Lithium-ion secondary battery

Abstract

To provide a lithium-ion secondary battery containing iron lithium phosphate as a positive electrode active material in which the iron elution from the positive electrode active material is sufficiently suppressed when stored at high temperatures, 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 contains a polyphosphoric acid ester, an electrolyte, and a solvent. The positive electrode includes iron lithium phosphate as a positive electrode active material.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 widely used as power sources for electronic devices such as smartphones and personal computers, and as power sources for automobiles. Lithium-ion secondary batteries having battery characteristics such as storage durability, safety, and charge-discharge characteristics suitable for their applications are adopted.

[0003] As a highly safe lithium-ion secondary battery, a lithium-ion secondary battery containing lithium iron phosphate as a positive electrode active material has attracted attention, and various studies have been conducted. For example, Patent Document 1 discloses a lithium-ion secondary battery containing lithium iron phosphate as a positive electrode active material, which has good low-temperature electrochemical performance and safety.

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, so it is desirable to be able to be stably stored even in a high-temperature environment, for example, at 80°C.

[0006] In view of the above circumstances, an object of the present invention is to provide a lithium-ion secondary battery containing lithium iron phosphate as a positive electrode active material, in which elution of iron from the positive electrode active material is sufficiently suppressed during storage at a high temperature, for example, at 80°C.

Means for Solving the Problems

[0007] The present disclosure provides, for example, a lithium-ion secondary battery described in [1] to [5] below. [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, and the positive electrode contains lithium iron phosphate as a positive electrode active material. [2] The lithium-ion secondary battery according to [1], 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. [3] The lithium-ion secondary battery according to [1] or [2], wherein the content of the polyphosphoric acid ester in the non-aqueous electrolyte is 0.01 to 5% by mass. [4] The lithium-ion secondary battery according to any one of [1] to [3], wherein the non-aqueous electrolyte further contains vinylene carbonate and / or lithium difluorophosphate. [5] The lithium-ion secondary battery according to any one of [1] to [4], wherein at least one selected from the group consisting of carbon dioxide, carbon monoxide, bicarbonate ions, and carbonate ions is dissolved in the non-aqueous electrolyte, and the total amount of the at least one selected from the group consisting of carbon dioxide, carbon monoxide, bicarbonate ions, and carbonate ions dissolved in the non-aqueous electrolyte is 20 to 5000 ppm by mass.

[0008] According to the lithium-ion secondary battery described in [1] to [5], elution of iron from the positive electrode active material is sufficiently suppressed during storage at high temperature, that is, the storage stability at high temperature is excellent. The reason is not necessarily clear, but the inventors consider 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 electrolyte on the positive electrode during storage at high temperature.

[0009] According to the lithium-ion secondary battery described in [1] to [5], further, self-discharge can be suppressed during storage at high temperatures. Thereby, a reduction in the remaining capacity due to storage at high temperatures can be suppressed.

Effects of the Invention

[0010] According to the present invention, it is possible to provide a lithium-ion secondary battery containing lithium iron phosphate as a positive electrode active material, in which elution of iron from the positive electrode active material is sufficiently suppressed during storage at 80°C.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Modes 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. Further, materials, components, or methods exemplified in this specification can be used alone or in combination of two or more unless otherwise specified.

[0013] The lithium-ion secondary battery according to the present embodiment has a non-aqueous electrolyte, a positive electrode, and a negative electrode. 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] A 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 condensed phosphoric acid compound having a structure formed by dehydration condensation of two or more phosphoric acids (H 3 PO 4 ). 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 formed by dehydration condensation of a plurality of phosphoric acids (including a condensed ring structure). 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 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 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 preferred.

[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] More preferably, the polyphosphoric acid ester is 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).

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

[0021] [Chemical formula] [In formula (1), R 1 is a hydrogen atom, an optionally substituted alkyl having 1 to 6 carbon atoms, an optionally substituted aryl having 6 to 10 carbon atoms, a group represented by the following formula (2) or a group represented by the following formula (3). A plurality of R 1 may be the same as or different from each other. X is hydroxy, an optionally substituted alkoxy having 1 to 6 carbon atoms, an optionally substituted aryloxy having 6 to 10 carbon atoms or a group represented by the following formula (4). Y 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). X and Y may together form a single bond. n is an integer of 2 or more.]

[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). 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). m is an integer of 1 or more.]

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

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

[0025] In this 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 general formula (1), from the viewpoint of further improving the storage stability at high temperatures, in general formula (1), R 1 is an optionally substituted alkyl group having 1 to 6 carbon atoms, a group represented by formula (2), or a group represented by formula (3). In formula (2), R 2 is an optionally substituted alkyl group having 1 to 6 carbon atoms or a group represented by formula (3), Z is an optionally substituted alkyl group having 1 to 6 carbon atoms, or a group represented by formula (3). In formula (3), R 3 is preferably an optionally substituted alkyl group having 1 to 6 carbon atoms. In this case, in general formula (1), X is an optionally substituted alkoxy group having 1 to 6 carbon atoms, or a group represented by formula (4), Y is an optionally substituted alkyl group having 1 to 6 carbon atoms or a group represented by formula (3), and in formula (4), R 4 is more preferably an optionally substituted alkyl group having 1 to 6 carbon atoms.

