A composite negative electrode for lithium-ion batteries and its preparation thereof.

Abstract

This invention provides a negative electrode composite electrode for lithium-ion batteries, its preparation, and its application in lithium-ion batteries. The negative electrode composite electrode includes a current collector, and on both sides of the current collector, a first active material layer, a binder layer, and a second active material layer are sequentially disposed in a direction away from the current collector. The first active material layer is composed of a first main material, a long-range conductive agent, and a first binder. The binder layer is composed of a second binder, a conductive agent, and a lithium conductivity enhancing material. The second active material layer is composed of a second main material, an oil-absorbing conductive agent, and a third binder. The negative electrode composite electrode exhibits low tortuosity, strong lithium-ion transport capacity, low rebound rate, and good adhesion between layers under high compaction density. Therefore, the negative electrode composite electrode has good reaction kinetics, a low risk of lithium plating, and a low risk of separation between layers. Consequently, the lithium-ion battery prepared with the negative electrode composite electrode has excellent cycle stability.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of batteries, and relates to a negative electrode composite electrode of a lithium ion battery, in particular to a negative electrode composite electrode of a lithium ion battery and preparation thereof and a lithium ion battery. BACKGROUND

[0002] With the development of lithium ion batteries, the requirement for energy density is getting higher and higher. The energy density is often increased by increasing the compaction density. However, with the increase of the compaction density, the path for lithium ion to embed and de-embed in the inside of the electrode sheet will become longer, and it will also cause the decrease of the electrode sheet porosity, the difficulty of electrolyte infiltration, the increase of the electrode sheet tortuosity, and the increase of the lithium ion diffusion resistance.

[0003] The conventional single-layer coating will have the above problems, therefore, in the prior art, a pore-forming agent is added to increase the electrode sheet porosity, and a pore is formed in the inside of the active material to buffer the volume change in the charging and discharging process, but the pore-forming agent cannot efficiently form an open pore, and the residual pore-forming agent affects the performance of the battery. In addition, in the prior art, double-layer coating is also used, the upper and lower layers of the slurry are changed to make the electrode sheet have a gradient porosity, and the electrolyte infiltration is increased, which is beneficial to the embedding and de-embedding of lithium ions, and improves the kinetic performance of the electrode sheet, but there may be a problem of insufficient adhesion between the upper and lower layers of the slurry, which affects the cycle performance of the battery.

[0004] CN116914148A discloses a negative electrode sheet, a preparation method thereof, and a lithium ion battery. The negative electrode sheet comprises a current collector, a first coating layer, a second coating layer, and a third coating layer which are sequentially stacked. The first coating layer comprises a first graphite active material, the D50 of the first graphite active material is 20-25 μm, and the BET is ≤1 m 2 / g; the second coating layer comprises a silicon-based active material and a second graphite active material, the surface of the silicon-based active material is coated with sodium polyacrylate; the surface of the second graphite active material is coated with carbon, the D50 of the second graphite active material is 10-15 μm, and the BET is 1 m 2 / g-1.5 m 2 / g; the third coating layer comprises a third graphite active material with a porosity of 35-40%.

[0005] CN113178543A discloses a negative electrode sheet and a lithium ion battery. The negative electrode sheet comprises a current collector, a first coating layer, a second coating layer, and a third coating layer. The current collector is provided with a tab on one side, and at least one surface of the current collector is provided with a first coating area and a second coating area, and the first coating area is arranged close to the tab. The first coating layer covers the first coating area, the second coating layer covers the second coating area, and the thickness of the first coating layer is greater than that of the second coating layer. The third coating layer is arranged on the side of the first coating layer away from the current collector and the side of the second coating layer away from the current collector.

[0006] The negative electrode sheet of the lithium ion battery disclosed in the prior art has certain defects, and has the problems of large tortuosity of the negative electrode sheet under high compaction density, high rebound rate or poor adhesion performance, thereby leading to insufficient kinetics of the negative electrode sheet, easy occurrence of lithium precipitation, or easy separation between the coatings, thereby leading to poor cycle performance of the battery prepared by the negative electrode sheet in the prior art. Therefore, it is crucial to develop and design a new type of negative composite electrode of lithium ion battery and its preparation and lithium ion battery. SUMMARY

[0007] In view of the deficiencies in the prior art, the purpose of the present application is to provide a negative composite electrode of lithium ion battery and its preparation and lithium ion battery. The negative composite electrode provided by the present application has smaller tortuosity of the negative electrode sheet under high compaction density, stronger lithium ion transport capacity, lower rebound rate and better adhesion performance between layers. Therefore, the negative composite electrode has better reaction kinetics, lower risk of lithium precipitation and lower risk of separation between layers. Therefore, the lithium ion battery prepared by the negative composite electrode has excellent cycle stability.

[0008] To achieve this purpose, the present application adopts the following technical solutions:

[0009] In a first aspect, the present application provides a negative composite electrode of lithium ion battery, which comprises a current collector, and a first active material layer, a bonding layer and a second active material layer arranged in sequence on both sides of the current collector in a direction away from the current collector.

[0010] The first active material layer is composed of a first main material, a long-range conductive agent and a first binder;

[0011] The bonding layer is composed of a second binder, a conductive agent and a lithium conduction enhancement material;

[0012] The second active material layer is composed of a second main material, an oil-absorbing conductive agent and a third binder.

[0013] The first active material layer of the negative electrode composite electrode comprises a long-range conductive agent with high conductivity, which can increase the amount of the binder while reducing the amount of the conductive agent, thereby enhancing the adhesion between the first active material layer and the current collector and the adhesion between the first active materials, and reducing the risk of separation between the first active material layer and the current collector and between the first active materials during the cycle process; the negative electrode composite electrode comprises a binder layer comprising a lithium ion conduction enhancing material between the first active material layer and the second active material layer, which can enhance the adhesion between the first active material layer and the second active material layer, improve the lithium ion transmission capacity of the negative electrode composite electrode, and reduce the tortuosity of the negative electrode composite electrode; the second active material layer comprises an oil-absorbing conductive agent, thereby improving the electrolyte retention capacity of the negative electrode composite electrode, ensuring sufficient electrolyte in the later cycle process, and thereby improving the cycle performance.

