Nonaqueous electrolyte secondary battery, its manufacturing method, and inspection method

Abstract

To provide a nonaqueous electrolyte solution secondary battery with improved thermal stability.SOLUTION: A nonaqueous electrolyte solution secondary battery includes an electrode body including a positive electrode with a positive electrode active material layer and a negative electrode with a negative electrode active material layer, and a nonaqueous electrolyte solution. The negative electrode active material layer has a width of 15 cm or more, includes a film containing a boron element, and has a black unevenness index of 12 or less.SELECTED DRAWING: Figure 12

Description

[Technical Field]

[0001] The present invention relates to a non-aqueous electrolyte secondary battery, a method for manufacturing the same, and a method for inspecting the same. [Background technology]

[0002] Conventionally, non-aqueous electrolyte secondary batteries have been known that include an electrode assembly including a positive electrode and a negative electrode, and a non-aqueous electrolyte. In non-aqueous electrolyte secondary batteries, part of the non-aqueous electrolyte decomposes during initial charging, and a coating containing the decomposition product (Solid Electrolyte Interface film: SEI film) is deposited on the surface of the negative electrode active material layer. This coating stabilizes the interface between the negative electrode and the non-aqueous electrolyte, which can improve battery performance. Related prior art documents include Patent Documents 1 and 2.

[0003] For example, Patent Document 1 describes that the initial output characteristics of a non-aqueous electrolyte secondary battery can be improved by adding an oxalate complex compound to the non-aqueous electrolyte and carrying out a predetermined aging treatment. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-250288 [Patent Document 2] Japanese Patent Publication No. 2022-154250 [Patent Document 3] Japanese Patent Application Publication No. 2017-022067 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the inventors' investigations have revealed that there is still room for improvement when applying the above technology to recent high-capacity nonaqueous electrolyte secondary batteries. Specifically, in high-capacity nonaqueous electrolyte secondary batteries, the width of the electrode assembly is large, e.g., 15 cm or more, making it difficult for additives to penetrate to the center of the electrode assembly in the width direction. Therefore, in the center of the electrode assembly, a coating containing decomposition products of the additive is less likely to form on the surface of the negative electrode active material layer during initial charging. In other words, the coating formed on the negative electrode is likely to be uneven in its state (e.g., quantity and quality) between the center and other parts of the electrode assembly. As a result, the reactivity of the negative electrode is increased in the center, locally reducing thermal stability and making it more susceptible to temperature rise, for example, during overcharging.

[0006] 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 with improved thermal stability, a method for manufacturing the same, and a method for inspecting the same. [Means for solving the problem]

[0007] The present invention provides a non-aqueous electrolyte secondary battery comprising: an electrode assembly including a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer; a non-aqueous electrolyte; and a battery case that accommodates the electrode assembly and the non-aqueous electrolyte, wherein the negative electrode active material layer has a width of 15 cm or more and is provided with a coating containing elemental boron, and has a black unevenness index of 12 or less as determined by the following (Steps 1) to (Step 7). The black unevenness index is calculated by: (Step 1) photographing the surface of the negative electrode with a camera so as to include the central portion of the negative electrode active material layer in the width direction, and obtaining photographed image data expressed in an RGB color model; (Step 2) extracting the Blue component of the RGB values ​​from the photographed image data and converting it into 256-level grayscale image data in which black is 0 and white is 255; (Step 3) extracting a line profile from the grayscale image data along the width direction of the negative electrode, with the X axis representing the distance from one end of the width direction and the Y axis representing the 256-level value; and (Step 4) linearly approximating the line profile and correcting the slope with the obtained slope to create a first correction profile. (Step 5) A second correction profile is created by quadratically approximating the first correction profile and performing curvature correction using the obtained quadratic approximation formula; (Step 6) A central portion of the X-axis is extracted from the second correction profile so as to include a peak passing through the minimum point where the Y-axis value is smallest, and then the extracted central portion is linearly approximated and tilt corrected using the obtained tilt to create a third correction profile; (Step 7) When the portion of the third correction profile excluding the peak passing through the minimum point is used as a base portion, the absolute value of the difference between the average value (base value) of the Y-axis at the base portion and the Y-axis value at the minimum point is calculated as the black unevenness index.

[0008] The present inventors have newly discovered that unevenness in the coating formation state at the widthwise center of the negative electrode active material layer manifests as color unevenness (black unevenness). Furthermore, when the degree (shade) of this color unevenness is defined as a "black unevenness index" using the above-described procedure, they have found a positive correlation between the black unevenness index and the amount of heat generated by the battery. That is, the higher the black unevenness index (in other words, the darker the black), the greater the amount of heat generated and the poorer the thermal stability. Therefore, in the present invention, the black unevenness index is adjusted to a predetermined value or less. This suppresses heat generation in the battery and improves thermal stability. Ultimately, it also suppresses temperature rise during overcharging, etc. Furthermore, by using an objective numerical value (black unevenness index) based on photographed image data as an index, accuracy is relatively less likely to vary compared to, for example, visually identifying color unevenness in the negative electrode active material layer. Therefore, thermal stability can be stably improved, resulting in a highly reliable battery.

[0009] The present invention also provides a method for producing a non-aqueous electrolyte secondary battery comprising an electrode assembly including a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer, a non-aqueous electrolyte, and a battery case containing the electrode assembly and the non-aqueous electrolyte, wherein the width of the negative electrode active material layer is 15 cm or more. The production method includes a construction step of constructing a battery assembly by housing the electrode assembly together with the non-aqueous electrolyte containing a solvent, an electrolyte salt, and a compound containing elemental boron in the battery case, an initial charging step of adjusting the state of charge (SOC) of the battery assembly to 5 to 50%, and an aging step of leaving the battery assembly after the initial charging step at a temperature above 50°C and not exceeding 70°C for 24 hours or more with the state of charge being 5 to 50%.

[0010] The inclusion of the aging step facilitates adjustment of the black unevenness index of the negative electrode active material layer, and the black unevenness index can be suitably suppressed to a predetermined value or less. Therefore, according to the present invention, heat generation is suppressed, and a highly reliable nonaqueous electrolyte secondary battery with excellent thermal stability can be suitably manufactured.

[0011] Although not related to the present invention, Patent Document 3 discloses a manufacturing method in which a battery assembly is charged to an SOC of 65% or more after initial charging, and then the battery assembly is subjected to low-temperature aging by being left at 15 to 30°C for 6 hours or more, and then to high-temperature aging by being left at 60°C for 20 hours or more.

[0012] The present invention also provides a method for inspecting a non-aqueous electrolyte secondary battery, including: a construction step of constructing a battery assembly by housing, in a battery case, an electrode body in which a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer are stacked with a separator interposed therebetween, and a non-aqueous electrolyte solution containing a solvent, an electrolyte salt, and a compound containing boron element; an initial charging step of adjusting the state of charge (SOC) of the battery assembly to 5 to 50%; an aging step of leaving the battery assembly, after the initial charging step, in a state of 5 to 50% charge at a temperature exceeding 50°C and not exceeding 70°C for 24 hours or more; a dismantling step of dismantling the battery assembly after the aging step; and a calculation step of determining, after the dismantling step, a black unevenness index of the negative electrode active material layer using (Steps 1) to (Step 7) above.