[0027] In general formula (1), R 1 The optionally substituted alkyl having 1 to 6 carbon atoms exemplified as 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.

[0028] In general formula (1), the optionally substituted alkoxy having 1 to 6 carbon atoms exemplified as X is preferably a linear alkoxy having 1 to 6 carbon atoms or a trifluoroalkoxy having 1 to 6 carbon atoms, more preferably a linear alkoxy having 1 to 3 carbon atoms or a trifluoroalkoxy having 1 to 3 carbon atoms, still more preferably ethoxy or trifluoroethoxy, and particularly preferably ethoxy.

[0029] In general formula (1), the optionally substituted alkyl having 1 to 6 carbon atoms exemplified as Y 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.

[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 are groups 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), R 2The optionally substituted alkyl having 1 to 6 carbon atoms exemplified as [alkyl] 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.

[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, and 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, and 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 in general formula (1) 1 (R 1(except for the case where it is a group represented by the formula (2)), a group obtained by removing the oxygen atom directly bonded to the phosphorus atom from X in the general formula (1), Y in the general formula (1), all R's in the formula (2) 2 and Z in the 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 more preferred.

[0039] The polyphosphate ester is, for example 31 By measuring the presence or absence of peaks indicating the bonds between the phosphorus atom and the surrounding groups and their abundance ratios (integration ratios of each peak) by means of 31P-NMR or the like, 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), etc. can be analyzed. The following general formulas (5-1) to (5-3) are shown 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 the peaks shown by the phosphorus atom 31 classified according to the chemical shift (ppm) of the peaks shown in 31P-NMR. 31 The measurement conditions for 31P-NMR may be those described in the examples described later.

[0040]

Chemical formula

[0041] [Chemical formula]

[0042] [Chemical formula]

[0043] Pt indicates 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 indicates 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 indicates 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 polyphosphoric acid 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 polyphosphoric acid 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 31P-NMR, the number (q + 2) of repeating units can be estimated by the following formula (5-4). However, when the polyphosphoric acid 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 the present embodiment, the content of the polyphosphate ester is preferably 0.01 to 5% by mass, more preferably 0.05 to 3% by mass, and still more preferably 0.1 to 2% by mass from the viewpoint of further improving the storage stability at high temperatures.

[0047] The method for producing the polyphosphate 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 hexamethylenedisiloxane, hexaisopropyldisiloxane, 1,3-di-tert-butyltetramethylenedisiloxane, or an ether compound such as diethyl ether is added and stirred, and then the solvent is removed to obtain it. In this case, by reducing the addition amount of the disiloxane compound or the ether compound, a polyphosphate 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 distilling off the solvent with an evaporator after filtering the solution obtained by stirring. 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 contained in the non-aqueous 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 (bis(trifluoromethylsulfonyl)imide)), LiPF 3 (C 2 F 5 ) 3 、LiBF(CF 3 ) 3 、LiAsF 6 、LiSbF 6 、LiFSO 3 、LiF and the like can be mentioned. As the electrolyte, from the viewpoint of further improving the storage stability at high temperatures, LiPF 6 or LiFSI is preferable.

[0050] In the non-aqueous electrolyte of the lithium ion secondary battery according to this embodiment, the total 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] The solvent contained in the non-aqueous electrolyte is not particularly limited, and known solvents used in lithium-ion secondary batteries can be used. For example, saturated cyclic carbonate (carbonate ester) solvents such as ethylene carbonate, propylene carbonate, ethylene 2,3-dimethyl carbonate, 1,2-butylene carbonate, and erythritan carbonate; chain carbonate (carbonate ester) solvents such as dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, diphenyl carbonate, and methyl phenyl carbonate; carboxylic acid ester solvents such as methyl propionate, ethyl propionate, butyl propionate, isopropyl propionate, propyl propionate, ethyl acetate, methyl acetate, propyl acetate, and isopropyl acetate; 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; 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, ethyl dimethyl phosphate, diethyl methyl 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, etc. can be mentioned.

[0052] From the viewpoint of further improving the storage stability at high temperatures, ethylene carbonate, ethyl methyl carbonate, or a combination thereof is preferable as the solvent.

[0053] The 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 cyclic carbonates (carbonic acid esters) having an unsaturated bond such as vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate, 2-vinyl ethylene carbonate, phenyl ethylene carbonate; lithium phosphates such as lithium monofluorophosphate and lithium difluorophosphate; carboxylic acid anhydrides such as succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, diglycolic anhydride, cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylic dianhydride, phenyl succinic 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 ester; 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 (amidosulfuric acid, H 3 NSO 3 ); sulfamic acid salts (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 ) and the like.

[0054] As the above additive, vinylene carbonate or lithium difluorophosphate is preferable from the viewpoint of further suppressing self-discharge at high temperatures.

[0055] When the non-aqueous electrolyte contains an additive, the content of the additive is preferably 0.1 to 10% by mass, more preferably 0.3 to 5% by mass, and still more preferably 0.3 to 3% by mass from the viewpoint of further suppressing self-discharge at high temperatures. 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.

