Negative electrode for nonaqueous electrolyte secondary batteries and nonaqueous electrolyte secondary battery

Abstract

Provided is a negative electrode for nonaqueous electrolyte secondary batteries, capable of improving rapid charge / discharge cycle characteristics.　A negative electrode (12) for non-aqueous electrolyte secondary batteries is characterized by comprising the negative electrode current collector (40) and a negative electrode active material layer (42) provided on one surface or both surfaces of the negative electrode current collector (40), and characterized in that: the negative electrode active material layer (42) includes a negative electrode active material and a binding material; the average glass transition temperature of the binding material in a region (42b) in the negative electrode active material layer (42), the region extending to a depth of 30% in the thickness direction from the surface away from the negative electrode current collector (40), is higher than the average glass transition temperature of the binding material in a region (42a) in the negative electrode active material layer (42), the region extending to a depth of 30% in the thickness direction from the surface facing the negative electrode current collector (40); and the mass per unit area of the negative electrode active material layer (42) is 100 g / m2 to 200 g / m2 per surface.

Description

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

[0001] The present disclosure relates to a negative electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery.

[0002] In recent years, non-aqueous electrolyte secondary batteries have been widely used as high-power, high-energy density secondary batteries. These batteries include a positive electrode, a negative electrode, and a non-aqueous electrolyte, and are charged and discharged by transferring lithium ions and the like between the positive electrode and the negative electrode.

[0003] For example, the following are known as negative electrodes for non-aqueous electrolyte secondary batteries.

[0004] For example, Patent Document 1 discloses a negative electrode having a current collector, a first mixture layer formed on the surface of the current collector, and a second mixture layer formed on the surface of the first mixture layer, wherein the first mixture layer and the second mixture layer contain the same binder and thickener, and the ratio B2 / B1 of the content B2 of the binder in the second mixture layer to the content B1 of the binder in the first mixture layer is 0.1 to 0.5.

[0005] Furthermore, for example, Patent Document 2 discloses a negative electrode having 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 containing a negative electrode active material and styrene-butadiene rubber, and the styrene-butadiene rubber in a region of the negative electrode active material layer that accounts for 10% of the thickness from the surface on the opposite side to the negative electrode current collector has a higher glass transition temperature (Tg) than the styrene-butadiene rubber in a region of the negative electrode active material layer that accounts for 10% of the thickness from the surface on the negative electrode current collector side.

[0006] JP 2011-192539 A International Publication No. 2020 / 110589

[0007] 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 are capable of improving rapid charge / discharge cycle characteristics.

[0008] A negative electrode for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes a negative electrode current collector and a negative electrode active material layer provided on one or both surfaces of the negative electrode current collector, the negative electrode active material layer including a negative electrode active material and a binder, the binder in a region of the negative electrode active material layer that is 30% in a thickness direction from a surface on the side opposite to the negative electrode current collector having a higher average glass transition temperature than the binder in a region of the negative electrode active material layer that is 30% in a thickness direction from a surface on the negative electrode current collector side, and the mass per unit area of ​​the negative electrode active material layer is 100 g / m per side. ２ Above, 200g / m ２ The present invention is characterized by the following:

[0009] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes the above-described negative electrode for a non-aqueous electrolyte secondary battery.

[0010] According to one aspect of the present disclosure, it is possible to improve rapid charge / discharge cycle characteristics.

[0011] 1 is a cross-sectional view of a non-aqueous electrolyte secondary battery according to an embodiment of the present invention;

[0012] A negative electrode for a non-aqueous electrolyte secondary battery is manufactured by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, and the like to a negative electrode current collector to form a negative electrode active material layer on the negative electrode current collector. In such a negative electrode for a non-aqueous electrolyte secondary battery, if the adhesion between the negative electrode current collector and the negative electrode active material layer is poor or if the electrolyte permeability from the outer surface of the negative electrode active material layer (the surface of the negative electrode active material layer opposite the negative electrode current collector) is poor, rapid charge-discharge cycle performance may be degraded. Therefore, the present inventors have conducted extensive research and discovered a negative electrode for a non-aqueous electrolyte secondary battery that can improve rapid charge-discharge cycle performance. Specifically, the following aspects are described.

