Non-aqueous electrolyte secondary battery

Abstract

To provide a nonaqueous electrolyte secondary battery in which resistance of a negative electrode is suppressed and which is superior in thermal stability.SOLUTION: In the nonaqueous electrolyte secondary battery disclosed herein, the negative electrode has a negative electrode active material layer and a plurality of negative electrode tabs, when a root portion of the negative electrode active material layer from which the negative electrode tab extends is measured with a spectrophotometer, an a * value in an L * a * b * color system is 1.3 or less, and when an amount of carbon element and an amount of boron element are measured by laser ablation ICP mass spectrometry along a width direction of the negative electrode active material layer, the value obtained by integrating the ratio (B / C) of the amount of the boron element to the amount of the carbon element within the range of ± 20 mm from the center in the width direction is not less than 28.SELECTED DRAWING: Figure 8

Description

[Technical Field]

[0001] The present invention relates to a non-aqueous electrolyte secondary battery. [Background technology]

[0002] Conventionally, non-aqueous electrolyte secondary batteries have been known, each comprising an electrode assembly including a positive electrode and a negative electrode, a positive electrode terminal electrically connected to the positive electrode, a negative electrode terminal electrically connected to the negative electrode, and a non-aqueous electrolyte. In non-aqueous electrolyte secondary batteries, typically, a portion of the non-aqueous electrolyte decomposes during initial charging, and a coating containing the decomposition product (solid electrolyte interface film: SEI film) is formed on the surface of the negative electrode active material layer. This coating stabilizes the interface between the negative electrode active material layer and the non-aqueous electrolyte (e.g., Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-165125 Summary of the Invention [Problem to be solved by the invention]

[0004] According to the inventors' research, in recent high-capacity non-aqueous electrolyte secondary batteries, the width of the electrode assembly has increased, making it difficult for the non-aqueous electrolyte to penetrate to the center in the width direction. As a result, it has been found that unevenness in the quantity and quality of the coating occurs in the center of the negative electrode active material layer, which tends to reduce thermal stability. Furthermore, some high-capacity non-aqueous electrolyte secondary batteries have a negative electrode with multiple negative electrode tabs, which are stacked and folded and electrically connected to the negative electrode terminal. In such a configuration, it has been found that unevenness in the quantity and quality of the coating occurs at the base of the extending negative electrode tabs, which tends to locally increase resistance.

[0005] The present invention has been made in view of the above circumstances, and has as its object to provide a non-aqueous electrolyte secondary battery in which the resistance of the negative electrode is suppressed and which has excellent thermal stability. [Means for solving the problem]

[0006] The present invention provides a non-aqueous electrolyte secondary battery comprising an electrode assembly including a positive electrode and a negative electrode, a positive electrode terminal electrically connected to the positive electrode, a negative electrode terminal electrically connected to the negative electrode, and a non-aqueous electrolyte. The negative electrode has a negative electrode active material layer containing a carbon material and having a width of 200 mm or more, and a plurality of negative electrode tabs provided at one end in the width direction. The plurality of negative electrode tabs are electrically connected to the negative electrode terminal in a stacked and folded state. When the base portion of the negative electrode tabs extending from the negative electrode active material layer is measured with a spectrophotometer, the L of the negative electrode tabs according to Japanese Industrial Standard JIS Z8781-4:2013 is 0.015. * a * b * a in color system * The value of the ratio (B / C) of the amount of boron element to the amount of carbon element is 1.3 or less, and the amount of carbon element and the amount of boron element are measured along the width direction of the negative electrode active material layer by laser ablation ICP mass spectrometry, and the integrated value of the ratio (B / C) of the amount of boron element to the amount of carbon element is 28 or more within a range of ±20 mm from the center in the width direction.

[0007] As a result of extensive research by the inventors, it was found that areas with high resistance appear as "color unevenness" and that L * a * b * Color system a * It was also found that the amount of heat generated by the battery correlates with the B / C ratio obtained by laser ablation ICP mass spectrometry. * The value is adjusted to a predetermined value or less, and the B / C ratio is adjusted to a predetermined value or more. According to the above configuration, a battery having reduced negative electrode resistance and excellent thermal stability can be provided. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a perspective view schematically showing a nonaqueous electrolyte secondary battery according to one embodiment. [Figure 2] FIG. 2 is a schematic vertical cross-sectional view taken along line II-II in FIG. [Figure 3] FIG. 3 is a schematic cross-sectional view taken along line III-III in FIG. [Figure 4] FIG. 4 is a perspective view that schematically shows an electrode assembly attached to a sealing plate. [Figure 5] FIG. 5 is a perspective view that schematically shows a wound electrode body according to one embodiment. [Figure 6] FIG. 6 is a schematic diagram showing the configuration of a wound electrode body according to one embodiment. [Figure 7] FIG. 7(a) shows the resistance distribution of Comparative Example 1, FIG. 7(b) shows the resistance distribution of Comparative Example 2, and FIG. 7(c) shows the resistance distribution of the Example. [Figure 8] FIG. 8 is a graph showing the relationship between the resistance ratio in the vicinity of the negative electrode tab and the a* value. [Figure 9] FIG. 9 is a schematic diagram of the negative electrode (measurement sample). [Figure 10] Figure 10(a) shows the distribution of the ratio (B / C) for Comparative Example 1, Figure 10(b) shows the distribution of the ratio (B / C) for Comparative Example 2, and Figure 10(c) shows the distribution of the ratio (B / C) for the Example. [Figure 11] FIG. 11 is a graph showing the relationship between the integral value of the ratio (B / C) at the center of the negative electrode active material layer and the amount of heat generated by the battery. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, some preferred embodiments of the technology disclosed herein will be described with reference to the drawings. It should be noted that matters other than those specifically mentioned in this specification but necessary for carrying out the present invention (for example, the general configuration and manufacturing process of a non-aqueous electrolyte secondary battery that does not characterize the present invention) can be understood as design matters for a person skilled in the art based on the prior art in the relevant field. The present invention can be carried out based on the contents disclosed in this specification and the technical common sense in the relevant field. In this specification, the expression "A to B" indicating a range includes not only the meaning of A or more and not more than B, but also the meanings of "greater than A" and "smaller than B."

[0010] In this specification, the term "nonaqueous electrolyte secondary battery" refers to a general electricity storage device that can be repeatedly charged and discharged by the movement of charge carriers between a positive electrode and a negative electrode via a nonaqueous electrolyte. The concept of nonaqueous electrolyte secondary battery encompasses not only so-called secondary batteries such as lithium ion secondary batteries and nickel-metal hydride secondary batteries, but also capacitors that utilize chemical reactions, such as lithium ion capacitors and pseudo-capacitor capacitors.

[0011] <Battery 100> FIG. 1 is a perspective view of a nonaqueous electrolyte secondary battery (hereinafter simply referred to as a battery) 100. FIG. 2 is a schematic longitudinal cross-sectional view taken along line II-II in FIG. 1. FIG. 3 is a schematic transverse cross-sectional view taken along line III-III in FIG. 1. In the following description, the symbols L, R, F, Rr, U, and D in the drawings represent left, right, front, rear, top, and bottom, and the symbols X, Y, and Z in the drawings represent the short side direction of the battery 100, the long side direction perpendicular to the short side direction, and the up-down direction perpendicular to the short side and long side directions, respectively. The long side direction Y is an example of the width direction. However, these directions are merely used for convenience of explanation and do not limit the installation form of the battery 100 in any way.

[0012] As shown in Fig. 2, the battery 100 includes a battery case 10, an electrode assembly 20, a positive electrode terminal 30, a negative electrode terminal 40, and a non-aqueous electrolyte (not shown). The battery 100 further includes a positive electrode current collector 50 and a negative electrode current collector 60. The battery 100 is a lithium ion secondary battery. The battery 100 is preferably a lithium ion secondary battery.

[0013] The battery case 10 is a housing that houses the electrode assembly 20 and the nonaqueous electrolyte. As shown in FIG. 1, the battery case 10 has a flat, bottomed, rectangular parallelepiped (rectangular) outer shape. The material of the battery case 10 may be the same as that conventionally used, and is not particularly limited. The battery case 10 is preferably made of metal, and more preferably made of, for example, aluminum, an aluminum alloy, iron, an iron alloy, or the like. As shown in FIG. 2, the battery case 10 includes an exterior body 12 having an opening 12h and a sealing plate (lid) 14 that closes the opening 12h.

[0014] As shown in Fig. 1, the exterior body 12 includes a substantially rectangular bottom wall 12a, a pair of long side walls 12b extending from the long sides of the bottom wall 12a and facing each other, and a pair of short side walls 12c extending from the short sides of the bottom wall 12a and facing each other. The bottom wall 12a faces an opening 12h. The area of ​​the long side walls 12b is larger than the area of ​​the short side walls 12c. In this specification, the term "substantially rectangular" is intended to encompass not only a perfect rectangular shape (rectangular shape), but also shapes in which the corners connecting the long and short sides of the rectangle are rounded or have notches at the corners.