[0013] By calculating the black unevenness index, the degree of heat generation of the battery can be accurately predicted or confirmed. [Brief explanation of the drawings]

[0014] [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 perspective view schematically showing an electrode assembly. [Figure 4] FIG. 4 is a schematic diagram showing the configuration of the electrode body. [Figure 5] FIG. 5 is an example of a line profile. [Figure 6] FIG. 6 is a first-order approximation of the line profile in FIG. [Figure 7] FIG. 7 shows the first correction profile after tilt correction. [Figure 8] FIG. 8 is a quadratic approximation of the first correction profile of FIG. [Figure 9] FIG. 9 shows the second correction profile after the curvature correction. [Figure 10] FIG. 10 is a first-order approximation of the partial extracted profile obtained by extracting a part of FIG. [Figure 11] FIG. 11 shows the third correction profile after tilt correction. [Figure 12] FIG. 12 is a graph showing the relationship between the black unevenness index and the amount of heat generated. [Figure 13] FIG. 13 is a graph showing the relationship between the retention time (number of days) and the black unevenness index. DETAILED DESCRIPTION OF THE INVENTION

[0015] 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."

[0016] 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 so-called storage batteries such as lithium ion secondary batteries, and capacitors such as lithium ion capacitors and electric double layer capacitors.

[0017] <Battery 100> FIG. 1 is a perspective view of a nonaqueous electrolyte secondary battery (hereinafter also simply referred to as battery) 100. FIG. 2 is a schematic longitudinal cross-sectional view taken along line II-II 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 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. However, these directions are merely used for convenience of description and do not in any way limit the installation form of battery 100.

[0018] 2, the battery 100 includes a battery case 10, an electrode assembly 20, a positive electrode terminal 30, a negative electrode terminal 40, a positive electrode current collector 50, a negative electrode current collector 60, a positive electrode insulating member 70, a negative electrode insulating member 80, and a nonaqueous electrolyte (not shown). The battery 100 here is a lithium ion secondary battery. The battery 100 is preferably a lithium ion secondary battery.

[0019] 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. The battery case 10 preferably includes an exterior body 12 and a sealing plate 14.

[0020] 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.

[0021] 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).

[0022] 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).

[0023] 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 and the negative electrode terminal 40 extend from the inside to the outside of the sealing plate 14 through the terminal lead-out holes 18, 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, 19 by a crimping process. 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).

[0024] As shown in Fig. 2, the positive electrode terminal 30 is electrically connected to the positive electrode 22 (see Fig. 4, specifically, the positive electrode tab group 23) of the electrode body 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, for example, aluminum or an aluminum alloy.

[0025] The negative electrode terminal 40 is electrically connected to the negative electrode 24 (see FIG. 4 , specifically, the negative electrode tab group 25) of the electrode body 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.

[0026] 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.

[0027] As shown in FIG. 2, the electrode assembly 20 is accommodated inside the battery case 10 (more specifically, inside the exterior housing 12). The number of electrode assemblies 20 arranged inside one battery case 10 is not particularly limited, and may be one, or two or more (plural). The electrode assembly 20 may be arranged inside the battery case 10 while covered with an insulating electrode assembly holder. In other words, the electrode assembly holder may be interposed between the electrode assembly 20 and the battery case 10 (more specifically, the exterior housing 12). The electrode assembly holder may be, for example, box-shaped, with the electrode assembly 20 arranged inside it. The electrode assembly holder is preferably made of resin.

[0028] FIG. 3 is a perspective view that schematically shows the electrode assembly 20. FIG. 4 is a schematic diagram that shows the configuration of the electrode assembly 20. As shown in FIG. 4, the electrode assembly 20 includes a positive electrode 22, a negative electrode 24, and a separator 26. In this example, the electrode assembly 20 is a wound electrode assembly. The electrode assembly 20 is configured by stacking a strip-shaped positive electrode 22 and a strip-shaped negative electrode 24 with a strip-shaped separator 26 interposed between them and winding them around a winding axis WL. Although not particularly limited, the number of windings of the electrode assembly 20 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.

[0029] The electrode assembly 20 is preferably a wound electrode assembly. As shown in Fig. 3, when the electrode assembly 20 is a wound electrode assembly, the nonaqueous electrolyte is supplied only from both ends in the long side direction Y (winding axis WL direction). Therefore, the center M in the long side direction Y (winding axis WL direction) Y The nonaqueous electrolyte is particularly unlikely to penetrate sufficiently into the central portion, including the center portion. Therefore, unevenness in the state of coating formation (e.g., quantity and quality) tends to occur between the central portion in the long side direction Y and other portions. Therefore, applying the technology disclosed herein is particularly effective.

[0030] As can be seen from FIGS. 2 and 3 , the electrode assembly 20 is disposed inside the battery case 10 with the winding axis WL oriented approximately parallel to the long side direction Y. The winding axis WL direction is the width direction of the positive electrode 22, the negative electrode 24, and the separator 26, and coincides with the long side direction Y here. The electrode assembly 20 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. The battery 100 here 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 20 in the winding axis WL direction (the left and right ends in FIGS. 2 and 3 ).

[0031] As shown in FIG. 3, the electrode body 20 has a flat outer shape. The electrode body 20 preferably has a flat outer shape. The electrode body 20 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 (the YZ plane in FIG. 3). 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.

[0032] 1 to 3, 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 20 is preferably disposed inside the battery case 10 such that the stacking direction (thickness direction) of the positive electrode 22 (see FIG. 4) and the negative electrode 24 (see FIG. 4) in the flat portions 20f coincides with the short side direction X (the direction perpendicular to the long side walls 12b).

[0033] The positive electrode 22 may be the same as a conventional one and is not particularly limited. As shown in FIG. 4, 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 may be omitted in other embodiments. The positive electrode current collector 22c 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. In this example, the positive electrode current collector 22c is a metal foil, specifically, an aluminum foil.

[0034] A plurality of positive electrode tabs 22t are provided at one end of the positive electrode current collector 22c in the long side direction Y (the left end in FIG. 4). The plurality of positive electrode tabs 22t protrude toward one side in the long side direction Y (the left side in FIG. 4). The plurality of positive electrode tabs 22t protrude further in the long side direction Y than the separator 26. Here, the positive electrode tab 22t is part of the positive electrode current collector 22c and is made of metal foil (aluminum foil). At least a portion of the positive electrode tab 22t is a collector exposed portion where the positive electrode active material layer 22a and the positive electrode protective layer 22p are not formed and the positive electrode current collector 22c is exposed. As shown in FIG. 3, the plurality of positive electrode tabs 22t are stacked at one end of the long side direction Y (the left end in FIG. 3) to form a positive electrode tab group 23. The positive electrode tab group 23 is electrically connected to the positive electrode terminal 30 via the positive electrode current collector 50. A positive electrode second current collecting portion 52 (described later) of the positive electrode current collecting portion 50 is attached (more specifically, joined) to the positive electrode tab group 23.

[0035] As shown in Fig. 4, 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 lithium nickel cobalt manganese 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. For example, a carbon material such as acetylene black (AB) can be used as the conductive material. For example, polyvinylidene fluoride (PVdF) can be used as the binder.