[0013] A negative electrode for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes a negative electrode current collector and a negative electrode active material layer provided on one or both surfaces of the negative electrode current collector, the negative electrode active material layer including a negative electrode active material and a binder, the binder in a region of the negative electrode active material layer that is 30% in a thickness direction from a surface on the side opposite to the negative electrode current collector having a higher average glass transition temperature than the binder in a region of the negative electrode active material layer that is 30% in a thickness direction from a surface on the negative electrode current collector side, and the mass per unit area of ​​the negative electrode active material layer is 100 g / m per side. ２ Above, 200g / m ２ The negative electrode active material layer is characterized by the following: In this way, by disposing a binder with a low average glass transition temperature in a region of 30% of the thickness from the surface of the negative electrode current collector side of the negative electrode active material layer, adhesion between the negative electrode active material layer and the negative electrode current collector is ensured. Furthermore, by disposing a binder with a high average glass transition temperature in a region of 30% of the thickness from the surface of the negative electrode active material layer opposite the negative electrode current collector, the blocking property of the outer surface of the negative electrode active material layer is alleviated or affinity with the electrolyte is increased, thereby improving the permeability of the electrolyte from the outer surface of the negative electrode active material layer. However, if the mass per unit area of ​​the negative electrode active material layer is too large, it may be difficult to ensure adhesion of the negative electrode active material layer or to allow a sufficient amount of electrolyte to permeate into the negative electrode active material layer. Note that if the mass per unit area of ​​the negative electrode active material layer is too small, it may be difficult to ensure sufficient battery capacity. Therefore, in the present disclosure, the average glass transition temperature of the binder in the region on the outer surface side of the negative electrode active material layer is made higher than the average glass transition temperature of the binder in the region on the negative electrode current collector side, and further, the mass per unit area of ​​the negative electrode active material layer is set within an appropriate range, thereby achieving both high adhesion between the negative electrode active material layer and the negative electrode current collector and high permeability of the electrolyte solution into the negative electrode active material layer, thereby improving the rapid charge-discharge cycle characteristics of the nonaqueous electrolyte secondary battery.

[0014] Hereinafter, an example of an embodiment will be described in detail with reference to the drawings. Note that the nonaqueous electrolyte secondary battery of the present disclosure is not limited to the embodiment described below. Furthermore, the drawings referred to in the description of the embodiment are schematic.

[0015] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment. The nonaqueous electrolyte secondary battery 10 shown in FIG. 1 includes a wound electrode assembly 14 formed by winding a positive electrode 11 and a negative electrode 12 with a separator 13 interposed therebetween, a nonaqueous electrolyte, insulating plates 18 and 19 disposed above and below the electrode assembly 14, respectively, and a battery case 15 for accommodating the above components. The battery case 15 is composed of a cylindrical case body 16 with a bottom and a sealing body 17 that closes the opening of the case body 16. Note that, instead of the wound electrode assembly 14, other electrode bodies may be used, such as a laminated electrode body formed by alternately stacking positive and negative electrodes with separators interposed therebetween. Examples of the battery case 15 include cylindrical, prismatic, coin-shaped, button-shaped, or other metal exterior cans, and pouch exteriors formed by laminating a resin sheet and a metal sheet.

[0016] The case body 16 is, for example, a cylindrical metal outer can with a bottom. A gasket 28 is provided between the case body 16 and the sealing body 17 to ensure airtightness inside the battery. The case body 16 has, for example, a protruding portion 22, which is a portion of the side surface that protrudes inward and supports the sealing body 17. The protruding portion 22 is preferably formed in an annular shape along the circumferential direction of the case body 16, and supports the sealing body 17 on its upper surface.

[0017] The sealing body 17 has a structure in which a filter 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in this order from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a disk or ring shape, and all components except for the insulating member 25 are electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected to each other at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the nonaqueous electrolyte secondary battery 10 increases due to heat generation caused by an internal short circuit or the like, for example, the lower valve body 24 deforms and ruptures, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 ruptures, and gas is discharged from the opening of the cap 27.

[0018] 1 , a positive electrode lead 20 attached to the positive electrode 11 passes through a through-hole in the insulating plate 18 and extends toward the sealing body 17, and a negative electrode lead 21 attached to the negative electrode 12 passes outside the insulating plate 19 and extends toward the bottom of the case body 16. The positive electrode lead 20 is connected by welding or the like to the underside of a filter 23, which is the bottom plate of the sealing body 17, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the filter 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected by welding or the like to the inner bottom surface of the case body 16, and the case body 16 serves as the negative electrode terminal.

[0019] Each component of the nonaqueous electrolyte secondary battery 10 will be described in detail below.

[0020] [Negative Electrode] Fig. 2 is a cross-sectional view of a negative electrode according to an embodiment. The negative electrode 12 includes a negative electrode current collector 40 and a negative electrode active material layer 42 provided on the negative electrode current collector 40. In the negative electrode 12 shown in Fig. 2, the negative electrode active material layer 42 is provided on one side of the negative electrode current collector 40, but is not limited thereto and may be provided on both sides of the negative electrode current collector 40.

[0021] The negative electrode current collector 40 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.

[0022] The negative electrode active material layer 42 contains a negative electrode active material and a binder.

[0023] In the negative electrode active material layer 42, the binder in a region 42b that occupies 30% of the thickness from the surface on the side opposite to the negative electrode current collector 40 has a higher average glass transition temperature than the binder in a region 42a that occupies 30% of the thickness from the surface on the negative electrode current collector 40 side. The mass per unit area of ​​the negative electrode active material layer 42 is 100 g / m per side. ２ Above, 200g / m ２ As described above, this configuration contributes to ensuring the adhesion between the negative electrode active material layer 42 and the negative electrode current collector 40 and improving the permeability of the electrolyte from the outer surface of the negative electrode active material layer 42, leading to improved rapid charge / discharge cycle characteristics of the non-aqueous electrolyte secondary battery. The lower limit of the mass per unit area of ​​the negative electrode active material layer 42 is 110 g / m per side from the viewpoint of increasing the capacity of the battery. ２ It is preferable that this is equal to or greater than this.

