Lithium-ion secondary battery

Abstract

Provided is a lithium-ion secondary battery with which, more than conventional configurations, it is possible to prevent the production of gas due to electrolyte degradation.　The invention includes a positive electrode section, a negative electrode section, and an electrolyte. The electrolyte contains an additive that includes a siloxane bond as a main chain and that includes, as at least two side chains of silicon atoms of the siloxane bond, saturated hydrocarbon groups in which hydrogen atoms are unsubstituted or are partially replaced with another atom, and which have carbon numbers that are the same or different.

Description

Lithium-ion rechargeable battery

[0001] This invention relates to a lithium-ion secondary battery.

[0002] In recent years, in order to maintain a high operating voltage while suppressing the decomposition of the electrolyte during charging and discharging, a core-shell type positive electrode active material has been developed for lithium-ion secondary batteries, in which lithium nickel manganese oxide (hereinafter also called LNMO), which has a high operating voltage, is used as the core part and lithium iron phosphate (hereinafter also called LFP) is used as the shell layer (for example, Patent Document 1). In the lithium-ion secondary battery of Patent Document 1, the LNMO core part is covered with LFP as the shell layer, so the core part is not directly exposed to the electrolyte, and the decomposition of the electrolyte can be suppressed significantly compared to when LNMO is used as the positive electrode active material.

[0003] International Publication No. 2023 / 149363

[0004] However, while the lithium-ion secondary battery described in Patent Document 1 can significantly suppress gas generation compared to the case where LNMO is used as the positive electrode active material, the gas generation suppression effect is not sufficient when charging and discharging is repeated over a long period of time, and there is room for further improvement.

[0005] Therefore, the present invention aims to provide a lithium-ion secondary battery that can suppress the generation of gas due to the decomposition of the electrolyte compared to conventional batteries.

[0006] One aspect of the present invention for solving the above-mentioned problems is a lithium-ion secondary battery comprising a positive electrode portion, a negative electrode portion, and an electrolyte, wherein the electrolyte comprises a siloxane bond as its main chain, and at least two side chains of the silicon atoms of the siloxane bond are saturated hydrocarbon groups that are unsubstituted or have some hydrogen atoms substituted with other atoms, and each additive has saturated hydrocarbon groups with the same or different number of carbon atoms.

[0007] According to this method, the generation of gas due to the decomposition of the electrolyte can be suppressed compared to conventional methods.

[0008] A preferred feature is that the additive has three or more silicon atoms constituting the siloxane bond.

[0009] A preferred aspect is that the additive contains a cyclic siloxane bond as its main chain.

[0010] A preferred configuration is that both saturated hydrocarbon groups constituting the two side chains are methyl groups.

[0011] A preferred configuration is that the positive electrode portion includes positive electrode particles having a core-shell structure with a first shell layer containing a lithium-containing oxide on the surface of a first core portion, and the lithium-containing oxide contains phosphorus.

[0012] A preferred configuration is that the positive electrode particle has a first core portion composed of lithium nickel manganese oxide.

[0013] A preferred configuration is that the electrolyte contains the additive in an amount of 0.1% to 20% relative to the weight of the electrolyte.

[0014] A preferred configuration is that the additive is at least one selected from the group consisting of octamethylcyclotetrasiloxane, octadecamethylcyclooctadecanenonasiloxane, 1,3,5-tris(trifluoropropylmethyl)cyclotrisiloxane, and tetrakis(trimethylsilyl) orthosilicate.

[0015] One aspect of the present invention is a lithium-ion secondary battery comprising a positive electrode portion, a negative electrode portion, and an electrolyte, wherein the electrolyte contains an additive having a cyclic siloxane bond as its main chain, and methyl groups as two side chains of silicon atoms in the cyclic siloxane bond.

[0016] According to this method, the generation of gas due to the decomposition of the electrolyte can be suppressed compared to conventional methods.

[0017] The aspects described above can be dependent on each other, refer to some of their components, or substitute for some of their components, as long as they fall within the technical scope of the present invention.

[0018] According to the present invention, the generation of gas due to the decomposition of the electrolyte can be suppressed compared to conventional methods.

[0019] This is a conceptual diagram illustrating a secondary battery according to the first embodiment of the present invention, where (a) is a perspective view of the secondary battery and (b) is a cross-sectional view taken along line A-A in (a). This is an explanatory diagram of the electrode portion of Figure 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.

[0020] Embodiments of the present invention will be described in detail below.

