CN117878382A

CN117878382A - Battery cell

Info

Publication number: CN117878382A
Application number: CN202410215930.2A
Authority: CN
Inventors: 刘建奇; 何柳青; 方嘉琳; 王烽
Original assignee: Zhuhai Cosmx Battery Co Ltd
Current assignee: Zhuhai Cosmx Battery Co Ltd
Priority date: 2024-02-27
Filing date: 2024-02-27
Publication date: 2024-04-12

Abstract

The invention relates to the technical field of lithium batteries, and provides a battery. The battery comprises a positive electrode, a negative electrode and electrolyte; the negative electrode comprises a modified silicon material, wherein the modified silicon material is a silicon material with the surface coated with cyano-containing polymer; the electrolyte comprises fluoroethylene carbonate. The negative electrode containing the modified silicon material and the electrolyte containing the FEC are combined to form a more stable SEI film on the surface of the negative electrode, so that the expansion and cracking of the silicon material are reduced, and the cycle performance of the battery is improved; and the use amount of FEC is reduced, and the stability and the safety of the battery in a high-temperature environment are improved.

Description

Battery cell

Technical Field

The invention relates to the technical field of lithium batteries, in particular to a battery.

Background

Lithium Ion Batteries (LIBs) are considered suitable power sources in mobile electronic devices, electric vehicles, and mass energy storage systems due to their high energy density and long cycle life. In order to increase the energy density of lithium ion batteries, researchers have developed electrode materials with high reversible capacities, including high voltage positive electrode materials and low operating potential negative electrode materials near 0V (relative to Li/Li).

Silicon anodes are considered to be one of the most promising anode materials for next generation lithium ion batteries. However, as silicon undergoes a large volume change during cycling, the silicon particles break up and create new active surface sites, which may lead to further irreversible electrolyte decomposition. Accordingly, in the related art, problems of silicon volume expansion and cycle stability are often alleviated by compounding or screening a matching binder with a material (e.g., graphite) that buffers the silicon volume expansion. However, the silicon material has very large volume change in the charge and discharge process, and the simple buffer material is compounded and matched with the binder to completely overcome the difficult problem of volume expansion.

Disclosure of Invention

The invention aims to overcome the problems in the prior art and provide a battery, wherein a negative electrode containing a modified silicon material and an electrolyte containing FEC are combined to form a more stable SEI film on the surface of the negative electrode, so that the expansion and rupture of the silicon material are reduced, and the cycle performance of the battery is improved; and the use amount of FEC is reduced, and the stability and the safety of the battery in a high-temperature environment are improved.

According to the research of the invention, on one hand, as the dependence of the carbon material in the anode material on FEC is not high, the surface of the anode material is coated with the cyano-containing polymer by modifying the silicon material, the cyano-containing polymer can perform specific dipole interaction with carbonyl groups of the FEC, the FEC is enriched on the surface of the silicon anode by the dipole interaction, and then a denser SEI film is formed, so that the expansion of silicon carbon particles is effectively prevented, and the cycle performance of the battery is improved; on the other hand, the modified silicon material can have strong interaction with the FEC, so that the use amount of the FEC in the silicon-carbon mixing system is reduced, and the stability and the safety of the battery in a high-temperature environment are improved.

In order to achieve the above object, the present invention provides a battery including a positive electrode, a negative electrode, and an electrolyte; the negative electrode comprises a modified silicon material, wherein the modified silicon material is prepared by coating a cyano-containing polymer on the surface of the silicon material; the electrolyte includes fluoroethylene carbonate.

The technical scheme adopted by the invention has the following beneficial effects:

(1) The battery provided by the invention is beneficial to forming a more stable SE I film on the negative electrode, reduces the expansion and cracking of a silicon material, keeps the cycle performance and the safety of the battery under the low-temperature condition, and improves the cycle life and the durability of the lithium ion battery.

(2) The battery provided by the invention is beneficial to reducing the risk of thermal failure of the battery in a high-temperature environment by reducing the use amount of FEC.

(3) The battery provided by the invention has higher performance and safety, and provides important support for technical progress and market development of electric automobiles, mobile electronic equipment and large-scale energy storage systems.

The endpoints and any values of the ranges disclosed herein are not limited to the precise range or value, and are understood to encompass values approaching those ranges or values. For numerical ranges, one or more new numerical ranges may be found between the endpoints of each range, between the endpoint of each range and the individual point value, and between the individual point value, in combination with each other, and are to be considered as specifically disclosed herein. Herein, unless otherwise specified, data ranges all include endpoints.

Drawings

Fig. 1 shows SEM images before and after modification of a silicon carbon material, wherein the left side is before modification of the silicon carbon material, and the right side is after modification of the silicon carbon material.

Fig. 2a shows a SEM cross-section of comparative example 1 before cycling.

Fig. 2b shows a SEM cross-section after cycling of comparative example 1.

Fig. 2c shows a SEM cross-section of example 1 before cycling.

Fig. 2d shows a SEM cross-section after cycling of example 1.

Fig. 3 shows XPS graphs after cycling of comparative example 1 and example 1.

FIG. 4 is a disassembled view of comparative example 1 and example 7 after 400T cycles.

Detailed Description

The following describes specific embodiments of the present invention in detail. It should be understood that the detailed description and specific examples, while indicating and illustrating the invention, are not intended to limit the invention.

Unless defined otherwise, all scientific and technical terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention relates.

