Lithium-ion secondary battery and charging method thereof, and electric device

Abstract

A lithium-ion secondary battery and a charging method thereof, and an electric device. The lithium-ion secondary battery comprises a positive electrode sheet and an electrolyte, wherein the positive electrode sheet comprises a positive electrode active material. The positive electrode active material comprises a first positive electrode active material and a second positive electrode active material, wherein the first positive electrode active material comprises a lithium-containing transition metal oxide, and the second positive electrode active material comprises a lithium-containing transition metal phosphate. The average particle size D1 of primary particles of the first positive electrode active material satisfies: 1.6 μm≤D1≤2.6 μm; and the average particle size D2 of primary particles of the second positive electrode active material satisfies: 40 nm≤D2≤250 nm. The electrolyte comprises a solvent, an additive and a lithium salt, with the additive comprising a cyclic sulfate compound. The lithium-ion secondary battery has a good power density and good high-temperature storage performance.

Description

Lithium-ion secondary batteries, their charging methods, and electrical devices Cross-references to related applications

[0001] This patent document claims priority and benefit to Chinese Patent Application No. 202510003877.4, filed on January 2, 2025, entitled "Lithium-ion Secondary Battery and Charging Method and Electrical Device Thereof". The entire contents of the aforementioned patent application are incorporated herein by reference as a part of the disclosure of this patent document. Technical Field

[0002] This application relates to the field of batteries, and more specifically, to a lithium-ion secondary battery and its charging method and electrical device. Background Technology

[0003] In recent years, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power plants, as well as in many fields such as power tools, electric bicycles, electric motorcycles, electric cars, military equipment, and aerospace, thus achieving great development.

[0004] The use of a mixture of lithium-containing transition metal oxides and lithium-containing transition metal phosphates can improve the power performance of lithium-ion secondary batteries, but it degrades their storage performance. Storage performance has a significant impact on the preservation, transportation, and lifespan of lithium-ion secondary batteries. Therefore, improving the storage performance of lithium-ion secondary batteries is a pressing technical problem that needs to be solved. Summary of the Invention

[0005] This application is made in view of the above-mentioned technical problems, and its purpose is to provide a lithium-ion secondary battery and its charging method and power device, wherein the lithium-ion secondary battery has both power performance and storage performance.

[0006] In a first aspect, a lithium-ion secondary battery is provided, comprising a positive electrode sheet and an electrolyte. The positive electrode sheet includes a positive current collector and a positive electrode film layer disposed on at least one side of the positive current collector. The positive electrode film layer includes a positive electrode active material, which includes a first positive electrode active material and a second positive electrode active material. The first positive electrode active material includes a lithium-containing transition metal oxide, and the second positive electrode active material includes a lithium-containing transition metal phosphate. The average particle size D1 of the primary particles of the first positive electrode active material satisfies: 1.6 μm ≤ D1 ≤ 2.6 μm; the average particle size D2 of the primary particles of the second positive electrode active material satisfies: 40 nm ≤ D2 ≤ 250 nm. The electrolyte includes a solvent, an additive, and a lithium salt, wherein the additive includes a cyclic sulfate compound.

[0007] In the embodiments of this application, the positive electrode active material includes both lithium-containing transition metal oxide and lithium-containing transition metal phosphate, and the particle sizes of the primary particles of the first positive electrode active material and the primary particles of the second positive electrode active material are controlled within a corresponding range. Meanwhile, the electrolyte includes cyclic sulfate additives. Thus, the side reactions between the positive electrode active material and the electrolyte can be improved while enhancing the power performance of the lithium-ion secondary battery, and the dissolution of transition metals in the positive electrode active material can be suppressed, so that the lithium-ion secondary battery can balance power performance and storage performance.

[0008] In some embodiments, the charging cutoff voltage v of the lithium-ion secondary battery satisfies: 4.3V≤v≤5V.

[0009] In some embodiments, the cyclic sulfate compound comprises at least one of a compound of formula (I) or a modified compound thereof: R1-R4 includes at least one of the following: halogenated or unhalogenated H, alkyl with 1-6 carbon atoms, alkenyl with 2-6 carbon atoms, alkoxy with 1-6 carbon atoms, acyloxy with 1-6 carbon atoms, ester with 2-6 carbon atoms, and cyclic sulfate ester with 4-membered to 7-membered rings.

[0010] In some embodiments, R1-R4 further include at least one group of formulas (II-I) to (II-IV): in, Indicating the bonding position, R5 includes at least one of halogen, alkyl group having 1-6 carbon atoms, haloalkyl group having 1-6 carbon atoms, and alkoxy group having 1-6 carbon atoms. The group of formula (II-I) to (II-IV) is bonded to R1 and R3, or to R2 and R4.

[0011] In some embodiments, the cyclic sulfate compound has a polycyclic structure.

[0012] Cyclic sulfates with multi-ring structures can form more dense interfacial films.

[0013] In some embodiments, the cyclic sulfate compound includes at least one of compounds with the molecular formula (II) to (I-XII):

[0014] In some embodiments, the mass fraction m1 of the cyclic sulfate compound, based on the total mass of the electrolyte, satisfies the following condition: 0.1% ≤ m1 ≤ 5%.

[0015] In some embodiments, 0.5% ≤ m1 ≤ 2%.

[0016] In some embodiments, the ionic conductivity σ of the electrolyte satisfies: 7.5 mS / cm ≤ σ ≤ 15 mS / cm.

[0017] In some embodiments, the lithium salt comprises at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorobis(oxalate)phosphate, lithium bis(oxalate)borate, lithium difluorooxalateborate, or lithium tetrafluorooxalate; optionally, the lithium salt comprises lithium hexafluorophosphate.

[0018] In some embodiments, the concentration c of the lithium salt in the electrolyte satisfies: 0.9M ≤ c ≤ 1.2M.

[0019] In some embodiments, the solvent includes at least one of carbonate solvents and carboxylic acid ester solvents.

[0020] In some embodiments, the carbonate solvent includes at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, dimethyl carbonate, and diethyl carbonate.

[0021] In some embodiments, the carboxylic acid ester solvent includes at least one of methyl formate, ethyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl acrylate, and ethyl acrylate.

[0022] In some embodiments, the carbonate solvent comprises ethylene carbonate based on the total mass of the electrolyte, and the mass fraction m2 of the ethylene carbonate satisfies: 10% ≤ m2 ≤ 30%.

[0023] In some embodiments, the additive includes fluoroethylene carbonate.

[0024] In some embodiments, in a cross-section of the positive electrode film layer along the thickness direction of the positive electrode sheet, primary particles of the first positive electrode active material surround an aggregate of at least a portion of the primary particles of the second positive electrode active material.

[0025] In some embodiments, the BET1 of the positive electrode active material satisfies: 0.5m 2 / g≤BET1≤4.5m 2 / g.

[0026] In some embodiments, the volume distribution particle size Dv50 of the positive electrode active material satisfies: 3.2μm≤Dv50≤6μm.

[0027] In some embodiments, the lithium-containing transition metal oxide comprises Li n1 (Ni x1 Co y1 Mn z1 )1-n G n O 2-m K m At least one of the compounds or their modified compounds, wherein G includes one or more of Zr, Al, Nb, Sr, B, Ba, Mg, Sn, Y, Na, Si, W and Ti, K includes one or more of P, F, N, S, F, Cl and I, 0.6≤n1≤1.5, 0.3≤x1≤0.7, 0.02≤y1≤0.15, x1+y1+z1=1, 0<n≤0.1, 0<m≤0.1, 0<r≤0.1.

[0028] In some embodiments, the average mass concentration of K element in the outer region corresponding to 1 / 6 radius from the surface of the primary particle containing lithium transition metal oxide is greater than the average mass concentration of K element in the core region corresponding to 1 / 6 radius from the center of the primary particle.

[0029] In some embodiments, the average mass concentration α of K element in the outer region corresponding to 1 / 6 radius of the surface of the primary particles containing lithium transition metal oxide satisfies: 0.1% ≤ a ≤ 0.5%.

[0030] In some embodiments, the average mass concentration percentage b of K element in the core region corresponding to 1 / 6 radius of the primary particle containing lithium transition metal oxide satisfies: 0.01% ≤ b ≤ 0.05%.

[0031] In some embodiments, the lithium-containing transition metal phosphate includes the molecular formula Li n2 Fe x2 Mn y2 P z2 O j Q q The compound or its modified compound, wherein Q includes at least one of Al, Na, K, Mg, Cu, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, 0.6≤n2≤1.15, 0<x2, 0≤y2, 0.9≤x2+y2≤1, 0.95≤z2≤1, 3.5≤j≤4, 0≤q≤0.1.

[0032] In some embodiments, the mass fraction m3 of the lithium transition metal phosphate, based on the total mass of the positive electrode active material, satisfies: 5% ≤ m3 ≤ 40%.

[0033] In some embodiments, when the lithium-containing transition metal phosphate includes lithium iron phosphate, 5% ≤ m3 ≤ 15%; or when the lithium-containing transition metal phosphate includes lithium manganese iron phosphate, 10% ≤ m3 ≤ 40%.

[0034] In some embodiments, the specific surface area BET2 of the positive electrode sheet satisfies: 2m² 2 / g≤BET2≤5m 2 / g.

[0035] In some embodiments, the porosity p of the positive electrode sheet, as measured by the true density method, satisfies: 22% ≤ p ≤ 28%.

[0036] In some embodiments, the compaction density ρ of the positive electrode sheet satisfies: 3.0 g / cm³ 3 ≤ρ≤3.5g / cm 3 .

[0037] In a second aspect, an electrical device is provided, the electrical device comprising a lithium-ion secondary battery as implemented in any of the embodiments of the first aspect.

[0038] Thirdly, a charging method for a lithium-ion secondary battery is provided, the charging method comprising charging the lithium-ion secondary battery at a preset rate Cset to a cutoff voltage v; and charging the lithium-ion secondary battery at the cutoff voltage v to a cutoff current i; wherein, 4.3V≤v≤5V, 0.02C≤i≤0.1C. Attached Figure Description

[0039] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the embodiments of this application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the drawings without creative effort.

[0040] Figure 1 is a cross-sectional view of a positive electrode sheet along its thickness.

[0041] Figure 2 is a partial view of a cross-section in the thickness direction of a positive electrode sheet.

[0042] Figure 3 is a schematic diagram of a lithium-ion secondary battery.

[0043] Figure 4 is a schematic diagram of a single battery cell.

[0044] Figure 5 is a schematic flowchart of a charging method for a lithium-ion secondary battery. Detailed Implementation

[0045] The embodiments of this application are hereby disclosed in detail with appropriate reference to the accompanying drawings. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of actually identical structures may be omitted. This is to avoid making the following description unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0046] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0047] In the description of this application, it should be noted that, unless otherwise stated, "a plurality of" means two or more; the terms "upper," "lower," "left," "right," "inner," "outer," etc., indicating orientation or positional relationships are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on this application. Furthermore, the terms "first," "second," "third," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0048] Unless otherwise specified, in this application, the phrase "A and / or B" means "A, B, or both A and B". More specifically, the condition "A and / or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0049] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0050] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0051] Unless otherwise specified, the following terms have the following meanings. Any undefined terms have their technically accepted meanings.

