A negative electrode sheet, a preparation method thereof, and a lithium-ion battery

Abstract

The present invention relates to a negative electrode sheet, a preparation method thereof, and a lithium ion battery. The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer, and the negative electrode current collector includes a positive temperature coefficient composite material; the positive temperature coefficient composite material includes a polymer, an indium-tin alloy, and a metal oxide. The negative electrode sheet provided by the present invention includes a negative electrode current collector with thermistor characteristics. When the lithium ion battery has its temperature increased due to abuse or external damage, the resistance of the negative electrode current collector with the positive temperature coefficient composite material will increase sharply, thereby blocking the transmission of electrons in the lithium ion battery, so that the battery stops working, effectively avoiding the possibility of safety accidents such as fires caused by the lithium ion battery under extreme conditions such as overcharge, over-discharge, short circuit, and puncture, and eliminating the safety hazards of the lithium ion battery at the battery level.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and relates to a negative electrode sheet, in particular to a negative electrode sheet and a preparation method thereof, and a lithium-ion battery. Background Art

[0002] Lithium-ion batteries are considered to be one of the most important energy storage tools in the world today. Fast charging and high energy density have always been important directions pursued by lithium batteries. With the gradual popularization of lithium batteries in various industries, frequent lithium battery safety accidents have affected consumers' nerves, and lithium battery safety has become the focus of research in the industry in recent years, especially the safety issues of fast-charging batteries.

[0003] Research shows that the thermal runaway behavior of lithium-ion batteries will change after fast charging cycles. Compared with fresh batteries, the thermal runaway trigger temperature of lithium-ion batteries will drop sharply after fast charging cycles. That is to say, used fast-charging lithium-ion batteries are more likely to undergo thermal runaway or even combustion and explosion.

[0004] Conventional methods for improving battery safety are as follows: 1) increasing battery safety structure components, such as cartridge batteries; 2) using materials with relatively higher safety performance to make batteries, such as lithium iron phosphate batteries and lithium manganate batteries; 3) improving the main materials of the positive and negative electrodes, electrolytes, separators, etc. of the battery, and improving battery safety by means of coating, doping, functionalization, developing new materials, etc.; 4) developing intrinsically safe solid-state batteries to completely solve the risk of thermal runaway.

[0005] At present, all of the above methods have development and application examples, but they will also cause other performance losses: (I) The increase in safety structure components will lead to an increase in the cost of the battery system, reduce market competitiveness, and at the same time will also lead to a decrease in the energy density of the system. Since this safety improvement is carried out from the perspective of the external structure design of the battery, it fails to essentially improve the safety of the battery itself, and there is still a risk of combustion and explosion under extreme conditions; (II) Lithium manganate and lithium iron phosphate batteries are relatively safer than ternary batteries, but their energy density is relatively low, which limits their future application prospects; (III) After the material modification, the safety performance has been improved to a certain extent, but it fails to essentially solve the risk of thermal runaway, and at the same time will bring disadvantages such as electrical performance loss and cost increase; (IV) Solid-state batteries are a research hotspot in the industry, but there are still technical problems such as poor interface contact and low ionic conductivity at present, and they cannot be put into practical application in the short term.

[0006] CN 108899576A discloses a battery structure for improving the safety performance of lithium-ion batteries, which includes a housing, a positive electrode plate, a negative electrode plate, an electrolyte, and a separator. The interior of the housing is divided into two independent chambers by the separator. The positive electrode plate and the negative electrode plate are respectively placed in the chambers, and the electrolyte fills the chambers. The positive electrode plate includes a positive current collector, a positive active material layer coated on the positive current collector, and a thermistor functional layer wrapped around the positive active material layer. The negative electrode plate includes a negative current collector and a negative active material layer coated on the negative current collector. The beneficial effects of this invention are that the thermistor functional layer effectively avoids safety accidents such as fires caused by lithium-ion batteries under extreme conditions such as overcharging, over-discharging, and short-circuiting, and eliminates potential safety hazards of lithium-ion batteries.

[0007] However, the above method requires an additional coating process, which will reduce the yield rate of the negative electrode plate and thus increase the cost of the battery. At the same time, the thermistor functional layer will inevitably have a certain thickness and weight, resulting in significant losses in the volume energy density and mass energy density of the battery. This has certain drawbacks for commercial batteries that pursue high energy density.

