Lithium ion secondary battery

Abstract

The invention discloses a lithium ion secondary battery, belongs to the technical field of batteries, and overcomes the defect that cobalt ions of a cobalt-containing positive electrode material are easy to dissolve out in the battery circulation process in the prior art, so that the cycle life of the battery is influenced. A positive electrode binder and a negative electrode binder of the lithium ion secondary battery both comprise cyano and ester functional groups; the positive electrode binder and the negative electrode binder both meet the following conditions: in an XRD pattern, a characteristic diffraction peak 1 exists at a position where 2theta is equal to 16-18 degrees, a characteristic diffraction peak 2 exists at a position where 2theta is equal to 22-30 degrees, and the intensity ratio of the characteristic diffraction peak 1 to the characteristic diffraction peak 2 is 1.3-2; s1 is the peak area of the characteristic diffraction peak 1, S2 is the peak area of the characteristic diffraction peak 2, Stotal = S1 + S2, and 60% < = S1 / Stotal < = 90%; the swelling ratio of the positive electrode binder and the swelling ratio of the negative electrode binder are respectively and independently 50%-200%. According to the lithium ion secondary battery, dissolution of cobalt ions is effectively inhibited, and the cycle performance is remarkably improved.

Description

Technical Field

[0001] This invention belongs to the field of battery technology, specifically relating to a lithium-ion secondary battery. Background Technology

[0002] Under high voltage, cobalt-containing cathode materials undergo phase transitions during charge and discharge, leading to decreased structural stability, vacancy defects, oxygen evolution, and structural slippage and collapse. Repeated lithium-ion insertion and extraction during charge and discharge cause volume changes and internal stress, resulting in structural collapse or even rupture of the cathode material. This exposes new, unstable surfaces that come into contact with the electrolyte, triggering side reactions and accelerating cobalt dissolution. During battery cycling, the dissolved cobalt ions migrate to the surface of the silicon-based anode and are reduced to highly catalytically active metallic cobalt. This metallic cobalt catalyzes a continuous reduction and decomposition reaction of the electrolyte on the anode surface. This decomposition reaction consumes a large amount of active lithium ions, leading to an irreversible decrease in battery capacity. To cover the newly exposed anode surface, a new, thicker SEI film is continuously generated. This thickened SEI film has poor ionic conductivity, increasing internal resistance and ultimately reducing battery capacity and deteriorating cycle life. Summary of the Invention

[0003] Therefore, the technical problem to be solved by the present invention is to overcome the defect in the prior art that cobalt-containing cathode materials are prone to cobalt ion dissolution during battery cycling, which affects the battery cycle life, and to provide a lithium-ion secondary battery.

[0004] To this end, the present invention provides the following technical solution.

[0005] This invention provides a lithium-ion secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte; The positive electrode sheet includes a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector; the positive active layer includes a cobalt-containing positive active material and a positive binder; The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer disposed on at least one side surface of the negative electrode current collector; the negative electrode active layer includes a negative electrode active material and a negative electrode binder, and the negative electrode active material includes a silicon-based material; Both the positive electrode binder and the negative electrode binder contain cyano and ester functional groups; Both the positive electrode binder and the negative electrode binder satisfy the following conditions: In the XRD patterns of the positive electrode binder and the negative electrode binder, there is a characteristic diffraction peak 1 at 2θ = 16~18° and a characteristic diffraction peak 2 at 2θ = 22~30°. The intensity ratio of characteristic diffraction peak 1 to characteristic diffraction peak 2 is 1.3~2. S1 is the peak area of ​​characteristic diffraction peak 1, and S2 is the peak area of ​​characteristic diffraction peak 2. 总 =S1+S2, 60%≤S1 / S 总 ≤90%.

[0006] In one possible implementation, the intensity ratio of characteristic diffraction peak 1 to characteristic diffraction peak 2 is 1.4 to 1.9; In one possible implementation, the swelling rates of the positive electrode binder and the negative electrode binder are each independently 50% to 200%, and optionally 80% to 150%.

[0007] In one possible implementation, the mass content of the positive electrode binder is 0.1% to 3% based on the mass of the positive electrode active layer; In one possible implementation, the mass content of the negative electrode binder is 2% to 5% based on the mass of the negative electrode active layer.

[0008] In one possible implementation, the negative electrode active layer includes a first negative electrode active layer and a second negative electrode active layer; the second negative electrode active layer is disposed on the side of the first negative electrode active layer away from the negative electrode current collector; Based on the total mass of the negative electrode active layer, the mass content of silicon is 1.5% to 40%. The mass content of silicon in the first negative electrode active layer within a region of 0-10 μm from the surface of the negative electrode current collector is lower than the mass content of silicon in the second negative electrode active layer within a region of 0-10 μm from the surface of the second negative electrode active layer; 0-10 μm does not include 0.

[0009] In one possible implementation, the surface of the second negative electrode active layer is provided with a plurality of recesses; the distance between two adjacent recesses is 0.5mm to 2mm, the width of a single recess is 50μm to 200μm, and the depth of a single recess is 85μm to 40μm; In one possible implementation, the thickness of the first negative electrode active layer is 10 μm to 30 μm; In one possible implementation, the thickness of the second negative electrode active layer is 10 μm to 50 μm.

[0010] In one possible implementation, the positive electrode binder and the negative electrode binder are polymers formed by copolymerization of a first monomer, a second monomer, and a third monomer, respectively. The first monomer includes at least one of acrylonitrile, methacrylonitrile, and fumaronitrile; The second monomer includes at least one of acrylate, methacrylate, and vinyl acetate; The third monomer includes at least one of a monomer containing an acidic group, a monomer containing a basic group, and a halogen-containing monomer. In one possible implementation, the mass content of the first monomer is 15-85 wt%, based on the total mass of all monomers. In one possible implementation, the second monomer content is 15-85 wt% based on the total mass of all monomers. In one possible implementation, the mass content of the third monomer is 1-10 wt% based on the total mass of all monomers. In one possible implementation, the degree of ordered alternation between the first monomer and the second monomer is ≥90%.

[0011] In one possible implementation, the cobalt content in the negative electrode active layer is less than 10,000 ppm at 0% SOC.

[0012] In one possible implementation, at 50% SOC, the impedance R1 of the positive electrode is less than the impedance R2 of the negative electrode, the impedance R1 of the positive electrode is in the range of 15~30mΩ, the impedance R2 of the negative electrode is in the range of 17~45mΩ, and R2-R1=2~20mΩ.

[0013] In one possible implementation, the electrolyte includes nitrile additives; In one possible implementation, the nitrile additive includes one or more of adiponitrile, succinate, acetonitrile, 3-methoxypropionitrile, and 1,3,6-hexanetrionitrile; In one possible implementation, the content of the nitrile additive is 0.1w% to 8wt% based on the total mass of the electrolyte, and optionally 0.5w% to 5wt%.

[0014] In one possible implementation, the thickness of the negative electrode sheet is 95μm to 140μm at 50% SOC; In one possible implementation, the thickness of the negative electrode at 100% SOC ranges from 100 μm to 170 μm. In one possible implementation, the cell thickness change rate is 0.5%~2% under ambient temperature cycling at 200T and 100% SOC. In one possible implementation, the cell thickness change rate is 2%~6% under ambient temperature cycling at 600T and 100% SOC.

[0015] The technical solution of this invention has the following advantages: 1. This invention provides a lithium-ion secondary battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte; the positive electrode comprises a positive current collector and a positive active layer disposed on at least one surface of the positive current collector; the positive active layer comprises a cobalt-containing positive active material and a positive binder; the negative electrode comprises a negative current collector and a negative active layer disposed on at least one surface of the negative current collector; the negative active layer comprises a negative active material and a negative binder, wherein the negative active material... The material includes silicon-based materials; both the positive and negative electrode binders contain cyano and ester functional groups; both the positive and negative electrode binders satisfy the following conditions: in the XRD patterns of the positive and negative electrode binders, a characteristic diffraction peak 1 exists at 2θ = 16~18°, a characteristic diffraction peak 2 exists at 2θ = 22~30°, and the intensity ratio of characteristic diffraction peak 1 to characteristic diffraction peak 2 is 1.3~2; S1 is the peak area of ​​characteristic diffraction peak 1, S2 is the peak area of ​​characteristic diffraction peak 2, and S... 总 =S1+S2, 60%≤S1 / S 总 ≤90%.

