Secondary battery and electric device

Abstract

The invention provides a secondary battery and a power utilization device. The electrode comprises an electrode assembly and an electrolyte, the electrode assembly comprises a positive pole piece and a negative pole piece; the size of the long side of the electrode assembly is 400 mm to 620 mm; the content of dimethyl carbonate in the electrolyte is 14 wt%-25 wt%. The secondary battery provided by the invention has good energy density, cycle performance and low-temperature performance.

Description

Technical Field

[0001] This application relates to the technical field of secondary batteries, and particularly to secondary batteries and electrical devices. Background Art

[0002] In recent years, with the development of lithium-ion secondary battery technology, lithium-ion secondary batteries have been widely used in energy storage power systems such as hydro, thermal, wind, and solar power stations, as well as in many fields such as power tools, electric bicycles, electric motorcycles, electric vehicles, and aerospace. With the popularization of the application of lithium-ion secondary batteries, users have put forward higher requirements for the cost performance and performance of the batteries. Therefore, preparing lithium-ion secondary batteries with high energy density, excellent low-temperature performance, and long cycle life has become a new challenge in the industry. Summary of the Invention

[0003] In view of the above problems, this application provides a secondary battery and an electrical device with high energy density, excellent low-temperature performance, and long cycle life.

[0004] In a first aspect, some embodiments of this application provide a secondary battery, which includes an electrode assembly and an electrolyte; the electrode assembly includes a positive electrode sheet and a negative electrode sheet; the long side dimension of the electrode assembly is 400 mm - 620 mm; the content of dimethyl carbonate in the electrolyte is 14 wt% - 25 wt%.

[0005] Increasing the long side dimension of the electrode assembly is beneficial to improving the energy density of the battery, but it will also lead to an increase in the transmission paths of ions and electrons, an increase in internal resistance, and deterioration of the cycle performance of the battery. Dimethyl carbonate has a low viscosity, which can improve the fluidity of the electrolyte and the wettability of the electrode sheet, thereby improving the cycle performance of the battery. However, dimethyl carbonate has a relatively high crystallization temperature and is prone to crystallization at low temperatures, which is not conducive to the performance of the battery at low temperatures. The above secondary battery uses an electrode assembly with a specific long side dimension, which is beneficial to balancing the energy density and cycle performance. By controlling the content of dimethyl carbonate in the electrolyte within the defined range, it is beneficial to reduce the viscosity of the electrolyte at normal and low temperatures, improve the conductivity and wetting ability of the electrolyte, thereby improving the cycle performance and low-temperature performance of the battery, and reducing the adverse impact on the energy density of the battery. Therefore, it has high energy density, excellent low-temperature performance, and long cycle life.

[0006] In some embodiments, the content of dimethyl carbonate in the electrolyte is 15 wt% - 20 wt%.

[0007] Controlling the content of dimethyl carbonate in the electrolyte within the above range is beneficial to reducing the viscosity of the electrolyte at normal and low temperatures, improving the conductivity and wetting ability of the electrolyte, thereby improving the cycle performance and low-temperature performance of the battery.

[0008] In some embodiments, the electrolyte further includes ethyl methyl carbonate; the total content of dimethyl carbonate and ethyl methyl carbonate in the electrolyte is 43 wt% - 71 wt%, optionally 50 wt% - 65 wt%.

[0009] Ethyl methyl carbonate has the characteristic of low melting point. Adding ethyl methyl carbonate to the electrolyte can significantly reduce the low-temperature viscosity of the electrolyte, thereby improving the ion migration rate and conductivity of the electrolyte at low temperature and improving the low-temperature performance of the battery. However, the viscosity of ethyl methyl carbonate is slightly higher than that of dimethyl carbonate. Adding too much will affect the normal-temperature viscosity of the electrolyte and further affect the wetting effect of the electrolyte on the electrode sheet, which is not conducive to the cycle performance. Controlling the content of dimethyl carbonate and ethyl methyl carbonate in the electrolyte within the above range is beneficial to reducing the viscosity of the electrolyte at normal temperature and low temperature, improving the conductivity and wetting ability of the electrolyte, and thus improving the cycle performance and low-temperature performance of the battery.

[0010] In some embodiments, in the electrolyte, the mass ratio of dimethyl carbonate to ethyl methyl carbonate is 0.27 - 0.56:1.

[0011] Dimethyl carbonate has low viscosity and high melting point, which is beneficial to reducing the normal-temperature viscosity of the electrolyte but not conducive to the low-temperature viscosity. While ethyl methyl carbonate has low melting point and slightly higher viscosity, which is beneficial to reducing the low-temperature viscosity of the electrolyte. Controlling the mass ratio of dimethyl carbonate to ethyl methyl carbonate within the above range is beneficial to reducing the viscosity of the electrolyte at normal temperature and low temperature, improving the conductivity and wetting ability of the electrolyte, and thus improving the cycle performance and low-temperature performance of the battery.

[0012] In some embodiments, the ratio of the long side dimension to the short side dimension of the electrode assembly is 3.3 - 6:1.

[0013] Increasing the ratio of the long side dimension to the short side dimension of the electrode assembly can improve the energy density and have better cost performance. However, increasing the ratio of the long side dimension to the short side dimension generally means an increase in the electrode thickness, resulting in poor wettability of the electrode sheet and an increase in the internal resistance of the battery, thereby deteriorating the cycle performance of the battery. Controlling the long side dimension and the short side dimension of the electrode assembly within the above range is beneficial to taking into account both the energy density and the cycle life.

[0014] In some embodiments, the ratio of the long side dimension to the thickness dimension of the electrode assembly is 10 - 40:1.

[0015] Increasing the ratio of the long side dimension to the thickness dimension of the electrode assembly is beneficial to improving the energy density of the battery, but it will also lead to an increase in the transmission paths of ions and electrons, an increase in the internal resistance, and deterioration of the cycle performance of the battery. Controlling the ratio of the long side dimension to the thickness dimension of the electrode assembly within the above range is beneficial to taking into account both the energy density and the cycle life.

[0016] In some embodiments, the short side dimension of the electrode assembly is 80 mm - 187 mm.

[0017] Reducing the short side dimension of the electrode assembly is beneficial for reducing the ion and electron transport paths, lowering the internal resistance of the battery, and improving the wettability of the electrolyte, but it is not conducive to the energy density. Controlling the short side dimension of the electrode assembly within the above range is beneficial for balancing the energy density, low-temperature performance, and cycle life.

[0018] In some embodiments, the thickness dimension of the electrode assembly is 12 mm - 62 mm.

[0019] Increasing the thickness dimension of the electrode assembly can improve the structural stability and energy density of the electrode assembly, but it will also increase the internal resistance and is not conducive to the cycle life. Controlling the thickness dimension of the electrode assembly within the above range is beneficial for balancing the energy density and cycle life.

[0020] In some embodiments, the capacity of the secondary battery is 130 Ah - 220 Ah.

[0021] Increasing the capacity of the battery is beneficial for improving the energy density and endurance time, but at the same time, the internal electrochemical reaction path becomes longer and the ion transport distance increases, which will lead to a gradual increase in the internal resistance of the battery and deterioration of the cycle performance of the battery. Controlling the capacity of the secondary battery within the above range is beneficial for balancing the energy density and cycle life.

[0022] In some embodiments, the porosity of the active layer of the positive electrode plate is 21% - 35%; it can be optionally 23% - 32%.

