Polymer coating material as well as preparation method and application thereof

Abstract

The invention provides a polymer coating material as well as a preparation method and application thereof. The polymer coating material comprises a heteroatom-containing polymer, the heteroatom-containing polymer comprises an active group, and the active group comprises a combination of a hydroxyl group and a boronic acid pinacol ester group. The polymer coating material provided by the invention is helpful for eliminating HF in an electrolyte, and avoids corrosion and damage of HF to a phase interface film on the surface of an electrode. Meanwhile, the polymer coating material and the silicon-based negative electrode material have good interfacial compatibility and strong binding capacity, and volume expansion of the silicon material can be effectively inhibited, so that the cycling stability of the silicon-based negative electrode material is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery materials, and in particular relates to a polymer coating material and a preparation method and application thereof. Background Art

[0002] With the development and utilization of renewable energy, lithium-ion batteries, owing to their high energy density, long cycle life, and low self-discharge, are considered one of the most important energy storage technologies. With the growing demand for lithium-ion batteries in applications such as new energy vehicles and energy storage, the market is placing higher demands on battery upgrades in terms of high voltage, high specific energy, and high power. Silicon-based anode materials, with their extremely high theoretical specific capacity, have become a key research direction for improving the energy density of lithium-ion batteries.

[0003] However, the repeated large-volume expansion and contraction of silicon-based negative electrode materials during the charge and discharge process will generate large stresses, causing internal cracks and even pulverization. In addition, its inherent low conductivity also affects the high-rate performance of lithium-ion batteries. The traditional SEI film on the surface of silicon-based negative electrode materials has poor stability and is prone to decomposition under high voltages. As a result, it cannot effectively suppress the volume expansion of silicon materials, causing the specific capacity of lithium-ion batteries to rapidly decay. At the same time, the charge transfer impedance of the SEI film increases accordingly under high-rate charge and discharge, which increases the polarization of the negative electrode during the charging process. These factors have severely limited the further application of silicon-based negative electrode materials in high-performance batteries.

[0004] In addition, during the operation of lithium-ion batteries, the water in the electrolyte easily undergoes hydrolysis reaction with lithium salts (such as LiPF6). The generated hydrogen fluoride (HF) will further destroy the stability of the SEI film, causing its structural degradation, thereby affecting the cycle life and safety of the lithium-ion battery.

[0005] Although some existing technologies have been developed to improve the SEI film of silicon-based anode materials, most of them have failed to effectively address all of the above-mentioned issues and are unable to meet the needs of practical applications. Therefore, there is an urgent need in the field to develop an SEI film material suitable for silicon-based anode materials to comprehensively improve the energy density, rate performance, and safety of lithium-ion batteries. Summary of the Invention

[0006] In response to the shortcomings of the prior art, the present invention aims to provide a polymer-coated material, a preparation method thereof, and its application. The polymer-coated material provided by the present invention helps eliminate HF in the electrolyte, preventing HF from corroding and damaging the interface film on the electrode surface. At the same time, the polymer-coated material has good interfacial compatibility and strong binding ability with the silicon-based negative electrode material, which can effectively inhibit the volume expansion of the silicon material, thereby improving the cycle stability of the silicon-based negative electrode material.

[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 polymer coating material, wherein the polymer coating material comprises a polymer containing heteroatoms, wherein the polymer containing heteroatoms comprises an active group, and the active group comprises a combination of a hydroxyl group and a pinacol borate ester group.

[0009] The polymer coating material provided by the present invention can specifically address the technical problems of electrolyte corrosion on silicon-based negative electrode materials, damage to the negative electrode structure caused by repeated volume expansion of silicon materials, and unstable SEI films. It improves the chemical and structural stability of silicon-based negative electrode materials and strengthens the interaction between the SEI film and the silicon-based negative electrode material at the interface, thereby meeting the market's urgent demand for lithium-ion batteries with high energy density, long cycle life, and high safety. Specifically, it is manifested in:

[0010] (1) The pinacol borate groups in the polymer coating material structure can absorb HF in the electrolyte, inhibit the corrosion of HF on the silicon material during the cycle, and reduce the side reactions at the interface between the electrolyte and the silicon material, thereby improving the chemical stability of the silicon-based negative electrode material and thus improving the cycle performance of silicon-based lithium-ion batteries under high voltage.

