JP2006252999A - Lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium secondary battery having high capacity and an excellent charge-discharge characteristic by improving a positive electrode, in a lithium secondary battery using a material containing silicon as a negative electrode active material. <P>SOLUTION: This lithium secondary battery includes: a negative electrode 2 containing silicon as the negative electrode active material; the positive electrode 1 containing a lithium transition metal composite oxide as a positive electrode active material; and a nonaqueous electrolyte. The lithium secondary battery is characterized by that electrolyte-retaining powder for retaining the nonaqueous electrolyte is added in the positive electrode 1; the average particle diameter of the electrolyte-retaining powder is 10 nm to 1 μm; and the BET specific surface area thereof is restricted to not less than 5 m<SP>2</SP>/g. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a lithium secondary battery using a material containing silicon as a negative electrode active material, and more particularly to improvement of the positive electrode.

In recent years, mobile information terminals such as mobile phones, notebook personal computers, and PDAs have been rapidly reduced in size and weight, and batteries as drive power sources are required to have higher capacities. A lithium secondary battery that performs charging / discharging by moving lithium ions between the positive and negative electrodes along with charging / discharging has a high energy density and high capacity. As widely used. In the future, with further miniaturization and higher functionality of these mobile information terminals, it is expected that the load on the lithium secondary battery as a power source will increase, and there is a demand for higher energy density of the lithium secondary battery. Is very expensive.
Here, to increase the energy density of the battery, it is effective to use a material having a larger energy density as the active material. Recently, in lithium secondary batteries, as an anode active material having a higher energy density, an alloy of elements such as Al, Sn, and Si that occludes lithium by an alloying reaction with lithium instead of graphite that has been put into practical use. The use of materials has been proposed and has been actively studied.
However, in the electrode using a material alloyed with lithium as the negative electrode active material,
Since the volume of the active material expands and contracts during the insertion and extraction of lithium, the negative electrode active material is pulverized and peeled off from the current collector. There is a problem that characteristics deteriorate.
In view of this, the present applicant, in a negative electrode using a material containing silicon as a negative electrode active material that is alloyed with lithium, has a negative electrode active material and a binder on the surface of a negative electrode current collector made of a conductive metal foil having irregularities on the surface. The negative electrode layer comprising a negative electrode mixture layer containing and sintered in a non-oxidizing atmosphere exhibits high current collection in the electrode due to high adhesion between the negative electrode mixture layer and the negative electrode current collector, and is good It has been found that excellent charge / discharge cycle characteristics can be obtained (see Patent Document 1 below).

Japanese Patent Laid-Open No. 2002-260637

However, even in this method, cracking occurs in the silicon active material due to the charge / discharge reaction, and the surface area of silicon is increased, so that a large amount of electrolyte in the battery system is retained in the negative electrode. There is a problem that the electrolyte is depleted and the charge / discharge characteristics are deteriorated.
Accordingly, the main object of the present invention is to provide a lithium secondary battery having high capacity and excellent charge / discharge characteristics by improving the positive electrode in a lithium secondary battery using a material containing silicon as the negative electrode active material. .

In order to achieve the above object, the invention according to claim 1 of the present invention includes a negative electrode containing silicon as a negative electrode active material, a positive electrode containing a lithium transition metal composite oxide as a positive electrode active material, and a non-aqueous electrolyte. In the lithium secondary battery having the above structure, a liquid retaining powder for retaining the nonaqueous electrolyte is added in the positive electrode, and the liquid retaining powder has an average particle diameter of 10 nm to 1 μm, and a BET The specific surface area is limited to 5 m ² / g or more.
Liquid retaining powders with an average particle diameter of 1 μm or less and a BET specific surface area of 5 m ² / g or more have high liquid retaining properties. Therefore, by adding this to the positive electrode, the liquid retaining properties of the positive electrode are improved. To do. Therefore, even when the negative electrode active material is cracked by the charge / discharge reaction and the surface area of silicon is increased, the non-aqueous electrolyte in the battery system is restrained from being retained only in the negative electrode, so the charge / discharge characteristics Can be improved.
The average particle size of the liquid-retaining powder is restricted to 10 nm or more and 1 μm or less because when the average particle size is less than 10 nm, the handling becomes difficult. This is because the liquid retainability is low and the charge / discharge characteristics cannot be sufficiently improved.
In addition, the BET specific surface area of the liquid retaining powder is regulated to 5 m ² / g or more because when the BET specific surface area is less than 5 m ² / g, the liquid retaining property of the electrolyte is low and the charge / discharge characteristics are low. This is because sufficient improvement cannot be achieved.

