A composite negative electrode sheet and its preparation method and battery

Abstract

The application provides a composite negative electrode sheet, a preparation method thereof and a battery. The composite negative electrode sheet comprises a current collector and a bottom layer, a transition layer and a surface layer which are sequentially arranged on at least one side surface of the current collector. The bottom layer comprises a graphite material, a first conductive agent and a first binder. The transition layer comprises a titanium-niobium oxide coated graphite material, a second conductive agent and a second binder. The surface layer comprises titanium-niobium oxide, a third conductive agent and a third binder. The composite negative electrode sheet can improve the mixed potential of the negative electrode surface by constructing a high-capacity bottom layer, a potential transition layer and a potential buffer surface layer, and stabilize the potential within a safe range. The possibility of lithium precipitation is completely eliminated from the thermodynamic point of view, and the problem of lithium dendrite precipitation caused by the excessively low potential of the negative electrode surface at the end of fast charging of the existing lithium ion battery is solved. Meanwhile, the battery energy density is not excessively sacrificed due to the introduction of low-capacity materials.

Description

Technical Field

[0001] This invention belongs to the field of battery technology, and relates to a composite negative electrode sheet, its preparation method, and a battery. Background Technology

[0002] With the continuous improvement of the energy density of lithium-ion batteries, the application of high-capacity anode materials (such as graphite and silicon-carbon) is becoming increasingly widespread. However, under high-rate fast charging conditions, the large-scale and rapid insertion of lithium ions into the anode leads to a sharp drop in the surface potential of the anode, which can easily approach 0V (vs Li / Li). + When the negative electrode potential drops below 0V, lithium ions will precipitate on the negative electrode surface in the form of metallic lithium, forming lithium dendrites.

[0003] To address the lithium plating problem at the end of fast charging, existing technologies mainly employ the following methods:

[0004] (1) Optimize graphite materials: improve the intrinsic rate performance of graphite through secondary granulation, surface coating and other means, and attempt to complete the rapid insertion of lithium ions at higher currents.

[0005] (2) Doping with fast-charging materials: In the negative electrode formulation, materials with excellent rate performance such as soft carbon, hard carbon or lithium titanate (LTO) are mixed in to slow down the desorption of lithium by utilizing their fast lithium-ion transport capability.

[0006] However, the above solutions have the following obvious limitations: the optimized graphite material still has a lithium intercalation potential plateau of around 0.1V, which does not fundamentally change the state of the low potential; while the addition of materials such as LTO, although the potential is high (~1.5V), its capacity is too low (~175mAh / g), which will significantly sacrifice the overall energy density of the battery.

[0007] Therefore, existing technologies face the problem of "high safety (high potential) and high capacity (low potential) being mutually exclusive", and there is an urgent need to provide a composite negative electrode that can provide a high potential safety buffer without significantly sacrificing capacity. Summary of the Invention

[0008] The purpose of this invention is to provide a composite negative electrode sheet, its preparation method, and a battery. The composite negative electrode sheet, by constructing a high-capacity bottom layer, a potential transition layer, and a potential buffer surface layer, raises the mixed potential of the negative electrode surface and stabilizes it within a safe range. This thermodynamically eliminates the possibility of lithium plating, solves the problem of lithium dendrite precipitation in existing lithium-ion batteries at the end of fast charging due to the excessively low potential of the negative electrode surface, and avoids excessive sacrifice of battery energy density due to the introduction of low-capacity materials.

[0009] To achieve this objective, the present invention adopts the following technical solution:

[0010] In a first aspect, the present invention provides a composite negative electrode sheet, the composite negative electrode sheet comprising a current collector and a bottom layer, a transition layer and a top layer sequentially stacked on at least one side surface of the current collector;

[0011] The bottom layer comprises graphite material, a first conductive agent, and a first binder; the transition layer comprises graphite material coated with titanium niobium oxide, a second conductive agent, and a second binder; and the top layer comprises titanium niobium oxide, a third conductive agent, and a third binder.

