Secondary battery, battery module, battery pack, and electrical device

Abstract

The present application provides a secondary battery, a battery module, a battery pack, and an electrical device. The secondary battery of the present application includes a negative electrode sheet and an electrolyte. The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer formed on the negative electrode current collector. The negative electrode film layer includes a binder. The electrolyte contains a linear carbonate and a linear carboxylate. When the sum of the mass percentage of the linear carbonate and the mass percentage of the linear carboxylate in the electrolyte is set as A%, and the content of the binder per unit area of the negative electrode film layer is set as S mg / dm<supgt;2< / supgt;, S and A satisfy the following relationship: 0.40A ≤ S ≤ 0.70A.

Description

Technical Field

[0001] The present application relates to the technical field of secondary batteries, and particularly to a secondary battery, a battery module, a battery pack, and an electric device. Background Art

[0002] In recent years, secondary batteries have been widely used in energy storage power systems such as hydroelectric, thermal, wind, and solar power stations, as well as in multiple fields such as power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, and aerospace.

[0003] Due to the great development of secondary batteries, higher requirements have also been put forward for their fast charging performance, cycle performance, and safety performance.

[0004] Therefore, there is an urgent need for a secondary battery that can balance good fast charging performance and cycle life. Summary of the Invention

[0005] In view of the problems existing in the prior art, the purpose of the present application is to provide a secondary battery that can balance good fast charging performance and cycle life, as well as a battery module, a battery pack, and an electric device using the secondary battery.

[0006] To achieve the above purpose, in the first aspect of the present application, a secondary battery is provided, which includes a negative electrode plate and an electrolyte.

[0007] The negative electrode plate includes the negative electrode current collector and a negative electrode film layer formed on the negative electrode current collector, and the negative electrode film layer includes a binder.

[0008] The electrolyte contains a linear carbonate and a linear carboxylate.

[0009] When the sum of the mass percentages of the linear carbonate and the linear carboxylate in the electrolyte is set as A%, and the content of the binder per unit area of the negative electrode film layer is set as S mg / dm 2 , S and A satisfy the following relationship:

[0010] 0.40A ≤ S ≤ 0.70A.

[0011] In the present application, by adjusting the relationship between the total content of the linear carbonate and the linear carboxylate in the electrolyte and the content of the binder per unit area of the negative electrode film layer, the fast charging performance and cycle life of the secondary battery can be balanced.

[0012] In any embodiment, S and A can satisfy the following relationship: 0.40A ≤ S ≤ 0.60A.

[0013] Thus, it is possible to further ensure a better balance between the high conductivity of the electrolyte and the low swelling property of the binder, resulting in better fast charging performance and cycle life of the secondary battery.

[0014] In any embodiment, the mass percentage of the linear carbonate in the above electrolyte may be 10% to 50%.

[0015] Thus, a relatively high conductivity of the electrolyte can be obtained, improving the kinetic performance of the liquid-phase transport of ions, which is beneficial to avoiding lithium deposition on the negative electrode during the fast charging process and long-term use, thereby improving the fast charging performance and cycle life. In addition, the oxidation potential of the linear carbonate is relatively high (compared with the cyclic carbonate), which is beneficial to increasing the electrochemical window of the electrolyte, reducing the side reactions at the negative electrode / electrolyte interface, and enhancing the fast charging performance and cycle life of the secondary battery.

[0016] In any embodiment, the mass percentage of the linear carboxylate in the above electrolyte may be 10% to 20%.

[0017] The linear carboxylate has a relatively low viscosity. By setting the content of the linear carboxylate within the above range, the liquid-phase transport resistance of ions can be further reduced, improving the fast charging performance and cycle performance of the secondary battery.

[0018] In any embodiment, the sum of the mass percentage of the linear carbonate and the mass percentage of the linear carboxylate in the above electrolyte may be: 20% ≤ A% ≤ 70%.

[0019] Thus, under the combined action of the linear carbonate and the linear carboxylate, the viscosity of the electrolyte significantly decreases, the conductivity is significantly improved, it has good kinetic performance, and has good electrochemical stability, which can further ensure that the secondary battery has good cycle stability.

[0020] In any embodiment, the content of the binder per unit area of the negative electrode film layer may be 8 to 60 mg / dm 2 .

[0021] Thus, by controlling the usage amount of the binder in the negative electrode film layer, it is possible to avoid the peeling off of the negative electrode film layer due to the expansion and contraction of the negative electrode plate during cyclic use, thereby deteriorating the cycle life of the battery.

[0022] In any embodiment, the conductivity of the electrolyte may be 9 to 15 mS / cm.

[0023] Thus, by controlling the conductivity of the electrolyte within a certain range, it is possible to ensure the fast charging performance and long-term cycle life of the battery.

[0024] The second aspect of the present application provides a battery module, which includes the secondary battery of the first aspect of the present application.

[0025] The third aspect of the present application provides a battery pack, which includes the battery module of the second aspect of the present application.

[0026] The fourth aspect of the present application provides an electrical device, which includes the secondary battery of the first aspect of the present application, the battery module of the second aspect of the present application, or the battery pack of the third aspect of the present application.