[0015] As shown in Fig. 1, the sealing plate 14 has a substantially rectangular shape in a plan view. As shown in Fig. 2, the sealing plate 14 is attached to the exterior body 12 so as to close the opening 12h of the exterior body 12. The sealing plate 14 faces the bottom wall 12a of the exterior body 12. The battery case 10 is integrated by joining (for example, welding) the sealing plate 14 to the periphery of the opening 12h of the exterior body 12. The battery case 10 is hermetically sealed (sealed).

[0016] As shown in FIG. 2, the sealing plate 14 is provided with a liquid inlet 15, a gas release valve 17, and two terminal holes 18 and 19. The liquid inlet 15 is for injecting a nonaqueous electrolyte after the sealing plate 14 is assembled to the exterior body 12. The sealing plate 14 preferably has the liquid inlet 15. The liquid inlet 15 is sealed with a sealing member 16. The gas release valve 17 is configured to break when the pressure inside the battery case 10 reaches or exceeds a predetermined value, thereby releasing gas inside the battery case 10 to the outside. The terminal holes 18 and 19 are formed at both ends of the sealing plate 14 in the long side direction Y (the left and right ends in FIG. 2). The terminal holes 18 and 19 penetrate the sealing plate 14 in the thickness direction (the vertical direction Z). The terminal pull-out holes 18, 19 have inner diameters large enough to allow the positive electrode terminal 30 and the negative electrode terminal 40 to be inserted therethrough before being attached to the sealing plate 14 (before being crimped).

[0017] The positive electrode terminal 30 and the negative electrode terminal 40 are each fixed to the sealing plate 14 of the battery case 10. The positive electrode terminal 30 is disposed on one side of the sealing plate 14 in the long side direction Y (the left side in FIGS. 1 and 2). The negative electrode terminal 40 is disposed on the other side of the sealing plate 14 in the long side direction Y (the right side in FIGS. 1 and 2). As shown in FIG. 2, the positive electrode terminal 30 extends from the inside to the outside of the sealing plate 14 through the terminal lead-out hole 18. The negative electrode terminal 40 extends from the inside to the outside of the sealing plate 14 through the terminal lead-out hole 19. The positive electrode terminal 30 and the negative electrode terminal 40 are preferably attached to the sealing plate 14. Here, the positive electrode terminal 30 and the negative electrode terminal 40 are crimped to the peripheral portions of the sealing plate 14 surrounding the terminal lead-out holes 18 and 19 by crimping. The positive electrode terminal 30 and the negative electrode terminal 40 have crimped portions 30c, 40c formed at their ends on the exterior body 12 side (lower ends in FIG. 2).

[0018] As shown in Fig. 2, the positive electrode terminal 30 is electrically connected to the positive electrode 22 (see Fig. 6, specifically, the positive electrode tab group 23) of the electrode body group 20 inside the battery case 10 via the positive electrode current collecting portion 50. The positive electrode terminal 30 is insulated from the sealing plate 14 by a positive electrode insulating member 70 and a gasket 90. The positive electrode terminal 30 is preferably made of metal, and more preferably made of aluminum or an aluminum alloy, for example.

[0019] The negative electrode terminal 40 is electrically connected to the negative electrode 24 (see FIG. 6 , specifically, the negative electrode tab group 25) of the electrode body group 20 via the negative electrode current collector 60 inside the battery case 10. The negative electrode terminal 40 is insulated from the sealing plate 14 by a negative electrode insulating member 80 and a gasket 90. The negative electrode terminal 40 is preferably made of metal, and more preferably made of copper or a copper alloy, for example. The negative electrode terminal 40 may be formed by joining two conductive members together. For example, the portion of the negative electrode terminal 40 that is connected to the negative electrode current collector 60 may be made of copper or a copper alloy, and the portion that is exposed on the outer surface of the sealing plate 14 may be made of aluminum or an aluminum alloy.

[0020] A plate-shaped positive electrode external conductive member 32 and a plate-shaped negative electrode external conductive member 42 are attached to the outer surface of the sealing plate 14. The positive electrode external conductive member 32 and the plate-shaped negative electrode external conductive member 42 are members to which bus bars are attached when electrically connecting multiple batteries 100 to each other. The positive electrode external conductive member 32 is electrically connected to the positive electrode terminal 30. The negative electrode external conductive member 42 is electrically connected to the negative electrode terminal 40. The positive electrode external conductive member 32 and the plate-shaped negative electrode external conductive member 42 are insulated from the sealing plate 14 by an external resin member 92. The positive electrode external conductive member 32 and the plate-shaped negative electrode external conductive member 42 are preferably made of metal, more preferably aluminum or an aluminum alloy. However, the positive electrode external conductive member 32 and the plate-shaped negative electrode external conductive member 42 are not essential and may be omitted in other embodiments.

[0021] As shown in FIG. 2, the electrode assembly group 20 is housed inside the battery case 10 (more specifically, inside the exterior body 12). FIG. 4 is a perspective view that schematically shows the electrode assembly group 20 attached to the sealing plate 14. Here, the electrode assembly group 20 has three electrode bodies 20a, 20b, and 20c. However, the number of wound electrode bodies arranged inside one battery case 10 is not particularly limited and may be two or more (plural), or may be one. The electrode assembly group 20 may be arranged inside the battery case 10 while covered with an insulating electrode assembly holder. In other words, an electrode assembly holder may be interposed between the electrode assembly group 20 and the battery case 10 (more specifically, the exterior body 12). The electrode assembly holder is preferably made of resin.

[0022] FIG. 5 is a perspective view schematically illustrating the electrode assembly 20a. FIG. 6 is a schematic diagram illustrating the configuration of the electrode assembly 20a. Note that the electrode assembly 20a will be described in detail below as an example, but the electrode assemblies 20b and 20c may also have a similar configuration. As shown in FIG. 6, the electrode assembly 20a includes a positive electrode 22 and a negative electrode 24. The positive electrode 22 and the negative electrode 24 are insulated by a separator 26. The electrode assembly 20a is a wound electrode assembly here. The electrode assembly 20a is configured by stacking a strip-shaped positive electrode 22 and a strip-shaped negative electrode 24 in an insulated state (for example, via a strip-shaped separator 26) and winding them around a winding axis WL. However, in other embodiments, the electrode assembly 20a may be a stacked electrode assembly in which multiple rectangular positive electrodes and multiple rectangular negative electrodes are stacked in an insulated state.

[0023] The electrode assembly 20a is preferably a wound electrode assembly. When the electrode assembly 20a is a wound electrode assembly, the nonaqueous electrolyte is supplied only from both ends in the direction of the winding axis WL. This makes it particularly difficult for the nonaqueous electrolyte to penetrate into the center of the electrode assembly 20a in the direction of the winding axis WL, making it more likely that the quantity and quality of the coating will be uneven in this center. Therefore, applying the technology disclosed herein is particularly effective.

[0024] Although not particularly limited, the number of windings (number of turns) of the electrode body 20a is preferably 20 turns or more, more preferably 30 turns or more, and even more preferably 50 turns or more, and can be, for example, 150 turns or less, or 100 turns or less.

[0025] 2 and 6, the electrode body 20a is disposed inside the battery case 10 with the winding axis WL oriented approximately parallel to the long side direction Y. The direction of the winding axis WL here coincides with the long side direction Y (width direction). The electrode body 20a is disposed inside the battery case 10 with the winding axis WL oriented parallel to the bottom wall 12a and perpendicular to the short side wall 12c.

[0026] Here, the battery 100 has a so-called horizontal tab structure in which the positive electrode tab group 23 and the negative electrode tab group 25 are located at both ends of the electrode assembly 20a in the winding axis WL direction (left and right in FIGS. 2 and 4). However, in other embodiments, the battery 100 may have a so-called top tab structure in which the positive electrode tab group 23 and the negative electrode tab group 25 are located at one end of the electrode assembly 20a in the winding axis WL direction (for example, the top end in FIGS. 2 and 4). In this case, the winding axis WL direction may coincide with the up-down direction Z.

[0027] As shown in FIG. 5, the electrode body 20a has a flat outer shape. The electrode body 20a preferably has a flat outer shape. The electrode body 20a has a pair of flat portions 20f extending along the long side direction Y (winding axis WL direction) and a pair of curved portions (R portions) 20r connecting the pair of flat portions 20f. The flat portions 20f have a flat outer surface (YZ plane in FIG. 5). The curved portions 20r have a curved outer surface. In this specification, the term "flat outer surface" is not limited to a completely flat surface, and includes cases where, for example, when viewed microscopically, there are slight steps, curves, recesses, protrusions, etc.

[0028] 2 and 5, in this embodiment, the pair of flat portions 20f face the pair of long side walls 12b of the exterior body 12. The flat portions 20f extend along the long side walls 12b. The pair of curved portions 20r face the bottom wall 12a and the sealing plate 14 of the exterior body 12. As in this embodiment, the electrode body 20a is preferably disposed inside the battery case 10 such that the stacking direction (thickness direction) of the positive electrode 22 (see FIG. 6) and the negative electrode 24 (see FIG. 6) in the flat portions 20f coincides with the short side direction X (the direction perpendicular to the long side walls 12b).

