Method for manufacturing lithium-ion secondary battery

Abstract

The present disclosure aims to provide a manufacturing method of a lithium-ion secondary battery that suppresses deposition of lithium metal on the surface of a negative electrode. The technology disclosed herein includes: a battery assembly preparation step in which a battery assembly in which an electrode body and a non-aqueous electrolyte are housed in a case is prepared; a first charging step in which the battery assembly is charged; a first aging step in which, after the first charging step, the battery assembly is subjected to an aging treatment; and a second charging step in which the battery assembly that has undergone the first aging step is charged until the state of charge (SOC) of the battery assembly becomes 100%. Here, in the second charging step, the charging of the battery assembly is performed in such a way that the Li saturation degree of the surface of the negative electrode is 0.98 or less.

Description

Technical Field

[0001] This disclosure relates to a method for manufacturing lithium-ion secondary batteries. Background Technology

[0002] In the manufacture of a typical lithium-ion secondary battery, a battery assembly containing electrodes and a non-aqueous electrolyte is prepared, and the battery assembly is initially charged and aged. From a production efficiency perspective, the battery assembly is required to be charged with a high current (high rate) and the charging time shortened during lithium-ion secondary battery manufacturing. For example, Japanese Patent Publication No. 2019-121556 discloses an initial charging method for a lithium-ion secondary battery, characterized by: initial charging performed in two phases, during coating formation and during charging; during coating formation, charging is performed by progressively increasing the target voltage using CCCV charging at a current rate of 5C or less; and during charging, charging is performed by CCCV charging at a current rate exceeding 5C but below 6.5C until the lithium-ion secondary battery is fully charged.

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Publication No. 2019-121556

[0006] However, when a lithium-ion secondary battery first reaches 100% State of Charge (SOC) (fully charged), the volume of the negative electrode active material easily expands due to lithium ion adsorption. Therefore, during the initial full charge, when charging at a high rate, the volume expansion of the negative electrode active material can sometimes cause cracks and peeling. This creates new surfaces within the negative electrode active material. Due to these new surfaces, the lithium ion acceptability of the negative electrode active material deteriorates, potentially leading to the deposition of lithium metal on the negative electrode surface. Summary of the Invention

[0007] The problem that the invention aims to solve

[0008] This disclosure was made in view of the above circumstances, and its main objective is to provide a method for manufacturing a lithium-ion secondary battery that suppresses the deposition of lithium metal on the negative electrode surface.

[0009] Methods for solving problems

[0010] The disclosed technology is a method for manufacturing a lithium-ion secondary battery. The lithium-ion secondary battery includes an electrode body and a non-aqueous electrolyte. The electrode body is formed by stacking a positive electrode and a negative electrode containing a negative electrode active material separated by a separator. The method for manufacturing the lithium-ion secondary battery includes: a battery assembly preparation step, in which a battery assembly containing the electrode body and the non-aqueous electrolyte is prepared in a housing; a first charging step, in which the battery assembly is charged; a first aging step, after the first charging step, in which the battery assembly is aged; and a second charging step, in which the battery assembly that has undergone the first aging step is charged until the state of charge (SOC) of the battery assembly reaches 100%. In the second charging step, the battery assembly is charged such that the Li saturation on the surface of the negative electrode is below 0.98.

[0011] Based on this structure, it is possible to manufacture lithium-ion secondary batteries that suppress the deposition of lithium metal on the negative electrode surface. Attached Figure Description

[0012] Figure 1 This is a perspective view schematically illustrating one embodiment of a lithium-ion secondary battery.

[0013] Figure 2 It is along Figure 1 A schematic cross-sectional view of line II-II.

[0014] Figure 3 This is a schematic diagram showing the structure of an electrode body according to one embodiment.

[0015] Figure 4 This is a flowchart of a method for manufacturing a lithium-ion secondary battery according to one embodiment.

[0016] Figure 5 This is a schematic diagram illustrating the movement of lithium ions in the negative electrode of a battery assembly.

[0017] Figure 6 This is a graph showing the state of charge (50%~100%) of the battery assembly and the shift in current value during charging in the second charging process of the first embodiment.

[0018] Figure 7 This is a diagram schematically illustrating the movement of lithium ions in the negative electrode of the battery assembly during the second charging process of the second embodiment.

[0019] Figure 8 This is a graph showing the shift in SOC (50~100%) and charging current value of the battery assembly during the second charging process of the second embodiment.

[0020] Figure 9 This is a photograph of the electrode (negative electrode) showing the results of the lithium metal deposition evaluation of the embodiment.

[0021] Figure 10 This is a photograph of the electrode (negative electrode) showing the results of the lithium metal deposition evaluation of the comparative example.

[0022] Explanation of reference numerals in the attached figures

[0023] 10. Shell

[0024] 12. Shell Body

[0025] 14 Sealing board

[0026] 20 Electrode Body

[0027] 22 Positive electrode

[0028] 22a Positive electrode active material layer

[0029] 22c Positive current collector

[0030] 24 Negative electrode

[0031] 24a Negative electrode active material layer

[0032] 24c negative current collector

[0033] 24s negative electrode surface

[0034] 26. Diaphragm

[0035] 30 Positive extremes

[0036] 40 Negative extremes

[0037] 50 Positive current collector

[0038] 60 Negative current collector

[0039] 100 Lithium-ion Secondary Battery

[0040] S10 Battery Assembly Preparation Process

[0041] S20 First Charging Process

[0042] S30 First Aging Process

[0043] S40 Internal Resistance Inspection Procedure

[0044] S50 Second Charging Process

[0045] S60 Second Aging Process

[0046] S70 Short circuit check procedure. Detailed Implementation

[0047] Hereinafter, some preferred embodiments of the present disclosure will be described with appropriate reference to the accompanying drawings. Matters not specifically mentioned in this specification and necessary for the implementation of this disclosure (e.g., the general structure and manufacturing process of lithium-ion secondary batteries not characterized by this disclosure) can be understood by those skilled in the art based on prior art. This disclosure can be implemented based on the content disclosed in this specification and common technical knowledge in the field.

[0048] Furthermore, in the following figures, components and parts that perform the same function are labeled with the same reference numerals, and sometimes repeated descriptions are omitted or simplified. Additionally, the expressions "A~B" indicating ranges in this specification include meanings of "above A and below B" and "above A and below B".

[0049] Furthermore, in the following description, the reference numerals L, R, F, Rr, U, and D in the accompanying drawings represent left, right, front, back, top, and bottom, respectively. The reference numerals X, Y, and Z in the accompanying drawings represent the short side direction, the long side direction orthogonal to the short side direction, and the vertical direction (also called the height direction) orthogonal to both the short and long side directions of the lithium-ion secondary battery 100, respectively. However, these directions are determined for ease of explanation and do not impose any limitation on the arrangement of the lithium-ion secondary battery 100.

[0050] Furthermore, in this specification, "lithium-ion secondary battery" refers to an energy storage device that uses lithium ions as charge carriers and achieves repeated charging and discharging through the movement of charge accompanying lithium ions between the positive and negative electrodes.

