Secondary battery negative electrode

Abstract

To provide a secondary battery negative electrode capable of improving the input-output characteristics of a secondary battery by smoothly moving charge carriers over the entire negative electrode active material layer having a multi-layer structure.SOLUTION: A secondary battery negative electrode includes a negative electrode collector and a negative electrode active material layer formed on the negative electrode collector and including a negative electrode active material. The negative electrode active material layer includes a first layer on the side of the negative electrode collector and a second layer on the surface side of the negative electrode active material layer; and when setting the aspect ratio of the negative electrode active material in the first layer to LD1, the perpendicularity of the negative electrode active material in the first layer to N1, the porosity of the first layer to S1, the aspect ratio of the negative electrode active material in the second layer to LD2, the perpendicularity of the negative electrode active material in the second layer to N2 and the porosity of the second layer to S2, the relations of the following formulae (1)-(3) are satisfied, LD1>LD2...the formula (1), N1>N2...the formula (2) and S1<S2...the formula (3).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a negative electrode for a secondary battery. [Background technology]

[0002] In recent years, secondary batteries such as lithium-ion secondary batteries and nickel-metal hydride batteries have been used as so-called portable power sources for personal computers, mobile terminals, etc., and as power sources for driving vehicles. In particular, lithium-ion secondary batteries, which are lightweight and have high energy density, are preferably used as high-output power sources for driving vehicles such as electric vehicles and hybrid vehicles. Secondary batteries are batteries that can be repeatedly charged and discharged as charge carriers move between the positive and negative electrodes.

[0003] A typical negative electrode used in this type of secondary battery is manufactured by applying a negative electrode mixture containing a material such as a negative electrode active material capable of reversibly absorbing and releasing charge carriers to a negative electrode current collector, followed by drying and rolling. The negative electrode manufactured in this manner has a configuration in which a negative electrode active material layer containing the negative electrode active material is formed on the negative electrode current collector. To improve the performance of such a negative electrode, a technology has been proposed in which different types of negative electrode active materials are used in each layer of a multi-layered negative electrode active material layer.

[0004] Patent Document 1 discloses a multilayered anode for a lithium secondary battery including an electrode current collector; a primary anode active material layer including a first anode active material formed on the electrode current collector; and a secondary anode active material layer formed on the primary anode active material layer and including a second anode active material having a relatively lower rolling density and a relatively larger average particle size than the first anode active material, and a lithium secondary battery including the same.

[0005] In this way, Patent Document 1 describes that a negative electrode for a lithium secondary battery includes a multilayer active material layer on an electrode current collector, the multilayer active material layer including two types of negative electrode active materials having different rolling densities and average particle sizes of the negative electrode active materials, thereby improving the porosity of the electrode surface even after the rolling process and improving ion mobility into the electrode, thereby improving the charging characteristics and life characteristics of the lithium secondary battery. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Special Publication No. 2015-511389 Summary of the Invention [Problem to be solved by the invention]

[0007] Incidentally, the smaller the average particle size of the negative electrode active material in the negative electrode active material layer formed on the negative electrode current collector, the more tortuous the migration path (hereinafter also referred to as the ion migration path) of charge carriers (e.g., lithium ions) within the negative electrode active material layer may be. In the technology described in Patent Document 1, the negative electrode active material (first negative electrode active material) in the layer (primary negative electrode active material layer) on the negative electrode current collector side of the negative electrode active material layer has a relatively small average particle size, which may increase the tortuousness of the ion migration path within the layer on the negative electrode current collector side of the negative electrode active material layer. Therefore, the technology described in Patent Document 1 has a problem in that charge carriers do not migrate smoothly deep into the negative electrode active material layer during charge and discharge, which may result in insufficient input / output characteristics of the secondary battery. In particular, the thicker the negative electrode active material layer, the longer the ion migration path in the thickness direction. Therefore, negative electrodes with thick negative electrode active material layers tend to be more susceptible to such problems.

[0008] The present disclosure has been made to solve such problems, and aims to provide a negative electrode for a secondary battery that improves the input / output characteristics of the secondary battery by allowing charge carriers to move smoothly throughout the entire negative electrode active material layer having a multilayer structure. [Means for solving the problem]

[0009] The negative electrode for a secondary battery according to one embodiment has a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector and containing a negative electrode active material. The negative electrode active material layer has a first layer on the negative electrode current collector side and a second layer on the surface side of the negative electrode active material layer. When the aspect ratio of the negative electrode active material in the first layer is LD1, the perpendicularity of the negative electrode active material in the first layer is N1, the porosity of the first layer is S1, the aspect ratio of the negative electrode active material in the second layer is LD2, the perpendicularity of the negative electrode active material in the second layer is N2, and the porosity of the second layer is S2, the negative electrode for a secondary battery satisfies the relationships of the following formulas (1) to (3). LD1 > LD2 ··· Formula (1) N1 > N2 ··· Formula (2) S1 < S2 ··· Formula (3) Here, the perpendicularity of the negative electrode active material is m1 / m2 when the number of negative electrode active materials with an inclination θn with respect to the surface of the negative electrode current collector being 60° ≤ θn ≤ 90° is m�1 and the number of negative electrode active materials with an inclination θn with respect to the surface of the negative electrode current collector being 0° ≤ θn ≤ 30° is m2.

Advantages of the Invention

[0010] According to the present disclosure, it is possible to provide a negative electrode for a secondary battery that improves the input / output characteristics of the secondary battery by enabling smooth movement of charge carriers throughout the entire negative electrode active material layer having a multilayer structure.

