Lithium secondary battery

Abstract

The present disclosure provides a means for reducing irreversible capacity in a lithium precipitation type lithium secondary battery having a negative electrode interlayer. A lithium secondary battery is provided, which has a power generation element having a positive electrode, a negative electrode having a negative electrode current collector that precipitates lithium metal upon charging, a solid electrolyte layer containing a solid electrolyte interposed between the positive electrode and the negative electrode, and a negative electrode interlayer containing a lithium reactive material and a binder interposed between the negative electrode current collector and the solid electrolyte layer, the negative electrode interlayer having a weight per unit area exceeding 0.1 mg / cm 2 and less than 1.0 mg / cm 2 , the content of the binder in the negative electrode interlayer exceeding 10 mass %, the lithium reactive material containing amorphous carbon having a DBP absorption amount of 60 [mL / 100g] or more and 240 [mL / 100g] or less.

Description

Technical Field

[0001] This invention relates to lithium secondary batteries. Background Technology

[0002] In recent years, research and development of all-solid-state lithium secondary batteries using oxide-based and / or sulfide-based solid electrolytes has flourished. Solid electrolytes are primarily composed of ion conductors capable of ion conduction in a solid state. Therefore, in principle, all-solid-state lithium secondary batteries do not suffer from the various problems caused by flammable organic electrolytes found in conventional liquid lithium secondary batteries. Furthermore, the use of high-potential / high-capacity positive electrode materials and high-capacity negative electrode materials can significantly improve the battery's output density and energy density.

[0003] As a type of all-solid-state lithium secondary battery, lithium deposition type lithium secondary batteries, also known as lithium-deposited lithium secondary batteries, are known to deposit lithium metal on the negative electrode current collector during charging. While lithium-deposited all-solid-state lithium secondary batteries exhibit excellent energy density and output characteristics, they are prone to short circuits due to dendrites originating from the lithium metal layer. To suppress dendrite growth, a technique has been proposed to place a layer of negative electrode active material (negative electrode interlayer) between the solid electrolyte layer and the negative electrode current collector, which forms an alloy or compound with lithium.

[0004] For example, Japanese Patent Application Publication No. 2020-167146 (corresponding to U.S. Patent Application Publication No. 2020 / 0313164) discloses a technology relating to an all-solid-state secondary battery having a positive electrode active material layer, a solid electrolyte layer, and the aforementioned negative electrode active material layer (negative electrode intermediate layer) in sequence. According to this document, by containing at least 33% by mass of amorphous carbon having at least one of a specified nitrogen adsorption specific surface area and a specified DBP oil absorption capacity in the negative electrode active material layer (negative electrode intermediate layer), and by setting the initial charging capacity of the positive electrode active material layer as a (mAh) and the initial charging capacity of the negative electrode active material layer (negative electrode intermediate layer) as b (mAh), the capacity is controlled to be 0.01. Summary of the Invention

[0005] The problem the invention aims to solve

[0006] However, the inventors conducted research and confirmed that the all-solid-state secondary batteries described in the above-mentioned literature sometimes have large irreversible capacities.

[0007] Therefore, the objective of this invention is to provide a means for reducing irreversible capacity in a lithium-deposition type lithium secondary battery having a negative electrode intermediate layer.

[0008] ​Solution for solving the problem

[0009] In view of the above-mentioned problems, the inventors conducted in-depth research and found that by reducing the unit area weight of the negative electrode intermediate layer and making the negative electrode intermediate layer contain amorphous carbon with a specified DBP absorption amount and a specified amount of binder, the above-mentioned problems can be solved, thus completing the present invention.

[0010] Specifically, one aspect of the present invention relates to a lithium secondary battery comprising a power generation element having: a positive electrode; a negative electrode having a negative current collector that deposits lithium metal during charging; a solid electrolyte layer disposed between the positive electrode and the negative electrode, containing a solid electrolyte; and a negative electrode interlayer disposed between the negative current collector and the solid electrolyte layer, containing a lithium reactive material and a binder. Furthermore, the lithium secondary battery is characterized in that the area weight of the negative electrode interlayer exceeds 0.1 mg / cm². 2 And less than 1.0 mg / cm 2 The binder content in the negative electrode intermediate layer exceeds 10% by mass, the lithium reactive material contains amorphous carbon, and the DBP absorption of the amorphous carbon is above 60 [mL / 100g] and below 240 [mL / 100g]. Attached Figure Description

[0011] Figure 1 This is a cross-sectional view schematically illustrating the overall structure of a stacked (internal parallel connection type) all-solid-state lithium secondary battery (stacked secondary battery) according to one embodiment of the present invention. Detailed Implementation

[0012] One aspect of the present invention is a lithium secondary battery comprising a power generation element having: a positive electrode; a negative electrode having a negative current collector that deposits lithium metal during charging; a solid electrolyte layer disposed between the positive electrode and the negative electrode, containing a solid electrolyte; and a negative electrode interlayer disposed between the negative current collector and the solid electrolyte layer, containing a lithium reactive material and a binder, wherein the weight per unit area of ​​the negative electrode interlayer exceeds 0.1 mg / cm³. 2 And less than 1.0 mg / cm 2 The binder content in the negative electrode interlayer exceeds 10% by mass, and the lithium reactive material contains amorphous carbon, wherein the DBP absorption of the amorphous carbon is 60 mL / 100g or more and 240 mL / 100g or less. According to this method, irreversible capacity can be reduced in a lithium deposition-type lithium secondary battery having a negative electrode interlayer.

