Secondary battery and power consuming device

Abstract

The present invention provides a secondary battery and a power consumption device, the secondary battery including a stacked electrode assembly, the electrode assembly including stacked positive and negative electrode pieces, and a separator disposed between the positive and negative electrode pieces, the outermost electrode piece of the electrode assembly being a single-sided positive electrode piece, the single-sided positive electrode piece including a first positive electrode current collector and a positive electrode active material layer, the first positive electrode current collector including a first polymer layer and a first conductive layer disposed on the surface of the first polymer layer facing the inside of the electrode assembly, the positive electrode active material layer being disposed on the first conductive layer, the positive electrode pieces other than the outermost electrode piece in the electrode assembly being double-sided positive electrode pieces, the double-sided positive electrode piece including a second positive electrode current collector and positive electrode active material layers disposed on both surfaces of the second positive electrode current collector, the second positive electrode current collector of at least one double-sided positive electrode piece being made of metal. The above overall structure of the secondary battery enhances the safety of the secondary battery.

Description

[Technical Field]

[0001] The present invention relates to the field of electrochemical technology, and in particular to secondary batteries and power consuming devices. [Background technology]

[0002] In recent years, secondary batteries, such as lithium-ion batteries, have been rapidly adopted and widely used in the market. However, due to the high activity of lithium itself, safety accidents frequently occur during the use of lithium-ion batteries, which is a major factor restricting the further widespread use of lithium-ion batteries. One of the most typical safety accidents involving lithium-ion batteries is when an external force, such as an impact, causes internal compression of the electrode assembly, resulting in a short circuit, leading to thermal runaway and fire. Because the principles and structures of sodium-ion batteries are similar to those of lithium-ion batteries, there is also a certain risk of thermal runaway due to an internal short circuit.

[0003] Research has shown that internal short circuits in lithium-ion and sodium-ion batteries primarily occur between the positive electrode active material and the positive electrode current collector, and between the negative electrode active material and the negative electrode current collector. A short circuit between the positive electrode current collector and the negative electrode active material is considered the most dangerous because it can lead to the combustion of commonly used carbon-based negative electrode active materials. It is also the primary short circuit pattern that causes thermal runaway in lithium-ion and sodium-ion batteries. Thermal runaway significantly reduces the impact pass rate of lithium-ion and sodium-ion batteries, affecting their safety. Summary of the Invention

[0004] The present invention provides a secondary battery and a power consuming device to enhance the safety of the secondary battery.

[0005] In the present invention, the present invention will be described using a lithium ion battery as an example of a secondary battery, but the secondary battery of the present invention is not limited to a lithium ion battery and can also be applied to a sodium ion battery, etc. The specific technical solution is as follows:

[0006] A first aspect of the present invention provides a secondary battery, comprising a stacked electrode assembly including positive and negative electrode pieces, and a separator disposed between the positive and negative electrode pieces, wherein the outermost electrode piece of the electrode assembly is a single-sided positive electrode piece, the single-sided positive electrode piece including a first positive electrode current collector and a positive electrode active material layer, the first positive electrode current collector including a first polymer layer and a first conductive layer disposed on a surface of the first polymer layer facing the interior of the electrode assembly, the positive electrode active material layer being disposed on the first conductive layer, and the positive electrode pieces other than the outermost electrode piece in the electrode assembly are double-sided positive electrode pieces, the double-sided positive electrode piece including a second positive electrode current collector and positive electrode active material layers disposed on both surfaces of the second positive electrode current collector, and the second positive electrode current collector of at least one double-sided positive electrode piece is made of metal. In the electrode assembly, by using a composite current collector including a first polymer layer and a first conductive layer in the outermost single-sided positive electrode piece, the thickness of the conductive material layer (i.e., the first conductive layer) in the first positive electrode current collector of the outermost electrode piece of the electrode assembly is reduced compared to the thickness when the entire positive electrode current collector is made of a conductive metal such as aluminum in the prior art. This reduces the proportion of the conductive material layer in the first positive electrode current collector and the proportion of the fracture surface of the secondary battery that is occupied by the conductive material layer in the first positive electrode current collector of the outermost electrode piece. In this way, under operating conditions such as when the secondary battery is impacted, the probability that the current collector of the outermost electrode piece (i.e., the first positive electrode current collector) will be involved in a dangerous short circuit (i.e., a short circuit between the positive electrode current collector and the negative electrode active material layer) is reduced, thereby reducing the risk of the secondary battery catching fire or exploding. This improves the safety of the secondary battery. In addition, the positive electrode pieces, excluding the outermost electrode piece, in the electrode assembly are double-sided positive electrode pieces, thereby ensuring the energy density of the electrode assembly. By using a metal material for the current collector of at least one double-sided positive electrode piece, i.e., a conventional metal current collector, costs can be reduced because the cost of composite current collectors is relatively high. It is well known to those skilled in the art that the outermost electrode piece of a laminated electrode assembly consists of two pieces, one above the other, in the thickness direction. That is, the single-sided positive electrode piece described in the present invention refers to two electrode pieces, one above the other, in the thickness direction of the laminated electrode assembly.

[0007] In some embodiments of the present invention, the second positive electrode current collectors of one to four double-sided positive electrode pieces adjacent to the single-sided positive electrode piece are double-sided composite current collectors, and the double-sided composite current collector includes a second polymer layer and a first conductive layer provided on both surfaces of the second polymer layer. The inventors discovered that in an impact test for a stacked battery, the positive electrode pieces closest to both ends of the electrode assembly in the thickness direction have a significant impact on the impact test pass rate, the outermost single-sided positive electrode piece has the greatest impact on the impact test pass rate, followed by the one to four double-sided positive electrode pieces adjacent to the single-sided positive electrode piece, and the positive electrode pieces further inside the electrode assembly have a very small impact on the impact pass rate. Therefore, by using composite current collectors for the one to four double-sided positive electrode pieces adjacent to the outermost single-sided positive electrode piece in the thickness direction of the electrode assembly, it is possible to significantly reduce costs while ensuring an impact pass rate (the current collectors of the other double-sided positive electrode pieces may be conventional metal current collectors).

[0008] In some embodiments of the present invention, the materials of the first and second polymer layers may independently comprise at least one of polyester, polyamide, modified polyolefin, olefin copolymer, and unsaturated olefin copolymer. Selecting these materials as the materials for the polymer layers of different composite current collectors can ensure that the strength, toughness, insulation, and formability of the polymer layers meet the requirements of the composite current collector, provide good support for the first conductive layer, and better protect the inner electrode assembly in the event of penetration or impact to the secondary battery, thereby improving the safety of the secondary battery. The materials and thicknesses of the first and second polymer layers may be the same or different.

[0009] In some embodiments of the present invention, the thickness T1 of the first polymer layer is 3 μm to 15 μm. When the thickness of the first polymer layer is in this range, the strength of the first positive electrode current collector and the energy density of the secondary battery can be taken into consideration while ensuring improved safety of the secondary battery.

[0010] In some embodiments of the present invention, the material of the first conductive layer includes at least one of aluminum, aluminum alloy, nickel, nickel alloy, and stainless steel, and the material of the metallic second positive electrode current collector includes at least one of aluminum, aluminum alloy, nickel, nickel alloy, and stainless steel. Selecting such a material for the first conductive layer provides the first positive electrode current collector and the second positive electrode current collector with good electrical conductivity, meeting the electron conduction performance requirements, while also providing certain strength and weldability, ensuring that the first positive electrode current collector and the second positive electrode current collector have sufficient strength during processing and making it convenient to weld a tab to the first conductive layer.

[0011] In some embodiments of the present invention, the thickness T2 of the first conductive layer is 0.5 μm to 5 μm. By controlling the thickness T2 of the first conductive layer within the above range, the safety of the secondary battery can be further improved, and the raw material cost, production cost, and energy density of the secondary battery can be taken into consideration.

