Lithium ion secondary battery

Abstract

The present disclosure provides a lithium ion secondary battery. The lithium ion secondary battery of the present disclosure comprises a positive electrode sheet; the positive electrode sheet comprises a positive electrode current collector, a positive electrode active layer, and a positive electrode protection layer arranged between the positive electrode current collector and the positive electrode active layer; the positive electrode protection layer comprises conductive particles; the positive electrode active layer comprises a positive electrode active material; the ratio of the thickness d1 of the positive electrode protection layer to the particle size dv50 of the conductive particles is greater than or equal to 4. According to the lithium ion secondary battery of the present disclosure, short-circuit contact between the positive electrode current collector and a negative electrode sheet can be avoided when the battery is mechanically abused, so that the lithium ion secondary battery has excellent safety performance; moreover, the active material in the positive electrode active layer can be prevented from being in direct contact with the positive electrode current collector, thereby reducing the local damage of the active material to the current collector during rolling, enhancing the mechanical strength of the current collector, solving the problem that an electrode sheet easily breaks, and improving the safety performance of the battery. In addition, the overall compaction density of the positive electrode sheet can be improved, and the energy density of the battery can be considered.

Description

A lithium-ion secondary battery Technical Field

[0001] The present disclosure relates to the technical field of lithium-ion batteries, and in particular to a lithium-ion secondary battery.

[0002] Background of the Invention

[0003] When a lithium-ion battery experiences mechanical damage, a short circuit between the positive current collector and the negative electrode can easily occur, leading to excessive heat generation and potentially fire and failure. Therefore, a protective layer is often applied to the positive current collector to reduce the likelihood of short circuits between the positive and negative electrodes and improve battery safety.

[0004] However, since the protective layer has a certain thickness, increasing the thickness of the pole piece will lead to a decrease in the energy density of the battery. Therefore, limiting the thickness of the protective layer or increasing the compaction density of the active material is often adopted to reduce the adverse effects of coating the protective layer on the positive electrode current collector on the battery energy density. However, after the pole piece is rolled to increase the compaction density, the sharp edges of the active material will cause local deformation and shrinkage of the current collector, which will lead to a decrease in the mechanical strength of the current collector and easily cause local damage to the pole piece. As the compaction density of the active material continues to increase, the pole piece may even break, seriously affecting the safety performance of the battery.

[0005] Therefore, the market is in urgent need of new lithium-ion batteries to solve this technical problem. Summary of the Invention

[0006] To improve the above-mentioned problems existing in the prior art, the present disclosure provides a lithium-ion secondary battery. The lithium-ion secondary battery disclosed herein includes a positive electrode sheet. By disposing a positive electrode protective layer between the positive electrode current collector and the positive electrode active layer, the positive electrode sheet can prevent the positive electrode current collector from short-circuiting with the negative electrode sheet when the battery is mechanically abused, thereby providing excellent safety performance. At the same time, by limiting the relative relationship between the particle size of the conductive particles in the positive electrode protective layer and the thickness of the positive electrode protective layer, the active material in the positive electrode active layer can be prevented from directly contacting the positive electrode current collector, thereby reducing local damage to the current collector by the active material during rolling, improving the mechanical strength of the current collector, solving the problem of easy breakage of the electrode sheet, improving the safety performance of the battery, and increasing the overall compaction density of the positive electrode sheet, while taking into account the energy density of the lithium-ion battery.

[0007] To achieve the above objectives, the technical solutions adopted in this disclosure are as follows:

[0008] A first aspect of the present disclosure provides a lithium ion secondary battery, comprising a positive electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector, a positive electrode active layer, and a positive electrode protective layer disposed between the positive electrode current collector and the positive electrode active layer;

[0009] The positive electrode protection layer includes conductive particles;

[0010] The positive electrode active layer includes a positive electrode active material;

[0011] The ratio of the thickness d1 of the positive electrode protection layer to the particle size dv50 of the conductive particles is d1 / dv50≥4.

[0012] Through the above technical solution, the present disclosure has at least the following advantages compared with the prior art:

[0013] (1) The positive electrode sheet provided by the present disclosure has a positive electrode protective layer disposed between the positive electrode current collector and the positive electrode active layer, which can prevent the positive electrode current collector from short-circuiting with the negative electrode sheet when the battery is mechanically abused, thereby providing excellent safety performance;

[0014] (2) In the positive electrode sheet provided by the present invention, there is a specific relationship between the particle size dv50 of the conductive particles and the thickness d1 of the positive electrode protective layer, which avoids direct contact between the active material in the positive electrode active layer and the positive electrode current collector, can reduce the local damage of the active material to the current collector during rolling, improve the mechanical strength of the current collector, solve the problem of easy breakage of the electrode sheet, and further improve the safety performance of the battery; and can reduce the thickness of the electrode sheet by increasing the overall compaction density of the positive electrode sheet, while improving the energy density of the lithium-ion battery.

[0015] Additional aspects and advantages of the embodiments of the present application will be described and shown in part in the subsequent description, or explained through the implementation of the embodiments of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIG1 is a cross-sectional SEM image of a positive electrode sheet without a positive electrode protective layer in an example provided by the present disclosure;

[0017] FIG2 is a cross-sectional SEM image of a positive electrode sheet with a positive electrode protection layer in an example provided by the present disclosure. DETAILED DESCRIPTION

[0018] The following describes the specific embodiments of the present disclosure in detail. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present disclosure and are not intended to limit the present disclosure.

