Secondary battery and electric device

Abstract

The embodiment of the invention provides a secondary battery and electric equipment. The secondary battery comprises a positive pole piece. The positive pole piece comprises a positive current collector, a first active material layer and a second active material layer. The first active material layer is arranged on at least one surface of the positive electrode current collector, the first active material layer comprises a positive electrode active composition, the positive electrode active composition comprises first manganese iron lithium phosphate and lithium nickel cobalt manganese oxide, and a particle size distribution curve of the positive electrode active composition comprises a first characteristic peak and a second characteristic peak, the particle size corresponding to the peak value of the first characteristic peak is in a range of 0.4-1.8 [mu] m, and the particle size corresponding to the peak value of the second characteristic peak is in a range of 3-6 [mu] m. The second active material layer is arranged on one side, deviating from the positive electrode current collector, of the first active material layer, and the second active material layer comprises second lithium manganese iron phosphate. The manganese-iron molar ratio of the first lithium manganese-iron phosphate is greater than that of the second lithium manganese-iron phosphate. Therefore, the rate capability and the cycle performance of the secondary battery can be improved.

Description

Technical Field

[0001] The exemplary embodiments of this application generally relate to the field of batteries, and particularly to secondary batteries and electrical devices. Background Technology

[0002] With the development of rechargeable batteries, the market demands higher and higher performance from them. The positive electrode of a rechargeable battery is crucial to its performance. In order to achieve high energy density, the positive electrode needs to have a high compaction density. However, high compaction density may increase the transport path of ions and electrons, affecting kinetic performance and leading to problems such as poor cycle performance. Summary of the Invention

[0003] In a first aspect of this application, a secondary battery is provided. The secondary battery includes a positive electrode sheet, comprising: a positive current collector; a first active material layer disposed on at least one surface of the positive current collector, the first active material layer comprising a positive active composition, the positive active composition comprising a first lithium manganese iron phosphate and lithium nickel cobalt manganese oxide, the particle size distribution curve of the positive active composition including a first characteristic peak and a second characteristic peak, the peak value of the first characteristic peak corresponding to a particle size in the range of 0.4 μm to 1.8 μm, and the peak value of the second characteristic peak corresponding to a particle size in the range of 3 μm to 6 μm; and a second active material layer disposed on the side of the first active material layer opposite to the positive current collector, the second active material layer comprising a second lithium manganese iron phosphate; the manganese-iron molar ratio of the first lithium manganese iron phosphate is greater than the manganese-iron molar ratio of the second lithium manganese iron phosphate.

[0004] According to the secondary battery provided in this application embodiment, by sequentially depositing a first active material layer and a second active material layer on the surface of the positive electrode current collector, and controlling the particle size distribution of the positive electrode active material in the first active material layer, the particle size distribution curve of the positive electrode active composition includes a first characteristic peak with a peak particle size in the range of 0.4 μm to 1.8 μm and a second characteristic peak with a peak particle size in the range of 3 μm to 6 μm. This allows the first active material layer to have a better pore structure, facilitating electrolyte wetting. The first and second characteristic peaks can be formed at least by first lithium manganese iron phosphate and lithium nickel cobalt manganese oxide through a gradation process. In this way, the pores in the first active material layer can be controlled and have a higher compaction density, resulting in better electrical performance of the secondary battery.

[0005] The manganese-iron molar ratio of the first lithium manganese iron phosphate is greater than that of the second lithium manganese iron phosphate. In other words, the manganese content in the second active material layer, which is in direct contact with the electrolyte, is lower. Therefore, the degree of manganese dissolution during battery cycling is lower, which can improve the cycle performance of the secondary battery.

[0006] In some embodiments, the median particle size D4 of the second lithium manganese iron phosphate is greater than the median particle size D2 of the first lithium manganese iron phosphate.

[0007] By controlling the particle size distribution of the first and second lithium manganese iron phosphate layers, the porosity of the first active material layer is made lower than that of the second active material layer. This allows the porosity of the positive electrode to decrease from the outside to the inside, which is beneficial for electrolyte wetting and, consequently, shortens the transport paths of ions and electrons, improving the battery's kinetic performance. Furthermore, the decreasing porosity gradient of the positive electrode also helps to increase the overall compaction density of the positive electrode, resulting in higher energy density and better cycle performance.

[0008] In some embodiments, the median particle size D2 of the first lithium manganese iron phosphate is 0.55 μm to 3 μm, and the median particle size D4 of the second lithium manganese iron phosphate is 0.8 μm to 5 μm.

[0009] In some embodiments, the median particle size D2 of the first lithium manganese iron phosphate can be a range of one or any two of 0.55 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm or 3 μm.

[0010] In some embodiments, the median particle size D4 of the second lithium manganese iron phosphate can be a range of one or any two of 0.8 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm or 5 μm.

[0011] In some embodiments, the median particle size D1 of the positive electrode active composition is 1 μm to 5 μm.

