Composition for forming a negative electrode active material layer in lithium-ion secondary batteries

Abstract

To provide a composition for forming a negative electrode active material layer that makes it possible to homogenize the reaction in a battery and manufacture a battery with a longer service life.SOLUTION: A composition for forming a negative electrode active material layer for a lithium-ion secondary battery contains a negative electrode active material and carbon nanotubes. The negative electrode active material contains amorphous carbon material. Assuming the total amount of the composition to be 100 mass%, the content of carbon nanotubes is 0.01 to 0.8 mass%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a composition for forming a negative electrode active material layer for lithium-ion secondary batteries. [Background technology]

[0002] The lithium-ion battery (LIB) business has achieved remarkable growth through continuous technological development aimed at miniaturizing, lightening, and enhancing the functionality (pursuit of convenience) of portable devices, leveraging its high energy density, high voltage, and safety features. With global environmental and resource issues now in the spotlight, policies such as promoting the spread of eco-cars and transitioning to renewable energy are expected to continue driving the growth of the lithium-ion battery market. On the other hand, if lithium-ion batteries continue to be applied to privately owned electric vehicles (EVs) and stationary energy storage (for leveling renewable energy, etc.), the amount of lithium-ion batteries needed by 2030 is estimated to be approximately 10 times that of 2018. This could lead to unfavorable situations for users from an economic standpoint, such as a sharp rise in battery costs due to resource shortages. Therefore, research and development on reuse, from the perspective of how to use a single battery for as long as possible, and recycling, from the perspective of resource recovery, are becoming more active, and extending the lifespan of batteries is also effective from the standpoint of life cycle assessment (LCA).

[0003] Furthermore, new proposals are being made regarding the future of mobility, including EVs. Concepts such as CASE and MaaS, aimed at improving user convenience, are becoming a reality, and there are signs that mobility, including EVs, will shift from individual ownership to sharing in the future. In this trend, AIEVs (Artificial Intelligence Electric Vehicles), which are expected to be deployed in the market in the future, are EVs equipped with AI, with automated driving, vehicle sharing, and the vehicles being managed by a management company rather than being individually owned. The management company will provide mobility services (CASE) and various services needed by the user inside the vehicle (MaaS). In addition, since the management company will be able to control driving and charging / discharging, it will be possible to operate the vehicle with consideration for battery life and safety. Moreover, vehicle sharing can reduce the burden on users to about 1 / 5 of that compared to individual ownership, making it economically superior. In other words, in the future, replacing private cars, which are predominantly gasoline-powered, with AIEVs (shared cars) is expected to contribute to the global environment, reduce traffic accidents and congestion, provide a new mode of transportation for aging and depopulated areas, reduce individual costs, and enable more effective use of travel time. Furthermore, AIEVs can also function as a massive energy storage system, automatically charging and discharging electricity generated from renewable energy sources with significant fluctuations, using a management system operated by the management company.

[0004] However, achieving this would require a reassessment of the development direction of lithium-ion batteries, which are currently the mainstream (Figure 1).

[0005] In particular, a dramatic increase in the lifespan of batteries used in electric vehicles is absolutely essential. To realize AIEVs, unlike when a vehicle is privately owned, the total driving range of the vehicle needs to be, for example, 500,000 km or more. If the battery life remains at the current level, multiple battery replacements will be necessary, which will impair economic efficiency and will not solve resource problems. The required battery performance will prioritize lifespan over the currently mainstream requirements of energy density (driving range) and fast charging. Specifically, an energy density of 400 Wh / L per cell is required, and the vehicle driving range is 200-300 km with a 20-30 kWh battery, but the target for actual operating lifespan needs to be more than five times that of the current battery. [Overview of the project] [Problems that the invention aims to solve]

[0006] While various studies have been conducted to extend the lifespan of batteries, if the goal is to achieve a practical operating life that is more than five times longer than the current lifespan, a new perspective is needed. In contrast to the many degradation factors that have been studied in the past, such as degradation due to oxidation and reduction of active materials and components, degradation due to the insertion and removal of lithium ions from the active material, consumption of lithium ions due to the decomposition of the electrolyte, and degradation due to electrode loosening, the inventors of this invention have focused on the overvoltage factor.

[0007] The inventors have explained phenomena such as low-temperature degradation and secondary degradation by defining a new degradation mechanism called "degradation due to overvoltage factor (current × resistance)." Degradation due to overvoltage factor is defined as a degradation factor affected by load current and battery resistance (DC resistance). For example, degradation during low-temperature cycles can be explained as being caused by an increase in resistance at low temperatures, and degradation due to rapid charging can be explained as being caused by an increase in load current. Furthermore, as a battery degrades, its resistance increases, and as its capacity degrades, the load on the active material (apparent load) increases when the same current as the initial load is applied. Moreover, if gas accumulates in the electrode group, since gas does not allow ions to pass through, a load is placed on the electrode parts and active material other than the gas reservoir. From this perspective, one factor affecting battery life as an overvoltage factor is the uneven distribution of reactions within the electrodes. If reactions do not occur uniformly within the electrodes, stress accumulates on some of the active materials used in the electrodes during long-term battery operation. For example, in the case of the negative electrode, this can lead to localized strong reduction, resulting in lithium ion consumption, and in the worst case, lithium electrodeposition. As a result, after repeated charge-discharge cycles, a sharp decrease in capacity and a rapid increase in resistance (secondary degradation) almost always occur. By homogenizing the reactions in the electrodes and suppressing the uneven distribution of reactions within the battery, this secondary degradation (sharp decrease in capacity) can be suppressed, and the battery life can be dramatically extended. For this reason, it is important to homogenize the reactions within the battery, but currently, there is little effort being made to homogenize the reactions within the battery.

[0008] To homogenize the reactions within the battery, it is crucial to create an electrode structure that allows for uniform ion movement within the electrodes. While methods such as increasing the electrode porosity or decreasing the electrode's apparent weight have been explored, these methods inevitably reduce energy density (distance traveled). Therefore, these methods alone have limitations in achieving the aforementioned 400 Wh / L level. Furthermore, eliminating factors that can inhibit ion movement within the electrodes—specifically, binders typically used to strengthen the electrodes, thickeners used to stabilize the electrode slurry, and dispersants—is considered effective in homogenizing ion movement within the electrodes. However, reducing or eliminating these components significantly reduces electrode strength, making it unable to follow volume changes during charging and discharging. This can lead to a lack of current collection paths and, consequently, a decrease in lifespan characteristics.

