Positive electrode active material for nonaqueous electrolyte secondary battery and nonaqueous electrolyte secondary battery

Abstract

A positive electrode active material for a nonaqueous electrolyte secondary battery contains: a compound having a rock-salt related structure, a composition formula Li a Mn b M c O 2‑X F x (in the formula, M is at least one metal element other than Li, Mn, and 2.000

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 same positive electrode active material. Background Art

[0002] In non-aqueous electrolyte secondary batteries such as lithium ion batteries, the positive electrode active material has a great influence on battery performance such as input / output characteristics, capacity, and cycle characteristics. As the positive electrode active material, for example, an NCM-based lithium transition metal composite oxide containing Ni, Co, and Mn is widely used. In recent years, as a next-generation positive electrode active material with high capacity, a Li-excess type material based on the rock salt structure of Li x Mn 1-x O2 has attracted much attention.

[0003] For example, Patent Document 1 discloses a positive electrode active material having a crystal structure belonging to the space group Fm-3m, including a lithium transition metal composite oxide represented by the composition formula Li 1+x Nb y Me z A p O2 (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, where Li 1+p Fe 1-q Nb q O2 is excluded, and 0.15 < p ≤ 0.3, 0 < q ≤ 0.3).

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Patent No. 6197029 Summary of the Invention

[0007] As described above, it is expected to use a material based on the rock salt structure of Li x Mn 1-x O2 as a high-capacity positive electrode active material. However, when it is put into practical use, various characteristics must be improved, especially the load characteristics must be improved. The same applies to the positive electrode active material disclosed in Patent Document 1, and the load characteristics must be improved.

[0008] The positive electrode active material for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes: having a rock salt-related structure, the composition formula Li a Mn b M c O 2-X F xA lithium transition metal composite oxide represented by (where 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).

[0009] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes: a positive electrode containing the above positive electrode active material; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte.

[0010] According to the present disclosure, a positive electrode active material with high capacity and high load characteristics can be provided. The positive electrode active material according to the present disclosure can improve the capacity and load characteristics of the non-aqueous electrolyte secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 It is a cross-sectional view of a non-aqueous electrolyte secondary battery as an example of an embodiment. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0012] As described above, it is expected to use a material based on Li x Mn 1-x O2 as a high-capacity positive electrode active material, but a non-aqueous electrolyte secondary battery using this material has a problem of low load characteristics. The inventors of the present invention conducted in-depth research to improve the load characteristics of the above material, and as a result, found that: by introducing a specified amount of fluoride ions and introducing Li into the tetrahedral sites of the crystal structure to form a cation-rich structure, the load characteristics are specifically improved. The mechanism of improving the load characteristics is not fully understood, but the technology of the present disclosure may become a breakthrough for the practical application of the next-generation positive electrode active material with high capacity and high load characteristics.

[0013] Hereinafter, an example of an embodiment of the positive electrode active material for a non-aqueous electrolyte secondary battery and the non-aqueous electrolyte secondary battery using the positive electrode active material according to the present disclosure will be described in detail with reference to the drawings. It should be noted that it is initially assumed to selectively combine multiple embodiments and modification examples described below.

[0014] Hereinafter, a cylindrical battery obtained by housing a wound electrode body 14 in a bottomed cylindrical outer can 16 is exemplified, but the outer casing is not limited to the cylindrical outer can. For example, it can be a square outer can (square battery), a coin-shaped outer can (coin-shaped battery), or an outer casing made of a laminate sheet including a metal layer and a resin layer (laminated battery). In addition, the electrode body can also be a laminated electrode body in which multiple positive electrodes and multiple negative electrodes are alternately laminated with a separator interposed therebetween.

