Positive Electrode Active Material for Non-Aqueous Electrolyte Secondary Battery and Non-Aqueous Electrolyte Secondary Battery

Abstract

This positive-electrode active material for a non-aqueous electrolyte secondary battery contains a lithium and transition metal composite oxide represented by the composition formula: LixMnyNizGeaMbO2-cFc (where, M is at least one element selected from Ti, Co, Si, Al, Nb, W, Mo, P, Ca, Mg, Sb, Na, B, V, Cr, Fe, Cu, Zn, Sr, Zr, Ru, K, and Bi, 1.0<x≤1.2, 0.4≤y≤0.8, 0≤z≤0.4, 0<a<0.01, 0<b<0.03, 0<c<0.1, and x+y+z+a+b≤2).

Description

[Technical Field]

[0001] The present disclosure relates to a positive electrode active material for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using the positive electrode active material. [Background technology]

[0002] In non-aqueous electrolyte secondary batteries such as lithium-ion batteries, the positive electrode active material significantly affects battery performance, including input / output characteristics, capacity, and cycle characteristics. Lithium transition metal composite oxides containing metal elements such as Ni, Co, Mn, and Al are generally used as positive electrode active materials. Because the properties of lithium transition metal composite oxides vary significantly depending on their composition, numerous studies have been conducted on the type and amount of added elements.

[0003] For example, Patent Document 1 describes a compound having the composition formula Li x Ni 1-y Co y-z M z O 2-a X b The positive electrode active material for a non-aqueous electrolyte secondary battery is represented by the formula: and has an a-axis lattice constant of 2.81 to 2.91 Å and a c-axis lattice constant of 13.7 to 14.4 Å, as measured by X-ray diffraction, and a ratio of the diffraction peak intensity of the (104) plane to the peak intensity of the (003) plane of 0.3 to 0.8. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 4197002 Summary of the Invention

[0005] Lithium-excess composite oxides with a molar ratio of Li to transition metal exceeding 1 have also been proposed. Lithium-excess composite oxides are expected as next-generation cathode active materials with high capacity, but there are problems such as easy elution of transition metals. It is known that adding F to lithium-excess composite oxides suppresses the elution of transition metals and improves durability, but there is a problem that the operating voltage decreases when charge and discharge are repeated, and this problem cannot be solved by adding F.

[0006] The cathode active material for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure has a composition formula Li x Mn y Ni z Ge a M b O 2-c F c (where M is at least one selected from Ti, Co, Si, Al, Nb, W, Mo, P, Ca, Mg, Sb, Na, B, V, Cr, Fe, Cu, Zn, Sr, Zr, Ru, K, Bi; 1.0 < x ≤ 1.2, 0.4 ≤ y ≤ 0.8, 0 ≤ z ≤ 0.4, 0 < a < 0.01, 0 < b < 0.03, 0 < c < 0.1, x + y + z + a + b ≤ 2), and contains a lithium transition metal composite oxide represented by this formula.

[0007] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes a cathode containing the above cathode active material, an anode, a separator interposed between the cathode and the anode, and a non-aqueous electrolyte.

[0008] The lithium-excess cathode active material according to the present disclosure has a high voltage retention rate and excellent cycle characteristics.

Brief Description of the Drawings

[0009] [Figure 1] FIG. 1 is a cross-sectional view of a non-aqueous electrolyte secondary battery which is an example of an embodiment.

Embodiments for Carrying Out the Invention

[0010] As described above, when F is added to a lithium-excess composite oxide, the elution of transition metals is suppressed and the durability of the composite oxide is improved, but the effect is not sufficient, and further improvement is required. As a result of the inventors' investigations, it was confirmed that the addition of Ge contributes to improving durability, but the improvement effect is small when Ge is added alone. Ge is thought to suppress oxygen desorption and stabilize the structure of the composite oxide.

[0011] As a result of further intensive research, the present inventors have found that durability is significantly improved and the voltage retention rate during charge and discharge is improved by adding Ge and one or more specific elements to a lithium-excess F-containing composite oxide containing at least Mn as a transition metal.

[0012] Hereinafter, with reference to the drawings, an example of an embodiment of a positive electrode active material for a non-aqueous electrolyte secondary battery according to the present disclosure and a non-aqueous electrolyte secondary battery using the positive electrode active material will be described in detail. Note that it is initially anticipated that multiple embodiments and modifications described below may be selectively combined.

