Electrolyte and nonaqueous electrolyte secondary battery

Abstract

This invention provides an electrolyte that does not easily experience a decrease in discharge capacity under high current conditions and can achieve a high capacity retention rate, as well as a non-aqueous electrolyte secondary battery using this electrolyte. The invention is an electrolyte containing a carboxylic acid ester and compounds having hydroxyl groups and / or compounds having ether groups, wherein the content of the aforementioned compounds having hydroxyl groups and / or compounds having ether groups is less than 50 ppm by mass.

Description

Technical Field

[0001] The present invention relates to an electrolyte and a non-aqueous electrolyte secondary battery having the electrolyte. Background Technology

[0002] With the rapid expansion of the market for laptops, mobile phones, electric vehicles, and other products, there is a growing demand for high-energy-density rechargeable batteries. As a new type of high-energy-density rechargeable battery, lithium-ion batteries, which utilize a non-aqueous electrolyte containing a solute dissolved in a non-aqueous solvent and leverage the high electromotive force of lithium oxidation and reduction, are beginning to gain traction.

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Application Publication No. 2005-50585 Summary of the Invention

[0006] The problem that the invention aims to solve

[0007] However, in the past, there was a problem that the negative electrode could not achieve sufficient preservation characteristics when stored in a charged state due to the reaction between the negative electrode and the non-aqueous electrolyte.

[0008] In particular, in recent years, lithium secondary batteries, as mentioned above, have begun to be used as power sources for backup storage in electronic devices such as mobile phones. However, there is a problem that the storage characteristics of lithium secondary batteries are further significantly reduced due to exposure to high temperatures.

[0009] In response to this, for example, Patent Document 1 discloses a method for adding at least one additive compound selected from alcohols, aldehydes and carboxylic acid esters to a non-aqueous electrolyte.

[0010] However, when additive compounds are added as described in Patent Document 1, there is a problem that the discharge capacity of the secondary battery decreases as the current value increases. In particular, there is a significant decrease in capacity retention at high current values.

[0011] The purpose of this invention is to provide an electrolyte that does not easily experience a decrease in discharge capacity under high current conditions and can achieve a high capacity retention rate, as well as a non-aqueous electrolyte secondary battery using the electrolyte.

[0012] Methods for solving problems

[0013] The present invention is an electrolyte containing a carboxylic acid ester and a compound having a hydroxyl group and / or a compound having an ether group, wherein the content of the compound having a hydroxyl group and / or the compound having an ether group is less than 50 ppm by mass.

[0014] The present invention will now be described in detail.

[0015] The inventors conducted in-depth research and discovered that by adding a specified amount of a compound with a specified functional group to a carboxylic acid ester, an electrolyte that does not easily experience a decrease in discharge capacity under high current conditions and can achieve a high capacity retention rate can be prepared, thus completing the present invention.

[0016] The electrolyte in one embodiment of the present invention contains a carboxylic acid ester. By featuring the presence of the aforementioned carboxylic acid ester in the electrolyte as one of its characteristics, a high capacity retention rate can be achieved not only at room temperature but also at low temperatures. The aforementioned carboxylic acid ester is not particularly limited, but preferably has the structure shown in the general formula R-COOR' (R represents a hydrogen atom or a straight-chain or branched hydrocarbon group having 1 to 10 carbon atoms, and R' represents a straight-chain or branched hydrocarbon group having 1 to 10 carbon atoms).

[0017] More preferably, R and R' are straight-chain alkyl groups. The carboxylic acid ester can be one type or a mixture of two or more types.

[0018] Furthermore, the number of carbon atoms in R and R' is preferably 1 or more and 8 or less, more preferably 5 or less, even more preferably 3 or less, and particularly preferably 2 or less.

[0019] Examples of the aforementioned carboxylic acid esters include saturated aliphatic carboxylic acid esters such as formate, acetate, propionate, and butyrate; and unsaturated aliphatic carboxylic acid esters such as acrylate, methacrylate, and maleate. Among these, acetate and propionate are preferred, methyl acetate and methyl propionate are more preferred, and methyl acetate is even more preferred.

