Lithium ion secondary battery

Abstract

The invention relates to the technical field of batteries, in particular to a lithium ion secondary battery. Comprising a positive plate, the positive plate comprises a positive current collector and a positive active layer arranged on at least one side surface of the positive current collector, the positive active layer comprises a positive active material, and the positive active material comprises lithium manganese iron phosphate and a ternary positive material; the lithium manganese iron phosphate is Li < y > Mn < x > Fe < w > M < 1 > z > PO4, and the ternary positive electrode material is Li Ni Co < c > Mn < d > M < 2 > e O < 2 >; in the positive electrode active layer, the mass content of Fe element is c2; the lithium ion secondary battery also comprises an electrolyte, and the electrolyte comprises lithium difluorophosphate; and the mass contents c1 and c2 of the lithium difluorophosphate in the electrolyte meet: # imgabs0 #. According to the battery disclosed by the invention, the safety performance of the battery can be effectively improved, and meanwhile, the problem of gas production caused by high-temperature storage of the battery is solved.

Description

Technical Field

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

[0002] In some specific usage scenarios, lithium-ion secondary batteries need to pass the needle penetration test. For example, in the presence of hard needle-shaped foreign objects such as iron nails, when the device containing the lithium-ion secondary battery is placed incorrectly, it may be pierced by the iron nail, causing internal short circuits in the lithium-ion secondary battery, local heating, thermal runaway, oxygen release and other problems, and may even catch fire and explode. At this time, lithium-ion secondary batteries that can pass the needle penetration test are crucial to the safety of the application scenario.

[0003] At present, most electric tools use high-nickel ternary materials as the main positive active materials to improve rate performance and energy density. However, the thermal runaway temperature of pure ternary materials will decrease with the increase of nickel content. For example, the thermal runaway temperature of nickel-cobalt-manganese ternary material NCM811 is about 200°C. When the heat generation temperature during acupuncture is higher than this temperature, it is easy to cause the decomposition and deoxygenation of the material and subsequent thermal runaway. Therefore, lithium-ion secondary batteries that only contain high-nickel ternary materials in the positive active materials have poor safety performance, low thermal runaway temperature, and cannot pass the acupuncture test. The thermal runaway temperature of lithium iron manganese phosphate materials is relatively high. Mixing it with high-nickel ternary materials can effectively improve the acupuncture pass rate of the battery. However, when lithium iron manganese phosphate is used as a positive active material, it is easy to cause the problem of belt breakage in the process of rolling the positive electrode sheet, and it is difficult to use it as a positive active material for batteries alone. These materials have obvious shortcomings when used alone as positive active materials, which limits their wider application in the field of positive electrode materials for lithium-ion batteries. Summary of the invention

[0004] The purpose of the present invention is to overcome the above problems existing in the prior art and provide a lithium ion secondary battery. The lithium ion secondary battery of the present invention (hereinafter referred to as the battery) improves the tolerance of the pure ternary system battery under the needle puncture and overcharge test by mixing lithium iron manganese phosphate with high nickel ternary positive electrode materials; at the same time, by adding lithium difluorophosphate to the electrolyte and regulating the relationship between the content of Fe element and lithium difluorophosphate in the positive electrode active layer, the conductivity, high temperature storage performance and cycle stability of the battery can be effectively improved.

[0005] In the prior art, when the thermal runaway temperature of the positive active material used in the battery is not high (for example, the thermal runaway temperature of high-nickel (the molar content of element Ni is greater than or equal to 0.8) ternary materials is about 200 °C, and the thermal runaway temperature of lithium cobaltate or other low-nickel materials is about 230 °C - 250 °C), the internal short circuit of the battery will be caused during the nail penetration test, resulting in a low passing rate of the nail penetration test or even failure to pass. This is because the steel nail used for nail penetration has a small resistance, a large current, and generates a lot of heat. The heat accumulates inside the battery, and the temperature around it can reach several hundred degrees, which easily triggers the decomposition and oxygen release of the battery active material and subsequent thermal runaway. The core of nail penetration is that the temperature rise in the steel nail area is too high, causing the decomposition and oxygen release of the positive active material particles nearby, and then spreading, thereby triggering a strong thermal runaway. The thermal runaway temperature of high-nickel ternary materials is relatively low. When the heat generation temperature during nail penetration is higher than this temperature, thermal runaway is easily triggered. Therefore, the battery with only high-nickel ternary materials in the positive active material has poor safety performance, a low thermal runaway temperature, and cannot pass the nail penetration test.

[0006] The positive active material of the positive electrode sheet of the battery of the present invention includes lithium iron manganese phosphate and high-nickel ternary positive electrode material, which can enable the battery to have a high passing rate of the nail penetration test, overcharge safety and energy density at the same time. This is because: a high nickel content in the ternary positive electrode material can provide a higher specific capacity for the material and improve the energy density of the battery. However, as the nickel content increases, the thermal runaway temperature of the ternary positive electrode material will decrease, and thermal runaway is more likely to occur. The thermal runaway temperature of lithium iron manganese phosphate material is high (≥300 °C). By adding lithium iron manganese phosphate material with a high thermal runaway temperature to the high-nickel ternary positive electrode material with a low thermal runaway temperature, the overall thermal runaway temperature of the positive electrode sheet can be increased. Taking safety as an example, a fire will occur after the high-nickel ternary positive electrode material is penetrated by a nail. Blending 10% of lithium iron manganese phosphate can prevent the battery from catching fire and only cause it to smoke. Therefore, mixing the high-nickel ternary positive electrode material with a high specific capacity but a low thermal runaway temperature with the lithium iron manganese phosphate material with a high thermal runaway temperature itself can balance the performance, prevent heat spread, increase the overall thermal runaway temperature of the positive electrode sheet, delay the occurrence of thermal runaway of the lithium-ion secondary battery, and make the battery have both safety and high energy density.

[0007] However, the specific capacity of lithium iron manganese phosphate material is relatively low, and there are problems such as a relatively large specific surface area and a low tap density. Therefore, using it alone as the positive active material will reduce the energy density of the battery. Moreover, after making it into a pole piece as the active material, the pole piece is prone to absorbing water, resulting in a risk of gas generation. This not only leads to a low yield in the manufacturing process but also causes the cycle stability and conductivity of the battery to deteriorate, the swelling rate to increase, and there is a problem of gas generation during high-temperature storage. Therefore, blending lithium iron manganese phosphate with high-nickel ternary materials can offset some of the adverse effects of lithium iron manganese phosphate on the battery, but it may still result in a relatively low ionic conductivity and poor high-temperature storage performance of the battery. This is because lithium iron manganese phosphate is a positive electrode material obtained by adding manganese elements on the basis of traditional lithium iron phosphate in order to obtain the high-voltage platform of lithium manganese phosphate. Therefore, lithium iron manganese phosphate maintains the stable olivine-type structure of lithium iron phosphate, and this structure determines the high thermal runaway temperature of lithium iron manganese phosphate and the one-dimensional lithium-ion transport channels. Therefore, adding lithium iron manganese phosphate to the battery may cause problems such as relatively low ionic conductivity and poor high-temperature storage performance of the battery. A relatively low ionic conductivity will lead to the occurrence of lithium plating on the negative electrode, which will cause a large thickness expansion of the battery during the cycle process and also squeeze the positive electrode sheet, resulting in the problem of the positive current collector breaking.

[0008] Through extensive research, the inventors of the present invention have found that by adding lithium difluorophosphate to the electrolyte and further regulating the relationship between the mass content c2 of Fe element in the positive electrode active layer and the mass ratio c1 of lithium difluorophosphate in the electrolyte, the problems of low ionic conductivity and poor high-temperature storage performance of the battery caused by the addition of lithium iron manganese phosphate can be improved. The reasons are as follows: First, lithium difluorophosphate can form a stable interfacial film on the surface of the positive electrode, reduce the interfacial impedance, and improve the lithium ion transport efficiency; and lithium difluorophosphate can improve the ionic conductivity of the electrolyte, thereby improving the overall conductivity of the battery. Second, lithium difluorophosphate is relatively stable at high temperatures, which can reduce the decomposition of the electrolyte and the occurrence of side reactions, and reduce the capacity attenuation of the battery at high temperatures; and the poor high-temperature storage performance of lithium iron manganese phosphate is mainly because manganese in it is easily dissolved, and the interfacial film formed by lithium difluorophosphate can inhibit this problem and keep the material structure stable; in addition, this interfacial film can also reduce the side reactions between the electrolyte and the positive electrode material at high temperatures, prevent gas generation and the destruction of the material structure. However, it is necessary to regulate c1 / c2. When c1 / c2 is too large (for example, greater than 1), the amount of lithium difluorophosphate exceeds the required amount, resulting in too high viscosity of the electrolyte system and too large battery impedance, which not only seriously affects the nail penetration and overcharge safety performance of the battery, but also causes a decrease in the overall rate performance of the battery and the occurrence of lithium plating; when c1 / c2 is too small (for example, less than 0.01), the film-forming stability of lithium difluorophosphate is low, which easily causes lithium plating problems, resulting in a decrease in the battery safety performance. Therefore, controlling the ratio of c1 and c2 within a suitable range can not only improve the safety performance and energy density of the battery, but also improve the conductivity, high-temperature storage performance and cycle stability of the battery.