[0012] The positive electrode active composition can be obtained by mixing at least lithium manganese iron phosphate and lithium nickel cobalt manganese oxide in a graded manner. Therefore, by adjusting the particle size and ratio of lithium manganese iron phosphate and lithium nickel cobalt manganese oxide, the median particle size of the positive electrode active composition can be controlled within the range of 1 μm to 5 μm. Adjusting the particle size and ratio of lithium manganese iron phosphate and lithium nickel cobalt manganese oxide can also cause an appropriate transverse shift of the first and second characteristic peaks in the particle size distribution curve of the positive electrode active composition. This makes the microstructure inside the first active material layer easier to characterize and represent. In actual production, by adjusting the particle size and ratio of lithium manganese iron phosphate and lithium nickel cobalt manganese oxide, and calibrating in conjunction with the particle size distribution curve of the positive electrode active composition, a positive electrode sheet with better electrical performance can be obtained.

[0013] In some embodiments, the median particle size D1 of the positive electrode active composition may be a range of one or both of 1 μm, 2 μm, 3 μm, 4 μm or 5 μm.

[0014] In some embodiments, the median particle size D3 of lithium nickel cobalt manganese oxide is 2 μm to 7 μm.

[0015] In some embodiments, the median particle size D3 of the lithium nickel cobalt manganese oxide can be a range of one or both of 2 μm, 3 μm, 4 μm, 5 μm, 6 μm or 7 μm.

[0016] In some embodiments, the porosity of the positive electrode sheet is 25% to 30%.

[0017] In some embodiments, the porosity of the positive electrode sheet is 26% to 28%.

[0018] In some embodiments, the porosity of the positive electrode sheet may be a range of one or both of 27%, 27.2%, 27.4%, 27.6%, 27.8%, or 28%.

[0019] The porosity of the positive electrode reflects, to a certain extent, the void structure on its surface. Optimizing the void structure of the positive electrode facilitates electrolyte wetting, thereby improving the electrical performance of the secondary battery. For example, it can reduce the internal resistance and improve the rate performance. Furthermore, suitable porosity can increase the energy density of the electrode and provide appropriate buffering for electrode expansion during charge and discharge, thus improving the cycle performance of the secondary battery.

[0020] In some embodiments, the manganese-iron molar ratio of the first lithium manganese phosphate is (1~4):1, and the manganese-iron molar ratio of the second lithium manganese phosphate is (0.6~1.5):1.

[0021] In some embodiments, the manganese-iron molar ratio k1 of the first lithium manganese iron phosphate can be a range of one or both of 1, 1.5, 2, 2.5, 3, 3.5 or 4.

[0022] In some embodiments, the manganese-iron molar ratio k2 of the second lithium manganese iron phosphate can be a range of one or both of 0.6, 0.8, 1, 1.2, or 1.5.

[0023] In some embodiments, the manganese-iron molar ratio in the first and second lithium manganese iron phosphate layers can be adjusted so that the manganese-iron molar ratio in the second active material layer is lower than that in the first active material layer. This reduces the degree of manganese dissolution in the second active material layer during secondary battery cycling, thereby minimizing its impact on the cycle performance of the secondary battery.

[0024] In some optional embodiments, the manganese-iron molar ratio k1 of the first lithium manganese iron phosphate is 1, and the manganese-iron molar ratio k2 of the second lithium manganese iron phosphate is 0.67; in some optional embodiments, the manganese-iron molar ratio k1 of the first lithium manganese iron phosphate is 2, and the manganese-iron molar ratio k2 of the second lithium manganese iron phosphate is 0.67; in some optional embodiments, the manganese-iron molar ratio k1 of the first lithium manganese iron phosphate is 4, and the manganese-iron molar ratio k2 of the second lithium manganese iron phosphate is 0.67; in some optional embodiments, the manganese-iron molar ratio k1 of the first lithium manganese iron phosphate is 2, and the manganese-iron molar ratio k2 of the second lithium manganese iron phosphate is 1; in some optional embodiments, the manganese-iron molar ratio k1 of the first lithium manganese iron phosphate is 2, and the manganese-iron molar ratio k2 of the second lithium manganese iron phosphate is 1.5; in some optional embodiments, the manganese-iron molar ratio k1 of the first lithium manganese iron phosphate is 4, and the manganese-iron molar ratio k2 of the second lithium manganese iron phosphate is 1.5.

[0025] In some embodiments, lithium nickel cobalt manganese oxide includes lithium with the chemical formula Li a1 Ni x1 Co y1 Mn z1 Me p Compounds of O2, wherein Me contains at least one of the elements B, Y, Nb, In, La, Zr, Ce, W, Al, Ti, Sr, Mg, Sb, V, Zn, Cu, Cr and Fe, and 0.95≤a1≤1.05, 0.5≤x1<1, 0<y1≤0.2, 0<z1≤0.3, 0≤p≤0.1.

