Secondary battery and electric device

Abstract

A secondary battery and an electric device. The secondary battery comprises a negative electrode sheet and an electrolyte, wherein the negative electrode sheet comprises a negative electrode current collector and a negative electrode active layer located on at least one surface of the negative electrode current collector, the negative electrode active layer comprises graphite, and the areal density of the negative electrode active layer is 0.09 mg / mm2 to 0.16 mg / mm2; the lithium ion conductivity of the electrolyte at 25ºC is 10 mS / cm to 20 mS / cm; and the electrolyte comprises a first solvent, the first solvent having a general structural formula of R1-COO-R2, where R1 and R2 are each independently selected from any one of C1-C5 alkyl and C1-C5 haloalkyl.

Description

Secondary batteries and electrical devices Technical Field

[0001] The present application relates to the technical field of secondary batteries, and in particular to a secondary battery and an electrical device. Background Art

[0002] The statements herein merely provide background information related to the present application and do not necessarily constitute prior art.

[0003] As the popularity of secondary batteries continues to increase, consumers are demanding higher performance from them, with battery cycle performance and charging speed becoming key concerns.

[0004] Summary of the Invention

[0005] In order to achieve the above objectives, the present application provides a secondary battery with good cycle performance.

[0006] The present application provides a secondary battery, comprising a negative electrode plate and an electrolyte; the negative electrode plate comprises a negative electrode current collector and a negative electrode active layer located on at least one surface of the negative electrode current collector, the negative electrode active layer comprises graphite, and the surface density of the negative electrode active layer is 0.09 mg / mm 2 ~0.16mg / mm 2 ; The lithium ion conductivity of the electrolyte at 25°C is 10mS / cm to 20mS / cm; and the electrolyte includes a first solvent, the first solvent having a general structural formula of R1-COO-R2, wherein R1 and R2 are independently selected from any one of a C1 to C5 alkyl group and a C1 to C5 halogenated alkyl group.

[0007] The present application provides a secondary battery, comprising a negative electrode plate and an electrolyte; the negative electrode plate comprises a negative electrode current collector and a negative electrode active layer located on at least one surface of the negative electrode current collector, the negative electrode active layer comprises graphite, and the surface density of the negative electrode active layer is ≤0.16 mg / mm2; the lithium ion conductivity of the electrolyte is ≤20 mS / cm.

[0008] The secondary battery can achieve both good cycle performance and good fast charging performance by appropriately matching the negative electrode plate and the electrolyte.

[0009] In some embodiments, the lithium ion conductivity of the electrolyte is 12 mS / cm to 20 mS / cm.

[0010] In some embodiments, the lithium ion conductivity of the electrolyte is 15 mS / cm to 20 mS / cm.

[0011] In some embodiments, the surface density of the negative electrode active layer is 0.12 mg / mm2 ~0.16mg / mm 2 .

[0012] In some embodiments, the first solvent comprises at least one of the following compounds:

[0013] In some embodiments, the mass percentage of the first solvent in the electrolyte is 5% to 55%, optionally 10% to 55%, and more optionally 25% to 55%.

[0014] In some embodiments, the electrolyte further includes an additive, and when forming the SEI film, at least a portion of the additive reacts before the first solvent.

[0015] In some embodiments, the additive satisfies at least one of the following characteristics:

[0016] (1) The additive relative to Li / Li + The reduction potential is 0.9V~1.7V;

[0017] (2) The additive includes one or more of vinylene carbonate, fluoroethylene carbonate, vinyl ethylene carbonate, vinyl sulfate, vinyl sulfite, 1,3-propane sultone, and 1,3-propylene sultone;

[0018] (3) The mass percentage of the additive in the electrolyte is 2% to 10%, optionally 3% to 8%.

[0019] In some embodiments, the mass d1 of the first solvent and the mass d2 of the additive satisfy: 0.05≤d2 / d1≤1; optionally, 0.05≤d2 / d1≤0.6; optionally, 0.05≤d2 / d1≤0.4.

[0020] In some embodiments, the mass d1 of the first solvent, the mass d2 of the additive, and the rated capacity A of the secondary battery satisfy: 0.1 g / Ah≤(d1+d2) / A≤2.3 g / Ah; optionally, 0.5 g / Ah≤(d1+d2) / A≤2.2 g / Ah.

[0021] In some embodiments, the graphite is artificial graphite, and the secondary battery satisfies: 0.05≤d2 / d1≤0.5, optionally 0.05≤d2 / d1≤0.35; and / or, 0.4g / Ah≤(d1+d2) / A≤2.3g / Ah, optionally 0.7g / Ah≤(d1+d2) / A≤2.0g / Ah.

[0022] In some embodiments, the graphite includes natural graphite, and the secondary battery satisfies: 0.1≤d2 / d1≤1, optionally 0.1≤d2 / d1≤0.35; and / or 0.1g / Ah≤(d1+d2) / A≤1.5g / Ah, optionally 0.5g / Ah≤(d1+d2) / A≤1.5g / Ah.

[0023] In some embodiments, the secondary battery satisfies at least one of the following conditions: the porosity P of the negative electrode active layer satisfies 20%≤P≤50%, and further optionally 25%≤P≤40%; the compaction density Cd of the negative electrode active layer satisfies Cd≥1.5g / cm 3 , further optionally, 1.5 g / cm 3 ≤Cd≤1.8g / cm 3 .

[0024] In some embodiments, the electrolyte includes a second solvent, and the second solvent includes at least one of a cyclic carbonate and a chain carbonate; optionally, the mass percentage of the second solvent in the electrolyte is 30% to 85%.

[0025] In some embodiments, the secondary battery satisfies: d3 / A≤3.5g / Ah, wherein d3 represents the mass of the electrolyte, in g; A represents the rated capacity of the secondary battery, in Ah; optionally, 2g / Ah≤d3 / A≤3.3g / Ah.

[0026] In some embodiments, the secondary battery satisfies: 0.8≤(d2 / d1) / (CW×P / Cd)≤230; wherein d1 represents the mass of the first solvent, in g; d2 represents the mass of the additive, in g; CW represents the surface density of the negative electrode active layer, in mg / mm 2 ; P represents the porosity of the negative electrode active layer; Cd represents the compacted density of the negative electrode active layer, unit g / cm 3 .

[0027] In some embodiments, the secondary battery further includes a positive electrode sheet, the positive electrode sheet including a positive electrode collector and a positive electrode active layer located on at least one surface of the positive electrode collector, the positive electrode active layer including an olivine-structured lithium-containing phosphate.

