Negative electrode composite material and preparation method thereof, negative electrode pole piece and lithium ion battery

Abstract

The invention relates to the technical field of battery materials, in particular to a negative electrode composite material and a preparation method thereof, a negative electrode plate and a lithium ion battery. The negative electrode composite material comprises bent nano graphene and graphite embedded into the bent nano graphene. The preparation method comprises the following steps: mixing the nano carbon material with graphite in proportion, and carrying out mechanical ball milling, so that the graphite is uniformly embedded into the nano carbon material to form a composite structure. The composite material combines the characteristics of large interlayer spacing and rapid ion transmission of the nano carbon material and the high capacity and good conductivity of graphite, and effectively improves the intercalation kinetics of lithium ions. When the composite material is applied to a negative pole piece and a lithium ion battery, the fast charging performance and the cycling stability of the battery can be remarkably improved, and meanwhile, the preparation process is simple and suitable for large-scale production.

Description

Negative electrode composite materials and their preparation methods, negative electrode sheets and lithium-ion batteries Technical Field

[0001] This invention relates to the field of battery materials technology, specifically to a negative electrode composite material and its preparation method, a negative electrode sheet, and a lithium-ion battery. Background Technology

[0002] As the core of current electrochemical energy storage systems, the continuous improvement of lithium-ion battery performance is crucial for driving the development of electric vehicles, portable electronic devices, and large-scale energy storage systems. Anode materials, as a key component of lithium-ion batteries, directly affect the battery's energy density, cycle life, fast-charging capability, and safety. Among various anode materials, graphite has become the most widely used anode material in commercial lithium-ion batteries due to its wide availability, low cost, stable electrochemical performance, and mature processing technology. However, with the increasing market demand for high energy density and fast charging, the limitations of traditional graphite anodes are gradually becoming apparent.

[0003] During fast charging, lithium ions in graphite anodes need to embed within their layered structure. However, due to the relatively narrow interlayer spacing of graphite, the diffusion kinetics of lithium ions are slow. Under high-current charging conditions, lithium ions tend to accumulate on the electrode surface and cannot embed into the graphite in time, leading to the precipitation of metallic lithium and the formation of lithium dendrites. Lithium dendrites not only consume active lithium, reduce battery capacity and cycle life, but may also puncture the separator, causing internal short circuits and posing serious safety hazards. Furthermore, the performance degradation of graphite anodes is significant at low temperatures, further limiting their application in wide-temperature-range, high-power scenarios.

[0004] To improve the fast-charging performance of graphite anodes, researchers have proposed various modification strategies, including surface coating, elemental doping, structural nanostructuring, and compositing with other materials. For example, compositing graphite with highly conductive carbon materials such as carbon nanotubes can improve the electrode's conductivity and ion transport rate to some extent; surface treatments such as oxidation and fluorination of graphite can also adjust its surface chemical properties and enhance its compatibility with electrolytes. However, these methods often involve complex synthesis processes, harsh reaction conditions, or the use of toxic reagents, resulting in high production costs and poor process reproducibility, making it difficult to meet the needs of large-scale industrial production.

[0005] In conclusion, developing a negative electrode composite material that is simple to manufacture, easy to scale up, and can effectively balance high capacity and excellent fast-charging performance is of great significance for promoting the development of next-generation high-power lithium-ion batteries. Summary of the Invention

[0006] Against the background described above, this invention proposes a composite negative electrode material based on bent nano-graphene and graphite, its preparation method, negative electrode sheet, and lithium-ion battery. The aim is to solve the problem of lithium plating of graphite under fast charging conditions through the effective coupling of the two structures and the synergistic effect of their performance, thereby improving the overall electrochemical performance and safety of the battery and enabling the battery to have both high rate and high capacity performance.

[0007] To achieve the above objectives, the present invention provides the following technical solution: In a first aspect, the present invention provides a negative electrode composite material, the negative electrode composite material comprising bent graphene nanoparticles and graphite embedded within the bent graphene nanoparticles.

[0008] As a preferred embodiment of the present invention, the mass ratio of the bent nanographene to graphite is (1.5:1) to (1:1.5).