[0036] Although not particularly limited, as shown in FIG. 4, in a high-capacity battery 100 used for in-vehicle applications, the width of the positive electrode active material layer 22a in the direction of the winding axis WL (average value, excluding the portion formed on the positive electrode tab 22t), in other words, the length Lc in the long side direction Y, is preferably 15 cm or more, more preferably 20 cm or more, and even more preferably 25 cm or more.

[0037] As shown in FIG. 4, 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 (the left end in FIG. 4) of the positive electrode current collector 22c in the long side direction Y. 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, and the like. 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.

[0038] As shown in FIG. 4, 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. The negative electrode current collector 24c is made of a conductive metal such as copper, a copper alloy, nickel, or stainless steel. In this example, the negative electrode current collector 24c is a metal foil, specifically a copper foil.

[0039] A plurality of negative electrode tabs 24t are provided at one end in the long side direction Y of the negative electrode current collector 24c (the right end in FIG. 4). The plurality of negative electrode tabs 24t each protrude toward one side in the long side direction Y (the right side in FIG. 4). The plurality of negative electrode tabs 24t protrude further in the long side direction Y than the separator 26. 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. As shown in FIG. 3, the plurality of 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 tab group 25 is electrically connected to the negative electrode terminal 40 via the negative electrode current collector 60. A negative electrode second current collecting portion 62 of the negative electrode current collecting portion 60, which will be described later, is attached (more specifically, joined) to the negative electrode tab group 25.

[0040] As shown in FIG. 4, 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 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).

[0041] In this embodiment, the negative electrode active material layer 24a includes a coating (SEI film) containing boron (B). This boron is a component derived from a coating-forming agent, specifically, a boron-based additive, added to the nonaqueous electrolyte solution during construction of the battery 100. The coating is a decomposition product containing the boron-based additive decomposed during initial charging. This can favorably improve battery characteristics (e.g., output characteristics, cycle characteristics, durability, etc.). The presence of a coating containing boron can be confirmed by detecting B using a conventionally known method, such as inductively coupled plasma (ICP) emission spectroscopy, ion chromatography, or X-ray absorption fine structure (XAFS).

[0042] As described above, in recent high-capacity electrode assemblies 20, it has become difficult for additives to penetrate to the center in the long side direction Y (winding axis WL direction, width direction). According to the inventors' investigations, this causes the coating formation state (e.g., quantity and quality) to differ from other portions in the center in the long side direction Y, resulting in color unevenness (black unevenness). That is, the negative electrode active material layer 24a has color unevenness (black unevenness). The inventors defined the degree (shade) of this black unevenness as a "black unevenness index" obtained by a predetermined procedure and quantified it. As a result, a positive correlation was found between this black unevenness index and the heat generation amount of the battery 100, as will be described in detail in the Examples below. Therefore, in this embodiment, the black unevenness index of the negative electrode active material layer 24a is set to 12 or less. This suppresses heat generation in the center in the long side direction Y, thereby improving the thermal stability of the battery 100. This in turn makes it possible to suppress the temperature rise of the battery 100 during overcharging and the like.

[0043] In this specification, the "black unevenness index" refers to a value determined by the following steps (Steps 1) to (Step 7) after disassembling the battery 100 and removing the negative electrode 24. That is, first, in Step 1, the surface of the negative electrode 24 is photographed with a camera so as to include the central portion of the negative electrode active material layer 24a in the long side direction Y, and photographed image data expressed in the RGB color model (Red-Green-Blue color model) is obtained. The photographing can be performed, for example, with a commercially available digital camera. When the electrode assembly 20 is a wound electrode assembly as in this embodiment, it is preferable to photograph the surface of the negative electrode 24 located in the central portion in the short side direction X (the thickness direction perpendicular to the winding axis WL direction) which is thought to be least susceptible to penetration of the nonaqueous electrolyte, for example, within five turns (for example, about the third turn) from the winding start end.

[0044] Next, in (Step 2), for example, commercially available image analysis software is used to extract only the blue component of the RGB values ​​from the captured image data and convert it into grayscale image data with 256 gradations, with black being 0 and white being 255. Next, in (Step 3), a line profile is extracted from the grayscale image data along the long side direction Y of the negative electrode 24. The line profile can be expressed as a distance from one end of the long side direction Y on the X axis and a value (gray value) of the 256 gradations on the Y axis. When the electrode assembly 20 is a flat wound electrode assembly as in this embodiment, it is preferable to extract the line profile so as to include the center of the flat portion 20f (YZ plane in FIG. 3 ) into which the nonaqueous electrolyte solution is less likely to penetrate, i.e., the center in the long side direction Y (winding axis WL direction) and the center in the up-down direction Z.

[0045] Next, in (Step 4), the line profile is linearly approximated (first-order approximation) to calculate a first-order approximation formula. Then, a first correction profile is created by correcting the slope of the line profile using the slope of the first-order approximation formula. Next, in (Step 5), the first correction profile is quadratically approximated to calculate a second-order approximation formula. Then, a second correction profile is created by curvature-correcting the first correction profile using the obtained second-order approximation formula. Then, in (Step 6), a predetermined range (e.g., 50 pixels) of the center portion in the long side direction Y is extracted from the second correction profile so as to include a peak passing through the minimum point where the Y-axis value is smallest (black is the darkest), and a partial extraction profile is created. Next, the partial extraction profile is linearly approximated (first-order approximation) to calculate a first-order approximation formula. Then, a third correction profile is created by correcting the slope of the partial extraction profile using the slope of the first-order approximation formula. Next, in (Step 7), when the portion of the third correction profile excluding the peak passing through the minimum point is taken as the base portion, the absolute value of the difference between the average value (base value) of the Y axis of the base portion and the Y axis value of the minimum point is calculated as the black unevenness index.

[0046] Although color unevenness (black unevenness) in the coating can be identified by visual inspection, for example, due to individual differences in human eyes, it is expected that the results of determining whether or not there is color unevenness will vary from person to person. In contrast, when an objective numerical value (black unevenness index) based on captured image data is used as an index, as in the technology disclosed herein, there is relatively little variation in accuracy. Furthermore, it is possible to distinguish color differences that are indistinguishable by the human eye. Therefore, it is possible to provide a battery 100 that can stably improve thermal stability and has high reliability.

[0047] From the viewpoint of achieving the effects of the technology disclosed herein at a high level, the black unevenness index is preferably 10 or less, more preferably 9 or less, and even more preferably 8 or less. For example, in recent high-capacity batteries 100, the black unevenness index is typically 1 or more, 2 or more, and may be, for example, 4 or more, 5 or more, 6 or more, or 7 or more. In embodiments where the black unevenness index is a predetermined value or more, applying the technology disclosed herein is particularly effective. The black unevenness index can be suitably adjusted not only by the amount of non-aqueous electrolyte injected during construction of the battery 100 and the concentration of the additive (a compound containing boron element) in the non-aqueous electrolyte, but also by the conditions of the aging step (step 5) after initial charging in the manufacturing method described below, particularly the retention time.