[0029] The positive electrode 22 may be the same as a conventional one and is not particularly limited. As shown in FIG. 6, the positive electrode 22 has a positive electrode current collector 22c, a positive electrode active material layer 22a and a positive electrode protective layer 22p adhered to at least one surface of the positive electrode current collector 22c. However, the positive electrode protective layer 22p is not essential and can be omitted in other embodiments. The positive electrode current collector 22c here is strip-shaped. The positive electrode current collector 22c is made of a conductive metal such as aluminum, an aluminum alloy, nickel, or stainless steel. Here, the positive electrode current collector 22c is a metal foil, specifically, an aluminum foil.

[0030] A plurality of positive electrode tabs 22t are provided at one end (left end in FIG. 6) in the long side direction Y (width direction, winding axis WL direction) of the positive electrode current collector 22c. Each of the plurality of positive electrode tabs 22t is convex and protrudes toward one side in the long side direction Y (left side in FIG. 6). The plurality of positive electrode tabs 22t extend further in the long side direction Y than the separator 26. The plurality of positive electrode tabs 22t are provided at intervals (intermittently) along the longitudinal direction of the positive electrode 22. Providing a plurality of positive electrode tabs 22t can reduce the resistance of the battery 100. Here, the positive electrode tab 22t is part of the positive electrode current collector 22c and is made of metal foil (aluminum foil).

[0031] As shown in FIG. 3, the positive electrode tabs 22t are stacked at one end in the long side direction Y (the left end in FIG. 3) to form a positive electrode tab group 23. The positive electrode tabs 22t are stacked and bent and curved so that their outer ends are aligned. This improves the fitment into the battery case 10, thereby enabling the battery 100 to be made more compact. It also improves the volumetric energy density of the battery 100. A later-described positive electrode second current collecting portion 52 of the positive electrode current collecting portion 50 is attached (more specifically, joined) to the positive electrode tab group 23. The positive electrode tabs 22t are connected to the positive electrode second current collecting portion 52 in a stacked and bent state. The positive electrode tab group 23 is electrically connected to the positive electrode terminal 30 via the positive electrode current collecting portion 50.

[0032] As shown in Fig. 6, the positive electrode active material layer 22a is provided in a strip shape along the longitudinal direction of the strip-shaped positive electrode current collector 22c. The positive electrode active material layer 22a contains a positive electrode active material (e.g., a lithium transition metal composite oxide such as a lithium nickel cobalt manganese-containing composite oxide) that can reversibly store and release charge carriers. The positive electrode active material layer 22a may contain optional components other than the positive electrode active material, such as a conductive material, a binder, and various additive components. As the conductive material, for example, a carbon material such as acetylene black (AB) can be used. As the binder, for example, polyvinylidene fluoride (PVdF) can be used.

[0033] Although not particularly limited, in a high-capacity battery 100 used in a vehicle or the like, the length Lc (average value, excluding the portion formed on the positive electrode tab 22t) of the positive electrode active material layer 22a in the long side direction Y (winding axis WL direction) is preferably 150 mm or more, more preferably 200 mm or more, and even more preferably 250 mm or more, as shown in Fig. 6. The length Lc is preferably the same as or shorter than the length La of the negative electrode active material layer 24a in the long side direction Y, which will be described later.

[0034] As shown in FIG. 6, the positive electrode protective layer 22p is provided between the positive electrode current collector 22c and the positive electrode active material layer 22a in the long side direction Y. Here, the positive electrode protective layer 22p is provided at one end of the positive electrode current collector 22c in the long side direction Y (the left end in FIG. 6). The positive electrode protective layer 22p is provided in a strip shape along the positive electrode active material layer 22a. The positive electrode protective layer 22p contains an inorganic filler (e.g., alumina). The positive electrode protective layer 22p may contain optional components other than the inorganic filler, such as a conductive material, a binder, various additive components, etc. The conductive material and binder may be the same as those exemplified as those that may be contained in the positive electrode active material layer 22a.

[0035] As shown in FIG. 6, the negative electrode 24 includes a negative electrode current collector 24c and a negative electrode active material layer 24a fixed to at least one surface of the negative electrode current collector 24c. The negative electrode current collector 24c is strip-shaped here. The negative electrode current collector 24c is made of a conductive metal such as copper, a copper alloy, nickel, or stainless steel. The negative electrode current collector 24c preferably contains copper or a copper alloy. Here, the negative electrode current collector 24c is a metal foil, specifically, a copper foil.

[0036] A plurality of negative electrode tabs 24t are provided at one end (the right end in FIG. 6 ) in the long side direction Y (width direction, winding axis WL direction) of the negative electrode current collector 24c. Each of the plurality of negative electrode tabs 24t is convex and protrudes toward one side in the long side direction Y (the right side in FIG. 6 ). The plurality of negative electrode tabs 24t extend further in the long side direction Y than the separator 26. The plurality of negative electrode tabs 24t are provided at intervals (intermittently) along the longitudinal direction of the negative electrode 24. Providing the plurality of negative electrode tabs 24t can reduce the resistance of the battery 100. Here, the negative electrode tab 24t is part of the negative electrode current collector 24c and is made of metal foil (copper foil). At least a portion of the negative electrode tab 24t is a current collector exposed portion where the negative electrode active material layer 24a is not formed and the negative electrode current collector 24c is exposed.

[0037] As shown in FIG. 3 , the negative electrode tabs 24t are stacked at one end in the long side direction Y (the right end in FIG. 3 ) to form a negative electrode tab group 25. The negative electrode tabs 24t are stacked and bent and curved so that their outer ends are aligned. This improves the fitment into the battery case 10, allowing the battery 100 to be downsized. It also improves the volumetric energy density of the battery 100. A negative electrode second current collecting portion 62 (described later) of the negative electrode current collecting portion 60 is attached (more specifically, joined) to the negative electrode tab group 25. The negative electrode tabs 24t are connected to the negative electrode second current collecting portion 62 in a stacked and bent state. The negative electrode tab group 25 is electrically connected to the negative electrode terminal 40 via the negative electrode current collecting portion 60.

[0038] As shown in FIG. 6, the negative electrode active material layer 24a is provided in a strip shape along the longitudinal direction of the strip-shaped negative electrode current collector 24c. The negative electrode active material layer 24a contains a negative electrode active material (e.g., a carbon material such as graphite) that can reversibly store and release charge carriers. When the total solid content of the negative electrode active material layer 24a is taken as 100 mass%, the negative electrode active material (e.g., graphite) may account for approximately 80 mass% or more, typically 90 mass% or more, for example, 95 mass% or more. The negative electrode active material layer 24a may contain optional components other than the negative electrode active material, such as a binder, a dispersant, and various additives. Examples of the binder include rubbers such as styrene butadiene rubber (SBR). Examples of the dispersant include celluloses such as carboxymethyl cellulose (CMC).

[0039] As shown in FIG. 6, the length La (average value, excluding the portion formed in the negative electrode tab 24t) of the negative electrode active material layer 24a in the long side direction Y (winding axis WL direction) is typically equal to or longer than the length Lc of the positive electrode active material layer 22a in the long side direction Y. Although not particularly limited, the length La of the negative electrode active material layer 24a is preferably 200 mm or more, more preferably 250 mm or more, from the viewpoint of increasing capacity, etc. In the electrode body 20a, the longer the length La, the shorter the central M in the long side direction Y. YThe nonaqueous electrolyte is less likely to penetrate into the central portion, including the center portion (see FIG. 5). As a result, the central portion in the long side direction Y is more likely to have unevenness in the amount and quality of the coating. Therefore, it is effective to apply the technology disclosed herein. The length La may be, for example, 1000 mm or less, or 500 mm or less. This allows the effects of the technology disclosed herein to be exerted to a high level.

[0040] 5, the height Ha of the negative electrode active material layer 24a located on the flat portion 20f of the electrode body 20a (the same as the height of the flat portion 20f) is preferably 110 mm or less, more preferably 50 to 110 mm, even more preferably 70 to 100 mm, and particularly preferably 70 to 90 mm. In the flat portion 20f, the ratio of the length La in the long side direction Y to the height Ha of the negative electrode active material layer 24a (horizontal / vertical ratio) is preferably 1 to 10, more preferably 2 to 7, and even more preferably 3 to 5. This allows the effects of the technology disclosed herein to be exerted at a high level.

[0041] The negative electrode active material layer 24a typically includes a coating (SEI film) containing boron (B). This boron is a component derived from a compound containing boron (B-element-containing compound), such as a coating agent described below, that is added to the non-aqueous electrolyte solution during construction of the battery 100. The coating is a decomposition product containing the B-element-containing compound that is decomposed during initial charging, for example. The coating containing boron has excellent stability, and can therefore suitably improve the durability and thermal stability of the battery 100.