[0027] Advantages of the Invention

[0028] In the secondary battery of the present application, by adjusting the relationship between the total content of linear carboxylate and linear carbonate in the electrolyte and the content of the binder per unit area of the negative electrode film layer, it is possible to ensure that the viscosity of the electrolyte is suitable for the liquid-phase transport kinetic performance of ions, improve the charge rate performance, and ensure the stability of the negative electrode plate during the recycling process, thus taking into account both the fast charge performance and the cycle life of the secondary battery. Specifically, as solvents in the electrolyte, linear carboxylate and linear carbonate have smaller molecular volumes compared to cyclic carbonate and cyclic carboxylate, and can reduce the viscosity of the electrolyte. Therefore, in the present application, in order to make the viscosity of the electrolyte suitable for the liquid-phase transport kinetic performance of ions, linear carboxylate and linear carbonate are added to the electrolyte. However, linear carboxylate and linear carbonate have a certain swelling effect on the binder in the negative electrode film layer. When their content reaches a certain level, the bonding performance of the binder may decrease, causing the negative electrode film layer to peel off, which will seriously affect the fast charge performance and cycle life of the secondary battery. However, when their content is too low, the liquid-phase transport kinetic performance of ions is insufficient, and lithium deposition is likely to occur at the negative electrode interface during charging, which will also affect the fast charge performance and cycle life of the secondary battery. Through a large number of experiments, the inventors of the present application found that: when the sum of the contents of linear carbonate and linear carboxylate in the electrolyte and the content of the binder per unit area of the negative electrode film layer satisfy the relationship of 0.40A ≤ S ≤ 0.70A, a higher conductivity of the electrolyte and good charge rate performance can be obtained, and the adhesion of the negative electrode film layer will not be reduced, so that a secondary battery that takes into account both fast charge performance and cycle life can be obtained. Description of the Drawings

[0029] Figure 1 is a schematic diagram of a secondary battery according to an embodiment of the present application.

[0030] Figure 2 is Figure 1 the exploded view of the secondary battery according to an embodiment of the present application shown in

[0031] Figure 3 is a schematic diagram of a battery module according to an embodiment of the present application.

[0032] Figure 4 It is a schematic diagram of a battery pack according to an embodiment of the present application.

[0033] Figure 5 is Figure 4 An exploded view of the battery pack according to an embodiment of the present application shown in the figure.

[0034] Figure 6 It is a schematic diagram of an electrical device using a secondary battery according to an embodiment of the present application as a power source.

[0035] Description of reference numerals:

[0036] 1: Battery pack; 2: Upper box body; 3: Lower box body; 4: Battery module; 5: Secondary battery; 51: Housing; 52: Electrode assembly; 53: Top cover assembly. Specific embodiments

[0037] Hereinafter, with appropriate reference to the drawings, embodiments of the secondary battery, battery module, battery pack, and electrical device of the present application specifically disclosed will be described in detail. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are 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.

[0038] The "range" disclosed in the present application is 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 limit and upper limit define the boundary of a particular range. The ranges defined in this way can include the end values or not include 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 the present application, unless otherwise specified, the numerical range "a-b" represents an abbreviated representation of any real number combination between a and b, where a and b are both real numbers. For example, the numerical range "0-5" represents that all real numbers between "0-5" have been fully listed herein, and "0-5" is only an abbreviated representation of these numerical combinations. In addition, when it is stated 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.

[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 the present application can be combined with each other to form a new technical solution.

[0041] Unless otherwise specified, all steps of the present application can be carried out in sequence or randomly, preferably in sequence. For example, the 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.

[0042] Unless otherwise specified, the terms "comprising" and "including" mentioned in the present application mean open-ended or can also be closed-ended. For example, the "comprising" and "including" can mean that other components not listed can also be included or comprised, or can also only include or comprise the listed components.

[0043] Unless otherwise specified, in the present 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) while B is true (or exists); or both A and B are true (or exist).

[0044] The present application provides a secondary battery, a battery module, a battery pack, and an electrical device. The secondary battery, battery module, battery pack, and electrical device of the present application will be described below with reference to the accompanying drawings as appropriate.

[0045] Secondary battery

[0046] With the accelerating pace of life, in addition to a strong demand for the cycle life of secondary batteries, the fast charging performance is also an issue that must be considered. To improve the fast charging performance, it is necessary to improve the liquid-phase transport ability of ions in the secondary battery. Usually, a low-viscosity solvent is added to increase the ion transport speed and improve the charge rate performance, thereby achieving the fast charging performance. However, the inventors of the present application found that the polarity of the low-viscosity solvent is comparable to that of the binder in the negative electrode film layer. According to the principle of "like dissolves like", the low-viscosity solvent easily enters the interior of the binder, causing the binder to swell and reducing the adhesive force, thus causing the negative electrode film layer to peel off from the negative electrode current collector, seriously affecting the cycle life of the secondary battery.

[0047] Therefore, in terms of ensuring a low viscosity of the electrolyte while not affecting the adhesion stability between the negative electrode film layer and the negative electrode current collector, the secondary batteries of the prior art still need to be improved. After careful research, the inventors of the present application have provided the following secondary battery. The secondary battery has been improved in terms of ensuring a low viscosity of the electrolyte while not affecting the adhesion stability between the negative electrode film layer and the negative electrode current collector, thereby taking into account good fast charging performance and cycle life.

