Composite negative electrode sheet, preparation method therefor and lithium ion battery

Abstract

A composite negative electrode sheet, a preparation method therefor and a lithium ion battery. The composite negative electrode sheet uses a double-layer coating design. A first active substance layer and a second active substance layer are successively provided on the surface of a negative electrode current collector, the active substance of the first active substance layer being first graphite, and the active substance of the second active substance layer being second graphite. The height-width ratio and OI value of the first graphite and the height-width ratio and OI value of the second graphite satisfy the following relationship: 2≤(B×OIa) / (A×OIb)≤30, where A is the height-width ratio of the first graphite and is 0.1 to 0.65, OIa is the OI value of the first graphite and is 8 to 20, B is the height-width ratio of the second graphite and is 0.8 to 0.99, and OIb is the OI value of the second graphite and is 2 to 10.

Description

Composite negative electrode sheet and preparation method thereof, lithium ion battery

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on January 5, 2024, with application number 2024100202242 and invention name "A composite negative electrode sheet and its preparation method, lithium-ion battery", the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The embodiments of the present invention relate to, but are not limited to, the technical field of lithium batteries, and specifically, to a composite negative electrode sheet and a preparation method thereof, and a lithium-ion battery. Background Art

[0003] With the rapid development of the electric vehicle industry, people are placing higher demands on the comprehensive performance of lithium-ion batteries, especially the growing demand for energy density and fast charging performance. From a technical perspective, lithium-ion batteries complete the charging and discharging process through the insertion and extraction of ions between the positive and negative active materials. Therefore, the negative electrode has a significant impact on battery performance.

[0004] Traditional graphite anode materials offer advantages such as high electronic conductivity, structural stability, and economical and environmentally friendly performance. However, their limited theoretical capacity makes it difficult to meet the growing energy density demands of lithium-ion batteries. While increasing the anode coating weight and compaction can improve some of this, it can degrade the fast-charging performance of the battery cell, making it impossible to achieve both energy density and fast-charging performance. Summary of the Invention

[0005] The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

[0006] An embodiment of the present invention discloses a composite negative electrode sheet, comprising a negative electrode current collector, a first active material layer, and a second active material layer, wherein the first active material layer is disposed on a surface of the negative electrode current collector, and the second active material layer is disposed on a surface of the first active material layer, wherein the first active material layer comprises a first graphite, and the second active material layer comprises a second graphite, wherein the aspect ratio and OI value of the first graphite and the aspect ratio and OI value of the second graphite satisfy the following relationship: 2≤(B×OIa) / (A×OIb)≤30;

[0007] Wherein, A is the aspect ratio of the first graphite, which is 0.1 to 0.65, OIa is the OI value of the first graphite, which is 8 to 20, B is the aspect ratio of the second graphite, which is 0.8 to 0.99, and OIb is the OI value of the second graphite, which is 2 to 10.

[0008] Optionally, the aspect ratio and OI value of the first graphite and the aspect ratio and OI value of the second graphite satisfy the following relationship: 2≤(B×OIa) / (A×OIb)≤10.

[0009] Optionally, the aspect ratio and OI value of the first graphite and the aspect ratio and OI value of the second graphite satisfy the following relationship: 3≤(B×OIa) / (A×OIb)≤6.

[0010] Optionally, the aspect ratio A of the first graphite is 0.3 to 0.6.

[0011] Optionally, the aspect ratio B of the second graphite is 0.8 to 0.95.

[0012] Optionally, the OI value OIa of the first graphite is 8 to 14.

[0013] Optionally, the OI value OIb of the second graphite is 2 to 6.

[0014] The present invention also discloses a method for preparing a composite negative electrode sheet, which is used to prepare the composite negative electrode sheet as described above, comprising:

[0015] Mixing a first graphite, a first dispersant, a first conductive agent, and a first binder in deionized water to prepare a first active material slurry; mixing a second graphite, a second dispersant, a second conductive agent, and a second binder in deionized water to prepare a second active material slurry; the relationship between the aspect ratio and OI value of the first graphite and the aspect ratio and OI value of the second graphite is: 2≤(B×OIa) / (A×OIb)≤30; wherein A is the aspect ratio of the first graphite, which is 0.1 to 0.65, OIa is the OI value of the first graphite, which is 8 to 20, B is the aspect ratio of the second graphite, which is 0.8 to 0.99, and OIb is the OI value of the second graphite, which is 2 to 10;

[0016] The first active material slurry is coated on the surface of the negative electrode current collector, and the second active material slurry is coated on the surface of the first active material slurry. After drying, the first active material slurry forms a first active material layer, and the second active material slurry forms a second active material layer. After cold pressing and cutting, a composite negative electrode sheet is obtained.