[0028] In some embodiments, the chemical formula of the olivine-structured lithium-containing phosphate is LiFe 1-x-y Mn x M yPO4, 0≤x≤1, 0≤y<1, M is selected from one or more of V, Nb, Ti, Co, Ni, Sc, Ge, Mg, Al, Zr, Mn, Hf, Ta, Mo, W, Ru, Ag, Sn and Pb.

[0029] The present application also provides an electrical device comprising the above-mentioned secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] To more clearly illustrate the technical solution of this application, the following is a brief introduction to the drawings used in this application. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the drawings without inventive effort.

[0031] FIG1 is a schematic diagram of a secondary battery according to one embodiment of the present application.

[0032] FIG. 2 is an exploded view of the secondary battery according to one embodiment of the present application shown in FIG. 1 .

[0033] FIG3 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present application.

[0034] Explanation of the accompanying symbols: 1. Secondary battery; 11. Housing; 12. Electrode assembly; 13. Cover plate; 2. Electric device.

[0035] In order to better describe and illustrate the embodiments and / or examples of the inventions disclosed herein, reference may be made to one or more of the accompanying drawings. The additional details or examples used to describe the accompanying drawings should not be considered to limit the scope of the disclosed inventions, the presently described embodiments and / or examples, or any of the best modes currently understood for these inventions. DETAILED DESCRIPTION

[0036] To facilitate understanding of the present application, a more comprehensive description of the present application will be provided below with reference to the accompanying drawings. The accompanying drawings illustrate preferred embodiments of the present application. However, the present application may be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and comprehensive understanding of the disclosure of the present application.

[0037] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this application pertains. The terms used herein in the specification of this application are for the purpose of describing specific embodiments only and are not intended to limit this application. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0038] The "ranges" disclosed herein are defined in terms of lower and upper limits, where 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 the particular range. Ranges defined in this manner can be inclusive or exclusive of the end values ​​and can be combined arbitrarily, i.e., any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60 to 120 and 80 to 110 are listed for a particular parameter, it is understood that ranges of 60 to 110 and 80 to 120 are also contemplated. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, the following ranges are all contemplated: 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4, and 2 to 5. In this application, unless otherwise indicated, the numerical range "a to b" is a shorthand representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0039] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0040] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0041] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0042] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0043] Unless otherwise specified, the term "or" is used in this application to be inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true or present, and B is false or absent; A is false or absent, and B is true or present; or both A and B are true, or both A and B are present.

[0044] Unless otherwise indicated, the terms used in this application have the commonly understood meanings generally understood by those skilled in the art. Unless otherwise indicated, the numerical values ​​of the various parameters mentioned in this application can be measured using various measurement methods commonly used in the art. For example, the tests can be performed using the methods described in the examples of this application.

[0045] One embodiment of the present application provides a secondary battery, comprising a negative electrode plate and an electrolyte. The negative electrode plate comprises a negative electrode current collector and a negative electrode active layer located on at least one surface of the negative electrode current collector. The negative electrode active layer comprises graphite, and the surface density of the negative electrode active layer is ≤0.16 mg / mm 2 , the lithium ion conductivity of the electrolyte is ≤20mS / cm, for example, the surface density is 0.09mg / mm 2 -0.16mg / mm 2 The electrolyte has a lithium ion conductivity of 10-20 mS / cm at 25°C. The first solvent has a general structural formula of R1-COO-R2, where R1 and R2 are independently selected from any one of a C1-C5 alkyl group and a C1-C5 halogenated alkyl group. In this secondary battery, by appropriately matching the negative electrode and electrolyte, the battery can achieve a high energy density while also maintaining good cycling performance and fast charging performance.

[0046] Increasing the surface density of the electrode in a secondary battery is beneficial to improving the energy density, but the increase in surface density may have an adverse effect on the fast charging performance of the battery. Moreover, under the premise of the same compaction density, the greater the surface density, the thicker the active material layer of the electrode, and therefore the concentration polarization of lithium ions caused by the liquid phase diffusion process of the electrolyte will also be aggravated. The concentration polarization problem may cause lithium to be deposited on the surface of the negative electrode, thereby affecting the fast charging performance and the life of the battery. Therefore, by increasing the lithium ion conductivity of the electrolyte and thus improving the liquid phase diffusion capacity, the fast charging performance of the battery can be improved. The use of compounds with an R1-COO-R2 structure can effectively improve the ionic conductivity of the electrolyte. However, excessive pursuit of an increase in ionic conductivity will significantly increase the content of R1-COO-R2 compounds in the electrolyte, resulting in a significant increase in battery gas production, which is not conducive to the cycle life of the battery. Therefore, by matching the negative electrode plate and the electrolyte accordingly, using an electrolyte containing the first solvent, and controlling the surface density of the plate and the lithium ion conductivity of the electrolyte within a certain range, it is possible to maintain good energy density and fast charging performance while taking into account safety and cycle performance.

[0047] The lithium ion conductivity measurement method is as follows: 100ml of sample is placed in a dry, clean, corrosion-resistant sample bottle, sealed, and placed in a constant temperature water bath at 25°C ± 0.5°C. Once the sample temperature is constant, a commercially available multi-temperature conductivity analyzer is used. The sample bottle cap is replaced with a rubber stopper with an electrode. When the temperature is within the range of 25°C ± 0.5°C, the reading on the conductivity meter is read to provide the conductivity of the sample being tested.

[0048] In some embodiments, the surface density of the negative electrode active layer is 0.09 mg / mm 2 ~0.16mg / mm 2 Specifically, it can be 0.12mg / mm 2 ~0.16mg / mm 2 Optionally, the surface density of the negative electrode active layer is 0.1 mg / mm 2 , 0.11mg / mm 2 、 0.12mg / mm 2 , 0.13mg / mm 2 , 0.14mg / mm 2 , 0.15mg / mm 2 wait.

[0049] In some embodiments, the lithium ion conductivity of the electrolyte is 10mS / cm to 20mS / cm. For example, it can be 12mS / cm to 20mS / cm, or 15mS / cm to 20mS / cm. Alternatively, the lithium ion conductivity of the electrolyte is 11mS / cm, 12mS / cm, 13mS / cm, 14mS / cm, 15mS / cm, 16mS / cm, 17mS / cm, 18mS / cm, 19mS / cm, etc. Too low lithium ion conductivity of the electrolyte may reduce the charging performance of the battery. Further optionally, the lithium ion conductivity of the electrolyte is 14mS / cm to 20mS / cm.