[0012] In some embodiments of the present invention, the single-sided positive electrode piece further includes an insulating layer, the insulating layer being disposed on a surface of the single-sided positive electrode piece away from the interior of the electrode assembly. The insulating layer includes an inert material, the inert material including at least one of boehmite, aluminum oxide, magnesium oxide, silicon oxide, titanium oxide, and magnesium hydroxide, and the mass percentage W3 of the inert material relative to the mass of the insulating layer is 30% to 70%. The insulating layer reduces the risk of short-circuiting between the first positive electrode current collector and the negative electrode active material in the outermost single-sided positive electrode piece in the event of penetration or impact, further improving the safety of the secondary battery. It also reduces, to a certain extent, the variation in stress distribution due to single-sided coating of the single-sided positive electrode piece, reduces the degree of curvature of the single-sided positive electrode piece during cold pressing, and improves the processability of the single-sided positive electrode piece. In some embodiments of the present invention, the thickness T3 of the insulating layer is 5 μm to 50 μm. By controlling the thickness T3 of the insulating layer within the above range, the safety of the secondary battery can be further improved, and the cost, processability, and energy density of the secondary battery can be taken into consideration.

[0013] In some embodiments of the present invention, the single-sided positive electrode sheet further includes a second conductive layer, the second conductive layer is provided between the first positive electrode current collector and the positive electrode active material layer, the second conductive layer contains a conductive agent, and the conductive agent contains at least one of conductive carbon black, carbon nanotubes, carbon fibers, conductive graphite, and graphene. The mass percentage W4 of the conductive agent with respect to the mass of the second conductive layer is 30% to 75%. By providing the second conductive layer, the adhesion between the positive electrode active material and the first positive electrode current collector can be increased, the interfacial resistance between the positive electrode active material and the first positive electrode current collector can be reduced, and the internal resistance of the secondary battery can be reduced.

[0014] In some embodiments of the present invention, the adhesion N1 between the second conductive layer and the first positive electrode current collector is 3 N / m to 120 N / m, and the adhesion N2 between the second conductive layer and the positive electrode active material layer is 3 N / m to 120 N / m. By controlling N1 and N2 within the above ranges, a good adhesion can be provided between the second conductive layer and the first positive electrode current collector and the positive electrode active material layer, and the risk of the positive electrode active material layer falling off during the cycling of the secondary battery can be reduced.

[0015] In some embodiments of the present invention, the thickness T4 of the second conductive layer satisfies 0 μm < T4 ≤ 4 μm. By controlling the thickness T4 of the second conductive layer within the above range, the adhesion between the positive electrode active material layer and the first positive electrode current collector can be increased, the secondary battery resistance can be reduced, and the energy density can be considered.

[0016] In some embodiments of the present invention, the first conductive layer is provided only on the surface of the electrode assembly of the first polymer layer facing the inside. In this way, the proportion of the conductive material layer in the first positive electrode current collector can be reduced, and the safety of the secondary battery can be further improved. <00000​​In some embodiments of the present invention, a first conductive layer is further provided on a surface of the first polymer layer away from the interior of the electrode assembly, i.e., the first positive electrode composite current collector uses a conventional "sandwich" structure current collector configuration, and by providing first conductive layers on both sides of the polymer layer, the strength of the first positive electrode current collector can be increased and tab welding is convenient. If the total area ratio of the first conductive layer to the cross section of the first positive electrode current collector remains the same, providing first conductive layers on both sides of the first polymer layer is also advantageous for heat dissipation of the electrode assembly.

[0018] The current collector of the negative electrode piece in the electrode assembly of the present invention may be of the type of negative electrode current collector commonly used in the prior art, for example, copper foil, a composite current collector for use in negative electrodes, or both.

[0019] A second aspect of the present invention provides a power consuming device including the secondary battery according to any one of the above embodiments, thereby ensuring good safety of the power consuming device.

[0020] The present invention provides a secondary battery and a power consumption device, the secondary battery including a stacked electrode assembly, the electrode assembly including stacked positive and negative electrode pieces, and a separator disposed between the positive and negative electrode pieces, the outermost electrode piece of the electrode assembly being a single-sided positive electrode piece, the single-sided positive electrode piece including a first positive electrode current collector and a positive electrode active material layer, the first positive electrode current collector including a first polymer layer and a first conductive layer disposed on the surface of the first polymer layer facing the inside of the electrode assembly, the positive electrode active material layer being disposed on the first conductive layer, the positive electrode pieces other than the outermost electrode piece in the electrode assembly being double-sided positive electrode pieces, the double-sided positive electrode piece including a second positive electrode current collector and positive electrode active material layers disposed on both surfaces of the second positive electrode current collector, the second positive electrode current collector of at least one double-sided positive electrode piece being made of metal. The overall structure of the secondary battery described above enhances the safety of the secondary battery. [Brief explanation of the drawings]

[0021] In order to more clearly describe the technical solutions of the embodiments of the present invention and the prior art, the following briefly describes the necessary drawings used in the embodiments and the prior art. Obviously, the drawings described below are only some embodiments of the present invention, and those skilled in the art can obtain other drawings based on these drawings.

[0022] [Figure 1] FIG. 1 is a structural schematic diagram of an electrode assembly according to some embodiments of the present invention. [Figure 2] FIG. 2 is a schematic cross-sectional view of an electrode assembly according to some embodiments of the present invention, taken along its thickness direction. [Figure 3] FIG. 3 is a schematic cross-sectional view of the single-sided positive electrode piece in FIG. 2 taken along its thickness direction. [Figure 4] FIG. 4 is a schematic diagram of the cross-sectional structure of a single-sided positive electrode piece in the thickness direction in some embodiments of the present invention. [Figure 5] FIG. 5 is a schematic cross-sectional view of a single-sided positive electrode piece taken along its thickness direction in some other embodiments of the present invention. [Figure 6] FIG. 6 is a schematic diagram of the cross-sectional structure of a first positive electrode current collector taken along its thickness direction in some embodiments of the present invention. [Figure 7] FIG. 7 is a schematic diagram of the cross-sectional structure of the double-sided positive electrode piece in FIG. 2 taken along its thickness direction. [Figure 8] FIG. 8 is a schematic cross-sectional view of a double-sided positive electrode piece according to some other embodiments of the present invention, taken along its thickness direction. DETAILED DESCRIPTION OF THE INVENTION

[0023] In order to clearly illustrate the objectives, technical solutions and advantages of the present invention, the present invention will be described in more detail below with reference to the drawings and examples. Obviously, the described examples are only some of the embodiments of the present invention, and not all of the embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention fall within the protection scope of the present invention.

[0024] In the specific embodiments, the present invention will be described using a lithium ion battery as an example of a secondary battery, but the secondary battery of the present invention is not limited to a lithium ion battery.

[0025] A first aspect of the present invention provides a secondary battery including an electrode assembly 10 with a stacked structure, and as shown in Fig. 1, for ease of understanding, a three-dimensional Cartesian coordinate system is created in which the width direction of the electrode assembly 10 is the X direction, the length direction of the electrode assembly 10 is the Y direction, and the thickness direction of the electrode assembly 10 is the Z direction. As shown in Figs. 2 and 3, the electrode assembly 10 includes a stacked positive electrode piece 20, a negative electrode piece 30, and a separator 40 provided between the positive electrode piece 20 and the negative electrode piece 30, and the positive electrode piece 20 includes a single-sided positive electrode piece 21 and a double-sided positive electrode piece 22. The outermost electrode piece of the electrode assembly 10 is a single-sided positive electrode piece 21, which includes a first positive electrode current collector 211 and a positive electrode active material layer 212. The first positive electrode current collector 211 includes a first polymer layer 112 and a first conductive layer 111. The first conductive layer 111 is provided on one surface 112a facing the inside of the electrode assembly 10, of the two surfaces 112a and 112b of the first polymer layer 112 that face each other along the thickness direction Z, and the positive electrode active material layer 212 is provided on the first conductive layer 111. As can be understood, the positive electrode active material layer 212 is provided on one surface 111a facing the inside of the electrode assembly 10, of the two surfaces 111a and 111b of the first conductive layer 111 that face each other along the thickness direction Z. As can be understood, the width, length, and thickness directions of the positive electrode pieces, negative electrode pieces, and separator in the present invention are the same as those of the electrode assembly. In the electrode assembly having a stacked structure in the secondary battery described above, the outermost electrode piece is a single-sided positive electrode piece, the first positive electrode current collector in the positive electrode piece is a composite current collector including a first polymer layer and a first conductive layer, and the positive electrode pieces other than the outermost electrode piece in the electrode assembly are double-sided positive electrode pieces, which include a second positive electrode current collector and positive electrode active material layers provided on both surfaces of the second positive electrode current collector, and the material of the second positive electrode current collector of at least one double-sided positive electrode piece is metal. By providing the above-described overall structure of the secondary battery, the thickness of the conductive material layer (i.e., the first conductive layer) in the first positive electrode current collector of the outermost electrode piece of the electrode assembly is reduced compared to the thickness when the entire positive electrode current collector is made of a conductive metal such as aluminum in the prior art, thereby reducing the proportion of the conductive material layer in the first positive electrode current collector and the proportion of the fracture surface of the secondary battery that is occupied by the conductive material layer.In this way, the probability that the first positive electrode current collector of the outermost electrode piece will be involved in a dangerous short circuit during operating conditions such as impact on the secondary battery is reduced, thereby reducing the risk of the secondary battery catching fire or exploding. This improves the safety of the secondary battery. Furthermore, by using double-sided positive electrode pieces for the positive electrode pieces other than the outermost electrode piece in the electrode assembly, the energy density of the electrode assembly can be ensured. By using a metal current collector for at least one double-sided positive electrode piece, i.e., a conventional metal current collector, costs can be reduced because composite current collectors are relatively expensive. It is well known to those skilled in the art that the outermost electrode piece of a laminated electrode assembly has two electrode pieces, one above the other in the thickness direction, i.e., the single-sided positive electrode piece described in the present invention refers to two electrode pieces, one above the other in the thickness direction of the laminated electrode assembly.

