Anti-deformation low-expansion lithium ion battery negative electrode and preparation method thereof

Abstract

The application discloses an anti-deformation low-expansion lithium ion battery negative electrode and a preparation method thereof. The negative electrode comprises a current collector and a negative active material layer coated on at least one surface of the current collector. The current collector is a modified copper foil treated by a corona, and the surface of the modified copper foil has a rough structure and is grafted with a hydroxyl functional group. The negative active material layer comprises a silicon-based active material, a conductive agent and a crosslinked composite binder. The crosslinked composite binder is a composite binder with a three-dimensional network structure formed by in-situ crosslinking of polyacrylic acid and carboxymethyl cellulose through esterification condensation reaction. Free carboxyl groups in the crosslinked composite binder respectively condense with the hydroxyl functional group on the surface of the modified copper foil and the silicon hydroxyl group on the surface of the silicon-based active material to form covalent bonds. Compared with the prior art, the application can effectively solve the delamination and pulverization failure problems caused by the expansion of the silicon-based negative electrode, and prolong the cycle life of the battery.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery preparation technology, and particularly relates to a lithium-ion battery anode that is resistant to deformation and has low expansion, and its preparation method. Background Technology

[0002] With the increasing prevalence of portable electronic devices and electric vehicles, the demand for high-energy-density lithium-ion batteries is growing. Silicon-based materials, due to their extremely high theoretical specific capacity (>4000mAh / g), are considered ideal candidates to replace traditional graphite anodes. However, silicon-based particles experience dramatic volume expansion during charge and discharge (expansion rates can reach over 300%).

[0003] This drastic volume change can lead to extremely serious consequences: First, silicon particles are prone to pulverization and breakage, resulting in the breakage of the conductive network; second, the electrode as a whole will undergo severe thickness expansion, generating huge internal mechanical stress; most fatally, the adhesion between the negative electrode active material layer and the copper foil current collector is prone to failure, leading to large-area delamination and peeling of the electrode.

[0004] Existing commercial anodes mostly use linear polymers such as polyvinylidene fluoride (PVDF) or polyacrylic acid (PAA) as binders. These rely solely on weak van der Waals forces or hydrogen bonds to bind with active particles, which cannot withstand the enormous mechanical stress generated by the dramatic expansion of silicon-based particles. After long-term cycling, the physically entangled bonding network will collapse, causing the electrode to expand and shed powder.

[0005] Therefore, there is an urgent need to develop a silicon-based anode that can establish a strong chemical bond between the current collector and silicon-based particles, while also possessing resistance to deformation and low expansion. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by providing a deformation-resistant and low-expansion lithium-ion battery anode and its preparation method, which can fundamentally solve the problems of delamination and pulverization failure caused by the expansion of silicon-based anodes and significantly extend the cycle life of the battery.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A deformation-resistant and low-expansion lithium-ion battery negative electrode includes a current collector and a negative electrode active material layer coated on at least one surface of the current collector. The current collector is a modified copper foil that has been corona treated. The surface of the modified copper foil has a rough structure and is grafted with hydroxyl functional groups. The negative electrode active material layer includes a silicon-based active material, a conductive agent, and a cross-linked composite binder; the cross-linked composite binder is a c-PAA-CMC composite binder with a three-dimensional network structure formed by in-situ cross-linking of polyacrylic acid and carboxymethyl cellulose through an esterification condensation reaction. The free carboxyl groups in the crosslinked composite adhesive that did not participate in the esterification condensation reaction react with the hydroxyl functional groups on the surface of the modified copper foil and the silanol groups on the surface of the silicon-based active material to form covalent bonds.

[0008] Preferably, the modified copper foil has a surface roughness Ra of 0.5~1.5μm and a surface water contact angle of <30°.

[0009] Preferably, the negative electrode active material layer further includes a dynamic crosslinking agent, which is a water-soluble metal salt containing free calcium ions; the calcium ions coordinate with the remaining free carboxyl groups in the crosslinked composite binder to form reversible dynamic ionic bonds.

[0010] Preferably, the water-soluble metal salt is one or more of calcium acetate and calcium citrate.

[0011] Preferably, the initial slurry for preparing the negative electrode active material layer comprises, by mass percentage: 80-90% silicon-based active material, 2-8% conductive agent, 4-10% polyacrylic acid and 1-5% carboxymethyl cellulose.

[0012] Preferably, the mass ratio of the polyacrylic acid to the carboxymethyl cellulose is (6~8):(2~4).