[0017] Optionally, the first dispersant includes carboxymethyl cellulose, the first conductive agent includes conductive carbon black, the first binder includes chloroprene rubber, the second dispersant includes carboxymethyl cellulose, the second conductive agent includes conductive carbon black, and the second binder includes chloroprene rubber.

[0018] An embodiment of the present invention further discloses a lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. The negative electrode sheet is the composite negative electrode sheet as described above.

[0019] The composite negative electrode sheet and preparation method thereof, and the lithium-ion battery of the embodiment of the present invention have the following beneficial effects: a double-layer coating design is adopted, and a first active material layer and a second active material layer are sequentially coated on the surface of the negative electrode current collector. At the same time, the two active material layers respectively use graphite with different aspect ratios and OI values. The aspect ratios and OI values ​​of the two layers of graphite form a mutually restrictive relationship, which jointly affects the energy density, fast charging performance and cycle performance of the composite negative electrode sheet. By jointly controlling the aspect ratio and OI value of the graphite in the two active material layers, they are selected within their respective value ranges and satisfy a certain proportional relationship, and ultimately a higher electrode sheet compaction density and a lower membrane resistance are obtained. The lithium-ion battery made using the composite negative electrode sheet of the embodiment of the present invention has the advantages of high energy density, high cycle performance and high fast charging performance.

[0020] Still other aspects will become apparent upon reading and understanding the detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG1 is a schematic structural diagram of a composite negative electrode sheet according to an embodiment of the present invention;

[0022] FIG2 is another schematic structural diagram of a composite negative electrode sheet according to an embodiment of the present invention;

[0023] FIG3 shows a method for preparing a composite negative electrode sheet according to an embodiment of the present invention.

[0024] Explanation of reference numerals: 1 - negative electrode current collector; 2 - first active material layer; 3 - second active material layer. DETAILED DESCRIPTION

[0025] The traditional graphite negative electrode sheet consists of a current collector and a negative electrode membrane. The negative electrode membrane contains graphite, a conductive agent and a binder. It is coated on the current collector at one time during the coating process. After drying and roller pressing, the negative electrode sheet is obtained, and the pores in the sheet are evenly distributed.

[0026] Research has shown that during battery charging, the diffusion pattern of lithium ions within the negative electrode plate follows a gradient. Specifically, the lithium ion diffusion rate is higher on the side away from the current collector and lower on the side close to the current collector. When the lithium ion concentration on the side away from the current collector exceeds the kinetic tolerance limit of graphite, metallic lithium will precipitate on the surface of the negative electrode plate, causing rapid battery capacity decay and posing a safety hazard.

[0027] Theoretically, the above-mentioned problem of metallic lithium precipitation can be effectively solved by using high-kinetic graphite to prepare negative electrode plates. However, the use of high-kinetic graphite to prepare negative electrode plates is often accompanied by a higher plate porosity, which is not conducive to the design of high-energy density battery cells. In addition, high-kinetic graphite and electrolyte produce more side reactions, which will cause the battery cell capacity to decay rapidly during the cycle process, which is not conducive to improving the overall performance of the battery cell.

[0028] Given that traditional graphite negative electrode sheets have low energy density and poor fast-charging performance, the embodiments of the present invention provide a composite negative electrode sheet that effectively improves the fast-charging performance of the negative electrode sheet while ensuring high energy density and excellent cycle performance. The energy density and fast-charging performance are significantly improved compared to traditional graphite negative electrode sheets.

[0029] In order to make the above-mentioned objects, features and advantages of the embodiments of the present invention more obvious and easy to understand, specific embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0030] Referring to Figures 1 and 2 , a composite negative electrode sheet according to an embodiment of the present invention includes a negative electrode current collector 1, a first active material layer 2, and a second active material layer 3. The first active material layer 2 is disposed on the surface of the negative electrode current collector 1, and the second active material layer 3 is disposed on the surface of the first active material layer 2. The first active material layer 2 includes a first graphite, and the second active material layer 3 includes a second graphite. The aspect ratio and OI value of the first graphite and the aspect ratio and OI value of the second graphite meet the following relationship: 2≤(B×OIa) / (A×OIb)≤30;

[0031] Wherein, A is the aspect ratio of the first graphite, which is 0.1 to 0.65, OIa is the OI value of the first graphite, which is 8 to 20, B is the aspect ratio of the second graphite, which is 0.8 to 0.99, and OIb is the OI value of the second graphite, which is 2 to 10.