[0048] As shown in FIG. 4 , the width of the negative electrode active material layer 24a in the winding axis WL direction (average value, excluding the portion formed on the negative electrode tab 24t), in other words, the length La in the long side direction Y, is typically equal to or longer than the length Lc in the long side direction Y of the positive electrode active material layer 22a. From the viewpoint of achieving high capacity, the length La is 15 cm or more. The length La is preferably 20 cm or more, and more preferably 25 cm or more. The longer the length La, the more difficult it is for the nonaqueous electrolyte to penetrate into the center portion in the long side direction Y, and the more likely it is that the state of the coating (e.g., quantity and quality) will be uneven. Therefore, applying the technology disclosed herein is particularly effective. The length La may be, for example, 100 cm or less, or 50 cm or less. This allows the effects of the technology disclosed herein to be exerted to a high level.

[0049] As shown in FIG. 4, 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 width of the separator 26 in the winding axis WL direction, in other words, the length Ls of the long side direction Y, 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 heat-resistant layer (HRL) or an adhesive layer on the surface of a substrate portion made of a porous resin sheet. The heat-resistant layer is, for example, a layer containing an inorganic filler and a binder. The adhesive layer is a layer containing a binder. The heat-resistant layer and the adhesive layer may have the same configuration as conventional ones.

[0050] As shown in FIG. 2 , the positive electrode current collector 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 collector 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 collector 50 includes a positive electrode first current collector 51 and a positive electrode second current collector 52. The positive electrode first current collector 51 is attached to the inner surface of the sealing plate 14. The positive electrode second current collector 52 extends along the short side wall 12c of the exterior body 12. The positive electrode second current collector 52 is attached to the positive electrode tab group 23 of the electrode body 20.

[0051] 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 and a negative electrode second current collector 62. 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. The negative electrode second current collector 62 is attached to the negative electrode tab group 25 of the electrode assembly 20.

[0052] 2, the positive electrode insulating member 70 is a member that insulates the sealing plate 14 from the positive electrode first current collecting part 51. The positive electrode insulating member 70 is made of a resin material that has electrical insulation properties and resistance to the electrolyte solution used, and is elastically deformable, and is preferably made of, for example, a polyolefin resin such as polypropylene (PP), a fluorinated resin such as tetrafluoroethylene-perfluoroalkoxyethylene copolymer (PFA), or polyphenylene sulfide (PPS).

[0053] 2, the negative electrode insulating member 80 is a member that insulates the sealing plate 14 from the negative electrode first current collecting portion 61. The negative electrode insulating member 80 is disposed symmetrically to the positive electrode insulating member 70 with respect to the center CL in the long side direction Y of the electrode body 20. The material, configuration, etc. of the negative electrode insulating member 80 may be the same as those of the positive electrode insulating member 70.

[0054] 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).

[0055] 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.

[0056] The non-aqueous electrolyte may further contain additional components (additives). As the additives, one or more of those conventionally known to be addible to non-aqueous electrolytes may be used. Examples of the additives include boron-based additives containing boron, such as lithium bis(oxalato)borate (LiBOB) and lithium difluoro(oxalato)borate (LiODFB); phosphorus-based additives containing phosphorus, such as lithium difluorophosphate (lithium difluorophosphate, LiPOF), lithium difluorooxalatophosphate (LiDFOP), and lithium (fluorosulfonyl)(difluorophosphonyl)imide; sulfur-based additives containing sulfur, such as lithium fluorosulfonate (LiSOF), lithium ethyl sulfate, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium (fluorosulfonyl)(difluorophosphonyl)imide; and carbonate-based additives containing a carbonate group (—O—(C═O)—O—), such as vinylene carbonate (VC), vinylethylene carbonate (VEC), and fluoroethylene carbonate (FEC). These additives may be so-called film-forming agents that decompose (at a low potential) before the non-aqueous solvent and / or electrolyte salt during initial charging and deposit as a film on the surface of the negative electrode active material layer 24a, etc.

[0057] The non-aqueous electrolyte preferably contains a boron-based additive (an additive containing boron element). This can suitably improve battery characteristics, such as output characteristics and cycle characteristics (high output retention rate and / or high capacity retention rate). The boron-based additive is preferably an oxalate complex compound containing boron element (an oxalate compound containing B element).

[0058] In addition to the boron-based additive, the nonaqueous electrolyte preferably further contains at least one of a phosphorus-based additive, a sulfur-based additive, and a carbonate-based additive. In particular, it is preferable to use a boron-based additive and a carbonate-based additive in combination. This can further improve battery characteristics, such as power output characteristics and cycle characteristics (high power retention rate and / or high capacity retention rate).

[0059] The additives in the non-aqueous electrolyte (for example, the above-mentioned boron-based additives, phosphorus-based additives, sulfur-based additives, and carbonate-based additives) are typically electrically decomposed by initial charging during battery production, etc., 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 may or may not contain (remain) the above-mentioned additives.

[0060] <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), an injection hole sealing step (step 4), and an aging step (step 5), in this order. However, the electrolyte impregnation step (step 2) is optional and can be omitted in other embodiments. In other embodiments, the order of the injection hole sealing step (step 4) and the aging step (step 5) can be reversed. Furthermore, other steps may be included at any stage.

[0061] In the construction step (step 1), the electrode body 20 and the non-aqueous electrolyte are placed in the battery case 10 in a glove box to construct a battery assembly. In this specification, the term "battery assembly" refers to an intermediate product that has been assembled up to the state before the initial charging step (step 3) is performed in the manufacturing process of the battery 100. The order in which the electrode body 20 and the non-aqueous electrolyte are placed in the battery case 10 is not particularly limited. For example, the electrode body 20 may be placed in the battery case 10, and then the non-aqueous electrolyte may be poured into the battery case 10.

[0062] 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 can 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) can be reversed, or the order of the welding and joining process (process 1-2) and the liquid pouring process (process 1-4) can be reversed. In other embodiments, the liquid pouring process (process 1-4) can be performed in multiple stages. In addition, other processes can be included at any stage.

[0063] In the placement step (step 1-1), the electrode assembly 20 is placed inside the exterior housing 12. Specifically, the electrode assembly 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 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 20 is removed. Moisture removal can be performed in a conventional manner using a heating and drying device, a vacuum drying device, or the like, by heating, reducing pressure, or the like, alone or in combination. The heating temperature is preferably set to a temperature that allows adequate evaporation of moisture under reduced pressure and does not cause thermal degradation of the separator of the electrode assembly 20. The heating temperature can be set, for example, within a range of 50 to 200°C.

[0064] Next, in the liquid injection step (step 1-4), a non-aqueous electrolyte solution is first prepared. The non-aqueous electrolyte solution contains a boron-based additive in addition to a non-aqueous solvent and an electrolyte salt. Although not particularly limited, the proportion of the boron-based additive in the entire non-aqueous electrolyte solution is preferably 0.1% by mass or more, and more preferably 0.5% by mass or more. On the other hand, from the viewpoint of suppressing an increase in battery resistance, the proportion of the boron-based additive in the entire non-aqueous electrolyte solution is preferably 5% by mass or less, more preferably 2% by mass or less, and even more preferably 1% by mass or less.