[0048] One embodiment of the present application provides a secondary battery, which includes a negative electrode plate and an electrolyte.

[0049] The above-mentioned negative electrode plate includes a negative electrode current collector and a negative electrode film layer formed on the negative electrode current collector, and the negative electrode film layer includes a binder.

[0050] The above-mentioned electrolyte contains a linear carbonate and a linear carboxylate.

[0051] When the sum of the mass percentages of the linear carbonate and the linear carboxylate in the above-mentioned electrolyte is set as A%, and the content of the binder per unit area of the above-mentioned negative electrode film layer is set as S mg / dm 2 , S and A satisfy the following relationship:

[0052] 0.4A ≤ S ≤ 0.7A.

[0053] Although the mechanism is not yet clear, the applicant has unexpectedly found that by adjusting the relationship between the total content of the linear carbonate and the linear carboxylate in the electrolyte and the content of the binder per unit area of the negative electrode film layer, the fast charging performance and cycle life of the secondary battery can be taken into account.

[0054] When the relationship between the total content of the linear carbonate and the linear carboxylate in the electrolyte and the content of the binder per unit area of the negative electrode film layer is 0.4A > S, due to the principle of like dissolves like, the binder in the negative electrode film layer of the negative electrode plate will swell, causing the negative electrode film layer to detach from the negative electrode current collector, resulting in a decrease in cycle life; when the relationship between the total content of the linear carbonate and the linear carboxylate in the electrolyte and the content of the binder per unit area of the negative electrode film layer is 0.7A < S, there is too much binder in the negative electrode film layer. Since the binder itself is not conductive, too much binder will cause a significant decrease in the conductivity of the negative electrode plate, an increase in the resistance of the negative electrode plate, and thus a decrease in the fast charging performance of the battery. The relationship between the total content of the linear carbonate and the linear carboxylate in the electrolyte and the content of the binder per unit area of the negative electrode film layer can be 0.40A ≤ S ≤ 0.60A, preferably 0.50A ≤ S ≤ 0.60A. Thereby, the fast charging performance and cycle life of the secondary battery can be further taken into account.

[0055] The secondary battery of the present application can be a lithium-ion secondary battery or the like. The secondary battery of the present application can also include a positive electrode plate and a separator. During the charging and discharging process of the battery, active ions are embedded and extracted back and forth between the positive electrode plate and the negative electrode plate. The electrolyte plays a role in conducting ions between the positive electrode plate and the negative electrode plate. The separator is disposed between the positive electrode plate and the negative electrode plate, mainly playing a role in preventing short circuit between the positive and negative electrodes, and at the same time allowing ions to pass through. The following will detail each component of the secondary battery.

[0056] [Positive Electrode Plate]

[0057] The positive electrode plate can include a positive electrode current collector and a positive electrode film layer provided on at least one surface of the positive electrode current collector. As an example, the positive electrode current collector has two surfaces opposite to each other in its own thickness direction, and the positive electrode film layer is provided on either one or both of the two opposite surfaces of the positive electrode current collector.

[0058] The above 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 base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (such as aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material substrate (such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0059] The positive electrode film layer includes a positive electrode active material. The positive electrode active material includes, but is not limited to, lithium cobaltate, lithium nickel manganese cobaltate, lithium nickel manganese aluminate, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate, lithium manganese silicate, spinel-type lithium manganate, spinel-type lithium nickel manganate, lithium titanate, etc. The positive electrode active material can use one or several of these.

[0060] The positive electrode film layer may also optionally include a binder. However, the type of the binder is not specifically limited, and those skilled in the art can select according to actual needs. As an example, the binder can include one or several of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0061] The positive electrode film layer may also optionally include a conductive agent. However, the type of the conductive agent is not specifically limited, and those skilled in the art can select according to actual needs. As an example, the conductive agent for the positive electrode film layer can be selected from one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0062] The preparation of the positive electrode tab can be carried out according to methods known in the art. As an example, the positive electrode active material, the conductive agent, and the binder can be dispersed in a solvent (such as N-methylpyrrolidone (NMP)) to form a uniform positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector, and after processes such as drying and cold pressing, the positive electrode tab is obtained.

[0063] [Negative electrode tab]

[0064] The negative electrode tab may include a negative electrode current collector and a negative electrode film layer provided on at least one surface of the negative electrode current collector, and the negative electrode film layer includes a negative electrode active material and a binder. As an example, the negative electrode current collector has two surfaces opposite to each other in its own thickness direction, and the negative electrode film layer is provided on any one or both of the two opposite surfaces of the negative electrode current collector.

[0065] In the secondary battery of the present application, the negative electrode active material can use the negative electrode active materials commonly used in the art for preparing the negative electrode of the secondary battery. As the negative electrode active material, artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate can be listed. The silicon-based materials can be selected from one or more of elemental silicon, silicon oxides (such as silicon monoxide), silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based materials can be selected from one or more of elemental tin, tin oxides, and tin alloys.

[0066] The negative electrode current collector can adopt a metal foil or a composite current collector. For example, as the metal foil, copper foil can be adopted. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be formed by forming a metal material (such as copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material substrate (such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0067] In the secondary battery of the present application, the negative electrode film layer can include a negative electrode active material, a binder, an optional conductive agent, and other optional additives, and is usually formed by coating and drying the slurry for the negative electrode film layer. The slurry for the negative electrode film layer is usually formed by dispersing the negative electrode active material, the binder, and the optional conductive agent, etc. in a solvent and stirring evenly. The solvent can be N-methylpyrrolidone (NMP) or deionized water.

