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

Abstract

The purpose of the present invention is to provide a negative electrode for lithium ion secondary batteries that can improve battery charging performance and long lifespan. A negative electrode for lithium ion secondary batteries according to the present invention comprises a negative electrode current collector and a negative electrode active material layer laminated on the negative electrode current collector. The negative electrode active material layer includes a negative electrode first active material layer laminated on the negative electrode current collector and a negative electrode second active material layer laminated on the negative electrode first active material layer. The negative electrode first active material layer contains a negative electrode first active material. The negative electrode second active material layer contains a negative electrode second active material. The BET specific surface area of the negative electrode second active material is greater than the BET specific surface area of the negative electrode first active material.

Description

[Technical Field]

[0001] The present invention relates to a negative electrode for a lithium-ion secondary battery and a lithium-ion secondary battery equipped with the negative electrode. [Background technology]

[0002] Lithium-ion secondary batteries having a negative electrode with multiple active material layers have been known for some time (see, for example, Patent Documents 1 to 6). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] International Publication No. 2011 / 114433 [Patent Document 2] Japanese Patent Publication No. 2009-009858 [Patent Document 3] Japanese Patent Publication No. 2013-246900 [Patent Document 4] Japanese Patent Publication No. 2014-229581 [Patent Document 5] Japanese Patent Publication No. 2015-187926 [Patent Document 6] Japanese Patent Publication No. 2019-185920 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] There is a need for the development of negative electrodes for lithium-ion secondary batteries that improve both the charging performance and durability of lithium-ion secondary batteries. [Means for solving the problem]

[0005] The negative electrode for a lithium-ion secondary battery of the present invention comprises a negative electrode current collector and a negative electrode active material layer laminated on the negative electrode current collector, wherein the negative electrode active material layer includes a first negative electrode active material layer laminated on the negative electrode current collector and a second negative electrode active material layer laminated on the first negative electrode active material layer, wherein the first negative electrode active material layer contains a first negative electrode active material, the second negative electrode active material layer contains a second negative electrode active material, and the BET specific surface area of ​​the second negative electrode active material is larger than the BET specific surface area of ​​the first negative electrode active material.

[0006] The negative electrode for a lithium-ion secondary battery of the present invention comprises a negative electrode current collector and a negative electrode active material layer laminated on the negative electrode current collector, wherein the negative electrode active material layer includes a first negative electrode active material layer laminated on the negative electrode current collector and a second negative electrode active material layer laminated on the first negative electrode active material layer, satisfying at least one of the following conditions: the first negative electrode active material layer contains a first negative electrode active material, the second negative electrode active material layer contains a second negative electrode active material, and the density of the second negative electrode active material layer is lower than the density of the first negative electrode active material layer; and the first negative electrode active material layer and the second negative electrode active material layer contain a conductive additive, and the ratio of the weight of the conductive additive to the total weight of the second negative electrode active material layer is greater than the ratio of the weight of the conductive additive to the total weight of the first negative electrode active material layer, wherein the first negative electrode active material contains a Si-based material. The Si-based material contained in the first negative electrode active material may be pre-doped with lithium.

[0007] The lithium-ion secondary battery of the present invention is a lithium-ion secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode is the negative electrode for the lithium-ion secondary battery of the present invention. [Effects of the Invention]

[0008] According to the lithium-ion secondary battery equipped with the negative electrode for lithium-ion secondary batteries of the present invention, improvements in charging performance and lifespan can be achieved. [Brief explanation of the drawing]

[0009] [Figure 1]It is a schematic perspective view showing a battery 1 including a negative electrode 120 of an example according to the first and second embodiments. [Figure 2] It is a schematic perspective view showing a charge / discharge body 100 of the battery 1 shown in FIG. 1. [Figure 3] It is a schematic perspective view showing a portion including a positive electrode tab 111b and a negative electrode tab 121b in the charge / discharge body 100 shown in FIG. 2, with the positions of the side ends of the positive electrode 110, the negative electrode 120, and the separator 130 being different. [Figure 4] It is a schematic side view showing a charge / discharge body 100 of an example of the battery 1 according to the first embodiment, which is the charge / discharge body 100 shown in FIG. 3. [Figure 5] It is a schematic enlarged view of a portion of a die head and a back roller used in the manufacture of the negative electrode. [Figure 6] It is a schematic cross-sectional view of a positive electrode and a negative electrode of a separatorless battery including a negative electrode of another example according to the first and second embodiments. [Figure 7] It is a schematic enlarged cross-sectional view of an interface between a second negative electrode active material layer and a negative electrode electron insulating layer of a negative electrode active material layer of a negative electrode of another example according to the first and second embodiments and its vicinity. [Figure 8] It is a schematic side view showing a charge / discharge body 100 of an example of the battery 1 according to the second embodiment, which is the charge / discharge body 100 shown in FIG. 3.

Modes for Carrying Out the Invention

[0010] Each embodiment of the present invention will be described with reference to the drawings. In order to facilitate understanding of each embodiment, the size and ratio of the constituent members may be exaggerated in each drawing. In each drawing, the same reference numerals are given to the same configurations. In each drawing, the lateral direction X (X-axis direction), the depth direction Y (Y-axis direction), and the height direction Z (Z-axis direction) of the battery 1 and the constituent members of the battery 1 are indicated by arrows. However, in each drawing, the lateral direction X, the depth direction Y, and the height direction Z indicate relative directional relationships. That is, for example, when the battery 1 is rotated 180 degrees to reverse the upper and lower surfaces, or when the battery 1 is rotated 90 degrees to dispose the upper surface as a side surface, the lateral direction X, the depth direction Y, and the height direction Z of the battery 1 change.

[0011] Hereinafter, the "negative electrode for a lithium ion secondary battery" may be abbreviated as "negative electrode". The "lithium ion secondary battery" may be abbreviated as "battery".

[0012] [First Embodiment] The negative electrode for a lithium ion secondary battery according to the first embodiment includes a negative electrode current collector and a negative electrode active material layer laminated on the negative electrode current collector. The negative electrode active material layer includes a negative electrode first active material layer laminated on the negative electrode current collector and a negative electrode second active material layer laminated on the negative electrode first active material layer. The negative electrode first active material layer contains a negative electrode first active material, the negative electrode second active material layer contains a negative electrode second active material, and the BET specific surface area of the negative electrode second active material is larger than the BET specific surface area of the negative electrode first active material. And the lithium ion secondary battery according to the first embodiment is a lithium ion secondary battery including a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode is the negative electrode for a lithium ion secondary battery according to the first embodiment.

[0013] (Configuration of a battery including an example negative electrode according to the first embodiment) The configuration of a battery including an example negative electrode according to the first embodiment will be described with reference to FIGS. 1 to 4.

[0014] A battery 1 equipped with a negative electrode 120 according to an example of the first embodiment is a lithium-ion secondary battery, and as shown in Figure 1, comprises a charge / discharge unit 100 on which power is charged and discharged, a container 200 housing the charge / discharge unit 100, and an external terminal 300 connected to the charge / discharge unit 100 and attached to the container 200.

[0015] As shown in Figures 2 to 4, the charge / discharge body 100 has a positive electrode 110, a negative electrode 120, and a separator 130. When the charge / discharge body 100 is housed in the container 200, the separator 130 is impregnated with an electrolyte solution in which a supporting salt (electrolyte) is dissolved. As shown in Figures 2 and 3, the charge / discharge body 100 is constructed by winding a long positive electrode 110 and a long negative electrode 120 around a long separator 130. When the components of the charge / discharge body 100 are wound, both ends are formed into a rectangular prism shape.

[0016] The positive electrode 110 is a positive electrode for a lithium-ion secondary battery and comprises a positive electrode current collector 111 and a positive electrode active material layer 112 laminated on the positive electrode current collector 111, as shown in Figures 3 and 4.

[0017] The positive electrode current collector 111 is formed in an elongated shape extending in the width direction X. As shown in Figures 3 and 4, the positive electrode current collector 111 includes a current collecting portion 111a and a positive electrode tab 111b. The current collecting portion 111a is elongated in the width direction X and is formed in a foil shape. As shown in Figures 3 and 4, the positive electrode tab 111b protrudes from the side edge 111c along the longitudinal direction of the current collecting portion 111a in the short direction (above the height direction Z) of the current collecting portion 111a. The positive electrode tab 111b is formed integrally with the current collecting portion 111a. One positive electrode tab 111b is formed on the current collecting portion 111a, for example. The current collecting portion 111a is formed of, for example, aluminum or an aluminum alloy, for example, an aluminum foil having a plate-like (sheet-like) shape.

[0018] As shown in Figure 4, the positive electrode active material layer 112 is bonded to the current collector portion 111a of the positive electrode current collector 111. The positive electrode active material layer 112 may be formed on both sides of the current collector portion 111a. The positive electrode active material layer 112 faces, for example, the entire area along the short side direction (height direction Z) of the current collector portion 111a.

[0019] The positive electrode active material layer 112 contains a positive electrode active material composed of a lithium-containing composite oxide. Examples of lithium-containing composite oxides include metallic elements such as nickel, cobalt, and manganese, along with lithium.

[0020] Examples of lithium-containing composite oxides that constitute the positive electrode active material include the following general composition formula Li 1+X M A O2(1) (In the formula, X satisfies -0.15 ≤ X ≤ 0.15, and M A (This represents a group of elements containing at least one element selected from the group consisting of Mn and Al, Ni, and Co.) A ternary lithium-containing composite oxide represented by the formula shown may also be used.

[0021] The ternary lithium-containing composite oxide represented by the above general composition formula (1) exhibits high thermal stability and stability under high potential conditions. By applying this oxide, the safety and various battery characteristics of battery 1 can be improved.

[0022] The positive electrode active material layer 112 further includes additives such as conductive additives and binders, in addition to the positive electrode active material.

[0023] As a conductive additive for the positive electrode active material layer 112, carbon-based materials can be used. As carbon-based materials, crystalline carbon, amorphous carbon, or mixtures thereof can be used. Examples of crystalline carbon include natural graphite (e.g., flake graphite), artificial graphite (synthetic graphite), carbon fibers, or mixtures thereof. Examples of amorphous carbon include carbon black (e.g., acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, or mixtures thereof). An example of carbon fiber is carbon nanotubes.

[0024] As the binder for the positive electrode active material layer 112, for example, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyethylene, polystyrene, polybutadiene, polyacrylonitrile, polyvinyl fluoride, polyfluoropropylene, polyfluorochloroprene, butyl rubber, nitrile rubber, styrene-butadiene rubber (SBR), polysulfur rubber, nitrocellulose, cyanoethylcellulose, various latexes, acrylic resins, or mixtures thereof can be used.