[0011] (2) The hydroxyl groups contained in the polymer coating material can form hydrogen bonds with the hydroxyl groups on the surface of the silicon-based negative electrode material, making the SEI film more tightly bonded to the silicon-based negative electrode material, thereby improving the structural stability of the silicon-based negative electrode material, thereby effectively solving the technical problem of poor performance of lithium-ion batteries caused by the volume expansion of the silicon-based negative electrode material during the charging and discharging process.

[0012] In summary, the present invention can comprehensively improve the rate performance, cycle performance, energy density and safety of lithium-ion batteries by regulating the composition of the coating material and the types of its active groups.

[0013] Preferably, the molar ratio of the hydroxyl group to the pinacol borate ester group is (1-3):(1-2), preferably 1:(1-2), for example, it can be 1:1, 1:2, 2:1, 3:1 or 3:2, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0014] The present invention further improves the comprehensive performance of the polymer coating material by regulating the molar ratio of the hydroxyl group to the pinacol borate group, thereby reducing the corrosion of HF in the electrolyte on the silicon-based negative electrode material and improving the interface stability. If a lower content of the pinacol borate group is used, an excessive amount of hydroxyl groups (-OH) will be introduced, which will form a dense network through hydrogen bonding, resulting in a significant increase in the rigidity of the coating layer, lacking suitable elasticity, and due to the low content of the pinacol borate group, it is impossible to fully absorb the HF in the electrolyte; if a higher content of the pinacol borate group is used, the hydroxyl content will be less, resulting in fewer hydrogen bonds formed between the polymer coating material and the surface of the silicon-based negative electrode material, which is not conducive to improving the stability of the SEI film. At the same time, a higher content of the pinacol borate group may have other adverse effects, such as the steric hindrance effect of the pinacol borate group inhibiting the close packing between molecules, causing the cross-linking density of the copolymer to decrease, and being unable to limit the disordered expansion of the silicon particles, causing the electrolyte to easily penetrate.

[0015] Preferably, the structure of the monomer forming the polymer containing heteroatoms is as shown in Formula 1:

[0016]

[0017] wherein R1 and R2 are each independently selected from any one of a hydrogen atom, a C1-C3 alkyl group substituted by a hydroxyl group, a C5-C7 cycloalkyl group substituted by a hydroxyl group, or a pinacol borate ester group, and at least one of R1 and R2 is a C1-C3 alkyl group substituted by a hydroxyl group or a C5-C7 cycloalkyl group substituted by a hydroxyl group, and R3 is selected from a hydrogen atom or a substituted or unsubstituted C1-C3 alkyl group.

[0018] In the present invention, the C1-C3 alkyl group exemplarily includes any one of methyl, ethyl, n-propyl or isopropyl.

[0019] In the present invention, the C5-C7 cycloalkyl group includes a cyclohexyl group.

[0020] In the present invention, the substituted group includes any one of a halogen atom, a carboxyl group or an aldehyde group.

[0021] Preferably, the monomers forming the heteroatom-containing polymer include (E)-2-methyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-3-buten-2-ol (CAS No.: 581802-26-8) and / or (Z)-(4-hydroxy-2-buten-2-yl)boronic acid pinacol ester (CAS No.: 1008752-03-1). By optimizing the types of the above-mentioned polymerizable monomers, the present invention enables the pinacol ester group and the hydroxyl group to be evenly distributed in the polymer chain, thereby further improving the overall performance of the polymer coating material.

