A gradient cathode sheet and a secondary battery
By utilizing a combination of single-crystal and polycrystalline particles in the gradient cathode of lithium-ion batteries, the problem of particle breakage under high voltage density was solved, achieving high cycle stability and high rate performance of the secondary battery and improving energy density.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- 惠州赣锋锂电科技有限公司
- Filing Date
- 2026-04-27
- Publication Date
- 2026-06-09
AI Technical Summary
Under high density conditions, existing lithium-ion batteries suffer from uneven stress on the active material particles, leading to easy breakage of the bottom particles and affecting the cycle life, rate performance, and energy density of the secondary battery.
A gradient positive electrode design is adopted, in which high-strength single-crystal particles are placed at the bottom layer close to the current collector, and low-strength but high-rate-performance polycrystalline particles are placed on the surface layer. By controlling the percentage of particles and compressive strength, a gradient structure is formed to match the stress distribution inside the electrode.
It effectively suppresses particle breakage, improves the cycle stability and rate performance of secondary batteries, balances high energy density and high rate performance, and significantly improves electrochemical performance.
Smart Images
Figure SMS_1
Abstract
Description
Technical Field
[0001] This invention belongs to the field of battery technology, specifically relating to a gradient positive electrode and a secondary battery. Background Technology
[0002] Increasing electrode compaction density is one of the most direct and effective techniques to improve the energy density of lithium-ion batteries. However, due to the high compaction density, the active material particles in the electrode are prone to breakage, especially for high-nickel ternary cathode materials. This breakage not only exposes fresh surfaces and exacerbates interfacial side reactions, but also damages the conductive network of the electrode, leading to rapid capacity decay in the lithium-ion battery.
[0003] Currently, researchers mostly use the following methods to alleviate the above problems, as detailed below: (1) Single crystallization: Single crystal particles are used because they have no grain boundaries, have high compressive strength, and are not easily broken.
[0004] (2) Secondary sphere strengthening: By optimizing the primary particle arrangement and sintering strength of the secondary spheres, the mechanical strength of the polycrystalline particles can be improved.
[0005] (3) Optimize the rolling process: adopt multi-stage rolling, heated rolling and other methods to make the particles slowly press and reduce breakage.
[0006] However, these methods still have the following limitations in practical applications: (a) Single-crystal materials have relatively poor rate performance and high cost.
[0007] (b) Polycrystalline particles enhance compressive strength through structural reinforcement, but their compressive strength still has an upper limit, and there is still a risk of breakage under high pressure density.
[0008] (c) Existing technologies do not take into account the differences in stress distribution along the electrode thickness direction. They typically distribute the same active material particles (whether single-crystal or polycrystalline) uniformly throughout the electrode, which is not the optimal mechanical design.
[0009] Therefore, there is an urgent need to develop an electrode structure design that can differentiate the mechanical properties of particles according to the internal stress distribution of the electrode, thereby improving the overall electrochemical performance of lithium-ion batteries. Summary of the Invention
[0010] To address the shortcomings of existing technologies, the present invention aims to provide a gradient positive electrode sheet and a secondary battery, thereby solving the technical problem that under high actual density, the uneven stress on the active material particles in existing positive electrode sheets leads to the easy breakage of the bottom particles, which in turn affects the cycle life, rate performance, and energy density of the secondary battery.
[0011] To achieve this objective, the present invention employs the following technical solution: In a first aspect, the present invention provides a gradient positive electrode sheet, the gradient positive electrode sheet comprising a current collector, a first positive active material layer and a second positive active material layer, wherein the first positive active material layer is disposed on at least one surface of the current collector, and the second positive active material layer is disposed on the surface of the first positive active material layer away from the current collector. The first positive electrode active material layer comprises a first positive electrode active material, which includes single-crystal particles; the percentage of single-crystal particles is 80% to 100% based on the total number of particles in the first positive electrode active material. The second positive electrode active material layer contains a second positive electrode active material, which includes polycrystalline particles; the polycrystalline particles account for 80% to 100% of the total number of particles in the second positive electrode active material.
[0012] This invention successfully suppresses particle breakage under high compaction density by placing single-crystal particles with high strength and a specific percentage content in the first positive electrode active material layer (located at the bottom layer) near the current collector, resulting in a lower cycle expansion rate for the secondary battery compared to batteries made from pure polycrystalline particles. Simultaneously, the second positive electrode active material layer (located at the surface layer) containing a specific percentage of polycrystalline particles ensures excellent rate performance, thus overcoming the poor rate performance of batteries made from pure single-crystal particles. Furthermore, compared to the existing technology's physical mixing of single-crystal and polycrystalline particles, the gradient electrode structure design provided by this invention can more precisely protect the most vulnerable bottom layer region and improve the overall electrochemical performance of the secondary battery.
