Negative electrode for lithium-ion secondary battery and lithium-ion secondary battery

Abstract

To provide an anode for lithium ion secondary batteries capable of making high energy density compatible with rate characteristics especially during high rate charge, and a lithium ion secondary battery.SOLUTION: An anode for lithium ion secondary batteries comprises an electrode collector and an anode mixture layer which is formed on the electrode collector. In the anode for lithium ion secondary batteries, the anode mixture layer does not contain a conduction assistant and contains an anode active material. The anode active material is a lithium titanium oxide. In the anode mixture layer, a weight ratio of the anode active material is 98% or more and 99.9% or less, and an electrode density is greater than 1.70 g / cc and less than 1.90 g / cc. In the anode mixture layer, a coating amount on one face is 30 g / m2 or more and 70 g / m2 or less, and ten-point average roughness Rz on an electrode surface satisfies 0<Rz<10 μm.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a negative electrode for lithium-ion secondary batteries and to lithium-ion secondary batteries. [Background technology]

[0002] Currently, lithium (Li)-ion secondary batteries are used in a wide range of applications, including small power supplies for mobile phones and laptops, and storage batteries for electric vehicles. In recent years, there has been a growing demand for higher input and output power in lithium-ion secondary batteries, particularly those used as power sources for IoT devices and wearable devices.

[0003] Lithium-ion secondary batteries can generally be made to have high input / output power by selecting positive and negative electrode active materials, miniaturizing the active material material or coating it with carbon, adding conductive additives, and using electrolytes and separators with good ion conductivity.

[0004] Lithium titanium oxide (hereinafter referred to as "LTO") is one of the candidate materials that can achieve rapid charging and discharging as a negative electrode for lithium-ion secondary batteries. LTO has a robust three-dimensional crystal structure, and is known to exhibit high cycle characteristics because there is almost no expansion or contraction of the crystal when lithium is intercalated or released. Furthermore, electrodes using LTO have a high potential relative to lithium of 1.5V, so the deposition of metallic lithium is unlikely to occur when charging and discharging is repeated, and the decomposition of the non-aqueous electrolyte hardly occurs, making them highly safe electrodes. In addition, the ionic conductivity in the crystal is high, resulting in excellent charge-discharge rate characteristics.

[0005] Generally, electrodes consist of an electrode current collector and an electrode composite layer. The electrode composite layer is composed of an active material, a conductive additive, and a binder. In this case, the conductive additive plays a role in improving the electronic conductivity in the electrode composite layer and providing good charge-discharge rate characteristics. On the other hand, increasing the proportion of the conductive additive in the electrode composite layer reduces the proportion of the active material in the electrode composite layer, which decreases the energy density. Thus, there is a trade-off relationship between rate characteristics and energy density, and it is difficult to achieve both. This is also true for lithium-ion secondary batteries that use LTO as the negative electrode active material, which is intended for high-rate applications.

[0006] Patent Document 1 describes that, in order to solve the above problems, it is possible to achieve both energy density and discharge rate characteristics by constructing the negative electrode composite layer with only LTO and a binder, and by making the electrode density 2.0 g / cc or higher. In other words, by not including conductive additives, the ratio of active material in the negative electrode composite layer is increased, and furthermore, by using LTO, which has excellent rate characteristics, as the negative electrode active material, both energy density and discharge rate characteristics are achieved. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2013-518376 [Overview of the project] [Problems that the invention aims to solve]

[0008] Incidentally, high input characteristics are also required for lithium-ion secondary batteries. However, Patent Document 1 does not adequately consider how to achieve both high energy density and charge rate characteristics at high charge rates exceeding 20C.

[0009] When attempting to increase the energy density by increasing the coating amount of the negative electrode composite layer, the charging rate characteristics deteriorate. Generally, when the negative electrode composite layer is thickly coated, lithium ion depletion occurs inside the bulk, and the charging rate characteristics decrease, making it difficult to achieve both high energy and charging rate characteristics.

[0010] The present invention has been made in view of the above, and an object thereof is to provide a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery that can achieve both a high energy density and rate characteristics, particularly during high-rate charging.