[0022] Preferably, the number average molecular weight of the polymer containing heteroatoms is 10,000 Da to 60,000 Da, preferably 20,000 Da to 50,000 Da, for example, it can be 10,000 Da, 15,000 Da, 20,000 Da, 25,000 Da, 30,000 Da, 35,000 Da, 40,000 Da, 45,000 Da, 50,000 Da, 55,000 Da or 60,000 Da, etc., but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0023] By regulating the number-average molecular weight of the heteroatom-containing polymer, the present invention balances the film-forming properties, mechanical properties, and lithium-ion transport rate of the polymer coating material, thereby better protecting the silicon-based negative electrode material. Using a heteroatom-containing polymer with a lower number-average molecular weight results in poor film-forming and mechanical properties of the polymer coating material, resulting in poor structural stability of the resulting silicon-based negative electrode material. Using a heteroatom-containing polymer with a higher number-average molecular weight results in impeded lithium-ion transport, ultimately impacting the rate performance of the lithium-ion battery.

[0024] In a second aspect, the present invention provides a method for preparing the polymer coating material according to the first aspect, the method comprising the following steps:

[0025] An unsaturated monomer including a combination of a hydroxyl group and a pinacol borate ester group, an initiator and an organic solvent are mixed, and after a polymerization reaction, the polymer coating material is obtained.

[0026] Preferably, the polymerization reaction is carried out under an inert atmosphere, which exemplarily includes argon and / or nitrogen.

[0027] Preferably, the polymerization reaction temperature is 60°C to 90°C, for example, 60°C, 65°C, 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.

[0028] 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.

[0029] In the present invention, the initiator exemplarily 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 0.3% to 1.5%, for example, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.2%, or 1.5%, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0030] In the present invention, the organic solvent illustratively includes at least one of benzene, tetrahydrofuran (THF), N-methylpyrrolidone (NMP) or N,N-dimethylformamide (DMF).

[0031] 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 polymer precipitate, and then washing and drying the polymer precipitate to obtain the polymer.

[0032] In the present invention, the precipitation solvent illustratively includes at least one of propanol, isopropanol or acetone.

[0033] In a third aspect, the present invention provides a negative electrode material, comprising a negative electrode active material and a polymer coating layer coated on the surface of the negative electrode active material, wherein the material of the polymer coating layer comprises the polymer coating material according to the first aspect.

[0034] Preferably, the negative electrode active material comprises a silicon-carbon material.

[0035] In the present invention, the average particle size of the silicon-carbon material is 3μm to 11μm, for example, it can be 3μm, 3.5μm, 4μm, 4.5μm, 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 or 11μm, etc., not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0036] 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 lithium-ion batteries. 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 lithium ion migration path, thereby degrading the rate performance of the lithium-ion battery.

[0037] In the present invention, 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, 2m 2 / 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.

[0038] In the present invention, the mass percentage of silicon material in the silicon-carbon material is 40% to 52%, for example, it can be 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51% or 52%, etc., and is not limited to the listed values. Other unlisted values ​​within this numerical range are also applicable.

[0039] The present invention achieves both high gram capacity and good cycling 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 lithium-ion battery. Using a silicon-carbon material with a higher silicon content results in a collapse of the negative electrode structure due to the volume expansion of the silicon, thus reducing the service life and stability of the material.

[0040] Preferably, based on the total mass of the negative electrode material as 100%, the mass percentage of the material of the polymer coating layer is 1% to 6%, 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% or 6%, etc., not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0041] The present invention regulates the mass percentage of the material of the polymer coating layer to form a complete and dense coating layer with suitable mechanical strength, thereby further optimizing the interface stability, corrosion resistance and ion conductivity, and reducing the occurrence of electrolyte decomposition and side reactions.

[0042] Preferably, the thickness of the polymer coating layer is 20nm to 90nm, for example, it can be 20nm, 22nm, 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 or 90nm, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0043] The present invention achieves comprehensive improvements in the cycle and rate performance of lithium-ion batteries by regulating the polymer coating to a moderate thickness, ensuring both good structural stability and high ionic conductivity. A thinner polymer coating can easily rupture due to mechanical stress, exposing new active surfaces and further accelerating the formation and consumption of the SEI film. A thicker polymer coating significantly increases the length of the lithium-ion transmission path, increasing the lithium-ion transmission impedance and subsequently reducing the cycle and rate performance of the lithium-ion battery.

[0044] The present invention also provides a method for preparing the negative electrode material, which comprises the following steps:

[0045] The negative electrode material is obtained by mixing the negative electrode active material with a solution containing the polymer coating material according to the first aspect and spray drying the mixture.

