Modified EVA (Ethylene Vinyl Acetate) and preparation method and application thereof

Abstract

The invention discloses modified EVA (Ethylene Vinyl Acetate) which has a structure as shown in a formula 1. The modified EVA has ionic boric acid ester dynamic crosslinking bonds and ionic conductivity, the conductivity can be improved due to electron deficiency of the boron element, meanwhile, the modified EVA is used for coating materials, the use amount is small, and the risk that the battery performance is affected is avoided, so that the process of additionally adding a conductive agent is avoided while the silicon volume change is controlled, and the service life of the battery is prolonged. The manufacturing cost of the battery is greatly reduced; meanwhile, conditions are mild, the method is simple, and industrial production is hopeful.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion batteries, and particularly relates to a modified EVA, a preparation method thereof, and an application thereof. Background Art

[0002] Lithium-ion batteries have received extensive attention and research from all sectors of society due to their rechargeability, high energy density, environmental friendliness, etc. Traditional carbon materials used as electrodes have characteristics such as good electrical conductivity, stable chemical properties, and low theoretical specific capacity (372 mAh / g). New silicon anode materials have characteristics such as severe volume expansion effect, high theoretical specific capacity (4200 mAh / g), poor electrical conductivity, and environmental friendliness. When used alone, neither of them can meet the power requirements of modern devices. Combining the advantages of both, silicon / carbon composite anode materials have emerged. However, in practical applications, although the silicon element in the composite material can greatly improve the specific capacity of the battery, the volume effect causes problems such as pulverization of active substances and poor electrical conductivity, resulting in problems such as rapid decline of battery capacity and poor rate performance, which are always difficult to solve.

[0003] To solve the problem of the volume expansion effect of silicon element, researchers use polyacrylic acid (PAA) containing more carboxyl groups in the structure as the binder for silicon-carbon composite materials. Because carboxyl groups will form hydrogen bonds with the hydroxyl groups on the silicon surface and the current collector surface and bind tightly, which can alleviate the pulverization of active substances. However, PAA has strong rigidity, and during the deformation of silicon, its long chain is easily broken, the bonding points fail, and finally the active substances will still pulverize and fall off. Alexander A. Pavlovskii believes that almost all carbonyl compounds are N-type electrode materials, which are reduced during the charge and discharge process, and the C=O functional group forms a negatively charged anion to carry out a reversible nucleophilic addition reaction with lithium ions to achieve the purpose of transporting lithium ions and electrons. At the same time, the structure is controllable and is expected to replace inorganic anode materials. However, such polymers often have poor electrical conductivity and low energy density. Han Weiqiang believes that ethylene-vinyl acetate copolymer (EVA) containing C=O has the advantages of adjustable structure and easy penetration of electrolytes. By adjusting the addition amounts of ethylene and vinyl acetate, EVA with strong elasticity and good mechanical properties can be obtained, and PAA and EVA are proportioned to form a composite binder. The use of this binder significantly improves the electrochemical performance of the lithium-ion battery anode (the anode material is silicon element). However, due to the large amount of EVA used and poor electrical conductivity, the addition amount of conductive carbon black increases, which undoubtedly increases the manufacturing cost and the volume of the battery.

[0004] Therefore, developing a modified EVA for use in anode materials to reduce the use of conductive agents and lower costs is an urgent problem to be solved by those skilled in the art. Summary of the Invention

[0005] In view of this, the present application provides a modified EVA having borate dynamic crosslinking bonds, which is applied to the anode material of a battery, can reduce the volume expansion of silicon, and enhance the conductivity effect on the basis of reducing the use of conductive agents.

[0006] The present application provides a modified EVA having the structure of Formula 1:

[0007]

[0008] Wherein, R1 and R2 are each independently hydrogen, an alkyl group with 1 to 10 carbon atoms, an ester group with 2 to 10 carbon atoms, an ether group with 2 to 10 carbon atoms, or a carbonyl group with 2 to 10 carbon atoms, and X and Y are each independently integers from 1 to 20. In some specific implementation manners, when R1 and R2 are each independently an ester group, R1 and R2 are each independently of the structure -COOC-, with its left side connected to the EVA molecular structure and its right side being a straight-chain or branched-chain alkane. In some specific implementation manners, when R1 and R2 are each independently an ether group, R1 and R2 are each independently of the structure -C-O-C-, with its left side connected to the EVA molecular structure and its right side being a straight-chain or branched-chain alkane.

