Negative plate and electrochemical device

Abstract

The invention discloses a negative plate and an electrochemical device, and belongs to the field of electrochemical devices. The negative plate comprises a current collector and a negative active layer, the current collector is covered with the negative active layer, the negative active layer comprises a coating missing area, the coating missing area is covered with a polymer layer, the hydrolysis degree of the polymer layer is 80%-90%, and the weight-average molecular weight of the polymer layer is 20000g / mol-40000g / mol. The polymer layer of the negative plate provided by the invention directly covers an unavoidable coating missing area in a coating process to form a physical barrier, so that direct contact between an electrolyte and an exposed current collector is isolated, lithium precipitation is avoided, and the cycling stability of the battery is improved.

Description

Technical Field

[0001] This invention relates to the field of electrochemical device technology, and particularly to a negative electrode and an electrochemical device. Background Technology

[0002] The coating of electrodes in secondary batteries is a process of uniformly coating a stable, viscous, and fluid electrode slurry onto the positive and negative current collectors. Defects are commonly found in the negative electrode coating of secondary batteries, leading to foil leakage and lithium plating, which further reduces battery capacity, shortens cycle life, and increases safety hazards. Currently, methods such as adjusting the fluid properties of the electrode slurry, recoating the electrode slurry, and designing the coating die structure have been developed to improve the coating effect. However, current negative electrode sheets still have areas with coating defects, posing risks of foil leakage and lithium plating. Summary of the Invention

[0003] To address the shortcomings of existing technologies, this invention provides a negative electrode sheet and its preparation method, as well as a negative electrode sheet and an electrochemical device, aiming to solve the problems that negative electrode sheets still have coating defects, foil leakage, and potential lithium plating issues.

[0004] To achieve the above objectives, the present invention proposes a negative electrode sheet, comprising a current collector and a negative electrode active layer, wherein the negative electrode active layer is coated on the current collector, the negative electrode active layer includes a coating defect area, the coating defect area is coated with a polymer layer, the degree of hydrolysis of the polymer layer is 80%~90%, and the weight average molecular weight of the polymer layer is 20000g / mol~40000g / mol.

[0005] In some embodiments, the polymer in the polymer layer includes at least one of polyvinyl acetate, a modified polyvinyl acetate, polyvinyl alcohol, and a modified polyvinyl alcohol.

[0006] In some embodiments, the polymer in the polymer layer includes at least one of polyvinyl alcohol, polyvinyl butyral, and boric acid crosslinked polyvinyl alcohol.

[0007] In some embodiments, the negative electrode active layer comprises the polymer, and the polymer has a higher mass percentage closer to the current collector in the thickness direction of the negative electrode sheet.

[0008] In some embodiments, the thickness of the polymer layer is denoted as H1 micrometers, and the thickness of the negative electrode active layer is denoted as H2 micrometers, where H1:H2 is 1:(100~200).

[0009] In some embodiments, the thickness H1 of the polymer layer is 0.5 micrometers to 1 micrometer.

[0010] In some embodiments, the negative electrode active layer includes an aqueous binder, wherein the mass ratio of the aqueous binder to the negative electrode active layer is a, where a = 1 wt% to 1.5 wt%.

[0011] In some embodiments, the water-based adhesive includes at least one of carboxymethyl cellulose, styrene-butadiene latex, and polyacrylic acid.

[0012] To achieve the above objectives, the present invention also proposes an electrochemical device comprising an electrolyte and the aforementioned negative electrode.

[0013] In some embodiments, the volume concentration of the additive in the electrolyte relative to the volume of the electrolyte is denoted as Cvol%, and the thickness of the polymer layer is H1 micrometers, where C and H1 satisfy the following relationship: 0≤C / H1≤0.6.

[0014] In some embodiments, the additive includes one of the following: halogenated silane dehydrating agents, alkoxysilane-isocyanate dehydrating agents, bismaleimide dehydrating agents, organosilicon-siloxane dehydrating agents, and phosphite / phosphonate dehydrating agents.

[0015] In some embodiments, the additive includes trimethylchlorosilane.