[0009] The present invention provides a lithium ion secondary battery, which includes a positive electrode sheet. The positive electrode sheet includes a positive electrode current collector and a positive electrode active layer provided on at least one surface of the positive electrode current collector. The positive electrode active layer includes a positive electrode active material, and the positive electrode active material includes lithium iron manganese phosphate and a ternary positive electrode material; the lithium iron manganese phosphate is Li y Mn x Fe w M 1 z PO4, where 0.2 ≤ w ≤ 0.75, 0.2 ≤ x ≤ 0.8, 0.95 ≤ y ≤ 1.05, 0 ≤ z ≤ 0.05, M 1 includes at least one of Mg, Ti, Zn, V, Al, Ca, C and Ba; the ternary positive electrode material is Li a Ni b Co c Mn d M 2 eO2, where 0.9 ≤ a ≤ 1.1, 0.8 ≤ b ≤ 0.99, 0.01 ≤ c ≤ 0.1, 0.01 ≤ d ≤ 0.1, 0 ≤ e ≤ 0.05, M 2 includes at least one of Al, Mg, Zr, B, Y, Sr, W, Ti, and Nb; in the positive electrode active layer, the mass content of Fe element is c2; the lithium-ion secondary battery further includes an electrolyte, and the electrolyte includes lithium difluorophosphate; the mass content of lithium difluorophosphate in the electrolyte is c1, and c1 and c2 satisfy:

[0010] Through the above technical solutions, the present invention has at least the following advantages compared with the prior art:

[0011] (1) The battery of the present invention has a high passing rate of the needle penetration test, overcharge safety, and energy density.

[0012] (2) The battery of the present invention has high rate performance and cycle stability, and can effectively improve the problems of lithium deposition on the negative electrode and disconnection of the positive electrode current collector.

[0013] (3) The battery of the present invention has a high capacity recovery rate and a low thickness expansion rate after high-temperature storage.

[0014] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein. Detailed Embodiments

[0015] The following provides a detailed description of the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only for explaining and illustrating the present invention, and are not used to limit the present invention.

[0016] The present invention provides a lithium-ion secondary battery, which includes a positive electrode sheet. The positive electrode sheet includes a positive electrode current collector and a positive electrode active layer provided on at least one surface of the positive electrode current collector. The positive electrode active layer includes a positive electrode active material, and the positive electrode active material includes lithium iron manganese phosphate and a ternary positive electrode material.

[0017] In the present invention, the lithium iron manganese phosphate is Li y Mn x Fe w M 1 zPO4, where 0.2 ≤ w ≤ 0.75 (such as 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 or 0.75), 0.2 ≤ x ≤ 0.8 (such as 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 or 0.8), 0.95 ≤ y ≤ 1.05 (such as 0.95, 0.96, 0.97, 0.98, 0.99, 1, 1.01, 1.02, 1.03, 1.04 or 1.05), 0 ≤ z ≤ 0.05 (such as 0, 0.01, 0.02, 0.03, 0.04 or 0.05), M 1 includes at least one of Mg, Ti, Zn, V, Al, Ca, C and Ba. The ternary cathode material is Li a Ni b Co c Mn d M 2 e O2, where 0.9 ≤ a ≤ 1.1 (such as 0.9, 0.95, 1.0, 1.05 or 1.1), 0.8 ≤ b ≤ 0.99 (such as 0.8, 0.85, 0.9, 0.95 or 0.99), 0.01 ≤ c ≤ 0.1 (such as 0.01, 0.02, 0.04, 0.06, 0.08 or 0.1), 0.01 ≤ d ≤ 0.1 (such as 0.01, 0.02, 0.04, 0.06, 0.08 or 0.1), 0 ≤ e ≤ 0.05 (such as 0, 0.01, 0.02, 0.03, 0.04 or 0.05), M 2 includes at least one of Al, Mg, Zr, B, Y, Sr, W, Ti and Nb; in the positive electrode active layer, the mass content of Fe element is c2.

[0018] In the present invention, the lithium ion secondary battery further includes an electrolyte, and the electrolyte includes lithium difluorophosphate. The mass content of lithium difluorophosphate in the electrolyte is c1, and c1 and c2 satisfy: such as 0.01, 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, 0.6, 0.7, 0.8, 0.9 or 1.

[0019] In one example,

[0020] In one example,

[0021] In one example, x is 0.4 - 0.6.

[0022] In the present invention, x can be obtained by testing through conventional methods in the art. For example, inductively coupled plasma (ICP).

[0023] In one example, b is 0.9 - 0.96.

[0024] A high nickel content in the pure ternary cathode material can provide a higher specific capacity to the material and improve the energy density of the battery. However, as the nickel content increases, the thermal runaway temperature of the pure ternary will decrease. When the heat generation temperature during pinprick is higher than this temperature, it is very easy to trigger the decomposition of the cathode active material, oxygen release, and subsequent thermal runaway. Therefore, when the nickel content in the ternary material is within a specific range, the battery can further balance high energy density and high safety performance.

[0025] In the present invention, b can be obtained by testing through conventional methods in the art. For example, ICP.

[0026] In the present invention, in the cathode active layer, the mass content c2 of Fe element is 2% - 15%, for example, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14% or 15%.

[0027] In one example, c2 is 5% - 7.5%.

[0028] In the present invention, in the cathode active layer, the mass content c2 of Fe element can be obtained by testing through conventional methods in the art. For example, ICP.

[0029] In the present invention, in the cathode active material, the mass content of lithium iron manganese phosphate is 20% - 60%, for example, 20%, 30%, 40%, 50% or 60%.

[0030] In one example, in the cathode active material, the mass content of lithium iron manganese phosphate is 30% - 50%.

[0031] If the mass content of lithium iron manganese phosphate in the cathode active material is too low (for example, <20%), the improvement of the battery safety performance is not obvious, and there may be occasional failures in the pinprick test, or failures in a more stringent test regime (wherein, a more stringent test means more severe test conditions, for example, test example 2 in the present invention is more stringent than test example 1); if the mass content of lithium iron manganese phosphate in the cathode active material is too high (for example, >60%), it will lead to a decrease in the voltage plateau, obvious loss of energy density, a decrease in the conductivity of the cathode active material, poor rate performance of the battery, and an increase in the sheet resistance of the electrode.

[0032] In the present invention, in the cathode active material, the mass content of lithium iron manganese phosphate can be obtained by testing through conventional methods in the art. For example, it can be obtained by ICP testing.

[0033] In the present invention, c1 is 0.1% - 5%, for example, 0.1%, 1%, 2%, 3%, 4% or 5%.

[0034] In one example, c1 is 1.5% - 2.5%.

[0035] Lithium difluorophosphate in the electrolyte can participate in the construction of the interface film. The formed interface film usually has lithium ion conductivity and can reduce the interface impedance. However, when the content of lithium difluorophosphate exceeds a certain range, the ionic conductivity will decrease with the increase in the amount of lithium difluorophosphate added. If c1 is too small (e.g., <0.1%), the film-forming effect is poor and the impedance of the interface film is too large; if c1 is too large (e.g., >5%), the viscosity of the electrolyte increases and the ionic conductivity decreases significantly, which is not conducive to the migration of lithium ions.

[0036] In the present invention, c1 can be obtained by testing through conventional methods in the art. For example, it can be obtained by gas chromatography (GC) testing.

[0037] In the present invention, in the positive electrode active layer, the mass content of Mn element is 2% - 15%, for example, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14% or 15%.

[0038] In one example, in the positive electrode active layer, the mass content of Mn element is 5% - 11.5%.

[0039] In the present invention, in the positive electrode active layer, the mass content of Mn element can be obtained by testing through conventional methods in the art. For example, it can be obtained by ICP testing.

[0040] In the present invention, in the positive electrode active layer, the ratio of the mass content of Fe element to the mass content of Mn element is 0.2 - 4, for example, 0.2, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 2, 2.5, 3, 3.5 or 4.

