Negative electrode for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery

Abstract

A negative electrode (30) for nonaqueous electrolyte secondary batteries according to the present invention comprises a negative electrode collector (32) and a negative electrode active material layer (34) that is provided on the negative electrode collector (32); and with respect to the negative electrode active material layer (34), first active material layers (X) and second active material layers (Y) are alternately arranged on the negative electrode collector (32), the first and second active material layers containing, as a negative electrode active material, graphite particles A and graphite particles B, while being different from each other in the content of the graphite particles A relative to the total mass of the graphite particles A and the graphite particles B. The internal void fraction of the graphite particles A is lower than the internal void fraction of the graphite particles B; and the content of the graphite particles A in the first active material layers (X) is higher than the content of the graphite particles A in the second active material layers (Y). The ratio of the width Wx of the first active material layers (X) to the width Wy of the second active material layers (Y), namely Wx / Wy is 0.03 to 3.13.

Description

[Technical Field]

[0001] Book Disclosure The present invention relates to a negative electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries that use carbon materials as negative electrode active materials are widely used as secondary batteries with high energy density. Furthermore, non-aqueous electrolyte secondary batteries used as power sources for electric vehicles (EVs) and the like are desired to have excellent charge-discharge cycle characteristics.

[0003] Patent Document 1 describes a nonaqueous electrolyte secondary battery in which a widthwise central portion of the active material layer of a strip-shaped negative electrode, where the packing density is low, and other portions, where the packing density is high, are arranged in stripes on a current collector. This configuration is said to reduce the occurrence of decomposition of the electrolyte and deterioration of the active material in the widthwise central portion of the strip-shaped negative electrode, thereby achieving excellent charge-discharge cycle characteristics.

[0004] Patent Document 2 describes a configuration in which grooves are formed on the surface of the negative electrode active material layer to improve the permeability of the electrolyte in a non-aqueous electrolyte secondary battery in order to improve the charge / discharge cycle.

[0005] Patent Document 3 describes a nonaqueous electrolyte secondary battery having a first active material portion containing active material particles with a large average particle size formed partially on a negative electrode current collector along multiple lines that cross the negative electrode active material layer, and a second active material portion containing active material particles with a small average particle size formed to cover the first active material portion. This configuration allows the electrolyte to easily penetrate into the center of the active material layer through the first active material portion, which has active material particles with a large average particle size, and is therefore less likely to reduce capacity retention in low-temperature environments. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-90980 [Patent Document 2] Japanese Patent Application Publication No. 9-298057 [Patent Document 3] Japanese Patent Application Laid-Open No. 2013-246900 Summary of the Invention [Problem to be solved by the invention]

[0007] However, conventional negative electrodes for non-aqueous electrolyte secondary batteries and non-aqueous electrolyte secondary batteries have room for improvement in terms of achieving both excellent charge-discharge cycle characteristics and high capacity. In the configuration described in Patent Document 1, a region with low packing density is formed in part of the active material layer of the negative electrode, resulting in a decrease in battery capacity.

[0008] Furthermore, in the configuration described in Patent Document 2, the packing density of the active material in the grooved portion of the negative electrode is high, making it difficult to increase the permeability of the electrolyte in the grooved portion, which leaves room for improvement in terms of obtaining excellent charge-discharge cycle characteristics.

[0009] In the configuration described in Patent Document 3, the surface of the negative electrode plate is covered with the small-particle active material of the second active material portion, which restricts the movement of electrolyte from the negative electrode active material layer to the positive electrode, thereby limiting the improvement of charge-discharge cycle characteristics. Furthermore, if an active material with large expansion and contraction, such as silicon, is used in combination with an active material with large particle size of the first active material portion in order to increase capacity, the number of contact points between the active material particles decreases with charge-discharge cycles. This makes it difficult to ensure contact points, i.e., electrical conductivity, due to the expansion and contraction of silicon, making it difficult to achieve both excellent charge-discharge cycle characteristics and high capacity.

[0010] Therefore, an object of the present disclosure is to provide a negative electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery that can achieve both excellent charge-discharge cycle characteristics and high capacity. [Means for solving the problem]

[0011] A negative electrode for a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector, the negative electrode active material layer including graphite particles A and graphite particles B as negative electrode active materials, and first active material layers X and second active material layers Y having different contents of graphite particles A relative to the total mass of graphite particles A and B are alternately arranged on the negative electrode current collector, the internal porosity of graphite particles A is smaller than the internal porosity of graphite particles B, the content of graphite particles A in first active material layer X is greater than the content of graphite particles A in second active material layer Y, and the ratio Wx / Wy of the width Wx of first active material layer X to the width Wy of second active material layer Y is 0.03 or more and 3.13 or less.