[0068] The binder can be selected from one or more of styrene-butadiene rubber, polyvinylidene fluoride, polyacrylic acid, polyacrylonitrile, acrylonitrile copolymer, polyvinyl alcohol, xanthan gum, gum arabic, sodium alginate, and sodium carboxymethyl cellulose. Since the polarities of the linear carbonates and linear carboxylates in the electrolyte are relatively close to the polarity of the binder in the negative electrode film layer, when selecting the binder, it is advisable to choose a binder with a relatively large difference in polarity from the linear carbonates and linear carboxylates, so as to reduce the binder swelling effect caused by similar polarities. Considering from the above viewpoints, the binder is preferably styrene-butadiene rubber, sodium carboxymethyl cellulose, and polyvinylidene fluoride. That is, by selecting such a binder, better stability can be obtained, thereby ensuring the cycle stability of the secondary battery.

[0069] In some embodiments, the content of the binder per unit area of the negative electrode film layer can be 8-60 mg / dm 2 . The main function of the binder is to enhance the interaction between the negative electrode active materials and increase the adhesion between the negative electrode film layer and the negative electrode current collector. When the content of the binder per unit area of the negative electrode film layer is too low, for example, lower than 8 mg / dm 2 , it is not conducive to generating an effective bonding effect, the adhesion between the negative electrode active materials will deteriorate, and the adhesion between the negative electrode film layer and the negative electrode current collector will also deteriorate, and phenomena such as film peeling are likely to occur, which will further lead to poor stability of the negative electrode plate during cyclic use and a significant decrease in the cycle performance of the battery. When the content of the binder per unit area of the negative electrode film layer is too high, for example, higher than 60 mg / dm 2 , since too much binder (usually the binder is non-conductive or has very low conductivity) will reduce the conductivity of the negative electrode plate, resulting in a higher film resistance of the negative electrode plate, thereby reducing the charging rate performance of the battery. By making the content of the binder per unit area of the negative electrode film layer within the above range, the stability of the negative electrode plate during cyclic use can be better ensured, and the charging rate performance of the battery can also be better ensured without being reduced. Considering from the viewpoint of further obtaining the above effects, the content of the binder per unit area of the negative electrode film layer is preferably 10-45 mg / dm 2 , more preferably 20-42 mg / dm 2 .

[0070] As an example, the conductive agent can be selected from one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0071] As an example, the auxiliary agent may optionally include a thickener.

[0072] The preparation of the negative electrode sheet can be carried out according to methods known in the art. As an example, the negative electrode active material, conductive agent, binder, and any other components can be dispersed in a solvent (such as N-methylpyrrolidone (NMP) or deionized water) to form a uniform negative electrode slurry; the slurry is coated on at least one surface of the negative electrode current collector through a conventional coating process, and after processes such as drying and cold pressing, the negative electrode sheet is obtained.

[0073] [Electrolyte solution]

[0074] The electrolyte solution plays a role in conducting ions between the positive electrode sheet and the negative electrode sheet. The electrolyte solution can include an electrolyte salt and a solvent.

[0075] As an example, the electrolyte salt can 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 difluorooxalate borate (LiDFOB), lithium bis(oxalato)borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluoro bis(oxalato)phosphate (LiDFOP), and lithium tetrafluorooxalate phosphate (LiTFOP). Among them, lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI) have good solubility, suitable ionic conductivity, suitable dissociation constants, good antioxidant properties, and good thermal stability. Therefore, at least one of lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI) is preferably used as the electrolyte salt.

[0076] In some embodiments, the concentration of the above-mentioned electrolyte salt in the electrolyte solution can be selected to be 0.8 M to 1.5 M. When the concentration of the electrolyte salt is too low, for example, lower than 0.8 M, it is not conducive to achieving the energy density required by the battery. When the concentration of the electrolyte salt is too high, for example, higher than 1.5 M, the production cost of the battery is too high, and lithium deposition is likely to form on the electrode sheet, destroying the stability of the battery.

[0077] In the present application, the solvent in the electrolyte solution includes linear carbonates and linear carboxylates.

[0078] In some embodiments, the linear carbonate can be selected from at least one of dimethyl carbonate, ethyl methyl carbonate, dipropyl carbonate, methyl propyl carbonate, diethyl carbonate, and ethyl propyl carbonate. Due to its straight-chain alkane structure, the linear carbonate can reduce the viscosity of the electrolyte solution, and the shorter the carbon chain, the more obvious the viscosity reduction effect. Dimethyl carbonate has a relatively short carbon chain structure, so it can be considered that its effect of reducing viscosity is more obvious.

[0079] In some embodiments, the linear carboxylic acid ester may be selected from at least one of methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, methyl butyrate, and ethyl butyrate. Similar to the linear carbonate, the shorter the carbon chain of the linear carboxylic acid ester, the more obvious the viscosity reduction. However, since the boiling point of ethyl acetate is higher than that of methyl formate and ethyl formate, the liquid temperature range of the solvent is relatively wide, so ethyl acetate is preferred.