[0013] In this invention, the percentage content of the single crystal particles and the polycrystalline particles is independently 80% to 100%, for example, it can be 80%, 85%, 90%, 95% or 100%, etc., and is not limited to the listed values. Other unlisted values within this range are also applicable.
[0014] In this invention, the percentage content of each of the single crystal particles and the polycrystalline particles can be obtained by scanning electron microscopy.
[0015] More preferably, the percentage of single crystal particles is 90% to 100% based on the total number of particles of the first positive electrode active material. For example, it can be 90%, 92%, 95%, 98%, or 100%, etc., and is not limited to the listed values. Other unlisted values within this range are also applicable.
[0016] This invention significantly improves the particle integrity and structural stability of the positive electrode sheet under high real density by further controlling the percentage of single crystal particles in the first positive electrode active material layer, thereby improving the energy density and cycle stability of the secondary battery.
[0017] More preferably, the percentage of polycrystalline particles is 90% to 100% based on the total number of particles of the second positive electrode active material. For example, it can be 90%, 92%, 95%, 98%, or 100%, etc., and is not limited to the listed values. Other unlisted values within this range are also applicable.
[0018] This invention effectively suppresses the volume expansion of the high-voltage density cathode sheet by further controlling the percentage content of polycrystalline particles in the second positive electrode active material layer, thereby improving the rate performance of the secondary battery.
[0019] Preferably, the average compressive strength of the single crystal particle is σ1, in MPa. The value of σ1 is greater than or equal to 500 MPa, for example, it can be 500 MPa, 550 MPa, 600 MPa, 650 MPa, 700 MPa, 750 MPa or 800 MPa, etc., and is not limited to the listed values. Other unlisted values within this range are also applicable.
[0020] Preferably, the average compressive strength of the polycrystalline particles is σ2, in MPa. The value of σ2 is less than or equal to 400 MPa, for example, it can be 100 MPa, 150 MPa, 200 MPa, 250 MPa, 300 MPa, 350 MPa, 380 MPa or 400 MPa, etc., and is not limited to the listed values. Other unlisted values within this range are also applicable.
[0021] In this invention, the average compressive strength of the single crystal particles and the polycrystalline particles can be obtained by means of nanoindentation, for example, by means of measuring the average compressive strength of not less than 100 particles.
[0022] Since the first positive electrode active material layer (bottom layer) closest to the current collector bears the greatest compressive stress, it is configured with single-crystal particles with high compressive strength; while since the second positive electrode active material layer (surface layer) far from the current collector bears less compressive stress, it is configured with polycrystalline particles with lower compressive strength and high rate performance.
[0023] More preferably, the average compressive strength of the single crystal particle is σ1, in MPa, and the value of σ1 ranges from 600MPa to 800MPa. For example, it can be 600MPa, 650MPa, 700MPa, 750MPa or 800MPa, etc., and is not limited to the listed values. Other unlisted values within this range are also applicable.
[0024] More preferably, the average compressive strength of the polycrystalline particles is σ2, in MPa, and the value of σ2 ranges from 200MPa to 350MPa. For example, it can be 200MPa, 250MPa, 300MPa or 350MPa, etc., and is not limited to the listed values. Other unlisted values within this range are also applicable.
[0025] This invention designs a gradient electrode structure based on the micromechanical properties of particles by further controlling the average compressive strength of single-crystal particles and polycrystalline particles, thereby enabling the secondary battery to have high energy density, excellent cycle performance and high rate performance.
[0026] Preferably, the maximum compressive stress at the interface between the first positive electrode active material layer and the current collector is P, in MPa. The gradient positive electrode sheet satisfies the following relationship: K=σ1 / P, which is the ratio of the average compressive strength of the single crystal particles in the first positive electrode active material layer to the maximum compressive stress actually experienced by the bottom layer during the rolling of the positive electrode sheet. The value of K is greater than or equal to 1.2, for example, it can be 1.2, 1.25, 1.3, 1.5, 1.8, 2, 2.5 or 5, etc., and is not limited to the listed values. Other unlisted values within this range are also applicable.
[0027] In this invention, the maximum compressive stress P borne at the interface between the first positive electrode active material layer and the current collector can be obtained by finite element simulation, or by actual measurement using a pressure testing membrane when the positive electrode sheet is rolled under the target compaction density and thickness, which is the maximum compressive stress borne at the interface between the first positive electrode active material layer and the current collector.
[0028] In this invention, the maximum compressive stress at the interface between the first positive electrode active material layer and the current collector can be obtained, for example, by adjusting the rolling process parameters (e.g., roll gap, temperature, etc.).
[0029] This invention ensures that the active material particles in the first positive electrode active material layer near the current collector maintain structural integrity during the manufacturing process by adjusting the value range of K to a suitable range.
