A lithium-ion secondary battery

Abstract

This invention relates to the technical field of lithium-ion batteries and provides a lithium-ion secondary battery. The electrolyte of this secondary battery includes a carboxylic acid ester solvent and an electrolyte lithium salt. The mass percentage of the carboxylic acid ester solvent is A, and the mass percentage of lithium bis(fluorosulfonyl)imide is B. The thickness of the first adhesive layer on the side of the separator closest to the positive electrode is C μm, and the thickness of the second adhesive layer on the side closest to the negative electrode is D μm. A, B, C, and D satisfy the relationship: 0.006 ≤ (D-C) / (A+B) ≤ 0.08. This lithium-ion secondary battery can effectively solve the problem of lithium-ion transport obstruction after separator adhesive layer blockage, improve the lithium-ion transport speed in the lithium battery, reduce the battery's DC internal resistance, improve the battery's cycle performance at low temperatures, and enhance the battery's overcharge safety performance.

Description

Technical Field

[0001] This invention relates to the technical field of lithium batteries, and specifically to a lithium-ion secondary battery. Background Technology

[0002] Lithium-ion batteries are widely used in 3C digital products, power tools, aerospace, energy storage, and electric vehicles due to their advantages such as high specific energy, no memory effect, and long cycle life. Lithium-ion batteries consist of four main materials: positive electrode material, negative electrode material, separator, and electrolyte. To improve battery performance, the separator typically uses a base film coated with ceramic and adhesive. The ceramic layer has good thermal stability, reducing thermal shrinkage of the separator and improving the safety performance of the lithium-ion battery. The adhesive layer enhances the adhesion between the electrode and the separator, ensuring smooth lithium-ion migration. In actual production, hot pressing is used to improve the contact between the electrode and the separator. However, the thermal expansion of the adhesive layer in the separator can cause pore blockage in the base film, hindering lithium-ion migration, resulting in poor electrolyte wetting, and lithium plating during cycling, posing a safety risk. Furthermore, when lithium-ion batteries cycle at low temperatures, base film blockage can cause reduced kinetics and increased DC internal resistance, leading to a drop in cycle life. Summary of the Invention

[0003] The purpose of this invention is to overcome the aforementioned problems in the prior art and provide a lithium-ion secondary battery. By adding a highly polar carboxylic acid ester solvent and a highly conductive lithium bis(fluorosulfonyl)imide to the electrolyte, and by adjusting the thickness difference between the first and second adhesive layers in the separator to meet a specific relationship, the problem of lithium-ion transport being blocked after the separator adhesive layer is blocked can be effectively solved, thereby improving the lithium-ion transport speed in the lithium battery, reducing the battery's DC internal resistance (DCR), improving the lithium battery's cycle performance at low temperatures, and improving the battery's overcharge safety performance.

[0004] To achieve the above objectives, the present invention provides a lithium-ion secondary battery, the battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator;

[0005] The electrolyte includes a carboxylic acid ester solvent; the mass percentage of the carboxylic acid ester solvent is denoted as A, based on the total mass of the electrolyte.

[0006] The electrolyte includes an electrolyte lithium salt, which includes lithium bis(fluorosulfonyl)imide; the mass percentage of lithium bis(fluorosulfonyl)imide based on the total mass of the electrolyte is denoted as B.

[0007] The separator includes a base film, a first adhesive layer, a ceramic layer, and a second adhesive layer; the first adhesive layer is disposed on the surface of the base film near the positive electrode, and the ceramic layer is disposed between the first adhesive layer and the base film; the second adhesive layer is disposed on the surface of the base film near the negative electrode; the thickness of the first adhesive layer is denoted as C μm, and the thickness of the second adhesive layer is denoted as D μm;

[0008] Among them, A, B, C and D satisfy the relationship: 0.006≤(DC) / (A+B)≤0.08.

[0009] The present invention, by adopting the above technical solution, has the following beneficial effects:

[0010] (1) The lithium-ion secondary battery provided by the present invention can reduce membrane pore blockage and effectively solve the problem of lithium-ion transport obstruction after membrane adhesive layer blockage, thereby improving the lithium-ion transport speed in the lithium battery.

[0011] (2) The lithium-ion secondary battery provided by the present invention can reduce the DC internal resistance (DCR) of the battery.

[0012] (3) The lithium-ion secondary battery provided by the present invention can improve the cycle performance of lithium batteries at low temperatures and improve the overcharge safety of batteries.

[0013] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and should be understood to include values ​​close to those ranges or values. For numerical ranges, endpoint values ​​of various ranges, endpoint values ​​of various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. In this document, unless otherwise specified, data ranges include endpoints. Attached Figure Description

[0014] Figure 1 The diagram shown is a schematic diagram of the diaphragm structure in an example of the present invention.

[0015] Reference numerals: 1-Separator; 11-Base film; 12-Ceramic layer; 13-First adhesive layer; 14-Second adhesive layer; 2-Negative electrode; 3-Positive electrode. Detailed Implementation

[0016] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0017] Unless otherwise defined, all scientific and technical terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art.

[0018] In this invention, the terms "battery", "lithium battery", "lithium-ion battery" and "lithium-ion secondary battery" all have the same meaning, referring to lithium-ion secondary batteries, which typically include electrode components (e.g., positive electrode, negative electrode and separator), a container (shell) housing the electrode components, and an electrolyte.

