Non-aqueous electrolyte, lithium secondary battery precursor, lithium secondary battery, and method for manufacturing a lithium secondary battery

Abstract

To provide a non-aqueous electrolyte that is resistant to discoloration even when stored for a long period of time in a high-temperature environment, and that can suppress an increase in DC resistance of a lithium secondary battery due to charging and discharging in a room-temperature environment.SOLUTION: A non-aqueous electrolyte contains at least one of lithium monofluorophosphate and lithium difluorophosphate, and a chain ether compound. The content of the chain ether compound is more than 0 mass ppm and 1000 mass ppm or less with respect to the total amount of the non-aqueous electrolyte.SELECTED DRAWING: None

Description

Technical Field

[0001] The present disclosure relates to a non-aqueous electrolyte, a lithium secondary battery precursor, a lithium secondary battery, and a method for manufacturing a lithium secondary battery.

Background Art

[0002] Conventionally, from the viewpoint of improving battery performance, various studies have been made on non-aqueous solvents and electrolytes used in non-aqueous electrolytes. It is known that when lithium difluorophosphate is added to a non-aqueous solvent, high-temperature storage characteristics, high-temperature cycle characteristics, etc. can be improved. Patent Document 1 discloses a non-aqueous electrolyte in which lithium difluorophosphate in an amount far exceeding the conventional solubility is uniformly and completely dissolved. In the non-aqueous electrolyte disclosed in Patent Document 1, lithium hexafluorophosphate is dissolved in a non-aqueous solvent. The non-aqueous solvent contains a cyclic carbonate and a chain carbonate. The non-aqueous electrolyte contains a chain ether compound having 2 or more carbon atoms and a methoxy group, which is a dissolution aid for lithium difluorophosphate. The ratio (volume% / mass%) of the ratio (volume%) of the dissolution aid to the total volume of the non-aqueous solvent with respect to the amount (mass%) of lithium difluorophosphate in the non-aqueous electrolyte is 0.1 or more and 5 or less. At 25°C, 1.2 mass% or more of lithium difluorophosphate is dissolved.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in a lithium secondary battery using a non-aqueous electrolyte as disclosed in Patent Document 1, there is a possibility that the DC resistance may easily increase when charging and discharging are repeated under an environment at room temperature (for example, 25°C). Furthermore, when a non-aqueous electrolyte such as that disclosed in Patent Document 1 is stored for a long period (e.g., one week) under a high-temperature environment (e.g., 60°C), there is a risk of coloring due to the generation of impurities. Such impurities may reduce the battery performance of the lithium secondary battery. In the non-aqueous electrolyte disclosed in Patent Document 1, when the above ratio (volume% / mass%) is 0.1 or more and 5 or less, and lithium difluorophosphate is dissolved at 1.2 mass% or more at 25°C, the content of the chain ether compound is calculated to be 1044 mass ppm or more based on the total amount of the non-aqueous electrolyte.

[0005] An object of one aspect of the present disclosure is to provide a non-aqueous electrolyte, a lithium secondary battery precursor, a lithium secondary battery, and a method for manufacturing a lithium secondary battery that are less likely to be colored even when stored for a long period under a high-temperature environment and can suppress an increase in the DC resistance of the lithium secondary battery due to charging and discharging under a normal-temperature environment.

Means for Solving the Problems

[0006] Means for solving the above problems include the following aspects. <1> A non-aqueous electrolyte containing at least one of lithium monofluorophosphate and lithium difluorophosphate, and a chain ether compound wherein the content of the chain ether compound is more than 0 mass ppm and 1000 mass ppm or less based on the total amount of the non-aqueous electrolyte. <2> The non-aqueous electrolyte according to <1>, which contains the lithium difluorophosphate. <3> The non-aqueous electrolyte according to <2>, wherein the content of the lithium difluorophosphate is 0.001 mass% to 5 mass% based on the total amount of the non-aqueous electrolyte. <4> The non-aqueous electrolyte according to any one of <1> to <3>, wherein the chain ether compound contains 1,2-dimethoxyethane. <5> A positive electrode, and a negative electrode containing a negative electrode active material capable of occluding and releasing lithium ions, ​ The non-aqueous electrolyte according to any one of <1> to <4> above and A lithium secondary battery precursor comprising the same. <6> A lithium secondary battery obtained by charging and discharging the lithium secondary battery precursor according to <5> above. <7> A step of preparing the lithium secondary battery precursor according to <5> above, A step of charging and discharging the lithium secondary battery precursor, and A method for manufacturing a lithium secondary battery including the same.

Advantages of the Invention

[0007] According to one aspect of the present disclosure, there are provided a non-aqueous electrolyte, a lithium secondary battery precursor, a lithium secondary battery, and a method for manufacturing a lithium secondary battery that are difficult to be colored even when stored for a long time in a high-temperature environment and can suppress an increase in the DC resistance of the lithium secondary battery due to charging and discharging in a normal-temperature environment.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0009] In this specification, a numerical range represented using "~" means a range including the numerical values described before and after "~" as a lower limit value and an upper limit value. In this specification, the amount of each component in the composition means the total amount of the plurality of substances corresponding to each component in the composition, unless otherwise specified, when there are a plurality of substances corresponding to each component in the composition.

[0010] [Non-aqueous electrolyte] The non-aqueous electrolyte of the present disclosure is at least one of lithium monofluorophosphate and lithium difluorophosphate, and a chain ether compound and is a non-aqueous electrolyte containing The content of the chain ether compound is more than 0 mass ppm and 1000 mass ppm or less with respect to the total amount of the non-aqueous electrolyte.

[0011] Hereinafter, lithium monofluorophosphate and lithium difluorophosphate may be collectively referred to as "lithium fluorophosphate compound".