[0020] The content of the aforementioned carboxylic acid ester in the electrolyte is preferably 3% by mass or more and 40% by mass or less. By making the content of the aforementioned carboxylic acid ester 3% by mass or more, the viscosity of the electrolyte can be reduced and the mobility of lithium ions in the electrolyte can be increased. By making it 40% by mass or less, stable charge and discharge can be performed without disrupting the solvation of the electrolyte. The content of the aforementioned carboxylic acid ester is more preferably 4% by mass or more and 35% by mass or less, even more preferably 6% by mass or more and 30% by mass or less, even more preferably 8% by mass or more and 25% by mass or less, and particularly preferably 10% by mass or more and 20% by mass or less.

[0021] The boiling point of the carboxylic acid ester in the electrolyte is preferably 35°C or higher and 90°C or lower, more preferably 40°C or higher and 80°C or lower, and even more preferably 50°C or higher and 70°C or lower. If the boiling point of the carboxylic acid ester in the electrolyte is within the above range, the viscosity of the electrolyte can be reduced, and a higher capacity retention rate can be achieved under high current conditions. If the boiling point of the carboxylic acid ester in the electrolyte is above the lower limit of the above range, the flash point of the electrolyte becomes higher, resulting in improved thermal safety.

[0022] It should be noted that the above boiling points can be determined at atmospheric pressure using commercially available boiling point measuring devices, such as the FP81HT / FP81C (manufactured by METTLER TOLEDO Co., Ltd.). Boiling points are described, for example, in documents such as the "CRC Handbook of Chemistry and Physics" and the "Aldrich Handbook of Fine Chemicals and Laboratory Equipment".

[0023] The melting point of the carboxylic acid ester in the electrolyte is preferably below -50°C, more preferably below -70°C, even more preferably below -80°C, and still more preferably below -90°C. The lower limit of the melting point is not particularly limited, but may be above -130°C or above -110°C. If the melting point of the carboxylic acid ester in the electrolyte is within the above range, the viscosity increase of the electrolyte can be suppressed even in environments below the freezing point, and a high capacity retention rate can be achieved even when used in cold regions. It should be noted that the melting point can be measured, for example, using a differential scanning calorimeter (DSC6220, manufactured by Seiko Instruments Co., Ltd.). During the determination, 10 mg of the sample was first added to an aluminum pan, and differential scanning calorimetry (DSC) was performed using a DSC6220 (manufactured by Seiko Instruments Co., Ltd.). The measurement conditions were as follows: the sample was heated from -130°C to 50°C at a rate of 2°C / min. After reaching each set temperature (-130°C, 50°C), the sample was held at the same set temperature for 3 minutes. The first peak that could be confirmed during the cooling process was taken as the melting point. Three cycles were performed, and the data from the last two cycles were used to calculate the average value.

[0024] As one embodiment of the present invention, the electrolyte contains a compound having a hydroxyl group and / or a compound having an ether group. By featuring the above-mentioned electrolyte as one of the above-mentioned compounds, it is possible to produce an electrolyte that is not prone to a decrease in discharge capacity under high current conditions and can achieve a high capacity retention rate.

[0025] In the electrolyte of one embodiment of the present invention, the content of the above-mentioned compounds having hydroxyl groups and / or compounds having ether groups is 50 ppm by mass or less. By setting it to 50 ppm by mass or less, the discharge capacity is less likely to decrease under high current conditions, and a high capacity retention rate can be achieved.

[0026] The content of the aforementioned compounds having hydroxyl groups and / or compounds having ether groups is preferably 1 ppm by mass or more and 40 ppm by mass or less, more preferably 1.2 ppm by mass or more and 35 ppm by mass or less, and even more preferably 1.5 ppm by mass or more and 30 ppm by mass or less. Furthermore, the content of the aforementioned compounds having hydroxyl groups and / or compounds having ether groups is even more preferably 1.7 ppm by mass or more and 25 ppm by mass or less, and particularly preferably 2 ppm by mass or more and 20 ppm by mass or less.

[0027] In this invention, "the content of compounds having hydroxyl groups and / or compounds having ether groups" refers to the content of only one of the compounds having hydroxyl groups and compounds having ether groups when the invention contains only one of the compounds having hydroxyl groups and compounds having ether groups, and to the total content of both when the invention contains both of the compounds having hydroxyl groups and compounds having ether groups.

[0028] In addition, the content of "compounds having hydroxyl groups and / or compounds having ether groups" can be determined, for example, by gas chromatography (GC). It should be noted that, besides being determined by directly measuring the content in the electrolyte, the above-mentioned content can also be determined by calculating it based on the composition ratio with other components after determining the content in a specific raw material.

