Negative electrode sheet, negative electrode sheet preparation method and battery

Abstract

This invention relates to the field of battery technology, and particularly to a negative electrode sheet. The negative electrode sheet includes a conductive layer and a first coating layer, a second coating layer, and a functional layer sequentially stacked on both sides of the conductive layer. The second coating layer comprises lithium lanthanum titanate. By adding a second coating layer containing lithium lanthanum titanate, the lithium lanthanum titanate component undergoes electrochemical conversion with lithium salts in the electrolyte to generate a functional layer rich in inorganic components such as lithium fluoride and lithium phosphate. This functional layer exhibits better chemical stability and physical support compared to naturally formed solid electrolyte interfacial films, effectively alleviating the stress caused by the volume expansion of the first coating layer and hindering its expansion tendency. This improves the mechanical strength and toughness of the negative electrode sheet surface, reduces the expansion rate of the negative electrode sheet, and extends its service life. This invention also provides a method for preparing the above-mentioned negative electrode sheet and a battery.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a negative electrode sheet, a method for preparing the negative electrode sheet, and a battery. Background Technology

[0002] With the rapid development of the consumer electronics sector, the demand for capacity and energy efficiency in batteries has also increased. To improve energy density, most current batteries use silicon-carbon anode materials to make anode plates; however, because the process of lithium ion insertion and extraction affects the crystal structure of the anode plate, the anode plate is prone to volume expansion during operation. Repeated volume changes generate significant mechanical stress within the anode plate, leading to cracks or pulverization and damage to the conductive network. Summary of the Invention

[0003] To address the problem that existing negative electrode sheets easily expand during operation, leading to particle pulverization, damage to the conductive network, and ultimately, failure of the negative electrode sheet, this invention provides a negative electrode sheet, a method for preparing the negative electrode sheet, and a battery.

[0004] The present invention provides a negative electrode sheet, comprising a conductive layer; two first coatings, each comprising a first main material and disposed on both sides of the conductive layer; two second coatings, each comprising a second main material and disposed on the side of each first coating away from the conductive layer; the second main material further comprising a lithium lanthanum titanate component; and two functional layers, each disposed on the side of each second coating away from the conductive layer, wherein the main components of the functional layers include lithium fluoride and lithium phosphate; when the second coating is placed in an external electrolyte, the lithium lanthanum titanate component in the second coating undergoes an electrochemical conversion reaction with the lithium hexafluorophosphate component in the external electrolyte, thereby generating the functional layer on the side of the second coating away from the conductive layer.

[0005] Preferably, in the second main material, the mass percentage of the lithium lanthanum titanate component ranges from 10% to 20%.

[0006] Preferably, both the first main material and the second main material comprise a negative electrode main material obtained by mixing silicon-carbon components and graphite components; in the negative electrode main material, the content of silicon-carbon components is 5% to 50%.

[0007] Preferably, the lithium lanthanum titanate component is distributed close to the side of the second coating away from the first coating.

[0008] Preferably, the negative electrode sheet further comprises the following components in weight percentage: 0.5%-1.5% conductive carbon black, 0.3%-1.0% single-walled carbon nanotubes, 0.4%-1.0% sodium hydroxymethyl cellulose, 0.5%-2.0% polyacrylic acid and 0.5%-2.0% styrene-butadiene rubber.

[0009] Preferably, the areal density of the first coating is 60-120 g / cm², and the thickness is 45-80 μm; the areal density of the second coating and the functional layer is 20-35 g / cm², and the thickness is 10-30 μm; the sum of the thicknesses of the first coating, the second coating, and the functional layer located on one side of the conductive layer is 55-110 μm.

[0010] This invention provides a method for preparing a negative electrode sheet, comprising the following steps: preparing a first main material and pretreating it, then coating the pretreated first main material onto both sides of a conductive layer to form a first coating; drying and rolling the first coating; preparing a second main material mixed with lithium lanthanum titanate, and after pretreatment, coating the second main material onto the side of the first coating away from the conductive layer after drying and rolling to form a second coating; drying and rolling the second coating; placing the second coating in an external electrolyte, wherein the lithium lanthanum titanate component in the second coating undergoes an electrochemical conversion reaction with the electrolyte to generate a functional layer.

[0011] Preferably, the pretreatment of the first main material includes the following steps: dry mixing graphite and conductive agent; preparing a pre-dispersion liquid and stirring; mixing the pre-dispersion liquid with the dry-mixed graphite and conductive agent, and then adding silicon carbon; adding a reinforcing agent and mixing; adding a binder and stirring at low speed; and vacuum degassing. The pretreatment of the second main material includes the following steps: dry mixing lithium lanthanum titanate powder and graphite, and then adding a conductive agent for dry mixing; preparing a pre-dispersion liquid and stirring; mixing the pre-dispersion liquid with the dry-mixed lithium lanthanum titanate, graphite and conductive agent, and then adding silicon carbon; adding a reinforcing agent and mixing; adding a binder and stirring at low speed; and vacuum degassing.

[0012] Preferably, the main components of the conductive agent are conductive carbon black and single-walled carbon nanotubes, the main component of the pre-dispersion liquid is sodium hydroxymethyl cellulose, the main component of the reinforcing agent is polyacrylic acid, and the main component of the binder is styrene-butadiene rubber.

[0013] The present invention also provides a battery comprising a positive electrode, an electrolyte, and a negative electrode as described above.

