Lithium-ion battery

Abstract

Provided is a lithium ion battery with a positive electrode having a positive electrode mixture layer that contains a positive electrode active material, and a negative electrode having a negative electrode mixture layer that contains a negative electrode active material, the lithium ion battery being charged and discharged by the movement of lithium ions between the positive electrode and the negative electrode. The negative electrode mixture layer contains a negative electrode active material represented by general formula M3Me2X7 (in the formula, M includes at least one of La and Ca; Me includes at least one of Mn, Ni, Fe, and Co; and X includes at least one of Ge, Si, Sn, and Al), and a binding agent that contains cyano groups, the ratio of the binding agent in the negative electrode mixture layer being 0.5-7.0 mass% (inclusive).

Description

Technical Field

[0001] The present disclosure relates to a lithium-ion battery including a positive electrode having a positive electrode mixture layer containing a positive electrode active material, a negative electrode having a negative electrode mixture layer containing a negative electrode active material, and a lithium-ion battery in which charge and discharge are performed by the movement of lithium ions between the positive electrode and the negative electrode.

Background Art

[0002] Lithium-ion (Li-ion) batteries in which charge and discharge are performed by the movement of lithium ions between a negative electrode and a positive electrode are widely used. As the negative electrode active material in the negative electrode mixture layer of this lithium-ion battery, many graphite-based materials are used. When a graphite-based negative electrode active material is used together with Si, the volume change during charge and discharge is large, the capacity retention characteristics are likely to deteriorate, and the cost is relatively high.

[0003] Therefore, non-graphite negative electrode active materials have also been proposed. For example, Patent Document 1 describes the use of an alloy having a La3Co2Sn7-type crystal structure as a negative electrode active material.

[0004] In addition, a binder is used in the negative electrode mixture layer to suppress the occurrence of peeling and cracks. However, when the amount of this binder increases, the efficiency of the battery reaction of the negative electrode active material decreases, so there is also a demand to reduce the amount of the binder. Patent Document 2 describes that the content of the binder is 0.5% by mass or more and 5.0% by mass.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

[0006] In Patent Document 1, polyvinylidene fluoride (PVDF) is used as a binder. However, as a result of experiments, it was found that when La3Ni2Sn7 is used as an active material and PVDF is used as a binder, the mixture slurry used for forming the negative electrode mixture layer gels due to the reaction between the two, making coating difficult. In order to reduce the reactivity between La3Ni2Sn7 and PVDF and enable coating, it is necessary to increase the particle size of the negative electrode active material. However, when the particle size of the negative electrode active material is increased, the reactivity between the negative electrode active material and Li decreases, and the capacity tends to decrease.

[0007] The lithium-ion battery according to the present disclosure is a lithium-ion battery having a positive electrode having a positive electrode mixture layer containing a positive electrode active material, a negative electrode having a negative electrode mixture layer containing a negative electrode active material, and lithium ions moving between the positive electrode and the negative electrode to perform charge and discharge. The negative electrode mixture includes a negative electrode active material represented by the general formula M3Me2X7 (where M includes at least one of La and Ca, Me includes at least one of Mn, Ni, Fe, and Co, and X includes at least one of Ge, Si, Sn, and Al), and a binder containing a cyano group, and the ratio of the binder in the negative electrode mixture layer is 0.5% by mass or more and 7.0% by mass or less.

[0008] In the present disclosure, a negative electrode active material represented by the general formula M3Me2X7 and a binder containing a cyano group are used as the negative electrode active material, and the addition amount of the binder is 0.5% by mass or more and 7.0% by mass or less. As a result, coating of the negative electrode mixture layer becomes possible, and the decrease in capacity can be suppressed by using a relatively small addition amount of the binder.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Best Mode for Carrying Out the Invention

[0010] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. Note that the present disclosure is not limited to the embodiments described herein.

[0011] "Regarding the negative electrode material" As the negative electrode material of a lithium-ion battery, a material that satisfies high energy density and low expansion is preferable. Therefore, various research and developments have been carried out, and it has been proposed to use an intermetallic compound represented by M3Me2X7 (M = La, Ca, Me = Mn, Ni, Fe, Co, X = Ge, Si, Sn, Al) such as La3Ni2Sn7 as the negative electrode active material. Since such an intermetallic compound performs lithium absorption and release by an intercalation reaction, it has a low expansion rate and is considered to be able to achieve a long life.

