JP6074728B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte secondary battery.

  In recent years, lithium primary batteries, lithium ion secondary batteries, lithium polymer secondary batteries, and the like have been put to practical use as high energy density power storage devices.

  In particular, lithium ion secondary batteries are attracting attention and are being developed not only as power sources for mobile devices, but also as storage batteries for applications such as hybrid vehicles and electric vehicles, and household power storage. These applications may be used in harsh environments such as low and high temperatures, and rapid charge / discharge characteristics and long-term reliability in such situations are required.

  Lithium ion secondary batteries use materials that can occlude and release lithium ions for the positive electrode and negative electrode, with the positive electrode and negative electrode facing each other through a separator to prevent electrical contact and a lithium ion conductive electrolyte. It has a configuration. In the battery, through the electrolyte, a reaction occurs in which lithium ions are released from the positive electrode during storage and stored in the negative electrode, and conversely, during discharge, the positive electrode stores lithium ions released from the negative electrode. At this time, electrons corresponding to the movement of lithium ions in the battery are transferred through an external circuit, and charging / discharging is performed.

Conventionally, a carbon-based material is generally used as a negative electrode active material capable of occluding and releasing lithium ions. However, in a negative electrode using a carbon material, the reaction potential of the insertion and release of lithium ions is as low as 0.1 V (vs. Li ^/ Li ⁺ ) or less, and is close to the dissolution and precipitation potential of lithium. Precipitation of metallic lithium on the negative electrode surface tends to occur during rapid charging in the environment. The deposition of metallic lithium on the negative electrode not only causes an internal short circuit, but also causes a decrease in capacity due to passivation, which may cause battery malfunction and performance deterioration.

  Thus, studies have been made on oxide-based materials as negative electrode active materials that can replace carbon materials. Oxide-based materials can be reversibly charged and discharged by inserting and extracting lithium ions into the sites in the crystal lattice consisting of anions and cations. Good characteristics are expected as an electrode material for repetitive secondary batteries.

  At the same time, the oxide-based material has a higher reaction potential with lithium than the carbon material, and is sufficiently noble than the dissolution precipitation potential of lithium. For this reason, in the oxide-based material, precipitation of metallic lithium, which is one of the problems with the above-described carbon material, can be suppressed, and the reliability of the battery can be further improved.

However, even if it is an oxide material, if the reaction potential with lithium is less than 0.8 V (vs. Li ^/ Li ⁺ ), the difference from the dissolution precipitation potential of lithium is small, and the carbon material In the same manner as above, precipitation of metallic lithium on the negative electrode is likely to occur. In addition, when a carbon material is added as a conductive agent, the carbon material causes a charge / discharge reaction, and the volume change of the negative electrode and the decomposition reaction of the electrolytic solution on the surface occur more significantly. It may be a factor to accelerate.

Here, lithium titanate will be described as an example of a typical oxide material that can be used as a negative electrode active material of a lithium ion secondary battery. Lithium titanate is a composite oxide having a spinel crystal structure similar to that of lithium manganate used as a positive electrode active material, and can reversibly occlude and release lithium ions. However, unlike lithium manganate, since it reacts reversibly with lithium at a low potential of about 1.55 V (vs. Li ^/ Li ⁺ ), it can be used as a negative electrode active material. Further, when lithium titanate is used at this potential, the volume change during the charge / discharge reaction is very small even when compared with general oxide materials. Therefore, lithium titanate can be expected to have particularly excellent characteristics as an electrode material for a secondary battery that repeats charging and discharging.

Specifically, a lithium titanate compound represented by the general formula Li _a Ti _3-a O ₄ (wherein a represents a number of 0 <a <3) is used for the negative electrode, and the general formula: LiCo _b Ni _1-b O ₂ (0 ≦ b ≦ 1), LiAl _c Co _d Ni _1-cd O ₂ (0 ≦ c ≦ 1, 0 ≦ d ≦ 1, 0 ≦ c + d ≦ 1) Non-aqueous secondary batteries used in the field have already been proposed.

However, in general, an oxide-based material such as lithium titanate has a lower conductivity than a carbon material used as a conventional negative electrode active material, and may be inferior in charge / discharge performance at a relatively large current. It is done. This is a problem in terms of high rate characteristics and power characteristics that require input / output with a large current, and therefore it is widely practiced to improve the current collection by adding a carbon material as a conductive agent during electrode construction. Yes. For example, in Patent Document 1, it is proposed to use an electrode to which a conductive agent containing carbon fiber is added as a means for improving the current collecting property. Further, in Patent Document 2, the characteristics are further improved by using a vapor-grown carbon fiber graphitized so that the lattice constant C _{0 in the} stacking direction is 6.7 mm or more and 6.8 mm or less as a conductive agent. It is disclosed that. The value of C ₀ shown above corresponds to a value of d ₀₀₂ widely used to define the structure of graphite, which is 0.335 nm (3.35 Å) or more and 0.340 nm (3.40 Å) or less. To do.

  On the other hand, in the lithium ion secondary battery, deterioration due to the reaction of the nonaqueous electrolyte on the positive electrode surface and the negative electrode surface may be a problem. This is because the non-aqueous electrolyte composed of a non-aqueous electrolyte causes a reduction reaction on the negative electrode surface and an oxidation reaction on the positive electrode surface, which causes not only deterioration of battery characteristics but also swelling of the battery due to gas generation. It also becomes. As a non-aqueous solvent for non-aqueous electrolytes, a cyclic carbonate having a high dielectric constant and a low-viscosity chain carbonate are generally used as a mixture, but deterioration occurs due to the use of propylene carbonate (propylene carbonate) among cyclic carbonates. The suppression effect is obtained. Patent Document 3 also discloses that propylene carbonate is rich in electrochemical redox stability.

