Lithium-ion rechargeable battery

Abstract

In a lithium-ion secondary battery having a laminated structure in which a positive electrode active material layer coated on the positive electrode foil and a negative electrode active material layer coated on the negative electrode foil face each other with a separator in between, and a non-aqueous electrolyte is injected between the electrode foils, the objective is to prevent the deposition of metallic lithium on the negative electrode as inexpensively as possible. [Solution] In a lithium-ion secondary battery, the positive electrode active material layer has a first layer on the positive electrode foil side and a second layer on the separator side, and the gas permeation rate of the second layer is slower than that of the first layer.

Description

[Technical Field]

[0001] This invention relates to a lithium-ion secondary battery, and more specifically, to the structure of the positive electrode of the battery. [Background technology]

[0002] In simple terms, secondary batteries such as lithium-ion secondary batteries have a laminated structure in which a positive electrode active material layer coated on a current collector (positive electrode foil), which may be a metal foil, and a negative electrode active material layer coated on a current collector (negative electrode foil), which may be a metal foil, face each other with a separator in between, and an electrolyte is injected between them. Various configurations have been proposed to address the various problems that may arise in such secondary batteries. For example, Patent Document 1 proposes controlling the water content of the electrode active material slurry, which includes an electrode active material, lithium salt, and non-aqueous solvent, to less than 500 ppm by mass in order to obtain a non-aqueous electrolyte secondary battery with reduced internal resistance, high capacity, and high cycle durability. Patent Document 2 proposes that, in order to obtain an electrode for a lithium secondary battery that has a large electrolyte capacity, high output, and at the same time ensures volume density and suppresses a decrease in electronic conductivity, an active material mixture layer formed on the electrode foil is formed with an inner layer on the electrode foil side and a surface layer located on the surface side, the average porosity of the inner layer is made higher than that of the surface layer, and an intermediate layer with a lower porosity than both ends is formed at an intermediate position in the thickness direction of the inner layer. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2019-186009 [Patent Document 2] International release 2020 / 179149 [Overview of the project] [Problems that the invention aims to solve]

[0004] The electrodes of the lithium-ion secondary battery described above are formed by applying a slurry containing an active material, a conductive material, and a binder onto a metal foil that serves as a current collector, drying it, and forming an active material layer. In the case of the positive electrode of such a lithium-ion secondary battery, if the moisture content in the positive electrode active material layer is high, a phenomenon is observed in which metallic lithium is deposited locally at the negative electrode during battery use, causing that area to heat up. More specifically, as schematically depicted in Figure 2, if the moisture content of the positive electrode active material layer 11b is high, when charging, the electrolyte and water decompose in the positive electrode active material layer, generating gases g such as oxygen, hydrogen, carbon dioxide, and carbon monoxide. These gases pass through the positive electrode active material layer and accumulate locally between it and the negative active material layer 10, increasing the distance between the electrodes. As a result, the electrical resistance increases in the area where the distance between the electrodes has increased, so the potential fluctuation of the negative electrode becomes larger during charging, and when the negative electrode potential reaches 0V, metallic lithium is deposited. Furthermore, if metallic lithium deposits form, that part of the battery will generate heat during use.

[0005] It has been found that the deposition of metallic lithium in the negative electrode, as described above, can be prevented by reducing the moisture content in the positive electrode active material layer (see experimental examples later). However, reducing the moisture content in the positive electrode active material layer requires high-temperature and long-duration drying when applying the slurry containing the positive electrode active material onto the electrode foil, or strengthening drying and moisture control back to the material manufacturing process (active material and auxiliary materials), which can make the process expensive. Furthermore, in battery systems with large electrode areas, a larger drying environment is also required, which can make the process even more expensive. Therefore, it is advantageous to have a configuration in the manufacturing of the positive electrode that can minimize the burden of processing to reduce the moisture content in the positive electrode active material layer.

[0006] In this regard, according to the inventor's research and development, it has been found that when forming a positive electrode active material layer on an electrode foil, if a certain thickness portion of the positive electrode active material layer on the separator side is prepared to have a slower gas permeation rate, the deposition of metallic lithium can be prevented even if the moisture content of the remaining portion of the positive electrode active material layer on the electrode foil side remains high to a certain extent. That is, by forming a positive electrode active material layer by laminating a second layer with a slower gas permeation rate than the first layer on the separator side above the first layer laminated on the electrode foil side, it becomes possible to prevent the deposition of metallic lithium on the negative electrode. This is thought to be because when gas is generated in the first layer and tries to escape to the separator side, the second layer makes it difficult for the gas to permeate, so the gas spreads in the planar direction and moves towards the separator side, preventing localized gas accumulation. This finding is utilized in the present invention.

[0007] Thus, the main objective of the present invention is to prevent the deposition of metallic lithium on the negative electrode in a lithium-ion secondary battery at the lowest possible cost. [Means for solving the problem]

[0008] According to the present invention, the above problem is solved by a lithium-ion secondary battery having a laminated structure in which a positive electrode active material layer coated on a positive electrode foil and a negative electrode active material layer coated on a negative electrode foil face each other with a separator in between, and a non-aqueous electrolyte is injected between the electrode foils. This is achieved by a lithium-ion secondary battery in which the positive electrode active material layer has a first layer on the positive electrode foil side and a second layer on the separator side, and the gas permeation rate of the second layer is lower than that of the first layer.

