Nonaqueous electrolyte secondary battery and method for inspecting negative electrode for nonaqueous electrolyte secondary battery

Abstract

To provide a nonaqueous electrolyte solution secondary battery with improved thermal stability.SOLUTION: A nonaqueous electrolyte solution secondary battery includes an electrode body in which a positive electrode and a negative electrode are stacked through a separator, a nonaqueous electrolyte solution, and a battery case that accommodates the electrode body and the nonaqueous electrolyte solution. The negative electrode includes a negative electrode current collector and a negative electrode active material layer fixed on the negative electrode current collector. The negative electrode active material layer includes a film containing a boron element. When the negative electrode active material existing in a central region of the electrode body is measured using a spectrometer, the b* value in the L*a*b* color system based on JIS Z8781-4:2013 is 3 or less.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to a non-aqueous electrolyte secondary battery and a method for inspecting a negative electrode for a non-aqueous electrolyte secondary battery. [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. This coating stabilizes the interface between the negative electrode and the non-aqueous electrolyte, which can improve battery performance. Patent Document 1 is an example of a related prior art document.

[0003] Patent Document 1 describes that the durability of a non-aqueous electrolyte secondary battery can be improved by adding an oxalate complex compound containing boron or an oxalate complex compound containing phosphorus to a non-aqueous electrolyte as an additive (a so-called film-forming agent). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-165125 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-010320 Summary of the Invention [Problem to be solved by the invention]

[0005] However, according to the inventors' research, there is still room for improvement when applying the above technology to recent high-capacity non-aqueous electrolyte secondary batteries. Specifically, in high-capacity non-aqueous electrolyte secondary batteries, for example, the width of the electrode assembly increases and the density of the active material layer increases, making it difficult for additives to penetrate to the center region of the electrode assembly. Therefore, during initial charging, a coating containing decomposition products of the additive is less likely to form on the surface of the negative electrode in the central region of the electrode assembly. In other words, the quantity and quality of the coating formed on the negative electrode are likely to vary between the central region and other regions of the electrode assembly. As a result, it has been newly discovered that the thermal stability of the negative electrode is likely to be locally reduced in the central region, making it more likely to experience temperature increases, for example, during overcharge.

[0006] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a non-aqueous electrolyte secondary battery with improved thermal stability and a method for inspecting a negative electrode for a non-aqueous electrolyte secondary battery. [Means for solving the problem]

[0007] The present invention provides a non-aqueous electrolyte secondary battery comprising an electrode assembly in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween, a non-aqueous electrolyte, and a battery case for accommodating the electrode assembly and the non-aqueous electrolyte. The negative electrode comprises a negative electrode current collector and a negative electrode active material layer fixed on the negative electrode current collector, the negative electrode active material layer comprising a coating containing elemental boron, and when the negative electrode active material layer located in the central region of the electrode assembly is measured using a spectrophotometer, it satisfies L 100% or less based on Japanese Industrial Standards JIS Z8781-4:2013. * a * b * b in color space * The value is less than or equal to 3.

[0008] The present inventors have found that unevenness in the quantity and quality of the coating film appears as "color unevenness." * It has been found that there is a positive correlation between the value of b and the amount of heat generated by the battery. *The value is adjusted to a predetermined value or less. This improves the thermal stability of the central region of the electrode assembly, thereby suppressing the temperature rise of the battery during overcharging, etc. Furthermore, by using objective numerical values ​​obtained by spectrophotometer measurement as an index, there is relatively little variation in accuracy compared to, for example, visually identifying color unevenness in the negative electrode active material layer. Therefore, thermal stability can be stably improved, and a highly reliable battery can be provided.

[0009] The present invention also provides a battery assembly comprising: a construction step of housing an electrode assembly in a battery case, the electrode assembly being formed by laminating a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer with a separator interposed therebetween, and a non-aqueous electrolyte solution containing a solvent, an electrolyte salt, and a compound containing boron element, the construction step of constructing a battery assembly; a charging step of charging the battery assembly until at least the compound containing boron element is decomposed; a disassembly step of disassembling the battery assembly after the charging step; and a disassembly step of measuring the L value of the negative electrode active material layer located in the central region of the electrode assembly using a spectrophotometer according to Japanese Industrial Standard JIS Z8781-4:2013 after the disassembly step. * a * b * b in color space * and a color measurement step of measuring a color value.

[0010] In the color measurement process, the spectrophotometer was used to measure the color of the negative electrode located in the center region of the electrode body. * By measuring this value, the degree of heat generation in the battery can be accurately predicted or confirmed.

[0011] Although not related to the present invention, Patent Document 2 discloses an inspection method for an electrode current collector, which uses a spectrophotometer to determine whether an electrode current collector made of copper or a copper alloy is good or bad. [Brief explanation of the drawings]

[0012] [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 a graph showing the relationship between the first aging period in the first aging step and the b* value of the negative electrode active material layer. [Figure 6] FIG. 6 is a graph showing the relationship between the amount of heat generated by a battery and the b* value of the negative electrode active material layer. [Figure 7] FIG. 7 is a graph showing the relationship between the b* value of the negative electrode active material layer and the b* value of the separator. [Figure 8] FIG. 8 is a graph showing the relationship between the amount of heat generated by the battery and the b* value of the separator. DETAILED DESCRIPTION OF THE INVENTION

[0013] 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."

