JP6847990B2 - Non-aqueous electrolyte and non-aqueous secondary battery - Google Patents

Description

This specification discloses a non-aqueous electrolyte solution and a non-aqueous secondary battery.

Conventionally, as a non-aqueous secondary battery, it has been desired to develop a lithium ion secondary battery having high battery characteristics and high energy density. As such a lithium ion secondary battery, a battery having a fully charged positive electrode potential of 4.5 V or more based on metallic lithium (hereinafter, also referred to as “high potential battery”) has been proposed (for example, Patent Document 1). reference). This high-potential battery can increase the amount of lithium occlusion / release per unit volume of the positive electrode as compared with the conventional lithium-ion battery, and can increase the battery voltage, so that the energy density of the battery can be increased. On the other hand, in this high-potential battery, since the positive electrode potential at the time of full charge becomes high, the oxidative decomposition reaction of the electrolytic solution is likely to occur, the capacity decreases due to the charge / discharge cycle, and the battery exterior swells due to the generation of the electrolytic solution decomposition gas. There is concern that such things will occur. Therefore, in order to suppress the oxidative decomposition reaction of the electrolytic solution, a battery using an electrolytic solution containing a halogen-containing compound as one component of a single solvent or a mixed solvent has been proposed (see, for example, Patent Document 2).

Japanese Unexamined Patent Publication No. 2004-281158 Japanese Unexamined Patent Publication No. 6-176768

However, in the battery of Patent Document 2, by halogenating the solvent molecule, the oxidative decomposition reaction of the electrolytic solution can be suppressed, while the LUMO of the molecule is lowered and the reduction potential is raised, so that the reduction resistance is lowered. There was a problem. This electrolytic solution may cause a reduction decomposition reaction on the negative electrode at a negative electrode potential of 0 to 3 V, and further improvement for preventing this may be desired.

The present disclosure has been made in view of such a problem, and an object of the present disclosure is to provide a non-aqueous secondary battery capable of further suppressing a decrease in capacity and further reducing internal resistance.

As a result of diligent research to achieve the above-mentioned object, the present inventors have found that when a predetermined trifluorovinyl compound is added to the electrolytic solution, the capacity decrease in repeated charging and discharging is further suppressed and the internal resistance is further reduced. We have found that we can do this, and have completed the invention disclosed in this specification.

That is, the non-aqueous electrolyte solution disclosed in this specification is
A non-aqueous electrolyte solution used in non-aqueous secondary batteries.
It contains one or more of the trifluorovinyl compounds of the following formulas (1) and (2).

The non-aqueous secondary batteries disclosed herein are:
Positive electrode with positive electrode active material and positive electrode
Negative electrode with a negative electrode active material and
The above-mentioned non-aqueous electrolyte solution that is interposed between the positive electrode and the negative electrode and conducts ions,
It is equipped with.

In the non-aqueous electrolyte solution and the non-aqueous secondary battery of the present disclosure, the decrease in capacity can be further suppressed and the internal resistance can be further reduced. The reason why such an effect can be obtained is considered as follows. For example, in this non-aqueous electrolyte solution, it is presumed that the trifluorovinyl compound is polymerized on the negative electrode to form a film having low electron conductivity on the negative electrode, and the decomposition of the non-aqueous electrolyte solution is further suppressed. To.

The schematic diagram which shows an example of the structure of the non-aqueous secondary battery 20. The relationship diagram of the capacity retention rate with respect to the number of cycles in a high temperature endurance test. Relationship diagram of IV resistance between initial and endurance.

The non-aqueous secondary battery described in the present embodiment includes a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and a non-aqueous electrolyte solution that is interposed between the positive electrode and the negative electrode and conducts lithium ions. ing. In this non-aqueous secondary battery, the carrier may be an alkali metal or an alkaline earth metal. Examples of the alkali metal include one or more of Li, Na, K and the like, and examples of the alkaline earth metal include one or more of Ca, Ba, Mg and the like. Here, for convenience of explanation, a lithium secondary battery including a non-aqueous electrolytic solution that conducts lithium ions, a positive electrode active material that occludes and releases lithium ions, a negative electrode active material, and the like will be mainly described.