[0008] Therefore, how to improve the safety performance without affecting the energy density of the battery is a technical problem that urgently needs to be solved in the field of lithium-ion battery technology. Summary of the Invention

[0009] To solve the above technical problems, the present invention provides a negative electrode plate, a preparation method thereof, and a lithium-ion battery. By using a positive temperature coefficient composite material to replace the conventional metal foil as the negative current collector, the possibility of safety accidents such as fires caused by lithium-ion batteries under extreme conditions such as overcharging, over-discharging, short-circuiting, and puncturing is effectively avoided, and potential safety hazards of lithium-ion batteries are eliminated at the battery level.

[0010] To achieve this purpose, the present invention adopts the following technical solutions:

[0011] In the first aspect, the present invention provides a negative electrode plate, which includes a negative current collector and a negative active material layer, and the negative current collector includes a positive temperature coefficient composite material;

[0012] The positive temperature coefficient composite material includes a polymer, an indium-tin alloy, and a metal oxide.

[0013] The negative electrode sheet provided by the present invention includes a negative electrode current collector with thermistor characteristics. When the temperature of a lithium-ion battery increases due to abuse or external damage, the resistance of the negative electrode current collector with a positive temperature coefficient composite material will increase sharply, thereby blocking the transmission of electrons in the lithium-ion battery, causing the battery to stop working, and effectively avoiding the possibility of safety accidents such as fires caused by the lithium-ion battery under extreme conditions such as overcharging, over-discharging, short-circuiting, and puncturing, eliminating potential safety hazards of the lithium-ion battery at the battery level.

[0014] The negative electrode sheet provided by the present invention is particularly suitable for improving the safety performance of fast-charging batteries. Because compared with conventional energy-type batteries, fast-charging batteries pay more attention to the rate performance of the battery cells in their cell designs, the specific surface area of the selected negative electrode material is relatively larger, the impedance of the SEI film is lower, and at the same time, the conductivity of the selected electrolyte is also higher. On the other hand, due to the large current during fast charging, the heat generation during the charging process is high, so the risk of battery failure is high, and the use of the negative electrode sheet provided by the present invention can effectively control the thermal runaway of fast-charging batteries.

[0015] Preferably, the metal element in the metal oxide includes any one or a combination of at least two of Nb, Ta, Bi, Sb, Y, or La. Typical but non-limiting combinations include the combination of Nb and Ta, the combination of Ta and Bi, the combination of Bi and Sb, the combination of Sb and Y, the combination of Y and La, the combination of Nb, Ta, and Bi, the combination of Ta, Bi, and Sb, the combination of Bi, Sb, and Y, the combination of Sb, Y, and La, the combination of Nb, Ta, Bi, and Sb, the combination of Ta, Bi, Sb, and Y, or the combination of Bi, Sb, Y, and La, preferably Nb.

[0016] Preferably, the mass of the indium-tin alloy accounts for 50-85 wt% of the mass of the positive temperature coefficient composite material. For example, it can be 50 wt%, 55 wt%, 60 wt%, 65 wt%, 70 wt%, 75 wt%, 80 wt%, or 85 wt%, but is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0017] When the percentage of the indium-tin alloy is between 50-85 wt%, the safety performance of the lithium-ion battery is good and it is not easy to have safety accidents; when the percentage of the indium-tin alloy is outside 50-85 wt%, there are potential safety hazards of safety accidents for the lithium-ion battery.

[0018] Preferably, the particle size range of the indium-tin alloy is 7-15 μm. For example, it can be 7 μm, 9 μm, 10 μm, 12 μm, or 15 μm, but is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0019] Preferably, the polymer includes any one or a combination of at least two of polyethylene, polypropylene, polyvinyl chloride, or polyvinylidene fluoride. Typical but non-limiting combinations include a combination of polyethylene and polypropylene, a combination of polypropylene and polyvinyl chloride, a combination of polyvinyl chloride and polyvinylidene fluoride, a combination of polyethylene, polypropylene, and polyvinyl chloride, a combination of polypropylene, polyvinyl chloride, and polyvinylidene fluoride, and a combination of polyethylene, polypropylene, polyvinyl chloride, and polyvinylidene fluoride. Preferably, it is polyethylene.

[0020] The temperature characteristic of a material with a positive temperature coefficient effect is that when the internal temperature of the material reaches a certain temperature, the resistivity rises sharply with the increase in temperature. Therefore, when a certain voltage is applied and within the sensitive temperature range, the heating power of the material will decrease with the increase in temperature, thus playing a protective role in the circuit.