[0009] In some specific implementation manners, when R1 and R2 are each independently a carbonyl group, R1 and R2 are each independently the structure, with its left side connected to the EVA molecular structure and its right side being a straight-chain or branched-chain alkane.

[0010] In some specific implementation manners, R1 and R2 are each independently hydrogen, an alkyl group with 1 to 5 carbon atoms, an ester group with 2 to 5 carbon atoms, an ether group with 2 to 5 carbon atoms, or a carbonyl group with 2 to 5 carbon atoms, and X and Y are each independently integers from 1 to 10.

[0011] In some specific implementation manners, the modified EVA has any one of the structures of Formula 2 to 4;

[0012]

[0013]

[0014] The modified EVA has ionic borate dynamic crosslinking bonds, has ionic conductivity, and the boron element itself is electron-deficient, which can cause an increase in conductivity. At the same time, the modified EVA is used as a coating material, with a small dosage, and there is no risk of affecting the battery performance. In this way, while controlling the volume change of silicon, the process of adding extra conductive agents is eliminated, greatly reducing the battery manufacturing cost.

[0015] The present application also provides a preparation method of the modified EVA, including:

[0016] The amination reaction is carried out between the EVA of formula 5 structure and the amino compound of formula 6 structure to obtain the compound shown in formula 7;

[0017]

[0018] The borate esterification is carried out between the compound shown in formula 7 and boric acid to obtain modified EVA.

[0019] Wherein, R1 and R2 are each independently hydrogen, an alkyl group with 1 to 10 carbon atoms, an ester group with 2 to 10 carbon atoms, an ether group with 2 to 10 carbon atoms, a carbonyl group with 2 to 10 carbon atoms or a phenyl group with 6 to 20 carbon atoms, and X and Y are each independently an integer from 1 to 20.

[0020] In some specific implementation manners, R1 and R2 are each independently hydrogen, an alkyl group with 1 to 5 carbon atoms, an ester group with 2 to 5 carbon atoms, an ether group with 2 to 5 carbon atoms, a carbonyl group with 2 to 5 carbon atoms or a phenyl group with 6 to 10 carbon atoms, and X and Y are each independently an integer from 1 to 10.

[0021] In some specific implementation manners, the amino compound has the structures of formula 8 - formula 10:

[0022]

[0023] In this application, EVA is first subjected to an amination reaction with an amino compound to obtain a compound of formula 7. In some specific implementation manners, the amination reaction is carried out in the presence of a solvent, and the solvent includes but is not limited to one or more of methanol, ethanol, n-propanol or isopropanol. This application has no special requirements for the selection of the solvent. In some specific implementation manners, the molar fraction of ethylene in the EVA is 20% to 60%. In some specific implementation manners, EVA:solvent (m:V) is (1 - 50):1, where V represents volume in mL and m represents mass in g. In some specific implementation manners, amino compound:solvent (m:V) is (0.1 - 5):1. In some specific implementation manners, the temperature of the amination reaction is 30°C to 80°C. In some specific implementation manners, the time of the amination is 1 h to 12 h. In some specific implementation manners, vacuum drying is also included after the amination reaction. The temperature of the vacuum drying is 30°C to 80°C, and the time of the vacuum drying is 1 h to 24 h. In some specific implementation manners, the preparation method of the EVA includes polymerizing vinyl acetate (Vac), ethylene, an initiator and a solvent to obtain EVA. In some specific implementation manners, the inhibitor in VAc is removed by vacuum distillation and stored in a sealed manner at 10°C; the initiator is purified by recrystallization, vacuum dried, protected from light and stored in a sealed manner at 10°C, and the solvent needs to be dehydrated by molecular sieve. In some specific implementation manners, the initiator is an azo-based, peroxide-based compound, organic or inorganic, including but not limited to one or more of azobisisobutyronitrile (AIBN), azobisisoheptonitrile (ABVN), benzoyl peroxide (BPO), diethylhexyl peroxydicarbonate (EHP), potassium persulfate, ammonium persulfate or hydroperoxide. This application has no special requirements for the selection of the initiator. The solvent includes but is not limited to one or more of diethyl ether, methanol, ethanol, tert-butanol, dimethyl carbonate (DMC), tetrahydrofuran, petroleum ether, acetone or hexane. This application has no special requirements for the selection of the solvent. In some specific implementation manners, VAc:initiator:solvent (V:m:V) is 1 to 100:1 to 100:1 to 100, where V represents volume in mL and m represents mass in g. In some specific implementation manners, the pressure of the ethylene is 0.01 to 10 MPa; the temperature of the polymerization reaction is 60 to 100°C; the time of the polymerization reaction is 0.5 to 10 h. The ethylene, VAc, solvent and initiator involved are all commercially available.