[0012] However, it has been found that further improvement is required when actually using this material. First, as described above, when PVDF is used as the binder, the negative electrode mixture slurry gels, making it difficult to coat the negative electrode mixture layer. In addition, if the gelation is suppressed by increasing the particle size of the binder, the battery reaction may be inhibited.

[0013] In the present disclosure, by using a binder containing a cyano group, for example, polyacrylonitrile (PAN), the gelation of the negative electrode mixture slurry is suppressed. In this case, the addition amount of the binder in the negative electrode mixture layer can be 2.0% by mass or more and 5.0% by mass or less.

[0014] In this way, by using a negative electrode active material having an M3Me2X7 type crystal structure and a binder containing a cyano group, it is possible to coat the negative electrode mixture layer and obtain a high energy density battery.

[0015] "Configuration of the Embodiment" FIG. 1 is a longitudinal sectional view of a cylindrical secondary battery 10 which is an example of an embodiment. In the secondary battery 10 shown in FIG. 1, an electrode body 14 and a non-aqueous electrolyte are housed in an exterior body 15. The electrode body 14 has a wound structure in which a positive electrode 11 and a negative electrode 12 are wound with a separator 13 interposed therebetween. As the non-aqueous solvent (organic solvent) of the non-aqueous electrolyte, carbonates, lactones, ethers, ketones, esters, etc. can be used, and these solvents can be used as a mixture of two or more kinds. When using a mixture of two or more solvents, it is preferable to use a mixed solvent containing a cyclic carbonate and a chain carbonate. For example, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc. can be used as the cyclic carbonate, and dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), etc. can be used as the chain carbonate. As the electrolyte salt of the non-aqueous electrolyte, LiPF6, LiBF4, LiCF3SO3, etc. and mixtures thereof can be used. The dissolution amount of the electrolyte salt with respect to the non-aqueous solvent can be, for example, 0.5 to 2.0 mol / L. Hereinafter, for convenience of explanation, the side of the sealing body 16 will be referred to as "upper", and the bottom side of the exterior body 15 will be referred to as "lower".

[0016] The opening end of the exterior body 15 is closed by the sealing body 16, so that the inside of the secondary battery 10 is sealed. Insulating plates 17 and 18 are respectively provided above and below the electrode body 14. The positive electrode lead 19 extends upward through the through hole of the insulating plate 17 and is welded to the lower surface of the filter 22 which is the bottom plate of the sealing body 16. In the secondary battery 10, the cap 26 which is the top plate of the sealing body 16 electrically connected to the filter 22 serves as the positive electrode terminal. On the other hand, the negative electrode lead 20 extends through the through hole of the insulating plate 18 to the bottom side of the exterior body 15 and is welded to the inner surface of the bottom of the exterior body 15. In the secondary battery 10, the exterior body 15 serves as the negative electrode terminal. When the negative electrode lead 20 is installed at the end portion, the negative electrode lead 20 extends through the outside of the insulating plate 18 to the bottom side of the exterior body 15 and is welded to the inner surface of the bottom of the exterior body 15.

[0017] The exterior body 15 is, for example, a metal exterior can having a bottomed cylindrical shape. A gasket 27 is provided between the exterior body 15 and the sealing body 16 to ensure the hermeticity inside the secondary battery 10. The exterior body 15 has, for example, a groove portion 21 that supports the sealing body 16 and is formed by pressing the side surface from the outside. The groove portion 21 is preferably formed in an annular shape along the circumferential direction of the exterior body 15, and supports the sealing body 16 via the gasket 27 on its upper surface.

[0018] The sealing body 16 has a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26, which are laminated in order from the electrode body 14 side. Each member constituting the sealing body 16 has, for example, a disc shape or a ring shape, and each member except the insulating member 24 is electrically connected to each other. The lower valve body 23 and the upper valve body 25 are connected to each other at their central portions, and the insulating member 24 is interposed between the peripheral edges of each. When the internal pressure of the battery rises due to abnormal heat generation, for example, the lower valve body 23 breaks, and as a result, the upper valve body 25 bulges toward the cap 26 side and separates from the lower valve body 23, thereby cutting off the electrical connection between the two. When the internal pressure further rises, the upper valve body 25 breaks, and gas is discharged from the opening 26a of the cap 26.

[0019] Hereinafter, the positive electrode 11, the negative electrode 12, and the separator 13 constituting the electrode body 14 will be described, and in particular, the negative electrode active material constituting the negative electrode 12 will be described.

