Positive electrode active material and method for manufacturing positive electrode active material

Abstract

The present invention provides a positive electrode active material that suppresses the decrease in capacity of non-aqueous secondary batteries, and a method for producing the positive electrode active material. [Solution] The method for producing the positive electrode active material involves making the surface of monoclinic NaMnO2 particles P2 type Na 0.7 The coating process (step S2) involves coating with MnO2, and then replacing Na with Li by electrochemical substitution, thereby creating an O2-type LiMnO2 particle surface. 0.7 The process includes a substitution step (step S3) to produce a material coated with MnO2.

Description

[Technical Field]

[0001] This invention relates to a positive electrode active material and a method for producing a positive electrode active material. [Background technology]

[0002] Patent Document 1 discloses a positive electrode active material having an O2-type structure used in lithium-ion batteries. The positive electrode active material having an O2-type structure exhibits excellent cycle characteristics during high-speed charge and discharge. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-90466 [Overview of the project] [Problems that the invention aims to solve]

[0004] By the way, the positive electrode active material described in Patent Document 1 above has an O2 type structure, Li 0.7 Lithium can only be filled up to the MnO2 composition. Therefore, the amount of lithium that can be charged and discharged is less than that of a monoclinic LiMnO2 cathode active material, resulting in a decrease in the actual capacity of the non-aqueous secondary battery. [Means for solving the problem]

[0005] The cathode active material that solves the above problem is one in which the surface of monoclinic LiMnO2 particles is coated with O2-type LiMnO2. Monoclinic LiMnO2 undergoes a spinel phase transition, where it transitions to a spinel structure during charge-discharge cycles, resulting in a decrease in capacity. On the other hand, O2-type LiMnO2 is less prone to the spinel phase transition, while Li 0.7Since lithium can only be filled up to the composition, the capacity of the non-aqueous secondary battery decreases. According to the above configuration, by coating the surface of the monoclinic LiMnO2 particles with O2-type LiMnO2, the bulk region of the positive electrode active material becomes monoclinic LiMnO2 particles, and the surface region of the positive electrode active material becomes O2-type LiMnO2. Therefore, the spinel phase transition of the monoclinic LiMnO2 particles can be suppressed, and the decrease in the capacity of the non-aqueous secondary battery can be suppressed.

[0006] The method for manufacturing a positive electrode active material for solving the above problems includes a coating step of coating the surface of monoclinic NaMnO2 particles with P2-type Na 0.7 MnO2, and a substitution step of generating a material in which the surface of the monoclinic LiMnO2 particles is coated with O2-type Li 0.7 MnO2 by exchanging Na with Li through electrochemical substitution.

[0007] Monoclinic LiMnO2 undergoes a spinel phase transition during charge-discharge cycles, resulting in a decrease in capacity. Also, while O2-type LiMnO2 is less likely to undergo a spinel phase transition, it can only be filled with lithium up to the composition, so the capacity of the non-aqueous secondary battery decreases. According to the above configuration, by coating the surface of the monoclinic LiMnO2 particles with O2-type LiMnO2, the bulk region of the positive electrode active material becomes monoclinic LiMnO2 particles, and the surface region of the positive electrode active material becomes O2-type LiMnO2. Therefore, the spinel phase transition of the monoclinic LiMnO2 particles can be suppressed, and the decrease in the capacity of the non-aqueous secondary battery can be suppressed.

[0008] The method for manufacturing a positive electrode active material for solving the above problems includes a first substitution step of generating monoclinic LiMnO2 particles by exchanging Na with Li through electrochemical substitution of monoclinic NaMnO2 particles, a coating step of coating the surface of the monoclinic LiMnO2 particles with P2-type Na 0.7 MnO2, and a second substitution step of generating O2-type Li 0.7 MnO2 by exchanging Na with Li through electrochemical substitution of P2-type Na 0.7 MnO2.