The FEC (fluoroethylene carbonate) film forming additive is added into the electrolyte of the lithium battery to promote the surface of the silicon negative electrode to form a compact SEI film, thereby preventing the silicon particles from swelling and cracking and improving the electrochemical performance of the silicon negative electrode (such as silicon-carbon blended graphite). However, the research of the invention finds that the addition of excessive FEC in the electrolyte is unfavorable for the high-temperature cycle performance and furnace temperature performance of the battery, and the addition of a small amount of FEC is easy to cause the shortage of the FEC content in the cycle process of the battery, and after the FEC is consumed, the cycle curve of the battery has inflection points, and then the battery has poor cycle performance due to capacity water jump. Therefore, how to effectively prevent the expansion and rupture of silicon particles and improve the high-temperature cycle performance and furnace temperature performance of the battery and the normal-temperature cycle performance is an urgent problem to be solved.

The first aspect of the present invention provides a battery comprising a positive electrode, a negative electrode, and an electrolyte; the negative electrode comprises a modified silicon material, wherein the modified silicon material is prepared by coating a cyano-containing polymer on the surface of the silicon material; the electrolyte includes fluoroethylene carbonate.

In the invention, cyano-containing polymer can perform specific dipole interaction with carbonyl of FEC (dipole moment interaction between other carbonate substances in electrolyte and cyano is weak), so as to form a compact SEI film with proper thickness, the F content on the surface of the SEI film can be increased, and some elastic polymer is formed to prevent silicon from expanding; in addition, the cyano-containing polymer can interact with FEC, the growth rate of the SEI film in the circulation process can be reduced, capacity loss caused by too thick SEI film is avoided, and the SEI film loses electrical contact and fails due to too thick SEI film; and the use amount of FEC is reduced, the cycle life and the safety of the battery in a high-temperature environment are improved, and the lithium separation effect is improved.

Further, by reducing the amount of FEC used, it helps to reduce the risk of thermal failure of the battery in high temperature environments, which is particularly important for battery systems operating in harsh environments, such as electric vehicles and large-scale energy storage systems. The invention provides important support for technical progress and market development of electric automobiles, mobile electronic equipment and large-scale energy storage systems, and promotes further development of the fields.

In some embodiments, the mass ratio of modified silicon material to fluoroethylene carbonate is (4-10): 1, which may be, for example, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, 10:1. When the mass ratio of the modified silicon material to the fluoroethylene carbonate is in the above range, the addition amount of the modified silicon material is too small to effectively improve the capacity and the cycle performance of the battery, the addition amount of the fluoroethylene carbonate is too small to be avoided, the cycle performance of the battery is easily influenced by the consumption, the addition amounts of the silicon material and the FEC are balanced, and the capacity and the cycle performance of the battery are comprehensively improved.

In some preferred embodiments, the mass ratio of the modified silicon material to fluoroethylene carbonate is (6-8): 1. Further preferably, the mass ratio of the modified silicon material to fluoroethylene carbonate can better balance the addition amount of the silicon material and FEC, and further comprehensively improve the capacity and the cycle performance of the battery.

In some embodiments, SEM of the modified silicon material before and after modification is compared with SEM of the silicon carbon particles before modification as shown in fig. 1, the left is SEM of the modified silicon carbon particles, and the right is SEM of the modified silicon carbon particles, and it can be seen that the surface of the silicon modified silicon carbon particles is rougher, and the surface is coated with cyano-containing polymer.

In some embodiments, the modified silicon material has a cyano-containing polymer mass ratio of 1% -8%, such as 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, preferably2% -6%. The mass ratio of the cyano-containing polymer is within the above range, so that the effect of inhibiting the expansion of the silicon material can be prevented from being influenced by too little coating amount of the cyano-containing polymer, and the effect of removing lithium from the anode can be prevented from being influenced by too thick coating layer due to too much coating amount of the cyano-containing polymer. The mass ratio of the cyano-containing polymer can be tested by adopting a thermogravimetric method, the cyano-containing polymer can be completely separated into gases when being sintered at 600 ℃ in an air atmosphere, and the silicon material can form SiO ₂ The protective film has no obvious mass loss after being sintered at 600 ℃, so that the mass ratio of the cyano-containing polymer can be calculated according to the mass loss after being sintered.

In some embodiments, the mass fraction of FEC in the electrolyte is 4% -10%, for example, may be 4%, 5%, 6%, 7%, 8%, 9%, 10%, preferably 5% -8%. In the related art, the mass ratio of fluoroethylene carbonate in the electrolyte is generally 12% -15%, and when the mass ratio is less than 8%, capacity jump of the battery after the FEC is consumed is easy to occur, so that the cycle performance is poor. The invention limits the FEC mass ratio so that most FEC is consumed on the surface of the modified silicon material, thereby reducing the use amount of FEC, avoiding water jump of the capacity of the battery, improving the cycle performance of the battery and improving the stability and safety of the battery in a high-temperature environment.