[0026] In some embodiments, the first lithium manganese iron phosphate and / or the second lithium manganese iron phosphate may also contain doping elements, such as Mg doping or Ti doping.

[0027] In some embodiments, based on the total mass of the positive electrode active composition, the proportion of lithium manganese iron phosphate is 10wt% to 40wt%, and the proportion of lithium nickel cobalt manganese oxide is 60wt% to 90wt%.

[0028] In some embodiments, the proportion of lithium manganese iron phosphate first can be one or any two of 10wt%, 20wt%, 30wt%, 40wt%, based on the total mass of the positive electrode active composition.

[0029] In some embodiments, the proportion of lithium nickel cobalt manganese oxide can be a range of 60 wt%, 70 wt%, 80 wt%, 90 wt%, or any two of these, based on the total mass of the positive electrode active composition.

[0030] In some embodiments, the positive electrode active composition may consist only of lithium iron manganese phosphate and lithium nickel cobalt manganese oxide. In some embodiments, the positive electrode active composition may include other active materials in addition to lithium iron manganese phosphate and lithium nickel cobalt manganese oxide, such as lithium iron phosphate, lithium manganese oxide, etc.

[0031] In some embodiments, the first active material layer further includes a conductive agent. The conductive agent includes at least one of carbon nanotubes, Super P, and carbon black.

[0032] In some embodiments, the first active material layer further includes an adhesive. The adhesive includes at least one selected from styrene-butadiene rubber, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, sodium polyacrylate, lithium polyacrylate, polyacrylamide, polyimide, polyvinylidene fluoride, and polytetrafluoroethylene.

[0033] In some embodiments, the second active material layer further includes a conductive agent. The conductive agent includes at least one of carbon nanotubes, Super P, and carbon black.

[0034] In some embodiments, the second active material layer further includes an adhesive. The adhesive includes at least one selected from styrene-butadiene rubber, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, sodium polyacrylate, lithium polyacrylate, polyacrylamide, polyimide, polyvinylidene fluoride, and polytetrafluoroethylene.

[0035] In some embodiments, the positive current collector may be aluminum foil or surface-treated aluminum foil. For example, the positive current collector may be carbon-coated aluminum foil.

[0036] In some embodiments, the positive electrode of the secondary battery can be prepared by the following method.

[0037] The first active slurry is obtained by mixing the first amount of lithium manganese iron phosphate, lithium nickel cobalt manganese oxide, conductive agent and binder, and then adding N-methylpyrrolidone and mixing evenly with a solid content of 50%~80%.

[0038] The second active slurry is obtained by mixing the formulated amounts of lithium manganese iron phosphate, conductive agent, and binder, adding N-methylpyrrolidone and mixing evenly, with a solid content of 50%~80%.

[0039] A first active slurry is coated on at least one surface of the positive electrode current collector to form a first active material layer, and a second active slurry is coated on the surface of the first active material layer opposite to the positive electrode current collector to form a second active material layer. This yields the positive electrode of the secondary battery.

[0040] In some embodiments, the secondary battery further includes a negative electrode sheet. The negative electrode sheet includes a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector, the negative electrode material layer containing a negative electrode material.

[0041] In some embodiments, the type of negative electrode material can be selected according to actual needs. For example, the negative electrode material may include at least one of artificial graphite, natural graphite, silicon-carbon composite material, silicon suboxide, hard carbon, lithium metal, and lithium titanate.

[0042] In some embodiments, the type of negative electrode current collector can also be selected according to actual needs. For example, copper foil or carbon-coated copper foil can be used as the negative electrode current collector.

[0043] In some embodiments, the secondary battery further includes a separator. The type of separator can be selected according to actual needs. Exemplarily, the separator can be a polypropylene membrane, a polyethylene membrane, a polyvinylidene fluoride membrane, a spandex membrane, an aramid membrane, or a coated and modified multilayer composite membrane.

[0044] In some embodiments, the secondary battery further includes an electrolyte, which may be an organic solution with a lithium salt as the solute. For example, dehydrated lithium hexafluorophosphate can be dissolved in an organic solvent composed of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate to obtain an electrolyte for the secondary battery.

[0045] In some embodiments, the secondary battery also includes an outer packaging, which can be rigid packaging, such as a hard plastic shell, aluminum shell, steel shell, etc. The outer packaging of the secondary battery can also be flexible packaging, such as a pouch-type soft pack, and the material of the soft pack can be polypropylene, polybutylene terephthalate, and / or polybutylene succinate.

[0046] In some embodiments, the secondary battery may be constrained by its outer packaging into any suitable shape. For example, the secondary battery as a whole may be cylindrical, prismatic, or other regular or irregular shapes to match the battery space of the electrical device.