[0021] The positive temperature coefficient composite material provided by the present invention has good electrical conductivity at room temperature and excellent positive temperature coefficient effect performance.

[0022] At room temperature, conductive particles such as indium tin alloy and metal oxide added form conductive chains in contact with each other in the polymer matrix, making the material have good electrical conductivity and a small resistivity. As the temperature rises, the volumes of the polymer and the conductive indium tin alloy particles expand, and the distance between the conductive particles increases, causing the conductive network formed at room temperature to be forced to disconnect. Therefore, the resistance shows an order-of-magnitude increase, that is, the positive temperature coefficient effect performance is excellent.

[0023] In the indium tin alloy provided by the present invention, its melting point is close to that of the polymer, but the surface tension of the polymer is much greater than that of the indium tin alloy. Therefore, the polymer melt will adhere to the surface of the indium tin alloy, preventing the tendency of the indium tin alloy particles to reaggregate and breaking the reformation of the conductive network. Thus, the second PTC (positive temperature coefficient) effect and the disappearance of the NTC (negative temperature coefficient) phenomenon of the material occur.

[0024] Preferably, the thickness of the negative electrode current collector is 80 - 120 μm. For example, it can be 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, but is not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0025] Preferably, the thickness of the negative electrode active material layer is 250 - 300 μm. For example, it can be 250 μm, 260 μm, 270 μm, 280 μm, 290 μm, or 300 μm, but is not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0026] Preferably, the compaction density of the negative electrode sheet is 1.5 - 2.5 g / cm 3 , for example, it can be 1.5 g / cm3 、 1.8 g / cm 3 、 2 g / cm 3 、 2.2 g / cm 3 、 2.5 g / cm 3 , but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0027] Preferably, the negative electrode active material layer includes a negative electrode active material, a negative electrode conductive agent, and a negative electrode binder.

[0028] Preferably, the negative electrode active material includes any one or a combination of at least two of artificial graphite, natural graphite, silicon carbide, lithium titanate, graphene, tin oxide, tin-based composite oxide, tin-based alloy, silicon-based alloy, germanium-based alloy, aluminum-based alloy, antimony-based alloy, or magnesium-based alloy. Typical but non-limiting combinations include a combination of artificial graphite and natural graphite, a combination of natural graphite and silicon carbide, a combination of silicon carbide and lithium titanate, a combination of lithium titanate and graphene, a combination of graphene and tin oxide, a combination of tin oxide and tin-based composite oxide, a combination of tin-based composite oxide and tin-based alloy, a combination of tin-based alloy and silicon-based alloy, a combination of silicon-based alloy and germanium-based alloy, a combination of germanium-based alloy and aluminum-based alloy, a combination of aluminum-based alloy and antimony-based alloy, or a combination of antimony-based alloy and magnesium-based alloy. Preferably, it is artificial graphite.

[0029] Preferably, the negative electrode conductive agent includes any one or a combination of at least two of conductive carbon black, Super P, KS6, VGCF, or carbon nanotubes. Typical but non-limiting combinations include a combination of conductive carbon black and Super P, a combination of Super P and KS6, a combination of KS6 and VGCF, a combination of VGCF and carbon nanotubes, a combination of conductive carbon black, Super P, and KS6, a combination of Super P, KS6, and VGCF, or a combination of KS6, VGCF, and carbon nanotubes.

[0030] Preferably, the negative electrode binder includes any one or a combination of at least two of sodium polymethylcellulose, lithium polymethylcellulose, styrene-butadiene rubber, polyacrylic acid, or polyacrylonitrile. Typical but non-limiting combinations include a combination of sodium polymethylcellulose and lithium polymethylcellulose, a combination of lithium polymethylcellulose and styrene-butadiene rubber, a combination of styrene-butadiene rubber and polyacrylic acid, a combination of polyacrylic acid and polyacrylonitrile, a combination of sodium polymethylcellulose, lithium polymethylcellulose, and styrene-butadiene rubber, a combination of lithium polymethylcellulose, styrene-butadiene rubber, and polyacrylic acid, or a combination of styrene-butadiene rubber, polyacrylic acid, and polyacrylonitrile.

