A negative electrode sheet, a secondary battery

Abstract

The application discloses a negative pole piece and a secondary battery, and belongs to the technical field of batteries.The negative pole piece can effectively reduce the volume effect of silicon elements, avoid the influence of active material crushing or falling off on the stability of the pole piece, reduce the reaction probability of the pole piece and electrolyte, reduce the consumption of active lithium, improve the cycle stability of the corresponding secondary battery, and maintain the deintercalation rate of lithium ions in the negative pole piece at a high level, so that ideal fast charging performance can be realized when the negative pole piece is applied to a secondary battery.

Description

Technical Field

[0001] This application relates to the field of battery technology, specifically to a negative electrode sheet and a secondary battery. Background Technology

[0002] Currently, to achieve high energy density and fast charging, batteries typically use silicon-containing negative electrode sheets because silicon has a high theoretical specific capacity. This means that, for the same volume or mass, silicon-containing negative electrode sheets can store more lithium ions. Furthermore, silicon exhibits excellent lithium intercalation kinetics, allowing lithium ions to quickly intercalate into the silicon lattice during fast charging, reducing charging time. However, due to the significant volume expansion / contraction effect of silicon during battery charging and discharging, the interface damage of the negative electrode sheet is high, and the probability of side reactions between the negative electrode sheet and the electrolyte is increased, ultimately leading to poor battery cycle performance and short lifespan. While modification methods such as coating can mitigate the volume effect of silicon in the negative electrode sheet to some extent, they weaken the electrode's kinetic performance, reduce fast charging performance, and limit its application range. Summary of the Invention

[0003] The purpose of this application is to overcome the shortcomings of the existing technology and provide a negative electrode sheet. By controlling the silicon content in the negative electrode material layer of the negative electrode sheet and simultaneously regulating the interface strength and impedance of the electrode sheet, it is possible to effectively reduce the silicon volume effect, avoid the impact of active material breakage or shedding on the stability of the electrode sheet, reduce the reaction probability between the electrode sheet and the electrolyte, reduce the consumption of active lithium, and improve the cycle stability of the battery. At the same time, it can also ensure that the lithium ion insertion / extraction rate in the negative electrode sheet is maintained at a high level, achieving ideal fast charging performance.

[0004] To achieve the above objectives, in a first aspect of this application, this application provides a negative electrode sheet, the negative electrode sheet comprising a negative electrode material layer, the negative electrode material layer comprising a negative electrode active material, the negative electrode active material containing silicon element;

[0005] The negative electrode plate satisfies: a×100 / (b×c)=0.0033~90;

[0006] Where a is the mass percentage of silicon in the negative electrode material layer, b is the Young's modulus of the negative electrode sheet, and c is the charge transfer impedance of the negative electrode sheet.

[0007] The beneficial effects of this application are as follows:

[0008] This application provides a negative electrode sheet. By controlling the silicon content in the negative electrode material layer of the negative electrode sheet and simultaneously regulating the interface strength and impedance of the electrode sheet, it can not only effectively reduce the silicon volume effect and avoid the impact of active material breakage or shedding on the stability of the electrode sheet, but also reduce the reaction probability between the electrode sheet and the electrolyte, reduce the consumption of active lithium, and improve the cycle stability of the battery. At the same time, it can also ensure that the lithium ion insertion / extraction rate in the negative electrode sheet is maintained at a high level, achieving ideal fast charging performance. Detailed Implementation

[0009] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0010] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0011] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0012] The present application is further illustrated below with specific embodiments:

[0013] A negative electrode sheet, the negative electrode sheet comprising a negative electrode material layer, the negative electrode material layer comprising a negative electrode active material, the negative electrode active material layer containing silicon element;

[0014] The secondary battery satisfies: a×100 / (b×c)=0.0033~90;

[0015] Where a is the mass percentage of silicon in the negative electrode active material layer, b is the Young's modulus of the negative electrode sheet, and c is the charge transfer impedance of the negative electrode sheet.

[0016] The silicon content in the negative electrode directly affects its cycle stability and fast-charging performance when used in secondary batteries. As the silicon content increases, its volume expansion during charging and discharging becomes more pronounced, leading to greater damage at the negative electrode interface and increasing the probability of reaction between the electrode and the electrolyte. To effectively control this trend, this application's technical solution simultaneously adjusts the Young's modulus of the negative electrode while controlling the silicon content. This enhances the interface strength of the negative electrode, ensuring it maintains structural stability even during silicon volume expansion, preventing active material breakage or detachment, reducing the probability of side reactions between the negative electrode and the electrolyte, and thus improving battery cycle stability. However, if the Young's modulus of the negative electrode is too high, the electrode material becomes too rigid and lacks sufficient toughness. Lithium ions cannot effectively dissipate stress during insertion / extraction, making the negative electrode material prone to cracking. These cracks hinder lithium ion transport, resulting in a decrease in fast charging performance. On the other hand, the aforementioned interface strength regulation of the negative electrode sheet also affects the lithium-ion transport rate on the negative electrode sheet to some extent, thereby reducing the fast-charging performance of the electrode sheet when used in secondary batteries. Simultaneously regulating the charge transfer impedance of the negative electrode sheet can effectively optimize the kinetic performance of the electrode sheet and improve lithium-ion transport efficiency. However, if the charge transfer impedance of the negative electrode sheet is too small, a large number of lithium ions will be embedded, resulting in high volume expansion stress of the negative electrode active material particles, making them prone to cracking or even pulverization. The SEI film will continuously rupture and regenerate, increasing the consumption of active lithium and leading to a decrease in the cycle stability of the battery. Therefore, the charge transfer impedance of the negative electrode sheet cannot be too small and needs to be simultaneously matched and regulated. Through the synergistic regulation of these three factors, the negative electrode sheet can achieve excellent lithium-ion transport efficiency, good kinetic performance, low DCR, and significantly improved cycle stability when used in secondary batteries.

[0017] In some implementations, a×100 / (b×c) = a range of one or any two of the following: 0.0033, 0.0035, 0.005, 0.01, 0.02, 0.03, 0.05, 0.1, 0.2, 0.5, 0.8, 1, 2, 2.5, 2.7, 3, 5, 10, 20, 50, 60, 80, 90.

[0018] More preferably, a×100 / (b×c) = 0.03~2.7.

[0019] As mentioned above, the silicon content, Young's modulus, and impedance of the negative electrode directly affect its application performance in terms of ion conduction efficiency and mechanical structural stability, ultimately impacting the battery's fast-charging performance and cycle stability. By synergistically regulating these three characteristics of the negative electrode and further optimizing them within the aforementioned range, a balance between the negative electrode's kinetic performance and stability can be achieved, resulting in a battery with high fast-charging efficiency and cycle performance.

[0020] In some implementations, a = 1.5~20%;

[0021] More preferably, a is a range of one or any two of the following: 1.5%, 2%, 2.5%, 3%, 5%, 7%, 10%, 12%, 15%, 18%, and 20%.

[0022] More preferably, a = 5~15%.