[0013] Preferably, the silicon-based active material is silicon-carbon composite particles, which are a mixture of nano-silicon-based particles and graphite particles in a mass ratio of 1:(0.5~1.5); the outer surface of the nano-silicon-based particles contains a native silicon dioxide layer and hydroxyl functional groups.

[0014] Preferably, the conductive agent is a mixture of single-walled carbon nanotubes and graphene in a mass ratio of 1:1.

[0015] Preferably, the negative electrode active material layer has a double-layer gradient distribution along the thickness direction, including an inner coating layer close to the current collector and an outer coating layer away from the current collector; the mass ratio of polyacrylic acid to carboxymethyl cellulose in the inner coating layer is (7~9):(1~3); the mass ratio of polyacrylic acid to carboxymethyl cellulose in the outer coating layer is (4~6):(4~6).

[0016] Furthermore, the present invention also provides a method for preparing the deformation-resistant and low-expansion lithium-ion battery negative electrode as described above, comprising the following steps: (1) Mix the polyacrylic acid aqueous solution and the carboxymethyl cellulose aqueous solution evenly at room temperature, then add the silicon-based active material and conductive agent, and disperse them at high speed at 20~30℃ to obtain the negative electrode slurry; (2) The copper foil was subjected to corona discharge treatment to obtain modified copper foil with hydroxyl functional groups grafted on its surface. (3) The negative electrode slurry is coated on at least one surface of the modified copper foil and pre-dried and dehydrated at a temperature of 80~100°C to obtain the initial electrode sheet; (4) The initial electrode is placed in a vacuum oven and kept at 140~160℃ and in a vacuum environment for 2~6 hours. During this process, the free carboxyl groups of polyacrylic acid and the hydroxyl groups of carboxymethyl cellulose undergo in-situ esterification condensation reaction to generate a three-dimensional cross-linked network. At the same time, the free carboxyl groups of polyacrylic acid undergo in-situ condensation reaction with the hydroxyl groups on the surface of the modified copper foil and the silanol groups on the surface of the silicon-based active material to form a covalent bonded network. After cooling and rolling, the lithium-ion battery negative electrode is obtained.

[0017] Preferably, in step (2), the conditions for the corona discharge treatment are: in an air atmosphere, the discharge power density is 50~200 W·min / m 2 The processing time is 5~30 seconds.

[0018] Compared with the prior art, the present invention has at least the following beneficial effects: This invention constructs a fully covalently bonded "molecular bridge" network structure of "current collector-binder-silicon-based particles" through interface activation and in-situ three-dimensional covalent cross-linking design.

[0019] 1) The corona treatment on the surface of the current collector not only increases the physical roughness (riveting effect), but also generates active hydroxyl groups (-OH) in situ on the copper surface.

[0020] 2) During the high-temperature vacuum baking stage of electrode preparation, PAA and CMC undergo in-situ esterification and crosslinking to form a highly crosslinked three-dimensional network (c-PAA-CMC). This three-dimensional network overcomes the defects of traditional linear polymers, such as easy slippage and swelling, and possesses extremely strong tensile strength and resistance to electrolyte swelling.

[0021] 3) The incompletely internally cross-linked PAA free carboxyl groups (-COOH) in the composite binder undergo esterification reactions with the hydroxyl groups (Cu-OH) on the copper foil surface and the silanol groups (Si-OH) on the silicon particle surface at high temperatures. These high-energy covalent bonds (CO-Cu and CO-Si) replace the traditional weak van der Waals forces, firmly locking the current collector, binder, and active particles in a unified rigid framework; effectively suppressing the violent movement of silicon-based particles, fundamentally preventing electrode thickness expansion and delamination. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the cross-sectional structure of the negative electrode in one embodiment of the present invention; Figure 2 This is a schematic diagram of the cross-sectional structure of the negative electrode in another embodiment of the present invention.

[0023] In the figure: 1. Current collector; 2. Negative electrode active material layer; 21. Inner coating layer; 22. Outer coating layer. Detailed Implementation

[0024] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0026] According to the first aspect of this application, Figure 1 As shown, this application provides a deformation-resistant and low-expansion lithium-ion battery negative electrode, including a current collector 1 and a negative electrode active material layer 2 coated on at least one surface of the current collector 1. The current collector 1 is a modified copper foil that has been corona treated. The surface of the modified copper foil has a rough structure and is grafted with hydroxyl functional groups. The negative electrode active material layer 2 includes a silicon-based active material, a conductive agent, and a cross-linked composite binder; the cross-linked composite binder is a c-PAA-CMC binder with a three-dimensional network structure formed by in-situ cross-linking of polyacrylic acid and carboxymethyl cellulose through an esterification condensation reaction. In the crosslinked composite adhesive, the free carboxyl groups that did not participate in the esterification condensation reaction react with the hydroxyl functional groups on the surface of the modified copper foil and the silanol groups on the surface of the silicon-based active material to form covalent bonds.