[0023] Controlling the porosity of the active layer of the positive electrode plate within the above range, the positive electrode plate simultaneously has a high tap density, good wetting effect, and appropriate ion diffusion rate, and can balance the energy density, cycle life, and low-temperature performance of the battery.

[0024] In some embodiments, the porosity of the active layer of the negative electrode plate is 21% - 35%; it can be optionally 23% - 32%.

[0025] Controlling the porosity of the active layer of the negative electrode plate within the above range, the negative electrode plate simultaneously has a high tap density, good wetting effect, and appropriate ion diffusion rate, and can balance the energy density, cycle life, and low-temperature performance of the battery.

[0026] In some embodiments, the tap density of the positive electrode plate is 2.25 g / cm 3 - 2.65 g / cm 3 ; it can be optionally 2.3 g / cm 3 - 2.45 g / cm 3 .

[0027] Increasing the compaction density of the positive electrode sheet is beneficial to improving the energy density. However, during the charge and discharge process of the electrode sheet, greater stress is generated, accelerating the degradation of the material structure, deteriorating the cycle life, and at the same time, the porosity of the active layer of the electrode sheet decreases, and the diffusion rate of ions becomes slower at low temperatures, resulting in the deterioration of the low-temperature performance of the battery. Controlling the compaction density of the positive electrode sheet within the above range is beneficial to taking into account the energy density, cycle performance, and low-temperature performance.

[0028] In some embodiments, the compaction density of the negative electrode sheet is 1.3 g / cm 3 -1.55 g / cm 3 ; optionally 1.4 g / cm 3 -1.5 g / cm 3 .

[0029] Controlling the compaction density of the negative electrode sheet within the above range is beneficial to taking into account the energy density, cycle performance, and low-temperature performance.

[0030] In some embodiments, the electrode assembly includes a stacked electrode assembly and / or a wound electrode assembly; optionally, the electrode assembly includes a stacked electrode assembly.

[0031] The wound electrode assembly has obvious advantages in terms of production efficiency, process maturity, cost, and consistency, while the stacked electrode assembly has significant advantages in terms of energy density, cycle life, and low-temperature performance. The corresponding electrode assembly can be adopted according to actual needs.

[0032] In some embodiments, the single-sided coating weight of the positive electrode sheet is 0.33 g / 1540.25 mm 2 -0.43 g / 1540.25 mm 2 ; optionally 0.36 g / 1540.25 mm 2 -0.4 g / 1540.25 mm 2 .

[0033] Increasing the coating weight of the electrode sheet is beneficial to increasing the compaction density and improving the energy density. However, it will reduce the porosity of the active layer of the electrode sheet, which is not conducive to the infiltration of the electrolyte. At the same time, increasing the coating weight will increase the lithium-ion transmission path in the thickness direction of the electrode sheet, resulting in an increase in the internal resistance of the battery cell and the deterioration of the battery cycle life; controlling the single-sided coating weight of the positive electrode sheet within the above range is beneficial to taking into account the energy density and cycle life.

[0034] In some embodiments, the single-sided coating weight of the negative electrode sheet is 0.15 g / 1540.25 mm 2 -0.21 g / 1540.25 mm 2 , optionally 0.17 g / 1540.25 mm 2 -0.20 g / 1540.25 mm2 。

[0035] Controlling the single-sided coating weight of the negative electrode sheet within the above range is beneficial to balance the energy density and cycle life.

[0036] In some embodiments, the active material of the positive electrode sheet includes lithium iron phosphate.

[0037] Lithium iron phosphate has good thermal stability, long cycle life, low cost, and good compatibility with the electrode assembly with a defined long side dimension. Using them in combination can balance the energy density, cycle performance, and low-temperature performance of the battery.

[0038] In a second aspect, some embodiments of the present application provide an electrical device including the secondary battery described in the first aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] To better describe and illustrate the embodiments or examples provided in the present application, one or more drawings can be referred to. Additional details or examples used to describe the drawings should not be considered as limiting the scope of any of the disclosed applications, the currently described embodiments or examples, and the currently understood best mode of these applications. Moreover, in all the drawings, the same reference numerals are used to represent the same components. In the drawings:

[0040] Figure 1 is a schematic diagram of a secondary battery according to an embodiment of the present application.

[0041] Figure 2 is Figure 1 an exploded view of the secondary battery according to an embodiment of the present application shown.

[0042] Figure 3 is a schematic diagram of an electrical device using the secondary battery according to an embodiment of the present application as a power source.

[0043] Description of the reference numerals in the drawings:

[0044] 1. Secondary battery; 11. Housing; 12. Electrode assembly; 13. Cover plate; 2. Electrical device. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0045] Hereinafter, embodiments of the secondary battery and the electrical device of the present application are specifically disclosed in detail with appropriate reference to the drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there may be cases where detailed descriptions of well-known matters and repeated descriptions of actually identical structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter recited in the claims.

[0046] The "ranges" disclosed in this application are defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, and the selected lower and upper limits define the boundaries of a particular range. The ranges defined in this way can include or exclude the end values, and can be combined arbitrarily, that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60 - 120 and 80 - 110 are listed for a specific parameter, ranges of 60 - 110 and 80 - 120 are also contemplated. In addition, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4, and 5 are listed, then the following ranges are all contemplated: 1 - 3, 1 - 4, 1 - 5, 2 - 3, 2 - 4, and 2 - 5. In this application, unless otherwise specified, the numerical range "a - b" represents an abbreviated representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range "0 - 5" means that all real numbers between "0 - 5" are fully listed herein, and "0 - 5" is only an abbreviated representation of these numerical combinations. Additionally, when stating that a certain parameter is an integer ≥2, it is equivalent to disclosing that the parameter is, for example, the integers 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0047] If there is no special instruction, all embodiments and optional embodiments of this application can be combined with each other to form a new technical solution.

[0048] If there is no special instruction, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0049] If there is no special instruction, all steps of this application can be carried out in sequence or randomly, and preferably in sequence. For example, a method includes steps (a) and (b), which means that the method can include steps (a) and (b) carried out in sequence, or can also include steps (b) and (a) carried out in sequence. For example, it is mentioned that the method may further include step (c), which means that step (c) can be added to the method in any order. For example, the method can include steps (a), (b), and (c), or can also include steps (a), (c), and (b), or can also include steps (c), (a), and (b), etc.

[0050] If there is no special instruction, the "including" and "comprising" mentioned in this application mean open-ended or can also be closed-ended. For example, "including" and "comprising" can mean that other components not listed can also be included or comprised, or can also mean only including or comprising the listed components.

[0051] If not specifically stated, in this application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0052] In this application, unless otherwise specified, A (such as B) means that B is a non-limiting example of A, and it can be understood that A is not limited to B.

[0053] In the present application, "plurality", "multiple" and the like, unless otherwise specified, refer to a number greater than or equal to 2. For example, "one or more" means one or greater than or equal to two.

[0054] In the present application, in "the first aspect", "the second aspect", "the third aspect", "the fourth aspect", "the fifth aspect", etc., the terms "first", "second", "third", "fourth", etc. are used only for descriptive purposes and cannot be understood as indicating or implying relative importance or quantity, nor can they be understood as implicitly indicating the importance or quantity of the indicated technical features. Moreover, "first", "second", "third", "fourth", etc. only serve the purpose of non-exhaustive enumeration and description, and it should be understood that they do not constitute a closed limitation on quantity.