[0026] In some embodiments of the present invention, the second positive electrode current collector of one to four double-sided positive electrode pieces adjacent to the single-sided positive electrode piece is a double-sided composite current collector that includes a second polymer layer and a first conductive layer provided on both surfaces of the second polymer layer (not shown), thereby balancing safety and cost.

[0027] In some embodiments of the present invention, the materials of the first polymer layer and the second polymer layer independently comprise at least one of polyester, polyamide, modified polyolefin, olefin copolymer, and unsaturated olefin copolymer. The present invention does not limit the types of polyester, polyamide, modified polyolefin, olefin copolymer, and unsaturated olefin copolymer, as long as the objectives of the present invention can be achieved. For example, polyesters include, but are not limited to, polybutylene terephthalate (PBT), polycarbonate (PC), and polyether polyester. Polyamides include, but are not limited to, polyamide 6 (PA6) and polyamide 66 (PA66). Modified polyolefins include, but are not limited to, polymaleic anhydride, polymethyl methacrylate, and glycidyl methacrylate-grafted polyethylene. Olefin copolymers include, but are not limited to, ethylene-propylene copolymer. Unsaturated olefin copolymers include, but are not limited to, methyl (meth)acrylate-vinyl (meth)acrylate copolymer. By selecting the above materials for the polymer layer, the strength, toughness, insulation, and formability of the polymer layer can be ensured to meet the requirements of the composite current collector, providing good support for the first conductive layer, and better protecting the inner electrode assembly in the event of penetration or impact on the secondary battery. This also reduces the risk of short circuits caused by contact between the first conductive layer and the case, thereby improving the safety of the secondary battery.

[0028] In some embodiments of the present invention, as shown in FIG. 3 , the thickness T1 of the first polymer layer 112 is 3 μm to 15 μm. For example, the thickness T1 is 3 μm, 5 μm, 7 μm, 9 μm, 11 μm, 13 μm, 15 μm, or any value between any two of the above values. By controlling the thickness T1 of the first polymer layer within this range, the first polymer layer has relatively high strength, reducing the risk of the positive electrode pieces breaking during rolling, pulling, or stretching, and the proportion of inactive material in the positive electrode pieces can be controlled within a range that does not affect the energy density of the secondary battery. In this way, it is advantageous to consider the strength of the first positive electrode current collector and the energy density of the secondary battery while further improving the safety of the secondary battery.

[0029] In some embodiments of the present invention, the thickness T5 of the second polymer layer is 3 μm to 15 μm. For example, the thickness T5 is 3 μm, 5 μm, 7 μm, 9 μm, 11 μm, 13 μm, 15 μm, or any value between any two of the above values. Controlling the thickness T5 of the second polymer layer within the above range is advantageous in terms of further improving the safety of the secondary battery while also considering the strength of the second positive electrode current collector and the energy density of the secondary battery.

[0030] In some embodiments of the present invention, the material of the first conductive layer includes at least one of aluminum, aluminum alloy, nickel, nickel alloy, and stainless steel. Selecting such materials for the first conductive layer provides the first positive electrode current collector 211 and the second positive electrode current collector 222 with good electrical conductivity, meeting the electronic conductivity requirements of the first positive electrode current collector 211 and the second positive electrode current collector 222, while providing certain strength and weldability, ensuring that the first positive electrode current collector 211 and the second positive electrode current collector 222 have sufficient strength during processing and making it convenient to weld a tab to the first conductive layer.

[0031] In some embodiments of the present invention, the second positive electrode current collector is made of a metal, and the material includes at least one of aluminum, an aluminum alloy, nickel, a nickel alloy, and stainless steel.

[0032] In some embodiments of the present invention, as shown in FIG. 3 , the thickness T2 of the first conductive layer 111 is 0.5 μm to 5 μm. For example, the thickness T2 is 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, or any value between any two of the above values. By controlling the thickness T2 of the first conductive layer within the above range, the first conductive layer has relatively high strength and good conductivity, and the risk of defects such as burn-through due to the thinness of the first conductive layer when welding a component such as a tab to the first positive electrode current collector is relatively low. The first conductive layer in the first positive electrode current collector accounts for a relatively small proportion of the fracture surface of the secondary battery, reducing the probability of the first conductive layer being involved in a dangerous short circuit when the secondary battery is subjected to an impact or other operating conditions. This is advantageous for further improving the safety of the secondary battery without affecting the volumetric energy density, raw material costs, and production costs of the secondary battery.

[0033] 2 and 4, the single-sided positive electrode piece 21 further includes an insulating layer 213, which is disposed on a surface of the single-sided positive electrode piece 21 away from the interior of the electrode assembly 10. The insulating layer includes an inert material, which may include at least one of boehmite, aluminum oxide, magnesium oxide, silicon oxide, titanium oxide, and magnesium hydroxide, and the mass percentage of the inert material relative to the mass of the insulating layer is 30% to 70%. For example, the mass percentage W3 of the inert material is 30%, 40%, 50%, 60%, 70%, or any value between any two of the above ranges. By selecting the above-mentioned type of inert material and controlling the mass percentage of the inert material within the above-mentioned range to prepare an insulating layer, and then applying the insulating layer to the single-sided positive electrode piece, it is possible to reduce the variation in stress distribution due to single-sided coating of the single-sided positive electrode piece, reduce the degree of curvature during cold pressing of the single-sided positive electrode piece, and improve the processability of the single-sided positive electrode piece. The provision of the insulating layer also reduces the chance of the first conductive layer being exposed in the event of penetration or impact on the secondary battery, thereby reducing the risk of a short circuit between the first positive electrode current collector and the negative electrode active material in the outermost single-sided positive electrode piece of the electrode assembly. Thus, the provision of the insulating layer further improves the safety of the secondary battery and improves the processability of the single-sided positive electrode piece.

[0034] In some embodiments of the present invention, the insulating layer further includes a first binder. The present invention is not particularly limited by the type or mass of the first binder, as long as the object of the present invention can be achieved. For example, the first binder includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, polypropylene, polyacrylic acid, ethyl polyacrylate, polyacrylonitrile, sodium carboxymethyl cellulose, styrene-butadiene rubber, and polyurethane, and the mass percentage of the first binder is 30% to 70% of the mass of the insulating layer.

[0035] In some embodiments of the present invention, as shown in FIG. 4 , the thickness T3 of the insulating layer 213 is 5 μm to 50 μm. For example, the thickness T3 is 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, or any value between any two of the above ranges. Controlling the thickness T3 of the insulating layer within the above range reduces the degree of curvature during cold pressing of the single-sided positive electrode piece without affecting the volumetric energy density of the secondary battery, reduces the chance of exposure of the first conductive layer, and is advantageous in reducing the risk of short-circuiting between the first positive electrode current collector and the negative electrode active material in the outermost single-sided positive electrode piece of the electrode assembly. Thus, providing the insulating layer can further improve the safety of the secondary battery and enhance the processability of the single-sided positive electrode piece.

[0036] The present invention does not limit the method for preparing the insulating layer, as long as the objectives of the present invention are achieved. For example, it can be prepared by the following steps: uniformly mixing an inactive material and a first binder, adding an organic solvent, and stirring uniformly to prepare an insulating layer coating. The insulating layer coating is applied to the surface of the single-sided positive electrode piece away from the interior of the electrode assembly and dried to obtain an insulating layer. The present invention does not limit the solid content of the insulating layer coating, as long as the objectives of the present invention are achieved. The present invention does not limit the type of organic solvent, as long as the objectives of the present invention are achieved. For example, the organic solvent is selected from N-methylpyrrolidone.