[0014] The negative electrode composite electrode provided by the application has a small tortuosity, a strong lithium ion transmission capacity, a low rebound rate, and good adhesion between layers at a high compaction density; therefore, the negative electrode composite electrode has good reaction kinetics, a low risk of lithium precipitation, and a low risk of separation between layers; therefore, the lithium ion battery prepared from the negative electrode composite electrode has excellent cycle stability.

[0015] Preferably, the first active material comprises any one of graphite, soft carbon, hard carbon, or graphene, or a combination of at least two thereof, and typical but non-limiting combinations include a combination of graphite and soft carbon, a combination of soft carbon and hard carbon, a combination of hard carbon and graphene, a combination of graphite, soft carbon, and hard carbon, or a combination of graphite, soft carbon, hard carbon, and graphene.

[0016] Preferably, the long-range conductive agent comprises carbon nanotubes and / or nanocarbon fibers.

[0017] Preferably, the first binder comprises any one of carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyacrylic acid (PAA), polyvinyl alcohol (PVA), or sodium alginate (SA), or a combination of at least two thereof, and typical but non-limiting combinations include a combination of CMC and SBR, a combination of SBR and PAA, a combination of PAA and PVA, a combination of PVA and SA, or a combination of CMC, SBR, and PAA.

[0018] Preferably, the mass ratio of the first active material, the long-range conductive agent, and the first binder in the first active material layer is (92-99):(2-6):(0.5-1.5).

[0019] The long-range conductive agent in the first active material layer has excellent conductive performance, so that the first active material layer has high conductivity while the amount of the long-range conductive agent is not high; in addition, the content of the first binder in the first active material layer is high, which enhances the bonding performance of the first active material layer, thereby improving the bonding force between the first active material layer and the current collector and the bonding force between the first main materials in the first active material layer, reducing the risk of separation of the first active material layer from the current collector during the cycle process, and also reducing the risk of separation of the first main materials in the first active material layer, thereby reducing the rebound rate of the first active material layer, and thus reducing the rebound rate of the negative electrode composite electrode.

[0020] The mass ratio of the first main material to the long-range conductive agent in the first active material layer is (92-99):(2-6), for example, it can be 92:2, 92:3, 92:5, 92:6, 93:2, 93:3, 93:5, 93:6, 95:2, 95:3, 95:5, 95:6, 99:2, 99:3, 99:5 or 99:6, but is not limited to the listed values, and other values not listed in the value range are also applicable.

[0021] The mass ratio of the long-range conductive agent to the first binder in the first active material layer is (2-6):(0.5-1.5), for example, it can be 2:0.5, 2:1, 2:1.5, 3:0.5, 3:1, 3:1.5, 5:0.5, 5:1, 5:1.5, 6:0.5 or 6:1, but is not limited to the listed values, and other values not listed in the value range are also applicable.

[0022] Preferably, the second binder includes a combination of at least two of lithiumated polyacrylic acid (PAA-Li), lithium carboxymethyl cellulose (CMC-Li), SBR, CMC or PAA, and typical but non-limiting combinations include a combination of PAA-Li and CMC-Li, a combination of SBR and CMC, a combination of CMC and PAA, or a combination of SBR, CMC and PAA; preferably, a combination of PAA-Li and CMC-Li.

[0023] When PAA-Li is included in the second binder in the bonding layer, PAA-Li can improve the lithium ion conductivity of the bonding layer while ensuring the bonding force; when CMC-Li is included in the second binder in the bonding layer, CMC-Li can play a dispersion role while ensuring the bonding force, thereby improving the uniformity of the bonding layer.

[0024] Preferably, the conductive agent in the bonding layer comprises a combination of at least two of conductive carbon black (SP), vapor grown carbon nanofiber (VGCF) or single-walled carbon nanotube (SWCNT), typically but not limitedly a combination of SP and VGCF, a combination of SP and SWCNT, or a combination of SP, VGCF and SWCNT; preferably a combination of SP and VGCF.

[0025] Preferably, the lithium-conducting reinforcing material comprises lithium carbonate.

[0026] The use of lithium carbonate as the lithium-conducting reinforcing material in the bonding layer described in the present application can improve the lithium-conducting capacity of the bonding layer, thereby improving the lithium ion-conducting capacity of the negative composite electrode.

[0027] Preferably, the mass ratio of the second binder, the conductive agent and the lithium-conducting reinforcing material in the bonding layer is (20-40):(30-70):(1-10).

[0028] The mass ratio of the second binder and the conductive agent in the bonding layer described in the present application is (20-40):(30-70), for example, it can be 20:30, 20:37, 20:47, 20:57, 20:67, 20:70, 27:30, 27:37, 27:47, 27:57, 27:67, 27:70, 37:30, 37:37, 37:47, 37:57, 37:67, 37:70, 40:30, 40:37, 40:47, 40:57, 40:67 or 40:70, but not limited to the listed values, other values not listed in this range are also applicable.

[0029] The mass ratio of the conductive agent and the lithium-conducting reinforcing material in the bonding layer described in the present application is (30-70):(1-10), for example, it can be 30:1, 30:3, 30:7, 30:10, 30:1, 37:3, 37:7, 37:10, 47:1, 47:3, 47:7, 47:10, 57:1, 57:3, 57:7, 57:10, 67:1, 67:3, 67:7, 67:10, 70:1, 70:3 or 70:10, but not limited to the listed values, other values not listed in this range are also applicable.

[0030] Preferably, the second main material comprises any one of graphite, soft carbon, hard carbon or graphene, or a combination of at least two of them, typically but not limitedly a combination of graphite and soft carbon, a combination of soft carbon and hard carbon, a combination of hard carbon and graphene, a combination of graphite, soft carbon and hard carbon, or a combination of graphite, soft carbon, hard carbon and graphene.