[0021] The secondary battery 1 of the first embodiment of the present invention is a lithium-ion secondary battery and, as shown in Figure 1, comprises a battery stack 2, electrode extraction members 3a and 3b, electrolyte 5, and an outer casing 6.

[0022] <Battery Stack 2> As shown in Figure 1(b), the battery stack 2 has a plurality of positive electrode portions 10, a plurality of negative electrode portions 11, and a plurality of separators 12. In this embodiment, the battery stack 2 has the electrode portions 10, 11 and separators 12 arranged alternately, such as ... / separator 12 / positive electrode portion 10 / separator 12 / negative electrode portion 11 / separator 12 / ..., with the separators 12 positioned on the outermost side in the stacking direction.

[0023] (Positive electrode section 10) As shown in Figure 2(a), the positive electrode section 10 is an intercalation electrode in which lithium ions can be inserted and removed, with a positive electrode active material layer 21 laminated on at least one main surface of the positive electrode current collector 20. The positive electrode active material layer 21 is a layered body having a positive electrode active material 22, a conductive additive, and a binder.

[0024] The positive electrode active material 22 is composed of a plurality of first core-shell particles 23 (positive electrode particles), as shown in the enlarged view of Figure 2(a). The first core-shell particles 23 have a first shell layer 26 covering the surface of a first core portion 25.

[0025] The first core portion 25 is made of a lithium-ion conductive oxide and is capable of inserting and removing lithium ions. The first core portion 25 has an average potential of lithium removal and insertion relative to the deposition potential of Li (vs. Li / Li ＋It is preferably 4.5 V or more and 5.0 V or less (as 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 metallic lithium alone. The potential of the lithium ion insertion / desorption reaction (hereinafter also referred to as voltage) (vs. Li / Li ＋ ), for example, can be obtained by measuring the charge-discharge characteristics of a half-cell with a working electrode using the first core part 25 and a metallic lithium counter electrode, and reading the voltage values at the start and end of the plateau. When there are two or more plateaus, it is sufficient if the plateau with the lowest voltage value is 4.5 V (vs. Li / Li ＋ or more, and it is sufficient if the plateau with the highest voltage value is 5.0 V (vs. Li / Li ＋ or less).

[0026] 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 O ４ ... (1) In the formula (1), x and y satisfy 0 ≤ x ≤ 0.2 and 0 < y ≤ 0.8, respectively, and M is at least one selected from the group consisting of Al, Mg, Zn, Ni, Co, Fe, Ti, Cu, and Cr.

[0027] Among the above formula (1), a lithium nickel manganese oxide (hereinafter also referred to as LNMO) in which M is Ni is preferable.

[0028] The first shell layer 26 is a film composed of a lithium ion conductive oxide and is composed of an intercalation material that functions as a positive electrode active material alone. The first shell layer 26 is a lithium-containing oxide containing phosphorus as an element. Specifically, the main component is lithium manganese phosphate (LiMnPO ４, hereinafter also referred to as LMP). Here, the "main component" refers to a component that occupies more than 50% of the total amount. The same shall apply hereinafter. That is, in the first shell layer 26, LMP occupies more than 50% of the total amount, preferably 90% or more, and more preferably 98% or more. The thickness of the first shell layer 26 is preferably thinner than the particle size 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 covers 95% or more, and preferably covers completely.

[0029] (Negative electrode portion 11) The negative electrode portion 11 is formed by laminating a negative electrode active material layer 31 on at least one main surface of a negative electrode current collector 30 as shown in FIG. 2(b), and is an intercalation electrode capable of inserting and extracting lithium ions. The negative electrode active material layer 31 includes a negative electrode active material 32, a conductive auxiliary agent, and a binder.

[0030] 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 shell layer 36 covering the surface of the second core portion 35.

[0031] The second core portion 35 is composed 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 second core portion 35 is preferably 0.5 V or more and 2.5 V or less with respect to the deposition potential of Li (vs. Li / Li ＋ also shown). That is, the second core portion 35 preferably has an operating potential of 0.5 V or more and 2.5 V or less based on lithium metal in a single body.

[0032] From the viewpoint of improving safety by making lithium precipitation difficult to occur, it is preferable to use lithium titanate (hereinafter also referred to as LTO) for the second core portion 35. Among lithium titanates, for the second core portion 35, lithium titanate having a spinel structure is particularly preferable because the expansion and contraction of the negative electrode active material 32 in the reaction of inserting and extracting lithium ions are small.

