A lithium-ion secondary battery

Abstract

The present invention relates to the technical field of lithium batteries and provides a lithium-ion secondary battery. The lithium-ion secondary battery includes a positive electrode sheet, a negative electrode sheet, and an electrolyte; the positive electrode sheet includes lithium iron phosphate primary particles with a Dv50 of a1 μm and lithium iron phosphate secondary particles with a Dv50 denoted as a2, where a1 < a2; based on the total mass of the positive electrode active material, the mass ratio of the lithium iron phosphate primary particles is denoted as w1; the electrolyte includes a first solvent, and the first solvent includes a linear organic solvent having C1-C4; based on the total mass of the solvent, the mass ratio of the first solvent is denoted as S1; wherein, a1, w1, and S1 satisfy the following relationship: By controlling the addition amount of the first solvent, when the Dv50 and mass ratio of the lithium iron phosphate primary particles of the positive electrode material meet the above conditions, the fast charging performance of the battery can be improved, the limit power of the battery can be increased, the cycle performance and capacity retention rate of the battery can be improved, the temperature rise during fast charging can be reduced, and the battery swelling can be reduced.

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 have advantages over traditional batteries, including higher energy density, longer cycle life, no pollution, and higher operating voltage. Therefore, they are widely used in portable electronic devices, energy storage, automotive power, and many other fields. With the increasing adoption of electric vehicles, the demand for fast-charging and safety performance of power batteries is growing.

[0003] To improve the fast-charging and safety performance of batteries, mainstream power batteries still use lithium iron phosphate (LFP) batteries, which are attributed to their higher safety performance compared to ternary lithium batteries. However, LFP cathodes have some inherent drawbacks, such as lower ion and electron conductivity, which results in higher battery temperature rise during fast charging, thus limiting rate performance and fast-charging capabilities. Summary of the Invention

[0004] The purpose of this invention is to overcome the aforementioned problems in the prior art and provide a lithium-ion secondary battery. In this lithium-ion secondary battery, by using a first solvent and controlling the amount of the first solvent added, and simultaneously controlling the Dv50 and mass ratio of the primary lithium iron phosphate particles of the positive electrode material to meet certain requirements... At the same time, it can improve battery fast charging performance, increase battery maximum power, improve battery cycle performance and capacity retention, reduce lithium plating, reduce temperature rise during fast charging, and reduce battery swelling.

[0005] To achieve the above objectives, the present invention provides a lithium-ion secondary battery, which includes a positive electrode, a negative electrode, and an electrolyte.

[0006] The positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one side surface of the positive current collector; the positive active material layer includes a positive active material; the positive active material includes primary lithium iron phosphate particles and secondary lithium iron phosphate particles, wherein the Dv50 of the primary lithium iron phosphate particles is denoted as a1μm, and the Dv50 of the secondary lithium iron phosphate particles is denoted as a2μm, a1... <a2;

[0007] The mass percentage of primary lithium iron phosphate particles is denoted as w1, based on the total mass of the positive electrode active material.

[0008] The electrolyte includes a solvent, which includes a first solvent, which is a linear organic solvent of C1-C4; the mass percentage of the first solvent is denoted as S1, based on the total mass of the solvent.

[0009] Where a1, w1, and S1 satisfy the following relationship:

[0010] The present invention, by adopting the above technical solution, has the following beneficial effects:

[0011] The lithium-ion secondary battery provided by this invention uses a linear organic solvent of C1-C4 with low viscosity. By using a low-viscosity first solvent, the transport impedance of lithium ions in the electrolyte and at the interface between the electrolyte and the positive electrode can be reduced. At the same time, by controlling the Dv50 and mass ratio of the primary lithium iron phosphate particles of the positive electrode material, the transport distance of lithium ions within the positive electrode material particles can be shortened. This enables rapid transport of lithium ions in the electrolyte, at the interface between the electrolyte and the positive electrode, and within the positive electrode particles. This improves the battery's fast-charging performance, increases the battery's maximum power, improves the battery's cycle performance and capacity retention, reduces lithium plating, reduces temperature rise during fast charging, and reduces battery expansion.

[0012] 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

[0013] Figure 1 The diagram shown is a schematic of the structure of a prismatic battery.

[0014] Figure 2 The diagram shows the structure of a square-shell battery at different angles.

[0015] Figure 3 The diagram shows a cross-sectional view of the battery casing in a prismatic battery.

[0016] Reference numerals: 1-Shell; 11-Third side; 12-Fourth side; 2-Top cover; 3-Positive electrode post; 4-Negative electrode post; 5-Injection hole; 6-Explosion-proof valve; 61-Explosion-proof plate; 62-Scratched groove; 7-Bottom wall. Detailed Implementation

[0017] 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.

[0018] 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.

[0019] 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.

[0020] In this invention, the term "Dv50" refers to the particle size corresponding to a sample when the cumulative volumetric particle size distribution percentage reaches 50%. Dv50 can be tested using a laser particle size analyzer.

[0021] In this invention, the term "particle size range" refers to the diameter of lithium iron phosphate particles falling within this range. This invention also allows for a certain margin of error; that is, when less than 5% of the total number of particles have a diameter outside the required range, it is still considered to meet the requirements, which can be measured using a transmission electron microscope (TEM).

[0022] In this invention, the term "diameter" refers to the maximum internal straight-line distance between two points on the connecting surface of a lithium iron phosphate particle.

[0023] In this invention, the term "low solvation energy" refers to a state in which the interaction between solvent molecules and dissolved ions or molecules in the electrolyte is relatively weak.

[0024] In this invention, the term "maximum power" refers to the maximum power that a lithium battery can output under specific conditions. For example, the power output by a lithium battery during a high-rate discharge in a short period of time when a car is started.

[0025] In this invention, the term "low-viscosity solvent" refers to a solvent with a viscosity of 0.5 mPa·s or less. The term "high-viscosity solvent" refers to a solvent with a viscosity greater than 0.5 mPa·s. The solvent viscosity is tested using the rotational method in the national standard GB / T 10247-2008, and the viscosity is measured using a rheometer.

[0026] This invention provides a lithium-ion secondary battery, which includes a positive electrode, a negative electrode, and an electrolyte.

[0027] The positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one side surface of the positive current collector; the positive active material layer includes a positive active material; the positive active material includes primary lithium iron phosphate particles and secondary lithium iron phosphate particles, wherein the Dv50 of the primary lithium iron phosphate particles is denoted as a1μm, and the Dv50 of the secondary lithium iron phosphate particles is denoted as a2μm, a1... <a2;

[0028] The mass percentage of primary lithium iron phosphate particles is denoted as w1, based on the total mass of the positive electrode active material.

[0029] The electrolyte includes a solvent, which includes a first solvent, which is a linear organic solvent of C1-C4; the mass percentage of the first solvent is denoted as S1, based on the total mass of the solvent.

