Secondary battery and method for manufacturing secondary battery

Abstract

Provided is a secondary battery using a negative electrode capable of improving cycle characteristics, increasing energy density, and improving safety of the secondary battery without providing irregularities on a negative electrode current collector. In the secondary battery, the negative electrode active material layer contains a silicon-containing compound. The negative electrode active material layer has a plurality of recessed portions recessed toward the negative electrode current collector side from a surface opposite to a surface adjacent to the negative electrode current collector, and protruding portions formed on both sides of the recessed portions. Both surfaces of the negative electrode current collector have flat surfaces without irregularities. The depth of the recess is 10% or more and less than 100% of the thickness of the negative electrode active material layer.

Description

Technical Field

[0001] The present invention relates to a secondary battery and a method for manufacturing the secondary battery. Background Art

[0002] Patent Document 1 describes a secondary battery having a silicon-containing compound in its negative electrode, and discloses a technique in which unevenness is provided on the negative electrode active material layer and the negative electrode current collector so that the unevenness overlaps.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: Japanese Patent Application Laid-Open No. 2016-85978 Summary of the Invention

[0006] Technical problem to be solved by the invention

[0007] However, since the negative electrode current collector also has irregularities, it is prone to breakage, which may reduce electron conductivity.

[0008] Furthermore, since the concavities and convexities of the negative electrode current collector are formed by rolling with a press cutter, the accuracy and regularity of the concavities and convexities are insufficient, making it difficult to control the basis weight, and possibly deteriorating the cycle characteristics.

[0009] Furthermore, when the current collector is exposed in the recessed portion, Li metal precipitation occurs at the exposed portion, and there is a possibility that the cycle characteristics may be further deteriorated.

[0010] The present disclosure has been made in view of the above circumstances, and an object of the present disclosure is to provide a secondary battery having improved energy density and cycle characteristics.

[0011] Technical solutions to technical problems

[0012] A secondary battery according to one embodiment of the present disclosure comprises: a positive electrode, a negative electrode, and a separator arranged between the positive electrode and the negative electrode, the negative electrode comprising a negative electrode collector and a negative electrode active material layer coated on the negative electrode collector, the negative electrode active material layer comprising a silicon-containing compound, the negative electrode active material layer comprising a plurality of recessed portions facing the negative electrode collector on the side opposite to the surface adjacent to the negative electrode collector, and convex portions formed on both sides of the recessed portions, the surface of the negative electrode collector corresponding to the boundary between the recessed portions and the convex portions comprising a flat surface without concave and convex portions, and the depth of the recessed portions being greater than 10% and less than 100% relative to the thickness of the negative electrode active material layer.

[0013] The manufacturing method of a secondary battery involved in other embodiments of the present disclosure is a manufacturing method of a secondary battery comprising a positive electrode, a negative electrode, and a separator arranged between the positive electrode and the negative electrode, and the process of manufacturing the negative electrode includes: a process of forming a negative electrode collector; a process of forming a negative electrode active material layer on the negative electrode collector; and a process of forming a recess on the negative electrode active material layer, wherein the recess is formed by laser ablation.

[0014] Effects of the Invention

[0015] According to the present disclosure, a secondary battery with improved energy density and cycle characteristics can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a perspective view showing the structure of a secondary battery according to an embodiment.

[0017] Figure 2 Yes Figure 1 A cross-sectional view of the structure of a battery element is shown.

[0018] Figure 3 It is an explanatory diagram for explaining the step of forming the recessed portion in the negative electrode active material layer according to the embodiment.

[0019] Figure 4 This is a cross-sectional view of a pattern in which recessed portions of a negative electrode active material layer are formed parallel to each other in one direction.

[0020] Figure 5 This is a cross-sectional view showing a negative electrode active material layer in which recessed portions are formed in a grid-like pattern in two directions.

[0021] Figure 6 This is a cross-sectional view showing a negative electrode active material layer in which recessed portions are formed in a grid-like pattern in two directions.

[0022] Figure 7 This is a cross-sectional view of a negative electrode active material layer in which recessed portions are formed in a triangular pattern in three directions.

[0023] Figure 8 This is a cross-sectional view of a pattern in which the recessed portions of the negative electrode active material layer are formed in a circular shape.

[0024] Figure 9 When recessed portions are formed on the negative electrode active material layer and the negative electrode current collector Figure 2 The region A shown is an enlarged cross-sectional view.

[0025] Figure 10 It is a cross-sectional SEM photograph showing an enlarged concave portion and a convex portion. DETAILED DESCRIPTION

[0026] Hereinafter, embodiments of the secondary battery of the present invention will be described in detail with reference to the accompanying drawings. It should be noted that the present invention is not limited to these embodiments. Each embodiment is merely an example, and it is naturally possible to partially replace or combine the structures shown in different embodiments.

[0027] (Secondary battery)

[0028] Figure 1 : is a perspective view showing the structure of a secondary battery according to an embodiment. Figure 1 As shown, the secondary battery 1 according to the embodiment includes a battery element 20, an outer film 30, a positive electrode lead 41, and a negative electrode lead 42. The secondary battery 1 according to this embodiment is a laminated film type non-aqueous electrolyte secondary battery that uses a flexible outer film 30 to house the battery element 20.

[0029] (Outer packaging film)

[0030] The outer film 30 houses the battery element 20. Figure 1 As shown, the outer film 30 comprises two separate, film-like components 30A and 30B. The components 30A and 30B overlap with each other via the battery element 20. Since the outer edges of the four sides of the outer film 30 are bonded to each other, adhesive portions are formed on the outer edges of the outer film 30. The outer film 30 has a bag-like structure capable of enclosing the battery element 20. Furthermore, the film component 30A is provided with a recessed portion 31 for accommodating the battery element 20.

