4.25V, then charged with a constant voltage of 4.25V to 0.05C, and then discharged with a constant current of 1C to 2.8V. This process is repeated 100 times, and the discharge capacity Qn (mAh) of the nth cycle is recorded. Capacity retention rate (%) = The lithium batteries prepared in each embodiment and comparative example were subjected to a pressure of 10 PSI at 25°C, charged at a constant current of 0.1C to 4.25V, and then charged at a constant voltage of 4.25V to 0.05C. The batteries were then placed in a 60°C hot chamber for 150 min, then heated at a rate of 5°C / min to 130°C and held for 30 min, then heated at a rate of 2°C / min to 150°C and held for 30 min, then heated at a rate of 2°C / min to 170°C and held for 30 min, and finally heated at a rate of 2°C / min to 180°C and held for 60 min. The temperature at which the voltage dropped to 4V was recorded as the internal short-circuit temperature. If the battery did not catch fire or explode, it passed the hot chamber test. The test results are shown in the table below:

[0085] Table 1: Test Results of Each Example and Comparative Example

[0086]

[0087] As can be seen, Comparative Example 1, which did not use thermal safety element 200, had an internal short-circuit temperature of 180℃, failing the hot box test. This is because the high-nickel ternary cathode releases oxygen at high temperatures, reacting with the anode to generate heat, leading to membrane rupture and thermal runaway.

[0088] Examples 1 to 4 and Example 6 all incorporated a thermal safety element 200, with an internal short-circuit temperature of 110°C, significantly lower than the 180°C of Comparative Example 1. In Example 1, the impedance layer 220 had a resistance of 5 mΩ, indicating an excessively severe internal short circuit, and failed the hot-box test. In Examples 2 to 4 and Example 6, the impedance layer 220 had resistances ranging from 15 mΩ to 5000 mΩ, and all passed the hot-box test. This indicates that the impedance layer 220 has a critical lower limit; when the resistance is controlled within the range of 15 mΩ to 5000 mΩ, a mild internal short circuit can be achieved, preventing thermal runaway.

[0089] The resistance of both Examples 3 and 5 was 500 mΩ, and both passed the hot box test, indicating that the safety effect depends on the resistance value rather than the spatial distribution.

[0090] Comparative Example 2 used a polypropylene porous membrane (heat resistance threshold 165℃), with an internal short-circuit temperature of 170℃, and failed the hot box test. Comparative Example 3 used a polyethylene oxide membrane (heat resistance threshold 80℃), which passed the hot box test, but its internal short-circuit temperature was too low and its capacity retention was only 61.2%. Therefore, it is reasonable for this invention to limit the heat resistance threshold of the insulating layer 210 to 100-130℃.

[0091] Observation of the test results of Comparative Example 1, Examples 1 to 4, and Comparative Example 2 shows that introducing the thermal safety element 200 on the positive electrode side reduces the cycle stability of the battery. The capacity retention rates of Examples 1 to 4 are 90.2%, 90.4%, 89.3%, and 91.0%, respectively, while the capacity retention rate of Comparative Example 1 is 95.4%. This is because when the base layer 230 (aluminum foil) of the thermal safety element 200 is charged to above 4V along with the positive electrode 110, it undergoes a side reaction with the electrolyte and produces byproducts in the absence of a delithiation reaction, thus leading to capacity decay.

[0092] As observed in Example 5, when the surface of the substrate 230 is not covered by the impedance layer 220, the side reaction between the substrate 230 and the electrolyte is more intense. The capacity retention rate of Example 5 is only 70.2%, significantly lower than 89.3% of Example 3, indicating that the impedance layer 220 covering the surface of the substrate 230 plays an important role in suppressing side reactions.

[0093] As observed in Example 6, connecting the thermal safety element 200 in parallel with the negative electrode can effectively suppress the side reactions between the base layer 230 and the electrolyte. The capacity retention rate of Example 6 is 94.7%, significantly higher than 89.3% of Example 3, and even close to 95.4% of Comparative Example 1. This is because the negative electrode potential is low, and it only reacts with the electrolyte during the first charge to form a stable solid electrolyte interlayer (SEI film), after which the side reactions are effectively suppressed.