[0014] 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.

[0015] <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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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).

[0020] 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).

[0021] The positive electrode terminal 30 and the negative electrode terminal 40 are each fixed to the sealing plate 14 of the battery case 10. The positive electrode terminal 30 is disposed on one side of the sealing plate 14 in the long side direction Y (the left side in FIGS. 1 and 2). The negative electrode terminal 40 is disposed on the other side of the sealing plate 14 in the long side direction Y (the right side in FIGS. 1 and 2). As shown in FIG. 2, the positive electrode terminal 30 extends from the inside to the outside of the sealing plate 14 through the terminal lead-out hole 18, and the negative electrode terminal 40 extends from the inside to the outside of the sealing plate 14 through the terminal lead-out hole 19. The positive electrode terminal 30 and the negative electrode terminal 40 are preferably attached to the sealing plate 14. Here, the positive electrode terminal 30 and the negative electrode terminal 40 are crimped to the peripheral portions of the sealing plate 14 surrounding the terminal lead-out holes 18 and 19 by crimping. The positive electrode terminal 30 and the negative electrode terminal 40 have crimped portions 30c, 40c formed at their ends on the exterior body 12 side (lower ends in FIG. 2).

[0022] 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.

[0023] Meanwhile, 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 collecting part 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 collecting part 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.

[0024] 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.

[0025] 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.

[0026] 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. Here, 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 therebetween and winding the stack around a winding axis WL. However, in other embodiments, the electrode assembly 20 may be a stacked electrode assembly that includes a plurality of rectangular positive electrodes and a plurality of rectangular negative electrodes that are insulated from each other by a separator interposed therebetween.

[0027] Although not particularly limited, the number of windings (number of turns) 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. The greater the number of windings, the more difficult it is for the nonaqueous electrolyte to penetrate into the central region CA (see FIG. 3 ) of the electrode assembly 20, particularly the central portion in the short side direction X, and the more likely it is that unevenness will occur in the quantity and quality of the coating. Therefore, applying the technology disclosed herein is particularly effective.

[0028] 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 nonaqueous electrolyte is particularly unlikely to sufficiently penetrate into the central region CA of the electrode assembly 20. This makes it easy for the quantity and quality of the coating to vary between the central region CA and other parts of the electrode assembly 20, for example, the center and ends in the long side direction Y. Therefore, applying the technology disclosed herein is particularly effective.

[0029] In this specification, the "central region CA of the electrode body 20" refers to the central region of the surface perpendicular to the stacking direction (thickness direction) as shown in Fig. 3. More specifically, the central region M in the long side direction Y (winding axis WL direction) Y The center part including the center M in the vertical direction Z Z In the case where the electrode assembly 20 is a wound electrode assembly as in this embodiment, the term further refers to the intermediate periphery in the winding direction (number of turns).

[0030] 2 and 3, the electrode body 20 is disposed inside the battery case 10 with the winding axis WL oriented approximately parallel to the long side direction Y. Here, the direction of the winding axis WL coincides with the long side direction Y. The electrode body 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.

[0031] Here, the battery 100 has a so-called horizontal tab structure in which the positive electrode tab group 23 and the negative electrode tab group 25 are located at both ends of the electrode assembly 20 in the winding axis WL direction (left and right in FIGS. 2 and 3). However, in other embodiments, the battery 100 may have a so-called top tab structure in which the positive electrode tab group 23 and the negative electrode tab group 25 are located at one end of the electrode assembly 20 in the winding axis WL direction (for example, the upper end in FIGS. 2 and 3). In this case, the winding axis WL direction may coincide with the up-down direction Z.

[0032] 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.

[0033] 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).

[0034] 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.

[0035] 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). 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 a positive electrode current collector 50. A later-described positive electrode second current collector 52 of the positive electrode current collector 50 is attached (more specifically, joined) to the positive electrode tab group 23.

[0036] 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.

[0037] Although not particularly limited, as shown in FIG. 4, in a high-capacity battery 100 used for in-vehicle applications, etc., the width of the positive electrode active material layer 22a in the winding axis WL direction (average value, excluding the portion formed on the positive electrode tab 22t), in other words, the length W1 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.

[0038] 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.

[0039] 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.

[0040] A plurality of negative electrode tabs 24t are provided at one end of the negative electrode current collector 24c in the long side direction Y (the right end in FIG. 4). Each of the plurality of negative electrode tabs 24t protrudes toward one side in the long side direction Y (the right side in FIG. 4). 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 of 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 a negative electrode current collector 60. A later-described negative electrode second current collector 62 of the negative electrode current collector 60 is attached (more specifically, joined) to the negative electrode tab group 25.

[0041] 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).

