Negative active material with surface modification layer as well as preparation method and application of negative active material

Abstract

The invention provides a negative electrode active material with a surface modification layer and a preparation method and application thereof. The negative electrode active material with the surface modification layer comprises negative electrode active particles and the surface modification layer covering the exposed surfaces of the negative electrode active particles, and the surface modification layer comprises polyamide containing at least two amide groups. The negative electrode active material with the surface modification layer provided by the invention not only can effectively reduce the polarization phenomenon of an electrode interface, but also can remarkably improve the capacity and the energy density of a secondary battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of negative electrodes, and in particular relates to a negative electrode active material with a surface modification layer, a preparation method and an application thereof. Background Art

[0002] With the advancement of technology, modern devices (such as smartphones, laptops, and electric vehicles) have placed higher demands on the battery life and portability of lithium-ion batteries. High-energy-density lithium-ion batteries can provide longer usage time in a smaller size or lighter weight, significantly improving user experience and device performance.

[0003] The graphite negative electrode material currently widely used in commercial lithium-ion batteries has a theoretical specific capacity of 372mAh / g, and an actual specific capacity close to 360-365mAh / g, which is close to the theoretical limit, thus limiting the further improvement of the battery energy density. Compared with graphite negative electrode materials, the theoretical specific capacity of silicon-based negative electrode materials is much higher than that of graphite, which can be as high as 4200mAh / g, more than 10 times that of graphite materials. However, silicon-based negative electrode materials will undergo a huge volume expansion (over 300%) during the lithium insertion process, resulting in a decrease in the cycle performance of lithium-ion batteries, and the first coulomb efficiency is low. Some lithium ions will form irreversible lithium oxides, thereby reducing the actual available capacity of lithium-ion batteries. At the same time, silicon-based negative electrode materials will produce a large polarization phenomenon due to the repeated rupture and repair of the SEI film.

[0004] Existing technologies disclose methods such as surface modification and electrolyte formulation optimization to optimize the interfacial properties between silicon-based anode materials and electrolytes, thereby improving the capacity and cycle performance of lithium-ion batteries. However, these methods still have some shortcomings: for example, some coating materials cannot effectively suppress the volume expansion of silicon materials, which can easily lead to pulverization of the anode active material; or some electrolyte additives can only improve a certain aspect of performance, making it difficult to achieve a comprehensive improvement in the overall performance of lithium-ion batteries.

[0005] Therefore, it is necessary to provide a silicon-based negative electrode material with good comprehensive performance to solve the above technical problems. Summary of the Invention

[0006] To address the shortcomings of the prior art, the present invention aims to provide a negative electrode active material with a surface modification layer, as well as its preparation method and application. The surface modification layer provided by the present invention not only effectively reduces polarization at the electrode interface but also significantly improves the capacity and energy density of secondary batteries.

[0007] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0008] In a first aspect, the present invention provides a negative electrode active material having a surface modification layer, wherein the negative electrode active material having a surface modification layer comprises negative electrode active particles and a surface modification layer covering the exposed surface of the negative electrode active particles, wherein the surface modification layer comprises a polyamide containing at least two amide groups.

[0009] The present invention utilizes a polyamide with a specific number of amide groups as the surface modification layer material, significantly improving the energy density of secondary batteries and reducing the polarization phenomenon of secondary batteries. On the one hand, the polyamide-modified material not only effectively protects the silicon-based negative electrode material, but the amide groups in its structure also possess a certain degree of electrochemical activity, capable of participating in the charge and discharge reactions of the secondary battery, thereby helping to increase the capacity and energy density of the secondary battery. On the other hand, the amide groups can further improve the charge transfer process of the silicon-based negative electrode material, ultimately reducing the polarization phenomenon of the secondary battery, thereby improving the overall electrochemical performance of the secondary battery.

[0010] In addition, the amide groups in the polyamide structure can form hydrogen bonds with the hydroxyl groups on the surface of the silicon-based negative electrode material, thereby enhancing the bonding force between the surface modification layer and the silicon-based negative electrode material.

[0011] Preferably, the monomers forming the polyamide include an olefin compound containing at least two amide groups.

