Lithium-ion secondary battery, method of manufacturing lithium-ion secondary battery, and electric device

Abstract

This application provides a lithium-ion secondary battery, a method for preparing a lithium-ion secondary battery, and an electrical device. The lithium-ion secondary battery includes a positive electrode sheet, which includes a positive electrode active material. The positive electrode active material includes a first positive electrode active material and a second positive electrode active material. The discharge curve of the first positive electrode active material relative to graphite at a discharge rate of 0.3C-1C includes a first discharge plateau voltage v1, and the discharge curve of the second positive electrode active material relative to graphite at a discharge rate of 0.3-1C includes a second discharge plateau voltage v2; 3V≤v1≤5V, 2V≤v2<3V. This lithium-ion secondary battery exhibits excellent power performance.

Description

Technical Field

[0001] This application relates to the field of batteries, and more specifically, to a lithium-ion secondary battery, a method for preparing a lithium-ion secondary battery, and an electrical device for using it. Background Technology

[0002] 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.

[0003] Improving the energy density of lithium-ion rechargeable batteries has always been a key research focus. However, for lithium-ion rechargeable batteries with specific applications, such as those used in power tools and electric vehicles, stable power output is required. Therefore, how to improve the power performance of lithium-ion rechargeable batteries is a technical problem that urgently needs to be solved. Summary of the Invention

[0004] This application is made in view of the above-mentioned technical problems, and its purpose is to provide a lithium-ion secondary battery, a method for preparing a lithium-ion secondary battery, and an electrical device for using the lithium-ion secondary battery, wherein the lithium-ion secondary battery has excellent power performance.

[0005] In a first aspect, a lithium-ion secondary battery is provided, comprising a positive electrode sheet, the positive electrode sheet comprising a positive electrode active material, the positive electrode active material comprising a first positive electrode active material and a second positive electrode active material; wherein, the discharge curve of the first positive electrode active material relative to graphite at a discharge rate of 0.3C-1C includes a first discharge plateau voltage v1, and the discharge curve of the second positive electrode active material relative to graphite at a discharge rate of 0.3-1C includes a second discharge plateau voltage v2; 3V≤v1≤5V, 2V≤v2<3V.

[0006] In the embodiments of this application, the positive electrode active material includes a first positive electrode active material having a first discharge plateau voltage and a second positive electrode active material having a second discharge plateau voltage. Through the dual plateau voltage effect of the two positive electrode active materials, the lithium-ion secondary battery can achieve stable power output even at low SOC, thereby improving the power performance of the lithium-ion secondary battery.

[0007] In some embodiments, 0.2V ≤ v1 - v2 ≤ 2V.

[0008] In some embodiments, the first positive electrode active material includes at least one of lithium-containing transition metal oxides and lithium-containing transition metal phosphates.

[0009] In some embodiments, the lithium-containing transition metal oxide includes at least one of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, or modified compounds thereof.

[0010] In some embodiments, the lithium-containing transition metal phosphate includes at least one of lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate, or modified compounds thereof.

[0011] In some embodiments, the second positive electrode active material comprises a lithium transition metal sulfide.

[0012] In some embodiments, the lithium transition metal sulfide comprises a compound with the molecular formula Li. 1+a Fe x Ti y M z S2 is a compound or a modified compound thereof, wherein -0.05≤a≤0.2, 0.3≤x, 0.3≤y, 0≤z≤0.1, a+x+y+z=1, and M includes at least one of Ni, Co, Mn, V, Zr, Y, Al, Mg, B, Sr, Na, K, Nb, La, and Si.

[0013] In some embodiments, the second positive electrode active material includes a doping element, wherein the doping element includes at least one of sodium and magnesium.

[0014] In some embodiments, the mass fraction w of the second positive electrode active material, based on the mass of the positive electrode active material, satisfies: 0.5% ≤ w ≤ 20%.

[0015] In some embodiments, in the discharge curve of the positive electrode relative to graphite at a discharge rate of 0.3C-1C, the ratio q of the discharge capacity of the second positive electrode active material to the total discharge capacity of the positive electrode active material satisfies: 3% ≤ q ≤ 30%.

[0016] In some embodiments, the Dv501 of the first positive electrode active material satisfies: 0.5μm≤Dv501≤20μm.

[0017] In some embodiments, the Dv502 of the second positive electrode active material satisfies: 2.5μm≤Dv502≤20μm.

[0018] In some embodiments, the areal density CW of the positive electrode sheet satisfies: 5 mg / cm² 2 ≤CW≤40mg / cm 2 .

[0019] In some embodiments, the compaction density PD of the positive electrode sheet satisfies: 1.5 g / cm³3 ≤PD≤4g / cm 3 .

[0020] In some embodiments, the film resistance R of the positive electrode sheet satisfies: 0.1Ω≤R≤1Ω.

[0021] In a second aspect, a method for preparing a lithium-ion secondary battery according to any embodiment of the first aspect is provided, the method comprising: providing a first positive electrode active material, and preparing a second positive electrode active material by the following steps: mixing lithium sulfide, titanium sulfide, ferrous sulfide and transition metal sulfides to form a mixture; sintering the mixture to obtain a sintered material; and crushing the sintered material to obtain the second positive electrode active material.

[0022] In some embodiments, the molar ratio of lithium to metal in the mixture is (0.95:1) to (1.2:1).