[0052] If mentioned, "charging cut-off voltage" refers to the highest charging voltage reached by a lithium-ion secondary battery during the charging process. When the lithium-ion secondary battery reaches this voltage, the charging process will stop to prevent overcharging, which could damage the lithium-ion secondary battery or cause safety issues.

[0053] As mentioned, "discharge plateau voltage" refers to the voltage corresponding to the "plateau region" in the voltage-capacity or voltage-time curve under constant current discharge conditions. In the voltage-capacity or voltage-time curve of the discharge process, the voltage first drops rapidly as the discharge capacity or discharge time increases, then remains almost constant or changes very little, and then drops rapidly to the cutoff voltage. The part of the curve where the voltage remains almost constant or changes very little is the "plateau region." The discharge plateau voltage can be the voltage corresponding to the plateau region (voltage almost constant) or the median of the voltage range corresponding to the plateau region (voltage changing). The discharge plateau voltage of a certain material is usually measured through a half-cell composed of that material and lithium metal.

[0054] If mentioned, "lithium-containing transition metal phosphates" refers to a class of salts that include lithium, transition metals, and phosphate ions. Examples include lithium iron phosphate materials and lithium manganese iron phosphate materials.

[0055] As mentioned, "lithium-containing transition metal oxides" refers to a class of oxides that include lithium and transition metal elements. Structurally, this includes ternary materials with layered structures, such as LiCoO2 and LiNiO2, as well as LiMn2O4 with a spinel structure. Ternary materials refer to lithium transition metal oxides containing three different transition metal elements. It should be understood that ternary materials can also be doped or coated with trace amounts of other transition metal elements; generally, ternary materials doped or coated with other transition metal elements are still considered ternary materials.

[0056] If mentioned, “cyclic sulfate compounds” refers to cyclic compounds that include the -O-SO2-O- group, wherein the -O-SO2-O- group is usually located on the ring of the cyclic compound.

[0057] The embodiments of this application will be described next.

[0058] A single cell in a lithium-ion secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator. During the charging and discharging process of a lithium-ion secondary battery, active ions repeatedly insert and extract between the positive and negative electrodes. The electrolyte acts as a conductor of active ions between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, prevents short circuits while allowing active ions to pass through, thus ensuring the normal conduction of the electrochemical reaction.

[0059] Improving the energy density of lithium-ion rechargeable batteries has always been a key research focus in the battery field. The positive electrode active material is one of the direct factors affecting the energy density of lithium-ion rechargeable batteries. Lithium-containing transition metal oxides have high specific capacity, which helps to improve the energy density of lithium-ion rechargeable batteries. However, lithium-containing transition metal oxides have poor power performance at low SOC. Specifically, in the later stages of discharge of lithium-ion rechargeable batteries with lithium-containing transition metal oxides in the positive electrode, at low SOC (e.g., 10% SOC and below), the voltage of the lithium-ion rechargeable battery experiences a sudden drop, resulting in poor power performance at low SOC and affecting the overall power density of the lithium-ion rechargeable battery. Lithium-containing transition metal phosphates, on the other hand, have a discharge plateau at low SOC, which can regulate the power performance of lithium-ion rechargeable batteries at low SOC. The primary particles of lithium-containing transition metal phosphates are typically nanoscale, with a small average particle size, short lithium-ion transport paths, fast ion transport rates, and good kinetic performance, which are more beneficial to the power performance of lithium-ion rechargeable batteries. Primary lithium transition metal oxide particles are typically micrometer-sized, with a relatively large average particle size. Combining these particles with smaller lithium transition metal oxide particles can increase the compaction density of the positive electrode, thereby improving the energy density of lithium-ion secondary batteries. Therefore, by compounding lithium transition metal phosphates and lithium transition metal oxides in the positive electrode active material and grading the particle sizes of the two substances, a lithium-ion secondary battery that balances energy density and power density can be obtained.

[0060] However, the applicant's research found that, although the principle is not yet clear, when the positive electrode active material includes lithium-containing transition metal oxides and lithium-containing transition metal phosphates, the lithium-containing transition metal oxides actually accelerate oxygen release and react with the electrolyte, causing the transition metal in the positive electrode active material to dissolve and deposit on the negative electrode, resulting in a deterioration in the storage performance of the lithium-ion secondary battery.

[0061] In view of this, embodiments of this application provide a lithium-ion secondary battery and its charging method and power device. The positive electrode active material of the lithium-ion secondary battery includes a lithium-containing transition metal oxide and a second positive electrode active material. The average particle size of the primary particles containing the lithium transition metal oxide and the average particle size of the primary particles containing the lithium transition metal phosphate are controlled within appropriate ranges. The electrolyte includes a cyclic sulfate compound, which enables the lithium-ion secondary battery to have good storage performance under high power density.

[0062] The lithium-ion secondary battery provided in this application will be described next.

[0063] [Lithium-ion rechargeable battery]

[0064] Firstly, a lithium-ion secondary battery is provided, comprising a positive electrode and an electrolyte. The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one side of the surface of the positive current collector. The positive electrode film layer includes a positive electrode active material, which includes a first positive electrode active material and a second positive electrode active material. The first positive electrode active material includes a lithium-containing transition metal oxide, and the second positive electrode active material includes a lithium-containing transition metal phosphate. The average particle size D1 of the primary particles of the first positive electrode active material satisfies: 1.6 μm ≤ D1 ≤ 2.6 μm; the average particle size D2 of the primary particles of the second positive electrode active material satisfies: 40 nm ≤ D2 ≤ 250 nm. The electrolyte includes a solvent, an additive, and a lithium salt, wherein the additive includes a cyclic sulfate compound.

[0065] The positive electrode active material includes lithium-containing transition metal oxides and lithium-containing transition metal phosphates, which enable lithium-ion secondary batteries to have good power density. However, this also makes lithium-containing transition metal oxides more prone to oxygen release and side reactions with the electrolyte. This may be because lithium-containing transition metal oxide particles are relatively large, while lithium-containing transition metal phosphate particles are typically small. This gradation increases the specific surface area of ​​the overall positive electrode active material and the surface area of ​​the positive electrode sheet. The larger contact area between the positive electrode active material and the electrolyte exacerbates the side reactions. These side reactions can lead to the dissolution of transition metals from the positive electrode active material, damaging the interfacial film between the positive electrode active material and the electrolyte, further intensifying the side reactions. Furthermore, the dissolved transition metals may also deposit on the negative electrode, hindering lithium-ion transport and affecting the electrochemical reactions of lithium ions. Therefore, this impacts the storage performance and lifespan of the lithium-ion secondary battery.

[0066] The lithium-ion secondary battery of this application, on the one hand, introduces cyclic sulfate compounds as additives into the electrolyte. Cyclic sulfate compounds can undergo ring-opening in the electrolyte and participate in the film-forming reactions of the positive and negative electrodes. The participation of cyclic sulfate compounds in the interfacial film-forming reaction can form a dense and uniform interfacial film. Its participation in film formation at the positive electrode can effectively reduce the contact area between the positive electrode active material and the electrolyte, inhibiting the dissolution of transition metals; its participation in film formation at the negative electrode can prevent dissolved metal ions from depositing at the negative electrode. On the other hand, the average particle size of the primary particles of the first positive electrode active material is controlled within the range of 1.6 μm-2.6 μm, and the average particle size of the primary particles of the second positive electrode active material is controlled within the range of 40 nm-250 nm. The above-mentioned particle size combination of primary particles in the mixed cathode system can not only achieve a high compaction density, but also enable the lithium transition metal phosphate particles to be distributed around the lithium transition metal oxide, which can physically isolate the lithium transition metal oxide from the electrolyte, thereby reducing the side reactions between the lithium transition metal oxide and the electrolyte, thus inhibiting the dissolution of transition metal and stabilizing the CEI film.

[0067] Therefore, when the positive electrode active material includes lithium-containing transition metal oxides and lithium-containing transition metal phosphates, by controlling the average particle size of the primary particles of the two positive electrode active materials and using cyclic sulfate compounds as additives in the electrolyte, the storage performance of lithium-ion secondary batteries can be effectively improved, enabling lithium-ion secondary batteries to achieve both good power density and storage performance.

[0068] For positive electrode active materials, the specific surface area refers to the total surface area per unit mass of material. In other words, the specific surface area of ​​a positive electrode active material is the ratio of the sum of the specific surface areas of all particles of the positive electrode active material to the mass of all particles.

[0069] The specific surface area of ​​the cathode active material increases after mixing lithium-containing transition metal oxides and lithium-containing transition metal phosphates. This may be due to two main reasons: firstly, lithium-containing transition metal phosphates themselves have smaller particles, resulting in a larger specific surface area than the larger particles of lithium-containing transition metal oxides. Mixing them increases the specific surface area of ​​the cathode active material. Secondly, during the preparation of the cathode active material, lithium-containing transition metal oxides and lithium-containing transition metal phosphates are typically physically mixed using methods such as ball milling. During this physical mixing process, the particles of both materials may be further broken down, increasing the number of small particles in the cathode active material and further increasing its specific surface area. It should be understood that the terms "larger particles" and "smaller particles" in this application refer to particle size.

[0070] Specifically, D1 can be 1.6μm, 1.7μm, 1.8μm, 1.9μm, 2μm, 2.1μm, 2.2μm, 2.3μm, 2.4μm, 2.5μm, or 2.6μm, or a value within the range obtained by any combination of the above two values. D2 can be 40nm, 50nm, 60nm, 70nm, 80nm, 90nm, 100nm, 110nm, 120nm, 130nm, 140nm, 150nm, 160nm, 170nm, 180nm, 190nm, 200nm, 210nm, 220nm, 230nm, 240nm, or 250nm, or a value within the range obtained by any combination of the above two values.

[0071] For primary particles containing lithium transition metal phosphates, a smaller average particle size results in a shorter lithium-ion transport path, faster transport speed, and better kinetic performance, which is beneficial for improving the power performance of lithium-ion rechargeable batteries. However, if the particle size of the primary particles containing lithium transition metal phosphates is too small, the increase in the specific surface area of ​​the entire positive electrode active material is significant. This can lead to a larger contact area between the positive electrode active material and the electrolyte, rapidly increasing the risk of side reactions and affecting the storage performance of the lithium-ion rechargeable battery. Therefore, controlling the average particle size of the primary particles containing lithium transition metal phosphates within the aforementioned range is more conducive to achieving a balance between power performance and storage performance in the battery cell.