[0019] In this invention, the term "average particle size" refers to the average diameter of a number of (n≥20) lithium iron phosphate particles, which can be measured by transmission electron microscopy (TEM).

[0020] This invention provides a lithium-ion secondary battery, the battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator;

[0021] The electrolyte includes a carboxylic acid ester solvent; the mass percentage of the carboxylic acid ester solvent is denoted as A, based on the total mass of the electrolyte.

[0022] The electrolyte includes an electrolyte lithium salt, which includes lithium bis(fluorosulfonyl)imide; the mass percentage of lithium bis(fluorosulfonyl)imide based on the total mass of the electrolyte is denoted as B.

[0023] The separator includes a base film, a first adhesive layer, a ceramic layer, and a second adhesive layer; the first adhesive layer is disposed on the surface of the base film near the positive electrode, and the ceramic layer is disposed between the first adhesive layer and the base film; the second adhesive layer is disposed on the surface of the base film near the negative electrode; the thickness of the first adhesive layer is denoted as C μm, and the thickness of the second adhesive layer is denoted as D μm;

[0024] Among them, A, B, C and D satisfy the relationship: 0.006≤(DC) / (A+B)≤0.08.

[0025] In the lithium-ion secondary battery of the present invention, a highly polar carboxylic acid ester solvent and a high-conductivity lithium bisfluorosulfonylimide (LiFSI) are added to the electrolyte, and the separator structure is defined as a first adhesive layer + a ceramic layer + a base film + a second adhesive layer (the first adhesive layer is close to the positive electrode, and the second adhesive layer is close to the negative electrode). The carboxylic acid ester solvent and LiFSI can improve the kinetic performance of the electrolyte, effectively alleviate the poor electrolyte wetting caused by separator pore blockage, and improve the lithium-ion transport performance. A higher A+B value results in better electrolyte kinetics and wettability, but also increases side reactions at high temperatures, leading to poor high-temperature performance and a higher risk of thermal runaway. Simultaneously, adjusting the DC difference to increase D while keeping C constant maintains the (DC) / (A+B) ratio within the range of 0.006-0.08. This reduces the amount of active particles on the positive and negative electrodes encapsulated by the first and second adhesive layers, thus increasing lithium-ion migration channels. Furthermore, increasing A+B leads to a larger solvation radius and lower HOMO energy level in the solvation structure formed by lithium ions with solvent coordination, resulting in better heat and pressure resistance. Increasing the DC difference further enhances lithium-ion migration channels, ensuring unaffected lithium-ion transport through the solvation structure. The increased solvation structure also improves electrolyte stability at high temperatures, reduces decomposition and gas generation, and mitigates the poor high-temperature performance caused by increasing the A+B value.

[0026] In summary, to improve electrolyte kinetics, reduce the adverse effects of excessively high or low levels of carboxylic acid ester solvents and LiFSI, and mitigate the problem of poor lithium-ion transport caused by separator pore blockage, this invention has found that when A, B, C, and D satisfy the relationship 0.006 ≤ (DC) / (A+B) ≤ 0.08, electrolyte kinetics and wettability can be effectively improved, separator pore blockage can be reduced, and the problem of impaired lithium-ion transport after separator adhesive layer blockage can be effectively solved. This improves the lithium-ion transport rate in lithium batteries, reduces the battery's DC internal resistance (DCR), enhances the low-temperature cycle performance of lithium batteries, and improves the battery's overcharge safety.

[0027] In some embodiments, the diaphragm structure is as follows: Figure 1 As shown, the separator 1 includes a base membrane 11, a ceramic layer 12 is disposed on the surface of the base membrane 11 near the positive electrode 3, a first adhesive layer 13 is disposed on the surface of the ceramic layer 12 near the positive electrode 3, that is, the ceramic layer 12 is disposed between the first adhesive layer 13 and the base membrane 11; a second adhesive layer 14 is disposed on the surface of the base membrane 11 near the negative electrode 2.

[0028] In some implementations, the ratio (DC) / (A+B) can be, for example, 0.006, 0.0062, 0.0065, 0.0068, 0.007, 0.008, 0.009, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.065, 0.07, 0.0769, 0.078, or 0.08, preferably 0.02 ≤ (DC) / (A+B) ≤ 0.065. When calculating the ratio (DC) / (A+B), the units for D and C are μm, and only the numbers are considered in the calculation of D and C; A and B are percentages, and only the numbers before the percentage sign are considered in the calculation. For example, if D = 3.0 μm, C = 2.0 μm, A = 40%, and B = 10%, then (DC) / (A+B) = (3-2) / (40+10) = 0.02.

[0029] In some implementations, D and C satisfy the relationship: 0.1 ≤ DC ≤ 1.5, and the difference of DC is, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, or 1.5. Preferably, the thickness C of the first adhesive layer is 0.5 μm to 2.5 μm, for example, C is 0.5 μm, 0.6 μm, 0.8 μm, 1.0 μm, 1.2 μm, 1.5 μm, 1.8 μm, 2.0 μm, or 2.5 μm. Preferably, the thickness D of the second adhesive layer is 0.6 μm to 4.0 μm, for example, D is 0.6 μm, 0.8 μm, 1.0 μm, 1.2 μm, 1.5 μm, 1.8 μm, 2.0 μm, 2.5 μm, 3.0 μm, 3.5 μm, or 4.0 μm.