[0025] The positive electrode 110 can be manufactured, for example, as follows. First, the materials to be contained in the positive electrode active material layer 112 (e.g., positive electrode active material, conductive additive, binder, etc.) are prepared. These materials may be in powder form. Next, these materials are mixed, and the resulting mixture is dispersed in a solvent (e.g., N-methyl-2-pyrrolidone (NMP) and / or water) to prepare a positive electrode slurry. Next, the positive electrode slurry is applied to the surface (one or both sides) of the positive electrode current collector 111 by known techniques, dried, and calendered as necessary to form the positive electrode active material layer 112. The positive electrode 110 is thus obtained.

[0026] The negative electrode 120 is a negative electrode for a lithium-ion secondary battery and, as shown in Figures 3 and 4, comprises a negative electrode current collector 121 and a negative electrode active material layer 122 laminated on the negative electrode current collector 121. The negative electrode active material layer 122 includes a first negative electrode active material layer 123 laminated on the negative electrode current collector 121 and a second negative electrode active material layer 124 laminated on the first negative electrode active material layer 123. In other words, the negative electrode 120 comprises a plurality of active material layers.

[0027] The negative electrode current collector 121 is formed in an elongated shape extending in the width direction X. As shown in Figures 3 and 4, the negative electrode current collector 121 includes a current collector portion 121a and a negative electrode tab 121b. The current collector portion 121a is elongated in the width direction X and is formed in a foil shape. As shown in Figure 4, the current collector portion 121a of the negative electrode 120 is wider in the short direction (height direction Z) compared with the current collector portion 111a of the positive electrode 110. Both ends of the current collector portion 111a of the positive electrode 110, along the short direction (from the upper end to the lower end in the height direction Z), are located within the range of the current collector portion 121a of the negative electrode 120 along the short direction (from the upper end to the lower end in the height direction Z), via the separator 130. As shown in Figures 3 and 4, the negative electrode tab 121b protrudes from the side edge 121c along the longitudinal direction of the current collector 121a in the short direction (above in the height direction Z) of the current collector 121a. When stacked with the positive electrode 110 via the separator 130, the negative electrode tab 121b protrudes in the same direction (above in the height direction Z) as the positive electrode tab 111b of the positive electrode 110. When stacked with the positive electrode 110 via the separator 130, the negative electrode tab 121b is separated from the positive electrode tab 111b of the positive electrode 110 in the width direction X. The negative electrode tab 121b is formed integrally with the current collector 121a. For example, one negative electrode tab 121b is formed on the current collector 121a. The current collector 121a is formed of, for example, copper or a copper alloy.

[0028] As shown in Figure 4, the first negative electrode active material layer 123 of the negative electrode active material layer 122 is bonded to the current collector portion 121a of the negative electrode current collector 121. The first negative electrode active material layer 123 may be formed on both sides of the current collector portion 121a. The first negative electrode active material layer 123 faces, for example, the entire area along the short side direction (height direction Z) of the current collector portion 121a. The second negative electrode active material layer 124 of the negative electrode active material layer 122 is bonded to the first negative electrode active material layer 123.

[0029] The first active material layer 123 of the negative electrode contains the first active material 123a. The first active material 123a contains pitch-coated natural graphite 123a1, natural graphite 123a2 whose surface is exposed without coating, and artificial graphite (synthetic graphite) 123a3. In addition to the first active material 123a, the first active material layer 123 further contains additives such as a conductive additive 123c and a binder 123b. The first active material layer 123 of the negative electrode is a high-capacity layer that can store a relatively large amount of lithium ions and generally corresponds to the negative electrode active material layer used in electric vehicles (BEVs: Battery Electric Vehicles).

[0030] The negative electrode second active material layer 124 includes the negative electrode second active material 124a, as shown in Figure 4. The negative electrode second active material 124a contains pitch-coated natural graphite 124a1 and natural graphite 124a2 whose surface is exposed without coating. In addition to the negative electrode second active material 124a, the negative electrode second active material layer 124 further contains additives such as a conductive additive 124c and a binder 124b. The negative electrode second active material layer 124 generally corresponds to the negative electrode active material layer used in hybrid electric vehicles (HEVs).

[0031] In the negative electrode active material layer 122, the average particle size of the negative electrode second active material 124a in the negative electrode second active material layer 124 is smaller than the average particle size of the negative electrode first active material 123a in the negative electrode first active material layer 123. As a result, the BET specific surface area of ​​the negative electrode second active material 124a in the negative electrode second active material layer 124 is larger than the BET specific surface area of ​​the negative electrode first active material 123a in the negative electrode first active material layer 123.

[0032] As shown in Figures 3 and 4, the separator 130 has an insulating function that insulates the positive electrode 110 and the negative electrode 120, preventing short circuits between them, and also has a function of holding the non-aqueous electrolyte. The separator 130 allows lithium ions to pass through the electrolyte. The separator 130 is formed in an elongated shape. As shown in Figure 4, the separator 130 is wider in the shorter direction (height direction Z) compared to the current collector 111a of the positive electrode 110 and the current collector 121a of the negative electrode 120. Both ends of the current collector 111a of the positive electrode 110 (from the upper end to the lower end in the height direction Z) and both ends of the current collector 121a of the negative electrode 120 (from the upper end to the lower end in the height direction Z) are located within the range of the separator 130 in the shorter direction (from the upper end to the lower end in the height direction Z). The separator 130 is made of a porous material. As the separator 130, porous sheets made of resins such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide, or laminated sheets made of these materials (for example, a three-layer sheet of PP / PE / PP) are used.

[0033] A layer containing an inorganic material (e.g., alumina particles) and a binder may be provided on one or both sides of the separator 130. This prevents the separator 130 from melting and maintains its insulating function even when the battery 1 is used under abnormal conditions (e.g., when the temperature of the lithium-ion secondary battery rises to 160°C or higher due to overcharging or crushing). Therefore, the safety of the battery 1 is improved.

[0034] The electrolyte is impregnated into the separator 130 and is in contact with the positive electrode 110 and the negative electrode 120. The electrolyte contains an organic solvent and a supporting salt (electrolyte), and may further contain additives such as an SEI film-forming agent. For example, a carbonate ester is used as the organic solvent. For example, a lithium salt is used as the supporting salt.

[0035] As shown in Figure 1, the container 200 houses the charge / discharge element 100. The container 200 includes a case 201 and a lid 202. The lid 202 is joined to the opening of the case 201 and together with the case 201 seals the charge / discharge element 100. The charge / discharge element 100, sealed by the case 201 and the lid 202, is filled with an electrolyte.

[0036] As shown in Figure 1, the external terminal 300 includes a positive terminal 301 and a negative terminal 302. The positive terminal 301 and the negative terminal 302 relay power input and output between the charge / discharge unit 100 and the external device. When multiple batteries 1 are used to form a battery pack, adjacent positive terminals 301 and adjacent negative terminals 302 are connected via a busbar. The positive terminal 301 is indirectly or directly connected to the positive tab 111b via a positive current collector plate. The negative terminal 302 is indirectly or directly connected to the negative tab 121b via a negative current collector plate. The positive terminal 301 and the negative terminal 302 are attached to the cover 202.

[0037] (Manufacturing method for a battery comprising an example of a negative electrode according to the first embodiment) A battery equipped with an example of a negative electrode according to the first embodiment can be manufactured using techniques known in the art of the present invention, except for the method of manufacturing the negative electrode.

[0038] An example of a negative electrode 120 according to the first embodiment can be manufactured, for example, as follows. First, the materials to be contained in the first active material layer 123 of the negative electrode (for example, the first active material of the negative electrode, additives such as conductive additives and binders) are prepared. These materials may be in powder form. Next, these materials are mixed, and the resulting mixture is dispersed in a solvent (for example, N-methyl-2-pyrrolidone (NMP) and / or water) to prepare the first slurry of the negative electrode. Next, the first slurry of the negative electrode is applied to the surface (one or both sides) of the negative electrode current collector 121 by known art, dried, and calendered as necessary to form the first active material layer 123 of the negative electrode.

[0039] Next, the materials to be contained in the second active material layer 124 of the negative electrode (for example, the second active material of the negative electrode, additives such as conductive additives and binders) are prepared. These materials may be in powder form. Next, these materials are mixed, and the resulting mixture is dispersed in a solvent (for example, N-methyl-2-pyrrolidone (NMP) and / or water) to prepare the second slurry of the negative electrode. Next, the second slurry of the negative electrode is applied to the surface (one or both sides) of the first active material layer 123 of the negative electrode by known techniques, dried, and calendered as necessary to form the second active material layer 124 of the negative electrode. The negative electrode 120 is obtained by the above manufacturing method. However, the negative electrode 120 is not limited to that manufactured by the above manufacturing method, and may be manufactured by other methods.

[0040] (Effects of a battery equipped with an example of a negative electrode according to the first embodiment) The effects of a battery equipped with an example of a negative electrode according to the first embodiment will be explained with reference to Figure 4.

[0041] In an example of the negative electrode 120 according to the first embodiment, the negative electrode active material layer 122 includes a negative electrode first active material layer 123 laminated on the negative electrode current collector 121 and a negative electrode second active material layer 124 laminated on the negative electrode first active material layer 123. The average particle size of the negative electrode second active material 124a in the negative electrode second active material layer 124 is smaller than the average particle size of the negative electrode first active material 123a in the negative electrode first active material layer 123. As a result, the BET specific surface area of ​​the negative electrode second active material 124a in the negative electrode second active material layer 124 is larger than the BET specific surface area of ​​the negative electrode first active material 123a in the negative electrode first active material layer 123. Therefore, in the negative electrode active material layer 122, the negative electrode second active material layer 124 containing the negative electrode second active material 124a, which has a large reaction area with lithium ions, is arranged on the separator 130 side, which is the lithium ion receiving side when the battery 1 is charged. As a result, the deposition of Li on the separator 130 side of the negative electrode active material layer 122 and the occurrence of side reactions associated with Li deposition are suppressed, thereby improving the durability of the battery 1 and achieving a longer battery life. Furthermore, the rapid charging performance of the battery 1 can be improved. On the other hand, on the opposite negative electrode current collector 121 side, the negative electrode first active material layer 123 containing the negative electrode first active material 123a, which has a small reaction area with lithium ions, is arranged, so the amount of lithium ions trapped in the negative electrode active material and no longer contributing to subsequent battery reactions can be suppressed. As a result, the cycle characteristics of the battery 1 can be improved and the storage durability of the battery 1 can be improved. Thus, a longer battery life can be achieved. Therefore, with a negative electrode of 120, improvements in charging performance and lifespan can be achieved.

