Secondary battery electrode and secondary battery

Abstract

A secondary battery electrode comprises: a sheet-shaped electrode current collector; an electrode mixture layer that is supported on a main surface of the electrode current collector, and contains an electrode active material; a first coating that covers at least some of an end surface of the electrode mixture layer; and a second coating that covers at least some of the surface of the first coating. The first coating contains a constituent element of the electrode active material, and the second coating contains carbon.

Description

Electrode for secondary battery and secondary battery Cross-reference to related applications

[0001] This disclosure claims the benefit of priority with respect to Japanese Patent Application No. 2024-167821, filed on September 26, 2024, with the Japan Patent Office, and the entire contents of the said patent application are incorporated herein by reference.

[0002] This disclosure relates to an electrode for a secondary battery and a secondary battery.

[0003] A secondary battery includes a pair of electrodes and an electrolyte. At least one of the pair of electrodes includes a sheet-like current collector and an electrode mixture layer supported on the main surface of the current collector. The electrode mixture layer contains an electrode active material that occludes and releases lithium ions.

[0004] Patent Document 1 proposes "an electrode for a battery characterized in that an active material is coated on the surface of a current collector and an insulating material or an electrode active material mixture is coated on the peripheral portion of the current collector."

[0005] Patent Document 2 proposes "an electrode including an active material layer, a groove is formed on the surface of the active material layer, at least a part of the inner surface of the groove is coated with a nanostructure layer, and the nanostructure layer includes nanostructures."

[0006] Japanese Patent Application Laid-Open No. 11-111302 Japanese Patent Application Laid-Open No. 2024-38714

[0007] It is required to suppress a decrease in the cycle characteristics of a secondary battery.

[0008] One aspect of this disclosure relates to an electrode for a secondary battery (electrode A) including a sheet-like current collector, an electrode mixture layer supported on the main surface of the current collector and containing an electrode active material, a first film covering at least a part of an end surface of the electrode mixture layer, and a second film covering at least a part of a surface of the first film, wherein the first film contains constituent elements of the electrode active material and the second film contains carbon.

[0009] Another aspect of the present disclosure relates to an electrode for a secondary battery (electrode B), which includes a sheet-like current collector, an electrode mixture layer carried on a main surface of the current collector and containing an electrode active material, a groove portion provided on a main surface of the electrode mixture layer, a first coating covering at least a part of an inner surface of the groove portion, and a second coating covering at least a part of a surface of the first coating. The first coating contains a constituent element of the electrode active material, and the second coating contains carbon.

[0010] Still another aspect of the present disclosure relates to a secondary battery, which includes a pair of electrodes and an electrolyte, and at least one of the pair of electrodes is the above-described electrode for a secondary battery (electrode A or electrode B).

[0011] According to the present disclosure, it is possible to suppress a decrease in cycle characteristics of a secondary battery. The novel features of the present invention are described in the appended claims. However, the present invention will be better understood from the following detailed description in combination with the drawings, with respect to both the configuration and the content, together with other objects and features of the present invention.

[0012] It is a top view schematically showing an example of an electrode for a secondary battery according to an embodiment of the present disclosure. It is a cross-sectional view of a main part schematically showing an example of an electrode for a secondary battery according to an embodiment of the present disclosure. It is a top view schematically showing another example of an electrode for a secondary battery according to an embodiment of the present disclosure. It is a cross-sectional view of a main part schematically showing another example of an electrode for a secondary battery according to an embodiment of the present disclosure. It is a schematic perspective view of a part of a secondary battery according to an embodiment of the present disclosure with a cutout.

[0013] Hereinafter, embodiments of the present disclosure will be described with examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values and materials may be exemplified, but other numerical values and materials may be applied as long as the effects of the present disclosure can be obtained. In this specification, the description "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less". In the following description, when the lower limit and the upper limit of a numerical value related to a specific physical property or condition are exemplified, any combination of any of the exemplified lower limits and any of the exemplified upper limits can be made as long as the lower limit is not more than the upper limit.

[0014] (Electrode for secondary battery) An electrode for a secondary battery (electrode A) according to one embodiment of the present disclosure comprises a sheet-shaped electrode current collector, an electrode composite layer supported on the main surface of the electrode current collector, a first coating covering at least a portion of the end face of the electrode composite layer, and a second coating covering at least a portion of the surface of the first coating. The electrode composite layer contains an electrode active material. The first coating contains constituent elements of the electrode active material, and the second coating contains carbon.

[0015] The strength is low near the edges of the electrode composite layer, and active material may detach from the edges of the electrode composite layer during charging and discharging, which can degrade the cycle characteristics. The method described in Patent Document 1, in which the edges of the electrode composite layer are covered with an insulating material or an electrode active material composite, is still insufficient in suppressing the detachment of active material during charging and discharging.

[0016] In contrast, in this disclosure, by covering at least a portion of the end face of the electrode composite layer with a first coating and covering at least a portion of the surface of the first coating with a second coating, the shedding of the active material and the resulting decrease in cycle characteristics can be significantly suppressed.

[0017] The first and second coatings may cover the end face of the electrode composite layer as well as at least a portion of the end face of the electrode current collector that is connected to the end face of the electrode composite layer.

[0018] An electrode for a secondary battery (electrode B) according to another embodiment of the present disclosure comprises a sheet-shaped electrode current collector, an electrode composite layer supported on the main surface of the electrode current collector, grooves provided on the main surface of the electrode composite layer, a first coating covering at least a portion of the inner surface of the grooves, and a second coating covering at least a portion of the surface of the first coating. The electrode composite layer contains an electrode active material. The first coating contains constituent elements of the electrode active material, and the second coating contains carbon.

[0019] While forming grooves on the main surface of the electrode composite layer can improve the permeability of the electrolyte into the electrode composite layer, the area near the inner surface of the grooves in the electrode composite layer has low strength, and active material may detach from the inner surface of the grooves during charging and discharging, thereby degrading the cycle characteristics. In the method described in Patent Document 2, in which the inner surface of the grooves is covered with a nanostructure layer, the suppression of active material detachment during charging and discharging is still insufficient.

