Negative electrode sheet and secondary battery

Abstract

The application provides a negative electrode sheet and a secondary battery thereof, comprising a negative electrode current collector and a negative electrode active material layer arranged on at least one side surface of the current collector; the negative electrode active material layer comprises a negative electrode active material, the negative electrode active material comprises at least one of a carbon material, a silicon-carbon material and a silicon material; the negative electrode active material is a low-hardness particle or a high-hardness particle, the low-hardness particle satisfies (T-Z) / G≤0.4, and the high-hardness particle satisfies (T-Z) / G>0.4; the compaction density P of the negative electrode active material layer satisfies the following relationship: 0.9g / cm 3 ≤T≤2.0g / cm 3 , 0.8g / cm 3 ≤P≤2.0g / cm 3 , 0.8g / cm 3 ≤Z≤1.2g / cm 3 , 1.4g / cm 3 ≤G≤2.4g / cm 3 ; the negative electrode sheet provided by the application has good electron and ion migration channels, low electrode impedance, improved cycle performance and rate performance of the battery, and improved cycle stability of the battery.

Description

Technical Field

[0001] This invention belongs to the field of battery technology, specifically relating to a negative electrode sheet and its secondary battery. Background Technology

[0002] As a component of an electrochemical device, the electrode consists of a current collector and an active material layer. The active material layer is composed of particles such as active material powder, conductive agent, and binder. The particle size, specific surface area, and surface morphology of the active material affect its distribution in the active material layer of the electrode.

[0003] Existing negative electrode sheets, including negative electrode active materials, consist of both low-hardness and high-hardness particles. The compaction density design of existing negative electrode sheets is generally based on experience, without considering whether the overall negative electrode active material is of low or high hardness. This leads to the following problems: 1) If the overall negative electrode active material is of high hardness, excessively low compaction results in low pressure, leading to high electrode porosity and reduced battery rate performance. Excessively high compaction, with its corresponding high pressure, can cause particle cracking, affecting battery cycle performance and rate performance. 2) If the overall negative electrode active material is of low hardness, excessively high compaction, with its corresponding high pressure, causes the low-hardness particles to deform and adhere, resulting in excessively low electrode porosity and reduced rate performance. Summary of the Invention

[0004] To address the problem that existing negative electrode sheets, which do not consider whether the overall hardness of the negative electrode active material is low or high, result in excessively high or low compaction density, leading to poor cycle performance and reduced rate performance of the battery, this application provides a negative electrode sheet and its secondary battery.

[0005] To solve the above-mentioned technical problems, this application provides a negative electrode sheet, including a negative electrode current collector and a negative electrode active material layer, wherein the negative electrode active material layer is disposed on at least one side surface of the current collector;

[0006] The negative electrode active material layer includes a negative electrode active material, which includes at least one of carbon material, silicon-carbon material, and silicon material; the negative electrode active material is low-hardness particles or high-hardness particles, wherein the low-hardness particles satisfy (TZ) / G≤0.4, and the high-hardness particles satisfy (TZ) / G>0.4;

[0007] The compaction density P of the negative electrode active material layer satisfies the following relationship:

[0008] ≤ ≤ ; and 0.9g / cm 3 ≤T≤2.0g / cm3 0.8g / cm 3 ≤Z≤1.2g / cm 3 1.4g / cm 3 ≤G≤2.4g / cm 3 ;

[0009] Where T is the compaction density of the negative electrode active material under 5T conditions, in g / cm³. 3 ;

[0010] Z represents the tap density of the negative electrode active material, in g / cm³. 3 ;

[0011] G represents the skeleton density of the negative electrode active material, in g / cm³. 3 ;

[0012] P is the compaction density of the negative electrode active material layer, in g / cm³. 3 .

[0013] Preferably, the compaction density T of the negative electrode active material under 5T conditions is 1.8 g / cm³. 3 ≤T≤2.0g / cm 3 .

[0014] Preferably, the tap density Z of the negative electrode active material is 0.9 g / cm³. 3 ≤Z≤1.2g / cm 3 .

[0015] Preferably, the framework density G of the negative electrode active material is 1.8 g / cm³. 3 ≤G≤2.2g / cm 3 .

[0016] Preferably, the Dv90, Dv50, and Dv10 of the negative electrode active material satisfy the following relationship:

[0017] F=(Dv90-Dv10) / Dv50, 0.7≤F≤1.5.

[0018] Preferably, the Dv90, Dv50, and Dv10 of the negative electrode active material satisfy the following relationship:

[0019] L=(Dv90-Dv50) / (Dv50-Dv10),1.4≤L≤1.8.

[0020] Preferably, the sphericity S of the negative electrode active material particles is ≥0.4; S is the minimum inscribed circle diameter / minimum circumscribed circle diameter of the negative electrode active material particles.

[0021] Preferably, the Dn10 of the negative electrode active material is greater than 1 μm;

[0022] Alternatively, the specific surface area B of the negative electrode active material is 0.7 m². 2 / g≤B≤2.5m 2 / g.

[0023] Preferably, the porosity of the negative electrode sheet is 20%-40%;

[0024] In the negative electrode active material layer, the mass content of the negative electrode active material is 90%~100%.