[0024] The present application then performs a borylation reaction on the compound shown in Formula 7 with boric acid to obtain modified EVA. In some specific implementation manners, the amination reaction is carried out in the presence of a solvent, and the solvent includes, but is not limited to, one or more of methanol, ethanol, n-propanol, or isopropanol. The present application has no special requirements for the selection of the solvent. In some specific implementation manners, the molar ratio of the compound of Formula 7 structure to boric acid is (1.5 - 2.5):1. In some specific implementation manners, the temperature of the borylation reaction is 30°C to 80°C, and the time of the borylation is 1 h to 12 h. In some specific implementation manners, vacuum drying is further included after the borylation reaction, the temperature of the vacuum drying is 30°C to 80°C, and the time of the vacuum drying is 1 h to 24 h.

[0025] The present application also provides a modified silicon, including silicon and modified EVA coated on the surface of the silicon; the modified EVA is the above-mentioned modified EVA or the modified EVA prepared by the above-mentioned preparation method. In some specific implementation manners, the particle size of the modified silicon is 20 μm to 100 μm, and it can be 20 μm, 22 μm, 24 μm, 26 μm, 28 μm, 30 μm, 32 μm, 34 μm, 36 μm, 38 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm, 80 μm, 85 μm, 90 μm, 95 μm, 96 μm, 98 μm, 100 μm. In order to solve the problems of severe volume expansion effect and poor conductivity of elemental silicon, modified EVA containing amine compounds and boric acid respectively crosslinked with thermoplastic modified EVA to obtain boric acid ester dynamic crosslinking bonds has ionic conductivity, and the boron element itself lacks electrons, which can cause an increase in conductivity. At the same time, the modified EVA is used as a coating material with a small amount, and there is no risk of affecting the battery performance. In this way, while controlling the volume change of silicon, the process of additionally adding a conductive agent is eliminated, greatly reducing the battery manufacturing cost.

[0026] The present application also provides a preparation method of a modified silicon, including: mixing and curing silicon with modified EVA to obtain modified silicon.

[0027] In some specific implementation manners, the modified EVA is dissolved in an alcohol solvent, silicon is added, and the mixture is transferred to a kneader and stirred evenly, followed by curing and ball milling to obtain modified silicon. In some specific implementation manners, the ratio of the modified EVA to the alcohol solvent (m:V) is (1 - 50):1. In some specific implementation manners, the particle size of the silicon is 1 - 100 μm. In some specific implementation manners, the mass ratio of the modified EVA to the silicon is 1:(1 - 100). In some specific implementation manners, the stirring time of the kneader is 1 - 12 h, and the rotation speed of the kneader is 100 - 800 r / min. In some specific implementation manners, the rotation speed of the ball milling is 200 - 1000 r / min, and the time of the ball milling is 1 - 12 h. In some specific implementation manners, the temperature of the curing is 25 - 80 °C, and the time of the curing is 1 - 12 h.

[0028] This application also provides a negative electrode material, including the above-mentioned modified silicon or the modified silicon prepared by the above-mentioned preparation method.

[0029] This application also provides a battery, including the above-mentioned negative electrode material.