[0013] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Furthermore, in the description of the drawings, the same elements are labeled with the same reference numerals, and repeated descriptions are omitted. It should be noted that the scale of the drawings is exaggerated for ease of explanation and may sometimes differ from the actual scale.

[0014] Figure 1 This is a schematic cross-sectional view illustrating the overall structure of a stacked (internal parallel connection type) all-solid-state lithium secondary battery (hereinafter also simply referred to as "stacked secondary battery") according to one embodiment of the present invention. It should be noted that... Figure 1 This represents the cross-section of a stacked secondary battery during charging. Figure 1 The stacked secondary battery 10a shown has a structure in which a generally rectangular power generation element 21, which actually performs the charge and discharge reaction, is encapsulated inside a laminated film 29, which serves as the outer packaging of the battery. Here, the power generation element 21 has a configuration where a negative electrode, a solid electrolyte layer 17, and a positive electrode are stacked. The negative electrode has a configuration where a negative electrode current collector 11' and a negative electrode active material layer 13, consisting of lithium metal deposited on the surface of the negative electrode current collector 11', are stacked. Furthermore, a negative electrode intermediate layer 14 is disposed in contact with both the negative electrode active material layer 13 and the solid electrolyte layer 17. The positive electrode has a configuration where a positive electrode active material layer 15 is disposed on the surface of the positive electrode current collector 11''. Thus, the negative electrode current collector 11', the negative electrode active material layer 13, the negative electrode intermediate layer 14, the solid electrolyte layer 17, the positive electrode active material layer 15, and the positive electrode current collector 11'' constitute a single-cell layer 19. Therefore, Figure 1 The stacked secondary battery 10a shown can be described as having a configuration in which multiple single-cell layers 19 are electrically connected in parallel. It has the following structure: a negative electrode current collector 25 and a positive electrode current collector 27, respectively connected to the respective electrodes (negative and positive), are mounted on the negative electrode current collector 11' and the positive electrode current collector 11'', and are led out to the outside of the laminated film 29 by being clamped at the end of the laminated film 29. In the stacked secondary battery 10a, a constraint pressure (not shown) is applied in the stacking direction of the power generation element 21 by a pressurizing member. Therefore, the volume of the power generation element 21 remains constant.

[0015] The main components of the lithium secondary battery involved in this method are described below.

[0016] [Current Collector]

[0017] A current collector (negative electrode current collector, positive electrode current collector) has the function of dispersing electrons from the electrode active material layer (negative electrode active material layer, positive electrode active material layer). There are no particular limitations on the materials used to construct the current collector. For example, metals such as aluminum, nickel, iron, stainless steel, titanium, and copper, as well as conductive resins, can be used. There are no particular limitations on the thickness of the current collector; for example, it can be 10–100 μm.

[0018] [Negative electrode active material layer]

[0019] This type of lithium-ion secondary battery is a lithium deposition type secondary battery, where lithium metal is deposited on the negative electrode current collector during charging. The layer of lithium metal deposited on the negative electrode current collector during this charging process constitutes the negative electrode active material layer of this lithium-ion secondary battery. Therefore, the thickness of the negative electrode active material layer increases as charging progresses and decreases as discharging progresses. The negative electrode active material layer may not be present during complete discharge, but depending on the circumstances, a certain degree of lithium metal negative electrode active material layer may be present during complete discharge. Furthermore, the thickness of the negative electrode active material layer (lithium metal layer) during complete charging is not particularly limited and is typically 0.1~1000 μm.

[0020] [Negative Electrode Intermediate Layer]

[0021] The negative electrode interlayer is a layer located between the negative electrode current collector and the solid electrolyte layer, containing lithium reactive materials and binders. By setting such a negative electrode interlayer, the precipitation / growth of lithium dendrites can be suppressed.

[0022] In this type of lithium-ion secondary battery, the lithium reactive material must contain amorphous carbon, which is capable of storing lithium during charging. There are no particular limitations on the amorphous carbon; examples include carbon black (specifically, acetylene black, Ketjen black, furnace black, tank black, thermal lamp black, etc.), hard carbon, etc. Carbon black is preferred, and acetylene black, Ketjen black, furnace black, tank black, and thermal lamp black are more preferred. One of these materials may be used alone, or two or more may be used in combination.