[0055] Changdao batteries have significant advantages in system grouping, which can effectively improve the energy density of the entire product. At the same time, by adopting thick coating and high-pressure sealing design, the energy density of the product can be further improved, providing customers with more cost-effective products. However, as the ratio of the long side dimension to the short side dimension of the Changdao battery's electrode assembly increases, the electron transmission path becomes longer accordingly, resulting in an increase in ohmic impedance, which not only reduces the energy efficiency of the battery, but also affects the full utilization of the capacity. In addition, thick coating and high-pressure sealing design will also have a negative impact on the wettability of the electrolyte in the pole piece, hindering the transmission of lithium ions and thus deteriorating battery performance.

[0056] In order to improve the above shortcomings, the present application, on the one hand, reduces the influence of ohmic impedance on electrical performance by reasonably controlling the long side size of the blade electrode assembly; on the other hand, it adopts a low-viscosity, high-conductivity electrolyte to improve the insufficient wetting problem caused by thick coating and high pressure density. At the same time, in order to ensure that the battery cell has excellent low-temperature performance and meet the needs of users in low-temperature areas, the present application optimizes the component content of the electrolyte; through the combination of the above approaches, high energy density, excellent low-temperature performance and long cycle life are achieved.

[0057] Based on this, the present application provides a secondary battery and an electrical device having good energy density, cycle performance and low temperature performance.

[0058] In a first aspect, some embodiments of the present application provide a secondary battery, comprising an electrode assembly and an electrolyte; the electrode assembly includes a positive electrode sheet and a negative electrode sheet; the long side dimension of the electrode assembly is 400 mm - 620 mm; the content of dimethyl carbonate in the electrolyte is 14 wt% - 25 wt%.

[0059] Increasing the long side dimension of the electrode assembly is beneficial to improving the energy density of the battery, but it will also lead to an increase in the transmission paths of ions and electrons, an increase in internal resistance, and deterioration of the battery's cycling performance. Dimethyl carbonate has a low viscosity, which can improve the fluidity of the electrolyte and the wettability of the electrode sheets, thereby improving the cycling performance of the battery. However, dimethyl carbonate has a relatively high crystallization temperature and is prone to crystallization at low temperatures, which is not conducive to the performance of the battery at low temperatures. For the above-mentioned secondary battery, using an electrode assembly with a specific long side dimension is beneficial to balancing the energy density and cycling performance. By controlling the content of dimethyl carbonate in the electrolyte within the defined range, it is beneficial to reduce the viscosity of the electrolyte at normal and low temperatures, improve the conductivity and wetting ability of the electrolyte, thereby improving the cycling performance and low-temperature performance of the battery, and reducing the adverse effects on the energy density of the battery. Therefore, it has a high energy density, excellent low-temperature performance, and a long cycle life.

[0060] In a specific example, the long side dimension of the electrode assembly can be selected from 480 mm - 620 mm, specifically, it can be 400 mm, 420 mm, 440 mm, 460 mm, 480 mm, 500 mm, 520 mm, 540 mm, 560 mm, 580 mm, 600 mm, 620 mm, etc.

[0061] In some embodiments, the content of dimethyl carbonate in the electrolyte is 15 wt% - 20 wt%.

[0062] Controlling the content of dimethyl carbonate in the electrolyte within the above range is beneficial to reducing the viscosity of the electrolyte at normal and low temperatures, improving the conductivity and wetting ability of the electrolyte, thereby improving the cycling performance and low-temperature performance of the battery.

[0063] In a specific example, the content of dimethyl carbonate in the electrolyte is 14 wt%, 15 wt%, 16 wt%, 17 wt%, 18 wt%, 19 wt%, 20 wt%, 21 wt%, 22 wt%, 23 wt%, 24 wt%, 25 wt%, etc.

[0064] In some embodiments, the electrolyte further includes ethyl methyl carbonate; the total content of dimethyl carbonate and ethyl methyl carbonate in the electrolyte is 43 wt% - 71 wt%, optionally 50 wt% - 65 wt%, and specifically can be 43 wt%, 45 wt%, 47 wt%, 49 wt%, 50 wt%, 55 wt%, 60 wt%, 65 wt%, 67 wt%, 69 wt%, 71 wt%, etc.

[0065] Ethyl methyl carbonate has the characteristic of low melting point. Adding ethyl methyl carbonate to the electrolyte can significantly reduce the low-temperature viscosity of the electrolyte, thereby improving the ion mobility and conductivity of the electrolyte at low temperature and improving the low-temperature performance of the battery. However, the viscosity of ethyl methyl carbonate is slightly higher than that of dimethyl carbonate. Adding too much will affect the normal-temperature viscosity of the electrolyte and further affect the wetting effect of the electrolyte on the electrode sheet, which is not conducive to the cycle performance. Controlling the content of dimethyl carbonate and ethyl methyl carbonate in the electrolyte within the above range is beneficial to reducing the viscosity of the electrolyte at normal temperature and low temperature, improving the conductivity and wetting ability of the electrolyte, and thus improving the cycle performance and low-temperature performance of the battery.

[0066] In some embodiments, in the electrolyte, the mass ratio of dimethyl carbonate to ethyl methyl carbonate is 0.27 - 0.56:1, such as 0.27:1, 0.28:1, 0.3:1, 0.32:1, 0.34:1, 0.36:1, 0.4:1, 0.42:1, 0.44:1, 0.46:1, 0.48:1, 0.5:1, 0.52:1, 0.54:1, 0.56:1, etc.

[0067] Dimethyl carbonate has low viscosity and high melting point, which is beneficial to reducing the normal-temperature viscosity of the electrolyte but not conducive to the low-temperature viscosity. While ethyl methyl carbonate has low melting point and slightly higher viscosity, which is beneficial to reducing the low-temperature viscosity of the electrolyte. Controlling the mass ratio of dimethyl carbonate to ethyl methyl carbonate within the above range is beneficial to reducing the viscosity of the electrolyte at normal temperature and low temperature, improving the conductivity and wetting ability of the electrolyte, and thus improving the cycle performance and low-temperature performance of the battery.

[0068] In some embodiments, the ratio of the long side dimension to the short side dimension of the electrode assembly is 3.3 - 6:1, such as 3.3:1, 3.4:1, 3.5:1, 3.6:1, 3.7:1, 3.8:1, 3.9:1, 4:1, 4.1:1, 4.2:1, 4.3:1, 4.4:1, 4.5:1, 4.6:1, 4.7:1, 4.8:1, 4.9:1, 5:1, 5.1:1, 5.2:1, 5.3:1, 5.4:1, 5.5:1, 5.6:1, 5.7:1, 5.8:1, 5.9:1, 6:1, etc.

[0069] Increasing the ratio of the long side dimension to the short side dimension of the electrode assembly can improve the energy density and have a better cost performance. However, increasing the ratio of the long side dimension to the short side dimension generally means an increase in the electrode thickness, resulting in poor wettability of the electrode sheet and an increase in the internal resistance of the battery, thus deteriorating the cycle performance of the battery. Controlling the long side dimension and the short side dimension of the electrode assembly within the above ranges is beneficial to balancing the energy density and the cycle life.

[0070] In some embodiments, the short side dimension of the electrode assembly is 80 mm - 187 mm, such as 80 mm, 90 mm, 100 mm, 110 mm, 120 mm, 130 mm, 140 mm, 150 mm, 160 mm, 170 mm, 180 mm, 187 mm, etc.