[0016] The positive electrode binder contains cyano and ester functional groups. The cyano group exhibits strong adsorption and passivation properties. On one hand, it anchors active particles; the cyano group, being highly polar, can form strong coordination bonds with metal oxides or metal ions (such as cobalt ions) on the surface of the positive electrode active material, stabilizing the surface lattice structure of the cobalt-containing positive electrode active material, suppressing phase transitions and oxygen release under high voltage, enhancing the binder's encapsulation of the positive electrode active particles, and reducing the contact area between particles and the electrolyte. On the other hand, it suppresses interfacial side reactions; the cyano group preferentially adsorbs onto the surface of the positive electrode active material. During battery cycling, the cyano-containing binder interacts with the electrolyte to form a dense protective layer, ensuring the integrity of the positive electrode active material structure, increasing the lithium nitride content in the CEI film of the positive electrode, reducing impedance, and blocking acidic substances such as HF in the electrolyte from affecting the positive electrode. The erosion of active materials; however, binders containing only cyano groups are prone to failure due to the hardness of the cyano segments. During battery cycling, the internal stress generated by repeated lithium insertion and extraction from the cathode material can easily cause the cyano segments to crack and fail, resulting in a decrease in the bonding stability between cathode active material particles. Therefore, it is necessary to combine them with ester functional groups with better flexibility. Ester groups have hydrophobicity and good flexibility. On the one hand, the hydrophobic barrier can form a low surface energy coating layer on the surface of cathode particles, reducing the penetration of side reactions of electrolyte solvent and trace water into the internal interface of cathode active material. On the other hand, the flexible segments of ester groups and the rigid segments of cyano groups form a soft-hard balance, giving the cathode binder high ductility, buffering the volume change of cathode particles during charging and discharging, reducing stress-induced cracking of cathode active material, and thus reducing the risk of cobalt dissolution in the cathode.

[0017] The negative electrode binder contains cyano and ester functional groups: On the one hand, the cyano group can anchor cobalt elements that migrate from the positive electrode active layer to the negative electrode active layer during battery cycling, preventing them from entering the silicon-based material and damaging the SEI film on the surface of the silicon-based negative electrode material, thus increasing the stability of the silicon-based material during battery cycling; on the other hand, the ester group has good adsorption properties for electrolyte, which can enhance the strength and stability of the SEI film on the surface of the silicon-based material, buffer the volume change of the silicon-based material during charging and discharging, prevent the silicon-based material from breaking and the SEI film from rupturing and exposing fresh interfaces, and also enhance the uniformity of electrolyte wetting of the negative electrode active material, reducing the migration resistance of lithium ions on the surface of the negative electrode active material.

[0018] The XRD pattern simultaneously exhibits sharp diffraction peaks (characteristic peak 1, crystalline, cyano group) and tufted peaks (characteristic peak 2, amorphous, ester group), and the intensity ratio of characteristic diffraction peak 1 to characteristic diffraction peak 2 is between 1.3 and 2. This indicates that the internal chain segments of the formed binder are relatively stable and possess both hardness and flexibility. This can address the stress caused by volume changes in the positive and negative electrode active materials during battery cycling, protecting the structural stability and film formation of the positive and negative electrode active materials, as well as protecting the interaction between the positive and negative electrode active materials and the positive and negative electrode current collectors. The adhesive properties enhance battery cycle performance. Furthermore, it indicates that the cyano and ester groups in the binder can form an ordered periodic network. By adjusting the relative content of cyano and ester groups on this periodic network, the density of the network structure can be controlled, thereby regulating the binder's swelling rate, improving adhesive stability and liquid absorption, and ultimately enhancing battery cycle performance. When the binder's order is low, the crystalline and amorphous networks are loose, with weak linkages, failing to form a stable periodic network. This results in only a broad, dome-shaped peak in the XRD pattern, and the swelling rate cannot be effectively adjusted. An excessively large or small intensity ratio between characteristic diffraction peak 1 and characteristic diffraction peak 2 leads to excessive rigidity or flexibility in the binder, deteriorating the structural stability of the positive and negative electrode sheets containing the binder, as well as the interfacial stability between the positive and negative electrode sheets and the separator. This also worsens the binder's stability at high voltages, thus impairing battery cycle performance.

[0019] The sum of the peak area S1 of characteristic diffraction peak 1 and the peak area S2 of characteristic diffraction peak 2 satisfies: 60% ≤ S1 / S 总When the concentration is ≤90%, controlling the area ratio of the characteristic peak of S1 can ensure that the intensity ratio of the two characteristic diffraction peaks meets the above range, while allowing the binder to appropriately expand and swell in the electrolyte. Electrolyte molecules can penetrate the binder polymer network, weakening the forces between chain segments. The polymer chain expansion facilitates the exposure of more cyano groups at the binder polymer / electrolyte interface, forming strong interactions with metal oxides or metal ions on the surface of the positive electrode active material. This inhibits the dissolution of cobalt from the positive electrode and its transfer to the negative electrode. The carbonyl group (C=O) in the ester group can interact with Lewis acid sites (such as Co) on the surface of the cobalt-containing positive electrode active material. 3+ Coordination occurs, forming a stable chemisorption layer, further reducing cobalt leaching. If S1 / S 总 If the ratio is below a certain range, it indicates that the strong polar interaction dominated by the cyano group is weak. This also affects the intensity ratio of characteristic diffraction peak 1 and characteristic diffraction peak 2, preventing the adhesive from forming an ordered periodic network. The strength / density of the physical cross-linking points in the periodic network decreases, the network "pore size" may increase, chain segment mobility increases, solvent penetration becomes excessive, and swelling is excessive. This causes excessive extension of the adhesive polymer chain, intermolecular forces (hydrogen bonds, van der Waals forces) are shielded by the solvent, adhesive strength decreases, and cobalt dissolution / reduction cannot be effectively suppressed, thus failing to maintain the structural stability of the positive and negative electrodes. If S1 / S 总 If the intermolecular forces are greater than a certain range, the periodic network becomes too dense (with more and stronger physical cross-linking points), limiting the penetration of electrolyte solvent molecules and the unfolding of chain segments, resulting in a low swelling rate. When the polymer network maintains its tightly coiled conformation in the dry state, a large number of cyano groups are wrapped inside the molecular chain and cannot fully contact the positive / negative electrode surface. Only a few cyano groups located at the end of the chain segment can participate in adsorption, resulting in a large number of unprotected active sites on the positive / negative electrode, which cannot play a role in protecting the electrode structure.

[0020] By simultaneously limiting the intensity ratio of characteristic diffraction peak 1 and characteristic diffraction peak 2, and if S1 / S 总 Within this range, the balance between the order, rigidity, and flexibility of the periodic network of the binder, as well as the connection and swelling rate between binder molecular chains, are effectively controlled to form an ordered periodic network. The swollen periodic network can form "high-speed ion channels," and the channel pore size can just accommodate Li + Solvation shell, reducing Li + Migration barrier; if the above range is not met, the order of the binder network is poor, the swelling is too large, and side reactions such as active layer detachment are triggered, or the binder swelling is too small, blocking ion transport; while random copolymer binders undergo local debonding under repeated strain, resulting in failure of active material connection.