[0019] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein.

[0020] A first aspect of the present disclosure provides a lithium-ion secondary battery, comprising a positive electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector, a positive electrode active layer, and a positive electrode protective layer arranged between the positive electrode current collector and the positive electrode active layer; the positive electrode protective layer comprises conductive particles; the positive electrode active layer comprises a positive electrode active material; and the ratio of the thickness d1 of the positive electrode protective layer to the particle size dv50 of the conductive particles is d1 / dv50≥4.

[0021] In one example, the positive electrode active layer is located on any one surface of the current collector, and the positive electrode protective layer is located between the positive electrode current collector and the positive electrode active layer on any one surface. In one example, the positive electrode active layer is located on both surfaces of the current collector, and the positive electrode protective layer is located between the positive electrode current collector and the positive electrode active layers on both surfaces, and the positive electrode protective layer is provided on both surfaces of the current collector. In one example, the positive electrode active layer is located on both surfaces of the current collector, and the positive electrode protective layer is located between the positive electrode current collector and the positive electrode active layer on any one surface, and the positive electrode protective layer is provided on only one surface of the current collector.

[0022] It is understood that dv50 is the particle size corresponding to the proportion of the conductive particles in the volume distribution being equal to 50%, and d1 is the average thickness of the positive electrode protective layer. In the present disclosure, in order to reduce the impact of the positive electrode protective layer on the battery energy density, the thickness of the positive electrode active layer can be reduced by increasing the compaction density of the positive electrode active material, thereby increasing the energy density of the battery; however, in conventional batteries, the active material is directly coated on the current collector. As shown in Figure 1, when the compaction density of the active material after roller pressing is large, the sharp edges of the active material will cause local deformation and shrinkage of the current collector, thereby reducing the mechanical strength of the current collector and making the pole piece prone to breakage. Therefore, when the active material is directly coated on the current collector, the compaction density of the pole piece usually has a limit value. The present invention coats a positive electrode protective layer between the positive electrode current collector and the positive electrode active layer, and makes the ratio of the thickness d1 of the positive electrode protective layer to the particle size dv50 of the conductive particles contained in the positive electrode protective layer d1 / dv50≥4, thereby ensuring that a certain number of conductive particles are present along the thickness direction of the positive electrode protective layer, effectively avoiding direct contact between the positive electrode active material and the positive electrode current collector, reducing local damage to the current collector by the sharp edges of the positive electrode active material during rolling, thereby improving the mechanical strength of the current collector and solving the problem of easy breakage of the electrode sheet. Moreover, after avoiding direct contact between the positive electrode active material and the positive electrode current collector, the thickness of the electrode sheet can be reduced by increasing the overall compaction density of the positive electrode sheet, thereby taking into account both the safety performance and energy density of the lithium-ion battery.

[0023] In one embodiment, the ratio d1 / dv50 of the thickness d1 of the positive electrode protective layer to the particle size dv50 of the conductive particles is 4 to 20. In one embodiment, the ratio d1 / dv50 of the thickness d1 of the positive electrode protective layer to the particle size dv50 of the conductive particles can be 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, or a range consisting of any two values. When the ratio d1 of the thickness of the positive electrode protective layer to the particle size dv50 of the conductive particles is greater than 20, the number of conductive particles along the thickness direction of the positive electrode protective layer is too large. Although this can solve the problem of easy breakage of the electrode sheet, the thickness of the electrode sheet will increase, affecting the energy density of the battery. Therefore, it is necessary to further control d1 / dv50≤20.

[0024] In one embodiment, d1 / dv50 is 6-10.

[0025] In one embodiment, the particle size dv50 of the conductive particles is 0.01 μm to 1 μm. In one embodiment, the particle size dv50 of the conductive particles can be 0.01 μm, 0.02 μm, 0.03 μm, 0.04 μm, 0.05 μm, 0.06 μm, 0.07 μm, 0.08 μm, 0.09 μm, 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, or a range consisting of any two values.

[0026] In one embodiment, the particle size dv50 of the conductive particles is 0.05 μm to 0.5 μm.

[0027] In one embodiment, the thickness d1 of the positive electrode protective layer is 0.1 μm to 5 μm. In one embodiment, the thickness d1 of the positive electrode protective layer can be 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, or a range consisting of any two values.

[0028] In one embodiment, the thickness d1 of the positive electrode protection layer is 0.5 μm to 2 μm.

[0029] It should be noted that the thickness d1 of the positive electrode protective layer can be measured using the following method: After discharging the battery, dissect and remove the positive electrode sheet. Use an ion grinder to cut a cross section of the sheet, and observe the cross section under a scanning electron microscope. Take a SEM photograph, calculate the area S of the protective layer using Image-Pro software, and measure the length L of the protective layer using a ruler. The thickness of the positive electrode protective layer can be calculated as d1′ = S / L. Repeat this test several times and take the average value, which is d1.