[0031] Preferably, the oil absorption value of the oil-absorbing conductive agent is ≥ 280 mL / 100 g, for example, it can be 280 mL / 100 g, 282 mL / 100 g, 284 mL / 100 g, 286 mL / 100 g, 288 mL / 100 g, 290 mL / 100 g, 293 mL / 100 g, 295 mL / 100 g, 297 mL / 100 g, 300 mL / 100 g, 305 mL / 100 g, 310 mL / 100 g, 315 mL / 100 g, 320 mL / 100 g, 330 mL / 100 g, or 350 mL / 100 g, but not limited to the listed values, other values not listed in the range are also applicable.

[0032] Preferably, the oil-absorbing conductive agent comprises SP.

[0033] Preferably, the third binder comprises any one or a combination of at least two of CMC, SBR, PAA, PVA, or sodium alginate, and a typical but non-limiting combination includes a combination of CMC and SBR, a combination of PAA and PVA, a combination of PVA and sodium alginate, a combination of CMC, SBR, and PAA, or a combination of CMC, SBR, PAA, and PVA.

[0034] Preferably, the mass ratio of the second main material, the oil-absorbing conductive agent, and the third binder in the second active material layer is (92-99):(0.5-3):(2.5-5).

[0035] Preferably, the mass ratio of the second main material and the oil-absorbing conductive agent in the second active material layer is (92-99):(0.5-3), for example, it can be 92:0.5, 93:0.5, 95:0.5, 97:0.5, 99:0.5, 92:1.3, 93:1.3, 95:1.3, 97:1.3, 99:1.3, 92:2.3, 93:2.3, 95:2.3, 97:2.3, 99:2.3, 92:3, 97:3, or 99:3, but not limited to the listed values, other values not listed in the range are also applicable.

[0036] Preferably, the mass ratio of the oil-absorbing conductive agent and the third binder in the second active material layer is (0.5-3):(2.5-5), for example, it can be 0.5:2.5, 1.3:2.5, 1.7:2.5, 2.3:2.5, 2.7:2.5, 0.5:3.3, 1.3:3.3, 1.7:3.3, 2.3:3.3, 2.7:3.3, 0.5:4.3, 1.3:4.3, 1.7:4.3, 2.3:4.3, 2.7:4.3, 1.3:5, 1.7:5, 2.3:5, or 2.7:5, but not limited to the listed values, other values not listed in the range are also applicable.

[0037] Preferably, the thickness of the first active material layer is 15-140 μm, for example, it can be 15 μm, 25 μm, 35 μm, 45 μm, 55 μm, 65 μm, 75 μm, 85 μm, 95 μm, 100 μm, 110 μm, 120 μm, 130 μm or 140 μm, but is not limited to the listed values, and other values not listed in the range are also applicable.

[0038] Preferably, the thickness of the adhesive layer is 1-10 μm, for example, it can be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm or 10 μm, but is not limited to the listed values, and other values not listed in the range are also applicable, preferably 1-5 μm.

[0039] Preferably, the thickness of the second active material layer is 15-140 μm, for example, it can be 15 μm, 25 μm, 35 μm, 45 μm, 55 μm, 65 μm, 75 μm, 85 μm, 95 μm, 100 μm, 110 μm, 120 μm, 130 μm or 140 μm, but is not limited to the listed values, and other values not listed in the range are also applicable.

[0040] Preferably, the compacted density of the negative electrode composite is 1.35-1.80 g / cm 3 , for example, it can be 1.35 g / cm 3 , 1.4 g / cm 3 , 1.45 g / cm 3 , 1.5 g / cm 3 , 1.55 g / cm 3 , 1.6 g / cm 3 , 1.65 g / cm 3 , 1.7 g / cm 3 , 1.75 g / cm 3 or 1.8 g / cm 3 , but is not limited to the listed values, and other values not listed in the range are also applicable.

[0041] In a second aspect, the present application provides a preparation method of the negative electrode composite of the first aspect, the preparation method comprising:

[0042] The first slurry containing the first main material, the long-range conductive agent and the first binder is prepared; the second slurry containing the second binder, the conductive agent and lithium carbonate is prepared; the third slurry containing the second main material, the SP and the third binder is prepared; the first coating layer, the second coating layer and the third coating layer are sequentially stacked on the two side surfaces of the current collector in sequence in the direction away from the surface of the current collector by taking the first slurry, the second slurry and the third slurry as raw materials, and then cold pressing is performed to obtain the negative electrode composite tab.

[0043] Preferably, the method for preparing the first coating layer, the second coating layer and the third coating layer on the two side surfaces of the current collector in sequence in the direction away from the surface of the current collector by taking the first slurry, the second slurry and the third slurry as raw materials comprises:

[0044] The first slurry is first coated on the surface of the current collector, and the first coating layer is obtained after drying; then the second slurry and the third slurry are coated on the surface of the first coating layer by using a double-layer coating process to obtain the second coating layer and the third coating layer which are sequentially stacked in the direction away from the current collector.

[0045] In the present application, after the first coating layer is prepared, the second slurry which is not completely dried is brought into contact with the first coating layer and the third slurry by using a double-layer coating method, so that the adhesion between the second coating layer and the first coating layer and the third coating layer is enhanced, and the structural stability of the negative electrode composite electrode is improved.

[0046] Preferably, the method for preparing the first coating layer, the second coating layer and the third coating layer on the two side surfaces of the current collector in sequence in the direction away from the surface of the current collector by taking the first slurry, the second slurry and the third slurry as raw materials comprises:

[0047] The first slurry is first coated on the surface of the current collector, and the first coating layer is obtained after drying; then the second slurry is coated on the surface of the first coating layer, and the second coating layer is obtained after drying; and then the third slurry is coated on the surface of the second coating layer, and the third coating layer is obtained after drying.