[0031] Film members 30A and 30B are three-layer laminated films, each composed of a welding layer, a metal layer, and a surface protection layer stacked sequentially from the inside. When film members 30A and 30B are stacked, the outer edges of the four sides of the welding layer are welded to each other. The welding layer comprises a polymer compound, such as polypropylene. The metal layer comprises a metal material, such as aluminum. The surface protection layer comprises a polymer compound, such as nylon. It should be noted that the outer edges of the four sides of the welding layer may also be bonded to each other using an adhesive.

[0032] The structure of the outer film 30 is not particularly limited, and may be a single layer, a double layer, or four or more layers.

[0033] A sealing film 33 is inserted between the outer film 30 and the positive electrode lead 41. A sealing film 34 is inserted between the outer film 30 and the negative electrode lead 42. Sealing films 33 and 34 are components for preventing the intrusion of external air and the like into the interior of the outer film 30, and each comprises one or more polymer compounds such as polyolefins that are adhesive to the positive electrode lead 41 and the negative electrode lead 42. Examples of polyolefins include polyethylene, polypropylene, modified polyethylene, and modified polypropylene. It should be noted that either or both of sealing films 33 and 34 may be omitted.

[0034] (Battery element)

[0035] like Figure 1 As shown, the battery element 20 is housed within the outer film 30. The battery element 20 includes a positive electrode 210, a negative electrode 220, a separator 230, and a non-aqueous electrolyte (not shown). The rectangular battery element 20 has a principal surface 20A and a principal surface 20B opposite the principal surface 20A. The principal surface 20A has longitudinal sides 20C and transverse sides 20D.

[0036] The battery element 20 is a structure in which positive electrodes 210 and negative electrodes 220 are alternately stacked with separators 230 interposed therebetween. Therefore, the positive electrodes 210 and the negative electrodes 220 face each other with the separators 230 interposed therebetween.

[0037] Figure 2 It will Figure 1 The region A shown is an enlarged cross-sectional view. Figure 2 It is a cross-sectional view showing a portion of the positive electrode 210 , the negative electrode 220 , and the separator 230 in an enlarged manner.

[0038] Next, detailed materials of the positive electrode 210 , the negative electrode 220 , the separator 230 , and the non-aqueous electrolyte will be described.

[0039] (positive electrode)

[0040] like Figure 2 As shown, the positive electrode 210 includes a positive electrode current collector 211 and a positive electrode active material layer 212 provided on one or both surfaces of the positive electrode current collector 211 .

[0041] The positive electrode active material layer 212 contains a positive electrode active material. The positive electrode active material is a positive electrode material that can insert and remove lithium ions, such as lithium cobalt oxide (LCO), nickel·cobalt·manganese (NCM), lithium nickel oxide (NCA), lithium iron phosphate (LFP), LiNiO2, LiCoO2, LiCo 0.98 Al 0.01 Mg 0.01 O2、LiNi 0.5 Co 0.2 Mn0 .3O2、LiNi 0.8 Co 0.15 Al 0.05 O2、LiNi 0.33 Co 0.33 Mn 0.33 O2、Li 1.2 Mn 0.52 Co 0.175 Ni 0.1 O2、Li 1.15 (Mn 0.65 Ni 0.22 Co 0.13 )O2 and LiMn2O4. Specific examples of phosphate compounds are LiFePO4, LiMnPO4, LiFe 0.5 Mn 0.5 PO4 and LiFe 0.3 Mn 0.7 PO4, etc. Specific examples of phosphoric acid compounds include LiFePO4, LiMnPO4, LiFe 0.5 Mn 0.5 PO4 and LiFe 0.3 Mn 0.7 PO 4 and other compounds. In addition, the positive electrode active material layer 212 may be housed in a shell-shaped positive electrode current collector 211, for example.

[0042] Furthermore, the positive electrode active material layer 212 may have a conductive additive such as carbon on the surface of the positive electrode active material layer 212 in order to improve the conductivity of the positive electrode active material.

[0043] The conductive additive used in the positive electrode active material layer 212 includes carbon materials such as acetylene black (AB), carbon black, carbon nanotubes (CNT), and carbon nanofibers (CNF). The conductive additive is not limited to a single type; a mixture of multiple conductive materials may be used. It should be noted that any conductive additive can be a metal or a conductive polymer, as long as it is conductive.

[0044] Furthermore, the positive electrode active material layer 212 may have a binding material (hereinafter referred to as a binder) on the surface of the positive electrode active material layer 212 in order to improve the adhesion of the positive electrode active material.

[0045] The binder for the positive electrode active material layer 212 may be polyvinylidene fluoride (PVDF), carboxymethyl cellulose sodium (CMC), or styrene-butadiene rubber (SBR). However, the binder is not limited to these materials; any binder containing one or more of a synthetic rubber and a polymer compound may be used.

[0046] The content of the conductive additive is preferably 1 wt % to 10 wt % inclusive relative to the total amount of the positive electrode active material layer 212. The content of the binder is preferably 1 wt % to 10 wt % inclusive relative to the total amount of the positive electrode active material layer 212.

[0047] (negative electrode)

[0048] like Figure 2 As shown, the negative electrode 220 includes a negative electrode current collector 221 and a negative electrode active material layer 222 provided on one or both surfaces of the negative electrode current collector 221. The negative electrode 220 is an electrode having a lower potential than the positive electrode 210.