[0029] Examples of compounds containing hydroxyl groups include at least one selected from primary, secondary, and tertiary alcohols. From the viewpoint of oxidizing power, primary alcohols are preferred. Additionally, polyols such as diols and triols can also be used as compounds containing hydroxyl groups.

[0030] Furthermore, the carbon number of the aforementioned compounds having hydroxyl groups is preferably 1 or more, and preferably 10 or less, more preferably 8 or less, even more preferably 5 or less, even more preferably 3 or less, and particularly preferably 2 or less.

[0031] These compounds containing hydroxyl groups can be used in one or in combination of two or more. The aforementioned compounds containing hydroxyl groups can be linear or branched. These compounds can have one, two, or three or more hydroxyl groups.

[0032] It should be noted that compounds containing a hydroxyl group can be represented by the general formula R-(OH). n (where R is an optional hydrocarbon group with substituents, and n is an integer of 1 or more). The compound having a hydroxyl group is preferably of the general formula R-OH (where R is an optional hydrocarbon group with substituents).

[0033] Examples of primary alcohols include methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, 1-heptanol, and 1-octanol. Methanol is preferred.

[0034] Examples of secondary alcohols mentioned above include isopropanol, 2-butanol, cyclopentanol, and cyclohexanol.

[0035] Examples of the aforementioned tertiary alcohols include 1-adamantanol, tert-butanol, and tert-amyl alcohol.

[0036] In the electrolyte according to one embodiment of the present invention, the content of the aforementioned compound having hydroxyl groups is preferably 1 ppm by mass or more and 40 ppm by mass or less. By setting it within the above range, an electrolyte with higher capacity retention can be provided. This is believed to be because a stable SEI film (Solid Electrolyte Interphase) can be formed on the surface of the negative electrode.

[0037] The content of the aforementioned hydroxyl-containing compound is more preferably 1.2 ppm or more and 35 ppm or less by mass, further preferably 1.5 ppm or more and 30 ppm or less by mass, even more preferably 1.7 ppm or more and 25 ppm or less by mass, and particularly preferably 2 ppm or more and 20 ppm or less by mass.

[0038] Examples of compounds containing an ether group include monoether compounds, diether compounds, and triether compounds. Among these, diether compounds are preferred.

[0039] In addition to non-cyclic ether compounds, cyclic ether compounds can also be used as the compounds with the ether group mentioned above.

[0040] Furthermore, the number of carbon atoms in the aforementioned compounds having an ether group is preferably 2 or more and 10 or less, more preferably 3 or more and 8 or less, and even more preferably 4 or more and 6 or less.

[0041] These compounds with ether groups can be used in one or more combinations.

[0042] In addition, the boiling point of the above-mentioned compounds having an ether group is preferably -30°C or higher and 150°C or lower, more preferably -10°C or higher and 120°C or lower, even more preferably 10°C or higher and 90°C or lower, and even more preferably 30°C or higher and 70°C or lower.

[0043] Examples of monoether compounds mentioned above include dialkyl ethers such as dimethyl ether, diethyl ether, diisopropyl ether, and dibutyl ether, as well as monoether compounds of polyalkylene glycols.

[0044] Examples of the aforementioned diether compounds include dimethoxymethane, methoxyethoxymethane, diethoxymethane, 1,1-dimethoxyethane, 1,2-dimethoxyethane, methoxyethoxyethane, diethoxyethane, ethylene glycol di-n-propyl ether, and ethylene glycol di-n-butyl ether. 1,1-dimethoxyethane is preferred.

[0045] Examples of the aforementioned triether compounds include trialkyl ethers of tri-membered alcohols. It should be noted that, as the aforementioned compounds containing ether groups, a structure in which multiple ether groups are bonded to the same carbon atom is preferred.

[0046] In the electrolyte according to one embodiment of the present invention, the content of the ether-containing compound is preferably 1 ppm by mass or more and 40 ppm by mass or less. By setting it within the above range, an electrolyte with higher capacity retention can be provided. This is believed to be because a stable SEI film (Solid Electrolyte Interphase) can be formed on the surface of the negative electrode.

[0047] The content of the above-mentioned ether-containing compound is more preferably 1.2 ppm or more and 35 ppm or less by mass, further preferably 1.5 ppm or more and 30 ppm or less by mass, even more preferably 1.7 ppm or more and 25 ppm or less by mass, and particularly preferably 2 ppm or more and 20 ppm or less by mass.

