Secondary batteries and electrical equipment

Abstract

The present application provides a secondary battery and an electric device, the secondary battery including a positive electrode sheet and a negative electrode sheet, the positive electrode sheet including a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector, and the negative electrode sheet including a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. The secondary battery satisfies 0.5≦(σ1 / σ2) / CB≦9.4. The present application limits the relationship between the areal density and the CB value to balance the lithium desorption speed of the positive / negative electrode sheets, prevent lithium deposition and a drop in battery capacity during operation of the secondary battery, and improve the life of the secondary battery.

Description

[Technical Field]

[0001] This application claims priority from a Chinese patent application filed with the China Patent Office on October 27, 2022, bearing application number 202211327819.X and entitled "Secondary Battery and Electrical Device," the entire contents of which are incorporated herein by reference.

[0002] The present application relates to the field of battery technology, and more particularly to secondary batteries and electrical devices. [Background technology]

[0003] Lithium-ion batteries offer significant advantages in the field of energy storage, yet several challenges remain before they can be put to practical use. For example, the service life of a lithium iron phosphate battery is 4,000-6,000 cycles, making it difficult to meet the 20-30-year service life (over 10,000 cycles) required for energy storage projects with high lifespan requirements. The loss of active lithium is a major factor in the lifespan degradation of lithium-ion energy storage batteries. During the lithium desorption process due to battery cycling, factors such as the expansion and contraction of graphite and the leaching of positive electrode transition metals cause the destruction and formation of the SEI, increasing the area and thickness of the SEI film. This depletes the battery system's limited active lithium, ultimately shortening the battery's service life. Dealing with and replenishing the lost active lithium during the battery degradation process is a key challenge for extending the lifespan of lithium iron phosphate batteries.

[0004] To address these issues, the most common approach is to "lithium replenish" the negative electrode. This approach includes physical and electrochemical methods. However, physical replenishment is prone to lithium deposition in the battery after subsequent cycling, potentially causing short circuits and posing safety risks. While electrochemical replenishment can address the shortcomings of physical replenishment, current methods are ineffective, complex, and expensive, making them unsuitable for practical production and widespread use. Researchers have also selected graphite anodes with special performance indicators, such as small specific surface area, narrow particle size distribution, and surface coating modifications. They have also selected electrolytes with good infiltration performance at room and high temperatures, and used appropriate battery composition design to mitigate the volumetric expansion and contraction of graphite during cycling, reducing the specific surface area of ​​graphite and the loss of active lithium during the SEI breakdown and formation process, thereby extending the service life of lithium iron phosphate batteries as much as possible. Therefore, there is a need to develop secondary batteries that have an ultra-long cycle life, excellent electrochemical performance, are easy to process and manufacture, and have lower operating costs.

[0005] The present application provides a secondary battery with the objective of preventing lithium deposition and a decrease in battery capacity during operation of the secondary battery and improving the life of the secondary battery by defining the relationship between the areal density and the CB value and balancing the lithium desorption rates of the positive and negative electrode sheets. Summary of the Invention [Means for solving the problem]

[0006] In a first aspect, an embodiment of the present application provides a secondary battery including a positive electrode sheet and a negative electrode sheet, wherein the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, and the secondary battery satisfies 0.5≦(σ1 / σ2) / CB≦9.4. Here, σ1 is the surface density of the positive electrode active material layer, and its unit is g / m 2 is. σ2 is the surface density of the negative electrode active material layer, and its unit is g / m 2 is. CB is the ratio of the capacity per unit area of ​​the negative electrode sheet to the capacity per unit area of ​​the positive electrode sheet.

[0007] In some embodiments, 140≦σ1 / CB≦750. In some embodiments, 75≦σ2 / CB≦360. In some embodiments, 0.8≦CB≦1.1. In some embodiments, the areal density σ of the positive electrode active material layer is 150 g / m 2 ≦σ1≦600 g / m 2 Meet the following. In some embodiments, the areal density σ of the negative electrode active material layer is 80 g / m 2 ≦σ2≦290 g / m 2 Meet the following. In some embodiments, the secondary battery satisfies 5≦σ1 / a≦20, where a is the porosity of the positive electrode active material layer, and is expressed in %.

[0008] In some embodiments, 28%≦a≦32%. In some embodiments, the secondary battery satisfies 2.5≦σ2 / b≦9.1, where b is the porosity of the negative electrode active material layer, and is expressed in %. In some embodiments, 30%≦b≦38%. In some embodiments, 1.0≦(σ1 / σ2) / CB≦6.0. In some embodiments, 2.0≦(σ1 / σ2) / CB≦3.0.

