A lithium ion battery and a preparation method thereof

Abstract

The application relates to the technical field of lithium ion batteries, in particular to a lithium ion battery and a preparation method. The lithium ion battery comprises a positive electrode sheet, a diaphragm, a negative electrode sheet and an electrolyte arranged in the lithium ion battery; the diaphragm is arranged between the positive electrode sheet and the negative electrode sheet; the positive electrode sheet comprises a positive electrode current collector, a positive electrode active material layer coated on the surface of the positive electrode current collector and a first CEI film layer covering the surface of the positive electrode active material layer; the first CEI film layer comprises a lithium salt compound; the electrolyte is added with a sulfur-containing additive; wherein the thickness of the first CEI film layer is H microns; the specific surface area of the positive electrode active material layer is A m 2 / g; the mass percentage content of the sulfur-containing additive in the electrolyte is W%; the lithium ion battery satisfies the following relationship: 0.5 <= (H*W) / A <= 20. The application controls the specific surface area A of the positive electrode active material, the thickness H of the artificial CEI film layer and the content W% of the sulfur-containing additive in the electrolyte to satisfy the following relationship: 0.5 <= (H*W) / A <= 20, so that the low-temperature discharge performance and the high-temperature cycle performance of the lithium ion battery are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a lithium ion battery and a preparation method. BACKGROUND

[0002] Lithium ion batteries are a common type of rechargeable lithium ion battery that is widely used in mobile devices, power tools, electric vehicles, and many other applications. They typically have high energy density, long life, and low self-discharge rates, making them an ideal choice for many applications. Lithium ion batteries contain a positive electrode, a negative electrode, an electrolyte, and a separator. During charging, lithium ions move from the positive electrode to the negative electrode, and vice versa during discharging. This movement of lithium ions causes the flow of electrons, resulting in an electric current.

[0003] The electrolyte in a lithium ion battery acts as an ion transport medium for the oxidation-reduction reactions of the positive and negative electrodes, playing an important role in both the thermodynamic and kinetic performance of the lithium ion battery. During the life cycle of a lithium ion battery, the electrolyte is a "consumable" that is continuously consumed and decomposed at the electrode / electrolyte interface during the cycling process of the lithium ion battery, and this is exacerbated by the increase in the surface area (reaction area) of the electrode material, the operating temperature, and the instability of the electrode material, resulting in poor low-temperature discharge performance and high-temperature cycle performance of the lithium ion battery. SUMMARY

[0004] The present application provides a lithium ion battery and a preparation method to solve the problem of poor low-temperature discharge performance and high-temperature cycle performance of the lithium ion battery during the cycling process of the lithium ion battery in the prior art.

[0005] The present application discloses a lithium ion battery, which comprises a positive electrode sheet, a separator, a negative electrode sheet and an electrolyte arranged inside the lithium ion battery; the separator is arranged between the positive electrode sheet and the negative electrode sheet;

[0006] The positive electrode sheet comprises a positive electrode current collector, a positive electrode active material layer coated on the surface of the positive electrode current collector, and a first CEI film layer covering the surface of the positive electrode active material layer; the first CEI film layer comprises a lithium salt compound;

[0007] The electrolyte contains a sulfur-containing additive;

[0008] The thickness of the first CEI film layer is H μm; the specific surface area of the positive electrode active material layer is A m 2 / g; the mass percentage of the sulfur-containing additive in the electrolyte is W%; the lithium ion battery satisfies the following relationship:

[0009] 0.5≤(H×W) / A≤20.

[0010] Optionally, the lithium salt compound comprises one or more of Li4SiO4, Li2SiO3, Li3N, LiAlH4, Li2S.

[0011] Optionally, 10≤H≤100.

[0012] Optionally, the sulfur-containing additive is selected from at least one of the following compounds:

[0013]

[0014]

[0015] Optionally, 0.5%≤W%≤5%.

[0016] Optionally, 5≤A≤20.

[0017] Optionally, the electrolyte further comprises an auxiliary additive, the mass percentage of the auxiliary additive in the electrolyte is 1% to 6%; the auxiliary additive comprises at least one of a carbonate additive, a phosphate additive, and a borate additive.

[0018] Optionally, the carbonate additive comprises at least one of fluoroethylene carbonate, vinylene carbonate, 3,3,3-trifluoropropylene carbonate, tetrafluoroethylene carbonate, and vinyl ethylene carbonate; and / or

[0019] The phosphate additive comprises at least one of tris(trimethylsilyl) phosphate, tris(trimethylsilyl) phosphite, tripropargyl phosphate, and diethyl fluorophosphate; and / or

[0020] The borate additive comprises at least one of tris(trimethylsilyl) borate, triallyl borate, and tris(triethylsilyl) borate.