JP 2001-196060 A JP 2005-317509 A JP-A-4-155775

As described above, when a graphite material having a high effect of improving the current collecting property in the electrode is used as the conductive agent, the reaction between propylene carbonate, which is a nonaqueous solvent for the electrolyte, and the graphite material becomes a problem. A negative electrode active material having a relatively high reaction potential of 1.55 V (vs. Li ^/ Li ⁺ ) such as lithium titanate does not cause a problem under normal use conditions. However, when the potential of the negative electrode is lowered due to some trouble, the above-described reaction between propylene carbonate and the graphite material occurs, and the battery characteristics are greatly impaired. In addition, when a negative electrode active material having a reaction potential of 0.8 V (vs. Li ^/ Li ⁺ ) or less is used, the reaction between propylene carbonate and graphite material is a problem even under normal charge / discharge conditions. End up.

  The object of the present invention is to provide a nonaqueous electrolyte secondary battery that improves such problems of the prior art, has excellent charge / discharge characteristics and reliability, and suppresses deterioration of characteristics during overcharge.

The non-aqueous electrolyte secondary battery of the present invention is
A positive electrode;
A negative electrode,
A separator disposed between the positive electrode and the negative electrode;
A non-aqueous electrolyte,
With
The negative electrode contains an active material that occludes and releases lithium at a reaction potential higher than 0.8 V on the basis of a dissolution and precipitation potential of lithium, and a conductive agent including a graphite material,
The nonaqueous electrolyte contains propylene carbonate, and further contains at least one compound selected from vinylene carbonate and fluoroethylene carbonate.

  In the nonaqueous electrolyte secondary battery of the present invention, the negative electrode containing an active material that occludes and releases lithium at a reaction potential higher than 0.8 V with respect to the dissolution and precipitation potential of lithium, and a conductive agent including a graphite material. Thus, a nonaqueous electrolyte containing propylene carbonate is used. In such a combination of the negative electrode and the non-aqueous electrolyte, in the non-aqueous electrolyte secondary battery of the present invention, the non-aqueous electrolyte further contains at least one compound selected from vinylene carbonate and fluoroethylene carbonate. Thereby, the non-aqueous electrolyte secondary battery of the present invention has excellent charge / discharge characteristics and reliability, and at the same time, suppresses the reaction between the non-aqueous electrolyte and the conductive agent containing the graphite material. Characteristic deterioration can also be suppressed.

(A) is a perspective view which shows the lithium ion secondary battery which is one Embodiment of the nonaqueous electrolyte secondary battery of this invention, (b) was along the II line | wire of Fig.1 (a). It is sectional drawing, (c) is sectional drawing which expands and shows the electrode group 13 shown to Fig.1 (a) and (b). FIG. 3 is a diagram showing dimensions of the positive electrode 1 produced in Example 1. FIG. 3 is a diagram showing dimensions of a negative electrode 2 produced in Example 1.

In the nonaqueous electrolyte secondary battery of the present invention, a negative electrode active material that occludes and releases lithium at a reaction potential higher than 0.8 V on the basis of the dissolution and precipitation potential of lithium (hereinafter, “charge / discharge reaction potential is 0.8 V ( vs. Li ^/ Li ⁺ ) ”) and a negative electrode containing a conductive agent including a graphite material. In the nonaqueous electrolyte secondary battery of the present invention, a nonaqueous electrolyte containing propylene carbonate as a nonaqueous solvent is used in combination with such a negative electrode. In the non-aqueous electrolyte secondary battery of the present invention, the non-aqueous electrolyte further includes 1,3-propane sultone (1,3-Propanesultone), vinylene carbonate (Vinylene carbonate, 1,3-Dioxo-2-one), and fluoroethylene. By including at least one kind of compound selected from carbonate (Fluoroethylene carbonate, 4-Fluoro-1,3-dioxolin-2-one), it is possible to suppress deterioration of characteristics during overcharge.

In this respect, a non-aqueous electrolyte secondary battery using a negative electrode containing a negative electrode active material having a charge / discharge reaction potential exceeding 0.8 V (vs. Li ^/ Li ⁺ ) and a conductive agent including a graphite material, The present inventors have studied the nonaqueous electrolyte and have arrived at the present invention. Hereinafter, embodiments of the present invention will be described in detail. The following embodiment is an example of the present invention, and the present invention is not limited to the following embodiment.

  First, a configuration example of a lithium ion secondary battery which is an embodiment of the nonaqueous electrolyte secondary battery of the present invention will be described.

  FIG. 1A is a perspective view of the lithium ion secondary battery of the present embodiment. FIG.1 (b) has shown the cross section along the II line | wire in Fig.1 (a).

  As shown in FIGS. 1A and 1B, the lithium ion secondary battery of the present embodiment includes an electrode group 13, a battery case 14 that houses the electrode group 13, and a non-filled battery case 14. A water electrolyte solution (non-aqueous electrolyte) 15. The positive electrode in the electrode group 13 is connected to the positive electrode lead 11. The negative electrode in the electrode group 13 is connected to the negative electrode lead 12. The positive electrode lead 11 and the negative electrode lead 12 are drawn out of the battery case 14.

  The non-aqueous electrolyte 15 contains propylene carbonate (PC) as a non-aqueous solvent. The non-aqueous solvent preferably contains 20% by weight or more, more preferably 40% by weight or more, because propylene carbonate can suppress deterioration of battery characteristics.

