Lithium-ion rechargeable battery

Abstract

This disclosure aims to provide a lithium-ion secondary battery with improved capacity retention. [Solution] The lithium-ion secondary battery according to this disclosure comprises a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and a non-aqueous electrolyte containing an electrolyte additive. The positive electrode active material contains nickel. The electrolyte additive is an electrolyte additive represented by the following chemical formula (1). In formula JPEG2026115749000010.jpg44170, R1, R2, and R3 may be the same or different from each other and are alkyl groups having 1 to 5 carbon atoms, R4 is an alkylene group having 1 to 5 carbon atoms, and R5 is a CN group, an NH2 group, an SH group, or a halogen element.

Description

[Technical Field]

[0001] This disclosure relates to lithium-ion secondary batteries. [Background technology]

[0002] In recent years, lithium-ion rechargeable batteries have come to be used not only in electronic devices such as smartphones and PCs, but also as power sources for transportation equipment such as automobiles, and there is a growing demand for even higher capacity.

[0003] Currently, lithium-ion secondary batteries using graphite as the negative electrode are close to their theoretical capacity, making significant increases in capacity difficult. Therefore, development of lithium-ion secondary batteries using materials that alloy with lithium, such as silicon, as the negative electrode is progressing. On the other hand, such negative electrodes experience large expansion and contraction in volume during charging and discharging. This volume change of the negative electrode can cause cracks in the film (SEI) at the electrode / electrolyte interface. When cracks occur in the film, electrolyte decomposition reactions and lithium ion consumption can progress. Electrolyte decomposition reactions and lithium ion consumption can lead to a decrease in charge-discharge cycle life and gas generation problems. In particular, with silicon negative electrodes, there is a trade-off relationship between capacity and lifespan, and the challenge is to achieve both high capacity and long lifespan.

[0004] Several methods have been developed to solve these problems. One approach to stabilizing a robust solid electrolyte interface (SEI) involves miniaturizing the negative electrode active material particles to nanoscale them and suppressing volume changes (Non-Patent Literature 1). Another approach involves using Si-based carbon composite materials as the negative electrode active material (Non-Patent Literature 2). [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Chen P, Xu J, Chen H, Zhou C. 2011. Hybrid silicon-carbon nanostructured composites as superior anodes for lithium ion batteries. Nano Res.. 4(3):290-296 [Non-Patent Document 2] Park M, Kim MG, Joo J, Kim K, Kim J, Ahn S, Cui Y, Cho J. 2009. Silicon Nanotube Battery Anodes. Nano Lett.. 9(11):3844-3847. [Summary of the Invention] [Problems to be Solved by the Invention]

[0006] However, although the prior art as described above has a certain effect on the stabilization of SEI, the capacity retention rate is still insufficient.

[0007] The present disclosure has been made to solve the problems of the prior art as described above, and an object thereof is to provide a lithium ion secondary battery with an improved capacity retention rate. [Means for Solving the Problems]

[0008] As a result of intensive studies on the above problems, the inventors have found that the capacity retention rate is improved in the case of a lithium ion secondary battery including a positive electrode containing a specific positive electrode active material, a negative electrode, and an electrolyte containing a specific electrolyte additive, and have completed the present invention.

[0009] In order to solve the above problems, the present disclosure includes the following configurations.

[0010] The lithium ion secondary battery according to the present embodiment includes a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and a non-aqueous electrolyte containing an electrolyte additive and is provided with The positive electrode active material contains nickel, The aforementioned electrolyte additive has the following chemical formula (1):

[0011] [ka]

[0012] (In the formula, R1, R2, and R3 may be the same or different from each other, and are alkyl groups having 1 to 5 carbon atoms.) R4 is an alkylene group having 1 to 5 carbon atoms. R5 is a CN group, an NH2 group, an SH group, or a halogen element. This is an electrolyte additive represented by [formula].

[0013] In one embodiment, the compound represented by the chemical formula (1) is the following chemical formula (2):

[0014] [ka]

[0015] (In the formula, R is a CN group, an NH2 group, an SH group, or a halogen element.) It may also be represented as follows.