[0009] The present invention aims to solve the above-mentioned problems and provides a composition for forming a negative electrode active material layer that can homogenize the battery reaction and produce batteries with a longer lifespan. [Means for solving the problem]

[0010] In view of the above-mentioned problems, the inventors have diligently conducted research. As a result, they have found that by using an amorphous carbon material as the negative electrode active material and using a very small amount of carbon nanotubes, it is possible to suppress the inhibition of lithium ion movement while forming a uniform electron conduction path, thereby homogenizing the reaction within the battery and extending the battery's lifespan. This invention was completed based on these findings and further research. In other words, this invention encompasses the following configuration.

[0011] Item 1. A composition for forming a negative electrode active material layer for lithium-ion secondary batteries, containing a negative electrode active material and carbon nanotubes, The negative electrode active material contains an amorphous carbon material, Assuming the total amount of the composition is 100% by mass, A composition for forming a negative electrode active material layer for a lithium ion secondary battery, wherein the content of carbon nanotubes is 0.01 to 0.8% by mass.

[0012] Item 2. Containing a conductive assistant other than the carbon nanotubes, The composition for forming a negative electrode active material layer for a lithium ion secondary battery according to Item 1, wherein the content of the conductive assistant other than the carbon nanotubes is 0.1 to 10.0% by mass with the total amount of the composition being 100% by mass.

[0013] Item 3. The composition for forming a negative electrode active material layer for a lithium ion secondary battery according to Item 1 or 2, wherein the content of the carbon nanotubes is 0.1 to 10.0% by mass with the total amount of the carbon nanotubes and the conductive assistant other than the carbon nanotubes being 100% by mass.

[0014] Item 4. The composition for forming a negative electrode active material layer for a lithium ion secondary battery according to any one of Items 1 to 3, wherein the content of the negative electrode active material is 79.2 to 99.8% by mass with the total amount of the composition being 100% by mass.

[0015] Item 5. Containing a negative electrode constituent material excluding the negative electrode active material, the carbon nanotubes, and the conductive assistant other than the carbon nanotubes, The composition for forming a negative electrode active material layer for a lithium ion secondary battery according to any one of Items 1 to 4, wherein the content of the negative electrode constituent material is 0.1 to 10.0% by mass with the total amount of the composition being 100% by mass.

[0016] Item 6. The composition for forming a negative electrode active material layer for a lithium ion secondary battery according to any one of Items 1 to 5, wherein the negative electrode active material is an amorphous layered carbon material and the average interlayer distance of the (002) plane is 0.350 nm or more.

[0017] Item 7. The composition for forming a negative electrode active material layer for a lithium ion secondary battery according to any one of Items 1 to 6, wherein the negative electrode active material contains hard carbon.

[0018] Item 8. The composition for forming a negative electrode active material layer for a lithium-ion secondary battery according to any one of items 1 to 7, wherein the carbon nanotube is a single-walled carbon nanotube.

[0019] Item 9. A composition for forming a negative electrode active material layer for a lithium-ion secondary battery, as described in any one of items 1 to 8, used to reduce the uneven distribution of reactions in a lithium-ion secondary battery.

[0020] Item 10. A composition for forming a negative electrode active material layer for lithium-ion secondary batteries, as described in any one of Items 1 to 9, for use in lithium-ion secondary batteries used in electric vehicles for car sharing.

[0021] Item 11. A negative electrode active material layer for a lithium-ion secondary battery, comprising the composition for forming a negative electrode active material layer for a lithium-ion secondary battery described in any one of items 1 to 10.

[0022] Item 12. A negative electrode active material layer for a lithium-ion secondary battery as described in Item 11, for use in a lithium-ion secondary battery used in an electric vehicle for car sharing.

[0023] Item 13. A negative electrode for a lithium-ion secondary battery, comprising a negative electrode active material layer for a lithium-ion secondary battery as described in Item 11 or 12.

[0024] Item 14. A negative electrode for a lithium-ion secondary battery as described in Item 13, for use in electric vehicles for car sharing.

[0025] Item 15. A lithium-ion secondary battery comprising the negative electrode for lithium-ion secondary batteries described in Item 13 or 14.

[0026] Item 16. The State of Charge (SOC) is given by the following formula (1): SOC (%) = Remaining capacity (Ah) / Full charge capacity (Ah) × 100 (1) Defined as, After discharging from a state of charge (SOC) of 100% to a state of charge (SOC) of 90% under conditions of 25°C and 3.0°C, the battery was left idle for 10 minutes, and the voltage increase during the idle period was measured. The following equation (2): Internal resistance = (Voltage increase during idle (V) / Current value during discharge (A)) × Opposite area of ​​positive and negative electrodes (cm²) 2 ) (2) The internal resistance calculated by this method is 1.0 to 25.0 Ω·cm 2 The lithium-ion secondary battery described in item 15.

[0027] Item 17. A lithium-ion secondary battery as described in Item 15 or 16, used in electric vehicles for car sharing. [Effects of the Invention]

[0028] According to the present invention, by using an amorphous carbon material as the negative electrode active material and using a very small amount of carbon nanotubes, it is possible to form a uniform electron conduction path while suppressing the inhibition of lithium ion movement, thereby homogenizing the reaction within the battery and extending the battery life. [Brief explanation of the drawing]

[0029] [Figure 1] This is a schematic diagram illustrating the direction of lithium-ion battery development, taking into account a world view in which the use and operation of automobiles will change significantly. [Figure 2] This graph outlines the method for analyzing internal resistance in the high-load rest period method used in Test Example 2. [Figure 3] This graph shows the results of the lifespan characteristics for Test Example 3. [Modes for carrying out the invention]

[0030] In this specification, "contains" is a concept that encompasses all of the following: "contains," "consist essentially of," and "consist of."

[0031] Furthermore, in this specification, the notation "A~B" means "greater than or equal to A and less than or equal to B".

[0032] 1. Composition for forming a negative electrode active material layer for lithium-ion secondary batteries The present invention relates to a composition for forming a negative electrode active material layer for lithium-ion secondary batteries, which contains a negative electrode active material and carbon nanotubes, wherein the negative electrode active material contains an amorphous carbon material, and the carbon nanotube content is 0.01 to 0.8% by mass, based on 100% by mass of the total amount of the composition.