[0071] Reducing the short side dimension of the electrode assembly is beneficial to reducing the ion and electron transmission paths, lowering the internal resistance of the battery, and improving the wettability of the electrolyte, but it is not conducive to the energy density. Controlling the short side dimension of the electrode assembly within the above ranges is beneficial to balancing the energy density, low-temperature performance, and cycle life.

[0072] In some embodiments, the ratio of the long side dimension to the thickness dimension of the electrode assembly is 10 - 40:1, such as 10:1, 15:1, 20:1, 25:1, 30:1, 35:1, 40:1, etc.

[0073] Increasing the ratio of the long side dimension to the thickness dimension of the electrode assembly is beneficial to improving the energy density of the battery, but it will also lead to an increase in the ion and electron transmission paths, an increase in the internal resistance, and deterioration of the cycle performance of the battery. Controlling the ratio of the long side dimension to the thickness dimension of the electrode assembly within the above ranges is beneficial to balancing the energy density and the cycle life.

[0074] In some embodiments, the thickness dimension of the electrode assembly is 12 mm - 62 mm, such as 12 mm, 14 mm, 16 mm, 18 mm, 20 mm, 25 mm, 30 mm, 35 mm, 40 mm, 45 mm, 50 mm, 55 mm, 60 mm, 62 mm, etc.

[0075] Increasing the thickness dimension of the electrode assembly can improve the structural stability and energy density of the electrode assembly, but it will also increase the internal resistance and is not conducive to the cycle life. Controlling the thickness dimension of the electrode assembly within the above ranges is beneficial to balancing the energy density and the cycle life.

[0076] In some embodiments, the capacity of the secondary battery is 130 Ah - 220 Ah, such as 130 Ah, 140 Ah, 150 Ah, 160 Ah, 170 Ah, 180 Ah, 190 Ah, 200 Ah, 210 Ah, 220 Ah, etc.

[0077] Increasing the capacity of the battery is beneficial to improving the energy density and endurance time. However, at the same time, the internal electrochemical reaction path becomes longer and the ion transport distance increases, which will lead to a gradual increase in the internal resistance of the battery and deteriorate the cycle performance of the battery. Controlling the capacity of the secondary battery within the above range is beneficial to taking into account both the energy density and the cycle life.

[0078] In some embodiments, the porosity of the active layer of the positive electrode plate is 21%-35%; it can be optionally 23%-32%, such as 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, etc.

[0079] Controlling the porosity of the active layer of the positive electrode plate within the above range, the positive electrode plate simultaneously has a high tap density, good wetting effect and appropriate ion diffusion rate, and can take into account the energy density, cycle life and low-temperature performance of the battery.

[0080] In some embodiments, the porosity of the active layer of the negative electrode plate is 21%-35%; it can be optionally 23%-32%, such as for example 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, etc.

[0081] Controlling the porosity of the active layer of the negative electrode plate within the above range, the negative electrode plate simultaneously has a high tap density, good wetting effect and appropriate ion diffusion rate, and can take into account the energy density, cycle life and low-temperature performance of the battery.

[0082] In some embodiments, the tap density of the positive electrode plate is 2.25 g / cm 3 -2.65 g / cm 3 ; it can be optionally 2.3 g / cm 3 -2.45 g / cm 3 , such as 2.25 g / cm 3 , 2.3 g / cm 3 , 2.35 g / cm 3 , 2.4 g / cm 3 , 2.45 g / cm 3 , 2.5 g / cm 3 , 2.55 g / cm 3 , 2.6 g / cm 3 , 2.65 g / cm 3 etc.

[0083] Increasing the compaction density of the positive electrode sheet is beneficial to improving the energy density. However, during the charge and discharge process of the electrode sheet, greater stress is generated, accelerating the degradation of the material structure, deteriorating the cycle life, and at the same time, the porosity of the active layer of the electrode sheet decreases, and the diffusion rate of ions at low temperatures becomes slower, resulting in the deterioration of the low-temperature performance of the battery. Controlling the compaction density of the positive electrode sheet within the above range is beneficial to taking into account the energy density, cycle performance, and low-temperature performance.

[0084] In some embodiments, the compaction density of the negative electrode sheet is 1.3 g / cm 3 -1.55 g / cm 3 ; optionally 1.4 g / cm 3 -1.5 g / cm 3 , for example 1.3 g / cm 3 , 1.35 g / cm 3 , 1.4 g / cm 3 , 1.45 g / cm 3 , 1.5 g / cm 3 , 1.55 g / cm 3 and so on.

[0085] Controlling the compaction density of the negative electrode sheet within the above range is beneficial to taking into account the energy density, cycle performance, and low-temperature performance.

[0086] In some embodiments, the electrode assembly includes a stacked electrode assembly and / or a wound electrode assembly; optionally, the electrode assembly includes a stacked electrode assembly.

[0087] The wound electrode assembly has obvious advantages in terms of production efficiency, process maturity, cost, and consistency, while the stacked electrode assembly has significant advantages in terms of energy density, cycle life, and low-temperature performance. The corresponding electrode assembly can be adopted according to actual needs.

[0088] In some embodiments, the single-sided coating weight of the positive electrode sheet is 0.33 g / 1540.25 mm 2 -0.43 g / 1540.25 mm 2 ; optionally 0.36 g / 1540.25 mm 2 -0.4 g / 1540.25 mm 2 , for example 0.33 g / 1540.25 mm 2 , 0.34 g / 1540.25 mm 2 , 0.35 g / 1540.25 mm 2 , 0.36 g / 1540.25 mm 2 , 0.37 g / 1540.25 mm 2 , 0.38 g / 1540.25 mm 2, 0.39 g / 1540.25 mm 2 , 0.40 g / 1540.25 mm 2 , 0.41 g / 1540.25 mm 2 , 0.42 g / 1540.25 mm 2 , 0.43 g / 1540.25 mm 2 etc.

[0089] Increasing the coating weight of the electrode sheet is beneficial to improving the compaction density and energy density, but it will reduce the porosity of the active layer of the electrode sheet, which is not conducive to the infiltration of the electrolyte. At the same time, increasing the coating weight will increase the lithium-ion transmission path in the thickness direction of the electrode sheet, resulting in an increase in the internal resistance of the battery cell and deterioration of the battery cycle life; controlling the single-sided coating weight of the positive electrode sheet within the above range is beneficial to balancing the energy density and cycle life.

[0090] In some embodiments, the single-sided coating weight of the negative electrode sheet is 0.15 g / 1540.25 mm 2 -0.21 g / 1540.25 mm 2 , and it can be optionally 0.17 g / 1540.25 mm 2 -0.20 g / 1540.25 mm 2 , for example, 0.15 g / 1540.25 mm 2 , 0.16 g / 1540.25 mm 2 , 0.17 g / 1540.25 mm 2 , 0.18 g / 1540.25 mm 2 , 0.19 g / 1540.25 mm 2 , 0.20 g / 1540.25 mm 2 , 0.21 g / 1540.25 mm 2 etc.

[0091] Controlling the single-sided coating weight of the negative electrode sheet within the above range is beneficial to balancing the energy density and cycle life.

[0092] In some embodiments, the active material of the positive electrode sheet includes lithium iron phosphate.

[0093] Lithium iron phosphate has good thermal stability, long cycle life, low cost, and good compatibility with the electrode assembly with a defined long side dimension. Using them together can balance the energy density, cycle performance, and low-temperature performance of the battery.

