Non-aqueous electrolyte and power storage device

Abstract

A non-aqueous electrolyte according to the present invention contains a non-aqueous solvent and a solute dissolved in the non-aqueous solvent. The non-aqueous solvent contains a first solvent having a viscosity of less than 0.6 mPa·s-1 at 25°C, and a fluorine-containing aromatic compound as a second solvent. The molar amount of the solute per unit volume of the first solvent is 2.4-5.0 mol / L. The activation energy determined from the conductivity at a temperature from 0-45°C is 10 kJ / mol or less.

Description

Non-aqueous electrolytes and energy storage devices Cross-reference of related applications

[0001] This disclosure claims priority rights to Japanese Patent Application No. 2025-013766, filed with the Japan Patent Office on 30 January 2025, and the entirety of the said patent application is incorporated herein by reference.

[0002] This disclosure relates to a non-aqueous electrolyte and an energy storage device.

[0003] Patent Document 1 proposes an electrolyte for a non-aqueous electrolyte secondary battery, comprising an electrolyte salt with a concentration of 2.0 mol / L or more and 5.0 mol / L or less, a base solvent for dissolving the electrolyte salt, and a cyclic carbonate-containing solvent, wherein the cyclic carbonate-containing solvent contains the cyclic carbonate in an amount of 1% to 15% by volume relative to the total volume of the base solvent and the cyclic carbonate, and the cyclic carbonate-containing solvent comprises a coordination solvent coordinated to ions ionized from the electrolyte salt and a free solvent not coordinated to ions ionized from the electrolyte salt, wherein the peak area ratio of the total free solvent, as determined by Raman spectroscopy, is 1% to 25%, the peak area ratio of the total free solvent is the ratio of the peak area of ​​the total free solvent to the total area of ​​the peak area of ​​the free solvent and the peak area of ​​the coordination solvent, as determined by Raman spectroscopy, and the peak area is the area enclosed by the peaks separated by the peak separation process and a preset baseline.

[0004] Patent Document 2 proposes a nonaqueous electrolyte comprising a nonaqueous solvent and a solute dissolved in the nonaqueous solvent, wherein the nonaqueous solvent contains a fluorine atom-containing aromatic compound and a cyclic diether.

[0005] Patent Document 3 proposes using a non-aqueous electrolyte in a composite element, comprising a non-aqueous solvent, a solute dissolved in the non-aqueous solvent, and an additive that can be reduced to lithium at a potential of 0.8 V or higher. Here, the non-aqueous solvent comprises ethylene carbonate, propylene carbonate, and a low-viscosity solvent, wherein the low-viscosity solvent has a lower viscosity than the ethylene carbonate and the propylene carbonate, and the content M of the ethylene carbonate in the total non-aqueous electrolyte is ＥＣ The amount is 5 to 30% by mass, and the content of the propylene carbonate is M ＰＣ The content of the low viscosity solvent is 15 to 60% by mass. ＬＶ The amount is 10 to 55% by mass, and the MADD content of the additive is 1 to 10% by mass.

[0006] Non-patent document 1 teaches that by using methyl acetate in a non-aqueous electrolyte, high ionic conductivity can be obtained, thereby improving the rapid charge / discharge characteristics and low-temperature output characteristics of the battery.

[0007] Japanese Patent Publication No. 2019-145325, Japanese Patent Publication No. 2013-137945, Japanese Patent Publication No. 2013-131374

[0008] The Journal of Physical Chemistry C 124, 23 (2020), 12269-12280.

[0009] The first aspect of this disclosure is a non-aqueous electrolyte for an energy storage device, comprising a non-aqueous solvent and a solute dissolved in the non-aqueous solvent, wherein the non-aqueous solvent has a pressure of 0.6 mPa·s at 25°C. －１ This invention relates to a non-aqueous electrolyte comprising a first solvent having a viscosity of less than 1, and a fluorine-containing aromatic compound as a second solvent, wherein the molar amount of the solute per unit volume of the first solvent is 2.4 mol / L or more and 5.0 mol / L or less, and the activation energy determined from the conductivity at a temperature of 0°C or more and 45°C or less is 10 kJ / mol or less.

[0010] A second aspect of this disclosure is a non-aqueous electrolyte for an energy storage device, comprising a non-aqueous solvent and a solute dissolved in the non-aqueous solvent, wherein the non-aqueous solvent has a pressure of 0.6 mPa·s at 25°C. －１A non-aqueous electrolyte includes a first solvent having a viscosity lower than that of a second solvent which is a fluorine-containing aromatic compound, wherein the volume ratio of the fluorine-containing aromatic compound in the non-aqueous solvent is 50% by volume or more and 70% by volume or less, and the molar amount of the solute per unit volume of the first solvent is 2.4 mol / L or more and 5.0 mol / L or less.

[0011] A third aspect of the present disclosure relates to a power storage device including a pair of electrodes and the non-aqueous electrolyte described above.

[0012] According to the present disclosure, a non-aqueous electrolyte capable of improving the low-temperature output characteristics and high-temperature storage characteristics of a power storage device can be provided.

[0013] FIG. is a longitudinal sectional view schematically showing a power storage device (non-aqueous electrolyte secondary battery) according to an embodiment of the present disclosure.

[0014] The novel features of the present disclosure are described in the appended claims. However, the present disclosure will be better understood from the following detailed description in conjunction with the drawings, in terms of both the structure and content, together with other objects and features of the present disclosure.

[0015] When a low-viscosity solvent such as methyl acetate is used in a non-aqueous electrolyte, high ionic conductivity is expected to be obtained, and excellent low-temperature output characteristics of a power storage device are expected to be obtained. However, in reality, due to low reduction resistance, it decomposes at the negative electrode and the reversible reactivity at the negative electrode is impaired. Therefore, in a power storage device using such a non-aqueous electrolyte, the low-temperature output characteristics are insufficient and the high-temperature characteristics (such as high-temperature storage characteristics) are also insufficient.