[0065] In addition to the boron-based additive, the nonaqueous electrolyte preferably further contains other additives, such as at least one of the above-mentioned phosphorus-based additives, sulfur-based additives, and carbonate-based additives. The concentrations of the various additives may be the same as the proportion of the boron-based additive. The prepared nonaqueous electrolyte is then injected into the battery case 10 through the injection hole 15 in the sealing plate 14. Injection is preferably performed under reduced pressure within the battery case 10 to improve the impregnation of the nonaqueous electrolyte into the electrode assembly 20.

[0066] In the electrolyte impregnation step (step 2), after the battery assembly construction step (more specifically, the liquid injection step), the battery assembly is impregnated with the electrolyte. In this step, to promote the impregnation, the battery assembly may be housed in a pressure-adjustable chamber, and pressurization and / or depressurization may be repeated a predetermined number of times with the liquid injection hole 15 open (in other words, with no pressure difference between the inside and outside of the battery case 10). This allows the nonaqueous electrolyte to be thoroughly impregnated into the inside of the electrode body 20, particularly the central portion in the long side direction Y, and further reduces unevenness of the coating in the long side direction Y.

[0067] In a preferred embodiment, pressurization and depressurization are repeated a predetermined number of times. The pressure during pressurization depends on factors such as the length La of the negative electrode active material layer 24a in the long side direction Y, but is preferably 0.5 MPa or more, and more preferably 0.8 MPa or more. The time for which the pressurized state is maintained depends on factors such as the length La of the negative electrode active material layer 24a in the long side direction Y, but is preferably 5 minutes or more, and more preferably 6 minutes or more. After pressurization, the pressure is released to return to normal pressure (approximately 0 MPa), and the time for maintaining the normal pressure state is preferably 1 minute or more. The number of times pressurization and depressurization are repeated is preferably 10 times or more, more preferably 15 times or more, and even more preferably 20 times or more. This process may be performed in an environment at room temperature (e.g., approximately 25°C±10°C, 25°C±5°C).

[0068] In the initial charging step (step 3), after the electrolyte impregnation step, the state of charge (SOC) of the battery assembly is adjusted to 5 to 50%. The battery assembly can be charged in the same manner as in the past. Typically, an external power source is connected between the positive and negative terminals of the battery assembly, and charging is performed until a predetermined state of charge (SOC) is achieved. The state of charge (SOC) is not particularly limited as long as it is 5 to 50%, but from the viewpoint of stably ensuring the charge amount necessary to decompose the additive, it is preferably 10% or more, and more preferably 15% or more. Furthermore, excessive charging in this step (in other words, before the formation of the coating in the aging step (step 5)) may accelerate solvent decomposition, so the state of charge (SOC) is preferably 40% or less, and more preferably 30% or less. The charge rate can be, for example, about 0.1 C to 2 C. Charging may be performed once, or it can be repeated two or more times, for example, with a discharge in between. This step may be carried out in a room temperature environment (for example, about 25°C ± 10°C, about 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.

[0069] During the initial charge, the additives in the non-aqueous electrolyte (at least the boron-based additive) are typically electrolyzed before other components in the non-aqueous electrolyte (the non-aqueous solvent and the electrolyte salt), forming a coating (SEI film) containing at least the decomposition product of the boron-based additive on the surface of the negative electrode active material layer 24a.

[0070] In the liquid injection hole sealing step (step 4), the liquid injection hole 15 is sealed with a sealing member 16. In a preferred embodiment, first, after the initial charging step, gas inside the battery case 10, such as air or gas generated by decomposition of the nonaqueous electrolyte solution during the initial charging step, is exhausted to the outside of the battery case 10. The gas can be exhausted, for example, by reducing the pressure inside the battery case 10. Next, the battery case 10 is airtightly sealed (hermetically sealed) while the pressure inside the battery case 10 remains at normal pressure or is reduced.

[0071] In the aging step (step 5), the battery assembly after initial charging is maintained in a temperature environment above 50°C and below 70°C for 24 hours or more while maintaining the state of charge (SOC) adjusted in the initial charging step (step 3), i.e., an SOC of 5 to 50%. This promotes film formation of the negative electrode active material layer 24a, and suitably adjusts the black unevenness index in the long side direction Y to a predetermined value or less. Therefore, heat generation is suppressed, and a highly reliable battery 100 with excellent thermal stability can be suitably manufactured. This step may be started within approximately one day, e.g., within one hour, after the initial charging step (step 3) or the liquid injection hole sealing step (step 4). The time for maintaining the battery assembly in a temperature environment of 50°C or less after the initial charging step (step 3) or the liquid injection hole sealing step (step 4) is preferably within one day, e.g., within one hour.

[0072] The battery assembly can be maintained at the above temperature conditions (high temperature conditions) by any suitable conventional heating means. For example, the battery assembly can be placed in a thermostatic chamber (temperature-controlled thermostatic chamber) set to a predetermined temperature, or the battery assembly can be heated externally using a heating means such as an infrared heater. The temperature at which the battery assembly is maintained is not particularly limited as long as it is greater than 50°C and equal to or less than 70°C, but is preferably 60 to 70°C.

[0073] Although details will be described in the Examples below, the inventors' investigations have revealed that there is a negative correlation between the holding temperature in this step and the black unevenness index of the negative electrode active material layer 24a. That is, the higher the holding temperature in this step, the smaller the black unevenness index tends to be. Therefore, by setting the holding temperature at a predetermined value or higher, the black unevenness index can be efficiently reduced. In addition, the time required for this step can be shortened, thereby improving production efficiency. Furthermore, by setting the temperature within the above range, decomposition of additives that were not completely decomposed in the initial charging step (step 3) is promoted, and a high-quality coating can be formed on the surface of the negative electrode active material layer 24a. Therefore, a battery 100 with excellent thermal stability can be realized.

[0074] The holding time of the battery assembly is not particularly limited as long as it is 24 hours (1 day) or more. According to the inventors' investigations, there is a negative correlation between the holding time in this step and the black unevenness index of the negative electrode active material layer 24a. That is, the longer the holding time, the smaller the black unevenness index tends to be. Therefore, the holding time 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 48 hours (2 days) or more, and more preferably 72 hours (3 days) or more. By setting the holding time to a predetermined time or more, it becomes easier to adjust the black unevenness index of the negative electrode active material layer 24a to the above-mentioned range (e.g., 12 or less). From the viewpoint of production efficiency, the holding time is preferably 240 hours (10 days) or less, and more preferably 144 hours (6 days) or less.

[0075] This step is preferably performed with the battery assembly restrained. This prevents gas (e.g., air or gas generated in the initial charging step (step 3)) from becoming trapped inside the electrode body 20. 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, and in this state, a restraining load is applied to the battery assembly from the short side direction X by the press. The restraining load is preferably 0.5 kN or more, more preferably 1 to 15 kN, and even more preferably 3 to 10 kN. However, this step may also be performed with the battery assembly not restrained (unrestrained state). In this manner, the battery 100 can be suitably manufactured.