[0048] Preferably, the thickness of the first coating layer is 30-150 μm, for example, can be 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm or 150 μm, but is not limited to the listed values, and other values not listed in the range are also applicable.

[0049] Preferably, the thickness of the second coating layer is 1-10 μm, for example, it can be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm or 10 μm, but is not limited to the listed values, other values not listed within the range are also applicable, preferably 1-5 μm.

[0050] Preferably, the thickness of the third coating layer is 30-150 μm, for example, it can be 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm or 150 μm, but is not limited to the listed values, other values not listed within the range are also applicable.

[0051] In a second aspect, the present application provides a lithium ion battery, which comprises the negative composite electrode of the first aspect.

[0052] The numerical ranges of the present application include not only the above-mentioned point values, but also any point values not mentioned within the above-mentioned numerical ranges. Due to the limited space and for the sake of simplicity, the present application does not list all the specific point values included in the range.

[0053] Compared with the prior art, the present application has the following beneficial effects:

[0054] (1) The negative composite electrode provided by the present application includes a long-range conductive agent with high conductivity in the first active material layer, which can increase the amount of binder while reducing the amount of conductive agent, thereby enhancing the adhesion between the first active material layer and the current collector, and the adhesion between the first main materials, and reducing the risk of separation between the first active material layer and the current collector, and the first main materials during the cycle process;

[0055] (2) The negative composite electrode provided by the present application inserts a binder layer containing lithium-conducting enhancement material between the first active material layer and the second active material layer, which can enhance the adhesion between the first active material layer and the second active material layer, improve the lithium ion transport capacity of the negative composite electrode, and reduce the tortuosity of the negative composite electrode;

[0056] (3) The negative composite electrode provided by the present application includes an oil-absorbing conductive agent in the second active material layer, thereby improving the electrolyte retention capacity of the negative composite electrode, ensuring sufficient electrolyte in the later cycle, and thereby improving the cycle performance. BRIEF DESCRIPTION OF DRAWINGS

[0057] Figure 1 FIG. 1 is a structural schematic diagram of the negative composite electrode of the lithium ion battery provided in embodiments 1-13 of the present application.

[0058] Wherein, 1-current collector; 2-first active material layer; 3-adhesive layer; 4-second active material layer. Detailed Implementation

[0059] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0060] Example 1

[0061] This embodiment provides a negative electrode composite electrode for a lithium-ion battery, such as... Figure 1 As shown, the compaction density of the negative electrode composite electrode is 1.55 g / cm³. 3 The negative electrode composite electrode includes a current collector 1 with a thickness of 6 μm, and a first active material layer 2 with a thickness of 45 μm, an adhesive layer 3 with a thickness of 2 μm, and a second active material layer 4 with a thickness of 45 μm, which are sequentially disposed on both sides of the current collector 1 in a direction away from the current collector 1.

[0062] The first active material layer 2 is composed of graphite, long-range conductive agent (VGCF and SWCNT in a mass ratio of 1:1) and first binder (CMC, SBR and PAA in a mass ratio of 1:1:3) in a mass ratio of 95:4:1.

[0063] The adhesive layer 3 is composed of a second adhesive (PAA-Li and CMC-Li in a mass ratio of 3:1), a conductive agent (SP and SWCNT in a mass ratio of 3:1, with SP having an oil absorption value of 290 mL / 100 g) in a mass ratio of 30:68:2, and lithium carbonate.

[0064] The second active material layer 4 is composed of graphite in a mass ratio of 95:2.5:2.5, SP with an oil absorption value of 290mL / 100g, and a third binder (CMC, SBR, and PAA in a mass ratio of 1:1:3).

[0065] The method for preparing the negative electrode composite electrode is as follows:

[0066] A first slurry containing graphite, a long-range conductive agent, and a first binder is prepared; a second slurry containing a second binder, a conductive agent, and lithium carbonate is prepared; a third slurry containing graphite, SP, and a third binder is prepared; the first slurry is first coated on the surface of the current collector 1, and after drying, a first coating with a thickness of 70 μm is obtained; then, the second slurry and the third slurry are coated on the surface of the first coating using a double-layer coating process, resulting in a second coating with a thickness of 3 μm and a third coating with a thickness of 70 μm stacked sequentially in a direction away from the current collector 1; then, cold pressing is performed to obtain the negative electrode composite electrode sheet.

[0067] Example 2

[0068] The present example provides a negative composite electrode of a lithium ion battery, the compaction density of the negative composite electrode is 1.55 g / cm 3 , the negative composite electrode comprises a current collector 1 with a thickness of 6 μm, and on both side surfaces of the current collector 1, a first active material layer 2 with a thickness of 45 μm, a bonding layer 3 with a thickness of 2 μm and a second active material layer 4 with a thickness of 45 μm are sequentially arranged in a direction away from the current collector 1;

[0069] The first active material layer 2 is composed of graphite, long-range conductive agent (VGCF and SWCNT with a mass ratio of 1:1) and first binder (CMC, SBR and PAA with a mass ratio of 1:2:3) with a mass ratio of 95:4:1;

[0070] The bonding layer 3 is composed of second binder (PAA-Li and CMC-Li with a mass ratio of 3:1), conductive agent (SP and SWCNT with a mass ratio of 3:1, the oil absorption value of SP is 290 mL / 100g) and lithium carbonate with a mass ratio of 27:70:2;

[0071] The second active material layer 4 is composed of graphite, SP with an oil absorption value of 290 mL / 100g and third binder (CMC, SBR and PAA with a mass ratio of 1:1:3) with a mass ratio of 95:2.5:2.5.

[0072] The preparation method of the negative composite electrode is as follows:

[0073] A first slurry containing a first main material, a long-range conductive agent and a first binder is prepared; a second slurry containing a second binder, a conductive agent and lithium carbonate is prepared; a third slurry containing a second main material, SP and a third binder is prepared; the first slurry is first coated on the surface of the current collector 1, and after drying, a first coating layer with a thickness of 70 μm is obtained; then the second slurry is coated on the surface of the first coating layer, and after drying, a second coating layer with a thickness of 3 μm is obtained; then the third slurry is coated on the surface of the second coating layer, and after drying, a third coating layer with a thickness of 70 μm is obtained, and then cold pressing is performed to obtain the negative composite electrode.

