JP2001176551A - Non-aqueous electrolyte and lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a secondary battery excel lent in battery characteristics, productivity, and safety. SOLUTION: The lithium secondary battery having an electrolytic layer including positive and negative electrodes and an electrolyte includes ketones of 0.01-10 weight % of the electrolyte.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a non-aqueous electrolyte and a lithium secondary battery using the same.

[0002]

2. Description of the Related Art As a non-aqueous electrolyte used for a lithium secondary battery or the like, a non-aqueous electrolyte containing an electrolyte salt and a non-aqueous organic solvent is known. The organic solvent used here must have a relatively high dielectric constant in order to ionize the supporting salt, and must not be decomposed by charge and discharge in order to be used as a battery. A preferable organic solvent used from such a viewpoint includes propylene carbonate. Propylene carbonate has a high ionic conductivity in a wide temperature range, and is excellent in that it has no problem of volatilization and liquid leakage even in use at a particularly high temperature because it is a high boiling point solvent, and is an extremely effective solvent. But,
On the other hand, there is a problem that it is susceptible to decomposition due to a reduction reaction on the negative electrode surface during charging.

With respect to such an oxidation-reduction reaction on the electrode surface during charge / discharge, it is possible to form a film on the electrode surface by adding a specific additive to the electrolytic solution, thereby improving the capacity with high charge / discharge efficiency. Done. The composition of the coating changes with the use of additives in the electrolyte. The additives are
The addition of a small amount of, for example, 0.01 to 10% by weight to the electrolytic solution has a great effect on the formation of the film, and the effect on the battery performance is very large depending on what kind of additive is used. .

[0004]

In a secondary battery such as a lithium secondary battery, not only the capacity is large, but also a sufficient capacity can be ensured even at a high rate (rate characteristic). It is required that the capacity does not decrease (high cycle characteristics) even if the above is performed, and various improvements have been made. However, in recent years, demands for higher performance have been increasing, and further improvements in these characteristics have been demanded. The above-mentioned additives have a large effect on these required properties, and more effective additives have been required.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and an object of the present invention is to provide a high-capacity battery having high rate characteristics and an electrolytic solution used for the same. The present inventors have conducted intensive studies to achieve the above object, and as a result, have found that a ketone compound may be used as the additive, and have completed the present invention. That is,
The gist of the present invention is characterized in that in a non-aqueous electrolyte containing an electrolyte salt and a non-aqueous organic solvent, the electrolyte further contains a ketone in an amount of 0.01 to 10% by weight based on the electrolyte. Non-aqueous electrolyte solution and a lithium secondary battery using the same.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION Ketones to be used as additives in the present invention may be those having two carbon atoms bonded to a carbon atom constituting a carbonyl group. What is necessary is just to have at least one inside. Examples of the ketones include a cyclic ketone in which a carbon atom of a ketone group forms a part of a cyclic structure of a molecule and a chain ketone in which a carbon atom of a ketone group forms a part of a chain structure of a molecule. it can. Further, an aromatic ketone having an aromatic group can also be used. Preferred are aliphatic cyclic ketones, aliphatic chain ketones and aromatic ketones. When a cyclic ketone is used, the ring having a ketone group is preferably an 8-membered ring or less in view of the effect of suppressing gas generated by decomposition of the electrolyte solvent at the time of initial charging.

The ketones used may have a substituent other than a ketone group such as an ester group or an ether group in the molecule. Further, those having two or more ketone groups in the molecule, such as diketones, or those having a conjugated ketone structure containing an unsaturated group conjugated to the ketone group may be used.
The molecular weight of the ketone used is usually 500 or less, preferably 300 or less. If the molecular weight is too large, the influence of the inhibiting factor on the charge / discharge due to the other structure is increased more than the effect of the ketone group, and the ion conduction may be inhibited, resulting in an adverse effect.
Further, from the viewpoint of suppressing the irreversible capacity during charge and discharge and improving the cycle life, those having a structure having no hydroxyl group, carboxyl group or amino group in the molecule are more effective in terms of efficiency.

Specific examples of ketones to be used include aliphatic cyclic ketones such as cyclobutanone, cyclopentanone, cyclohexanone and cycloheptanone; and aliphatic chain ketones such as 4-heptanone and 4-methyl-2-pentanone. Aromatic ketones such as acetophenone and 2'-acetonaphthone; conjugated ketones such as isophorone;
Diketones such as cyclohexanedione and acetonylacetone; ketoesters such as ethyl levulinate and ethylbenzoyl formate; ether ketones such as 2-methoxycyclohexanone and 4H-pyran-4-one.