[0020] In contrast, in this disclosure, by covering at least a portion of the inner surface of the groove with a first coating and covering at least a portion of the surface of the first coating with a second coating, the shedding of active material and the resulting decrease in cycle characteristics can be significantly suppressed.

[0021] (First and Second Coatings) The first and second coatings synergistically provide protection for the end faces (or grooves) of the electrode composite layer. The detailed reasons for this are not clear, but the following can be inferred: The first coating enhances the adhesion between the electrode composite layer and the second coating. From the viewpoint of improving the adhesion between the electrode composite layer and the second coating, the first coating is preferably a dense film. The second coating has good flexibility and excellent ability to follow the expansion and contraction of the electrode composite layer. The first and second coatings described above maintain the effect of suppressing the shedding of active material over long cycles.

[0022] The first coating contains the constituent elements of the electrode active material. The first coating may contain the same components as the electrode composite layer, or it may contain the electrode active material. The first coating may have a lower oxygen content than the electrode composite layer. The oxygen content (mass%) of the first coating may be, for example, 0.5 times or more and less than 1 time (or 0.95 times or less) of the oxygen content (mass%) of the electrode composite layer. The oxygen composition ratio of the electrode active material (e.g., lithium-containing transition metal oxide) may be lower in the first coating than in the electrode composite layer. For example, the oxygen composition ratio (molar ratio) of the lithium-containing transition metal oxide in the first coating may be about 5% to 50% lower than in the electrode composite layer. At least some of the oxygen can be released from the electrode active material at the surface portion (combined region of the first and second coatings) of the cut portion (or grooved portion) by laser processing, plasma processing, gas cutting, etc.

[0023] The first coating may contain carbon, and the carbon content of the first coating may be lower than that of the second coating, and also lower than that of the electrode composite layer and the second coating. The carbon may also be a constituent element of the components contained in the electrode composite layer (e.g., conductive material, binder, etc.).

[0024] The second coating contains carbon. The second coating may contain carbon as its main component. The carbon content in the second coating may be, for example, 50% by mass or more, or 50% by mass or more and 99% by mass or less. The carbon content of the second coating may be higher than that of the electrode composite layer. The carbon content (by mass) of the second coating may be, for example, 2 to 100 times the carbon content (by mass) of the electrode composite layer.

[0025] The second coating may contain constituent elements of the electrode active material, or it may contain the same components as the electrode composite layer. The second coating may have a lower oxygen composition ratio of the electrode active material (e.g., lithium-containing transition metal oxide) than the electrode composite layer, and may have a lower oxygen content than the electrode composite layer.

[0026] In processed areas formed by laser processing, plasma processing, gas cutting, etc., oxygen and carbon concentration gradients may occur. This can be utilized to form a first and second coating. On the surface of the processed area formed by laser processing, plasma processing, gas cutting, etc., some oxygen is released from the components contained in the electrode composite layer (for example, electrode active materials such as lithium-containing transition metal oxides). Also, the carbon concentration tends to be higher on the outer surface of the surface. In this case, the carbon mainly originates from the conductive material, but may also originate from other components such as binders. By appropriately adjusting the processing conditions, a first coating with a lower oxygen content than the electrode composite layer can be formed. On the surface of the first coating, a second coating can be formed with a lower oxygen content than the electrode composite layer and a higher carbon content than both the electrode composite layer and the first coating.

[0027] The thickness T2 of the second coating may be 0.01 μm or more and 30 μm or less, or 0.1 μm or more and 10 μm or less. The ratio of the thickness T2 of the second coating to the thickness T1 of the first coating, T2 / T1, may be 0.001 or more and 100 or less, or 0.01 or more and 10 or less.

[0028] For the analysis of the first and second coatings, for example, X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), secondary ion mass spectrometry (SIMS), etc., can be used.

[0029] The electrode composite layer may be formed on both main surfaces (first main surface and second main surface) of the electrode current collector. In the case of electrode A, it is preferable that the end faces of the first electrode composite layer supported on the first main surface and the end faces of the second electrode composite layer supported on the second main surface are covered with the first and second coatings, respectively. In the case of electrode B, it is preferable that a first groove is formed on the main surface of the first electrode composite layer supported on the first main surface, and a second groove is formed on the main surface of the second electrode composite layer supported on the second main surface. It is preferable that the inner surfaces of the first and second grooves are covered with the first and second coatings, respectively.

[0030] Furthermore, the electrode composite layer may be formed on one main surface of the electrode current collector. In the case of electrode A, the end face of the electrode composite layer supported on one main surface of the electrode current collector may be covered with the first and second coatings. In the case of electrode B, a groove may be formed on the main surface of the electrode composite layer supported on one main surface of the electrode current collector, and the inner surface of the groove may be covered with the first and second coatings.

[0031] The electrodes can be manufactured, for example, by applying an electrode composite slurry to the main surface of a current collector sheet (e.g., metal foil or alloy foil), drying it to form a coating, and rolling the coating as needed to form a laminate of the current collector sheet and the electrode composite layer. The electrode composite layer may be formed on one main surface of the current collector sheet, or on both main surfaces of the current collector sheet. The end faces of the electrodes (electrode composite layer) are formed, for example, by cutting the laminate to a predetermined size using a predetermined cutting blade (e.g., a slitting blade). The grooves can also be formed by grooving the electrode composite layer with a predetermined cutting blade.

[0032] Cutting of the laminate or grooving of the electrode composite layer may be performed by laser processing, plasma processing, or gas cutting. In this case, the melting and solidification of the cut portion of the laminate or the grooved portion of the electrode composite layer may be used to form the first and second coatings on the inner surface of the cut surface or groove. By appropriately adjusting the conditions of the cutting or grooving process, the first and second coatings can be formed in the cutting or grooving process. In this case, the first and second coatings contain components contained in the electrode composite layer (e.g., electrode active material, conductive material, binder, etc.) or constituent elements of said components.

