Secondary battery and electric device

Abstract

This application discloses a secondary battery and an electrical device. The secondary battery of this application uses primary particles containing doped elements and secondary spherical lithium manganese iron phosphate as the positive electrode active material. The average particle size of the secondary spherical lithium manganese iron phosphate, the true density and particle size distribution of the positive electrode active material, the compaction density of the positive electrode sheet, and the content and type of doped elements are controlled to reduce the voids between the positive electrode active material particles and optimize the kinetic performance of the positive electrode active material. This improves the powder compaction density and specific capacity of the positive electrode active material, thereby achieving the goal of improving the energy density, low-temperature performance, and cycle performance of the secondary battery.

Description

Technical Field

[0001] This invention belongs to the field of batteries, specifically relating to secondary batteries and electrical devices. Background Technology

[0002] The positive electrode active material plays a decisive role in the energy density, power, safety, and lifespan of secondary batteries. Currently, the main positive electrode active materials used in secondary batteries are lithium nickel cobalt manganese oxide, lithium iron phosphate, and lithium manganese iron phosphate. Among them, lithium nickel cobalt manganese oxide has a high energy density, but it is very expensive and cannot be reduced in cost. Lithium iron phosphate is relatively inexpensive, has high safety, and good cycle performance, but its energy density is relatively low, and due to the material itself, it is difficult to increase the energy density, which limits its application.

[0003] Lithium manganese iron phosphate (LMP) has the same theoretical specific capacity as lithium iron phosphate (LFP) (170 mAh / g), and its lithium-to-lithium potential reaches up to 4.1V, significantly higher than LFP's 3.4V. This higher voltage platform gives secondary batteries using LMP as the positive electrode active material the potential advantage of higher energy density, while maintaining similar safety and cost to LFP. However, LMP has a relatively wide band gap between the conduction and valence bands, resulting in a higher activation energy during lithium-ion conduction. This leads to significantly lower electronic and ionic conductivity compared to LFP, resulting in lower specific capacity, lower power performance, and poorer low-temperature performance. Furthermore, the presence of Mn in LMP further complicates the process. 3+ The presence of Mn makes it prone to the Jahn-Teller effect, resulting in significant lattice volume changes. 3+ Disproportionation reaction and Mn 2+ Dissolution leads to deterioration of high-temperature cycling performance. Summary of the Invention

[0004] The purpose of this application is to overcome the shortcomings of existing lithium manganese iron phosphate secondary batteries, such as low energy density, poor low-temperature performance, and poor cycle performance, and to provide a secondary battery and power supply device to improve the energy density, low-temperature performance, and cycle performance of lithium manganese iron phosphate secondary batteries.

[0005] To achieve the above objectives, in a first aspect, this application provides a secondary battery 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, and the positive electrode active material containing element M;

[0006] The element M includes at least one of Mg, Ti, V, Zn, Al, Co, W, Mo, Y, Nb, In, La, Zr, Ce, Sr, and Sb.

[0007] The first positive electrode active material comprises lithium manganese iron phosphate with a secondary spherical morphology, which is formed by the aggregation of primary particles;

[0008] The second positive electrode active material comprises primary particles of lithium manganese iron phosphate;

[0009] The positive electrode plate satisfies the following relationship:

[0010] 1.85≤D1×D2×ρ×PD×(Dv 90 -Dv 10 )÷Dv 50 ÷c≤7.42;

[0011] Wherein, D1 nm is the average particle size of the primary particles in the secondary spherical lithium manganese iron phosphate;

[0012] D2μm is the average particle size of the lithium manganese iron phosphate with the secondary spherical morphology;

[0013] ρg / cm 3 The true density of the positive electrode active material;

[0014] PD g / cm 3 The compaction density of the positive electrode sheet;

[0015] Dv 10 μm is the particle size corresponding to a volume cumulative particle size distribution percentage of the positive electrode active material reaching 10%;

[0016] Dv 50 μm is the particle size corresponding to a volume cumulative particle size distribution percentage of 50% for the positive electrode active material;

[0017] Dv 90 μm is the particle size corresponding to the volume cumulative particle size distribution percentage of the positive electrode active material reaching 90%;

[0018] c ppm represents the content of element M in the positive electrode active material.

[0019] As an embodiment of this application, the D1 nm satisfies: 50nm ≤ D1 nm ≤ 200nm.

[0020] As an embodiment of this application, the D2μm satisfies: 4.1μm≤D2μm≤9μm.

[0021] As an implementation of this application, the Dv 50 μm satisfies: 0.8μm≤Dv 50 μm≤10.5μm.

[0022] As an implementation of this application, the Dv90 μm satisfies: 4.5μm≤Dv 90 μm≤30μm.

[0023] As an implementation of this application, the Dv 10 μm satisfies: 0.25μm≤Dv 10 μm≤2.6μm.

