Method for producing positive electrode active material and method for producing secondary battery

Abstract

The present invention provides a method for producing a positive electrode active material and a method for producing a secondary battery capable of improving the capacity retention rate and suppressing the generation of gas due to the decomposition of the electrolyte as compared with the related art.SOLUTION: A method for producing a positive electrode active material including first core-shell particles each including a first core portion and a first shell layer covering a surface of the first core portion, a main component of the first shell layer being lithium manganese phosphate having an olivine crystal structure, the method including a first core-shell forming step of covering the first core portion with first precursor particles to form the first shell layer, the first precursor particles having a first peak attributed to a (111) plane in a range of 2 θ from 24.5 ° to 26.5 ° in a diffraction pattern obtained by X-ray diffraction measurement using a Cu-K α ray, in addition, the crystallite size determined from the half width of the first peak by the Scherrer equation is 25nm or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for producing a positive electrode active material and a method for producing a secondary battery. [Background technology]

[0002] BACKGROUND ART Lithium nickel manganese oxide (hereinafter also referred to as LNMO) has been known as a positive electrode active material for lithium ion secondary batteries (for example, Patent Document 1). LNMO has an operating voltage of 4.7 V based on the lithium metal deposition potential, which is higher than conventional lithium insertion materials used as positive electrode active materials (for example, lithium cobalt oxide, which has a voltage of 4 V), and is therefore expected to contribute to achieving higher energy density. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-051987 Summary of the Invention [Problem to be solved by the invention]

[0004] In lithium-ion secondary batteries, the crystal structure of the active material changes during the insertion and desorption of lithium ions, and distortion occurs within the crystal structure, which can lead to instability.

[0005] In such cases, transition metal ions that constitute the crystalline structure of the positive electrode active material are eluted, and in the vicinity of the positive electrode, the bond between the transition metal and oxygen is broken, causing oxygen to be released, generating oxygen gas or reacting with the electrolyte to generate carbon dioxide gas.

[0006] In particular, in a lithium ion secondary battery using LNMO as a positive electrode active material as in Patent Document 1, the operating voltage is high and the reaction proceeds in an oxidizing atmosphere near the positive electrode, which causes the problem of significant gas generation as described above.

[0007] Therefore, an object of the present invention is to provide a method for producing a positive electrode active material that, when used as a positive electrode of a lithium ion secondary battery, can improve the capacity retention rate and suppress the generation of gas due to decomposition of the electrolyte compared to conventional methods, and to provide a method for producing a secondary battery that can improve the capacity retention rate and suppress the generation of gas due to decomposition of the electrolyte compared to conventional methods. [Means for solving the problem]

[0008] One aspect of the present invention for solving the above-mentioned problems is a method for producing a positive electrode active material having first core-shell particles with a first core portion and a first shell layer covering the surface of the first core portion, wherein the main component of the first shell layer is lithium manganese phosphate having an olivine crystal structure, the method including a first core-shell formation step of covering the first core portion with a first precursor particle to form the first shell layer, wherein the first precursor particle has a first peak at 2θ in the range of 24.5° to 26.5°, which is assigned to the (111) plane, in a diffraction pattern obtained by X-ray diffraction measurement using Cu-Kα radiation, and the crystallite diameter calculated from the half-width of the first peak by the Scherrer equation is 25 nm or less.

[0009] The term "major component" as used here means that it accounts for more than 50% of the total, and the same applies below.

[0010] According to this aspect, when used in the positive electrode of a lithium ion secondary battery, the capacity retention rate can be improved and gas generation due to electrolyte decomposition can be suppressed compared to conventional cases.

[0011] In a preferred aspect, the first precursor particles have a second peak in the diffraction pattern at 2θ in the range of 28.5° to 30.5°, and the peak intensity of the first peak of the first precursor particles is 0.8 to 1.0 times the peak intensity of the second peak.

[0012] In a preferred aspect, the first core portion contains lithium nickel manganese oxide having a spinel-type crystal structure.

[0013] In a preferred aspect, the first precursor particles have a crystallite diameter of 20 nm or less.

[0014] In a preferred aspect, the first precursor particles have a ratio of crystallite diameter to BET specific surface area converted diameter calculated from the specific surface area determined by the BET method of 0.14 or more and 0.22 or less.

[0015] A preferred aspect includes a first milling step in which lithium manganese phosphate having an olivine-type crystal structure is milled in a bead mill to form the first precursor particles.

[0016] One aspect of the present invention is a method for manufacturing a secondary battery including a positive electrode active material, a negative electrode active material, and an electrolyte, wherein the negative electrode active material comprises second core-shell particles having second core portions and second shell layers covering the surfaces of the second core portions, and the second shell layers are mainly composed of lithium iron phosphate having an olivine crystal structure; the method includes: a positive electrode active material layer formation step of forming the positive electrode active material using the above-described method for manufacturing a positive electrode active material; and a second core-shell formation step of covering the second core portions with second precursor particles to form the second shell layer; and the second precursor particles have a third peak at 2θ in the range of 24.5° to 26.5° assigned to the (111) plane in a diffraction pattern obtained by X-ray diffraction measurement using Cu-Kα radiation, and a crystallite diameter calculated from the half-width of the third peak by the Scherrer equation is 25 nm or less.

[0017] According to this aspect, the capacity retention rate can be improved and gas generation due to decomposition of the electrolyte can be suppressed compared to conventional methods.

[0018] In a preferred aspect, the electrolyte is a non-aqueous electrolytic solution, and the method includes a step of adding an electrolyte additive, wherein the electrolyte additive is a vinyl group-containing cyclic siloxane.

[0019] In a preferred aspect, the second precursor particles have a ratio of crystallite diameter to BET specific surface area converted diameter calculated from the specific surface area determined by the BET method of 1.20 or more and 1.60 or less.

[0020] In a preferred aspect, the second core portion contains lithium titanate having a spinel-type crystal structure.

[0021] A preferred aspect includes a second milling step of milling lithium iron phosphate having an olivine-type crystal structure with a bead mill to form the second precursor particles.