[0009] In some embodiments, the positive electrode active material of the positive electrode sheet includes one or more of lithium iron phosphate, lithium cobalt oxide, lithium nickel oxide, and lithium manganese oxide. In some embodiments, the negative electrode active material of the negative electrode sheet includes one or more of artificial graphite, natural graphite, amorphous carbon, carbon nanotubes, and mesocarbon microspheres.

[0010] In a second aspect, an embodiment of the present application provides an electrical device including the secondary battery, wherein the secondary battery functions as a power supply for the electrical device.

[0011] Compared to conventional technologies, the secondary battery of the present application includes a positive electrode sheet and a negative electrode sheet. The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector. The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. The secondary battery satisfies 0.5≦(σ1 / σ2) / CB≦9.4. The present application ensures a redundancy design for the positive electrode and balances the lithium desorption rates of the positive and negative electrode sheets, preventing lithium deposition and capacity loss during secondary battery operation and improving the lifespan of the secondary battery. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present application provides a secondary battery and an electric device, and in order to make the purpose, technical solution and effects of the present application clearer and more explicit, the present application will be described in more detail below with reference to examples. It should be understood that the specific examples described herein are used only to interpret the present application and are not intended to limit the present application.

[0013] secondary battery The present application provides a secondary battery, which includes a positive electrode sheet and a negative electrode sheet. The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector. The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. The secondary battery satisfies 0.5≦(σ1 / σ2) / CB≦9.4, where σ1 is the areal density of the positive electrode active material layer, and is expressed in g / m. 2 σ2 is the surface density of the negative electrode active material layer, and its unit is g / m 2 CB is the ratio of the capacity per unit area of ​​the negative electrode sheet to the capacity per unit area of ​​the positive electrode sheet.

[0014] When the capacity per gram of the positive and negative electrode materials is constant, the greater the areal density of the positive and negative electrode active material, the higher the reversible capacity of the positive and negative electrode sheets, so σ1 / σ2 is correlated with the design CB value of the secondary battery. On the other hand, the lithium desorption rate of the positive and negative electrode sheets is correlated with the areal density of the active material layer of the positive and negative electrode sheets. The higher the coating area density of the electrode sheets, the greater the polarization of the system, the greater the resistance to lithium desorption, and the relatively slower the rate.

[0015] This application ensures a positive electrode redundancy design by limiting the ratio (σ1 / σ2) / CB to ensure sufficient lithium is available to replenish the active lithium lost during secondary battery operation. At the same time, it balances the lithium desorption rates of the positive and negative electrode sheets, preventing lithium deposition and capacity loss during secondary battery operation and improving the cycle life of the secondary battery. If (σ1 / σ2) / CB<0.5, the relative coating weight of the negative electrode sheet is too high, slowing the lithium desorption rate, increasing the risk of lithium deposition in the secondary battery system, increasing side reactions, and accelerating battery life degradation. If (σ1 / σ2) / CB>9.4, the relative coating weight of the positive electrode sheet is too high, increasing positive electrode polarization, slowing the lithium desorption rate, and insufficient lithium ion desorption during high-rate charging, resulting in a decrease in secondary battery capacity and energy density.

[0016] In some embodiments, 1.0≦(σ1 / σ2) / CB≦6.0. In some embodiments, 2.0≦(σ1 / σ2) / CB≦3.0.

[0017] By further optimizing the surface density of the positive / negative electrodes and the combination of CB, the present application has reliably improved the cycle life of the secondary battery, while also improving the energy efficiency and energy density of the secondary battery.

[0018] In some embodiments, the value of (σ1 / σ2) / CB is 0.5, 0.8, 1.0, 1.1, 1.3, 1.5, 1.7, 2.0 、2It can be any of the following values: 0.2, 2.5, 2.8, 3.0, 3.3, 3.5, 3.8, 4.0, 4.3, 4.5, 4.8, 5.0, 5.3, 5.5, 5.8, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 8.7, 9.0, 9.4, or in a range consisting of any two values. In some embodiments, the unit area of ​​the positive and negative electrode sheets is equal when calculating the CB value. In some embodiments, the capacity per unit area of ​​the negative electrode sheet is the charge capacity per unit area of ​​the negative electrode sheet. In some embodiments, the capacity per unit area of ​​the positive electrode sheet is discharge capacity per gram per unit area of ​​the positive electrode sheet.