[0015] Figure 1This is a cross-sectional view of a non-aqueous electrolyte secondary battery 10, as an example of an embodiment. (See attached image.) Figure 1 The example non-aqueous electrolyte secondary battery 10 includes: a wound electrode body 14, a non-aqueous electrolyte, and a housing 16 for housing the electrode body 14 and the electrolyte. The electrode body 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 in between. The housing 16 is a bottomed cylindrical metal container with an opening on one axial side, and the opening of the housing 16 is sealed by a sealing body 17. Hereinafter, for ease of explanation, the side with the sealing body 17 of the battery will be referred to as the top, and the bottom side of the housing 16 will be referred to as the bottom.

[0016] Non-aqueous electrolytes comprise: a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and mixtures of two or more of these. The non-aqueous solvent may also contain halogen substitutes, formed by replacing at least a portion of the hydrogen atoms of these solvents with halogen atoms such as fluorine. Examples of electrolyte salts include lithium salts such as LiPF6. It should be noted that the electrolyte is not limited to a liquid electrolyte and can also be a solid electrolyte.

[0017] The positive electrode 11, negative electrode 12, and spacer 13 constituting the electrode body 14 are all strip-shaped elongated bodies, and are spirally wound, thereby being alternately stacked along the radial direction of the electrode body 14. To prevent lithium deposition, the negative electrode 12 is formed to be one size larger than the positive electrode 11. That is, the negative electrode 12 is longer in both the length and width (lateral) directions compared to the positive electrode 11. Two spacers 13 are formed to be at least one size larger than the positive electrode 11, and are arranged, for example, to clamp the positive electrode 11. 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.

[0018] Insulating plates 18 and 19 are respectively arranged above and below the electrode body 14. Figure 1 In the example shown, the positive lead 20 extends towards the sealing body 17 through the through hole in the insulating plate 18, and the negative lead 21 extends towards the bottom of the outer casing 16 through the outer side of the insulating plate 19. The positive lead 20 is connected to the lower surface of the internal terminal plate 23 of the sealing body 17 by welding or the like, and the top plate, i.e., the cover 27, of the sealing body 17, which is electrically connected to the internal terminal plate 23, becomes the positive terminal. The negative lead 21 is connected to the inner bottom surface of the outer casing 16 by welding or the like, and the outer casing 16 becomes the negative terminal.

[0019] A gasket 28 is provided between the outer casing 16 and the sealing body 17 to ensure the airtightness of the battery interior. A grooved portion 22 is formed on the outer casing 16, a portion of which protrudes inward to support the sealing body 17. The grooved portion 22 is preferably formed in a ring shape along the circumferential direction of the outer casing 16, and its upper surface supports the sealing body 17. The sealing body 17 is fixed to the upper part of the outer casing 16 by the grooved portion 22 and the open end of the outer casing 16 to which the sealing body 17 is riveted.

[0020] 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 cover 27 are stacked sequentially from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a circular or annular shape, and all components except the insulating member 25 are electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected at their respective central portions, and the insulating member 25 is sandwiched between their respective peripheral portions. When the internal pressure of the battery rises due to abnormal heat dissipation, the lower valve body 24 deforms and breaks by pushing the upper valve body 26 towards the cover 27 side, thereby blocking the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure rises further, the upper valve body 26 breaks, and gas is discharged from the opening of the cover 27.

[0021] The following describes in detail the positive electrode 11, negative electrode 12, separator 13 constituting the electrode body 14, and especially the positive electrode active material constituting the positive electrode 11.

[0022] [positive electrode]

[0023] The positive electrode 11 comprises: a positive electrode core and a positive electrode composite material layer disposed on the surface of the positive electrode core. The positive electrode core can be a foil of a metal stable within the potential range of the positive electrode 11, such as aluminum or an aluminum alloy, or a thin film of the metal disposed on its surface. The positive electrode composite material layer comprises a positive electrode active material, a conductive material, and a binder material, and is preferably disposed on both sides of the positive electrode core. The positive electrode 11 can be manufactured, for example, by coating a positive electrode composite material slurry comprising a positive electrode active material, a conductive material, and a binder material onto the positive electrode core, drying the coating, and then compressing it to form a positive electrode composite material layer on both sides of the positive electrode core.