[0022] The above aspects may be made dependent on each other, or some of the configurations may be quoted or substituted for each other, as long as they are included in the technical scope of the present invention. [Effects of the Invention]

[0023] According to the present invention, the capacity retention rate can be improved and gas generation due to decomposition of the electrolyte can be suppressed compared to the prior art. [Brief explanation of the drawings]

[0024] [Figure 1] 1A and 1B are explanatory diagrams conceptually illustrating a secondary battery according to a first embodiment of the present invention, in which (a) is a perspective view of the secondary battery, and (b) is a cross-sectional view taken along the line AA in (a). [Figure 2] 2A and 2B are explanatory views of the electrode portion of FIG. 1, where (a) is a cross-sectional view of the positive electrode portion and (b) is a cross-sectional view of the negative electrode portion. [Figure 3] 2A and 2B are examples of XRD patterns of first precursor particles of the secondary battery of FIG. 1, where (a) is the XRD pattern of the first precursor particles, and (b) is an enlarged view of a main part of (a). [Figure 4]2A and 2B are examples of XRD patterns of second precursor particles of the secondary battery of FIG. 1, where (a) is the XRD pattern of the second precursor particles, and (b) is an enlarged view of a main part of (a). DETAILED DESCRIPTION OF THE INVENTION

[0025] Hereinafter, embodiments of the present invention will be described in detail.

[0026] The secondary battery 1 of the first embodiment of the present invention is a lithium ion secondary battery, and includes a battery stack 2, electrode lead-out members 3a and 3b, an electrolyte 5, and an exterior body 6, as shown in FIG.

[0027] <Battery stack 2> The battery stack 2 has a plurality of positive electrode parts 10, a plurality of negative electrode parts 11, and a plurality of separators 12, as shown in FIG. 1(b). In the battery stack 2 of this embodiment, the electrode parts 10, 11 and the separators 12 are arranged alternately, as in ... / separator 12 / positive electrode part 10 / separator 12 / negative electrode part 11 / separator 12 / ..., with the separators 12 being arranged on the outermost sides in the stacking direction.

[0028] (Positive electrode part 10) As shown in FIG. 2(a), the positive electrode part 10 is an intercalation electrode in which a positive electrode active material layer 21 is laminated on at least one main surface of a positive electrode current collector 20, and lithium ions can be inserted and extracted. The positive electrode active material layer 21 is a layered body having a positive electrode active material 22, a conductive additive, and a binder.

[0029] The positive electrode active material 22 is composed of a plurality of first core-shell particles 23, as shown in the enlarged view of FIG. 2(a). The first core-shell particle 23 has a first core portion 25 whose surface is coated with a first shell layer 26 .

[0030] The first core portion 25 is made of a lithium ion conductive oxide and is capable of inserting and extracting lithium ions. The average potential of lithium desorption and insertion in the first core part 25 is preferably 4.5 V or more and 5.0 V or less with respect to the deposition potential of Li (vs. Li + / Li is also shown). That is, the first core part 25 preferably has an operating potential of 4.5 V or more and 5.0 V or less based on lithium metal alone. The potential of the lithium ion insertion / desorption reaction (hereinafter also referred to as voltage) (vs. Li + / Li) can be obtained, for example, by measuring the charge-discharge characteristics of a half-cell with the working electrode using the first core part 25 and lithium metal as the counter electrode, and reading the voltage values at the start and end of the plateau. When there are two or more plateaus, it is only necessary that the plateau with the lowest voltage value is 4.5 V (vs. Li + / Li) or more, and the plateau with the highest voltage value is 5.0 V (vs. Li + / Li) or less.

[0031] The first core part 25 is not particularly limited as long as it has ion conductivity, but a spinel-type lithium manganese-based oxide represented by the following formula (1) is preferable. Li 1+x M y Mn 2-x-y O4···(1) In the above formula (1), x and y each satisfy 0 ≦ x ≦ 0.2 and 0 < y ≦ 0.8, and M is at least one selected from the group consisting of Al, Mg, Zn, Ni, Co, Fe, Ti, Cu, and Cr.

[0032] Among the above formula (1), lithium nickel manganese oxide (hereinafter also referred to as LNMO) in which M is Ni is preferable.

[0033] The first shell layer 26 is a film composed of a lithium ion conductive oxide having an olivine-type crystal structure, and is composed of an intercalation material that functions as a positive electrode active material alone. Specifically, the main component of the first shell layer 26 is lithium manganese phosphate (LiMnPO4, hereinafter also referred to as LMP) having an olivine-type crystal structure. The thickness of the first shell layer 26 is preferably thinner than the particle diameter of the first core portion 25 . The first shell layer 26 covers at least a part of the surface of the first core portion 25, more preferably 95% or more, and preferably completely.

[0034] (negative electrode part 11) As shown in FIG. 2(b), the negative electrode part 11 is an intercalation electrode in which a negative electrode active material layer 31 is laminated on at least one main surface of a negative electrode current collector 30, and lithium ions can be inserted and extracted. The negative electrode active material layer 31 includes a negative electrode active material 32, a conductive additive, and a binder.

[0035] The negative electrode active material 32 is composed of a plurality of second core-shell particles 33, as shown in the enlarged view of FIG. 2(b). The second core-shell particle 33 has a second core portion 35 whose surface is coated with a second shell layer 36 .

[0036] It is preferable to use lithium titanate (hereinafter also referred to as LTO) for the second core portion 35, as this makes lithium deposition less likely to occur and improves safety. For the second core portion 35, among lithium titanates, lithium titanate with a spinel structure is particularly preferred because the expansion and contraction of the negative electrode active material 32 during the lithium ion insertion and desorption reaction is small.

[0037] The second shell layer 36 is a coating made of a lithium ion conductive oxide containing phosphorus as an element, and is made of an intercalation material. Specifically, the second shell layer 36 is mainly composed of lithium iron phosphate (LiFePO4, hereinafter also referred to as LFP) having an olivine type crystal structure. The thickness of the second shell layer 36 is preferably thinner than the particle diameter of the second core portion 35 . The second shell layer 36 covers at least a portion of the surface of the second core portion 35, more preferably 95% or more, and preferably completely.