[0009] In the above configuration, the lithium-ion secondary battery may be a lithium-ion secondary battery using a non-aqueous electrolyte in a conventional manner, except for the configuration of the positive electrode active material layer as described above. The positive electrode foil (positive foil) and the negative electrode foil (negative foil) may be current collectors made of metal foil in a conventional manner, and the positive electrode active material layer and the negative electrode active material layer are prepared by coating the positive electrode foil and the negative electrode foil with a slurry in which the active material, conductive material, and binder are dispersed in a solvent, and then drying it. The positive electrode active material is typically lithium salt particles such as lithium iron phosphate LFP (LiFePO4), and may be dispersed in a non-aqueous solvent such as NMP (N-methyl-2-pyrrolidone) or an aqueous solvent such as water to form a slurry. The negative electrode active material is graphite, etc., and may be dispersed in water to form a slurry. The separator and electrolyte may also be a separator and a non-aqueous electrolyte in a conventional manner.

[0010] In short, according to the above configuration, even if gas is generated in a part of the first layer of the positive electrode active material layer and moves toward the separator, when the gas reaches the second layer, because the gas permeation rate of the second layer is low, it does not proceed in the thickness direction of the positive electrode active material layer but spreads out in the planar direction. As a result, gas reservoirs where gas is locally stored are not formed between the outer surface of the positive electrode active material layer and the separator, and thus, an increase in the localized distance between electrodes and a resulting increase in localized electrical resistance does not occur, and the deposition of metallic lithium on the negative electrode surface can be prevented. That is, according to the above configuration of the present invention, in the positive electrode active material layer, if the moisture content of the second layer of the positive electrode active material layer is low and gas generation in the second layer is suppressed, then it is acceptable for the moisture content of the first layer of the positive electrode active material layer to be somewhat high and for gas to be generated. Therefore, it is not necessary to reduce the moisture content throughout the entire positive electrode active material layer, and consequently, the cost of forming the positive electrode active material layer can be reduced accordingly.

[0011] In the above configuration, the second layer, which has a low gas permeation rate, can be formed in various ways. In one way, the active material of the second layer may have a particle size smaller than that of the active material of the first layer. By reducing the particle size of the active material of the second layer, the gas permeation rate of the second layer is reduced compared to that of the first layer. According to the inventor's experiments, which will be described in the section on embodiments below, it has been found that when the particle size of the active material of the second layer is 70% or less of that of the active material of the first layer, the deposition of metallic lithium is not observed. In another way, the density of the second layer may be greater than that of the first layer, thereby reducing the gas permeation rate of the second layer compared to that of the first layer. According to the inventor's experiments, which will be described in the section on embodiments below, it has been found that when the density of the second layer is 20% or more of the density of the first layer, the deposition of metallic lithium is not observed. In yet another way, the amount of binder in the second layer may be greater than that of the first layer, thereby reducing the gas permeation rate of the second layer compared to that of the first layer. According to the inventor's experiments, which will be described in the section on embodiments below, it has been found that when the amount of binder in the second layer is 20% or more of the amount of binder in the first layer, the deposition of metallic lithium is not observed.

[0012] Furthermore, as will be explained in the section on embodiments below, experiments conducted by the inventors of the present invention have shown that when the moisture content of the entire positive electrode active material layer is 500 ppm or less, no deposition of metallic lithium is observed. Conversely, if the moisture content of the second layer is 500 ppm or less, no deposition of metallic lithium is observed even if the moisture content of the first layer exceeds 500 ppm. Therefore, in the above-described configuration of the present invention, the moisture content of the second layer may be 500 ppm or less, and the moisture content of the first layer may be greater than 500 ppm. In fact, it has been confirmed that no deposition of metallic lithium occurs when the moisture content of the second layer is between 50 ppm and 500 ppm. However, when the moisture content of the second layer is 400 ppm, deposition of metallic lithium was observed when the moisture content of the first layer reached 1500 ppm. Therefore, it is preferable that the moisture content of the first layer be less than 1500 ppm. The moisture content of the active material layer can be adjusted by moisture control in the active material storage process, moisture control in the electrode storage process, and the magnitude of the lithium metal ratio in the active material manufacturing process.

[0013] Furthermore, in the configuration of the present invention described above, the basis weight (weight per unit area) of the positive electrode active material layer is 25 to 90 mg / cm². 2 It has been confirmed that the deposition of metallic lithium is prevented when this condition is met. Furthermore, it has been confirmed that the deposition of metallic lithium is prevented when the basis weight ratio of the first and second layers is 4:6 to 9:1 in the above configuration. [Effects of the Invention]

[0014] Thus, in the configuration of the present invention, in the positive electrode active material layer, since the gas permeation rate of the second layer is lower than that of the first layer, the water content of the first layer is high. Even if gas is generated in the first layer during battery use, when they move toward the separator side, they spread in the plane direction in the second layer, thereby avoiding the formation of local gas accumulation on the outer surface of the positive electrode active material layer. Thus, the precipitation of metallic lithium can be prevented. Therefore, in the configuration of the present invention, it is not necessary to use a slurry of a positive electrode active material with a reduced water content in all of the thickness direction of the positive electrode active material layer applied on the positive electrode foil. For the first layer, since the process of reducing the water content of the slurry of the positive electrode active material is alleviated, the cost required for forming the positive electrode active material layer can be reduced.