[0030] In some specific implementation manners, the preparation method of the battery includes: mixing the modified silicon, graphite, acetylene black, PAA, and sodium carboxymethyl cellulose, coating the mixture on the surface of a copper foil, and performing vacuum drying to form a negative electrode material. In some specific implementation manners, the time of the vacuum drying is 10 h to 20 h, and the temperature of the vacuum drying is 80 °C to 120 °C. Then, a button CR2025 half-cell is assembled. In some specific implementation manners, the mass ratio of the modified silicon, graphite, acetylene black, PAA, and sodium carboxymethyl cellulose is 1:(5 - 7):(0.5 - 2):(0.5 - 2):(0.5 - 2), preferably 1:(5.5 - 6.5):(0.8 - 1.2):(0.8 - 1.2):(0.8 - 1.2).

[0031] In this application, an amine compound and boric acid are respectively cross-linked with thermoplastic modified EVA to obtain modified EVA with ionic borate dynamic cross-linking bonds. This modified EVA has ionic conductivity, and the boron element itself lacks electrons, which can cause an increase in conductivity. At the same time, the modified EVA is used as a coating material with a small amount, and there is no risk of affecting the battery performance. In this way, while controlling the volume change of silicon, the process of adding an additional conductive agent is eliminated, greatly reducing the battery manufacturing cost; at the same time, the conditions are mild and the method is simple, showing promise for industrial production. Description of the Drawings

[0032] Figure 1 It is the infrared spectrum diagram of the modified EVA provided in Embodiments 1 - 3 of this application. Detailed Embodiments

[0033] It should be understood that the expression "one or more of..." individually includes each of the objects recited after the said expression and various different combinations of two or more of the recited objects, unless otherwise understood from the context and usage. The expression "and / or" in combination with three or more recited objects should be understood to have the same meaning, unless otherwise understood from the context.

[0034] The terms "comprising", "having" or "including", including the use of their grammatical synonyms, should generally be understood as open-ended and non-restrictive, for example, not excluding other unrecited elements or steps, unless specifically stated otherwise or otherwise understood from the context.

[0035] It should be understood that as long as this application remains operable, the order of steps or the order of performing certain actions is not important. In addition, two or more steps or actions can be carried out simultaneously.

[0036] The use of any and all examples or exemplary language such as "for example" or "including" in this document is merely intended to better illustrate this application and does not limit the scope of this application unless a claim is made. No language in this specification should be construed as indicating that any unclaimed element is essential for the practice of this application.

[0037] In addition, the numerical ranges and parameters used to define this application are approximate values. The relevant values in the specific embodiments have been presented as precisely as possible herein. However, any value inherently and inevitably contains standard deviations due to individual testing methods. Therefore, unless otherwise clearly stated, it should be understood that all ranges, quantities, values and percentages used in this disclosure are modified by "about". Here, "about" generally means that the actual value is within plus or minus 10%, 5%, 1% or 0.5% of a specific value or range.

[0038] This application provides a modified EVA having the structure of Formula 1:

[0039]

[0040] Wherein, R1 and R2 are each independently hydrogen, an alkyl group with 1 to 10 carbon atoms, an ester group with 2 to 10 carbon atoms, an ether group with 2 to 10 carbon atoms, a carbonyl group with 2 to 10 carbon atoms or a phenyl group with 6 to 20 carbon atoms, and X and Y are each independently an integer from 1 to 20.

[0041] In this application, an amine compound and boric acid are respectively crosslinked with thermoplastic modified EVA to obtain modified EVA with ionic borate dynamic crosslinking bonds. This modified EVA has ionic conductivity, and the boron element itself is electron-deficient, which can cause an increase in conductivity. At the same time, the modified EVA is used as a coating material with a small amount, and there is no risk of affecting battery performance. In this way, while controlling the volume change of silicon, the process of adding an additional conductive agent is eliminated, greatly reducing the battery manufacturing cost; at the same time, the conditions are mild and the method is simple, which is expected to be industrially produced.

[0042] The following further elaborates on this application in combination with embodiments. The protection scope of this application is not limited by the following embodiments.