The nonaqueous electrolytic solution 15 further contains at least one compound selected from 1,3-propane sultone (PS), vinylene carbonate (VC), and fluoroethylene carbonate (FEC). As the supporting electrolyte salt contained in the nonaqueous electrolytic solution 15, those usable as the supporting electrolyte salt of the lithium ion secondary battery can be used. For example, a solution in which LiPF ₆ (commercial battery grade) is dissolved in propylene carbonate (commercial battery grade) as a supporting electrolyte salt at a concentration of 1 mol / L, which is 1,3-propane sultone, vinylene carbonate and fluoro A solution to which at least one compound selected from ethylene carbonate is further added can be used as the non-aqueous electrolyte 15. In the present embodiment, as an example of the non-aqueous electrolyte solution 15, a combination of the non-aqueous solvent and the supporting electrolyte salt is used. However, it is a nonaqueous electrolyte containing propylene carbonate and at least one compound selected from 1,3-propane sultone, vinylene carbonate, and fluoroethylene carbonate, and used for a lithium ion secondary battery. However, other combinations than the combination of the non-aqueous solvent and the supporting electrolyte salt may be used. For example, cyclic carbonates such as ethylene carbonate and chain carbonates such as dimethyl carbonate and ethyl methyl carbonate may further be contained as a non-aqueous solvent. Further, a cyclic ester such as gamma butyrolactone and a chain ester such as methyl propionate may further be contained as a non-aqueous solvent. As the supporting electrolyte salt, other lithium salts may be used as long as they can be used as a supporting electrolyte salt of an electrolyte solution for lithium ion batteries, such as lithium tetrafluoroborate.

FIG. 1C shows an enlarged cross section of the electrode group 13. As shown in FIG. 1C, the electrode group 13 includes a positive electrode 1, a negative electrode 2, and a separator 3 provided between the positive electrode 2 and the negative electrode 2. The positive electrode 1 includes, for example, a positive electrode current collector 1a made of an aluminum foil, and a positive electrode mixture layer 1b containing LiNi _0.80 Co _0.15 Al _0.05 O ₂ disposed on the surface of the positive electrode current collector 1a. . On the other hand, the negative electrode 2 includes, for example, a negative electrode current collector 2a made of an aluminum foil, and a negative electrode mixture layer 2b containing lithium titanate (Li ₄ Ti ₅ O ₁₂ ) disposed on the surface of the negative electrode current collector 2a. have. The separator 3 is made of a nonwoven fabric sheet made of polypropylene, for example. However, when a separator using polyolefin such as polypropylene and a non-aqueous solvent mainly composed of propylene carbonate are used in combination, the separator is subjected to a surface treatment for improving wettability such as corona discharge treatment and plasma discharge treatment. It is preferable to apply. Moreover, a paper-like sheet | seat which consists of a cellulose fiber etc. can also be used for a separator, In this case, surface treatment is not required.

As the positive electrode active material of the positive electrode mixture layer 1b, a lithium-containing transition metal oxide other than LiNi _0.80 Co _0.15 Al _0.05 O ₂ may be used. For _{example, Li x CoO 2, Li x} NiO 2, Li x MnO 2, Li x Co y Ni 1-y O 2, Li x Co y M 1-y O z, Li x Ni 1-y M y O z, Li _x Mn ₂ O ₄ , Li _x Mn _{2 -y My} O ₄ (M is Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb and B X, y, and z satisfy x = 0 to 1.2, y = 0 to 0.9, and z = 1.7 to 2.3. Other than these materials, any material can be used as the positive electrode active material as long as the potential of the positive electrode 1 during charging exceeds 4 V on the basis of lithium. A plurality of different materials may be mixed and used as the positive electrode active material. When the positive electrode active material is a powder, the average particle diameter is not particularly limited, but for example, a powder having an average particle diameter of 0.1 to 30 μm is preferable. The positive electrode mixture layer 1b usually has a thickness of about 50 to 200 μm, but the thickness is not particularly limited. The positive electrode mixture layer 1b may have a thickness of 0.1 to 50 μm. Here, the average particle diameter is a value measured by a laser diffraction / scattering method.

  The positive electrode mixture layer 1b may include both a conductive agent and a binder other than the oxide, or may include only one of them. Alternatively, the positive electrode mixture layer 1b does not include either a conductive agent or a binder, and may be composed of only a positive electrode active material.

  The conductive agent used for the positive electrode mixture layer 1 b may be anything as long as it is an electron conductive material that does not cause a chemical change at the charge / discharge potential of the positive electrode 1. For example, using conductive fibers such as graphites, carbon blacks, carbon fibers and metal fibers, metal powders, conductive whiskers, conductive metal oxides, or organic conductive materials alone. It may also be used as a mixture thereof. Although the addition amount of a electrically conductive agent is not specifically limited, 1-50 weight% is preferable with respect to a positive electrode active material, and 1-30 weight% is especially preferable.

  A thermoplastic resin and a thermosetting resin can be used for the binder used for the positive electrode mixture layer 1b. Preferred binders include, for example, polyolefin resins such as polyethylene and polypropylene, fluorine resins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and hexafluoropropylene (HFP), and copolymer resins thereof, And polyacrylic acid and copolymer resins thereof.

  In addition to the conductive agent and the binder, a filler, a dispersant, an ionic conductor, a pressure enhancer, and other various additives can be used. The filler may be any fibrous material that does not cause a chemical change in the lithium ion secondary battery.

  The material of the positive electrode current collector 1a may be any electronic conductor that does not cause a chemical change at the charge / discharge potential of the positive electrode 1. For example, stainless steel, aluminum, titanium, carbon, conductive resin, and the like can be used. Moreover, you may give an unevenness | corrugation to the surface of the positive electrode electrical power collector 1a by surface treatment. The shape may be any of a film, a sheet, a net, a punching metal, an expanded metal, a porous body, a foam, a fiber group, a nonwoven fabric, and a molded body thereof, in addition to the foil. The thickness of the positive electrode current collector 1a is not particularly limited, but is generally 1 to 500 μm.