[0076] <Battery 100 inspection method> The battery assembly or battery 100 that has undergone steps 1 to 5 of the above-described manufacturing method may be subjected to quality control of thermal stability, for example, in the form of a random sampling inspection. Therefore, in the inspection method disclosed herein, the battery assembly is further subjected to the following steps, in this order: a charge / discharge step (step 6) of charging and discharging the battery assembly, a disassembly step (step 7) of disassembling the battery assembly, and a calculation step (step 8) of calculating the black unevenness index of the negative electrode active material layer 24a. However, the charge / discharge step (step 6) is optional and may be omitted in other embodiments. Furthermore, in this embodiment, an evaluation step (step 9) of evaluating the thermal stability of the battery assembly is also performed. Furthermore, other steps may be included at any stage.

[0077] In the charge / discharge step (step 6), the battery assembly after the aging step is charged and discharged at least once. Although not particularly limited, in one example, the battery assembly is charged until the state of charge (SOC) reaches 80% or more, preferably 90% or more, for example, 100%, and then discharged until the state of charge (SOC) reaches 20% or less, preferably 10% or less, for example, 0%. This step can also be understood as a capacity confirmation step for the battery assembly. The charge / discharge rate can be, for example, about 0.1 C to 2 C. Charging and discharging can be performed once, or can be repeated two or more times. This step can be performed in a room temperature environment (for example, about 25°C ± 10°C, 25°C ± 5°C).

[0078] In the disassembly step (step 7), the battery assembly is disassembled after the charge / discharge step (or aging step). Disassembly of the battery assembly is preferably carried out in a dry air atmosphere (for example, with a dew point of about −50°C), for example, in a glove box, to avoid deterioration of the negative electrode 24. 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 body 20 from inside the exterior body 12. The removed electrode body 20 can then be unwound to separate it into the positive electrode 22, the negative electrode 24, and the separator 26.

[0079] In the calculation step (step 8), after the disassembly step, the black unevenness index of the negative electrode active material layer 24a is calculated according to the above-mentioned (step 1) to (step 7). As described above, there is a positive correlation between the black unevenness index of the negative electrode active material layer 24a and the amount of heat generated by the battery 100. Therefore, by calculating the black unevenness index of the negative electrode active material layer 24a, the thermal stability (heat generation behavior) of the battery 100 can be predicted or confirmed. Furthermore, compared to directly measuring the amount of heat generated by the battery using, for example, a calorimeter, the thermal stability (heat generation behavior) of the battery 100 can be grasped relatively easily.

[0080] In the evaluation step (step 9), the thermal stability of the battery assembly or the battery 100 is evaluated. In a preferred embodiment, the correlation between the black unevenness index and the heat generation amount of the battery 100 is expressed in advance by a formula (e.g., an approximate curve shown in the Examples) through a preliminary test or the like, and the heat generation amount of the battery 100 is predicted by substituting the black unevenness index into the formula. In another preferred embodiment, the quality of the battery 100 is determined based on the black unevenness index of the negative electrode active material layer 24a. For example, if the black unevenness index of the negative electrode active material layer 24a is equal to or less than a predetermined value (e.g., 12 or less), the battery assemblies are determined to be good quality. In this case, the battery assemblies determined to be good quality may have reduced heat generation and little variation in thermal stability. This allows the upper limit of the heat generation amount of the battery 100 to be suitably controlled, and highly reliable batteries 100 can be supplied to the market.

[0081] <Uses of Battery 100> Battery 100 can be used for a variety of purposes, but for example, because of its high capacity 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.

[0082] Hereinafter, several examples of the present invention will be described, but it is not intended that the present invention be limited to these examples.

[0083] <Test Example I (Correlation between Black Spot Index and Heat Generation Amount of Battery)> First, in the construction step (step 1), a flat wound electrode body and a non-aqueous electrolyte were housed in a battery case to construct battery assemblies (Examples 1 to 6). The flat portion of each negative electrode active material layer had a length of 285 mm in the winding axis direction (width direction) and a length (height) of 90 mm in the vertical direction. The non-aqueous electrolyte used in each case was a mixed solvent containing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 30:30:40, to which 0.8 mass % of lithium bis(oxalato)borate (LiBOB) as a boron-based additive and 1.0 mass % of vinylene carbonate (VC) as a carbonate-based additive were added, and LiPF6 as an electrolyte salt was dissolved at a concentration of 1.15 mol / L.

[0084] Next, in the electrolyte impregnation step (step 2), the battery assembly was left standing until the electrolyte permeated the entire electrode body. Next, in the initial charging step (step 3), the battery assembly was charged at a constant current to an SOC of 12% (corresponding to a voltage of 3.4 V). Next, in the liquid injection hole sealing step (step 4), the liquid injection hole was sealed with a sealing member while the battery case was kept at normal pressure. Next, in the aging step (step 5), the battery assembly was held in a temperature environment of 25°C with a restraining load of 1 kN applied to it for the holding time shown in Table 1. Next, in the charge / discharge step (step 6), the battery assembly was charged to an SOC of 90% and then discharged to an SOC of 0%. In this way, nonaqueous electrolyte secondary batteries (Examples 1 to 6) were constructed, with only the holding time in the aging step being different.

[0085] Next, in the disassembly step (step 7), the nonaqueous electrolyte secondary battery of each example was disassembled in a glove box under an Ar atmosphere, and the electrode assembly was removed from the battery case. The electrode assembly removed from the battery case was then unwound, and the positive electrode, negative electrode, and separator were separated.

[0086] <Evaluation of Black Unevenness Index> Next, in the calculation step (step 8), first, a digital camera (Canon PowerShot SX620 HS) was used to photograph the surface of the negative electrode so as to include substantially the entire width of the negative electrode active material layer, and photographed image data for each example was obtained (step 1). The photographing position was set to include the center (the center in the width direction and the center in the vertical direction) of the flat portion located three turns from the winding starting end. Next, using commercially available image analysis software, the blue component of the RGB values ​​was extracted from the photographed image data and converted into a 256-level grayscale image, with black being 0 and white being 255 (step 2). Next, a line profile (X axis: distance from one end in the width direction (pixels); Y axis: value of the 256 levels (gray value)) was extracted from the grayscale image data along the width direction so as to include the center in the width direction (step 3). Note that the extraction position was set to the center in the vertical direction. Figure 5 shows an example of a line profile.

[0087] Next, the line profile was linearly approximated to calculate a first-order approximation equation. Figure 6 shows the approximation equation for the line profile in Figure 5. In this example, the first-order approximation equation was y = -0.3104x + 153.72. The line profile was then tilt-corrected using the obtained tilt (-0.3104). Specifically, the product of the tilt (-0.3104) and the X-axis value was subtracted from the Y-axis value of each point in the line profile in Figure 5. This created a first correction profile (Step 4). Figure 7 shows the first correction profile.

[0088] Next, the first correction profile was subjected to quadratic approximation to calculate a quadratic approximation formula. Figure 8 shows the quadratic approximation formula for the first correction profile in Figure 7. In this example, the quadratic approximation formula is y=-0.0019x 2+0.4507x+135.69. Then, the first correction profile was subjected to curvature correction using the obtained quadratic approximation formula. Specifically, the values ​​obtained by substituting each X-axis value into the quadratic approximation formula were subtracted from the Y-axis value of each point on the line profile in Figure 7. This created a second correction profile (Step 5). Figure 9 shows the second correction profile.