[0015] Other objects and advantages of the present invention will become apparent from the following description of the preferred embodiments of the present invention.

Brief Description of the Drawings

[0016] [Figure 1] FIG. 1 is a schematic partial cross-sectional view of a lithium-ion secondary battery to which the present embodiment is applied. [Figure 2] FIGS. 2(A) to (C) are schematic partial cross-sectional views of a conventional lithium-ion secondary battery. (A) shows a state where gas is generated in the positive electrode active material layer, (B) shows a state where local gas accumulation is formed on the outer surface of the positive electrode active material layer, and (C) shows a state where metallic lithium is deposited on the negative electrode facing the local gas accumulation formed on the outer surface of the positive electrode active material layer.

Explanation of Reference Numerals

[0017] 1... Lithium-ion secondary battery, 10... Negative electrode, 11... Positive electrode, 11a... Positive electrode foil, 11b... Positive electrode active material layer, 11b_1... First layer of the positive electrode active material layer, 11b_2... Second layer of the positive electrode active material layer, 11c... Positive electrode active material particles (first layer), 11d... Positive electrode active material particles (second layer), 12... Separator, 13... Gas accumulation, 14 Deposited metallic lithium, g... Gas flow

Best Mode for Carrying Out the Invention

[0018] The present invention will be described in detail below with reference to the attached figures, with reference to several preferred embodiments. In the figures, the same reference numerals indicate the same parts.

[0019] Basic structure of lithium-ion rechargeable batteries As schematically depicted in Figures 1 and 2(A), in the lithium-ion secondary battery 1 to which this embodiment is applied, a single cell is formed by a laminated structure in which a negative electrode active material layer coated on the surface of the electrode foil (negative electrode foil) (not shown) of the negative electrode 10 and a positive electrode active material layer 11b coated on the surface of the electrode foil (positive electrode foil) 11a of the positive electrode 11 face each other with a separator 12 in between. Although not shown, multiple such cells may be stacked on top of each other, in which case the positive electrode foil 11a of each cell may be bonded to the negative electrode foil of the cell adjacent to it on the lower side in the figure to form a "bipolar electrode". The positive electrode foil 11a and the negative electrode foil may each be a commonly used metal foil with a thickness of several tens of micrometers, such as aluminum foil, nickel foil, or copper foil. The positive electrode active material layer 11b may basically be formed by dispersing a mixture of commonly used lithium salts for positive electrodes, such as NCM (nickel-cobalt-manganate lithium), LFP (lithium iron phosphate), and LMFP (lithium iron manganese phosphate), with conductive additives such as carbon black and CNT (carbon nanotubes), and a binder such as PVdF (polyvinylidene fluoride), in a non-aqueous solvent such as NMP (N-methyl-2-pyrrolidone), forming a slurry, coating it onto the positive electrode foil, and drying it to form a layer with a thickness of about 0.1 mm. Furthermore, if moisture can be properly removed during the drying of the slurry, the positive electrode active material layer 11b may be formed by dispersing a mixture of commonly used lithium salt positive electrode active materials such as LFP (lithium iron phosphate) or LMFP (lithium manganese iron phosphate), conductive additives such as carbon black and CNT (carbon nanotubes), thickeners such as CMC (carboxymethylcellulose), and binders such as SBR (styrene-butadiene rubber) and SAR (styrene-acrylic rubber) in an aqueous solvent such as pure water to form a slurry, which is then applied to the positive electrode foil and dried to form a layer with a thickness of about 0.1 mm.The negative electrode active material layer 5 is basically formed by dispersing a mixture of commonly used negative electrode active materials such as graphite (natural or artificial), hard carbon, SiO, and SiC with conductive additives such as carbon black and CNTs, thickeners such as CMC, and binders such as SBR and SAR in water to form a slurry, which is then coated onto the negative electrode foil and dried to form a layer with a thickness of about 0.1 mm. The separator 12 may be a resin film such as PE (polyethylene) with a thickness of about 16 to 20 μm. The space on both sides of the separator 12 between the positive electrode and the negative electrode is filled with electrolyte. The electrolyte is appropriately selected depending on the type of battery, and may be, for example, a non-aqueous solvent in which LiPF6 (lithium hexafluoride phosphate) is dissolved at about 1 M in a mixture of EC (ethyl carbonate), EMC (ethyl methyl carbonate), and DMC (dimethyl carbonate). Although not shown in the diagram, the outer edges of the positive electrode, separator, and negative electrode may be held and fixed by a sealing portion made of any resin with appropriate rigidity, such as polyethylene. Charging and discharging of the battery involves electrically connecting the positive and negative electrode foils to a power source or load.