[0016] Low-viscosity non-aqueous solvents such as methyl acetate and dimethyl carbonate have relatively high dielectric constants. Therefore, when such non-aqueous solvents are used as the non-aqueous electrolyte in energy storage devices, high conductivity of charge carrier ions (hereinafter referred to as carrier ions) is easily obtained. High ionic conductivity of the non-aqueous electrolyte is advantageous for rapid charging of energy storage devices. However, low-viscosity non-aqueous solvents have relatively low molecular weights and low resistance to reduction and oxidation, and tend to decompose on electrodes, forming thick films. Such film formation impairs the main electrochemical reactions of the energy storage device and their reversibility, which tends to degrade properties such as low-temperature characteristics (output characteristics, etc.) or high-temperature characteristics (high-temperature storage characteristics, etc.). One reason for this is that the above-mentioned non-aqueous solvents readily solvate carrier ions.

[0017] The inventors conducted various studies to obtain high low-temperature power output characteristics and high high-temperature storage characteristics while using the low-viscosity solvent described above. As a result, they found that the interaction between the low-viscosity solvent and the carrier ions can be weakened by increasing the interaction between the cations (carrier ions) constituting the solute and the anions, or by using a solvent that interacts with the low-viscosity solvent in combination. Furthermore, it became clear that by selecting the type of solvent, reducing the activation energy determined from the conductivity of the non-aqueous electrolyte in a specific temperature range, or adjusting the volume ratio of the solvent, it is possible to achieve both high low-temperature power output characteristics and high high-temperature storage characteristics in an energy storage device.

[0018] Technical (1) In view of the above, the non-aqueous electrolyte relating to the first aspect of this disclosure is a non-aqueous electrolyte for an energy storage device, comprising a non-aqueous solvent and a solute dissolved in the non-aqueous solvent. The non-aqueous solvent has a pressure of 0.6 mPa·s at 25°C. －１The solution comprises a first solvent having a viscosity of less than 1 / 4, and a fluorine-containing aromatic compound as a second solvent. The molar amount of the solute per unit volume of the first solvent is 2.4 mol / L or more and 5.0 mol / L or less. The activation energy, determined from the conductivity at temperatures between 0°C and 45°C, is 10 kJ / mol or less. Here, the unit "mol / L" does not refer to molar concentration, but rather to the presence of 1 L of the first solvent and 2.4 mol or more and 5.0 mol or less of solute.

[0019] Thus, in this disclosure, the molar amount of solute relative to the first solvent is relatively large in the non-aqueous electrolyte, and a fluorine-containing aromatic compound is used as the second solvent. Furthermore, the activation energy of the non-aqueous electrolyte is set to 10 kJ / mol or less. As a result, even while using a low-viscosity first solvent, the solvation of the first solvent with respect to carrier ions can be weakened. In addition, by increasing the molar amount of solute relative to the first solvent, the anions constituting the solute become more easily reductively decomposed on the electrode than in the first solvent. This makes it easier for a film with excellent film quality derived from anions to form on the electrode surface. The formation of a thick film derived from the first solvent is suppressed, and the inhibition of the main electrochemical reaction on the electrode is suppressed. As a result, the decomposition of the first solvent is suppressed, and a non-aqueous electrolyte with low viscosity and high conductivity can be obtained while suppressing the decrease in the capacity of the energy storage device. As a result, excellent output characteristics at low temperatures and excellent high-temperature storage characteristics are obtained. The non-aqueous electrolyte of this disclosure exhibits suppressed coagulation even at low temperatures such as -40°C and achieves high ionic conductivity even at low temperatures, enabling rapid charging when used in energy storage devices. Furthermore, it exhibits high storage characteristics even at high temperatures such as 55°C, resulting in excellent durability for energy storage devices.

[0020] The viscosity of the first solvent is the viscosity measured using a cone-plate viscometer at a rotation speed of 60 rpm.

[0021] The activation energy of a non-aqueous electrolyte can be determined by measuring the conductivity of the non-aqueous electrolyte at three arbitrary temperatures within the range of 0°C to 45°C, and then using the following Arrhenius equation based on the relationship between these conductivity values ​​and temperatures.

[0022]

[0023] σ(T): Reaction rate constant; A: Frequency factor; Ea: Activation energy (unit: kJ / mol －１ ), T: Temperature (absolute temperature, unit: K); R: Gas constant (8.314 J / mol －１ K －１ )

[0024] Incidentally, the conductivity of the non-aqueous electrolyte can be determined, for example, by the following procedure. Set a conductivity meter using the alternating current two-electrode method in a thermostat and set the thermostat to a predetermined temperature. Wash the conductivity cell successively with distilled water and ethanol and dry it. Fill the dried conductivity cell with the non-aqueous electrolyte using a syringe and set it on the conductivity meter. Start the conductivity meter and continue measuring the resistivity (unit: Ω·m) of the non-aqueous electrolyte until a stable value is obtained (about 5 minutes). From the measured resistivity, the conductivity (electrical conductivity) (unit: mS / cm), which is the reciprocal of this resistivity, is obtained. The measurement of the conductivity (and resistivity) is performed at three temperatures (for example, 25°C, 30°C, and 40°C) in the range of 0°C or higher and 40°C or lower.

[0025] Technique (2) In the above technique (1), the volume ratio of the fluorine-containing aromatic compound in the non-aqueous solvent is preferably 50% by volume or more and 70% by volume or less. In this case, it is easy to keep the activation energy of the non-aqueous electrolyte low, and it is easy to ensure high fluidity and high ionic conductivity at low temperatures. Therefore, higher input / output characteristics at low temperatures can be obtained. Also, since the solvation of the first solvent to the carrier ions is easily relaxed, the formation of a reduction film on the electrode of the first solvent is suppressed, and the main electrochemical reaction at the electrode proceeds smoothly. Since the decrease in capacity is easily suppressed, the storage characteristics at high temperatures can be further enhanced.

[0026] Technique (3) The non-aqueous electrolyte for a power storage device according to the second aspect of the present disclosure includes a non-aqueous solvent and a solute dissolved in the non-aqueous solvent. The non-aqueous solvent has a viscosity of 0.6 mPa·s at 25°C －１The solution comprises a first solvent having a viscosity of less than 1 / 3, and a fluorine-containing aromatic compound as a second solvent. The volume ratio of the fluorine-containing aromatic compound in the non-aqueous solvent is 50% by volume or more and 70% by volume or less. The molar amount of the solute per unit volume of the first solvent is 2.4 mol / L or more and 5.0 mol / L or less.