[0023] In some embodiments, the sintering temperature is 600℃-900℃, the sintering time is 10h-40h, and the sintering atmosphere is an inert atmosphere.

[0024] In some embodiments, the inert atmosphere includes at least one of nitrogen, argon, and helium.

[0025] In some embodiments, the crushing method includes at least one of ball milling and air jet milling.

[0026] Thirdly, an electrical device is provided, comprising a lithium-ion secondary battery as described in any embodiment of the first aspect, and / or a lithium-ion secondary battery prepared by the method described in any embodiment of the second aspect. Attached Figure Description

[0027] 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.

[0028] Figure 1 This is a schematic diagram of a lithium-ion secondary battery.

[0029] Figure 2 This is a schematic diagram of a single battery cell.

[0030] Figure 3 This is an XRD pattern of a second positive electrode active material.

[0031] Figure 4 This is a morphology diagram of a second positive electrode active material.

[0032] Figure 5 This is a laser particle size distribution diagram of a second positive electrode active material.

[0033] Figure 6 This is a capacity-voltage curve of a second positive electrode active material. Detailed Implementation

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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).

[0038] 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.

[0039] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0040] Unless otherwise specified, the following terms have the following meanings. Any undefined terms have their technically accepted meanings.

[0041] As mentioned, the "discharge plateau voltage" is the voltage corresponding to the "plateau region" in the battery's voltage-capacity curve. In the voltage-capacity or voltage-time curve of the discharge process, the voltage decreases rapidly as the discharge capacity or discharge time increases, then remains almost constant or changes very little, before rapidly decreasing to the cutoff voltage. The portion of this 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 (where the voltage remains almost constant) or the median of the voltage range corresponding to the plateau region (where the voltage changes). The discharge plateau voltage of a particular material is usually measured using a half-cell composed of that material and a counter electrode. Common counter electrodes include graphite and lithium metal.

[0042] 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.

[0043] 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.

[0044] If mentioned, "lithium-containing transition metal sulfides" refers to a class of sulfides that include lithium and transition metal elements. Examples include lithium molybdenum sulfides, lithium titanium sulfides, and lithium iron sulfides.

[0045] The embodiments of this application will be described next.

[0046] 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.

[0047] The positive electrode active material is one of the direct factors affecting the power performance of lithium-ion secondary batteries. Lithium transition metal phosphates and lithium transition metal oxides are commonly used positive electrode active materials. However, current batteries are prone to voltage drops and sudden shutdowns at low SOC levels, resulting in poor power performance at low SOCs (e.g., 10% SOC and below).

[0048] In view of this, this application provides a lithium-ion secondary battery, a method for preparing a lithium-ion secondary battery, and an electrical device for using it. This lithium-ion secondary battery exhibits excellent power performance.

[0049] The lithium-ion secondary battery provided in this application will be described next.

[0050] [Lithium-ion rechargeable battery]

[0051] Firstly, a lithium-ion secondary battery is provided, including a positive electrode sheet, which includes a positive electrode active material, and the positive electrode active material includes a first positive electrode active material and a second positive electrode active material; wherein, the discharge curve of the first positive electrode active material relative to graphite at a discharge rate of 0.3C-1C includes a first discharge plateau voltage v1, and the discharge curve of the second positive electrode active material relative to graphite at a discharge rate of 0.3-1C includes a second discharge plateau voltage v2; 3V≤v1≤5V, 2V≤v2<3V.

[0052] For example, v1 can be 3V, 3.1V, 3.2V, 3.3V, 3.4V, 3.5V, 3.6V, 3.7V, 3.8V, 3.9V, 4V, 4.1V, 4.2V, 4.3V, 4.4V, 4.5V, 4.6V, 4.7V, 4.8V, 4.9V, or 5V, or a value within the range obtained by any two of the above values. v2 can be 2V, 2.1V, 2.2V, 2.3V, 2.4V, 2.5V, 2.6V, 2.7V, 2.8V, 2.9V, or 3V, or a value within the range obtained by any two of the above values.

[0053] For lithium-ion secondary batteries, the voltage drops rapidly at low SOC. Typically, when the voltage of a lithium-ion secondary battery drops below 2V, the battery management system detects this voltage and considers the battery inoperable. However, in the embodiments of this application, by combining a first positive electrode active material and a second positive electrode active material, a dual voltage plateau exists during the discharge process of the lithium-ion secondary battery, constructed from the first and second positive electrode active materials. This allows the lithium-ion secondary battery to experience a second discharge plateau voltage of 2V-3V from the second positive electrode active material, even when the voltage is rapidly decreasing at low SOC. This reduces the probability of a sudden voltage drop, improves the power performance of the lithium-ion secondary battery at low SOC, and allows the battery's capacity to be more fully released. Therefore, the power performance of the lithium-ion secondary battery is improved.

[0054] In some embodiments, 0.2V ≤ v1 - v2 ≤ 2V.

[0055] v1-v2 is the difference between the first and second discharge plateau voltages, representing the voltage difference between the two discharge plateaus. For example, v1-v2 can be 0.2V, 0.3V, 0.4V, 0.5V, 0.6V, 0.7V, 0.8V, 0.9V, 1V, 1.1V, 1.2V, 1.3V, 1.4V, 1.5V, 1.6V, 1.7V, 1.8V, 1.9V, 2V, or a value within the range obtained by any combination of the above two values.

