A secondary battery and electrical device

Abstract

This invention provides a secondary battery and an electrical device. The secondary battery of this invention includes a positive electrode, an electrolyte, a separator, and a negative electrode. The positive electrode includes a positive current collector and a positive electrode film disposed on at least one surface of the positive current collector and comprising a positive active material. The positive active material comprises lithium iron manganese oxide and lithium nickel cobalt manganese oxide. The secondary battery satisfies the following relationship: 1 <C LMFP ·PD·[(D v90 -D v10 ) / D v50 ]·[(D n90 -D n10 ) / D n50 ]·A EL ≤30. This invention satisfies the above conditions by rationally combining positive electrode active materials, adjusting the particle size distribution of positive electrode active materials, optimizing the compaction density of positive electrode sheets, and rationally designing the electrolyte. This results in a significant improvement in the energy density and cycle performance of the obtained secondary battery, while significantly reducing the battery manufacturing cost.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a secondary battery and an electrical device. Background Technology

[0002] Secondary batteries have outstanding advantages such as high energy density, high power density, long service life and no memory effect, and have been widely used in electric vehicles.

[0003] Compared to lithium iron phosphate (LFP) cathode materials, lithium manganese iron phosphate (LMP) cathode materials offer a higher voltage platform and greater safety, attracting widespread attention from researchers in the power battery application field. Since LMP and ternary materials share similar electrochemical windows, blending the lower-cost LMP with ternary materials can improve the safety performance of secondary batteries to some extent. However, improving the energy density and cycle performance of secondary batteries using LMP-T as the cathode active material remains a significant challenge for the entire power battery industry. Summary of the Invention

[0004] The purpose of this invention is to address the issues of the need for further improvement in energy density and cycle performance, as well as the high cost of existing secondary batteries, and to provide a new type of secondary battery. This invention effectively improves the cycle performance and energy density of the battery by effectively controlling the particle size distribution of the composite positive electrode active material, the compaction density of the positive electrode sheet, and the parameters of the electrolyte, even with a relatively low content of ternary positive electrode active material.

[0005] To achieve the above objectives, a first aspect of the present invention provides a secondary battery, comprising a positive electrode, an electrolyte, a separator, and a negative electrode. The positive electrode includes a positive current collector and a positive electrode film disposed on at least one surface of the positive current collector and comprising a positive active material. The positive active material comprises lithium iron manganese oxide and lithium nickel cobalt manganese oxide. The secondary battery satisfies the following relationship:

[0006] 1 <C LMFP ·PD·[(D v90 -D v10 ) / D v50 ]·[(D n90 -D n10 ) / D n50 ]·A EL ≤30;

[0007] Among them, C LMFP The weight percentage of lithium iron manganese oxide in the positive electrode active material;

[0008] PD is the compaction density of the positive electrode, expressed in g / cm³. 3 ;

[0009] D v10 D v50 D v90 These are the particle sizes corresponding to the cumulative volume percentage of positive electrode active material reaching 10%, 50%, and 90%, respectively, in μm;

[0010] D n10 D n50 D n90 The particle size corresponds to the cumulative percentage of positive electrode active material reaching 10%, 50%, and 90%, in μm.

[0011] A EL The electrolyte weight per unit capacity of the secondary battery is expressed in g / Ah.

[0012] As an embodiment of the present invention, the secondary battery satisfies the following relationship: 9.5 ≤ C LMFP ·PD·[(D v90 -D v10 ) / D v50 ]·[(D n90 -D n10 ) / D n50 ]·A EL ≤14.

[0013] As an embodiment of the present invention, the weight percentage of lithium iron phosphate oxide in the positive electrode active material is 10% ≤ C LMFP ≤90%.

[0014] As an embodiment of the present invention, the compaction density of the positive electrode sheet is 2.2–3.7 g / cm³. 3 .

[0015] As an embodiment of the present invention, in the relationship, (D v90 -D v10 ) / D v50 It ranges from 1.1 to 2.5.

[0016] As an embodiment of the present invention, in the relationship, (D n90 -D n10 ) / D n50 The value ranges from 0.5 to 2.5.

[0017] As an embodiment of the present invention, the electrolyte weight A per unit capacity EL The concentration is 2.0–5.0 g / Ah, more preferably 2.5–4.5 g / Ah.