[0014] Compared with the prior art, the negative electrode sheet, the method for preparing the negative electrode sheet, and the battery of the present invention have the following advantages: 1. The negative electrode sheet of the present invention includes a conductive layer and a first coating layer, a second coating layer, and a functional layer sequentially stacked on both sides of the conductive layer, wherein the second coating layer comprises lithium lanthanum titanate. By adding a second coating layer containing lithium lanthanum titanate, the lithium lanthanum titanate component undergoes electrochemical conversion with lithium salts in the electrolyte, mainly lithium hexafluorophosphate, to generate a functional layer rich in inorganic components such as lithium fluoride and lithium phosphate. Compared with naturally formed solid electrolyte interface films, this functional layer has better chemical stability and physical support capabilities, effectively alleviating the stress generated by the volume expansion of the first coating layer and hindering its expansion trend, improving the mechanical strength and toughness of the negative electrode sheet surface, reducing the expansion rate of the negative electrode sheet, and extending its service life.

[0015] 2. In the second main material of the present invention, the mass percentage of the lanthanum lithium titanate component ranges from 10% to 20%. By controlling the content of the lanthanum lithium titanate component to be moderate, the corresponding functional layer has a low-strain three-dimensional skeleton, which further suppresses the expansion of the first coating and effectively ensures the integrity of the conductive network of the negative electrode sheet.

[0016] 3. Both the first and second main materials of the present invention comprise a negative electrode main material obtained by mixing silicon-carbon components and graphite components. In the negative electrode main material, the content of silicon-carbon components is 5% to 50%. On the one hand, this component content is beneficial to forming a continuous rigid network of graphite while maintaining the capacity of the negative electrode main material, thereby constraining the expansion space of silicon particles. On the other hand, it is beneficial to uniformly disperse expansion hot spots, improve the uniformity of stress distribution, and prevent local stress concentration from causing cracking of the negative electrode sheet.

[0017] 4. The lanthanum lithium titanate component of the present invention is distributed close to the second coating on the side away from the first coating, which is beneficial to induce the formation of a thin and dense functional layer on the surface of the second coating on the side away from the first coating, reducing irreversible lithium consumption and improving the first efficiency of the negative electrode sheet; at the same time, this positional distribution also avoids the lanthanum lithium titanate affecting the energy density of the first coating, thereby avoiding interference with the conductive network of the negative electrode sheet.

[0018] 5. The negative electrode sheet of the present invention further comprises the following components in weight percentage: 0.5%-1.5% conductive carbon black, 0.3%-1.0% single-walled carbon nanotubes, 0.4%-1.0% sodium carboxymethyl cellulose, 0.5%-2.0% polyacrylic acid, and 0.5%-2.0% styrene-butadiene rubber. By controlling the content of the above components, the negative electrode sheet enhances its overall toughness while ensuring sufficient electron conductivity, which helps to suppress pulverization and excessive growth of the functional layer during use, thereby extending the cycle life of the negative electrode sheet.

[0019] 6. The negative electrode preparation method of the present invention includes the following steps: preparing a first main material and pretreating it, then coating the pretreated first main material onto both sides of the conductive layer to form a first coating; drying and rolling the first coating; preparing a second main material mixed with lithium lanthanum titanate, pretreating it, and then coating the second main material onto the first coating to form a second coating; drying and rolling the second coating; placing the second coating in an external electrolyte, where the lithium lanthanum titanate component in the second coating undergoes an electrochemical conversion reaction with the electrolyte to generate a functional layer. The layered double-coating preparation method helps improve the alignment accuracy of the first and second coatings, avoids interlayer misalignment between the two coatings, and thus ensures that the functional layer formed on the second coating can completely cover and protect the first coating and the conductive layer.

[0020] 7. The pretreatment of the first main material of the present invention includes the following steps: dry mixing graphite and conductive agent; preparing a pre-dispersion liquid and stirring; mixing the pre-dispersion liquid with the dry-mixed graphite and conductive agent, and then adding silicon carbon; adding a reinforcing agent and mixing; adding a binder and stirring at low speed; vacuum degassing; the pretreatment of the second main material is carried out before the dry mixing of graphite and conductive agent step, by first dry mixing lithium lanthanum titanate with graphite. Through pretreatment, the dispersion uniformity of the internal components of the first and second main materials is improved, which is conducive to the construction of a uniform conductive network and enhances its stress resistance, thereby improving the overall anti-expansion ability of the negative electrode sheet.

[0021] 8. The present invention also provides a battery comprising a positive electrode, an electrolyte and a negative electrode as described above, having the same beneficial effects as the negative electrode described above, which will not be elaborated here. Attached Figure Description

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

[0023] Figure 1 This is a schematic diagram of the negative electrode sheet provided in the first embodiment of the present invention.

[0024] Figure 2 This is a flowchart of the steps in the negative electrode preparation method provided in the second embodiment of the present invention.

[0025] Figure 3 This is a block diagram of a battery provided in the third embodiment of the present invention.

[0026] Explanation of reference numerals in the attached diagram: 1. Negative electrode; 2. Conductive layer; 3. First coating; 4. Second coating; 5. Functional layer; 6. Battery; 61. Positive electrode; 62. Electrolyte. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0028] It should be noted that when a component is said to be "fixed to" another component, it can be directly attached to the other component or there may be an intervening component. When a component is said to be "connected to" another component, it can be directly connected to the other component or there may be an intervening component. The terms "vertical," "horizontal," "left," "right," and similar expressions used in this document are for illustrative purposes only.

