Negative electrode active material and lithium ion battery

Abstract

Provided are: a negative electrode active substance having a La3Ni2Sn7 crystal structure and a high gravimetric energy density; and a lithium-ion battery using the same. The negative electrode active material according to one embodiment of the present disclosure is included in the negative electrode of a lithium-ion battery, and is represented by general formula M3Me2X7 (where: M includes La and / or Ca; Me includes at least one element selected from the group consisting of Mn, Ni, Fe and Co; X includes at least one element selected from the group consisting of Ge, Si, Sn, and Al). The dislocation density of the negative electrode active material is at least 1×1015 cm-2.

Description

[Technical Field]

[0001] The present disclosure relates to a negative electrode active material and a lithium ion battery. [Background technology]

[0002] Lithium-ion batteries, which are charged and discharged by the movement of lithium ions (Li ions) between the positive and negative electrodes, have become widespread. In recent years, graphite and silicon, which has a higher capacity than graphite, have been used as the active material in the negative electrode of these lithium-ion batteries. However, silicon undergoes a large volume change during charging and discharging, which makes it prone to a decrease in battery capacity due to repeated charging and discharging.

[0003] Therefore, various alloys other than Si are being considered as negative electrode active materials for high-capacity lithium-ion batteries. For example, Non Patent Document 1 describes the use of an alloy having a La3Ni2Sn7 type crystal structure as the negative electrode active material. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Shinsuke Matsuno et al, La3Ni2Sn7 Ternary Intermetallic Phase for Lithium Insertion and Deinsertion, Electrochemical and Solid-State Letters, volume 8(4), A234-A236, (2005). Summary of the Invention [Problem to be solved by the invention]

[0005] However, the alloy having the La3Ni2Sn7 type crystal structure described in Non-Patent Document 1 has a high volumetric energy density but a low weight energy density compared to graphite.

[0006] An object of the present disclosure is to provide a negative electrode active material that has a La3Ni2Sn7 type crystal structure and a high weight energy density, and a lithium ion battery using the same. [Means for solving the problem]

[0007] A negative electrode active material according to one embodiment of the present disclosure is a negative electrode active material contained in a negative electrode of a lithium ion battery, and is represented by the general formula M3Me2X7 (wherein M includes at least one of La and Ca, Me includes at least one element selected from the group consisting of Mn, Ni, Fe, and Co, and X includes at least one element selected from the group consisting of Ge, Si, Sn, and Al), and the dislocation density of the negative electrode active material is 1×10 15 cm -2 The present invention is characterized in that:

[0008] A lithium ion battery according to one embodiment of the present disclosure includes a negative electrode containing the above-described negative electrode active material, a positive electrode, and a non-aqueous electrolyte. [Effects of the Invention]

[0009] The negative electrode active material according to the present disclosure can increase the weight energy density. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a longitudinal cross-sectional view of a cylindrical lithium ion battery according to an embodiment of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0011] To increase the capacity of lithium-ion batteries, a negative electrode active material made of graphite mixed with Si is used. However, Si is expensive, and the inclusion of Si can deteriorate the cycle characteristics of lithium-ion batteries. Therefore, various alloys other than Si are attracting attention as negative electrode active materials. For example, as a negative electrode active material for high-capacity lithium-ion batteries, NonPatent Document 1 discloses an alloy having a La3Ni2Sn7 type crystal structure. Non The alloy disclosed in Patent Document 1 has a higher volumetric energy density but a lower weight energy density than graphite. 15 cm -2 Dislocations are formed from defects and voids, so if the dislocation density is 1×10 15 cm -2 Defects and voids are formed in the crystal structure, which is larger than this, and it is presumed that Li ions are inserted into these defects and voids.

[0012] An example of an embodiment of a negative electrode active material according to the present disclosure and a lithium-ion battery using the negative electrode active material will be described in detail below. In a lithium-ion battery, charging and discharging are performed by the movement of lithium ions between the positive electrode and the negative electrode. A cylindrical battery in which a wound electrode assembly is housed in a cylindrical outer can with a bottom is exemplified below. However, the outer can is not limited to a cylindrical outer can and may be, for example, a rectangular outer can or an outer can made of a laminate sheet including a metal layer and a resin layer. The electrode assembly may also be a laminated electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween. Furthermore, while a liquid nonaqueous electrolyte is exemplified below, the nonaqueous electrolyte is not limited to a liquid and may also be a solid.

