Secondary battery, battery pack and electric equipment

Abstract

The invention discloses a secondary battery, a battery pack and electric equipment, and belongs to the technical field of batteries. The secondary battery comprises at least one battery cell, the at least one battery cell comprises a positive pole piece, a negative pole piece and a diaphragm arranged between the positive pole piece and the negative pole piece, at least one of the positive pole piece and the negative pole piece comprises a composite current collector, and the composite current collector comprises a polymer base material layer and a conductive layer, the polymer base material layer comprises a polymer base material and a thermal decomposition additive, the thermal decomposition additive generates gas when reaching a decomposition temperature, and the conductive layer is arranged on the polymer base material layer. According to the present invention, the thermal decomposition additive is introduced into the composite current collector, such that the gas can be generated when the battery is locally overheated, the high pressure can be formed in the composite current collector, the composite current collector is caused to swell and even fracture, the short circuit or the open circuit can be formed in the cell, the thermal runaway occurrence possibility is reduced, and the safety of the secondary battery is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a secondary battery, a battery pack and a power consumption device. BACKGROUND

[0002] With the increasing demand for energy density and capacity of secondary batteries, the use of multi-cell parallel integrated design has become a mainstream technology direction. Such a battery contains multiple cells made by winding or laminating process, each cell is composed of a positive electrode sheet, a negative electrode sheet and a separator placed therebetween, and each cell is connected in parallel to a common current collector through the tab to achieve cooperative work. Among them, the separator can prevent internal short circuit caused by direct contact of positive and negative electrode sheets through physical isolation, and the microporous structure in the separator can ensure the migration of lithium ions in the electrolyte and maintain the stable progress of electrochemical reaction.

[0003] However, this structure has hidden dangers when facing local short circuit risks. Needle test, metal impurities introduced during battery manufacturing or cycling, lithium dendrite growth caused by improper charging and discharging, or electrode sheet burrs caused by internal stress changes, etc. can all induce a large amount of heat in the local cell, which in turn causes the separator to melt and shrink, thereby destroying its physical isolation function, aggravating the contact between the positive and negative electrode sheets and triggering thermal runaway. This problem is particularly serious in secondary batteries including multiple cells, because the thermal runaway triggered by the local failure of the separator in a single cell will quickly spread to adjacent cells through heat conduction mechanism, causing a chain reaction of separator shrinkage and thermal runaway, and ultimately leading to the failure of the entire battery system.

[0004] In the prior art, the thickness of the separator is increased to improve the thermal stability of the separator and reduce the possibility of separator shrinkage and short circuit. However, thickening the separator will hinder the migration path of ions, leading to increased battery resistance, decreased ion conduction efficiency, and thus reduced battery rate performance and energy density. Moreover, the method of increasing the thickness of the separator can delay the thermal destruction process of a single cell, but it is difficult to block the energy transfer and spread of thermal runaway between multiple cells. SUMMARY

[0005] The present application discloses a secondary battery, a battery pack and a power consumption device to solve the problem of battery thermal runaway caused by local short circuit in the prior art.

[0006] To solve the above technical problems, the present application is implemented as follows: In a first aspect, the present application discloses a secondary battery, comprising at least one cell, and wherein at least one of the cells comprises a positive electrode sheet, a negative electrode sheet, and a separator disposed between the positive electrode sheet and the negative electrode sheet, at least one of the positive electrode sheet and the negative electrode sheet comprises a composite current collector, and the composite current collector comprises: a polymer substrate layer, the polymer substrate layer comprises a polymer substrate and a thermal decomposition additive, the thermal decomposition additive generates gas when reaching a decomposition temperature; and a conductive layer disposed on the polymer substrate layer.

[0007] In some embodiments, in the polymer substrate layer, the mass percentage of the thermal decomposition additive is 1% to 1.7%.

[0008] In some embodiments, the decomposition temperature is 60 to 230℃.

[0009] In some embodiments, the particle size of the thermal decomposition additive is ≤1μm.

[0010] In some embodiments, the thermal decomposition additive comprises at least one of an organic acid, a chemical foaming agent, an inorganic acid, and a crystal-containing hydrate.

[0011] In some embodiments, the organic acid comprises at least one of citric acid, tartaric acid, and oxalic acid.

[0012] In some embodiments, the chemical foaming agent comprises at least one of azodicarbonamide, 4,4'-oxybisbenzenesulfonylhydrazide, p-toluenesulfonylurea, and sodium bicarbonate.

[0013] In some embodiments, the crystal-containing hydrate comprises at least one of copper sulfate pentahydrate.

[0014] In some embodiments, the inorganic acid comprises borax.

[0015] In some embodiments, the composite current collector comprises a first part and a second part, one side of the first part is used to connect a tab, the second part is connected to the side of the first part away from the tab; the mass of the thermal decomposition additive in the second part is less than or equal to the mass of the thermal decomposition additive in the first part, and / or the unit area additive mass of the thermal decomposition additive in the second part is less than or equal to the unit area additive mass of the thermal decomposition additive in the first part; the mass of the thermal decomposition additive in the first part is 0.5% to 1.5% of the mass of the polymer substrate layer; the mass of the thermal decomposition additive in the second part is 0.5% to 1% of the mass of the polymer substrate layer.

[0016] In some embodiments, the composite current collector further comprises a third portion connected to a side of the second portion away from the first portion, a mass of the thermal decomposition additive in the third portion is less than a mass of the thermal decomposition additive in the second portion, and / or a unit area additive mass of the thermal decomposition additive in the third portion is less than a unit area additive mass of the thermal decomposition additive in the second portion; the mass of the thermal decomposition additive in the third portion is 0-0.2% of the mass of the polymer substrate layer.

[0017] In some embodiments, the composite current collector has a second direction and a third direction perpendicular to each other, along the second direction, a size of the first portion accounts for 30-35% of a size of the composite current collector, along the second direction, a size of the second portion accounts for 30-35% of the size of the composite current collector, and along the second direction, a size of the third portion accounts for 30-35% of the size of the composite current collector. Along the third direction, the size of the first portion, the size of the second portion, and the size of the third portion are all equal to the size of the composite current collector.

[0018] In some embodiments, the thickness of the polymer substrate layer is 2-4 μm; and / or the thickness of the conductive layer is 2-4 μm.

[0019] In some embodiments, the secondary battery has a first direction, the cell comprises a first cell and a second cell, at least one of the first cell and the second cell comprises the composite current collector, the secondary battery comprises a middle portion and side portions located on both sides of the middle portion along the first direction; the first cell is located in the side portion, and the second cell is located in the middle portion; a relationship of capacities of the first cell and the second cell is represented as: 0.7≤Q1 / Q2≤1.3, wherein Q1 represents the capacity of the first cell, and Q2 represents the capacity of the second cell.