[0074] Example 3

[0075] The present example provides a negative composite electrode of a lithium ion battery, the compaction density of the negative composite electrode is 1.55 g / cm 3, the negative composite electrode comprises a current collector 1 with a thickness of 6 μm, and on both side surfaces of the current collector 1, a first active material layer 2 with a thickness of 45 μm, a bonding layer 3 with a thickness of 2 μm and a second active material layer 4 with a thickness of 45 μm are sequentially arranged in a direction away from the current collector 1;

[0076] The first active material layer 2 is composed of graphite, a long-range conductive agent (VGCF and SWCNT with a mass ratio of 1:1) and a first binder (CMC, SBR and PAA with a mass ratio of 1:2:3) with a mass ratio of 95:4:1;

[0077] The bonding layer 3 is composed of a second binder (PAA-Li and CMC-Li with a mass ratio of 3:1), a conductive agent (SP and SWCNT with a mass ratio of 3:1, and the oil absorption value of SP is 290 mL / 100g) and lithium carbonate with a mass ratio of 32:66:2;

[0078] The second active material layer 4 is composed of graphite, SP with an oil absorption value of 290 mL / 100g and a third binder (CMC, SBR and PAA with a mass ratio of 1:1:3) with a mass ratio of 95:2.5:2.5.

[0079] The preparation method of the negative composite electrode is as follows:

[0080] A first slurry containing a first main material, a long-range conductive agent and a first binder is prepared, a second slurry containing a second binder, a conductive agent and lithium carbonate is prepared, and a third slurry containing a second main material, SP and a third binder is prepared. The first slurry is first coated on the surface of the current collector 1, and after drying, a first coating layer with a thickness of 70 μm is obtained. Then, the second slurry and the third slurry are coated on the surface of the first coating layer by using a double-layer coating process to obtain a second coating layer with a thickness of 3 μm and a third coating layer with a thickness of 70 μm which are sequentially arranged in a direction away from the current collector 1. Finally, cold pressing is performed to compact the second coating layer and the third coating layer to obtain the negative composite electrode tab.

[0081] Example 4

[0082] The negative composite electrode of the lithium ion battery provided in this embodiment is the same as that in Example 1, except that the thickness of the first active material layer 2 is 45 μm, the thickness of the bonding layer 3 is 1.6 μm, and the thickness of the second active material layer 4 is 45 μm.

[0083] That is, in the preparation method of the negative composite electrode, the thickness of the first coating layer is 70 μm, the thickness of the second coating layer is 2.5 μm, and the thickness of the third coating layer is 70 μm, and the rest are the same as those in Example 1.

[0084] Example 5

[0085] The embodiment provides a negative composite electrode of a lithium ion battery, wherein, in addition to a first active material layer 2 with a thickness of 45 μm, a bonding layer 3 with a thickness of 2.3 μm and a second active material layer 4 with a thickness of 45 μm which are sequentially arranged in a direction away from the current collector 1,

[0086] That is, in the preparation method of the negative composite electrode, the thickness of the first coating layer is 70 μm, the thickness of the second coating layer is 3.5 μm, the thickness of the third coating layer is 70 μm, and the rest are the same as in embodiment 1.

[0087] Embodiment 6

[0088] The embodiment provides a negative composite electrode of a lithium ion battery, wherein, in addition to a first active material layer 2 with a thickness of 45 μm, a bonding layer 3 with a thickness of 2.3 μm and a second active material layer 4 with a thickness of 45 μm which are sequentially arranged in a direction away from the current collector 1,

[0089] That is, in the preparation method of the negative composite electrode, the thickness of the first coating layer is 70 μm, the thickness of the second coating layer is 3.5 μm, the thickness of the third coating layer is 70 μm, and the rest are the same as in embodiment 1.

[0090] Embodiment 7

[0091] The embodiment provides a negative composite electrode of a lithium ion battery, wherein the compacted density of the negative composite electrode is 1.35 g / cm 3 , the negative composite electrode comprises a current collector 1 with a thickness of 6 μm, and a first active material layer 2 with a thickness of 111 μm, a bonding layer 3 with a thickness of 3.7 μm and a second active material layer 4 with a thickness of 111 μm which are sequentially arranged in a direction away from the current collector 1 on two side surfaces of the current collector 1;

[0092] The first active material layer 2 is composed of graphene, a long-range conductive agent (carbon nanotube) and a first bonding agent (PVA or sodium alginate with a mass ratio of 1:1) with a mass ratio of 92:6:1.5;

[0093] The bonding layer 3 is composed of a second bonding agent (PAA-Li and CMC-Li with a mass ratio of 3:1), a conductive agent (SP and SWCNT with a mass ratio of 3:1) and lithium carbonate with a mass ratio of 40:30:10;

[0094] The second active material layer 4 is composed of graphene, SP with an oil absorption value of 300 mL / 100 g and a third bonding agent (CMC and SBR with a mass ratio of 1:1) with a mass ratio of 99:0.5:2.5.

[0095] The preparation method of the negative composite electrode is as follows:

[0096] A first slurry containing graphene, a long-range conductive agent and a first binder is prepared; a second slurry containing a second binder, a conductive agent and lithium carbonate is prepared; a third slurry containing graphene, SP and a third binder is prepared; the first slurry is first coated on the surface of the current collector 1, and after drying, a first coating layer with a thickness of 150 μm is obtained; then the second slurry and the third slurry are coated on the surface of the first coating layer by a double-layer coating process, to obtain a second coating layer with a thickness of 5 μm and a third coating layer with a thickness of 150 μm, which are sequentially stacked away from the current collector 1; and then cold pressing is performed to compact, to obtain the negative composite electrode sheet.