[0080] In some embodiments, the mass percentage of the linear carbonate in the electrolyte may be 10% - 50%. The linear carbonate has the effect of reducing the viscosity of the electrolyte. When the content of the linear carbonate is too low, such as less than 10%, the decrease in the viscosity of the solvent is small, the conductivity of the secondary battery is low, which is not conducive to meeting the requirements of ion kinetic diffusion performance, and the fast charging performance of the secondary battery is poor. When the content of the linear carbonate is too high, such as higher than 50%, the viscosity of the solvent decreases significantly, but the dielectric constant of the electrolyte also decreases significantly, and it is more likely to be miscible with the binder in the negative electrode film layer, resulting in an obvious swelling reaction of the binder, a decrease in the cycle capacity retention rate of the battery, and affecting the service life of the battery. Moreover, when the content of the linear carbonate is too high, the flash point of the electrolyte decreases significantly. Since the battery heats up significantly during use, the low solvent flash point cannot effectively guarantee the use safety of the battery. From the perspective of further obtaining the above effects, the mass percentage of the linear carbonate in the electrolyte is preferably 20% - 40%, and more preferably 25% - 40%.

[0081] In some embodiments, the mass percentage of the linear carboxylic acid ester in the electrolyte may be 10% - 20%. The linear carboxylic acid ester has a lower viscosity, lower freezing point, and smaller surface tension compared to the linear carbonate. When used as the solvent of the electrolyte, it makes the liquid temperature range of the electrolyte wider and the low-temperature performance of the battery better. When the content of the linear carboxylic acid ester is too low, such as less than 10%, the decrease in the viscosity of the solvent is small, the conductivity of the secondary battery is low, which is not conducive to meeting the requirements of ion kinetic diffusion performance, and the fast charging performance of the battery is poor. When the content of the linear carboxylic acid ester is too high, such as higher than 20%, the viscosity of the electrolyte decreases significantly. However, due to the compatibility between the linear carboxylic acid ester and the binder, the structure of the negative electrode sheet of the battery is damaged due to the swelling of the binder. Therefore, after cyclic use, the capacity of the battery decreases significantly. At the same time, since the increase in the content of the linear carboxylic acid ester leads to a decrease in the freezing point of the electrolyte and a decrease in the low-temperature discharge capacity, it is not suitable to add too much linear carboxylic acid ester. From the perspective of further obtaining the above effects, the mass percentage of the linear carboxylic acid ester in the electrolyte is preferably 15% - 20%.

[0082] In some embodiments, the sum of the mass percentages of the linear carbonate and the linear carboxylate in the electrolyte can be 20% ≤ A% ≤ 70%. As described above, under the combined action of the linear carbonate and the linear carboxylate, the viscosity of the electrolyte significantly decreases, enabling the electrolyte to have good kinetic performance. However, when the sum of the mass percentages of the linear carbonate and the linear carboxylate in the electrolyte is too low, for example, less than 20%, it is not conducive to ensuring that the overall viscosity of the electrolyte meets the requirements of ionic kinetic diffusion performance. When the sum of the mass percentages of the linear carbonate and the linear carboxylate in the electrolyte is too high, for example, more than 70%, the interaction with the binder is significant, causing the binder on the negative electrode sheet to undergo a swelling reaction, reducing the cycle retention rate of the battery. Moreover, the flash point and freezing point of the electrolyte will also decrease, affecting the use safety of the battery. From the perspective of further achieving the above effects, the sum of the mass percentages of the linear carbonate and the linear carboxylate in the electrolyte is preferably 30% - 60%.

[0083] In addition to the above-mentioned linear carbonate and linear carboxylate, a certain content of cyclic carbonate can also be added to the solvent system of the electrolyte. The cyclic carbonate can increase the flash point and boiling point of the electrolyte and broaden the liquid temperature range. The mass percentage of the cyclic carbonate in the electrolyte can be 10% - 50%. When the content of the cyclic carbonate is too low, for example, less than 10%, the flash point and boiling point of the electrolyte are relatively low, and the liquid temperature range is narrow. When the content of the cyclic carbonate is too high, for example, more than 50%, the viscosity of the solvent increases, and the kinetic conduction performance of the ions is affected. The type of the cyclic carbonate is not particularly limited and can be selected from at least one of ethylene carbonate, propylene carbonate, butylene carbonate, and fluoroethylene carbonate. From the perspective of further achieving the above effects, the mass percentage of the cyclic carbonate in the electrolyte is preferably 15% - 30%.

[0084] In addition, the electrolyte can also contain solvents such as γ-butyrolactone and tetrahydrofuran. By adding an organic reagent with relatively stable chemical properties and strong solubility as a solvent, the chemical stability of the electrolyte can be improved, and the compatibility with the electrode material can be enhanced. However, ethers have poor antioxidant properties and are easily oxidized and decomposed at low potentials. Therefore, the addition amount is generally small, and the mass percentage of γ-butyrolactone or tetrahydrofuran in the electrolyte is preferably 5% - 10%.

