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-transition metal composite oxide that has a rock salt-related structure and is represented by the compositional formula LiaMnbMcO2-XFx (in the formula: M is at least one metal element excluding Li and Mn; 2.000 < a + b + c ≤ 2.195; 1.0 < a ≤ 1.4; 0.4 ≤ b ≤ 0.9; 0 ≤ c ≤ 0.2; and 0.2 ≤ x ≤ 0.6). The metal element M preferably is at least one selected from Ni, Sn, Mo, Ti, W, Zn, and Al.

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 Art

[0002] In non-aqueous electrolyte secondary batteries such as lithium-ion batteries, the positive electrode active material greatly affects battery performance such as input / output characteristics, capacity, and cycle characteristics. As the positive electrode active material, for example, NCM-based lithium transition metal composite oxides containing Ni, Co, and Mn are widely used. In recent years, as a next-generation high-capacity positive electrode active material, a Li-excess type material based on rock salt structure Li x Mn 1-x O 2 has attracted attention.

[0003] For example, Patent Document 1 discloses a positive electrode active material containing a lithium transition metal composite oxide having a crystal structure belonging to the space group Fm-3m and represented by the composition formula Li 1+x Nb y Me z A p O 2 (Me is a transition metal containing Fe and / or Mn, 0 < x < 1, 0 < y < 0.5, 0.25 ≦ z < 1, A is an element other than Nb and Me, 0 ≦ p ≦ 0.2, provided that Li 1+p Fe 1-q Nb q O 2 is excluded, where 0.15 < p ≦ 0.3 and 0 < q ≦ 0.3).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

[0005] As described above, Li of rock salt structure x Mn 1-x O2 Materials based on it are expected to be high-capacity cathode active materials, but various properties need to be improved for practical use, and in particular, the load characteristics need to be improved. The same applies to the cathode active materials disclosed in Patent Document 1, and it is necessary to improve the load characteristics.

[0006] The cathode active material for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure has a rock salt-related structure, and the composition formula is Li a Mn b M c O 2-X F x (In the formula, M is at least one metal element other than Li and Mn, and 2.000 < a + b + c ≤ 2.195, 1.0 < a ≤ 1.4, 0.4 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.2, 0.2 ≤ x ≤ 0.6), and includes a lithium transition metal composite oxide represented by

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

[0008] According to the present disclosure, a cathode active material with high capacity and high load characteristics can be provided. According to the cathode active material according to the present disclosure, the capacity and load characteristics of a non-aqueous electrolyte secondary battery can be improved.

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, Li with a rock salt structure x Mn 1-x O 2Materials based on fluoride ions are expected to be high-capacity positive electrode active materials, but nonaqueous electrolyte secondary batteries using such materials have the problem of low load characteristics. The present inventors have conducted extensive research to improve the load characteristics of such materials and have found that the load characteristics are specifically improved by introducing a predetermined amount of fluoride ions and Li into the tetrahedral sites of the crystal structure to form a cation-rich structure. Although the mechanism of the improvement in load characteristics is not well understood, the technology disclosed herein may be a breakthrough that leads to the practical application of next-generation positive electrode active materials with high capacity and high load characteristics.

[0011] Hereinafter, with reference to the drawings, an example of an embodiment of a positive electrode active material for a nonaqueous electrolyte secondary battery according to the present disclosure and a nonaqueous electrolyte secondary battery using the positive electrode active material will be described in detail. Note that it is initially assumed that the multiple embodiments and modified examples described below will be selectively combined.

[0012] In the following, a cylindrical battery in which a wound electrode body 14 is housed in a cylindrical exterior can 16 with a bottom is exemplified, but the exterior can is not limited to a cylindrical exterior can, and may be, for example, a square exterior can (square battery) or a coin-shaped exterior can (coin battery), or may be an exterior body (laminated battery) made of a laminate sheet including a metal layer and a resin layer. The electrode body may also be a laminated type electrode body in which multiple positive electrodes and multiple negative electrodes are alternately laminated with separators between them.

[0013] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As illustrated in FIG. 1, the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte, and an exterior can 16 that accommodates the electrode assembly 14 and the 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 wound in a spiral shape with the separator 13 interposed therebetween. The exterior can 16 is a cylindrical metal container with a bottom that is open on one axial side, and the opening of the exterior can 16 is closed by a sealing body 17. In the following description, for convenience of explanation, the sealing body 17 side of the battery is referred to as the top, and the bottom side of the exterior can 16 is referred to as the bottom.

