Secondary batteries and electronic devices

Abstract

This application provides a secondary battery and electronic device. By selecting high-nickel lithium nickel cobalt manganese oxide (LCM) that combines monocrystalline and polycrystalline LCM as the positive electrode active material, and controlling the values ​​of a, W1, b, and ab / K within the aforementioned ranges, the synergistic effect between the positive electrode active material, electrolyte, and rupture valve in this application can improve the thermal safety performance of the secondary battery.

Description

Technical Field

[0001] This application relates to the field of electrochemical technology, and in particular to a secondary battery and electronic device. Background Technology

[0002] Cylindrical secondary batteries (such as lithium-ion batteries) have advantages such as standardization, high automation, low cost, and good consistency. They can also be used in irregularly shaped casings, and are widely used, especially when paired with high-nickel systems, due to their good energy density. However, cylindrical secondary batteries also have their own problems. For example, radial heat transfer is difficult in cylindrical secondary batteries, leading to temperature rise, and they are prone to thermal runaway at the end of their lifespan. Increased internal temperature also leads to increased internal pressure, posing a risk of explosion. Therefore, developing a new technical solution to improve the thermal safety performance of cylindrical secondary batteries has become a pressing technical problem for those skilled in the art. Summary of the Invention

[0003] The purpose of this application is to provide a secondary battery and an electronic device to improve the thermal safety performance of the secondary battery.

[0004] It should be noted that while this application uses lithium-ion batteries as an example of secondary batteries to explain the invention, the secondary batteries in this application are not limited to lithium-ion batteries. The specific technical solution is as follows:

[0005] The first aspect of this application provides a secondary battery, including an electrode assembly, an electrolyte, a housing, and a rupture valve. The electrode assembly and the electrolyte are housed in the housing, and the rupture valve is disposed in the housing. The electrode assembly includes a positive electrode, a negative electrode, and a separator disposed between the positive and negative electrode. The electrode assembly is formed by stacking and winding the positive electrode, the separator, and the negative electrode. The positive electrode includes a positive active material layer, which includes lithium nickel cobalt manganese oxide. The lithium nickel cobalt manganese oxide has a nickel content of 80% to 97% by mass, and the lithium nickel cobalt manganese oxide includes monocrystalline nickel cobalt manganese. Lithium oxide and polycrystalline nickel-cobalt-manganese oxide; based on the mass of lithium nickel-cobalt-manganese oxide, the mass percentage of monocrystalline lithium nickel-cobalt-manganese oxide is W1, and the mass percentage of polycrystalline lithium nickel-cobalt-manganese oxide is W2, W1 / W2=a, 0.01≤a≤0.7, 1%≤W1≤41%; the electrolyte includes solvent, which includes carbonate and carboxylic acid ester, based on the mass of the electrolyte, the sum of the mass percentages of carbonate and carboxylic acid ester is b, 70%≤b≤95%, and the mass ratio of carbonate to carboxylic acid ester is (50 to 80):(20 to 50); the upper limit pressure for the burst valve to burst is K Pa, and the numerical relationship between a, b, and K satisfies: 1×10 -8 ≤ab / K≤5×10 -6This application uses high-nickel lithium nickel cobalt manganese oxide (LCA) as the positive electrode active material by combining monocrystalline and polycrystalline LCA, and by controlling the values ​​of a, W1, b, and ab / K within the aforementioned ranges. The synergistic effect between the positive electrode active material, electrolyte, and rupture valve in this application can improve the thermal safety performance of the secondary battery, and enable the secondary battery to have high energy density, good cycle performance, and kinetic performance.

[0006] In one embodiment of this application, 1×10 -7 ≤ab / K≤5×10 -6 Adjusting the value of ab / K within the above range is beneficial to improving the thermal safety performance of secondary batteries.

[0007] In one embodiment of this application, 1×10 5 ≤K≤1×10 6 Adjusting the value of K within the above range is beneficial to improving the thermal safety performance of secondary batteries.

[0008] In one embodiment of this application, 0.1 ≤ a ≤ 0.4. Adjusting the value of a within the above range is beneficial to improving the thermal safety performance of the secondary battery.

[0009] In one embodiment of this application, 75% ≤ b ≤ 90%. Adjusting the value of b within the above range is beneficial to improving the thermal safety performance of the secondary battery.

[0010] In one embodiment of this application, 2×10 5 ≤K≤7×10 5 Adjusting the value of K within the above range is beneficial to improving the thermal safety performance of secondary batteries.