[0042] According to the inventors' investigations, the base portion of the negative electrode tab 24t of the negative electrode active material layer 24a from which the negative electrode tab 24t extends (hereinafter simply referred to as the "neighborhood of the negative electrode tab 24t") is prone to localized high resistance due to the bending and curvature of the multiple negative electrode tabs 24t. While this is not intended to be a particularly restrictive interpretation, for example, if the negative electrode tab 24t is bent and curved (in other words, if an external force is applied to the negative electrode tab 24t), the interelectrode distance between the positive electrode 22 and the negative electrode 24 is likely to increase locally near the negative electrode tab 24t. As a result, nonaqueous electrolyte is likely to accumulate in this area. It is believed that initial charging in this state promotes decomposition of the nonaqueous solvent, increasing the amount of organic coating derived from the nonaqueous solvent and increasing resistance.

[0043] Therefore, in the technology disclosed herein, when the base portion (near the negative electrode tab 24t) of the negative electrode active material layer 24a where the negative electrode tab 24t extends is measured with a spectrophotometer, the L * a * b * a in color system * The value is set to 1.3 or less. * a * b * In the color system, it is possible to separate the coordinate axes of black and white (brightness) and yellow, blue, red, and green (chromaticity). As will be described in detail in the examples below, according to the study by the inventors, the high resistance portion of the negative electrode active material layer 24a appears as "color unevenness." * a * b * Color space a * It can be distinguished by the value (redness). * By adjusting the value to a predetermined value or less, it is possible to prevent the vicinity of the negative electrode tab 24t from becoming high in resistance, which in turn improves the battery characteristics.

[0044] Furthermore, while the shade of "color unevenness" can be distinguished by visual inspection, for example, due to individual differences in human eyes, the results of determining whether or not there is color unevenness may differ from person to person. In contrast, when objective numerical values ​​obtained by spectrophotometer measurement are used as indicators, as in the technology disclosed herein, there is relatively little variation in accuracy. Furthermore, it is possible to distinguish color differences that cannot be distinguished by the human eye. Therefore, it becomes easier to stably suppress resistance.

[0045] In this specification, the term "root portion from which the negative electrode tab extends" refers to a range of approximately 40 mm from the negative electrode tab 24t in the long side direction Y (width direction). The above measurement may be performed at multiple points in the root portion in consideration of variations. In this case, the a * It is more preferable that all of the values ​​are equal to or less than a predetermined value. Furthermore, when the electrode body 20a is a wound electrode body, the above measurement is carried out at one point or two or more points at the base portion of each turn, and the a * It is more preferable that all of the values ​​are equal to or less than a predetermined value.

[0046] a at the base of the negative electrode tab 24t * From the viewpoint of achieving the effects of the technology disclosed herein at a high level, the value of a is preferably 1.2 or less, more preferably 1.1 or less, even more preferably 1.0 or less, and particularly preferably 0.9 or less. * The value is typically 0.1 or greater, and may be, for example, 0.6 or greater, or 0.7 or greater.

[0047] Furthermore, according to the study by the present inventors, the nonaqueous electrolyte solution is less likely to penetrate into the central portion of the negative electrode active material layer 24a in the long side direction Y (width direction). Therefore, unevenness in the amount and quality of the coating is likely to occur in the central portion of the negative electrode active material layer 24a in the long side direction Y. This may result in a decrease in thermal stability.

[0048] Therefore, in the technique disclosed herein, the carbon element amount and the boron element amount are measured along the long side direction Y (width direction) of the negative electrode active material layer 24a by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and the central M Y The integral of the ratio (B / C) of the amount of boron element to the amount of carbon element within a range of ±20 mm from the negative electrode active material layer 24a (see FIG. 5) is set to 28 or more. The larger the integral of the ratio (B / C), the more boron (B) element is present in the central portion of the negative electrode active material layer 24a in the long side direction Y. As will be described in detail in the Examples below, according to the inventors' studies, the integral of the B / C ratio correlates with the amount of heat generated by the battery 100. Specifically, the larger the integral of the B / C ratio, the more the amount of heat generated by the battery 100 is suppressed. Therefore, by adjusting the B / C ratio to a predetermined value or more, the amount of heat generated is suppressed, and a battery 100 with excellent thermal stability can be provided.

[0049] From the viewpoint of achieving the effects of the technology disclosed herein at a high level, the integral value of the ratio (B / C) is preferably 30 or more, more preferably 35 or more, even more preferably 40 or more, and particularly preferably 50 or more. From the viewpoint of suppressing resistance, etc., the integral value of the ratio (B / C) is preferably 100 or less, more preferably 80 or less.

[0050] In addition, the above a * The value and / or the integral value of the ratio (B / C) can be suitably adjusted not only by the amount of non-aqueous electrolyte injected during construction of battery 100 and the concentration of the additive (a compound containing boron element) in the non-aqueous electrolyte, but also by, for example, the conditions of the electrolyte impregnation step (step 2) in the manufacturing method described below, particularly the conditions of the pressurized / depressurized impregnation step (step 2-1), and the conditions of the initial charging step (step 3), such as the pressing force (constraint load).

[0051] As shown in FIG. 6, the separator 26 is a member that insulates the positive electrode active material layer 22a of the positive electrode 22 from the negative electrode active material layer 24a of the negative electrode 24. The length Ls of the separator 26 in the long side direction Y (winding axis WL direction) is typically equal to or longer than the length La of the negative electrode active material layer 24a in the long side direction Y. The separator 26 is preferably a porous resin sheet made of a polyolefin resin such as polyethylene (PE) or polypropylene (PP). The separator 26 may have a functional layer, such as an adhesive layer or a heat-resistant layer (HRL), on the surface of the substrate portion made of a porous resin sheet. The adhesive layer is a layer containing a binder. The heat-resistant layer is a layer containing an inorganic filler such as alumina, silica, boehmite, magnesia, or titania, and a binder such as PVdF. The heat-resistant layer can also serve as an adhesive layer. The heat-resistant layer and adhesive layer may have the same configuration as conventional ones.

[0052] As shown in FIG. 2 , the positive electrode current collecting part 50 forms a conductive path that electrically connects the positive electrode tab group 23, which is made up of multiple positive electrode tabs 22t, to the positive electrode terminal 30. The positive electrode current collecting part 50 may be made of the same metal as the positive electrode current collector 22c, such as a conductive metal such as aluminum, an aluminum alloy, nickel, or stainless steel. The positive electrode current collecting part 50 includes a positive electrode first current collecting part 51 connected to the positive electrode terminal 30 and a positive electrode second current collecting part 52 connected to the positive electrode tab group 23. The positive electrode first current collecting part 51 is attached to the inner surface of the sealing plate 14.

[0053] The positive electrode second current collecting portion 52 extends along the short side wall 12c of the exterior body 12. The positive electrode second current collecting portion 52 is attached to the positive electrode tab group 23 of the electrode body 20a. As shown in FIG. 3, a joint J with the positive electrode tab group 23 is formed in the positive electrode second current collecting portion 52. The joint J is a welded joint formed by welding, such as ultrasonic welding, resistance welding, or laser welding, with the multiple positive electrode tabs 22t stacked together. The joint J is arranged by positioning the multiple positive electrode tabs 22t toward one side (the front side in FIG. 3) in the short side direction X of the electrode bodies 20a, 20b, and 20c. This allows the multiple positive electrode tabs 22t to be bent appropriately in a stacked state, thereby stably forming a curved positive electrode tab group 23.

[0054] As shown in FIG. 2 , the negative electrode current collector 60 forms a conductive path that electrically connects the negative electrode tab group 25, which is made up of multiple negative electrode tabs 24t, to the negative electrode terminal 40. The negative electrode current collector 60 may be made of the same metal as the negative electrode current collector 24c, such as a conductive metal such as copper, a copper alloy, nickel, or stainless steel. The negative electrode current collector 60 includes a negative electrode first current collector 61 connected to the negative electrode terminal 40 and a negative electrode second current collector 62 connected to the negative electrode tab group 25. The configuration and arrangement of the negative electrode first current collector 61 and the negative electrode second current collector 62 may be the same as those of the positive electrode first current collector 51 and the positive electrode second current collector 52 of the positive electrode current collector 50.

[0055] The negative electrode second current collecting portion 62 is attached to the negative electrode tab group 25 of the electrode body 20a. As shown in FIG. 3, a joint J with the negative electrode tab group 25 is formed on the negative electrode second current collecting portion 62. As with the positive electrode side, the joint J is a welded joint formed by welding, such as ultrasonic welding, resistance welding, or laser welding, with multiple negative electrode tabs 24t stacked together. The joint J is arranged such that the multiple negative electrode tabs 24t are closer to one side (the front side in FIG. 3) in the short side direction X of the electrode bodies 20a, 20b, and 20c. This allows the multiple negative electrode tabs 24t to be bent in an appropriate manner in a stacked state, thereby stably forming a curved negative electrode tab group 25.