[0033] The method for forming the coatings (first and second coatings) is not particularly limited. In addition to the above method, the first coating may be formed by applying the first treatment solution to the electrode end face (or the inner surface of the groove) and drying the coating. Subsequently, the second treatment solution may be applied to the surface of the first coating and drying the coating to form the second coating. The first treatment solution includes, for example, an electrode composite material and a dispersion medium or solvent. The second treatment solution includes, for example, a conductive carbon material and a dispersion medium or solvent. N-methyl-2-pyrrolidone (NMP) or the like can be used as the dispersion medium or solvent. The application method is not particularly limited. Examples of application methods include the dispenser method and the spray method. The drying method is not particularly limited. It may be air-dried or dried using a drying oven. Examples of electrode composite materials include the positive electrode composite material or the negative electrode composite material described later. In order to form the coating (especially the first coating) as a dense film, the coating may be compressed as needed.

[0034] Furthermore, the coating (especially the first coating) may be formed by known film formation methods (e.g., vapor phase methods). A dense film is easily obtained. Examples include chemical vapor deposition (CVD) and atomic layer deposition (ALD).

[0035] Here, Figure 1 is a schematic top view showing an example of an electrode (electrode A) for a secondary battery according to an embodiment of the present disclosure. The electrode in Figure 1 is strip-shaped. In Figure 1, LD indicates the length direction of the strip-shaped electrode 10 (winding direction when used in a wound electrode group), and WD indicates the width direction of the strip-shaped electrode 10. The shaded areas in Figure 1 are protective coatings 13 and 23 for the ends of the electrode 10. Figure 2 is a schematic cross-sectional view of a main part showing an example of an electrode for a secondary battery according to an embodiment of the present disclosure. Figure 2 shows a cross-section in the thickness direction near the end ES1 of the electrode 10 in Figure 1. In Figure 2, TD indicates the thickness direction of the electrode current collector 11 (electrode 10). Note that the electrode for a secondary battery according to an embodiment of the present disclosure is not limited to this.

[0036] The strip-shaped electrode 10 comprises a sheet-shaped electrode current collector 11 and electrode composite material layers 12a and 12b supported on both sides of the electrode current collector 11. The electrode composite material layers 12a and 12b contain electrode active material. The electrode 10 has two ends ES1 and ES2 in the width direction (WD direction) and two ends ES3 and ES4 in the length direction (LD direction). The end faces of ends ES1 to ES4 each have an end face 11E of the electrode current collector 11 and end faces 12AE and 12BE of the electrode composite material layers 12a and 12b connected to the end face 11E. The two ends ES1 and ES2 in the width direction are formed, for example, by slitting a large electrode with a cutting blade.

[0037] As shown in Figures 1 and 2, coatings 13 and 23 are formed on the end faces ES1 and ES2 of the electrode 10, respectively. The coatings 13 and 23 each comprise a first coating 14 and a second coating 15. The coating 13 formed on the end face of end ES1 will be described in detail below, but the coating 23 formed on the end face of end ES2 is formed in the same manner as the coating 13 on end ES1.

[0038] As shown in Figure 2, the electrode 10 comprises a first coating 14 that covers at least a portion of the end faces 12AE and 12BE of the electrode composite layers 12a and 12b, and a second coating 15 that covers at least a portion of the surface of the first coating 14. The first coating 14 contains constituent elements of the electrode active material, and the second coating 15 contains carbon. As shown in Figure 2, from the viewpoint of easily protecting the end faces 12AE and 12BE of the electrode composite layers 12a and 12b, it is preferable that the coating 13 (first coating 14 and second coating 15) further covers at least a portion of the end face 11E of the electrode current collector 11.

[0039] The coating 13, which is placed on the ends ES1 and ES2 of the electrode 10 shown in Figures 1 and 2, is formed entirely in the LD direction and TD direction, but may also be formed only partially.

[0040] The coverage rate of the coating 13 on the end faces 12AE (end face 12BE) of the electrode composite layers 12a and 12b in the thickness direction (TD direction) of the electrode 10 (hereinafter also referred to as "TD coverage rate") is approximately 100%, but is not limited to this. The TD coverage rate of the end faces 12AE (end face 12BE) may be 70% or more, or 90% or more.

[0041] When the electrode 10 is viewed from the direction normal to its main surface, the ratio of the length of the coating 13 covering the end ES1 (end ES2) in the LD direction to the length of the electrode in the LD direction (hereinafter referred to as "LD coverage rate") is approximately 100%, but is not limited to this. The LD coverage rate of the end ES1 (end ES2) may be, for example, 70% or more, or 90% or more.

[0042] Furthermore, the strip-shaped electrode 10 has both ends ES3 and ES4 in the longitudinal direction (LD direction). In the wound electrode group, one of the ends ES3 and ES4 of the electrode 10 is the starting end of the winding, and the other end ES3 and ES4 of the electrode 10 is the ending end of the winding. A coating may also be formed on the ends ES3 and ES4 of the electrode 10 in the same way as on the ends ES1 and ES2.

[0043] Here, Figure 3 is a schematic top view showing another example of an electrode (electrode B) for a secondary battery according to an embodiment of the present disclosure. The electrode in Figure 3 is strip-shaped. In Figure 3, LD indicates the length direction of the strip-shaped electrode 10 (winding direction when used in a wound electrode group), and WD indicates the width direction of the strip-shaped electrode 10. Figure 3 is a top view of the electrode 10 as seen from the electrode composite layer 12a side of Figure 4. The shaded area is a coating 13a that covers the inner surface of the groove 16a provided on the main surface of the electrode composite layer 12a of the electrode 10. Figure 4 is a schematic cross-sectional view of a main part showing another example of an electrode for a secondary battery according to an embodiment of the present disclosure, and is a cross-sectional view of the electrode 10 in the thickness direction. The electrode for a secondary battery according to an embodiment of the present disclosure is not limited thereto.