[0041] In one example, in the positive electrode active layer, the ratio of the mass content of Fe element to the mass content of Mn element is 0.6 - 1.1.

[0042] The inventors of the present invention further found that by regulating the ratio of the mass content of Fe element to the mass content of Mn element in the positive electrode active layer, the battery can further balance the energy density and the needle-punching safety performance. The reasons are as follows: First, the voltage platform of traditional lithium iron phosphate is relatively low. After introducing Mn element, its voltage platform can be raised to a higher level. By regulating the ratio of the two in the positive electrode active coating, the overall voltage platform of lithium iron manganese phosphate and the ternary positive electrode material can be increased, thereby improving the energy density. Moreover, the olivine structure of lithium iron manganese phosphate has excellent thermodynamic stability. When the ratio of the two is within a specific range, lithium iron manganese phosphate can alleviate the lattice oxygen precipitation and the collapse of the layered structure of the ternary positive electrode material at high voltages, thus allowing the ternary positive electrode material to work stably at a higher cut-off voltage and release more capacity. Second, lithium iron manganese phosphate with an olivine structure is not easily decomposed at high temperatures. The presence of Fe element further inhibits oxygen release (the Fe-O bond is more stable than the Mn-O bond), slows down thermal runaway. By regulating the ratio of the two, the oxygen precipitation caused by the dissolution of transition metals at high temperatures can be reduced, thereby inhibiting the decomposition of the electrolyte and the occurrence of thermal runaway.

[0043] In the present invention, the specific surface area of the ternary positive electrode material is 0.3 m 2 / g - 1.5 m 2 / g, for example, 0.3 m 2 / g, 0.5 m 2 / g, 0.7 m 2 / g, 0.9 m 2 / g, 1.1 m 2 / g, 1.3 m 2 / g or 1.5 m 2 / g.

[0044] In one example, the specific surface area of the ternary positive electrode material is 0.5 m 2 / g - 1.2 m 2 / g.

[0045] In the present invention, the specific surface area of the lithium iron manganese phosphate is 10 m 2 / g - 30 m 2 / g, for example, 10 m 2 / g, 15 m 2 / g, 20 m 2 / g, 25 m 2 / g or 30 m 2 / g.

[0046] In one example, the specific surface area of the lithium iron manganese phosphate is 15 m 2 / g - 25 m 2 / g.

[0047] In the present invention, the specific surface area of the positive electrode active material is 3 m 2 / g - 20 m 2 / g, for example, 3 m 2 / g, 5 m 2 / g, 10 m 2 / g, 15 m 2 / g or 20 m 2 / g.

[0048] In one example, the specific surface area of the positive electrode active material is 4.5 m 2 / g - 13.5 m 2 / g.

[0049] The specific surface area of the positive electrode active material is controlled by changing the specific surface areas of the ternary positive electrode material and the lithium iron manganese phosphate material. If the specific surface area of the positive electrode active material is too large (e.g., > 20 m 2 / g), water absorption and agglomeration are likely to occur during the slurry preparation process. Hard granular bodies will agglomerate into clusters, and during the rolling process, the positive electrode current collector (e.g., aluminum foil) may be extruded, resulting in perforations in the positive electrode current collector. At the same time, the uneven electrode sheet may have uneven stress, leading to the problem of fracture of the positive electrode current collector; if the specific surface area of the positive electrode active material is too small (e.g., < 3 m 2 / g), it indicates that the dispersibility of the material is poor, and it cannot be evenly dispersed with the conductive agent (conductive carbon particles and / or single- and multi-walled carbon nanotubes), resulting in poor conductivity of the positive electrode sheet.

[0050] In the present invention, the specific surface area of the positive electrode active material, the specific surface areas of the ternary positive electrode material and the lithium iron manganese phosphate can be obtained by testing with conventional methods in the art, such as the specific surface area analyzer - nitrogen adsorption method.

[0051] In the present invention, the particle size Dv10 of the lithium iron manganese phosphate ≥ 0.2 μm (e.g., 0.2 μm, 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm or 15 μm), the particle size Dv50 is 5 μm - 12 μm (e.g., 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm or 12 μm), and the particle size Dv99 ≥ 18 μm (e.g., 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm or 25 μm).

[0052] In one example, the particle size Dv10 of the lithium iron manganese phosphate is 0.5 μm - 15 μm.

[0053] In one example, the particle size Dv10 of the lithium iron manganese phosphate ≥ 2 μm.

[0054] In one example, the Dv10 of the lithium iron manganese phosphate ranges from 2 μm to 5 μm.

[0055] Due to the relatively small primary particles of the lithium iron manganese phosphate material, it is prone to agglomerate into large secondary particles during the sintering process. If the Dv10 is too small (e.g., <0.2 μm), on the one hand, it will lead to an increase in the specific surface area of the lithium iron manganese phosphate material and an increase in water absorption, resulting in poor K value and problems with gas generation during high-temperature storage; on the other hand, it will also cause excessive consumption of electrolyte components, especially lithium salts, to form the CEI (Cathode-Electrolyte Interphase) film on the positive electrode surface. At the same time, the number of side reaction sites on the material surface increases, leading to more serious gas generation during high-temperature storage. If the Dv10 is too large (e.g., >15 μm), it will cause weakening of conductivity due to contact between particles, an increase in the migration paths of lithium ions and electrons, and a decrease in conductivity, further reducing the rate performance of the battery. Since the ionic conductivity of lithium iron manganese phosphate is relatively low, the Dv10 of the lithium iron manganese phosphate needs to be controlled within an appropriate range.

[0056] In the present invention, the Dv10 of the ternary positive electrode material ranges from 1.8 μm to 3 μm (e.g., 1.8 μm, 1.9 μm, 2 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm, 2.5 μm, 2.6 μm, 2.7 μm, 2.8 μm, 2.9 μm, or 3 μm), the Dv50 of the ternary positive electrode material ranges from 2.5 μm to 7.5 μm (e.g., 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, or 7.5 μm), and the Dv99 of the ternary positive electrode material ≥ 8 μm (e.g., 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, or 15 μm).

[0057] In the present invention, the Dv10, Dv50, and Dv99 of the lithium iron manganese phosphate and the ternary positive electrode material can be obtained by using conventional methods in the art, such as a laser particle size analyzer.

[0058] In the present invention, the lithium-ion secondary battery further includes a separator, the separator includes a porous base film, and the pore size h (unit: nm) of the porous base film, the thickness t (unit: μm) of the separator, and the Dv10 (unit: μm) of the lithium iron manganese phosphate satisfy: For example, 2×10 -3 、3×10 -3 、4×10 -3 、5×10 -3 、6×10 -3 、7×10 -3, 8×10 -3 , 9×10 -3 , 1×10 -2 , 2×10 -2 , 3×10 - 2 , 4×10 -2 , 5×10 -2 or 6×10 -2 .

[0059] In one example,

[0060] Due to the low ionic conductivity of lithium iron manganese phosphate, it is necessary to control the relationship between h, t and the particle size Dv10 of lithium iron manganese phosphate. This is because: the smaller the pore size of the porous base film, the thicker the separator, and the larger the particle size Dv10 of lithium iron manganese phosphate, the worse the conductivity of the battery. First, a smaller pore size of the porous base film (e.g., <1 nm) will limit the migration speed of lithium ions, increase the internal resistance of the battery, and thus reduce the ionic conductivity. Second, an appropriate separator thickness is crucial for optimizing battery performance. The separator thickness is usually inversely proportional to the ion diffusion rate. Therefore, too thick a separator (e.g., t > 20 μm) will reduce the migration speed of ions, increase the internal resistance of the battery, and reduce the battery conductivity. In addition, if the particle size Dv10 of lithium iron manganese phosphate is too large (e.g., >15 μm), it will lead to less contact between particles, resulting in weakened conductivity and increased migration paths of lithium ions and electrons, leading to a decrease in conductivity. Therefore, by controlling h, t, and Dv10 within an appropriate range, the migration path of lithium ions can be shortened, conductivity can be improved, the internal resistance of the battery can be reduced, and there will not be a significant loss in safety. At the same time, the reduction of internal resistance can reduce the consumption of lithium ions during high-temperature storage of the battery and improve the energy recovery rate during high-temperature storage.

[0061] In the present invention, the pore size h is 1 nm - 100 nm, for example, 1 nm, 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm or 100 nm.

[0062] In one example, h is 10 nm - 50 nm.

