A composite positive electrode and battery

Abstract

This invention discloses a composite positive electrode sheet and a battery. The composite positive electrode sheet includes a positive current collector, and at least one side of the positive current collector is provided with a positive active coating. The positive active coating includes a first positive active coating, a second positive active coating, and a third positive active coating. The first, second, and third positive active coatings are all disposed on the positive current collector, and are sequentially connected along the width direction of the positive current collector. The first positive active coating comprises one or two of polycrystalline lithium nickel cobalt manganese oxide and monocrystalline lithium nickel cobalt manganese oxide. The second and third positive active coatings each comprise polycrystalline lithium nickel cobalt manganese oxide, monocrystalline lithium nickel cobalt manganese oxide, and monocrystalline lithium manganese iron phosphate. The composite positive electrode sheet enables the battery to have higher energy density and rate performance, while improving the charging capability of the battery cell.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology and relates to a composite positive electrode and a battery. Background Technology

[0002] Lithium-ion batteries mainly consist of a positive electrode, a negative electrode, a separator, and an electrolyte. During charging and discharging, lithium ions repeatedly insert and extract between the two electrodes. Lithium-ion batteries possess three key characteristics for battery industry development: high volumetric energy density and high gravimetric energy density, rechargeability, and zero pollution. Current lithium-ion battery positive electrodes include ternary systems, lithium iron phosphate systems, and lithium manganese iron phosphate systems. Among these, lithium iron phosphate, as the positive electrode active material, exhibits good safety and cycle performance, but its energy density and high-temperature performance are insufficient. Lithium manganese iron phosphate systems, on the other hand, have high energy density, primarily due to battery impedance affecting their performance.

[0003] To improve overall battery performance, Chinese patent document CN 115995528 A discloses a composite cathode, which mainly involves coating a lithium manganese iron phosphate coating with a lithium nickel manganese oxide coating. The layered mixing of lithium manganese iron phosphate and lithium nickel manganese oxide improves cycle life and safety performance. However, due to edge effects and uneven current distribution at the electrode edges, lithium plating is more likely to occur at the edges compared to the central region of the electrode. This is especially true under high energy density requirements, where higher coating amounts and compaction densities increase the risk of lithium plating at the edges.

[0004] In the aforementioned patent literature, all the composite cathode sheets studied use a double-layer coating design, which reduces the energy density of the battery cell. Furthermore, the charging capability at the edge position is still inferior to that at the center position, which is the bottleneck of charging capability. Summary of the Invention

[0005] In view of the problems existing in the prior art, the present invention provides a composite positive electrode and a battery. The composite positive electrode can enable the battery to have higher energy density and rate performance, while improving the charging capability of the battery cell.

[0006] This invention provides a composite positive electrode sheet, comprising a positive current collector 4, wherein at least one side of the positive current collector 4 is provided with a positive active coating, the positive active coating comprising a first positive active coating 1, a second positive active coating 2, and a third positive active coating 3; the first positive active coating 1, the second positive active coating 2, and the third positive active coating 3 are all disposed on the positive current collector 4, and the second positive active coating 2, the first positive active coating 1, and the third positive active coating 3 are sequentially connected along the width direction of the positive current collector 4, wherein the first positive active coating 1 comprises one or two of polycrystalline lithium nickel cobalt manganese oxide and monocrystalline lithium nickel cobalt manganese oxide; the second positive active coating 2 and the third positive active coating 3 each comprise polycrystalline lithium nickel cobalt manganese oxide, monocrystalline lithium nickel cobalt manganese oxide, and monocrystalline lithium manganese iron phosphate.

[0007] Preferably, in the second positive electrode active coating 2 and the third positive electrode active coating 3, the average particle size of polycrystalline lithium nickel cobalt manganese oxide is D1, the average particle size of single-crystal lithium nickel cobalt manganese oxide is D2, and the average particle size of single-crystal lithium manganese iron phosphate is D3, wherein D1×0.4≥D2≥D1×0.1, and D2×0.5≥D3≥D2×0.15.

[0008] Preferably, in the second positive electrode active coating 2 and the third positive electrode active coating 3, D1 ranges from 6 to 20 μm, preferably from 8 to 15 μm; D2 ranges from 1 to 6 μm, preferably from 2 to 4 μm; and D3 ranges from 0.3 to 1.5 μm, preferably from 0.5 to 1 μm.