[0033] Amorphous carbon materials have low electronic conductivity compared to graphite materials, so in order to maintain uniform reactions within the battery, it is necessary to form uniform electron conduction paths. On the other hand, when using conductive additives with a small aspect ratio, such as carbon black, the amount of conductive additive required to form the above electron conduction paths increases, which may inhibit the movement of lithium ions. On the other hand, carbon nanotubes with a high aspect ratio can form uniform electron conduction paths in small amounts. Thus, by using amorphous carbon materials and carbon nanotubes in combination, it is possible to maintain uniform reactions within the battery by forming uniform electron conduction paths while suppressing the inhibition of lithium ion movement. Furthermore, although carbon nanotubes are substances that can inhibit the movement of lithium ions, the carbon nanotube content in this invention is very small, so it is possible to maintain uniform reactions within the battery, and the synergistic effect with amorphous carbon materials can extend the battery life. In other words, the negative electrode active material layer forming composition for lithium-ion secondary batteries of the present invention can be used to reduce the uneven distribution of reactions in lithium-ion secondary batteries.

[0034] (1-1) Negative electrode active material As an amorphous carbon material used as a negative electrode active material, an amorphous layered carbon material is preferred from the viewpoint of easily homogenizing the reaction within the battery and extending the battery's lifespan.

[0035] The interlayer distance of the (002) plane of amorphous layered carbon material is preferably 0.35 nm or greater, and more preferably 0.36 nm or greater, from the viewpoint of facilitating homogenization of reactions within the battery and extending the battery life. There is no particular upper limit to the interlayer distance of the (002) plane of amorphous layered carbon material, but it is usually 0.40 nm. The interlayer distance of amorphous layered carbon material is measured by X-ray diffraction.

[0036] The average particle size of the amorphous layered carbon material is preferably 1 to 10 μm, and more preferably 3 to 8 μm, from the viewpoint of easily homogenizing the reaction within the battery and extending the battery life. The average particle size of the amorphous layered carbon material is measured by laser diffraction and scattering.

[0037] Examples of amorphous carbon materials that satisfy the above conditions include hard carbon (carbon material that is difficult to graphitize), soft carbon (carbon material that is easily graphitized), and mesophase pitch carbide. These amorphous carbon materials can be used individually or in combination of two or more types. Among these, hard carbon is preferred from the viewpoint of easily homogenizing the reaction in the battery and extending the battery life. In this invention, hard carbon means an amorphous carbon material in which the interlayer distance of the (002) plane does not become less than 0.34 nm when fired at 3000°C. Soft carbon means an amorphous carbon material in which the interlayer distance of the (002) plane becomes less than 0.34 nm when fired at 3000°C.

[0038] There are no particular restrictions on the shape of the negative electrode active material; various shapes such as spherical, flaky, lumpy, fibrous, whisker-like, and crushed can be used. Furthermore, a combination of negative electrode active materials of multiple shapes can be used. Note that "spherical" can refer to a perfect sphere, an elliptical shape, or other shapes.

[0039] The present invention provides a composition for forming a negative electrode active material layer for lithium-ion secondary batteries. By using an amorphous carbon material as the negative electrode active material and incorporating a very small amount of carbon nanotubes, it is possible to form a uniform electron conduction path while suppressing the inhibition of lithium ion movement, thereby homogenizing the reaction within the battery and extending the battery life. In this invention, the content of the negative electrode active material is preferably 79.2 to 99.8% by mass, more preferably 83.5 to 96.4% by mass, and even more preferably 77.8 to 94.9% by mass. When using multiple negative electrode active materials, it is preferable to adjust the total amount so that it falls within the above range. In the present invention, the total amount of the negative electrode active material, carbon nanotubes, conductive additives other than carbon nanotubes, and negative electrode constituent materials excluding the negative electrode active material, carbon nanotubes, and conductive additives other than carbon nanotubes (other negative electrode constituent materials) (total amount of composition) is 100% by mass.

[0040] (1-2) Carbon nanotubes The present invention provides a composition for forming a negative electrode active material layer for lithium-ion secondary batteries. By using an amorphous carbon material as the negative electrode active material and incorporating a very small amount of carbon nanotubes, it is possible to form a uniform electron conduction path while suppressing the inhibition of lithium ion movement, thereby homogenizing the reaction within the battery and extending the battery life. Although carbon nanotubes are substances that inhibit lithium ion movement, in small amounts they do not inhibit lithium ion movement, thus homogenizing the reaction within the battery and extending the battery life.

[0041] Carbon nanotubes are hollow carbon materials formed by closing a graphite sheet (i.e., a carbon atom plane with a graphite structure or a single layer of graphene sheet) into a tubular shape. Their diameter is on the nanometer scale, and their wall structure has a graphite structure. Carbon nanotubes in which the wall structure is a single graphite sheet (single layer of graphene sheet) and the tube is closed are called single-walled carbon nanotubes. On the other hand, carbon nanotubes in which multiple graphite sheets are each closed into tubes and nested together are called nested multi-walled carbon nanotubes. In this invention, both single-walled and multi-walled carbon nanotubes can be used, but single-walled carbon nanotubes are preferred from the viewpoint of facilitating current collection between negative electrode active materials, suppressing inhibition of lithium ion movement, forming a uniform electron conduction path, thereby facilitating uniform reactions within the battery and extending the battery life.

[0042] Such carbon nanotubes can be used individually or in combination of two or more types.

[0043] From the viewpoint of easily increasing the number of carbon nanotubes per unit volume, reducing the carbon nanotube content, and minimizing the obstruction of lithium ion movement, a small average outer diameter of carbon nanotubes is preferable. For this reason, the average outer diameter of carbon nanotubes is preferably 0.43 to 20 nm, and more preferably 0.43 to 10 nm. The average outer diameter of carbon nanotubes is measured by electron microscopy (TEM) observation. For carbon nanotubes having such an average outer diameter, the average inner diameter is set according to the average outer diameter.

[0044] While longer carbon nanotubes facilitate current collection between negative electrode active materials, shorter average lengths are preferable from the viewpoint of improving dispersibility and minimizing interference with lithium ion movement. Therefore, the average length of carbon nanotubes is preferably 0.5 to 200 μm, and more preferably 1 to 50 μm. The average length of carbon nanotubes is measured by electron microscopy (SEM) observation.