[0020] Deposition of metallic lithium on the negative electrode during charging and prevention of such deposition. Referring to Figure 2(A), as explained in the section on the summary of the invention, during use of the lithium-ion secondary battery 1 described above, in the positive electrode active material layer 11b, the electrolyte and water decompose, generating gases g such as oxygen, hydrogen, carbon dioxide, and carbon monoxide (in particular, heat tends to accumulate in the deeper regions of the active material layer, making it easier for gas to be generated), and these gases move toward the separator 12. At that time, if the gas permeability characteristics are uniform in the thickness direction of the positive electrode active material layer 11b, the gases g will proceed straight through the positive electrode active material layer 11b in the thickness direction from the point of generation and reach the outer surface of the positive electrode active material layer 11b, and as shown in Figure 2(B), gas will accumulate in such a way that it locally pushes the separator 12 toward the negative electrode side (formation of a gas reservoir 13). As a result, the distance between the positive and negative electrodes widens locally at the site where the gas reservoir 13 is formed, increasing the electrical resistance. This causes the potential fluctuation of the negative electrode to become large, and when the potential reaches 0V, metallic lithium deposits onto the negative electrode surface, as shown in Figure 2(C). This metallic lithium deposited on the negative electrode generates heat during battery use, potentially causing battery failure.

[0021] It has been found that the generation of gas in the positive electrode active material layer 11b that causes the deposition of metallic lithium as described above can be avoided by reducing the moisture content in the positive electrode active material layer 11b. In fact, according to experiments by the inventors of this embodiment, the moisture content of the entire positive electrode active material layer 11b is 500 ppm or less (based on a weight of 35 mg / cm³). 2 In this case, no deposition of metallic lithium was observed.

[0022] However, keeping the moisture content in the positive electrode active material layer 11b low requires costly processing. As mentioned earlier, the positive electrode active material layer 11b is formed by coating the positive electrode foil with a slurry obtained by dispersing a mixture containing the positive electrode active material in a solvent. To suppress the moisture content in the slurry, it is necessary to increase the temperature and duration of drying during slurry coating, and to strengthen drying and moisture control by going back to the material manufacturing process (active material and auxiliary materials). Furthermore, in battery systems with a large electrode area, it is also necessary to increase the drying area. As a result, the more slurry whose moisture content needs to be suppressed, the higher the processing costs become. Therefore, in order to keep processing costs as low as possible, it is preferable to minimize the processing required to suppress the moisture content of the slurry.

[0023] In this regard, the inventors of this embodiment conducted various studies and found that in the positive electrode active material layer 11b, if the gas permeation rate of the separator-side layer 11b_2 is lower than that of the positive electrode foil-side layer 11b_1, then metallic lithium deposition is not observed even if the moisture content of the positive electrode foil-side layer is high. This is because, as schematically depicted in Figure 1, the presence of a layer 11b_2 with a low gas permeation rate on the separator side causes gas g generated in the positive electrode foil-side layer 11b_1 within the positive electrode active material layer 11b and directed toward the separator side to spread in the planar direction as shown in the figure, rather than continuing to move in the thickness direction, when it reaches the layer 11b_2 with a low gas permeation rate. This is thought to prevent the formation of localized gas reservoirs 13 as shown in Figure 2(B) on the separator-side surface of the positive electrode active material layer 11b. In other words, if the positive electrode active material layer 11b has a layer with a low gas permeation rate as the separator-side layer 11b_2 (second layer), the moisture content of the positive electrode foil-side layer 11b_1 (first layer) can be relatively high. This reduces the amount of treatment required to suppress the moisture content of the slurry for the positive electrode active material layer 11b, thereby reducing processing costs.

[0024] Thus, in this embodiment, in the active material layer of the positive electrode of a lithium-ion secondary battery, a second layer 11b_2 with a low gas permeation rate is laminated on the separator side of the first layer 11b_1 laminated on the positive electrode foil side, thereby reducing the amount of treatment required to suppress the water content of the slurry while preventing the deposition of metallic lithium.

[0025] Composition of the positive electrode active material layer In this embodiment, as shown in Figure 1, the positive electrode active material layer 11b is composed of a first layer 11b_1 on the positive electrode foil 11a side and a second layer 11b_2 on the separator 12 side. As described above, the second layer 11b_2 is a layer with a lower gas permeation rate than the first layer 11b_1. When gas is generated from the second layer 11b_2, localized gas reservoirs may form on the surface of the positive electrode active material layer 11b. Therefore, it is preferable that the moisture content of the second layer 11b_2 is suppressed so that gas is not significantly generated from it. In the experimental example described later, if the moisture content of the second layer 11b_2 is 500 ppm or less, no deposition of metallic lithium is observed. Therefore, when constructing a lithium-ion secondary battery with the materials used in the experimental example, a layer with a moisture content of 500 ppm or less may be used for the second layer 11b_2 (however, the moisture content condition that the second layer 11b_2 must satisfy may change depending on the materials used to construct the lithium-ion secondary battery). For the first layer 11b_1, the moisture content may exceed 500 ppm, and therefore, the treatment required to suppress the moisture content of the slurry can be reduced. In other words, according to this embodiment, it is not necessary to perform a treatment to suppress the moisture content of the entire positive electrode active material layer 11b.