[0038] The current collectors 20 and 30 are not particularly limited, but are preferably made of aluminum or an aluminum alloy because they are stable in the positive electrode reaction atmosphere and the negative electrode reaction atmosphere. The current collectors 20 and 30 may be made of a metal other than aluminum (copper, SUS, nickel, titanium, and alloys thereof) coated with a metal that does not react with the potential of the positive electrode part 10 and the negative electrode part 11.

[0039] The conductive additive used in the active material layers 21 and 31 is not particularly limited, but is preferably a carbon material. The carbon material is preferably at least one selected from natural graphite, artificial graphite, vapor-grown carbon fiber, carbon nanotube, acetylene black, ketjen black, and furnace black. The amount of the conductive additive contained in the active material layers 21 and 31 is preferably 1 part by weight or more and 30 parts by weight or less per 100 parts by weight of the active materials 22 and 32. Within the above range, the conductivity of the active material layers 21 and 31 is ensured, while the adhesiveness to the binder is maintained, and sufficient adhesiveness to the current collectors 20 and 30 can be obtained.

[0040] The binder used in the active material layers 21 and 31 is not particularly limited, but for both the active material layers 21 and 31, at least one selected from the group consisting of polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber, polyimide, and derivatives thereof can be used. The amount of binder contained in the active material layers 21 and 31 is preferably 1 part by weight or more and 30 parts by weight or less per 100 parts by weight of the active materials 22 and 32. Within the above range, the adhesiveness between the active materials 22, 32 and the conductive additive can be maintained, and sufficient adhesiveness between the active materials 22, 32 and the current collectors 20, 30 can be obtained.

[0041] <Electrode extraction member 3a, 3b> As shown in FIG. 1(b), the positive electrode lead-out member 3a is electrically connected to at least one of the positive electrode sections 10 constituting the battery stack 2 within the exterior body 6, and is a positive electrode terminal that extends inside and outside the exterior body 6. The negative electrode lead-out member 3b is electrically connected to at least one of the negative electrode sections 11 that make up the battery stack 2 within the exterior case 6, and is a negative electrode terminal that extends inside and outside the exterior case 6. The electrode extraction members 3a and 3b are conductive plate-like bodies, and are not particularly limited as long as they are conductive. For example, metals such as aluminum and alloys thereof can be used.

[0042] <Electrolyte 5> The electrolyte 5 is not particularly limited as long as it has lithium ion conductivity, and may be a non-aqueous electrolyte solution in which a solute is dissolved in a non-aqueous solvent, or a gel electrolyte in which a polymer is impregnated with a non-aqueous electrolyte solution in which a solute is dissolved in a non-aqueous solvent, etc. The electrolyte 5 may be in a solid state or may be a solid electrolyte. The electrolyte 5 of this embodiment is a non-aqueous electrolyte solution.

[0043] <Separator 12> The separator 12 is disposed between the positive electrode part 10 and the negative electrode part 11 and may have any structure as long as it is insulating and can contain the electrolyte 5 . Examples of the separator 12 include woven fabrics, nonwoven fabrics, and microporous membranes made of nylon, cellulose, polysulfone, polyethylene, polypropylene, polybutene, polyacrylonitrile, polyimide, polyamide, polyethylene terephthalate, and composites of two or more of these. The separator 12 may contain various plasticizers, antioxidants, and flame retardants, and may be coated with metal oxides or the like.

[0044] <Exterior body 6> As shown in FIG. 1(b), the exterior body 6 has an internal space 40, and is a sealing member that houses and seals the battery stack 2 and the electrolyte 5 in the internal space 40, and is chemically stable against the electrolyte 5 and has water vapor barrier properties. As shown in FIG. 1, the exterior body 6 is made up of a first exterior film 41 and a second exterior film 42, and the battery stack 2 and the electrolyte 5 are sandwiched between the exterior films 41 and 42 to form a seal. The exterior films 41 and 42 are made of a laminate film containing a laminate resin.

[0045] Next, a method for manufacturing the secondary battery 1 of this embodiment will be described.

[0046] The method for manufacturing the secondary battery 1 of this embodiment mainly includes a positive electrode part forming step, a negative electrode part forming step, and a secondary battery assembling step.

[0047] The positive electrode part forming process is composed of a first core-shell forming process for forming first core-shell particles 23, and a positive electrode active material layer forming process for coating a positive electrode active material 22 containing a plurality of first core-shell particles 23 onto a positive electrode current collector 20 to form a positive electrode active material layer 21.

[0048] The first core-shell forming step is a step of forming a first shell layer 26 on the surface of the first core portion 25 to form a first core-shell particle 23.

[0049] Specifically, the first core-shell forming step is composed of a pulverizing step (first pulverizing step), a fluid forming step, a ground product forming step, and a removing step.

[0050] Specifically, first, lithium manganese phosphate (LMP) is put into a solvent and stirred, and then crushed and disintegrated in a wet bead mill, and the beads are removed to form first precursor particles of LMP (crushing step).

[0051] In this case, the beads used in the bead mill preferably have a diameter of 0.015 mm or more and 2.0 mm or less. The processing time in the bead mill can be appropriately set depending on the bead diameter and the particle size of the target LMP, but is preferably, for example, from 30 minutes to 24 hours, and more preferably from 1 hour to 24 hours. The treatment in the bead mill may be terminated when the integrated load power (Wh / g) reaches a predetermined amount. In this case, the solvent is not particularly limited as long as it can disperse lithium manganese phosphate without reacting with it. For example, it is preferable to use one or more alcohol solutions, and ethanol is more preferable from the viewpoints of volatility and safety. The bead mill may be a dry bead mill.

[0052] Subsequently, the first precursor particles are pulverized in the pulverization step, and the first precursor particles dispersed in the solvent are concentrated as necessary to form a first precursor fluid (fluid formation step).

