Secondary battery, charging method for the secondary battery, and battery module that can achieve the charging method

Abstract

To provide a secondary battery that achieves high capacity retention, a charging method for the secondary battery, and a battery module that can achieve the charging method.SOLUTION: A secondary battery that achieves high capacity retention, a charging method for the secondary battery, and a battery module that uses the charging method are provided according to the art disclosed herein. This charging method is a charging method for a secondary battery including an electrode body including a positive electrode 22 and a negative electrode 24. The positive electrode 22 includes a positive electrode core body 22c, and a positive electrode active material layer 22b formed on a surface of the positive electrode core body 22c. The negative electrode 24 includes a negative electrode core body 24c, and a negative electrode active material layer 24a formed on a surface of the negative electrode core body 24c. The negative electrode active material layer 24a includes at least a negative electrode active material 24b and a magnetic body 24d. The secondary battery is charged with the secondary battery disposed in a space where a magnetic field is applied.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to a secondary battery, a charging method for the secondary battery, and a battery module capable of implementing the charging method. [Background technology]

[0002] Secondary batteries such as lithium-ion secondary batteries are widely used as power sources for driving vehicles such as BEVs (electric vehicles), HEVs (hybrid electric vehicles), and PHEVs (plug-in hybrid electric vehicles). Due to their usage, secondary batteries used as vehicle power sources are required to have various performance characteristics compared to secondary batteries used in conventional mobile phones, mobile PCs, etc. Examples of such performance characteristics include high charge / discharge efficiency, improved charge / discharge cycle characteristics, and improved capacity retention. For example, Patent Document 1 describes a secondary battery in which the electrical resistance and reaction resistance of the positive and negative electrodes are controlled to control the ratio of electrical resistivities between the positive and negative electrodes, with the aim of preventing a decrease in constant input / output time and a decrease in capacity retention rate in a low-temperature environment when short-term or long-term input / output and charge / discharge are repeated. Patent Documents 2 and 3 also describe secondary batteries that are said to have excellent charge / discharge efficiency and charge / discharge cycle characteristics. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-10888 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-092928 [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-252945 Summary of the Invention [Problem to be solved by the invention]

[0004] Incidentally, one problem encountered when using secondary batteries such as lithium-ion secondary batteries is the deterioration of the secondary battery due to rapid charging, such as a decrease in capacity retention rate. One factor that is thought to cause this decrease in capacity retention rate is the precipitation of charge carriers (lithium in the case of lithium-ion batteries) at the negative electrode that accompanies rapid charging.

[0005] The present disclosure has been made in consideration of the above circumstances, and its main purpose is to provide a secondary battery that achieves a high capacity retention rate, a charging method for the secondary battery, and a battery module that can realize the charging method. [Means for solving the problem]

[0006] To achieve the above object, the secondary battery disclosed herein is a secondary battery including an electrode assembly including a positive electrode and a negative electrode. The positive electrode includes a positive electrode core and a positive electrode active material layer formed on the surface of the positive electrode core, and the negative electrode includes a negative electrode core and a negative electrode active material layer formed on the surface of the negative electrode core. The negative electrode active material layer includes at least a negative electrode active material and a magnetic material, and when the negative electrode active material layer is divided into two equal parts in the thickness direction into a surface side and a negative electrode core side, the content of the magnetic material on the surface side is greater than the content of the magnetic material on the negative electrode core side.

[0007] The secondary battery having the above configuration includes a magnetic material in the negative electrode active material layer. Therefore, by placing the secondary battery in a space where a magnetic field is applied during charging, the resistance of the negative electrode can be selectively improved during charging. This can suppress precipitation of charge carriers in the negative electrode, thereby improving the capacity retention rate. In particular, the secondary battery having the above configuration includes more magnetic material on the surface side of the negative electrode active material layer than on the negative electrode substrate side, thereby increasing the resistance of the surface side of the negative electrode active material layer. This can more effectively suppress precipitation of charge carriers on the surface side of the negative electrode active material layer, where precipitation of charge carriers is more likely to occur, thereby improving the capacity retention rate.

[0008] In a preferred embodiment of the secondary battery disclosed herein, the secondary battery includes a plurality of layers in the thickness direction, each having a different content of the magnetic material, and the content of the magnetic material in each layer increases sequentially from the layer closest to the negative electrode core toward the layer on the surface side, thereby achieving an improvement in the capacity retention rate.

[0009] In a preferred embodiment of the secondary battery disclosed herein, the negative electrode active material layer has three layers in the thickness direction, and when these three layers are designated as layer a, layer b, and layer c from the negative electrode substrate side, the ratio c / a of the content of the magnetic material in layer a to layer c is at least 2. This preferably realizes an improvement in capacity retention rate.

[0010] In a preferred embodiment of the secondary battery disclosed herein, the negative electrode active material layer contains a permalloy alloy (iron-nickel) as the magnetic material, and by placing the secondary battery in a space to which a magnetic field is applied during charging, the resistance value of the negative electrode can be suitably increased, thereby achieving a high capacity retention rate.

[0011] In a preferred embodiment of the charging method disclosed herein, the secondary battery is placed in a space to which a magnetic field is applied, and then the secondary battery is charged. By placing the secondary battery in a space to which a magnetic field is applied during charging, the battery can be charged in a state in which the resistance value of the negative electrode is increased, thereby achieving a high capacity retention rate.

[0012] In a preferred embodiment of the charging method disclosed herein, the space to which the magnetic field is applied is formed by an electromagnet, which allows for efficient selective application of a magnetic field during charging and favorably achieves a high capacity retention rate.

[0013] In a preferred embodiment of the charging method disclosed herein, the strength of the magnetic field is 0.1 T or more and 10 T or less. This makes it possible to more effectively increase the resistance value of the negative electrode during charging and discharging, and to achieve a higher level of high capacity retention.