[0042] More specifically, the reaction area of ​​the negative electrode active material per unit volume in the negative electrode active material layer is relatively larger in the negative electrode second active material layer, which is a high input / output layer, compared with the negative electrode first active material layer, which is a high capacity layer. Compared to the negative electrode first active material layer, the diffusion path of lithium ions is relatively shorter in the negative electrode second active material layer. Therefore, the charging characteristics of the battery, especially the rapid charging characteristics, can be improved in the negative electrode second active material layer. On the other hand, the reaction area of ​​the negative electrode active material per unit volume in the negative electrode active material layer is relatively smaller in the negative electrode first active material layer, which is a high capacity layer, compared with the negative electrode second active material layer, which is a high input / output layer. Therefore, the cycle durability and lithium ion storage durability when the battery is repeatedly charged and discharged can be improved in the negative electrode first active material layer, which is a high capacity layer.

[0043] Furthermore, in the negative electrode 120, the negative electrode first active material 123a contained in the negative electrode first active material layer 123 contains pitch-coated natural graphite 123a1, natural graphite 123a2 whose surface is exposed without coating, and artificial graphite 123a3. Because the surface of the pitch-coated natural graphite 123a1 is pitch-coated, it has higher conductivity and a smaller reaction area with lithium ions than ordinary natural graphite (natural graphite whose surface is exposed without coating) 123a2. However, because it is hard, if the negative electrode first active material 123a is composed only of pitch-coated natural graphite 123a1, the press-formability during the formation of the negative electrode first active material layer 123 decreases. In contrast, in the negative electrode 120, in addition to the pitch-coated natural graphite 123, ordinary natural graphite (natural graphite with an uncoated surface exposed) 123a2, which is more flexible than the pitch-coated natural graphite 123a1, is further incorporated into the negative electrode first active material 123a. This suppresses the reaction area of ​​the negative electrode first active material 123a, ensuring sufficient conductivity of the negative electrode first active material 123a while improving the press-formability of the negative electrode first active material layer 123 during formation. This further improves the cycle characteristics of the battery 1 and increases the energy density of the battery 1. Furthermore, since the ease of pressing can be arbitrarily controlled, it is easy to set the density of the negative electrode first active material layer to a predetermined level or set its thickness to a predetermined level.

[0044] Similar to the first negative electrode active material 123a, the second negative electrode active material 124a contained in the second negative electrode active material layer 124 also contains pitch-coated natural graphite 124a1 and natural graphite 124a2 whose surface is exposed without coating. Therefore, similarly, the reaction area of ​​the second negative electrode active material 124a can be suppressed, and the press-formability of the second negative electrode active material layer 124 can be improved while ensuring sufficient conductivity of the second negative electrode active material 124a. This further improves the cycle characteristics of the battery 1 and further increases the energy density of the battery 1. Furthermore, since the ease of pressing can be arbitrarily controlled, it is easy to set the density of the second negative electrode active material layer to a predetermined level or set its thickness to a predetermined level.

[0045] Furthermore, in a battery 1 equipped with a negative electrode 120, if the electrolyte further contains an SEI film-forming agent, the cycle characteristics of the battery 1 can be further improved and the storage durability of the battery 1 can be further improved by suppressing the reaction between the surface of the negative electrode active material and the electrolyte.

[0046] Next, the configuration of the negative electrode for a lithium-ion secondary battery and the lithium-ion secondary battery equipped with the negative electrode according to the first embodiment will be described in more detail.

[0047] 1. Negative electrode for lithium-ion secondary batteries The negative electrode for a lithium-ion secondary battery according to the first embodiment comprises a negative electrode current collector and a negative electrode active material layer laminated on the negative electrode current collector, wherein the negative electrode active material layer includes a first negative electrode active material layer laminated on the negative electrode current collector and a second negative electrode active material layer laminated on the first negative electrode active material layer.

[0048] (1)Negative electrode first active material layer The above-mentioned negative electrode first active material layer contains a negative electrode first active material. The negative electrode first active material is not particularly limited as long as it contains a negative electrode active material that allows for the insertion and removal of lithium ions, but for example, it contains at least one selected from the group consisting of natural graphite, artificial graphite, hard carbon, soft carbon, and graphite coated with amorphous carbon.

[0049] As the first active material of the negative electrode, it is preferable to have one that contains pitch-coated natural graphite and natural graphite whose surface is exposed without coating, and in particular, one that contains pitch-coated natural graphite, natural graphite whose surface is exposed without coating, and artificial graphite. This is because it is possible to suppress the reaction area of ​​the first active material of the negative electrode, ensure sufficient conductivity of the first active material of the negative electrode, and improve the press-formability when forming the first active material layer of the negative electrode. In addition, the properties of the negative electrode active material can be improved by using artificial graphite with high purity and high crystalline uniformity.

[0050] The negative electrode first active material layer is not particularly limited as long as it contains the negative electrode first active material, but it is preferable that, in addition to the negative electrode first active material, it further contains at least one additive selected from the group consisting of conductive additives and binders.

[0051] As a conductive additive for the first active material layer of the negative electrode, carbon-based materials can be used. As carbon-based materials, crystalline carbon, amorphous carbon, or mixtures thereof can be used. Examples of crystalline carbon include natural graphite (e.g., flake graphite), artificial graphite, carbon fibers, or mixtures thereof. Examples of amorphous carbon include carbon black (e.g., acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, or mixtures thereof). An example of carbon fiber is carbon nanotubes.

[0052] As the binder for the first active material layer of the negative electrode, for example, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyethylene, polystyrene, polybutadiene, polyacrylonitrile, polyvinyl fluoride, polypropylene fluoride, polychloroprene fluoride, butyl rubber, nitrile rubber, styrene-butadiene rubber (SBR), polysulfur rubber, nitrocellulose, cyanoethylcellulose, various latexes, acrylic resins, polyamide-imide, polyimide, or mixtures thereof can be used.

[0053] The ratio of the weight of the first negative electrode active material to the total weight of the first negative electrode active material layer is preferably, for example, 80% by weight or more and 99% by weight or less.

[0054] The thickness on one side of the negative electrode first active material layer in the stacking direction (for example, the depth direction Y in Figure 4) (for example, the first thickness T1 in Figure 4) may be, for example, an average thickness of 5 μm or more and 500 μm or less, or for example, an average thickness of 10 μm or more and 300 μm or less.

[0055] (2)Negative electrode second active material layer The above-mentioned negative electrode second active material layer contains a negative electrode second active material. The negative electrode second active material is not particularly limited as long as it contains a negative electrode active material that allows for the insertion and removal of lithium ions, but for example, it contains at least one selected from the same group as the negative electrode first active material.

[0056] As the second active material of the negative electrode, it is preferable to have one that contains pitch-coated natural graphite and natural graphite whose surface is exposed without coating, and in particular, one that contains pitch-coated natural graphite, natural graphite whose surface is exposed without coating, and artificial graphite. This is because it is possible to suppress the reaction area of ​​the first active material of the negative electrode, ensure sufficient conductivity of the first active material of the negative electrode, and improve the press-formability when forming the first active material layer of the negative electrode. In addition, the properties of the negative electrode active material can be improved by using artificial graphite with high purity and high crystalline uniformity.

[0057] The negative electrode second active material layer is not particularly limited as long as it contains the negative electrode second active material, but it is preferable that, in addition to the negative electrode second active material, it further contains at least one additive selected from the group consisting of conductive additives and binders. As the conductive additive for the negative electrode second active material layer, for example, the same as that used for the negative electrode first active material layer is used. As the binder for the negative electrode second active material layer, for example, the same as that used for the negative electrode first active material layer is used.

[0058] The ratio of the weight of the second negative electrode active material to the total weight of the second negative electrode active material layer is preferably, for example, 80% by weight or more and 99% by weight or less.

[0059] The thickness on one side of the negative electrode second active material layer in the stacking direction (for example, the depth direction Y in Figure 4) (for example, the first thickness T2 in Figure 4) may be, for example, an average thickness of 5 μm or more and 500 μm or less, or for example, an average thickness of 10 μm or more and 300 μm or less.

[0060] (3) Negative electrode active material layer In the above-mentioned negative electrode active material layer, the BET specific surface area of ​​the second negative electrode active material is larger than that of the first negative electrode active material. Here, the BET specific surface area can be calculated, for example, from the BET method using a pore distribution analyzer. The BET specific surface area of ​​the first negative electrode active material can be determined, for example, by extracting only the first negative electrode active material as a sample from a part of the first negative electrode active material layer and measuring the BET specific surface area of ​​that sample. Alternatively, the BET specific surface area of ​​the first negative electrode active material can also be determined, for example, by measuring the BET specific surface area of ​​the powdered material of the first negative electrode active material used when forming the first negative electrode active material layer. The BET specific surface area of ​​the second negative electrode active material can be determined in the same way as the BET specific surface area of ​​the first negative electrode active material.

[0061] The negative electrode active material layer is not particularly limited as long as the BET specific surface area of ​​the second negative electrode active material is greater than the BET specific surface area of ​​the first negative electrode active material. For example, if the BET specific surface area of ​​the first negative electrode active material is 1 m² 2 / g or more 6m 2 The value is less than or equal to / g, and the BET specific surface area of ​​the negative electrode second active material is 4m². 2 / g or more 10m 2 It is preferable that the concentration be less than or equal to / g. This is because it effectively suppresses side reactions associated with Li deposition on the separator side of the negative electrode active material layer, thereby effectively improving the rapid charging performance of the battery.

[0062] The negative electrode active material layer is not particularly limited as long as the BET specific surface area of ​​the second negative electrode active material is larger than the BET specific surface area of ​​the first negative electrode active material. However, it is preferable that the average particle size of the second negative electrode active material is smaller than the average particle size of the first negative electrode active material. This is because simply making the average particle size of the second negative electrode active material smaller than that of the first negative electrode active material makes the BET specific surface area of ​​the second negative electrode active material larger than that of the first negative electrode active material, thus easily improving the rapid charging performance of the battery.

[0063] Here, the average particle size is, for example, the median diameter (D50), which is the diameter of the particle when the cumulative value in the particle size distribution measurement, measured by the laser diffraction scattering particle size distribution method, is 50%. Alternatively, the median diameter (D50) may be determined by measuring the equivalent circle diameter of the projected area of ​​100 or more randomly selected active material particles based on microscopic observation images of each active material layer in the stacked cross-section of the battery, and then determining the diameter of the particle when the cumulative value in the particle size distribution of the active material obtained from these measurement results is 50%.