[0012] This invention introduces a surface layer as a high-potential buffer layer, containing titanium niobium oxide as a high-potential buffer material. This material boasts a theoretical capacity of ~380 mAh / g, far exceeding that of lithium titanate (~175 mAh / g), while maintaining a high lithium intercalation potential of ~1.6V. This allows the surface layer to act as a "safety valve" while also contributing considerable capacity, solving the problem of the trade-off between high safety and high capacity. Specifically, this invention sets a functional layer with a high lithium intercalation potential (~1.6V) on the negative electrode surface. At the end of fast charging, when the potential of the underlying high-capacity material (graphite material) approaches the danger threshold (0V), the high-potential material on the surface layer preferentially and extensively intercalates lithium ions, bearing the main current load. This "raises" the mixed potential on the negative electrode surface and stabilizes it within a safe range (>0.08V), thermodynamically eliminating the possibility of lithium plating. Meanwhile, in order to reduce the potential difference at the interface between the bottom and the surface layers and avoid the problem of local lithium plating caused by sudden potential jumps leading to drastic changes in lithium ion concentration at the interface, the present invention introduces a transition layer of graphite material coated with titanium niobium oxide. This makes lithium ion transport smoother, ensuring high capacity, reducing potential difference, and further buffering effect. Therefore, the present invention avoids the problem of lithium plating at the negative electrode without excessive loss of energy density.

[0013] In addition, the graphite material coated with titanium niobium oxide in the transition layer of the present invention has a core-shell structure. Compared with the blend of titanium niobium oxide and graphite material, titanium niobium oxide is coated on the surface of graphite material. Graphite material acts as the core to provide high capacity, while titanium niobium oxide acts as the shell on the surface, which can preferentially embed lithium ions, thereby raising the potential of the graphite material coated with titanium niobium oxide as a whole, and forming a uniform potential buffer transition layer.

[0014] Preferably, the ratio of the thickness of the surface layer to the total thickness of the bottom layer, transition layer and surface layer is 0.15-0.25, for example, it can be 0.15, 0.17, 0.19, 0.21, 0.23 or 0.25, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0015] In order to maximize the buffering effect of the surface layer without significantly losing energy density, the present invention preferably places the thickness of the surface layer within a specific range. That is, if the thickness of the surface layer is too small, the buffering effect will decrease, but if the thickness of the surface layer is too large, the overall energy density will be reduced.

[0016] Preferably, the ratio of the thickness of the transition layer to the total thickness of the bottom layer, the transition layer and the top layer is 0.05-0.15, for example, it can be 0.05, 0.07, 0.09, 0.11, 0.13 or 0.15, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0017] The thickness ratio of the transition layer described in this invention affects its performance. Preferably, within a certain range, if the thickness ratio of the transition layer is too small, its interface potential transition buffering effect will decrease. However, if the thickness ratio of the transition layer is too large, the energy density of the composite negative electrode will be lost.

[0018] Preferably, the total thickness of the bottom layer, transition layer and top layer is 60μm-100μm, for example, it can be 60μm, 65μm, 70μm, 75μm, 80μm, 85μm, 95μm or 100μm, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0019] Preferably, in the bottom layer, the compacted density of the graphite material is ≥1.7 g / cm³. 3 For example, it could be 1.7 g / cm³ 3 1.72g / cm 3 1.74 g / cm 3 1.76 g / cm 3 1.78g / cm 3 Or 1.8g / cm 3 However, this does not limit the listed values; other unlisted values ​​within the range are also applicable.

[0020] Preferably, in the bottom layer, the particle size D50 of the graphite material is 12μm-18μm, for example, it can be 12μm, 14μm, 16μm or 18μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0021] The selection of the particle size D50 of the graphite material described in this invention can ensure both capacity and kinetics. If the particle size D50 is too small, the specific surface area will be large, which will increase the side reactions. However, if the particle size D50 is too large, the rate performance will be deteriorated.

[0022] Preferably, in the bottom layer, the content of the graphite material is 94wt%-98wt%, for example, it can be 94wt%, 95wt%, 96wt%, 97wt% or 98wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0023] Preferably, in the bottom layer, the content of the first conductive agent is 1wt%-3wt%, for example, it can be 1wt%, 1.5wt%, 2wt%, 2.5wt% or 3wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0024] Preferably, in the bottom layer, the content of the first adhesive is 1wt%-3wt%, for example, it can be 1wt%, 1.5wt%, 2wt%, 2.5wt% or 3wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0025] Preferably, the titanium niobium oxide content in the graphite material coated with titanium niobium oxide is 5wt%-12wt%, for example, it can be 5wt%, 7wt%, 9wt%, 11wt% or 12wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0026] In the titanium niobium oxide-coated graphite material of the present invention, the coating amount of titanium niobium oxide can provide a potential boosting effect without hindering lithium ions from entering the graphite core due to excessive coating thickness.