[0044] The strip-shaped electrode 10 comprises a sheet-shaped electrode current collector 11 and electrode composite material layers 12a and 12b supported on both sides of the electrode current collector 11. The electrode composite material layers 12a and 12b contain electrode active material. The electrode 10 has two ends ES1 and ES2 in the width direction (WD direction) and two ends ES3 and ES4 in the length direction (LD direction).

[0045] As shown in Figure 3, the electrode 10 comprises grooves 16a and 16b provided on the main surfaces of the electrode composite layers 12a and 12b, respectively; first coatings 14a and 14b covering at least a portion of the inner surfaces of the grooves 16a and 16b; and second coatings 15a and 15b covering at least a portion of the surfaces of the first coatings 14a and 14b. The first coatings 14a and 14b contain constituent elements of the electrode active material, and the second coatings 15a and 15b contain carbon. Coatings 13a and 13b are formed on the inner surfaces of the grooves 16a and 16b of the electrode 10, respectively, and the coatings 13a and 13b comprise the first coatings 14a and 14b and the second coatings 15a and 15b, respectively.

[0046] As shown in Figure 3, a plurality of grooves 16a are formed on the main surface of the electrode composite layer 12a. Each of the grooves 16a extends along the width direction (WD direction) and is formed at a constant interval N1 in the length direction (LD direction). Similarly, a plurality of grooves 16b are formed on the main surface of the electrode composite layer 12b, each extending along the width direction (WD direction) and being formed at a constant interval N2 in the length direction (LD direction). The intervals N1 and N2 between the grooves 16a and 16b may be, for example, 1 mm or more and 100 mm or less, or 5 mm or more (or 10 mm or more) and 80 mm or less. As shown in Figure 4, the grooves 16a and 16b are formed to overlap via the electrode current collector 11, but they may also be formed with a staggered position relative to each other.

[0047] In Figure 3, each of the multiple grooves 16a is covered with the coating 13a, but this is not limited to this. The ratio of the number of grooves covered with the coating 13a to the total number of multiple grooves 16a may be, for example, 70% or 80% or more, or 90% or more. In Figure 3, almost the entirety of one groove 16a is covered with the coating 13a, but this is not limited to this. The ratio of the length of the portion of one groove 16a covered with the coating 13a to the length of one groove 16a may be, for example, 70% or 80% or more, or 90% or more. The same applies to multiple grooves 16b.

[0048] The depths DA and DB of the grooves 16a and 16b may, for example, be 0.5 μm or more and 50 μm or less, or 1 μm or more and 10 μm or less. The thicknesses TA and TB of the electrode composite layers 12a and 12b may, for example, be 20 μm or more and 100 μm or less, or 50 μm or more and 80 μm or less. The ratios DA / TA and DB / TB may, for example, be 0.005 or more and 2.5 or less, or 0.0125 or more and 0.2 or less. The widths WA and WB of the grooves 16a and 16b may, for example, be 1 μm or more and 10000 μm or less, or 10 μm or more and 1000 μm or less.

[0049] The thickness of the electrode 10 in Figures 1 and 3 is, for example, 100 μm or more and 300 μm or less. The thickness of the electrode current collector 11 is, for example, 5 μm or more and 30 μm or less.

[0050] (Secondary Battery) A secondary battery according to an embodiment of the present disclosure comprises a pair of electrodes and an electrolyte. At least one of the pair of electrodes is an electrode for a secondary battery according to an embodiment of the present disclosure. One of the pair of electrodes is a positive electrode, and the other of the pair of electrodes is a negative electrode. The positive electrode and the negative electrode are wound or stacked, for example, via a separator.

[0051] Examples of secondary batteries include lithium-ion secondary batteries, lithium metal secondary batteries, and non-aqueous electrolyte secondary batteries such as solid batteries containing gel electrolytes or solid electrolytes. In other words, a secondary battery may be a liquid-type secondary battery containing an electrolyte solution, or an all-solid-state secondary battery containing a solid electrolyte.

[0052] The following details each component of the secondary battery. [Positive Electrode] The positive electrode comprises a positive electrode current collector and a positive electrode composite layer supported on the positive electrode current collector. The positive electrode composite layer is composed of a positive electrode composite material. The positive electrode composite layer is supported on one or both main surfaces of the positive electrode current collector.

[0053] The positive electrode composite material contains a positive electrode active material as an essential component and may contain binders, conductive materials, thickeners, etc. as optional components. The positive electrode active material may be a material that reversibly intercepts and releases lithium ions. The positive electrode active material may be, for example, a lithium-containing transition metal oxide. Examples of transition metals include Ni, Co, Mn, etc. Representative examples of lithium-containing transition metal oxides include lithium cobalt oxide and lithium nickel oxide, which have a layered crystal structure and are of the rock salt type.

[0054] The positive electrode composite layer can be formed, for example, by applying a positive electrode composite slurry containing a positive electrode composite and a dispersion medium to the surface of the positive electrode current collector and drying it. The dried coating may be rolled if necessary. The positive electrode composite layer may be formed on one surface of the positive electrode current collector or on both surfaces. N-methyl-2-pyrrolidone (NMP) or the like can be used as the dispersion medium for the positive electrode composite slurry.

[0055] As the lithium-containing transition metal oxide, for example, Li ａ CoO ２ , Li ａ NiO ２ , Li ａ MnO ２ , Li ａ Co ｂ Ni １－ｂ O ２ , Li ａ Co ｂ M １－ｂ O ｃ , Li ａ Ni １－ｂ M ｂ O ｃ , Li ａ Mn ２ O ４ , Li ａ Mn ２－ｂ M ｂ O ４ , LiMPO ４ , Li ２ MPO ４ F (M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B. ) can be mentioned. Here, 0 < a ≤ 1.2, 0 < b ≤ 0.9, 2.0 ≤ c ≤ 2.3. The a value indicating the molar ratio of lithium increases or decreases by charge and discharge.