[0042] Although not particularly limited, the density of the negative electrode active material layer 24a is set to, for example, 0.5 g / cm from the viewpoint of increasing capacity. 3 More than 1.0 g / cm is preferable. 3 The density of the negative electrode active material layer 24a is more preferably 2.0 g / cm 3 from the viewpoint of achieving the effects of the technology disclosed herein at a high level. 3 Preferably less than 1.5 g / cm 3 The following is more preferred:

[0043] In this embodiment, the negative electrode active material layer 24a includes a coating (SEI film) containing boron (B). The boron is a component derived from a coating-forming agent added to the nonaqueous electrolyte during construction of the battery 100, specifically, a compound containing boron (B-containing compound). The coating is a decomposition product containing the B-containing compound that is decomposed during initial charging. The coating containing boron is highly stable, and can therefore suitably improve the durability of the battery 100. 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).

[0044] In the central region CA of the electrode body 20, the boron concentration ratio (B concentration ratio) of the negative electrode active material layer 24a is preferably 0.010% or more, for example, 0.010 to 0.020%, and more preferably 0.012% or more. The B concentration ratio can be measured and calculated by the following procedure. First, the battery 100 is disassembled, the negative electrode 24 is removed, and the negative electrode 24 is decompressed (for example, at -95 kPa for 10 minutes) to remove volatile components (for example, EMC, DMC, and VC, which will be described later). Next, two pieces of a predetermined size (for example, 10 x 60 mm) are punched out of the negative electrode 24, and these are immersed in GBL (γ-butyrolactone) to extract the non-volatile components. Next, the extract is diluted with HCl and pure water, and the amount of boron element (mg / cm) is measured using ICP atomic emission spectrometry. 2 Then, the abundance ratio relative to the ideal composition based on the phosphorus element, that is, the abundance ratio based on LiPF6 (100%) as the electrolyte salt described below, can be calculated.

[0045] In this embodiment, when the negative electrode active material layer 24a (typically the surface) located in the central region CA of the electrode assembly 20 is measured using a spectrophotometer, the L * a * b *b in color space * The value is 3 or less. * a * b * In the color system, it is possible to separate coordinate axes of black and white (brightness) and yellow, blue, red, and green (chromaticity). * There is a positive correlation between the value of b of the negative electrode active material layer 24a located in the central region CA of the electrode body 20 and the amount of heat generated by the battery 100. * By adjusting the value to a predetermined value or less, the amount of heat generated in the central area CA can be suppressed, and the thermal stability of the battery 100 can be improved.

[0046] According to the study by the inventors, the boron concentration ratio (B concentration ratio) of the negative electrode active material layer 24a located in the central region CA of the electrode body 20 does not show a strong correlation with the amount of heat generated by the battery 100. For example, even if the B concentration ratio is the same, * Therefore, as in the technology disclosed herein, b * It is important to evaluate it by value.

[0047] Furthermore, while the shade of the coating can be visually identified as "color unevenness," for example, due to individual differences in human eyes, the results of determining whether or not there is color unevenness may differ from person to person. In contrast, when objective numerical values ​​obtained by spectrophotometer measurement are used as indicators, as in the technology disclosed herein, there is relatively little variation in accuracy. Furthermore, color differences that cannot be distinguished by the human eye can be distinguished. Therefore, it is possible to provide a battery 100 that can stably improve thermal stability and has high reliability.

[0048] b of the negative electrode active material layer 24a * From the viewpoint of achieving the effects of the technology disclosed herein at a high level, the value of b is preferably 2.5 or less, more preferably 2 or less, and even more preferably 1.5 or less. * The value may typically be 1 or more, for example, 1.2 or more. *The value 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, for example, the conditions of the electrolyte impregnation step (step 2) in the manufacturing method described below, the conditions of the first aging step (step 5) after initial charging, or the conditions of the subsequent second aging step (step 6), etc. In particular, it can be suitably adjusted by the first aging period in the first aging step and the second aging period in the second aging step.

[0049] 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 W2 in the long side direction Y, is equal to or longer than the length W1 of the positive electrode active material layer 22a in the long side direction Y. While not particularly limited, from the viewpoint of achieving high capacity, the length W2 is preferably 15 cm or more, more preferably 20 cm or more, and even more preferably 25 cm or more. The longer the length W2, the more difficult it is for the nonaqueous electrolyte to penetrate into the central region CA of the electrode body 20, particularly the central portion in the long side direction Y, and the more likely it is that the quantity and quality of the coating will be uneven. Therefore, applying the technology disclosed herein is particularly effective. The length W2 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.

[0050] In addition, when the length W2 in the long side direction Y of the negative electrode active material layer 24a (width in the direction of the winding axis WL) is long, for example, 15 cm or more, or even 20 cm or more, the end portion in the long side direction Y is typically larger than the central portion in the long side direction Y by b * The value is relatively small. At the end of the long side direction Y, b * The value may be less than 3, for example, 1 / 2 or less of the central portion in the long side direction Y.

[0051] 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 W3 in the long side direction Y, is equal to or longer than the length W2 in the long side direction Y of the negative electrode active material layer 24a. 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.

[0052] Similarly to the negative electrode active material layer 24a, when the separator 26 (particularly the surface facing the negative electrode active material layer 24a) located in the central region CA of the electrode assembly 20 was measured using a spectrophotometer, the L * a * b * b in color space * It is preferable that the value of b is 2 or less. * As with the value, the separator 26, b * The value of b of the negative electrode active material layer 24a also has a positive correlation with the amount of heat generated. * The value plus the separator 26, b * By adjusting the value to a predetermined value or less, the amount of heat generated in the central area CA can be more stably suppressed.

