Lithium-ion secondary battery

Abstract

To provide a lithium-ion secondary battery using a ternary-system positive electrode active material with a high nickel content in which the elution of nickel from the positive electrode active material and the increase in DC resistance are suppressed when stored at a high temperature, such as 80°C.SOLUTION: The lithium-ion secondary battery has a non-aqueous electrolyte, a positive electrode, and a negative electrode. The non-aqueous electrolyte includes a polyphosphoric acid ester, an electrolyte, and a solvent. A polyphosphoric acid ester content is 0.01 to 1 mass% based on a total amount of the non-aqueous electrolyte. A propylene carbonate content in the solvent is less than 10 volume%. The positive electrode contains a positive electrode active material represented by the following formula (A). LivNixCoyMnzO(2+w)...(A) [In formula (A), 0.2≤v≤1.2, 0.6≤x≤0.9, 0<y≤0.3, 0<z<0.4, x+y+z=1, and -0.2≤w≤0.2.]SELECTED DRAWING: None

Description

Technical Field

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

Background Art

[0002] Lithium-ion secondary batteries are used as power sources for electronic devices such as smartphones and personal computers, and as power sources for automobiles. In these applications, since lithium-ion secondary batteries are used for a long time, they are required to have a high energy density. Therefore, in order to increase the energy density, increasing the nickel content in the positive electrode active material has been studied. However, such lithium-ion secondary batteries have a problem in storage stability at high temperatures, such as nickel in the positive electrode active material being easily eluted during storage at high temperatures.

[0003] On the other hand, Patent Document 1 discloses a high-nickel content ternary lithium-ion secondary battery that uses an electrolytic solution containing a predetermined amount of a specific compound such as trialkylsilyl phosphate and improves the generation of gas in the battery when stored at 50°C.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] By the way, with the expansion of the applications of lithium-ion secondary batteries, the storage environments are diverse. Therefore, it is desirable to be able to store the batteries even in an environment at a temperature higher than 50°C, for example, 80°C. However, when the present inventors examined the storage stability of the ternary lithium-ion secondary battery described in Patent Document 1, it became clear that there is room for improvement in the storage stability at 80°C.

[0006] In view of the above circumstances, an object of the present invention is to provide a lithium-ion secondary battery using a ternary cathode active material with a high nickel content, in which elution of nickel from the cathode active material and an increase in DC resistance are sufficiently suppressed during storage at a high temperature, for example, 80°C.

Means for Solving the Problems

[0007] The present disclosure provides a lithium-ion secondary battery described in, for example, the following [1] to [3]. [1] A lithium-ion secondary battery having a non-aqueous electrolyte, a positive electrode, and a negative electrode, wherein the non-aqueous electrolyte contains a polyphosphoric acid ester, an electrolyte, and a solvent, the content of the polyphosphoric acid ester is 0.01 to 1% by mass based on the total amount of the non-aqueous electrolyte, the content of propylene carbonate in the solvent is less than 10% by volume, and the positive electrode contains a positive electrode active material represented by the following formula (A). Li v Ni x Co y Mn z O (2+w) …(A) [In formula (A), 0.2 ≦ v ≦ 1.2, 0.6 ≦ x ≦ 0.9, 0 < y ≦ 0.3, 0 < z < 0.4, x + y + z = 1, and -0.2 ≦ w ≦ 0.2.] [2] A lithium-ion secondary battery having a non-aqueous electrolyte, a positive electrode, and a negative electrode, wherein the non-aqueous electrolyte contains a polyphosphoric acid ester, an electrolyte, and a solvent, the content of the polyphosphoric acid ester is 0.01 to 1% by mass based on the total amount of the non-aqueous electrolyte, The above-mentioned solvent contains at least one solvent selected from the group consisting of ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, methyl propionate, ethyl propionate, butyl propionate, isopropyl propionate, propyl propionate, ethyl acetate, methyl acetate, propyl acetate, and isopropyl acetate, in a proportion of 95% by volume or more based on the total amount of the solvent. The lithium ion secondary battery, wherein the positive electrode contains a positive electrode active material represented by the following formula (A). Li v Ni x Co y Mn z O (2+w) …(A) [In formula (A), 0.2 ≦ v ≦ 1.2, 0.6 ≦ x ≦ 0.9, 0 < y ≦ 0.3, 0 < z < 0.4, x + y + z = 1, and -0.2 ≦ w ≦ 0.2.] [3] The lithium ion secondary battery according to [1] or [2], wherein the polyphosphoric acid ester is at least one selected from the group consisting of trimethylsilyl polyphosphate, ethyl polyphosphate, triisopropylsilyl polyphosphate, and tert-butyldimethylsilyl polyphosphate.

[0008] According to the lithium ion secondary battery described in [1] to [3], elution of nickel from the positive electrode active material and an increase in DC resistance during storage at high temperature are sufficiently suppressed, that is, the storage stability at high temperature is excellent. The reason is not necessarily clear, but the inventors believe that one of the reasons is that a film derived from the polyphosphoric acid ester is densely formed on the positive electrode, suppressing side reactions involving the electrolytic solution on the positive electrode during storage at high temperature.

[0009] According to the lithium ion secondary battery described in [1] to [3], furthermore, one or more selected from the group consisting of elution of metals (for example, metals derived from the positive electrode active material such as nickel, cobalt, and manganese) from the positive electrode active material, reduction of the open circuit voltage, and increase in the volume of the lithium ion secondary battery during storage at high temperature can be sufficiently suppressed.

Effects of the Invention

[0010] According to the present invention, it is possible to provide a lithium-ion secondary battery using a ternary cathode active material with a high nickel content, in which elution of nickel from the cathode active material and an increase in DC resistance during storage at 80°C are sufficiently suppressed.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0012] Hereinafter, embodiments of the present disclosure will be described in detail. However, the present disclosure is not limited to the following embodiments. When a numerical range is indicated as X to Y, it means X or more and Y or less. For example, "5 to 100 nm" means 5 nm or more and 100 nm or less. Also, materials, components, or methods exemplified in this specification can be used alone or in combination of two or more without particular notice.

[0013] The lithium-ion secondary battery according to this embodiment has a non-aqueous electrolyte, a cathode, and an anode. Hereinafter, each component will be described.

[0014] (Non-aqueous electrolyte) The non-aqueous electrolyte contains a polyphosphoric acid ester, an electrolyte, and a solvent.

[0015] The polyphosphoric acid ester refers to a compound in which one or more hydrogen atoms in polyphosphoric acid are replaced by organic groups. Polyphosphoric acid refers to a compound in which two or more phosphoric acids (H 3 PO 4It refers to a condensed phosphoric acid compound having a structure in which [[ID=]] is dehydrated and condensed. The upper limit of the number of phosphoric acids in polyphosphoric acid is not particularly limited, but it can be, for example, 200 or less.

[0016] The polyphosphoric acid ester may be a chain polyphosphoric acid ester in which phosphoric acids are connected in a chain, or a polyphosphoric acid ester having a cyclic structure such as a 6-membered ring or a 12-membered ring (including a condensed ring structure) formed by dehydration condensation of a plurality of phosphoric acids. The above-mentioned chain polyphosphoric acid ester may be a linear polyphosphoric acid ester having no branched structure, or a branched-chain polyphosphoric acid ester having a branched structure branched from one or more phosphorus atoms in the linear polyphosphoric acid ester. The polyphosphoric acid ester having the above-mentioned cyclic structure may be a cyclic polyphosphoric acid ester in which both ends of the chain polyphosphoric acid ester are dehydrated and condensed, or a cyclic polyphosphoric acid ester having a branched structure branched from one or more phosphorus atoms in the cyclic polyphosphoric acid ester, or a cyclic polyphosphoric acid ester further having one or more cyclic polyphosphoric acid esters as a branched structure. Among these, from the viewpoint of further improving the storage stability at high temperatures, a branched-chain polyphosphoric acid ester is preferable.

[0017] When the polyphosphoric acid ester contains a plurality of organic groups, it is preferable that all the organic groups in the polyphosphoric acid ester are the same.

[0018] It is preferable that all hydrogen atoms in the polyphosphoric acid are replaced by organic groups in the polyphosphoric acid ester.

[0019] The polyphosphoric acid ester is more preferably a polyphosphoric acid silyl ester in which the organic group is a silyl group. Examples of the silyl group include a group represented by the following formula (3) described later.

[0020] The polyphosphoric acid ester may be a compound represented by the following general formula (1) (excluding polyphosphoric acid).