[0056] On the one hand, the larger the difference between the first and second discharge plateau voltages, the better the buffering effect on the lithium-ion secondary battery at low SOC. This is likely because, with the first discharge plateau at 3V-5V, a smaller difference between the first and second discharge plateau voltages means the second plateau is closer to the first. During the discharge process, the lithium-ion secondary battery's discharge behavior may converge to a single plateau, limiting its effectiveness in buffering the voltage drop at low SOC. On the other hand, the difference between the first and second discharge plateau voltages cannot be too large. With the first discharge plateau at 3V-5V, a large difference implies a smaller second discharge plateau voltage. An excessively low voltage during discharge is also detrimental to the lithium-ion secondary battery's discharge performance.

[0057] Therefore, by controlling v1-v2 within a suitable range, the voltage drop problem of lithium-ion secondary batteries at low SOC can be further improved, thereby enhancing the power performance of lithium-ion secondary batteries.

[0058] In some embodiments, the first positive electrode active material includes at least one of lithium-containing transition metal oxides and lithium-containing transition metal phosphates.

[0059] The discharge plateau voltage of lithium-containing transition metal oxides and lithium-containing transition metal phosphates is typically in the range of 3V-5V. Lithium-containing transition metal oxides have high specific capacity, which helps to improve the energy density of lithium-ion secondary batteries. Lithium-containing transition metal phosphates have good cycle stability, which helps to improve the cycle performance of lithium-ion secondary batteries.

[0060] Accordingly, when the first positive electrode active material comprises only lithium transition metal oxide, the discharge platform of the lithium-ion secondary battery includes the discharge platform of the lithium transition metal oxide and the discharge platform of the second positive electrode active material. When the first positive electrode active material comprises only lithium transition metal phosphate, the discharge platform of the lithium-ion secondary battery includes the discharge platform of the lithium transition metal phosphate and the discharge platform of the second positive electrode active material. When the first positive electrode active material comprises both lithium transition metal oxide and lithium transition metal phosphate, the discharge platform of the lithium-ion secondary battery includes the discharge platform of the lithium transition metal oxide, the discharge platform of the lithium transition metal phosphate, and the discharge platform of the second positive electrode active material.

[0061] In some embodiments, lithium transition metal oxides include 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 Mn1 / 3 O2 (also known as NCM333), LiNi 0.5 Co 0.2 Mn 0.3 O2 (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.

[0062] In some embodiments, lithium-containing transition metal phosphates include 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. During the charging and discharging process, Li undergoes insertion / extraction and consumption, resulting in different molar contents of Li in the positive electrode active material when the battery is discharged to different states. 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. When the positive electrode active material is applied to the battery system, the molar content of Li changes after charge-discharge cycles. 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.

[0063] In some embodiments, the second positive electrode active material comprises a lithium transition metal sulfide.

[0064] The discharge plateau voltage of lithium transition metal sulfides is typically in the range of 2V-3V, which can provide a discharge plateau for lithium-ion secondary batteries in the range of 2V-3V, thereby improving the voltage drop problem of lithium-ion secondary batteries at low SOC.

[0065] In some embodiments, lithium transition metal sulfides include those with the molecular formula Li. 1+a Fe x Ti y M zS2 is a compound or a modified compound thereof, wherein -0.05≤a≤0.2, 0.3≤x, 0.3≤y, 0≤z≤0.1, a+x+y+z=1, and M includes at least one of Ni, Co, Mn, V, Zr, Y, Al, Mg, B, Sr, Na, K, Nb, La, and Si.

[0066] The molecular formula is Li 1+a Fe x Ti y M z S2 lithium transition metal sulfides have a layered structure, which is beneficial for the intercalation and deintercalation of lithium ions in electrochemical reactions.

[0067] For positive electrode active materials including Li 1+a Fe x Ti y M z The S2 lithium-ion secondary battery, on the one hand, when the voltage of the lithium-ion secondary battery drops to the second discharge plateau voltage at a low SOC, lithium ions react with Li... 1+a Fe x Ti y M z S2 reacts, suppressing the voltage drop in lithium-ion secondary batteries. On the other hand, Li... 1+a Fe x Ti y M z S2 also helps improve the discharge capacity of lithium-ion secondary batteries at low temperatures and their high-power discharge capacity. The possible principle is that under unconventional discharge conditions such as low temperature and high power, the risk of a sudden voltage drop is high. During discharge, a sudden voltage drop means that the active sites in the first positive electrode active material are not yet fully intercalated with lithium (capacity not fully released). The voltage drops sharply to the second discharge plateau voltage, at which point lithium ions will intercalate into the active sites in the second positive electrode active material, gradually increasing the lithium ion concentration in the second positive electrode active material until it exceeds that of the first positive electrode active material. This creates a lithium ion concentration difference between the two materials, allowing lithium ions to migrate from the second to the first, thus releasing some of the capacity lost due to the voltage drop. Simultaneously, Li... 1+a Fe x Ti y M z The layered structure of S2 also facilitates the efficient migration of lithium ions to the first positive electrode active material.

[0068] Therefore, by using Li 1+a Fe x Ti y Mz As the second positive electrode active material, S2 not only improves the power output of lithium-ion secondary batteries at low SOC, but also enhances their high-power discharge capability at low temperatures, thereby further improving the power output of lithium-ion secondary batteries at low SOC and low temperatures.

[0069] In some embodiments, the second positive electrode active material includes a doping element, which includes at least one of sodium and magnesium.