[0018] As an embodiment of the present invention, the lithium iron manganese oxide includes a compound with the molecular formula Li a Mn x Fe 1-x M1-a A compound of PO4, where 0 < x < 1, 0.95 ≤ a ≤ 1.1, and M includes at least one of In, La, Zr, Ce, W, Al, Ti, Sr, Mg, Sb, V, Zn, Cu, and Cr.

[0019] As an embodiment of the present invention, the lithium nickel cobalt manganese oxide includes a compound with the molecular formula Li b (Ni y Co z Mn 1-y-z ) 1- c A c O2, where 0.95 ≤ b ≤ 1.1, 0 < y < 1, 0 < z < 1, 0 ≤ c ≤ 0.1, and A includes at least one of the elements Zr, Sr, W, Al, Ti, Mg, Ce, Y, and B.

[0020] In a second aspect of the present invention, there is provided an electrical device including the above secondary battery.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] By effectively regulating the particle size distribution of the composite cathode active material, the compaction density of the cathode electrode sheet, and the parameters of the electrolyte, the present invention can effectively improve the cycle performance and energy density of the battery under the condition of a lower content of the ternary cathode active material. Detailed Embodiments

[0023] To better illustrate the purpose, technical solutions, and advantages of the present invention, the following will further illustrate the present invention with specific examples, but the examples do not impose any form of limitation on the present invention. Unless otherwise specified, the reagents, methods, and equipment used in the present invention are conventional reagents, methods, and equipment in the technical field. Unless otherwise specified, the reagents and materials used in the present invention are commercially available.

[0024] An embodiment of the present invention provides a secondary battery, which includes a cathode electrode sheet, an electrolyte, a separator, and an anode electrode sheet. The cathode electrode sheet includes a cathode current collector and a cathode film provided on at least one surface of the cathode current collector and including a cathode active material. The cathode active material includes manganese iron lithium oxide and lithium nickel cobalt manganese oxide. The secondary battery satisfies the following relationship:

[0025] 1 < C LMFP ·PD·[(D v90 -D v10 ) / D v50 ·[(D n90 -D n10 ) / D n50 ·A EL ≤ 30;

[0026] Among them, C LMFP The weight percentage of lithium iron manganese oxide in the positive electrode active material;

[0027] PD is the compaction density of the positive electrode, expressed in g / cm³. 3 ;

[0028] D v10 D v50 D v90 These are the particle sizes corresponding to the cumulative volume percentage of positive electrode active material reaching 10%, 50%, and 90%, respectively, in μm;

[0029] D n10 D n50 D n90 The particle size corresponds to the cumulative percentage of positive electrode active material reaching 10%, 50%, and 90%, in μm.

[0030] A EL The electrolyte weight per unit capacity of the secondary battery is expressed in g / Ah.

[0031] This invention has found that the capacity utilization of the positive electrode active material is related to the proportion of lithium iron manganese oxide in the positive electrode active material, the particle size distribution of the positive electrode active material, the compaction density of the positive electrode sheet, and the rational design of the electrolyte (i.e., "C"). LMFP ·PD·[(D v90 -D v10 ) / D v50 ]·[(D n90 -D n10 ) / D n50 ]·A EL There is a clear correlation between these materials and the battery's performance. Through a reasonable combination of the above materials, the energy density and cycle stability of the battery can be significantly improved even with the addition of a small amount of lithium nickel cobalt manganese oxide (also commonly referred to in this field as "layered ternary cathode active material"). At the same time, the addition of a small amount of lithium nickel cobalt manganese oxide can also greatly reduce the cost of the battery, making it suitable for large-scale production and application.

[0032] It should be noted that the proportion of lithium manganese iron phosphate in the positive electrode active material of the secondary battery of the present invention can be calculated by ICP elemental analysis. Specifically, as follows: 1) ICP elemental analysis is performed on the positive electrode sample prepared by mixing lithium nickel cobalt manganese oxide and lithium manganese iron phosphate oxide to obtain the mass ratio of Li and P, thereby calculating the proportion of lithium manganese iron phosphate oxide.

[0033] In this invention, unless otherwise specified, D refers to... v10 D v50 Dv90 D n10 D n50 D n90 Both refer to the particle size of a mixture of positive electrode active materials containing lithium iron manganese oxide and lithium nickel cobalt manganese oxide.