[0050] In some embodiments, the compaction density Cd of the negative electrode active layer may satisfy Cd≥1.5 g / cm 3 , optionally, 1.5 g / cm 3 ≤Cd≤1.8g / cm 3 The compaction density of the negative electrode active layer can maintain a high energy density within this range. Further optionally, the compaction density Cd of the negative electrode active layer is 1.5 g / cm 3 , 1.6g / cm 3 , 1.7g / cm 3 , 1.8g / cm 3 wait.

[0051] In some embodiments, the porosity P of the negative electrode active layer may satisfy the following: P ≤ 50%, optionally 20% ≤ P ≤ 50%. Alternatively, 25% ≤ P ≤ 40%. A porosity within this range of the negative electrode active layer can provide a suitable transport path for lithium ions, thereby improving the transport efficiency of lithium ions and, in turn, the cycling performance of the secondary battery. Furthermore, optionally, the porosity P of the negative electrode active layer is 20%, 30%, 40%, 50%, etc.

[0052] When the compaction density and / or porosity of the negative electrode active layer meets the above requirements, the infiltration speed of the electrolyte in the active layer can be increased while ensuring the energy density of the negative electrode plate, which is beneficial to further improve the fast charging performance of the secondary battery.

[0053] In some embodiments, the first solvent included in the electrolyte can play a role in improving the lithium ion conductivity of the electrolyte. In order to further improve the safety performance of the electrolyte, the first solvent can be made to have a relative high conductivity to Li / Li +The reduction potential is ≤1.3V. During the charging process of the battery, when the solvent participates in the formation of the SEI film, there may be a certain gas production problem, which will have an adverse effect on the cycle performance of the battery. In this embodiment, the reduction potential of the first solvent is relatively small, which can delay the time when the first solvent participates in the formation of the SEI film during the charging process, reduce the gas production of the battery, and thus improve the cycle performance of the battery. Optionally, the first solvent is relatively large relative to Li / Li + The reduction potential of the first solvent is ≤1.2 V. Further optionally, the first solvent is + The reduction potential of the first solvent is ≤1.1 V. + The reduction potential is ≤1V.

[0054] In some embodiments, the first solvent has the general formula R1-COO-R2, where R1 and R2 are independently selected from any one of a C1-C5 alkyl group and a C1-C5 halogenated alkyl group. Introducing a carboxylate-type first solvent into the electrolyte can improve the battery's energy density and fast-charging performance. Optionally, R1 and R2 can be the same or different.

[0055] It is understood that an alkyl group refers to a saturated hydrocarbon containing primary (normal) carbon atoms, secondary carbon atoms, tertiary carbon atoms, quaternary carbon atoms, or a combination thereof. Phrases containing this term, such as "C1-C5 alkyl," refer to an alkyl group containing 1 to 5 carbon atoms, each occurrence of which may independently be a C1 alkyl, a C2 alkyl, a C3 alkyl, a C4 alkyl, or a C5 alkyl. Alternatively, the alkyl group may be a straight-chain or branched alkyl group.

[0056] It is also understood that the number of halogen atoms in a haloalkyl group may be one or more. When there are multiple halogen atoms in a haloalkyl group, the multiple halogen atoms may be the same or different.

[0057] In some embodiments, R1 and R2 are independently selected from any one of methyl, ethyl, propyl, butyl, fluoromethyl, fluoroethyl, fluoropropyl, and fluorobutyl. In this case, the electrolyte can maintain a suitable viscosity and lithium ion conductivity.

[0058] As some specific examples of the first solvent, the first solvent includes at least one of the following compounds:

[0059] As some examples of the amount of the first solvent used, the mass percentage of the first solvent in the electrolyte is 5% to 55%. More specifically, it can be 10% to 55, more specifically 25%-55%. If the mass percentage of the first solvent in the electrolyte is too small, the lithium ion conductivity of the electrolyte may be too small, thereby limiting the improvement of the fast charging performance. When the mass percentage of the first solvent in the electrolyte is too large, the gas production may increase, thereby limiting the improvement of the cycle performance of the battery. Optionally, the mass percentage of the first solvent in the electrolyte is 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50% or 55%, etc. Further optionally, it is 10% to 50%.

[0060] In some embodiments, the content of the first solvent can be matched with the porosity and compaction density of the electrode. Specifically, by adjusting the content of the first solvent and the porosity and / or compaction density of the negative electrode, the infiltration speed of the first solvent on the electrode can be increased, thereby improving the fast charging performance of the secondary battery. In some embodiments, the mass percentage of the first solvent in the electrolyte is 5% to 55%, and the surface density of the negative electrode active layer is 0.12 mg / mm 2 ~0.14mg / mm 2 , the porosity of the negative electrode active layer is 20-40%. When the above parameters of the secondary battery meet the above ranges, the secondary battery can have good fast charging performance, short fast charging time, less gas production, high battery safety performance, and maintain good cycle performance.

[0061] In some embodiments, the electrolyte further includes an additive. During SEI film formation, at least a portion of the additive may react before the first solvent. The additive exhibiting the aforementioned properties may react before the first solvent during secondary battery operation, for example, during SEI film formation. This may delay the first solvent's decomposition reaction and reduce the amount of gas generated by the first solvent due to decomposition. Consequently, secondary batteries containing the additive may exhibit improved cycle performance.

[0062] In some embodiments, the reduction potential of the additive is greater than the reduction potential of the first solvent. + The reduction potential of the first solvent is greater than that of Li / Li +When the reduction potential of the additive is greater than that of the first solvent, the additive can preferentially participate in the formation of the SEI film during charging, further delaying the time when the first solvent participates in the formation of the SEI film during charging. This helps further improve the gas production problem of the battery and thus improves the battery's cycling performance. Furthermore, the performance of the SEI formed with the participation of the first solvent is relatively poor. In this embodiment, the additive preferentially participates in the formation of the SEI film, which can improve the performance of the SEI film and further improve the battery's cycling performance.

[0063] Optionally, the additive is relative to Li / Li + The reduction potential of the additive is 0.9V to 1.7V. + The reduction potential of the additive is 0.9 V, 1 V, 1.1 V, 1.2 V, 1.3 V, 1.4 V, 1.5 V, 1.6 V, 1.7 V, etc. As some optional examples of the additive, the additive includes one or more of vinylene carbonate, fluoroethylene carbonate, vinyl ethylene carbonate, vinyl sulfate, vinyl sulfite, and 1,3-propane sultone.