[0048] In the above electrolyte, the mass ratio of the hydroxyl-containing compound to the ether-containing compound (content of the hydroxyl-containing compound / content of the ether-containing compound) is preferably 0.1 or more and 10 or less. By setting it within the above range, an electrolyte with higher capacity retention can be provided. Although not limited to a specific theory, it is believed that this is because an SEI film that facilitates lithium ion insertion and detachment can be formed on the surface of the negative electrode.

[0049] The above-mentioned mass ratio is more preferably 0.12 or more and 8 or less, even more preferably 0.15 or more and 5 or less, even more preferably 0.17 or more and 3 or less, particularly preferably 0.2 or more and 2 or less, and particularly more preferably 0.25 or more and 1 or less.

[0050] As the solvent used in the electrolyte, a non-aqueous solvent can be used, and examples of non-aqueous solvents include organic solvents.

[0051] As for the aforementioned organic solvents, there are no particular limitations as long as they are non-protic solvents. Examples include carbonates, esters, ethers, lactones, nitriles, amides, and sulfones. More specifically, examples include propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, dimethoxyethane, acetonitrile, propionitrile, tetrahydrofuran, 2-methyltetrahydrofuran, dioxane, nitromethane, N,N-dimethylformamide, dimethyl sulfoxide, sulfolane, and γ-butyrolactone. From the viewpoint of ionic conductivity, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, and methyl ethyl carbonate are preferred; from the viewpoint of salt dissociation, ethylene carbonate is more preferred. One or more of the aforementioned solvents may be used.

[0052] Among these, carbonates are preferred as solvents, and mixed solvents containing ethylene carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) are more preferred. Furthermore, when using the above-mentioned mixed solvent, the mixing ratio (EC:EMC:DEC) is preferably 2–4 / 2–4 / 2–6 by volume, more preferably 2.5–3.5 / 2.5–3.5 / 3–5, and even more preferably 2.8–3.2 / 2.8–3.2 / 3.6–4.4, for example, a ratio of 3:3:4.

[0053] It should be noted that the solvents mentioned above are different from the carboxylic acid esters, compounds with ether groups, and compounds with hydroxyl groups mentioned above.

[0054] The electrolyte used in the above-mentioned electrolyte solution is not particularly limited as long as it has the effects of the present invention. For example, metal ions or their salts can be cited. Among them, metal ions or their salts belonging to Group 1 or Group 2 of the periodic table are preferred. Specifically, lithium salts, potassium salts, sodium salts, calcium salts, magnesium salts, etc. can be cited. Among them, lithium salts are preferred from the viewpoint of output power.

[0055] There are no particular limitations on the lithium salts mentioned above, and examples include lithium hexafluoride phosphate (LiPF6), lithium boron tetrafluoride (LiBF4), lithium bis(trifluoromethanesulfonyl)imide (LiN(SO2CF3)2), lithium bis(pentafluoroethanesulfonyl)imide (LiN(SO2CF2CF3)2), lithium perchlorate (LiClO4), lithium trifluoromethanesulfonate (LiCF3SO3), lithium antimony hexafluoride (LiSbF6), lithium arsenic hexafluoride (LiAsF6), lithium tetraphenylborate (LiB(C6H5)4), LiC(SO2CF3)3, LiPF4(CF3)2, LiPF3(CF3)3, LiPF3(CF2CF3)3, LiPF3(CF(CF3)CF3)3, and LiPF5(CF(CF3)CF3). From the viewpoint of ionic conductivity, lithium hexafluoride phosphate (LiPF6) is preferred.

[0056] The electrolytes mentioned above can be used in one or more ways.

[0057] It should be noted that the concentration of the electrolyte in the above-mentioned electrolyte solution is preferably 0.005-5 mol / L, more preferably 0.01-4.5 mol / L, even more preferably 0.05-4 mol / L, even more preferably 0.1-3.5 mol / L, and particularly preferably 0.5-3 mol / L.

[0058] The electrolyte may contain additives other than those described above without impairing the effects of the present invention.

[0059] Examples of such additives include gas generators (so-called overcharge additives), SEI (Solid Electrolyte Interphase) film-forming agents, halogen-based, phosphorus-based, and other flame retardants.

[0060] Examples of phosphorus-based flame retardants include trimethyl phosphate (TMP), triethyl phosphate (TEP), (2,2,2-trifluoroethyl) phosphate (TFP), triphenyl phosphate (TPP), and tricresyl phosphate (TTP).