[0027] Preferably, in the transition layer, the content of the graphite material coated with titanium niobium oxide is 91wt%-95wt%, for example, it can be 91wt%, 92wt%, 93wt%, 94wt% or 95wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0028] Preferably, in the transition layer, the content of the second conductive agent is 4wt%-6wt%, for example, it can be 4wt%, 4.5wt%, 5wt%, 5.5wt% or 6wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0029] Preferably, in the transition layer, the content of the second adhesive is 1wt%-3wt%, for example, it can be 1wt%, 1.5wt%, 2wt%, 2.5wt% or 3wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0030] Preferably, the content of titanium niobium oxide in the surface layer is 91wt%-95wt%, for example, it can be 91wt%, 92wt%, 93wt%, 94wt% or 95wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0031] Preferably, the content of the third conductive agent in the surface layer is 4wt%-6wt%, for example, it can be 4wt%, 4.5wt%, 5wt%, 5.5wt% or 6wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0032] Preferably, the content of the third adhesive in the surface layer is 1wt%-3wt%, for example, it can be 1wt%, 1.5wt%, 2wt%, 2.5wt% or 3wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0033] Preferably, the particle size D50 of the titanium niobium oxide is 1μm-4μm, for example, it can be 1μm, 2μm, 3μm or 4μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0034] Preferably, the chemical formula of the titanium niobium oxide is Ti. x Nb 2-x O7 (preferably TiNb2O7), wherein 0≤x≤0.5, for example, can be 0, 0.2, 0.4 or 0.5, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0035] Preferably, the first conductive agent, the second conductive agent, and the third conductive agent are each independently one or a combination of at least two of carbon nanotubes, conductive carbon black, or graphene.

[0036] Preferably, the first adhesive comprises any one or a combination of at least two of styrene-butadiene rubber, sodium carboxymethyl cellulose, or polyacrylic acid.

[0037] Preferably, the second adhesive and the third adhesive are each independently one or a combination of at least two of polyvinylidene fluoride, styrene-butadiene rubber, sodium carboxymethyl cellulose, or polyacrylic acid.

[0038] Preferably, when the composite negative electrode is charged to full charge at 4C constant current, the potential of its surface relative to lithium metal is always ≥0.08V, for example, it can be 0.08V, 0.1V, 0.12V or 0.15V, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0039] Preferably, after the composite negative electrode sheet is assembled into a full cell and subjected to 500 cycles at a 4C / 1C rate, the thickness rebound rate of the composite negative electrode sheet is ≤15%, for example, it can be 15%, 13%, 11% or 9%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0040] In a second aspect, the present invention provides a method for preparing a composite negative electrode sheet as described in the first aspect, the method comprising the following steps:

[0041] The graphite material, the first conductive agent, the first binder, and the first solvent (deionized water) are mixed to obtain the bottom slurry;

[0042] A transition layer slurry is obtained by mixing titanium niobium oxide-coated graphite material, a second conductive agent, a second binder, and a second solvent (N-methylpyrrolidone).

[0043] A surface slurry is obtained by mixing titanium niobium oxide, a third conductive agent, a third binder, and a third solvent (N-methylpyrrolidone).

[0044] The bottom slurry, transition slurry, and surface slurry are sequentially layered and coated on at least one side of the current collector, and the composite negative electrode sheet is obtained after drying and rolling.

[0045] Preferably, the preparation method of the titanium niobium oxide-coated graphite material includes the following steps:

[0046] After mixing graphite material and titanium niobium oxide, heat treatment is performed to obtain graphite material coated with titanium niobium oxide.

[0047] The heat treatment temperature is 600℃-800℃ (preferably 650℃-750℃), for example, it can be 600℃, 650℃, 700℃, 750℃ or 800℃, and the time is 2h-8h (preferably 4h-6h), for example, it can be 2h, 4h, 6h or 8h, and the atmosphere includes nitrogen and / or inert gas (such as argon).

[0048] Thirdly, the present invention provides a battery comprising the composite negative electrode sheet as described in the first aspect.