[0053] The separator 26 is relatively less likely to form a coating than the negative electrode active material layer 24a. * The value is typically b * Smaller than the value of separator 26 *From the viewpoint of achieving the effects of the technology disclosed herein at a high level, the value is preferably 2 or less, more preferably 1.5 or less, even more preferably 1 or less, and particularly preferably 0.5 or less. * The value of b * As with the value, it can be suitably adjusted by, for example, the conditions of the electrolyte impregnation step (step 2), the first aging step (step 5) after initial charging, or the conditions of the subsequent second aging step (step 6) in the manufacturing method described below. In particular, it can be suitably adjusted by the first aging period in the first aging step or the second aging period in the second aging step.

[0054] 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.

[0055] 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.

[0056] 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).

[0057] 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.

[0058] The non-aqueous electrolyte 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 carbonates 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).

[0059] 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.

[0060] The nonaqueous electrolyte solution may further contain additional components (additives). Examples include boron-based additives containing boron, phosphorus-based additives containing phosphorus, sulfur-based additives containing sulfur, and carbonate-based additives. These additives may be so-called film-forming agents that decompose (at a low potential) prior to the nonaqueous solvent and / or electrolyte salt during initial charging and deposit as a film on the surface of the negative electrode active material layer 24a.

[0061] The non-aqueous electrolyte preferably contains a boron-based additive, i.e., a compound containing boron (B-element-containing compound). As the B-element-containing compound, one or more compounds that have been known to be additives to non-aqueous electrolytes (e.g., compounds described in Patent Document 1) can be used. Examples of the B-element-containing compound include oxalato complex compounds containing boron (B-element-containing oxalato compounds), such as lithium bisoxalatoborate (LiBOB) and lithium difluoro(oxalato)borate (LiODFB).

[0062] The non-aqueous electrolyte preferably further contains a phosphorus-based additive, i.e., a compound containing phosphorus (P-element-containing compound). As the P-element-containing compound, one or more compounds that have been known to be additives to non-aqueous electrolytes (e.g., compounds described in Patent Document 1) can be used. Examples of the P-element-containing compound include oxalate complex compounds containing phosphorus (P-element-containing oxalate compounds), such as lithium difluorophosphate (lithium difluorophosphate, LiPO2F2) and lithium difluorooxalate phosphate (LiDFOP).

[0063] The non-aqueous electrolyte preferably further contains a sulfur-based additive, i.e., a compound containing sulfur element (S-element-containing compound). As the S-element-containing compound, one or more compounds known to be additives to non-aqueous electrolytes can be used. Examples of the S-element-containing compound include lithium fluorosulfonate (LiSO3F), lithium ethyl sulfate, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and lithium bis(fluorosulfonyl)imide (LiFSI).

[0064] The non-aqueous electrolyte may further contain a carbonate-based additive. As the carbonate-based additive, for example, one or more of those known to be able to be added to non-aqueous electrolytes as a film-forming agent can be used. Examples of carbonates include vinylene carbonate (VC), vinyl ethylene carbonate (VEC), and fluoroethylene carbonate (FEC).

[0065] 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.

[0066] <Method of manufacturing the battery 100> The battery 100 can be manufactured by a manufacturing method including, for example, the following steps: a battery assembly construction step (step 1), an electrolyte impregnation step (step 2), an initial charging step (step 3), a degassing step (step 4), a first aging step (step 5), and a second aging step (step 6), in this order. However, the electrolyte impregnation step (step 2) and the degassing step (step 4) are optional and may be omitted in other embodiments. Furthermore, other steps may be included at any stage.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] Next, in the liquid injection step (step 1-4), a nonaqueous electrolyte solution is first prepared. The nonaqueous electrolyte solution contains a boron-based additive (a B-element-containing compound) in addition to a nonaqueous solvent and an electrolyte salt. Although not particularly limited, the concentration of the B-element-containing compound in the nonaqueous electrolyte solution is preferably 0.01 mol / L or more, and more preferably 0.05 mol / L or more, because this facilitates the formation of a coating of an appropriate quantity or quality on the surface of the negative electrode active material layer 24a in the central region CA. On the other hand, from the viewpoint of suppressing an increase in battery resistance, the concentration of the B-element-containing compound in the nonaqueous electrolyte solution is preferably 1 mol / L or less, more preferably 0.5 mol / L or less, and even more preferably 0.1 mol / L or less.

[0071] 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 concentration 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 inside the battery case 10 to improve the impregnation of the nonaqueous electrolyte into the electrode assembly 20.

[0072] In the electrolyte impregnation step (step 2), after the battery assembly construction step (more specifically, the liquid injection step), the impregnation of the electrode body 20, particularly the central portion in the long side direction Y, with the nonaqueous electrolyte is enhanced. This step may be performed in a room temperature environment (approximately 25°C ± 10°C). In a preferred embodiment, this step includes a first impregnation step (step 2-1), a reduced pressure impregnation step (step 2-2), and a second impregnation step (step 2-3) in this order. Furthermore, other steps may be further included at any stage. The required time for this step (total time for the first impregnation step, reduced pressure impregnation step, and second impregnation step) is preferably 45 to 200 hours, more preferably 49 to 140 hours.