[0020] [Positive Electrode] The positive electrode 11 has a positive electrode core body and a positive electrode mixture layer provided on the surface of the positive electrode core body. For the positive electrode core body, a foil of a metal stable within the potential range of the positive electrode 11 such as aluminum, a film having the metal disposed on the surface layer, etc. can be used. The thickness of the positive electrode core body is, for example, 10 μm to 30 μm. The positive electrode mixture layer contains a positive electrode active material, a binder, and a conductive material, and is preferably provided on both surfaces of the positive electrode core body except for the portion to which the positive electrode lead 19 is connected. The positive electrode 11 can be manufactured, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a binder, a conductive material, etc. on the surface of the positive electrode core body, drying the coating film, and then compressing it to form the positive electrode mixture layer on both surfaces of the positive electrode core body.

[0021] The positive electrode active material contains a lithium transition metal oxide as a main component. The positive electrode active material may be substantially composed of only the lithium transition metal oxide, or may be one in which inorganic compound particles such as aluminum oxide and lanthanoid-containing compounds are fixed on the particle surface of the lithium transition metal oxide. One type of lithium transition metal oxide may be used, or two or more types may be used in combination.

[0022] Examples of the metal elements contained in the lithium transition metal oxide include nickel (Ni), cobalt (Co), manganese (Mn), aluminum (Al), boron (B), magnesium (Mg), titanium (Ti), vanadium (V), chromium (Cr), iron (Fe), copper (Cu), zinc (Zn), gallium (Ga), strontium (Sr), zirconium (Zr), niobium (Nb), indium (In), tin (Sn), tantalum (Ta), tungsten (W), and the like. An example of a suitable lithium transition metal oxide is the general formula: Li α Ni x M (1―x) O2 (0.1 ≤ α ≤ 1.2, 0.3 ≤ x < 1, M contains at least one of Co, Mn, and Al), which is a composite oxide. For example, as the positive electrode material, NCA in which part of nickel is replaced by cobalt and aluminum is added is used.

[0023] Examples of the conductive material contained in the positive electrode binder layer include carbon materials such as carbon black, acetylene black, ketjen black, carbon nanotubes, carbon nanofibers, and graphite. Examples of the binder contained in the positive electrode binder layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide resin, acrylic resin, and polyolefin resin. These resins may be used in combination with cellulose derivatives such as carboxymethyl cellulose (CMC) or its salts, polyethylene oxide (PEO), and the like.

[0024] [Negative electrode] The negative electrode 12 has a negative electrode core body and a negative electrode mixture layer provided on the surface of the negative electrode core body. For the negative electrode core body, a foil of a metal stable within the potential range of the negative electrode 12 such as copper, a film having the metal disposed on the surface layer, or the like can be used. The thickness of the negative electrode core body is, for example, 5 μm to 15 μm. The negative electrode mixture layer contains a negative electrode active material and a binder, and is preferably provided on both surfaces of the negative electrode core body excluding, for example, the portion to which the negative electrode lead 20 is connected. The negative electrode 12 can be produced, for example, by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, etc. on the surface of the negative electrode core body, drying the coating film, and then compressing it to form the negative electrode mixture layer on both surfaces of the negative electrode core body. Also, a conductive material may be added to the negative electrode mixture slurry. The conductive path can be made uniform by the conductive material. Further, the negative electrode mixture layer may contain a conductive material such as acetylene black, similar to the positive electrode mixture layer.

[0025] The negative electrode mixture layer contains, as a negative electrode active material, an intermetallic compound represented by the general formula M3Me2X7 (where M contains at least one of La and Ca, Me contains at least one of Mn, Ni, Fe, and Co, and X contains at least one of Ge, Si, Sn, and Al) (an alloy of M3Me2X7 type crystal). Specific examples of suitable negative electrode active materials include La3Co2Sn7, La3Mn2Sn7, and La3Ni2Sn7. Among them, from the viewpoint of increasing the capacity, La3Co2Sn7 or La3Ni2Sn7 is preferable, and La3Ni2Sn7 is particularly preferable.