[0097] Example 8

[0098] The present embodiment provides a negative composite electrode of a lithium ion battery, the compaction density of the negative composite electrode is 1.80 g / cm 3 , the negative composite electrode comprises a current collector 1 with a thickness of 6 μm, and on the two side surfaces of the current collector 1, a first active material layer 2 with a thickness of 17 μm, a bonding layer 3 with a thickness of 1 μm and a second active material layer 4 with a thickness of 17 μm are sequentially arranged away from the current collector 1;

[0099] The first active material layer 2 is composed of hard carbon, a long-range conductive agent (nanometer carbon fiber) and a first binder (SBR and PAA with a mass ratio of 1:1) with a mass ratio of 99:2:0.5;

[0100] The bonding layer 3 is composed of a second binder (PAA-Li and CMC-Li with a mass ratio of 3:1), a conductive agent (SP and SWCNT with a mass ratio of 3:1) and lithium carbonate with a mass ratio of 20:70:1;

[0101] The second active material layer 4 is composed of hard carbon, SP with an oil absorption value of 320 mL / 100 g and a third binder (PAA, PVA and sodium alginate with a mass ratio of 3:1:1) with a mass ratio of 92:3:5.

[0102] The preparation method of the negative composite electrode is as follows:

[0103] A first slurry containing hard carbon, a long-range conductive agent and a first binder is prepared; a second slurry containing a second binder, a conductive agent and lithium carbonate is prepared; a third slurry containing hard carbon, SP and a third binder is prepared; the first slurry is first coated on the surface of the current collector 1, and after drying, a first coating layer with a thickness of 30 μm is obtained; then the second slurry is coated on the surface of the first coating layer, and after drying, a second coating layer with a thickness of 1 μm is obtained; then the third slurry is coated on the surface of the second coating layer, and after drying, a third coating layer with a thickness of 30 μm is obtained; and then cold pressing is performed to compact, to obtain the negative composite electrode sheet.

[0104] Example 9

[0105] This example provides a negative composite electrode of a lithium ion battery, which is identical to Example 1 except that the mass ratio of the long-range conductive agent to the first binder in the first active material layer 2 is 2:2.5.

[0106] Example 10

[0107] This example provides a negative composite electrode of a lithium ion battery, which is identical to Example 1 except that the mass ratio of the long-range conductive agent to the first binder in the first active material layer 2 is 6:0.3.

[0108] Example 11

[0109] This example provides a negative composite electrode of a lithium ion battery, which is identical to Example 1 except that the second binder in the adhesive layer 3 is replaced by PAA of the same mass.

[0110] Example 12

[0111] This example provides a negative composite electrode of a lithium ion battery, which is identical to Example 1 except that the second binder in the adhesive layer 3 is replaced by CMC of the same mass.

[0112] Example 13

[0113] This example provides a negative composite electrode of a lithium ion battery, which is identical to Example 1 except that the conductive agent in the adhesive layer 3 is replaced by SWCNT of the same mass.

[0114] Comparative Example 1

[0115] This comparative example provides a negative composite electrode of a lithium ion battery, which is identical to Example 1 except that the adhesive layer between the first active material layer and the second active material layer is omitted.

[0116] Comparative Example 2

[0117] This comparative example provides a negative composite electrode of a lithium ion battery, which has a compacted density of 1.55 g / cm 3 , and includes a current collector having a thickness of 6 μm, and active material layers having a thickness of 90 μm covering both side surfaces of the current collector 1;

[0118] The active material layer is composed of graphite, a conductive agent (SP and VGCF at a mass ratio of 3:1, the oil absorption value of SP being 290 mL / 100 g), and a binder (CMC, SBR, and PAA at a mass ratio of 1:1:3) at a mass ratio of 95:3:2.

[0119] The preparation method of the negative electrode composite electrode is as follows:

[0120] A slurry containing graphite, a conductive agent and a binder is prepared; the prepared slurry is coated on the surface of the current collector 1, and after drying, a coating layer with a thickness of 140 μm is obtained, and then cold pressing is performed to compact the coating layer, thereby obtaining the negative electrode composite electrode sheet.

[0121] Comparative Example 3

[0122] The comparative example provides a negative electrode composite electrode of a lithium ion battery, the compaction density of the negative electrode composite electrode is 1.55 g / cm 3 , the negative electrode composite electrode comprises a current collector with a thickness of 6 μm, and an active material layer with a thickness of 90 μm covering the two side surfaces of the current collector.

[0123] The active material layer is composed of graphite, a conductive agent (SP and VGCF with a mass ratio of 3:1, the oil absorption value of SP is 200 mL / 100 g) and a binder (CMC, SBR and PAA with a mass ratio of 1:1:3) with a mass ratio of 95:3:2.

[0124] The preparation method of the negative electrode composite electrode is as follows:

[0125] A slurry containing graphite, a conductive agent and a binder is prepared; the prepared slurry is coated on the surface of the current collector, and after drying, a coating layer with a thickness of 140 μm is obtained, and then cold pressing is performed to compact the coating layer, thereby obtaining the negative electrode composite electrode sheet.

[0126] Comparative Example 4

[0127] The comparative example provides a negative electrode composite electrode of a lithium ion battery, except that the long-range conductive agent in the first active material layer is replaced by SP with the same mass, and the rest is the same as Example 1.

[0128] Comparative Example 5

[0129] The comparative example provides a negative electrode composite electrode of a lithium ion battery, except that the lithium carbonate in the binder layer is omitted, and the rest is the same as Example 1.

[0130] Comparative Example 6

[0131] The comparative example provides a negative electrode composite electrode of a lithium ion battery, except that the SP with an oil absorption value of 290 mL / 100 g in the second active material layer is replaced by SP with an oil absorption value of 240 mL / 100 g, and the rest is the same as Example 1.

[0132] The negative composite electrode provided in the above examples and comparative examples was assembled into a lithium ion battery: the negative composite electrode provided in the above examples and comparative examples was prepared into a negative electrode sheet through slitting and die cutting processes.