[0023] The lithium-ion secondary battery of this method is characterized by a DBP absorption of amorphous carbon of 60 [mL / 100g] or more and 240 [mL / 100g] or less. Here, "DBP absorption" refers to the value expressed as the absorption of DBP (dibutyl phthalate) relative to 100g of carbon black (mL / 100g), serving as an indicator of the degree of development of the carbon material's structure (the connection between particles). The DBP absorption of amorphous carbon in this specification is the value determined according to JIS K 6217-4:2017 "Carbon black for rubber - basic properties - Part 4: method for determining oil absorption (including compression test specimens)". When using two or more materials as amorphous carbon, the weighted average value obtained by weighting the DBP absorption of each material by mass ratio is taken as the DBP absorption of the amorphous carbon. If the DBP absorption is less than 60 [mL / 100g], the amount of binder between the amorphous carbon structures increases, thus increasing the resistance, and consequently, potentially increasing the irreversible capacity. If the DBP absorption exceeds 240 mL / 100g, the binder is absorbed by the voids within the amorphous carbon aggregates, thereby reducing the amount of binder that holds the structures together and potentially decreasing the strength of the negative electrode interlayer. This can lead to cracks in the negative electrode interlayer, and consequently, an increase in irreversible capacity. From the viewpoint of further reducing irreversible capacity, the DBP absorption is preferably 100 mL / 100g or more and 240 mL / 100g or less, more preferably 100 mL / 100g or more and 230 mL / 100g or less, even more preferably 100 mL / 100g or more and 200 mL / 100g or less, and particularly preferably 100 mL / 100g or more and 150 mL / 100g or less. It should be noted that the manufacturing conditions of the carbon material can be changed to vary the DBP absorption value; for details, please refer to the technical knowledge in this field. In addition, the value of DBP absorption can also be varied by using two or more materials with different DBP absorption values ​​and adjusting the mixing amount of each material.

[0024] The content of amorphous carbon in the negative electrode intermediate layer (referring to the total content of two or more materials when they are used together) is not particularly limited. From the viewpoint of further reducing irreversible capacity, it is preferably 45% by mass or more and 70% by mass or less relative to the total mass of the negative electrode intermediate layer, more preferably 49% by mass or more and 65% by mass or less, and even more preferably 54% by mass or more and 60% by mass or less.

[0025] The lithium reactive material can be used alone or in combination with two or more. It is also preferred to use the aforementioned amorphous carbon and metallic materials (e.g., metals capable of alloying with lithium). That is, according to a preferred embodiment of the invention, the lithium reactive material comprises at least one of the aforementioned amorphous carbon and a metallic material selected from the group consisting of materials capable of alloying with lithium during charging. This ensures sufficient mechanical strength and / or lithium-ion conductivity of the negative electrode interlayer. More specifically, it is preferred to use amorphous carbon (especially carbon black) exhibiting the aforementioned specified DBP absorption amount and nanoparticles composed of In, Si, Sn, and Ag; more preferably, it is preferred to use amorphous carbon (especially carbon black) exhibiting the aforementioned specified DBP absorption amount and nanoparticles composed of Ag. The mixing ratio (mass ratio) of the aforementioned amorphous carbon and the lithium-alloyable metal is not particularly limited, but the mass ratio of amorphous carbon to lithium-alloyable metal is preferably 10:1 to 1:1, more preferably 5:1 to 2:1. There are no particular restrictions on the mixing ratio (volume ratio) of the amorphous carbon and the metal that can be alloyed with lithium as specified above. The volume ratio of amorphous carbon to the metal that can be alloyed with lithium is preferably 99:1 to 70:30, and more preferably 95:5 to 75:25.

[0026] When the amorphous carbon is in particulate form, its average particle size (average primary particle size) is, for example, 10 nm or more and 200 nm or less, preferably 15 nm or more and 150 nm or less, more preferably 20 nm or more and 100 nm or less, and even more preferably 25 nm or more and 70 nm or less. Similarly, when the metal is in particulate form, its average particle size is, for example, 10 nm or more and 500 nm or less, preferably 20 nm or more and 300 nm or less, more preferably 30 nm or more and 200 nm or less, and even more preferably 40 nm or more and 100 nm or less. If the average particle size of the amorphous carbon particles (preferably metal particles) is within the above range, it is easy to control the unit area weight of the negative electrode intermediate layer within a specified range. It should be noted that, in this specification, the average particle size of amorphous carbon and metal particles refers to the 50% cumulative diameter (D) of the particle size observed in several to dozens of fields of view when observing a cross-section of a layer containing particles using a scanning electron microscope (SEM), based on the number of particles. The diameter is the maximum distance between any two points on the observed particle outline. 50 ).

[0027] The content of lithium reactive material in the negative electrode intermediate layer (referring to the total content of two or more materials when they are used together) is not particularly limited. From the viewpoint of suppressing dendrite precipitation / growth and reducing irreversible capacity, it is preferably 66% by mass or more and 86% by mass or less relative to the total mass of the negative electrode intermediate layer, more preferably 68% by mass or more and 86% by mass or less, and even more preferably 70% by mass or more and 79% by mass or less.

[0028] In addition to the lithium reactive material, the negative electrode intermediate layer must also contain a binder. There are no particular restrictions on the type of binder; binders known in the art can be appropriately used. Examples include styrene-butadiene rubber (SBR), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF) (compounds containing hydrogen atoms replaced by other halogen elements), and carboxymethyl cellulose (CMC). Among these, styrene-butadiene rubber, PTFE, and PVDF are preferred, and PTFE and PVDF are more preferred. One type of binder can be used alone, or two or more can be used in combination.