[0009] As a preferred embodiment of the present invention, the bent graphene nanoparticles are needle-shaped crystals with a length of 1-3 μm, a width of 50-300 nm, and a thickness of 10-100 nm.

[0010] As a preferred embodiment of the present invention, the mass ratio of the bent nanographene to graphite is 1:1.

[0011] As a preferred embodiment of the present invention, the bent graphene nanoparticles include one or more of twisted hexabenzo[a]cobalamin (cHBC), chlorinated twisted hexabenzo[a]cobalamin (Cl-cHBC), and fluorinated twisted hexabenzo[a]cobalamin (F-cHBC).

[0012] Twisted hexabenzo[a]cobalt (cHBC) is a highly crystalline, curved graphene nanoparticle. Its superior stability in organic electrolytes and large interlayer spacing (4.6 Å, significantly higher than the 3.35 Å of traditional graphite) provide a more spacious lithium-ion intercalation channel. Simultaneously, its curved structure shortens the lithium-ion diffusion path, resulting in excellent rate performance. Furthermore, cHBC can be modified with halogens (such as fluorine and chlorine) to obtain chlorinated twisted hexabenzo[a]cobalt (Cl-cHBC) and fluorinated twisted hexabenzo[a]cobalt (F-cHBC). The repulsive forces between halogen elements not only increase the interlayer spacing but also modulate the electronic structure of the curved graphene nanoparticles, accumulating charge at graphene voids and defects, increasing the number of active lithium-ion storage sites, and further enhancing its rate performance.

[0013] As a preferred embodiment of the present invention, the graphite includes one or more of artificial graphite and natural graphite.

[0014] As a preferred embodiment of the present invention, the graphite has a particle size of 1 to 10 μm.

[0015] Secondly, the present invention provides a method for preparing the aforementioned negative electrode composite material, comprising the following steps: mixing bent nano-graphene and graphite at a predetermined mass ratio; and mechanically ball-milling the mixed material to obtain the negative electrode composite material.

[0016] As a preferred embodiment of the present invention, the parameters of the mechanical ball mill include: a rotation speed of 200-500 r / min, a ball milling time of 2-5 h, a ball-to-material ratio of (6:1)-(10:1), a ball milling atmosphere of air, and a temperature of room temperature; the ball milling media includes grinding balls with a diameter of 10 mm and a diameter of 5 mm, with a mass ratio of 6:4.

[0017] After ball milling, bent graphene nanoparticles and graphite are uniformly embedded within the bent graphene nanoparticles, forming a new composite material without significant phase separation, as shown in Figure 1. Since the average lithium binding energy of graphene is approximately 0.4 eV higher than that of graphite (graphene's lithium binding energy is approximately 1.2 eV, while graphite's is approximately 0.8 eV), it indicates that bent graphene nanoparticles have a stronger affinity for lithium ions. During charging, sequential lithium intercalation occurs as lithium ions intercalate from the positive electrode to the negative electrode: Stage I: Lithium preferentially intercalates between the layers of bent graphene nanoparticles and at the interface between graphene and graphite; Stage II: As lithium intercalation continues, lithium ions enter the inner layers of the bent graphene nanoparticle material and begin to penetrate into the graphite layers; Stage III: Lithium intercalation mainly occurs within the graphite layers.

[0018] Since lithium ions are first inserted into graphene with a large interlayer spacing and then transferred to the graphite layer, the large interlayer spacing provides a key structural basis for the rapid transport of lithium ions during the insertion / extraction process. Therefore, the composite material fully combines the advantages of the large interlayer spacing and high rate performance of bent nano-graphene with the excellent conductivity and high specific capacitance of graphite, which can effectively solve the problem of lithium plating in graphite under fast charging.

[0019] Thirdly, the present invention provides a negative electrode sheet, the negative electrode sheet comprising a negative electrode material, wherein the negative electrode material comprises the aforementioned negative electrode composite material or a negative electrode composite material prepared by the method described above.

[0020] Fourthly, the present invention provides a lithium-ion battery, comprising a positive electrode, a separator, an electrolyte, and the aforementioned negative electrode.

[0021] As a preferred embodiment of the present invention, the active material of the positive electrode is selected from one or more of lithium iron phosphate, NCM523, NCM613, and NCM811.