[0056] Further, as the lithium-containing transition metal oxide, Li ａ Ni ｂ M １－ｂ O ２ (M is at least one selected from the group consisting of Mn, Co and Al, 0 < a ≤ 1.2, 0.3 ≤ b < 1. ) may be used. From the viewpoint of increasing the capacity, it is more preferable to satisfy 0.85 ≤ b < 1. From the viewpoint of the stability of the crystal structure, Li containing Co and Al as M ａ Ni ｂ Co ｃ Al ｄ O ２ (0 < a ≤ 1.2, 0.85 ≤ b < 1, 0 < c < 0.15, 0 < d ≤ 0.1, b + c + d = 1) may be used.

[0057] Examples of binders include resin materials such as fluororesins like polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins like polyethylene and polypropylene; polyamide resins like aramid resins; polyimide resins like polyimide and polyamideimide; acrylic resins like polyacrylic acid, methyl polyacrylate, and ethylene-acrylic acid copolymers; vinyl resins like polyacrylonitrile and polyvinyl acetate; polyvinylpyrrolidone; and polyethersulfone. A single binder may be used alone, or two or more may be used in combination.

[0058] Examples of conductive materials include carbon materials such as graphite, carbon black such as acetylene black, and carbon fibers (carbon nanotubes (CNTs), carbon fibers other than CNTs). Conductive materials may be used individually or in combination of two or more types.

[0059] As the positive electrode current collector, a non-porous conductive substrate (such as metal foil) or a porous conductive substrate (such as mesh, net, or perforated sheet) can be used. Examples of materials for the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium. The thickness of the positive electrode current collector is not particularly limited, but is preferably 1 to 50 μm, and more preferably 5 to 20 μm.

[0060] [Negative electrode] The negative electrode may be a negative electrode that deposits lithium metal during charging, or a negative electrode that absorbs lithium ions during charging.

[0061] The negative electrode comprises a strip-shaped negative electrode current collector. The negative electrode may also comprise a negative electrode current collector and a negative electrode composite layer supported on the negative electrode current collector. The negative electrode composite layer is composed of a negative electrode composite material. The negative electrode composite layer is supported on one or both main surfaces of the negative electrode current collector.

[0062] The negative electrode composite material contains a negative electrode active material as an essential component and may contain binders, conductive materials, thickeners, etc. as optional components. The negative electrode composite layer can be formed, for example, by applying a negative electrode composite slurry containing the negative electrode composite material and a dispersion medium to the surface of the negative electrode current collector and drying it. The dried coating may be rolled if necessary. The negative electrode composite layer may be formed on one surface of the negative electrode current collector or on both surfaces.

[0063] The negative electrode active material may be a material that reversibly intercepts and releases lithium ions. Furthermore, the negative electrode active material may be lithium metal or a lithium alloy. That is, the negative electrode composite layer may be a negative electrode active material layer consisting of foil-shaped lithium metal or lithium alloy.

[0064] Examples of negative electrode active materials that intercept and release lithium ions include carbon materials, metallic materials such as Si and Sn, alloy materials containing Si and Sn, metallic compounds containing Si and Sn, and lithium-containing metal oxides. Examples of lithium-containing metal oxides include spinel-type lithium titanium oxide and spinel-type lithium manganese oxide.

[0065] The carbon material can be graphite, easily graphitizable carbon (soft carbon), or difficult-to-graphitize carbon (hard carbon). Among these, graphite is preferred because it has excellent charge / discharge stability and low irreversible capacity.

[0066] Graphite refers to a carbon material in which the interplanar spacing d002 of (002) planes, as measured by X-ray diffraction, is, for example, 0.340 nm or less. Furthermore, the crystallite size Lc(002) of graphite, as measured by X-ray diffraction, may be, for example, 5 nm or more, 5 nm or more and 300 nm or less, or 10 nm or more and 200 nm or less.

[0067] Furthermore, the negative electrode active material may be a composite material containing Si. Si-containing composite materials are suitable as negative electrode active materials due to their high capacity. This composite material contains a silicon phase. Silicon can reversibly form alloys with lithium. This composite material is capable of reversibly intercalating and releasing lithium ions.

[0068] The composite material comprises a silicon phase and a matrix phase in which the silicon phase is dispersed. The matrix phase may be composed of a material having lithium-ion conductivity. For example, the matrix phase may include at least one selected from the group consisting of a silicon oxide phase and a carbon phase.

[0069] The silicon oxide phase contains Si and O, and may also contain a third element other than Si and O. ２ It may be composed of, or lithium silicate, or both of these. Lithium silicate is, for example, Li ２ｙ SiO ２＋ｙ It can be expressed as (0 < y < 2). The silicon oxide phase is SiO ２ The composite material composed of SiO ｘ This can be expressed as (0.5 ≤ x ≤ 1.6).

[0070] When carbon materials and composite materials are used in combination, the proportion of composite materials in the negative electrode active material (total of carbon materials and composite materials) can be, for example, 1% by mass or more and 20% by mass or less, 3% by mass or more and 15% by mass or less, or 3% by mass or more and 10% by mass or less. In this case, a good balance between improved cycle characteristics and increased capacity can be easily obtained.

[0071] Examples of binders include resin materials such as fluororesins like polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins like polyethylene and polypropylene; polyamide resins like aramid resin; polyimide resins like polyimide and polyamideimide; acrylic resins like polyacrylic acid, methyl polyacrylate, and ethylene-acrylic acid copolymer; vinyl resins like polyacrylonitrile and polyvinyl acetate; polyvinylpyrrolidone; polyethersulfone; and rubber-like materials like styrene-butadiene copolymer rubber (SBR). A single binder may be used alone, or two or more may be used in combination.

[0072] Examples of conductive materials include carbon compounds such as acetylene black, carbon fibers (carbon nanotubes (CNTs), carbon fibers other than CNTs), metal fibers, and metal powders such as aluminum. Conductive materials may be used individually or in combination of two or more types.