[0025] Secondly, this application provides a secondary battery, including a positive electrode, a separator, and the aforementioned negative electrode.

[0026] The negative electrode sheet provided in this application uses a framework density representing the theoretical limit of particle packing, a tap density reflecting the packing efficiency of particles, and a compaction density representing the densification ability of the negative electrode active material under pressure, reflecting the maximum density that the electrode sheet can achieve. The value of TZ represents the change in pressure of the negative electrode active material; a larger TZ value indicates a greater change in pressure. The value of GZ represents the packing ability of the negative electrode active material; a larger GZ value indicates a weaker packing ability. The compaction density T, tap density Z, framework density G, and compaction density P of the negative electrode active material satisfy the following relationship: ≤ ≤ And 0.9g / cm 3 ≤T≤2.0g / cm 3 0.8g / cm 3 ≤Z≤1.2g / cm 3 1.4g / cm 3 ≤G≤2.4g / cm 3 The compaction density of the negative electrode sheet is obtained based on the skeleton density, compaction density, and vibration density of the negative electrode active material. Considering whether the overall negative electrode active material is a high-hardness or low-hardness particle, the pressure used on the negative electrode sheet is moderate to avoid problems such as cracking of high-hardness particles or deformation and adhesion of low-hardness particles due to excessive pressure, which would reduce the rate performance of the battery. The resulting negative electrode sheet has good electron and ion migration channels, low electrode impedance, high cycle capacity retention rate, and low thickness expansion rate, thereby improving the cycle performance and rate performance of the battery and ensuring high energy density. Detailed Implementation

[0027] To make the technical problems solved, the technical solutions, and the beneficial effects of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0028] In a first aspect, this application provides a negative electrode sheet, including a negative electrode current collector and a negative electrode active material layer, wherein the negative electrode active material layer is disposed on at least one surface of the current collector;

[0029] The negative electrode active material layer includes a negative electrode active material, which includes at least one of carbon materials and silicon-carbon materials; the negative electrode active material is low-hardness particles or high-hardness particles, wherein the low-hardness particles satisfy (TZ) / G≤0.4, and the high-hardness particles satisfy (TZ) / G>0.4;

[0030] The compaction density P of the negative electrode active material layer satisfies the following relationship:

[0031] ≤ ≤ ; and 0.9g / cm 3 ≤T≤2.0g / cm 3 0.8g / cm 3 ≤Z≤1.2g / cm 3 1.4g / cm 3 ≤G≤2.4g / cm 3 ;

[0032] Where T is the compaction density of the negative electrode active material under 5T conditions, in g / cm³. 3 ;

[0033] Z represents the tap density of the negative electrode active material, in g / cm³. 3 ;

[0034] G represents the skeleton density of the negative electrode active material, in g / cm³. 3 ;

[0035] P is the compaction density of the negative electrode active material layer, in g / cm³. 3 .

[0036] Specifically, based on the overall compaction density, vibration density, and framework density of the negative electrode active material, the material can be classified as either low-hardness or high-hardness particles. Low-hardness particles are defined as those whose compaction density T is measured under 5T conditions; if (TZ) / G ≤ 0.4, then these particles are considered low-hardness particles. High-hardness particles are defined as those whose compaction density T is measured under 5T conditions; if (TZ) / G > 0.4, then these particles are considered high-hardness particles.

[0037] The negative electrode sheet provided in this application uses a framework density representing the theoretical limit of particle packing, a tap density reflecting the packing efficiency of particles, and a compaction density representing the densification ability of the negative electrode active material under pressure, reflecting the maximum density that the electrode sheet can achieve. The value of TZ represents the change in pressure of the negative electrode active material; a larger TZ value indicates a greater change in pressure. The value of GZ represents the packing ability of the negative electrode active material; a larger GZ value indicates a weaker packing ability. The compaction density T, tap density Z, framework density G, and compaction density P of the negative electrode active material satisfy the following relationship: ≤ ≤ And 0.9g / cm 3 ≤T≤2.0g / cm 3 0.8g / cm 3 ≤Z≤1.2g / cm 3 1.4g / cm 3 ≤G≤2.4g / cm 3 The compaction density of the negative electrode sheet is obtained based on the skeleton density, compaction density, and vibration density of the negative electrode active material. Considering whether the overall negative electrode active material is a high-hardness or low-hardness particle, the pressure used on the negative electrode sheet is moderate to avoid problems such as cracking of high-hardness particles or deformation and adhesion of low-hardness particles due to excessive pressure, which would reduce the rate performance of the battery. The resulting negative electrode sheet has good electron and ion migration channels, low electrode impedance, high cycle capacity retention rate, and low thickness expansion rate, thereby improving the cycle performance and rate performance of the battery and ensuring high energy density.

[0038] In a specific embodiment, the compaction density T of the negative electrode active material can be 0.9 g / cm³. 3 0.95g / cm 3 1.0g / cm 3 1.2g / cm 3 1.3g / cm 3 1.5g / cm 3 1.8g / cm 3 2.0g / cm 3 Or within the range of either of the above.

[0039] In some preferred embodiments, the compaction density T of the negative electrode active material under 5T conditions is 0.9 g / cm³. 3 ≤T≤2.0g / cm 3 .