[0023] When the silicon content in the negative electrode material layer changes, in addition to changes in kinetic properties, the stability of the electrode also changes. When the silicon content is preferably within the above range, the volume expansion / contraction effect of silicon is effectively controlled, avoiding breakage or shedding of active materials. The stability of the electrode can be maintained at a high level, the probability of side reactions with electrolyte is low, and the cycle stability of the secondary battery is high. At the same time, it can also improve the lithium ion conduction efficiency in the electrode, thereby improving the fast charging performance of the corresponding secondary battery.

[0024] It should be noted that the silicon content in the negative electrode material layer described in this application can be controlled by the amount of silicon in the negative electrode active material. Specifically, it can be controlled during the preparation of the negative electrode active material. For example, when the negative electrode active material is prepared by a gas phase method, it can be controlled by the amount of silicon-containing gas phase introduced or the reaction time.

[0025] In some embodiments, the mass percentage 'a' of silicon in the negative electrode material layer can be confirmed, but is not limited to, in the following manner: the secondary battery is discharged at 0.33C to a voltage of 2.5V, then the negative electrode sheet is disassembled, soaked in dimethyl carbonate (DMC) for 2 hours, dried, and the negative electrode material layer powder is scraped off. A certain amount of powder is weighed and placed in a nickel crucible pre-filled with potassium hydroxide. A small amount of potassium hydroxide is added to cover the sample surface, and two drops of ethanol are added. The mixture is heated on an electric furnace until the potassium hydroxide melts and dehydrates. Then, it is transferred to a muffle furnace at 1100°C and kept at the melting temperature for 8 hours. The nickel crucible is removed, allowed to cool slightly, and then placed in a 300 mL plastic beaker. Hot water is added for extraction. After the reaction, the crucible is removed, and hydrochloric acid is added to the beaker for acidification. Hydrogen peroxide is added dropwise. After cooling, other impurities are filtered out, and the filtered solution is transferred to a 100 mL volumetric flask, diluted to volume, and shaken well. After standing, a portion of the solution is transferred to another 100 mL volumetric flask, diluted to volume, shaken well, and allowed to stand until clear before analysis. Simultaneously, a blank solution is prepared (without sample). The solution is then obtained following the steps described above. Based on the sample characteristics and the element to be detected, appropriate ICP instrument operating conditions are set, including a gas flow rate of 0.5 L / min, a power of 1150 W, and a Si element measurement wavelength of 288.158 nm. The Si content is determined by ICP analysis, thus obtaining the a.

[0026] In some implementations, b = 0.1~5.

[0027] More preferably, b is a value within the range of one or any two of the following: 0.1, 0.15, 0.2, 0.25, 0.5, 0.8, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, and 5.

[0028] More preferably, b = 0.5~3.5.

[0029] Adjusting the Young's modulus of the negative electrode can effectively optimize its interfacial strength, ensuring high integrity even after silicon volume expansion and reducing the tendency for side reactions with the electrolyte. On the other hand, excessively high Young's modulus can lead to excessive rigidity and insufficient toughness, preventing effective stress dissipation during lithium ion insertion / extraction and causing cracks in the negative electrode material. These cracks hinder lithium ion transport, resulting in reduced fast charging performance. When the Young's modulus of the negative electrode is preferably within the aforementioned range, the lithium ion transport efficiency of the negative electrode can be maintained at a high level, ultimately achieving a balance between cycle performance and fast charging performance.

[0030] It should be noted that the Young's modulus of the negative electrode sheet described in this application can be controlled by adjusting the tensile strength of the current collector on the negative electrode sheet and the particle size of the negative electrode material layer, and is not limited to this. When the negative electrode sheet comes into contact with and is wetted by the electrolyte, the SEI film layer formed thereon will also affect the Young's modulus of the negative electrode sheet. Therefore, the composition and content of the electrolyte, such as the setting and content of FEC in the electrolyte, can also control the Young's modulus of the negative electrode sheet. As long as it does not affect the final effect of the secondary battery, it is acceptable.

[0031] In some embodiments, the Young's modulus b of the negative electrode sheet can be confirmed in, but is not limited to, the following manner: the secondary battery is discharged at a rate of 0.33C to a voltage of 2.5V, the negative electrode sheet is disassembled, dried, and then the negative electrode sheet is flatly attached to a glass slide. It is first placed in a constant temperature and humidity environment (25°C, humidity ≤50%) for at least 1 hour. Then, the area function of the indenter of the tester is calibrated using a standard reference block (such as monocrystalline silicon, fused silica) to ensure the accuracy of the tip geometry parameters (cone angle, radius of curvature), with an error of ≤1%. Load-displacement calibration was performed to verify the instrument's load accuracy (error ≤1%) and displacement resolution (typically below 0.1 nm) using standard sensors. After calibration, the electrode sample was fixed on the sample stage, and the target area was pre-positioned using an optical microscope, avoiding defects or boundaries. Then, the indenter was slowly brought close to the sample surface with a low load (≤10 μN), and the initial contact point (zero displacement) was recorded to avoid impact damage to the sample. The maximum load was set to 1 mN, the loading rate to 0.05 1 / s, the pause to 5 s, and the sinus frequency and amplitude to 10 Hz and 0.2 mN, respectively. The elastic modulus (Poisson's ratio 0.3) was calculated using the Oliver & Pharr model, which yielded the Young's modulus of the negative electrode. The testing instrument was a nanoindenter (model: Anton Paar UNHT, Austria; indenter type: berkvioch; mode: continuous stiffness mode).

[0032] In some implementations, c = 2 to 100.

[0033] More preferably, c is a range of one or any two of the following: 2, 5, 10, 15, 20, 25, 30, 50, 60, 80, 100.

[0034] More preferably, c = 10~50.

[0035] Adjusting the charge transfer impedance of the negative electrode can effectively improve its kinetic performance when applied to a battery. However, if the adjustment is excessive, the charge transfer impedance will be too small, resulting in a large number of lithium ions being inserted. This leads to high stress due to the volume expansion of the negative electrode material particles, making them prone to cracking or even pulverization. The SEI film will continuously rupture and regenerate, increasing the consumption of active lithium, which will also affect the electrochemical cycle stability of the battery. When the charge transfer impedance c of the negative electrode is preferably within the above range, the battery can achieve a better balance between cycle stability and kinetic performance.

[0036] In some embodiments, the charge transfer impedance c of the negative electrode sheet can be confirmed by, but is not limited to, the following methods: The secondary battery is discharged at a rate of 0.33C to 2.5V for venting treatment. The negative electrode sheet is disassembled, cleaned and dried using DMC solvent. A solvent is prepared by mixing EC (ethylene carbonate), EMC (ethyl methyl carbonate), and DEC (diethyl carbonate) in a volume ratio of 1:1:1. Lithium hexafluorophosphate is then added to obtain an electrolyte with a concentration controlled at 1 mol / L. A half-cell is assembled with lithium metal sheets and the electrolyte is adjusted to 20% SOC at 25°C and ≤70% humidity. Subsequently, an electrochemical workstation and control software are used to perform half-cell impedance testing in EIS constant focus mode. The AC amplitude is 5 mV, and the frequency range is set to 0.01-1000000 Hz. Zview software is used to fit the measured EIS data using an equivalent circuit model to analyze the charge transfer impedance c of the negative electrode sheet.