[0021]

Chemical formula

[0022] [Chemical formula] [In formula (2), R 2 is a hydrogen atom, an optionally substituted alkyl having 1 to 6 carbon atoms, an optionally substituted aryl having 6 to 10 carbon atoms or a group represented by the following formula (3), and a plurality of R 2 may be the same as or different from each other, Z is a hydrogen atom, an optionally substituted alkyl having 1 to 6 carbon atoms, an optionally substituted aryl having 6 to 10 carbon atoms or a group represented by the following formula (3), and m is an integer of 1 or more.]

[0023] -Si(R 3 ) 3 …(3) [In formula (3), R 3 is an optionally substituted alkyl having 1 to 6 carbon atoms, an optionally substituted aryl having 6 to 10 carbon atoms, an optionally substituted alkoxy having 1 to 6 carbon atoms or an optionally substituted aryloxy having 6 to 10 carbon atoms, and the three R 3 may be the same as or different from each other.]

[0024] -OSi(R 4 ) 3…(4) [In formula (4), R 4 is optionally substituted alkyl having 1 to 6 carbon atoms, optionally substituted aryl having 6 to 10 carbon atoms, optionally substituted alkoxy having 1 to 6 carbon atoms, or optionally substituted aryloxy having 6 to 10 carbon atoms, and three R 4 may be the same as or different from each other.]

[0025] In the present specification, "optionally substituted" means that one or more hydrogen atoms may be substituted by a fluorine atom, a chlorine atom, an alkoxy group, a cyano group, or the like.

[0026] In the compound represented by the general formula (1), from the viewpoint of further improving the storage stability at high temperatures, in the general formula (1), R 1 is optionally substituted alkyl having 1 to 6 carbon atoms, a group represented by the formula (2), or a group represented by the formula (3). In the formula (2), R 2 is optionally substituted alkyl having 1 to 6 carbon atoms or a group represented by the formula (3), Z is optionally substituted alkyl having 1 to 6 carbon atoms, or a group represented by the formula (3). In the formula (3), R 3 is preferably optionally substituted alkyl having 1 to 6 carbon atoms. In this case, in the general formula (1), X is optionally substituted alkoxy having 1 to 6 carbon atoms, or a group represented by the formula (4), Y is optionally substituted alkyl having 1 to 6 carbon atoms or a group represented by the formula (3), and in the formula (4), R 4 is more preferably optionally substituted alkyl having 1 to 6 carbon atoms.

[0027] In the general formula (1), the optionally substituted alkyl having 1 to 6 carbon atoms exemplified as R 1 is preferably linear alkyl having 1 to 6 carbon atoms or trifluoroalkyl having 1 to 6 carbon atoms, more preferably linear alkyl having 1 to 3 carbon atoms or trifluoroalkyl having 1 to 3 carbon atoms, still more preferably ethyl or trifluoroethyl, and particularly preferably ethyl.

[0028] In general formula (1), the optionally substituted C1-C6 alkoxy exemplified as X is preferably a linear C1-C6 alkoxy or a C1-C6 trifluoroalkoxy, more preferably a linear C1-C3 alkoxy or a C1-C3 trifluoroalkoxy, still more preferably ethoxy or trifluoroethoxy, and particularly preferably ethoxy.

[0029] In general formula (1), the optionally substituted C1-C6 alkyl exemplified as Y is preferably a linear C1-C6 alkyl or a C1-C6 trifluoroalkyl, more preferably a linear C1-C3 alkyl or a C1-C3 trifluoroalkyl, still more preferably ethyl or trifluoroethyl, and particularly preferably ethyl.

[0030] In general formula (1), from the viewpoint of further improving the storage stability at high temperatures, it is preferable that one or more R 1 is a group represented by formula (2). That is, the compound represented by general formula (1) preferably has a branched structure.

[0031] In general formula (1), n is not particularly limited as long as it is an integer of 2 or more, and is not particularly limited. For example, it may be an integer of 2 or more and 200 or less, an integer of 2 or more and 50 or less, an integer of 2 or more and 20 or less, or an integer of 2 or more and 10 or less.

[0032] In formula (2), the optionally substituted C1-C6 alkyl exemplified as R 2 is preferably a linear C1-C6 alkyl or a C1-C6 trifluoroalkyl, more preferably a linear C1-C3 alkyl or a C1-C3 trifluoroalkyl, still more preferably ethyl or trifluoroethyl, and particularly preferably ethyl.

[0033] In formula (2), the optionally substituted alkyl having 1 to 6 carbon atoms exemplified as Z is preferably a linear alkyl having 1 to 6 carbon atoms or a trifluoroalkyl having 1 to 6 carbon atoms, more preferably a linear alkyl having 1 to 3 carbon atoms or a trifluoroalkyl having 1 to 3 carbon atoms, still more preferably ethyl or trifluoroethyl, and particularly preferably ethyl.

[0034] In formula (2), m is not particularly limited as long as it is an integer of 1 or more, and may be, for example, an integer of 1 or more and 100 or less, an integer of 1 or more and 25 or less, an integer of 1 or more and 10 or less, or an integer of 1 or more and 5 or less.

[0035] In formula (3), R 3 is preferably an alkyl having 1 to 6 carbon atoms, more preferably an alkyl having 1 to 4 carbon atoms. The group represented by formula (3) is preferably trimethylsilyl, triethylsilyl, triisopropylsilyl or tert-butyldimethylsilyl.

[0036] In formula (4), R 4 is preferably an alkyl having 1 to 6 carbon atoms, more preferably an alkyl having 1 to 4 carbon atoms. The group represented by formula (4) is preferably trimethylsiloxy, triethylsiloxy, triisopropylsiloxy or tert-butyldimethylsiloxy.

[0037] In the compound represented by general formula (1), all R 1 (except for the case where R 1 is the group represented by formula (2)), the group excluding the oxygen atom directly bonded to the phosphorus atom from X in general formula (1), Y in general formula (1), all R 2 in formula (2), and Z in formula (2) are preferably the same group.

[0038] Examples of the compound represented by the general formula (1) include ethyl polyphosphate, trimethylsilyl polyphosphate, triethylsilyl polyphosphate, triisopropylsilyl polyphosphate, tert-butyldimethylsilyl polyphosphate, tert-butyldiphenylsilyl polyphosphate, trimethoxysilyl polyphosphate, triethoxysilyl polyphosphate, triisopropoxysilyl polyphosphate, tert-butoxydimethoxysilyl polyphosphate, tert-butoxydiphenoxysilyl polyphosphate, etc. From the viewpoint of further improving the storage stability at high temperatures, trimethylsilyl polyphosphate, ethyl polyphosphate, triisopropylsilyl polyphosphate, and tert-butyldimethylsilyl polyphosphate are preferred, trimethylsilyl polyphosphate is more preferred, and trimethylsilyl polyphosphate having a branched structure is even more preferred.

[0039] Examples of the polyphosphate ester include 31 By measuring the presence or absence of peaks indicating the bonds between phosphorus atoms and the surrounding groups and their abundance ratios (integration ratios of each peak) by means of 31P-NMR or the like, it is possible to analyze the presence or absence, abundance ratios, etc. of a chain structure represented by the following general formula (5-1), a cyclic structure represented by the following general formula (5-2), a branched structure represented by the following general formula (5-3), and the like. The following general formulas (5-1) to (5-3) are shown by taking trimethylsilyl polyphosphate as an example. TMS is a trimethylsilyl group, (l + 2) is the number of repeating units, and Pt, Pm, and Pb are phosphorus atoms 31 The peaks shown in 31P-NMR are classified according to the chemical shift (ppm). 31 The measurement conditions for 31P-NMR may be those described in the examples below.

[0040]

Chemical formula

[0041]

Chemical formula

[0042]

Chemical formula

[0043] Pt represents the peak of a phosphorus atom at the terminal (including the terminal of the branch), specifically, among the oxygen atoms directly bonded to the phosphorus atom, the peak of the phosphorus atom where one oxygen atom is directly bonded to another phosphorus atom. Pm represents the peak of a phosphorus atom other than the terminal (including the terminal of the branch) and the branch point in the branched structure, specifically, among the oxygen atoms directly bonded to the phosphorus atom, the peak of the phosphorus atom where two oxygen atoms are directly bonded to another phosphorus atom. Pb represents the peak of the phosphorus atom at the branch point in the branched structure, specifically, among the oxygen atoms directly bonded to the phosphorus atom, the peak of the phosphorus atom where three oxygen atoms are directly bonded to another phosphorus atom.

[0044] As described above, the polyphosphate ester preferably has a branched structure. In this case, as the integral value of Pm and Pb with respect to the integral value of Pt, it is preferable that (integral value of Pm) / (integral value of Pt) is 3.0 to 9.0, and (integral value of Pb) / (integral value of Pt) is 0.2 to 3.0, and more preferably (integral value of Pm) / (integral value of Pt) is 6.0 to 7.0, and (integral value of Pb) / (integral value of Pt) is 0.7 to 1.2.