[0030] More preferably, the maximum compressive stress at the interface between the first positive electrode active material layer and the current collector is P, in MPa, and the gradient positive electrode sheet satisfies the following relationship: K=σ1 / P, where the value of K ranges from 1.2 to 2.0, for example, it can be 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9 or 2.0, etc., and is not limited to the listed values. Other unlisted values within this range are also applicable.
[0031] By further adjusting the value range of K to a suitable range, this invention can not only solve the problem of particle breakage under high density, but also enable the secondary battery to achieve the comprehensive electrochemical performance of "bottom high density (monocrystalline contribution) + surface high rate performance (polycrystalline contribution)".
[0032] Preferably, the ratio of the compaction density of the first positive electrode active material layer to the compaction density of the second positive electrode active material layer is 1:1 to 1.2:1, for example, it can be 1:1, 1.12:1, 1.15:1, 1.18:1 or 1.2:1, etc., and is not limited to the listed values. Other unlisted values within this range are also applicable, thereby comprehensively improving the energy density and electrochemical performance of the secondary battery.
[0033] Preferably, the ratio of the thickness of the first positive electrode active material layer to the thickness of the second positive electrode active material layer is 5:5 to 8:2, for example, it can be 5:5, 6:4, 7:3 or 8:2, etc., and is not limited to the listed values. Other unlisted values within this range are also applicable, thereby comprehensively improving the energy density and electrochemical performance of the secondary battery.
[0034] Preferably, the first positive electrode active material further includes polycrystalline particles.
[0035] Preferably, the percentage of polycrystalline particles is 0% to 20% based on the total number of particles of the first positive electrode active material. For example, it can be 0%, 5%, 10%, 15% or 20%, etc. It is not limited to the listed values. Other unlisted values within this range are also applicable, thereby further improving the rate performance of the secondary battery.
[0036] Preferably, the second positive electrode active material further includes single crystal particles.
[0037] Preferably, the percentage of single crystal particles is 0% to 20% based on the total number of particles of the second positive electrode active material. For example, it can be 0%, 5%, 10%, 15% or 20%, etc. It is not limited to the listed values. Other unlisted values within this range are also applicable, thereby comprehensively improving the energy density of the secondary battery.
[0038] Preferably, the compaction density of the gradient positive electrode sheet is 3.5 g / cm³. 3 ~3.7g / cm 3 For example, it can be 3.5g / cm³. 3 3.55g / cm 3 3.6g / cm 3 3.65g / cm 3 Or 3.7g / cm 3 The term "etc." applies not only to the listed values, but also to other unlisted values within the range. Preferably, the areal density of the gradient positive electrode sheet on one side is 200 g / m². 2 ~300g / m 2 For example, it can be 200g / m 2 220g / m 2 250g / m 2 280g / m 2 Or 300g / m 2 The term "etc." is not limited to the listed values; it also applies to other unlisted values within the range.
[0039] In this invention, the first positive electrode active material and the second positive electrode active material are each independently selected from ternary positive electrode materials, preferably high-nickel ternary positive electrode materials, thereby further improving the energy density of the secondary battery.
[0040] In a second aspect, the present invention provides a method for preparing a gradient positive electrode sheet as described in the first aspect, the method comprising the following steps: The first positive electrode active material containing single-crystal particles, a first conductive agent, a first binder, and a first solvent are mixed to obtain a first positive electrode slurry, wherein the percentage of single-crystal particles is 80% to 100% based on the total number of particles in the first positive electrode active material. The second mixing of the second positive electrode active material containing polycrystalline particles, the second conductive agent, the second binder, and the second solvent yields a second positive electrode slurry, wherein the percentage of polycrystalline particles, based on the total number of particles in the second positive electrode active material, is 80% to 100%. The first positive electrode slurry and the second positive electrode slurry are coated on at least one side of the current collector to form a first positive electrode active material layer and a second positive electrode active material layer, thereby obtaining the gradient positive electrode sheet, wherein the first positive electrode active material layer is located between the current collector and the second positive electrode active material layer.
[0041] Thirdly, the present invention provides a secondary battery, the secondary battery comprising a positive electrode, a negative electrode and an electrolyte, wherein the positive electrode comprises a gradient positive electrode as described in the first aspect.
[0042] In this invention, the electrolyte includes, but is not limited to, liquid electrolyte, gel electrolyte and solid electrolyte.
[0043] The numerical range described in this invention includes not only the point values listed above, but also any point values within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values included in the range.
[0044] Compared with the prior art, the present invention has the following beneficial effects: This invention provides a gradient positive electrode sheet. By placing single-crystal particles with high strength and a specific percentage near the current collector in a first positive electrode active material layer (located at the bottom layer), particle breakage under high compaction density is successfully suppressed, resulting in a lower cycle expansion rate for the secondary battery compared to batteries made from pure polycrystalline particles. Simultaneously, a second positive electrode active material layer (located at the surface layer) containing a specific percentage of polycrystalline particles ensures excellent rate performance, thus overcoming the poor rate performance of batteries made from pure single-crystal particles. Furthermore, compared to the existing technology employing a physical mixture of single-crystal and polycrystalline particles, the gradient electrode structure design provided by this invention can more precisely protect the most vulnerable bottom layer region, which is beneficial for improving the overall electrochemical performance of the secondary battery.