According to a second aspect of the present invention, in the first aspect of the present invention, the liquid retaining powder does not have a lithium storage capacity.
In this way, if the liquid retaining powder does not have lithium storage capacity, side reactions can be prevented from occurring between the nonaqueous electrolyte and the liquid retaining powder, so that the charge / discharge characteristics do not deteriorate. Because.

A third aspect of the invention is characterized in that, in the first or second aspect of the invention, the liquid retaining powder is uniformly dispersed in the positive electrode.
With the above configuration, since the liquid retaining powder is uniformly distributed around the positive electrode active material, there is sufficient nonaqueous electrolyte around the positive electrode active material in any part. It will be. Accordingly, since lithium ions are smoothly supplied to the positive electrode active material in any part, the charge / discharge characteristics can be further improved.
In order to uniformly mix and disperse the positive electrode active material and the liquid retaining powder, the positive electrode active material powder, the liquid retaining powder, and the positive electrode conductive agent powder are uniformly mixed in the positive electrode binder solution. A method of preparing a positive electrode mixture layer by preparing a mixed and dispersed positive electrode mixture slurry and then drying the slurry.
Since it is necessary for the positive electrode active material to sufficiently obtain the performance of the positive electrode active material, it is necessary to maintain the state in which the secondary particles before the slurry preparation are formed even after the preparation of the positive electrode mixture slurry. As a method for preparing the slurry, first, the liquid retaining powder and the positive electrode conductive agent powder are uniformly mixed and dispersed in the positive electrode binder solution, and then mixed with the positive electrode active material powder. A method is preferred in which the particles are mixed with such a force that the particles are not destroyed and all are mixed and dispersed uniformly.
Examples of the apparatus used to uniformly mix and disperse the liquid retaining powder in the binder solution before mixing the positive electrode active material include a mortar, a mixer, a homogenizer, a dissolver, and a kneader. A stirring mixer and a disperser such as a roll mill, a sand mill, and a ball mill can be preferably used.

According to a fourth aspect of the present invention, in the first to third aspects of the present invention, as the liquid retaining powder, at least one oxide selected from the group consisting of yttrium oxide, cerium oxide, silicon oxide, and tin oxide is used. It is characterized by using.
Among liquid retaining powders, in particular, oxides such as yttrium oxide, cerium oxide, silicon oxide, and tin oxide have a high liquid retaining property and do not have lithium storage capability. Charge / discharge characteristics can be further improved. However, the liquid retaining powder is not limited to these, and may be, for example, a powder made of a carbon material.

The invention according to claim 5 is characterized in that, in the invention according to claim 1, an addition ratio of the liquid retaining powder to the lithium transition metal composite oxide is regulated to 1% by mass or more and 5% by mass or less. To do.
The amount of liquid retaining powder is too small when the amount of liquid retaining powder added to the positive electrode is less than 1% by mass of the lithium transition metal composite oxide. On the other hand, when the content of the lithium transition metal composite oxide exceeds 5% by mass, the ratio of the positive electrode active material contained in the positive electrode is decreased, and the battery capacity is decreased. .

According to a sixth aspect of the present invention, in the first to fifth aspects of the present invention, carbon dioxide is dissolved in the non-aqueous electrolyte.
Thus, if carbon dioxide is dissolved in the non-aqueous electrolyte, the non-aqueous electrolyte is more permeable into the electrode, so that lithium ions can be supplied more smoothly to the surface of the positive and negative electrode active materials. This is because the discharge characteristics can be further improved.
The amount of carbon dioxide dissolved is preferably 0.01% by mass or more, more preferably 0.1% by mass or more of the nonaqueous electrolyte. The upper limit is not particularly limited, but the saturated dissolution amount in the nonaqueous electrolyte is the upper limit.

The invention according to claim 7 is the invention according to any one of claims 1 to 6, wherein the positive electrode includes a positive electrode active material containing the lithium transition metal composite oxide, the liquid retaining powder, a positive electrode binder, and a positive electrode conductive agent. The positive electrode material mixture layer containing a metal foil is characterized in that it is arranged on a positive electrode current collector made of a conductive metal foil.
Here, examples of the lithium transition metal composite oxide include LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiMn ₂ , LiCo _0.5 Ni _0.5 O ₂ , LiNi _0.33 Co _0.33 Mn _0.34 O _2, etc. LiCoO ₂ and a lithium transition metal composite oxide containing Li, Ni, Mn, and Co having a layered structure can be preferably used.