[0020] In some embodiments, a relationship of physical resistances of the first cell and the second cell is represented as: 0.8≤r1 / r2≤1.2, wherein r1 represents the physical resistance of the first cell, and r2 represents the physical resistance of the second cell.

[0021] In some embodiments, a relationship of direct current resistances of the first cell and the second cell is represented as: 0.75≤R1 / R2≤1.35, wherein R1 represents the direct current resistance of the first cell, and R2 represents the direct current resistance of the second cell.

[0022] In some embodiments, 1.4≤(r1 / r2)×(R1 / R2)≤3.8, where r1 represents the physical resistance of the first battery cell, r2 represents the physical resistance of the second battery cell; R1 represents the DC resistance of the first battery cell, and R2 represents the DC resistance of the second battery cell.

[0023] In some embodiments, 0.86≤D1 / D2≤1, where D1 represents the thickness of the first battery cell, and D2 represents the thickness of the second battery cell, the thickness of the first battery cell being the dimension of the first battery cell in the first direction, and the thickness of the second battery cell being the dimension of the second battery cell in the first direction.

[0024] In some embodiments, the secondary battery further comprises: a housing, the housing being wrapped around the outer periphery of the first battery cell, the housing having a first side and a second side opposite to the first side in the first direction; the distance between the first side and the second side being related to the thickness of the first battery cell and the thickness of the second battery cell as follows: 0.89≤(nD1+mD2) / h≤0.94; where h represents the distance between the first side and the second side, D1 represents the thickness of the first battery cell, D2 represents the thickness of the second battery cell, n represents the number of the first battery cells, and m represents the number of the second battery cells, 2≤n≤4, and 1≤m≤4.

[0025] In a second aspect, the present application discloses a battery pack comprising the secondary battery of the first aspect.

[0026] In a third aspect, the present application discloses a use-electric device comprising the secondary battery of the first aspect or the battery pack of the second aspect, the secondary battery and the battery pack being capable of providing electric energy to the use-electric device.

[0027] The application discloses a secondary battery, a battery pack and a power consumption device, the secondary battery comprising at least one battery cell, and at least one of the battery cell comprises a positive electrode sheet, a negative electrode sheet and a separator arranged between the positive electrode sheet and the negative electrode sheet, at least one of the positive electrode sheet and the negative electrode sheet comprises a composite current collector, the composite current collector comprises a polymer substrate layer and a conductive layer arranged on the polymer substrate layer, the polymer substrate layer comprises a polymer substrate and a thermal decomposition additive, the thermal decomposition additive generates gas when reaching a decomposition temperature. In the application, the composite current collector comprises a polymer substrate layer and a conductive layer arranged on the polymer substrate layer, the polymer substrate layer is dispersed with the thermal decomposition additive which can be decomposed to release gas when the temperature in the battery cell exceeds the working temperature of the battery cell. When the battery cell is locally overheated to exceed the working temperature of the battery cell under overcharge or needle puncture, the thermal decomposition additive is decomposed and generates gas, high pressure is formed in the composite current collector, the composite current collector is swollen and even broken, and the internal circuit of the battery cell is broken. At the same time, the tab is a current collection point, the temperature rising rate of the part of the composite current collector connected with the tab is high, the thermal decomposition additive in the part of the composite current collector is preferentially decomposed, the shape of the conductive layer in the part of the composite current collector is deformed until the conductive layer is separated from the tab, so that the internal circuit of the battery cell is broken, and the continuous and violent release of energy is blocked. Moreover, under the needle puncture test, the thermal decomposition additive near the needle puncture point is decomposed and generates gas, a gap is formed between the composite current collector and the needle, and part of the conductive layer is broken, so that the contact resistance is increased, the short-circuit current is inhibited, and the local heat generation is reduced. At the same time, the gas generated by the decomposition of the thermal decomposition additive can also cause the pressure in the battery cell to rise, the explosion-proof valve is opened in advance, and the heat dissipation in the battery cell is accelerated. Therefore, by introducing the thermal decomposition additive into the composite current collector, the internal short circuit or open circuit of the battery cell can be caused when the battery cell is locally overheated, the possibility of thermal runaway is reduced, and the safety of the secondary battery is improved. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 A structure schematic diagram of the electrode sheet in the embodiment of the application is shown; Figure 2 A structure schematic diagram of the secondary battery in the embodiment of the application is shown; Reference signs: 1, polymer substrate layer; 2, thermal decomposition additive; 3, tab; 4, ceramic layer; 5, first battery cell; 6, second battery cell; 7, shell; X, first direction; Y, second direction; Z, third direction. DETAILED DESCRIPTION

[0029] With reference to the drawings of the embodiments of the present application, the technical solutions in the embodiments of the present application will be described clearly and completely. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments of the present application, all the other embodiments obtained by those skilled in the art without creative work belong to the fixed scope of the present application.

[0030] The embodiments of the present application provide a secondary battery, which comprises at least one cell, and at least one cell comprises a positive electrode sheet, a negative electrode sheet and a separator arranged between the positive electrode sheet and the negative electrode sheet. Referring to Figure 1 As shown, at least one of the positive electrode sheet and the negative electrode sheet comprises a composite current collector, and the composite current collector comprises a polymer substrate layer 1 and a conductive layer, wherein the polymer substrate layer comprises a polymer substrate and a thermal decomposition additive 2, and the thermal decomposition additive 2 generates gas when reaching a decomposition temperature; the conductive layer is arranged on the polymer substrate layer 1.

[0031] In the embodiments, the secondary battery comprises at least one cell, and the cell comprises a positive electrode sheet, a negative electrode sheet and a separator, and the separator is arranged between the positive electrode sheet and the negative electrode sheet. The positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer arranged on at least one side of the current collector, and the negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer arranged on at least one side of the negative electrode current collector. At least one of the positive electrode current collector and the negative electrode current collector in the embodiments of the present application is a composite current collector.

[0032] The composite current collector comprises a polymer substrate layer and a conductive layer, and the polymer substrate layer comprises a polymer substrate and a thermal decomposition additive, and the conductive layer is arranged on the polymer substrate layer. The polymer substrate layer serves as the structural framework of the entire composite current collector, provides an adhesion surface for the active material layer and the conductive layer, guarantees the structural integrity of the electrode sheet during manufacturing and cycling, and is also a carrier of the thermal decomposition additive.

[0033] During charging and discharging, the conductive layer is in close contact with the active material, collects the electrons generated or consumed by the active material layer during electrochemical reaction from the active material layer, and transports the electrons to the tab position of the composite current collector, and finally flows into or out of the external circuit through the tab.