[0064] The median diameters of the first and second active materials of the negative electrode are preferably such that the median diameter of the second active material is smaller than that of the first active material. In particular, it is preferable that the median diameter of the first active material is 10 μm or more and 35 μm or less, and the median diameter of the second active material is 2 μm or more and 15 μm or less. This is because it effectively suppresses side reactions associated with Li deposition on the separator side of the negative electrode active material layer, thereby effectively improving the rapid charging performance of the battery.

[0065] As for the negative electrode active material layer, the surface smoothness of the negative electrode second active material may be less than that of the negative electrode first active material, so that the BET specific surface area of ​​the negative electrode second active material is greater than that of the negative electrode first active material. Examples of such a negative electrode active material layer include one in which the negative electrode first active material contains pitch-coated natural graphite, and the negative electrode second active material contains natural graphite whose surface is exposed without being coated.

[0066] (4) Manufacturing method As a method for manufacturing a negative electrode for a lithium-ion secondary battery according to the first embodiment, a manufacturing method may be used in which the first negative electrode active material layer and the second negative electrode active material layer are formed by simultaneous coating. This manufacturing method will be described below with reference to Figure 5.

[0067] In this manufacturing method, the materials to be contained in the first negative electrode active material layer (e.g., the first negative electrode active material, additives such as conductive additives and binders) are prepared. These materials are mixed, and the resulting mixture is dispersed in a solvent (e.g., N-methyl-2-pyrrolidone (NMP) and / or water) to obtain the first negative electrode slurry. The materials to be contained in the second negative electrode active material layer (e.g., the second negative electrode active material, additives such as conductive additives and binders) are prepared. These materials are mixed, and the resulting mixture is dispersed in a solvent (e.g., N-methyl-2-pyrrolidone (NMP) and / or water) to obtain the second negative electrode slurry.

[0068] Next, the first negative electrode slurry and the second negative electrode slurry are simultaneously applied onto the negative electrode current collector 34a using a die head 50, for example, as shown in Figure 5. The die head 50 has an outlet block 57, a three-dimensional shim 58, and an inlet block 59. Inside the die head 50 are a manifold 52 for the second negative electrode slurry and a manifold 51 for the first negative electrode slurry. The second negative electrode slurry and the first negative electrode slurry are simultaneously discharged from each manifold 52 and 51 toward the negative electrode current collector 34a being conveyed along the back roller 56. This forms the second negative electrode slurry layer 33d and the first negative electrode slurry layer 33b. Next, the solvents contained in the first negative electrode slurry layer 33b and the second negative electrode slurry layer 33d are evaporated using a drying oven or the like to dry the first negative electrode slurry layer 33b and the second negative electrode slurry layer 33d. As a result, a first negative electrode active material layer (not shown) and a second negative electrode active material layer (not shown) are formed on one side of the negative electrode current collector 34a. Next, the negative electrode current collector 34a, the first negative electrode active material layer, and the second negative electrode active material layer are press-formed. Specifically, the laminate containing the negative electrode current collector 34a, the first negative electrode active material layer, and the second negative electrode active material layer is pressed between rolls at 0 to 120°C and pressure is applied. After that, the laminate is slit to a predetermined width. This yields the negative electrode.

[0069] In a battery equipped with a negative electrode manufactured using the above-described two-layer simultaneous coating method, a layer of mixed negative electrode active material is formed at the interface between the first negative electrode active material layer and the second negative electrode active material layer. This mixed layer acts as a buffer layer that mitigates the difference in expansion and contraction between the first negative electrode active material layer and the second negative electrode active material layer, thereby reducing the separation between the first negative electrode active material layer and the second negative electrode active material layer during charging and discharging.

[0070] Furthermore, the interface between the first active material layer of the negative electrode (first slurry layer 33b) and the second active material layer of the negative electrode (second slurry layer 33d) is not pressed by the rolls. For example, the interface of the second active material layer of the negative electrode (second slurry layer 33d) opposite to the first active material layer of the negative electrode (first slurry layer 33b) is pressed by the rolls. As a result, the interface between the first active material layer of the negative electrode (first slurry layer 33b) and the second active material layer of the negative electrode (second slurry layer 33d) has greater irregularities than the interface on the opposite side of the second active material layer of the negative electrode (second slurry layer 33d). This gives the first active material layer of the negative electrode (first slurry layer 33b) a large surface area. This is therefore preferable for ion conduction. Furthermore, it is preferable to create larger irregularities at the interface between the first negative electrode active material layer (first negative electrode slurry layer 33b) and the second negative electrode active material layer (second negative electrode slurry layer 33d) than at the surface of the second negative electrode active material layer (second negative electrode slurry layer 33d) facing the roll, as this results in better adhesion and stable ion conductivity.

[0071] 2. Lithium-ion rechargeable batteries The lithium-ion secondary battery according to the first embodiment is a lithium-ion secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode is a negative electrode for a lithium-ion secondary battery according to the first embodiment.

[0072] The lithium-ion secondary battery according to the first embodiment is not particularly limited, but for example, it comprises a charge / discharge body having a positive electrode, a negative electrode, and a separator, with an electrolyte impregnated into the separator. The lithium-ion secondary battery according to the first embodiment comprises an electrolyte solution in which the above electrolyte is dissolved, and the electrolyte solution may further contain additives such as an SEI film-forming agent, and among these, it is preferable that the electrolyte solution contains an SEI film-forming agent. The SEI film-forming agent can protect the negative electrode. Therefore, the cycle durability of the negative electrode active material can be improved when the battery is repeatedly charged and discharged.

[0073] Here, the SEI film is an organic film called SEI (Solid Electrolyte Interface) formed on the surface of the negative electrode active material layer 122, and it plays a role in suppressing the excessive decomposition of the electrolyte solution and preventing the deterioration of the cycle characteristics of the battery 1. The SEI film forming agent refers to an additive added to the electrolyte solution so that the SEI film is formed. As the SEI film forming agent, for example, vinylene carbonate (VC), fluoroethylene carbonate (FEC), etc. are used.

[0074] Furthermore, as the lithium ion secondary battery according to the first embodiment, as the electrolyte, a battery including a solid electrolyte may be used, which has a positive electrode, a negative electrode, and a solid electrolyte layer including a solid electrolyte, and the solid electrolyte layer is interposed between the positive electrode and the negative electrode and includes a charge and discharge body.

[0075] Since a battery including such a solid electrolyte does not need to include an electrolyte solution, it can have high safety. Also, in a battery including such a solid electrolyte, since the reactivity on the surface of the active material particles is high, it can contribute to stable ion conduction. Such effects can be obtained. Note that the interface between the solid electrolyte layer of the negative electrode second active material layer is preferably more uneven in the thickness direction than the interface on the opposite side of the solid electrolyte layer from the negative electrode second active material layer. This is preferable for lithium ion movement because the adhesion is high.

[0076] Examples of the solid electrolyte include sulfide-based solid electrolytes, for example, Li 10 GeP2S 12 , Li6PS5Cl, Li2S-P2S5-based glass, Li2S-SiS2-based glass, Li2S-P2S5-GeS2-based glass, Li2S-B2S3-based glass, oxide-based solid electrolytes, for example, Li7La3Zr2O 12 , LiLaTiO3, LiTi(PO4)3, LiGe(PO4)3, and complex hydride-based solid electrolytes, for example, LiBH4-LiI, LiBH4-LiNH2, and mixtures of two or more of these.

[0077] 3. Others Another example of a battery with a negative electrode according to the first embodiment may be a separatorless battery that has a positive electrode electronic insulating layer provided on the positive electrode and a negative electrode electronic insulating layer provided on the negative electrode instead of a separator.

[0078] (Configuration of a separatorless battery with a negative electrode, as in another example according to the embodiment) The configuration of such a separator-less battery will be explained below with reference to Figures 6 and 7.

[0079] As shown in Figure 6, in a separatorless battery 1 (lithium-ion secondary battery) with a negative electrode 32 of another example according to the embodiment, the positive electrode 34 comprises a positive electrode current collector 34a, a positive electrode active material layer 34b bonded to both sides of the positive electrode current collector 34a, and a positive electrode electronic insulating layer 34d bonded to each of the positive electrode active material layer 34b (positive electrode mixture layer). The negative electrode 32 comprises a negative electrode current collector 32a and a negative electrode active material layer 32b (negative electrode mixture layer) bonded to both sides of the negative electrode current collector 32a, and the negative electrode active material layer 32b includes a negative electrode first active material layer 32b1 bonded to both sides of the negative electrode current collector 32a, and a negative electrode second active material layer 32b2 bonded to each of the negative electrode first active material layer 32b1. The negative electrode 32 further comprises a negative electrode electronic insulating layer 32d bonded to each of the negative electrode second active material layers 32b2.

[0080] One end of the positive electrode current collector 34a is provided with a portion 34c that is not covered by either the positive electrode active material layer 34b or the positive electrode electronic insulating layer 34d (hereinafter referred to as the "exposed portion of the positive electrode current collector"). The exposed portion of the positive electrode current collector 34c is provided on the end face of the winding group (not shown) and in its vicinity. The exposed portion of the positive electrode current collector 34c faces and is electrically connected to the positive electrode side connection end (not shown) of the positive electrode current collector plate (not shown). Similarly, one end of the negative electrode current collector 32a is provided with a portion 32c that is not covered by either the negative electrode active material layer 32b or the negative electrode electronic insulating layer 32d (hereinafter referred to as the "exposed portion of the negative electrode current collector"). The exposed portion of the negative electrode current collector 32c is provided on the end face of the winding group and in its vicinity. The exposed portion 32c of the negative electrode current collector faces and is electrically connected to the negative electrode side connection end (not shown) of the negative electrode current collector plate (not shown).

[0081] The positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d have the function of preventing short circuits between the positive electrode active material layer 34b and the negative electrode active material layer 32b, and the function of conducting ions between the positive electrode active material layer 34b and the negative electrode active material layer 32b. The positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d may be porous layers made of an electrically insulating (i.e., electronically insulating and ionic insulating) material. The porous layer can hold an electrolyte in its pores, and ions can be conducted between the positive electrode active material layer 34b and the negative electrode active material layer 32b through this electrolyte.

[0082] The porous positive electrode electronic insulating layer 34d and negative electrode electronic insulating layer 32d may also have the function of buffering the expansion and contraction of the positive electrode active material layer 34b and negative electrode active material layer 32b that occur during charging and discharging of the lithium-ion secondary battery 100. The expansion and contraction of the negative electrode active material layer 32b that occurs during charging and discharging of the battery 1 is generally greater than that of the positive electrode active material layer 34b. Therefore, in order to buffer the larger expansion and contraction of the negative electrode active material layer 32b, the negative electrode electronic insulating layer 32d may have an average pore diameter that is larger than the average pore diameter of the positive electrode electronic insulating layer 34d. In this application, the average pore diameter of the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d refers to the average value of the volume-based pore diameter measured by the mercury intrusion method.