[0049] Compared with the prior art, the present invention has the following beneficial effects:

[0050] This invention introduces a surface layer as a functional buffer layer with a high lithium intercalation potential (~1.6V). The titanium niobium oxide contained within acts as a high-potential buffer material. At the end of fast charging, when the potential of the underlying high-capacity material (graphite) approaches the danger threshold (0V), the high-potential material on the surface layer preferentially and extensively intercalates lithium ions, bearing the main current load. This "raises" and stabilizes the mixed potential at the negative electrode surface within a safe range (>0.08V), thermodynamically eliminating the possibility of lithium plating. Simultaneously, to reduce the potential difference at the interface between the underlying and surface layers and avoid drastic changes in lithium ion concentration caused by potential jumps, which could lead to localized lithium plating, a transition layer of graphite material coated with titanium niobium oxide is introduced. This smooths lithium ion transport, ensuring high capacity, reducing the potential difference, and further buffering the effect. Therefore, this invention avoids the problem of negative electrode lithium plating without excessive loss of energy density. Detailed Implementation

[0051] 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.

[0052] Example 1

[0053] This embodiment provides a composite negative electrode sheet, which includes a current collector (8μm thick copper foil) and a bottom layer, a transition layer and a top layer sequentially stacked on one side surface of the current collector. The thickness of the top layer is 0.2 times the total thickness of the bottom layer, the transition layer and the top layer, and the ratio of the thickness of the transition layer to the total thickness of the bottom layer, the transition layer and the top layer is 0.1 times. The total thickness of the bottom layer, the transition layer and the top layer is 85μm.

[0054] The bottom layer comprises 96 wt% graphite material (particle size D50 of 15 μm, compaction density of 1.75 g / cm³). 3 ), 1.5 wt% conductive carbon black, 1.5 wt% styrene-butadiene rubber and 1 wt% carboxymethyl cellulose;

[0055] The transition layer comprises 93 wt% TiNb2O7-coated graphite material (TiNb2O7 content is 8 wt%), 5 wt% conductive carbon black, and 2 wt% polyvinylidene fluoride.

[0056] The surface layer comprises 93 wt% TiNb2O7 (particle size D50 of 2 μm), 3 wt% carbon nanotubes, 2 wt% conductive carbon black and 2 wt% polyvinylidene fluoride.

[0057] The method for preparing the composite negative electrode sheet includes the following steps:

[0058] According to the formula, graphite material, conductive carbon black, styrene-butadiene rubber, carboxymethyl cellulose and deionized water are mixed to obtain the bottom slurry;

[0059] According to the formula, TiNb2O7-coated graphite material, conductive carbon black, polyvinylidene fluoride and N-methylpyrrolidone are mixed to obtain a transition layer slurry; the TiNb2O7-coated graphite material is obtained by mixing TiNb2O7 and graphite material according to the formula, and then heat-treating at 700°C for 5 hours under argon atmosphere.

[0060] According to the formula, TiNb2O7, carbon nanotubes, conductive carbon black, polyvinylidene fluoride and N-methylpyrrolidone are mixed to obtain the surface slurry;

[0061] The bottom slurry, transition slurry, and surface slurry are sequentially layered and coated on one side of the current collector. After drying and rolling, the composite negative electrode sheet is obtained.

[0062] Example 2

[0063] This embodiment provides a composite negative electrode sheet, which includes a current collector (8μm thick copper foil) and a bottom layer, a transition layer and a top layer sequentially stacked on one side surface of the current collector. The ratio of the thickness of the top layer to the total thickness of the bottom layer, the transition layer and the top layer is 0.2, the ratio of the thickness of the transition layer to the total thickness of the bottom layer, the transition layer and the top layer is 0.05, and the total thickness of the bottom layer, the transition layer and the top layer is 75μm.

[0064] The bottom layer comprises 97 wt% graphite material (particle size D50 of 12 μm, compaction density of 1.78 g / cm³). 3 ), 1 wt% conductive carbon black, 1 wt% styrene-butadiene rubber and 1 wt% carboxymethyl cellulose;

[0065] The transition layer comprises 95 wt% TiNb2O7-coated graphite material (TiNb2O7 content is 5 wt%), 4 wt% carbon nanotubes, and 1 wt% polyvinylidene fluoride.

[0066] The surface layer comprises 92 wt% TiNb2O7 (particle size D50 is 1 μm), 3 wt% carbon nanotubes, 2 wt% conductive carbon black and 3 wt% polyvinylidene fluoride.