[0034] Referring to Figure 1As shown, the composite current collector has a second direction Y and a third direction Z perpendicular to each other, and a tab 3 is connected to one side of the composite current collector along the second direction Y. When the battery cell is subjected to extrusion or impact, the tab root is prone to bending and deformation, and then stretching and causing deformation of the composite current collector. Therefore, the ceramic layer 4 is arranged on the side of the composite current collector close to the tab 3 in the embodiment, and the width of the ceramic layer 4 is less than the thickness of the composite current collector, which does not affect the electrical connection between the tab 4 and the conductive layer, and at the same time, can enhance the tensile strength of the composite current collector and improve the cycle life of the battery.

[0035] In the embodiment, the polymer substrate layer includes at least one of polyethylene terephthalate (PET), polyimide (PI), polyethylene naphthalate (PEN), polyether ether ketone (PEEK), and polyaryl sulfone (PT).

[0036] Among them, PET has high tensile strength, and good dimensional stability, creep resistance and wear resistance, but is not resistant to strong acid and strong base, and is easily corroded by electrolyte at high temperature, resulting in weakened battery cycle performance.

[0037] PI is a thin film type insulation material with good comprehensive performance, and has excellent mechanical properties, electrical properties, chemical stability, and high radiation resistance, high temperature resistance and low temperature resistance.

[0038] PEN is obtained by polycondensation of 2,6-dimethyl naphthalene dicarboxylate and ethylene glycol, and has more excellent mechanical properties, gas barrier properties and high temperature resistance, because the naphthalene ring in the molecular chain replaces the benzene ring structure of PET.

[0039] PEEK is an engineering plastic with semi-crystalline and thermoplastic properties, and its long-term use temperature can reach 260℃. Even in a steam environment, it can still maintain a high level of bending strength and tensile strength, so PEEK has strong high temperature resistance, chemical resistance and hydrolysis resistance.

[0040] PT is an amorphous high-temperature engineering plastic, and its molecular main chain contains both sulfone groups and biphenyl structures, and has excellent heat resistance. PT also has high mechanical strength, dimensional stability and creep resistance. In terms of chemical stability, it has good resistance to lithium ion battery electrolyte and can resist swelling and decomposition.

[0041] In the preparation of the polymer substrate layer, the thermal decomposition additive is blended with the polymer substrate to form a polymer co-extrusion liquid, and then the polymer co-extrusion liquid is laid on a workbench, and then elongated by a cold drawing process, and finally the polymer substrate layer is obtained; or a concave-convex roller is used to make grooves on the surface of the polymer substrate, and then a thermal decomposition additive particle with a particle size of less than 1 μm is adhered or filled in the grooves by a coating or screen shaking process. After the polymer substrate layer is cured, a uniform and light conductive layer is prepared on the polymer substrate layer by a process such as evaporation, ion sputtering or electrolysis.

[0042] In some embodiments, the mass percentage of the thermal decomposition additive in the polymer substrate layer is 1% to 1.7%. In this concentration range, the thermal decomposition additive is dispersed in the polymer substrate, ensuring that when the battery cell encounters heat, overcharge, needle puncture and other thermal abuses, sufficient gas can be generated rapidly, and the composite current collector is caused to swell, break or cause the composite current collector and the tab to separate, thereby actively creating a controllable internal circuit, reducing the possibility of thermal runaway and improving the safety of the battery. At the same time, the gas generated by the decomposition of the thermal decomposition additive can also cause the internal pressure of the battery cell to rise, prompting the explosion-proof valve to open early and accelerating the dissipation of heat inside the battery cell. Exemplarily, the mass percentage of the thermal decomposition additive in the polymer substrate layer is 1%, 1.2%, 1.4%, 1.5% or 1.7%.

[0043] In the preparation of the polymer substrate layer, the thickness of the polymer substrate layer can be controlled to be 2 to 4 μm. In this thickness range, the polymer substrate layer can provide sufficient mechanical support for the pole piece, ensuring the integrity of the pole piece structure during subsequent processing such as coating, rolling and cell forming. At the same time, when the thickness of the polymer substrate layer is in this thickness range, the space for filling the active material layer in the battery cell is larger under the condition that the volume of the battery cell is unchanged, which can further improve the energy density of the battery.

[0044] In the preparation of the polymer substrate layer, the thickness of the polymer substrate layer can be controlled to be 2 to 4 μm. In this thickness range, the polymer substrate layer can provide sufficient mechanical support for the pole piece, ensuring the integrity of the pole piece structure during subsequent processing such as coating, rolling and cell forming. At the same time, when the thickness of the polymer substrate layer is in this thickness range, the space for filling the active material layer in the battery cell is larger under the condition that the volume of the battery cell is unchanged, which can further improve the energy density of the battery.

[0045] In some embodiments, the thermal decomposition additive can generate gas when reaching the decomposition temperature, which is determined by the physicochemical properties of the thermal decomposition additive. Since the optimal working temperature of the battery is usually 15 to 35℃. If the local temperature continuously exceeds 60℃, it indicates that the battery has begun to be abnormal. Therefore, the decomposition temperature of the thermal decomposition additive selected in the embodiments of the present application is between 60℃ and 230℃.

[0046] Since thermal runaway in battery cells is typically triggered by a chain of exothermic reactions, the thermal decomposition of the solid electrolyte interphase (SEI) film at approximately 100°C is a key initiating step. Given this mechanism, if a thermally decomposable additive can be used to decompose and release gases at or above 100°C, causing the composite current collector to bulge and fracture, thereby actively creating a controllable physical circuit break within the battery cell, residual electrical energy can be consumed and subsequent violent chain reactions can be blocked, thus inhibiting or delaying the full-scale occurrence of thermal runaway. Therefore, the decomposition temperature of the thermally decomposable additive can be controlled between 100°C and 230°C.

[0047] In some embodiments, the thermally decomposable additive includes at least one of organic acids, chemical foaming agents, inorganic acids, and crystalline hydrates.

[0048] In some embodiments, the organic acid includes at least one selected from citric acid, tartaric acid, and oxalic acid. Citric acid begins to melt and decompose at approximately 135°C, producing carbon dioxide, carbon monoxide, water, and oxygen-containing organic matter. Tartaric acid undergoes thermal decomposition at a temperature between 135°C and 140°C, producing gaseous products such as carbon dioxide and water vapor. Oxalic acid sublimates and begins to decompose at approximately 102°C, producing carbon monoxide, carbon dioxide, formic acid, and water.