[0033] The second shell layer 36 is a film made of a lithium ion conductive oxide and is composed of an intercalation material. The second shell layer 36 is a lithium-containing oxide containing phosphorus as an element. Specifically, the main component is lithium iron phosphate (LiFePO ４ , hereinafter also referred to as LFP). That is, in the second shell layer 36, LFP occupies more than 50% of the total amount, preferably occupies 90% or more, and more preferably occupies 98% or more. It is preferable that the main component of the second shell layer 36 is the same type as the main component of the first shell layer 26. The thickness of the second shell layer 36 is preferably thinner than the particle size of the second core portion 35. The second shell layer 36 covers at least a part of the surface of the second core portion 35, more preferably covers 95% or more, and preferably covers it completely.

[0034] The current collectors 20 and 30 are not particularly limited, but since they are stable in the positive electrode reaction atmosphere and the negative electrode reaction atmosphere, they are preferably aluminum or an aluminum alloy. The current collectors 20 and 30 can also be those in which a metal that does not react at the potentials of the positive electrode portion 10 and the negative electrode portion 11 is coated on the surface of a metal other than aluminum (copper, SUS, nickel, titanium, and their alloys).

[0035] The conductive assistant used in the active material layers 21 and 31 is not particularly limited, but a carbon material is preferable. 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 assistant contained in the active material layers 21 and 31 is preferably 1 part by weight or more and 30 parts by weight or less with respect to 100 parts by weight of the active materials 22 and 32. Within the above range, while ensuring the conductivity of the active material layers 21 and 31, the adhesiveness with the binder can be maintained, and sufficient adhesiveness with the current collectors 20 and 30 can be obtained.

[0036] The binder used in the active material layers 21 and 31 is not particularly limited, but for either of the active material layers 21 or 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. Preferably, the amount of binder contained in the active material layers 21 and 31 is 1 part by weight or more and 30 parts by weight or less per 100 parts by weight of the active material 22 and 32. Within this range, the adhesion between the active material 22 and 32 and the conductive additive can be maintained, and sufficient adhesion to the current collectors 20 and 30 can be obtained.

[0037] <Electrode Extraction Members 3a, 3b> The positive electrode extraction member 3a is a positive electrode terminal that extends inside and outside the outer casing 6 and is at least electrically connected to one of the positive electrode portions 10 that make up the battery stack 2, as shown in Figure 1(b). The negative electrode extraction member 3b is a negative electrode terminal that extends inside and outside the outer casing 6 and is at least electrically connected to one of the negative electrode portions 11 that make up the battery stack 2, as well as within the outer casing 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 or their alloys can be used.

[0038] <Electrolyte 5> The electrolyte 5 is a non-aqueous electrolyte having lithium ion conductivity to which one or more additives are added. Preferably, the electrolyte 5 contains the additives in an amount of 0.1% to 20% relative to the weight of the electrolyte 5, and more preferably in an amount of 1% to 15%.

[0039] (Non-aqueous electrolyte) A non-aqueous electrolyte is a solution in which an electrolyte is dissolved in a non-aqueous solvent.

[0040] The non-aqueous solvent is not particularly limited, but for example, cyclic aprotic solvents and / or linear aprotic solvents can be used. Examples of cyclic aprotic solvents include cyclic carbonates, cyclic esters, cyclic sulfones, and cyclic ethers. Examples of linear aprotic solvents include linear carbonates, linear carboxylic acid esters, linear ethers, and acetonitrile.

[0041] The electrolyte is not particularly limited, but for example, LiBF ４ LiPF ６ LiAsF ６ LiCF ３ SO ３ , LiN (SO ２ CF ３ ) ２ Fluorine-containing lithium can be used.

[0042] (Additives) The additives include siloxane bonds as the main chain, and at least two side chains of the silicon atoms of the siloxane bonds are saturated hydrocarbon groups that are unsubstituted or have some hydrogen atoms substituted with other atoms, and each has saturated hydrocarbon groups with the same or different numbers of carbon atoms. The other atoms are not particularly limited as long as they can be substituted with hydrogen atoms, but examples include halogen atoms such as fluorine atoms, chlorine atoms, bromine atoms, and iodine atoms.

[0043] The siloxane bonds in the main chain may be linear or cyclic, but it is preferable that they have cyclic siloxane bonds represented by the following general formula (1).