[0049] The negative electrode active material layer 222 contains a negative electrode active material. For example, the negative electrode active material includes a carbon material such as graphite. More specifically, the carbon material used in the negative electrode active material is at least one of graphitizable carbon, non-graphitizable carbon, and graphite (natural graphite and artificial graphite). In particular, the negative electrode active material contains a silicon-containing compound, which increases the energy density of the secondary battery.

[0050] Furthermore, the negative electrode active material layer 222 may have a conductive additive such as carbon on the surface of the negative electrode active material layer 222 in order to improve the conductivity of the negative electrode active material.

[0051] The conductive agent in the negative electrode active material layer 222 is the same material as that in the positive electrode active material layer 212. The conductive agent in the negative electrode active material layer 222 may be the same material as that in the positive electrode active material layer 212 or a different material.

[0052] The negative electrode active material layer 222 may have a binding material (hereinafter referred to as a binder) on the surface of the negative electrode active material layer 222 in order to improve the adhesion of the negative electrode active material.

[0053] The binder of the negative electrode active material layer 222 is the same material as the binder contained in the positive electrode active material layer 212. The binder contained in the negative electrode active material layer 222 may be the same material as the binder contained in the positive electrode active material layer 212, or a different material.

[0054] The content of the conductive additive is preferably 1 wt % to 10 wt % inclusive relative to the total amount of the negative electrode active material layer 222 . The content of the binder is preferably 1 wt % to 10 wt % inclusive relative to the total amount of the negative electrode active material layer 222 .

[0055] (diaphragm)

[0056] The separator 230 separates the positive electrode 210 and the negative electrode 220, and allows lithium ions to pass through while preventing short circuits caused by contact between the two electrodes. Figure 2 In the illustrated example, the separator 230 is provided between the positive electrode active material layer 212 of the positive electrode 210 and the negative electrode active material layer 222 of the negative electrode 220 .

[0057] The separator 230 is formed of a film containing a polyolefin-based polymer compound such as polypropylene (PP) or polyethylene (PE). However, the separator 230 is not limited thereto and may be formed of a porous film formed of other resin materials.

[0058] (Non-aqueous electrolyte)

[0059] The non-aqueous electrolyte solution is impregnated in each of the positive electrode 210 , the negative electrode 220 , and the separator 230 , and contains a solvent and an electrolyte salt (lithium salt). The non-aqueous electrolyte solution may also contain additives and the like as needed.

[0060] (Solvent)

[0061] The non-aqueous electrolyte solution contains one or more non-aqueous solvents (organic solvents). An electrolyte solution containing a non-aqueous solvent is called a non-aqueous electrolyte solution. Non-aqueous solvents include esters and ethers. More specifically, they include carbonate compounds, carboxylate compounds, and lactone compounds.

[0062] Carbonate compounds include cyclic carbonates and chain carbonates, etc. Examples of cyclic carbonates include ethylene carbonate and propylene carbonate, etc. Examples of chain carbonates include dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0063] Carboxylate compounds include chain carboxylates. Examples of chain carboxylates include methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl trimethylacetate, ethyl trimethylacetate, methyl butyrate, and ethyl butyrate. Chain carboxylates preferably have a boiling point of 100°C or higher and a viscosity of 0.9 mPa·s or lower at 25°C.

[0064] The nonaqueous solvent can comprise one or both parties in chain carboxylate and cyclic carbonate (cyclic carbonate compound) and chain carbonate (chain carbonate compound). In the present embodiment, the nonaqueous solvent comprises cyclic carbonate compound and chain carboxylate at least. The kind of cyclic carbonate can be only one, or can be two or more. Similarly, the kind of chain carbonate can be only one, or can be two or more.

[0065] The non-aqueous electrolyte solution may contain a solvent other than the solvent. Examples of lactone compounds include lactones, etc. Specific examples of lactones include γ-butyrolactone and γ-valerolactone.

[0066] It should be noted that ethers may be compounds in which a portion of the ether is fluorinated. Examples of ethers include 1,2-dimethoxyethane, tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane, and 1,1,2-tetrafluoroethyl-2,2,2,3,3-tetrafluoropropyl ether.

[0067] (Electrolyte Salt)

[0068] In addition to the solvent, the electrolyte solution may also contain an electrolyte salt. The electrolyte salt includes a light metal salt such as a lithium salt. The electrolyte salt includes any one or more of the lithium salts. Lithium salts include, for example, lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(fluorosulfonyl)imide (LiN(FSO2)2), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF3SO2)2), lithium tris(trifluoromethanesulfonyl)methylide (LiC(CF3SO2)3), lithium bis(oxalato)borate (LiB(C2O4)2), lithium monofluorophosphate (Li2PFO3), and lithium difluorophosphate (LiPF2O2). The content of the electrolyte salt (LiFSI) is, for example, greater than or equal to 0.8 mol / kg and less than or equal to 1.2 mol / kg relative to the non-aqueous solvent. More preferably, the content of the electrolyte salt (LiFSI) is 0.9 mol / kg or more and 1.2 mol / kg or less.

[0069] (additive)

[0070] The non-aqueous electrolyte contains additives. The non-aqueous electrolyte may also contain any one or more of the additives. This is because the electrochemical stability of the electrolyte is improved, and therefore, in a lithium-ion secondary battery using the electrolyte, the decomposition reaction of the electrolyte is suppressed. The additives are not particularly limited, and examples include unsaturated cyclic carbonates, fluorinated cyclic carbonates, sulfonic acid esters, phosphates, acid anhydrides, and isocyanate compounds.