[0037] 2 and 5, the single-sided positive electrode piece 21 further includes a second conductive layer 214 disposed between the first positive electrode current collector 211 and the positive electrode active material layer 212. The second conductive layer includes a conductive agent, the conductive agent including at least one of conductive carbon black, carbon nanotubes, carbon fiber, conductive graphite, and graphene. The mass percentage W4 of the conductive agent is 30% to 75% of the mass of the second conductive layer. For example, the mass percentage of the conductive agent is 30%, 35%, 40%, 50%, 60%, 70%, 75%, or any value between any two of the above ranges. By selecting the above type of conductive agent, controlling the mass percentage of the conductive agent within the above range, preparing a second conductive layer, and providing the second conductive layer on a single-sided positive electrode piece, it acts together with the first positive electrode current collector to increase the riveting bond between the positive electrode active material, and the binder (i.e., the second binder) in the second conductive layer acts to increase the adhesion between the positive electrode active material and the first positive electrode current collector, reducing the interfacial resistance between the positive electrode active material and the first positive electrode current collector, and further reducing the internal resistance of the secondary battery.

[0038] In some embodiments of the present invention, the second conductive layer further comprises a second binder. The present invention does not particularly limit the type or mass of the second binder, as long as the objectives of the present invention are achieved. For example, the second binder may comprise at least one of polyvinylidene fluoride, polytetrafluoroethylene, polypropylene, polyacrylic acid, polyethyl acrylate, polyacrylonitrile, sodium carboxymethyl cellulose, styrene-butadiene rubber, and urethane. The mass percentage of the second binder relative to the mass of the second conductive layer is 25% to 70%. Optionally, the second conductive layer may further comprise another active material, such as lithium iron phosphate, which has relatively high safety and conductivity and therefore improves the safety and cycle performance of the secondary battery. The present invention does not particularly limit the mass percentage of the other active material, as long as the objectives of the present invention are achieved. For example, the mass percentage of the other active material relative to the mass of the second conductive layer is 50% to 90%.

[0039] In some embodiments of the present invention, as shown in FIG. 5, the adhesion force N1 between the second conductive layer 214 and the first positive current collector 211 is 3 N / m to 120 N / m, and the adhesion force N2 between the second conductive layer 214 and the positive electrode active material layer 212 is 3 N / m to 120 N / m. For example, N1 is 3 N / m, 10 N / m, 20 N / m, 40 N / m, 60 N / m, 80 N / m, 100 N / m, 120 N / m or any one numerical value within the range between any two of the above numerical values. N2 is 3 N / m, 10 N / m, 20 N / m, 40 N / m, 60 N / m, 80 N / m, 100 N / m, 120 N / m or any one numerical value within the range between any two of the above numerical values. When N1 and N2 are controlled within the above ranges, a good adhesion force can be provided between the second conductive layer and the first positive current collector and the positive electrode active material layer, so that when the secondary battery is impacted, the exposure of the first positive current collector can be reduced, thereby reducing the direct involvement of the first positive current collector in a short circuit and improving the short circuit caused by a specific short circuit pattern (contact between the first positive current collector and the surface of the negative electrode active material layer). Thereby, the safety of the secondary battery is enhanced. The risk of dropout of the positive electrode material layer during the cycle of the secondary battery can also be reduced.

[0040] In some embodiments of the present invention, as shown in FIG. 5, the thickness T4 of the second conductive layer 214 satisfies 0 μm < T4 ≤ 4 μm. For example, the thickness T4 is 0.1 μm, 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 4 μm or any one numerical value within the range between any two of the above numerical values. When the thickness T4 of the second conductive layer is controlled within the above range, while increasing the adhesion force between the positive electrode active material layer and the first positive current collector and reducing the secondary battery resistance, it is more advantageous in consideration of the energy density of the secondary battery.

[0041] The present invention does not limit the method for preparing the second conductive layer, as long as the objectives of the present invention can be achieved. For example, the second conductive layer can be prepared by the following steps: uniformly mixing the conductive agent and the second binder, adding an organic solvent, and stirring uniformly to prepare a coating material for the second conductive layer. The coating material for the second conductive layer is applied to the surface of the first positive electrode current collector facing the interior of the electrode assembly and dried to obtain a second conductive layer. The present invention does not limit the solid content of the coating material for the second conductive layer, as long as the objectives of the present invention can be achieved. The present invention does not limit the type of organic solvent, as long as the objectives of the present invention can be achieved. For example, the organic solvent is N-methylpyrrolidone. Optionally, the second conductive layer slurry can further contain other active materials.

[0042] In some embodiments of the present invention, as shown in FIGS. 2 to 5 , the first conductive layer 111 is provided only on the surface 112a of the first polymer layer 112 facing the inside of the electrode assembly 10. That is, the first positive electrode current collector 211 is composed of the first polymer layer 112 and one first conductive layer 111, and the first conductive layer 111 is provided on one of the two opposing surfaces 112a and 112b of the first polymer layer 112 in the thickness direction Z, the surface 112a facing the inside of the electrode assembly 10. When the above-described first positive electrode current collector is selected and applied to a single-sided positive electrode piece to provide good conductivity to the first positive electrode current collector, it is advantageous to reduce the proportion of the fracture surface of the secondary battery that is occupied by the first conductive layer. In this way, the probability that the first conductive layer will be involved in a dangerous short circuit during operating conditions such as impact of the secondary battery is reduced, reducing the risk of the secondary battery catching fire or exploding. This improves the safety of the secondary battery.

[0043] 6 , the first conductive layer 111 is further provided on a surface of the first polymer layer 112 that is remote from the interior of the electrode assembly 10. That is, the first positive electrode current collector 211 is composed of the first polymer layer 112 and two first conductive layers 111. The first polymer layer 112 has two surfaces 112a and 112b that face each other along the thickness direction Z of the first polymer layer 112. One surface facing the interior of the electrode assembly 10 is surface 112a, and the other surface remote from the interior of the electrode assembly 10 is surface 112b. The two first conductive layers 111 are provided on surfaces 112a and 112b, respectively. In this manner, the strength of the first positive electrode current collector 211 can be increased, and it is convenient for welding a tab. When the proportion of the cross section of the first positive electrode current collector occupied by the total area of ​​the first conductive layers remains unchanged, providing first conductive layers on both sides of the polymer layer is advantageous for heat dissipation of the electrode assembly, does not increase the proportion of the fracture surface of the secondary battery occupied by the first conductive layers, and does not increase the probability that the entire first conductive layer will be involved in a dangerous short circuit when the secondary battery is impacted, etc. Furthermore, the first positive electrode current collector described in the above embodiment can also be used as the second positive electrode current collector in a double-sided positive electrode piece.

[0044] In some embodiments of the present invention, as shown in FIGS. 2, 7, and 8, the positive electrode pieces 20 other than the outermost electrode piece in the electrode assembly 10 are double-sided positive electrode pieces 22. The double-sided positive electrode pieces 22 include a second positive electrode current collector 222 and a positive electrode active material layer 212 provided on both surfaces of the second positive electrode current collector 222. The second positive electrode current collector 222 is a double-sided composite current collector (as shown in FIG. 8) or an aluminum foil. As shown in FIG. 8, the second positive electrode current collector 222, i.e., a double-sided composite current collector, includes a second polymer layer 122 and a first conductive layer 111 provided on both surfaces of the second polymer layer 122. The aluminum foil used in the present invention is an aluminum foil known in the art for use as a positive electrode current collector.

[0045] In the present invention, there are no particular limitations on the method for preparing the first positive electrode current collector and the double-sided composite current collector, and any preparation method known in the art can be used as long as the object of the present invention can be achieved.

[0046] In the present invention, the positive electrode active material layers in the single-sided positive electrode piece and the double-sided positive electrode piece may be the same or different, but the present invention is not particularly limited thereto as long as the object of the present invention can be achieved.