[0063] In order to enable the battery to operate continuously and stably, it is required that the current density in the battery be kept uniform and stable. Therefore, it is required that the separator has an appropriate pore size. If the pore size is too small (e.g., <1 nm), the permeability of lithium ions will be restricted, thereby increasing the internal resistance of the battery and reducing the overall performance of the battery; if the pore size is too large (e.g., >100 nm), while increasing the permeability of lithium ions, it is also easily affected by the growth of lithium ion dendrites piercing the separator, resulting in safety problems such as battery short circuit or even explosion.

[0064] In the present invention, the pore diameter h can be obtained by testing through conventional methods in the art. For example, by directly observing with a scanning electron microscope (SEM), at least 20 pores are selected in the electron microscope image, the pore diameter of each pore is measured, and the average value is taken. If the pore is a regular circle, the diameter of this regular circle is the pore diameter; if the pore is a non-"regular circle" (such as an ellipse, an irregular curved-edge polygon, etc.), the diameter of the regular circle with the same area as this non-"regular circle" is the pore diameter.

[0065] In the present invention, the diaphragm thickness t is 5 μm - 20 μm, for example, 5 μm, 8 μm, 11 μm, 14 μm, 17 μm or 20 μm.

[0066] In one example, t is 7.5 μm - 12.5 μm.

[0067] If the diaphragm is too thick (for example, t > 20 μm), the lithium ion migration path will increase accordingly, resulting in a decrease in the lithium ion diffusion rate, a decrease in the specific capacity of the battery, and a deterioration of the charge-discharge rate; if the diaphragm is too thin (for example, t < 5 μm), the mechanical properties of the diaphragm will deteriorate, the tensile strength will decrease, the diaphragm is prone to deformation, and at the same time the puncture resistance of the diaphragm will decrease, resulting in a deterioration of the safety performance of the battery. Due to the low thermal stability and poor safety of the high-nickel ternary material, it is necessary to reduce the diaphragm thickness as much as possible under the condition of meeting the mechanical strength to improve the battery performance.

[0068] In the present invention, the diaphragm thickness t can be obtained by testing through conventional methods in the art. For example, 10 different sites on the diaphragm are taken respectively, and the thickness of each site is measured with a conventional thickness gauge, and the average value is taken.

[0069] In the present invention, the spacing between the pores is 50 nm - 500 nm, for example, 50 nm, 100 nm, 200 nm, 300 nm, 400 nm or 500 nm.

[0070] When the spacing between the pores in the porous base film is too large (for example, greater than 500 nm) or too small (for example, less than 50 nm), it will cause an uneven lithium ion flux, resulting in an uneven distribution of lithium ions on the electrode surface, uneven nucleation of lithium during deposition, and the formation of lithium dendrites. As the number of battery cycles increases, the lithium dendrites continue to grow and eventually pierce the diaphragm, causing a battery short circuit and triggering serious safety problems. Therefore, the spacing between the pores needs to be controlled within an appropriate range.

[0071] In the present invention, the spacing between the pores refers to the shortest distance between the edges of two adjacent pores, and can be obtained by testing through conventional methods in the art. For example, by directly observing with SEM, at least 20 groups of adjacent pores are selected in the electron microscope image, the spacing between each group of adjacent pores is measured, and the average value is taken.

[0072] In the present invention, the separator includes the porous base film, the ceramic layer, and the adhesive layer.

[0073] In one example, the battery further includes a negative electrode sheet, and the ceramic layer faces at least the negative electrode sheet. When the ceramic layer faces the negative electrode sheet, the safety performance of the battery can be effectively improved. When the battery fails, lithium ions on the surface of the negative electrode sheet cannot be embedded and are likely to deposit lithium on the surface, existing in the form of lithium dendrites. The continuous growth of lithium dendrites may pierce the separator, resulting in short-circuiting between the positive and negative electrodes and serious safety problems such as fire and explosion. When the ceramic layer of the separator faces the negative electrode, it can effectively block the lithium dendrites from piercing the separator and improve the battery safety.

[0074] In one example, the adhesive layer includes at least one of polyvinylidene fluoride, PVDF-HFP (polyvinylidene fluoride-hexafluoropropylene copolymer), and polymethyl methacrylate.

[0075] In one example, the separator includes the porous base film, the ceramic layers respectively located on both surface sides of the porous base film, and the adhesive layer located on the outer surface of one of the ceramic layers.

[0076] In one example, the separator includes the porous base film, the ceramic layer located on one surface side of the porous base film, the adhesive layer located on the outer surface of the ceramic layer, and the adhesive layer located on the other surface side of the porous base film.

[0077] In one example, the porous base film may include conventional polymer materials in the art, such as polyethylene and / or polypropylene. The ceramic layer includes inorganic particles. The inorganic particles include at least one of boehmite (hydrated aluminum hydroxide), magnesium oxide, magnesium hydroxide, barium sulfate, calcium silicate, dicalcium silicate, aluminum oxide, and titanium dioxide.

[0078] In the present invention, the thickness of the porous base film is 3 μm - 20 μm, such as 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 15 μm, or 20 μm.

[0079] In the present invention, the thickness of the ceramic layer is 0.5 μm - 5 μm, such as 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, or 5 μm. The thickness of the ceramic layer refers to the thickness of one side.

[0080] In the present invention, the thickness of the adhesive layer is 0.5 μm - 5 μm, such as 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, or 5 μm. The thickness of the adhesive layer refers to the thickness of one side.

[0081] In the present invention, the thicknesses of the porous substrate, the ceramic layer, and the adhesive layer can be obtained by testing using conventional methods in the art. For example, 10 different sites are taken on the porous substrate, the ceramic layer, and the adhesive layer respectively, and the thickness of each site is measured by a conventional thickness gauge, and the average value is taken.

[0082] In the present invention, the porosity of the separator is 30% - 70%, for example, 30%, 40%, 50%, 60%, or 70%.

[0083] In one example, the porosity of the separator is 40% - 60%.

[0084] The porosity of the separator refers to the ratio of the pore volume to the total volume. When the separator has a suitable and uniform porosity, it can prevent local polarization of the electrode and local lithium deposition. If the porosity is too low (e.g., <30%), it will lead to a decrease in the liquid storage capacity of the separator and an increase in the migration distance of lithium ions in the separator, resulting in poor electrolyte wettability of the separator, reduced liquid storage capacity, poor ionic conductivity, and problems such as local polarization and lithium deposition (formation of lithium dendrites) may occur; if the porosity is too high (e.g., >70%), it will lead to a decrease in the mechanical strength of the separator and an increase in the thermal shrinkage, which may cause short circuit between the positive and negative electrodes and even safety problems such as thermal runaway.

[0085] In the present invention, the porosity of the separator can be obtained by testing using conventional methods in the art, such as by a porosity tester for lithium battery thin film separators.

[0086] In the present invention, the liquid absorption rate of the separator is 10% - 60%, for example, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, or 60%.

[0087] In one example, the liquid absorption rate of the separator is 20% - 50%.

[0088] In a lithium-ion battery, the liquid absorption rate of the separator can reflect the electrolyte wettability of the separator, which will significantly affect the performance of the lithium-ion battery. A separator with good wettability can provide sufficient channels for ion transport, facilitating the uniform distribution of current on the electrode surface, thereby providing a smaller effective current density. While a separator with poor electrolyte wettability will affect the battery performance mainly in the following aspects: (1) increasing the internal resistance and reducing the battery rate performance; (2) affecting the discharge capacity of the battery; (3) reducing the liquid storage capacity, resulting in easy drying of the electrolyte during long cycles; (4) causing the growth of lithium dendrites.

[0089] Due to the small particle size of the lithium iron manganese phosphate material in the positive electrode active material and its tendency to agglomerate, with a relatively large contact area with the electrolyte, a relatively high liquid absorption rate of the separator and electrolyte wettability are required in practical applications to support the formation of the solid electrolyte film on the surface of the active material and the transport of lithium ions. The wettability and liquid absorption rate of the separator with respect to the electrolyte depend on the physical and chemical properties of the separator material, including parameters such as the thickness, pore size and distribution, and porosity of the separator. The thicker the separator thickness, the smaller the pore size, the more uniform the distribution, and the higher the porosity, the higher the corresponding liquid absorption rate of the separator.

[0090] In the present invention, the liquid absorption rate (EU) of the separator can be obtained by calculation using the formula: EU(%) = (W wet - W dry ) / W dry × 100%. Wherein, W dry is the weight of the original separator, and W wet is the weight of the separator after immersion in the electrolyte. After immersing the separator in the electrolyte for 2 h and then taking it out, wiping the excess electrolyte on the surface of the separator with filter paper gives the weight of the separator after immersion in the electrolyte.

[0091] In the present invention, the positive electrode sheet further includes a positive electrode tab extending from the positive electrode current collector.