[0071] Specific examples of unsaturated cyclic carbonates include vinylene carbonate, vinyl ethylene carbonate, and methylene ethylene carbonate. Specific examples of fluorinated cyclic carbonates include monofluoroethylene carbonate and difluoroethylene carbonate. Specific examples of sulfonic acid esters include propane sultone and propene sultone. Specific examples of phosphates include trimethyl phosphate and triethyl phosphate. Specific examples of acid anhydrides include succinic anhydride, 1,2-ethanedisulfonic anhydride, and 2-sulfobenzoic anhydride. Specific examples of isocyanate compounds include hexamethylene diisocyanate.

[0072] (Method for Manufacturing Secondary Battery)

[0073] Next, a description will be given of a method for manufacturing the secondary battery 100 according to Embodiment 1. In the method for manufacturing the secondary battery 100, after the positive electrode 210 and the negative electrode 220 are manufactured, the secondary battery 100 is manufactured using the positive electrode 210, the negative electrode 220, and the electrolyte.

[0074] (Positive Electrode Manufacturing Process)

[0075] First, the positive electrode current collector 211 is formed of a conductive material such as aluminum (Al), but is not limited to aluminum and may be made of other conductive materials such as nickel or stainless steel.

[0076] First, 97.5 wt% of a positive electrode active material (e.g., LCO), 1.0 wt% of a conductive additive, and 1.5 wt% of a binder are mixed and dispersed in a dispersant (e.g., N-methyl-2-pyrrolidone (NMP)) to prepare a paste-like positive electrode mixture slurry.

[0077] Next, the positive electrode mixture slurry is applied to both sides of the positive electrode current collector 211 and dried, and then compression-molded using a roll press to form positive electrode active material layers 212 on both sides of the positive electrode current collector 211, thereby manufacturing the positive electrode 210. It should be noted that the method for manufacturing the positive electrode 210 is not limited to the coating method, and for example, a vapor phase method, a liquid phase method, a thermal spraying method, a firing method (sintering method), or a combination of these methods may also be used.

[0078] (Negative Electrode Manufacturing Process)

[0079] Next, a description will be given of a process for manufacturing the negative electrode 220 according to the embodiment. Figure 3 1 is an explanatory diagram for explaining the process of forming a recessed portion on the negative electrode active material layer according to the embodiment. Figure 3 As shown, the manufacturing process of the negative electrode 220 includes the steps of forming a negative electrode current collector 221, forming a negative electrode active material layer 222 on the negative electrode current collector 221, and forming a recessed portion 223 in the negative electrode active material layer 222. Each step will be described in detail below.

[0080] First, the negative electrode current collector 221 is formed of a conductive material such as copper (Cu) (step ST101 ). The material of the negative electrode current collector 221 is not limited to copper, and may be other conductive materials such as nickel or stainless steel.

[0081] Next, 95.0 wt% of the negative electrode active material (e.g., a mixture of artificial graphite and Si oxide in a ratio of 7:3), 1.0 wt% of a conductive additive, and 4.0 wt% of a binder are mixed and dispersed in water to produce a paste-like negative electrode mixture slurry. The negative electrode mixture slurry is applied to both surfaces of the negative electrode current collector 221 (e.g., Cu), dried, and then compression-molded using a roller press to produce the negative electrode 220 (step ST102). It should be noted that the method for producing the negative electrode 220 is not limited to coating methods; for example, a vapor phase method, a liquid phase method, thermal spraying, a firing method (sintering method), or a combination of these methods can be used.

[0082] Next, after step ST102, a plurality of recesses 223 are formed in the negative electrode active material layer 222, with the surface opposite the surface adjacent to the negative electrode current collector 221 being recessed toward the negative electrode current collector 221 (step ST103). In this case, protrusions 224 of the negative electrode active material layer are formed on both sides of the recesses 223. Because the processing to form the recesses 223 does not affect both surfaces of the negative electrode current collector 221, the surface of the negative electrode current collector 221 corresponding to the boundary between the recesses 223 and the protrusions 224 is flat and free of unevenness. This ensures that there is no height difference between the surface of the negative electrode current collector 221 at locations overlapping the recesses 223 and the surface of the negative electrode current collector 221 at locations overlapping the protrusions 224.

[0083] Laser ablation is used as a processing method for the recess 223. Through laser ablation, laser photons cut the interatomic bonds in the negative electrode active material layer, thereby removing only the portion where the laser light is absorbed, and forming a recess with minimal thermal impact and sharp edges. The removed material is heated by the absorbed laser energy, evaporating or sublimating. The processing of forming the recess 223 through laser ablation does not affect both surfaces of the negative electrode current collector 221, and the surface of the negative electrode current collector 221 corresponding to the boundary between the recess 223 and the protrusion 224 becomes a flat surface without unevenness.

[0084] As the laser used, IR laser, excimer laser, YAG laser, CO2 laser, etc. can be used, but the laser is not limited thereto, and a plurality of lasers may be selected.

[0085] Laser ablation can adjust the mode, wavelength and output of the laser to adjust the width, depth or pattern of the recess.