[0011] In one embodiment of this application, 59% ≤ W2 ≤ 99%. Adjusting the value of W2 within the above range is beneficial for improving the thermal safety performance of the secondary battery.

[0012] In one embodiment of this application, the monocrystalline lithium nickel cobalt manganese oxide includes at least one of monocrystalline Ni90, monocrystalline Ni91, monocrystalline Ni92, monocrystalline Ni93, monocrystalline Ni94, monocrystalline Ni95, or monocrystalline NCM811. Using the above-mentioned types of monocrystalline lithium nickel cobalt manganese oxide enables the secondary battery to have high energy density and good cycle stability.

[0013] In one embodiment of this application, the polycrystalline lithium nickel cobalt manganese oxide includes at least one of polycrystalline Ni90, polycrystalline Ni91, polycrystalline Ni92, polycrystalline Ni93, polycrystalline Ni94, polycrystalline Ni95, or polycrystalline NCM811. The above-mentioned types of polycrystalline lithium nickel cobalt manganese oxide are selected.

[0014] In one embodiment of this application, the electrolyte further includes a lithium salt, which includes at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetraphenylborate, lithium methanesulfonate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonyl)imide, lithium tris(trifluoromethanesulfonyl)methyl, lithium hexafluorosilicate, lithium dioxalateborate, or lithium difluorooxalateborate; the mass percentage of the lithium salt is 5% to 30% based on the mass of the electrolyte. Using the above-mentioned types of lithium salts and controlling the content of the lithium salt in the electrolyte within the above-mentioned range is beneficial to improving the cycle performance of the secondary battery.

[0015] A second aspect of this application provides an electronic device comprising the secondary battery described in any of the foregoing embodiments. Therefore, the electronic device has good safety performance.

[0016] The beneficial effects of this application are:

[0017] This application provides a secondary battery and electronic device. By selecting high-nickel lithium nickel cobalt manganese oxide (LCO) that combines monocrystalline and polycrystalline LCO as the positive electrode active material, and controlling the values ​​of a, W1, b, and ab / K within the aforementioned ranges, the positive electrode active material, electrolyte, and rupture valve of this application work synergistically to improve the thermal safety performance of the secondary battery, and enable the secondary battery to have high energy density, good cycle performance, and kinetic performance.

[0018] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Detailed Implementation

[0019] The technical solution of this application will be clearly and completely described below with reference to the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0020] It should be noted that, in the specific embodiments of this application, a lithium-ion battery is used as an example of a secondary battery to explain this application; however, the secondary battery in this application is not limited to lithium-ion batteries. The specific technical solution is as follows:

[0021] The first aspect of this application provides a secondary battery, including an electrode assembly, an electrolyte, a housing, and a rupture valve. The electrode assembly and the electrolyte are housed in the housing, and the rupture valve is disposed in the housing. The electrode assembly includes a positive electrode, a negative electrode, and a separator disposed between the positive and negative electrode. The electrode assembly is formed by stacking and winding the positive electrode, the separator, and the negative electrode. The positive electrode includes a positive active material layer, which includes lithium nickel cobalt manganese oxide. The lithium nickel cobalt manganese oxide has a nickel content of 80% to 97% by mass, and the lithium nickel cobalt manganese oxide includes monocrystalline nickel cobalt manganese. Lithium oxide and polycrystalline nickel-cobalt-manganese oxide; based on the mass of lithium nickel-cobalt-manganese oxide, the mass percentage of monocrystalline lithium nickel-cobalt-manganese oxide is W1, and the mass percentage of polycrystalline lithium nickel-cobalt-manganese oxide is W2, W1 / W2=a, 0.01≤a≤0.7, 1%≤W1≤41%; the electrolyte includes solvent, which includes carbonate and carboxylic acid ester, based on the mass of the electrolyte, the sum of the mass percentages of carbonate and carboxylic acid ester is b, 70%≤b≤95%, and the mass ratio of carbonate to carboxylic acid ester is (50 to 80):(20 to 50); the upper limit pressure for the burst valve to burst is K Pa, and the numerical relationship between a, b, and K satisfies: 1×10 -8 ≤ab / K≤5×10 -6 .