[0027] This application constructs a fully covalently bonded "molecular bridge" network of "current collector-binder-silicon-based particles" through interface activation and in-situ three-dimensional covalent cross-linking design.

[0028] 1) The corona treatment on the surface of the current collector not only increases the physical roughness (micro-mechanical riveting effect), but also generates active hydroxyl groups (-OH) in situ on the copper surface, which activates the originally chemically inert copper foil surface.

[0029] 2) During the final high-temperature vacuum baking stage of electrode preparation, PAA and CMC undergo in-situ dehydration and esterification crosslinking to form a highly crosslinked three-dimensional network (c-PAA-CMC). This three-dimensional covalent network overcomes the fatal defects of traditional linear polymers (such as PVDF, pure PAA, etc.) that are prone to slippage and swelling by electrolyte, and possesses extremely strong tensile mechanical properties.

[0030] 3) Serving as a "molecular bridge" anchoring interface: The incompletely internally cross-linked PAA free carboxyl groups (-COOH) directly undergo esterification condensation reactions with the hydroxyl groups (Cu-OH) on the copper foil surface and the silanol groups (Si-OH) on the silicon particle surface at high temperatures. These high-bond-energy interfacial covalent bonds (CO-Cu and CO-Si) replace the traditional weak van der Waals forces and weak hydrogen bonds, firmly locking the current collector, binder, and active silicon particles in a unified rigid framework. This fundamentally prevents the relative slippage of silicon particles under huge expansion stress, thereby effectively solving the problems of electrode thickness expansion, pulverization and detachment failure.

[0031] In one embodiment according to this application, the surface roughness Ra of the modified copper foil is 0.5~1.5μm, and the surface water contact angle is <30°. Controlling Ra within this range provides optimal physical bonding force while avoiding excessive roughness that could lead to uneven coating or excessive localized internal resistance. A water contact angle <30° indicates that the surface is in an extremely hydrophilic state, which greatly promotes the spreading and deep wetting of the aqueous PAA / CMC slurry on the copper foil surface. This ensures sufficient molecular-level contact between the polymer chains and the reaction sites (hydroxyl groups) on the copper foil surface, thereby maximizing the number of interfacial "covalent molecular bridges" formed and significantly improving the peel strength of the electrode.

[0032] In one embodiment of this application, the negative electrode active material layer 2 further includes a dynamic crosslinking agent, which is a water-soluble metal salt containing free calcium ions; the calcium ions coordinate with the remaining free carboxyl groups in the c-PAA-CMC binder to form reversible dynamic ionic bonds.

[0033] In one embodiment of this application, the water-soluble metal salt is one or more of calcium acetate and calcium citrate.

[0034] In this process, water-soluble metal salts are introduced to allow free Ca to... 2+ Coordinate with the remaining free carboxylate groups in c-PAA-CMC to form reversible dynamic ionic bonds; this design endows the rigid covalent three-dimensional network with a dynamic self-healing capability that combines rigidity and flexibility. When silicon particles expand violently and generate extremely high internal stress, the relatively weaker ionic bonds act as "sacrificial bonds" and break preferentially to dissipate local stress and protect the main covalent framework from fracture; during particle discharge and contraction, electrostatic interactions drive the broken ionic bonds to spontaneously recombine and rearrange. This dual mechanism of "dynamic ionic bond buffering + strong covalent bond anchoring" endows the electrode with extremely strong mechanical toughness and resistance to strain fatigue, significantly improving the electrode's cycle life.

[0035] In one embodiment of this application, the initial slurry for preparing the negative electrode active material layer 2 comprises, by mass percentage: 80-90% silicon-based active material, 2-8% conductive agent, 4-10% polyacrylic acid and 1-5% carboxymethyl cellulose.

[0036] In one embodiment of this application, the mass ratio of polyacrylic acid to carboxymethyl cellulose is (6~8):(2~4).

[0037] PAA contains abundant carboxyl groups, primarily responsible for esterification crosslinking and providing covalent molecular bridge anchors; CMC possesses excellent film-forming properties, thickening and dispersing properties, and ion-conducting properties. Excessive PAA leads to over-crosslinking of the network, causing the electrode to become extremely brittle and prone to cracking; excessive CMC fails to provide a sufficient number of free carboxyl groups for interfacial bonding, drastically increasing the risk of delamination. The specific ratio defined in this invention achieves a perfect balance between the degree of crosslinking reactivity, the number of interfacial anchor points, and the electrode's inherent flexibility, thus balancing high adhesion with high flexibility.