[0014] The battery module disclosed herein is a battery module having a secondary battery with an electrode assembly including a positive electrode and a negative electrode. The positive electrode includes a positive electrode core and a positive electrode active material layer formed on the surface of the positive electrode core. The negative electrode includes a negative electrode core and a negative electrode active material layer formed on the surface of the negative electrode core. The negative electrode active material layer includes at least a negative electrode active material and a magnetic material. The battery module further includes a magnetic field switching means that can selectively apply a magnetic field to the space in which the secondary battery is placed only during charging.

[0015] In a preferred aspect of the battery module disclosed herein, the magnetic field switching means includes an electromagnet, and is configured to energize the electromagnet when the secondary battery is being charged and not energize the electromagnet when the secondary battery is being discharged, thereby enabling efficient application of a magnetic field selectively during charging and achieving a high capacity retention rate.

[0016] In a preferred embodiment of the battery module disclosed herein, the strength of the magnetic field is not less than 0.1 T and not more than 10 T. This makes it possible to suitably increase the capacity retention rate of each secondary battery (single cell) by applying a magnetic field during charging of the battery module. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a perspective view schematically illustrating a nonaqueous electrolyte secondary battery according to one embodiment. [Figure 2] FIG. 2 is a schematic longitudinal sectional view taken along line II-II in FIG. [Figure 3] FIG. 2 is a perspective view schematically showing an electrode assembly attached to a sealing plate. [Figure 4] FIG. 2 is a perspective view schematically showing an electrode assembly to which a positive electrode second current collecting portion and a negative electrode second current collecting portion are attached. [Figure 5] FIG. 2 is a schematic diagram showing the configuration of a wound electrode body. [Figure 6] FIG. 2 is a diagram schematically illustrating a cross section of an electrode body according to one embodiment. [Figure 7] FIG. 2 is a schematic diagram illustrating a charging method according to an embodiment. [Figure 8] FIG. 1 is a perspective view schematically illustrating a battery module according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0018] Preferred embodiments of the technology disclosed herein are described below. Matters necessary for implementing the technology disclosed herein, other than those specifically mentioned in this specification, can be understood as design matters for a person skilled in the art based on the prior art in the relevant field. The technology disclosed herein can be implemented based on the content disclosed in this specification and the technical common sense in the relevant field. In the following description, "A to B (where A and B are any values)" includes the values ​​of A and B (upper and lower limits). In the drawings described in this specification, components and parts that perform the same function are denoted by the same reference numerals, and redundant descriptions may be omitted or simplified. The dimensional relationships (length, width, thickness, etc.) in each drawing do not necessarily reflect the actual dimensional relationships.

[0019] 1. Secondary battery In this specification, the term "secondary battery" refers to a general term for an electricity storage device that can be repeatedly charged and discharged with the movement of charge carriers between positive and negative electrodes, and is a concept that encompasses so-called storage batteries (chemical batteries) such as lithium ion secondary batteries and sodium ion secondary batteries, and capacitors (physical batteries) such as lithium ion capacitors (LIC). Below, each of the main constituent materials of the secondary battery according to the present disclosure will be described. It should be noted that conventionally known materials can be used for constituent materials of secondary batteries not described here.

[0020] Fig. 1 is a perspective view of a nonaqueous electrolyte secondary 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 (thickness direction), the long side direction (width direction) perpendicular to the short side direction, and the up-down direction (height direction) of the nonaqueous electrolyte secondary battery 100, respectively. However, these directions are merely used for the convenience of explanation and do not in any way limit the installation form of the nonaqueous electrolyte secondary battery 100.

[0021] 2, the nonaqueous electrolyte secondary 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, and a negative electrode current collector 60. Although not shown, the nonaqueous electrolyte secondary battery 100 further includes a nonaqueous electrolyte solution. The nonaqueous electrolyte secondary battery 100 is a lithium ion secondary battery.

[0022] The battery case 10 is a housing that houses the electrode assembly 20. Here, 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.

[0023] As shown in FIG. 1 , the exterior body 12 includes a bottom wall 12a, a pair of long side walls 12b extending from the bottom wall 12a and facing each other, and a pair of short side walls 12c extending from the bottom wall 12a and facing each other. The bottom wall 12a is generally rectangular. The bottom wall 12a faces an opening 12h. The area of ​​the short side wall 12c is smaller than the area of ​​the long side wall 12b. The long side wall 12b and the short side wall 12c are examples of the first side wall and second side wall disclosed herein. In this embodiment, the direction in which the long side walls 12b face each other is the thickness direction of the battery. 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 sealing plate 14 is generally rectangular in plan view. The battery case 10 is integrated by joining (for example, welding) a sealing plate 14 to the periphery of the opening 12h of the exterior body 12. The battery case 10 is airtightly sealed (sealed).

[0024] As shown in FIG. 2 , the sealing plate 14 is provided with a liquid inlet 15, a gas release valve 17, and two terminal outlet holes 18 and 19. The liquid inlet 15 is for injecting an electrolyte after the sealing plate 14 is assembled to the exterior body 12. 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 outlet holes 18 and 19 are formed at both ends of the sealing plate 14 in the long side direction Y. The terminal outlet holes 18 and 19 penetrate the sealing plate 14 in the up-down direction Z. The terminal outlet holes 18 and 19 each have an inner diameter large enough to insert the positive electrode terminal 30 and the negative electrode terminal 40 before they are attached to the sealing plate 14 (before crimping).

[0025] The positive electrode terminal 30 and the negative electrode terminal 40 are each fixed to the sealing plate 14. 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. 1, the positive electrode terminal 30 and the negative electrode terminal 40 are exposed on the outer surface of the sealing plate 14. 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. 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 crimping. The positive electrode terminal 30 and the negative electrode terminal 40 have crimped portions at their ends on the exterior body 12 side (lower ends in FIG. 2).