[0024] As an implementation scheme of this application, the PD g / cm 3 Satisfying: 2.1 g / cm³ 3 ≤PD g / cm 3 ≤2.55g / cm 3 .

[0025] As an implementation scheme of this application, the ρg / cm 3 Satisfying: 3.4g / cm 3 ≤ρg / cm 3 ≤3.6g / cm 3 .

[0026] As an embodiment of this application, the c ppm satisfies: 2000ppm≤c ppm≤8000ppm.

[0027] As an embodiment of this application, the areal density of the positive electrode sheet is 0.125–0.35 g / 1540.25 mm². 2 .

[0028] As an embodiment of this application, the average particle size of the primary particles of lithium manganese iron phosphate in the second positive electrode active material is 100-400 nm.

[0029] In a second aspect, this application also provides an electrical device comprising the aforementioned secondary battery. The electrical device, including the secondary battery of this application, has high energy density, low internal resistance, good low-temperature performance, and excellent cycle performance, making it more suitable for practical applications.

[0030] The beneficial effects of this application are as follows: The secondary battery of this application uses two active materials containing doped elements as the overall positive electrode active material. The second active material mainly exists in the form of primary particles, while the first active material mainly exists in the form of secondary spherical morphology. The average particle size of lithium manganese iron phosphate with secondary spherical morphology in the first positive electrode active material, the true density and particle size distribution of the positive electrode active material, the compaction density of the positive electrode sheet, and the content and type of doped elements are controlled to reduce the voids between the positive electrode active material particles and optimize the kinetic performance of the positive electrode active material. This improves the powder compaction density and specific capacity of the positive electrode active material, thereby achieving the goal of improving the energy density, low-temperature performance, and cycle performance of the secondary battery. Attached Figure Description

[0031] Figure 1 This is a surface morphology diagram of the positive electrode sheet.

[0032] Figure 2 This is a cross-sectional morphology diagram of the positive electrode sheet. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0034] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0035] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0036] Unless otherwise specified, all reagents or instruments used in this application are commercially available products.

[0037] The present application is further illustrated below with specific embodiments:

[0038] This application provides a secondary battery, which includes a positive electrode sheet, the positive electrode sheet 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, and the positive electrode active material contains element M;

[0039] The element M includes at least one of Mg, Ti, V, Zn, Al, Co, W, Mo, Y, Nb, In, La, Zr, Ce, Sr, and Sb.

[0040] The first positive electrode active material comprises lithium manganese iron phosphate with a secondary spherical morphology, which is formed by the aggregation of primary particles;

[0041] The second positive electrode active material comprises primary particles of lithium manganese iron phosphate;

[0042] The positive electrode plate satisfies the following relationship:

[0043] 1.85≤D1×D2×ρ×PD×(Dv 90 -Dv 10 )÷Dv 50 ÷c≤7.42;

[0044] Wherein, D1 nm is the average particle size of the primary particles in the secondary spherical lithium manganese iron phosphate;

[0045] D2μm is the average particle size of the lithium manganese iron phosphate with the secondary spherical morphology;

[0046] ρg / cm 3 The true density of the positive electrode active material;

[0047] PD g / cm 3 The compaction density of the positive electrode sheet;

[0048] Dv 10 μm is the particle size corresponding to a volume cumulative particle size distribution percentage of the positive electrode active material reaching 10%;

[0049] Dv 50 μm is the particle size corresponding to a volume cumulative particle size distribution percentage of 50% for the positive electrode active material;

[0050] Dv 90 μm is the particle size corresponding to the volume cumulative particle size distribution percentage of the positive electrode active material reaching 90%;

[0051] c ppm represents the content of element M in the positive electrode active material.

[0052] This application uses lithium manganese iron phosphate with secondary spherical morphology and primary particles containing doped elements as the positive electrode active material, and controls the average particle size of the secondary spherical lithium manganese iron phosphate, the true density and particle size distribution of the positive electrode active material, the compaction density of the positive electrode sheet, and the content and type of doped elements, in order to reduce the voids between the positive electrode active material particles and optimize the kinetic performance of the positive electrode active material, improve the powder compaction density and specific capacity of the positive electrode active material, and achieve the purpose of improving the energy density, low temperature performance and cycle performance of the secondary battery.

[0053] In some embodiments, the D1 nm satisfies: 50nm ≤ D1 nm ≤ 200nm. For example, the D1 nm is a range of one or any two of 50nm, 75nm, 100nm, 125nm, 150nm, 175nm, and 200nm.