[0051] <Lithium-ion secondary batteries>

[0052] The following describes an example of a lithium-ion secondary battery that is the subject of the manufacturing method disclosed herein. Figure 1 This is a perspective view schematically showing one embodiment of a lithium-ion secondary battery 100. Figure 2 It is along Figure 1 A schematic longitudinal section view of line II-II. Figure 1 or Figure 2 The lithium-ion secondary battery 100 shown includes a casing 10, an electrode body 20, a positive terminal 30, a negative terminal 40, a positive current collector 50, a negative current collector 60, and a non-aqueous electrolyte (not shown). The following is a detailed description of each component.

[0053] The housing 10 is a housing that contains the electrode body 20 and the non-aqueous electrolyte. For example... Figure 1As shown, the housing 10 has a flat, bottomed cuboid shape (square). The material of the housing 10 can be the same as conventionally used materials, and there is no particular limitation. The housing 10 is preferably made of metal, and more preferably, for example, aluminum, aluminum alloy, iron, iron alloy, etc. However, in other embodiments, the housing 10 may also be, for example, a laminated bag shape.

[0054] like Figure 2 As shown, the housing 10 includes a housing body 12 with an opening 12h and a sealing plate (cover) 14 that seals the opening 12h. The sealing plate 14 is installed on the housing body 12 in such a way that it seals the opening 12h of the housing body 12. The housing 10 is integrally formed by joining (e.g., welding) the sealing plate 14 to the periphery of the opening 12h of the housing body 12. The housing 10 is hermetically sealed (airtight).

[0055] like Figure 1 As shown, the main body 12 of the housing includes a bottom 12a, a pair of long sidewalls 12b extending from the bottom 12a and facing each other, and a pair of short sidewalls 12c extending from the bottom 12a and facing each other. The bottom 12a is generally rectangular in shape. The bottom 12a faces the opening 12h. The long sidewalls 12b are flat. The long sidewalls 12b extend from the long side of the bottom 12a. The short sidewalls 12c extend from the short side of the bottom 12a. In plan view, the area of ​​the long sidewalls 12b is larger than the area of ​​the short sidewalls 12c.

[0056] like Figure 2 As shown, the sealing plate 14 is provided with an injection hole 15 and two terminal outlet holes 18 and 19. The injection hole 15 is used to inject a non-aqueous electrolyte into the housing 10 after the sealing plate 14 is assembled into the housing body 12. Preferably, the sealing plate 14 is provided with an injection hole 15. The injection hole 15 is sealed by a sealing member 16. The terminal outlet holes 18 and 19 are respectively formed at both ends of the sealing plate 14 in the long side direction Y ( Figure 2 (Left and right ends). Terminal lead-out holes 18 and 19 penetrate the sealing plate 14 in the thickness direction (vertical direction Z).

[0057] like Figure 2 As shown, the positive terminal 30 and the negative terminal 40 extend from the inside of the sealing plate 14 to the outside through the terminal lead-out holes 18 and 19. Here, the positive terminal 30 and the negative terminal 40 are riveted to the peripheral portion surrounding the terminal lead-out holes 18 and 19 of the sealing plate 14 by a riveting process. At the ends of the positive terminal 30 and the negative terminal 40 on the housing body 12 side ( Figure 2 The lower end of the component has riveting portions 30c and 40c.

[0058] The positive terminal 30 is electrically connected to the positive electrode tab assembly 23 of the electrode body 20 via the positive current collector 50 inside the housing 10. The positive terminal 30 is insulated from the sealing plate 14 by the positive electrode insulating member 70 and the gasket 90. The positive terminal 30 is preferably made of metal, more preferably, for example, aluminum or an aluminum alloy. The negative terminal 40 is electrically connected to the negative electrode tab assembly 25 of the electrode body 20 via the negative current collector 60 inside the housing 10. The negative terminal 40 is insulated from the sealing plate 14 by the negative electrode insulating member 80 and the gasket 90. The negative terminal 40 is preferably made of metal, more preferably, for example, copper or a copper alloy.

[0059] like Figure 2 As shown, the electrode body 20 is housed inside the housing 10 (more specifically, inside the housing body 12). The number of electrode bodies 20 disposed inside a housing 10 is not particularly limited; there may be one or more (multiple) such bodies.

[0060] Figure 3 This is a schematic diagram showing the structure of an electrode body 20 according to one embodiment. (As shown...) Figure 3 As shown, the electrode body 20 includes a positive electrode 22, a negative electrode 24, and a separator 26. The electrode body 20 is a flat, wound electrode body. The electrode body 20 is constructed by stacking strip-shaped positive electrodes 22 and strip-shaped negative electrodes 24 separated by a strip-shaped separator 26 and wound around a winding axis WL. However, the electrode body 20 can also be a stacked electrode body formed by stacking square (typically rectangular) positive electrodes and square (typically rectangular) negative electrodes in an insulated state.

[0061] The positive electrode 22 can be the same as before and is not particularly limited. The positive electrode 22 has a positive current collector 22c and a positive active material layer 22a and a positive protective layer 22p fixed to at least one surface of the positive current collector 22c. However, the positive protective layer 22p is not necessary and can be omitted in other embodiments. The positive current collector 22c is strip-shaped. The positive current collector 22c is made of conductive metals such as aluminum, aluminum alloy, nickel, and stainless steel. The positive current collector 22c is a metal foil, specifically an aluminum foil.

[0062] At one end of the long side Y of the positive current collector 22c ( Figure 3 Multiple positive electrode tabs 22t are provided at one end of the positive electrode 22c. The positive electrode tabs 22t are part of the positive current collector 22c and are made of metal foil (aluminum foil). The multiple positive electrode tabs 22t are located at one end in the long side direction Y. Figure 3 (The left end) is stacked, and as Figure 2 The positive electrode tab assembly 23 is configured as shown. A positive current collector 50 is attached (more specifically, connected) to the positive electrode tab assembly 23. The positive electrode tab assembly 23 is electrically connected to the positive terminal 30 via the positive current collector 50.

[0063] The positive electrode active material layer 22a is arranged in a strip shape along the long side of the strip-shaped positive electrode current collector 22c. The positive electrode active material layer 22a contains a positive electrode active material capable of reversibly absorbing and releasing charge carriers. Examples of positive electrode active materials include lithium transition metal composite oxides such as lithium nickel cobalt manganese composite oxides.

[0064] The positive electrode active material layer 22a may also contain any components other than the positive electrode active material, such as binders, conductive materials, and various additives. For example, polyvinylidene fluoride (PVdF) may be used as a binder. For example, carbon materials such as acetylene black (AB) may be used as a conductive material.

[0065] A positive electrode protective layer 22p is disposed between the positive electrode current collector 22c and the positive electrode active material layer 22a in the long side direction Y. The positive electrode protective layer 22p is disposed 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 also contain any component other than the inorganic filler, such as conductive materials, binders, various additives, etc. The conductive materials and binders may be the same as those exemplified as materials that can be contained in the positive electrode active material layer 22a.

[0066] The negative electrode 24 has 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 conductive metals such as copper, copper alloy, nickel, and stainless steel.

[0067] The negative current collector 24c is a metal foil, specifically a copper foil.