[0009] ​Monoclinic LiMnO₂ undergoes a spinel phase transition during charge-discharge cycles, resulting in a decrease in capacity. In addition, while O₂-type LiMnO₂ is less likely to undergo a spinel phase transition, it can only be filled with lithium up to the Li 0.7 composition, leading to a decrease in the capacity of the non-aqueous secondary battery. According to the above configuration, by coating the surface of the monoclinic LiMnO₂ particles with O₂-type LiMnO₂, the bulk region of the positive electrode active material becomes monoclinic LiMnO₂ particles, and the surface region of the positive electrode active material becomes O₂-type LiMnO₂. Therefore, the spinel phase transition of the monoclinic LiMnO₂ particles can be suppressed, and the decrease in the capacity of the non-aqueous secondary battery can be suppressed.

Advantages of the Invention

[0010] According to the present invention, a decrease in the capacity of a non-aqueous secondary battery can be suppressed.

Brief Description of the Drawings

[0011] [Figure 1] It is a perspective view showing a schematic configuration of a non-aqueous secondary battery of the first embodiment. [Figure 2] It is a diagram showing a part of the electrode body of the same embodiment developed. [Figure 3] It is a diagram showing a change in the crystal structure of monoclinic LiMnO₂ before and after charge-discharge cycles. [Figure 4] It is a diagram showing a change in the crystal structure of O₂-type LiMnO₂ before and after charge-discharge cycles. [Figure 5] It is a diagram showing a change in the crystal structure of monoclinic LiMnO₂ coated with P2-type Na₀.₇MnO₂ on the surface of the same embodiment before and after charge-discharge cycles. [Figure 6] It is a diagram showing the interface between monoclinic Li₀.₅MnO₂ and O₂-type Li₀.₅MnO₂. [Figure 7] It is a diagram showing the interface between spinel-type Li₀.₅MnO₂ and O₂-type Li₀.₅MnO₂. [Figure 8] It is a flowchart showing a method for manufacturing the positive electrode active material of the same embodiment. [Figure 9]This figure shows the interface between monoclinic NaMnO2 and P2-type Na0.7MnO2 in the same embodiment. [Figure 10] This figure shows the interface between monoclinic LiMnO2 and O2-type Li0.7MnO2 in the same embodiment. [Modes for carrying out the invention]

[0012] [This Circumstance] Hereinafter, an embodiment of the positive electrode active material and a method for manufacturing the positive electrode active material will be described with reference to Figures 1 to 10.

[0013] [Lithium-ion rechargeable battery 10] As shown in Figure 1, the lithium-ion secondary battery 10, which is a non-aqueous secondary battery, is a cell battery that forms a battery pack when multiple lithium-ion secondary batteries 10 are combined and enclosed in a resin or metal case. The battery pack is used in hybrid vehicles and electric vehicles.

[0014] The lithium-ion secondary battery 10 comprises a battery case 11 and a cover 12. The battery case 11 is rectangular in shape with an opening on its upper side. The cover 12 seals the opening of the battery case 11. The battery case 11 and the cover 12 are made of a metal such as aluminum or an aluminum alloy. The lithium-ion secondary battery 10 is formed as a sealed battery case by attaching the cover 12 to the battery case 11.

[0015] The cover 12 is provided with two positive external terminals 13A and a negative external terminal 13B. The positive external terminals 13A and the negative external terminals 13B are used for charging and discharging power. Multiple wound bodies 20 are housed inside the battery case 11. The wound bodies 20 are electrode bodies. In this embodiment, three wound bodies 20 are housed in the battery case 11. The positive electrode side current collector portion 20A, which is the positive electrode end of the wound body 20, is electrically connected to the positive external terminal 13A via the positive electrode side current collector member 14A. The negative electrode side current collector portion 20B, which is the negative electrode end of the wound body 20, is electrically connected to the negative external terminal 13B via the negative electrode side current collector member 14B. In addition, a non-aqueous electrolyte is injected into the battery case 11 through an injection hole (not shown). Note that the shapes of the positive external terminal 13A and the negative external terminal 13B are not limited to those shown in Figure 1, but may be any shape.