[0046] Preferably, the mass concentration of the solution containing the polymer coating material according to the first aspect is 8% to 22%, for example, it can be 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21% or 22%, etc., not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0047] In the present invention, the solution containing the polymer coating material according to the first aspect includes a polymer coating material and a solvent, and the solvent exemplarily includes at least one of benzene, tetrahydrofuran (THF), N-methylpyrrolidone (NMP) or N,N-dimethylformamide (DMF).

[0048] Preferably, the mixing temperature is 60°C to 110°C, for example, it can be 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, 95°C, 100°C or 110°C, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0049] 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.

[0050] Preferably, the inlet temperature of the spray drying is 120°C to 200°C, for example, it can be 120°C, 140°C, 150°C, 160°C, 170°C, 180°C, 190°C or 200°C, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0051] Preferably, the outlet temperature of the spray drying is 60°C to 90°C, for example, it can be 60°C, 65°C, 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 this numerical range are also applicable.

[0052] In a fourth aspect, the present invention provides a secondary battery comprising a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator, wherein the negative electrode sheet comprises a negative electrode active material, and the negative electrode active material comprises the negative electrode material according to the third aspect.

[0053] In the present invention, the electrolyte includes a lithium salt additive and a water and acid removal additive.

[0054] Preferably, the lithium salt additive includes lithium bis(fluorosulfonyl)imide (LiFSI).

[0055] Preferably, the concentration of the lithium salt additive in the electrolyte is 0.2 mol / L to 0.8 mol / L, preferably 0.5 mol / L, for example, it can be 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L or 0.8 mol / L, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0056] In the present invention, the lithium salt additive can promote the dissociation of the lithium salt and has a synergistic effect with the polymer coating material, thereby improving the lithium ion conductivity.

[0057] Preferably, the water and acid removal additive includes a silane compound, and the silane compound exemplarily includes vinyltrimethoxysilane.

[0058] Preferably, the mass percentage of the water-removing and acid-removing additive in the electrolyte is 0.5% to 2%, preferably 1%, for example, it can be 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8% or 2%, etc., and is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0059] In the present invention, the water-removing and acid-removing additive can react with water and HF in the electrolyte to remove impurities, and has a synergistic effect with the polymer coating material, thereby jointly improving the electrochemical performance and safety of the lithium-ion battery.

[0060] 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.

[0061] Compared with the prior art, the present invention has the following beneficial effects:

[0062] The present invention provides a polymer coating material that can specifically address the technical problems of electrolyte corrosion on silicon-based negative electrode materials, damage to the negative electrode structure caused by repeated volume expansion of silicon materials, and instability of the SEI film. It improves the chemical and structural stability of silicon-based negative electrode materials and strengthens the interaction between the SEI film and the silicon-based negative electrode material, thereby meeting the market's urgent demand for lithium-ion batteries with high energy density, long cycle life, and high safety. Specifically, it is manifested in:

[0063] (1) The pinacol borate groups in the polymer coating material structure can absorb HF in the electrolyte, inhibit the corrosion of HF on the silicon material during the cycle, and reduce the side reactions at the interface between the electrolyte and the silicon material, thereby improving the chemical stability of the silicon-based negative electrode material and thus improving the cycle performance of silicon-based lithium-ion batteries under high voltage.

[0064] (2) The hydroxyl groups contained in the polymer coating material can form hydrogen bonds with the hydroxyl groups on the surface of the silicon-based negative electrode material, making the SEI film more tightly bonded to the silicon-based negative electrode material, thereby improving the structural stability of the silicon-based negative electrode material, thereby effectively solving the technical problem of poor performance of lithium-ion batteries caused by the volume expansion of the silicon-based negative electrode material during the charging and discharging process.

[0065] In summary, the present invention can comprehensively improve the rate performance, cycle performance, energy density and safety of lithium-ion batteries by regulating the composition of the coating material and the types of its active groups. DETAILED DESCRIPTION

[0066] 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.