[0029] In this invention, the terms "upper," "lower," "left," "right," "front," "rear," "top," "bottom," "inner," "outer," "middle," "vertical," "horizontal," "lateral," and "longitudinal" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. These terms are primarily for the purpose of better describing the invention and its embodiments, and are not intended to limit the indicated devices, elements, or components to having a specific orientation, or to be constructed and operated in a specific orientation.

[0030] Furthermore, in addition to indicating direction or positional relationship, some of the aforementioned terms may also have other meanings. For example, the term "above" may also be used in certain situations to indicate a dependency or connection. Those skilled in the art can understand the specific meaning of these terms in this invention based on the specific circumstances.

[0031] Furthermore, the terms "installation," "setup," "equipped with," "connection," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral structure; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium, or an internal connection between two devices, components, or parts. Those skilled in the art can understand the specific meaning of these terms in this invention based on the specific circumstances.

[0032] Please see Figure 1The first embodiment of the present invention provides a negative electrode sheet 1, comprising a conductive layer 2, two first coatings 3, two second coatings 4, and two functional layers 5. The first coatings 3 comprise a first main material and are respectively disposed on both sides of the conductive layer 2; the second coatings 4 comprise a second main material and are disposed on the side of each first coating 3 away from the conductive layer 2; the second main material further comprises lithium lanthanum titanate (chemical formula: [missing information]). The functional layer 5 is disposed on the side of each second coating 4 away from the conductive layer 2, and the main component of the functional layer 5 includes lithium fluoride (chemical formula: LLTO). ) and lithium phosphate (chemical formula is When the second coating 4 is placed in the external electrolyte, the lithium lanthanum titanate component in the second coating 4 undergoes an electrochemical conversion reaction with the lithium hexafluorophosphate component in the external electrolyte, and a functional layer 5 is generated on the side of the second coating 4 away from the conductive layer 2.

[0033] Understandably, the first and second main materials also include components such as conductive agents, reinforcing agents, and binders, which effectively improve the capacitance of the first coating 3 and the second coating 4 and stabilize their charging and discharging efficiency. At the same time, it also ensures the tight connection between the first coating 3 and the second coating 4 and the remaining layers, preventing the first coating 3 or the second coating 4 from peeling off, thereby improving the integrity and safety of the negative electrode sheet 1.

[0034] Generally, external electrolytes are mainly composed of organic solvents and lithium salts, with lithium hexafluorophosphate (Chemical formula: lithium...) being the primary component of the lithium salt. During the first charge of a lithium-ion battery, the organic solvent and lithium salt in the electrolyte spontaneously react on the surface of the negative electrode, depositing to form a solid electrolyte interface film. This solid electrolyte interface film can isolate electrolyte molecules while allowing lithium ions to pass through normally, thus protecting the electrode from continuous corrosion. However, the main component of the naturally formed solid electrolyte interface film is mostly alkyl lithium carbonate (chemical formula: ...) that undergoes reduction and decomposition with the organic solvent. (where R represents alkyl), lithium carbonate (chemical formula is...) ) and alkyllithium (chemical formula is Organic components such as alkyl groups (where R represents alkyl groups) have problems such as being porous and having low mechanical strength, which can easily lead to the continuous decomposition of the electrolyte and repeated thickening of the naturally formed solid electrolyte interface film.

[0035] Understandably, the lithium lanthanum titanate component in the second coating 4 mainly undergoes an electrochemical reaction with the lithium salt in the external electrolyte to generate a functional layer 5 with lithium fluoride and lithium phosphate as the main components.

[0036] More specifically, lithium lanthanum titanate mainly undergoes an electrochemical reaction with lithium hexafluorophosphate in the lithium salt to form functional layer 5: First, the lithium salt in the external electrolyte undergoes a side reaction with water, such as lithium hexafluorophosphate reacting with water to produce lithium fluoride and phosphorus pentafluoride (chemical formula: ) and hydrogen fluoride (chemical formula is Then, lithium lanthanum titanate combines with the by-reaction products of lithium salt to form a fluoride precipitate. The main chemical reaction formula is as follows: - Compounds; understandably, the aforementioned fluoride precipitate is the main inorganic component constituting functional layer 5.

[0037] Understandably, the main components of lithium fluoride enhance the mechanical strength of functional layer 5 while ensuring its low permeability, thus giving it strong rigidity and density. On the one hand, the dense functional layer 5 effectively prevents external electrolyte from seeping into the first coating 3 along the pores of the second coating 4, avoiding electrochemical reactions between the electrolyte and the first coating 3 and the continuous formation of a solid electrolyte interface film, which would lead to a continuous increase in the thickness of the negative electrode sheet. On the other hand, since some lithium ions irreversibly embed into the first coating 3, causing it to expand, the rigid functional layer 5 can also apply pressure to the first coating 3 and the second coating 4 to a certain extent. This helps to limit the volume expansion of the first coating 3 and alleviate the stress caused by the volume expansion, preventing the negative electrode sheet 1 from pulverizing due to the expansion of its internal structure and the destruction of its own conductive network.

[0038] Understandably, the main components of lithium phosphate enhance the chemical stability of functional layer 5 and passivate the surface interface of functional layer 5, significantly reducing the possibility of irreversible side reactions in functional layer 5 itself. This reduces the impact of side reaction products on the tightness of the bond between functional layer 5 and the second coating layer 4, and prevents local peeling of functional layer 5 during the first charge-discharge and cycle of negative electrode 1, which could lead to the failure of the protective function of functional layer 5.