[0047] In some embodiments, the preparation of a secondary battery includes: stacking a positive electrode, a separator, and a negative electrode in sequence, with the separator positioned between the positive and negative electrodes to provide insulation. The positive electrode, separator, and negative electrode are then wound into a bare cell of appropriate shape and filled into an outer packaging. Subsequently, the cell is baked and dried at a temperature of 65°C to 95°C, injected with electrolyte, and sealed. After standing, formation, and capacity testing, a secondary battery is obtained.

[0048] In a second aspect of this application, an electrical appliance is provided. The electrical appliance includes a secondary battery provided according to the first aspect of this application, the secondary battery serving as a power source for the electrical appliance.

[0049] In some embodiments, the electrical equipment can be application devices such as vehicles, mobile phones, portable devices, laptops, ships, spacecraft, electric toys, and power tools. Vehicles can be new energy vehicles, including pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles; spacecraft include airplanes, rockets, space shuttles, and spacecraft; electric toys include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys; 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. This application does not impose special limitations on the above-mentioned devices.

[0050] It should be understood that the content described in the summary section of this invention is not intended to limit the key or essential features of the embodiments of this application, nor is it intended to limit the scope of this application. Other features of this application will become readily apparent from the following description. Attached Figure Description

[0051] The above and other features, advantages, and aspects of the embodiments of this application will become more apparent from the accompanying drawings and the following detailed description. In the drawings, the same or similar reference numerals denote the same or similar elements, wherein: Figure 1 A schematic diagram of a positive electrode sheet according to some embodiments of this application is shown; and Figure 2 A particle size distribution diagram of the first active composition according to Example 7 of this application is shown. Detailed Implementation

[0052] Embodiments of this application will now be described in more detail with reference to the accompanying drawings. While some embodiments of this application are shown in the drawings, it should be understood that this application can be implemented in various forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of this application. It should be understood that the drawings and embodiments of this application are for illustrative purposes only and are not intended to limit the scope of protection of this application.

[0053] It should be noted that the headings of any section / subsection provided herein are not limiting. Various embodiments are described throughout this document, and embodiments of any type may be included under any section / subsection. Furthermore, embodiments described in any section / subsection may be combined in any way with any other embodiments described in the same section / subsection and / or different sections / subsections.

[0054] In the description of embodiments of this application, the term "comprising" and similar terms should be understood as open-ended inclusion, i.e., "including but not limited to". The term "based on" should be understood as "at least partially based on". The term "one embodiment" or "the embodiment" should be understood as "at least one embodiment". The term "some embodiments" should be understood as "at least some embodiments". Other explicit and implicit definitions may also be included below. The terms "first", "second", etc., may refer to different or the same objects. Other explicit and implicit definitions may also be included below.

[0055] The present application will be further described below with reference to specific embodiments. It should be noted that the following implementation methods are further explanations of the present application and should not be construed as limiting the present application.

[0056] Preparation of secondary batteries Example 1 This embodiment provides a secondary battery, including a positive electrode, a separator, a negative electrode, an electrolyte, and an outer packaging shell.

[0057] Figure 1 A schematic diagram of the layer structure of a positive electrode sheet according to some embodiments of this application is shown. For example... Figure 1 As shown, the positive electrode includes a positive current collector 1, a first active material layer 2 disposed on at least one surface of the positive current collector, and a second active material layer 3 disposed on the side of the first active material layer facing away from the positive current collector. The first active material layer 2 is formed by coating a first active slurry onto the surface of the positive current collector 1, and the second active material layer 3 is formed by coating a second active slurry onto the surface of the first active material layer facing away from the positive current collector 1.

[0058] Preparation of the positive electrode sheet: The positive current collector is a carbon-coated aluminum foil with a thickness of 13 μm. The carbon coating is applied to both sides of the aluminum foil, with a thickness of 1 μm on each side.

[0059] Lithium manganese iron phosphate and lithium nickel cobalt manganese oxide were mixed in a mass ratio (m1:m2) of 6:4 to obtain a positive electrode active composition, wherein the median particle size D2 of lithium manganese iron phosphate was 0.95 μm and the median particle size D3 of lithium nickel cobalt manganese oxide was 3.34 μm.

[0060] The chemical formula of lithium manganese iron phosphate is LiFe 0.5 Mn 0.5 PO4, the manganese-iron molar ratio k1 of lithium manganese iron phosphate is 1:1 (that is, the stoichiometric ratio of Mn and Fe in the chemical formula); the chemical formula of lithium nickel cobalt manganese oxide is LiNi 0.6 Co 0.2 Mn 0.2O2.

[0061] The positive electrode active composition, binder polyvinylidene fluoride (PVDF), conductive agent carbon black, and conductive agent carbon nanotubes are mixed in a mass ratio of 96:2:1:1. N-methylpyrrolidone (NMP) is added to adjust the solid content to 58%. The mixture is then stirred evenly to obtain the first active slurry.