The positive electrode may have a positive electrode active material that occludes and releases lithium ions. Examples of the positive electrode active material include a composite compound containing lithium and a transition metal element. Specifically, a lithium manganese composite oxide having a basic composition formula of Li _(1-x) MnO ₂ (0 <x <1, etc., the same applies hereinafter) or Li _(1-x) Mn ₂ O _{4 or the like, basic composition.} Lithium cobalt composite oxide whose formula is Li _(1-x) CoO _2, etc., Lithium nickel composite oxide whose basic composition formula is Li _(1-x) NiO _2, etc., basic composition formula is Li _{(1-x) Ni a Co b Mn c O 2} (a + b + c = 1) lithium-nickel-cobalt-manganese composite oxide, and the like, the basic formula lithium vanadium composite oxide, and the like LiV ₂ O _3, the basic formula V ₂ O _5, etc. It is possible to use a transition metal oxide having a basic composition formula of LiFePO ₄ or the like, or lithium iron phosphate having a basic composition formula of LiFePO 4. Of these, lithium transition metal composite oxides, such as LiCoO ₂ , LiNiO ₂ , and LiMnO ₂ , are preferred. The "basic composition formula" means that other elements may be contained. The positive electrode active material may have a lithium reference potential of 5 V or higher. Such a positive electrode active material may be, for example, a spinel-type lithium manganese composite oxide in which a part of manganese is replaced with another transition metal, or as a composite oxide containing Li and at least Ni and Mn. It may be a composite oxide containing an additive element (for example, Mg, Al, etc.). Specific examples thereof include a spinel-type lithium nickel-manganese composite oxide having a basic composition formula of Li _(1-x) Ni _a Mn _b O _{4 (a + b = 2).} Examples of this composite oxide include those having a basic composition formula of LiNi _0.5 Mn _1.5 O ₄ .

For this positive electrode, for example, a positive electrode active material, a conductive material, and a binder are mixed, and an appropriate solvent is added to form a paste-like positive electrode material, which is applied and dried on the surface of a current collector, and if necessary. It may be formed by compression in order to increase the electrode density. The conductive material is not particularly limited as long as it is an electron conductive material that does not adversely affect the battery performance of the positive electrode, and for example, graphite such as natural graphite (scaly graphite, scaly graphite) or artificial graphite, acetylene black, carbon black, etc. One or a mixture of one or more of Ketjen black, carbon whisker, graphite coke, carbon fiber, metal (copper, nickel, aluminum, silver, gold, etc.) can be used. Among these, carbon black and acetylene black are preferable as the conductive material from the viewpoint of electron conductivity and coatability. The binder plays a role of binding the active material particles and the conductive material particles, and is, for example, a fluororesin such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), fluororubber, or polypropylene. A thermoplastic resin such as polyethylene, ethylene propylene diene monomer (EPDM) rubber, sulfonated EPDM rubber, natural butyl rubber (NBR) and the like can be used alone or as a mixture of two or more kinds. Further, an aqueous dispersion of cellulose-based binder or styrene-butadiene rubber (SBR), which is an aqueous binder, can also be used. Examples of the solvent for dispersing the positive electrode active material, the conductive material, and the binder include N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methylethylketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, N, N-dimethylaminopropylamine. , Ethylene oxide, tetrahydrofuran and other organic solvents can be used. Further, a dispersant, a thickener or the like may be added to water, and the active material may be slurried with a latex such as SBR. As the thickener, for example, polysaccharides such as carboxymethyl cellulose and methyl cellulose can be used alone or as a mixture of two or more kinds. Examples of the coating method include roller coating such as applicator roll, screen coating, doctor blade method, spin coating, bar coater, and the like, and any of these can be used to obtain an arbitrary thickness and shape. Collectors include aluminum, titanium, stainless steel, nickel, iron, calcined carbon, conductive polymers, conductive glass, etc., as well as aluminum, copper, etc. for the purpose of improving adhesiveness, conductivity, and oxidation resistance. The surface of the above can be treated with carbon, nickel, titanium, silver or the like. For these, it is also possible to oxidize the surface. Examples of the shape of the current collector include a foil shape, a film shape, a sheet shape, a net shape, a punched or expanded body, a lath body, a porous body, a foam body, and a fiber group forming body. As the thickness of the current collector, for example, one having a thickness of 1 to 500 μm is used.