[0016] In one embodiment, the electrolyte additive represented by chemical formula (1) may be included in an amount of 0.1% by mass or more and 5.0% by mass or less, relative to the total mass of the non-aqueous electrolyte.

[0017] In one embodiment, the non-aqueous electrolyte may further contain cyclic carbonates and linear carbonates.

[0018] In one embodiment, the non-aqueous electrolyte may further contain a lithium salt.

[0019] In one embodiment, the nickel content in the positive electrode active material may be 40% by mass or more and 90% by mass or less, relative to the total amount of the positive electrode active material.

[0020] In one embodiment, the negative electrode active material may contain the element Si.

[0021] In one embodiment, the lithium-ion secondary battery according to the present disclosure may have a charging voltage of 4.3V or higher. [Effects of the Invention]

[0022] According to this disclosure, it is possible to provide a lithium-ion secondary battery with improved capacity retention. [Modes for carrying out the invention]

[0023] The following provides a more detailed explanation of this disclosure.

[0024] Terms and words used in this specification and in the claims should not be interpreted in a limited sense, either in their ordinary or dictionary sense, but rather in a sense and concept consistent with the technical idea of ​​this disclosure, in accordance with the principle that inventors can appropriately define the concepts of terms in order to best describe their invention.

[0025] A lithium-ion secondary battery according to one embodiment of the present disclosure comprises a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and a non-aqueous electrolyte containing an electrolyte additive, wherein the positive electrode active material contains nickel, and the electrolyte additive is represented by the following chemical formula (1).

[0026] [ka]

[0027] (In the formula, R1, R2, and R3 may be the same or different, and are alkyl groups having 1 to 5 carbon atoms; R4 is an alkylene group having 1 to 5 carbon atoms; and R5 is a CN group, an NH2 group, an SH group, or a halogen element.)

[0028] [Electrolyte] The lithium-ion secondary battery according to this disclosure comprises a non-aqueous electrolyte, and the non-aqueous electrolyte contains a compound represented by the following chemical formula (1).

[0029] [ka]

[0030] In chemical formula (1), R1, R2, and R3 may be the same or different, and are alkyl groups having 1 to 5 carbon atoms; R4 is an alkylene group having 1 to 5 carbon atoms; and R5 is a CN group, an NH2 group, an SH group, or a halogen element.

[0031] Preferably, the compound represented by chemical formula (1) may be the compound represented by the following chemical formula (2).

[0032] [ka]

[0033] (In the formula, R is a CN group, an NH2 group, an SH group, or a halogen element.)

[0034] While not bound by theory, the lithium-ion secondary battery according to this disclosure contains a compound represented by chemical formula (1) in the electrolyte, which causes the transition metal in the positive electrode active material, particularly nickel, to form a complex with the compound represented by chemical formula (1), thereby promoting the formation of a Cathode Electrolyte Interphase (CEI) on the positive electrode surface. As a result, the elution of the transition metal from the positive electrode active material is suppressed. This suppression of the reaction between the transition metal and the solid electrolyte interface (SEI) on the negative electrode surface is thought to lead to improved capacity retention and cycle life of the lithium-ion secondary battery.

[0035] In chemical formula (1), R1, R2, and R3 may be the same or different, and may be alkyl groups having 1 to 5 carbon atoms. Preferably, R1, R2, and R3 may be the same. Preferably, R1, R2, and R3 may be alkyl groups having 1 to 3 carbon atoms, and more preferably, R1, R2, and R3 may be ethyl groups. Although not bound by theory, when R1, R2, and R3 are alkyl groups having 1 to 5 carbon atoms, the -OR1, -OR2, and -OR3 groups can be easily removed from the compound represented by chemical formula (1), which is more advantageous for CEI formation.

[0036] In chemical formula (1), R4 may be an alkylene group having 1 to 5 carbon atoms. Preferably, R4 may be an alkylene group having 1 to 3 carbon atoms. More preferably, R4 may be an ethylene group. Although not bound by theory, when R4 is an alkylene group having 1 to 5 carbon atoms, CEI formation is more advantageous.