[0013] FIG. 1 is a longitudinal cross-sectional view of a cylindrical lithium-ion battery 10 according to an embodiment. The lithium-ion battery 10 shown in FIG. 1 includes an electrode assembly 14 and a nonaqueous electrolyte (not shown) housed in an exterior case 15. The electrode assembly 14 has a wound structure in which a positive electrode 11 and a negative electrode 12 are wound with a separator 13 interposed therebetween. Examples of solvents (organic solvents) for the nonaqueous electrolyte include carbonates, lactones, ethers, ketones, and esters, and two or more of these solvents can be mixed. When two or more solvents are mixed, a mixed solvent containing a cyclic carbonate and a chain carbonate is preferably used. For example, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and the like can be used as the cyclic carbonate, and dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and the like can be used as the chain carbonate. Examples of electrolyte salts that can be used for the non-aqueous electrolyte include LiPF, LiBF, LiCF, SO, and mixtures thereof. The amount of electrolyte salt dissolved in the solvent can be, for example, 0.5 to 2.0 mol / L. For ease of explanation, the following description will refer to the sealing body 16 side as "top" and the bottom side of the exterior body 15 as "bottom."

[0014] The open end of the exterior body 15 is sealed with the sealing body 16, thereby sealing the interior of the lithium-ion battery 10. Insulating plates 17 and 18 are provided above and below the electrode body 14. The positive electrode lead 19 extends upward through a through hole in the insulating plate 17 and is welded to the underside of a filter 22, which is the bottom plate of the sealing body 16. In the lithium-ion battery 10, a cap 26, which is the top plate of the sealing body 16 and is electrically connected to the filter 22, serves as the positive electrode terminal. On the other hand, the negative electrode lead 20 extends through a through hole in the insulating plate 18 toward the bottom of the exterior body 15 and is welded to the inner bottom surface of the exterior body 15. In the lithium-ion battery 10, the exterior body 15 serves as the negative electrode terminal. Note that if the negative electrode lead 20 is installed at the terminal end, the negative electrode lead 20 passes outside the insulating plate 18, extends toward the bottom of the exterior body 15, and is welded to the inner bottom surface of the exterior body 15.

[0015] Exterior body 15 is, for example, a cylindrical metal exterior can with a bottom. A gasket 27 is provided between exterior body 15 and sealing body 16, ensuring the sealing of the interior of lithium-ion battery 10. Exterior body 15 has a grooved portion 21 that supports sealing body 16, formed, for example, by pressing the side surface from the outside. Grooved portion 21 is preferably formed in an annular shape along the circumferential direction of exterior body 15, and supports sealing body 16 on its upper surface via gasket 27.

[0016] The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26, which are stacked in this order from the electrode body 14 side. Each component of the sealing body 16 has, for example, a disk or ring shape, and all components except for 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 centers, with the insulating member 24 interposed between their respective peripheral edges. If the internal pressure of the battery increases due to abnormal heat generation, for example, the lower valve body 23 may break, causing the upper valve body 25 to bulge toward the cap 26 and separate from the lower valve body 25, thereby cutting off the electrical connection between them. If the internal pressure continues to increase, the upper valve body 25 may break, and gas may be released from the opening 26a of the cap 26.

[0017] The positive electrode 11, negative electrode 12, and separator 13 that constitute the electrode assembly 14, and in particular the negative electrode active material that constitutes the negative electrode 12, will be described in detail below.

[0018] [Positive electrode] The positive electrode 11 has, for example, 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 a foil of a metal such as aluminum that is stable within the potential range of the positive electrode 11, or a film with such a metal disposed on the surface. 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 agent, and is preferably provided on both sides of the positive electrode core except for the portion to which the positive electrode lead 19 is connected. The positive electrode 11 can be produced, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a binder, a conductive agent, etc. to the surface of the positive electrode core, drying the coating, and then compressing it to form positive electrode mixture layers on both sides of the positive electrode core.

