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 safety primer layer coated on the surface of the positive electrode current collector and a positive electrode active material layer covering the surface of the positive electrode safety primer layer; the positive electrode safety primer layer comprises at least one active material in lithium iron phosphate, lithium manganese iron phosphate and lithium titanate; the electrolyte is added with a sulfur-containing additive; wherein the thickness of the positive electrode safety primer layer is H micrometers; the mass percentage of the positive electrode active material in the positive electrode active material layer in the total mass of the positive electrode active material and the positive electrode safety primer layer active material is A%; the mass percentage of the sulfur-containing additive in the electrolyte is W%; the lithium ion battery satisfies the following relationship: 0.0313 <= (H*W) / A <= 0.375, so that the low-temperature discharge performance and the thermal shock 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] A lithium ion battery is a common type of secondary battery, also known as a rechargeable battery or accumulator. It uses lithium ions as the positive electrode material, and generates current through the migration of lithium ions between the positive and negative electrodes. This type of battery is widely used in many electronic devices, such as smartphones, laptops, tablets, power tools, and electric vehicles, etc. Compared with traditional nickel-hydrogen batteries, lithium ion batteries have many advantages, including high energy density, small size, light weight, no memory effect, high charging and discharging efficiency, etc. This makes it an ideal choice for many portable electronic devices.

[0003] The electrolyte in a lithium ion battery serves as an ion transport medium for the oxidation-reduction reactions of the positive and negative electrodes, and plays an important role in 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. Moreover, with the increase in the surface area (reaction area) of the electrode material, the working temperature, and the instability of the electrode material, the low-temperature discharge performance and thermal shock performance of the lithium ion battery are not good. 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 thermal shock 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; the positive electrode sheet comprises a positive electrode current collector, a positive electrode safety primer layer coated on the surface of the positive electrode current collector, and a positive electrode active material layer covering the surface of the positive electrode safety primer layer; the positive electrode safety primer layer comprises at least one active material selected from lithium iron phosphate, lithium manganese iron phosphate and lithium titanate; and the electrolyte contains a sulfur-containing additive.

[0006] In the formula, the thickness of the positive electrode safety primer layer is H μm; the mass percentage of the positive electrode active material in the positive electrode active material layer in the total mass of the positive electrode active material and the positive electrode safety primer layer active material is A%; the mass percentage of the sulfur-containing additive in the electrolyte is W%; and the lithium ion battery satisfies the following relationship:

[0007] 0.0313≤(H×W) / A≤0.375.

[0008] Optionally, the thickness H of the positive electrode safety undercoat layer is 5≤H≤30.

[0009] Optionally, the sulfur-containing additive is selected from at least one of the following compounds:

[0010]

[0011]

[0012] Optionally, 0.5%≤W%≤5%.

[0013] 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.

[0014] 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

[0015] The phosphate additive comprises at least one of tris(trimethylsilyl) phosphate, tris(trimethylsilyl) phosphite, tripropargyl phosphate, and diethyl fluorophosphate; and / or

[0016] The borate additive comprises at least one of tris(trimethylsilyl) borate, triallyl borate, and tris(triethylsilyl) borate.

[0017] Optionally, A% is 80%≤A%≤95%.

[0018] Optionally, the positive electrode active material layer comprises a positive electrode active material, a binder, and a conductive agent.

[0019] Optionally, the positive electrode active material comprises at least one of lithium cobaltate, lithium manganate, lithium nickelate, lithium nickel cobalt manganate, lithium-rich manganese-based, and lithium nickel cobalt aluminum.

[0020] Optionally, the negative electrode sheet comprises a negative electrode current collector and a negative electrode active material, and the negative electrode active material is coated on the surface of the negative electrode current collector.

[0021] The application further discloses a preparation method of the lithium ion battery.

[0022] At least one of lithium iron phosphate, lithium iron manganese phosphate, and lithium titanate is mixed with an undercoat conductive agent and an undercoat binder in an undercoat organic solvent to obtain an undercoat slurry;

[0023] The undercoat slurry is coated on the positive electrode current collector, and then is dried and cold-pressed to form a positive electrode safety undercoat layer on the surface of the positive electrode current collector.