[0012] Preferably, the molar ratio of the amide group to the alkenyl group in the structure of the olefin compound containing at least two amide groups is (2 to 6):1, preferably (2 to 4):1, for example, it can be 2:1, 3:1, 4:1, 5:1 or 6:1, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0013] The present invention achieves good flexibility and strong binding ability with negative electrode active particles by regulating the molar ratio of amide groups to alkenyl groups in the structure of an olefin compound containing at least two amide groups. Using a lower number of amide groups results in weaker interaction between the polyamide and the surface of the negative electrode active particles; using a higher number of amide groups results in reduced flexibility.

[0014] Preferably, the structure of the olefin compound containing at least two amide groups is as shown in Formula 1:

[0015]

[0016] Wherein, X1 is selected from any one of allyl group and butenyl group, and X2 is selected from any one of isopropylidene group, isobutylidene group and isopentylidene group.

[0017] Preferably, the olefin compound containing at least two amide groups includes allyl isopropyl urea acetate. The present invention further suppresses the volume expansion phenomenon of silicon-based negative electrode materials and improves the interfacial compatibility between the surface modification layer and the negative electrode active particles by preferably using allyl isopropyl urea acetate monomer. Among them, the flexible chain segments of isopropyl and allyl groups can adapt to the volume changes of silicon-based negative electrode materials, thereby alleviating mechanical stress, and the urea groups in the structure can form hydrogen bonds with the hydroxyl groups on the surface of the silicon-based negative electrode material, ultimately increasing the interaction between the two.

[0018] Preferably, the number average molecular weight of the polyamide is 15,000 Da to 40,000 Da, preferably 20,000 Da to 35,000 Da, for example, it can be 15,000 Da, 18,000 Da, 20,000 Da, 22,000 Da, 25,000 Da, 28,000 Da, 30,000 Da, 32,000 Da, 35,000 Da, 38,000 Da or 40,000 Da, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0019] The present invention regulates the number average molecular weight of the polyamide to provide the polyamide with good film-forming properties, processing properties, and interface compatibility. If a polyamide with a lower number average molecular weight is used, the polyamide's film-forming properties and structural stability are both poor, and a continuous and complete surface modification layer cannot be formed on the surface of the silicon-based negative electrode material. This results in insufficient protection of the silicon-based negative electrode material, resulting in a decrease in the cycle stability and rate performance of the secondary battery. If a polyamide with a higher number average molecular weight is used, the viscosity of the polyamide solution increases significantly, making it difficult to disperse evenly during the subsequent coating process. Furthermore, the thickness of the formed surface modification layer is easily uneven, ultimately affecting the consistency of the secondary battery's performance and increasing the internal resistance of the secondary battery, thereby reducing its charge and discharge efficiency.

[0020] Preferably, based on the total mass of the negative electrode active material having a surface modification layer as 100%, the mass percentage of the material of the surface modification layer is 1% to 7%, for example, it can be 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.2%, 4.5%, 4.8%, 5%, 5.2%, 5.5%, 5.8%, 6%, 6.2%, 6.5%, 6.8% or 7%, etc., not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0021] The present invention regulates the mass percentage of the material of the surface modification layer to form a coating layer with suitable thickness and uniform composition, thereby avoiding local current concentration, further optimizing interface stability and ion conductivity, and reducing the occurrence of electrolyte decomposition and side reactions.

[0022] Preferably, the thickness of the surface modification layer is 25nm to 99nm, for example, it can be 25nm, 28nm, 30nm, 32nm, 35nm, 38nm, 40nm, 42nm, 45nm, 48nm, 50nm, 52nm, 55nm, 58nm, 60nm, 62nm, 65nm, 68nm, 70nm, 72nm, 75nm, 78nm, 80nm, 82nm, 85nm, 88nm, 90nm, 95nm or 99nm, etc., but is not limited to the enumerated numerical values, and other unenumerated numerical values ​​within this numerical range are equally applicable.

[0023] The present invention regulates the thickness of the surface modification layer to ensure both good structural stability and high ion conductivity, thereby comprehensively improving the electrochemical performance of the assembled secondary battery. A thinner surface modification layer can easily break, leading to loss of active ions. A thicker surface modification layer can result in poor ion transport performance, and the cycle performance and rate capability of the assembled secondary battery can be reduced.

[0024] Preferably, the negative electrode active particles include silicon-carbon material.