[0034] In some embodiments of the present invention, the secondary battery satisfies the following relationship: 9.5 ≤ C LMFP ·PD·[(D v90 -D v10 ) / D v50 ]·[(D n90 -D n10 ) / D n50 ]·A EL Within the range of ≤14, the resulting secondary batteries exhibit superior cycle performance and energy density.

[0035] In secondary batteries, the particle size distribution of the positive electrode active material, the proportion of lithium iron manganese oxide, the compaction density of the positive electrode sheet, and the interaction between electrolytes all affect the energy density and cycle performance of the battery.

[0036] In some embodiments of the present invention, (D) v90 -D v10 ) / D v50 Within the range of 1.1 to 2.5, for example (D) v90 -D v10 ) / D v50 It can be 1.1, 1.3, 1.5, 1.7, 1.9, 2.1, 2.3, 2.5, or a range consisting of any two of the aforementioned values. This is beneficial for maximizing the kinetic performance of the secondary battery while also considering the battery's energy density.

[0037] In some embodiments of the present invention, (D) n90 -D n10 ) / D n50 Within the range of 0.5 to 2.5. For example (D n90 -D n10 ) / D n50 It can be 0.5, 0.7, 0.9, 1.1, 1.3, 1.5, 1.7, 1.9, 2.1, 2.3, 2.5, or a range consisting of any two of the aforementioned values. When (D n90 -D n10 ) / D n50 Within the range of 0.5 to 2.5, it is beneficial to the performance of the secondary battery dynamics while taking into account the battery energy density.

[0038] In some embodiments of the present invention, D v90 The size is 4–14 μm. For example, D v90It can be 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm or a range consisting of any two of the aforementioned values.

[0039] In some embodiments of the present invention, D v10 The thickness ranges from 0.7 to 1.8 μm. For example, D... v10 It can be 0.7μm, 0.8μm, 0.9μm, 1.0μm, 1.1μm, 1.2μm, 1.3μm, 1.4μm, 1.5μm, 1.6μm, 1.7μm, 1.8μm, or a range consisting of any two of the aforementioned values.

[0040] In some embodiments of the present invention, D v50 The size is 2–10 μm. For example, D v50 It can be 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm or a range consisting of any two of the aforementioned values.

[0041] In some embodiments of the present invention, D n90 The range is 0.5–1.4 μm. For example, D n90 It can be 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm, 1.0μm, 1.1μm, 1.2μm, 1.3μm, 1.4μm, or a range consisting of any two of the aforementioned values.

[0042] In some embodiments of the present invention, D n10 The thickness is 0.20–0.35 μm. For example, D n10 It can be 0.20μm, 0.22μm, 0.24μm, 0.26μm, 0.28μm, 0.30μm, 0.33μm, 0.35μm, or a range consisting of any two of the aforementioned values.

[0043] In some embodiments of the present invention, D n50 The range is 0.29–0.56 μm. For example, D n50 It can be 0.29μm, 0.32μm, 0.35μm, 0.38μm, 0.41μm, 0.43μm, 0.46μm, 0.49μm, 0.52μm, 0.56μm, or a range consisting of any two of the aforementioned values.

[0044] This invention has found that the mass percentage C of lithium iron phosphate oxide in the positive electrode active material is... LMFPThis affects the battery's energy density and watt-hour cost. Among positive electrode active materials, lithium iron phosphate oxide typically performs worse than layered ternary positive electrode active materials (lithium nickel cobalt manganese oxide), but its cost is significantly lower. Therefore, when C... LMFP At higher C values, it can reduce the cost of secondary batteries, but the battery's specific capacity also decreases accordingly; when C... LMFP At lower temperatures, because lithium iron phosphate (LFP) oxide particles are smaller than ternary material particles, during the rolling process, LFP particles can enter the gaps between ternary material particles, thus increasing the compaction density of the positive electrode sheet. This increased compaction density leads to a slight improvement in battery energy density while maintaining the same capacity, but its effect on reducing battery cost is limited. Therefore, in some embodiments of this invention, the mass percentage of LFP oxide in the positive electrode active material satisfies 10% ≤ C. LMFP ≤90%. For example, C LMFP It can be 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or a range consisting of any two of the aforementioned values.