[0068] At one end of the long side Y of the negative current collector 24c ( Figure 3 Multiple negative electrode tabs 24t are provided at one end of the negative electrode 24c. The negative electrode tabs 24t are part of the negative current collector 24c and are made of metal foil (copper foil). The multiple negative electrode tabs 24t are located at one end in the long side direction Y. Figure 3 (The right end) is stacked, and as Figure 2 The negative electrode tab assembly 25 is configured as shown. A negative current collector 60 is attached (more specifically, connected) to the negative electrode tab assembly 25. The negative electrode tab assembly 25 is electrically connected to the negative terminal 40 via the negative current collector 60.

[0069] The negative electrode active material layer 24a is arranged in a strip shape along the long side of the strip-shaped negative electrode current collector 24c. The negative electrode active material layer 24a contains a negative electrode active material capable of reversibly absorbing and releasing charge carriers. Examples of negative electrode active materials include carbon materials such as graphite and Si-containing materials containing silicon.

[0070] The negative electrode active material layer 24a may also contain any components other than the negative electrode active material, such as binders, dispersants, conductive materials, and various additives. For example, rubbers such as styrene-butadiene rubber (SBR) and acrylic resins such as polyacrylic acid (PAA) may be appropriately used as binders. For example, cellulose-based materials such as carboxymethyl cellulose (CMC) may be appropriately used as dispersants. For example, carbon materials such as carbon fibers and carbon nanotubes may be appropriately used as conductive materials.

[0071] While not limited to this, the positive-to-negative electrode capacity ratio (negative electrode capacity / positive electrode capacity) of the electrode body 20 is preferably 1.0 or more, preferably 1.025 or more, and more preferably 1.05 or more. This allows for more appropriate suppression of lithium metal deposition during charging. On the other hand, from the viewpoint of increasing the capacity of lithium-ion batteries, the positive-to-negative electrode capacity ratio of the electrode body 20 is preferably 1.6 or less, preferably 1.3 or less, more preferably 1.1 or less, and even more preferably 1.075 or less. Furthermore, in this specification, the "positive-to-negative electrode capacity ratio" can be calculated separately for the positive electrode capacity and the negative electrode capacity using the following formula: Positive-to-negative electrode capacity ratio = Negative electrode capacity / Positive electrode capacity. Here, the positive-to-negative electrode capacity ratio can be easily adjusted, for example, by changing the unit area weight of the positive and negative electrodes or by changing the types of active materials in the positive and negative electrodes. Furthermore, "unit area weight" refers to the value obtained by dividing the mass of the electrode active material layer by the area of ​​the formed region (mass of the electrode active material layer / area of ​​the formed region). Furthermore, the positive electrode capacity can be calculated, for example, as the capacity of a half-cell with Li metal as the counter electrode during initial charging within a voltage range corresponding to a full cell (e.g., around 2 to 4.2 V) (i.e., the initial positive electrode charging capacity). And, the negative electrode capacity can be calculated, for example, by combining the initial negative electrode discharge capacity and the operating voltage range of the negative electrode.

[0072] like Figure 3 As shown, the width W2 of the negative electrode active material layer 24a is longer than the width W1 of the positive electrode active material layer 22a. In other words, the negative electrode 24 in this embodiment has an opposing region 24o facing the positive electrode 22 and a non-opposing region 24n not facing the positive electrode 22. Therefore, the capacity of the negative electrode 24 can be larger than that of the positive electrode 22 over the entire surface of the negative electrode (in other words, the positive-to-negative electrode capacity ratio is greater than 1.0). This allows for more appropriate suppression of lithium metal deposition during charging.

[0073] The separator 26 is a component 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 separator 26 can be the same as conventional separators and is not particularly limited. For example, a porous sheet made of a resin such as polyethylene (PE) or polypropylene (PP) is preferred as the separator 26. The separator 26 may also have a heat resistance layer (HRL) and an adhesive layer on the surface of the substrate portion made of the resin porous sheet. The structure of the heat resistance layer and the adhesive layer can be the same as conventional separators.

[0074] The non-aqueous electrolyte can be the same as before and is not particularly limited. A non-aqueous electrolyte is typically a liquid non-aqueous electrolyte containing a non-aqueous solvent (organic solvent) and a supporting salt (electrolyte salt). Examples of non-aqueous solvents include cyclic carbonates such as ethylene carbonate (EC) and ethylene monofluorocarbonate (FEC), and chain carbonates such as dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC). Examples of supporting salts include lithium salts such as lithium hexafluoride phosphate (LiPF6) and sodium salts. The non-aqueous electrolyte may also further contain additives such as film-forming agents. In other embodiments, the non-aqueous electrolyte may also be in solid or gel form.

[0075] <Manufacturing Method of Lithium-ion Secondary Batteries>

[0076] Next, the manufacturing method of the lithium-ion secondary battery disclosed herein will be described. The manufacturing method of the lithium-ion secondary battery disclosed herein is characterized by including a battery assembly preparation step, a first charging step, a first aging step, and a second charging step. In addition to the above-mentioned steps, the manufacturing method of the energy storage device disclosed herein may further include other steps at any stage. Figure 4 This is a flowchart illustrating a method for manufacturing a lithium-ion secondary battery 100 according to one embodiment. Figure 4 As shown, the manufacturing method of this embodiment includes a battery assembly preparation step S10, a first charging step S20, a first aging step S30, an internal resistance inspection step S40, a second charging step S50, a second aging step S60, and a short circuit inspection step S70.

[0077] (Battery assembly preparation process S10)

[0078] The battery assembly preparation step S10 is a step to prepare a battery assembly in which the electrode body 20 and the non-aqueous electrolyte are housed in the casing 10. In this embodiment, this step sequentially includes the electrode body preparation step S12, the electrode body housing step S14, and the electrolyte injection step S16. However, the order of the electrode body housing step S14 and the electrolyte injection step S16 is not particularly limited. In addition, when the non-aqueous electrolyte is in solid form (solid electrolyte), typically, the solid electrolyte and the electrode body 20 are integrated. Therefore, the electrolyte injection step S16 can be omitted. Furthermore, this step may further include other steps at any stage. In addition, the term "battery assembly" in this specification refers to all intermediate components, including the casing 10, the electrode body 20, and the non-aqueous electrolyte, in the manufacture of the lithium-ion secondary battery 100, and its structure is not particularly limited.

[0079] In the electrode preparation step S12, electrode body 20 is prepared. Electrode body 20 can be a commercially available product or it can be manufactured by oneself. For example, when preparing a wound electrode body as electrode body 20, the wound electrode body can be manufactured by stacking and winding a strip-shaped positive electrode 22 and a strip-shaped negative electrode 24 in an insulated state (e.g., separated by a strip-shaped diaphragm). The wound electrode body can be manufactured, for example, by using a conventionally known winding device and winding the strip-shaped positive electrode 22, the strip-shaped negative electrode 24, and the strip-shaped diaphragm 26 into a roller shape with the winding shaft WL as the center.