[0043] Example 1

[0044] This example provides a modified EVA, and the preparation method of the modified EVA includes:

[0045]

[0046] Under the protection of high-purity N2, VAc, AIBN, and anhydrous methanol are mixed evenly according to the ratio of 30:10:50 (V:m:V). The clarified solution is transferred into a reaction kettle with a polytetrafluoroethylene lining, and ethylene at 2 MPa is filled. After the pressure is stable, the circulating pump is turned on for heating, the polymerization temperature is 60 °C, and after polymerization for 2 h, the material is taken out while it is hot and vacuum-dried at 60 °C for 24 h to remove small molecules, including solvents and unreacted VAC, to obtain transparent modified EVA for retention and testing.

[0047] 5 g of modified EVA and 50 mL of ethanol are stirred and mixed evenly. 1 g of dihydroxyethylmethylamine (shown in Formula 8) and 5 mL of ethanol are stirred and mixed evenly. Then the two solutions are uniformly mixed and stirred at 40 °C for 5 h. The mixed solution is transferred to n-hexane for precipitation, and the solvent and n-hexane are removed by rotary evaporation and vacuum-dried at 60 °C for 12 h to obtain slightly yellow transparent aminated modified EVA for retention.

[0048] 3 g of aminated modified EVA is added to 30 mL of ethanol under stirring to obtain a clarified solution. 1 g of boric acid is added to 5 mL of ethanol under stirring to obtain a clarified solution. Then the two solutions are uniformly mixed and stirred at 50 °C for 5 h. The mixed solution is transferred to n-hexane for precipitation, and the solvent and n-hexane are removed by rotary evaporation and vacuum-dried at 60 °C for 12 h to obtain slightly yellow transparent modified EVA for retention. The modified EVA (Structure of Formula 2) provided in this example is subjected to infrared testing, and the obtained infrared spectrum is as Figure 1 shown.

[0049] This example also provides micron-sized silicon coated with modified EVA, and the preparation method of the micron-sized silicon coated with modified EVA includes:

[0050] Dissolve 1 g of boric acid esterified modified EVA in ethanol. After stirring evenly, slowly add 100 g of micron-sized silicon with an average particle size of 20 μm to this solution, and continue stirring. Then transfer this slightly viscous turbid liquid to a kneader and stir rapidly at 60 °C for 5 h to obtain micron-sized silicon doped with EVA. Place this micron-sized silicon in an oven for curing. The curing temperature is 60 °C and the curing time is 2 h. Let it cool naturally to room temperature to shape the EVA copolymer coated on the surface of the micron-sized silicon. Crush it through a ball mill to obtain micron-sized silicon with an average particle size of 30 μm. The ball milling time is 2 h and the ball milling speed is 500 r / min to obtain micron-sized silicon coated with modified EVA.

[0051] This example also provides a lithium-ion battery. The preparation method of the lithium-ion battery includes:

[0052] Weigh accurately the masses of modified EVA-coated micron-sized silicon / graphite / acetylene black / PAA / sodium carboxymethyl cellulose (CMC) according to the ratio of 1:6:1:1:1 (m / m / m / m / m), and mix them. Dropwise add deionized water into a small weighing bottle until an appropriate amount is added to make the solution have a moderate viscosity. Place this solution on a magnetic stirrer and stir for more than 5 h to make it uniform. Evenly coat the electrode mixture on the cut copper foil with a flat coater. Before coating, wipe both sides of the copper foil with ethanol to prevent dirt. After coating, write down the number and pre-dry it under an ultraviolet lamp. After a few minutes, transfer it to a vacuum drying oven and dry it at 100 °C for 12 h. Cut the dried copper foil coated with the mixture into round thin slices with a diameter of 1.2 cm using a slicing machine, and use it as the negative electrode material of LIBs. Then, cut the blank copper foil and the electrode-coated slices respectively with a cutting machine, slice them, accurately weigh the mass, number them, record them, and calculate the mass of the active substance contained in each slice. For the parameter setting when measuring the electrochemical performance later, finally place them in a glove box for use in assembling the battery, and assemble the lithium-ion battery in a glove box filled with high-purity argon. Compress the assembled lithium-ion battery with a button cell sealer, let it stand for several hours, and then measure its electrochemical properties on a charge-discharge instrument. The data is shown in Table 1.