[0012] Since the non-aqueous electrolyte of the present disclosure has the above configuration, it is difficult to be colored even when stored for a long time in a high-temperature environment, and it is possible to suppress an increase in the DC resistance of a lithium secondary battery due to charging and discharging in a normal-temperature environment. The coloring state of the non-aqueous electrolyte can be quantitatively evaluated by the APHA color described later. The measurement method of the APHA color may be a method conforming to JIS K0071-1:2017. The measurement method of the APHA color may be visual measurement or measurement using a colorimeter. From the viewpoint that the difference in coloring is easy to confirm, the measurement using a colorimeter is preferable as the measurement method of the APHA color. The quantitative evaluation method for the increase in the DC resistance of a lithium secondary battery due to charging and discharging in a normal-temperature environment is the same as the evaluation method described in the examples described later.

[0013] Hereinafter, each component that can be contained in the non-aqueous electrolyte of the present disclosure will be described.

[0014] [Chain ether compound] The non-aqueous electrolyte of the present disclosure contains a chain ether compound. The chain ether compound is preferably a chain ether compound having 2 or more carbon atoms with a methoxy group, and more preferably a chain ether compound containing 4 or more carbon atoms, 10 or more hydrogen atoms, and 2 or more oxygen atoms. Specific examples of the chain ether compound include 1,2-dimethoxyethane, alkylene glycol dimethyl ether, etc. Examples of the alkylene glycol dimethyl ether include triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, etc. These chain ether compounds may be used alone or in combination of two or more. Among them, the chain ether compound preferably contains 1,2-methoxyethane. Thereby, the viscosity of the non-aqueous electrolyte can be lowered and the mobility of the electrolyte can be increased. As a result, the DC resistance of the lithium secondary battery can be lowered.

[0015] The content of the chain ether compound is more than 0 mass ppm and 1000 mass ppm or less based on the total amount of the non-aqueous electrolyte. If the content of the chain ether compound is within the above range, an increase in the DC resistance of the lithium secondary battery due to charging and discharging can be suppressed in a normal temperature environment. Furthermore, if the content of the chain ether compound is within the above range, coloring of the non-aqueous electrolyte can be further suppressed when the non-aqueous electrolyte is stored for a long period (for example, 1 week) in a high temperature environment (for example, 60 °C) (hereinafter referred to as "long-term storage at high temperature"). In other words, the progress of the decomposition reaction of the non-aqueous electrolyte can be suppressed. In the charge-discharge cycle of the lithium secondary battery, the components generated by the decomposition reaction of the non-aqueous electrolyte are considered to be factors that promote side reactions that are not the original battery reaction. The battery reaction indicates a reaction in which lithium ions enter and exit (intercalate) the positive and negative electrodes. Side reactions include the reduction decomposition reaction of the electrolyte by the negative electrode, the oxidation decomposition reaction of the electrolyte by the positive electrode, and the elution of metal elements in the positive electrode active material. As a result, even if the lithium secondary battery is stored at high temperature for a long time, the battery performance of the lithium secondary battery is less likely to deteriorate. The upper limit of the content of the chain ether compound is preferably 500 mass ppm or less, more preferably 300 mass ppm or less, based on the total amount of the non-aqueous electrolyte. The lower limit of the content of the chain ether compound is preferably more than 0 mass ppm, more preferably 40 mass ppm or more, based on the total amount of the non-aqueous electrolyte. In particular, the content of the chain ether compound is preferably more than 0 mass ppm and 300 mass ppm or less, more preferably 40 mass ppm or more and 300 mass ppm or less. Thereby, coloring of the non-aqueous electrolyte due to long-term storage at high temperature can be further suppressed. As a result, even when the lithium secondary battery is stored at high temperature for a long time, the battery performance of the lithium secondary battery is less likely to deteriorate.

[0016] The product of lithium difluorophosphate (hereinafter simply referred to as "product") may contain 1,2-dimethoxyethane. When the product is used as a raw material for lithium difluorophosphate, the content of the chain ether compound can be adjusted within a desired range according to the content of 1,2-dimethoxyethane contained in the product. For example, when the content of 1,2-dimethoxyethane contained in the product is small, the content of the chain ether compound can be adjusted within a desired range by adding a chain ether compound to the non-aqueous electrolyte. Also, when the content of 1,2-dimethoxyethane contained in the product is large, the content of the chain ether compound can be adjusted within a desired range by using a purified product obtained by purifying the product by a known method as a raw material for lithium difluorophosphate.

[0017] <Lithium fluorophosphate compound> The non-aqueous electrolyte of the present disclosure contains at least one selected from the group consisting of lithium monofluorophosphate and lithium difluorophosphate. The non-aqueous electrolyte of the present disclosure preferably contains lithium difluorophosphate. By including lithium difluorophosphate in the non-aqueous electrolyte, the initial and post-storage DC resistance of the lithium secondary battery can be improved. The initial DC resistance of the lithium secondary battery refers to, for example, the DC resistance of the lithium secondary battery obtained by subjecting the lithium secondary battery precursor to the aging treatment of the examples described later. The DC resistance of the lithium secondary battery after storage refers to, for example, the DC resistance of the lithium secondary battery obtained by subjecting the lithium secondary battery precursor to the aging treatment and the initial charge-discharge treatment of the examples described later.

[0018] The upper limit of the content of the lithium fluorophosphate compound is preferably 5% by mass or less, more preferably 3% by mass or less, and still more preferably 2% by mass or less with respect to the total amount of the non-aqueous electrolyte. If the upper limit of the content of the lithium fluorophosphate compound is within the above range, the solubility of lithium fluorophosphate in the non-aqueous solvent can be ensured. The lower limit of the content of the lithium fluorophosphate compound is preferably 0.001% by mass or more, more preferably 0.01% by mass or more, and still more preferably 0.1% by mass or more with respect to the total amount of the non-aqueous electrolyte. If the lower limit of the content of the lithium fluorophosphate compound is within the above range, the DC resistance of the lithium secondary battery can be further reduced.

[0019] <Additive> The non-aqueous electrolyte of the present disclosure may contain an additive. Thereby, in the charge-discharge cycle of the lithium secondary battery, the progress of side reactions can be suppressed. As a result, the battery performance of the lithium secondary battery is improved. The non-aqueous electrolyte may contain the additive alone or in combination of two or more.