[0072] For lithium-containing transition metal oxides, a larger average particle size results in a smaller specific surface area, fewer side reactions with the electrolyte, a more stable CEI film, and a higher reversible capacity of the lithium-ion secondary battery. Larger lithium-containing transition metal oxide particles within a suitable particle size range have a more complete crystal structure and stronger stress resistance, enabling the lithium-ion secondary battery to operate at higher voltages. Therefore, in typical mixed cathode active materials, the primary particle size of lithium-containing transition metal oxides is usually set to be relatively large. However, in this application, in the mixed cathode active material, when the average particle size of the primary lithium-containing transition metal phosphate particles is in the range of 40 nm-250 nm, and the electrolyte includes cyclic sulfate compounds, the average particle size of the primary lithium-containing transition metal oxide particles can be set to be relatively small, controlled within the range of 1.6 μm-2.6 μm. This is due to the physical isolation of lithium transition metal phosphates with a particle size range of 40nm-250nm from lithium transition metal oxides with a particle size range of 1.6μm-2.6μm, and the film-forming reaction involving cyclic sulfate compounds in the electrolyte. The combined effect of these two factors effectively reduces the side reactions between the entire positive electrode active material and the electrolyte.

[0073] In summary, this application achieves a balance between high power performance and good storage performance in lithium-ion secondary batteries through the comprehensive design of positive electrode active materials and electrolytes.

[0074] In addition, by controlling the average particle size of primary particles containing lithium transition metal phosphate and the average particle size of primary particles containing lithium transition metal oxide within appropriate ranges, the BET of the positive electrode active material can also be adjusted, so that the Dv50 and BET of the positive electrode active material and the BET of the positive electrode sheet are within appropriate ranges, thereby improving the power performance and storage performance of lithium-ion secondary batteries.

[0075] In some embodiments, the charging cutoff voltage v of the lithium-ion secondary battery satisfies: 4.3V≤v≤5V.

[0076] Specifically, v can be 4.3V, 4.35V, 4.4V, 4.45V, 4.5V, 4.55V, 4.6V, 4.65V, 4.7V, 4.75V, 4.8V, 4.85V, 4.9V, 4.95V, 5V, or a value within the range obtained by any combination of the above two values.

[0077] Lithium-containing transition metal oxides possess high specific capacity, which helps improve the energy density of lithium-ion secondary batteries. However, during the actual charging process of lithium-ion secondary batteries, not all lithium ions can be extracted from the lithium-containing transition metal oxide material. For the same mass or volume, the higher the charging cut-off voltage of the lithium-ion secondary battery, the more lithium ions can be extracted from the positive electrode active material, thereby increasing the specific capacity of the positive electrode active material and resulting in a higher energy density for the lithium-ion secondary battery.

[0078] Meanwhile, for lithium-containing transition metal oxides, lithium-containing transition metals are more likely to release oxygen at higher charging cutoff voltages, and the structure of lithium-containing transition metal phosphates may become unstable.

[0079] In the embodiments of this application, thanks to the cyclic sulfate compounds in the electrolyte, a stable interfacial film can be formed between the solid and liquid interfaces to reduce side reactions of lithium-containing transition metal oxides at high charging cutoff voltages. This interfacial film also ensures that the lithium-containing transition metal phosphates maintain a stable structure at high charging cutoff voltages. Therefore, the lithium-ion secondary battery of this application can achieve a charging cutoff voltage of 4.3V-5V, further improving the energy density of the lithium-ion secondary battery.

[0080] It should be understood that, considering the normal error of test conditions, if the charging cut-off voltage under the above conditions falls within the range of 25℃±2℃ and falls within the range defined in this application, it can be considered equivalent to the test result at 25℃ and thus fall within the range of this application.

[0081] The discharge plateau voltage of active materials is related to the type of material, its crystal structure, and its lithium storage mechanism. During the cycling process of a lithium-ion secondary battery, the discharge plateau voltage of the active material will vary with changes in the active material. The charging cutoff voltage of a lithium-ion secondary battery can be set within the aforementioned range based on factors such as the electrolyte voltage window, the potential difference between the positive and negative electrodes, and the discharge plateau voltage of the active material. The electrolyte voltage window refers to a voltage range that allows the electrolyte to maintain a constant state without decomposition; the electrolyte composition affects the voltage window. The potential difference between the positive and negative electrodes is related to the type of active material.

[0082] In some embodiments, the cyclic sulfate compounds include at least one of compounds of formula (I) or modified compounds thereof: R1-R4 includes at least one of the following: halogenated or unhalogenated H, alkyl with 1-6 carbon atoms, alkenyl with 2-6 carbon atoms, alkoxy with 1-6 carbon atoms, acyloxy with 1-6 carbon atoms, ester with 2-6 carbon atoms, and cyclic sulfate ester with 4-membered to 7-membered rings.

[0083] In some embodiments, R1-R4 further include at least one group of formulas (II-I) to (II-IV): in, Indicating the bonding position, R5 includes at least one of halogen, alkyl group having 1-6 carbon atoms, haloalkyl group having 1-6 carbon atoms, and alkoxy group having 1-6 carbon atoms. The group of formula (II-I) to (II-IV) is bonded to R1 and R3, or to R2 and R4.

[0084] In some embodiments, the cyclic sulfate compounds have a polycyclic structure. In other words, the cyclic sulfate compounds have two or more multi-membered rings. For example, the cyclic sulfate compounds have two or more five-membered rings.

[0085] Compared to cyclic sulfate compounds with a single ring, cyclic sulfate compounds with a multi-ring structure can form a more complete and dense interfacial film, which can more effectively inhibit the dissolution of transition metals and prevent the dissolved metal ions from depositing on the negative electrode, thereby further improving the storage performance of lithium-ion secondary batteries and enabling lithium-ion secondary batteries to have good power density and storage performance.

[0086] In some embodiments, the cyclic sulfate compounds include at least one of the compounds with the molecular formulas (II) to (I-XII):

[0087] Cyclic sulfates can undergo ring-opening decomposition in electrolytes to generate sulfur-containing polymers, which helps to improve the integrity and density of the interfacial film, thereby reducing side reactions at the solid-liquid interface.

[0088] In some embodiments, the mass fraction m1 of the cyclic sulfate compound, based on the total mass of the electrolyte, satisfies: 0.1% ≤ m1 ≤ 5%; optionally, 0.5% ≤ m1 ≤ 2%.

[0089] Specifically, m1 can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2 The concentrations of cyclic sulfate compounds in the electrolyte are 0.7%, 2.8%, 2.9%, 3%, 3.1%, 3.2%, 3.3%, 3.4%, 3.5%, 3.6%, 3.7%, 3.8%, 3.9%, 4%, 4.1%, 4.2%, 4.3%, 4.4%, 4.5%, 4.6%, 4.7%, 4.8%, 4.9%, and 5%, or values ​​within any combination of the above values. A higher concentration of cyclic sulfate compounds in the electrolyte is more conducive to the formation of the interfacial film. However, excessive amounts of cyclic sulfate compounds may result in an overly thick interfacial film, hindering the extraction of lithium ions from the active material and potentially worsening the internal impedance of the lithium-ion secondary battery, thus affecting its lifespan. Therefore, by controlling the mass fraction of cyclic sulfate compounds in the electrolyte within a specific range, it is possible to improve the storage performance of the lithium-ion secondary battery while simultaneously extending its lifespan.

[0090] In some embodiments, the ionic conductivity σ of the electrolyte satisfies: 7.5 mS / cm ≤ σ ≤ 15 mS / cm.

[0091] Specifically, σ can be 7.5 mS / cm, 8 mS / cm, 8.5 mS / cm, 9 mS / cm, 9.5 mS / cm, 10 mS / cm, 10.5 mS / cm, 11 mS / cm, 11.5 mS / cm, 12 mS / cm, 12.5 mS / cm, 13 mS / cm, 13.5 mS / cm, 14 mS / cm, 14.5 mS / cm, or 15 mS / cm, or a value within the range obtained by any combination of the above two values. Higher ionic conductivity of the electrolyte is more beneficial to the lithium-ion kinetics within the lithium-ion secondary battery, enabling rapid lithium-ion transport within the electrolyte. This allows the lithium-ion secondary battery to achieve a larger charge or discharge rate, further improving the power density. However, excessively high ionic conductivity of the electrolyte is detrimental to its stability, potentially increasing side reactions within the lithium-ion battery and affecting its storage performance. Therefore, by controlling the ionic conductivity of the electrolyte within a suitable range, it is helpful to improve the lithium-ion kinetics during the secondary battery cycle in some embodiments, thereby helping to improve the power density and storage performance of the secondary battery in some embodiments.

[0092] In some embodiments, the lithium salt includes at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorobis(oxalate)phosphate, lithium bis(oxalate)borate, lithium difluorooxalateborate, or lithium tetrafluorooxalate; optionally, the lithium salt includes lithium hexafluorophosphate.

[0093] In the embodiments of this application, when the charging cutoff voltage of the lithium-ion secondary battery is between 4.3V and 5V, lithium hexafluorophosphate is preferably used as the lithium salt. Lithium hexafluorophosphate maintains good chemical stability under high voltage and is not prone to side reactions with other components in the lithium-ion secondary battery.

[0094] In some embodiments, the concentration c of lithium salt in the electrolyte satisfies: 0.9M ≤ c ≤ 1.2M.

[0095] Specifically, it can be 0.9M, 0.95M, 1M, 1.1M, 1.15M, 1.2M, or a value within the range obtained by any combination of the above two values.

[0096] A higher lithium salt concentration results in a greater number of lithium ions in the electrolyte, which helps improve the electrolyte's ionic conductivity and also enhances lithium-ion kinetics within the lithium-ion battery. For example, during discharge, when the lithium-ion battery is in a low SOC state, the lithium-ion concentration in the negative electrode active material is low, making lithium-ion extraction difficult and resulting in a slow lithium-ion migration rate. In the same amount of time, lithium ions cannot escape from the negative electrode to reach the positive electrode, while electrons moving through the external circuit have already reached the positive electrode. Lithium ions in the electrolyte can quickly reach the positive electrode and embed themselves in the positive electrode active material, increasing the lithium-ion migration rate within the lithium-ion battery. Conversely, an excessively high lithium salt concentration may increase the electrolyte viscosity, which is detrimental to the rapid movement of lithium ions within the electrolyte and affects lithium-ion kinetics. Therefore, by controlling the lithium salt concentration within a specific range, the lithium-ion kinetics within the lithium-ion battery can be improved, enabling the lithium-ion battery to stably output power even at low SOC states, further increasing the power density of the lithium-ion battery.

[0097] In some embodiments, the solvent includes at least one of carbonate solvents and carboxylic acid ester solvents.

[0098] In some embodiments, the carbonate solvent includes at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, dimethyl carbonate, and diethyl carbonate.

[0099] Carbonate solvents help improve the ionic conductivity of the electrolyte, thereby helping to reduce the internal impedance of lithium-ion secondary batteries and further improving their power performance.

[0100] In some embodiments, the carboxylic acid ester solvent includes at least one of methyl formate, ethyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl acrylate, and ethyl acrylate.