As the negative electrode active material of the negative electrode mixture layer 2b, a material other than lithium titanate (Li ₄ Ti ₅ O ₁₂ ) is used if the charge / discharge reaction potential is higher than 0.8 V (vs. Li / Li ⁺ ). it can. The negative electrode active material may be an oxide containing Ti, for example. Examples thereof include LiTi ₂ O ₄ , H ₂ Ti ₁₂ O ₂₅ and TiO ₂ . A plurality of different materials may be mixed and used as the negative electrode active material. When the negative electrode active material is a powder, the average particle diameter is not particularly limited, but for example, a powder having an average particle diameter of 0.05 μm to 30 μm is preferable. The negative electrode mixture layer 2b usually has a thickness of about 50 to 200 μm, but the thickness is not particularly limited. The negative electrode mixture layer 2b may have a thickness of 0.1 to 50 μm. Here, the average particle diameter is a value measured by a laser diffraction / scattering method.

  The negative electrode mixture layer 2b further contains a conductive agent including a graphite material. The negative electrode mixture layer 2b may contain a substance other than the graphite material that functions as a conductive agent. However, as will be described in detail below, in order to impart higher conductivity to the electrode, it is preferable that 50% by weight or more of the conductive agent contained in the negative electrode mixture layer 2b is a graphite material, and 70% by weight or more. Is more preferably a graphite material, and the conductive agent may be composed of only a graphite material.

  As the negative electrode current collector 2a, an aluminum foil is preferable. For example, a copper foil, a nickel foil, or a stainless steel foil may be used. The negative electrode current collector 2a may have the same shape as the positive electrode current collector 1a.

  As the separator 3, not only a nonwoven fabric sheet made of polypropylene, but also a microporous sheet made of polyethylene, a microporous sheet made of polypropylene, a nonwoven fabric sheet, a sheet beaten and made with cellulose fibers, and a composite of these Sheet can be used. Moreover, you may perform the surface treatment for wettability improvement as needed. Of course, in addition to these, materials that can be used as separators for lithium ion secondary batteries can be used.

  Next, the conductive agent contained in the electrode, the graphite material used as the conductive agent, and the non-aqueous electrolyte used in combination with this conductive agent will be described.

  In general, an active material made of an oxide material is often slightly inferior in terms of conductivity compared to an active material made of a metal material and a carbon material. In the charge / discharge reaction, the active material exchanges lithium ions via the electrolyte, and at the same time, electrons corresponding to the amount of reaction electricity are transferred from the current collector through the inside of the electrode. In general, the ease with which electrons of the active material are transferred as described above is expressed as current collection.

  As an electrode of a lithium ion secondary battery, an electrode obtained by forming a mixture layer containing a powdered oxide material as an active material on a current collector made of a metal foil or the like is widely used. Yes. However, in such a configuration, the active material particles that are not in direct contact with the current collector in the electrode perform electron transfer through the adjacent active material particles having poor conductivity. . Accordingly, the current collecting property becomes insufficient, and the charge / discharge performance is lowered. As a technique for improving the current collecting property for the active material particles in such an electrode, it is generally practiced to mix a carbon material powder having excellent conductivity in the electrode mixture as a conductive agent. It has been broken.

  As described above, carbon material powder is widely used as the conductive agent to be mixed with the electrode mixture. Among them, higher conductivity can be imparted to the electrode by using a graphite material having a high degree of crystallinity and excellent electronic conductivity. As the graphite material, graphite materials having various shapes such as scales, lumps, spheres, and fibers can be used.

  On the other hand, as one of the problems of the lithium ion secondary battery, there are battery swelling and characteristic deterioration due to gas generation. A lithium ion secondary battery is composed of a combination of a positive electrode having a high reaction potential and a negative electrode having a low reaction potential, and generates a higher battery voltage than other battery systems due to a large potential difference therebetween. However, this very high positive electrode potential and very low negative electrode potential may cause the non-aqueous electrolyte in contact with these electrodes to be oxidized and reduced, resulting in a deterioration reaction involving gas generation. is there. In general, carbonate-based compounds used as solvents for non-aqueous electrolytes in lithium ion secondary batteries are materials that have a relatively wide potential window. Resulting in. Therefore, depending on the material constituting the electrode and the usage environment and conditions of the battery, deterioration may accumulate and become apparent. However, when propylene carbonate is used among the carbonate compounds, the above-described deterioration reaction can be suppressed due to its good redox stability.

However, when the negative electrode contains a graphite material as a conductive agent as described above, the reaction between the graphite material and propylene carbonate may become a problem when the solvent of the non-aqueous electrolyte contains propylene carbonate. In this reaction, propylene carbonate molecules are inserted between the crystal layers of graphite, and the crystal structure of graphite is destroyed. At the same time, deterioration of the electrolytic solution due to the decomposition reaction of propylene carbonate also occurs. Since the above reaction occurs at a potential in the vicinity of 0.8 V (vs. Li ^/ Li ⁺ ), there is a problem when a negative electrode active material having a charge / discharge reaction potential lower than that is used.

Therefore, for example, when lithium titanate (Li ₄ Ti ₅ O ₁₂ ) is used as the negative electrode active material, the charge / discharge reaction potential is as high as 1.55 V (vs. Li / Li ⁺ ). There is no problem with the conditions. However, when an overcharged state is reached, the negative electrode potential may be reduced to 0.8 V (vs. Li ^/ Li ⁺ ) or less. Therefore, even when lithium titanate (Li ₄ Ti ₅ O ₁₂ ) is used as the negative electrode active material, the reaction between the above-mentioned graphite material and propylene carbonate may occur, leading to deterioration of battery characteristics. In addition, if the negative electrode active material has a reaction potential of 0.8 V (vs. Li ^/ Li ⁺ ) or less, this reaction becomes a problem even under normal use conditions.

If a low crystalline carbon material such as carbon black having a low degree of graphitization is used as the conductive agent, the above problem does not occur, but it is desirable to use a graphite material from the viewpoint of conductivity. The graphite material used as the conductive agent preferably has a d ₀₀₂ obtained from an X-ray diffraction measurement as a value representing the interlayer spacing of the graphite crystals of 0.335 nm to 0.340 nm. If it is 0.340 nm or more, the degree of graphitization is low and the conductivity is poor. 0.335 nm is the theoretical value of d ₀₀₂ of graphite.