[0009] Preferably, the width of the second positive electrode active coating 2 and the width of the third positive electrode active coating 3 are 3 to 20 mm, and more preferably 5 to 10 mm.

[0010] Preferably, the mass ratio of the active material in the second positive electrode active coating 2 and the mass ratio of the active material in the third positive electrode active coating 3 satisfy: 3 ≤ (C 多晶镍钴锰酸锂 ×M 多晶镍钴锰酸锂 +C 单晶镍钴锰酸锂 ×M 单晶镍钴锰酸锂 ) / (C 单晶磷酸锰铁锂 ×M 单晶磷酸锰铁锂 ≤20; C is the discharge capacity in mAh / g, M is the weight percentage in %. 单晶镍钴锰酸锂 +M 多晶镍钴锰酸锂 +M 单晶磷酸锰铁锂 =1.

[0011] Preferably, the first positive electrode active coating 1 comprises polycrystalline lithium nickel cobalt manganese oxide and monocrystalline lithium nickel cobalt manganese oxide; the mass ratio of polycrystalline lithium nickel cobalt manganese oxide to monocrystalline lithium nickel cobalt manganese oxide in the second positive electrode active coating 2 and the mass ratio of polycrystalline lithium nickel cobalt manganese oxide to monocrystalline lithium nickel cobalt manganese oxide in the third positive electrode active coating 3 are respectively less than the mass ratio of polycrystalline lithium nickel cobalt manganese oxide to monocrystalline lithium nickel cobalt manganese oxide in the first positive electrode active coating 1.

[0012] Preferably, all positive electrode active coatings contain conductive agents, and the content of conductive agents in the second positive electrode active coating 2 and the third positive electrode active coating 3 is greater than the content of conductive agents in the first positive electrode active coating 1.

[0013] Preferably, the composite positive electrode further includes a tab, which is fixed at one end of the positive current collector 4 in the width direction.

[0014] This invention provides a method for preparing the composite positive electrode sheet as described above, comprising the following steps:

[0015] The active materials of the first positive electrode active coating, the second positive electrode active coating, and the third positive electrode active coating are coated and formed in one step in the corresponding area of ​​the positive electrode current collector to obtain a composite positive electrode sheet.

[0016] The present invention provides a battery comprising a separator, a negative electrode, and the composite positive electrode described above.

[0017] Preferably, the active material of the negative electrode is one or more of hard carbon, graphite, single-crystal silicon, and silicon suboxide.

[0018] Compared with the prior art, the present invention proposes a composite positive electrode sheet with a composite coating structure. The composite positive electrode sheet includes a positive current collector, on which at least one side is provided with a first positive active coating, a second positive active coating, and a third positive active coating, which are connected in the width direction. The first positive active coating contains one or two of polycrystalline lithium nickel cobalt manganese oxide and monocrystalline lithium nickel cobalt manganese oxide; the second and third positive active coatings each contain polycrystalline lithium nickel cobalt manganese oxide, monocrystalline lithium nickel cobalt manganese oxide, and monocrystalline lithium manganese iron phosphate. In the embodiments of the present invention, the positive active material coated on the positive current collector is divided into three regions: a central main region (corresponding to the first positive active coating), and edge regions at both ends (corresponding to the second and third positive active coatings); the two edge regions are coated with the same active material, namely polycrystalline lithium nickel cobalt manganese oxide, monocrystalline lithium nickel cobalt manganese oxide, and monocrystalline lithium manganese iron phosphate, while the central main region is coated with another active material (one or two of polycrystalline lithium nickel cobalt manganese oxide and monocrystalline lithium nickel cobalt manganese oxide). This invention focuses on improving the charging capability at the electrode edge without degrading the performance of the main electrode area. The electrode edge region utilizes a blending of three cathode materials to reduce the kinetic performance of the cathode edge, thereby solving the lithium plating problem. Furthermore, the specific capacity of the material in the main electrode area is not reduced, so the energy density of the battery cell remains largely unaffected. Since the main electrode area is not doped with monocrystalline lithium manganese iron phosphate, the impedance of the battery cell is not degraded, and the power capability of the battery cell is not affected. In addition, the composite cathode of this invention has a single-layer coating, allowing the electrode to be coated and formed in a single step.