The specific kind of the cyano group-containing polymer is not limited, and in order to further promote the interaction between the cyano group-containing polymer and the carbonyl group of FEC, the cyano group-containing polymer includes a structure represented by the following formula I:

wherein R is ₁ 、R ₂ 、R ₃ And R is ₄ Independently comprises H, cyano, phenylcyano, substituted or unsubstituted C1-10 alkyl, substituted or unsubstituted C1-10 alkoxy, substituted or unsubstituted C2-10 alkenyl, substituted or unsubstituted C1-10 alkyl-O-C1-10 alkyl, substituted or unsubstituted C1-10 alkyl-C (O) -C1-10 alkyl, substituted or unsubstituted C4-10 heteroaryl, substitutedOr at least one of unsubstituted C4-10 heterocyclic group and substituted or unsubstituted C6-10 aryl group, wherein the substituent is cyano or phenylcyano; r is R ₁ 、R ₂ 、R ₃ And R is ₄ At least one of which contains cyano groups; a is an integer of 0 to 5, b is an integer of 0 to 10, and n is an integer of 20 to 5000.

The substitution position and number of the cyano groups are not limited, and the cyano groups can realize specific dipole interaction between the cyano-containing polymer and the carbonyl groups of the FEC. In order to further promote the dipole action of both, it is further preferred that the cyano group containing polymer has the structure:

in some embodiments, the cyano-containing polymer includes a structure as shown in formula I-1 below:

(formula I-1), R ₁ And R is ₂ At least one of which contains cyano groups, R ₁ 、R ₂ A, b and n have the meanings as defined above.

In some embodiments, the cyano-containing polymer includes a structure as shown in formula I-2 below:

(formula I-2), R ₁ And R is ₂ At least one of which contains cyano groups, R ₁ 、R ₂ B and n have the meanings given above.

In some embodiments, the cyano-containing polymer includes a structure as shown in formula I-3 below:

(formula I-2), R ₁ And R is ₂ At least one of which contains cyano groups, R ₁ 、R ₂ And n has the meaning as defined above.

In some embodiments, the cyano-containing polymer includes at least one of poly 1, 1-dicyano compound ethylene, poly 4-cyanostyrene, poly 5-vinylbenzene-1, 3-dinitrile, nitrile rubber, poly (2-butenenitrile), poly (3-butenenitrile), acrylonitrile- (2-butenenitrile) copolymer, acrylonitrile- (3-butenenitrile) copolymer, (2-butenenitrile) - (3-butenenitrile) copolymer, acrylonitrile- (2-butenenitrile) - (3-butenenitrile) terpolymer.

In some embodiments, the cyano-containing polymer comprises a polymer as shown below:

is an integer of 20 to 5000.

In some embodiments, the cyano-containing polymer has a molecular weight of 1×10 ³ Da～1×10 ⁶ Da. For example, it may be 1X 10 ³ Da～1×10 ⁴ Da、1×10 ⁴ Da～1×10 ⁵ Da、1×10 ⁵ Da～1×10 ⁶ Da. When the molecular weight of the polymer is in the range, the cyano group number is more, the whole structure steric hindrance is smaller, the interaction between the modified silicon material and the FEC can be further promoted, and the cycle performance of the battery is better improved.

In some embodiments, the silicon material comprises at least one of silicon, silicon oxide, silicon carbon and silicon alloy, preferably silicon carbon, and the silicon carbon cathode has high gram capacity, high initial charge and discharge efficiency, low cost and mature process relative to other silicon-based cathode materials. The type of the silicon material is optimized, so that the smooth modification of the silicon material can be promoted, the interaction between the modified silicon material and the FEC is further promoted, and the electrochemical performance of the battery is improved (for example, the cycle performance of the battery is improved, and the impedance of the battery is reduced).

In the invention, the modified silicon material can adopt a mechanical force and/or high temperature action mode to coat the cyano-containing polymer on the surface of the silicon material. For example, the modified silicon material may be obtained by distributing a cyano group-containing polymer on the outer surface of the silicon material by using mechanical forces such as extrusion, impact, shearing, friction and the like and at a relatively high temperature to allow various components to infiltrate and diffuse into each other, thereby forming a cyano group-containing polymer coating layer on the surface of the silicon material.

The invention also provides a method for preparing the modified silicon material, which comprises the following steps:

(1) Synthesis of cyano-containing polymers: placing the high-pressure reaction kettle in a cold trap with a magnetic stirrer, adding a first solvent, starting the stirrer, adding a cyano-containing monomer, introducing inert gas to pressurize to 8-12atm, introducing an initiator into the system after the monomer is completely dissolved, stirring for 30-60min, and adding ethanol to terminate the reaction; filtering the product, washing with a small amount of ethanol, washing with water to neutrality, and drying;

(2) Preparing a modified silicon material: adding a silicon material into a ball milling tank of a planetary ball mill, adding a cyano-containing polymer, adding deionized water as a dispersing agent, introducing high-purity inert gas, setting the rotating speed to 300-500r/min, and ball milling for 1-3h; the sample was then transferred to a vacuum oven for drying.

In step (1), the first solvent may be an organic solvent commonly used in polymerization of monomers, such as tetrahydrofuran. The initiator may include sodium methoxide and/or butyllithium, etc. The inert gas includes nitrogen and/or argon.

In some embodiments, the cyano-containing monomer may include, for example, at least one of acrylonitrile, 1-dicyano compound ethylene, 4-cyanostyrene, 5-vinylbenzene-1, 3-dinitrile, 2-butenenitrile, 3-butenenitrile, acrylonitrile- (2-butenenitrile), acrylonitrile- (3-butenenitrile), (2-butenenitrile) - (3-butenenitrile), acrylonitrile- (2-butenenitrile) - (3-butenenitrile).