[0101] Carboxylic acid ester solvents also help improve the ionic conductivity of the electrolyte, thereby helping to reduce the internal impedance of secondary lithium-ion batteries and further improve the power performance of lithium-ion batteries.

[0102] In some embodiments, the carbonate solvent includes ethylene carbonate, and the mass fraction m2 of ethylene carbonate, based on the total mass of the electrolyte, satisfies: 10% ≤ m2 ≤ 30%.

[0103] Specifically, m2 can be 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, or a value within the range obtained by any combination of the above two values.

[0104] Ethylene carbonate has a high dielectric constant, which helps to dissociate lithium salts in the electrolyte, improves the ionic conductivity of the electrolyte, and further helps to improve the power performance of lithium-ion secondary batteries.

[0105] In some embodiments, the additive includes fluoroethylene carbonate.

[0106] By introducing fluoroethylene carbonate into the additives, the fluoroethylene carbonate can also participate in the film-forming reaction of the interfacial film. This helps to increase the inorganic components in the interfacial film and combine them with the organic components formed by cyclic sulfate esters. The resulting interfacial film is more stable, further improving the storage performance of lithium-ion secondary batteries.

[0107] In some embodiments, in a cross-section of the positive electrode film layer along the thickness direction of the positive electrode sheet, primary particles of the first positive electrode active material surround an aggregate of at least a portion of the primary particles of the second positive electrode active material.

[0108] Figure 1 is a cross-sectional view of a positive electrode sheet along the thickness direction according to this application. Figure 2 is a partial cross-sectional view of a positive electrode sheet along the thickness direction.

[0109] As shown in Figure 1-2, the primary particles of lithium transition metal oxides have a larger particle size and a shape closer to a blocky structure; the primary particles of lithium transition metal phosphates have a smaller particle size and form agglomerates, which are closer to circular or elliptical shapes. The primary particles of lithium transition metal oxides surround at least part of the agglomerates of primary particles of lithium transition metal phosphates, resulting in fewer pores between particles, which is beneficial for the compaction of the positive electrode film.

[0110] In some embodiments, the BET1 of the positive electrode active material satisfies: 0.5m 2 / g≤BET1≤4.5m 2 / g.

[0111] Specifically, BET1 can be 0.5m 2 / g、1m 2 / g, 1.5m 2 / g、2m 2 / g, 2.5m 2 / g、3m 2 / g, 3.5m 2 / g、4m 2 / g, 4.5m 2 / g, or a value within the range obtained by combining any two of the above values. As mentioned earlier, lithium transition metal oxide particles are generally large, while lithium transition metal phosphate particles are generally small. The combination of the two increases the specific surface area of ​​the positive electrode active material, exacerbating side reactions between the positive electrode active material and the electrolyte. Therefore, in the embodiments of this application, when the positive electrode active material includes lithium transition metal phosphate and lithium transition metal oxide, controlling the specific surface area of ​​the positive electrode active material within a suitable range can help improve the side reactions between the positive electrode active material and the electrolyte, thereby improving the storage performance of the lithium-ion secondary battery.

[0112] The BET (Benefit Element) of positive electrode active materials can be tested using methods known in the art. For example, the lithium-ion secondary battery under test can be disassembled to obtain the positive electrode sheet. After sintering to remove conductive agents, binders, and other components, the remaining components on the positive electrode current collector can be scraped off for BET testing to obtain the BET of the positive electrode active material. It should be understood that some positive electrode active materials may contain a small amount of carbon, which may be removed during the sintering process. However, due to the small content, its impact on the BET of the positive electrode active material is generally considered negligible.

[0113] In some embodiments, the volume distribution particle size Dv50 of the positive electrode active material satisfies: 3.2μm≤Dv50≤6μm.

[0114] Specifically, Dv50 can be 3.2μm, 3.4μm, 3.6μm, 3.8μm, 4μm, 4.2μm, 4.4μm, 4.6μm, 4.8μm, 5μm, 5.2μm, 5.4μm, 5.6μm, 5.8μm, 6μm, or a value within the range obtained by any combination of the above two values. The larger the volumetric particle size distribution of the positive electrode active material, the smaller its specific surface area. Therefore, by controlling the volumetric particle size distribution of the positive electrode active material, the specific surface area of ​​the positive electrode active material can be regulated within the above range, helping to improve the side reactions between the positive electrode active material and the electrolyte, and enhancing the storage performance of lithium-ion secondary batteries.

[0115] In some embodiments, the BET of the primary particles of the first positive electrode active material is 0.5 m. 2 / g-1m 2 / g; and / or the BET of the primary particles of the second positive electrode active material is 7m. 2 / g-16m 2 / g.

[0116] In some embodiments, lithium-containing transition metal oxides include those with the molecular formula Li n1 (Ni x1 Co y1 Mn z1 ) 1-n G n O 2-m K r At least one of the compounds or their modified compounds, wherein G includes one or more of Zr, Al, Nb, Sr, B, Ba, Mg, Sn, Y, Na, Si, W and Ti, K includes one or more of P, F, N, S, F, Cl and I, 0.6≤n1≤1.5, 0.3≤x1≤0.7, 0.02≤y1≤0.15, x1+y1+z1=1, 0<n≤0.1, 0<m≤0.1, 0<r≤0.1.

[0117] By selecting lithium-containing transition metal oxides with a nickel content of 0.3-0.7%, the problem of lithium-nickel mixing at higher charging cutoff voltages can be mitigated. Doping with G and K elements further stabilizes the crystal lattice of the lithium-containing transition metal oxides, further improving the lithium-nickel mixing problem at higher charging cutoff voltages.

[0118] In some embodiments, the average mass concentration of K element in the outer region corresponding to 1 / 6 radius from the surface of the primary particle containing lithium transition metal oxide is greater than the average mass concentration of K element in the core region corresponding to 1 / 6 radius from the center of the primary particle.

[0119] Modification with potassium (K) can stabilize the crystal structure of lithium-containing transition metal oxides, ensuring structural stability under high voltages of 4.3V-5V. In this application, modification of lithium-containing transition metal oxides with K can specifically manifest as doping and / or coating. In some embodiments, the modification is manifested as coating.

[0120] When the average mass concentration of potassium (K) on the surface of primary particles containing lithium transition metal oxides is greater than that at the center of the primary particles, K accumulates on the surface of the material particles. Inert coating can alleviate the side reactions at the cathode interface, ensure that the material structure is not damaged under high voltage, and enable the secondary battery to have better high-temperature cycle performance and high-temperature storage performance.

[0121] In some embodiments, the average mass concentration α of K element in the outer region corresponding to 1 / 6 radius of the surface of the primary particles containing lithium transition metal oxide satisfies: 0.1% ≤ a ≤ 0.5%.

[0122] When the mass concentration of K element on the surface of primary particles containing lithium transition metal oxide is 0.1%-0.5%, the lithium-ion secondary battery of this application has good high-temperature storage performance.

[0123] In some embodiments, the average mass concentration percentage b of K element in the core region corresponding to 1 / 6 radius of the center of the primary particle containing lithium transition metal oxide satisfies: 0.01% ≤ b ≤ 0.05%.

[0124] When the mass concentration of K element at the center of the primary particles containing lithium transition metal oxide is 0.01%-0.05%, the lithium-ion secondary battery of this application has good high-temperature storage performance.

[0125] In some embodiments, the phosphorus content at the surface of the primary lithium transition metal oxide particles is greater than the phosphorus content at the center of the primary particles.

[0126] When the phosphorus content on the surface of the primary particles of lithium transition metal oxide is greater than that in the center of the primary particles, a good coating layer will be formed due to the high phosphorus content at the grain boundaries. This passivates the surface of the lithium transition metal oxide material, provides a good physical barrier between the electrode and the electrolyte, reduces the side reactions between the positive electrode active material and the electrolyte, ensures that the material structure is not damaged under high voltage, and enables the secondary battery to have good high-temperature storage performance and cycle performance.

[0127] In other embodiments, examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also known as NCM333), LiNi 0.5 Co 0.2 Mn 0.3O2 (also known as NCM523), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM211), LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM622), LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM811), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.15 Al 0.05 At least one of O2 and its modified compounds.

[0128] It should be understood that, for the aforementioned positive electrode active materials, lithium-ion secondary batteries experience Li intercalation / deintercalation and consumption during charging and discharging. Therefore, the molar content of Li in the positive electrode active material varies depending on the discharge state of the lithium-ion secondary battery. In the examples of positive electrode active materials in this application, the molar content of Li refers to the initial state of the material, i.e., the state before feeding. After charge-discharge cycles, the molar content of Li changes when the positive electrode active material is applied to the battery system. Similarly, in the examples of positive electrode active materials in this application, the molar content of O is only an ideal value. Lattice oxygen release causes changes in the molar content of O, and the actual molar content of O will fluctuate.

[0129] In some embodiments, lithium-containing transition metal phosphates include those with the molecular formula Li. n2 Fe x2 Mn y2 P z2 O j Q q The compound or its modified compound, wherein Q includes at least one of Al, Na, K, Mg, Cu, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, 0.6≤n2≤1.15, 0<x2, 0≤y2, 0.9≤x2+y2≤1, 0.95≤z2≤1, 3.5≤j≤4, 0≤q≤0.1.

[0130] In other embodiments, examples of lithium phosphates may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0131] In some embodiments, the mass fraction m3 of the second positive electrode active material, based on the mass of the positive electrode active material, satisfies: 5% ≤ m3 ≤ 40%.

[0132] Specifically, m3 can be 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, or a value within the range obtained by any combination of the above two values.

[0133] In some embodiments, in Li z (Ni a Co b Mn c ) d M e O f A g When c is less than 0.4, lithium iron phosphate can be selected as the second positive electrode active material. m3 can be in the range of 5%-15%, and 10% is optional. By mixing 5%-15% lithium iron phosphate, a good power density of lithium-ion secondary batteries can be achieved.

[0134] In some embodiments, in Li z (Ni a Co b Mn c ) d M e O f A g When c is greater than or equal to 0.4, lithium manganese iron phosphate can be selected as the second positive electrode active material. m3 can be in the range of 10%-40%, and can be selected as 30%. By mixing 10%-40% lithium manganese iron phosphate, lithium-ion secondary batteries can have good power density.

[0135] By mixing 5%-40% lithium-containing transition metal phosphates into the positive electrode active material, the power density of lithium-ion secondary batteries has been improved. Simultaneously, the applicant also noted that compared to mixing small amounts (e.g., less than 5%) of lithium-containing transition metal phosphates, when the positive electrode active material includes 5%-40% lithium-containing transition metal phosphates, the specific surface area of ​​the positive electrode active material is larger, and the side reaction problem with the electrolyte is more severe. Therefore, cyclic sulfate compounds with polycyclic structures are preferred as additives, as they can form a denser interfacial film compared to monocyclic cyclic sulfate compounds. Thus, by using a high proportion of lithium-containing transition metal phosphates and cyclic sulfate compounds with polycyclic structures, the power density and storage performance of lithium-ion secondary batteries can be further improved.