[0032] The inventors found through extensive experimental research and data analysis that the aspect ratio and OI value (graphite orientation) of the second graphite and the first graphite in the composite negative electrode sheet are the main factors affecting its energy density, fast charging performance and cycle performance.

[0033] The aspect ratio represents the ratio of the minimum Feret diameter to the maximum diameter of the particle, and is usually used to screen slender particles or spherical particles. The aspect ratio reflects the morphology of the graphite particles. The lower the aspect ratio, the flatter the graphite. The particles are more densely stacked during the rolling process, and the prepared negative electrode sheet has a higher compaction density. At the same time, a low aspect ratio will also lead to obvious anisotropy on the surface of the graphite particles, or in other words, the degree of isotropy on the graphite surface is low, which is not conducive to the rapid insertion and removal of lithium ions, and the fast charging performance is poor. The higher the aspect ratio, the higher the degree of spheroidization of the graphite, the more obvious its surface isotropy, the lower the charge transfer resistance, which is conducive to the rapid insertion and removal of lithium ions, and the better fast charging performance. At the same time, a high aspect ratio will also cause the graphite particles to be loosely stacked during the rolling process, and the prepared negative electrode sheet has a lower compaction density.

[0034] The OI value of graphite is an indicator used to measure the conductive properties of graphite materials. It represents the ratio of the resistivity of the graphite material to the vacuum resistivity. The OI value of graphite reflects the crystal plane orientation of the graphite particles. The higher the OI value, the closer the graphite crystal structure is to parallel arrangement, the greater the true density of the particles, the fewer lithium ion insertion channels, the longer the insertion path, and the poorer the fast-charging performance. The lower the OI value, the closer the graphite crystal structure is to vertical arrangement, the lower the true density of the particles, the more lithium ion insertion channels, the shorter the insertion path, and the better the fast-charging performance. It can be seen that the OI value directly affects the fast-charging performance of graphite.

[0035] During their research and analysis, the inventors also found that by jointly controlling the aspect ratio and OI value of the first graphite and the second graphite in the composite negative electrode sheet, the negative electrode sheet can simultaneously have high energy density, high fast charging performance and high cycle performance.

[0036] Specifically, when the lower limit of (B×OIa) / (A×OIb) is lower than 2 due to the aspect ratio B of the second graphite being too low, or the OI value OIa of the first graphite being too low, or the aspect ratio A of the first graphite being too high, or the OI value OIb of the second graphite being too high, the overall performance of the battery is poor. This is because, when the aspect ratio B of the second graphite is too low, its surface isotropy is low, and there are fewer active sites for lithium ion intercalation and deintercalation, resulting in poor battery fast charging performance. When the OI value OIb of the second graphite is too high, there are fewer active sites for lithium ions to be embedded in the second graphite and the intercalation path is longer, which also results in poor battery fast charging performance. When the aspect ratio A of the first graphite is too high, the first graphite particles are too loosely stacked, which is not conducive to improving the energy density of the battery, and there are more side reactions between the first graphite and the electrolyte, resulting in poor battery cycle performance. When the OI value OIa of the first graphite is too low, there are more sites on the graphite surface available for lithium ion embedding, which will also lead to an increase in side reactions between the first graphite and the electrolyte, resulting in a decrease in battery cycle performance.

[0037] When the upper limit of (B×OIa) / (A×OIb) exceeds 30 due to the OI value OIa of the first graphite being too high, or the aspect ratio A of the first graphite being too low, or the OI value OIb of the second graphite being too low, the overall performance of the battery is also poor. This is because, when the OI value OIa of the first graphite is too high, the first graphite has too few active sites for lithium ion embedding and the path is too long, resulting in poor fast charging performance of the battery. When the aspect ratio A of the first graphite is too low, it will also result in too few active sites for lithium ion embedding in the first graphite, and the fast charging performance will deteriorate. When the OI value OIb of the second graphite is too low, there are too many active sites on the surface of the second graphite for lithium ion embedding, resulting in an increase in side reactions between the electrolyte and graphite, and poor cycle performance.