[0049] In some embodiments, the chemical blowing agent includes at least one selected from azodicarbonamide, 4,4'-oxobisbenzenesulfonylhydrazine, p-toluenesulfonamide, and sodium bicarbonate. Azodicarbonamide decomposes at a temperature of 190°C to 205°C, producing nitrogen, carbon monoxide, and a small amount of carbon dioxide. 4,4'-oxobisbenzenesulfonylhydrazine begins to decompose at approximately 130°C and undergoes vigorous decomposition in the range of 140°C to 170°C, producing nitrogen and water vapor. p-Toluenesulfonamide decomposes at a temperature of 225°C to 235°C, producing primarily nitrogen and carbon dioxide. Sodium bicarbonate begins to decompose at approximately 85°C and completely decomposes at 140°C, producing carbon dioxide, water, and sodium carbonate.

[0050] In some embodiments, the hydrated component includes copper sulfate pentahydrate, which begins to gradually dehydrate at about 110°C, generating water vapor, and eventually forming anhydrous copper sulfate. The inorganic acid includes borax, which begins to lose its water of crystallization at about 300°C, generating water vapor.

[0051] In practical applications, one of these thermal decomposition additives can be added to the polymer substrate to form a polymer substrate layer, or two or more of these thermal decomposition additives with different decomposition temperatures can be compounded and added to the polymer substrate to form a polymer substrate layer.

[0052] In some embodiments, the thermal decomposition additive can be uniformly dispersed in the polymer substrate to form a polymer substrate layer. The thermal decomposition additive uniformly dispersed in the polymer substrate layer can ensure that no matter where the thermal runaway starts, there is sufficient thermal decomposition additive around the thermal runaway starting point to decompose and generate gas, which expands sharply in the sealed space between the polymer substrate layer and the conductive layer, and the resulting stress can cause the composite current collector to swell, tear, or even physically break, preventing the spread of thermal runaway.

[0053] In other embodiments, the composite current collector has a second direction Y and a third direction Z perpendicular to each other. Along the second direction Y, the composite current collector includes a first portion and a second portion, one side of the first portion is used to connect the tab, and the second portion is connected to the side of the first portion away from the tab. The mass of the thermal decomposition additive in the second portion is less than or equal to the mass of the thermal decomposition additive in the first portion. The mass of the thermal decomposition additive in the first portion is 0.5% to 1.5% of the mass of the polymer substrate layer. The mass of the thermal decomposition additive in the second portion is 0.5% to 1% of the mass of the polymer substrate layer.

[0054] It should be noted that the function of the tab is to collect and conduct current. In the composite current collector, the polymer substrate layer mainly assumes the functions of mechanical support and insulation, and itself is an insulator and cannot establish an effective electron conduction path. The conductive layer is responsible for electron transport, so the tab needs to form a low-resistance ohmic contact with the conductive layer to ensure that electrons can flow in or out.

[0055] In specific applications, part or all of the composite current collector is divided into zones along the second direction Y. For example, the composite current collector is divided into a first portion and a second portion, the first portion connects the tab, and the second portion is connected to the side of the first portion away from the tab. In the case where the first portion and the second portion have equal areas, the mass of the thermal decomposition additive in the first portion and the second portion is set as follows: when the mass of the thermal decomposition additive in the first portion is equal to the mass of the thermal decomposition additive in the second portion, the first portion and the second portion constitute uniform protection; when the mass of the thermal decomposition additive in the first portion is greater than the mass of the thermal decomposition additive in the second portion, the first portion near the tab, where the current is collected and the temperature rises faster, can produce more gas, accelerating the internal short circuit or open circuit of the battery cell, and the second portion produces gas at a slower speed and in a smaller amount.

[0056] However, when the first part and the second part are not equal in area, the use of mass or mass ratio cannot accurately reflect the concentration difference of the thermal decomposition additive in the first part and the second part. Therefore, the unit area additive mass is introduced. Whether the area of the first part and the second part is equal or not, the unit area additive mass of the second part should be less than or equal to that of the first part. In the case where the area of the first part and the second part is not equal, the unit area additive mass of the thermal decomposition additive in the first part and the second part is set: when the unit area additive mass of the thermal decomposition additive in the first part is equal to that in the second part, the gas production of the first part and the second part is the same; when the unit area additive mass of the thermal decomposition additive in the first part is greater than that in the second part, the first part near the tab has more gas production due to faster current collection and temperature rise, so as to accelerate the internal circuit breaking of the battery cell.

[0057] Specifically, since the tab is the collection point of current, under the conditions of overcharge, heating, needle puncture and other thermal runaway, the first part near the tab will first rise in temperature due to the Joule heating effect of resistance, and it is the area with the highest temperature of the entire composite current collector. Therefore, the mass or unit area additive mass of the thermal decomposition additive in the first part is slightly higher than that in other parts of the composite current collector, which can be set to 0.5% to 1.5% of the mass of the polymer matrix layer, so as to realize rapid response of gas production. Once thermal abuse occurs, the thermal decomposition additive in the first part will rapidly decompose and produce gas due to the local high temperature in the first part, which preferentially causes the conductive layer near the connection between the composite current collector and the tab to swell, peel off or even break, thereby creating a physical breaking point, cutting off the circuit, reducing the further accumulation of heat and energy, and reducing the risk of thermal runaway spreading. At the same time, the thermal decomposition additive in the first part preferentially decomposes, causing the conductive layer in the first part to deform in shape until it separates from the tab, thereby achieving internal circuit breaking of the battery cell and blocking the continuous and intense release of energy. Exemplarily, the mass of the thermal decomposition additive in the first part is 0.5%, 0.8%, 1.0%, 1.2%, 1.4% or 1.5% of the mass of the polymer matrix layer. Correspondingly, the second part is designed as a secondary safety protection zone because it is far away from the tab, and the mass of the thermal decomposition additive in the second part is 0.5% to 1% of the mass of the polymer matrix layer, which is to cope with the intensification of thermal abuse and the diffusion of heat from the tab area to the first part and then to the second part. The thermal decomposition additive in the second part acts as a second line of defense and decomposes to produce gas when the temperature reaches its decomposition temperature, causing the conductive layer in the second part to swell and inducing a controllable local short circuit in the battery cell, thereby dissipating the residual electrical energy in the battery cell and preventing the spread of thermal runaway.

[0058] Exemplarily, the mass of the thermal decomposition additive in the second part is 0.5%, 0.6%, 0.75% or 1% of the mass of the polymer matrix layer.

[0059] In some embodiments, the composite current collector further comprises a third portion connected to the second portion away from the first portion, the mass of the thermal decomposition additive in the third portion is less than the mass of the thermal decomposition additive in the second portion, and / or the unit area additive mass of the thermal decomposition additive in the third portion is less than the unit area additive mass of the thermal decomposition additive in the second portion; the mass of the thermal decomposition additive in the third portion is 0-0.2% of the mass of the polymer base material layer.