[0045] This invention has found that the compaction density (PD) of the positive electrode sheet also affects the energy density of the battery. In some embodiments of this invention, the compaction density (PD) of the positive electrode sheet is 2.2–3.7 g / cm³. 3 For example, the compaction density (PD) of the positive electrode sheet can be 2.2 g / cm³. 3 2.4g / cm 3 2.6g / cm 3 2.8g / cm 3 2.9g / cm 3 3.1g / cm 3 3.3g / cm 3 3.5g / cm 3 3.7g / cm 3 This ensures good wettability between the positive electrode active material and the electrolyte, guarantees an appropriate number of redox reaction sites, thereby ensuring the full utilization of the positive electrode capacity and improving the battery energy density.

[0046] This invention has found that the weight A of the electrolyte per unit capacity EL This will affect the energy density of the secondary battery. Specifically, when the proportion of smaller lithium iron phosphate particles in the positive electrode is high, they are more likely to adsorb electrolyte. Therefore, the weight A of electrolyte per unit capacity should be appropriately increased. EL When the proportion of smaller manganese iron lithium oxide particles in the positive electrode is low, the electrolyte A per unit capacity should be appropriately reduced. ELThe dosage can effectively avoid side reactions of the electrolyte and, to a certain extent, also play a role in reducing costs. However, if the electrolyte dosage per unit capacity is too low, it will lead to serious capacity attenuation caused by electrolyte drying during repeated charging of the battery. Therefore, the electrolyte dosage in the secondary battery should also be within a suitable range. In some embodiments of the present invention, the weight A of the electrolyte per unit capacity EL within the range of 2.0 - 5.0 g / Ah can meet the performance requirements of the battery; A EL The secondary battery obtained within the range of 2.5 - 4.5 g / Ah has a higher energy density. The concentration of the electrolyte in the electrolyte, calculated based on lithium element, is within the range of 0.9 - 1.1 mol / L.

[0047] For A in the secondary battery EL , the capacity of the battery can be obtained by performing one capacity grading, then the electrolyte of the battery is drained and weighed, and the weight of the electrolyte can be calculated, and then the A of the battery can be calculated EL value. In the secondary battery, the A value of the secondary battery can be controlled by controlling the injection amount of the electrolyte in the secondary battery. EL value.

[0048] In the secondary battery of the present invention, the positive electrode film (which can also be referred to as the positive electrode active material layer) containing the positive electrode active material can be disposed on one surface of the positive electrode current collector or on both surfaces of the positive electrode current collector. The positive electrode film may further include a conductive agent and a binder, and the types and contents of the conductive agent and the binder are not specifically limited and can be selected according to actual needs. The type of the positive electrode current collector is also not specifically limited and can be selected according to actual needs.

[0049] In the secondary battery of the present invention, the positive electrode active materials lithium manganese iron oxide (i.e., lithium iron phosphate manganese) and lithium nickel cobalt manganese oxide can also be doped and modified and / or coated and modified. The types and contents of the doped elements are not limited. It can be doped with one element or multiple elements can be doped together. Commonly used doped elements in the art can be used in the present invention; the type, content and thickness of the surface coating layer are also not limited. The coating layer can be at least one of commonly used carbon materials or metal oxides in the art.

[0050] In some embodiments of the present invention, the manganese iron lithium oxide includes a compound with the molecular formula Li a Mn x Fe 1-x M 1-a PO4, where 0 < x < 1, 0.95 ≤ a ≤ 1.1, and M includes at least one of In, La, Zr, Ce, W, Al, Ti, Sr, Mg, Sb, V, Zn, Cu, Cr.

[0051] In some embodiments of the present invention, the lithium nickel cobalt manganese oxide includes a compound with the molecular formula Li b (Ni y Co z Mn 1-y-z ) 1-c A c O2, where 0.95 ≤ b ≤ 1.1, 0 < y < 1, 0 < z < 1, 0 ≤ c ≤ 0.1, and A includes at least one of the elements Zr, Sr, W, Al, Ti, Mg, Ce, Y, B.

[0052] In the secondary battery of the present invention, the negative electrode film sheet containing the negative electrode active material (this negative electrode film sheet can also be referred to as the negative electrode active material layer) can be disposed on one surface of the negative electrode current collector or on both surfaces of the negative electrode current collector. The negative electrode film sheet may further include a conductive agent and a binder, and the types and contents of the conductive agent and the binder are not specifically limited and can be selected according to actual needs. The type of the negative electrode current collector is also not specifically limited and can be selected according to actual needs.