[0053] The first precursor particles removed from the solvent have a diffraction pattern obtained by X-ray diffraction measurement (hereinafter also referred to as XRD) using Cu-Kα radiation shown in Figure 3, which shows a first peak at 2θ in the range of 24.5° to 26.5° assigned to the (111) plane and a second peak at 2θ in the range of 28.5° to 30.5° assigned to the (211) plane. The first precursor particles preferably have a crystallite diameter D of 10 nm or more, which is calculated from the half width β of the first peak shown in FIG. 3 by the Scherrer equation (2) below. The first precursor particles have a crystallite diameter D calculated from the half width β of the first peak by the Scherrer formula of 25 nm or less, and preferably 20 nm or less.

[0054] D=0.9λ / βcosθ (2) Here, λ is the wavelength of the X-ray used, β is the half-width, and θ is the flag angle.

[0055] In the first precursor particles, the peak intensity of the first peak is preferably 0.8 to 1.0 times the peak intensity of the second peak.

[0056] The first precursor particles preferably have a BET specific surface area equivalent diameter (hereinafter also referred to as dBET) of 90 nm or more, more preferably 100 nm or more. The first precursor particles preferably have a dBET of 120 nm or less, more preferably 110 nm or less. Note that dBET is the particle size calculated by the nitrogen adsorption BET specific surface area determined by the single-point nitrogen adsorption method in accordance with the BET method specified in JIS Z8830 (2013), and then using the formula dBET = 6 / (density × BET specific surface area). The same applies hereinafter.

[0057] Next, at least one type of energy, including shear force, compression force, collision force, and centrifugal force, is applied to the lithium ion conductive oxide constituting the core portion 25 and / or the first precursor particles in the first precursor fluid constituting the shell layer 26, and a mechanical coating method is used to mechanically contact the core portion 25 with the first precursor particles in the first precursor fluid, thereby forming a shell layer 26 composed of LMP on the surface of the core portion 25. In this embodiment, the surface of the core portion 25 is ground by a grinding device such as a grinding mill, and is bonded to a fine particle fluid to form a ground product (ground product forming step).

[0058] At this time, the first precursor particles (LMP) in the fine particle fluid fed into the milling device are preferably 0.5 wt % or more. At this time, the first precursor particles (LMP) in the fine particle fluid fed into the milling device are preferably 2.5 wt % or less, more preferably 1.6 wt % or less, and particularly preferably 1.2 wt % or less. The processing temperature in the milling device is preferably 5°C or higher and 100°C or lower, more preferably 10°C or higher and 90°C or lower, and even more preferably 20°C or higher and 80°C or lower. The processing time in the milling device is preferably 5 minutes or more and 90 minutes or less, and more preferably 10 minutes or more and 60 minutes or less. The processing by the milling device may be terminated when the integrated load power (Wh / g) reaches a predetermined amount. The atmosphere in the milling device at this time is preferably an inert gas atmosphere or an air atmosphere.

[0059] Subsequently, the ground product is subjected to a heat treatment to remove the dispersion solvent from the ground product, forming the first shell layer 26 and forming the first core-shell particles 23 (removal step).

[0060] The heat treatment temperature at this time is preferably 300° C. or higher. If the heat treatment temperature is below 300° C., the adhesion between the core portion 25 and the shell layer 26 will be insufficient, which may cause the shell layer 26 to peel off during charging and discharging of the battery, leading to a decrease in the long-term reliability of the battery. On the other hand, if the heat treatment temperature is too high, the crystal structure of the shell layer 26 may change, the lithium ion conductivity may decrease, and the battery may not be charged or discharged normally. Therefore, the heat treatment temperature is preferably 850°C or less. The heat treatment time can be appropriately set depending on the heat treatment temperature, and can be, for example, 30 minutes or more and 150 minutes or less.

[0061] The positive electrode active material layer forming step is composed of a paste application step and a firing step. Specifically, the first core-shell particles 23 constituting the positive electrode active material 22 are mixed with a conductive material and a binder, dispersed in a solvent to form a paste, and a slurry is formed. The slurry is then applied to the positive electrode current collector 20 to form a slurry-coated body (paste application process).

[0062] The solvent used in the paste application step is not particularly limited as long as it can dissolve or disperse the first core-shell particles 23, the conductive material, and the binder, and examples of solvents that can be used include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, methyl acetate, ethyl acetate, and tetrahydrofuran. A dispersant and a thickener may also be added to these solvents.

[0063] Subsequently, the slurry-coated body formed in the paste coating step is fired at a predetermined firing temperature for a predetermined firing time to form the positive electrode active material layer 21, thereby forming the positive electrode part 10 (firing step).

[0064] At this time, the baking time is not particularly limited as long as the temperature is such that the slurry-coated body is baked and the slurry solidifies, but it is preferably, for example, from 1 minute to 1 hour, and more preferably 20 minutes or more. The firing temperature is not particularly limited as long as it is a temperature at which the slurry-coated body is fired and the slurry solidifies, but is preferably, for example, 80°C or higher and 200°C or lower.

[0065] This concludes the description of the positive electrode part forming step.

[0066] The negative electrode part formation process is similar to the positive electrode part formation process described above, except that in the crushing step (second crushing step) of the first core-shell formation process, lithium iron phosphate (LFP) is crushed using a bead mill or a ball mill to form second precursor particles, the second precursor particles are used instead of the first precursor particles in each step, and lithium titanate (LTO) is used as the core part 25 in the crushed material formation step of the first core-shell formation process. Therefore, a description thereof will be omitted. That is, the negative electrode part forming process is composed of a second core-shell forming process and a negative electrode active material layer forming process, and the second core-shell forming process is composed of a second crushing process, a fluid forming process, a ground material forming process, and a removing process, and the negative electrode active material layer forming process is composed of a paste applying process and a firing process.

[0067] In addition, the second precursor particles have a third peak attributable to the (111) plane in the 2θ range of 24.5° to 26.5° and a fourth peak attributable to the (211) plane in the 2θ range of 29.0° to 30.5° in the diffraction pattern obtained by X-ray diffraction measurement using Cu-Kα radiation as shown in Figure 4. The second precursor particles preferably have a crystallite diameter of 10 nm or more, which is calculated from the half width β of the third peak shown in FIG. 4 by the Scherrer formula (2). The second precursor particles preferably have a crystallite diameter of 25 nm or less, more preferably 20 nm or less, as determined by the Scherrer equation from the half-width β of the third peak. The second precursor particles preferably have a dBET of 5 nm or more, and preferably have a dBET of 20 nm or less.

