Secondary battery electrode and secondary battery

Abstract

This secondary battery electrode comprises: a sheet-shaped electrode current collector; an electrode mixture layer supported on a main surface of the electrode current collector; and needle-shaped crystals covering at least a part of an end surface of the electrode mixture layer.

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 - 230993, filed on December 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 - shaped electrode current collector and an electrode mixture layer carried on the main surface of the electrode current collector and containing an electrode active material.

[0004] In Patent Document 1, "a power storage device having a current collector and an active material layer on the current collector, the current collector having titanium and the active material layer having crystalline silicon, the active material layer covering the entire upper surface of the current collector, the active material layer having a first region in contact with the upper surface of the current collector and a second region on the first region, the second region having a plurality of protrusions, and any one of the plurality of protrusions having irregularities on the surface" is proposed.

[0005] Japanese Unexamined Patent Application Publication No. 2017 - 84797

[0006] Further improvement in the cycle characteristics of secondary batteries is required.

[0007] One aspect of this disclosure relates to an electrode for a secondary battery (electrode A) including a sheet - shaped electrode current collector, an electrode mixture layer carried on the main surface of the electrode current collector, and acicular crystals covering at least a part of the end face of the electrode mixture layer.

[0008] Another aspect of this disclosure relates to an electrode for a secondary battery (electrode B) including a sheet - shaped electrode current collector, an electrode mixture layer carried on the main surface of the electrode current collector, a groove portion provided on the main surface of the electrode mixture layer, and acicular crystals covering at least a part of the inner surface of the groove portion.

[0009] Still another aspect of the present disclosure relates to a secondary battery including a pair of electrodes and an electrolyte, wherein at least one of the pair of electrodes is the electrode for a secondary battery (electrode A or electrode B) described above.

[0010] According to the present disclosure, the cycle characteristics of the secondary battery can be improved. The novel features of the present invention are described in the appended claims, but the present invention will be better understood from the following detailed description in combination with the drawings, with reference to both the configuration and the content, as well as other objects and features of the present invention.

[0011] 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 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.

[0012] 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 regarding 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.

[0013] (Electrode for Secondary Battery) The electrode for a secondary battery (electrode A) according to an embodiment of the present disclosure includes a sheet-like electrode current collector, an electrode mixture layer supported on the main surface of the electrode current collector, and acicular crystals covering at least a part of the end face of the electrode mixture layer. The electrode mixture layer contains an electrode active material.

[0014] When the edge surfaces of the electrode composite layer are covered with needle-shaped crystals, the specific surface area at the edge surfaces of the electrode composite layer increases, and capillary action occurs. As a result, the permeability of the electrolyte into the electrode composite layer is greatly improved, and the cycle characteristics (capacity retention rate) of the secondary battery are improved.

[0015] Furthermore, in secondary batteries, since the end face of the electrode composite layer does not face the separator, contact between the needle-shaped crystals and the separator is suppressed, thereby suppressing the occurrence of internal short circuits due to the above-mentioned contact. The protrusions described in Patent Document 1 are formed of crystalline silicon and are formed on the main surface of the electrode composite layer facing the separator, so internal short circuits may occur due to contact between these protrusions and the separator.

[0016] When electrode A is strip-shaped, it is preferable that the end faces of the electrode composite layer at both ends in the width direction of electrode A are covered with needle-shaped crystals. When such electrode A is wound to form a columnar electrode group, the permeability of the electrolyte is efficiently increased from the inner circumference to the outer circumference of the electrode group.

[0017] The electrode composite layer may be formed on both main surfaces of the electrode current collector. That is, the main surface of the electrode current collector may have a first main surface and a second main surface opposite to the first main surface, and the electrode composite layer may have a first layer supported on the first main surface and a second layer supported on the second main surface. In this case, the end face of the electrode composite layer has a first end face of the first layer and a second end face of the second layer, and at least one of the first end face and the second end face is covered with needle-shaped crystals. It is preferable that each of the first end face and the second end face is covered with needle-shaped crystals.