The carbon material modification method provided by the invention can be applied to silicon cathodes and other cathode materials (such as lithium metal) depending on FEC, improves the electrochemical performance of the cathodes, has wide application potential, and can be applied to various types of lithium ion batteries.

In some embodiments, the negative electrode further comprises a carbon material, and the carbon material is matched with the silicon material for use, so that the effect of buffering the volume expansion of the silicon is achieved. The specific type of the silicon material is not limited, and carbon negative electrode materials commonly used in the art may be selected, and for example, artificial graphite, natural graphite, soft carbon, hard carbon, and the like may be used.

In some embodiments, the mass ratio of silicon material to carbon material is (0.2-0.5): 1, which may be, for example, 0.2:1, 0.25:1, 0.3:1, 0.35:1, 0.4:1, 0.45:1, 0.5:1. When the mass ratio of the silicon material to the carbon material is in the range, the volume expansion of silicon can be relieved, and the cycling stability of the battery can be improved; the reversible capacity of the battery can be improved, and the energy density of the battery can be improved.

In some embodiments, the anode includes an anode current collector and an anode active material layer coated on one or both side surfaces of the anode current collector. The anode active material layer includes a modified silicon material, and the anode active material layer may be disposed in the entire region of the anode current collector or in a partial region of the anode current collector, for example, the anode active material layer including the modified silicon material is disposed in the tail region of the wound battery, and has an effect of improving tail lithium precipitation.

In some embodiments, the mass ratio of the electrolyte to the negative electrode material is 1 (1.5-4), e.g., may be 1:1.5, 1:2, 1:2.5, 1:3, 1:3.5, 1:4. The term "mass of the negative electrode material" refers to the total mass of the negative electrode active material (silicon material and carbon material), the binder, the conductive agent, and other auxiliary agents. When the mass ratio of the electrolyte to the cathode material is in the above range, too little electrolyte can be avoided, the residual liquid coefficient of the battery core can be improved, and the cycle performance of the battery can be improved.

In some embodiments, the positive electrode comprises a positive electrode active material comprising a ternary material of lithium cobaltate, sodium-containing lithium cobaltate, lithium manganate, nickel cobalt manganese (formula Li _a Ni _x Co _y Mn _z A _k O ₂ Wherein 0 is<x<1，0<y<1，0<z<A is more than or equal to 1.9 and less than or equal to 1.1, k is more than or equal to 0 and less than or equal to 0.1; wherein a is a dopant comprising the following elements: co, cu, zn, fe, al, mg, ti, zr, Y, B, la, mo, nb, P, mn or a combination thereof), nickel cobalt aluminum ternary material (chemical formula is Li _a Ni _x Co _y Al _z A _k O ₂ ，0<x<1.1，0<y<1，0<z<1.9.ltoreq.a.ltoreq.1, 0.ltoreq.k.ltoreq.0.1, A being a dopant comprising: co, cu, zn, fe, al, mg, ti, zr, Y, B, la, mo, nb, P, mn or a combination thereof), lithium iron phosphate, lithium-rich manganese-based materials, and manganese iron phosphateAt least one of lithium or lithium titanate, lithium nickel manganese oxide, lithium nickel oxide, lithium manganese oxide and nickel manganese binary materials.

In some embodiments, the positive and negative electrodes further comprise a conductive agent and a binder. The conductive agent includes at least one of conductive carbon black (SP), acetylene black, ketjen black, graphene, conductive carbon fibers, 350G, carbon Nanotubes (CNTs), metal powder, and carbon fibers. The binder comprises at least one of sodium carboxymethyl cellulose, styrene Butadiene Rubber (SBR), polytetrafluoroethylene and polyethylene oxide.

In some embodiments, the battery further comprises an electrolyte comprising an electrolyte salt, an organic solvent, and optionally an additive.

In some embodiments, the electrolyte salt may be selected from LiPF ₆ Lithium hexafluorophosphate, liBF ₄ Lithium tetrafluoroborate, liClO ₄ (lithium perchlorate), liAsF ₆ (lithium hexafluoroarsenate), liFeSI (lithium bis-fluorosulfonyl imide), liTFSI (lithium bis-trifluoromethanesulfonyl imide), liTFS (lithium trifluoromethanesulfonate), liDFOB (lithium difluorooxalato borate), liBOB (lithium bisoxalato borate), liPO ₂ F ₂ (lithium difluorophosphate), liDFOP (lithium difluorodioxalate phosphate) and LiTFOP (lithium tetrafluorooxalate phosphate).

In some embodiments, the mass ratio of the fluoroethylene carbonate to the organic solvent satisfies the following formula: M1/M2 is more than or equal to 1/8 and less than or equal to 1/30,

wherein M1 is the mass of the fluoroethylene carbonate; m2 is the mass of the organic solvent.

The mass ratio of the fluoroethylene carbonate to the organic solvent may be, for example, 1:8, 1:10, 1:12, 1:15, 1:18, 1:20, 1:22, 1:25, 1:28, 1:30. When the mass ratio of the fluoroethylene carbonate to the organic solvent meets the relation, the influence of the excessive content of the fluoroethylene carbonate on the furnace temperature performance of the battery and the influence of the excessive content of the fluoroethylene carbonate on the normal-temperature cycle performance of the battery can be avoided.