[0021] This application employs positive and negative electrode binders with orderly copolymerization of cyano and ester groups and moderate swelling ratios. It suppresses cobalt leaching through a triple mechanism of "chemical anchoring-hydrophobic barrier-dynamic complexation". In addition, specific binders are used simultaneously on the positive and negative electrodes to suppress cobalt leaching at the source and retain cobalt in the migration path at the positive electrode, and to suppress the expansion of silicon at the negative electrode, thereby enhancing interface stability and improving battery cycle stability. Attached Figure Description

[0022] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0023] Figure 1 The image shows the XRD pattern of the positive electrode binder prepared in Example 1. Detailed Implementation

[0024] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0025] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0026] Combining cobalt-containing active materials in the positive electrode active layer with silicon-based negative electrode materials can improve the energy density of the battery. However, during cycling, on the one hand, the volume change caused by lithium insertion / extraction in the positive electrode active material generates stress, leading to stress accumulation between grains and causing particle breakage, resulting in the dissolution of cobalt from the positive electrode active material. On the other hand, LiPF6 in the electrolyte readily reacts with trace amounts of water or decomposes at high temperatures, generating strong acidic substances such as HF and PF5. HF attacks the surface of the positive electrode active material, reducing lattice stability and causing cobalt to dissolve as Co. 2+ Lithium cobalt oxide undergoes a transformation from a layered structure to a spinel or rock salt phase, particularly under high temperature and high pressure environments (a battery charging cut-off voltage greater than or equal to 4.5V is considered high pressure). This transformation leads to the release of lattice oxygen and changes in the cobalt valence state (e.g., Co). 3+ →Co 2+ / Co 4+ ), generating low-priced Co2+ More soluble in electrolyte, high-priced Co 4+ It is unstable; high temperatures not only promote HF formation but also accelerate lattice oxygen escape and phase transition processes, further exacerbating the destruction of the cathode active material structure and accelerating cobalt dissolution (the above applies to lithium cobalt oxide cathode active materials); in ternary materials, Li + with Ni 2+ With similar radii, cation mixing is likely to occur, reducing lattice stability and making cobalt more easily exposed and dissolved. After cobalt dissolves, cobalt ions migrate from the positive electrode to the surface of the negative electrode, damaging the structure of the negative electrode active material and the SEI film of the negative electrode. This easily accelerates the capacity decay of silicon-based materials, resulting in poor battery cycle performance.

[0027] To address the above problems, the present invention provides a lithium-ion secondary battery comprising a positive electrode and a negative electrode. The positive electrode sheet includes a positive current collector and a positive active layer disposed on at least one surface of the positive current collector; the positive active layer includes a cobalt positive active material and a positive binder; The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer disposed on at least one side surface of the negative electrode current collector; the negative electrode active layer includes a negative electrode active material and a negative electrode binder, and the negative electrode active material includes a silicon-based material; Both the positive electrode binder and the negative electrode binder contain cyano and ester functional groups; Both the positive electrode binder and the negative electrode binder satisfy the following conditions: In the XRD patterns of the positive electrode binder and the negative electrode binder, there is a characteristic diffraction peak 1 at 2θ = 16~18° and a characteristic diffraction peak 2 at 2θ = 22~30°. The intensity ratio of characteristic diffraction peak 1 to characteristic diffraction peak 2 is 1.3~2, preferably 1.4~1.9. S1 is the peak area of ​​characteristic diffraction peak 1, and S2 is the peak area of ​​characteristic diffraction peak 2. 总 =S1+S2, 60%≤S1 / S 总 ≤90%.

[0028] The positive electrode binder contains cyano and ester functional groups. The cyano group exhibits strong adsorption and passivation properties. On one hand, it anchors active particles; the cyano group, being highly polar, can form strong coordination bonds with metal oxides or metal ions (such as cobalt ions) on the surface of the positive electrode active material, stabilizing the surface lattice structure of the cobalt-containing positive electrode active material, suppressing phase transitions and oxygen release under high voltage, enhancing the binder's encapsulation of the positive electrode active particles, and reducing the contact area between particles and the electrolyte. On the other hand, it suppresses interfacial side reactions; the cyano group preferentially adsorbs onto the surface of the positive electrode active material. During battery cycling, the cyano-containing binder interacts with the electrolyte to form a dense protective layer, ensuring the integrity of the positive electrode active material structure, increasing the lithium nitride content in the CEI film of the positive electrode, reducing impedance, and blocking acidic substances such as HF in the electrolyte from affecting the positive electrode. The erosion of active materials; however, binders containing only cyano groups are prone to failure due to the hardness of the cyano segments. During battery cycling, the internal stress generated by repeated lithium insertion and extraction from the cathode material can easily cause the cyano segments to crack and fail, resulting in a decrease in the bonding stability between cathode active material particles. Therefore, it is necessary to combine them with ester functional groups with better flexibility. Ester groups have hydrophobicity and good flexibility. On the one hand, the hydrophobic barrier can form a low surface energy coating layer on the surface of cathode particles, reducing the penetration of side reactions of electrolyte solvent and trace water into the internal interface of cathode active material. On the other hand, the flexible segments of ester groups and the rigid segments of cyano groups form a soft-hard balance, giving the cathode binder high ductility, buffering the volume change of cathode particles during charging and discharging, reducing stress-induced cracking of cathode active material, and thus reducing the risk of cobalt dissolution in the cathode.

[0029] The negative electrode binder contains cyano and ester functional groups: On the one hand, the cyano group can anchor cobalt elements that migrate from the positive electrode active layer to the negative electrode active layer during battery cycling, preventing them from entering the silicon-based material and damaging the SEI film on the surface of the silicon-based negative electrode material, thus increasing the stability of the silicon-based material during battery cycling; on the other hand, the ester group has good adsorption properties for electrolyte, which can enhance the strength and stability of the SEI film on the surface of the silicon-based material, buffer the volume change of the silicon-based material during charging and discharging, prevent the silicon-based material from breaking and the SEI film from rupturing and exposing fresh interfaces, and also enhance the uniformity of electrolyte wetting of the negative electrode active material, reducing the migration resistance of lithium ions on the surface of the negative electrode active material.

[0030] The XRD pattern simultaneously exhibits sharp diffraction peaks (characteristic peak 1, crystalline, cyano group) and tufted peaks (characteristic peak 2, amorphous, ester group), and the intensity ratio of characteristic diffraction peak 1 to characteristic diffraction peak 2 is between 1.3 and 2. This indicates that the internal chain segments of the formed binder are relatively stable and possess both hardness and flexibility. This can address the stress caused by volume changes in the positive and negative electrode active materials during battery cycling, protecting the structural stability and film formation of the positive and negative electrode active materials, as well as protecting the interaction between the positive and negative electrode active materials and the positive and negative electrode current collectors. The adhesive properties enhance battery cycle performance. Furthermore, it indicates that the cyano and ester groups in the binder can form an ordered periodic network. By adjusting the relative content of cyano and ester groups on this periodic network, the density of the network structure can be controlled, thereby regulating the binder's swelling rate, improving adhesive stability and liquid absorption, and ultimately enhancing battery cycle performance. When the binder's order is low, the crystalline and amorphous networks are loose, with weak linkages, failing to form a stable periodic network. This results in only a broad, dome-shaped peak in the XRD pattern, and the swelling rate cannot be effectively adjusted. An excessively large or small intensity ratio between characteristic diffraction peak 1 and characteristic diffraction peak 2 leads to excessive rigidity or flexibility in the binder, deteriorating the structural stability of the positive and negative electrode sheets containing the binder, as well as the interfacial stability between the positive and negative electrode sheets and the separator. This also worsens the binder's stability at high voltages, thus impairing battery cycle performance.