[0016] The beneficial effects of this invention are: This invention provides a polymer layer for the negative electrode that possesses insulating properties. The polymer layer directly covers the unavoidable coating defects during the coating process, forming a physical barrier that isolates the electrolyte from direct contact with the exposed current collector. This fundamentally prevents preferential and disordered deposition of lithium ions in the defective areas, improving the battery's cycle stability. Furthermore, by controlling the weight-average molecular weight range of the polymer layer, it is possible to ensure that the polymer layer has good film-forming properties, mechanical strength, and flexibility, allowing it to firmly adhere to the coating defects. Simultaneously, limiting the degree of hydrolysis of the polymer layer ensures that the polymer has sufficiently high hydrophilicity. This high hydrophilicity allows the polymer layer to be effectively wetted by the electrolyte, facilitating lithium ion transport at the polymer layer / electrolyte interface and within the polymer layer, without negatively impacting the battery's rate performance. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the structure of a negative electrode sheet according to an embodiment of the present invention. Reference numerals: 1-current collector, 2-negative electrode active layer, 3-polymer layer; The realization of the purpose, functional features and advantages of this application will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0018] To make the technical solutions and advantages of the present invention clearer, the present invention and its beneficial effects will be described in further detail below in conjunction with specific embodiments. The accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter recorded in the claims.

[0019] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60~120 and 80~110 are listed for a specific parameter, it is also expected that ranges of 60~110 and 80~120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1~3, 1~4, 1~5, 2~3, 2~4, and 2~5. In this application, unless otherwise stated, the numerical range "a~b" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0~5" indicates that all real numbers between "0~5" have been listed in this article; "0~5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0020] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0021] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0022] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0023] Electrode coating in electrochemical devices involves uniformly coating a stable, viscous, and fluid electrode slurry onto the positive and negative electrode current collectors. Negative electrode coating in electrochemical devices commonly suffers from coating defects, leading to foil leakage and lithium plating, which further reduces battery capacity, shortens cycle life, and increases safety hazards. Therefore, methods such as adjusting the fluid properties of the electrode slurry, secondary coating of the electrode slurry, and designing the coating die structure have been developed to optimize the coating effect. However, current negative electrode coatings still suffer from foil leakage due to coating issues.

[0024] In view of this, the present invention proposes a negative electrode sheet, and the embodiments are described below. Figure 1 The present invention includes a current collector 1 and a negative electrode active layer 2, wherein the negative electrode active layer 2 is coated on the current collector 1, the negative electrode active layer 2 includes a coating defect area, the coating defect area is coated with a polymer layer 3, the degree of hydrolysis of the polymer layer 3 is 80%~90%, and the weight average molecular weight of the polymer layer is 20000g / mol~40000g / mol.

[0025] The polymer layer 3 of this solution is insulating. The polymer layer 3 directly covers the coating defects that are unavoidable in the coating process, forming a physical barrier that isolates the electrolyte from direct contact with the exposed current collector 1. This fundamentally prevents lithium ions from preferentially and disorderly depositing in the foil defects, thereby improving the cycle stability of the battery.

[0026] The degree of hydrolysis of polymer layer 3 is controlled at 80%~90%, ensuring that the polymer has sufficiently high hydrophilicity. High hydrophilicity allows the polymer layer to be effectively wetted by the electrolyte, which is beneficial for the transport of lithium ions at the polymer layer / electrolyte interface and within the polymer layer, without significantly negatively impacting the rate performance of the battery. In some embodiments, the degree of hydrolysis of the polymer layer is any value within the range of 80%~90%, such as 81.1%, 83%, 85.2%, 87%, 89.5%.

[0027] The weight-average molecular weight of polymer layer 3 is ≥20000g / mol, which ensures that the polymer layer has good film-forming properties, mechanical strength and flexibility. This allows it to adhere firmly to the foil area and is not easy to break or fall off during the volume expansion and contraction of the negative electrode active material during battery charging and discharging, thus maintaining long-term effectiveness. The weight-average molecular weight of polymer layer is ≤40000g / mol to avoid the polymer viscosity being too high due to excessive weight-average molecular weight, which makes it difficult to coat or reduces flexibility and makes it brittle. At the same time, it is also conducive to maintaining good compatibility between polymer layer and surrounding electrode materials and electrolyte.