[0064] As some examples of the amount of additives used, the mass percentage of the additive in the electrolyte is 2% to 10%. When the mass percentage of the additive in the electrolyte is too small, the effect of improving the gas production of the battery is poor. When the mass percentage of the additive in the electrolyte is too large, the amount of the first solvent may be reduced accordingly, which may reduce the kinetic properties of the electrolyte, such as reducing the lithium ion conductivity of the electrolyte, thereby limiting the improvement of the fast charging performance of the battery. Optionally, the mass percentage of the additive in the electrolyte is 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%, etc. Further optionally, the mass percentage of the additive in the electrolyte is 3% to 8%.

[0065] Since the additive proposed in this application can form an SEI film in preference to the first solvent, the electrolyte, particularly the additive, can be increased infiltration rate on the electrode surface by adjusting the additive content, as well as parameters such as the electrode porosity and compaction density. Specifically, there will be a concentration gradient of the additive inside and outside the electrode. Therefore, selecting the appropriate additive type and content, electrode porosity, and electrode compaction density can increase the additive infiltration rate in thickly coated electrodes, which is more conducive to film formation inside the electrode and reduces gas production.

[0066] In some embodiments, the content of the additive and the content of the first solvent can also be matched to each other. For example, the content of the additive can be increased as the content of the carboxylate as the first solvent increases. First solvents such as carboxylate have poor compatibility with the negative electrode and are more prone to gas production. Therefore, when the content of the first solvent in the electrolyte increases, more additives can be added, thereby improving the additive's ability to maintain interface performance and reducing gas production. Controlling the total amount of the first solvent and additive can achieve a better balance between the fast charging performance and safety performance of the secondary battery, thereby achieving fast charging while controlling the amount of gas production and improving battery safety. Specifically, when the total injection amount of the first solvent and additive is small, it is easy to affect the fast charging performance of the secondary battery. When the total injection amount of the first solvent and additive is large, gas production increases, and the life and safety of the secondary battery are reduced. In some embodiments, the mass d1 (unit: g) of the first solvent and the mass d2 (unit: g) of the additive can satisfy the following requirements: 0.05≤d2 / d1≤1. When the content of the first solvent and additive meets the above requirements, the secondary battery can have both good fast charging performance and safety performance. At this time, the amount of the additive and the amount of the first solvent are appropriately proportioned, which can more fully exert the effects of the additive and the first solvent. For example, when the amount of the additive and the first solvent meets this condition, it can promote the improvement of the electrolyte ion conductivity while forming a good SEI film, so that the battery can maintain good fast charging performance, while effectively reducing gas production, thereby further improving the battery's cycle performance. Optionally, 0.05≤d2 / d1≤0.6; optionally, 0.05≤d2 / d1≤0.4, optionally, d2 / d1 can be 0.05, 0.06, 0.07, 0.08, 0.09, 0.1.

[0067] In some embodiments, the rated capacity A (unit Ah) of the secondary battery and the mass d1 (unit g) of the first solvent and the mass d2 (unit g) of the additive can satisfy the following conditions: 0.1g / Ah≤(d1+d2) / A≤2.3g / Ah. Optionally, 0.5g / Ah≤(d1+d2) / A≤2.2g / Ah. When the content of the first solvent and the additive and the rated capacity A of the secondary battery meet the above requirements, a good balance can be maintained between fast charging performance and cycle performance. While being able to improve the lithium ion conductivity of the electrolyte to improve the fast charging performance, the components in the electrolyte, especially the gas production problem caused by the decomposition of the first solvent, are controlled, thereby maintaining the secondary battery with good cycle life and safety performance. In general, when the secondary battery meets the above conditions, the secondary battery can have good electrode-electrolyte interface performance, the electrolyte has a faster wetting speed on the electrode, and the electrolyte can maintain good kinetic properties, so the secondary battery can have good comprehensive performance.

[0068] For purposes of this application, the rated capacity of a secondary battery can be determined using conventional techniques in the art. For example, the rated capacity is determined by charging the battery to 3.65V at a 0.33C charge rate, which corresponds to the battery's nominal capacity. This is followed by constant voltage charging at 3.65V to 0.05C, allowing the battery to rest for 10 minutes, and then discharging the battery to 2.0V at a 0.33C discharge rate. The nominal capacity is typically indicated on the device itself.

[0069] In some embodiments, the content of the first solvent and the additive can also be adjusted according to the composition of the negative electrode. For example, the graphite may include artificial graphite and does not contain natural graphite. Specifically, the graphite is artificial graphite. In this case, 0.05≤d2 / d1≤0.5 can be set, and 0.05≤d2 / d1≤0.35 can be selected. Further, 0.4g / Ah≤(d1+d2) / A≤2.3g / Ah can be set, and 0.7g / Ah≤(d1+d2) / A≤2.0g / Ah can be selected. The surface of artificial graphite has fewer defects. Therefore, when the negative electrode does not contain natural graphite, the content of the first solvent in the electrolyte can be appropriately reduced, and the content of the additive can be adjusted accordingly.

[0070] In some embodiments, the graphite includes natural graphite. In this case, 0.1≤d2 / d1≤1 can be set, and 0.1≤d2 / d1≤0.35 can be optionally set. Furthermore, 0.1g / Ah≤(d1+d2) / A≤1.5g / Ah can be set. Alternatively, 0.5g / Ah≤(d1+d2) / A≤1.5g / Ah can be set. Natural graphite has relatively many surface defects, so the content of the first solvent can be appropriately increased to compensate for the consumption of the first solvent by the natural graphite. Similarly, the content of the additive can also be adjusted accordingly.

[0071] In some embodiments, the secondary battery satisfies: 0.8≤(d2 / d1) / (CW×P / Cd)≤230; wherein d1 represents the mass of the first solvent, in g; d2 represents the mass of the additive, in g; CW represents the surface density of the negative electrode active layer, in mg / mm 2 ; P represents the porosity of the negative electrode active layer; Cd represents the compacted density of the negative electrode active layer, unit is g / cm 3 . During the charging process of the secondary battery, there is a certain concentration gradient of the first solvent and the additive in the thickness direction of the electrode. The closer to the negative electrode current collector, the worse the quality of the SEI film formed, and the more likely it is to have side reactions and produce gas. When the secondary battery meets 0.8≤(d2 / d1) / (CW×P / Cd)≤230, the concentration gradient can be better balanced, and the concentration uniformity of the first solvent and the additive in the thickness direction of the electrode can be improved, thereby reducing gas production and further improving the cycle performance of the battery. Optionally, the secondary battery meets 0.8≤(d2 / d1) / (CW×P / Cd)≤30.