[0031] The sum of the peak area S1 of characteristic diffraction peak 1 and the peak area S2 of characteristic diffraction peak 2 satisfies: 60% ≤ S1 / S 总 When the concentration is ≤90%, controlling the area ratio of the characteristic peak of S1 allows the intensity ratio of the two characteristic diffraction peaks to meet the above range while enabling the binder segments to appropriately extend in the electrolyte. This promotes the penetration of electrolyte molecules into the binder polymer network, weakens the interaction forces between segments, and allows more cyano groups to be exposed at the binder polymer / electrolyte interface. These cyano groups then form strong interactions with metal oxides or metal ions on the surface of the positive electrode active material, inhibiting the dissolution of cobalt from the positive electrode and its transfer to the negative electrode. The carbonyl group (C=O) in the ester group can interact with Lewis acid sites (such as Co) on the surface of the cobalt-containing positive electrode active material. 3+ Coordination occurs, forming a stable chemisorption layer, further reducing cobalt leaching. If S1 / S 总If the ratio is below a certain range, it indicates that the strong polar interaction dominated by the cyano group is weak. This also affects the intensity ratio of characteristic diffraction peak 1 and characteristic diffraction peak 2, preventing the adhesive from forming an ordered periodic network. The strength / density of the physical cross-linking points in the periodic network decreases, the network "pore size" may increase, chain segment mobility increases, solvent penetration becomes excessive, and swelling is excessive. This causes excessive extension of the adhesive polymer chain, intermolecular forces (hydrogen bonds, van der Waals forces) are shielded by the solvent, adhesive strength decreases, and cobalt dissolution / reduction cannot be effectively suppressed, thus failing to maintain the structural stability of the positive and negative electrodes. If S1 / S 总 If the intermolecular forces are greater than a certain range, the periodic network becomes too dense (with more and stronger physical cross-linking points), limiting the penetration of electrolyte solvent molecules and the unfolding of chain segments, resulting in a low swelling rate. When the polymer network maintains its tightly coiled conformation in the dry state, a large number of cyano groups are wrapped inside the molecular chain and cannot fully contact the positive / negative electrode surface. Only a few cyano groups located at the end of the chain segment can participate in adsorption, resulting in a large number of unprotected active sites on the positive / negative electrode, which cannot play a role in protecting the electrode structure.

[0032] By simultaneously limiting the intensity ratio of characteristic diffraction peak 1 and characteristic diffraction peak 2, and if S1 / S 总 Within a certain range, the balance between the order, rigidity, and flexibility of the periodic network of the binder, as well as the swelling rate, can be effectively controlled to form an ordered periodic network. The swollen periodic network can form "high-speed ion channels," and the channel pore size can just accommodate Li + Solvation shell, reducing Li + Migration barrier; if the above range is not met, the order of the binder network is poor, the swelling is too large, and side reactions such as active layer detachment are triggered, or the binder swelling is too small, blocking ion transport; while random copolymer binders undergo local debonding under repeated strain, resulting in failure of active material connection.

[0033] This application employs positive and negative electrode binders with orderly copolymerization of cyano and ester groups and moderate swelling ratios. It suppresses cobalt leaching through a triple mechanism of "chemical anchoring-hydrophobic barrier-dynamic complexation". In addition, specific binders are used simultaneously on the positive and negative electrodes to suppress cobalt leaching at the source and retain cobalt in the migration path at the positive electrode, and to suppress the expansion of silicon at the negative electrode, thereby enhancing interface stability and improving battery cycle stability.

[0034] For example, the intensity ratio of characteristic diffraction peak 1 to characteristic diffraction peak 2 can be any value among 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, and 2. S1 / S 总 It can be any value among 60%, 35%, 70%, 75%, 80%, 85%, and 90%; The swelling rates of the positive electrode binder and the negative electrode binder are each independently 50% to 200%, preferably 80% to 150%; In this application, unless otherwise specified, the numerical range includes the endpoints. For example, a swelling ratio of 50% to 200% includes both 50% and 200%.

[0035] The swelling ratios of the positive and negative electrode binders are independently any value among 50%, 60%, 70%, 80%, 90%, 100%, 110%, 120%, 130%, 140%, 150%, 160%, 170%, 180%, 190%, and 200%. If the swelling ratio of the positive and negative electrode binders is too large (exceeding 200%), the bonding strength between the positive and negative electrode active material particles will be weakened, which is not conducive to suppressing the shedding and detachment of the positive and negative electrode active material particles, and is not conducive to improving the battery cycle performance. If the swelling ratio of the positive and negative electrode binders is too small (less than 50%), it is not conducive to their proper adsorption of electrolyte, which is also not conducive to improving the battery cycle performance.

[0036] In one possible implementation, the positive and negative electrode binders of this application are fluorine-free binders, which avoids the fluorine release reaction when traditional PVDF is used as a binder (polyvinylidene fluoride), reduces the formation of an acidic environment in the battery from the source, and inhibits the leaching of Co in the positive electrode and the damage to the positive and negative electrode materials caused by the HF generated by the fluorine in the fluorine-containing binder.

[0037] In one possible implementation, the mass content of the positive electrode binder is 0.1% to 3% based on the mass of the positive electrode active layer. Within this range, the positive electrode binder can uniformly coat the positive electrode active material particles, effectively inhibiting cobalt leaching. If the amount of positive electrode binder added is too small, the binder's adhesion will be insufficient, which may lead to the detachment of the positive electrode active material particles, poor contact between particles, and breakage of the electron conduction network. If the positive electrode binder does not completely cover the positive electrode active material particles, the electrolyte will directly corrode the defective areas, exacerbating cobalt leaching. If the amount of positive electrode binder added is too large, the positive electrode binder will cover too much of the surface of the positive electrode active material particles, hindering lithium-ion conduction. The proportion of positive electrode active material will decrease, resulting in a loss of energy density. Excessive positive electrode binder will adsorb more electrolyte, increasing the risk of increased interfacial side reactions. For example, based on the mass of the positive electrode active layer, the mass content of the positive electrode binder can be any value among 0.1%, 0.2%, 0.5%, 0.8%, 1.0%, 1.3%, 1.5%, 1.8%, 2.0%, 2.3%, 2.5%, 2.7%, and 3.0%.

[0038] In one possible implementation, the mass content of the negative electrode binder is 2% to 5% based on the mass of the negative electrode active layer. Within this range, the negative electrode binder and the conductive agent form an elastic conductive network coating the negative electrode active material particles. If the amount added is too small, the negative electrode adhesion is insufficient, the negative electrode active material pulverizes, and the conductive network / electrode structure collapses. During battery cycling, the negative electrode active material, being unconstrained, is prone to breakage, exposing new surfaces that continuously react with the electrolyte, repeatedly forming an SEI film and consuming lithium ions. If the amount added is too large, the negative electrode binder will cover too much of the surface of the negative electrode active material particles, hindering lithium ion conduction. It will also reduce the proportion of negative electrode active material, resulting in a loss of energy density. Furthermore, excessive adhesion increases the brittleness of the negative electrode sheet, making it unable to adapt to volume changes during charging and discharging, and prone to breakage. For example, based on the mass of the negative electrode active layer, the mass content of the negative electrode binder can be any value among 2.0%, 2.3%, 2.5%, 2.7%, 3.0%, 3.5%, 4.0%, 4.5%, and 5.0%.

[0039] In one possible implementation, the negative electrode active layer includes a first negative electrode active layer and a second negative electrode active layer; the second negative electrode active layer is disposed on the side of the first negative electrode active layer away from the negative electrode current collector; Based on the total mass of the negative electrode active layer, the mass content of silicon element is 1.5% to 40%, including the endpoint value; The mass content of silicon in the first negative electrode active layer within a region of 0-10 μm from the surface of the negative electrode current collector is lower than the mass content of silicon in the second negative electrode active layer within a region of 0-10 μm from the surface of the second negative electrode active layer. In one possible implementation, the silicon element content is 0% to 5% based on the total mass of the first negative electrode active layer, including the endpoint value. In one possible implementation, the silicon content is 1.5% to 40% by mass, including endpoint values, based on the total mass of the second negative electrode active layer. The negative electrode active layer adopts a double-layer structure to disperse and transfer stress, improving the structural stability and conductivity of the negative electrode sheet during cycling. The high silicon content of the second negative electrode active layer is conducive to absorbing more lithium ions and preventing lithium ion accumulation on the negative electrode surface, thus preventing lithium plating. The high silicon content of the second negative electrode active layer means that even with large volume changes, it will not have a significant impact on the material structure of the bottom first negative electrode active layer, ensuring stable contact between the bottom active layer and the current collector, thereby ensuring the overall structural stability of the negative electrode sheet. The lower silicon content of the first negative electrode active layer can optimize the conductive network and reduce the interface resistance, enhance the interfacial bonding strength between the active layer and the current collector, and provide a rigid support foundation for the second negative electrode active layer.