[0031] Preferably, the mass ratio of the negative electrode active material, the negative electrode conductive agent, and the negative electrode binder is (94 to 96):(2 to 4):(1 to 3). For example, it can be 94:4:2, 95:3:2, 95:2:3, 96:2:2, or 96:3:1, but is not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0032] In a second aspect, the present invention provides a method for preparing the negative electrode sheet according to the first aspect. The preparation method includes the following steps:

[0033] Mix a polymer, an indium-tin alloy, and a metal oxide, and hot-press them into a sheet to obtain a negative electrode current collector;

[0034] Mix the negative electrode active material, the negative electrode conductive agent, the negative electrode binder, and a solvent to obtain a negative electrode slurry;

[0035] Coat the obtained negative electrode slurry on the obtained negative electrode current collector, and dry it to obtain the negative electrode sheet.

[0036] Preferably, the temperature of the hot-pressing is 150°C to 190°C. For example, it can be 150°C, 160°C, 170°C, 180°C, or 190°C, but is not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0037] Preferably, the solvent includes deionized water.

[0038] Preferably, the solid content in the negative electrode slurry is 50 to 70 wt%. For example, it can be 50 wt%, 55 wt%, 60 wt%, 65 wt%, or 70 wt%, but is not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0039] Preferably, the temperature of the drying is 80°C to 100°C. For example, it can be 80°C, 85°C, 90°C, 95°C, or 100°C, but is not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0040] Preferably, the time of the drying is 22 to 26 h. For example, it can be 22 h, 23 h, 24 h, 25 h, or 26 h, but is not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0041] In a third aspect, the present invention provides a lithium-ion battery, and the lithium-ion battery contains the negative electrode sheet as described in the first aspect.

[0042] Preferably, the lithium-ion battery further includes a housing, a positive electrode sheet, an electrolyte, and a separator.

[0043] Preferably, the housing includes an aluminum-plastic film and / or a metal housing.

[0044] Preferably, the structure of the lithium-ion battery includes winding or laminating.

[0045] Preferably, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer coated on the positive electrode current collector.

[0046] Preferably, the positive electrode current collector includes aluminum foil and / or aluminum alloy foil.

[0047] Preferably, the positive electrode active material layer includes a positive electrode active material, a positive electrode conductive agent, and a positive electrode binder.

[0048] Preferably, the positive electrode active material includes any one or a combination of at least two of lithium cobaltate, lithium iron phosphate, lithium iron manganese phosphate, or a ternary positive electrode material. Typical but non-limiting combinations include a combination of lithium cobaltate and lithium iron phosphate, a combination of lithium iron phosphate and lithium iron manganese phosphate, a combination of lithium iron manganese phosphate and a ternary positive electrode material, a combination of lithium cobaltate, lithium iron phosphate, and lithium iron manganese phosphate, or a combination of lithium iron phosphate, lithium iron manganese phosphate, and a ternary positive electrode material.

[0049] Preferably, the positive electrode conductive agent includes any one or a combination of at least two of conductive carbon black, Super P, KS6, VGCF, or carbon nanotubes. Typical but non-limiting combinations include a combination of conductive carbon black and Super P, a combination of Super P and KS6, a combination of KS6 and VGCF, a combination of VGCF and carbon nanotubes, a combination of conductive carbon black, Super P, and KS6, a combination of Super P, KS6, and VGCF, or a combination of KS6, VGCF, and carbon nanotubes.

[0050] Preferably, the positive electrode binder includes polyvinylidene fluoride.

[0051] Compared with the prior art, the present invention has at least the following beneficial effects:

[0052] (1) The negative electrode sheet provided by the present invention includes a negative electrode current collector with thermistor characteristics. When the temperature of the lithium-ion battery increases due to abuse or external damage, the resistance of the negative electrode current collector with a positive temperature coefficient composite material will increase sharply, thereby blocking the transmission of electrons in the lithium-ion battery, causing the battery to stop working, and effectively avoiding the possibility of safety accidents such as fires caused by the lithium-ion battery under extreme conditions such as overcharging, over-discharging, short-circuiting, and puncturing, eliminating the safety hazards of the lithium-ion battery at the battery level.

[0053] (2) The lithium-ion battery provided by the present invention does not add additional process steps during manufacturing, and ensures the energy density of the battery, effectively improving the safety performance of the fast-charging battery, and can effectively control the thermal runaway of the fast-charging battery. Detailed implementation manners

[0054] For ease of understanding of the present invention, the following are examples of the present invention. Those skilled in the art should understand that the said examples are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention.