In some embodiments, the organic solvent may be selected from one or more of Ethylene Carbonate (EC), propylene Carbonate (PC), ethylmethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), butylene Carbonate (BC), fluoroethylene carbonate (FEC), methyl Formate (MF), methyl Acetate (MA), ethyl Acetate (EA), propyl Acetate (PA), methyl Propionate (MP), ethyl Propionate (EP), propyl Propionate (PP), methyl Butyrate (MB), ethyl Butyrate (EB), 1, 4-butyrolactone (GBL), sulfolane (SF), dimethylsulfone (MSM), methylsulfone (EMS) and diethylsulfone (ESE), preferably a combination of EC and DMC.

In some embodiments, the organic solvent is a combination of EC and DMC, and the mass ratio of the fluoroethylene carbonate to the organic solvent satisfies the following formula: 1/8 < M (FEC)/[ M (EC) +M (DMC) ] < 1/30.EC and DMC exhibit good electrochemical stability in lithium batteries, are resistant to electrolyte decomposition and oxidation reactions of the electrodes, and help to extend the cycle life of the battery.

In some embodiments, additives such as polynitrile compounds may also be added to the electrolyte, and in order to distinguish between cyano-containing polymers in the modified silicon material and polynitrile compounds in the electrolyte, the electrolyte may be removed by dipping with DMC and then detected by infrared spectroscopy, with cyano wavenumbers in the range 2000-2200.

In some embodiments, the separator includes a polymer film and a coating disposed on a surface of the polymer film. The coating comprises one or more of polyvinylidene fluoride, aluminum oxide and boehmite.

In some embodiments, the battery is a lithium ion battery.

In some embodiments, the battery is a lithium ion battery of laminated structure, i.e., a laminated battery. The space utilization of the battery core of the laminated battery is higher, and the energy density is improved.

In some embodiments, the battery is a wound battery, wherein the negative electrode includes a first coated region near a winding start and a second coated region near a winding end; the second coating zone is provided with the modified silicon material. The negative electrode paste containing the modified silicon material is arranged in the second coating area close to the winding tail end, so that the problem of lithium precipitation at the tail of the battery can be effectively solved, and the cycle performance of the battery is improved.

Preferably, the length of the first coating area is L1, and the length of the second coating area is L2, L2/(L1+L2) _1/10. The length of the negative electrode sheet along the winding direction can be understood as L1+L2, L2 occupies at least 1/10 of the length of the negative electrode sheet, and the tail lithium precipitation area can be better covered, so that the problem of tail lithium precipitation of the winding type battery is further effectively restrained.

In some embodiments, doctor blade and spray are controlled, and the spray heads 1,3,5 are used to spray a common negative paste (without modified silicon material) on the head, and the spray heads 2,4,6 are changed to spray the negative paste (with modified silicon material) of the invention when the tail is folded by 1-10. The negative electrode paste is coated in this way, so that the area of tail lithium precipitation can be better covered, and the problem of tail lithium precipitation of a winding type battery can be effectively restrained.

The components (e.g., positive electrode sheet, separator, electrolyte, etc.) and assembly of the battery of the present invention other than the negative electrode sheet may be performed in a manner conventional in the art, and will not be described in detail herein.

The technical solutions in the embodiments of the present invention will be clearly and completely described in the following in conjunction with the embodiments of the present invention, and it is obvious that the described embodiments are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.

Materials, reagents and the like used in the examples described below are commercially available unless otherwise specified.

The invention is described in detail below in connection with specific embodiments, which are intended to be illustrative rather than limiting.

Preparation example 1 preparation of modified silicon Material

(1) Synthesis of cyano-containing polymers

The synthetic route is as follows:

the specific operation steps are as follows: the polymerization reactor was evacuated and purged with nitrogen several times. 25mL of anhydrous methanol is added by a syringe, 2g of metallic sodium is added under the protection of nitrogen, the mixture is heated, the reflux reaction is carried out for 1h, the heating is stopped, the colorless sodium methoxide solution is obtained, and the initiator is obtained by sealing.

A50 mL autoclave was placed in a cold trap with a magnetic stirrer, 20mL of tetrahydrofuran was added, the stirrer was started, and 5g of monomeric 1, 1-dicyano-ethylene was addedNitrogen was introduced under pressure to 10atm, and after the monomers were completely dissolved, 2ml of sodium methoxide was introduced into the system, and after stirring for 45 minutes, 10ml of ethanol was added to terminate the reaction. Filtering the product, washing with a small amount of ethanol, washing with water to neutrality, and drying to obtain cyano-containing polymer ∈>

(2) Preparation of modified silicon materials

10g of silicon carbide with Dv50 of 20 mu m is added into a ball milling tank of a planetary ball mill, 0.42g of the cyano-containing polymer in the step (1) is added, deionized water is added as a dispersing agent, high-purity argon is introduced, the rotating speed is set to 400r/min, and the ball milling is carried out for 2 hours. And then transferring the sample into a vacuum drying oven, and drying at 50 ℃ for 24 hours to obtain the modified silicon material, wherein the cyano-containing polymer is obtained by testing with the mass ratio of about 4%.

PREPARATION EXAMPLE 2 group

Reference was made to preparation 1, except that the types of monomers were changed as follows:

preparation example 2a: equivalent amount of 4-cyanostyrene monomerSubstituted for original monomer 1, 1-dicyano compound ethylene to obtain cyano group-containing polymer +.>

Preparation example 2b: copolymerization of commercially available acrylonitrile and butadieneArticle (B)(CN-3) or by self-synthesis according to preparation example 1.