[0024] The average glass transition temperature of the binder in the region means the glass transition temperature (Tg) of the binder when the region contains one type of binder, and means the sum of the glass transition temperatures (Tg) of the binders multiplied by the mass fraction of each binder when the region contains two or more types of binders (weighted average value of the glass transition temperatures). For example, when the region 42a (or region 42b) contains a binder having a glass transition temperature of Tg １ The first binder of X １ Parts by mass and glass transition temperature Tg ２ The second binder of X ２ When a binder containing 1 part by mass is present, the average glass transition temperature (Tg ０ ) is calculated by the following formula: ０ = (Tg １ ×X １ + Tg ２ ×X ２ ) / (X １ +X ２ )

[0025] The glass transition temperature (Tg) of each binder can be measured in accordance with the provisions of JIS K7121-1987 (Method for measuring transition temperature of plastics). Specifically, the glass transition temperature (Tg) can be determined as the midpoint of the intersection of a line equidistant in the vertical axis direction from the line extending each baseline and the curve of the stepwise change in the glass transition in a DSC curve obtained by differential scanning calorimetry (DSC).

[0026] The binder contained in the region 42a and the region 42b of the negative electrode active material layer 42 can be extracted, for example, as follows. The region 42a (or the region 42b) of the negative electrode active material layer 42 is scraped off from the negative electrode 12, and an organic solvent is added to the solid material of the scraped off region 42a (or the region 42b). This allows the binder in the solid material to be extracted into the organic solvent. Examples of organic solvents for extracting the binder include N-methyl-2-pyrrolidone (NMP), ethanol, acetone, hexane, methyl ethyl ketone, and toluene. The organic solvent is then volatilized, and the glass transition temperature of the binder in the obtained solid content is measured. Furthermore, the binder in the obtained solid content is identified and quantified, whereby the average glass transition temperature of the binder in the region 42a (or the region 42b) can be determined. The methods for measuring the glass transition temperature and calculating the average glass transition temperature are as described above. Furthermore, the method for identifying the binder is not particularly limited, and examples thereof include FT-IR (Fourier transform infrared spectroscopy) measurement, GPC (gel permeation chromatography), solution NMR (nuclear magnetic resonance), solid-state NMR, and Py-GC / MS (pyrolysis gas chromatography mass spectrometry). It is preferable to identify the binder by combining a plurality of analytical techniques. It is also not particularly limited, and examples thereof include TG (thermogravimetry), combustion ion chromatography using a tubular combustion method, Py-GC / MS (pyrolysis gas chromatography mass spectrometry), GPC, and solution NMR measurement. It is preferable to quantify the binder by combining a plurality of analytical techniques.

[0027] In terms of further improving the rapid charge-discharge cycle characteristics of the nonaqueous electrolyte secondary battery, it is preferable that the binder in region 42 f occupying 50% of the thickness from the surface opposite to the negative electrode current collector 40 has a higher average glass transition temperature than the binder in region 42 e occupying 50% of the thickness from the surface on the negative electrode current collector 40 side.

[0028] The average glass transition temperature of the binder in region 42b (or region 42f) is preferably 60°C or higher, and more preferably 70°C or higher, in order to further improve the permeability of the electrolyte solution into the negative electrode active material layer 42 and the rapid charge / discharge cycle characteristics. The upper limit of the average glass transition temperature is not particularly limited, but may be, for example, 100°C or lower.

[0029] The average glass transition temperature of the binder in region 42a (or region 42e) is preferably 70° C. or lower, and more preferably 60° C. or lower, in order to further increase the adhesion between the negative electrode current collector 40 and the negative electrode active material layer 42 and to further improve the rapid charge / discharge cycle characteristics. The lower limit of the average glass transition temperature is not particularly limited, but may be, for example, 30° C. or higher.

[0030] The content of the binder in the anode active material layer 42 is preferably, for example, 0.3% by mass to 5.0% by mass. By making the content of the binder in the anode active material layer 42 0.3% by mass or more, for example, the adhesion between particles of the anode active material and the adhesion between the anode active material layer 42 and the anode current collector 40 can be improved. Furthermore, if the content of the binder in the anode active material layer 42 is 5.0% by mass or less, the permeability of the electrolyte solution into the anode active material layer 42 can be improved. Furthermore, the content of the binder in each region of the anode active material layer 42 is preferably, for example, 0.2% by mass to 3.5% by mass with respect to the total mass of the anode active material layer 42.