[0080] In the electrode housing process S14, the electrode 20 obtained above is housed in the housing 10. In one example, the electrode 20 is housed from the opening 12h of the housing body 12 into the interior of the housing body 12. Next, a sealing plate 14 is welded to the periphery of the opening 12h of the housing body 12, so that the housing body 12 and the sealing plate 14 are integrated.

[0081] In the electrolyte injection process S16, firstly, a non-aqueous electrolyte is prepared as a non-aqueous electrolyte. Then, the prepared non-aqueous electrolyte is injected into the housing 10. In one example, the prepared non-aqueous electrolyte is injected from the injection hole 15 of the sealing plate 14 into the interior of the housing 10 containing the electrode body 20. The battery assembly is prepared as described above.

[0082] (First charging process S20)

[0083] In the first charging step S20, the battery assembly is charged. The first charging step S20 can be performed according to known methods. For example, an external power source is connected between the positive terminal 30 and the negative terminal 40 of the battery assembly, and charging is performed at a constant current under a specified temperature environment until a specified voltage is reached between the terminals. As a result, a portion of the non-aqueous electrolyte is decomposed, and an SEI (Solid Electrolyte Interface) coating is formed on the surface of the negative electrode 24 (or, in more detail, the surface of the negative electrode active material layer 24a).

[0084] The temperature environment for the first charging process S20 can be either at room temperature (e.g., 25℃±10℃, preferably around 25℃±5℃) or at a high temperature of around 45℃.

[0085] While not particularly limited, from the viewpoint of effectively forming an SEI coating on the surface of the negative electrode 24, it is preferable to charge the battery assembly in the first charging step S20 such that its SOC is 15% or higher, and more preferably 20% or higher. On the other hand, from the viewpoint of higher safety in the first charging step S20, it is preferable to charge the battery assembly in the first charging step S20 such that its SOC is 80% or lower, and more preferably 50% or lower.

[0086] The charging rate of the first charging step S20 can be, for example, around 0.1C to 2C. This step can be performed either by constant current (CC) charging or by maintaining a constant voltage for a specified time after reaching a predetermined voltage. That is, constant current-constant voltage (CCCV) charging can also be performed in this step. In addition, charging can be performed once, or it can be repeated more than twice with discharge in between. Furthermore, "1C" refers to the current value that can charge the battery to the predicted capacity (Ah) based on the theoretical capacity of the positive electrode active material within 1 hour.

[0087] While not limited to this, it is preferable that, in the first charging step S20, charging is performed while the battery assembly housing 10 is constrained and under load. This facilitates the venting of gases generated during charging out of the housing 10. The constraint on the battery assembly can be implemented, for example, by applying a predetermined load to the long sidewall 12b of the housing body 12 along the short side direction X. The load applied to the housing 10 is not particularly limited, but can be approximately 3 kN to 25 kN, preferably 15 kN to 23 kN. Furthermore, in some embodiments, the process can proceed to steps after the first charging step S20 (first aging step S30 to short-circuit check step S70) while the battery assembly housing 10 is constrained and under load. In other words, in some embodiments, multiple steps, including the first charging step S20, can be performed while the assembly housing 10 is constrained and under load.

[0088] In this embodiment, during the first charging process S20, the liquid injection hole 15 of the sealing plate 14 is in an open state. Then, after the first charging process S20 is completed (in other words, before the first aging process S30 begins), the liquid injection hole 15 is sealed by the sealing member 16. As a result, the housing 10 is airtightly sealed (sealed).

[0089] (First aging process S30)

[0090] In the first aging process S30, the battery assembly is subjected to an aging treatment. This modifies the SEI coating on the negative electrode surface for 24 seconds. The first aging process S30 can be performed, for example, by placing the battery assembly in a constant temperature bath or similar environment set to maintain a predetermined temperature.

[0091] The aging temperature in the first aging process S30 is not particularly limited, but from the viewpoint of promoting the modification of the SEI coating, it is preferably 40°C or higher, more preferably 50°C or higher, and even more preferably 55°C or higher. On the other hand, from the viewpoint of properly modifying the SEI coating, the aging temperature in the first aging process S30 is preferably 85°C or lower, more preferably 80°C or lower, and even more preferably 75°C or lower.

[0092] Since the aging time in the first aging process S30 also depends on the aging temperature, battery size, etc., it cannot be generalized, but it is preferably 5 hours or more, more preferably 10 hours or more. From the viewpoint of efficiency, the aging time is preferably 120 hours or less, and more preferably 72 hours or less. Preferably, the battery assembly maintains the voltage reached in the first charging process S20 during this process.

[0093] (Internal resistance inspection procedure S40)

[0094] In the internal resistance inspection step S40, the internal resistance (IV resistance) of the battery assembly after the first aging step S30 is inspected. While not limited to this, in some preferred embodiments, the internal resistance inspection step S40 is preferably performed. This allows confirmation of the internal resistance characteristics of the battery assembly. The inspection method for the internal resistance inspection step S40 is not particularly limited, and conventionally known inspection methods and apparatus can be used. For example, at a specified temperature (e.g., 25°C), the battery assembly after the first aging step S30 is subjected to a constant current discharge for 10 seconds, starting from a state where the SOC of the battery assembly reaches a specified value (e.g., 15% or more and less than 80%). By dividing the voltage drop ΔV over this period (0.1 to 10 seconds) by the current value I, the internal resistance (IV resistance) can be determined. Furthermore, the constant current value at this time is preferably 1C or more.

[0095] (Second charging process S50)

[0096] In the second charging step S50, the battery assembly that has undergone the first aging step S30 is charged until the state of charge (SOC) of the battery assembly reaches 100%. The second charging step is characterized by charging the battery assembly in such a way that the Li saturation at the negative electrode surface is below 0.98 after 24 seconds. Figure 5 This is a schematic diagram illustrating the movement of lithium ions in the negative electrode of a battery assembly. Figure 5 In the figure, as shown in the legend, the Li saturation of the negative electrode active material layer 24a is represented by concentration. Additionally, in... Figure 5 In the diagram, dashed lines represent the division of the negative electrode active material layer 24a. Furthermore, regarding the "Li saturation of the negative electrode surface" in this specification, the negative electrode active material layer 24a is divided into 20 parts along the thickness t direction. At this time, the outermost layer (in other words, the region of the negative electrode active material layer 24a from the surface to t / 20 when viewed along the thickness direction of the negative electrode 24) is (refer to...) Figure 5 Let 24s be the negative electrode surface. Furthermore, Li saturation indicates the degree of charge in that region (here, the negative electrode surface 24s). For example, in the negative electrode surface 24s, a state where the negative electrode surface 24s is charged to 100% can be defined as "Li saturation of the negative electrode surface 24s is 1", and a state where it is not charged (0%) can be defined as "Li saturation of the negative electrode surface 24s is 0". That is, "Li saturation of the negative electrode surface 24s is 0.98 or less" means that the state of charge of the negative electrode surface 24s is 98% or less.