[0038] In this embodiment, the aspect ratio A of the first graphite is 0.1 to 0.65, and the aspect ratio B of the second graphite is 0.8 to 0.99. The first active material layer is designed as high-density flat graphite to obtain a high-energy density electrode, and the second active material layer is designed as low-density spherical graphite to obtain a high-fast charging electrode. By rationally matching the aspect ratios of the graphites in the first and second active material layers, the electrode sheet achieves high energy density. In this embodiment, the OI value of the first graphite is 8 to 20, and the OI value of the second graphite is 2 to 10. The rational matching of the OI values ​​of the graphites in the first and second active material layers facilitates uniform diffusion of lithium ions within the negative electrode sheet, thereby achieving excellent fast charging performance for the electrode sheet.

[0039] The inventors further discovered that the aspect ratio A of the first graphite, the OI value OIa of the first graphite, the aspect ratio B of the second graphite, and the OI value OIb of the second graphite form a mutually restrictive relationship, jointly affecting the energy density, fast charging performance, and cycle performance of the composite negative electrode plate, and cannot be too large or too small at the same time. Through extensive research, the inventors further discovered that when the aspect ratio A of the first graphite, the OI value OIa of the first graphite, the aspect ratio B of the second graphite, and the OI value OIb of the second graphite in the composite negative electrode plate satisfy 2≤(B×OIa) / (A×OIb)≤30, the prepared lithium-ion battery has better overall performance, with higher energy density, better fast charging performance, and better cycle performance.

[0040] The composite negative electrode plate of this embodiment adopts a double-layer coating design, and the first active material layer 2 and the second active material layer 3 are coated on the surface of the negative electrode current collector 1 in sequence. The first active material layer 2 can be coated on one side (as shown in Figure 1) or both sides (as shown in Figure 2) of the negative electrode current collector 1, and the second active material layer 3 is coated on the surface of the first active material layer 2. The active material of the first active material layer 2 is a first graphite, and the active material of the second active material layer 3 is a second graphite. By jointly controlling the aspect ratio and OI value of the graphite in the two active material layers, a higher electrode compaction density and a lower membrane resistance are obtained at the same time. The lithium-ion battery made using the composite negative electrode plate of this embodiment has the characteristics of high energy density, excellent cycle performance and fast charging performance.

[0041] In an optional embodiment, in order to better balance the relationship between the aspect ratio A of the first graphite, the OI value OIa of the first graphite, the aspect ratio B of the second graphite, and the OI value OIb of the second graphite, the composite negative electrode plate satisfies 2≤(B×OIa) / (A×OIb)≤10. In another optional embodiment, the composite negative electrode plate satisfies 3≤(B×OIa) / (A×OIb)≤6. Meeting the above conditions can enable the lithium-ion battery to have higher energy density, better fast charging performance, and better cycle performance.

[0042] In some embodiments, the aspect ratio A of the first graphite is 0.3 to 0.6, the aspect ratio B of the second graphite is 0.8 to 0.95, the OI value of the first graphite is 8 to 14, and the OI value of the second graphite is 2 to 6.

[0043] The inventors have found that by making the aspect ratio and OI value of the first graphite and the second graphite satisfy the above-mentioned ratio relationship conditions, and controlling the aspect ratio of the first graphite and the second graphite within the above-mentioned range, the negative electrode plate made by combining high-fast charging low-compaction spherical graphite and high-energy density high-compaction flat graphite has higher energy density and better fast charging performance than traditional graphite negative electrode plates, among which the energy density is increased by 20% to 30%, and the fast charging capability is increased by more than 20%.

[0044] In some embodiments, the negative electrode current collector is copper foil. The type of the negative electrode current collector is not particularly limited and can be selected according to actual needs, for example, the copper foil mentioned above can be selected.

[0045] The present invention also provides a method for preparing a composite negative electrode sheet. Referring to FIG3 , the method includes:

[0046] A first graphite, a first dispersant, a first conductive agent, and a first binder are mixed in deionized water to prepare a first active material slurry; a second graphite, a second dispersant, a second conductive agent, and a second binder are mixed in deionized water to prepare a second active material slurry; wherein the first graphite and the second graphite are selected according to the following relationship: 2≤(B×OIa) / (A×OIb)≤30, A is 0.1 to 0.65, B is 0.8 to 0.99, OIa is 8 to 20, and OIb is 2 to 10. The dispersant, conductive agent, and binder used in the two slurries can be the same or different. For ease of description, the following description will uniformly use dispersant, conductive agent, and binder, without distinguishing between first and second.