[0056] The non-aqueous electrolyte may dissolve at least one selected from the group consisting of carbon dioxide (CO 2 ), carbon monoxide (CO), hydrogen carbonate ion (HCO 3 - ), and carbonate ion (CO 3 2- ).

[0057] From the viewpoint of further suppressing self-discharge at high temperatures, it is preferable that the non-aqueous electrolyte dissolves carbon dioxide.

[0058] The total dissolved amount of at least one selected from the group consisting of carbon dioxide, carbon monoxide, hydrogen carbonate ion, and carbonate ion in the non-aqueous electrolyte is preferably 20 to 5000 ppm by mass, and more preferably 2000 to 4000 ppm by mass from the viewpoint of further suppressing self-discharge at high temperatures.

[0059] The method of dissolving at least one selected from the group consisting of carbon dioxide, carbon monoxide, hydrogen carbonate ion, and carbonate ion in the non-aqueous electrolyte is not particularly limited, and a known method may be used. For example, the dissolution of carbon dioxide in the non-aqueous electrolyte may be performed by allowing the non-aqueous electrolyte to stand for 10 to 50 minutes in a carbon dioxide atmosphere of 0.2 to 0.4 MPa and 90 to 100% by volume.

[0060] The method for measuring the dissolved amount of at least one selected from the group consisting of carbon dioxide, carbon monoxide, hydrogen carbonate ions, and carbonate ions in the non-aqueous electrolyte is not particularly limited, and it may be performed by a known method such as gas chromatography. Specifically, the above measurement method by gas chromatography may be performed by the method described in the examples below.

[0061] (Positive electrode) The positive electrode of the lithium ion secondary battery according to this embodiment may be one in which a positive electrode composite material layer is formed on a positive electrode current collector.

[0062] The above positive electrode contains lithium iron phosphate. That is, the above positive electrode composite material may contain a positive electrode active material represented by the following formula (A).

[0063] Lithium iron phosphate may be represented by the following formula (A). Li x Fe y (PO 4 ) z …(A) (In the formula, 0.95 ≦ x ≦ 1.05, y = 1, 0.95 ≦ z ≦ 1.05)

[0064] From the viewpoint of improving the output characteristics and electrical characteristics of the lithium ion secondary battery according to this 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 assistant. Examples of the conductive assistant 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 this embodiment, the content of the conductive assistant in the positive electrode composite material layer is preferably 0.5 to 20% by mass, and more preferably 4 to 8% by mass.

[0067] The above positive electrode composite material layer may further contain a binder. Examples of the binder include fluororesins such as polyvinylidene fluoride, polyvinylidene fluoride fluoride, and polytetrafluoroethylene; synthetic rubbers such as styrene-butadiene rubber and nitrile butadiene rubber; polyamide resins such as polyamideimide; polyolefin resins such as polyethylene and polypropylene; poly(meth)acrylic resins; polyacrylic acid; cellulose 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 4 to 8% by mass.

[0069] The above positive electrode composite material layer may further contain other components as required. 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-mentioned positive electrode is not particularly limited and can be manufactured by a known method. For example, a positive electrode active material, a conductive assistant, and a binder can be dispersed in a solvent to form a slurry, which is then coated on a positive electrode current collector and dried, followed by roll pressing.

[0073] Examples of the above-mentioned 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-mentioned negative electrode composite material layer may contain, as a negative electrode active material, carbon materials such as graphite, anthracite, and mesophase sintered bodies made from petroleum pitch, such as artificial graphite and natural graphite, non-graphitizable carbon, Si-based negative electrode materials such as Si, Si alloys, and SiO, 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 area ratio of the peak area of the D band to the peak area of the G band of the above-mentioned graphite (hereinafter, also referred to as "D / G ratio") is not particularly limited, but from the viewpoint of further improving the storage stability at high temperatures, it is preferably 0.8 or less, more preferably 0.7 or less, still more preferably 0.5 or less, and particularly preferably 0.2 or less. Here, the G band and the D band are peaks observed at around 1580 cm -1 and around 1350 cm -1 respectively in Raman spectroscopic measurement. The G band is a peak derived from the in-plane stretching vibration of the six-membered ring structure of carbon atoms in graphite. The D band is a peak derived from the defect structure of the six-membered ring structure of the above-mentioned carbon atoms. A low D / G ratio indicates high crystallinity of graphite. The above Raman spectroscopic measurement may be performed by a known method or may be performed by the method described in the examples.

[0077] The full-width at half-maximum of the G band of the graphite is not particularly limited, but from the viewpoint of further improving the storage stability at high temperatures, it is preferably 35 or less, more preferably 28 or less, and still more preferably 23 or less. The full-width at half-maximum of the G band may be obtained by a known method by Raman spectroscopic measurement, or may be obtained by the method described in the examples. Here, a small full-width at half-maximum of the G band indicates high orientation of the graphite.

[0078] In the negative electrode composite material layer, the content of the negative electrode active material is preferably 80 to 99% by mass, and more preferably 90 to 98% by mass.