[0136] In some embodiments, the specific surface area BET2 of the positive electrode sheet satisfies: 2m² 2 / g≤BET2≤5m 2 / g.

[0137] Specifically, BET2 can be 2m 2 / g, 2.5m 2 / g、3m 2 / g, 3.5m 2 / g、4m 2 / g, 4.5m 2 / g、5m 2 / g, or a value within the range obtained by any combination of the above two values. A larger specific surface area of ​​the electrode improves the wettability of the electrolyte, which provides an ion conduction path for the material, improving kinetic performance and thus enhancing the power performance of the lithium-ion secondary battery. However, it also increases the risk of side reactions. Therefore, the lithium-ion secondary battery of this application, by controlling the specific surface area of ​​the positive electrode within a suitable range, can achieve high power density and good storage performance.

[0138] In some embodiments, the porosity p of the positive electrode sheet, measured by the true density method, satisfies: 22% ≤ p ≤ 28%.

[0139] Specifically, p can be 22%, 23%, 24%, 25%, 26%, 27%, 28%, or a value within the range obtained by any combination of the above two values. Higher electrode porosity improves electrolyte wettability, providing an ion conduction pathway for the material and improving kinetic performance, thereby enhancing the power performance of the lithium-ion secondary battery. However, it also increases the risk of side reactions. Therefore, the lithium-ion secondary battery of this application, by controlling the porosity of the positive electrode within a suitable range, can achieve high power density and good storage performance.

[0140] In some embodiments, the compaction density ρ of the positive electrode sheet satisfies: 3.0 g / cm³ 3 ≤ρ≤3.5g / cm 3 .

[0141] Specifically, ρ can be 3.0 g / cm³. 3 3.1g / cm 3 3.2g / cm 3 3.3g / cm 3 3.4g / cm 3 3.5g / cm 3 The density of the positive electrode sheet, or its value, falls within the range obtained by combining any two of the above values. A higher compaction density of the positive electrode sheet is more beneficial for improving the energy density of lithium-ion secondary batteries. However, excessively high compaction density may affect the wetting of the positive electrode film by the electrolyte, reducing the contact between the electrolyte and the positive electrode active material but severely impacting lithium-ion kinetics. Therefore, by controlling the compaction density of the positive electrode sheet within a suitable range, lithium-ion secondary batteries can achieve higher energy density, power density, and storage performance.

[0142] Next, using a lithium-ion secondary battery as a specific example, a detailed description of the positive electrode, negative electrode, separator, and electrolyte in a lithium-ion secondary battery will be provided. It should be understood that the lithium-ion secondary battery is only an example, and the solution provided in this application can also be applied to other types of secondary batteries, such as sodium-ion batteries, magnesium-ion batteries, and lithium-sulfur batteries.

[0143] [Negative electrode plate]

[0144] A negative electrode typically includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector. The negative electrode film layer includes a negative electrode active material.

[0145] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0146] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0147] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. Silicon-based materials may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0148] In some embodiments, the negative electrode active material is a silicon-containing material. The silicon-containing material includes at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, silicon-containing alloys, or silicon-oxygen-carbon composites. By selecting a silicon-containing material as the negative electrode active material, it is beneficial to further improve the volumetric energy density of the battery cell 20. Combined with the structural design of the battery cell 20 in the aforementioned embodiments, the battery cell 20 can possess both high energy density and excellent safety performance.

[0149] In one embodiment, the negative electrode film layer further includes an adhesive. The adhesive may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0150] In some embodiments, the negative electrode film layer further includes a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0151] In some embodiments, the negative electrode film layer also includes other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0152] In some embodiments, the negative electrode sheet can be prepared by forming a negative electrode slurry using the components described above. For example, the negative electrode active material, conductive agent, binder, and any other components are dispersed in a solvent (e.g., N-methylpyrrolidone) to form a negative electrode slurry. The negative electrode slurry is then coated onto a negative electrode current collector, and after drying, cold pressing, and other processes, the negative electrode sheet is obtained.

[0153] [Positive electrode plate]

[0154] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector. The positive electrode film layer includes a positive electrode active material, which includes a first positive electrode active material and a second positive electrode active material.

[0155] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0156] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0157] In some embodiments, the positive electrode film layer further includes a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0158] In some embodiments, the positive electrode film layer further includes a conductive agent. As an example, the conductive agent may also include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, graphene, and carbon nanofibers.

[0159] In some embodiments, the positive electrode sheet can be prepared by forming a positive electrode slurry from the components described above. For example, the positive electrode active material, conductive agent, binder, and any other components are dispersed in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry. The positive electrode slurry is then coated onto a positive electrode current collector, and after drying, cold pressing, and other processes, the positive electrode sheet is obtained.

[0160] Electrolyte

[0161] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements.

[0162] In some embodiments, the electrolyte includes a solvent, which may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0163] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0164] [Isolation membrane]

[0165] This application does not impose any particular restrictions on the type of separator membrane. For example, any well-known porous separator membrane with good chemical and mechanical stability can be selected.

[0166] In some embodiments, the separator includes a porous substrate, the material of which may be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The porous substrate may be a single-layer film or a multi-layer composite film, without particular limitation. When the porous substrate is a multi-layer composite film, the materials of each layer may be the same or different, without particular limitation.

[0167] In some embodiments, the separator further includes a porous coating. The porous coating can serve as a heat-resistant and / or adhesive layer.

[0168] In some embodiments, the porous coating includes heat-resistant particles. The heat-resistant particles may include at least one of inorganic particles and organic particles.

[0169] In some embodiments, inorganic particles may include one or more of the following: inorganic particles having a dielectric constant of 5 or greater, inorganic particles having ion conductivity but not storing ions, or inorganic particles capable of undergoing electrochemical reactions.

[0170] In some embodiments, inorganic particles having a dielectric constant of 5 or higher may include boehmite, alumina, barium sulfate, magnesium oxide, magnesium hydroxide, silicon oxides, tin dioxide, titanium oxide, calcium oxide, zinc oxide, zirconium oxide, yttrium oxide, nickel oxide, hafnium dioxide, cerium oxide, zirconium titanate, barium titanate, magnesium fluoride, aluminum hydroxide, barium oxide, silicon carbide, boron carbide, aluminum nitride, silicon nitride, boron nitride, calcium fluoride, barium fluoride, magnesium aluminum silicate, lithium magnesium silicate, sodium magnesium silicate, bentonite, hydropyrite, Pb(Zr,Ti)O3 (abbreviated as PZT), Pb1-mLamZr1-nTinO3 (abbreviated as PLZT, 0 < m < 1, 0 < n < 1), Pb(Mg3Nb) 2 / 3 The inorganic particles can be selected from one or more of PbTiO3 (PMN-PT) and their respective modified inorganic particles. Optionally, the modification of each inorganic particle can be chemical modification and / or physical modification.

[0171] In some embodiments, inorganic particles that are ion-conductive but do not store ions may include Li3PO4, lithium titanium phosphate (Li3PO4), etc. x1 Ti y1 (PO4)3, Lithium aluminum titanium phosphate (Li) x2 Al y2 Ti z1 (PO4)3、(LiAlTiP) x3 O y3 Type glass, lithium lanthanum titanate (Li) x4 La y4 TiO3, lithium germanium thiophosphate Li x5 Ge y5 P z2 S w Lithium nitride (Li) x6 N y6 SiS2 type glass Li x7 Si y7 S z3 and P2S5 type glass Li x8 P y8 S z4 One or more of the following are given: 0 < x1 < 2, 0 < y1 < 3, 0 < x2 < 2, 0 < y2 < 1, 0 < z1 < 3, 0 < x3 < 4, 0 < y3 < 13, 0 < x4 < 2, 0 < y4 < 3, 0 < x5 < 4, 0 < y5 < 1, 0 < z2 < 1, 0 < w < 5, 0 < x6 < 4, 0 < y6 < 2, 0 < x7 < 3, 0 < y7 < 2, 0 < z3 < 4, 0 < x8 < 3, 0 < y8 < 3, 0 < z4 < 7. This can improve the ion conductivity of the separator.

[0172] In some embodiments, the inorganic particles capable of undergoing electrochemical reactions may include one or more of lithium-containing transition metal oxides, lithium-containing phosphates, carbon-based materials, silicon-based materials, tin-based materials, and lithium-titanium compounds.

[0173] In some embodiments, the organic particles may include at least one of a thermoplastic resin polymer, a thermosetting resin polymer, or a crosslinked polymer.

[0174] In some embodiments, the thermoplastic resin polymer may include one or more of the following: polycarbonate organic particles, polymethyl methacrylate organic particles, polyoxymethylene organic particles, polyamide organic particles, styrene-acrylonitrile copolymer, polyphenylene sulfide organic particles, polyether ether ketone organic particles, polyimide organic particles, polysulfone organic particles, polyether sulfone organic particles, polyphenylene sulfone organic particles, polybenzimidazole organic particles, polyamide-imide organic particles, and polyethyleneimine organic particles.

[0175] In some embodiments, the thermosetting resin polymer may include one or more of the following: phenolic resin organic particles, polymer particles containing triazine ring structural units, epoxy resin organic particles, unsaturated polyester resin organic particles, urea-formaldehyde resin organic particles, and furan resin organic particles.

[0176] In some embodiments, the crosslinking polymer may include one or more of crosslinked styrene organic particles and silicon-containing organic crosslinked resin particles.

[0177] In some embodiments, the porous coating includes binder particles. The binder particles may include homopolymers or copolymers of acrylate monomer units, homopolymers or copolymers of acrylic monomer units, homopolymers or copolymers of styrene monomer units, polyurethane compounds, rubber compounds, homopolymers or copolymers of fluorinated alkenyl monomer units, homopolymers or copolymers of olefinic monomer units, homopolymers or copolymers of unsaturated nitrile monomer units, homopolymers or copolymers of epoxide monomer units, and one or more of the modified compounds of the above homopolymers or copolymers.

[0178] In some embodiments, the adhesive particles may include copolymers of acrylate monomer units and styrene monomer units, copolymers of acrylate monomer units and styrene monomer units, copolymers of acrylate monomer units, acrylate monomer units, and styrene monomer units, copolymers of styrene monomer units and unsaturated nitrile monomer units, copolymers of styrene monomer units, olefin monomer units, and unsaturated nitrile monomer units, polytetrafluoroethylene, polychlorotrifluoroethylene, polyvinylidene fluoride, polyvinylidene fluoride, polyethylene, polypropylene, polyacrylonitrile, polyethylene oxide, copolymers of different fluorinated alkenyl monomer units, copolymers of fluorinated alkenyl monomer units and vinyl monomer units, copolymers of fluorinated alkenyl monomer units and acrylate monomer units, copolymers of fluorinated alkenyl monomer units and acrylate monomer units, and one or more of the modified compounds of the above homopolymers or copolymers.