[0068] The secondary battery assembly process includes a battery stack formation process, an interface electrolyte injection process, a voltage application process, and an electrolyte injection process. Specifically, a separator 12 is sandwiched between a positive electrode part 10 and a negative electrode part 11 to form a battery stack 2 (battery stack formation step).

[0069] Next, electrode extraction members 3a and 3b are attached to the battery stack 2, and the battery stack 2 is placed inside the exterior body 6, which has been partially opened in advance, and the interface electrolyte is injected into the exterior body 6 (interface electrolyte injection process).

[0070] The interface electrolyte used in this case may be the electrolyte 5 to which an electrolyte additive containing a silicon-containing compound has been added. As the electrolyte additive, for example, a vinyl group-containing cyclic siloxane such as 2,4,6,8-tetravinyl-2,4,6,8-tetramethylcyclotetrasiloxane (hereinafter also referred to as 4VC4S) can be used.

[0071] Subsequently, a voltage is applied between the electrode extraction members 3a and 3b (voltage application step).

[0072] The applied voltage at this time is a voltage at which the silicon-containing compound in the interface electrolyte is precipitated on the surface of the shell layer 26, and is 4.5 V or more and 5 V or less (vs. Li + / Li).

[0073] Next, if necessary, the interface electrolyte is discharged from inside the exterior body 6, the electrolyte 5 is injected into the exterior body 6 (electrolyte injection process), the exterior body 6 is sealed, and the battery stack 2 is sealed, thereby completing the secondary battery 1.

[0074] The secondary battery 1 of this embodiment can improve the capacity retention rate and suppress the generation of gas due to decomposition of the electrolyte 5 compared to conventional batteries.

[0075] In the above-described embodiment, the interface electrolyte injection step and the voltage application step are performed before the electrolyte injection step, but the present invention is not limited to this. The interface electrolyte injection step and the voltage application step may be omitted.

[0076] In the above-described embodiment, the negative electrode active material 32 is composed of second core-shell particles 33 in which the surfaces of the second core portions 35 are coated with the second shell layers 36, but the present invention is not limited to this. The negative electrode active material 32 may be composed of only the second core portions 35. In other words, the second core portions 35 do not have to be coated with the second shell layer 36.

[0077] In the above-described embodiments, each component can be freely substituted or added between the respective embodiments as long as it falls within the technical scope of the present invention. [Example]

[0078] The present invention will be specifically described below with reference to experimental examples. Note that the present invention is not limited to the following experimental examples, and can be practiced with appropriate modifications within the scope of the present invention.

[0079] (Experimental Example 1) (a) Preparation of positive electrode active material First, lithium manganese phosphate (LMP) powder with an olivine crystal structure was mixed with a specified amount of ethanol as a solvent, and the mixture was milled for 3 hours using beads with a diameter of 0.2 mm. After removing the beads from the milled mixture, some of the ethanol was removed to obtain a slurry containing 16.4 wt% of the first precursor particles (fine LMP powder).

[0080] The first core of the positive electrode was made of spinel-type lithium nickel manganese oxide (LiNi 0.5 Mn 1.5 O4, hereafter also referred to as LNMO) was used. 30 g of LNMO was placed in a grinding mill, and while rotating at 2600 rpm with a clearance of 0.8 mm and a rotor load power of 1.5 kW, an ethanol dispersion slurry of the first precursor particles was added in two portions so that the amount of the first precursor particles added was 1.2 wt%. Then, with the rotor rotation speed kept in the range of 2600 rpm to 3000 rpm, the mixture was treated at room temperature in an air atmosphere for 10 minutes, and then heat-treated at 350°C for 1 hour, to obtain a cathode active material containing first core-shell particles in which the surface of LMNO (first core portion) was coated with LMP (first shell layer).

[0081] (b) Preparation of the positive electrode A mixture containing the obtained positive electrode active material, acetylene black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder at solid concentrations of 90 parts by weight, 6 parts by weight, and 4 parts by weight, respectively, was dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a slurry. The binder was prepared as an N-methyl-2-pyrrolidone (NMP) solution with a solid concentration of 5% by weight, and NMP was further added to adjust the viscosity to facilitate coating.

[0082] The slurry was applied to a 15 μm aluminum foil and then dried in an oven at 120° C. This operation was performed on both sides of the aluminum foil, and then the foil was further dried in a vacuum at 170° C. to prepare a positive electrode part.

[0083] (c) Preparation of negative electrode active material First, lithium iron phosphate (LFP) powder with an olivine crystal structure was mixed with a specified amount of ethanol as a solvent, and the mixture was ball milled for 3 hours using zirconia balls with a ball diameter of 0.5 mm. After removing the zirconia balls from the mixture, some of the ethanol was removed to obtain a slurry containing 16.4 wt% of the second precursor particles (LFP fine powder).

[0084] The second core of the negative electrode was made of spinel-type lithium titanate (Li4Ti5O 12 , hereinafter referred to as LTO) was used. 30 g of LTO was placed in a grinding mill, and while rotating at 2600 rpm with a clearance of 0.8 mm and a rotor load power of 1.5 kW, an ethanol dispersion slurry of second precursor particles was added in two portions so that the amount of second precursor particles added was 2.4 wt%. Then, with the rotor rotation speed kept in the range of 2600 rpm to 3000 rpm, the mixture was treated at room temperature in an air atmosphere for 10 minutes, and then heat-treated at 350°C for 1 hour, to obtain a negative electrode active material containing second core-shell particles in which the surface of the LTO (second core portion) was coated with LFP (second shell layer).

[0085] (d) Preparation of the negative electrode A mixture containing the obtained negative electrode active material, acetylene black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder at solid concentrations of 92 parts by weight, 3 parts by weight, and 5 parts by weight, respectively, was dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a slurry. The binder was prepared as an N-methyl-2-pyrrolidone (NMP) solution with a solid concentration of 5% by weight, and NMP was further added to adjust the viscosity to facilitate coating.