[0079] The negative electrode composite material layer may further contain a conductive assistant. The conductive assistant may be the same as that in the positive electrode composite material layer, and is preferably carbon fiber. In the negative electrode composite material layer, the content of the conductive assistant is preferably 0.1 to 10% by mass, and more preferably 0.5 to 4% by mass.

[0080] The negative electrode composite material layer may further contain a binder. The binder may be the same as that in the positive electrode composite material layer, and is preferably styrene-butadiene rubber and carboxymethyl cellulose. In the negative electrode composite material layer, the content of the binder is preferably 0.1 to 10% by mass, and more preferably 0.5 to 4% by mass.

[0081] The negative electrode composite material layer may further contain other components as necessary. The other components may be the same as those in the positive electrode composite material layer. In the negative electrode composite material layer, the content of the other components may be the same as the content in the positive electrode composite material layer.

[0082] The negative electrode current collector may be the same as the positive electrode current collector, and is preferably copper.

[0083] The 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 preferable as the solvent.

[0084] (Separator) The lithium ion secondary battery according to this embodiment may include a separator. The separator is arranged so as 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 holding 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, and the like. Examples of the material of the porous sheet include polyethylene, polypropylene, and a laminate having a three-layer structure of polypropylene / polyethylene / polypropylene. Examples of the material of the non-woven fabric separator include cotton, rayon, acetate, nylon, polyester, polypropylene, polyethylene, polyimide, aramid, glass, and the like. As the separator, a porous sheet made of polyethylene is preferable.

[0085] (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 include an expanded metal, a fuse, an overcurrent prevention element such as a PTC element, a lead plate, etc. as necessary to prevent an increase in pressure inside the battery and overcharge / discharge.

[0086] 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 rectangular shape, a laminated shape, a coin shape, a large size, etc.

[0087] The rated charging voltage of the lithium ion secondary battery according to this embodiment is not particularly limited, but from the viewpoint of increasing the energy density, it may be 3.6 V or more, or may be 4.2 V or more. From the viewpoint of enhancing safety, the rated charging voltage may be 4.6 V or less.

Examples

[0088] Examples are given below to explain the present invention in more detail, but the present invention is not limited only to these examples. Note that the concentration (M) described in the electrolyte columns of Tables 2 to 7 is the amount of substance of the electrolyte per unit volume of the non-aqueous electrolyte (mol / L). The concentration (wt%) of the polyphosphoric acid ester in Tables 2 to 7 is the concentration (mass%) of the polyphosphoric acid ester based on the total amount of the non-aqueous electrolyte.

[0089] In Examples 1 to 6 described below, the following methods were used to manufacture a lithium-ion secondary battery, evaluate various battery characteristics after storage at 80°C, perform Raman spectroscopic measurement of the graphite used for the negative electrode, perform 31 P-NMR analysis of the polyphosphoric acid ester, and measure the amount of dissolved carbon dioxide in the non-aqueous electrolyte.

[0090] <Manufacture of laminated lithium-ion secondary battery> (Manufacture of positive electrode) LiFePO 4 (Commercially available product), acetylene black (manufactured by Denka), graphite (manufactured by Nippon Graphite, SP270), and polyvinylidene fluoride (manufactured by Kuraray, product number KF Polymer 7208, solid content 8%) were weighed at a solid content ratio of 89:3:3:5, dispersed in N-methylpyrrolidone, and a positive electrode slurry was prepared. The obtained positive electrode slurry was coated on a carbon-coated Al foil (manufactured by Showa Denko) at a coating weight of 17.61 mg / cm 2 and dried on a hot plate at 130°C, and then further dried in a vacuum drying furnace set at 130°C for 12 hours. After drying, it was pressed with a roll press machine so that the composite density became 1.9 g / mL to produce a positive electrode.

[0091] (Manufacture of negative electrode 1) A 3% aqueous solution of carboxymethyl cellulose was prepared using carboxymethyl cellulose (commercial product). Graphite (manufactured by Hitachi Chemical, MAGE), VGCF (vapor grown carbon fiber, registered trademark, manufactured by Showa Denko), the above 3% aqueous solution of carboxymethyl cellulose, and styrene-butadiene rubber (commercial product) were weighed at a solid content ratio of 100:2:1:1, dispersed in water, and a negative electrode slurry was prepared. The obtained negative electrode slurry was applied to a copper foil with a thickness of 15 μm at a coating weight of 7.4 mg / cm 2 and dried on a hot plate at 70 °C, and then further dried in a vacuum drying furnace set at 80 °C for 12 hours. After drying, it was pressed with a roll press to a composite density of 1.5 g / mL to produce Negative Electrode 1.

[0092] (Manufacture of Negative Electrode 2) Negative Electrode 2 was produced in the same manner as described above for the "Manufacture of Negative Electrode 1", except that SLP50 manufactured by Imerys was used instead of MAGE manufactured by Hitachi Chemical as the graphite.

[0093] (Manufacture of Negative Electrode 3) Negative Electrode 3 was produced in the same manner as described above for the "Manufacture of Negative Electrode 1", except that O-MAC manufactured by Osaka Gas Chemical was used instead of MAGE manufactured by Hitachi Chemical as the graphite.