[0040] Specifically, the compaction density T of the negative electrode active material is determined by its crystal structure, particle morphology, and mechanical properties. The range varies significantly among different systems, directly affecting the electrode compaction density. When the compaction density T of the negative electrode active material meets the above range, it reflects the maximum density that the electrode can achieve. The resulting electrode has good compaction density and ion / electron migration channels, giving the negative electrode good ion / electron migration ability and improving battery cycle stability.

[0041] It should be noted that the compaction density T of the above-mentioned negative electrode active material under 5T conditions refers to the compaction density of the negative electrode active material powder obtained by testing under 5 tons of pressure.

[0042] If the compaction density T of the negative electrode active material is too low, the contact resistance between particles is high, the electrode sheet is loose, and the particles are prone to relative displacement during cycling, resulting in reduced battery cycle performance. If the electrode porosity is too high, the electrolyte is prone to volatilization, affecting cycle performance and rate performance. If the compaction density T of the negative electrode active material is too high, high-hardness materials will experience interlayer delamination, and low-hardness materials will experience particle adhesion. Stress concentration will occur inside the electrode sheet, making it prone to cracking during cycling and reducing battery cycle performance.

[0043] In a specific embodiment, the tap density Z of the negative electrode active material can be 0.8 g / cm³. 3 0.85g / cm 3 0.9g / cm 3 0.95g / cm 3 1.0g / cm 3 1.05g / cm 3 1.1g / cm 3 1.15g / cm 3 1.2g / cm 3 Or within the range of either of the above.

[0044] In some preferred embodiments, the tap density Z of the negative electrode active material is 0.9 g / cm³. 3 ≤Z≤1.2g / cm 3 .

[0045] Specifically, the tap density determines the initial packing efficiency of the particles, which in turn affects the degree of densification after pressurization. When the tap density Z of the negative electrode active material is within the above range, the particles have "fewer gaps" in their packing, making it easier to achieve densification of the negative electrode sheet. This is beneficial for improving the compaction density of the electrode sheet and enhancing the cycle stability of the battery.

[0046] If the tap density Z of the negative electrode active material is too low, the poor packing ability indicates a complex particle structure with large voids, which is detrimental to improving energy density. If the tap density Z of the negative electrode active material is too high, the particle distribution is too wide, and the strong packing ability improves energy density. However, the insufficient voids reduce the electrode liquid ion channels, and the wide particle size distribution increases side reactions in the small particle portion and results in insufficient kinetics in the large particle portion.

[0047] TZ represents the compaction density and vibration density of the negative electrode active material. The value of TZ can characterize the effect of pressure change on the negative electrode active material. The larger the value of TZ, the greater the pressure change on the negative electrode active material.

[0048] In a specific embodiment, the compaction density P of the negative electrode active material layer can be 0.8 g / cm³. 3 0.9g / cm 3 1g / cm 3 1.4g / cm 3 1.5g / cm 3 1.6g / cm 3 1.7g / cm 3 1.8g / cm 3 1.9g / cm 3 2.0g / cm 3 Or within the range of either of the above.

[0049] In a specific embodiment, the skeleton density G of the negative electrode active material can be 1.4 g / cm³. 3 1.5g / cm 3 1.6g / cm 3 1.8g / cm 3 1.9g / cm 3 2.0g / cm 3 2.1g / cm 3 2.2g / cm 3 2.4g / cm 3 Or within the range of either of the above.

[0050] In some embodiments, the skeleton density G of the negative electrode active material is 1.8 g / cm³. 3 ≤G≤2.2g / cm 3 .

[0051] Specifically, the skeletal density, including the density of closed-cell materials, is G, which is the material mass divided by (material volume + closed-cell volume), and is expressed in g / cm³. 3 .

[0052] The skeleton density includes the density of closed-cell materials. The compaction density of the electrode sheet is higher than the skeleton density of the negative electrode active material. The higher the skeleton density, the higher the safe upper limit of the compaction density of the electrode sheet, which is beneficial to improving the volumetric energy density of the battery.

[0053] If the skeleton density of the negative electrode active material is too high, the crystal structure is highly ordered, the ion insertion / extraction channels become narrower, diffusion resistance increases, electrode impedance increases, and battery cycle performance and rate performance decrease. If the skeleton density of the negative electrode active material is too low, the crystal structure becomes more disordered, defects increase, electronic conductivity decreases, battery expansion rate increases, cycle performance decreases, and rate performance deteriorates.

[0054] In some embodiments, the Dv90, Dv50, and Dv10 of the negative electrode active material satisfy the following relationship:

[0055] F=(Dv90-Dv10) / Dv50, 0.7≤F≤1.5.

[0056] Specifically, Dv90 refers to the particle size corresponding to a cumulative volume percentage of 90% in the cumulative volume distribution curve of the material. Dv50 refers to the particle size corresponding to a cumulative volume percentage of 50% in the cumulative volume distribution curve of the material. Dv10 refers to the particle size corresponding to a cumulative volume percentage of 10% in the cumulative volume distribution curve of the material.