[0073] Examples of thickening agents include carboxymethylcellulose (CMC) and its modified forms (including salts such as Na salts), cellulose derivatives such as methylcellulose (cellulose ethers, etc.), and saponified polymers having vinyl acetate units such as polyvinyl alcohol. The thickening agents may be used individually or in combination of two or more.

[0074] As the negative electrode current collector, a non-porous conductive substrate (such as metal foil) or a porous conductive substrate (such as mesh, net, or perforated sheet) can be used. Examples of materials for the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, and copper alloy. The thickness of the negative electrode current collector is not particularly limited, but is preferably 1 to 50 μm, and more preferably 5 to 20 μm.

[0075] [Electrolyte] The electrolyte may be a liquid electrolyte (electrolyte solution), a gel electrolyte, or a solid electrolyte. A liquid electrolyte is, for example, an electrolyte solution containing a non-aqueous solvent and a salt dissolved in the non-aqueous solvent. The concentration of the salt in the electrolyte solution is, for example, 0.5 mol / L or more and 2 mol / L or less. The electrolyte solution may contain known additives.

[0076] The gel-like electrolyte comprises a salt and a matrix polymer, or a salt, a non-aqueous solvent, and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. Examples of polymer materials include fluororesins, acrylic resins, polyether resins, and polyethylene oxide.

[0077] As the solid electrolyte, for example, materials known for use in all-solid-state lithium-ion secondary batteries (e.g., oxide-based solid electrolytes, sulfide-based solid electrolytes, halide-based solid electrolytes, etc.) are used.

[0078] For example, liquid non-aqueous electrolytes are prepared by dissolving a salt in a non-aqueous solvent. The salt is an electrolyte salt that undergoes ion dissociation in the electrolyte, and may include, for example, lithium salts. Various additives may be included in the electrolyte. Electrolytes are usually used in liquid form, but their fluidity may be restricted by gelling agents or the like.

[0079] Examples of non-aqueous solvents include cyclic carbonate esters, linear carbonate esters, cyclic carboxylic acid esters, and linear carboxylic acid esters. Examples of cyclic carbonate esters include propylene carbonate (PC) and ethylene carbonate (EC). Cyclic carbonate esters having unsaturated bonds, such as vinylene carbonate (VC), may also be used. Cyclic carbonate esters having fluorine atoms, such as fluoroethylene carbonate (FEC), may also be used. Examples of linear carbonate esters include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of linear carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. The non-aqueous solvent may be used alone or in combination of two or more types.

[0080] Examples of lithium salts include LiClO ４ LiBF ４ LiPF ６ LiAlCl ４ LiSbF ６ , LiSCN, LiCF ３ SO ３ LiCF ３ CO ２ LiAsF ６ LiB １０ Cl １０Examples include lithium lower aliphatic carboxylates, LiCl, LiBr, LiI, borates, and imide salts. Examples of borates include lithium bis(1,2-benzenediolate(2-)-O,O')borate, lithium bis(2,3-naphthalenedioleate(2-)-O,O')borate, lithium bis(2,2'-biphenyldiolate(2-)-O,O')borate, and lithium bis(5-fluoro-2-oleate-1-benzenesulfonic acid-O,O')borate. Examples of imide salts include lithium bisfluorosulfonylimide (LiN(FSO) ２ ) ２ ), bistrifluoromethanesulfonate lithium (LiN(CF ３ SO ２ ) ２ ), trifluoromethanesulfonic acid nonafluorobutanesulfonic acid imide lithium (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), bispentafluoroethanesulfonate lithium (LiN(C) ２ F ５ SO ２ ) ２ Examples include the following. A single lithium salt may be used alone, or two or more may be used in combination. The concentration of the lithium salt in the non-aqueous electrolyte is, for example, 0.5 mol / L or more and 2 mol / L or less.

[0081] [Separator] It is desirable to interpose a separator between the positive electrode and the negative electrode. The separator should have high ion permeability and appropriate mechanical strength and insulating properties. As the separator, a microporous thin film, woven fabric, nonwoven fabric, etc., can be used. As the material of the separator, polyolefins such as polypropylene and polyethylene are preferred.

[0082] Hereinafter, the structure of a rectangular secondary battery will be described as an example of a secondary battery according to the embodiments of this disclosure, with reference to Figure 6. Figure 6 is a schematic perspective view in which a part of a secondary battery according to one embodiment of this disclosure is cut out.

[0083] The battery comprises a bottomed rectangular battery case 4, an electrode group 1 housed within the battery case 4, and a non-aqueous electrolyte (not shown). The electrode group 1 has a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator interposed between them to prevent direct contact. The electrode group 1 is formed by winding the negative electrode, positive electrode, and separator around a flat core and then removing the core. At least one of the positive electrode and the negative electrode is an electrode for a secondary battery according to the embodiment of this disclosure.

[0084] One end of the negative electrode lead 3 is attached to the negative electrode current collector by welding or the like. The other end of the negative electrode lead 3 is electrically connected to the negative electrode terminal 6 provided on the sealing plate 5 via a resin insulating plate (not shown). The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. One end of the positive electrode lead 2 is attached to the positive electrode current collector by welding or the like. The other end of the positive electrode lead 2 is connected to the back surface of the sealing plate 5 via an insulating plate. That is, the positive electrode lead 2 is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The insulating plate separates the electrode group 1 from the sealing plate 5 and also separates the negative electrode lead 3 from the battery case 4. The periphery of the sealing plate 5 is fitted into the open end of the battery case 4, and the fitted portion is laser welded. In this way, the opening of the battery case 4 is sealed by the sealing plate 5. The injection hole for the non-aqueous electrolyte provided in the sealing plate 5 is closed by a seal 8.