Brief Description of the Drawings

[0011] [Figure 1] It is a cross-sectional view schematically showing the negative electrode for a secondary battery according to Embodiment 1. [Figure 2] It is a diagram showing the manufacturing conditions of the negative electrodes according to the examples and comparative examples and the evaluation results of each evaluation cell. [Figure 3] It is a diagram showing the transition of the 1.0C / 0.1C capacity retention rate for each evaluation cell.

Modes for Carrying Out the Invention

[0012] Embodiment 1 Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. However, the present disclosure is not limited to the following embodiments. Furthermore, for clarity of explanation, the following description and drawings have been simplified as appropriate. What is shown in the drawings is only a part of the whole, and in reality, many other configurations not shown are included. In the following description, the same or equivalent elements are given the same reference numerals, and redundant explanations will be omitted.

[0013] FIG. 1 is a cross-sectional view schematically illustrating a negative electrode for a secondary battery according to a first embodiment. The term "secondary battery" generally refers to a battery that can be repeatedly charged, and includes so-called storage batteries such as lithium-ion secondary batteries and nickel-metal hydride batteries, as well as capacitors such as electric double-layer capacitors. Hereinafter, one preferred embodiment of the negative electrode for a secondary battery disclosed herein will be specifically described as negative electrode 1, which is a negative electrode for a lithium-ion secondary battery.

[0014] A lithium ion secondary battery is a secondary battery that uses lithium ions as a charge carrier and realizes charging and discharging by the transfer of charge associated with the lithium ions between a positive electrode and a negative electrode 1. Such lithium ion secondary batteries are used as power sources for driving vehicles such as battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

[0015] A lithium ion secondary battery has a configuration in which, for example, an electrode body in which a positive electrode, a negative electrode 1, and a separator interposed between the positive electrode and the negative electrode 1 are stacked is housed in a battery case together with a non-aqueous electrolyte. The electrode body may be a wound electrode body in which a stack of a positive electrode, a negative electrode 1, and a separator is wound, or may be a stacked electrode body in which a plurality of positive electrodes, a plurality of negative electrodes 1, and a plurality of separators are stacked.

[0016] The non-aqueous electrolyte is, for example, an electrolytic solution containing a non-aqueous solvent and a supporting salt. Such an electrolytic solution is a so-called non-aqueous electrolytic solution that does not use water as a solvent. As the non-aqueous solvent, various organic solvents that can be used in non-aqueous electrolytic solutions for lithium ion secondary batteries can be used without particular limitation. Examples of such organic solvents include aprotic solvents such as carbonates, ethers, esters, nitriles, sulfones, and lactones. Among them, carbonates such as ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) can be preferably used as the non-aqueous solvent. Such non-aqueous solvents may be used alone or as a mixed solvent of two or more.

[0017] The supporting salt may be any lithium salt that can be used in a non-aqueous electrolyte solution for a lithium ion secondary battery, such as LiPF, LiBF, LiClO, LiAsF, Li(CFSO)N, or LiCFSO, without any particular limitation. These may be used alone or in combination of two or more.

[0018] The nonaqueous electrolyte may contain components other than the nonaqueous solvent and supporting salt, such as various additives including oxalate complexes, film-forming agents such as vinylene carbonate (VC), gas generating agents such as biphenyl (BP) and cyclohexylbenzene (CHB), and thickeners.

[0019] The separator is disposed so as to insulate the positive electrode from the negative electrode 1. Examples of the separator include porous resin sheets (films) made of resins such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. Among these, polyolefin-based porous resin sheets (for example, PE and PP) are preferred.

[0020] The positive electrode has, for example, a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and containing a positive electrode active material. The positive electrode has a positive electrode active material layer formed on one or both sides of the positive electrode current collector. The positive electrode current collector can be made of a conductive material made of a metal with good conductivity, such as aluminum, an aluminum alloy, or stainless steel. The shape of the positive electrode current collector can be the same as the shape of the negative electrode current collector 10 described below. The shape and dimensions of the positive electrode current collector are not particularly limited and may be determined appropriately depending on the design of the secondary battery.

[0021] The positive electrode active material is a material capable of absorbing and releasing lithium ions. As the positive electrode active material, any positive electrode active material that can be used for the positive electrode of a lithium ion secondary battery can be used without any particular limitation. As the positive electrode active material, for example, a lithium-containing compound containing lithium element and one or more transition metal elements (for example, lithium transition metal composite oxide) can be used. Examples of such lithium-containing compounds include lithium nickel composite oxides (for example, LiNiO2), lithium cobalt composite oxides (for example, LiCoO2), lithium manganese composite oxides (for example, LiMn2O4), and lithium nickel cobalt manganese composite oxides (for example, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2), and other ternary lithium-containing composite oxides.

[0022] Alternatively, a polyanionic compound (e.g., LiFePO4, LiMnPO4, LiFeVO4, LiMnVO4, Li2FeSiO4, Li2MnSiO4, Li2CoSiO4) having a general formula such as LiMPO4, LiMVO4, or Li2MSiO4 (wherein M is at least one element selected from the group consisting of Co, Ni, Mn, and Fe) may be used as the positive electrode active material.

[0023] Furthermore, the positive electrode active material layer may contain other optional components such as a binder, a conductive material, etc., in addition to the positive electrode active material. The binder may be any binder that can be used in the positive electrode of a lithium-ion secondary battery, without any particular limitation. For example, when preparing an aqueous positive electrode mixture, the same binder as that used in the negative electrode 1 described below may be appropriately used. When preparing a solvent-based positive electrode mixture, a polymer material that dissolves in an organic solvent (non-aqueous solvent), such as polyvinylidene fluoride (PVDF) or polyvinylidene chloride (PVDC), may be used.

[0024] Here, the term "solvent-based positive electrode mixture" refers to a composition in which the dispersion medium for the positive electrode active material is primarily an organic solvent, such as N-methylpyrrolidone (NMP).