[0014] 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 of these solvents is replaced with a halogen atom such as fluorine. The electrolyte salt may include, for example, LiPF 6 The electrolyte is not limited to a liquid electrolyte, and may be a solid electrolyte.

[0015] The positive electrode 11, the negative electrode 12, and the separator 13 constituting the electrode body 14 are all strip-shaped long bodies, and are alternately stacked in the radial direction of the electrode body 14 by being wound in a spiral shape. 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 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 to sandwich the positive electrode 11, for example. The electrode body 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.

[0016] 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 casing 16. The positive electrode lead 20 is connected to the lower surface of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is a top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, serves as a positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom inner surface of the outer casing 16 by welding or the like, and the outer casing 16 serves as a negative electrode terminal.

[0017] A gasket 28 is provided between the exterior can 16 and the sealing body 17 to ensure airtightness inside the battery. The exterior can 16 has a grooved portion 22 formed therein, with part of the side surface protruding inward, to support the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the exterior can 16, and supports the sealing body 17 on its upper surface. The sealing body 17 is fixed to the top of the exterior can 16 by the grooved portion 22 and the open end of the exterior can 16 which is crimped against the sealing body 17.

[0018] The sealing body 17 has a structure in which an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are laminated in this order from the electrode body 14 side. Each member constituting the sealing body 17 has, for example, a disk shape or a ring shape, and each member except the insulating member 25 is electrically connected to each other. 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 so as to push the upper valve body 26 toward the cap 27, thereby cutting off 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.

[0019] The positive electrode 11, the negative electrode 12, and the 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.

[0020] [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 such as aluminum or an aluminum alloy that is stable in the potential range of the positive electrode 11, or a film with the metal disposed on the surface layer, 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 produced, for example, by applying a positive electrode composite slurry containing a positive electrode active material, a conductive material, and a binder, and then drying the coating, and compressing it to form a positive electrode composite layer on both sides of the positive electrode core.

[0021] 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, and polyethylene oxide (PEO).

[0022] The positive electrode active material has a rock salt-related structure and has a composition formula Li a Mn b M c O 2-X F x (wherein M is at least one metal element other than Li and Mn, 2.000 < a + b + c ≦ 2.195, 1.0 < a ≦ 1.4, 0.4 ≦ b ≦ 0.9, 0 ≦ c ≦ 0.2, 0.2 ≦ x ≦ 0.6), and contains a lithium transition metal composite oxide represented by the formula. The composite oxide is a Li-excess type material in which the molar ratio of Li exceeds 1, and is further a cation-excess (cation-rich) composite oxide in which the molar ratio of at least the metal elements containing Li and Mn exceeds 2. In addition, a predetermined amount of fluoride ions is introduced, and a part of O is substituted with F.)

[0023] The positive electrode active material is mainly composed of the composite oxide represented by the above composition formula. Here, the main component means the component having 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 composition 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).

[0024] The composite oxide represented by the above compositional formula has a rock-salt related structure similar to the rock-salt structure, in which Li ions are introduced into the tetrahedral sites existing around the octahedral sites in addition to the octahedral sites of the rock-salt structure. The crystal structure of the composite oxide represented by the above compositional formula is similar to the rock-salt type crystal structure, but is not assigned to the space group Fm-3m. It should be noted that the fact that the composite oxide has a rock-salt related structure with Li ions introduced into the tetrahedral sites is identified from the X-ray diffraction pattern measured using a powder X-ray diffractometer (Desktop X-ray Diffractometer MiniFlex manufactured by Rigaku Corporation, X-ray source: CuKα).

[0025] The lithium transition metal composite oxide which is one aspect of the embodiment contains substantially only Li and Mn as metal elements. Further, the lithium transition metal composite oxide which is another aspect of the embodiment contains, in addition to Li and Mn, another metal element M. Examples of the other metal element M include at least one selected from Ni, Co, Fe, Al, Sn, Cu, Nb, Mo, Bi, Ti, V, Cr, Y, Zr, Zn, Na, K, Ca, Mg, Pt, Au, Ag, Ru, Ta, W, La, Ce, Pr, Sm, Eu, Dy, Er. Among them, at least one selected from Ni, Sn, Mo, Ti, W, Zn, Al is preferable.