[0072] It should be noted that porosity is usually expressed as a percentage. For example, the porosity of the negative electrode active layer is 20%, 30%, 40%, 50%, etc. When calculating (d2 / d1) / (CW×P / Cd), the value of P is substituted into the porosity value for the calculation. For example, when the porosity P of the negative electrode active layer is 50%, P = 0.5 is substituted into (d2 / d1) / (CW×P / Cd) for the calculation. When the porosity P of the negative electrode active layer is 20%, P = 0.2 is substituted into (d2 / d1) / (CW×P / Cd) for the calculation.

[0073] In some embodiments, the secondary battery can meet the following requirements: the surface density of the negative electrode active layer is 0.12 mg / mm 2 ~0.14mg / mm 2 The lithium ion conductivity of the electrolyte is 15mS / cm to 19mS / cm; the porosity of the negative electrode active layer is 20% to 40%, and the compaction density of the negative electrode active layer is 1.5g / cm 3 ~1.8g / cm 3 The electrolyte includes a first solvent and an additive, wherein the first solvent includes a carboxylic acid ester and the additive includes at least one of vinylene carbonate, 1,3-propylene sultone, and fluoroethylene carbonate. The first solvent content in the electrolyte is 20-50%, and the additive content is 5-10%.

[0074] In some embodiments, the electrolyte includes a second solvent, and the second solvent includes at least one of a cyclic carbonate and a linear carbonate. Optionally, the second solvent accounts for 30% to 85% by mass of the electrolyte. Further optionally, the second solvent accounts for 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, or 85% by mass of the electrolyte.

[0075] In some embodiments, the secondary battery satisfies the following relationship: d3 / A≤3.5g / Ah, where d3 represents the mass of the electrolyte in g, specifically the total mass of the electrolyte, including the total mass of the lithium salt, the first and second solvents, other solvents, and additives; and A represents the rated capacity of the secondary battery in Ah. Alternatively, d3 / A may be 1g / Ah, 1.2g / Ah, 1.5g / Ah, 1.8g / Ah, 2g / Ah, 2.2g / Ah, 2.5g / Ah, 2.8g / Ah, 3g / Ah, or 3.2g / Ah, etc. Further, optionally, 2g / Ah≤d3 / A≤3.3g / Ah. When the mass d3 of the electrolyte and the rated capacity A satisfy the above relationship, the secondary battery may have better performance.

[0076] It is understood that the quality of the first solvent and additives can be tested by the following methods:

[0077] S101: Weigh the battery and record the mass as M0.

[0078] S102: disassemble the battery, pour out the free electrolyte, and take the free electrolyte to test the electrolyte composition.

[0079] S103: Take out the internal battery cells and separate the positive electrode sheets, negative electrode sheets, separators and mechanical parts, and use DMC solvent to soak and clean the positive electrode sheets, negative electrode sheets, separators and mechanical parts for 24 to 48 hours, and repeat the soaking for more than 3 times.

[0080] S104: Place the positive electrode sheet, negative electrode sheet, separator and mechanical parts in an oven at 100°C for more than 24 hours until they are completely dried.

[0081] S105: Weigh the dried positive electrode sheet, negative electrode sheet, separator and mechanical parts, and record the mass as M1.

[0082] S106: The weight of the electrolyte in the battery d3 = M0 - M1.

[0083] S107: Use IC ion chromatograph to test the inorganic content in the electrolyte, weigh a quantitative electrolyte (the dilution concentration is in the middle of the standard curve), dilute to 100 mL with ultrapure water, perform automatic sampling detection on the ion chromatogram, test the inorganic ion chromatogram, compare the corresponding inorganic species according to the chromatogram peak position, calculate the corresponding inorganic ion concentration according to the peak area, and calculate the inorganic mass M2 in the electrolyte through the mass of the electrolyte.

[0084] S108: Use acetonitrile to dilute the above-mentioned free electrolyte to 3 to 10 times to obtain the electrolyte dilution solution to be tested, use GC-MS 3100 organic component gas chromatograph, place the above-mentioned electrolyte dilution solution in the instrument for full scan qualitative analysis, the injection port temperature is 250℃, the scanning range is: 35 to 270u, and the test is completed to obtain the total ion current chromatogram of each organic matter. According to the peak position of the chromatogram, the corresponding organic matter type is compared, and the corresponding content percentage of each organic matter is calculated according to the peak area. The mass of each organic matter in the electrolyte d3-M2 and the content percentage of each organic matter can be calculated. For example, the mass d1 of the first solvent in the electrolyte and the mass d2 of the additive in the electrolyte can be calculated.

[0085] In some embodiments, the secondary battery further comprises a positive electrode plate, the positive electrode plate comprising a positive electrode current collector and a positive electrode active layer located on at least one surface of the positive electrode current collector, the positive electrode active layer comprising an olivine-structured lithium-containing phosphate. Optionally, the chemical formula of the olivine-structured lithium-containing phosphate is LiFe 1-x-y Mn x M yPO4, 0≤x≤1, 0≤y<1, M is selected from one or more of V, Nb, Ti, Co, Ni, Sc, Ge, Mg, Al, Zr, Mn, Hf, Ta, Mo, W, Ru, Ag, Sn and Pb.

[0086] Optionally, the positive electrode active layer density is 0.15 mg / mm 2 ~0.35mg / mm 2 For example, the surface density of the positive electrode active layer is 0.15 mg / mm 2 , 0.2mg / mm 2 , 0.25mg / mm 2 , 0.3mg / mm 2 or 0.35 mg / mm 2 wait.

[0087] Further optionally, the compaction density of the positive electrode active layer is ≥2.4 g / cm 3 For example, the compaction density of the positive electrode active layer is 2.4g / cm 3 ~2.8g / cm 3 Specifically, the compaction density of the positive electrode active layer can be 2.4 g / cm 3 , 2.5g / cm 3 , 2.6g / cm 3 , 2.7g / cm 3 or 2.8g / cm 3 wait.

[0088] In some embodiments, during the cycle of the secondary battery, the gas generation internal pressure of the battery is ≤0.35 MPa.

[0089] The present application also provides an electric device in an embodiment. The electric device includes the secondary battery described above.

[0090] The secondary battery and the electric device of the present application will be described below with reference to the accompanying drawings as appropriate.