[0067] Example 1

[0068] This embodiment provides a polymer coating material, which is coated on the surface of the silicon-carbon active material to obtain a negative electrode material.

[0069] The polymer coating material is poly (E)-2-methyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl) 3-butene-2-ol with a number average molecular weight of 35,000 Da. The thickness of the polymer coating layer is 55 nm. Based on the total mass of the negative electrode material as 100%, the mass percentage of the polymer coating layer is 3.5%. The average particle size of the silicon-carbon active material is 7 μm, and the specific surface area is 4.5 m 2 / g, and the mass percentage of silicon material in the silicon-carbon active material is 46%.

[0070] This embodiment provides a method for preparing the above-mentioned polymer coating material and the negative electrode material thereof, which comprises the following steps:

[0071] (E)-2-methyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-3-butene-2-ol was added to tetrahydrofuran solvent, and then azobisisobutyronitrile initiator was added, and the mixture was heated to 75° C. under the protection of argon atmosphere to carry out polymerization reaction for 8 hours to obtain a reaction solution;

[0072] 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;

[0073] The polymer was dissolved in tetrahydrofuran to obtain a 15% polymer solution. The silicon-carbon material was added to the polymer solution and stirred at 85°C for 7 hours to obtain a mixed solution. The mixed solution was spray-dried at an inlet temperature of 160°C and an outlet temperature of 75°C to obtain the negative electrode material.

[0074] Example 2

[0075] This embodiment provides a polymer coating material, which is coated on the surface of the silicon-carbon active material to obtain a negative electrode material.

[0076] The polymer coating material is poly (E)-2-methyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl) 3-butene-2-ol with a number average molecular weight of 50,000 Da. The thickness of the polymer coating layer is 80 nm. Based on the total mass of the negative electrode material as 100%, the mass percentage of the polymer coating layer is 5%. The average particle size of the silicon-carbon active material is 7 μm, and the specific surface area is 4.5 m 2 / g, and the mass percentage of silicon material in the silicon-carbon active material is 50%.

[0077] This embodiment provides a method for preparing the above-mentioned polymer coating material and the negative electrode material thereof, which comprises the following steps:

[0078] (E)-2-methyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-3-butene-2-ol was added to tetrahydrofuran solvent, and then azobisisobutyronitrile was added as an initiator. The mixture was heated to 85° C. under an argon atmosphere to carry out a polymerization reaction for 7 hours to obtain a reaction solution.

[0079] 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;

[0080] The polymer was dissolved in tetrahydrofuran to obtain a 20% polymer solution. The silicon-carbon material was added to the polymer solution and stirred at 85°C for 7 hours to obtain a mixed solution. The mixed solution was spray-dried at an inlet temperature of 160°C and an outlet temperature of 75°C to obtain the negative electrode material.

[0081] Example 3

[0082] This embodiment provides a polymer coating material, which is coated on the surface of the silicon-carbon active material to obtain a negative electrode material.

[0083] The polymer coating material is poly (E)-2-methyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl) 3-butene-2-ol with a number average molecular weight of 20,000 Da. The thickness of the polymer coating layer is 25 nm. Based on the total mass of the negative electrode material as 100%, the mass percentage of the polymer coating layer is 2%. The average particle size of the silicon-carbon active material is 7 μm, and the specific surface area is 4.5 m 2 / g, and the mass percentage of silicon material in the silicon-carbon active material is 42%.

[0084] This embodiment provides a method for preparing the above-mentioned polymer coating material and the negative electrode material thereof, which comprises the following steps:

[0085] (E)-2-methyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-3-butene-2-ol was added to tetrahydrofuran solvent, and then azobisisobutyronitrile was added as an initiator. The mixture was heated to 70° C. under an argon atmosphere to carry out a polymerization reaction for 10 hours to obtain a reaction solution.

[0086] 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;

[0087] The polymer was dissolved in tetrahydrofuran to obtain a 12% polymer solution. The silicon-carbon material was added to the polymer solution and stirred at 85°C for 7 hours to obtain a mixed solution. The mixed solution was spray-dried at an inlet temperature of 160°C and an outlet temperature of 75°C to obtain the negative electrode material.