[0054] In this application, the positive electrode active material of the secondary battery includes two types of lithium manganese iron phosphate, designated as the first positive electrode active material and the second positive electrode active material, respectively. The first positive electrode active material mainly exists in a secondary spherical morphology, and the secondary spherical lithium manganese iron phosphate consists of spherical or near-spherical particles formed by the aggregation of primary particles. The second positive electrode active material mainly exists in the form of primary particles, which refer to primary particles that have not agglomerated or been artificially sphericalized. The first and second positive electrode active materials in this application each independently contain a dopant element M. In particular, when the M element is selected from at least one of Mg, Ti, V, Zn, Al, Co, W, Mo, Y, Nb, In, La, Zr, Ce, Sr, and Sb, the structural stability and kinetic performance of lithium manganese iron phosphate can be effectively improved. Furthermore, the combination of primary lithium manganese iron phosphate particles and secondary spherical particles is beneficial for balancing specific capacity and compaction density, thereby improving the energy density, low-temperature performance, and cycle performance of the secondary battery.

[0055] In some embodiments, M comprises at least one of Ti, Mg, Co, W, V, Nb, and In.

[0056] This application involves nano-sizing of the first positive electrode active material, which helps to shorten the lithium-ion diffusion distance, further improves the charging capacity and kinetic performance of the positive electrode active material, enhances the specific capacity of the positive electrode active material, and thus improves the energy density, low-temperature performance and cycle performance of the secondary battery.

[0057] In some embodiments, the D2μm satisfies: 4.1μm ≤ D2μm ≤ 9μm. Exemplarily, the D2μm is a value within the range of one or both of 4.1μm, 5μm, 6μm, 7μm, 8μm, and 9μm. By controlling the average particle size of the secondary spherical lithium manganese iron phosphate within the range of 4.1μm to 9μm, the specific surface area of ​​the particles can be reduced, particle stability improved, and processing defects caused by the nano-sizing of the primary particles of the first positive electrode active material can be mitigated, further increasing the energy density of the secondary battery. Furthermore, the secondary particle size of the first positive electrode active material in this application is relatively suitable and can be combined with the second positive electrode active material to achieve particle size distribution, improve particle packing effect, and enhance the energy density, cycle performance, and low-temperature performance of the secondary battery.

[0058] In some embodiments, the Dv 50 μm satisfies: 0.8μm≤Dv 50 μm ≤ 10.5μm. For example, the Dv 50 μm is a range of one or any two of the following: 0.8μm, 1.0μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, and 10.5μm.

[0059] In some embodiments, the Dv 90 μm satisfies: 4.5μm≤Dv 90 μm≤30μm. For example, the Dv 90 μm is a range of one or any two of the following: 4.5μm, 7.5μm, 10μm, 12.5μm, 15μm, 20μm, 22.5μm, 25μm, 27.5μm, and 30μm.

[0060] In some embodiments, the Dv 10 μm satisfies: 0.25μm≤Dv 10 μm ≤ 2.6μm. For example, the Dv 10 μm is a range of one or any two of the following: 0.25μm, 0.5μm, 0.75μm, 1.0μm, 1.25μm, 1.5μm, 1.75μm, 2.0μm, 2.25μm, and 2.6μm.

[0061] In some embodiments, the PD g / cm 3 Satisfying: 2.1 g / cm³ 3 ≤PD g / cm 3 ≤2.55g / cm 3 For example, the PD g / cm 3 2.10 g / cm 3 2.20g / cm 3 2.30g / cm 3 2.40 g / cm 3 2.55g / cm 3 The range of one or any two of them.

[0062] Controlling the Dv of the positive electrode active material 10 Dv 10 Dv 90 Within the scope of this application, a reasonable particle size distribution can be achieved, reducing the voids between positive electrode active material particles and increasing the particle packing density, thereby increasing the compaction density of the positive electrode sheet, and consequently improving the energy density, low-temperature performance, and cycle performance of the secondary battery. Furthermore, it also helps to ensure the processing performance of the positive electrode slurry and positive electrode sheet, as well as the capacity utilization and power performance of the secondary battery.

[0063] In some embodiments, the ρg / cm 3 Satisfying: 3.4g / cm 3 ≤ρg / cm 3 ≤3.6g / cm 3 For example, the ρg / cm 3 3.40 g / cm 33.45g / cm 3 3.50g / cm 3 3.55g / cm 3 3.60g / cm 3 The true density of the positive electrode active material can be controlled within the range of one or both of the above values ​​by controlling the proportion of the first positive electrode active material and the second positive electrode active material. This is beneficial to improving the specific capacity and kinetic performance of the positive electrode active material.

[0064] In some embodiments, c ppm satisfies: 2000ppm ≤ c ppm ≤ 8000ppm. Exemplarily, c ppm is a range of one or any two of 2000ppm, 3000ppm, 4000ppm, 5000ppm, 6000ppm, 7000ppm, and 8000ppm. When the amount of element M added in this application is selected within the above concentration range, the lithium-ion diffusion coefficient and stability of the positive electrode active material can be effectively improved, the kinetic performance and lifespan of the positive electrode active material can be enhanced, and the cycle performance of the secondary battery can be improved.