[0030] In one example, a ratio of the particle size dv90 of the conductive particles to the particle size Dv10 of the positive electrode active material (dv90 / Dv10) is ≤0.5.

[0031] It is understood that dv90 is the particle size corresponding to 90% of the conductive particles in the volume distribution, and Dv10 is the particle size corresponding to 10% of the positive electrode active material in the volume distribution. In order to enable the positive electrode current collector to withstand a higher compaction density of the positive electrode active layer, the present disclosure further limits the ratio of the particle size dv90 of the conductive particles to the particle size Dv10 of the positive electrode active material to dv90 / Dv10≤0.5, which can ensure that the conductive particles have a smaller particle size than the positive electrode active material, so that the local extrusion and damage of the positive electrode protective layer on the current collector is far less than that of the positive electrode active layer, thereby enabling the current collector to withstand a higher compaction density of the positive electrode active layer, thereby further reducing the thickness of the positive electrode active layer and improving the energy density of the battery. Specifically, as shown in Figures 1 and 2, Figure 1 is a cross-sectional SEM morphology of a positive electrode sheet without a positive electrode protective layer in an example provided by the present disclosure, that is, a positive electrode sheet without a positive electrode protective layer, the current collector is squeezed and bent by the active material particles with larger particle size, and the thickness range of the current collector is large, which reduces the mechanical strength and makes it impossible to roll the electrode sheet to a lower thickness. Figure 2 is a cross-sectional SEM morphology of a positive electrode sheet with a positive electrode protective layer in an example provided by the present disclosure, that is, a positive electrode sheet with a specifically set positive electrode protective layer, the current collector is relatively flat as a whole, has strong mechanical strength, and the electrode sheet can be rolled to a lower thickness, thereby reducing the thickness of the entire electrode sheet, improving the energy density of the battery, while reducing the risk of electrode breakage and improving the safety performance of the battery.

[0032] In one embodiment, the ratio of the particle size dv90 of the conductive particles to the particle size Dv10 of the positive electrode active material, dv90 / Dv10, is 0.02 to 0.5. In one embodiment, the ratio of the particle size dv90 of the conductive particles to the particle size Dv10 of the positive electrode active material, dv90 / Dv10, can be 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.2, 0.3, 0.4, 0.5, or a range consisting of any two values.

[0033] In one embodiment, dv90 / Dv10 is 0.1-0.4.

[0034] In one embodiment, the particle size dv90 of the conductive particles is 0.1 μm to 5 μm. In one embodiment, the particle size dv90 of the conductive particles can be 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, or a range consisting of any two values.

[0035] In one embodiment, the particle size dv90 of the conductive particles is 0.5 μm to 3 μm.

[0036] In one embodiment, the particle size Dv10 of the positive electrode active material is 1 μm to 10 μm. In one embodiment, the particle size Dv10 of the positive electrode active material can be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, or a range consisting of any two values.

[0037] In one embodiment, the particle size Dv10 of the positive electrode active material is 3 μm to 8 μm.

[0038] In one example, the conductive particles include at least one of conductive metal oxides and inorganic particles coated with conductive metal oxides.

[0039] In one example, the conductive metal oxide includes at least one of antimony-doped tin oxide, fluorine-doped tin oxide, tin-doped indium oxide, and aluminum-doped zinc oxide.

[0040] In one example, the conductive metal oxide-coated inorganic particles include antimony-doped tin oxide-coated aluminum oxide, antimony-doped tin oxide-coated titanium oxide, antimony-doped tin oxide-coated silicon oxide, antimony-doped tin oxide-coated magnesium oxide, antimony-doped tin oxide-coated zinc oxide, antimony-doped tin oxide-coated boehmite, fluorine-doped tin oxide-coated aluminum oxide, fluorine-doped tin oxide-coated titanium oxide, fluorine-doped tin oxide-coated silicon oxide, fluorine-doped tin oxide-coated magnesium oxide, fluorine-doped tin oxide-coated zinc oxide, fluorine-doped tin oxide, and fluorine-doped tin oxide. At least one of tin-coated boehmite, tin-doped indium oxide-coated aluminum oxide, tin-doped indium oxide-coated titanium oxide, tin-doped indium oxide-coated silicon oxide, tin-doped indium oxide-coated magnesium oxide, tin-doped indium oxide-coated zinc oxide, tin-doped indium oxide-coated boehmite, aluminum-doped zinc oxide-coated aluminum oxide, aluminum-doped zinc oxide-coated titanium oxide, aluminum-doped zinc oxide-coated silicon oxide, aluminum-doped zinc oxide-coated magnesium oxide, aluminum-doped zinc oxide-coated zinc oxide, and aluminum-doped zinc oxide-coated boehmite.

[0041] In one example, the average particle size of the conductive metal oxide is 0.01 μm to 0.1 μm. For example, the average particle size of the conductive metal oxide is 0.01 μm, 0.02 μm, 0.03 μm, 0.04 μm, 0.05 μm, 0.06 μm, 0.07 μm, 0.08 μm, 0.09 μm, 0.1 μm, or a range consisting of any two values. In one example, the average particle size of the conductive metal oxide is 0.05 μm to 0.1 μm. The average particle size of the conductive metal oxide refers to the particle size corresponding to 50% of the conductive metal oxide in the volume distribution.