[0057] In the negative electrode active material layer, the distribution of negative electrode active material particles of different sizes can achieve different kinetic properties: small particles have better kinetics than large particles due to their large specific surface area and short ion diffusion paths. The Dv90, Dv50, and Dv10 of the negative electrode active material satisfy the relationship: F=(Dv90-Dv10) / Dv50, 0.7≤F≤1.5. Large particles form the framework, medium particles fill the gaps between large particles, and small particles fill the gaps between medium particles. The resulting electrode has low porosity and can achieve a high compaction density. By optimizing the packing density and pore structure of the electrode, the negative electrode has good ion / electron migration ability, improving the battery's cycle performance and rate performance.

[0058] If F is too low, the difference between Dv90 and Dv10 will be too small, or Dv50 will be too large, resulting in an excessive proportion of small particles that are prone to agglomeration, reducing stacking efficiency and affecting electrode compaction density. If F is too high, the difference between Dv90 and Dv10 will be too large, or Dv50 will be too small, resulting in an excessive proportion of large particles that cannot effectively fill the gaps, forming large voids. This leads to increased ion transport resistance, intensified polarization, and affects battery charge and discharge performance.

[0059] In a specific embodiment, the value of F can be 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5 or any two of the above.

[0060] In some preferred embodiments, 0.9 ≤ F ≤ 1.5.

[0061] Specifically, within the above-mentioned preferred range, F results in high compaction density, low void ratio, and low porosity of the electrode, leading to a negative electrode with good ion / electron migration capability, effectively improving battery cycle performance and rate performance, and resulting in higher battery energy density.

[0062] In some embodiments, the Dv90, Dv50, and Dv10 of the negative electrode active material satisfy the following relationship:

[0063] L=(Dv90-Dv50) / (Dv50-Dv10),1.4≤L≤1.8.

[0064] Specifically, L satisfies the above formula, which reflects the particle size distribution of the negative electrode active material particles. The range of L affects the pore structure of the electrode after stacking. L satisfies the range of 1.4≤L≤1.8. By controlling the gradient distribution of particle size, the stacking density, pore structure, mechanical stability and conductive network of the electrode can be optimized, ultimately improving the battery cycle performance and rate performance.

[0065] If L is too large, the particle size distribution is too low, and fine particles can fill the gaps between coarse particles, resulting in a denser packing. This leads to lower electrode porosity, narrower ion migration channels, and increased battery impedance. Conversely, if L is too low, the particle size distribution is too high, making it difficult to fill the gaps between particles during packing. This results in higher electrode porosity, more ion migration channels, reduced ion migration capacity, increased impedance, and decreased rate performance.

[0066] In some embodiments, the sphericity S of the negative electrode active material particles is ≥ 0.4; S is the minimum inscribed circle diameter / minimum circumscribed circle diameter of the negative electrode active material particles.

[0067] Specifically, a sphericity S ≥ 0.4 for the negative electrode active material particles is a characterization of the morphology of the negative electrode active material. A high packing effect and close packing result in a higher electrode compaction density compared to irregular particles, increasing the compaction density of the negative electrode and thus improving the battery's energy density. Simultaneously, the uniform contact points between spherical particles and the conductive agent form a continuous conductive path, providing the electrode with good ion / electron migration channels and improving battery electrical performance. If S is too low, the negative electrode active material particles have an irregular morphology, resulting in low electrode compaction density and low electron migration ability, negatively impacting battery electrical performance.

[0068] In some embodiments, the Dn10 of the negative electrode active material is greater than 1 μm.

[0069] Specifically, Dn10 refers to the particle size corresponding to 10% of the total number of particles in the cumulative distribution. Dn10 > 1μm can effectively suppress the agglomeration of small particles, improve the stability and value of electrode compaction density, and enhance the energy density, cycle life, and rate performance of the battery.

[0070] If Dn10≤1μm, small particles are prone to agglomeration, leading to a decrease in electrode compaction density and structural instability, which affects the battery's electrical performance.

[0071] In some embodiments, the specific surface area B of the negative electrode active material is 0.7 m². 2 / g≤B≤2.5m 2 / g.

[0072] Specifically, when the specific surface area (B) of the negative electrode active material is within the aforementioned range, a large specific surface area reduces particle agglomeration, improves stacking efficiency, increases the compaction density of the negative electrode sheet, and enhances the battery's cycle performance and rate performance. If the specific surface area (B) of the negative electrode active material is too low, the particle specific surface area is small, resulting in low interparticle friction, but electrolyte wetting is difficult, and there are insufficient ion insertion sites, affecting battery cycle performance. If the specific surface area (B) of the negative electrode active material is too high, there are many interparticle contact sites, resulting in high friction, reduced electrode densification, and decreased electrode compaction density.

[0073] In a specific embodiment, the specific surface area B of the negative electrode active material can be 0.7 m². 2 / g, 0.8m 2 / g, 0.9m 2 / g, 1.0m 2 / g, 1.2m 2 / g, 1.5m 2 / g, 1.8m 2 / g, 2.0m 2 / g, 2.2m 2 / g, 2.3m 2 / g, 2.5m 2 / g or either of the above.

[0074] In some embodiments, the porosity of the negative electrode sheet is 20%-40%.

[0075] The porosity of the negative electrode is within the above range. High porosity means the electrode has good compaction density and ion / electron migration channels, which improves the ion / electron migration capability of the electrode and enhances the rate performance and cycle performance of the battery.