[0016] [Wound body 20] As shown in Figure 2, the wound body 20 is a flat electrode body formed by winding a laminate in which a long positive electrode sheet 21 and a negative electrode sheet 24 are laminated with a separator 27 in between. The positive electrode sheet 21, the negative electrode sheet 24, and the separator 27 are laminated so that their respective longitudinal directions coincide with the longitudinal direction D1. Before winding, the laminate is laminated in the order of positive electrode sheet 21, separator 27, negative electrode sheet 24, separator 27. The positive electrode sheet 21 and the negative electrode sheet 24 are electrode sheets.

[0017] [Positive electrode sheet 21] The positive electrode sheet 21 comprises a positive electrode current collector 22 and a positive electrode composite layer 23. The positive electrode current collector 22 is a foil-shaped positive electrode substrate formed in an elongated shape. The positive electrode composite layer 23 is provided on each of two opposing surfaces of the positive electrode current collector 22. The positive electrode current collector 22 has an uncoated positive electrode side portion 22A at one end in the width direction D2 where the positive electrode composite layer 23 is not formed and the positive electrode current collector 22 is exposed.

[0018] The positive electrode current collector 22 is made of a metal foil composed of aluminum or an alloy mainly composed of aluminum. The positive electrode current collector 22 functions as a current collector at the positive electrode. The unpainted positive electrode side portion 22A of the positive electrode current collector 22 is pressed against each other by opposing surfaces in the state of the wound body 20, forming the positive electrode side current collector portion 20A.

[0019] The positive electrode composite layer 23 is a cured form of a liquid positive electrode composite paste. The positive electrode composite paste contains a positive electrode active material, a positive electrode solvent, a positive electrode conductive material, and a positive electrode binder. The positive electrode composite layer 23 is formed when the positive electrode composite paste dries and the positive electrode solvent vaporizes. Therefore, the positive electrode composite layer 23 contains a positive electrode active material, a positive electrode conductive material, and a positive electrode binder.

[0020] The positive electrode active material is a lithium-containing composite oxide capable of intercalating and releasing lithium ions, which are charge carriers in the lithium-ion secondary battery 10. The lithium-containing composite oxide is an oxide containing lithium and other metallic elements other than lithium. The other metallic elements other than lithium are, for example, at least one selected from the group consisting of nickel, cobalt, manganese, vanadium, magnesium, molybdenum, niobium, titanium, tungsten, aluminum, and iron contained as iron phosphate in the lithium-containing composite oxide.

[0021] For example, lithium-containing composite oxides include lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), and lithium manganeseate (LiMn2O4). For example, a lithium-containing composite oxide is a ternary lithium-containing composite oxide containing nickel, cobalt, and manganese, and is nickel-cobalt-manganate lithium (LiNiCoMnO2). For example, a lithium-containing composite oxide is lithium iron phosphate (LiFePO4). The positive electrode active material of this embodiment is lithium manganeseate.

[0022] The positive electrode solvent is an NMP (N-methyl-2-pyrrolidone) solution, which is an example of an organic solvent. Examples of positive electrode conductive materials include carbon black such as acetylene black and Ketjenblack, carbon fibers such as carbon nanotubes and carbon nanofibers, and graphite. The positive electrode binder is an example of a resin component contained in the positive electrode paste. Examples of positive electrode binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), and styrene-butadiene rubber (SBR).

[0023] The positive electrode sheet 21 may have an insulating layer at the boundary between the uncoated portion 22A on the positive electrode side and the positive electrode composite layer 23. The insulating layer contains an inorganic component having insulating properties and a resin component that functions as a binder. The inorganic component is at least one selected from the group consisting of powdered boehmite, titania, and alumina. The resin component is at least one selected from the group consisting of PVDF, PVA, and acrylic.

[0024] [Negative electrode sheet 24] The negative electrode sheet 24 comprises a negative electrode current collector 25 and a negative electrode composite layer 26. The negative electrode current collector 25 is a foil-shaped negative electrode substrate formed in an elongated shape. The negative electrode composite layer 26 is provided on each of two opposing surfaces of the negative electrode current collector 25. The negative electrode current collector 25 has a negative electrode side unpainted portion 25A at one end in the width direction D2, which is located opposite the positive electrode side unpainted portion 22A, where the negative electrode composite layer 26 is not formed and the negative electrode current collector 25 is exposed.