[0045] The average aspect ratio of carbon nanotubes, defined as the ratio of the average length to the average outer diameter of the carbon nanotubes mentioned above, is preferably 25 to 200,000, and more preferably 100 to 50,000, from the viewpoint of making it easier to collect current between negative electrode active materials with a lower carbon nanotube content, suppressing the inhibition of lithium ion movement, forming a uniform electron conduction path, thereby making it easier to homogenize the reaction within the battery and extend the battery life. The average aspect ratio of carbon nanotubes is calculated from the average outer diameter measured by electron microscopy (TEM) observation and the average length measured by electron microscopy (SEM) observation.

[0046] In this invention, carbon nanotubes can be used individually or as bundles of multiple carbon nanotubes, which tend to exhibit greater strength as a bundle. In either case, a very small amount of carbon nanotubes can suppress the inhibition of lithium ion movement while forming a uniform electron conduction path, thereby homogenizing the reaction within the battery and extending the battery's lifespan.

[0047] Furthermore, having few defects in the graphene structure of carbon nanotubes, that is, having a high G / D ratio in the Raman spectrum, is considered desirable from the viewpoint of suppressing the reactivity between carbon nanotubes and the electrolyte. For this reason, in the present invention, the carbon nanotubes preferably have a G / D ratio of 1 to 200 in the Raman spectrum, and more preferably 50 to 150.

[0048] The present invention provides a composition for forming a negative electrode active material layer for lithium-ion secondary batteries. By using an amorphous carbon material as the negative electrode active material and incorporating a very small amount of carbon nanotubes, it is possible to form a uniform electron conduction path while suppressing the inhibition of lithium ion movement, thereby homogenizing the reaction within the battery and extending the battery life. Therefore, the carbon nanotube content is small. In this invention, the carbon nanotube content is 0.01 to 0.8 mass%, preferably 0.02 to 0.5 mass%, and more preferably 0.05 to 0.2 mass%. When using multiple carbon nanotubes, it is preferable to adjust the total amount so that it falls within the above range. Although carbon nanotubes are substances that inhibit lithium ion movement, if the amount is around 0.8 mass% or less, it is less likely to inhibit lithium ion movement, and the reaction within the battery can be kept uniform. In the present invention, the composition for forming a negative electrode active material layer for lithium-ion secondary batteries is such that the total amount of the negative electrode active material, carbon nanotubes, conductive additives other than carbon nanotubes, and negative electrode constituent materials other than the negative electrode active material, carbon nanotubes, and conductive additives other than carbon nanotubes (the total amount of the composition) is 100% by mass.

[0049] (1-3) Conductive additives other than carbon nanotubes The present invention provides a composition for forming a negative electrode active material layer for lithium-ion secondary batteries. By using an amorphous carbon material as the negative electrode active material and incorporating a very small amount of carbon nanotubes, it is possible to form a uniform electron conduction path while suppressing the inhibition of lithium ion movement, thereby homogenizing the reactions within the battery and extending the battery's lifespan.

[0050] However, depending on the particle morphology, particle size, particle shape, and electronic conductivity of the amorphous carbon material, the composition for forming the negative electrode active material layer of the present invention may also contain a certain amount of a conductive additive in addition to a very small amount of carbon nanotubes, from the viewpoint of maintaining current collection between particles of the active material. For example, when using an amorphous carbon material with spherical particle shape as the negative electrode active material, it is preferable to include a certain amount of a conductive additive in addition to carbon nanotubes.

[0051] Examples of conductive additives other than carbon nanotubes in this invention include carbon black such as acetylene black, furnace black, and Ketjenblack; flake graphite; graphene; and amorphous carbon obtained by heat treatment of organic matter. These conductive additives other than carbon nanotubes can be used individually or in combination of two or more. Among these, carbon black is preferred from the viewpoint of not hindering lithium ion movement, making it easier to homogenize reactions within the battery, and extending the battery life.

[0052] In the present invention, when a conductive additive other than carbon nanotubes is included, the content of the conductive additive other than carbon nanotubes is preferably 0.1 to 10.0% by mass, more preferably 1.0 to 8.0% by mass, and even more preferably 2.0 to 6.0% by mass, from the viewpoint of easily homogenizing the reaction in the battery and easily extending the battery life. When multiple conductive additives other than carbon nanotubes are used, it is preferable to adjust the total amount so that it falls within the above range. In the composition for forming a negative electrode active material layer for lithium-ion secondary batteries of the present invention, the total amount of the negative electrode active material, carbon nanotubes, conductive additives other than carbon nanotubes, and negative electrode constituent materials other than the negative electrode active material, carbon nanotubes, and conductive additives other than carbon nanotubes (other negative electrode constituent materials) (total amount of composition) is 100% by mass.

[0053] Furthermore, for similar reasons, with the total amount of carbon nanotubes and conductive additives other than carbon nanotubes being 100% by mass, the carbon nanotube content is preferably 0.1 to 100.0% by mass, more preferably 0.5 to 50% by mass, and even more preferably 1.0 to 20.0% by mass.

[0054] (1-4) Negative electrode constituent materials excluding negative electrode active material, carbon nanotubes, and conductive additives other than carbon nanotubes (other negative electrode constituent materials) The present invention provides a composition for forming a negative electrode active material layer for lithium-ion secondary batteries. By using an amorphous carbon material as the negative electrode active material and incorporating a very small amount of carbon nanotubes, it is possible to form a uniform electron conduction path while suppressing the inhibition of lithium ion movement, thereby homogenizing the reactions within the battery and extending the battery's lifespan.

[0055] In this invention, the negative electrode constituent materials (other negative electrode constituent materials), excluding the negative electrode active material, carbon nanotubes, and conductive additives other than carbon nanotubes, are a general term for substances that inhibit the movement of lithium ions (lithium ion movement inhibiting substances), excluding carbon nanotubes and conductive additives other than carbon nanotubes, such as substances that have adhesion to the negative electrode active material and negative electrode current collector (binding agents), dispersants, etc.