[0019] In some embodiments, the CB value is determined by the following method. The capacity per unit area of ​​the negative electrode sheet is obtained as follows: hold active material on one side of a unit area of ​​negative electrode sheet, assemble it with a lithium sheet, separator, and electrolyte to form a button battery, discharge it at 0.1C to 0.005V, discharge it at 0.05mA to 0.005V, discharge it at 0.02mA to 0.005V, and charge it at 0.1C to 2V, and the resulting charge capacity is the capacity per unit area of ​​the negative electrode sheet. The capacity per unit area of ​​the positive electrode sheet is obtained as follows: hold active material on one side of a unit area of ​​positive electrode sheet, assemble it with a lithium sheet, separator, and electrolyte to form a button battery, charge it to 3.7 V at 0.1 C, apply a constant voltage of 50 μA, and discharge it to 2.0 V at 0.1 C. The resulting discharge capacity is the capacity per unit area of ​​the positive electrode sheet.

[0020] In some embodiments, 140≦σ1 / CB≦750. The range of σ1 / CB for secondary batteries has been further limited, ensuring processing performance while improving the energy density of the secondary batteries. In some embodiments, the value of σ1 / CB is any of the following values, or a range consisting of any two of: 140, 150, 180, 200, 230, 250, 270, 290, 300, 320, 330, 340, 350, 370, 380, 390, 400, 420, 430, 450, 470, 480, 500, 520, 530, 540, 550, 560, 580, 600, 630, 650, 680, 700, 750.

[0021] In some embodiments, 75≦σ2 / CB≦360. In some embodiments, the value of σ / CB is 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200, 205, 210, 215, 220, 225, It can be any of the values ​​230, 235, 240, 245, 250, 255, 260, 265, 270, 275, 280, 285, 290, 295, 300, 305, 310, 315, 320, 325, 330, 335, 340, 345, 350, 355, 360, or in a range consisting of any two of these values. The range of σ2 / CB for secondary batteries has been further limited, ensuring processing performance while improving the energy density of secondary batteries.

[0022] In some embodiments, the range of CB values ​​is 0.8≦CB≦1.1. This application utilizes a surplus design for the positive electrode sheet to increase the reversible capacity of the battery's positive electrode sheet, and uses a constant capacity method with charge measurement control to allow the positive electrode sheet to release only a portion of the lithium ions during battery operation. The surplus lithium ions are used as a reserve to replenish the active lithium lost during secondary battery operation, thereby keeping the active lithium within an appropriate range at all times and improving the storage capacity recovery rate and energy efficiency.

[0023] In some embodiments, the CB value is any one of the following values, or a range consisting of any two of the following values: 0.8, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, 0.90, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1.0, 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.1. The method for measuring areal density is as follows: Specific area S m 2 If the positive or negative electrode sheet is taken and weighed as m1g, the positive or negative electrode active layer on both sides of the electrode sheet is scraped off, and the weight of the foil material is weighed as m2g, the areal density is (m1-m2) / S g / m 2 This becomes:

[0024] In some embodiments, the areal density σ of the positive electrode active material layer is 150 g / m 2 ≦σ1≦600 g / m 2 Meet the following. In some embodiments, σ ​​(units: g / m 2 ) is any one of the following values, or a range consisting of any two of the following values: 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, 300, 310, 320, 330, 340, 350, 360, 370, 380, 390, 400, 410, 420, 430, 440, 450, 460, 470, 480, 490, 500, 510, 520, 530, 540, 550, 560, 570, 580, 590, 600, 610, 620, 630, 640, 650, 660, 670, 680, 690, 700.

[0025] In some embodiments, the areal density σ of the negative electrode active material layer is 80 g / m 2 ≦σ2≦290 g / m 2 Meet the following. In some embodiments, σ ​​(units: g / m 2) is any one of 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, and 290, or is in a range consisting of any two of these values.

[0026] The present invention further limits the surface densities of the positive electrode active material layer and the negative electrode active material layer while ensuring the processing performance of the positive electrode sheet, thereby improving the energy density of the secondary battery and, at the same time, matching the lithium desorption rates of the positive and negative electrodes and improving the cycle life. In some embodiments, the secondary battery satisfies 5≦σ1 / a≦20, where a is the porosity of the positive electrode active material layer, and is expressed in %.