[0019] 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 a lithium transition metal oxide, or may be composed of lithium transition metal oxide particles having inorganic compound particles such as aluminum oxide or a lanthanoid-containing compound adhered to the surface of the lithium transition metal oxide particles. One type of lithium transition metal oxide may be used, or two or more types may be used in combination.

[0020] Examples of 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), and tungsten (W). An example of a suitable lithium transition metal oxide is represented by the general formula: Li α Ni x M (1―x) It is 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).

[0021] Examples of conductive agents contained 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 contained 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 be used in combination with cellulose derivatives such as carboxymethyl cellulose (CMC) or its salts, polyethylene oxide (PEO), and the like.

[0022] [Negative electrode] The negative electrode 12 has, for example, 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 a foil of a metal such as copper that is stable within the potential range of the negative electrode 12, or a film with such a metal disposed 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 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, a conductive agent, etc. to the surface of the negative electrode core, drying the coating, and then compressing it to form a negative electrode mixture layer on both sides of the negative electrode core. The negative electrode 12 may also be produced by mixing a negative electrode active material and copper powder and compressing the mixture into a pellet.

[0023] The negative electrode active material contained in the negative electrode 12 includes an alloy represented by the general formula M3Me2X7 (wherein M includes at least one of La and Ca, Me includes at least one element selected from the group consisting of Mn, Ni, Fe, and Co, and X includes at least one element selected from the group consisting of Ge, Si, Sn, and Al) (hereinafter, this may be referred to as an M3Me2X7 alloy). An example of an M3Me2X7 alloy is La3Ni2Sn7. The negative electrode 12 may also include a negative electrode active material other than the M3Me2X7 alloy. The negative electrode active material other than the M3Me2X7 alloy is not particularly limited as long as it can reversibly absorb and release lithium ions. For example, graphite (natural graphite or artificial graphite), metals that alloy with lithium, such as silicon (Si) and tin (Sn), or oxides containing metal elements such as Si and Sn, can be used.

[0024] The dislocation density of an M3Me2X7 type alloy is, for example, 1×10 15 cm -2 This makes it possible to increase the gravimetric energy density of the M3Me2X7 type alloy. The dislocation density of the M3Me2X7 type alloy is 1×10 17 cm -2 More than 1×10 is preferable. 19 cm -2The above is more preferable. The dislocation density can be calculated, for example, by observing a cross section of the M3Me2X7 type alloy using a TEM (Transmission Electron Microscope). Incidentally, the dislocation density of a general M3Me2X7 type alloy is about 1×10 6 ~1×10 8 , and the dislocation density of the M3Me2X7 type alloy according to the present disclosure is higher than that of a general M3Me2X7 type alloy.

[0025] The volume-based median diameter (D50) of the M3Me2X7 type alloy may be, for example, 0.1 μm to 50 μm, or may be 1 μm to 10 μm. The D50 of the M3Me2X7 type alloy can be made smaller, for example, by pulverizing with a ball mill, and can be adjusted according to the pulverization conditions. D50 means the particle diameter at which the cumulative frequency in the volume-based particle size distribution becomes 50% from the smaller particle diameter side, and is also called the median diameter. The particle size distribution of the M3Me2X7 type alloy can be measured using a laser diffraction type particle size distribution measuring device (for example, MT3000II manufactured by Microtrac Bell Co., Ltd.) with water as a dispersion medium.

[0026] The M3Me2X7 type alloy according to the present disclosure can be produced, for example, as follows.

[0027] <Production method of M3Me2X7 type alloy> (1) As raw materials, M metal, Me metal, and X metal are prepared, and after mixing these raw materials at a predetermined ratio, they are arc melted to produce a lump of the M3Me2X7 type alloy. (2) After the produced lump of the M3Me2X7 type alloy is vacuum sealed in a quartz tube, annealing is performed in a slow cooling furnace. The annealing conditions are, for example, the annealing temperature is 400°C to 1000°C, and the holding time is 10 hours to 720 hours. (3) The annealed lump of the M3Me2X7 type alloy is pulverized, for example, with a mortar or a planetary ball mill. The ball mill treatment conditions are, for example, the rotation speed is 100 rpm to 500 rpm, and the treatment time is 1 hour to 720 hours. (4) After the ball milling process, the M3Me2X7 type alloy is classified using a mesh, for example, to remove coarse particles.