[0070] By introducing doping elements into the second cathode active material, the structural stability of lithium transition metal sulfides with layered structures can be improved, enabling the second cathode active material to maintain structural stability during lithium ion insertion / extraction and migration. This not only improves the power problem of lithium-ion secondary batteries under low SOC, but also helps to enhance the cycle performance of lithium-ion secondary batteries.

[0071] In some embodiments, the mass fraction w of the second positive electrode active material, based on the mass of the positive electrode active material, satisfies: 0.5% ≤ w ≤ 20%.

[0072] For example, w can be 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, 10.5%, 11%, 11.5%, 12%, 12.5%, 13%, 13.5%, 14%, 14.5%, 15%, 15.5%, 16%, 16.5%, 17%, 17.5%, 18%, 18.5%, 19%, 19.5%, 20%, or a value within the range obtained by any combination of the above two values.

[0073] By controlling the mass fraction of the second positive electrode active material within the total positive electrode active material, the ratio of the discharge capacity of the second positive electrode active material to the total discharge capacity of the lithium-ion secondary battery can be adjusted. By controlling the mass fraction of the second positive electrode active material within the range of 0.5%-20%, it is possible to improve the power performance of the lithium-ion secondary battery at low SOC while simultaneously increasing its energy density.

[0074] In some embodiments, in the discharge curve of the positive electrode relative to graphite at a discharge rate of 0.3C-1C, the proportion q of the discharge capacity of the second positive electrode active material to the total discharge capacity of the positive electrode active material satisfies: 3% ≤ q ≤ 30%.

[0075] For example, q can be 3%, 4%, 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%, or a value within the range obtained by any combination of the above two values.

[0076] In some embodiments, the Dv501 of the first positive electrode active material satisfies: 0.5μm≤Dv501≤20μm.

[0077] For example, Dv501 can be 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm, 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, or a value within the range obtained by any combination of the above two values.

[0078] In some embodiments, the Dv502 of the second positive electrode active material satisfies: 2.5μm≤Dv502≤20μm.

[0079] For example, Dv502 can be 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, 5μm, 5.5μm, 6μm, 6.5μm, 7μm, 7.5μm, 8μm, 8.5μm, 9μm, 9.5μm, 10μm, 10.5μm, 11μm, 11.5μm, 12μm, 12.5μm, 13μm, 13.5μm, 14μm, 14.5μm, 15μm, 15.5μm, 16μm, 16.5μm, 17μm, 17.5μm, 18μm, 18.5μm, 19μm, 19.5μm, 20μm, or a value within the range obtained by any combination of the above two values.

[0080] By matching the particle sizes of the first and second positive electrode active materials, the compaction density of the positive electrode sheet can be improved, thereby improving the power performance of the lithium-ion secondary battery at low SOC and further increasing the energy density of the lithium-ion secondary battery.

[0081] In some embodiments, the areal density CW of the positive electrode sheet satisfies: 5 mg / cm² 2 ≤CW≤40mg / cm 2 .

[0082] For example, CW can be 5 mg / cm 2 8mg / cm 210mg / cm 2 12mg / cm 2 14mg / cm 2 16mg / cm 2 18mg / cm 2 20mg / cm 2 22mg / cm 2 24mg / cm 2 26mg / cm 2 28mg / cm 2 30mg / cm 2 32mg / cm 2 34mg / cm 2 36mg / cm 2 38mg / cm 2 40mg / cm 2 , or its value is within the range obtained by combining any two of the above values.

[0083] The areal density of the positive electrode sheet can be tested using the following method: Randomly select 30 unit areas on the positive electrode sheet to be tested with coating on one side. Take the positive electrode sheet to be tested from each unit area, weigh the mass of the material on the positive electrode sheet excluding the positive current collector, sum the mass and divide by 30 to obtain the areal density of the positive electrode sheet to be tested.

[0084] In some embodiments, the compaction density PD of the positive electrode sheet satisfies: 1.5 g / cm³ 3 ≤PD≤4g / cm 3 .

[0085] For example, PD can be 1.5 g / cm³. 3 1.8g / cm 3 2.0g / cm 3 2.2g / cm 3 2.4g / cm 3 2.6g / cm 3 2.8g / cm 3 3.0g / cm 3 3.2g / cm 3 3.4g / cm 3 3.6g / cm 3 3.8g / cm 3 4.0g / cm 3 , or its value is within the range obtained by combining any two of the above values.

[0086] The compaction density of the positive electrode sheet can be tested using the following method: Randomly select 30 unit areas on the positive electrode sheet to be tested. Take the positive electrode sheet of each unit area and weigh the mass m1 of the material (excluding the positive current collector) on that unit area of ​​the positive electrode sheet. Measure the thickness H1 of the positive electrode sheet and the thickness H0 of the current collector. The compaction density of each unit area of ​​the positive electrode sheet is calculated as m1 / (H1-H0). Sum the compaction densities of the 30 unit areas of the positive electrode sheet and divide by 30 to obtain the compaction density of the positive electrode sheet to be tested. The measurement deviation of the compaction density is within ±0.05 g / cm³. 3 Within the range.

[0087] In some embodiments, the film resistance R of the positive electrode sheet satisfies: 0.1Ω≤R≤1Ω.