[0027] The non-aqueous electrolyte relating to the second aspect provides high fluidity and ionic conductivity by containing a first solvent. Further containing a second solvent in the non-aqueous electrolyte, and ensuring the molar amount of solute relative to the first solvent is within a specific range, can weaken the solvation of the first solvent with respect to carrier ions. Furthermore, by containing a specific first solvent in the non-aqueous electrolyte, and ensuring the volume ratio of the second solvent in the non-aqueous solvent is within a specific range, the solvation of the first solvent with respect to carrier ions can be further relaxed. These factors allow the activation energy of the non-aqueous electrolyte to be kept low. As a result, high low-temperature power characteristics and high high-temperature storage characteristics are obtained.

[0028] Technology (4) In any one of the above technologies (1) to (3), it is preferable that the first solvent contains at least one selected from the group consisting of carbonate esters and fatty acid esters. Although such a first solvent is easily subjected to reductive decomposition or oxidative decomposition, according to this disclosure, even when such a first solvent is included, the formation of a film derived from the first solvent on the electrode can be suppressed, and high high-temperature storage characteristics can be ensured.

[0029] Technology (5) In Technology (4) above, the carbonate ester is preferably dimethyl carbonate. The fatty acid ester is preferably at least one selected from the group consisting of methyl acetate, ethyl acetate, and methyl propionate. These first solvents have low viscosity and high dielectric constant, making it easier to obtain higher input / output characteristics at low temperatures. Although these first solvents have relatively low resistance to reduction and oxidation, according to this disclosure, even when such a first solvent is included in the non-aqueous electrolyte, the decomposition of the first solvent is suppressed, and high durability of the energy storage device can be obtained.

[0030] Technology (6) In any one of the above technologies (1) to (5), it is preferable that the molar amount of solute per unit volume of the non-aqueous solvent is 0.9 mol / L or more and 2.5 mol / L or less. In this case, it is easy to achieve both high fluidity and high ionic conductivity of the non-aqueous electrolyte and high electrochemical stability of the non-aqueous electrolyte. Note that the unit "mol / L" here does not mean a unit of molar concentration, but rather means that there is 1 L of non-aqueous solvent and 0.9 mol or more and 2.5 mol or less of solute.

[0031] Technology (7) In any one of the above technologies (1) to (6), it is preferable that the solute contains a lithium salt of fluorine-containing acid imide. In this case, it is easier to ensure high fluidity and high ionic conductivity of the non-aqueous electrolyte at low temperatures, and the anion stability is high even at high temperatures. Therefore, the input / output characteristics of the energy storage device at low temperatures can be further improved, and higher durability can be obtained.

[0032] Technology (8) In any one of the above technologies (1) to (7), it is preferable that the fluorine-containing aromatic compound includes at least one selected from the group consisting of fluorobenzene and fluorotoluene. These second solvents interact with the first solvent to a moderate degree, while having relatively low interaction with the carrier ion. Therefore, it is easy to ensure higher conductivity of the carrier ion while weakening the solvation of the first solvent to the carrier ion. Thus, higher input / output characteristics can be obtained at low temperatures. Note that fluorobenzene is a benzene having a fluorine atom directly bonded to the benzene ring. Fluorotoluene is a toluene that includes at least one selected from the group consisting of a fluorine atom directly bonded to the benzene ring of toluene and a fluorine atom bonded to the methyl group of toluene.

[0033] Technology (9) The present disclosure also includes energy storage devices comprising the non-aqueous electrolyte described above. More specifically, an energy storage device relating to a second aspect of the present disclosure comprises a pair of electrodes and a non-aqueous electrolyte described in any one of the technologies (1) to (8) described above.

[0034] Technology (10) In Technology (9) above, one of the pair of electrodes may be a negative electrode. The negative electrode contains a negative electrode active material, and it is preferable that the negative electrode active material contains a silicon-containing material and graphite. Such a negative electrode is advantageous for increasing the capacity of energy storage devices, but a reduction film of the first solvent is easily formed. In this disclosure, the solvation of the first solvent with respect to carrier ions is weakened, so even when such a negative electrode is used, a film with excellent film quality derived from the anions constituting the solute is formed on the negative electrode surface. This maintains high input / output characteristics and suppresses capacity degradation.

[0035] Technology (11) In the above technology (10), the silicon-containing material may include a composite material. The composite material may include an ionic conductive phase and a silicon phase dispersed in the ionic conductive phase. Preferably, the content of the silicon phase in the composite material is 25% by mass or more and 60% by mass or less. Even when such a negative electrode is used, in this disclosure, the formation of a film derived from the first solvent is suppressed, thereby maintaining high input / output characteristics and suppressing capacity degradation.

[0036] In this specification, qualitative and quantitative analysis of each component in a non-aqueous electrolyte is performed, for example, using gas chromatography, under the following conditions. For quantitative analysis, the non-aqueous electrolyte before being supplied to the energy storage device may be used, or the non-aqueous electrolyte extracted from the energy storage device may be used. When the non-aqueous electrolyte is extracted from the energy storage device for analysis, it is preferable that the energy storage device from which the non-aqueous electrolyte is extracted is an initial energy storage device. An initial energy storage device may be an energy storage device after break-in charge / discharge or after aging, and in the case of commercially available energy storage devices, it may be an unused energy storage device or an energy storage device after the first charge. Analysis conditions: Measuring device: Shimadzu GC-2010 Plus Column: J&W HP-1 (diameter 1 μm, length 60 m) Linear velocity: 30.0 cm / sec Inlet temperature: 270°C Detector: FID 290°C (ses. 10 １ )

[0037] The non-aqueous electrolyte and energy storage device of this disclosure will be described in more detail below, including the above technologies (1) to (11), with reference to drawings as necessary. To the extent that it is not technically inconsistent, at least one of the above technologies (1) to (11) may be combined with at least one of the elements described below. Note that the figures are schematic representations, and the proportions of the dimensions (e.g., thickness) of each component may differ from those of the actual components.