[0030] Where a1, w1, and S1 satisfy the following relationship:

[0031] In this invention, A higher value generally results in better maximum power output for lithium-ion rechargeable batteries, but it can also worsen cycle performance or increase battery swelling. This may be because a higher proportion of primary lithium iron phosphate particles increases the number of active sites on the positive electrode surface, leading to severe decomposition of the primary solvent in the electrolyte and resulting in gas production. Therefore, it is necessary to control... Achieving a balance between cycle performance and maximum power in lithium-ion batteries requires a w1 range of 0.2-3.5. Increasing w1 indicates the use of more primary lithium iron phosphate particles (small particles) and fewer secondary lithium iron phosphate particles (large particles) in the cathode material. Increasing the value of a1 improves the fast-charging performance of the positive electrode, allowing for a reduction in the use of low-viscosity solvents (reducing S1) and thus achieving longer battery cycle life. Similarly, decreasing a1... As the value of a1 increases, the Dv50 of the cathode material particles used becomes smaller, the lithium-ion transport distance in the cathode solid phase decreases, the fast-charging performance of the cathode improves, and the use of low-viscosity first solvent is reduced (i.e., S1 decreases), thus achieving longer battery cycle performance. Conversely, when w1 decreases, a1 increases. As the value decreases, the fast-charging performance of the positive electrode weakens, requiring the use of more primary solvent, i.e., increasing the amount of primary solvent used (increasing S1), to reduce the lithium-ion transport impedance at the interface. Therefore, by adjusting the Dv50 and mass ratio of the primary lithium iron phosphate particles and the mass ratio of the primary solvent, the desired balance can be achieved. This allows lithium-ion secondary batteries to have a good maximum power output without deteriorating fast-charging cycle performance.

[0032] In summary, linear organic solvents of C1-C4 have low viscosity (viscosity ≤ 0.5 mPa·s). By using a low-viscosity first solvent, this invention can reduce the transport impedance of lithium ions in the electrolyte bulk and at the interface between the electrolyte and the positive electrode. At the same time, by controlling the Dv50 and mass ratio of the primary lithium iron phosphate particles of the positive electrode material, the transport distance of lithium ions within the positive electrode material particles can be shortened. This enables rapid transport of lithium ions in the electrolyte bulk, at the interface between the electrolyte and the positive electrode, and within the positive electrode particles, thereby improving the fast charging performance of the battery.

[0033] In this invention, the positive electrode active material comprises a mixture of primary lithium iron phosphate particles with a smaller Dv50 and secondary lithium iron phosphate particles with a larger Dv50. The combination of particles of different sizes shortens the solid-phase transport distance of lithium ions from the particle surface to the particle interior, resulting in better fast-charging performance of the positive electrode. The secondary lithium iron phosphate particles prevent the compaction density of the positive electrode powder from being too low, which would reduce the energy density of the battery. Thus, the mixture of primary and secondary lithium iron phosphate particles can balance the fast-charging performance and energy density of the battery.

[0034] In some implementations... The values ​​can be, for example, 0.2, 0.3, 0.5, 0.6, 0.8, 1.0, 1.2, 1.5, 1.6, 1.8, 2.0, 2.2, 2.4, 2.5, 2.6, 2.8, 3.0, 3.2, and 3.5, preferably...

[0035] In some embodiments, the Dv50a1 of the primary lithium iron phosphate particles satisfies: 0.1μm ≤ a1 ≤ 0.4μm, for example, a1 can be 0.1μm, 0.2μm, 0.3μm, or 0.4μm. When a1, w1, and S1 satisfy... When adjusting the value of a1 to meet the above range, the smaller the Dv50 of the primary lithium iron phosphate particles, the better the kinetics. A smaller Dv50 of the positive electrode active material (positive electrode particles) means a shorter solid-phase transport distance for lithium ions from the particle surface to the particle interior, resulting in better fast-charging performance and higher capacity retention. However, when the Dv50 of the primary lithium iron phosphate particles is less than 0.1 μm, current positive electrode processes are difficult to achieve, and it increases the number of active sites on the positive electrode surface, leading to increased battery gas production. If w1 and S1 remain unchanged, when the Dv50 of the primary lithium iron phosphate particles is greater than 0.4 μm, the fast-charging performance of the lithium battery deteriorates, and the kinetics are lower, making it prone to lithium plating. After lithium plating, the capacity retention decreases, and it also easily causes battery gas production.

[0036] In some embodiments, the Dv50 of the lithium iron phosphate secondary particles satisfies: 1μm ≤ a2 ≤ 2μm, for example, a2 can be 1μm, 1.1μm, 1.2μm, 1.3μm, 1.4μm, 1.5μm, 1.6μm, 1.7μm, 1.8μm, 1.9μm, or 2.0μm. The smaller the Dv50 of the lithium iron phosphate secondary particles, the smaller the Dv50 of the positive electrode active material (positive electrode particles), the shorter the solid-phase transport distance of lithium ions from the particle surface to the particle interior, resulting in better fast-charging performance and improving the battery's fast-charging performance. When the Dv50 of the lithium iron phosphate secondary particles is less than 1μm, the compaction density of the positive electrode powder is too low, leading to a decrease in battery energy density. Conversely, when the Dv50 of the lithium iron phosphate secondary particles is greater than 2μm, the positive electrode lithium iron phosphate secondary particles are too large, resulting in a long transport path for lithium ions in the second lithium iron phosphate, causing a decrease in the rate performance of the lithium battery.

[0037] In some embodiments, the particle size range of the primary lithium iron phosphate particles is 0.01 μm-1 μm, for example, 0.01 μm, 0.05 μm, 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, or 1 μm. The particle size range of the secondary lithium iron phosphate particles is 1 μm-10 μm, for example, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, or 10 μm. When the cathode material particles are a mixture of primary and secondary lithium iron phosphate particles, adjusting the particle size range of both can ensure that the Dv50 of both particles is within the protection scope of this invention, thereby improving the fast-charging performance and increasing the energy density of the battery.

[0038] In some embodiments, the mass percentage w1 of primary lithium iron phosphate particles satisfies the following condition: 10% ≤ w1 ≤ 60%. For example, w1 can be 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, or 60%, preferably 20% ≤ w1 ≤ 50%. Increasing the mass percentage w1 of primary lithium iron phosphate particles increases the battery's maximum power, but also increases battery expansion; a too small mass percentage w1 easily leads to lithium plating in the battery. When a1, w1, and S1 satisfy... When w1 is adjusted to meet the above range, it can avoid the low compaction density of the positive electrode due to w1 being too large, which would reduce the energy density of the battery and improve the energy density of the battery; it can also avoid the increase of surface active sites due to w1 being too large, which would lead to severe electrolyte decomposition and gas production and reduce battery expansion; it can also avoid w1 being too small, which would not be able to effectively shorten the solid phase transport distance of lithium ions, improve the fast charging performance of the battery and reduce lithium plating.