The lithium transition metal composite oxide preferably has a BET specific surface area of 3 m ² / g or less. This is because, when the BET specific surface area of the lithium transition metal composite oxide exceeds 3 m ² / g, the contact area with the non-aqueous electrolyte is too large, so the reactivity with the non-aqueous electrolyte increases, and the decomposition of the non-aqueous electrolyte occurs. This is because side reactions such as gas generation due to the reaction are likely to occur.

  Furthermore, the lithium transition metal composite oxide preferably has an average particle size (average particle size of secondary particles) of 20 μm or less. This is because when the average particle diameter exceeds 20 μm, the movement distance of lithium in the lithium transition metal composite oxide particles is increased, so that the charge / discharge cycle characteristics are deteriorated.

As the positive electrode conductive agent, various known conductive agents can be used. For example, a conductive carbon material can be preferably used, and in particular, acetylene black or ketjen black can be preferably used.
The amount of the positive electrode conductive agent with respect to the positive electrode mixture layer is preferably 1% by mass or more and 5% by mass or less. This is because when the amount of the positive electrode conductive agent with respect to the positive electrode mixture layer is less than 1% by mass, the amount of the positive electrode conductive agent is too small to form a sufficient conductive network around the positive electrode active material. On the other hand, when the amount of the positive electrode conductive agent with respect to the positive electrode mixture layer exceeds 5% by mass, the amount of the positive electrode conductive agent is too large. The binder is consumed for the adhesion of the positive electrode conductive agent, the adhesion of the positive electrode active material between the positive electrode active material particles and the positive electrode current collector is reduced, the positive electrode active material is easily detached, and the charge / discharge characteristics are improved. It is because it falls.

As the positive electrode binder, various known binders can be used without limitation as long as they do not dissolve in the non-aqueous electrolyte solvent in the present invention, for example, a fluorine resin such as polyvinylidene fluoride, a polyimide resin, Polyacrylonitrile and the like can be preferably used.
The amount of the positive electrode binder with respect to the positive electrode mixture layer is preferably 1% by mass or more and 5% by mass or less. This is because when the amount of the positive electrode binder with respect to the positive electrode mixture layer is less than 1% by mass, the contact area between the positive electrode active material particles increases and the contact resistance decreases, but the amount of the positive electrode binder is too small. The adhesion of the positive electrode active material between the positive electrode active material particles and the positive electrode current collector is reduced, the positive electrode active material is easily detached, and the charge / discharge characteristics are deteriorated, while the amount of the positive electrode binder with respect to the positive electrode mixture layer When the amount exceeds 5% by mass, the adhesion of the positive electrode active material between the positive electrode active material particles and the positive electrode current collector is improved, but the contact area between the positive electrode active material particles is large because the amount of the positive electrode binder is too large. This is because the contact resistance increases due to a decrease in charge / discharge characteristics.

  The conductive metal foil constituting the positive electrode current collector can be used without any limitation as long as it does not dissolve in the electrolyte at the potential applied to the positive electrode during charging and discharging, and for example, an Al foil is preferably used. be able to.

The density of the positive electrode mixture layer is preferably 3.0 g / cm ³ or more. This is because when the density of the positive electrode mixture layer is 3.0 g / cm ³ or more, the contact area between the positive electrode active materials is increased and the current collecting property in the positive electrode mixture layer is improved. This is because the discharge characteristics can be obtained.

The invention according to claim 8 is the invention according to any one of claims 1 to 7, wherein the negative electrode is a negative electrode in which a negative electrode mixture layer containing a negative electrode active material containing silicon and / or a silicon alloy and a negative electrode binder is made of a conductive metal foil. The structure is arranged on the surface of the current collector.
Examples of the silicon alloy include solid solutions of silicon and one or more other elements, intermetallic compounds of silicon and one or more other elements, and eutectic alloys of silicon and one or more other elements. Can be mentioned. In addition, as the negative electrode active material in the present invention, particles of silicon alone can be preferably used.