[0037] It should be noted that the charge transfer impedance of the negative electrode sheet described in this application can be controlled by the areal density of the negative electrode sheet and by setting a coating layer on the surface of the negative electrode active material and by varying the thickness of the coating layer on the surface of the negative electrode active material. However, it is not limited to this, and those skilled in the art can also control it in other ways.

[0038] In some embodiments, the volumetric cumulative distribution particle size D of the negative electrode material layer v10 The size is 2~10μm;

[0039] Furthermore, the cumulative volume distribution particle size D of the negative electrode material layer v10 The range is one or any two of the following values: 2μm, 3μm, 3.5μm, 4μm, 5μm, 5.5μm, 6μm, 8μm, and 10μm.

[0040] The particle size of the negative electrode material layer has a certain impact on the performance of the negative electrode sheet when applied to a secondary battery. When the particle size of the negative electrode material layer is small, its particle strength is low, which can easily cause volume effects during charging and discharging, affecting the integrity of the negative electrode sheet and increasing the probability of side reactions between the negative electrode sheet and the electrolyte. When the particle size of the negative electrode material layer is preferably within the above range, the lithium ion transport path can be shortened, the fast charging performance of the battery can be improved, and the stability of the secondary battery during charging and discharging can be effectively improved, resulting in better cycle performance.

[0041] In some embodiments, the particle size D of the negative electrode material layer v10 Confirmation can be made through, but is not limited to, the following methods: Discharge the secondary battery to 2.5V at a rate of 0.33C for a complete discharge process, disassemble the negative electrode, soak it in dimethyl carbonate (DMC) for 2 hours, and then dry it. Scrape off the powder from the negative electrode material layer. Then, use a laser particle size distribution analyzer (Mastersizer 3000) to measure the particle size distribution using laser diffraction (specific steps refer to GB / T19077-2016). The particle size distribution that accumulates to 10% of the volume distribution is considered the Dsize of the negative electrode material layer. v10 .

[0042] In some embodiments, the volumetric cumulative distribution particle size D of the negative electrode material layer v50 The particle size ranges from 9 to 18 μm, with a cumulative volume distribution particle size D. v90 The value is 19~31μm.

[0043] In some embodiments, the negative electrode material layer includes a negative electrode active material layer, wherein the negative electrode active material contains silicon.

[0044] In some embodiments, the negative electrode active material includes a silicon-based material;

[0045] More preferably, the silicon-based material includes at least one of elemental silicon, silicon oxide, silicon-carbon composite material, and silicon alloy.

[0046] In some embodiments, the negative electrode active material further includes graphite material;

[0047] More preferably, the graphite material includes at least one of artificial graphite and natural graphite.

[0048] By introducing graphite materials into the silicon-containing anode material layer, the overall conductivity of the anode active material can be effectively improved, thereby enhancing the kinetic performance of the electrode and improving the fast charging performance of the secondary battery, resulting in a lower DCR.

[0049] In some embodiments, a coating layer is further provided on the surface of the negative electrode active material;

[0050] More preferably, the coating layer includes at least one of a metal compound layer, a carbon layer, and an organic polymer layer;

[0051] More preferably, the metal compound layer includes at least one of metal oxides, metal nitrides, metal sulfides, and metal salts.

[0052] The metal element includes, but is not limited to, at least one of aluminum, titanium, and zinc. For example, the metal element can be aluminum, the metal oxide can be aluminum oxide, and the metal nitride can be aluminum nitride, but is not limited to these.

[0053] More specifically, the metal element may also be lithium, and the metal compound may also be a common solid electrolyte, such as at least one of lithium aluminum titanium phosphate (LATP), lithium lanthanum zirconium oxide (LLZO), and lithium nitride.

[0054] The carbon layer can be at least one of a doped carbon layer and an undoped carbon layer.

[0055] Furthermore, the carbon layer includes at least one of graphite carbon layer and amorphous carbon layer.

[0056] By coating and modifying the negative electrode active material, it is possible to effectively suppress the impact of the volume expansion effect of active particles on the stability of the negative electrode sheet during charging and discharging, improve the interfacial strength of the negative electrode sheet, and also effectively improve the ion conduction efficiency of the negative electrode active material.

[0057] In some embodiments, the average thickness of the coating layer is 2 to 10 nm.

[0058] After a coating layer is applied to the surface of the negative electrode active material layer, the thickness of the coating layer will affect the stability of the negative electrode active material and the lithium ion transport performance. Controlling the thickness within a certain range can not only ensure the transport of lithium ions, but also reduce the side reactions between the electrolyte and the negative electrode active material, thus balancing the fast charging performance and cycle performance of the battery.

[0059] In some embodiments, the porosity of the negative electrode sheet is 10-50%.

[0060] The porosity of the negative electrode sheet affects the structural strength and interfacial strength of the sheet itself, as well as its wettability when in contact with the electrolyte, thus affecting the lithium-ion transport efficiency. By controlling the porosity of the negative electrode sheet to be within the above-mentioned range, not only can high structural stability of the negative electrode sheet be ensured and high cycle stability of the battery be achieved, but also the wettability of the electrolyte to the negative electrode sheet can be improved, while also taking into account the fast charging performance of the battery.

[0061] It should be noted that the porosity of the negative electrode sheet described in this application can be confirmed by, but is not limited to, the following methods:

[0062] The secondary battery was discharged at a rate of 0.33C to a voltage of 2.5V. The negative electrode was then disassembled and cut into circular pieces with a diameter of D=12mm. Simultaneously, a thickness gauge was used to measure the thickness of the electrode and the current collector, recording them as h1 and h2 respectively. The formula V1=πR 2 Calculate the volume V1 of the positive electrode active material layer (h1-h2), then weigh the electrode and record the mass as m1. Next, immerse the electrode completely in a sealed container of hexadecane for 1 hour (the volume of hexadecane in the sealed container is not critical, but the amount must be sufficient to completely submerge the electrode). Remove the electrode and dry it with filter paper until a constant weight is achieved (generally about 1 hour). Weigh the electrode and record the weight as m2. Calculate the porosity of the negative electrode using the formula porosity% = (m2-m1 / ρ) / V1 × 100%, where ρ is the density of hexadecane, 0.7734 g / cm³. 3 .

[0063] In some embodiments, the negative electrode sheet further includes a current collector, the negative electrode material layer is disposed on at least one side of the current collector, and the tensile strength of the current collector is 200~800MPa.

[0064] More preferably, the current collector includes a metal foil, specifically a metal element, such as copper, or a metal alloy, such as at least one of copper alloys and copper-lithium alloys.

[0065] When the tensile strength of the current collector of the negative electrode sheet is preferably within the above range, the mechanical relationship between the current collector and the negative electrode material layer disposed on the current collector can be effectively balanced. This avoids the risk of the negative electrode material layer falling off due to the inconsistent deformation degree of the current collector and the negative electrode material layer when the silicon element in the negative electrode sheet undergoes volume expansion caused by either the current collector or the negative electrode material layer being too strong or too weak. At the same time, it can also suppress the influence of the extension deformation of the current collector on the negative electrode material layer, ensuring the structural stability of the negative electrode sheet during the cycling process.