[0073] In the first impregnation step (step 2-1), the battery assembly is left (held) under atmospheric pressure for a first impregnation time from the completion of the injection in the battery assembly construction step (specifically, the injection step). The first impregnation time is preferably 25 hours or longer. This reduces the viscosity of the nonaqueous electrolyte, which can promote impregnation of the nonaqueous electrolyte into the interior of the electrode assembly 20, particularly into the center portion in the long side direction Y.

[0074] Next, in the reduced-pressure impregnation step (step 2-2), after the first impregnation step, the battery assembly is housed in a pressure-adjustable chamber, and with the liquid inlet 15 open (in other words, with no pressure difference between the inside and outside of the battery case 10), the chamber is depressurized at least once and maintained in a reduced-pressure state for a predetermined time. The reduced-pressure state is preferably −0.05 MPa or less, for example, approximately −0.09 MPa, depending on, for example, the length W2 of the negative electrode active material layer 24a in the long side direction Y. The maintained reduced-pressure state is preferably 600 seconds or more, depending on, for example, the length W2 of the negative electrode active material layer 24a in the long side direction Y. The reduced-pressure may be performed once, or may be performed two or more times, for example, with pressure release in between. For example, after the first reduced-pressure, the battery case 10 may be depressurized to return to normal pressure (−0.01 MPa or more), maintained at normal pressure for a predetermined time, and then a second reduced-pressure may be performed. The holding time at normal pressure is preferably 60 seconds or more, more preferably 100 seconds or more, and the number of times of decompression is preferably two or more, more preferably four or more.

[0075] Next, in the second impregnation step (step 2-3), after the reduced pressure impregnation step, the battery assembly is left (held) at atmospheric pressure for a second impregnation time. The second impregnation time is preferably 20 hours or more. The second impregnation time may be shorter than the first impregnation time. This can further promote impregnation of the nonaqueous electrolyte solution into the interior of the electrode body 20, particularly into the central portion in the long side direction Y.

[0076] In the initial charging step (step 3), the battery assembly is charged at least once after the electrolyte impregnation step. Charging of the battery assembly can be performed in the same manner as conventional methods. Typically, an external power source is connected between the positive and negative electrode terminals of the battery assembly, and charging is performed until a predetermined ultimate voltage is reached between the positive and negative electrode terminals. The ultimate voltage is set so that at least the B element-containing compound in the non-aqueous electrolyte is electrically decomposed. For example, when the negative electrode active material is a carbon material, the ultimate voltage is set to approximately 2.5 V or higher, preferably 3 V or higher, for example, 3.5 V or higher, 4 V or higher. The charge rate can be, for example, about 0.1 C to 2 C. Charging can be performed once, or can be repeated two or more times, for example, with a discharge interval in between.

[0077] During the initial charge, the additives in the non-aqueous electrolyte (at least the B-element-containing compound) are typically electrolyzed before other components in the non-aqueous electrolyte (the non-aqueous solvent and the electrolyte salt), resulting in the formation of a coating (SEI film) containing at least the decomposition product of the B-element-containing compound on the surface of the negative electrode active material layer 24a.

[0078] In the degassing step (step 4), after the initial charging step, gases inside the battery case 10, such as air and gases generated by decomposition of the nonaqueous electrolyte solution during the initial charging step, are exhausted to the outside of the battery case 10. The gases can be exhausted, for example, by reducing the pressure inside the battery case 10. Then, preferably with the pressure inside the battery case 10 reduced, the liquid injection hole 15 is sealed with a sealing member 16. This allows the battery case 10 to be airtightly sealed (hermetically sealed).

[0079] In the first aging step (step 5), the battery assembly after initial charging is restrained and maintained in a predetermined temperature environment with a predetermined restraining load applied in the short side direction X (thickness direction of the electrode body 20) for a predetermined first aging period. The temperature environment is preferably 15 to 40°C, and may be, for example, room temperature (approximately 25°C ± 10°C). The restraining load is preferably 1 to 6 kN. In a preferred embodiment, first, a cell press equipped with a pair of restraining plates is prepared. Next, the battery assembly after initial charging is placed between the pair of restraining plates so that the pair of long side walls 12b of the battery case 10 face the restraining plates. In this state, a restraining load is applied to the battery assembly after initial charging using the press, and the battery is maintained for a predetermined first aging period. Note that in this step, the voltage adjusted in the initial charging step may be maintained. In this step, the battery's state of charge (SOC) is typically lower than that in the second aging step (step 6) described below, and may be, for example, 20% or less, or 10% or less.

[0080] Although details will be described in the Examples below, according to the study by the present inventors, the first aging period of this step and the b of the negative electrode active material layer 24a obtained by spectrophotometric colorimetry * That is, as the first aging period of this step becomes longer, the b * Similarly, the b value of the separator 26 tends to be small. * The value of b tends to decrease as the first aging period of this step increases. Therefore, the first aging period may vary depending on, for example, the length W2 of the long side direction Y of the negative electrode active material layer 24a and the conditions of the electrolyte impregnation step (step 2), but is preferably about 5 days or more, more preferably 6 days or more. By setting the first aging period to a predetermined period or more, the b * The value can be easily adjusted to the above range (for example, 3 or less). * This makes it easier to adjust the value to the above range (for example, 2 or less).