[0076] In specific embodiments, the porosity of the negative electrode sheet can be 20%, 21%, 23%, 24%, 25%, 28%, 30%, 32%, 35%, 36%, 38%, 40%, or within any two of the above ranges.

[0077] In some embodiments, the mass content of the negative electrode active material in the negative electrode active material layer is 90% to 100%.

[0078] Specifically, in the negative electrode active material layer, when the mass content of the negative electrode active material is 100%, the negative electrode active material layer is a pure silicon layer. When the mass content of the negative electrode active material is in the range of 90% to 100%, but not exceeding 100%, the negative electrode active material is selected from carbon materials and silicon-carbon materials.

[0079] In some embodiments, the carbon material includes one of artificial graphite and natural graphite.

[0080] In some embodiments, the negative electrode sheet further includes a negative electrode conductive agent, a negative electrode binder, and a negative electrode dispersant. The negative electrode conductive agent includes at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The negative electrode binder includes at least one selected from styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethyl methacrylate (PMAA), carboxymethyl chitosan (CMCS), polyurethane (PU), and polyacrylonitrile (PAN). The negative electrode dispersant includes sodium carboxymethyl cellulose.

[0081] It includes one or more of graphene, conductive graphite, and carbon nanotubes.

[0082] Secondly, this application provides a secondary battery, including a positive electrode, a separator, and the aforementioned negative electrode.

[0083] The secondary battery provided in this application includes the aforementioned negative electrode, which has good electron and ion migration channels, low electrode impedance, improves the battery's cycle performance and rate performance, has high energy density, and improves the battery's cycle stability.

[0084] The positive electrode sheet includes a positive electrode active material, a positive electrode conductive agent, and a positive electrode binder. The positive electrode active material is selected from existing technologies, such as including at least one of lithium-containing compounds and sodium-containing compounds. Lithium-containing compounds include lithium nickel cobalt manganese oxide compounds, lithium iron phosphate compounds, lithium-rich manganese-based compounds, lithium cobalt oxide, lithium manganese oxide, etc.

[0085] The positive electrode conductive agent includes at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0086] The positive electrode binder includes at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-tetrafluoroethylene-propylene terpolymer, ethylene-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0087] It should be noted that secondary batteries can be sodium-ion batteries, lithium-ion batteries, solid-state batteries, semi-solid-state batteries, etc.

[0088] The framework density G of the negative electrode active material can be obtained by gas displacement method or specific gravity bottle method. The core is to accurately measure the volume of the solid framework (including closed pores) by replacing the open pores inside the particles with inert gas, and calculate the framework density by combining the sample mass; that is, G = material mass / (material volume + closed pore volume).

[0089] The framework density G of the negative electrode active material was measured using the gas volume displacement method, according to the national standard (GB / T40401-2021 / ISO12154:2014). The specific principle and method are as follows.

[0090] Principle: Based on Archimedes' principle of gas displacement, helium or nitrogen is selected as the displacement medium (gas molecules have small diameters, can quickly penetrate the open pores inside the particles, and are not adsorbed by the material or react with the material): by measuring the volume of the displacement gas in the sample, the skeletal volume of the particles is obtained, and the skeletal density is calculated in combination with the sample mass.

[0091] The testing system consists of a "sample cell" and a "reference cell," and the known volumes of the two cells are determined through calibration.

[0092] After the sample is pretreated, the air in the gaps between particles and in the open pores is replaced by an inert gas. At this time, the measured "sample volume" is the volume of the solid skeleton plus the volume of the closed pores.

[0093] Based on the ideal gas law PV=nRT, the true skeleton volume of the sample is calculated by comparing the pressure change after the sample cell is filled with gas with the calibration data of the reference cell.

[0094] The skeleton density G is calculated based on G = material mass / (material volume + closed-cell volume).

[0095] The testing method for G can be implemented using the following procedures.

[0096] S1. Take a negative electrode active material powder sample and vacuum dry it at 105℃±5℃ for 4 hours to remove the moisture and air adsorbed on the sample surface (to avoid affecting the gas replacement efficiency). Cool it to room temperature for later use.

[0097] S2. Use high-purity helium (purity ≥ 99.999%) or nitrogen (purity ≥ 99.99%) as the replacement medium;

[0098] S3. Use a standard sphere of known volume to calibrate the actual volumes of the reference cell and the sample cell (to eliminate equipment system errors).

[0099] S4. Accurately weigh the mass m of the pretreated sample and slowly place it into the calibrated sample cell to ensure that the sample is evenly spread and to avoid gas retention caused by particle agglomeration.

[0100] S5. Seal the sample cell, start the instrument to evacuate (vacuum degree ≤1kPa) to remove air from the sample cell; fill the sample cell and reference cell with inert gas at a set pressure (usually 0.1~0.3MPa), let stand for 10~15min to ensure that the gas fully penetrates the open pores inside the sample and reaches pressure and temperature equilibrium.

[0101] S6. Record the equilibrium pressure and temperature data of the two pools, calculate the true skeleton volume (Vs) of the sample according to the ideal gas law, and obtain the skeleton density according to the formula G=m / Vs.

[0102] S7. Samples from the same batch should be tested at least 3 times, and the average value should be taken as the final skeleton density.