[0066] In some embodiments, the tensile strength of the current collector in the negative electrode sheet can be confirmed by, but is not limited to, the following methods: The secondary battery is disassembled in a glove box, and a 500mm long current collector sample is cut from the obtained negative electrode sheet using a ceramic knife. After cleaning and drying the sample, five strips are cut longitudinally. The sampling position of the strips must cover the entire width of the foil. When sampling from the edge, the sampling position must be within 5mm of the edge. A universal testing machine is used for testing: the cylinder clamp is replaced, the sensor is confirmed, and the machine is connected to the network. The "Copper Foil Tensile Test Method" is selected. Set the gauge length to 100mm, width to 15mm, and tensile rate to 50mm / min (copper foil); set the thickness parameter to the nominal thickness T corresponding to the foil in the test batch; set the test force zero point, clamp the sample on the testing machine, and ensure that the clamped sample is perpendicular to the clamp under axial tensile force. Click "Start Automatic Test". The sample breaks, the test ends automatically, and the software displays the test results. After the test, it is necessary to confirm whether the sample breaks in the middle. If the break is near the two ends of the clamp, the test result is invalid and a new sample needs to be taken for testing.

[0067] In some embodiments, the areal density of the negative electrode sheet is 70~300 g / cm³. 2 .

[0068] In some embodiments, the compaction density of the negative electrode sheet is 1.3~1.65 g / cm³. 3 .

[0069] In some embodiments, the areal density of the negative electrode sheet in the technical solution of this application can be confirmed by, but is not limited to, the following methods: Discharge the secondary battery to 2.5V at a rate of 0.33C, disassemble the secondary battery to obtain the negative electrode sheet, soak the negative electrode sheet in DMC at room temperature (25°C) for 60 minutes, take it out and dry it; use a punching machine to punch the pretreated negative electrode sheet into circular pieces of fixed area, the area is recorded as S0, take three circular pieces as parallel samples, and then use an electronic balance to weigh the mass of the three circular pieces respectively, take the average value and record it as M1; add an appropriate amount of deionized water to the three circular pieces respectively, gently wipe the coating on the circular pieces with lint-free paper to expose the current collector, let it stand at room temperature (or dry) for 10 minutes, after the current collector is dry, weigh the mass of the three current collectors respectively, take the average value and record it as M0, and calculate the areal density of the negative electrode sheet according to the areal density B=(M1-M0) / S0.

[0070] In some embodiments, the negative electrode active material includes graphite materials and silicon-based materials.

[0071] In some embodiments, the negative electrode material layer may also include a conductive agent, a thickener, and a binder.

[0072] The conductive agent in the negative electrode material layer is used to provide conductivity. Any conductive agent can be used without particular limitation, as long as it has suitable electronic conductivity and does not significantly cause adverse chemical changes in the battery. For example, the conductive agent includes, but is not limited to, at least one of carbon nanotubes, carbon black, graphite, carbon fibers, activated carbon, mesoporous carbon, and fullerenes, wherein carbon fibers are, for example, carbon nanofibers; and carbon black is, for example, SP, acetylene black, Ketjen black, etc.

[0073] In some embodiments, the mass percentage of the conductive agent in the negative electrode material layer is 0.4% to 5%, such as 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, 3%, 4%, 5%, or any range formed by any two of the above values.

[0074] The thickener and binder in the negative electrode material layer are used to improve the adhesion between the negative electrode active material particles and the adhesion between the negative electrode active material and the negative electrode current collector. Any binder can be used without particular limitation, as long as it has suitable adhesion and does not significantly cause adverse chemical changes in the battery. For example, the thickener and / or binder includes, but is not limited to, at least one of carboxymethyl cellulose (CMC), styrene-butadiene rubber, polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl butyral, and aqueous acrylic resin.

[0075] In some embodiments, the mass percentage of the binder in the negative electrode material layer is 1.0% to 5%, such as 1.0%, 1.2%, 1.5%, 1.8%, 2.0%, 2.2%, 2.5%, 2.7%, 3.0%, 3.2%, 3.5%, 3.8%, 4.0%, 4.2%, 4.5%, 4.8%, 5%, or any range formed by any two of the above values. The mass percentage of the thickener in the negative electrode material layer is 0% to 0.2%, such as 0.01%, 0.02%, 0.05%, 0.08%, 0.1%, 0.12%, 0.15%, 0.18%, 0.2%, or any range formed by any two of the above values.

[0076] In some embodiments, the negative electrode material layer in the negative electrode sheet includes a negative electrode active material, a binder, and a conductive agent, and the mass percentage of the negative electrode active material in the negative electrode material layer is 90-99%.

[0077] In some embodiments, the negative electrode active material includes silicon-carbon composite material and graphite material, wherein the mass percentage of silicon-carbon composite material in the negative electrode material layer is 3-40%, and the mass percentage of graphite material is 56-93%.

[0078] In some embodiments, the negative electrode active material includes a silicon-carbon composite material, which comprises a silicon-carbon material and a coating layer disposed on the surface of the silicon-carbon material. It can be prepared by, but is not limited to, the following method: porous carbon is added to a rotary kiln as a matrix, nitrogen is introduced to remove oxygen, and the mixture is heated and calcined. The nitrogen flow rate, calcination temperature and heating rate, and rotary kiln rotation speed are common parameters in the art; for example, the nitrogen flow rate can be set to 10 L / min, the calcination heating rate can be 10 °C / min, the temperature can be 480~540 °C, and the rotary kiln rotation speed can be 1 rpm. Subsequently, while maintaining a constant calcination temperature, a mixed gas is introduced. The mixture includes nitrogen and silane in a volume ratio of 1:(1~4). The flow rate of the mixed gas can be set to 2~6 L / min, and the flow rate and introduction time are determined according to the requirements. After the reaction is completed, the silane is turned off, and only nitrogen is introduced. The mixture is calcined in a nitrogen atmosphere at a temperature of 500~600℃. When the temperature fluctuation gradually stabilizes, acetylene gas is introduced to carry out the deposition reaction (if a carbon coating layer is required). The flow rate of the introduced acetylene gas can be set to 1~3 L / min. The deposition time is 14~18h. After the reaction is completed, the gas is turned off, and the mixture is allowed to cool naturally in a nitrogen atmosphere. After the cooling is complete, the material is taken out and crushed, sieved, and demagnetized to obtain the silicon-carbon composite material.

[0079] As mentioned above, the silicon content of the negative electrode material layer in this application can be controlled by the process parameters during preparation. For example, when preparing silicon-carbon material in the negative electrode sheet using the above process, the content can be controlled by the silane flow rate and the inlet time. Furthermore, the value of c can also be controlled by the deposition conditions, and no specific limitation is made thereto.

[0080] In addition, those skilled in the art can also use other methods to prepare silicon-carbon composite materials, and set specific parameters to configure the coating layer. For example, silicon-carbon materials can be subsequently coated with metal compounds and / or solid electrolytes to prepare silicon-carbon composite materials. The specific steps are as follows: dispersing silicon-carbon materials in water or an organic solvent, adding metal oxides and / or solid electrolytes, and sealing the reaction vessel. Then, reacting at 100~220℃ and a pressure below 3.0MPa for 5~15h, the silicon-carbon composite material is obtained.