[0042] In one embodiment, the conductive metal oxide has a specific surface area of ​​20 m 2 / g~200m 2 / g, illustratively, the specific surface area of ​​the conductive metal oxide is 20m 2 / g、30m 2 / g, 40m 2 / g, 50m 2 / g, 60m 2 / g、70m 2 / g、80m 2 / g、90m 2 / g、100m 2 / g、110m 2 / g, 120m 2 / g, 130m 2 / g, 140m 2 / g, 150m 2 / g, 160m 2 / g, 170m 2 / g, 180m 2 / g, 190m 2 / g, 200m 2 / g or a range consisting of any two points.

[0043] In one embodiment, the specific surface area of ​​the conductive metal oxide is 20 to 50 m 2 / g.

[0044] In one example, the average particle size of the inorganic particles coated with the conductive metal oxide is 0.1 μm to 1 μm. For example, the average particle size of the inorganic particles coated with the conductive metal oxide is 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, or a range consisting of any two values. In one example, the average particle size of the inorganic particles coated with the conductive metal oxide is 0.1 to 0.5 μm. The average particle size of the inorganic particles coated with the conductive metal oxide refers to the particle size corresponding to 50% of the inorganic particles coated with the conductive metal oxide in the volume distribution.

[0045] In one embodiment, the specific surface area of ​​the conductive metal oxide-coated inorganic particles is 5 m 2 / g~50m 2 / g, illustratively, the specific surface area of ​​the inorganic particles coated with the conductive metal oxide is 5m 2 / g、10m 2 / g、15m 2 / g, 20m 2 / g, 25m 2 / g、30m 2 / g、35m 2 / g, 40m 2 / g、45m 2 / g, 50m 2 / g or a range consisting of any two values. In one embodiment, the specific surface area of ​​the inorganic particles coated with the conductive metal oxide is 10m 2 / g~50m 2 / g.

[0046] In one example, the positive electrode active layer includes not only the positive electrode active material but also a conductive agent and a first binder.

[0047] In one embodiment, the positive electrode active material is a lithium-containing transition metal oxide, such as lithium cobaltate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium manganese oxide, and lithium-rich manganese-based materials. The positive electrode active material can be at least one of the above materials.

[0048] In one example, the conductive agent includes at least one of carbon black, carbon nanotubes, graphene, and carbon fiber.

[0049] In one example, the first binder includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, polyhexafluoropropylene, a copolymer of vinylidene fluoride and hexafluoropropylene, polyacrylate, polyacrylonitrile, polyacrylic acid, polyacrylate, polyamide, polyvinylpyrrolidone, and polymethyl methacrylate.

[0050] In one embodiment, the positive electrode active layer may be one or more layers, for example, 2 layers, 3 layers, 4 layers, 5 layers, or a range consisting of any two of these values. In one embodiment, each layer of the multi-layer positive electrode active layer may include the same or different positive electrode active materials as described above, as well as the binder and the conductive agent.

[0051] In one example, the positive electrode current collector is one of aluminum foil, carbon-coated aluminum foil, porous aluminum foil or aluminum-polymer composite current collector.

[0052] In one embodiment, the positive electrode protective layer further includes a second binder, wherein the second binder includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, polyhexafluoropropylene, a copolymer of vinylidene fluoride and hexafluoropropylene, polyacrylate, polyacrylonitrile, polyacrylic acid, a polyacrylate, polyamide, polyvinylpyrrolidone, and polymethyl methacrylate. The second binder is selected independently of the first binder and may be the same or different.

[0053] In order to further solve the problem of gassing caused by high-temperature floating charge of the battery, the present disclosure provides a lithium-ion secondary battery, further comprising an electrolyte, wherein the electrolyte comprises a nitrile additive;

[0054] The surface density of the positive electrode protective layer is W1, the mass percentage m of the conductive particles in the positive electrode protective layer, the specific surface area n of the conductive particles, and the mass percentage p of the nitrile additive in the electrolyte satisfy the relationship W1*m*n / 1000≤p, wherein the W1, m, n, and p in the relationship are dimensionless values. The conductive particles with a large specific surface area in the positive electrode protective layer have a significant impact on the side reactions of the battery under high temperature and high pressure. Therefore, by utilizing the cyano group in the nitrile additive, it can be complexed with the conductive particle material in the positive electrode protective layer earlier than other components in the electrolyte, preventing the high-voltage conductive particles on the positive electrode side from oxidizing and decomposing the electrolyte when the battery is fully charged, thereby improving the stability of the lithium-ion battery. More importantly, by combining the nitrile additive with the positive electrode protective layer set on the positive electrode current collector, it is possible to further improve the high-temperature floating charge performance of the battery on the basis of providing battery safety performance, and solve the problem of gas generation during high-temperature floating charge of the battery.

[0055] In one embodiment, the surface density W1 of the positive electrode protective layer is 0.3 g / cm 2 ~1.5g / cm 2 For example, the surface density W1 of the positive electrode protective layer can be 0.3 g / cm 2 , 0.4g / cm 2 , 0.5g / cm 2 , 0.6g / cm 2 , 0.7g / cm 2 , 0.8g / cm2 , 0.9g / cm 2 , 1g / cm 2 , 1.1g / cm 2 , 1.2g / cm 2 , 1.3g / cm 2 , 1.4g / cm 2 , 1.5g / cm 2 Or a range consisting of any two values. In one embodiment, the surface density W1 of the positive electrode protective layer is 0.3 to 0.9 g / cm 2 .