[0067] The method for preparing the composite negative electrode sheet includes the following steps:

[0068] According to the formula, graphite material, conductive carbon black, styrene-butadiene rubber, carboxymethyl cellulose and deionized water are mixed to obtain the bottom slurry;

[0069] According to the formula, TiNb2O7-coated graphite material, conductive carbon black, polyvinylidene fluoride and N-methylpyrrolidone are mixed to obtain a transition layer slurry; the TiNb2O7-coated graphite material is obtained by mixing TiNb2O7 and graphite material according to the formula, and then heat-treating at 600°C for 8 hours under a nitrogen atmosphere.

[0070] According to the formula, TiNb2O7, carbon nanotubes, conductive carbon black, polyvinylidene fluoride and N-methylpyrrolidone are mixed to obtain the surface slurry;

[0071] The bottom slurry, transition slurry, and surface slurry are sequentially layered and coated on one side of the current collector. After drying and rolling, the composite negative electrode sheet is obtained.

[0072] Example 3

[0073] This embodiment provides a composite negative electrode sheet, which includes a current collector (8μm thick copper foil) and a bottom layer, a transition layer and a top layer sequentially stacked on one side surface of the current collector. The thickness of the top layer is 0.2 times the total thickness of the bottom layer, the transition layer and the top layer, and the total thickness of the top layer is 0.15 times the total thickness of the bottom layer, the transition layer and the top layer, and the total thickness of the top layer, the transition layer and the top layer is 95μm.

[0074] The bottom layer comprises 94 wt% graphite material (particle size D50 of 18 μm, compaction density of 1.7 g / cm³). 3 ), 3 wt% conductive carbon black, 1.5 wt% styrene-butadiene rubber and 1.5 wt% carboxymethyl cellulose;

[0075] The transition layer comprises 91 wt% TiNb2O7-coated graphite material (TiNb2O7 content is 12 wt%), 6 wt% conductive carbon black, and 3 wt% polyvinylidene fluoride.

[0076] The surface layer comprises 95 wt% TiNb2O7 (particle size D50 of 4 μm), 2 wt% carbon nanotubes, 2 wt% conductive carbon black and 1 wt% polyvinylidene fluoride.

[0077] The method for preparing the composite negative electrode sheet includes the following steps:

[0078] According to the formula, graphite material, conductive carbon black, styrene-butadiene rubber, carboxymethyl cellulose and deionized water are mixed to obtain the bottom slurry;

[0079] According to the formula, TiNb2O7-coated graphite material, conductive carbon black, polyvinylidene fluoride and N-methylpyrrolidone are mixed to obtain a transition layer slurry; the TiNb2O7-coated graphite material is obtained by mixing TiNb2O7 and graphite material according to the formula, and then heat-treating at 800°C for 2 hours under argon atmosphere.

[0080] According to the formula, TiNb2O7, carbon nanotubes, conductive carbon black, polyvinylidene fluoride and N-methylpyrrolidone are mixed to obtain the surface slurry;

[0081] The bottom slurry, transition slurry, and surface slurry are sequentially layered and coated on one side of the current collector. After drying and rolling, the composite negative electrode sheet is obtained.

[0082] Example 4

[0083] This embodiment provides a composite negative electrode sheet, which is the same as that in Embodiment 1 except that the ratio of the thickness of the surface layer to the total thickness of the bottom layer, transition layer and surface layer is 0.15.

[0084] Example 5

[0085] This embodiment provides a composite negative electrode sheet, which is the same as that in Embodiment 1 except that the ratio of the thickness of the surface layer to the total thickness of the bottom layer, transition layer and surface layer is 0.25.

[0086] Example 6

[0087] This embodiment provides a composite negative electrode sheet, which is the same as that in Embodiment 1 except that the ratio of the thickness of the surface layer to the total thickness of the bottom layer, transition layer and surface layer is 0.1.

[0088] Example 7

[0089] This embodiment provides a composite negative electrode sheet, which is the same as that in Embodiment 1 except that the ratio of the thickness of the surface layer to the total thickness of the bottom layer, transition layer and surface layer is 0.3.

[0090] Example 8

[0091] This embodiment provides a composite negative electrode sheet, which is the same as that in Embodiment 1 except that the ratio of the thickness of the transition layer to the total thickness of the bottom layer, the transition layer and the surface layer is 0.02.

[0092] Example 9

[0093] This embodiment provides a composite negative electrode sheet, which is the same as that in Embodiment 1 except that the ratio of the thickness of the transition layer to the total thickness of the bottom layer, the transition layer and the surface layer is 0.2.