[0045] Further, when the polyphosphate ester has a chain structure (a structure in which no Pb peak is detected) represented by the general formula (5-1), for example, 31 Based on Pt and Pm obtained by P-NMR, the number (q + 2) of repeating units can be estimated by the following formula (5-4). However, when the polyphosphate trimethylsilyl contains a cyclic structure, the actual number of repeating units is estimated to be smaller than the number (q + 2) of repeating units calculated by the following formula (5-4). q + 2 = 2 × {1 + (integral value of Pm) / (integral value of Pt)} …(5-4)

[0046] In the non-aqueous electrolyte of the lithium-ion secondary battery according to this embodiment, the content of the polyphosphoric acid ester is 0.01 to 1% by mass. When the content of the polyphosphoric acid ester is less than 0.01% by mass, the additive effect of the sufficient polyphosphoric acid ester is not exhibited. When the content of the polyphosphoric acid ester exceeds 1% by mass, the DC resistance after storage at high temperature becomes high. From the viewpoint of further improving the storage stability at high temperature, the content of the polyphosphoric acid ester is preferably 0.03 to 1% by mass.

[0047] The production method of the polyphosphoric acid ester is not particularly limited, and it can be produced by a known method. For example, after dispersing diphosphorus pentoxide in a solvent, while stirring, a disiloxane compound such as hexamethylene disiloxane, hexaisopropyldisiloxane, 1,3-di-tert-butyltetramethylene disiloxane, or an ether compound such as diethyl ether is added and stirred, and then the solvent may be removed. In this case, by reducing the addition amount of the disiloxane compound or the ether compound, a polyphosphoric acid ester having many branched structures and / or cyclic structures can be produced.

[0048] Examples of the above solvent include toluene, chloroform, dichloromethane, etc. The above stirring may be performed, for example, at room temperature or at 35°C. The above stirring may be performed, for example, for 1 day or for 3 days. The removal of the above solvent may be performed, for example, by distilling off the solvent with an evaporator and then performing vacuum drying, or by filtering the solution obtained by stirring and then distilling off the solvent with an evaporator. More specifically, trimethylsilyl polyphosphate, ethyl polyphosphate, triisopropylsilyl polyphosphate, and tert-butyldimethylsilyl polyphosphate can be produced by the method described in the examples below.

[0049] The electrolyte is not particularly limited, and a known electrolyte used in a lithium-ion secondary battery can be used. For example, LiPF 6 , LiN(SO 2 F) 2(LiFSI, Lithium bis(fluorosulfonyl)imide), LiBF 4 、LiN(SO 2 CF 3 ) 2 (Lithium (bistrifluoromethylsulfonyl)imide), LiPF 3 (C 2 F 5 ) 3 、LiBF(CF 3 ) 3 、LiAsF 6 、LiSbF 6 、LiFSO 3 、LiF, LiPO 2 F 2 and the like.

[0050] In the non-aqueous electrolyte of the lithium-ion secondary battery according to this embodiment, the content of the electrolyte is not particularly limited and may be 0.12 to 12 mol / L, or may be 0.3 to 4.8 mol / L.

[0051] In one aspect of this embodiment, the content of propylene carbonate in the solvent contained in the non-aqueous electrolyte is less than 10% by volume. When the content of propylene carbonate is 10% by volume or more, the storage stability at high temperatures decreases. From the viewpoint of further improving the storage stability at high temperatures, the content of propion carbonate in the above solvent is preferably 5% by volume or less, and more preferably 0% by volume.

[0052] In another aspect of the present embodiment, the solvent contained in the non-aqueous electrolyte contains at least one solvent selected from the group consisting of ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, methyl propionate, ethyl propionate, butyl propionate, isopropyl propionate, propyl propionate, ethyl acetate, methyl acetate, propyl acetate, and isopropyl acetate, in a proportion of 95% by volume or more based on the total amount of the solvent. If the content of other solvents exceeds 5% by volume, the storage stability at high temperatures will decrease. From the perspective of further improving the storage stability at high temperatures, the above solvents may be contained in a proportion of 98% by volume or more, or even 100% by volume.

[0053] Also, from the perspective of further improving the storage stability at high temperatures, the above solvent may contain at least one solvent selected from the group consisting of ethylene carbonate, ethyl methyl carbonate, methyl propionate, and dimethyl carbonate, in a proportion of 95% by volume or more, 98% by volume or more, or even 100% by volume based on the total amount of the solvent.

[0054] As long as the solvent contained in the non-aqueous electrolyte satisfies the above requirements, it may contain other organic solvents. Specific examples thereof include saturated cyclic carbonate (carbonate ester) solvents such as ethylene 2,3-dimethyl carbonate, 1,2-butylene carbonate, and erythritan carbonate; chain carbonate (carbonate ester) solvents such as diphenyl carbonate and methyl phenyl carbonate; ether solvents such as ethylene glycol dimethyl ether, ethylene glycol diethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 2,6-dimethyltetrahydrofuran, tetrahydropyran, crown ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,4-dioxane, and 1,3-dioxolane; cyclic carbonate (carbonate ester) solvents having an unsaturated bond such as vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate, 2-vinyl ethylene carbonate, and phenyl ethylene carbonate; fluorine-containing cyclic carbonate (carbonate ester) solvents such as fluoroethylene carbonate, 4,5-difluoroethylene carbonate, and trifluoropropylene carbonate; aromatic carboxylic acid ester solvents such as methyl benzoate and ethyl benzoate; lactone solvents such as γ-butyrolactone, γ-valerolactone, and δ-valerolactone; phosphate ester solvents such as trimethyl phosphate, diethylmethyl phosphate, ethyl dimethyl phosphate, and triethyl phosphate; nitrile solvents such as acetonitrile, propionitrile, methoxypropionitrile, glutaronitrile, adiponitrile, 2-methylglutaronitrile, valeronitrile, butyronitrile, and isobutyronitrile; sulfur compound solvents such as dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, sulfolane, 3-methylsulfolane, and 2,4-dimethylsulfolane; aromatic nitrile solvents such as benzonitrile and tolunitrile; nitromethane, 1,3-dimethyl-2-imidazolidinone, 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone, 3-methyl-2-oxazolidinone, and the like.

[0055] The above non-aqueous electrolyte may further contain an additive for the purpose of improving various characteristics of the lithium-ion secondary battery. Examples of the additive include carboxylic acid anhydrides such as succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, diglycolic anhydride, cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylic dianhydride, phenylsuccinic anhydride; sulfur-containing compounds such as ethylene sulfite, 1,3-propane sultone, 1,4-butane sultone, methyl methanesulfonate, busulfan, sulfolene, tetramethylthiuram monosulfide, trimethylene glycol sulfate; nitrogen-containing compounds such as 1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone, N-methylsuccinimide; saturated hydrocarbon compounds such as heptane, octane, cycloheptane; sulfamic acid (amide sulfuric acid, H 3 NSO 3 ); sulfamates (for example, alkali metal salts such as lithium salt, sodium salt, potassium salt; alkaline earth metal salts such as calcium salt, strontium salt, barium salt; other metal salts such as manganese salt, copper salt, zinc salt, iron salt, cobalt salt, nickel salt; ammonium salt; guanidine salt, etc.); fluorosulfonic acid compounds such as sodium fluorosulfonate (NaFSO 3 ), potassium fluorosulfonate (KFSO 3 ), magnesium fluorosulfonate (Mg(FSO 3 ) 2 ), etc.

[0056] When the above non-aqueous electrolyte contains an additive, the content of the additive may be 0.1 to 10% by mass, or may be 0.3 to 5% by mass. When the content of the additive is 0.1% by mass or more, the effects derived from the additive tend to be easily obtained. When the content of the additive is 10% by mass or less, an increase in the viscosity of the non-aqueous electrolyte can be suppressed, and an excessive additive that hardly obtains an effect commensurate with the addition amount can be reduced.

[0057] The above non-aqueous electrolyte contains carbon dioxide (CO 2 ), carbon monoxide (CO), hydrogen carbonate ion (HCO 3- ) and carbonate ions (CO 3 2- ) may be dissolved at least one selected from the group consisting of.

[0058] When carbon dioxide (CO 2 ), carbon monoxide (CO), hydrogen carbonate ions (HCO 3 - ) and carbonate ions (CO 3 2- ) are dissolved at least one selected from the group consisting of, carbon dioxide (CO 2 ) in the non-aqueous electrolyte, carbon monoxide (CO), hydrogen carbonate ions (HCO 3 - ) and carbonate ions (CO 3 2- ) may be the total amount of at least one selected from the group consisting of dissolved, 20 mass ppm or more, may be 100 mass ppm or more, may be 250 mass ppm or more, may be below the saturation solubility at 25 ° C.