[0020] There is no particular limitation on the additive, and known ones can be arbitrarily used. As the additive, for example, the additives described in paragraphs 0042 to 0055 of JP-A-2019-153443 can be used.

[0021] When the non-aqueous electrolyte of the present disclosure contains an additive, the content of the additive is preferably 0.001% by mass to 10% by mass, more preferably 0.005% by mass to 5% by mass, still more preferably 0.01% by mass to 2% by mass, particularly preferably 0.1% by mass to 2% by mass, and even more preferably 0.1% to 1% by mass, based on the total amount of the non-aqueous electrolyte.

[0022] Next, other components of the non-aqueous electrolyte will be described. The non-aqueous electrolyte generally contains an electrolyte and a non-aqueous solvent.

[0023] <Electrolyte> The electrolyte in the non-aqueous electrolyte of the present disclosure preferably contains a lithium salt, and more preferably contains LiPF6. When the electrolyte contains LiPF6, the ratio of LiPF6 in the electrolyte is preferably 10% by mass to 100% by mass, more preferably 50% by mass to 100% by mass, and still more preferably 70% by mass to 100% by mass.

[0024] The concentration of the electrolyte in the non-aqueous electrolyte of the present disclosure is preferably 0.1 mol / L to 3 mol / L, and more preferably 0.5 mol / L to 2 mol / L. Also, the concentration of LiPF6 in the non-aqueous electrolyte of the present disclosure is preferably 0.1 mol / L to 3 mol / L, and more preferably 0.5 mol / L to 2 mol / L.

[0025] When the electrolyte contains LiPF6, the electrolyte may contain a compound other than LiPF6. Compounds other than LiPF6 include: (C2H5)4NPF6, (C2H5)4NBF4, (C2H5)4NClO4, (C2H5)4NAsF6, (C2H5)4N2SiF6, (C2H5)4NOSO2C k F (2k+1) (where k is an integer from 1 to 8), (C2H5)4NPF n [C k F (2k+1) (6-n) (where n = 1 to 5 and k is an integer from 1 to 8), etc., tetraalkylammonium salts; ​LiBF4, LiClO4, LiAsF6, Li2SiF6, LiOSO2C k F (2k+1) (k is an integer from 1 to 8), LiPF n [C k F (2k+1) (6-n) (n = 1 to 5, k is an integer from 1 to 8), LiC(SO2R 7 )(SO2R 8 )(SO2R 9 )、LiN(SO2OR 10 )(SO2OR 11 )、LiN(SO2R 12 )(SO2R 13 (where R 7 ~R 13 may be the same as or different from each other and is a fluorine atom or a perfluoroalkyl group having 1 to 8 carbon atoms), etc. lithium salts (i.e., lithium salts other than LiPF6); and the like can be mentioned.

[0026] <Non-aqueous solvent> The non-aqueous solvent in the non-aqueous electrolyte of the present disclosure may be only one kind or two or more kinds. As the non-aqueous solvent, various known ones can be appropriately selected. As the non-aqueous solvent, for example, the non-aqueous solvents described in paragraphs 0069 to 0087 of JP-A-2017-45723 can be used.

[0027] The non-aqueous solvent preferably contains a cyclic carbonate compound and a chain carbonate compound. In this case, the cyclic carbonate compound and the chain carbonate compound contained in the non-aqueous solvent may each be only one kind or two or more kinds.

[0028] Examples of the cyclic carbonate compound include ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, and the like. ​Among these, ethylene carbonate and propylene carbonate, which have a high dielectric constant, are preferred. In the case of a battery using a negative electrode active material containing graphite, it is more preferable that the non-aqueous solvent contains ethylene carbonate.

[0029] Examples of the chain carbonate compound include dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, ethyl propyl carbonate, dipropyl carbonate, methyl butyl carbonate, ethyl butyl carbonate, dibutyl carbonate, methyl pentyl carbonate, ethyl pentyl carbonate, dipentyl carbonate, methyl heptyl carbonate, ethyl heptyl carbonate, diheptyl carbonate, methyl hexyl carbonate, ethyl hexyl carbonate, dihexyl carbonate, methyl octyl carbonate, ethyl octyl carbonate, dioctyl carbonate, and the like.

[0030] As combinations of cyclic carbonates and chain carbonates, specifically, ethylene carbonate and dimethyl carbonate, ethylene carbonate and methyl ethyl carbonate, ethylene carbonate and diethyl carbonate, propylene carbonate and dimethyl carbonate, propylene carbonate and methyl ethyl carbonate, propylene carbonate and diethyl carbonate, ethylene carbonate and propylene carbonate and methyl ethyl carbonate, ethylene carbonate and propylene carbonate and diethyl carbonate, ethylene carbonate and dimethyl carbonate and methyl ethyl carbonate, ethylene carbonate and dimethyl carbonate and diethyl carbonate, ethylene carbonate and methyl ethyl carbonate and diethyl carbonate, ethylene carbonate and dimethyl carbonate and methyl ethyl carbonate and diethyl carbonate, ethylene carbonate and propylene carbonate and dimethyl carbonate and methyl ethyl carbonate, ethylene carbonate and propylene carbonate and dimethyl carbonate and diethyl carbonate, ethylene carbonate and propylene carbonate and methyl ethyl carbonate and diethyl carbonate, ethylene carbonate and propylene carbonate and dimethyl carbonate and methyl ethyl carbonate and diethyl carbonate, etc. can be mentioned.

[0031] The mixing ratio of the cyclic carbonate compound and the chain carbonate compound, expressed as a mass ratio, is such that cyclic carbonate compound:chain carbonate compound is, for example, 5:95 to 80:20, preferably 10:90 to 70:30, more preferably 15:85 to 55:45. By setting such a ratio, an increase in the viscosity of the non-aqueous electrolyte can be suppressed, and the degree of dissociation of the electrolyte can be increased. Therefore, the conductivity of the non-aqueous electrolyte related to the charge and discharge characteristics of the battery can be increased. Also, the solubility of the electrolyte can be further increased. Thus, a non-aqueous electrolyte excellent in electrical conductivity at normal temperature or low temperature can be obtained, and the load characteristics of the battery from normal temperature to low temperature can be improved.