[0056] Examples of negative electrode constituent materials (other negative electrode constituent materials) in this invention, excluding the negative electrode active material, carbon nanotubes, and conductive additives other than carbon nanotubes, include fluorine-based polymers (polyvinylidene fluoride resin, polytetrafluoroethylene resin, vinylidene fluoride-hexafluoropropylene copolymer, etc.), polyolefin resins (styrene-butadiene copolymer resin, ethylene vinyl alcohol copolymer resin, etc.), synthetic rubbers (styrene-butadiene rubber, acrylonitrile-butadiene rubber, ethylene propylene diene rubber, etc.), polyacrylonitrile, polyamide, polyimide, polyacrylic acid, polyacrylic acid ester, polyvinyl ether, carboxymethylcellulose, carboxymethylcellulose sodium salt, carboxymethylcellulose ammonium, polyurethane, hydroxypropylcellulose, hydroxyethylcellulose, methylcellulose, etc., used as binders, thickeners, or dispersants.

[0057] The present invention provides a composition for forming a negative electrode active material layer for lithium-ion secondary batteries. By using an amorphous carbon material as the negative electrode active material and incorporating a very small amount of carbon nanotubes, it is possible to form a uniform electron conduction path while suppressing the inhibition of lithium ion movement, thereby homogenizing the reaction within the battery and extending the battery life. From this viewpoint, it is preferable that the content of negative electrode constituent materials (other negative electrode constituent materials), excluding the negative electrode active material, carbon nanotubes, and conductive additives other than carbon nanotubes, be small. On the other hand, from the viewpoint of easily maintaining sufficient strength to preserve the shape of the electrode, it is preferable that the content be above a certain level. For this reason, in the present invention, the content of negative electrode constituent materials (other negative electrode constituent materials), excluding the negative electrode active material, carbon nanotubes, and conductive additives other than carbon nanotubes, is preferably 0.1 to 10.0% by mass, more preferably 1.0 to 8.0% by mass, and even more preferably 2.0 to 6.0% by mass. Furthermore, when using multiple negative electrode constituent materials (other negative electrode constituent materials) excluding the negative electrode active material, carbon nanotubes, and conductive additives other than carbon nanotubes, it is preferable to adjust the total amount so that it falls within the above range.In the composition for forming a negative electrode active material layer for lithium-ion secondary batteries of the present invention, the total amount of the negative electrode active material, carbon nanotubes, conductive additives other than carbon nanotubes, and negative electrode constituent materials (other negative electrode constituent materials) excluding the negative electrode active material, carbon nanotubes, and conductive additives other than carbon nanotubes (total amount of composition) is 100% by mass.

[0058] (1-5) Composition for forming a negative electrode active material layer for lithium-ion secondary batteries In the present invention's composition for forming a negative electrode active material layer for lithium-ion secondary batteries, when mixing a negative electrode active material, carbon nanotubes, a conductive additive other than carbon nanotubes if necessary, and negative electrode constituent materials (other negative electrode constituent materials) excluding the negative electrode active material, carbon nanotubes, and conductive additives other than carbon nanotubes if necessary, to form a paste composition for forming a negative electrode active material layer for lithium-ion secondary batteries, it is also possible to make it into a paste by including one or more organic solvents such as water, alcohol (methanol, ethanol, n-propyl alcohol, isopropyl alcohol, etc.), acetone, N-methylpyrrolidone, dimethyl sulfoxide, and dimethylformamide. In this case, the content of each component is a value with the total amount of the negative electrode active material, carbon nanotubes, a conductive additive other than carbon nanotubes if necessary, and negative electrode constituent materials (other negative electrode constituent materials) excluding the negative electrode active material, carbon nanotubes, and conductive additives other than carbon nanotubes if necessary, i.e., the total amount of solids is taken as 100% by mass.

[0059] The method for producing the composition for forming a negative electrode active material layer for lithium-ion secondary batteries of the present invention is not particularly limited. For example, the composition for forming a negative electrode active material layer for lithium-ion secondary batteries of the present invention can be produced by mixing each of the above-mentioned components by a conventional method. The mixing can be done by mixing all the components simultaneously or by mixing them sequentially.

[0060] 2. Negative electrode active material layer for lithium-ion secondary batteries The negative electrode active material layer for lithium-ion secondary batteries of the present invention contains the composition for forming the negative electrode active material layer for lithium-ion secondary batteries of the present invention.

[0061] The thickness of the negative electrode active material layer for the lithium-ion secondary battery of the present invention is not particularly limited. However, a thinner layer is preferable in terms of ensuring lithium ion penetration and electrical conductivity, thereby facilitating a more uniform reaction. On the other hand, since the present invention aims to improve battery life by reducing factors that inhibit lithium ion movement and thus uniformizing the battery reaction, a thicker layer is also possible for increased energy density per electrode. For this reason, the thickness of the negative electrode active material layer for the lithium-ion secondary battery of the present invention is preferably 1 to 300 μm, more preferably 10 to 150 μm, and even more preferably 50 to 100 μm.

[0062] Such a negative electrode active material layer for a lithium-ion secondary battery of the present invention can be manufactured by forming the above-described composition for forming a negative electrode active material layer for a lithium-ion secondary battery into layers. For example, if the composition for forming a negative electrode active material layer for a lithium-ion secondary battery of the present invention is a paste composition for forming a negative electrode active material layer for a lithium-ion secondary battery, the paste composition can be dried by a conventional method and formed into layers.

[0063] 3. Negative electrode for lithium-ion secondary battery The negative electrode for a lithium-ion secondary battery of the present invention comprises the negative electrode active material layer for a lithium-ion secondary battery of the present invention described above, but more specifically, it is preferable that it comprises a negative electrode current collector and the negative electrode active material layer for a lithium-ion secondary battery of the present invention disposed on the negative electrode current collector.

[0064] The negative electrode current collector is preferably made of a material that is electrochemically stable at the potential used and has high electronic conductivity, such as copper, stainless steel, nickel, or carbon material. This negative electrode current collector can be in the form of a foil, mesh, or other similar component.

[0065] When manufacturing the negative electrode for a lithium-ion secondary battery according to the present invention, it can be manufactured by forming the above-described composition for forming a negative electrode active material layer for a lithium-ion secondary battery in a layer on a negative electrode current collector. For example, if the composition for forming a negative electrode active material layer for a lithium-ion secondary battery according to the present invention is a paste composition for forming a negative electrode active material layer for a lithium-ion secondary battery, the negative electrode for a lithium-ion secondary battery according to the present invention can be manufactured by drying the paste composition on the negative electrode active material by a conventional method and forming it in a layer.

[0066] 4. Lithium-ion rechargeable batteries The lithium-ion secondary battery of the present invention is equipped with the negative electrode for lithium-ion secondary batteries of the present invention described above. In addition to the negative electrode for lithium-ion secondary batteries of the present invention, the lithium-ion secondary battery of the present invention may also be equipped with a positive electrode, an electrolyte, and a container for housing these, which are applicable to known lithium-ion secondary batteries.