[0088] For example, R can be 0.1Ω, 0.2Ω, 0.3Ω, 0.4Ω, 0.5Ω, 0.6Ω, 0.7Ω, 0.8Ω, 0.9Ω, 1Ω, or a value within the range obtained by any combination of the above two values.

[0089] The film resistance of the positive electrode can be tested using the following method: Using the four-probe method, place the positive electrode to be tested on the testing device, ensuring its surface is flat. Adjust the pressure gauge to fix the positive electrode to be tested, ensuring good contact between the probe and the surface of the positive electrode. Start the testing device, introduce a known current into the positive electrode to be tested, and record the voltage change. Calculate the film resistance value of the positive electrode according to Ohm's law.

[0090] [Preparation Method]

[0091] This application also provides a method for preparing the lithium-ion secondary battery described in the foregoing embodiments, the method comprising preparing a second positive electrode active material by the following steps:

[0092] Step 1: Mix lithium sulfide, titanium sulfide, ferrous sulfide, and transition metal sulfides to form a mixture.

[0093] Step 2: Sinter the mixture to obtain a sintered product.

[0094] Step 3: Crush the sintered material to obtain the second positive electrode active material.

[0095] In some embodiments, the molar ratio of lithium to metal in the mixture is (1:1) to (1.2:1).

[0096] In some embodiments, the sintering temperature is 600℃-900℃, and the sintering atmosphere is an inert atmosphere.

[0097] In some embodiments, the inert atmosphere includes at least one of nitrogen, argon, and helium.

[0098] By controlling the molar ratio of lithium to metal elements in the mixture (i.e., raw materials), the sintering temperature, and the sintering atmosphere, Li with a layered structure as described in the previous examples can be prepared. 1+a Fe x Ti y M z S2 transition metal sulfides.

[0099] In some embodiments, the crushing method includes at least one of ball milling and air jet milling.

[0100] By crushing the sintered material and controlling the number of crushing processes and the time, the particle size range of lithium transition metal sulfides can be controlled, thereby preparing lithium transition metal sulfides with a particle size of 0.5μm-20μm.

[0101] 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.

[0102] [Negative electrode plate]

[0103] 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.

[0104] 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.

[0105] 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.).

[0106] 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.

[0107] 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).

[0108] 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.

[0109] In some embodiments, the negative electrode film layer also includes other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0110] 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.

[0111] [Positive electrode plate]

[0112] 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.

[0113] 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.

[0114] 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.).

[0115] In one embodiment, the positive electrode active material may further include positive electrode active materials known in the art for use in batteries, but this application is not limited to these materials. These positive electrode active materials may be used alone or in combination of two or more.

[0116] 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.

[0117] 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.

[0118] 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.

[0119] Electrolyte

[0120] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not specify any particular type of electrolyte; it can be selected according to requirements. The electrolyte includes electrolyte salts and solvents.

[0121] In some embodiments, the solvent 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.

[0122] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0123] 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.

[0124] [Isolation membrane]

[0125] 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.

[0126] 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.

[0127] In some embodiments, the separator further includes a porous coating. The porous coating can serve as a heat-resistant and / or adhesive layer.

[0128] 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.

[0129] 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.

[0130] 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.

[0131] 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 e 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 can be used: 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 < e < 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.

[0132] 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.

[0133] In some embodiments, the organic particles may include at least one of a thermoplastic resin polymer, a thermosetting resin polymer, or a crosslinked polymer.

[0134] 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.

[0135] 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.

[0136] In some embodiments, the crosslinking polymer may include one or more of crosslinked styrene organic particles and silicon-containing organic crosslinked resin particles.

[0137] 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.

[0138] 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.

[0139] 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.

[0140] 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. As an example, the dispersant may include, but is not limited to, one or more of sodium carboxymethyl cellulose, sodium polyacrylate, and ammonium polyacrylate.

[0141] Figure 1 This is a schematic diagram of a lithium-ion secondary battery according to this application. Figure 2 This is a schematic diagram of a battery cell according to this application. Figure 1-2As shown, the lithium-ion secondary battery 10 may include one or more battery cells 20 to meet different power usage 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.

[0142] 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 mixed configuration 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 within, for example, by the shape of the combination of the multiple battery cells 20 housed within. At least one of the first housing portion 101 and the second housing portion 102 may have an opening. For example, as... Figure 1 As shown, 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 one opening and the first housing portion 101 as a plate-shaped 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.

[0143] For example, unlike Figure 1 As shown, 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 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.

[0144] 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.

[0145] 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.

[0146] [Electrical appliances]

[0147] This application provides an electrical device including the lithium-ion secondary battery 10 described in the above embodiments.

[0148] 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.

[0149] This application provides an electrical device, which is a vehicle.

[0150] 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.

[0151] 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.

[0152] [Examples and Comparative Examples]

[0153] Example 1

[0154] (1) Preparation of positive electrode active material

[0155] The raw materials Li2S, TiS2, FeS and NiS were mixed evenly in a molar ratio of 0.565:0.3:0.5:0.07 and placed into a sintering furnace. The mixture was sintered at 750℃ for 30 hours in an N2 atmosphere. After crushing, a second positive electrode active material with a Dv50 of 10μm was obtained.

[0156] Lithium iron phosphate (LFP) was selected as the second positive electrode active material. The first positive electrode active material and the second positive electrode active material were mixed evenly at a mass ratio of 9:1 to prepare the positive electrode active material.