[0086] The slurry was applied to a 15 μm aluminum foil and then dried in an oven at 120° C. This operation was performed on both sides of the aluminum foil, and then the foil was further dried in a vacuum at 170° C. to prepare a negative electrode part.

[0087] (e) Fabrication of lithium-ion secondary batteries A battery was fabricated using the positive electrode part and negative electrode part fabricated in (b) and (d) above and a 20 μm polypropylene separator according to the following procedure. First, the positive and negative electrode sections were dried under reduced pressure at 80°C for 12 hours. Next, 15 positive electrodes and 16 negative electrodes were stacked in the order of negative electrode section / separator / positive electrode section to form a battery stack. The outermost layers of the battery stack were both separators. Next, aluminum tabs were vibration-welded to the positive and negative electrode sections at both ends.

[0088] Two sheets of aluminum laminate film were prepared as exterior materials, and a depression to become the battery portion and a depression to become the gas collection portion were formed by pressing, after which the battery stack was inserted. The outer periphery, leaving a space for electrolyte injection, was heat-sealed at 180°C for 7 seconds, and a non-aqueous electrolyte was poured into the unsealed area. The non-aqueous electrolyte was made by dissolving LiPF6 at a ratio of 1 mol / L in a solvent made by mixing ethylene carbonate, propylene carbonate, and ethyl methyl carbonate in a volume ratio of ethylene carbonate / propylene carbonate / ethyl methyl carbonate = 15 / 15 / 70. 3 wt% of D4Vi was added to the non-aqueous electrolyte as an electrolyte additive to prepare an additive-containing electrolyte. The unsealed area was then heat-sealed at 180°C for 7 seconds while the pressure was reduced. The resulting battery was charged at a constant current equivalent to 0.2 C until the battery voltage reached a cut-off voltage of 3.4 V, at which point charging was stopped. After leaving the battery at rest for 24 hours in a 60°C environment, it was discharged at a constant current equivalent to 0.2 C, and discharge was stopped when the battery voltage reached 2.5 V. After discharging was stopped, the gas trapped in the gas collection section was removed and the battery was resealed. A lithium-ion secondary battery for evaluation was produced using the above procedures.

[0089] (Experimental Example 2) (b) In the preparation of the positive electrode part, LMNO without being coated with LMP was used as the positive electrode active material, (d) in the preparation of the negative electrode part, LTO without being coated with LFP was used as the negative electrode active material, and (e) in the preparation of the lithium ion secondary battery, no electrolyte additive was added to the non-aqueous electrolyte, except for the above, which was designated as Experimental Example 2.

[0090] (Experimental Example 3) (b) In the preparation of the positive electrode part, LMNO without coating with LMP was used as the positive electrode active material, and (e) in the preparation of the lithium ion secondary battery, the same procedure as in Experimental Example 1 was carried out except that no electrolyte additive was added to the non-aqueous electrolyte. This was designated Experimental Example 3.

[0091] (Experimental Example 4) (b) In the preparation of the positive electrode part, the same procedure as in Experimental Example 1 was carried out except that LMNO without being coated with LMP was used as the positive electrode active material. This was designated Experimental Example 4.

[0092] (Experimental Example 5) (a) In the preparation of the positive electrode active material, spinel-type LNMO with a median diameter of 8.8 μm was used as the positive electrode active material, and beads with a bead diameter of 0.1 mm were used and bead milled for 7 hours. (c) In the preparation of the negative electrode active material, instead of ball milling, bead milling was performed for 16 hours to form second precursor particles, which were LFP fine powders, and an ethanol-dispersed slurry of the second precursor particles was added so that the amount of the second precursor particles added was 3.6 wt%. Other than these, the same procedure as in Experimental Example 1 was repeated, and this was designated Experimental Example 5.

[0093] (Experimental Example 6) (a) In the preparation of the positive electrode active material, the same procedure as in Experimental Example 5 was carried out except that the bead mill treatment was carried out for 1 hour, and this was designated Experimental Example 6.

[0094] (Experimental Example 7) (c) Experimental Example 7 was prepared in the same manner as Experimental Example 5, except that the bead mill treatment was carried out for 2 hours in the preparation of the negative electrode active material.

[0095] (Experimental Example 8) (d) In the preparation of the negative electrode part, the same procedure as in Experimental Example 5 was carried out, except that LTO without being coated with LFP was used as the negative electrode active material. This was designated Experimental Example 8.

[0096] (Experimental Example 9) (b) In the preparation of the positive electrode part, LMNO without being coated with LMP was used as the positive electrode active material, (d) in the preparation of the negative electrode part, LTO without being coated with LFP was used as the negative electrode active material, and (e) in the preparation of the lithium ion secondary battery, no electrolyte additive was added to the non-aqueous electrolyte, except for the above, the same procedures as in Experimental Example 5 were carried out. This was designated Experimental Example 9.

[0097] (Experimental Example 10) (b) In the preparation of the positive electrode part, LMNO without being coated with LMP was used as the positive electrode active material, and (d) in the preparation of the negative electrode part, LTO without being coated with LFP was used as the negative electrode active material, except that the same procedure as in Experimental Example 5 was carried out. This was designated Experimental Example 10.

[0098] (Gas generation rate measurement) The amount of gas generated from the lithium ion secondary battery before and after the cycle performance evaluation in each of Experimental Examples 1 to 10 was evaluated using the Archimedes method, that is, the buoyancy of the lithium ion secondary battery. The evaluation was performed as follows.

[0099] First, the weight of the lithium-ion secondary battery was measured using an electronic balance. Next, the weight in water was measured using a hydrometer (manufactured by Alpha Mirage, product number: MDS-3000). The buoyancy was calculated by taking the difference between these weights. This buoyancy was calculated based on the density of water (1.0 g / cm 3 The volume of the lithium ion secondary battery was calculated by dividing the volume by the aging time. The amount of gas generated was calculated by comparing the volume after aging with the volume after the cycle characteristic evaluation described below.