[0038] The following descriptions may include examples of embodiments of the Disclosure, but the Disclosure is not limited to these examples. The following descriptions may include specific numerical values ​​and materials, but other numerical values ​​and materials may be used as long as the effects of the Disclosure are achieved. In the following descriptions, when lower and upper limits of numerical values ​​relating to specific physical properties or conditions are given as examples, any combination of either of the given lower limits and either of the given upper limits may be used, as long as the lower limit does not exceed the upper limit. When multiple materials are given as examples, one may be selected and used alone, or two or more may be used in combination.

[0039] [Non-aqueous electrolyte] The non-aqueous electrolytes relating to the first and second aspects of this disclosure are used, for example, in non-aqueous energy storage devices (or electrochemical devices) such as non-aqueous electrolyte secondary batteries (such as lithium-ion secondary batteries). The non-aqueous electrolyte comprises a non-aqueous solvent and a solute dissolved in the non-aqueous solvent. The non-aqueous electrolyte may further contain additives.

[0040] (Non-aqueous solvent) The non-aqueous solvent comprises at least a first solvent and a second solvent, and may optionally further contain a third solvent other than the first and second solvents. Here, the first solvent has a pressure of 0.6 mPa·s at 25°C. －１ It has a viscosity of less than . The second solvent is a fluorine-containing aromatic compound. However, the first solvent does not include the second solvent. As for the third solvent, for example, it has a viscosity of 0.6 mPa·s at 25°C. －１ This includes at least one selected from the group consisting of the above non-aqueous solvents and fluorine-containing non-aqueous solvents other than fluorine-containing aromatic compounds.

[0041] The inclusion of a non-aqueous solvent as the first solvent provides high fluidity and high ionic conductivity to the non-aqueous electrolyte. Further inclusion of a second non-aqueous solvent weakens the solvation effect of the first solvent on carrier ions. This suppresses reductive and oxidative decomposition of the first solvent, allowing the main electrochemical reaction on the electrode to proceed smoothly and preventing capacity degradation. As a result, high input / output characteristics are obtained at low temperatures, and excellent properties (storage characteristics, cycle characteristics, etc.) are also obtained at high temperatures.

[0042] The first solvent has a viscosity of 0.6 mPa·s at 25°C. －１ A non-aqueous solvent with a dielectric constant of less than 1 is preferred. The first solvent is preferably at least one selected from the group consisting of carbonate esters and fatty acid esters (such as acetate esters and propionic acid esters). Among the carbonate esters and fatty acid esters, alkyl esters are preferred. The number of carbon atoms in the alkyl portion is preferably 1 to 3, and preferably 1 or 2. Dimethyl carbonate (DMC) is preferred as the carbonate ester. Among the fatty acid esters, at least one selected from the group consisting of methyl acetate (MA), ethyl acetate, and methyl propionate is preferred. Using these non-aqueous solvents makes it easier to obtain higher low-temperature power characteristics and higher high-temperature storage characteristics. The non-aqueous solvent may contain only one of the first solvents, or it may contain a combination of two or more.

[0043] In this disclosure, it is also preferable that the first solvent contains at least a fatty acid ester. The first solvent may contain a fatty acid ester and a carbonate ester. When the first solvent contains a fatty acid ester, it tends to be easier to obtain higher low-temperature input / output characteristics and higher high-temperature storage characteristics.

[0044] The second solvent, a fluorine-containing aromatic compound, is a compound containing a fluorine atom and an aromatic ring. The aromatic ring is preferably a benzene ring. The number of fluorine atoms in the fluorine-containing aromatic compound is preferably 1 to 5 per molecule, and more preferably 1 to 3. This is because it allows for easy interaction with the first solvent while suppressing the interaction of the second solvent with the carrier ion. The fluorine atom may be directly bonded to the aromatic ring, or a substituent containing a fluorine atom may be bonded to the aromatic ring. Examples of such substituents include alkyl fluorides and alkoxy fluorides. The number of carbon atoms in the substituent is preferably 1 to 4, and more preferably 1 or 2.

[0045] The fluorine-containing aromatic compound preferably contains at least one selected from the group consisting of fluorobenzene and fluorotoluene. Fluorobenzene includes monofluorobenzene (FB), 1,3-difluorobenzene, 1,4-difluorobenzene, 1,3,5-trifluorobenzene, and the like. Fluorotoluene includes 4-fluorotoluene, 2,3-difluorotoluene, benzomonofluoride, benzodifluoride, trifluorotoluene (TFT), and the like. It is more preferable that the fluorine-containing aromatic compound contains at least one selected from the group consisting of FB and TFT.

[0046] The volume ratio of the fluorine-containing aromatic compound (second solvent) in the non-aqueous solvent (in other words, in the entire non-aqueous solvent) is preferably 50% by volume or more and 70% by volume or less. Having the volume ratio of the second solvent within this range allows for mitigation of the solvation of the first solvent with respect to the carrier ions. Furthermore, it is advantageous in mitigating the solvation of the first solvent with respect to the carrier ions because it makes it easier to reduce the activation energy of the non-aqueous electrolyte. The volume ratio of the second solvent in the non-aqueous solvent (in the entire non-aqueous solvent) may be greater than 50% by volume and 70% by volume or less, or 60% by volume or more and 70% by volume or less. This is because it makes it easier to keep the activation energy low and to mitigate the solvation of the first solvent with respect to the carrier ions. In particular, it is preferable that the volume ratio of the second solvent be within this range when the non-aqueous solvent contains a carbonate ester.

[0047] Examples of the third solvent include carbonate esters, carboxylic acid esters, and ethers. These third solvents may be cyclic or linear. The third solvent may contain fluorine atoms. The non-aqueous electrolyte may contain one third solvent or two or more.

[0048] Examples of cyclic carbonate esters include ethylene carbonate (EC), propylene carbonate, butylene carbonate, fluoroethylene carbonate (FEC), vinylene carbonate (VC), and vinylethylene carbonate. Examples of chain carbonates include diethyl carbonate (DEC) and ethyl methyl carbonate (EMC).

[0049] Examples of cyclic carboxylic acid esters include γ-butyrolactone and γ-valerolactone. Examples of linear carboxylic acid esters include propyl acetate, ethyl propionate, and methyl fluoropropionate.