[0037] In chemical formula (1), R5 may be a CN group, an NH2 group, an SH group, or a halogen element. Preferably, the halogen element may be F, Cl, Br, or I. Preferably, R5 may be an SH group. Although not bound by theory, when R5 is a CN group, an NH2 group, an SH group, or a halogen element, the bonding force between R5 and the transition metal in the positive electrode active material, especially nickel, is high, which is advantageous for CEI formation and can suppress the elution of the transition metal in the positive electrode active material.

[0038] The ratio of the mass of the compound represented by chemical formula (1) to the mass of the non-aqueous electrolyte is calculated according to the following formula 1.

[0039] [Formula 1] (Ratio of the mass of the compound represented by chemical formula (1) to the mass of the non-aqueous electrolyte) (mass%) = (mass of the compound represented by chemical formula (1)) (g) / (total mass of the non-aqueous electrolyte) (g)

[0040] Preferably, the ratio of the mass of the compound represented by chemical formula (1) to the mass of the non-aqueous electrolyte may be 0.01% by mass or more and 5.0% by mass or less. For example, the content of the compound represented by chemical formula (1) in the non-aqueous electrolyte can be identified by gas chromatography, nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FT-IR), etc. In one embodiment, the ratio of the mass of the compound represented by chemical formula (1) to the mass of the non-aqueous electrolyte may be 0.05% by mass or more and 3.0% by mass or less. The lower limit of this mass ratio may be 0.01% by mass or more, 0.05% by mass or more, 0.1% by mass or more, or 0.3% by mass or more. The upper limit of this mass ratio may be 5.0% by mass or less, 3.0% by mass or less, 2.5% by mass or less, or 1% by mass or less.

[0041] The non-aqueous electrolyte used in the lithium-ion secondary battery according to this disclosure preferably contains at least one selected from cyclic carbonates and linear carbonates as a solvent. The non-aqueous electrolyte more preferably contains cyclic carbonates and linear carbonates.

[0042] Cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate (VC), methylvinylene carbonate, ethylvinylene carbonate, 1,2-diethylvinylene carbonate, vinylethylene carbonate (VEC), 1-methyl-2-vinylethylene carbonate, 1-ethyl-2-vinylethylene carbonate, 1-methyl-2-vinylethylene carbonate, and 1,1-divinylethylene carbonate. Examples include polyethylene carbonate, 1,2-divinylethylene carbonate, 1,1-dimethyl-2-methyleneethylene carbonate, 1,1-diethyl-2-methyleneethylene carbonate, ethynylethylene carbonate, 1,2-diethynylethylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, chloroethylene carbonate, and combinations thereof.

[0043] Examples of linear carbonates include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), methyl isopropyl carbonate, methyl butyl carbonate, diethyl carbonate (DEC), ethyl propyl carbonate, ethyl butyl carbonate, dipropyl carbonate, propyl butyl carbonate, and combinations thereof.

[0044] The non-aqueous electrolyte may also contain carbonates containing fluorine atoms, such as cyclic carbonates or chain carbonates. Examples of carbonates containing fluorine atoms include fluorovinylene carbonate, trifluoromethylvinylene carbonate, fluoroethylene carbonate, 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, trifluoromethylethylene carbonate, 4-fluoro-1,3-dioxolan-2-one, trans or cis-4,5-difluoro-1,3-dioxolan-2-one, 4-ethynyl-1,3-dioxolan-2-one, methyl-2,2,2-trifluoroethyl carbonate, and combinations thereof.

[0045] In particular, among carbonates, cyclic carbonates such as ethylene carbonate, propylene carbonate, and fluoroethylene carbonate are high-viscosity organic solvents. Ethylene carbonate and propylene carbonate have high dielectric constants and readily dissociate lithium salts in electrolytes. By mixing such cyclic carbonates with low-viscosity, low-dielectric-constant chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate in appropriate proportions, an electrolyte with high electrical conductivity can be produced.

[0046] The non-aqueous electrolyte of this disclosure may contain a mixture of cyclic carbonates and linear carbonates. The proportion of cyclic carbonates to the total of cyclic carbonates and linear carbonates is preferably 1% to 95% by volume, more preferably 1% to 50% by volume, and most preferably 5% to 30% by volume.