[0086] In step ST102, the following may be formed: Figures 4 to 8 The formation pattern of the concave portion 223 of the negative electrode active material layer shown in any one of the figures. Figure 4 This is a cross-sectional view of a pattern in which recessed portions of a negative electrode active material layer are formed parallel to each other in one direction. Figure 5 This is a cross-sectional view showing a negative electrode active material layer in which recessed portions are formed in a grid-like pattern in two directions. Figure 6 This is a cross-sectional view showing a negative electrode active material layer in which recessed portions are formed in a grid-like pattern in two directions. Figure 7 This is a cross-sectional view of a negative electrode active material layer in which recessed portions are formed in a triangular pattern in three directions. Figure 8 This is a cross-sectional view of a pattern in which the recessed portions of the negative electrode active material layer are formed in a circular shape.

[0087] like Figure 4 As shown, the grooves of adjacent recesses 223 are formed in parallel in the same direction at equal intervals. Figure 5 As shown, the grooves of the adjacent recesses 223 are formed in a plurality of intersecting manner at equal intervals. The portion surrounded by the recesses 223 is square in a plan view. Figure 6 As shown, the grooves of the adjacent recesses 223 are formed in a plurality of intersecting manner at equal intervals. The portion of the recess 223 surrounded by the grooves is diamond-shaped when viewed from above. Figure 7 As shown, the grooves of the adjacent recesses 223 are formed in a plurality of intersecting manner at equal intervals. The portion surrounded by the recesses 223 is triangular in a plan view. Figure 8 As shown, a plurality of grooves of the adjacent recessed portions 223 are formed at equal intervals. The grooves of the recessed portions 223 are circular in a plan view.

[0088] (Assembly of secondary batteries)

[0089] First, the positive electrodes 210 and negative electrodes 220 are alternately stacked with separators 230 interposed therebetween to form a laminate. The laminate has the same structure as the battery element 20, except that none of the positive electrodes 210, negative electrodes 220, and separators 230 are impregnated with electrolyte. Next, the positive electrode lead 41 is connected to the positive electrode current collector 211. Separately, the negative electrode lead 42 is connected to the negative electrode current collector 221. The joining and connecting methods are not particularly limited and may be any one, or two or more, of ultrasonic welding, resistance welding, and soldering.

[0090] After the laminate is housed within the recessed portion 31, the film members 30A and 30B are stacked with the laminate interposed therebetween. The outer peripheral edges of the three sides of each of the film members 30A and 30B, excluding one side, are then bonded together, thereby housing the laminate within the bag-shaped outer film 30. The method for bonding the film members 30A and 30B is not particularly limited; either heat fusion or adhesive may be used.

[0091] After the electrolyte is injected into the interior of the bag-shaped outer film 30, the outer peripheral edge portions of the remaining sides of each of the film members 30A and 30B are bonded together, thereby sealing the outer film 30. In this case, the sealing film 33 is inserted between the outer film 30 and the positive electrode lead 41, and the sealing film 34 is inserted between the outer film 30 and the negative electrode lead 42. Thus, the electrolyte is impregnated into the laminate, thereby producing the battery element 20. The positive electrode lead 41 and the negative electrode lead 42 are led out from the interior of the outer film 30, while the battery element 20 is housed within the outer film 30. Thus, the battery element 20 is sealed within the outer film 30. Thus, the secondary battery 10 is produced.

[0092] As described above, both surfaces of the negative electrode current collector 221 are flat, without any unevenness. The negative electrode active material layer 222 has a recessed portion 223 on the side opposite to the surface adjacent to the negative electrode current collector 221, which is recessed toward the negative electrode current collector 221. Because the recessed portion 223 does not affect the surface of the negative electrode current collector 221, wrinkles are not formed on the surface of the negative electrode current collector 221. As a result, the expansion of the negative electrode active material is absorbed by the recessed portion 223, suppressing expansion in the thickness direction. This increases the energy density and improves the cycle characteristics.

[0093] The depth of the recess 223 is greater than or equal to 10% and less than 100% of the thickness of the negative electrode active material layer 222. The deeper the recess 223 formed on one surface of the negative electrode active material layer 222, the easier it is to absorb the expansion of the negative electrode active material. Therefore, when the recess 223 is present only on the surface of the negative electrode active material layer, and the depth of the recess 223 is greater than or equal to 10% and less than 100% of the thickness of the negative electrode active material layer 222, the cycle retention rate can be improved compared to a case where no recess 223 is provided on the surface of the negative electrode active material layer 222.

[0094] Preferably, the ratio of oxygen to the constituent elements on the surface of the concave portion 223 is greater than the ratio of oxygen to the constituent elements on the surface of the convex portion 224. Forming the concave portion 223 on one surface of the negative electrode active material layer 222 creates a difference in oxygen ratio between the surface of the concave portion 223 and the surface of the convex portion 224 of the negative electrode active material layer 222. Since an oxygen-rich layer is formed on the surface of the concave portion 223 of the negative electrode active material layer 222 compared to the surface of the convex portion 224, the cycle retention rate can be improved compared to a case where the surface of the negative electrode active material layer 222 is not provided with the concave portion 223.

[0095] Preferably, the width of the recess 223 after full charge is greater than or equal to 0.01 μm and less than or equal to 20 μm. The larger the width of the recess after full charge, the lower the energy density. However, when the width is greater than or equal to 0.01 μm and less than or equal to 20 μm, the width is not too large, so the energy density can be increased. Here, full charge means that when the operating voltage range of the secondary battery is specified so that the potential of the negative electrode 220 is less than or equal to 0.1 V based on lithium metal, charging is performed within this range until the charge state of the secondary battery reaches 100%.