[0088] Example 4

[0089] This embodiment provides a polymer coating material, which is coated on the surface of the silicon-carbon active material to obtain a negative electrode material.

[0090] The polymer coating material is poly (E)-2-methyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl) 3-butene-2-ol with a number average molecular weight of 10,000 Da. The thickness of the polymer coating layer is 20 nm. Based on the total mass of the negative electrode material as 100%, the mass percentage of the polymer coating layer is 1%. The average particle size of the silicon-carbon active material is 3 μm, and the specific surface area is 8 m 2 / g, and the mass percentage of silicon material in the silicon-carbon active material is 40%.

[0091] This embodiment provides a method for preparing the above-mentioned polymer coating material and the negative electrode material thereof, which comprises the following steps:

[0092] (E)-2-methyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-3-butene-2-ol was added to tetrahydrofuran solvent, and then azobisisobutyronitrile was added as an initiator. The mixture was heated to 60° C. under an argon atmosphere to carry out a polymerization reaction for 12 hours to obtain a reaction solution.

[0093] 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;

[0094] The polymer was dissolved in tetrahydrofuran to obtain an 8% polymer solution. The silicon-carbon material was added to the polymer solution and stirred at 85°C for 7 hours to obtain a mixed solution. The mixed solution was spray-dried at an inlet temperature of 165°C and an outlet temperature of 75°C to obtain the negative electrode material.

[0095] Example 5

[0096] This embodiment provides a polymer coating material, which is coated on the surface of the silicon-carbon active material to obtain a negative electrode material.

[0097] The polymer coating material is poly (E)-2-methyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl) 3-butene-2-ol with a number average molecular weight of 60,000 Da. The thickness of the polymer coating layer is 90 nm. Based on the total mass of the negative electrode material as 100%, the mass percentage of the polymer coating layer is 6%. The average particle size of the silicon-carbon active material is 11 μm, and the specific surface area is 1 m 2 / g, and the mass percentage of silicon material in the silicon-carbon active material is 52%.

[0098] This embodiment provides a method for preparing the above-mentioned polymer coating material and the negative electrode material thereof, which comprises the following steps:

[0099] (E)-2-methyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-3-butene-2-ol was added to tetrahydrofuran solvent, and then azobisisobutyronitrile was added as an initiator. The mixture was heated to 90° C. under an argon atmosphere to carry out a polymerization reaction for 5 hours to obtain a reaction solution;

[0100] 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;

[0101] The polymer was dissolved in tetrahydrofuran to obtain a polymer solution with a mass concentration of 22%. The silicon-carbon material was added to the polymer solution and stirred at 85°C for 7 hours to obtain a mixed solution. The mixed solution was spray-dried at an inlet temperature of 165°C and an outlet temperature of 75°C to obtain the negative electrode material.

[0102] Example 6

[0103] The difference between this embodiment and Example 1 is that the (E)-2-methyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)3-butene-2-ol monomer is replaced with an equal amount of (Z)-(4-hydroxy-2-butene-2-yl)boronic acid pinacol ester monomer, and the rest is the same as Example 1.

[0104] Example 7

[0105] The difference between this embodiment and Example 1 is that the (E)-2-methyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)3-butene-2-ol monomer is replaced with an equal amount of (E)-1-(1-hydroxycyclohexyl)ethylene-1,2-diboronic acid di(pinacol) ester monomer (CAS No.: 264144-69-6), and the rest is the same as Example 1.

[0106] Example 8

[0107] The difference between this embodiment and Example 1 is that the (E)-2-methyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)3-butene-2-ol monomer is replaced with an equal amount of 4-hydroxycyclohexene-1-boronic acid pinacol ester monomer (CAS No.: 1310384-24-7), and the rest is the same as Example 1.

[0108] Example 9

[0109] The difference between this embodiment and embodiment 1 is that the number average molecular weight of the polymer is 5000 Da, wherein the change of 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.

[0110] Example 10

[0111] The difference between this embodiment and embodiment 1 is that the number average molecular weight of the polymer is 90,000 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.