[0012] A nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure includes a negative electrode for a nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure, a positive electrode, and a separator between the negative electrode for a nonaqueous electrolyte secondary battery and the positive electrode. [Effects of the Invention]

[0013] According to one aspect of the present disclosure, it is possible to achieve both excellent charge-discharge cycle characteristics and high capacity for a nonaqueous electrolyte secondary battery. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a perspective view of a nonaqueous electrolyte secondary battery according to an embodiment; [Figure 2] FIG. 1 is a schematic perspective view showing an electrode assembly including a negative electrode according to an embodiment. [Figure 3] FIG. 2 is a cross-sectional view of a negative electrode according to an embodiment of the present invention. [Figure 4] FIG. 2 is a cross-sectional view of a graphite particle in a negative electrode active material layer. [Figure 5] FIG. 1 is a graph showing the relationship between the width ratio Wx / Wy of the active material layers X and Y and the capacity retention rate (cycle retention rate) at the 300th cycle, based on measurement results using nonaqueous electrolyte secondary batteries of Examples and Comparative Examples. [Figure 6] FIG. 10 is a schematic perspective view showing an electrode assembly including a negative electrode according to another embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0015] (Findings that formed the basis of this disclosure) In the negative electrode, the negative electrode active material is placed on a negative electrode current collector and compressed with a roller or the like to increase the packing density and energy density. This compression crushes the graphite particles, reducing the voids between the active material, thereby increasing the packing density and energy density. Meanwhile, during charging, the active material of the negative electrode expands, expelling the non-aqueous electrolyte present between the active material layers. During discharge, the active material of the negative electrode contracts, allowing the non-aqueous electrolyte to penetrate back into the active material layer. However, if the voids between the active material are reduced in order to achieve high capacity, the non-aqueous electrolyte will have difficulty penetrating into the active material layer.

[0016] When graphite with a relatively small internal porosity is used as the negative electrode active material, it is possible to ensure voids between the active material when compressed. This allows for faster penetration of the non-aqueous electrolyte during charge-discharge cycles, thereby alleviating any partial deficiency of the non-aqueous electrolyte. Therefore, excellent charge-discharge cycle characteristics can be expected. On the other hand, when graphite with a relatively small internal porosity is used, the active material is less likely to be crushed during compression, making it difficult to increase the packing density. Furthermore, because the contact area with the surrounding active material is small during compression, it is difficult to maintain contact with the surrounding active material when the active material expands and contracts during charge and discharge, and the reduced contact area increases electrical resistance.

[0017] When graphite with a relatively large internal porosity is used as the negative electrode active material, the active material is crushed and deformed during compression, increasing the packing density. On the other hand, when graphite with a relatively large internal porosity is used, the gaps between the active material are reduced, making it difficult for the non-aqueous electrolyte to penetrate during charge-discharge cycles, resulting in a partial shortage of non-aqueous electrolyte.

[0018] Furthermore, although the negative electrode current collector constituting the negative electrode and the graphite particles disposed thereon are bound together by a binder or the like, graphite particles with low internal porosity are less likely to be crushed when forming the electrode, and the binding strength between the negative electrode current collector and the graphite particles is likely to decrease. Therefore, depending on the state of charge, the graphite particles tend to peel off from the negative electrode current collector, and therefore, simply using graphite particles with low internal porosity may not effectively prevent a decrease in charge-discharge cycle performance.

[0019] The present inventors have discovered that by alternately arranging layers containing many graphite particles with small internal porosity and layers containing many graphite particles with large internal porosity in an appropriate ratio in a negative electrode active material layer, the nonaqueous electrolyte can penetrate through the layers containing many graphite particles with small internal porosity to the layers containing many graphite particles with large internal porosity, thereby improving charge-discharge cycle characteristics without reducing the packing density of the active material in the negative electrode active material layer and achieving both high capacity and excellent charge-discharge cycle characteristics. Based on this discovery, the present inventors have conceived the following negative electrode for a nonaqueous electrolyte secondary battery.

[0020] A negative electrode for a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector, the negative electrode active material layer including graphite particles A and graphite particles B as negative electrode active materials, and first active material layers X and second active material layers Y having different contents of graphite particles A relative to the total mass of graphite particles A and B are alternately arranged on the negative electrode current collector, the internal porosity of graphite particles A is smaller than the internal porosity of graphite particles B, the content of graphite particles A in first active material layer X is greater than the content of graphite particles A in second active material layer Y, and the ratio Wx / Wy of the width Wx of first active material layer X to the width Wy of second active material layer Y is 0.03 or more and 3.13 or less.

[0021] An example of an embodiment will be described in detail below with reference to the drawings. Note that the nonaqueous electrolyte secondary battery of the present disclosure is not limited to the embodiments described below. Furthermore, the drawings referred to in the description of the embodiments are schematic. Below, a case where the nonaqueous electrolyte secondary battery is a laminated battery will be described, but the configuration of the present disclosure is not limited to laminated batteries and can be applied to various battery forms, such as flat prismatic batteries or cylindrical batteries. Furthermore, below, a case where the electrode body is a laminated type will be described, but the configuration of the present disclosure can also be applied to a battery form having a wound type electrode body.

[0022] FIG. 1 shows a nonaqueous electrolyte secondary battery 10 according to an embodiment. Perspective view 2 is a schematic perspective view showing the electrode assembly 14. The nonaqueous electrolyte secondary battery 10 shown in FIG. 1 includes a battery case 11 made up of two laminate films 11a and 11b. The power generating elements (electrode assembly 14 and electrolyte), which will be described later, are housed in the internal space of a housing portion 12 formed between the laminate films 11a and 11b. The battery case 11 has a sealing portion 13 formed by joining the outer peripheries of the laminate films 11a and 11b, which seals the internal space housing the power generating elements.

[0023] The battery case 11 is pouch-shaped and made up of two laminate films 11a and 11b. Each of the laminate films 11a and 11b has a resin layer laminated on the surface of, for example, aluminum foil. Examples of resin materials that make up the resin layer include polyethylene, polypropylene, polyethylene terephthalate, and nylon. These resin layers may consist of only one layer, or two or more layers may be laminated together.