Means for Solving the Problems

[0011] In order to solve the above-described problems and achieve the object, the negative electrode for a lithium ion secondary battery according to the present invention is a negative electrode for a lithium ion secondary battery including an electrode current collector and a negative electrode composite layer formed on the electrode current collector, wherein the negative electrode composite layer does not contain a conductive assistant and contains a negative electrode active material, the negative electrode active material is lithium titanate, the weight ratio of the negative electrode active material in the negative electrode composite layer is 98% or more and 99.9% or less, the electrode density is greater than 1.70 g / cc and less than 1.90 g / cc, and the coating amount on one side in the negative electrode composite layer is 30 g / m 2 or more and 70 g / m 2 or less, and the ten-point average roughness Rz on the electrode surface satisfies 0 < Rz < 10 μm, which is characterized by this.

[0012] Further, the negative electrode for a lithium ion secondary battery according to the present invention is characterized in that, in the above invention, the binder in the negative electrode composite layer is an organic binder.

[0013] Further, the negative electrode for a lithium ion secondary battery according to the present invention is characterized in that, in the above invention, the negative electrode composite layer consists only of the negative electrode active material and a binder.

[0014] Further, the negative electrode for a lithium ion secondary battery according to the present invention is characterized in that, in the above invention, the lithium titanate is of the spinel type.

[0015] The lithium-ion secondary battery according to the present invention further includes a positive electrode having a positive electrode current collector and a positive electrode composite material layer coated on the positive electrode current collector and containing a positive electrode active material, a negative electrode current collector, a negative electrode composite material layer coated on the negative electrode current collector and containing a negative electrode active material, a separator interposed between the positive electrode and the negative electrode, an electrolytic solution, and a case for housing the positive electrode, the negative electrode, the separator, and the electrolytic solution. The negative electrode composite material layer does not contain a conductive auxiliary agent and contains a negative electrode active material. The negative electrode active material is lithium titanate. The weight ratio of the negative electrode active material in the negative electrode composite material layer is 98% or more and 99.9% or less. The electrode density of the negative electrode is greater than 1.70 g / cc and less than 1.90 g / cc. The coating amount on one side in the negative electrode composite material layer is 30 g / m 2 to 70 g / m 2 or less, and the ten-point average roughness Rz on the electrode surface of the negative electrode satisfies 0 < Rz < 10 μm.

[0016] The lithium-ion secondary battery according to the present invention is the above invention, and the charging rate during use of the lithium-ion secondary battery is 20C or more.

Advantages of the Invention

[0017] According to the present invention, it is possible to obtain a negative electrode for a lithium-ion secondary battery and a lithium-ion secondary battery that can achieve both a high energy density and particularly good rate characteristics during high-rate charging.

Brief Description of the Drawings

[0018] [Figure 1] FIG. 1 is an exploded perspective view for explaining the configuration of a lithium-ion secondary battery including a negative electrode for a lithium-ion secondary battery according to an embodiment of the present invention.

Embodiments for Carrying Out the Invention

[0019] The embodiments of the present invention will be described below, but the present invention is not limited to the following description. Furthermore, various modifications or improvements can be made to these embodiments, and such modified or improved forms may also be included in the present invention. Figure 1 shows an example configuration of a coin-type lithium-ion secondary battery (coin cell) as an example of an embodiment, but the shape of the lithium-ion secondary battery in the present invention is not particularly limited and may be flat, cylindrical, prismatic, or laminated. Furthermore, the casing of the lithium-ion secondary battery is not particularly limited and known materials such as laminate film, aluminum, aluminum alloy, and stainless steel can be used.

[0020] (Embodiment) Figure 1 is an exploded perspective view illustrating the configuration of a lithium-ion secondary battery equipped with a negative electrode for a lithium-ion secondary battery according to one embodiment of the present invention.

[0021] The lithium-ion secondary battery 10 comprises a case 1, a leaf spring 2, a spacer 3, a positive electrode 4, a separator 5, a negative electrode 6, a gasket 7, and a cap 8. The positive electrode 9 is formed by a positive electrode current collector 3 and a positive electrode composite layer 4, making it a coin-type lithium-ion secondary battery.

[0022] In the lithium-ion secondary battery 10, the case 1 and cap 8 are fixed together by crimping or the like, and the inside is filled with a non-aqueous electrolyte. In the non-aqueous electrolyte secondary battery 10, the case 1, gasket 7, and cap 8 provide a liquid-tight seal. In addition, the spacer 3, positive electrode 4, separator 5, and negative electrode 6 are biased toward the cap 8 by the leaf spring 2. This maintains a state in which each component is in close contact with the others.