[0091] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During the battery's charge and discharge processes, active ions are inserted and removed between the positive and negative electrodes. The electrolyte conducts ions between the positive and negative electrodes. The separator, located between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0092] Positive electrode

[0093] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode film layer includes a positive electrode active material.

[0094] As an example, the positive electrode current collector has two surfaces facing each other in its thickness direction, and the positive electrode active material layer is provided on either or both of the two facing surfaces of the positive electrode current collector.

[0095] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. 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 formed by forming a metal material on a polymer material substrate. Alternatively, the metal material may include but is not limited to one or more of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy. Alternatively, the polymer material substrate may include but is not limited to one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS) and polyethylene (PE).

[0096] As an example, the positive electrode active material may include positive electrode active materials for batteries that are well known in the art. As an example, the positive electrode active material may include at least one 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 traditional materials that can be used as positive electrode active materials for batteries 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 at least one of lithium cobalt oxide, 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. Optionally, lithium cobalt oxide includes LiCoO2. Lithium nickel oxide includes LiNiO2. Lithium manganese oxide includes at least one of LiMnO2 and LiMn2O4. Lithium nickel cobalt manganese oxide includes LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2(NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2(NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM 622 ) and LiNi 0.8 Co 0.1 Mn 0.1O2(NCM 811 ) at least one of. Lithium nickel cobalt aluminum oxide includes LiNi 0.85 Co 0.15 Al 0.05 O2. Examples of olivine-structured lithium-containing phosphates may include, but are not limited to, at least one of 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, and a composite material of lithium manganese iron phosphate and carbon. Optionally, the lithium iron phosphate includes LiFePO4 (LFP). The lithium manganese phosphate includes LiMnPO4. The weight ratio of the positive electrode active material in the positive electrode film layer is 80 to 100 weight percent, based on the total weight of the positive electrode film layer.

[0097] In some embodiments, the positive electrode film layer may further optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin. The weight ratio of the binder in the positive electrode film layer is 0 to 20 weight%, based on the total weight of the positive electrode film layer.

[0098] In some embodiments, the positive electrode film layer may also optionally include a conductive agent. For example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The weight ratio of the conductive agent in the positive electrode film layer is 0 to 20% by weight, based on the total weight of the positive electrode film layer.

[0099] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent to form a positive electrode slurry. Optionally, the solvent includes N-methylpyrrolidone. The positive electrode slurry has a solid content of 40 to 80 wt %, and the viscosity at room temperature is adjusted to 5000 to 25000 mPa·s. The positive electrode slurry is coated on the surface of the positive electrode current collector, dried, and then cold-pressed by a cold rolling mill to form a positive electrode sheet; the positive electrode powder coating unit surface density is 0.15-0.35 mg / mm 2 The compaction density of the positive electrode is 2.4-3.6g / cm 3 Optionally, when the positive electrode active material is a lithium-containing phosphate with an olivine structure, the compaction density is 2.4 to 2.8 g / cm 3 , can be selected as 2.5~2.7g / cm 3 Optionally, when the positive electrode active material is a lithium transition metal oxide, the compaction density is 3.0-3.6 g / cm 3 , optional 3.4-3.6g / cm3 The calculation formula of the compaction density is: compaction density = coating surface density / (thickness of the electrode after extrusion - thickness of the current collector).

[0100] Negative electrode

[0101] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer provided on at least one surface of the negative electrode current collector, wherein the negative electrode film layer includes a negative electrode active material.

[0102] As an example, the negative electrode current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0103] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base material. The composite current collector may be formed by forming a metal material on a polymer base material. Optionally, the metal material includes at least one of copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy. The polymer material includes at least one of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS) and polyethylene (PE).

[0104] In some embodiments, the negative electrode active material may adopt the negative electrode active material for batteries that is well known in the art. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxides, and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more. The weight ratio of the negative electrode active material in the negative electrode film layer is 70 to 100 weight %, based on the total weight of the negative electrode film layer.

[0105] In some embodiments, the negative electrode film layer may further include a binder. The binder may be selected from at least one 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). The weight ratio of the binder in the negative electrode film layer is 0 to 30% by weight, based on the total weight of the negative electrode film layer.

[0106] In some embodiments, the negative electrode film layer may further include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The weight ratio of the conductive agent in the negative electrode film layer is 0 to 20% by weight, based on the total weight of the negative electrode film layer.

[0107] In some embodiments, the negative electrode film layer may further include other additives, such as a thickener. The weight ratio of the other additives in the negative electrode film layer is 0 to 15% by weight, based on the total weight of the negative electrode film layer. Optionally, the thickener includes sodium carboxymethylcellulose (CMC-Na).

[0108] In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent to form a negative electrode slurry. Optionally, the solvent includes deionized water. The solid content of the negative electrode slurry is 30 to 70 wt%, and the viscosity at room temperature is adjusted to 2000 to 10000 mPa·s; the obtained negative electrode slurry is coated on the negative electrode current collector, and after a drying process, cold pressing, such as rolling, is performed to obtain a negative electrode sheet. The negative electrode powder coating unit area density is 0.09-0.16 mg / mm 2 The compaction density of the negative electrode is 1.5 to 1.8 g / m 3 .

[0109] electrolytes

[0110] The electrolyte conducts ions between the positive and negative electrodes. This application does not specify the type of electrolyte, and the electrolyte can be selected based on the needs. For example, the electrolyte can be liquid, gel, or solid.

[0111] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0112] In some embodiments, the electrolyte salt may be selected from 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 difluorooxalatoborate (LiDFOB), lithium bis(oxalatoborate) (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorobis(oxalatophosphate) (LiDFOP), and lithium tetrafluorooxalatophosphate (LiTFOP). The concentration of the electrolyte salt is typically 0.5 to 5 mol / L.

[0113] In some embodiments, the solvent can be selected from one or more of fluoroethylene carbonate (FEC), ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS) and diethyl sulfone (ESE).

[0114] In some embodiments, the electrolyte may further include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0115] Isolation film

[0116] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0117] In some embodiments, the material of the separator can be selected from at least one 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.

[0118] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0119] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0120] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery can also be a soft package, such as a bag-type soft package. The material of the soft package can be plastic. Examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate. This application does not particularly limit the shape of the secondary battery, which can be cylindrical, square, or any other shape. For example, Figure 1 shows a secondary battery 1 with a square structure as an example.