[0031] Examples of binders include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide resins, acrylic resins, polyolefin resins, carboxymethyl cellulose (CMC), nitrile-butadiene rubber (NBR), styrene-butadiene rubber (SBR), polyacrylic acid (PAA), and polyvinyl alcohol (PVA). Incidentally, if SBR has a low Tg, the adhesive area increases, making it more susceptible to clogging. Therefore, by increasing the Tg of SBR, clogging of the flow paths in the negative electrode active material layer can be physically prevented, thereby improving liquid permeability. However, generally, as the Tg of SBR increases, adhesion deteriorates, resulting in a trade-off with battery performance. Furthermore, as the Tg of CMC increases, the polarity of its molecular chain increases. This increased polarity increases affinity with the electrolyte, and these chemical properties improve the permeability of the electrolyte into the negative electrode active material layer. On the other hand, to ensure adhesion, a high molecular weight CMC is required, and as a result, the Tg of the CMC needs to be low. That is, a CMC with a high Tg increases the permeability of the electrolyte, but there is a trade-off in terms of adhesion, so a CMC with a low Tg is required to ensure adhesion. It is also presumed that increasing the Tg of the CMC also deteriorates adhesion by changing the dispersion state and compatibility with the negative electrode active material.

[0032] To make the average glass transition temperature of the binder in region 42b higher than that of the binder in region 42a, for example, a first binder and a second binder having a higher glass transition temperature than that of the first binder may be used, and the content of the second binder in region 42b may be higher than that of the first binder, and the content of the first binder in region 42a may be higher than that of the second binder. The content of the first binder in region 42b and the content of the second binder in region 42a may both be zero. Furthermore, as long as the average glass transition temperature of the binder in region 42b can be maintained higher than that of the binder in region 42a, a binder having a glass transition temperature different from that of the first binder or the second binder may be contained in each of regions 42b and 42a.

[0033] The glass transition temperature of the first binder is preferably in the range of 60°C to 100°C, for example, from the viewpoint of improving the permeability of the electrolyte solution from the outer surface of the negative electrode active material layer 42. The glass transition temperature of the second binder is preferably higher than the glass transition temperature of the first binder and in the range of 35°C to 70°C, for example, from the viewpoint of improving the adhesion between the negative electrode current collector 40 and the negative electrode active material layer 42.

[0034] The glass transition temperature of the binder varies depending on the type of material used as the binder, but even for the same type of material, it also varies depending on the molecular weight of the material and the degree of substitution (DS) of the introduced substituent. Therefore, the first binder and the second binder can be made of the same type of material but with different glass transition temperatures. For example, it is preferable to use carboxymethyl cellulose, which has different glass transition temperatures, for the first binder and the second binder.

[0035] Examples of the negative electrode active material contained in the negative electrode active material layer 42 include carbon materials such as graphite particles, non-graphitizable carbon particles, and graphitizable carbon particles, Si-based materials, and Sn-based materials. Examples of the Si-based materials include Si, alloys containing Si, and SiO Ｘ Examples of silicon oxides include SiO Ｘ The volume change caused by charging and discharging is smaller than that of Si, so Si-based materials are considered to have SiO Ｘ It is particularly preferable to use SiO Ｘ is, for example, amorphous SiO ２ The negative electrode active material has a structure in which fine Si particles are dispersed in a matrix of the above. Only one of these materials may be used, or a mixture of two or more of them may be used.

[0036] When the negative electrode active material contains graphite particles, the graphite particles contained in region 42b of negative electrode active material layer 42 preferably have a 10% yield strength of 5 MPa or more. By using graphite particles having a 10% yield strength of 5 MPa or more as the graphite particles contained in region 42b of negative electrode active material layer 42, good output characteristics may be obtained. The pressure at 10% yield strength can be measured using a microcompression tester (MCT-211, manufactured by Shimadzu Corporation) or the like.

[0037] Furthermore, when the negative electrode active material contains graphite particles, the graphite particles contained in the region 42b of the negative electrode active material layer 42 preferably contain 1% by mass to 5% by mass of an amorphous component. By using graphite particles containing 1% by mass to 5% by mass of an amorphous component as the graphite particles contained in the region 42b of the negative electrode active material layer 42, good output characteristics may be obtained. The amount of amorphous components in the graphite particles can be quantified by Raman spectroscopy. Specifically, by Raman spectroscopy, －１ The G-band peak, which is derived from the graphite structure, is located near 1350 cm －１ A D-band peak due to defects is detected in the vicinity, and the amount of amorphous components in the graphite particles can be determined from the peak intensity ratio of D-band / G-band.

[0038] Furthermore, when the negative electrode active material contains a Si-based material, it is preferable that the content of the Si-based material be smaller in region 42b than in region 42a of negative electrode active material layer 42. This may reduce the amount of Si-based material that falls off from negative electrode active material layer 42, thereby suppressing a decrease in battery performance.

[0039] An example of a suitable Si-based material is an amorphous silicon oxide phase having a sea-island structure in which fine Si is dispersed uniformly, and the overall structure is represented by the general formula SiO ｘ The silicon oxide may be mainly composed of silicon dioxide. The silicon oxide phase may be doped with Li. The oxygen to Si content (x) is, for example, 0.5≦x<2.0, and preferably 0.8≦x≦1.5.