[0081] Next, in the second aging step (step 6), the battery assembly is charged to a predetermined SOC, preferably 10% to 30%, and then maintained for a predetermined second aging period in a temperature environment higher than that used in the first aging step (step 5). The temperature environment in this step is preferably 50 to 90°C, and may be, for example, 70°C ± 10°C. The second aging period may vary depending on, for example, the length W2 of the negative electrode active material layer 24a in the long side direction Y, the conditions of the electrolyte impregnation step (step 2), and the first aging period of the first aging step (step 5), but is preferably approximately 6 to 24 hours. The SOC (State of Charge) refers to the state of charge (charging rate) based on the voltage range in which the battery 100 is normally used. In this manner, the battery 100 can be suitably manufactured.

[0082] <Method of inspecting the negative electrode 24 or the separator 26> For example, quality control of thermal stability can be performed in the form of a sampling inspection on the battery assembly after the initial charge or the battery 100 that has undergone the second aging step. In the sampling inspection, the negative electrode 24 or the separator 26 can be inspected. Therefore, in the inspection method disclosed herein, the battery assembly (or the battery 100) that has undergone at least the construction step (step 1), the initial charging step (step 3), the first aging step (step 5), and the second aging step (step 6) of the manufacturing method is subjected to the following steps: a disassembly step (step 7) of disassembling the battery assembly; * A color measurement step (step 8) for measuring the color value is performed in this order. In this embodiment, an evaluation step (step 9) for evaluating the thermal stability of the battery assembly or the battery 100 is also performed. Furthermore, other steps may be included at any stage.

[0083] In the disassembly step (step 7), the battery assembly is disassembled. Disassembly of the battery assembly is preferably carried out in an atmosphere of dry air (for example, with a dew point of about −50° C.), for example, in a glove box, to prevent deterioration of the negative electrode 24 or separator 26. The battery assembly can be disassembled, for example, by first cutting the battery case 10 with a tool such as an end mill or a laser, separating the sealing plate 14 from the exterior body 12, and then removing the electrode 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.

[0084] In the color measurement step (step 8), after the disassembly step, a spectrophotometer is used to measure the L based on Japanese Industrial Standard JIS Z8781-4:2013 for the negative electrode active material layer 24a or the separator 26 (typically the surface) located in the central region CA of the electrode assembly 20. * a * b * b in color space * The measurement may be carried out multiple times to take into account variations. In this case, the arithmetic mean of the multiple measurements is taken as b * As described above, the b value of the negative electrode active material layer 24a in the central region CA of the electrode body 20 can be used. * There is a positive correlation between the value of b of the separator 26 in the central region CA of the electrode assembly 20 and the amount of heat generated by the battery 100. * There is a positive correlation between the value of b of the negative electrode active material layer 24a or the separator 26 and the amount of heat generated by the battery 100. * By measuring the value, the thermal stability (heat generation behavior) of the battery 100 can be predicted or confirmed.

[0085] In the evaluation step (step 9), the thermal stability of the battery assembly or battery 100 is evaluated. In a preferred embodiment, the thermal stability of the battery assembly or battery 100 is evaluated in advance by a preliminary test or the like. * The correlation between the value and the heat generation amount of the battery 100 is expressed by an equation (for example, a linear function shown in the examples), and the equation includes b * In another preferred embodiment, the amount of heat generated by the battery 100 is predicted by substituting the value of b * Value and / or separator 26 b* For example, the negative electrode active material layer 24a is judged to be a non-defective product based on the value of b * Value and / or separator 26 b * If the value is equal to or less than a predetermined value, the product is determined to be non-defective. * If the value is 3 or less, it is determined to be a good product. * Value of b * If the value is 2 or less, the battery assembly is determined to be a good product. In this case, a battery assembly determined to be a good product may have reduced heat generation and little variation in thermal stability. This allows the upper limit of the heat generation of the battery 100 to be suitably controlled, and highly reliable batteries 100 can be supplied to the market.

[0086] <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.

[0087] Hereinafter, several examples of the present invention will be described, but it is not intended that the present invention be limited to these examples.

[0088] <Test Example I (Examples 1 and 2, Comparative Examples 1 to 5)> <Evaluation of Color Unevenness> First, in the construction step (step 1), a battery assembly was constructed by placing the electrode body and a nonaqueous electrolyte solution having the composition shown in Table 1 in a battery case. The flat portion of the negative electrode active material layer had a length of 285 mm in the winding axis direction (width direction) and a height of 90 mm. Next, in the electrolyte impregnation step (step 2), the battery assembly was left standing for 25 hours (first impregnation time) from the completion of the electrolyte injection. Next, the pressure inside the battery case was reduced to -0.09 MPa, and this state was maintained for 600 seconds. After that, the pressure inside the battery case was returned to 0 MPa and this state was maintained for 60 seconds. This operation was repeated four times in total. Next, the battery assembly was left standing for 20 hours (second impregnation time).