[0022] For example, the mass percentage of nickel in lithium nickel cobalt manganese oxide is 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, or any value between any two of the above ranges. For example, the value of 'a' is 0.01, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, or any value between any two of the above ranges. For example, W1 is 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 41%, or any value between any two of the above ranges. For example, b is 70%, 75%, 80%, 85%, 90%, 95%, or any value between any two of the above ranges. The mass ratio of carbonate to carboxylic acid ester is 50:50, 60:40, 70:30, 80:20, or any ratio within any two of these ranges. For example, the value of ab / K is 1 × 10⁻⁶. -8 5×10 -8 1×10 -7 5×10 -7 1×10 -6 5×10 -6 Or any value between any two of the above ranges.

[0023] Lithium nickel cobalt manganese oxide (LCO) materials with a nickel content within the scope of this application are selected. These materials possess high theoretical capacity and good structural stability, exhibiting excellent cycle stability at both room and high temperatures. They also possess a high voltage platform, enabling the secondary battery to maintain good cycle stability at high voltages. This results in advantages such as high energy density and high cycle performance. The secondary battery utilizes both monocrystalline and polycrystalline LCO as cathode active materials. At the end of the charge-discharge cycle, polycrystalline LCO exhibits more gas production due to surface side reactions, but its high specific capacity ensures sufficient energy at the end of high-rate discharge. Monocrystalline LCO has a relatively lower specific capacity compared to polycrystalline LCO, but it exhibits fewer side reactions and less gas production at the end of the charge-discharge cycle, thus providing more capacity at the end of the cycle. A value of a less than 0.01 or W1 less than 1% indicates an insufficient relative content of monocrystalline nickel-cobalt-manganese lithium oxide, which will affect the cycle life of the secondary battery. A value of a greater than 0.7 or W1 greater than 41% indicates an insufficient relative content of polycrystalline nickel-cobalt-manganese lithium oxide, which will affect the kinetic performance of the secondary battery. A value of b less than 70% indicates an insufficient content of carbonate and carboxylic acid esters in the electrolyte, resulting in inadequate electrolyte kinetics and affecting the kinetic performance of the secondary battery. A value of b greater than 95% indicates an excessively high content of carbonate and carboxylic acid esters in the electrolyte, resulting in high electrolyte reactivity. This makes the electrolyte more prone to reacting with positive and negative electrode active materials in the secondary battery, producing more gas at high temperatures, which will affect the storage performance and safety performance of the secondary battery. A value of ab / K less than 1×10 -8 If the upper limit pressure of the rupture valve is too high, it will make it more difficult to depressurize the secondary battery, causing the internal positive and negative electrode plates to break and resulting in an internal short circuit, thereby increasing the risk of explosion and thermal failure of the secondary battery; the value of ab / K is greater than 5×10 -6 If the upper limit pressure at which the rupture valve explodes is too low, the secondary battery will fail due to the pressure release caused by the explosion. Therefore, this application uses a combination of monocrystalline and polycrystalline lithium nickel cobalt manganese oxide with high-nickel lithium nickel cobalt manganese oxide as the positive electrode active material, and controls the values ​​of a, W1, b, and ab / K within the aforementioned ranges. The synergistic effect between the positive electrode active material, electrolyte, and rupture valve in this application improves the thermal safety performance of the secondary battery and enables it to possess high energy density, good cycle performance, and kinetic performance.

[0024] In this application, room temperature refers to 20°C to 25°C, high temperature refers to a temperature greater than or equal to 45°C, high voltage refers to a working voltage greater than or equal to 2.5V, and high rate refers to a discharge rate greater than or equal to 6C.

[0025] In one embodiment of this application, 1×10 -7 ≤ab / K≤5×10 -6 For example, the value of ab / K is 1 × 10.-7 5×10 -7 1×10 -6 5×10 -6 Or any value between any two of the above ranges. Adjusting the value of ab / K within the above range allows for synergistic effects between the positive electrode active material, electrolyte, and rupture valve, which is beneficial for improving the thermal safety performance of the secondary battery. Furthermore, it enables the secondary battery to possess high energy density, good cycle performance, and kinetic performance.

[0026] In one embodiment of this application, 1×10 5 ≤K≤1×10 6 In one embodiment of this application, 2×10 5 ≤K≤7×10 5 For example, the value of K is 1 × 10. 5 2×10 5 4×10 5 6×10 5 7×10 5 8×10 5 1×10 6 Or any value between any two of the above ranges. Adjusting the value of K within the above range ensures that the upper limit pressure at which the rupture valve will burst is within a suitable range, which is beneficial for improving the thermal safety performance of the secondary battery. Furthermore, the secondary battery exhibits high energy density, good cycle performance, and kinetic performance.