[0026] 2, the positive electrode terminal 30 is electrically connected to the positive electrode tab group 23 of the electrode assembly 20 via a positive electrode current collector 50 inside the exterior housing 12. The negative electrode terminal 40 is electrically connected to the negative electrode tab group 25 of the electrode assembly 20 via a negative electrode current collector 60 inside the exterior housing 12. The positive electrode terminal 30 and the negative electrode terminal 40 are examples of terminals disclosed herein.

[0027] The positive electrode terminal 30 is preferably made of a metal, more preferably made of aluminum or an aluminum alloy, for example. The negative electrode terminal 40 is preferably made of a metal, 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 connected to the negative electrode current collecting part 60 may be made of copper or a copper alloy, and the portion exposed on the outer surface of the sealing plate 14 may be made of aluminum or an aluminum alloy.

[0028] As shown in FIG. 1 , 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 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 negative electrode external conductive member 42 are members to which bus bars are attached when electrically connecting multiple nonaqueous electrolyte secondary batteries 100 to each other. The positive electrode external conductive member 32 and the negative electrode external conductive member 42 are preferably made of metal, more preferably aluminum or an aluminum alloy. The positive electrode external conductive member 32 and the negative electrode external conductive member 42 are insulated from the sealing plate 14 by an external insulating member 92. However, the positive electrode external conductive member 32 and the negative electrode external conductive member 42 are not essential and may be omitted in other embodiments.

[0029] FIG. 3 is a perspective view schematically illustrating an electrode assembly 20 attached to a sealing plate 14. FIG. 4 is a perspective view schematically illustrating an electrode assembly 20a. In a nonaqueous electrolyte secondary battery 100 according to this embodiment, an electrode assembly 20 having multiple electrode assemblies 20a, 20b, and 20c is housed inside a battery case 10. However, the number of electrode assemblies arranged inside one exterior housing 12 is not particularly limited and may be two or more (plural), or may be one. While the detailed structure will be described later, each of the electrode assemblies 20a, 20b, and 20c has a positive electrode tab group 23 composed of multiple positive electrode tabs 22t and a negative electrode tab group 25 composed of multiple negative electrode tabs 24t. The positive electrode current collector 50 forms a conductive path electrically connecting the positive electrode tab group 23 and the positive electrode terminal 30. The negative electrode current collector 60 forms a conductive path electrically connecting the negative electrode tab group 25 and the negative electrode terminal 40. Here, the electrode body 20a has a positive electrode tab group 23 at one end and a negative electrode tab group 25 at the other end.

[0030] As shown in FIG. 2, the positive electrode current collecting portion 50 includes a positive electrode first current collecting portion 51, which is a plate-shaped conductive member extending along the inner surface of the sealing plate 14, and a positive electrode second current collecting portion 52, which is a plate-shaped conductive member extending along the vertical direction Z. The lower end of the positive electrode terminal 30 extends toward the inside of the battery case 10 through the terminal lead-out hole 18 of the sealing plate 14 and is connected to the positive electrode first current collecting portion 51 (see FIG. 2). On the other hand, as shown in FIG. 2, one end of the positive electrode second current collecting portion 52 is connected to the positive electrode first current collecting portion 51, and the other end is connected to the positive electrode tab group 23 of the electrode body group 20. Here, the positive electrode tab group 23 of the electrode body group 20 is bent so that the positive electrode second current collecting portion 52 faces the side of the electrode bodies 20a, 20b, and 20c having the positive electrode tab group 23. This allows the width of the positive electrode tab group 23 in the long side direction Y to be reduced. As a result, it is possible to increase the coating width in the long side direction Y of the positive electrode active material layer 22a and the negative electrode active material layer 24a of the electrode body group 20 described below, thereby achieving a high capacity of the nonaqueous electrolyte secondary battery 100. The positive electrode first current collecting part 51 and the positive electrode second current collecting part 52 are preferably made of metal, and may be made of, for example, aluminum, an aluminum alloy, nickel, stainless steel, or the like.

[0031] As shown in FIG. 2, the negative electrode current collecting part 60 includes a negative electrode first current collecting part 61, which is a plate-shaped conductive member extending along the inner surface of the sealing plate 14, and a negative electrode second current collecting part 62, which is a plate-shaped conductive member extending in the up-down direction Z. The lower end of the negative electrode terminal 40 passes through the terminal lead-out hole 19 of the sealing plate 14 and extends toward the inside of the battery case 10, and is connected to the negative electrode first current collecting part 61 (see FIG. 2). On the other hand, as shown in FIG. 2, one end of the negative electrode second current collecting part 62 is connected to the negative electrode first current collecting part 61, and the other end is connected to the negative electrode tab group 25 of the electrode body group 20. Here, the negative electrode tab group 25 of the electrode body group 20 is bent so that the negative electrode second current collecting part 62 faces the side of the electrode bodies 20a, 20b, and 20c having the negative electrode tab groups 25. This, like the above-described configuration in which the positive electrode tab group 23 is folded, makes it possible to achieve a high capacity of the nonaqueous electrolyte secondary battery 100. The negative electrode first current collecting portion 61 and the negative electrode second current collecting portion 62 are preferably made of metal, and may be made of, for example, copper, a copper alloy, nickel, stainless steel, or the like.

[0032] In the nonaqueous electrolyte secondary battery 100 according to this embodiment, various insulating members are attached between any of the members to prevent conduction between the members.

[0033] An insulating member that prevents electrical conduction is attached between the battery case 10 and the electrode assembly 20. Specifically, an external insulating member 92 is interposed between the positive electrode external conductive member 32 (or the negative electrode external conductive member 42) and the outer surface of the sealing plate 14 (see FIG. 2). This prevents electrical conduction between the positive electrode external conductive member 32 or the negative electrode external conductive member 42 and the sealing plate 14.