[0025] The conductive material can be any conductive material that can be used for the positive electrode of a lithium ion secondary battery, without any particular limitation. Examples of the conductive material that can be used include carbon materials such as carbon powder and carbon fiber. Examples of the carbon powder that can be used include various carbon blacks (e.g., acetylene black (AB), furnace black, ketjen black, etc.) and graphite powder. These materials may be used alone or in combination of two or more.

[0026] The positive electrode can be produced, for example, by applying a positive electrode mixture, which is a mixture of a positive electrode active material and, if necessary, a binder, a conductive material, and the like, to a positive electrode current collector, drying the mixture to form a positive electrode active material layer, and then rolling the mixture as necessary.

[0027] 1, the negative electrode 1 has a negative electrode current collector 10 and a negative electrode active material layer 11 formed on the negative electrode current collector 10 and containing a negative electrode active material 20. While FIG. 1 shows an example in which the negative electrode active material layer 11 is formed on one side of the negative electrode current collector 10, the negative electrode active material layer 11 may be formed on both sides of the negative electrode current collector 10.

[0028] The negative electrode current collector 10 can be a conductive member made of a metal with good conductivity, such as copper, a copper alloy, nickel, titanium, or stainless steel. Copper is preferred as the negative electrode current collector 10. The negative electrode current collector 10 may have various shapes, such as a rod, a plate, a sheet, or a foil. The shape and dimensions of the negative electrode current collector 10 are not particularly limited and may be determined appropriately depending on the design of the secondary battery. The thickness of the negative electrode current collector 10 is preferably, for example, about 5 to 50 μm.

[0029] The negative electrode active material 20 is a material capable of absorbing and releasing lithium ions. A particulate carbon material having at least a graphite structure (layer structure) in at least a portion thereof can be used as the negative electrode active material 20. Examples of the carbon material include natural graphite, artificial graphite, natural graphite at least partially coated with amorphous carbon, graphite, non-graphitizable carbonaceous material (hard carbon), easily graphitizable carbonaceous material (soft carbon), and composite materials thereof. These materials may be used alone or in combination of two or more.

[0030] Because they have excellent conductivity and can provide high energy density, at least one of natural graphite, artificial graphite, and natural graphite coated with amorphous carbon can be preferably used as the negative electrode active material 20. Artificial graphite is particularly preferable as the negative electrode active material 20 because it can be easily oriented by applying a magnetic field, has higher purity and lower resistance than natural graphite, and is of stable quality.

[0031] The average particle size of the negative electrode active material 20 is not particularly limited, but is preferably, for example, about 1 to 30 μm from the viewpoint of achieving both high energy density and high input / output characteristics at a high level. In this embodiment, the average particle size of the negative electrode active material 20 is a particle size corresponding to a cumulative 50% on a volume basis based on a laser diffraction / light scattering method.

[0032] The shape of the negative electrode active material 20 may be, for example, substantially spherical, slightly distorted spherical, scale-like, flat, etc. The aspect ratio of the negative electrode active material 20 is not particularly limited as long as it satisfies the formula (1) described below.

[0033] The aspect ratio is the average value of the ratio of the major axis diameter to the minor axis diameter (major axis diameter / minor axis diameter) of the negative electrode active material 20. The major axis diameter is the maximum diameter of the particles of the negative electrode active material 20, and the minor axis diameter is the maximum diameter among the diameters perpendicular to the major axis diameter. The aspect ratio can be confirmed, for example, by obtaining and observing an SEM image of at least a part of the cross section of the negative electrode active material layer 11 using a scanning electron microscope (SEM). To obtain the aspect ratio more accurately, it is preferable to calculate the aspect ratio by observing multiple cross sections (e.g., five or more) perpendicular to the surface of the negative electrode current collector 10. Note that the multiple cross sections perpendicular to the negative electrode current collector 10 may be set so as to divide approximately 360° evenly in a plan view, and SEM images of these multiple cross sections may be prepared.

[0034] Alternatively, in the SEM images of a plurality of cross sections, for example, a predetermined number (e.g., 10 or more) of negative electrode active materials 20 may be extracted from the negative electrode active materials 20 having a large apparent cross-sectional area, and the aspect ratio may be calculated for the extracted negative electrode active materials 20. In this embodiment, a predetermined number of negative electrode active materials 20 are extracted from each of the first layer 12 and the second layer 13.

[0035] The perpendicularity of the negative electrode active material 20 is not particularly limited as long as it satisfies the formula (2) described below. The perpendicularity of the negative electrode active material 20 can be adjusted by the strength and direction of the magnetic field applied in the orientation step when manufacturing the negative electrode 1.

[0036] The perpendicularity of the negative electrode active material 20 is m1 / m2, where m1 is the number of negative electrode active material 20 having an inclination θn of 60°≦θn≦90° with respect to the surface of the negative electrode current collector 10, and m2 is the number of negative electrode active material 20 having an inclination θn of 0°≦θn≦30° with respect to the surface of the negative electrode current collector 10. The inclination θn is the angle formed by the major axis diameter of the negative electrode active material 20 with respect to the surface of the negative electrode current collector 10.