[0027] In one embodiment of this application, 0.1 ≤ a ≤ 0.4. For example, the value of a is 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, or any value between any two of the above ranges. Adjusting the value of a within the above range allows for the synergistic effect of monocrystalline and polycrystalline lithium nickel cobalt manganese oxide, which is beneficial for improving the thermal safety performance of the secondary battery. Furthermore, the secondary battery exhibits high energy density, good cycle performance, and kinetic performance.

[0028] In one embodiment of this application, 75% ≤ b ≤ 90%. For example, b is 75%, 80%, 85%, 90%, or any value between any two of the above ranges. By adjusting the value of b within the above range, the carbonate and carboxylic acid esters can provide sufficient power to the electrolyte, enabling the secondary battery to have good cycle performance and kinetic performance while improving thermal safety.

[0029] In one embodiment of this application, 59% ≤ W2 ≤ 99%. For example, W2 is 59%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 99%, or any value between any two of the above ranges. Controlling the value of W2 within the above range helps to control the value of a within the above range, thereby improving the thermal safety performance of the secondary battery.

[0030] In one embodiment of this application, the monocrystalline nickel-cobalt-manganese lithium oxide includes at least one of monocrystalline Ni90, monocrystalline Ni91, monocrystalline Ni92, monocrystalline Ni93, monocrystalline Ni94, monocrystalline Ni95, or monocrystalline NCM811. The above-mentioned monocrystalline nickel-cobalt-manganese lithium oxides possess high capacity and structural stability, exhibiting excellent cycle performance. When applied in secondary batteries, they enable the secondary batteries to achieve high energy density and good cycle stability.

[0031] In one embodiment of this application, the polycrystalline nickel-cobalt-manganese lithium oxide includes at least one of polycrystalline Ni90, polycrystalline Ni91, polycrystalline Ni92, polycrystalline Ni93, polycrystalline Ni94, polycrystalline Ni95, or polycrystalline NCM811. The above-mentioned types of polycrystalline nickel-cobalt-manganese lithium oxide possess high capacity and structural stability, exhibiting excellent cycle performance. When applied in secondary batteries, they enable the secondary batteries to have high energy density and good cycle stability.

[0032] This application does not impose any particular limitation on the types of carbonates and carboxylic esters, as long as they can achieve the purpose of this application. For example, carbonates include at least one of dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, dipropyl carbonate, methyl propyl carbonate, fluoroethylene carbonate, difluoroethylene carbonate, pentafluoropropyl ethylene carbonate, methyl trifluoroethyl carbonate, trifluoromethyl ethylene carbonate, or di(2,2,2-trifluoroethyl) carbonate, and carboxylic esters include at least one of propyl propionate, ethyl propionate, ethyl acetate, ethyl formate, methyl acetate, methyl propionate, propyl acetate, butyl butyrate, ethyl difluoroacetate, difluoroethyl acetate, ethyl trifluoroacetate, trifluoroethyl acetate, or methyl trifluoropropionate.

[0033] In one embodiment of this application, the electrolyte further includes a lithium salt, which includes at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetraphenylborate, lithium methanesulfonate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonyl)imide, lithium tris(trifluoromethanesulfonyl)methyl, lithium hexafluorosilicate, lithium dioxalate borate, or lithium difluorooxalate borate; the mass percentage of the lithium salt is 5% to 30% based on the mass of the electrolyte. For example, the mass percentage of the lithium salt is 5%, 10%, 15%, 20%, 25%, 30%, or any value between any two of the above ranges. Selecting the above-mentioned types of lithium salts and controlling the content of the lithium salt in the electrolyte within the above ranges is beneficial to improving the ionic conductivity of the electrolyte, thereby improving the cycle performance of the secondary battery.

[0034] This application does not impose any particular restrictions on the shape, size, type, model, material, or installation method of the rupture valve, as long as the upper limit pressure for causing the rupture valve to burst is within the range of this application and the purpose of this application can be achieved.

[0035] The positive electrode sheet of this application also includes a positive current collector, and a positive active material layer is disposed on at least one surface of the positive current collector. This application does not particularly limit the type of positive current collector, as long as it achieves the purpose of this application. For example, the positive current collector may include aluminum foil, aluminum alloy foil, etc. In this application, there is no particular limitation on the thickness of the positive current collector and the positive active material layer, as long as it achieves the purpose of this application. For example, the thickness of the positive current collector is 5 μm to 20 μm. The thickness of the positive active material layer is 30 μm to 120 μm. Optionally, the positive active material layer may also include a positive conductive agent and a positive binder. This application does not particularly limit the types of positive conductive agents and positive binders in the positive active material layer, as long as they achieve the purpose of this application. This application does not particularly limit the mass ratio of the positive active material, positive conductive agent, and positive binder in the positive active material layer; those skilled in the art can choose according to actual needs, as long as the purpose of this application is achieved. For example, the mass ratio of positive electrode active material, positive electrode conductive agent and positive electrode binder in the positive electrode active material layer is (94-98):(0.5-2.8):(1.5-3.4).