[0053] Example 2

[0054] This example provides a modified EVA. The preparation method of the modified EVA includes:

[0055]

[0056] Under the protection of high-purity N2, VAc, AIBN, and anhydrous methanol were mixed evenly according to the ratio of 30:10:50 (V:m:V). The clarified solution was transferred into a reaction kettle with a polytetrafluoroethylene lining, and ethylene at 2 MPa was charged. After the pressure was stabilized, the circulation pump was turned on for heating. The polymerization temperature was 60 °C. After 2 h of polymerization, the material was taken out while it was hot and dried in vacuum at 60 °C for 24 h to remove small molecules, obtaining transparent modified EVA for retention and testing.

[0057] 5 g of modified EVA and 50 mL of ethanol were stirred and mixed evenly. 1 g of 1-(methylamino)propane-2,2-diol (shown in Structure 9) and 5 mL of ethanol were stirred and mixed evenly. Then the two solutions were evenly mixed and stirred at 40 °C for 5 h. The mixed solution was transferred into n-hexane for precipitation. The solvent and n-hexane were removed by rotary evaporation, and dried in vacuum at 60 °C for 12 h to obtain slightly yellow transparent aminated modified EVA for retention.

[0058] 3 g of aminated modified EVA was added to 30 mL of ethanol under stirring to obtain a clarified solution. 1 g of boric acid was added to 5 mL of ethanol under stirring to obtain a clarified solution. Then the two solutions were evenly mixed and stirred at 50 °C for 5 h. The mixed solution was transferred into n-hexane for precipitation. The solvent and n-hexane were removed by rotary evaporation, and dried in vacuum at 60 °C for 12 h to obtain slightly yellow transparent boric acid esterified modified EVA for retention. The infrared test was carried out on the modified EVA (Structure 3) provided in this example, and the obtained infrared spectrum was as Figure 1 shown.

[0059] This example also provides a micron silicon coated with modified EVA. The preparation method of the micron silicon coated with modified EVA includes:

[0060] 1 g of boric acid esterified modified EVA was dissolved in ethanol. After stirring evenly, 100 g of micron silicon with an average particle size of 20 μm was slowly added to this solution, and stirring was continued. Then this slightly viscous turbid liquid was transferred to a kneader and rapidly stirred at 60 °C for 5 h to obtain micron silicon doped with EVA. This micron silicon was placed in an oven for curing. The curing temperature was 60 °C, and the curing time was 2 h. It was naturally cooled to room temperature to shape the EVA copolymer coated on the surface of the micron silicon. It was pulverized by a ball mill to obtain micron silicon with an average particle size of 30 μm. The ball milling time was 2 h, and the ball milling speed was 500 r / min, obtaining micron silicon coated with crosslinked modified EVA.

[0061] This example also provides a lithium-ion battery. The preparation method of the lithium-ion battery is the same as that in Example 1. The assembled battery was compacted using a button battery sealer, left standing for several hours, and then placed on a charge-discharge instrument to measure its electrochemical properties. The data are shown in Table 1.

[0062] Example 3

[0063] This embodiment provides a modified EVA. The preparation method of the modified EVA includes:

[0064]

[0065] Under the protection of high-purity N2, VAc, AIBN, and anhydrous methanol were mixed evenly according to the ratio of 30:10:50 (V:m:V). The clarified solution was transferred into a reaction kettle with a polytetrafluoroethylene lining, and ethylene at 2 MPa was charged. After the pressure was stable, the circulating pump was turned on for heating. The polymerization temperature was 60 °C. After 2 h of polymerization, the material was taken out while it was hot and vacuum-dried at 60 °C for 24 h to remove small molecules, obtaining transparent modified EVA for retention and testing.

[0066] 5 g of modified EVA and 50 mL of ethanol were stirred and mixed evenly. 1 g of dihydroxyethylacetamide (shown in Formula 10) and 5 mL of ethanol were stirred and mixed evenly. Then the two solutions were uniformly mixed and stirred at 40 °C for 5 h. The mixed solution was transferred into n-hexane for precipitation. The solvent and n-hexane were removed by rotary evaporation, and vacuum-dried at 60 °C for 12 h to obtain slightly yellow transparent aminated modified EVA for retention.