[0021] Optionally, the first CEI film layer further comprises a plasticizer.

[0022] Optionally, the positive active material layer comprises a positive active material, a binder, and a conductive agent, the chemical formula of the positive active material is LiNi x Co y Mn (1-x-y) MzO2, 0.5≤x≤0.9, x+y<1, 0≤z<0.08, and M is one of Al, Mg, Zr, and Ti.

[0023] The application further discloses a preparation method of the lithium ion battery.

[0024] The positive electrode slurry is prepared, the positive electrode slurry is coated on at least one side surface of the positive electrode current collector to form a positive active material layer, and the positive electrode sheet intermediate product is obtained through drying and cold pressing.

[0025] A first CEI film precursor slurry is prepared, and the first CEI film precursor slurry is deposited on the surface of the positive active material layer of the positive electrode sheet intermediate product to form a first CEI film layer, thereby obtaining a positive electrode sheet.

[0026] The positive electrode sheet, the separator, the negative electrode sheet, and the electrolyte are prepared into a lithium ion battery.

[0027] Optionally, the step of preparing the first CEI film precursor slurry specifically includes:

[0028] The lithium salt compound and the plasticizer are dispersed in the solvent to obtain the first CEI film precursor slurry.

[0029] Compared with the prior art, the positive electrode sheet provided by the embodiment of the present application has the beneficial effects that in the lithium ion battery of the present application, the surface of the positive active material layer is covered with an artificial CEI film layer; a sulfur-containing additive is added to the electrolyte, and the thickness of the artificial CEI film layer is H μm, the specific surface area of the positive active material layer is A m 2 / g, and the mass percentage content of the sulfur-containing additive in the electrolyte is W%, which satisfies the relationship 0.5≤(H×W) / A≤20. The inventor of the present application has found that by controlling the specific surface area A of the positive active material, the thickness H of the artificial CEI film layer, and the content W% of the sulfur-containing additive in the electrolyte to satisfy the relationship 0.5≤(H×W) / A≤20, the low-temperature discharge performance and the high-temperature cycle performance of the lithium ion battery can be improved. DETAILED DESCRIPTION

[0030] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The preferred embodiments of the present application will be described in detail.

[0031] The embodiment of the present application provides a lithium ion battery. The lithium ion battery comprises a positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte arranged in the inside of the lithium ion battery; the separator is arranged between the positive electrode sheet and the negative electrode sheet. The positive electrode sheet comprises a positive current collector, a positive active material layer coated on the surface of the positive current collector, and a first CEI film layer covering the surface of the positive active material layer; the first CEI film layer comprises a lithium salt compound. A sulfur-containing additive is added to the electrolyte.

[0032] The thickness of the first CEI film layer is H μm; the specific surface area of the positive active material layer is A m 2 / g; the mass percentage content of the sulfur-containing additive in the electrolyte is W%; and the lithium ion battery satisfies the following relationship:

[0033] 0.5≤(H×W) / A≤20.

[0034] In the lithium ion battery of the present application, the surface of the positive active material layer is covered with a first CEI film layer; a sulfur-containing additive is added to the electrolyte, and the thickness of the first CEI film layer is H μm, the specific surface area of the positive active material layer is A m 2 / g, and the mass percentage content of the sulfur-containing additive in the electrolyte is W%, which satisfies the relationship 0.5≤(H×W) / A≤20. The inventors of the present application have found that by controlling the specific surface area A of the positive active material, the thickness H of the first CEI film layer, and the content W% of the sulfur-containing additive in the electrolyte to satisfy the relationship 0.5≤(H×W) / A≤20, the low-temperature discharge performance and high-temperature cycle performance of the lithium ion battery can be improved.