[0081] Specifically, the metal compound may be, but is not limited to, at least one of aluminum oxide and titanium dioxide, and the solid electrolyte may be, but is not limited to, at least one of LLZO, LiAlO2, and Li3N.

[0082] In some embodiments, the secondary battery further includes an electrolyte.

[0083] In some embodiments, the electrolyte comprises a lithium salt and a solvent.

[0084] Furthermore, the solvent includes at least one of carbonate solvents, carboxylic acid ester solvents, ether solvents, sulfone solvents, nitrile solvents, and phosphate ester solvents.

[0085] Exemplary examples include, but are not limited to, at least one of propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC); carboxylic acid ester solvents include, but are not limited to, at least one of ethyl acetate, methyl formate, and 1,4-butyrolactone; ether solvents include, at least one of dimethyltetrahydrofuran, tetrahydrofuran, and 1,2-dimethoxyethane; sulfone solvents include, at least one of methyl sulfone and dimethyl sulfoxide; nitrile solvents include, at least one of propionitrile, butyronitrile, 1-(2-cyanoethyl)pyrrole, and 1,3,6-hexanetrionitrile; and phosphate ester solvents include, at least one of trimethyl triphosphate and triethyl phosphate.

[0086] More preferably, the electrolyte further includes additives, including fluoroethylene carbonate (FEC).

[0087] In the electrolyte of a secondary battery, when fluoroethylene carbonate is selected as the solvent component and the ratios of the preferred technical features are controlled within the above-mentioned range, a stable SEI film layer rich in high-modulus inorganic components (such as lithium carbonate) can be formed on the surface of the negative electrode. This reduces the probability of side reactions between the electrolyte and the negative electrode, reduces battery gas production, improves the expansion effect of the negative electrode, enhances the cycle performance of the battery, increases the Young's modulus of the overall electrode, reduces interfacial impedance, increases the lithium-ion transport rate, and ultimately improves the fast-charging performance of the secondary battery.

[0088] In some embodiments, the fluoroethylene carbonate has a mass percentage of 1-5% in the electrolyte.

[0089] In some embodiments, the lithium salt includes at least one of hexafluorophosphate, imide salt, and oxalate borate.

[0090] More preferably, the lithium salt includes at least one of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetrafluoroborate, lithium dioxalate borate, lithium difluorooxalate borate, lithium trifluoromethanesulfonate, lithium difluoromethanesulfonylimide, lithium ditrifluoromethanesulfonylimide, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0091] More preferably, the lithium salt comprises lithium hexafluorophosphate.

[0092] Using lithium hexafluorophosphate as the lithium salt will generate hydrofluoric acid to a certain extent. This acidic substance can further enhance the neutralization effect of the electrolyte on the residual alkali on the surface of lithium nickel cobalt manganese oxide in the positive electrode active material, reduce the gas generation caused by residual alkali catalyzing the decomposition of the electrolyte, alleviate the expansion of the negative electrode sheet, and improve the structural stability of the negative electrode sheet.

[0093] In some embodiments, the concentration of the lithium salt in the electrolyte is 0.6~1.6 mol / L.

[0094] In some embodiments, the secondary battery further includes a positive electrode plate;

[0095] In some embodiments, the positive electrode sheet includes a positive electrode material layer, which includes a positive electrode active material, a binder, and a conductive agent, wherein the positive electrode active material has a mass percentage content of 92-99% in the positive electrode material layer.

[0096] In some embodiments, the binder is used to improve the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Any binder can be used without particular limitation, as long as it has suitable adhesion and does not significantly cause adverse chemical changes in the battery. For example, the binder includes fluorinated polyolefin binders, including but not limited to polyvinylidene fluoride (PVDF), PVDF copolymers, or their modified derivatives (e.g., modified with carboxylic acids, acrylic acid, acrylonitrile, etc.).

[0097] Specifically, the adhesive is selected from polytetrafluoroethylene or polyvinylidene fluoride.

[0098] In some embodiments, the mass percentage of the binder in the positive electrode material layer is 0.5% to 4.0%, such as 0.5%, 1%, 1.2%, 1.5%, 1.8%, 2.0%, 2.2%, 2.5%, 2.7%, 3.0%, 3.2%, 3.5%, 3.8%, 4.0%, or any range formed by any two of the above values.

[0099] In some embodiments, the conductive agent is used to provide conductivity. Any conductive agent can be used without particular limitation, as long as it has suitable electronic conductivity and does not significantly cause adverse chemical changes in the battery. Exemplary examples of conductive agents in the positive electrode material layer include, but are not limited to, at least one of carbon nanotubes, carbon black, graphite, carbon fibers, activated carbon, mesoporous carbon, and fullerenes, wherein carbon fibers are, for example, carbon nanofibers; and carbon black is, for example, SP (Super P), acetylene black, Ketjen black, etc.

[0100] In some embodiments, the mass percentage of the conductive agent in the positive electrode material layer is 0.5% to 4%, such as 0.5%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.5%, 3.0%, 4.0%, or any range formed by any two of the above values.

[0101] In some embodiments, the positive electrode active material includes at least one of lithium iron phosphate and nickel-cobalt ternary materials.

[0102] Lithium iron phosphate (LFP) is a positive electrode active material with an olivine-type crystal structure, which has the advantages of low cost and high safety.

[0103] In some embodiments, the general chemical formula of the lithium iron phosphate may be LiFe. 1- x M x PO y Q z Where x≤0.1, 3.85≤y≤4, 0≤z≤0.05.

[0104] In some embodiments, the general chemical formula of the nickel-cobalt ternary material can be Li h Ni i Co j M1 k M2 l O m R n Wherein, 0.75≤h≤1.2,0<i<1;0<j<1,0<k<1,i+j+k=1;0≤l≤0.2,1≤m≤2.5,0≤n≤1,m+n≤3; R includes, but is not limited to, at least one of N, F, S, and Cl.

[0105] More preferably, the positive electrode active material includes lithium iron phosphate, and the secondary battery satisfies: a×100 / (b×c)=1.2~2.7.

[0106] More preferably, the positive electrode active material includes a nickel-cobalt ternary material, and the secondary battery satisfies: a×100 / (b×c)=0.03~1.5.

[0107] When lithium iron phosphate (LFP) cathode materials and nickel-cobalt ternary cathode materials are used as the positive electrode active materials and negative electrode sheets in the secondary battery of this application, their ion transport efficiency during lithium-ion intercalation / deintercalation and their impact on the negative electrode sheet differ, thus affecting the stability of the negative electrode sheet. LFP has a gentler deintercalation / deintercalation potential, resulting in a weaker impact of lithium-ions on the negative electrode sheet during deintercalation / deintercalation. This leads to higher stability of the SEI film layer on the surface of the negative electrode sheet and better stability in terms of electrode size and structure. Therefore, the silicon content in the negative electrode sheet can be appropriately increased, or the modulus or impedance of the negative electrode sheet can be appropriately reduced. Further optimization within these ranges can improve the fast-charging performance of the secondary battery. The wide insertion / extraction potential range of nickel-cobalt ternary materials results in a large concentration gradient of lithium ions during insertion / extraction, leading to a strong impact on the negative electrode. This causes the SEI film on the surface of the negative electrode to be easily damaged and remodeled, and the size and structural stability of the electrode is relatively low. It is necessary to coordinately control the silicon content, modulus, and impedance of the negative electrode to ensure the cycle stability of the secondary battery. By simultaneously controlling the silicon content, interface strength, and impedance of the negative electrode in the secondary battery, it is possible to effectively ensure that the positive electrode active material and the negative electrode are matched, ensuring that the structural stability and ion transport efficiency of the negative electrode are both at a high level. The secondary battery can achieve better cycle stability and fast charging performance.