[0037] m1 is the number of negative electrode active material particles 20 that are relatively upright with respect to the negative electrode current collector 10. Furthermore, m2 is the number of negative electrode active material particles 20 that are relatively lying flat with respect to the negative electrode current collector 10. In this way, the perpendicularity of the negative electrode active material particles 20 is evaluated by (the number of negative electrode active material particles 20 that are relatively upright with respect to the negative electrode current collector 10) / (the number of negative electrode active material particles 20 that are relatively lying flat with respect to the negative electrode current collector 10). Therefore, the perpendicularity of the negative electrode active material particles 20 can be used as an index for evaluating the degree to which the negative electrode active material particles 20 are upright with respect to the negative electrode current collector 10 in the negative electrode active material layer 11. That is, a perpendicularity of 1 indicates that the number of negative electrode active material particles 20 that are relatively upright with respect to the negative electrode current collector 10 is the same as the number of negative electrode active material particles 20 that are relatively lying flat with respect to the negative electrode current collector 10. On the other hand, the greater the perpendicularity is above 1, the more the negative electrode active material particles 20 are evaluated to be upright with respect to the negative electrode current collector 10. On the other hand, it can be evaluated that the smaller the perpendicularity is from 1, the more the negative electrode active material 20 lies flat relative to the negative electrode current collector 10 .

[0038] The perpendicularity can be confirmed, for example, by obtaining an X-ray CT image of at least a part of the cross section of the negative electrode active material layer 11 using an X-ray computed tomography apparatus (X-ray CT apparatus) and analyzing the image using analysis software. In order to obtain the perpendicularity more accurately, it is preferable to calculate the perpendicularity by analyzing multiple cross sections (for example, five or more) perpendicular to the surface of the negative electrode current collector 10.

[0039] The analysis of the multiple cross sections may be performed, for example, by extracting the distribution of the negative electrode active material 20 from a binarized image obtained by binarizing X-ray CT images of the multiple cross sections, and calculating the perpendicularity of the negative electrode active material 20 that has been ellipse-fitted. Furthermore, when calculating the perpendicularity, for example, the inclination θn of the negative electrode active material 20 relative to the surface of the negative electrode current collector 10 is determined based on a straight line along the longest distance of the negative electrode active material 20. The number of negative electrode active materials 20 whose inclination θn is 60°≦θn≦90° is defined as m1, and the number of negative electrode active materials 20 whose inclination θn is 0°≦θn≦30° is defined as m2, and the perpendicularity (m1 / m2) of the negative electrode active material 20 may be calculated. In this embodiment, the perpendicularity of the negative electrode active material 20 is determined for each of the first layer 12 and the second layer 13.

[0040] The negative electrode active material layer 11 has voids between particles of the negative electrode active material 20, etc. The voids in the negative electrode active material layer 11 can be impregnated with an electrolyte solution. The porosity of the negative electrode active material layer 11 indicates the proportion of voids in the negative electrode active material layer 11. The porosity of the negative electrode active material layer 11 is not particularly limited as long as it satisfies formula (3), but it is preferably 26 to 50%, for example. When the porosity of the negative electrode active material layer 11 is in the above range, lithium ions move more smoothly during charging and discharging of the secondary battery, thereby improving the input and output characteristics of the secondary battery. The porosity of the negative electrode active material layer 11 can be adjusted by, for example, the pressure applied to the negative electrode active material layer 11 in the rolling step when manufacturing the negative electrode 1.

[0041] The porosity of the negative electrode active material layer 11 is, for example, the apparent volume Va (cm 3 ) and the volume Vb (cm ) of the voids formed inside the negative electrode active material layer 11. 3 ) and the value calculated by the following formula (a): (Vb / Va) × 100 Equation (a)

[0042] The apparent volume Va is, for example, the area (cm ) of the negative electrode active material layer 11 in a plan view. 2 ) and the thickness (cm) of the negative electrode active material layer 11. The volume Vb can be measured, for example, using a mercury porosimeter that measures the pore distribution of a porous body by mercury intrusion porosimetry.

[0043] The method for measuring the porosity of the negative electrode active material layer 11 is not limited to the above method. The porosity of the negative electrode active material layer 11 can be calculated, for example, by multiplying the mass W (g) of the negative electrode active material layer 11 by the apparent volume V (cm 3 ) and the true density ρ of the negative electrode active material layer 11 (a value obtained by dividing the mass W by the actual volume excluding voids) by the following formula (b). (1-W / ρV)×100...Equation (b)

[0044] The true density ρ can be measured, for example, by a density measuring device such as a constant volume expansion method (gas displacement pycnometer method).

[0045] The porosity of the negative electrode active material layer 11 may be a value calculated, for example, by the following formula (c) from the area Sb occupied by voids contained per unit cross-sectional area of ​​the negative electrode active material layer 11 in a cross-sectional sample of the negative electrode active material layer 11 and the apparent cross-sectional area Sa of the negative electrode active material layer 11. (Sb / Sa) × 100 Equation (c)

[0046] The areas Sa and Sb can be measured, for example, by observing the cross section of the negative electrode active material layer 11 using an SEM. From the SEM image, voids formed in the cross section of the constituent material of the negative electrode active material layer 11 can be identified based on differences in color tone and shading. A sample for observation can be cut out from the negative electrode 1, solidified with resin, and then polished with a cross section polisher (CP) to obtain a precise cross section. The more samples there are, the more accurately the void ratio can be determined.

[0047] Furthermore, the negative electrode active material layer 11 may contain other optional components such as a binder 30 and a thickener in addition to the negative electrode active material 20. In Fig. 1, the negative electrode active material 20 and the binder 30 in the negative electrode active material layer 11 are shown enlarged and schematically so as to clarify the structure of the negative electrode active material layer 11.

[0048] The binder 30 can be any binder that can be used for the negative electrode of a lithium-ion secondary battery, without any particular limitation. For example, when preparing an aqueous negative electrode mixture, a polymer material that dissolves or disperses in water can be suitably used as the binder 30. Examples of water-soluble (water-soluble) polymer materials include cellulose-based polymers such as carboxymethyl cellulose (CMC), methyl cellulose (MC), cellulose acetate phthalate (CAP), and hydroxypropyl methyl cellulose (HPMC); polyvinyl alcohol (PVA); and the like. Examples of water-dispersible (water-dispersible) polymer materials include fluorine-based resins such as polytetrafluoroethylene (PTFE); vinyl acetate copolymers; and rubbers such as styrene-butadiene rubber (SBR). The polymer materials exemplified above can be used not only as binders but also as thickeners or other additives for the negative electrode mixture.