[0094] In a second aspect, some embodiments of the present application provide an electrical device including the secondary battery described in the first aspect.

[0095] Generally, a secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. During the charge and discharge process of the battery, active ions are inserted into and extracted from between the positive electrode sheet and the negative electrode sheet. The electrolyte functions to conduct ions between the positive electrode sheet and the negative electrode sheet. The separator is disposed between the positive electrode sheet and the negative electrode sheet, mainly functioning to prevent short - circuit between the positive and negative electrodes, and at the same time allowing ions to pass through.

[0096] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer provided on at least one surface of the positive electrode current collector, and the positive electrode film layer includes a positive electrode active material.

[0097] As a non - restrictive example, the positive electrode current collector has two surfaces opposite to each other in its own thickness direction, and the positive electrode active material layer is provided on either one or both of the two opposite surfaces of the positive electrode current collector.

[0098] In some embodiments, the positive electrode current collector can be a metal foil or a composite current collector. For example, as the metal foil, aluminum foil can be used. The composite current collector can include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be obtained by forming a metal material on the polymer material substrate. In the positive electrode current collector, non - restrictive examples of the metal material can include one or more of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc. In the positive electrode current collector, non - restrictive examples of the polymer material substrate can include one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0099] In some of these embodiments, the positive electrode active material may be a positive electrode active material for batteries known in the art. As a non-limiting example, the positive electrode active material may include one or more of the following materials: lithium-containing phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, the present application is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials may also be used. These positive electrode active materials may be used alone or in combination of two or more. Among them, examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and their modified compounds, etc. Non-limiting examples of lithium-containing phosphates with an olivine structure may include, but are not limited to, lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite material of lithium manganese iron phosphate and carbon, etc. Non-limiting examples of lithium cobalt oxide may include LiCoO2; non-limiting examples of lithium nickel oxide may include LiNiO2; non-limiting examples of lithium manganese oxide may include LiMnO2, LiMn2O4, etc.; non-limiting examples of lithium nickel cobalt manganese oxide may include LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (which may also be abbreviated as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (which may also be abbreviated as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2 (which may also be abbreviated as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2 (which may also be abbreviated as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2 (which may also be abbreviated as NCM 811 ), etc. Non-limiting examples of lithium nickel cobalt aluminum oxide may include LiNi 0.8 Co 0.15 Al 0.05 O2.

[0100] In some of these embodiments, the positive electrode active material layer may further optionally include a binder. As a non-limiting example, the binder may include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0101] In some of these embodiments, the positive electrode active material layer may further optionally include a conductive agent. As a non-limiting example, the conductive agent may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0102] In some of these embodiments, the positive electrode plate can be prepared by the following method: dispersing the components for preparing the positive electrode plate, such as the positive electrode active material, the conductive agent, the binder, and any other components, in a solvent to form a positive electrode slurry; coating the positive electrode slurry on at least one surface of the positive electrode current collector, and after processes such as drying and cold pressing, the positive electrode plate can be obtained. The type of the solvent can be selected from, but not limited to, any one of the foregoing embodiments, such as N-methylpyrrolidone (NMP). The surface of the positive electrode current collector on which the positive electrode slurry is coated can be a single surface of the positive electrode current collector or both surfaces of the positive electrode current collector. The solid content of the positive electrode slurry can be 40 wt% - 80 wt%. The viscosity (initial viscosity) of the positive electrode slurry at room temperature can be adjusted to 5000 mPa·s - 25000 mPa·s.

[0103] The negative electrode plate 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, and the negative electrode active material layer includes a negative electrode active material.

[0104] As a non-limiting example, the negative electrode current collector has two surfaces opposite to each other in its own thickness direction, and the negative electrode active material layer is provided on either or both of the two opposite surfaces of the negative electrode current collector.

[0105] In some of these embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, as the metal foil, copper foil may be used. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector may be obtained by forming a metal material on the polymer material base layer. In the negative electrode current collector, non-limiting examples of the metal material may include one or more of copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc. In the negative electrode current collector, non-limiting examples of the polymer material base layer may include one or more of base layers such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0106] In some of these embodiments, the negative electrode active material may be a negative electrode active material for a battery known in the art. As non-limiting examples, the negative electrode active material may include one or more of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based materials may include one or more of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based materials may include one or more of elemental tin, tin oxides, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as the negative electrode active material of the battery may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0107] In some of these embodiments, in the negative electrode active material layer, the content of the negative electrode active material is 90wt%-98wt%, optionally 94wt%-98wt%, for example 90wt%, 91wt%, 92wt%, 93wt%, 94wt%, 94.5wt%, 95wt%, 95.5wt%, 96wt%, 96.5wt%, 97wt%, 97.5wt%, 98wt%, etc.

[0108] In some of these embodiments, the negative electrode active material layer may also optionally include a binder. The binder may include one or more of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0109] In some of these embodiments, in the negative electrode active material layer, the dosage of the binder is 0.5wt%-4wt%, for example 0.5wt%, 1wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%, etc.

[0110] In some of these embodiments, the negative electrode active material layer may further optionally include a conductive agent. The conductive agent may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0111] In some of these embodiments, in the negative electrode active material layer, the content of the conductive agent is 0.1 wt% - 3 wt%, such as 0.1 wt%, 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 3 wt%, etc.

[0112] In some of these embodiments, the negative electrode active material layer may further optionally include other additives, such as thickeners and dispersants.

[0113] In some of these embodiments, the negative electrode plate can be prepared in the following manner: Disperse the components for preparing the negative electrode plate described above, such as the negative electrode active material, conductive agent, binder, and any other components, in a solvent (a non-limiting example of the solvent is deionized water) to form a negative electrode slurry; or adopt stepwise mixing. For example, mix the negative electrode active material, conductive agent, cellulose derivative with a part of the solvent, then add an unsaturated carboxylate and mix, and finally add the binder and the remaining solvent to mix to form a negative electrode slurry. Then coat the negative electrode slurry on at least one surface of the negative electrode current collector, and after processes such as drying and cold pressing, the negative electrode plate can be obtained. The surface of the negative electrode current collector coated with the negative electrode slurry can be a single surface of the negative electrode current collector or two surfaces of the negative electrode current collector. The solid content of the negative electrode slurry can be 40 wt% - 70 wt%. The viscosity (initial viscosity) of the negative electrode slurry at room temperature can be adjusted to 2000 mPa·s - 13000 mPa·s.

[0114] In some of these embodiments, the drying temperature of the negative electrode plate is 70°C - 150°C, such as 70°C, 80°C, 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, etc.

[0115] The electrolyte functions to conduct ions between the positive electrode plate and the negative electrode plate. The present application does not particularly limit the type of the electrolyte, and it can be selected according to requirements. For example, the electrolyte can be liquid, gel-like, or all-solid-state.

[0116] In some embodiments, the electrolyte uses an electrolytic solution. The primary electrolytic solution includes an electrolyte salt and a solvent.

[0117] In some embodiments, the electrolyte salt may include one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorophosphate (LiPO2F2), lithium difluoro(oxalato)borate (LiDFOB), lithium bis(oxalato)borate (LiBOB), lithium difluoro bis(oxalato)phosphate (LiDFOP), and lithium tetrafluoro(oxalato)phosphate (LiTFOP).