[0047] The non-aqueous electrolyte of this disclosure may further contain ester compounds. Examples of ester compounds include carboxylic acid esters. Examples of carboxylic acid esters include methyl formate, ethyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, methyl valerate, ethyl valerate, propyl valerate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, ε-caprolactone, compounds in which some of the hydrogen atoms of these carboxylic acid esters are substituted with fluorine, and combinations thereof.

[0048] In addition to the foregoing, the non-aqueous electrolytes of this disclosure may also include, without limitation, other solvents, such as ether compounds including cyclic ethers or linear ethers, polyethers, sulfur-containing solvents, and phosphorus-containing solvents, as long as they do not impair the purpose of this disclosure.

[0049] Examples of cyclic ethers include tetrahydrofuran and 2-methyltetrahydrofuran. The non-aqueous electrolyte of this disclosure may further contain linear ethers. Examples of linear ethers include dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, and ethyl propyl ether.

[0050] The non-aqueous electrolyte used in the lithium-ion secondary battery according to this disclosure may include electrolytes commonly used in lithium-ion secondary batteries. The electrolyte acts as a transport medium for ions involved in electrochemical reactions within the lithium-ion secondary battery. Preferably, the non-aqueous electrolyte used in the lithium-ion secondary battery according to this disclosure contains a lithium salt as the electrolyte.

[0051] Lithium salts contained in the non-aqueous electrolyte used in the lithium-ion secondary battery relating to this disclosure include, for example, LiPF6, LiBF4, and LiB 12 F 12 , LiAsF6, LiFSO3, Li2SiF6, LiCF3CO2, LiCH3CO2, LiCF3SO3, LiC4F9SO3, LiCF3CF2SO3, LiCF3(CF2)7SO3, LiCF3CF2(CF3)2CO, Li(CF3SO2)2CH, LiNO3, Li N(CN)2, LiN(FSO2)2, LiN(F2SO2)2, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiC(CF3SO2)3, LiP(CF3)6, LiPF(CF3)5, LiPF2(CF3)4, LiPF3(CF3)3, LiPF4(CF3)2 This may include LiPF4(C2F5)2, LiPF4(CF3SO2)2, LiPF4(C2F5SO2)2, LiBF2C2O4, LiBC4O8, LiBF2(CF3)2, LiBF2(C2F5)2, LiBF2(CF3SO2)2, LiBF2(C2F5SO2)2, LiSbF6, LiAlO4, LiAlF4, LiSCN, LiClO4, LiCl, LiF, LiBr, LiI, LiAlCl4, lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), etc. In one embodiment, the electrolyte used in the lithium-ion secondary battery according to this disclosure contains LiTFSI as a lithium salt. One type of lithium salt can be used alone, or a combination of multiple lithium salts can be used.

[0052] The electrolyte content is not particularly limited, but it may be included in an amount of 0.1 mol / L to 5 mol / L, preferably 0.5 mol / L to 3 mol / L, and more preferably 0.5 mol / L to 2 mol / L, relative to the total amount of the non-aqueous electrolyte. By setting the electrolyte amount within the above range, sufficient battery characteristics can be obtained.

[0053] In the lithium-ion secondary battery relating to this disclosure, the non-aqueous electrolyte may contain at least one further additive. Examples of further additives include flame retardants, wetting agents, stabilizers, corrosion inhibitors, gelling agents, overcharge inhibitors, and negative electrode film-forming additives.

[0054] [Negative electrode] The negative electrode used in the lithium-ion secondary battery of this disclosure can be manufactured, for example, by coating a negative electrode slurry containing a negative electrode active material, a binder, a conductive material, and a solvent onto a negative electrode current collector, followed by drying and rolling.

[0055] The negative electrode current collector may generally have a thickness of 3 μm to 500 μm. The negative electrode current collector is not particularly limited as long as it has high conductivity without inducing chemical changes in the lithium-ion secondary battery of this disclosure. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface treatment with carbon, nickel, titanium, and silver, and aluminum-cadmium alloy can be used as the negative electrode current collector. Furthermore, similar to the positive electrode current collector, the negative electrode current collector may have fine irregularities formed on its surface to strengthen the bonding force of the negative electrode active material, and may be used in various forms such as films, sheets, foils, meshes, porous materials, foams, and nonwoven fabrics.