[0032] The non-aqueous solvent may contain other compounds other than cyclic carbonate compounds and chain carbonate compounds. In this case, the other compounds contained in the non-aqueous solvent may be only one kind or two or more kinds. Examples of the other compounds include cyclic carboxylic acid ester compounds (such as γ-butyrolactone), cyclic sulfone compounds, cyclic ether compounds, chain carboxylic acid ester compounds, chain ether compounds, chain phosphate ester compounds, amide compounds, chain carbamate compounds, cyclic amide compounds, cyclic urea compounds, boron compounds, polyethylene glycol derivatives, and the like. Regarding these compounds, the descriptions in paragraphs 0069 to 0087 of JP-A-2017-45723 can be appropriately referred to.

[0033] The proportion of the cyclic carbonate compound and the chain carbonate compound in the non-aqueous solvent is preferably 80% by mass or more, more preferably 90% by mass or more, and still more preferably 95% by mass or more. The proportion of the cyclic carbonate compound and the chain carbonate compound in the non-aqueous solvent may be 100% by mass.

[0034] The proportion of the non-aqueous solvent in the non-aqueous electrolyte is preferably 60% by mass or more, more preferably 70% by mass or more. The upper limit of the proportion of the non-aqueous solvent in the non-aqueous electrolyte depends on the contents of other components (electrolytes, additives, etc.), but the upper limit is, for example, 99% by mass, preferably 97% by mass, and still more preferably 90% by mass.

[0035] The non-aqueous electrolyte of the present disclosure can be used as a non-aqueous electrolyte for a battery, and among the non-aqueous electrolytes for a battery, it can be particularly preferably used as a non-aqueous electrolyte for a lithium secondary battery.

[0036] 〔Lithium secondary battery precursor, lithium secondary battery〕 The lithium secondary battery precursor of the present disclosure is a positive electrode, A negative electrode containing a negative electrode active material capable of occluding and releasing lithium ions, the non-aqueous electrolyte of the present disclosure described above, and includes.

[0037] The lithium secondary battery of the present disclosure is a lithium secondary battery obtained by charging and discharging the lithium secondary battery precursor of the present disclosure.

[0038] In the present disclosure, the lithium secondary battery precursor means a lithium secondary battery after manufacture and before charging and discharging. In other words, in the present disclosure, the lithium secondary battery means a battery in which charging and discharging have been performed on the lithium secondary battery precursor.

[0039] The lithium secondary battery precursor of the present disclosure includes the non-aqueous electrolyte of the present disclosure described above. Therefore, according to the lithium secondary battery precursor and the non-aqueous electrolyte of the present disclosure, effects similar to those of the non-aqueous electrolyte of the present disclosure described above are achieved.

[0040] Hereinafter, each component that can be included in the lithium secondary battery precursor of the present disclosure will be described. Each component that can be included in the lithium secondary battery of the present disclosure is basically the same as each component that can be included in the lithium secondary battery precursor of the present disclosure. In the lithium secondary battery of the present disclosure, an SEI (Solid Electrolyte Interface) film is preferably formed on the surface of the positive electrode (particularly the positive electrode active material) and / or the negative electrode (particularly the negative electrode active material). The SEI film is a film that can be formed by charging and discharging the lithium secondary battery precursor. The SEI film contains components in the non-aqueous electrolyte and / or products (e.g., decomposition products) derived from the above components.

[0041] <Negative electrode> The negative electrode may include a negative electrode active material and a negative electrode current collector. As the negative electrode active material in the negative electrode, at least one selected from the group consisting of metallic lithium, lithium-containing alloys, metals or alloys capable of alloying with lithium, oxides capable of doping and dedoping lithium ions, transition metal nitrides capable of doping and dedoping lithium ions, and carbon materials capable of doping and dedoping lithium ions (which may be used alone or as a mixture containing two or more of these) can be used. Examples of metals or alloys capable of alloying with lithium (or lithium ions) include silicon, silicon alloys, tin, tin alloys, and the like. Examples of oxides capable of doping and dedoping lithium ions include lithium titanate, silicon oxide (preferably SiOx (X represents 0.5 or more and less than 1.6), more preferably SiO), and the like. Among these, from the viewpoint of further improving the film-forming property on the negative electrode and further reducing the resistance of the battery after initial use and / or storage, a carbon material capable of doping and dedoping lithium ions is preferable. Examples of such carbon materials include carbon black, activated carbon, graphite materials (artificial graphite, natural graphite), amorphous carbon materials, and the like. The form of the above carbon materials may be any of fibrous, spherical, potato-shaped, and flaky forms.

[0042] Specific examples of the above amorphous carbon materials include hard carbon, coke, mesocarbon microbeads (MCMB) fired at 1500 °C or lower, mesophase pitch carbon fiber (MCF), and the like. Examples of the above graphite materials include natural graphite and artificial graphite. As artificial graphite, graphitized MCMB, graphitized MCF, and the like are used. In addition, as the graphite material, those containing boron can also be used. Further, as the graphite material, those coated with metals such as gold, platinum, silver, copper, and tin, those coated with amorphous carbon, and those mixed with amorphous carbon and graphite can also be used.

[0043] These carbon materials may be used alone or in combination of two or more. As the carbon material, a carbon material having an interlayer spacing d(002) of the (002) plane measured by X-ray analysis of 0.340 nm or less is particularly preferable. Further, as the carbon material, graphite having a true density of 1.70 g / cm 3 or a highly crystalline carbon material having properties similar thereto is also preferable. When using a carbon material as described above, the energy density of the battery can be made higher.

[0044] There is no particular limitation on the material of the negative electrode current collector in the negative electrode, and known materials can be arbitrarily used. Specific examples of the negative electrode current collector include metal materials such as copper, nickel, stainless steel, and nickel-plated steel. Among them, copper is particularly preferable from the viewpoint of ease of processing.