[0023] (Negative electrode) According to one embodiment of the present invention, a negative electrode for a lithium secondary battery is provided, comprising at least a negative electrode current collector and a negative electrode composite material layer provided on one or both sides of the negative electrode current collector. In this embodiment, the negative electrode composite material layer is composed of a negative electrode active material and a binder, and does not contain a conductive additive. The negative electrode 6 is formed by rolling with a roll press machine.

[0024] (Negative electrode current collector) There is no particular limitation on the material constituting the negative electrode current collector, but it is preferable to use a metal. Specifically, copper, aluminum, nickel, stainless steel, titanium, other alloys, etc. may be mentioned. Among them, aluminum is preferable from the viewpoints of electron conductivity and battery operating potential.

[0025] (Negative electrode active material) Lithium titanate (hereinafter, sometimes referred to as "LTO") is used as the negative electrode active material. As the lithium titanate, various compounds satisfying the general formula (1) can be used alone or in combination of two or more. Li a Ti 3-a O4 (0 < a < 3) ···(1) Among them, in particular, the compound represented by the following formula (1-1) is an active material with a small expansion - contraction amount during charge and discharge and a relatively long cycle life of the electrode. Li 4 / 3 Ti 5 / 3 O4···(1-1)

[0026] The negative electrode composite material layer is composed only of a negative electrode active material and an organic binder. The negative electrode active material is lithium titanate, and the weight ratio of the negative electrode active material in the negative electrode composite material layer is 98% or more and 99.9% or less. The electrode density in the negative electrode 6 is greater than 1.70 g / cc and less than 1.90 g / cc.

[0027] The lithium titanate is not particularly limited, but it is more preferable that the crystal is of the spinel type, and even more preferably, it is a crystal represented by the crystal formula Li4Ti5O 12 is a crystal represented by. The lithium titanate is Li2TiO3 or Li4Ti5O 12These are crystals, such as those shown above, and have a high potential and are less prone to decomposition reactions with the electrolyte, making them materials with high cycle characteristics. Among them, spinel-type lithium titanium oxide has a strong crystal structure and maintains a stable crystal structure even when lithium is absorbed, making it a material with even higher cycle characteristics. Furthermore, among spinel-type lithium titanium oxides, Li4Ti5O 12 This material exhibits almost no volume expansion in the crystal before and after lithium storage, making it less susceptible to degradation due to electrode expansion and contraction, and thus possessing superior cycle characteristics.

[0028] (binder) Organic binders are used as binders in the negative electrode composite layer, and examples include one of the following, or mixtures thereof: polyethylene, polypropylene, ethylene propylene polymer, butadiene rubber, styrene butadiene rubber, butyl rubber, polytetrafluoroethylene, poly(meth)acrylate, polyvinylidene fluoride, polyethylene oxide, polypropylene oxide, polyepichlorohydrin, polyphasfazen, polyacrylonitrile, etc.

[0029] As a result of diligent experiments conducted by the inventors, they discovered that in electrodes using only LTO and binder in the negative electrode composite layer without conductive additives, there is an inflection point in the reciprocal relationship between the rate value and capacity. Furthermore, they found that the rate value at which the inflection point occurs varies depending on the amount of coating; in other words, they found that the inflection point exists at a coating amount above a certain amount. This inflection point is presumed to be mainly caused by concentration polarization, and is thought to indicate a state where lithium ions are depleted, i.e., the critical current density, particularly in the deep part of the electrode (the current collector side in the thickness direction of the negative electrode composite layer) on the current collector side of the electrode. More specifically, when comparing electrodes with the same electrode density but different coating amounts, electrodes with a larger coating amount have a larger negative electrode composite layer thickness, resulting in a longer diffusion distance of lithium ions in the electrolyte, which is thought to cause a concentration distribution and a phenomenon where the supply of lithium ions to the active material cannot keep up. Therefore, it is thought that when the critical current density is exceeded, the ion diffusion reaction becomes the rate-limiting reaction rather than the charge transfer reaction. In other words, it is thought that the rate-determining reaction changes from a charge transfer reaction to an ion diffusion reaction around the inflection point that appears as the rate value increases, and especially in the reaction process on the high-rate side, it is thought that the rate-determining reaction is ion diffusion rather than charge transfer. From the experimental results, 30 g / m 2 More than 70g / m 2 An inflection point was observed at 20C for coating amounts within the following range. In other words, within the above coating amount range, ion diffusion becomes the rate-limiting reaction at high rates above 20C. Therefore, it has been confirmed that the rate characteristics are advantageous when the negative electrode composite layer does not contain conductive additives such as acetylene black to promote electron conductivity, and only LTO, which has excellent ionic conductivity, is used as the negative electrode active material.