[0034] A gasket 90 is attached to each of the terminal lead-out holes 18, 19 of the sealing plate 14. This prevents the positive electrode terminal 30 (or the negative electrode terminal 40) inserted into the terminal lead-out holes 18, 19 from becoming electrically connected to the sealing plate 14.

[0035] An internal insulating member 94 is disposed between the positive electrode first current collecting portion 51 (or the negative electrode first current collecting portion 61) and the inner surface of the sealing plate 14. This internal insulating member 94 has a plate-shaped base portion 94a interposed between the positive electrode first current collecting portion 51 (or the negative electrode first current collecting portion 61) and the inner surface of the sealing plate 14. This prevents the positive electrode first current collecting portion 51 or the negative electrode first current collecting portion 61 from being electrically connected to the sealing plate 14. Furthermore, the internal insulating member 94 has a protrusion 94b that protrudes from the inner surface of the sealing plate 14 toward the electrode assembly group 20. This restricts movement of the electrode assembly group 20 in the up-down direction Z and prevents direct contact between the electrode assembly group 20 and the sealing plate 14.

[0036] The material of each of the insulating members described above is not particularly limited as long as it has the required insulating properties. For example, synthetic resin materials such as polyolefin resins (e.g., polypropylene (PP) and polyethylene (PE)) and fluorine-based resins (e.g., perfluoroalkoxyalkane (PFA) and polytetrafluoroethylene (PTFE)) can be used.

[0037] Fig. 5 is a schematic diagram showing the configuration of the electrode assembly 20a, and Fig. 6 is a schematic diagram showing a cross section of the electrode assembly 20a. The electrode assembly 20a will be described in detail below as an example, but the electrode assemblies 20b and 20c can also have a similar configuration. As shown in Fig. 5, the electrode assembly 20a has a positive electrode 22 and a negative electrode 24. The electrode assembly 20a here is a flat wound electrode assembly in which a strip-shaped positive electrode 22 and a strip-shaped negative electrode 24 are stacked with a strip-shaped separator 26 interposed therebetween and wound around a winding axis WL.

[0038] The electrode assembly 20a is disposed inside the exterior housing 12 with its winding axis WL parallel to the long-side direction Y. In other words, the electrode assembly 20a is disposed inside the exterior housing 12 with its winding axis WL parallel to the bottom wall 12a and perpendicular to the short side wall 12c. The end face of the electrode assembly 20a (in other words, the stacking surface where the positive electrode 22, the negative electrode 24, and the separator 26 are stacked; the end face in the long-side direction Y in FIG. 5) faces the short side wall 12c. However, the electrode assembly 20a may be a laminated electrode assembly formed by stacking multiple square-shaped (typically rectangular) positive electrodes and multiple square-shaped (typically rectangular) negative electrodes in an insulated state. In this specification, the "thickness direction of the electrode assembly" refers to the stacking direction of the electrode plates. In the electrode assembly 20a of this embodiment, the thickness direction of the electrode assembly refers to the direction in which the wide surfaces perpendicular to the stacking end faces face each other (the X direction in FIG. 4). In this embodiment, the thickness direction of the battery and the thickness direction of the electrode assembly coincide with each other.

[0039] As shown in FIG. 5, the positive electrode 22 has a positive electrode core 22c and a positive electrode active material layer 22a formed on at least one surface (both surfaces in this case) of the positive electrode core 22c.

[0040] The positive electrode substrate 22c is strip-shaped and made of a conductive metal such as aluminum, an aluminum alloy, nickel, stainless steel, etc. The positive electrode substrate 22c here is a metal foil, specifically an aluminum foil.

[0041] 6, the positive electrode active material layer 22a is provided in a strip shape along the longitudinal direction of the strip-shaped positive electrode core 22c, and contains at least a positive electrode active material 22b that can reversibly store and release charge carriers. From the viewpoint of maintaining battery performance, the thickness of the positive electrode active material layer 22a is 250 μm or less, preferably 220 μm or less, and more preferably 200 μm or less.

[0042] The positive electrode active material 22b preferably contains at least one of Ni, Co, and Mn, and may be, for example, a lithium transition metal composite oxide such as a lithium nickel cobalt manganese composite oxide. When the entire solid content of the positive electrode active material layer 22a is taken as 100 mass%, the positive electrode active material 22b may account for approximately 80 mass% or more, typically 90 mass% or more, for example 95 mass% or more.

[0043] The positive electrode active material layer 22a may contain any component other than the positive electrode active material 22b, such as a conductive material, a binder, various additive components, etc. The conductive material may be, for example, a carbon material such as carbon black (e.g., acetylene black (AB)) or carbon nanotubes. The binder may be, for example, PVdF.

[0044] As shown in FIG. 5, the negative electrode 24 has a negative electrode core 24c and a negative electrode active material layer 24a formed on at least one surface (both surfaces in this case) of the negative electrode core 24c.

[0045] The negative electrode substrate 24c is strip-shaped and made of a conductive metal such as copper, a copper alloy, nickel, stainless steel, etc. Here, the negative electrode substrate 24c is a metal foil, specifically a copper foil.

[0046] 6, the negative electrode active material layer 24a is provided in a strip shape along the longitudinal direction of a strip-shaped negative electrode core 24c, and contains at least a negative electrode active material 24b and a magnetic material 24d. From the viewpoint of maintaining battery performance, the thickness of the negative electrode active material layer 24a is 250 μm or less, preferably 220 μm or less, and more preferably 200 μm or less.

[0047] The negative electrode active material 24b is a negative electrode active material 24b (for example, a carbon material such as graphite, or a silicon-based material such as Si or SiO) that can reversibly absorb and release charge carriers, and when the entire solid content of the negative electrode active material layer 24a is taken as 100 mass%, the negative electrode active material 24b may account for approximately 80 mass% or more, typically 90 mass% or more, for example 95 mass% or more.