[0096] Preferably, the recesses 223 are formed by laser ablation. Laser ablation allows the recesses 223 to be formed uniformly across the entire surface of the negative electrode active material layer 222, compared to methods such as rolling. This can increase energy density and improve cycle characteristics. It should be noted that the oxygen ratio in the oxygen-rich layer on the surface of the recesses 223 of the negative electrode active material layer 222 can be appropriately varied depending on the laser processing conditions, laser pulse width, laser output, and processing time.

[0097] A preferred embodiment further includes a step of charging while pressurizing the entire secondary battery 1. By charging while pressurizing the entire secondary battery, the recessed portion can absorb the expansion of the negative electrode active material layer 222, thereby increasing energy density and improving cycle characteristics.

[0098] (evaluate)

[0099] The evaluation tests described below were performed on Examples 1 to 7 and Comparative Examples 1 to 4.

[0100] [Capacity evaluation test]

[0101] Secondary battery 1 manufactured using the aforementioned secondary battery manufacturing method was subjected to constant current charging at 0.1 C, the designed capacity, to 4.4 V while being pressurized in the thickness direction of the secondary battery. This was followed by constant voltage charging at 4.40 V to 0.025 C. The initial capacity was then measured by constant current discharge at 0.1 C to 3.0 V. The pressurization conditions can be, for example, 1.0 MPa.

[0102] [Energy density evaluation test]

[0103] After charging, the thickness of the secondary battery was measured using a μ meter. The volume was calculated from the area of ​​the negative electrode 220 and the thickness of the secondary battery 1. The energy density was calculated by dividing the discharge capacity calculated in the above-mentioned capacity evaluation test by the calculated volume.

[0104] [Cyclic evaluation test]

[0105] The battery was charged at a constant current of 0.5C to 4.4V at the designed capacity, then charged at a constant voltage of 4.40V to 0.025C, and discharged at a constant current of 0.5C to 3.0V. This cycle was repeated for 100 cycles. The retention rate at 100 cycles was calculated.

[0106] [Composition evaluation]

[0107] As a measuring apparatus, Hitachi FE-SEM S-4800 was used. Figure 10 This is a cross-sectional SEM photograph showing the magnified concave and convex parts of the negative electrode active material. Figure 10 As shown, the composition of the bottom surface of the concave portion 223 and the surface of the convex portion 224 before charge and discharge was evaluated using a scanning electron microscope (SEM). The oxygen concentration was calculated based on the constituent elements other than the metal of the negative electrode current collector 221. The oxygen concentration in the concave portion 223 was designated as X1, and the oxygen concentration in the convex portion 224 was designated as X2.

[0108] [Electrode surface observation]

[0109] As a measuring device, Hitachi FE-SEM S-4800 was used. Constant current charging was performed at 0.1C of the designed capacity to 4.4V, and then constant voltage charging was performed at 4.40V to 0.025C. This state was considered to be fully charged, and the charged secondary battery was disassembled to observe the negative electrode surface and measure the width of the concave portion 223. Figure 10 As shown, the width of the recess 223 is set to W1.

[0110] [Electrode breaking strength evaluation test]

[0111] A Series 5560 manufactured by INSTRON was used as the measuring apparatus. A tensile strength test was conducted on the negative electrode 220, and the fracture strain was measured from the stress-strain curve. The fracture strain is the strain required to fracture under increased stress. Therefore, a larger value indicates a difficulty in fracture, while a smaller value indicates an increased likelihood of fracture.

[0112] (Example 1)

[0113] The formation pattern of the recessed portions 223 in Example 1 is a pattern in which the recessed portions of the negative electrode active material layer are formed in a grid pattern in two directions (see Figure 5 In Example 1, recess 223 was formed by laser ablation. The depth of recess 223 was 10% of the thickness of negative electrode active material layer 222. The width W1 of recess 223 was 20 μm. Note that laser ablation is referred to as "laser" in the "Fabrication Method" column of Table 1.

[0114] (Example 2)

[0115] In Example 2, a secondary battery 1 was manufactured in the same manner as in Example 1, except that the depth of the recess 223 was 50% of the thickness of the negative electrode active material layer 222 , and the above-mentioned various evaluation tests were performed.

[0116] (Example 3)

[0117] In Example 3, a secondary battery 1 was manufactured in the same manner as in Example 1 except that the depth of the recessed portion 223 was 90% of the thickness of the negative electrode active material layer 222 , and the aforementioned various evaluation tests were performed.

[0118] (Example 4)

[0119] In Example 4, a secondary battery 1 was manufactured in the same manner as in Example 1, except that the oxygen concentration was set to X1=X2 and the recess 223 was processed by rolling using a mesh. The various evaluation tests described above were performed. The results are shown in Table 1. A mesh is a processing tool that forms a grid pattern of stainless steel wire. It should be noted that rolling using a mesh is referred to as rolling in the "Production Method" column of Table 1.

[0120] (Example 5)

[0121] In Example 5, a secondary battery 1 was manufactured in the same manner as in Example 1, except that the width W1 of the recess 223 was 40 μm and the depth of the recess 223 was 90% of the thickness of the negative electrode active material layer 222 . The above-mentioned various evaluation tests were performed. The obtained results are shown in Table 1.

[0122] (Example 6)

[0123] In Example 6, a secondary battery 1 was manufactured in the same manner as in Example 1, except that the width W1 of the recess 223 was 10 μm and the depth of the recess 223 was 90% of the thickness of the negative electrode active material layer 222 . The above-mentioned various evaluation tests were performed. The obtained results are shown in Table 1.