[0026] Reducing the gas permeation rate in the second layer 11b_2 can be achieved in several ways. Specifically, in one way, the active material of the second layer 11b_2 may have a particle size smaller than that of the active material of the first layer 11b_1. As schematically shown in Figure 1, by making the diameter of the particles 11d of the active material of the second layer 11b_2 smaller than the diameter of the particles 11c of the active material of the first layer 11b_1, the gas permeation rate of the second layer 11b_2 is reduced compared to that of the first layer 11b_1. According to experimental examples described later, it has been found that when the particle size of the active material of the second layer is 70% or less of the particle size of the active material of the first layer, the deposition of metallic lithium is not observed. In another way, the density of the second layer 11b_2 may be greater than that of the first layer 11b_1, thereby reducing the gas permeation rate of the second layer compared to that of the first layer. An increase in the density of the second layer 11b_2 can be achieved, for example, by adding an active material that is less prone to collapse (has low porosity) to the second layer. As shown in the experimental examples described later, it has been found that no deposition of metallic lithium is observed when the density of the second layer is 20% or more of the density of the first layer. In yet another embodiment, the amount of binder in the second layer 11b_2 may be greater than the amount of binder in the first layer 11b_1, thereby reducing the gas permeation rate of the second layer compared to the first layer. As shown in the experimental examples described later, it has been found that no deposition of metallic lithium is observed when the amount of binder in the second layer is 20% or more of the amount of binder in the first layer.

[0027] In the configuration of this embodiment, the basis weight of the positive electrode active material layer may be selected as appropriate. According to the experimental examples described later, the basis weight is 25 to 90 mg / cm³. 2 It has been confirmed that the deposition of metallic lithium is prevented in this condition.

[0028] In the configuration of this embodiment, the basis weight ratio of the first layer to the second layer may be 4:6 to 9:1. According to experimental examples described later, it has been confirmed that the deposition of metallic lithium is prevented in the above ratio.

[0029] The formation of the positive electrode active material layer of the lithium-ion secondary battery described above is achieved by applying a slurry containing the positive electrode active material onto the positive electrode foil to an appropriately set basis weight, drying it, and pressing it. In particular, in this embodiment, when applying the slurry, a slurry for the first layer and a slurry for the second layer are prepared, the slurry for the first layer is applied onto the positive electrode foil to an appropriately set basis weight using a die coater or the like, then the slurry for the second layer is applied to an appropriately set basis weight using a die coater or the like, and then dried.

[0030] Furthermore, this embodiment is advantageous for a configuration in which the inside of the lithium-ion secondary battery is depressurized and the cell is held at atmospheric pressure, rather than using a restraining plate that clamps and restrains the cell from both sides. In this regard, when a lithium-ion secondary battery is used in a state where the cell is clamped from both sides by a restraining plate, the distance between the positive and negative electrodes is maintained by the restraining plate, so the formation of localized gas pockets due to gas generation is hardly a problem. However, because a restraining plate is required, the weight of the equipment required for using the lithium-ion secondary battery increases, which is disadvantageous, for example, when mounting the lithium-ion secondary battery on a mobile device. On the other hand, when a lithium-ion secondary battery is used in a state where the inside of the cell is depressurized and the cell is held at atmospheric pressure, a restraining plate is not required, so the weight of the equipment is reduced. However, as shown in Figure 2, the distance between the positive and negative electrodes may increase due to the formation of localized gas pockets due to gas generation. However, according to this embodiment, the formation of localized gas pockets due to gas generation can be prevented, and the deposition of metallic lithium can be avoided, thus achieving both weight reduction and prevention of metallic lithium deposition simultaneously, which is advantageous.

[0031] Experimental example The effectiveness of this embodiment was confirmed by the following experimental examples. It should be understood that these experimental examples are illustrative of the effectiveness of this embodiment and do not limit the scope of the present invention.

[0032] In the experiment, lithium-ion secondary batteries with various positive electrode active material layers were fabricated using the following procedure.

[0033] (1) Preparation of the positive electrode - LFP (LiFePO4) as the positive electrode active material, CNT as the conductive material, and PVdF as the binder were dispersed in NMP (dispersion medium) in a mass fraction of approximately 97:1:2 (this may vary depending on the sample). The slurry was then coated onto a 15 μm thick Al foil using a die coater, dried, and pressed to create a 15 μm thick negative electrode active material layer. When the positive electrode active material layer is formed in two layers, the positive electrode active material of the first layer had an average primary particle size (crystal grain size) of 60 nm, an average secondary particle size (aggregated particle size) of 8 or 10 μm, and a specific surface area of ​​15 m². 2 Using a material of / g, the positive electrode active material for the second layer has a primary particle size of an average of 150 nm, a secondary particle size of an average of 4, 5, 5.6, 7, 7.2, 9 or 10 μm, and a specific surface area of ​​9 m². 2 A suitable amount of each substance was selected and used, and slurries were prepared separately. The first layer of slurry was applied to the aluminum foil, followed by the second layer of slurry, and then dried. The basis weight ratio of the first and second layers was varied within the range of 2:8 to 9.5:0.5. The total basis weight (sum of the first and second layers) was 15 to 95 mg / cm³. 2 It was changed within the range.