[0092] In one example, the number of the positive electrode tabs is greater than or equal to 2.

[0093] The multi-tab structure refers to the number of positive electrode tabs in the battery being greater than or equal to 2, or the total number of positive electrode tabs and negative electrode tabs being greater than 2. When the number of positive electrode tabs is greater than or equal to 2, the energy density and high-temperature cycle performance of the battery can be further improved. The reason is that due to the low ionic conductivity of lithium iron manganese phosphate and the ternary positive electrode material, and the easy breakage of the electrode sheet, resulting in a reduced lithium ion transport rate or an isolated path, while the multi-tab structure has a larger current-carrying area, which is beneficial for increasing heat dissipation and improving the electron transport rate, can effectively reduce the internal resistance of the positive electrode sheet / negative electrode sheet, prevent an obvious loss of battery capacity, improve the conductivity of the electrode sheet, make up for the problem of insufficient conductivity of the positive electrode material, enhance the electrical contact of the electrode sheet, and improve the power discharge performance. And the multi-tab structure can also reduce the heat concentration of the tabs, reduce the risk of thermal runaway of the battery under abnormal conditions such as overcharge and over-discharge, and improve the safety of the battery.

[0094] However, the battery with a multi-tab structure has a large charge and discharge current, a high charge and discharge rate of the battery, and a faster lithium ion extraction speed, resulting in more vacancies in the positive electrode structure and an unstable delithiated structure of the positive electrode material, which is prone to side reactions and causes the problem of high-temperature gas generation; therefore, it is necessary to coordinately control the contents of lithium iron manganese phosphate and lithium difluorophosphate and the thickness and pore size of the separator to form a more stable protective film on the surface of the positive electrode sheet to solve the problem of high-temperature gas generation caused by the multi-tabs.

[0095] In the present invention, the positive electrode sheet and the negative electrode sheet are wound to form a core.

[0096] In one example, the positive electrode sheet, the negative electrode sheet and the separator are wound to form a core.

[0097] In the present invention, the electrolyte may further include a lithium salt. The lithium salt may include at least one of lithium hexafluorophosphate, lithium difluoro(oxalato)borate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium difluorobis(oxalato)phosphate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium hexafluoroantimonate, lithium hexafluoroarsenate, lithium bis(pentafluoroethylsulfonyl)imide, lithium tris(trifluoromethylsulfonyl)methyl, and lithium bis(trifluoromethylsulfonyl)imide. Based on the total weight of the electrolyte, the content of the lithium salt is ≤21%, for example, 21%, 20%, 15%, 10% or 5%. The lithium salt determines the basic physical and chemical properties of the electrolyte and is the most important component in the electrolyte composition that affects the characteristics of the lithium-ion battery. It can conduct lithium ions and increase the conductivity of the electrolyte.

[0098] In the present invention, the electrolyte may further include an additive that can form a film on the surface of the positive electrode / negative electrode to protect the positive electrode / negative electrode. The additive may include at least one of fluoroethylene carbonate, 1,3-propane sultone, 1,3-acrylonitrile lactone, vinylene carbonate, ethylene vinylene carbonate, succinonitrile, and 1,3,6-hexanetricarbonitrile. Based on the total weight of the electrolyte, the content of the additive is ≤25%, for example, 25%, 20%, 15%, 10% or 5%.

[0099] Due to the electron-withdrawing effect of fluorine atoms, replacing hydrogen atoms with fluorine atoms can lower the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of the solvent, thereby enhancing the antioxidant property of the solvent and its ability to form a SEI at the negative electrode. Compared with the C—H bond, the C—F bond has a higher bond energy. Therefore, fluorinated solvents (such as lithium difluorophosphate and fluoroethylene carbonate) are ideal choices for high-voltage lithium-ion batteries. In addition, introducing fluorine atoms can eliminate hydrogen radicals generated during the combustion of the electrolyte and reduce the flammability of the electrolyte. Fluorine-containing additives can decompose on the surface of the graphite negative electrode to form a SEI (Solid Electrolyte Interphase) film rich in lithium fluoride. This SEI film can uniformly and densely cover the graphite surface, inhibit side reactions at the interface, and remain stable during cycling. The higher the stability of the surface SEI film, the better the high-temperature storage performance.

[0100] In the present invention, the electrolyte may further include an organic solvent, which may include at least one of ethylene carbonate, propylene carbonate, propyl propionate, ethyl propionate, ethyl butyrate, ethyl acetate, diethyl carbonate, and ethyl methyl carbonate. Among them, ethylene carbonate can participate in the formation of the SEI film during the first charge-discharge cycle of the battery, which can improve the rate of subsequent lithium ion insertion and extraction at the negative electrode and reduce the occurrence of side reactions; propylene carbonate has a high dielectric constant, which can promote the dissociation of lithium salts and greatly increase the ionic conductivity in the solution.

[0101] In the present invention, the positive electrode sheet further includes a certain proportion of conductive agents (such as carbon black, single-walled and multi-walled carbon nanotubes, etc.) to improve the conductivity of the positive electrode sheet, and after coating, rolling is carried out to make the positive electrode active material and the conductive agent particles contact more closely, so as to improve the electronic conductivity and improve the battery cycle stability.

[0102] It should be noted that in the present invention, the numerical representations such as "first" and "second" are only used to distinguish different substances or usage methods, and do not represent the difference in order.

[0103] The present invention will be described in detail below through examples. The examples described in the present invention are only a part of the examples of the present invention, rather than all the examples. All other examples obtained by those of ordinary skill in the art based on the examples in the present invention without making creative efforts fall within the scope of protection of the present invention.

[0104] In the following examples, unless otherwise specified, the materials used are commercially available analytical pure.

[0105] The following examples are used to illustrate the lithium ion secondary battery of the present invention.

[0106] Example 1

[0107] The battery was prepared according to the following method:

[0108] (1) Preparation of the positive electrode sheet

[0109] The ternary positive electrode material (LiNi 0.93 Co 0.03 Mn 0.02 Al 0.02 O2, specific surface area of 0.8 m 2 / g) and lithium manganese iron phosphate (LiMn 0.5 Fe 0.5 PO4, specific surface area of 20 m 2 / g, particle size Dv10 of 2.7 μm) were mixed evenly in a high-speed mixer according to a mass ratio of 3:2 to obtain the positive electrode active material;

[0110] Mix the positive electrode active material, polyvinylidene fluoride, conductive carbon black, and multi-walled carbon nanotubes evenly in a mass ratio of 96.3:1.2:1.2:1.3, add N-methylpyrrolidone (NMP) to obtain a positive electrode slurry with a solid content of 65%; evenly coat the positive electrode slurry on an aluminum foil with a thickness of 10 μm using a coater, dry, roll, die-cut, and slice to obtain a positive electrode sheet;

[0111] Among them, the mass content c2 of Fe element in the positive electrode active layer is 6.25%, the mass content of Mn element is 8.02%, and the ratio of the two is 0.78; the specific surface area of the positive electrode active material is 8.5 m 2 / g.

[0112] (2) Prepare the negative electrode sheet

[0113] Mix artificial graphite, single-walled carbon nanotubes, multi-walled carbon nanotubes, styrene-butadiene rubber, and sodium carboxymethyl cellulose evenly in a mass ratio of 96.1:0.25:0.15:2.9:0.6, add ethylene carbonate accounting for 1% of the total mass of the above materials, and then add deionized water to obtain a negative electrode slurry with a cobalt content of 45%; evenly coat the negative electrode slurry on a high-strength carbon-coated copper foil with a thickness of 6 μm, dry, roll, die-cut, and slice to obtain a negative electrode sheet.

[0114] (3) Prepare the electrolyte

[0115] In a glove box filled with argon (H2O < 0.1 ppm, O2 < 0.1 ppm), mix ethylene carbonate, propylene carbonate, diethyl carbonate, and propyl propionate evenly in a mass ratio of 15:10:10:65 to obtain an organic solvent; quickly add lithium hexafluorophosphate that has been fully dried and accounts for 13% of the total mass of the electrolyte to it, then add lithium difluorophosphate (c1 is 1.5%) and 0.5% of succinonitrile based on the total mass of the electrolyte, stir evenly, and after passing the moisture and free acid tests, obtain the electrolyte;

[0116] Among them, is 0.24.