[0035] Specifically, the inventors of the present application found that during high-temperature cycling of lithium ion batteries, the positive electrode CEI film is unstable and prone to decomposition, consuming active lithium and leading to capacity attenuation. The sulfur-containing additive in the electrolyte of the present application can participate in the film formation on the positive electrode sheet, thereby forming a second CEI film layer covering the first CEI film layer. For simplicity of subsequent expression, the first CEI film layer and the second CEI film layer are collectively referred to as the CEI film. The second CEI film layer formed by the sulfur-containing additive is on the outer layer of the two-layer CEI film consisting of the first CEI film layer and the second CEI film layer, and has the characteristics of high temperature resistance, good chemical stability and few side reactions. The first CEI film is on the inner layer, has the characteristic of low impedance, and the first CEI film layer and the second CEI film layer complement each other, improving the rate and chemical stability of the interface film. The first CEI film formed is more dense and thin, which is more conducive to the transmission of lithium ions, and the interface film formed has better chemical stability and is not prone to decomposition at high temperature. Compared with traditional positive electrode film-forming additives such as 1,3-propanesulfonic acid lactone and ethylene sulfate, the film formation on the positive electrode sheet is more stable. The first CEI film layer in the present application can improve the transmission capacity of lithium ions at the interface, thereby reducing the interface impedance of the lithium ion battery. The first CEI film layer can reduce the reconstruction of the CEI film during formation and cycling (the second CEI film layer will continuously consume and decompose during the charging and discharging process of the battery, and then re-form a film). Lithium salt compounds are the main components of the first CEI film layer, which are equivalent to substances that play a protective role on the surface of the positive electrode sheet. They have high electrical conductivity, better chemical and thermal stability, avoid more side reactions at the interface, can build a protective layer with better chemical and mechanical stability at the interface, and reduce the consumption of active lithium. For example, compounds such as lithium nitride have strong ion-conducting ability and low interface impedance, which are beneficial to the rate performance of lithium ion batteries. Compounds such as LiAlH4 have strong mechanical properties and chemical stability, and the protective layer is not easily broken down and consumes active lithium during the cycling process of the lithium ion battery. At the same time, by controlling the specific surface area of the positive active material layer, the consumption of electrolyte during the cycling process of the lithium ion battery can be controlled, thereby ensuring that the active lithium content in the lithium ion battery system is at a reasonable level, thereby improving the low-temperature discharge and high-temperature cycling performance of the lithium ion battery.

[0036] The larger the specific surface area of the positive active material layer, the more interfaces between the electrolyte and the active material, the more opportunities for reaction, and the faster the consumption of electrolyte. If the specific surface area is too low, the contact between the electrolyte and the active material will be less, the transmission path of lithium ions will be less, and the consumption of electrolyte will be slowed down, but the rate performance will be severely lost.

[0037] In combination with the above relationship, H is the first CEI film thickness, the greater H, the more initial film formation. The mass percentage of sulfur-containing additives in the electrolyte is W%, which participates in the formation of the CEI film, the greater W, the better the film stability. The greater A, the greater the specific surface area of the positive active material layer, the more side reactions. The first two H and W are protection for the latter A, so the content of HxW relative to A is related to the protection of H and W to A.

[0038] In some optional embodiments, the value of (HxW) / A can be any value in the range of greater than or equal to 0.5 and less than or equal to 20, for example, it can be any one of 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20.

[0039] Specifically, the lithium salt compound includes one or more of Li4SiO4, Li2SiO3, Li3N, LiAlH4, Li2S. Compounds such as lithium nitride have strong ion conductivity and low interface impedance, which is beneficial to the rate performance of lithium ion batteries. Compounds such as LiAlH4 have strong mechanical properties and chemical stability, and in the lithium ion battery cycle process, the protective layer is not easy to break and recombine to consume active lithium.

[0040] Specifically, 10≤H≤100. The thickness H can be any value in the range of greater than or equal to 10 μm and less than or equal to 100 μm, for example, it can be any one of 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 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, 100 μm. The thickness of the first CEI film layer in this range can improve the transmission capacity of lithium ions at the interface, thereby reducing the interface impedance of the lithium ion battery. In addition, the first CEI film layer can reduce the consumption of active lithium during the reconstruction of the CEI film during formation and cycling. By controlling the thickness of the first CEI film layer within a certain range (10 μm≤H≤100 μm), on the one hand, the first CEI film layer is too thick, resulting in high impedance and high cost, on the other hand, the first CEI film layer is too low, which cannot improve the performance of the lithium ion battery.

[0041] Further, the first CEI film layer further comprises a plasticizer. The plasticizer can increase the viscosity and adhesion of the first CEI film layer slurry, so that the first CEI film layer is better adhered to the positive active material layer. The plasticizer can be carboxymethyl cellulose (Carboxymethyl Cellulose, CMC).

[0042] Specifically, the sulfur-containing additive is selected from at least one of the following compounds:

[0043]

[0044] Preferably, the sulfur-containing additive is compound 1-1 and / or compound 1-2, which is more stable in nature and easier to form film and more stable in film formation compared to other traditional film-forming agents.

[0045] Specifically, 0.5%≤W%≤5%. The specific value of W% can be any value in the range of greater than or equal to 0.5% and less than or equal to 5%, for example, it can be any one of 0.5%, 0.7%, 0.9%, 1.1%, 1.3%, 1.5%, 1.7%, 1.9%, 2.0%, 2.3%, 2.5%, 2.7%, 2.9%, 3.0%, 3.3%, 3.5%, 3.7%, 3.9%, 4.0%, 4.3%, 4.5%, 4.7%, 5%. By controlling the mass percentage of the sulfur-containing additive in the electrolyte within this range, the stability of the positive electrode CEI film is improved, while effectively avoiding the loss of active lithium in the positive and negative electrode materials and the electrolyte, thereby improving the low-temperature discharge performance and high-temperature cycle performance of the lithium ion battery.