[0044]

[0045] In general formula (1), n ​​is a natural number between 3 and 10, R1 is a saturated hydrocarbon group that is unsubstituted or in which some hydrogen atoms are substituted with the other atoms mentioned above, and R2 is a saturated hydrocarbon group that is unsubstituted or in which some hydrogen atoms are substituted with the other atoms mentioned above. It is preferable that n is a natural number of 4 or more, and preferably a natural number of 10 or less.

[0046] In the above general formula (1), R1 is preferably an unsubstituted or partially substituted methyl, ethyl, propyl, and isopropyl group, and R2 is preferably an unsubstituted or partially substituted methyl, ethyl, and propyl group. In the above general formula (1), the additive is preferably such that R1 and R2 are both the same functional group, more preferably such that R1 and R2 are both methyl groups, and more preferably octamethylcyclotetrasiloxane or octadecamethylcyclononasiloxane. The additive is preferably such that, in the above general formula (1), R1 is a methyl group and R2 is one of a methyl group, an ethyl group, or a propyl group in which some hydrogen atoms are replaced with fluorine atoms, and more preferably 1,3,5-tris(trifluoropropylmethyl)cyclotrisiloxane.

[0047] As an additive, tetrakis(trimethylsilyl) orthosilicate can be used, where the siloxane bond in the main chain is a linear siloxane bond.

[0048] In addition to the additives mentioned above, it is preferable to add an antioxidant to the electrolyte 5. The antioxidant is preferably a radical chain inhibitor that scavenges radicals and has an effect of preventing auto-oxidation, and is more preferably a phenolic antioxidant. As a phenolic antioxidant, for example, butylated hydroxytoluene (hereinafter also referred to as BHT) can be used.

[0049] <Separator 12> The separator 12 is installed between the positive electrode portion 10 and the negative electrode portion 11 and may have an insulating structure that can contain the electrolyte 5. Examples of materials for the separator 12 include woven fabrics, nonwoven fabrics, and microporous membranes made from nylon, cellulose, polysulfone, polyethylene, polypropylene, polybutene, polyacrylonitrile, polyimide, polyamide, polyethylene terephthalate, and composites of two or more of these materials.

[0050] <Outer casing 6> As shown in Figure 1(b), the outer casing 6 has an internal space 40 and is a sealing member that houses and seals the battery stack 2 and electrolyte 5 in the internal space 40. It is chemically stable to the electrolyte 5 and has water vapor barrier properties. As shown in Figure 1, the outer casing 6 is composed of a first outer film 41 and a second outer film 42, and can be sealed by sandwiching the battery stack 2 and electrolyte 5 between the outer films 41 and 42. The outer films 41 and 42 are made of laminate films containing a laminate resin.

[0051] The secondary battery 1 of this embodiment includes a positive electrode portion 10, a negative electrode portion 11, and an electrolyte 5. The electrolyte 5 contains a siloxane bond as its main chain, and at least two side chains of the silicon atoms of the siloxane bond are saturated hydrocarbon groups that are unsubstituted or have some hydrogen atoms substituted with other atoms, and each contains an additive having saturated hydrocarbon groups with the same or different numbers of carbon atoms. Therefore, the generation of gas due to the decomposition of the electrolyte 5 can be suppressed compared to conventional batteries.

[0052] In the secondary battery 1 of this embodiment, it is preferable that the additive has three or more silicon atoms constituting the siloxane bond. This suppresses the generation of aging gas, which will be described later.

[0053] In the secondary battery 1 of this embodiment, it is preferable that the additive contains a cyclic siloxane bond as the main chain. This suppresses the generation of gas due to the decomposition of the electrolyte 5 over a long period of time.

[0054] In the secondary battery 1 of this embodiment, it is preferable that both saturated hydrocarbon groups constituting the two side chains of the additive are methyl groups. This suppresses decomposition in the side chains of the additive.

[0055] In the secondary battery 1 of this embodiment, the positive electrode portion 10 includes first core-shell particles 23 having a core-shell structure with a first shell layer 26 containing a lithium-containing oxide on the surface of a first core portion 25 as the positive electrode active material, and it is preferable that the lithium-containing oxide contains phosphorus. This makes it less likely for the first core portion 25 to be exposed to the electrolyte 5, and the decomposition of the electrolyte 5 by the first core portion 25 can be suppressed.

[0056] In the secondary battery 1 of this embodiment, it is preferable that the first core portion 25 of the first core shell particle 23 is composed of lithium nickel manganese oxide. In the secondary battery 1 of this embodiment, even if lithium nickel manganese oxide with a high operating voltage is used as the first core portion 25, the decomposition of the electrolyte 5 can be suppressed.