[0030] Under the condition that A, B, C, and D satisfy 0.006≤(DC) / (A+B)≤0.08, further adjusting the difference in DC and ensuring that the values ​​of D and C are within the above range can control the thickness of the second adhesive layer to be greater than that of the first adhesive layer, and keep the thickness difference within a suitable range. This reduces the phenomenon of pore blockage in the separator adhesive layer, lowers the DCR of the battery, improves the lithium-ion transport performance, and thus improves the fast-charging cycle performance of the battery. It also avoids the situation where the difference in DC is too small, i.e., when D and C are both large, the thickness of the first and second adhesive layers is large, resulting in a longer lithium-ion migration channel, reduced kinetics, and a sharp drop in lithium plating during low-temperature cycling. It also avoids the situation where the difference in DC is too large, i.e., when D is large and C is small, resulting in weak adhesion between the positive electrode and the separator, causing ion channels to be blocked and failing to effectively improve lithium-ion transport.

[0031] In some embodiments, A and B satisfy the relationship: 13 ≤ A + B ≤ 81; for example, the value of A + B can be 13, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 80.5, or 81. Preferably, the mass percentage A of the carboxylic acid ester solvent is 10% to 70%, for example, 10%, 20%, 30%, 40%, 50%, 60%, or 70%, more preferably 20% to 50%. Preferably, the mass percentage B of the lithium bis(fluorosulfonyl)imide is 1.5% to 14%, for example, 1.5%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 10.5%, 11%, 12%, 13%, 13.5%, or 14%, more preferably 3.0% to 11%.

[0032] Under the condition that A, B, C, and D satisfy 0.006≤(DC) / (A+B)≤0.08, further adjusting the value of A+B, and ensuring that the values ​​of A and B are within the above range, can better improve the kinetics and wettability of the electrolyte, enhance the lithium-ion transport performance when the separator is blocked, thereby improving the fast-charge cycle performance of the battery and reducing lithium plating. It also avoids the situation where the value of A+B is too high (both A and B are too high), which results in poor high-temperature performance of the battery, reducing the risk of battery combustion and thus improving battery safety. Furthermore, it avoids the situation where the value of A+B is too low (both A and B are too low), which results in insufficient electrolyte kinetics and an inability to improve lithium-ion transport.

[0033] In some embodiments, the carboxylic acid ester solvent includes at least one of MA (methyl acetate), EA (ethyl acetate), EP (ethyl propionate), and EB (ethyl butyrate).

[0034] In some embodiments, the electrolyte further includes a carbonate solvent, which includes at least two selected from ethylene carbonate, propylene carbonate, methyl ethyl carbonate, dimethyl carbonate, and diethyl carbonate. Under the condition that A, B, C, and D satisfy 0.006 ≤ (DC) / (A+B) ≤ 0.08, adding a carbonate solvent to the electrolyte can make the interface formed at the negative electrode more stable, improve the battery's cycle performance, and enhance its safety performance.

[0035] In some embodiments, the base film comprises a polyolefin material, such as at least one selected from polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyimide, and PE and PP composite structures. Preferably, the base film thickness is 3 μm-20 μm, and the porosity of the base film is 20%-60%.

[0036] In some embodiments, the ceramic layer material comprises one or more of the following: alumina, boehmite, magnesium oxide, magnesium hydroxide, barium sulfate, zinc oxide, calcium oxide, silicon dioxide, silicon carbide, and nickel oxide. Preferably, the thickness of the ceramic layer is 1 μm to 5 μm. Preferably, the Dv50 of the ceramic layer material is 0.5 μm to 5.0 μm.

[0037] In some embodiments, the first and second adhesive layers are conventional spray-applied adhesive layers. For example, the first and second adhesive layers independently include one or more of the following: polytetrafluoroethylene, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene modified and copolymerized forms, polyimide, butyl acrylate-acrylonitrile copolymer, polyacrylonitrile, aramid resin, polymethyl methacrylate, polyacrylic acid, ethylene-acrylic acid copolymer, polymethyl acrylate, polyethyl acrylate, styrene-butadiene rubber (SBR), sodium carboxymethyl cellulose, polyvinyl alcohol and its copolymerized modified polyvinyl alcohol.

[0038] In some embodiments, the peel force between the positive electrode and the separator is denoted as X N / m, and the peel force between the negative electrode and the separator is denoted as Y N / m; wherein B, X, and Y satisfy the relationship: 1.5 ≤ B / (X / Y) ≤ 14, and the ratio of B / (X / Y) can be, for example, 1.5, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, or 14. When calculating the ratio of B / (X / Y), B is a percentage, and only the number before the percentage sign is considered in the calculation; the units of X and Y are N / m, and only the numbers are considered in the calculation of X and Y. For example, when B = 5%, X = 5 N / m, and Y = 4 N / m, B / (X / Y) = 5 / (5 / 4) = 4. The test method for the peel force between the electrode and the separator includes performing a T-type peel test using a universal tensile testing machine.