[0085] In some embodiments, the electrolyte may also optionally include additives. For example, the electrolyte may include negative electrode film-forming additives, positive electrode film-forming additives, additives for improving the overcharge performance of the battery, additives for improving the high-temperature performance of the battery, additives for improving the low-temperature performance of the battery, etc. Specifically, examples include vinylene carbonate (VC), fluoroethylene carbonate (FEC), ethylene sulfate (DTD), 1,3-propane sultone (PS), lithium difluoro(oxalato)phosphate (LiDFOP), lithium difluoro(oxalato)borate (LiDFOB), lithium bis(oxalato)borate (LiBOB), lithium difluorophosphate (LiPO2F2), etc.

[0086] In some embodiments, the conductivity of the electrolyte can be 9 - 15 mS / cm. The conductivity of the electrolyte is not only affected by the viscosity of the solvent, but also by the diffusion kinetics of the ions themselves. The conductivity of the electrolyte has a direct impact on the fast charging performance and the charging rate performance of the battery. When the conductivity of the electrolyte is lower than 9 mS / cm, the fast charging performance and the charging rate performance of the battery become low, and the energy density decreases. When the conductivity of the electrolyte is higher than 15 mS / cm, a large amount of heat is easily generated during the cyclic use of the battery, causing a significant temperature rise, which will reduce the use safety of the battery, and the fast charging performance also significantly decreases. The conductivity of the electrolyte is preferably 10 - 14 mS / cm.

[0087] [Separator]

[0088] The separator separates the positive electrode sheet from the negative electrode sheet, preventing internal short circuit of the battery, and at the same time enabling active ions to pass through the separator and move between the positive and negative electrodes. In the secondary battery of the present application, there is no particular limitation on the type of the separator, and any publicly known porous structure separator with good chemical stability and mechanical stability can be selected.

[0089] In some embodiments, the material of the separator can be selected from one or several of glass fiber membranes, non-woven membranes, polyethylene (PE) membranes, polypropylene (PP) membranes, polyvinylidene fluoride membranes, and multilayer composite membranes containing one or more of them. The separator can be a single-layer separator or a multilayer composite separator, without particular limitation. When the separator is a multilayer composite separator, the materials of each layer can be the same or different, without particular limitation.

[0090] In some 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.

[0091] In some embodiments, the secondary battery may include an outer package. The outer package can be used to encapsulate the electrode assembly and the electrolyte as described above.

[0092] In some embodiments, the outer package of the secondary battery can be a hard case, such as a hard plastic case, an aluminum case, a steel case, etc. The outer package 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. As plastics, polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS) can be listed, etc.

[0093] This application has no particular limitation on the shape of the secondary battery, and it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 is a secondary battery 5 with a square structure as an example.

[0094] In some embodiments, referring to Figure 2 , the outer package can include a housing 51 and a top cover assembly 53. Among them, the housing 51 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 51 has an opening communicating with the receiving cavity, and the top cover assembly 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet, and the separator can form an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0095] Battery module

[0096] In some embodiments, the secondary batteries can be assembled into a battery module. The number of secondary batteries contained in the battery module can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0097] Figure 3 is a battery module 4 as an example. Referring to Figure 3 , in the battery module 4, multiple secondary batteries 5 can be arranged in sequence along the length direction of the battery module 4. Of course, they can also be arranged in any other arbitrary manner. Further, the multiple secondary batteries 5 can be fixed by fasteners.

[0098] Optionally, the battery module 4 can further include a housing with a receiving space, and multiple secondary batteries 5 are accommodated in the receiving space.

[0099] In some embodiments, the above battery module can also be assembled into a battery pack. Those skilled in the art can select the number of battery modules contained in the battery pack according to the application and capacity of the battery pack.

[0100] Figure 4 and Figure 5 is a battery pack 1 as an example. Referring to Figure 4 and Figure 5, a battery pack 1 may include a battery box and a plurality of battery modules 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can cover the lower box body 3 and form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0101] Electrically operated device

[0102] This application also provides an electrical device, which includes the secondary battery, battery module, or battery pack provided by this application. The secondary battery, battery module, or battery pack 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, but is not limited to, 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, satellites, energy storage systems, etc.

[0103] As an electrical device, the secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0104] Figure 6 is an electrical device as an example. The electrical device is a pure electric vehicle, hybrid electric vehicle, or 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 battery module can be used.

[0105] As another example of an electrical device, it can be a mobile phone, tablet computer, laptop computer, etc. This electrical device usually requires being thin and light, and a secondary battery can be used as the power source.

[0106] Example

[0107] Hereinafter, 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 regarding specific technologies or conditions, they shall be carried out according to the technologies or conditions described in the literature in the art or according to the product specifications. For reagents or instruments not specified by the manufacturer, they are all conventional products that can be obtained through commercial procurement.

[0108] Embodiment 1

[0109] Preparation of positive electrode sheet

[0110] Taking LiNi as the positive electrode active material 0.5 Co 0.2 Mn 0.3O2, Super P (TIMCAL) as the conductive agent, and polyvinylidene fluoride (PVDF) as the binder were uniformly mixed in N-methylpyrrolidone (NMP) to prepare the positive electrode slurry. The solid content in the positive electrode slurry was 50 wt%, and in the solid components, LiNi 0.5 Co 0.2 Mn 0.3 O2, Super P, and PVDF had a mass ratio of 95:3:2. The positive electrode slurry was coated on the positive electrode current collector aluminum foil, dried at 85 °C, cold-pressed, and then trimmed, sliced, and slit. After that, it was dried under vacuum at 85 °C for 4 hours to prepare the positive electrode plate.