[0028] In some embodiments, coating defects include chatter, ribs, streaks, bubbles, etc.

[0029] In some embodiments, the method for preparing the negative electrode sheet includes the following steps: S1: Coat the current collector with an aqueous solution of polymer to obtain a wetted polymer layer; S2: Before the moist polymer layer is completely dry and cured, a negative electrode slurry is applied, and then dried and cured to obtain a negative electrode sheet.

[0030] Before the moist polymer layer is completely dry and cured, polymer layer 3 is in a swellable state. The aqueous solvent in the negative electrode slurry will dissolve it, exposing the surface of the negative electrode current collector, achieving direct contact between the negative electrode slurry and the negative electrode current collector. That is, the negative electrode active layer 2 can adhere tightly to the negative electrode current collector 1, ensuring good conductivity in the negative electrode slurry coating area. In the coating gaps not covered by the negative electrode slurry, polymer layer 3 is completely preserved, and its insulation can prevent lithium ions from depositing in the empty foil area. The above coating process for the negative electrode sheet is simple and easy to control, and is well compatible with existing battery manufacturing processes.

[0031] In some embodiments, the aqueous solution of the polymer is a solution of 0.5 wt% to 1.2 wt%.

[0032] In some embodiments, the negative electrode slurry is applied within 10 minutes of obtaining the wet polymer layer before the wet polymer layer has completely dried and cured.

[0033] In some embodiments, the drying and curing temperature is 75°C to 85°C.

[0034] In some embodiments, the polymer in the polymer layer includes at least one of polyvinyl acetate, a modified polyvinyl acetate, polyvinyl alcohol, and a modified polyvinyl alcohol.

[0035] When the polymer layer 3 is selected from the above-mentioned materials, the polymer layer is compatible with a variety of electrolytes and will not undergo adverse chemical reactions with the electrolyte. Furthermore, the numerous hydrophilic hydroxyl groups can absorb moisture from the battery cell and electrolyte, reducing the adverse effects of moisture on the electrolyte decomposition reaction and improving electrical performance and safety.

[0036] In some embodiments, the polymer in the polymer layer includes at least one of polyvinyl alcohol, polyvinyl butyral, and boric acid crosslinked polyvinyl alcohol.

[0037] The polyvinyl alcohol (PVA) layer, with its higher density of hydroxyl groups, provides strong hydrophilicity and controllable swelling, which is more conducive to direct contact between the negative electrode slurry and the negative electrode current collector, and also facilitates rapid wetting of the electrolyte and electrode sheet after electrolyte injection. Furthermore, PVA raw materials are widely available and inexpensive, exhibiting good film-forming properties. PVA butyral offers superior mechanical strength and interfacial adhesion, inhibiting cyclic cracking at the junction with the negative electrode active layer in the coating area, and is more adaptable to changes in electrode volume. In boric acid-crosslinked PVA, borate ions form a reversible crosslinking network with PVA hydroxyl groups, enhancing the high-temperature stability of the polymer layer.

[0038] In some embodiments, refer to Figure 1 The negative electrode active layer 2 includes the polymer, and the mass percentage of the polymer is higher the closer it is to the current collector 1 in the thickness direction of the negative electrode sheet.

[0039] During the preparation process, before the moist polymer layer 3 is completely dried and cured, the polymer layer 3 is in a swellable state. The aqueous solvent in the negative electrode slurry will dissolve it, exposing the surface of the negative electrode current collector, thus achieving direct contact between the negative electrode slurry and the negative electrode current collector. At the same time, the polymer is incorporated into the negative electrode slurry, creating a gradient concentration in the thickness direction of the negative electrode active layer 2. This results in a higher mass ratio of the polymer in the final product in the thickness direction of the negative electrode sheet, closer to the current collector 1.