[0117] (4) Prepare the battery

[0118] The positive electrode sheet prepared in step (1), the negative electrode sheet prepared in step (2), and the separator (including a polyethylene film, a boehmite ceramic layer and a polyvinylidene fluoride adhesive layer respectively located on both surface sides of the polyethylene film, and a polyvinylidene fluoride adhesive layer located on the outer surface of the boehmite ceramic layer, wherein the thickness of the polyethylene film is 6 μm, the thickness of the boehmite ceramic layer is 1 μm, the thickness of the polyvinylidene fluoride adhesive layer is 1.5 μm, and the thickness t of the separator is 10 μm; the pore diameter h of the porous base film is 33 nm, and the pore spacing is 342 nm; the porosity of the separator is 52%, and the liquid absorption rate is 33%) are wound by a winding machine to obtain a battery core in which the positive and negative electrode sheets are separated by the separator, and then the battery is obtained through processes such as welding, encapsulation, liquid injection, formation, airbag cutting, and sorting;

[0119] Among them, is 8.18×10 -3 , the boehmite ceramic layer faces the negative electrode sheet, and the battery is a multi-tab core structure.

[0120] Example 2

[0121] Prepare the battery according to the following method:

[0122] (1) Prepare the positive electrode sheet

[0123] Mix the ternary positive electrode material (LiNi 0.9 Co 0.03 Mn 0.04 Ti 0.03 O2, specific surface area is 0.5 m 2 / g) and lithium manganese iron phosphate (LiMn 0.4 Fe 0.55 C 0.05 PO4, specific surface area is 15 m 2 / g, Dv10 particle size is 2.5 μm) evenly in a high-speed mixer to obtain the positive electrode active material;

[0124] Mix the positive electrode active material, polyvinylidene fluoride, conductive carbon black and multi-walled carbon nanotubes evenly according to a mass ratio of 96.3:1.2:1.2:1.3, add N-methylpyrrolidone (NMP) to obtain a positive electrode slurry with a solid content of 65%; use a coater to evenly coat the positive electrode slurry on an aluminum foil with a thickness of 10 μm, dry, roll, die-cut, and cut into pieces to obtain the positive electrode sheet;

[0125] Among them, the mass content c2 of Fe element in the positive electrode active layer is 5.24%, the mass content of Mn element is 5.18%, and the ratio of the two is 1.01; the specific surface area of the positive electrode active material is 4.9 m 2 / g.

[0126] (2) Prepare the negative electrode sheet

[0127] Mix artificial graphite, single-walled carbon nanotubes, multi-walled carbon nanotubes, styrene-butadiene rubber, and sodium carboxymethylcellulose evenly according to a mass ratio of 96.1:0.25:0.15:2.9:0.6. Add ethylene carbonate accounting for 1% of the total mass of the above materials, and then add deionized water to obtain a negative electrode paste with a cobalt content of 45%. Coat the negative electrode paste evenly on a high-strength carbon-coated copper foil with a thickness of 6 μm, dry, roll, die-cut, and slice to obtain a negative electrode sheet.

[0128] (3) Prepare the electrolyte

[0129] In a glove box filled with argon (H2O < 0.1 ppm, O2 < 0.1 ppm), mix ethylene carbonate, propylene carbonate, diethyl carbonate, and propyl propionate evenly according to a mass ratio of 15:10:10:65 to obtain an organic solvent. Quickly add lithium hexafluorophosphate that has been fully dried and accounts for 13% of the total mass of the electrolyte to it, then add lithium difluorophosphate (c1 is 2%) and 0.5% of succinonitrile based on the total mass of the electrolyte, stir evenly, and after passing the moisture and free acid tests, obtain the electrolyte;

[0130] Among them, is 0.382.

[0131] (4) Prepare the battery

[0132] Wind the positive electrode sheet prepared in step (1), the negative electrode sheet prepared in step (2), and the separator (including a polyethylene film, boehmite ceramic layers and polyvinylidene fluoride adhesive layers on both surfaces of the polyethylene film respectively, and a polyvinylidene fluoride adhesive layer on the outer surface of the boehmite ceramic layer. Among them, the thickness of the polyethylene film is 3 μm, the thickness of the boehmite ceramic layer is 0.5 μm, the thickness of the polyvinylidene fluoride adhesive layer is 2 μm, and the thickness t of the separator is 7.5 μm; the pore diameter h of the porous base film is 12 nm, and the pore spacing is 53 nm; the porosity of the separator is 60%, and the liquid absorption rate is 48%) through a winding machine to obtain a battery core in which the positive and negative electrode sheets are separated by the separator, and then obtain the battery through processes such as welding, encapsulation, liquid injection, formation, air bag cutting, and sorting;

[0133] Among them, is 2.78×10 -2 , the boehmite ceramic layer faces the negative electrode sheet, and the battery is a multi-tab core structure.

[0134] Example 3

[0135] Prepare the battery according to the following method:

[0136] (1) Prepare the positive electrode sheet

[0137] Mix the ternary positive electrode material (LiNi 0.96 Co0.01 Mn 0.02 Zr 0.01 O2, with a specific surface area of 1.2 m 2 / g) and lithium iron manganese phosphate (LiMn 0.6 Fe 0.4 PO4, with a specific surface area of 25 m 2 / g and a particle size Dv10 of 4.8 μm) are mixed evenly in a high-speed mixer according to a mass ratio of 1:1 to obtain the positive electrode active material;

[0138] The positive electrode active material, polyvinylidene fluoride, conductive carbon black, and multi-walled carbon nanotubes are mixed evenly according to a mass ratio of 96.3:1.2:1.2:1.3, and N-methylpyrrolidone (NMP) is added to obtain a positive electrode slurry with a solid content of 65%; the positive electrode slurry is evenly coated on an aluminum foil with a thickness of 10 μm using a coater, dried, rolled, die-cut, and sliced to obtain the positive electrode sheet;

[0139] Among them, the mass content c2 of Fe element in the positive electrode active layer is 7.36%, the mass content of Mn element is 11.21%, and the ratio of the two is 0.66; the specific surface area of the positive electrode active material is 13.1 m 2 / g.

[0140] (2) Preparation of the negative electrode sheet

[0141] Artificial graphite, single-walled carbon nanotubes, multi-walled carbon nanotubes, styrene-butadiene rubber, and sodium carboxymethyl cellulose are mixed evenly according to a mass ratio of 96.1:0.25:0.15:2.9:0.6, 1% of ethylene carbonate based on the total mass of the above materials is added, and then deionized water is added to obtain a negative electrode slurry with a cobalt content of 45%; the negative electrode slurry is evenly coated on a high-strength copper foil with a thickness of 6 μm, dried, rolled, die-cut, and sliced to obtain the negative electrode sheet.

[0142] (3) Preparation of the electrolyte

[0143] In a glove box filled with argon (H2O < 0.1 ppm, O2 < 0.1 ppm), ethylene carbonate, propylene carbonate, diethyl carbonate, and propyl propionate are mixed evenly according to a mass ratio of 15:10:10:65 to obtain an organic solvent; 13% of fully dried lithium hexafluorophosphate based on the total mass of the electrolyte is quickly added thereto, and then 2.5% of lithium difluorophosphate (c1 is 2.5%) and 0.5% of succinonitrile based on the total mass of the electrolyte are added, stirred evenly, and after passing the moisture and free acid tests, the electrolyte is obtained;

[0144] Among them, is 0.34.

[0145] (4) Preparation of the battery

[0146] The positive electrode sheet prepared in step (1), the negative electrode sheet prepared in step (2), and the separator (including a polyethylene film, a boehmite ceramic layer and a polyvinylidene fluoride adhesive layer on both surface sides of the polyethylene film respectively, and a polyvinylidene fluoride adhesive layer on the outer surface of the boehmite ceramic layer, wherein the thickness of the polyethylene film is 8 μm, the thickness of the boehmite ceramic layer is 3.5 μm, the thickness of the polyvinylidene fluoride adhesive layer is 0.5 μm, and the thickness t of the separator is 12.5 μm; the pore diameter h of the porous base film is 49 nm, and the pore spacing is 496 nm; the porosity of the separator is 41%, and the liquid absorption rate is 21%) are wound by a winding machine to obtain a battery core in which the positive and negative electrode sheets are separated by the separator, and then through processes such as welding, encapsulation, liquid injection, formation, air bag cutting, and sorting to obtain a battery;

[0147] Among them, is 7.8×10 -3 , the boehmite ceramic layer faces the negative electrode sheet, and the battery is a multi-pole ear core structure.

[0148] Example 4 group

[0149] This group of examples is used to verify the influence brought about by the change of "ternary cathode material".

[0150] This group of examples is carried out with reference to Example 1. The difference is that the ternary cathode material is changed, specifically as follows:

[0151] Example 4a, the ternary cathode material is replaced with the same mass of LiNi 0.8 Co 0.12 Mn 0.05 Al 0.03 O2;

[0152] Example 4b, the ternary cathode material is replaced with the same mass of LiNi 0.98 Co 0.01 Mn 0.01 O2.