[0029] The lithium-ion secondary battery of this method is characterized in that the binder content in the negative electrode interlayer exceeds 10% by mass relative to the total mass of the negative electrode interlayer. When the binder content is less than 10% by mass, the strength of the negative electrode interlayer decreases, resulting in cracks, etc., and as a result, the irreversible capacity may increase. The binder content in the negative electrode interlayer is preferably 14% by mass or more and 35% by mass or less, more preferably 20% by mass or more and 30% by mass or less. It should be noted that the binder content in the negative electrode interlayer can be 15% by mass or more (e.g., 15% by mass or more and 35% by mass or less, 15% by mass or more and 32% by mass or less, 15% by mass or more and 30% by mass or less, etc.). When the binder content is 14% by mass or more, there is more binder binding the amorphous carbon structures to each other, the gaps between the structures decrease, so lithium is difficult to be captured in the gaps, and the irreversible capacity is further reduced. On the other hand, when the binder content is below 35% by mass, the resistance increase caused by excessive binder dosage between amorphous carbon structures is suppressed, and as a result, the irreversible capacity is further reduced.

[0030] Furthermore, the lithium secondary battery of this method is characterized by having a unit area weight of more than 0.1 mg / cm³ for the negative electrode intermediate layer. 2 And less than 1.0 mg / cm 2 If the weight per unit area is 0.1 mg / cm² 2 Below this, it may be impossible to maintain the strength of the negative electrode intermediate layer. If the unit area weight is 1.0 mg / cm² 2The increased amount of amorphous carbon makes it possible for lithium to be trapped in the voids within the amorphous carbon, and for irreversible capacity increases caused by side reactions between lithium and oxygen-based functional groups (e.g., -COOH, -C=O) contained in the amorphous carbon. From the perspective of balancing improved strength of the negative electrode interlayer with reduced irreversible capacity, the preferred weight per unit area of ​​the negative electrode interlayer is 0.2 mg / cm³. 2 Above and below 1.0 mg / cm³ 2 More preferably 0.2 mg / cm³ 2 Above and 0.5 mg / cm 2 The following is a further preferred value: 0.2 mg / cm³ 2 Above and below 0.5 mg / cm 2 .

[0031] According to the inventors' research, in lithium deposition type lithium secondary batteries with a negative electrode interlayer, by keeping the unit area weight of the negative electrode interlayer within the aforementioned range, making the negative electrode interlayer contain amorphous carbon exhibiting the aforementioned DBP absorption, and further making the binder content in the negative electrode interlayer greater than 10% by mass, the irreversible capacity of the lithium secondary battery is significantly reduced compared to the prior art. The mechanism by which the lithium secondary battery of this method achieves the aforementioned effect is not entirely clear and is not limited to any theory, but the following mechanism is hypothesized. First, if the unit area weight of the negative electrode interlayer is too low (0.1 mg / cm²),... 2 If the negative electrode interlayer has excessive weight per unit area (1.0 mg / cm³), its strength will decrease, potentially leading to cracking and preventing it from fully realizing its intended function. Conversely, if the negative electrode interlayer has excessive weight per unit area (1.0 mg / cm³), it will also weaken. 2 The above will lead to an irreversible increase in capacity. This problem is believed to arise in lithium reactive materials where lithium is trapped in the voids of amorphous carbon, a material capable of storing lithium during charging, and is caused by side reactions between the oxygen-based functional groups (e.g., -COOH, -C=O) in the amorphous carbon and lithium. Based on this understanding, by achieving a weight per unit area of ​​less than 1.0 mg / cm²... 2 Based on this, the DBP absorption of amorphous carbon is controlled within the range of 60 [mL / 100g] to 240 [mL / 100g] and the binder content is made to be more than 10% by mass, which can fully maintain the strength of the negative electrode intermediate layer and achieve the reduction of irreversible capacity, thereby improving the energy density.

[0032] [Solid electrolyte layer]

[0033] A solid electrolyte layer lies between the positive and negative electrodes and contains a solid electrolyte (usually the main component). There are no particular limitations on the solid electrolyte contained in the solid electrolyte layer; any solid electrolyte known in this art can be appropriately used. Examples include LPS (Li₂S-P₂S₅), Li₆PS₅X (where X is Cl, Br, or I), and Li₇P₃S₅. 11 Li 3.2 P 0.96 Sulfide solid electrolytes such as S and Li3PS4. These sulfide solid electrolytes have excellent lithium-ion conductivity and low bulk modulus, thus they can follow the volume change of the electrode active material during charging and discharging, making them preferred. One type of these solid electrolytes can be used alone, or two or more can be used in combination. It should be noted that solid electrolytes other than those mentioned above can also be used.

[0034] The content of solid electrolyte in the solid electrolyte layer is preferably 50% by mass or more and 100% by mass or less, more preferably 90% by mass or more and 99% by mass or less.

[0035] In addition to the solid electrolyte, the solid electrolyte layer may further contain a binder. The binder that can be used in the solid electrolyte layer is the same as the binder described in the negative electrode intermediate layer above.

