lithium-ion batteries

Abstract

A lithium-ion battery is provided, comprising: a positive electrode having a positive electrode mixture layer containing a positive electrode active material; and a negative electrode having a negative electrode mixture layer containing a negative electrode active material, wherein charging and discharging are performed by the movement of lithium ions between the positive and negative electrodes. The negative electrode mixture layer comprises: a negative electrode active material represented by the general formula M3Me2X7 (wherein M comprises at least one of La and Ca, Me comprises at least one of Mn, Ni, Fe, and Co, and X comprises at least one of Ge, Si, Sn, and Al); and a binder containing a cyano group, wherein 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.

Description

Technical Field

[0001] The present disclosure relates to a lithium ion battery comprising a positive electrode having a positive electrode mixture layer containing a positive electrode active material and a negative electrode having a negative electrode mixture layer containing a negative electrode active material, wherein charging and discharging are performed by the movement of lithium ions between the positive electrode and the negative electrode. Background Art

[0002] Lithium-ion batteries, which charge and discharge by the movement of lithium ions (Li ions) between the negative and positive electrodes, are widely used. The negative electrode active material in the negative electrode mixture layer of these lithium-ion batteries often uses a graphite-based material. Graphite-based negative electrode active materials are sometimes used together with silicon, which results in significant volume changes during charge and discharge, which can deteriorate capacity retention and increase costs.

[0003] Here, non-graphite-based negative electrode active materials have also been proposed. For example, Patent Document 1 describes a proposal to use an alloy having a La 3 Co 2 Sn 7 type crystal structure as a negative electrode active material.

[0004] Furthermore, a binder is used in the negative electrode mixture layer to prevent delamination and cracking. However, if the amount of binder increases, the efficiency of the battery reaction of the negative electrode active material decreases. Therefore, there is also a demand to reduce the amount of binder. Patent Document 2 describes a solution to set the binder content to 0.5% by mass or more and 5.0% by mass or less.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent No. 4127692

[0008] Patent Document 2: Japanese Patent Application Laid-Open No. 2007-258127 Summary of the Invention

[0009] In Patent Document 1, polyvinylidene fluoride (PVDF) is used as a binder. However, it was found that when La3Ni2Sn7 was used as the negative electrode active material in the experiment and PVDF was used as the binder, the mixture slurry used in the formation of the negative electrode mixture layer would gel through the reaction between the two, and coating would become difficult. In order to reduce the reactivity of La3Ni2Sn7 and PVDF and make coating possible, 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 of the negative electrode active material with Li will decrease, and the capacity will be easily reduced.

[0010] The lithium-ion battery disclosed herein comprises: a positive electrode having a positive electrode mixture layer containing a positive electrode active material, and a negative electrode having a negative electrode mixture layer containing a negative electrode active material, and charging and discharging are performed by the movement of lithium ions between the positive electrode and the negative electrode, the negative electrode mixture comprising: a negative electrode active material represented by the general formula M3Me2X7 (wherein M comprises at least one of La and Ca, Me comprises at least one of Mn, Ni, Fe, and Co, and X comprises 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 greater than or equal to 0.5% by mass and less than or equal to 7.0% by mass.

[0011] In the present disclosure, a negative electrode active material represented by the general formula M3Me2X7 and a cyano group-containing binder are used as the negative electrode mixture layer, with the binder added in an amount of 0.5% by mass to 7.0% by mass. This allows for coating of the negative electrode mixture layer and reduces the amount of binder added, thereby suppressing capacity reduction. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 It is a longitudinal cross-sectional view of a cylindrical secondary battery 10 as an example of an embodiment.

[0013] Figure 2 It is a graph showing the initial efficiency of Examples 1 to 5 and Comparative Example 1.

[0014] Figure 3 This is a graph showing the initial discharge capacity of Examples 1 to 5. DETAILED DESCRIPTION

[0015] Hereinafter, embodiments of the present disclosure will be described based on the accompanying drawings. However, it should be noted that the present disclosure is not limited to the embodiments described herein.