[0036] This application places no particular restrictions on the negative electrode sheet, as long as the objectives of this application can be achieved. For example, the negative electrode sheet 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. This application places no particular restrictions on the negative electrode current collector, as long as the objectives of this application can be achieved. For example, the negative electrode current collector can include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam or copper foam, etc. The negative electrode active material layer of this application contains a negative electrode active material. This application places no particular restrictions on the type of the negative electrode active material, as long as the objectives of this application can be achieved. For example, the negative electrode active material can include natural graphite, artificial graphite, mesocarbon microbeads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiO x (0 < x < 2), Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel-structured lithium titanate Li4Ti5O 12 , Li-Al alloy or at least one of metallic lithium. In this application, there are no particular restrictions on the thickness of the negative electrode current collector and the negative electrode active material layer, as long as the objectives of this application can be achieved. For example, the thickness of the negative electrode current collector is 6 μm to 10 μm, and the thickness of the negative electrode active material layer is 30 μm to 130 μm. Optionally, the negative electrode active material layer may further include at least one of a thickening agent or a negative electrode binder. This application places no particular restrictions on the types of the thickening agent and the negative electrode binder in the negative electrode active material layer, as long as the objectives of this application can be achieved. This application places no particular restrictions on the mass ratio of the negative electrode active material, the thickening agent and the negative electrode binder in the negative electrode active material layer, as long as the objectives of this application can be achieved. For example, the mass ratio of the negative electrode active material, the thickening agent and the negative electrode binder in the negative electrode active material layer is (96 - 98):(0 - 2.0):(1.0 - 2.0).

[0037] This application places no particular restrictions on the separator, as long as the objectives of this application can be achieved. For example, the material of the separator can include, but is not limited to, at least one of polyolefin (PO) such as polyethylene (PE) and polypropylene (PP), polyester (for example, polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex or aramid. The type of the separator can include at least one of a woven film, a non-woven film, a microporous film, a composite film, a rolled film or a spun film.

[0038] This application places no particular restrictions on the housing, as long as it is provided with a bursting valve within the scope of this application and can achieve the objectives of this application. For example, the housing can include, but is not limited to, a steel shell and an aluminum shell.

[0039] This application places no particular restrictions on the shape of the housing, as long as the objectives of this application can be achieved. For example, the shape of the housing includes, but is not limited to, cylindrical, rectangular or oval.

[0040] This application does not impose any particular limitation on the type of secondary battery, which may include any device in which an electrochemical reaction occurs. For example, secondary batteries may include, but are not limited to: lithium metal secondary batteries, lithium-ion secondary batteries (lithium-ion batteries), sodium-ion secondary batteries (sodium-ion batteries), lithium polymer secondary batteries, and lithium-ion polymer secondary batteries.

[0041] This application does not impose any particular limitation on the preparation method of the secondary battery. Any preparation method known in the art can be used, as long as it can achieve the purpose of this application. For example, the preparation method of the secondary battery includes, but is not limited to, the following steps: stacking the positive electrode, separator, and negative electrode in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain a secondary battery; or stacking the positive electrode, separator, and negative electrode in sequence, then fixing the four corners of the entire stacked structure to obtain a stacked electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain a secondary battery.

[0042] A second aspect of this application provides an electronic device comprising the secondary battery described in any of the foregoing embodiments. Therefore, the electronic device has good safety performance.

[0043] The electronic device described in this application is not particularly limited and can be any electronic device known in the prior art. For example, the electronic device may include, but is not limited to: laptop computers, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, household large-capacity batteries, and lithium-ion capacitors.

[0044] Example

[0045] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below.

[0046] Test methods and equipment:

[0047] Electrolyte composition and content testing:

[0048] The electrolyte was obtained by disassembling a lithium-ion battery (0% SOC, operating voltage 2.5V). The mass percentage of solvent in the electrolyte was determined by gas chromatography-mass spectrometry (GCMS), and the mass percentage of lithium salt was determined by ion chromatography-mass spectrometry.