[0089] Next, from the second correction profile, a 50-pixel central portion of the X-axis (within the frame in Figure 9) was extracted, centered on the minimum point Min, so as to include peak P passing through the minimum point Min, where the Y-axis value is the smallest. The Y-axis value at the minimum point Min was then zeroed and the X-axis value at the minimum point Min was centered to create a partial extraction profile. Figure 10 shows the partial extraction profile in Figure 9. Next, the partial extraction profile was linearly approximated to calculate a first-order approximation. Figure 9 also shows the approximation. In this example, the first-order approximation was y = 0.1878x - 10.475. The obtained slope (0.1878) was used to correct the slope of the partial extraction profile. Specifically, the product of the slope (0.1878) and the X-axis value was subtracted from the Y-axis value of each point in the line profile in Figure 10. This created a third correction profile (Step 6). Figure 11 shows the third correction profile. Next, the portion of the third correction profile excluding the peak P was taken as the base portion, and the absolute value of the difference between the average value of the Y axis of the base portion (base value, BL in FIG. 11) and the Y axis value of the minimum point was calculated as the black unevenness index (step 7). The results are shown in Table 1.

[0090] <Evaluation of Heat Generation> First, a positive electrode active material layer and a negative electrode active material layer were each cut out to a size of 30 mm x 30 mm from the center (the center in the width direction and the center in the vertical direction) of the flat portion of the electrode assembly. Next, one positive electrode active material layer (30 mm x 30 mm), one negative electrode material layer (30 mm x 30 mm), and 250 μL of electrolyte were placed in a sample container. The nonaqueous electrolyte used was a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of EC:DMC:EMC = 3:4:3, with LiPF6 dissolved as the electrolyte salt at a concentration of 1.1 mol / L (no additives added).

[0091] Next, the sample container was press-sealed under 20 MPa and placed in a Calve calorimeter (SETARAM, model: CALVET HT) together with a standard substance (Al2O3, 2 mg). The temperature was then increased from 25°C to 400°C in an inert atmosphere using the following heating program, and the calorific value (J) between 75 and 200°C was calculated by integration. The results are shown in Table 1. [Heating program] Total 434 minutes (1) 25°C to 30°C, heating rate: 1.5°C / min (4 min) (2) Keep at 30°C (60 minutes) (3) 30°C to 400°C, heating rate: 1°C / min (370 min)

[0092] <Evaluation of the amount of organic coating> A piece of the negative electrode active material layer measuring 30 mm x 30 mm was cut from the center of the flat part of the electrode body (the center in the width direction and the center in the vertical direction) and immersed in water to extract the components. After filtering the extract, the amount of organic coating was measured using a nuclear magnetic resonance (NMR) device under the following conditions. The results are shown in Table 1. Equipment: 600MHz NMR manufactured by JEOL Ltd. (JEOL) Measurement conditions: 1H-NMR

[0093] [Table 1]

[0094] Figure 12 shows the relationship between the black unevenness index of the negative electrode active material layer and the amount of heat generated by the battery. As shown in Figure 12, a positive correlation was observed between the black unevenness index and the amount of heat generated. Furthermore, when the black unevenness index was small (in other words, the black color was light), the amount of heat generated was small and the thermal stability was high. Furthermore, based on the above approximation curve, it was found that by setting the black unevenness index to 12 or less, the amount of heat generated could be kept to approximately 20 J or less, and the thermal stability of the battery could be improved.

[0095] FIG. 13 shows the relationship between the holding time (number of days) and the black unevenness index. As shown in FIG. 13, the longer the holding time in the aging step (step 5), the smaller the black unevenness index of the negative electrode active material layer. In other words, a negative correlation was observed between the holding time and the black unevenness index. Therefore, it was found that the black unevenness index can be suitably adjusted by lengthening or shortening the holding time in the aging step. When the holding temperature in the aging step is 25°C, it was thought that a holding time of 3.5 days or more was necessary to achieve a black unevenness index of 12 or less.

[0096] Furthermore, as shown in Table 1, it was found that the larger the black unevenness index, the greater the amount of organic coating. 2 A very strong positive correlation of 0.05% = 1 was observed. The amount of organic coating is thought to be due to side reactions such as the decomposition of solvent components. From this, it was inferred that if the retention time in the aging process is insufficient and maturation is insufficient, the amount of organic coating increases and the "quality" of the coating decreases, resulting in a higher black unevenness index (darker black) and reduced thermal stability.

[0097] <Test Example II (Examples 7 to 9)> When the holding temperature in the aging step was 25°C, a holding time of 3.5 days or more was required to achieve a black unevenness index of 12 or less. Therefore, in this test example, the holding temperature in the aging step was set to 60°C in order to improve production efficiency. The black unevenness index of the negative electrode active material layer was calculated in the same manner as in Test Example I, except that the battery assembly was held for the holding time shown in Table 2. The results are shown in Table 2.

[0098] [Table 2]

[0099] As shown in Table 2, by setting the holding temperature in the aging process to 60°C, it was possible to reduce the black unevenness index in a much shorter time than when the temperature was 25°C. In other words, production efficiency was dramatically improved. Furthermore, when the holding temperature in the aging process was 60°C, the black unevenness index could be reduced to 10 or less by setting the holding time to 24 hours (1 day) or more as in Example 7, and the black unevenness index could be reduced even further to 8 or less by setting the holding time to 72 hours (3 days) or more as in Example 8.

[0100] 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.

[0101] For example, in the above-described embodiment, the electrode assembly 20 is a wound electrode assembly. However, this is not limited to this. In another embodiment, the electrode assembly 20 may be a stacked electrode assembly in which multiple rectangular positive electrodes and multiple rectangular negative electrodes are stacked in a state insulated by a separator. In this case, the "center in the width direction" into which the electrolyte solution is less likely to penetrate is the center in the width direction of one side of the square (preferably in the long side direction in the case of a rectangle).

[0102] For example, in the above-described embodiment, the electrode assembly 20 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 in the winding axis WL direction, and is housed in the battery case 10 so that the winding axis WL direction coincides with the long side direction Y. However, this is not limited to this. 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 in the winding axis WL direction of the electrode assembly 20 (e.g., the upper end in Figures 2 and 3). In this case, the winding axis WL direction may coincide with the vertical direction Z. In this case, the "center in the width direction" into which the electrolyte solution is less likely to permeate is the center in the vertical direction Z.