[0024] In the nonaqueous electrolyte secondary battery 10, a positive electrode lead 15 connected to a positive electrode 20 (FIG. 2) of an electrode assembly 14 (described below) and a negative electrode lead 16 connected to a negative electrode 30 (FIG. 2) are drawn out from a battery case 11. The nonaqueous electrolyte secondary battery 10 includes the electrode assembly 14 and an electrolyte (not shown) as power generating elements. As described above, the power generating elements are housed in a housing 12 sealed by a sealing part 13. As the electrolyte, for example, a nonaqueous electrolyte containing a nonaqueous solvent and an electrolyte salt such as a lithium salt dissolved in the nonaqueous solvent is used.

[0025] As shown in FIG. 2, the electrode assembly 14 has a laminated structure in which positive electrodes 20 and negative electrodes 30 are alternately laminated with separators (not shown) interposed therebetween.

[0026] A current collecting portion consisting of an uncoated portion of each positive electrode 20 protrudes in the same direction from one widthwise end (left end in FIG. 2) of one longitudinal end (upper end in FIG. 2) of a rectangular positive electrode main body (not shown) that forms the positive electrode current collector. The portions protruding from one longitudinal end of each positive electrode main body are stacked to form a positive electrode current collecting tab 21 that is integral with the electrode body 14. A positive electrode lead 15 (FIG. 1) is superimposed on one surface of the positive electrode current collecting tab 21 and electrically connected by joining by welding or the like.

[0027] A current collecting portion consisting of an uncoated portion of each negative electrode 30 protrudes in the same direction from one longitudinal end (the upper end in FIG. 2) of a rectangular negative electrode main body 35 that forms the negative electrode current collector 32, at the other widthwise end (the right end in FIG. 2). The portions of each negative electrode main body 35 that protrude from one longitudinal end are stacked to form a negative electrode current collecting tab 31 that is integral with the electrode body 14. A negative electrode lead 16 (FIG. 1) is superimposed on one surface of the negative electrode current collecting tab 31, and the two are electrically connected by joining by welding or the like.

[0028] Each component of the nonaqueous electrolyte secondary battery 10 will be described in detail below.

[0029] [Negative electrode] 3 is a cross-sectional view of an example of an embodiment of a negative electrode 30. The negative electrode 30 is a negative electrode for a secondary battery having a negative electrode current collector 32 and a negative electrode active material layer 34 provided on the negative electrode current collector 32.

[0030] The negative electrode current collector 32 may be, for example, a foil of a metal such as copper that is stable in the potential range of the negative electrode, or a film having such a metal disposed on the surface thereof.

[0031] The negative electrode active material layer 34 contains graphite particles as the negative electrode active material. The negative electrode active material layer 34 preferably also contains a binder and the like. The negative electrode 30 can be produced, for example, by preparing a negative electrode mixture slurry containing the negative electrode active material, the binder, and the like, applying this negative electrode mixture slurry onto the negative electrode current collector 32, drying it to form the negative electrode active material layer 34, and rolling this negative electrode active material layer 34. The method for producing the negative electrode active material layer 34 will be described in detail below.

[0032] Fig. 4 is a cross-sectional view of a graphite particle 40 in a negative electrode active material layer. As shown in Fig. 4, in the cross-sectional view of graphite particle 40, graphite particle 40 has closed voids 44 that do not connect the interior of the particle to the particle surface (hereinafter referred to as internal voids 44), and voids 46 that connect the interior of the particle to the particle surface (hereinafter referred to as external voids 46).

[0033] Graphite particles 40 in this embodiment include graphite particles A and graphite particles B, and the internal porosity of graphite particles A is smaller than that of graphite particles B. For example, graphite particles 40 include graphite particles A with an internal porosity of 5% or less and graphite particles B with an internal porosity of 8% to 20%. The internal porosity of graphite particles A may be 5% or less, from the viewpoint of suppressing a deterioration in charge-discharge cycle characteristics, but is preferably 1% to 5%, and more preferably 3% to 5%. The internal porosity of graphite particles B may be 8% to 20%, from the viewpoint of suppressing a deterioration in charge-discharge cycle characteristics, but is preferably 10% to 18%, and more preferably 12% to 16%. Here, the internal porosity of a graphite particle is a two-dimensional value calculated from the ratio of the area of ​​internal voids 44 of the graphite particle to the cross-sectional area of ​​the graphite particle. The internal porosity of a graphite particle can be calculated using the following procedure.