The use amount of these additives is 0.01% by weight or more, preferably 0.05% by weight, based on the whole electrolyte.
As described above, the content is more preferably 0.07% by weight or more, and 10% by weight or less, preferably 7% by weight or less. If the amount is too large, the ketones may act as an inhibitor of lithium ion migration in the electrolyte, resulting in a decrease in ionic conductivity, resulting in a decrease in capacity at high rates. Conversely, if the amount used is too small, sufficient effects will not be exhibited, and gas will be generated due to decomposition of the electrolyte solvent particularly at the time of initial charging, resulting in an increase in resistance during charging and a decrease in charge / discharge capacity. Sometimes.

[0010] The electrolytic solution of the present invention contains an electrolyte salt and a non-aqueous organic solvent. The electrolyte salt is not particularly limited as long as it is a lithium salt having lithium as a cation. LiPF ₆ , LiCl ₄ , LiBF ₄ , LiCF ₃
Although SO _{3 and the} like can be exemplified, among them, LiPF ₆ is particularly preferable in that it provides high ionic conductivity and high-rate discharge characteristics. The concentration of these electrolyte salts contained in the electrolytic solution is preferably in the range of 0.5 mol or more and 2.0 mol or less per 1 L of the electrolytic solution in order to provide high ionic conductivity. As the non-aqueous organic solvent used for the electrolyte,
Ethylene carbonate, propylene carbonate, γ-
Organic solvents such as butyrolactone, dimethyl carbonate, diethyl carbonate, 1,2-dimethoxyethane, and 1,2-diethoxyethane can be exemplified. Preferably, the solvent contains propylene carbonate. As a result, a high ionic conductivity can be obtained in a wide temperature range, and the effect that there is little problem of volatilization and liquid leakage even at a high temperature can be obtained. Particularly preferably, the solvent contains propylene carbonate and ethylene carbonate. Further, as a solvent, a mixed solvent of a high-boiling solvent such as ethylene carbonate, propylene carbonate, and γ-butyrolactone and a low-boiling solvent such as dimethyl carbonate, diethyl carbonate, 1,2-dimethoxyethane, and 1,2-diethoxyethane. Can also be used. The electrolytic solution of the present invention can be used for a lithium secondary battery.

The lithium secondary battery of the present invention has a positive electrode, a negative electrode, and an electrolyte, and is used in an amount of 0.01 to 10 with respect to the electrolyte.
Contains ketones by weight. Ketones are usually contained in the electrolytic solution, but may be contained in the positive electrode or the negative electrode. In particular, it is effective to include ketones in the negative electrode in terms of forming a film on the active material surface of the negative electrode. The amount of ketones used is preferably 0.0
It is at least 5% by weight, more preferably at least 0.07% by weight, and preferably at most 7% by weight. If the amount is too large, the ketones may act as an inhibitor of lithium ion migration in the electrolyte, resulting in a decrease in ionic conductivity, resulting in a decrease in capacity at high rates. Conversely, if the amount used is too small, sufficient effects will not be exhibited, and gas will be generated due to decomposition of the electrolyte solvent particularly at the time of initial charging, resulting in an increase in resistance during charging and a decrease in charge / discharge capacity. Sometimes.

The positive electrode and the negative electrode used in the secondary battery of the present invention may be appropriately selected according to the type of the battery, and contain at least an active material corresponding to the positive electrode and the negative electrode. Further, a binder for fixing the active material may be contained. Examples of the positive electrode active material that can be used in the lithium secondary battery of the present invention include oxides having transition metals such as Fe, Co, Ni, and Mn, composite oxides with lithium, and inorganic compounds such as sulfides. . Specifically, Mn
Transition metal oxides such as O, V ₂ O ₅ , V ₆ O ₁₃ and TiO ₂ ; composite oxides of lithium and transition metals such as lithium nickelate, lithium cobaltate and lithium manganate; TiS ₂ and FeS; Transition metal sulfides. In addition, examples of the positive electrode active material include organic compounds such as a conductive polymer such as polyaniline.
Of course, a plurality of the above active materials may be mixed and used.
When the active material is granular, the particle size is usually 1 to 30 μm, and preferably about 1 to 10 μm, from the viewpoint of excellent battery characteristics such as rate characteristics and cycle characteristics.