[0024] Examples of conductive materials included in the positive electrode composite layer include carbon materials such as carbon black, acetylene black, Ketjen black, and graphite. Examples of binder materials included in the positive electrode composite layer include fluoropolymers such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide resins, acrylic resins, and polyolefin resins. These resins can also be used in combination with cellulose derivatives such as carboxymethyl cellulose (CMC) or its salts, and polyethylene oxide (PEO).

[0025] The positive electrode active material includes: a rock salt-related structure with the composition Lia Mn b M c O 2-X F x (where M is at least one metal element other than Li and Mn, and the lithium transition metal composite oxide is represented by 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). This composite oxide is a Li-excess system material with a molar ratio of Li exceeding 1, and is also a cation-excess (cation-rich) composite oxide with a molar ratio of metal elements containing at least Li and Mn exceeding 2. In addition, a prescribed amount of fluoride ions is introduced to replace a part of O with F.)

[0026] The positive electrode active material has the composite oxide represented by the above compositional formula as the main component. 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 system, a composite compound that does not contain fluoride ions) may also be used in combination, and the content of the above composite oxide is preferably 50% by mass or more, and may be substantially 100% by mass. It should be noted that the composition of the composite oxide can be measured using an ICP emission spectroscopic analyzer (iCAP6300 manufactured by ThermoFisher Scientific).

[0027] 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 not only into the octahedral sites of the rock salt structure but also into the tetrahedral sites surrounding the octahedral sites. The crystal structure of the composite oxide represented by the above compositional formula is similar to the crystal structure of the rock salt type, but does not belong to the space group Fm-3m. It should be noted that the rock salt-related structure in which Li ions are introduced into the tetrahedral sites of the composite oxide is identified by the X-ray diffraction pattern measured using a powder X-ray diffractometer (tabletop X-ray diffractometer MiniFlex manufactured by Rigaku Co., Ltd., X-ray source: CuKα).

[0028] As one mode of the embodiment, the lithium transition metal composite oxide substantially contains only Li and Mn as metal elements. Further, as another mode of the embodiment, the lithium transition metal composite oxide contains another metal element M in addition to Li and Mn. As the other metal element M, 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 can be cited. Among them, at least one selected from Ni, Sn, Mo, Ti, W, Zn, Al is preferred.

[0029] 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 as 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 this range, the high-rate characteristics are specifically improved, and a battery with high load characteristics can be obtained.

[0030] 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 this range, the improvement effect of the load characteristics is more significant.

[0031] 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 improvement effect of the load characteristics is more significantly demonstrated. It should be noted that the molar ratio (c) of the other metal element M other than Li and Mn is 0.2 or less, preferably 0.15 or less, more preferably 0.12 or less.

[0032] The aforementioned lithium transition metal composite oxide can be synthesized, for example, by using lithium fluoride (LiF), lithium manganese oxide (LiMnO2), and lithium oxide (Li2O) as raw materials, and mixing them in a planetary ball mill under an inert gas atmosphere such as Ar. Alternatively, a mixer capable of applying the same stirring and shearing force to the powder can be used instead of a planetary ball mill, and the powder can also be heated during the mixing process. The composition of the composite oxide can be adjusted to the target range, for example, by changing the mixing ratio of the raw materials and the mixing conditions (rotation speed, processing time, processing temperature, etc.).

[0033] [negative electrode]

[0034] The negative electrode 12 comprises: a negative electrode core and a negative electrode composite material layer disposed on the surface of the negative electrode core. The negative electrode core may be a foil of a metal stable within the potential range of the negative electrode 12, such as copper, or a thin film of the metal disposed on its surface. The negative electrode composite material layer comprises a negative electrode active material and a binder material, and is preferably disposed on both sides of the negative electrode core. The negative electrode 12 may be manufactured, for example, by coating the surface of the negative electrode core with a negative electrode composite material slurry comprising a negative electrode active material, a conductive material, and a binder material, drying the coating, and then compressing it to form a negative electrode composite material layer on both sides of the negative electrode core.