[0027] The lithium desorption rate of the positive electrode sheet is negatively correlated with its areal density and positively correlated with the porosity of the electrode sheet. In this application, by limiting σ1 / a and controlling the lithium desorption rate of the positive electrode sheet, a certain balance with the lithium desorption rate of the negative electrode sheet is achieved, preventing lithium deposition due to an imbalance in lithium desorption rates during operation of the secondary battery and ensuring improved cycle life.

[0028] In some embodiments, the values ​​of σ1 / a are 5.0, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6, 5.7, 5.8, 5.9, 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, 7.0, 7.1, 7.2, 7.3, 7.4, 7.5, 7.6, 7.7, 7.8, 7.9, 8.0, 8.1, 8.2, 8.3, 8.4, 8.5, 8.6, 8.7, 8.8, 8.9, 9.0, 9.1, 9.2, 9.3, 9.4, 9.5, 9.6, 9.7, 9.8, 9 ...3, 9.4, 9.5, 9.6, 9.7, 9.8, 9.9, 9. 2, 9.3, 9.4, 9.5, 9.6, 9.7, 9.8, 9.9, 10.0, 10.1, 10.2, 10.3, 10.4, 10.5, 10.6, 10.7, 10.8, 10.9, 11.0, 11.1, 11.2, 11.3, 11.4, 11.5, 11.6, 11.7, 11.8, 11.9, 12.0, 12.1, 12.2, 12.3, 12.4, 12.5, 12.6, 12.7, 12.8, 12.9, 13.0, 13.1 , 13.2, 13.3, 13.4, 13.5, 13.6, 13.7, 13.8, 13.9, 14.0, 14.1, 14.2, 14.3, 14.4, 14.5, 14.6, 14.7, 14.8, 14.9, 15.0, 15.1, 15.2, 15.3, 15.4, 15.5, 15.6, 15.7, 15.8, 15.9, 16.0, 16.1, 16.2, 16.3, 16.4, 16.5, 16.6, 16.7, 16.8, 16. 9, 17.0, 17.1, 17.2, 17.3, 17.4, 17.5, 17.6, 17.7, 17.8, 17.9, 18.0, 18.1, 18.2, 18.3, 18.4, 18.5, 18.6, 18.7, 18.8, 18.9, 19.0, 19.1, 19.2, 19.3, 19.4, 19.5, 19.6, 19.7, 19.8, 19.9, 20.0, or in a range consisting of any two of these values. In some embodiments, 28%≦a≦32%.

[0029] The lithium desorption rate of the positive electrode sheet is positively correlated with the porosity of the electrode sheet. In the present invention, by further limiting the porosity a of the positive electrode sheet and controlling the lithium desorption rate of the positive electrode sheet, a certain degree of balance with the lithium desorption rate of the negative electrode sheet is achieved, preventing lithium deposition due to an imbalance in lithium desorption rates during operation of the secondary battery and improving the cycle life. In some embodiments, the value of a (in %) is any one of the following values, or a range consisting of any two of the following values: 28.0, 28.1, 28.2, 28.3, 28.4, 28.5, 28.6, 28.7, 28.8, 28.9, 29.0, 29.1, 29.2, 29.3, 29.4, 29.5, 29.6, 29.7, 29.8, 29.9, 30.0, 30.1, 30.2, 30.3, 30.4, 30.5, 30.6, 30.7, 30.8, 30.9, 31.0, 31.1, 31.2, 31.3, 31.4, 31.5, 31.6, 31.7, 31.8, 31.9, 32.0.

[0030] In some embodiments, 2.5≦σ2 / b≦9.1, where b is the porosity of the negative electrode active material layer, expressed in %. The lithium desorption rate of the negative electrode sheet is negatively correlated with its surface density and positively correlated with the porosity of the electrode sheet. By limiting σ2 / b and controlling the lithium desorption rate of the negative electrode sheet, a certain degree of balance with the lithium desorption rate of the positive electrode sheet can be achieved, preventing lithium deposition due to an imbalance in lithium desorption rate during operation of the secondary battery and improving the cycle life.

[0031] In some embodiments, the value of σ2 / b is 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4.0, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, 5.0, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6, 5.7, 5.8, 5.9, It is any one of the values ​​6.0, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, 7.0, 7.1, 7.2, 7.3, 7.4, 7.5, 7.6, 7.7, 7.8, 7.9, 8.0, 8.1, 8.2, 8.3, 8.4, 8.5, 8.6, 8.7, 8.8, 8.9, 9.0, and 9.1, or is in a range consisting of any two of these values. In some embodiments, 30%≦b≦38%.