[0060] In the present embodiment, the composite current collector is divided into the first portion, the second portion and the third portion along the second direction Y, the first portion is close to the tab, the second portion is located on the side of the first portion away from the tab, and the third portion is connected to the second portion away from the third portion, and the distance between the first portion, the second portion and the third portion and the tab gradually increases. Therefore, when the areas of the first portion, the second portion and the third portion are equal, the mass of the thermal decomposition additive in the third portion is less than the mass of the thermal decomposition additive in the second portion. In specific applications, the mass of the thermal decomposition additive in the third portion is 0-0.2% of the mass of the polymer base material layer, for example, the mass of the thermal decomposition additive in the third portion is 0.1%, 0.15% or 0.2% of the mass of the polymer base material layer. When the areas of the first portion, the second portion and the third portion are not equal, the unit area additive mass of the thermal decomposition additive in the third portion is less than the unit area additive mass of the thermal decomposition additive in the second portion.

[0061] In some embodiments, the composite current collector has a second direction Y and a third direction Z perpendicular to each other, along the second direction Y, the size of the first portion accounts for 30-35% of the size of the composite current collector, along the second direction Y, the size of the second portion accounts for 30-35% of the size of the composite current collector, and along the second direction Y, the size of the third portion accounts for 30-35% of the size of the composite current collector. Along the third direction Z, the size of the first portion, the size of the second portion and the size of the third portion are equal to the size of the composite current collector.

[0062] In a specific application, when the battery cell is subjected to thermal abuse, heat is generated from the tab where current is collected and diffuses outward, forming a clear temperature gradient, i.e. the closer to the tab, the higher the temperature and the faster the temperature rises, and vice versa. Therefore, the concentration of the thermal decomposition additive in the first part, the second part and the third part is gradually decreased in a gradient. When a fault occurs, the first part with the highest temperature will trigger protection first, and the high concentration of the additive will decompose rapidly, which is beneficial to quickly achieve physical disconnection; if the heat continues to diffuse outward, the thermal decomposition additive in the second part will be activated to dissipate energy and prevent the spread of thermal runaway by gas swelling; and the third part of the additive content is reduced to a very low level or even zero, which ensures that the composite current collector can still maintain its structural integrity and normal current collecting and conducting function in the area far from the tab where the heat may not have reached, thereby maximizing the performance reliability and stability of the battery cell in a non-fault state while achieving safety protection.

[0063] Exemplarily, the second direction Y is the width of the composite current collector, the third direction Z is the length of the composite current collector, the length of the first part, the second part and the third part is equal to the length of the composite current collector, the width of the first part accounts for 33.3% of the width of the composite current collector, the width of the second part accounts for 33.3% of the width of the composite current collector, the width of the third part accounts for 33.3% of the width of the composite current collector, and the mass of the thermal decomposition additive in the first part is 1% of the mass of the polymer substrate layer, the mass of the thermal decomposition additive in the second part is 0.5% of the mass of the polymer substrate layer, and the mass of the thermal decomposition additive in the third part is 0.2% of the mass of the polymer substrate layer.

[0064] Referring to Figure 2 In some embodiments, the secondary battery has a first direction X, the secondary battery includes a battery cell, the battery cell includes a first battery cell 5 and a second battery cell 6, at least one of the first battery cell 5 and the second battery cell 6 includes the above-mentioned composite current collector, and the secondary battery includes a middle part and side parts located on both sides of the middle part along the first direction X; the first battery cell 5 is located in the side part, and the second battery cell 6 is located in the middle part; the capacity relationship of the first battery cell 5 and the second battery cell 6 is represented as: 0.7≤Q1 / Q2≤1.3, wherein Q1 represents the capacity of the first battery cell, and Q2 represents the capacity of the second battery cell.

[0065] It should be noted that, in an embodiment, as Figure 2As shown, the thickness direction of the secondary battery is the first direction X, which is also equivalent to the arrangement direction of the first cell 5 and the second cell 6. The secondary battery comprises the cells and a shell 7, the shell 7 is provided with a receiving cavity, the receiving cavity is divided into a middle part and side parts located on both sides of the middle part along the thickness direction of the secondary battery, the cells comprise the first cell 5 and the second cell 6, and the first cell 5 and the second cell 6 are connected in series or parallel. The first cell 5 is located in the side part, and the second cell 6 is located in the middle part. The current collector in the first cell 5 and / or the second cell 6 is the composite current collector described above, and the beneficial effects of the composite current collector in the first cell 5 and / or the second cell 6 are the same as the structure of the composite current collector in the above embodiment, and the beneficial effects are similar, which will not be repeated here.

[0066] In the embodiments of the present application, the capacity ratio of the cells at different positions in the battery is also controlled. Specifically, each side part is provided with a first cell, the first cell comprises at least one, the second cell is located in the middle part, the second cell comprises at least one, and the capacity ratio of the first cell to the second cell is controlled to be between 0.7 and 1.3. This is because the first cell located in the side part is exposed to the environment, and the heat dissipation condition is better than the second cell which is sandwiched and has limited heat dissipation.

[0067] When the capacity ratio of the first cell and the second cell is controlled to be between 0.7 and 1.3, it is helpful to keep the state of charge and voltage plateau of the first cell and the second cell in the charging and discharging process synchronous, and at the same time, since the side part provided with the first cell of moderate capacity has better heat dissipation condition, it is beneficial to make the first cell dissipate the heat generated during operation to the external environment in time, reduce the generation of excessive local current density and heat accumulation, and further improve the uniformity of the internal heat distribution of the battery, and improve the cycle life of the battery.

[0068] In some embodiments, the capacity relationship of the first cell and the second cell is represented as: 0.9≤Q1 / Q2≤1.2, which can better balance long cycle life, high energy density and excellent manufacturing process, so that the battery reaches a balanced state in terms of thermal management and electrical performance. Exemplarily, Q1 / Q2 is 0.7, 0.8, 0.9, 1.0, 1.1, 1.2 or 1.3.

[0069] It should be noted that the measurement method of the cell capacity comprises: disassembling the cell, taking the positive plate for battery test, obtaining the capacity Q a of the part of the positive plate, measuring the area S1 of the part of the positive plate and the area S2 of the positive plate, and calculating the cell capacity Q=S2 / S1×Q a .

[0070] In some embodiments, the relationship between the physical resistances of the first and second cells is represented as: 0.8≤r1 / r2≤1.2, where r1 represents the physical resistance of the first cell and r2 represents the physical resistance of the second cell.