[0065] In some embodiments, the areal density of the positive electrode sheet on one side is 0.125–0.35 g / 1540.25 mm. 2 For example, the areal density of the positive electrode sheet on one side can be 0.125g / 1540.25mm. 2 0.150g / 1540.25mm 2 0.175g / 1540.25mm 2 0.200g / 1540.25mm 2 0.225g / 1540.25mm 2 0.250g / 1540.25mm 2 0.275g / 1540.25mm 2 0.300g / 1540.25mm 2 0.325g / 1540.25mm 2 0.350g / 1540.25mm 2 The range of one or both of these values. When the areal density of the positive electrode is maintained within the above range, it is not only beneficial to improve energy density, low-temperature performance and cycle performance, but also to minimize the amount used and reduce costs while ensuring the electrochemical performance of the secondary battery.

[0066] In some embodiments, the average particle size of lithium manganese iron phosphate in the primary particles of the second positive electrode active material is expressed as D3nm, and the average particle size D3nm of lithium manganese iron phosphate in the primary particles of the second positive electrode active material is 100-400nm. Exemplarily, the average particle size of lithium manganese iron phosphate in the primary particles of the second positive electrode active material is a range of one or both of 100nm, 150nm, 200nm, 300nm, and 400nm. When the primary particles in the second positive electrode active material are maintained within the above range, the particle size of the primary particles of the second positive electrode active material is smaller, which is beneficial for shortening the lithium-ion diffusion distance, improving the kinetic performance of the positive electrode active material, and can be combined with the first active material to achieve particle size distribution, thereby increasing the particle packing density and further improving the energy density, low-temperature performance, and cycle performance of the secondary battery.

[0067] In some embodiments, in the positive electrode sheet, the mass percentage of the first positive electrode active material is 1-95%, and the mass percentage of the second positive electrode active material is 1-95%. Exemplarily, the mass percentage of the first positive electrode active material is a range of one or both of 1%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, and 95%, and the mass percentage of the second positive electrode active material is a range of one or both of 1%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, and 95%.

[0068] In some embodiments, the positive electrode sheet further includes a conductive agent and a binder.

[0069] In some embodiments, the secondary battery further includes a negative electrode, a separator, and an electrolyte.

[0070] In some embodiments, the negative electrode sheet includes a negative electrode active material, which includes natural graphite, artificial graphite, mesophase microcarbon spheres (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiO, LiSn alloy, LiSnO alloy, Sn, SnO, SnO2, and spinel-structured lithium titanate Li4Ti5O. 12 At least one of LiAl alloy and metallic lithium.

[0071] In some embodiments, the diaphragm comprises a porous sheet-like or nonwoven resin or glass fiber, wherein the resin comprises at least one of polyolefin, aromatic polyamide, polytetrafluoroethylene, and polyethersulfone.

[0072] In a second aspect, this application also provides an electrical device comprising the aforementioned secondary battery.

[0073] The present application is further illustrated below with specific embodiments:

[0074] Example 1

[0075] A secondary battery, the method for preparing the secondary battery includes the following steps:

[0076] S1. Preparation of positive electrode active materials: There are no special restrictions on the preparation methods of the first positive electrode active material and the second positive electrode active material in this application. For example, the first positive electrode active material and the second positive electrode active material can be synthesized by liquid phase grinding and high temperature solid phase secondary sintering process. Alternatively, other conventional methods in the art can be used to obtain the first positive electrode active material and the second positive electrode active material required in this application.

[0077] The following describes in detail, using Example 1 as an example, the preparation of the first positive electrode active material and the second positive electrode active material by liquid phase grinding and high-temperature solid phase secondary sintering process. The first positive electrode active material and the second positive electrode active material in other examples and comparative examples are prepared by referring to this preparation method.

[0078] Preparation method of the first positive electrode active material:

[0079] (1) Weigh out 6.045 kg FePO4, 4.585 kg Mn3O4, 6.248 kg LiH2PO4, 1.573 kg Li2CO3, 0.026 kg TiO2, 0.047 kg MgO, 0.010 kg CoO, and 0.197 kg glucose respectively; mix the above raw materials evenly, and grind them in a ball mill jar with deionized water as solvent until the particle size Dv50 is 500 nm;

[0080] (2) The product is then spray-dried and sintered (490℃, 8h) to obtain the first sintered product;

[0081] (3) Weigh 0.01 kg WO3 and 0.512 kg glucose respectively, then mix the first sintering product, WO3 and glucose evenly, and then perform a second grinding until the particle size Dv50 is 250 nm.

[0082] (4) The product is then spray-dried and sintered (700℃, 24h) to obtain the second sintered product; the second sintered product is then sieved and iron removed to obtain the first active material. The chemical formula of the first positive electrode active material is LiMn. 0.6 Fe 0.4 PO4.

[0083] D1 and D2 can be controlled by changing the grinding, spraying and sintering steps in the above preparation process. For details of the first positive electrode active material used in this application, please refer to Tables 1 to 2.