[0097] In the manufacturing of lithium-ion secondary batteries, from a production efficiency perspective, it is required to charge the battery assembly with a high current (high rate) and shorten the charging time. However, when the battery assembly's State of Charge (SOC) first reaches 100% (fully charged), the volume expansion of the negative electrode active material is prone to occur due to lithium ion adsorption. Therefore, during the initial full charge, under high-rate charging conditions, the volume expansion of the negative electrode active material can sometimes cause cracks and peeling. This results in the formation of new surfaces within the negative electrode active material. The SEI coating on these new surfaces is not fully formed and remains in a high-resistivity state. Consequently, lithium ion acceptability deteriorates, and lithium ions may not be completely adsorbed and may precipitate as lithium metal on the surface of the negative electrode active material layer.

[0098] To address this issue, the inventors focused on the formation of new surfaces in the negative electrode active material during the initial full charge. Therefore, the inventors discovered that when the Li saturation on the negative electrode surface exceeds 0.98 and the battery assembly is charged, new surfaces are formed in the negative electrode active material due to volume expansion. Furthermore, the inventors' dedicated research resulted in the manufacturing method disclosed herein.

[0099] In the second charging step S50, the battery assembly is charged such that the Li saturation of the negative electrode surface 24s is 0.98 or less (preferably 0.97 or less, more preferably 0.96 or less). By controlling the Li saturation of the negative electrode surface 24s within the above range, the volume expansion of the negative electrode active material can be appropriately controlled, and the formation of new surfaces on the negative electrode active material can be prevented. This suppresses the deposition of lithium metal on the surface of the negative electrode active material layer. On the other hand, while not limited to this, from the viewpoint of production efficiency, it is preferable to charge the battery assembly such that the Li saturation of the negative electrode surface 24s is 0.9 or more (more preferably 0.95 or more).

[0100] As an example of a method for charging such that the Li saturation at the negative electrode surface is 0.98 or less after 24 s, the following method (first embodiment) can be cited. Furthermore, the method of the second charging step S50 of this disclosure is not limited to the following method. Figure 6 This is a graph showing the shift in the SOC (50%~100%) of the battery assembly and the charging current value during the second charging step S50 of the first embodiment. Figure 6 The table shows the threshold value Ix (hereinafter also referred to as "threshold value Ix") of the current value when the Li saturation of the negative electrode surface is 0.98 after 24s, the current value Ic of the prior art, and the current value Ia of this embodiment.

[0101] like Figure 5 As shown, during battery assembly charging, lithium ions (Li) are received from the surface of the negative electrode active material layer 24a (the surface on the negative electrode surface 24s side). + The lithium ions are adsorbed and retained on the negative electrode surface for 24 seconds. The adsorbed lithium ions diffuse from the negative electrode surface 24 seconds into the interior of the negative electrode active material layer 24a. During high-rate charging, the adsorption rate of lithium ions sometimes exceeds the diffusion rate. Therefore, during high-rate charging, the Li saturation at the negative electrode surface 24 seconds is higher than the Li saturation in other regions of the negative electrode active material layer 24a. Furthermore, as the SOC of the battery assembly increases, the Li saturation at the negative electrode surface 24 seconds increases, while the diffusion rate of lithium ions cannot keep up with the adsorption rate. Therefore, as... Figure 6 As shown, in the second charging step S50, the threshold value Ix for the current value at which the Li saturation of the negative electrode surface 24s reaches 0.98 (SOC) becomes smaller, with the threshold value Ix as the dividing line. On the other hand, as with the current value Ic in the prior art, the current value Ic sometimes exceeds the threshold value Ix and the battery assembly is charged while maintaining the current value without considering the threshold value Ix. Therefore, when the Li saturation of the negative electrode surface 24s exceeds 0.98, a new surface will be generated in the negative electrode active material, and lithium metal may be deposited on the surface of the negative electrode active material layer 24a.

[0102] Therefore, in some preferred embodiments, such as Figure 6 As shown, as the SOC of the battery assembly increases (in other words, as the charging of the battery assembly progresses), the battery assembly can be charged while the current value Ia is gradually decreased. In other words, the battery assembly can be charged while the current value Ia is gradually decreased (controlled) so as not to exceed the threshold Ix of a current value where the Li saturation at the negative electrode surface is 0.98 after 24 seconds. Thus, the battery assembly can be charged in a manner that keeps the Li saturation at the negative electrode surface below 0.98 after 24 seconds. Furthermore, the current value Ia can decrease in stages, either in two or more stages or in three or more stages. On the other hand, the number of stages in which the current value Ia decreases can, for example, be 10 stages or less.

[0103] Furthermore, the threshold value Ix for the current value with a Li saturation of 0.98 can be determined, for example, through simulation. This simulation can be performed using the method described in Japanese Patent Application Publication No. 2021-125423. For example, design values ​​for the lithium-ion secondary battery 100 to be manufactured are input (e.g., the coating area of ​​the positive and negative electrode active material layers, the resistance of the positive and negative terminals, the exchange current density, the lithium-ion diffusion coefficient, etc.), and the lithium-ion balance and the positive and negative electrode potentials at Δt (t: time) are simulated. The threshold value Ix for the current value with a Li saturation of 0.98 can be determined by calculation based on this simulation.

[0104] While not limited to this, in the first embodiment, the battery assembly is charged in multiple stages (periods) based on a threshold Ix for a current value with a Li saturation of 0.98 calculated through the above simulation. For example, firstly, in the first stage, constant current charging (CC charging) is performed at a predetermined current value (predetermined current of the first stage) until the voltage of the battery assembly reaches a predetermined voltage (predetermined voltage of the first stage). When the battery assembly reaches the predetermined voltage, the process moves to the next stage, and the current value Ia is reduced to a predetermined current value (predetermined current of the second stage). Then, CC charging is performed at the predetermined current of the second stage until the voltage of the battery assembly reaches the predetermined voltage (predetermined voltage of the second stage). This operation is repeated in multiple stages (in... Figure 6 (There are six stages in total). Then, in the final stage (the sixth stage in this embodiment), after the CC charge reaches 98% of the SOC of the battery assembly, the battery assembly is charged with a constant voltage (CV charge) until a predetermined termination current is reached. In this way, charging can be performed until the SOC of the battery assembly reaches 100% (fully charged).

[0105] The current value (charging rate) at the start of the second charging step S50 can be higher, the same, or lower than the charging value at the start of the first charging step S20. Furthermore, while not limited to this, from the viewpoint of shortening charging time, it is preferable to charge the battery assembly at a higher current value (charging rate) before the threshold value Ix of the current value begins to decrease after the start of the second charging step S50. From this viewpoint, it is preferable to charge the battery assembly at a charging rate of 0.1C or higher (more preferably 0.5C or higher, and even more preferably 1C or higher) from the start of the second charging step S50 until the threshold value Ix of the current value begins to decrease. On the other hand, the upper limit of this charging rate is not particularly limited, but can be, for example, 2C or lower. Furthermore, "before the threshold value Ix of the current value begins to decrease" in this specification refers to the period from the start of the second charging step S50 until the Li saturation at the negative electrode surface first reaches 0.98 (preferably 0.97, more preferably 0.96) after 24 seconds.