[0055] Example 1

[0056] This example provides a lithium-ion battery. The structure of the lithium-ion battery is a wound type. The lithium-ion battery includes a housing, a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator; the housing is an aluminum-plastic film of a soft-pack battery.

[0057] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer coated on the positive electrode current collector. The positive electrode current collector is aluminum foil. The positive electrode active material layer includes lithium nickel cobalt manganese oxide 622, conductive carbon black, and polyvinylidene fluoride (KYNAR 340) with a mass ratio of 96:2:2.

[0058] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer coated on the negative electrode current collector; the thickness of the negative electrode current collector is 90 μm, the thickness of the negative electrode active material layer is 270 μm, and the compaction density of the negative electrode sheet is 2.0 g / cm 3 , and the negative electrode active material layer includes artificial graphite, conductive carbon black, and sodium carboxymethyl cellulose with a mass ratio of 95:3:2.

[0059] The negative electrode current collector is a positive temperature coefficient material; the positive temperature coefficient material includes polyethylene (Dow Chemical, USA, KT10000UE), indium-tin alloy, and Nb2O3. The mass of the indium-tin alloy accounts for 65 wt% of the mass of the positive temperature coefficient composite material. The particle size range of the indium-tin alloy is 7 - 15 μm. The mass of polyethylene accounts for 25 wt% of the mass of the positive temperature coefficient composite material, and the balance is Nb2O3.

[0060] The preparation method of the negative electrode sheet includes the following steps:

[0061] (1) Mix polyethylene, indium-tin alloy, and Nb2O3, ball-mill them into a mixed powder, and hot-press them into a sheet at a temperature of 180 °C to obtain the negative electrode current collector;

[0062] (2) Mix the negative electrode active material, negative electrode conductive agent, negative electrode binder, and deionized water to obtain a negative electrode slurry, where the solid content of the negative electrode slurry is 60 wt%;

[0063] (3) Coat the negative electrode slurry obtained in step (2) on the negative electrode current collector obtained in step (1), and dry it at 90 °C for 24 h to obtain the negative electrode sheet.

[0064] Example 2

[0065] This embodiment provides a lithium-ion battery. The structure of the lithium-ion battery is a stacked type. The lithium-ion battery includes a housing, a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. The housing is the metal casing of an aluminum shell battery.

[0066] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer coated on the positive electrode current collector. The positive electrode current collector is an aluminum alloy foil. The positive electrode active material layer includes lithium cobaltate, carbon nanotubes, and polyvinylidene fluoride with a mass ratio of 95:3:2.

[0067] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer coated on the negative electrode current collector. The thickness of the negative electrode current collector is 120 μm, the thickness of the negative electrode active material layer is 300 μm, and the compaction density of the negative electrode sheet is 2.5 g / cm 3 , and the negative electrode active material layer includes silicon carbon, carbon nanotubes, and lithium polymethylcellulose with a mass ratio of 96:2:2.

[0068] The negative electrode current collector is a positive temperature coefficient material. The positive temperature coefficient material includes polypropylene (Lee Chang Yung Chemical Industry, ST868M), indium-tin alloy, and Ta2O3. The mass of the indium-tin alloy accounts for 50 wt% of the mass of the positive temperature coefficient composite material. The particle size range of the indium-tin alloy is 7 - 15 μm. The mass of polypropylene accounts for 40 wt% of the mass of the positive temperature coefficient composite material, and the remainder is Ta2O3.

[0069] The preparation method of the negative electrode sheet includes the following steps:

[0070] (1) Mix polypropylene, indium-tin alloy, and Ta2O3, ball mill them into a mixed powder, and hot press them into a sheet at a temperature of 150 °C to obtain the negative electrode current collector.

[0071] (2) Mix the negative electrode active material, negative electrode conductive agent, negative electrode binder, and deionized water to obtain a negative electrode slurry, where the solid content of the negative electrode slurry is 50 wt%.

[0072] (3) Coat the negative electrode slurry obtained in step (2) on the negative electrode current collector obtained in step (1), and dry it at 80 °C for 26 h to obtain the negative electrode sheet.

[0073] Example 3

[0074] This embodiment provides a lithium-ion battery. The structure of the lithium-ion battery is a wound type. The lithium-ion battery includes a housing, a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. The housing is the metal casing of a cylindrical battery.

[0075] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer coated on the positive electrode current collector. The positive electrode current collector is aluminum foil, and the positive electrode active material layer includes lithium iron phosphate, Super P, and polyvinylidene fluoride with a mass ratio of 96:2:2.