[0108] In some embodiments, the molar ratio of nickel to total transition metal elements in the nickel-cobalt ternary material is ≥0.8.

[0109] Increasing the nickel content can further improve the overall energy density of the positive electrode active material, but it will also exacerbate the volume change during the delithiation process of the positive electrode material, leading to stress concentration inside the electrode and the formation of microcracks. The formation of cracks increases the contact area between the electrode and the electrolyte, further promoting gas generation and electrode expansion. However, in the solution described in this application, the stress and charge conduction performance of the negative electrode are synergistically controlled, which can effectively alleviate these problems in the positive electrode active material and further improve battery performance.

[0110] In some embodiments, the molar ratio of nickel to total transition metal elements in the nickel-cobalt ternary material can be determined by the following method:

[0111] The secondary battery was discharged to 2.5V at a rate of 0.33C and disassembled to obtain the positive electrode. The positive electrode was immersed in DMC (dimethyl carbonate) at 25℃ for 60 minutes, then removed and air-dried at room temperature with humidity ≤15%. The positive electrode material powder was scraped off the surface of the current collector, and a certain amount of powder was accurately weighed and dispersed in 20mL of water. Then, 10mL of nitric acid was added, and the mixture was heated until the positive electrode material powder dissolved. The material was then diluted with water to 100mL to obtain the test solution. The test solution was subjected to ICP testing. Before the test, a standard solution must be prepared. The linear correlation coefficient of the standard concentration must be above 0.999 to be used as a normal standard. The 1000 mg / L standard solution was diluted with deionized water to different concentrations (generally 0, 1 mg / 100 mL, 2 mg / 100 mL, 3 mg / 100 mL; this standard is also a solution containing the target analyte element, and the standard solution is commercially available). The detection wavelength of the transition metal element was selected, and the experimental conditions were set: according to the characteristics of the sample and the element to be detected, the appropriate ICP instrument operating conditions were set: gas flow rate 0.5 L / min, power 1150 W, and the test wavelength of nickel and other transition metal elements was selected (for example, the wavelength of nickel is 231.60 nm). The content of nickel and other transition metal elements in the sample can be read by the self-analysis function of the ICP test software. Then, the molar content is converted and the molar ratio of nickel in the total transition metal elements is calculated.

[0112] In some embodiments, the positive electrode material layer includes a positive electrode active material, which includes a nickel-cobalt ternary material, and the cumulative volume distribution particle size D of the positive electrode active material is... v90 The thickness is 5~12μm;

[0113] When nickel-cobalt ternary materials are selected as the positive electrode active material to match the silicon-containing negative electrode described in this application to construct a secondary battery, the large particles in the nickel-cobalt ternary materials will affect the lithium-ion transport efficiency in the secondary battery. In this case, the D-phase of the nickel-cobalt ternary materials... v90 By optimizing and adjusting within the above range, the negative electrode can be effectively matched, and the efficiency of lithium ions being extracted from the positive electrode can be improved, ultimately achieving better overall dynamic performance, better fast charging performance of the secondary battery, and a smaller DCR.

[0114] In some embodiments, the cumulative volumetric particle size D of the positive electrode material layer v50 The particle size is 2~4μm, and the cumulative volume distribution particle size D v10 The thickness ranges from 0.8 to 2 μm.

[0115] In some embodiments, the cumulative volumetric particle size D of the positive electrode material layer v10 D v50 and D v90You can confirm this in the following ways:

[0116] The secondary battery was discharged to 2.5V at a rate of 0.33C for venting treatment. The positive electrode was disassembled, soaked in dimethyl carbonate (DMC) for 2 hours, and then dried. The positive electrode material layer was scraped to remove powder. Subsequently, the particle size distribution was measured using a laser particle size analyzer (Mastersizer 3000) according to the laser diffraction method for particle size distribution (specific steps refer to GB / T19077-2016). The particle size that reached 10% of the cumulative volume distribution was defined as D. v10 The particle size reaches 50%, which is D. v50 The particle size reaches 90%, which is D. v90 .

[0117] In some embodiments, the secondary battery further includes a separator.

[0118] In the secondary battery described in this application, a separator is disposed between the positive and negative electrode plates to separate them and prevent short circuits caused by contact. The separator can be at least one of glass fiber, non-woven fabric, polyethylene (PE), polypropylene (PP), and polyvinylidene fluoride. A coating can also be provided on the surface of the separator, which can be an inorganic coating and / or an organic coating. The inorganic coating material includes at least one of alumina, silicon oxide, titanium oxide, magnesium oxide, zirconium oxide, and boehmite; the organic coating includes at least one of aramid coating and polyvinylidene fluoride (PVDF) coating.

[0119] More preferably, the air permeability of the diaphragm is 100~500s / 100mL.

[0120] It should be noted that the air permeability of the diaphragm described in this application can be confirmed in the following way:

[0121] The secondary battery was discharged at 0.33C to the lower limit voltage of 2.5V. Then, the empty battery was disassembled, the separator was removed, and soaked in DMC solution for 2 hours. After drying, the separator permeability was measured using a separator permeability meter. In a low-humidity, normal-pressure environment, with the instrument applying a pressure of 1.21 kPa, the permeability of 100 mL of air was measured to be 6.45 cm². 2 The time required for the sample (the cut diaphragm) to be processed is the air permeability of the diaphragm.

[0122] In the technical solution of this application, the negative electrode sheet can effectively balance the stability of the electrode sheet itself and the ion transport efficiency by adjusting the silicon content, interface strength and impedance. When the permeability of the separator of the secondary battery is further optimized within the above range, it can not only control the side reactions between the electrolyte and the positive and negative electrode sheets, reduce the amount of gas produced, alleviate the expansion of the negative electrode sheet and improve the cycle performance of the battery, but also ensure the full wetting of the electrolyte, so that lithium ions can be fully transported between the positive and negative electrodes through the separator, resulting in better fast charging performance.