[0025] Preferably, the average particle size of the silicon-carbon material is 5μm to 12μm, for example, it can be 5μm, 5.2μm, 5.5μm, 5.8μm, 6μm, 6.2μm, 6.5μm, 6.8μm, 7μm, 7.2μm, 7.5μm, 7.8μm, 8μm, 8.2μm, 8.5μm, 8.8μm, 9μm, 9.2μm, 9.5μm, 9.8μm, 10μm, 10.5μm, 11μm, 11.5μm or 12μm, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0026] The present invention regulates the average particle size of the silicon-carbon material to facilitate subsequent processing, thereby better balancing the cycle performance and rate performance of the secondary battery. Using a silicon-carbon material with a smaller average particle size may cause the particles to agglomerate; using a silicon-carbon material with a larger average particle size may lengthen the migration path of active ions, thereby degrading the rate performance of the secondary battery.

[0027] Preferably, the specific surface area of ​​the silicon-carbon material is 1m 2 / g~8m 2 / g, for example, 1m 2 / g, 1.2m 2 / g, 1.5m 2 / g, 1.8m 2 / g, 2m2 / g, 2.2m 2 / g, 2.5m 2 / g, 2.8m 2 / g、3m 2 / g, 3.2m 2 / g, 3.5m 2 / g, 3.8m 2 / g、4m 2 / g, 4.2m 2 / g, 4.5m 2 / g, 4.8m 2 / g、5m 2 / g, 5.2m 2 / g, 5.5m 2 / g, 5.8m 2 / g、6m 2 / g, 6.2m 2 / g, 6.5m 2 / g, 6.8m 2 / g、7m 2 / g, 7.2m 2 / g, 7.5m 2 / g, 7.8m 2 / g or 8m 2 / g, etc., are not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0028] Preferably, the mass percentage of silicon material in the silicon-carbon material is 40% to 55%, for example, it can be 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54% or 55%, etc., and is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0029] The present invention achieves both high gram capacity and good cycle performance by regulating the mass percentage of silicon in the silicon-carbon material. Using a silicon-carbon material with a lower silicon content results in a lower gram capacity, thus reducing the energy density of the secondary battery. Using a silicon-carbon material with a higher silicon content results in a volume expansion effect of the silicon material, which can cause the negative electrode material structure to collapse, reducing the service life and stability of the silicon-carbon material.

[0030] In a second aspect, the present invention provides a method for preparing the negative electrode active material having a surface modification layer according to the first aspect, the method comprising the following steps:

[0031] The negative electrode active particles are mixed with a polyamide solution containing at least two amide groups, and the mixture is spray-dried to obtain the negative electrode active material having a surface modification layer.

[0032] Preferably, the mass concentration of the polyamide solution is 5% to 15%, for example, it can be 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14% or 15%, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0033] Preferably, the method for preparing the polyamide in the polyamide solution comprises the following steps:

[0034] An olefin compound monomer containing at least two amide groups, an initiator and an organic solvent are subjected to polymerization reaction to obtain the polyamide.

[0035] Preferably, the polymerization reaction is carried out under an inert atmosphere.

[0036] In the present invention, the inert atmosphere illustratively includes argon and / or nitrogen.

[0037] In the present invention, the initiator illustratively includes at least one of azobisisobutyronitrile, azobisisoheptanenitrile, or dibenzoyl peroxide (BPO). Furthermore, based on the total mass of the monomers as 100%, the mass percentage of the initiator is 0.5% to 1%, for example, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or 1%, etc., and is not limited to the values ​​listed above. Other values ​​not listed within this numerical range are also applicable.

[0038] In the present invention, the organic solvent illustratively includes at least one of benzene, tetrahydrofuran (THF), N-methylpyrrolidone (NMP) or N,N-dimethylformamide (DMF).

[0039] In the present invention, after the polymerization reaction, the following steps are further included: adding the reaction solution after the polymerization reaction into a precipitation solvent to obtain a polyamide precipitate, and then washing and drying the polyamide precipitate to obtain the polyamide.

[0040] In the present invention, the precipitation solvent illustratively includes at least one of propanol, isopropanol or acetone.