[0048] The magnetic material is a substance that becomes magnetic when placed in a space where a magnetic field is applied (e.g., permalloy alloy (iron-nickel), iron-based alloy (iron-cobalt-magnesium oxide), manganese-based alloy (manganese-cobalt-iron), Sn-doped indium antimonide (InSb), etc.). The content of the magnetic material 24d is suitably approximately 1% by mass or more, preferably 1.5% by mass or more, more preferably 2% by mass or more, and may be 3% by mass or more, 5% by mass or more, or 7% by mass or more, when the entire solid content of the negative electrode active material layer 24a is taken as 100% by mass. On the other hand, if the content of the magnetic material 24d is too high, the internal resistance of the battery may become excessively high and the content of the active material may become relatively low, which is undesirable. For example, when the total solid content of the negative electrode active material layer 24a is taken as 100% by mass, the magnetic material 24d is suitably approximately 15% by mass or less, and preferably 10% by mass or less. Although there are no particular restrictions on the particle size of the magnetic material 24d contained in each layer, the average particle size (50% cumulative particle size on a volume basis based on laser diffraction and scattering) is suitably 0.01 μm or more, preferably 0.1 μm or more, and suitably 15 μm or less, preferably 10 μm or less.

[0049] When the above magnetic body 24d bisects the negative electrode active material layer 24a in its thickness direction, the content of the magnetic body 24d on the surface side is configured to be contained more than that on the negative electrode core 24c side. For example, as shown in FIG. 6, in the thickness direction of the negative electrode active material layer 24a, it preferably has a layered structure in which the content of the magnetic body 24d gradually increases from the negative electrode core 24c side toward the surface side. Thereby, since the precipitation of charge carriers can be more effectively suppressed on the surface side of the negative electrode active material layer 24a where the precipitation of charge carriers tends to occur, an improvement in the capacity retention rate can be realized.

[0050] The number of layers of the above layered structure is not particularly limited as long as it is two or more, but from the viewpoint of the formation efficiency (lamination efficiency) of the negative electrode active material layer 24a, 5 or less is appropriate, and about 2 to 3 layers is particularly preferable.

[0051] In the above layered structure, the thickness of each layer may be substantially uniform. For example, the difference between the arithmetic mean value of the thickness of each layer and the thickness of each layer may be within 10% of the arithmetic mean, preferably within 5%.

[0052] For example, when the above layered structure is formed of three layers, when it is the a layer, b layer, and c layer from the negative electrode core 24c side (preferably composed of the a layer, b layer, and c layer with uniform thickness), when the ratio of the content (mass%) of the magnetic body 24d in the c layer to the content (mass%) of the magnetic body 24d in the a layer is c / a, from the viewpoint of improving the capacity retention rate, c / a is preferably 2 or more, particularly preferably 5 or more, and from the viewpoint of suppressing an excessive increase in the resistance value, 25 or less is preferable and 20 or less is particularly preferable. Specifically, the content of the magnetic body 24d in the a layer is, for example, 0.5 mass% or more and 2 mass% or less. The content of the magnetic body 24d in the b layer is, for example, 1 mass% or more and 10 mass% or less, 1.5 mass% or more and 5 mass% or less, or 3 mass% or more and 5 mass% or less. The content of the magnetic body 24d in the c layer is, for example, 1.5 mass% or more and 20 mass% or less, 2 mass% or more and 15 mass% or less, or 5 mass% or more and 15 mass% or less. It is preferable that the content of the magnetic body 24d is a layer < b layer < c layer.

[0053] The negative electrode active material layer 24a may contain optional components other than the negative electrode active material 24b and the magnetic material 24d, such as a binder, a dispersant, and various additives. Examples of the binder that can be used include rubbers such as styrene butadiene rubber (SBR). Examples of the dispersant that can be used include celluloses such as carboxymethyl cellulose (CMC).

[0054] 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. A porous resin sheet made of a polyolefin resin such as polyethylene (PE) or polypropylene (PP) is suitable for the separator 26. The separator 26 may have a heat resistance layer (HRL) containing an inorganic filler provided on the surface of the resin sheet. Examples of inorganic fillers that can be used include alumina, boehmite, aluminum hydroxide, and titania. An adhesive layer is preferably provided on one or both surfaces of the separator 26. The adhesive layer improves adhesion to the contacting positive electrode active material layer 22a or negative electrode active material layer 24a. The adhesive layer contains, for example, polyvinylidene fluoride (PVdF) as an adhesive component. The adhesive layer may also contain inorganic particles such as alumina and boehmite. The adhesive layer may be provided on the surface of the resin sheet or on the surface of the HRL.

[0055] The nonaqueous electrolyte may be the same as conventional ones and is not particularly limited. The nonaqueous electrolyte contains, for example, a nonaqueous solvent and a supporting salt. The nonaqueous solvent contains, for example, carbonates such as ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate. The supporting salt is, for example, a fluorine-containing lithium salt such as LiPF6. However, the nonaqueous electrolyte may be in a solid state (solid electrolyte) and integrated with the electrode assembly 20.

[0056] 2. Charging and discharging secondary batteries (1) Charging the secondary battery The magnetic field applied to the space when charging the secondary battery will be described below.

[0057] When magnets are brought close to each other, they repel each other when the same poles are brought close to each other, and when opposite poles are brought close to each other, they attract each other. In this specification, the term "magnetic field" refers to a space in which the above phenomenon can occur, i.e., a space in which magnetic field lines are generated.

[0058] In the above-mentioned magnetic field, the higher the density of magnetic field lines in an area, the stronger the magnetic field strength, and the more likely the above phenomenon is to occur. The degree to which this phenomenon occurs is called "magnetic field strength." In this specification, the magnetic flux density of 1 Wb (weber) per square meter of a surface perpendicular to the direction of the magnetic flux is defined as 1 T (tesla).