[0049] Here, the aqueous negative electrode mixture is a concept that refers to a composition in which the dispersion medium of the negative electrode active material 20 is water or a mixed solvent (aqueous solvent) mainly composed of water. As the solvent other than water that constitutes such a mixed solvent, one or more organic solvents (lower alcohols, lower ketones, etc.) that can be uniformly mixed with water can be appropriately selected and used.

[0050] The proportion of the negative electrode active material 20 in the entire negative electrode active material layer 11 is 50% by mass or more, for example, preferably 90 to 99% by mass, and particularly preferably 95 to 99% by mass, from the viewpoint of realizing a high energy density. When a binder 30 is used, the proportion of the binder 30 in the entire negative electrode active material layer 11 is 0.5 to 10% by mass, for example, preferably 0.5 to 5% by mass, from the viewpoint of ensuring mechanical strength (shape retention). When a thickener is used, the proportion of the thickener in the entire negative electrode active material layer 11 is 0.5 to 10% by mass, and preferably 0.5 to 5% by mass.

[0051] The negative electrode active material layer 11 has a multilayer structure including a plurality of layers stacked in the thickness direction of the negative electrode 1. The negative electrode active material layer 11 has a first layer 12 on the negative electrode current collector 10 side, and a second layer 13 on the electrode surface side, which is the surface of the negative electrode active material layer 11 opposite to the negative electrode current collector 10.

[0052] The first layer 12 is disposed between the negative electrode current collector 10 and the second layer 13. The first layer 12 is disposed closer to the negative electrode current collector 10 than the second layer 13. The first layer 12 includes, for example, the back surface of the negative electrode active material layer 11. The first layer 12 is, for example, a layer of the negative electrode active material layer 11 whose thickness from the negative electrode current collector 10 is 0 to 50%.

[0053] The second layer 13 is disposed closer to the electrode surface than the first layer 12. The electrode surface is the surface facing the positive electrode active material layer in a secondary battery. The second layer 13 includes, for example, the electrode surface. The second layer 13 is laminated on, for example, the first layer 12. The second layer 13 is, for example, a layer of the negative electrode active material layer 11 whose thickness from the negative electrode current collector 10 is 50 to 100%. In other words, the second layer 13 is a layer of the negative electrode active material layer 11 whose thickness from the electrode surface is 0 to 50%.

[0054] 1 has a two-layer structure including a first layer 12 formed on the negative electrode current collector 10 and a second layer 13 formed on the first layer 12. Note that the negative electrode active material layer 11 may have a multi-layer structure of three or more layers including another layer (not shown) different from the first layer 12 and the second layer 13 between the first layer 12 and the second layer 13, as long as it includes the first layer 12 on the negative electrode current collector 10 side and the second layer 13 on the electrode surface side.

[0055] The negative electrode 1 according to this embodiment satisfies the relationships of the following formulas (1) to (3), where LD1 is the aspect ratio of the negative electrode active material 20 in the first layer 12, N1 is the perpendicularity of the negative electrode active material 20 in the first layer 12, S1 is the porosity of the first layer 12, LD2 is the aspect ratio of the negative electrode active material 20 in the second layer 13, N2 is the perpendicularity of the negative electrode active material 20 in the second layer 13, and S2 is the porosity of the second layer 13. LD1>LD2...Formula (1) N1 > N2 ··· Formula (2) S1 < S2 ··· Formula (3)

[0056] In the negative electrode 1 satisfying the above formulas (1) to (3), since the first layer 12 contains the negative electrode active material 20 having a relatively larger aspect ratio and perpendicularity than the negative electrode active material 20 contained in the second layer 13, when rolling the negative electrode 1 during the rolling process for manufacturing, the voids in the second layer 13 on the electrode surface side are difficult to be filled (difficult to decrease). And in the negative electrode 1 satisfying the above formulas (1) to (3), since the negative electrode active material 20 having a large aspect ratio contained in the first layer 12 is oriented so as to approach perpendicular to the surface of the negative electrode current collector 10, the degree of bending of the lithium ion movement path (hereinafter also referred to as the ion movement path) along the thickness direction in the first layer 12 is reduced.

[0057] Thus, in the negative electrode 1 according to the present embodiment, the decrease in the voids in the second layer 13 on the electrode surface side is suppressed, and the increase in the degree of bending of the ion movement path along the thickness direction in the first layer 12 is also suppressed. Thereby, the movement of lithium ions is smoothly performed within each layer of the first layer 12 on the negative electrode current collector 10 side and the second layer 13 on the electrode surface side. Therefore, according to the negative electrode 1 according to the present embodiment, the movement of lithium ions is smoothly performed over the entire negative electrode active material layer 11 having a multilayer structure, so that the input / output characteristics of the secondary battery can be improved.

[0058] Furthermore, in the negative electrode 1 satisfying the above formulas (1) to (3), when the total negative electrode active material contained in the first layer 12 is 100% by mass, it is preferable that the ratio of the negative electrode active material 20 having an aspect ratio of 1.8 or more is 70% by mass or more. Thereby, since the increase in the degree of bending of the ion movement path along the thickness direction in the first layer 12 is preferably suppressed, the input / output characteristics of the secondary battery can be surely improved.

[0059] Furthermore, in the negative electrode 1 satisfying the above formulas (1) to (3), the proportion of the negative electrode active material 20 having an aspect ratio of 1.8 or more in the second layer 13 is preferably less than 70 mass % when the total amount of the negative electrode active material contained in the second layer 13 is taken as 100 mass %. This suitably suppresses a decrease in the voids in the second layer 13 on the electrode surface side, thereby reliably improving the input / output characteristics of the secondary battery.