[0061] It should be noted that, as the flame retardant mentioned above, the phosphate ester compound described in the GS Yuasa Technical Report (June 2005, Vol. 2, No. 1, pp. 26-31) is preferred.

[0062] The electrolyte described above can be obtained by mixing a carboxylic acid ester, a compound having a hydroxyl group and / or a compound having an ether group, and a solvent and electrolyte as needed. The order of addition is not limited; for example, it can be prepared by adding a compound having a hydroxyl group and / or a compound having an ether group to the carboxylic acid ester, followed by mixing the solvent and electrolyte. More specifically, in the case of preparing an electrolyte with a concentration of 1 mol / L, the electrolyte can be obtained by adding a solvent to a solution obtained by mixing a carboxylic acid ester, a compound having a hydroxyl group and / or a compound having an ether group in a prescribed ratio, and then dissolving an electrolyte such as lithium hexafluoride phosphate (LiPF6) to achieve a concentration of 1 mol / L.

[0063] In the above-mentioned electrolyte, when the constituent materials other than the electrolyte are set to 100% by mass, the solvent content is preferably 65-97% by mass, more preferably 70-96% by mass, even more preferably 75-95% by mass, further preferably 75-94% by mass, and particularly preferably 75-90% by mass.

[0064] It should be noted that, in the constituent materials other than the electrolyte, the content of carboxylic acid ester is preferably 3 to 35% by mass, more preferably 4 to 30% by mass, even more preferably 5 to 25% by mass, further preferably 6 to 25% by mass, and particularly preferably 10 to 25% by mass.

[0065] It should be noted that, in the constituent materials other than the electrolyte, the content of compounds having hydroxyl groups and / or compounds having ether groups is preferably less than 50 ppm by mass, more preferably 1 to 47 ppm by mass, more preferably 2 to 43 ppm by mass, more preferably 3 to 38 ppm by mass, even more preferably 3 to 30 ppm by mass, and particularly preferably 3 to 25 ppm by mass.

[0066] In the constituent materials other than the electrolyte, the content of the compound having hydroxyl groups is preferably 1 ppm or more and 40 ppm or less by mass, more preferably 1.2 ppm or more and 35 ppm or less by mass, even more preferably 1.5 ppm or more and 30 ppm or less by mass, even more preferably 1.7 ppm or more and 25 ppm or less by mass, and particularly preferably 2 ppm or more and 20 ppm or less by mass.

[0067] In addition, among the constituent materials other than the electrolyte, the content of the compound having an ether group is preferably 1 ppm or more and 40 ppm or less by mass, more preferably 1.2 ppm or more and 35 ppm or less by mass, even more preferably 1.5 ppm or more and 30 ppm or less by mass, even more preferably 1.7 ppm or more and 25 ppm or less by mass, and particularly preferably 2 ppm or more and 20 ppm or less by mass.

[0068] Based on the aforementioned electrolyte, a positive electrode, a negative electrode, and a spacer are also used, thereby enabling the fabrication of a non-aqueous electrolyte secondary battery. This type of non-aqueous electrolyte secondary battery with an electrolyte is also one aspect of this invention.

[0069] Examples of non-aqueous electrolyte secondary batteries include lithium-ion batteries.

[0070] As another embodiment of the present invention, the non-aqueous electrolyte secondary battery can be manufactured according to known methods. For example, it can be manufactured in a glove-operated box or in a dry air atmosphere using the above-described electrolyte, positive electrode, negative electrode, spacer, etc.

[0071] The aforementioned positive electrode contains at least a positive electrode active material. Examples of such positive electrode active materials include LiCoO2, LiNiO2, LiMnO2, LiMn2O4, and LiNiO2. 0.5 Mn 0.3 Co 0.2 O2, LiNi 0.8 Co 0.15 Al 0.05 O2, LiFePO4, etc. The aforementioned positive electrode may contain conductive additives. Examples of such conductive additives include carbon black (CB) and acetylene black (AB). Additionally, examples of such binders include polyvinylidene fluoride (PVdF).

[0072] The aforementioned negative electrode contains at least a negative electrode active material. Examples of such negative electrode active materials include graphite (artificial graphite, etc.), soft carbon, hard carbon, silicon, silicon oxide, silicon-based alloys, tin, tin oxide, and tin-based alloys. In addition to using PVdF as a binder, carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) can also be used.