[0045] The negative electrode may include a negative electrode current collector and a negative electrode active material layer provided on at least a part of the surface of the negative electrode current collector. The negative electrode active material layer contains at least one kind of negative electrode active material. The negative electrode active material in the negative electrode active material layer preferably contains the carbon material described above. The content of the carbon material in the negative electrode active material layer is preferably 70% by mass or more, more preferably 80% by mass or more, and still more preferably 90% by mass or more, based on the total amount of the negative electrode active material layer, from the viewpoint of further improving the film-forming property with respect to the negative electrode and further reducing the resistance of the battery after initial use and / or storage. The negative electrode active material layer may further contain at least one kind of binder. As the binder, at least one selected from the group consisting of styrene-butadiene (SBR) rubber (for example, SBR latex), acrylonitrile-butadiene rubber, acrylonitrile-butadiene-styrene rubber, carboxymethyl cellulose (CMC), hydroxypropyl methyl cellulose, polyvinyl alcohol, hydroxypropyl cellulose, and diacetyl cellulose is preferable. The binder preferably contains SBR latex and carboxymethyl cellulose. The content of the binder in the negative electrode active material layer is preferably 1% by mass to 20% by mass, more preferably 1% by mass to 10% by mass, and still more preferably 1% by mass to 5% by mass with respect to the total amount of the negative electrode active material layer.

[0046] The content of Si with respect to the entire negative electrode is preferably 5% by mass or less. When the content of Si with respect to the entire negative electrode is 5% by mass or less, the formability of the film with respect to the negative electrode is further improved, and the resistance of the battery in the initial stage and / or after storage is further reduced.

[0047] <Positive Electrode> The positive electrode may include a positive electrode active material and a positive electrode current collector. Examples of the positive electrode active material in the positive electrode include transition metal oxides or transition metal sulfides such as MoS2, TiS2, MnO2, and V2O5, LiCoO2, LiMnO2, LiMn2O4, LiNiO2, LiNi X Co (1-X) O2 [0 < X < 1], Li having an α-NaFeO2 type crystal structure 1+α Me 1-α O2 (Me is a transition metal element including Mn, Ni, and Co, 1.0 ≦ (1 + α) / (1 - α) ≦ 1.6), LiNi x Co y Mn z O2 [x + y + z = 1, 0 < x < 1, 0 < y < 1, 0 < z < 1] (for example, LiNi 0.33 Co 0.33 Mn 0.33 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, etc.), composite oxides composed of lithium and transition metals such as LiFePO4 and LiMnPO4, conductive polymer materials such as polyaniline, polythiophene, polypyrrole, polyacetylene, polyacene, dimercaptothiadiazole, and polyaniline composites. Among these, composite oxides composed of lithium and transition metals are particularly preferred. When the negative electrode is a lithium metal or a lithium alloy, a carbon material can also be used as the positive electrode. Further, a mixture of a composite oxide of lithium and a transition metal and a carbon material can also be used as the positive electrode. The positive electrode active material may be used alone or in combination of two or more kinds. When the positive electrode active material has insufficient conductivity, it can be used together with a conductivity aid to form a positive electrode. Examples of the conductivity aid include carbon materials such as carbon black, amorphous whiskers, and graphite.

[0048] There is no particular limitation on the material of the positive electrode current collector in the positive electrode, and known materials can be arbitrarily used. Specific examples of the positive electrode current collector include, for example, metal materials such as aluminum, aluminum alloy, stainless steel, nickel, titanium, and tantalum; carbon materials such as carbon cloth and carbon paper; and the like.

[0049] The positive electrode may include a positive electrode current collector and a positive electrode active material layer provided on at least a part of the surface of the positive electrode current collector. The positive electrode active material layer contains at least one kind of positive electrode active material. The positive electrode active material in the positive electrode active material layer preferably contains a composite oxide composed of lithium and a transition metal. The content of the composite oxide in the positive electrode active material layer is preferably 70% by mass or more, more preferably 80% by mass or more, based on the total amount of the positive electrode active material layer. The positive electrode active material layer may further contain at least one kind of the above-mentioned conductivity aid. The positive electrode active material layer may further contain at least one kind of binder. Examples of the binder include polyvinyl acetate, polymethyl methacrylate, nitrocellulose, fluororesin, rubber particles, etc. Examples of the fluororesin include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), vinylidene fluoride-hexafluoropropylene copolymer, etc. Examples of the rubber particles include styrene-butadiene rubber particles, acrylonitrile rubber particles, etc. Among these, from the viewpoint of improving the oxidation resistance of the positive electrode active material layer, fluororesin is preferred. The content of the binder in the positive electrode active material layer is preferably 1% by mass to 20% by mass, more preferably 1% by mass to 10% by mass, based on the total amount of the positive electrode active material layer.

[0050] <Separator> The lithium secondary battery precursor of the present disclosure preferably includes a separator between the negative electrode and the positive electrode. The separator is a film that electrically insulates the positive electrode and the negative electrode and allows lithium ions to permeate, and examples thereof include a porous film and a polymer electrolyte. As the porous film, a microporous polymer film is preferably used, and examples of the material include polyolefin, polyimide, polyvinylidene fluoride, polyester, and the like. In particular, a porous polyolefin is preferable, and specifically, a porous polyethylene film, a porous polypropylene film, or a multilayer film of a porous polyethylene film and a polypropylene film can be exemplified. Another resin excellent in thermal stability may be coated on the porous polyolefin film. Examples of the polymer electrolyte include a polymer in which a lithium salt is dissolved and a polymer swollen with an electrolyte solution. The non-aqueous electrolyte of the present disclosure may be used for the purpose of swelling a polymer to obtain a polymer electrolyte.

[0051] <Configuration of Lithium Secondary Battery Precursor> The lithium secondary battery precursor of the present disclosure can take various known shapes and can be formed into a cylindrical shape, a coin shape, a square shape, a laminate shape, a film shape, or any other shape. However, the basic structure of the lithium secondary battery precursor is the same regardless of the shape, and design changes can be made according to the purpose. Regarding the basic structure of the lithium secondary battery of the present disclosure, the basic structure of the lithium secondary battery precursor of the present disclosure can also be referred to.