[0094] Example 10

[0095] This embodiment provides a composite negative electrode sheet, which is the same as in Example 1 except that the content of TiNb2O7 in the graphite material coated with TiNb2O7 is 3wt%.

[0096] Example 11

[0097] This embodiment provides a composite negative electrode sheet, which is the same as in Example 1 except that the content of TiNb2O7 in the graphite material coated with TiNb2O7 is 18wt%.

[0098] Comparative Example 1

[0099] This comparative example provides a negative electrode sheet, which is the same as that in Example 1 except that it does not include a transition layer and a surface layer, and the thickness of the bottom layer is 85 μm.

[0100] Comparative Example 2

[0101] This comparative example provides a negative electrode sheet, which is the same as that in Example 1 except that it does not include the bottom layer and transition layer, and the thickness of the surface layer is 85 μm.

[0102] Comparative Example 3

[0103] This comparative example provides a negative electrode sheet, which is the same as in Example 1 except that the TiNb2O7 is replaced by lithium titanate and the graphite material coated with TiNb2O7 is replaced by graphite material coated with lithium titanate.

[0104] Comparative Example 4

[0105] This comparative example provides a negative electrode sheet, which is the same as that in Example 1 except that it does not contain a transition layer.

[0106] The electrode sheets obtained in the above embodiments and comparative examples were subjected to performance testing. The testing methods are as follows:

[0107] (1) Three-electrode battery test: Assemble a three-electrode soft-pack battery with NCM811 as the positive electrode, the electrode under test as the negative electrode, and lithium metal as the reference electrode. Charge at a constant current rate of 4C, and monitor and record the potential change curve of the negative electrode relative to lithium in real time through the reference electrode, and record the lowest potential value.

[0108] (2) Cyclic electrode rebound rate test: After the three-electrode battery is cycled 500 times at 4C / 1C rate, the battery is disassembled, the negative electrode is taken out, and the thickness of the central area is measured after cleaning with DMC. The formula for calculating the rebound rate is: |(Cyclic electrode thickness - initial electrode thickness) / initial electrode thickness|×100%.

[0109] (3) Full cell cycle test: Assemble the NCM811-test electrode soft pack full cell, perform charge and discharge cycles at 4C / 1C rate, and record the capacity retention rate after 500 cycles.

[0110] (4) Energy density loss rate test compared to Comparative Example 1: The full cell with NCM811 positive electrode and test electrode (with consistent liquid injection volume and N / P ratio) was tested at a 0.2C rate, and the energy density (Wh / kg) based on the total mass of the battery was recorded; the energy density loss rate compared to Comparative Example 1 was calculated as follows: Energy density loss rate = (energy density of Comparative Example 1 battery - energy density of Example 1 / energy density of Comparative Example 1 battery) / energy density of Comparative Example 1 battery × 100%.

[0111] The test results are shown in Table 1 below:

[0112] Table 1

[0113]

[0114] As can be seen from Table 1 above:

[0115] As shown in Examples 1-5 and Comparative Example 1, during 4C fast charging, the minimum potential remained stable above 0.08V, far exceeding the lithium plating potential, successfully achieving potential buffering. In contrast, the potential of Comparative Example 1 dropped to 0.002V, inevitably leading to lithium plating. Furthermore, the energy density of Examples 1-5 did not show a significant loss compared to Comparative Example 1, while also exhibiting excellent cycle performance. As shown in Examples 1 and Comparative Example 2, when only the surface layer is used as the active layer, although the potential is high, the energy density loss is as high as 35%. As shown in Examples 1 and Comparative Example 3, the titanium niobium oxide used in this invention has a higher lithium intercalation potential and higher theoretical capacity than lithium titanate, without significant energy density loss. As shown in Examples 1 and Comparative Example 4, the transition layer setting in this invention can further improve the 4C fast charging performance. The lowest charging potential of C is achieved without causing significant energy density loss, while also improving cycle performance. As shown in Examples 1 and 4-7, the thickness of the surface layer is preferably within a specific range, which can simultaneously ensure the buffering effect of the surface layer without significantly losing energy density. As shown in Examples 1 and 8-9, the thickness of the transition layer is preferably within a specific range, which can further improve the transition effect without causing significant energy density loss. As shown in Examples 1 and 10-11, in the titanium niobium oxide-coated graphite material of the present invention, the coating amount of titanium niobium oxide is preferably within a specific range, which can provide a potential boosting effect without hindering lithium ions from entering the graphite core due to excessive coating thickness, thereby ensuring the overall performance of the battery.