[0094] (Manufacture of Non-aqueous Electrolyte) The non-aqueous electrolyte was prepared by dissolving an electrolyte in a mixed solvent (manufactured by Kishida Chemical Co., Ltd.) having a composition of ethylene carbonate:ethyl methyl carbonate = 3:7 (volume ratio) so as to have the compositions shown in Tables 2 to 7, and adding a polyphosphoric acid ester and an additive to the obtained solution and stirring for 1 day. However, as shown in Tables 2 to 7, no polyphosphoric acid ester was added in Comparative Examples 1 to 9. When carbon dioxide was dissolved in the non-aqueous electrolyte, the non-aqueous electrolyte produced by the above method was placed in an autoclave connected to a carbon dioxide cylinder, the inside of the autoclave was replaced with carbon dioxide, and then pressurized at 3 atm (0.3 MPa) for 30 minutes to prepare a non-aqueous electrolyte in which carbon dioxide was dissolved. As the electrolyte, lithium bis(fluorosulfonyl)imide (manufactured by Nippon Shokubai Co., Ltd., hereinafter also referred to as "LiFSI"), and LiPF 6 (manufactured by Stella Chemifa) were used. As the additives, vinylene carbonate (manufactured by Kishida Chemical Co., Ltd., hereinafter also referred to as "VC"), and LiPO 2 F 2 (manufactured by Tokyo Chemical Industry Co., Ltd.) were used. As the polyphosphate ester, those described in Examples 1 to 6 described later were used.

[0095] (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 by an ultrasonic welder. Each of the fabricated negative electrodes 1 to 3 was cut at an effective area of 13.44 cm 2 and the polarity lead-out lead was welded to each of the cut negative electrodes 1 to 3 by ultrasonic waves. These positive electrodes and each of the negative electrodes 1 to 3 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 an unassembled cell using each of the negative electrodes 1 to 3. For each unassembled cell, 700 μL of the above-mentioned non-aqueous electrolyte was injected from the unsealed side, and a laminated lithium-ion battery with a capacity of 22 mAh was fabricated. After injecting the electrolyte, the battery was charged at a constant current of 2.2 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 22 mA, 3.6 V, and termination at 0.44 mA was performed to reach a fully charged state, and the evaluation battery was completed. Conditioning conditions First cycle Charge: Constant current constant voltage charging at 11 mA and 3.6 V for 5 hours ⇒ Discharge: Constant current discharge at 4.4 mA with a termination voltage of 2.0 V Second cycle Charge: Constant current constant voltage charging at 11 mA and 3.6 V for 5 hours ⇒ Discharge: Constant current discharge at 22 mA with a termination voltage of 2.0 V 3rd cycle charging: Constant current and constant voltage charging at 11 mA and 3.6 V for 5 hours ⇒ Discharging: Constant current discharging at 44 mA until the cut-off voltage of 2.0 V

[0096] <Evaluation of various battery characteristics after storage at 80 °C> The evaluation battery was stored in an 80 °C thermostat for 14 days. After leaving it at 25 °C for 6 hours or more after storage, constant current discharging was performed at 25 °C and 4.4 mA until the cut-off voltage of 2.0 V, and the remaining capacity (mAh) was measured. The remaining capacity (mAh) obtained was divided by the mass of the positive electrode active material to calculate the remaining capacity (mAh / g) per 1 g of the positive electrode active material. After measuring the remaining capacity, the cell was disassembled in a glove box. The separator and the negative electrode were washed with ethyl methyl carbonate and then dried. 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. After filtering the obtained nitric acid solutions respectively, they were diluted with ultrapure water, and the diluted solutions were analyzed by ICP to analyze the elution amount (μg / cell) of iron from the positive electrode active material.

[0097] <Raman spectroscopy measurement of the graphite used for the negative electrode> The Raman spectroscopy measurement was carried out using JASCO NRS-3100 (manufactured by JASCO Corporation) under the following conditions. From the obtained Raman spectrum, the peak area ratio of the D band to the peak area of the G band (area ratio (D / G ratio)) and the full width at half maximum of the G band were calculated. Conditions for Raman spectroscopy measurement · Laser wavelength: 532 nm · Exposure time: 5 s × 4 times · Central wave number: 2250 cm -1 · Slit: φ0.2 mm · Neutral density filter: OD1 (laser output 0.7 mW) · Objective lens: 20× · Baseline correction (linear correction between 400 cm -1 ~2400 cm -1 during)

[0098] <P-NMR analysis of polyphosphate ester 31 > 31 The 31P-NMR measurement was carried out using a JNM-ECA500 manufactured by JEOL (Japan Electron) with the inverse-gated decoupling method. The measurement was performed 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 in one of the tubes set as 0 ppm.