[0084] Method for preparing the second positive electrode active material:

[0085] (1) Weigh out 6.045 kg FePO4, 4.585 kg Mn3O4, 6.248 kg LiH2PO4, 1.573 kg Li2CO3, 0.026 kg TiO2, 0.047 kg MgO, 0.010 kg CoO and 0.197 kg glucose respectively, mix the above raw materials evenly, and grind them in a ball mill jar with deionized water as solvent until the particle size Dv50 is 800 nm;

[0086] (2) The product is then spray-dried and sintered (490℃, 8h) to obtain the first sintered product;

[0087] (3) Weigh 0.01 kg WO3 and 0.512 kg glucose respectively, then mix the first sintering product, WO3 and glucose evenly, and perform a second grinding until the particle size Dv50 is 350 nm.

[0088] (4) Re-spray drying and sintering (725℃, 24h) yields a second sintered product. The second sintered product is then crushed, sieved, and iron removed to obtain a second active material. The chemical formula of the second positive electrode active material is LiMn. 0.6 Fe 0.4 PO4.

[0089] D3 can be controlled by changing the grinding, sintering and crushing steps in the above preparation process. For details of the second positive electrode active material used in this application, please refer to Tables 1 to 2.

[0090] The first positive electrode active material and the second positive electrode active material are mixed evenly according to the proportions in Table 2 to obtain the positive electrode active material.

[0091] Because the first positive electrode active material contains a secondary spherical morphology, and compared with the preparation method of the first positive electrode active material, the second positive electrode active material involves a crushing process, the secondary spherical morphology is destroyed, resulting in a primary particle morphology without a secondary spherical morphology. Due to differences in morphology, particle size (Dv10, Dv50, Dv90), and ρ between the two positive electrode active materials, and because the positive electrode active material is composed of a mixture of the first and second positive electrode active materials with different mass percentages, the respective proportions of the first and second active materials will lead to differences in Dv10, Dv50, Dv90, and ρ of the positive electrode active material.

[0092] S2. The positive electrode active material, binder PVDF, conductive carbon black, and solvent NMP are mixed and stirred evenly to form a positive electrode slurry. After sieving, the positive electrode slurry is evenly coated onto the positive electrode current collector aluminum foil. Then, the positive electrode sheet is obtained through baking, rolling, slitting, and cutting processes. The weight ratio of positive electrode active material, conductive carbon black, and binder PVDF in the positive electrode sheet is 96:2.5:1.5. During the preparation process, the compaction density (P) of the positive electrode sheet is controlled by controlling the rolling pressure and the roller gap, and the single-sided surface density (CW) is changed by controlling the weight of the coated positive electrode slurry. The specific values ​​are detailed in Tables 1-2.

[0093] S3. Preparation of negative electrode sheet: The negative electrode active material graphite, the thickener sodium carboxymethyl cellulose, the binder styrene-butadiene rubber, and the conductive agent conductive carbon black are mixed in a mass ratio of 97:1:1:1, deionized water is added, and a negative electrode slurry is obtained under the action of a vacuum stirrer; the negative electrode slurry is uniformly coated on the negative electrode current collector copper foil; the coated electrode sheet is transferred to an oven to dry, and then cold-pressed and slit to obtain the negative electrode sheet;

[0094] S4. Preparation of electrolyte: Ethyl carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed evenly to obtain an organic solvent, wherein the volume ratio of EC, EMC, and DEC is 20:20:60; then, in an argon atmosphere glove box with a water content of <10ppm, fully dried lithium salt LiPF6 is dissolved in the organic solvent prepared above and mixed evenly to obtain an electrolyte, wherein the concentration of lithium salt in the electrolyte is 1mol / L;

[0095] S5. The diaphragm is a commercially available nonwoven polyolefin resin membrane;

[0096] S6. The positive and negative electrode sheets are wound and hot-pressed together with a commercial separator, and the tabs are welded. Then, they are placed in an aluminum-plastic film and baked at 85°C for 24 hours. Electrolyte is injected, and the cells are allowed to stand, form, and be capacity tested. The components are then assembled into a secondary battery.

[0097] Examples 2-11 and 16

[0098] The secondary batteries in Examples 2-11 and 16 differ from those in Example 1 in that the mass percentages (ω1) of the first positive electrode active material and (ω2) of the second positive electrode active material are different, resulting in different Dv values ​​in the positive electrode active material. 10 Dv 50 Dv 90 The compaction density PD of the positive electrode sheet varies with different values ​​of ρ and ρ, and the compaction density PD of the positive electrode sheet varies by controlling the roller pressure and the roller gap, as shown in Table 1.

[0099] Examples 12-13

[0100] The secondary batteries in Examples 12 and 13 differ from those in Example 1 in that the grinding particle size in steps (1) and (3) of the preparation method of the first positive electrode active material is changed, so that D1 is different and PD in Example 12 is different.