[0179] In some embodiments, the adhesive particles may include one or more of the following: butyl acrylate-styrene copolymer, butyl methacrylate-isooctyl methacrylate copolymer, isooctyl methacrylate-styrene copolymer, methacrylate-methacrylate-styrene copolymer, methyl acrylate-isooctyl methacrylate-styrene copolymer, butyl acrylate-isooctyl methacrylate-styrene copolymer, butyl acrylate-isooctyl methacrylate-styrene copolymer, butyl methacrylate-isooctyl methacrylate-styrene copolymer, butyl methacrylate-isooctyl methacrylate-styrene copolymer, styrene-acrylonitrile copolymer, styrene-butadiene-acrylonitrile copolymer, methyl acrylate-styrene-acrylonitrile copolymer, isooctyl methacrylate-styrene-acrylonitrile copolymer, styrene-vinyl acetate copolymer, styrene-vinyl acetate-pyrrolidone copolymer, vinylidene fluoride-trifluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-trifluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-hexafluoropropylene-acrylic acid copolymer, vinylidene fluoride-hexafluoropropylene-acrylate copolymer, and a modified compound of the above copolymers.

[0180] In some embodiments, the coating may further include a dispersant, such as one or more of alkylphenol polyoxyethylene ethers, polyacrylic acid dispersants, and cellulose dispersants, including but not limited to. For example, the dispersant may include one or more of sodium carboxymethyl cellulose, sodium polyacrylate, and ammonium polyacrylate.

[0181] Figure 3 is a schematic diagram of a lithium-ion secondary battery according to this application. Figure 4 is a schematic diagram of a single battery cell according to this application. As shown in Figures 3-4, the lithium-ion secondary battery 10 may include one or more battery cells 20 to meet different power requirements. When the lithium-ion secondary battery 10 includes only one battery cell 20, the lithium-ion secondary battery 10 is simply the battery cell 20.

[0182] The lithium-ion secondary battery 10 may further include a housing with a hollow interior, housing multiple battery cells 20. For example, multiple battery cells 20 may be connected in parallel, series, or a combination thereof and then placed inside the housing. The housing may include a first housing portion 101 and a second housing portion 102, which are fitted together to form the housing. The shapes of the first housing portion 101 and the second housing portion 102 may be determined by the shape of the components housed inside, for example, by the shape of the combination of the multiple battery cells 20 housed inside. At least one of the first housing portion 101 and the second housing portion 102 may have an opening. For example, as shown in FIG3, only one of the first housing portion 101 and the second housing portion 102 may be a hollow cuboid with an opening, while the other may be plate-shaped to cover the opening. Taking the second housing portion 102 as a hollow cuboid with an opening, and the first housing portion 101 as a plate as an example, the first housing portion 101 covers the opening of the second housing portion 102 to form a housing with a closed chamber, which can be used to accommodate multiple battery cells 20.

[0183] For example, unlike that shown in Figure 3, the first housing portion 101 and the second housing portion 102 can both be hollow cuboids with one open side each. The openings of the first housing portion 101 and the second housing portion 102 are arranged opposite to each other, and the first housing portion 101 and the second housing portion 102 are interlocked to form a housing with a closed chamber, which can be used to accommodate multiple battery cells 20. The multiple battery cells 20 are connected in parallel, series, or mixed and placed in the housing formed by the interlocking of the first housing portion 101 and the second housing portion 102.

[0184] In some embodiments, the lithium-ion secondary battery 10 may further include other components. For example, the lithium-ion secondary battery 10 may further include a busbar component, which can be used to realize electrical connections between multiple battery cells 20, such as in parallel, series, or mixed connections. Specifically, the busbar component can realize electrical connections between battery cells 20 by connecting to the electrode terminals of the battery cells 20; or, the busbar component can also realize electrical connections between battery cells 20 by connecting to other components of the battery cells 20. The busbar component can be fixed to corresponding components of the battery cells 20 by welding, for example, by welding to electrode terminals, sealing structures, or housings, etc., and the embodiments of this application are not limited thereto.

[0185] The battery cells 20 can be directly assembled into a lithium-ion secondary battery 10, or they can be first assembled into a battery module, and then multiple battery modules can be assembled into a lithium-ion secondary battery 10.

[0186] [Electrical appliances]

[0187] This application provides an electrical device including the lithium-ion secondary battery 10 described in the above embodiments.

[0188] Electrical devices can include vehicles, mobile phones, portable devices, laptops, ships, spacecraft, electric toys, and power tools, etc. Vehicles can be gasoline-powered cars, natural gas-powered cars, or new energy vehicles; new energy vehicles can be pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles, etc. Spacecraft include airplanes, rockets, space shuttles, and spacecraft, etc. Electric toys include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc. Power tools include metal cutting power tools, grinding power tools, assembly power tools, and railway power tools, such as electric drills, electric grinders, electric wrenches, electric screwdrivers, electric hammers, impact drills, concrete vibrators, and electric planers, etc. This application does not impose any special limitations on the above-mentioned electrical devices.

[0189] This application provides an electrical device, which is a vehicle.

[0190] The vehicle can be a gasoline-powered vehicle, a natural gas-powered vehicle, or a new energy vehicle. New energy vehicles can be pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles, etc. The vehicle's interior can house a motor, a controller, and a lithium-ion secondary battery 10. The controller is used to control the lithium-ion secondary battery 10 to supply power to the motor. For example, the lithium-ion secondary battery 10 can be located at the bottom, front, or rear of the vehicle. The lithium-ion secondary battery 10 can be used to power the vehicle; for example, it can serve as the vehicle's operating power source for the vehicle's electrical system, such as meeting the power requirements for starting, navigation, and operation. In another embodiment of this application, the lithium-ion secondary battery 10 can not only serve as the vehicle's operating power source but also as its driving power source, replacing or partially replacing gasoline or natural gas to provide driving power to the vehicle.

[0191] [Charging Method]

[0192] This application also provides a charging method for a lithium-ion secondary battery 10. Figure 5 is a schematic flowchart of a charging method for a lithium-ion secondary battery 10 according to this application.

[0193] As shown in Figure 5, the charging method 300 includes:

[0194] S301 charges the lithium-ion secondary battery to the cutoff voltage v at a preset rate Cset.

[0195] S302, charges the lithium-ion secondary battery to the cutoff current i at the cutoff voltage v.

[0196] Wherein, 4.3V≤v≤5V, 0.02C≤i≤0.1C.

[0197] It should be understood that the preset multiplier Cset can be any preset value, such as 0.1C, 0.2C, 0.5C, 1C, 2C, 3C, etc.

[0198] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0199] [Examples and Comparative Examples]

[0200] Example 1

[0201] (1) Preparation of positive electrode sheet

[0202] The positive electrode active material, binder polyvinylidene fluoride, and conductive agent Super P are mixed in a weight ratio of 97:1.5:1.5, and N-methylpyrrolidone (NMP) solvent is added. The mixture is stirred to form a positive electrode slurry. The slurry is then coated onto a current collector aluminum foil, dried, and subjected to cold pressing, slitting, and cutting to produce the positive electrode sheet for a lithium-ion battery. The positive electrode active material includes a first positive electrode active material, Li(Ni), comprising 90% by mass. 55 Co 12 Mn 33 ) 0.98 Zr 0.02 O 1.97 P 0.03 The second positive electrode active material, LiFePO4, has a mass fraction of 10%, and the BET1 of the positive electrode active material is 2.1m. 2 / g. The BET2 of the positive electrode is 2.9m. 2 / g, the compacted density of the positive electrode sheet ρ=3.25g / cm³ 3 .

[0203] (2) Preparation of negative electrode sheet

[0204] The negative electrode active material graphite, conductive agent carbon black, thickener sodium carboxymethyl cellulose (CMC-Na), and binder styrene-butadiene rubber (SBR) are mixed in a weight ratio of 97:0.5:1:1.5, added to deionized water as a solvent, and stirred to form a negative electrode slurry. The negative electrode slurry is then obtained under the action of a vacuum mixer. The slurry is then coated onto a current collector copper foil, dried, and cold-pressed, slit, and cut into sheets to form the negative electrode sheet of a lithium-ion battery.

[0205] (3) Preparation of electrolyte

[0206] In an argon-atmosphere glove box with a water content <1 ppm and an oxygen content <1 ppm, a non-aqueous solvent, ethylene carbonate (EC), and ethyl methyl carbonate (EMC), were mixed at a mass ratio of 3:7 to obtain a mixed solvent. Additive II-I and FEC were added to the mixed solvent, and finally 1 M lithium salt LiPF6 was added. The mixture was stirred until dissolved. Based on the total mass of the electrolyte, the mass percentage of additive II-I (m1) was 1%, and the mass percentage of FEC was 3%. The ionic conductivity of this electrolyte was 8.5 mS / cm.

[0207] (4) Separating membrane

[0208] Polyethylene is used as the separator.

[0209] (5) Preparation of secondary battery cells

[0210] The positive electrode, separator, and negative electrode are stacked in sequence and wound to obtain an electrode assembly. The electrode assembly is placed in a housing, electrolyte is injected, and then it is encapsulated to obtain a secondary battery cell.

[0211] Examples 2-13, Comparative Examples 1-3

[0212] Compared with Example 1, the difference is that the parameters of the positive electrode active material or the electrolyte are different from those in Example 1, as detailed in Table 1.

[0213] Product parameters of Examples 1-13 and Comparative Examples 1-3. Table 1: Product parameters of Examples 1-13 and Comparative Examples 1-3

[0214] In Table 1, "Additives" indicates the type of additives in the electrolyte; "m1" indicates the mass fraction of cyclic sulfate compounds in the electrolyte; "m'" indicates the mass fraction of fluoroethylene carbonate in the electrolyte; "Cathode Material 2" indicates the second cathode active material; "m3" indicates the mass fraction of the second cathode active material in the cathode active material; "Dv50" indicates the volume average particle size of the cathode active material; "BET1" indicates the specific surface area of ​​the cathode active material; "BET2" indicates the specific surface area of ​​the cathode electrode; "p" indicates the porosity of the cathode electrode; and "FEC" indicates fluoroethylene carbonate.

[0215] Battery performance tests were conducted on Examples 1-13 and Comparative Examples 1-3 in Table 1, and the battery performance data are shown in Table 2.

[0216] Table 2: Battery performance of Examples 1-13 and Comparative Examples 1-3

[0217] In Table 2, "Power Density" represents the power density of a lithium-ion secondary battery at 10% SOC in 30 seconds; "High Temperature Storage Capacity Retention Rate" represents the capacity retention rate of a single secondary battery cell after 60 days of storage at 60°C.

[0218] Comparative analysis of the examples and comparative examples shows that the power density and high-temperature storage performance of Examples 1-13 are superior to those of Comparative Examples 1-3. This demonstrates that when the positive electrode active material includes lithium-containing transition metal oxides and lithium-containing transition metal phosphates, and when the electrolyte includes cyclic sulfate additives, lithium-ion secondary batteries can simultaneously possess good power performance and high-temperature storage performance.