[0085] [Note] The following technologies are disclosed based on the above description of embodiments. (Technology 1) An electrode for a secondary battery comprising: a sheet-shaped electrode current collector; an electrode composite layer containing an electrode active material supported on the main surface of the electrode current collector; a first coating covering at least a portion of the end surface of the electrode composite layer; and a second coating covering at least a portion of the surface of the first coating, wherein the first coating contains constituent elements of the electrode active material, and the second coating contains carbon. (Technology 2) An electrode for a secondary battery comprising: a sheet-shaped electrode current collector; an electrode composite layer containing an electrode active material supported on the main surface of the electrode current collector; a groove provided on the main surface of the electrode composite layer; a first coating covering at least a portion of the inner surface of the groove; and a second coating covering at least a portion of the surface of the first coating, wherein the first coating contains constituent elements of the electrode active material, and the second coating contains carbon. (Technology 3) The electrode for a secondary battery according to Technology 1 or 2, wherein the thickness T2 of the second film is 0.1 μm or more and 10 μm or less. (Technology 4) The electrode for a secondary battery according to any one of Technology 1 to 3, wherein the ratio of the thickness T2 of the second film to the thickness T1 of the first film: T2 / T1 is 0.01 or more and 10 or less. (Technology 5) The electrode for a secondary battery according to any one of Technology 1 to 4, wherein the second film contains the constituent elements of the electrode active material. (Technology 6) The electrode for a secondary battery according to any one of Technology 1 to 5, wherein the first film has a lower oxygen content than the electrode composite layer. (Technology 7) The electrode for a secondary battery according to any one of Technology 1 to 6, wherein the second film has a higher carbon content than the electrode composite layer. (Technology 8) A secondary battery comprising a pair of electrodes and an electrolyte, wherein at least one of the pair of electrodes is the electrode for a secondary battery according to any one of Technology 1 to 7.

[0086] The present disclosure will be described below in detail based on examples, but the present disclosure is not limited to the following examples.

[0087] 《Secondary Batteries A1-A5》 (Preparation of Positive Electrode) A rock salt-type lithium-containing transition metal oxide (NCA: positive electrode active material) containing Li, Ni, Co, and Al (with a molar ratio of Li to the total of Ni, Co, and Al being 1.0) and having a layered structure was prepared. This lithium-containing transition metal oxide (NCA), acetylene black (AB: conductive material), and polyvinylidene fluoride (PVdF: binder) were mixed in a mass ratio of NCA:AB:PVdF = 95:2.5:2.5, and then an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added and stirred to prepare a positive electrode mixture slurry.

[0088] A positive electrode composite slurry was applied to both sides of a strip-shaped aluminum foil (15 μm thick), which served as the positive electrode current collector, and then dried to form a coating of the positive electrode composite. Next, the coating was rolled using a roller to form a positive electrode composite layer. The resulting laminate of the positive electrode current collector and the positive electrode composite layer was cut into strips of a predetermined size using a cutting blade. In this way, a positive electrode was obtained. The thickness of each side of the positive electrode composite layer was 80 μm.

[0089] (Formation of the first and second coatings) Coatings 13 and 23 (first coating 14 and second coating 15) shown in Figures 1 and 2 were formed on almost the entire end faces of the positive electrode ends ES1 and ES2 as follows.

[0090] A first coating containing the same compound (NCA) as the positive electrode active material was formed on the end faces ES1 and ES2 at both ends in the width direction of the strip-shaped positive electrode by a vapor phase method. In this way, as shown in Figure 2, the first coating 14 was formed on the end faces of the positive electrode (end face 11E of the positive electrode current collector 11 and end faces 12AE and 12BE of the positive electrode composite layers 12a and 12b in Figure 2). The thickness T1 of the first coating was 5 μm.

[0091] Next, acetylene black and polyvinylidene fluoride were mixed in a 1:1 mass ratio, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added and stirred to prepare a coating slurry. The coating slurry was applied to the surface of the first coating, and then dried to form the second coating. In this way, as shown in Figure 2, the second coating 15 was formed on the surface of the first coating 14. The thickness of the second coating was set to the values ​​shown in Table 1. The thickness of the second coating was adjusted by the amount of coating slurry applied.

[0092] (Preparation of the negative electrode) A negative electrode slurry was prepared by kneading 100 parts by mass of artificial graphite, 1 part by mass of styrene-butadiene copolymer rubber (SBR), 1 part by mass of carboxymethylcellulose (CMC), and an appropriate amount of water.

[0093] A negative electrode mixture slurry was applied to both sides of a strip-shaped Cu foil (negative electrode current collector), and then dried to form a coating of the negative electrode mixture. Next, the coating was rolled using a roller to form a negative electrode mixture layer. Finally, the resulting laminate of the negative electrode current collector and the negative electrode mixture layer was cut to a predetermined size to obtain the negative electrode.

[0094] (Preparation of non-aqueous electrolyte) Ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed in a volume ratio of EC:DMC = 30:70. LiPF was added to the resulting mixed solvent. ６ The concentration becomes 1 mol / L, and LiBF ２ (C ２ O ４ A liquid non-aqueous electrolyte was prepared by dissolving these substances so that their concentration was 0.1 mol / L.

[0095] (Battery Fabrication) One end of an aluminum positive electrode lead was attached to the positive electrode obtained above. One end of a nickel negative electrode lead was attached to the negative electrode obtained above. A wound electrode group was fabricated by winding the positive and negative electrodes with a polyethylene separator in between. The electrode group was housed in a bottomed cylindrical battery case that also served as the negative electrode terminal. At this time, an upper insulating plate and a lower insulating plate were placed at the top and bottom of the electrode group, respectively. Next, a non-aqueous electrolyte was injected into the battery case, and the opening of the battery case was closed by placing a metal sealing body that also served as the positive electrode terminal at the opening of the battery case. At this time, a resin insulating gasket was interposed between the sealing body and the opening end of the battery case. The other end of the positive electrode lead was connected to the sealing body, and the other end of the negative electrode lead was connected to the inner bottom surface of the battery case. In this way, cylindrical non-aqueous electrolyte secondary batteries A1 to A5 (diameter 18 mm, height 65 mm) were fabricated. Note that A1 to A5 in Table 1 are examples, and B1 to B3 are comparative examples.

[0096] Battery B1 was manufactured in the same manner as Battery A1, except that the first and second coatings were not formed on the end faces of both ends ES1 and ES2 in the width direction of the strip-shaped positive electrode.