[0056] In one embodiment, the mass percentage m of the conductive particles in the positive electrode protective layer is 2% to 99%. For example, the mass percentage m of the conductive particles in the positive electrode protective layer can be 2%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or a range consisting of any two values. In one embodiment, the mass percentage m of the conductive particles in the positive electrode protective layer is 50% to 97%.

[0057] In one embodiment, the specific surface area n of the conductive particles is 5m 2 / g~200m 2 / g, illustratively, the specific surface area n of the conductive particles can be 5m 2 / g、10m 2 / g, 20m 2 / g、30m 2 / g, 40m 2 / g, 50m 2 / g, 60m 2 / g、70m 2 / g、80m 2 / g、90m 2 / g、100m 2 / g、110m 2 / g, 120m 2 / g, 130m 2 / g, 140m 2 / g, 150m 2 / g, 160m 2 / g, 170m 2 / g, 180m 2 / g, 190m 2 / g, 200m 2 / g or a range consisting of any two values. In one embodiment, the specific surface area n of the conductive particles is 10m 2 / g~50m 2 / g.

[0058] In one example, the mass percentage p of the nitrile additive in the electrolyte is ≤ 10%. It is understood that, in one possible embodiment, the above relationship satisfies W1*m*n / 1000≤p≤10%. By further controlling the amount of the nitrile additive in the electrolyte, it is possible to ensure that the nitrile additive content meets the requirements of the electrolyte side reactions. At the same time, the nitrile additive content should not be too high, as excessive nitrile additive content may lead to excessive complexation with the positive electrode, resulting in increased battery impedance and reduced charge and discharge capacity.

[0059] In one example, the nitrile additive includes at least one of a dicyano compound, a polycyano compound, and a polycyano compound containing other functional groups.

[0060] In one example, the dicyano compound includes at least one of succinonitrile, glutaronitrile, adiponitrile, pimelonitrile, and suberonitrile.

[0061] In one example, the polycyano compound includes at least one of 1,3,5-pentanetrinitrile and 1,3,6-hexanetrinitrile.

[0062] In one example, the polycyano compound containing other functional groups includes at least one of sulfonyldipropionitrile and 1,2-bis(cyanoethoxy)ethane.

[0063] In one embodiment, the electrolyte is a non-aqueous solvent electrolyte comprising a lithium salt, a solvent, and an additive. The lithium salt comprises one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonyl imide), lithium bis(oxalatoborate), and lithium bis(fluorosulfonyl imide). The solvent comprises at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, propyl propionate, and ethyl propionate.

[0064] In one example, the electrolyte further includes other additives, and the other additives include at least one of fluoroethylene carbonate, vinylene carbonate, vinyl ethylene carbonate, 1,3-propane sultone, and biphenyl.

[0065] The lithium-ion secondary battery provided by the present disclosure also includes a negative electrode sheet and a separator.

[0066] In one example, the negative electrode sheet includes a negative electrode active material, and the negative electrode active material includes at least one of natural graphite, artificial graphite, silicon-carbon material, silicon-oxygen material, hard carbon, and soft carbon.

[0067] In one example, the separator may include a substrate layer and a surface coating layer, wherein the substrate layer is a non-woven fabric, a film, or a composite film having a porous structure.

[0068] In one example, the material of the substrate layer includes at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide.

[0069] In one example, the surface coating layer is disposed on at least one surface of the substrate layer.

[0070] In one example, the surface coating includes a polymer layer, an inorganic layer, or a mixture of the two.

[0071] In one example, the inorganic layer includes inorganic particles and a third binder.

[0072] In one example, the inorganic particles include at least one of aluminum oxide, silicon oxide, titanium oxide, magnesium oxide, zinc oxide, zirconium oxide, boehmite, aluminum hydroxide, and magnesium hydroxide.

[0073] In one embodiment, the third binder includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, polyhexafluoropropylene, a copolymer of vinylidene fluoride and hexafluoropropylene, polyacrylate, polyacrylonitrile, polyacrylic acid, polyacrylate, polyamide, polyvinylpyrrolidone, and polymethyl methacrylate. The third binder is selected independently from the first binder and the second binder and may be the same or different.

[0074] In one example, the polymer layer contains a polymer, and the polymer material includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, polyhexafluoropropylene, a copolymer of vinylidene fluoride and hexafluoropropylene, polyacrylate, polyacrylonitrile, polyacrylic acid, polyacrylate, polyamide, polyvinylpyrrolidone, and polymethyl methacrylate.

[0075] The preparation of lithium-ion batteries is described below using lithium-ion batteries as an example and in combination with specific embodiments. Those skilled in the art will understand that the preparation method described in this application is only an example, and any other suitable preparation method is within the scope of this application.

[0076] The following describes performance evaluations of examples and comparative examples of lithium ion secondary batteries according to the present disclosure.