[0026] In the composite oxide represented by the above compositional formula, the molar ratio (a + b + c) of Li, Mn, and the optionally added metal element M, which are cation components, is 2.000 < a + b + c ≦ 2.195, preferably 2.000 < a + b + c ≦ 2.150. By using the cation-rich F-containing composite oxide in which the molar ratio (a + b + c) of the cation components is within the above range, the high-rate characteristics are specifically improved, and a battery with high-load characteristics can be obtained.

[0027] In the composite oxide represented by the above compositional formula, the molar ratio (a + b) of Li and Mn is preferably 1.900 < x ≦ 2.195, more preferably 1.950 ≦ x ≦ 2.150. Further, the molar ratio (a) of Li is 1.0 < a ≦ 1.4, preferably 1.10 ≦ a ≦ 1.35, more preferably 1.15 ≦ a ≦ 1.35. The molar ratio (b) of Mn is 0.4 ≦ b ≦ 0.9, preferably 0.65 ≦ b ≦ 0.89, more preferably 0.70 ≦ b ≦ 0.88. If the molar ratio of Li and Mn is within the above range, the effect of improving the loading characteristics is more remarkable.

[0028] In the composite oxide represented by the above compositional formula, the molar ratio (x) of F is 0.2 ≦ x ≦ 0.6, preferably 0.2 ≦ x ≦ 0.5, more preferably 0.20 ≦ x ≦ 0.35. In this case, the effect of improving the loading characteristics appears more remarkably. In addition, the molar ratio (c) of other metal elements M other than Li and Mn is 0.2 or less, preferably 0.15 or less, more preferably 0.12 or less.

[0029] The above lithium transition metal composite oxide can be synthesized, for example, by using lithium fluoride (LiF), lithium manganate (LiMnO 2 ), and lithium oxide (Li 2 O) as raw materials and performing a mixing treatment with a planetary ball mill in an inert gas atmosphere such as Ar. Further, instead of the planetary ball mill, a mixer capable of applying the same shearing force to the powder may be used, and the powder may be heated during the mixing treatment. The composition of the composite oxide can be adjusted to the target range, for example, by changing the mixing ratio of the raw materials, mixing conditions (rotation speed, treatment time, treatment temperature, etc.).

[0030] [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. For the negative electrode core, a foil of a metal such as copper that is stable in the potential range of the negative electrode 12, a film with the metal disposed on the surface layer, or the like can be used. 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, and the like 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.

[0031] 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 are graphites such as natural graphite, such as flake graphite, lump graphite, and earthy graphite, and artificial graphite, such as lump artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB). In addition, the negative electrode active material may be a Si-based active material composed of at least one of Si and a Si-containing compound, or a carbon-based active material and a Si-based active material may be used in combination.

[0032] As the conductive material contained in the negative electrode mixture layer, carbon materials such as carbon black, acetylene black, ketjen black, and graphite can be used, as in the case of the positive electrode 11. As the binder contained in the negative electrode mixture layer, fluororesin, PAN, polyimide, acrylic resin, polyolefin, and the like can be used, as in the case of the positive electrode 11, but styrene-butadiene rubber (SBR) is preferably used. In addition, the negative electrode mixture layer preferably further contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), and the like. Among them, it is preferable to use SBR in combination with CMC or a salt thereof, and PAA or a salt thereof.

[0033] [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. The material of the separator 13 is preferably a polyolefin such as polyethylene, polypropylene, a copolymer of ethylene and α-olefin, or 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, a polyimide, or a polyamideimide may be formed on the surface of the separator 13.

[0034] <Example> The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0035] <Example 1> [Synthesis of positive electrode active material] Lithium fluoride (LiF) and lithium manganese oxide (LiMnO 2 ) and lithium oxide (Li 2 The mixed powder was placed in a planetary ball mill (Fritsch Premium-Line P7, rotation speed: 600 rpm, container: 45 mL, ball: Zr ball of φ3 mm) and treated in an Ar atmosphere at room temperature for 35 hours (35 cycles of 1 hour operation followed by 10 minutes of rest) to obtain a powder with the composition formula Li 1.318 Mn 0.825 O 1.792 F 0.208 The X-ray diffraction pattern of the obtained composite oxide confirmed that the crystal structure of the composite oxide was similar to a rock salt structure in which Li ions were introduced into the tetrahedral sites of the rock salt structure.