[0106] (Second aging process S60)

[0107] In the second aging process S60, the battery assembly following the second charging process S50 undergoes further aging treatment. While not limited to this, in some preferred embodiments, the second aging process S60 is preferably performed. This further promotes the diffusion of lithium ions from the non-facing region 24n to the facing region 24o in the negative electrode 24. Additionally, when performing the short-circuit check process S70 described later, the accuracy of the short-circuit check can be further improved. Similar to the first aging process S30, the second aging process S60 can be implemented, for example, by placing the battery assembly in a constant temperature bath or similar environment set to maintain a predetermined temperature.

[0108] While not particularly limited, from the viewpoint of promoting lithium-ion diffusion, the aging temperature in the second aging process S60 is preferably 40°C or higher, more preferably 50°C or higher, and even more preferably 55°C or higher. On the other hand, from the viewpoint of suppressing degradation, the aging temperature in the second aging process S60 is preferably 85°C or lower, more preferably 80°C or lower, and even more preferably 75°C or lower.

[0109] Since the aging time in the second aging process S60 also depends on the aging temperature, the size of the battery, etc., it cannot be generalized. However, it can be about 6 hours to 72 hours, preferably 12 hours to 48 hours, and more preferably 18 hours to 24 hours.

[0110] (Short circuit inspection procedure S70)

[0111] In the short-circuit inspection step S70, a short-circuit inspection is performed on the battery assembly. While not limited to this, in some preferred embodiments, it is preferable to perform the short-circuit inspection step S70 on the battery assembly that has undergone the second charging step S50. This allows for the detection of minute short circuits in the battery assembly. Furthermore, "battery assembly that has undergone the second charging step S50" in this specification refers to a battery assembly that has undergone charging treatment based on the second charging step S50. Therefore, other steps can also be performed after the second charging step S50. In this embodiment, the short-circuit inspection step S70 is performed on the battery assembly that has undergone the second aging step S60 after the second charging step S50. The inspection method for the short-circuit inspection step S70 is not particularly limited, and conventionally known inspection methods can be used. For example, for the battery assembly, it can be allowed to self-discharge by being left for a certain period of time, thereby measuring the voltage drop. Then, a short circuit can be checked based on whether the voltage drop is below a reference value.

[0112] As described above, a lithium-ion secondary battery 100 can be appropriately manufactured. According to this manufacturing method, a lithium-ion secondary battery 100 in which lithium metal deposition is suppressed on the negative electrode surface can be provided.

[0113] The first embodiment of the manufacturing method disclosed herein has been described above. Furthermore, the technology disclosed herein is not limited to the above embodiment. For example, as another example of a method for charging a battery assembly in such a way that the Li saturation at the negative electrode surface for 24 s is 0.98 or less, the following embodiments can also be listed. Other embodiments of the technology disclosed herein will be described below.

[0114] <Second Implementation>

[0115] Figure 7 This is a diagram schematically illustrating the movement of lithium ions in the negative electrode of the battery assembly during the second charging step S50 of the second embodiment. Figure 7 In the figure, as shown in the legend, the Li saturation of the negative electrode active material layer 24a is represented by concentration. Additionally, in... Figure 7 In the diagram, dashed lines represent the division of the negative electrode active material layer 24a. Figure 8 This is a graph showing the shift in the SOC (50~100%) of the battery assembly and the charging current value during the second charging step S50 of the second embodiment. Figure 8 The text shows the threshold value Ix2 (hereinafter also referred to as "the threshold value Ix2 of the current value when the Li saturation is 0.98 under the condition of setting a charging pause period") and the shift of the current value Ia in the embodiment. Furthermore, in... Figure 8For reference, the shift of the threshold value Ix2´ for a Li saturation of 0.98 is also shown when no charging pause period is set. Furthermore, the manufacturing method of the second embodiment is the same as that of the above embodiment, except for the difference in the charging method of the second charging step S50. Therefore, the description of other steps is omitted below.

[0116] In the second embodiment, in the second charging step S50, at least one charging pause period is provided before the SOC of the battery assembly reaches 100%. In other words, in some preferred embodiments of the second charging step S50, it is preferable to provide at least one charging pause period before the SOC of the battery assembly reaches 100%. Furthermore, in this specification, "charging pause period" refers to a period during which neither charging nor discharging of the battery assembly is performed.

[0117] As described above, in the negative electrode active material layer 24a, during high-rate charging, the Li saturation of the negative electrode surface 24s is higher than that of other regions. Here, as... Figure 7 As shown, when a charging pause period is provided, lithium ion reception in the negative electrode 24 is paused (interrupted) during the charging pause period. On the other hand, lithium ion diffusion continues from the negative electrode surface 24s into the interior of the negative electrode active material layer 24a. Consequently, the Li saturation at the negative electrode surface 24s gradually decreases compared to before the charging pause. Therefore, as... Figure 8 As shown, the threshold value Ix2 of the current value when a charging pause period is set is higher than the threshold value Ix2´ of the current value when a charging pause period is not set (recovery). Therefore, compared with the case where no charging pause period is set, the current value Ia after the charging pause period can be set to be relatively higher. As a result, the time of the second charging step S50 can be shortened, and lithium-ion secondary batteries can be manufactured more efficiently.

[0118] Furthermore, the simulation method in the second embodiment can be the same as that in the first embodiment. Alternatively, in the second embodiment, the threshold value Ix2 of the current value when a charging rest period is set can be updated by performing the simulation again for each charging rest period. Alternatively, in the second embodiment, the following method can be used: performing simulation based on a pre-planned charging rest period (the timing, time, number of times the charging rest period is set, etc.), and calculating the threshold value Ix2 of the current value when a charging rest period is set.

[0119] When a charging pause period is provided in the second charging step S50, the number of charging pause periods can be either one or multiple (more than two). Although not particularly limited, from the viewpoint of allowing lithium ions to diffuse into the interior of the negative electrode active material layer 24a, the charging pause period is preferably one or more times, more preferably two or more times. On the other hand, the charging pause period is, for example, 10 times or less, preferably 8 times or less.

[0120] The duration of each charge pause is not particularly limited, but from the viewpoint of allowing lithium ions to diffuse from the surface of the negative electrode active material layer 24a into its interior, it can be at least 0.2 minutes, preferably 0.5 minutes or more, and more preferably 1 minute or more. On the other hand, from the viewpoint of manufacturing efficiency, the duration of each charge pause is preferably 10 minutes or less, and more preferably 5 minutes or less. Typically, as the SOC of the battery assembly increases, the concentration gradient of Li ions in the thickness direction of the negative electrode active material layer decreases. Therefore, as the SOC of the battery assembly increases, the rate of decrease in Li saturation at the negative electrode surface 24s accompanying the charge pause slows down. Therefore, it is also possible to set the duration of each charge pause to be longer as the SOC of the battery assembly increases.

[0121] As long as the current value does not exceed the threshold Ix2 of the current value under the condition of a charging pause period, the current value Ia after the charging pause can be either the same as before the charging pause or set to be lower than before the charging pause. As mentioned above, as the SOC of the battery assembly increases, the rate of decrease in Li saturation at the negative electrode surface for 24 seconds accompanying the charging pause slows down. Therefore, from the viewpoint of shortening the charging pause period, it is preferable to charge the battery assembly while gradually decreasing the charging current value Ia as the SOC of the battery assembly increases.