[0025] The negative electrode current collector 25 is made of metal foil composed of copper or an alloy mainly composed of copper. The negative electrode current collector 25 functions as a current collector at the negative electrode. In the state of the wound body 20, the unpainted negative electrode side portion 25A has opposing surfaces pressed against each other to form the negative electrode side current collector portion 20B.

[0026] The negative electrode composite layer 26 is a cured body of a liquid negative electrode composite paste. The negative electrode composite paste contains a negative electrode active material, a lithium salt, a negative electrode solvent, a negative electrode thickener, and a negative electrode binder. The negative electrode composite layer 26 is formed when the negative electrode composite paste dries and the negative electrode solvent vaporizes. Therefore, the negative electrode composite layer 26 contains the negative electrode active material, a lithium salt, and further, as additives, a negative electrode thickener and a negative electrode binder. The negative electrode composite layer 26 may further contain additives such as a conductive material.

[0027] The negative electrode active material is a material capable of intercalating and releasing lithium ions. Examples of negative electrode active materials include carbon materials such as graphite, poorly graphitizable carbon, and easily graphitizable carbon. The negative electrode solvent is, for example, water. The lithium salt can be one or more lithium compounds (lithium salts) selected from LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, LiC4F9SO3, LiN(CF3SO2)2, LiC(CF3SO2)3, LiI, LiBOB (lithium bisoxalate borate), etc. As an example of a negative electrode thickener, CMC (carboxymethylcellulose) can be used as a thickener containing a sodium salt. The negative electrode binder can be the same as that used for the positive electrode binder. As an example of a negative electrode binder, SAR (styrene-acrylic acid copolymer) can be used as a binder containing a sodium salt.

[0028] [Separator 27] The separator 27 prevents contact between the positive electrode sheet 21 and the negative electrode sheet 24, and holds the non-aqueous electrolyte between the positive electrode sheet 21 and the negative electrode sheet 24. When the wound body 20 is immersed in the non-aqueous electrolyte, the non-aqueous electrolyte penetrates from the ends in the width direction D2 of the separator 27 toward the center.

[0029] The separator 27 is a nonwoven fabric made of polypropylene or the like. As the separator 27, for example, porous polymer membranes such as porous polyethylene membranes, porous polyolefin membranes, and porous polyvinyl chloride membranes, and ion-conductive polymer electrolyte membranes can be used.

[0030] [Nonaqueous electrolyte] A non-aqueous electrolyte is a composition containing a supporting salt in a non-aqueous solvent. As the non-aqueous solvent, one or more materials selected from the group consisting of propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, etc., can be used. As the supporting salt, one or more lithium compounds (lithium salts) selected from LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, LiC4F9SO3, LiN(CF3SO2)2, LiC(CF3SO2)3, LiI, etc., can be used.

[0031] [Cathode active material] Here, we will describe lithium manganese oxide (LiMnO), which is the positive electrode active material used in this embodiment. As shown in Figure 3, when monoclinic LiMnO2 undergoes repeated charge-discharge cycles, lithium detachment occurs, and since the spinel-type LiMn2O4 is the stable phase, a spinel phase transition occurs, transforming the LiMnO2 into a spinel structure. Because the spinel phase transition occurs from the surface inward, monoclinic LiMnO2 remains near the center. When monoclinic LiMnO2 changes to spinel-type LiMn2O4, the capacity of the lithium-ion secondary battery 10 decreases by about 50%.

[0032] As shown in Figure 4, O2-type LiMnO2 is less prone to spinel phase transitions even after repeated charge-discharge cycles. However, O2-type LiMnO2 is Li 0.7 Since lithium can only be filled up to its composition, the capacity of the lithium-ion secondary battery 10 is limited to about 70%.

[0033] Therefore, as shown in Figure 5, in this embodiment, a positive electrode active material is used in which the surface of monoclinic LiMnO2 particles is coated with O2-type LiMnO2. O2-type LiMnO2 is Li 0.7Since lithium can only be filled up to its composition, the bulk region of the positive electrode active material is monoclinic LiMnO2, and the surface region of the positive electrode active material is O2-type LiMnO2. Monoclinic LiMnO2 coated with O2-type LiMnO2 is less prone to spinel phase transitions even after repeated charge-discharge cycles, thus maintaining the capacity of the lithium-ion secondary battery 10 at nearly 100% and suppressing capacity degradation.