[0039] Test method for detecting the mass percentage of primary lithium iron phosphate particles in a battery: The volume fraction of primary and secondary lithium iron phosphate particles can be obtained by measuring with a Malvern laser particle size analyzer. For lithium iron phosphate materials, the chemical structure is the same and the crystal density is the same. Mass = volume * density. Since the density is the same, the mass ratio = the volume ratio. Therefore, the volume percentage is the mass percentage.

[0040] In some implementations, if the positive electrode active material consists only of primary lithium iron phosphate particles and secondary lithium iron phosphate particles, then the mass percentage of the secondary lithium iron phosphate particles is 1-w1.

[0041] In some embodiments, the mass percentage S1 of the first solvent satisfies: 10% ≤ S1 ≤ 90%, for example, S1 can be 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, or 90%, more preferably 20% ≤ S1 ≤ 70%. The more low-viscosity first solvent there is, the lower the impedance of lithium-ion transport in the electrolyte liquid phase. A lower solvation energy of the first solvent can reduce the lithium-ion interfacial transport impedance. When a1, w1, and S1 satisfy... When adjusting the value of S1 to meet the above range, it can reduce the impedance of lithium ion migration in the electrolyte and reduce the transmission impedance of lithium ion at the interface, thereby further improving the fast charging performance of the battery. At the same time, it avoids the situation where too much S1 is used, resulting in less high-viscosity solvent, which leads to instability of the electrolyte at the negative electrode interface, causing continuous decomposition of the electrolyte, increased gas production, and a higher battery expansion rate.

[0042] In some embodiments, the viscosity of the first solvent is ≤0.5 mPa·s, and the first solvent includes at least one or more of ethyl acetate EA (0.426 mPa·s), methyl acetate MA (0.364 mPa·s), methyl formate MF (0.328 mPa·s), dimethyl glycol (0.46 mPa·s), dimethyl formaldehyde DMM (0.33 mPa·s), and acetonitrile AN (0.35 mPa·s), preferably ethyl acetate.

[0043] In some embodiments, the solvent further includes a second solvent, which comprises a C1-C10 cyclic organic solvent and / or a linear organic solvent with more than 4 carbon atoms, and the viscosity of the second solvent is >0.5 mPa·s. When the present invention uses a low-viscosity first solvent and blends primary and secondary lithium iron phosphate particles, adjusting the solvent to include a high-viscosity second solvent can make the electrolyte interface at the negative electrode more stable, reduce electrolyte decomposition and gas production, and decrease battery expansion.

[0044] In some embodiments, the second solvent comprises at least one or more of ethylene carbonate EC (1.9 mPa·s, 40°C), propylene carbonate PC (2.53 mPa·s), butenyl carbonate BC (3.2 mPa·s), γ-butyrolactone (1.73 mPa·s), diethyl carbonate DEC (0.748 mPa·s), ethyl methyl carbonate EMC (0.65 mPa·s), dimethyl carbonate DMC (0.58 mPa·s), propyl propionate PP (0.68 mPa·s), and ethyl propionate EP (0.502 mPa·s).

[0045] In some embodiments, the positive electrode active material layer further includes a conductive agent; the conductive agent includes carbon nanotubes, the aspect ratio of which is denoted as a3. Although the present invention uses a low-viscosity first solvent and a mixture of primary and secondary lithium iron phosphate particles, a1, w1, and S1 satisfy... While this method enables rapid lithium-ion transport at the interface and within the solid phase of the cathode particles, it does not significantly improve the electronic conductivity of the cathode material particles. Rapid electron transfer is still required for the electrochemical system to achieve fast energy conversion in the battery system. To further enhance the electronic conductivity of the cathode material and reduce the battery's bulk impedance, this invention also incorporates carbon nanotube conductive agents into the cathode active material layer. The carbon nanotubes include multi-walled carbon nanotubes and / or single-walled carbon nanotubes.

[0046] In some embodiments, the aspect ratio a3 of the carbon nanotubes, the mass percentage w2 of the carbon nanotubes, and the mass percentage S1 of the first solvent satisfy the relationship: 0.4 ≤ a3 * w2 * S1 ≤ 8, for example, it can be 0.4, 0.5, 0.6, 0.8, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, preferably 0.5 ≤ a3 * w2 * S1 ≤ 6. When a1, w1, and S1 satisfy... When a3, w2, and S1 are adjusted simultaneously to satisfy 0.4 ≤ a3*w2*S1 ≤ 8, the battery exhibits lower bulk impedance reduction and excellent limiting power, while also enabling long-term cycling at high rates. A larger a3*w2*S1 value allows for rapid charge transfer in the positive electrode current collector and positive electrode membrane, as well as rapid charge transfer in the lithium-ion electrolyte liquid phase, improving the fast-charging performance of the lithium-ion battery. In the formula 0.4 ≤ a3*w2*S1 ≤ 8, increasing the aspect ratio a3 of the carbon nanotubes and increasing the amount of carbon nanotubes used (w2), i.e., increasing the a3*w2 value, improves the conductivity of the positive electrode. Increasing the amount of low-viscosity solvent used (i.e., increasing S1) reduces the lithium-ion transport impedance in the electrolyte liquid phase, thereby reducing the battery's bulk impedance. When the positive electrode has good conductivity (i.e., a3*w2 is large), the use of S1 needs to be reduced to prevent electrolyte oxidation and gas generation at the positive electrode, which could lead to battery failure.

[0047] In some embodiments, the aspect ratio a3 of the carbon nanotubes satisfies: 200 ≤ a3 ≤ 1500. For example, a3 can be 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, or 1500, more preferably 500 ≤ a3 ≤ 1000. A larger aspect ratio results in stronger conductivity; when using carbon nanotubes with a larger aspect ratio, the conductivity of the positive electrode is better. However, a very large aspect ratio exposes more active sites, making it easier to react with components in the electrolyte, leading to severe gas generation in the electrolyte. When a1, w1, and S1 satisfy... When a3, w2, and S1 satisfy 0.4≤a3*w2*S1≤8, adjusting the aspect ratio a3 of the carbon nanotubes within the above range can improve the conductivity of the positive electrode, enhance the ion conduction of the positive electrode material particles, and at the same time avoid the battery from generating gas due to an excessively large aspect ratio of the carbon nanotubes, thus reducing battery expansion.