  The average particle diameter of the negative electrode active material is not particularly limited, but is preferably 100 μm or less, more preferably 50 μm or less, and most preferably 10 μm or less. This is because when the negative electrode active material particles having a small particle diameter are used, the absolute amount of volume expansion and contraction of the negative electrode active material particles due to insertion and extraction of lithium during charging and discharging is reduced, so that the electrode during charging and discharging is reduced. The absolute amount of distortion between the negative electrode active material particles in the inside also becomes small, and the negative electrode binder is hardly broken. As a result, it is possible to suppress a decrease in the current collecting ability in the electrode and to obtain excellent charge / discharge characteristics.

  The particle size distribution of the negative electrode active material is preferably as narrow as possible. This is because, in the case of a wide particle size distribution, there is a large difference in the absolute amount of volume expansion and contraction associated with lithium storage and release between negative electrode active material particles having greatly different particle sizes. Distortion occurs in the agent layer. As a result, the negative electrode binder is broken, and the current collecting property in the electrode is lowered, so that the charge / discharge characteristics are lowered.

  The conductive metal foil as the negative electrode current collector preferably has a surface roughness Ra of 0.2 μm or more on the surface on which the negative electrode mixture layer is disposed. By using the conductive metal foil having such a surface roughness Ra as the negative electrode current collector, the negative electrode binder enters the uneven surface portion of the negative electrode current collector, and the anchor effect is provided between the negative electrode binder and the negative electrode current collector. Therefore, high adhesion can be obtained. For this reason, peeling of the negative electrode mixture layer from the negative electrode current collector due to expansion and contraction of the volume of the negative electrode active material particles accompanying lithium occlusion and release is suppressed. In addition, when arrange | positioning a negative mix layer on both surfaces of a negative electrode collector, it is preferable that surface roughness Ra of both surfaces of a negative electrode collector is 0.2 micrometer or more.

  The surface roughness Ra and the average interval S between the local peaks are preferably 100Ra ≧ S. The surface roughness Ra and the average interval S between the local peaks are defined in Japanese Industrial Standard (JIS B 0601-1994), and can be measured by, for example, a surface roughness meter.

  In order to set the surface roughness Ra of the conductive metal foil to 0.2 μm or more, the conductive metal foil may be roughened. Examples of such roughening treatment include a plating method, a vapor phase growth method, an etching method, and a polishing method.

  Examples of the conductive metal foil include metal foil such as copper, nickel, iron, titanium, and cobalt, or an alloy foil made of a combination thereof.

  It is particularly preferable that the negative electrode current collector has high mechanical strength. This is because the negative electrode current collector has high mechanical strength, so that even when stress generated by volume change of the negative electrode active material during insertion and extraction of lithium is applied to the negative electrode current collector, Since the electric body can be mitigated without causing breakage or plastic deformation, the peeling of the negative electrode mixture layer from the negative electrode current collector is suppressed, the current collecting property within the electrode is maintained, and excellent charge / discharge characteristics are achieved. Because it can be obtained.

Although the thickness of the said negative electrode collector is not specifically limited, It is preferable that it is the range of 10 micrometers-100 micrometers.
The upper limit of the surface roughness Ra of the negative electrode current collector is not particularly limited. However, since the thickness of the negative electrode current collector is preferably in the range of 10 to 100 μm as described above, substantially, The upper limit of the surface roughness Ra is 10 μm or less.

  In the negative electrode of the present invention, the thickness X of the negative electrode mixture layer preferably has a relationship of 5Y ≧ X, 250Ra ≧ X with the thickness Y and surface roughness Ra of the negative electrode current collector. This is because, when the thickness X of the negative electrode mixture layer is 5Y or 250 Ra or more, the expansion and contraction of the volume of the negative electrode mixture layer during charge / discharge increases, so depending on the unevenness on the surface of the negative electrode current collector, This is because the adhesion with the negative electrode current collector cannot be maintained, and the negative electrode mixture layer peels from the negative electrode current collector.

  Although the thickness X of the said negative mix layer is not specifically limited, 1000 micrometers or less are preferable, More preferably, they are 10 micrometers-100 micrometers.

  The negative electrode binder preferably has high mechanical strength and is excellent in elasticity. This is because the negative electrode binder has excellent mechanical properties, so that even when the volume of the negative electrode active material changes during insertion and extraction of lithium, the negative electrode binder does not break, and the negative electrode active material changes in volume. This is because the negative electrode mixture layer that follows can be deformed, so that the current collecting property in the electrode is maintained, and excellent charge / discharge characteristics can be obtained. As the negative electrode binder having such high mechanical properties, a polyimide resin can be preferably used. Moreover, fluorine-type resins, such as polyvinylidene fluoride and polytetrafluoroethylene, can also be used preferably.