[0036] The thickness of the solid electrolyte layer varies depending on the composition of the target lithium secondary battery, and is typically 0.1 μm or more and 1000 μm or less, preferably 10 μm or more and 40 μm or less.

[0037] [Positive electrode active material layer]

[0038] The positive electrode active material layer must contain positive electrode active material, and may include solid electrolyte, binder, and / or conductive additives as needed. A typical positive electrode active material layer looks like this... Figure 1 As shown, it is disposed on the surface of the positive current collector. However, if the positive active material layer 15 itself has sufficient conductivity, the positive current collector may not be used and the positive active material layer itself may constitute the positive electrode.

[0039] There are no particular limitations on the type of positive electrode active material contained in the positive electrode active material layer, but lithium-containing metal oxides are preferred. That is, according to a preferred embodiment of the present invention, the positive electrode active material contains at least one type selected from lithium-containing metal oxides. According to a more preferred embodiment of the present invention, the positive electrode active material is composed of only at least one type selected from lithium-containing metal oxides. Specific examples of lithium-containing metal oxides include layered rock salt type active materials such as LiCoO2, LiMnO2, LiNiO2, and Li(Ni-Mn-Co)O2, as well as LiMn2O4 and LiNi... 0.5 Mn1.5 Spinel-type active materials such as O4, olivine-type active materials such as LiFePO4 and LiMnPO4, and Si-containing active materials such as Li2FeSiO4 and Li2MnSiO4 are also examples. In addition, as oxide active materials other than those mentioned above, Li4Ti5O4 is another example. 12 LiVO2. Preferably, Li(Ni-Mn-Co)O2 and substances in which a portion of these transition metals are replaced by other elements (NMC composite oxides) are used as positive electrode active materials. These positive electrode active materials can be used individually or in combination of two or more.

[0040] The shapes of positive electrode active materials can include, for example, particulate (spherical, fibrous) and thin film forms. When the positive electrode active material is in particulate form, its average particle size (D...) 50 For example, the particle size is preferably in the range of 1 nm to 100 μm, more preferably in the range of 10 nm to 50 μm, even more preferably in the range of 100 nm to 20 μm, and particularly preferably in the range of 1 to 20 μm. It should be noted that, in this specification, the average particle size (D) of the positive electrode active material... 50 The value of can be determined by laser diffraction scattering.

[0041] The content of the positive electrode active material is not particularly limited. From the viewpoint of energy density, relative to the total mass of the positive electrode active material layer, it is, for example, 50% by mass or more and 99% by mass or less, preferably 70% by mass or more and 99% by mass or less, and more preferably 80% by mass or more and 99% by mass or less.

[0042] In addition to the positive electrode active material, the positive electrode active material layer may further contain a solid electrolyte, a binder, and / or a conductive additive. Here, the solid electrolyte that can be used in the positive electrode active material layer is the same as the solid electrolyte described in the solid electrolyte layer above. The binder that can be used in the positive electrode active material layer is the same as the binder described in the negative electrode intermediate layer above. Examples of conductive additives include metals such as aluminum, stainless steel (SUS), silver, gold, copper, and titanium, alloys or metal oxides containing these metals; carbon fibers (specifically, vapor-grown carbon fibers (VGCF), polyacrylonitrile-based carbon fibers, pitch-based carbon fibers, rayon-based carbon fibers, activated carbon fibers, etc.), carbon nanotubes (CNTs), and carbon black (specifically, acetylene black, Ketjen black (registered trademark), furnace black, channel black, thermal lamp black, etc.), but are not limited to these. Furthermore, materials formed by coating the aforementioned metal materials around particulate ceramic materials or resin materials through plating or the like can also be used as conductive additives.

[0043] The thickness of the positive electrode active material layer varies depending on the composition of the target lithium secondary battery, for example, it is 0.1 μm or more and 1000 μm or less, preferably 30 μm or more and 300 μm or less, more preferably 50 μm or more and 200 μm or less, and even more preferably 70 μm or more and 150 μm or less.

[0044] [Positive current collector and negative current collector]

[0045] The materials used to construct the current collectors (25, 27) are not particularly limited, and well-known highly conductive materials conventionally used in current collectors for secondary batteries can be used. For example, aluminum, copper, titanium, nickel, stainless steel (SUS), and their alloys are preferred materials for the current collectors. From the viewpoints of lightweight, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. It should be noted that the same material or different materials can be used in the positive current collector 27 and the negative current collector 25.

[0046] [Positive and negative leads]

[0047] Furthermore, although the illustration is omitted, the current collectors (11'', 11') and the current collector plates (27, 25) can be electrically connected via positive and negative leads. The materials used in lithium-ion batteries can also be used as the constituent materials for the positive and negative leads. It should be noted that the portion removed from the outer packaging is preferably covered with heat-resistant and insulating heat-shrink tubing to prevent contact with surrounding equipment or wiring, which could lead to leakage and damage to the product (e.g., automotive parts, especially electronic devices).