This negative electrode may have a negative electrode active material that occludes and releases lithium. Examples of the negative electrode active material include inorganic compounds such as lithium metals, lithium alloys, and tin compounds, carbonaceous materials capable of occluding and releasing lithium ions, composite oxides containing a plurality of elements, and conductive polymers. .. Examples of the carbonaceous material include cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, carbon fibers and the like. Of these, graphites such as artificial graphite and natural graphite have an operating potential close to that of metallic lithium and can be charged and discharged at a high operating voltage. When lithium salt is used as the supporting salt, self-discharge is suppressed. Moreover, it is preferable because the irreversible capacity at the time of charging can be reduced. Examples of the composite oxide include a lithium titanium composite oxide and a lithium vanadium composite oxide. Of these, the carbonaceous material is preferable as the negative electrode active material from the viewpoint of safety. This negative electrode may be formed by bringing the negative electrode active material and the current collector into close contact with each other. For example, the negative electrode active material, the conductive material, and the binder are mixed, and an appropriate solvent is added to form a paste-like negative electrode. The material may be applied and dried on the surface of the current collector, and if necessary, compressed to increase the electrode density. As the conductive material, the binder, the solvent and the like used for the negative electrode, those exemplified for the positive electrode can be used. The current collector of the negative electrode includes copper, nickel, stainless steel, titanium, aluminum, calcined carbon, conductive polymer, conductive glass, Al-Cd alloy, etc., as well as improved adhesiveness, conductivity and reduction resistance. For the purpose, for example, one in which the surface of copper or the like is treated with carbon, nickel, titanium, silver or the like can also be used. For these, it is also possible to oxidize the surface. The shape of the current collector can be the same as that of the positive electrode.

As the non-aqueous electrolyte solution of this non-aqueous secondary battery, an organic solvent containing a supporting salt or the like can be used. Examples of the organic solvent include carbonic acid ester and fluorine-containing carbonic acid ester. Examples of the carbonic acid ester include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate, vinylene carbonate, butylene carbonate and chloroethylene carbonate, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate and ethyl-. Examples thereof include chain carbonates such as n-butyl carbonate, methyl-t-butyl carbonate, di-i-propyl carbonate and t-butyl-i-propyl carbonate. As the fluorine-containing carbonic acid ester, for example, one or more hydrogens of the above-mentioned carbonic acid ester such as fluorinated cyclic carbonate and fluorinated chain carbonate may be replaced with fluorine. The organic solvent preferably contains one or more of the fluorinated cyclic carbonate and the fluorinated chain carbonate, and more preferably contains both. Specifically, examples of the fluorine-containing carbonic acid ester include trifluoropropylene carbonate, monofluoroethylene carbonate, difluoroethylene carbonate, fluoromethylmethyl carbonate, difluoromethylmethyl carbonate, trifluoromethylmethylcarbonate, and fluoromethyldifluoromethylcarbonate. Can be mentioned. Of these, trifluoropropylene carbonate and trifluoroethylmethyl carbonate are preferable. In addition to the carbonic acid ester, one or more other solvents such as esters, ethers, nitriles, furans, sulfolanes, and dioxolanes may be added to the non-aqueous electrolyte solution. The other solvent may not be contained in the electrolytic solution, or may be added in a small amount (for example, 10% by volume or less) so as not to change the properties of the electrolytic solution.