The ratio of the mass of the negative electrode binder to the total mass of the negative electrode mixture layer is preferably 5% or more, and the ratio of the volume of the negative electrode binder to the total volume of the negative electrode mixture layer is preferably 5% or more.
This is because when the ratio of the mass of the negative electrode binder to the total mass of the negative electrode mixture layer is less than 5% and the ratio of the volume of the negative electrode binder to the total volume of the negative electrode mixture layer is less than 5%, the negative electrode active material Since the amount of the negative electrode binder relative to the particles is too small, the adhesion within the electrode due to the negative electrode binder becomes insufficient, while when the amount of the negative electrode binder is increased too much, the resistance in the electrode increases, so the initial charge is reduced. It becomes difficult. Therefore, the ratio of the mass of the negative electrode binder to the total mass of the negative electrode mixture layer is preferably 50% or less, and the ratio of the volume of the negative electrode binder to the total volume of the negative electrode mixture layer is preferably 50% or less. The total volume of the negative electrode mixture layer is the sum of the volumes of the negative electrode active material and the negative electrode binder contained in the negative electrode mixture layer, and there are voids in the negative electrode mixture layer. In this case, the volume that closes the gap is not included.

  In the negative electrode of the present invention, conductive powder may be mixed in the negative electrode mixture layer. This is because by mixing the conductive powder, a conductive network of the conductive powder is formed around the active material particles, so that the current collecting property in the electrode can be further improved. As the conductive powder, a material similar to that of the conductive metal foil can be preferably used. Specifically, it is an alloy or a mixture made of a metal such as copper, nickel, iron, titanium, cobalt, or a combination thereof. In particular, copper powder is preferably used as the metal powder. Also, conductive carbon powder can be preferably used.

  The ratio of the mass of the conductive powder to the total mass of the negative electrode mixture layer is preferably 50% or less, and the ratio of the volume of the conductive powder to the total volume of the negative electrode mixture layer is preferably 20% or less. This is because when the amount of the conductive powder mixed is too large, the proportion of the negative electrode active material in the negative electrode mixture layer is relatively reduced, and the charge / discharge capacity of the negative electrode is reduced. In this case, since the ratio of the negative electrode binder amount compared to the total amount of the negative electrode active material and the conductive powder in the negative electrode mixture layer is decreased, the strength of the negative electrode mixture layer is decreased, and the charge / discharge characteristics are decreased. This is because there is a problem of doing so.

  The average particle size of the conductive powder is not particularly limited, but is preferably 100 μm or less, more preferably 50 μm or less, and most preferably 10 μm or less.

  In the negative electrode according to the present invention, a negative electrode mixture slurry in which particles containing silicon and / or silicon alloy as a negative electrode active material are uniformly mixed and dispersed in a negative electrode binder solution is used as a negative electrode current collector. It is preferable to manufacture by coating on the surface of the metal foil. This is because the negative electrode mixture layer formed using the slurry in which the negative electrode active material particles are uniformly mixed and dispersed in the negative electrode binder solution has a structure in which the negative electrode binder is uniformly distributed around the negative electrode active material particles. This is because the mechanical characteristics of the negative electrode binder are utilized to the maximum and high electrode strength is obtained, so that excellent charge / discharge characteristics can be obtained.

The invention according to claim 9 is the invention according to claim 8, wherein the negative electrode is disposed by sintering the negative electrode mixture layer on a surface of the negative electrode current collector.
As described above, when the negative electrode mixture layer is disposed on the current collector surface by sintering, the adhesion between the negative electrode active material particles and the adhesion between the negative electrode mixture layer and the negative electrode current collector are improved by the sintering effect. This is because it is greatly improved, and even when the volume change of the negative electrode active material occurs during insertion and extraction of lithium, the current collecting property of the negative electrode mixture layer is maintained, and excellent charge / discharge characteristics can be obtained.

  In this case, the negative electrode binder is particularly preferably thermoplastic. For example, in the case where the negative electrode binder has a glass transition temperature, the negative electrode binder is made to have a negative electrode active by performing a heat treatment to sinter and dispose the negative electrode mixture layer on the surface of the negative electrode current collector at a temperature higher than the glass transition temperature. The material particles and the negative electrode current collector are heat-sealed, and the adhesion between the negative electrode active material particles and between the negative electrode mixture layer and the negative electrode current collector is further greatly improved, and the current collecting property in the electrode is greatly improved. And further excellent charge / discharge characteristics can be obtained.