[0013] In the following, a cylindrical battery in which a wound electrode assembly 14 is housed in a cylindrical outer can 16 with a bottom is exemplified, but the outer can is not limited to a cylindrical outer can and may be, for example, a prismatic outer can (prismatic battery) or a coin-shaped outer can (coin battery), or may be an outer can (laminated battery) 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.

[0014] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As shown in FIG. 1, the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte, and an outer can 16 that houses the electrode assembly 14 and the nonaqueous electrolyte. The electrode assembly 14 has a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The outer can 16 is a cylindrical metal container that is open on one axial side and has a bottom, and the opening of the outer can 16 is closed by a sealing member 17. For ease of explanation, the sealing member 17 side of the battery will be referred to as the top, and the bottom side of the outer can 16 will be referred to as the bottom.

[0015] The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents are substituted with halogen atoms such as fluorine. The electrolyte salt may be, for example, a lithium salt such as LiPF6. The non-aqueous electrolyte is not limited to a liquid electrolyte, but may also be a solid electrolyte.

[0016] The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all long, strip-like bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in both the longitudinal direction and the width direction (short direction). The two separators 13 are formed to be at least slightly larger than the positive electrode 11 and are arranged, for example, to sandwich the positive electrode 11. The electrode assembly 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0017] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in Fig. 1 , the positive electrode lead 20 passes through a through-hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom side of the outer can 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal.

[0018] A gasket 28 is provided between the outer can 16 and the sealing body 17 to ensure airtightness inside the battery. The outer can 16 has a grooved portion 22 formed on its side surface that protrudes inward and supports the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer can 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the outer can 16 by the grooved portion 22 and the open end of the outer can 16 that is crimped to the sealing body 17.

[0019] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are layered. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0020] The positive electrode 11, negative electrode 12, and separator 13 that constitute the electrode assembly 14, and in particular the positive electrode active material that constitutes the positive electrode 11, will be described in detail below.

[0021] [Positive electrode] The positive electrode 11 has a positive electrode core and a positive electrode composite layer provided on the surface of the positive electrode core. For the positive electrode core, a foil of a metal stable within the potential range of the positive electrode 11 such as aluminum or an aluminum alloy, a film having the metal disposed on the surface layer, etc. can be used. The positive electrode composite layer contains a positive electrode active material, a conductive material, and a binder, and is preferably provided on both sides of the positive electrode core. The positive electrode 11 can be manufactured, for example, by applying a positive electrode composite slurry containing a positive electrode active material, a conductive material, a binder, etc. on the positive electrode core, drying the coating film, and then compressing it to form the positive electrode composite layer on both sides of the positive electrode core.

[0022] Examples of the conductive material contained in the positive electrode composite layer include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. Examples of the binder contained in the positive electrode composite 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), etc.

[0023] The positive electrode active material has a composition formula Li x Mn y Ni z Ge a M b O 2-c F c (where M is at least one selected from Ti, Co, Si, Al, Nb, W, Mo, P, Ca, Mg, Sb, Na, B, V, Cr, Fe, Cu, Zn, Sr, Zr, Ru, K, Bi; 1.0 < x ≤ 1.2, 0.4 ≤ y ≤ 0.8, 0 ≤ z ≤ 0.4, 0 < a < 0.01, 0 < b < 0.03, 0 < c < 0.1, x + y + z + a + b ≤ 2) and contains a lithium transition metal composite oxide represented by. The composite oxide is a Li-excess system material in which the molar ratio of Li to the transition metal exceeds 1, a predetermined amount of fluoride ions is introduced, and a part of O is substituted by F.

[0024] The positive electrode active material is mainly composed of a composite oxide represented by the above compositional formula. Here, the main component means the component with the highest mass ratio among the constituent components of the composite oxide. In the positive electrode 11, as the positive electrode active material, a composite oxide other than the composite oxide represented by the above compositional formula (for example, a composite oxide that is not a Li-excess type or a composite compound that does not contain fluoride ions) may be used in combination, but the content of the above composite oxide is preferably 50% by mass or more, and may be substantially 100% by mass. The composition of the composite oxide can be measured using an ICP emission spectroscopic analyzer (iCAP6300 manufactured by Thermo Fisher Scientific).

[0025] The lithium transition metal composite oxide represented by the above compositional formula may contain Ni in addition to Li, Mn, and Ge. Further, it contains two or more elements selected from Ti, Co, Si, Al, Nb, W, Mo, P, Ca, Mg, Sb, Na, B, V, Cr, Fe, Cu, Zn, Sr, Zr, Ru, K, Bi as essential components. Among them, Ti, Co, Nb, Sr, Mg, Al, Si, W are preferable.