[0099] <Measurement of Carbon Dioxide Dissolved Amount in Non-Aqueous Electrolyte> The measurement of the carbon dioxide dissolved amount in the non-aqueous electrolyte was carried out using gas chromatography (apparatus: GC-2010 plus, manufactured by Shimadzu Corporation, column: Micropacked ST, manufactured by Shinwa Chemical Industries Co., Ltd.). Specifically, in order to prevent air from entering the measurement system, the inside of the gas chromatography apparatus was purged with nitrogen, and then the non-aqueous electrolyte was directly injected into the apparatus and measured under the following conditions. The calibration curve was created using multiple types of standard helium gases with known carbon dioxide mixing ratios. However, the injection amount of the standard helium gas was set to 1 mL. Conditions of Gas Chromatography · Column temperature program: Hold at 37°C (for 2.5 minutes from the start) ⇒ 37°C to 250°C (heating at 20°C / min) ⇒ 250°C to 270°C (heating at 15°C / min) ⇒ Hold at 270°C (for 5.42 minutes) · Vaporization chamber temperature: 130°C · Detector temperature: 300°C (BID) · Carrier gas: Helium (column flow rate 1.33 mL / min) · Injection amount: 1 μL (split method, split ratio: 5.0).

[0100] [Results of Raman Spectroscopy Measurement of Graphite Used for Negative Electrode] When the graphite used for negative electrodes 1 to 3 (MAGE manufactured by Hitachi Chemical, SLP50 manufactured by Imerys, and O-MAC manufactured by Osaka Gas Chemical) was subjected to the above Raman spectroscopy measurement, the G band and D band were confirmed in all the graphite. The peak area of the D band with respect to the peak area of the G band (hereinafter also referred to as "D / G ratio") and the full width at half maximum of the G band are shown in Table 1 below.

[0101]

Table 1

[0102] From Table 1, it was shown that MAGE manufactured by Hitachi Chemical had the smallest D / G ratio and the smallest half-value width of the G band, indicating the highest orientation and crystallinity. O-MAC manufactured by Osaka Gas Chemical had the largest D / G ratio and the largest half-value width of the G band, indicating the lowest orientation and crystallinity.

[0103] [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 above non-aqueous electrolyte.

[0104] PPSE_1 was subjected to the above 31 When subjected to 31P-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 was estimated that PPSE_1 is trimethylsilyl polyphosphate having no branched structure and containing a large amount of linear structure, and the average number of repeating units of PPSE_1 was calculated to be (2 + 2 × Pm / Pt) = 4.86.

[0105] According to the method for manufacturing the above laminate-type lithium ion secondary battery, laminate-type lithium ion secondary batteries of Examples 1-1 to 1-112 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 2.

[0106] [Comparative Examples 1 to 9] According to the method for manufacturing the laminated lithium ion secondary battery, laminated lithium ion secondary batteries of Comparative Examples 1 to 9 were manufactured. For these, according to the method for evaluating various battery characteristics after storage at 80°C, the various battery characteristics after storage at 80°C were evaluated. The results are shown in Table 2.

[0107]

Table 2

[0108] From Table 2, in a lithium ion secondary battery containing LiFePO 4 as a positive electrode active material, when the non-aqueous electrolyte contains PPSE_1 (Examples 1-1 to 1-12), compared with the case where the non-aqueous electrolyte does not contain PPSE_1 (Comparative Examples 1 to 9), the elution amount of iron after storage at 80°C was small, and the remaining capacity after storage at 80°C was large. The improvement in storage stability at these high temperatures is considered to be partly due to the fact that the film derived from PPSE_1 was densely formed on the positive electrode, suppressing the side reaction of the electrolyte on the positive electrode. Also, as an additive, a lithium ion secondary battery containing VC or LiPO 2 F 2 (Examples 1-11 to 1-12), compared with a lithium ion secondary battery (Example 1-10) under the same conditions except that these additives are not contained, the elution amount of iron after storage at 80°C was less or about the same, and the remaining capacity after storage at 80°C was larger. Also, from the comparison of Example 1-2, Example 1-7, and Example 1-9, and the comparison of Example 1-5, Example 1-8, and Example 1-10, it was shown that the lower the D / G ratio of the graphite contained in the negative electrode and the smaller the half-value width of the G band, the less the elution amount of iron after storage at 80°C.

[0109] [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 while stirring, followed by stirring 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 non-aqueous electrolyte.

[0110] 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 one branched structure, and the average number of repeating units of PPSE_2 was calculated to be (Pt + Pm + Pb) = 8.73.

[0111] 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 2-12 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 3.

[0112]

Table 3

[0113] From Table 3, as the positive electrode active material, LiFePO 4In a lithium-ion secondary battery containing [something], when the non-aqueous electrolyte contains PPSE_2 (Examples 2-1 to 2-12), compared with the case where the non-aqueous electrolyte does not contain PPSE_2 (Comparative Examples 1 to 9), the elution amount of iron after storage at 80 °C was less, and the remaining capacity after storage at 80 °C was larger. The improvement in storage stability at high temperatures is considered to be due in part to the fact that the film derived from PPSE_2 was densely formed on the positive electrode, suppressing the side reaction of the electrolyte on the positive electrode. Also, although the reason is unclear, when comparing Examples 1-1 to 1-12 and Examples 2-1 to 2-12 with the same electrolytes, concentrations of polyphosphate esters, and additives, PPSE_2, which is presumed to have a branched structure, had less elution amount of iron after storage at 80 °C than PPSE_1, which is presumed not to have a branched structure, in any case. Therefore, PPSE_2 was shown to be superior in storage stability at high temperatures compared to PPSE_1. Also, as an additive, VC or LiPO 2 F 2 The lithium-ion secondary battery containing [something] (Examples 2-11 to 2-12), compared with the lithium-ion secondary battery under the same conditions except for not containing these additives (Example 2-10), had less or comparable elution amount of iron after storage at 80 °C and a larger remaining capacity after storage at 80 °C. Also, from the comparison of Examples 2-2, 2-7, and 2-9, and the comparison of Examples 2-5, 2-8, and 2-10, it was shown that the lower the D / G ratio of the graphite contained in the negative electrode and the smaller the half-width of the G band, the less the elution amount of iron after storage at 80 °C.