[0059] In the above magnetic field, the tangential direction of the magnetic field lines at any point is referred to in this specification as the "direction of the magnetic field." When the magnetic field lines are straight, the direction parallel to the magnetic field lines is referred to as the "direction of the magnetic field."

[0060] 7 is a diagram schematically illustrating a method for charging a secondary battery according to one embodiment, in which only the main components are shown and some of the constituent materials are omitted.

[0061] In one aspect of the charging method disclosed herein, a secondary battery is placed in a space to which a magnetic field is applied and then charged. The secondary battery includes a magnetic material in the negative electrode active material layer. The secondary battery is preferably the nonaqueous electrolyte secondary battery 100 described above. In one embodiment of the charging method for a secondary battery, a magnetic field is applied using a magnetic field switching means 200 during charging. The magnetic field switching means 200 is not limited as long as it is a means capable of switching between application and non-application of a magnetic field, and examples thereof include an electromagnet. In FIG. 7 , a Helmholtz coil is used as the magnetic field switching means 200. In one embodiment of the charging method for a secondary battery, a Helmholtz coil is placed between a pair of coils during charging. Then, a magnetic field is applied between the pair of coils by passing current through the pair of coils. The strength of the magnetic field may depend on the distance between the pair of coils, the number of coil turns, and the current passing through the coil.

[0062] From the viewpoint of significantly increasing the resistance value of the negative electrode, the strength of the applied magnetic field is, for example, 0.1 T or more, preferably 1 T or more, and more preferably 5 T or more. From the viewpoint of suppressing an excessive increase in the internal resistance of the battery, the strength is preferably 10 T or less, and more preferably 8 T or less.

[0063] The direction of the magnetic field applied to the space in which the secondary battery is placed is preferably the direction of the magnetic field lines in the thickness direction of the electrode body 20a of the placed battery. This allows a more uniform magnetic field to be applied to a wide area on the surface side (the side closer to the separator) of the negative electrode active material layer, where charge carrier precipitation is likely to occur. This can more evenly suppress charge carrier precipitation, thereby improving the capacity retention rate.

[0064] The charge rate is not particularly limited, but for example, the maximum charge rate during charging to 80% SOC may be 1C or higher, 2C or higher, or 3C or higher, and the maximum charge rate may be, for example, 20C or lower, or 10C or lower. This technology can achieve a high level of improvement in capacity retention even in charging with a high maximum charge rate (so-called rapid charging). In this specification, the charge rate "1C" refers to the current value that changes the SOC from 0% to 100% in one hour.

[0065] According to one embodiment of the charging method disclosed herein, a secondary battery containing a magnetic material in a negative electrode active material layer is charged while a magnetic field is applied. This increases the resistance of the negative electrode during charging, thereby reducing the current to the negative electrode. As a result, lithium deposition in the negative electrode can be suppressed, improving the capacity retention rate. Note that the above explanation of the mechanism of action of the present technology is speculative and does not limit the present technology. On the other hand, since discharging can be performed in a state where the strength of the magnetic field applied to the space is weaker than during charging, the effect of improving the output characteristics of the secondary battery is also expected.

[0066] (2) Discharge of secondary batteries Next, a discharging method according to one embodiment will be described.

[0067] In one embodiment, the discharging method is performed in a state where the strength of the magnetic field between the two coils of the Helmholtz coils, which are the magnetic field switching means 200, is relatively weaker than during charging. Preferably, the discharging is performed in a state where no magnetic field is applied between the two coils of the Helmholtz coils 200, which are the magnetic field switching means. For example, the above-described discharging can be achieved by weakening the strength of the magnetic field generated by the Helmholtz coils 200, or by not generating a magnetic field.

[0068] The charge / discharge method according to one embodiment operates by using an electromagnet to alternately apply and disable a magnetic field to a space. The secondary battery exhibits a high capacity retention rate and can be suitably used in various applications, including automotive batteries.

[0069] <Battery module> FIG. 8 is a schematic diagram illustrating a battery module according to one embodiment. The battery module disclosed herein includes the above-described secondary battery and a means for charging and discharging the secondary battery using the above-described charging and discharging method. The battery module 400 shown in FIG. 8 includes a plurality of secondary batteries 100 arranged in a predetermined direction and a plurality of magnetic field switching means 200 arranged at both ends of the arrangement direction. The battery module 400 can selectively apply a magnetic field to the space in which the secondary batteries 100 are arranged during charging by the magnetic field switching means 200. The battery module 400 can apply a magnetic field in the arrangement direction of the secondary batteries 100 (thickness direction of the electrode body 20a). The magnetic field switching means 200 and the charging and discharging method for the secondary batteries can be similar to those described above. Note that the number of secondary batteries and magnetic field switching means 200 used in one battery module is not particularly limited and may be two or more, or may be one. Furthermore, the magnetic field switching means 200 may be arranged in a direction perpendicular to the arrangement direction of the secondary batteries 100 (e.g., vertically relative to the secondary batteries 100). This allows a more uniform magnetic field to be applied to the multiple secondary batteries 100 included in the battery module 400. The charging and discharging process itself in a vehicle using this module may be the same as the conventional process, except that a magnetic field of a predetermined magnetic force is applied using the magnetic field switching means 200 during charging.

[0070] The battery module 400 shown in FIG. 8 has a restraining mechanism 300. The restraining mechanism 300 is configured to hold a plurality of secondary batteries 100 and a plurality of magnetic field switching means 200 therein. Here, the restraining mechanism 300 is configured with a pair of end plates 310, a pair of side plates 320, and a plurality of screws 330. The pair of end plates 310 are aligned in the arrangement direction of the secondary batteries 100. The pair of end plates 310 are arranged at both ends of the battery module 400 in the arrangement direction of the secondary batteries 100. The pair of side plates 320 bridge the pair of end plates 310. The plurality of secondary batteries 100 and the plurality of magnetic field switching means 200 are arranged between a pair of end plates 310 along the arrangement direction of the secondary batteries 100. This allows the plurality of secondary batteries 100 and the plurality of magnetic field switching means 200 to be integrally held within the restraint mechanism 300. However, the restraint mechanism 300 is not limited to this. The restraint mechanism 300 may include, for example, a plurality of restraint bands, bind bars, etc. instead of the side plates 320.