[0094] As observed in Comparative Example 3, using polyethylene oxide as the insulating layer 210 severely impacts battery performance. The capacity retention rate of Comparative Example 3 is only 61.2%, significantly lower than other examples. This is because polyethylene oxide has a very low heat resistance threshold, making it easily soluble in the electrolyte in the battery operating environment and causing side reactions with the positive electrode.

[0095] The above results demonstrate that the present invention, through the three-layer structure and parameter control of the thermal safety element 200, achieves active triggering of a mild internal short circuit at the warning temperature, effectively improving the safety of high-nickel ternary lithium batteries. Furthermore, by connecting the thermal safety element 200 in parallel with the negative electrode and ensuring that the impedance layer 220 completely covers the surface of the base layer 230, excellent safety protection can be achieved without affecting the normal operating performance of the battery.

[0096] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A lithium battery, comprising a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, characterized in that, Also includes: Thermal safety element, the thermal safety element comprising: The base layer is electrically connected to the positive electrode or the negative electrode. An impedance layer is disposed on at least one side of the base layer, the impedance layer being ion-insulating and having resistance; An insulating layer is disposed outside the base layer and the impedance layer and at least covers the impedance layer. The insulating layer is insulating against electrons and ions at room temperature and has a heat resistance threshold. Specifically, when the battery temperature is below the heat resistance threshold, the insulating layer remains in an insulating state, and the thermal safety element does not participate in the electrochemical reaction of the battery; when the battery temperature reaches or exceeds the heat resistance threshold, the insulating layer loses its insulating function, and the impedance layer directly contacts the positive electrode and the negative electrode through the failure area of ​​the insulating layer to form an electrical connection, thereby forming an internal short circuit inside the battery.

2. A lithium battery according to claim 1, characterized in that, The thermal safety element is disposed between adjacent positive and negative electrodes, the insulating layer is in direct contact with one of the positive and negative electrodes, and the base layer is electrically connected to the other of the positive and negative electrodes.

3. A lithium battery according to claim 1, characterized in that, The impedance layer is not ionicly conductive, and the insulating layer is completely insulated from electrons and ions when the temperature is below the heat resistance threshold, so that the thermal safety element is electrically insulated from the electrochemical reaction circuit of the battery during normal charging and discharging.

4. A lithium battery according to claim 1, characterized in that, The insulating layer is an electronic insulating material layer that loses its insulating properties in a temperature range of 100°C to 130°C.

5. A lithium battery according to claim 4, characterized in that, The insulating layer is a porous polyethylene membrane with a heat resistance threshold of 110°C.

6. A lithium battery according to claim 1, characterized in that, The impedance layer comprises a polymer matrix and an electronically conductive filler uniformly dispersed in the polymer matrix, and the impedance layer provides a resistance of 15 mΩ to 5000 mΩ.

7. A lithium battery according to claim 6, characterized in that, The polymer matrix is ​​electrochemically stable within the battery operating voltage window, and the conductive filler is a carbon material.

8. A lithium battery according to claim 1, characterized in that, The base layer is a metal foil; when the base layer is electrically connected to the positive electrode, the metal foil is an aluminum foil; when the base layer is electrically connected to the negative electrode, the metal foil is a copper foil.

9. A method for preparing a lithium battery, applicable to the lithium battery according to any one of claims 1 to 8, characterized in that, include: The polymer matrix and conductive filler are dispersed in a solvent to form a coating solution; The coating liquid is applied to at least one side surface of the base layer and dried to form an impedance layer, thus obtaining a base layer-impedance layer unit. The base-impedance layer unit is covered with an insulating layer to form a thermal safety element; The positive electrode, negative electrode, separator, and thermal safety element are stacked to form a bare battery; The bare battery is packaged, tabs are welded, electrolyte is injected, and formation is performed to obtain a lithium battery.

10. A method for preparing a lithium battery according to claim 9, characterized in that, When the base layer is electrically connected to the negative electrode, the stacking sequence includes: negative electrode, separator, positive electrode, thermal safety element, positive electrode, separator, negative electrode, separator, positive electrode, separator, negative electrode; and the positive electrode adjacent to the thermal safety element has no positive electrode active material on the side close to the thermal safety element.

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