[0124] (Example 7)

[0125] In Example 7, a secondary battery 1 was manufactured in the same manner as in Example 1, except that the width W1 of the recess 223 was 0.01 μm and the depth of the recess 223 was 90% of the thickness of the negative electrode active material layer 222 . The above-mentioned various evaluation tests were performed. The obtained results are shown in Table 1.

[0126] (Comparative Example 1)

[0127] In Comparative Example 1, a secondary battery 1 was manufactured in the same manner as in Example 1, except that the recess 223 was not formed, and the various evaluation tests described above were performed. The results are shown in Table 1. Note that the case where the recess 223 was not formed is indicated as "no processing" in the "Fabrication Method" column in Table 1.

[0128] (Comparative Example 2)

[0129] In Comparative Example 2, a secondary battery 1 was manufactured in the same manner as in Example 1, except that a recess 225 was provided in the negative electrode current collector 221, the depth of the recess 223 was 90% of the thickness of the negative electrode active material layer 222, and the recesses 223 and 225 were processed by rolling using a mesh. The aforementioned various evaluation tests were performed. The results are shown in Table 1. Figure 9 When recessed portions are formed on the negative electrode active material layer and the negative electrode current collector Figure 2 The cross-sectional view of the area A shown is enlarged. Figure 9 As shown, if the process for forming the concave portion 223 is rolling, the surface of the negative electrode current collector 221 is easily affected, and a height difference is generated between the surface of the negative electrode current collector 221 at the position overlapping with the concave portion 223 and the surface of the negative electrode current collector 221 at the position overlapping with the convex portion 224, thereby forming a concave portion 225. Figure 9 As shown, by forming the concavo-convex portions on the negative electrode current collector 221 , the negative electrode current collector 221 is easily broken.

[0130] (Comparative Example 3)

[0131] In Comparative Example 3, a secondary battery 1 was manufactured in the same manner as in Example 1, except that the recesses 225 were provided only in the negative electrode current collector 221 and the recesses 225 were formed by rolling using a mesh. The above-mentioned various evaluation tests were performed. The obtained results are shown in Table 1.

[0132] (Comparative Example 4)

[0133] In Comparative Example 4, a secondary battery 1 was manufactured in the same manner as in Example 1, except that the depth of the recess 223 was 5% of the thickness of the negative electrode active material layer 222 , and the aforementioned various evaluation tests were performed.

[0134] (Comparative Example 5)

[0135] In Comparative Example 5, a secondary battery 1 was manufactured in the same manner as in Example 1, except that the depth of the recess 223 was 100% of the thickness of the negative electrode active material layer 222 , and the above-mentioned various evaluation tests were performed.

[0136] [Table 1]

[0137]

[0138] It should be noted that the above-mentioned embodiment is for easy understanding of the present invention and is not intended to limit the interpretation of the present invention. The present invention can be changed / improved without departing from its main purpose, and its equivalent is also included in the present invention. In addition, taking into account deviation, the depth and width of the recess are represented by average value.

[0139] The results of electrode surface observation indicated that recesses were observed as A, and that no recesses were observed (flat) as B. The A rating was indicated by “◯” in Table 1, and the B rating was indicated by “×” in Table 1.

[0140] Comparing Example 1 and Example 4, in the case of Example 4, X1=X2, but since rolling is performed using a mesh as a method for forming the recess on the negative electrode current collector, the oxygen ratio remains unchanged and is 1.0. In the case of Example 1, X1>X2, but X1 is, for example, 16.2%, and X2 is, for example, 11.5%. The ratio of oxygen concentration is 1.4. This is because, in Example 1, the recess 223 is formed by laser ablation, and therefore, by the thermal history of the laser remaining on the bottom surface of the recess, an oxygen-rich layer can be formed on the bottom surface of the recess 223, and the oxygen concentration of the bottom surface of the recess 223 is higher than that of the surface of the convex portion 224. As a result, the cycle retention rate is improved.

[0141] The depths of the recesses 223 in Examples 1, 2, and 3 were 10%, 50%, and 90%, respectively, relative to the thickness of the negative electrode active material layer 222 in which the recesses 223 were provided. Compared to Comparative Example 1, the energy density ratio was increased and the cycle retention rate was improved. Note that the energy density ratio is expressed as the E density ratio in Table 1.

[0142] On the other hand, in Comparative Example 4, the depth of the recess 223 is 5% relative to the thickness of the negative electrode active material layer 222. This results in a lower energy density ratio than in Example 1. This is because shallow recesses of approximately 5% are not very effective in suppressing expansion caused by charge and discharge cycles.

[0143] In Comparative Example 5, the depth of the recessed portion 223 is 100% of the thickness of the negative electrode active material layer 222. Compared with Comparative Example 1, the energy density ratio is lowered and the cycle retention rate is poor.

[0144] Therefore, by setting the depth of the recess 223 to be 10% or more and less than 100% of the thickness of the negative electrode active material layer 222 , the energy density can be increased and the cycle characteristics can be improved.

[0145] In Comparative Example 2, a mesh was used to roll the concave portions 225 in the negative electrode current collector 221. Consequently, the regularity and precision of the concave and convex portions were insufficient compared to Comparative Example 1, resulting in a lower energy density ratio and a poorer cycle retention rate. Furthermore, the presence of the concave and convex portions in the collector foil resulted in a low fracture strain, making the collector foil of the negative electrode current collector 221 susceptible to fracture.

[0146] In Comparative Example 3, a mesh was used to roll the recessed portions 225 in the negative electrode current collector 221, resulting in a lower energy density ratio and a poorer cycle retention rate than in Comparative Example 1. Furthermore, the presence of recesses and protrusions on the collector foil reduced the fracture strain, making the collector foil of the negative electrode current collector 221 more susceptible to fracture.