[0123] The present invention is further illustrated below with specific embodiments, which should not be construed as limiting the scope of protection claimed by the present invention:

[0124] Example 1

[0125] A secondary battery, the method for preparing the secondary battery includes the following steps:

[0126] (1) Preparation of positive electrode: The positive electrode active material lithium nickel cobalt manganese oxide (LiNi) is prepared. 0.8 Co 0.1 Mn 0.1 O2, conductive agent CNT, conductive agent SWCNT, and binder PVDF were dispersed in N-methylpyrrolidone at a mass ratio of 98.2:0.65:0.05:1.1 to prepare a slurry for the positive electrode active material layer. The slurry was then placed in a coating machine and coated onto both surfaces of an aluminum foil current collector. After drying, the coating was rolled and slit to obtain positive electrode sheets with an areal density of 350 g / m³. 2 The compacted density is 1.35 g / cm³. 3 ;

[0127] (2) Preparation of silicon-carbon composite: Porous carbon material is added to a fluidized bed / rotary furnace as a carbon matrix, nitrogen is introduced to remove oxygen, and the temperature is raised for calcination. For example, the nitrogen flow rate is set to 10 L / min, the calcination heating rate is 10℃ / min, and the temperature is 480℃. Then, the calcination temperature is kept constant, and a mixed gas is introduced. The mixed gas includes silane and nitrogen with a volume ratio of 1:4, and is introduced at a flow rate of 4.5 L / min for 16 h. After the reaction is completed, the silane is turned off, and only nitrogen is introduced. The mixture is calcined in a nitrogen atmosphere at a calcination temperature of 550℃. Acetylene is introduced for deposition reaction. The volume ratio of the acetylene to nitrogen mixed gas is 1:3, and the flow rate is 2 L / min. The mixture is deposited for 3 h for coating. After the reaction is completed, the gas is turned off, and the mixture is cooled naturally in a nitrogen atmosphere. After the cooling is complete, the material is taken out and subjected to air jet milling, sieving, and demagnetization to obtain the silicon-carbon composite material.

[0128] (3) Preparation of negative electrode sheet: A mixture of natural graphite and silicon-carbon composite material (mass ratio of 56.98:38.32), conductive agent (a mixture of SP and SWCNT, mass ratio of 6:1), and binder (a mixture of carboxymethyl cellulose, polyacrylic acid, and styrene-butadiene rubber, mass ratio of 0.4:0.58:3.02) were mixed and dispersed in water at a mass ratio of 95.3:0.7:4 to prepare a negative electrode active material layer slurry. The negative electrode material layer slurry was placed in a coating machine and coated on both surfaces of the current collector copper foil (the tensile strength of the copper foil is 200 MPa; copper foils with different tensile strengths can be obtained commercially). After drying, the negative electrode sheet was obtained by rolling and cutting using a roller press. The areal density of the negative electrode sheet is 90 g / m³. 2 The compacted density is 1.5 g / cm³. 3 ;

[0129] (4) Preparation of electrolyte: EC, EMC and DEC are mixed in a ratio of 1:1:1 to obtain a solvent. Then, 2% VC and 3% FEC are added. LiPF6 is then added and the lithium salt concentration is controlled to be 1 mol / L to obtain the electrolyte.

[0130] (5) Preparation of the separating membrane

[0131] The diaphragm is commercially available. Its base membrane is a PE membrane with a thickness of 9μm. One side surface of the base membrane is sequentially provided with an alumina coating with a thickness of 3μm and a PVDF coating with a thickness of 2μm. The other side surface of the base membrane is provided with a PVDF coating with a thickness of 2μm.

[0132] (6) Assembly and formation

[0133] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. The cells are then wound to obtain a bare cell. The bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a lithium-ion battery is obtained.

[0134] The parameters of the negative electrode sheet during preparation and the results and parameters obtained from the product testing are shown in Tables 1-2.

[0135] Examples 2-23, Comparative Examples 1-4

[0136] A secondary battery differs from Example 1 only in the manufacturing process of each secondary battery.

[0137] The parameters during the preparation of each negative electrode sheet and the results and parameters obtained from the product testing are shown in Tables 1-2. The current collector copper foil of the negative electrode sheet is a commercially available product with different specifications and its tensile strength varies.

[0138] The silicon-carbon composite material described in Example 16 is further subjected to the following reaction:

[0139] Using aluminum isopropoxide as the aluminum source precursor and anhydrous ethanol as the solvent, aluminum isopropoxide was first dispersed in anhydrous ethanol in a stirred reactor with reflux condensation. The silicon-carbon composite material powder obtained after calcination was then added, and the mixture was stirred to achieve uniform dispersion. Deionized water was then slowly added dropwise using a peristaltic pump, and the mixture was stirred at a constant temperature of 60°C for 4 hours to allow the aluminum isopropoxide to hydrolyze and condense to form amorphous aluminum hydroxide, which then coated the surface of the silicon-carbon composite material particles in situ. Subsequently, deionized water was added dropwise to the resulting mixture, and the solid content was adjusted to 20%. The mixture was filtered and sent to a spray dryer, where the inlet temperature was set to 200°C, the outlet temperature to 90°C, and the atomizer speed to 20,000 r / min for granulation, resulting in silicon-carbon composite material particles coated with pre-cured aluminum hydroxide. The particles were then placed in a tubular sintering furnace, and after purging the air with nitrogen, the temperature was increased to 600°C at 5°C / min and calcined for 2 hours. After cooling in the furnace, the particles were passed through a 200-mesh sieve to remove large agglomerated particles, yielding the coated silicon-carbon composite material.

[0140] The silicon-carbon composite material described in Example 17 was further subjected to the following reaction:

[0141] The silicon-carbon composite material was ultrasonically dispersed in ethanol to form a stable suspension, ensuring a clean surface and preventing agglomeration. Lithium nitrate and aluminum nitrate were dissolved in a deionized water / ethanol (50:50 volume ratio) mixture at a Li:Al molar ratio of 1:1 to form a homogeneous solution with a total metal ion concentration of 0.5 mol / L. A small amount of acetylacetone or citric acid was added as a chelating agent to regulate the hydrolysis rate and prevent local precipitation. The mixture was magnetically stirred at 60°C for 3 hours to promote precursor hydrolysis and condensation, forming a transparent sol. The suspension containing silicon-carbon composite material particles was slowly added to this transparent sol, and stirring was continued for 2 hours. Finally, the resulting slurry was vacuum dried at 80°C for 12 hours to obtain a gel precursor powder. This powder was then calcined at 700°C for 3 hours under a nitrogen atmosphere at a heating rate of 5°C / min to obtain the coated silicon-carbon composite material.

[0142] In Examples 19-21, the positive electrode active material was replaced with lithium iron phosphate (LiFePO4) during the preparation of the positive electrode sheet.

[0143] Table 1

[0144]

[0145] Continued from Table 1

[0146]

[0147] Continued from Table 1

[0148]

[0149] Table 2

[0150]

[0151] Example of effect

[0152] The lithium-ion batteries obtained in each embodiment and comparative example were tested as follows:

[0153] (1) DCR test: Place the secondary battery in a 25℃ temperature chamber until thermal equilibrium is reached.

[0154] Then, it is charged at a constant current rate of 0.33C to the upper limit voltage (the upper limit voltage of the product with lithium iron phosphate as the positive electrode active material is 3.65V, and the upper limit voltage of other products with lithium nickel cobalt manganese oxide as the positive electrode active material is 4.25V), charged at a constant voltage until the current is less than 0.05C, and then discharged at 0.33C to the lower limit voltage (2.5V). This process is repeated more than 3 times to obtain the discharge capacity of the battery, and the battery capacity C1 is recorded. The battery is left to stand for 5 minutes; the battery is adjusted to 50% SOC at a discharge rate of 0.33C; and discharged at a current rate of 1C for 18 seconds. The battery voltage U2 and current I before the discharge stops and the battery voltage U1 after the battery voltage stabilizes are recorded. The DC internal resistance R1 is calculated according to the formula R1=(U2-U1) / I. This R1 is the discharge DCR of the secondary battery at 25°C at a 1C rate.