[0101] Examples 14-15

[0102] The secondary batteries in Examples 14 and 15 differ from those in Example 1 in that the parameters of the spraying process in step (4) of the preparation method of the first positive electrode active material are changed, resulting in different values ​​for D2 and Dv. 10 Dv 50 Dv 90 different.

[0103] Examples 17-18

[0104] The secondary batteries in Examples 17-18 differ from those in Example 1 in that the mass of TiO2, MgO, CoO, and WO3 weighed in steps (1) and (3) of the preparation methods of the first and second positive electrode active materials is changed to adjust the c. In addition, the doping amount of the same element in the first and second positive electrode active materials of each example is the same.

[0105] Examples 19-20

[0106] The secondary batteries in Examples 19 and 20 differ from those in Example 1 in that the grinding particle size in steps (1) and (3) of the preparation method of the second positive electrode active material is changed, resulting in different D3 and different PD in Example 20.

[0107] Examples 21-22

[0108] The secondary batteries in Examples 21 and 22 differ from those in Example 1 in that the PD is different, wherein the PD is controlled by the rolling pressure and the roll gap.

[0109] Examples 23-24

[0110] The secondary batteries in Examples 23 and 24 differ from those in Example 1 in that the CW is different by changing the weight of the coated positive electrode slurry.

[0111] Example 25

[0112] The secondary battery in Example 25 differs from that in Example 1 in that the mass of FePO4 and Mn3O4 weighed in step (1) of the preparation method of the first positive electrode active material is changed, resulting in a different Mn / Fe molar ratio (N1) of the first positive electrode active material. The chemical formula of the first positive electrode active material in this example is LiMn 0.5 Fe0.5 PO4.

[0113] Example 26

[0114] The secondary battery in Example 26 differs from that in Example 1 in that the mass of FePO4 and Mn3O4 weighed in step (1) of the preparation method of the second positive electrode active material is changed, resulting in a different Mn / Fe molar ratio (N2) of the second positive electrode active material. In this example, the chemical formula of the second positive electrode active material is LiMn. 0.5 Fe 0.5 PO4.

[0115] Example 27

[0116] The difference between this embodiment and Embodiment 1 is that in this embodiment, MgO in step (1) of the preparation method of the first positive electrode active material and the second positive electrode active material is replaced with vanadium oxide, and the contents of Mg and V are equal.

[0117] Example 28

[0118] The difference between this embodiment and Embodiment 1 is that in this embodiment, in the preparation methods of the first positive electrode active material and the second positive electrode active material, MgO in step (1) is replaced with niobium oxide, WO3 in step (3) is replaced with indium oxide, and the contents of Mg and Nb elements are equal, and the contents of W and In elements are equal.

[0119] Example 29

[0120] The difference between this embodiment and embodiment 1 is that in the preparation method of the first positive electrode active material and the second positive electrode active material in this embodiment, step (1) does not weigh MgO and CoO, but increases the content of TiO2 so that the increased content of Ti element is equal to the total content of Mg element and Co element.

[0121] Comparative Example 1

[0122] The secondary battery in this comparative example differs from that in Example 1 in that the grinding particle size in steps (1) and (3) of the preparation method of the first positive electrode active material is changed, so that D1 is different.

[0123] Comparative Example 2

[0124] The secondary battery in this comparative example differs from that in Example 1 in that the grinding particle size of steps (1) and (3) in the preparation method of the first positive electrode active material is changed to make D1 different, and the PD is made different by the rolling pressure and the roll gap.

[0125] Comparative Example 3

[0126] The difference between the secondary battery in this comparative example and Example 1 is that the positive electrode active material in this comparative example contains only the second positive electrode active material and does not contain the first positive electrode active material.

[0127] Comparative Example 4

[0128] The difference between the secondary battery in this comparative example and Example 1 is that the positive electrode active material in this comparative example contains only the first positive electrode active material and does not contain the second positive electrode active material.

[0129] The performance testing methods for secondary batteries are as follows:

[0130] (1) Average particle size (D1) of the primary particles of the first positive electrode active material: The positive electrode sheet was cut by argon ion cutting and the cross-section was tested by field emission scanning electron microscopy (SEM). Five secondary spherical particles were randomly selected from the 3K magnification SEM image of the cross-section. Then, the five secondary spherical particles were magnified to 30K magnification. Ten primary particles with diameters of 50nm to 400nm were randomly selected from the 30K magnification SEM image of each particle. The specific size of the primary particles was measured. Then, the average size of the 50 particles was calculated to obtain the average particle size D1 of the primary particles of the first positive electrode active material.