[0032] The lithium desorption rate of the negative electrode sheet is positively correlated with the porosity of the electrode sheet. By limiting the porosity b of the negative electrode sheet and controlling the lithium desorption rate of the negative electrode sheet, a certain degree of balance with the lithium desorption rate of the positive electrode sheet can be achieved, preventing lithium deposition due to an imbalance in the lithium desorption rate during operation of the secondary battery and improving the cycle life. In some embodiments, the value of b (in %) is 30.0, 30.1, 30.2, 30.3, 30.4, 30.5, 30.6, 30.7, 30.8, 30.9, 31.0, 31.1, 31.2, 31.3, 31.4, 31.5, 31.6, 31.7, 31.8, 31.9, 32.0, 32.1, 32.2, 32.3, 32.4, 32.5, 32.6, 32.7, 32.8, 32.9, 33.0, 33.1, 33.2, 33.3, 33.4, 33.5, 33.6, 33.7, 33.8, 33.9, 34.0, 34.1, 34.2, 34.3, 34.4, 34.5, 34.6, 34.7, 34.8, 34.9, 35.0, 35.1, 35.2, 35.3, 35.4, 35.5, 35.6, 35.7, 35.8, 35.9, 36.0, 36.1, 36.2, 36.3, 36.4, 36.5, 36.6, 36.7, 36.8, 36.9, 37.0, 37.1, 37.2, 37.3, 37.4, 37.5, 37.6, 37.7, 37.8, 37.9, 38.1, 38.2, 38.3, .1, 34.2, 34.3, 34.4, 34.5, 34.6, 34.7, 34.8, 34.9, 35.0, 35.1, 35.2, 35.3, 35.4, 35.5, 35.6, 35.7, 35.8, 35.9, 36.0, 36.1, 36.2, 36.3, 36.4, 36.5, 36.6, 36.7, 36.8, 36.9, 37.0, 37.1, 37.2, 37.3, 37.4, 37.5, 37.6, 37.7, 37.8, 37.9, 38.0, or in a range consisting of any two of these values.

[0033] In the present application, both the areal density and the porosity can be measured by existing measurement methods. In some embodiments, the areal density of the positive electrode active material layer is measured by the following method: specific area (S1, unit: m 2 The weight (g) of the positive electrode sheet is measured, and the weight (g) of the positive electrode current collector is subtracted to obtain the applied weight (g) of the positive electrode active material layer. This is then divided by the area S1 of the positive electrode sheet to obtain the surface density σ1 (unit: g / m 2 ) to get the

[0034] In some embodiments, the areal density of the negative electrode active material layer is measured by the following method: 2) and subtract the weight (g) of the negative electrode current collector to obtain the applied weight (g) of the negative electrode active material layer. This is then divided by the area S2 of the negative electrode sheet to obtain the surface density σ2 (unit: g / m 2 ) to get the

[0035] In some embodiments, the porosity of the positive electrode active material layer and the porosity of the negative electrode active material layer are measured by mercury intrusion porosimetry. Positive electrode sheet In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material disposed on the positive electrode current collector. In some embodiments, the active cathode material includes one or more of lithium iron phosphate, lithium cobalt oxide, lithium nickel oxide, and lithium manganese oxide.

[0036] In some embodiments, the positive electrode active material is lithium iron phosphate, lithium cobalt oxide, lithium nickel oxide, or lithium manganese oxide, and the manufacturing method can be any known technique, such as a high-temperature solid-state method, a carbothermal reduction method, a spray-drying method, a template method, or a hydrothermal synthesis method. Specifically, lithium iron phosphate, which is the positive electrode active material, is produced by the following method: a lithium source (Li2CO3), an iron source (FePO4), and a carbon source are mixed and pretreated at 300 to 500°C, and then sintered at 600 to 850°C to produce lithium iron phosphate as the positive electrode active material.

[0037] In some embodiments, the positive electrode active material is lithium cobalt oxide. The manufacturing method can be any of the existing techniques such as high temperature solid phase method and sol-gel method. Specifically, the positive electrode active material, lithium cobalt oxide, is produced by the following method: mixing a lithium source (Li2CO3), a cobalt source (Co3O4), and ethanol, polishing the mixture, pretreating it at 300°C, sintering it at 600°C, and then carrying out a solid-state reaction at 800°C to produce lithium cobalt oxide.