[0122] The lithium-ion secondary batteries obtained by the manufacturing method disclosed herein can be used for various applications, such as being suitable as a power source (drive power supply) for motors in vehicles such as cars and trucks. The type of vehicle is not particularly limited; examples include plug-in hybrid electric vehicles (PHEVs), hybrid electric vehicles (HEVs), and battery electric vehicles (BEVs). The lithium-ion secondary batteries obtained by the manufacturing method disclosed herein can also be suitable for the construction of battery packs.

[0123] [Experimental Example]

[0124] The following describes test examples related to the technology disclosed herein, but these test examples are not intended to limit the technology disclosed herein.

[0125] <Battery Assembly Preparation Process>

[0126] (Example)

[0127] Lithium nickel cobalt manganese composite oxide (NCM) as the positive electrode active material, polyvinylidene fluoride (PVDF) as the binder, and carbon nanotubes (CNTs) as the conductive material were weighed at a mass ratio of NCM:PVDF:CNT = 97.5:1.5:1 and mixed in N-methyl-2-pyrrolidone (NMP) to prepare the positive electrode slurry. This positive electrode slurry was coated onto both sides of a strip-shaped positive electrode current collector (aluminum foil) and dried. By cutting it to a specified size and rolling it using a rolling mill, a positive electrode sheet with positive electrode active material layers on both sides of the positive electrode current collector was obtained.

[0128] Natural graphite (C) as the negative electrode active material, styrene-butadiene rubber (SBR) as the binder, and carboxymethyl cellulose (CMC) as the thickener were weighed in a mass ratio of C:SBR:CMC = 98.5:1:0.5 and mixed in water to prepare the negative electrode slurry. This negative electrode slurry was coated onto both sides of a strip-shaped negative electrode current collector and allowed to dry. By cutting it to a specified size and rolling it using a roller press, a negative electrode sheet with negative electrode active material layers on both sides of the negative electrode current collector was obtained.

[0129] A three-layer separator composed of PE / PP / PE was prepared. Then, a wound electrode body was fabricated by overlapping and winding the positive electrode, separator, and negative electrode in that order. Furthermore, the positive and negative electrode sheets of the wound electrode body were wound 35 times (in layers).

[0130] After attaching the electrode terminals to the electrode body fabricated above, it is housed together with the non-aqueous electrolyte within a casing. This provides the battery assembly of the embodiment. Furthermore, as the non-aqueous electrolyte, a non-aqueous electrolyte containing LiPF6 as a supporting salt dissolved in a mixed solvent comprising ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) at a volume ratio of EC:DMC:EMC = 3:3:4 at a concentration of 1.15 mol / L was used. This also provides the battery assembly of the embodiment.

[0131] <First Charging Process>

[0132] For the battery assembly prepared in the battery assembly preparation step, CC charging was performed at 100 A (0.5 C) in a 25°C environment until the SOC of the battery assembly reached 80%. Here, the process from the first charging step to the second charging step described later was performed in a state where the casing of the battery assembly was restrained and a load was applied at a constant pressure of 6 kN. In addition, the restraint on the casing of the battery assembly was continued until the short circuit inspection step.

[0133] <First Aging Step>

[0134] The battery assembly after the first charging step was held in a 60°C environment for 6 hours.

[0135] <Internal Resistance Inspection Step>

[0136] The battery assembly after the first aging step was subjected to an internal resistance inspection. Specifically, discharge was performed for 10 seconds at a current value I=100 A (0.5 C) in a 25°C environment. Then, the internal resistance (IV resistance) was calculated by the following formula (I) based on the voltage drop amount ΔV (voltage difference) from 0.1 to 10 seconds and the discharge current value. As a result, the internal resistance of the battery assembly of the example was equal to or less than the reference value, so it was determined as acceptable and transferred to the second charging step.

[0137] Internal resistance = ΔV / 100A ··· (I)

[0138] <Second Charging Step>

[0139] In the battery assembly of the example, in a 25°C environment at Figure 6The battery assembly was charged at a current value as indicated by the current value Ia of the present embodiment. First, based on the design values of the battery assembly prepared above, the lithium ion balance and positive and negative electrode potentials at Δt (t: time) of the battery assembly were obtained by simulation. In addition, the above simulation was performed according to the method described in Japanese Unexamined Patent Publication No. 2021-125423. Based on the obtained values, the threshold Ix of the current value at which the Li saturation degree is 0.98 was calculated. Then, the battery assembly was charged while the current value Ia during charging was decreased stepwise so that the current value Ia during charging was equal to or less than the threshold Ix of the above current value. In this test example, the battery assembly was charged in six stages according to stages (periods) 1 to 6 shown in Table 1. Specifically, in periods 1 to 5, CC charging was performed at the current value Ia shown in Table 1 until the voltage of the battery assembly reached a predetermined voltage ("end voltage" in Table 1). Then, when the voltage of the battery assembly reached the predetermined voltage, the process proceeded to the next period. In period 6, after CC charging was performed at 50A ("current value Ia during charging" in Table 1) until the voltage of the battery assembly reached 4.12V ("end voltage" in Table 1), CV charging was performed until the current value Ia of the battery assembly reached 5A, whereby CCCV charging was performed. In this way, the battery assembly was charged such that the Li saturation degree on the negative electrode surface was 0.98 or less.

[0140] Table 1

[0141]

[0142] <Second Aging Step>

[0143] The battery assembly after the second charging step was held in an environment of 60°C for 6 hours.

[0144] <Short Circuit Inspection Step>

[0145] The battery assembly after the second aging step was subjected to a short circuit inspection. Specifically, the battery assembly after the second aging step was left to stand for 48 hours in an environment of 25°C to cause self-discharge. Then, the voltage drop amount was calculated from the voltage of the battery assembly before self-discharge and the voltage of the battery assembly after self-discharge. As a result, the voltage drop amount of the battery assembly of the example was equal to or less than the reference value, and thus was determined to be acceptable. In this way, the evaluation battery of the example was prepared.

[0146] (Comparative Example)

[0147] <Battery Assembly Preparation Step to Internal Resistance Inspection Step>

[0148] In the comparative example, the steps from the battery assembly preparation process to the internal resistance inspection process are the same as those in the example. That is, in the comparative example, a battery assembly having the same structure as that in the example was prepared, and the first charging process, the first aging process, and the internal resistance inspection process were performed under the same conditions as those in the example. In addition, as a result of the internal resistance inspection, the internal resistance of the battery assembly of the comparative example was equal to or less than the reference value, so it was determined as acceptable and transferred to the second charging process.

[0149] <Second Charging Process>

[0150] In the battery assembly of the comparative example, Figure 6 the battery assembly was charged at a current value as indicated by the prior art current value Ic. Specifically, CCCV charging was performed at 100 A (0.5 C) in an environment of 25°C until the SOC of the battery assembly reached 100%. The charging time was the same as that in the example.

[0151] <Second Aging Process and Short Circuit Inspection Process>

[0152] In the comparative example, the second aging process and the short circuit inspection process were performed on the battery assembly after the second charging process under the same conditions as those in the example. As a result of the short circuit inspection process, the voltage drop amount of the battery assembly of the comparative example was equal to or less than the reference value, so it was determined as acceptable. In this way, the evaluation battery of the comparative example was prepared.