[0039] The X / Y ratio represents the ratio between the peel force between the positive and negative electrodes and the separator. Since a ceramic layer is also provided on the positive electrode side of this invention, the peel force between the positive electrode and the separator is greater, and the X / Y ratio is usually greater than 1. However, the larger the X / Y ratio, the greater the difference in adhesion between the positive and negative electrodes and the separator. This results in a larger X value, which means a smaller thickness of the first adhesive layer. Consequently, the positive electrode particles are surrounded by the separator adhesive layer, blocking the lithium-ion migration channels, reducing kinetics, and causing a sharp drop in lithium plating during low-temperature cycling. Therefore, under the condition that A, B, C, and D satisfy 0.006≤(DC) / (A+B)≤0.08, further adjusting the relationship between LiFSI content and peel force in the electrolyte to satisfy that B, X, and Y satisfy 1.5≤B / (X / Y)≤14 can improve the kinetics of the electrolyte and enhance the low-temperature cycle performance of the battery; avoid the B / (X / Y) ratio being too small, the B content being too low, or the X / Y ratio being too large, which would affect the kinetics of the electrolyte; and avoid the B / (X / Y) ratio being too large, which would result in poor high-temperature performance of the battery.

[0040] In some implementations, X satisfies 0.2 N / m ≤ X ≤ 10 N / m, for example, X can be 0.2 N / m, 0.5 N / m, 0.8 N / m, 1 N / m, 2 N / m, 3 N / m, 4 N / m, 5 N / m, 6 N / m, 7 N / m, 8 N / m, 9 N / m, or 10 N / m. In some implementations, Y satisfies 0.1 N / m ≤ Y ≤ 10 N / m, for example, Y can be 0.1 N / m, 0.2 N / m, 0.5 N / m, 1 N / m, 2 N / m, 3 N / m, 4 N / m, 5 N / m, 6 N / m, 7 N / m, 8 N / m, 9 N / m, or 10 N / m. When the relationship between LiFSI content and peeling force in the electrolyte satisfies the condition that B, X, and Y satisfy 1.5≤B / (X / Y)≤14, further adjusting the values ​​of X and Y within the above range can improve the adhesion between the positive and negative electrodes and the separator, improve the integrity of the electrochemical system, reduce the resistance during lithium ion migration, improve the lithium ion transport performance, and thus improve the fast charging performance of the battery.

[0041] In some embodiments, the porosity of the negative electrode is denoted as E; wherein B, D, and E satisfy the relationship: 0.3 ≤ B / (D+E) ≤ 12; the value of B / (D+E) can be, for example, 0.3, 0.5, 0.8, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, preferably 0.85 ≤ B / (D+E) ≤ 8.5. Preferably, the porosity E of the negative electrode is 20% to 40%, for example, E can be 20%, 22%, 25%, 28%, 30%, 32%, 35%, 38%, 40%. When calculating the ratio B / (D+E), B is a percentage, and only the digits before the percentage sign are considered in the calculation; the unit of D is μm, and only the digits are considered in the calculation of D; E is a percentage, but it is written in decimal form when calculated. For example, when B = 5%, D = 1 μm, and E = 20%, B / (D+E) = 5 / (1+0.2) = 4.17.

[0042] The test method for the porosity of the negative electrode sheet refers to the measurement method and conditions for the true density of powder in the national standard GB / T 5162-2006 "Method for Measurement of True Density of Powder".

[0043] The thinner the second adhesive layer of the separator is after hot pressing, the tighter it adheres to the negative electrode, which further clogs the pores (i.e., the porosity of the separator decreases, meaning D decreases, and E also decreases). This obstructs the electrolyte wetting path, leading to poorer kinetics and a drop in battery performance during low-temperature cycling. Therefore, under the condition that A, B, C, and D satisfy 0.006≤(DC) / (A+B)≤0.08, B, D, and E should be further adjusted to satisfy 0.3≤B / (D+E)≤12. The larger B / (D+E) is, the higher the LiFSI content B, which is beneficial to improving electrolyte kinetics. At the same time, increasing D+E can reduce the problem of pore clogging on the negative electrode side of the separator. However, if B / (D+E) is too large, for example, B / (D+E) exceeds 12, the LiFSI content B is too high, which can easily lead to poor battery overcharge safety performance. When B / (D+E) is too small, meaning the LiFSI content is low, and D+E is also reduced, insufficient electrolyte kinetics can easily occur, leading to pore blockage of the negative electrode diaphragm. This makes it impossible to mitigate the problems of low-temperature cycle failure, increased DCR, and overcharge safety risks caused by diaphragm blockage. Therefore, when B, D, and E satisfy the condition 0.3≤B / (D+E)≤12, electrolyte kinetics can be improved, electrolyte wettability can be enhanced, and problems such as decreased internal resistance and cycle failure caused by diaphragm blockage can be reduced.

[0044] In some embodiments, the negative electrode sheet includes a negative electrode active material, and the Dv50 of the negative electrode active material is denoted as F; the first adhesive layer includes a first polymer, and the average particle size of the first polymer is denoted as G; G and F satisfy: 50*G < F. Preferably, the Dv50 of the negative electrode active material is 5 μm to 20 μm, for example, the Dv50 can be 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, 18 μm, 20 μm; preferably, the average particle size G of the first polymer is 0.1 μm to 0.5 μm, for example, G can be 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm. When calculating 50*G < F, the unit of G is μm and the unit of F is μm, and the unit can be not considered in the calculation. For example, G = 0.2 μm, F = 15 μm, 50*0.2 < 15.