[0040] Another example of a suitable Si-based material is a composite particle having a sea-island structure in which fine Si particles are uniformly dispersed in an amorphous silicate phase. A suitable silicate phase is a lithium silicate phase containing Li. The lithium silicate phase can be, for example, represented by the general formula Li ２ｚ SiO （２＋ｚ） (0<z<2). The lithium silicate phase contains Li ４ SiO ４ It is preferable that (Z=2) is not included. ４ SiO ４is an unstable compound and reacts with water to become alkaline, which may cause Si to change and lead to a decrease in charge / discharge capacity. The lithium silicate phase is considered to be a suitable phase for Li, from the viewpoints of stability, productivity, Li ion conductivity, etc. ２ SiO ３ (Z=1) or Li ２ Si ２ O ５ It is preferable that (Z=1 / 2) is used as the main component.

[0041] Another example of a suitable Si-based material is composite particles having a sea-island structure in which fine Si particles are substantially uniformly dispersed in a carbon phase. The carbon phase is preferably an amorphous carbon phase. The carbon phase may contain a crystalline phase component, but preferably contains a larger amount of amorphous phase components. The composite material may contain particles whose ion-conducting phase is an amorphous carbon phase. The amorphous carbon phase is, for example, composed of a carbon material having an average interplanar spacing of (002) planes of more than 0.34 nm as measured by X-ray diffraction. Note that the composite material containing a carbon phase may or may not have a conductive layer separate from the carbon phase.

[0042] The content of the Si-based material in region 42b of negative electrode active material layer 42 is, for example, preferably 20 mass% or less, and more preferably 15 mass% or less, relative to the total mass of the negative electrode active material. The content of the Si-based material in region 42a of negative electrode active material layer 42 is, for example, preferably in the range of 5 mass% to 25 mass% and more preferably in the range of 8 mass% to 15 mass%, relative to the total mass of the negative electrode active material.

[0043] When polyacrylic acid (PAA) is contained as a binder in the negative electrode active material layer 42, it is preferable that the content of polyacrylic acid is higher in the region 42a of the negative electrode active material layer 42 than in the region 42b. Note that the region 42b does not necessarily contain polyacrylic acid. Increasing the content of polyacrylic acid in the region 42a of the negative electrode active material layer 42 may improve the adhesion between the negative electrode active material layer 42 and the negative electrode current collector 40.

[0044] The negative electrode active material layer 42 may contain a conductive material. The conductive material may be any material having a higher conductivity than the negative electrode active material, such as carbon black (CB), acetylene black (AB), ketjen black, or carbon nanotubes. The shape (form) of the conductive material is not limited to particulate form and may be, for example, fibrous. In particular, it is preferable that the region 42a of the negative electrode active material layer 42 contains a fibrous conductive material. By containing a fibrous conductive material in this region, for example, conductivity between particles is maintained even when volume changes occur during charge and discharge, making the current collection structure within the electrode less likely to be destroyed, and may further improve rapid charge and discharge cycle characteristics.

[0045] An example of a method for manufacturing the negative electrode 12 according to this embodiment will be described. First, a first negative electrode composite slurry is prepared, including a negative electrode active material, a binder including a first binder, a solvent such as water, and the like. Separately, a second negative electrode composite slurry is prepared, including a negative electrode active material, a binder including a second binder having a higher glass transition temperature than the first binder, and a solvent such as water, and the like. The first negative electrode composite slurry is then applied to a negative electrode current collector 40 and dried. After that, the second negative electrode composite slurry is applied to the coating film derived from the first negative electrode composite slurry and dried to form a coating film derived from the second negative electrode composite slurry, thereby obtaining the negative electrode 12 according to this embodiment. The thicknesses of the coating films derived from the first negative electrode composite slurry and the second negative electrode composite slurry may be set as appropriate. In any case, the thicknesses of the respective layers should be set so that the binder in region 42b, which occupies 30% of the thickness of negative electrode active material layer 42 from the surface on the side opposite negative electrode current collector 40, has a higher average glass transition temperature than the binder in region 42a, which occupies 30% of the thickness of negative electrode active material layer 42 from the surface on the side facing negative electrode current collector 40. Note that in the above method, the first negative electrode composite slurry is applied and dried, and then the second negative electrode composite slurry is applied; however, a method in which the second negative electrode composite slurry is applied after the first negative electrode composite slurry is applied and before drying may also be used.

[0046] [Positive Electrode] The positive electrode 11 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 in 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 includes, for example, a positive electrode active material, a binder, a conductive material, etc.

[0047] The positive electrode 11 can be produced, for example, by applying a positive electrode composite slurry containing a positive electrode active material, a binder, a conductive material, etc. onto a positive electrode current collector, drying the slurry to form a positive electrode active material layer, and then rolling the positive electrode active material layer.