[0038] In some embodiments, the positive electrode sheet further includes a conductive agent and an adhesive, and the types and contents of the conductive agent and adhesive are not particularly limited and may be selected according to actual needs. In some embodiments, the conductive agent may include conductive carbon black, carbon nanotubes, graphene, etc., and the adhesive may include polyvinylidene fluoride. In some embodiments, the cathode sheet is manufactured by dispersing the cathode active material described above or the cathode active material obtained by the above method, a conductive agent, and an adhesive in a solvent, N-methylpyrrolidone (NMP), at a specific ratio, stirring uniformly, applying the resulting slurry to an aluminum foil, and then manufacturing the cathode sheet through processes such as rolling, stripping, and cutting. By changing the rolling parameters, the corresponding properties and parameters of the cathode sheet can be controlled. The cathode active material layer of the present application may be controlled to satisfy the above characteristics.

[0039] Negative electrode sheet In some embodiments, the negative electrode sheet includes a negative electrode current collector, a negative electrode active material coated on the negative electrode current collector, an adhesive, and a conductive agent. The types and contents of the negative electrode active material, adhesive, and conductive agent are not particularly limited and may be selected according to actual needs. In some embodiments, the negative electrode active material includes one or more of artificial graphite, natural graphite, amorphous carbon, carbon nanotubes, and mesocarbon microspheres.

[0040] electrolyte In some embodiments, the main components of the electrolyte solution include a lithium salt, an organic solvent, and an additive. Here, the type and composition of the lithium salt and the organic solvent are not particularly limited and may be selected according to actual needs. Here, the lithium salt may include lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, etc., the solvent may include ethylene carbonate, methyl ethyl carbonate, dimethyl carbonate, propyl propanoate, etc., and the additive may include lithium difluorophosphate, lithium bis(oxalato)borate, succinonitrile, etc.

[0041] Separator In some embodiments, the type of separator is not particularly limited and may be selected according to actual needs. The separator may be a polypropylene film, a polyethylene film, a polyvinylidene fluoride film, a spandex film, an aramid film, or a multi-layer composite film modified by a coating. In some embodiments, a secondary battery is manufactured as follows: A positive electrode sheet, a separator, and a negative electrode sheet are stacked in this order, with a separator positioned between the positive and negative electrode sheets to provide isolation, and then wound to form a rectangular core. This is then inserted into a battery case, dried at 65 to 95°C to remove moisture, and then an electrolyte is poured into the battery case, which is then sealed. The secondary battery is then obtained through processes such as standing, hot and cold curing, chemical conversion, fixing, and capacity selection.

[0042] In some embodiments, fabricating a secondary battery includes the following steps: (1) Lithium iron phosphate, a positive electrode conductive agent, a positive electrode adhesive, and a positive electrode solvent are mixed to prepare a positive electrode slurry, which is then applied to the surface of a positive electrode current collector to obtain a positive electrode sheet. (2) Graphite, anode dispersant, anode conductive agent, anode adhesive, and anode solvent are mixed to prepare anode slurry, which is then applied to the surface of anode current collector to obtain anode sheet. (3) In the manufacturing process of a lithium-ion battery, the positive electrode sheet, negative electrode sheet, and separator manufactured in this application are assembled with other battery components, and then undergo processes such as molding, drying, packaging, liquid injection, chemical formation, and capacity selection to obtain a long-life lithium iron phosphate lithium-ion battery, and battery types include softbank, cylindrical, and aluminum case, etc. Here, the design CB value of the battery satisfies 0.8≦CB≦1.1, i.e., the reversible capacity of the negative electrode sheet of the battery / the reversible capacity of the positive electrode sheet of the battery is 0.8 to 1.10, and satisfies 0.5≦(σ1 / σ2) / CB≦9.4. In some embodiments, the secondary battery types include softbank batteries, cylindrical batteries, aluminum case batteries, and the like.

[0043] Electrical Equipment The electrical device of the present application includes the above-described secondary battery. In some embodiments, the electrical device of the present application may be, but is not limited to, a backup power supply, a motor, an electric vehicle, an electric motorcycle, a power-assisted bicycle, a bicycle, a power tool, a large-scale home battery, and the like.