[0057] In the secondary battery 1 of this embodiment, it is preferable that the electrolyte 5 contains an additive in an amount of 0.1% to 20% relative to the weight of the electrolyte 5. By doing so, it is possible to maintain the initial capacity while expecting a gas generation suppression effect from the additive.

[0058] The secondary battery 1 of this embodiment includes a positive electrode portion 10, a negative electrode portion 11, and an electrolyte 5. The electrolyte 5 may also include an additive having a cyclic siloxane bond as its main chain, and methyl groups as two side chains of silicon atoms in the cyclic siloxane bond. This suppresses the generation of gas due to the decomposition of the electrolyte 5.

[0059] In the embodiment described above, the secondary battery 1 was a laminated type secondary battery in which the battery stack 2 was sealed with outer films 41 and 42, but the present invention is not limited thereto. Other types of secondary batteries may also be used. For example, a cylindrical secondary battery in which a wound battery stack 2 is sealed in a cylindrical housing may also be used.

[0060] In the embodiments described above, the positive electrode active material 22 was composed of first core-shell particles 23 having a core-shell structure, but the present invention is not limited thereto. The positive electrode active material 22 may be composed of general positive electrode active material particles. For example, it may be composed of positive electrode active material particles that do not have a core-shell structure, such as LNMO.

[0061] In the embodiments described above, the negative electrode active material 32 was composed of second core-shell particles 33 having a core-shell structure, but the present invention is not limited thereto. The negative electrode active material 32 may be composed of general negative electrode active material particles. For example, it may be composed of negative electrode active material particles that do not have a core-shell structure, such as LTO.

[0062] In the embodiments described above, the components can be freely substituted or added between each embodiment, as long as they fall within the technical scope of the present invention.

[0063] The present invention will be specifically described below with reference to examples. However, the present invention is not limited to the following examples and can be implemented with appropriate modifications without altering its essence.

[0064] (Example 1) (a) Preparation of positive electrode active material First, a predetermined amount of ethanol, which is used as a solvent, was mixed with lithium manganese phosphate (LMP) powder having an olivine-type crystal structure, and the mixture was treated with a bead mill for 3 hours using beads with a diameter of 0.2 mm. After removing the beads from the mixture, a portion of the ethanol was removed to obtain a slurry containing 16.4 wt% of first precursor particles (LMP fine powder).

[0065] As the first core portion of the positive electrode, a spinel-type lithium nickel manganese oxide (LiNi) with a median diameter of 8.8 μm is used. ０．５ Mn １．５ O ４(hereinafter also referred to as LNMO) was used. 30 g of LNMO was placed in a grinding mill, and while rotating at a clearance of 0.8 mm, rotor load power of 1.5 kW, and 2600 rpm, an ethanol-dispersed slurry of first precursor particles was added in two portions so that the amount of first precursor particles added was 1.2 wt%. After that, the rotor rotation speed was maintained in the range of 2600 rpm to 3000 rpm and the material 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 positive electrode active material containing first core-shell particles in which the surface of LNMO (first core portion) was coated with LMP (first shell layer).

[0066] (b) Preparation of the positive electrode A slurry was prepared by dispersing a mixture containing 90 parts by weight, 6 parts by weight, and 4 parts by weight of the obtained positive electrode active material, acetylene black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder, respectively, in N-methyl-2-pyrrolidone (NMP) in solid content concentrations. The binder used was an N-methyl-2-pyrrolidone (NMP) solution with a solid content concentration of 5 wt%, and further NMP was added to adjust the viscosity to facilitate coating.

[0067] The slurry was coated onto 15 μm aluminum foil and then dried in an oven at 120°C. After performing this operation on both sides of the aluminum foil, the positive electrode portion was fabricated by further vacuum drying at 170°C.

[0068] (c) Preparation of negative electrode active material First, lithium iron phosphate (LFP) powder having an olivine-type crystalline structure was mixed with a predetermined 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, a portion of the ethanol was removed to obtain a slurry containing 16.4 wt% of second precursor particles (LFP fine powder).

[0069] As the second core of the negative electrode, a spinel-type lithium titanate (Li) with a median diameter of 8.5 μm is used. ４ Ti ５ O １２(hereinafter also referred to as LTO) was used. 30 g of LTO was placed in a grinding mill, and while rotating at a clearance of 0.8 mm, rotor load power of 1.5 kW, and 2600 rpm, an ethanol-dispersed slurry of second precursor particles was added in two portions so that the amount of second precursor particles added was 3.6 wt%. After that, the rotor rotation speed was maintained in the range of 2600 rpm to 3000 rpm and the material 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).