[0062] The second active slurry was prepared by mixing lithium manganese iron phosphate, polyvinylidene fluoride (PVDF) binder, carbon black conductive agent, and carbon nanotube conductive agent in a mass ratio of 97:1.5:1:0.5, adding N-methylpyrrolidone (NMP) to adjust the solid content to 58%, and mixing evenly under the action of a stirrer.

[0063] Among them, the chemical formula of lithium manganese iron phosphate is LiFe. 0.6 Mn 0.4 The manganese-iron molar ratio k2 of lithium manganese iron phosphate is 0.67 (that is, the stoichiometric ratio of Mn and Fe in the chemical formula), and the median particle size D4 of lithium manganese iron phosphate is 3.95 μm.

[0064] A first active slurry is coated onto at least one surface of the positive electrode current collector to form a first active material layer. A second active slurry is coated onto the surface of the first active material layer opposite to the positive electrode current collector to form a second active material layer. The coating amount of the first active slurry is 178 mg / 1540.25 mm. 2 The coating amount of the second active slurry is 80 mg / 1540.25 mm. 2 The coated positive current collector is placed in an oven to dry, and then rolled and slit to obtain the positive electrode sheet.

[0065] Preparation of negative electrode sheet The negative electrode current collector is made of copper foil with a thickness of 6μm.

[0066] Artificial graphite (anode material), carbon black (conductive agent), sodium carboxymethyl cellulose (CMC) (binder), and styrene-butadiene rubber (SBR) (binder) were mixed in a mass ratio of 94.3:1.5:1.2:3. Deionized water was added to adjust the solid content to 49%. The mixture was stirred evenly under the action of a mixer to obtain a cathode slurry. The cathode slurry was then uniformly coated onto the copper foil of the cathode current collector. The coating amount of the cathode slurry was 131 mg / 1540.25 mm. 2 The coated negative electrode current collector is placed in an oven to dry, then rolled and slit to obtain the negative electrode sheet.

[0067] Preparation of electrolyte Ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate were mixed in a volume ratio of 1:1:1 to obtain an organic solvent. In a glove box under an argon atmosphere with a water content below 10 ppm, thoroughly dried lithium hexafluorophosphate was dissolved in the organic solvent, and the concentration of lithium hexafluorophosphate was controlled at 1 mol / L to obtain the electrolyte.

[0068] Assembly of secondary batteries The positive electrode, polypropylene separator, and negative electrode are stacked in sequence and wound to obtain a bare cell. The bare cell is placed in an outer packaging shell, vacuum dried, and then injected with electrolyte. After standing, formation, and capacity testing, a secondary battery is obtained.

[0069] Example 2-16 The difference between Examples 2-16 and Example 1 is that, in the preparation of the positive electrode sheet, the median particle size D2 of the first lithium manganese iron phosphate, the median particle size D3 of the lithium nickel cobalt manganese oxide, the median particle size D4 of the second lithium manganese iron phosphate, the manganese-iron molar ratio k1 of the median particle size of the first lithium manganese iron phosphate, the manganese-iron molar ratio k2 of the second lithium manganese iron phosphate, and the ratio of the mass (m1) of the first lithium manganese iron phosphate to the mass (m2) of the lithium nickel cobalt manganese oxide are different.

[0070] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that, in the preparation of the positive electrode sheet, Comparative Example 1 mixes the first and second active slurries prepared according to the proportions in Example 1, and directly coats the mixed active slurry onto the surface of the positive electrode current collector. The coating amount of the mixed active slurry is 258 mg / 1540.25 mm. 2 .

[0071] Comparative Examples 2 and 3 The difference between Comparative Examples 2 and 3 and Example 1 is that, in the preparation of the positive electrode sheet, the median particle size D2 of the first lithium manganese iron phosphate, the median particle size D3 of lithium nickel cobalt manganese oxide, and the median particle size D4 of the second lithium manganese iron phosphate are different.

[0072] The raw material formulations for Examples 1-16 and Comparative Examples 1-3 are shown in Table 1: Table 1

[0073] In Table 1, D2 represents the median particle size of the first lithium manganese iron phosphate, D3 represents the median particle size of lithium nickel cobalt manganese oxide, D4 ​​represents the median particle size of the second lithium manganese iron phosphate, k1 represents the manganese-iron molar ratio of the first lithium manganese iron phosphate, k2 represents the manganese-iron molar ratio of the second lithium manganese iron phosphate, and m1 / m2 is the mass ratio of the first lithium manganese iron phosphate to lithium nickel cobalt manganese oxide.

[0074] Performance testing Test content (1) Detect the particle size distribution and median particle size D1 of the positive electrode active composition. The particle size distribution curves of the cathode compositions of Examples 1-16 and Comparative Examples 1-3 were obtained using a laser diffractometer. The particle sizes corresponding to the first and second characteristic peaks in the particle size distribution curves were determined, as well as the median particle size D1 of the cathode active composition. Test method: The sample was mixed with a dispersant and stirred for 1 min. Then, 30 ml of deionized water was added. The mixture was sonicated externally at 53 kHz and 100% power for 3 min, with an internal sonication power of 15.00 kHz, from the start to the end of the test. 800 mL of water was added to a Hydro MU2000 syringe, and the sample was stirred and injected at 2400 r / min to obtain the median particle size.