[0048] In some embodiments, the mass percentage w2 of the carbon nanotubes satisfies: 0.1% ≤ w2 ≤ 2%, for example, w2 can be 0.1%, 0.2%, 0.4%, 0.5%, 0.6%, 0.8%, 1.0%, 1.2%, 1.4%, 1.5%, 1.6%, 1.8%, 2.0%, more preferably 0.2% ≤ w2 ≤ 1%. When the carbon nanotube content is low, the improvement effect on the ion conduction performance of the cathode material particles is not good, and the fast charging performance of the battery deteriorates; the higher the carbon nanotube content, the better the conductivity of the cathode and the lower the bulk impedance of the battery, but excessive use will cause the electrolyte to decompose on the carbon nanotubes, resulting in electrolyte consumption and capacity decay. When a1, w1, and S1 satisfy... When a3, w2, and S1 satisfy 0.4≤a3*w2*S1≤8, adjusting the mass ratio w2 of carbon nanotubes within the above range can improve the electronic conductivity of the cathode material particles, reduce the battery's bulk impedance, and further enhance the battery's fast charging performance and reduce battery gas expansion.

[0049] Although the present invention uses a low-viscosity first solvent, and blends primary and secondary lithium iron phosphate particles, a1, w1, and S1 satisfy... While this method enables rapid lithium-ion transport at the interface and within the solid phase of the cathode particles, it does not significantly improve the electronic conductivity of the cathode material particles. The electrochemical system still requires rapid electron transfer to achieve fast energy conversion in the battery system. To further enhance the electronic conductivity of the cathode material and reduce the battery's bulk impedance, this invention also provides a carbon layer on at least one side of the surface of the cathode current collector.

[0050] In some embodiments, a carbon layer is disposed on at least one side of the surface of the positive electrode current collector, and the positive electrode active material layer is disposed on the surface of the carbon layer and / or the surface of the positive electrode current collector. For example, carbon layers are disposed on both opposite sides of the positive electrode current collector, and the positive electrode active material layer is disposed on the surface of the carbon layer away from the positive electrode current collector; or, a carbon layer is disposed on one side of the surface of the positive electrode current collector, and the positive electrode active material layer is disposed on the surface of the carbon layer away from the positive electrode current collector, while the positive electrode active material layer is also disposed on the surface of the positive electrode current collector where no carbon layer is disposed.

[0051] In some embodiments, the thickness of the carbon layer is denoted as a4 μm, where a4 satisfies: 0.2 ≤ a4 ≤ 2. For example, a4 can be 0.2, 0.3, 0.5, 0.6, 0.8, 1.0, 1.2, 1.4, 1.5, 1.6, 1.8, or 2.0, and more preferably 0.5 ≤ a4 ≤ 1.5. A thicker carbon layer results in better contact between the positive electrode current collector and the positive electrode active material layer, stronger electronic conductivity, lower impedance of the positive electrode sheet, and stronger fast-charging capability of the battery. However, excessively thick carbon layers increase the mass ratio of the positive electrode current collector, reducing the battery's energy density. Furthermore, a larger carbon layer can lead to oxidation at the positive electrode, generating carbon dioxide and causing battery swelling and failure. When a1, w1, and S1 satisfy... When a carbon layer is placed on the surface of the current collector and the thickness of the carbon layer is adjusted within the above range, the conductivity of the positive electrode is improved and the impedance of the positive electrode is lowered, thereby further improving the fast charging performance of the battery, while reducing the energy density loss of the battery and reducing the gas expansion of the battery.

[0052] Furthermore, the present invention can solve the problem of low electronic conductivity of lithium iron phosphate cathode material by setting a carbon layer on the surface of the positive electrode current collector and using carbon nanotube conductive agent. When used with a low-viscosity first solvent, it can improve the charge transfer rate of the liquid phase, thereby reducing the battery's bulk impedance and solving the temperature rise problem.

[0053] When a1, w1, and S1 satisfy... When a3, w2, and S1 satisfy 0.4 ≤ a3*w2*S1 ≤ 8, the battery can achieve good maximum power and fast charging performance. However, the electrolyte interface formed at the negative electrode is unstable, causing continuous electrolyte decomposition and affecting the battery's long-cycle performance. To meet the requirements for long-cycle performance, the additives in the electrolyte need to be designed.

[0054] The design scheme for additives in the electrolyte is as follows:

[0055] In some embodiments, the electrolyte further includes a first additive; the mass percentage of the first additive, denoted as b1, is based on the total mass of the electrolyte, and b1 satisfies: 0.1% ≤ b1 ≤ 8%, for example, b1 can be 0.1%, 0.2%, 0.4%, 0.5%, 0.8%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, preferably 2% ≤ b1 ≤ 5%. When the amount of the first additive is small, the negative electrode interface is not stable enough, and the battery performance decreases; when the amount of the first additive is large, it will lead to a decrease in the viscosity of the electrolyte and an increase in the battery impedance. When a1, w1, and S1 satisfy... When a3, w2, and S1 satisfy 0.4≤a3*w2*S1≤8, adding the first additive to the electrolyte and adjusting the mass ratio of the first additive within the above range can improve the stability of the negative electrode interface of the battery, reduce the battery impedance, and thus improve the cycle performance and capacity retention of the battery.

[0056] In some embodiments, the first additive includes one or more of vinylene carbonate (VC), vinyl ethylene carbonate (VEC), fluoroethylene carbonate (FEC), vinyl sulfate, ethylene sulfite, fluorobenzene, thiophene, and difluorothiophene.

[0057] In some embodiments, the electrolyte further includes a second additive; the mass percentage of the second additive, denoted as b2, is the total mass of the electrolyte; b1, b2, and S1 satisfy the following relationship: Preferably, the mass percentage b2 of the second additive satisfies: 0.1% ≤ b2 ≤ 4%, more preferably 0.5% ≤ b2 ≤ 3%. When the amount of the second additive is low, the battery will experience rapid capacity decay during fast charging cycles. When the amount of the second additive is high, it will lead to increased battery impedance, deterioration of the battery's maximum power, poor battery kinetics, and consequently, lithium plating and cycle failure. When a1, w1, and S1 satisfy... When a3, w2, and S1 satisfy 0.4≤a3*w2*S1≤8, adding a second additive to the electrolyte and adjusting the mass ratio of the second additive within the above range can improve the battery's kinetic performance, reduce battery impedance, increase the battery's maximum power, improve the battery's cycle performance, and reduce lithium plating.

[0058] In some embodiments, the second additive comprises one or more of the following: tris(trimethylsilane)phosphate TMSP, tris(trimethylsilane)borate TMSB, vinyl sulfate DTD, methylene disulfonate MMDS, 1,3-propane sulpholol PS, lithium difluorophosphate LiPO2F2, lithium bis(oxalate)borate LiBOB, lithium difluorooxalateborate LiODFB, lithium difluorodioxalate phosphate LiODFP, and lithium tetrafluoroborate LiBF4, preferably DTD+LiPO2F2.