[0052] An example of the lithium secondary battery precursor or the lithium secondary battery of the present disclosure is a laminate battery. FIG. 1 is a schematic perspective view showing an example of a laminated battery, which is an example of a lithium secondary battery precursor or a lithium secondary battery of the present disclosure, and FIG. 2 is a schematic cross-sectional view in the thickness direction of a laminated electrode body housed in the laminated battery shown in FIG. 1. The laminated battery shown in FIG. 1 includes a laminated outer package 1 in which a non-aqueous electrolyte (not shown in FIG. 1) and a laminated electrode body (not shown in FIG. 1) are housed, and the periphery is sealed so that the inside is sealed. As the laminated outer package 1, for example, a laminated outer package made of aluminum is used. As shown in FIG. 2, the laminated electrode body housed in the laminated outer package 1 includes a laminate in which a positive electrode plate 5 and a negative electrode plate 6 are alternately laminated via a separator 7, and a separator 8 surrounding the periphery of this laminate. The positive electrode plate 5, the negative electrode plate 6, the separator 7, and the separator 8 are impregnated with the non-aqueous electrolyte of the present disclosure. A plurality of positive electrode plates 5 in the laminated electrode body are all electrically connected to a positive electrode terminal 2 via a positive electrode tab (not shown), and a part of the positive electrode terminal 2 protrudes outward from the peripheral end of the laminated outer package 1 (FIG. 1). The portion where the positive electrode terminal 2 protrudes at the peripheral end of the laminated outer package 1 is sealed by an insulating seal 4. Similarly, a plurality of negative electrode plates 6 in the laminated electrode body are all electrically connected to a negative electrode terminal 3 via a negative electrode tab (not shown), and a part of the negative electrode terminal 3 protrudes outward from the peripheral end of the laminated outer package 1 (FIG. 1). The portion where the negative electrode terminal 3 protrudes at the peripheral end of the laminated outer package 1 is sealed by an insulating seal 4. In the laminated battery according to the above example, the number of positive electrode plates 5 is 5, the number of negative electrode plates 6 is 6, and the positive electrode plate 5 and the negative electrode plate 6 are laminated via a separator 7 in such an arrangement that the outermost layers on both sides are both negative electrode plates 6. However, it goes without saying that the number of positive electrode plates, the number of negative electrode plates, and the arrangement in the laminated battery are not limited to this example, and various modifications may be made.

[0053] As another example of the lithium secondary battery precursor or the lithium secondary battery of the present disclosure, a coin-type battery is also included. FIG. 3 is a schematic perspective view showing an example of a coin-type battery which is another example of the lithium secondary battery precursor or the lithium secondary battery of the present disclosure. In the coin-type battery shown in FIG. 3, a disk-shaped negative electrode 12, a separator 15 injected with a non-aqueous electrolyte, a disk-shaped positive electrode 11, and, if necessary, spacer plates 17 and 18 such as stainless steel or aluminum are laminated in this order and housed between a positive electrode can 13 (hereinafter also referred to as "battery can") and a sealing plate 14 (hereinafter also referred to as "battery can lid"). The positive electrode can 13 and the sealing plate 14 are caulked and sealed via a gasket 16. In this example, the non-aqueous electrolyte of the present disclosure is used as the non-aqueous electrolyte injected into the separator 15.

[0054] The use of the lithium secondary battery precursor or the lithium secondary battery of the present disclosure is not particularly limited, and it can be used for various known uses. For example, it can be widely used for various devices regardless of whether they are small portable devices such as notebook computers, mobile computers, mobile phones, headphone stereos, video movies, liquid crystal TVs, handy cleaners, electronic notebooks, calculators, radios, backup power supplies, motors, automobiles, electric vehicles, motorcycles, electric motorcycles, bicycles, electric bicycles, lighting fixtures, game machines, watches, power tools, cameras, etc., or large devices.

[0055] 〔Method for manufacturing lithium secondary battery〕 The method for manufacturing the lithium secondary battery of the present disclosure includes a step of preparing the lithium secondary battery precursor of the present disclosure described above (hereinafter also referred to as "preparation step"), and a step of charging and discharging the lithium secondary battery precursor. and includes.

[0056] The preparation step may be a step of simply preparing the lithium secondary battery precursor of the present disclosure manufactured in advance for use in the step of charging and discharging, or may be a step of manufacturing the lithium secondary battery precursor of the present disclosure. The lithium secondary battery precursor is as described above.

[0057] In the process of charging and discharging, the charging and discharging of the lithium secondary battery precursor can be performed according to a known method. In this process, the charging and discharging cycles may be repeated a plurality of times for the lithium secondary battery precursor. As described above, preferably, a SEI film is formed on the surface of the positive electrode (especially the positive electrode active material) and / or the negative electrode (especially the negative electrode active material) in the lithium secondary battery precursor by this charging and discharging.

Example

[0058] Hereinafter, embodiments according to the present disclosure will be described in detail with reference to examples. Note that the present disclosure is not limited to the descriptions of these examples at all.

[0059] 〔Example 1〕 A lithium secondary battery precursor was produced as follows.

[0060] <Preparation of non-aqueous electrolyte> Ethylene carbonate (EC), dimethyl carbonate (DMC), and methyl ethyl carbonate (EMC) were mixed at a ratio of 30:35:35 (volume ratio), respectively. Thereby, a mixed solvent as a non-aqueous solvent was obtained. LiPF6 as an electrolyte was dissolved in the obtained mixed solvent so that the concentration of LiPF6 in the finally obtained non-aqueous electrolyte was 1.2 mol / L, and a first solution was obtained. Lithium difluorophosphate as an additive was added to the obtained first solution so that the concentration of lithium difluorophosphate in the finally obtained non-aqueous electrolyte was 1.0 mass%, and a second solution was obtained. 1,2-Dimethoxyethane as a chain ether solvent was added to the obtained second solution so that the concentration of 1,2-dimethoxyethane in the finally obtained non-aqueous electrolyte was 30 mass ppm, and a non-aqueous electrolyte was obtained.