[0045] Furthermore, the present invention further optimizes the average compressive strength of single-crystal particles, the average compressive strength of polycrystalline particles, or the gradient positive electrode sheet to satisfy a specific relationship, which has the following advantages: (1) Significantly improves particle integrity under high compaction density: After high-pressure rolling, the single-crystal particle structure in the first positive electrode active material layer near the current collector remains intact and free of microcracks, and the overall conductive network of the positive electrode sheet is more stable; (2) Effectively suppresses positive electrode sheet expansion: The polycrystalline particles on the surface absorb part of the volume change stress through grain boundary slip, avoiding the warping of the positive electrode sheet caused by stress concentration; (3) The secondary battery takes into account both high energy density and high rate performance: The single-crystal particles at the bottom layer ensure high compaction density (≥3.5g / cm). 3 High capacity, and the large specific surface area of the polycrystalline particles on the surface provides excellent rate performance (discharge capacity retention >90% at 3C rate). Detailed Implementation
[0046] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention.
[0047] Example 1 This embodiment provides a gradient positive electrode sheet, which includes an aluminum foil current collector, a first positive active material layer, and a second positive active material layer. The first positive active material layer is disposed on one surface of the aluminum foil current collector, and the second positive active material layer is disposed on the surface of the first positive active material layer away from the aluminum foil current collector.
[0048] The first positive electrode active material layer comprises a first positive electrode active material, SuperP conductive agent, and polyvinylidene fluoride (PVDF) binder in a mass ratio of 96.5:2:1.5. The first positive electrode active material is single-crystal NCM622 particles, and the D of the single-crystal NCM622 particles... v The particle size was 4 μm, and the average compressive strength σ1 was obtained by nanoindentation method.
[0049] The second positive electrode active material layer comprises a second positive electrode active material with a mass ratio of 96.5:2:1.5, a Super P conductive agent, and a polyvinylidene fluoride (PVDF) binder. The second positive electrode active material is polycrystalline NCM622 particles, and the D of the polycrystalline NCM622 particles... v The particle size was 10 μm, and the average compressive strength σ2 was obtained by nanoindentation method.
[0050] The ratio of the compaction density of the first positive electrode active material layer to the compaction density of the second positive electrode active material layer is 1.12:1, and the ratio of the thickness of the first positive electrode active material layer to the thickness of the second positive electrode active material layer is 7:3. Finite element simulation shows that the maximum compressive stress P at the interface between the first positive electrode active material layer and the aluminum foil current collector is 450MPa. The above gradient positive electrode sheet satisfies the following relationship: K=σ1 / P=620 / 450≈1.38>1.2.
[0051] This embodiment also provides a method for preparing the above-mentioned gradient positive electrode sheet, the method comprising the following steps: The first positive electrode slurry was obtained by mixing single-crystal NCM622 particles, Super P conductive agent, PVDF binder and N-methylpyrrolidone solvent according to the formula.
[0052] The second positive electrode slurry is obtained by mixing polycrystalline NCM622 particles, Super P conductive agent, PVDF binder and N-methylpyrrolidone solvent according to the formula.
[0053] The first and second positive electrode slurries were simultaneously coated onto an aluminum foil current collector using a double-layer coating machine. After drying, they were rolled at the target compaction density and thickness to form the first and second positive electrode active material layers, resulting in a single-sided surface density of 250 g / m². 2 The target compaction density is 3.65 g / cm³. 3 The gradient positive electrode.
[0054] Example 2 This embodiment provides a gradient positive electrode sheet, which includes an aluminum foil current collector, a first positive active material layer, and a second positive active material layer. The first positive active material layer is disposed on one surface of the aluminum foil current collector, and the second positive active material layer is disposed on the surface of the first positive active material layer away from the aluminum foil current collector.
[0055] The first positive electrode active material layer comprises a first positive electrode active material, SuperP conductive agent, and polyvinylidene fluoride (PVDF) binder in a mass ratio of 96.5:2:1.5. The first positive electrode active material is single-crystal NCM622 particles, and the D of the single-crystal NCM622 particles... v The particle size was 5.5 μm, and the average compressive strength σ1 was obtained by nanoindentation method.
[0056] The second positive electrode active material layer comprises a second positive electrode active material with a mass ratio of 96.5:2:1.5, a Super P conductive agent, and a polyvinylidene fluoride (PVDF) binder. The second positive electrode active material is polycrystalline NCM622 particles, and the D of the polycrystalline NCM622 particles... v The particle size was 13 μm, and the average compressive strength σ2 was obtained by nanoindentation method.