[0070] (d) Preparation of the negative electrode A slurry was prepared by dispersing a mixture containing 92 parts by weight, 3 parts by weight, and 5 parts by weight of the obtained negative electrode active material, acetylene black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder, respectively, in N-methyl-2-pyrrolidone (NMP) at solid content concentrations. The binder used was an N-methyl-2-pyrrolidone (NMP) solution with a solid content concentration of 5 wt%, and further NMP was added to adjust the viscosity to facilitate coating.

[0071] The slurry was coated onto 15 μm aluminum foil and then dried in an oven at 120°C. After performing this operation on both sides of the aluminum foil, the negative electrode portion was fabricated by further vacuum drying at 170°C.

[0072] (e) Fabrication of a lithium-ion secondary battery Using the positive electrode and negative electrode parts fabricated in (b) and (d) above, and a 20 μm polypropylene separator, a battery was fabricated by the following procedure. First, the positive electrode and negative electrode parts 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 / separator / positive electrode to form a battery stack. The outermost layer of both battery stacks was made of separator. Next, aluminum tabs were vibrated and welded to the positive electrode and negative electrode parts at both ends.

[0073] Two aluminum laminate films were prepared to serve as the outer casing. After forming recesses for the battery section and gas collection section by pressing, the battery stack was placed inside. The outer periphery, leaving space for electrolyte injection, was heat-sealed at 180°C for 7 seconds. From the unsealed area, ethylene carbonate, propylene carbonate, and ethyl methyl carbonate were mixed by volume in a solvent of ethylene carbonate / propylene carbonate / ethyl methyl carbonate = 15 / 15 / 70, and LiPF was added. ６ A non-aqueous electrolyte solution prepared by dissolving the substance at a concentration of 1 mol / L was added, and 5 wt% of octamethylcyclotetrasiloxane (hereinafter also referred to as D4Me) was added to the non-aqueous electrolyte solution as an additive to create an electrolyte solution with additives. The unsealed areas were then heat-sealed at 180°C for 7 seconds under reduced pressure.

[0074] (f) Aging of the lithium-ion secondary battery The obtained battery was charged with a constant current value equivalent to 0.2C until the battery voltage reached the termination voltage of 3.4V, and then the charging was stopped. After that, it was left to stand in a 60°C environment for 24 hours, and then discharged with a constant current value equivalent to 0.2C, and the discharge was stopped when the battery voltage reached 2.5V. After the discharge was stopped, the gas accumulated in the gas collection section was extracted as aging gas, and the section was resealed. A lithium-ion secondary battery for evaluation was fabricated by the above procedure.

[0075] (Example 2) The process for producing the lithium-ion secondary battery described in (e) above was the same as in Example 1, except that 10 wt% of D4Me was added as an additive to the non-aqueous electrolyte to make an electrolyte with an additive. This was designated as Example 2.

[0076] (Example 3) In the preparation of the lithium-ion secondary battery described in (e) above, 5 wt% of 1,3,5-Tris(3,3,3-trifluoropropyl)-1,3,5-trimethylcyclotrisiloxane (hereinafter referred to as D3TFP) was added to the non-aqueous electrolyte as an additive to create an electrolyte with an additive. Otherwise, the procedure was the same as in Example 1, and this was designated as Example 3.

[0077] (Example 4) In the preparation of the lithium-ion secondary battery described above (e), 10 wt% of D3TFP was added to the non-aqueous electrolyte to make an electrolyte with an additive. The procedure was the same as in Example 1, and this was designated as Example 4.

[0078] (Example 5) In the preparation of the lithium-ion secondary battery described in (e) above, the procedure was the same as in Example 1, except that 3 wt% of octadecamethylcyclooctadecanonasiloxane (hereinafter referred to as D9Me) was added to the non-aqueous electrolyte as an additive to create an electrolyte with an additive. This was designated as Example 5.

[0079] (Example 6) In the preparation of the lithium-ion secondary battery described in (e) above, the procedure was the same as in Example 1, except that 3 wt% of tetrakis(trimethylsilyl) orthosilicate (hereinafter referred to as TTOS) was added to the non-aqueous electrolyte as an additive to create an electrolyte with an additive, and this was designated as Example 6.