The supporting salts contained in this non-aqueous electrolyte solution include, for example, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiSO ₃ CF ₃ , LiN (SO ₂ F) ₂ , LiN (SO ₂ CF _{3 ) 2} , and LiC (SO). _{2 CF 3) 3, LiSbF 6} , LiSiF 6, LiAlF 4, LiSCN, LiClO 4, LiCl, LiF, LiBr, LiI, and the like LiAlCl _4. Of these, inorganic salts such as _{LiPF 6} , LiBF ₄ , LiAsF ₆ , and LiClO _{4 , and LiSO 3} CF ₃ , LiN (SO ₂ F) ₂ , LiN (SO ₂ CF ₃ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ It is preferable to use one kind or a combination of two or more kinds of salts selected from the group consisting of organic salts such as, from the viewpoint of electrical characteristics. The concentration of this supporting salt in the non-aqueous electrolytic solution is preferably 0.1 mol / L or more and 5 mol / L or less, and more preferably 0.5 mol / L or more and 2 mol / L or less. When the concentration at which the supporting salt is dissolved is 0.1 mol / L or more, a sufficient current density can be obtained, and when it is 5 mol / L or less, the electrolytic solution can be made more stable.

This non-aqueous electrolyte solution contains one or more of the following formulas (1) and (2) of a trifluorovinyl compound (hereinafter, also simply referred to as a trifluorovinyl compound). ^{However, R 1} and R ² in the formulas (1) and (2) are independently hydrogen, an alkyl group having 1 or more and 5 or less carbon atoms, an alkoxy group having 1 or more and 5 or less carbon atoms, and an aryl having 12 or less carbon atoms. It may be one of the groups. The aryl group may have 6 or more carbon atoms. The aryl group has one or more of an alkyl group having 1 to 5 carbon atoms, an alkoxy group having 1 to 5 carbon atoms, and an amino group having an alkyl group having 0 to 5 carbon atoms as a substituent. It may be a thing. Examples of the alkyl group include a methyl group, an ethyl group and a propyl group. Examples of the alkoxy group include a methoxy group and an ethoxy group. Examples of the aryl group and its derivatives include a phenyl group, an o-methylphenyl group, a p-methylphenyl group, an m-methylphenyl group, an o-methoxyphenyl group, an o-dimethylaminophenyl group and the like. Further, this trifluorovinyl compound may be, for example, a compound represented by the formula (3). This trifluorovinyl compound is preferably contained in, for example, a non-aqueous electrolytic solution in the range of 0.05 mol / L or more and 0.1 mol / L or less. When the trifluorovinyl compound is contained in the non-aqueous electrolyte solution in this range, it is preferable because the addition effect of further suppressing the volume decrease due to repeated charging and discharging and further reducing the internal resistance can be exhibited. ..

This non-aqueous secondary battery may include a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as it has a composition that can withstand the range of use of a non-aqueous secondary battery. For example, a polymer non-woven fabric such as a polypropylene non-woven fabric or a polyphenylene sulfide non-woven fabric, or a thin olefin resin such as polyethylene or polypropylene is used. Examples include microporous membranes. These may be used alone or in combination of two or more.

The shape of this non-aqueous secondary battery is not particularly limited, and examples thereof include a coin type, a button type, a sheet type, a laminated type, a cylindrical type, a flat type, and a square type. Further, it may be applied to a large-sized vehicle used for an electric vehicle or the like. FIG. 1 is a cross-sectional view showing an outline of the configuration of a coin-type non-aqueous secondary battery 20. The non-aqueous secondary battery 20 has a cup-shaped case 21, a positive electrode 22 having a positive electrode active material and provided at the lower part of the case 21, and a negative electrode active material via a separator 24 with respect to the positive electrode 22. It is provided with a negative electrode 23 provided at a position facing each other, a gasket 25 formed of an insulating material, and a sealing plate 26 disposed in an opening of the case 21 and sealing the case 21 via the gasket 25. .. The non-aqueous secondary battery 20 includes a non-aqueous electrolytic solution 27 in the space between the positive electrode 22 and the negative electrode 23. In this non-aqueous secondary battery 20, the positive electrode 22 may contain a positive electrode active material having a lithium reference potential of 4.5 V or more. Further, the non-aqueous electrolyte solution 27 contains one or more of the trifluorovinyl compounds of the formulas (1) and (2).