[0030] Furthermore, it is desirable that the active material ratio in the negative electrode composite layer be between 98% and 99.9%. If the active material ratio is less than 98%, the binder component will be in excess and act as a resistive component, causing a significant decrease in charge rate characteristics. Conversely, if the active material ratio is greater than 99.9%, the amount of binder will be insufficient, and the adhesion between the negative electrode composite and the current collector will be significantly reduced.

[0031] Furthermore, it is desirable that the electrode density be greater than 1.70 g / cc and less than 1.90 g / cc. Moreover, it is preferable that the electrode density be between 1.75 g / cc and 1.85 g / cc. Conventionally, carbon black is commonly used as a conductive additive in the negative electrode composite layer. This conductive additive has a smaller particle size than the negative electrode active material and penetrates the gaps between the negative electrode active materials, acting as a buffer in the electrode. Therefore, even when the electrode is pressed to a high density of 2.0 g / cc or more, the active material particles are not crushed, and the conductive paths in the composite are maintained, making it possible to obtain good rate characteristics. On the other hand, since the present invention is characterized by not containing a conductive additive, if the electrodes are pressed to a high density, the active material particles themselves will be crushed. As a result, the distortion and collapse of the LTO particles reduce the contact area between LTO particles, leading to the disruption of ion conduction paths and, consequently, a decrease in rate characteristics. Therefore, while conventional electrode densities have been considered better at higher densities, as shown in Patent Document 1, in the present invention, the electrode density is set to a range greater than 1.70 g / cc and less than 1.90 g / cc, as this is the range in which the LTO active material is not crushed because the negative electrode composite layer does not contain a conductive additive. Furthermore, when the electrode density is within the above range, the charge rate characteristics are particularly improved. The reason for this is that the amount of electrolyte present in the negative electrode composite layer has a significant influence. During charging, a reaction occurs in which lithium ions are inserted into the LTO particles of the negative electrode active material, and these lithium ions are supplied from the electrolyte near the active material. Therefore, it is necessary for there to be a sufficient amount of electrolyte around the active material. Here, electrode density greatly affects the amount of voids within the negative electrode composite layer, i.e., the amount of electrolyte that can penetrate into the negative electrode composite layer. If the electrode density is too high, the amount of voids within the negative electrode composite layer decreases, and as a result, the amount of electrolyte penetration also decreases. When the charging speed increases, the amount of lithium ions consumed during the reaction increases, but if the amount of electrolyte penetration is low, the amount of lithium ions that can be supplied will be insufficient. As a result, when the lithium ion concentration near the electrodes becomes 0%, further charging becomes impossible, and the charging rate characteristics deteriorate. Also, if the electrode density is too low, the active materials do not come into contact with each other, so ion conduction paths cannot be formed. Therefore, at densities greater than 1.70 g / cc and less than 1.90 g / cc, the LTO active materials do not collapse, allowing them to come into contact with each other and be sufficiently immersed in the electrolyte, resulting in good charging rate characteristics. During discharge, a reaction occurs in which lithium ions are released from the active materials, and this lithium is released into the electrolyte near the active materials. In that case, there are plenty of lithium ions in the active material and they will not be depleted, so it is less of a problem than during charging.