[0056] The non-aqueous electrolyte typically contains a non-aqueous solvent and an electrolyte salt (supporting salt). As the non-aqueous solvent, one or more of those known to be usable in non-aqueous electrolyte secondary batteries can be used. Examples of the non-aqueous solvent include organic solvents such as carbonates, ethers, esters, nitriles, sulfones, and lactones. The non-aqueous solvent preferably contains a carbonate. Examples of the carbonate include linear carbonates such as ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC), and cyclic carbonates such as propylene carbonate (PC).

[0057] The electrolyte salt is not particularly limited as long as it contains a charge carrier (typically lithium ion), and one or more of those known to be usable in non-aqueous electrolyte secondary batteries can be used. Examples of the electrolyte salt include fluorine-containing lithium salts such as LiPF6 and LiBF4. The electrolyte salt preferably contains LiPF6.

[0058] The nonaqueous electrolyte may further contain additional components (additives). One or more additives known to be additives to nonaqueous electrolytes may be used. Examples include boron-based additives containing boron, such as lithium bis(oxalato)borate (LiBOB) and lithium difluoro(oxalato)borate (LiODFB); and phosphorus-based additives containing phosphorus, such as lithium difluorophosphate (lithium difluorophosphate, LiPOF) and lithium difluorooxalatophosphate (LiDFOP). These additives may act as so-called film-forming agents, which decompose (at a lower potential) than the nonaqueous solvent and / or electrolyte salt during initial charging and deposit as a film on the surface of the negative electrode active material layer 24a.

[0059] The non-aqueous electrolyte preferably contains a compound containing boron (B) (B-element-containing compound), such as a lithium salt containing boron (B-element-containing lithium salt). Examples of the B-element-containing compound (e.g., B-element-containing lithium salt) include oxalate complex compounds containing boron (B-element-containing oxalate compounds), such as LiBF4 exemplified above as the supporting salt, and LiBOB and LiODFB exemplified above as the boron-based additive.

[0060] It should be noted that additives (for example, the above-mentioned boron-based additives) added to the non-aqueous electrolyte solution during production are electrically decomposed by initial charging or the like and consumed to form a coating on the negative electrode active material layer 24a, etc. Therefore, in the state of the battery 100, the non-aqueous electrolyte solution may or may not contain (remain) the above-mentioned additives.

[0061] <Method of manufacturing the battery 100> The battery 100 can be manufactured by a manufacturing method including, for example, the following steps: a battery assembly construction step (step 1), an electrolyte impregnation step (step 2), an initial charging step (step 3), a degassing step (step 4), a liquid injection hole sealing step (step 5), and an aging step (step 6), in this order. However, the degassing step (step 4) is optional and may be omitted in other embodiments. Furthermore, other steps may be included at any stage. For example, an activation step may be further included after the aging step (step 6).

[0062] In the construction step (step 1), a battery assembly is constructed by housing an electrode body group 20 (electrode bodies 20a, 20b, 20c) and a non-aqueous electrolyte in a battery case 10, typically in a glove box. In this specification, the term "battery assembly" refers to an intermediate product that has been assembled up to the state prior to the initial charging step (step 3) in the manufacturing process of the battery 100. The order in which the electrode body group 20 and the non-aqueous electrolyte are housed in the battery case 10 is not particularly limited, but it is preferable to house the electrode body group 20 in the battery case 10 and then inject the non-aqueous electrolyte into the battery case 10.

[0063] In a preferred embodiment, this process includes a placement process (process 1-1), a welding and joining process (process 1-2), a drying process (process 1-3), and a liquid pouring process (process 1-4), typically in this order. However, the drying process (process 1-3) is optional and may be omitted in other embodiments. In other embodiments, the order of the welding and joining process (process 1-2) and the drying process (process 1-3) may be reversed. Furthermore, other processes may be included at any stage.

[0064] In the arrangement step (step 1-1), the electrode assembly group 20 is arranged inside the exterior housing 12. Specifically, the electrode assembly group 20 is accommodated inside the exterior housing 12 through the opening 12h. Next, in the welding and joining step (step 1-2), the sealing plate 14 is welded to the periphery of the opening 12h of the exterior housing 12 to integrate the exterior housing 12 and the sealing plate 14. Next, in the drying step (step 1-3), the exterior housing 12 accommodating the electrode assembly group 20 is dried with the liquid inlet 15 open to remove moisture from inside the exterior housing 12. In particular, moisture inside the electrode assembly group 20 is removed. Moisture removal can be performed in a conventional manner using, for example, a heating and drying device or a vacuum drying device, by using heating and / or decompression alone or in combination as appropriate. The heating temperature is preferably set to a temperature that can adequately evaporate moisture under reduced pressure and that does not thermally deteriorate the separators and other components of the electrode assembly group 20. The heating temperature can be set, for example, within a range of 50 to 200°C.

[0065] Next, in the liquid injection step (step 1-4), a nonaqueous electrolyte solution is first prepared. The nonaqueous electrolyte solution preferably contains a B-element-containing compound (e.g., a B-element-containing lithium salt) as described above. For example, the nonaqueous electrolyte solution preferably contains a boron-based additive in addition to a nonaqueous solvent and an electrolyte salt. While not particularly limited, the concentration of the boron-based additive in the nonaqueous electrolyte solution is preferably 0.01 mol / L or more, more preferably 0.05 mol / L or more, because this facilitates the formation of a coating of an appropriate quantity or quality on the surface of the negative electrode active material layer 24a. On the other hand, from the viewpoint of suppressing an increase in battery resistance, the concentration of the boron-based additive in the nonaqueous electrolyte solution is preferably 1 mol / L or less, more preferably 0.5 mol / L or less, and even more preferably 0.1 mol / L or less. The prepared nonaqueous electrolyte solution is then injected into the battery case 10 through the liquid injection hole 15 in the sealing plate 14. The injection is preferably carried out under reduced pressure inside the battery case 10 in order to improve the impregnation of the non-aqueous electrolyte into the electrode assembly 20 (electrode bodies 20a, 20b, 20c).

[0066] In the electrolyte impregnation step (step 2), after the battery assembly construction step (more specifically, the liquid injection step), the impregnation of the electrode body group 20, particularly the central portion in the long side direction Y, with the nonaqueous electrolyte is enhanced. This step may be performed in a room temperature environment (approximately 25°C ± 10°C). In a preferred embodiment, this step includes a pressurized / depressurized impregnation step (step 2-1) and a second impregnation step (step 2-2) in this order. Furthermore, other steps may be included at any stage. The required time for this step (total time for the pressurized / depressurized impregnation step and the second impregnation step) is preferably 10 to 200 hours. This facilitates the formation of a coating of suitable quantity or quality on the surface of the negative electrode active material layer 24a, thereby enabling the effects of the technology disclosed herein to be exerted to a high level.

[0067] In the pressurized / depressurized impregnation step (step 2-1), the inside of the battery assembly is pressurized and depressurized. In one example, the battery assembly is first housed in a pressure-adjustable chamber with the liquid inlet 15 open (in other words, with no pressure difference between the inside and outside of the battery case 10). Then, (1) a pressurization operation is performed in which the inside of the chamber is pressurized and the pressurized state is maintained for a predetermined time, and (2) a depressurization operation is performed in which the inside of the chamber is depressurized and the depressurized state is maintained for a predetermined time. The order of the pressurization operation and the depressurization operation is not particularly limited, but in one example, it is preferable to perform the depressurization operation after the pressurization operation.

[0068] The pressurization and depressurization conditions, such as the pressure and retention time, are preferably adjusted appropriately depending on the length Lc of the long side direction Y of the negative electrode active material layer 24a. In one example, when the length Lc of the long side direction Y of the negative electrode active material layer 24a is 200 mm or more, the pressurization pressure (degree of pressurization) in this step is preferably 0.60 MPa or more, more preferably 0.80 MPa or more. The retention time under pressure is preferably 30 minutes or more (for example, 30 to 120 minutes), more preferably 40 minutes or more.

[0069] According to the inventors' investigations, a positive correlation is found between the holding time under pressure in this step and the integral value of the ratio (B / C) at the center of the negative electrode active material layer 24a. That is, as the holding time under pressure increases, the integral value of the ratio (B / C) at the center of the negative electrode active material layer 24a tends to increase. In other words, the amount of boron (B) element tends to increase at the center in the long side direction Y. Therefore, by setting the holding time under pressure to a predetermined value or more, it is easy to adjust the integral value of the ratio (B / C) to the above-mentioned range (e.g., 28 or more).

[0070] The reduced pressure (degree of reduced pressure) in this step is preferably -0.070 to -0.098 MPa, more preferably -0.080 to -0.090 MPa. The time held under reduced pressure is preferably shorter than the time held under pressurized pressure. The time held under reduced pressure is preferably 1 to 10 minutes, and for example, preferably 5 minutes or more.