[0153] <Evaluation Test>

[0154] The evaluation batteries of each example were discharged until the SOC reached 0%. After discharging, the evaluation batteries of each example were disassembled and the electrode bodies were taken out. Then, the taken-out electrode bodies were disassembled, and the presence or absence of lithium metal deposition on the surface of the negative electrode (negative electrode active material layer) was visually confirmed. Figure 9 is a photograph of the electrode body (negative electrode) showing the results of the lithium metal deposition evaluation of the example. Figure 10 is a photograph of the electrode body (negative electrode) showing the results of the lithium metal deposition evaluation of the comparative example. In addition, Figure 9 and Figure 10 are images obtained by photographing the surface of the negative electrode (negative electrode active material layer) at the 27th to 30th layers from the outermost periphery of the electrode body of each example.

[0155] As Figure 10 shown, in the evaluation battery of the comparative example, deposition of lithium metal was observed on the surface of the negative electrode (for example, at Figure 10 the upper end portion). On the other hand, in the evaluation battery of the example, for example, as shown in Figure 9As shown, no lithium metal deposition was found on the negative electrode surface in any layer. This can be attributed to the fact that, in the embodiments, the battery assembly was charged such that the Li saturation on the negative electrode surface was below 0.98, thereby suppressing the formation of new surfaces in the negative electrode active material.

[0156] The above description illustrates some embodiments of the technology disclosed herein, but these embodiments are merely examples. The technology disclosed herein can also be implemented in various other forms. The technology disclosed herein can be implemented based on the content disclosed in this specification and common technical knowledge in the field. The technology described in the claims includes technologies obtained by various modifications and alterations to the embodiments illustrated above. For example, a portion of the above embodiments can be replaced with other modifications, or other modifications can be added to the above embodiments. Furthermore, if a technical feature is not described as an essential technical feature, it can be appropriately omitted.

[0157] As described above, specific solutions to the technologies disclosed herein can be listed in the following items.

[0158] <Project 1>

[0159] A method for manufacturing a lithium-ion secondary battery, the lithium-ion secondary battery comprising an electrode body and a non-aqueous electrolyte, wherein the electrode body is formed by stacking a positive electrode and a negative electrode containing a negative electrode active material separated by a separator, wherein the method for manufacturing the lithium-ion secondary battery includes:

[0160] The battery assembly preparation process involves preparing a battery assembly in which the electrode body and the non-aqueous electrolyte are housed in a casing.

[0161] The first charging process involves charging the battery assembly.

[0162] The first aging process, following the first charging process, involves aging the battery assembly.

[0163] In the second charging process, the battery assembly that has undergone the first aging process is charged until the state of charge (SOC) of the battery assembly reaches 100%.

[0164] In the second charging process, the battery assembly is charged in such a way that the Li saturation on the negative electrode surface is below 0.98.

[0165] <Project 2>

[0166] According to the manufacturing method of the lithium-ion secondary battery described in Project 1, wherein,

[0167] In the second charging process, as the SOC of the battery assembly increases, the charging current value is reduced in stages while the battery assembly is being charged.

[0168] <Project 3>

[0169] According to the manufacturing method of lithium-ion secondary batteries described in Project 1 or 2, wherein,

[0170] In the second charging process, at least one charging pause is set before the SOC of the battery assembly reaches 100%.

[0171] <Project 4>

[0172] The method for manufacturing a lithium-ion secondary battery according to any one of items 1 to 3, wherein,

[0173] After the first aging process, the manufacturing method of the lithium-ion secondary battery further includes an internal resistance inspection process, in which the internal resistance of the battery assembly is inspected.

[0174] <Project 5>

[0175] The method for manufacturing a lithium-ion secondary battery according to any one of items 1 to 4, wherein,

[0176] The manufacturing method of the lithium-ion secondary battery further includes a second aging process, in which the battery assembly after the second charging process is further aged.

[0177] <Project 6>

[0178] The method for manufacturing a lithium-ion secondary battery according to any one of items 1 to 5, wherein,

[0179] The manufacturing method of the lithium-ion secondary battery further includes a short-circuit inspection step, in which the battery assembly that has undergone the second charging step is subjected to a short-circuit inspection.

[0180] <Project 7>

[0181] The method for manufacturing a lithium-ion secondary battery according to any one of items 1 to 6, wherein,

[0182] The first charging process is performed such that the SOC of the battery assembly is 15% or more and 80% or less.

[0183] <Project 8>

[0184] The method for manufacturing a lithium-ion secondary battery according to any one of items 1 to 7, wherein,

[0185] The ratio of the positive and negative electrode capacities (negative electrode capacity / positive electrode capacity) of the electrode body is greater than 1.0 and less than 1.6.

Claims

1. A method for manufacturing a lithium-ion secondary battery, wherein the lithium-ion secondary battery comprises an electrode body and a non-aqueous electrolyte, the electrode body being formed by stacking a positive electrode and a negative electrode containing a negative electrode active material separated by a separator, wherein... The method for manufacturing the lithium-ion secondary battery includes: The battery assembly preparation process involves preparing a battery assembly in which the electrode body and the non-aqueous electrolyte are housed in a casing. The first charging process involves charging the battery assembly. The first aging process, following the first charging process, involves aging the battery assembly. In the second charging process, the battery assembly that has undergone the first aging process is charged until the state of charge (SOC) of the battery assembly reaches 100%. In the second charging process, the battery assembly is charged in such a way that the Li saturation on the negative electrode surface is below 0.

98.

2. The method for manufacturing a lithium-ion secondary battery according to claim 1, wherein, In the second charging process, as the SOC of the battery assembly increases, the charging current value is reduced in stages while the battery assembly is being charged.

3. The method for manufacturing a lithium-ion secondary battery according to claim 1 or 2, wherein, In the second charging process, at least one charging pause is set before the SOC of the battery assembly reaches 100%.

4. The method for manufacturing a lithium-ion secondary battery according to claim 1 or 2, wherein, After the first aging process, the manufacturing method of the lithium-ion secondary battery further includes an internal resistance inspection process, in which the internal resistance of the battery assembly is inspected.

5. The method for manufacturing a lithium-ion secondary battery according to claim 1 or 2, wherein, The manufacturing method of the lithium-ion secondary battery further includes a second aging process, in which the battery assembly after the second charging process is further aged.

6. The method for manufacturing a lithium-ion secondary battery according to claim 1 or 2, wherein, The manufacturing method of the lithium-ion secondary battery further includes a short-circuit inspection step, in which the battery assembly that has undergone the second charging step is subjected to a short-circuit inspection.

7. The method for manufacturing a lithium-ion secondary battery according to claim 1 or 2, wherein, The first charging process is performed such that the SOC of the battery assembly is 15% or more and 80% or less.

8. The method for manufacturing a lithium-ion secondary battery according to claim 1 or 2, wherein, The ratio of the positive and negative electrode capacities of the electrode body, i.e., the negative electrode capacity / positive electrode capacity, is greater than 1.0 and less than 1.6.

Images