[0041] Preferably, the polymerization reaction temperature is 60°C to 100°C, for example, it can be 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, 95°C or 100°C, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0042] Preferably, the polymerization reaction time is 5 h to 12 h, for example, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h or 12 h, etc., and is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0043] Preferably, the mixing temperature is 60°C to 100°C, for example, it can be 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, 95°C or 100°C, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0044] Preferably, the mixing time is 5 h to 10 h, for example, 5 h, 6 h, 7 h, 8 h, 9 h or 10 h, etc., and is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0045] Preferably, the inlet temperature of the spray drying is 100°C to 200°C, for example, 100°C, 120°C, 150°C, 180°C or 200°C, etc., and is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0046] Preferably, the outlet temperature of the spray drying is 70°C to 90°C, for example, 70°C, 75°C, 80°C, 85°C or 90°C, etc., and is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0047] In a third aspect, the present invention provides a negative electrode plate, comprising a negative electrode current collector and a negative electrode active material layer arranged on at least one side of the negative electrode current collector, wherein the negative electrode active material layer comprises the negative electrode active material with a surface modification layer according to the first aspect.

[0048] In a fourth aspect, the present invention provides a secondary battery comprising a positive electrode sheet, a negative electrode sheet, and an electrolyte, wherein the negative electrode sheet is the negative electrode sheet according to the third aspect.

[0049] Preferably, the electrolyte includes a lithium salt additive.

[0050] Preferably, the lithium salt additive includes lithium difluorophosphate (LiPO2F2).

[0051] Preferably, the mass percentage of the lithium salt additive in the electrolyte is 0.2% to 1%, for example, it can be 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9% or 1%, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0052] In the present invention, the lithium salt additive has a synergistic effect with the polyamide modified layer in the negative electrode active particles, thereby further reducing the interface impedance between the negative electrode active particles and the electrolyte.

[0053] The numerical range described in the present invention includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0054] Compared with the prior art, the present invention has the following beneficial effects:

[0055] The present invention provides a negative electrode active material with a surface modification layer. By using a polyamide having a specific number of amide groups as the material for the surface modification layer, the negative electrode active material significantly improves the energy density of the secondary battery and reduces the polarization phenomenon of the secondary battery. On the one hand, the polyamide modified material not only effectively protects the silicon-based negative electrode material, but the amide groups in its structure also have certain electrochemical activity and can participate in the charge and discharge reactions of the secondary battery, thereby helping to increase the capacity and energy density of the secondary battery. On the other hand, the amide groups can further improve the charge transfer process of the silicon-based negative electrode material, ultimately reducing the polarization phenomenon of the secondary battery, thereby improving the overall electrochemical performance of the secondary battery.

[0056] In addition, the amide groups in the polyamide structure can form hydrogen bonds with the hydroxyl groups on the surface of the silicon-based negative electrode material, thereby enhancing the bonding force between the surface modification layer and the silicon-based negative electrode material. DETAILED DESCRIPTION

[0057] The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0058] Example 1

[0059] This embodiment provides a negative electrode active material with a surface modification layer. The negative electrode active material with a surface modification layer includes a silicon-carbon material and a surface modification layer covering the exposed surface of the silicon-carbon material. The material of the surface modification layer includes polyallyl isopropyl urea acetate with a number average molecular weight of 27,500 Da.

[0060] The total mass of the negative electrode active material with the surface modification layer is 100%, the mass percentage of the surface modification layer is 4%, and the thickness of the surface modification layer is 62nm. The average particle size of the silicon-carbon material is 8.5μm, and the specific surface area is 4.5m 2 / g, and the mass percentage of silicon material in the silicon-carbon material is 48%.

[0061] This embodiment provides a method for preparing the above-mentioned negative electrode active material having a surface modification layer, the preparation method comprising the following steps:

[0062] Allyl isopropyl urea acetate was added to tetrahydrofuran solvent, and then azobisisobutyronitrile initiator was added, and the mixture was heated to 80° C. under argon atmosphere for 9 hours to obtain a reaction solution;

[0063] The reaction solution is added to a propanol solvent to precipitate a polymer, and the polymer precipitate is washed and dried to obtain a polymer;

[0064] The polymer was dissolved in tetrahydrofuran to obtain a 10% polymer solution. The silicon-carbon material was added to the polymer solution and stirred at 80°C for 7 hours to obtain a mixed solution. The mixed solution was spray-dried at an inlet temperature of 150°C and an outlet temperature of 80°C to obtain a negative electrode active material with a surface modification layer.

[0065] Example 2

[0066] This embodiment provides a negative electrode active material with a surface modification layer. The negative electrode active material with a surface modification layer includes a silicon-carbon material and a surface modification layer covering the exposed surface of the silicon-carbon material. The material of the surface modification layer includes polyallyl isopropyl urea acetate with a number average molecular weight of 35,000 Da.