PREPARATION EXAMPLE 3 group

Reference was made to preparation 1, except that the weight of the monomers was changed as follows:

preparation example 3a: the addition amount of the polymer containing cyano groups is 0.22g, and the mass ratio of the polymer containing cyano groups in the modified silicon material is about 2%;

preparation example 3b: the addition amount of the polymer containing cyano groups is 0.62g, and the mass ratio of the polymer containing cyano groups in the modified silicon material is about 6%;

preparation example 3c: the addition amount of the polymer containing cyano groups is 0.12g, and the mass ratio of the polymer containing cyano groups in the modified silicon material is about 1%;

preparation example 3d: the addition amount of the polymer containing cyano groups is 0.82g, and the mass ratio of the polymer containing cyano groups in the modified silicon material is about 8%;

preparation example 3e: the addition amount of the polymer containing cyano groups is 0.085g, and the mass ratio of the polymer containing cyano groups in the modified silicon material is about 0.8%;

preparation example 3f: the cyano group-containing polymer was added in an amount of 1.02g, and the mass ratio of the cyano group-containing polymer in the modified silicon material was about 10%.

Example 1

(1) Preparation of positive plate

LiCoO is added with ₂ : conductive carbon black: PVDF is prepared from the following components in percentage by mass: 2:1, followed by stirring on a magnetic stirrer for 12h; and uniformly coating the positive electrode paste on aluminum foil with the diameter of 10 mu m, drying in an oven at 60 ℃ for 24 hours, and cutting to obtain the positive electrode plate.

(2) Preparation of negative plate

Modified silicon carbon: graphite: conductive carbon black: SBR was prepared at mass ratio of 2:6:1: mixing at a ratio of 1, dissolving in N-methyl-2-pyrrolidone, coating on copper foil with a thickness of 8 μm, drying in oven at 60deg.C for 24 hr, and cutting to obtain the negative electrode sheet.

(3) Electrolyte composition: the volume ratio of the organic solvent to the EC/DMC is 1:1, the total mass ratio of the FEC to the EC and DMC is 1:19 (FEC accounts for 5% of the total mass of the electrolyte), and the LiPF is 1mol/L ₆ 。

(4) A diaphragm: the polyethylene diaphragm with the thickness of 5 mu m is coated with a layer of coating with the thickness of 2 mu m on two sides, wherein the coating comprises polyvinylidene fluoride and aluminum oxide (the mass ratio is 1:2).

(5) Winding the positive plate, the negative plate and the diaphragm into a winding core; and then packaging the obtained winding core in a film shell, and injecting electrolyte, vacuum packaging, aging, formation, secondary packaging, capacity sorting and other working procedures to obtain the corresponding soft-package laminated lithium ion battery, wherein the mass ratio of the electrolyte to the cathode material is 1:2.

The lithium ion batteries of the remaining examples and comparative examples were each prepared by referring to the method of example 1, except that the modified silicon material in the negative electrode was different and the FEC content was different,the differences are shown in Table 1。

TABLE 1

Example 5

Reference example 1 was made, except that the modified silicon carbon was replaced with an equal amount of a mixture of ordinary silicon carbon and modified silicon carbon, the mass ratio of ordinary silicon carbon to modified silicon carbon was 0.25:0.08, and acrylonitrile as a polynitrile compound additive was added to the electrolyte.

Comparative example 1

Reference example 1 was performed except that the modified silicon carbon was replaced with an equal amount of ordinary silicon carbon.

Comparative example 2

Reference example 1 was made, except that the modified silicon carbon was replaced with an equal amount of ordinary silicon carbon, and acrylonitrile as a polynitrile compound additive was added to the electrolyte.

The batteries in the above examples and comparative examples were subjected to performance tests, and the test methods are described below:

(1) 400T cycle test: charging with 2C to 4.4V, constant voltage of 4.4V to multiplying power 1C, charging 1C to 4.5V, constant voltage to multiplying power 0.1C. The discharge regime was 0.7C to 3V, to 0.2C to 3V, and the results after testing are reported in Table 2.

(2) Furnace temperature test: and (3) placing the full-charged battery into a test box, heating the test box at a heating rate of 5 ℃/min, keeping the temperature constant after the temperature in the test box reaches 130+/-2 ℃, and keeping the temperature for one hour for no abrupt change, wherein the temperature of the battery body is abrupt change to no passage even if the battery passes.

(3) EIS (electrochemical impedance spectroscopy) after 25 ℃ cycling: the discharge capacity of the battery was recorded for the last cycle, charged to 50% soc with a rate of 0.1C. EIS tests were performed at an amplitude of 5mV at a frequency of 30mHz-10KHz to test ohmic, membrane and load impedances, and the results are reported in Table 2.

400T cycle test: charging with 2C to 4.4V, constant voltage of 4.4V to multiplying power 1C, charging 1C to 4.5V, constant voltage to multiplying power 0.1C. The discharge system is that 0.7C is discharged to 3V, and 0.2C is changed to 3V.

TABLE 2

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The results in table 2 show that when comparative example 1 uses unmodified silicon carbon blended graphite negative electrode, the capacity retention rate recycled to 400T cell is lower than 70%, whereas example 1, example 2-1 and example 2-2, after recycling to 400 times, the capacity retention rates are 81.57%, 78.40% and 79.57%, respectively, which indicates that the mass ratio of cyano-containing polymer in the modified silicon material is more suitable at 4%, and that too much doping of cyano-containing polymer may result in too thick coating layer, affecting lithium deintercalation. Comparative example 1 and example 1-1 found that the cycle performance was enhanced after the cyano group was introduced more, illustrating the important role of the cyano group.