[0032] Also, it is desirable that the ten-point average roughness Rz on the surface of the electrode (here, the negative electrode 6) satisfies 0 < Rz < 10 μm. At this time, it is more preferable that the ten-point average roughness Rz on the electrode surface is 9 μm or less. When the ten-point average roughness Rz is 10 μm or more, it indicates that the density becomes locally non-uniform, that is, there are sites where the electrode density is excessively high or excessively low. As described above, where the electrode density is high, when the electrode is pressed, the active material particles are crushed, and due to the distortion and collapse of the LTO particles, the contact area between the LTO particles decreases, and the ion conduction path is interrupted. Therefore, the ion conduction path between the LTO particles cannot be established, and good rate characteristics cannot be maintained. Also, where the electrode density is low, when the charging rate increases, the amount of lithium consumed during the reaction increases, so the amount of lithium ions that can be supplied is insufficient. As a result, when the lithium ion concentration near the electrode becomes 0%, further charging cannot proceed, so the charging rate characteristics deteriorate. On the other hand, when the ten-point average roughness Rz on the electrode surface is 0, new manufacturing control items such as controlling the particle size of all the LTO particles to be the same are required, making it difficult to apply to general production processes. Therefore, setting Rz to 0 is not realistic from the perspective of productivity. When the ten-point average roughness is 0 < Rz < 10 μm, the electrode density becomes uniform, so the charging rate characteristics are improved. In particular, in consideration of the immersion of the electrolyte, it is more preferable that the ten-point average roughness is 8 < Rz < 10 μm. From the above, in the LTO negative electrode, by defining the weight ratio of the negative electrode active material in the negative electrode composite layer, the electrode density, the coating amount, and the ten-point average roughness on the electrode surface, it becomes possible to obtain good rate characteristics during high-rate charging.

[0033] Also, when fabricating the electrode, during slurry preparation, first disperse only LTO using a planetary mixer, then add PVDF and NMP, and mix with a solid content of 70% or more. By doing so, the secondary particles in LTO can be broken down, and it is possible to adjust the average ten-point roughness Rz of the electrode surface after pressing to be 0 μm < Rz < 10 μm. Also, the solid content can be adjusted by the amount of NMP added. At this time, increasing the amount of NMP decreases the solid content, and decreasing the amount of NMP increases the solid content. Here, the solid content refers to the components other than the non-aqueous electrolyte in the slurry, and it means the content ratio of the solid content when the weight of the slurry is 100%. Note that the weights of lithium and lithium-containing compounds are not included in the weight of the slurry.

[0034] Here, the average ten-point roughness was measured by using a microscope to measure the surface of the electrode. From the roughness curve obtained by the microscope, only a reference length in the direction of the average line was extracted, and the average value of the absolute values of the elevations (Yp) of the top five peaks and the average value of the absolute values of the elevations (Yv) of the bottom five valleys measured in the direction of the vertical magnification from the average line of this extracted part were calculated. The sum of these values, expressed in micrometers (μm), was defined as the average ten-point roughness.

[0035] The negative electrode composite material layer has a coating amount of 30 g / m 2 or more and 70 g / m 2By following the above, ion diffusion becomes the rate-limiting factor at high charging rates of 20C or higher. Therefore, conductive additives such as acetylene black (AB) that promote electronic conductivity are not included, and only LTO and a binder with excellent ion conductivity are used as the negative electrode active material. Furthermore, by setting the active material ratio to 98% or more and 99.9% or less, it is possible to reduce the resistance component due to the binder. In this case, by setting the electrode density to be greater than 1.7 g / cc and less than 1.9 g / cc, good charge rate characteristics can be maintained even when only LTO and a binder are used in the negative electrode composite layer. Moreover, when the ten-point average roughness of the electrode is Rz < 10 μm, the electrode density becomes uniform, thus enabling the maintenance of good charge rate characteristics. From the above, it is possible to provide an electrode that enables both high energy density and good charge rate characteristics when LTO is used as the negative electrode active material during high-rate charging.

[0036] Furthermore, in this embodiment, the binder is an organic binder. The LTO active material reacts with even a small amount of moisture, generating gas. This causes the battery to swell, degrading its performance. Therefore, in this embodiment, it is preferable to use an organic binder rather than a water-based binder. For example, styrene-butadiene rubber, polytetrafluoroethylene, poly(meth)acrylate, polyvinylidene fluoride, etc., can be used as organic binders.

[0037] (positive electrode) The positive electrode 4 consists of a positive electrode current collector and a positive electrode composite material layer formed on one or both sides of the positive electrode current collector.

[0038] The positive electrode current collector is not limited in terms of the material it is made of, but it is preferable to use a metal. Specifically, examples include aluminum, nickel, stainless steel, titanium, and other alloys. Among these, aluminum is preferred from the viewpoint of electronic conductivity and battery operating potential. Furthermore, the positive electrode current collector is preferably 1 to 50 μm thick.