[0018] Furthermore, the electrode composite layer may be formed on one main surface of the electrode current collector, and the end face of the electrode composite layer formed on one main surface of the electrode current collector may be covered with needle-shaped crystals.

[0019] The needle-shaped crystals may be formed in layers. In electrode A, the thickness of the layered needle-shaped crystals is preferably 0.4 μm or more and 1.0 μm or less, and more preferably 0.5 μm or more and 0.9 μm or less. When the thickness of the needle-shaped crystals is 0.4 μm or more (or 0.5 μm or more), the effects of needle-shaped crystal formation are easily obtained. When the thickness of the needle-shaped crystals is 1.0 μm or less (or 0.9 μm or less), contact between the needle-shaped crystals and the separator is easily suppressed. The thickness of the layered needle-shaped crystals is determined by measuring the thickness of the layered needle-shaped crystals at any 10 to 20 locations using an SEM image of the electrode cross-section including the end face of the electrode composite layer, and averaging them.

[0020] An electrode for a secondary battery (electrode B) according to another embodiment of the present disclosure comprises a sheet-like 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, and needle-shaped crystals covering at least a portion of the inner surface of the grooves. The electrode composite layer contains an electrode active material.

[0021] 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 improvement in electrolyte permeability remains insufficient. In contrast, in this disclosure, the inner surface of the grooves is covered with needle-shaped crystals, thereby increasing the specific surface area on the inner surface of the grooves and inducing capillary action. This significantly improves the permeability of the electrolyte into the electrode composite layer, thereby improving the cycle characteristics (capacity retention rate) of the secondary battery.

[0022] Furthermore, since the grooves in the electrode composite layer are formed recessed from the main surface of the electrode composite layer, and needle-shaped crystals are formed on the inner surface of the grooves, contact between the needle-shaped crystals and the separator facing the main surface of the electrode composite layer is suppressed in the secondary battery, thereby suppressing the occurrence of internal short circuits due to the aforementioned contact.

[0023] The inner surface of the groove may have an inner bottom surface and an inner surface. In this case, at least one of the inner bottom surface and the inner surface is covered with needle-shaped crystals. It is preferable that each of the inner bottom surface and the inner surface is covered with needle-shaped crystals.

[0024] The arrangement of grooves on the main surface of the electrode composite layer is not particularly limited. When the electrode B is strip-shaped, one groove extending in the longitudinal direction may be arranged linearly in the center in the width direction, or two or more grooves extending in the longitudinal direction may be arranged parallel to each other with spaces between them. When such strip-shaped electrodes B are wound to form a columnar electrode group, the permeability of the electrolyte is efficiently increased from the inner circumference to the outer circumference of the electrode group.

[0025] The electrode composite layer may be formed on both main surfaces of the electrode current collector. That is, the main surface of the electrode current collector has a first main surface and a second main surface opposite to the first main surface, and the electrode composite layer may have a first layer supported on the first main surface and a second layer supported on the second main surface. In this case, grooves are provided on at least one of the first main surface of the first layer and the second main surface of the second layer. It is preferable that grooves are provided on each of the first main surface of the first layer and the second main surface of the second layer. That is, it is preferable that a first groove is provided on the first main surface of the first layer and a second groove is provided on the second main surface of the second layer. In this case, at least one of the inner surface of the first groove and the inner surface of the second groove is covered with needle-shaped crystals. It is preferable that each of the inner surface of the first groove and the inner surface of the second groove is covered with needle-shaped crystals.

[0026] Furthermore, the electrode composite layer may be formed on one main surface of the electrode current collector, or grooves may be provided on the main surface of the electrode composite layer formed on one main surface of the electrode current collector, and the inner surface of the grooves may be covered with needle-shaped crystals.