[0147] The width W1 of the recess 223 in Examples 3, 6, and 7 is 20 μm, 10 μm, and 0.01 μm, respectively. Compared with Comparative Example 1, the energy density ratio is improved and the cycle retention rate is improved.

[0148] On the other hand, the width W1 of the recessed portion 223 of Example 5 is 40 μm. Compared with Comparative Example 1, the energy density ratio is lowered and the cycle retention rate is deteriorated.

[0149] Therefore, by setting the width W1 of the recess 223 to be greater than or equal to 0.01 μm and less than or equal to 20 μm, the energy density can be increased and the cycle characteristics can be improved.

[0150] It should be noted that the present disclosure may also be a combination of the following structures.

[0151] (1) A secondary battery comprising a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode.

[0152] The negative electrode includes a negative electrode current collector and a negative electrode active material layer coated on the negative electrode current collector.

[0153] The negative electrode active material layer contains a silicon-containing compound.

[0154] The negative electrode active material layer has a plurality of recessed portions that are recessed toward the negative electrode current collector on the side opposite to the surface adjacent to the negative electrode current collector, and convex portions formed on both sides of the recessed portions.

[0155] The surface of the negative electrode current collector corresponding to the boundary between the concave portion and the convex portion has a flat surface without concavities and convexities.

[0156] The depth of the recess is not less than 10% and less than 100% of the thickness of the negative electrode active material layer.

[0157] (2) In the secondary battery according to (1), a ratio of oxygen in the constituent elements of the concave portion is greater than a ratio of oxygen in the constituent elements of the convex portion.

[0158] (3) In the secondary battery according to (1) or (2), a width of the recessed portion when fully charged is 0.01 μm or more and 20 μm or less.

[0159] (4) A method for manufacturing a secondary battery comprising a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode.

[0160] The process of manufacturing the negative electrode includes:

[0161] a step of forming a negative electrode current collector;

[0162] forming a negative electrode active material layer on the negative electrode current collector; and

[0163] forming a concave portion on the negative electrode active material layer;

[0164] The recess is formed by laser ablation.

[0165] (5) The method for manufacturing a secondary battery according to (4), further comprising the step of assembling the negative electrode manufactured in the step of manufacturing the negative electrode, the positive electrode, and the separator, and charging the secondary battery while pressurizing the entire secondary battery impregnated with the electrolyte in the thickness direction.

[0166] (6) The method for manufacturing a secondary battery according to (4) or (5), wherein the ratio of oxygen in the constituent elements of the concave portion is greater than the ratio of oxygen in the constituent elements of the convex portions formed on both sides of the concave portion.

[0167] (7) The method for manufacturing a secondary battery according to any one of (4) to (6), wherein a width of the recessed portion when fully charged is 0.01 μm or more and 20 μm or less.

[0168] (8) The method for manufacturing a secondary battery according to any one of (4) to (7), wherein the depth of the recessed portion is 10% or more and less than 100% of the thickness of the negative electrode active material layer.

[0169] Description of Reference Numerals

[0170] 1: secondary battery; 210: positive electrode; 211: positive electrode current collector; 212: positive electrode active material layer; 220: negative electrode; 221: negative electrode current collector; 222: negative electrode active material layer; 223: concave portion; 224: convex portion; 225: concave portion; 230: separator; W1: width of the concave portion.

Claims

1. A secondary battery comprising a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode. The negative electrode includes a negative electrode current collector and a negative electrode active material layer coated on the negative electrode current collector. The negative electrode active material layer contains a silicon-containing compound. The negative electrode active material layer has a plurality of recessed portions that are recessed toward the negative electrode current collector on the side opposite to the surface adjacent to the negative electrode current collector, and convex portions formed on both sides of the recessed portions. The surface of the negative electrode current collector corresponding to the boundary between the concave portion and the convex portion has a flat surface without concavities and convexities. The depth of the recess is not less than 10% and less than 100% of the thickness of the negative electrode active material layer.

2. The secondary battery according to claim 1, wherein The ratio of oxygen in the constituent elements of the concave portion is greater than the ratio of oxygen in the constituent elements of the convex portion.

3. The secondary battery according to claim 1 or 2, wherein The width of the recessed portion when fully charged is 0.01 μm or more and 20 μm or less.

4. A method for manufacturing a secondary battery comprising a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, The process of manufacturing the negative electrode includes: a step of forming a negative electrode current collector; forming a negative electrode active material layer on the negative electrode current collector; and forming a concave portion on the negative electrode active material layer; The recess is formed by laser ablation.

5. The method for manufacturing a secondary battery according to claim 4, wherein: The secondary battery manufacturing method further includes assembling the negative electrode manufactured in the negative electrode manufacturing step, the positive electrode, and the separator, and charging the secondary battery while pressurizing the entire secondary battery impregnated with the electrolyte in a thickness direction.

6. The method for manufacturing a secondary battery according to claim 4 or 5, wherein: The ratio of oxygen in the constituent elements of the concave portion is higher than the ratio of oxygen in the constituent elements of the convex portions formed on both sides of the concave portion.

7. The method for manufacturing a secondary battery according to any one of claims 4 to 6, wherein: The width of the recessed portion when fully charged is 0.01 μm or more and 20 μm or less.

8. The method for manufacturing a secondary battery according to any one of claims 4 to 7, wherein: The depth of the recess is not less than 10% and less than 100% of the thickness of the negative electrode active material layer.

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