[0019] Furthermore, by controlling the D50 particle size ratio between active materials and controlling the mass ratio of active materials, this invention enables the battery to still have an energy density level and rate performance comparable to ternary batteries, while reducing the rate performance at the edge of the positive electrode. At the same time, the charging capability of the cell is greatly improved, and no lithium plating occurs at the edge after cycling. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the composite positive electrode sheet in some embodiments of the present invention. Detailed Implementation

[0021] The technical solutions in the embodiments of this application are described clearly and completely below. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0022] This invention provides a composite positive electrode sheet, comprising a positive current collector 4, wherein at least one side of the positive current collector 4 is provided with a positive active coating, the positive active coating comprising a first positive active coating 1, a second positive active coating 2, and a third positive active coating 3; the first positive active coating 1, the second positive active coating 2, and the third positive active coating 3 are all disposed on the positive current collector 4, and the second positive active coating 2, the first positive active coating 1, and the third positive active coating 3 are sequentially connected along the width direction of the positive current collector 4, wherein the first positive active coating 1 comprises one or two of polycrystalline lithium nickel cobalt manganese oxide and monocrystalline lithium nickel cobalt manganese oxide; the second positive active coating 2 and the third positive active coating 3 each comprise polycrystalline lithium nickel cobalt manganese oxide, monocrystalline lithium nickel cobalt manganese oxide, and monocrystalline lithium manganese iron phosphate.

[0023] The composite cathode sheet provided by this invention enables the battery to have higher energy density and rate performance, while improving the charging capability of the battery cell.

[0024] See Figure 1 , Figure 1 This is a schematic diagram of the composite positive electrode sheet in some embodiments of the present invention. Figure 1 In the diagram, 1 represents the first positive electrode active coating, 2 represents the second positive electrode active coating, 3 represents the third positive electrode active coating, and 4 represents the positive electrode current collector.

[0025] The composite positive electrode sheet described in this embodiment of the invention includes a positive electrode current collector 4. The positive electrode current collector 4 serves both as a carrier for the positive electrode active material and as a positive electrode electron collector and conductor. Its function is to collect the current generated by the battery active material to produce a larger output current. In lithium-ion batteries, it mainly consists of metal foils such as copper foil and aluminum foil, and may also include tabs.

[0026] In an embodiment of the present invention, both surfaces of the positive electrode current collector 4 are coated with a coating containing a positive electrode active material, referred to as the positive electrode active coating, which is divided into a first positive electrode active coating 1, a second positive electrode active coating 2, and a third positive electrode active coating 3, corresponding to three regions: region 1, region 2, and region 3, respectively. Regions 2 and 3 are coated with the same active material, while region 1 is coated with a different active material. It should be noted that the positive electrode active coating may also be disposed only on one side of the positive electrode current collector.

[0027] This invention proposes designs for active materials in different coating regions; the first positive electrode active coating 1, i.e., coating region 1 (covering the central main region of the positive electrode current collector), uses a conventionally designed active material. Depending on the fast-charging capability of the battery cell, one or a mixture of polycrystalline lithium nickel cobalt manganese oxide and monocrystalline lithium nickel cobalt manganese oxide can be used. Lithium nickel cobalt manganese oxide (NCM) has the chemical formula LiNi. x Co y Mn 1-x-yO2 has a layered structure; polycrystalline NCM exhibits high kinetic properties, while single-crystal NCM exhibits low kinetic properties.

[0028] In this embodiment of the invention, the coatings at the edge of the composite positive electrode, namely the second positive electrode active coating 2 (region 2) and the third positive electrode active coating 3 (region 3), are both made of a mixture of three active materials: polycrystalline lithium nickel cobalt manganese oxide, monocrystalline lithium nickel cobalt manganese oxide, and monocrystalline lithium manganese iron phosphate. By utilizing the different crystal kinetic properties, the kinetic properties at the edge of the positive electrode are reduced, thereby solving the problem of lithium deposition at the edge. Among them, lithium manganese iron phosphate (LiFe) x Mn 1-x PO4 (abbreviated as LMFP) has an ordered and regular olivine-type crystal structure.