[0103] Tap density refers to the bulk density (g / cm³) of powder materials when they reach a close packing state under mechanical vibration. 3 The tap density Z of a negative electrode active material is the mass divided by its volume after the dry powder has been compacted through regular vibration. This tap density can be obtained using an automatic vibration density meter, for example, by taking a sample of the negative electrode active material and weighing it (m1). Then, place the sample into a clean, dry standard graduated cylinder (e.g., a 250mL graduated cylinder) of known volume. Fix the graduated cylinder to the automatic vibration density meter. The instrument vibrates vertically at a set frequency (e.g., 250 times / min ± 15 times) and amplitude (e.g., 3mm ± 0.2mm) for a certain period (e.g., 10 minutes) until the powder volume no longer decreases significantly. After vibration, read the final powder volume (V1) shown on the graduated cylinder.

[0104] The formula for calculating the tap density Z is: Tap density Z = m1 / V1.

[0105] Compacted density typically refers to the density of powder after it has been compressed into sheet or block form under external force, emphasizing the compressibility and formability of the material in electrode manufacturing. The test method for the compacted density T of negative electrode active materials under 5T conditions is as follows:

[0106] Sample preparation: A certain mass of negative electrode active material powder is placed into a mold of known area.

[0107] Apply pressure: Use a tablet press (such as a hydraulic press) to apply a pressure of 5T to the powder and maintain it for a period of time to make the powder particles tightly packed together.

[0108] Dimensional Measurement: After depressurization, remove the pressed sheet sample and use tools such as vernier calipers to accurately measure its thickness and diameter (or side length) to calculate the volume of the pressed object.

[0109] Calculation: The formula for calculating compacted density T is: Compacted density T = Powder mass / Compacted volume.

[0110] It should be noted that T, Z, and G are physical characteristics of the negative electrode active material. The compacted density, tapped density, and skeleton density of the negative electrode active material can be obtained according to the above testing methods, and the values ​​of T, Z, and G of the selected negative electrode active material can be substituted into the formula. ≤ ≤ The compaction density P of the negative electrode active material layer in the corresponding negative electrode sheet is obtained.

[0111] The specific embodiments of the present invention will be further explained and illustrated below through examples, but this does not mean that the scope of protection of the present invention is limited to the scope described in the examples.

[0112] Example 1

[0113] (1) Preparation of the positive electrode:

[0114] Lithium cobalt oxide, conductive carbon black, carbon nanotubes, and polyvinylidene fluoride were mixed in a mass ratio of 97:1:0.5:1.5. N-methylpyrrolidone (NMP) was added, and the mixture was stirred under vacuum until it formed a homogeneous positive electrode slurry. The positive electrode slurry was then uniformly coated onto the positive electrode current collector aluminum foil. After drying, cold pressing, cutting, and slitting, the positive electrode sheet was obtained.

[0115] (2) Preparation of negative electrode:

[0116] The negative electrode active material, conductive carbon black, sodium carboxymethyl cellulose, styrene-butadiene rubber, and PAA are mixed in a mass ratio of 96:1:0.5:1:1.5. The mixture is then added to a deionized water solvent and stirred thoroughly to obtain a negative electrode slurry. The negative electrode slurry is then uniformly coated onto the negative electrode current collector copper foil. After drying, cold pressing, cutting, and slitting, the negative electrode sheet is obtained.

[0117] The negative electrode active material is graphite. The values ​​of Dv10, Dv50, Dv90, and Dn10 of the negative electrode active material are obtained using the formulas F=(Dv90-Dv10) / Dv50 and L=(Dv90-Dv50) / (Dv50-Dv10). The sphericity S value of the negative electrode active material particles is shown in Table 1. The tap density Z, specific surface area B, compaction density T, framework density G, and compaction density P of the negative electrode active material layers are also detailed. 实际 Specific values ​​are shown in Table 2. The porosity of the negative electrode is shown in Table 2.

[0118] (3) Preparation of lithium-ion batteries

[0119] PE porous polymer film is used as the separator.

[0120] The positive electrode, separator, and the prepared negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes for isolation. The stacked electrodes and separator are then wound together to form a core. The core is placed in a pre-formed aluminum-plastic film bag, and electrolyte is injected. After vacuum sealing, settling, and formation processes, the battery is complete. The electrolyte is obtained through purchase.

[0121] Examples 2-20 and Comparative Examples 1-10 are largely the same as Example 1, except that Dv10, Dv50, Dv90, Dn10, F, L, and S are different (see Table 1 for details); Z, B, T, G, and P are also different. 实际 The porosity of the negative electrode varies depending on the specific characteristics, as detailed in Table 2. The rest is the same as in Example 1.

[0122] Table 1

[0123]

[0124] Continued from Table 1

[0125]

[0126] Table 2

[0127]

[0128] Continued from Table 2

[0129]

[0130] Battery performance test:

[0131] The batteries obtained from the above embodiments and comparative examples were subjected to the following performance tests:

[0132] (1) Battery mass energy density: At 25°C, the battery was charged to 4.2V at a constant current of 1C, and then charged at a constant voltage of 4.3V with a cutoff current of 0.05C. The battery was then discharged to 3.0V at a constant current of 1C. The discharge capacity C1 of the battery was tested, and the mass energy density of the battery was calculated. The specific data are shown in Table 3.