[0016] About negative electrode materials

[0017] Lithium-ion battery anode materials preferably meet the requirements of high energy density and low expansion. Consequently, various research and development efforts have led to the use of intermetallic compounds represented by M3Me2X7 (M = La, Ca, Me = Mn, Ni, Fe, Co, X = Ge, Si, Sn, Al), such as La3Ni2Sn7, as anode active materials. These intermetallic compounds absorb and release lithium through intercalation reactions, resulting in low expansion coefficients and the potential for extended battery life.

[0018] However, it was discovered that this material requires further improvement for practical use. First, as mentioned above, when PVDF is used as a binder, the negative electrode mixture slurry will gel, making the application of the negative electrode mixture layer difficult. In addition, increasing the binder particle size to inhibit gelation may hinder the battery reaction.

[0019] In the present disclosure, the use of a cyano group-containing substance, such as polyacrylonitrile (PAN), as a binder can suppress gelation of the negative electrode mixture slurry. In this case, the amount of the binder added to the negative electrode mixture layer can be set to 2.0% by mass or more and 5.0% by mass or less.

[0020] Thus, by using a negative electrode active material having an M3Me2X7 type crystal structure and a binder containing a cyano group, coating of the negative electrode mixture layer becomes possible, and a battery with a high energy density can be obtained.

[0021] "Structure of Implementation Methods"

[0022] Figure 1 It is a longitudinal cross-sectional view of a cylindrical secondary battery 10 as an example of an embodiment. Figure 1 In the secondary battery 10 shown, the electrode body 14 and the non-aqueous electrolyte are housed in an outer shell 15. The electrode body 14 has a wound structure in which the positive electrode 11 and the negative electrode 12 are wound with the separator 13. As a non-aqueous solvent (organic solvent) for the non-aqueous electrolyte, carbonates, lactones, ethers, ketones, esters, etc. can be used, and two or more of these solvents can be mixed and used. When two or more solvents are mixed and used, it is preferable to use a mixed solvent containing cyclic carbonates and chain carbonates. For example, as cyclic carbonates, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc. can be used, and as chain carbonates, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) can be used. As electrolyte salts for the non-aqueous electrolyte, LiPF6, LiBF4, LiCF3SO3, etc. and mixtures thereof can be used. The amount of the electrolyte salt dissolved in the non-aqueous solvent can be set to, for example, 0.5 to 2.0 mol / L. It should be noted that, for convenience of description, the sealing body 16 side is referred to as “upper” and the bottom side of the outer shell 15 is referred to as “lower” in the following description.

[0023] The open end of the outer shell 15 is sealed by the sealing body 16, thereby hermetically sealing the interior of the secondary battery 10. Insulating plates 17 and 18 are provided above and below the electrode body 14, respectively. The positive lead 19 extends upward through the through hole of the insulating plate 17 and is welded to the lower surface of the partially opened metal plate 22 serving as the bottom plate of the sealing body 16. In the secondary battery 10, the cover 26 serving as the top plate of the sealing body 16, which is electrically connected to the partially opened metal plate 22, becomes the positive terminal. On the other hand, the negative lead 20 extends toward the bottom side of the outer shell 15 through the through hole of the insulating plate 18 and is welded to the bottom inner surface of the outer shell 15. In the secondary battery 10, the outer shell 15 becomes the negative terminal. It should be noted that when the negative lead 20 is provided at the terminal portion, the negative lead 20 extends toward the bottom side of the outer shell 15 through the outside of the insulating plate 18 and is welded to the bottom inner surface of the outer shell 15.

[0024] The outer shell 15 is, for example, a cylindrical metal outer can with a bottom. A gasket 27 is provided between the outer shell 15 and the sealing member 16 to ensure the airtightness of the interior of the secondary battery 10. The outer shell 15 has, for example, a groove 21 formed by applying pressure to the side surface from the outside to support the sealing member 16. The groove 21 is preferably formed in an annular shape along the circumference of the outer shell 15, and the upper surface thereof supports the sealing member 16 via the gasket 27.

[0025] The sealing body 16 has a partially open metal plate 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cover 26 stacked in sequence from the electrode body 14 side. The components constituting the sealing body 16 have, for example, a disc shape or a ring shape, and the components except the insulating member 24 are electrically connected to each other. The lower valve body 23 and the upper valve body 25 are connected to each other at their respective central portions, and an insulating member 24 is sandwiched between their respective peripheral portions. When the internal pressure of the battery rises due to abnormal heat, for example, the lower valve body 23 will break, whereby the upper valve body 25 expands toward the cover 26 side and away from the lower valve body 23, thereby cutting off the electrical connection between the two. When the internal pressure rises further, the upper valve body 25 breaks, and the gas is discharged from the opening 26a of the cover 26.