[0118] In some embodiments, the concentration of the electrolyte salt is 0.5 mol / L - 2 mol / L, and may be optionally 0.8 mol / L - 1.5 mol / L.

[0119] In some embodiments, the solvent includes ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC).

[0120] In some embodiments, the solvent may further include at least one of an ether solvent, an ester solvent, and a sulfone solvent; as an example, the ether solvent may include at least one of ethylene glycol dimethyl ether (DME), diethylene glycol dimethyl ether (DEGDME), triethylene glycol dimethyl ether (TRGDME), tetraethylene glycol dimethyl ether (TEGDME), and 1,3-dioxolane (DOL);

[0121] As an example, the ester solvent may include at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), fluoroethylene carbonate (FEC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), γ-butyrolactone (BL), 1,3-propane sultone (1,3-PS), methyl propionate (MP), methyl butyrate (MB), ethyl acetate (EA), ethyl propionate (EP), propyl propionate (PP), and ethyl butyrate (EB);

[0122] As an example, in the electrolyte, the total content of the ether solvent, the ester solvent, and the sulfone solvent is 16 wt% - 45 wt%, such as 16 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt%, etc.

[0123] In some embodiments, the electrolyte includes dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and vinylene carbonate (VC).

[0124] In some of these embodiments, the content of the solvent in the electrolyte is 80 wt% - 88 wt%, such as 80 wt%, 81 wt%, 82 wt%, 83 wt%, 84 wt%, 85 wt%, 86 wt%, 87 wt%, 88 wt%, etc.

[0125] In some of these embodiments, in the electrolyte, the content of ethylene carbonate is 15 wt% - 40 wt%, such as 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, etc.

[0126] In some of these embodiments, in the electrolyte, the content of vinylene carbonate is 1 wt% - 5 wt%, such as 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 3 wt%, 3.5 wt%, 4 wt%, 4.5 wt%, 5 wt%.

[0127] In some of these embodiments, the electrolyte may optionally further include additives. For example, the additives may include negative electrode film - forming additives, positive electrode film - forming additives, and may also include additives that can improve certain battery performance, such as additives for improving the over - charge performance of the battery, additives for improving the high - temperature or low - temperature performance of the battery, etc.

[0128] In some embodiments, the additives in the electrolyte may include, but are not limited to, one or more of fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), trifluoromethyl ethylene carbonate (TFPC), etc.

[0129] In some of these embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of the separator, and any well - known porous - structure separator with good chemical stability and mechanical stability can be selected.

[0130] In some of these embodiments, the material of the separator may include one or more of glass fiber, non - woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single - layer film or a multi - layer composite film, without particular limitation. When the separator is a multi - layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0131] In some embodiments, the thickness of the separator is 6 μm - 40 μm, and can be optionally 12 μm - 20 μm.

[0132] In some of these embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be made into an electrode assembly by a winding process or a stacking process.

[0133] In some of these embodiments, the secondary battery may include an outer package. The outer package can be used to encapsulate the above - mentioned electrode assembly and electrolyte.

[0134] In some embodiments, the outer packaging of the secondary battery can be a hard case, such as a hard plastic case, an aluminum case, a steel case, etc. The outer packaging of the secondary battery can also be a soft package, such as a pouch soft package. The material of the soft package can be plastic. Further, non-limiting examples of the plastic can include one or more of polypropylene, polybutylene terephthalate, and polybutylene succinate, etc.

[0135] The secondary battery includes at least one battery cell. The secondary battery can include one or more battery cells.

[0136] In this application, unless otherwise specified, a "battery cell" refers to a basic unit capable of converting chemical energy and electrical energy into each other. Further, generally, it includes at least a positive electrode plate, a negative electrode plate, and an electrolyte. During the charging and discharging process of the battery, active ions are embedded and removed back and forth between the positive electrode plate and the negative electrode plate. The electrolyte plays a role in conducting active ions between the positive electrode plate and the negative electrode plate.

[0137] This application has no particular limitation on the shape of the battery cell, and it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 is a secondary battery 1 with a square structure as an example.

[0138] In some of these embodiments, with reference to Figure 2 , the outer packaging can include a housing 11 and a cover plate 13. Among them, the housing 11 can include a bottom plate and side plates connected to the bottom plate, and the bottom plate and the side plates enclose to form a receiving cavity. The housing 11 has an opening communicating with the receiving cavity, and the cover plate 13 can be covered on the opening to close the receiving cavity. The positive electrode plate, the negative electrode plate, and the separator can be formed into an electrode assembly 12 through a winding process or a stacking process. The electrode assembly 12 is encapsulated in the receiving cavity. The electrode assembly 12 is immersed in the electrolyte. The number of electrode assemblies 12 included in the secondary battery 1 can be one or more, and those skilled in the art can select according to actual needs.

[0139] The secondary battery can be a battery device or a battery pack.

[0140] The battery device includes at least one battery cell. The number of battery cells included in the battery device can be one or more, and those skilled in the art can select a suitable number according to the application and capacity of the battery device.

[0141] In the battery device, multiple battery cells can be arranged in sequence along the length direction of the battery device. Of course, they can also be arranged in any other arbitrary manner. Further, the multiple battery cells can be fixed by fasteners.

[0142] Optionally, the battery device can further include a housing having a receiving space, and multiple battery cells are received in the receiving space.

[0143] In some of these embodiments, the above battery device can also be assembled into a battery pack. The number of battery devices included in the battery pack can be one or more, and those skilled in the art can select an appropriate number according to the application and capacity of the battery pack.

[0144] The battery pack may include a battery box and a plurality of battery devices disposed in the battery box. The battery box includes an upper box body and a lower box body. The upper box body can cover the lower box body and form a closed space for accommodating the battery devices. The plurality of battery devices can be arranged in the battery box in any manner.

[0145] In addition, the secondary battery can be used as the power source of the electrical device or as the energy storage unit of the electrical device. The electrical device can include mobile devices, electric vehicles, electric trains, ships, satellites, energy storage systems, etc., but is not limited thereto. Among them, the mobile device can be, for example, a mobile phone, a laptop computer, etc.; the electric vehicle can be, for example, a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc., but is not limited thereto.

[0146] As the electrical device, the secondary battery can be selected according to its usage requirements.

[0147] Figure 3 Shown is electrical device 2 as an example. The electrical device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the high power and high energy density requirements of the electrical device for the secondary battery, a battery pack or a battery device can be adopted.

[0148] Another example of the device can be a mobile phone, a tablet computer, a laptop computer, etc. This device usually requires being thin and light, and a secondary battery can be used as the power source.

[0149] Hereinafter, some embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application and should not be construed as a limitation to the present application. For those not specified in the embodiments in terms of techniques or conditions, they shall be carried out according to the descriptions above, or according to the techniques or conditions described in the literature in this field or according to the product specifications. For the reagents or instruments not specified in terms of the manufacturer, they are all conventional products that can be obtained through commercial purchase, or can be synthesized from commercially available products in a conventional manner.

[0150] In the following embodiments, "room temperature" refers to 20°C - 30°C, and further, it can be 25°C.

[0151] Embodiment

[0152] The following describes the embodiments of the present application. The embodiments described below are exemplary and are only used to explain the present application, and should not be construed as a limitation of the present application. For those technologies or conditions not specified in the embodiments, they shall be carried out according to the technologies or conditions described in the literature in the field or according to the product specifications. For the reagents or instruments whose manufacturers are not specified, they are all conventional products that can be obtained through commercial purchase.