[0112] Comparative Example 1

[0113] The difference between this comparative example and Example 1 is that the silicon-carbon material is not subjected to polymer 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.

[0114] Comparative Example 2

[0115] The difference between this comparative example and Example 1 is that the (E)-2-methyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)3-butene-2-ol monomer is replaced with an equal amount of trans-2-phenylethyleneboronic acid pinacol ester monomer (CAS No.: 83947-56-2), and the rest are the same as Example 1.

[0116] Comparative Example 3

[0117] The difference between this comparative example and Example 1 is that the (E)-2-methyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)3-butene-2-ol monomer is replaced with an equal amount of vinylboronic acid pinacol ester monomer (CAS No.: 75927-49-0), and the rest is the same as Example 1.

[0118] Application Examples 1-10 and Comparative Application Examples 1-3

[0119] The silicon-carbon negative electrode materials provided in Examples 1 to 10 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:

[0120] Preparation of negative electrode sheet:

[0121] The silicon-carbon negative electrode material provided in the above examples and comparative examples, conductive carbon black Super-P, single-walled carbon nanotubes (SWCNTs) and polyacrylic acid binder (PAA) 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.

[0122] Preparation of positive electrode:

[0123] 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. The positive electrode slurry is then coated on an aluminum foil through a coating process, and then dried and cold pressed to obtain a positive electrode sheet.

[0124] Electrolyte:

[0125] Ethylene carbonate, dimethyl carbonate, diethyl carbonate, and fluoroethylene carbonate are mixed in a volume ratio of 20:40:30:10 to obtain an organic solvent. The fully dried lithium salt LiPF6 is then dissolved in the organic solvent to prepare an electrolyte with a lithium salt concentration of 1 mol / L. In addition, the electrolyte also includes a LiFSI additive and a vinyltrimethoxysilane additive. The concentration of the LiFSI additive in the electrolyte is 0.5 mol / L, and the mass percentage of the vinyltrimethoxysilane additive is 1% based on the total mass of the electrolyte as 100%.

[0126] Preparation of lithium-ion batteries:

[0127] 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.

[0128] Comparative Application Example 4

[0129] The difference between this comparative application example and application example 1 is that the LiFSI additive is replaced with LiPF6 lithium salt of equal concentration, and the rest is the same as application example 1.

[0130] Comparative Application Example 5

[0131] The difference between this comparative application example and application example 1 is that no vinyltrimethoxysilane additive is added, and the content of the organic solvent is adaptively adjusted so that the total content of the electrolyte system is 100%. The rest is the same as application example 1.

[0132] Test conditions

[0133] The lithium-ion batteries provided in Application Examples 1 to 10 and Comparative Application Examples 1 to 5 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:

[0134] (1) First Coulombic efficiency

[0135] 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 first coulombic efficiency of the lithium-ion battery was calculated.

[0136] First coulombic efficiency (%)=(first discharge total capacity of the lithium-ion battery at a rate of 0.33C / first charge total capacity of the lithium-ion battery at a rate of 0.33C)×100%.

[0137] (2) Capacity retention after 1000 cycles at 1C / 2C at room temperature

[0138] 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.

[0139] 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.

[0140] (3) Room temperature 6C rate performance - constant current charging ratio

[0141] 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%.

[0142] (4) 1C / 8C discharge capacity retention rate at room temperature

[0143] 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%.

[0144] (5) Initial expansion rate of negative electrode

[0145] 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.

[0146] 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;

[0147] 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;

[0148] ④The initial expansion rate of the negative electrode sheet is: (h2-h1) / (h1-h0)×100%.

[0149] The test results are shown in Table 1:

[0150] Table 1

[0151]

[0152]

[0153] As can be seen from Table 1, compared with Comparative Application Example 1, the specific types of polymer coating materials provided in Application Examples 1 to 7 of the present invention can not only eliminate HF in the electrolyte and avoid HF corrosion and damage to the interface film on the electrode surface, but also have good interfacial compatibility and strong binding ability with the silicon-carbon negative electrode material, thereby effectively inhibiting the volume expansion of the silicon material, and ultimately improving the cycle performance, rate performance and safety of the lithium-ion battery.