[0047] The first active material slurry is coated on the surface of the negative electrode current collector 1, and the second active material slurry is coated on the surface of the first active material slurry. After drying, the first active material slurry forms a first active material layer 2, and the second active material slurry forms a second active material layer 3, thereby obtaining a composite negative electrode sheet.

[0048] In a specific example, a double-die extrusion coater can be used to coat the active material slurry on one or both sides of the negative electrode current collector copper foil. For example, the first active material slurry is coated on the surface of the copper foil, and the second active material slurry is coated on the surface of the first active material slurry. The coated electrode sheets are dried, cold pressed, and cut to obtain composite negative electrode sheets.

[0049] In some embodiments, the dispersant is CMC (carboxymethyl cellulose), the conductive agent is conductive carbon black, and the binder is SBR (chloroprene rubber). The first active material slurry and the second active material slurry use the same dispersant, conductive agent, and binder. Taking the first active material slurry as an example, its preparation method is as follows:

[0050] 70% CMC and conductive carbon black were added to deionized water and stirred at low speed for 2 hours to form a conductive adhesive. The first graphite was added to the conductive adhesive and stirred at low viscosity and high speed for 1 hour to form a first mixed solution. SBR was added to the first mixed solution and stirred at high viscosity and high speed for 1 hour to form a second mixed solution. Ethanol and the remaining 30% CMC powder were added to the second mixed solution and stirred at low viscosity and high speed for 2 hours to prepare the first active material slurry. The preparation method of the second active material slurry was the same as that of the first active material slurry, except that the second graphite was added to the conductive adhesive.

[0051] The preparation of the first active material slurry and the second active material slurry is collectively referred to as the preparation of the negative electrode slurry. In this embodiment, only one optional preparation method is provided. The specific preparation method of the negative electrode slurry can also be selected from other optional preparation methods, which is not limited in this embodiment.

[0052] An embodiment of the present invention further provides a lithium-ion battery, which includes a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator, wherein the negative electrode sheet adopts the composite negative electrode sheet mentioned above.

[0053] The following will further illustrate the beneficial effects achieved by the embodiments of the present invention in conjunction with examples and comparative examples. The lithium secondary batteries of the embodiments and comparative examples were prepared according to the following method.

[0054] A method for preparing a lithium secondary battery comprises the following steps:

[0055] (1) Preparation of positive electrode sheet

[0056] The positive electrode active material NCM811 (a type of ternary lithium battery), the conductive agent carbon nanotubes, and the binder PVDF (polyvinylidene fluoride) are mixed in a mass ratio of 94:3:4, and the solvent NMP (nitrogen methyl pyrrolidone) is added. The mixture is stirred evenly to obtain a positive electrode slurry; the positive electrode slurry is evenly coated on both sides of the positive electrode current collector aluminum foil, dried in an oven, and then cold pressed and cut to obtain positive electrode sheets.

[0057] (2) Preparation of composite negative electrode sheets

[0058] A first active material slurry and a second active material slurry were prepared. The mass ratio of the negative electrode active material, conductive carbon black, CMC, and SBR in the active material slurry was 93:2:2:3, and the solid content was 50%. The specific preparation method of the active material slurry was as follows: deionized water, conductive carbon black, and 70% of CMC powder were added to a stirring tank and stirred at low speed for 2 hours to form a conductive adhesive; the negative electrode active material was added to the conductive adhesive and stirred at low viscosity and high speed for 1 hour to form a first mixed solution; SBR was added to the first mixed solution and stirred at high viscosity and high speed for 1 hour to form a second mixed solution; ethanol and the remaining 30% of CMC powder were added to the second mixed solution and stirred at low viscosity and high speed for 2 hours to prepare the negative electrode slurry. The volume ratio of deionized water to ethanol in the slurry was 9:1.

[0059] The negative electrode active material selects the first graphite or the second graphite according to the principles in Table 1, the first active material slurry selects the first graphite as the negative electrode active material, and the second active material slurry selects the second graphite as the active material; a double-die extrusion coater is used to evenly coat the active material slurry on both sides of the negative electrode current collector copper foil, wherein the first active material slurry is coated on the surface of the copper foil, and the second active material slurry is coated on the surface of the first active material slurry. The coated electrode is dried in an oven and then cold pressed and cut to obtain a composite negative electrode electrode.