[0039] The positive electrode composite layer can be one used in conventional lithium-ion secondary batteries. Preferably, the positive electrode composite layer contains a positive electrode active material consisting of one selected from lithium metal oxide having a layered rock salt structure or spinel structure, lithium metal phosphate having an olivine structure, or a mixture thereof. Furthermore, known materials such as conductive additives, graphite, binders, and other additives may be used.

[0040] (Separator) The separator 5 is provided between the positive electrode 4 and the negative electrode 6 and has a porous nature that allows components of the non-aqueous electrolyte to pass through. Examples of materials used for the separator 5 include porous sheet separators made of polymers or fibers, and nonwoven fabric separators. The separator 5 preferably has a pore size of 0.01 to 10 μm and a thickness of 5 to 30 μm. In addition, the separator 5 may have a ceramic layer laminated on the porous substrate as a heat-resistant insulating layer.

[0041] (electrolyte) A non-aqueous electrolyte is used. The non-aqueous electrolyte contains a lithium salt and a non-aqueous solvent.

[0042] Examples of lithium salts include one or more selected from LiBF4, LiPF6, Li(FSO2)2N, Li(CF3SO2)2N, etc., but are not necessarily limited to these.

[0043] The non-aqueous solvent is not particularly limited, but examples include one or more mixed solvents selected from dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), methyl propionate, methyl acetate, methyl formate, methyl butyrate, dioxolane, 2-methyltetrahydrofuran, tetrahydrofuran, dimethoxyethane, γ-butyrolactone, acetonitrile, benzonitrile, etc. In particular, DMC, DEC, DPC, EMC, EC, and PC are preferred, and in particular, the inclusion of EC is preferred from the viewpoint of good film formation on the negative electrode active material.

[0044] Furthermore, evaluation half-cells for evaluating the lithium-ion secondary battery 10 can be manufactured by the following method, regardless of their dimensions or type. First, a commercially available lithium-ion secondary battery is disassembled, and one of the negative electrode composite material layers on both sides of the extracted negative electrode is peeled off. Next, the negative electrode is punched out in a circular shape to a certain area. The punched-out circular negative electrode 6 is used as the working electrode, and the positive electrode 4 is used as the counter electrode. An evaluation half-cell is assembled by stacking the working electrode and counter electrode with a separator 5 in between. During assembly, the negative electrode composite material layer of the working electrode is positioned to face the counter electrode via the separator 5. Also, the area of ​​the counter electrode used as the counter electrode is larger than the area of ​​the working electrode and is assembled to cover the area of ​​the working electrode. Specifically, the area of ​​the counter electrode facing the working electrode is assembled to be equal to the area of ​​the working electrode. After that, a spacer 3 and a leaf spring 2 are placed on the back surface of the counter electrode, it is placed in a case 1 with a gasket 7, a cap 8 is placed on top, and it is sealed by crimping. The evaluation half-cell only needs to have at least a working electrode, a counter electrode, a separator, a non-aqueous electrolyte, and an outer casing, and the separator, non-aqueous electrolyte, and outer casing can be arbitrarily selected from those used for lithium-ion secondary batteries.

[0045] In this embodiment, in the negative electrode 6, which is a negative electrode for a lithium secondary battery, the negative electrode composite layer is composed of a negative electrode active material and a binder, the weight ratio of the negative electrode active material in the negative electrode composite layer is 98% or more and 99.9%, the electrode density is greater than 1.7 g / cc and less than 1.9 g / cc, and the coating amount on one side is 30 g / m². 2 More than 70g / m 2 The following was determined. According to this embodiment, by employing a negative electrode that satisfies the above conditions in a lithium secondary battery, it is possible to achieve both high energy density and rate characteristics, especially during high-rate charging, in the lithium secondary battery. [Examples]

[0046] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way by the following examples.

[0047] (Example 1) <Negative electrode> The negative electrode was fabricated as follows. Lithium titanium oxide (hereinafter referred to as LTO) was used as the positive electrode active material, and polyvinylidene fluoride (hereinafter referred to as PVDF) was used as the binder. These materials were mixed in a weight ratio of positive electrode active material (LTO):binder (PVDF) = 99:1, and then dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a negative electrode slurry with a solid content of 70%. Subsequently, the obtained negative electrode slurry was coated onto one side of an aluminum foil at a rate of 45 g / m². 2 The material was coated and then dried. After drying, the electrodes were pressed using a roll press to adjust the electrode density to 1.80 g / cc and fabricated. The average roughness of the obtained electrodes was measured at 10 points using a microscope, and Rz was found to be 9.24 μm. Furthermore, the coating amount of the negative electrode was determined by cutting out a negative electrode from a fixed area, peeling the negative electrode composite layer from the current collector foil and weighing it, and dividing the resulting weight by the area. Similarly, the electrode density was determined by cutting out a negative electrode from a fixed area, measuring the thickness of the negative electrode composite layer after removing the current collector foil, peeling the negative electrode composite layer from the current collector foil and weighing it, and dividing the resulting weight by the volume obtained by multiplying the thickness by the area.