[0045] In the first adhesive layer of the separator, when 50 times the average particle size of the first polymer is greater than the D50 of the negative electrode material (i.e., 50*G > F, when the condition of 50*G < F of the present invention is not satisfied), after hot pressing, the first polymer undergoes thermal swelling and wraps the negative electrode active material particles, resulting in the blockage of the lithium insertion channels. Lithium ions obtain electrons on the surface of the negative electrode and are reduced to lithium单质, and further form lithium dendrites piercing the separator as the battery charges, leading to battery fire and triggering safety accidents. Therefore, when the present invention adjusts 50*G < F, it can further reduce lithium plating in the battery, improve the cycle performance of the battery, and improve the safety performance of the battery.

[0046] In some embodiments, the coverage rate of the first adhesive layer is 5% to 96%, for example, it can be 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 96%, and more preferably 20 - 40%.

[0047] In some embodiments, the coverage rate of the second adhesive layer on the base film is 5% to 96%, for example, it can be 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 96%, and more preferably 20 - 40%.

[0048] The test method for the coverage rate of the first adhesive layer and the coverage rate of the second adhesive layer includes taking a separator of a unit area and testing the covered area of the adhesive layer on the base film with a scanning electron microscope, which is denoted as the coverage rate.

[0049] When A, B, C, and D satisfy the condition of 0.006 ≤ (D - C) / (A + B) ≤ 0.08, when further adjusting the coverage rate of the first adhesive layer and the coverage rate of the second adhesive layer within the above range, the problem of pore blockage of the separator adhesive layer can be reduced, the DCR of the battery can be effectively reduced, and the problems of hindered lithium ion migration caused by pore blockage of the adhesive layer, resulting in low-temperature cycle diving and high overcharge safety risks, can be reduced.

[0050] In some embodiments, the electrolyte includes a sulfate additive; the mass percentage of the sulfate additive is 0.1% to 2% based on the total mass of the electrolyte, for example, 0.1%, 0.2%, 0.5%, 0.8%, 1.0%, 1.2%, 1.4%, 1.5%, 1.6%, 1.8%, or 2%. Under the condition that A, B, C, and D satisfy 0.006 ≤ (DC) / (A+B) ≤ 0.08, the addition of the sulfate additive to the electrolyte allows the sulfate additive to react with the electrolyte to generate lithium sulfite, which increases ionic conductivity, reduces battery impedance, and facilitates smoother lithium-ion transport within the battery, thereby improving the battery's high-temperature fast-charging performance.

[0051] In some embodiments, the sulfate additive includes one or more of the following compounds:

[0052]

[0053]

[0054] In some embodiments, the electrolyte includes carbonate additives; the mass percentage of carbonate additives is 0.1% to 2% based on the total mass of the electrolyte, for example, 0.1%, 0.2%, 0.5%, 0.8%, 1.0%, 1.2%, 1.4%, 1.5%, 1.6%, 1.8%, or 2%. Under the condition that A, B, C, and D satisfy 0.006 ≤ (DC) / (A+B) ≤ 0.08, the addition of carbonate additives to the electrolyte allows the carbonate additives to preferentially reduce the SEI film at the negative electrode compared to ethyl acetate solvent, suppressing side reactions on the electrode surface, reducing the formation of side reaction products, and improving the high-temperature performance of the battery.

[0055] In some embodiments, the carbonate additive includes at least one of vinylene carbonate (VC), fluoroethylene carbonate (FEC), ethylene ethylene carbonate (VEC), trifluoromethyl ethylene carbonate (TFEC), 1,2-difluoroethylene carbonate (DFEC), and ethylene dicarbonate (EC).

[0056] In some embodiments, the electrolyte further includes a lithium salt additive; the mass percentage of the lithium salt additive, based on the total mass of the electrolyte, is 0.1% to 1%, for example, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or 1%. Preferably, the lithium salt additive includes at least one of lithium difluorophosphate (LiPO2F2), lithium bis(oxalato)borate (LiBOB), lithium difluorooxalato)borate (LiODFB), lithium difluorooxalato)phosphate (LiODFP), lithium tetrafluoroborate (LiBF4), and lithium tetrafluorooxalato)phosphate (LiTFOP). When A, B, C, and D satisfy 0.006 ≤ (DC) / (A+B) ≤ 0.08, adjusting the electrolyte by adding the above-mentioned lithium salt additive can improve the ionic conductivity of the electrolyte, making the migration rate of lithium ions in the electrolyte faster, thereby improving the fast-charging performance of the battery.

[0057] In some embodiments, the electrolyte lithium salt further includes lithium hexafluorophosphate.

[0058] In some embodiments, the lithium-ion secondary battery includes a positive electrode sheet comprising a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector; the positive active material layer comprises a positive active material; and the positive active material comprises lithium iron phosphate. Preferably, the lithium iron phosphate has a Dv50 of 0.5 μm-1.5 μm and a specific surface area BET of 5 m². 2 / g-15m 2 / g.