[0047] The positive electrode active material layer of the present invention includes a positive electrode active material. The type of positive electrode active material is not particularly limited as long as the objectives of the present invention are achieved. For example, the positive electrode active material may include at least one of lithium nickel-cobalt manganese oxide (e.g., commonly known as NCM811, NCM622, NCM523, or NCM111), lithium nickel-cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO), lithium manganese oxide, lithium iron manganese phosphate, and lithium titanate. In the present invention, the positive electrode active material may include nonmetallic elements such as fluorine, phosphorus, boron, chlorine, silicon, and sulfur, which can further enhance the stability of the positive electrode active material. Optionally, the positive electrode active material layer further includes a conductive agent and a binder. The type of conductive agent and binder in the positive electrode active material layer is not particularly limited as long as the objectives of the present invention are achieved. In the present invention, there are no particular limitations on the mass ratio of the positive electrode active material, conductive agent, and binder in the positive electrode active material layer, and those skilled in the art can select the mass ratio as needed as long as the object of the present invention can be achieved. For example, in the positive electrode active material layer, the mass ratio of the positive electrode active material:conductive agent:binder is (97.5 to 97.9):(0.9 to 1.7):(1.0 to 2.0).

[0048] In the present invention, there is no particular limitation on the thickness of the positive electrode active material layer, as long as the object of the present invention can be achieved. For example, the thickness of the positive electrode active material layer is 30 μm to 120 μm.

[0049] The negative electrode piece of the present invention includes a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector. The "negative electrode active material layer provided on at least one surface of the negative electrode current collector" refers to the negative electrode active material layer being provided on one surface of the negative electrode current collector along its thickness direction, or on both surfaces of the negative electrode current collector along its thickness direction. Here, the "surface" may refer to the entire area of ​​the negative electrode current collector or a portion of the area of ​​the negative electrode current collector. The present invention is not particularly limited as long as the objective of the present invention can be achieved. The negative electrode current collector is not particularly limited as long as the objective of the present invention can be achieved. For example, the negative electrode current collector may include copper foil, copper alloy foil, nickel foil, titanium foil, foamed nickel, or foamed copper. The negative electrode active material layer includes a negative electrode active material. The present invention is not particularly limited as to the type of negative electrode active material being provided as long as the objective of the present invention can be achieved. For example, the negative electrode active material may include at least one of natural graphite, artificial graphite, soft carbon, hard carbon, mesocarbon microbeads, tin-based materials, silicon-based materials, lithium titanate, transition metal nitrides, and natural flake graphite. Optionally, the negative electrode active material layer further includes at least one of a conductive agent, a thickener, and a binder. The present invention does not particularly limit the types of conductive agent, thickener, and binder in the negative electrode active material layer, as long as the objective of the present invention is achieved. The present invention does not particularly limit the mass ratio of the negative electrode active material, conductive agent, thickener, and binder in the negative electrode active material layer, as long as the objective of the present invention is achieved. For example, in the negative electrode active material layer, the mass ratio of negative electrode active material:conductive agent:thickener:binder is (97-98):(0-1.5):(0.5-1.5):(1.0-1.9).

[0050] In the present invention, there are no particular limitations on the thickness of the negative electrode current collector and the negative electrode active material layer, as long as the object of the present invention can be achieved. For example, the thickness of the negative electrode current collector is 5 μm to 20 μm, and the thickness of the negative electrode active material layer is 30 μm to 120 μm.

[0051] The separator of the present invention is not particularly limited as long as it achieves the objectives of the present invention. For example, the separator material may include, but is not limited to, at least one of polyethylene (PE), polypropylene (PP), primarily polyolefin (PO), polyester (e.g., polyethylene terephthalate (PET)), cellulose, polyimide (PI), polyamide (PA), spandex, and aramid. The separator type may include, but is not limited to, at least one of woven film, nonwoven film, microporous film, composite film, separator paper, laminate film, and spun film. For example, the separator may include a substrate layer and a surface treatment layer. The substrate layer may be a nonwoven fabric, film, or composite film having a porous structure, and the material of the substrate layer may include at least one of polyethylene, polypropylene, polyethylene terephthalate, polyimide, etc. Optionally, a polypropylene porous film, a polyethylene porous film, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite film may be used. Optionally, a surface treatment layer is provided on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed from a mixture of a polymer and an inorganic material. For example, the inorganic layer contains inorganic particles and a binder. The inorganic particles are not particularly limited and may be at least one selected from aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate. The binder is not particularly limited and may be at least one selected from polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyamide, polyacrylonitrile, polyacrylic acid ester, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene.The polymer layer includes a polymer, and the polymer material includes at least one of polyamide, polyacrylonitrile, acrylic ester polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride, and poly(vinylidene fluoride-hexafluoropropylene), and the like.

[0052] The electrode assembly in the present invention has a laminated structure, and the electrode assembly includes multiple tabs, with one positive electrode tab and one negative electrode tab drawn out from each layer of positive and negative electrode pieces, respectively. Finally, one laminated electrode assembly includes multiple groups of positive and negative electrode tabs, which may be welded to metal pieces by transfer welding, respectively, to draw out the positive and negative electrode tabs.

[0053] In the present invention, the above-mentioned "tab" generally refers to a metal conductor drawn from a positive electrode piece or a negative electrode piece, and is used to connect other parts of an electrochemical device in series or parallel. The positive electrode tab is drawn from the positive electrode piece, and the negative electrode tab is drawn from the negative electrode piece. The material of the tab of the present invention is not particularly limited as long as it can achieve the objectives of the present invention. For example, a tab material known in the art can be used.

[0054] The secondary battery of the present invention further comprises an electrolyte. The electrolyte is not particularly limited in the present invention; those skilled in the art can select any electrolyte as needed, provided that the objectives of the present invention are achieved. For example, at least one of ethylene carbonate (also known as ethylene carbonate, abbreviated as EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl propionate (EP), propyl propionate (PP), methyl ethyl carbonate (EMC), dimethyl carbonate (DMC), vinylene carbonate (VC), and fluoroethylene carbonate (FEC) may be mixed in a specific mass ratio to obtain a nonaqueous organic solvent, and then a lithium salt may be added, dissolved, and mixed uniformly. The present invention is not particularly limited in the mass ratio, provided that the objectives of the present invention are achieved. The present invention is not particularly limited in the type of lithium salt, provided that the objectives of the present invention are achieved. For example, the lithium salt may include at least one of LiPF, LiBF, LiAsF, LiClO, LiB(C6H5), LiCH3SO, LiCF3SO, LiN(SO2CF3)2, LiC(SO2CF3)3, LiSiF, lithium bis(oxalato)borate (LiBOB), and lithium difluoroborate. The present invention does not particularly limit the concentration of the lithium salt in the electrolyte solution, as long as the object of the present invention can be achieved. For example, the concentration of the lithium salt is 1.0 mol / L to 2.0 mol / L.

[0055] The secondary battery of the present invention further includes a case. The present invention does not particularly limit the case, and those skilled in the art can select one as needed as long as the object of the present invention can be achieved. For example, the case may include an aluminum-plastic film.

[0056] The secondary battery of the present invention is not limited to, and may include any device in which an electrochemical reaction occurs. For example, the secondary battery may include, but is not limited to, a lithium metal secondary battery, a lithium ion secondary battery (lithium ion battery), a sodium ion secondary battery, a lithium polymer secondary battery, and a lithium ion polymer secondary battery.

[0057] The present invention does not limit the method for preparing a secondary battery, and any preparation method known in the art can be used as long as the objectives of the present invention can be achieved. For example, methods for preparing a secondary battery include, but are not limited to, stacking a positive electrode piece, a separator, and a negative electrode piece in this order, fixing the four corners of the entire stack structure to obtain a stacked electrode assembly, placing the electrode assembly in a case, injecting an electrolyte into the case, and sealing it to obtain an electrochemical device. Furthermore, if necessary, an overcurrent protection element, lead plates, etc. may be placed in the case to prevent pressure buildup within the electrochemical device and overcharging and discharging.

[0058] A second aspect of the present invention provides a power consuming device including the secondary battery according to any one of the above embodiments.

[0059] The power consuming device of the present invention is not limited to any particular type and may be any known power consuming device used in the prior art. For example, the power consuming device may include, but is not limited to, a notebook computer, a pen-input computer, a mobile computer, an electronic book player, a mobile phone, a portable facsimile machine, a portable copier, a portable printer, a stereo headset, a video recorder, an LCD television, a portable vacuum cleaner, a portable CD player, a minidisc, a walkie-talkie, an electronic organizer, a calculator, a memory card, a portable tape recorder, a radio, a backup power supply, a motor, an automobile, a motorcycle, an electric bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, an electric tool, a flashing lamp, a camera, a large household storage battery, and a lithium ion capacitor. [Example]

[0060] The present invention will be described in more detail below with reference to examples and comparative examples. Tests and evaluations are carried out according to the following methods.