The nonaqueous electrolytic solution of this embodiment contains at least one kind of compound selected from 1,3-propane sultone, vinylene carbonate, and fluoroethylene carbonate. These compounds cause a reduction reaction at a potential higher than about 0.8 V (vs. Li ^/ Li ⁺ ) at which the graphite material reacts with propylene carbonate to form a film made of the reaction product on the negative electrode. Since this film is also formed on the surface of the graphite material that is a conductive agent and functions as a protective layer, there is an effect of suppressing the reaction between the graphite material and propylene carbonate.

  The contents of 1,3-propane sultone, vinylene carbonate, and fluoroethylene carbonate in the nonaqueous electrolytic solution are each preferably 0.1% by weight or more and 5.0% by weight or less. If it is less than 0.1% by weight, the surface protecting effect on the graphite material is small. In addition, since the coating of the above compound is formed not only on the graphite material but also on the surface of the active material, if it exceeds 5.0% by weight, the amount of coating on the surface of the active material increases and the charge / discharge reaction is hindered and the characteristics are deteriorated. May invite.

  In the case of using fluoroethylene carbonate, details of the mechanism are unclear, but at the same time, by adding dinitrile compounds such as adiponitrile and succinonitrile, it may be added in excess of 25% by weight. Is possible.

  In the present embodiment, the sheet-type lithium ion secondary battery has been described as an example. However, the lithium ion secondary battery of the present embodiment may have other shapes. For example, the lithium ion secondary battery of the present embodiment may have a cylindrical shape and a rectangular shape. Moreover, you may have a large sized shape used for an electric vehicle etc.

  The lithium ion secondary battery of this embodiment can be used for a portable information terminal, a portable electronic device, a small electric power storage device for home use, a motorcycle, an electric vehicle, a hybrid electric vehicle, and the like. Moreover, it can be used for devices other than these.

  Examples of the present invention will be described in detail below.

1. Production of battery (Example 1)
<Preparation of positive electrode>
First, LiNi _0.80 Co _0.15 Al _0.05 O ₂ was prepared as a positive electrode active material. The preparation method of LiNi _0.80 Co _0.15 Al _0.05 O ₂ is as follows.

  A predetermined ratio of cobalt sulfate was added to a nickel sulfate aqueous solution having a concentration of 1 mol / l to prepare a metal salt aqueous solution. While stirring this aqueous metal salt solution at 50 ° C. at a low speed, an alkaline solution containing 30% by weight of sodium hydroxide was added dropwise so as to have a pH of 12 to obtain a hydroxide precipitate. The precipitate was filtered and washed with water, and then dried by heating to 80 ° C. in air.

Next, the obtained hydroxide was stirred in water in a reaction tank at 30 ° C., and a predetermined amount of NaAlO ₂ was added to the reaction tank and sufficiently stirred. Thereafter, the solution in the reaction vessel was neutralized with sulfuric acid until the pH reached 9. As a result, aluminum hydroxide, which is a compound containing Al, was uniformly deposited on the surface of the hydroxide. Thereafter, moisture was removed, and calcination was performed in an air atmosphere at 700 ° C. for 10 hours to obtain [Ni _0.80 Co _0.15 Al _0.05 ] O which is a ternary oxide. It was confirmed by powder X-ray diffraction (manufactured by Rigaku) that the obtained oxide had a single phase.

Further, lithium hydroxide monohydrate powder was mixed with the obtained oxide so that the ratio of the total number of moles of Ni, Co and Al to the number of moles of Li was 1: 1. This mixture was heated to 750 ° C. in an oxygen atmosphere over 10 hours, and heat-treated at 750 ° C. for 36 hours to obtain the target LiNi _0.80 Co _0.15 Al _0.05 O ₂ . It was confirmed by powder X-ray diffraction (manufactured by Rigaku) that the obtained LiNi _0.80 Co _0.15 Al _0.05 O ₂ had a single-phase hexagonal layered structure. After pulverizing and classifying treatment, secondary particles of almost spherical or ellipsoidal shape are formed by aggregation of many primary particles of about 0.2 μm to 1.0 μm from observation with a scanning electron microscope (manufactured by Hitachi High-Technologies). Was confirmed to form. In addition, the average particle diameter was calculated | required using the scattering type particle size distribution measuring apparatus (made by HORIBA).

Next, a positive electrode was produced using the obtained LiNi _0.80 Co _0.15 Al _0.05 O ₂ powder. LiNi _0.80 Co _0.15 Al _0.05 O ₂ , acetylene black (AB) as a conductive agent, and polyvinylidene fluoride (PVdF) as a binder in a weight ratio of LiNi _0.80 Co _0.15 Al _0.05 O ₂ : AB: PVdF = 100: 3: 4, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added and stirred and mixed to obtain a slurry-like positive electrode mixture.

  Next, as shown in FIG. 2, by applying the slurry-like positive electrode mixture to one side of the current collector 1a made of an aluminum foil having a thickness of 20 μm, drying the coating film, and rolling with a roller, A positive electrode mixture layer 1b was obtained.

  The obtained electrode plate was punched out to the dimensions shown in FIG. 2, and the positive electrode mixture layer 1 b at the tab portion as the lead attachment portion was peeled off to obtain the positive electrode 1. The positive electrode current collector 1a provided with the positive electrode mixture layer 1b had a rectangular shape of 30 mm × 40 mm.

<Preparation of electrolyte>
To propylene carbonate (PC), lithium hexafluorophosphate (LiPF ₆ ) is added and dissolved to a concentration of 1.0 M, and further 0.05 wt%, 0.1 wt%, 1.5 wt% 1,3-propane sultone (PS) was added and dissolved so as to be 5.0% by weight to prepare an electrolytic solution.