[0050] Examples of cyclic ethers include 1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, and furan. Examples of linear ethers include diethyl ether, dipropyl ether, methylphenyl ether, methoxytoluene, 1,1-diethoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether, and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE).

[0051] The total volume ratio of the first and second solvents in the non-aqueous solvent (in other words, in the entire non-aqueous solvent) is preferably 80% by volume or more, and more preferably 90% by volume or more. When the volume ratio of the first and second solvents is within this range, higher low-temperature power characteristics and higher high-temperature storage characteristics are more easily obtained. The total volume ratio of the first and second solvents in the non-aqueous solvent is 100% by volume or less. The non-aqueous solvent may consist only of the first and second solvents.

[0052] (Solute) Depending on the type of energy storage device, a salt of a cation and anion that act as charge carriers is selected as the solute. For example, in lithium-ion secondary batteries, lithium salts are used as the solute. Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO2). ４ LiAlCl ４ LiB １０ Cl １０ (e.g.), lithium salts of fluorine-containing acids (LiPF) ６ LiPF ２ O ２ LiBF ４ LiSbF ６ LiAsF ６ LiCF ３ SO ３ LiCF ３ CO ２ These include lithium salts of fluorine-containing acidimides, oxalate complex salts (lithium bis(oxalate)borate, lithium difluoro(oxalate)borate, lithium tetrafluoro(oxalate)phosphate, lithium difluorobis(oxalate)phosphate), lithium halides (LiCl, LiBr, LiI, etc.). Lithium salts may be used individually or in combination of two or more.

[0053] The interaction of the anions constituting the solute with the cations (carrier ions) is appropriate, which makes it easier to ensure high ionic conductivity of the carrier ions while mitigating the solvation of the first solvent to the carrier ions. From this viewpoint, it is preferable that the solute contains a lithium salt (first salt) of a fluorine-containing acid imide. Furthermore, since the first salt easily forms a good quality film on the electrode, it can suppress the decomposition of the first solvent on the electrode, suppress electrode degradation, and allow the main electrochemical reaction to proceed smoothly. In addition, the first salt easily provides high fluidity and high ionic conductivity even at low temperatures and is highly stable even at high temperatures. The solute may contain the first salt and a second salt other than the first salt. The second salt may be at least one selected from the above lithium salts other than the first salt. Imide salts tend to corrode electrodes (especially Al contained in the current collector of the electrode, etc.) during high-potential sweeps, but in this disclosure, such electrode corrosion can also be suppressed.

[0054] For example, the first salt is LiN(SO4). ２ R １ ) (SO ２ R ２ ) (However, R １ and R ２ Each of them is independently C ｎ F ２ｎ＋１ Examples of such imide salts include LiN(SO4), where n is a non-negative integer. ２ F) ２ (LiFSI), LiN(SO ２ CF ３ ) ２ , LiN (SO ２ CF ３ ) (SO ２ C ４ F ９ ), LiN (SO ２ C ２ F ５ ) ２ These are some examples. The solute preferably contains LiFSI, as this further enhances the low-temperature power output characteristics and high-temperature storage characteristics.

[0055] The molar ratio of the first salt to the total solute is preferably 60 mol% or more, more preferably 80 mol% or more, and even more preferably 90 mol% or more. The molar ratio of the first salt to the total solute is 100 mol% or less. The solute may consist only of the first salt. When the molar ratio of the first salt is within this range, the solvation of the first solvent with respect to the carrier ions is easily relaxed, making it easier to obtain higher low-temperature power characteristics and higher high-temperature storage characteristics.

[0056] In this disclosure, the molar amount of solute per unit volume of the first solvent is also important. The molar amount of solute per unit volume of the first solvent is 2.4 mol / L or more and 5.0 mol / L or less. When the non-aqueous electrolyte contains such a relatively large amount of solute relative to the first solvent, the interaction between the anions and cations (carrier ions) constituting the solute is increased, and the solvation of the carrier ions by the first solvent is mitigated. From the viewpoint of easily obtaining higher high-temperature storage characteristics, the molar amount of solute per unit volume of the first solvent may be 3.0 mol / L or more and 5.0 mol / L or less.

[0057] The molar amount of solute per unit volume of the non-aqueous solvent is preferably 0.9 mol / L or more and 2.5 mol / L or less, and may also be 0.9 mol / L or more and 2.0 mol / L or less. When the molar amount of solute per unit volume of the non-aqueous solvent is within this range, high ionic conductivity is easily obtained, and the input / output characteristics at low temperatures can be further improved.

[0058] (Activation Energy) In the non-aqueous electrolyte according to the first aspect of this disclosure, it is also important that the activation energy is 10 kJ / mol or less. This ensures high fluidity and high ionic conductivity of the non-aqueous electrolyte, suppresses the decomposition of the first solvent, and allows the main electrochemical reaction on the electrode to proceed smoothly. As a result, high input / output characteristics are obtained at low temperatures and high storage characteristics are obtained at high temperatures. The energy storage device can be operated over a wide temperature range from low to high temperatures. The above activation energy can be determined from the conductivity at temperatures between 0°C and 45°C, as described above. Note that the non-aqueous electrolyte according to the second aspect of this disclosure can also obtain an activation energy within the above range. The activation energy may be, for example, 1 kJ / mol or more, or 3 kJ / mol or more.

[0059] (Other) The non-aqueous electrolyte may be gelled. For example, the non-aqueous electrolyte may be gelled by compounding it with a matrix polymer. The matrix polymer can be any polymer that absorbs a non-aqueous solvent and gels. Examples of such polymer materials include fluororesins, acrylic resins, and polyether resins.

[0060] [Energy Storage Device] An energy storage device according to the third aspect of this disclosure comprises a pair of electrodes and the non-aqueous electrolyte. One of the pair of electrodes is a positive electrode, and the other of the pair of electrodes is a negative electrode. Examples of energy storage devices include non-aqueous electrolyte secondary batteries (lithium-ion secondary batteries, lithium-metal secondary batteries, etc.), electric double-layer capacitors, lithium-ion capacitors, etc.