[0053] The negative electrode active material is a common material in the art, including but not limited to at least one of carbon materials and silicon-based materials; the carbon materials can be selected from at least one of graphite, hard carbon, soft carbon, carbon fiber or mesophase carbon microspheres, and the graphite can be selected from one or several of artificial graphite and natural graphite; the silicon-based materials can be selected from one or several of elemental silicon, silicon oxides, silicon-carbon composites, and silicon alloys.

[0054] It should be noted that when the positive electrode film sheet and the negative electrode film sheet are respectively disposed on the two surfaces of the positive electrode current collector and the negative electrode current collector, as long as the positive electrode film sheet on any one surface of the positive electrode current collector and the negative electrode film sheet on any one surface of the negative electrode current collector meet the present invention, it is considered that the battery falls within the protection scope of the present invention. At the same time, the parameters of the positive and negative electrode film sheets given in the present invention also refer to the parameters of the single-sided positive and negative electrode film sheets.

[0055] In the secondary battery of the present invention, the separator is disposed between the positive and negative electrode plates and serves to isolate the positive and negative electrodes. The type of the separator is not specifically limited and can be any separator material used in existing batteries, such as polyethylene, polypropylene, polyvinylidene fluoride, and their multilayer composite films, but not limited to these.

[0056] In the secondary battery of the present invention, the type of electrolyte is not specifically limited. The electrolyte includes an electrolyte salt and an organic solvent, and the specific types of the electrolyte salt and organic solvent are not specifically limited and can be selected according to actual needs. The electrolyte may also include additives, and the type of additives is not particularly limited. These additives can be film-forming additives for the positive and / or negative electrodes, or additives that can improve certain battery performance, such as additives that improve the battery's high or low temperature performance.

[0057] The present invention also protects electrical devices that include the aforementioned secondary batteries.

[0058] The following are specific embodiments of the present invention, and the technical solution of the present invention will be further described in conjunction with the embodiments. However, the present invention is not limited to these embodiments. Unless otherwise specified, the reagents, methods and equipment used in the present invention are all conventional reagents, methods and equipment in this technical field.

[0059] Examples 1-32, Comparative Examples 1-9

[0060] The secondary batteries described in the embodiments and comparative examples of the present invention are prepared by a method comprising the following steps:

[0061] Preparation of positive electrode sheet

[0062] The positive electrode active material, binder polyvinylidene fluoride (PVDF), and conductive agent acetylene black are mixed in a mass ratio of 96:3:1, and N-methylpyrrolidone solvent is added. The mixture is then stirred in a vacuum mixer until the system is homogeneous to obtain the positive electrode slurry. The positive electrode slurry is uniformly coated onto both sides of the positive electrode current collector (carbon-coated aluminum foil). The coated electrode is dried in an oven and then cold-pressed and slit to obtain the positive electrode sheet.

[0063] Preparation of negative electrode sheet

[0064] The negative electrode active material, thickener sodium carboxymethyl cellulose, binder styrene-butadiene rubber, and conductive agent acetylene 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 mixer. 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.

[0065] Preparation of electrolyte

[0066] The organic solvent is a mixture containing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC), wherein the volume ratio of EC, EMC, and DEC is 20:20:60. In an argon-atmosphere glove box with a water content of <10 ppm, thoroughly dried lithium salt LiPF6 is dissolved in the organic solvent and mixed thoroughly to obtain the electrolyte. The concentration of lithium salt in the electrolyte is 1 mol / L.

[0067] Assembly of lithium secondary batteries

[0068] The positive electrode, separator (polypropylene), and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. After being wound into a square bare cell, it is placed in a casing, baked at 80°C to remove water, injected with electrolyte, and sealed. After processes such as standing, hot and cold pressing, formation, clamping, and capacity testing, a lithium secondary battery is obtained.

[0069] The specific selection and parameters of the positive and negative electrode active materials are detailed in Table 1.

[0070] For ease of description, LiMn is used in the embodiments and comparative examples of the present invention. 0.6 Fe 0.4 PO4 is abbreviated as LMFP-64, LiMn 0.7 Fe 0.3 PO4 is abbreviated as LMFP-73; LiNi 0.6 Co 0.1 Mn 0.3 O2 is abbreviated as NCM-613, LiNi 0.8 Co 0.1 Mn 0.1 O2 is abbreviated as NCM-811.