[0153] Example 1

[0154] Preparation of lithium-ion battery:

[0155] 1. Preparation of positive electrode sheet:

[0156] The positive electrode active material, polyvinylidene fluoride, and conductive carbon black are mixed at a weight ratio of 97:2.2:0.8 and then added to the solvent N-methylpyrrolidone, stirred evenly, and the viscosity is adjusted to form a positive electrode slurry with a solid content of 65%. The positive electrode slurry is coated on both surfaces of the positive electrode current collector aluminum foil to form a positive electrode film layer (the single-sided coating weight is 0.37 g / 1540.25 mm 2 ), and after drying and hot pressing, a positive electrode sheet is obtained with a porosity of 29% and a tap density of 2.38 g / cm 3 .

[0157] 2. Negative electrode sheet:

[0158] The negative electrode active material, conductive agent (conductive carbon black), binder (styrene-butadiene rubber (SBR)), and thickener (sodium carboxymethyl cellulose (CMC, molecular weight 150,000 g / mol, degree of substitution 0.8)) are mixed at a mass ratio of 96.4:0.4:2.2:1.0, added with deionized water and stirred, and dispersed to make a negative electrode slurry with a solid content of 53%. Then the negative electrode slurry is coated on both surfaces of the Cu foil (the single-sided coating weight is 0.183 g / 1540.25 mm 2 ), and after drying, compaction, slitting, and sheet making, a negative electrode sheet is prepared with a porosity of 28% and a tap density of 1.4 g / cm 3 .

[0159] 3. Separator:

[0160] A 7-μm polyethylene film was used as the base film. The ceramic material boehmite powder and the binder material polyvinylidene fluoride (PVDF) were added to the solvent N-methylpyrrolidone (the mass ratio of alumina:PVDF:solvent was 5:1:10) and mixed evenly to form a ceramic layer slurry. The ceramic layer slurry was coated on both sides of the base film and dried to form a ceramic layer. Polyvinylidene fluoride (PVDF) was added to the solvent N-methylpyrrolidone and mixed evenly, and then polyethylene glycol (PEG) as a pore-forming agent was added to prepare a binder layer solution, where the mass content of PVDF was 20% and the mass content of PEG was 10%. The binder layer solution was coated on the ceramic layer, pre-volatilized at 80 °C and dried at 110 °C, and then immersed in deionized water to dissolve polyethylene glycol, obtaining a separator membrane.

[0161] 4. Preparation of electrolyte:

[0162] In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), the organic solvents ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed evenly. Then lithium hexafluorophosphate was added and dissolved in the organic solvents, so that the concentration of lithium hexafluorophosphate in the electrolyte was 1.09 mol / L (accounting for 13.8% of the electrolyte mass), and vinylene carbonate (VC) was added and stirred evenly to obtain the electrolyte. The specific composition is shown in Table 1-2.

[0163] 5. Battery assembly:

[0164] The above-mentioned positive electrode sheet, separator membrane, and negative electrode sheet were stacked in sequence, with the separator membrane placed between the positive electrode sheet and the negative electrode sheet to play a role in isolation. Then an electrode assembly was formed through the lamination process. The size of the electrode assembly is shown in Table 1. The electrode assembly was placed in the outer package, dried, and the above-mentioned electrolyte was injected. After vacuum packaging, standing, forming, shaping and other processes, a lithium-ion battery was obtained.

[0165] Examples 2-18 and Comparative Examples 1-5

[0166] The difference is only that the electrode assembly size, electrolyte, positive electrode sheet parameters or negative electrode sheet parameters are different. The specific details are shown in Table 1-2, and the others are the same as in Example 1.

[0167] Test examples

[0168] 1. Determination of the porosity of the active layer of the electrode sheet:

[0169] The porosity is tested by the gas displacement method with reference to GB / T 24586-2009. After the electrode sheet / separator is immersed in DMC for cleaning and drying, it is tested using a true density meter AccuPyc II 1340. Among them, the percentage of the pore volume in the electrode sheet accounting for the total volume of the electrode sheet is the porosity of the electrode sheet, and the calculation formula is: porosity = (V - V0) / V × 100%, where V0 is the true volume, V is the apparent volume, and the unit is %.

[0170] 2. Determination of battery capacity

[0171] At 25°C, the lithium-ion battery is charged at a constant current of 0.33C to 3.65V, charged at a constant voltage of 3.65V to 0.05C, and left standing for 10 min; then discharged at a constant current of 0.33C to 2.5V, left standing for 10 min, and the discharge capacity is recorded, which is the capacity C0 of the battery, and the unit is Ah.

[0172] 3. Determination of tap density

[0173] In this application, the tap density of the electrode sheet refers to the tap density of the electrode sheet of the battery cell at 0% state of charge (SOC), and it can be detected by the following method. Disassemble the electrode sheet from the battery cell at 0% state of charge (SOC), and measure the tap density of the electrode sheet (the unit is g / cm 3 ). For example, take a single-sided coated positive electrode sheet (if it is a double-sided coated electrode sheet, the positive electrode film layer on one side can be wiped off first), punch it into small round pieces with an area of S1, weigh it, record it as M1, and measure its thickness H1. Then wipe off the positive electrode film layer of the above-mentioned weighed positive electrode sheet, weigh the weight of the positive electrode current collector, record it as M0, and measure its thickness H0. The single-sided coating weight of the positive electrode sheet = (the weight M1 of the positive electrode sheet - the weight M0 of the positive electrode current collector) / S1, the thickness of the positive electrode film layer = the thickness H1 of the positive electrode sheet - the thickness H0 of the positive electrode current collector, and the tap density of the positive electrode sheet = the single-sided coating weight of the positive electrode film layer / the thickness of the positive electrode film layer.

[0174] 4. Method for cycle performance test:

[0175] At 25°C, the lithium-ion battery is subjected to charge and discharge cycle tests on a charge and discharge instrument. First, the battery is charged at a constant power of 0.5P to 3.65V and left standing for 30 min; then the battery is discharged at a constant power of 0.5P to 2.5V and left standing for 30 min, and the discharge capacity of the first cycle is recorded. Cycle the above steps 2000 times, then the capacity retention rate after 2000 cycles at 25°C = (the discharge capacity after the 2000th cycle / the discharge capacity of the first cycle) × 100%.

[0176] 5. Method for low-temperature performance test:

[0177] At 25°C, charge the lithium-ion battery at 1 / 3C to 3.65V, then charge it at a constant voltage until the current reaches 0.05C, and then let it stand for 10 minutes. Discharge it at 1 / 3C to 2V, and record the discharge capacity as D0. Subsequently, charge the lithium-ion battery at 1 / 3C to 3.65V, charge it at a constant voltage until the current reaches 0.05C, and let it stand for 10 minutes; place the lithium-ion battery at -10°C and let it stand for 2 hours, then discharge it at 1 / 3C to 2V, and record the discharge capacity as D1; then the low-temperature capacity retention rate of the lithium-ion battery at -10°C = D1 / D0×100%.

[0178] 6. Volume energy density

[0179] At 25°C, charge the battery cell at a constant current of 0.33C until the cut-off voltage of 3.65V, and then charge it at a constant voltage of 3.65V until the current reaches 0.05C. At this time, the secondary battery is in a fully charged state. After letting the fully charged secondary battery stand for 5 minutes, discharge it at a constant current of 0.33C until the cut-off voltage of 2.5V, and record the discharge energy as Q0.