[0061] The following describes the configuration of a non-aqueous electrolyte secondary battery.

[0062] A non-aqueous electrolyte secondary battery comprises a positive electrode, a negative electrode, and the non-aqueous electrolyte described above. A separator is usually placed between the positive electrode and the negative electrode.

[0063] (Negative electrode) The negative electrode includes at least a negative electrode current collector and may comprise a negative electrode mixture layer formed on the surface of the negative electrode current collector and containing a negative electrode active material. Lithium metal secondary batteries do not need to have a negative electrode mixture layer. Lithium ion secondary batteries have a negative electrode mixture layer. The negative electrode mixture layer can be formed by coating a negative electrode slurry, in which the negative electrode mixture is dispersed in a dispersion medium, onto the surface of the negative electrode current collector and drying it. The dried coating may be rolled if necessary. The negative electrode mixture layer may be formed on one surface of the negative electrode current collector or on both surfaces.

[0064] The negative electrode mixture contains a negative electrode active material as an essential component and may contain optional components such as binders, conductive agents, and thickeners. The optional components may include those described later. The negative electrode active material contains a material that electrochemically intercepts and releases lithium ions. As the electrochemically intercepting and releasing lithium ions, carbon materials, Si-containing materials, etc., can be used. As the Si-containing material, silicon oxide (SiO₂) is used. ｘ Examples include composite materials containing an ionic conductive phase (such as a silicate phase, silicon oxide phase, graphite phase, or amorphous carbon phase) and a silicon phase (such as silicon particles) dispersed within the ionic conductive phase, where x is 0.5 ≤ x ≤ 1.5.

[0065] Examples of carbon materials include graphite, easily graphitizable carbon (soft carbon), and difficult-to-graphitize carbon (hard carbon). Among these, graphite is preferred because it has excellent charge-discharge stability and low irreversible capacity. Graphite refers to materials having a graphite-type crystal structure and includes natural graphite, artificial graphite, and graphitized mesophase carbon particles. Carbon materials may be used individually or in combination of two or more types.

[0066] The negative electrode active material preferably contains a silicon-containing material and graphite. The silicon-containing material may be a composite material containing an ionic conduction phase and a silicon phase dispersed in the ionic conduction phase. In the composite material, the content of the silicon phase is preferably 25% by mass or more and 60% by mass or less, and more preferably 30% by mass or more and 60% by mass or less. In the negative electrode active material, the content of the silicon-containing material (for example, the above composite material) is preferably 25% by mass or more and 60% by mass or less, and more preferably 25% by mass or more and 45% by mass or less.

[0067] For the negative electrode current collector, metal foil, mesh, net, or perforated sheet may be used. Examples of materials for the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, and copper alloy.

[0068] (Positive electrode) The positive electrode comprises, for example, a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector and containing a positive electrode active material. The positive electrode mixture layer can be formed by applying a positive electrode slurry, obtained by dispersing the positive electrode mixture in a dispersion medium, to the surface of the positive electrode current collector and drying it. The dried coating may be rolled if necessary. The positive electrode mixture layer may be formed on one surface of the positive electrode current collector or on both surfaces.

[0069] The positive electrode mixture contains a positive electrode active material as an essential component and may include binders, conductive agents, etc., as optional components. The positive electrode active material contains a material that electrochemically intercepts and releases lithium ions. Lithium transition metal composite oxides are preferred as the material that electrochemically intercepts and releases lithium ions. Examples of lithium transition metal composite oxides include layered compounds with a rock salt crystal structure, spinel compounds, and polyanionic compounds. Among these, layered compounds containing Ni, in which the proportion of Ni to all metal elements other than Li is 90 mol% or more, are preferred in that they exhibit high capacity.

[0070] A preferred example of a layered compound is one with the composition of formula (C): Li α Ni(1-x1-x2-x3-y)Co ｘ１ Mn ｘ２ Al ｘ３ M ｙ O ２＋βExamples of lithium transition metal composite oxides are given by the following equations: where equation (C) satisfies 0.95 ≤ α ≤ 1.05, 0.8 ≤ 1-x1-x2-x3-y ≤ 0.99, 0 ≤ x1 ≤ 0.1, 0 ≤ x2 ≤ 0.1, 0 ≤ x3 ≤ 0.1, 0 ≤ y ≤ 0.1, and -0.05 ≤ β ≤ 0.05. M is at least one selected from the group consisting of Ti, Zr, Nb, Mo, W, Fe, Zn, B, Si, Mg, Ca, Sr, and Y.

[0071] For the positive electrode current collector, metal foil can be used, and examples of materials include stainless steel, aluminum, aluminum alloy, and titanium.

[0072] Examples of binders for each electrode include resin materials such as fluororesins like polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins such as polyethylene and polypropylene; polyamide resins such as aramid resin; polyimide resins such as polyimide and polyamideimide; acrylic resins such as polyacrylic acid, polyacrylate salts (e.g., lithium polyacrylate), methyl polyacrylate, and ethylene-acrylic acid copolymers; vinyl resins such as polyacrylonitrile and polyvinyl acetate; polyvinylpyrrolidone; polyethersulfone; and rubber-like materials such as styrene-butadiene copolymer rubber (SBR). These may be used individually or in combination of two or more.

[0073] Examples of conductive materials for each electrode include carbon blacks such as acetylene black; conductive fibers such as carbon fibers, carbon nanotubes (single-walled carbon nanotubes, multi-walled carbon nanotubes, etc.), and metal fibers; carbon fluoride; metal powders such as aluminum; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and organic conductive materials such as phenylene derivatives. These may be used individually or in combination of two or more.

[0074] Examples of thickening agents include cellulose derivatives such as cellulose ether. Examples of cellulose derivatives include carboxymethylcellulose (CMC) and its modified forms, and methylcellulose. A single thickening agent may be used alone, or two or more may be used in combination.

[0075] The dispersion medium used to form each electrode is not particularly limited, but examples include water, alcohol, and N-methyl-2-pyrrolidone (NMP).

[0076] (Separator) Typically, a separator is interposed between the positive and negative electrodes. The separator has high ion permeability and possesses appropriate mechanical strength and insulating properties. Microporous thin films, woven fabrics, nonwoven fabrics, etc., can be used as separators. Polyolefins such as polypropylene and polyethylene are preferred as the material of the separator.