[0076] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer coated on the negative electrode current collector; the thickness of the negative electrode current collector is 80 μm, the thickness of the negative electrode active material layer is 250 μm, and the compaction density of the negative electrode sheet is 1.5 g / cm 3 , and the negative electrode active material layer includes lithium titanate, VGCF, and polyacrylic acid (Aladdin) with a mass ratio of 95:3:2.

[0077] The negative electrode current collector is a positive temperature coefficient material; the positive temperature coefficient material includes polyvinyl chloride (Guangzhou Dongcao, TG-1000S), indium-tin alloy, and La2O3. The mass of the indium-tin alloy accounts for 85 wt% of the mass of the positive temperature coefficient composite material. The particle size range of the indium-tin alloy is 7 - 15 μm. The mass of polyvinyl chloride accounts for 10 wt% of the mass of the positive temperature coefficient composite material, and the balance is La2O3.

[0078] The preparation method of the negative electrode sheet includes the following steps:

[0079] (1) Mix polyvinyl chloride, indium-tin alloy, and La2O3, ball mill them into a mixed powder, and hot press them into a sheet at a temperature of 200 °C to obtain the negative electrode current collector;

[0080] (2) Mix the negative electrode active material, negative electrode conductive agent, negative electrode binder, and deionized water to obtain a negative electrode slurry, where the solid content of the negative electrode slurry is 70 wt%;

[0081] (3) Coat the negative electrode slurry obtained in step (2) on the negative electrode current collector obtained in step (1), and dry it at 100 °C for 22 h to obtain the negative electrode sheet.

[0082] Example 4

[0083] This example provides a lithium-ion battery. Except that the mass of the indium-tin alloy accounts for 40 wt% of the mass of the positive temperature coefficient composite material, the contents of the other components are the same as those in Example 1.

[0084] Example 5

[0085] This example provides a lithium-ion battery. Except that the mass of the indium-tin alloy accounts for 95 wt% of the mass of the positive temperature coefficient composite material, the mass of polyethylene accounts for 2.5 wt% of the mass of the positive temperature coefficient composite material, and the balance is Nb2O3, the contents of the other components are the same as those in Example 1.

[0086] Comparative Example 1

[0087] This comparative example provides a lithium-ion battery, which is the same as Example 1 except that the negative current collector is replaced with a conventional copper foil.

[0088] Comparative Example 2

[0089] This comparative example provides a lithium-ion battery, which is the same as Example 1 except that the negative current collector is replaced with a polyvinyl PTC thermosensitive material filled with 6.66% graphite powder / carbon black (1:1) conductive particles.

[0090] Comparative Example 3

[0091] This comparative example provides a lithium-ion battery, which is the same as Example 1 except that the negative current collector is replaced with a barium titanate-based PTC ceramic material doped with 0.6 mol% Nb2O5.

[0092] Put the above lithium-ion battery into a safety explosion-proof box and connect it to a battery charge and discharge device. Discharge the battery completely, and then perform a 20C high-current charge. The experimental results are as follows:

[0093] Table 1

[0094] Test number Test result Example 1 The battery cell is fully charged within 18 minutes, and then the current drops suddenly within 5 minutes. The battery cell is normal Example 2 The battery cell is fully charged within 20 minutes, and then the current drops suddenly within 5 minutes. The battery cell is normal Example 3 The battery cell is fully charged within 19 minutes, and then the current drops suddenly within 5 minutes. The battery cell is normal Example 4 The battery cell is fully charged within 30 minutes, and then emits thick smoke within 10 minutes and catches fire within 20 minutes Example 5 The battery cell is fully charged within 15 minutes, and then emits thick smoke within 20 minutes without catching fire Comparative example 1 The battery cell is fully charged within 6 minutes, and then catches fire and explodes within 11 minutes Comparative example 2 The room temperature resistance of the battery cell is too large, affecting normal use Comparative example 3 The room temperature resistance of the battery cell is too large, affecting normal use

[0095] The following conclusions can be drawn from Table 1:

[0096] (1) As can be seen from Examples 1-3, the lithium-ion battery provided by the present invention does not add additional process steps in manufacturing, and ensures the energy density of the battery, effectively improving the safety performance of the fast-charging battery, and can effectively control the thermal runaway of the fast-charging battery.