Furthermore, comparative examples 1 and 3-3 found that the cycling performance was better when the mass ratio of modified silicon material to FEC was 6:1, mainly due to the low silicon content, the FEC was more adequate, the performance was moderate when the ratio was increased to 8:1, and a rapid decay had occurred when the ratio was increased to 10:1, indicating that the FEC content was too low and had been consumed. The content of modified silicon carbon in example 5 is too small, which is not within the scope of the present invention, and the negative electrode swells during cycling, and the SEI film becomes thick, resulting in failure of the battery.

From the results of table 2, it can be seen that the use of the silicon carbon material without cyano groups in comparative example 1 has very large load transfer resistance after recycling, which may be that the SEI film is relatively thick, affecting lithium ion transfer. While other embodiments have a relatively small impedance, particularly embodiment 1, with a minimal impedance, indicating that cyano groups have the effect of reducing the load transfer impedance during cycling.

The present invention also investigated the SEI film thickness before and after cycling of comparative example 1 and example 1, and the results are shown in Table 3 and FIG. 2.

TABLE 3 Table 3

Cycle number	Comparative example 1	Example 1
			75T	398.8nm	340.6nm
150T	676.2nm	409.2nm
			233T	1383.5nm	539.3nm
400T	—	613.3nm

Fig. 2a is a SEM cross-sectional view before the cycle of comparative example 1, fig. 2b is a SEM cross-sectional view after the cycle of comparative example 1, fig. 2c is a SEM cross-sectional view before the cycle of example 1, and fig. 2d is a SEM cross-sectional view after the cycle of example 1. The results of fig. 2 show that the SEI film on the surface of the silicon carbon particles after 233T of the comparative example 1 has reached a thickness of approximately 2 μm, whereas the SEI film after the surface of the modified silicon particles circulates only about 600 nm. In addition, the present invention also collected SEI film thicknesses corresponding to different numbers of cycles of comparative example 1 and example 1, as shown in Table 3, the average value of the increase in the SEI film thickness of comparative example 1 was far more than that of example 1 as cycles proceeded. The reason for this phenomenon may be that the SEI film formed at the initial stage of the surface of the unmodified silicon particles has poor mechanical strength, and when charging is performed, the silicon particles expand to cause the SEI film to be broken, the fresh surface is exposed, and then the SEI film reacts with the electrolyte to form a thick SEI film. However, after cyano modification is introduced into the surface, the FEC is enriched on the surface of the silicon particles due to dipole interaction of the cyano and the FEC, and an SEI film with good mechanical strength is formed at the initial stage of charging, so that the expansion of the silicon-carbon particles is prevented.

The invention also investigated XPS (X-ray photoelectron spectroscopy) comparison after cycling of comparative example 1 and example 1, as shown in Table 4 and FIG. 3.

TABLE 4 Table 4

FEC was reduced to form LiF and some crosslinked polymer on the surface of the anode at the time of charging, the atomic concentration ratio of LiF after the cycle of comparative example 1 was lower, and the surface Li was _x PF _y And Li (lithium) _x PO _y F _z (LiPF ₆ Reduced product, liPF ₆ Is reduced in preference to FEC, so Li is formed on the surface _x PO _y F _z ) The higher the content of (c) indicates that the decomposition of the lithium salt has occurred. While example 1 shows a relatively high LiF content on the surface, the introduction of cyano groups attracts FEC and reduces to LiF on the surface during the cycle, inhibiting the lithium salt LiPF ₆ And decomposing to finally realize the improvement of the cycle performance of the battery. As the voltage increases FEC begins to decompose, forming a polymer that inhibits silicon swelling and LiPF ₆ Decompose, thus subsequent cycling Li in example 1 _x PO _y F _z The content will be lower.

Example 6

The battery of comparative example 1 was disassembled, and it was found that there was serious lithium precipitation at the tail of the battery, in order to prove that the modified silicon carbon has the beneficial effect of inhibiting lithium precipitation, a unique coating method, doctor blade and spray coating were adopted, the negative electrode paste of comparative example 1 was sprayed on the head by using spray heads 1,3,5, the negative electrode paste of example 1 (with the head part having hollow foil) was sprayed on the spray heads 2,4,6 when the reciprocal of the tail was folded by 1-10, and then the soft-pack battery was formed through the steps of rolling, slicing, winding, formation packaging and the like. Example 6 was obtained by this procedure. Wherein the negative electrode sheet was folded at the front 11 to be the negative electrode paste of comparative example 1, and at the rear 10 to be the negative electrode paste of example 1.

The capacity retention rates of the batteries of comparative example 1 and example 6 were measured, and the results are recorded in table 5, and the case of lithium precipitation is shown in fig. 4.

TABLE 5

Group of	Silicon carbon: FEC quality	400T capacity retention	Tail lithium precipitation degree
				Comparative example 1	8:1	<70％	Is very serious
Example 6	8:1	78.5％	Slight

FIG. 4 is a disassembled view after 400T cycles of comparative example 1 (up) and example 6 (down). After 400T circulation, the tail is obviously improved by disassembly, and the severity of lithium precipitation is greatly reduced.