[0077] (Other) One example of a secondary battery structure is a structure in which an electrode group, in which the positive and negative electrodes are wound around a separator, and a non-aqueous electrolyte are housed in an outer casing. Alternatively, other forms of electrode groups may be used instead of the wound electrode group, such as a laminated electrode group in which the positive and negative electrodes are stacked around a separator. Secondary batteries may take any form, such as cylindrical, prismatic, coin-type, button-type, or laminated type.

[0078] The non-aqueous electrolyte relating to this disclosure is suitable for energy storage devices used in a wide temperature range from low to high temperatures. The energy storage devices relating to this disclosure are useful in a variety of applications, including electronic devices such as mobile phones, smartphones, and tablet devices, as well as hybrid vehicles, plug-in hybrid vehicles, and electric vehicles. However, the applications of the non-aqueous electrolyte and energy storage devices relating to this disclosure are not limited to these.

[0079] The structure of the energy storage device of this disclosure will be described below with reference to Figure 1. Figure 1 is a longitudinal cross-sectional view of a cylindrical non-aqueous electrolyte secondary battery 10, which is an example of this embodiment. However, this disclosure is not limited to the following configuration.

[0080] The non-aqueous electrolyte secondary battery 10 comprises an electrode group 18, an electrolyte (not shown), and a bottomed cylindrical battery case 22 that houses these. A sealing body 11 is crimped and fixed to the opening of the battery case 22 via a gasket 21. This seals the inside of the battery. The sealing body 11 comprises a valve body 12, a metal plate 13, and an annular insulating member 14 interposed between the valve body 12 and the metal plate 13. The valve body 12 and the metal plate 13 are connected to each other at their respective centers. A positive electrode lead 15a, which is led out from the positive electrode plate 15, is connected to the metal plate 13. Therefore, the valve body 12 functions as an external terminal of the positive electrode. A negative electrode lead 16a, which is led out from the negative electrode plate 16, is connected to the inner surface of the bottom of the battery case 22. An annular groove 22a is formed near the open end of the battery case 22. A first insulating plate 23 is positioned between one end face of the electrode group 18 and the annular groove 22a. A second insulating plate 24 is positioned between the other end face of the electrode group 18 and the bottom of the battery can 22. The electrode group 18 is formed by winding a positive electrode plate 15 and a negative electrode plate 16 with a separator 17 in between.

[0081] [Examples] The present disclosure will be described in detail below based on examples and comparative examples. The present disclosure is not limited to the following examples.

[0082] Examples 1-11 and Comparative Examples 1-8: Cylindrical non-aqueous electrolyte secondary batteries (batteries E1-E11 in Examples 1-11, and batteries C1-C8 in Comparative Examples 1-8) were fabricated and evaluated according to the following procedure.

[0083] (1) Preparation of the negative electrode A negative electrode slurry was obtained by mixing the negative electrode active material with 1.0 part by mass of carboxymethylcellulose (CMC), 3.0 parts by mass of styrene-butadiene rubber (SBR), 5.0 parts by mass of single-walled carbon nanotubes, and an appropriate amount of water. As the negative electrode active material, a mixture of graphite (Gr) and particles of a composite material in which Si phase (Si particles) are dispersed in a porous carbon phase (amorphous carbon phase) was used. In the composite material, the Si phase content was set to 50% by mass, and the composite material content in the negative electrode active material was set to 30% by mass. Next, the negative electrode slurry was applied to the surface of the copper foil, the coating film was dried, and then it was rolled to form a negative electrode slurry layer on both sides of the copper foil (total thickness 120 μm, density 1.3 g / cm³). ３) was formed, and the negative electrode was obtained.

[0084] (2) Preparation of the positive electrode Lithium-containing composite oxide (LiNi ０．８８ Co ０．０７ Mn ０．０５ O ２ 95 parts by mass of acetylene black, 2.5 parts by mass of polyvinylidene fluoride, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) were mixed to obtain a positive electrode slurry. Next, the positive electrode slurry was applied to the surface of the aluminum foil, the coating was dried, and then the foil was rolled to create a positive electrode mixture layer on both sides of the aluminum foil (total thickness 160 μm, density 3.6 g / cm³). ３ A positive electrode was obtained by forming a positive electrode.

[0085] (3) Preparation of Non-Aqueous Electrolyte A non-aqueous electrolyte was prepared by dissolving a lithium salt as a solute in a non-aqueous solvent. The lithium salts used were those shown in Table 1, with the molar amounts per liter of non-aqueous solvent being the values ​​shown in Table 1. The non-aqueous solvent was those shown in Table 1, in the volume ratios (volume ratios in the total non-aqueous solvent) shown in Table 1. In addition, VC was added to the non-aqueous electrolyte at a concentration of 1.5% by mass. Table 1 also shows the molar amount (mol / L) of lithium salt per unit volume of the first solvent for each example of the electrolyte.

[0086] (4) Fabrication of a non-aqueous electrolyte secondary battery An Al positive electrode lead was attached to the positive electrode obtained above, and a Ni negative electrode lead was attached to the negative electrode obtained above. In an inert gas atmosphere, the positive electrode and negative electrode were wound in a spiral shape with a polyethylene thin film (separator) in between to fabricate a wound electrode group. The electrode group was housed in a cylindrical outer casing, the non-aqueous electrolyte was injected, and then the outer casing was sealed to fabricate a cylindrical 2170 battery with a rated capacity of 5 Ah.

[0087] (5) Evaluation The following evaluation was performed using the non-aqueous electrolyte secondary battery obtained in (4) above or the non-aqueous electrolyte obtained in (3) above.

[0088] (5-1) Activation Energy Following the procedure described above, the conductivity of the non-aqueous electrolyte was measured, and the activation energy was determined using the Arrhenius equation based on this conductivity. For each non-aqueous electrolyte, the conductivity was determined at three temperatures: 5°C, 25°C, and 40°C.

[0089] (5-2) Low-temperature characteristics (a) Non-aqueous electrolyte that solidified at -40°C was cooled to -40°C and evaluated as A if it did not solidify and B if it did solidify.