[0100] (Evaluation of cycle characteristics of lithium-ion secondary batteries) The lithium ion secondary batteries prepared in each of Experimental Examples 1 to 10 were connected to a charge / discharge device (HJ1005SD8, manufactured by Hokuto Denko Corporation) and cycled. In a 45°C environment, constant current charging was performed at a current value equivalent to 1.0 C until the battery voltage reached a cut-off voltage of 3.4 V, and charging was then stopped. Subsequently, constant current discharging was performed at a current value equivalent to 1.0 C, and discharging was stopped when the battery voltage reached 2.5 V. This constituted one cycle, and charging and discharging were repeated. The stability of the cycle characteristics was evaluated as a capacity retention rate (%), where the first discharge capacity was set to 100 and the 400th discharge capacity was calculated.

[0101] First, the evaluation results of Experimental Examples 1 to 4 are shown in Table 1.

[0102] [Table 1]

[0103] In Experimental Example 1, by using a bead mill for the first grinding process in forming the positive electrode portion, XRD measurements of the first precursor particles showed a peak for the (111) plane in the 2θ range of 24.5° to 26.5° and a peak for the (211) plane in the 2θ range of 28.5° to 30.5°, and it was found that the crystallite diameter of the first precursor particles could be calculated using the peak for the (111) plane. As shown in Table 1, Experimental Example 1 showed a 78% reduction in gas generation compared to Experimental Example 2, in which no shell layer was provided on either the positive electrode active material in the positive electrode portion or the positive electrode active material in the negative electrode portion, and no electrolyte additive was added, and also showed a capacity retention rate of 93%, which was 17% higher than Experimental Example 2.

[0104] In Experimental Example 1, in which an LMP shell layer was formed in the positive electrode active material of the positive electrode portion, the amount of gas generation was reduced by 43% and the capacity retention rate was improved by 9%, as shown in Table 1, compared to Experimental Example 4, in which no shell layer was formed in the positive electrode active material of the positive electrode portion. This suggests that by forming an LMP shell layer in the positive electrode active material of the positive electrode portion, the amount of gas generation is suppressed and the capacity retention rate is improved.

[0105] In Experimental Example 3, in which an LFP shell layer was formed in the negative electrode active material of the negative electrode section, the amount of gas generation was reduced by 13% and the capacity retention rate was improved by 7% compared to Experimental Example 2, in which a shell layer was not formed in the negative electrode active material of the negative electrode section. This suggests that by forming an LFP shell layer in the negative electrode active material of the negative electrode section, the amount of gas generation can be suppressed while improving the capacity retention rate.

[0106] In Experimental Example 4, in which a vinyl group-containing cyclic siloxane electrolyte additive was added, the amount of gas generated was reduced by 22% compared to Experimental Example 3, in which no electrolyte additive was added, and the capacity retention rate was also improved by 1%. This suggests that the addition of an electrolyte additive of vinyl group-containing cyclic siloxane improves the amount of gas generated and the capacity retention rate.

[0107] These results suggest that by constructing the positive electrode active material with core-shell particles with LMP as the shell layer, gas generation can be significantly suppressed and the capacity retention rate can be improved. Furthermore, by constructing the negative electrode active material with core-shell particles with LFP as the shell layer, gas generation can be further suppressed and the capacity retention rate can be improved. Furthermore, it was suggested that adding a vinyl-group-containing cyclic siloxane electrolyte additive further suppressed gas generation and improved the capacity retention rate.

[0108] Next, Table 2 shows the evaluation results of Experimental Examples 5 to 10, in which the particle size of LNMO in the positive electrode active material of the positive electrode portion was made smaller than that of Experimental Examples 1 to 4.

[0109] [Table 2]

[0110] In Experimental Examples 5 to 8, in which the particle size of LNMO was reduced, the first grinding step in forming the positive electrode active material for the positive electrode portion was performed using a bead mill. In the XRD measurement of the first precursor particles, peaks of the (211) plane appeared in the 2θ range of 24.5° to 26.5° and 2θ range of 28.5° to 30.5°, and it was found that the crystallite diameter of the first precursor particles could be calculated using the peak of the (111) plane. Furthermore, in Experimental Examples 5 to 7, by using a bead mill for the second grinding process when forming the negative electrode active material for the negative electrode portion, XRD measurements revealed a peak for the (111) plane in the 2θ range of 24.5° to 26.5° and a peak for the (211) plane in the 2θ range of 29.0° to 30.5°, and it was found that the crystallite diameter of the second precursor particles could be calculated using the peak for the (111) plane.

[0111] In Experimental Examples 5 to 7, the amount of gas generated was reduced by 24% or more compared to Experimental Example 8 in which the negative electrode active material in the negative electrode portion was not provided with a second shell layer. In Experimental Examples 5 to 7, the amount of gas generated was reduced by 65% ​​or more, and the capacity retention rate was improved by 13% or more compared to Experimental Example 9, in which no shell layer was provided on either the positive electrode active material in the positive electrode section or the negative electrode active material in the negative electrode section, and no electrolyte additive was added. In Experimental Examples 5 to 7, in which the crystallite diameter of the first precursor particles when forming the first shell layer was 25 nm or less and the ratio of the crystallite diameter to dBET was 0.22 or less, and the crystallite diameter of the second precursor particles when forming the second shell layer was 25 nm or less and the ratio of the crystallite diameter to dBET was 1.50 or less, the amount of gas generated was reduced by 37% or more compared to Experimental Example 10, in which the shell layers were not coated with either the positive electrode active material or the negative electrode active material. In Experimental Example 8, in which a shell layer was provided in the positive electrode active material of the positive electrode part, the amount of gas generation was reduced by 13% and the capacity retention rate was improved by 4% or more compared to Experimental Example 10, in which a shell layer was not provided in either the positive electrode active material of the positive electrode part or the negative electrode active material of the negative electrode part. In Experimental Example 10, in which an electrolyte additive was added, the amount of gas generated was suppressed compared to Experimental Example 9, in which no electrolyte additive was added, and the capacity retention rate was improved by 13%. These results suggest that even when the particle size of the LNMO core of the positive electrode active material is reduced, the amount of gas generation can be significantly reduced and the capacity retention rate can be improved by constructing the positive electrode active material with core-shell particles in which the shell layer is LMP. Furthermore, it was suggested that the addition of a vinyl group-containing cyclic siloxane electrolyte additive further reduced the amount of gas generation and improved the capacity retention rate.