<Production of negative electrode>
First, Li ₄ Ti ₅ O ₁₂ (average particle size ≦ 1 μm) was prepared as a negative electrode active material. The preparation method of Li ₄ Ti ₅ O ₁₂ used as the negative electrode active material is as follows. In addition, the average particle diameter here is a value measured with a scattering type particle size distribution measuring apparatus (manufactured by HORIBA). The reaction potential for charging / discharging Li ₄ Ti ₅ O ₁₂ is about 1.55 V (vs. Li / Li ⁺ ).

LiOH · H ₂ O and TiO ₂ raw material powders, which are commercially available reagents, were weighed so that the Li / Ti molar mixing ratio was slightly more Li than the stoichiometric ratio, and these were mixed in a mortar. As the raw material TiO ₂ , one having an anatase type crystal structure was used.

The mixed raw material powder was put in an Al ₂ O ₃ crucible and subjected to heat treatment at 850 ° C. for 12 hours in an air atmosphere to obtain the target Li ₄ Ti ₅ O ₁₂ .

The heat-treated material was taken out of the crucible and pulverized in a mortar to obtain a Li ₄ Ti ₅ O ₁₂ coarse powder. When the obtained Li ₄ Ti ₅ O ₁₂ coarse powder was measured by powder X-ray diffraction (manufactured by Rigaku), a single-phase diffraction pattern having a spinel structure in which the space group was attributed to Fd3m was obtained.

Subsequently, the obtained Li ₄ Ti ₅ O ₁₂ coarse powder was used for jet mill pulverization and classification. It was confirmed that the obtained powder was pulverized into single particles having a particle size of about 0.7 μm from observation with a scanning electron microscope (manufactured by Hitachi High-Technologies).

Next, a negative electrode was produced using the Li ₄ Ti ₅ O ₁₂ powder obtained by the above method. Li ₄ Ti ₅ O ₁₂ , a vapor-grown carbon fiber of graphite material (VGCF, made by Showa Denko) as a conductive agent, and polyvinylidene fluoride (PVdF) as a binder in a weight ratio of Li ₄ Ti ₅ O ₁₂ : VGCF: PVdF = 100: 7: 5 Weighed so that an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added, and stirred and mixed to obtain a slurry-like negative electrode mixture. .

  Next, as shown in FIG. 3, by applying the slurry-like negative electrode mixture on one side of the current collector 2a made of an aluminum foil having a thickness of 20 μm, drying the coating film, and rolling with a roller, A negative electrode mixture layer 2b was obtained.

  The obtained electrode plate was punched out to the dimensions shown in FIG. 3, and the negative electrode mixture layer 2 b at the tab portion as a lead attachment portion was peeled off to obtain the negative electrode 2. The negative electrode current collector 2a provided with the negative electrode mixture layer 2b had a rectangular shape of 30 mm × 40 mm.

<Assembly>
The obtained positive electrode 1 and negative electrode 2 were laminated via a separator 3 to produce an electrode group 13 as shown in FIG. As the separator, a polypropylene nonwoven fabric sheet having a surface treatment of 80 μm in thickness was used.

  Next, as shown in FIG. 1A, an aluminum positive electrode lead 11 was welded to the positive electrode 1 of the electrode group 13, and an aluminum negative electrode lead 12 was welded to the negative electrode 2. Thereafter, the electrode group 13 was accommodated in a battery case 14 made of an aluminum laminate film having a thickness of 0.12 mm opened in three directions, and fixed to the inner surface of the battery case 14 with a polypropylene tape. The opening including the opening from which the positive electrode lead 11 and the negative electrode lead 12 protrude is thermally welded, and only one opening is left without being thermally welded, so that the battery case 14 has a bag shape. Each electrolyte solution prepared as the electrolyte solution 15 was injected from the opening portion that was not thermally welded, and the inside of the battery was sealed by thermally welding the opening portion under reduced pressure after depressurization and deaeration. The designed capacity when the assembled battery was charged at 2.8 V was 40 mAh. Regarding the obtained battery, batteries obtained by using an electrolyte solution having a content of 1,3-propane sultone of 0.1% by weight, 1.5% by weight, and 5.0% by weight were respectively designated as battery A1. , A2 and A3.

(Example 2)
A battery A4 was produced in the same manner as in Example 1 except that the electrolytic solution was different. In the electrolyte solution, lithium hexafluorophosphate (a solution of 1.0M in a mixed solvent in which propylene carbonate (PC) and diethyl carbonate (DEC) were mixed at a volume ratio of 4: 1 to a concentration of 1.0M was used. LiPF ₆ ) was added and dissolved, and 1,3-propane sultone (PS) was added and dissolved so as to be 1.5% by weight.

(Example 3)
A battery was produced in the same manner as in Example 1 except that the electrolytic solution was different. In the electrolyte, lithium hexafluorophosphate (LiPF ₆ ) was added and dissolved in propylene carbonate (PC) to a concentration of 1.0 M, and further 0.1 wt%, 1.5 wt%, What added and mixed vinylene carbonate (VC) so that it might become 5.0 weight% was used. Batteries obtained using electrolyte solutions having vinylene carbonate contents of 0.1 wt%, 1.5 wt%, and 5.0 wt% were designated as batteries A5, A6, and A7, respectively.

Example 4
A battery was produced in the same manner as in Example 1 except that the electrolytic solution was different. In the electrolyte, lithium hexafluorophosphate (LiPF ₆ ) was added and dissolved in propylene carbonate (PC) to a concentration of 1.0 M, and further 0.1 wt%, 1.5 wt%, A mixture prepared by adding fluoroethylene carbonate (FEC) so as to be 5.0% by weight was used. Batteries obtained using electrolytic solutions having fluoroethylene carbonate contents of 0.1% by weight, 1.5% by weight, and 5.0% by weight were designated as batteries A8, A9, and A10, respectively.