[0071] <Evaluation> 1. Test example Test examples relating to the technology disclosed herein will be described below, but it is not intended that the technology disclosed herein be limited to such test examples.

[0072] (1) Examples 1 to 3 and Comparative Examples 1 to 3 Lithium nickel cobalt manganese composite oxide (NCM) as the positive electrode active material, polyvinylidene fluoride (PVdF) as the binder, and carbon nanotubes (CNT) as the conductive material were weighed out in a mass ratio of NCM:PVdF:CNT = 97.5:1.5:1 and mixed in N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode slurry. This positive electrode slurry was applied to both sides of a long strip-shaped positive electrode substrate (aluminum foil, 12 μm thick) and dried. This was cut to a predetermined size and rolled using a roll press to obtain a positive electrode sheet with a 150 μm thick positive electrode active material layer on both sides of the positive electrode substrate.

[0073] Next, natural graphite (C) as the negative electrode active material, styrene butadiene rubber (SBR) as a binder, carboxymethyl cellulose (CMC) as a thickener, and 47Ni-Fe as a permalloy alloy were weighed out to a mass ratio of C:SBR:CMC:47Ni-Fe=98.5:1:0.5 (the ratio of 47Ni-Fe is shown in Table 1), and mixed in water to prepare a negative electrode slurry. This negative electrode slurry was applied to both sides of a long strip-shaped negative electrode core (copper foil, 9 μm) and dried. After this process, a negative electrode active material layer was formed on the negative electrode core, and then a b layer was formed on the a layer and a c layer on the b layer by the same process, and the layers were rolled out using a roll press. The negative electrode core Double-sided A negative electrode sheet was obtained having negative electrode active material layers with a total thickness of 180 μm, each layer consisting of layer a, layer b, and layer c, each 60 μm thick.

[0074] Next, a reference electrode was obtained, which was mainly made of platinum wire and had a conductive part and an LFP-coated part.

[0075] A 14 μm-thick separator with a three-layer structure made of PE / PP / PE was prepared, and the positive electrode sheet, separator, reference electrode, separator, and negative electrode sheet were stacked in this order. After attaching electrode terminals to the prepared electrode assembly, it was housed in a battery case together with a nonaqueous electrolyte and sealed with an aluminum / PP laminate film so that each conductive part was exposed, producing a laminated three-electrode cell. Terminals were connected to the reference electrode and negative electrode of the laminated three-electrode cell, and the cell was pre-charged at 0.6 C to a state of charge (SOC) of 50%. The non-aqueous 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 = 3:3:4, with LiPF6 dissolved as a supporting electrolyte at a concentration of 1.15 mol / L.

[0076] (2) Comparative Examples 4 and 5 The negative electrode was prepared in the same manner as in Examples 1 to 3, except that no gradation of the magnetic material was provided in the negative electrode active material layer.

[0077] (3) Comparative Example 6 The negative electrode active material layer was prepared in the same manner as in Examples 1 to 3, except that no magnetic material was added to the negative electrode active material layer.

[0078] 2. Evaluation test (1) Resistance measurement during charging and discharging Charge and discharge In Examples 1 to 3 and Comparative Example 4 only, during charging, current was passed through a pair of coils of the Helmholtz coils, and a magnetic field of strength 1 T was applied between the coils. Then, charging was performed with the secondary battery placed between the pair of coils. On the other hand, during discharging, no current was passed between the pair of coils, and discharging was performed with no magnetic field applied between the pair of coils. Resistance measurement The above magnetic field was applied at 25°C, and AC impedance measurement (frequency range: 10 mHz to 10 kHz, applied voltage: 30 mV) was performed to measure the resistance value. The resistance value during charging was measured using an AC impedance measuring device. On the other hand, the resistance value during discharge was measured using the same procedure as above, except that no current was passed between the pair of coils during resistance measurement. The resistance value between 50 mHz and 1 kHz was taken as the positive electrode resistance value / negative electrode resistance value, and the area (cm) of the positive and negative electrode sheets was calculated. 2 ) to obtain the resistance value (Ω / cm 2 ) was calculated. (2) Capacity retention rate measurement After soaking in a constant temperature bath at 25°C for 1 hour, the secondary battery was charged and discharged for two cycles at 0.1C, 4.2V-3.0V cut, and the discharge capacity obtained in the second cycle was designated as initial discharge capacity a. Next, for the secondary battery after measuring the initial capacity, 100 cycles of charge and discharge were repeated at 3.0C charge and 1.0C discharge, 4.2V-3.0V cut, and then one cycle of charge and discharge at 0.1C, 4.2V-3.0V cut, and the obtained discharge capacity was designated as post-cycle discharge capacity b. The capacity retention rate was calculated as b / a×100 [%].

[0079] 3. Evaluation Results The test results for each sample are summarized in Table 1. When the capacity retention rate was 80% or more, the evaluation was given as "Good", and otherwise it was given as "Poor".

[0080] [Table 1]

[0081] From the above results, it was confirmed that Examples 1 to 3 achieved high capacity retention rates. On the other hand, Comparative Example 4 did not achieve a suitable improvement in capacity retention rate, even though a magnetic field was applied during charging. One reason for this is thought to be that the magnetic material contained in the negative electrode active material layer was uniform. In Comparative Examples 1 to 3, 5, and 6, no magnetic field was applied during charging, and therefore no increase in resistance value during charging or improvement in capacity retention rate was achieved regardless of the distribution of the magnetic material in the negative electrode active material layer. From the above results, it was confirmed that in order to preferably achieve a high capacity retention rate, it is effective to place more magnetic material on the surface side of the negative electrode active material layer than on the negative electrode substrate side, and to apply a magnetic field to the space in which the secondary battery is placed during charging.