[0089] Next, in the initial charging step (step 3), constant current charging was performed at a charge rate of 1C up to 3V. Next, in the degassing step (step 4), the pressure inside the battery case was reduced to -0.09 MPa. Next, in the first aging step (step 5), the battery assembly after the initial charge was held in a temperature environment of 25°C with a restraint load of 4 kN applied to it for the first aging period shown in Table 1. Next, after the first aging step (step 5), the battery assembly was charged until the SOC reached 20%, and in the second aging step (step 6), it was left in a temperature environment of 70°C for a predetermined second aging period. Then, in the disassembly step (step 7), the battery assembly after the second aging step was disassembled in a glove box, and the electrode assembly was removed from the battery case. Then, the electrode assembly removed from the battery case was unwound, and the positive electrode, negative electrode, and separator were separated.

[0090] Next, in the color measurement process (process 8), the color unevenness of the negative electrode active material layer in the center region of the electrode body is visually observed by a person, and the surface of the negative electrode active material layer in the center region of the electrode body is measured using a diffused illumination type spectrophotometer (model: CM-26dG) manufactured by Konica Minolta using the SCI (Specular Component Include) method, which captures specular reflected light by trapping specular reflected light, and the L based on Japanese Industrial Standard JIS Z8781-4:2013 is obtained. * a * b * b in color space *The values ​​were measured. The visual observation position and the spectrophotometer measurement position were the flat part located at the 15th turn (intermediate turn) from the winding start end. In some examples, the separator was also observed and measured in the same way as the negative electrode active material layer. The results are shown in Table 1.

[0091] <Evaluation of Heat Generation> First, a sample for DSC measurement was prepared. Specifically, a laminate cell was constructed by placing a positive electrode having a positive electrode active material layer (20 mm x 20 mm) and a negative electrode having a negative electrode active material layer (22 mm x 22 mm) opposite each other with a separator interposed therebetween in a dry air atmosphere (dew point: -50°C), and housing them together with 0.4 mL of a nonaqueous electrolyte solution having the composition shown in Table 1.

[0092] Next, the laminated cell fabricated as described above was charged at a constant current of 8 mA to 4.25 V, followed by 5 hours of constant voltage charging. The charged laminated cell was then disassembled in a glove box (Ar atmosphere). The electrolyte was then collected, and the positive and negative electrodes were removed. The positive electrode composite was peeled from the central region of the positive electrode active material layer, and the negative electrode composite was peeled from the central region of the negative electrode active material layer. Then, 1 mg of the positive electrode composite peeled from the positive electrode, 2 mg of the negative electrode composite peeled from the negative electrode, and 4 mg of the collected electrolyte were placed in a sample container. The sample container was press-sealed at 20 MPa and then placed in a differential scanning calorimetry (DSC) along with a reference material (2 mg of Al2O3). The temperature was then increased from 25° C. to 350° C. at a rate of 2° C. / min in an inert atmosphere, and the calorific value (J) between 100 and 200° C. was calculated by integration. The results are shown in Table 1.

[0093] [Table 1]

[0094] As shown in Table 1, in Comparative Example 1, in which the first aging period was 0 days (none), and in Comparative Example 2, in which the first aging period was 1 day, strong color unevenness was observed in the negative electrode active material layer by visual inspection. Therefore, measurement with a spectrophotometer was omitted. In addition, in Comparative Example 3, in which the first aging period was 2 days, strong color unevenness was also observed in the negative electrode active material layer. On the other hand, in Comparative Examples 4 and 5, the presence or absence of color unevenness was judged differently depending on the person.

[0095] FIG. 5 shows the relationship between the first aging period of the first aging step (step 5) and the b * As shown in FIG. 5, the relationship between the first aging period and the b value of the negative electrode active material layer is * Between the values, R 2 A strong negative correlation of b = 0.9 or more was observed. In other words, the longer the aging period, * Therefore, the value of b * It was found that the value of b can be suitably adjusted. In addition, by setting the first aging period to 5 days as in Example 1, the b * The value can be set to 3 or less, and by setting the first aging period to 6 days as in Example 2, the b * The value could be reduced to 2 or less.

[0096] FIG. 6 shows the relationship between the amount of heat generated by the battery and the b value of the negative electrode active material layer in the central region of the electrode body for Comparative Examples 3 to 5 and Examples 1 and 2. * As shown in Figure 6, the relationship between the amount of heat generated by the battery and the b value of the negative electrode active material layer is * Between the values, R 2 A very strong positive correlation of 0.93 or more was observed. * The larger the value, the larger the amount of heat generated. * It was found that by setting the value to a predetermined value or less (for example, 3 or less), the amount of heat generated can be suppressed (for example, 17 J or less, preferably 15 J or less), and the thermal stability of the battery can be improved.

[0097] Furthermore, although a detailed explanation is omitted, when the inventors measured the boron concentration ratio (B concentration ratio) for the negative electrode active material layer in the central region of the electrode body in each example, they did not find a correlation as strong as that in Figure 6 above between the calorific value and the B concentration ratio. The reason for this is unclear, but from the results of ICP optical emission spectroscopy of non-volatile components, it is thought that, for example, the negative electrode active material layer in the central region of the electrode body contains a large amount of coating resulting from the decomposition of the solvent, and the quality of the coating is different. From the above, b * It was suggested that the value is a parameter that reflects not only the boron concentration in the coating but also the formation state of the coating (the quality of the coating). Therefore, simply measuring the B concentration ratio is insufficient. * It was considered important to evaluate the value.