[0097] (2) As can be seen from the comparison between Examples 4 and 5 and Example 1, when the mass of indium-tin alloy does not account for 50-85 wt% of the mass of the positive temperature coefficient composite material, a safety accident occurs in the lithium-ion battery, and at this time, the safety performance of the fast-charging battery cannot be improved, and the thermal runaway of the fast-charging battery cannot be effectively controlled.

[0098] (3) As can be seen from the comparison between Comparative Example 1 and Example 1, when the negative current collector is replaced with a conventional copper foil, a safety accident occurs in the lithium-ion battery, and at this time, the safety performance of the fast-charging battery cannot be improved, and the thermal runaway of the fast-charging battery cannot be effectively controlled.

[0099] (4) As can be seen from the comparison between Comparative Examples 2 and 3 and Example 1, when the negative current collector is replaced with other positive temperature coefficient materials, the room temperature resistance of the battery cell is too large, affecting normal use. At this time, the safety performance of the fast-charging battery cannot be improved, and the thermal runaway of the fast-charging battery cannot be effectively controlled.

[0100] In summary, the negative electrode sheet provided by the present invention includes a negative current collector with thermistor characteristics. When the temperature of a lithium-ion battery increases due to abuse or external damage, the resistance of the negative current collector with a positive temperature coefficient composite material will increase sharply, thereby blocking the transmission of electrons in the lithium-ion battery, causing the battery to stop working, effectively avoiding the possibility of safety accidents such as fires caused by the lithium-ion battery under extreme conditions such as overcharging, over-discharging, short-circuiting, and puncturing, and eliminating the safety hazards of the lithium-ion battery at the battery level.

[0101] The present invention uses the above embodiments to illustrate the detailed process flow of the present invention. However, the present invention is not limited to the above detailed process flow, that is, it does not mean that the present invention must rely on the above detailed process flow to be implemented. Those skilled in the art should understand that any improvement to the present invention, the equivalent replacement of each raw material of the product of the present invention, the addition of auxiliary components, and the selection of specific methods, etc., all fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A negative electrode sheet for a lithium-ion battery, characterized in that, The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer, and the negative electrode current collector is a positive temperature coefficient composite material; The positive temperature coefficient composite material includes a polymer, an indium-tin alloy, and a metal oxide; The metal oxide is a conductive particle; The mass of the indium-tin alloy accounts for 50-85 wt% of the mass of the positive temperature coefficient composite material; and / or, the mass of the polymer accounts for 10-40 wt% of the mass of the positive temperature coefficient composite material; The polymer includes any one or a combination of at least two of polyethylene, polypropylene, polyvinyl chloride, or polyvinylidene fluoride.

2. The negative electrode sheet according to claim 1, wherein The metal element in the metal oxide includes any one or a combination of at least two of Nb, Ta, Bi, Sb, Y, or La.

3. The negative electrode sheet according to claim 1 or 2, characterized in that The particle size range of the indium-tin alloy is 7-15 μm.

4. The negative electrode sheet according to claim 3, characterized in that, The thickness of the negative electrode current collector is 80-120 μm; and / or, the thickness of the negative electrode active material layer is 250-300 μm; And / or, the compaction density of the negative electrode sheet is 1.5 to 2.5 g / cm 3 .

5. The negative electrode sheet according to claim 4, characterized in that, The negative electrode active material layer includes a negative electrode active material, a negative electrode conductive agent, and a negative electrode binder; The mass ratio of the negative electrode active material, the negative electrode conductive agent, and the negative electrode binder is (94-96):(2-4):(1-3).

6. A method for preparing a negative electrode sheet according to any one of claims 1-5, characterized in that, The preparation method includes the following steps: Mix the polymer, the indium-tin alloy, and the metal oxide, and hot press them into a sheet to obtain the negative electrode current collector; Mix the negative electrode active material, the negative electrode conductive agent, the negative electrode binder, and a solvent to obtain a negative electrode slurry; Coat the obtained negative electrode slurry on the obtained negative electrode current collector, and dry it to obtain the negative electrode sheet.

7. The preparation method according to claim 6, characterized in that, The temperature of the hot pressing is 150°C - 190°C; and / or, the solid content in the negative electrode slurry is 50-70 wt%.

8. The preparation method according to claim 6 or 7, characterized in that, The temperature of the drying is 80-100°C; and / or, the drying time is 22-26 h.

9. A lithium-ion battery, characterized in that, The lithium-ion battery contains the negative electrode sheet according to any one of claims 1-5.