[0082] As described above, this specification includes the disclosures set forth in the following sections. Section 1: A secondary battery having an electrode assembly including a positive electrode and a negative electrode, the positive electrode includes a positive electrode core and a positive electrode active material layer formed on a surface of the positive electrode core, the negative electrode includes a negative electrode core and a negative electrode active material layer formed on a surface of the negative electrode core, the negative electrode active material layer contains at least a negative electrode active material and a magnetic material, In the negative electrode active material layer, when the active material layer is divided into two equal parts in the thickness direction into a surface side and a negative electrode core side, the content of the magnetic material on the surface side is greater than the content of the magnetic material on the negative electrode core side. Secondary battery.

[0083] Section 2: Item 2. The secondary battery according to item 1, wherein the negative electrode active material layer has a plurality of layers having different contents of the magnetic material in the thickness direction, and the content of the magnetic material in each layer increases sequentially from the layer closest to the negative electrode core toward the layer on the surface side.

[0084] Section 3: Item 3. The secondary battery according to item 1 or 2, wherein the negative electrode active material layer has three layers in the thickness direction, and when the three layers are layer a, layer b, and layer c from the negative electrode substrate side, a ratio c / a of the content of the magnetic material in layer a to layer c is at least 2.

[0085] Section 4: 4. The secondary battery according to any one of items 1 to 3, wherein the negative electrode active material layer contains a permalloy alloy as the magnetic material.

[0086] Section 5: A method for charging the secondary battery according to any one of items 1 to 4, A method for charging a secondary battery, comprising charging the secondary battery in a state in which the secondary battery is placed in a space to which a magnetic field is applied.

[0087] Item 6: Item 6. The method for charging a secondary battery according to item 5, wherein the space to which the magnetic field is applied is formed by an electromagnet.

[0088] Section 7: Item 7. The method for charging a secondary battery according to item 5 or 6, wherein the strength of the magnetic field is 0.1 T or more and 10 T or less.

[0089] Section 8: The secondary battery according to any one of items 1 to 4, A magnetic field switching means is provided that can selectively apply a magnetic field to the space in which the secondary battery is placed only during charging. Battery module.

[0090] Section 9: Item 9. The battery module according to item 8, wherein the magnetic field switching means includes an electromagnet, and is configured to energize the electromagnet when the secondary battery is being charged and not energize the electromagnet when the secondary battery is being discharged.

[0091] Section 10: Item 10. The battery module according to item 8 or 9, wherein the strength of the magnetic field is 0.1 T or more and 10 T or less. [Explanation of symbols]

[0092] 10 Battery case 12 Exterior body 14 Sealing plate 20 Electrode group 20a, 20b, 20c electrode body 22 Positive electrode 22a Cathode active material layer 22b Cathode active material 22c positive electrode core 22t Positive electrode tab 24 Negative electrode 24a Negative electrode active material layer 24b Negative electrode active material 24c negative electrode core 24d magnetic material 24t negative electrode tab 26 Separator 30 Positive terminal 40 Negative terminal 50 Positive electrode current collector 60 Negative electrode current collector 100 Nonaqueous electrolyte secondary battery 200 Magnetic field switching means 300 Restraint mechanism 310 End Plate 320 Side Plate 330 bis 400 battery module

Claims

1. A secondary battery having an electrode assembly including a positive electrode and a negative electrode, the positive electrode includes a positive electrode core and a positive electrode active material layer formed on a surface of the positive electrode core, the negative electrode includes a negative electrode core and a negative electrode active material layer formed on a surface of the negative electrode core, the negative electrode active material layer contains at least a negative electrode active material and a magnetic material, In the negative electrode active material layer, when the active material layer is divided into two equal parts in the thickness direction into a surface side and a negative electrode core side, the content of the magnetic material on the surface side is greater than the content of the magnetic material on the negative electrode core side. Secondary battery.

2. 2. The secondary battery according to claim 1, wherein the negative electrode active material layer includes a plurality of layers having different contents of the magnetic material in the thickness direction, and the contents of the magnetic material in each layer are configured to increase sequentially from a layer closest to the negative electrode core toward a layer on the surface side.

3. 3. The secondary battery according to claim 2, wherein the negative electrode active material layer has three layers in the thickness direction, and when the three layers are designated as layer a, layer b, and layer c from the negative electrode substrate side, a ratio c / a of the content of the magnetic material in layer a to that in layer c is at least 2.

4. The secondary battery according to claim 1 , wherein the negative electrode active material layer contains a permalloy alloy as the magnetic material.

5. A method for charging the secondary battery according to any one of claims 1 to 4, comprising: A method for charging a secondary battery, comprising charging the secondary battery in a state in which the secondary battery is placed in a space to which a magnetic field is applied.

6. 6. The method for charging a secondary battery according to claim 5, wherein the space to which the magnetic field is applied is formed by an electromagnet.

7. 7. The method for charging a secondary battery according to claim 6, wherein the strength of the magnetic field is 0.1 T or more and 10 T or less.

8. The secondary battery according to any one of claims 1 to 4. A magnetic field switching means is provided that can selectively apply a magnetic field to the space in which the secondary battery is placed only during charging. Battery module.

9. 9. The battery module according to claim 8, wherein the magnetic field switching means includes an electromagnet, and is configured to energize the electromagnet when the secondary battery is being charged and not energize the electromagnet when the secondary battery is being discharged.

10. The battery module according to claim 9 , wherein the strength of the magnetic field is 0.1 T or more and 10 T or less.

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