[0046] Further, 5≤A≤20. The specific value of A can be any value in the range of greater than or equal to 5m 2 / g and less than or equal to 20m 2 / g, for example, it can be any one of 5m 2 / g, 6m 2 / g, 7m 2 / g, 8m 2 / g, 9m 2 / g, 10m 2 / g, 11m 2 / g, 12m 2 / g, 13m 2 / g, 14m 2 / g, 15m 2 / g, 16m 2 / g, 17m 2 / g, 18m 2 / g, 19m 2 / g, 20m 2 / g. By controlling the specific surface area of the positive electrode active material layer, the efficiency of electron conduction and ion mass transfer can be ensured, the side reactions of the electrolyte are reduced, the consumption of the electrolyte during the cycle process of the lithium ion battery is controlled, thereby ensuring the active lithium content in the lithium ion battery system at a reasonable level, thereby improving the cycle performance of the lithium ion battery. If the specific surface area of the lithium ion active material is too small (less than 5m 2 / g), the lithium ion active material particles are large, the electron conduction and ion mass transfer are limited, leading to decreased kinetic performance, increased DCR, and corresponding increased voltage polarization, further causing increased electrolyte side reaction, and thus decreased cycle performance. If the specific surface area of the lithium ion active material is too large (greater than 20 m 2 / g), the electrolyte and the positive electrode contact surface increase, the degree of side reaction increases, the electrolyte consumption accelerates, the active lithium content cannot be maintained at a reasonable level, and thus the cycle performance decreases.

[0047] Further, the electrolyte further comprises an auxiliary additive, the mass percentage of the auxiliary additive in the electrolyte is 1% to 6%; the auxiliary additive comprises at least one of a carbonate additive, a phosphate additive, and a borate additive. The electrolyte of the present application further adds an auxiliary additive on the basis of adding a sulfur-containing additive, and the auxiliary additive can further improve the cycle performance. The auxiliary additive and the sulfur-containing additive together form a CEI film, improve the compactness of the CEI film, effectively avoid the loss of protection of active lithium, improve the ion conductivity of the CEI film, and thus further improve the cycle performance and improve the low-temperature discharge performance. Especially when the auxiliary additive is a phosphate additive, it can form a more compact CEI film with the lithium salt compound in the first CEI film layer, inhibit the decomposition of the electrolyte solvent, slow down the consumption rate of the electrolyte, make the film thinner and more uniform, reduce the mass transfer resistance of lithium ions in the CEI film, further improve the ion conductivity of the CEI film, and at the same time have the adsorption capacity of the positive electrode surface singlet oxygen, thereby further improving the cycle performance and improving the low-temperature discharge performance. The mass percentage of the auxiliary additive in the electrolyte can be any value in the range of greater than or equal to 1% and less than or equal to 6%, for example, it can be any one of 1%, 1.2%, 1.5%, 1.6%, 1.8%, 2%, 2.2%, 2.4%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.6%, 3.8%, 4%, 4.2%, 4.5%, 4.6%, 4.8%, 5%, 5.2%, 5.5%, 5.6%, 5.8%, 6%.

[0048] Specifically, the carbonate additive comprises at least one of fluoroethylene carbonate, vinylene carbonate, 3,3,3-trifluoroallyl carbonate, tetrafluoroethylene carbonate, and vinyl ethylene carbonate.

[0049] Specifically, the phosphate additive comprises at least one of tris(trimethylsilyl) phosphate, tris(trimethylsilyl) phosphite, tripropargyl phosphate, and diethyl fluorophosphate.

[0050] Specifically, the borate additive comprises at least one of tris(trimethylsilyl) borate, triallyl borate, and tris(triethylsilyl) borate.

[0051] The electrolyte further includes a solvent, and the solvent includes at least one of ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl propyl carbonate, diphenyl carbonate, methyl acetate, methyl propionate, methyl butyrate, methyl acetate, ethyl acetate, propyl acetate, ethyl butyrate, propyl propionate, γ-butyrolactone, 1,3-dioxolane, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether. It should be noted that the present application does not make a special limitation on the preparation method of the electrolyte, and a person skilled in the art can prepare the electrolyte according to conventional technical means, for example, by uniformly mixing the raw materials of the electrolyte according to the proportion.

[0052] Further, the positive electrode active material layer includes a positive electrode active material, a binder, and a conductive agent, and is prepared into a positive electrode active slurry by a solvent in the preparation process. The positive electrode active material has a chemical formula of LiNixCoyMn(1-x-y)MzO2, 0.5≤x≤0.9, x+y<1, 0≤z<0.08, and M is one of Al, Mg, Zr, and Ti. The positive electrode current collector is provided with the positive electrode active material layer on at least one surface thereof.