[0056] In the lithium-ion secondary battery according to this disclosure, the negative electrode active material preferably contains silicon (Si). Examples of Si-containing materials include Si and SiO x(0 < x < 2), an Si-A alloy (where A is an element selected from the group consisting of an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a transition metal, a rare earth element, and combinations thereof and is not Si), and a mixture of at least one of these with SiO2 can be used. Preferably, the silicon-containing material is Si or SiO x (0 < x < 2). Most preferably, the silicon-containing material is Si. Without being bound by theory, when Si is included as the negative electrode active material, the expansion and contraction of the negative electrode volume are large and the SEI on the negative electrode surface is easily damaged. Therefore, when the compound represented by Chemical Formula (1) according to the present disclosure is included in the electrolytic solution, the transition metal outflow from the positive electrode is suppressed, and thus effects such as a significant improvement in the capacity retention rate and an improvement in the cycle life can be exhibited.

[0057] The negative electrode active material is preferably contained in an amount of 80% by mass or more and 99% by mass or less, more preferably 90% by mass or more and 99% by mass or less, based on the total mass of the solid content in the negative electrode slurry.

[0058] The binder is a component that helps bind between the conductive material, the negative electrode active material, and the current collector. The binder is preferably contained in an amount of 1% by mass or more and 30% by mass or less, more preferably 1% by mass or more and 10% by mass or less, based on the total mass of the solid content in the negative electrode slurry. Examples of the binder include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), and fluorine rubber. The binder can be used alone or in combination of a plurality of compounds.

[0059] The conductive material is a component that further improves the conductivity of the negative electrode active material. The conductive material can be included in an amount of 0.1% to 20% by mass relative to the total mass of solids in the negative electrode slurry, for example, 0.5% to 10% by mass, or for example, 1% to 3% by mass. The conductive material is not particularly limited as long as it is conductive without inducing a chemical change in the lithium-ion secondary battery, and examples include graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; metal powders such as aluminum and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0060] The solvent used in the negative electrode slurry is not particularly limited as long as it can form a slurry using the negative electrode active material, binder, and conductive material as negative electrode materials. For example, water, NMP, and organic solvents such as alcohol can be used. Furthermore, the amount used can be such that the negative electrode slurry has a suitable viscosity. For example, it can be used in an amount such that the solid content concentration in the slurry is 50% by mass or more and 75% by mass or less, preferably 50% by mass or more and 65% by mass or less.

[0061] [Positive electrode] The positive electrode used in the lithium-ion secondary battery of this disclosure can be manufactured, for example, by coating a positive electrode slurry containing a positive electrode active material, a binder, a conductive material, and a solvent onto a positive electrode current collector, followed by drying and rolling.

[0062] The positive electrode current collector is not particularly limited as long as it is conductive and does not induce any chemical changes in the lithium-ion secondary battery of this disclosure. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, and silver can be used as the positive electrode current collector.