[0039] In summary, when the second coating 4 undergoes an electrochemical conversion reaction with the electrolyte at its surface, the addition of lithium lanthanum titanate to induce the formation of the functional layer 5 has several advantages. Firstly, the functional layer 5, mainly composed of inorganic components, possesses good mechanical strength and surface support, which helps to withstand the internal stress generated by the negative electrode 1 during charging and discharging. This solves the problem that the naturally formed organic solid electrolyte interface film is relatively brittle and prone to cracking when the negative electrode 1 undergoes volume expansion and contraction, leading to the failure of the protection function and continuous consumption of electrolyte. Secondly, compared with the natural solid electrolyte interface film, the functional layer 5 has a smaller thickness and higher density, which facilitates the rapid conduction of lithium ions through the functional layer 5. At the same time, it effectively isolates electrons in the external electrolyte, solving the problem that the natural solid electrolyte interface film has generally low ionic conductivity and limited electron blocking ability, leading to continuous decomposition of the electrolyte at the interface.

[0040] Specifically, lithium lanthanum titanate has a perovskite structure. The perovskite structure is a highly symmetrical cubic structure. In the lithium lanthanum titanate composition, lithium ions or lanthanum ions are located at the vertices of the cube, titanium ions are located at the center of the cube, and oxygen ions are located at the center of each face of the cube.

[0041] Understandably, the cubic framework composed of titanium ions and oxygen ions provides structural rigidity, and lanthanum ions form a bottleneck at the vertices of the cube and restrict lattice expansion. Therefore, the lithium lanthanum titanate component has good low strain characteristics. When lithium ions enter the second coating 4, the space near the lithium ions and lanthanum ions in the lithium lanthanum titanate is used to accommodate lithium ions. That is, the embedding of lithium ions does not affect the stable three-dimensional framework structure of lithium lanthanum titanate.

[0042] Furthermore, the lithium lanthanum titanate component itself can act as a fast ion conductor, and the functional layer 5 it induces also has a high efficiency in lithium-ion transport. At the same time, it is beneficial to homogenize the lithium-ion flow in the negative electrode 1, thereby inhibiting the growth of lithium dendrites and preventing the internal structure of the negative electrode 1 from being damaged. In addition, the lithium lanthanum titanate located in the second coating 4 also plays a role in preventing the electrolyte from directly contacting the first coating 3, thereby reducing the solid electrolyte interface film naturally generated on the surface of the first coating 3 and improving the protective effect of the second coating 4 on the first coating 3.

[0043] More specifically, since the lithium lanthanum titanate component itself can also provide lithium ions, the lithium ions in the lithium lanthanum titanate can be used for electrochemical conversion reaction when the functional layer 5 is formed, thereby reducing the consumption of lithium ions in the external electrolyte during the formation of the functional layer 5, which is beneficial to improving the first-stage efficiency of the negative electrode 1.

[0044] Understandably, the conductive layer 2 is a layered structure made of metal current collector, carbon nanotubes or graphene; specifically, in this embodiment, the conductive layer 2 is a copper foil.

[0045] Furthermore, in the second main material, the mass percentage of the lithium lanthanum titanate component ranges from 10% to 20%.

[0046] Understandably, by limiting the mass percentage of the lithium lanthanum titanate component to a moderate level, the functional layer 5 induced by lithium lanthanum titanate has a three-dimensional framework structure. This structure has low strain and good toughness, which helps to suppress the expansion amplitude of the first coating 3 when it expands due to lithium ion insertion, thus preventing the integrity of the conductive network of the negative electrode 1 from being destroyed by its own expansion and improving the service life of the negative electrode 1.

[0047] Understandably, when the content of lithium lanthanum titanate is low, the continuity of the functional layer 5 formed by the second coating 4 is poor, the continuity of interface protection decreases, and the probability of electrolyte penetration into the coating increases, which in turn leads to a decrease in the limiting effect of the functional layer 5 on the silicon expansion of the first coating 3. When the content of lithium lanthanum titanate is high, since the intrinsic ionic conductivity of lithium lanthanum titanate at room temperature is typically approximately S / cm, meaning that lithium lanthanum titanate has a low intrinsic conductivity, a high doping amount can easily disrupt the continuity of the original conductive network in the second coating 4, blocking the electron transport path in the second coating 4, thereby affecting the charging and discharging efficiency of the negative electrode 1 and increasing its polarization.

[0048] Specifically, when the mass percentage of lithium lanthanum titanate component in the second coating 4 exceeds 20%, the relative content of active material and conductive component in the second coating 4 is excessively diluted by the lithium lanthanum titanate component. Understandably, on the one hand, the reduced proportion of active material limits the maximum achievable lithium-ion storage capacity of the second coating 4 itself, and the second coating 4 also struggles to provide sufficient chemical driving force for lithium ions migrating towards the first coating 3, resulting in a significant decrease in the number of lithium ions entering the first coating 3 through the second coating 4; on the other hand, the reduced proportion of conductive component disrupts the integrity of the conductive network of the negative electrode 1, exacerbating electrode polarization, and consequently leading to a significant capacity loss in the negative electrode 1.

[0049] More specifically, in this embodiment, the total mass percentage of lithium lanthanum titanate in the negative electrode 1 ranges from 1% to 10%.

[0050] The negative electrode 1 provided by this invention, along with the positive electrode, electrolyte, and other components, were assembled into complete batteries A1, A2, and A3 according to conventional procedures. Control samples B1, B2, and B3 were also provided. Electrochemical performance tests were performed on each battery, and the parameters and corresponding test results for each battery are shown in the table below. The unit for single-sided areal density is... The unit of compaction density is The unit of internal resistance is mΩ.