[0029] In embodiments of the present invention, the median particle size D50 of the three active materials in regions 2 and 3 needs to be strictly controlled, especially the D50 of monocrystalline lithium nickel cobalt manganese oxide and monocrystalline lithium manganese iron phosphate. If the particle size is too large, the diffusion path of lithium ions in large-sized particles is longer, and the resistance to diffusion is greater, resulting in a serious deterioration of charging capacity. If the particle size is too small, on the one hand, the specific surface area of ​​the material is often larger, the oxidation activity is higher, and the surface side reactions will increase; on the other hand, the material is prone to agglomeration. Monocrystalline lithium nickel cobalt manganese oxide and monocrystalline lithium manganese iron phosphate agglomerate in large quantities in the particle size gaps of polycrystalline lithium nickel cobalt manganese oxide, which will block the lithium ion transport channels. The average particle size (D50) of polycrystalline lithium nickel cobalt manganese oxide is denoted as D1, the average particle size of monocrystalline lithium nickel cobalt manganese oxide is D2, and the average particle size of monocrystalline lithium manganese iron phosphate is D3. Preferably, D1×0.4≥D2≥D1×0.1, and D2×0.5≥D3≥D2×0.15. Specifically, D1 ranges from 6 to 20 μm, preferably from 8 to 15 μm; D2 ranges from 1 to 6 μm, preferably from 2 to 4 μm; and D3 ranges from 0.3 to 1.5 μm, preferably from 0.5 to 1 μm.

[0030] In the second positive electrode active coating 2 and the third positive electrode active coating 3 of the present invention, the mass ratio of active materials satisfies: 3 ≤ (C 多晶NCM ×M 多晶NCM +C 单晶NCM ×M 单晶NCM ) / (C 单晶LMFP ×M 单晶LMFP )≤20, with an optimal value between 5 and 10. Among them, C 多晶NCM C 单晶NCM C 单晶LMFP These represent the discharge quantile capacity of the three active materials, in mAh / g; M 多晶NCM M 单晶NCM M 单晶LMFP The weight percentages of the three active materials are shown in %, M. 多晶NCM +M 单晶NCM +M 单晶LMFP=1. The lower this ratio, the greater the amount of monocrystalline LMFP doping, resulting in a greater improvement in edge charging capability, but correspondingly, a greater loss in cell energy density. The higher this ratio, the smaller the amount of monocrystalline LMFP doping, the smaller the difference between the edge region and the central region of the electrode, and the greater the difference in charging capability.

[0031] The present invention preferably controls the D50 particle size ratio between active materials and the mass ratio of active materials, so that the battery still has an energy density level and rate performance comparable to ternary batteries, reduces the rate performance at the edge of the positive electrode, and significantly improves the charging capability of the cell.

[0032] Furthermore, the second positive electrode active coating 2, the first positive electrode active coating 1, and the third positive electrode active coating 3 are sequentially connected along the width direction of the positive electrode current collector 4. In this embodiment of the invention, the total coating thickness is between 80 μm and 160 μm, and the total width is generally between 80 mm and 600 mm, depending on the cell model. The width of the second region is denoted as L2, and the width of the third region is denoted as L3. L2 and L3 need to have a certain width range to improve the lithium plating window at the edge without significantly degrading the cell's energy density. Specifically, L2 and L3 are preferably 3–20 mm, and more preferably 5–10 mm.

[0033] Furthermore, in the embodiments of the present invention, the mass ratio of polycrystalline NCM to monocrystalline NCM in the coating at the edge of the composite cathode can be different from that at the center; the proportion at the edge can be lower than the corresponding proportion at the center, that is, the proportion of polycrystalline NCM at the edge is lower, thereby reducing the rate of lithium ion insertion / extraction at the edge of the cathode.

[0034] In addition to the active material, the composite positive electrode coating of this invention includes a conductive agent and a binder. Furthermore, the conductive agent content in the coating at the edge of the composite positive electrode can be higher than that in the center, thereby improving conductivity and charging capability at the edge. The conductive agent includes one or more composites of carbon black (such as acetylene black), graphite, graphene, and carbon nanotubes. For example, acetylene black is carbon black obtained by continuous pyrolysis of acetylene with a purity of over 99%, obtained by decomposing and refining byproduct gases from the calcium carbide method or naphtha pyrolysis. The conductive agent typically accounts for 2-4% of the total mass of the coating; the higher the proportion of the conductive agent in the coating, the better the conductivity of the coating. The binder generally uses PVDF (polyvinylidene fluoride), a bonding agent well-known to those skilled in the art, and typically accounts for 1-3% of the total mass of the coating.

[0035] The composite positive electrode sheet described in this embodiment of the invention also includes a tab, which is fixedly disposed at one end of the positive current collector 4 in the width direction. The tab is a metallic conductor that leads the positive and negative electrodes out of the battery cell; it can be welded to one end of the positive current collector 4 in the width direction or integrally formed with the current collector. This application does not impose any special restrictions on the structure and composition of the tab; conventional tab components can be used.