[0034] <Method for measuring internal porosity> (1) Exposing the cross section of the negative electrode active material layer. For example, a method for exposing the cross section includes cutting out a part of the negative electrode and processing it with an ion milling device (e.g., IM4000PLUS manufactured by Hitachi High-Technologies Corporation) to expose the cross section of the negative electrode active material layer. (2) A backscattered electron image of the cross section of the exposed negative electrode active material layer is taken using a scanning electron microscope at a magnification of 3,000 to 5,000 times. (3) The cross-sectional image obtained as described above is imported into a computer and binarized using image analysis software (e.g., ImageJ, manufactured by the National Institutes of Health, USA) to obtain a binarized image in which the particle cross-sections in the cross-sectional image are colored black and the voids present in the particle cross-sections are colored white. (4) From the binarized image, graphite particles A and B with particle sizes of 5 μm to 50 μm are selected, and the cross-sectional area of ​​the graphite particles and the area of ​​the internal voids present in the cross-sectional area of ​​the graphite particles are calculated. Here, the cross-sectional area of ​​the graphite particles refers to the area of ​​the region surrounded by the outer periphery of the graphite particles, i.e., the area of ​​the entire cross-sectional portion of the graphite particles. Furthermore, for voids present in the cross-sectional area of ​​the graphite particles with a width of 3 μm or less, it may be difficult to distinguish between internal and external voids in image analysis, so voids with a width of 3 μm or less may be considered internal voids. Then, the internal porosity of the graphite particles (area of ​​internal voids in the cross-sectional area of ​​the graphite particles × 100 / area of ​​the cross-sectional area of ​​the graphite particles) is calculated from the calculated cross-sectional area of ​​the graphite particles and the area of ​​the internal voids in the cross-sectional area of ​​the graphite particles. The internal porosity of graphite particles A and B is the average value of 10 graphite particles A and B, respectively.

[0035] The graphite particles A and B are produced, for example, as follows. <Graphite particles A with internal porosity of 5% or less> For example, coke (precursor), which is the main raw material, is crushed to a predetermined size, agglomerated with a binder, and then fired at a temperature of 2600°C or higher to graphitize the particles, followed by sieving to obtain graphite particles A of a desired size. Here, the internal porosity can be adjusted to 5% or less by adjusting the particle size of the crushed precursor or the particle size of the agglomerated precursor. For example, the average particle size (volume-based median diameter D50) of the crushed precursor is preferably in the range of 12 μm to 20 μm. Furthermore, when the internal porosity is reduced to a range of 5% or less, it is preferable to increase the particle size of the crushed precursor. <Graphite particles B with internal porosity of 8% to 20%> For example, the coke (precursor) that is the main raw material is crushed to a predetermined size, agglomerated with a binder, and then pressed into a block shape. This block is then fired at a temperature of 2600°C or higher to be graphitized. The graphitized block is crushed and sieved to obtain graphite particles B of the desired size. The internal porosity can be adjusted to 8% to 20% by adjusting the amount of volatile components added to the block. If a portion of the binder added to the coke (precursor) volatilizes during firing, the binder can be used as the volatile component. Pitch is an example of such a binder.

[0036] The graphite particles A and B used in this embodiment may be natural graphite, artificial graphite, or the like, but are not particularly limited thereto. However, artificial graphite is preferred in terms of ease of adjusting the internal porosity. The interplanar spacing (d 002 ) is, for example, preferably 0.3354 nm or more, more preferably 0.3357 nm or more, and preferably less than 0.340 nm, more preferably 0.338 nm or less. The crystallite size (Lc(002)) of the graphite particles A and B used in this embodiment, determined by X-ray diffraction, is, for example, preferably 5 nm or more, more preferably 10 nm or more, and preferably 300 nm or less, more preferably 200 nm or less. The interplanar spacing (d 002When the surface roughness (Lc(002)) and the crystallite size (Lc(002)) satisfy the above ranges, the battery capacity of the non-aqueous electrolyte secondary battery tends to be larger than when the above ranges are not satisfied. It is preferable that at least a portion of the surface of the graphite particles A is coated with amorphous carbon. This improves the low-temperature characteristics of the non-aqueous electrolyte secondary battery.

[0037] In this embodiment, the negative electrode active material layer 34 shown in FIG. 3 contains graphite particles A and graphite particles B as negative electrode active materials, and the internal porosity of the graphite particles A is smaller than the internal porosity of the graphite particles B. Furthermore, the negative electrode active material layer 34 includes first active material layers X and second active material layers Y, each having a different content of graphite particles A relative to the total mass of the graphite particles A and B, which are alternately arranged on the negative electrode current collector 32 along the width direction (left-right direction in FIG. 3 ) of the negative electrode main body 35 that forms the negative electrode current collector 32. In FIGS. 2 and 3 , the first active material layers X are indicated by black portions, and the second active material layers Y are indicated by gray portions. Each first active material layer X and each second active material layer Y extends in the longitudinal direction of the negative electrode main body 35, reaching both longitudinal ends of the negative electrode main body 35. In particular, the content of graphite particles A in the first active material layer X is greater than the content of graphite particles A in the second active material layer Y. Furthermore, the ratio Wx / Wy of the width Wx of the first active material layer X to the width Wy of the second active material layer Y is 0.03 or more and 3.13 or less. This allows the nonaqueous electrolyte to penetrate into the second active material layer Y, which contains a large amount of graphite particles with a large internal porosity, via the first active material layer X, which contains a large amount of graphite particles with a small internal porosity, during the charge-discharge cycle of the nonaqueous electrolyte secondary battery 10. This eliminates the partial lack of nonaqueous electrolyte in the negative electrode 30, thereby improving the charge-discharge cycle characteristics of the nonaqueous electrolyte secondary battery 10 without reducing the packing density of the negative electrode active material in the negative electrode active material layer 34. This allows the nonaqueous electrolyte secondary battery 10 to achieve both high capacity and excellent charge-discharge cycle characteristics.