In the non-aqueous secondary battery described in detail above, the trifluorovinyl compound is polymerized on the negative electrode to form a film having low electron conductivity on the negative electrode, and the decomposition of the non-aqueous electrolytic solution is further suppressed, so that the capacity is reduced. Can be further suppressed and the internal resistance can be further reduced.

It goes without saying that the present disclosure is not limited to the above-described embodiment, and can be implemented in various embodiments as long as it belongs to the technical scope of the present disclosure.

For example, in the above-described embodiment, the non-aqueous secondary battery provided with the non-aqueous electrolytic solution has been described, but the non-aqueous electrolytic solution may be used. If this non-aqueous electrolyte solution is used in a non-aqueous secondary battery, the same effect as that of the above-mentioned non-aqueous secondary battery can be obtained.

Hereinafter, an example in which a non-aqueous secondary battery is specifically manufactured will be described as an example.

[Example 1]
A solvent containing trifluoropropylene carbonate as a fluorinated cyclic carbonate and trifluoroethylmethyl carbonate as a fluorinated chain carbonate in a volume ratio of 30/70 was used as a solvent. _{LiPF 6} was dissolved in this solvent at 1 mol / L as a supporting salt, and the trifluorovinyl compound of the formula (3) was dissolved at 0.05 mol / L as an additive. The obtained product was used as the non-aqueous electrolyte solution of Example 1.

[Comparative Example 1]
The electrolyte obtained through the same steps as in Example 1 except that the trifluorovinyl compound was not added was used as the electrolytic solution of Comparative Example 1.

[Examples 2 and 3]
The electrolytic solution of Example 2 was obtained through the same steps as in Example 1 except that the trifluorovinyl compound was 0.1 mol / L. Further, a solution obtained through the same steps as in Example 1 except that 0.05 mol / L of vinylene carbonate was added was used as the electrolytic solution of Example 3.

(Preparation of bipolar evaluation cell)
The positive electrode sheet was prepared as follows. _{85% by mass of LiNi 0.5} Mn _1.5 O ₄ as the positive electrode active material, 10% by mass of carbon black as the conductive material, 5% by mass of polyvinylidene fluoride as the binder, and an appropriate amount of N-methyl-2-pyrrolidone as the solvent. It was added to disperse the positive electrode active material and the like to obtain a slurry-like mixture. This slurry-like mixture was uniformly applied to a 15 μm-thick aluminum foil current collector and dried by heating to prepare a coating sheet. After that, the coating sheet was passed through a roll press to increase the density, and the mixture coating portion was cut into a shape having a width of 40 mm and a length of 40 mm to obtain a positive electrode sheet. The negative electrode sheet was prepared as follows. 95% by mass of graphite was mixed as the negative electrode active material, and 5% by mass of polyvinylidene fluoride was mixed as the binder to prepare a slurry-like mixture in the same manner as the positive electrode. This slurry-like mixture was uniformly applied to a copper foil current collector having a thickness of 10 μm and dried by heating to prepare a coating sheet. After that, the coating sheet was passed through a roll press to increase the density, and the mixture coating portion was cut into a shape having a width of 42 mm and a length of 42 mm to obtain a negative electrode sheet. The obtained positive electrode sheet and the negative electrode sheet were opposed to each other with a 25 μm-thick polyethylene separator interposed therebetween to prepare a laminated electrode body. This electrode body was sealed in an aluminum laminated bag, impregnated with the non-aqueous electrolytic solution prepared above, and then sealed to prepare a lithium ion secondary battery.

The produced lithium ion secondary battery was activated (charged / discharged). At a temperature of 25 ° C., the battery voltage was oxidized (charged) to 4.9 V at 0.2 C, and then reduced (discharged) to 3.5 V at 0.2 C for 3 cycles. The discharge capacity in the third cycle was defined as the initial capacity Qi.