[0036] Accordingly, embodiments of the present invention provide a method for preparing the composite positive electrode sheet described above, comprising the following steps:

[0037] The active materials of the first positive electrode active coating, the second positive electrode active coating, and the third positive electrode active coating are coated and formed in one step in the corresponding area of ​​the positive electrode current collector to obtain a composite positive electrode sheet.

[0038] Specifically, in this embodiment of the invention, 94-96% of the positive electrode active material, 2-4% of the conductive agent, and 1-3% of the binder are thoroughly stirred in a solvent such as N-methylpyrrolidone (NMP) to prepare a positive electrode slurry. Slurries for different regions can be prepared separately. The main slurry prepared in the above steps is coated onto the main area (region 1) of the aluminum foil using a main coating head, and another positive electrode slurry prepared using the same method is coated onto regions 2 and 3 using an edge coating head.

[0039] In an embodiment of the present invention, the single-sided coated electrode sheet can be dried in a high-temperature oven at 80-90°C, and then a second coating can be performed. The coating method, area, and materials are the same as those described in the single-sided coating step above. The coated double-sided electrode sheet is then dried in a high-temperature oven at 80-90°C. Finally, the double-sided coated sample is cold-pressed, slit, cut, and the positive electrode tabs are welded to obtain a composite positive electrode sheet.

[0040] The present invention also provides a battery comprising a negative electrode, a composite positive electrode, an insulating negative electrode, and a separator for the composite positive electrode.

[0041] In embodiments of the present invention, the active material of the negative electrode sheet can be one or more of carbon materials (including hard carbon, artificial graphite, natural graphite, etc.) and silicon materials (monocrystalline silicon, silicon suboxide, etc.). Hard carbon is usually a high-molecular-weight pyrolytic carbon that is difficult to graphitize at temperatures above 2500℃; natural graphite comes from graphite deposits, or can be made into artificial graphite from petroleum coke, pitch coke, etc., through a series of processes.

[0042] This invention does not impose any special restrictions on the composition of the raw materials for the negative electrode; conventional materials and structures can be used. Specifically, the negative electrode active material, conductive agent, binder, and optionally dispersant can be thoroughly mixed in water at a certain weight ratio to prepare a uniform negative electrode slurry. Then, the prepared negative electrode slurry can be coated evenly onto a current collector, transferred to an oven for drying, and then cold-pressed and slit to obtain the negative electrode sheet.

[0043] The electrolyte preparation and cell assembly of the battery described in this embodiment of the invention can be performed using conventional techniques in the field. For example, the separator is a ceramic-coated polyethylene (PE) material separator. The electrolyte can be composed of three types of substances: lithium salt, solvent, and additives. The lithium salt is mainly LiPF6, and the solvent is often a mixture of ethylene carbonate (EC), polycarbonate (PC), dimethyl carbonate (DMC), and diethyl carbonate (DEC).

[0044] In this embodiment of the invention, the composite positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the composite positive and negative electrodes to provide isolation. Each electrode assembly is placed in a packaging shell, injected with electrolyte, and sealed. After formation, the final lithium-ion battery is produced.

[0045] The lithium-ion battery comprising the above-mentioned composite positive electrode sheet was prepared according to the embodiments of the present invention, and its performance was tested according to the following test methods:

[0046] (1) Lithium plating window test:

[0047] First, the electrochemical device is fully discharged. Then, a specific temperature is set (e.g., 25°C), and conventional charging (constant current + constant voltage) is performed at different rates, such as 1C, 1.1C, 1.2C, etc., according to the design of the electrochemical device. That is, charging is performed at a specific rate until the battery cutoff voltage, followed by constant voltage charging until the 0.05C cutoff charging. After charging, it is fully discharged at 0.33C. This charge-discharge process is repeated 20 times. Finally, the battery is fully charged and disassembled to observe whether lithium plating occurs at the edge of the negative electrode. The maximum current without lithium plating (no white spots on the negative electrode, only grayish-brown lithium plating) is defined as the maximum non-lithiation rate of the battery, which is also the lithium plating window.