[0060] In addition, in negative electrode 1 satisfying the above formulae (1) to (3), the porosity of first layer 12 is preferably 80% or less of the porosity of second layer 13. This suitably suppresses a decrease in the porosity in second layer 13 on the electrode surface side, thereby reliably improving the input / output characteristics of the secondary battery.

[0061] Here, the thickness and density of the negative electrode active material layer 11 can be adjusted by the rolling process during the production of the negative electrode 1. The thickness of the negative electrode active material layer 11 is preferably, for example, 10 to 100 μm. The density of the negative electrode active material layer 11 is preferably, for example, 1.1 to 1.6 g / cm. 3 The weight of the negative electrode active material layer 11 per surface of the negative electrode current collector 10 is preferably 20 mg / cm 2 , for example. 2 The weight per unit area of ​​the negative electrode active material layer 11 is the mass of the negative electrode active material layer 11 provided per unit area of ​​the negative electrode current collector 10.

[0062] In the negative electrode 1 that satisfies the above formulas (1) to (3), the negative electrode 1 has a negative electrode active material layer 11 in which at least one of the above properties (thickness, density, and basis weight) satisfies the above range, and therefore an ion migration path can be easily secured within the negative electrode active material layer 11, allowing lithium ions to smoothly migrate deep into the negative electrode active material layer 11, thereby further improving the input / output characteristics of the secondary battery.

[0063] Next, an example of a method for manufacturing the negative electrode 1 according to this embodiment will be described. The method for manufacturing the negative electrode 1 includes, for example, the following preparation step, active material layer formation step, and rolling step. The preparation step is a step of preparing a negative electrode mixture. In the preparation step, a first negative electrode mixture is prepared by mixing a negative electrode active material 20 having an aspect ratio LD1, a binder 30, and a solvent, and a second negative electrode mixture is prepared by mixing a negative electrode active material 20 having an aspect ratio LD2 smaller than the aspect ratio LD1, a binder 30, and a solvent. The active material layer formation step is a step of applying the negative electrode mixtures (first negative electrode mixture and second negative electrode mixture) prepared in the preparation step to the negative electrode current collector 10, thereby forming a negative electrode active material layer 11 on the negative electrode current collector 10. The rolling step is a step of rolling the negative electrode active material layer 11 formed on the negative electrode current collector 10.

[0064] First, the active material layer forming process may include, for example, a first coating process, an orientation process, a first drying process, a second coating process, and a second drying process. The first coating process is a process of coating a first negative electrode mixture on a negative electrode current collector 10. The first coating process forms a first layer 12 on the negative electrode current collector 10. The orientation process is a process of applying a magnetic field to the first layer 12 formed on the negative electrode current collector 10 to adjust the orientation of the negative electrode active material 20 in the first layer 12. The orientation process aligns the negative electrode active material 20 in the first layer 12 with respect to the negative electrode current collector 10 so that 60°≦θn≦90° is satisfied. The first drying process is a process of drying the first layer 12 after the orientation process. The second coating process is a process of coating a second negative electrode mixture on the first layer 12 after the first drying process. The second coating process forms a second layer 13 on the first layer 12. The second drying step is a step of drying the second layer 13 formed on the first layer 12. In the second drying step, the negative electrode active material layer 11 formed on the negative electrode current collector 10 is dried.

[0065] According to this active material layer forming process, the first layer 12 formed by applying the first anode mixture in the first applying process is dried in the first drying process, and then the second anode mixture is applied in the second applying process, so that mixing of the first anode mixture and the second anode mixture can be suppressed. Furthermore, according to this active material layer forming process, the anode active material 20 in the first layer 12 is oriented in the orienting process so that the angle θn is 60°≦θn≦90° with respect to the anode current collector 10, and then the first layer 12 is dried in the first drying process, so that the perpendicularity of the anode active material 20 in the first layer 12 can be largely maintained.

[0066] After the second drying step, the negative electrode active material layer 11 including the first layer 12 and the second layer 13 is rolled in the rolling step. The rolling step forms a negative electrode active material layer 11 in which the porosity of the second layer 13 on the electrode surface side is relatively higher than the porosity of the first layer 12 on the negative electrode current collector 10 side. In the rolling step, it is preferable to adjust the rolling ratio so that the perpendicularity of the negative electrode active material 20 in the first layer 12 does not become smaller than necessary. This makes it possible to reduce the perpendicularity of the negative electrode active material 20 in the second layer 13 while maintaining a large perpendicularity of the negative electrode active material 20 in the first layer 12.

[0067] In addition, when the negative electrode active material layer 11 has a multi-layer structure of three or more layers including another layer different from the first layer 12 and the second layer 13, the active material layer forming step may further include a step of forming another layer on the first layer 12 after the first drying step and before the second application step.

[0068] Next, the present disclosure will be described more specifically based on examples with reference to FIGS. 2 and 3. However, the present disclosure is not limited to these examples. FIG. 2 shows the manufacturing conditions of the negative electrodes according to the examples and comparative examples, and the evaluation results of each evaluation cell. FIG. 3 is a graph showing the progress of the 1.0C / 0.1C capacity retention rate for each evaluation cell. The vertical axis of the graph in FIG. 3 shows the voltage (V) of the evaluation cell, and the horizontal axis of the graph in FIG. 3 shows the 1.0C / 0.1C capacity retention rate (% ) is shown.

[0069] <Preparation of evaluation cell> Evaluation cells were fabricated using the negative electrodes of Example and Comparative Examples 1 and 2, and the tortuosity of the ion migration path and the 1.0 C / 0.1 C capacity retention rate (%) of each of the fabricated evaluation cells were evaluated. Each evaluation cell was a laminated cell with a rated capacity of 155 mAh.