[0067] The total mass of the negative electrode active material with the surface modification layer is 100%, the mass percentage of the surface modification layer is 7%, and the thickness of the surface modification layer is 99nm. The average particle size of the silicon-carbon material is 8.5μm, and the specific surface area is 4.5m 2 / g, and the mass percentage of silicon material in the silicon-carbon material is 55%.

[0068] This embodiment provides a method for preparing the above-mentioned negative electrode active material having a surface modification layer, the preparation method comprising the following steps:

[0069] Allyl isopropyl urea acetate was added to tetrahydrofuran solvent, and then azobisisobutyronitrile initiator was added, and the mixture was heated to 90° C. under argon atmosphere for 7 hours to perform polymerization reaction to obtain a reaction solution;

[0070] The reaction solution is added to a propanol solvent to precipitate a polymer, and the polymer precipitate is washed and dried to obtain a polymer;

[0071] The polymer was dissolved in tetrahydrofuran to obtain a polymer solution with a mass concentration of 15%. The silicon-carbon material was added to the polymer solution and stirred at 80°C for 7 hours to obtain a mixed solution. The mixed solution was spray-dried at an inlet temperature of 150°C and an outlet temperature of 80°C to obtain a negative electrode active material with a surface modification layer.

[0072] Example 3

[0073] This embodiment provides a negative electrode active material with a surface modification layer. The negative electrode active material with a surface modification layer includes a silicon-carbon material and a surface modification layer covering the exposed surface of the silicon-carbon material. The material of the surface modification layer includes polyallyl isopropyl urea acetate with a number average molecular weight of 20,000 Da.

[0074] The total mass of the negative electrode active material with the surface modification layer is 100%, the mass percentage of the material of the surface modification layer is 1%, and the thickness of the surface modification layer is 25nm. The average particle size of the silicon-carbon material is 8.5μm, and the specific surface area is 4.5m 2 / g, and the mass percentage of silicon material in the silicon-carbon material is 40%.

[0075] This embodiment provides a method for preparing the above-mentioned negative electrode active material having a surface modification layer, the preparation method comprising the following steps:

[0076] Allyl isopropyl urea acetate was added to tetrahydrofuran solvent, and then azobisisobutyronitrile initiator was added, and the mixture was heated to 70° C. under argon atmosphere for polymerization reaction for 10 hours to obtain a reaction solution;

[0077] The reaction solution is added to a propanol solvent to precipitate a polymer, and the polymer precipitate is washed and dried to obtain a polymer;

[0078] The polymer was dissolved in tetrahydrofuran to obtain a 5% polymer solution. The silicon-carbon material was added to the polymer solution and stirred at 80°C for 7 hours to obtain a mixed solution. The mixed solution was spray-dried at an inlet temperature of 150°C and an outlet temperature of 80°C to obtain a negative electrode active material with a surface modification layer.

[0079] Example 4

[0080] The difference between this embodiment and embodiment 1 is that, based on the total mass of the negative electrode active material with the surface modification layer being 100%, the mass percentage of the material of the surface modification layer is 10%. Other aspects are the same as those of embodiment 1.

[0081] Example 5

[0082] The difference between this embodiment and embodiment 1 is that the number average molecular weight of the polymer is 10,000 Da, wherein the change in the number average molecular weight of the polymer is achieved by adjusting the temperature and time of the polymerization reaction. The rest is the same as embodiment 1.

[0083] Example 6

[0084] The difference between this embodiment and embodiment 1 is that the number average molecular weight of the polymer is 80,000 Da, wherein the change in the number average molecular weight of the polymer is achieved by adjusting the temperature and time of the polymerization reaction. The rest is the same as embodiment 1.

[0085] Example 7

[0086] The difference between this embodiment and embodiment 1 is that the mass percentage of silicon material in the silicon-carbon material is 30%, and the rest is the same as embodiment 1.

[0087] Comparative Example 1

[0088] The difference between this comparative example and Example 1 is that the silicon-carbon material is not subjected to surface modification layer coating treatment, and only one silicon-carbon material is provided, and the average particle size, specific surface area and mass percentage of the silicon material provided are the same as those in Example 1.

[0089] Comparative Example 2

[0090] The difference between this comparative example and Example 1 is that allyl isopropyl urea acetate is replaced with an equal amount of allyl urea monomer (CAS No.: 557-11-9), and the other contents are the same as those in Example 1.