[0133] (2) 25℃ ambient temperature cycling test:

[0134] The battery was charged at 1C constant current to 4.2V at room temperature of 25℃, then charged at 4.2V constant voltage with a cutoff current of 0.05C, and then discharged at 1C constant current to 3.0V. The discharge capacity and thickness of the battery were tested in the first cycle. This cycle was repeated for 800 cycles, and the discharge capacity and thickness of the battery in the 800th cycle were tested.

[0135] Calculate the 800-cycle capacity retention rate = discharge capacity in week 800 / discharge capacity in week 1 × 100%.

[0136] Calculate the thickness expansion rate over 800 weeks = (thickness of week 800 - thickness of week 1) / thickness of week 1 × 100%.

[0137] The specific test results are shown in Table 3.

[0138] (3) Ratio performance test method

[0139] 1. Discharge at 0.2C to 2.75V and let stand for 10 minutes;

[0140] 2. Charge at a constant current and constant voltage of 1.0C to the upper limit voltage (4.50V), cut off current of 0.05C, let stand for 10 minutes, and take two data points: the charging capacity during the constant current stage and the total charging capacity.

[0141] 1.0C constant current charging ratio = constant current stage charging capacity / total charging capacity.

[0142] The test results are shown in Table 3.

[0143] Table 3

[0144]

[0145] Continued from Table 3

[0146]

[0147] As shown in Tables 1-3, compared with Comparative Examples 1-10, the compaction density P of the negative electrode material layer in Comparative Examples 1-4 does not satisfy the relationship in Examples 1-20. ≤ ≤ The battery has poor rate performance or poor cycle performance, and a high cycle thickness expansion rate. In Comparative Example 5, Z is less than 0.8 g / cm. 3 ≤Z≤1.2g / cm 3 Within a certain range, the rate performance of the battery is poor; in Comparative Example 6, Z is greater than 0.8 g / cm³. 3 ≤Z≤1.2g / cm 3 The battery exhibits poor rate performance, excessive gas production, and high cycle thickness expansion rate; in Comparative Example 7, T is less than 0.9 g / cm³. 3 ≤T≤2.0g / cm 3 The battery cycle capacity retention rate is low, and in Comparative Example 8, T is greater than 0.9 g / cm³. 3 ≤T≤2.0g / cm 3 The battery exhibits poor cycle performance and rate performance; in Comparative Example 9, the g / cm³ is less than 1.4 g / cm³. 3 ≤G≤2.4g / cm 3 Within this range, the battery's cycle performance is poor; in Comparative Example 10, G is greater than 1.4 g / cm³. 3 ≤G≤2.4g / cm 3 The battery's rate performance is poor. Through comparison, it is shown that the compaction density T, tap density Z, and skeleton density G of the negative electrode active material, and the compaction density P of the negative electrode material layer satisfy the following relationship: ≤ ≤ And 0.9g / cm 3 ≤T≤2.0g / cm 3 0.8g / cm 3 ≤Z≤1.2g / cm 3 1.4g / cm 3 ≤G≤2.4g / cm 3 The resulting negative electrode sheet, based on the skeleton density, compaction density, and vibration density of the negative electrode active material, has a compaction density that takes into account whether the overall negative electrode active material consists of high-hardness or low-hardness particles. The pressure applied to the negative electrode sheet is moderate to avoid problems such as cracking of high-hardness particles or deformation and adhesion of low-hardness particles due to excessive pressure, which would reduce the battery's rate performance. The resulting negative electrode sheet has good electron and ion migration channels, low electrode impedance, high cycle capacity retention, and low thickness expansion rate, improving the battery's cycle performance and rate performance, and ensuring high energy density.

[0148] A comparison of Examples 1-8, 10-11, and 19 with Examples 9, 18, and 20 shows that T is 1.8 g / cm³. 3 ≤T≤2.0g / cm 3 P is 1.5 g / cm³ 3 ≤P≤1.9g / cm 3Z is 0.9 g / cm 3 ≤Z≤1.2g / cm 3 G is 1.8 g / cm³ 3 ≤G≤2.2g / cm 3 The battery has good overall rate performance, cycle performance, and energy density.

[0149] Comparing Examples 1-5 with Example 13, F in Example 13 does not satisfy the condition F=(Dv90-Dv10) / Dv50, 0.7≤F≤1.5, resulting in low cycle capacity retention and low energy density. This indicates that Dv90, Dv50, and Dv10 of the negative electrode active material satisfy the following relationship: F=(Dv90-Dv10) / Dv50, 0.7≤F≤1.5. Large particles form the framework, medium particles fill the gaps between large particles, and small particles fill the gaps between medium particles, resulting in uniform particle distribution, which helps improve the cycle performance and energy density of the battery. Comparing Examples 6-11 with Example 14, L in Example 14 does not satisfy the condition L=(Dv90-Dv50) / (Dv50-Dv10), 1.4≤L≤1.8, and the specific surface area B of the negative electrode active material does not meet the requirement of 0.7m². 2 / g≤B≤2.5m 2 Under the given conditions, the battery exhibits poor rate performance, low cycle capacity retention, and high expansion rate. This indicates that the negative electrode active materials Dv90, Dv50, and Dv10 satisfy the following relationship: L = (Dv90 - Dv50) / (Dv50 - Dv10), 1.4 ≤ L ≤ 1.8. By controlling the gradient distribution of particle size, the packing density, pore structure, mechanical stability, and conductive network of the electrode sheets can be optimized, thereby improving the battery's cycle performance and rate performance.