[0024] The positive electrode active material, the positive electrode conductive agent and the positive electrode binder are mixed by stirring in a positive electrode organic solvent to obtain a positive electrode active paste, the positive electrode active paste is coated on the positive electrode safety primer layer, and then drying, cold pressing are performed to form a positive electrode active material layer on the surface of the positive electrode safety primer layer, thereby obtaining a positive electrode sheet.

[0025] The positive electrode sheet, the negative electrode sheet, the separator and the shell are assembled into a lithium ion battery.

[0026] Compared with the prior art, the lithium ion battery provided by the embodiment of the present application has the beneficial effects that in the lithium ion battery, the positive electrode safety primer layer is coated between the positive electrode active material layer and the positive electrode current collector, the sulfur-containing additive is added in the electrolyte, and the thickness of the positive electrode safety primer layer is H μm, the mass percentage of the positive electrode active material layer in the positive electrode sheet is A %, and the mass percentage of the sulfur-containing additive in the electrolyte is W %, which satisfy the relationship 0.0313≤(H×W) / A≤0.375. The inventor of the present application has found that by controlling the mass percentage of the positive electrode active material layer, the thickness of the positive electrode safety primer layer and the sulfur-containing additive in the electrolyte to satisfy the relationship 0.5≤(H×W) / A≤20, the low-temperature discharge performance and the thermal shock performance of the lithium ion battery can be improved. DETAILED DESCRIPTION

[0027] 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.

[0028] 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 electrode current collector, a positive electrode safety primer layer coated on the surface of the positive electrode current collector and a positive electrode active material layer covering the surface of the positive electrode safety primer layer; the positive electrode safety primer layer comprises at least one active material selected from lithium iron phosphate, lithium manganese iron phosphate and lithium titanate; and the electrolyte is added with a sulfur-containing additive.

[0029] The thickness of the positive electrode safety primer layer is H μm; the mass percentage of the positive electrode active material in the positive electrode active material layer in the total mass of the positive electrode active material and the active material of the positive electrode safety primer layer is A %; the mass percentage of the sulfur-containing additive in the electrolyte is W %; and the lithium ion battery satisfies the following relationship:

[0030] 0.0313≤(H×W) / A≤0.375.

[0031] In the lithium ion battery of the present application, a positive electrode safety primer layer is coated between the positive electrode active material layer and the positive electrode current collector, a sulfur-containing additive is added to the electrolyte, and the thickness of the positive electrode safety primer layer is H microns, the mass percentage of the positive electrode active material layer in the positive electrode sheet is A%, and the mass percentage of the sulfur-containing additive in the electrolyte is W%, which satisfies the relationship 0.0313≤(H×W) / A≤0.375. The inventors of the present application have found that by controlling the mass percentage of the positive electrode active material layer, the thickness of the positive electrode safety primer layer, and the sulfur-containing additive in the electrolyte to satisfy the relationship 0.0313≤(H×W) / A≤0.375, the low-temperature discharge performance and thermal shock performance of the lithium ion battery can be improved.

[0032] Specifically, the inventors of the present application have found that when the lithium ion battery is subjected to thermal shock testing, the CEI film of the positive electrode sheet is rapidly decomposed, causing the positive electrode active material layer to be exposed to the electrolyte, accelerating the dissolution of metal elements from the positive electrode active material, such as cobalt atoms from the lithium cobaltate positive electrode active material, which accelerate the side reaction with the electrolyte, release oxygen, and intensify the temperature rise, ultimately leading to battery fire and thermal shock failure.

[0033] The sulfur-containing additive in the present application can participate in the film formation of the positive electrode sheet, forming a more dense and thinner CEI film that is beneficial to the transmission of lithium ions, has low impedance, and has better chemical stability of the interface film, which is more stable than the traditional positive electrode film-forming additives 1,3-propanesulfonic acid lactone and ethylene sulfate in the film formation of the positive electrode sheet.

[0034] The lithium iron phosphate, lithium manganese iron phosphate, and lithium titanate in the positive electrode safety primer layer have much better safety performance than the positive electrode active materials such as lithium cobaltate in the positive electrode active material layer. The contact position between the positive electrode active material layer and the positive electrode current collector is the area with the largest current, the most generation, and the best thermal shock performance. Therefore, the positive electrode safety primer layer between the positive electrode active material layer and the positive electrode current collector can effectively improve the thermal shock performance of the lithium ion battery.