[0061] Measurement methods and equipment Measurement of adhesive strength N1 and N2 A sample of positive electrode strips measuring 30 mm wide x 120 mm long was cut and attached to a steel plate with 20 mm wide x 100 mm long double-sided tape, with the measurement surface facing down. One end of a paper tape the same width as the positive electrode strip but 150 mm longer than the sample was inserted under the positive electrode strip and secured with crepe tape. The steel plate was fixed to the bottom of a high-temperature tensile tester, and the other end of the paper tape was fixed to the tester's jig. The tester was then started. The tensile force data obtained after the tensile force stabilized was used as the adhesive strength.

[0062] Impact pass rate measurement After fully charging the lithium-ion batteries of each example and comparative example, they were placed on a flat iron plate as a sample, and a round rod with a diameter of 15.8±0.1 mm and a length of at least 6 cm was placed perpendicular to the sample. A weight of 9.1±0.1 kg was dropped vertically and freely at a point 61±2.5 cm from the intersection of the round rod and the sample. Judgment criteria: A lithium-ion battery that does not catch fire or explode is considered to have passed. 100 lithium ion batteries were measured for each example or comparative example, and the impact pass rate (%) = number of passed batteries / 100 x 100%.

[0063] Example 1-1 <Preparation of positive electrode piece> (1) Preparation of single-sided positive electrode pieces The first positive electrode current collector 211 shown in FIG. 6 was selected, which includes a first polymer layer 112 and a first conductive layer 111 provided on both surfaces of the first polymer layer 112. The polymer layer is made of PBT (weight average molecular weight is 3×10 4 The first conductive layer was made of an aluminum alloy (grade number 1235) and had a thickness T2 of 1 μm.

[0064] The positive electrode active material is lithium iron phosphate, and the positive electrode binder is polyvinylidene fluoride (abbreviated as PVDF, with a weight-average molecular weight of 5 × 10 5The cathode conductive agent, conductive carbon black (SP), was mixed in a mass ratio of 97.6:1.3:1.1, N-methylpyrrolidone was added, and the mixture was uniformly stirred using a vacuum mixer to obtain a cathode slurry with a solid content of 75 wt%. The cathode slurry was uniformly coated onto one surface of a 10 μm-thick first cathode current collector, dried at 85°C, and then dried, cold-pressed, cut, and slit to obtain a single-sided cathode piece measuring 38 mm x 58 mm.

[0065] (2) Preparation of double-sided positive electrode pieces The positive electrode active material, lithium iron phosphate, the positive electrode binder, PVDF, and the positive electrode conductive agent, SP, were mixed in a mass ratio of 97.6:1.3:1.1, N-methylpyrrolidone was added, and the mixture was uniformly stirred using a vacuum mixer to obtain a positive electrode slurry with a solids content of 75 wt%. The positive electrode slurry was uniformly coated onto one surface of a 10 μm-thick aluminum foil second positive electrode current collector, and the aluminum foil was dried at 90°C. The above steps were then repeated on the other surface of the aluminum foil to obtain a double-sided positive electrode piece. After cold pressing, cutting, and slitting, a double-sided positive electrode piece measuring 38 mm x 58 mm was obtained.

[0066] <Preparation of negative electrode pieces> The negative electrode active material is artificial graphite, and the thickener is carboxymethyl cellulose sodium (abbreviated as CMC-Na, with a weight-average molecular weight of 3 × 10 5 ) and the binder, styrene butadiene rubber (abbreviated as SBR, with a weight average molecular weight of 2 × 10 5 The two materials were mixed in a mass ratio of 98:1:1, deionized water was added, and the mixture was stirred uniformly using a vacuum mixer to obtain a negative electrode slurry with a solids content of 75 wt%. The negative electrode slurry was uniformly coated on one surface of a 12 μm-thick copper foil negative electrode current collector and then dried at 85°C to obtain a negative electrode coated with a negative electrode active material layer on one side and a coating layer thickness of 130 μm. The above steps were repeated on the other surface of the negative electrode current collector to obtain a negative electrode piece coated with a negative electrode active material layer on both sides. After drying at 85°C for 1 hour, the piece was cold-pressed, cut, and slit to obtain a negative electrode piece measuring 41 mm x 61 mm.

[0067] <Preparation of electrolyte> EC, DEC, PC, PP, and VC were mixed in a mass ratio of 20:30:20:28:2 to obtain a non-aqueous organic solvent, and then the electrolyte was prepared by mixing the lithium salt LiPF6 and the non-aqueous organic solvent in a mass ratio of 8:92.

[0068] <Preparation of separator> A porous polyethylene film with a thickness of 7 μm was used.

[0069] <Preparation of lithium-ion batteries> The double-sided positive and negative electrode pieces prepared above were stacked alternately, with the negative electrode pieces at the start and end, and a separator was interposed between the double-sided positive and negative electrode pieces. The single-sided positive electrode pieces were stacked on the outermost layer of the semi-finished electrode assembly, with the positive electrode active material layer of the single-sided positive electrode piece facing the interior of the electrode assembly and the four corners fixed, to obtain a stacked electrode assembly. The single-sided positive electrode pieces had two layers, the double-sided positive electrode pieces had 15 layers, the negative electrode pieces had 16 layers, and the separator had 32 layers.

[0070] The electrode assembly was placed in an aluminum plastic film case, moisture was removed at 80°C, the formulated electrolyte was injected, and the lithium ion battery was obtained after processes such as vacuum packaging, standing, formation, and gas extraction.

[0071] Examples 1-2 to 1-14 The same as Example 1-1, except that the relevant preparation parameters were adjusted according to Table 1.

[0072] Example 2-1 <Preparation of positive electrode piece> (1) Preparation of single-sided positive electrode pieces The positive electrode active material (lithium iron phosphate), the positive electrode binder (PVDF), and the positive electrode conductive agent (SP) were mixed in a mass ratio of 97.6:1.3:1.1, and N-methylpyrrolidone was added. The mixture was uniformly stirred using a vacuum mixer to obtain a positive electrode slurry with a solid content of 75 wt%. The inactive material (boehmite) and the first binder (PVDF) were mixed in a mass ratio of 30:70, and N-methylpyrrolidone was added. The mixture was uniformly stirred using a vacuum mixer to obtain an insulating layer slurry with a solid content of 30 wt%.

[0073] The positive electrode slurry was uniformly applied to one surface of a 10 μm-thick first positive electrode current collector and dried at 85° C. The insulating layer slurry was uniformly applied to the other surface of the first positive electrode current collector and dried at 85° C. to obtain an insulating layer with a thickness of 10 μm. After cold pressing, cutting, and slitting, a single-sided positive electrode piece measuring 38 mm × 58 mm was obtained. Other than the above, the results were the same as in Example 1-1.

[0074] Examples 2-2 to 2-3 The same as Example 2-1, except that the relevant preparation parameters were adjusted according to Table 2.

[0075] Examples 2-4 to 2-7 The same as Example 2-1, except that the mass percentage W3 of the inactive material was adjusted according to Table 2, the mass percentage of the first binder was changed accordingly, and the sum of both mass percentages was 100%.

[0076] Examples 2-8 to 2-12 The same as Example 2-1, except that the relevant preparation parameters were adjusted according to Table 2.

[0077] Example 3-1 <Preparation of positive electrode piece> (1) Preparation of single-sided positive electrode pieces The positive electrode active material, lithium iron phosphate, the positive electrode binder, PVDF, and the positive electrode conductive agent, SP, were mixed in a mass ratio of 97.6:1.3:1.1, and N-methylpyrrolidone was added. The mixture was uniformly stirred using a vacuum mixer to obtain a positive electrode slurry with a solids content of 75 wt%. The inactive material, boehmite, and the first binder, PVDF, were mixed in a mass ratio of 30:70, and N-methylpyrrolidone was added. The mixture was uniformly stirred using a vacuum mixer to obtain an insulating layer slurry with a solids content of 30 wt%. The conductive agent, SP, and the second binder, PVDF, were mixed in a mass ratio of 35:65, and N-methylpyrrolidone was added. The mixture was uniformly stirred using a vacuum mixer to obtain a second conductive layer slurry with a solids content of 30 wt%.