[0111] Preparation of negative electrode sheet

[0112] Graphite as the negative electrode active material, Super P (TIMCAL) as the conductive agent, and styrene-butadiene rubber as the binder were mixed uniformly in deionized water according to a mass ratio of 95:2:3 to prepare the negative electrode slurry. The solid content in the negative electrode slurry was 30 wt%. Through a conventional process, the negative electrode slurry was uniformly coated on the negative electrode current collector copper foil with a coating amount of 18 mg / dm of the binder per unit area of the negative electrode film layer, dried at 85 °C, and then cold-pressed, trimmed, sliced, and slit. Then, it was dried under vacuum at 120 °C for 12 hours to obtain the negative electrode plate. 2 of the coating amount, and then dried at 85 °C, cold-pressed, trimmed, sliced, and slit. Then, it was dried under vacuum at 120 °C for 12 hours to obtain the negative electrode plate.

[0113] Preparation of electrolyte

[0114] In a glove box under an argon atmosphere, dimethyl carbonate and diethyl carbonate as linear carbonates, ethyl acetate as a linear carboxylate ester, and ethylene carbonate (EC) as other solvents were mixed uniformly. Then, 1 M lithium hexafluorophosphate (LiPF6) was slowly added dropwise as the electrolyte salt, and stirred thoroughly. After the electrolyte salt was completely dissolved, 100 g of the electrolyte solution was obtained. Among them, the mass percentage of dimethyl carbonate in the electrolyte solution was 15%, the mass percentage of diethyl carbonate in the electrolyte solution was 15%, the mass percentage of ethyl acetate in the electrolyte solution was 15%, the mass percentage of lithium hexafluorophosphate (LiPF6) in the electrolyte solution was 12.5%, and the rest was ethylene carbonate (EC).

[0115] Preparation of battery

[0116] A 16-μm polyethylene film is used as the separator. The positive electrode plate obtained in the above steps, the separator, and the negative electrode plate obtained in the above steps are stacked in this order, with the separator placed between the positive electrode plate and the negative electrode plate to isolate the positive and negative electrodes. Then, it is wound to obtain a bare battery. An electrode tab is welded onto the bare battery, and the bare battery is placed into an aluminum case. The assembled battery is baked at 100 °C to remove water, and then the electrolyte obtained in the above steps is injected to obtain a non-charged battery. The non-charged battery is successively subjected to standing, hot and cold pressing, formation, shaping, and capacity testing to obtain a secondary battery (the thickness of the secondary battery is 4.0 mm, the width is 60 mm, and the length is 140 mm). For the obtained secondary battery, the conductivity, the adhesion of the negative electrode film layer, the charge rate performance, and the cycle capacity retention rate are measured. The composition of the secondary battery is shown in Table 1, and the performance measurement results of the secondary battery are shown in Table 2.

[0117] Measurement method

[0118] (1) Method for measuring the content of the binder per unit area of the negative electrode film layer

[0119] Use a scale to measure a negative electrode plate with an area of c. Place the negative electrode plate into a beaker containing deionized water and perform ultrasonic treatment. After about 1 hour, the negative electrode film layer completely detaches from the negative electrode current collector. Since the active material and conductive agent in the negative electrode film layer are insoluble in water, they can be removed by suction filtration. The binder soluble in water can be obtained by evaporating the water, and the mass m of the binder is weighed. The mass of the binder per unit area of the negative electrode film layer can be calculated as m / c, and the unit is: mg / dm 2 。

[0120] (2) Method for measuring the conductivity

[0121] The conductivity measurement of the electrolyte can be carried out with reference to HG-T 4067-2015.

[0122] (3) Method for measuring the adhesion between the negative electrode film layer and the negative electrode current collector

[0123] Take a fresh battery and discharge it at 1C to 2.8V. Then disassemble the battery and extract the negative electrode plate. After the electrolyte on the surface of the negative electrode is completely volatilized, measure the peel strength of the negative electrode film layer on the negative electrode plate according to the following steps. That is, take a negative electrode plate with a length of 20 cm and a width of 2 cm. One side of the electrode plate is adhered to a steel plate with double-sided tape, and the film layer is gently torn so that about 2 cm of the film layer is peeled off from the current collector. One end of the steel plate is fixed to the fixture of a tensile testing machine and clamped in the fixture of the tensile testing machine, and the peeled film layer is fixed to the clip at the upper end of the tensile testing machine. Use an Instron 3365 high-speed tensile testing machine to pull at a speed of 30 mm / min, and the film layer slowly peels off from the current collector. The tensile value when the tensile force is stable is displayed on the computer, which is the peel strength value of the film layer.

[0124] (4) Method for measuring 2C charge rate performance

[0125] At 25°C, connect the battery to a Neware charger, let it stand for 5 min, discharge it at 1C to 2.8V, let it stand for 5 min, charge it at 0.5C to 4.2V, record the charging capacity C0, let it stand for 5 min, discharge it at 1C to 2.8V, let it stand for 5 min, charge it at 1C to 4.2V, let it stand for 5 min, discharge it at 1C to 2.8V, let it stand for 5 min, charge it at 2C to 4.2V, record the charging capacity as C1, and 2C charge rate = C1 / C0.