It should be noted that, in this document, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one … …" does not exclude the presence of other like elements in a process, method, article, or apparatus that comprises the element. Furthermore, it should be noted that the scope of the methods and apparatus in the embodiments of the present application is not limited to performing the functions in the order shown or discussed, but may also include performing the functions in a substantially simultaneous manner or in an opposite order depending on the functions involved, e.g., the described methods may be performed in an order different from that described, and various steps may also be added, omitted, or combined. Additionally, features described with reference to certain examples may be combined in other examples.

The foregoing description of the preferred embodiments of the invention is not intended to be limiting, but rather is to be construed as including any modifications, equivalents, and alternatives falling within the spirit and principles of the invention.

Claims

1. A battery comprising a positive electrode, a negative electrode, and an electrolyte;

the negative electrode comprises a modified silicon material, wherein the modified silicon material is prepared by coating a cyano-containing polymer on the surface of the silicon material;

the electrolyte includes fluoroethylene carbonate.

2. The battery according to claim 1, characterized in that the mass ratio of the modified silicon material to fluoroethylene carbonate is (4:10): 1, preferably (6-8): 1.

3. The cell according to claim 1, wherein the modified silicon material has a cyano-containing polymer mass ratio of 1% to 8%, preferably 2% to 6%.

4. The battery according to claim 1, characterized in that the mass ratio of fluoroethylene carbonate in the electrolyte is 4-10%, preferably 5-8%.

5. The battery of any one of claims 1-4, wherein the cyano-containing polymer comprises a structure according to formula I:

wherein R is ₁ 、R ₂ 、R ₃ And R is ₄ Independently comprises at least one of H, cyano, phenylcyano, substituted or unsubstituted C1-10 alkyl, substituted or unsubstituted C1-10 alkoxy, substituted or unsubstituted C2-10 alkenyl, substituted or unsubstituted C1-10 alkyl-O-C1-10 alkyl, substituted or unsubstituted C1-10 alkyl-C (O) -C1-10 alkyl, substituted or unsubstituted C4-10 heteroaryl, substituted or unsubstituted C4-10 heterocyclyl, substituted or unsubstituted C6-10 aryl, and the substituent is cyano or phenylcyano;

R ₁ 、R ₂ 、R ₃ and R is ₄ At least one of which contains cyano groups;

a is an integer of 0-5, b is an integer of 0-10, and n is an integer of 20-5000;

preferably, R ₁ 、R ₂ 、R ₃ And R is ₄ Independently comprises at least one of H, cyano, phenylcyano, substituted or unsubstituted C1-3 alkyl, substituted or unsubstituted C1-3 alkoxy, substituted or unsubstituted C2-3 alkenyl, substituted or unsubstituted C1-3 alkyl-O-C1-3 alkyl, substituted or unsubstituted C1-3 alkyl-C (O) -C1-3 alkyl, substituted or unsubstituted C4-6 heteroaryl, substituted or unsubstituted C4-6 heterocyclyl, substituted or unsubstituted C6-8 aryl, and the substituent is cyano or phenylcyano;

preferably, R ₁ 、R ₂ 、R ₃ And R is ₄ At least two of which contain cyano groups;

preferably, a is an integer from 0 to 3, b is an integer from 0 to 5, and n is an integer from 20 to 5000.

6. The battery of claim 5, wherein the cyano-containing polymer comprises at least one of poly 1, 1-dicyano compound ethylene, poly 4-cyanostyrene, poly 5-vinylbenzene-1, 3-dinitrile, nitrile rubber, poly (2-butenenitrile), poly (3-butenenitrile), acrylonitrile- (2-butenenitrile) copolymer, acrylonitrile- (3-butenenitrile) copolymer, (2-butenenitrile) - (3-butenenitrile) copolymer, and acrylonitrile- (2-butenenitrile) - (3-butenenitrile) terpolymer;

preferably, the cyano-containing polymer has a molecular weight of 1X 10 ³ Da～1×10 ⁶ Da。

7. The battery of any one of claims 1-4, wherein the negative electrode further comprises a carbon material;

preferably, the mass ratio of the silicon material to the carbon material is (0.2-0.5): 1;

preferably, the carbon material comprises at least one of artificial graphite, natural graphite, soft carbon, hard carbon.

8. The battery according to any one of claims 1 to 4, wherein the mass ratio of the electrolyte to the anode material is 1 (1.5 to 4), the anode material including at least the modified silicon material;

preferably, the silicon material comprises at least one of silicon, silicon oxide, silicon carbon and silicon alloy, preferably silicon carbon;

preferably, the positive electrode includes a positive electrode active material including at least one of lithium cobaltate, sodium-containing lithium cobaltate, lithium manganate, nickel cobalt manganese ternary material, nickel cobalt aluminum ternary material, lithium iron phosphate, lithium-rich manganese-based material, lithium iron phosphate or lithium titanate, lithium nickel manganate, lithium nickelate, lithium manganate, nickel manganese binary material, preferably lithium cobaltate.

9. The battery of any one of claims 1-4, wherein the battery is a wound battery, wherein the negative electrode comprises a first coated region near a winding start and a second coated region near a winding end; the second coating region is provided with the modified silicon material;

preferably, the length of the first coating area is L1, and the length of the second coating area is L2, L2/(L1+L2) _1/10.

10. The battery of any one of claims 1-4, wherein the electrolyte further comprises an organic solvent;

preferably, the mass ratio of the fluoroethylene carbonate to the organic solvent satisfies the following formula:

1/8≤M1/M2≤1/30，