[0098] 7 shows the results of the negative electrode active material layer b for Comparative Examples 3 and 5 and Example 2. * Value and separator b * As shown in FIG. 7, although there were three measurement points, the b * Value and separator b * Between the values, R 2 A very strong correlation of b = 0.99 or more was observed. * Instead of the value, use the separator b * I found it possible to use the value of the separator b * The value is the b * It was found that the value was smaller than the

[0099] FIG. 8 shows the relationship between the heat generation amount of the battery and the separator b for Comparative Examples 3, 5, and Example 2. * As shown in Figure 8, the relationship between the amount of heat generated by the battery and the b value of the separator * Between the values, R 2 = 0.97 or more. That is, as in the case of the negative electrode active material layer, * The larger the value, the greater the heat generation. *It was found that by setting the value to a predetermined value or less (for example, 2 or less, further 1.5 or less), the amount of heat generated can be suppressed (for example, 17 J or less, preferably 15 J or less), and the thermal stability of the battery can be improved.

[0100] <Test Example II (Examples 3-5)> The color unevenness and heat generation amount of the negative electrode active material layer located in the center region of the electrode assembly were measured in the same manner as in Example 2 of Test Example I, except that a nonaqueous electrolyte solution having the composition shown in Table 2 was used. The results are shown in Table 2. Table 2 also shows the results of Example 2 of Test Example I.

[0101] [Table 2]

[0102] As shown in Table 2, in Example 3, in which lithium fluorosulfonate (LiSO3F) was not added, Example 4, in which lithium difluorophosphate (LiPO2F2) was not added, and Example 5, in which vinylene carbonate (VC) was not added, the calorific value was approximately the same as that of Example 2. Therefore, it was shown that the non-aqueous electrolyte only needs to contain a compound containing boron element (here, lithium bis(oxalato)borate (LiBOB)) as an additive, and other additives are not essential. In addition, b of the negative electrode active material layer * It was found that the value may vary somewhat depending on the composition of the non-aqueous electrolyte (for example, the type and amount of additives).

[0103] 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.

[0104] As described above, specific aspects of the technology disclosed herein include those described in the following sections. Item 1: A battery comprising an electrode assembly in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween, a non-aqueous electrolyte, and a battery case that accommodates the electrode assembly and the non-aqueous electrolyte, wherein the negative electrode has a negative electrode current collector and a negative electrode active material layer fixed on the negative electrode current collector, the negative electrode active material layer having a coating containing elemental boron, and when the negative electrode active material layer located in a central region of the electrode assembly is measured using a spectrophotometer, the negative electrode active material layer exhibits an L * a * b * b in color space * A non-aqueous electrolyte secondary battery having a value of 3 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: When the separator located in the central region of the electrode assembly is measured using a spectrophotometer, * Item 3. The nonaqueous electrolyte secondary battery according to item 1 or 2, wherein the value is 2 or less. Item 4: The nonaqueous electrolyte secondary battery according to any one of Items 1 to 3, wherein the nonaqueous electrolyte contains a compound containing elemental boron. Item 5: A construction step of constructing a battery assembly by housing an electrode assembly 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; a charging step of charging the battery assembly at least until the compound containing boron element is decomposed; a disassembly step of disassembling the battery assembly after the charging step; and a spectrophotometer for measuring the L value of the negative electrode active material layer located in the central region of the electrode assembly based on Japanese Industrial Standard JIS Z8781-4:2013 after the disassembly step. * a * b * b in color space *and a color measurement step of measuring a color value. [Explanation of symbols]

[0105] 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. 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; a charging step of charging the battery assembly at least until the compound containing elemental boron is decomposed; a disassembly step of disassembling the battery assembly after the charging step; After the disassembly step, the negative electrode active material layer located in the central region of the electrode assembly was measured using a spectrophotometer according to L ＊ a ＊ b ＊ b in the color system ＊ a color measurement step for measuring a color value; A method for inspecting a negative electrode for a non-aqueous electrolyte secondary battery, comprising:

2. A battery comprising: an electrode assembly in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween; a non-aqueous electrolyte; and a battery case that accommodates the electrode assembly and the non-aqueous electrolyte; the negative electrode has a negative electrode current collector and a negative electrode active material layer fixed onto the negative electrode current collector, the negative electrode active material layer has a coating containing elemental boron, a method for producing a nonaqueous electrolyte secondary battery, wherein, when the negative electrode active material layer located in a central region of the electrode assembly is measured using a spectrophotometer, the b* value in the L*a*b* color system based on Japanese Industrial Standards JIS Z8781-4:2013 is 3 or less; an initial charging step of charging a battery assembly, which is formed by housing an electrode body and a non-aqueous electrolyte in a battery case, at least once; a first aging step of holding the battery assembly in a temperature environment of 15°C to 40°C for 5 days or more after the initial charging step; A method for manufacturing a non-aqueous electrolyte secondary battery, comprising:

3. The method further includes a second aging step of holding the battery assembly in a temperature environment higher than that in the first aging step. The method of claim 2.

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