[0049] Testing of lithium nickel cobalt manganese oxide content in monocrystalline and polycrystalline forms:

[0050] After the lithium-ion batteries were left to stand for 10 minutes, they were charged at a constant current of 0.5C to 4.2V, then charged at a constant voltage of 0.025C, and left to stand for 5 minutes to achieve a 100% SOC (State of Charge). Thirty-two lithium-ion batteries at 100% SOC were randomly selected. The positive electrode sheets were disassembled and immersed in dimethyl carbonate (DMC) for 5 minutes. The composition and content of monocrystalline and polycrystalline lithium nickel cobalt manganese oxide (LCO) in the positive electrode active material layer on the surface of the positive electrode sheet were tested using Mallows's Cp (CP). Specifically, in the CP test, 20 images were stitched together using image recognition, with each image magnified at 2kX, to classify particle sizes. The particle size range of monocrystalline LCO was 10±2μm, and the particle size range of polycrystalline LCO was 4±2μm. Since the true density of monocrystalline and polycrystalline lithium nickel cobalt manganese oxide (NMC) particles is consistent, and the particle area ratio is approximately equal to the mass ratio, the mass ratio is characterized by the particle area ratio. The content (%) of monocrystalline NMC particles = (area of ​​monocrystalline NMC particles + area of ​​polycrystalline NMC particles) × 100%, and the content (%) of polycrystalline NMC particles = (area of ​​polycrystalline NMC particles + area of ​​monocrystalline NMC particles) × 100%. Thirty-two lithium-ion batteries were tested in each example and comparative example, and the average value was taken as the final content of monocrystalline and polycrystalline NMC.

[0051] Testing the upper limit pressure at which the rupture valve will burst:

[0052] The measurements were taken according to the national standard GB / T 3836.1-2021 "Explosive Atmospheres - Part 1: Equipment - General Requirements".

[0053] Testing of the thermal safety performance of lithium-ion batteries:

[0054] Thermal stability was tested using lithium-ion batteries with 60% SOH (State of Health). 60% SOH refers to the ratio of the capacity released at a 7.5C rate at the end of the lithium-ion battery's life to its initial capacity.

[0055] Thermal stability test procedure: (1) Charge the lithium-ion battery at a constant current of 2C to 4.2V, and then charge it at a constant voltage of 4.2V to 0.05C to fully charge the lithium-ion battery; (2) Place the fully charged lithium-ion battery in an oven and heat it to 130±2℃ at a rate of 5±2℃ / min and keep it at that temperature for 30min. If the lithium-ion battery does not catch fire or explode, it passes the test.

[0056] Each embodiment and comparative example used 20 lithium-ion batteries for testing. The pass rate = number of batteries that passed the test / 20. The pass rate is used to characterize the thermal safety performance of lithium-ion batteries. The more batteries that pass the test, the better the thermal safety performance of the lithium-ion batteries.

[0057] Example 1

[0058] <Preparation of the positive electrode>

[0059] Lithium nickel cobalt manganese oxide (LCM), polyvinylidene fluoride (PVDF) (weight average molecular weight 600,000), and conductive carbon black (Super P) were mixed in a mass ratio of 94.8:2.8:2.4. N-methylpyrrolidone (NMP) was added as a solvent, and the mixture was stirred under vacuum until a homogeneous positive electrode slurry with a solid content of 75 wt% was obtained. The positive electrode slurry was uniformly coated onto one surface of a 10 μm thick aluminum foil current collector and dried at 105 °C to obtain a single-sided coated positive electrode sheet. The above steps were then repeated on the other surface of the aluminum foil to obtain a double-sided coated positive electrode sheet. After cold pressing, cutting, and welding of tabs, a 74 mm × 851 mm positive electrode sheet was obtained for use. The coating weight of the positive electrode active material layer was 18 mg / cm³. 2 The compacted density is 2.69 g / cm³. 3 The positive electrode active material lithium nickel cobalt manganese oxide includes monocrystalline NCM811 and polycrystalline NCM811. Based on the mass of lithium nickel cobalt manganese oxide, the mass percentage of monocrystalline NCM811 is W1 = 23%, and the mass percentage of polycrystalline NCM811 is W2 = 77%.