[0116] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the scope of protection and disclosure of the present invention.

Claims

1. A composite negative electrode sheet, characterized in that, The composite negative electrode sheet includes a current collector and a bottom layer, a transition layer and a top layer that are sequentially stacked on at least one side surface of the current collector; The bottom layer comprises graphite material, a first conductive agent, and a first binder; the transition layer comprises graphite material coated with titanium niobium oxide, a second conductive agent, and a second binder; and the top layer comprises titanium niobium oxide, a third conductive agent, and a third binder.

2. The composite negative electrode sheet according to claim 1, characterized in that, The ratio of the thickness of the surface layer to the total thickness of the bottom layer, transition layer, and surface layer is 0.15-0.25; And / or, the ratio of the thickness of the transition layer to the total thickness of the bottom layer, the transition layer and the top layer is 0.05-0.15; And / or, the total thickness of the bottom layer, transition layer and top layer is 60μm-100μm.

3. The composite negative electrode sheet according to claim 1 or 2, characterized in that, In the bottom layer, the compacted density of the graphite material is ≥1.7 g / cm³. 3 ; And / or, in the underlying layer, the particle size D50 of the graphite material is 12μm-18μm; And / or, in the underlying layer, the content of the graphite material is 94wt%-98wt%; And / or, in the underlying layer, the content of the first conductive agent is 1wt%-3wt%; And / or, in the underlying layer, the content of the first adhesive is 1wt%-3wt%.

4. The composite negative electrode sheet according to claim 1 or 2, characterized in that, In the titanium niobium oxide-coated graphite material, the content of titanium niobium oxide is 5wt%-12wt%; And / or, in the transition layer, the content of the graphite material coated with titanium niobium oxide is 91wt%-95wt%; And / or, in the transition layer, the content of the second conductive agent is 4wt%-6wt%; And / or, in the transition layer, the content of the second adhesive is 1wt%-3wt%.

5. The composite negative electrode sheet according to claim 1 or 2, characterized in that, In the surface layer, the content of titanium niobium oxide is 91wt%-95wt%; And / or, in the surface layer, the content of the third conductive agent is 4wt%-6wt%; And / or, in the surface layer, the content of the third adhesive is 1wt%-3wt%.

6. The composite negative electrode sheet according to claim 1 or 2, characterized in that, The particle size D50 of the titanium niobium oxide is 1μm-4μm; And / or, the chemical formula of the titanium niobium oxide is Ti x Nb 2-x O7, where 0 ≤ x ≤ 0.

5.

7. The composite negative electrode sheet according to claim 1 or 2, characterized in that, The first conductive agent, the second conductive agent, and the third conductive agent are each independently one or a combination of at least two of carbon nanotubes, conductive carbon black, or graphene; And / or, the first adhesive comprises any one or a combination of at least two of styrene-butadiene rubber, sodium carboxymethyl cellulose, or polyacrylic acid; And / or, the second adhesive and the third adhesive are each independently any one or a combination of at least two of polyvinylidene fluoride, styrene-butadiene rubber, sodium carboxymethyl cellulose, or polyacrylic acid.

8. A method for preparing a composite negative electrode sheet as described in any one of claims 1-7, characterized in that, The preparation method includes the following steps: The graphite material, the first conductive agent, the first binder, and the first solvent are mixed to obtain the bottom slurry; A transition layer slurry is obtained by mixing titanium niobium oxide-coated graphite material, a second conductive agent, a second binder, and a second solvent. Titanium niobium oxide, a third conductive agent, a third binder, and a third solvent are mixed to obtain a surface slurry; The bottom slurry, transition slurry, and surface slurry are sequentially layered and coated on at least one side of the current collector, and the composite negative electrode sheet is obtained after drying and rolling.

9. The preparation method according to claim 8, characterized in that, The preparation method of the titanium niobium oxide-coated graphite material includes the following steps: After mixing graphite material and titanium niobium oxide, heat treatment is performed to obtain graphite material coated with titanium niobium oxide. The heat treatment is performed at a temperature of 600℃-800℃ for 2-8 hours, and the atmosphere includes nitrogen and / or inert gas.

10. A battery, characterized in that, The battery includes a composite negative electrode sheet as described in any one of claims 1-7.

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