[0026] In the above compositional formula, it is particularly preferable that M is at least one selected from Co and Al. That is, M is (1) Co, (2) Al, or (3) either Co or Al. Also, the molar ratio (b) of M is preferably 0 < b < 0.02, more preferably 0.001 ≤ b ≤ 0.015, and particularly preferably 0.0002 ≤ b ≤ 0.010. When the element M is selected from the above (1) to (3), the effect of improving the voltage retention rate appears more significantly.

[0027] In the above compositional formula, the molar ratio (x) of Li is 1.0 < x ≤ 1.2, preferably 1.1 ≤ x ≤ 1.2. The molar ratio (y) of Mn is 0.4 ≤ y ≤ 0.8, preferably 0.45 ≤ y ≤ 0.6. The molar ratio (a) of Ge is 0 < a < 0.01, preferably 0.001 ≤ a ≤ 0.007, and more preferably 0.002 ≤ a ≤ 0.005. If the molar ratios of Li, Mn, and Ge are within the above ranges, the improvement effect on the voltage retention rate will be more显著. Ni is an optional component, but for example, it is preferably contained in an amount of 0.05 ≤ z ≤ 0.3.

[0028] In the lithium transition metal composite oxide represented by the above compositional formula, the total molar amount (x + y + z + a + b) of Li, Mn, Ni, Ge, and M is 2 or less, preferably 2. That is, the composite oxide is preferably a Li-excess type composite oxide and not a cation-excess type composite oxide. Also, the molar ratio (c) of F is 0 < c ≤ 0.1, preferably 0.05 ≤ x ≤ 0.085. By adding a predetermined amount of F, the elution of transition metals is suppressed and the durability is improved.

[0029] Specific examples of suitable lithium transition metal composite oxides are Li-excess type F-containing composite oxides containing Mn, Ni, Ge, and at least one selected from Co and Al. The composite oxide substantially does not contain elements other than, for example, Mn, Ni, Ge, Co, Al, Li, O, and F. The molar ratio of each of Co and Al is preferably 0.01 or less, more preferably 0.001 to 0.007, and particularly preferably 0.002 to 0.005, for example, less than the molar ratio of Ge.

[0030] The lithium transition metal composite oxide of this embodiment can be synthesized, for example, by mixing a carbonate containing Mn and Ni, compounds containing Ge, Co, Al, etc. (such as germanium oxide, cobalt sulfate, aluminum hydroxide, etc.), and lithium fluoride (LiF), and firing the mixture. An example of the firing conditions is 700 - 900 °C × 10 - 30 hours.

[0031] [Negative electrode] The negative electrode 12 has a negative electrode core and a negative electrode composite 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 negative electrode composite layer contains a negative electrode active material and a binder, and is preferably provided on both sides of the negative electrode core. The negative electrode 12 can be produced, for example, by applying a negative electrode composite slurry containing a negative electrode active material, a conductive material, a binder, etc. to the surface of the negative electrode core, drying the coating, and then compressing it to form a negative electrode composite layer on both sides of the negative electrode core.

[0032] The negative electrode mixture layer contains, as the negative electrode active material, for example, a carbon-based active material that reversibly absorbs and releases lithium ions. Suitable carbon-based active materials include natural graphite such as flake graphite, lump graphite, and amorphous graphite, and artificial graphite such as lump graphite (MAG) and graphitized mesophase carbon microbeads (MCMB). The negative electrode active material may also be a Si-based active material composed of at least one of Si and a Si-containing compound, or a combination of a carbon-based active material and a Si-based active material.

[0033] As in the case of the positive electrode 11, the conductive material contained in the negative electrode mixture layer can be a carbon material such as carbon black, acetylene black, ketjen black, or graphite. As in the case of the positive electrode 11, the binder contained in the negative electrode mixture layer can be a fluororesin, PAN, polyimide, acrylic resin, polyolefin, or the like, but it is preferable to use styrene-butadiene rubber (SBR). Furthermore, it is preferable that the negative electrode mixture layer further contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like. Among these, it is preferable to use a combination of SBR with CMC or a salt thereof, or PAA or a salt thereof.

[0034] [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 thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene, polypropylene, and copolymers of ethylene and α-olefins, and cellulose. The separator 13 may have either a single-layer structure or a laminated structure. A heat-resistant layer containing inorganic particles, or a heat-resistant layer made of a highly heat-resistant resin such as an aramid resin, polyimide, or polyamideimide, may be formed on the surface of the separator 13.