[0038] In one embodiment of this application, the silicon-based active material is silicon-carbon composite particles, which are a mixture of nano-silicon-based particles and graphite particles in a mass ratio of 1:(0.5~1.5); the outer surface of the nano-silicon-based particles contains a native silicon dioxide layer and hydroxyl functional groups.

[0039] Among them, graphite particles serve as a robust conductive buffer framework, which alleviates the expansion of nano-silicon from a physical perspective. More importantly, the native silica layer on the surface of nano-silicon is rich in active silanol groups (Si-OH), which provides chemical cross-linking sites for the carboxyl groups in PAA. This ensures that each silicon particle is firmly "tethered" to the polymer network, maintaining the permanent connectivity of the conductive network, avoiding the phenomenon of isolated silicon particles (dead silicon), and improving the utilization rate of active materials.

[0040] In one embodiment of this application, the negative electrode active material layer 2 further contains 0.5-5% silicon dioxide by mass of the total negative electrode active material layer 2. The addition of silicon dioxide as a filler not only provides mechanical support, preventing the silicon-based active material from expanding and slipping, but also allows the silanol groups on the silicon dioxide surface to undergo a condensation reaction with the free carboxyl groups of the crosslinked composite binder to form chemical bonds, further enhancing the overall deformation resistance of the negative electrode active material layer 2.

[0041] In one embodiment according to this application, such as Figure 2As shown, the negative electrode active material layer 2 exhibits a double-layer gradient distribution along the thickness direction, including an inner coating layer 21 close to the current collector 1 and an outer coating layer 22 far from the current collector 1; the mass ratio of polyacrylic acid to carboxymethyl cellulose in the inner coating layer 21 is (7~9):(1~3); the mass ratio of polyacrylic acid to carboxymethyl cellulose in the outer coating layer 22 is (4~6):(4~6).

[0042] The inner coating 21 is closer to the current collector 1. Increasing the PAA content can provide more carboxyl groups, which can fully react with the hydroxyl groups on the surface of the corona copper foil, effectively improving the interfacial bonding force (peel strength) between the negative electrode active material layer 2 and the current collector 1. The outer coating 22 is closer to the electrolyte side. Increasing the CMC content can increase the flexibility of the electrode surface (prevent surface cracking and powdering). At the same time, the excellent hydrophilicity of CMC can promote electrolyte penetration, accelerate the lithium ion insertion and extraction transport rate on the surface, and reduce interfacial impedance.

[0043] According to a second aspect of this application, this application also provides a method for preparing the deformation-resistant and low-expansion lithium-ion battery negative electrode as described above, comprising the following steps: (1) Mix the polyacrylic acid aqueous solution and the carboxymethyl cellulose aqueous solution evenly at room temperature, then add the silicon-based active material and conductive agent, and disperse them at high speed at 20~30℃ to obtain the negative electrode slurry; (2) The copper foil was subjected to corona discharge treatment to obtain modified copper foil with hydroxyl functional groups grafted on its surface. (3) The negative electrode slurry is coated on at least one surface of the modified copper foil and pre-dried at a temperature of 80~100℃ to obtain the initial electrode sheet; (4) The initial electrode is placed in a vacuum oven and kept at 140~160℃ and in a vacuum environment for 2~6h. During this process, the free carboxyl groups of polyacrylic acid and the hydroxyl groups of carboxymethyl cellulose undergo in-situ esterification condensation reaction to generate a c-PAA-CMC crosslinking network. At the same time, the free carboxyl groups of polyacrylic acid undergo in-situ condensation reaction with the hydroxyl groups on the surface of the modified copper foil and the silanol groups on the surface of the silicon-based active material to form a covalent bond network. After cooling and rolling, the negative electrode of the lithium-ion battery is obtained.

[0044] The present invention provides a method for preparing a deformation-resistant and low-expansion lithium-ion battery negative electrode, which ingeniously integrates the slurry dispersion coating process, the electrode deep dehydration process, and the binder in-situ chemical crosslinking process. It has significant advantages in terms of process continuity and ease of industrial scale-up. The specific mechanism is as follows: 1. Room temperature dispersion avoids gelation, ensuring excellent coating processability: Step (1) involves high-speed dispersion of the slurry at room temperature (20~30℃). Within this temperature range, only weak physical hydrogen bonding exists between the free carboxyl groups of PAA and the hydroxyl groups of CMC, and the activation energy is insufficient to trigger esterification crosslinking. This avoids the slurry thickening and gelation problems that are very likely to occur when using traditional "pre-crosslinking binders," ensuring that the negative electrode slurry has excellent rheological properties and long-term storage stability, enabling continuous and uniform thin-layer coating.