[0121] In some embodiments, referring to Figure 2, the outer packaging may include a shell 11 and a cover plate 13. The shell 11 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 11 has an opening connected to the receiving cavity, and the cover plate 13 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can be formed into an electrode assembly 12 through a winding process or a lamination process. The electrode assembly 12 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 12. The number of electrode assemblies 12 contained in the secondary battery 1 can be one or more, and those skilled in the art can select according to specific actual needs.

[0122] In addition, the present application also provides an electrical device, which includes the secondary battery provided in the present application. The secondary battery can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but is not limited thereto.

[0123] As the electrical device, a secondary battery can be selected according to its usage requirements.

[0124] Figure 3 shows an example of an electric device 2. The electric device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the electric device's requirements for high power and high energy density of secondary batteries, a battery pack or battery module can be used.

[0125] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a secondary battery as a power source.

[0126] Example

[0127] In order to make the technical problems, technical solutions and beneficial effects solved by this application clearer, the application will be further described in detail below with reference to the embodiments and drawings. Obviously, the described embodiments are only a part of the embodiments of this application, rather than all the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is in no way intended to limit the present application and its applications. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0128] If no specific techniques or conditions are specified in the examples, the experiments were carried out according to the techniques or conditions described in the literature in the field or according to the product instructions. All reagents or instruments used without specifying the manufacturer are commercially available conventional products.

[0129] Example 1

[0130] Preparation of positive electrode sheet.

[0131] The positive electrode active material lithium iron phosphate, the binder polyvinylidene fluoride, and the conductive agent acetylene black are mixed in a ratio of 97:2:1, and then the solvent N-methylpyrrolidone (NMP) is added and stirred to form a positive electrode slurry. The positive electrode slurry is evenly coated on the positive electrode current collector aluminum foil, dried, and cold pressed to obtain a positive electrode sheet. Among them, the surface density of the positive electrode active layer of the positive electrode sheet is 0.27 mg / mm 2 The compaction density of the positive electrode active layer is 2.55g / cm 3 .

[0132] Preparation of negative electrode sheet.

[0133] The negative electrode active material graphite, conductive agent acetylene black, binder styrene butadiene rubber and thickener sodium carboxymethyl cellulose were mixed in a mass ratio of 96:1:2:1, and then deionized water was added as a solvent and stirred to form a negative electrode slurry. The negative electrode slurry was evenly coated on the negative electrode current collector copper foil, dried and cold pressed to obtain a negative electrode sheet. The surface density of the negative electrode active layer of the negative electrode sheet is 0.12 mg / mm 2 The compaction density of the negative electrode active layer is 1.6g / cm 3 The porosity of the negative electrode active layer is 30%.

[0134] Electrolyte preparation.

[0135] In an argon atmosphere glove box with a water content of less than 10 ppm, ethyl acetate (EA), ethylene carbonate (EC), and ethyl methyl carbonate (EMC) were uniformly mixed in a mass ratio of 20:30:50 to create the electrolyte solvent. 15% by mass of lithium hexafluorophosphate (LiPF6) was slowly added as a lithium salt and stirred thoroughly until completely dissolved. After returning to room temperature, the additives vinylene carbonate (3%), fluoroethylene carbonate (1%), 1,3-propylene sultone (0.5%), vinyl sulfite (0.5%), and lithium difluorobis(oxaloyl)phosphate (1%) were added sequentially in mass percentages, respectively, and mixed thoroughly to create the electrolyte.

[0136] Preparation of isolation membrane.

[0137] A polyethylene (PE) film coated with nano-aluminum oxide was used as the separator.

[0138] Preparation of secondary batteries.

[0139] The positive electrode sheet, separator, and negative electrode sheet are stacked and wound in sequence to form a wound electrode assembly. The electrode assembly is placed in a square aluminum casing, dried, and then filled with electrolyte. After packaging, standing, formation, aging, secondary packaging, and capacity measurement, a secondary battery is obtained. The secondary battery has a liquid retention coefficient (d3 / A) of 3.0 g / Ah.

[0140] Example 2 to Example 15

[0141] Compared with Example 1, the differences between Examples 2 to 15 are shown in Table 1.

[0142] Comparative Example 1 to Comparative Example 3

[0143] Compared with Example 1, the differences between Comparative Examples 1 to 3 are shown in Table 1.

[0144] The types of additives in the examples and comparative examples are the same, except that the mass percentages of some materials are different, as shown in Table 1.

[0145] Test Case

[0146] The secondary batteries in the embodiment and comparative example were subjected to a fast charge time test, a cycle number test, and a gas production internal pressure test. The test results are shown in Table 1.

[0147] (1) The fast charge test method is: the stacked three-electrode test method, the positive and negative electrodes of the hard shell battery cell are taken out and soaked in DMC solvent for more than 72 hours. After the electrolyte solvent, lithium salt and additives are completely leached, the electrodes are dried in a vacuum oven, and then the positive and negative electrodes are assembled into a stacked three-electrode battery cell, in which the copper wire is used as the reference electrode. The stacked three-electrode battery cell is then tested for lithium deposition charge rate at each SOC of the stacked battery cell at 25°C. The maximum charge rate is terminated when the reference electrode potential drops to 0mV, and the maximum charge rate at this SOC is recorded. According to this method, with 5% SOC as a point, the maximum charge rate at each 5% SOC is tested, such as 5% SOC, 10% SOC, 15% SOC to 100% SOC. According to the maximum charge rate at 10% SOC to 80% SOC obtained from this test, the continuous charging time at 10% to 80% SOC is calculated as the fast charge time.

[0148] (2) The cycle test method is as follows: at 25°C, charge the battery to 3.65V at a charge rate of 0.5C of the nominal capacity, then charge it to 0.05C at a constant voltage of 3.65V, let it stand for 10 minutes, and then discharge it to 2.5V at a discharge rate of 1C, let it stand for 10 minutes. The above charge and discharge is one cycle. The test is stopped until the battery capacity decays to 80% of the nominal capacity, which is recorded as the cycle number @80% SOH.

[0149] (3) The method for testing the internal gas pressure is as follows: before disassembling the fixture, connect the cycled battery cell to the gas pressure gauge using a needle with a diameter of 1 to 10 mm. Insert the needle into the battery cell through the explosion-proof valve of the battery cell. Observe the reading on the gas pressure gauge, which is the internal gas pressure of the battery cell.