[0034] (2) Preparation of the negative electrode - The negative electrode was prepared by dispersing synthetic graphite (negative electrode active material), CMC (thickener), CNT (conductive material), and SBR (binder) in water (dispersion medium) in a mass fraction of 97:0.5:0.5:2, and coating the slurry onto a 10 μm thick Cu foil (negative electrode foil) with a die coater, drying, and pressing to form a 15 μm thick negative electrode active material layer.

[0035] (3) The separator was a single layer of PE resin film with a thickness of 16 μm.

[0036] (4) The electrolyte was prepared by mixing EC, DMC, and EMC in a 3:3:4 (volume ratio) and dissolving LiPF6 at a concentration of 1.1 mol / L.

[0037] (5) In assembling the cell, the positive electrode, separator, and negative electrode were stacked, and external terminals were attached to the positive and negative electrodes. A laminate film pouch was prepared as a case, the stacked components were placed inside the case, the electrolyte was injected into the case, and then the case was sealed to create a test battery. The battery had a capacity of 6000mAh, and was configured so that the capacity ratio of the positive and negative electrodes was 1:1.1.

[0038] The moisture content of the positive electrode active material layer was detected using a Karl Fischer moisture meter under a set temperature of 200°C. For detection, the moisture content of a positive electrode coated only with the first layer was first detected as the moisture content of the first layer, h1. The moisture content of the second layer, h2, was then calculated using the following formula, after detecting the moisture content of a positive electrode coated with both the first and second layers, ht. h2 = (ht - h1 × w1) / w2 Here, w1 and w2 are the weight ratios of the first and second layers. (w1 + w2 = 1) Furthermore, the moisture content of the positive electrode active material layer obtained by sequentially coating a mixture of 700 ppm and 400 ppm positive electrode active materials in a 1:1 ratio was detected as 550 ppm using the Karl Fischer moisture meter described above. Since this value is the average of 700 ppm and 400 ppm, it was confirmed that the moisture content of the second layer detected by the above method is correct.

[0039] The presence or absence of metallic lithium deposition in the positive electrode active material layer was detected as follows. First, the test battery formed as described above underwent activation charging and aging treatments. In the activation charging treatment, the battery was fully charged and then completely discharged. In the aging treatment, the battery was charged to 80% of its state of charge (SOC) and left at 65°C for 24 hours. After that, with the cell unrestrained from both sides by the restraining plate, the gas inside the cell was discharged by reducing the pressure to -90kPa, and in that state (reduced pressure restraint), 100 charge-discharge cycles were performed using the following process (i) to (iv). Under an ambient temperature of 45°C, (i) Charge from 0% to 80% SOC at a charging rate of 0.1C. (ii) Pause 1 minute (iii) Discharge from SOC 80% to 0% at a discharge rate of 2C. (iv) Rest for 1 minute Thus, after executing the charge-discharge cycle described above, the cell was disassembled in a glove box, and the presence or absence of white deposits on the negative electrode was visually confirmed. Furthermore, it was confirmed by ESR (electron spin resonance) that the deposit was metallic lithium.

[0040] In the results, first, when the positive electrode active material layer was formed in one layer, the presence or absence of metallic lithium deposition was as follows. In the table, No. is the sample number, h1 is the moisture content of the first layer, D1 is the average particle size of the active material in the first layer, B1 is the weight percentage of the binder in the first layer, C1 is the density of the first layer, wt / a is the basis weight of the positive electrode active material layer, Li is the presence or absence of metallic lithium deposition, Y indicates "deposition present", and N indicates "no deposition". (The same applies hereinafter)

Table 1

[0041] Next, the positive electrode active material layer is formed in a first and second layer. The presence or absence of metallic lithium deposition when the gas permeation rate of the second layer is changed by changing the average particle size of the second layer is as follows. In the table, h2 is the moisture content of the second layer, D2 is the average particle size of the active material in the second layer, and D2 / D1 is the average particle size of the active material in the second layer divided by the average particle size of the first layer (the same applies below). In the following example, the moisture content of the first layer is 700 ppm, the weight percentage of the binder in the first and second layers is 2 wt%, and the density is 2.0 g / cm³. 3 The estimated amount is 35 mg / cm³. 2 The ratio of the weight of the first and second layers is 7:3. [Table 2] In the above samples, the total moisture content of the positive electrode active material layer was 610-628 ppm (based on a moisture content of 0.0214-0.0220 mg / cm³). 2) If the positive electrode active material layer is uniform, the deposition of metallic lithium should be observed. Also, when the moisture content of the first layer is 700 ppm, if it is a single layer, the deposition of metallic lithium should be observed. However, when the average particle size of the active material in the second layer becomes 70% or less of the average particle size of the active material in the first layer, the deposition of metallic lithium is no longer observed. As described above, the smaller the average particle size of the active material in the second layer is compared to the average particle size of the active material in the first layer, the smaller the gaps in the second layer become, the lower the gas permeability in the second layer becomes, and the lower the gas permeation rate. Therefore, according to the above results, even if the moisture content of the first layer and the moisture content of the entire positive electrode active material layer are high enough to cause the deposition of metallic lithium, it has been shown that the deposition of metallic lithium can be prevented by setting the average particle size of the active material in the second layer to 70% or less of the average particle size of the active material in the first layer, and by making the gas permeation rate of the second layer lower than that of the first layer. In other words, according to this embodiment, it has been shown that the deposition of metallic lithium can be prevented without reducing the moisture content of the entire positive electrode active material layer, or without reducing the moisture content of the first layer.