[0063] The positive electrode active material is a compound capable of reversibly occluding and releasing lithium, and specifically, may include a lithium composite metal oxide containing one or more metals such as cobalt, manganese, nickel, or aluminum and lithium. More specifically, the lithium composite metal oxide is a lithium-manganese-based oxide (for example, LiMnO2, LiMn2O4, etc.), a lithium-cobalt-based oxide (for example, LiCoO2, etc.), a lithium-nickel-based oxide (for example, LiNiO2, etc.), a lithium-nickel-manganese-based oxide (for example, LiNi 1-y1 Mn y1 O2 (where 0 < y1 < 1), LiMn 2-z1 Ni z O4 (where 0 < Z1 < 2), etc.), a lithium-nickel-cobalt-based oxide (for example, LiNi 1-y2 Co y2 O2 (where 0 < y2 < 1), etc.), a lithium-manganese-cobalt-based oxide (for example, LiCo 1-y3 Mn y3 O2 (where 0 < y3 < 1), LiMn 2-z2 Co z2 O4 (where 0 < Z2 < 2), etc.), a lithium-nickel-manganese-cobalt-based oxide (for example, Li(Ni p1 Co q1 Mn r1 )O2 (where 0 < p1 < 1, 0 < q1 < 1, 0 < r1 < 1, p1 + q1 + r1 = 1), or Li(Ni p2 Co q2 Mn r2 )O4 (where 0 < p2 < 2, 0 < q2 < 2, 0 < r2 < 2, p2 + q2 + r2 = 2), etc.), or a lithium-nickel-cobalt-transition metal (M) oxide (for example, Li(Ni p3 Co q3 Mn r3 M S3)O2 (where M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, and p3, q3, r3, and s3 are the atomic fractions of independent elements, respectively, where 0 < p3 < 1, 0 < q3 < 1, 0 < r3 < 1, 0 < s3 < 1, and p3 + q3 + r3 + s3 = 1), etc.) and the like can be mentioned, and these may be included alone or two or more of them may be included.

[0064] Preferably, from the viewpoint of being able to enhance the capacity characteristics and stability of the battery, the lithium composite metal oxide may be a lithium composite metal oxide containing a metal containing nickel and lithium. Specifically, lithium-nickel-based oxides (for example, LiNiO2, etc.), lithium-nickel-manganese-cobalt (NCM) oxides (for example, Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2, or Li(Ni 0.8 Mn 0.1 Co 0.1 )O2, etc.), lithium-nickel-cobalt-aluminum (NCA) oxides (for example, Li(Ni 0.8 Co 0.15 Al 0.05 )O2, etc.), or lithium-nickel-cobalt-manganese-aluminum (NCMA) oxides (for example, Li[Ni 0.90 Co 0.045 Mn 0.045 Al 0.01 O2), etc. can be used. In particular, it is preferable from the cost aspect to use lithium-nickel-manganese-cobalt oxide or lithium-nickel-cobalt-aluminum oxide, which are nickel-cobalt-manganese (NCM) or nickel-cobalt-aluminum (NCA) ternary system materials.

[0065] The positive electrode active material is preferably present in an amount of 80% to 99% by mass, and more preferably 90% to 99% by mass, relative to the total mass of solids in the positive electrode slurry. By setting the content of the positive electrode active material within the above range, high energy density and capacity can be obtained.

[0066] The binder is a component that helps to bond the positive electrode active material to conductive materials and to the current collector. Preferably, the binder is included in an amount of 1% to 30% by mass relative to the total mass of solids in the positive electrode slurry. Examples of binders include polyvinylidene fluoride (PVdF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, and fluororubber.

[0067] The conductive material is a substance that imparts conductivity to the lithium-ion secondary battery of this disclosure without inducing any chemical changes. The conductive material is preferably included in an amount of 0.5% to 50% by mass relative to the total mass of solids in the positive electrode slurry, and more preferably in an amount of 1% to 20% by mass, for example, 1% to 5% by mass. By including the conductive material in the above range, the electrical conductivity of the positive electrode is improved. Furthermore, by including the conductive material in the above range, a lithium-ion secondary battery with high energy density and capacity can be obtained.

[0068] Examples of conductive materials include carbon powders such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; graphite powders such as natural graphite, artificial graphite, and graphite with well-developed crystalline structures; conductive fibers such as carbon fibers and metal fibers; metal powders such as aluminum and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0069] The solvent for the positive electrode slurry is not limited as long as it can form a slurry using the positive electrode active material, binder, and conductive material as the positive electrode material. For example, organic solvents such as NMP (N-methyl-2-pyrrolidone), DMF (dimethylformamide), acetone, dimethylacetamide, and water can be used. Furthermore, the amount used can be such that the positive electrode slurry has a suitable viscosity. For example, it can be used in an amount such that the solid content concentration in the slurry is 10% by mass or more and 60% by mass or less, preferably 20% by mass or more and 50% by mass or less.

[0070] [Separator] In the lithium-ion secondary battery relating to this disclosure, a separator may be interposed between the positive electrode and the negative electrode.