[0090] (b) Low-temperature power characteristics: A non-aqueous electrolyte secondary battery was charged with a constant current of 1.5A from a discharged state to 3.28V (equivalent to a State of Charge (SOC) of 10%), and then held at a constant voltage of 3.28V until the charging current was 10mA or less. Next, the DC internal resistance (DCIR) of the battery was measured at -20°C (low temperature) after it had been left in an open circuit state for 6 hours. More specifically, at -20°C, the voltage drop (ΔV) when a constant power discharge current of 15W was passed through the battery for 10 seconds was divided by the current value after 10 seconds (DCIR, unit: mΩ), and this value was used as an index for evaluating the low-temperature power characteristics.

[0091] (5-3) High-temperature storage characteristics A non-aqueous electrolyte secondary battery was charged at 25°C under the following charging conditions and discharged under the following discharge conditions. The discharge capacity at this time (initial discharge capacity c) １ The discharge capacity (discharge capacity c after storage) was measured. After measurement, the battery was recharged under the following charging conditions. The recharged battery was stored in a constant temperature bath at 55°C for one month. Then, the battery was stored at 25°C for one day. After this storage, the battery was charged under the following charging conditions, and then discharged under the following discharge conditions, and the discharge capacity at this time (discharge capacity c after storage) was measured. １Ｍ ) was measured. And c １Ｍ / c １ The capacity retention rate (%) was calculated using the formula ×100 and used as an indicator of high-temperature storage characteristics.

[0092] (Charging) Constant current charging was performed at a current of 0.3C until the voltage reached 4.2V, and then constant voltage charging was performed until the current value reached 0.02C at 4.2V. (Discharging) Discharging was performed at a constant current value of 0.2C until the voltage reached 2.5V.

[0093] The results of the examples and comparative examples are shown in Table 1.

[0094]

[0095] As shown in Table 1, when a non-aqueous electrolyte containing the first solvent but not the second solvent is used, the activation energy remains high even when the molar amount of solute relative to the first solvent is increased, resulting in poor low-temperature power output characteristics and high-temperature storage characteristics (see C3, C4, C7, and C8). Lowering the molar amount of solute relative to the first solvent reduces the activation energy, but the Al contained in the electrode corrodes, making it impossible to evaluate the characteristics (see C1 and C2). Even with a non-aqueous electrolyte containing both the first and second solvents, if the activation energy is high or the volume ratio of the second solvent is low, the low-temperature power output characteristics and high-temperature storage characteristics will be poor (see C5-C6). In contrast to the results of these comparative examples, the non-aqueous electrolyte in the example does not solidify even at -40°C. Furthermore, the battery in the example obtains excellent low-temperature power output characteristics at -20°C and excellent high-temperature cycle characteristics at 55°C (E1-E11).

[0096] Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention.

[0097] 10: Energy storage device (non-aqueous electrolyte secondary battery) 11: Sealing body 12: Valve body 13: Metal plate 14: Insulating material 15: Positive electrode plate 15a: Positive electrode lead 16: Negative electrode plate 16a: Negative electrode lead 17: Separator 18: Electrode group 21: Gasket 22: Battery can 22a: Groove 23: First insulating plate 24: Second insulating plate

Claims

1. A non-aqueous electrolyte for an energy storage device, comprising a non-aqueous solvent and a solute dissolved in the non-aqueous solvent, wherein the non-aqueous solvent has a pressure of 0.6 mPa·s at 25°C. －１ A non-aqueous electrolyte comprising a first solvent having a viscosity of less than 1, and a fluorine-containing aromatic compound as a second solvent, wherein the molar amount of the solute per unit volume of the first solvent is 2.4 mol / L or more and 5.0 mol / L or less, and the activation energy determined from the conductivity at a temperature of 0°C or more and 45°C or less is 10 kJ / mol or less.

2. The non-aqueous electrolyte according to claim 1, wherein the volume ratio of the fluorine-containing aromatic compound in the non-aqueous solvent is 50% by volume or more and 70% by volume or less.

3. A non-aqueous electrolyte for an energy storage device, comprising a non-aqueous solvent and a solute dissolved in the non-aqueous solvent, wherein the non-aqueous solvent has a pressure of 0.6 mPa·s at 25°C. －１ A non-aqueous electrolyte comprising a first solvent having a viscosity of less than 1 / 3 and a fluorine-containing aromatic compound as a second solvent, wherein the volume ratio of the fluorine-containing aromatic compound in the non-aqueous solvent is 50% by volume or more and 70% by volume or less, and the molar amount of the solute per unit volume of the first solvent is 2.4 mol / L or more and 5.0 mol / L or less.

4. The non-aqueous electrolyte according to any one of claims 1 to 3, wherein the first solvent comprises at least one selected from the group consisting of carbonate esters and fatty acid esters.

5. The non-aqueous electrolyte according to claim 4, wherein the carbonate ester is dimethyl carbonate, and the fatty acid ester is at least one selected from the group consisting of methyl acetate, ethyl acetate, and methyl propionate.

6. The non-aqueous electrolyte according to any one of claims 1 to 3, wherein the molar amount of the solute per unit volume of the non-aqueous solvent is 0.9 mol / L or more and 2.5 mol / L or less.

7. The non-aqueous electrolyte according to any one of claims 1 to 3, wherein the solute comprises a lithium salt of a fluorine-containing acid imide.

8. The non-aqueous electrolyte according to any one of claims 1 to 3, wherein the fluorine-containing aromatic compound comprises at least one selected from the group consisting of fluorobenzene and fluorotoluene.

9. An energy storage device comprising a pair of electrodes and a non-aqueous electrolyte according to any one of claims 1 to 3.

10. The energy storage device according to claim 9, wherein one of the pair of electrodes is a negative electrode, the negative electrode includes a negative electrode active material, and the negative electrode active material includes a silicon-containing material and graphite.

11. The energy storage device according to claim 10, wherein the silicon-containing material comprises a composite material, the composite material comprises an ionic conductive phase and a silicon phase dispersed in the ionic conductive phase, and the content of the silicon phase in the composite material is 25% by mass or more and 60% by mass or less.

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