[0153] Example 5 group

[0154] This group of examples is used to verify the influence brought about by the change.

[0155] This group of examples is carried out with reference to Example 2 and Example 3 respectively. The difference is that by changing the mass content c1 of lithium difluorophosphate in the electrolyte to regulate Specifically as follows:

[0156] Example 5a, carried out with reference to Example 2. The difference is that c1 is 2.5%, is 0.477;

[0157] Example 5b was carried out with reference to Example 3, except that c1 was 1.5%, being 0.204.

[0158] Example 6 group

[0159] This group of examples was used to verify the influence brought about by the change of "lithium iron manganese phosphate".

[0160] This group of examples was carried out with reference to Example 1, except that the lithium iron manganese phosphate was changed as follows:

[0161] Example 6a: The lithium iron manganese phosphate was replaced with the same mass of LiMn 0.2 Fe 0.8 PO4; among them, the mass content c2 of Fe element in the positive electrode active layer was 11.57%, the mass content of Mn element was 2.98%, and the ratio of the two was 3.88;

[0162] Example 6b: The lithium iron manganese phosphate was replaced with the same mass of LiMn 0.8 Fe 0.2 PO4; among them, the mass content c2 of Fe element in the positive electrode active layer was 2.51%, the mass content of Mn element was 11.94%, and the ratio of the two was 0.21.

[0163] Example 7 group

[0164] This group of examples was used to verify the influence brought about by the change of "the mass content of lithium iron manganese phosphate in the positive electrode active material".

[0165] This group of examples was carried out with reference to Example 1, except that the mass content of lithium iron manganese phosphate in the positive electrode active material was changed as follows:

[0166] Example 7a: The mass content of lithium iron manganese phosphate in the positive electrode active material was 20%;

[0167] Example 7b: The mass content of lithium iron manganese phosphate in the positive electrode active material was 60%.

[0168] Example 8 group

[0169] This group of examples was used to verify the influence brought about by the change of "the specific surface area of the positive electrode active material".

[0170] This group of examples was carried out with reference to Example 1, except that the specific surface area of the positive electrode active material was regulated by changing the specific surface area of the ternary positive electrode material and the specific surface area of the lithium iron manganese phosphate as follows:

[0171] Example 8a: The specific surface area of the ternary positive electrode material was 0.3 m 2 / g, and the specific surface area of the lithium iron manganese phosphate was 10 m2 / g (the particle size Dv10 is 3.9 μm), the specific surface area of the positive electrode active material is 4.2 m 2 / g;

[0172] In Example 8b, the specific surface area of the ternary positive electrode material is 1.5 m 2 / g, and the specific surface area of lithium iron manganese phosphate is 30 m 2 / g (the particle size Dv10 is 1.2 μm), the specific surface area of the positive electrode active material is 12.9 m 2 / g.

[0173] Example 9 group

[0174] This group of examples is used to verify the influence brought about by the change of "the mass content c1 of lithium difluorophosphate in the electrolyte".

[0175] This group of examples is carried out with reference to Example 1. The difference is that c1 is changed as follows:

[0176] In Example 9a, c1 is 0.1%, is 0.016;

[0177] In Example 9b, c1 is 5%, is 0.8.

[0178] Example 10 group

[0179] This group of examples is used to verify the influence brought about by the change of "the pore size h of the porous base film".

[0180] This group of examples is carried out with reference to Example 1. The difference is that h is regulated by changing the separator as follows:

[0181] In Example 10a, h is 5 nm;

[0182] In Example 10b, h is 98 nm.

[0183] Example 11 group

[0184] This group of examples is used to verify the influence brought about by the change of "the thickness t of the separator".

[0185] This group of examples is carried out with reference to Example 1. The difference is that t is regulated by changing the thickness of the polyethylene film, the boehmite ceramic layer and the polyvinylidene fluoride adhesive layer as follows:

[0186] In Example 11a, the thickness of the polyethylene film is 3 μm, the thickness of the boehmite ceramic layer is 1 μm, the thickness of the polyvinylidene fluoride adhesive layer is 0.5 μm, and the thickness t of the separator is 5 μm;

[0187] Example 11b, the thickness of the polyethylene film is 10 μm, the thickness of the boehmite ceramic layer is 4 μm, the thickness of the polyvinylidene fluoride adhesive layer is 3 μm, and the thickness t of the separator is 20 μm.

[0188] Example 12 group

[0189] This group of examples is used to verify the influence brought by the change of

[0190] This group of examples is carried out with reference to Examples 2 and 3 respectively. The difference is that the separator is changed to regulate Specifically as follows:

[0191] Example 12a, carried out with reference to Example 2. The difference is that the separator in Example 3 is used. Among them, Dv10 is 2.5 μm, t is 12.5 μm, and h is 49 nm; is 4.08×10 -3 ;

[0192] Example 12b, carried out with reference to Example 3. The difference is that the separator in Example 2 is used. Among them, Dv10 is 4.8 μm, t is 7.5 μm, and h is 12 nm; is 3.89×10 -2 .

[0193] Example 13

[0194] It is used to verify the influence brought by the change of "the ceramic layer faces the negative electrode sheet".

[0195] Carried out with reference to Example 1. The difference is that the boehmite ceramic layer faces the positive electrode sheet.

[0196] Example 14

[0197] It is used to verify the influence brought by the change of "the number of tab ears in the winding core".

[0198] Carried out with reference to Example 1. The difference is that it is a conventional tab ear, that is, the number of positive tab ears and the number of negative tab ears are both 1, and the positive tab ear and the negative tab ear are located in the empty foil area and are welded to form.

[0199] In the above examples, the ternary cathode material all meets the following: the particle size Dv10 is 1.8 μm - 3 μm, the particle size Dv50 is 2.5 μm - 7.5 μm, and the particle size Dv99 is ≥ 8 μm; the lithium iron manganese phosphate all meets the following: the particle size Dv10 ≥ 0.2 μm, the particle size Dv50 is 5 μm - 12 μm, and the particle size Dv99 ≥ 18 μm.

[0200] Comparative Example 1

[0201] Used to verify the impact brought about by the change of "the positive electrode active material includes lithium iron manganese phosphate and ternary positive electrode material".

[0202] Carried out with reference to Example 1, except that the positive electrode active material was changed as follows: the positive electrode active material was all ternary positive electrode material LiNi 0.93 Co 0.03 Mn 0.02 Al 0.02 O2.

[0203] Comparative Example 2

[0204] Used to verify the impact brought about by "whether lithium difluorophosphate is contained in the electrolyte".

[0205] Carried out with reference to Example 1, except that lithium difluorophosphate was not added to the electrolyte.

[0206] Comparative Example 3 group

[0207] This group of comparative examples is used to verify the impact brought about by the change.

[0208] This group of comparative examples was carried out with reference to Example 1, except that by changing c1 and the mass content of lithium iron manganese phosphate in the positive electrode active material to regulate Specifically as follows:

[0209] Comparative Example 3a, c1 is 5%, the mass content of lithium iron manganese phosphate in the positive electrode active material is 20%, c2 is 3.56%, is 1.405;

[0210] Comparative Example 3b, c1 is 0.1%, the mass content of lithium iron manganese phosphate in the positive electrode active material is 60%, c2 is 10.67%, is 0.0094.

[0211] Test Example

[0212] (1) Penetration test

[0213] Under the environment of 25°C ± 5°C, the batteries prepared in the examples and comparative examples were charged to the upper limit voltage (4.2V) at a rate of 0.7C, the cut-off current was 0.02C, and they were left standing for 10 min; iron nails 1 (with a diameter of 10 mm and a tip length of 15 mm) and iron nails 2 (with a diameter of 8 mm and a tip length of 10 mm) were used respectively; the iron nails were installed upside down, and the flat head surfaces of the iron nails were used to pass through the center positions of the batteries respectively, the needle speed was 30 mm / s, and the nails remained in the batteries; the holding time was 5 min. Five batteries were tested for each example and comparative example. If there was no smoking, no fire, and no explosion, it was considered passed. Record the number of passes / the number of tests as n / 5 in Table 1. The larger this value is, the higher the penetration safety of the battery is proved.

[0214] (2) Overcharge Test

[0215] Under the environment of 25℃±5℃, discharge the batteries prepared in the examples and comparative examples at 0.5C to the lower cut-off voltage (2.5V); charge the batteries to 6V cut-off at 8C or 10C respectively; continue charging for more than 24h and then stop the experiment. Among them, if there is no smoking, no fire and no explosion, it is considered to pass. Record the number of passes / the number of tests as m / 5 in Table 1. The larger this value is, the higher the overcharge safety performance of the battery is proved.