(Example 5)
A battery A11 was produced in the same manner as in Example 1 except that the electrolytic solution was different. In the electrolytic solution, lithium hexafluorophosphate (LiPF ₆ ) is added to propylene carbonate (PC) to a concentration of 1.0 M and dissolved, and further, fluoroethylene carbonate is added to 25.0 wt%. (FEC) and 1.5 wt% adiponitrile added and mixed were used.

(Example 6)
A battery A12 was produced in the same manner as in Example 1 except that different negative electrode active materials were used and that 1,3-propane sultone (PS) added to the electrolyte was changed to 1.5% by weight. LiTi ₂ O ₄ prepared by the following method was used as the negative electrode active material. The reaction potential for charging and discharging LiTi ₂ O ₄ is about 1.3 V (vs. Li ^/ Li ⁺ ).

Raw material powders of Li ₂ CO ₃ and TiO ₂ , which are commercially available reagents, were weighed so that the molar mixing ratio of Li / Ti was slightly more Li than the stoichiometric ratio, and these were mixed in a mortar.

The mixed raw material powder was placed in an Al ₂ O ₃ crucible and subjected to heat treatment at 850 ° C. for 12 hours in an air atmosphere to obtain the target LiTi ₂ O ₄ .

The heat-treated material was taken out of the crucible and pulverized in a mortar to obtain a coarse powder of LiTi ₂ O ₄ . When measurement of powder X-ray diffraction (manufactured by Rigaku) of the obtained LiTi ₂ O ₄ coarse powder was performed, a single-phase diffraction pattern having a spinel structure in which the space group was attributed to Fd3m was obtained.

Subsequently, jet mill grinding and classification were performed using the obtained LiTi ₂ O ₄ coarse powder. It was confirmed that the obtained powder was pulverized into single particles having a particle size of about 1.0 μm from observation with a scanning electron microscope (manufactured by Hitachi High-Technologies).

Next, using the LiTi ₂ O ₄ powder obtained by the above method as the negative electrode active material, a negative electrode was prepared. A negative electrode was produced in the same manner as in Example 1 except that the negative electrode active material was different.

(Example 7)
A battery A13 was produced in the same manner as in Example 1 except that different negative electrode active materials were used and 1,3-propane sultone (PS) added to the electrolytic solution was changed to 1.5% by weight. As the negative electrode active material, H ₂ Ti ₁₂ O ₂₅ prepared by the following method was used. Note that the charge / discharge reaction potential of H ₂ Ti ₁₂ O ₂₅ is about 1.55 V (vs. Li ^/ Li ⁺ ).

Raw material powders of Na ₂ CO ₃ and TiO ₂ , which are commercially available reagents, were weighed so that the molar mixing ratio of Na / Ti was a stoichiometric ratio, and these were mixed in a mortar.

The mixed raw material powder was put into an Al ₂ O ₃ crucible and subjected to heat treatment at 850 ° C. for 12 hours in an air atmosphere to obtain the target Na ₂ Ti ₃ O ₇ .

The obtained Na ₂ Ti ₃ O ₇ was dispersed in hydrochloric acid having a concentration of 0.5 mol / L for 7 days and then sufficiently dried to obtain H ₂ Ti ₃ O ₇ .

Further, the obtained H ₂ Ti ₃ O ₇ was heat-treated at 200 ° C. for 12 hours in an air atmosphere to obtain the target H ₂ Ti ₁₂ O ₂₅ .

The heat-treated material was taken out from the crucible and pulverized in a mortar to obtain a coarse powder of H ₂ Ti ₁₂ O ₂₅ . When measurement of powder X-ray diffraction (manufactured by Rigaku) of the obtained H ₂ Ti ₁₂ O ₂₅ coarse powder was performed, a single-phase diffraction pattern in which the space group was attributed to P2 / m was obtained.

Subsequently, the obtained H ₂ Ti ₁₂ O ₂₅ coarse powder was used for jet mill pulverization and classification. It was confirmed that the obtained powder was pulverized into particles having a particle size of about 0.8 μm from observation with a scanning electron microscope (manufactured by Hitachi High-Technologies).

Next, using the H ₂ Ti ₁₂ O ₂₅ powder obtained by the above method as the negative electrode active material, a negative electrode was prepared. A negative electrode was produced in the same manner as in Example 1 except that the negative electrode active material was different.

(Example 8)
A battery A14 was produced in the same manner as in Example 1, except that a different conductive agent was used for the negative electrode and that 1,3-propane sultone (PS) added to the electrolyte was changed to 1.5% by weight. Artificial graphite powder (manufactured by Timcal, KS-4) was used as the conductive agent.

Example 9
Batteries A15 and A16 were produced in the same manner as in Example 1, except that 1,3-propane sultone (PS) added to the electrolytic solution was changed to 0.05 wt% and 5.5 wt%.

(Example 10)
Batteries A17 and A18 were produced in the same manner as in Example 1 except that vinylene carbonate (VC) added to the electrolytic solution was changed to 0.05 wt% and 5.5 wt%.

(Example 11)
Batteries A19 and A20 were produced in the same manner as in Example 1 except that fluoroethylene carbonate (FEC) added to the electrolytic solution was changed to 0.05 wt% and 5.5 wt%.

(Comparative Example 1)
A battery B1 was produced in the same manner as in Example 1 except that 1,3-propane sultone (PS) was not added to the electrolytic solution.

(Comparative Example 2)
A battery B2 was produced in the same manner as in Example 1 except that the electrolytic solution was different. The electrolyte solution was lithium hexafluorophosphate in a mixed solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 1: 3 to a concentration of 1.0M. What was obtained by adding (LiPF ₆ ) and dissolving was used. That is, in the electrolyte solution of Comparative Example 2, propylene carbonate (PC) was not used as a nonaqueous solvent, and 1,3-propane sultone (PS), vinylene carbonate (VC), and fluoroethylene carbonate (FEC) were used. ) Was not added.