[0131] (2) Average particle size (D2) of the secondary particles of the first positive electrode active material: The positive electrode sheet was cut by argon ion cutting and the cross-section was tested by field emission scanning electron microscopy (SEM). 15 particles with diameters of 2μm to 10μm were randomly selected from the 3K magnification SEM image of the cross-section. The specific size of the 15 particles was measured and then the average value of the 15 particle sizes was calculated to obtain the average particle size of the secondary particles of the first positive electrode active material.

[0132] (3) Referring to GB-T 19077-2016, the particle size of the positive electrode active material was measured by a particle size analysis laser diffractometer to obtain Dv. 10 Dv 50 Dv 90 .

[0133] (4) True density (ρ) of positive electrode active material: The true density of positive electrode active material was tested using a true density meter, referring to the test method of true density in GB / T 24533-2019.

[0134] (5) Test of the type and content of element M in the positive electrode active material: Cut the positive electrode sheet into small round pieces with a diameter of 12 mm, take 20 small round pieces for ICP test, and test the type and content of element M by referring to EPA6010D-2018 inductively coupled plasma atomic emission spectrometry.

[0135] (6) Detection of the molar ratio of manganese and iron (N1, N2) of the first positive electrode active material and the second positive electrode active material: ICP test was performed on the first positive electrode active material and the second active material respectively. The manganese and iron ratio was tested according to EPA 6010D-2018 inductively coupled plasma atomic emission spectrometry.

[0136] (7) Average particle size (D3) of primary particles in the second positive electrode active material: Argon ion cutting is performed on the positive electrode sheet, and the cross-section is tested by field emission scanning electron microscopy (SEM). Five regions of non-secondary spherical particles are randomly selected on the 3K magnification SEM image of the cross-section. Then, the above five regions are magnified to 30K magnification. Ten primary particles with diameters of 50nm to 600nm are randomly selected on the 30K magnification SEM image. The specific size of the primary particles is measured. Then, the average value of the 50 particle sizes is calculated to obtain the average particle size of the primary particles in the second positive electrode active material.

[0137] (8) Testing of the compaction density (PD) of the positive electrode sheet: The compaction density of the positive electrode sheet is calculated using the formula: PD = 2 × CW ÷ d; where CW is the areal density of a single side of the positive electrode sheet, in g / 1540.25 mm. 2 d represents the thickness of the positive electrode active material layer after rolling (the thickness of the positive electrode active material layer is the thickness of the positive electrode sheet minus the thickness of the aluminum foil current collector), in mm; PD is in g / cm³. 3 .

[0138] (9) Test of the surface density (CW) of the positive electrode sheet: 1540.25 mm² was measured by weighing. 2 The mass of the positive electrode sheet with a small area is obtained by subtracting the weight of the aluminum foil current collector. This yields the mass of the positive electrode active material layer. Dividing this by 2 gives the mass of the single-sided positive electrode active material layer. The unit is g / 1540.25mm. 2 .

[0139] (10) Specific capacity test: At 25°C, the secondary battery was left to stand for 10 minutes, then charged at 1 / 3C constant current to 4.35V, constant voltage to 0.05C, left to stand for 10 minutes, and then discharged at 1 / 3C constant current to 2.5V. The capacity of the secondary battery was obtained by testing, and was calculated by Specific capacity = Capacity (mAh) / Mass of active material (g).

[0140] (11) Room temperature DCR test: At 25℃, the secondary battery is fully charged to 4.35V, then discharged at the actual 1C capacity for 30min, and adjusted to 50% SOC; then it is left to stand at 25℃ for 30min, discharged at 5C constant current for 10s, and left to stand for 5min. Calculate DCR = (voltage before pulse discharge - voltage after pulse discharge) / discharge current.

[0141] (12) -20℃ low temperature capacity retention rate test: The secondary battery is fully charged to 4.35V at 25℃, and the discharge capacity C1 of the secondary battery at 25℃ is recorded; then it is left to stand at -20℃ for 120min, and discharged to 2.0V at a constant current of 1C, and the discharge capacity C2 of the secondary battery is recorded; -20℃ capacity retention rate = C2 / C1*100%.

[0142] (13) -20℃ low temperature pulse charging time test: The secondary battery was fully charged to 4.35V at 25℃, then discharged at the actual 1C capacity for 18min, adjusted to 70% SOC, left to stand at -20℃ for 60min, discharged at 0.25C constant current for 10s, left to stand for 40s, charged at 0.25C constant current to 4.35V or 10s (whichever comes first), left to stand for 5min, and the -20℃ low temperature pulse charging time was recorded.

[0143] (14) Cyclic test of secondary battery at 45℃: After the secondary battery is clamped, it is placed in a constant temperature chamber at 45℃ for 2 hours, and then a cyclic test is carried out with a charge-discharge rate of 1C. That is, the cyclic test temperature is 45℃, the voltage range is 2.5V~4.25V, the depth of discharge is 100% DOD, and the charge-discharge rate is 1C. The test ends when the capacity retention rate of the lithium-ion battery is ≤80%. The capacity retention rate of the secondary battery after 500 cycles is calculated and recorded.