[0042] Next, the positive electrode active material layer is formed in a first and second layer. The presence or absence of metallic lithium deposition when the gas permeation rate of the second layer is changed by changing the amount of binder in the second layer is as follows. In the table, B2 is the weight percentage of the binder in the second layer, and B2 / B1 is the ratio of the amount of binder in the second layer to the amount of binder in the first layer. In the following example, the moisture content of the first layer is 700 ppm, the moisture content of the second layer is 400 ppm, the average particle size of the active material in both the first and second layers is 10 μm, and the basis weight is 35 mg / cm³. 2 The ratio of the first layer to the second layer is 7:3. [Table 3] In the above sample, the total moisture content of the positive electrode active material layer was 610 ppm (based on a moisture content of 0.0214 mg / cm³). 2If the positive electrode active material layer is uniform, the deposition of metallic lithium should be observed. Also, if the moisture content of the first layer is 700 ppm, the deposition of metallic lithium should be observed if it is a single layer. However, when the amount of binder in the second layer is 20% or more of that of the first layer, the deposition of metallic lithium is no longer observed. As described above, as the amount of binder in the second layer increases compared to the amount of binder in the first layer, the gap in the second layer becomes smaller, the gas permeability in the second layer decreases, and the gas permeation rate decreases. Therefore, according to the above results, even if the moisture content of the first layer and the total moisture content of the positive electrode active material layer are high enough to cause the deposition of metallic lithium, the deposition of metallic lithium can be prevented by setting the amount of binder in the second layer to 20% or more of that of the first layer, and by making the gas permeation rate of the second layer lower than that of the first layer. In other words, according to this embodiment, it has been shown that the deposition of metallic lithium can be prevented without reducing the moisture content of the entire positive electrode active material layer, or without reducing the moisture content of the first layer.

[0043] Furthermore, the positive electrode active material layer is formed in two layers, and the presence or absence of metallic lithium deposition when the gas permeation rate of the second layer is changed by changing the density of the second layer was as follows. In the table, C2 is the density of the second layer, and C2 / C1 is the ratio of the density of the second layer to the density of the first layer. In the following example, the moisture content of the first layer is 700 ppm, the moisture content of the second layer is 400 ppm, the average particle size of the active material in both the first and second layers is 10 μm, the binder amount in both the first and second layers is 2.0 wt%, and the basis weight is 35 mg / cm³. 2 The ratio of the first layer to the second layer is 7:3. [Table 4] Furthermore, in order to increase the density of the second layer, single crystals of the non-crushing NCM (Ni1 / 3:Co1 / 3:Mn1 / 3) active material were added to the second layer at a concentration of 5% in No. 18, 10% in No. 19, and 15% in No. 20 relative to the total basis weight of the positive electrode. In the above samples, the total moisture content of the positive electrode active material layer was 610 ppm. If the positive electrode active material layer were uniform, the deposition of metallic lithium should have been observed. Also, when the moisture content of the first layer was 700 ppm, if it were a single layer, the deposition of metallic lithium should have been observed. However, when the density of the second layer exceeded 20% of that of the first layer, the deposition of metallic lithium was no longer observed. As described above, the higher the density of the second layer compared to the density of the first layer, the smaller the gaps in the second layer become, which reduces the gas permeability in the second layer and lowers the gas permeation rate. Therefore, the above results show that even if the moisture content of the first layer and the moisture content of the entire positive electrode active material layer are high enough to cause the deposition of metallic lithium, the deposition of metallic lithium can be prevented by setting the density of the second layer to 20% or more of the density of the first layer and lowering the gas permeation rate of the second layer compared to the first layer. In other words, this embodiment shows that the deposition of metallic lithium can be prevented without reducing the moisture content of the entire positive electrode active material layer, or even without reducing the moisture content of the first layer.