[0028] The M3Me2X7 type alloy produced by the above method has a dislocation density of 1×10 15 cm -2 It is believed that the crushing process creates defects and voids in the crystal structure of the M3Me2X7 type alloy, increasing the dislocation density of the M3Me2X7 type alloy.

[0029] The binder contained in the negative electrode mixture layer can be, for example, a fluororesin, PAN, polyimide resin, acrylic resin, or polyolefin resin, as in the case of the positive electrode 11. When preparing the mixture slurry using an aqueous solvent, it is preferable to use CMC or a salt thereof, styrene-butadiene rubber (SBR), polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol, or the like.

[0030] The negative electrode mixture layer may contain a conductive agent. The conductive agent can make the conductive path uniform. Examples of the conductive agent contained in the negative electrode mixture layer include carbon-based particles such as carbon black (CB), acetylene black (AB), ketjen black, carbon nanotubes (CNT), graphene, and graphite. These may be used alone or in combination of two or more. The conductive agent preferably contains CNT. The CNT may be either single-walled carbon nanotubes (SWCNT) or multi-walled carbon nanotubes (MWCNT). Since a smaller amount of SWCNT than MWCNT can form a conductive path in the negative electrode mixture layer, it is preferable that the CNT contains SWCNT.

[0031] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous membrane, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include olefin resins such as polyethylene and polypropylene, and cellulose. 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 heat-resistant materials include polyamide resins such as aliphatic polyamides and aromatic polyamides (aramids), and polyimide resins such as polyamideimides and polyimides. [Example]

[0032] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0033] Example 1 [Preparation of negative electrode active material] The raw materials used were lanthanum (La) metal pieces, nickel (Ni) wire, and tin (Sn) metal pieces. The lanthanum (La) metal pieces were made of yttrium and 3N purity. They were arc-melted, degassed, and then polished with a metal brush. The nickel (Ni) wire was manufactured by Kojundo Kagaku and had a diameter of 3mm and a purity of 3N. The tin (Sn) metal pieces were manufactured by Furuuchi Kagaku and had a shot shape and a purity of 4N. These raw materials were mixed in a molar ratio of La:Ni:Sn = 3:2:7 and then arc-melted to obtain an alloy ingot with a La3Ni2Sn7 crystal structure. This ingot was then sealed in a quartz tube under vacuum and annealed in a muffle furnace. The annealing was performed by increasing the temperature from room temperature at a rate of 300°C / hour, reaching 680°C, and maintaining this temperature for 48 hours, followed by natural cooling. The annealed alloy mass was then manually ground in a mortar for 1 hour, then placed in a zirconia container with zirconia balls and ball milled for a predetermined time at a predetermined rotation speed using a planetary ball mill. The ball milling was performed with a 5-minute break every 5 minutes. The powder after ball milling was classified using a 45 μm mesh to remove coarse particles. The volume-based median diameter (D50) of the prepared negative electrode active material (powder La3Ni2Sn7) was 6.8 μm.

[0034] [Preparation of negative electrode] The negative electrode active material and Cu powder were mixed in a mass ratio of 4:6 and then compressed to prepare a pellet having a diameter of 7.5 mm and a height of 0.8 mm, which was used as a negative electrode.

[0035] [Preparation of counter electrode] To examine the characteristics of the negative electrode active material, metallic lithium (Li) was used as the counter electrode instead of the above-mentioned positive electrode. Generally, lithium-ion batteries use lithium transition metal oxides such as LiNiO2 (typically containing transition metals such as Co, Mn, and Ni) as the positive electrode active material. However, in this study, to examine the characteristics of the negative electrode active material itself, which is independent of the positive electrode active material, a lithium metal foil cut to a diameter of 17 mm was used as the counter electrode instead of the positive electrode active material typically used for electrodes. This type of method is often used to evaluate active materials.