[0059] (Positive electrode) The positive electrode of the lithium ion secondary battery according to the present embodiment may be one in which a positive electrode mixture layer is formed on a positive electrode current collector.

[0060] The positive electrode contains a positive electrode active material represented by the following formula (A). That is, the positive electrode mixture may contain a positive electrode active material represented by the following formula (A). Li v Ni x Co y Mn z O (2+w) …(A) [In the formula (A), 0.2 ≦ v ≦ 1.2, 0.6 ≦ x ≦ 0.9, 0 <y ≦ 0.3, 0 <z <0.4, x + y + z = 1, -0.2 ≦ w ≦ 0.2.]

[0061] In the formula (A), v is preferably 0.5 or more and 1.2 or less, more preferably 0.8 or more and 1.1 or less, and still more preferably 1.

[0062] In formula (A), w is preferably not less than -0.1 and not more than 0.1, and more preferably 0.

[0063] Examples of the positive electrode active material represented by formula (A) include LiNi 0.6 Co 0.2 Mn 0.2 O 2 、LiNi 0.7 Co 0.2 Mn 0.1 O 2 、or LiNi 0.8 Co 0.1 Mn 0.1 O 2 are preferred, and LiNi 0.6 Co 0.2 Mn 0.2 O 2 、or LiNi 0.8 Co 0.1 Mn 0.1 O 2 are more preferred.

[0064] From the viewpoint of improving the output characteristics and electrical characteristics of the lithium-ion secondary battery according to the present embodiment, the content of the positive electrode active material in the positive electrode composite material layer is preferably 75 to 99% by mass, and more preferably 85 to 95% by mass.

[0065] The positive electrode composite material layer may further contain a conductive aid. Examples of the conductive aid include carbon blacks such as ketjen black and acetylene black, carbon fibers, graphite, etc., and acetylene black and graphite are preferred.

[0066] From the viewpoint of improving the output characteristics and electrical characteristics of the lithium-ion secondary battery according to the present embodiment, the content of the conductive aid in the positive electrode composite material layer is preferably 0.5 to 20% by mass, and more preferably 1 to 5% by mass.

[0067] The above positive electrode composite material layer may further contain a binder. Examples of the binder include fluorine-based resins such as polyvinylidene fluoride, polyvinylidene fluoride fluoride, and polytetrafluoroethylene; synthetic rubbers such as styrene-butadiene rubber and nitrile butadiene rubber; polyamide-based resins such as polyamideimide; polyolefin-based resins such as polyethylene and polypropylene; poly(meth)acrylic resins; polyacrylic acid; cellulose-based resins such as carboxymethyl cellulose; etc. Polyvinylidene fluoride is preferred.

[0068] In the above positive electrode composite material layer, the content of the above binder is preferably 0.5 to 20% by mass, and more preferably 1 to 5% by mass.

[0069] The above positive electrode composite material layer may further contain other components as needed. Examples of other components include polymers such as non-fluorine-based polymers such as (meth)acrylic polymers, nitrile-based polymers, and diene-based polymers, and fluorine-based polymers such as polytetrafluoroethylene; emulsifiers such as anionic emulsifiers, nonionic emulsifiers, and cationic emulsifiers; dispersants such as polymer dispersants such as styrene-maleic acid copolymers and polyvinylpyrrolidone; thickeners such as carboxymethyl cellulose, hydroxyethyl cellulose, polyvinyl alcohol, polyacrylic acid (salt), and alkali-soluble type (meth)acrylic acid-(meth)acrylic acid ester copolymer; preservatives, etc. may be contained.

[0070] In the above positive electrode, the content of the above other components may be 0 to 15% by mass, or may be 0 to 10% by mass.

[0071] Examples of the above positive electrode current collector include iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, platinum, etc. Aluminum is preferred.

[0072] The above positive electrode is not particularly limited and can be manufactured by a known method. For example, a positive electrode active material, a conductive auxiliary agent, and a binder may be dispersed in a solvent to form a slurry, which is then coated on a positive electrode current collector and roll-pressed after drying.

[0073] Examples of the above solvent include N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, tetrahydrofuran, acetonitrile, acetone, ethanol, ethyl acetate, water, etc., and N-methylpyrrolidone is preferred.

[0074] (Negative electrode) The negative electrode of the lithium-ion secondary battery according to this embodiment may be one in which a negative electrode composite material layer is formed on a negative electrode current collector.

[0075] The above negative electrode composite material layer may contain, as a negative electrode active material, carbon materials such as graphite such as artificial graphite and natural graphite, coal, mesophase sintered bodies made from petroleum pitch, non-graphitizable carbon, etc., Si-based negative electrode materials such as Si, Si alloys, SiO, etc., Sn-based negative electrode materials such as Sn alloys, lithium metal, lithium alloys such as lithium-aluminum alloys, etc., and preferably contains graphite.

[0076] The content of the above negative electrode active material in the above negative electrode composite material layer is preferably 80 to 99% by mass, and more preferably 90 to 98% by mass.

[0077] The above negative electrode composite material layer may further contain a conductive assistant. The conductive assistant may be the same as that in the above positive electrode composite material layer, and is preferably carbon fiber. The content of the above conductive assistant in the above negative electrode composite material layer is preferably 0.1 to 10% by mass, and more preferably 1 to 5% by mass.

[0078] The above negative electrode composite material layer may further contain a binder. The binder may be the same as that in the above positive electrode composite material layer, and is preferably styrene-butadiene rubber and carboxymethyl cellulose. The content of the above binder in the above negative electrode composite material layer is preferably 0.1 to 10% by mass, and more preferably 1 to 5% by mass.

[0079] The above negative electrode composite material layer may further contain other components as required. The other components may be the same as those in the positive electrode composite material layer. The content of the other components in the negative electrode composite material layer may be the same as that in the positive electrode composite material layer.

[0080] The above negative electrode current collector may be the same as the positive electrode current collector, and is preferably copper.

[0081] The above negative electrode is not particularly limited and can be manufactured by a known method. For example, it may be manufactured in the same manner as the positive electrode. At this time, water is preferably used as the solvent.

[0082] (Separator) The lithium-ion secondary battery according to this embodiment may include a separator. The separator is arranged to separate the positive electrode and the negative electrode. Examples of the separator include a porous sheet made of a polymer capable of absorbing and retaining a non-aqueous electrolyte (for example, a polyolefin-based microporous separator, a cellulose-based separator, etc.), a non-woven fabric separator, a porous metal body, etc. Examples of the material of the porous sheet include polyethylene, polypropylene, a laminate having a three-layer structure of polypropylene / polyethylene / polypropylene, etc. Examples of the material of the non-woven fabric separator include cotton, rayon, acetate, nylon, polyester, polypropylene, polyethylene, polyimide, aramid, glass, etc. As the separator, a porous sheet made of polyethylene is preferred.

[0083] (Battery exterior material) The lithium-ion secondary battery according to this embodiment may be housed in a battery exterior material. The material of the battery exterior material is not particularly limited, and any conventionally known exterior material can be used. The battery exterior material may be provided with an overcurrent prevention element such as expanded metal, a fuse, a PTC element, etc., and a lead plate as required to prevent an increase in pressure inside the battery and overcharge / discharge.

[0084] The shape of the lithium-ion secondary battery according to this embodiment is not particularly limited and can be a known shape, such as a cylindrical shape, a square shape, a laminate shape, a coin shape, or a large size.

[0085] The rated charging voltage of the lithium-ion secondary battery according to this embodiment is not particularly limited. However, from the viewpoint of increasing the energy density, it may be 3.6 V or more, or 4.2 V or more. From the viewpoint of enhancing safety, the rated charging voltage may be 4.6 V or less.

Examples

[0086] Examples are given below to explain the present invention in more detail, but the present invention is not limited to these examples only. Note that the concentration (M) described in the electrolyte columns of Tables 1 to 6 is the amount of substance (mol / L) of the electrolyte per unit volume of the non-aqueous electrolyte. The ratios described in the solvent compositions of Tables 1 to 6 are the volume ratios of the respective solvents. The concentration (%) of the polyphosphoric acid ester in Tables 1 to 6 is the concentration (mass%) of the polyphosphoric acid ester based on the total amount of the non-aqueous electrolyte.