[0035] The active material capacity of the positive electrode safety primer layer is low, and the low-temperature rate performance is poor. The CEI film formed by the sulfur-containing additive in the electrolyte of the present application improves the transmission ability of lithium ions and has low impedance. Therefore, the use of the positive electrode safety primer layer in combination with the sulfur-containing additive in the electrolyte improves the low-temperature performance of the positive electrode safety primer layer.

[0036] Further combining the above relationship, H is the thickness of the positive electrode safety primer layer, the larger H is, the more material that can be directly contacted with the electrolyte (the electrolyte will be soaked into the positive electrode safety primer layer) and has better safety and better thermal stability. The mass percentage of the sulfur-containing additive in the electrolyte is W%, which participates in the formation of the CEI film, the larger W is, the better the film stability is. The larger A is, the more unstable positive electrode active material and the more side reactions. The former two H and W are protective for the latter A, and therefore the content of HxW relative to A is related to the protection of H and W on A. The positive electrode safety primer layer with a thickness of H has relatively poor rate performance, more content, deteriorated low temperature, and the W% of the sulfur-containing additive needs to be in an appropriate range to improve the low temperature, and the W% is too high, the viscosity is large, and the low temperature is deteriorated.

[0037] In some optional embodiments, the value of (HxW) / A can be any value in the range of greater than or equal to 0.0313 and less than or equal to 0.375, for example, can be any one of 0.0313, 0.318, 0.323, 0.328, 0.333, 0.338, 0.343, 0.348, 0.353, 0.358, 0.363, 0.368, 0.373, 0.375.

[0038] Specifically, the thickness H of the positive electrode safety primer layer is 5≤H≤30. The specific value of the thickness H can be any value in the range of greater than or equal to 5 μm and less than or equal to 30 μm, for example, can be any one of 5 μm, 7 μm, 10 μm, 11 μm, 13 μm, 15 μm, 17 μm, 19 μm, 20 μm, 23 μm, 25 μm, 27 μm, 29 μm, 30 μm. Controlling the thickness of the positive electrode safety primer layer to be greater than 5 μm can ensure that the positive electrode sheet will not collapse and heat or form strong oxidizing substances at high temperature or overcharge. Controlling the thickness of the positive electrode safety primer layer to be less than 30 μm can avoid the problem of poor conductivity of the positive electrode sheet caused by the positive electrode safety primer layer being too thick.

[0039] The sulfur-containing additive is selected from at least one of the following compounds:

[0040]

[0041]

[0042] Preferably, the sulfur-containing additive is compound 1-1 and / or compound 1-2, which is more stable in nature and easier to form a film than other traditional film formers.

[0043] 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, 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%. The present scheme controls the mass percentage content of the sulfur-containing additive in the electrolyte, and within this range, the formed CEI film is more dense, thinner, more conducive to the transmission of lithium ions, and the formed interface film has better chemical stability and is not easy to decompose at high temperature, thus improving the thermal shock performance and low-temperature discharge performance of the lithium ion battery.

[0044] The electrolyte further comprises an auxiliary additive, the mass percentage content of the auxiliary additive in the electrolyte is 1%-6%; the auxiliary additive comprises at least one of a carbonate additive, a phosphate additive, and a borate additive. The mass percentage content 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, 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%. The electrolyte of the present application further adds an auxiliary additive on the basis of adding a sulfur-containing additive, which can further improve the cycle performance. The auxiliary additive and the sulfur-containing additive together form a CEI film, improve the density of the CEI film, effectively avoid the loss of protecting active lithium, improve the ion conductivity of the CEI film, and thus further improve the cycle performance and the thermal shock performance and low-temperature discharge performance of the battery.

[0045] In particular, when the auxiliary additive is a phosphate additive, the carbonate additive can form a film on the surface of the positive plate, inhibit the decomposition of the electrolyte solvent, slow down the consumption rate of the electrolyte, make the film thinner, more uniform and denser, reduce the mass transfer resistance of lithium ions in the CEI film, and form a more dense CEI film with the sulfur-containing additive, further improve the ion conductivity of the CEI film, and thus further improve the thermal shock performance and low-temperature discharge performance of the battery.