[0060] (3) Preparation of electrolyte

[0061] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then fully dried lithium salt LiPF6 is dissolved in the mixed organic solvent at a concentration of 1 mol / L to obtain an electrolyte after full dissolution.

[0062] (4) Preparation of isolation membrane

[0063] Polyethylene film was selected as the separator.

[0064] (5) Preparation of lithium secondary batteries

[0065] The positive electrode sheet, separator, and composite negative electrode sheet are stacked in order, with the separator placed between the positive and negative electrode sheets to serve as an isolation, and then wound to obtain a bare cell; the bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte, and after vacuum packaging, standing, forming, shaping and other processes, a lithium secondary battery is obtained.

[0066] According to Table 1, different first graphites and second graphites are respectively selected, and the lithium secondary batteries prepared in the embodiments and comparative examples are subjected to performance tests, and the performance tests include kinetic performance tests, cycle performance tests and energy density tests. It can be understood that the quality of the battery's kinetic performance will directly affect the fast charging performance of the battery. If the battery's kinetic performance is good, it will be able to accept charge faster during fast charging and maintain performance more stably during the charging process. On the contrary, if the battery's kinetic performance is poor, problems such as insufficient internal battery reaction rate and difficulty in charge transfer may occur during fast charging, resulting in a decrease in fast charging performance and may even cause safety hazards. Therefore, this embodiment tests the fast charging performance of the battery by testing the battery's kinetic performance.

[0067] The specific test methods for each of the above tests are as follows:

[0068] (1) Dynamic performance test: At 25°C, the lithium secondary batteries prepared in the examples and comparative examples were fully charged at a 4C rate and fully discharged at a 1C rate for 10 times, and then the lithium-ion batteries were fully charged at a 4C rate. The composite negative electrode plates were then disassembled and the lithium deposition on the surface of the composite negative electrode plates was observed. A lithium deposition area of ​​less than 5% on the surface of the composite negative electrode plate was considered to be mild lithium deposition, a lithium deposition area of ​​5% to 40% on the surface of the composite negative electrode plate was considered to be moderate lithium deposition, and a lithium deposition area of ​​more than 40% on the surface of the composite negative electrode plate was considered to be severe lithium deposition.

[0069] (2) Cycling performance test: At 25°C, the lithium secondary batteries prepared in the examples and comparative examples were charged at a 2C rate and discharged at a 1C rate, and a full charge-discharge cycle test was performed until the capacity of the lithium secondary battery decayed to 80% of the initial capacity. The number of cycles was recorded.

[0070] (3) Actual energy density test: At 25°C, the lithium-ion batteries prepared in the examples and comparative examples were fully charged at a 1C rate and fully discharged at a 1C rate, and the actual discharge energy at this time was recorded; at 25°C, the lithium-ion batteries were weighed using an electronic balance; the ratio of the 1C actual discharge energy of the lithium-ion battery to the weight of the lithium-ion battery was the actual energy density of the lithium-ion battery. When the actual energy density was less than 80% of the target energy density, the actual energy density of the battery was considered to be very low; when the actual energy density was greater than or equal to 80% of the target energy density and less than 95% of the target energy density, the actual energy density of the battery was considered to be low; when the actual energy density was greater than or equal to 95% of the target energy density and less than 105% of the target energy density, the actual energy density of the battery was considered to be moderate; when the actual energy density was greater than or equal to 105% of the target energy density and less than 120% of the target energy density, the actual energy density of the battery was considered to be high; when the actual energy density was greater than 120% of the target energy density, the actual energy density of the battery was considered to be very high.

[0071] The following describes the test methods for graphite parameters in the examples and comparative examples.

[0072] Aspect ratio of graphite: obtained by using an R-3000 particle size and shape analyzer. The powder sample is evenly dispersed on the surface of an optical glass plate and photographed using a telecentric zoom lens. The aspect ratio data of the graphite is obtained by statistically analyzing the ratio of the minimum and maximum Feret diameters of the graphite particles.

[0073] OI of graphite: The OI of graphite was determined using an X-ray powder diffractometer (X'pertPRO). An X-ray diffraction pattern was obtained in accordance with the general principles of X-ray diffraction analysis and the method for determining the lattice parameters of graphite (JIS K 0131-1996 and JB / T 4220-2011). The OI of graphite was calculated using the formula VOI = C004 / C110, where C004 is the peak area of ​​the 004 characteristic diffraction peak, and C110 is the peak area of ​​the 110 characteristic diffraction peak.