[0048] Examples of the positive electrode active material include lithium transition metal oxides containing transition metal elements such as Co, Mn, and Ni. ｘ CoO ２ , Li ｘ NiO ２ , Li ｘ MnO ２ , Li ｘ Co ｙ Ni １－ｙ O ２ , Li ｘ Co ｙ M １－ｙ O ｚ , Li ｘ Ni １－ｙ M ｙ O ｚ , Li ｘ Mn ２ O ４ , Li ｘ Mn ２－ｙ M ｙ O ４ , LiMPO ４ , Li ２ MPO ４ F (M: at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B, 0<x≦1.2, 0<y≦0.9, 2.0≦z≦2.3). These may be used alone or in combination. In terms of increasing the capacity of the non-aqueous electrolyte secondary battery, the positive electrode active material is preferably Li ｘ NiO ２ , Li ｘ Co ｙ Ni１－ｙ O ２ , Li ｘ Ni １－ｙ M ｙ O ｚ It is preferable that the lithium-nickel composite oxide contains a lithium-nickel composite oxide such as (M; at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B; 0<x≦1.2, 0<y≦0.9, 2.0≦z≦2.3).

[0049] The binder may be, for example, the same material as the binder used in the negative electrode 12. The conductive material may be any material that is more conductive than the positive electrode active material, and examples thereof include carbon black (CB), acetylene black (AB), ketjen black, carbon nanotubes, and graphite.

[0050] [Separator] The separator 13 may be, for example, a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets 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 13 may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin-based resin. Alternatively, the separator 13 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.

[0051] [Non-aqueous electrolyte] The non-aqueous electrolyte may be, for example, a non-aqueous electrolyte solution containing a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent 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 compound in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine.

[0052] 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.

[0053] 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.

[0054] As the halogen-substituted compound, it is preferable to use fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, fluorinated chain carboxylates such as methyl fluoropropionate (FMP), and the like.

[0055] The electrolyte salt is preferably a lithium salt. Examples of lithium salts include LiBF ４ , LiClO ４ , LiPF ６ , LiAsF ６ , LiSbF ６ , LiAlCl ４ , LiSCN, LiCF ３ SO ３ , LiCF ３ CO ２ , Li(P(C ２ O ４ ) F ４ ), 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, Li ２ B ４ O ７ , Li(B(C ２ O ４ ) F ２ ), borates such as LiN(SO ２ CF ３ ) ２ , LiN(C １ F ２ｌ＋１ SO ２ ) (C ｍ F ２ｍ＋１ SO ２ ) {l and m are integers of 0 or more}. The lithium salt may be used alone or in combination of two or more. Among these, LiPF ６ The concentration of the lithium salt is preferably 0.8 to 1.8 mol per liter of the solvent.

[0056] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0057] Example 1 [Fabrication of Positive Electrode] Aluminum-containing lithium nickel cobalt oxide (LiNi) was used as the positive electrode active material. ０．８８ Co０．０９ Al ０．０３ O ２ ) was used. 100 parts by mass of the above positive electrode active material, 1 part by mass of graphite as a conductive material, and 0.9 parts by mass of polyvinylidene fluoride powder as a binder were mixed, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was further 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), and after drying the coating, the coating was rolled with a rolling roller to produce a positive electrode in which a positive electrode active material layer was formed on both sides of the positive electrode current collector.

[0058] [Preparation of Negative Electrode] 97 parts by weight of graphite particles as the negative electrode active material, 3 parts by weight of a Si-based material, and 2.3 parts by weight of a first slurry binder were mixed, and an appropriate amount of water was added to prepare a first negative electrode composite slurry. The first slurry binder contained 1.1% by weight of carboxymethyl cellulose (CMC, average glass transition temperature 124°C) and 1.2% by weight of styrene-butadiene rubber (glass transition temperature -10°C). The average glass transition temperature of the first slurry binder was 54°C.

[0059] A second negative electrode composite slurry was prepared by mixing 100 parts by weight of graphite particles as the negative electrode active material and 2.5 parts by weight of a second slurry binder, and then adding an appropriate amount of water. The second slurry binder contained 1.7% by weight of carboxymethyl cellulose (CMC, average glass transition temperature 129°C) and 0.8% by weight of styrene-butadiene rubber (glass transition temperature -10°C). The average glass transition temperature of the second slurry binder was 84°C.

[0060] The first negative electrode composite slurry was applied to both sides of a negative electrode current collector made of copper foil and dried. Next, the second negative electrode composite slurry was applied to the coating film derived from the first negative electrode composite slurry and dried. The coating thickness ratio of the second negative electrode composite slurry / the first negative electrode composite slurry was 50 / 50, and the mass per unit area of ​​the coating film (negative electrode active material layer) was 140 g / m per side. ２The coating film was then rolled using a rolling roller to produce a negative electrode in which a negative electrode active material layer was formed on both sides of the negative electrode current collector. In the produced negative electrode, a 30% region in the thickness direction from the surface of the negative electrode active material layer opposite the negative electrode current collector (region on the outer surface side) was derived from the second negative electrode composite slurry, and therefore the average glass transition temperature of the binder in this region was 84°C. In addition, a 30% region in the thickness direction from the surface on the negative electrode current collector side (region on the current collector side) was derived from the first negative electrode composite slurry, and therefore the average glass transition temperature of the binder in this region was 54°C.