[0071] The separator in the lithium-ion secondary battery of this disclosure serves to block internal short circuits between the two electrodes and impregnate them with electrolyte. The separator may be formed by mixing a polymer resin, a filler, and a solvent to produce a separator composition, and then directly coating and drying the separator composition onto the top of the electrodes. Alternatively, the separator may be formed by casting and drying the separator composition onto a support, and then laminating the separator film, which has been peeled off the support, onto the top of the electrodes.

[0072] As a separator, conventional porous polymer films used as separators, such as porous polymer films made from polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, may be used alone or in laminates thereof. Alternatively, conventional porous nonwoven fabrics, such as nonwoven fabrics made from high-melting-point glass fibers or polyethylene terephthalate fibers, may be used, but are not limited to these.

[0073] The pore size of the porous separator is generally between 0.01 μm and 50 μm, and the porosity may be between 5% and 95%. The thickness of the porous separator may also generally be in the range of 5 μm to 300 μm.

[0074] [Lithium-ion rechargeable battery] The external shape of the lithium-ion secondary battery of this disclosure is not particularly limited, but may be cylindrical, prismatic, pouch-shaped, or coin-shaped.

[0075] <1. Battery Construction> (1) Examples 1 to 8 and Comparative Example 1 (i) Fabrication of the positive electrode plate NCM(811) ternary cathode material (Li(Ni)) is used as the cathode active material. 0.8 Co 0.1 Mn 0.1 A positive electrode slurry was prepared by dispersing 96.5% by mass of O2, 1.5% by mass of acetylene black as a conductive material, and 2% by mass of polyvinylidene fluoride (PVdF) as a binder in N-methyl-2-pyrrolidone solvent. The obtained positive electrode slurry was uniformly coated onto an Al foil, heated and vacuum dried, and then pressed to obtain a positive electrode plate with a predetermined film thickness and mixture density.

[0076] (ii) Fabrication of the negative electrode plate A negative electrode slurry was prepared by dispersing 97.0% by mass of a mixture of graphite and Si in a mass ratio of 90:10 as the negative electrode active material, and 3.0% by mass of styrene-butadiene rubber (SBR) and carboxymethylcellulose (CMC) as binders in water. The obtained negative electrode slurry was uniformly coated onto a Cu foil, heated and vacuum dried, and then pressed to obtain a negative electrode plate with a predetermined film thickness and mixture density.

[0077] (iii) Preparation of electrolyte A mixture of EC (cyclic carbonate) and EMC (chain carbonate) in a volume ratio of 10:90 was used as the solvent, and LiFSI was dissolved in it as a solute to a salt concentration of 1 M to obtain the base electrolyte. To the base electrolyte, an electrolyte additive represented by the following chemical formula (1), having the structure shown in Table 1, was added at the concentrations shown in Table 1 to obtain the electrolyte.

[0078] [ka]

[0079] (4) Battery assembly The above-mentioned positive electrode plate, negative electrode plate, and electrolyte, as well as a polyolefin film (polyethylene separator) as the separator, with a facing area of ​​approximately 12 cm². 2 Pouch cells were fabricated. The fabricated batteries were designated as Examples 1 to 8 and Comparative Example 1.

[0080] (2) Comparative Example 2 LiCo 0.5 Mn 0.5 The battery was prepared in the same manner as in Example 2, except that O2 was used.

[0081] (3) Comparative Example 3 LiCo 0.5 Mn 0.5 The battery was prepared in the same manner as in Comparative Example 1, except that O2 was used.