[0083] The total content of Na and Fe in the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d may be 300 ppm or less, based on the weight of the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d. The amount of each element contained in the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d can be measured by the ICP (Inductive Coupled Plasma) method.

[0084] The positive electrode electronic insulating layer 34d may contain positive electrode electronic insulating particles, and the negative electrode electronic insulating layer 32d may contain negative electrode electronic insulating particles. Hereinafter, the positive electrode electronic insulating particles and the negative electrode electronic insulating particles will be collectively referred to as electronic insulating particles. The electronic insulating particles may be electrical insulating particles. Ceramic particles are an example of electrical insulating particles. The ceramic particles may contain at least one selected from the group consisting of alumina (Al2O3), boehmite (Al2O3 hydrate), magnesia (MgO), zirconia (ZrO2), titania (TiO2), iron oxide, silica (SiO2), and barium titanate (BaTiO2), and preferably contain at least one selected from the group consisting of alumina, boehmite, magnesia, zirconia, and titania. The electronic insulating particles may have an average particle size in the range of 0.7 to 1.1 μm. The average particle diameter of the electronic insulating particles can be determined by calculating the arithmetic mean of the projected area equivalent diameter of 100 or more randomly selected electronic insulating particles based on microscopic images of the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d. The electronic insulating particles may contain at least one of the following, based on the weight of the electronic insulating particles: 100-200 ppm of Na, 50-100 ppm of Fe, or 50-100 ppm of Ca.

[0085] The positive electrode electron insulating layer 34d and the negative electrode electron insulating layer 32d may further contain a binder. The binder may be dispersed or dissolved in an aqueous solvent or a non-aqueous solvent (e.g., N-methyl-2-pyrrolidone (NMP)) and may contain at least one selected from the group consisting of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), and carboxymethylcellulose (CMC).

[0086] The positive electrode electron insulating layer 34d and the negative electrode electron insulating layer 32d may further contain a dispersant. The dispersant may contain at least one selected from the group consisting of carboxylic acid compounds and phosphoric acid compounds.

[0087] The interface 34e between the positive electrode electronic insulating layer 34d and the positive electrode active material layer 34b has an uneven shape, and the height of the unevenness is 2 μm or more, preferably in the range of 2 to 4 μm. The interface 32e between the negative electrode electronic insulating layer 32d and the negative electrode active material layer 32b (negative electrode second active material layer 32b2) also has an uneven shape, and the height of the unevenness is 2 μm or more, preferably in the range of 2 to 4 μm. Since the height of the unevenness of the interface 34e between the positive electrode electronic insulating layer 34d and the positive electrode active material layer 34b, and the interface 32e between the negative electrode electronic insulating layer 32d and the negative electrode active material layer 32b is 2 μm or more, the adhesion between the positive electrode electronic insulating layer 34d and the positive electrode active material layer 34b, and the adhesion between the negative electrode electronic insulating layer 32d and the negative electrode active material layer 32b can be improved. This prevents or reduces the peeling of the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d from the positive electrode active material layer 34b and the negative electrode active material layer 32b, respectively, thereby improving the reliability of the lithium-ion secondary battery 100.

[0088] The height of the unevenness of the interface 34e between the positive electrode active material layer 34b and the positive electrode electronic insulating layer 34d can be controlled, for example, by the particle size of the positive electrode active material particles (positive electrode active material) contained in the positive electrode active material layer 34b and the positive electrode electronic insulating particles contained in the positive electrode electronic insulating layer 34d. As shown in Figure 7, if the average particle size of the positive electrode active material particles 34bp (positive electrode active material) contained in the positive electrode active material layer 34b is larger than the average particle size of the positive electrode electronic insulating particles 34dp contained in the positive electrode electronic insulating layer 34d, the positive electrode electronic insulating particles 34dp will enter the gaps between the positive electrode active material particles 34bp, and the interface 34e between the positive electrode active material layer 34b and the positive electrode electronic insulating layer 34d will have an uneven shape. For example, by using spherical positive electrode active material particles 34bp having an average particle diameter in the range of 4.5 to 5.5 μm, and positive electrode electronic insulating particles 34dp having an average particle diameter in the range of 0.7 to 1.1 μm, the unevenness height of the interface 34e between the positive electrode active material layer 34b and the positive electrode electronic insulating layer 34d can be set to 2 μm or more, preferably in the range of 2 to 4 μm.

[0089] Similarly, the height of the unevenness at the interface 32e between the negative electrode active material layer 32b (negative electrode second active material layer 32b2) and the negative electrode electronic insulating layer 32d can be controlled by the particle size of the negative electrode second active material particles (negative electrode second active material) contained in the negative electrode second active material layer 32b2 and the negative electrode electronic insulating particles contained in the negative electrode electronic insulating layer 32d. For example, by using flaky negative electrode second active material particles having an average particle size in the range of 9 to 11 μm, and negative electrode electronic insulating particles having an average particle size in the range of 0.7 to 1.1 μm, the height of the unevenness at the interface 32e between the negative electrode active material layer 32b and the negative electrode electronic insulating layer 32d can be set to 2 μm or more, preferably in the range of 2 to 4 μm.

[0090] In this application, the surface height of the interface 34e between the positive electrode electronic insulating layer 34d and the positive electrode active material layer 34b, and the surface height of the interface 32e between the negative electrode active material layer 32b (negative electrode second active material layer 32b2) and the negative electrode electronic insulating layer 32d are measured as follows: Cross-sectional SEM images of any three locations on the positive electrode 34 or negative electrode 32 are obtained using a scanning electron microscope (SEM), and the distance from any 10 or more points on the interfaces 34e and 32e to a predetermined reference plane is measured in each cross-sectional SEM image (for example, the distance from any 10 or more points on the interfaces 34e and 32e to the surface 34f of the positive electrode electronic insulating layer 34d and 32f of the negative electrode electronic insulating layer 32d, i.e., the thickness of the positive electrode electronic insulating layer 34d and the thickness of the negative electrode electronic insulating layer 32d at any 10 or more locations). The standard deviation of the obtained distance values ​​is taken as the surface height of the interfaces 34e and 32e. Furthermore, the surface 34f of the positive electrode electronic insulating layer 34d and the surface 32f of the negative electrode electronic insulating layer 32d are opposing surfaces and may be sufficiently flat compared to the interfaces 34e and 32e. For example, the unevenness heights of the surfaces 34f and 32f of the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d may be one-tenth or less of the unevenness heights of the interfaces 34e and 32e, respectively.

[0091] The statement "the interface 34e between the positive electrode electronic insulating layer 34d and the positive electrode active material layer 34b has an uneven shape" can also be rephrased as "there is a positive electrode mixed layer between the positive electrode electronic insulating layer 34d and the positive electrode active material layer 34b that contains a positive electrode active material and an electronic insulating material." Similarly, the statement "the interface 32e between the negative electrode electronic insulating layer 32d and the negative electrode active material layer 32b has an uneven shape" can be rephrased as "there is a negative electrode mixed layer between the negative electrode electronic insulating layer 32d and the negative electrode active material layer 32b that contains a negative electrode active material and an electronic insulating material." The thickness of the positive electrode mixed layer is 2 μm or more, preferably in the range of 2 to 4 μm. The thickness of the negative electrode mixed layer is 2 μm or more, preferably in the range of 2 to 4 μm. The thickness of the positive electrode mixed layer and the negative electrode mixed layer can be measured in the same way as the height of the irregularities of the interfaces 34e and 32e between the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d and the positive electrode active material layer 34b and the negative electrode active material layer 32b described above.

[0092] The positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d may be in contact with each other. Preferably, the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d may be in contact with each other without being fixed to each other. By not fixing the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d to each other, it is possible to alleviate the stress caused by the expansion and contraction of the negative electrode active material layer 32b and the positive electrode active material layer 34b during charging and discharging of the lithium-ion secondary battery 100, and to prevent or reduce the growth of dendrites that could cause a short circuit between the positive electrode active material layer 34b and the negative electrode active material layer 32b, which would penetrate the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d.

[0093] The peel strength of the positive electrode electronic insulating layer 34d from the positive electrode active material layer 34b, and the peel strength of the negative electrode electronic insulating layer 32d from the negative electrode active material layer 32b, may be greater than the peel strength of the positive electrode electronic insulating layer 34d from the negative electrode electronic insulating layer 32d. The peel strength can be measured, for example, by a 180° tape peel test in accordance with JIS C 0806-3 1999.

[0094] The height of the surface irregularities at the interface 34e between the positive electrode active material layer 34b and the positive electrode electronic insulating layer 34d can be controlled not only by the particle size of the positive electrode active material particles and positive electrode electronic insulating particles as described above, but also by the type and viscosity of the solvent used in the positive electrode slurry used to form the positive electrode active material layer 34b and the positive electrode electronic insulating material slurry used to form the positive electrode electronic insulating layer 34d. Similarly, the height of the surface irregularities at the interface 32e between the negative electrode active material layer 32b (negative electrode second active material layer 32b2) and the negative electrode electronic insulating layer 32d can also be controlled by the type and viscosity of the solvent used in the negative electrode slurry (negative electrode second slurry) used to form the negative electrode active material layer 32b (negative electrode second active material layer 32b2) and the negative electrode electronic insulating material slurry used to form the negative electrode electronic insulating layer 32d.

[0095] The average pore diameter of the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d can be controlled by the particle size of the electronic insulating particles and the press pressure during press processing. Specifically, the higher the press pressure, the smaller the average pore diameter, and the smaller the particle size of the electronic insulating particles, the smaller the average pore diameter.

[0096] In the separator-less battery 1 equipped with the positive electrode 34 of the other examples described above, stable ion conduction is achieved because of the high adhesion between the negative electrode active material layer 32b and the negative electrode electronic insulating layer 32d. Furthermore, such other examples of battery 1 can contribute to providing a battery with high energy density and long life by incorporating electrodes manufactured using the manufacturing method to which the two-layer simultaneous coating described above is applied, or electrodes manufactured using the manufacturing method to which the three-layer simultaneous coating described later is applied.

[0097] (modified version) Furthermore, in a modified example of the separator-less battery 1 (lithium-ion secondary battery) having the negative electrode 32 of the other examples described above, each of the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d is a layer containing a solid electrolyte (i.e., an electronically insulating and ionically conductive material). This modified battery (lithium-ion secondary battery) does not need to contain an electrolyte, and therefore can have high safety. In this modified example, the electronically insulating particles contained in the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d may be solid electrolyte particles. Since solid electrolytes can be well molded by press molding, in this case, it is not essential that the positive electrode electronic insulating layer 34d and the negative electrode electronic insulating layer 32d contain a binder and a dispersant.