[0051]

[0052] As can be seen from the above data, firstly, the expansion rate of the battery assembled using the negative electrode sheet 1 provided by the present invention is greatly reduced. The expansion rate after 500 cycles is smaller than the increase in the initial expansion rate, and the cycle capacity retention rate is higher. This effectively suppresses the self-expansion of the negative electrode sheet 1 during operation, which is conducive to maintaining the integrity of its interface state after long-term use and avoiding the situation where the negative electrode sheet 1 expands violently and damages its own conductive structure. Secondly, the battery assembled using the negative electrode sheet 1 provided by the present invention has good initial battery efficiency, good conductivity, and a small increase in internal resistance under long-term use. Its charge and discharge efficiency and range are greatly improved, and it is also conducive to reducing battery heat generation, improving battery life and safety.

[0053] Furthermore, both the first and second main materials contain a negative electrode material obtained by mixing silicon-carbon components and graphite components; in the negative electrode material, the content of silicon-carbon components is 5% to 50%.

[0054] Generally speaking, pure graphite structures have good support, but their theoretical specific capacity is low, making it difficult to meet the current demand for long-lasting devices. Silicon-carbon components are silicon-carbon composite materials formed through nano-sizing and carbon coating technologies. Pure silicon-carbon structures significantly improve battery capacity, but while they are effective in limiting volume expansion, they can still easily cause the solid electrolyte interface film on the surface of the negative electrode to be ruptured.

[0055] Understandably, the design of the negative electrode material, which combines silicon-carbon and graphite, effectively combines the high capacity of silicon with the stability of carbon. This helps control the volume expansion of silicon, thereby increasing the energy density of the negative electrode 1. By controlling the content of silicon-carbon components to 5% to 50%, on the one hand, it avoids the problem that if the content of silicon-carbon components is too low, the high capacity advantage of silicon materials will be diluted, resulting in a significant decrease in the energy density improvement effect of the negative electrode 1. On the other hand, by controlling the maximum content of silicon-carbon components, it avoids that excessive silicon-carbon components will damage the rigid network structure of graphite components. This would cause the expansion of the first coating 3 to lose its buffering limit, resulting in a drastic volume change in the negative electrode 1 during operation, leading to particle pulverization and breakage, and the coating peeling off from the conductive layer 2.

[0056] Understandably, by controlling the capacity of silicon-carbon components in the first and second main materials and the graphite components, a continuous rigid network structure dominated by graphite is constructed within the negative electrode 1. This facilitates the application of spatial constraints to silicon particles at the microscale, thereby limiting the expansion amplitude of silicon particles in each direction when lithium ions are embedded in the negative electrode 1, compressing the volume change range of silicon particles, and thus limiting the expansion volume of the negative electrode 1. Furthermore, under the condition of moderate silicon-carbon content, the uniformity of the interface distribution between graphite particles and silicon particles is improved, which is conducive to the uniform spatial dispersion of expansion hotspots inside the negative electrode 1. This makes the stress distribution of the negative electrode 1 tend to be balanced during cycling, reducing the local stress peak inside the negative electrode 1, and effectively preventing the peeling and cracking of the first coating 3 or the second coating 4.

[0057] Specifically, based on the mass percentage of lithium lanthanum titanate in the first main material and the second main material, in the first main material, the mass of the negative electrode main material accounts for 96% of the total mass of the first main material; in the second main material, the mass of the negative electrode main material accounts for 76%-86% of the total mass of the second main material.

[0058] Furthermore, the lithium lanthanum titanate component is distributed close to the side of the second coating 4 away from the first coating 3.

[0059] Understandably, the distribution of the lithium lanthanum titanate component restricts the space for the second coating 4 to contact the electrolyte and undergo electrochemical conversion reactions within a narrow region. This is beneficial for the second coating 4 to induce the formation of a thin and dense functional layer 5, which limits the excessive decomposition of the electrolyte at the interface of the second coating 4. This reduces irreversible lithium loss caused by side reactions of the electrolyte and improves the initial efficiency of the negative electrode 1.

[0060] Understandably, the distribution of the lithium lanthanum titanate component also helps to control the electrochemical conversion reaction to occur at a location far from the first coating 3, avoiding the dilution of the high-capacity active material in the first coating 3 or the occupation of lithium ion storage sites by lithium lanthanum titanate and its products, thereby ensuring that the first coating 3 maintains a high energy density and effectively improving the efficiency and stability of the conductive network of the negative electrode 1.

[0061] Furthermore, the negative electrode 1 also contains the following components by mass percentage: 0.5%-1.5% conductive carbon black, 0.3%-1.0% single-walled carbon nanotubes, 0.4%-1.0% sodium carboxymethyl cellulose, 0.5%-2.0% polyacrylic acid and 0.5%-2.0% styrene-butadiene rubber.

[0062] Understandably, conductive carbon black and single-walled carbon nanotubes constitute the conductive structure of the negative electrode 1. The above ratio ensures the electronic conduction capability of the conductive structure, avoids the obstruction of electronic conduction, and prevents the excessive content of conductive carbon black and single-walled carbon nanotubes from diluting the active material.