[0067] Any positive electrode capable of supplying lithium ions to the negative electrode will suffice, and well-known positive electrodes can be used.

[0068] Examples of materials that can be used as the positive electrode current collector include aluminum, stainless steel, and carbon materials, which are electrochemically stable at the potential being used and have high electronic conductivity.

[0069] Furthermore, as the positive electrode active material constituting the positive electrode, a material capable of intercalating and releasing lithium ions is usually used. Examples include lithium transition metal composite oxides having an α-NaFeO2 type crystal structure, lithium transition metal oxides having a spinel type crystal structure, polyanionic compounds, chalcogen compounds, sulfur, etc. As an example of a lithium transition metal composite oxide having an α-NaFeO2 type crystal structure, Li[Li x1 Ni γ1 Mn β1 Co (1-x1-γ1-β1) ]O2(0≦x1<0.5, 0≦γ1≦1, 0≦β1≦1, 0≦γ1+β1≦1), Li[Li x2 Ni γ2Co β2 Al (1-x2-γ2-β2) O2 (0 ≦ x2 < 0.5, 0 ≦ γ2 ≦ 1, 0 ≦ β2 ≦ 1, 0 ≦ γ2 + β2 ≦ 1), etc. are included. As the lithium transition metal oxide having a spinel crystal structure, Li x3 Mn2O4 (0.9 ≦ x3 < 1.5), Li x4 Ni γ4 Mn (2-γ4) O4 (0.9 ≦ x4 < 1.5, 0 ≦ γ4 ≦ 2), etc. are included. As the polyanion compound, LiFePO4, LiMnPO4, LiNiPO4, LiCoPO4, Li3V2(PO4)3, Li2MnSiO4, Li2CoPO4F, etc. are included. As the chalcogen compound, titanium disulfide, molybdenum disulfide, molybdenum dioxide, etc. are included. Atoms or polyanions in these materials may be partially substituted with atoms or anion species composed of other elements. These cathode active materials can be used alone or in combination of two or more. Among these cathode active materials, from the viewpoint of high energy density, the above lithium transition metal composite oxide is preferable.

[0070] As the cathode constituent material other than the cathode active material constituting the cathode, the same materials as those of the anode constituent materials other than the anode active material and carbon nanotubes described above can be used, and the content can also be the same as that of the anode constituent materials other than the anode active material and carbon nanotubes in the anode.

[0071] Further, the electrolytic solution is an electrolytic solution in which a salt is dissolved in an aprotic organic solvent, is disposed between the cathode and the anode, and is preferably impregnated and held in a separator made of, for example, a non-woven fabric for preventing short circuit between the cathode and the anode.

[0072] Examples of aprotic organic solvents that constitute the electrolyte mentioned above include esters such as ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, γ-butyrolactone, methyl formate, and methyl acetate; furans such as tetrahydrofuran and 2-methyltetrahydrofuran; ethers such as dioxolane, diethyl ether, dimethoxyethane, diethoxyethane, and methoxyethoxyethane; dimethyl sulfoxide; sulforanes such as sulfolane and methylsulfolane; and acetonitrile. These aprotic organic solvents may be used individually or in combination of two or more.

[0073] On the other hand, examples of lithium salts that dissolve in such aprotic organic solvents include lithium perchlorate, lithium borofluoride, lithium hexafluoride phosphate, lithium hexafluoride arsenate, lithium trifluoromethanesulfonate, lithium halides, lithium aluminate chloride, and lithium bis(fluorosulfonyl)imide. These salts may be used individually or in combination of two or more.

[0074] Because the lithium-ion secondary battery of the present invention uses the composition for forming the negative electrode active material layer of the present invention in the negative electrode, it can suppress the obstruction of lithium ion movement and form a uniform electron conduction path, thereby homogenizing the reaction within the battery and extending the battery life. For this reason, this lithium-ion secondary battery is expected to be deployed in the market in the future and can be effectively used in electric vehicle applications for car sharing, particularly in AIEV (Artificial Intelligence Electric Vehicle) applications, where further extended lifespan will be required.

[0075] 5. Method for evaluating the internal resistance and reaction distribution of lithium-ion secondary batteries. Generally, the reaction resistance of a battery is measured by AC impedance measurement. However, AC impedance measurement shows that the reaction resistance of the battery remains almost the same even when the content of each negative electrode component is changed. In other words, the reaction resistance measured by AC impedance measurement is an index that does not include the effect of reaction unevenness, and it is not possible to evaluate the degree of reaction unevenness. Thus, even when the content of each negative electrode component is changed, the reaction resistance of the battery remains almost the same, and it was not possible to evaluate the reaction unevenness.

[0076] On the other hand, under high-load conditions, reaction uneven distribution is likely to occur. Therefore, by measuring the internal resistance under high-load conditions, the inhibition of lithium ion transfer can be evaluated.

[0077] However, under high-load conditions during the charging process, there is a concern that lithium electrodeposition may occur at the negative electrode, making it difficult to accurately measure the resistance. Therefore, it is necessary to measure the internal resistance under high-load conditions during the discharge process.

[0078] Therefore, in this invention, the state of charge (SOC) is given by the following formula (1): SOC (%) = Remaining capacity (Ah) / Full charge capacity (Ah) × 100 (1) Defined as, After discharging from a state of charge (SOC) of 100% to a state of charge (SOC) of 90% under conditions of 25°C and 2.5C or higher, the device is left idle for 10 minutes, and the voltage rise during the idle period is measured. Then, the following equation (2): Internal resistance = (Voltage increase during idle (V) / Current value during discharge (A)) × Opposite area of ​​positive and negative electrodes (cm²) 2 ) (2) The internal resistance is calculated using this method.

[0079] As described above, the discharge rate is set to 2.5C or higher (preferably 2.7 to 10.0C), which is a high-load condition, because this condition makes it easier to evaluate the uneven distribution of reactions.

[0080] Furthermore, the reason for setting the State of Charge (SOC) at the start of discharge to 100% is to ensure that the state before measurement is free from reaction unevenness by sufficiently accumulating lithium ions within the negative electrode.