In Examples 1 to 11 and Comparative Examples 1 and 2, the solvents (PC, DEC, EC, EMC), LiPF ₆ , PS, VC, and FEC used for preparing the electrolyte were all lithium battery grade. It was a commercial product.

2. Battery evaluation <Evaluation of battery characteristics>
The battery characteristics were evaluated using each produced battery.

  In an environment of 25 ° C., constant current charging was performed up to 2.8 V at a current value of 2 mA. Thereafter, constant current discharge was performed up to 1.5 V at a current value of 2 mA, and it was confirmed that all the batteries were operating at almost the designed capacity.

Next, each battery was subjected to constant current charging to 3.5 V at a current value of 2 mA in an environment of 25 ° C. to be in an overcharged state. Thereafter, constant current discharge was performed to 1.5 V at a current value of 2 mA. Subsequently, constant current charging was performed up to 2.8 V at a current value of 2 mA, and then constant current discharging was performed up to 1.5 V at a current value of 2 mA. The percentage of the discharge capacity after overcharging with respect to the discharge capacity before overcharging was calculated as the capacity maintenance rate after overcharging by the following formula. The results are shown in Table 1. Note that the potential of the negative electrode in an overcharged state of 3.5 V was 0.5 V (vs. Li ^/ Li ⁺ ).
Capacity maintenance ratio after overcharge (%) = (discharge capacity after overcharge / discharge capacity before overcharge) × 100

<High temperature storage test>
A 60 ° C. storage test was performed using each battery for which the battery characteristics were evaluated.

  Each battery after performing the characteristic evaluation including overcharge up to 3.5 V as described above was subjected to constant current charge up to 2.8 V at a current value of 2 mA in an environment of 25 ° C. Thereafter, the charged battery was placed in a high-temperature bath in an open circuit state, and kept at 60 ° C. for one week. Then, it took out from the high temperature tank and analyzed the generated gas.

The gas accumulated in each battery after high temperature storage was collected and analyzed by gas chromatography. Table 2 shows the amount of CO ₂ gas accumulated in each battery.

  In battery B1 using propylene carbonate (PC) as a solvent and using an electrolyte solution that does not contain 1,3-propane sultone (PS), vinylene carbonate (VC), or fluoroethylene carbonate (FEC), The capacity maintenance rate has dropped significantly. This is because the fibrous graphite material (VGCF) used as the conductive agent and propylene carbonate (PC) react to destroy the current collecting structure in the electrode.

Further, in the comparison of CO ₂ gas generation by storage at 60 ° C. in the charged state, the amount of gas generated in the battery B1 is large. This is also due to the reaction between the fibrous graphite material (VGCF) and propylene carbonate (PC).

Even in the battery B2 that does not use propylene carbonate (PC) as the solvent of the electrolytic solution, the CO ₂ gas generation amount is increased. This is considered to be due to the oxidation / reduction reaction of the solvent.

As described above, in a battery using a negative electrode containing a negative electrode active material having a charge / discharge reaction potential exceeding 0.8 V (vs. Li ^/ Li ⁺ ) and a graphite material as a conductive agent, the electrolytic solution is In the case of a combination containing propylene carbonate (PC) as a solvent and at least one compound selected from 1,3-propane sultone (PS), vinylene carbonate (VC) and fluoroethylene carbonate (FEC), It was possible to provide a battery with little performance deterioration due to overcharge and less gas generation.

  Moreover, among batteries A1 to A20, the contents of 1,3-propane sultone (PS), vinylene carbonate (VC), and fluoroethylene carbonate (FEC) added to the electrolyte solution are each 0.1% by weight or more. A battery (battery A11) having 5.0 wt% or less (batteries A1 to A10 and batteries A12 to A14) and 25 wt% fluoroethylene carbonate (FEC) added to the electrolyte together with adiponitrile The capacity retention rate after charging exceeded 90%. From this result, the content of 1,3-propane sultone (PS), vinylene carbonate (VC) and fluoroethylene carbonate (FEC) added to the electrolytic solution is 0.1 wt% or more and 5.0 wt% or less. By doing so, it has been found that the performance deterioration due to overcharging can be more reliably suppressed. In addition, when the compound added to the electrolytic solution is fluoroethylene carbonate (FEC), the addition of a dinitrile compound such as adiponitrile can provide a high performance deterioration suppressing effect even if it exceeds 5.0% by weight. I understand.

According to the present invention, excellent charge and discharge characteristics, less lithium ion secondary battery characteristics deterioration during overcharge can be realized. The present invention is particularly used for a lithium ion secondary battery using an active material that occludes and releases lithium at a reaction potential higher than 0.8 V with respect to the dissolution and precipitation potential of lithium.

DESCRIPTION OF SYMBOLS 1 Positive electrode 1a Positive electrode collector 1b Positive electrode mixture layer 2 Negative electrode 2a Negative electrode collector 2b Negative electrode mixture layer 3 Separator 11 Positive electrode lead 12 Negative electrode lead 13 Electrode group 14 Battery case 15 Electrolytic solution (nonaqueous electrolyte)

Claims (3)

A positive electrode;
A negative electrode,
A separator disposed between the positive electrode and the negative electrode;
A non-aqueous electrolyte,
With
The negative electrode contains a negative electrode active material made of lithium at a high reaction potential than 0.8V dissolution deposition potential of lithium as a reference material for absorbing and releasing, and a conductive agent containing graphite material,
In the negative electrode, the content of the negative electrode active material is greater than the content of the conductive agent,
The nonaqueous electrolyte contains propylene carbonate, containing full Oro ethylene carbonate Natick preparative Furthermore,
Non-aqueous electrolyte secondary battery. The active material contained in the negative electrode is an oxide containing Ti,
The nonaqueous electrolyte secondary battery according to claim 1 . The active material contained in the negative electrode is lithium titanate represented by the general formula Li ₄ Ti ₅ O ₁₂ .
The nonaqueous electrolyte secondary battery according to claim 1 or 2 .

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