[0044] Example 1 The manufacturing method of the positive electrode sheet is as follows: Lithium iron phosphate, conductive carbon black (SP), and PVDF are mixed in a mass ratio of 97:0.7:2.3, then added to NMP and mixed thoroughly to form a uniform mixture. The mixture is then applied to both sides of a (12+1+1) μm carbon-coated aluminum foil, and the electrode sheet is then dried, rolled, stripped, and cut to obtain the positive electrode sheet. The manufacturing method of the negative electrode sheet is as follows: Graphite, conductive carbon black (SP), CMC, and SBR are mixed in a mass ratio of 96.3:0.7:1.1:1.9, then added to water and mixed thoroughly. After uniform mixing, the mixture is applied to both sides of 6 μm copper foil, and the electrode sheet is then dried, rolled, stripped, and cut to obtain the negative electrode sheet.

[0045] The separator is a polyethylene film. The electrolyte solution is prepared as follows: ethylene carbonate, methyl ethyl carbonate, and dimethyl carbonate, which are organic solvents, are mixed in a volume ratio of 1:1:1. In a glove box with an argon atmosphere and a moisture content of less than 10 ppm, thoroughly dried lithium hexafluorophosphate is dissolved in the organic solvent to a concentration of 1 mol / L, and the mixture is mixed uniformly to obtain the electrolyte solution.

[0046] In the manufacturing process of lithium-ion batteries, the positive electrode sheet, negative electrode sheet, and separator manufactured in this application are assembled with other battery components, and then undergo processes such as molding, drying, packaging, liquid injection, chemical formation, and capacity selection to obtain long-life lithium iron phosphate lithium-ion batteries, and battery types include prismatic case batteries. Examples 2-14 and Comparative Examples 1-2 were both manufactured according to the method of Example 1, except that the amount of active material applied and the thickness of the active material layer on the electrode sheet were adjusted to obtain the desired design CB value and areal density. The parameters of the manufactured batteries are shown in Table 1.

[0047] Battery performance measurement The cycle capacity retention rate is calculated as follows: the nominal capacity of the secondary battery is C1, and the battery is subjected to a corresponding number of 1C / 1C cycles at a specified temperature (25°C) to determine the discharge capacity C2, where the capacity retention rate = C2 / C1 × 100%. Energy efficiency is the percentage ratio of the energy output when discharging a lithium-ion battery to the energy input during the previous charge.

[0048] The method for measuring energy efficiency is as follows: (1) The secondary battery is charged to the nominal capacity of the battery at a constant current of 1C, and the charging energy is E1. (2) Leave it for 30 minutes. (3) Discharge the battery at a constant current of 1C down to the lower voltage limit (2.5V), and let the discharge energy be E2. (4) The secondary battery energy efficiency value is E2 / E1.

[0049] The method for measuring the recovery rate of storage capacity is as follows. (1) At room temperature, charge the secondary battery at a constant current of 1C up to the battery's nominal capacity of x1Ah. (2) The secondary battery is placed in an oven at 60°C and stored for 400 days. (3) Discharge the secondary battery at room temperature at 1C to 2.5V. (4) Leave it for 30 minutes. (5) At room temperature, the secondary battery is charged to 3.65 V at a constant current and voltage of 1 C, and the cutoff current is 0.05 C. (6) Leave it for 30 minutes. (7) At room temperature, discharge the secondary battery to 2.5 V at a constant current of 1 C, and the discharge capacity is X2 Ah. (8) The capacity recovery rate of a secondary battery is X2 / X1.

[0050] Table 1 shows the performance parameters of the secondary batteries manufactured in Examples 1-14 and Comparative Examples 1-2. [Table 1]

[0051] Table 2 shows the measurement results for Examples 1-14 and Comparative Examples 1-2. [Table 2]

[0052] As can be seen from Tables 1 and 2, the present invention limits the relationship between the positive and negative electrode areal densities and the design CB value, and limits the porosity and areal densities of the positive and negative electrode sheets to ensure a redundancy design of the positive electrode, so that sufficient lithium can replenish the active lithium lost during operation of the secondary battery. At the same time, it balances the lithium desorption rate of the positive and negative electrode sheets, preventing lithium deposition and capacity loss during operation of the secondary battery, thereby significantly improving the cycle life, storage life, and energy efficiency of the secondary battery.