[0157] (2) Preparation of positive electrode sheet

[0158] The positive electrode active material, conductive agent carbon black, and binder PVDF are dispersed in NMP solvent at a mass ratio of 96:2:2 to obtain a positive electrode slurry. The positive electrode slurry is uniformly coated onto a positive electrode current collector aluminum foil, dried, and cold-pressed to obtain a positive electrode sheet.

[0159] (3) Preparation of negative electrode sheet

[0160] The negative electrode active material graphite, the thickener sodium carboxymethyl cellulose, the binder styrene-butadiene rubber, and the conductive agent acetylene black were dispersed in deionized water at a mass ratio of 96.5:1.5:1:1 to obtain a negative electrode slurry. The negative electrode slurry was uniformly coated onto the negative electrode current collector copper foil, dried, and cold-pressed to obtain the negative electrode sheet.

[0161] (4) Preparation of secondary battery cells

[0162] 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, an electrolyte is injected, and then it is encapsulated to obtain a lithium-ion secondary battery.

[0163] Examples 2-9, Comparative Examples 1-2

[0164] Compared with Example 1, the difference is that the parameters of the positive electrode active material are different from those in Example 1, as detailed in Table 1.

[0165] Examples 10-17

[0166] Compared with Example 1, the difference is that the preparation parameters of the second positive electrode active material are different from those in Example 1, as detailed in Table 2.

[0167] Product parameters and performance parameters of Examples 1-9 and Comparative Examples 1-2.

[0168] Table 1: Product parameters and performance parameters of Examples 1-9 and Comparative Examples 1-2

[0169] In Table 1, "Material 1" represents the first positive electrode active material; "Material 2" represents the second positive electrode active material; "w1" represents the mass fraction of the first positive electrode active material in the positive electrode active material; "w2" represents the mass fraction of the second positive electrode active material in the positive electrode active material; "Specific Capacity" represents the specific capacity of the positive electrode active material; "v1" represents the first discharge plateau voltage in the discharge curve of the first positive electrode active material relative to graphite at a discharge rate of 0.3C-1C; "v2" represents the second discharge plateau voltage in the discharge curve of the second positive electrode active material relative to graphite at a discharge rate of 0.3-1C; "v1-v2" represents the difference between the first and second discharge plateau voltages; "q" represents the proportion of the discharge capacity of the second positive electrode active material to the total discharge capacity of the positive electrode active material in the discharge curve of the positive electrode relative to graphite at a discharge rate of 0.3C-1C; and "Discharge Time" represents the discharge time of the lithium-ion battery measured in the -20℃ discharge time test. For a detailed description of the test method, please refer to the following section for the specific test procedure.

[0170] Comparative analysis of the embodiments and comparative examples shows that the discharge times of Examples 1-9 are all better than those of Comparative Examples 1-2, indicating that the lithium-ion batteries of Examples 1-9 can still achieve stable power output at low SOC (10% SOC), resulting in a better discharge time than those of Comparative Examples 1-2. Therefore, this application effectively improves the power performance of lithium-ion secondary batteries at low SOC by utilizing the dual-plateau voltage effect of two positive electrode active materials.

[0171] As can be seen from Examples 1-4, when the type of the first positive electrode active material is fixed, by adjusting the mass fraction of the second positive electrode active material, the specific capacity of the positive electrode active material can be adjusted, and the ratio of the discharge capacity of the second positive electrode active material to the total discharge capacity of the positive electrode active material can be affected, thereby affecting the power performance of the lithium-ion battery at low SOC, which is reflected in the different discharge times in Examples 1-4.

[0172] As shown in Examples 1 and 5-9, by combining different types of first and second positive electrode active materials, the specific capacity of the positive electrode active material can be controlled, and the ratio of the discharge capacity of the second positive electrode active material to the total discharge capacity of the positive electrode active material can be affected, thereby affecting the power performance of the lithium-ion battery at low SOC, as reflected in the different discharge times of Examples 1 and 5-8. In Example 7, the mass ratio of LFP (lithium iron phosphate) to LFMP (lithium manganese iron phosphate) is 1:1, meaning that the mass fraction of LFP in the positive electrode active material is 45%, and the mass fraction of LFMP is 45%. In Example 8, LFP (lithium iron phosphate) and NCM523 (LiNi)... 0.5 Co 0.2 Mn 0.3 The mass ratio of O2 is 1:1, meaning that the mass fraction of LFP in the positive electrode active material is 45%, and the mass fraction of NCM523 is 45%.

[0173] Table 2: Preparation parameters and product parameters of Examples 1, 10-17

[0174] In Table 2, "raw materials" refers to the raw materials used to prepare the second positive electrode active material; "molar ratio" refers to the molar ratio of the raw materials used to prepare the second positive electrode active material; "sintering temperature" refers to the sintering temperature at which the mixture formed from the raw materials is sintered; "sintering time" refers to the sintering time at which the mixture formed from the raw materials is sintered; "material 2" refers to the second positive electrode active material obtained; "v2" refers to the second discharge plateau voltage in the discharge curve of the second positive electrode active material relative to graphite at a discharge rate of 0.3-1C; and "specific capacity of material 2" refers to the specific capacity of the second positive electrode active material.

[0175] Figure 3 The XRD pattern of the second positive electrode active material in Example 1 is shown. Figure 4 SEM images of the second positive electrode active material in Example 1 are shown. Figure 5 The laser particle size distribution map of the second positive electrode active material in Example 1 is shown. Figure 6 The capacity-voltage curve of the second positive electrode active material in Example 1 is shown.