[0080] (Comparative Example 1) The process for producing the lithium-ion secondary battery described above (e) was the same as in Example 1, except that no additives were added to the non-aqueous electrolyte. This was designated as Comparative Example 1.

[0081] (Comparative Example 2) The process for producing the lithium-ion secondary battery described in (e) above was the same as in Example 1, except that 3 wt% of 1,1,1,5,5,5-Hexamethyl-3-[(trimethylsilyl)oxy]-3-vinyltrisiloxane (hereinafter referred to as LS4V1) was added to the non-aqueous electrolyte as an additive to create an electrolyte with an additive. This was designated as Comparative Example 2.

[0082] (Comparative Example 3) The process for producing the lithium-ion secondary battery described in (e) above was the same as in Example 1, except that 3 wt% of 1,3-divinyltetramethyldisiloxane (hereinafter referred to as LS2V2) was added to the non-aqueous electrolyte as an additive to create an electrolyte with an additive. This was designated as Comparative Example 3.

[0083] (Evaluation of Cycle Characteristics of Lithium-ion Secondary Batteries) The lithium-ion secondary batteries prepared in each of Examples 1 to 6 and Comparative Examples 1 to 3 were connected to a charge / discharge device (HJ1005SD8, manufactured by Hokuto Denko Co., Ltd.) and cycle operation was performed. Under a 45°C environment, constant current charging was performed at a current value equivalent to 1.0C until the battery voltage reached the termination voltage of 3.4V, and then charging was stopped. Subsequently, constant current discharge was performed at a current value equivalent to 1.0C, and discharge was stopped when the battery voltage reached 2.5V. This was considered one cycle, and the charge / discharge process was repeated.

[0084] (Gas Generation Measurement) The gas generation amount of lithium-ion secondary batteries before and after aging and before and after cycle characteristic evaluation in each of Examples 1 to 6 and Comparative Examples 1 to 3 was evaluated using the Archimedes method, i.e., the buoyancy of the lithium-ion secondary battery. The evaluation was performed as follows.

[0085] 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 (Alpha Mirage Co., Ltd., model number: MDS-3000), and the buoyancy was calculated by taking the difference between these weights. This buoyancy was calculated using the density of water (1.0 g / cm³). ３ The volume of the lithium-ion secondary battery was calculated by dividing by ( ). The amount of gas generated was calculated by comparing the volume before aging with the volume after aging, or the volume after aging with the volume after 800 cycles of cycle characteristic evaluation.

[0086] Table 1 shows the evaluation results for each of Examples 1 to 6 and Comparative Examples 1 to 3. In Comparative Example 3, since a large amount of aging gas was generated during the aging of the lithium-ion secondary battery (f) above, gas generation amount measurement and cycle characteristic evaluation were not performed, and only the initial capacity (initial discharge capacity) was measured. Also, the gas generation amount after 800 cycles in Table 1 is normalized so that the gas generation amount in Comparative Example 1 is 1.00.

[0087]

[0088] In Examples 1 to 6, where an additive containing a siloxane bond as the main chain, and in which the side chains of each silicon atom in the siloxane bond are unsubstituted or in which some hydrogen atoms are substituted with fluorine atoms, was added to the non-aqueous electrolyte, the initial volume was similar to or improved compared to Comparative Example 1, in which no additive was added to the non-aqueous electrolyte, and the amount of gas generated after 800 cycles was suppressed.

[0089] In Comparative Example 3, where an additive containing siloxane bonds as its main chain, with two silicon atoms in the siloxane bonds, was added to the non-aqueous electrolyte, the amount of aging gas generated increased significantly compared to Comparative Example 1, where no additive was added to the non-aqueous electrolyte. Furthermore, in Examples 1 to 6 and Comparative Example 2, where an additive containing siloxane bonds as its main chain, with three or more silicon atoms in the siloxane bonds, was added to the non-aqueous electrolyte, the amount of aging gas generated was at the same level or reduced compared to Comparative Example 1, where no additive was added to the non-aqueous electrolyte. These results suggest that even when an additive is added to a non-aqueous electrolyte, using an additive with three or more silicon atoms constituting the siloxane bonds can suppress the amount of aging gas generated to the same level as when no additive is added to the non-aqueous electrolyte.

[0090] In Examples 1-5, where an additive having a cyclic siloxane bond was added to the non-aqueous electrolyte, the amount of aging gas generated was suppressed compared to Example 6, where an additive having a linear siloxane bond was added to the non-aqueous electrolyte, and the amount of gas generated after 800 cycles was particularly suppressed. This suggests that adding an additive having a cyclic siloxane bond to the non-aqueous electrolyte can suppress gas generation over the long term.