[0063] Understandably, sodium hydroxymethyl cellulose, polyacrylic acid, and styrene-butadiene rubber mainly serve to bind the internal structure of the negative electrode sheet 1. This arrangement gives the first and second main materials a good thickening effect, which is conducive to the stable suspension of particles within them, preventing the sedimentation of silicon carbon particles, and thus enhancing the interlayer bonding force and bonding stability of the negative electrode sheet 1. In addition, this arrangement also improves the overall toughness of the negative electrode sheet 1, which is conducive to suppressing the pulverization of the negative electrode sheet and the excessive growth of the functional layer during use, thereby extending the cycle life of the negative electrode sheet.

[0064] Specifically, in this embodiment, both the first main material and the second main material contain the above-mentioned components, and the mass percentage of each component in the two main materials is equal respectively; more specifically, in this embodiment, the preferred mass percentage of each component is: 0.6% conductive graphite, 0.4% single-walled carbon nanotubes, 0.5% sodium hydroxymethyl cellulose, 1.5% polyacrylic acid and 1% styrene-butadiene rubber.

[0065] Furthermore, the areal density of the first coating 3 is 60-120 g / cm², and the thickness is 45-80 μm; the areal density of the second coating 4 and the functional layer 5 is 20-35 g / cm², and the thickness is 10-30 μm; the sum of the thicknesses of the first coating 3, the second coating 4, and the functional layer 5 located on one side of the conductive layer 2 is 55-110 μm.

[0066] Specifically, the compaction density of the first coating 3 is between 1.5 and 1.7. Between these two layers, the compaction density of the second coating 4 and the functional layer 5 is between 1.3 and 1.6. Between; the total areal density of the first coating 3, the second coating 4, and the functional layer 5 on one side of the conductive layer 2 is between 80 and 150. between.

[0067] Understandably, the larger surface density and thickness of the first coating 3 are beneficial to increasing the mass of conductive material that can be accommodated per unit area of ​​the first coating 3, and controlling the thickness of the first coating 3 to balance the capacity of the first coating 3 and the expansion range during lithium ion insertion. While ensuring that the first coating 3 has good conductivity, it avoids the occurrence of internal stress concentration and powder shedding of the negative electrode sheet 1 due to excessive density or thickness of the first coating 3.

[0068] Understandably, the smaller areal density and thickness of the second coating 4 and the functional layer 5 reduce the overall weight of both and decrease their impact on the specific capacity of the negative electrode 1. It also helps to ensure the continuity of the protective coverage of the functional layer 5 and avoids the accidental failure of its protective function.

[0069] More specifically, since the thickness of the conductive layer 2 is usually within 10 μm, the total thickness of the negative electrode 1 is 110-230 μm; the thickness ratio of the sum of the thicknesses of the second coating 4 and the functional layer 5 to the thickness of the first coating 3 is between 1:2 and 1:6.

[0070] Please see Figure 1 and Figure 2 The second embodiment of the present invention provides a method for preparing a negative electrode sheet, comprising the following steps: S1: Prepare the first main material and pre-treat it, then coat the pre-treated first main material onto both sides of the conductive layer 2 to form the first coating layer 3.

[0071] Specifically, in this embodiment, the viscosity of the first main material after pretreatment is between 2000-4000 mPa·s, and its output solid content is 50±5%.

[0072] S2: Dry and roll the first coating 3.

[0073] Understandably, the first coating 3 is dried using a gradient drying method, which reduces the side reactions between the components in the first coating 3 and water during the drying process, avoids the generation of gas or accidental expansion in the first coating 3, and facilitates the thorough drying of the moisture inside the first coating 3. Specifically, in this embodiment, the first coating 3 is dried under a gradient temperature increase condition of 80-120°C.

[0074] S3: Prepare a second main material mixed with lithium lanthanum titanate. After pretreatment, coat the second main material onto the side of the first coating 3 away from the conductive layer 2 after drying and rolling to form the second coating 4.

[0075] Specifically, in this embodiment, the viscosity of the pretreated second main material is between 1500-3000 mPa·s, and its output solid content is 40±5%.

[0076] S4: Dry and roll the second coating 4.

[0077] Understandably, since the lithium lanthanum titanate component in the second coating 4 is sensitive to moisture, the second coating 4 is dried at a lower temperature to avoid unwanted side reactions of lithium lanthanum titanate in a water-containing state; in addition, similarly to the first coating 3, the second coating 4 is dried using a gradient drying method.

[0078] More specifically, in this embodiment, the second coating 4 is dried under a gradient temperature increase of 70-110°C.

[0079] Understandably, the above-described film preparation sequence effectively prevents the lithium lanthanum titanate particles in the second coating 4 from being pressed into the deeper position, thereby preventing the lithium lanthanum titanate particles from entering the first coating 3 and ensuring that the first coating 3 maintains a high energy density.

[0080] S5: The second coating 4 is placed in the external electrolyte, and the lithium lanthanum titanate component in the second coating 4 undergoes an electrochemical conversion reaction with the electrolyte to generate the functional layer 5.

[0081] Understandably, the above preparation method involves coating and curing the first coating 3 before coating the second coating 4. This step-by-step preparation method is beneficial for the first coating 3 to have a stable boundary and shape when the second coating 4 is applied. It is also beneficial for the second coating 4 to align with the edge contour of the first coating 3 and to avoid relative flow or penetration between the two coatings. This fundamentally prevents interlayer misalignment between the first coating 3 and the second coating 4, effectively ensuring that the second coating 4 completely covers the first coating 3 and the conductive layer. In turn, it ensures that the functional layer 5 formed on the second coating 4 can completely cover and protect the first coating 3 and the conductive layer 2.

[0082] Specifically, the negative electrode 1 is prepared using a double-layer slit coating machine equipped with two independent feeding systems, with the two feeding systems respectively filling the first main material and the second main material.