[0078] The second conductive layer slurry was uniformly coated on one surface of the first positive electrode current collector and dried at 85°C to form a 1 μm thick second conductive layer, the positive electrode slurry was uniformly coated on the surface of the second conductive layer and dried at 85°C, and the insulating layer slurry was uniformly coated on the other surface of the first positive electrode current collector and dried at 85°C to form a 10 μm thick insulating layer. After cold pressing, cutting, and slitting, a single-sided positive electrode piece measuring 38 mm x 58 mm was obtained. Other than the above, the results were the same as in Example 1-1.

[0079] Example 3-2 The same as Example 3-1, except that the relevant preparation parameters were adjusted according to Table 3.

[0080] Examples 3-3 to 3-6 The same as Example 3-1, except that the mass percentage W4 of the conductive agent was adjusted according to Table 3, the mass percentage of the second binder was changed accordingly, and the sum of both mass percentages was 100%.

[0081] Examples 3-7 to 3-9 The same as Example 3-1, except that the relevant preparation parameters were adjusted according to Table 3.

[0082] Example 4-1 <Preparation of positive electrode piece> (1) Preparation of single-sided positive electrode pieces A first positive electrode current collector 211 shown in FIG. 3 was selected, which includes a first polymer layer 112 and a first conductive layer 111 provided on one surface 112a of the first polymer layer 112. Other than the above, the results were the same as in Example 1-1.

[0083] Example 4-2 The same as Example 4-1, except that the relevant preparation parameters were adjusted according to Table 4.

[0084] Example 4-3 The procedure was the same as in Example 2-1, except that the same first positive electrode current collector as in Example 4-1 was selected.

[0085] Example 4-4 The procedure was the same as in Example 3-1, except that the same first positive electrode current collector as in Example 4-1 was selected.

[0086] Examples 4-5 <Preparation of positive electrode piece> (2) Preparation of double-sided positive electrode pieces The double-sided positive electrode piece closest to the single-sided positive electrode piece was the second positive electrode current collector 222 shown in FIG. 8, which includes a second polymer layer made of the same material and size as the first polymer layer 112 in Example 3-1, and first conductive layers 111 provided on both surfaces of the second polymer layer. The second polymer layer was made of PBT (weight average molecular weight 3×10 4 ) and had a thickness T5 of 6 μm, and the first conductive layer was made of an aluminum alloy (No. 1235) and had a thickness T2 of 1 μm.

[0087] The positive electrode active material, lithium iron phosphate, the positive electrode binder, PVDF, and the positive electrode conductive agent, SP, were mixed in a mass ratio of 97.6:1.3:1.1, N-methylpyrrolidone was added, and the mixture was uniformly stirred using a vacuum mixer to obtain a positive electrode slurry with a solids content of 75 wt%. The positive electrode slurry was uniformly coated onto one surface of a 10 μm-thick second positive electrode current collector, and the aluminum foil was dried at 90°C. The above steps were then repeated on the other surface of the second positive electrode current collector to obtain a double-sided positive electrode piece. After cold pressing, cutting, and slitting, a double-sided positive electrode piece measuring 38 mm x 58 mm was obtained. The results were the same as in Example 3-1.

[0088] Examples 4-6 to 4-8 The same as Example 4-5 was used except that the number of layers of double-sided positive electrode pieces using composite current collectors adjacent to the single-sided positive electrode pieces was different.

[0089] Comparative Example 1 The same procedure as in Example 1-1 was carried out except that the first positive electrode current collector was changed to the same aluminum foil as the second positive electrode current collector.

[0090] Comparative Example 2 The same procedure as in Example 2-1 was carried out except that the first positive electrode current collector was changed to the same aluminum foil as the second positive electrode current collector.

[0091] Comparative Example 3 The same procedure was followed as in Example 3-1, except that the first positive electrode current collector was changed to the same aluminum foil as the second positive electrode current collector.

[0092] Tables 1 to 4 show the preparation parameters and performance parameters of each of the examples and comparative examples.

[0093] [Table 1] Note: "\" in Table 1 indicates that there is no associated preparation parameter.

[0094] As can be seen from Examples 1-1 to 1-14 and Comparative Example 1, the secondary batteries of the present invention are designed with an electrode assembly whose overall structure includes a single-sided positive electrode piece as the outermost electrode piece of the electrode assembly. The first positive electrode current collector in the single-sided positive electrode piece includes a first polymer layer and a first conductive layer on the surface of the first polymer layer facing the interior of the electrode assembly. The thickness of the first conductive layer in the outermost electrode piece of the electrode assemblies in Examples 1-1 to 1-14 is reduced compared to the thickness of the aluminum foil in Comparative Example 1, thereby reducing the proportion of the fracture surface of the secondary battery that is occupied by the first conductive layer. The insulating properties of the first polymer layer also reduce the possibility of contact between the first conductive layer and the negative electrode active material in the negative electrode piece. Impact tests performed on the secondary batteries in the examples showed higher impact pass rates, indicating the safety of the secondary batteries of the present invention.

[0095] The thickness T1 of the first polymer layer generally affects the safety of secondary batteries. As can be seen from Examples 1-1 to 1-6, secondary batteries with a first polymer layer thickness T1 within the range of the present invention have a relatively high impact pass rate, indicating good safety. Furthermore, when the T1 thickness is within the range of 3 μm to 15 μm, the impact pass rate of the secondary battery decreases with increasing first polymer layer thickness T1, indicating a decrease in safety. This is because the greater the first polymer layer thickness T1, the greater the elongation of the first positive electrode current collector during the impact test, resulting in a more chaotic fracture surface. This increases the risk of contact between the conductive material of the first conductive layer and the negative electrode piece at the fracture surface of the secondary battery, thereby decreasing the impact pass rate of the secondary battery and affecting its safety. Therefore, Example 1-6 has a relatively lower impact pass rate than Example 1-5. Although the thickness of T1 in Example 1-5 is relatively thin, the strength of the electrode piece is too low, making the first positive electrode current collector prone to fracture during impact, and therefore the impact pass rate is not the highest. Furthermore, the strength of the current collector is too low, making the electrode piece prone to wrinkles during production, and causing pits to form in the electrode piece when the active material is cold-pressed, so the thickness of the first polymer layer in Example 1-5 is undesirable.

[0096] The thickness T2 of the first conductive layer generally affects the safety of secondary batteries. As can be seen from Examples 1-1, 1-7, and 1-12, secondary batteries having a first conductive layer thickness T2 within the range of the present invention exhibit a relatively high impact pass rate, indicating good safety. Furthermore, as the thickness T2 of the first conductive layer increases, the impact pass rate of the secondary battery decreases, indicating a decrease in safety. This is because, as the thickness T2 of the first conductive layer increases, the proportion of the conductive material of the first conductive layer that occupies the secondary battery's fracture surface increases, increasing the risk of contact between the first conductive layer and the negative electrode piece, thereby decreasing the impact pass rate of the secondary battery and affecting the safety of the secondary battery. Note that the first conductive layer T2 of Example 1-11 was relatively thin and achieved a relatively good impact pass rate, but the current collector strength during its production process was relatively low, making production relatively difficult. Furthermore, when a tab was welded onto it, burn-through was likely to occur, making the thickness of the first conductive layer undesirable.

[0097] The material of the polymer layer generally affects the safety of the secondary battery. As can be seen from Examples 1-1 and 1-13, secondary batteries whose polymer layer materials are within the range of the present invention have a relatively high impact pass rate, indicating good safety.

[0098] The material of the first conductive layer generally affects the safety of the secondary battery. As can be seen from Examples 1-1 and 1-14, secondary batteries whose first conductive layer materials are within the range of the present invention have a relatively high impact pass rate, indicating good safety.

[0099] [Table 2] Note: "\" in Table 2 indicates that there are no relevant preparation parameters. The difference between Comparative Example 2 and Example 2-1 is that the first positive electrode current collector in Comparative Example 2 is aluminum foil.

[0100] As can be seen from Examples 1-1, 2-1 to 2-12, and Comparative Example 2, the secondary battery of the present invention has an electrode assembly with a single-sided positive electrode piece as the outermost electrode piece, and the first positive electrode current collector in the single-sided positive electrode piece includes a first polymer layer and a first conductive layer on the surface of the first polymer layer facing the inside of the electrode assembly. The single-sided positive electrode piece also includes an insulating layer, which further increases the impact pass rate and safety of the secondary battery. In contrast, Comparative Example 2 does not use the first positive electrode current collector of the present invention, which indicates that the impact pass rate cannot be increased and safety cannot be improved.