[0027] The needle-shaped crystals may be formed in layers. In electrode B, the thickness of the layered needle-shaped crystals is preferably 0.1 μm or more and 0.5 μm or less, and more preferably 0.2 μm or more and 0.45 μm or less. When the thickness of the needle-shaped crystals is 0.1 μm or more (or 0.2 μm or more), the effects of needle-shaped crystal formation are easily obtained. When the thickness of the needle-shaped crystals is 0.5 μm or less (or 0.45 μm or less), contact between the needle-shaped crystals and the separator is easily suppressed. The thickness of the layered needle-shaped crystals is determined by measuring the thickness of the layered needle-shaped crystals at any 10 to 20 locations using an SEM image of the electrode cross-section including the groove portion of the electrode composite layer, and averaging them.

[0028] The needle-shaped crystals may contain constituent elements of the electrode composite material (e.g., electrode active material, conductive material, etc.), or they may contain constituent elements of the electrode active material. The constituent elements of the electrode active material include metal elements, oxygen, etc. The components of the needle-shaped crystals include metal oxides such as aluminum oxide, manganese oxide, cobalt oxide, and nickel oxide. Needle-shaped crystals mainly composed of metal oxides (e.g., aluminum oxide) have low conductivity, and even if the needle-shaped crystals fall off, the occurrence of internal short circuits is suppressed. The protrusions described in Patent Document 1 are formed from crystalline silicon, and if the protrusions detach from the electrode, internal short circuits may occur.

[0029] Needle-shaped crystals can be formed, for example, on the end face of the electrode composite layer or on the inner surface of the groove in the electrode composite layer by laser irradiation, plasma irradiation, etc. Needle-shaped crystals can be formed by appropriately adjusting the irradiation conditions (e.g., wavelength, power, repetition frequency, etc.). In this case, the needle-shaped crystals contain constituent elements of the materials contained in the electrode composite layer (e.g., electrode active material, conductive material, etc.).

[0030] Furthermore, needle-shaped crystals may be formed by chemical vapor deposition (CVD) or the like. In chemical vapor deposition, needle-shaped crystals may be formed on the end face of the electrode mixture layer or on the inner surface of the grooves in the electrode mixture layer by heating under a raw material gas atmosphere. In this case, the composition of the needle-shaped crystals can be easily controlled.

[0031] Needle-shaped crystals can be observed using a scanning electron microscope (SEM) or a transmission electron microscope (TEM). For analysis of needle-shaped crystals, energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), etc., can be used.

[0032] The electrode composite layer contains an electrode active material and may also contain other components such as a conductive material and a binder. For example, the electrode active material may be a material capable of intercalating and releasing lithium ions. The electrode 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, rolling the coating as needed to form a laminate of the current collector sheet and the electrode composite layer, and then cutting the laminate to a predetermined size. 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 laminate can be cut using a predetermined cutting blade (e.g., a slitting blade). Grooves can be made in the electrode composite layer using a predetermined cutting blade.

[0033] 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 needle-shaped crystals 13a and 23a that cover the end faces of the electrode composite layer 12a at the ends ES1 and ES2 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 cross-sections in the width direction and thickness direction of the electrode 10 in Figure 1. Note that the electrode for a secondary battery according to an embodiment of the present disclosure is not limited to this.

[0034] 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.

[0035] As shown in Figures 1 and 2, the electrode 10 includes needle-shaped crystals 13a covering at least a portion of the end face 12AE of the electrode composite layer 12a at end ES1, and needle-shaped crystals 13b covering at least a portion of the end face 12BE of the electrode composite layer 12b at end ES1. The electrode 10 also includes needle-shaped crystals 23a covering at least a portion of the end face 12AE of the electrode composite layer 12a at end ES2, and needle-shaped crystals 23b covering at least a portion of the end face 12BE of the electrode composite layer 12b at end ES2. Needle-shaped crystals 13a and 13b will be described in detail below, but needle-shaped crystals 23a and 23b are formed in the same manner as needle-shaped crystals 13a and 13b.