[0035] The negative electrode composite material layer includes, for example, a carbon-based active material that reversibly absorbs and releases lithium ions as the negative electrode active material. Ideal carbon-based active materials include natural graphite such as flake graphite, block graphite, and amorphous graphite, as well as artificial graphite such as blocky graphite (MAG) and graphitized mesophase carbon microspheres (MCMB). Alternatively, the negative electrode active material can be a Si-based active material composed of at least one of Si and Si-containing compounds, or a combination of carbon-based and Si-based active materials can be used.

[0036] Similar to the case of the positive electrode 11, carbon materials such as carbon black, acetylene black, Ketjen black, and graphite can be used as the conductive material contained in the negative electrode composite layer. Similarly to the case of the positive electrode 11, fluoropolymers, PAN, polyimide, acrylic resins, and polyolefins can also be used as the binder material contained in the negative electrode composite layer, but styrene-butadiene rubber (SBR) is preferred. Furthermore, the negative electrode composite layer preferably also contains CMC or its salts, polyacrylic acid (PAA) or its salts, and polyvinyl alcohol (PVA). Ideally, SBR should be used in combination with CMC or its salts, and PAA or its salts.

[0037] [Separator]

[0038] The separator 13 is made of a porous sheet with ion permeability and insulation. Specific examples of porous sheets include microporous films, woven fabrics, and nonwoven fabrics. Ideal materials for the separator 13 include polyethylene, polypropylene, copolymers of ethylene and α-olefins, and cellulose. The separator 13 can be a single-layer structure or a multilayer structure. A heat-resistant layer containing inorganic particles, an aromatic polyamide resin, polyimide, polyamide-imide, or other resins with high heat resistance can be formed on the surface of the separator 13.

[0039] <Example>

[0040] The present disclosure will be further described below with reference to embodiments, but the present disclosure is not limited to these embodiments.

[0041] <Example 1>

[0042] Synthesis of positive electrode active materials

[0043] Lithium fluoride (LiF) was mixed with lithium manganese oxide (LiMnO2) and lithium oxide (Li2O) at a specified mass ratio. The mixture was then placed in a planetary ball mill (Fritsch Premium-Line P7, speed: 600 rpm, container: 45 mL, balls: ... The mixed powder was added to Zr pellets and treated in an Ar atmosphere at room temperature for 35 hours (35 cycles of running for 1 hour, then pausing for 10 minutes) to obtain Li. 1.318 Mn 0.825 O 1.792 F 0.208 The lithium transition metal composite oxide shown is an example. The X-ray diffraction pattern of the obtained composite oxide confirms that its crystal structure is similar to that of rock salt, with Li ions introduced into the tetrahedral sites of the rock salt structure.

[0044] [The production of the positive electrode]

[0045] The obtained positive electrode active material was mixed with acetylene black and polyvinylidene fluoride at a solid component mass ratio of 7:2:1, and N-methyl-2-pyrrolidone (NMP) was used as the dispersion medium to prepare a positive electrode composite slurry. Next, the positive electrode composite slurry was coated onto a positive electrode core formed from aluminum foil. After the coating was dried and compressed, it was cut into the specified electrode size to obtain the positive electrode.

[0046] [Preparation of non-aqueous electrolytes]

[0047] Ethyl carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) are mixed in a specified volume ratio. LiPF6 is then added to this mixed solvent to obtain a non-aqueous electrolyte.

[0048] [Fabrication of the Experimental Battery Cell]

[0049] The positive electrode and the negative electrode formed of lithium metal foil are arranged opposite each other through a separator to form an electrode body, which is then housed in a coin-shaped outer casing. After injecting the non-aqueous electrolyte into the outer casing, the casing is sealed to obtain a coin-shaped experimental battery unit (non-aqueous electrolyte secondary battery).