  In this case, it is preferable that the negative electrode binder remains without being completely decomposed even after the heat treatment for disposing the negative electrode mixture layer on the surface of the negative electrode current collector. This is because, when the negative electrode binder is completely decomposed after the heat treatment, the adhesive effect by the negative electrode binder is lost, and the current collecting property in the electrode is greatly reduced, resulting in poor charge / discharge characteristics. .

  Sintering for disposing the negative electrode mixture layer on the surface of the negative electrode current collector is preferably performed in a vacuum, a nitrogen atmosphere, or an inert gas atmosphere such as argon. Further, it may be performed in a reducing atmosphere such as a hydrogen atmosphere. The temperature of the heat treatment at the time of sintering is equal to or lower than the thermal decomposition start temperature of the binder resin because the negative electrode binder preferably remains without being completely decomposed even after the heat treatment for sintering as described above. It is preferable. Further, as a sintering method, a discharge plasma sintering method or a hot press method may be used.

In order to achieve the above object, the invention according to claim 10 of the present invention includes a negative electrode containing silicon as a negative electrode active material, a positive electrode containing a lithium transition metal composite oxide as a positive electrode active material, In a lithium secondary battery provided with a water electrolyte, at least one oxide powder selected from the group consisting of yttrium oxide, cerium oxide, silicon oxide, and tin oxide is added to the positive electrode. Features.
If it is the said structure, the effect similar to the effect of Claim 4 can be obtained.

The invention described in claim 11 is the invention described in claim 10, characterized in that the oxide powder is uniformly dispersed in the positive electrode.
If it is the said structure, the effect similar to the effect of Claim 3 can be obtained.

The invention of claim 12 is characterized in that, in the invention of claim 10 or 11, the addition ratio of the oxide powder to the lithium transition metal composite oxide is regulated to 1 mass% or more and 5 mass% or less. And
If it is the said structure, the effect similar to the effect of Claim 5 can be obtained.

A thirteenth aspect of the invention is characterized in that in the inventions of the tenth to twelfth aspects, carbon dioxide is dissolved in the non-aqueous electrolyte.
If it is the said structure, the effect similar to the effect of Claim 6 can be obtained.

The invention according to claim 14 is the invention according to claims 10 to 13, wherein the positive electrode comprises a positive electrode active material containing the lithium transition metal composite oxide, the oxide powder, a positive electrode binder, and a positive electrode conductive agent. The positive electrode material mixture layer to be included has a structure arranged on a positive electrode current collector made of a conductive metal foil.
If it is the said structure, the effect similar to the effect of Claim 7 can be obtained.

According to a fifteenth aspect of the present invention, in the negative electrode according to the tenth to fourteenth aspects, the negative electrode includes a negative electrode active material containing silicon and / or a silicon alloy, and a negative electrode mixture layer containing a negative electrode binder. It is the structure arrange | positioned on the surface of the negative electrode collector which becomes.
If it is the said structure, the effect similar to the effect of Claim 8 can be obtained.

The invention according to claim 16 is the invention according to claim 15, characterized in that the negative electrode is arranged by sintering the negative electrode mixture layer on the surface of the negative electrode current collector.
If it is the said structure, the effect similar to the effect of Claim 9 can be obtained.

(Other matters related to non-aqueous electrolytes, which are the main components of batteries)
(A) Although the solvent of a nonaqueous electrolyte is not specifically limited, Cyclic carbonates, such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, and chain carbonates, such as dimethyl carbonate, methyl ether carbonate, and diethyl carbonate And esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 1,2-dioxane , Ethers such as 2-methyltetrahydrofuran, nitriles such as acetonitrile, amides such as dimethylformamide, and the like can be used alone or in combination. In particular, a mixed solvent of a cyclic carbonate and a chain carbonate can be preferably used.

(B) The solute of the non-aqueous electrolyte is not particularly limited, but is a chemical formula LiXF _y such as LiPF ₆ , LiBF ₄ , LiAsF ₆ , where X is P, As, Sb, B, Bi, Al , Ga, or In, and when X is P, As, or Sb, y is 6, and when X is B, Bi, Al, Ga, or In, y is 4. LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) ₃ , Lithium compounds such as LiC (C ₂ F ₅ SO ₂ ) ₃ , LiClO ₄ , Li ₂ B ₁₀ Cl ₁₀ , and Li ₂ B ₁₂ Cl ₁₂ can be used. Among these, LiPF ₆ can be particularly preferably used.