[0059] When a1, w1, and S1 satisfy... When a3, w2, and S1 satisfy 0.4≤a3*w2*S1≤8, the battery can have good ultimate power and fast charging performance. However, in order to meet the long cycle performance of the battery, the electrolyte lithium salt in the electrolyte needs to be designed to meet the long cycle performance of the battery.

[0060] The design scheme for the lithium salt electrolyte in the electrolyte is as follows:

[0061] In some embodiments, the electrolyte further includes a lithium electrolyte salt; the mass percentage of the lithium electrolyte salt, denoted as b3, is based on the total mass of the electrolyte, and b3 satisfies the following condition: 8% ≤ b3 ≤ 20%. For example, b3 can be 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, or 20%. When b3 is too small, the lithium electrolyte salt content is too low, resulting in low electrolyte conductivity and poor battery cycle performance. When b3 is too large, i.e., the lithium electrolyte salt content is too high, the cations and anions in the electrolyte combine, reducing ionic conductivity and hindering the improvement of fast-charging performance. When a1, w1, and S1 satisfy... When a3, w2, and S1 satisfy 0.4≤a3*w2*S1≤8, adjusting the electrolyte lithium salt content within the above range not only improves the battery's fast charging performance and maximum power, but also helps to improve the electrolyte's conductivity, enhance the battery's long-cycle performance, and increase the battery's capacity retention.

[0062] In some embodiments, the electrolyte lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium hexafluorophosphate (LiPF6). Preferably, the mass ratio of lithium bis(fluorosulfonyl)imide to lithium hexafluorophosphate is denoted as w3, and S1 and w3 satisfy: 0.02 ≤ S1 * w3 ≤ 3, more preferably 0.2 ≤ S1 * w3 ≤ 0.5. Preferably, w3 satisfies: 0.01 ≤ w3 ≤ 2, for example, w3 can be 0.01, 0.1, 0.2, 0.3, 0.5, 0.6, 0.8, 1.0, 1.2, 1.4, 1.5, 1.6, 1.8, 2.0, and preferably 0.3 ≤ w3 ≤ 1.

[0063] When the lithium salt content of the electrolyte is fixed, increasing the LiFSI content (i.e., increasing w3) increases the ionic conductivity of the electrolyte. However, excessively high w3 leads to insufficient LiPF6, which cannot stabilize the negative electrode interface and passivate the positive electrode current collector. Conversely, decreasing the LiFSI content (i.e., decreasing w3) reduces the electrolyte conductivity, resulting in insufficient electrolyte kinetics and, in severe cases, lithium plating during fast charging. When a1, w1, and S1 satisfy... When adjusting S1 and w3 to satisfy 0.1≤S1*w3≤1 and when adjusting w3 within the above range, when S1 decreases, w3 can be increased, that is, the relative content of LiFSI can be increased, to improve conductivity and thus meet the fast charging cycle requirements; when S1 increases, the interface is not stable enough, and the fast charging cycle of the battery will deteriorate. Decreasing w3, that is, increasing the relative content of LiPF6, can generate a more stable interface and improve the cycle performance of the battery.

[0064] By defining the formula above, the lithium-ion secondary battery of the present invention has good fast-charging cycle performance and good maximum power. However, during fast charging, the battery body may overheat, and the separator may shrink at high temperatures, posing a safety hazard of short circuit. Therefore, to improve the battery's safety performance, the present invention also includes a battery casing, which can quickly conduct heat and prevent heat accumulation.

[0065] The design scheme for the battery casing is as follows.

[0066] The battery casing satisfies at least one of the following conditions, preferably all of the following conditions:

[0067] (i) The thickness d1 of the battery casing on the side with the largest area satisfies: 0.2mm≤d1≤3mm, for example, d1 can be 0.2mm, 0.4mm, 0.5mm, 0.6mm, 0.8mm, 1.0mm, 1.2mm, 1.5mm, 1.6mm, 1.8mm, 2.0mm, 2.2mm, 2.5mm, 2.8mm, 3mm;

[0068] (ii) The thickness d2 of the battery casing satisfies: 20mm ≤ d2 ≤ 60mm. The thickness d2 of the battery casing refers to the thickness of the smaller sides of the battery casing, excluding the side with the largest area. For example, d2 can be 20mm, 25mm, 30mm, 35mm, 40mm, 45mm, 50mm, 55mm, 58mm, 60mm, 70mm, 80mm, 90mm, or 100mm.

[0069] (iii) The width d3 of the battery casing satisfies: 50mm≤d3≤400mm, for example, d3 can be 50mm, 100mm, 120mm, 140mm, 160mm, 180mm, 200mm, 220mm, 240mm, 260mm, 280mm, 300mm, 320mm, 350mm, 380mm, 400mm;

[0070] (iv) The height d4 of the battery casing satisfies: 50mm≤d4≤400mm, for example, d4 can be 50mm, 70mm, 90mm, 100mm, 120mm, 140mm, 160mm, 180mm, 200mm, 220mm, 250mm, 280mm, 300mm, 320mm, 350mm, 380mm, or 400mm.

[0071] The smaller the thickness d1 of the battery casing on the side with the largest area, the faster heat dissipates from the inside of the battery to the outside, preventing heat accumulation and helping to reduce the temperature rise during fast charging. The larger the thickness d2 of the battery casing, the greater the heat accumulation and the higher the temperature rise. The larger the width d3 and height d4 of the battery casing, the larger the casing area, the faster the heat transfer and the smaller the temperature rise. This is achieved when a1, w1, and S1 satisfy... When the parameters of the battery casing are adjusted to the above range, heat can be quickly conducted, preventing heat accumulation. This helps to reduce the temperature rise during fast charging, reduce safety issues caused by battery overheating, and improve battery safety performance.

[0072] In some embodiments, d1 and S1 satisfy the condition: 0.4 ≤ d1 / S1 ≤ 20. For example, the ratio can be 0.2, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20. The larger the d1 / S1 ratio, the larger d1 becomes. When S1 remains constant, the battery heat dissipates more slowly, leading to greater battery expansion. Conversely, when d1 remains constant, a smaller S1 results in poorer improvement of lithium-ion transport impedance in the electrolyte liquid phase, affecting the battery's fast-charging performance. Furthermore, an excessively large d1 / S1 ratio can negatively impact the battery's energy density. When a1, w1, and S1 satisfy the condition... When d1 and S1 are adjusted to meet the above conditions, the fast charging performance of the battery can be improved, the battery swelling can be reduced, and excessive loss of battery energy density can be avoided.