[0074] The parameters and test results of Examples 1-11 and Comparative Examples 1-5 are shown in Table 1.

[0075] Table 1

[0076] Overall, in Examples 1-11, the aspect ratio A of the first graphite, the OI value OIa of the first graphite, the aspect ratio B of the second graphite, and the OI value OIb of the second graphite in the composite negative electrode sheet all satisfy 2≤(B×OIa) / (A×OIb)≤30, while Comparative Examples 1-4 do not satisfy the above conditions. By comparing the test results of Examples 1-10 and Comparative Examples 1-4, it can be seen that when the aspect ratio A of the first graphite, the OI value OIa of the first graphite, the aspect ratio B of the second graphite, and the OI value OIb of the second graphite in the composite negative electrode sheet satisfy 2≤(B×OIa) / (A×OIb)≤30, the energy density, kinetic performance, and cycle performance of the lithium-ion battery are better, as shown by having a higher or moderate energy density, no or slight lithium deposition on the surface of the negative electrode sheet, and a higher number of cycles. This is mainly due to the good combination of the first graphite and the second graphite in the composite negative electrode sheet provided in this embodiment. The second graphite with a high aspect ratio and a low OI value provides excellent dynamic performance for the composite negative electrode sheet, and the first graphite with a low aspect ratio and a high OI value provides a higher energy density and excellent cycle performance for the composite negative electrode sheet, ultimately making the prepared battery have excellent overall performance.

[0077] However, when the OI value OIa of the first graphite in the composite negative electrode sheet is too high, the aspect ratio A of the first graphite is too low, or the OI value OIb of the second graphite is too low, resulting in the upper limit of (B×OIa) / (A×OIb) exceeding 30, the overall performance of the battery is poor. For example, in Comparative Examples 1 and 2, (B×OIa) / (A×OIb) is far greater than 30, resulting in severe lithium deposition on the surface of the battery negative electrode sheet, and the cycle number of Comparative Example 2 is only 2098 cycles.

[0078] When the lower limit of (B×OIa) / (A×OIb) falls below 2 due to the second graphite aspect ratio B in the composite negative electrode sheet, the first graphite OI value OIa is too low, the first graphite aspect ratio A is too high, or the second graphite OI value OIb is too high, the overall battery performance is also poor. For example, in Comparative Examples 3 and 4, (B×OIa) / (A×OIb) is less than 2, the battery cell energy density is very low, and the cycle life is only 1133 and 1351 cycles.

[0079] Specifically, from Example 1 to Example 5, the aspect ratio and OI value of the first graphite did not change, the aspect ratio of the second graphite gradually increased, and the OI value of the second graphite gradually decreased, so that (B×OIa) / (A×OIb) gradually increased. From Example 6 to Example 10, the aspect ratio and OI value of the second graphite did not change, the aspect ratio of the first graphite gradually increased, and the OI value of the second graphite gradually decreased, so that (B×OIa) / (A×OIb) gradually decreased. A larger aspect ratio and a smaller OI value are conducive to improving the fast charging performance of the battery, but are not conducive to improving the compaction density of the negative electrode sheet. Therefore, from Example 1 to Example 5, and from Example 6 to Example 10, the energy density and the number of cycles tend to decrease, but the lithium deposition on the surface of the negative electrode sheet is improved.

[0080] According to Comparative Examples 1, 2, and 5, it can be seen that in order to ensure good overall performance of the battery, both condition 1 (2≤(B×OIa) / (A×OIb)≤30) and condition 2 (A is 0.1 to 0.65, OIa is 8 to 20, B is 0.8 to 0.99, and OIb is 2 to 10) are indispensable. Specific analysis:

[0081] In Comparative Examples 1 and 2, the aspect ratio A of the first graphite, the aspect ratio B of the second graphite, the OI value OIa of the first graphite, and the OI value OIb of the second graphite each satisfy the value ranges, i.e., A is 0.1 to 0.65, OIa is 8 to 20, B is 0.8 to 0.99, and OIb is 2 to 10. However, (B×OIa) / (A×OIb) is greater than 30, which does not meet the requirement of 2≤(B×OIa) / (A×OIb)≤30. As a result, the negative electrode sheets of Comparative Examples 1 and 2 exhibit severe lithium deposition on their surfaces and exhibit low cycle times. Thus, although the aspect ratios and OI values ​​of the first and second graphites are selected within their respective value ranges, the results are poor because the ratio relationship between them is not satisfied, and the goal of achieving both high energy density, high cycle performance, and excellent kinetic performance cannot be achieved.