[0087] In addition, the abbreviations in the examples indicate the following materials. NCM111: LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O 2 (manufactured by Umicore) NCM523: LiNi 0.5 Co 0.2 Mn 0.3 O 2 (manufactured by Beijing Representative Office) NCM622: LiNi 0.6 Co 0.2 Mn 0.2 O 2 (manufactured by Beijing Representative Office) NCM811: LiNi 0.8 Co 0.1 Mn 0.1 O 2 (manufactured by Beijing Representative Office) EC: Ethylene carbonate MEC: Ethyl methyl carbonate PM: Methyl propionate DMC: Dimethyl carbonate PC: Propylene carbonate LiFSI: Lithium bis(fluorosulfonyl)imide

[0088] In Examples 1 to 6 described below, the lithium-ion secondary battery was manufactured, the polyphosphate ester was 31 analyzed by P-NMR, and various battery characteristics after storage at 80 °C were evaluated by the following methods.

[0089] <Manufacture of laminated lithium-ion secondary battery> (Manufacture of positive electrode) As a ternary positive electrode active material, NCM111, NCM523, NCM622, or NCM811 shown in Tables 1 to 6, acetylene black (manufactured by Denka, Denka Black), graphite (manufactured by Nippon Carbon, SP270), and polyvinylidene fluoride (PVdF, #1120, commercially available product) were weighed at a mass ratio of 100:3:3:3, and dispersed in N-methyl-2-pyrrolidone (NMP, commercially available product) to prepare a slurry. The prepared slurry was applied to one side of an aluminum foil (the coating weight of NCM111 is 19.7 mg / cm 2 , the coating weights of NCM523 and NCM622 are 19.5 mg / cm 2 , and the coating weight of NCM811 is 15.7 mg / cm 2 ), and after drying, roll pressing was performed to produce a positive electrode.

[0090] (Manufacture of negative electrode) An aqueous slurry of graphite (O-MAC, manufactured by Osaka Gas Chemical Co., Ltd.), carbon fiber (VGCF, manufactured by Showa Denko KK), styrene-butadiene rubber (SBR, commercially available product), and carboxymethyl cellulose (CMC, commercially available product) at a mass ratio of 100:2:1:1 was prepared, and applied to one side of a copper foil (the coating weight is 9.8 mg / cm 2 ), and after drying, roll pressing was performed to produce a negative electrode.

[0091] (Manufacture of non-aqueous electrolyte) The non-aqueous electrolyte was produced by dissolving an electrolyte in a solvent so as to have the compositions shown in Tables 1 to 6, adding a polyphosphoric acid ester to the resulting solution, and stirring for one day. However, as shown in Tables 1 to 6, there are also comparative examples in which no polyphosphoric acid ester was added. As the solvent, a mixed solvent in which EC, MEC, PM, DMC and / or PC were mixed at a predetermined volume ratio was used. In addition, for example, a mixed solvent in which each solvent is mixed at a predetermined volume ratio and which is available from Kishida Chemical Co., Ltd. may be used. As the electrolyte, LiFSI (manufactured by Nippon Shokubai Co., Ltd.) and LiPF 6 (manufactured by Stella Chemifa Corporation) were used. As the polyphosphoric acid ester, those described in Examples 1 to 6 described later were used.

[0092] (Manufacture of laminated lithium ion secondary battery) The fabricated positive electrode was cut at an effective area of 12 cm 2 and the polarity lead-out lead was welded to the cut positive electrode using an ultrasonic welder. The fabricated negative electrode was cut to an effective area of 13.44 cm 2 and the polarity lead-out lead was welded to the cut negative electrode using ultrasonic waves. These positive and negative electrodes were opposed to each other through a polyethylene separator with a thickness of 25 μm, and three sides were sealed with a laminated exterior to fabricate a non-injected cell. 700 μL of the electrolyte was injected from the unsealed side to fabricate a 30 mAh laminated lithium ion battery. After injecting the electrolyte, the battery was charged at a constant current of 3 mA for 3 hours, one piece was cracked, and degassing was performed by vacuum sealing again. After the degassed cell was stored at 25 °C for 48 hours, charge and discharge were repeated under the following conditioning conditions, and then constant current constant voltage charging at 30 mA, 4.2 V, and 0.6 mA cut-off was performed to make it in a fully charged state, completing the evaluation battery. Conditioning conditions First cycle Charge: Constant current constant voltage charging at 3 mA, 4.2 V, cut-off at 0.3 mA ⇒ Discharge: Discharge at 6 mA, cut-off at 2.75 V Second cycle Charge: Constant current constant voltage charging at 15 mA, 4.2 V, cut-off at 0.6 mA ⇒ Discharge: Discharge at 6 mA, terminate at 2.75 V 3rd cycle Charge: Constant current and constant voltage charge at 15 mA, 4.2 V, terminate at 0.6 mA ⇒ Discharge: Discharge at 30 mA, terminate at 2.75 V 4th cycle Charge: Constant current and constant voltage charge at 15 mA, 4.2 V, terminate at 0.6 mA ⇒ Discharge: Discharge at 60 mA, terminate at 2.75 V

[0093] <Evaluation of various battery characteristics after storage at 80°C> For the evaluation battery, the open circuit voltage (V) (hereinafter also referred to as "OCV") of the cell before storage at 80°C was measured, and volume measurement based on the Archimedes method was performed. Then, it was stored in an 80°C constant temperature bath for 14 days, and after leaving it to stand at 25°C for 6 hours or more after storage, the OCV (V) was measured, and the reduction amount of OCV due to storage at 80°C was calculated from the OCV before and after storage at 80°C. "ΔV" described in the OCV column in the table indicates the calculated reduction amount of OCV due to storage at 80°C. The volume of the cell after OCV measurement was measured based on the Archimedes method in the same manner as before storage, and the volume expansion rate was calculated from the volumes before and after storage at 80°C. However, for the comparative examples where the "endurance temperature" in Tables 1 to 6 is 60°C, they were stored in a 60°C constant temperature bath instead of an 80°C constant temperature bath. Thereafter, the battery was subjected to constant current discharge at 25°C with 6 mA and terminated at 2.75 V, and then constant current and constant voltage charge at 30 mA (1C), 4.2 V, and terminated at 0.6 mA was performed to bring it to a fully charged state. Thereafter, the DC resistance (Ω) (hereinafter also referred to as "DCR") of the battery was measured at 25°C. In the DCR measurement, after waiting for 30 minutes after completion of full charge, it was discharged at 6 mA (0.2C) for 10 seconds, then after waiting for 30 minutes, it was discharged at 30 mA (1C) for 10 seconds, and finally, after waiting for 30 minutes, it was discharged at 90 mA (3C) for 10 seconds. An I-V straight line was created from the relationship between the voltage difference and current immediately before the start of discharge and 10 seconds later at each discharge current, and its slope was calculated as the DCR. The battery after DCR measurement was subjected to constant current discharge at 6 mA and a cut-off voltage of 2.75 V at 25°C. After discharge, the cell was disassembled in a glove box, and the separator and the negative electrode were washed and dried with MEC. The negative electrode composite material peeled from the copper foil was immersed in nitric acid for 24 hours to dissolve it. The separator was also immersed in nitric acid for 24 hours. The obtained respective nitric acid solutions were filtered, diluted with ultrapure water, and the diluted solutions were analyzed by ICP to analyze the elution amounts of cobalt, manganese, and nickel from the positive electrode active material.

[0094] <Of polyphosphate 31 P-NMR analysis> 31 P-NMR measurement was performed using JNM-ECA500 manufactured by JEOL (JEOL Ltd.) with the inverse gated decoupling method. Measurement was carried out using a double sample tube as the sample tube, and the chemical shift was determined with the peak of phosphorus in H 3 PO 4 added to one tube set as 0 ppm.

[0095] In Examples 1 to 6, when the reduction amount of OCV due to storage at 80°C was 0.1650 V or less, it was evaluated that the reduction of OCV due to storage at 80°C was sufficiently suppressed. Also, in Examples 1 to 6, when the volume expansion rate during storage at 80°C was 3.0% or less, it was evaluated that the increase in volume due to storage at 80°C was sufficiently suppressed. Further, in Examples 1 to 6, when the DCR after storage at 80°C was 5.00 Ω or less, it was evaluated that the increase in DCR due to storage at 80°C was sufficiently suppressed. Also, in Examples 1 to 6, regarding the amount of metal precipitation after storage at 80°C, when the precipitation amount of nickel was 50 μg / cell or less, it was evaluated that the elution of nickel from the positive electrode active material due to storage at 80°C was sufficiently suppressed. Further, in Examples 1 to 6, regarding the amount of metal precipitation after storage at 80°C, when the precipitation amount of cobalt was 3.5 μg / cell or less, the precipitation amount of manganese was 7.0 μg / cell or less, and the precipitation amount of nickel was 50 μg / cell or less, it was evaluated that the elution of metal from the positive electrode active material due to storage at 80°C was sufficiently suppressed.