[0048] The electrochemical properties were evaluated using the evaluation half-cell described above. In this test, a 300 μm thick lithium metal foil was attached to a 100 μm thick stainless steel foil current collector as the counter electrode. A non-aqueous electrolyte was used, prepared by dissolving LiPF6 as the lithium salt at a concentration of 1.3 mol / L in a mixed solvent of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 2:5:3.

[0049] Using the negative electrode, counter electrode, electrolyte, and glass filter as separator described above, a 2032 type coin-type lithium secondary battery (hereinafter referred to as a coin-type battery) was fabricated as an evaluation half-cell. The coin-type battery was fabricated under an argon atmosphere with a dew point of -50°C or lower.

[0050] The coin-type batteries described above were transferred to a constant temperature bath set to 25°C, and a charge-discharge test (initial activation process) consisting of 5 cycles of discharge and charge was performed. The charge-discharge conditions were constant current discharge with a discharge current of 0.2C and a discharge termination voltage of 2.0V for the discharge condition, and constant current charging with a charging current of 0.2C and a cutoff voltage of 2.0V for the charge condition. Subsequently, a rate characteristic test was performed at a charge rate of 30C. In the rate characteristic test, the charge capacity at 0.2C and 30C, and the charge capacity ratio of 30C / 0.2C were calculated. Table 1 shows the physical properties and test results for Example 1. Table 1 shows the weight ratio (LTO:AB:PVDF) of the positive electrode active material and binder with acetylene black (hereinafter referred to as AB) added. In Example 1, LTO:AB:PVDF = 99:0:1. [Table 1]

[0051] (Example 2) Example 2 is the same as Example 1, except that the weight ratio (LTO:AB:PVDF) was set to 99:0:2. The physical properties and test results for Example 2 are shown in Table 1.

[0052] (Example 3) Example 3 is the same as Example 1, except that the electrode density was set to 1.75 g / cc. The physical properties and test results for Example 3 are shown in Table 1.

[0053] (Example 4) Example 4 is the same as Example 1, except that the electrode density was set to 1.85 g / cc. The physical properties and test results for Example 4 are shown in Table 1.

[0054] (Example 5) Example 5 involves coating one side of the aluminum foil with the negative electrode slurry at a rate of 30 g / m². 2 Except for the above, it is the same as Example 1. The physical properties and test results for Example 5 are shown in Table 1.

[0055] (Example 6) Example 6 involved coating one side of the aluminum foil with the negative electrode slurry at a rate of 70 g / m². 2 Except for the above, it is the same as Example 1. The physical properties and test results for Example 6 are shown in Table 1.

[0056] (Example 7) Example 7 is the same as Example 1, except that the weight ratio (LTO:AB:PVDF) was set to 99.5:0:0.5. The physical properties and test results for Example 7 are shown in Table 1.

[0057] (Example 8) Example 8 involves coating one side of the aluminum foil with the negative electrode slurry at a rate of 70 g / m². 2 The method is the same as in Example 1, except that the solid content was set to 80%. The physical properties and test results for Example 8 are shown in Table 1.

[0058] (Comparative Example 1) Comparative Example 1 is the same as Example 1 except that the weight ratio (LTO:AB:PVDF) was 95:0:5. The physical properties and test results for Comparative Example 1 are shown in Table 1.

[0059] (Comparative Example 2) Comparative Example 2 is the same as Example 1, except that acetylene black was used as a conductive additive and the weight ratio (LTO:AB:PVDF) was 98:1:1. The physical properties and test results for Comparative Example 2 are shown in Table 1.

[0060] (Comparative Example 3) Comparative Example 3 is the same as Example 1, except that the solid content of the negative electrode slurry was 60%. The physical properties and test results for Comparative Example 3 are shown in Table 1.