[0082] In Comparative Example 5, although the aspect ratio and OI value of the first graphite and the second graphite meet the requirement of 2≤(B×OIa) / (A×OIb)≤30, the OI value OIa of the first graphite is not within the range of 8 to 20. Even though the ratio relationship between the aspect ratio and the OI value of the first graphite and the second graphite is met, the overall performance of the battery is still poor. As can be seen from Table 1, the energy density of Comparative Example 5 is low, lithium deposition is moderate on the surface of the negative electrode, and the number of cycles is only 1573.

[0083] Although the embodiments of the present invention are disclosed above, the protection scope of the embodiments of the present invention is not limited thereto. Those skilled in the art may make various changes and modifications without departing from the spirit and scope of the embodiments of the present invention, and these changes and modifications will fall within the protection scope of the embodiments of the present invention.

Claims

1. A composite negative electrode plate, comprising a negative current collector, a first active material layer, and a second active material layer. The first active material layer is disposed on the surface of the negative current collector, and the second active material layer is disposed on the surface of the first active material layer. The first active material layer includes first graphite, and the second active material layer includes second graphite. The aspect ratio and OI value of the first graphite and the aspect ratio and OI value of the second graphite satisfy the following relationship: 2 ≤ (B × OIa) / (A × OIb) ≤ 30; Among them, A is the aspect ratio of the first graphite, ranging from 0.1 to 0.65, OIa is the OI value of the first graphite, ranging from 8 to 20, B is the aspect ratio of the second graphite, ranging from 0.8 to 0.99, and OIb is the OI value of the second graphite, ranging from 2 to 10.

2. The composite negative electrode sheet according to claim 1, wherein, The aspect ratio and OI value of the first graphite and the aspect ratio and OI value of the second graphite satisfy the following relationship: 2 ≤ (B × OIa) / (A × OIb) ≤ 10.

3. The composite negative electrode sheet according to claim 1, wherein, The aspect ratio and OI value of the first graphite and the aspect ratio and OI value of the second graphite satisfy the following relationship: 3 ≤ (B × OIa) / (A × OIb) ≤ 6.

4. The composite negative electrode sheet according to claim 1, wherein, The aspect ratio A of the first graphite ranges from 0.3 to 0.

6.

5. The composite negative electrode sheet according to claim 1, wherein, The aspect ratio B of the second graphite ranges from 0.8 to 0.

95.

6. The composite negative electrode sheet according to claim 1, wherein, The OI value OIa of the first graphite ranges from 8 to 14.

7. The composite negative electrode sheet according to claim 1, wherein, The OI value OIb of the second graphite ranges from 2 to 6.

8. A method for preparing a composite negative electrode plate for preparing the composite negative electrode plate according to any one of claims 1-7, comprising: Mixing first graphite, a first dispersant, a first conductive agent, and a first binder in first deionized water to prepare a first active material slurry; Mixing second graphite, a second dispersant, a second conductive agent, and a second binder in second deionized water to prepare a second active material slurry; the relationship between the aspect ratio and OI value of the first graphite and the aspect ratio and OI value of the second graphite is: 2 ≤ (B × OIa) / (A × OIb) ≤ 30; wherein, A is the aspect ratio of the first graphite, ranging from 0.1 to 0.65, OIa is the OI value of the first graphite, ranging from 8 to 20, B is the aspect ratio of the second graphite, ranging from 0.8 to 0.99, and OIb is the OI value of the second graphite, ranging from 2 to 10; Coating the first active material slurry on the surface of the negative current collector, and coating the second active material slurry on the surface of the first active material slurry. After drying, the first active material slurry forms a first active material layer, and the second active material slurry forms a second active material layer, obtaining a composite negative electrode plate.

9. The preparation method of the composite negative electrode sheet according to claim 8, wherein, The first dispersant includes carboxymethyl cellulose, the first conductive agent includes conductive carbon black, the first binder includes chloroprene rubber, the second dispersant includes carboxymethyl cellulose, the second conductive agent includes conductive carbon black, and the second binder includes chloroprene rubber.

10. A lithium-ion battery, comprising a positive electrode plate, a negative electrode plate, an electrolyte, and a separator. The negative electrode plate uses the composite negative electrode plate according to any one of claims 1-7.

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