[0026] The particle size of M3Me2X7 as the negative electrode active material is preferably 1 to 30 μm, more preferably 2 to 20 μm, and particularly preferably 2 to 10 μm. If the particle size of M3Me2X7 becomes too large, the reactivity with Li decreases, and the contact area between particles becomes small, resulting in an increase in resistance. On the other hand, if the particle size becomes too small, it is assumed that the packing density of the negative electrode active material decreases and the capacity decreases. The average particle size of M3Me2X7 is, for example, 3 to 15 μm, or 5 to 10 μm. The particle size of M3Me2X7 is measured as the diameter of the circumscribed circle of M3Me2X7 particles in the cross-sectional image of the negative electrode mixture layer observed by a scanning electron microscope (SEM). The average particle size is calculated by averaging the particle sizes of 100 arbitrary particles.

[0027] The intermetallic compound represented by M3Me2X7 can be formed by arc melting, and it is preferable to anneal after arc melting. Also, regarding M, it may be substituted for La up to about 50%. For example, in the case where about 40% of La is substituted with Ca, a large charge-discharge capacity (initial charge capacity: 301 mAh / g, initial discharge capacity: 223 mAh / g (1718 mAh / cc)) and a small volume change rate (0.5% or less) were obtained.

[0028] The negative electrode active material contains M3Me2X7 as the main component (the component with the highest mass ratio), and may be substantially composed of only M3Me2X7. On the other hand, other active materials such as intermetallic compounds other than M3Me2X7, carbon-based active materials such as graphite, or Si-based active materials containing Si may be used in combination with the negative electrode active material. For example, when graphite is used in combination, the content of graphite may be 50 to 90% by mass based on the mass of the negative electrode active material.

[0029] A compound containing a cyano group is adopted as the binder contained in the negative electrode binder layer. When using the above M3Me2X7 as the negative electrode active material, if a binder such as polyvinylidene fluoride (PVDF), which is commonly used, is used, the negative electrode binder slurry gels and it becomes difficult to coat the slurry. However, by using a binder containing a cyano group, the dispersibility of the negative electrode active material is improved and the gelation of the slurry is suppressed. Also, the binder containing a cyano group has a high affinity with M3Me2X7 and functions sufficiently as a binder even in a small amount.

[0030] Specific examples of the binder containing a cyano group include polyacrylonitrile (PAN), polymethacrylonitrile, poly-α-chloroacrylonitrile, poly-α-ethylacrylonitrile, and the like. Among them, PAN or polymethacrylonitrile is preferred, and PAN is particularly preferred. The binder containing a cyano group is synthesized, for example, by polymerization of a cyano group-containing monomer having 5 or less carbon atoms, but may contain a copolymerization component not containing a cyano group as long as the object of the present disclosure is not impaired. Further, only one type of the binder containing a cyano group may be used, or two or more types may be used in combination.

[0031] The mass ratio of the binder containing a cyano group in the negative electrode binder layer is 0.5% by mass to 7.0% by mass. When the content of the binder exceeds 7.0% by mass, the initial charge-discharge efficiency significantly decreases. On the other hand, when the content of the binder is less than 0.5% by mass, it becomes difficult to ensure the binding force between active material particles and the binding force between active material particles and the core. An example of a suitable content is 1.0% by mass to 5.0% by mass, or 2.0% by mass to 3.0% by mass. Note that the negative electrode binder layer may contain a binder not containing a cyano group as long as the object of the present disclosure is not impaired.

[0032] [Separator] For the separator 13, a porous sheet having ion permeability and insulation is used. Specific examples of the porous sheet include a microporous membrane, a woven fabric, a non-woven fabric, and the like. As the material of the separator 13, an olefin resin such as polyethylene or polypropylene, cellulose, or the like is suitable. The separator 13 may have either a single-layer structure or a laminated structure. A heat-resistant layer containing a heat-resistant material may be formed on the surface of the separator 13. Examples of the heat-resistant material include polyamide resins such as aliphatic polyamide and aromatic polyamide (aramid), and polyimide resins such as polyamideimide and polyimide.

[0033] [Examples] Hereinafter, the present disclosure will be further described by way of examples, but the present disclosure is not limited to these examples.

[0034] <Example 1> [Fabrication of Negative Electrode] As the negative electrode active material, La3Ni2Sn7 with a particle size of 2 to 20 μm was used. As the binder, polyacrylonitrile (PAN) was used. As the conductive material, acetylene black was used. The negative electrode active material, the binder, and the conductive material were mixed at a mass ratio of 96:3:1, and N-methyl-2-pyrrolidone (NMP) was used as the dispersion medium to prepare a negative electrode mixture slurry. Next, the negative electrode mixture slurry was applied onto a negative electrode core made of copper foil. After drying and compressing the coating film, it was cut into a predetermined electrode size to obtain a negative electrode.