[0022] As a preferred embodiment of the present invention, the electrolyte comprises a solvent, a lithium salt, and an additive. The solvent comprises ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 1:1:1. The lithium salt comprises lithium hexafluorophosphate (LiPF6) with a concentration of 1–1.3 mol / L. The additive comprises at least one of vinylene carbonate (VC), vinyl sulfate (DTD), and 1,3-propanesulfonate lactone (PS).

[0023] Compared with existing technologies, the beneficial effects of this invention include: This invention proposes a method for preparing a negative electrode sheet for fast-charging lithium-ion batteries. This involves forming a composite negative electrode by simply mechanically mixing highly crystalline bent graphene nanoparticles with excellent kinetic properties and graphite with excellent conductivity and capacity. Given that these two materials are complementary yet significantly different in performance, by optimizing their ratio, the resulting composite negative electrode can simultaneously possess the advantages of both materials, combining the excellent kinetic properties of bent graphene nanoparticles with the excellent conductivity and capacity of graphite. This synergistic effect not only improves the overall electrochemical performance of the battery but also effectively alleviates problems such as lithium metal deposition, and provides higher safety and durability in practical lithium-ion battery applications. Furthermore, this composite material can be produced simply by mechanical ball milling, a simple synthesis process with significant advantages in mass production. When this composite negative electrode is used in NCM613 ternary lithium-ion batteries, it exhibits high rate performance and excellent cycle performance when the mass ratio of the two components is 1:1. Attached Figure Description

[0024] Figure 1 is a schematic diagram of the composite negative electrode morphology after bending graphene nanoparticles and graphite ball milling. Detailed Implementation

[0025] To enable those skilled in the art to better understand the technical solutions of the present invention, the preferred embodiments of the present invention are described below in conjunction with specific examples. However, these should not be construed as limiting the present invention and are merely examples.

[0026] Unless otherwise specified, the test methods or experimental methods described in the following examples are conventional methods; unless otherwise specified, the reagents and materials are obtained from conventional commercial sources or prepared by conventional methods.

[0027] The present invention will now be described in further detail with reference to specific embodiments and comparative examples.

[0028] Example 1: This example provides a negative electrode composite material, including bent graphene nanoparticles and graphite, wherein the bent graphene nanoparticles are chlorinated twisted hexaphenyl crown ether (Cl-cHBC) with dimensions of 2 μm in length, 200 nm in width, and 40 nm in thickness; the graphite is artificial graphite (Gr) with a particle size of 6 μm.

[0029] This embodiment also provides a method for preparing the above-mentioned negative electrode composite material, including the following steps: mixing Cl-cHBC and Gr in a ball mill at a mass ratio of 1:1; and mechanically ball milling the mixed material to obtain the negative electrode composite material (as shown in Figure 1).

[0030] The ball milling parameters were as follows: rotation speed 300 r / min; milling time 3 h; ball ratio (Φ10 mm: Φ5 mm = 6:4) 8:1; atmosphere: air; temperature: room temperature.

[0031] Example 2: The difference between Example 2 and Example 1 is that the mass ratio of the two is Cl-cHBC:Gr=1:1.5, and the rest is the same as Example 1.

[0032] Example 3: The difference between Example 3 and Example 1 is that the mass ratio of the two is Cl-cHBC:Gr=1.5:1, and the rest is the same as Example 1.

[0033] Example 4: The difference between Example 4 and Example 1 is that Cl-cHBC is replaced with cHBC, and the rest is the same as Example 1.

[0034] Example 5: The difference between Example 5 and Example 1 is that Cl-cHBC is replaced with F-cHBC, and the rest is the same as Example 1.

[0035] Comparative Example 1: The negative electrode of Comparative Example 1 does not contain bent graphene nanoparticles for composite formation, and is pure graphite.

[0036] Comparative Example 2: The difference between Comparative Example 2 and Example 1 is that the mass ratio of the two is Cl-cHBC:Gr=2:1, and the rest is the same as Example 1.

[0037] Comparative Example 3: The difference between Comparative Example 3 and Example 1 is that the mass ratio of the two is Cl-cHBC:Gr=1:2, and the rest is the same as Example 1.