[0038] In this embodiment, the first active material layer X only needs to contain a larger amount of graphite particles A than the second active material layer Y. The first active material layer X may contain only graphite particles A, or both the first active material layer X and the second active material layer Y, or only the first active material layer X, may contain both graphite particles A and graphite particles B. On the other hand, in order to ensure adhesion between the negative electrode current collector 32 and the graphite particles, the first active material layer X preferably contains both graphite particles A and graphite particles B. In this case, the mass ratio of the graphite particles A to the graphite particles B in the first active material layer X is preferably in the range of 10:0 to 2:8, and more preferably 6:4 to 3:7, from the viewpoints of charge / discharge cycle characteristics and high capacity.

[0039] A specific method for increasing the content of graphite particles A in the first active material layer X compared to the second active material layer Y will be described. For example, first, a negative electrode active material containing graphite particles A (and graphite particles A and B as necessary), a binder, and a solvent such as water are mixed to prepare a negative electrode mixture slurry for the first active material layer X. Separately, a negative electrode active material containing graphite particles B (and graphite particles A and B as necessary) and having a lower content of graphite particles A than the negative electrode mixture slurry for the first active material layer X, a binder, and a solvent such as water are mixed to prepare a negative electrode mixture slurry for the second active material layer Y. Then, the negative electrode mixture slurry for the first active material layer X and the negative electrode mixture slurry for the second active material layer Y are alternately applied to both surfaces of a negative electrode current collector along the surface direction and dried, thereby forming the negative electrode active material layer 34.

[0040] The negative electrode active material may contain, in addition to the graphite particles A and B used in this embodiment, other materials capable of reversibly absorbing and releasing lithium ions, such as Si-based materials. Examples of Si-based materials include Si, alloys containing Si, and SiO XExamples of suitable materials include silicon oxides such as silicon dioxide (X) (where X is 0.8 to 1.6). Si-based materials are anode materials that can improve battery capacity compared to graphite particles. However, their large volume expansion during charge and discharge makes them unsuitable for charge-discharge cycle performance. However, in anode active material layers containing graphite particles A and B and a Si-based material, the particle size of the graphite particles does not need to be excessively large. This increases the number of contact points with the Si-based material, thereby ensuring electrical conductivity and ensuring contact points with the expansion and contraction of the Si-based material. This effectively prevents deterioration of charge-discharge cycle performance. The content of the Si-based material is preferably 1% to 10% by mass, and more preferably 3% to 7% by mass, relative to the mass of the anode active material, from the viewpoints of improving battery capacity and preventing deterioration of charge-discharge cycle performance.

[0041] Other examples of the other material capable of reversibly absorbing and releasing lithium ions include metals that alloy with lithium, such as tin (Sn), or alloys or oxides containing metal elements such as Sn. The negative electrode active material may contain the other material, and the content of the other material is preferably, for example, 10 mass % or less relative to the mass of the negative electrode active material.

[0042] Examples of binders include fluorine-based resins, PAN, polyimide-based resins, acrylic-based resins, polyolefin-based resins, styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), carboxymethyl cellulose (CMC) or a salt thereof, polyacrylic acid (PAA) or a salt thereof (PAA-Na, PAA-K, etc., or a partially neutralized salt), polyvinyl alcohol (PVA), etc. These may be used alone or in combination of two or more.

[0043] [Positive electrode] The positive electrode 20 is composed of a positive electrode current collector, such as a metal foil, and a positive electrode active material layer formed on the positive electrode current collector. The positive electrode current collector can be a foil of a metal, such as aluminum, that is stable within the potential range of the positive electrode, or a film with such a metal disposed on the surface layer. The positive electrode active material layer contains, for example, a positive electrode active material, a binder, a conductive agent, etc.

[0044] The positive electrode 20 can be produced, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a binder, a conductive agent, etc. onto a positive electrode current collector, drying to form a positive electrode active material layer, and then rolling this positive electrode active material layer.

[0045] Examples of the positive electrode active material include lithium transition metal oxides containing transition metal elements such as Co, Mn, Ni, etc. The lithium transition metal oxide is, for example, Li x CoO2, Li x NiO2, Li x MnO2, Li x Co y Ni 1-y O2, Li x Co y M 1-y O z 、Li[[ID=2�]] x Ni 1-y M y O z 、Li x Mn2O4, Li x Mn 2-y M y O4, LiMPO4, Li2MPO4F (M; at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B, 0 < x ≤ 1.2, 0 < y ≤ 0.9, 2.0 ≤ z ≤ 2.3). These may be used alone or in combination of multiple kinds. In terms of achieving a higher capacity of the non-aqueous electrolyte secondary battery, the positive electrode active material preferably contains lithium nickel composite oxides such as Li x NiO2, Li x Co y Ni 1-y O2, Li x Ni 1-y M y O z (M; at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B, 0 < x ≤ 1.2, 0 < y ≤ 0.9, 2.0 ≤ z ≤ 2.3), etc.

[0046] Examples of the conductive agent include carbon particles such as carbon black (CB), acetylene black (AB), ketjen black, graphite, etc. These may be used alone or in combination of two or more.

[0047] Examples of binders include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide-based resins, acrylic-based resins, polyolefin-based resins, etc. These may be used alone or in combination of two or more.

[0048] [Separator] The separator may be, for example, a porous sheet having ion permeability and insulating properties. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator include olefin-based resins such as polyethylene and polypropylene, and cellulose. The separator may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin-based resin. Alternatively, the separator may be a multilayer separator including a polyethylene layer and a polypropylene layer, and a separator whose surface is coated with a material such as an aramid-based resin or ceramic may be used.