[0091] Comparative Example 3

[0092] The difference between this comparative example and Example 1 is that allyl isopropyl urea acetate is replaced with an equal amount of N-isopropyl acrylamide monomer (CAS No.: 2210-25-5), and the other contents are the same as those in Example 1.

[0093] Application Examples 1-7 and Comparative Application Examples 1-3

[0094] The silicon-carbon negative electrode materials provided in Examples 1 to 7 and Comparative Examples 1 to 3 were used to prepare negative electrode sheets, and then assembled to obtain lithium-ion batteries. The specific preparation method is as follows:

[0095] Preparation of negative electrode sheet:

[0096] The silicon-carbon negative electrode material, conductive carbon black Super-P, single-walled carbon nanotubes (SWCNTs) and polyacrylic acid binder (PAA) provided in the above embodiments and comparative examples were mixed and stirred uniformly with water in a mass ratio of 80:9:1:10 to obtain a negative electrode slurry, and the solid content was controlled to be 30%. The negative electrode slurry was then coated on a copper foil current collector through a coating process, and then vacuum dried and cold pressed to obtain a negative electrode sheet.

[0097] Preparation of positive electrode:

[0098] The ternary cathode material NCM811 (LiNi 0.8 Co 0.1 Mn 0.1 The active material O2, polyvinylidene fluoride binder (PVDF) and conductive carbon black Super-P are mixed and stirred evenly with N-methylpyrrolidone solvent in a mass ratio of 96:2:2 to obtain a positive electrode slurry, and then the positive electrode slurry is coated on an aluminum foil through a coating process, and then dried and cold pressed to obtain a positive electrode sheet.

[0099] Electrolyte:

[0100] Ethylene carbonate, dimethyl carbonate, diethyl carbonate, and fluoroethylene carbonate are mixed in a volume ratio of 20:40:30:10 to prepare an organic solvent. Thoroughly dried lithium salt LiPF6 is then dissolved in the organic solvent to prepare an electrolyte solution having a lithium salt concentration of 1 mol / L. The electrolyte solution also includes a lithium difluorophosphate additive, with the weight percentage of the lithium difluorophosphate additive being 0.6% based on the total weight of the electrolyte being 100%.

[0101] Preparation of lithium-ion batteries:

[0102] The positive electrode sheet, separator (including a polyethylene-based film and a ceramic coating provided on one side of the polyethylene-based film) and negative electrode sheet are stacked in order, with the separator placed between the positive electrode sheet and the negative electrode sheet to play an isolating role, and then wound to obtain a bare battery cell; the bare battery cell is placed in an outer packaging shell, dried, and then injected with electrolyte, and after vacuum packaging, standing, forming and shaping processes, a lithium-ion battery is obtained.

[0103] Comparative Application Example 4

[0104] The difference between this comparative application example and application example 1 is that the lithium difluorophosphate additive is replaced with an equal content of LiPF6 lithium salt, and the rest is the same as application example 1.

[0105] Test conditions

[0106] The lithium-ion batteries provided in Application Examples 1 to Application Examples 7 and Comparative Application Examples 1 to Comparative Application Examples 4 were subjected to performance tests. The rate performance of the batteries was tested under lithium-ion battery test conditions. The tests were conducted on the LAND battery test system of Wuhan Jinnuo Electronics Co., Ltd. at room temperature (25°C). The charge and discharge voltages were limited to 2.5V to 4.2V. The test conditions are as follows:

[0107] (1) Energy density

[0108] At 25°C, the lithium-ion battery was charged to 4.2V at a constant current and constant voltage rate of 0.33C and allowed to stand for 10 minutes. Then, the lithium-ion battery was discharged to 2.5V at a constant current rate of 0.33C and allowed to stand for 10 minutes. The energy density at the 0.33 rate discharge was calculated.

[0109] Energy density at 0.33C rate = energy at 0.33C rate discharge / weight of lithium-ion battery.

[0110] (2) Capacity retention after 1000 cycles at 1C / 2C at room temperature

[0111] At 25°C, the lithium-ion battery is charged to 4.2V at a constant current and constant voltage rate of 1C, with a cut-off current of 0.05C, and allowed to stand for 10 minutes. Then, the lithium-ion battery is discharged to 2.5V at a constant current rate of 2C and allowed to stand for 10 minutes. This is considered one charge and discharge cycle. The lithium-ion battery is charged and discharged 1200 times according to the above method. The capacity retention rate of the lithium-ion battery after 1000 charge and discharge cycles at 1C / 2C is calculated.