[0048] [Battery outer packaging material]

[0049] As a battery outer packaging material, in addition to the well-known metal can shell, other materials such as... Figure 1 The bag-shaped housing, as shown, covers the power generation element and incorporates an aluminum-containing laminated film 29. This laminated film can be, for example, a three-layer structure consisting of sequentially stacked PP, aluminum, and nylon, but is not limited to these. From the viewpoint of high output, excellent cooling performance, and suitability for large-scale battery applications in EVs and HEVs, the laminated film is preferred. Furthermore, from the perspective of easily adjusting the packing voltage applied externally to the power generation element, an aluminum-containing laminated film is more preferable as the outer packaging material.

[0050] The lithium-ion secondary battery described in this method has a structure consisting of multiple single-cell layers connected in parallel, resulting in high capacity and excellent cycle durability. Therefore, the lithium-ion secondary battery of this method is suitable for use as a power source for EVs and HEVs.

[0051] The above describes one embodiment of the lithium secondary battery of the present invention. However, the present invention is not limited to the configuration described in the foregoing embodiment and can be appropriately modified based on the claims.

[0052] For example, as a type of lithium secondary battery to which the present invention is applied, a bipolar type battery can also be cited, which includes a bipolar electrode having a positive electrode active material layer that is electrically bonded to one side of a current collector and a negative electrode active material layer that is electrically bonded to the opposite side of the current collector.

[0053] Furthermore, the lithium secondary battery involved in this method does not necessarily have to be all-solid-state. That is, the solid electrolyte layer may further contain a conventionally known liquid electrolyte. There is no particular limitation on the amount of liquid electrolyte that the solid electrolyte layer may contain, but it is preferable to have an amount that maintains the shape of the solid electrolyte layer formed by the solid electrolyte and prevents leakage of the liquid electrolyte.

[0054] It should be noted that the following embodiments are also included within the scope of the present invention: the lithium secondary battery of claim 1 having the features of claim 2; the lithium secondary battery of claim 1 or 2 having the features of claim 3; the lithium secondary battery of any one of claims 1 to 3 having the features of claim 4; the lithium secondary battery of any one of claims 1 to 4 having the features of claim 5; the lithium secondary battery of any one of claims 1 to 5 having the features of claim 6; the lithium secondary battery of any one of claims 1 to 6 having the features of claim 7; and the lithium secondary battery of any one of claims 1 to 7 having the features of claim 8.

[0055] Example

[0056] The present invention will be further described in detail below through examples. However, the technical scope of the present invention is not limited to the following examples. It should be noted that the following operations are carried out in a glove box with a dew point below -68°C. In addition, the utensils and devices used in the glove box are thoroughly dried beforehand.

[0057] <Example of battery cell fabrication for evaluation>

[0058] [Example 1]

[0059] (The production of the positive electrode)

[0060] The NMC composite oxide (LiNi) used as the positive electrode active material was weighed in a mass ratio of 85:15:5. 0.8 Mn 0.1 Co 0.1O2), carbon fiber as a conductive additive, and silver sulfide-germanium sulfide solid electrolyte (Li6PS5Cl) as a solid electrolyte were mixed using an agate mortar and then further stirred using a planetary ball mill. 2 parts by mass of polytetrafluoroethylene (PTFE) as a binder were added to 100 parts by mass of the resulting mixed powder and mixed. The resulting mixture was overlapped with aluminum foil as the positive electrode current collector and pressed to obtain a positive electrode with a positive electrode active material layer (100 μm thick) on the surface of the positive electrode current collector.

[0061] (Fabrication of the solid electrolyte layer)

[0062] Compared to silver-germanium sulfide solid electrolytes (Li6PS5Cl) as solid electrolytes, the average particle size (D) 50 A solid electrolyte slurry was prepared by mixing 100 parts by weight of styrene-butadiene rubber (SBR) as a binder and 2 parts by weight of mesitylene as a solvent. The solid electrolyte slurry was then coated onto the surface of a stainless steel foil used as a support and dried to obtain a solid electrolyte layer (40 μm thick).

[0063] (Fabrication of the negative electrode intermediate layer)

[0064] Weigh silver nanoparticles (average particle size (D) 50 19 parts by mass of 60nm) and 57 parts by mass of carbon black (DBP absorbance: 175 [mL / 100g]) (Ag:C = 1:3 (mass ratio)) were mixed. 76 parts by mass of the resulting mixture were then mixed with 24 parts by mass of polyvinylidene fluoride (PVDF) as a binder and N-methyl-2-pyrrolidone (NMP) as a solvent to prepare the negative electrode interlayer slurry. The negative electrode interlayer slurry was coated onto the surface of a stainless steel foil serving as the negative electrode current collector and dried to obtain the negative electrode interlayer (0.3 mg / cm²). 2 ).