[0026] Hereinafter, the positive electrode 11 , the negative electrode 12 , and the separator 13 constituting the electrode assembly 14 , and in particular, the negative electrode active material constituting the negative electrode 12 will be described.

[0027] The positive electrode 11 has a positive electrode core and a positive electrode mixture layer provided on the surface of the positive electrode core. The positive electrode core can be made of a foil of a metal such as aluminum that is stable within the potential range of the positive electrode 11, a thin film having the metal provided on the surface, or the like. The thickness of the positive electrode core 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 sides of the positive electrode core except for the portion connected to the positive electrode lead 19. The positive electrode 11 can be produced, for example, as follows: a positive electrode mixture slurry containing a positive electrode active material, a binder, and a conductive material is applied to the surface of the positive electrode core, the coating is dried and then compressed, and the positive electrode mixture layers are formed on both sides of the positive electrode core.

[0028] The positive electrode active material contains a lithium transition metal oxide as a main component. The positive electrode active material may consist essentially solely of the lithium transition metal oxide, or may contain inorganic compound particles such as aluminum oxide or a lanthanide-containing compound attached to the surface of the lithium transition metal oxide particles. A single lithium transition metal oxide may be used, or two or more may be used in combination.

[0029] Examples of metal elements contained in lithium transition metal oxides 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), and tungsten (W). An example of an ideal lithium transition metal oxide is a general formula: Li α Ni x M (1-x) A composite oxide represented by O2 (0.1≤α≤1.2, 0.3≤x<1, M contains at least one of Co, Mn, and Al). For example, NCA, in which a portion of nickel is replaced with cobalt and aluminum is added, can be used as a positive electrode material.

[0030] Examples of conductive materials included in the positive electrode mixture layer include carbon materials such as carbon black, acetylene black, Ketjen black, carbon nanotubes, carbon nanofibers, and graphite. Examples of binders included in the positive electrode mixture layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide resins, acrylic resins, and polyolefin resins. These resins may also be used in combination with cellulose derivatives such as carboxymethyl cellulose (CMC) or its salts, polyethylene oxide (PEO), and the like.

[0031] [negative electrode]

[0032] The negative electrode 12 includes a negative electrode core and a negative electrode mixture layer provided on the surface of the negative electrode core. The negative electrode core can be made of a foil of a metal such as copper that is stable within the potential range of the negative electrode 12, or a thin film having the metal provided on the surface. The thickness of the negative electrode core 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 sides of the negative electrode core except for the portion connected to the negative electrode lead 20. The negative electrode 12 can be produced, for example, by applying a negative electrode mixture slurry containing a negative electrode active material and a binder to the surface of the negative electrode core, drying the coating, and then compressing the coating to form negative electrode mixture layers on both sides of the negative electrode core. In addition, a conductive material can also be added to the negative electrode mixture slurry. The conductive material can make the conductive path uniform. In addition, like the positive electrode mixture layer, the negative electrode mixture layer can also contain a conductive material such as acetylene black.

[0033] The negative electrode mixture layer includes, as a negative electrode active material, an intermetallic compound (an alloy of M3Me2X7 type crystals) represented by the general formula M3Me2X7 (wherein M comprises at least one of La and Ca, Me comprises at least one of Mn, Ni, Fe, and Co, and X comprises at least one of Ge, Si, Sn, and Al). Specific examples of ideal negative electrode active materials include La3Co2Sn7, La3Mn2Sn7, and La3Ni2Sn7. Among these, La3Co2Sn7 and La3Ni2Sn7 are preferred from the perspective of increasing capacity, and La3Ni2Sn7 is particularly preferred.