[0073] In some implementations, such as Figure 1 and Figure 2 As shown, the battery casing includes a housing 1, which includes a first side and a second side arranged opposite each other along the X-axis (i.e., the length direction of the housing). The first side is, for example, a top cover 2, and the second side is, for example, a bottom wall 7. A positive electrode post 3 and a negative electrode post 4 are arranged opposite each other at both ends along the first direction of the housing 1 on the top cover 2. An injection hole 5 is also provided on the top cover 2, located between the positive electrode post 3 and the negative electrode post 4. An explosion-proof valve 6 is provided in the central part of the bottom wall 7. The explosion-proof valve 6 can also be provided on other side walls of the housing 1. The number of explosion-proof valves 6 can be selected and adjusted according to the gas production and voltage conditions of the battery. When at least two explosion-proof valves 6 are provided, they can be provided on the bottom wall 7 and other side walls.

[0074] In some implementations, such as Figure 1 , Figure 3 As shown, the shell 1 is enclosed by six rectangular faces. Along the Y-axis, the two opposite rectangular faces with the largest areas are denoted as the third side 11 and the fourth side 12. The shell thickness of both the third side 11 and the fourth side 12 is d1, and the shell thickness of the remaining sides is d2, where d1 is less than d2. The distance along the Y-axis of the shell 1 is denoted as the width of the shell, represented by d3; the distance along the Z-axis of the shell 1 is denoted as the height of the shell, represented by d4.

[0075] By defining the formula above, the lithium-ion secondary battery of the present invention has good fast-charging cycle performance and good maximum power. However, during fast charging, the battery body may overheat, and the separator may shrink at high temperatures, posing a safety hazard of battery short circuit. To further enhance the battery's safety performance and use a safer separator, the present invention also designs the separator's structure and strength to reduce separator shrinkage and avoid battery short circuit.

[0076] The diaphragm design scheme is as follows:

[0077] In some embodiments, the battery further includes a separator, which comprises a base film, an adhesive layer, and a ceramic layer; the ceramic layer is disposed on at least one surface of the base film; the adhesive layer is disposed on at least one surface of the base film, or on the surface of the ceramic layer away from the base film. For example, the separator structure can be adhesive layer + ceramic + base film, adhesive layer + ceramic + base film + adhesive layer, or adhesive layer + ceramic + base film + ceramic + adhesive layer. When a1, w1, and S1 satisfy... When the structure of the separator is adjusted within the above range, the structural stability of the separator can be improved, the thermal shrinkage rate of the separator can be reduced, and the risk of short circuit in the battery can be reduced, thereby improving the safety performance of the battery.

[0078] In some embodiments, the base film comprises at least one of polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyimide, and a PE and PP composite structure. And / or, the ceramic layer comprises one or more of alumina (Al₂O₃), aluminum hydroxide, magnesium oxide, alumina, zirconium oxide, titanium oxide, and silicon oxide. And / or, the adhesive layer is a conventional spray-applied adhesive layer, such as one or more of polyvinylidene fluoride (PVDF), ethylene-vinylidene fluoride-hexafluoropropylene copolymer, polyacrylic acid resin, polymethyl acrylate (PMMA), butyl acrylate-acrylonitrile copolymer, polyacrylonitrile, ethylene-acrylic acid copolymer, ethyl polyacrylate, or sodium carboxymethyl cellulose.

[0079] In some implementations, the lithium-ion secondary battery can be a power lithium battery, a consumer lithium battery, or an energy storage lithium battery.

[0080] 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.

[0081] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0082] The batteries in the embodiments and comparative examples of this invention were prepared according to the following preparation method. The differences from Example A1 are shown below.

[0083] Example A1

[0084] (1) Preparation of positive electrode sheet

[0085] Different ternary materials, lithium iron phosphate powder, polyvinylidene fluoride, acetylene black, and carbon nanotubes (aspect ratio a3 of 706), were added to a vacuum mixer in a mass ratio of 96:2:1.5:0.5. N-methylpyrrolidone (NMP) was added, and the mixture was thoroughly mixed under vacuum until a uniform and fluid positive electrode slurry with a solid content of 55 wt% was formed. The positive electrode slurry was uniformly coated onto a 12 μm thick carbon-coated aluminum foil (the carbon layer thickness a4 was 1 μm), with a double-sided areal density of 22 mg / cm2. The coating was then dried, rolled, slit, and punched to obtain the positive electrode sheet.

[0086] (2) Preparation of negative electrode sheet

[0087] Graphite, styrene-butadiene rubber, sodium carboxymethyl cellulose, and acetylene black were added to a vacuum mixer in a mass ratio of 96.5:1:1:1.5. Deionized water was added, and the mixture was thoroughly mixed under vacuum to form a uniform and fluid negative electrode slurry with a solid content of 45 wt%. The negative electrode slurry was then uniformly coated onto a 6 μm thick copper foil, dried, rolled, and die-cut to obtain the negative electrode sheet.

[0088] (3) Preparation of electrolyte

[0089] In an argon-filled glove box with a water content of <0.1 ppm and an oxygen content of <0.1 ppm, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and ethyl acetate (EA) were mixed uniformly at a mass ratio of 3:2:5. Fully dried lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI) were added and stirred to dissolve. The mass of LiPF6 and LiFSI added was 10% and 5% of the total electrolyte mass, respectively. 2% (based on the total electrolyte mass) of vinylene carbonate (VC), 2% (based on the total electrolyte mass) of fluoroethylene carbonate (FEC), 1.0% (based on the total electrolyte mass) of vinyl sulfate (DTD), and 1.0% (based on the total electrolyte mass) of lithium difluorophosphate (LiPO2F2) were added. After stirring until homogeneous and passing physical property testing, the electrolyte was obtained.

[0090] (4) Battery fabrication

[0091] The positive electrode obtained in step (1), the negative electrode obtained in step (2), and the separator (the separator has a structure of adhesive layer + ceramic + base film + adhesive layer, the adhesive layer is PVDF, and the positive electrode side of the separator contains ceramic Al2O3) are wound together to obtain a bare cell; the bare cell is welded with tabs and placed in the battery casing; the electrolyte prepared in step (3) is injected into the dried and qualified cell; and the battery is obtained after processes such as standing, aging, formation, degassing, aging, and sorting.

[0092] (5) Test the battery

[0093] Electrochemical performance testing: Electrochemical performance was tested using the Blue Electric charge-discharge test cabinet and the following test methods:

[0094] (1) Fast charging cycle performance test

[0095] Fast charging cycle performance test: At 25℃, the battery thickness d1 before cycling was measured with vernier calipers. The battery, after capacity testing, was charged at 1C constant current and constant voltage to 3.65V, with a cutoff current of 0.05C. Then, it was discharged at 1C constant current to 2.5V, and the initial discharge capacity C0 was recorded. The battery was then charged at 1C to 20% SOC, followed by 5C charging to 80% SOC, and finally charged at 1C constant current and constant voltage to 100% SOC, with a cutoff current of 0.05C. Then, it was discharged at 1C constant current to 2.5V. This cycle was repeated 1000 times. After 1000 charge-discharge cycles, the capacity retention rate on the 1000th cycle was calculated, and the battery thermal thickness d2 was recorded. The calculation formula is as follows:

[0096] Cycle capacity retention rate (%) at 1000th week = (Cycle discharge capacity at 1000th week / Initial cycle discharge capacity) × 100%.