[0073] Porous membranes or the like can be used as the aforementioned spacers. Alternatively, porous membranes made of polyethylene (PE), polypropylene (PP), or the like can be used.

[0074] The aforementioned spacers can be single-layer or multi-layer structures.

[0075] Invention Effects

[0076] According to the present invention, an electrolyte that is not prone to a decrease in discharge capacity under high current conditions and can achieve a high capacity retention rate, as well as a non-aqueous electrolyte secondary battery using the electrolyte, can be provided. Detailed Implementation

[0077] The present invention will be further described in detail below with reference to the embodiments provided; however, the present invention is not limited to these embodiments.

[0078] (Example 1)

[0079] (Preparation of carboxylic acid ester compositions)

[0080] First, the prepared methyl acetate (manufactured by Sigma Aldrich) was analyzed using GC (manufactured by Shimadzu Corporation, GC-2010) to confirm that it did not contain hydroxyl groups (R-(OH)). n Compounds containing ether groups, where R is an optional hydrocarbon group with substituents and n is an integer greater than or equal to 1.

[0081] Then, relative to the methyl acetate mentioned above, methanol and 1,1-dimethoxyethane were added to prepare a carboxylic acid ester composition by adding methanol to a content of 23 ppm by mass and 1,1-dimethoxyethane to a content of 115 ppm by mass. It should be noted that the contents of methanol and 1,1-dimethoxyethane were determined using a GC (Shimadzu Corporation, GC-2010).

[0082] (Electrolyte preparation)

[0083] A mixed solvent was prepared by mixing lithium-ion battery grade (LBG, Kishida Chemical Company) ethylene carbonate (EC) / diethyl carbonate (DEC) / ethyl methyl carbonate (EMC) in a volume ratio of 3:4:3.

[0084] The resulting mixed solvent is then mixed with the carboxylic acid ester composition at a mass ratio of 9:1 to produce the electrolyte solvent.

[0085] Subsequently, lithium hexafluoride phosphate (LiPF6, manufactured by Kishida Chemical Company) was dissolved in the electrolyte solvent to make the salt concentration 1 mol / L, thus preparing the electrolyte.

[0086] It should be noted that the contents of methyl acetate, methanol, and 1,1-dimethoxyethane in the electrolyte (total electrolyte) were calculated based on the composition ratio of the carboxylic acid ester composition, mixed solvent, and salt (electrolyte). Additionally, the contents of methyl acetate, methanol, and 1,1-dimethoxyethane in the constituent materials other than the electrolyte (excluding the electrolyte) were calculated. The calculated contents (total electrolyte) and contents (excluding the electrolyte) are shown in Table 1.

[0087] (Preparation of positive electrode active material)

[0088] LiNi, as a positive electrode active material, was prepared by referring to the method described in the non-patent literature (Journal of PowerSources, Vol. 146, pp. 636-639 (2005)). 0.5 Mn0.3 Co 0.2 O2.

[0089] Specifically, lithium hydroxide is first mixed with a ternary hydroxide containing nickel, manganese, and cobalt in a molar ratio of 5:3:2 to obtain a mixture. Then, the mixture is heated and granulated at 1000°C in air to produce the positive electrode active material.

[0090] (The production of the positive electrode)

[0091] The obtained positive electrode active material (LiNi) was 27.6g. 0.5 Mn 0.3 Co 0.2 A slurry was prepared by mixing 1.2g of O2, 1.2g of carbon black (DENKABLACK, manufactured by DENKA Corporation), 15g of binder (PVdF, 8wt% solids concentration, NMP solution), and 8g of NMP. This slurry was then coated onto an aluminum foil (20μm thick), heated in a forced-air oven at 80°C for 10 minutes to remove the solvent, and then vacuum-dried at 150°C for 12 hours. Finally, the foil was pressed using a roller press to form the positive electrode sheet.

[0092] The capacity of the obtained positive electrode was calculated based on the mass of the positive electrode active material per unit area and the theoretical capacity of the positive electrode active material (150 mAh / g). The result was that the capacity of the positive electrode was 1.2 mAh / cm². 2 .

[0093] (Making the negative electrode)

[0094] First, 12.5g of binder (PVdF, 12% by mass solids, NMP solution) was added to 28.5g of negative electrode active material (artificial graphite) and mixed to prepare a slurry. Then, the slurry was coated onto copper foil (20μm), heated in a forced-air oven at 80°C for 10 minutes to remove the solvent, and then vacuum-dried at 150°C for 12 hours. Finally, the negative electrode sheet was produced by pressing using a roller press.