[0059] In some embodiments, the lithium-ion secondary battery includes a negative electrode sheet comprising a negative electrode current collector and a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector; the negative electrode active material layer comprises a negative electrode active material; the negative electrode active material comprises at least one of artificial graphite, natural graphite, hard carbon, soft carbon, mesophase carbon microspheres, silicon-based negative electrode material, and lithium-containing metal composite oxide material.

[0060] In some implementations, the lithium-ion secondary battery can be a power lithium battery, a consumer lithium battery, or an energy storage lithium battery.

[0061] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0062] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0063] The present invention will now be described in detail with reference to specific embodiments, which are intended to understand rather than limit the invention.

[0064] The batteries in the embodiments and comparative examples of this invention were prepared according to the following preparation method. The differences from those in Examples 1-1 are shown below.

[0065] Example 1-1

[0066] (1) Preparation of the positive electrode sheet:

[0067] Lithium iron phosphate (LiFePO4), conductive carbon (Super-P), and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of approximately 97:1:2 in the solvent N-methylpyrrolidone and stirred until homogeneous to obtain a positive electrode slurry. The slurry was coated onto an aluminum foil with a thickness of approximately 12 μm, dried, cold-pressed, and then cut and welded with tabs to obtain the positive electrode sheet.

[0068] (2) Preparation of negative electrode sheet:

[0069] Graphite, conductive carbon (Super-P), styrene-butadiene rubber, and sodium carboxymethyl cellulose (CMC) were mixed in deionized water at a mass ratio of approximately 96.5:1.5:1:1 and stirred until homogeneous to obtain a negative electrode slurry. The slurry was coated onto a copper foil with a thickness of approximately 8 μm, dried, cold-pressed, and then cut and welded with tabs to obtain the negative electrode sheet.

[0070] (3) Electrolyte preparation

[0071] In an argon-filled glove box (moisture <10ppm, oxygen <1ppm), ethylene carbonate (EC), ethyl methyl carbonate (EMC), and carboxylic acid ester solvent EA are mixed evenly (wherein, EA accounts for A% of the electrolyte, EC accounts for 30% of the electrolyte, and the content of EMC in the electrolyte is determined by calculation). Then, 14.5wt% fully dried lithium hexafluorophosphate, 2.5wt% vinylene carbonate, and other additives (specific amounts and selections are shown in Table 1) based on the total mass percentage of the electrolyte are quickly added to the mixed solution, and the mixture is stirred evenly to obtain the electrolyte.

[0072] (4) Preparation of lithium-ion batteries

[0073] The prepared positive electrode, separator, and negative electrode are stacked using a Z-type stacking machine to obtain bare cells without electrolyte injection. The bare cells are then placed in outer packaging foil, and the prepared electrolyte is injected into the dried bare cells. After vacuum sealing, settling, formation, secondary sealing, and sorting, the desired lithium-ion battery is obtained. The thicknesses C of the first adhesive layer and D of the second adhesive layer in the separator are shown in Table 1.

[0074] (5) Test the battery

[0075] (i) -10℃ Cyclic Test

[0076] The batteries obtained in the above embodiments and comparative examples were placed in an environment of (-10±2)℃ and left to stand for 2-3 hours. When the battery body reached (-10±2)℃, the battery was charged at 0.5C to 3.65V and cut off at 0.05C, left to stand for 5 minutes, discharged at 0.5C to 2.5V, and left to stand for 5 minutes. The highest discharge capacity of the first 3 cycles was recorded as the initial capacity Q. When the required number of cycles was reached, the discharge capacity Q1 of the battery in the last cycle was recorded. The calculation formula used is as follows: Capacity retention rate (%) = Q1 / Q × 100%.

[0077] (ii) -10℃ 50% SOC DCR test

[0078] The battery was placed in an environment of -10±2℃ and left to stand for 120 minutes. It was then discharged at a constant current of 0.5C to 2.5V, left to stand for 30 minutes, and charged at a constant current and constant voltage of 0.5C to 3.65V with a cutoff current of 0.05C. After another 30 minutes of rest, it was discharged at a constant current of 0.5C to 2.5V, and the discharge capacity C0 was recorded. After another 30 minutes of rest, it was charged at a constant current and constant voltage of 1C to 3.65V with a cutoff current of 0.05C. After another 30 minutes of rest, it was discharged at a 1C to 50% SOC, i.e., C0*(1-50% SOC). In an environment of -10℃±1℃, it was left to stand for 120 minutes and discharged at 3C for 10 seconds (sampling time 100ms). The voltage at the end of the rest period was recorded as V1, the voltage at the end of the discharge period was recorded as V2, and the actual discharge current was recorded as I. The formula for calculating the 50% SOCDCR at 25℃ is: DCR=(V1-V2) / I;

[0079] (iii) 45℃ Cyclic Test

[0080] Place the battery in an environment of 45±2℃ and let it rest for 180 minutes. Then, discharge it at a constant current of 1C to 2.5V, let it rest for 30 minutes, charge it at a constant current and constant voltage of 1C to 3.65V, cut off the current at 0.05C, let it rest for 30 minutes, discharge it at a constant current of 1C to 2.5V, and record the discharge capacity C0. Let it rest for 30 minutes, charge it at a constant current and constant voltage of 1.0C0 to 3.65V, cut off the current at 0.05C0, let it rest for 30 minutes, and discharge it at a constant current of 1.0C0 to 2.5V. When the cycle reaches n, record the discharge capacity Cn of the last cycle. The formula for calculating the capacity retention rate of the nth cycle is: Capacity retention rate (%) = Cn / C0 × 100%

[0081] (iv) 25℃ 5.5V overcharge test

[0082] Place the battery in an environment of 25±2℃ and leave it for 180 minutes. Then overcharge it to 5.5V with a constant current of 1C and observe whether the battery catches fire during this process.