[0040] In some embodiments, the thickness of the polymer layer is denoted as H1 micrometers, and the thickness of the negative electrode active layer is denoted as H2 micrometers, where H1:H2 is 1:(100~200). A polymer layer thickness within this range can effectively suppress lithium plating at the negative electrode and simultaneously reduce the amount of electrolyte water-absorbing additives and negative electrode binders used. In some embodiments, H1:H2 is any value from 1:(100~200), such as 1:100, 1:110, 1:120, 1:130, 1:150, 1:180, 1:190, etc. Preferably, H1:H2 is 1:(100~150).

[0041] In some embodiments, the thickness H1 of the polymer layer is 0.5 micrometers to 1 micrometer. In some embodiments, H1 is any value among 0.5 micrometers to 1 micrometer, such as 0.6 micrometers, 0.7 micrometers, 0.8 micrometers, 0.9 micrometers, etc. Preferably, the thickness H1 of the polymer layer is 0.75 micrometers to 1 micrometer.

[0042] In some embodiments, the negative electrode active layer includes an aqueous binder, wherein the mass ratio of the aqueous binder to the negative electrode active layer is a, where a = 1 wt% to 1.5 wt%.

[0043] In some embodiments, the negative electrode active layer further includes a negative electrode active material and a conductive agent; the negative electrode active material includes natural graphite, artificial graphite, as well as mesophase carbon microspheres, hard carbon, soft carbon and graphene, titanium-based materials, silicon-based materials, tin-based materials, nitrides and metallic lithium, etc.; the conductive agent includes conductive graphite, conductive carbon black, conductive carbon fiber, graphene, carbon nanotubes, etc.

[0044] Typically, the binder ratio in the negative electrode active layer is 2wt%. After adding the polymer coating of this solution, the polymer swells and enters the negative electrode slurry, which can reduce the amount of binder in the slurry, reducing the amount of binder to 1wt%~1.5wt%, thereby increasing the mass ratio of the negative electrode active material and further improving the energy density of the negative electrode active layer.

[0045] In some embodiments, the aqueous binder includes at least one of carboxymethyl cellulose, styrene-butadiene latex, and polyacrylic acid. In some embodiments, preferably, the polymer layer is a polyvinyl alcohol layer, and the aqueous binder includes styrene-butadiene latex, as polyvinyl alcohol can more harmoniously synergize with and replace the binding effect of styrene-butadiene latex in the negative electrode slurry.

[0046] To address the aforementioned problems, this invention also proposes an electrochemical device. The battery includes an electrolyte and the aforementioned negative electrode. The electrochemical device includes at least one of a lithium-ion electrochemical device, a sodium-ion electrochemical device, and a potassium-ion electrochemical device. It should be noted that regardless of the conventional selection of the positive electrode, separator, electrolyte, battery assembly method, etc., as long as the negative electrode of this solution is used, the battery can possess the beneficial effects claimed by incorporating the negative electrode.

[0047] In some embodiments, the electrolyte includes additives. These additives are used to remove water from the electrolyte; they reduce the water content and acidity of the electrolyte by chemically consuming water or reacting with HF, thereby improving battery cycle stability.

[0048] In some embodiments, the volume concentration of the additive in the electrolyte relative to the volume of the electrolyte is denoted as Cvol%, and the thickness of the polymer layer is H1 micrometers, where C and H1 satisfy the following relationship: C = 0.6 - 0.6·H1, and C > 0, H1 > 0.

[0049] The volume concentration percentage of dehydrating additives in the electrolyte is typically 1 vol% ± 0.2 vol%. In some embodiments of this solution, the abundant hydrophilic hydroxyl groups in the polymer layer can absorb moisture from the battery cell and electrolyte, reducing the amount of dehydrating agent used. When the polymer coating thickness is 0.5 μm, the volume concentration percentage of the dehydrating agent in the electrolyte can be reduced to 0.3 vol%. The thicker the polymer coating H1, the better the water absorption, and the lower the required content of dehydrating agent in the electrolyte, thus avoiding the impact of adding dehydrating agent on the conductivity, chemical stability, and other properties of the electrolyte. When the polymer layer thickness in this solution is 1 μm, no dehydrating agent needs to be added.