[0112] Capacity retention rate (%) of a lithium-ion battery after N cycles = (discharge capacity at the Nth cycle / initial discharge capacity) × 100%, where N is the number of cycles of the lithium-ion battery.

[0113] (3) Room temperature 6C rate performance - constant current charging ratio

[0114] At 25°C, the lithium-ion battery was discharged at a constant current rate of 1C to 2.5V and allowed to stand for 10 minutes. The lithium-ion battery was then charged at a constant current and constant voltage rate of 6C to 4.2V with a cut-off current of 0.05C and allowed to stand for 10 minutes. The constant current charging capacity Q1 and the total constant current and constant voltage charging capacity Q2 of the lithium-ion battery were recorded. The constant current charging ratio of the 6C rate charging was calculated according to the following formula: 6C rate charging constant current charging ratio = (constant current charging capacity Q1 / total constant current and constant voltage charging capacity Q2) × 100%.

[0115] (4) 1C / 8C discharge capacity retention rate at room temperature

[0116] At 25°C, the divided lithium-ion battery is charged to 4.2V at a constant current and constant voltage rate of 1C, with a cut-off current of 0.05C; it is left to stand for 10 minutes; then the lithium-ion battery is discharged to 2.5V at a constant current rate of 1C, and its discharge capacity Q is recorded. 1C As the initial discharge capacity; then at 25 ° C, the lithium ion battery is charged to 4.2V at a constant current and constant voltage rate of 1C, with a cut-off current of 0.05C; let it stand for 10 minutes; then the fully charged lithium ion battery is discharged to 2.5V at a constant current rate of 8C, and its discharge capacity Q is recorded. 8C ; Calculate the discharge capacity retention rate (%) of lithium-ion batteries at 1C / 8C rate = (discharge capacity Q at 8C rate) 8C / Discharge capacity Q at 1C rate 1C )×100%.

[0117] (5) Initial expansion rate of negative electrode

[0118] ① Before assembling the lithium-ion battery, use a micrometer to measure the initial thickness of the negative electrode sheet and record it as h1. The thickness of the negative electrode current collector is recorded as h0.

[0119] ② Fully charged: At 25°C, after assembling the battery cells, charge the lithium-ion battery at a constant current and constant voltage rate of 0.33C to 4.2V, with a cut-off current of 0.05C, and let it stand for 120 minutes;

[0120] ③ Disassemble the fully charged lithium-ion battery to obtain the negative electrode sheet, clean the negative electrode sheet with dimethyl carbonate, and measure the thickness of the cleaned negative electrode sheet, which is recorded as h2;

[0121] ④The initial expansion rate of the negative electrode is: (h2-h1) / (h1-h0)×100%.

[0122] The test results are shown in Table 1:

[0123] Table 1

[0124]

[0125]

[0126] As can be seen from Table 1, compared with Comparative Application Example 1, the specific types of polyamide surface modification layers provided in Application Examples 1 to 3 of the present invention, while being able to effectively suppress the volume expansion phenomenon of the silicon-carbon material and having good bonding ability with the silicon-carbon material, not only improve the energy density of the lithium-ion battery, but also improve the charge transfer process of the silicon-carbon material, ultimately reducing the polarization phenomenon of the lithium-ion battery, thereby improving the overall electrochemical performance of the lithium-ion battery.

[0127] By comparing Application Example 1 with Application Example 4, it can be seen that the present invention achieves a better coating effect by regulating the mass percentage of the material of the surface modification layer, thereby optimizing the interface stability and ion conductivity, and reducing the occurrence of electrolyte decomposition and side reactions.

[0128] Comparing Application Examples 1, 5, and 6 shows that the number-average molecular weight of the polymer significantly influences the film-forming properties, structural stability, and interfacial properties of the resulting surface modification layer, thereby affecting the bonding between the surface modification layer and the silicon-carbon material, as well as the polarization at the electrode interface. Whether using polymers with too low or too high a number-average molecular weight, lithium-ion batteries cannot achieve good cycling and rate performance.

[0129] Comparing Application Example 1 with Application Example 7, it can be seen that the present invention optimizes the mass percentage of silicon material in the silicon-carbon material, thereby further improving the comprehensive performance of the lithium-ion battery while ensuring that the lithium-ion battery has a high energy density.