[0048] (2) Battery energy density test:

[0049] The battery under test was left to stand at 25°C for 30 minutes, then charged at a constant current rate of 0.33C until the voltage reached the rated voltage. Subsequently, it was charged at a constant voltage until the charge / discharge rate reached 0.05C, at which point charging was stopped. The battery was then left at room temperature for 30 minutes, and then discharged at a rate of 0.33C to 2.5V. The obtained capacity is taken as the actual battery energy C, in Wh. This is then divided by the cell weight W, in kg, to obtain the battery's energy density VED = C / W, in Wh / kg.

[0050] Test results show that the battery described in this embodiment of the invention has high energy density and rate performance, and the battery cell has good charging capability.

[0051] The embodiments of the present invention are further illustrated below, but the present invention is not limited to these embodiments. The raw materials used in the embodiments of the present invention are commercially available.

[0052] Example 1: Preparation of Lithium-ion Batteries

[0053] 1. Positive electrode partitioning coating

[0054] 1) Take 9.5 parts of monocrystalline lithium manganese phosphate (LMFP), 3 parts of conductive carbon black (SP), 2 parts of PVDF, and NMP, mix them, and stir to obtain a pre-slurry. Add 66.5 parts of polycrystalline NCM and 19 parts of monocrystalline NCM, and stir to obtain the cathode slurry. Polycrystalline NCM, monocrystalline NCM, and monocrystalline LMFP are combined using different particle sizes (D50). The general formula for the cathode active materials, polycrystalline NCM and monocrystalline NCM, is LiNi. x Co y Mn 1-x- y O2, where x = 0.8, y = 0.1; positive electrode active material LMFP, general formula LiMn m Fe n M 1-m-n PO4, where m = 0.65 and n = 0.35.

[0055] 2) Single-sided coating: The positive electrode slurry prepared in step 1) is uniformly coated onto regions 2 and 3 of the aluminum foil using an edge coating head, and a different positive electrode slurry prepared using the same method is uniformly coated onto the main body region (region 1) of the aluminum foil using a main coating head. The coating in region 1 contains polycrystalline NCM, single-crystalline NCM, conductive carbon black SP, and PVDF. Specifically, 73.9 parts of polycrystalline NCM, 21.1 parts of single-crystalline NCM, 3 parts of conductive carbon black SP, and 2 parts of PVDF are mixed with NMP and stirred to obtain a slurry. The D50 particle sizes of the polycrystalline NCM and single-crystalline NCM are combined using different particle sizes. The positive electrode active materials, polycrystalline NCM and single-crystalline NCM, have the general formula LiNi. x Co y Mn 1-x-y O2, where x = 0.8 and y = 0.1.

[0056] Double-sided coating: The coated single-sided electrode sheet is dried in a high-temperature oven at 90°C, and a second coating is applied. The coating method, area, and materials are the same as described in step 1). The coated double-sided electrode sheet is then dried in a high-temperature oven at 90°C. Afterward, it is cold-pressed, slit, cut, and the positive electrode tabs are welded to obtain the composite positive electrode sheet.

[0057] 2. Negative electrode coating:

[0058] The negative electrode slurry is prepared by thoroughly mixing 95% artificial graphite, 2% conductive agent, 2.6% SBR binder, and 0.4% CMC in deionized water according to a certain weight ratio.

[0059] The negative electrode slurry was uniformly coated onto the negative electrode current collector, transferred to an oven for drying, and then cold-pressed and slit to obtain the negative electrode sheet. The weight of the single-sided active material coated on the surface of the negative electrode current collector was 18 mg / cm³. 2 The current collector is coated with 36 mg / cm² on both sides. 2 .

[0060] The conductive agent is the same as that used in the positive electrode coating; the SBR adhesive is styrene-butadiene latex for conventional bonding, and the CMC is sodium carboxymethyl cellulose for conventional bonding.

[0061] 3. Electrolyte preparation & cell assembly

[0062] Lithium salt LiPF6 was dissolved in a solvent to prepare a 1 mol / L electrolyte. The organic solvent was ethylene carbonate (EC): diethyl carbonate (DEC): propylene carbonate (PC): ethylene carbonate (VC) = 30:40:28:2, mass ratio).

[0063] The separator is made of ceramic-coated polyethylene (PE) material. The PE separator has a porosity of 42%, a base film thickness of 9μm, and a 1.5μm ceramic coating on both sides. The outermost layer is coated with PVDF adhesive.

[0064] The negative electrode, separator, and composite positive electrode are stacked in sequence, with the separator positioned between the composite positive and negative electrodes to provide insulation. The electrode assembly is then placed in a packaging shell, injected with electrolyte, and sealed. After formation, the final lithium-ion battery is manufactured.