[0034] The particle size of M3Me2X7 as a negative electrode active material is preferably 1 to 30 μm, more preferably 2 to 20 μm, and particularly preferably 2 to 10 μm. When the particle size of M3Me2X7 is too large, the reactivity with Li will decrease, and the contact area between the particles will decrease, and the resistance will increase. On the other hand, when the particle size is too small, the packing density of the negative electrode active material is expected to decrease and the capacity will decrease. 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 the M3Me2X7 particles in the cross-sectional image of the negative electrode mixture layer observed using a scanning electron microscope (SEM). The particle size is calculated by averaging the particle sizes of any 100 particles.

[0035] The intermetallic compound represented by M3Me2X7 can be formed by arc melting, preferably followed by annealing. Furthermore, M can be replaced with La by approximately 50% to 50%. For example, replacing approximately 40% of La with Ca yields a high charge / discharge capacity (initial charge capacity 301 mAh / g, initial discharge capacity 223 mAh / g (1718 mAh / cc)) and a low volume change rate (less than 0.5%).

[0036] The negative electrode active material contains M3Me2X7 as the main component (the component with the highest mass ratio), or may be substantially composed of 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 also be used in the negative electrode active material. For example, when graphite is used, the content of graphite can be 50 to 90 mass % relative to the mass of the negative electrode active material.

[0037] The binder contained in the negative electrode mixture layer can be a compound containing a cyano group. When using the above-mentioned M3Me2X7 as the negative electrode active material, when using a binder such as polyvinylidene fluoride (PVDF) that is commonly used, the negative electrode mixture slurry will gel, and the coating of the slurry becomes difficult. 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. In addition, the binder containing a cyano group has a high affinity with M3Me2X7, and even a small amount can fully function as a binder.

[0038] Specific examples of cyano group-containing binders include polyacrylonitrile (PAN), polymethacrylonitrile, poly-α-chloroacrylonitrile, and poly-α-ethylacrylonitrile. Of these, PAN and polymethacrylonitrile are preferred, with PAN being particularly preferred. The cyano group-containing binder is synthesized, for example, by polymerizing a cyano group-containing monomer having 5 or fewer carbon atoms. It may also contain a copolymer component that does not contain a cyano group, as long as the purpose of the present disclosure is not impaired. Furthermore, a single cyano group-containing binder may be used alone, or two or more may be used in combination.

[0039] The mass ratio of the binder containing a cyano group in the negative electrode mixture layer is 0.5% to 7.0% by mass. When the content of the binder is greater than 7.0% by mass, the initial charge and discharge efficiency will be greatly reduced. On the other hand, when the content of the binder is less than 0.5% by mass, it is difficult to ensure the binding force between the active material particles and the binding force between the active material particles and the core. An example of an ideal content is 1.0% to 5.0% by mass, or 2.0% to 3.0% by mass. It should be noted that, within the scope of not impairing the purpose of the present disclosure, the negative electrode mixture layer may also contain a binder that does not contain a cyano group.

[0040] [Separator]

[0041] The separator 13 can use a porous sheet with ion permeability and insulation. Specific examples of the porous sheet include microporous membranes, woven fabrics, non-woven fabrics, and the like. As the material of the separator 13, olefin resins such as polyethylene and polypropylene, cellulose, and the like are ideal. The separator 13 can be a single-layer structure or a laminated structure. A heat-resistant layer containing a heat-resistant material can also be formed on the surface of the separator 13. As heat-resistant materials, polyamide resins such as aliphatic polyamides and aromatic polyamides (aramid), polyamide-imides, polyimide, and polyimide resins can be exemplified.

[0042] <Example>

[0043] Hereinafter, the present disclosure will be further described with reference to examples, but the present disclosure is not limited to these examples.

[0044] <Example 1>

[0045] [Production of negative electrode]

[0046] La3Ni2Sn7 with a particle size of 2 to 20 μm is used as the negative electrode active material, polyacrylonitrile (PAN) is used as the binder, and acetylene black is used as the conductive material. The negative electrode active material, binder, and conductive material are mixed in a mass ratio of 96:3:1, and N-methyl-2-pyrrolidone (NMP) is used as the dispersion medium to prepare the negative electrode mixture slurry. Next, the negative electrode mixture slurry is applied to the negative electrode core formed of copper foil. The coating is dried, compressed, and cut into the specified electrode size to obtain the negative electrode.