[0091] In Example 6, where an additive having linear siloxane bonds and side chains of the silicon atoms in the siloxane bonds being saturated hydrocarbon groups was added to a non-aqueous electrolyte, the amount of aging gas was suppressed, the initial volume increased, and the amount of gas generated after 800 cycles was suppressed compared to Comparative Examples 2 and 3, where an additive having linear siloxane bonds and side chains of the siloxane bonds being unsaturated hydrocarbon groups was added to the non-aqueous electrolyte. Furthermore, in Comparative Example 3, where an additive with two unsaturated hydrocarbon groups was added to the non-aqueous electrolyte, a large amount of aging gas was generated, whereas in Comparative Example 2, where an additive with one unsaturated hydrocarbon group was added to the non-aqueous electrolyte, the generation of aging gas was suppressed. This suggests that the number of unsaturated hydrocarbon groups in the functional group connected to the silicon atom affects the generation of aging gas. This is likely because, during aging, the presence of unsaturated hydrocarbon groups causes the additive itself to decompose, generating aging gas.

[0092] From the above results, the following (1) to (4) were found: (1) By adding an additive to the non-aqueous electrolyte that contains a siloxane bond as the main chain and in which the side chains of each silicon atom in the siloxane bond are unsubstituted or in which some hydrogen atoms are substituted with fluorine atoms, the amount of gas generated after 800 cycles was suppressed while maintaining the initial volume, compared to when no additive was added to the non-aqueous electrolyte. (2) By setting the number of silicon atoms in the siloxane bond of the additive to three or more, the amount of aging gas generated was suppressed to the same extent as when no additive was added to the non-aqueous electrolyte. (3) By adding an additive having a cyclic siloxane bond to the non-aqueous electrolyte, the amount of gas generated after 800 cycles was particularly suppressed compared to when no additive was added or when an additive having a linear siloxane bond was added to the non-aqueous electrolyte. (4) It was found that the presence of an unsaturated hydrocarbon group makes it easier for aging gas to be generated.

[0093] 1 Secondary battery 5 Electrolyte 10 Positive electrode 11 Negative electrode 22 Positive electrode active material 23 First core-shell particles (positive electrode particles) 25 First core 26 First shell layer

Claims

1. A lithium-ion secondary battery comprising a positive electrode portion, a negative electrode portion, and an electrolyte, wherein the electrolyte contains a siloxane bond as its main chain, and at least two side chains of the silicon atoms of the siloxane bond are saturated hydrocarbon groups that are unsubstituted or have some hydrogen atoms substituted with other atoms, and each contains an additive having saturated hydrocarbon groups with the same or different number of carbon atoms.

2. The lithium-ion secondary battery according to claim 1, wherein the additive has three or more silicon atoms constituting the siloxane bond.

3. The lithium-ion secondary battery according to claim 2, wherein the additive comprises a cyclic siloxane bond as the main chain.

4. The lithium-ion secondary battery according to any one of claims 1 to 3, wherein both saturated hydrocarbon groups constituting the two side chains are methyl groups.

5. The lithium-ion secondary battery according to any one of claims 1 to 3, wherein the positive electrode portion includes positive electrode particles having a core-shell structure with a first shell layer containing a lithium-containing oxide on the surface of a first core portion, and the lithium-containing oxide contains phosphorus.

6. The lithium-ion secondary battery according to claim 5, wherein the positive electrode particle has a first core portion composed of lithium nickel manganese oxide.

7. The lithium-ion secondary battery according to any one of claims 1 to 3, wherein the electrolyte contains the additive in an amount of 0.1% or more and 20% or less relative to the weight of the electrolyte.

8. The lithium-ion secondary battery according to any one of claims 1 to 3, wherein the additive is at least one selected from the group consisting of octamethylcyclotetrasiloxane, octadecamethylcyclooctadecanenonasiloxane, 1,3,5-tris(trifluoropropylmethyl)cyclotrisiloxane, and tetrakis(trimethylsilyl) orthosilicate.

9. A lithium-ion secondary battery comprising a positive electrode portion, a negative electrode portion, and an electrolyte, wherein the electrolyte contains a cyclic siloxane bond as its main chain, and an additive having methyl groups as two side chains of silicon atoms in the cyclic siloxane bond.

Images