[0114] [Example 3] (Synthesis of trimethylsilyl polyphosphate) 1.553 g of phosphorus pentoxide was dispersed in 10 mL of toluene as a solvent, and 1.710 g of hexamethylenedisiloxane was gradually added dropwise while stirring. After that, the mixture was stirred at room temperature for about one day. Then, the solvent was distilled off to synthesize trimethylsilyl polyphosphate (hereinafter also referred to as "PPSE_3"). In Example 3, PPSE_3 was used as the phosphate ester in the production of the non-aqueous electrolyte.

[0115] PPSE_3 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 3). The integral 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.

[0116] According to the method for manufacturing the above laminate-type lithium-ion secondary battery, laminate-type lithium-ion secondary batteries of Examples 3-1 to 3-12 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 4.

[0117]

Table 4

[0118] From Table 4, in a lithium-ion secondary battery containing LiFePO 4 as a positive electrode active material, when the non-aqueous electrolyte contains PPSE_3 (Examples 3-1 to 3-12), compared with the case where the non-aqueous electrolyte does not contain PPSE_3 (Comparative Examples 1 to 9), the elution amount of iron after storage at 80°C was less, and the remaining capacity after storage at 80°C was larger. The improvement in storage stability at these high temperatures is considered to be partly due to the fact that the film derived from PPSE_3 was densely formed on the positive electrode, suppressing the side reaction of the electrolyte on the positive electrode. Also, although the reason is unclear, when comparing Examples 1-1 to 1-12 and Examples 3-1 to 3-12 with the same electrolyte, polyphosphate ester concentration, and additives, in any case, the elution amount of iron after storage at 80°C was less for PPSE_3, which is presumed to have a branched structure, than for PPSE_1, which is presumed not to have a branched structure. Therefore, PPSE_3 was shown to be superior in storage stability at high temperatures compared to PPSE_1. Also, as additives, VC or LiPO 2 F 2 The lithium-ion secondary batteries containing (Examples 3-11 to 3-12) had a larger remaining capacity after storage at 80°C compared to the lithium-ion secondary batteries (Example 3-10) under the same conditions except for not containing these additives. Also, from the comparison of Example 3-2, Example 3-7, and Example 3-9, and the comparison of Example 3-5, Example 3-8, and Example 3-10, it was shown that the lower the D / G ratio of the graphite contained in the negative electrode and the smaller the half-width of the G band, the less the elution amount of iron after storage at 80°C.

[0119] [Example 4] (Synthesis of ethyl polyphosphate) 10 g of diphosphorus 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.

[0120] PPE was used in the above 31When subjected to P-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 integration ratio of the three peaks was Pt:Pm:Pb = 1.00:3.59:0.42. From this analysis result, PPE is presumed to be ethyl polyphosphate containing a branched structure.

[0121] According to the method for manufacturing the above laminate-type lithium ion secondary battery, the laminate-type lithium ion secondary batteries of Examples 4-1 to 4-9 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. Also, in Example 4-9, according to the method for measuring the dissolved amount of carbon dioxide in the above non-aqueous electrolyte, the evaluation of the dissolved amount of carbon dioxide contained in the non-aqueous electrolyte was carried out. The results are shown in Table 5.

[0122]

Table 5

[0123] From Table 5, in a lithium ion secondary battery containing LiFePO 4 when the non-aqueous electrolyte contains PPE (Examples 4-1 to 4-9), compared with the case where the non-aqueous electrolyte does not contain PPE (Comparative Examples 1 to 9), the elution amount of iron after storage at 80°C was less and the residual capacity after storage at 80°C was larger. The improvement in storage stability at these high temperatures is considered to be partly due to the fact that the film derived from PPE was densely formed on the positive electrode, suppressing the side reaction of the electrolyte on the positive electrode. Also, as an additive, VC or LiPO 2 F 2The lithium-ion secondary batteries (Examples 4-7 and 4-8) containing [specific component] and the lithium-ion secondary battery (Example 4-9) with carbon dioxide dissolved in the non-aqueous electrolyte had a larger residual capacity after storage at 80°C compared to the lithium-ion secondary battery (Example 4-6) under the same conditions, except that they did not contain these additives and did not have carbon dioxide dissolved in them. Also, from the comparison between Example 4-1 and Example 4-5, and the comparison between Example 4-3 and Example 4-6, it was shown that the lower the D / G ratio of the graphite contained in the negative electrode and the smaller the half-width of the G band, the less the amount of iron eluted after storage at 80°C.