[0081] After discharge under these conditions, the voltage rises due to the pause in the reaction. Normally, the voltage rise saturates after 10 minutes and maintains a constant voltage, so the voltage rise during the pause can be measured after 10 minutes, and the following equation (2): Internal resistance = (Voltage increase during idle (V) / Current value during discharge (A)) × Opposite area of ​​positive and negative electrodes (cm²) 2 ) (2) The internal resistance can be calculated using this method.

[0082] As a result, a high internal resistance value indicates a large uneven distribution of reactions and a short lifetime, while a low internal resistance value indicates a small uneven distribution of reactions and a longer lifetime.

[0083] Specifically, when using the lithium-ion secondary battery negative electrode active material layer formation composition of the present invention, the internal resistance calculated as described above under the condition of 3.0C is 1.0 to 25.0 Ω·cm. 2 Preferably, 1.0 to 24.0 Ω·cm 2 More preferably, 1.0~23.0Ω·cm 2 This is even more preferable. The lower limit of the internal resistance is, as stated above, 1.0 Ω·cm. 2 This is preferable, but 2.0Ω·cm 2 or 3.0Ω·cm 2 It is also possible to set the same as the lower limit.

[0084] Furthermore, in the charging process to bring the SOC to 100% as described above, charging at a high load condition of 3.0C raises concerns about lithium electrodeposition at the negative electrode; therefore, it is preferable not to charge under high load conditions. Also, since the objective is to ensure that the state before measurement is free from reaction unevenness by sufficiently intercalating lithium ions in the negative electrode, it is preferable to charge using constant current constant voltage (CCCV charging) up to the upper limit voltage equivalent to 100% SOC in the charging process to bring the SOC to 100% as described above. The charging rate is preferably 0.01 to 1.0C, and more preferably 0.01 to 0.75C. As described above, the lower limit of the charging rate is preferably 0.01C, but it is also possible to set the lower limit to 0.02C or 0.03C. [Examples]

[0085] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[0086] Comparative Example 1 As the negative electrode active material, 90.0% by mass of hard carbon (a non-graphitizable carbon material; particle shape: spherical; average particle diameter 5 μm) was added and kneaded with 5.0% by mass of acetylene black (AB) as a conductive additive other than carbon nanotubes, 5.0% by mass of polyvinylidene fluoride (PVdF) as other negative electrode constituent materials, and an appropriate amount of N-methylpyrrolidone (NMP) to form a slurry. This slurry was then applied to copper foil (thickness 10 μm) and dried with a doctor blade. The weight per unit area of ​​the negative electrode active material layer after drying was 10.6 mg / cm². 2 After coating and drying at 100°C, the density of the negative electrode active material layer is 1.0 g / cm³. 3 The material was roll-pressed in this manner, dried under reduced pressure at 170°C, and a negative electrode was obtained.

[0087] Example 1 As the negative electrode active material, 90.0 mass% of hard carbon (a carbon material that is difficult to graphitize; particle shape: spherical; average particle diameter 5 μm) was added, along with 0.1 mass% of single-walled carbon nanotubes (bundled single-walled CNT aggregates; per single-walled CNT, average outer diameter 2 nm, average length > 5 μm, G / D: 80~150), along with 4.9 mass% of acetylene black (AB) as a conductive additive other than carbon nanotubes, and 5.0 mass% of polyvinylidene fluoride (PVdF) and an appropriate amount of N-methylpyrrolidone (NMP) as other negative electrode constituent materials. These were mixed together to form a slurry. After drying this slurry on the copper foil using a doctor blade, the weight per unit area of ​​the negative electrode active material layer was 10.6 mg / cm². 2 After coating and drying at 100°C, the density of the negative electrode active material layer is 1.0 g / cm³. 3 The material was roll-pressed in this manner, dried under reduced pressure at 170°C, and a negative electrode was obtained.

[0088] The compositions of each example and comparative example are shown in Table 1.

[0089] [Table 1]

[0090] Manufacturing example: Manufacturing of lithium-ion rechargeable batteries As the negative electrode, the negative electrodes obtained in Example 1 and Comparative Example 1 were used.

[0091] With respect to the total weight of the cathode composition, LiNi 0.8 Co 0.15 Al 0.05 92.0% by mass of O2 (NCA; average particle size 6 μm), 4.0% by mass of polyvinylidene fluoride (PVdF) and 4.0% by mass of acetylene black (AB) as other cathode constituent materials, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) were added and kneaded to form a slurry. This slurry was then applied to aluminum foil (thickness 17 μm) and dried with a doctor blade. The mass per unit area of ​​the cathode active material layer after drying was 16.2 mg / cm². 2After coating and drying at 100°C, the density of the positive electrode active material layer is 3.0 g / cm³. 3 The material was roll-pressed in this manner, dried under reduced pressure at 170°C, and the positive electrode was obtained.

[0092] The electrolyte used consisted of a mixture of ethylene carbonate (EC) and methyl ethyl carbonate (MEC) as solvents in a volume ratio of 3:7, and 1 mol / L lithium hexafluoride phosphate (LiPF6) as the salt. This electrolyte was impregnated into a polyethylene porous film, which served as the separator.

[0093] A lithium-ion secondary battery was fabricated using the negative electrode, positive electrode, electrolyte, and separator described above. The opposing surface area of ​​the positive and negative electrodes of the fabricated lithium-ion secondary battery was 2.8 cm². 2 That's what I decided.

[0094] Test Example 1: Initial Charge / Discharge Characteristics Charge-discharge tests were conducted on the lithium-ion secondary batteries of Example 1 and Comparative Example 1, which were fabricated, with the aim of confirming their initial charge-discharge capacity.

[0095] The charging and discharging process involved constant current constant voltage (CCCV) charging at a charging rate of 0.1C until the current value reached 0.05C up to an upper voltage limit of 4.05V, followed by a 10-minute pause. Then, discharge was performed at a discharge rate of 0.1C down to a lower voltage limit of 2.0V, followed by another 10-minute pause. As a result, Example 1 showed a charging capacity of 7.68mAh, a discharge capacity of 5.14mAh, and a Coulomb efficiency of 66.9%, while Comparative Example 1 showed a charging capacity of 7.76mAh, a discharge capacity of 4.90mAh, and a Coulomb efficiency of 63.1%.

[0096] It was confirmed that including a small amount of carbon nanotubes to form a uniform electron conduction path improved the initial Coulomb efficiency and increased the discharge capacity.