[0049] [Non-aqueous electrolyte] The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent that can be used include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted product in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine.

[0050] Examples of the esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone and γ-valerolactone; and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate.

[0051] Examples of the ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, cyclic ethers such as crown ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, and methyl phenyl ether. and chain ethers such as ethyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0052] As the halogen-substituted compound, it is preferable to use a fluorinated cyclic carbonate such as fluoroethylene carbonate (FEC), a fluorinated chain carbonate, a fluorinated chain carboxylate such as methyl fluoropropionate (FMP), or the like.

[0053] The electrolyte salt is preferably a lithium salt. Examples of the lithium salt include LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiAlCl4, LiSCN, LiCF3SO3, LiCF3CO2, Li(P(C2O4)F4), LiPF 6-x (C n F 2n+1 ) x (1 < x < 6, n is 1 or 2), LiB 10 Cl 10 、LiCl、LiBr、LiI、lithium chloroborane, lithium lower aliphatic carboxylate, borate salts such as Li2B4O7, Li(B(C2O4)F2), imide salts such as LiN(SO2CF3)2, LiN(C1F 2l+1 SO2)(C m F 2m+1 SO2){l, m are integers of 1 or more}, etc. The lithium salt may be used alone or in combination of multiple kinds. Among these, from the viewpoints of ionic conductivity, electrochemical stability, etc., it is preferable to use LiPF6. The concentration of the lithium salt is preferably 0.8 to 1.8 mol per 1 L of the solvent.

Example

[0054] Hereinafter, the present disclosure will be further described by examples, but the present disclosure is not limited to these examples.

[0055] [[ID=3]]<Example 1> [Preparation of positive electrode] As the positive electrode active material, lithium nickel cobalt aluminate containing aluminum (LiNi 0.88 Co 0.09 Al 0.03 O2) was used. 100 parts by mass of the above positive electrode active material, 0.8 parts by mass of carbon black as a conductive agent, and 0.7 parts by mass of polyvinylidene fluoride powder as a binder were mixed so that the amounts became as such, and further an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode mixture slurry. This slurry was applied to both sides of a positive electrode current collector made of aluminum foil (thickness: 15 μm) by the doctor blade method, and the coating film was dried. At this time, the total coating amount of the mixture on both sides was 560 g / m2 The coating was then rolled using a pressure roller and cut to a predetermined electrode size to produce a positive electrode in which a positive electrode active material layer was formed on both sides of the positive electrode current collector. The rolling was performed so that the electrode plate thickness was 161 μm.

[0056] [Preparation of graphite particles A] Coke was pulverized to an average particle size (median diameter D50) of 12 μm. Pitch was added as a binder to the pulverized coke, and the coke was agglomerated to an average particle size (median diameter D50) of 17 μm. The agglomerates were graphitized by firing at a temperature of 2800°C, and then sieved using a 250-mesh sieve to obtain graphite particles A with an average particle size (median diameter D50) of 23 μm.

[0057] [Preparation of graphite particles B] The coke was crushed to an average particle size (median diameter D50) of 15 μm, and then pitch was added to the crushed coke as a binder to agglomerate it. Then, the coke was further agglomerated under an isotropic pressure of 1.6 g / cm. 3 ~1.9g / cm 3 A block-shaped compact having a density of 1000 MPa was produced. This block-shaped compact was graphitized by firing at a temperature of 2800°C. The graphitized block-shaped compact was then pulverized and sieved using a 250 mesh sieve to obtain graphite particles B having an average particle size (median diameter D50) of 23 μm.

[0058] [Preparation of negative electrode] Graphite particles A were mixed in an amount of 47.5 parts by mass, graphite particles B in an amount of 47.5 parts by mass, and SiO in an amount of 5 parts by mass to form a negative electrode active material G1 for the first active material layer X. The negative electrode active material G1 was mixed with 1 part by mass of carboxymethyl cellulose (CMC) and water. This mixture was mixed with 1 part by mass of styrene-butadiene copolymer rubber (SBR) and water to form a negative electrode mixture slurry for the first active material layer X. Furthermore, graphite particles B were mixed with 95 parts by mass and 5 parts by mass of SiO in an amount of 5 parts by mass to form a negative electrode active material G2 for the second active material layer Y. The negative electrode active material G2 was mixed with 1 part by mass of carboxymethyl cellulose (CMC) and water. This mixture was mixed with 1 part by mass of styrene-butadiene copolymer rubber (SBR) and water to form a negative electrode mixture slurry for the second active material layer Y.

[0059] The negative electrode mixture slurry for the first active material layer X and the negative electrode mixture slurry for the second active material layer Y were simultaneously applied to both sides of a negative electrode current collector made of copper foil using a die coater so that the application width was 0.1 mm and the application width was 3.2 mm, respectively, and the coating film was dried. At this time, the total amount of the mixture applied to both sides was 282 g / m 2 The coating was then rolled using a pressure roller and cut to a predetermined electrode size to produce a negative electrode in which a negative electrode active material layer was formed on both sides of the negative electrode current collector. The rolling was performed so that the electrode plate thickness was 161 μm.

[0060] [Preparation of non-aqueous electrolyte] A non-aqueous electrolyte was prepared by adding 2 parts by mass of vinylene carbonate (VC) to a non-aqueous solvent prepared by mixing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 2:6:2, and dissolving LiPF6 as an electrolyte at a concentration of 1.3 mol / L.