[0053] In Examples 1-7, the porosities a% and b% of the positive and negative electrode sheets were constant at 30% and 32%, respectively. However, with an increase in (σ1 / σ2) / CB and σ1 / a and a decrease in σ2 / b, the secondary battery's capacity retention at 6,000 1C / 1C cycles at 25°C, storage capacity recovery at 60°C, and energy efficiency at 1C / 1C at 25°C all tended to initially increase and then decrease. This is primarily because, as (σ1 / σ2) / CB increases, the designed CB value decreases, and the sufficient excess designed amount of positive electrode material allows for more active lithium to replenish the secondary battery's operation. This is beneficial for improving cycle and storage life. However, as (σ1 / σ2) / CB increases, the positive electrode surface density σ1 increases or the negative electrode surface density σ2 decreases, resulting in a deterioration in positive electrode kinetics or an improvement in negative electrode kinetics, resulting in inconsistent lithium desorption rates between the positive and negative electrode sheets. Therefore, when (σ1 / σ2) / CB increases to a certain extent, the cycle life, storage life and energy efficiency of the secondary battery begin to decline.

[0054] In Examples 8 to 14, when the design CB values ​​of the core, σ1, σ2, and (σ1 / σ2) / CB, were unchanged, the capacity retention rate at 6,000 1C / 1C cycles at 25°C, the storage capacity recovery rate at 60°C, and the energy efficiency at 1C / 1C at 25°C all tended to initially increase and then decrease with an increase in a and b and a decrease in σ1 / a and σ2 / b. This is primarily because an increase in a and b increases the porosity of the positive and negative electrode sheets, improving the kinetics of the positive and negative electrode sheets of the secondary battery, which is beneficial for improving cycle life, storage life, and energy efficiency. However, as the porosity of the positive and negative electrode sheets increases, the lithium desorption kinetics of the positive and negative electrode sheets become inconsistent, increasing the likelihood of lithium deposition during secondary battery operation. Therefore, when a and b increase to a certain extent, the cycle life, storage life, and energy efficiency of the secondary battery begin to decline.

[0055] In the above embodiments, the description of each embodiment has its own focus, and the parts of the embodiments that are not described in detail can refer to the relevant descriptions of other embodiments. The above has provided a detailed description of the secondary battery and electrical device according to the embodiments of the present application. The specific examples are intended to illustrate the principles and implementation of the present invention, i.e., are used only to facilitate understanding of the method and core idea of ​​the present invention. At the same time, those skilled in the art may change the specific embodiments and application scope based on the concept of the present application. In summary, it should be understood that the above description is not a limitation on the present application.

Claims

1. a positive electrode sheet including a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector, the positive electrode active material of the positive electrode active material layer including lithium iron phosphate; a negative electrode sheet including a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector; Including, 0.5≦(σ １ / σ ２ ) / CB≦9.4 and 0.8≦CB<1.1, where σ １ is the surface density of the positive electrode active material layer, and its unit is g / m ２ and σ ２ is the surface density of the negative electrode active material layer, and its unit is g / m ２ and CB is the ratio of the capacity per unit area of ​​the negative electrode sheet to the capacity per unit area of ​​the positive electrode sheet, A secondary battery characterized by:

2. 1.0≦(σ １ / σ ２ ) / CB≦6.0; 2. The secondary battery according to claim 1 .

3. 2.0≦(σ １ / σ ２ ) / CB≦3.0; 2. The secondary battery according to claim 1 .

4. 140≦σ １ / CB≦750; 2. The secondary battery according to claim 1 .

5. 75≦σ ２ / CB≦360; 2. The secondary battery according to claim 1 .

6. 5≦σ １ / a≦20 is satisfied, Here, a is the porosity of the positive electrode active material layer, and the unit is %.

2. The secondary battery according to claim 1 .

7. 2≦σ ２ / b≦9 is satisfied, Here, b is the porosity of the negative electrode active material layer, and the unit is %.

2. The secondary battery according to claim 1 .

8. The areal density σ of the positive electrode active material layer １ is 150≦σ １ Satisfying ≦600, 2. The secondary battery according to claim 1 .

9. The surface density σ of the negative electrode active material layer ２ is 80≦σ ２ Satisfying ≦290, 2. The secondary battery according to claim 1 .

10. The areal density σ of the positive electrode active material layer １ is 300≦σ １ ≦600, or the areal density σ of the negative electrode active material layer is ２ is 110≦σ ２ Satisfying ≦290, 2. The secondary battery according to claim 1 .

11. 28%≦a≦32%; 7. The secondary battery according to claim 6.

12. 30%≦b≦38%; 8. The secondary battery according to claim 7.

13. The negative electrode active material of the negative electrode sheet includes one or more of artificial graphite, natural graphite, amorphous carbon, carbon nanotubes, and mesocarbon microspheres; 2. The secondary battery according to claim 1 .

14. The secondary battery according to any one of claims 1 to 13, An electrical device characterized by:

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