[0050] In some embodiments, the additive includes one of the following: halosilane dehydrating agents, alkoxysilane-isocyanate dehydrating agents, bismaleimide dehydrating agents, organosilicon-siloxane dehydrating agents, and phosphite / phosphonate dehydrating agents. Preferably, the additive includes halosilanes or bismaleimides; halosilane dehydrating agents have a faster dehydration rate, while bismaleimide dehydrating agents have higher interfacial stability.

[0051] In some embodiments, the additive comprises trimethylchlorosilane. Trimethylchlorosilane has a rapid hydrolysis rate, making it more suitable for high-speed coating lines, and its rapid removal of moisture can reduce the corrosion of electrodes by HF.

[0052] Example 1 I. Preparation of negative electrode sheet A suitable amount of polyvinyl alcohol with a hydrolysis degree of 88% and a weight-average molecular weight of 20,000-30,000 g / mol was dissolved in distilled water to prepare a 1 wt% solution. The coating speed was set to 150-220 m / min, and the solution was uniformly coated on the surface of an 8 μm thick copper foil negative electrode current collector to obtain a polymer layer with a thickness of 0.5 μm. After 5 minutes, before the polyvinyl alcohol coating was completely dried and cured, the negative electrode slurry was coated. The negative electrode slurry consisted of 96 wt% graphite, 1.25 wt% styrene-butadiene latex, 0.8 wt% carbon nanotube conductive agent, and the balance being an aqueous solvent. The coated electrode sheet was placed in an oven for drying. The negative electrode slurry and the polyvinyl alcohol coating were dried and cured simultaneously at 80°C. The thickness ratio of the polymer layer to the negative electrode active layer was 1:150. Then, the electrode sheet was rolled and cut to obtain the negative electrode sheet.

[0053] II. Preparation of Lithium-ion Battery Cells Positive electrode sheet: Lithium cobalt oxide, superconducting carbon (Super-P) as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder are placed in a planetary mixer at a mass ratio of 96.5:1.2:2.3. The mixture is first dry-mixed for 10 minutes, then N-methylpyrrolidone (NMP) is slowly added until the solid content reaches 70%. High-speed mixing continues for 2 hours to obtain a uniform, particle-free positive electrode slurry. The slurry is uniformly coated onto the surface of an aluminum foil current collector, dried at 85°C, cold-pressed for shaping, then slit and wound for later use.

[0054] Separator: Using PE base film as the base, Al2O3 / PVDF slurry is uniformly coated on both sides, dried and cured, then cut and rolled up for later use.

[0055] Electrolyte: Ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) are premixed at a mass ratio of 1:2:1, with a moisture content ≤10ppm. In a glove box with a dew point ≤–40℃, add LiPF6 to... Stir until completely dissolved. The electrolyte also contains 0.3 vol% trimethylchlorosilane as a dehydrating agent.

[0056] The positive electrode, the aforementioned negative electrode, and the separator are assembled, rolled into a soft-pack battery, vacuum baked, and then filled with electrolyte. Finally, the battery cell is obtained through formation.

[0057] Polyvinyl alcohol and trimethylchlorosilane were purchased from MerckKGaA in Darmstadt, Germany; graphite was purchased from Qingdao Lianchuang Lida Graphite Co., Ltd.; styrene-butadiene latex was purchased from Shandong Gaoshi Science and Industry Co., Ltd.; and carbon nanotubes were purchased from Defang Nano.

[0058] Example 2 The polyvinyl alcohol polymer layer is 1 μm thick, and no dehydrating agent is added. For the rest, refer to Example 1.

[0059] Example 3 The polymer layer is a polyvinyl acetate layer, and the rest is as described in Example 1.

[0060] Example 4 The polymer layer is a polyvinyl butyral layer, and the rest is as described in Example 1.

[0061] Example 5 The polymer layer is a boric acid crosslinked polyvinyl alcohol layer, and the rest is as described in Example 1.