[0150] Comparing Examples 1-5 with Example 12, and Examples 6-11 with Example 15, F does not satisfy the condition F=(Dv90-Dv10) / Dv50, 0.7≤F≤1.5, and L does not satisfy the condition L=(Dv90-Dv50) / (Dv50-Dv10), 1.4≤L≤1.8. The battery's rate performance and cycle performance are poor. This indicates that by simultaneously satisfying the conditions F=(Dv90-Dv10) / Dv50, 0.7≤F≤1.5 and L=(Dv90-Dv50) / (Dv50-Dv10), 1.4≤L≤1.8, the battery's cycle performance and rate performance can be effectively improved by controlling particle size and particle size distribution, optimizing the electrode packing density and pore structure, and increasing the porosity of the negative electrode.

[0151] Comparing Examples 1-11 and Example 16, the sphericity S of the negative electrode active material particles in Example 16 is less than 0.4, resulting in poor rate performance of the battery. This indicates that the sphericity S of the negative electrode active material particles is ≥0.4, which improves the energy density of the battery. At the same time, the spherical particles have uniform contact points with the conductive agent, forming a continuous conductive path, which makes the electrode have good ion / electron gap migration channels and improves the battery's electrical performance.

[0152] A comparison of Examples 1-11 with Examples 14 and 17 demonstrates that the specific surface area B of the negative electrode active material is 0.7 m². 2 / g≤B≤2.5m 2 A range of / g is beneficial for improving the rate performance and cycle performance of batteries.

[0153] Comparing Examples 1-11 with Examples 14 and 20, when the porosity of the negative electrode sheet is less than 20%, the rate performance of the battery is poor and gas production increases. When the porosity of the negative electrode sheet is greater than 40%, the rate performance of the battery is poor, the cycle capacity retention rate is low, and the energy density is low. This indicates that a porosity of 20%-40% for the negative electrode sheet is beneficial to improving the rate performance and cycle performance of the battery and ensuring that the battery has a high energy density.

[0154] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A negative electrode sheet, characterized in that, It includes a negative electrode current collector and a negative electrode active material layer, wherein the negative electrode active material layer is disposed on at least one surface of the current collector; The negative electrode active material layer includes a negative electrode active material, which includes at least one of carbon material, silicon-carbon material, and silicon material; the negative electrode active material is low-hardness particles or high-hardness particles, wherein the low-hardness particles satisfy (TZ) / G≤0.4, and the high-hardness particles satisfy (TZ) / G>0.4; the compaction density P of the negative electrode active material layer satisfies the following relationship: ≤ ≤ ; and 0.9 g / cm 3 ≤T≤2.0 g / cm 3 ,0.8 g / cm 3 ≤Z≤1.2 g / cm 3 ,1.4 g / cm 3 ≤G≤2.4 g / cm 3 ; in, T represents the compaction density of the negative electrode active material under a 5T condition, in g / cm³. 3 ; Z represents the tap density of the negative electrode active material, in g / cm³. 3 ; G represents the skeleton density of the negative electrode active material, in g / cm³. 3 ; P is the compaction density of the negative electrode active material layer, in g / cm³. 3 .

2. The negative electrode sheet according to claim 1, characterized in that, The compaction density T of the negative electrode active material under 5T conditions is 1.8 g / cm³. 3 ≤T≤2.0g / cm 3 .

3. The negative electrode sheet according to claim 1, characterized in that, The tap density Z of the negative electrode active material is 0.9 g / cm³. 3 ≤Z≤1.2g / cm 3 .

4. The negative electrode sheet according to claim 1, characterized in that, The skeleton density G of the negative electrode active material is 1.8 g / cm³. 3 ≤G≤2.2g / cm 3 .

5. The negative electrode sheet according to claim 1, characterized in that, The Dv90, Dv50, and Dv10 of the negative electrode active material satisfy the following relationship: F=(Dv90-Dv10) / Dv50, 0.7≤F≤1.

5.

6. The negative electrode sheet according to claim 1 or 5, characterized in that, The Dv90, Dv50, and Dv10 of the negative electrode active material satisfy the following relationship: L=(Dv90-Dv50) / (Dv50-Dv10),1.4≤L≤1.

8.

7. The negative electrode sheet according to claim 1, characterized in that, The sphericity S of the negative electrode active material particles is ≥ 0.4; S is the minimum inscribed circle diameter / minimum circumscribed circle diameter of the negative electrode active material particles.

8. The negative electrode sheet according to claim 1, characterized in that, The negative electrode active material has a Dn10 > 1 μm; Alternatively, the specific surface area B of the negative electrode active material is 0.7 m². 2 / g≤B≤2.5m 2 / g.

9. The negative electrode sheet according to claim 1, characterized in that, The porosity of the negative electrode sheet is 20%-40%; In the negative electrode active material layer, the mass content of the negative electrode active material is 90%~100%.

10. A secondary battery, characterized in that, It includes a positive electrode, a separator, and a negative electrode as described in any one of claims 1-9.

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