[0036] The coverage rate of the end face 12AE of the electrode composite layer 12a at end ES1 by the needle-shaped crystals 13a is not particularly limited and may be 30% or more, 50% or more, 70% or more, or 80% or more. The coverage rate is the ratio of the area of ​​the region where the end face 12AE at end ES1 is covered by the needle-shaped crystals 13a to the area of ​​the end face 12AE at end ES1. The same applies to the coverage rate of the end face 12BE of the electrode composite layer 12b at end ES1 by the needle-shaped crystals 13b. The end faces 12AE and 12BE of the electrode composite layers 12a and 12b covered by the needle-shaped crystals 13a and 13b do not face the other electrode when the electrode group is constructed, so the formation of the needle-shaped crystals 13a and 13b has almost no effect on the internal resistance of the battery.

[0037] The thickness of the layered needle-shaped crystals 13a is preferably 0.5 μm or more and 0.9 μm or less. The thickness of the electrode composite layer 12a may be, for example, 20 μm or more and 100 μm or less, or 50 μm or more and 80 μm or less. The same applies to the thickness of the layered needle-shaped crystals 13b and the electrode composite layer 12b.

[0038] 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. Needle-shaped crystals may be further formed on the end faces 12AE and 12BE of the electrode composite layers 12a and 12b at the ends ES3 and ES4 of the electrode 10.

[0039] 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 needle-shaped crystals 13a covering 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 in the thickness direction of the electrode 10. The electrode for a secondary battery according to an embodiment of the present disclosure is not limited thereto.

[0040] 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).

[0041] As shown in Figure 3, the electrode 10 comprises a groove 16a provided on the main surface of the electrode composite layer 12a, a groove 16b provided on the main surface of the electrode composite layer 12b, needle-shaped crystals 13a covering at least a portion of the inner surface of the groove 16a, and needle-shaped crystals 13b covering at least a portion of the inner surface of the groove 16b.

[0042] In Figure 3, a single groove 16a is formed linearly along the length direction (LD direction) in the center of the width direction (WD direction) on the main surface S1 of the electrode composite layer 12a. Similarly, a single groove 16b is formed linearly along the length direction (LD direction) in the center of the width direction (WD direction) on the main surface of the electrode composite layer 12b.

[0043] In Figure 3, one groove is arranged on the main surface of one electrode composite layer, but multiple grooves may be spaced apart from each other and arranged parallel to each other along the length direction (LD direction). In Figure 4, grooves 16a and 16b are formed to overlap via the electrode current collector 11, but they may be formed with their positions offset from each other.

[0044] Since the proportion of grooves 16a to the main surface S1 of the electrode composite layer 12a is sufficiently small, the effect on electrode resistance due to the inner surface of the grooves 16a being covered with needle-shaped crystals 13a of metal oxide is small. When the electrode 10 is viewed from the side of the main surface S1 of the electrode composite layer 12a, the ratio of the total area of ​​grooves 16a arranged on the main surface S1 of the electrode composite layer 12a to the area of ​​the main surface S1 of the electrode composite layer 12a including the grooves 16a may be, for example, 10% or less, or 5% or less. The same applies to grooves 16b arranged on the electrode composite layer 12b.

[0045] The coverage rate of the needle-shaped crystals 13a on the inner surface of the groove 16a is not particularly limited, but from the viewpoint of improving the permeability of the electrolyte, the coverage rate may be 30% or more, 50% or more, 70% or more, or 80% or more. Furthermore, the above coverage rate may be 80% or less, or 60% or less. The above coverage rate is the ratio of the area of ​​the region where the inner surface of the groove 16a is covered with needle-shaped crystals 13a to the area of ​​the inner surface of the groove 16a. The same applies to the coverage rate of the needle-shaped crystals 13b on the inner surface of the groove 16b.

[0046] The thickness Y1 of the layered needle-shaped crystals 13a covering the inner surface of the groove 16a is preferably 0.2 μm or more and 0.45 μm or less. The thickness Y1 of the layered needle-shaped crystals 13a may be 1 / 4 or less (or 1 / 8 or less) of the depth D1 of the groove 16a. The thickness Y1 of the layered needle-shaped crystals 13a may be 1 / 4 or less (or 1 / 8 or less) of the width W1 of the groove 16a.