[0065] Comparative Example 1:

[0066] In the conventional design, the coating slurry used at the edges and center of the electrode does not contain lithium manganese iron phosphate.

[0067] Examples 2-9:

[0068] The D50 particle sizes of polycrystalline NCM, monocrystalline NCM, and monocrystalline LMFP were matched with different values, and the rest of the operation was the same as in Example 1.

[0069] Following the testing methods described above, the prepared battery was subjected to lithium plating window and energy density tests, and the results are as follows.

[0070] Table 1. Battery performance test results obtained by the embodiments of the present invention (different particle sizes in the edge region of the composite positive electrode).

[0071]

[0072] By comparing Example 1 and Comparative Example 1, it can be seen that the addition of LMFP to the edge coating area increases the lithium plating window by 0.4C, thereby improving the overall charging capability of the cell, while the energy density only decreases by 1Wh / kg, which is within the acceptable design range.

[0073] Comparing Examples 1 to 3, it can be seen that both excessively high and excessively low polycrystalline NCM D50 particle size will reduce the improvement in charging capability. In Example 2, D50 of 12μm is relatively superior.

[0074] Similarly, by comparing Example 1 with Examples 4-6, and Example 1 with Examples 7-9, the particle size D50 of single-crystal NCM and single-crystal LMFP cannot be too large or too small.

[0075] According to the comparison of the embodiments, the average particle size of polycrystalline lithium nickel cobalt manganese oxide is D1, the average particle size of monocrystalline lithium nickel cobalt manganese oxide is D2, and the average particle size of monocrystalline lithium manganese iron phosphate is D3, wherein D1*0.4≥D2≥D1*0.1&D2*0.5≥D3≥D2*0.15. D1 ranges from 6 to 20 μm, preferably 8 to 15 μm; D2 ranges from 1 to 6 μm, preferably 2 to 4 μm; D2 ranges from 0.3 to 1.5 μm, preferably 0.5 to 1 μm.

[0076] Examples 10-16:

[0077] Based on Example 2 in Table 1, the D50 of the three positive electrode active materials were selected as 12μm, 3.2μm, and 0.8μm. By adjusting the mass ratio of different active materials, corresponding composite positive electrode sheets were obtained, and then batteries were assembled.

[0078] The test results are as follows:

[0079] Table 2. Battery performance test results obtained from the embodiments of the present invention (different ratios in the edge region of the composite positive electrode).

[0080]

[0081]

[0082] By comparing Example 2 with Examples 10-13, (C) 多晶NCM ×M 多晶NCM +C 单晶NCM ×M 单晶NCM ) / (C 单晶LMFP ×M 单晶LMFPThe ratio should not be too high or too low; around 10 is relatively optimal, with the overall ratio controlled between 3 and 20. When the ratio is 3 or 24.6, the lithium plating window shows virtually no improvement. Using a different NCM ratio, comparisons were made in Examples 14-16, and C... 多晶NCM ×M 多晶NCM +C 单晶NCM ×M 单晶NCM ) / (C 单晶LMFP ×M 单晶LMFP When the lithium plating window ratio is 4.5 and 21.3, the lithium plating window ratio decreases by 0.2 to 0.3 C when the ratio is 10.1.

[0083] The positive electrode active materials used in the experiment were polycrystalline NCM and single-crystal NCM, with the general formula LiNi. x Co y Mn 1-x-y O2, where x = 0.8, y = 0.1; positive electrode active material LMFP, general formula LiMn m Fe n M 1-m-n PO4, where m = 0.65 and n = 0.35; the preferred D50 particle sizes of the three active materials are 10.5 μm, 3.2 μm, and 0.8 μm, respectively.

[0084] As can be seen from the above embodiments, the present invention uses a mixture of three active materials—polycrystalline lithium nickel cobalt manganese oxide, monocrystalline lithium nickel cobalt manganese oxide, and monocrystalline lithium manganese iron phosphate—for coating at the edge of the composite cathode. By controlling the D50 particle size ratio and the mass ratio of the active materials, the battery still has an energy density level and rate performance comparable to that of a ternary battery, while reducing the rate performance at the edge of the composite cathode. At the same time, the charging capability of the cell is greatly improved, and no lithium plating occurs at the edge after rapid cycling.