[0216] (3) High-temperature Storage Test

[0217] Place the batteries prepared in the examples and comparative examples in a constant-temperature environment of 25℃, charge them to the upper limit voltage of 4.2V at 0.2C with a cut-off current of 0.02C; discharge them to 2.5V at a rate of 0.2C and record the capacity data C1. Store them in an oven at 60℃ for 30 days. Record the full-charge thickness of the battery before and after storage, which are H1 and H2 respectively. The expansion rate = (H2 - H1) / H1×100%. After cooling the battery, place it in a constant-temperature environment of 25℃, charge it to the upper limit voltage of 4.2V at 0.2C with a cut-off current of 0.02C; discharge it to 2.5V at a rate of 0.2C and record the capacity data C2. The capacity recovery rate = (C2 - C1) / C1×100%. Record the results in Table 1.

[0218] (4) Rate Test

[0219] Place the batteries prepared in the examples and comparative examples in a constant-temperature environment of 25℃, charge them at 0.2C with a cut-off voltage range of 2.5V - 4.2V, and then discharge them to 2.5V at rates of 0.2C and 3C respectively. Record the capacity data during the process. Divide the discharge capacity at 3C rate by the discharge capacity at 0.2C rate to obtain the rate retention rate. Record the results in Table 2.

[0220] (5) Weight Energy Density Test

[0221] Charge the batteries prepared in the examples and comparative examples to the upper limit voltage (4.2V) at 0.2C with a cut-off current of 0.02C, and discharge them to the lower limit voltage (2.5V) at 0.2C. Output the discharge capacity and working voltage. Measure the mass of the battery using a balance. Calculate the weight energy density through the formula: energy density = discharge capacity × working voltage / mass. Record the results in Table 2.

[0222] (6) Cycle Test

[0223] The batteries prepared in the examples and comparative examples were placed in a constant temperature environment of 45°C and subjected to charge and discharge tests at a rate of 3C / 10C. The cut-off voltage range was 2.5V - 4.2V, and the cycle was repeated 300 times. The cyclic discharge capacity was recorded and divided by the discharge capacity of the first cycle to obtain the cyclic capacity retention rate. After cycling, the battery was charged to 4.2V at 0.2C and then disassembled to observe the states of the positive and negative electrodes. It was judged whether there was lithium deposition on the negative electrode and whether there was belt breakage on the positive electrode. The presence of lithium deposition or belt breakage was marked with Y, and the absence of lithium deposition or belt breakage was marked with N. The results are presented in Table 2.

[0224] Table 1

[0225]

[0226]

[0227] Comparative Example 1 used pure ternary high-nickel material. In the high-temperature storage test, a large amount of oxidizing gas was generated in the ternary material, resulting in an increase in the thickness expansion rate. In the examples of the present invention, the ternary high-nickel material was used in combination with lithium iron manganese phosphate. In the high-temperature storage test, there was an electron neutralization effect between the high oxidizing property of the ternary material and the induced system reducibility of lithium iron manganese phosphate, making the battery system more stable, reducing gas production, and significantly reducing the thickness expansion rate.

[0228] Table 2

[0229]

[0230]

[0231] As can be seen from Table 1 and Table 2, compared with the comparative examples, the batteries of the present invention can balance a relatively high passing rate of the needle penetration test, overcharge test, energy density, rate performance, and cycle stability, and have a relatively high capacity recovery rate and a relatively low thickness expansion rate after high-temperature storage.

[0232] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A lithium ion secondary battery, characterized in that: The lithium-ion secondary battery comprises a positive electrode sheet, the positive electrode sheet comprises a positive electrode current collector and a positive electrode active layer disposed on at least one side surface of the positive electrode current collector, the positive electrode active layer comprises a positive electrode active material, and the positive electrode active material comprises lithium manganese iron phosphate and a ternary positive electrode material; The lithium manganese iron phosphate is Li y Mn x Fe w M 1 z PO4, where 0.2≤w≤0.75, 0.2≤x≤0.8, 0.95≤y≤1.05, 0≤z≤0.05, M 1 including at least one of Mg, Ti, Zn, V, Al, Ca, C and Ba; The ternary positive electrode material is Li a Ni b Co c Mn d M 2 e O2, where 0.9≤a≤1.1, 0.8≤b≤0.99, 0.01≤c≤0.1, 0.01≤d≤0.1, 0≤e≤0.05, M 2 including at least one of Al, Mg, Zr, B, Y, Sr, W, Ti and Nb; In the positive electrode active layer, the mass content of Fe element is c2; The lithium-ion secondary battery further comprises an electrolyte, wherein the electrolyte comprises lithium difluorophosphate; The mass content of lithium difluorophosphate in the electrolyte is c1, and c1 and c2 satisfy:

2. The lithium ion secondary battery according to claim 1, wherein c2 is 2%-15%; preferably 5%-7.5%; and / or, c1 is 0.1%-5%; preferably 1.5%-2.5%; and / or, Preferably, And / or, in the positive electrode active material, the mass content of the lithium manganese iron phosphate is 20%-60%, preferably 30%-50%.

3. The lithium ion secondary battery according to claim 1 or 2, wherein: In the positive electrode active layer, the mass content of Mn element is 2%-15%, preferably 5%-11.5%; Preferably, in the positive electrode active layer, the ratio of the mass content of the Fe element to the mass content of the Mn element is 0.2-4; more preferably 0.6-1.

1.

4. The lithium ion secondary battery according to claim 1 or 2, wherein: x is 0.4-0.6; And / or, b is 0.9-0.

96.

5. The lithium ion secondary battery according to claim 1 or 2, wherein: The specific surface area of ​​the ternary positive electrode material is 0.3 m 2 / g-1.5m 2 / g; preferably 0.5m 2 / g-1.2m 2 / g; And / or, the specific surface area of ​​the lithium manganese iron phosphate is 10m 2 / g-30m 2 / g; preferably 15m 2 / g-25m 2 / g; And / or, the specific surface area of ​​the positive electrode active material is 3m 2 / g-20m 2 / g; preferably 4.5m 2 / g-13.5m 2 / g.

6. The lithium ion secondary battery according to claim 1 or 2, wherein: The particle size Dv10 of the lithium iron manganese phosphate is ≥0.2 μm, the particle size Dv50 is 5 μm-12 μm, and the particle size Dv99 is ≥18 μm; preferably, the particle size Dv10 of the lithium iron manganese phosphate is ≥2 μm; more preferably, the particle size Dv10 of the lithium iron manganese phosphate is 2 μm-5 μm; And / or, the particle size Dv10 of the ternary positive electrode material is 1.8 μm-3 μm, the particle size Dv50 is 2.5 μm-7.5 μm, and the particle size Dv99 is ≥8 μm.

7. The lithium ion secondary battery according to claim 1 or 2, wherein: The lithium-ion secondary battery further comprises a diaphragm, wherein the diaphragm comprises a porous base film; the pore size h of the porous base film, in nm, the thickness t of the diaphragm, in μm, and the particle size Dv10 of the lithium iron manganese phosphate, in μm, satisfy: Preferably, 8. The lithium ion secondary battery according to claim 7, wherein: The pore diameter h is 1nm-100nm; And / or, the thickness t of the diaphragm is 5 μm-20 μm; And / or, the porosity of the diaphragm is 30%-70%, preferably 40%-60%.

9. The lithium ion secondary battery according to claim 7, wherein: The diaphragm comprises the porous base film, a ceramic layer and a glue layer; Preferably, the thickness of the porous base film is 3 μm-20 μm; Preferably, the thickness of the ceramic layer is 0.5 μm-5 μm; Preferably, the thickness of the adhesive layer is 0.5 μm-5 μm; Preferably, the ceramic layer comprises inorganic particles, and the inorganic particles comprise at least one of boehmite, magnesium oxide, magnesium hydroxide, barium sulfate, calcium silicate, dicalcium silicate, aluminum oxide and titanium dioxide; Preferably, the adhesive layer includes at least one of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer and polymethyl methacrylate.

10. The lithium ion secondary battery according to claim 1 or 2, wherein: The positive electrode sheet also includes a positive electrode tab extending from the positive electrode current collector; Preferably, the number of the positive electrode tabs is greater than or equal to 2; Preferably, the lithium-ion secondary battery further includes a negative electrode sheet and a separator, and the negative electrode sheet, the separator and the positive electrode sheet are wound to form a winding core.

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