[0045] 2. The environmental friendliness and high efficiency of corona treatment: Step (2) uses corona discharge to treat copper foil. Compared with the traditional strong acid / strong alkali chemical etching surface modification method, corona treatment is a dry plasma modification method that does not produce any waste liquid. The process is extremely environmentally friendly and can be connected to the existing roll-to-roll continuous coating production line of electrode sheets, with extremely high processing efficiency.

[0046] 3. Step-by-step heat treatment, taking into account both film morphology and chemical cross-linking: The function and mechanism of the stepped heat treatment process of 80~100℃ initial drying and 140~160℃ vacuum high temperature in this invention are as follows: 1) Stable type: The initial drying and dehydration in step (3) can gently and quickly evaporate the free water in the slurry, so that the electrode can be initially solidified and shaped. If the high temperature stage is entered directly, the violent boiling of water will cause a large number of pores or even peeling off the coating on the surface of the electrode.

[0047] 2) Thermodynamic and kinetic synergistic driving of in-situ crosslinking: The vacuum insulation at 140~160℃ in step (4) is the core of this preparation method. From the perspective of reaction kinetics, the high temperature of 140~160℃ breaks through the activation energy barrier of the esterification condensation reaction between free carboxyl groups and hydroxyl groups, triggering the generation of three-dimensional network structure and interfacial covalent molecular bridges; from the perspective of chemical thermodynamics (Le Chatelier's principle), the esterification condensation reaction will generate water as a byproduct, and the vacuum environment can remove the water molecules generated by the reaction in real time and continuously, thereby breaking the reaction equilibrium and strongly driving the esterification condensation reaction in the forward direction (i.e., the crosslinking direction).

[0048] 3) Simplified Process: This step cleverly utilizes the essential "vacuum baking and dehydration" process in lithium battery manufacturing, combining physical dehydration with chemical crosslinking. No additional toxic chemical crosslinking agents or thermal initiators need to be added to the slurry, nor is additional complex manufacturing equipment required, significantly reducing production costs and ensuring full compatibility with existing lithium battery manufacturing processes.

[0049] In one embodiment of this application, in step (2), the conditions for corona discharge treatment are: in an air atmosphere, the discharge power density is 50~200 W·min / m 2 The processing time is 5~30 seconds.

[0050] This parameter range allows for precise control of the surface state of the copper foil. If the power density is too low or the time is too short, the amount of hydroxyl groups generated on the surface will be insufficient, failing to provide enough chemical anchors. If the power is too high or the time is too long, it will lead to excessive oxidation of the copper foil surface, forming a fragile copper oxide layer. This not only reduces conductivity but also makes the fragile layer prone to self-peeling under stress. This limited parameter ensures an extremely high hydroxyl grafting rate while perfectly preserving the excellent mechanical strength and conductivity of the copper substrate.

[0051] Ordinary copper foil has a chemically inert surface, making it difficult to form strong bonds with polymers. This invention employs a high-voltage corona discharge plasma treatment device, where high-energy electrons and oxygen free radicals bombard the copper foil surface in an air atmosphere. On one hand, this breaks the smooth crystal lattice of the surface, forming a micro-nano-scale rough structure (Ra = 0.5~1.5 μm); on the other hand, moisture and oxygen in the air, under the action of the plasma, graft a large number of active hydroxyl groups (Cu-OH) onto the surface of the copper atoms.

[0052] In this application, the c-PAA-CMC is not synthesized in advance in a slurry tank. When PAA and CMC are mixed at room temperature (20~30℃), only weak physical hydrogen bonds exist, and no condensation reaction occurs. Therefore, the slurry has good flowability and coatability. Only after coating is completed and the mixture enters the high-temperature vacuum baking stage at 140~160℃, the thermal energy provided by the environment breaks through the activation energy barrier. At the same time, the vacuum environment continuously removes the water molecules generated in the reaction, causing the -COOH of PAA and the -OH of CMC to undergo an esterification reaction (-COO-), generating an irreversible c-PAA-CMC three-dimensional framework in situ.

[0053] The present invention will be further described below with reference to specific embodiments and comparative examples.