[0035] <Example> The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0036] Example 1 [Synthesis of lithium transition metal composite oxides] A carbonate containing Mn and Ni in a molar ratio of 2:1 was mixed with germanium oxide, aluminum hydroxide, and lithium fluoride, and the mixture was baked at 800°C for 20 hours in an oxygen stream to produce a crystalline ... 1.167 Mn 0.55 Ni 0.275 Ge 0.02 Al 0.002 O 1.92 F 0.08 A lithium transition metal composite oxide represented by the formula:

[0037] [Preparation of positive electrode] The lithium transition metal composite oxide was used as the positive electrode active material. The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed in a solid mass ratio of 7:2:1, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode composite slurry. Next, the positive electrode composite slurry was applied to a positive electrode core made of aluminum foil, the coating was dried and compressed, and then cut to the specified electrode size to obtain a positive electrode.

[0038] [Preparation of non-aqueous electrolyte] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a predetermined volume ratio, and LiPF6 was added to the mixed solvent to obtain a non-aqueous electrolyte solution.

[0039] [Test cell construction] The positive electrode and the negative 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 the non-aqueous electrolyte solution was poured into the outer can, the outer can was sealed to obtain a coin-shaped test cell (non-aqueous electrolyte secondary battery).

[0040] The voltage retention rate of the test cells was evaluated by the following method, and the evaluation results are shown in Table 1 together with the composition of the positive electrode active material.

[0041] [Evaluation of voltage maintenance rate] The test cell was charged and discharged under the following charge and discharge conditions, and the voltage retention rate was calculated from the average operating voltage at the 20th cycle (V20) and the average operating voltage at the 1st cycle (V1) using the following formula.

[0042] Voltage maintenance rate = (V20 / V1) x 100 Charge / discharge conditions: CC charging was performed at a constant current of 0.05 C up to a battery voltage of 5.2 V, followed by a 20-minute pause and CC discharging at a constant current of 0.05 C down to a battery voltage of 2.5 V. This charge / discharge cycle was repeated 20 times.

[0043] <Example 2, Comparative Example 1> In the synthesis of the lithium transition metal composite oxide, the types of raw materials and the mixing ratio of the raw materials were changed so as to obtain the composition shown in Table 1 (the contents of Ni and Mn were the same as in Example 1). Except for this, test cells were prepared in the same manner as in Example 1, and the voltage retention ratio was evaluated.

[0044] [Table 1]

[0045] As shown in Table 1, the test cells of the examples, which used a lithium-excess F-containing composite oxide containing Mn, Ni, and Ge to which at least one element selected from Co and Al was added as a positive electrode active material, had higher voltage retention ratios than the test cells of the comparative examples. In particular, the positive electrode active material of Example 2, which contains Ge, Al, and Co, significantly improved the voltage retention ratio of the test cells.

[0046] As described above, by adding at least one specific element selected from Co, Al, and the like together with Ge to a lithium-excess F-containing composite oxide containing at least Mn as a transition metal, the voltage retention ratio can be significantly improved. [Explanation of symbols]

[0047] 10 Nonaqueous electrolyte secondary battery 11 Positive electrode 12 Negative electrode 13 Separator 14 Electrode body 16 outer can 17 Sealing body 18,19 Insulating plate 20 Positive lead 21 Negative lead 22 Grooved part 23 Internal terminal board 24 Lower valve body 25 Insulating material 26 Superior valve 27 Cap 28 Gasket

Claims

1. Compositional formula Li ｘ Mn ｙ Ni ｚ Ge ａ M ｂ O ２－ｃ F ｃ (In the formula, M is at least one selected from Co and Al, 1.0 < x ≤ 1.2, 0.4 ≤ y ≤ 0.8, 0 ≤ z ≤ 0.4, 0 < a < 0.01, 0 < b < 0.03, 0 < c < 0.1, x + y + z + a + b ≤ 2), a positive electrode active material for a non-aqueous electrolyte secondary battery containing a lithium transition metal composite oxide represented by the formula.

2. Compositional Li ｘ Mn ｙ Ni ｚ Ge ａ M ｂ O ２－ｃ F ｃ In this case, the molar ratio (b) of M is 0 < b < 0.

02. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1.

3. Compositional formula Li ｘ Mn ｙ Ni ｚ Ge ａ M ｂ O ２－ｃ F ｃ In the case of, the molar ratio (a) of Ge is 0.002 ≦ a ≦ 0.005, the positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 or 2.

4. A non-aqueous electrolyte secondary battery comprising a positive electrode containing a positive electrode active material according to any one of Claims 1 to 3, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte.

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