[0154] Comparing Application Example 1 with Application Example 8, it can be seen that the present invention optimizes the types of polymerizable monomers so that the boric acid pinacol ester groups and hydroxyl groups can be evenly distributed in the polymer chain, thereby being more conducive to improving the overall performance of the polymer coating material.

[0155] By comparing Application Example 1, Application Example 9 and Application Example 10, it can be seen that the present invention, by regulating the number average molecular weight of the polymer to an appropriate range, enables the lithium-ion battery to have better rate performance and cycle life while better protecting the silicon-carbon negative electrode material.

[0156] From Comparative Application Examples 1, 2 and 3, it can be seen that the combination of the pinacol borate group and the hydroxyl group has a better coating effect. If only a polymer prepared using a monomer containing a certain group is used, it is impossible to achieve a comprehensive improvement in the overall performance of the lithium-ion battery, indicating that there is a synergistic effect between the pinacol borate group and the hydroxyl group in the polymer structure.

[0157] Comparing Application Examples 1, 4, and 5, we can see that Comparative Example 4 demonstrates that lithium bis(fluorosulfonyl)imide can promote the dissociation of lithium salts and has a synergistic effect with the polymer coating material, thereby improving lithium ion conductivity. Comparative Example 5 demonstrates that the vinyltrimethoxysilane additive can react with water and HF in the electrolyte to remove impurities and has a synergistic effect with the polymer coating material, thereby improving the electrochemical performance and safety of lithium-ion batteries.

[0158] 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 polymer coating material, characterized in that: The polymer coating material includes a polymer containing heteroatoms, wherein the polymer containing heteroatoms comprises an active group, and the active group includes a combination of a hydroxyl group and a pinacol borate ester group.

2. The polymer coating material according to claim 1, characterized in that The molar ratio of the hydroxyl group to the pinacol borate ester group is (1-3):(1-2), preferably 1:(1-2).

3. The polymer coating material according to claim 1 or 2, characterized in that: The structure of the monomer forming the polymer containing heteroatoms is shown in Formula 1: wherein R1 and R2 are each independently selected from any one of a hydrogen atom, a C1-C3 alkyl group substituted by a hydroxyl group, a C5-C7 cycloalkyl group substituted by a hydroxyl group, or a pinacol borate ester group, and at least one of R1 and R2 is a C1-C3 alkyl group substituted by a hydroxyl group or a C5-C7 cycloalkyl group substituted by a hydroxyl group, and R3 is selected from a hydrogen atom or a substituted or unsubstituted C1-C3 alkyl group.

4. The polymer coating material according to claim 3, characterized in that The monomers forming the polymer containing heteroatoms include (E)-2-methyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)3-buten-2-ol and / or (Z)-(4-hydroxy-2-buten-2-yl)boronic acid pinacol ester.

5. The polymer coating material according to any one of claims 1 to 4, characterized in that The number average molecular weight of the polymer containing heteroatoms is 10,000 Da to 60,000 Da, preferably 20,000 Da to 50,000 Da.

6. A method for preparing the polymer coating material according to any one of claims 1 to 5, characterized in that: The method comprises the following steps: An unsaturated monomer including a combination of a hydroxyl group and a pinacol borate ester group, an initiator and an organic solvent are mixed, and after a polymerization reaction, the polymer coating material is obtained.

7. The method according to claim 6, characterized in that The polymerization reaction is carried out under an inert atmosphere; Preferably, the polymerization reaction temperature is 60°C to 90°C; Preferably, the polymerization reaction time is 5 h to 12 h.

8. A negative electrode material, characterized in that The negative electrode material includes a negative electrode active material and a polymer coating layer coated on the surface of the negative electrode active material, and the material of the polymer coating layer includes the polymer coating material according to any one of claims 1 to 5.

9. The negative electrode material according to claim 8, characterized in that The negative electrode active material includes a silicon-carbon material; Preferably, based on the total mass of the negative electrode material being 100%, the mass percentage of the material of the polymer coating layer is 1% to 6%; Preferably, the thickness of the polymer coating layer is 20 nm to 90 nm.

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

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