[0077] Example 1

[0078] The first step: prepare the protective layer slurry, mix 90% by mass of antimony-doped tin oxide-coated titanium oxide (the mass proportion of antimony in antimony-doped tin oxide is 10%) and 10% by mass of the second binder polyvinylidene fluoride, add a certain amount of N-methylpyrrolidone, adjust the solid content of the slurry to 40%, and stir to prepare the positive electrode protective layer slurry.

[0079] Step 2: Prepare the positive electrode slurry by mixing 96% by mass of lithium cobalt oxide, 1% by mass of carbon black + 1% by mass of carbon nanotubes, and 2% by mass of the first binder PVDF, adding a certain amount of NMP, adjusting the solid content of the slurry to 70%, and stirring to prepare the positive electrode slurry.

[0080] Step 3: Prepare the negative electrode slurry by mixing 96% by mass of artificial graphite, 1% by mass of carbon black, 1.5% by mass of styrene-butadiene rubber + 1.5% by mass of sodium carboxymethyl cellulose, adding deionized water, adjusting the solid content of the slurry to 40%, and stirring to prepare the negative electrode slurry.

[0081] Step 4: Preparation of positive electrode sheet: coating the protective layer slurry of step 1 on the positive electrode current collector, drying it, and then coating the positive electrode active material slurry of step 2 on the protective layer through an extrusion coater, and drying it to obtain the positive electrode sheet.

[0082] Step 5: Preparation of negative electrode sheet: coating the negative electrode slurry in step 3 on the negative electrode collector through an extrusion coating process to obtain a negative electrode sheet.

[0083] Step 6: Use a roller press to roll the positive and negative electrodes to the designed thickness.

[0084] Step 7: Weld the tabs on the positive and negative electrodes respectively.

[0085] Step 8: Place the diaphragm between the positive and negative electrodes and wind it up to obtain a core, then fix it with adhesive tape.

[0086] Step 9: Use a punching mold to punch out the aluminum-plastic film, then use the punched aluminum-plastic film to encapsulate the roll core to obtain the battery cell, bake it until the moisture content is qualified, and inject the electrolyte.

[0087] Step 10: Use lithium-ion battery formation equipment to charge and discharge the battery cells to harden them and sort out the capacity of the battery cells.

[0088] Step 11: The battery cell is sealed for the second time and folded to make the battery cell basically formed.

[0089] Step 12: Perform OCV test on the battery to determine the K value of the battery and select products with qualified K value.

[0090] Examples 2-9

[0091] Examples 2-9 were carried out with reference to Example 1, except that at least one of the thickness of the positive electrode protective layer, the particle size of the conductive particles, and the particle size of the positive electrode active material was different from that of Example 1. The rest were the same, as shown in Table 1.

[0092] Comparative Example 1

[0093] Comparative Example 1 was carried out with reference to Example 1, except that no positive electrode protective layer was provided. See Table 1 for details.

[0094] Comparative Examples 2-3

[0095] Comparative Examples 2-3 were carried out with reference to Example 1, except that at least one of the thickness of the positive electrode protective layer, the particle size of the conductive particles, and the particle size of the positive electrode active material was different from that of Example 1. The rest were the same, as shown in Table 1.

[0096] The above Examples 1-9 and Comparative Examples 1-5 were tested as follows, and the data were recorded in Table 1, specifically:

[0097] (1) Extreme difference in thickness of positive electrode current collector:

[0098] After the battery is discharged, the positive electrode is removed by dissection. An ion grinder is used to cut a cross section of the electrode, and the cross section is observed under a scanning electron microscope. The maximum and minimum thicknesses of the current collector in the observation area are measured using a microscope. The difference is the current collector thickness range (μm). The current collector thickness range can be used to characterize the mechanical strength of the current collector. A larger current collector thickness range indicates lower mechanical strength and a greater likelihood of breakage during actual manufacturing. Conversely, a smaller current collector thickness range indicates a lower likelihood of breakage.

[0099] (3) Minimum curvature radius of the positive electrode current collector surface:

[0100] After the battery is discharged, the positive electrode is removed by dissection. An ion grinder is used to cut a cross section of the electrode, and the cross section is observed under a scanning electron microscope. A microscope is used to photograph the cross section, and the location of the maximum curvature of the current collector is identified. The minimum radius of curvature (μm) at this location is calculated. The minimum radius of curvature of the current collector surface can also be used to characterize the mechanical strength of the current collector. The smaller the minimum radius of curvature of the current collector surface, the lower the mechanical strength and the greater the likelihood of breakage during actual manufacturing. Conversely, a smaller minimum radius of curvature indicates a lower likelihood of breakage.

[0101] (4) Compaction density of pole piece

[0102] After the battery is fully discharged, disassemble it. Take the positive electrode sheet, cut it into small pieces with a length and width of 10 cm, weigh it, and record the weight as M in g. Use an ion beam to cut the cross section of the positive electrode sheet, place it under a scanning electron microscope for observation, and measure the thickness of the electrode sheet D and the thickness of the current collector d in cm. The compaction density of the electrode sheet PD = (M-2.7*100*d) / (Dd) / 100, in g / cm 3 .

[0103] (5)Battery energy density:

[0104] After the battery is fully charged, it is discharged at 0.2C to the lowest voltage (usually 3.0V). The discharge energy is E, and the energy density is E / (L*W*H), where L is the battery length, W is the battery width, and H is the battery height.