[0036] [Preparation of non-aqueous electrolyte] A non-aqueous electrolyte was prepared by dissolving 1.0 mol / L of LiPF6 as an electrolyte salt in a non-aqueous solvent prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 1:3.

[0037] [Test cell construction] The negative electrode and a positive electrode made of lithium metal foil were arranged opposite each other with a separator interposed therebetween to form an electrode assembly, which was then housed in a coin-shaped outer can. After a non-aqueous electrolyte was poured into the outer can, the outer can was sealed to obtain a coin-shaped test cell (nonaqueous electrolyte secondary battery).

[0038] <Example 2> A test cell was produced in the same manner as in Example 1, except that in the preparation of the negative electrode active material, the time of ball mill treatment using a planetary ball mill was extended.

[0039] <Comparative Example> A test cell was produced in the same manner as in Example 1, except that the ball mill treatment using a planetary ball mill was not carried out in the production of the negative electrode active material.

[0040] For each test cell of the examples and comparative examples, the battery capacity (charge capacity and discharge capacity) was evaluated by the following method. The evaluation results are shown in Table 1. Table 1 also shows the dislocation density of the negative electrode active material and the charge / discharge efficiency.

[0041] [Evaluation of Battery Capacity] In this evaluation, "discharge" refers to discharge in a battery that combines the negative electrode active material of the examples and comparative examples with a commonly used positive electrode, such as LiNiO2. In this evaluation, since the coin-type battery uses the negative electrode as the working electrode and metallic lithium (Li) as the counter electrode, the actual term should be "charge." However, in accordance with the charge / discharge behavior of the negative electrode in a battery that combines a commonly used positive electrode and a negative electrode, the term "charge / discharge" is used, which refers to the opposite charge / discharge direction. In other words, "charge" refers to the flow of current to lower the potential of the negative electrode, which serves as the working electrode, and "discharge" refers to the flow of current to raise the potential of the negative electrode, which serves as the working electrode.

[0042] The battery was charged at a constant current of 2.6 mA until the battery voltage reached 0.01 V, and then discharged at a constant current of 2.6 mA until the battery voltage reached 1.5 V. This charge-discharge cycle was repeated twice, and the charge capacity and discharge capacity of the second cycle were measured. Note that the descriptions of charge and discharge are written in the reverse order as described above. That is, charging refers to flowing a current to decrease the potential of the working electrode until the battery voltage reaches 0 V, and discharging refers to flowing a current to increase the potential of the working electrode until the battery voltage reaches 1 V.

[0043] [Table 1]

[0044] As shown in Table 1, the dislocation density is 1×10 15 cm -2 The test cells of the examples containing the above negative electrode active materials had a dislocation density of 1×10 15 cm -2 The battery capacity was higher than that of the test cell of the comparative example containing a negative electrode active material of less than 1000 kJ / cm2. Furthermore, the battery of the example also had higher charge / discharge efficiency than the battery of comparative example 1. [Explanation of symbols]

[0045] 10 Lithium-ion battery, 11 Positive electrode 、1 2 negative electrode, 13 separator, 14 electrode body, 15 exterior body, 16 sealing body, 17, 18 insulating plate, 19 positive electrode lead, 20 negative electrode lead, 21 grooved portion, 22 filter, 23 lower valve body, 24 insulating member, 25 upper valve body, 26 cap, 26a opening, 27 gasket

Claims

1. A negative electrode active material contained in a negative electrode of a lithium ion battery, The negative electrode active material is represented by the general formula M ３ Me ２ X ７ (wherein M contains at least one of La and Ca, Me contains at least one element selected from the group consisting of Mn, Ni, Fe, and Co, and X contains at least one element selected from the group consisting of Ge, Si, Sn, and Al), The dislocation density of the negative electrode active material is 1×10 cm －２ This is the negative electrode active material.

2. a negative electrode comprising the negative electrode active material according to claim 1; A positive electrode and and a non-aqueous electrolyte.

3. 3. The lithium ion battery according to claim 2, wherein the non-aqueous electrolyte is a solid.

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