[0083] The following examples were conducted, with the following changes: in Example 1-1 to Example 1-4 and Comparative Example 1, the content of EA was changed; in Examples 1-5 to Example 1-7, the types of carboxylic acid esters were changed (MA in Example 1-5, EP in Example 1-6, and EB in Example 1-7); in Example 2, the content of LiFSI was changed; in Example 3, the thickness of the first adhesive layer and the thickness of the second adhesive layer were changed; and in Comparative Example 2, EA and LiFSI were not added. The specific differences are shown in Table 1.

[0084] Table 1

[0085]

[0086]

[0087] Note: " / " indicates that the corresponding parameter was not tested.

[0088] As shown in Table 1, when A, B, C, and D satisfy 0.006≤(DC) / (A+B)≤0.08, they can effectively improve the cycle performance of lithium batteries at low and high temperatures, reduce the DC internal resistance (DCR) of the battery, and improve the overcharge safety performance of the battery.

[0089] The process was carried out in accordance with Examples 1-1. In Example 4, the peeling force of the positive and negative electrode sheets was adjusted by changing the thickness of the adhesive layer. The specific differences are shown in Table 2.

[0090] Table 2

[0091]

[0092]

[0093] As shown in Table 2, adjusting the relationship between the LiFSI content in the electrolyte and the peeling force to satisfy B, X, and Y as 1.5≤B / (X / Y)≤14 can improve the kinetics of the electrolyte and enhance the low-temperature and high-temperature cycling performance of the battery.

[0094] The experiment was conducted according to Examples 1-1. In Example 5, the thickness of the second adhesive layer and the porosity of the negative electrode were changed. The specific differences are shown in Table 3.

[0095] Table 3

[0096]

[0097] As can be seen from Table 3, when B, D, and E satisfy the condition of 0.3 ≤ B / (D + E) ≤ 12, the kinetics of the electrolyte can be better improved, the wettability of the electrolyte can be enhanced, problems such as the reduction of internal resistance caused by diaphragm pore blockage and cycle diving can be reduced, and the low-temperature cycle performance and high-temperature cycle performance of the battery can be improved.

[0098] Referring to Example 1-1, in 6 groups of examples, the first adhesive layer coverage rate and the second adhesive layer coverage rate are adjusted by changing the thickness during the coating of the adhesive layer, and in 7 groups of examples, the Dv50 of the negative active material and the average particle size of the first polymer are changed. The specific differences are shown in Table 4.

[0099] Table 4

[0100]

[0101]

[0102] As can be seen from Table 4, when the coverage rate of the first adhesive layer and the coverage rate of the second adhesive layer in the 6 groups of examples are within the preferred range, the DCR of the battery can be reduced, and the problem of low-temperature cycle diving caused by the hindered migration of lithium ions due to the blockage of the adhesive layer pores can be reduced. In the 7 groups of examples, when 50*G < F is adjusted, the lithium plating of the battery can be further reduced, the cycle performance of the battery can be improved, and the safety performance of the battery can be improved.

[0103] Carried out by referring to Example 1-1, in the 8 groups of examples, the types of additives in the electrolyte are changed. The specific differences are shown in Table 5.

[0104] Table 5

[0105]

[0106] Note: " / " means that the corresponding additive is not added.

[0107] As can be seen from Table 5, adding sulfate additives, carbonate additives, and lithium salt additives to the electrolyte can improve the low-temperature and high-temperature cycle performance of the battery and reduce the internal resistance.

[0108] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element. Furthermore, it should be noted that the scope of the methods and apparatuses in the embodiments of this application is not limited to performing functions in the order shown or discussed, but may also include performing functions substantially simultaneously or in the reverse order, depending on the functions involved. For example, the described methods may be performed in a different order than described, and various steps may be added, omitted, or combined. Additionally, features described with reference to certain examples may be combined in other examples.

[0109] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A lithium-ion secondary battery, characterized in that, The battery includes a positive electrode, a negative electrode, an electrolyte, and a separator; The electrolyte includes a carboxylic acid ester solvent; the mass percentage of the carboxylic acid ester solvent is denoted as A, based on the total mass of the electrolyte. The electrolyte includes an electrolyte lithium salt, which includes lithium bis(fluorosulfonyl)imide; the mass percentage of lithium bis(fluorosulfonyl)imide based on the total mass of the electrolyte is denoted as B. The separator includes a base film, a first adhesive layer, a ceramic layer, and a second adhesive layer; the first adhesive layer is disposed on the surface of the base film near the positive electrode, and the ceramic layer is disposed between the first adhesive layer and the base film; the second adhesive layer is disposed on the surface of the base film near the negative electrode. The thickness of the first adhesive layer is denoted as C μm, and the thickness of the second adhesive layer is denoted as D μm; Among them, A, B, C and D satisfy the relationship: 0.006≤(DC) / (A+B)≤0.