As the negative electrode active material that can be used in the lithium secondary battery of the present invention, lithium metal or lithium alloy can be used. However, since the use of additives has a large effect of forming a film, lithium It is particularly preferable to use a carbonaceous substance such as coke, acetylene black, mesophase microbeads, and graphite as the compound capable of inserting and extracting ions. The particle size of the granular negative electrode active material is usually about 1 to 50 μm, preferably about 15 to 30 μm from the viewpoint of excellent battery characteristics such as initial efficiency, rate characteristics, and cycle characteristics.

As the binder that can be used for the positive electrode and the negative electrode, various materials are used from the viewpoint of weather resistance, chemical resistance, heat resistance, flame retardancy and the like. Specifically, inorganic compounds such as silicate and glass, alkane polymers such as polyethylene, polypropylene and poly-1,1-dimethylethylene; unsaturated polymers such as polybutadiene and polyisoprene; polystyrene, polymethylstyrene, Polymers having a ring such as polyvinylpyridine, poly-N-vinylpyrrolidone; polymethyl methacrylate, polyethyl methacrylate, polybutyl methacrylate, polymethyl acrylate, polyethyl acrylate, polyacrylic acid, polymethacrylic acid, poly Acrylic derivative polymers such as acrylamide; fluorine resins such as polyvinyl fluoride, polyvinylidene fluoride, and polytetrafluoroethylene; CN group-containing polymers such as polyacrylonitrile and polyvinylidene cyanide; polyvinyl acetate Polyvinyl alcohol polymers such as polyvinyl alcohol;
Halogen-containing polymers such as polyvinyl chloride and polyvinylidene chloride; conductive polymers such as polyaniline can be used. Further, a mixture of the above-mentioned polymers, a modified product, a derivative, a random copolymer, an alternating copolymer, a graft copolymer, a block copolymer, or the like can also be used. The weight average molecular weight of these resins is usually 10,000
-300000, preferably 100000-1000
It is about 000. If it is too low, the strength of the electrode tends to decrease. On the other hand, if it is too high, the viscosity becomes high, and it may be difficult to form an electrode. Preferred binder resins are fluororesins and polymers containing CN groups.

The amount of the binder to be used per 100 parts by weight of the active material is usually at least 0.1 part by weight, preferably at least 1 part by weight, and is usually at most 30 parts by weight, preferably at most 20 parts by weight. If the amount of the binder is too small, the strength of the electrode tends to decrease, and if the amount of the binder is too large, the ion conductivity tends to decrease. In order to improve the conductivity and mechanical strength of the electrode, an electrode that exhibits various functions such as a conductive material and a reinforcing material, a powder, a filler, and the like may be contained in the electrode. The conductive material is not particularly limited as long as it is capable of imparting conductivity by mixing an appropriate amount with the above-mentioned active material, but usually, acetylene black, carbon black, carbon powder such as graphite, and various metal fibers, Foil and the like. The DBP oil absorption of the carbon powder conductive material is preferably 120 cc / 100 g or more,
In particular, 150 cc / 100 g or more is preferable because it holds the electrolytic solution. Additives include trifluoropropylene carbonate, 1,6-Dioxaspiro
[4,4] nonane-2,7-dione, 12-
Crown-4-ether, vinylene carbonate, catechol carbonate, and the like can be used to increase the stability and life of the battery. As the reinforcing material, various inorganic or organic spherical or fibrous fillers can be used.

The electrode can be formed by applying and drying a paint containing components such as an active material and a binder and a solvent. The thickness of the electrode is usually at least 1 μm, preferably at least 10 μm, more preferably at least 20 μm, most preferably at least 40 μm, and usually at least 20 μm.
It is 0 μm or less, preferably 150 μm or less, more preferably 100 μm or less. If the thickness is too thin, application becomes difficult and uniformity is hardly ensured, and the battery capacity may be too small. On the other hand, if the thickness is too large, the rate characteristics may be too low.