[0144] In Tables 1 and 2 below, D1 nm is the average particle size of the primary particles of the first positive electrode active material; D2 μm is the average particle size of the secondary particles of the first positive electrode active material; ρg / cm 3 True density of the positive electrode active material; PD g / cm³ 3 Dv is the compaction density of the positive electrode sheet. 10 μm is the particle size corresponding to a volumetric cumulative particle size distribution percentage of 10% for the positive electrode active material; Dv 50 μm is the particle size corresponding to a volumetric cumulative particle size distribution percentage of 50% for the positive electrode active material; Dv 90 μm is the particle size corresponding to a volumetric cumulative particle size distribution percentage of 90% for the positive electrode active material; c ppm is the content of element M in the positive electrode active material; N1 and N2 are the molar ratios of manganese and iron in the first and second positive electrode active materials, respectively; Y = D1 × D2 × ρ × PD × (Dv 90 -Dv 10 )÷Dv 50 ÷c, ω1 and ω2 refer to the mass percentage of the first and second positive electrode active materials in the positive electrode active material, respectively. D3 is the average particle size of the primary particles of the second positive electrode active material. CW is the surface density of the positive electrode sheet on one side. R is the film resistance of the positive electrode sheet.

[0145] Table 1

[0146]

[0147]

[0148] Table 2

[0149]

[0150]

[0151] The performance test results of the secondary batteries in the above embodiments and comparative examples are shown in Table 3.

[0152] Table 3

[0153]

[0154] As can be seen from the above embodiments and comparative examples, when the positive electrode plate satisfies the following relationship: 1.85≤D1×D2×ρ×PD×(Dv) 90 -Dv 10 )÷Dv 50 When ÷c≤7.42, the secondary battery has higher energy density, lower internal resistance, better low-temperature performance (low-temperature charging capability, low-temperature capacity retention) and better cycle performance.

Claims

1. A secondary battery, characterized in that, The device includes a positive electrode sheet, which includes a positive electrode active material, which includes a first positive electrode active material and a second positive electrode active material, and the positive electrode active material contains element M. The element M includes at least one of Mg, Ti, V, Zn, Al, Co, W, Mo, Y, Nb, In, La, Zr, Ce, Sr, and Sb. The first positive electrode active material comprises lithium manganese iron phosphate with a secondary spherical morphology, which is formed by the aggregation of primary particles; The second positive electrode active material comprises primary particles of lithium manganese iron phosphate; The positive electrode plate satisfies the following relationship: 1.85≤D1×D2×ρ×PD×(Dv 90 -Dv 10 )÷Dv 50 ÷c≤7.42; Wherein, D1 nm is the average particle size of the primary particles in the secondary spherical lithium manganese iron phosphate; D2μm is the average particle size of the lithium manganese iron phosphate with the secondary spherical morphology; ρg / cm 3 The true density of the positive electrode active material; PD g / cm 3 The compaction density of the positive electrode sheet; Dv 10 μm is the particle size corresponding to a volume cumulative particle size distribution percentage of the positive electrode active material reaching 10%; Dv 50 μm is the particle size corresponding to a volume cumulative particle size distribution percentage of 50% for the positive electrode active material; Dv 90 μm is the particle size corresponding to the volume cumulative particle size distribution percentage of the positive electrode active material reaching 90%; c ppm represents the content of element M in the positive electrode active material.

2. The secondary battery as described in claim 1, characterized in that, The D1 nm satisfies the following condition: 50nm ≤ D1nm ≤ 200nm.

3. The secondary battery as described in claim 1, characterized in that, The D2μm satisfies: 4.1μm≤D2μm≤9μm.

4. The secondary battery as described in claim 1, characterized in that, The Dv 50 μm satisfies: 0.8μm≤Dv 50 μm≤10.5μm.

5. The secondary battery as described in claim 1, characterized in that, The PD g / cm 3 Satisfying: 2.1 g / cm³ 3 ≤PD g / cm 3 ≤2.55g / cm 3 .

6. The secondary battery as described in claim 1, characterized in that, The positive electrode sheet satisfies at least one of the following: A、4.5μm≤Dv 90 μm≤30μm; B、0.25μm≤Dv 10 μm≤2.6μm; C、3.4g / cm 3 ≤ρg / cm 3 ≤3.6g / cm 3 。 7. The secondary battery as described in claim 1, characterized in that, The areal density of the positive electrode sheet is 0.125–0.35 g / 1540.25 mm. 2 .

8. The secondary battery as described in claim 1, characterized in that, 2000ppm≤c ppm≤8000ppm.

9. The secondary battery as described in claim 1, characterized in that, The average particle size of the primary lithium manganese iron phosphate particles in the second positive electrode active material is 100-400 nm.

10. An electrical appliance, characterized in that, It includes the secondary battery as described in any one of claims 1 to 9.

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