[0101] The type of inert material in the insulating layer generally affects the safety of the secondary battery. As can be seen from Examples 2-1 to 2-3, secondary batteries in which the type of inert material selected falls within the range of the present invention have a relatively high impact pass rate, indicating that the secondary batteries have good safety.

[0102] The mass percentage W3 of the inactive material in the insulating layer generally affects the safety of the secondary battery. As can be seen from Examples 2-1, 2-4, and 2-7, secondary batteries in which the mass percentage W3 of the inactive material selected falls within the range of the present invention exhibited relatively high impact pass rates, indicating that the secondary batteries have good safety. The lithium-ion battery of Example 2-7 exhibited a relatively high impact pass rate, but the relatively high mass percentage W3 of the inactive material in the insulating layer resulted in a relatively low content of the first binder, which resulted in poor adhesion between the insulating layer and the first positive electrode current collector, undesirably affecting the productivity of the positive electrode pieces and the operating performance of the secondary battery.

[0103] The thickness T3 of the insulating layer generally affects the safety of a secondary battery. As can be seen from Examples 2-1, 2-8, and 2-12, secondary batteries with selected insulating layer thicknesses T3 within the range of the present invention have relatively high impact pass rates, indicating that the secondary batteries have good safety. The lithium-ion battery of Example 2-12 has a relatively high impact pass rate, but its insulating layer thickness T3 is relatively large, which increases the volume and thereby impairs the energy density of the secondary battery, making it undesirable.

[0104] [Table 3] Note: "\" in Table 3 indicates that there are no relevant preparation parameters. The difference between Comparative Example 3 and Example 3-1 is that the first positive electrode current collector in Comparative Example 3 is aluminum foil.

[0105] As can be seen from Examples 2-1, 3-1 to 3-9, and Comparative Example 3, the secondary battery of the present invention has an electrode assembly with a single-sided positive electrode piece as the outermost electrode piece, and the first positive electrode current collector in the single-sided positive electrode piece includes a first polymer layer and a first conductive layer on the surface of the first polymer layer facing the inside of the electrode assembly. The single-sided positive electrode piece also includes an insulating layer and a second conductive layer, which further increases the impact pass rate and safety of the secondary battery. In contrast, Comparative Example 3 does not use the first positive electrode current collector of the present invention, which indicates that the impact pass rate cannot be increased and safety cannot be improved.

[0106] The type of conductive agent in the second conductive layer generally affects the safety of the secondary battery. As can be seen from Examples 3-1 and 3-2, secondary batteries in which the type of conductive agent selected falls within the scope of the present invention have a relatively high impact pass rate, indicating that the secondary batteries have good safety.

[0107] The mass percentage W4 of the conductive agent in the second conductive layer generally affects the safety of the secondary battery. As can be seen from Examples 3-1, 3-3, and 3-6, secondary batteries in which the mass percentage W4 of the conductive agent selected is within the range of the present invention have a relatively high impact pass rate, indicating that the secondary batteries have good safety.

[0108] The thickness T4 of the second conductive layer generally affects the safety of the secondary battery. As can be seen from Examples 3-1, 3-7, and 3-9, secondary batteries in which the selected thickness T4 of the second conductive layer is within the range of the present invention have a relatively high impact pass rate, indicating that the secondary batteries have good safety.

[0109] [Table 4]

[0110] The type of first positive electrode current collector generally affects the safety of the secondary battery. As can be seen from Examples 1-1, 4-1 and 4-2, 2-1 and 4-3, and 3-1 and 4-4, when a first positive electrode current collector in which the first conductive layer is formed only on the surface of the first polymer layer facing the inside of the electrode assembly and a first positive electrode current collector in which the first conductive layer is formed on the surface of the first polymer layer facing the inside of the electrode assembly and also on the surface of the first polymer layer away from the inside of the electrode assembly are selected and applied to a secondary battery, the secondary battery has a relatively high impact pass rate and good safety. The type of second positive electrode current collector generally affects the safety of the secondary battery. As can be seen from Examples 3-1 and 4-5 to 4-8, changing the current collector of the double-sided positive electrode piece adjacent to the single-sided positive electrode piece from aluminum foil to a composite current collector can further increase the impact pass rate of the battery. However, if the changed double-sided positive electrode current collector has more than four layers, the impact pass rate cannot be further increased.

[0111] The above description is only a preferred embodiment of the present invention, and is not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the scope of protection of the present invention.

Claims

1. A secondary battery, A laminated electrode assembly is included, The electrode assembly includes a positive electrode piece, a negative electrode piece, and a separator provided between the positive electrode piece and the negative electrode piece, which are arranged in a stacked manner; the outermost electrode piece of the electrode assembly is a single-sided positive electrode piece; the single-sided positive electrode piece includes a first positive electrode current collector and a positive electrode active material layer, the first positive electrode current collector includes a first polymer layer and a first conductive layer provided on a surface of the first polymer layer facing the inside of the electrode assembly, the positive electrode active material layer being provided on the first conductive layer; The positive electrode pieces other than the outermost electrode piece in the electrode assembly are double-sided positive electrode pieces, the double-sided positive electrode piece includes a second positive electrode current collector and the positive electrode active material layer provided on both surfaces of the second positive electrode current collector, and the second positive electrode current collector of at least one of the double-sided positive electrode pieces is made of a metal.

2. the second positive electrode current collector of one to four double-sided positive electrode pieces adjacent to the single-sided positive electrode piece is a double-sided composite current collector; The secondary battery according to claim 1 , wherein the double-sided composite current collector includes a second polymer layer and the first conductive layer provided on both surfaces of the second polymer layer.

3. 2. The secondary battery according to claim 1, wherein the materials of the first polymer layer and the second polymer layer independently include at least one of polyester, polyamide, modified polyolefin, olefin copolymer, and unsaturated olefin copolymer.

4. The thickness T of the first polymer layer １ The secondary battery according to claim 1, wherein is 3 μm to 15 μm.

5. a material of the first conductive layer including at least one of aluminum, an aluminum alloy, nickel, a nickel alloy, and stainless steel; The secondary battery according to claim 1 , wherein the second positive electrode current collector is made of a metal and the metal comprises at least one of aluminum, an aluminum alloy, nickel, a nickel alloy, and stainless steel.

6. The thickness T of the first conductive layer ２ The secondary battery according to claim 1, wherein the thickness is 0.5 μm to 5 μm.

7. The single-sided positive electrode piece further includes an insulating layer, the insulating layer being provided on a surface of the single-sided positive electrode piece that is remote from the interior of the electrode assembly; the insulating layer includes an inert material, the inert material including at least one of boehmite, aluminum oxide, magnesium oxide, silicon oxide, titanium oxide, and magnesium hydroxide; The mass percentage W of the inactive material relative to the mass of the insulating layer ３ The secondary battery according to claim 1, wherein the ratio is 30% to 70%.

8. The thickness T of the insulating layer ３ The secondary battery according to claim 7, wherein is 5 μm to 50 μm.

9. the single-sided positive electrode piece further includes a second conductive layer, the second conductive layer being disposed between the first positive electrode current collector and the positive electrode active material layer; the second conductive layer includes a conductive agent, the conductive agent including at least one of conductive carbon black, carbon nanotubes, carbon fibers, conductive graphite, and graphene; The mass percentage W of the conductive agent relative to the mass of the second conductive layer ４ The secondary battery according to claim 7, wherein the ratio is 30% to 75%.

10. Adhesion strength N between the second conductive layer and the first positive electrode current collector １ is 3 N / m to 120 N / m, and the adhesive strength N between the second conductive layer and the positive electrode active material layer is ２ The secondary battery according to claim 9, wherein the resistance is 3 N / m to 120 N / m.

11. The thickness T of the second conductive layer ４ is 0 μm<T ４ The secondary battery according to claim 9 , wherein the thickness satisfies the following condition:≦4 μm.

12. 2. The secondary battery according to claim 1, wherein the first conductive layer is provided only on a surface of the first polymer layer facing the inside of the electrode assembly.

13. The secondary battery according to claim 1 , wherein the first conductive layer is further provided on a surface of the first polymer layer away from the interior of the electrode assembly.

14. A power consuming device comprising the secondary battery according to any one of claims 1 to 13.

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