[0061] [Preparation of non-aqueous electrolyte] Five parts by mass of vinylene carbonate (VC) was added to a non-aqueous solvent in which ethylene carbonate (EC) and dimethyl carbonate were mixed in a volume ratio of 1:3, and LiPF ６ was dissolved in the solution at a concentration of 1.5 mol / L. This was used as a non-aqueous electrolyte.

[0062] [Fabrication of Non-Aqueous Electrolyte Secondary Battery] (1) A positive electrode lead was attached to the positive electrode current collector, and a negative electrode lead was attached to the negative electrode current collector. A separator made of a microporous polyethylene film was then interposed between the positive and negative electrodes, and the resulting structure was wound to fabricate a wound electrode assembly. (2) Insulating plates were placed above and below the electrode assembly, and the negative electrode lead was welded to the case body and the positive electrode lead was welded to a sealing member, and the electrode assembly was then housed within the case body. (3) A non-aqueous electrolyte was injected into the case body under reduced pressure, and the opening of the case body was then sealed with a sealing member via a gasket. This resulted in a non-aqueous electrolyte secondary battery.

[0063] Example 2 A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that in the production of the negative electrode, a first slurry binder containing 1.1 mass % carboxymethyl cellulose (CMC, average glass transition temperature 126°C) and 1.2 mass % styrene-butadiene rubber (glass transition temperature -10°C) and having an average glass transition temperature of 55°C was used, and a second slurry binder containing 1.7 mass % carboxymethyl cellulose (CMC, average glass transition temperature 127°C) and 0.8 mass % styrene-butadiene rubber (glass transition temperature -10°C) and having an average glass transition temperature of 83°C was used. In the produced negative electrode, the average glass transition temperature of the binder in a region (outer surface region) covering 30% of the thickness of the negative electrode active material layer from the surface on the side opposite the negative electrode current collector was 83°C, and the average glass transition temperature of the binder in a region (current collector side region) covering 30% of the thickness of the negative electrode active material layer from the surface on the negative electrode current collector side was 55°C.

[0064] Example 3 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that in fabricating the negative electrode, the coating thickness ratio of the second negative electrode composite slurry / the first negative electrode composite slurry was set to 30 / 70.

[0065] Example 4 The mass per unit area of ​​the coating film (negative electrode active material layer) was 100 g / m per side. ２ A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that the above-mentioned

[0066] Example 5 The mass per unit area of ​​the coating film (negative electrode active material layer) was 200 g / m per side. ２ A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that the above-mentioned

[0067] Comparative Example 1 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that in fabricating the negative electrode, the second negative electrode composite slurry was applied to both surfaces of a negative electrode current collector made of copper foil and dried, and then the first negative electrode composite slurry was applied to the coating film derived from the second negative electrode composite slurry and dried. In the fabricated negative electrode, the average glass transition temperature of the binder in a region (outer surface region) that accounted for 30% of the thickness from the surface on the side opposite the negative electrode current collector in the negative electrode active material layer was 54°C, and the average glass transition temperature of the binder in a region (current collector side region) that accounted for 30% of the thickness from the surface on the negative electrode current collector side was 84°C.

[0068] Comparative Example 2 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that the second negative electrode composite slurry was applied to both sides of a negative electrode current collector made of copper foil, dried, and then the coating was rolled with a rolling roller to fabricate a negative electrode in which a negative electrode active material layer was formed on both sides of the negative electrode current collector. In the fabricated negative electrode, the average glass transition temperature of the binder in a region (outer surface region) that covered 30% of the thickness of the negative electrode active material layer from the surface on the side opposite the negative electrode current collector, and the average glass transition temperature of the binder in a region (current collector side region) that covered 30% of the thickness of the negative electrode active material layer from the surface on the negative electrode current collector side were both 84°C.

[0069] Comparative Example 3 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that the first negative electrode composite slurry was applied to both sides of a negative electrode current collector made of copper foil, dried, and then the coating was rolled with a rolling roller to fabricate a negative electrode in which a negative electrode active material layer was formed on both sides of the negative electrode current collector. In the fabricated negative electrode, the average glass transition temperature of the binder in a region (outer surface region) that covered 30% of the thickness of the negative electrode active material layer from the surface on the side opposite the negative electrode current collector, and the average glass transition temperature of the binder in a region (current collector side region) that covered 30% of the thickness of the negative electrode active material layer from the surface on the negative electrode current collector side were both 54°C.