[0098] Furthermore, the positive electrode active material layer 34b may further contain a solid electrolyte in addition to the active material and optionally selected binders, conductive additives, and dispersants. This can improve the ionic conductivity of the positive electrode active material layer 34b.

[0099] At least one of the first negative electrode active material layer 32b1 and the second negative electrode active material layer 32b2 contained in the negative electrode active material layer 32b may further contain a solid electrolyte in addition to the electrode active material and an optional binder, conductive additive, and dispersant. This makes it possible to improve the ionic conductivity of at least one of the first negative electrode active material layer 32b1 and the second negative electrode active material layer 32b2.

[0100] In the modified batteries described above, there is no need to include an electrolyte, and since the strength of the electronic insulating layers (positive electrode electronic insulating layer 34d and negative electrode electronic insulating layer 32d) is higher than that of the separator, a high level of safety can be achieved. Furthermore, in particular, by incorporating electrodes manufactured using the manufacturing method to which the two layers of simultaneous coating described above are applied, or electrodes manufactured using the manufacturing method to which the three layers of simultaneous coating described later are applied, these modified batteries can contribute to providing batteries with high energy density and long lifespan.

[0101] (Method for manufacturing a separatorless battery with a negative electrode, as in another example according to the embodiment) Such separator-less batteries (lithium-ion secondary batteries) can be manufactured using techniques known in the art of the present invention, except for the method of manufacturing the negative electrode as in other examples of the embodiment.

[0102] In another example of the embodiment, the negative electrode 32 can be manufactured by simultaneously coating the negative electrode first active material layer 32b1 and the negative electrode second active material layer 32b2 and the negative electrode electronic insulating layer 32d of the negative electrode active material layer 32b, for example, as follows.

[0103] First, the materials to be contained in the first negative electrode active material layer 32b1 (e.g., negative electrode active material, conductive additive, binder, etc.) are prepared. These materials are mixed, and the resulting mixture is dispersed in a solvent (e.g., N-methyl-2-pyrrolidone (NMP) and / or water) to obtain the first negative electrode slurry. Next, the materials to be contained in the second negative electrode active material layer 32b2 (e.g., negative electrode active material, conductive additive, binder, etc.) are prepared. These materials are mixed, and the resulting mixture is dispersed in a solvent (e.g., N-methyl-2-pyrrolidone (NMP) and / or water) to obtain the second negative electrode slurry. Furthermore, the materials to be contained in the negative electrode electronic insulating layer 34d (e.g., negative electrode electronic insulating particles, binder, dispersant, etc.) are prepared. These materials are mixed, and the resulting mixture is dispersed in a solvent (e.g., N-methyl-2-pyrrolidone (NMP) and / or water) to obtain the negative electrode electronic insulating material slurry.

[0104] Next, the first negative electrode slurry, the second negative electrode slurry, and the negative electrode electronic insulating material slurry are simultaneously applied to the negative electrode current collector. This forms the first negative electrode slurry layer, the second negative electrode slurry layer, and the negative electrode electronic insulating material slurry layer. Next, the solvents contained in the first negative electrode slurry layer, the second negative electrode slurry layer, and the negative electrode electronic insulating material slurry layer are evaporated using a drying oven or the like to dry them. This forms the first negative electrode active material layer 32b1, the second negative electrode active material layer 32b2, and the negative electrode electronic insulating material slurry layer 32d on one side of the negative electrode current collector. Next, the negative electrode current collector, the first negative electrode active material layer, the second negative electrode active material layer 32b2, and the negative electrode electronic insulating material layer 32d are press-formed. Specifically, a laminate containing a negative electrode current collector, a first negative electrode active material layer 32b1, a second negative electrode active material layer 32b2, and a negative electrode electronic insulating layer 32d is sandwiched between rolls heated to 60-120°C and pressure is applied. Then, this laminate is slit to a predetermined width. This process yields the negative electrode.

[0105] In a battery 1 equipped with a negative electrode 32 manufactured using the above-described simultaneous coating method, the structure of the positive electrode 32 is shown in Figure 6. Because the interfaces of each layer have irregularities, they adhere tightly, resulting in high safety and reliability. Furthermore, if this manufacturing method is applied to the lithium-ion secondary battery of the above modification, it is not necessary to include an electrolyte, resulting in even higher safety and reliability.

[0106] Furthermore, the interface between the negative electrode active material layer 32b (negative electrode second active material layer 32b2) and the negative electrode electronic insulating layer 32d is not pressed by the roll. For example, the interface of the negative electrode electronic insulating layer 32d opposite to the negative electrode active material layer 32b is pressed by the roll. As a result, the interface between the negative electrode active material layer 32b and the negative electrode electronic insulating layer 32d has greater irregularities than the opposite interface of the negative electrode electronic insulating layer 32d. This gives the negative electrode active material layer 32b a large surface area. This is preferable for ion conduction. Also, forming greater irregularities at the interface between the negative electrode electronic insulating layer 32d and the negative electrode active material layer 32b than at the surface of the negative electrode electronic insulating layer 32d facing the roll is preferable because it provides good adhesion and stable ion conduction.

[0107] [Second Embodiment] The negative electrode for a lithium-ion secondary battery according to the second embodiment comprises a negative electrode current collector and a negative electrode active material layer laminated on the negative electrode current collector, wherein the negative electrode active material layer includes a first negative electrode active material layer laminated on the negative electrode current collector and a second negative electrode active material layer laminated on the first negative electrode active material layer, satisfying at least one of the following conditions: the first negative electrode active material layer contains a first negative electrode active material, the second negative electrode active material layer contains a second negative electrode active material, and the density of the second negative electrode active material layer is lower than the density of the first negative electrode active material layer; and the first negative electrode active material layer and the second negative electrode active material layer contain a conductive additive, and the ratio of the weight of the conductive additive to the total weight of the second negative electrode active material layer is greater than the ratio of the weight of the conductive additive to the total weight of the first negative electrode active material layer, wherein the first negative electrode active material contains a Si-based material. The Si-based material may be pre-doped with lithium. The lithium-ion secondary battery according to the second embodiment is a lithium-ion secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode is the negative electrode for the lithium-ion secondary battery according to the second embodiment.

[0108] (A battery equipped with a negative electrode as an example according to the second embodiment) The configuration of a battery with a negative electrode according to an example of the second embodiment will be described with reference to Figure 8, focusing on the differences from the configuration of a battery with a negative electrode according to an example of the first embodiment.

[0109] In an example of the negative electrode 120 according to the second embodiment, similar to the example of the negative electrode 120 according to the first embodiment, the negative electrode active material layer 122 includes a first negative electrode active material layer 123 laminated on the negative electrode current collector 121 and a second negative electrode active material layer 124 laminated on the first negative electrode active material layer 123. In other words, the negative electrode 120 comprises a plurality of active material layers.

[0110] The first active material layer 123 of the negative electrode contains the first active material 123a, as shown in Figure 8. The first active material 123a contains a lithium-predoped Si-based material 123a3. In addition to the first active material 123a, the first active material layer 123 further contains additives such as a conductive additive 123c and a binder 123b. The first active material layer 123 contains carbon nanotubes as the conductive additive 123c. The first active material layer 123 is a high-capacity layer that can store a relatively large amount of lithium ions and generally corresponds to the negative electrode active material layer used in electric vehicles (BEVs: Battery Electric Vehicles). The second active material layer 124 of the negative electrode generally corresponds to the negative electrode active material layer used in hybrid electric vehicles (HEVs: Hybrid Electric Vehicles).

[0111] The negative electrode second active material layer 124 includes the negative electrode second active material 124a, as shown in Figure 8. The negative electrode second active material 124a contains at least one selected from the group consisting of, for example, natural graphite, artificial graphite, hard carbon, soft carbon, and graphite coated with amorphous carbon. In addition to the negative electrode second active material 124a, the negative electrode second active material layer 124 further contains additives such as a conductive additive 124c and a binder 124b.

[0112] In the negative electrode active material layer 122, the density of the negative electrode second active material layer 124 is lower than the density of the negative electrode first active material layer 123. Furthermore, the ratio of the weight of the conductive additive 124c to the total weight of the negative electrode second active material layer 124 is greater than the ratio of the weight of the conductive additive 123c to the total weight of the negative electrode first active material layer 123.

[0113] Furthermore, in the negative electrode active material layer 122, the average particle size of the negative electrode second active material 124a in the negative electrode second active material layer 124 is smaller than the average particle size of the negative electrode first active material 123a in the negative electrode first active material layer 123. As a result, the BET specific surface area of ​​the negative electrode second active material 124a in the negative electrode second active material layer 124 is larger than the BET specific surface area of ​​the negative electrode first active material 123a in the negative electrode first active material layer 123.

[0114] The configuration of the battery 1 equipped with an example of a negative electrode 120 according to the second embodiment is the same as the configuration of the battery 1 equipped with an example of a negative electrode 120 according to the first embodiment, except for the points described above.

[0115] (Manufacturing method for a battery equipped with an example of a negative electrode according to the second embodiment) Furthermore, the method for manufacturing a battery comprising an example negative electrode according to the second embodiment is the same as the method for manufacturing a battery comprising an example negative electrode according to the first embodiment, except that the material comprising an example according to the second embodiment is used as the material contained in the first active material layer 123 of the negative electrode to prepare the first negative electrode slurry, and the material comprising an example according to the second embodiment is used as the material contained in the second active material layer 124 of the negative electrode to prepare the second negative electrode slurry. In the method for manufacturing a battery comprising an example negative electrode according to the second embodiment, in order to make the density of the second active material layer 124 lower than the density of the first active material layer 123 of the negative electrode, for example, a method is used in which the press pressure during the formation of the second active material layer 124 is lower than the press pressure during the formation of the first active material layer 123.

[0116] (Effects of a battery equipped with an example negative electrode according to the second embodiment) The effect of a battery equipped with an example of a negative electrode according to the second embodiment will be explained with reference to Figure 8.

[0117] In an example of the negative electrode 120 according to the second embodiment, the negative electrode active material layer 122 includes a negative electrode first active material layer 123 laminated on the negative electrode current collector 121 and a negative electrode second active material layer 124 laminated on the negative electrode first active material layer 123. The density of the negative electrode second active material layer 124 is lower than the density of the negative electrode first active material layer 123. As a result, in the negative electrode active material layer 122, the negative electrode second active material layer 124, which has good electrolyte circulation, is positioned on the separator 130 side, which is the lithium ion receiving side when the battery 1 is charged, thereby improving the rapid charging performance of the battery 1. In addition, the ratio of the weight of conductive additive 124c to the total weight of the negative electrode second active material layer 124 is greater than the ratio of the weight of conductive additive 123c to the total weight of the negative electrode first active material layer 123. Since the highly conductive negative electrode second active material layer 124 is placed on the separator 130 side, which is the lithium ion receiving side when charging battery 1, the rapid charging performance of battery 1 can be further improved.