[0044] Furthermore, when the positive electrode active material layer is formed with a first layer and a second layer with a lower gas permeation rate (when the average particle size of the active material in the second layer is reduced), the presence or absence of metallic lithium deposition when the moisture content of the second layer is changed was as follows. In the table, ht is the moisture content of the entire positive electrode active material layer. In the following example, the average particle sizes of the active material in the first and second layers are 10 μm and 7 μm, respectively, the binder amount in both the first and second layers is 2.0 wt%, and the density is 2.0 g / cm³. 3 The basis weight is 35 mg / cm³. 2 The ratio of the first layer to the second layer is 7:3. [Table 5] In the above example, the estimated amount is 35 mg / cm³. 2In this case, if the moisture content of the second layer was 500 ppm or less, the deposition of metallic lithium was no longer observed. In other words, comparing this with the results for No. 2 in Table 1, it was shown that, according to the configuration of this embodiment, if the moisture content of the second layer is lower than the moisture content at which metallic lithium is not deposited when the entire positive electrode active material layer is a single layer, the deposition of metallic lithium can be prevented even if the overall moisture content and the moisture content of the first layer are high.

[0045] Furthermore, when the positive electrode active material layer is formed with a first layer and a second layer with a lower gas permeation rate (when the average particle size of the active material in the second layer is reduced), the presence or absence of metallic lithium deposition when the basis weight of the positive electrode active material layer is changed was as follows. In the following example, the average particle sizes of the active material in the first and second layers were 10 μm and 7 μm, respectively; the moisture content of the first and second layers was 700 ppm and 420 ppm, respectively; the binder amount of both the first and second layers was 2.0 wt%; and the density was 2.0 g / cm³. 3 The ratio of the first layer to the second layer is 7:3. [Table 6] In the example above, the basis weight is 90 mg / cm³. 2 No metallic lithium deposition was observed under the following conditions. Since the total moisture content of the positive electrode active material layer at this time was 616 ppm, the amount at which metallic lithium deposition was not observed was 90 mg / cm³. 2 In this case, the moisture content per unit area is 0.055 mg. In contrast, referring to No. 4 in Table 1, when the positive electrode active material layer is uniform, the moisture content is 700 ppm and the basis weight is 25 mg / cm³. 2 At a moisture content of 0.018 mg per unit area, the deposition of metallic lithium was already observed. This demonstrates that, according to this embodiment, even if the positive electrode active material layer has a moisture content that would cause the deposition of metallic lithium to be observed if it were uniform, the deposition of metallic lithium can be prevented.

[0046] Furthermore, when the positive electrode active material layer is formed with a first layer and a second layer with a lower gas permeation rate (when the average particle size of the active material in the second layer is reduced), the presence or absence of metallic lithium deposition when the basis weight ratio of the first and second layers is changed was as follows. In the table, W1;W2 are the basis weight ratio of the first and second layers. In the following example, the average particle size of the active material in the first and second layers is 10 μm and 7 μm, respectively, the moisture content of the first and second layers is 700 ppm and 420 ppm, respectively, the binder amount of both the first and second layers is 2.0 wt%, and the basis weight is 35 mg / cm³. 2 The density is 2.0 g / cm³. 3 In addition, for samples No. 32 and 33, the electrolyte penetration was poor, resulting in poor electrolyte distribution to the electrodes and a battery capacity below 90% of the theoretical capacity. Therefore, the presence or absence of metallic lithium deposition was not checked. [Table 7] In the results described above, no deposition of metallic lithium was observed when the basis weight ratio of the first and second layers was in the range of 4:6 to 9:1. Therefore, this embodiment demonstrates that deposition of metallic lithium can be prevented when the basis weight ratio of the first and second layers is in the range of 4:6 to 9:1.

[0047] Furthermore, when the moisture content of the first layer was 1500 ppm and the moisture content of the second layer was 400 ppm, the deposition of metallic lithium could not be prevented even when the average particle size of the second layer was reduced, the amount of binder increased, and the density increased.

[0048] Thus, in the configuration of this embodiment described above, by composing the positive electrode active material layer with a first layer on the electrode foil side and a second layer on the separator side with a lower gas permeation rate, it becomes possible to reduce the treatment required to reduce the moisture content of the positive electrode active material layer in order to prevent the deposition of metallic lithium, and thus the cost required to form the positive electrode active material layer can be reduced.

[0049] While the above description is made in relation to embodiments of the present invention, many modifications and changes are readily possible for those skilled in the art, and it will be clear that the present invention is not limited to the embodiments illustrated above, but can be applied to various devices without departing from the concept of the present invention.

Claims

1. A lithium-ion secondary battery having a laminated structure in which a positive electrode active material layer coated on a positive electrode foil and a negative electrode active material layer coated on a negative electrode foil face each other with a separator in between, and a non-aqueous electrolyte is injected between the electrode foils, A lithium-ion secondary battery in which the positive electrode active material layer has a first layer on the positive electrode foil side and a second layer on the separator side, and the gas permeation rate of the second layer is lower than that of the first layer.

2. A lithium-ion secondary battery according to claim 1, wherein the moisture content of the second layer is 500 ppm or less, and the moisture content of the first layer is greater than 500 ppm.

3. A lithium-ion secondary battery according to claim 1, wherein the particle size of the active material in the second layer is smaller than the particle size of the active material in the first layer.

4. A lithium-ion secondary battery according to claim 1, wherein the density of the second layer is greater than the density of the first layer.

5. A lithium-ion secondary battery according to claim 1, wherein the amount of binder in the second layer is greater than the amount of binder in the first layer.

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