[0047] The depth D1 of the groove 16a may be, for example, 0.5 μm or more and 50 μm or less, or 1 μm or more and 10 μm or less. The width W1 of the groove 16a may be, for example, 1 μm or more and 10,000 μm or less, or 10 μm or more and 1,000 μm or less. The thickness T1 of the electrode composite layer 12a may be, for example, 20 μm or more and 100 μm or less, or 50 μm or more and 80 μm or less. D1 / T1 may be, for example, 0.005 or more and 2.5 or less, or 0.0125 or more and 0.2 or less, respectively.

[0048] The same can be said for the thickness Y2 of the layered needle-shaped crystals 13b, the depth D2 and width W2 of the groove portion 16b, and the thickness T2 of the electrode composite layer 12b.

[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. The electrodes according to an embodiment of the present disclosure may be used as the positive electrode of a secondary battery or as the negative electrode of a secondary battery. The electrolyte may be a liquid electrolyte or a gel electrolyte.

[0051] Examples of secondary batteries include non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries and lithium metal secondary batteries.

[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 by, for example, applying a positive electrode composite slurry containing a positive electrode composite and a dispersion medium onto the surface of a positive electrode current collector and drying it. The dried coating film 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. As the dispersion medium of the positive electrode composite slurry, N-methyl-2-pyrrolidone (NMP) or the like is used.

[0055] Examples of the lithium-containing transition metal oxide include, 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. 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] In addition, examples of the lithium-containing transition metal oxide include Li ａ Ni ｂ M １－ｂ O ２(M may be at least one selected from the group consisting of Mn, Co, and Al, with 0 < a ≤ 1.2 and 0.3 ≤ b < 1.) From the viewpoint of increasing capacity, it is more preferable that 0.85 ≤ b < 1 is satisfied. From the viewpoint of crystal structure stability, 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) is also acceptable.

[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) or a gel 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] 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.

[0078] 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.

[0079] Examples of lithium salts include LiClO ４ LiBF ４ LiPF ６ LiAlCl ４ LiSbF ６ , LiSCN, LiCF ３ SO ３ LiCF ３ CO ２LiAsF ６ LiB １０ Cl １０ Examples 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 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.

[0080] [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.

[0081] 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 5. Figure 5 is a schematic perspective view in which a part of a secondary battery according to one embodiment of this disclosure is cut out.

[0082] 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.

[0083] 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.

[0084] [Note] The above description of embodiments discloses the following technologies: (Technology 1) An electrode for a secondary battery comprising: a sheet-shaped electrode current collector; an electrode composite layer supported on the main surface of the electrode current collector; and needle-shaped crystals covering at least a portion of the end face of the electrode composite layer. (Technology 2) The electrode for a secondary battery according to Technology 1, wherein the main surface of the electrode current collector has a first main surface and a second main surface opposite to the first main surface; the electrode composite layer has a first layer supported on the first main surface and a second layer supported on the second main surface; the end face of the electrode composite layer has a first end face of the first layer and a second end face of the second layer; and at least one of the first end face and the second end face is covered with the needle-shaped crystals. (Technology 3) The electrode for a secondary battery according to Technology 2, wherein each of the first end face and the second end face is covered with the needle-shaped crystals. (Technology 4) An electrode for a secondary battery according to any one of Techniques 1 to 3, wherein the needle-shaped crystals are formed in layers, and the thickness of the layered needle-shaped crystals is 0.5 μm or more and 0.9 μm or less. (Technology 5) An electrode for a secondary battery comprising: a sheet-shaped electrode current collector; an electrode composite layer supported on the main surface of the electrode current collector; a groove provided on the main surface of the electrode composite layer; and needle-shaped crystals covering at least a part of the inner surface of the groove. (Technology 6) An electrode for a secondary battery according to Technique 5, wherein the inner surface of the groove has an inner bottom surface and an inner surface, and at least one of the inner bottom surface and the inner surface is covered with the needle-shaped crystals. (Technology 7) An electrode for a secondary battery according to Technique 6, wherein each of the inner bottom surface and the inner surface is covered with the needle-shaped crystals. (Technology 8) An electrode for a secondary battery according to any one of Technology 5 to 7, wherein the needle-shaped crystals are formed in layers, and the thickness of the layered needle-shaped crystals is 0.2 μm or more and 0.45 μm or less. (Technology 9) An electrode for a secondary battery according to any one of Technology 1 to 8, wherein the needle-shaped crystals contain a metal oxide. (Technology 10) An electrode for a secondary battery according to any one of Technology 1 to 8, wherein the needle-shaped crystals contain the constituent elements of the electrode active material. (Technology 11) A secondary battery comprising a pair of electrodes and an electrolyte, wherein at least one of the pair of electrodes is an electrode for a secondary battery according to any one of Technology 1 to 10.