[0054] Example 1

[0055] A method for preparing a deformation-resistant and low-expansion lithium-ion battery negative electrode includes the following steps: (1) An aqueous solution of polyacrylic acid (PAA, molecular weight 450,000) and an aqueous solution of carboxymethyl cellulose (CMC, degree of substitution 0.7) were mixed at room temperature, with a dry weight ratio of PAA to CMC of 7:3. Subsequently, a silicon-carbon composite material (SiOx / C, particle size 150 nm) and a conductive agent (single-walled carbon nanotubes to graphene in a 1:1 mass ratio) were added. The solid-phase dry weight ratio was: silicon-carbon composite material 85%, conductive agent 5%, and binder 10%. The mixture was dispersed at high speed at 25°C for 3 hours to obtain the negative electrode slurry. (2) A corona discharge treatment machine was used to treat bare copper foil with a thickness of 8 μm, and the discharge power density was set to 100 W·min / m. 2The processing time was 15s, and a modified copper foil with hydroxyl-rich surface was prepared (Ra=1.0μm, water contact angle 20°). (3) The negative electrode slurry is uniformly coated on the surface of the modified copper foil and dried in an atmospheric oven at 90°C for 30 minutes to remove most of the free water, thus obtaining the initial electrode sheet.

[0056] (4) The initial electrode was transferred to a vacuum oven and heated to 150℃ for 4 hours under a vacuum of ≤-0.09MPa. During this period, in-situ dehydration condensation esterification reactions of PAA with CMC, PAA with Cu-OH, and PAA with Si-OH were triggered. Afterward, it was cooled and rolled to a compaction density of 1.5 g / cm³. 3 Thus, the lithium-ion battery negative electrode of Example 1 is obtained.

[0057] Example 2

[0058] The only difference from Example 1 is the introduction of a dynamic crosslinking agent in step (1): calcium acetate (Ca(CH3COO)2) at 3% of the total mass of the binder is added when mixing the PAA and CMC aqueous solution. Since the polymer chains are not fixed at room temperature, Ca... 2+ Uniformly dispersed in the slurry. After forming the c-PAA-CMC covalent network through vacuum baking at 150℃, the remaining free -COO - With Ca 2+ Automatic coordination forms dynamic ionic bonds (-COO) throughout a three-dimensional network. - ···Ca 2+ ··· - OOC-). The remaining steps are the same as in Example 1.

[0059] Example 3

[0060] The difference from Example 2 is the use of a two-layer gradient coating process: Prepare the inner coating slurry and the outer coating slurry separately.

[0061] Inner coating slurry binder ratio: PAA:CMC=8:2 (strengthens the covalent ester bond anchoring force on the current collector side); Outer coating slurry binder ratio: PAA:CMC=5:5 (improves ionic conductivity and toughness on the electrolyte side).

[0062] Calcium acetate was added in equal proportions. Using a double-layer extrusion coating machine, the inner coating (accounting for 30% of the total electrode thickness) was first coated, followed by the outer coating (accounting for 70% of the total electrode thickness). The subsequent drying and 150°C vacuum crosslinking steps were the same as in Example 2.

[0063] Comparative Example 1

[0064] The difference from Example 1 is that: in step (2), the copper foil was not corona treated, and the untreated smooth inert bare copper foil was directly used for coating.

[0065] Comparative Example 2

[0066] The difference from Example 1 is that the baking conditions in step (4) are changed to drying in a vacuum oven at 100°C for 10 hours. Since the temperature does not reach the activation temperature required for the esterification reaction (above 140°C), at this time the PAA, CMC, copper foil, and silicon particles are physically mixed and weakly hydrogen bonded, and no covalently cross-linked c-PAA-CMC and molecular bridges are formed.

[0067] Comparative Example 3

[0068] The negative electrode active material layer adopts a conventional commercial system: silicon-carbon composite material, conductive agent, and PVDF binder are mixed in NMP solvent at a mass ratio of 85:5:10. Untreated bare copper foil is used, and after coating, it is conventionally baked and dried at 100°C.

[0069] Comparative Example 4

[0070] The difference from Example 1 is that the dry weight ratio of PAA to CMC in the adhesive system is 9:1 (PAA is extremely excessive and CMC is too little).

[0071] Comparative Example 5

[0072] The difference from Example 1 is that the dry weight ratio of PAA to CMC in the adhesive system is 2:8 (PAA is very little and CMC is in excess).

[0073] The negative electrode and battery of the above embodiments and comparative examples were subjected to the following performance tests, and the test results are shown in Table 1.