[0040] In one possible implementation, the silicon-based material includes one or more of silicon-carbon composite materials and silicon-oxygen materials; In one possible implementation, the negative electrode active material further includes graphite.

[0041] In one possible implementation, the thickness of the first negative electrode active layer is 10 μm to 30 μm, including the endpoint values; In one possible implementation, the thickness of the second negative electrode active layer is 10 μm to 50 μm, including the endpoint values; In one possible implementation, the silicon-based material includes one or more of spherical silicon-based materials and bulk silicon-based materials; In one possible implementation, the first negative electrode active layer and the second negative electrode active layer independently comprise one or more of spherical silicon-based materials and bulk silicon-based materials.

[0042] In one possible implementation, the surface of the second negative electrode active layer is provided with a plurality of recesses; the distance between two adjacent recesses is 0.5 mm to 2 mm, including the endpoint value; the width of a single recess is 50 μm to 200 μm, including the endpoint value; and the depth of a single recess is 8 μm to 40 μm, including the endpoint value. In one possible implementation, the recess is obtained by laser grooving or mechanical grooving on the surface of the second negative electrode active layer; the recess can improve the dynamic performance of the negative electrode and alleviate negative electrode expansion, increase its lithium intercalation speed, and improve negative electrode lithium plating.

[0043] In one possible implementation, the positive electrode binder and the negative electrode binder are polymers formed by copolymerization of a first monomer, a second monomer, and a third monomer, respectively. The first monomer includes at least one of acrylonitrile, methacrylonitrile, and fumaronitrile; The second monomer includes at least one of acrylate, methacrylate and vinyl acetate; The third monomer includes at least one of a monomer containing an acidic group, a monomer containing a basic group, and a halogen-containing monomer. In one possible implementation, the mass content of the first monomer is 15-85 wt%, based on the total mass of all monomers. In one possible implementation, the second monomer content is 15-85 wt% based on the total mass of all monomers. In one possible implementation, the mass content of the third monomer is 1-10 wt% based on the total mass of all monomers. In one possible implementation, the acrylate includes at least one of methyl acrylate, ethyl acrylate, butyl acrylate, and 2-ethylhexyl acrylate; In one possible implementation, methacrylates include at least one of methyl 2-methacrylate and ethyl 2-methacrylate; In one possible implementation, the degree of ordered alternation between the first monomer and the second monomer is ≥90%.

[0044] The degree of orderly alternation between the first and second monomers refers to the percentage of the total molar amount of the first and second monomer units that are orderly and alternately distributed along the polymer backbone in the polymer relative to the total molar amount of all monomer units.

[0045] For example, if the first monomer is A, the second monomer is B, and the third monomer is C, the monomer arrangement sequence in the entire binder molecule is AB-AB-AB-AB-AB-BB-AA-AAB-AAB-C-........., among which the units arranged in the AB form are the most numerous. The degree of orderly alternation between the first and second monomers refers to the percentage of the sum of the molar amounts of A and B arranged in the AB form to the total molar amounts of A, B, and C.

[0046] For example: AAB-AAB-AAB-AB-AB-AAB-AAB-BB-AA-AAB-AAB-C-........, where the units arranged in the AAB form are the most numerous. The degree of orderly alternation between the first and second monomers refers to the percentage of the sum of the moles of A and B arranged in the AAB form to the total moles of A, B, and C.

[0047] The arrangement of the first and second monomers includes, but is not limited to, AB, AAB, AAAB, AABA, etc. The degree of order in the arrangement of the first and second monomers is mainly determined by the binder preparation process, such as adjustments to the reaction temperature, reaction time, monomer types, and the amount of each monomer used.

[0048] Quantitative testing of the degree of alternation of order by nuclear magnetic resonance spectroscopy (NMR): (1) Sample preparation and data acquisition: Dissolve approximately 20 mg of the test sample in 0.5 mL of a suitable deuterated solvent to ensure complete dissolution of the sample and obtain a clear and transparent solution; use 13 C NMR spectrum; (2) Spectral analysis and sequence assignment: Analyze the nitrile carbon (-C≡N) of the first monomer unit and the carbonyl carbon (-C=O) of the second monomer unit, assign the peaks and perform integral calculations to obtain the degree of ordered alternation.

[0049] In one possible implementation, at 0% SOC, the cobalt content in the negative electrode active layer is less than 10,000 ppm. Both the positive and negative electrodes employ functional groups containing cyano and ester groups, which suppresses cobalt dissolution from the positive electrode active material, enhances the structural stability of the positive electrode material, and reduces irreversible capacity loss. The stability of the negative electrode active material is improved, reducing damage during cycling. The stability of the SEI film of the negative electrode is enhanced, mitigating the phenomena of excessive SEI film thickness and Co migration and deposition damaging the SEI film. Cobalt dissolution is reduced, and cobalt dissolution-related side reactions / electrolyte consumption are reduced, decreasing the possibility of local short circuits / overheating. Low-temperature performance is optimized to prevent dissolved Co from blocking lithium-ion transport paths, improving the battery's cycle performance and safety performance.

[0050] In one possible implementation, at 50% SOC, the impedance R1 of the positive electrode is less than the impedance R2 of the negative electrode, the impedance R1 of the positive electrode is in the range of 15~30mΩ, the impedance R2 of the negative electrode is in the range of 17~45mΩ, and R2-R1=2~20mΩ.

[0051] ① The highly polar cyano group in the specific binder of this application may facilitate interaction with the electrolyte, promoting local lithium-ion transport. Moreover, the moderate swelling capacity of the specific binder helps to adsorb a certain amount of electrolyte, further accelerating the lithium insertion / extraction process and reducing transport impedance. The ionic conduction assisting effect of the cyano group and the bonding / flexibility enhanced by the ester group together construct a better ion / electron hybrid conductive network, reducing ohmic polarization. Furthermore, the highly polar cyano group in the specific binder can cause weak solvation of lithium ions. When it acts on the positive electrode, it can increase the lithium nitride content in the CEI film on the positive electrode surface. The CEI film containing lithium nitride has high ionic conductivity and stable mechanical strength, which can reduce ion transfer impedance, thereby reducing the impedance of the positive electrode sheet. The cyano group in the negative electrode binder reduces the damage to the negative electrode surface caused by cobalt extraction from the positive electrode. In addition, the moderate hardness of the specific binder can maintain the structural integrity of the negative electrode active material and the integrity of the conductive structure of the negative electrode sheet, thereby ensuring that the impedance of the negative electrode sheet is within a suitable range.

[0052] In the same system environment, the use of the same binder for the positive and negative electrodes of the battery means that the chemical environment (functional group interaction) experienced by lithium ions at the interface between the active material and the binder is similar. This reduces the mismatch in lithium insertion / extraction rates caused by differences in interface chemistry. More consistent electrode states, better structural stability (positive and negative electrodes), and a better conductive network (positive and negative electrodes) mean that the polarization growth of the positive and negative electrodes tends to be more consistent under long-term cycling or high-rate conditions. This helps to maintain the durability of rate matching and is beneficial to reducing the impedance of the positive and negative electrodes.