[0095] The capacity of the resulting negative electrode was calculated based on the mass of the active material per unit area and the theoretical capacity of the active material (350 mAh / g). The result was that the capacity of the negative electrode was 1.4 mAh / cm². 2 .

[0096] (Manufacturing of non-aqueous electrolyte secondary batteries)

[0097] The positive electrode is made of a material that is punched into a circle with a diameter of 12mm, and the negative electrode is made of a material that is punched into a circle with a diameter of 14mm.

[0098] Then, electrolyte is impregnated into the positive and negative electrodes and the spacer (a polypropylene microporous membrane, 16μm, 16mm in diameter), so that the positive and negative electrodes are sandwiched between the spacer and the positive electrode. The positive electrode, the spacer and the negative electrode are then stacked and housed in the casing, thereby producing the 2032 button cell battery.

[0099] (Example 2)

[0100] Except that methanol was added in a manner that made the content of 1,1-dimethoxyethane 46 ppm by mass and the content of 1,1-dimethoxyethane 115 ppm by mass in the (preparation of carboxylic acid ester composition), the 2032 button cell was prepared in the same manner as in Example 1.

[0101] (Example 3)

[0102] Except that methanol was added in a manner that made the content of 1,1-dimethoxyethane 92 ppm by mass and 1,1-dimethoxyethane 115 ppm by mass in the (preparation of carboxylic acid ester composition), the 2032 button cell was prepared in the same manner as in Example 1.

[0103] (Example 4)

[0104] Except that methanol was added to the (preparation of the carboxylic acid ester composition) in such a way as to make the content of methanol 180 ppm by mass and the content of 1,1-dimethoxyethane 90 ppm by mass, the 2032 button cell was prepared in the same manner as in Example 1.

[0105] (Example 5)

[0106] Except that methanol was added in a manner that made the content of 1,1-dimethoxyethane 360 ​​ppm by mass and 90 ppm by mass in the (preparation of carboxylic acid ester composition), the 2032 button cell was prepared in the same manner as in Example 1.

[0107] (Example 6)

[0108] Except that the mixed solvent and the carboxylic acid ester composition were mixed at a mass ratio of 8:2 in the preparation of the electrolyte, the 2032 button cell was prepared in the same manner as in Example 3.

[0109] (Example 7)

[0110] Except that methanol and 1,1-dimethoxyethane were added in a manner that made the content of the carboxylic acid ester composition 180 ppm by mass and 1,1-dimethoxyethane 180 ppm by mass, the 2032 button cell was prepared in the same manner as in Example 1.

[0111] (Example 8)

[0112] Except that methanol was added to the carboxylic acid ester composition to a content of 90 ppm by mass and 1,1-dimethoxyethane to a content of 45 ppm by mass in the preparation of the carboxylic acid ester composition, and the resulting mixed solvent was mixed with the carboxylic acid ester composition in a mass ratio of 8:2 in the preparation of the electrolyte, the 2032 button cell was prepared in the same manner as in Example 1.

[0113] (Example 9)

[0114] Except that methanol was added in a manner that made the content of the carboxylic acid ester composition 46 ppm by mass and 1,1-dimethoxyethane was not added, the 2032 button cell was prepared in the same manner as in Example 1.

[0115] (Example 10)

[0116] Except that 1,1-dimethoxyethane was added in a manner that resulted in a content of 90 ppm by mass during the (preparation of the carboxylic acid ester composition) and methanol was not added, the 2032 button cell was prepared in the same manner as in Example 1.

[0117] (Example 11)

[0118] Except that methanol was added to the carboxylic acid ester composition to a content of 90 ppm by mass and 1,1-dimethoxyethane to a content of 180 ppm by mass in the preparation of the carboxylic acid ester composition, and the resulting mixed solvent was mixed with the carboxylic acid ester composition in a mass ratio of 9.5:0.5 in the preparation of the electrolyte, the 2032 button cell was prepared in the same manner as in Example 1.

[0119] (Comparative Example 1)

[0120] Except that the carboxylic acid ester composition was not mixed in the electrolyte preparation and the mixed solvent was set as the electrolyte solvent, the 2032 button cell was prepared in the same manner as in Example 1.