[0057] The ratio of the compaction density of the first positive electrode active material layer to the compaction density of the second positive electrode active material layer is 1:1, and the ratio of the thickness of the first positive electrode active material layer to the thickness of the second positive electrode active material layer is 5:5. Finite element simulation shows that the maximum compressive stress P at the interface between the first positive electrode active material layer and the aluminum foil current collector is 400MPa. The above gradient positive electrode sheet satisfies the following relationship: K=σ1 / P=500 / 400=1.25>1.2.
[0058] This embodiment also provides a method for preparing the above-mentioned gradient positive electrode sheet, the method comprising the following steps: The first positive electrode slurry was obtained by mixing single-crystal NCM622 particles, Super P conductive agent, PVDF binder and N-methylpyrrolidone solvent according to the formula.
[0059] The second positive electrode slurry is obtained by mixing polycrystalline NCM622 particles, Super P conductive agent, PVDF binder and N-methylpyrrolidone solvent according to the formula.
[0060] The first and second positive electrode slurries were simultaneously coated onto an aluminum foil current collector using a double-layer coating machine. After drying, they were rolled at the target compaction density and thickness to form the first and second positive electrode active material layers, resulting in a single-sided surface density of 200 g / m². 2 The target compaction density is 3.55 g / cm³. 3 The gradient positive electrode.
[0061] Example 3 This embodiment provides a gradient positive electrode sheet, which includes an aluminum foil current collector, a first positive active material layer, and a second positive active material layer. The first positive active material layer is disposed on one surface of the aluminum foil current collector, and the second positive active material layer is disposed on the surface of the first positive active material layer away from the aluminum foil current collector.
[0062] The first positive electrode active material layer comprises a first positive electrode active material, SuperP conductive agent, and polyvinylidene fluoride (PVDF) binder in a mass ratio of 96.5:2:1.5. The first positive electrode active material is single-crystal NCM622 particles, and the D of the single-crystal NCM622 particles... v The particle size was 2.5 μm, and the average compressive strength σ1 was obtained by nanoindentation method.
[0063] The second positive electrode active material layer comprises a second positive electrode active material with a mass ratio of 96.5:2:1.5, a Super P conductive agent, and a polyvinylidene fluoride (PVDF) binder. The second positive electrode active material is polycrystalline NCM622 particles, and the D of the polycrystalline NCM622 particles... v The particle size was 7 μm, and the average compressive strength σ2 was obtained by nanoindentation method.
[0064] The ratio of the compaction density of the first positive electrode active material layer to the compaction density of the second positive electrode active material layer is 1.2:1, and the ratio of the thickness of the first positive electrode active material layer to the thickness of the second positive electrode active material layer is 8:2. Finite element simulation shows that the maximum compressive stress P at the interface between the first positive electrode active material layer and the aluminum foil current collector is 470MPa. The above gradient positive electrode sheet satisfies the following relationship: K=σ1 / P=800 / 470=1.7>1.2.
[0065] This embodiment also provides a method for preparing the above-mentioned gradient positive electrode sheet, the method comprising the following steps: The first positive electrode slurry was obtained by mixing single-crystal NCM622 particles, Super P conductive agent, PVDF binder and N-methylpyrrolidone solvent according to the formula.
[0066] The second positive electrode slurry is obtained by mixing polycrystalline NCM622 particles, Super P conductive agent, PVDF binder and N-methylpyrrolidone solvent according to the formula.
[0067] The first and second positive electrode slurries were simultaneously coated onto an aluminum foil current collector using a double-layer coating machine. After drying, they were rolled at the target compaction density and thickness to form the first and second positive electrode active material layers, resulting in a single-sided surface density of 300 g / m². 2 The target compaction density is 3.70 g / cm³. 3 The gradient positive electrode.
[0068] Example 4 This embodiment provides a gradient positive electrode sheet, which includes an aluminum foil current collector, a first positive active material layer, and a second positive active material layer. The first positive active material layer is disposed on one surface of the aluminum foil current collector, and the second positive active material layer is disposed on the surface of the first positive active material layer away from the aluminum foil current collector.
[0069] The first positive electrode active material layer comprises a first positive electrode active material, SuperP conductive agent, and polyvinylidene fluoride (PVDF) binder in a mass ratio of 96.5:2:1.5. The first positive electrode active material includes monocrystalline NCM622 particles and polycrystalline NCM622 particles. The monocrystalline NCM622 particles have a D... v 50 particles with a diameter of 4 μm, after nanoindentation testing, yielded an average compressive strength σ1 = 620 MPa. The D... v 50 Particle size = 10μm. Based on the total number of particles of the first positive electrode active material, the percentage of single-crystal NCM622 particles is 90%, and the percentage of polycrystalline NCM622 particles is 10%.