At least one of the positive electrode and the negative electrode is
Usually formed on a current collector. Various collectors can be used, but usually a metal or an alloy is used. Specifically, the current collector of the positive electrode includes aluminum, nickel, and SUS, and the current collector of the negative electrode includes copper, nickel, and SUS. Preferably, aluminum is used as the current collector of the positive electrode, and copper is used as the current collector of the negative electrode. Since the effect of binding to the positive and negative electrode layers is improved, it is preferable that the surfaces of these current collectors are previously subjected to a roughening treatment. Examples of the surface roughening method include a method such as blasting and rolling with a rough roll, a polishing cloth paper to which abrasive particles are fixed, a grindstone, an emery buff, a wire brush provided with a steel wire, or the like. A mechanical polishing method for polishing the surface, an electrolytic polishing method, a chemical polishing method, and the like can be given.

In order to reduce the weight of the battery, that is, to increase the weight energy density, a perforated current collector such as an expanded metal or a punched metal may be used. In this case, the weight can be freely changed by changing the aperture ratio. In addition, when an active material is present on both surfaces of such a perforated current collector, peeling of the coating film tends to be less likely to occur due to the rivet effect of the coating film through this hole, but the aperture ratio is too large. When it becomes higher, the contact area between the coating film and the current collector becomes smaller, so that the adhesive strength may be lowered.

The thickness of the current collector is usually 1 μm or more, preferably 5 μm or more, and usually 100 μm or less, preferably 50 or less. If it is too thick, the overall capacity of the battery will be too low, and if it is too thin, handling may be difficult. The electrolytic solution of the present invention may be gelled by a polymer to be in a semi-solid state. The amount of the electrolyte used in the semi-solid electrolyte is usually at least 30% by weight, preferably 5% by weight, based on the total amount of the semi-solid electrolyte.
The content is 0% by weight or more, more preferably 75% by weight or more, and usually 99.95% by weight or less, preferably 99% by weight or less, more preferably 98% by weight or less. If the amount is too large, it is difficult to hold the electrolytic solution, and liquid leakage tends to occur. Conversely, if the amount is too small, the charge / discharge efficiency and capacity may be insufficient.

When the above-mentioned semi-solid electrolyte is used, the amount of the ketones used is usually at least 0.01% by weight, preferably at least 0.05% by weight, more preferably at least 0.05% by weight, based on the semi-solid electrolyte. Preferably at least 0.07% by weight,
Further, it is usually at most 10% by weight, preferably at most 7% by weight. If the amount is too large, the ketones may act as inhibitors of lithium ion migration in the electrolyte, lowering ionic conductivity and increasing internal resistance, which may result in lower capacity at high rates. Conversely, if the amount used is too small, the effect of the present invention will be insufficient, and gas will be generated due to decomposition of the electrolyte solvent during the initial charging, and as a result, the resistance during charging will increase and the charge / discharge capacity will decrease. May be invited.

It is preferable that a porous spacer is provided between the positive electrode and the negative electrode in order to prevent a short circuit.
That is, in this case, the electrolytic solution is used by being impregnated in a porous spacer. As a material for the spacer, polyolefin such as polyethylene or polypropylene, polytetrafluoroethylene, polyether sulfone, or the like can be used, but polyolefin is preferable. The thickness of the spacer is usually 1 μm or more, preferably 5 μm or more, more preferably 10 μm or more.
It is 0 μm or less, preferably 40 μm or less, and more preferably 30 μm or less. If the porous film is too thin, the insulating properties and mechanical strength may deteriorate. If the porous film is too thick, not only the battery performance such as rate characteristics may deteriorate, but also the energy density of the whole battery may decrease. The porosity of the spacer is usually 20% or more, preferably 35% or more, more preferably 45% or more, and usually 90% or more.
%, Preferably 85% or less, more preferably 75% or less.
% Or less. If the porosity is too small, the film resistance tends to increase and the rate characteristics tend to deteriorate. On the other hand, if it is too large, the mechanical strength of the film tends to decrease, and the insulating property tends to decrease.
The average pore size of the spacer is usually 0.5 μm or less, preferably 0.2 μm or less, and usually 0.05 μm or more. If it is too large, a short circuit is likely to occur, and if it is too small, the film resistance increases and the rate characteristics may deteriorate.