[0060] <Preparation of Negative Electrode Sheets>

[0061] Artificial graphite (negative electrode active material), carboxymethyl cellulose (CMC) (weight average molecular weight 60,000), and styrene-butadiene rubber (SBR, weight average molecular weight 300,000) (negative electrode binder) were mixed in a mass ratio of 97:1.7:1.3. Deionized water was then added as a solvent, and the mixture was stirred until a homogeneous negative electrode slurry with a solid content of 55 wt% was obtained. The negative electrode slurry was uniformly coated onto one surface of a 6 μm thick copper foil current collector and dried at 90°C to obtain a negative electrode sheet with a single-sided coating of negative electrode material. The above steps were then repeated on the other surface of the same copper foil to obtain a negative electrode sheet with a double-sided coating of negative electrode material. After cold pressing, cutting, and welding of tabs, a negative electrode sheet with dimensions of 76 mm × 867 mm was obtained for later use. The coating weight of the negative electrode active material layer was 8.26 mg / cm³. 2 The compacted density is 1.6 g / cm³. 3 .

[0062] <Preparation of the diaphragm>

[0063] A porous polypropylene (PP) film with a thickness of 7 μm was used.

[0064] <Preparation of Electrolyte>

[0065] In an environment with a water content of less than 10 ppm, dimethyl carbonate and propyl propionate, solvents, are mixed at a mass ratio of 50:50. Lithium salt (LiPF6) is then added to the solvent, dissolved, and mixed thoroughly to obtain the electrolyte. Based on the mass of the electrolyte, the sum of the mass percentages of the carbonate and carboxylic acid esters (b) is 80%, and the mass percentage of the lithium salt is 20%.

[0066] <Preparation of Lithium-ion Batteries>

[0067] The positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to provide insulation. The electrode assembly is then wound to form the electrode assembly. This assembly is placed in a cylindrical steel shell (made of 45# carbon steel), dried, and then injected with electrolyte. Following vacuum sealing, settling, formation, capacity testing, degassing, and trimming, a lithium-ion battery is obtained. The shell is equipped with a rupture valve, 18mm in diameter, made of polytetrafluoroethylene (PTFE). The upper limit pressure for the rupture valve to burst is K = 3 × 10⁻⁶. 5 Pa.

[0068] Examples 2 to 17

[0069] Except for adjusting the relevant preparation parameters according to Table 1, everything else is the same as in Example 1.

[0070] In the <Preparation of Electrolyte>, when the sum of the mass percentages b of carbonate and carboxylic acid esters changes, the mass percentage of lithium salt also changes, and the sum of b and the mass percentage of lithium salt is 100%.

[0071] Comparative Examples 1 to 6

[0072] Except for adjusting the relevant preparation parameters according to Table 1, everything else is the same as in Example 1.

[0073] In the <Preparation of Electrolyte>, when the sum of the mass percentages b of carbonate and carboxylic acid esters changes, the mass percentage of lithium salt also changes, and the sum of b and the mass percentage of lithium salt is 100%.

[0074] The preparation and performance parameters of each embodiment and comparative example are shown in Table 1.

[0075] Table 1

[0076]

[0077]

[0078] As can be seen from Examples 1 to 17 and Comparative Examples 1 to 6, the secondary batteries in this application, by using high-nickel lithium nickel cobalt manganese oxide (LCO) combined with monocrystalline and polycrystalline LCO as the positive electrode active material, and by controlling the values ​​of a, W1, b, and ab / K within the aforementioned ranges, exhibit a higher thermal stability test pass rate, indicating that the secondary batteries have higher thermal safety performance. In contrast, the secondary batteries in the comparative examples, where at least one of the values ​​of a, W1, b, and ab / K is not within the range of this application, exhibit a lower thermal stability test pass rate, indicating that the secondary batteries have lower thermal safety performance.

[0079] The ab / K value typically affects the thermal safety performance of secondary batteries. As can be seen from Examples 1 to 13, secondary batteries using ab / K values ​​within the scope of this application exhibit a high pass rate in thermal stability tests, indicating that the secondary batteries possess good thermal safety performance.

[0080] The value of 'a' typically affects the thermal safety performance of secondary batteries. As can be seen from Examples 1 to 5, secondary batteries with 'a' values ​​within the scope of this application exhibit a high pass rate in thermal stability tests, indicating that the secondary batteries have good thermal safety performance.

[0081] The value of b typically affects the thermal safety performance of secondary batteries. As can be seen from Examples 1, 6 to 9, secondary batteries with b values ​​within the scope of this application exhibit a high pass rate in thermal stability tests, indicating that the secondary batteries have good thermal safety performance.