[0075] (2) Detecting the porosity of the positive electrode sheet Take the coated positive electrode sheet and punch out small round discs with a diameter of 14 mm using a punching machine. Place 35 discs into a sample cup. Record the number of discs and calculate the apparent volume V2. Place the sample cup containing the discs into a true density analyzer, seal the testing system, and introduce helium gas according to the program. By detecting the gas pressure in the sample chamber and expansion chamber, and then calculating the true volume V1 according to the ideal gas law (PV = nRT), the porosity of the sample to be tested can be obtained. Porosity P = (V2 - V1) / V2 × 100%, apparent volume V2 = S × H × A; Where S represents the area (cm²) 2 H represents the thickness (cm); A represents the number of electrode discs; V1 represents the actual volume of the disc (cm). 3 V2 represents the apparent volume (cm²) of the small electrode disc. 3 ).

[0076] (3) Testing the rate performance of secondary batteries Under a 25℃ testing environment, S1: Charge the secondary battery at a constant current and constant voltage of 0.2C to 4.4V, cut off at 0.05C, and let it rest for 10 minutes. S2: Discharge at 0.2C to 2.8V, record the discharge capacity C0, and let it rest for 10 minutes. S3: Charge the secondary battery at a constant current and constant voltage of 0.2C to 4.4V, cut off at 0.05C, and let it rest for 10 minutes. S4: Discharge at 2C to 2.8V, record the discharge capacity C1, and let it rest for 10 minutes. The 2C discharge capacity retention rate is (C1 / C0)*100%.

[0077] (4) Test the cycle capacity retention rate of the secondary battery. In a 25℃ testing environment, the secondary battery was charged at 0.2C for 3 cycles to obtain the average capacity C0. Then, it was charged at a constant rate of 1C0 to 4.4V and allowed to rest for 10 minutes. It was then discharged at a constant rate of 1C0 to 2.5V and allowed to rest for 10 minutes. This charge-discharge cycle was repeated for 1500 cycles. The battery's cycle capacity retention rate was calculated using the following formula: Cycle capacity retention = Discharge capacity over 1500 cycles / Discharge capacity over the first cycle × 100%; The detection results of Examples 1-16 and Comparative Examples 1-3 are shown in Table 2: Table 2

[0078] In Table 2, D1 represents the median particle size of the positive electrode active composition.

[0079] Referring to Table 2 and in conjunction with Table 1, the secondary batteries prepared in Examples 1-16, by sequentially coating the surface of the positive electrode current collector with a first active material layer and a second active material layer having different manganese-iron ratios, and with the first active material layer having a higher manganese-iron molar ratio than the second active material layer, can reduce the manganese content on the surface of the positive electrode, thereby weakening the reduction in the degree of manganese dissolution on the surface of the positive electrode during the cycling process of the secondary battery. In this way, the rate of manganese dissolution on the surface of the positive electrode can be reduced, thereby improving the 2C rate performance and cycle capacity retention of the secondary battery. The secondary batteries provided according to the embodiments of this application can achieve a 2C rate performance of over 82.5% and a cycle capacity retention of over 79%.

[0080] Combining Examples 1-16 and Comparative Example 1, the secondary batteries prepared in Examples 1-16 can achieve a 2C rate performance of over 82.70% and a cycle capacity retention of over 78.50%. Specifically, Example 1 achieves a 2C rate performance of 85.10% and a cycle capacity retention of 80.60%; Example 9 achieves a 2C rate performance of 85.90% and a cycle capacity retention of 82.90%. In contrast, in Comparative Example 1, because the first and second active slurries are mixed and the resulting mixed active slurry is directly coated onto the surface of the positive electrode current collector, the manganese content on the surface of the positive electrode does not exhibit a gradient decrease from the inside to the outside. Comparative Example 1 achieves a 2C rate performance of 81.70% and a cycle capacity retention of 78.1%. The rate performance and cycle capacity retention of Examples 1-16 are significantly better than those of Comparative Example 1.

[0081] Examples 1-16 further control the particle size distribution of the positive electrode active composition in the first active material layer so that the particle size distribution diagram of the positive electrode active composition includes a first characteristic peak with a peak particle size in the range of 0.4 μm to 1.8 μm and a second characteristic peak with a peak particle size in the range of 3 μm to 6 μm. Figure 2 The particle size distribution curve of the positive electrode active composition prepared according to Example 9 of this application is shown. Figure 2 Two characteristic peaks are clearly visible along the horizontal axis (particle size). These peaks are influenced by the particle size distribution and ratio of the first lithium manganese iron phosphate and lithium nickel cobalt manganese oxide. This reflects the particle size distribution of the positive electrode active composition and also affects the pore structure within the first active material layer. Optimizing the particle size distribution of the positive electrode active composition optimizes the pore structure within the first active material layer, thus facilitating electrolyte wetting and improving the electrical performance of the secondary battery.