[0047] [Fabrication of test battery cells]

[0048] The negative electrode and the positive electrode formed of lithium metal foil were positioned opposite each other with a separator interposed therebetween to form an electrode assembly, which was then housed in a coin-shaped outer can. A prescribed non-aqueous electrolyte solution was then injected into the outer can and the can was sealed to produce a coin-shaped test cell (non-aqueous electrolyte secondary battery).

[0049] [Charge and discharge test (capacity evaluation)]

[0050] The resulting test cell was CC-charged at a constant current of 0.15C to a cell voltage of 4.5V at room temperature, and then CC-discharged at a constant current of 0.15C to a cell voltage of 2.5V. This charge-discharge cycle was repeated three times, and the charge and discharge capacities were measured for each cycle. The evaluation results are shown in Table 1, along with the particle size of the negative electrode active material, the coating capability of the negative electrode mixture slurry, and the type and amount of binder added.

[0051] <Examples 2 to 5 and Comparative Examples 1 to 4>

[0052] In the preparation of the negative electrode mixture slurry, the amount of binder added, the type of binder, and the particle size of the negative electrode active material were changed as shown in Table 1. A test cell was prepared in the same manner as in Example 1, and a charge-discharge test was conducted. It should be noted that in Comparative Example 2, polyimide (PI) was used instead of PAN as the binder, and in Comparative Example 3, polyvinylidene fluoride (PVDF) was used instead of PAN.

[0053] "result"

[0054] Table 1 is a graph showing the charge and discharge test results of Examples 1 to 5 and Comparative Examples 1 to 4.

[0055] [Table 1]

[0056]

[0057] In Examples 1 to 5, high charge and discharge efficiency was shown in all of the first, second, and third charge and discharge cycles. On the other hand, in Comparative Example 1, the charge capacity was significantly reduced in the first, second, and third cycles. Figure 2 This is a graph showing the initial efficiency of Examples 1 to 5 and Comparative Example 1. It can be seen that the initial efficiency of Comparative Example 1 is significantly lower than that of Examples 1 to 5. This is presumably because the excessive amount of binder prevented sufficient transfer of Li to the active material.

[0058] in addition, Figure 3 This is a graph showing the initial discharge capacity of Examples 1 to 5. It is clear that the discharge capacity of Example 5 is smaller than that of Examples 1 to 4. Therefore, it is clear that the binder amount is more preferably 1% to 5% by mass, rather than 1% to 7% by mass.

[0059] In Comparative Example 4, the negative electrode mixture slurry was gelled due to the reaction between La 3 Ni 2 Sn 7 and PVDF, and coating was not possible.

[0060] It can be seen from this that an ideal 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% by mass to 7% by mass (more preferably 1% by mass to 5% by mass).

[0061] Description of Reference Numerals

[0062] 10 Secondary Batteries

[0063] 11. Positive electrode

[0064] 12 negative electrode

[0065] 13 Dividers

[0066] 14 Electrode body

[0067] 15 outer shell

[0068] 16 Sealing body

[0069] 17, 18 insulation board

[0070] 19 Positive lead

[0071] 20 Negative lead

[0072] 21 grooved part

[0073] 22 Metal plates with partial openings

[0074] 23 Lower valve body

[0075] 24 Insulation components

[0076] 25 Upper valve body

[0077] 26 Cover

[0078] 26a Opening

[0079] 27 gasket

Claims

1. A lithium-ion battery comprising: a positive electrode having a positive electrode mixture layer containing a positive electrode active material; and a negative electrode having a negative electrode mixture layer containing a negative electrode active material, wherein charging and discharging are performed by the movement of lithium ions between the positive electrode and the negative electrode. The negative electrode mixture layer comprises: A negative electrode active material represented by La3Ni2Sn7; and A binder, wherein the binder is a binder containing a cyano group, The cyano group-containing binder is at least one selected from polyacrylonitrile, polymethacrylonitrile, poly-α-chloroacrylonitrile and poly-α-ethylacrylonitrile, The ratio of the binder in the negative electrode mixture layer is 1.0 mass % or more and less than 3.0 mass %.

2. The lithium-ion battery according to claim 1, wherein The binder containing cyano group is polyacrylonitrile.

3. The lithium ion battery according to claim 2, wherein The ratio of polyacrylonitrile is 2.0 mass % or more and less than 3.0 mass %.

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