[0083] More specifically, in this embodiment, the first main material and the second main material are coated on the conductive layer 2 and the first coating layer 3 respectively at a coating speed of 15-25 m / min. Understandably, this coating speed is lower than the usual single-layer coating speed, which helps to ensure the positional accuracy of the alignment with the upper layer structure during coating, thereby improving the accuracy of the preparation of the negative electrode sheet 1.

[0084] Further, step S1 specifically includes the following steps: dry mixing graphite and conductive agent; preparing a pre-dispersion liquid and stirring; mixing the pre-dispersion liquid with the dry-mixed graphite and conductive agent, and then adding silicon carbide; adding a reinforcing agent and mixing; adding a binder and stirring at low speed; and vacuum degassing.

[0085] Further, step S3 specifically includes the following steps: dry mixing lithium lanthanum titanate powder and graphite, then adding a conductive agent for dry mixing; preparing a pre-dispersion liquid and stirring; mixing the pre-dispersion liquid with the dry-mixed lithium lanthanum titanate, graphite and conductive agent, then adding silicon carbon; adding a reinforcing agent and mixing; adding a binder and stirring at low speed; vacuum degassing.

[0086] Understandably, the liquid environment formed by the pre-dispersion liquid is conducive to the uniform dispersion of each component particle in the pre-dispersion liquid, and the viscosity of the pre-dispersion liquid itself also provides sufficient buoyancy and shear force for the component particles, avoiding their sedimentation and resulting in uneven dispersion of components.

[0087] Understandably, when the internal components of the first and second main materials are well dispersed, graphite and conductive agent are evenly distributed and form a continuous three-dimensional conductive network. Furthermore, since reinforcing agents and binders are added to the first and second main materials, the stress resistance of the first and second main materials is further enhanced, and the stability of the internal three-dimensional conductive network is further improved, effectively enhancing the overall anti-expansion ability and electrical performance stability of the negative electrode sheet 1.

[0088] Furthermore, the main components of the conductive agent are conductive carbon black and single-walled carbon nanotubes, the main component of the pre-dispersion liquid is sodium hydroxymethyl cellulose, the main component of the reinforcing agent is polyacrylic acid, and the main component of the binder is styrene-butadiene rubber.

[0089] Please see Figure 1 and Figure 3 The third embodiment of the present invention also provides a battery 6, including a negative electrode 1, a positive electrode 61 and an electrolyte 62.

[0090] Compared with the prior art, the negative electrode sheet, the method for preparing the negative electrode sheet, and the battery of the present invention have the following advantages: 1. The negative electrode sheet of the present invention includes a conductive layer and a first coating layer, a second coating layer, and a functional layer sequentially stacked on both sides of the conductive layer, wherein the second coating layer comprises lithium lanthanum titanate. By adding a second coating layer containing lithium lanthanum titanate, the lithium lanthanum titanate component undergoes electrochemical conversion with lithium salts in the electrolyte, mainly lithium hexafluorophosphate, to generate a functional layer rich in inorganic components such as lithium fluoride and lithium phosphate. Compared with naturally formed solid electrolyte interface films, this functional layer has better chemical stability and physical support capabilities, effectively alleviating the stress generated by the volume expansion of the first coating layer and hindering its expansion trend, improving the mechanical strength and toughness of the negative electrode sheet surface, reducing the expansion rate of the negative electrode sheet, and extending its service life.

[0091] 2. In the second main material of the present invention, the mass percentage of the lanthanum lithium titanate component ranges from 10% to 20%. By controlling the content of the lanthanum lithium titanate component to be moderate, the corresponding functional layer has a low-strain three-dimensional skeleton, which further suppresses the expansion of the first coating and effectively ensures the integrity of the conductive network of the negative electrode sheet.

[0092] 3. Both the first and second main materials of the present invention comprise a negative electrode main material obtained by mixing silicon-carbon components and graphite components. In the negative electrode main material, the content of silicon-carbon components is 5% to 50%. On the one hand, this component content is beneficial to forming a continuous rigid network of graphite while maintaining the capacity of the negative electrode main material, thereby constraining the expansion space of silicon particles. On the other hand, it is beneficial to uniformly disperse expansion hot spots, improve the uniformity of stress distribution, and prevent local stress concentration from causing cracking of the negative electrode sheet.

[0093] 4. The lanthanum lithium titanate component of the present invention is distributed close to the second coating on the side away from the first coating, which is beneficial to induce the formation of a thin and dense functional layer on the surface of the second coating on the side away from the first coating, reducing irreversible lithium consumption and improving the first efficiency of the negative electrode sheet; at the same time, this positional distribution also avoids the lanthanum lithium titanate affecting the energy density of the first coating, thereby avoiding interference with the conductive network of the negative electrode sheet.

[0094] 5. The negative electrode sheet of the present invention further comprises the following components in weight percentage: 0.5%-1.5% conductive carbon black, 0.3%-1.0% single-walled carbon nanotubes, 0.4%-1.0% sodium carboxymethyl cellulose, 0.5%-2.0% polyacrylic acid, and 0.5%-2.0% styrene-butadiene rubber. By controlling the content of the above components, the negative electrode sheet enhances its overall toughness while ensuring sufficient electron conductivity, which helps to suppress pulverization and excessive growth of the functional layer during use, thereby extending the cycle life of the negative electrode sheet.