[0082] <2. Battery Evaluation> (1) 45°C charge / discharge cycle test Using the lithium-ion secondary batteries of Examples 1 to 8 and Comparative Examples 1 to 3, charge-discharge cycle tests were performed at 45°C with a maximum charge voltage of 4.3V and a minimum discharge voltage of 2.8V. Before starting the cycle, the initial discharge capacity was measured under a 0.1C condition. Furthermore, at 300 cycles, a test was performed using a constant current of 0.1C to confirm the accurate capacity. The capacity retention rate was calculated from the measured capacity according to Equation 2 below. [Formula 2] (Capacity retention rate) (%) = (Discharge capacity after 300 cycles (mAh·g) -1 ) / Initial discharge capacity (mAh·g -1 )) × 100

[0083] Furthermore, the initial resistance and the resistance after 300 cycles were measured under the condition of SOC 50%, and the resistivity was calculated according to Equation 3 below. [Formula 3] (Resistivity) (%) = (Resistance after 300 cycles (Ω) / Initial resistance (Ω)) × 100

[0084] Furthermore, the volume change of the pouch cell before and after the test was measured, and the volume expansion rate was calculated according to Equation 4 below to evaluate the effect of gas generation due to side reactions. The volume of the pouch cell was measured by the Archimedes method. [Formula 4] (Volume expansion coefficient) (%) = (Volume of pouch cell after 300 cycles (cm³) 3 ) / Pouch cell volume before cycle test (cm³ 3 )) × 100

[0085] (2) High-temperature storage test The batteries of Examples 1 to 8 and Comparative Examples 1 to 3, charged under a condition of 0.1C, had their resistance measured in the charged state. After measurement, the batteries were left in a 60°C environment for two weeks, and then their resistance was measured again to perform a high-temperature storage test. Based on the measured resistance values, the resistivity was calculated according to Equation 5 below. [Formula 5] (Resistivity)(%)=(Resistance after high temperature storage (Ω) / Initial resistance (Ω))×100

[0086] The results obtained from each evaluation are shown in Table 1.

[0087] [Table 1]

[0088] Table 1 shows that the batteries of Examples 1 to 8 had higher capacity retention rates, lower resistivity, and lower volume expansion rates after cycle testing compared to the battery of Comparative Example 1. Furthermore, it was found that their resistivity after high-temperature storage testing was also lower.

[0089] Furthermore, comparative examples 2 and 3 in Table 1 show that even when the electrolyte contains an additive represented by chemical formula (1), the capacity retention rate improvement effect does not appear when Ni is not included in the positive electrode active material. [Industrial applicability]

[0090] The lithium-ion secondary battery described herein is useful because it suppresses the outflow of transition metals from the positive electrode active material, thereby improving the capacity retention rate and charge-discharge cycle characteristics of the lithium-ion secondary battery.

Claims

1. Positive electrode containing positive electrode active material, A negative electrode containing a negative electrode active material, and Non-aqueous electrolyte containing electrolyte additives Equipped with, The positive electrode active material contains nickel, The aforementioned electrolyte additive has the following chemical formula (1): 【Chemistry 1】 (In the formula, R １ , R ２ and R ３ These may be the same or different, and are alkyl groups having 1 to 5 carbon atoms. R ４ This is an alkylene group having 1 to 5 carbon atoms. R ５ is a CN group, NH ２ (It is a group, an SH group, or a halogen element.) Lithium-ion secondary batteries, which are electrolyte additives represented by [this formula].

2. The compound represented by the above chemical formula (1) is the following chemical formula (2): 【Chemistry 2】 (In the formula, R is a CN group, NH ２ (It is a group, an SH group, or a halogen element.) A lithium-ion secondary battery according to claim 1, represented as shown.

3. The lithium-ion secondary battery according to claim 1, wherein the electrolyte additive represented by the chemical formula (1) is contained in an amount of 0.1% by mass or more and 5.0% by mass or less based on the total mass of the non-aqueous electrolyte.

4. The lithium-ion secondary battery according to claim 1, wherein the non-aqueous electrolyte further comprises a cyclic carbonate and a chain carbonate.

5. The lithium-ion secondary battery according to claim 1, wherein the non-aqueous electrolyte further comprises a lithium salt.

6. A lithium-ion secondary battery according to any one of claims 1 to 5, wherein the nickel content in the positive electrode active material is 40% by mass or more and 90% by mass or less with respect to the total amount of the positive electrode active material.

7. The lithium-ion secondary battery according to any one of claims 1 to 5, wherein the negative electrode active material contains the element Si.

8. A lithium-ion secondary battery according to any one of claims 1 to 5, wherein the charging voltage is 4.3V or higher.

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