[0067] 3 g of aminated modified EVA was added to 30 mL of ethanol under stirring to obtain a clarified solution. 1 g of boric acid was added to 5 mL of ethanol under stirring to obtain a clarified solution. Then the two solutions were uniformly mixed and stirred at 50 °C for 5 h. The mixed solution was transferred into n-hexane for precipitation. The solvent and n-hexane were removed by rotary evaporation, and vacuum-dried at 60 °C for 12 h to obtain slightly yellow transparent boric acid esterified modified EVA for retention. Infrared testing was performed on the modified EVA (structure of Formula 4) provided in this embodiment, and the obtained infrared spectrum is as Figure 1 shown.

[0068] This embodiment also provides micron-sized silicon coated with modified EVA. The preparation method of the micron-sized silicon coated with modified EVA includes:

[0069] 1 g of boric acid esterified modified EVA was dissolved in ethanol and stirred evenly. Then 100 g of micron-sized silicon with an average particle size of 20 μm was slowly added to this solution and stirred continuously. Then this slightly viscous turbid liquid was transferred to a kneader and rapidly stirred at 60 °C for 5 h to obtain micron-sized silicon doped with EVA. This micron-sized silicon was placed in an oven for curing. The curing temperature was 60 °C and the curing time was 2 h. It was naturally cooled to room temperature to shape the EVA copolymer coated on the surface of the micron-sized silicon. It was crushed by a ball mill to obtain micron-sized silicon with an average particle size of 30 μm. The ball milling time was 2 h and the ball milling speed was 500 r / min, obtaining micron-sized silicon coated with crosslinked modified EVA.

[0070] This embodiment also provides a lithium-ion battery. The preparation method of the lithium-ion battery is the same as that of Example 1. The assembled battery is compacted using a button battery sealer, left to stand for several hours, and then placed on a charge-discharge instrument to measure its electrochemical properties. The data is shown in Table 1.

[0071] Comparative Example 1

[0072] This comparative example provides a micron-sized silicon coated with crosslinked modified EVA. The preparation method of the micron-sized silicon coated with crosslinked modified EVA includes:

[0073] Under the protection of high-purity N2, VAc, AIBN, and anhydrous methanol are mixed evenly according to the ratio of 30:10:50 (V:m:V). The clarified solution is transferred into a reaction kettle with a polytetrafluoroethylene liner, and 2 MPa of ethylene is charged. After the pressure is stable, the circulating pump is turned on for heating. The polymerization temperature is 60 °C. After 2 h of polymerization, the material is taken out while it is hot and dried in vacuum at 60 °C for 24 h to remove small molecules, obtaining transparent modified EVA for later use and testing.

[0074] 1 g of modified EVA is dissolved in ethanol and stirred evenly. Then, 100 g of micron-sized silicon with an average particle size of 20 μm is slowly added to this solution and stirred continuously. Then, this slightly viscous and turbid liquid is transferred to a kneader and rapidly stirred at 60 °C for 5 h to obtain micron-sized silicon doped with EVA. This micron-sized silicon is placed in an oven for curing. The curing temperature is 60 °C, and the curing time is 2 h. It is naturally cooled to room temperature to shape the EVA copolymer coated on the surface of the micron-sized silicon. It is pulverized by a ball mill to obtain micron-sized silicon with an average particle size of 30 μm. The ball milling time is 2 h, and the ball milling speed is 500 r / min, obtaining micron-sized silicon coated with crosslinked modified EVA.

[0075] This comparative example also provides a lithium-ion battery. The preparation method of the lithium-ion battery is the same as that of Example 1. The assembled battery is compacted using a button battery sealer, left to stand for several hours, and then placed on a charge-discharge instrument to measure its electrochemical properties. The data is shown in Table 1.