[0061] [Fabrication of non-aqueous electrolyte secondary battery] A laminated electrode assembly was fabricated by stacking five positive electrodes and six negative electrodes, with two negative electrodes positioned on the outside and a 20 μm-thick separator made of a polyethylene microporous film interposed between the positive and negative electrodes. A positive electrode lead was attached to the positive electrode current collecting tab, and a negative electrode lead was attached to the negative electrode current collecting tab. The electrode assembly and 3.5 g of nonaqueous electrolyte were then housed in a battery case made of an aluminum laminate film, and the opening of the battery case was sealed to fabricate the nonaqueous electrolyte secondary battery of Example 1.

[0062] <Example 2> The negative electrode mixture slurry for the first active material layer X was applied with a width of 0.2 mm, and the negative electrode mixture slurry for the second active material layer Y was applied with a width of 3.1 mm, and this was repeated to apply the slurry to both sides of the negative electrode current collector. Otherwise, the nonaqueous electrolyte secondary battery of Example 2 was fabricated in the same manner as Example 1.

[0063] Example 3 The negative electrode mixture slurry for the first active material layer X was applied with a width of 1.0 mm, and the negative electrode mixture slurry for the second active material layer Y was applied with a width of 2.3 mm, and this was repeated to apply the slurry to both sides of the negative electrode current collector. Otherwise, the nonaqueous electrolyte secondary battery of Example 3 was fabricated in the same manner as Example 1.

[0064] Example 4 The negative electrode mixture slurry for the first active material layer X was applied to a width of 2.0 mm, and the negative electrode mixture slurry for the second active material layer Y was applied to a width of 1.3 mm, and this was repeated to apply the slurry to both sides of the negative electrode current collector. Otherwise, the nonaqueous electrolyte secondary battery of Example 4 was fabricated in the same manner as in Example 1.

[0065] <Example 5> The negative electrode mixture slurry for the first active material layer X was applied with a width of 2.5 mm, and the negative electrode mixture slurry for the second active material layer Y was applied with a width of 0.8 mm, and this was repeated to apply the slurry to both sides of the negative electrode current collector. Otherwise, the nonaqueous electrolyte secondary battery of Example 5 was fabricated in the same manner as in Example 1.

[0066] <Comparative Example 1> The negative electrode mixture slurry for the first active material layer X was applied with a width of 3.0 mm, and the negative electrode mixture slurry for the second active material layer Y was applied with a width of 0.3 mm, and this was repeated to apply the slurry to both sides of the negative electrode current collector. Otherwise, a nonaqueous electrolyte secondary battery of Comparative Example 1 was fabricated in the same manner as in Example 1.

[0067] <Comparative Example 2> Only the negative electrode mixture slurry for the second active material layer Y was applied to both sides of the negative electrode current collector. At this time, the application width of the negative electrode mixture slurry was 29.7 mm. Otherwise, a nonaqueous electrolyte secondary battery of Comparative Example 2 was fabricated in the same manner as in Example 1.

[0068] <Comparative Example 3> When preparing the negative electrode mixture slurry, 95 parts by mass of graphite particles A and 5 parts by mass of SiO were mixed to prepare a negative electrode active material G3 for the third active material layer Z. The negative electrode active material G3 was mixed with 1 part by mass of carboxymethyl cellulose (CMC) and water. This mixture was mixed with 1 part by mass of styrene-butadiene copolymer rubber (SBR) and water to prepare a negative electrode mixture slurry for the third active material layer Z. Only this negative electrode mixture slurry for the third active material layer Z was applied to both sides of the negative electrode current collector. At this time, the application width of the negative electrode mixture slurry was 29.7 mm. A nonaqueous electrolyte secondary battery of Comparative Example 3 was fabricated in the same manner as in Example 1 except for the above.

[0069] <Comparative Example 4> Only the negative electrode mixture slurry for the first active material layer X was applied to both sides of the negative electrode current collector. At this time, the application width of the negative electrode mixture slurry was 29.7 mm. Otherwise, a nonaqueous electrolyte secondary battery of Comparative Example 4 was fabricated in the same manner as in Example 1.

[0070] [Measurement of capacity retention rate during charge / discharge cycles] At an ambient temperature of 25°C, the nonaqueous electrolyte secondary batteries of each Example and Comparative Example were charged at a constant current of 1 C (4600 mA) until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current was reduced to 1 / 50 C. They were then discharged at a constant current of 0.5 C until the voltage reached 2.5 V. This cycle of charge and discharge constitutes one cycle, and 300 cycles were performed. The capacity retention rate of each Example and Comparative Example during the charge and discharge cycles was then calculated using the following formula. Capacity retention rate = (discharge capacity at 300th cycle / discharge capacity at 1st cycle) x 100

[0071] Table 1 summarizes the results of the capacity retention rate during charge-discharge cycles of the nonaqueous electrolyte secondary batteries of each Example and Comparative Example. A higher value of the capacity retention rate during charge-discharge cycles indicates better charge-discharge cycle characteristics. Fig. 5 shows the relationship between the width ratio Wx / Wy of the first active material layer X to the second active material layer Y and the capacity retention rate (cycle retention rate) at the 300th cycle, based on the measurement results using the nonaqueous electrolyte secondary batteries of each Example and Comparative Example. In Fig. 5, black circles represent Examples 1 to 5, and open diamonds represent Comparative Examples 1 and 2.