[0046] Specifically, the carbonate-based additive includes at least one of fluoroethylene carbonate, vinylene carbonate, 3,3,3-trifluoro propylene carbonate, tetrafluoroethylene carbonate, and vinyl ethylene carbonate; and / or the phosphate-based additive includes at least one of tris(trimethylsilyl) phosphate, tris(trimethylsilyl) phosphite, tripropargyl phosphate, and diethyl fluorophosphate; and / or the borate-based additive includes at least one of tris(trimethylsilyl) borate, triallyl borate, and tris(triethylsilyl) borate.

[0047] Specifically, A% is 80%≤A%≤95%. The mass percentage of the positive electrode active material layer to the positive electrode sheet can be any value in the range of greater than or equal to 80% and less than or equal to 95%, for example, can be any one of 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, and 95%. By controlling the content of the positive electrode active material, the lithium ion battery can achieve a certain capacity, while taking into account the low-temperature rate performance and safety performance. If the content of the lithium ion active material is higher than 95%, the safety performance of the lithium ion battery is poor; if the content of the lithium ion active material is lower than 80%, the low-temperature rate performance of the lithium ion battery is poor, and the battery capacity is low.

[0048] Further, the positive electrode active material layer is arranged on at least one surface of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material, a positive electrode binder, and a positive electrode conductive agent. The positive electrode active material includes at least one of lithium cobaltate, lithium manganate, lithium nickelate, lithium nickel cobalt manganate, lithium-rich manganese-based, and lithium nickel cobalt aluminum.

[0049] In the present application, the type of the positive electrode current collector is not particularly limited, which can be any known material suitable for use as a positive electrode current collector, for example, the positive electrode current collector includes metal materials such as aluminum, stainless steel, nickel plating layer, titanium, tantalum, and carbon materials such as carbon cloth and carbon paper. Specifically, the positive electrode current collector is a metal material. The carbon material can be one or more of natural graphite, artificial graphite, mesophase carbon microbeads, hard carbon, soft carbon, silicon, and silicon-carbon composite. In one embodiment,

[0050] 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. The negative electrode active material layer includes a negative electrode active material, a negative electrode binder, and a negative electrode conductive agent; the negative electrode active material includes at least one of natural graphite, artificial graphite, mesophase carbon microbeads, hard carbon, soft carbon, silicon, silicon-carbon composite, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel-structured lithiated TiO2-Li4Ti5O 12 , Li-Al alloy.

[0051] In the present application, the type of negative current collector is not particularly limited, and can be any material known to be suitable for use as a negative current collector, as long as the object of the present application can be achieved. For example, it can be a copper foil, a copper alloy foil, a nickel foil, a stainless steel foil, a titanium foil, a foamed nickel, a foamed copper, or a composite current collector, etc. In one embodiment, the type of positive and negative conductive agents mentioned in the present application is not limited, and any known conductive agent can be used. The positive conductive agent can be at least one of acetylene black, carbon nanotubes, and graphene. The negative conductive agent can be at least one of natural graphite, artificial graphite, and needle coke. The type of positive and negative binders mentioned in the present application is not limited, and any known binder can be used. The positive binder can be at least one of polyethylene, polypropylene, and polyethylene terephthalate, and the negative binder can be at least one of polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, and nitrocellulose.

[0052] In the present application, in order to prevent short circuit, a separator is provided between the positive electrode and the negative electrode. The material and shape of the separator are not particularly limited in the present application. In one embodiment, the separator includes a porous sheet or non-woven fabric-like substance having 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. The lithium ion battery includes an outer package that can be used to package the above-mentioned electrode assembly and electrolyte.

[0053] In one embodiment, 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 particular limitation on the preparation method of the electrolyte, and those skilled in the art can prepare it into an electrolyte according to conventional technical means, for example, by uniformly mixing the raw materials according to the ratio.

[0054] The lithium ion battery can include an outer package for packaging the above-mentioned positive electrode sheet, negative electrode sheet, separator, and 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 shape of the lithium ion battery is not particularly limited in the present application, and it can be cylindrical, square, or any other shape.

[0055] Further, the application also discloses a preparation method of the lithium ion battery.

[0056] S1, at least one active material of lithium iron phosphate, lithium manganese iron phosphate and lithium titanate, a primer conductive agent and a primer binder are stirred and mixed in a primer organic solvent to obtain a primer slurry;

[0057] S2, the primer slurry is coated on the positive current collector, and then drying and cold pressing are performed, so that the primer slurry forms a positive safety primer layer on the surface of the positive current collector.