[0070] <Positive electrode of evaluation cell> The positive electrode of the evaluation cell used an aluminum foil with a thickness of 30 μm as a positive electrode current collector. The mass ratio of the solid content of the positive electrode mixture prepared for forming the positive electrode active material layer was positive electrode active material:conductive material:binder=97.8:0.8:1.4. The positive electrode active material was LiNi 0.8 Co 0.1 Mn 0.1 O2 (lithium nickel cobalt manganese composite oxide) particles were used. The same positive electrode active material was used for each evaluation cell. AB was used as the conductive material. PVDF was used as the binder.

[0071] <Negative electrode of evaluation cell> The negative electrode of the evaluation cell used a 15 μm-thick copper foil as the negative electrode current collector. The mass ratio of the solid content of the negative electrode mixture prepared for forming the negative electrode active material layer was negative electrode active material: thickener: binder = 97:0.6:2.4. The negative electrode active material used was at least one of flake graphite, a type of natural graphite, and spherical graphite obtained by processing flake graphite into spheres by mechanical processing. CMC was used as the thickener. SBR was used as the binder.

[0072] <Evaluation cell separator> A polyethylene (PE) resin sheet was used as the separator.

[0073] <Electrolyte for evaluation cell> The electrolyte used was a non-aqueous electrolyte prepared by mixing EC, DEC, and EMC in a volume ratio of 1:1:1 and adding LiPF6 as a supporting salt at a concentration of about 1 mol / liter.

[0074] <Bending degree> The tortuosity τ of the ion migration path in the negative electrode active material layer of the negative electrodes according to Example and Comparative Examples 1 and 2 was calculated using the following formula (4). The results are shown in FIG. Curvature degree τ=(Rion A K ε) / 2d...Equation (4)

[0075] Rion is the ionic resistance, A is the area of ​​the negative electrode, K is the ionic conductivity of the electrolyte, ε is the porosity of the negative electrode active material layer, and d is the thickness of the negative electrode active material layer.

[0076] The ionic resistance Rion was calculated by measuring the symmetrical cell impedance of a symmetrical model cell and deriving it from the real component of the measured symmetrical cell impedance at the lowest frequency (=ionic resistance Rion / 3). The ionic conductivity K of the electrolyte was calculated from the measured resistance at 25°C and 10 kHz of a sample in which the electrolyte of the above composition was sealed in a cell containing Li metal. The porosity ε of the negative electrode active material layer was measured using mercury intrusion porosimetry.

[0077] <1.0C / 0.1C capacity maintenance rate> The evaluation cell was charged at a constant current of 0.1 C in a voltage range of 3.0 V to 4.25 V at 25°C, followed by a constant current discharge at 0.1 C. Next, the evaluation cell was charged at a constant current of 1.0 C in a voltage range of 3.0 V to 4.25 V at 25°C, followed by a constant current discharge at 1.0 C. The 1.0 C / 0.1 C capacity retention rate (%) was calculated from the discharge capacity (mAh) at 0.1 C and the discharge capacity (mAh) at 1.0 C using the following formula (5). The results are shown in Figures 2 and 3. (1.0C discharge capacity / 0.1C discharge capacity)×100(%)...Formula (5) It can be said that the higher the value of this 1.0C / 0.1C capacity retention rate (%), the higher the electronic conductivity and ionic conductivity of the negative electrode active material layer, and the better the input / output characteristics.

[0078] (Example) First, in the preparation step, flake graphite, CMC, and SBR were mixed in the above-mentioned mass ratio, and water was added to this mixture to prepare a first negative electrode mixture having a solid content ratio of 53.5 mass%. Also, in the preparation step, spherical graphite, CMC, and SBR were mixed in the above-mentioned mass ratio, and water was added to this mixture to prepare a second negative electrode mixture having a solid content ratio of 52.5 mass%. Next, in the first application step, the basis weight of the first layer 12 was 13 mg / cm. 2 The first negative electrode mixture was applied to one side of the copper foil so that the first layer 12 was formed by applying the negative electrode mixture. Next, in a magnetic field orientation step, a magnetic field was applied to the first layer 12 formed by applying the negative electrode mixture under magnetic field application conditions of 2 T and 5 minutes. Next, in a first drying step, the first layer 12 after the magnetic field application was dried by heating under drying conditions of 50°C for 0.5 hours.

[0079] Next, in the second application step, the basis weight of the second layer 13 is 13 mg / cm 2 The second negative electrode mixture was applied onto the first layer 12 formed on the copper foil so that the thickness of the first layer 12 was 216 μm and the density of the second layer 13 formed on the first layer 12 were the same as those of the first layer 12. Next, in a second drying step, the second layer 13 was dried by heating at 50° C. for 0.5 hours under drying conditions. Next, in a rolling step, the negative electrode active material layer formed on the copper foil was rolled so that the thickness of the negative electrode active material layer was 216 μm and the density was 1.2 g / cc, thereby producing the negative electrode 1 according to this example shown in FIG.

[0080] The negative electrode 1 according to the example thus fabricated includes a copper foil and a two-layer negative electrode active material layer 11 formed on the copper foil. As shown in FIG. 2 , in the negative electrode 1 according to the example, the aspect ratio of the flake graphite in the first layer 12 was 2.3, the perpendicularity of the flake graphite in the first layer 12 was 0.22, and the porosity of the first layer 12 was 27.5%. In addition, in the negative electrode 1 according to the example, the aspect ratio of the spherical graphite in the second layer 13 was 1.6, the perpendicularity of the spherical graphite in the second layer 13 was 0.18, and the porosity of the second layer 13 was 36.1%. That is, the negative electrode 1 according to the example satisfies the formulas (1) to (3).