[0085] The above description of the embodiments is only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A composite positive electrode, characterized in that, It includes a positive current collector (4), and at least one side of the positive current collector (4) is provided with a positive active coating, the positive active coating including a first positive active coating (1), a second positive active coating (2) and a third positive active coating (3). The first positive electrode active coating (1), the second positive electrode active coating (2), and the third positive electrode active coating (3) are all disposed on the positive electrode current collector (4), and the second positive electrode active coating (2), the first positive electrode active coating (1), and the third positive electrode active coating (3) are sequentially connected along the width direction of the positive electrode current collector (4). The first positive electrode active coating (1) contains one or two of polycrystalline lithium nickel cobalt manganese oxide and monocrystalline lithium nickel cobalt manganese oxide; the second positive electrode active coating (2) and the third positive electrode active coating (3) each contain polycrystalline lithium nickel cobalt manganese oxide, monocrystalline lithium nickel cobalt manganese oxide, and monocrystalline lithium manganese iron phosphate. In the second positive electrode active coating (2) and the third positive electrode active coating (3), the average particle size of polycrystalline lithium nickel cobalt manganese oxide is D1, the average particle size of single-crystal lithium nickel cobalt manganese oxide is D2, and the average particle size of single-crystal lithium manganese iron phosphate is D3, wherein D1×0.4≥D2≥D1×0.1, and D2×0.5≥D3≥D2×0.15; In the second positive electrode active coating (2) and the third positive electrode active coating (3), D1 ranges from 6 to 20 μm; D2 ranges from 1 to 6 μm; and D3 ranges from 0.3 to 1.5 μm. The mass ratio of the active material in the second positive electrode active coating (2) and the mass ratio of the active material in the third positive electrode active coating (3) satisfy: 3 ≤ (C 多晶镍钴锰酸锂 ×M 多晶镍钴锰酸锂 + C 单晶镍钴锰酸锂 ×M 单晶镍钴锰酸锂 ) / (C 单晶磷酸锰铁锂 ×M 单晶磷酸锰铁锂 ≤20; C is the discharge capacity in mAh / g, M is the weight percentage in %. 单晶镍钴锰酸锂 + M 多晶镍钴锰酸锂 +M 单晶磷酸锰铁锂 =1.

2. The composite positive electrode sheet according to claim 1, characterized in that, In the second positive electrode active coating (2) and the third positive electrode active coating (3), D1 ranges from 8 to 15 μm; D2 ranges from 2 to 4 μm; and D3 ranges from 0.5 to 1 μm.

3. The composite positive electrode sheet according to claim 1, characterized in that, The width of the second positive electrode active coating (2) and the width of the third positive electrode active coating (3) are 3~20mm respectively.

4. The composite positive electrode sheet according to claim 3, characterized in that, The width of the second positive electrode active coating (2) and the width of the third positive electrode active coating (3) are 5~10mm respectively.

5. The composite positive electrode sheet according to claim 1, characterized in that, The first positive electrode active coating (1) contains polycrystalline lithium nickel cobalt manganese oxide and monocrystalline lithium nickel cobalt manganese oxide; the mass ratio of polycrystalline lithium nickel cobalt manganese oxide to monocrystalline lithium nickel cobalt manganese oxide in the second positive electrode active coating (2) and the mass ratio of polycrystalline lithium nickel cobalt manganese oxide to monocrystalline lithium nickel cobalt manganese oxide in the third positive electrode active coating (3) are respectively less than the mass ratio of polycrystalline lithium nickel cobalt manganese oxide to monocrystalline lithium nickel cobalt manganese oxide in the first positive electrode active coating (1).

6. The composite positive electrode sheet according to claim 1, characterized in that, The positive electrode active coatings all contain conductive agents, and the content of conductive agents in the second positive electrode active coating (2) and the third positive electrode active coating (3) is greater than the content of conductive agents in the first positive electrode active coating (1).

7. The composite positive electrode sheet according to any one of claims 1-3, characterized in that, The composite positive electrode also includes a tab, which is disposed at one end of the positive current collector (4) in the width direction.

8. A battery, characterized in that, It includes a separator, a negative electrode, and a composite positive electrode as described in any one of claims 1-7, wherein the separator is disposed between the negative electrode and the composite positive electrode.

9. The battery according to claim 8, characterized in that, The active material of the negative electrode is one or more of hard carbon, graphite, monocrystalline silicon, and silicon suboxide.

Images