[0058] S3, a positive active material, a positive conductive agent and a positive binder are stirred and mixed in a positive organic solvent to obtain a positive active slurry, the positive active slurry is coated on the positive safety primer layer, and then drying and cold pressing are performed, so that the positive active slurry forms a positive active material layer on the surface of the positive safety primer layer, thereby obtaining a positive sheet.

[0059] The positive sheet, the negative sheet, the diaphragm and the shell are assembled into the lithium ion battery.

[0060] The technical scheme of the application will be described in detail below through specific embodiments.

[0061] Embodiment 1

[0062] The lithium ion battery of the embodiment 1 of the application is prepared by the following steps.

[0063] S1, an active material LiFePO4, a primer conductive agent CNT (Carbon Nanotube) and a primer binder PVDF (polyvinylidene fluoride) are fully stirred and mixed in a primer organic solvent N-methyl pyrrolidone at a mass ratio of 97:1.5:1.5 to form a uniform primer slurry, the primer slurry is coated on a positive current collector made of an aluminum foil, and then drying and cold pressing are performed on the primer slurry, so that a positive safety primer layer is formed on the surface of the positive current collector.

[0064] S2, a positive active material LiCoO2, a positive conductive agent CNT and a positive binder PVDF are fully stirred and mixed in a positive organic solvent N-methyl pyrrolidone at a mass ratio of 97:1.5:1.5 to form a uniform positive active slurry, the positive active slurry is coated on the positive safety primer layer, and then drying and cold pressing are performed, so that a positive active material layer is formed on the surface of the positive safety primer layer, thereby obtaining a positive sheet.

[0065] S31, negative electrode sheet preparation: the negative electrode active material graphite, silicon, conductive agent acetylene black, adhesive styrene-butadiene rubber SBR, thickening agent sodium carboxymethyl cellulose CMC were mixed in a mass ratio of 90:6:1.2:1.5:1.3 in a proper amount of deionized water solvent, and fully stirred to form a uniform negative electrode slurry; the negative electrode slurry was uniformly coated on the surface of the negative electrode current collector copper foil, and after drying and cold pressing, a negative electrode sheet coated with a negative electrode active material layer on both sides was obtained, which met the winding requirements.

[0066] S32, preparation of electrolyte: mixing ethylene carbonate EC, propylene carbonate PC, propyl propionate PP in a mass ratio of 1:1:3 to obtain a solvent. Then, based on the total mass of the electrolyte, the additives were added in the mass percentage shown in Table 1, and then lithium salt LiPF6 was added, and the electrolyte was prepared after mixing.

[0067] S33, using PE porous polymer film as a separator, the positive electrode sheet, the separator and the negative electrode sheet were stacked in order, the separator was between the positive electrode sheet and the negative electrode sheet, the separator played a role of isolation for the positive electrode sheet and the negative electrode sheet, then the stacked positive electrode sheet, negative electrode sheet and separator were wound to obtain a roll core, and the roll core was baked and dried, and then placed in a shell. The shell is an aluminum plastic film bag formed by punching. Then the electrolyte was injected into the shell, and the shell was vacuum packaged, placed, and formed, and the preparation of the lithium ion battery was completed.

[0068] The thickness of the positive electrode safety primer, the mass percentage of the positive electrode active material LiCoO2, the type and amount of the sulfur-containing additive, and the content and amount of the auxiliary additive are shown in Table 1.

[0069] The lithium ion batteries of Examples 2-18 and Comparative Examples 1-9 were prepared by the same preparation method as Example 1, and the differences are shown in Table 1. That is, the thickness of the positive electrode safety primer, the mass percentage of the positive electrode active material LiCoO2, the type and amount of the sulfur-containing additive, and the content and amount of the auxiliary additive of Examples 1-18 and Comparative Examples 1-9 are shown in Table 1.

[0070] Table 1

[0071]

[0072]

[0073]

[0074] Battery performance test

[0075] Thermal shock test: 1.5C constant current constant voltage charge to 4.35V at room temperature, 0.5C constant current constant voltage charge to 4.5V, 0.05C cut-off current, stand for 2H, record the internal resistance and voltage of the battery; the full battery is placed in an oven, the temperature is raised to 130℃±2℃ at a rate of(5℃±2℃) / min, then kept for 60min, the battery does not catch fire, does not explode, it is passed.