[0150] Table 1

[0151] It is understood that in Table 1, the unit of the areal density CW is mg / mm 2 The unit of compacted density Cd is g / cm 3 The unit of porosity P is %. The unit of reduction potential is V. The unit of lithium ion conductivity is mS / cm. The unit of fast charge time is min. The unit of cycle number is lap. The unit of gas production internal pressure is MPa.

[0152] It should also be understood that in Table 1, the mass percentage of the first solvent represents the mass percentage of the first solvent in the electrolyte. The mass percentage of the additive represents the mass percentage of the material in the corresponding "Type" column of the additive in the electrolyte. The first solvent C4H8O2 represents ethyl acetate, and C4H7FO2 represents ethyl fluoroacetate.

[0153] It can be seen from Table 1 that the surface density of the negative electrode active layer is ≤0.16 mg / mm 2 When the lithium ion conductivity of the electrolyte is ≤20mS / cm, the battery can have both good cycle performance and good fast charging performance.

[0154] The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0155] The above-described embodiments merely represent several implementation methods of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art could make various modifications and improvements without departing from the spirit of the present application, all of which fall within the scope of protection of the present application. Therefore, the scope of protection of the present patent application shall be determined by the appended claims.

Claims

1. A secondary battery, characterized in that: The invention comprises a negative electrode sheet and an electrolyte; the negative electrode sheet comprises a negative electrode current collector and a negative electrode active layer located on at least one surface of the negative electrode current collector, the negative electrode active layer comprises graphite, and the surface density of the negative electrode active layer is 0.09 mg / mm 2 ~0.16mg / mm 2 ; The lithium ion conductivity of the electrolyte at 25°C is 10mS / cm to 20mS / cm; and the electrolyte includes a first solvent, the first solvent has a general structural formula of R1-COO-R2, wherein R1 and R2 are independently selected from any one of a C1-C5 alkyl group and a C1-C5 halogenated alkyl group.

2. The secondary battery according to claim 1, characterized in that: The lithium ion conductivity of the electrolyte is 12 mS / cm to 20 mS / cm.

3. The secondary battery according to claim 2, characterized in that: The lithium ion conductivity of the electrolyte is 15 mS / cm to 20 mS / cm.

4. The secondary battery according to any one of claims 1 to 3, characterized in that: The surface density of the negative electrode active layer is 0.12 mg / mm 2 ~0.16mg / mm 2 .

5. The secondary battery according to any one of claims 1 to 4, characterized in that: The first solvent comprises at least one of the following compounds:

6. The secondary battery according to any one of claims 1 to 5, characterized in that: The mass percentage of the first solvent to the electrolyte is 5% to 55%, optionally 10% to 55%, and more optionally 25% to 55%.

7. The secondary battery according to any one of claims 1 to 6, characterized in that: The electrolyte further includes an additive, and when the SEI film is formed, at least a portion of the additive reacts prior to the first solvent.

8. The secondary battery according to claim 7, characterized in that: The additive meets at least one of the following characteristics: (1) The additive relative to Li / Li + The reduction potential is 0.9V~1.7V; (2) the additive includes one or more of vinylene carbonate, fluoroethylene carbonate, vinyl carbonate, vinyl sulfate, vinyl sulfite, 1,3-propane sultone and 1,3-propylene sultone; (3) The mass percentage of the additive in the electrolyte is 2% to 10%, and optionally 3% to 8%.

9. The secondary battery according to claim 7 or 8, characterized in that: The mass d1 of the first solvent and the mass d2 of the additive satisfy: 0.05≤d2 / d1≤1; optionally, 0.05≤d2 / d1≤0.6; optionally, 0.05≤d2 / d1≤0.

4.

10. The secondary battery according to any one of claims 7 to 9, characterized in that: The mass d1 of the first solvent, the mass d2 of the additive, and the rated capacity A of the secondary battery satisfy: 0.1g / Ah≤(d1+d2) / A≤2.3g / Ah; optionally, 0.5g / Ah≤(d1+d2) / A≤2.2g / Ah.

11. The secondary battery according to any one of claims 7 to 10, characterized in that: The graphite is artificial graphite, and the secondary battery satisfies: 0.05≤d2 / d1≤0.5, optionally 0.05≤d2 / d1≤0.35; and / or, 0.4g / Ah≤(d1+d2) / A≤2.3g / Ah, optionally 0.7g / Ah≤(d1+d2) / A≤2.0g / Ah.

12. The secondary battery according to any one of claims 7 to 10, characterized in that: The graphite includes natural graphite, and the secondary battery satisfies: 0.1≤d2 / d1≤1, optionally 0.1≤d2 / d1≤0.35; and / or, 0.1g / Ah≤(d1+d2) / A≤1.5g / Ah, which can be optionally 0.5g / Ah≤(d1+d2) / A≤1.5g / Ah.

13. The secondary battery according to any one of claims 1 to 12, characterized in that: The secondary battery satisfies at least one of the following conditions: The porosity P of the negative electrode active layer satisfies 20%≤P≤50%, and further optionally 25%≤P≤40%; The compaction density Cd of the negative electrode active layer satisfies Cd≥1.5g / cm 3 , further optionally, 1.5 g / cm 3 ≤Cd≤1.8g / cm 3 .

14. The secondary battery according to any one of claims 1 to 13, characterized in that: The electrolyte includes a second solvent, and the second solvent includes at least one of a cyclic carbonate and a chain carbonate; Optionally, the mass percentage of the second solvent in the electrolyte is 30% to 85%.

15. The secondary battery according to any one of claims 1 to 14, characterized in that: The secondary battery satisfies: d3 / A ≤ 3.5 g / Ah, wherein d3 represents the mass of the electrolyte, in g; A represents the rated capacity of the secondary battery, in Ah; Optionally, 2g / Ah≤d3 / A≤3.3g / Ah.

16. The secondary battery according to any one of claims 1 to 15, characterized in that: The secondary battery further comprises a positive electrode plate, wherein the positive electrode plate comprises a positive electrode current collector and a positive electrode active layer located on at least one surface of the positive electrode current collector, wherein the positive electrode active layer comprises a lithium-containing phosphate having an olivine structure; Optionally, the chemical formula of the olivine-structured lithium-containing phosphate is LiFe 1-x-y Mn x M y PO4, 0≤x≤1, 0≤y<1, M is selected from one or more of V, Nb, Ti, Co, Ni, Sc, Ge, Mg, Al, Zr, Mn, Hf, Ta, Mo, W, Ru, Ag, Sn and Pb.

17. An electrical device, characterized in that: A secondary battery comprising the secondary battery according to any one of claims 1 to 16.

Images