[0072] [Table 1]

[0073] As can be seen from Table 1 and FIG. 5, when the width ratio Wx / Wy was 0.03 or more and 3.13 or less as in Examples 1 to 5, the capacity retention rate during charge-discharge cycling was improved compared to Comparative Examples 1 to 4. On the other hand, in Comparative Example 1, where the width ratio Wx / Wy was 10.00, the cycle retention rate was almost the same as that of Comparative Example 3, in which only graphite particles A were used in the negative electrode active material layer, and no significant improvement was observed. Furthermore, in Examples 2 to 4, where the width ratio Wx / Wy was 0.06 or more and 1.6 or less, the cycle retention rate was 80% or more, confirming a significant improvement in charge-discharge cycle characteristics. On the other hand, as can be seen from FIG. 5, when the width ratio Wx / Wy exceeds 4.0, it is believed that the cycle retention rate decreases significantly. This is thought to be because when the width ratio Wx / Wy exceeds 4.0, the effect of improving the cycle retention rate due to the improved permeability of the nonaqueous electrolyte in the negative electrode active material layer is outweighed by the effect of capacity degradation due to the increased first active material layer X.

[0074] Furthermore, the first active material layer X contains both graphite particles A and graphite particles B. As a result, the presence of graphite particles B, which are appropriately crushed during electrode formation, ensures better adhesion between the negative electrode current collector and the graphite particles than when the first active material layer X is formed only with graphite particles A, which are less likely to be crushed. As a result, the graphite particles are less likely to peel off from the negative electrode current collector during charge-discharge cycles, making it easier to ensure excellent charge-discharge cycle characteristics.

[0075] FIG. 6 is a schematic perspective view showing an electrode assembly 14a including a negative electrode 30a according to another embodiment. In the configuration of this example, unlike the embodiments shown in FIGS. 1 to 4, first active material layers X and second active material layers Y are alternately arranged on a negative electrode current collector 32 along the longitudinal direction (the vertical direction in FIG. 6) of a negative electrode main body 35 that forms the negative electrode current collector 32. Each first active material layer X and each second active material layer Y extends in the width direction (the horizontal direction in FIG. 6) of the negative electrode main body 35, reaching both ends of the negative electrode main body 35 in the width direction. The content of graphite particles A in the first active material layer X is greater than the content of graphite particles A in the second active material layer Y. Furthermore, the ratio Wx / Wy of the width Wx of the first active material layer X to the width Wy of the second active material layer Y is 0.03 or more and 3.13 or less. The configuration of this example also makes it possible to achieve both excellent charge-discharge cycle characteristics and high capacity for the nonaqueous electrolyte secondary battery, similar to the configurations of Figures 1 to 4. In this example, the other configurations and functions are the same as those of Figures 1 to 4.

[0076] 1 to 4 and 6, the electrode assembly is described as being of a laminated type. However, the electrode assembly may be of a wound type in which a positive electrode and a negative electrode are wound with a separator interposed therebetween. In this case, too, the first active material layer X and the second active material layer Y are alternately disposed on the negative electrode current collector so that the ratio Wx / Wy is 0.03 or more and 3.13 or less. This allows the nonaqueous electrolyte secondary battery to achieve both excellent charge-discharge cycle characteristics and high capacity. [Explanation of symbols]

[0077] 10 non-aqueous electrolyte secondary battery, 11 battery case, 11a, 11b laminate film, 12 housing portion, 13 sealing portion, 14 electrode body, 15 positive electrode lead, 16 negative electrode lead, 20 positive electrode, 21 positive electrode current collecting tab, 30 negative electrode, 31 negative electrode current collecting tab, 32 negative electrode current collector, 34 negative electrode active material layer, 35 negative electrode main body portion, 40 graphite particles, 44 internal void, 46 external void.

Claims

1. A negative electrode for a non-aqueous electrolyte secondary battery, comprising: a negative electrode current collector; and a negative electrode active material layer provided on the negative electrode current collector, the negative electrode active material layer includes graphite particles A and graphite particles B as negative electrode active materials, and first active material layers X and second active material layers Y, the contents of which relative to the total mass of the graphite particles A and the graphite particles B are different from each other, are alternately arranged on the negative electrode current collector; the graphite particles A have an internal porosity smaller than that of the graphite particles B, and the content of the graphite particles A in the first active material layer X is greater than the content of the graphite particles A in the second active material layer Y; a ratio Wx / Wy of a width Wx of the first active material layer X to a width Wy of the second active material layer Y is 0.03 or more and 3.13 or less;

2. 2. The negative electrode for a non-aqueous electrolyte secondary battery in accordance with claim 1, wherein the graphite particles A have an internal porosity of 5% or less, and the graphite particles B have an internal porosity of 8% or more and 20% or less.

3. 3. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein a ratio Wx / Wy of a width Wx of the first active material layer X to a width Wy of the second active material layer Y is 0.06 or more and 1.6 or less.

4. 4. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode active material layer further contains a Si-based material.

5. 5. A non-aqueous electrolyte secondary battery comprising: the negative electrode for a non-aqueous electrolyte secondary battery according to claim 1; a positive electrode; and a separator between the negative electrode for a non-aqueous electrolyte secondary battery and the positive electrode.

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