[0070] [Adhesion Strength Test] The negative electrode active material layer surface of a 100 mm x 25 mm negative electrode was attached to a 120 mm x 30 mm acrylic plate via double-sided tape (Nichiban Co., Ltd., Nicetack NW-20). Using a small benchtop testing machine (FGS-TV and FGP-5) manufactured by Nidec-Shimpo Corporation, 90° peeling was performed at a measurement temperature of 25°C and a tensile speed of 50 mm / min to measure the adhesion strength between the negative electrode active material layer and the negative electrode current collector, and the results are summarized in Table 1. In Table 1, the adhesion strength of Example 1 is taken as the reference (100), and the adhesion strengths of the other Examples and Comparative Examples are shown as relative values.

[0071] [Measurement of Capacity Retention Rate in Rapid Charge 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 to 4.2 V, and then charged at a constant voltage of 4.3 V to 1 / 50 C. Subsequently, the batteries were discharged at a constant current of 1 C to 2.5 V. This charge / discharge cycle constituted one cycle, and 200 cycles were performed. The capacity retention rate in rapid charge cycles of the nonaqueous electrolyte secondary batteries of each Example and Comparative Example was calculated using the following formula, and the results are summarized in Table 1. In Table 1, the capacity retention rate in rapid charge cycles of Example 1 was set as the reference (100), and the capacity retention rates in rapid charge cycles of the other Examples and Comparative Examples were shown as relative values. Capacity retention rate = (discharge capacity at 200th cycle / discharge capacity at 1st cycle) × 100

[0072]

[0073] As can be seen from Table 1, all of Examples 1 to 5 exhibited higher values ​​for capacity retention rate in rapid charging cycles than Comparative Examples 1 to 3. Therefore, the binder in the region of 30% in the thickness direction from the surface opposite the negative electrode current collector in the negative electrode active material layer had a higher average glass transition temperature than the binder in the region of 30% in the thickness direction from the surface on the negative electrode current collector side, and the mass per unit area of ​​the negative electrode active material layer was 100 g / m per side. ２ Above, 200g / m ２ It can be said that the use of the negative electrode having the following properties can improve the rapid charge-discharge cycle characteristics. Note that all of Examples 1 to 5 exhibited higher values ​​of adhesion strength between the negative electrode active material layer and the negative electrode current collector than Comparative Examples 1 to 3.

[0074] [Notes] (1) A negative electrode active material layer includes a negative electrode current collector and a negative electrode active material layer provided on one or both surfaces of the negative electrode current collector, the negative electrode active material layer including a negative electrode active material and a binder, the binder in a region of the negative electrode active material layer that is 30% in the thickness direction from the surface on the side opposite to the negative electrode current collector has a higher average glass transition temperature than the binder in a region of the negative electrode active material layer that is 30% in the thickness direction from the surface on the negative electrode current collector side, and the mass per unit area of ​​the negative electrode active material layer is 100 g / m per side. ２ Above, 200g / m ２ (2) The negative electrode for a non-aqueous electrolyte secondary battery according to (1), wherein the binder includes a first binder and a second binder having a higher glass transition temperature than the first binder, and the content of the second binder is higher than that of the first binder in a 30% region of the negative electrode active material layer in the thickness direction from the surface on the side opposite the negative electrode current collector, and the content of the first binder is higher than that of the second binder in a 30% region of the negative electrode active material layer in the thickness direction from the surface on the negative electrode current collector side. (3) The negative electrode for a non-aqueous electrolyte secondary battery according to (2), wherein the first binder and the second binder are carboxymethyl cellulose. (4) A non-aqueous electrolyte secondary battery comprising the negative electrode for a non-aqueous electrolyte secondary battery according to any one of (1) to (3).

[0075] 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 15 Battery case, 16 Case body, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Protruding portion, 23 Filter, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 40 Negative electrode current collector, 42 Negative electrode active material layer, 42a, 42b, 42e, 42f Regions.

Claims

1. A negative electrode active material layer comprising a negative electrode current collector and a negative electrode active material layer provided on one or both sides of the negative electrode current collector, the negative electrode active material layer including a negative electrode active material and a binder, the binder in a 30% region in a thickness direction from a surface of the negative electrode active material layer opposite to the negative electrode current collector has a higher average glass transition temperature than the binder in a 30% region in a thickness direction from a surface on the negative electrode current collector side, and the mass per unit area of ​​the negative electrode active material layer is 100 g / m per side. ２ Above, 200g / m ２ The negative electrode for a non-aqueous electrolyte secondary battery is as follows:

2. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the binder comprises a first binder and a second binder having a higher glass transition temperature than the first binder, and a 30% region in a thickness direction from a surface of the negative electrode active material layer opposite to the negative electrode current collector has a greater content of the second binder than the first binder, and a 30% region in a thickness direction from a surface of the negative electrode active material layer on the negative electrode current collector side has a greater content of the first binder than the second binder.

3. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 2, wherein the first binder and the second binder are carboxymethyl cellulose.

4. A non-aqueous electrolyte secondary battery comprising the negative electrode for non-aqueous electrolyte secondary batteries according to claim 1 or 2.

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