[0062] Example 6 The ratio of the polymer layer thickness to the negative electrode active layer thickness is 1:100, and the rest is as described in Example 1.

[0063] Example 7 The ratio of the polymer layer thickness to the negative electrode active layer thickness is 1:200, and the rest is as described in Example 1.

[0064] Example 8 The negative electrode slurry contains 1 wt% styrene-butadiene latex, and the rest is as described in Example 1.

[0065] Example 9 The negative electrode slurry includes 1.5 wt% styrene-butadiene latex, and the remainder is as described in Example 1.

[0066] Example 10 The degree of hydrolysis of the polyvinyl alcohol polymer layer is 90%, the thickness of the polymer layer H1 is 0.8 μm, the volume concentration C of trimethylchlorosilane in the electrolyte is 0.12 vol%, and the rest are as described in Example 1.

[0067] Example 11 The degree of hydrolysis of the polyvinyl alcohol polymer layer is 80%, and the rest are as described in Example 1.

[0068] Example 12 The weight-average molecular weight of the polyvinyl alcohol polymer layer is 30,000 g / mol to 40,000 g / mol, and the rest is as described in Example 1.

[0069] Example 13 The dehydrating agent in the electrolyte is a bismaleimide dehydrating agent; for the rest, refer to Example 1.

[0070] Comparative Example 1: Without setting a polymer layer, the negative electrode slurry is conventionally coated onto the current collector copper foil, and the rest is as described in Example 1.

[0071] Comparative Example 2: The degree of hydrolysis of the polyvinyl alcohol polymer layer is 75%, and the rest are as described in Example 1.

[0072] Comparative Example 3: The weight-average molecular weight of the polyvinyl alcohol polymer layer is 42,000 g / mol to 47,000 g / mol, and the rest is as described in Example 1.

[0073] Performance testing (1) Lithium plating test The battery was placed in a constant temperature chamber at -10℃±1℃ and left to stand for 2 hours. It was then charged to the upper limit voltage with a constant current of 1C and the cutoff current was set to 0.05C. The battery was disassembled in a glove box, the negative electrode was removed, and the battery was rinsed three times with DMC solvent to remove electrolyte residue. The battery was then vacuum dried and observed under an optical microscope for the presence of gray metallic luster spots.

[0074] (2) Coulomb efficiency test After formation, the battery cell was left to stand at 25°C for 2 hours, then charged at a constant current of 0.5C to the upper limit voltage, and then charged at a constant voltage of 0.05C, recording the charging capacity Qc. Subsequently, it was discharged at 0.5C to the lower limit voltage, recording the discharge capacity Qd. This cycle was repeated 10 times, with each cycle following the same charging and discharging steps. The single-cycle coulombic efficiency CE was calculated as Qd / Qc × 100%, and the average value of the 10 cycles was taken as the test result.

[0075] (3) 1000-cycle capacity retention test After the battery cell is left to stand at 25℃ for 2 hours, it is charged at a constant current of 1C to 4.2V, and then discharged at a constant voltage of 0.05C to 2.8V. The discharge capacity C0 of the first cycle is recorded. This cycle is repeated for 1000 cycles (with a 5-minute break between charge and discharge). The discharge capacity C1 is recorded every 100 cycles to ensure that the capacity decay curve reflects the battery's healthy state. This continues until the C1000 value is measured. The capacity retention rate is calculated as C1000 / C0 × 100%, and a retention rate of ≥80% is considered acceptable.

[0076] The technical features of Examples 1-13 and Comparative Examples 1-3 are recorded in Table 1, and the performance test results are recorded in Table 2.

[0077] Table 1. Table 2. Regarding lithium plating, no lithium plating occurred in any of the examples, while lithium plating occurred in Comparative Example 1. This indicates that polymers such as PVA and its derivatives, and polyvinyl acetate can effectively cover the leaked foil area and prevent dendrite formation.