[0103] As described above, specific aspects of the technology disclosed herein include those described in the following sections. Item 1: A non-aqueous electrolyte secondary battery comprising: an electrode assembly including a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer; a non-aqueous electrolyte; and a battery case that accommodates the electrode assembly and the non-aqueous electrolyte, wherein the negative electrode active material layer has a width of 15 cm or more and is provided with a coating that contains elemental boron, and the black unevenness index obtained by the above (Steps 1) to (Steps 7) is 12 or less. Item 2: The nonaqueous electrolyte secondary battery according to Item 1, wherein the electrode body is a wound electrode body formed by stacking a strip-shaped positive electrode and a strip-shaped negative electrode with a strip-shaped separator interposed therebetween and winding them, and the width of the negative electrode active material layer in the winding axis direction of the wound electrode body is 15 cm or more. Item 3: The nonaqueous electrolyte secondary battery according to Item 1 or 2, wherein the nonaqueous electrolyte contains an additive containing elemental boron. Item 4: The nonaqueous electrolyte secondary battery according to any one of Items 1 to 3, wherein the black unevenness index is 5 or more. Item 5: A method for producing a non-aqueous electrolyte secondary battery comprising: an electrode assembly including a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer; a non-aqueous electrolyte; and a battery case that accommodates the electrode assembly and the non-aqueous electrolyte, wherein the width of the negative electrode active material layer is 15 cm or more, the method comprising: a construction step of accommodating the electrode assembly in the battery case together with the non-aqueous electrolyte containing a solvent, an electrolyte salt, and a compound containing boron element to construct a battery assembly; an initial charging step of adjusting the state of charge (SOC) of the battery assembly to 5 to 50%, and an aging step of leaving the battery assembly, after the initial charging step, at a temperature exceeding 50°C and not exceeding 70°C for 24 hours or more in a state where the state of charge is 5 to 50%. Item 6: The method for producing a nonaqueous electrolyte secondary battery according to Item 5, wherein the temperature condition in the aging step is 60° C. or higher and 70° C. or lower. Item 7: A method for inspecting a non-aqueous electrolyte secondary battery, comprising: a construction step of constructing a battery assembly by housing, in a battery case, an electrode body in which a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer are stacked with a separator interposed therebetween, and a non-aqueous electrolyte solution containing a solvent, an electrolyte salt, and a compound containing boron element; an initial charging step of adjusting the state of charge (SOC) of the battery assembly to 5 to 50%; an aging step of leaving the battery assembly, after the initial charging step, in a state in which the state of charge is 5 to 50% at a temperature exceeding 50°C and not exceeding 70°C for 24 hours or more; a dismantling step of dismantling the battery assembly after the aging step; and a calculation step of determining, after the dismantling step, a black unevenness index of the negative electrode active material layer by (Step 1) to (Step 7) above. [Explanation of symbols]

[0104] 10 Battery case 20 Electrode body (wound electrode body) 22 Positive electrode 24 Negative electrode 24a Negative active material layer 24c negative electrode current collector 26 Separator 100 batteries

Claims

1. an electrode assembly including a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer; a non-aqueous electrolyte; and a battery case that accommodates the electrode assembly and the non-aqueous electrolyte; The negative electrode active material layer has a width of 15 cm or more, includes a coating containing elemental boron, and is formed by the following (Step 1) to (Step 7): (Step 1) photographing the surface of the negative electrode with a camera so as to include the central portion of the negative electrode active material layer in the width direction, and acquiring photographed image data expressed in an RGB color model; (Step 2) Extract the blue component of the RGB values ​​from the captured image data and convert it into grayscale image data with 256 gradations, with black being 0 and white being 255; (Step 3) extracting a line profile from the grayscale image data along the width direction of the negative electrode, the line profile having an X axis representing the distance from one end in the width direction and a Y axis representing the 256 gradation values; (Step 4) A first correction profile is created by linearly approximating the line profile and correcting the tilt using the obtained tilt; (Step 5) quadratically approximating the first correction profile and correcting the curvature with the obtained quadratic approximation formula to create a second correction profile; (Step 6) A central portion of the X-axis is extracted from the second correction profile so as to include a peak passing through the minimum point where the Y-axis value is smallest, and then the extracted central portion is linearly approximated, and a third correction profile is created by correcting the slope using the obtained slope; (Step 7) When a portion of the third correction profile excluding the peak passing through the minimum point is defined as a base portion, the absolute value of the difference between the average value (base value) of the Y axis of the base portion and the Y axis value of the minimum point is calculated as a black unevenness index; The black unevenness index calculated by the above formula is 12 or less. Nonaqueous electrolyte secondary battery.

2. the electrode body is a wound electrode body formed by stacking a strip-shaped positive electrode and a strip-shaped negative electrode with a strip-shaped separator interposed therebetween and winding them, The width of the negative electrode active material layer in the winding axis direction of the wound electrode body is 15 cm or more. The nonaqueous electrolyte secondary battery according to claim 1 .

3. The non-aqueous electrolyte contains an additive containing elemental boron.

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

4. The black unevenness index is 5 or more.

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

5. A method for manufacturing a non-aqueous electrolyte secondary battery comprising: an electrode assembly including a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer; a non-aqueous electrolyte; and a battery case that accommodates the electrode assembly and the non-aqueous electrolyte, wherein the width of the negative electrode active material layer is 15 cm or more, a construction step of housing the electrode body in the battery case together with a non-aqueous electrolyte solution containing a solvent, an electrolyte salt, and a compound containing boron element to construct a battery assembly; an initial charging step of adjusting the state of charge (SOC) of the battery assembly to 5 to 50%; an aging step of leaving the battery assembly at a state of charge of 5 to 50% for 24 hours or more at a temperature exceeding 50°C and not exceeding 70°C after the initial charging step; Including, A method for manufacturing a non-aqueous electrolyte secondary battery.

6. The temperature condition of the aging step is 60°C or higher and 70°C or lower. The method for producing the nonaqueous electrolyte secondary battery according to claim 5 .

7. a construction step of constructing a battery assembly by housing an electrode body in which a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer are stacked with a separator interposed therebetween, and a non-aqueous electrolyte solution containing a solvent, an electrolyte salt, and a compound containing boron element, in a battery case; an initial charging step of adjusting the state of charge (SOC) of the battery assembly to 5 to 50%; an aging step of leaving the battery assembly at a state of charge of 5 to 50% for 24 hours or more at a temperature exceeding 50°C and not exceeding 70°C after the initial charging step; a disassembly step of disassembling the battery assembly after the aging step; After the disassembly process, the following steps (Step 1) to (Step 7): (Step 1) photographing the surface of the negative electrode with a camera so as to include the central portion of the negative electrode active material layer in the width direction, and acquiring photographed image data expressed in an RGB color model; (Step 2) Extract the blue component of the RGB values ​​from the captured image data and convert it into grayscale image data with 256 gradations, with black being 0 and white being 255; (Step 3) extracting a line profile from the grayscale image data along the width direction of the negative electrode, the line profile having an X axis representing the distance from one end in the width direction and a Y axis representing the 256 gradation values; (Step 4) Linearly approximate the line profile and correct the tilt using the obtained tilt to create a first correction profile; (Step 5) quadratically approximating the first correction profile and correcting the curvature with the obtained quadratic approximation formula to create a second correction profile; (Step 6) A central portion of the X-axis is extracted from the second correction profile so as to include a peak passing through the minimum point where the Y-axis value is smallest, and then the extracted central portion is linearly approximated, and a third correction profile is created by correcting the slope using the obtained slope; (Step 7) When a portion of the third correction profile excluding the peak passing through the minimum point is defined as a base portion, the absolute value of the difference between the average value (base value) of the Y axis of the base portion and the Y axis value of the minimum point is calculated as a black unevenness index; a calculation step of determining a black unevenness index of the negative electrode active material layer by Including, Inspection method for non-aqueous electrolyte secondary battery.

Images