[0071] In this step, it is preferable to perform the pressurization operation and the depressurization operation once each. According to the investigations of the present inventors, if the number of repetitions of the pressurization operation and the depressurization operation increases, the inter-electrode distance between the positive electrode 22 and the negative electrode 24 in the vicinity of the negative electrode tab 24t increases, and as a result, the resistance tends to increase. By performing the pressurization operation and the depressurization operation once each, it is possible to prevent the inter-electrode distance between the positive electrode 22 and the negative electrode 24 from increasing, and thus to suppress an increase in resistance. This allows the a * This makes it easier to adjust the value to the above range (for example, 1.3 or less).

[0072] Next, in the second impregnation step (step 2-2), the battery assembly is left (held) under atmospheric pressure, which can further promote impregnation of the nonaqueous electrolyte solution into the interiors of the electrode bodies 20a, 20b, and 20c, particularly into the central portions in the long side direction Y.

[0073] In the initial charging step (step 3), the battery assembly is charged at least once after the electrolyte impregnation step. During the initial charging, additives in the non-aqueous electrolyte (e.g., boron-based additives) are typically electrolyzed before other components in the non-aqueous electrolyte (e.g., non-aqueous solvent and electrolyte salt). This results in the formation of a coating (SEI film) on the surface of the negative electrode active material layer 24a. For example, if the non-aqueous electrolyte contains a B-element-containing compound (typically, a B-element-containing lithium salt), a coating (SEI film) containing a decomposition product of the B-element-containing compound is formed on the surface of the negative electrode active material layer 24a.

[0074] The initial charge is preferably performed with a predetermined region of the battery case 10 pressed. Specifically, it is preferably performed with a portion of the flat portion 20f where the positive electrode active material layer 22a and the negative electrode active material layer 24a face each other pressed. In particular, it is preferable to perform the initial charge with the vicinity of the positive electrode tab 22t and the negative electrode tab 24t pressed (the portion where the inter-electrode distance between the positive electrode 22 and the negative electrode 24 is large and where the non-aqueous electrolyte is likely to accumulate). This makes it possible to prevent the inter-electrode distance from becoming locally large near the positive electrode tab 22t and the negative electrode tab 24t, and for example, a portion at the base of the negative electrode tab 24t is pressed. * This makes it easier to adjust the value to the above range (for example, 1.3 or less).

[0075] In a preferred embodiment, a cell press equipped with a pair of restraint plates is first prepared. A pressing member for pressing a predetermined region of the battery case 10 is also prepared. The size of the pressing member is preferably smaller than the long side wall 12b of the battery case 10 and preferably smaller than the electrode assembly 20a inside the battery case 10. The pressing member preferably has a length in the long side direction Y that does not significantly press the positive electrode tab group 23 and the negative electrode tab group 25, and is capable of pressing from the base of the positive electrode tab group 23 to the base of the negative electrode tab group 25. In the long side direction Y, the length of the pressing member is preferably equal to or longer than the length of the portion where the positive electrode active material layer 22a and the negative electrode active material layer 24a face each other (here, the length Lc of the positive electrode active material layer 22a). This effectively prevents the interelectrode distance from becoming locally large. The length of the pressing member is preferably longer than the length La of the negative electrode active material layer 24a. The pressing member preferably has a length that allows it to press almost the entire negative electrode active material layer 24a in the long side direction Y. The length of the pressing member is preferably 150 mm or more, more preferably 200 mm or more, and even more preferably 250 mm or more.

[0076] In this embodiment, the height of the pressing member in the vertical direction Z is smaller than the overall height of the electrode body 20a and is also smaller than the height Ha of the negative electrode active material layer 24a (the height of the flat portion 20f). This ensures a flow path for the nonaqueous electrolyte, allowing initial charging to be performed in a state in which the electrode body group 20 (electrode bodies 20a, 20b, 20c) is sufficiently impregnated with the nonaqueous electrolyte. The height of the pressing member is preferably 100 mm or less, more preferably 50 to 100 mm, even more preferably 60 to 90 mm, and particularly preferably 70 to 80 mm.

[0077] In this step, the pair of long side walls 12b of the battery assembly are then sandwiched between two pressing members from the short side direction X. Specifically, the battery assembly and the pressing members are opposed to each other so that the center of the long side walls 12b of the battery case 10 coincides with the center of the pressing members. In this state, the battery assembly is placed between a pair of restraining plates of a press machine, and charging is performed with a predetermined pressing force (restraining load) applied to the battery assembly. From the viewpoint of achieving the effects of the technology disclosed herein at a high level, the restraining load is preferably 10 kN or more, and more preferably 15 kN or more (e.g., 17 kN).

[0078] With the battery case 10 pressed in this manner, the battery assembly is charged. Charging of the battery assembly can be performed in a conventional manner. Typically, an external power source is connected between the positive and negative terminals of the battery assembly, and charging is performed until a predetermined voltage is reached between the positive and negative terminals. When the nonaqueous electrolyte contains an additive, charging is preferably performed at least up to the decomposition potential of the additive. For example, when the negative electrode active material is a carbon material such as graphite, the voltage reached may be set to approximately 3 V or higher, typically 3.5 V or higher, e.g., 4 V or higher. The charge rate may be, for example, about 0.1 C to 2 C. Charging may be performed once, or may be repeated two or more times, for example, with a discharge interval in between. This process may be performed in a room temperature environment (e.g., about 25°C ± 10°C, 25°C ± 5°C) or in a high-temperature environment, for example, about 45°C. Charging in a high-temperature environment can promote film formation.

[0079] In the degassing step (step 4), after the initial charging step, gases inside the battery case 10, such as air and gases generated by decomposition of the nonaqueous electrolyte solution during the initial charging step, are exhausted to the outside of the battery case 10. The gases can be exhausted, for example, by reducing the pressure inside the battery case 10. Then, in the liquid injection hole sealing step (step 5), the liquid injection hole 15 is sealed with a sealing member 16 while the pressure inside the battery case 10 remains at normal pressure or is reduced. This allows the battery case 10 to be airtightly sealed (hermetically sealed).

[0080] In the aging step (step 6), the battery assembly after initial charging is restrained and maintained in a predetermined temperature environment with a predetermined restraining load applied in the short side direction X (thickness direction of the electrode body group 20) for a predetermined aging period. The temperature environment is preferably 15 to 40°C, and may be, for example, room temperature (approximately 25°C ± 10°C). The restraining load is preferably 1 to 6 kN. In a preferred embodiment, first, a cell press equipped with a pair of restraining plates is prepared. Next, the battery assembly after initial charging is placed between the pair of restraining plates so that the pair of long side walls 12b of the battery case 10 face the restraining plates. In this state, a restraining load is applied to the battery assembly after initial charging using the press, and the battery is maintained for a predetermined aging period. The aging period may vary depending on, for example, the length La of the negative electrode active material layer 24a in the long side direction Y and the conditions of the electrolyte impregnation step (step 2), but is preferably approximately 5 days or more, more preferably 6 days or more. In this step, the voltage adjusted in the initial charging step may be maintained. In this manner, the battery 100 can be suitably manufactured.

[0081] <Inspection method for negative electrode 24> For example, quality control of resistance variations and thermal stability can be performed by sampling inspection on the battery assembly after the initial charge or the battery 100 that has undergone the aging step (step 6). The negative electrode 24 can be inspected during the sampling inspection. Therefore, in the inspection method disclosed herein, the battery assembly (or battery 100) that has undergone at least the construction step (step 1), the electrolyte impregnation step (step 2), and the initial charging step (step 3) of the manufacturing method is subjected to the following steps in this order: a disassembly step (step 7) of disassembling the battery assembly or battery 100, and a measurement step (step 8). In this embodiment, the measurement step (step 8) is followed by an evaluation step (step 9) of evaluating the resistance and thermal stability of the battery assembly or battery 100. Furthermore, other steps may be included at any stage.

[0082] In the disassembly step (step 7), the battery assembly is disassembled. Disassembly of the battery assembly is preferably carried out in an atmosphere of dry air (for example, with a dew point of about −50°C), for example, in a glove box, to prevent deterioration of the negative electrode 24 or separator 26. The battery assembly can be disassembled, for example, by first cutting the battery case 10 with a tool such as an end mill or a laser, separating the sealing plate 14 from the exterior body 12, and then removing the electrode assembly group 20 from inside the exterior body 12. The electrode body 20a is then separated from the removed electrode assembly group 20, and the wound structure is unwound, allowing the positive electrode 22, negative electrode 24, and separator 26 to be separated.