[0219] Examples 1-4 demonstrate that various cyclic sulfate additives can enable lithium-ion secondary batteries to simultaneously possess good power performance and high-temperature storage performance.

[0220] Comparative analysis of Examples 1 and 5-8 shows that within the range of 0.1%-2%, a higher mass fraction of the cyclic sulfate additive results in a higher power density and better high-temperature storage performance for the lithium-ion secondary battery. Within the range of 2%-5%, a higher mass fraction of the cyclic sulfate additive leads to a higher power density but a slight decrease in high-temperature storage performance. This may be because a higher additive mass fraction results in a thicker interfacial film, affecting the internal impedance of the lithium-ion secondary battery. Therefore, controlling the mass fraction of the cyclic sulfate additive within the range of 0.1%-5% can help improve the power performance and high-temperature storage performance of the lithium-ion secondary battery.

[0221] According to the comparative analysis of Examples 1, 9, and 11, by adjusting the mass ratio of the first positive electrode active material and the second positive electrode active material, the specific surface area and Dv50 of the positive electrode active material can be adjusted, thereby keeping the specific surface area and porosity of the positive electrode sheet within a suitable range.

[0222] Comparative analysis of Examples 1, 4, and 9-10 shows that the power density of Examples 1 and 4 is superior to that of Examples 9-10, indicating that a higher mass percentage of the second positive electrode active material is more beneficial to the power performance of the lithium-ion secondary battery. In Examples 9-10, where the mass percentage of the second positive electrode active material is relatively low, both additives of Formula II with a monocyclic structure and additives of Formulas I-VII with polycyclic structures can effectively improve the high-temperature storage performance of the lithium-ion secondary battery. In Examples 1 and 4, where the mass percentage of the second positive electrode active material is relatively high, the improvement effect of additives of Formula II with a monocyclic structure is limited, and the high-temperature storage performance of Example 1 is inferior to that of Example 4. Therefore, it is demonstrated that when the mass percentage of the second positive electrode active material is relatively high (e.g., 5% or more), combining it with a cyclic sulfate compound with a polycyclic structure can further improve the power density and storage performance of the lithium-ion secondary battery.

[0223] Example 12 verifies that, when the second positive electrode active material includes lithium manganese iron phosphate, the solution of this application can also improve the power performance and high-temperature storage performance of lithium-ion secondary batteries.

[0224] Comparative analysis of Examples 1 and 13 shows that introducing FEC into the electrolyte can further improve the high-temperature storage performance of lithium-ion secondary batteries.

[0225] Compared with Example 9, Examples 14-17 and Comparative Examples 4-6 have different primary particle sizes for lithium transition metal oxides and lithium transition metal phosphates. See Table 3 for details.

[0226] Table 3: Parameters and performance data of Examples 9, 14-17 and Comparative Examples 4-6.

[0227] In Table 3, "D1" represents the average particle size of the primary particles of the first positive electrode active material. "D2" represents the average particle size of the primary particles of the second positive electrode active material. "Power density" represents the power density of the lithium-ion secondary battery at 10% SOC in 30 seconds; "High-temperature storage capacity retention" represents the capacity retention rate of the secondary battery cells after 60 days of storage at 60°C.

[0228] According to the comparative analysis of the data in Table 3, compared with the comparative examples, the average particle size of the primary particles containing lithium transition metal oxides and lithium transition metal phosphates in Examples 9 and 14-17 was controlled within the range set in this application, resulting in superior power density and storage performance of the lithium-ion secondary battery. This indicates that the particle size matching in this application can effectively reduce side reactions between the positive electrode active material and the electrolyte, while providing good kinetic performance for the lithium-ion secondary battery, enabling it to achieve both good storage performance and excellent power density. In Comparative Example 4, the particle size of the primary particles containing lithium transition metal oxides was relatively large, resulting in a significant decrease in the power performance of the lithium-ion secondary battery. In Comparative Example 5, the particle size of the primary particles containing lithium transition metal phosphates was too small, leading to an overall small average particle size of the positive electrode active material, increasing the risk of side reactions with the electrolyte, and significantly reducing the storage performance of the lithium-ion secondary battery.

[0229] Compared to the examples and Comparative Examples 4-5, the power performance and storage performance of the lithium-ion secondary battery in Comparative Example 6 showed a significant further deterioration. In the absence of cyclic sulfate compounds in the electrolyte, the primary particles containing lithium transition metal oxides had a larger average particle size, while the primary particles containing lithium transition metal phosphates had a smaller average particle size. This resulted in a further increase in side reactions between the mixed positive electrode active material and the electrolyte. The excessively small lithium transition metal phosphate particles could not physically isolate the larger lithium transition metal oxide particles, leading to an increase in side reactions between them and the electrolyte. Furthermore, the larger lithium transition metal oxide particles were more prone to fragmentation during the reaction, further exacerbating the side reactions. Simultaneously, the absence of cyclic sulfate compounds in the electrolyte resulted in a more porous CEI film structure formed by the electrolyte solvent, which could not effectively suppress the side reactions between the electrolyte and the positive electrode active material. This further exacerbated the side reactions during high-rate charge and discharge of the lithium-ion secondary battery, leading to a combined deterioration in power and storage performance. This demonstrates that the present application, through the comprehensive design of the particle size distribution of the mixed positive electrode active material and the electrolyte additives, can improve the power performance of lithium-ion secondary batteries while simultaneously enhancing their storage performance.

[0230] The following is a brief description of the testing methods for the physicochemical and performance parameters involved in the embodiments of this application. It should be understood that the following testing methods are only examples, and other testing methods known in the art can also be used for testing.

[0231] 1. Test methods for mass content

[0232] The Fe content can be determined by inductively coupled plasma atomic emission spectrometry (ICP, Ametek, model: SPECTROARCOSICP-OES) according to standards YS / T1006.2-2014, GB / T23367.2-2009 or YS / T1028.5-2015.

[0233] The mass fraction of additives in the electrolyte can be determined using nuclear magnetic resonance spectroscopy (NMR). The specific testing procedure is as follows: Add 500 μL of deuterated reagent to an NMR tube in a nitrogen-filled glove box. Add 100 μL of the non-aqueous electrolyte sample to the NMR tube. Shake the NMR tube to dissolve the non-aqueous electrolyte in the deuterated reagent. The test is performed using an Oxford Instruments X-Pulse benchtop NMR spectrometer. Because the non-aqueous electrolyte is highly sensitive to moisture, both the NMR test and sample preparation are conducted in a nitrogen atmosphere (H₂O content less than 0.1 ppm, O₂ content less than 0.1 ppm). Simultaneously, all instruments used in the test must be pre-washed with pure water and dried in a vacuum environment at 60°C for at least 48 hours. The deuterated reagent was prepared as follows: Deuterated dimethyl sulfoxide (DMSO-d6), deuterated acetonitrile, and trifluoromethylbenzene were dried using a 4A molecular sieve at a temperature above 25°C for at least 3 days, ensuring that the water content of all reagents was less than 3 ppm. A Metrohm 831KF coulometric moisture analyzer was used for moisture testing. Then, 10 mL of dried DMSO-d6 and 300 μL of dried internal standard trifluoromethylbenzene were mixed thoroughly in a nitrogen-filled glove box to obtain the first solution. 10 mL of dried deuterated acetonitrile and 300 μL of dried internal standard trifluoromethylbenzene were then mixed thoroughly to obtain the second solution. The first and second solutions were then mixed thoroughly to obtain the deuterated reagent.

[0234] The reference standard GB / T 9722-2006 specifies the quantitative analysis of solvent content in electrolytes using organic gas chromatography.

[0235] The lithium salt content in the electrolyte is quantitatively analyzed by ion chromatography, according to the reference standard JY / T020-1996.

[0236] 2. Dv50 Testing Method

[0237] According to standard GB / T 19077-2016, the volume average particle size Dv50 of the material can be obtained by testing the material using a laser particle size analyzer (such as Malvern Master Sizer 3000).

[0238] Calculation: Calculate the particle size at the 50% position of the volume distribution curve from smallest to largest, which is Dv50.

[0239] 3. BET Testing Methods

[0240] The specific surface area of ​​solid materials was determined by gas adsorption BET method using a specific surface area and porosity analyzer, with reference to standard GB / T 19587-2017 or GB / T19587-2004.

[0241] Pretreatment: Take an appropriate amount of sample in a special sample tube, heat and degas for 2 hours, and weigh the total weight after cooling to room temperature. Subtract the mass of the sample tube to obtain the sample mass.

[0242] Test: The sample tube is loaded into the workstation and the amount of gas adsorbed on the solid surface under different adsorption pressures is measured at a constant low temperature. Based on the BET multilayer adsorption theory and its formula, the amount of monolayer adsorption of the sample is obtained, and the specific surface area of ​​the solid sample per unit mass is calculated.

[0243] Adsorbed gas: nitrogen; Adsorption pressure points: 0.05 / 0.10 / 0.15 / 0.20 / 0.25 / 0.30; Test atmosphere: high-purity liquid nitrogen atmosphere.

[0244] 4. Methods for testing ionic conductivity

[0245] The test method follows HG / T 4067-2015. The ionic conductivity of the electrolyte to be tested is measured using a conductivity meter: Take about 100 mL of the sample to be tested in a dry, clean, corrosion-resistant sample bottle, seal it and place it in a constant temperature water bath at 25±0.5℃. When the temperature of the sample to be tested is constant, replace the cap of the sample bottle with a rubber stopper with an electrode inserted. When the temperature is within the range of 25±0.5℃, read the data, which is the ionic conductivity of the sample to be tested.

[0246] 5. Power density test method

[0247] Room temperature power density test method: At 25℃, charge the secondary battery with a constant current of 1C to 4.5V, then charge with a constant voltage of 4.5V to the cutoff current of 0.05C, let it stand for 10 minutes, and then discharge with a constant current of 0.5C to 2.5V. Record the discharge capacity D0 and discharge energy density W0 at this time. Then, let the battery stand for 30 minutes, charge with a constant current of 1C to 4.5V, then charge with a constant voltage of 4.5V to the cutoff current of 0.05C, let it stand for 10 minutes, and then discharge the battery with a constant current of 0.5C to 0.9 times the capacity of D0 (at this time, it is at 10% SOC). After standing for 30 minutes, discharge with a test current for 30 seconds until it discharges to 2.5±0.05V. Record the terminal voltage V1 and the maximum discharge current I1.

[0248] Battery power density = (terminal voltage V1 * maximum discharge current I1) / battery discharge energy density W0.