[0082] The value of K typically affects the thermal safety performance of secondary batteries. As can be seen from Examples 1, 10 to 13, secondary batteries with K values ​​selected within the scope of this application exhibit a high pass rate in thermal stability tests, indicating that the secondary batteries have good thermal safety performance.

[0083] The mass ratio of carbonate to carboxylic acid ester in the electrolyte typically affects the thermal safety performance of a secondary battery. As can be seen from Examples 1, 14, and 15, the secondary battery using the appropriate mass ratio of carbonate to carboxylic acid ester in the electrolyte within the scope of this application exhibits a high pass rate in thermal stability tests, indicating that the secondary battery possesses good thermal safety performance.

[0084] Polycrystalline lithium nickel cobalt manganese oxide (LiCO2) typically affects the thermal safety performance of secondary batteries. As can be seen from Examples 1 to 5, secondary batteries using polycrystalline LiCO2 within the scope of this application exhibit a high pass rate in thermal stability tests, indicating that the secondary batteries possess good thermal safety performance.

[0085] The type of lithium nickel cobalt manganese oxide (LCO) – monocrystalline or polycrystalline – typically affects the thermal safety performance of secondary batteries. As can be seen from Examples 1, 16, and 17, secondary batteries using monocrystalline or polycrystalline LCO within the scope of this application exhibit a high pass rate in thermal stability tests, indicating that the secondary batteries possess good thermal safety performance.

[0086] It should be noted that, in this document, the terms “comprising,” “including,” or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0087] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0088] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A secondary battery, comprising an electrode assembly, an electrolyte, a housing, and a burst valve, wherein the electrode assembly and the electrolyte are housed in the housing, the burst valve is disposed in the housing, the electrode assembly comprises a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, the electrode assembly being formed by stacking and winding the positive electrode, the separator, and the negative electrode; The positive electrode sheet includes a positive active material layer, which includes lithium nickel cobalt manganese oxide. The lithium nickel cobalt manganese oxide contains 80% to 97% nickel by mass. The lithium nickel cobalt manganese oxide includes monocrystalline lithium nickel cobalt manganese oxide and polycrystalline lithium nickel cobalt manganese oxide. Based on the mass of the lithium nickel cobalt manganese oxide, the mass percentage of the monocrystalline lithium nickel cobalt manganese oxide is W1, the mass percentage of the polycrystalline lithium nickel cobalt manganese oxide is W2, W1 / W2=a, 0.01≤a≤0.7, 1%≤W1≤41%; The electrolyte includes a solvent, which includes carbonates and carboxylic acid esters. Based on the mass of the electrolyte, the sum of the mass percentages of the carbonates and carboxylic acid esters is b, where 70% ≤ b ≤ 95%, and the mass ratio of the carbonates to the carboxylic acid esters is (50 to 80):(20 to 50). The upper limit pressure at which the burst valve bursts is K Pa, and the numerical relationships between a, b, and K satisfy: 1×10 -8 ≤ab / K≤5×10 -6 .

2. The secondary battery according to claim 1, wherein, 1×10 -7 ≤ab / K≤5×10 -6 。 3. The secondary battery according to claim 1, wherein, 1×10 5 ≤K≤1×10 6 。 4. The secondary battery according to claim 1, wherein, 0.1≤a≤0.4。 5. The secondary battery according to claim 1, wherein, 75％≤b≤90％。 6. The secondary battery according to claim 1, wherein, 59％≤W2≤99％。 7. The secondary battery according to claim 1, wherein, The single-crystal lithium nickel cobalt manganese oxide includes at least one of single-crystal Ni90, single-crystal Ni91, single-crystal Ni92, single-crystal Ni93, single-crystal Ni94, single-crystal Ni95 or single-crystal NCM811.

8. The secondary battery according to claim 1, wherein, The polycrystalline lithium nickel cobalt manganese oxide includes at least one of polycrystalline Ni90, polycrystalline Ni91, polycrystalline Ni92, polycrystalline Ni93, polycrystalline Ni94, polycrystalline Ni95 or polycrystalline NCM811.

9. The secondary battery according to claim 1, wherein, The electrolyte further includes lithium salts, which include at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetraphenylborate, lithium methanesulfonate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonyl)imide, lithium tris(trifluoromethanesulfonyl)methyl lithium, lithium hexafluorosilicate, lithium dioxalateborate, or lithium difluorooxalateborate. Based on the mass of the electrolyte, the lithium salt has a mass percentage content of 5% to 30%.

10. An electronic device, wherein, The electronic device includes a secondary battery as described in any one of claims 1 to 9.

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