[0097] Battery expansion rate (%) after 1000 cycles = (Battery thickness d2 after 1000 cycles / Battery thickness d1 before cycling) × 100%.

[0098] (2) Limiting power test

[0099] At 25℃, the battery, after capacity testing, was charged to 3.65V using a 1C constant current and constant voltage method, with a cutoff current of 0.05C. Then, it was discharged to 2.5V using a 1C constant current method, and the initial discharge capacity C0 was recorded. The battery was then discharged to 50% SOC at 1C, and after standing for 4 hours, the battery temperature T0 was recorded. Next, the battery was discharged at a 10C rate to the lower limit voltage of 2.5V, and the discharge time and the maximum temperature T1 during the discharge process were recorded. The temperature rise T1-T0 was calculated; the longer the discharge time, the better the ultimate power.

[0100] (3) Determination of lithium plating in batteries

[0101] At 25°C, the capacity-graded battery was charged at 1C constant current and constant voltage to 3.65V, with a cutoff current of 0.05C. It was then discharged at 1C constant current to 2.5V, and the initial discharge capacity C0 was recorded. The battery was then charged at 1C to 20% SOC, followed by a 6C charge to 80% SOC, and finally charged at 1C constant current and constant voltage to 100% SOC, with a cutoff current of 0.05C. This cycle was repeated 20 times. Afterward, the battery was charged at 1C to 20% SOC, followed by a 6C charge to 80% SOC, and then charged at 1C constant current and constant voltage to 100% SOC to achieve full charge. Finally, the fully charged battery was dissected in a drying room to observe whether lithium plating was present on the surface, categorized as no lithium plating, slight lithium plating, lithium plating, or severe lithium plating.

[0102] Examples A, B, and C, as well as Comparative Examples N1-N4, were performed in accordance with Example A1, with differences shown in Table 1. Performance test data are recorded in Table 2. Specifically, in Example A, the Dv50 (a1) of the primary lithium iron phosphate particles was changed; in Example B, the Dv50 (a2) of the secondary lithium iron phosphate particles was changed; in Example C, the mass percentage (w1) of the primary lithium iron phosphate particles was changed; in Example D, the mass percentage (S1) of the first solvent was changed; and in Example D4, an equal amount of MA replaced EA. In Comparative Examples N1 and N2, the Dv50 of the primary lithium iron phosphate particles was changed; in Comparative Example N3, the mass percentage of the primary lithium iron phosphate particles was changed; and in Comparative Example N4, the mass percentage of the first solvent was changed.

[0103] Table 1

[0104]

[0105]

[0106] Table 2

[0107]

[0108]

[0109] As shown in Table 2, this invention achieves its goals by using a low-viscosity first solvent and controlling the amount of the first solvent added, while simultaneously controlling the Dv50 and mass ratio of the primary lithium iron phosphate cathode material particles to meet the requirements. This can improve battery fast charging performance, increase battery maximum power, improve battery cycle performance and capacity retention, reduce temperature rise during fast charging, and reduce battery expansion. In Example D4, MA viscosity is lower, battery maximum power is better, and expansion rate is increased.

[0110] Examples E, F, and G were performed in accordance with Example A1, with specific differences shown in Table 3. Performance test data are recorded in Table 4. In Example E, the aspect ratio a3 of the carbon nanotubes was changed; in Example F, the mass percentage w2 of the carbon nanotubes was changed; and in Example G, the carbon layer thickness a4 was changed.

[0111] Table 3

[0112]

[0113] Table 4

[0114]

[0115]

[0116] As shown in Table 4, the results of Example E indicate that a longer aspect ratio of carbon nanotubes leads to better conductivity and kinetics. However, a large aspect ratio exposes more active sites, resulting in severe electrolyte gas production and a higher battery expansion rate. The results of Example F show that increased use of carbon nanotubes increases the maximum power discharge time, but also increases electrolyte gas production and leads to a higher battery expansion rate. The results of Example G show that increased carbon layer thickness increases the maximum power discharge time, but also increases gas production and leads to a higher battery expansion rate.

[0117] Examples H, I, J, K, L, and M were performed according to Example A1, with specific differences shown in Table 5. Performance test data are recorded in Table 6. In Example H, the mass percentage (b1) of the first additive was changed; in Example I, the type of the first additive was changed, except in Example I3 where the first additive was not added; in Example J, the mass percentage (b2) of the second additive was changed, except in Example J4 where the second additive was not added; in Example K, the type of the second additive was changed. In Example L, the lithium salt content (b3) was changed. In Example M, the mass ratio (w3) of LiFSI to LiPF6 was changed in Examples M1-M4; Example M5 contained only LiFSI, and Example M6 contained only LiPF6.

[0118] Table 5

[0119]

[0120]

[0121] Table 6

[0122]

[0123]

[0124] As shown in Table 6, in Example H, increasing the content of the first additive shortens the maximum power discharge time, increases capacity retention, and reduces battery swelling. In Example I, VC+VEC leads to increased battery impedance and decreased maximum power, but reduces battery swelling. In Example J, increasing the content of the second additive increases battery impedance, decreases maximum power, worsens kinetics, and reduces capacity retention; excessive increase in the second additive content can cause lithium plating and cycle failure. In Example L, increasing lithium salt content causes a decrease in maximum power, while decreasing lithium salt content reduces cycle capacity retention. In Example M, increasing LiFSI increases the battery's maximum power, but also increases battery swelling.

[0125] The O group and P group of Examples were carried out in accordance with Example A1, and the main differences are shown in Table 7.

[0126] Table 7

[0127]

[0128] As can be seen from Table 7, when d1 and S1 satisfy 0.4≤d1 / S1≤20, battery expansion can be reduced.

[0129] Example Q group was conducted in accordance with Example A1. In Example Q1, the diaphragm structure was adhesive layer + ceramic + base membrane + ceramic + adhesive layer; in Example Q2, the diaphragm structure was adhesive layer + base membrane + ceramic + adhesive layer. Performance test records are shown in Table 8.

[0130] Table 8

[0131]

[0132] As can be seen from Table 8, changes in the structure of the separator will affect the battery's expansion rate; fast charging performance will be slightly better when there is no ceramic layer.