[0076] Comparative Example 2

[0077] This comparative example provides a lithium-ion battery. The preparation method of the lithium-ion battery includes:

[0078] Micron silicon / graphite / acetylene black / PAA / sodium carboxymethyl cellulose (CMC) were accurately weighed and mixed according to the ratio of 1:6:1:1:1 (m / m / m / m / m). Deionized water was added dropwise into a small weighing bottle until an appropriate amount was added to make the solution have a moderate viscosity. The solution was stirred on a magnetic stirrer for more than 5 h to make it uniform. The electrode mixture was evenly coated on the cut copper foil on a flat coater. Before coating, both sides of the copper foil were wiped clean with ethanol to prevent dirt. After coating, the number was written down and it was pre-dried under an ultraviolet lamp. After a few minutes, it was transferred to a vacuum drying oven and dried at 100 °C for 12 h. The dried copper foil coated with the mixture was cut into circular slices with a diameter of 1.2 cm using a slicing machine and used as the negative electrode material of LIBs. Then, the blank copper foil and the electrode-coated slices were cut with a cutting machine, sliced, accurately weighed, numbered, recorded, and the mass of the active substance contained in each slice was calculated. For the parameter setting when measuring the electrochemical performance later, finally, it was placed in a glove box for assembling the battery, and a button CR2025 half-cell was assembled in a glove box filled with high-purity argon. The assembled battery was compacted using a button battery sealer and left for several hours, and then its electrochemical properties were measured on a charge-discharge instrument. The data are shown in Table 1.

[0079] Table 1

[0080]

[0081] In Examples 1-3 of the present application, by using modified EVA to coat micron silicon and graphite as the negative electrode material of the lithium-ion battery, the initial discharge specific capacity of the battery is still above 890 mAh / g, the initial Coulomb efficiency is above 95.3%, and after 100 cycles, the Coulomb efficiency can still be maintained above 99%, which is better than Comparative Examples 1 and 2, and the cycle stability is better and the internal resistance is also smaller. While the electrochemical performance is improved, after 100 cycles, the change in the thickness of the electrode sheet is significantly smaller than that of Comparative Examples 1 and 2, indicating that the modified EVA can significantly improve the volume expansion effect of micron silicon during the cycling process. This is because the modified EVA can increase the cohesion and adhesion of the electrode sheet and reduce the risk of binder failure, which is also the reason for the improvement of the electrochemical performance.

[0082] The above is only a preferred specific embodiment of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present application, according to the technical solution of the present application and its application concept, makes an equivalent replacement or change, and should be covered by the protection scope of the present application.

Claims

1. A modified EVA, characterized in that, Having the structure of Formula 1: Wherein, R1 and R2 are each independently hydrogen, an alkyl group having 1 to 10 carbon atoms, an ester group having 2 to 10 carbon atoms, an ether group having 2 to 10 carbon atoms, a carbonyl group having 2 to 10 carbon atoms, or a phenyl group having 6 to 20 carbon atoms, and X and Y are each independently an integer from 1 to 20.

2. The modified EVA according to claim 1, wherein R1 and R2 are each independently hydrogen, an alkyl group having 1 to 5 carbon atoms, an ester group having 2 to 5 carbon atoms, an ether group having 2 to 5 carbon atoms, a carbonyl group having 2 to 5 carbon atoms, or a phenyl group having 6 to 10 carbon atoms, and X and Y are each independently an integer from 1 to 10.

3. The modified EVA according to claim 1, wherein, The modified EVA has any one of the structures of Formula 2 to 4; 4. A preparation method of modified EVA, characterized in that, Including: Subjecting the EVA of Formula 5 structure to an amination reaction with an amino compound of Formula 6 structure to obtain a compound shown in Formula 7; Subjecting the compound shown in Formula 7 to a boric acid esterification reaction with boric acid to obtain the modified EVA.

5. The preparation method according to claim 4, characterized in that, The amino compound has the structures of Formula 8 to Formula 10:

6. A modified silicon, characterized in that, Including silicon and modified EVA coated on the surface of the silicon; the modified EVA is the modified EVA described in any one of Claims 1 to 3 or the modified EVA prepared by the preparation method described in Claim 4 or 5.

7. The modified silicon according to claim 6, wherein The particle size of the modified silicon is 20 μm to 100 μm.

8. A method for preparing modified silicon, characterized in that, Including: Mixing and curing silicon with the modified EVA to obtain the modified silicon.

9. A negative electrode material, characterized in that, Including the modified silicon described in Claim 6 or 7 or the modified silicon prepared by the preparation method described in Claim 8.

10. A battery, characterized in that, Including the negative electrode material described in Claim 9.

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