[0130] Comparing Application Examples 1, 2, and 3, we can see that Example 2 demonstrates that the flexible isopropyl and allyl segments can adapt to the volume changes of the silicon-carbon material, thereby alleviating mechanical stress. Example 3 demonstrates that the number of amide groups significantly influences the interaction between the formed polyamide and the surface of the negative electrode active particles, thereby affecting the energy density, cycle performance, and rate capability of the lithium-ion battery.

[0131] Comparing Application Example 1 with Comparative Application Example 4, it can be seen that the specific type of polyamide provided by the present invention has a synergistic effect with the lithium difluorophosphate additive in the electrolyte, thereby further reducing the interfacial impedance between the negative electrode active particles and the electrolyte, and optimizing the overall performance of the lithium-ion battery.

[0132] The applicant declares that the above is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the scope of protection and disclosure of the present invention.

Claims

1. A negative electrode active material having a surface modification layer, characterized in that: The negative electrode active material with a surface modification layer includes negative electrode active particles and a surface modification layer covering the exposed surface of the negative electrode active particles. The surface modification layer includes a polyamide containing at least two amide groups.

2. The negative electrode active material having a surface modification layer according to claim 1, characterized in that: The monomers forming the polyamide include an olefin compound containing at least two amide groups; Preferably, the molar ratio of the amide group to the alkenyl group in the structure of the olefin compound containing at least two amide groups is (2-6):1, preferably (2-4):1; Preferably, the structure of the olefin compound containing at least two amide groups is as shown in Formula 1: wherein X1 is selected from any one of allyl or butenyl, and X2 is selected from any one of isopropylidene, isobutylidene or isopentylidene; Preferably, the olefin compound containing at least two amide groups comprises allyl isopropyl urea acetate.

3. The negative electrode active material having a surface modification layer according to claim 1 or 2, characterized in that: The number average molecular weight of the polyamide is 15,000 Da to 40,000 Da, preferably 20,000 Da to 35,000 Da.

4. The negative electrode active material having a surface modification layer according to any one of claims 1 to 3, characterized in that: Based on the total mass of the negative electrode active material having the surface modification layer being 100%, the mass percentage of the material of the surface modification layer is 1% to 7%; Preferably, the thickness of the surface modification layer is 25 nm to 99 nm.

5. The negative electrode active material having a surface modification layer according to any one of claims 1 to 4, characterized in that: The negative electrode active particles include silicon-carbon material; Preferably, the average particle size of the silicon-carbon material is 5 μm to 12 μm; Preferably, the specific surface area of ​​the silicon-carbon material is 1m 2 / g~8m 2 / g; Preferably, the mass percentage of silicon material in the silicon-carbon material is 40% to 55%.

6. A method for preparing a negative electrode active material having a surface modification layer according to any one of claims 1 to 5, characterized in that: The method comprises the following steps: The negative electrode active particles are mixed with a polyamide solution containing at least two amide groups, and the mixture is spray-dried to obtain the negative electrode active material having a surface modification layer.

7. The method according to claim 6, characterized in that The mass concentration of the polyamide solution is 5% to 15%; Preferably, the method for preparing the polyamide in the polyamide solution comprises the following steps: Conducting a polymerization reaction of an olefin compound monomer containing at least two amide groups, an initiator and an organic solvent to obtain the polyamide; Preferably, the polymerization reaction is carried out under an inert atmosphere; Preferably, the polymerization reaction temperature is 60°C to 100°C; Preferably, the polymerization reaction time is 5h to 12h; Preferably, the mixing temperature is 60°C to 100°C; Preferably, the mixing time is 5h to 10h; Preferably, the inlet temperature of the spray drying is 100°C to 200°C; Preferably, the outlet temperature of the spray drying is 70°C to 90°C.

8. A negative electrode plate, characterized in that: The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one side of the negative electrode current collector. The negative electrode active material layer includes the negative electrode active material with a surface modification layer according to any one of claims 1 to 5.

9. A secondary battery, characterized in that: The secondary battery includes a positive electrode sheet, a negative electrode sheet, and an electrolyte, and the negative electrode sheet is the negative electrode sheet according to claim 8.

10. The secondary battery according to claim 9, wherein The electrolyte includes a lithium salt additive; Preferably, the lithium salt additive includes lithium difluorophosphate; Preferably, the mass percentage of the lithium salt additive in the electrolyte is 0.2% to 1%.

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