(Evaluation of capacity retention rate and IV resistance increase rate after high temperature durability)
Under the temperature condition of 60 ° C., the above-mentioned prepared secondary battery is charged with a constant current of 2C to a charge upper limit voltage of 4.75V, and then discharged with a constant current of 2C to a discharge lower limit voltage of 3.5V. One cycle was set, and this cycle was performed for a total of 150 cycles. After adjusting the battery after the cycle to a charged state (SOC = 60%) of 60% of the battery capacity, a current of 0.5C, 1C, 2C, 5C, and 10C is applied at a measurement temperature of -10 ° C., and 10 seconds later. The battery voltage was measured. The applied current and voltage were linearly approximated, and the IV resistance, that is, the internal resistance of the battery was obtained from the slope. The rate of increase in IV resistance after high temperature endurance was calculated using the formula {(resistance after 150 cycles-initial resistance) / initial resistance x 100%}. The capacity retention rate after high temperature durability was calculated using the formula (discharge capacity after 150 cycles / initial discharge capacity × 100%).

(Results and discussion)
Addition amount (mol / L) of additives of Examples 1 to 3 and Comparative Example 1, capacity retention rate after high temperature endurance (%), IV resistance (Ω) after initial and high temperature endurance, IV resistance after high temperature endurance The rate of increase (%) is summarized in Table 1. FIG. 2 is a diagram showing the relationship between the capacity retention rate and the number of cycles in the high temperature durability test. FIG. 3 is a diagram showing the relationship between the initial IV resistance and the high temperature endurance. As shown in FIG. 2, in the cell of Comparative Example 1 in which the upper limit voltage of charging was increased to 4.75 V, the capacity retention rate tended to gradually decrease as the charge / discharge cycle continued. It was speculated that this was related to the decomposition of the electrolytic solution. On the other hand, in Examples 1 to 3 to which the trifluorovinyl compound was added, it was clarified that the decrease in the capacity retention rate was further suppressed as compared with Comparative Example 1. Further, as shown in FIG. 3, it was found that in Examples 1 to 3, the IV resistance after high temperature endurance was lower than the initial resistance. It is presumed that the reason for this is that a film having low electron conductivity based on a trifluorovinyl compound is formed on the negative electrode by charging / discharging added to the electrolytic solution, and this film suppresses the subsequent reductive decomposition of the electrolytic solution. ..

It should be noted that the present disclosure is not limited to the above-described examples, and it goes without saying that the present disclosure can be carried out in various aspects as long as it belongs to the technical scope of the present disclosure.

The non-aqueous electrolyte solution and the non-aqueous secondary battery disclosed in the present specification can be used in the technical field of the secondary battery.

20 non-aqueous secondary battery, 21 case, 22 positive electrode, 23 negative electrode, 24 separator, 25 gasket, 26 sealing plate, 27 non-aqueous electrolyte solution.

Claims (6)

A non-aqueous electrolyte solution used in non-aqueous secondary batteries.
The trifluorovinyl compound containing any one or more of the following formulas (1) and (2) is contained.
Non-aqueous electrolyte.

The non-aqueous electrolyte solution according to claim 1, which contains the compound of the formula (3) as the trifluorovinyl compound.

The non-aqueous electrolyte solution according to claim 1 or 2, which contains the trifluorovinyl compound in the range of 0.05 mol / L or more and 0.1 mol / L or less. The non-aqueous electrolyte solution according to any one of claims 1 to 3, which contains one or more of a fluorinated cyclic carbonate and a fluorinated chain carbonate as an organic solvent. Positive electrode with positive electrode active material and positive electrode
Negative electrode with a negative electrode active material and
The non-aqueous electrolyte solution according to any one of claims 1 to 4, which is interposed between the positive electrode and the negative electrode and conducts ions.
Non-water-based rechargeable battery equipped with. The non-aqueous secondary battery according to claim 5, wherein the positive electrode has the positive electrode active material having a fully charged positive electrode potential of 4.5 V or more at a lithium reference potential.

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