[0053] In the present application, the type of the positive electrode current collector is not particularly limited, and it can be any material known to be suitable for use as a positive electrode current collector. In one embodiment, the positive electrode current collector includes metal materials such as aluminum, stainless steel, nickel plating, titanium, tantalum, and carbon materials such as carbon cloth and carbon paper. In one embodiment, the positive electrode current collector is a metal material.

[0054] In one embodiment, the carbon material can be one or more of natural graphite, artificial graphite, mesocarbon microbeads, hard carbon, soft carbon, silicon, and silicon-carbon composite.

[0055] In one embodiment, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer, and the negative electrode active material layer is coated on the surface of the negative electrode current collector.

[0056] In the present application, the negative electrode current collector is not particularly limited as long as it can achieve the purpose of the present application, and it can be, for example, a copper foil, a copper alloy foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foam, or a composite current collector.

[0057] In one embodiment, the negative electrode active material layer includes a negative electrode active material, a conductive agent, and a binder, and is prepared into a negative electrode active slurry by a solvent in the preparation process.

[0058] In one embodiment, the type of the conductive agent in the positive active material layer and the negative active material layer is not limited, and any known conductive agent can be used. The conductive agent in the positive active material layer can be at least one of acetylene black, carbon nanotube, and graphene. The conductive agent in the negative active material layer can be at least one of natural graphite, artificial graphite, and needle coke.

[0059] In one embodiment, the type of the binder in the positive active material layer and the negative active material layer is not limited, and any known binder can be used. The binder in the positive active material layer can be at least one of polyethylene, polypropylene, and polyethylene terephthalate. The binder in the negative active material layer can be at least one of polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, and nitrocellulose.

[0060] In the lithium ion battery mentioned in the present application, a separator is provided between the positive electrode and the negative electrode to prevent short circuit. The material and shape of the separator are not particularly limited. In one embodiment, the separator includes a porous sheet or non-woven fabric-like substance with excellent liquid retention, etc. The material of the resin or glass fiber separator includes, but is not limited to, polyolefin, aromatic polyamide, polytetrafluoroethylene, polyether sulfone, etc.

[0061] The lithium ion battery can include an outer package for packaging the positive electrode sheet, the negative electrode sheet, the separator, and the electrolyte. In some embodiments, the outer package of the lithium ion battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package can also be a soft package, such as a bag-type soft package. The material of the soft package can be plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate, etc. The present application does not particularly limit the shape of the lithium ion battery, which can be cylindrical, square, or any other shape.

[0062] Further, the present application also discloses a preparation method of the lithium ion battery mentioned above, comprising the steps of:

[0063] Preparation of a positive electrode slurry, coating the positive electrode slurry on at least one side surface of the positive electrode current collector to form a positive active material layer, and obtaining a positive electrode sheet intermediate product through drying and cold pressing;

[0064] Preparation of a first CEI film precursor slurry, deposition of the first CEI film precursor slurry on the surface of the positive active material layer of the positive electrode sheet intermediate product to form a first CEI film layer, and preparation of a positive electrode sheet;

[0065] Preparation of a lithium ion battery from the positive electrode sheet, the separator, the negative electrode sheet, and the electrolyte.

[0066] Specifically, the step of preparing the first CEI film precursor slurry is specifically: uniformly mixing the lithium salt compound and the plasticizer to obtain the first CEI film precursor slurry.

[0067] More specifically, the positive electrode sheet of the embodiment of the present application, the preparation method thereof comprises the following steps:

[0068] (1) uniformly mixing the positive electrode active material, the binder and the conductive agent into the solvent to obtain the positive electrode slurry, coating the positive electrode slurry on at least one side surface of the positive electrode current collector to form the positive electrode active material layer, and then drying and cold-pressing to obtain the positive electrode sheet intermediate product.

[0069] (2) uniformly mixing the lithium salt compound and the plasticizer to obtain the first CEI film precursor slurry, depositing the first CEI film precursor slurry on the surface of the positive electrode active material layer of the positive electrode sheet intermediate product to form the first CEI film layer, and obtaining the positive electrode sheet.

[0070] In step (2), the plasticizer is first uniformly dispersed in the solvent to obtain a precursor slurry solution, and the lithium salt compound is uniformly dispersed into the precursor slurry solution to obtain the first CEI film precursor slurry; then the first CEI film precursor slurry is continuously and uniformly deposited on the surface of the positive electrode active material layer under non-vacuum conditions by atomic layer deposition technology, and the positive electrode sheet is obtained after drying.