[0038] Comparative Example 4: The difference between Comparative Example 3 and Example 1 is that Cl-cHBC is replaced with ordinary graphene (such as graphene oxide), and the rest is the same as Example 1.

[0039] The experimental example uses the negative electrode materials prepared in Examples 1 to 5 and Comparative Examples 1 to 4 to make lithium-ion batteries. The specific preparation method is as follows: (1) The positive electrode active material (NCM613), conductive agent (SP), and binder (PVDF) are mixed evenly in a mass ratio of 97:1:2. Solvent (NMP) is added and stirred to form a uniform slurry. The slurry is uniformly coated on a 13μm carbon-coated aluminum foil using a transfer coating machine, and then rolled and cut to form a positive electrode sheet.

[0040] (2) The negative electrode material, conductive agent (SP), dispersant (CMC) and binder (SBR) are mixed evenly in a mass ratio of 96.5:0.5:1.2:1.8. Solvent (H2O) is added and stirred to form a uniform slurry. The slurry is then uniformly coated onto a 6μm copper foil using a transfer coating machine, and then rolled and cut to form a negative electrode sheet.

[0041] (3) The positive electrode, negative electrode and separator are stacked to form a core package, and then the electrolyte in the above examples and comparative examples is injected to assemble the battery cell. After baking, electrolyte injection, high temperature standing, formation and capacity testing, a lithium-ion soft pack battery is obtained.

[0042] The following performance tests were conducted on the soft-pack batteries corresponding to each embodiment and comparative example: (1) Rate charging performance The soft-pack batteries were subjected to charge and discharge tests at different rates. The specific test steps are as follows: The initial test temperature was 25±2℃. The cells were charged at different rates and then discharged at 1C. The specific charging rates were 1C / 2C / 3C / 4C. The discharge capacity and capacity retention rate of the cells at different charging rates were recorded (with 1C / 1C discharge capacity as 100%). The capacity retention rate = discharge capacity at different charging rates / discharge capacity at 1C charging. The test results are shown in Table 1 below: Table 1 As can be seen from the above, when comparing the capacity retention rate under fast charging rate, the capacity retention rates of Examples 1-3 and Comparative Examples 2-3 are all higher than that of Comparative Example 1, indicating that the fast charging performance of the material is improved after adding the curved nano-graphene composite.

[0043] Examples 1-3 and Comparative Example 1 show that when the graphene to graphite mixing ratio is between 1:1.5 and 1.5:1, a uniform and dense composite material can be obtained during mechanical mixing, fully combining the complementary advantages of the two components, resulting in a significant improvement in both discharge capacity and fast-charging capacity retention. The performance is optimal when the ratio is 1:1. Beyond the above ratio range, i.e., Comparative Examples 2 and 3, the composite material cannot fully utilize the advantages of both components, exhibiting the characteristics of the component with a higher proportion. For example, in Comparative Example 2, when the graphene:graphite ratio is 2:1, although the rate performance of the material is improved, the excessive addition of low-capacity graphene leads to a very low capacity utilization of the composite material. In Comparative Example 3, when the graphene:graphite ratio is 1:2, although the rate performance is improved compared to Comparative Example 1, the capacity is also reduced accordingly, thus the overall performance shows a downward trend.

[0044] Examples 1, 4, 5 and Comparative Example 4 compared the effects of different types of bent nanographene and ordinary graphene on the battery cell. Twisted hexabenzo[a]cobalt (cHBC), chlorinated twisted hexabenzo[a]cobalt (Cl-cHBC) modified with halogens (such as fluorine and chlorine), and fluorinated twisted hexabenzo[a]cobalt (F-cHBC) can all be mixed with graphite to obtain composite anode materials that balance capacity and rate capability. However, ordinary graphene, due to its lack of large interlayer spacing and low diffusion path, cannot achieve the sequential lithium intercalation mechanism, and its addition has some negative impact on the performance of graphite.