[0070] The second positive electrode active material layer comprises a second positive electrode active material with a mass ratio of 96.5:2:1.5, a Super P conductive agent, and a polyvinylidene fluoride (PVDF) binder. The second positive electrode active material includes polycrystalline NCM622 particles and monocrystalline NCM622 particles, with the polycrystalline NCM622 particles having a D... v 50 particles with a diameter of 10 μm, after nanoindentation testing, yielded an average compressive strength σ² = 280 MPa. The D50 of the single-crystal NCM622 particles... v50 Particle size = 4μm. In terms of the total number of particles in the second positive electrode active material, the percentage of polycrystalline NCM622 particles is 90%, and the percentage of monocrystalline NCM622 particles is 10%.
[0071] The ratio of the compaction density of the first positive electrode active material layer to the compaction density of the second positive electrode active material layer is 1.12:1, and the ratio of the thickness of the first positive electrode active material layer to the thickness of the second positive electrode active material layer is 7:3. Finite element simulation shows that the maximum compressive stress P at the interface between the first positive electrode active material layer and the aluminum foil current collector is 430 MPa. The above gradient positive electrode sheet satisfies the following relationship: K=σ1 / P=620 / 430=1.4>1.2.
[0072] This embodiment also provides a method for preparing the above-mentioned gradient positive electrode sheet, the method comprising the following steps: The first positive electrode slurry is obtained by mixing single-crystal NCM622 particles, polycrystalline NCM622 particles, Super P conductive agent, PVDF binder and N-methylpyrrolidone solvent according to the formula.
[0073] The second positive electrode slurry is obtained by mixing polycrystalline NCM622 particles, single-crystal NCM622 particles, Super P conductive agent, PVDF binder and N-methylpyrrolidone solvent according to the formula.
[0074] The first and second positive electrode slurries were simultaneously coated onto an aluminum foil current collector using a double-layer coating machine. After drying, they were rolled at the target compaction density and thickness to form the first and second positive electrode active material layers, resulting in a single-sided surface density of 250 g / m². 2 The target compaction density is 3.65 g / cm³. 3 The gradient positive electrode.
[0075] Example 5 The difference between this embodiment and Embodiment 1 is that, in the first positive electrode active material layer, the first positive electrode active material is single-crystal NCM622 particles, and the D of the single-crystal NCM622 particles... v The particle size was 7.5 μm, and the average compressive strength σ1 was 450 MPa after testing with nanoindentation method. All other parameters were the same as in Example 1.
[0076] Example 6 The difference between this embodiment and Embodiment 1 is that, in the first positive electrode active material layer, the first positive electrode active material is single-crystal NCM622 particles, and the D of the single-crystal NCM622 particles... vThe particle size was 1.5 μm, and the average compressive strength σ1 was 1000 MPa after testing with nanoindentation method. All other parameters were the same as in Example 1.
[0077] Example 7 The difference between this embodiment and Embodiment 1 is that, in the second positive electrode active material layer, the second positive electrode active material is polycrystalline NCM622 particles, and the D of the polycrystalline NCM622 particles... v The particle size was 5.5 μm, and the average compressive strength σ2 was 450 MPa after testing with nanoindentation method. All other parameters were the same as in Example 1.
[0078] Example 8 The difference between this embodiment and Embodiment 1 is that, in the second positive electrode active material layer, the second positive electrode active material is polycrystalline NCM622 particles, and the D of the polycrystalline NCM622 particles... v The particle size was 19 μm, and the average compressive strength σ2 was 80 MPa after testing with nanoindentation method. All other parameters were the same as in Example 1.
[0079] Example 9 The difference between this embodiment and Embodiment 1 is that, by adjusting the roll gap in the rolling process, the ratio of the compaction density of the first positive electrode active material layer to the compaction density of the second positive electrode active material layer is 2:1, and the ratio of the thickness of the first positive electrode active material layer to the thickness of the second positive electrode active material layer is 7:3. Finite element simulation shows that the maximum compressive stress P at the interface between the first positive electrode active material layer and the aluminum foil current collector is 620MPa. The above-mentioned gradient positive electrode sheet satisfies the following relationship: K=σ1 / P=620 / 620=1, and all other aspects are the same as in Embodiment 1.
[0080] Example 10 The difference between this embodiment and Embodiment 1 is that, by adjusting the roll gap in the rolling process, the ratio of the thickness of the first positive electrode active material layer to the thickness of the second positive electrode active material layer is 7:3. Finite element simulation shows that the maximum compressive stress P at the interface between the first positive electrode active material layer and the aluminum foil current collector is 1240MPa. The above-mentioned gradient positive electrode sheet satisfies the following relationship: K=σ1 / P=620 / 1240=0.5. All other aspects are the same as in Embodiment 1.