[0071] Specifically, in step (1), uniformly mixing the positive electrode active material, the binder and the conductive agent into the solvent helps to form a uniform slurry, ensures uniform distribution of the electrode material, facilitates coating and subsequent drying process, and improves the conductivity and mechanical stability of the electrode. Drying can remove the solvent, solidify the electrode material, and prevent the solvent residue from affecting the battery performance. Cold pressing can increase the density and mechanical strength of the electrode, which helps to improve the cycle stability of the battery.

[0072] In step (2), the lithium salt compound and the plasticizer are uniformly mixed, and the plasticizer can improve the flexibility and adhesion of the film and improve the protection effect of the film. The formed first CEI film layer can improve the transmission capacity of lithium ions at the interface, thereby reducing the interface impedance of the lithium ion battery. The added lithium salt compound is the main component of the first CEI film layer, which is equivalent to a substance that plays a protective role on the surface of the positive electrode sheet. It has high electrical conductivity, better chemical stability and thermal stability, avoids more side reactions of electrolyte at the interface, can build a protective layer with better chemical stability and mechanical stability at the interface, and reduces the consumption of active lithium. Further, the atomic layer deposition technology can realize uniform film deposition under non-vacuum conditions, ensure the uniformity of the CEI film, and improve the overall performance and life of the battery. After the first CEI film precursor slurry is deposited on the surface of the positive electrode active material layer and dried, the first CEI film can be solidified to ensure its adhesion with the positive electrode active material layer and improve the long-term stability of the battery.

[0073] The technical solutions of the present application are described in detail below through specific examples.

[0074] Example 1

[0075] The lithium ion battery of Example 1 of the present application is prepared according to a method comprising the following steps:

[0076] (1) Preparation of electrolyte

[0077] Vinyl carbonate EC, propylene carbonate PC, propyl propionate PP are mixed in a mass ratio of 1:1:3 to obtain a solvent. Then, based on the total mass of the electrolyte, the sulfur-containing additive and the auxiliary additive are added in the mass percentage amounts shown in Table 1, respectively, and then the lithium salt LiPF6 is added, and after uniform mixing, the electrolyte is prepared.

[0078] (2) Preparation of positive electrode sheet

[0079] The positive electrode active material lithium cobaltate (LiCoO2), the conductive agent carbon nanotube CNT, and the binder polyvinylidene fluoride PVDF are mixed in a mass ratio of 97:1.5:1.5 in N-methyl pyrrolidone NMP solvent and stirred thoroughly to form a uniform positive electrode slurry; the positive electrode slurry is uniformly coated on the surface of the positive electrode current collector to form a positive electrode active material layer; and after drying, cold pressing, slitting, welding, and adhesive application, a positive electrode sheet intermediate product is prepared.

[0080] 5% of the plasticizer CMC powder by mass percentage is uniformly dispersed in N-methyl pyrrolidone NMP solvent to obtain a precursor slurry solution I, and 95% of the lithium salt compound powder shown in Table 1 by mass percentage is uniformly dispersed into the precursor slurry solution I to obtain a first CEI film precursor slurry; the obtained first CEI film precursor slurry is continuously and uniformly deposited on the surface of the positive electrode active material layer of the first positive electrode sheet under non-vacuum conditions by atomic layer deposition technology, and after drying, a positive electrode sheet meeting the winding requirements is obtained.

[0081] (3) Preparation of negative electrode sheet

[0082] The negative electrode active material graphite, silicon, conductive agent acetylene black, adhesive styrene butadiene rubber SBR, and thickening agent sodium carboxymethyl cellulose CMC are mixed in a mass ratio of 90:6:1.2:1.5:1.3 in an appropriate amount of deionized water solvent and stirred thoroughly to form a uniform negative electrode slurry; the negative electrode slurry is uniformly coated on the surface of the negative electrode current collector copper foil, and after drying and cold pressing, a negative electrode sheet having negative electrode active material layers coated on both sides is obtained, which meets the winding requirements.

[0083] (4) Preparation of lithium ion battery

[0084] The positive electrode sheet, the separator and the negative electrode sheet are stacked in order with the separator between the positive electrode sheet and the negative electrode sheet to play a separating role, and then the stacked electrode sheet and the separator are wound to obtain a bare battery cell. The bare battery cell is placed in an aluminum plastic film shaped by punching, and top side sealing is completed. The electrolyte prepared above is injected into the battery cell after drying by baking, vacuum packaging, standing, and formation according to the formation current and temperature parameters in Table 1, to complete the preparation of the lithium ion battery.

[0085] The lithium ion batteries of Examples 2-18 and Comparative Examples 1-9 were prepared by the same preparation method as Example 1, with the differences shown in Table 1. That is, the thickness of the first CEI film layer, the specific surface area of the positive active material, the type and amount of sulfur-containing additives, and the type and amount of auxiliary additives of Examples 1-18 and Comparative Examples 1-9 are shown in Table 1.