[0176] As shown in Examples 1, 10-11, and 17, different second positive electrode active materials can be prepared by adjusting the molar ratio and raw materials. These second positive electrode active materials all have a discharge plateau voltage between 2V and 3V, which can effectively improve the power performance of lithium-ion batteries at low SOC. As shown in Examples 1, 13-16, sintering temperature and sintering time affect the specific capacity of the second positive electrode active material. Therefore, by controlling the sintering time and sintering temperature within a suitable range, a second positive electrode active material with a discharge plateau voltage and specific capacity that meet the design requirements can be prepared. As shown in Example 18, when the molar ratio of lithium to other metal elements in the raw materials is not within the specified range, lithium transition metal sulfides cannot be prepared. Only a composite phase formed by lithium sulfide and other metal sulfides can be obtained, which does not have a layered structure and has a significantly reduced specific capacity. When applied to lithium-ion batteries, compared with layered lithium transition metal sulfides, lithium ion extraction is more difficult, and the specific capacity is not conducive to the energy density of lithium-ion batteries. The overall performance is not as good as that of Examples 1, 10-16.

[0177] 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.

[0178] 1. Testing of the types of positive electrode active materials

[0179] The lithium-ion battery under test is disassembled to obtain the positive electrode sheet. The binder is washed away with NMP, and the type of positive electrode active material can then be determined by XRD and SEM. Specifically, the characteristic peaks of the positive electrode active material can be obtained by XRD. After excluding the characteristic peaks of the conductive carbon, the type of positive electrode active material is determined by comparing the remaining characteristic peaks.

[0180] For example, XRD testing can be performed under the following conditions: the powder is placed on the sample stage, compacted with a glass slide, and placed in an XRD testing instrument: tube voltage 40kV, tube current 20mA, Cu target, scanning speed 2.000 (d·min-1), scanning range 10°-80°.

[0181] 2. Testing of discharge plateau voltage

[0182] At 25℃, unused lithium-ion batteries under test were charged at a constant current and constant voltage rate of 0.33C to the cutoff voltage (LFP system -3.8V, NCM523 -4.4V, LFMP -4.2V, LNMO -4.8V), with a cutoff current of 0.05C. Subsequently, they were discharged at a rate of 0.33C, with a discharge cutoff voltage of 2V. Charge-discharge curves (capacity-voltage curves) of the lithium-ion batteries were obtained. The discharge capacity of the lithium-ion batteries between the upper cutoff voltage of 3V and the upper cutoff voltage was statistically analyzed; this is the discharge capacity of the first discharge plateau (which can be equivalent to the discharge capacity of the first positive electrode active material). The average voltage corresponding to this portion of the capacity is the first discharge plateau voltage v1. The discharge capacity of the lithium-ion batteries between 2-3V was statistically analyzed; this is the discharge capacity of the second discharge plateau (which can be equivalent to the discharge capacity of the second positive electrode active material). The average voltage corresponding to this portion of the capacity is the second voltage plateau v2.

[0183] 3. Testing the ratio of the discharge capacity of the second positive electrode active material to the total discharge capacity of the positive electrode active material.

[0184] At 25℃, fresh battery cells are charged at a constant current and constant voltage rate of 0.33C to the cutoff voltage (LFP system -3.8V, NCM523 -4.4V, LFMP -4.2V, LNMO -4.8V), with a cutoff current of 0.05C. They are then discharged at a rate of 0.33C, with a discharge cutoff voltage of 2V. The discharge capacity between 2-3V is calculated, and then divided by the total discharge capacity to obtain the discharge capacity ratio of the second cathode material.

[0185] 4. Testing the Dv50 of the material

[0186] Unless otherwise specified, the particle size distribution parameters of the positive electrode active material, such as Dv50, determined by particle size distribution measurements in this application are determined using a particle size analyzer-laser diffraction method. Specifically, refer to standard GB / T19077-2016, which uses a laser diffraction scattering particle size analyzer and is measured according to the manufacturer's instructions.

[0187] Testing equipment: Particle size analyzer.

[0188] Pretreatment: Take a clean beaker, add an appropriate amount of the sample to be tested, add a surfactant and then add a dispersant, and sonicate at 120W / 5min to ensure that the sample is completely dispersed in the dispersant.

[0189] Test: After the sample is poured into the injection tower, it circulates with the solution to the test optical path system. Under the irradiation of the laser beam, the particle size distribution characteristics can be obtained by receiving and measuring the energy distribution of the scattered light (shading degree: 8-12%).

[0190] 5. Testing the specific capacity of positive electrode active materials

[0191] At 25℃, the lithium-ion secondary battery is charged at a constant current and voltage rate of 0.33C to 100% SOC, i.e., charged at a constant current rate of 0.33C to the upper limit cutoff voltage, 3.8V (LFP), and then charged at the upper limit voltage (LFP-3.8V) to the cutoff current of 0.05C. It is then discharged at a constant current rate of 0.33C to a cutoff voltage of 2V. This charge-discharge cycle is repeated three times. The capacity of the third cycle is taken as the cell test capacity C1. Combined with the mass of the positive electrode active material, the specific capacity of the positive electrode active material can be calculated.