[0076] Low temperature discharge test: the test method is: in a 25±2℃ constant temperature box, the lithium ion battery is charged to 4.5V at 1C constant current constant voltage, the cut-off current is 0.05C, and 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, the lithium ion battery is charged to 4.5V at 1C constant current constant voltage, the cut-off current is 0.05C, and then discharged to 3V at 0.2C, and the discharge capacity R2 is recorded.

[0077] -30℃-0.2C rate discharge capacity retention rate (%) = R2 / R1*100%

[0078] Each group of 5 batteries is recorded in Table 1 by taking the average value.

[0079] From the data of Comparative Example 2 and Comparative Example 1, it can be seen that the addition of sulfur-containing additives alone in Comparative Example 2 can improve the low temperature discharge performance and thermal shock performance of lithium ion batteries, which is due to the fact that the sulfur-containing additives can form a more dense, thinner and more chemically stable CEI film on the positive electrode.

[0080] From the data of Comparative Example 3 and Comparative Example 1, it can be seen that when the content of sulfur-containing additives in Comparative Example 3 is too high, the low temperature discharge performance is deteriorated, which is due to the fact that excessive additives will cause the interface impedance to rise sharply, deteriorating the battery performance.

[0081] From the data of Examples 1-3, it can be seen that when the content of sulfur-containing additives is gradually increased between 0.5% and 5%, the low temperature discharge performance and thermal shock performance are improved, which is due to the fact that the sulfur-containing additives can form a more dense, thinner and more chemically stable CEI film on the positive electrode.

[0082] From the data of Example 4, Example 5-7, it can be seen that when the content of positive active material LiCo2 is increased in the range of 80%-95%, the low temperature discharge performance is improved, but the thermal shock is deteriorated.

[0083] From the data of Example 4, Example 5-7, it is shown that the mass percentage A% of the first positive electrode active material in the positive electrode sheet is 80%≤A%≤95%, by controlling the content of the positive electrode active material, the battery can reach a certain capacity, while taking into account the low temperature rate performance and safety performance. From the data of Comparative Example 5, it is shown that if the content of the lithium ion active material is higher than 95%, the safety performance of the battery is poor; from the data of Comparative Example 4, it is shown that if the content of the lithium ion active material is lower than 80%, the low temperature rate performance of the battery is poor, and the battery capacity is low.

[0084] From the data of Example 4, Example 8-10, it is shown that when the thickness of the positive electrode safety primer layer is increased in the range of 5-30 μm, the low temperature discharge is poor, because the active material such as lithium iron phosphate will cause the conductivity to be poor. However, when the thickness of the positive electrode safety primer layer is increased, the thermal shock performance of the battery is improved. When the thickness of the positive electrode safety primer layer in Comparative Example 10 is 4 μm, due to the insufficient thickness of the positive electrode safety primer layer, the low temperature discharge is poorer than that of Example 4.

[0085] From the data of Comparative Examples 6-7, Comparative Example 1, it is shown that when the thickness of the positive electrode safety primer layer H=5 μm, the thermal shock pass rate is slightly better than H=1 μm, and worse than H=50 μm, the-30℃-0.2C rate discharge retention rate is better than H=50 μm, and worse than H=1 μm.

[0086] From the data of Example 4, Example 13-15, it is shown that when the sulfur-containing additive is other several kinds involved in the present application, the improvement effect is consistent. From the data of Example 4, Example 16, it is shown that when the positive electrode active material of the positive electrode safety primer layer is other materials, the improvement effect is consistent.

[0087] From the data of Example 11-12 and Example 4, it is shown that on the basis of the sulfur-containing additive, the addition of the auxiliary additive can further improve the battery performance, wherein the carbonate 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, more uniform and denser, and reduce the mass transfer resistance of lithium ions in the CEI film.