[0074] 1. Peel strength test: Cut the rolled electrode into strips of 20mm×100mm, fix them to the stainless steel plate with 3M double-sided tape, and use a universal tensile testing machine to perform a 180° peel test at a speed of 50mm / min. Record the average peel strength (N / m).

[0075] 2. Electrode flexibility and crack resistance test: The electrode is bent 180° along a cylindrical steel rod with a diameter of 4mm, and the surface of the electrode at the bend is observed with an optical microscope to see if there are macroscopic cracks or powder falling off.

[0076] 3. Thickness Expansion Rate Test: Assemble the electrode sheets into a coin cell and perform 100 complete cycles at a 0.5C charge / discharge rate. Disassemble the battery, wash off the surface electrolyte and dry it. Measure the electrode thickness H1 after cycling with a micrometer, and calculate the electrode thickness expansion rate using the formula (H1-H0) / H0×100%.

[0077] 4. Cycle capacity retention: At room temperature, within a voltage range of 2.8~4.2V, 500 charge-discharge cycles are performed at a rate of 1C. The percentage of the discharge capacity on the 500th cycle relative to the discharge capacity on the first cycle is recorded.

[0078] Table 1

[0079]

[0080] The analysis of the above test results is as follows: (1) A comparison of Example 1 with Comparative Examples 1-3 shows that: The peel strength of Example 1 reached 40.6 N / m, while that of conventional PVDF (Comparative Example 3) was only 9.8 N / m. Comparative Example 1 used in-situ crosslinking but did not undergo corona treatment, resulting in a peel strength of only 18.4 N / m; Comparative Example 2 used corona-treated copper foil but did not trigger crosslinking at 150°C, resulting in a peel strength of only 23.5 N / m. This fully demonstrates that both the introduction of hydroxyl groups by corona treatment and the high-temperature triggering of in-situ covalent molecular bridges in PAA are indispensable. Their synergistic effect is necessary to firmly bond the negative electrode active material layer to the current collector, causing the electrode thickness expansion rate to plummet from 82.5% in Comparative Example 3 to 18.5% in Example 1, and significantly improving the cycle retention rate.

[0081] (2) A comparison between Example 2 and Example 1 shows that: Example 2, based on Example 1, added calcium acetate, further reducing the expansion rate to 14.2% and increasing the retention rate after 500 cycles to 89.7%. The possible reason is that although the covalent bonds are extremely strong, they are too rigid. Adding Ca... 2+ The resulting dynamic ionic bonds act as "molecular springs." When silicon particles expand to extreme sizes, the ionic bonds reversibly break, releasing localized stress and preventing the main covalent bonds from being broken; during contraction, they spontaneously recombine. This dynamic self-healing mechanism greatly enhances resistance to deformation fatigue.

[0082] (3) A comparison between Example 3 and Example 2 shows that: Example 3, through gradient control of the ratio of inner and outer adhesives, further improved the interfacial adhesion of the bottom current collector (peel strength increased to 48.3 N / m), while the outer layer, rich in CMC, improved interfacial ion conductivity and flexibility. Therefore, its expansion rate was suppressed to a minimum of 11.5%, achieving optimal cycling performance.

[0083] (4) A comparison of Example 1 with Comparative Examples 4 and 5 shows that: In Comparative Example 4, the PAA was excessively excessive, resulting in an overly dense and rigid cross-linked network. The electrode completely lost its flexibility and fractured brittlely during testing. The inability to release expansion stress led to poor battery cycle performance. In Comparative Example 5, the CMC was severely excessive, preventing the system from providing enough free -COOH to construct molecular bridges to anchor the copper foil. The peel strength dropped to 20.1 N / m, and the electrode pulverized and peeled off during cycling. Therefore, the mass ratio range of polyacrylic acid and carboxymethyl cellulose defined in this invention is a key critical parameter for achieving a balance between rigidity and flexibility.

[0084] In summary, the deformation-resistant and low-expansion lithium-ion battery anode and its preparation method provided by this invention successfully construct in-situ covalent bonding and a three-dimensional self-healing network at the current collector interface using molecular engineering methods. Its expansion rate is extremely low, and its crack resistance and cycle stability are significantly better than those of existing technologies.

[0085] It should be noted that the contents not described in detail in this specification are existing technologies known to those skilled in the art, and will not be elaborated here.

[0086] Based on the disclosure and teachings of the foregoing specification, those skilled in the art can make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the specific embodiments described above, and any obvious improvements, substitutions, or modifications made by those skilled in the art based on the present invention are within the scope of protection of the present invention. Furthermore, although some specific terms are used in this specification, these terms are only for convenience of explanation and do not constitute any limitation on the present invention.