[0081] (Comparative Example 1) Only flake graphite was used as the negative electrode active material, and the coating weight of the negative electrode active material layer in the first coating step was 26 mg / cm 2 The negative electrode of Comparative Example 1 shown in FIG. 2 was fabricated in the same manner as in the Examples, except that the first negative electrode mixture was applied to the copper foil so that the first negative electrode mixture was 0.23, and the second application step and the second drying step were omitted. The negative electrode of Comparative Example 1 fabricated in this manner had a copper foil and a single-layer negative electrode active material layer formed on the copper foil. As shown in FIG. 2, in the negative electrode of Comparative Example 1, the aspect ratio of the flake graphite in the negative electrode active material layer was 2.3, the perpendicularity of the flake graphite in the negative electrode active material layer was 0.23, and the porosity of the negative electrode active material layer was 33.2%. In other words, Comparative Example 1 is a negative electrode that does not satisfy formulas (1) to (3).

[0082] (Comparative Example 2) The negative electrode of Comparative Example 2 shown in FIG. 2 was fabricated in the same manner as Comparative Example 1, except that the orientation step was omitted. The negative electrode of Comparative Example 2 fabricated in this manner had a copper foil and a single-layer negative electrode active material layer formed on the copper foil. As shown in FIG. 2, in the negative electrode of Comparative Example 2, the aspect ratio of the flake graphite in the negative electrode active material layer was 2.4, the perpendicularity of the flake graphite in the negative electrode active material layer was 0.12, and the porosity of the negative electrode active material layer was 33.0%. In other words, Comparative Example 2 is a negative electrode that does not satisfy formulas (1) to (3).

[0083] <Evaluation results> As shown in Figure 2, the negative electrode of Comparative Example 1, which used only flake graphite as the negative electrode active material in a single-layer negative electrode active material layer, did not have second layer 13, and therefore the tortuosity of the ion migration path in the negative electrode active material layer was as large as 4.0. Furthermore, as shown in Figures 2 and 3, the 1.0 C / 0.1 C capacity retention rate of the evaluation cell using the negative electrode of Comparative Example 1 was 64%.

[0084] 2, in the negative electrode of Comparative Example 2, the flake graphite was not oriented, and therefore the degree of perpendicularity of the flake graphite was reduced compared to the negative electrode of Comparative Example 1, resulting in an increase in the degree of tortuosity of the ion migration path in the negative electrode active material layer to 5.06. This prevented smooth migration of lithium ions in the negative electrode active material layer, and as shown in FIGS. 2 and 3, the 1.0 C / 0.1 C capacity retention rate of the evaluation cell using the negative electrode of Comparative Example 2 was a low value of 48%.

[0085] In contrast, the negative electrode 1 according to the example, in which flake graphite and spherical graphite were used as the negative electrode active material 20 in the first layer 12 and the second layer 13 of the two-layered negative electrode active material layer 11, had a small tortuosity of 1.8 for the ion migration path within the negative electrode active material layer 11. This allowed for smooth migration of lithium ions within the negative electrode active material layer 11, and as shown in Figures 2 and 3, the 1.0 C / 0.1 C capacity retention rate of the evaluation cell using the negative electrode 1 according to the example was a high value of 66%.

[0086] In the negative electrode 1 according to the example, the perpendicularity N1 of the flake graphite having a relatively large aspect ratio LD1 in the first layer 12 on the copper foil side was made larger than the perpendicularity N2 of the spherical graphite having a relatively small aspect ratio LD2 in the second layer 13 on the electrode surface side, thereby obtaining a negative electrode active material layer 11 in which the porosity S1 of the first layer 12 was smaller than the porosity S2 of the second layer 13 after the rolling process. In this way, it was confirmed that the negative electrode 1 according to the example, in which both the decrease in voids in the second layer 13 on the electrode surface side and the increase in the tortuosity of the ion migration path in the first layer 12 were suppressed, realized an evaluation cell with superior input / output characteristics compared to the negative electrodes according to Comparative Examples 1 and 2.

[0087] As is clear from these evaluation results, according to this embodiment, lithium ions can be smoothly transferred throughout the entire negative electrode active material layer 11 having a multi-layer structure, thereby providing a negative electrode 1 that improves the input / output characteristics of a secondary battery.

[0088] The present disclosure is not limited to the above-described embodiment, and can be modified as appropriate within the scope of the present disclosure. [Explanation of symbols]

[0089] 1 Negative electrode 10 Negative electrode current collector 11 Negative electrode active material layer 12 1st layer 13 2nd layer 20 negative electrode active material 30 binder

Claims

[Claim 1] a negative electrode current collector; a negative electrode active material layer formed on the negative electrode current collector and containing a negative electrode active material, The negative electrode active material layer is a first layer on the negative electrode current collector side; a second layer on the surface side of the negative electrode active material layer, an aspect ratio of the negative electrode active material in the first layer is defined as LD1; the perpendicularity of the negative electrode active material in the first layer is N1; The porosity of the first layer is S1, the aspect ratio of the negative electrode active material in the second layer is defined as LD2; the perpendicularity of the negative electrode active material in the second layer is N2; When the porosity of the second layer is S2, A negative electrode for a secondary battery that satisfies the relationships of the following formulas (1) to (3): LD1>LD2...Formula (1) N1>N2...Formula (2) S1<S2...Formula (3) Here, the perpendicularity of the negative electrode active material is m1 / m2, where m1 is the number of the negative electrode active materials having an inclination θn of 60°≦θn≦90° with respect to the surface of the negative electrode current collector, and m2 is the number of the negative electrode active materials having an inclination θn of 0°≦θn≦30° with respect to the surface of the negative electrode current collector.

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