[0053] In one possible implementation, the electrolyte includes nitrile additives; The cyano groups in the binder and the nitrile groups (RC≡N) in the nitrile additives in the electrolyte are both strongly polar groups, and there is a strong dipole-dipole attraction between them, which can form a dynamic physical bonding network; the long-chain structure of the nitrile additives can be intercalated between the polymer chains of the binder, achieving physical entanglement through van der Waals forces; Li + Coordination bridging allows both cyano and nitrile groups to act as Lewis bases, coordinating with Li+ ions in the electrolyte. This coordination creates a dynamic cross-linked network at the cathode interface, linking the binder and nitrile additives. Free protons (H+) in the electrolyte (e.g., H+ from hydrofluoric acid generated in a side reaction) readily react with the lone pairs of electrons in the -CN group of the nitrile additives, effectively removing moisture from the electrolyte, reducing cobalt dissolution, and maintaining the stability of the cathode structure. Improved cathode interface stability leads to preferential oxidative decomposition of the nitrile additives on the cathode surface, synergistically constructing a nitrogen-rich / cyano-rich CEI film with the cyano binder. This CEI film exhibits high ionic conductivity and excellent mechanical toughness. The dynamic network combining cyano and nitrile groups can reduce Li+ ions... + Desolvation energy barrier, increasing Li + The transmission speed; if the amount of nitrile additives added is too small, the aforementioned effect cannot be achieved. However, the addition of nitrile additives will have a certain corrosive effect on the silicon-based anode, so too much cannot be added. Therefore, its content must be controlled within the above range. In one possible implementation, the nitrile additive includes one or more of adiponitrile, succinate, acetonitrile, 3-methoxypropionitrile, and 1,3,6-hexanetrionitrile; In one possible implementation, the content of the nitrile additive is 0.1w% to 8wt% based on the total mass of the electrolyte, and optionally 0.5w% to 5wt%.

[0054] In one possible implementation, the thickness of the negative electrode at 50% SOC is in the range of 95μm to 140μm. In one possible implementation, the thickness of the negative electrode at 100% SOC ranges from 100 μm to 170 μm. It should be noted that in some possible implementations, after the battery is formed and packaged, the thickness of the negative electrode sheet ranges from 65μm to 125μm.

[0055] The thickness of the negative electrode sheet refers to the total thickness of the negative electrode current collector and the negative electrode active layer disposed on both sides of the negative electrode current collector.

[0056] In one possible implementation, the cell thickness change rate is 0.5%~2% under ambient temperature cycling at 200T and 100% SOC. In one possible implementation, under ambient temperature cycling at 600T, the battery deformation rate is 2%~6%. In one possible implementation, the thickness of the negative electrode sheet is 100μm to 170μm under ambient temperature cycling, 200T, and 100% SOC. In one possible implementation, the thickness of the negative electrode sheet is 110μm to 180μm under ambient temperature cycling, 600T, and 100% SOC. In one possible implementation, the thickness of the prepared negative electrode sheet is 98±10 μm at 0 SOC% before charging and discharging.

[0057] The rigidity of the cyano group provides high modulus, while the flexibility of the ester group allows for appropriate absorption and swelling in the electrolyte. This can buffer the volume strain of the positive and negative electrode materials. The binder with a specific ordered periodic structure forms an interpenetrating network after swelling, maintaining good adhesion while suppressing the relative compression of particle volume changes during charging and discharging. In contrast, the random copolymer binder undergoes local debonding under repeated strain, leading to more severe particle agglomeration and expansion, thereby reducing the degree of electrode thickness change and reducing the overall deformation of the battery cell.

[0058] Example 1 This embodiment provides a lithium-ion secondary battery, the preparation method of which includes the following steps: (1) Positive electrode plate: Lithium cobalt oxide (LCO), single-walled carbon nanotubes (SWCNTs), and a positive electrode binder were mixed uniformly at a mass ratio of 97.2:1.8:1. The mixture was then thoroughly stirred in an appropriate amount of N-methylpyrrolidone solvent to obtain a positive electrode active slurry with a solid content of 70%. This active slurry was coated onto both sides of a positive electrode current collector (aluminum foil, 9 μm). After drying and cold pressing, a positive electrode active layer with a thickness of 70 μm (the sum of the thicknesses of the two layers) was obtained. The overall thickness of the positive electrode sheet was 79 μm, and the areal density of one side was 0.01570 g / cm³. 2 The positive electrode plate.

[0059] The positive electrode binder is a copolymer of a first monomer, a second monomer, and a third monomer. Based on the total mass of the monomers, the first monomer is acrylonitrile with a content of 55 wt%, the second monomer is methyl acrylate with a content of 40 wt%, and the third monomer is acrylic acid with a content of 5 wt%.

[0060] The preparation method of the positive electrode binder includes: ① Add 110g acrylonitrile monomer, 80g methyl acrylate monomer, 10g acrylic acid monomer and 200g deionized water to the reactor, purge with nitrogen and stir at room temperature for 1 hour (stirring speed: 500 rpm). ② Heat to 60℃, add 1g initiator (ammonium sulfate), 1g emulsifier (sodium dodecyl sulfate), and 2g buffer (sodium bicarbonate), and polymerize for 24 hours (stirring speed: 300 rpm). ③ After the reaction is complete, the positive electrode binder is obtained by centrifugation, filtration, washing, and drying.

[0061] The XRD pattern of the positive electrode binder in this embodiment is shown below. Figure 1 The intensity ratio of characteristic diffraction peak 1 to characteristic diffraction peak 2 is 1.8; S1 / S 总 The swelling rate is 80% and the swelling ratio is 90%. In other embodiments, the intensity ratio and swelling ratio of the characteristic diffraction peak 1 and characteristic diffraction peak 2 of the positive electrode binder are adjusted by adjusting the type of monomer and the amount of each monomer.

[0062] (2) Negative electrode plate Spherical silicon-carbon composite material (with a silicon content of 47% by mass), graphite, single-walled carbon nanotubes, and negative electrode binder were mixed uniformly in a mass ratio of 14:81.5:1.5:3. Deionized water solvent was added, and the mixture was thoroughly stirred according to a known batching process. The mixture was then passed through a 150-mesh sieve to prepare a negative electrode slurry with a solid content of 40% by mass. This slurry was uniformly coated onto a copper foil current collector and dried to obtain the negative electrode active layer. The thickness of the negative electrode active layer on one side was 45 μm (the total thickness of the negative electrode active layer on both sides of the current collector was 90 μm), and the areal density of the surface on one side was 0.006 g / cm³. -2 The copper foil thickness is 8 μm, and the negative electrode thickness is 98 μm. The silicon content in the negative electrode active layer is 6.6% by mass. The silicon content in the negative electrode active layer can be changed by altering the proportion of silicon-carbon composite material or the silicon content of the silicon-carbon composite material particles; the specific implementation method is not limited here.

[0063] The negative electrode binder is a copolymer of a first monomer, a second monomer, and a third monomer. Based on the total mass of the monomers, the first monomer is acrylonitrile with a content of 55 wt%, the second monomer is methyl acrylate with a content of 40 wt%, and the third monomer is acrylic acid with a content of 5 wt%. The intensity ratio range of characteristic diffraction peak 1 to characteristic diffraction peak 2 is 1.8. The preparation method of the negative electrode binder is the same as that of the positive electrode binder.

[0064] Laser wire bonding is performed on the surface of the negative electrode active layer, with a weight loss rate of 1%, a wire spacing of 1.4 mm, a wire width of 70 μm, and a wire depth of 20 μm. (3) Preparation of lithium-ion batteries: Lithium-ion battery fabrication: The positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes for isolation. This is then wound to obtain a bare cell. The bare cell is placed in outer packaging, vacuum dried, injected with electrolyte, and sealed. After formation and sorting processes, a lithium-ion battery is obtained.

[0065] A 7μm porous polyethylene membrane is used as the separator. The electrolyte includes a solvent, LiPF6, and additives adiponitrile and fluoroethylene carbonate (FEC). The solvent is a mixture of ethylene carbonate (EC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC) in a weight ratio of 2:3:5. The concentration of LiPF6 in the electrolyte is approximately 1 mol / L. Based on the total weight of the electrolyte, adiponitrile accounts for 5 wt% of the electrolyte, and fluoroethylene carbonate accounts for 10 wt% of the electrolyte.

[0066] The parameters for Examples 1-27 and Comparative Examples 1-4 are shown in Tables 1-4.

[0067] Table 1

[0068] Table 2

[0069] Table 3

[0070] Table 4

[0071] Example 24 is basically the same as Example 1, except that the negative electrode active layer includes a first negative electrode active layer and a second negative electrode active layer.