[0061] <Fabrication of the positive electrode> As the positive electrode active material, Li(Ni 0.5 Co 0.2 Mn 0.3 O2) 94% by mass, 3% by mass of carbon black as a conductive assistant, and 3% by mass of polyvinylidene fluoride (PVdF) as a binder were added to obtain a mixture. The obtained mixture was dispersed in an N-methylpyrrolidone solvent to obtain a positive electrode mixture slurry. An aluminum foil with a thickness of 20 μm was prepared as the positive electrode current collector. The obtained positive electrode mixture slurry was coated on the aluminum foil, dried, and then rolled with a press to obtain a sheet-shaped positive electrode. The obtained positive electrode consists of a positive electrode current collector and a positive electrode active material layer.

[0062] <Fabrication of the negative electrode> As the negative electrode active material, graphite (96% by mass), 1% by mass of carbon black as a conductive assistant, 1% by mass of sodium carboxymethyl cellulose dispersed in pure water as a thickener, and 2% by mass of styrene-butadiene rubber (SBR) dispersed in pure water as a binder were mixed to obtain a negative electrode mixture slurry. A copper foil with a thickness of 10 μm was prepared as the negative electrode current collector. The obtained negative electrode mixture slurry was coated on the copper foil, dried, and then rolled with a press to obtain a sheet-shaped negative electrode. The obtained negative electrode consists of a negative electrode current collector and a negative electrode active material layer.

[0063] <Preparation of the separator> A porous polyethylene film was prepared as the separator.

[0064] <Fabrication of the coin-type battery> The negative electrode was punched into a disk shape with a diameter of 14 mm, the positive electrode was punched into a disk shape with a diameter of 13 mm, and the separator was punched into a disk shape with a diameter of 17 mm. As a result, a coin-shaped negative electrode, a coin-shaped positive electrode, and a coin-shaped separator were obtained respectively. The obtained coin-shaped negative electrode, coin-shaped separator, and coin-shaped positive electrode were laminated in this order inside a stainless steel battery can (size: 2032 size). Next, 20 μL of a non-aqueous electrolyte was injected into this battery can, and the separator, positive electrode, and negative electrode were impregnated with the non-aqueous electrolyte. Next, a plate made of aluminum (thickness: 1.2 mm, diameter: 16 mm) and a spring were placed on the positive electrode, and the battery can lid was caulked through a gasket made of polypropylene to seal the battery. As described above, a coin-type lithium secondary battery precursor having the configuration shown in Figure 3 was obtained. The size of the coin-type lithium secondary battery precursor was 20 mm in diameter and 3.2 mm in height.

[0065] [Examples 2 to 5, Comparative Example 1] In the preparation of the non-aqueous electrolyte described above, a non-aqueous electrolyte and a lithium secondary battery precursor were obtained in the same manner as in Example 1, except that the content of 1,2-dimethoxyethane as an additive in the non-aqueous electrolyte was added to be the amount shown in Table 1.

[0066] [High-temperature storage test] Using the non-aqueous electrolyte obtained in the <Preparation of non-aqueous electrolyte> described above, a high-temperature storage test was conducted. The non-aqueous electrolyte used in the high-temperature storage test was different from the non-aqueous electrolyte injected into the battery can in the <Manufacture of coin-type battery> described above. The non-aqueous electrolyte was placed in a sealed container and left in a constant temperature bath at 60 °C for one week to obtain an electrolyte after high-temperature storage in the non-aqueous electrolyte. The APHA color of the obtained electrolyte after high-temperature storage was measured with a petroleum product color tester "OME 2000" (manufactured by Nippon Denshoku Industries Co., Ltd.), and the coloring state of the electrolyte after high-temperature storage was evaluated. Note that the APHA color indicates the Hazen unit color number, which is an index indicating the strength of the color tone. The Hazen unit color number indicates the scale defined in JIS K0071-1:2017, which is "taking the color of a solution containing 1 mg of platinum in the form of hexachloroplatinate ions and 2 mg of cobalt(II) chloride hexahydrate in 1 L as 1". The APHA color quantitatively indicates the coloring state of the electrolyte after high-temperature storage. The larger the numerical value of the APHA color of the electrolyte after high-temperature storage, the greater the coloring of the electrolyte after high-temperature storage.

[0067] After high-temperature storage, the APHA was defined as the relative value of the APHA color of the electrolyte after high-temperature storage to the APHA color of the electrolyte after high-temperature storage in Comparative Example 1, as shown in the following formula (X1), and was designated as "APHA after high-temperature storage [%]".

[0068] APHA after high-temperature storage [relative value; %] = (APHA color of the electrolyte after high-temperature storage / APHA color of the electrolyte after high-temperature storage in Comparative Example 1) × 100…(X1)

[0069] The measurement results of the electrolyte storage test are shown in Table 1.

[0070] 〔Battery evaluation test〕 Using the above-mentioned lithium secondary battery precursor, a battery evaluation test was conducted. The following aging treatment was performed on the lithium secondary battery precursor to obtain a first battery. The obtained first battery was subjected to the following initial charge-discharge treatment to obtain a second battery. The obtained second battery was subjected to the following DC resistance evaluation treatment to obtain a third battery. Using the obtained third battery, the battery resistance (%) was measured by the following measurement method. The measurement results are shown in Table 1.

[0071] <Aging treatment> The following aging treatment was performed on the lithium secondary battery precursor to obtain a first battery.

[0072] The battery precursor was charged in the temperature range of 25 to 70 °C and the termination voltage range of 1.5 V to 3.5 V, and then rested in the range of 5 to 50 hours. Next, the battery precursor was charged in the temperature range of 25 to 70 °C and the termination voltage range of 3.5 V to 4.2 V, and held in the range of 5 to 50 hours. Next, the battery precursor was charged to 4.2 V in the temperature range of 25 to 70 °C, and then discharged to 2.5 V.

[0073] <Initial charge-discharge treatment> The following initial charge-discharge treatment was performed on the first battery to obtain a second battery.