08.

2. The lithium-ion secondary battery according to claim 1, characterized in that, A, B, C, and D satisfy the following relationship: 0.02 ≤ (DC) / (A + B) ≤ 0.

065.

3. The lithium-ion secondary battery according to claim 1, characterized in that, D and C satisfy the relationship: 0.1≤DC≤1.

5.

4. The lithium-ion secondary battery according to claim 1, characterized in that, A and B satisfy the relationship: 13≤A+B≤81.

5. The lithium-ion secondary battery according to claim 2, characterized in that, The mass percentage A of the carboxylic acid ester solvent is 10% to 70%.

6. The lithium-ion secondary battery according to claim 2, characterized in that, The mass percentage B of the lithium difluorosulfonyl imide is 1.5% to 14%.

7. The lithium-ion secondary battery according to claim 2, characterized in that, The thickness C of the first adhesive layer is 0.5 μm to 2.5 μm.

8. The lithium-ion secondary battery according to claim 2, characterized in that, The thickness D of the second adhesive layer is 0.6 μm to 4.0 μm.

9. The lithium-ion secondary battery according to claim 1, characterized in that, The peel force between the positive electrode and the separator is denoted as XN / m, and the peel force between the negative electrode and the separator is denoted as YN / m; wherein, B, X and Y satisfy the relationship: 1.5≤B / (X / Y)≤14.

10. The lithium-ion secondary battery according to claim 9, characterized in that, X satisfies 0.2 N / m ≤ X ≤ 10 N / m.

11. The lithium-ion secondary battery according to claim 9, characterized in that, Y satisfies 0.1 N / m ≤ Y ≤ 10 N / m.

12. The lithium-ion secondary battery according to claim 1, characterized in that, The porosity of the negative electrode sheet is denoted as E; where B, D and E satisfy the relationship: 0.3≤B / (D+E)≤12.

13. The lithium-ion secondary battery according to claim 12, characterized in that, The porosity E of the negative electrode sheet is 20%~40%.

14. The lithium-ion secondary battery according to claim 1, characterized in that, The negative electrode sheet includes a negative electrode active material, the Dv50 of which is denoted as F; the first adhesive layer includes a first polymer, the average particle size of which is denoted as G; G and F satisfy: 50 × G <F。 15. The lithium-ion secondary battery according to claim 14, characterized in that, The Dv50 F of the negative electrode active material is 5 μm to 20 μm.

16. The lithium-ion secondary battery according to claim 14, characterized in that, The average particle size G of the first polymer is 0.1 μm to 0.5 μm.

17. The lithium-ion secondary battery according to any one of claims 1-16, characterized in that, The coverage of the first adhesive layer and the second adhesive layer on the base film is independently 5% to 96%.

18. The lithium-ion secondary battery according to any one of claims 1-16, characterized in that, The coverage of the first adhesive layer and the second adhesive layer on the base film is independently 20%-40%.

19. The lithium-ion secondary battery according to any one of claims 1-16, characterized in that, The carboxylic acid ester solvent includes at least one of methyl acetate, ethyl acetate, ethyl propionate, and ethyl butyrate.

20. The lithium-ion secondary battery according to any one of claims 1-16, characterized in that, The electrolyte also includes a carbonate solvent, which includes at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, dimethyl carbonate, and diethyl carbonate.

21. The lithium-ion secondary battery according to any one of claims 1-16, characterized in that, The electrolyte includes a sulfate ester additive; the sulfate ester additive has a mass percentage content of 0.1% to 2% based on the total mass of the electrolyte.

22. The lithium-ion secondary battery according to claim 21, characterized in that, The sulfate ester additive includes one or more of the following compounds: (Formula I-1), (Formula I-2), (Formula I-3), (Formula I-4), (Formula I-5), (Formula I-6), (Formula I-7), (Formula I-8), (Formula I-9), (Formula I-10), (Formula I-11), (Equation I-12).

23. The lithium-ion secondary battery according to any one of claims 1-16, characterized in that, The electrolyte includes carbonate additives; the carbonate additives account for 0.1% to 2% of the total mass of the electrolyte.

24. The lithium-ion secondary battery according to claim 23, characterized in that, The carbonate additives include at least one of fluoroethylene carbonate, ethylene carbonate, ethylene ethylene carbonate, trifluoromethyl ethylene carbonate, 1,2-difluoroethylene carbonate, and diethylene carbonate.

25. The lithium-ion secondary battery according to any one of claims 1-16, characterized in that, The electrolyte also includes lithium salt additives; the mass percentage of lithium salt additives is 0.1% to 1% based on the total mass of the electrolyte.

26. The lithium-ion secondary battery according to claim 25, characterized in that, The lithium salt additive includes at least one of lithium difluorophosphate, lithium bis(oxalate)borate, lithium difluorooxalateborate, lithium difluorooxalate phosphate, lithium difluorooxalate phosphate, and lithium tetrafluorooxalate phosphate.

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