[0045] (2) Fast charging cycle performance: The above-described embodiments and comparative sample cells were subjected to a 2C / 1C cycle test at 25℃. The specific test steps are as follows: The initial test temperature was 25±2℃. Each group of cells was charged to 4.3V at 2C, then charged to 0.05C at constant voltage, rested for 10 minutes, and then discharged to 2.8V at 1C, rested for 10 minutes. The above steps were repeated until the capacity dropped to 80% of the initial capacity. The cycle data are shown in Table 2 below: Table 2 Similar conclusions to those drawn from the above can be drawn regarding the fast charging performance. In comparison of fast charging cycle performance, the capacity retention rates of Examples 1-3 and Comparative Examples 2-3 are all higher than that of Comparative Example 1, indicating that the fast charging cycle performance of the material is improved after the addition of bent nano-graphene composite.

[0046] Examples 1-3 and Comparative Example 1 show that when the graphene:graphite mixing ratio is between 1:1.5 and 1.5:1, the composite material fully combines the complementary advantages of the two components, resulting in a significant improvement in both the initial discharge capacity and the cycle capacity retention rate. The performance is optimal when the ratio is 1:1. Beyond the above ratio range, i.e., Comparative Examples 2 and 3, the composite material cannot fully utilize the advantages of both components, exhibiting the characteristics of the component with a higher proportion. For example, in Comparative Example 2, when the graphene:graphite ratio is 2:1, although the fast-charge cycle retention rate of the material is improved, the excessive addition of low-capacity graphene leads to a very low initial cycle capacity of the composite material. In Comparative Example 3, when the graphene:graphite ratio is 1:2, although the cycle performance is improved compared to Comparative Example 1, the initial capacity is also reduced accordingly, thus the overall performance shows a downward trend.

[0047] Examples 1, 4, 5 and Comparative Example 4 show that different types of bent graphene nanoparticles have no significant effect on battery cycle performance, while ordinary graphene, which does not have a similar structure and properties to bent graphene nanoparticles, has some negative impact on the cycle performance of graphite when added.

[0048] The above are merely preferred embodiments of the present invention. It should be noted that the above preferred embodiments should not be considered as limitations on the present invention, and the scope of protection of the present invention should be determined by the scope defined in the claims. For those skilled in the art, several improvements and modifications can be made without departing from the spirit and scope of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A negative electrode composite material, characterized in that, The negative electrode composite material includes bent graphene nanoparticles and graphite embedded within the bent graphene nanoparticles.

2. The negative electrode composite material according to claim 1, characterized in that, The mass ratio of the bent graphene nanoparticles to graphite is (1.5:1) to (1:1.5).

3. The negative electrode composite material according to claim 1, characterized in that, The bent graphene nanoparticles include one or more of twisted hexabenzo[a]cobalamin, chlorinated twisted hexabenzo[a]cobalamin, and fluorinated twisted hexabenzo[a]cobalamin.

4. The negative electrode composite material according to claim 3, characterized in that, The bent graphene nanoparticles are needle-shaped crystals with a length of 1–3 μm, a width of 50–300 nm, and a thickness of 10–100 nm.

5. The negative electrode composite material according to claim 1, characterized in that, The graphite includes one or more of artificial graphite and natural graphite.

6. The negative electrode composite material according to claim 1 or 5, characterized in that, The graphite has a particle size of 1–10 μm.

7. A method for preparing a negative electrode composite material as described in any one of claims 1 to 6, characterized in that, Includes the following steps: Bending graphene nanoparticles are mixed with graphite at a predetermined mass ratio; the mixture is then mechanically ball-milled to obtain the negative electrode composite material.

8. The preparation method according to claim 6, characterized in that, The parameters of the mechanical ball mill include: rotation speed of 200-500 r / min, ball milling time of 2-5 h, ball-to-material ratio of (6:1)-(10:1), ball milling atmosphere of air, and temperature of room temperature; the ball milling media include grinding balls with diameters of 10 mm and 5 mm, with a mass ratio of 6:

4.

9. A negative electrode sheet, characterized in that, The negative electrode sheet includes a negative electrode material, wherein the negative electrode material includes the negative electrode composite material according to any one of claims 1 to 6 or the negative electrode composite material prepared by the method according to any one of claims 7 to 8.

10. A lithium-ion battery, characterized in that, It includes a positive electrode, a separator, an electrolyte, and a negative electrode as described in claim 9.

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