[0124] [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 carry out the reaction. After adding 30 mL of hexane to the reaction solution and allowing it 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 carry out the reaction. The reaction solution was filtered and concentrated under reduced pressure to synthesize 2.10 g of triisopropylsilyl polyphosphate (hereinafter also referred to as "PPSE(TIPS)"), which is a viscous liquid. In Example 5, PPSE(TIPS) was used as the phosphate ester in the production of the above non-aqueous electrolyte.

[0125] 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 5-9 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. Also, in Example 5-9, according to the method for measuring the amount of carbon dioxide dissolved in the non-aqueous electrolyte, the evaluation of the amount of carbon dioxide dissolved in the non-aqueous electrolyte was carried out. The results are shown in Table 6.

[0126]

Table 6

[0127] From Table 6, in a lithium-ion secondary battery containing LiFePO 4 as the positive electrode active material, when the non-aqueous electrolyte contains PPSE(TIPS) (Examples 5-1 to 5-9), the elution amount of iron after storage at 80 °C was less and the remaining capacity after storage at 80 °C was larger than when the non-aqueous electrolyte did not contain PPE (Comparative Examples 1 to 9). The improvement in storage stability at high temperatures is considered to be partly due to the suppression of side reactions of the electrolyte on the positive electrode by the formation of a dense film derived from PPSE(TIPS) on the positive electrode. Also, as additives, lithium-ion secondary batteries containing VC or LiPO 2 F 2 (Examples 5-7, 5-8) and a lithium-ion secondary battery in which carbon dioxide was dissolved in the non-aqueous electrolyte (Example 5-9) had a smaller elution amount of iron after storage at 80 °C and a larger remaining capacity after storage at 80 °C compared to a lithium-ion secondary battery (Example 5-6) under the same conditions except that these additives were not contained and carbon dioxide was not dissolved. Also, from the comparison between Example 5-1 and Example 5-5, and the comparison between Example 5-3 and Example 5-6, it was shown that the lower the D / G ratio of graphite contained in the negative electrode and the smaller the half-value width of the G band, the less the elution amount of iron after storage at 80 °C.

[0128] [Example 6] (Synthesis of tert-butyldimethylsilyl polyphosphate) Tert-butyldimethylsilyl polyphosphate was synthesized in the same manner as the method described for the synthesis of triisopropylsilyl polyphosphate 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.

[0129] According to the method for manufacturing the laminated lithium ion secondary battery, the laminated lithium ion secondary batteries of Examples 6-1 to 6-9 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. Further, in Example 6-9, according to the method for measuring the amount of carbon dioxide dissolved in the non-aqueous electrolyte, the amount of carbon dioxide dissolved in the non-aqueous electrolyte was evaluated. The results are shown in Table 7.

[0130]

Table 7

[0131] From Table 7, in a lithium ion secondary battery containing LiFePO 4 as a positive electrode active material, when the non-aqueous electrolyte contains PPSE(TBDMS) (Examples 6-1 to 6-9), compared with the case where the non-aqueous electrolyte does not contain PPE (Comparative Examples 1 to 9), the amount of iron eluted after storage at 80°C was small, and the remaining capacity after storage at 80°C was large. The improvement in storage stability at these high temperatures is considered to be due in part to the fact that the film derived from PPSE(TBDMS) was densely formed on the positive electrode, suppressing the side reaction of the electrolyte on the positive electrode. Also, as additives, lithium ion secondary batteries containing VC or LiPO 2 F 2 (Examples 6-7, 6-8) and lithium ion secondary batteries in which carbon dioxide was dissolved in the non-aqueous electrolyte (Example 6-9) had a larger remaining capacity after storage at 80°C compared to lithium ion secondary batteries (Example 6-6) under the same conditions except that they did not contain these additives and did not dissolve carbon dioxide. Also, from the comparison between Example 6-1 and Example 6-5, and the comparison between Example 6-3 and Example 6-6, it was shown that the lower the D / G ratio of the graphite contained in the negative electrode and the smaller the half-value width of the G band, the less the amount of iron eluted after storage at 80°C.

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, and the positive electrode contains lithium iron phosphate as a positive electrode active material. The lithium-ion secondary battery.

2. The lithium-ion secondary battery according to claim 1, 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.

3. The lithium-ion secondary battery according to claim 1 or 2, wherein the content of the polyphosphoric acid ester in the non-aqueous electrolyte is 0.01 to 5% by mass.

4. The lithium-ion secondary battery according to claim 1 or 2, wherein the non-aqueous electrolyte further contains vinylene carbonate and / or lithium difluorophosphate.

5. At least one selected from the group consisting of carbon dioxide, carbon monoxide, hydrogen carbonate ions, and carbonate ions is dissolved in the non-aqueous electrolyte, and the total amount of at least one selected from the group consisting of carbon dioxide, carbon monoxide, hydrogen carbonate ions, and carbonate ions dissolved in the non-aqueous electrolyte is 20 to 5000 ppm by mass. The lithium-ion secondary battery according to claim 1 or 2.

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