[0085] The present disclosure will be described in detail below based on examples, but the present disclosure is not limited to the following examples.

[0086] 《Secondary Batteries A1-A6》 (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 an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added and stirred to prepare a positive electrode mixture slurry.

[0087] 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 positive electrode composite layers (first layer, second 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 strip-shaped positive electrode was obtained. The thickness of each side of the positive electrode composite layer was 80 μm.

[0088] (Formation of needle-shaped crystals) Needle-shaped crystals were formed on the end faces of the positive electrode composite layer by laser irradiation. Specifically, needle-shaped crystals were formed on at least one of the first end face (end face 12AE) of the first layer (electrode composite layer 12a) and the second end face (end face 12BE) of the second layer (electrode composite layer 12b) at both ends ES1 and ES2 in the width direction of the strip-shaped positive electrode (Figures 1 and 2). At least one of needle-shaped crystals 13a, 23a and needle-shaped crystals 13b, 23b was formed. By appropriately adjusting the laser irradiation conditions (wavelength, power, frequency, etc.), needle-shaped crystals were formed in layers, and the thickness of the needle-shaped crystals was set to the values ​​shown in Table 1. The components of the needle-shaped crystals included metal oxides (aluminum oxide, etc.) containing the constituent elements of the positive electrode active material (NCA).

[0089] (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.

[0090] 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.

[0091] (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.

[0092] (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 assembly was fabricated by winding the positive and negative electrodes with a polyethylene separator in between. The electrode assembly 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 assembly, 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, a cylindrical non-aqueous electrolyte secondary battery (diameter 18 mm, height 65 mm) was fabricated.

[0093] Battery B1 was manufactured in the same manner as Battery A1, except that needle-shaped crystals were not formed on the end faces of the positive electrode composite layer at both ends ES1 and ES2 in the width direction of the strip-shaped positive electrode.

[0094] [Evaluation] The following evaluations were performed on each battery.

[0095] (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.

[0096] (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.

[0097] (Discharge) A constant current discharge was performed with a current of 700mA until the voltage reached 3V.

[0098] 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.

[0099] The evaluation results are shown in Table 1. Note that A1 to A6 in Table 1 are examples, and B1 is a comparative example.

[0100]

[0101] Batteries A1 to A6 showed higher capacity retention rates compared to battery B1. In particular, batteries A2 to A4, in which both the edges of the first and second layers were covered with needle-shaped crystals with a thickness of 0.5 to 0.9 μm, showed even greater capacity retention rates.

[0102] 《Batteries A7-A13》 (Formation of grooves) A ​​strip-shaped positive electrode was fabricated in the same manner as in battery A1. Then, grooves were formed on the main surfaces of the positive electrode composite layers (first layer, second layer) formed on both sides of the positive electrode current collector using a cutting blade. Specifically, groove 16a was formed on the main surface of the first layer (electrode composite layer 12a), and groove 16b was formed on the main surface of the second layer (electrode composite layer 12b) (Figures 3 and 4). The thickness of each side of the positive electrode composite layer was 70 μm. The grooves 16a and 16b each had a width of 100 μm and a depth of 30 μm. The grooves 16a and 16b were each formed linearly in the center of the width direction along the length direction (LD direction).