[0249] 6. High-Temperature Storage Performance Test

[0250] At 25℃, the secondary battery was charged at a constant current of 1C to 4.5V, then charged at a constant voltage of 4.5V to the cutoff current of 0.05C. After resting for 10 minutes, it was discharged at a constant current of 0.5C to 2.5V, and the discharge capacity D0 was recorded. The secondary battery was stored in an environment of 60℃ for 60 days. Afterward, the secondary battery was removed and cooled to a surface temperature of 25℃. Subsequently, at 25℃, it was discharged at a constant current of 0.5C to 2.5V, then charged at a constant current of 1C to 4.5V, then charged at a constant voltage of 4.5V to the cutoff current of 0.05C, rested for 10 minutes, and discharged at a constant current of 0.5C to the termination voltage of 2.5V, obtaining the discharge capacity D1 after high-temperature storage. High-temperature storage capacity retention rate (%) = (D1 / D0) × 100%.

[0251] 7. Test method for membrane compaction density

[0252] To obtain a processable battery positive electrode sheet, a stamping press was used to punch out a positive electrode sheet with a diameter of 14mm. The mass (m) of the positive electrode sheet was measured using an electronic balance and a benchtop digital thickness gauge. c Thickness d c A sufficient number of aluminum foil substrates with a diameter of 14mm were punched out using a punching machine. The mass (m) of the aluminum foil substrates was measured using an electronic balance and a benchtop digital thickness gauge. Al Thickness d Al .

[0253] Compacted density of positive electrode sheet

[0254] Where: ρ is the compaction density of the positive electrode sheet, in grams per cubic centimeter (g / cm³). 3 );m c The mass of the positive electrode is expressed in grams (g); m Al The mass of the aluminum foil substrate is expressed in grams (g). d represents the diameter of the positive electrode plate, in millimeters (mm); c The thickness of the positive electrode is expressed in micrometers (μm); d Al The thickness of the aluminum foil substrate is expressed in micrometers (μm).

[0255] 8. Test method for the average particle size of primary particles

[0256] The particle morphology and size of the sample were obtained using SEM. The "LIBMAS Lithium-ion Battery Material Microscopic Intelligent Analysis System" software was used for automatic AI identification of primary particles, drawing particle outlines, and obtaining the number, number, area, and maximum caliper diameter of each particle. 2000 particles were collected, and the average was calculated. The average particle diameter was calculated as: (Sum of all measured particle sizes (longest diameter)) / (Sum of all measured particle numbers).

[0257] Although this application has been described with reference to preferred embodiments, various modifications can be made thereto and components can be replaced with equivalents without departing from the scope of this application. In particular, the technical features mentioned in the various embodiments can be combined in any manner, provided there is no structural conflict. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A lithium-ion secondary battery, characterized in that, It includes a positive electrode sheet and an electrolyte, wherein the positive electrode sheet includes a positive current collector and a positive electrode film layer disposed on at least one side of the positive current collector; The positive electrode film layer includes a positive electrode active material, which includes a first positive electrode active material and a second positive electrode active material. The first positive electrode active material includes a lithium-containing transition metal oxide, and the second positive electrode active material includes a lithium-containing transition metal phosphate. Wherein, the average particle size D1 of the primary particles of the first positive electrode active material satisfies: 1.6μm≤D1≤2.6μm; The average particle size D2 of the primary particles of the second positive electrode active material satisfies: 40nm≤D2≤250nm; The electrolyte includes a solvent, additives, and a lithium salt, wherein the additives include cyclic sulfate compounds.

2. The lithium-ion secondary battery according to claim 1, characterized in that, The charging cutoff voltage v of the lithium-ion secondary battery satisfies: 4.3V≤v≤5V.

3. The lithium-ion secondary battery according to claim 1 or 2, characterized in that, The cyclic sulfate compounds include at least one of compounds with the molecular formula (I) or modified compounds thereof: R1-R4 includes at least one of the following: halogenated or unhalogenated H, alkyl with 1-6 carbon atoms, alkenyl with 2-6 carbon atoms, alkoxy with 1-6 carbon atoms, acyloxy with 1-6 carbon atoms, ester with 2-6 carbon atoms, and cyclic sulfate ester with 4-membered to 7-membered rings.

4. The lithium-ion secondary battery according to claim 3, characterized in that, R1-R4 also include at least one of the groups with the molecular formulas (II-I) to (II-IV): in, Indicating the bonding position, R5 includes at least one of halogen, alkyl group having 1-6 carbon atoms, haloalkyl group having 1-6 carbon atoms, and alkoxy group having 1-6 carbon atoms. The group of formula (II-III)-(II-IV) is bonded to R1 and R3, or to R2 and R4.

5. The lithium-ion secondary battery according to claim 3 or 4, characterized in that, The cyclic sulfate compounds have a polycyclic structure.

6. The lithium-ion secondary battery according to any one of claims 3-5, characterized in that, The cyclic sulfate compounds include at least one of the compounds with the molecular formula (II) to (I-XII):

7. The lithium-ion secondary battery according to any one of claims 1-6, characterized in that, Based on the total mass of the electrolyte, the mass fraction m1 of the cyclic sulfate ester compound satisfies: 0.1% ≤ m1 ≤ 5%.

8. The lithium-ion secondary battery according to claim 7, characterized in that, 0.5％≤m1≤2％。 9. The lithium-ion secondary battery according to any one of claims 1-8, characterized in that, The ionic conductivity σ of the electrolyte satisfies: 7.5 mS / cm ≤ σ ≤ 15 mS / cm.

10. The lithium-ion secondary battery according to any one of claims 1-9, characterized in that, The lithium salt includes at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorobis(oxalate)phosphate, lithium bis(oxalate)borate, lithium difluorooxalateborate, or lithium tetrafluorooxalate; optionally, the lithium salt includes lithium hexafluorophosphate.

11. The lithium-ion secondary battery according to any one of claims 1-10, characterized in that, The concentration c of the lithium salt in the electrolyte satisfies: 0.9M ≤ c ≤ 1.2M.

12. The lithium-ion secondary battery according to any one of claims 1-11, characterized in that, The solvent includes at least one of carbonate solvents and carboxylic acid ester solvents.

13. The lithium-ion secondary battery according to claim 12, characterized in that, The carbonate solvents include at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, dimethyl carbonate, and diethyl carbonate.

14. The lithium-ion secondary battery according to claim 12, characterized in that, The carboxylic acid ester solvents include at least one of methyl formate, ethyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl acrylate, and ethyl acrylate.

15. The lithium-ion secondary battery according to any one of claims 12-14, characterized in that, The carbonate solvent includes ethylene carbonate, and the mass fraction m2 of ethylene carbonate, based on the total mass of the electrolyte, satisfies the following condition: 10% ≤ m2 ≤ 30%.

16. The lithium-ion secondary battery according to any one of claims 1-15, characterized in that, The additives include fluoroethylene carbonate.

17. The lithium-ion secondary battery according to any one of claims 1-16, characterized in that, In the cross-section of the positive electrode film layer along the thickness direction of the positive electrode sheet, the primary particles of the first positive electrode active material surround at least a portion of the primary particles of the second positive electrode active material.

18. The lithium-ion secondary battery according to any one of claims 1-17, characterized in that, The BET1 of the positive electrode active material satisfies: 0.5m 2 / g≤BET1≤4.5m 2 / g.

19. The lithium-ion secondary battery according to any one of claims 1-18, characterized in that, The volume distribution particle size Dv50 of the positive electrode active material satisfies: 3.2μm≤Dv50≤6μm.

20. The lithium-ion secondary battery according to any one of claims 1-19, characterized in that, The lithium-containing transition metal oxide includes those with the molecular formula Li. n1 (Ni x1 Co y1 Mn z1 ) 1-n G n O 2-m K r At least one of the compounds or their modified compounds, wherein G includes one or more of Zr, Al, Nb, Sr, B, Ba, Mg, Sn, Y, Na, Si, W and Ti, K includes one or more of P, F, N, S, F, Cl and I, 0.6≤n1≤1.5, 0.3≤x1≤0.7, 0.02≤y1≤0.15, x1+y1+z1=1, 0<n≤0.1, 0<m≤0.1, 0<r≤0.

1.

21. The lithium-ion secondary battery according to claim 20, characterized in that, The average mass concentration of K element in the outer region corresponding to 1 / 6 radius from the surface of the primary particle containing lithium transition metal oxide is greater than the average mass concentration of K element in the core region corresponding to 1 / 6 radius from the center of the primary particle.

22. The lithium-ion secondary battery according to claim 20, characterized in that, The average mass concentration percentage 'a' of K element in the outer region corresponding to 1 / 6 radius of the surface of the primary particles containing lithium transition metal oxide satisfies the following condition: 0.1% ≤ a ≤ 0.5%.

23. The lithium-ion secondary battery according to claim 20, characterized in that, The average mass concentration percentage b of K element in the core region corresponding to 1 / 6 radius of the primary particle of the lithium-containing transition metal oxide satisfies: 0.01% ≤ b ≤ 0.05%.

24. The lithium-ion secondary battery according to any one of claims 1-23, characterized in that, The lithium-containing transition metal phosphate includes those with the molecular formula Li. n2 Fe x2 Mn y2 P z2 O j Q q The compound or its modified compound, wherein Q includes at least one of Al, Na, K, Mg, Cu, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, 0.6≤n2≤1.15, 0<x2, 0≤y2, 0.9≤x2+y2≤1, 0.95≤z2≤1, 3.5≤j≤4, 0≤q≤0.

1.

25. The lithium-ion secondary battery according to any one of claims 1-24, characterized in that, Based on the total mass of the positive electrode active material, the mass fraction m3 of the lithium transition metal phosphate satisfies: 5% ≤ m3 ≤ 40%.

26. The lithium-ion secondary battery according to claim 25, characterized in that, In the case where the lithium-containing transition metal phosphate includes lithium iron phosphate, 5% ≤ m3 ≤ 15%; or In the case where the lithium-containing transition metal phosphate includes lithium manganese iron phosphate, 10% ≤ m3 ≤ 40%.

27. The lithium-ion secondary battery according to any one of claims 1-26, characterized in that, The specific surface area BET2 of the positive electrode sheet satisfies: 2m 2 / g≤BET2≤5m 2 / g.

28. The lithium-ion secondary battery according to any one of claims 1-27, characterized in that, The porosity p of the positive electrode sheet, measured by the true density method, satisfies the following condition: 22% ≤ p ≤ 28%.

29. The lithium-ion secondary battery according to any one of claims 1-28, characterized in that, The compaction density ρ of the positive electrode sheet satisfies: 3.0 g / cm³ 3 ≤ρ≤3.5g / cm 3 .

30. An electrical device, characterized in that, The electrical device includes the lithium-ion secondary battery as described in any one of claims 1-29.

31. A method for charging a lithium-ion secondary battery, characterized in that, The charging method includes: The lithium-ion secondary battery is charged to the cutoff voltage v at a preset rate Cset. The lithium-ion secondary battery is charged to the cutoff current i at the cutoff voltage v. Wherein, 4.3V≤v≤5V, 0.02C≤i≤0.1C.

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