[0034] In Figure 6, the surface is O2-type Li 0.5 Monoclinic Li coated with MnO2 0.5 It shows MnO2. Monoclinic Li 0.5 MnO2 and O2-type Li 0.5 The interfacial energy with MnO2 is 0.46 [J / m 2 Interfacial energy is the energy possessed by a unit area of ​​an interface, and it is the energy required to change the interface from a stable state to a different state.

[0035] Figure 7 shows the surface of O2-type Li 0.5 This shows spinel-type LiMn2O4 coated with MnO2. Figure 6 shows monoclinic Li 0.5 This shows a state where MnO2 is replaced by spinel-type LiMn2O4. Spinel-type LiMn2O4 and O2-type Li 0.5 The interfacial energy with MnO2 is 0.85 [J / m 2 Therefore, the surface is O2 type Li 0.5 Monoclinic Li coated with MnO2 0.5 MnO2 is O2-type Li 0.5 It is thermodynamically more stable than spinel-type LiMn2O4 coated with MnO2. Therefore, monoclinic Li 0.5 O2-type Li coated on the surface of MnO2 0.5 The spinel phase transition of MnO2 is suppressed.

[0036] [Manufacturing method] Next, the manufacturing method of the positive electrode active material will be described with reference to Figures 8 to 10. The positive electrode active material has an O2 type Li surface. 0.5 Monoclinic Li coated with MnO2 0.5These are MnO2 particles.

[0037] As shown in Figure 8, monoclinic NaMnO2 is synthesized (Step S1). That is, Na2CO3 and Mn2O3 are mixed and formed into pellets. Then, the pellets are fired in air at a high temperature of 800°C for 12 hours to synthesize monoclinic NaMnO2.

[0038] Next, P2-type Na is placed on the surface of monoclinic NaMnO2 particles. 0.7 The MnO2 is coated (Step S2). This is done using the sol-gel method. A sol is formed by dissolving NaNO3 and Mn2NO3 in deionized water and stirring. Then, NH4NO3 is added to adjust the pH. After heating overnight at 80°C, the formed gel is dried at 130°C. Subsequently, it is calcined in air starting at 500°C and increasing to 900°C to form P2-type Na on the surface of monoclinic NaMnO2 particles. 0.7 The material is coated with MnO2. Step S2 corresponds to the coating process.

[0039] Next, Na is replaced with Li by electrochemical substitution (step S3). That is, P2 type Na 0.7 Ion exchange between Na and Li is performed by heating monoclinic NaMnO2 particles coated with MnO2 in a molten salt of LiCl. Note that the molten salt may be other molten salts such as LiBrLiNO3 or mixtures thereof. Therefore, O2-type Li 0.7 Monoclinic LiMnO2 particles coated with MnO2 can be obtained. Step S3 corresponds to the substitution step.

[0040] The ion exchange between Na and Li in step S3 will be explained. In Figure 9, the surface is P2 type Na 0.7 This shows monoclinic NaMnO2 coated with MnO2. Monoclinic NaMnO2 and P2-type Na 0.7 The interfacial energy with MnO2 is 1.16 [J / m²]. 2 ]

[0041] Figure 10 shows the surface as O2-type Li 0.7This shows monoclinic LiMnO2 coated with MnO2. Monoclinic LiMnO2 and O2-type Li 0.7 The interfacial energy with MnO2 is 0.74 [J / m²]. 2 Therefore, the surface is O2 type Li 0.7 Monoclinic LiMnO2 coated with MnO2 has a P2-type Na coating on its surface. 0.7 It is thermodynamically more stable than monoclinic NaMnO2 coated with MnO2. Therefore, the surface is P2 type Na 0.7 From monoclinic NaMnO2 coated with MnO2, the surface is converted to O2-type Li 0.7 It is possible to convert it to monoclinic LiMnO2 coated with MnO2.