[0097] Battery B2 was manufactured in the same manner as Battery A1, except that the second coating was not formed on the surface of the first coating.

[0098] Battery B3 was manufactured in the same manner as Battery A3, except that the second coating was formed on the end faces of both ends ES1 and ES2 in the width direction of the strip-shaped positive electrode, without forming the first coating.

[0099] [Evaluation] The following evaluations were performed on each battery.

[0100] (Cycle Test) Each obtained battery was subjected to 500 charge-discharge cycles under the following conditions. The cycle test was conducted in an environment of 25°C. A 20-minute pause was taken between charge and discharge cycles.

[0101] (Charging) Constant current charging was performed with a current of 700mA until the voltage reached 4.2V, and then constant voltage charging was performed with a voltage of 4.2V until the current reached 35mA.

[0102] (Discharge) A constant current discharge was performed with a current of 700mA until the voltage reached 3V.

[0103] The ratio of the discharge capacity after 500 cycles to the discharge capacity after 1 cycle was calculated as the capacity retention rate after 500 cycles.

[0104] The evaluation results are shown in Table 1.

[0105]

[0106] Batteries A1 to A5 showed a significant improvement in capacity retention compared to batteries B1 to B3.

[0107] 《Batteries A6 to A10》 (Formation of grooves) The positive electrode was manufactured in the same manner as in battery A1, and then grooves were formed on the main surface of the positive electrode composite layer using a cutting blade. Specifically, multiple grooves 16a and 16b (width 100 μm, depth 30 μm) shown in Figures 3 and 4 were formed on the main surface of the positive electrode composite layers 12a and 12b (thickness 70 μm) formed on both sides of the positive electrode current collector. Each of the multiple grooves 16a and 16b extends along the width direction and is formed at a constant interval in the length direction. The interval between grooves 16a and 16b was 1 mm.

[0108] (Formation of the first and second coatings) Coatings 13a and 13b (first coatings 14a and 14b and second coatings 15a and 15b) shown in Figures 3 and 4 were formed on almost the entire inner surface of the multiple grooves 16a and 16b as follows.

[0109] A first coating containing the same compound (NCA) as the positive electrode active material was formed on the inner surface of the grooves by a vapor phase method. In this way, as shown in Figure 4, the first coatings 14a and 14b were formed on the inner surfaces of the grooves 16a and 16b. The thickness T1 of the first coating was 5 μm.

[0110] A coating slurry was prepared in the same manner as for battery A1. After applying the coating slurry to the surface of the first coating, the second coating was formed by drying. In this way, as shown in Figure 4, the second coatings 15a and 15b were formed on the surfaces of the first coatings 14a and 14b. The thickness of the second coating was set to the values ​​shown in Table 2. The thickness of the second coating was adjusted by the amount of second coating slurry applied.

[0111] Batteries A6 to A10 were manufactured and evaluated in the same manner as battery A1, except that instead of using a positive electrode in which the end faces of both ends ES1 and ES2 in the width direction were covered with the first and second coatings, a positive electrode in which the inner surface of the groove was covered with the first and second coatings was used.

[0112] Battery B4 was manufactured and evaluated in the same manner as Battery A6, except that the first and second coatings were not formed on the inner surface of the grooves.

[0113] Battery B5 was manufactured and evaluated in the same manner as Battery A6, except that the second coating was not formed on the surface of the first coating.

[0114] Battery B6 was manufactured and evaluated in the same manner as Battery A8, except that a second coating was formed on the inner surface of the groove without forming a first coating.

[0115] The evaluation results are shown in Table 2. In Table 2, A6 to A10 are examples, and B4 to B6 are comparative examples.

[0116]

[0117] Batteries A6 to A10 showed a significant improvement in capacity retention compared to batteries B4 to B6.

[0118] The secondary battery described herein is useful as a primary power source for mobile communication devices, portable electronic devices, electric vehicles, and the like.

[0119] Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention.

[0120] 1: Electrode group, 2: Positive lead, 3: Negative lead, 4: Battery case, 5: Sealing plate, 6: Negative terminal, 7: Gasket, 8: Sealing plug, 10: Electrode, 11: Electrode current collector, 12a, 12b: Electrode composite layer, 13: Coating, 14: First coating, 15: Second coating, 16a, 16b: Grooves

Claims

1. An electrode for a secondary battery comprising: a sheet-shaped electrode current collector; an electrode composite layer containing an electrode active material supported on the main surface of the electrode current collector; a first coating covering at least a portion of the end surface of the electrode composite layer; and a second coating covering at least a portion of the surface of the first coating, wherein the first coating contains constituent elements of the electrode active material, and the second coating contains carbon.

2. An electrode for a secondary battery comprising: a sheet-shaped electrode current collector; an electrode composite layer containing an electrode active material supported on the main surface of the electrode current collector; grooves provided on the main surface of the electrode composite layer; a first coating covering at least a portion of the inner surface of the grooves; and a second coating covering at least a portion of the surface of the first coating, wherein the first coating contains constituent elements of the electrode active material, and the second coating contains carbon.

3. The electrode for a secondary battery according to claim 1 or 2, wherein the thickness T2 of the second coating is 0.1 μm or more and 10 μm or less.

4. The ratio of the thickness T2 of the second coating to the thickness T1 of the first coating: T2 / T1 is 0.01 or more and 10 or less, the electrode for a secondary battery according to claim 1 or 2.

5. The electrode for a secondary battery according to claim 1 or 2, wherein the second coating contains the constituent elements of the electrode active material.

6. The electrode for a secondary battery according to claim 1 or 2, wherein the first coating has a lower oxygen content than the electrode composite layer.

7. The electrode for a secondary battery according to claim 1 or 2, wherein the second coating has a higher carbon content than the electrode composite layer.

8. A secondary battery comprising a pair of electrodes and an electrolyte, wherein at least one of the pair of electrodes is the electrode for a secondary battery according to claim 1 or 2.

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