[0096] [Example 1] In Example 1, trimethylsilyl polyphosphate (hereinafter also referred to as "PPSE_1", manufactured by Sigma-Aldrich) was used as the polyphosphate ester in the production of the non-aqueous electrolyte.

[0097] PPSE_1 was subjected to the above 31 When subjected to P-NMR analysis, two peaks were confirmed: a peak (Pt) appearing at a chemical shift of -28 ppm to -33 ppm and a peak (Pm) appearing at -35 ppm to -41 ppm (Figure 1). The integral ratio of these two peaks was Pt:Pm = 1.00:1.43. On the other hand, a peak (Pb) appearing at -41 ppm to -45 ppm was not confirmed. Therefore, it is estimated that PPSE_1 is trimethylsilyl polyphosphate containing a large amount of linear structure and having no branched structure, and the average number of repeating units of PPSE_1 is calculated to be (2 + 2 × Pm / Pt) = 4.86.

[0098] According to the method for manufacturing the laminated lithium-ion secondary battery, laminated lithium-ion secondary batteries of Examples 1-1 to 1-14 and Comparative Examples 1-1 to 1-15 were manufactured. For these, evaluations of various battery characteristics after storage at 80°C were performed according to the method for evaluating various battery characteristics after storage at 80°C. The results are shown in Table 1.

[0099]

Table 1

[0100] From Table 1, for the lithium-ion secondary batteries (Examples 1-1 to 1-14) in which the content of PPSE_1 was 0.01 to 1% by mass based on the total amount of the non-aqueous electrolyte, the content of propylene carbonate in the solvent was less than 10% by volume, and NMC811 or NMC622 was used as the positive electrode active material, the reduction amount of OCV due to storage at 80 °C was 0.1650 V or less, and the reduction of OCV due to storage at 80 °C was sufficiently suppressed. Further, for the above lithium-ion secondary batteries, the volume expansion rate during storage at 80 °C was 3.0% or less, and the volume expansion due to storage at 80 °C was sufficiently suppressed. Further, for the above lithium-ion secondary batteries, the DCR after storage at 80 °C was 5.00 Ω or less, and the increase in DCR due to storage at 80 °C was sufficiently suppressed. Further, for the above lithium-ion secondary batteries, regarding the amount of metal precipitation after storage at 80 °C, the precipitation amount of cobalt was 3.5 μg / cell or less, the precipitation amount of manganese was 7.0 μg / cell or less, and the precipitation amount of nickel was 50 μg / cell or less, and the elution of nickel and the elution of metal due to storage at 80 °C were sufficiently suppressed. As the reasons for the above suppression of OCV reduction, suppression of volume expansion, suppression of increase in DCR, and suppression of metal elution, it is considered that one of the reasons is that the film derived from PPSE_1 was densely formed on the positive electrode, thereby suppressing the side reaction of the electrolyte on the positive electrode. On the other hand, for the lithium-ion secondary batteries (Comparative Examples 1-1, 1-2, and 1-6) that did not contain PPSE_1 in the non-aqueous electrolyte, the suppression of OCV reduction, suppression of volume expansion, suppression of increase in DCR, and suppression of metal elution during storage at 80 °C were not sufficient. Further, for the lithium-ion secondary batteries (Comparative Examples 1-3 and 1-7) in which the content of PPSE_1 was 2% by mass based on the total amount of the non-aqueous electrolyte, the suppression of the increase in DCR due to storage at 80 °C was not sufficient. It is considered that one of the reasons is that the film derived from PPSE_1 was formed on the positive electrode with excessive denseness. Further, for the lithium-ion secondary batteries (Comparative Examples 1-4 and 1-8) in which the content of propylene carbonate in the solvent was 10% by volume, the suppression of OCV reduction, suppression of volume expansion, suppression of increase in DCR, and suppression of metal elution during storage at 80 °C were not sufficient.

[0101] [Example 2] (Synthesis of Trimethylsilyl Polyphosphate) 1.553 g of phosphorus pentoxide was dispersed in 10 mL of methylene chloride as a solvent, and 1.710 g of hexamethyldisiloxane was gradually added dropwise with stirring. After that, the mixture was stirred at room temperature for about 1 day. Then, the solvent was distilled off to synthesize trimethylsilyl polyphosphate (hereinafter also referred to as "PPSE_2"). In Example 2, PPSE_2 was used as the phosphate ester in the production of the above non-aqueous electrolyte.

[0102] PPSE_2 was subjected to the above 31 When subjected to P-NMR analysis, three peaks were confirmed: a peak (Pt) appearing at a chemical shift of -28 ppm to -33 ppm, a peak (Pm) appearing at -35 ppm to -41 ppm, and a peak (Pb) appearing at -41 ppm to -45 ppm (Figure 2). The integral ratio of the three peaks was Pt:Pm:Pb = 1.00:6.73:1.00. From this analysis result, it was estimated that PPSE_2 is a chain-like trimethylsilyl polyphosphate having no cyclic structure and having one branched structure, and the average number of repeating units of PPSE_2 was calculated to be (Pt + Pm + Pb) = 8.73.

[0103] According to the method for manufacturing the above laminate-type lithium-ion secondary battery, laminate-type lithium-ion secondary batteries of Examples 2-1 to 2-14 and Comparative Examples 2-1 to 2-15 were manufactured. For these, according to the method for evaluating various battery characteristics after storage at 80°C, the evaluation of various battery characteristics after storage at 80°C was performed. The results are shown in Table 2.

[0104]

Table 2

[0105] From Table 2, in the lithium-ion secondary batteries (Examples 2-1 to 2-14) where the content of PPSE_2 is 0.01 to 1% by mass based on the total amount of the non-aqueous electrolyte, the content of propylene carbonate in the solvent is less than 10% by volume, and NMC811 or NMC622 is used as the positive electrode active material, the reduction amount of OCV due to storage at 80°C is 0.1650 V or less, and the reduction of OCV due to storage at 80°C is sufficiently suppressed. Further, the volume expansion rate of the lithium-ion secondary battery during storage at 80°C is 3.0% or less, and the volume expansion due to storage at 80°C is sufficiently suppressed. Further, the DCR of the lithium-ion secondary battery after storage at 80°C is 5.00 Ω or less, and the increase in DCR due to storage at 80°C is sufficiently suppressed. Further, regarding the amount of metal precipitation after storage of the lithium-ion secondary battery at 80°C, the precipitation amount of cobalt is 3.5 μg / cell or less, the precipitation amount of manganese is 7.0 μg / cell or less, and the precipitation amount of nickel is 50 μg / cell or less, and the elution of nickel and the elution of metal due to storage at 80°C are sufficiently suppressed. As the reasons for the suppression of the reduction of OCV, the suppression of volume expansion, the suppression of the increase in DCR, and the suppression of the elution of metal above, it is considered that one of the reasons is that the film derived from PPSE_2 is densely formed on the positive electrode, thereby suppressing the side reaction of the electrolyte on the positive electrode. Also, although the reason is unknown, when comparing Examples 1-1 to 1-14 and Examples 2-1 to 2-14 with the same solvent composition and the same concentration of polyphosphoric acid ester, PPSE_2, which is presumed to have a branched structure, has a smaller reduction amount of OCV due to storage at 80°C, a smaller DCR after storage at 80°C, and less elution of manganese and nickel after storage at 80°C than PPSE_1, which is presumed not to have a branched structure, in any case. In the above comparison, the elution amount of cobalt after storage at 80°C was about the same in Examples 1-6 and 2-6, but in other cases, PPSE_2 had less elution than PPSE_1. Therefore, PPSE_2 was shown to be superior in storage stability at high temperatures compared to PPSE_1. On the other hand, lithium-ion secondary batteries (Comparative Examples 2-1, 2-2, and 2-6) that do not contain PPSE_2 in the non-aqueous electrolyte were not sufficient in suppressing the reduction of OCV, suppressing volume expansion, suppressing the increase in DCR, and suppressing the elution of metal during storage at 80°C. In addition, lithium-ion secondary batteries (Comparative Examples 2-3 and 2-7) in which the content of PPSE_2 is 2% by mass based on the total amount of the non-aqueous electrolyte were not sufficient in suppressing the increase in DCR due to storage at 80°C. This is presumably because a film derived from PPSE_2 is formed on the positive electrode with excessive denseness. In addition, lithium-ion secondary batteries (Comparative Examples 2-4 and 2-8) in which the content of propylene carbonate in the solvent is 10% by volume were not sufficient in suppressing the reduction of OCV, suppressing volume expansion, suppressing the increase in DCR, and suppressing the elution of metal during storage at 80°C.