[0061] (Comparative Example 4) Comparative Example 4 is the same as Example 1 except that the electrode density was 2.00 g / cc. The physical properties and test results for Comparative Example 4 are shown in Table 1.

[0062] (Comparative Example 5) Comparative Example 5 is the same as Example 1 except that the electrode density was 1.70 g / cc. The physical properties and test results for Comparative Example 5 are shown in Table 1.

[0063] (Comparative Example 6) Comparative Example 6 involves a coating amount of 20 g / m² of the negative electrode slurry on one side of the aluminum foil. 2 Except for the above, it is the same as Example 1. The physical properties and test results for Comparative Example 6 are shown in Table 1.

[0064] (Comparative Example 7) Comparative Example 7 shows a coating amount of 80 g / m² of the negative electrode slurry on one side of the aluminum foil. 2 Except for the above, it is the same as Example 1. The physical properties and test results for Comparative Example 8 are shown in Table 1.

[0065] (Comparative Example 8) Comparative Example 8 is the same as Example 1, except that the solid content of the negative electrode slurry was 65%. The physical properties and test results for Comparative Example 8 are shown in Table 1.

[0066] (Comparative Example 9) Comparative Example 9 is the same as Example 1 except that the electrode density was 1.90 g / cc. The physical properties and test results for Comparative Example 9 are shown in Table 1.

[0067] In Examples 1 to 8, optimizing the electrodes—specifically, the mixing ratio, electrode density, coating amount, and the ten-point roughness of the electrode surface—made it possible to achieve both high rate characteristics and energy density during high-rate charging. On the other hand, in Comparative Examples 1 to 9, because the electrodes—specifically, the mixing ratio, electrode density, coating amount, and the ten-point roughness of the electrode surface—were not optimized, it became impossible to achieve both high rate characteristics and energy density during high-rate charging. [Explanation of symbols]

[0068] 1 case 2 leaf springs 3 Spacers 4 Positive electrode 5 Separators 6 negative electrode 7 Gasket 8 caps 10 Lithium-ion rechargeable batteries

Claims

1. A negative electrode for a lithium-ion secondary battery comprising an electrode current collector and a negative electrode composite layer formed on the electrode current collector, The aforementioned negative electrode composite layer does not contain a conductive additive and contains a negative electrode active material. The negative electrode active material is lithium titanium oxide. The weight ratio of the negative electrode active material in the negative electrode composite layer is 98% or more and 99.9% or less. The electrode density is greater than 1.70 g / cc and less than 1.90 g / cc. The coating amount on one side of the aforementioned negative electrode composite layer is 30 g / m². ２ 70g / m or more ２ The following: The ten-point average roughness Rz on the surface of the negative electrode composite layer satisfies 0 < Rz < 10 μm. A negative electrode for lithium-ion secondary batteries characterized by the following features.

2. The binder in the aforementioned negative electrode composite layer is an organic binder. The negative electrode for a lithium-ion secondary battery according to feature 1.

3. The negative electrode composite layer consists only of the negative electrode active material and the binder. The negative electrode for a lithium-ion secondary battery according to feature 1.

4. The lithium titanium oxide is of the spinel type. The negative electrode for a lithium-ion secondary battery according to feature 1.

5. A positive electrode having a positive electrode current collector and a positive electrode composite layer containing a positive electrode active material coated on the positive electrode current collector, A negative electrode comprising a negative electrode current collector and a negative electrode composite layer containing a negative electrode active material coated on the negative electrode current collector, A separator interposed between the positive electrode and the negative electrode, Electrolyte and A case housing the positive electrode, the negative electrode, the separator, and the electrolyte, Equipped with, The aforementioned negative electrode composite layer does not contain a conductive additive and contains a negative electrode active material. The negative electrode active material is lithium titanium oxide. The weight ratio of the negative electrode active material in the negative electrode composite layer is 98% or more and 99.9% or less. The electrode density at the negative electrode is greater than 1.70 g / cc and less than 1.90 g / cc. The coating amount on one side of the aforementioned negative electrode composite layer is 30 g / m². ２ 70g / m or more ２ The following: The ten-point average roughness Rz on the surface of the negative electrode on the negative electrode composite layer side satisfies 0 < Rz < 10 μm. A lithium-ion secondary battery characterized by the following features.

6. The lithium-ion secondary battery in question has a charge rate of 20C or higher during use. The lithium-ion secondary battery according to feature 5.

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