[0071] The particle size distribution of the positive electrode active material used in the embodiments and comparative examples of the present invention was tested using a Malvern 3000 laser particle size analyzer, and the dispersant used in the test was deionized water.

[0072] Table 1. Positive electrode active materials and their parameters for secondary batteries in the examples and comparative examples.

[0073]

[0074]

[0075] The performance of the secondary batteries obtained in the above embodiments and comparative examples was tested. The specific test items, test methods, and results are as follows (see Table 2):

[0076] 1. Compacted density PD of the positive electrode (unit: g / cm³) 3): 1) First, weigh out a unit area of ​​1540.25 mm². 2 The mass of the empty aluminum foil is m1, in grams; 2) Weigh out a piece with a unit area of ​​1540.25 mm². 2 The mass of the positive electrode sheet with a uniformly coated positive electrode active material layer on both sides is m2, in grams. Therefore, CW = (m2 - m1) / 2, in grams / 1540.25mm. 2 3) Measure the thickness L1 of the aluminum foil using a micrometer, in μm; measure the thickness L2 of the double-sided uniform positive electrode active material, in μm. Therefore, the compaction density of the electrode is PD = 2*CW / (L2-L1), in g / cm³. 3 .

[0077] 2. Electrolyte weight A per unit capacity EL (Unit: g / Ah): The battery's design capacity is b Ah, and the weight of the injected electrolyte is ag, so A EL = a / b(g / Ah).

[0078] 3. Discharge performance test of secondary batteries:

[0079] 3.1 0.33C First Discharge Capacity (mAh / g) Test :

[0080] ① Adjust the temperature of the insulation box to 25℃ and let it stand for 2 hours; ② Charge it with a constant current of 0.33C to 3.65V, and then charge it with a constant voltage to the cutoff current of 0.05C; ③ Let it stand for 5 minutes; ④ Discharge it with a constant current of 0.33C to 2.5V; ⑤ Let it stand for 5 minutes.

[0081] 3.2 4C rate discharge performance test :

[0082] ① Adjust the temperature of the insulation box to 25℃ and let it stand for 2 hours; ② Charge at a constant current of 0.33C to 3.65V, then charge at a constant voltage to the cutoff current of 0.05C; ③ Let it stand for 5 minutes; ④ Discharge at a constant current of 0.33C to 2.5V; ⑤ Let it stand for 5 minutes; ⑥ Charge at a constant current of 0.33C to 3.65V, then charge at a constant voltage to the cutoff current of 0.05C; ⑦ Let it stand for 5 minutes; ⑧ Discharge at a constant current of 4C to 2.5V; ⑨ Let it stand for 5 minutes; Wherein, the 4C discharge capacity retention rate (%) = discharge capacity at 4C rate / initial discharge capacity at 0.33C × 100%.

[0083] 3.3 Cyclic capacity retention test :

[0084] The cyclic steps are as follows: ① Adjust the temperature of the insulation box to 25℃ and let it stand for 2 hours; ② Charge at a constant current of 0.33C to 3.65V, then charge at a constant voltage to the cutoff current of 0.05C; ③ Let it stand for 5 minutes; ④ Discharge at a constant current of 0.33C to 2.5V; ⑤ Let it stand for 5 minutes; ⑥ Charge at a constant current of 1C to 3.65V, then charge at a constant voltage to the cutoff current of 0.05C; ⑦ Let it stand for 5 minutes; ⑧ Discharge at a constant current of 1C to 2.5V; ⑨ Let it stand for 5 minutes; ⑩ Repeat steps ⑥ to ⑨ until 4000 cycles are completed. The capacity retention rate (%) = discharge capacity after 4000 cycles / discharge capacity on the first cycle at 1C × 100%.

[0085] 4. Actual energy density of secondary batteries:

[0086] At 25°C, the secondary batteries prepared in the examples and comparative examples were fully charged and discharged at a 1C rate, and the actual discharge energy was recorded. At 25°C, the battery was weighed using an electronic scale. The ratio of the actual 1C discharge energy of the battery to the battery weight is the actual energy density of the battery.

[0087] Specifically, 1) when the actual energy density is less than 80% of the target energy density, the battery's actual energy density is considered very low; 2) when 80% of the target energy density is less than or equal to the actual energy density and less than 95% of the target energy density, the battery's actual energy density is considered low; 3) when 95% of the target energy density is less than or equal to the actual energy density and less than 105% of the target energy density, the battery's actual energy density is considered moderate; 4) when 105% of the target energy density is less than or equal to the actual energy density and less than 120% of the target energy density, the battery's actual energy density is considered relatively high; and 5) when 120% of the target energy density is less than or equal to the actual energy density, the battery's actual energy density is considered very high.