[0106] [Example 3] (Synthesis of trimethylsilyl polyphosphate) 1.553 g of diphosphorus pentoxide was dispersed in 10 mL of toluene as a solvent, and 1.710 g of hexamethylenedisiloxane was gradually added dropwise while stirring, followed by stirring at room temperature for about one day. Thereafter, trimethylsilyl polyphosphate (hereinafter also referred to as "PPSE_3") was synthesized by distilling off the solvent. In Example 3, PPSE_3 was used as the polyphosphate ester in the production of the non-aqueous electrolyte.

[0107] PPSE_3 was subjected to the above-mentioned 31 P-NMR analysis. As a result, three peaks were confirmed: a peak (Pt) appearing at a chemical shift of -28 ppm to -33 ppm, a peak (Pm) appearing at -35 ppm to -41 ppm, and a peak (Pb) appearing at -41 ppm to -45 ppm (Figure 3). The integration ratio of the three peaks was Pt:Pm:Pb = 1.00:6.51:0.81. From this analysis result, it is presumed that PPSE_3 is trimethylsilyl polyphosphate containing a branched structure.

[0108] According to the method for manufacturing the laminated lithium ion secondary battery, laminated lithium ion secondary batteries of Examples 3-1 to 3-14 and Comparative Examples 3-1 to 3-15 were manufactured. For these, according to the method for evaluating various battery characteristics after storage at 80°C, the evaluation of various battery characteristics after storage at 80°C was carried out. The results are shown in Table 3.

[0109]

Table 3

[0110] From Table 3, in the lithium ion secondary battery (Examples 3-1 to 3-14) in which the content of PPSE_3 was 0.01 to 1% by mass based on the total amount of the non-aqueous electrolyte, the content of propylene carbonate in the solvent was less than 10% by volume, and NMC811 or NMC622 was used as the positive electrode active material, the reduction amount of OCV due to storage at 80°C was 0.1650 V or less, and the reduction of OCV due to storage at 80°C was sufficiently suppressed. Further, the volume expansion rate of the lithium ion secondary battery during storage at 80°C was 3.0% or less, and the volume expansion due to storage at 80°C was sufficiently suppressed. Further, the DCR of the lithium ion secondary battery after storage at 80°C was 5.00 Ω or less, and the increase in DCR due to storage at 80°C was sufficiently suppressed. Further, regarding the amount of metal deposition after storage of the lithium ion secondary battery at 80°C, the deposition amount of cobalt was 3.5 μg / cell or less, the deposition amount of manganese was 7.0 μg / cell or less, and the deposition amount of nickel was 50 μg / cell or less, and the elution of nickel and the elution of metal due to storage at 80°C were sufficiently suppressed. As the reasons for the suppression of the reduction of OCV, the suppression of volume expansion, the suppression of the increase in DCR, and the suppression of the elution of metal, it is considered that one factor is that the film derived from PPSE_3 is densely formed on the positive electrode, thereby suppressing the side reaction of the electrolyte on the positive electrode. Also, although the reason is unclear, when comparing those with the same solvent composition and concentration of polyphosphate ester among Examples 1-1 to 1-14 and Examples 3-1 to 3-14, in any case, the reduction amount of OCV due to storage at 80°C was smaller for PPSE_3, which is presumed to have a branched structure, than for PPSE_1, which is presumed not to have a branched structure. In the above comparison, the DCR after storage at 80°C was comparable in Examples 1-11 and 3-11, but for the rest, it was smaller for PPSE_3 than for PPSE_1. The elution amount of cobalt after storage at 80°C was comparable in Examples 1-4 and 3-4, but for the rest, it was less for PPSE_3 than for PPSE_1. The elution amount of manganese after storage at 80°C was less for PPSE_3 than for PPSE_1, except in Examples 1-4 and 3-4. The elution amount of nickel after storage at 80°C was less for PPSE_3 than for PPSE_1, except in Examples 1-9 and 3-9, and Examples 1-10 and 3-10. Therefore, PPSE_3 was shown to be superior in storage stability at high temperatures compared to PPSE_1. On the other hand, lithium-ion secondary batteries (Comparative Examples 3-1, 3-2, and 3-6) that did not contain PPSE_3 in the non-aqueous electrolyte were not sufficient in suppressing the reduction of OCV, suppressing volume expansion, suppressing the increase in DCR, and suppressing the elution of metals during storage at 80°C. Also, lithium-ion secondary batteries (Comparative Examples 3-3 and 3-7) in which the content of PPSE_3 was 2% by mass based on the total amount of the non-aqueous electrolyte were not sufficient in suppressing the increase in DCR due to storage at 80°C. This is presumably because a film derived from PPSE_3 was formed on the positive electrode with excessive denseness. Also, lithium-ion secondary batteries (Comparative Examples 3-4 and 3-8) in which the content of propylene carbonate in the solvent was 10% by volume were not sufficient in suppressing the reduction of OCV, suppressing volume expansion, suppressing the increase in DCR, and suppressing the elution of metals during storage at 80°C.

[0111] [Example 4] (Synthesis of ethyl polyphosphate) 10 g of phosphorus pentoxide, 10 g of chloroform as a solvent, and 20 g of diethyl ether were placed in a test tube and stirred at 900 rpm and 35°C for 3 days. Then, the solvent was distilled off using an evaporator and vacuum dried for 24 hours to synthesize ethyl polyphosphate (hereinafter also referred to as "PPE"). In Example 4, PPE was used as the polyphosphate ester in the production of the above non-aqueous electrolyte.

[0112] PPE was subjected to the above 31 When subjected to 31P-NMR analysis, three peaks were confirmed: a peak (Pt) appearing at a chemical shift of -12 ppm to -15 ppm, a peak (Pm) appearing at -25 ppm to -31 ppm, and a peak (Pb) appearing at -39 ppm to -46 ppm. The integral ratio of the three peaks was Pt:Pm:Pb = 1.00:3.59:0.42. From this analysis result, it is presumed that PPE is ethyl polyphosphate containing a branched structure.

[0113] According to the method for manufacturing the above laminate-type lithium-ion secondary battery, laminate-type lithium-ion secondary batteries of Examples 4-1 to 4-11 and Comparative Examples 4-1 to 4-4 were manufactured. For these, evaluations of various battery characteristics after storage at 80°C were performed according to the method for evaluating various battery characteristics after storage at 80°C. The results are shown in Table 4.

[0114]

Table 4

[0115] From Table 4, in the lithium-ion secondary batteries (Examples 4-1 to 4-11) where the content of PPE was 0.01 to 1% by mass based on the total amount of the non-aqueous electrolyte, the content of propylene carbonate in the solvent was less than 10% by volume, and NMC811 or NMC622 was used as the positive electrode active material, the reduction amount of OCV due to storage at 80°C was 0.1650 V or less, and the reduction of OCV due to storage at 80°C was sufficiently suppressed. Further, in the above lithium-ion secondary battery, the volume expansion rate during storage at 80°C was 3.0% or less, and the volume expansion due to storage at 80°C was sufficiently suppressed. Further, in the above lithium-ion secondary battery, the DCR after storage at 80°C was 5.00 Ω or less, and the increase in DCR due to storage at 80°C was sufficiently suppressed. Further, regarding the amount of metal precipitation after storage at 80°C in the above lithium-ion secondary battery, the precipitation amount of cobalt was 3.5 μg / cell or less, the precipitation amount of manganese was 7.0 μg / cell or less, and the precipitation amount of nickel was 50 μg / cell or less, and the elution of nickel and the elution of metal due to storage at 80°C were sufficiently suppressed. As the reasons for the suppression of the reduction of the above OCV, the suppression of volume expansion, the suppression of the increase in DCR, and the suppression of the elution of metal, it is considered that one of the reasons is that the film derived from PPE was densely formed on the positive electrode, thereby suppressing the side reaction of the electrolyte on the positive electrode. On the other hand, in the lithium-ion secondary batteries (Comparative Examples 4-1 and 4-3) that did not contain PPE in the non-aqueous electrolyte, the suppression of the reduction of OCV, the suppression of volume expansion, the suppression of the increase in DCR, and the suppression of the elution of metal during storage at 80°C were not sufficient. Further, in the lithium-ion secondary batteries (Comparative Examples 4-2 and 4-4) where the content of PPE was 2% by mass based on the total amount of the non-aqueous electrolyte, the suppression of the increase in DCR due to storage at 80°C was not sufficient. It is considered that one of the reasons is that the film derived from PPE was formed on the positive electrode with excessive denseness.