[0180] Measure the length, width, and thickness of the battery cell, and then calculate the volume of the battery cell, which is recorded as V; then the volume energy density of the battery = Q0 / V, unit: Wh / L.

[0181] Table 1 Battery parameters of examples and comparative examples

[0182]

[0183] Table 2 Battery parameters and performance of examples and comparative examples

[0184]

[0185] Among them, the coating weight refers to the single-sided coating weight of the positive electrode plate or the negative electrode plate, and the unit is g / 1540.25mm 2 ; the unit of the compaction density at 0% SOC is g / cm 3 .

[0186] As can be seen from Tables 1-2, the lithium-ion batteries of the embodiments of the present application have both high energy density, excellent low-temperature performance, and long cycle life.

[0187] Comparing Example 1, Example 18, and Comparative Examples 1-3, it can be seen that the DMC content has a great influence on the battery performance. When the DMC content in the electrolyte is not within the limited range, good cycle life and low-temperature performance cannot be obtained simultaneously.

[0188] Comparing Example 1, Example 19, and Comparative Example 4, it can be seen that the length dimension of the electrode assembly of the lithium-ion battery has a great influence on the battery performance. When the length dimension of the electrode assembly is too large, it is impossible to balance the energy density, cycle life, and low-temperature performance.

[0189] Comparing Example 1 with Examples 14-17, it can be seen that controlling the porosity of the active layer of the positive electrode plate and the porosity of the active layer of the negative electrode plate has a great influence on the performance of the battery. Within the optional range, it is more conducive to taking into account the energy density, cycle life and low-temperature performance.

[0190] It should be noted that this application is not limited to the above embodiments. The above embodiments are only examples, and embodiments having the same composition as the technical idea and achieving the same effect within the technical solution scope of this application are included in the technical scope of this application. In addition, within the scope not departing from the gist of this application, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways constructed by combining some of the constituent elements in the embodiments are also included in the scope of this application.

Claims

1. A secondary battery, characterized in that, It includes an electrode assembly and an electrolyte; the electrode assembly includes a positive electrode sheet and a negative electrode sheet; the long side dimension of the electrode assembly is 400 mm - 620 mm; the content of dimethyl carbonate in the electrolyte is 14 wt% - 25 wt%. The electrolyte further includes ethyl methyl carbonate; the total content of dimethyl carbonate and ethyl methyl carbonate in the electrolyte is 43 wt% - 71 wt%.

2. The secondary battery according to claim 1, wherein The content of dimethyl carbonate in the electrolyte is 15 wt% - 20 wt%.

3. The secondary battery according to claim 2, characterized in that, The total content of dimethyl carbonate and ethyl methyl carbonate in the electrolyte is 50 wt% - 65 wt%.

4. The secondary battery according to claim 1, wherein In the electrolyte, the mass ratio of dimethyl carbonate to ethyl methyl carbonate is 0.27 - 0.56:

1.

5. The secondary battery according to any one of claims 1 to 4, characterized in that, The ratio of the long side dimension to the short side dimension of the electrode assembly is 3.3 - 6:

1.

6. The secondary battery according to any one of claims 1 to 4, characterized in that, The ratio of the long side dimension to the thickness dimension of the electrode assembly is 10 - 40:

1.

7. The secondary battery according to any one of claims 1 to 4, characterized in that, The short side dimension of the electrode assembly is 80 mm - 187 mm.

8. The secondary battery according to any one of claims 1 to 4, characterized in that, The thickness dimension of the electrode assembly is 12 mm - 62 mm.

9. The secondary battery according to any one of claims 1 to 4, characterized in that, The capacity of the secondary battery is 130 Ah - 220 Ah.

10. The secondary battery according to any one of claims 1 to 4, characterized in that, The porosity of the active layer of the positive electrode sheet is 21% - 35%.

11. The secondary battery according to claim 10, characterized in that, The porosity of the active layer of the positive electrode sheet is 23% - 32%.

12. The secondary battery according to any one of claims 1 to 4 and 11, characterized in that, The porosity of the active layer of the negative electrode sheet is 21% - 35%.

13. The secondary battery according to claim 12, wherein, The porosity of the active layer of the negative electrode sheet is 23% - 32%.

14. The secondary battery according to any one of claims 1-4, 11, and 13, characterized in that, The tap density of the positive electrode sheet is 2.25 g / cm 3 -2.65 g / cm 3 .

15. The secondary battery according to claim 14, wherein, The compaction density of the positive electrode sheet is 2.3 g / cm 3 - 2.45 g / cm 3 .

16. The secondary battery according to any one of claims 1-4, 11, 13, and 15, characterized in that, The compaction density of the negative electrode plate is 1.3 g / cm 3 - 1.55 g / cm 3 .

17. The secondary battery according to claim 16, characterized in that, The compaction density of the negative electrode sheet is 1.4 g / cm 3 - 1.5 g / cm 3 .

18. The secondary battery according to any one of claims 1-4, 11, 13, 15, and 17, characterized in that, The electrode assembly includes a stacked electrode assembly and / or a wound electrode assembly.

19. The secondary battery according to claim 18, wherein The electrode assembly includes a stacked electrode assembly.

20. The secondary battery according to any one of claims 1-4, 11, 13, 15, 17, and 19, characterized in that, The single-sided coating weight of the positive electrode sheet is 0.33 g / 1540.25 mm 2 -0.43 g / 1540.25 mm 2 .

21. The secondary battery according to claim 20, characterized in that, The single-sided coating weight of the positive electrode sheet is 0.36 g / 1540.25 mm 2 -0.4 g / 1540.25 mm 2 .

22. The secondary battery according to any one of claims 1-4, 11, 13, 15, 17, 19 and 21, characterized in that, The single-sided coating weight of the negative electrode sheet is 0.15 g / 1540.25 mm 2 -0.21 g / 1540.25 mm 2 .

23. The secondary battery according to claim 22, characterized in that, The single-sided coating weight of the negative electrode plate is 0.17 g / 1540.25 mm 2 -0.20 g / 1540.25 mm 2 .

24. The secondary battery according to any one of claims 1-4, 11, 13, 15, 17, 19, 21 and 23, characterized in that, The active material of the positive electrode sheet includes lithium iron phosphate.

25. The secondary battery according to any one of claims 1-4, 11, 13, 15, 17, 19, 21, and 23, characterized in that, The electrolyte further includes at least one of an ether solvent, an ester solvent, and a sulfone solvent. The ether solvent includes at least one of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and 1,3 - dioxolane. The ester solvent includes at least one of ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, γ - butyrolactone, 1,3 - propane sultone, methyl propionate, methyl butyrate, ethyl acetate, ethyl propionate, propyl propionate, and ethyl butyrate. In the electrolyte, the total content of the ether solvent, the ester solvent, and the sulfone solvent is 16 wt% - 45 wt%.

26. The secondary battery according to any one of claims 1-4, 11, 13, 15, 17, 19, 21 and 23, characterized in that, The electrolyte includes dimethyl carbonate, ethyl methyl carbonate, ethylene carbonate, and vinylene carbonate. In the electrolyte, the content of ethylene carbonate is 15 wt% - 40 wt%. In the electrolyte, the content of vinylene carbonate is 1 wt% - 5 wt%.

27. An electric device, characterized in that, It includes the secondary battery according to any one of claims 1 - 26.

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