[0103] (Formation of needle-shaped crystals) Needle-shaped crystals 13a and 13b were formed on the inner surfaces (at least one of the inner bottom surface and inner surface) of the grooves 16a and 16b by laser irradiation (Figures 3 and 4). By appropriately adjusting the laser irradiation conditions (wavelength, power, frequency, etc.), needle-shaped crystals were formed in layers, and the thickness of the needle-shaped crystals was set to the values ​​shown in Table 2. The components of the needle-shaped crystals included metal oxides (aluminum oxide, etc.) containing the constituent elements of the positive electrode active material (NCA).

[0104] Batteries A7 to A13 were fabricated and evaluated in the same manner as battery A1, except that instead of a positive electrode in which the end face of the positive electrode composite layer was covered with needle-shaped crystals, a positive electrode in which the inner surface of the groove was covered with needle-shaped crystals was used.

[0105] Battery B2 was fabricated and evaluated in the same manner as Battery A7, except that needle-shaped crystals were not formed on the inner surface of the grooves.

[0106] The evaluation results are shown in Table 2. In Table 2, A7 to A13 are examples, and B2 is a comparative example.

[0107]

[0108] Batteries A7 to A13 showed a higher capacity retention rate compared to battery B2. In particular, batteries A8 to A10, in which both the inner bottom surface and inner surface of the groove were covered with needle-shaped crystals with a thickness of 0.20 to 0.45 μm, showed an even greater capacity retention rate.

[0109] The secondary battery described herein is useful as a primary power source for mobile communication devices, portable electronic devices, electric vehicles, and the like.

[0110] 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.

[0111] 1: Electrode group, 2: Positive electrode lead, 3: Negative electrode lead, 4: Battery case, 5: Sealing plate, 6: Negative electrode terminal, 7: Gasket, 8: Sealing plug, 10: Electrode, 11: Electrode current collector, 12a, 12b: Electrode composite layer, 13a, 13b, 23a, 23b: Needle crystal layer, 16a, 16b: Groove

Claims

1. An electrode for a secondary battery comprising: a sheet-shaped electrode current collector; an electrode composite layer supported on the main surface of the electrode current collector; and needle-shaped crystals covering at least a portion of the end face of the electrode composite layer.

2. The electrode for a secondary battery according to claim 1, wherein the main surface of the electrode current collector has a first main surface and a second main surface opposite to the first main surface, the electrode composite layer has a first layer supported on the first main surface and a second layer supported on the second main surface, the end surface of the electrode composite layer has a first end surface of the first layer and a second end surface of the second layer, and at least one of the first end surface and the second end surface is covered with the needle-shaped crystals.

3. The electrode for a secondary battery according to claim 2, wherein each of the first end face and the second end face is covered with the needle-shaped crystals.

4. The electrode for a secondary battery according to claim 1, wherein the needle-shaped crystals are formed in layers, and the thickness of the layered needle-shaped crystals is 0.5 μm or more and 0.9 μm or less.

5. An electrode for a secondary battery comprising: 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; and needle-shaped crystals covering at least a portion of the inner surface of the grooves.

6. The electrode for a secondary battery according to claim 5, wherein the inner surface of the groove has an inner bottom surface and an inner surface, and at least one of the inner bottom surface and the inner surface is covered with the needle-shaped crystals.

7. The electrode for a secondary battery according to claim 6, wherein the inner bottom surface and the inner surface are each covered with the needle-shaped crystals.

8. The electrode for a secondary battery according to claim 5, wherein the needle-shaped crystals are formed in layers, and the thickness of the layered needle-shaped crystals is 0.2 μm or more and 0.45 μm or less.

9. The electrode for a secondary battery according to claim 1 or 5, wherein the needle-shaped crystal contains a metal oxide.

10. The electrode for a secondary battery according to claim 1 or 5, wherein the needle-shaped crystals contain the constituent elements of the electrode active material.

11. 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 5.

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