[0082] Porosity can describe the void structure of the first active material layer to a certain extent. According to the secondary battery provided in the embodiments of this application, the porosity of its positive electrode is 25%~30%. More specifically, the porosity of the secondary batteries prepared in Examples 1-16 can be controlled within the range of 27.37%-27.95%, which is higher than the porosity of the secondary batteries prepared in Comparative Examples 1-3.

[0083] Combining Examples 1 and Comparative Examples 2 and 3, the particle size distribution of the positive electrode active composition in Example 1 shows a particle size of 0.42 μm at the first characteristic peak and 3.31 μm at the second characteristic peak. In contrast, the particle size distribution of the positive electrode active composition in Comparative Example 2 shows a particle size of 0.36 μm at the first characteristic peak and 2.5 μm at the second characteristic peak. Comparative Example 2 exhibits a 2C rate performance of 83.10% and a cycle capacity retention of 77.30%. The particle size distribution of the positive electrode active composition in Comparative Example 3 shows a particle size of 2 μm at the first characteristic peak and 6.6 μm at the second characteristic peak. Comparative Example 3 exhibits a 2C rate performance of 82.90% and a cycle capacity retention of 78.00%. In comparison, the cycle capacity retention of Examples 1-16 is significantly better than that of Comparative Examples 2 and 3.

[0084] Referring to Examples 1 and 3-5, the variation of the median particle size D2 of the first lithium manganese iron phosphate has a significant impact on the particle size distribution of the positive electrode active composition, especially the particle size corresponding to the first characteristic peak in the particle size distribution. By changing the median particle size D2 of the first lithium manganese iron phosphate, the pore structure of the first active material layer can be adjusted, thereby optimizing the electrical performance of the secondary battery. In Example 9, the median particle size D2 of the first lithium manganese iron phosphate is 0.95 μm, and the 2C rate performance of the secondary battery can reach 85.90%, and the cycle capacity retention rate can reach 82.90%.

[0085] Referring to Reference Examples 5, 9, 10 and Comparative Examples 2, 3, the change in the median particle size D3 of the lithium nickel cobalt manganese oxide has a significant impact on the particle size distribution of the positive electrode active composition, especially the particle size corresponding to the second characteristic peak in the particle size distribution diagram. By changing the median particle size D3 of the lithium nickel cobalt manganese oxide, the pore structure of the first active material layer can be adjusted, thereby optimizing the electrical performance of the secondary battery.

[0086] If the median particle size D3 of the lithium nickel cobalt manganese oxide is within the range of 2 μm to 7 μm, as the median particle size D3 of the lithium nickel cobalt manganese oxide changes, the particle size corresponding to the second characteristic peak also changes accordingly. And when the median particle size of the lithium nickel cobalt manganese oxide reaches 3.34 μm, the electrical performance of the secondary battery (for example, 2C rate performance and cycle capacity retention rate) can be improved significantly. If the median particle size D3 of the lithium nickel cobalt manganese oxide is less than 2 μm, such as 1.5 μm shown in Comparative Example 2, the particle size corresponding to the second characteristic peak is 2.5 μm, and the electrical performance of the secondary battery will be significantly reduced. Similarly, if the median particle size D3 of the lithium nickel cobalt manganese oxide is greater than 7 μm, such as 8 μm shown in Comparative Example 3, the particle size corresponding to the second characteristic peak is 6.6 μm, and the electrical performance of the secondary battery will be significantly reduced.

[0087] In addition, the mass ratio m1 / m2 of the first lithium iron phosphate and the lithium nickel cobalt manganese oxide can also affect the particle size distribution of the first active material layer, causing the first characteristic peak and the second characteristic peak to shift horizontally in varying degrees. Referring to Reference Examples 13, 15 and 16, the particle sizes of the first characteristic peak and the second characteristic peak can be controlled within the corresponding ranges by the mass ratio m1 / m2 of the first lithium iron phosphate and the lithium nickel cobalt manganese oxide. Thereby, the secondary battery can have excellent electrical performance.

[0088] Referring to Reference Examples 1, 3-5 and 7, by adjusting the median particle size D2 of the first lithium iron phosphate and the median particle size D4 of the second lithium iron phosphate, D2 < D4. This can make the porosity of the first active material layer lower than that of the second active material layer, thereby enabling the porosity of the positive electrode sheet to decrease in a gradient from the outside to the inside, which is beneficial to the infiltration of the electrolyte, shortening the transmission paths of ions and electrons, and improving the kinetic performance of the battery. Specifically, referring to Reference Examples 1, 3, 4, in Examples 3 and 4, the median particle size D4 of the first lithium iron phosphate > the median particle size D4 of the lithium nickel cobalt manganese oxide. Example 1 has better 2C rate performance and cycle capacity retention rate than Examples 3 and 4. Referring to Reference Examples 3, 5 and 7, in Examples 5 and 7, the median particle size D2 of the first lithium iron phosphate < the median particle size D4 of the lithium nickel cobalt manganese oxide. Examples 5 and 7 have higher 2C rate performance and cycle capacity retention rate than Example 3.