[0133] The obtained negative electrode sheet was subjected to a sheet tortuosity test: the obtained negative electrode sheet was assembled into a symmetrical battery, and EIS was tested by an electrochemical workstation, with a frequency range of 100000 Hz-0.1 Hz and an amplitude of 5 mV; the Nyquist diagram obtained was fitted by an equivalent circuit, to obtain ion impedance Rion; the tortuosity was calculated by the formula τ = RionAεκ / t, wherein Rion is the liquid phase transmission impedance of ions in the electrode pores, A is the apparent area of the electrode, ε is the porosity of the electrode, κ is the ionic conductivity of the electrolyte, and t is the total thickness of the two electrodes; the tortuosity of the negative electrode sheet was tested and is shown in Table 1.

[0134] The obtained negative electrode sheet was subjected to a peeling force test: three groups of negative electrode sheets were taken from each example, respectively, and a CBH-1 type mechanical testing machine was used for a 180° peeling test, with a rate of 200 mm / min; the average of three measurements was the peeling force of the electrode sheet; the peeling force of the obtained negative electrode sheet is shown in Table 1.

[0135] The obtained negative composite electrode was subjected to a 24h rebound rate test: the thickness of the negative composite electrode obtained after the cold pressing and compaction step was immediately measured by a micrometer, and the average of 10 points was taken as the initial thickness; after 24h, the thickness was again measured by a micrometer, and the average of 10 points was taken as the thickness after 24h; the 24h electrode sheet rebound rate = thickness after 24h / initial thickness; the 24h electrode sheet rebound rate obtained is shown in Table 1.

[0136] The obtained negative composite electrode was subjected to a full charge rebound rate test: the thickness of the negative composite electrode obtained after the cold pressing and compaction step was immediately measured by a micrometer, and the average of 10 points was taken as the initial thickness; a negative electrode sheet was prepared from the negative composite electrode, and a lithium ion soft pack battery was prepared from the negative electrode sheet; the lithium ion soft pack battery after formation and capacity measurement was fully charged, then disassembled, and the thickness of the electrode sheet after disassembly was measured by a micrometer, and the average of 10 points was taken as the full charge electrode sheet thickness; the full charge rebound rate = full charge electrode sheet thickness / initial thickness; the full charge rebound rate obtained is shown in Table 1.

[0137] The negative electrode sheet prepared from the negative composite electrode was used as a negative electrode, lithium iron phosphate was used as a positive electrode, and 1M LiPF6 (EC:DEC:DMC = 1:1:1, volume ratio) was used as an electrolyte; the electrode sheets were stacked, liquid was injected, packaged, and vacuum sealed to obtain a lithium ion soft pack battery; the obtained lithium ion soft pack battery was subjected to a cycle stability test; the cycle stability test included a cycle stability test at 25°C and a cycle stability test at 45°C.

[0138] 25℃ under the cyclic stability test: at 25℃, the lithium iron phosphate battery was tested by 1C / 1C cycle test, the voltage interval was 2.5-3.65V, the capacity retention rate was calculated by the capacity after 500 cycles divided by the initial capacity, and the 25℃ cycle capacity retention rate was shown in Table 2.

[0139] 45℃ under the cyclic stability test: at 45℃, the lithium iron phosphate battery was tested by 1C / 1C cycle test, the voltage interval was 2.5-3.65V, the capacity retention rate was calculated by the capacity after 500 cycles divided by the initial capacity, and the 45℃ cycle capacity retention rate was shown in Table 2.

[0140] Table 1

[0141]

[0142] Table 2

[0143]

[0144] From Table 1, it can be seen that:

[0145] (1) The negative composite electrode of the lithium ion battery provided in Examples 1-8 has a lower tortuosity, a stronger peeling force, and a smaller 24h rebound rate and full charge rebound rate. The lithium ion soft package battery prepared from the negative composite electrode exhibits strong cycle stability;

[0146] (2) From the comparison of Example 1 with Examples 9 and 10, it can be seen that the mass ratio of the long-range conductive agent to the first binder in the first active material layer 2 will affect the performance of the negative composite electrode and the lithium ion soft package battery. When the mass ratio of the long-range conductive agent to the first binder is (2-6):(0.5-1.5), it is beneficial to obtain a negative composite electrode and a lithium ion soft package battery with better performance. This is because the appropriate amount of long-range conductive agent forms a three-dimensional conductive network between the graphite particles, promoting the transmission of electrons during charging and discharging. The appropriate amount of adhesive makes the three-dimensional conductive network able to withstand the expansion and contraction of the graphite particles during charging and discharging, and still maintains the integrity of the structure. Excessive long-range conductive agent is difficult to disperse and forms agglomerates, which cannot form an effective conductive network. Too little conductive network is incomplete. Too little adhesive results in weak adhesion, which cannot guarantee the integrity of the conductive network during the cycle process. Too much adhesive increases the electrode resistance and affects the transmission of electrons;

[0147] (3) Through the comparison of Example 1 and Examples 11 and 12, it can be seen that the second binder in the bonding layer 3 of the application can affect the performance of the negative composite electrode and the lithium ion soft package battery; when the binder is a combination of PAA-Li and CMC-Li, it is beneficial to obtain a negative composite electrode and a lithium ion soft package battery with better performance, because PAA-Li can improve the lithium ion transmission capacity of the bonding layer 3 while ensuring the bonding force, and CMC-Li can play a dispersing role while ensuring the bonding force, thereby improving the uniformity of the bonding layer 3;

[0148] (4) Through the comparison of Example 1 and Example 13, it can be seen that the conductive agent in the bonding layer 3 of the application can affect the performance of the negative composite electrode and the lithium ion soft package battery; when the conductive agent is a combination of SP and SWCNT, it is beneficial to obtain a negative composite electrode and a lithium ion soft package battery with better performance, because the proportion of the binder in the bonding layer 3 is large, and a long-diameter conductive agent SWCNT is needed to form a conductive network between the first active layer 2 and the second active layer 3 to ensure smooth electron transmission; in addition, due to the problem of difficult infiltration of electrolyte in the inner layer, the addition of SP with high oil absorption value makes the electrolyte in the bonding layer 3 sufficient, so that the first active layer 2 is more easily infiltrated by the electrolyte, and SP has the ability to preserve electrolyte, which can provide electrolyte for the first active layer in the later stage of the cycle, thereby improving the cycle performance;