[0083] The measurement step (step 8) is * The method includes a first measuring step (step 8a) of measuring the B / C ratio and a second measuring step (step 8b) of measuring the B / C ratio and calculating an integral value. In the first measuring step (step 8a), after the disassembly step, a spectrophotometer is used to measure the B / C ratio in the vicinity of the negative electrode tab 24t of the negative electrode active material layer 24a (the base portion where the negative electrode tab 24t extends). * The measurement may be carried out multiple times to take into account variations. In this case, the arithmetic mean of the multiple measurements is taken as a * As mentioned above, the above a * There is a positive correlation between the value and the resistance value. * By measuring the B / C ratio, it is possible to easily identify whether or not there is a high resistance portion. In the second measurement step (step 8b), after the disassembly step, the B / C ratio is measured at the center of the negative electrode active material layer 24a in the long side direction Y using LA-ICP-MS, and an integrated value is calculated. As described above, there is a negative correlation between the integrated value of the B / C ratio and the heat generation amount of the battery 100. Therefore, by measuring the B / C ratio and calculating the integrated value, the thermal stability (heat generation behavior) of the battery 100 can be easily predicted or confirmed.

[0084] In the evaluation step (step 9), the resistance and thermal stability of the battery assembly or battery 100 are evaluated. * The product is judged to be good based on the value and the integral value of the B / C ratio. *If the value is equal to or less than a predetermined value (e.g., 1.3 or less) and the integral value of the B / C ratio is equal to or greater than a predetermined value (e.g., 28 or more), the battery assembly or battery 100 is determined to be a good product. In this case, a battery assembly or battery 100 determined to be a good product may have low resistance, excellent thermal stability, and little variation in quality. This allows highly reliable batteries 100 to be suitably supplied to the market.

[0085] <Uses of Battery 100> Battery 100 can be used for a variety of purposes, but for example, because of its high capacity, low resistance, and excellent thermal stability, it can be suitably used as a power source (driving power source) for motors mounted on vehicles such as passenger cars and trucks. The type of vehicle is not particularly limited, and examples include plug-in hybrid electric vehicles (PHEVs), hybrid electric vehicles (HEVs), and battery electric vehicles (BEVs). Battery 100 can also be suitably used as a battery pack formed by arranging a plurality of batteries 100 in a predetermined arrangement direction and applying a load from the arrangement direction using a restraining mechanism.

[0086] Hereinafter, several examples of the present invention will be described, but it is not intended that the present invention be limited to these examples.

[0087] <Preparation of evaluation battery> In the construction step (step 1), battery assemblies (Example, Comparative Examples 1 and 2) with the same configuration were constructed. Specifically, first, a lithium nickel cobalt manganese composite oxide (LiNi 0.6 Co 0.2 Mn 0.2O2, NCM) was prepared. A positive electrode active material layer containing this positive electrode active material, a carbon material (AB) as a conductive material, and PVdF as a binder in a mass ratio of NCM:AB:PVdF = 97.5:1.5:1.0 was provided on aluminum foil as a positive electrode current collector to form a strip-shaped positive electrode sheet. A negative electrode active material layer containing graphite (C) as a negative electrode active material and SBR and CMC as binders in a mass ratio of C:(SBR+CMC) = 98.5:1.5 was provided on copper foil as a negative electrode current collector to form a strip-shaped negative electrode sheet.

[0088] Next, the positive electrode sheet and the negative electrode sheet prepared above were placed opposite each other with a separator sheet interposed therebetween and wound into a flat shape to produce a wound electrode body. The separator sheet used was a PE substrate with a heat-resistant layer (also serving as an adhesive layer) containing alumina and PVdF on its surface. The length La of the negative electrode active material layer in the long side direction (winding axis direction) was 290 mm, and the height Ha was 90 mm.

[0089] Next, a non-aqueous electrolyte solution was prepared by dissolving LiPF in a mixed solvent of EC, EMC, and DMC, and further adding LiBOB as an additive to a concentration of 0.05 mol / L. The wound electrode body and the non-aqueous electrolyte solution were then housed in a rectangular parallelepiped battery case to construct a battery assembly.

[0090] In the electrolyte impregnation step (step 2), first, for each example, the battery assembly was placed in a pressure-adjustable chamber with the inlet port open. Then, in the examples, as shown in Table 1, the battery case was pressurized to 0.8 MPa and held at that pressure for 50 minutes. Then, the battery case was depressurized to -90 kPa and held at that pressure for 5 minutes. The battery case was then restored to 0 MPa (pressurized / depressurized impregnation step (step 2-1)). In contrast, in Comparative Example 1, only the depressurization operation was performed under the conditions shown in Table 1, and in Comparative Example 2, only the pressurization operation was performed under the conditions shown in Table 1. In Comparative Example 2, the battery case was pressurized to 0.8 MPa, held at that pressure for 6 minutes, and then the battery case was restored to 0 MPa. This operation was repeated 20 times. Then, for each example, the battery assembly was left as it was (second impregnation step (step 2-2)). Table 1 also shows the ratio (relative value) of the total time required for the electrolyte impregnation step (step 2).

[0091] In the initial charging step (step 3), a pressing member was first prepared. Next, the pressing member was positioned based on the center of the long side wall of the constructed battery assembly, and the battery assembly and pressing member were restrained with the restraining load shown in Table 1. As a result, the pressing member restrained the flat portion of the wound electrode body where the positive electrode active material layer and the negative electrode active material layer faced each other. In the long side direction, the flat portion of the wound electrode body was pressed by the pressing member from the base of the positive electrode tab group to the base of the negative electrode tab group. Next, the battery assembly with the restraining load applied was charged to an SOC (State of Charge) of 12% at a charge rate of 0.2 C. Next, in the degassing step (step 4), the pressure inside the battery case was reduced to -0.09 MPa. Next, in the liquid injection hole sealing step (step 5), the liquid injection hole was sealed with a sealing member while the pressure inside the battery case 10 was reduced. Next, in the aging step (step 6), the battery assembly after initial charging was held for 5 days in a 25°C environment with a restraining load of 4 kN applied. Then, activation treatment was performed and the battery was removed at the point indicated by the arrow. In this manner, evaluation batteries (Example and Comparative Examples 1 and 2) were produced.

[0092] [Table 1]

[0093] <Disassembly of the evaluation battery> In the disassembly step (step 7), the test battery after the activation step was discharged until the voltage reached 3.0 V, and then disassembled in a dry air atmosphere (for example, with a dew point of about -50°C), and the wound electrode body was removed from the battery case. Then, the wound electrode body was unwound, and the negative electrode was separated.

[0094] <Checking for color unevenness on the negative electrode and measuring resistance> First, the negative electrode was cut into an appropriate size along its long side and washed with DMC to prepare a test specimen for resistance measurement. The cut-out position was the flat portion located 15 turns (intermediate circumference) from the winding starting point. Next, color unevenness in the negative electrode active material layer (particularly near the negative electrode tab) was visually observed. The results are shown in Table 1. As shown in Table 1, no color unevenness was observed in the negative electrode active material layer in Comparative Example 1, where only the decompression operation was performed in the electrolyte impregnation step, or in the Example, where the pressurization operation and the decompression operation were performed once each in the electrolyte impregnation step. On the other hand, in Comparative Example 2, where the pressurization operation was repeated in the electrolyte impregnation step, dark and light areas of color unevenness were visually observed. This is thought to be due to the fact that the repeated pressurization in the electrolyte impregnation step opened up the gap between the positive and negative electrodes, causing localized accumulation of nonaqueous electrolyte, resulting in the formation of a large amount of organic coating derived from the nonaqueous solvent during initial charging.

[0095] Next, a resistance testing device was prepared, which included a mounting section for accommodating a test specimen and a non-aqueous electrolyte, a probe to be brought into contact with the measurement point, and an AC impedance measuring section. The configuration of the resistance testing device can be seen, for example, in Japanese Patent Application Laid-Open No. 2014-25850. The probe includes, for example, a cylindrical main body that accommodates a non-aqueous electrolyte and a counter electrode (metallic Li), and a measuring section that connects to the bottom end of the main body and contacts a portion (measurement point) of the negative electrode active material layer of the test specimen, and is configured to be movable along the long side direction (width direction) of the test specimen. The diameter of the measuring section is, for example, approximately Φ1 mm to 10 mm. The AC impedance measuring section is configured to measure impedance by inputting an AC current or AC voltage between the working electrode in contact with the negative electrode tab and the measurement point (counter electrode) where the measuring section of the probe is in contact.

[0096] Next, the mounting portion of the resistance testing device was filled with a nonaqueous electrolyte (containing only a nonaqueous solvent and a supporting salt, with no additives), and a test specimen was placed in the mounting portion. Next, with the working electrode in contact with the negative electrode tab, a probe was moved at measurement intervals of approximately 10 mm along the long side (width direction) from the base of the negative electrode tab of the negative electrode active material layer, and the resistance of the surface of the negative electrode active material layer was measured in spots using an AC impedance method. Specifically, for each measurement point, the difference in resistance (ΔΩ) from the DC to the end of the impedance semicircle was obtained. Then, a resistance distribution was created showing the relationship between the measurement position and the difference in resistance (ΔΩ). The resistance distribution of Comparative Example 1 is shown in FIG. 7(a), the resistance distribution of Comparative Example 2 is shown in FIG. 7(b), and the resistance distribution of the Example is shown in FIG. 7(c).