[0086] Table 1

[0087]

[0088]

[0089] Lithium ion battery performance test: 45℃ cycle test:

[0090] The test method is as follows: the lithium ion battery is charged to 4.5V at 1C constant current and constant voltage in a 45±2℃ constant temperature box, the cutoff current is 0.05C, then discharged to 3V at 1C, and the above conditions are repeated for multiple charge and discharge cycles. The capacity retention rate of the lithium ion battery after 600 cycles is calculated, and each group has 5 lithium ion batteries.

[0091] Capacity retention rate (%) = discharge capacity (mAh) corresponding to the cycle number / third week cycle discharge capacity (mAh) * 100%

[0092] The capacity retention rate of each group of 5 lithium ion batteries after different cycle times is averaged and recorded in Table 1 above.

[0093] -30℃-0.2C rate discharge test:

[0094] The test method is as follows: the lithium ion battery is charged to 4.5V at 1C constant current and constant voltage in a 25±2℃ constant temperature box, the cutoff current is 0.05C, then discharged to 3V at 0.2C, and the discharge capacity R1 is recorded; then the battery is placed in a -30±2℃ constant temperature box, and the lithium ion battery is charged to 4.5V at 1C constant current and constant voltage, the cutoff current is 0.05C, then discharged to 3V at 0.2C, and the discharge capacity R2 is recorded.

[0095] -30℃-0.2C rate discharge capacity retention rate (%) = R2 / R1*100%

[0096] The lithium ion batteries of each group of 5 were recorded in Table 1 by taking the average value.

[0097] From the data of Comparative Example 2 and Comparative Example 1, it is shown that the lithium ion battery performance is improved by adding the sulfur-containing additive alone in Comparative Example 2, which is due to the fact that the sulfur-containing additive can form a more compact, thinner and more chemically stable CEI film on the positive electrode.

[0098] From the data of Comparative Example 3 and Comparative Example 1, it is shown that the lithium ion battery performance is deteriorated when the content of the sulfur-containing additive is too high in Comparative Example 3, which is due to the fact that the excess sulfur-containing additive can cause a sharp rise in interfacial impedance, deteriorating the performance of the lithium ion battery.

[0099] From the data of Examples 1-3, it is shown that the lithium ion battery performance is improved when the content of the sulfur-containing additive is gradually increased between 0.5% and 5%, which is due to the fact that the sulfur-containing additive can form a more compact, thinner and more chemically stable CEI film on the positive electrode.

[0100] From the data of Examples 4-7, it is shown that the lithium ion battery performance is improved when the specific surface area of the positive electrode active material layer is increased within the range of 5m 2 / g-20m 2 / g, which is due to the fact that the contact area between the electrolyte and the active material is increased, which is beneficial to the transport of lithium ions;

[0101] From the data of Comparative Examples 4-5, it is shown that if the specific surface area of the positive electrode active material layer is too small (less than 5), the lithium ion active material particles are large, the electron conduction and ion mass transfer are limited, resulting in a decrease in kinetic performance, an increase in DCR, and a corresponding increase in voltage polarization, further causing an increase in the degree of electrolyte side reaction, and thus a decrease in cycle performance. If the specific surface area of the positive electrode active material layer is too large (greater than 20), the contact area between the electrolyte and the positive electrode is increased, the degree of side reaction is increased, the consumption of electrolyte is accelerated, and the active lithium content cannot be maintained at a reasonable level, thus leading to a decrease in cycle performance. When the specific surface area A of the positive electrode active material layer is 5≤A≤20, the specific surface area of the positive electrode active material layer is controlled, which can ensure the efficiency of electron conduction and ion mass transfer, reduce the side reaction of electrolyte, control the consumption of electrolyte during the cycle process of the lithium ion battery, and thus ensure the active lithium content in the lithium ion battery system at a reasonable level, thereby improving the cycle performance and low-temperature discharge performance of the lithium ion battery.

[0102] From the data of Comparative Example 1, Comparative Examples 6-7, it is shown that the 45°C cycle capacity retention rate is poor when the thickness of the first CEI film layer is too small or too large, and the thickness of the first CEI film layer is slightly better when the thickness is 10μm;

[0103] The data of examples 4, 8-10 show that when the thickness of the first CEI film layer is increased in the range of 10-100 μm, the low-temperature discharge and high-temperature cycle performance is improved, the increase of the first CEI film layer helps to improve the transport of lithium ions in the electrolyte, reduces the consumption of lithium source by interface film reconstruction, and thus improves the performance of the lithium ion battery.