[0088] Table 2. Performance test results of the secondary batteries prepared in the examples and comparative examples.

[0089]

[0090]

[0091] The results above show that:

[0092] Based on the above examples and comparative examples, the mass percentage (C) of lithium manganese iron phosphate in the positive electrode active material is... LMFP The particle size distribution width of the positive electrode active material, the compaction density design of the positive electrode sheet, and the weight of electrolyte per unit capacity all affect the battery's electrical performance and actual energy density.

[0093] The positive electrode active materials in Comparative Examples 1-4 contained only lithium iron manganese oxide or lithium nickel cobalt manganese oxide, resulting in secondary batteries with low discharge capacity or significantly poor rate performance. In Comparative Examples 5-9, C... LMFP ·PD·[(D v90 -D v10 ) / D v50 ]·[(D n90 -D n10 ) / D n50 ]·A EL If the value is less than 1 or greater than 30, the discharge capacity, rate performance, cycle performance, and energy density of the resulting secondary battery are significantly worse than those of the embodiments of the present invention.

[0094] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A secondary battery, comprising a positive electrode, an electrolyte, a separator, and a negative electrode, characterized in that, The positive electrode includes a positive current collector and a positive electrode film disposed on at least one surface of the positive current collector and comprising a positive active material. The positive active material includes lithium iron manganese oxide and lithium nickel cobalt manganese oxide. The secondary battery satisfies the following relationship: 1 <C LMFP ·PD·[(D v90 -D v10 ) / D v50 ]·[(D n90 -D n10 ) / D n50 ]·HAS EL ≤30; Among them, C LMFP The weight percentage of lithium iron manganese oxide in the positive electrode active material; PD is the compaction density of the positive electrode, expressed in g / cm³. 3 ; D v10 D v50 D v90 These are the particle sizes corresponding to the cumulative volume percentage of positive electrode active material reaching 10%, 50%, and 90%, respectively, in μm; D n10 D n50 D n90 These are the particle sizes corresponding to the cumulative percentage of positive electrode active material reaching 10%, 50%, and 90%, respectively, in μm; A EL The electrolyte weight per unit capacity of the secondary battery is expressed in g / Ah.

2. The secondary battery according to claim 1, characterized in that, The secondary battery satisfies the following relationship: 9.5 ≤ C LMFP ·PD·[(D v90 -D v10 ) / D v50 ]·[(D n90 -D n10 ) / D n50 ]·A EL ≤14.

3. The secondary battery according to claim 1, characterized in that, The weight percentage of lithium iron phosphate oxide in the positive electrode active material is 10% ≤ C LMFP ≤90%.

4. The secondary battery according to claim 1, characterized in that, The compaction density of the positive electrode sheet is 2.2–3.7 g / cm³. 3 .

5. The secondary battery according to claim 1, characterized in that, In the aforementioned relationship, (D) v90 -D v10 ) / D v50 It ranges from 1.1 to 2.

5.

6. The secondary battery according to claim 1, characterized in that, In the aforementioned relationship, (D) n90 -D n10 ) / D n50 The value ranges from 0.5 to 2.

5.

7. The secondary battery according to claim 1, characterized in that, The weight A of the electrolyte per unit capacity EL It ranges from 2.5 to 4.5 g / Ah.

8. The secondary battery according to claim 1, characterized in that, The lithium manganese iron oxide includes a compound with the molecular formula Li a Mn x Fe 1-x M 1-a PO4, where 0 < x < 1, 0.95 ≤ a ≤ 1.1, and M includes at least one of In, La, Zr, Ce, W, Al, Ti, Sr, Mg, Sb, V, Zn, Cu, Cr.

9. The secondary battery according to claim 1, characterized in that, The lithium nickel cobalt manganese oxide includes a compound with the molecular formula Li b (Ni y Co z Mn 1-y-z ) 1-c A c O2, where 0.95 ≤ b ≤ 1.1, 0 < y < 1, 0 < z < 1, 0 ≤ c ≤ 0.1, and A includes at least one of the elements Zr, Sr, W, Al, Ti, Mg, Ce, Y, B.

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

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