[0081] Comparative Example 1 This comparative example provides a positive electrode sheet, which includes an aluminum foil current collector and a positive active material layer disposed on one surface of the aluminum foil current collector.
[0082] The positive electrode active material layer comprises positive electrode active material, Super P conductive agent, and polyvinylidene fluoride (PVDF) binder in a mass ratio of 96.5:2:1.5. The positive electrode active material is single-crystal NCM622 particles, and the D of the single-crystal NCM622 particles... v The particle size was 4 μm, and the average compressive strength σ1 was obtained by nanoindentation method.
[0083] This embodiment also provides a method for preparing the above-mentioned positive electrode sheet, the method comprising the following steps: The positive electrode slurry is obtained by mixing single-crystal NCM622 particles, Super P conductive agent, PVDF binder and N-methylpyrrolidone solvent according to the formula.
[0084] The above-mentioned positive electrode slurry was coated onto an aluminum foil current collector, dried, and then rolled at the target compaction density to form a positive electrode active material layer, resulting in a target compaction density of 3.65 g / cm³. 3 The positive electrode plate mentioned above.
[0085] Comparative Example 2 This comparative example provides a positive electrode sheet, which includes an aluminum foil current collector and a positive active material layer disposed on one surface of the aluminum foil current collector.
[0086] The positive electrode active material layer comprises positive electrode active material, Super P conductive agent, and polyvinylidene fluoride (PVDF) binder in a mass ratio of 96.5:2:1.5. The positive electrode active material is polycrystalline NCM622 particles, and the D of the polycrystalline NCM622 particles... v The particle size was 10 μm, and the average compressive strength σ1 was obtained by nanoindentation method.
[0087] This embodiment also provides a method for preparing the above-mentioned positive electrode sheet, the method comprising the following steps: The positive electrode slurry is obtained by mixing polycrystalline NCM622 particles, Super P conductive agent, PVDF binder and N-methylpyrrolidone solvent according to the formula.
[0088] The above-mentioned positive electrode slurry was coated onto an aluminum foil current collector, dried, and then rolled at the target compaction density to form a positive electrode active material layer, resulting in a target compaction density of 3.65 g / cm³. 3 The positive electrode plate mentioned above.
[0089] Comparative Example 3 This comparative example provides a positive electrode sheet, which includes an aluminum foil current collector and a positive active material layer disposed on one surface of the aluminum foil current collector.
[0090] The positive electrode active material layer comprises positive electrode active material, Super P conductive agent, and polyvinylidene fluoride (PVDF) binder in a mass ratio of 96.5:2:1.5. The positive electrode active material includes single-crystal NCM622 particles and polycrystalline NCM622 particles in a mass ratio of 7:3. The single-crystal NCM622 particles have a D... v 50 particles with a diameter of 4 μm, after nanoindentation testing, yielded an average compressive strength σ1 = 620 MPa; the D of polycrystalline NCM622 particles... v The particle size was 10 μm, and the average compressive strength σ1 was obtained by nanoindentation method.
[0091] This embodiment also provides a method for preparing the above-mentioned positive electrode sheet, the method comprising the following steps: The positive electrode slurry is obtained by mixing single-crystal NCM622 particles, polycrystalline NCM622 particles, Super P conductive agent, PVDF binder and N-methylpyrrolidone solvent according to the formula.
[0092] The above-mentioned positive electrode slurry was coated onto an aluminum foil current collector, dried, and then rolled at the target compaction density to form a positive electrode active material layer, resulting in a target compaction density of 3.65 g / cm³. 3 The positive electrode plate mentioned above.
[0093] Test conditions The positive electrode sheets provided in the above embodiments and comparative examples were subjected to performance tests under the following conditions: Particle integrity after rolling: The cross-section of the electrode was observed by SEM, and the proportion of cracks in the particles in the bottom layer region (within 20μm of the current collector) was statistically analyzed.
[0094] The positive electrode sheets provided in the above embodiments and comparative examples were used to prepare lithium-ion batteries, and then performance tests were conducted, as detailed below: Preparation of negative electrode sheet: Artificial graphite, Super P conductive agent, styrene-butadiene rubber binder and sodium carboxymethyl cellulose thickener are mixed in a mass ratio of 90:3:5:2, deionized water is added and stirred thoroughly to obtain negative electrode slurry. Then, the negative electrode slurry is coated onto copper foil, dried and rolled to obtain negative electrode sheet.
[0095] Preparation of lithium-ion batteries: The positive electrode, separator, negative electrode, and electrolyte provided in the above examples and comparative examples are assembled to obtain a soft-pack full battery. The electrolyte is prepared by dissolving lithium hexafluorophosphate (LiPF6) in a mixed solvent, which is prepared by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 1:1.