[0112] In Experimental Example 5, in which the crystallite diameter of the first precursor particles was 20 nm or less and the ratio of crystallite diameter to dBET was 0.20 or less, the amount of gas generated was reduced by 7% and the capacity retention rate was improved by 4% compared to Experimental Example 6, in which the crystallite diameter of the first precursor particles was more than 20 nm and the ratio of crystallite diameter to dBET was more than 0.20. This suggests that by making the crystallite diameter of the first precursor particles 20 nm or less and the ratio of the crystallite diameter to dBET 0.20 or less, the amount of gas generated can be further suppressed and the capacity retention rate can be further improved.

[0113] In Experimental Example 5, in which the crystallite diameter of the second precursor particles was 15 nm or less and the ratio of the crystallite diameter to dBET was 1.30 or more and 1.60 or less, the amount of gas generated was reduced by 4% compared to Experimental Example 7, in which the crystallite diameter of the second precursor particles was more than 15 nm and the ratio of the crystallite diameter to dBET was less than 1.30. In addition, although the capacity retention rate decreased by 3%, it was still maintained at 90% or more. This suggests that the amount of gas generated can be further suppressed by making the crystallite diameter of the second precursor particles 15 nm or less and the ratio of the crystallite diameter to dBET 1.30 or more.

[0114] From the above results, the following (1) to (4) were found. (1) By using a bead mill in the grinding process for forming positive electrode active material and negative electrode active material particles, the first precursor particles and the second precursor particles each exhibit a (111) plane peak in XRD measurement, making it possible to calculate the crystallite diameters of the first precursor particles and the second precursor particles by XRD. (2) By using core-shell particles with LMP as the positive electrode active material in the positive electrode, the amount of gas generation is significantly reduced and the capacity retention rate is improved. (3) In the negative electrode section, the negative electrode active material is made into core-shell particles with LFP as the shell layer, which further suppresses the amount of gas generation and improves the capacity retention rate. (4) The addition of a vinyl group-containing cyclic siloxane electrolyte additive further suppresses gas generation and improves capacity retention. [Explanation of symbols]

[0115] 1. Secondary battery (lithium-ion secondary battery) 5 Electrolytes 22 Cathode active material 23 First core-shell particle 25 First Core Section 26 First shell layer 32 Negative electrode active material 33 Secondary core-shell particles 35 Second Core Section 36 Second shell layer

Claims

1. A method for producing a positive electrode active material comprising first core-shell particles having a first core portion and a first shell layer covering a surface of the first core portion, wherein a main component of the first shell layer is lithium manganese phosphate having an olivine-type crystal structure, the method comprising: a first core-shell forming step of coating the first core portion with first precursor particles to form the first shell layer, the first precursor particles have a first peak attributable to a (111) plane in a 2θ range of 24.5° to 26.5° in a diffraction pattern obtained by X-ray diffraction measurement using Cu-Kα radiation, and a crystallite diameter calculated from the half width of the first peak by the Scherrer equation is 25 nm or less.

2. the first precursor particles have a second peak in the 2θ range of 28.5° to 30.5° in the diffraction pattern; The method for producing a positive electrode active material according to claim 1 , wherein the first peak of the first precursor particles has a peak intensity that is 0.8 to 1.0 times the peak intensity of the second peak.

3. The method for producing a positive electrode active material according to claim 1 , wherein the first core portion contains lithium nickel manganese oxide having a spinel-type crystal structure.

4. The method for producing a positive electrode active material according to claim 1 , wherein the first precursor particles have a crystallite diameter of 20 nm or less.

5. 2 . The method for producing a positive electrode active material according to claim 1 , wherein the first precursor particles have a ratio of a crystallite diameter to a BET specific surface area equivalent diameter calculated from a specific surface area determined by a BET method of 0.14 to 0.

22.

6. 2 . The method for producing a positive electrode active material according to claim 1 , comprising a first pulverization step of pulverizing lithium manganese phosphate having an olivine-type crystal structure with a bead mill to form the first precursor particles.

7. A method for manufacturing a secondary battery having a positive electrode active material, a negative electrode active material, and an electrolyte, comprising: the negative electrode active material includes second core-shell particles having a second core portion and a second shell layer covering a surface of the second core portion, and the second shell layer is mainly composed of lithium iron phosphate having an olivine-type crystal structure; a positive electrode active material layer forming step of forming the positive electrode active material using the method for producing a positive electrode active material according to any one of claims 1 to 6; a second core-shell forming step of coating the second core portion with second precursor particles to form the second shell layer, the second precursor particles have a third peak at 2θ in the range of 24.5° to 26.5°, which is attributed to a (111) plane, in a diffraction pattern obtained by X-ray diffraction measurement using Cu-Kα radiation, and a crystallite diameter calculated from the half width of the third peak by the Scherrer equation is 25 nm or less.

8. the electrolyte is a non-aqueous electrolyte solution, an additive addition step of adding an electrolyte additive; The method for producing a secondary battery according to claim 7 , wherein the electrolyte additive is a vinyl group-containing cyclic siloxane.

9. 8. The method for manufacturing a secondary battery according to claim 7, wherein the second precursor particles have a ratio of a crystallite diameter to a BET specific surface area equivalent diameter calculated from a specific surface area determined by a BET method of 1.20 or more and 1.60 or less.

10. The method for manufacturing a secondary battery according to claim 7 , wherein the second core portion contains lithium titanate having a spinel-type crystal structure.

11. 8. The method for producing a secondary battery according to claim 7, further comprising a second pulverization step of pulverizing lithium iron phosphate having an olivine-type crystal structure with a bead mill to form the second precursor particles.

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