[0118] On the other hand, if the density of the negative electrode second active material layer 124 is low, or if the weight ratio of the conductive additive 124c to the total weight of the negative electrode second active material layer 124 is high, the energy density of the battery 1 may decrease. In contrast, in the negative electrode 120, the negative electrode first active material 123a contained in the negative electrode first active material layer 123 contains lithium-predoped Si-based material 123a3. Therefore, it is possible to suppress the consumption of lithium ions contained in the positive electrode active material layer 112 of the positive electrode 110 during the first charge and discharge of the battery 1, thereby preventing them from contributing to subsequent battery reactions and suppressing a decrease in the charge and discharge capacity of the battery 1. On the other hand, since the Si-based material 123a3 expands and contracts significantly with the insertion and removal of lithium ions, the inclusion of Si-based material 123a3 in the negative electrode first active material 123a may reduce the conductivity of the negative electrode first active material layer 123, potentially causing deformation of the negative electrode 120. However, the first active material layer 123 of the negative electrode contains carbon nanotubes as a conductive additive 123c, which have a particularly high conductivity-enhancing effect, and also contains a binder 123b that binds the Si-based materials 123a3 together. Therefore, the decrease in conductivity of the first active material layer 123 of the negative electrode can be suppressed, and the deformation of the negative electrode 120 can be suppressed. Thus, the negative electrode 120 can suppress the reduction in energy density of the battery 1 and improve the cycle characteristics of the battery 1. Consequently, the negative electrode 120 can achieve improved charging performance and longer lifespan.

[0119] More specifically, the area of ​​the negative electrode active material that can come into contact with the electrolyte per unit volume in the negative electrode active material layer is relatively larger in the negative electrode second active material layer, which is a high input / output layer, compared with the negative electrode first active material layer, which is a high capacity layer. In other words, the electrolyte comes into contact with the negative electrode active material more easily in the negative electrode second active material layer than in the negative electrode first active material layer. Therefore, the charging characteristics of the battery, especially the rapid charging characteristics, can be improved in the negative electrode second active material layer, which is a high input / output layer. Here, the negative electrode first active material is, for example, silicon oxide (Li-SiO) in which lithium is pre-doped into silicon. Since silicon takes in and retains a certain amount of lithium as the battery charges, the amount of positive electrode active material in the positive electrode can be reduced by pre-doping it with lithium. The expansion and contraction of the negative electrode active material per unit volume due to charging and discharging of the battery is relatively larger in the negative electrode first active material layer, which is a high capacity layer, compared with the negative electrode second active material layer, which is a high input / output layer. In particular, Si-based materials contained in the first active material of the negative electrode exhibit greater expansion and contraction compared to carbon-based materials. However, the carbon nanotubes and binder contained in the first active material layer of the negative electrode can absorb the expansion and contraction of the first active material. Therefore, in the first active material layer, which is a high-capacity layer, the cycle durability of the first active material and the storage durability of lithium ions can be improved when the battery undergoes repeated charging and discharging.

[0120] When the ratio of the weight of the conductive additive to the total weight of the negative electrode second active material layer is greater than the ratio of the weight of the conductive additive to the total weight of the negative electrode first active material layer, the conductivity of the negative electrode second active material layer, which is a high input / output layer, is relatively greater than that of the negative electrode first active material layer, which is a high capacity layer. Therefore, the charging characteristics of the battery, especially the rapid charging characteristics, can be improved in the negative electrode second active material layer. On the other hand, since the ratio of the conductive additive in the negative electrode first active material layer is smaller than that of the negative electrode second active material layer, the reaction area is relatively smaller. Therefore, the cycle durability of the negative electrode first active material and the storage durability of lithium ions can be improved in the negative electrode first active material layer when the battery is repeatedly charged and discharged.

[0121] Furthermore, the average particle size of the negative electrode second active material 124a in the negative electrode second active material layer 124 is smaller than the average particle size of the negative electrode first active material 123a in the negative electrode first active material layer 123. As a result, the BET specific surface area of ​​the negative electrode second active material 124a in the negative electrode second active material layer 124 is larger than the BET specific surface area of ​​the negative electrode first active material 123a in the negative electrode first active material layer 123. Therefore, in the negative electrode active material layer 122, the negative electrode second active material layer 124 containing the negative electrode second active material 124a, which has a large reaction area with lithium ions, is positioned on the separator 130 side, which is the lithium ion receiving side when the battery 1 is charged, thus further improving the rapid charging performance of the battery 1.

[0122] Furthermore, in a battery 1 equipped with a negative electrode 120, if the electrolyte further contains an SEI film-forming agent, the cycle characteristics of the battery 1 can be further improved and the storage durability of the battery 1 can be further improved by suppressing the reaction between the surface of the negative electrode active material and the electrolyte.

[0123] Next, the configuration of the negative electrode for a lithium-ion secondary battery and the lithium-ion secondary battery equipped with the negative electrode according to the second embodiment will be described in more detail.

[0124] 1. Negative electrode for lithium-ion secondary batteries The negative electrode for a lithium-ion secondary battery according to the second embodiment comprises a negative electrode current collector and a negative electrode active material layer laminated on the negative electrode current collector, wherein the negative electrode active material layer includes a first negative electrode active material layer laminated on the negative electrode current collector and a second negative electrode active material layer laminated on the first negative electrode active material layer.

[0125] (1)Negative electrode first active material layer The above-mentioned negative electrode first active material layer contains a negative electrode first active material. The above-mentioned negative electrode first active material contains a Si-based (silicon-based) material as a negative electrode active material material capable of insertion and removal of lithium ions. The Si-based material may be pre-doped with lithium.

[0126] Here, lithium-predoped Si-based materials refer to negative electrode active materials in which lithium is predoped into a Si-based material. Examples of lithium-predoped Si-based materials include negative electrode active materials in which lithium is predoped into elemental Si (elemental silicon) or Si compounds (silicon compounds) such as SiO2 and SiO2.

[0127] The first negative electrode active material is not particularly limited as long as it contains a Si-based material. However, in addition to the Si-based material, the negative electrode active material may also contain at least one material selected from the group consisting of, for example, natural graphite, artificial graphite, hard carbon, soft carbon, and graphite coated with amorphous carbon. This is because the inclusion of these materials, which are more flexible than Si-based materials, can suppress damage to the negative electrode.

[0128] As the first negative electrode active material, among those containing at least one selected from the above group, it is preferable that it contains pitch-coated natural graphite and natural graphite whose surface is exposed without coating, and in particular, it is preferable that it contains pitch-coated natural graphite, natural graphite whose surface is exposed without coating, and artificial graphite.

[0129] The first active material layer of the negative electrode is not particularly limited as long as it contains the first active material of the negative electrode, but it is preferable that, in addition to the first active material of the negative electrode, it further contains at least one additive selected from the group consisting of conductive additives and binders. The conductive additive of the first active material layer of the negative electrode is the same as that of the first active material layer of the first embodiment. The binder of the first active material layer of the negative electrode is the same as that of the first active material layer of the first embodiment.

[0130] Among the negative electrode first active material layers, those containing carbon nanotubes as a conductive additive and a binder are preferred.

[0131] The ratio of the weight of the first negative electrode active material to the total weight of the first negative electrode active material layer is preferably, for example, 80% by weight or more and 99% by weight or less.

[0132] The thickness of one side of the negative electrode first active material layer in the stacking direction (for example, the depth direction Y in Figure 8) (for example, the first thickness T1 in Figure 8) is the same as that of the negative electrode first active material layer according to the first embodiment, so its explanation is omitted here.

[0133] (2)Negative electrode second active material layer The above-mentioned negative electrode second active material layer contains a negative electrode second active material. The negative electrode second active material is not particularly limited as long as it contains a negative electrode active material that allows for the insertion and removal of lithium ions, but for example, it contains at least one selected from the group consisting of natural graphite, artificial graphite, hard carbon, soft carbon, and graphite coated with amorphous carbon.

[0134] The negative electrode second active material may, for example, contain at least one selected from the above group, but it is preferable to contain pitch-coated natural graphite and natural graphite with an uncoated surface exposed, and in particular, it is preferable to contain pitch-coated natural graphite, natural graphite with an uncoated surface exposed, and artificial graphite.

[0135] The negative electrode second active material layer is not particularly limited as long as it contains the negative electrode second active material, but it is preferable that, in addition to the negative electrode second active material, it further contains at least one additive selected from the group consisting of conductive additives and binders. As the conductive additive for the negative electrode second active material layer, for example, the same as that used for the negative electrode first active material layer is used. As the binder for the negative electrode second active material layer, for example, the same as that used for the negative electrode first active material layer is used.

[0136] The ratio of the weight of the second negative electrode active material to the total weight of the second negative electrode active material layer is preferably, for example, 80% by weight or more and 99% by weight or less.

[0137] The thickness of one side of the negative electrode second active material layer in the stacking direction (for example, the depth direction Y in Figure 8) (for example, the second thickness T2 in Figure 8) is the same as that of the negative electrode second active material layer according to the first embodiment, so its explanation is omitted here.

[0138] (3) Negative electrode active material layer The negative electrode active material layer satisfies at least one of the following conditions: firstly, the density of the second negative electrode active material layer is lower than the density of the first negative electrode active material layer; and secondly, the first negative electrode active material layer and the second negative electrode active material layer contain a conductive additive, and the ratio of the weight of the conductive additive to the total weight of the second negative electrode active material layer is greater than the ratio of the weight of the conductive additive to the total weight of the first negative electrode active material layer.

[0139] The negative electrode active material layer is not particularly limited as long as it satisfies at least one of the first and second conditions described above. However, among negative electrode active material layers that satisfy the first condition, it is preferable that the proportion of voids in the second negative electrode active material layer is higher than the proportion of voids in the first negative electrode active material layer. This is because the electrolyte circulation in the second negative electrode active material layer tends to be better than in the first negative electrode active material layer.

[0140] An example of a negative electrode active material layer that satisfies the above first condition is one in which the density of the first negative electrode active material layer is 1.4 g / cm³. 3 More than 2.0g / cm 3 The following conditions apply, and the density of the negative electrode second active material layer is 1.0 g / cm³. 3 More than 1.6g / cm 3 The following are preferable: A high density in the first active material layer of the negative electrode suppresses the reaction with the electrolyte, thereby improving its lifespan. A low density in the second active material layer of the negative electrode promotes the reaction with the electrolyte, thereby improving its charging performance.