[0071] It should be noted that the physical resistance of a cell is used to represent the transmission resistance of electrons inside the cell, reflecting the difficulty of current passing. The smaller the physical resistance value of the cell, the better the conductivity of the cell. The measurement method of the physical resistance of the first and second cells includes: measuring the physical resistance r of the battery at 1000 Hz with an alternating resistance meter k , disassembling the battery shell, keeping the connection of the first and second cells and the pole, then disassembling the connection between the first cell and the pole, and measuring the physical resistance r2 of the second cell at 1000 Hz with an alternating resistance meter. k

[0072] In this embodiment, the physical resistance ratio r1 / r2 of the first and second cells is controlled in the range of 0.8 to 1.2, which can promote the uniform distribution of current inside the battery, reduce the current bottleneck caused by the too high physical resistance of a certain cell, thereby causing local overheating or inconsistent charge and discharge depth, and further alleviate the decay rate of a single cell, improve the rate performance and cycle life of the battery, and also improve the stability of the output voltage of the battery. For example, r1 / r2 is 0.8, 0.9, 1.0, 1.1 or 1.2.

[0073] In some embodiments, the relationship between the direct current resistances of the first and second cells is represented as: 0.75≤R1 / R2≤1.35, where R1 represents the direct current resistance of the first cell and R2 represents the direct current resistance of the second cell.

[0074] It should be noted that the direct current resistance is used to represent the resistance ability of the cell to direct current, and the lower the direct current resistance value, the better the conductive path and the better the dynamic performance of the cell. The measurement method of the direct current resistance of the first and second cells includes: disassembling the battery to obtain the first and second cells, keeping the tabs of the first and second cells, and electrically connecting the tabs with aluminum sheets and nickel sheets, then packaging with aluminum plastic film, recording the initial voltage U1 of the first cell, applying a constant direct current I to the first cell, recording the voltage U2 before and after applying the direct current, and calculating the direct current resistance R1 of the first cell as (U1-U2) / I; recording the initial voltage U3 of the second cell, applying a constant direct current I to the second cell, recording the voltage U4 before and after applying the direct current, and calculating the direct current resistance R2 of the second cell as (U3-U4) / I.

[0075] ​In the embodiment, the ratio of the direct current resistance of the first cell located at the side part to the direct current resistance of the second cell located at the middle part is controlled in the range of 0.75 to 1.35, which is beneficial to the first cell and the second cell with different heat dissipation conditions to work cooperatively under current load, reduces the accelerated aging of the second cell with poor heat dissipation condition due to the mismatch of direct current resistance, and reduces the problem of uneven current distribution caused by too large difference in direct current resistance, thereby improving the cycle life of the battery. For example, R1 / R2 is 0.75, 0.9, 1.05, 1.2 or 1.35.

[0076] In some embodiments, 1.4≤(r1 / r2)×(R1 / R2)≤3.8, wherein r1 represents the physical resistance of the first cell, r2 represents the physical resistance of the second cell, R1 represents the direct current resistance of the first cell, and R2 represents the direct current resistance of the second cell.

[0077] In the embodiment, (r1 / r2)×(R1 / R2) reflects the matching degree of static conductive capacity and dynamic reaction kinetics between the first cell located at the side part and the second cell located at the middle part, and the value can simultaneously regulate the current distribution and thermal management performance of the battery.

[0078] Specifically, by controlling (r1 / r2)×(R1 / R2) in the range of 1.4 to 3.8, an appropriate amount of current can be guided to pass through the first cell with better heat dissipation condition preferentially, while ensuring that the second cell bears an appropriate amount of current load, thereby reducing the premature decay of the second cell due to heat accumulation and improving the cycle life of the battery. For example, (r1 / r2)×(R1 / R2) is 1.4, 1.8, 2.2, 2.6, 3.0, 3.4 or 3.8.

[0079] In some embodiments, 0.86≤D1 / D2≤1, wherein D1 represents the thickness of the first cell, and D2 represents the thickness of the second cell. The thickness of the first cell refers to the size of the first cell in the first direction X, and the thickness of the second cell refers to the size of the second cell in the first direction X.

[0080] In the embodiment, D1 / D2 is controlled in the range of 0.86 to 1, which indicates that the thicknesses of the first cell and the second cell are relatively uniform, and when D1 / D2 is less than 1, the first cell with better heat dissipation condition is slightly thinner than the second cell located at the middle part, which can promote the rapid heat dissipation of the first cell and further promote the heat loss of the second cell, thereby prolonging the cycle life. At the same time, it also reduces the excessive thinning of the first cell for the purpose of heat dissipation, and realizes the balance between heat dissipation and battery energy density. For example, D1 / D2 is 0.86, 0.90, 0.94, 0.98 or 1.

[0081] Referring toFigure 2 As shown, in some embodiments, the secondary battery further comprises a shell 7, the shell 7 is wrapped around the outer periphery of the first battery cell 6, along the first direction X, the shell 7 has a first side and a second side opposite to the first side; the distance between the first side and the second side is related to the thickness of the first battery cell 6 and the thickness of the second battery cell 7, which is expressed as: 0.89≤(nD1+mD2) / h≤0.94; wherein, h represents the distance between the first side and the second side, D1 represents the thickness of the first battery cell, D2 represents the thickness of the second battery cell, n represents the number of the first battery cell, m represents the number of the second battery cell, 2≤n≤4, 1≤m≤4.

[0082] It should be noted that the thickness of the first battery cell 6 refers to the size of the first battery cell 6 in the first direction X, and the thickness of the second battery cell 7 refers to the size of the second battery cell 7 in the first direction X.

[0083] In the present embodiment, (nD1+mD2) / h is between 0.89 and 0.94, indicating that the total thickness of the first battery cell and the second battery cell occupies most of the space inside the shell, but still reserves the necessary buffer gap, which provides accommodation area for the normal volume expansion of the first battery cell and the second battery cell in long-term cycling, reduces the expansion force caused by the rigid restriction of the shell due to the expansion of the first battery cell and the second battery cell, which is the main reason for the failure of active material contact, the increase of internal resistance and even the decrease of cycle life. At the same time, (nD1+mD2) / h is between 0.89 and 0.94, which is also conducive to ensuring the high energy density of the battery. Exemplarily, n takes a value of 2, 3 or 4, m takes a value of 1, 2, 3 or 4, (nD1+mD2) / h is 0.89, 0.90, 0.91, 0.92, 0.93 or 0.94.

[0084] The present application also provides a battery pack comprising the above-mentioned secondary battery.

[0085] In some embodiments, the above-mentioned secondary battery can be assembled into a battery pack, and the number of secondary batteries included in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0086] The present application also provides a power consuming device comprising the above-mentioned secondary battery or the above-mentioned battery pack, which can provide power to the power consuming device.