[0116] [Example 5] (Synthesis of Triisopropylsilyl Polyphosphate) After dissolving 0.53 g of indium(III) bromide in 30 mL of tetrahydrofuran, 4.75 g of triisopropylsilane was added, and the mixture was stirred at room temperature for 1 day to conduct the reaction. When 30 mL of hexane was added to the solution after the reaction and allowed to stand, it separated into two layers. The upper layer recovered by liquid separation was concentrated under reduced pressure to obtain 4.01 g of hexaisopropyldisiloxane, which is a colorless liquid. Subsequently, 1.55 g of phosphorus pentoxide and 1.70 g of the hexaisopropyldisiloxane obtained above were added to 10 mL of dichloromethane, and the mixture was stirred at 35 °C for 3 days to conduct the reaction. The solution after the reaction was filtered and concentrated under reduced pressure to synthesize 2.10 g of triisopropylsilyl polyphosphate (hereinafter, also referred to as "PPSE(TIPS)") as a viscous liquid. In Example 5, PPSE(TIPS) was used as the polyphosphate ester in the production of the above non-aqueous electrolyte.

[0117] According to the method for manufacturing the above laminate-type lithium-ion secondary battery, laminate-type lithium-ion secondary batteries of Examples 5-1 to 5-11 and Comparative Examples 5-1 to 5-4 were manufactured. For these, according to the method for evaluating various battery characteristics after storage at 80 °C, the evaluation of various battery characteristics after storage at 80 °C was conducted. The results are shown in Table 5.

[0118]

Table 5

[0119] From Table 5, in the lithium-ion secondary batteries (Examples 5-1 to 5-11) where the content of PPSE(TIPS) is 0.01 to 1% by mass based on the total amount of the non-aqueous electrolyte, the content of propylene carbonate in the solvent is less than 10% by volume, and NMC811 or NMC622 is used as the positive electrode active material, the reduction amount of OCV due to storage at 80°C is 0.1650 V or less, and the reduction of OCV due to storage at 80°C is sufficiently suppressed. Also, in the above lithium-ion secondary batteries, the volume expansion rate during storage at 80°C is 3.0% or less, and the volume expansion due to storage at 80°C is sufficiently suppressed. Further, in the above lithium-ion secondary batteries, the DCR after storage at 80°C is 5.00 Ω or less, and the increase in DCR due to storage at 80°C is sufficiently suppressed. Also, regarding the amount of metal precipitation after storage at 80°C in the above lithium-ion secondary batteries, the precipitation amount of cobalt is 3.5 μg / cell or less, the precipitation amount of manganese is 7.0 μg / cell or less, and the precipitation amount of nickel is 50 μg / cell or less, and the elution of nickel and the elution of metal due to storage at 80°C are sufficiently suppressed. As the reasons for the above suppression of OCV reduction, suppression of volume expansion, suppression of increase in DCR, and suppression of metal elution, it is considered that one factor is that the film derived from PPSE(TIPS) is densely formed on the positive electrode, thereby suppressing the side reaction of the electrolyte on the positive electrode. On the other hand, in the lithium-ion secondary batteries (Comparative Examples 5-1 and 5-3) that do not contain PPSE(TIPS) in the non-aqueous electrolyte, the suppression of OCV reduction, suppression of volume expansion, suppression of increase in DCR, and suppression of metal elution during storage at 80°C were not sufficient. Also, in the lithium-ion secondary batteries (Comparative Examples 5-2 and 5-4) where the content of PPSE(TIPS) is 2% by mass based on the total amount of the non-aqueous electrolyte, the suppression of the increase in DCR due to storage at 80°C was not sufficient. This is considered to be due in part to the fact that the film derived from PPSE(TIPS) is formed on the positive electrode with excessive denseness.

[0120] [Example 6] (Synthesis of tert-Butyldimethylsilyl Polyphosphate) Poly-tert-butyldimethylsilyl phosphate was synthesized in the same manner as described for the synthesis of poly-isopropylsilyl phosphate in Example 5 above, except that triisopropylsilane was changed to an equal amount of tert-butyldimethylsilane. In Example 6, PPSE(TBDMS) was used as the phosphate ester in the production of the non-aqueous electrolyte described above.

[0121] According to the method for manufacturing the above laminate-type lithium ion secondary battery, laminate-type lithium ion secondary batteries of Examples 6-1 to 6-11 and Comparative Examples 6-1 to 6-4 were manufactured. For these, the various battery characteristics after storage at 80°C were evaluated according to the method for evaluating the various battery characteristics after storage at 80°C described above. The results are shown in Table 6.

[0122]

Table 6

[0123] From Table 6, in the lithium-ion secondary batteries (Examples 6-1 to 6-11) where the content of PPSE(TBDMS) is 0.01 to 1% by mass based on the total amount of the non-aqueous electrolyte, the content of propylene carbonate in the solvent is less than 10% by volume, and NMC811 or NMC622 is used as the positive electrode active material, the reduction amount of OCV due to storage at 80°C was 0.1650 V or less, and the reduction of OCV due to storage at 80°C was sufficiently suppressed. Also, the volume expansion rate of the above lithium-ion secondary battery during storage at 80°C was 3.0% or less, and the volume expansion due to storage at 80°C was sufficiently suppressed. Further, the DCR of the above lithium-ion secondary battery after storage at 80°C was 5.00 Ω or less, and the increase in DCR due to storage at 80°C was sufficiently suppressed. Also, regarding the amount of metal precipitation after storage at 80°C of the above lithium-ion secondary battery, the precipitation amount of cobalt was 3.5 μg / cell or less, the precipitation amount of manganese was 7.0 μg / cell or less, and the precipitation amount of nickel was 50 μg / cell or less, and the elution of nickel and the elution of metal due to storage at 80°C were sufficiently suppressed. As the reasons for the suppression of the reduction of OCV, the suppression of volume expansion, the suppression of the increase in DCR, and the suppression of the elution of metal described above, it is considered that it is partly because the film derived from PPSE(TBDMS) is densely formed on the positive electrode, thereby suppressing the side reaction of the electrolyte on the positive electrode. On the other hand, in the lithium-ion secondary batteries (Comparative Examples 6-1 and 6-3) that do not contain PPSE(TBDMS) in the non-aqueous electrolyte, the suppression of the reduction of OCV, the suppression of volume expansion, the suppression of the increase in DCR, and the suppression of the elution of metal during storage at 80°C were not sufficient. Also, in the lithium-ion secondary batteries (Comparative Examples 6-2 and 6-4) where the content of PPSE(TBDMS) is 2% by mass based on the total amount of the non-aqueous electrolyte, the suppression of the increase in DCR due to storage at 80°C was not sufficient. This is considered to be partly because the film derived from PPSE(TBDMS) is formed on the positive electrode with excessive denseness.

Claims

1. A lithium-ion secondary battery having a non-aqueous electrolyte, a positive electrode, and a negative electrode, wherein the non-aqueous electrolyte contains a polyphosphoric acid ester, an electrolyte, and a solvent, the content of the polyphosphoric acid ester is 0.01 to 1% by mass based on the total amount of the non-aqueous electrolyte, the content of propylene carbonate in the solvent is less than 10% by volume, and the positive electrode contains a positive electrode active material represented by the following formula (A). Li ｖ Ni ｘ Co ｙ Mn ｚ O （２＋ｗ） … (A) [In formula (A), 0.2 ≦ v ≦ 1.2, 0.6 ≦ x ≦ 0.9, 0 < y ≦ 0.3, 0 < z < 0.4, x + y + z = 1, and -0.2 ≦ w ≦ 0.2.]

2. A lithium-ion secondary battery having a non-aqueous electrolyte, a positive electrode, and a negative electrode, wherein the non-aqueous electrolyte contains a polyphosphoric acid ester, an electrolyte, and a solvent, the content of the polyphosphoric acid ester is 0.01 to 1% by mass based on the total amount of the non-aqueous electrolyte, the solvent contains at least one solvent selected from the group consisting of ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, methyl propionate, ethyl propionate, butyl propionate, isopropyl propionate, propyl propionate, ethyl acetate, methyl acetate, propyl acetate, and isopropyl acetate in a proportion of 95% by volume or more based on the total amount of the solvent, and the positive electrode contains a positive electrode active material represented by the following formula (A). Li ｖ Ni ｘ Co ｙ Mn ｚ O （２＋ｗ） … (A) [In formula (A), 0.2 ≦ v ≦ 1.2, 0.6 ≦ x ≦ 0.9, 0 < y ≦ 0.3, 0 < z < 0.4, x + y + z = 1, and -0.2 ≦ w ≦ 0.2.]

3. The lithium-ion secondary battery according to claim 1 or 2, wherein the polyphosphoric acid ester is at least one selected from the group consisting of trimethylsilyl polyphosphate, ethyl polyphosphate, triisopropylsilyl polyphosphate, and tert-butyldimethylsilyl polyphosphate.

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