[0036] [Preparation of positive electrode] The obtained positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed in a solid content mass ratio of 7:2:1, and a positive electrode mixture slurry was prepared using N-methyl-2-pyrrolidone (NMP) as a dispersion medium. Next, the positive electrode mixture slurry was applied onto a positive electrode core made of aluminum foil, and the coating was dried and compressed, and then cut into a predetermined electrode size to obtain a positive electrode.

[0037] [Preparation of non-aqueous electrolyte] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a predetermined volume ratio. LiPF 6 to obtain a non-aqueous electrolyte solution.

[0038] [Preparation of test cell] 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. The nonaqueous electrolyte was poured into the outer can, and the outer can was then sealed to obtain a coin-shaped test cell (nonaqueous electrolyte secondary battery).

[0039] The load characteristics of the test cells were evaluated by the following method, and the evaluation results are shown in Table 1 together with the composition of the positive electrode active material.

[0040] [Evaluation of load characteristics] Charge / discharge conditions for the first cycle: The test cell was CC charged at a constant current of 0.05C in a room temperature environment up to a battery voltage of 5.2V, then left for 20 minutes and CC discharged at a constant current of 0.05C down to a battery voltage of 2.5V.

[0041] Charge and discharge conditions for the second cycle: The test cell was CC charged at a constant current of 0.2C in a room temperature environment up to a battery voltage of 5.2V, then left for 20 minutes, and CC discharged at a constant current of 0.2C down to a battery voltage of 2.5V.

[0042] The discharge capacities at the first and second cycles were measured, and the load characteristics were calculated using the following formula.

[0043] Load characteristic (%) = (Discharge capacity in the second cycle / Discharge capacity in the first cycle) × 100 <Examples 2 to 10, Comparative Examples 1 to 4> In the synthesis of the lithium transition metal composite oxide, LiF, LiMnO were used so that the compositions shown in Table 1 could be obtained. 2 , and Li 2 O (for Examples 5, 6, and 17, nickel oxide was further added), test cells were fabricated in the same manner as in Example 1 except that the mixing ratios were changed, and the load characteristics were evaluated.

[0044]

Table 1

[0045] As shown in Table 1, all of the test cells of the examples are superior in load characteristics to the test cells of the comparative examples. In Comparative Example 1 and Examples 1 to 6 where the molar ratio of F is 0.20 to 0.25, in particular, the compositions of the positive electrode active materials of Comparative Example 1 and Example 4 are similar, but there is a large difference in the load characteristics of the test cells using these. Also, the compositions of the positive electrode active materials of Comparative Example 3 and Example 12, and the compositions of the positive electrode active materials of Comparative Example 4 and Example 16 are extremely similar, but there is also a large difference in the load characteristics of the test cells. In addition, in the test cell of Comparative Example 2 using a positive electrode active material with a total amount of cation components of 2.199 moles, the load characteristics were significantly reduced compared to the test cells of the examples.

[0046] As described above, by using a cation-rich F-containing composite oxide in which the total amount (a + b + c) of the cation components is within the range of 2.000 < a + b + c ≤ 2.195, the load characteristics of the battery are specifically improved.

Explanation of Symbols

[0047] 10 Non-aqueous 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 materials 26 Upper valve 27 Cap 28 Gasket

Claims

1. It has a rock salt-related structure, Composition formula Li ａ Mn ｂ M ｃ O ２－Ｘ F ｘ (wherein M is at least one metal element other than Li and Mn, and the lithium transition metal composite oxide is represented by the formula: 2.000<a+b+c≦2.195, 1.0<a≦1.4, 0.4≦b≦0.9, 0≦c≦0.12, and 0.2≦x≦0.6), The metal element M is at least one selected from the group consisting of Ni, Sn, Mo, Ti, W, Zn, and Al.

2. Composition formula Li ａ Mn ｂ M ｃ O ２－Ｘ F ｘ 2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the metal element M comprises Ni.

3. 3. A non-aqueous electrolyte secondary battery comprising: a positive electrode containing the positive electrode active material according to claim 1; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte.

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