[0035] [Fabrication of Test Cell] A positive electrode made of a lithium metal foil was arranged opposite to the above negative electrode via a separator to form an electrode body, and the electrode body was accommodated in a coin-shaped exterior can. After injecting a predetermined non-aqueous electrolyte into the exterior can, the exterior can was sealed to obtain a coin-shaped test cell (non-aqueous electrolyte secondary battery).

[0036] [Charge and Discharge Test (Evaluation of Capacity)] The obtained test cell was charged at a constant current of 0.15C to a battery voltage of 4.5V under normal temperature environment, and then discharged at a constant current of 0.15C to a battery voltage of 2.5V. This charge and discharge cycle was repeated 3 times, and the charge capacity and discharge capacity were measured in each cycle. The evaluation results are shown in Table 1 together with the particle size of the negative electrode active material, the applicability of the coating of the negative electrode mixture slurry, and the type and addition amount of the binder.

[0037] <Examples 2 to 5 and Comparative Examples 1 to 4> In the preparation of the negative electrode mixture slurry, test cells were fabricated and charge and discharge tests were conducted in the same manner as in Example 1, except that the addition amount of the binder, the type of the binder, and the particle size of the negative electrode active material were changed as shown in Table 1. Note that as the binder, polyimide (PI) was used instead of PAN in Comparative Example 2, and polyvinylidene fluoride (PVDF) was used instead of PAN in Comparative Example 3.

[0038] "Results" Table 1 is a diagram showing the charge and discharge test results of Examples 1 to 5 and Comparative Examples 1 to 4.

[0039]

Table 1

[0040] In Examples 1 to 5, relatively high charge-discharge efficiency was shown in any of the first (1st), second (2nd), and third (3rd) charge-discharges. On the other hand, in Comparative Example 1, the charge capacities of the first, second, and third times decreased significantly. Figure 2 is a graph showing the initial efficiency for Examples 1 to 5 and Comparative Example 1. Thus, in Comparative Example 1, the initial efficiency is very low compared to Examples 1 to 5. This is considered to be because the amount of the binder is too large and sufficient Li transfer to the active material cannot be performed.

[0041] Also, Figure 3 is a graph showing the initial discharge capacity for Examples 1 to 5. Thus, the discharge capacity of Example 5 is smaller compared to Examples 1 to 4. Therefore, it can be seen that the amount of the binder is more preferably 1 mass% to 5 mass% compared to 1 mass% to 7 mass%.

[0042] Also, in Comparative Example 4, due to the reaction between La3Ni2Sn7 and PVDF, the negative electrode mixture slurry gelled and coating could not be performed.

[0043] From this, it was found that a suitable non-aqueous electrolyte secondary battery can be obtained by using La3Ni2Sn7 as the negative electrode active material, PAN as the binder, and setting the amount of PAN to 1 mass% to 7 mass% (more preferably 1 mass% to 5 mass%).

Explanation of Signs

[0044] 10 Secondary battery 11 Positive electrode 12 Negative electrode 13 Separator 14 Electrode body 15 Outer package 16 Sealing body 17, 18 Insulating plate 19 Positive electrode lead 20 Negative electrode lead 21 Groove inlet 22 Filter 23 Lower valve body 24 Insulating member 25 Upper valve body 26 Cap 26a Opening 27 Gasket

Claims

Claim 1 A lithium-ion battery comprising a positive electrode having a positive electrode mixture layer containing a positive electrode active material, a negative electrode having a negative electrode mixture layer containing a negative electrode active material, and lithium ions moving between the positive electrode and the negative electrode to perform charge and discharge, wherein the negative electrode mixture layer contains a negative electrode active material which is either La3Ni2Sn7 or La3Co2Sn7, a binder containing a cyano group, and, the ratio of the binder in the negative electrode mixture layer is 0.5% by mass or more and 7.0% by mass or less, a lithium-ion battery. Claim 2 the binder containing a cyano group is polyacrylonitrile, and the ratio of polyacrylonitrile is 1.0% by mass or more and 5.0% by mass or less, the lithium-ion battery according to Claim 1. Claim 3 the ratio of polyacrylonitrile is 2.0% by mass or more and 3.0% by mass or less, the lithium-ion battery according to Claim 2.

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