[0087] Specifically, the above-mentioned power consuming device can be, but is not limited to, an electric vehicle, an electric scooter, a mobile phone, a tablet, a notebook computer, an electric toy, a ship, a spacecraft, etc. The electric toy can include fixed or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric plane toys, etc. The spacecraft can include airplanes, rockets, space shuttles, and spaceships, etc.

[0088] The technical solutions of the present application are further illustrated below in combination with comparative examples and examples.

[0089] The batteries were prepared based on the parameters shown in Table 1 respectively and according to the following methods. Step 1: Preparation of negative electrode sheet PT and thermal decomposition additive were mixed uniformly to prepare a polymer base layer with a thickness of 4 μm, and then a conductive layer was evaporated on the surface of the polymer base layer to obtain a composite current collector. The material of the conductive layer was copper.

[0090] Graphite, conductive carbon black, butadiene styrene rubber and sodium carboxymethyl cellulose were mixed and added into a solvent to prepare a negative electrode slurry, the negative electrode slurry was coated on the composite current collector, and the negative electrode sheet was prepared by drying, rolling and slicing. The mass ratio of graphite, conductive carbon black, butadiene styrene rubber and sodium carboxymethyl cellulose was 96:1:1.8:1.2.

[0091] Step 2: Preparation of positive electrode sheet Lithium iron phosphate, conductive carbon black and polyvinylidene fluoride were mixed and added into a solvent to prepare a positive electrode slurry, the positive electrode slurry was coated on an aluminum foil, and the positive electrode sheet was prepared by drying, rolling and slicing. The mass ratio of lithium iron phosphate, conductive carbon black and polyvinylidene fluoride was 97:0.8:2.2.

[0092] Step 3: Preparation of battery cell The positive electrode sheet, the separator and the negative electrode sheet were stacked in sequence to prepare a wound bare battery cell by a winding process. Then, the wound bare battery cell was heat-pressed to stabilize its structure, and the thickness of the wound bare battery cell after heat-pressing was measured by a micrometer. If there was a deviation between the measured thickness and the designed thickness, the winding process parameters (such as tension, alignment, etc.) were adjusted and the winding and heat-pressing were performed again until a bare battery cell meeting the design specifications was obtained. Then, an electrolyte was injected into the bare battery cell to obtain a battery cell. The electrolyte was a 1 mol / L LiPF6 solution, wherein the solvent was a mixed solvent composed of ethylene carbonate, dimethyl carbonate and diethyl carbonate, and the additives were fluoroethylene carbonate and vinylene carbonate. It should be noted that the mass ratio of ethylene carbonate, dimethyl carbonate, diethyl carbonate, fluoroethylene carbonate and vinylene carbonate was 0.3:0.3:0.3:0.07:0.03. The separator was a polypropylene separator.

[0093] Step 4: Preparation of battery Four battery cells were assembled in a shell to prepare a battery including four battery cells. The two battery cells located at the side were first battery cells, and the two battery cells located in the middle were second battery cells.

[0094] It should be noted that the type of thermal decomposition additive, the mass ratio w of the thermal decomposition additive in the polymer substrate layer, and the thickness L of the conductive layer of each battery cell in the examples and comparative examples are shown in Table 1, the capacity Q1, the physical resistance r1, the direct current resistance R1 and the thickness D1 of the first battery cell, and the capacity Q2, the physical resistance r2, the direct current resistance R2 and the thickness D2 of the second battery cell, and the distance h between the first side and the second side of the shell are shown in Table 1.

[0095] Q1 / Q2, r1 / r2, R1 / R2, (r1 / r2) x (R1 / R2), D1 / D2 and (2D1+2D2) / h of each example and comparative example were calculated according to the parameters in Table 1, and the results are shown in Table 2. The batteries prepared from the examples and comparative examples were tested for performance, and the test results are shown in Table 3, and the test methods include: (1) Cycle 500 cycle capacity retention rate test: at 25±2℃, the battery is subjected to 1 to 3 complete charge and discharge cycles to activate and stabilize its performance, and the discharge capacity of the last cycle is recorded as the initial capacity C0. Subsequently, enter the cycle test phase, charge to the upper limit voltage with 1C constant current constant voltage, and then discharge to the lower limit voltage with 1C constant current, repeat the step 500 times. After the completion of the 500th cycle, record the discharge capacity C 500 of the battery, calculate the cycle 500 cycle capacity retention rate / %=(C 500 / C0) x 100.

[0096] (2) Needle test: the battery is fully charged, a high-temperature resistant steel needle with a diameter of 5mm is used to pierce the battery at a speed of 0.1mm / s, the piercing depth is 30% of the thickness of the battery, and the high-temperature resistant steel needle is left in the battery for 1 hour after piercing the battery, which is considered to complete the test. If no fire or explosion occurs during the test, it is considered to pass (pass). Three independent needle tests were performed on the batteries of the same example or comparative example.

[0097] (3) Overcharge test: perform initialization charging, connect the battery with the charge and discharge equipment, connect the voltage and temperature data sampling line of the charge and discharge device with the battery according to the requirements; then charge with constant current until the voltage reaches 1.5 times the battery charging cutoff voltage or the time reaches 1h, stop charging, observe after 1h, complete the test, and if the battery does not catch fire or explode during the test, it is considered to pass (pass). Three independent process tests were performed on the batteries of the same example or comparative example.

[0098] (4) Heating test: the fully charged battery was directly placed on a flat or rod-shaped heating device; the heating device was controlled to start a heating program, and the temperature was raised to 130±2℃ at a rate of 5℃ / min, and kept at this temperature for 30 minutes. During the temperature rising process and the subsequent 30 minutes of holding, the battery did not catch fire or explode, and was considered to pass. Three independent heating tests were performed on the batteries of the same example or comparative example.

[0099] (5) Overcharge heating test: the battery was directly placed on a flat or rod-shaped heating device after being fully charged; the heating device was controlled to start a heating program, and the temperature was raised to 130±2℃ at a rate of 5℃ / min, and kept stable at this temperature. At the same time of starting the heating program, the overcharge test was immediately applied to the battery, i.e. charging at a constant current until the voltage reached 1.5 times the battery charging cutoff voltage or the time reached 1h, and the test was completed after 1h of observation. During the test, the battery did not catch fire or explode, and was considered to pass. Three independent overcharge heating tests were performed on the batteries of the same example or comparative example.

[0100] Table 1

[0101] Table 2

[0102] Table 3

[0103] As can be seen from the above table, compared with the comparative examples, the capacity retention rate of the battery is improved, and the pass rate of each test performance is improved.

[0104] It should be understood that the reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described is included in at least one embodiment of the application. Therefore, appearances of “in one embodiment” or “in an embodiment” at various places throughout the specification are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, or characteristics can be combined in any suitable manner in one or more embodiments.