[0078] Regarding coulombic efficiency, the initial coulombic efficiencies of all samples were very close, with a range of <0.5%, indicating that the polymer layer itself is electrochemically stable and does not introduce serious side reactions or consume a limited lithium source. Coulombic efficiency is mainly affected by the active material graphite itself, the conductive agent, the styrene-butadiene latex, and the formation process. The 0.9% difference between the examples and Comparative Example 1 may be due to the polymer layer improving the interface and reducing dead lithium formation, thus slightly improving efficiency.

[0079] Regarding the capacity retention rate after 1000 cycles, the capacity retention rate of Comparative Example 1 without the polymer layer was significantly lower than that of all other examples, demonstrating that the polymer layer effectively mitigates the loss of active lithium and the decomposition of the electrolyte by inhibiting lithium plating and stabilizing the interface, which is key to improving cycle life.

[0080] Among the polymer materials, the polyvinyl alcohol and boric acid crosslinked PVA in Examples 4 and 5 performed the best and were the most stable, while the polyvinyl acetate in Example 3 performed slightly worse, which may be related to its lower adhesion, hydrophilicity or mechanical strength compared to PVA.

[0081] The above performance results demonstrate that the polymer layer of the negative electrode has insulating properties. The polymer layer directly covers the coating defects that are unavoidable in the coating process, forming a physical barrier that isolates the electrolyte from direct contact with the exposed current collector. This fundamentally prevents lithium ions from preferentially and randomly depositing in the foil defects, thereby improving the cycle stability of the battery.

[0082] Without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of the different embodiments or examples.

[0083] The above description is only a part or preferred embodiment of the present invention. Neither the text nor the drawings should limit the scope of protection of the present invention. All equivalent structural transformations made using the content of the present invention specification and drawings under the overall concept of the present invention, or direct / indirect applications in other related technical fields, are included within the scope of protection of the present invention.

Claims

1. A negative electrode sheet, comprising a current collector and a negative electrode active layer, characterized in that, The negative electrode active layer is coated on the current collector. The negative electrode active layer includes a coating defect area, which is covered with a polymer layer. The degree of hydrolysis of the polymer layer is 80%~90%, and the weight-average molecular weight of the polymer layer is 20000g / mol~40000g / mol.

2. The negative electrode sheet according to claim 1, characterized in that, The polymer in the polymer layer includes at least one of polyvinyl acetate, polyvinyl acetate modified, polyvinyl alcohol, and polyvinyl alcohol modified.

3. The negative electrode sheet according to claim 2, characterized in that, The polymer in the polymer layer includes at least one of polyvinyl alcohol, polyvinyl butyral, and boric acid crosslinked polyvinyl alcohol.

4. The negative electrode sheet according to claim 3, characterized in that, The negative electrode active layer includes the polymer, and the polymer has a higher mass ratio closer to the current collector in the thickness direction of the negative electrode sheet.

5. The negative electrode sheet according to claim 1, characterized in that, The thickness of the polymer layer is denoted as H1 micrometers, and the thickness of the negative electrode active layer is denoted as H2 micrometers, where H1:H2 is 1:(100~200).

6. The negative electrode sheet according to claim 5, characterized in that, The thickness H1 of the polymer layer is 0.5 micrometers to 1 micrometer.

7. The negative electrode sheet according to claim 3, characterized in that, The negative electrode active layer includes an aqueous binder, and the mass ratio of the aqueous binder to the negative electrode active layer is a, where a = 1wt%~1.5wt%.

8. An electrochemical device, characterized in that, The electrochemical device includes an electrolyte and a negative electrode according to any one of claims 1 to 7.

9. The electrochemical device according to claim 8, characterized in that, The volume concentration of the additive in the electrolyte relative to the volume of the electrolyte is denoted as C vol%, and the thickness of the polymer layer is H1 micrometers. C and H1 satisfy the following relationship: C = 0.6 - 0.6·H1, and C > 0, H1 > 0.

10. The electrochemical device according to claim 9, characterized in that, The additives include one of the following: halogenated silane dehydrating agents, alkoxysilane-isocyanate dehydrating agents, bismaleimide dehydrating agents, organosilicon-siloxane dehydrating agents, and phosphite / phosphonate dehydrating agents.

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