[0133] 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 lithium-ion secondary battery includes a positive electrode, a negative electrode, and an electrolyte; The positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one side surface of the positive current collector; the positive active material layer includes a positive active material; the positive active material includes primary lithium iron phosphate particles and secondary lithium iron phosphate particles, wherein the Dv50 of the primary lithium iron phosphate particles is denoted as a1 µm, and the Dv50 of the secondary lithium iron phosphate particles is denoted as a2 µm, a1 µm. <a2; The mass percentage of primary lithium iron phosphate particles is denoted as w1, based on the total mass of the positive electrode active material. The electrolyte includes a solvent, which includes a first solvent, which is a linear organic solvent of C1-C4; the mass percentage of the first solvent is denoted as S1, based on the total mass of the solvent. Where a1, w1, and S1 satisfy the following relationship: 。 2. The lithium-ion secondary battery according to claim 1, characterized in that, a1, w1, and S1 satisfy the following relationship: 。 3. The lithium-ion secondary battery according to claim 1, characterized in that, The Dv50 of the primary lithium iron phosphate particles satisfies: 0.1 µm ≤ a1 ≤ 0.4 µm; and / or, the Dv50 of the secondary lithium iron phosphate particles satisfies: 1 µm ≤ a2 ≤ 2 µm.

4. The lithium-ion secondary battery according to claim 1, characterized in that, The primary lithium iron phosphate particles have a particle size range of 0.01 µm to 1 µm; and / or, the secondary lithium iron phosphate particles have a particle size range of 1 µm to 10 µm.

5. The lithium-ion secondary battery according to claim 1, characterized in that, The mass percentage w1 of primary lithium iron phosphate particles satisfies: 10% ≤ w1 ≤ 60%; And / or, the mass percentage S1 of the first solvent satisfies: 10% ≤ S1 ≤ 90%.

6. The lithium-ion secondary battery according to claim 5, characterized in that, The mass percentage w1 of primary lithium iron phosphate particles satisfies: 20% ≤ w1 ≤ 50%; And / or, the mass percentage S1 of the first solvent satisfies: 20% ≤ S1 ≤ 70%.

7. The lithium-ion secondary battery according to claim 1, characterized in that, The solvent further includes a second solvent, which includes cyclic organic solvents of C1-C10 and / or linear organic solvents with more than 4 carbon atoms.

8. The lithium-ion secondary battery according to claim 7, characterized in that, The viscosity of the second solvent is >0.5 mPa·s, and the second solvent includes at least one or more of ethylene carbonate, propylene carbonate, butene carbonate, γ-butyrolactone, diethyl carbonate, methyl ethyl carbonate, dimethyl carbonate, propyl propionate, and ethyl propionate.

9. The lithium-ion secondary battery according to claim 1, characterized in that, The viscosity of the first solvent is ≤0.5 mPa·s, and the first solvent includes at least one or more of ethyl acetate, methyl acetate, methyl formate, dimethyl ethylene glycol, dimethyl alcohol, and acetonitrile.

10. The lithium-ion secondary battery according to claim 9, characterized in that, The first solvent is selected from ethyl acetate.

11. The lithium-ion secondary battery according to claim 1, characterized in that, The positive electrode active material layer also includes a conductive agent; the conductive agent includes carbon nanotubes, and the aspect ratio of the carbon nanotubes is denoted as a3.

12. The lithium-ion secondary battery according to claim 11, characterized in that, The mass percentage of the carbon nanotubes is denoted as w2, based on the total mass of the positive electrode active material. Among them, a3, w2 and S1 satisfy the relationship: 0.4≤a3×w2×S1≤8.

13. The lithium-ion secondary battery according to claim 11, characterized in that, a3, w2, and S1 satisfy the following relationship: 0.5 ≤ a3 × w2 × S1 ≤ 6; And / or, the aspect ratio a3 of the carbon nanotubes satisfies: 200≤a3≤1500; And / or, the mass percentage w2 of the carbon nanotubes satisfies: 0.1% ≤ w2 ≤ 2%.

14. The lithium-ion secondary battery according to claim 13, characterized in that, The aspect ratio a3 of the carbon nanotubes satisfies: 500≤a3≤1000; And / or, the mass percentage w2 of the carbon nanotubes satisfies: 0.2% ≤ w2 ≤ 1%.

15. The lithium-ion secondary battery according to any one of claims 1-14, characterized in that, A carbon layer is disposed on at least one side of the surface of the positive electrode current collector, and the positive electrode active material layer is disposed on the surface of the carbon layer and / or the surface of the positive electrode current collector.

16. The lithium-ion secondary battery according to claim 15, characterized in that, The thickness of the carbon layer is denoted as a4 µm, where a4 satisfies: 0.2≤a4≤2.

17. The lithium-ion secondary battery according to claim 16, characterized in that, a4 satisfies: 0.5≤a4≤1.

5.

18. The lithium-ion secondary battery according to any one of claims 1-14, characterized in that, The electrolyte also includes a first additive; the mass percentage of the first additive is denoted as b1 based on the total mass of the electrolyte, and b1 satisfies: 0.1%≤b1≤8%.

19. The lithium-ion secondary battery according to claim 18, characterized in that, b1 satisfies: 2% ≤ b1 ≤ 5%.

20. The lithium-ion secondary battery according to claim 18, characterized in that, The first additive includes one or more of vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate, vinyl sulfate, ethylene sulfite, fluorobenzene, thiophene, and 2-fluorothiophene.

21. The lithium-ion secondary battery according to claim 18, characterized in that, The electrolyte also includes a second additive; the mass percentage of the second additive, based on the total mass of the electrolyte, is denoted as b2; b1, b2, and S1 satisfy the following relationship: 。 22. The lithium-ion secondary battery according to claim 21, characterized in that, The mass percentage of the second additive, b2, satisfies the following condition: 0.1% ≤ b2 ≤ 4%.

23. The lithium-ion secondary battery according to claim 22, characterized in that, The mass percentage of the second additive, b2, satisfies the following condition: 0.5% ≤ b2 ≤ 3%.

24. The lithium-ion secondary battery according to claim 21, characterized in that, The second additive includes one or more of tris(trimethylsilane) phosphate, tris(trimethylsilane) borate, vinyl sulfate, methylene disulfonate, 1,3-propane sulphol, lithium difluorophosphate, lithium bis(oxalato)borate, lithium difluorooxalatoborate, lithium difluorodi(oxalato)phosphate, and lithium tetrafluoroborate.

25. The lithium-ion secondary battery according to claim 1, characterized in that, The lithium-ion secondary battery also includes a battery casing.

26. The lithium-ion secondary battery according to claim 25, characterized in that, The battery casing satisfies at least one of the following conditions: (i) The thickness d1 of the battery casing on the side with the largest area satisfies: 0.2 mm ≤ d1 ≤ 3 mm; (ii) The thickness d2 of the battery casing satisfies: 20 mm ≤ d2 ≤ 100 mm; (iii) The width d3 of the battery casing satisfies: 50 mm ≤ d3 ≤ 400 mm; (iv) The height d4 of the battery casing satisfies: 50 mm ≤ d4 ≤ 400 mm.

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