[0074] The first battery was held in a temperature environment of 25°C for 12 hours. Subsequently, the first battery was charged at a charging rate of 0.2C to 4.2V (SOC (State Of Charge): 100%) by constant current constant voltage charging (0.2C-CCCV), then rested for 30 minutes, and then discharged at a discharge rate of 0.2C to 2.5V by constant current discharge (0.2C-CC). This was performed for 3 cycles to stabilize the battery. Thereafter, it was charged at a charging rate of 0.2C to 4.2V by constant current constant voltage charging (0.5C-CCCV), then rested for 30 minutes, and then discharged at a discharge rate of 1C to 2.5V by constant current discharge (1C-CC). Thereby, a second battery was obtained.

[0075] <DC Resistance Evaluation Process> The following DC resistance evaluation process was performed on the second battery to obtain a third battery.

[0076] The DC resistance evaluation process was carried out in a temperature environment of 25°C. The second battery was discharged at a discharge rate of 0.2C to 2.5V by CC discharge and charged at a charging rate of 0.2C to 3.7V by CCCV charging. "CCCV charging" means charging at constant current constant voltage.

[0077] Subsequently, the second battery was subjected to CC 10s discharge at a discharge rate of 0.2C and CC 10s charging at a charging rate of 0.2C. "CC 10s discharge" means discharging at constant current for 10 seconds. "CC 10s charging" means charging at constant current for 10 seconds.

[0078] Subsequently, the second battery was subjected to CC 10s discharge at a discharge rate of 0.5C and CC 25s charging at a charging rate of 0.2C. Subsequently, the second battery was subjected to CC 10s discharge at a discharge rate of 1C and CC 50s charging at a charging rate of 0.2C. Subsequently, the second battery was subjected to CC 10s discharge at a discharge rate of 2C and CC 100s charging at a charging rate of 0.2C. Thereby, a third battery was obtained.

[0079] As shown in the following formula (X1), the relative value of the DC resistance of the third battery with respect to the DC resistance of the third battery in Comparative Example 1 was defined as "battery resistance [%]".

[0080] Battery resistance [relative value; %] = (DC resistance of the third battery [Ω] / DC resistance of the third battery in Comparative Example 1 [Ω]) × 100…(X1)

[0081] The DC resistance was measured by the following method. Based on the voltage drop amount (= voltage before discharge - voltage at the 10th second after the start of discharge) due to "CC10s discharge" at each discharge rate of 0.2C to 2C and each current value (i.e., each current value corresponding to the discharge rate of 0.2C to 2C), the DC resistance (Ω) of the third battery was determined.

[0082]

Table 1

[0083] In Table 1, "LiMFP" represents lithium monofluorophosphate, "LiDFP" represents lithium difluorophosphate, and "DME" represents 1,2-dimethoxyethane.

[0084] The non-aqueous electrolytes of Examples 1 to 5 contained lithium difluorophosphate and 1,2-dimethoxyethane as a chain ether compound. The content of 1,2-dimethoxyethane was more than 0 mass ppm and 1000 mass ppm or less with respect to the total amount of the non-aqueous electrolyte. Therefore, the battery resistance of Examples 1 to 5 was 72% or less. Thus, it was found that the non-aqueous electrolytes of Examples 1 to 5 can suppress an increase in the DC resistance of a lithium secondary battery due to charging and discharging under normal temperature environment.

[0085] Furthermore, for the non-aqueous electrolytes of Examples 1 to 5, the APHA was 95% or less after high-temperature storage. As a result, it was found that the non-aqueous electrolytes of Examples 1 to 5 are less likely to be colored even when stored for a long time in a high-temperature environment. In other words, it was found that the progress of the decomposition reaction of the non-aqueous electrolytes of Examples 1 to 5 is suppressed more than that of the non-aqueous electrolyte of Comparative Example 1. As a result, it was found that even when the lithium secondary batteries of Examples 1 to 5 are stored at high temperature for a long time, the decrease in the battery performance of the lithium secondary batteries can be suppressed more than that of the lithium secondary battery of Comparative Example 1. In the non-aqueous electrolytes of Example 1, Example 2, and Example 3, the content of 1,2-dimethoxyethane was in the range of more than 0 mass ppm and 300 mass ppm or less. Therefore, it was found that the progress of the decomposition reaction of the non-aqueous electrolytes of Example 1, Example 2, and Example 3 is suppressed more than that of the non-aqueous electrolytes of Example 4 and Example 5. In particular, in the non-aqueous electrolytes of Example 2 and Example 3, the content of 1,2-dimethoxyethane was in the range of 40 mass ppm or more and 300 mass ppm or less. Therefore, it was found that the progress of the decomposition reaction of the non-aqueous electrolytes of Example 2 and Example 3 is suppressed more than that of the non-aqueous electrolytes of Example 1, Example 4, and Example 5.

Explanation of Signs

[0086] 1 Laminate exterior body 2 Positive electrode terminal 3 Negative electrode terminal 4 Insulating seal 5 Positive electrode plate 6 Negative electrode plate 7, 8 Separator 11 Positive electrode 12 Negative electrode 13 Positive electrode can 14 Sealing plate 15 Separator 16 Gasket 17, 18 Spacer plate

Claims

1. Lithium difluorophosphate and, a chain ether compound and a non-aqueous electrolyte containing the same, wherein the content of the chain ether compound is more than 0 mass ppm and 1000 mass ppm or less with respect to the total amount of the non-aqueous electrolyte, the content of lithium difluorophosphate is 0.1 mass % to 3 mass % with respect to the total amount of the non-aqueous electrolyte, and the chain ether compound is 1,2-dimethoxyethane, the non-aqueous electrolyte.

2. a positive electrode, a negative electrode containing a negative electrode active material capable of occluding and releasing lithium ions, and the non-aqueous electrolyte according to Claim 1 a lithium secondary battery precursor comprising the same.

3. A lithium secondary battery obtained by charging and discharging the lithium secondary battery precursor according to Claim 2.

4. A method for manufacturing a lithium secondary battery, comprising the step of preparing the lithium secondary battery precursor according to Claim 2, and the step of charging and discharging the lithium secondary battery precursor. ​

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