[0096] Electrochemical performance testing: The prepared lithium-ion batteries were subjected to electrochemical performance testing at a test voltage of 3.0V~4.3V. The capacity retention rate and battery expansion rate of the lithium-ion batteries after 200 charge-discharge cycles at 45℃ and 1C rate were tested, as well as the discharge capacity retention rate at 25℃ and 3C rate.
[0097] The test results are shown in Table 1.
[0098] Table 1 As shown in Table 1, compared with Comparative Examples 1 to 3, the gradient positive electrode sheets provided in Examples 1 to 4 of this invention successfully suppress particle breakage under high compaction by placing single-crystal particles with high strength and a specific percentage content in the first positive electrode active material layer (located at the bottom layer) near the current collector. This also results in a lower cycle expansion rate for lithium-ion batteries compared to batteries made from pure polycrystalline particles. Simultaneously, the second positive electrode active material layer (located at the surface layer) containing a specific percentage content of polycrystalline particles ensures excellent rate performance, thereby overcoming the poor rate performance of batteries made from pure single-crystal particles.
[0099] Comparing Examples 1, 5 to 8, it can be seen that the present invention enables lithium-ion batteries to have high energy density, excellent high-temperature cycle performance and high rate performance by adjusting the average compressive strength of single crystal particles and polycrystalline particles.
[0100] Comparing Examples 1, 9 and 10, it can be seen that the present invention, by further adjusting the value range of K to a suitable range, ensures that the active material particles in the first positive electrode active material layer near the current collector maintain structural integrity during the manufacturing process.
[0101] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.
Claims
1. A gradient positive electrode, characterized in that, The gradient positive electrode includes a current collector, a first positive active material layer, and a second positive active material layer. The first positive active material layer is disposed on at least one surface of the current collector, and the second positive active material layer is disposed on the surface of the first positive active material layer away from the current collector. The first positive electrode active material layer comprises a first positive electrode active material, which includes single-crystal particles; the percentage of single-crystal particles is 80% to 100% based on the total number of particles in the first positive electrode active material. The second positive electrode active material layer contains a second positive electrode active material, which includes polycrystalline particles; the polycrystalline particles account for 80% to 100% of the total number of particles in the second positive electrode active material.
2. The gradient positive electrode sheet according to claim 1, characterized in that, Based on the total number of particles of the first positive electrode active material, the percentage of single crystal particles is 90% to 100%. And / or, based on the total number of particles of the second positive electrode active material, the percentage of the number of polycrystalline particles is 90% to 100%.
3. The gradient positive electrode sheet according to claim 1 or 2, characterized in that, The average compressive strength of the single crystal particle is σ1, in MPa, and the value of σ1 is greater than or equal to 500 MPa. And / or, the average compressive strength of the polycrystalline particles is σ2, in MPa, and the value of σ2 is less than or equal to 400 MPa.
4. The gradient positive electrode sheet according to claim 1 or 2, characterized in that, The average compressive strength of the single crystal particles is σ1, in MPa, and the value of σ1 ranges from 600 MPa to 800 MPa. And / or, the average compressive strength of the polycrystalline particles is σ2, in MPa, and the value of σ2 ranges from 200MPa to 350MPa.
5. The gradient positive electrode according to claim 3, characterized in that, The maximum compressive stress at the interface between the first positive electrode active material layer and the current collector is P, in MPa. The gradient positive electrode sheet satisfies the following relationship: K=σ1 / P, where the value of K is greater than or equal to 1.
2.
6. The gradient positive electrode according to claim 3, characterized in that, The maximum compressive stress at the interface between the first positive electrode active material layer and the current collector is P, in MPa. The gradient positive electrode sheet satisfies the following relationship: K=σ1 / P, where the value of K ranges from 1.2 to 2.
0.
7. The gradient positive electrode according to claim 5, characterized in that, The ratio of the compaction density of the first positive electrode active material layer to the compaction density of the second positive electrode active material layer is 1:1 to 1.2:1; And / or, the ratio of the thickness of the first positive electrode active material layer to the thickness of the second positive electrode active material layer is 5:5 to 8:
2.
8. The gradient positive electrode sheet according to claim 1, characterized in that, The first positive electrode active material also includes polycrystalline particles; And / or, based on the total number of particles of the first positive electrode active material, the percentage of the polycrystalline particles is 0% to 20%; And / or, the second positive electrode active material further includes single crystal particles; And / or, based on the total number of particles of the second positive electrode active material, the percentage of the number of single crystal particles is 0% to 20%.
9. The gradient positive electrode sheet according to claim 1, characterized in that, The compaction density of the gradient positive electrode sheet is 3.5 g / cm³. 3 ~3.7g / cm 3 ; And / or, the areal density of the gradient positive electrode sheet on one side is 200 g / m². 2 ~300g / m 2 .
10. A secondary battery, characterized in that, The secondary battery includes a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode includes a gradient positive electrode according to any one of claims 1 to 9.