[0141] The method for calculating the void ratio between the first and second negative electrode active material layers is not particularly limited, but it can be calculated using, for example, a 3D-SEM. A set of 2D images of the first and second negative electrode active material layers in the stacked cross-section of the battery is obtained. Then, the area of ​​the voids present in the set of 2D images is calculated, and this area is integrated to calculate the volume of the voids in the 3D region. Finally, the void ratio can be calculated by calculating the volume of the voids relative to the total volume of the 3D region.

[0142] As a negative electrode active material layer that satisfies the second condition above, it is preferable that, for example, the weight ratio of the conductive additive to the total weight of the first negative electrode active material layer is 0.5% by weight or more and 10% by weight or less, and the weight ratio of the conductive additive to the total weight of the second negative electrode active material layer is 1% by weight or more and 15% by weight or less. This is because if the weight ratio is too large, the energy density will decrease, and if the weight ratio is too small, the conductivity inside the electrode will deteriorate.

[0143] As for the negative electrode active material layer, it is preferable that the BET specific surface area of ​​the second negative electrode active material is larger than the BET specific surface area of ​​the first negative electrode active material. The method for determining the BET specific surface areas of the first negative electrode active material and the second negative electrode active material, as well as the preferred ranges for the BET specific surface areas of the first negative electrode active material and the second negative electrode active material, are the same as in the first embodiment.

[0144] As for the negative electrode active material layer, among those in which the BET specific surface area of ​​the second negative electrode active material is larger than that of the first negative electrode active material, it is preferable, for example, that the average particle size of the second negative electrode active material is smaller than that of the first negative electrode active material. This is because simply making the average particle size of the second negative electrode active material smaller than that of the first negative electrode active material makes the BET specific surface area of ​​the second negative electrode active material larger than that of the first negative electrode active material, thus easily improving the rapid charging performance of the battery. The definition of average particle size, and the preferred ranges for the median diameter of the first negative electrode active material and the median diameter of the second negative electrode active material are the same as in the first embodiment.

[0145] (4) Manufacturing method As a method for manufacturing a negative electrode for a lithium-ion secondary battery according to the second embodiment, a manufacturing method may be used in which the first negative electrode active material layer and the second negative electrode active material layer of the negative electrode active material layer are formed by simultaneous coating. This manufacturing method is the same as the manufacturing method for forming the first negative electrode active material layer and the second negative electrode active material layer of the negative electrode active material layer by simultaneous coating according to the first embodiment.

[0146] (5) Negative electrode for lithium-ion secondary battery As the negative electrode for the lithium-ion secondary battery according to the second embodiment, other examples of negative electrodes may be used in which the BET specific surface area and average particle size of the active materials in the first and second negative electrode active material layers are adjusted.

[0147] In other examples of anodes, the first active material layer of the anode contains a first active material containing a Si-based material such as lithium-predoped silicon oxide (SiO), carbon nanotubes, and a binder. The second active material layer of the anode contains a carbon-based second active material. The BET specific surface area of ​​the second active material in the second active material layer is greater than that of the first active material in the first active material layer. The average particle size of the second active material in the second active material layer is smaller than that of the first active material in the first active material layer.

[0148] With this configuration, the reaction area of ​​the negative electrode active material per unit volume in the negative electrode active material layer is relatively larger in the negative electrode second active material layer, which is a high input / output layer, compared with the negative electrode first active material layer, which is a high capacity layer. Therefore, the battery's charging characteristics, especially its rapid charging characteristics, can be improved in the negative electrode second active material layer, which is a high input / output layer. On the other hand, the expansion and contraction of the negative electrode active material per unit volume due to battery charging and discharging is relatively larger in the negative electrode first active material layer, which is a high capacity layer, compared with the negative electrode second active material layer, which is a high input / output layer. In other words, silicon-based negative electrode first active material expands and contracts more than carbon-based negative electrode second active material. However, in the negative electrode first active material layer, the expansion and contraction of the negative electrode first active material can be absorbed by carbon nanotubes and a binder. Therefore, in the negative electrode first active material layer, which is a high capacity layer, the cycle durability of the negative electrode first active material and the storage durability of lithium ions can be improved when the battery is repeatedly charged and discharged.

[0149] 2. Lithium-ion rechargeable batteries The lithium-ion secondary battery according to the second embodiment is a lithium-ion secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode is a negative electrode for a lithium-ion secondary battery according to the second embodiment.

[0150] The lithium-ion secondary battery according to the second embodiment is not particularly limited, but for example, it comprises a charge / discharge body having a positive electrode, a negative electrode, and a separator, with an electrolyte impregnated into the separator. The lithium-ion secondary battery according to the second embodiment comprises an electrolyte solution in which the above electrolyte is dissolved, and the electrolyte solution may further contain additives such as an SEI film-forming agent, and among these, it is preferable that the electrolyte solution contains an SEI film-forming agent. The SEI film and SEI film-forming agent are the same as in the first embodiment.

[0151] Furthermore, the lithium-ion secondary battery according to the second embodiment may be a battery that includes a solid electrolyte as the electrolyte, and comprises a positive electrode, a negative electrode, and a solid electrolyte layer containing the solid electrolyte, wherein the solid electrolyte layer is interposed between the positive electrode and the negative electrode. Examples of such a battery with a solid electrolyte and the solid electrolyte are the same as in the first embodiment.

[0152] 3. Others Another example of a battery with a negative electrode according to the second embodiment may be a separatorless battery having a positive electrode electronic insulating layer provided on the positive electrode and a negative electrode electronic insulating layer provided on the negative electrode instead of a separator. The configuration, modifications, and manufacturing method of such a separatorless battery are the same as in the first embodiment.

[0153] The present invention includes the following embodiments. [Section 1] A positive electrode comprising a positive electrode current collector, a positive electrode mixture layer provided on the positive electrode current collector, and a positive electrode electronic insulating layer provided on the positive electrode mixture layer, A negative electrode comprising a negative electrode current collector, a negative electrode mixture layer provided on the negative electrode current collector, and a negative electrode electronic insulating layer provided on the negative electrode mixture layer, Equipped with, The surface irregularity height at the interface between the positive electrode mixture layer and the positive electrode electronic insulating layer is 2 μm or more. A lithium-ion secondary battery in which the surface irregularity height of the interface between the negative electrode mixture layer and the negative electrode electronic insulating layer is 2 μm or more. [Section 2] The lithium-ion secondary battery according to item 1, wherein the positive electrode electronic insulating layer and the negative electrode electronic insulating layer are in contact with each other. [Section 3] The lithium-ion secondary battery according to claim 1 or 2, wherein the positive electrode electronic insulating layer and the negative electrode electronic insulating layer are in contact with each other but are not fixed to each other. [Explanation of symbols]

[0154] 1 Battery (lithium-ion secondary battery), 100 Charge / discharge element, 110 Positive electrode, 111 Positive electrode current collector, 111a Current collector, 111b Positive electrode tab, 111c Side edge, 112 Positive electrode active material layer, 120 Negative electrode (negative electrode for lithium-ion secondary battery), 121 Negative electrode current collector, 121a Current collector, 121b Negative electrode tab, 121c Side edge, 122 Negative electrode active material layer, 123 Negative electrode first active material layer, 124 Negative electrode second active material layer, 130 Separator, 200 Container, 201 Case, 202 Lid, 300 External terminals, 301 Positive electrode terminal, 302 Negative electrode terminal, X Width direction of battery 1, Y Depth direction of battery 1, Z Height direction of battery 1. All publications, patents, and patent applications cited herein shall be incorporated herein by direct reference.

Claims

1. Negative electrode current collector and The negative electrode current collector comprises a negative electrode active material layer laminated thereon, The negative electrode active material layer includes a first negative electrode active material layer laminated on the negative electrode current collector and a second negative electrode active material layer laminated on the first negative electrode active material layer. The negative electrode first active material layer includes the negative electrode first active material, The negative electrode second active material layer contains the negative electrode second active material, The BET specific surface area of ​​the second negative electrode active material is greater than the BET specific surface area of ​​the first negative electrode active material. A negative electrode for a lithium-ion secondary battery, wherein at least one of the first negative electrode active material and the second negative electrode active material contains pitch-coated natural graphite and natural graphite whose surface is exposed without coating.

2. The negative electrode for a lithium-ion secondary battery according to claim 1, wherein the average particle size of the second negative electrode active material is smaller than the average particle size of the first negative electrode active material.

3. Negative electrode current collector and The negative electrode current collector comprises a negative electrode active material layer laminated thereon, The negative electrode active material layer includes a first negative electrode active material layer laminated on the negative electrode current collector and a second negative electrode active material layer laminated on the first negative electrode active material layer. The negative electrode first active material layer includes the negative electrode first active material, The negative electrode second active material layer contains the negative electrode second active material, The first condition is that the density of the second active material layer of the negative electrode is lower than the density of the first active material layer of the negative electrode, and the second condition is that the first active material layer of the negative electrode and the second active material layer of the negative electrode contain a conductive additive, and the ratio of the weight of the conductive additive to the total weight of the second active material layer of the negative electrode is greater than the ratio of the weight of the conductive additive to the total weight of the first active material layer of the negative electrode, satisfying at least one of these conditions. The negative electrode first active material contains a Si-based material, A negative electrode for a lithium-ion secondary battery, wherein at least one of the first negative electrode active material and the second negative electrode active material contains pitch-coated natural graphite and natural graphite whose surface is exposed without coating.

4. The negative electrode for a lithium-ion secondary battery according to claim 3, wherein the Si-based material contained in the first negative electrode active material is pre-doped with lithium.

5. The negative electrode for a lithium-ion secondary battery according to claim 3 or 4, wherein the negative electrode first active material layer contains carbon nanotubes as the conductive additive and contains a binder.

6. The negative electrode for a lithium-ion secondary battery according to claim 3 or 4, wherein the BET specific surface area of ​​the second negative electrode active material is greater than the BET specific surface area of ​​the first negative electrode active material.

7. The negative electrode for a lithium-ion secondary battery according to claim 6, wherein the average particle size of the second negative electrode active material is smaller than the average particle size of the first negative electrode active material.

8. A lithium-ion secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode is a lithium-ion secondary battery negative electrode according to any one of claims 1 to 4.

9. The lithium-ion secondary battery according to claim 8, comprising an electrolyte in which the aforementioned electrolyte is dissolved, wherein the electrolyte contains an SEI film-forming agent.

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