[0042] O2-type Li coating the surface of monoclinic LiMnO2 particles 0.7 Let's explain the thickness of MnO2. The theoretical capacity of LiMnO2 is 285 [mAh / g], which is 4.1% higher than the theoretical capacity of LiCoO2, a common cathode active material, which is 274 [mAh / g]. In order to maintain this advantage, O2-type Li 0.7 The thickness of the MnO2 layer should ideally be within 2.1% of the particle diameter, so that the volume reduction due to Li deficiencies in the O2-type structure is within 4.1%. Specifically, if the average particle diameter of monoclinic LiMnO2 is 2 μm, the thickness of the O2-type LiMnO2 layer should ideally be within 42 nm.

[0043] [Effects of this embodiment] Next, the effects of this embodiment will be described. (1) By coating the surface of monoclinic LiMnO2 particles with O2-type LiMnO2, the bulk region of the positive electrode active material becomes monoclinic LiMnO2 particles, and the surface region of the positive electrode active material becomes O2-type LiMnO2. Therefore, the spinel phase transition of monoclinic LiMnO2 particles can be suppressed, and the decrease in capacity of the lithium-ion secondary battery 10 can be suppressed.

[0044] [Other embodiments] The above embodiment can be implemented with the following modifications. The above embodiment and the following modifications can be combined with each other to the extent that they do not contradict each other technically.

[0045] In the above embodiment, the surface of monoclinic NaMnO2 particles is P2 type Na 0.7 After coating with MnO2, Na is replaced with Li by electrochemical substitution, thereby creating an O2-type LiMnO2 particle surface. 0.7 The manufacturing method was designed to produce particles coated with MnO2. However, the first substitution step involves replacing Na with Li in monoclinic NaMnO2 particles by electrochemical substitution to produce monoclinic LiMnO2 particles, and the surface of the monoclinic LiMnO2 particles is coated with P2 type Na 0.7 A coating process in which MnO2 is used, and P2 type Na 0.7 By electrochemical substitution, Na is replaced with Li in MnO2, resulting in O2-type Li 0.7 The manufacturing method may also include a second substitution step for generating MnO2. Even with such a manufacturing method, the same effects as in the above embodiment will be achieved.

[0046] The lithium-ion secondary battery 10 may be installed in automated transport machines, special vehicles for cargo handling, electric vehicles, hybrid vehicles, etc., as well as in computers and other electronic devices, or it may constitute a system other than those mentioned above. For example, it may be installed in mobile objects such as ships and aircraft, or it may be part of a power supply system that supplies electricity from a power plant to buildings and homes where the secondary battery is installed via a substation or the like. [Explanation of Symbols]

[0047] 10…Lithium-ion rechargeable battery 11…Battery case 12... Lid 13A... Positive external terminal 13B…Negative external terminal 14A... Positive electrode current collector 14B... Negative electrode current collector 20...Wound body 20A... Positive electrode current collector 20B... Negative electrode current collector 21…Positive electrode sheet 22...Positive electrode current collector 22A...Unpainted area on the positive electrode side 23…Positive electrode composite layer 24... Negative electrode sheet 25...Negative electrode current collector 25A...Unpainted area on the negative electrode side 26…Negative electrode composite material layer 27... Separator

Claims

1. Monoclinic LiMnO ２ The surface of the particles is O2-type LiMnO ２ Covered with Cathode active material.

2. Monoclinic NaMnO ２ P2 type Na on the surface of the particles ０．７ MnO ２ A coating process in which the material is applied, By replacing Na with Li through electrochemical substitution, monoclinic LiMnO ２ The surface of the particles is O2 type Li ０．７ MnO ２ A substitution step that produces a coated material, A method for manufacturing a positive electrode active material.

3. Monoclinic NaMnO ２ By electrochemically replacing Na with Li in the particles, monoclinic LiMnO ２ A first substitution step for generating particles, and Monoclinic LiMnO ２ P2 type Na on the surface of the particles ０．７ MnO ２ A coating process in which the material is applied, P2 type Na ０．７ MnO ２ By replacing Na with Li through electrochemical substitution, O2-type Li is produced. ０．７ MnO ２ A second substitution step that generates a second substitution step, including A method for manufacturing a positive electrode active material.

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