[0089] Referring to Examples 10-14, changes in the manganese-iron molar ratio in the positive electrode (i.e., the manganese-iron molar ratio k1 of the first lithium manganese iron phosphate and the manganese-iron molar ratio k2 of the second lithium manganese iron phosphate) also affect the electrical performance of the secondary battery. Specifically, when the manganese-iron molar ratio k1 of the first lithium manganese iron phosphate is controlled at (1~4):1, the secondary battery can have better 2C rate performance and cycle capacity retention. In Example 11, the manganese-iron molar ratio k1 of the first lithium manganese iron phosphate is 2:1, and its secondary battery can achieve a 2C rate performance of 85.60% and a cycle capacity retention of 82.30%. When the manganese-iron molar ratio k2 of the second lithium manganese iron phosphate is controlled at (0.6~1.5):1, the secondary battery can have better 2C rate performance and cycle capacity retention. In Example 13, the manganese-iron molar ratio k2 of the second lithium manganese iron phosphate is 1:1, and its secondary battery can achieve a 2C rate performance of 85.10% and a cycle capacity retention of 80.30%.

[0090] The various implementations of this application have been described above. The foregoing description is exemplary and not exhaustive, nor is it limited to the disclosed implementations. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described implementations. The terminology used herein is chosen to best explain the principles, practical applications, or improvements to technology in the market, or to enable others skilled in the art to understand the various implementations disclosed herein.

Claims

1. A secondary battery, characterized in that, Includes a positive electrode plate, wherein the positive electrode plate comprises: Positive current collector; A first active material layer is disposed on at least one surface of the positive electrode current collector. The first active material layer comprises a positive electrode active composition, which includes lithium manganese iron phosphate and lithium nickel cobalt manganese oxide. The particle size distribution curve of the positive electrode active composition includes a first characteristic peak and a second characteristic peak. The particle size corresponding to the peak value of the first characteristic peak is in the range of 0.4 μm to 1.8 μm, and the particle size corresponding to the peak value of the second characteristic peak is in the range of 3 μm to 6 μm. The second active material layer is disposed on the side of the first active material layer away from the positive current collector, and the second active material layer includes a second lithium manganese iron phosphate. The manganese-iron molar ratio of the first lithium manganese iron phosphate is greater than that of the second lithium manganese iron phosphate.

2. The secondary battery according to claim 1, characterized in that, The median particle size D4 of the second lithium manganese iron phosphate is greater than the median particle size D2 of the first lithium manganese iron phosphate.

3. The secondary battery according to claim 2, characterized in that, The median particle size D2 of the first lithium manganese iron phosphate is 0.55 μm to 3 μm, and the median particle size D4 of the second lithium manganese iron phosphate is 0.8 μm to 5 μm.

4. The secondary battery according to claim 1 or 2, characterized in that, The median particle size D1 of the positive electrode active composition is 1 μm to 5 μm.

5. The secondary battery according to claim 1, characterized in that, The median particle size D3 of the lithium nickel cobalt manganese oxide is 2 μm to 7 μm.

6. The secondary battery according to claim 1, characterized in that, The porosity of the positive electrode sheet is 25%~30%.

7. The secondary battery according to claim 1, characterized in that, The first lithium manganese iron phosphate has a manganese iron molar ratio of (1~4):1, and the second lithium manganese iron phosphate has a manganese iron molar ratio of (0.6~1.5):

1.

8. The secondary battery according to any one of claims 1-3 and 5, 6, characterized in that, The lithium nickel cobalt manganese oxide includes the chemical formula Li a1 Ni x1 Co y1 Mn z1 Me p Compounds of O2, among which Me contains at least one of the elements B, Y, Nb, In, La, Zr, Ce, W, Al, Ti, Sr, Mg, Sb, V, Zn, Cu, Cr, and Fe, with 0.95 ≤ a1 ≤ 1.05, 0.5 ≤ x1 < 1, 0 < y1 ≤ 0.2, 0 < z1 ≤ 0.3, and 0 ≤ p ≤ 0.

1.

9. The secondary battery according to any one of claims 1-4 and 6, characterized in that, Based on the total mass of the positive electrode active composition, the first lithium manganese iron phosphate accounts for 10wt%~40wt%, and the lithium nickel cobalt manganese oxide accounts for 60wt%~90wt%.

10. An electrical appliance, characterized in that, include: The secondary battery according to any one of claims 1-9 serves as the power supply for the electrical equipment.

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