[0072] Methods for preparing the negative electrode active layer include: First negative electrode active layer: Graphite, single-walled carbon nanotubes, graphene, and negative electrode binder are mixed evenly in a mass ratio of 95:2:1:2. Deionized water solvent is added, and the mixture is thoroughly stirred according to a known batching process. The mixture is then passed through a 150-mesh sieve to prepare a negative electrode slurry with a solid content of 40%. This slurry is uniformly coated on both sides of a copper foil and dried. The thickness T1 of the first negative electrode active layer is 15 μm (30 μm on both sides), and the areal density is 0.0023 g / cm³. -2 ; Second negative electrode active layer: Spherical silicon-carbon composite material (with a silicon content of 47% by mass), graphite, single-walled carbon nanotubes, and negative electrode binder are mixed uniformly in a mass ratio of 14:81.5:1.5:3. Deionized water solvent is added, and the mixture is thoroughly stirred according to a known batching process. The mixture is then passed through a 150-mesh sieve to prepare a negative electrode slurry with a solid content of 40% by mass. This slurry is uniformly coated onto the first negative electrode active layer and dried to obtain the second negative electrode active layer. The second negative electrode active layer has a silicon content of 6.6% by mass, a thickness T1 of 30 μm (60 μm on both sides), and an areal density of 0.0045 g / cm³. -2 .

[0073] Example 25 is basically the same as Example 1, except that the negative electrode active layer includes a first negative electrode active layer and a second negative electrode active layer. The preparation method includes: First negative electrode active layer: Bulk silicon-carbon composite material (with a silicon content of 46% by mass), graphite, single-walled carbon nanotubes, graphene, and negative electrode binder are mixed uniformly in a mass ratio of 7:88:2:1:2. Deionized water solvent is added, and the mixture is thoroughly stirred according to a known batching process. The mixture is then passed through a 150-mesh sieve to prepare a negative electrode slurry with a solid content of 40% by mass. This slurry is then uniformly coated onto both sides of a copper foil and dried. The thickness T1 of the first negative electrode active layer is 15 μm (30 μm on both sides), and the areal density is 0.0025 g / cm³. -2 The mass content of silicon in the first negative electrode active layer is 3.22%.

[0074] Second negative electrode active layer: Spherical silicon-carbon composite material (with a silicon content of 47% by mass), graphite, single-walled carbon nanotubes, and negative electrode binder are mixed uniformly in a mass ratio of 14:81.5:1.5:3. Deionized water solvent is added, and the mixture is thoroughly stirred according to a known batching process. The mixture is then passed through a 150-mesh sieve to prepare a negative electrode slurry with a solid content of 40% by mass. This slurry is then uniformly coated onto the first active layer and dried. The thickness T1 of the second negative electrode active layer is 30 μm (60 μm on both sides), and the areal density is 0.0051 g / cm³. -2 The silicon content in the second negative electrode active layer is 6.6% by mass.

[0075] Test case (1) The intensity ratio of characteristic diffraction peak 1 to characteristic diffraction peak 2 and S1 / S 总 : X-ray diffraction (XRD) was used to test the positive or negative electrode binder using a Shimadzu XRD-6100 X-ray diffractometer. The sample amount used for testing was 0.5 g / cm³. 2The Kα line of Cu was used as the incident X-ray. The working voltage of the X-ray source was 40 kV, the test power was 2 kW, 2θ was used as the abscissa and the unit was °, the signal intensity was used as the ordinate, the test range was 10~80°, the scanning rate was 4° / min, and the data point interval was 0.02°.

[0076] (2) The cobalt content in the negative electrode active layer under 0% SOC condition: At 25℃±5℃, the battery was discharged at 0.2C to the lower limit voltage of 3V, and then discharged to 0% SOC. After standing for 10 min, the battery was disassembled and ICP testing was performed using inductively coupled plasma optical emission spectrometry (ICP-OES) with a PerkinElmer Optima 8300 spectrometer. After disassembling the battery, approximately 2g of active layer was scraped from the electrode with a spoon as the sample to be tested. The sample was pretreated by microwave digestion (HNO3:H2O2=3:1, 180℃ / 30min) and the volume was adjusted to 50mL (2% HNO3 medium). Test conditions: plasma power 1.4 kW, nebulizer gas flow rate 0.8 L / min, auxiliary gas flow rate 0.2 L / min, cooling gas flow rate 15 L / min. Axial observation mode was used, and the sample lift rate was 1.5 mL / min. The calibration curve was configured with 5 gradients (0.1~10 mg / L), and Y internal standard solution (1 mg / L) was added to compensate for matrix effects. Each sample was measured three times, with RSD controlled to be <2%, and background interference was subtracted by blank correction.

[0077] (3) At 50% SOC, the impedances of the positive electrode R1 and the negative electrode R2 are: Three-electrode fabrication: During the winding process of the positive electrode, negative electrode, and separator, copper wire with a diameter of 0.1~0.2mm is selected and lithiated to form a Li-Cu alloy reference electrode (potential 0.6V vs. Li⁺ / Li). Copper wire is inserted during winding: except for the tip 1~2mm, the copper wire is covered with an insulating layer and placed parallel between the positive electrode and separator or the negative electrode and separator. Insulating tape is applied to both sides to prevent short circuits. The copper wire is led out from the aluminum-plastic film sealing area. Connection method, positive electrode impedance: positive electrode → working electrode (WE), negative electrode → counter electrode (CE), copper wire → reference electrode (RE); negative electrode impedance: negative electrode → WE, positive electrode → CE, copper wire → RE; 1. Discharge to the lower limit voltage at 0.2C under 25℃±5℃ environment; let stand for 10min; 2. Adjust SOC: charge to 50% SOC at 0.7C constant current; 3. Let stand for 2h at 25℃±5℃; perform EIS test: parameter settings: frequency 100 kHz~10 mHz, amplitude 10 mV, temperature 25±0.5℃, let stand before test until voltage fluctuation <0.1 mV / min; 4. Fit the EIS test data of positive electrode and negative electrode to obtain the positive electrode impedance R1 and negative electrode impedance R2.

[0078] (4) Thickness of negative electrode at 50% SOC: At 25°C, the battery was discharged to 3V at a constant current of 0.2C and left to stand for 10 minutes; charged to 4.5V at a constant current and constant voltage of 0.5C and left to stand for 10 minutes; discharged to 50% SOC at a constant current of 0.5C; the battery that has reached the target SOC was disassembled and the thickness of the negative electrode was measured using a micrometer.

[0079] (5) Thickness of negative electrode at 100% SOC: The battery was discharged to 3V at 0.2C constant current at 25℃ and left to stand for 10 minutes; it was charged to 4.5V at 0.5C constant current and constant voltage, with a cutoff current of 0.05C and left to stand for 10 minutes; the battery was disassembled under full charge and the thickness of the negative electrode was measured using a micrometer.

[0080] (6) Cell thickness change rate at room temperature cycling, 200T or 600T, 100% SOC: First, measure the T of the battery cell. 极耳位厚度 Then, at 25°C, the battery is discharged at a constant current of 0.2C to 3V and left to stand for 10 minutes; it is then charged at a constant current and constant voltage of 0.5C to 4.5V, with a cutoff current of 0.05C, and left to stand for 10 minutes; the above charging and discharging steps are repeated for 200 or 600 cycles, and the thickness T of the cell in a fully charged state is measured. 电芯本体厚度 .

[0081] Cell thickness change rate = (T 电芯本体厚度 -T 极耳位厚度 ) / T 电芯本体厚度 ; T 电芯本体厚度 This refers to the maximum thickness of the entire battery cell after cycling, measured using an automatic constant voltage thickness gauge. T 极耳位厚度 This refers to the thickness of the entire cell at the tab position before cycling, measured using a digital micrometer.