[0088] From the data of Example 11, Example 17-19, it is shown that when the content of the auxiliary additive is increased in the range of 1%-6%, the-30℃-0.2C rate discharge retention rate is improved, and the thermal shock pass rate remains stable. From Table 1, it can be seen that by controlling the mass percentage of the positive electrode active material, the mass percentage of the sulfur-containing additive in the electrolyte, and the thickness of the positive electrode safety primer layer, the present application satisfies the following relationship: 0.0313≤(H×W) / A≤0.375, and at the same time satisfies 0.5%≤W%≤5%, 5 μm≤H≤30 μm, 80%≤A%≤95%, which can significantly improve the low temperature discharge performance and thermal shock performance of the lithium ion battery.

[0089] It should be understood that the above examples are only used to illustrate the technical solutions of the present application, but not limit the technical solutions of the present application. Those skilled in the art can make modifications to the technical solutions described in the above examples, or make equivalent replacements to some of the technical features. All these modifications and replacements shall fall within the protection scope of the present application.

Claims

1. A lithium-ion battery, characterized by, The lithium ion battery comprises a positive electrode sheet, a separator, a negative electrode sheet and an electrolyte arranged inside; the separator 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 safety primer layer coated on the surface of the positive electrode current collector, and a positive electrode active material layer covering the surface of the positive electrode safety primer layer; the positive electrode safety primer layer comprises at least one active material selected from lithium iron phosphate, lithium manganese iron phosphate and lithium titanate; The electrolyte is added with a sulfur-containing additive; The thickness of the positive electrode safety primer layer is H pm; the mass percentage of the positive electrode active material in the positive electrode active material layer in the total mass of the positive electrode active material and the positive electrode safety primer layer is A%; the mass percentage of the sulfur-containing additive in the electrolyte is W%; the lithium ion battery satisfies the following relationship: 0.0313≤(H×W) / A≤0.375; The thickness H of the positive electrode safety primer layer is 5≤H≤30; the sulfur-containing additive is at least one selected from the following compounds:

2. The lithium-ion battery of claim 1, wherein, 0.5%≤W%≤5%。 3. The lithium-ion battery of claim 1, wherein, The electrolyte further comprises an auxiliary additive, and the mass percentage of the auxiliary additive in the electrolyte is 1%-6%; the auxiliary additive comprises at least one selected from carbonate-based additives, phosphate-based additives and borate-based additives.

4. The lithium-ion battery of claim 3, wherein, The carbonate-based additives comprise at least one selected from fluoroethylene carbonate, vinylene carbonate, 3,3,3-trifluoroallyl carbonate, tetrafluoroethylene carbonate and vinyl ethylene carbonate; and / or The phosphate-based additives comprise at least one selected from tris(trimethylsilyl) phosphate, tris(trimethylsilyl) phosphite, tripropargyl phosphate and diethyl fluorophosphate; and / or The borate-based additives comprise at least one selected from tris(trimethylsilyl) borate, triallyl borate and tris(triethylsilyl) borate.

5. The lithium-ion battery of claim 1, wherein, The A% is 80%≤A%≤95%.

6. The lithium-ion battery of claim 1, wherein, The positive electrode active material layer comprises a positive electrode active material, a binder and a conductive agent.

7. The lithium-ion battery of claim 6, wherein, The positive electrode active material comprises at least one selected from lithium cobaltate, lithium manganate, lithium nickelate, lithium nickel cobalt manganate, lithium-rich manganese-based and lithium nickel cobalt aluminum.

8. The lithium-ion battery of claim 1, wherein, The negative electrode sheet comprises a negative electrode current collector and a negative electrode active material coated on the surface of the negative electrode current collector.

9. The method of producing a lithium-ion battery according to any one of claims 1 to 8, characterized in that, The method comprises the following steps: at least one active material selected from lithium iron phosphate, lithium manganese iron phosphate and lithium titanate, a primer conductive agent and a primer binder are stirred and mixed in a primer organic solvent to obtain a primer slurry; the primer slurry is coated on the positive electrode current collector, and then drying and cold pressing are performed to form a positive electrode safety primer layer on the surface of the positive electrode current collector; a positive electrode active material, a positive electrode conductive agent and a positive electrode binder are stirred and mixed in a positive electrode organic solvent to obtain a positive electrode active slurry, the positive electrode active slurry is coated on the positive electrode safety primer layer, and then drying and cold pressing are performed to form a positive electrode active material layer on the surface of the positive electrode safety primer layer to obtain a positive electrode sheet; the positive electrode sheet, the negative electrode sheet, the separator and a shell are assembled into a lithium ion battery.

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