[0104] The data of examples 4, 13-15 show that when the sulfur-containing additive is one of the other several additives involved in the present application, the improvement effect is consistent. The data of examples 4, 16 show that when the first CEI film composition is other lithium-containing compounds, the improvement effect is consistent. The data of examples 11-12 and example 4 show that the present application can further improve the cycle performance on the basis of adding the sulfur-containing additive, and adding an auxiliary additive, wherein the phosphate ester additive can form a film on the surface of the positive electrode, inhibit the decomposition of the electrolyte solvent, slow down the consumption rate of the electrolyte, make the film thinner and more uniform, reduce the mass transfer resistance of lithium ions in the CEI film, and at the same time have the adsorption capacity of the positive electrode surface singlet oxygen.

[0105] The data of examples 11, 17-19 show that when the content of the auxiliary additive is increased in the range of 1%-6%, the low-temperature discharge and high-temperature cycle performance is improved.

[0106] As can be seen from Table 1, by controlling the specific surface area of the positive active material and the mass percentage content of the sulfur-containing additive in the electrolyte to satisfy the following relationship: 0.5≤(H×W) / A≤20, 0.5%≤W%≤5%, 10≤H≤100, 5≤A≤20, the low-temperature discharge performance and high-temperature cycle performance of the sodium ion lithium ion battery can be significantly improved.

[0107] It should be understood that the above examples are only used to illustrate the technical solutions of the present application, and are not limited thereto. For those skilled in the art, the technical solutions described in the above examples can be modified, or some technical features can be replaced by equivalents; all these modifications and replacements shall fall within the protection scope of the appended claims of the present application.

Claims

1. A lithium-ion battery, characterized in that, The lithium-ion battery includes a positive electrode, a separator, a negative electrode, and an electrolyte disposed therein; the separator is disposed between the positive electrode and the negative electrode. The positive electrode includes a positive current collector, a positive active material layer coated on the surface of the positive current collector, and a first CEI film layer covering the surface of the positive active material layer; the first CEI film layer includes a lithium salt compound. The electrolyte contains sulfur-containing additives; The thickness of the first CEI film is H μm; the specific surface area of ​​the positive electrode active material layer is A m. 2 / g; the sulfur-containing additive has a mass percentage content of W% in the electrolyte; the lithium-ion battery satisfies the following relationship: 0.5≤(H×W) / A≤20; The lithium salt compound includes one or more of Li4SiO4, Li2SiO3, Li3N, LiAlH4, and Li2S; The sulfur-containing additive is selected from at least one of the following compounds:

2. The lithium-ion battery according to claim 1, characterized in that, 10≤H≤100。 3. The lithium-ion battery according to claim 1 or 2, characterized in that, 0.5％≤W%≤5％。 4. The lithium-ion battery according to claim 1 or 2, characterized in that, 5≤A≤20。 5. The lithium-ion battery according to claim 1 or 2, characterized in that, The electrolyte also includes auxiliary additives, which have a mass percentage of 1% to 6% in the electrolyte; the auxiliary additives include at least one of carbonate additives, phosphate additives, and borate additives.

6. The lithium-ion battery according to claim 5, characterized in that, The carbonate additives include at least one of fluoroethylene carbonate, vinylene carbonate, propylene 3,3,3-trifluorocarbonate, ethylene tetrafluorocarbonate, and vinyl ethylene carbonate; and / or The phosphate ester additives include at least one of tris(trimethylsilyl)phosphate, tris(trimethylsilane)phosphite, and triargyl phosphate; and / or The borate ester additives include at least one of tris(trimethylsilane)borate, triallyl borate, and tris(triethylsilane)borate.

7. The lithium-ion battery according to claim 1 or 2, characterized in that, The first CEI film layer also includes a plasticizer.

8. The lithium-ion battery according to claim 1 or 2, characterized in that, The positive electrode active material layer includes a positive electrode active substance, a binder, and a conductive agent, wherein the chemical formula of the positive electrode active substance is LiNi. x Co y Mn (1-x-y) MzO2, 0.5≤x≤0.9, x+y<1, 0≤z<0.08, where M is one of Al, Mg, Zr and Ti.

9. The method for preparing a lithium-ion battery according to any one of claims 1 to 8, characterized in that, Including the following steps: Prepare a positive electrode slurry, coat the positive electrode slurry onto at least one side of the surface of the positive electrode current collector to form a positive electrode active material layer, and obtain a positive electrode intermediate product by drying and cold pressing; A first CEI film precursor slurry is prepared, and the first CEI film precursor slurry is deposited on the surface of the positive active material layer of the positive electrode intermediate to form a first CEI film layer, thereby obtaining a positive electrode. A lithium-ion battery is made by combining a positive electrode, a separator, a negative electrode, and an electrolyte.

10. The method for preparing a lithium-ion battery according to claim 9, characterized in that, The specific steps for preparing the first CEI membrane precursor slurry are as follows: The lithium salt compound and plasticizer are dispersed in a solvent to obtain the first CEI membrane precursor slurry.

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