[0022]

EXAMPLES [Production of Positive Electrode] A current collector made of aluminum having a thickness of 20 μm was coated with lithium cobaltate (having an average particle diameter of 5 μm).
μm) 90% by weight and polyvinylidene fluoride (PVdF)
A coating containing 5% by weight and 5% by weight of acetylene black was applied and dried to obtain a positive electrode. [Production of Negative Electrode] A current collector made of copper having a thickness of 20 μm was provided with 87.4% by weight of mesocarbon particles (average particle size: 6 μm) and 9.7% by weight of PVdF.
And a coating containing 2.9% by weight of acetylene black 2 was applied and dried to obtain a negative electrode. [Manufacture of lithium secondary battery] LiPF ₆ at 1 mol / L
Of propylene carbonate containing ethylene carbonate in a proportion of 93% by weight of a mixed solvent (mixing volume ratio of 1: 1) containing propylene carbonate and ethylene carbonate at a ratio of 4.67% by weight of polyethylene glycol diacrylate and trimethylol 2.33% by weight of propane ethylene oxide-modified triacrylate and 0.1% by weight of a polymerization initiator were further added to obtain a semi-solid electrolyte precursor.

The positive electrode, the negative electrode, and a film thickness of 16 μm;
The semisolid electrolyte precursor was applied and impregnated on a polyethylene biaxially stretched porous membrane film having a porosity of 45% and an average pore diameter of 0.05 μm, and these were laminated.
By heating at 5 ° C. for 5 minutes, a battery element having an electrolyte layer made of a non-fluid electrolyte was obtained. The obtained battery element was vacuum-sealed while protruding the terminals of the positive electrode and the negative electrode from a laminate film in which both surfaces of an aluminum layer were covered with a resin layer, to obtain a lithium secondary battery for evaluation.

[Evaluation of battery characteristics] Initial characteristics and load characteristics: Discharge amount per hour of lithium cobalt oxide was 120 mA.
h / g, a discharge rate of 1 C was determined from the ratio of this to the amount of the active material of the positive electrode of the lithium secondary battery for evaluation, and the rate was set. The initial capacity was determined at the time of charging and at the time of discharging, respectively. The initial efficiency was determined from these ratios. Next, the battery was charged at 1 C and then discharged at 2 C, and the obtained discharge capacity was set to a high rate capacity. Furthermore, the obtained high rate capacity and the 0.2C
The capacity retention ratio was determined from the ratio to the discharge capacity at the time.

Further, the presence or absence of unevenness due to gas generation appearing outside the laminate film at the time of initial charging of the lithium secondary battery was visually observed. Those that could be observed were NG, and those that could not be observed were OK. Examples 1 to 22 and Comparative Example 1 The additives shown in Table 1 were used as additives in an electrolytic solution (Li
The PF ₆ and propylene carbonate the total amount of the additive and a mixed solvent of ethylene carbonate) with respect to the results of using the amounts described in Table 1 are shown in Table 1. As is clear from Table 1, it can be seen that the addition of ketones provides a high initial capacity, high initial efficiency, and excellent rate characteristics.

[0026]

[Table 1]

[0027]

According to the present invention, a secondary battery having high capacity and excellent rate characteristics can be obtained, and a secondary battery excellent in productivity and safety can be obtained.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Masamichi Onuki 1000 Kamoshita-cho, Aoba-ku, Yokohama-shi, Kanagawa Prefecture Mitsubishi Chemical Corporation Yokohama Research Laboratory F-term (reference) 5H029 AJ03 AJ05 AK02 AK03 AK05 AK16 AL06 AL07 AL12 AM03 AM04 AM05 AM07 HJ01 HJ10

Claims (7)

[Claims] 1. A non-aqueous electrolytic solution containing an electrolyte salt and a non-aqueous organic solvent, characterized in that the electrolytic solution further contains 0.01 to 10% by weight of ketones based on the electrolytic solution. Non-aqueous electrolyte. 2. The method according to claim 1, wherein the ketone is a cyclic ketone.
The electrolyte according to any one of the above. 3. The method according to claim 1, wherein the ketone is a chain ketone.
The electrolyte according to any one of the above. 4. The electrolytic solution according to claim 1, wherein the non-aqueous organic solvent contains propylene carbonate. 5. The electrolytic solution according to claim 4, wherein the non-aqueous organic solvent contains propylene carbonate and ethylene carbonate. 6. A lithium secondary battery using the electrolytic solution according to claim 1. 7. The lithium secondary battery according to claim 6, wherein a negative electrode containing a negative electrode active material made of a carbonaceous material is used.