[0155] (2) Cyclic performance test:

[0156] Each secondary battery was left to stand at 25°C for 120 minutes, then charged at a constant current rate of 0.33C to the upper limit voltage (3.65V for products with lithium iron phosphate as the positive electrode active material, and 4.25V for other products with lithium nickel cobalt manganese oxide as the positive electrode active material), and charged at a constant voltage until the cutoff current ≤0.05C. After standing for 10 minutes, it was discharged at a constant current rate of 0.33C to 2.5V. This constitutes one cycle. The initial discharge capacity was recorded. The above steps were repeated until 80% SOH (i.e., 80% of the initial capacity) was reached, and the number of cycles was recorded.

[0157] The test results for each secondary battery are shown in Table 3.

[0158] Table 3

[0159]

[0160] As can be seen from Table 3:

[0161] (1) The secondary battery described in this application is based on the content of silicon in the negative electrode material layer in the negative electrode sheet, and the interface strength and impedance of the electrode sheet are controlled simultaneously. This can not only effectively improve the interface strength of the negative electrode sheet, so that the silicon in the electrode sheet can still maintain a certain structural stability when the volume expands, but also reduce the probability of side reactions between the negative electrode sheet and the electrolyte. The lithium ion transport rate on the negative electrode sheet can be maintained at a high level. Therefore, the secondary battery can not only achieve a low DCR (which can be controlled within 60mΩ), but also has good cycle stability, with a cycle count of more than 450 cycles and a maximum of more than 2000 cycles.

[0162] (2) The silicon content, Young's modulus and impedance of the negative electrode sheet directly affect the application effect of the negative electrode sheet in terms of ion conduction efficiency and mechanical structure stability. After the three characteristics of the negative electrode sheet of the secondary battery are coordinated and regulated, and further optimized to a×100 / (b×c)=0.03~2.7, the balance between the dynamic performance and stability of the negative electrode sheet can be taken into account, so that the fast charging efficiency and cycle performance of the battery are at a higher level.

[0163] (3) In addition, as can be seen from the embodiments, when the silicon content in the negative electrode active material layer changes, in addition to the change in kinetic performance, the stability of the electrode will also change. When it is further optimized to be in the range of 5~15%, the stability of the electrode can be higher, the side reaction with the electrolyte can be less, and the ion conduction efficiency can be better, thus improving the fast charging performance. At the same time, the Young's modulus of the negative electrode is preferably set to 0.5~3.5GPa, which can effectively optimize the interface strength of the negative electrode, so that the electrode can still maintain a high integrity after the silicon element undergoes volume expansion, and the lithium ion transport efficiency of the negative electrode can also be kept at a high level. Adjusting the charge transfer impedance of the negative electrode can effectively improve the kinetic performance of the negative electrode when it is applied to the battery. However, if the adjustment is excessive, it will affect the electrochemical cycle stability of the battery. After limiting the characteristics of the above three factors by a×100 / (b×c), the charge transfer impedance of the negative electrode is further optimized to be preferably set to 10~50mΩ, which can further improve the overall performance of the secondary battery.

Claims

1. A negative electrode sheet, characterized in that, The negative electrode sheet includes a negative electrode material layer, the negative electrode material layer includes a negative electrode active material, and the negative electrode active material contains silicon. The negative electrode plate satisfies: a×100 / (b×c)=0.0033~90; Where a is the mass percentage of silicon in the negative electrode material layer, a = 1.5~20%; b GPa is the Young's modulus of the negative electrode sheet, b = 0.5~3.5; c mΩ is the charge transfer resistance of the negative electrode sheet, c = 2~100.

2. The negative electrode sheet as described in claim 1, characterized in that, The value of a×100 / (b×c) is 0.03~2.

7.

3. The negative electrode sheet as described in claim 1, characterized in that, a = 5~15%, and / or c = 10~50%.

4. The negative electrode sheet as described in claim 1, characterized in that, The cumulative volume distribution particle size D of the negative electrode material layer v10 The value is 2~10μm.

5. The negative electrode sheet as described in claim 1, characterized in that, The negative electrode active material includes silicon-based materials; the silicon-based materials include at least one of elemental silicon, silicon oxide, silicon-carbon composite materials, and silicon alloys.

6. The negative electrode sheet as described in claim 5, characterized in that, The negative electrode active material also includes graphite materials.

7. The negative electrode sheet as described in claim 1, characterized in that, The surface of the negative electrode active material is also provided with a coating layer.

8. The negative electrode sheet as described in claim 7, characterized in that, The coating layer includes at least one of a metal compound layer, a carbon layer, and an organic polymer layer.

9. The negative electrode sheet as described in claim 8, characterized in that, The metal compound layer includes at least one of metal oxides, metal nitrides, metal sulfides, and metal salts, and / or the carbon layer includes at least one of doped carbon layers and undoped carbon layers.

10. The negative electrode sheet as described in claim 7, characterized in that, The average thickness of the coating layer is 2~10 nm.

11. The negative electrode sheet as described in claim 1, characterized in that, The porosity of the negative electrode sheet is 10-50%.

12. The negative electrode sheet as described in claim 1, characterized in that, The negative electrode sheet also includes a current collector, and the negative electrode material layer is disposed on at least one side of the current collector, wherein the tensile strength of the current collector is 200~800MPa.

13. A secondary battery, characterized in that, The secondary battery includes the negative electrode sheet according to any one of claims 1 to 12; the secondary battery further includes an electrolyte; the electrolyte includes a lithium salt and a solvent; the electrolyte further includes an additive, the additive including fluoroethylene carbonate.

14. The secondary battery as described in claim 13, characterized in that, The fluoroethylene carbonate has a mass percentage of 1-5% in the electrolyte.

15. The secondary battery as described in claim 13, characterized in that, The secondary battery further includes a positive electrode sheet; the positive electrode sheet includes a positive electrode material layer, the positive electrode material layer includes a positive electrode active material; the positive electrode active material includes at least one of lithium iron phosphate and nickel-cobalt ternary materials.

16. The secondary battery as described in claim 15, characterized in that, The positive electrode active material includes lithium iron phosphate, and the secondary battery satisfies: a×100 / (b×c)=1.2~2.

7.

17. The secondary battery as described in claim 15, characterized in that, The positive electrode active material includes a nickel-cobalt ternary material, and the secondary battery satisfies: a×100 / (b×c)=0.03~1.

5.

18. The secondary battery as described in claim 17, characterized in that, The molar ratio of nickel to total transition metal elements in the nickel-cobalt ternary material is ≥0.

8.

19. The secondary battery as described in claim 15, characterized in that, The positive electrode active material includes a nickel-cobalt ternary material, and the cumulative volume distribution particle size D of the positive electrode material layer is... v90 The value is 5~12μm.

20. The secondary battery as described in claim 13, characterized in that, The secondary battery also includes a separator; the air permeability of the separator is 100~500s / 100mL.

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