[0082] (7) Swelling rate: Preparation of adhesive film: Dissolve the adhesive in NMP to prepare a 5%wt homogeneous solution; pour the solution into a polytetrafluoroethylene mold and dry it thoroughly in an oven at 60°C; cut the dried adhesive film into uniform small pieces (3cm x 3cm) and weigh them as W0; In the immersion experiment, the adhesive film sample was placed in a container containing an electrolyte (including solvent, LiPF6, and additives adiponitrile and fluoroethylene carbonate (FEC); the solvent was a mixture of ethylene carbonate (EC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC) in a weight ratio of 2:3:5; the concentration of LiPF6 in the electrolyte was approximately 1 mol / L; the mass content of adiponitrile in the electrolyte was 5 wt% and the mass content of fluoroethylene carbonate in the electrolyte was 10 wt%), completely immersed, sealed, and placed in a constant temperature incubator. To calculate the swelling rate, after soaking for 7 days, remove the adhesive film from the container, remove excess electrolyte adhering to the film surface with filter paper, and weigh the adhesive film as W1; Swelling rate = (W1-W0) / W0.

[0083] (8) Mass content of silicon in the negative electrode active layer: The mass percentage of silicon in the total mass of the negative electrode active layer can be determined using conventional methods in the art. For example, after discharging the battery to 0% SOC, the negative electrode sheet is disassembled, soaked in dimethyl carbonate (DMC) solvent for 12 hours, and then rinsed with DMC solvent to remove lithium salts adhering to the negative electrode sheet. After drying, the negative electrode sheet is subjected to high-temperature treatment at 400°C in an inert atmosphere for 2 hours (e.g., in a tube furnace under nitrogen or argon atmosphere). The negative electrode active coating can then be peeled off from the negative electrode current collector, and the negative electrode active layer is collected as a test sample. Using a thermogravimetric analyzer (e.g., a TGA 550 thermogravimetric analyzer), the test sample amount is 5mg-15mg. Under an air or oxygen atmosphere, the temperature is increased from room temperature (25°C) to 900°C at a rate of 10°C / min, and held at 900°C for 40 minutes. This allows the non-silicon components in the negative electrode active coating to volatilize while silicon is fully oxidized to silicon dioxide. The remaining substance is the ash content of the negative electrode active coating. The mass content of silicon in the negative electrode active coating can be calculated based on the mass of ash. The calculation formula is as follows: Based on the total mass of the negative electrode active material layer, the mass percentage of silicon = 7 × mass of ash / (15 × mass of test sample).

[0084] (9) Battery high-temperature cycle performance: At 45℃, the battery is charged at a constant current of 2C to 4.55V, then at a constant current of 1C to 4.58V, and then charged at a constant voltage to the cutoff current of 0.05C. After resting for 5 minutes, it is discharged at a constant current of 0.7C to 3.0V and then rested for 5 minutes. This is the first cycle. The above process is repeated 600 times. The capacity retention rate (%) after 600 cycles = discharge capacity after 600 cycles / discharge capacity after the first cycle × 100%.

[0085] The test results are shown in Tables 1-6.

[0086] Table 5

[0087] Table 6

[0088] As shown in Tables 5 and 6, the cycle performance and safety performance of the lithium-ion secondary battery in this application are significantly improved.

[0089] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A lithium-ion secondary battery, characterized by comprising: The battery comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte; The positive electrode sheet comprises a positive electrode current collector and a positive electrode active layer arranged on at least one side surface of the positive electrode current collector; the positive electrode active layer comprises a positive electrode active material containing cobalt and a positive electrode binder; The negative electrode sheet comprises a negative electrode current collector and a negative electrode active layer arranged on at least one side surface of the negative electrode current collector; the negative electrode active layer comprises a negative electrode active material and a negative electrode binder, and the negative electrode active material comprises a silicon-based material; The positive electrode binder and the negative electrode binder both comprise cyano and ester functional groups; The positive electrode binder and the negative electrode binder both satisfy: In the XRD pattern of the positive electrode binder and the negative electrode binder, there is a characteristic diffraction peak 1 at 2θ=16-18° and a characteristic diffraction peak 2 at 2θ=22-30°, and the intensity ratio of the characteristic diffraction peak 1 to the characteristic diffraction peak 2 is 1.3-2; S1 is the peak area of characteristic diffraction peak 1, S2 is the peak area of characteristic diffraction peak 2, S 总 =S1+S2, 60%≤S1 / S 总 ≤90%.

2. The lithium-ion secondary battery according to claim 1, characterized by At least one of the following conditions is satisfied: (1) The intensity ratio of the characteristic diffraction peak 1 to the characteristic diffraction peak 2 is 1.4-1.9; (2) The swelling rate of the positive electrode binder and the negative electrode binder is independently 50%-200%, and optionally 80%-150%.

3. The lithium-ion secondary battery according to claim 1, characterized by The mass content of the positive electrode binder is 0.1%-3% based on the mass of the positive electrode active layer; And / or The mass content of the negative electrode binder is 2%-5% based on the mass of the negative electrode active layer.

4. The lithium-ion secondary battery according to claim 1, characterized by The negative electrode active layer comprises a first negative electrode active layer and a second negative electrode active layer; the second negative electrode active layer is arranged on the side of the first negative electrode active layer away from the negative electrode current collector; The mass content of silicon is 1.5%-40% based on the total mass of the negative electrode active layer; The mass content of silicon in the first negative electrode active layer in the region 0-10μm away from the surface of the negative electrode current collector is lower than the mass content of silicon in the second negative electrode active layer in the region 0-10μm away from the surface of the second negative electrode active layer; Optionally, the surface of the second negative electrode active layer is provided with a plurality of recesses; the spacing between adjacent two recesses is 0.5mm-2mm, the width of a single recess is 50μm-200μm, and the depth of a single recess is 8μm-40μm; Optionally, the thickness of the first negative electrode active layer is 10μm-30μm; Optionally, the thickness of the second negative electrode active layer is 10μm-50μm.

5. The lithium-ion secondary battery according to any one of claims 1 to 4, characterized by The positive electrode binder and the negative electrode binder are polymers formed by copolymerization of a first monomer, a second monomer and a third monomer; The first monomer comprises at least one of acrylonitrile, methacrylonitrile and fumaronitrile; The second monomer comprises at least one of acrylate, methacrylate and vinyl acetate; The third monomer comprises at least one of an acid group-containing monomer, an alkaline group-containing monomer and a halogen-containing monomer; Optionally, the mass content of the first monomer is 15-85wt% based on the total mass of all monomers; Optionally, the mass content of the second monomer is 15-85wt% based on the total mass of all monomers; Optionally, the mass content of the third monomer is 1-10 wt% based on the total mass of all monomers; Optionally, the order degree of the ordered alternation of the first monomer and the second monomer is greater than or equal to 90%.

6. The lithium-ion secondary battery according to any one of claims 1 to 4, characterized by The content of cobalt element in the negative active layer is less than 10,000 ppm at 0% SOC.

7. The lithium-ion secondary battery according to any one of claims 1 to 4, characterized by At 50% SOC, the impedance R1 of the positive plate is less than the impedance R2 of the negative plate, the impedance R1 of the positive plate ranges from 15 to 30 mΩ, the impedance R2 of the negative plate ranges from 17 to 45 mΩ, and R2-R1=2-20 mΩ.

8. The lithium-ion secondary battery according to any one of claims 1 to 4, characterized by The electrolyte comprises a nitrile additive; Optionally, the nitrile additive comprises one or more of adiponitrile, butanedinitrile, acetonitrile, 3-methoxypropionitrile, and 1,3,6-hexanetricarbonitrile.

9. The lithium-ion secondary battery according to claim 8, characterized by The content of the nitrile additive is 0.1 wt%-8 wt% based on the total mass of the electrolyte. Optionally, the content of the nitrile additive is 0.5 wt%-5 wt% based on the total mass of the electrolyte.

10. The lithium-ion secondary battery according to any one of claims 1 to 4, characterized by At least one of the following conditions is met: (1) the thickness of the negative plate at 50% SOC is 95-140 μm; (2) the thickness of the negative plate at 100% SOC ranges from 100 to 170 μm; (3) at room temperature, the thickness change rate of the battery cell at 100% SOC is 0.5%-2% after 200T cycles; (4) at room temperature, the thickness change rate of the battery cell at 100% SOC is 2%-6% after 600T cycles.

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