[0149] (5) Through the comparison of Example 1 and Comparative Examples 1-6, it can be seen that the first active material layer 2 in the negative composite electrode provided by the application includes a long-range conductive agent with high conductivity, which can increase the amount of the binder while reducing the amount of the conductive agent, thereby enhancing the bonding force between the first active material layer 2 and the current collector 1, and the bonding force between the first main materials, and reducing the risk of separation between the first active material layer 2 and the current collector 1, and between the first main materials during the cycle; by inserting the bonding layer 3 containing lithium transmission enhancing material between the first active material layer 2 and the second active material layer 4 in the negative composite electrode, the bonding force between the first active material layer 2 and the second active material layer 4 can be enhanced, the lithium ion transmission capacity of the negative composite electrode can be improved, and the tortuosity of the negative composite electrode can be reduced; the second active material layer 4 in the negative composite electrode includes an oil-absorbing conductive agent, thereby improving the electrolyte preservation capacity of the negative composite electrode, ensuring sufficient electrolyte in the middle and late stages of the cycle, and thereby improving the cycle performance.

[0150] The above merely describes specific embodiments of the application, but the protection scope of the application is not limited thereto, and those skilled in the art should understand that any changes or replacements within the technical scope disclosed by the application can be easily thought of by those skilled in the art, and all fall within the protection scope and disclosure scope of the application.

Claims

1. A negative composite electrode for a lithium-ion battery, characterized by comprising: The negative composite electrode comprises a current collector, and on both sides of the current collector, a first active material layer, a bonding layer and a second active material layer are sequentially arranged in a direction away from the current collector; The first active material layer is composed of a first main material, a long-range conductive agent and a first bonding agent; The bonding layer is composed of a second bonding agent, a conductive agent and a lithium conduction enhancement material; The second bonding agent is a combination of PAA-Li and CMC-Li; The second active material layer is composed of a second main material, an oil-absorbing conductive agent and a third bonding agent; The oil absorption value of the oil-absorbing conductive agent is greater than or equal to 280 mL / 100 g.

2. The negative composite electrode according to claim 1, characterized by The first main material comprises any one or a combination of at least two of graphite, soft carbon, hard carbon or graphene.

3. The negative composite electrode according to claim 1, characterized by The long-range conductive agent comprises carbon nanotubes and / or nanocarbon fibers.

4. The negative composite electrode according to claim 1, characterized by The first bonding agent comprises any one or a combination of at least two of CMC, SBR, PAA, PVA or SA.

5. The negative composite electrode according to claim 1, wherein The mass ratio of the first main material, the long-range conductive agent and the first bonding agent in the first active material layer is (92-99):(2-6):(0.5-1.5).

6. The negative composite electrode according to claim 1, wherein The conductive agent in the bonding layer comprises a combination of at least two of SP, VGCF or SWCNT.

7. The negative composite electrode according to claim 1, wherein The conductive agent in the bonding layer is a combination of SP and VGCF.

8. The negative composite electrode according to claim 1, wherein The lithium conduction enhancement material comprises lithium carbonate.

9. The negative composite electrode according to claim 1, wherein The mass ratio of the second bonding agent, the conductive agent and the lithium conduction enhancement material in the bonding layer is (20-40):(30-70):(1-10).

10. The negative composite electrode according to claim 1, wherein The second main material comprises any one or a combination of at least two of graphite, soft carbon, hard carbon or graphene.

11. The negative composite electrode according to claim 1, wherein The oil-absorbing conductive agent comprises SP.

12. The negative composite electrode according to claim 1, wherein The third bonding agent comprises any one or a combination of at least two of CMC, SBR, PAA, PVA or sodium alginate.

13. The negative composite electrode according to claim 1, wherein The mass ratio of the second main material, the oil-absorbing conductive agent and the third bonding agent in the second active material layer is (92-99):(0.5-3):(2.5-5).

14. The negative composite electrode according to claim 1, wherein The thickness of the first active material layer is 15-140 μm.

15. The negative composite electrode according to claim 1, wherein The thickness of the bonding layer is 1-10 μm.

16. The negative composite electrode according to claim 1, wherein The thickness of the bonding layer is 1-5 μm.

17. The negative composite electrode according to claim 1, wherein The thickness of the second active material layer is 15-140 μm.

18. The negative composite electrode according to claim 1, wherein The compacted density of the negative composite electrode is 1.35-1.80 g / cm 3 .

19. A method of producing the negative composite electrode according to any one of claims 1 to 18, characterized by, The preparation method comprises: A first slurry containing a first main material, a long-range conductive agent and a first bonding agent is prepared; a second slurry containing a second bonding agent, a conductive agent and lithium carbonate is prepared; a third slurry containing a second main material, SP and a third bonding agent is prepared; then, using the first slurry, the second slurry and the third slurry as raw materials, a first coating layer, a second coating layer and a third coating layer are sequentially arranged on both sides of the current collector in a direction away from the surface of the current collector; and then, cold pressing and compaction are performed to obtain the negative composite electrode tab.

20. The method of claim 19, wherein, The thickness of the first coating layer is 30-150 μm.

21. The method of claim 19, wherein, The thickness of the second coating layer is 1-10 μm.

22. The preparation method according to claim 19, characterized in that, The thickness of the second coating layer is 1-5 μm.

23. The preparation method according to claim 19, characterized in that, The thickness of the third coating layer is 30-150 μm.

24. A lithium-ion battery, characterized by, The lithium ion battery comprises the negative composite electrode according to any one of claims 1-18.

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