[0105] It should be noted that each of the embodiments in the present specification is described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same and similar parts between embodiments can be referred to each other.

[0106] While optional embodiments of the application have been described, those skilled in the art will recognize that many modifications and variations of the described implementations can be made without departing from the scope of the application. It is therefore intended that the appended claims cover all such modifications and variations as fall within the scope of the application.

[0107] Finally, it should be noted that, in the description above, relative terms such as first and second, etc. are used merely to distinguish one entity from another without necessarily requiring or implying any actual such relationship or order between or among these entities. Also, the terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process or method that comprises a list of elements does not include only those elements but can also include other elements not expressly listed or inherent to such process or method. An element proceeded by "comprises a" does not, without more constraints, foreclose the existence of additional identical elements in the process or method.

[0108] The above detailed description of the application has been presented for the purposes of clarity and understanding. It is not intended to be exhaustive or to limit the application to the precise form described. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the application be limited not with this detailed description, but rather by the claims appended hereto.

Claims

1. A secondary battery, characterized in that, The device includes at least one battery cell, wherein at least one of the battery cells includes a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, and at least one of the positive electrode and the negative electrode includes a composite current collector, the composite current collector comprising: A polymer substrate layer, the polymer substrate layer comprising a polymer substrate and a thermally decomposable additive, the thermally decomposable additive generating gas when a decomposition temperature is reached; A conductive layer is disposed on the polymer substrate layer.

2. The secondary battery according to claim 1, characterized in that, Satisfy at least one of the following: In the polymer substrate layer, the thermally decomposable additive accounts for 1% to 1.7% by mass; The decomposition temperature is 60~230℃; The particle size of the thermally decomposable additive is ≤1μm.

3. The secondary battery according to claim 1, characterized in that, The thermal decomposition additive includes at least one of organic acids, chemical foaming agents, inorganic acids, and crystalline hydrates.

4. The secondary battery according to claim 3, characterized in that, Satisfy at least one of the following: The organic acid includes at least one of citric acid, tartaric acid and oxalic acid; The chemical foaming agent includes at least one of azodicarbonamide, 4,4'-oxobisbenzenesulfonylhydrazine, p-toluenesulfonamide and sodium bicarbonate; The crystalline hydrate includes copper sulfate pentahydrate; The inorganic acid includes borax.

5. The secondary battery according to claim 1, characterized in that, The composite current collector includes a first part and a second part, one side of the first part is used to connect to the electrode tab, and the second part is connected to the side of the first part opposite to the electrode tab; The mass of the thermal decomposition additive in the second part is less than or equal to the mass of the thermal decomposition additive in the first part, and / or the mass of the thermal decomposition additive per unit area in the second part is less than or equal to the mass of the thermal decomposition additive per unit area in the first part. The mass of the thermally decomposable additive in the first part is 0.5% to 1.5% of the mass of the polymer substrate layer; The mass of the thermally decomposable additive in the second part is 0.5% to 1% of the mass of the polymer substrate layer.

6. The secondary battery according to claim 5, characterized in that, The composite current collector further includes a third part, which is connected to the side of the second part away from the first part. The mass of the thermal decomposition additive in the third part is less than the mass of the thermal decomposition additive in the second part, and / or the mass of the thermal decomposition additive per unit area in the third part is less than the mass of the thermal decomposition additive per unit area in the second part. The mass of the thermally decomposable additive in the third part is 0 to 0.2% of the mass of the polymer substrate layer.

7. The secondary battery according to claim 6, characterized in that, The composite current collector has a second direction and a third direction perpendicular to each other. Along the second direction, the size of the first part accounts for 30% to 35% of the size of the composite current collector. Along the second direction, the size of the second part accounts for 30% to 35% of the size of the composite current collector. Along the second direction, the size of the third part accounts for 30% to 35% of the size of the composite current collector. Along the third direction, the dimensions of the first part, the second part, and the third part are all equal to the dimensions of the composite current collector.

8. The secondary battery according to claim 1, characterized in that, The thickness of the polymer substrate layer is 2~4μm; And / or, the thickness of the conductive layer is 2~4μm.

9. The secondary battery according to claim 1, characterized in that, The secondary battery has a first orientation, the battery cell includes a first battery cell and a second battery cell, at least one of the first battery cell and the second battery cell includes the composite current collector, and the secondary battery includes a middle portion and side portions located on both sides of the middle portion along the first orientation. The first battery cell is located on the side, and the second battery cell is located in the middle. The relationship between the capacities of the first battery cell and the second battery cell is expressed as: 0.7≤Q1 / Q2≤1.3, where Q1 represents the capacity of the first battery cell and Q2 represents the capacity of the second battery cell.

10. The secondary battery according to claim 9, characterized in that, Satisfy at least one of the following: The relationship between the physical resistances of the first battery cell and the second battery cell is expressed as: 0.8≤r1 / r2≤1.2, where r1 represents the physical resistance of the first battery cell and r2 represents the physical resistance of the second battery cell; The relationship between the DC resistance of the first battery cell and the second battery cell is expressed as: 0.75≤R1 / R2≤1.35, where R1 represents the DC resistance of the first battery cell and R2 represents the DC resistance of the second battery cell; 1.4≤(r1 / r2)×(R1 / R2)≤3.8, where r1 represents the physical resistance of the first battery cell, r2 represents the physical resistance of the second battery cell, and R1 represents the DC resistance of the first battery cell, and R2 represents the DC resistance of the second battery cell; 0.86≤D1 / D2≤1, where D1 represents the thickness of the first battery cell, D2 represents the thickness of the second battery cell, the thickness of the first battery cell is the dimension of the first battery cell in the first direction, and the thickness of the second battery cell is the dimension of the second battery cell in the first direction.

11. The secondary battery according to claim 9 or 10, characterized in that, The secondary battery also includes: A housing that covers the outer periphery of the first battery cell, and along the first direction, the housing has a first side and a second side opposite to the first side; The relationship between the distance between the first side and the second side and the thickness of the first cell and the second cell is expressed as: 0.89≤(nD1+mD2) / h≤0.94; Wherein, h represents the distance between the first side and the second side, D1 represents the thickness of the first battery cell, D2 represents the thickness of the second battery cell, n represents the number of the first battery cells, m represents the number of the second battery cells, 2≤n≤4, 1≤m≤4.

12. A battery pack, characterized in that, Includes the secondary battery as described in any one of claims 1-11.

13. An electrical appliance, characterized in that, The device includes a secondary battery as described in any one of claims 1-11 or a battery pack as described in claim 12, wherein the secondary battery and the battery pack are capable of providing electrical energy to the electrical device.

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