[0065] <Evaluation of the fabrication of battery cells>

[0066] A positive electrode active material layer formed on the surface of an aluminum foil (positive current collector) and a solid electrolyte layer formed on the surface of a stainless steel foil are overlapped with the exposed surfaces of the positive electrode active material layer and the solid electrolyte layer facing each other. This is then pressed at 700 MPa for 1 minute using cold isostatic pressing (CIP) (first pressing step). This transfers the solid electrolyte layer to the exposed surface of the positive electrode active material layer. After peeling off the stainless steel foil adjacent to the solid electrolyte layer, the solid electrolyte layer is overlapped with a negative electrode intermediate layer formed on the surface of the stainless steel foil (negative current collector) with the exposed surfaces of the solid electrolyte layer and the negative electrode intermediate layer facing each other. This is then pressed at 500 MPa for 1 minute using cold isostatic pressing (CIP) (second pressing step). This transfers the negative electrode intermediate layer to the exposed surface of the solid electrolyte layer. Finally, aluminum positive electrode tabs and nickel negative electrode tabs are joined to aluminum foil (positive current collector) and stainless steel foil (negative current collector) respectively using an ultrasonic welding machine. The resulting laminate is placed inside an aluminum laminate and vacuum sealed, thereby obtaining the lithium deposition type all-solid-state lithium secondary battery of this embodiment, i.e., the battery cell for evaluation.

[0067] [Example 2]

[0068] Perform the above (fabrication of the negative electrode intermediate layer) using the following method.

[0069] Weigh silver nanoparticles (average particle size (D) 50 21.5 parts by weight of 60nm) and 64.5 parts by weight of carbon black (DBP absorbance: 175 [mL / 100g]) (Ag:C = 1:3 (mass ratio)) were mixed. 86 parts by weight of the resulting mixture were added to 14 parts by weight of polyvinylidene fluoride (PVDF) as a binder, and N-methyl-2-pyrrolidone (NMP) as a solvent, and mixed to prepare the negative electrode interlayer slurry. The negative electrode interlayer slurry was coated onto the surface of a stainless steel foil serving as the negative electrode current collector and dried to obtain the negative electrode interlayer (0.5 mg / cm²). 2 );

[0070] In addition, the evaluation battery cell of this embodiment was obtained by the same method as in Example 1.

[0071] [Example 3]

[0072] In the above (fabrication of the negative electrode intermediate layer), the unit area weight of the negative electrode intermediate layer is changed to 0.5 mg / cm³. 2 In addition, the evaluation battery cell of this embodiment is obtained by the same method as in Example 1.

[0073] [Example 4]

[0074] In the above (fabrication of the negative electrode intermediate layer), the silver nanoparticles (average particle size (D)) were weighed.50 16.5 parts by mass of 60nm) and 49.5 parts by mass of carbon black (DBP absorption: 175 [mL / 100g]) (Ag:C = 1:3 (mass ratio)) were mixed; 66 parts by mass of the resulting mixture were added with 34 parts by mass of polyvinylidene fluoride (PVDF) as a binder, and N-methyl-2-pyrrolidone (NMP) was added as a solvent and mixed to prepare a negative electrode intermediate layer slurry; otherwise, the evaluation battery cell of this embodiment was obtained by the same method as in Example 2.

[0075] [Example 5]

[0076] In the above (preparation of the negative electrode intermediate layer), as carbon black, 50 parts by mass of carbon black with a DBP absorption of 175 [mL / 100g] and 50 parts by mass of carbon black with a DBP absorption of 29 [mL / 100g] were used (the DBP absorption of this mixture is 102 [mL / 100g]). Otherwise, the evaluation battery cell of this embodiment was obtained by the same method as in Example 2.

[0077] [Example 6]

[0078] In the above (preparation of the negative electrode intermediate layer), as carbon black, a mixture of 35 parts by mass of carbon black with a DBP absorption of 175 [mL / 100g] and 65 parts by mass of carbon black with a DBP absorption of 29 [mL / 100g] (the DBP absorption of this mixture is 80 [mL / 100g]) was used. Otherwise, the evaluation battery cell of this embodiment was obtained by the same method as in Example 2.

[0079] [Comparative Example 1]

[0080] In the above (fabrication of the negative electrode intermediate layer), the unit area weight of the negative electrode intermediate layer is changed to 0.1 mg / cm³. 2 In addition, the negative electrode intermediate layer could not be fabricated by using the same method as in Example 1.

[0081] [Comparative Example 2]

[0082] In the above (fabrication of the negative electrode intermediate layer), the unit area weight of the negative electrode intermediate layer is changed to 1.0 mg / cm³. 2 In addition, the evaluation battery cell of this comparative example was obtained by the same method as in Example 2.

[0083] [Comparative Example 3]

[0084] In the above (fabrication of the negative electrode intermediate layer), the unit area weight of the negative electrode intermediate layer is changed to 1.0 mg / cm³. 2In addition, the evaluation battery cell of this comparative example was obtained by the same method as in Example 1.

[0085] [Comparative Example 4]

[0086] In the above (preparation of the negative electrode intermediate layer), carbon black with a DBP absorption of 247 [mL / 100g] was used as the carbon black. Otherwise, the evaluation battery cell of this comparative example was obtained by the same method as in Example 3.

[0087] [Comparative Example 5]

[0088] In the above (preparation of the negative electrode intermediate layer), carbon black with a DBP absorption of 29 [mL / 100g] was used as the carbon black. Otherwise, the evaluation battery cell of this comparative example was obtained by the same method as in Example 2.