[0095] 6. The negative electrode preparation method of the present invention includes the following steps: preparing a first main material and pretreating it, then coating the pretreated first main material onto both sides of the conductive layer to form a first coating; drying and rolling the first coating; preparing a second main material mixed with lithium lanthanum titanate, pretreating it, and then coating the second main material onto the first coating to form a second coating; drying and rolling the second coating; placing the second coating in an external electrolyte, where the lithium lanthanum titanate component in the second coating undergoes an electrochemical conversion reaction with the electrolyte to generate a functional layer. The layered double-coating preparation method helps improve the alignment accuracy of the first and second coatings, avoids interlayer misalignment between the two coatings, and thus ensures that the functional layer formed on the second coating can completely cover and protect the first coating and the conductive layer.

[0096] 7. The pretreatment of the first main material of the present invention includes the following steps: dry mixing graphite and conductive agent; preparing a pre-dispersion liquid and stirring; mixing the pre-dispersion liquid with the dry-mixed graphite and conductive agent, and then adding silicon carbon; adding a reinforcing agent and mixing; adding a binder and stirring at low speed; vacuum degassing; the pretreatment of the second main material is carried out before the dry mixing of graphite and conductive agent step, by first dry mixing lithium lanthanum titanate with graphite. Through pretreatment, the dispersion uniformity of the internal components of the first and second main materials is improved, which is conducive to the construction of a uniform conductive network and enhances its stress resistance, thereby improving the overall anti-expansion ability of the negative electrode sheet.

[0097] 8. The present invention also provides a battery comprising a positive electrode, an electrolyte and a negative electrode as described above, having the same beneficial effects as the negative electrode described above, which will not be elaborated here.

[0098] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A negative electrode sheet, characterized in that, include: Conductive layer; Two first coatings, each comprising a first main material, are respectively disposed on both sides of the conductive layer; Two second coatings, each comprising a second main material, are respectively disposed on the surface of each first coating away from the conductive layer; the second main material further comprises a lithium lanthanum titanate component; Two functional layers are disposed on the side of each second coating layer away from the conductive layer, and the main components of the functional layers include lithium fluoride and lithium phosphate; When the second coating is placed in an external electrolyte, the lithium lanthanum titanate component in the second coating undergoes an electrochemical conversion reaction with the lithium hexafluorophosphate component in the external electrolyte, forming the functional layer on the side of the second coating away from the conductive layer.

2. The negative electrode sheet as described in claim 1, characterized in that: In the second main material, the mass percentage of the lithium lanthanum titanate component ranges from 10% to 20%.

3. The negative electrode sheet as described in claim 2, characterized in that: Both the first main material and the second main material comprise a negative electrode material obtained by mixing silicon-carbon components and graphite components; in the negative electrode material, the content of silicon-carbon components is 5% to 50%.

4. The negative electrode sheet as described in claim 1, characterized in that: The lithium lanthanum titanate component is distributed close to the side of the second coating away from the first coating.

5. The negative electrode sheet as described in claim 1, characterized in that: The negative electrode sheet also contains the following components by mass percentage: 0.5%-1.5% conductive carbon black, 0.3%-1.0% single-walled carbon nanotubes, 0.4%-1.0% sodium carboxymethyl cellulose, 0.5%-2.0% polyacrylic acid and 0.5%-2.0% styrene-butadiene rubber.

6. The negative electrode sheet as described in claim 1, characterized in that: The first coating has a surface density of 60-120 g / cm² and a thickness of 45-80 μm; the second coating and the functional layer have a surface density of 20-35 g / cm² and a thickness of 10-30 μm. The sum of the thicknesses of the first coating, the second coating, and the functional layer located on one side of the conductive layer is 55-110 μm.

7. A method for preparing a negative electrode sheet, characterized in that, Includes the following steps: Prepare a first main material and pretreat it, then coat the pretreated first main material onto both sides of the conductive layer to form a first coating layer; The first coating is dried and rolled. A second main material mixed with lithium lanthanum titanate is prepared. After pretreatment, the second main material is coated on the side away from the conductive layer of the first coating after drying and rolling to form a second coating. The second coating is dried and rolled. The second coating is placed in an external electrolyte, where the lithium lanthanum titanate component in the second coating undergoes an electrochemical conversion reaction with the electrolyte to generate a functional layer.

8. The method for preparing the negative electrode sheet as described in claim 7, characterized in that: The pretreatment of the first main material includes the following steps: dry mixing graphite and conductive agent; preparing a pre-dispersion liquid and stirring; mixing the pre-dispersion liquid with the dry-mixed graphite and conductive agent, and then adding silicon carbon to it; Add reinforcing agent and mix; add binder and stir at low speed; vacuum degassing; The pretreatment of the second main material includes the following steps: dry mixing lithium lanthanum titanate powder and graphite, then adding a conductive agent for dry mixing; preparing a pre-dispersion liquid and stirring; mixing the pre-dispersion liquid with the dry-mixed lithium lanthanum titanate, graphite and conductive agent, and then adding silicon carbon. Add reinforcing agent and mix; add binder and stir at low speed; vacuum degas.

9. The method for preparing the negative electrode sheet as described in claim 8, characterized in that: The main components of the conductive agent are conductive carbon black and single-walled carbon nanotubes, the main component of the pre-dispersion liquid is sodium hydroxymethyl cellulose, the main component of the reinforcing agent is polyacrylic acid, and the main component of the binder is styrene-butadiene rubber.

10. A battery, characterized in that, It includes a positive electrode, an electrolyte, and a negative electrode as described in any one of claims 1-6.

Images