[0121] (Comparative Example 2)

[0122] Except that methanol and 1,1-dimethoxyethane were not added to methyl acetate in the (preparation of carboxylic acid ester composition) process, the 2032 button cell was prepared in the same manner as in Example 1.

[0123] (Comparative Example 3)

[0124] The 2032 button cell was prepared in the same manner as in Example 1, except that methyl formate was used instead of methyl acetate in the (preparation of the carboxylic acid ester composition) and methanol and 1,1-dimethoxyethane were not added to the methyl formate.

[0125] (Comparative Example 4)

[0126] Except that the mixed solvent and the carboxylic acid ester composition were mixed in a mass ratio of 7:3 during the preparation of the electrolyte, the 2032 button cell was prepared in the same manner as in Example 3.

[0127] <Evaluation>

[0128] The 2032 button cells obtained in the examples and comparative examples were evaluated as follows. The results are shown in Table 1.

[0129] (Evaluation of the rate characteristics of 2032 button cell [non-aqueous electrolyte secondary cell])

[0130] The obtained 2032 button cell battery was connected to a charge-discharge test chamber (TOSCAT3100, manufactured by Toyo Systems Co., Ltd.) and placed in a constant temperature bath at 25°C for 12 hours without current flow. Then, it was repeatedly charged and discharged three times under the conditions of constant current and constant voltage (CCCV) charging at 0.2C (charging termination voltage: 4.25V, CV STOP: 3 hours, or current value reaching 0.02C, pause time after charging: 10 minutes) and constant current (CC) discharging at 0.2C (discharge termination voltage: 2.5V, pause time after discharging: 10 minutes) to confirm whether it could function as a battery.

[0131] Next, rate characteristics were evaluated at 25°C and -10°C.

[0132] Regarding the rate characteristic evaluation at 25℃, starting from the state of CCCV charging with a current of 0.2C, CC discharge was performed with currents of 0.2C, 1C, 4C, 8C, and 16C respectively, repeated 3 times, and the discharge capacity at each discharge current was obtained. The capacity retention rate was calculated using these discharge capacities and the following equation (1).

[0133] Regarding the rate characteristic evaluation at -10℃, starting from the state of CCCV charging with a current of 0.2C, CC discharge was performed with currents of 1C, 4C, and 8C respectively, repeated 3 times, and the discharge capacity at each discharge current was obtained. The capacity retention rate was calculated using these discharge capacities and the following equation (1).

[0134] [Average discharge capacity obtained at each current (C) / Average discharge capacity obtained at 0.2C] × 100 (1) [Table 1]

[0135]

[0136] Regarding the electrolyte of this invention, the discharge capacity is not easily reduced even under high current conditions, and a high capacity retention rate can be achieved. In particular, when comparing high currents (8C→16C), the reduction in capacity retention rate is minimal.

[0137] Furthermore, when using the electrolyte described above, a high capacity retention rate can be achieved not only at room temperature but also at low temperatures.

[0138] Industrial availability

[0139] According to the present invention, an electrolyte that is not prone to a decrease in discharge capacity under high current conditions and can achieve a high capacity retention rate, as well as a non-aqueous electrolyte secondary battery using the electrolyte, can be provided.

Claims

1. An electrolyte for a non-aqueous secondary battery, comprising: Carboxylic esters, and Compounds containing hydroxyl groups and compounds containing ether groups, The total content of the compound having a hydroxyl group and the compound having an ether group is less than 50 ppm by mass. The mass ratio of the compound having a hydroxyl group to the compound having an ether group is 0.1 or more and 10 or less. The carboxylic ester has the structure shown in the general formula R-COOR', where R represents a hydrogen atom or a straight-chain or branched hydrocarbon group having 1 to 10 carbon atoms, and R' represents a straight-chain or branched hydrocarbon group having 1 to 10 carbon atoms. The compound containing a hydroxyl group is a primary alcohol. The compounds having an ether group include diether compounds.

2. The electrolyte for a non-aqueous secondary battery according to claim 1, wherein, The content of carboxylic acid esters is 3% or more by mass and less than 40% by mass.

3. The non-aqueous electrolyte for secondary batteries according to any one of claims 1 to 2, wherein, Compounds with an ether group have 3 or more but less than 10 carbon atoms.

4. A non-aqueous electrolyte secondary battery, comprising the non-aqueous electrolyte secondary battery electrolyte according to any one of claims 1 to 3.

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