[0126] (5) Cycle capacity retention rate

[0127] At 25°C, charge the battery at a constant current of 2C to 4.2V, then charge it at a constant voltage of 4.2V until the current is 0.05C, and then discharge it at a constant current of 1C to 2.8V. This is one charge-discharge cycle. Taking the capacity of the first discharge as 100%, calculate the capacity retention rate of the battery after 500 cycles. The capacity retention rate (%) of the battery after 500 cycles = discharge capacity of the 500th cycle / discharge capacity of the first cycle × 100%.

[0128] Examples 2 to 6 and Comparative Examples 1 to 4

[0129] Change the types and contents of the linear carbonates and linear carboxylates in the electrolyte, as well as the types and contents of the binders, as shown in Table 1. Except for this, operate in the same manner as in Example 1 to fabricate secondary batteries of Examples 2 to 6 and Comparative Examples 1 to 4. For the obtained secondary batteries, measure the conductivity, adhesion of the negative electrode film layer, charge rate performance, and cycle capacity retention rate. The performance measurement results of the secondary batteries are shown in Table 2.

[0130] Table 1

[0131]

[0132] Table 2

[0133] Conductivity (mS / cm) Adhesion force (N / m) 2C charge rate performance Cycle capacity retention rate Example 1 10.2 10.4 98.3％ 96.0％ Example 2 9.0 10.7 98.6％ 96.5％ Example 3 9.7 10.5 99.0％ 97.0％ Example 4 10.5 10.2 97.0％ 94.3％ Example 5 14.0 10.6 99.5％ 97.7％ Example 6 13.0 10.4 99.3％ 97.5％ Comparative example 1 10.0 10.9 86.3％ 82.4％ Comparative example 2 15.3 11.1 84.2％ 80.3％ Comparative example 3 10.0 8.2 80.0％ 75.4％ Comparative example 4 8.8 6.2 78.2％ 72.5％

[0134] As can be seen from the data in Table 1 and Table 2, in Examples 1 to 6, the total content of the linear carboxylate and the linear carbonate in the electrolyte and the content of the binder per unit area of the negative electrode film layer satisfy the relationship of 0.40A ≤ S ≤ 0.70A. The conductivity of the electrolyte and the adhesion between the negative electrode film layer and the negative electrode current collector are good. The 2C charge rate of the battery reaches more than 97%, and the cycle capacity retention rate reaches more than 94%. In contrast, in Comparative Examples 1 to 4, the total content of the linear carboxylate and the linear carbonate in the electrolyte and the content of the binder per unit area of the negative electrode film layer do not satisfy the relationship of 0.40A ≤ S ≤ 0.70A. The 2C charge rate is lower than 90%, and the cycle capacity retention rate is lower than 90%. From the comparison between Examples 1 to 6 and Comparative Examples 1 to 4, by adjusting the relationship between the total content of the linear carboxylate and the linear carbonate in the electrolyte and the content of the binder per unit area of the negative electrode film layer, it is possible to ensure that the viscosity of the electrolyte is suitable for the ion kinetic conduction performance and to ensure the stability of the negative electrode sheet during the cyclic use process, thus taking into account the fast charging performance and the cycle life of the secondary battery.

[0135] It should be noted that the present application is not limited to the above embodiments. The above embodiments are only examples, and embodiments having the same constitution and the same effect as the technical idea within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, within the scope not departing from the gist of the present application, various modifications that can be conceived by those skilled in the art to the embodiments and other ways constructed by combining some constituent elements of the embodiments are also included in the scope of the present application.

Claims

1. A secondary battery, characterized in that, it includes a negative electrode plate and an electrolyte, the negative electrode plate includes a negative electrode current collector and a negative electrode film layer formed on the negative electrode current collector, and the negative electrode film layer includes a binder, the electrolyte contains a linear carbonate and a linear carboxylate, When the sum of the mass percentage of the linear carbonate and the mass percentage of the linear carboxylate in the electrolyte is set to A%, and the content of the binder per unit area of the negative electrode film layer is set to S mg / dm 2 , S and A satisfy the following relational expression: 0.40A ≤ S ≤ 0.70A.

2. The secondary battery according to claim 1, characterized in that, 0.40A ≤ S ≤ 0.60A.

3. The secondary battery according to claim 1 or 2, characterized in that, the mass percentage of the linear carbonate in the electrolyte is 10% - 50%.

4. The secondary battery according to claim 1 or 2, characterized in that, the mass percentage of the linear carboxylate in the electrolyte is 10% - 20%.

5. The secondary battery according to claim 1 or 2, characterized in that, 20％≤A％≤70％。 6. The secondary battery according to claim 1 or 2, characterized in that, The content of the binder per unit area of the negative electrode film layer is 8 to 60 mg / dm 2 .

7. The secondary battery according to claim 1 or 2, characterized in that, the conductivity of the electrolyte is 9 - 15 mS / cm.

8. A battery module, characterized in that, It includes the secondary battery according to any one of claims 1 to 7.

9. A battery pack, characterized in that, It includes the battery module according to claim 8.

10. An electrical device, characterized in that, It includes at least one selected from the secondary battery according to any one of claims 1 to 7, the battery module according to claim 8, or the battery pack according to claim 9.

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