[0105] (6) Battery puncture rate:

[0106] The batteries obtained in the above examples and comparative examples were fully charged and then placed on the test bench of a needle penetration tester. A tungsten steel needle with a diameter of 3 mm and a tip length of 3.62 mm was inserted through the center of the battery at a speed of 100 mm / s. Batteries that did not catch fire or explode were considered to have passed the test. The needle penetration pass rate (%) was calculated by dividing the number of passing needles by the number of tests × 100%.

[0107] Table 1

[0108] Since the positive electrode protection layer is not provided in Comparative Example 1 in Table 1, the parameters related to the positive electrode protection layer are represented by “\”.

[0109] From the analysis of Table 1, it can be seen that the d1 / dv50 of Examples 1-8 is greater than 4, dv90 / Dv10 is less than 0.5, the maximum difference in the thickness of the current collector is less than 2 μm, and the minimum radius of curvature of the current collector surface is greater than 5 μm. In contrast, in Comparative Examples 1-3, the maximum difference in the thickness of the current collector is greater than 2 μm, and the minimum radius of curvature of the current collector surface is less than 5 μm. This indicates that the mechanical strength of the current collector of Comparative Examples 1-3 is significantly worse than that of Examples 1-8. The weaker the mechanical strength of the current collector, the greater the possibility of the current collector breaking during the actual production process. In addition, since the mechanical strength of the current collector of Examples 1-8 is better, the corresponding electrode compaction density will be larger, and the thickness of the entire electrode will be thinner than that of Comparative Examples 2-3. Therefore, the energy density of the battery obtained in Examples 1-8 can be significantly improved compared to the energy density of Comparative Examples 2-3. Finally, although the energy density loss is small or slightly improved compared to Comparative Example 1, the battery needle penetration rate of Examples 1 to 8 is significantly improved, and the safety of the battery is significantly improved.

[0110] Example 10

[0111] The preparation steps of the lithium-ion battery of Example 10 are the same as those of Example 3, except that during the preparation process, the following parameters are tested and recorded: the surface density W1 of the positive electrode protective layer, the mass percentage m of the conductive particles in the positive electrode protective layer, the specific surface area n of the conductive particles, and the mass percentage p of the nitrile additive in the electrolyte, as shown in Table 2.

[0112] Examples 11-18

[0113] Examples 11-18 refer to Example 10, except that at least one of the components of the protective layer, the surface density W1 of the positive electrode protective layer, the mass percentage m of the conductive particles in the positive electrode protective layer, the specific surface area n of the conductive particles, and the mass percentage p of the nitrile additive in the electrolyte are the same. See Table 2 for details.

[0114] Comparative Examples 4-6

[0115] Comparative Examples 4-6 refer to Example 10, except that at least one of the components of the protective layer, the surface density W1 of the positive electrode protective layer, the mass percentage m of the conductive particles in the positive electrode protective layer, the specific surface area n of the conductive particles, and the mass percentage p of the nitrile additive in the electrolyte are specifically shown in Table 2.

[0116] The above Examples 10-18 and Comparative Examples 4-6 were subjected to a high-temperature float charge test. Specifically, the lithium-ion battery was fully charged and then placed in a constant temperature environment of 45°C ± 3°C. Constant voltage charging was maintained for 42 days. The battery thickness was measured every 3 days and the battery status was observed. If the thickness growth was less than 10% and there was no bulging, the test was considered to have passed. Otherwise, it was considered to have failed. The data were recorded in Table 2.

[0117] Table 2

[0118] Table 2 is a comparison of the protective layer density, conductive particle ratio, conductive particle specific surface area, content of nitrile additives in the electrolyte, and high-temperature storage test of Examples 10-18 and Comparative Examples 4-6.

[0119] In Examples 10-18, the surface density of the protective layer, the content of the conductive particles, the specific surface area of ​​the conductive particles, and the content of the nitrile additive in the electrolyte satisfy the relationship W1*m*n / 1000≤p, which can ensure that the battery has good high-temperature float charge performance.

[0120] The content of nitrile additives in the electrolyte of Comparative Examples 4-6 does not satisfy W1*m*n / 1000≤p relative to the surface density of the positive electrode protective layer, the content of the conductive particles, and the specific surface area of ​​the conductive particles, and the high-temperature floating charge performance is NG.

[0121] References throughout this specification to "an embodiment," "part of an embodiment," "one embodiment," "another example," "an example," "a specific example," or "a portion of an example" mean that at least one embodiment or example in this application includes the specific features, structures, materials, or characteristics described in that embodiment or example. Therefore, descriptions appearing throughout this specification, such as, "in some embodiments," "in an embodiment," "in one embodiment," "in another example," "in an example," "in a specific example," or "an example," are not necessarily references to the same embodiment or example in this application. In addition, the specific features, structures, materials, or characteristics described herein may be combined in any suitable manner in one or more embodiments or examples.

[0122] Although illustrative embodiments have been shown and described, those skilled in the art should understand that the above embodiments should not be construed as limitations on the present application, and that changes, substitutions, and modifications may be made to the embodiments without departing from the spirit, principles, and scope of the present application.