[0089] <Charge and Discharge Test>

[0090] Positive and negative leads were connected to the positive and negative tabs of the evaluation battery cell (before initial charging), respectively. A constraint pressure of 3 MPa was applied in the stacking direction of the evaluation battery cell using a pressure-applying component, and the cell was charged and discharged according to the following charge and discharge test conditions.

[0091] (Charge and discharge test conditions)

[0092] Evaluation temperature: 333K (60℃)

[0093] Voltage range: 2.5~4.3V

[0094] Charging process (1): CC (25 hours until completion)

[0095] Charging rate (1): 0.01C

[0096] Charging process (2): CC

[0097] Charging rate (2): 0.05C

[0098] Discharge process: CC

[0099] Discharge rate: 0.1C

[0100] After charging and discharging, pause for 30 minutes each time.

[0101] The battery cells were evaluated using a charge-discharge tester. In a constant temperature bath set to the evaluation temperature described above, during the charging process (1) (lithium metal deposition onto the negative electrode current collector), the cells were charged at 0.01C for 25 hours in constant current (CC) mode. Then, during the charging process (2), the cells were also charged at constant current (CC) mode to 4.3V. Then, during the discharging process (dissolution of lithium metal on the negative electrode current collector), the cells were discharged at 0.1C to 2.5V in constant current (CC) mode. Here, 1C refers to the current value at which the battery is fully charged (100% charged) after charging for 1 hour. The initial coulombic efficiency ((discharge capacity / charge capacity) × 100) was calculated from the capacity obtained through the above charging and discharging. The higher the value of the initial coulombic efficiency, the further the irreversible capacity decreases. The results are shown in Table 1 below.

[0102] [Table 1]

[0103] Table 1

[0104]

[0105] As shown in Table 1, according to the present invention, by reducing the unit area weight of the negative electrode interlayer, and making the negative electrode interlayer contain amorphous carbon with a specified DBP absorption and a specified amount of binder, the initial coulombic efficiency is improved (irreversible capacity is reduced) in a lithium deposition type lithium secondary battery having a negative electrode interlayer.

[0106] Comparing Examples 2, 5, and 6, it is evident that the initial coulombic efficiency is further improved when the DBP absorption is in the range of 100 mL / 100g or more and 150 mL / 100g or less. Comparing Examples 2-4, it is evident that the initial coulombic efficiency is further improved when the binder dosage is in the range of 20% by mass or more and 30% by mass.

[0107] This application is based on Japanese Patent Application No. 2023-220340, filed on December 27, 2023, the disclosure of which is incorporated herein by reference in its entirety.

[0108] Explanation of reference numerals in the attached figures

[0109] 10A stacked secondary battery

[0110] 11' negative current collector,

[0111] 11'' Positive current collector,

[0112] 13 negative electrode active material layer,

[0113] 14 Negative electrode intermediate layer

[0114] 15 positive electrode active material layers

[0115] 17. Solid electrolyte layer

[0116] 19 single-cell layers

[0117] 21 power generation components

[0118] 25 negative current collector,

[0119] 27 Positive current collector,

[0120] 29-layer lamination.

Claims

1. A lithium secondary battery comprising a power generation element, said power generation element having: positive electrode; The negative electrode has a negative current collector, which deposits lithium metal during charging; A solid electrolyte layer, situated between the positive and negative electrodes, contains a solid electrolyte; and The negative electrode intermediate layer, located between the negative electrode current collector and the solid electrolyte layer, contains lithium reactive materials and a binder. The weight per unit area of ​​the negative electrode intermediate layer exceeds 0.1 mg / cm³. 2 And less than 1.0 mg / cm 2 , The binder content in the negative electrode intermediate layer exceeds 10% by mass. The lithium reactive material contains amorphous carbon, and the DBP absorption of the amorphous carbon is above 60 [mL / 100g] and below 240 [mL / 100g].

2. The lithium secondary battery according to claim 1, wherein, The content of the binder in the negative electrode intermediate layer is more than 20% by mass and less than 30% by mass.

3. The lithium secondary battery according to claim 1 or 2, wherein, The amorphous carbon has a DBP absorption capacity of 100 mL / 100g or more and 240 mL / 100g or less.

4. The lithium secondary battery according to claim 1 or 2, wherein, The amorphous carbon has a DBP absorption capacity of 100 mL / 100g or more and 150 mL / 100g or less.

5. The lithium secondary battery according to claim 1 or 2, wherein, The weight per unit area of ​​the negative electrode intermediate layer is 0.2 mg / cm³. 2 Above and below 1.0 mg / cm³ 2 .

6. The lithium secondary battery according to claim 1 or 2, wherein, The lithium reactive material contains a metallic material that can alloy with lithium during charging.

7. The lithium secondary battery according to claim 1 or 2, wherein, The content of amorphous carbon in the negative electrode intermediate layer is more than 45% by mass and less than 70% by mass.

8. The lithium secondary battery according to claim 1 or 2, wherein, The adhesive comprises at least one selected from styrene-butadiene rubber, polytetrafluoroethylene, and polyvinylidene fluoride.

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