[0097] Table 1 also shows the resistance values ​​of the negative electrode active material layer near the negative electrode tab (part (1) in FIGS. 7(a) and 7(c)) for Comparative Example 1 and the Example, and the resistance values ​​of the negative electrode active material layer near the negative electrode tab, in the part with color unevenness (parts (1) to (3) in FIG. 7(b)), for Comparative Example 2. As shown in Table 1, it was found that the resistance value was relatively high in the part where color unevenness was observed visually by a human. Table 1 also shows the resistance ratio when the resistance of the negative electrode active material layer near the negative electrode tab (end of the negative electrode active material layer) of Comparative Example 1 is set as the reference (1.0). As can be seen from this, the resistance ratio was less than 2 times in the Example, whereas the resistance ratio exceeded 2 times across the board in Comparative Example 2.

[0098] <Measurement using a spectrophotometer> In the first measurement step (step 8a), a diffused illumination type spectrophotometer (model: CM-26dG) manufactured by Konica Minolta was used to measure the surface of the portion of the negative electrode active material layer where the resistance value was determined (i.e., portion (1) in Figure 7(a) and Figure 7(c) and portions (1) to (3) in Figure 7(b)) using the SCI (Specular Component Include) method, which captures specular reflected light by trapping the specular reflected light, and the L * a * b * a in color system * The values ​​were measured and the results are shown in Table 1.

[0099] Figure 8 shows the relationship between the resistance ratio of the negative electrode active material layer near the negative electrode tab and a * As shown in Figure 8, the relationship between * It was found that if the value is 1.3 or less, there is no color unevenness near the negative electrode tab and resistance can be reduced.

[0100] <Checking for color unevenness on the negative electrode and measuring resistance> In the second measurement step (step 8b), a negative electrode (measurement sample) as shown in Figure 9 was first prepared. Then, using a laser ablation ICP mass spectrometer, a laser was irradiated onto the sample along the long side direction (width direction) from the vicinity of the negative electrode tab of the negative electrode active material layer, and ICP mass spectrometry was continuously performed while microparticulating the sample at the laser irradiated locations. The measurement range in the width direction was designated as the "measurement range (0 to 180 mm)" in Figure 9. The arrow in Figure 9 indicates the laser travel direction.

[0101] The ratio (B / C) of the amount of boron (B) to the amount of carbon (C) was then calculated, and a graph was created with the measurement position (mm) on the horizontal axis and the ratio (B / C) on the vertical axis. The distribution of the ratio (B / C) for Comparative Example 1 is shown in FIG. 10(a), the distribution of the ratio (B / C) for Comparative Example 2 is shown in FIG. 10(b), and the distribution of the ratio (B / C) for the Example is shown in FIG. 10(c). Table 1 also lists the values ​​integrated within the "integral range (0-100 mm)" in FIG. 9. As shown in Table 1, Comparative Example 2, in which pressure was repeatedly applied in the electrolyte impregnation step, had a relatively larger integral value of the ratio (B / C) than Comparative Example 1, in which only pressure reduction was applied in the electrolyte impregnation step. This indicates that the amount of B element increased in the center of the negative electrode active material layer. Furthermore, in the examples in which the pressurization operation and the decompression operation were each performed once in the electrolyte impregnation process, the integral value of the above ratio (B / C) became even larger, and it was found that the amount of B element in the central part of the negative electrode active material layer was significantly increased.

[0102] <Evaluation of battery heat generation> First, a sample for DSC measurement was prepared. Specifically, a laminate cell was constructed by placing a positive electrode having a positive electrode active material layer (20 mm x 20 mm) and a negative electrode having a negative electrode active material layer (22 mm x 22 mm) opposite each other with a separator interposed therebetween in a dry air atmosphere (dew point: -50°C) and housing them together with 0.4 mL of nonaqueous electrolyte. Next, the laminate cell prepared above was charged at a constant current of 8 mA to 4.25 V, followed by constant voltage charging for 5 hours. Next, the charged laminate cell was disassembled in a glove box (Ar atmosphere). Next, the electrolyte was collected, and the positive and negative electrodes were removed. The positive electrode composite was peeled off from the central region of the positive electrode active material layer, and the negative electrode composite was peeled off from the central region of the negative electrode active material layer. Then, 1 mg of the positive electrode composite removed from the positive electrode, 2 mg of the negative electrode composite removed from the negative electrode, and 4 mg of the collected electrolyte were placed in a sample container. This sample container was press-sealed at 20 MPa and then placed in a differential scanning calorimetry (DSC) along with a reference material (Al2O3, 2 mg). The temperature was then increased from 25°C to 350°C at a rate of 2°C / min in an inert atmosphere, and the calorific value (J) between 75°C and 200°C was calculated by integration. The results are shown in Table 1.

[0103] Figure 11 shows the relationship between the integral value of the ratio (B / C) at the center of the negative electrode active material layer and the amount of heat generated by the battery. As shown in Figure 11, a correlation was observed between the integral value of the ratio (B / C) and the amount of heat generated by the battery. That is, the larger the integral value of the ratio (B / C), in other words, the greater the amount of B element, the smaller the amount of heat generated by the battery. Therefore, it was found that if the integral value of the ratio (B / C) is 28 or more, the amount of heat generated can be kept small (for example, to 20 J or less, preferably to 18 J or less), and the thermal stability of the battery can be improved.

[0104] Although several embodiments of the present invention have been described above, the above embodiments are merely examples. The present invention can be implemented in various other forms. The present invention can be implemented based on the contents disclosed in this specification and the common general technical knowledge in the relevant field. The technology described in the claims includes various modifications and alterations of the above-exemplified embodiments. For example, it is possible to replace part of the above-described embodiments with other modifications, or to add other modifications to the above-described embodiments. Furthermore, if a technical feature is not described as essential, it may be deleted as appropriate.

[0105] As described above, specific aspects of the technology disclosed herein include those described in the following sections. Item 1: An electrode assembly including a positive electrode and a negative electrode, a positive electrode terminal electrically connected to the positive electrode, a negative electrode terminal electrically connected to the negative electrode, and a non-aqueous electrolyte, wherein the negative electrode includes a negative electrode active material layer containing a carbon material and having a width of 200 mm or more, and a plurality of negative electrode tabs provided at one end in the width direction of the negative electrode active material layer, the plurality of negative electrode tabs being electrically connected to the negative electrode terminal in a stacked and folded state, and wherein when a base portion from which the negative electrode tabs extend of the negative electrode active material layer is measured with a spectrophotometer, the L * a * b * a in color system * a value of 1.3 or less, and the carbon element amount and the boron element amount are measured along the width direction of the negative electrode active material layer by laser ablation ICP mass spectrometry, and the integrated value of the ratio of the boron element amount to the carbon element amount (B / C) within a range of ±20 mm from the center in the width direction is 28 or more. Item 2: The nonaqueous electrolyte secondary battery according to Item 1, wherein the electrode body is a wound electrode body in which the strip-shaped positive electrode and the strip-shaped negative electrode are stacked and wound in an insulated state. Item 3: The nonaqueous electrolyte secondary battery according to item 1 or 2, wherein the width direction coincides with the winding axis direction of the wound electrode body. Item 4: The nonaqueous electrolyte secondary battery according to any one of Items 1 to 3, wherein the nonaqueous electrolyte contains a compound containing elemental boron. [Explanation of symbols]

[0106] 10 Battery case 20a, 20b, 20c electrode body (wound electrode body) 22 Positive electrode 24 Negative electrode 24a Negative active material layer 24c negative electrode current collector 24t negative electrode tab 30 Positive terminal 40 Negative terminal 100 batteries

Claims

1. an electrode assembly including a positive electrode and a negative electrode, a positive electrode terminal electrically connected to the positive electrode, a negative electrode terminal electrically connected to the negative electrode, and a non-aqueous electrolyte; the negative electrode has a negative electrode active material layer containing a carbon material and having a width of 200 mm or more, and a plurality of negative electrode tabs provided at one end in the width direction; the plurality of negative electrode tabs are stacked and folded and electrically connected to the negative electrode terminal; When the base portion of the negative electrode active material layer from which the negative electrode tab extends is measured with a spectrophotometer, the L ＊ a ＊ b ＊ a in the color system ＊ The value is 1.3 or less, and the carbon element amount and the boron element amount are measured along the width direction of the negative electrode active material layer by laser ablation ICP mass spectrometry, and the integrated value of the ratio of the boron element amount to the carbon element amount (B / C) within a range of ±20 mm from the center in the width direction is 28 or more; Nonaqueous electrolyte secondary battery.

2. The electrode body is a wound electrode body in which the strip-shaped positive electrode and the strip-shaped negative electrode are stacked and wound in an insulated state. The nonaqueous electrolyte secondary battery according to claim 1 .

3. The width direction is a direction that coincides with the winding axis direction of the wound electrode body. The nonaqueous electrolyte secondary battery according to claim 2 .

4. The non-aqueous electrolyte solution contains a compound containing boron element.

3. The nonaqueous electrolyte secondary battery according to claim 1.

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