[0097] Test Example 2: Internal Resistance under High Load Conditions In the lithium-ion secondary batteries of Example 1 and Comparative Example 1, the internal resistance was measured using the high-load rest method to confirm the degree of reaction segregation.

[0098] Specifically, the lithium-ion secondary batteries of Example 1 and Comparative Example 1 were charged at 25°C under conditions of a charge rate of 0.5C and a cut-off current of 0.05C, using constant current constant voltage (CCCV charging) up to an upper voltage limit of 4.05V, which corresponds to 100% SOC. Next, after a 10-minute pause, they were discharged to 90% SOC (2 minutes) under conditions of a discharge rate of 3.0C, and then paused for another 10 minutes. The charge level (SOC) is given by the following formula (1): SOC (%) = Remaining capacity (Ah) / Full charge capacity (Ah) × 100 (1) It is defined as follows.

[0099] Furthermore, the following formula: Internal resistance = (Voltage rise during idle period "ΔV (10 min)" / Current value during discharge "3C current value") × Opposing area of ​​positive and negative electrodes "2.8 cm²" 2 " The internal resistance was calculated using the following method. A schematic of the analysis method in the high-load rest method is shown in Figure 2.

[0100] As a result of the high-load resting method, the internal resistance of Example 1 was 21.67 Ω·cm. 2 The internal resistance of Comparative Example 1 is 28.25 Ω·cm. 2 That was the case.

[0101] As a result, it was confirmed that by using hard carbon and adding a small amount of carbon nanotubes, the internal resistance under high-load conditions was dramatically reduced, meaning that the uneven distribution of electrode reactions was suppressed.

[0102] Test Example 3: Lifetime Characteristics Charge-discharge cycle tests were performed on the lithium-ion secondary batteries of Example 1 and Comparative Example 1 at 25°C. The charge-discharge conditions were as follows: constant current constant voltage (CCCV) charging was performed at a charge rate of 0.5C until the current value reached 0.05C up to an upper voltage limit of 4.05V, followed by a 10-minute pause, and then discharge at a discharge rate of 0.5C down to a lower voltage limit of 2.0V, followed by another 10-minute pause. These charge-discharge processes constituted one cycle, and the charge-discharge cycle test was performed accordingly.

[0103] As shown in Figure 3, the capacity retention rate (150 cycles) for Example 1 was 93.0%, and the capacity retention rate (150 cycles) for Comparative Example 1 was 79.7%.

[0104] As a result, it was confirmed that the charge-discharge cycle characteristics were improved by using hard carbon and adding a small amount of carbon nanotubes.

Claims

1. A composition for forming a negative electrode active material layer for a lithium-ion secondary battery, comprising an amorphous carbon material negative electrode active material and carbon nanotubes, Assuming the total amount of the composition is 100% by mass, A composition for forming a negative electrode active material layer for lithium-ion secondary batteries, wherein the content of the negative electrode active material is 79.2 to 99.8% by mass, and the content of the carbon nanotubes is 0.01 to 0.8% by mass.

2. It contains conductive additives other than the aforementioned carbon nanotubes, The composition for forming a negative electrode active material layer for a lithium-ion secondary battery according to claim 1, wherein the content of conductive additives other than carbon nanotubes is 0.1 to 10.0% by mass, with the total amount of the composition being 100% by mass.

3. The composition for forming a negative electrode active material layer for a lithium-ion secondary battery according to claim 1 or 2, wherein the total amount of the carbon nanotubes and conductive additives other than carbon nanotubes is 100% by mass, and the content of the carbon nanotubes is 0.1 to 10.0% by mass.

4. The negative electrode constituent material contains the negative electrode active material, the carbon nanotubes, and conductive additives other than the carbon nanotubes, A composition for forming a negative electrode active material layer for a lithium-ion secondary battery, according to any one of claims 1 to 3, wherein the content of the negative electrode constituent material is 0.1 to 10.0% by mass, with the total amount of the composition being 100% by mass.

5. The composition for forming a negative electrode active material layer for a lithium-ion secondary battery according to any one of claims 1 to 4, wherein the negative electrode active material is an amorphous layered carbon material and the interlayer distance of the (002) plane is 0.350 nm or more.

6. The composition for forming a negative electrode active material layer for a lithium-ion secondary battery according to any one of claims 1 to 5, wherein the negative electrode active material contains hard carbon.

7. The composition for forming a negative electrode active material layer for a lithium-ion secondary battery according to any one of claims 1 to 6, wherein the carbon nanotube is a single-walled carbon nanotube.

8. A composition for forming a negative electrode active material layer for a lithium-ion secondary battery, according to any one of claims 1 to 7, used to reduce the uneven distribution of reactions in a lithium-ion secondary battery.

9. A composition for forming a negative electrode active material layer for a lithium-ion secondary battery, according to any one of claims 1 to 8, for use in lithium-ion secondary batteries used in electric vehicles for car sharing.

10. A negative electrode active material layer for a lithium-ion secondary battery, comprising the composition for forming a negative electrode active material layer for a lithium-ion secondary battery according to any one of claims 1 to 9.

11. The negative electrode active material layer for a lithium-ion secondary battery according to claim 10, which is for a lithium-ion secondary battery used in an electric vehicle for car sharing.

12. A negative electrode for a lithium-ion secondary battery, comprising the negative electrode active material layer for a lithium-ion secondary battery according to claim 10 or 11.

13. The negative electrode for a lithium-ion secondary battery according to claim 12, which is for a lithium-ion secondary battery used in an electric vehicle for car sharing.

14. A lithium-ion secondary battery comprising the negative electrode for a lithium-ion secondary battery according to claim 12 or 13.

15. The state of charge (SOC) is given by the following formula (1): SOC (%) = Remaining capacity (Ah) / Full charge capacity (Ah) × 100 (1) Defined as, After discharging from a state of charge (SOC) of 100% to a state of charge (SOC) of 90% under conditions of 25°C and 3.0°C, the system was left idle for 10 minutes, and the voltage increase during the idle period was measured. The following equation (2): Internal resistance = (Voltage increase during idle (V) / Current value during discharge (A)) × Opposite area of ​​positive and negative electrodes (cm²) ２ ) (2) The internal resistance calculated by this method is 1.0 to 25.0 Ω·cm ２ The lithium-ion secondary battery according to claim 14.

16. A lithium-ion secondary battery according to claim 14 or 15, used in an electric vehicle for car sharing.

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