Claims

1. A lithium-ion secondary battery, characterized in that: The positive electrode sheet includes a positive electrode current collector, a positive electrode active layer, and a positive electrode protection layer disposed between the positive electrode current collector and the positive electrode active layer; The positive electrode protection layer includes conductive particles; The positive electrode active layer includes a positive electrode active material; The ratio of the thickness d1 of the positive electrode protection layer to the particle size dv50 of the conductive particles is d1 / dv50≥4.

2. The lithium-ion secondary battery according to claim 1, wherein The ratio of the particle size dv90 of the conductive particles to the particle size Dv10 of the positive electrode active material is dv90 / Dv10≤0.

5.

3. The lithium-ion secondary battery according to claim 1 or 2, characterized in that The particle size dv50 of the conductive particles is 0.01 μm to 1 μm.

4. The lithium-ion secondary battery according to any one of claims 1 to 3, characterized in that The thickness d1 of the positive electrode protection layer is 0.1 μm to 5 μm.

5. The lithium-ion secondary battery according to any one of claims 1 to 4, characterized in that The particle size dv90 of the conductive particles is 0.1 μm to 5 μm.

6. The lithium-ion secondary battery according to any one of claims 1 to 5, characterized in that The particle size Dv10 of the positive electrode active material is 1 μm to 10 μm.

7. The lithium-ion secondary battery according to any one of claims 1 to 6, characterized in that The conductive particles include at least one of conductive metal oxides and inorganic particles coated with conductive metal oxides.

8. The lithium-ion secondary battery according to claim 7, wherein The conductive metal oxide includes at least one of antimony-doped tin oxide, fluorine-doped tin oxide, tin-doped indium oxide, and aluminum-doped zinc oxide.

9. The lithium-ion secondary battery according to claim 7, characterized in that The conductive metal oxide coated inorganic particles include antimony-doped tin oxide coated aluminum oxide, antimony-doped tin oxide coated titanium oxide, antimony-doped tin oxide coated silicon oxide, antimony-doped tin oxide coated magnesium oxide, antimony-doped tin oxide coated zinc oxide, antimony-doped tin oxide coated boehmite, fluorine-doped tin oxide coated aluminum oxide, fluorine-doped tin oxide coated titanium oxide, fluorine-doped tin oxide coated silicon oxide, fluorine-doped tin oxide coated magnesium oxide, fluorine-doped tin oxide coated zinc oxide ... at least one of aluminum-doped zinc oxide coated boehmite, tin-doped indium oxide coated aluminum oxide, tin-doped indium oxide coated titanium oxide, tin-doped indium oxide coated silicon oxide, tin-doped indium oxide coated magnesium oxide, tin-doped indium oxide coated zinc oxide, tin-doped indium oxide coated boehmite, aluminum-doped zinc oxide coated aluminum oxide, aluminum-doped zinc oxide coated titanium oxide, aluminum-doped zinc oxide coated silicon oxide, aluminum-doped zinc oxide coated magnesium oxide, aluminum-doped zinc oxide coated zinc oxide, and aluminum-doped zinc oxide coated boehmite.

10. The lithium-ion secondary battery according to claim 7, wherein The conductive metal oxide has an average particle size of 0.01 μm to 0.1 μm and a specific surface area of ​​20 m 2 / g~200m 2 / g; Preferably, the inorganic particles coated with the conductive metal oxide have an average particle size of 0.1 μm to 1 μm and a specific surface area of ​​5 m 2 / g~50m 2 / g.

11. The lithium-ion secondary battery according to any one of claims 1 to 10, characterized in that: The lithium-ion secondary battery further includes an electrolyte, wherein the electrolyte includes a nitrile additive; The surface density of the positive electrode protective layer is W1, the mass percentage m of the conductive particles in the positive electrode protective layer, the specific surface area n of the conductive particles and the mass percentage p of the nitrile additive in the electrolyte satisfy the relationship W1*m*n / 1000≤p, wherein the W1, m, n, and p in the relationship are dimensionless values.

12. The lithium-ion secondary battery according to any one of claims 1 to 11, characterized in that: The surface density W1 of the positive electrode protective layer is 0.3 g / cm 2 ~1.5g / cm 2 .

13. The lithium-ion secondary battery according to any one of claims 1 to 12, characterized in that: The mass percentage m of the conductive particles in the positive electrode protection layer is 2% to 99%.

14. The lithium-ion secondary battery according to any one of claims 1 to 13, characterized in that: The specific surface area n of the conductive particles is 5m 2 / g~200m 2 / g.

15. The lithium-ion secondary battery according to any one of claims 1 to 14, characterized in that: The mass percentage p of the nitrile additive in the electrolyte is ≤10%; the nitrile additive includes at least one of a dicyano compound, a polycyano compound, and a polycyano compound containing other functional groups; Preferably, the dicyano compound comprises at least one of succinonitrile, glutaronitrile, adiponitrile, pimelonitrile, and suberonitrile; Preferably, the polycyano compound includes at least one of 1,3,5-pentanetrinitrile and 1,3,6-hexanetrinitrile; Preferably, the polycyano compound containing other functional groups includes at least one of sulfonyldipropionitrile and 1,2-bis(cyanoethoxy)ethane.

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