Claims

1. A deformation-resistant and low-expansion lithium-ion battery negative electrode, comprising a current collector and a negative electrode active material layer coated on at least one surface of the current collector, characterized in that: The current collector is a modified copper foil that has been corona treated. The surface of the modified copper foil has a rough structure and is grafted with hydroxyl functional groups. The negative electrode active material layer includes a silicon-based active material, a conductive agent, and a cross-linked composite binder; the cross-linked composite binder is a composite binder with a three-dimensional network structure formed by in-situ cross-linking of polyacrylic acid and carboxymethyl cellulose through an esterification condensation reaction. The free carboxyl groups in the crosslinked composite adhesive undergo condensation reactions with the hydroxyl functional groups on the surface of the modified copper foil and the silanol groups on the surface of the silicon-based active material to form covalent bonds.

2. The lithium-ion battery negative electrode with resistance to deformation and low expansion according to claim 1, characterized in that: The modified copper foil has a surface roughness Ra of 0.5~1.5μm and a surface water contact angle of <30°.

3. The deformation-resistant and low-expansion lithium-ion battery negative electrode according to claim 1, characterized in that: The negative electrode active material layer also contains a dynamic crosslinking agent, which is a water-soluble metal salt containing free calcium ions; the calcium ions coordinate with the remaining free carboxyl groups in the crosslinked composite binder to form reversible dynamic ionic bonds.

4. The deformation-resistant and low-expansion lithium-ion battery negative electrode according to claim 3, characterized in that: The water-soluble metal salt is one or more of calcium acetate and calcium citrate.

5. The deformation-resistant and low-expansion lithium-ion battery negative electrode according to claim 1, characterized in that: The initial slurry for preparing the negative electrode active material layer comprises, by mass percentage: 80-90% silicon-based active material, 2-8% conductive agent, 4-10% polyacrylic acid and 1-5% carboxymethyl cellulose.

6. The deformation-resistant and low-expansion lithium-ion battery negative electrode according to claim 5, characterized in that: The mass ratio of the polyacrylic acid to the carboxymethyl cellulose is (6~8):(2~4).

7. The deformation-resistant and low-expansion lithium-ion battery negative electrode according to claim 1, characterized in that: The silicon-based active material is silicon-carbon composite particles, which are a mixture of nano-silicon-based particles and graphite particles in a mass ratio of 1:(0.5~1.5); the outer surface of the nano-silicon-based particles contains a native silicon dioxide layer and hydroxyl functional groups.

8. The deformation-resistant and low-expansion lithium-ion battery negative electrode according to claim 1, characterized in that: The negative electrode active material layer exhibits a double-layer gradient distribution along the thickness direction, including an inner coating layer close to the current collector and an outer coating layer away from the current collector; the mass ratio of polyacrylic acid to carboxymethyl cellulose in the inner coating layer is (7~9):(1~3); the mass ratio of polyacrylic acid to carboxymethyl cellulose in the outer coating layer is (4~6):(4~6).

9. A method for preparing a deformation-resistant and low-expansion lithium-ion battery negative electrode as described in any one of claims 1 to 8, characterized in that, Includes the following steps: (1) Mix the polyacrylic acid aqueous solution and the carboxymethyl cellulose aqueous solution evenly at room temperature, then add the silicon-based active material and conductive agent, and disperse them at high speed at 20~30℃ to obtain the negative electrode slurry; (2) The copper foil was subjected to corona discharge treatment to obtain modified copper foil with hydroxyl functional groups grafted on its surface. (3) The negative electrode slurry is coated on at least one surface of the modified copper foil and pre-dried and dehydrated at a temperature of 80~100°C to obtain the initial electrode sheet; (4) The initial electrode is placed in a vacuum oven and kept at 140~160℃ and in a vacuum environment for 2~6 hours. During this process, the free carboxyl groups of polyacrylic acid and the hydroxyl groups of carboxymethyl cellulose undergo in-situ esterification condensation reaction to generate a three-dimensional cross-linked network. At the same time, the free carboxyl groups of polyacrylic acid undergo in-situ condensation reaction with the hydroxyl groups on the surface of the modified copper foil and the silanol groups on the surface of the silicon-based active material to form a covalent bonded network. After cooling and rolling, the lithium-ion battery negative electrode is obtained.

10. The method for preparing the deformation-resistant and low-expansion lithium-ion battery negative electrode according to claim 9, characterized in that: In step (2), the conditions for the corona discharge treatment are: in an air atmosphere, the discharge power density is 50~200 W·min / m 2 The processing time is 5~30 seconds.

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