[0050] For the test battery cells, the load characteristics were evaluated using the following method, and the evaluation results are shown in Table 1 together with the composition of the positive electrode active material.

[0051] [Evaluation of load characteristics]

[0052] The charge / discharge conditions for the first cycle are as follows: For the test battery cell, under normal temperature conditions, CC charge is performed at a constant current of 0.05C until the battery voltage reaches 5.2V. Then, after a 20-minute pause, CC discharge is performed at a constant current of 0.05C until the battery voltage reaches 2.5V.

[0053] The charging and discharging conditions for the second cycle are as follows: For the test battery cell, under normal temperature conditions, CC charging is performed at a constant current of 0.2C until the battery voltage reaches 5.2V. Then, the charging is paused for 20 minutes, and CC discharging is performed at a constant current of 0.2C until the battery voltage reaches 2.5V.

[0054] The discharge capacity of the first and second cycles was measured, and the load characteristics were calculated using the following formula.

[0055] Load characteristic (%) = (Discharge capacity in the second cycle / Discharge capacity in the first cycle) × 100

[0056] <Examples 2-10, Comparative Examples 1-4>

[0057] In the synthesis of lithium transition metal composite oxides, the mixing ratio of LiF, LiMnO2, and Li2O was changed to obtain the composition shown in Table 1 (for Examples 5, 6, and 17, nickel oxide was further added). Otherwise, test battery cells were fabricated in the same manner as in Example 1, and the load characteristics were evaluated.

[0058] [Table 1]

[0059]

[0060] As shown in Table 1, the test battery cells of the embodiments are excellent in load characteristics compared with those of the comparative examples. Among Comparative Example 1 and Embodiments 1 to 6 where the molar ratio of F is 0.20 to 0.25, particularly, the compositions of the positive electrode active materials of Comparative Example 1 and Embodiment 4 are similar, but there are significant differences in the load characteristics of the test battery cells using them. In addition, the compositions of the positive electrode active materials of Comparative Example 3 and Embodiment 12, and those of Comparative Example 4 and Embodiment 16 are also extremely similar, but similarly, there are significant differences in the load characteristics of the test battery cells. It should be noted that in the test battery cell of Comparative Example 2 using the positive electrode active material with a total amount of cation components of 2.199 moles, the load characteristics are significantly reduced compared with those of the test battery cells of the embodiments.

[0061] As described above, by using the cation-rich F-containing composite oxide in which the total amount (a + b + c) of the cation components is in the range of 2.000 < a + b + c ≤ 2.195, the load characteristics of the battery are specifically improved.

[0062] Explanation of reference numerals in the attached figures

[0063] 10 Non-aqueous electrolyte secondary battery

[0064] 11 Positive electrode

[0065] 12 Negative electrode

[0066] 13 Separator

[0067] 14 Electrode body

[0068] 16 Outer can

[0069] 17 Sealing body

[0070] 18, 19 Insulating plate

[0071] 20 Positive electrode lead

[0072] 21 Negative electrode lead

[0073] 22 Grooved portion

[0074] 23 Internal terminal board

[0075] 24 Lower valve body

[0076] 25 Insulating member

[0077] 26 Upper valve body

[0078] 27 Cover

[0079] 28 Gasket

Claims

1. A positive electrode active material for a non-aqueous electrolyte secondary battery, comprising: It possesses a cation-rich rock salt-related structure with Li ions introduced into a crystal structure based on Fm-3m, and a compositional formula of Li. a Mn b M c O 2-X F x The lithium transition metal composite oxide shown is In the formula, M represents Ni, 2.000 <a+b+c≤2.195、1.0<a≤1.4、0.4≤b≤0.9、0≤c≤0.15、0.2≤x≤0.6。 2. A non-aqueous electrolyte secondary battery, comprising: a positive electrode containing the positive electrode active material as described in claim 1; a negative electrode; a separator sandwiched between the positive electrode and the negative electrode; and a non-aqueous electrolyte.

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