[0192] The mass of the positive electrode active material can be determined by disassembling the lithium-ion battery under test, washing away the binder with NMP, and weighing the remaining material. The carbon content (conductive agent) is then tested using a CS analyzer, and the mass of the positive electrode active material is calculated by subtracting the carbon content from the mass of the remaining material. Alternatively, the content of the target element in the remaining material can be directly tested. For example, if both the first and second positive electrode active materials contain Fe, the mass of the entire positive electrode active material can be calculated from the Fe content.

[0193] 6. Discharge time test at -20℃

[0194] The lithium-ion secondary battery under test was first tested for capacity at 25°C (upper limit voltage of 2-3.8V). After determining the capacity, it was discharged to 10% SOC. The cell was then left to stand at -20°C for 120 minutes and discharged to 1.5V with a constant power of 1264W. The time of this low-temperature discharge stage was recorded.

[0195] 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, The lithium-ion secondary battery includes: A positive electrode sheet, wherein the positive electrode sheet includes a positive active material, and the positive active material includes a first positive active material and a second positive active material; The discharge curve of the first positive electrode active material relative to graphite at a discharge rate of 0.3C-1C includes a first discharge plateau voltage v1, and the discharge curve of the second positive electrode active material relative to graphite at a discharge rate of 0.3-1C includes a second discharge plateau voltage v2. 3V≤v1≤5V, 2V≤v2<3V.

2. The lithium-ion secondary battery according to claim 1, characterized in that, 0.2V≤v1-v2≤2V.

3. The lithium-ion secondary battery according to claim 1 or 2, characterized in that, The first positive electrode active material includes at least one of lithium-containing transition metal oxides and lithium-containing transition metal phosphates.

4. The lithium-ion secondary battery according to claim 3, characterized in that, The lithium-containing transition metal oxide includes at least one of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, or modified compounds thereof.

5. The lithium-ion secondary battery according to claim 3 or 4, characterized in that, The lithium-containing transition metal phosphate includes at least one of lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate, or modified compounds thereof.

6. The lithium-ion secondary battery according to any one of claims 1-5, characterized in that, The second positive electrode active material includes lithium transition metal sulfide.

7. The lithium-ion secondary battery according to claim 6, characterized in that, The lithium transition metal sulfide includes those with the molecular formula Li. 1+a Fe x Ti y M z At least one of the compounds of S2 or its modified compounds; Wherein, -0.05≤a≤0.2, 0.3≤x, 0.3≤y, 0≤z≤0.1, a+x+y+z=1, and M includes at least one of Ni, Co, Mn, V, Zr, Y, Al, Mg, B, Sr, Na, K, Nb, La, and Si.

8. The lithium-ion secondary battery according to claim 7, characterized in that, The second positive electrode active material includes a doping element, which includes at least one of sodium and magnesium.

9. The lithium-ion secondary battery according to any one of claims 1-8, characterized in that, Based on the mass of the positive electrode active material, the mass fraction w of the second positive electrode active material satisfies: 0.5% ≤ w ≤ 20%.

10. The lithium-ion secondary battery according to any one of claims 1-9, characterized in that, In the discharge curve of the positive electrode relative to graphite at a discharge rate of 0.3C-1C, the proportion q of the discharge capacity of the second positive electrode active material to the total discharge capacity of the positive electrode active material satisfies: 3% ≤ q ≤ 30%.

11. The lithium-ion secondary battery according to any one of claims 1-10, characterized in that, The first positive electrode active material Dv501 satisfies: 0.5μm≤Dv501≤20μm.

12. The lithium-ion secondary battery according to any one of claims 1-11, characterized in that, The second positive electrode active material Dv502 satisfies: 2.5μm≤Dv502≤20μm.

13. The lithium-ion secondary battery according to any one of claims 1-12, characterized in that, The areal density CW of the positive electrode sheet satisfies: 5 mg / cm³ 2 ≤CW≤40mg / cm 2 .

14. The lithium-ion secondary battery according to any one of claims 1-13, characterized in that, The compaction density PD of the positive electrode sheet satisfies: 1.5 g / cm³ 3 ≤PD≤4g / cm 3 .

15. The lithium-ion secondary battery according to any one of claims 1-14, characterized in that, The film resistance R of the positive electrode sheet satisfies: 0.1Ω≤R≤1Ω.

16. A method for preparing a lithium-ion secondary battery as described in any one of claims 1-15, characterized in that, The method includes: providing the first positive electrode active material, and preparing the second positive electrode active material through the following steps: Lithium sulfide, titanium sulfide, ferrous sulfide, and transition metal sulfides are mixed to form a mixture; The mixture is sintered to obtain a sintered product; The sintered material is crushed to obtain the second positive electrode active material.

17. The method according to claim 16, characterized in that, In the mixture, the molar ratio of lithium to other metal elements is (0.95:1)-(1.2:1).

18. The method according to claim 16 or 17, characterized in that, The sintering temperature is 600℃-900℃, the sintering time is 10h-40h, and the sintering atmosphere is an inert atmosphere.

19. The method according to claim 18, characterized in that, The inert atmosphere includes at least one of nitrogen, argon, and helium.

20. The method according to any one of claims 16-19, characterized in that, The crushing method includes at least one of ball milling and air jet milling.

21. An electrical appliance, characterized in that, The lithium-ion secondary battery includes any one of claims 1-15, and / or a lithium-ion secondary battery prepared by the method of any one of claims 16-20.

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