(C) Examples of the non-aqueous electrolyte include a gel polymer electrolyte obtained by impregnating a polymer electrolyte such as polyethylene oxide and polyacrylonitrile with an electrolytic solution, and an inorganic solid electrolyte such as LiI and Li ₃ N. The non-aqueous electrolyte in the present invention can be used without limitation as long as the lithium compound as a solute that develops lithium ion conductivity and the solvent that dissolves and retains the lithium compound do not decompose during charge / discharge or storage of the battery.

  ADVANTAGE OF THE INVENTION According to this invention, there exists an outstanding effect that the charge / discharge characteristic in the lithium secondary battery using the material containing silicon as a negative electrode active material can be improved dramatically.

  Hereinafter, the present invention will be described in more detail. However, the present invention is not limited to the following best modes, and can be appropriately modified and implemented without departing from the scope of the present invention.

[Production of positive electrode]
In a ₂ L bead mill container made of Al ₂ O _3, 500 g of a 6% N-methyl-2-pyrrolidone solution of polyvinylidene fluoride, 30 g of acetylene black, 1.8 kg of ZrO ₂ beads having a diameter of 1.5 mm, The mixture was stirred for 15 minutes by rotating a disc-shaped stirring blade made of Al ₂ O ₃ at 1500 rpm, and then all the ZrO ₂ beads were removed, so that N-methyl-2 of polyvinylidene fluoride was removed. -A positive electrode paste in which acetylene black powder was uniformly mixed and dispersed in a pyrrolidone solution was obtained.

Next, Li ₂ CO ₃ and CoCO ₃ are mixed in a mortar so that the molar ratio of Li and Co is 1: 1, heat-treated in an air atmosphere at 800 ° C. for 24 hours, and then pulverized. Thus, a lithium cobalt composite oxide represented by LiCoO ₂ having an average particle diameter of 8 μm and a BET specific surface area of 0.27 m ² / g was obtained. The particle diameter was measured by a laser diffraction method. Further, the particle diameters described later were all measured by a laser diffraction method.

A mixture of LiCoO ₂ powder as the positive electrode active material, the positive electrode paste, yttrium oxide powder having an average particle diameter of 33 nm and a BET specific surface area of 35 m ² / g at a mass ratio of 94: 53: 3 The mixture is kneaded for 30 minutes in a raking machine, and the LiCoO ₂ powder, polyvinylidene fluoride, acetylene black, and silicon oxide powder in a N-methyl-2-pyrrolidone solution of polyvinylidene fluoride are in a mass ratio of 94: 3: 3. : A positive electrode mixture slurry mixed in 3 was obtained.

  This positive electrode mixture slurry was applied to one side of an aluminum foil (thickness 15 μm) as a positive electrode current collector, dried, and then rolled. This was cut out into a 20 × 20 mm square shape, a current collecting tab was attached, and a positive electrode was produced.

(Production of negative electrode)
N-methyl-2-pyrrolidone as a dispersion medium, silicon powder having an average particle size of 3 μm as a negative electrode active material (purity 99.9%), a glass transition temperature 190 ° C. as a negative electrode binder, and a density of 1.1 g / cm ³ Thermoplastic polyimide was mixed so that the mass ratio of the active material and the binder was 90:10 to obtain a negative electrode mixture slurry.

  This negative electrode mixture slurry was applied to one side (rough surface) of an electrolytic copper foil (thickness 35 μm) having a surface roughness Ra of 1.0 μm, which was a negative electrode current collector, and dried. The obtained product was cut out into a 25 × 30 mm rectangular shape and rolled, and then heat treated at 400 ° C. for 1 hour in an argon atmosphere, and sintered to prepare a negative electrode.

(Preparation of electrolyte)
Carbon dioxide was blown into a solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 3: 7 and 1 mol / liter of LiPF ₆ was dissolved to dissolve the carbon dioxide, thereby preparing an electrolyte. When the dissolved amount of carbon dioxide in the electrolytic solution was determined by a mass method, it was 0.37% of the total mass of the electrolytic solution.

[Production of battery]
The lithium secondary battery was fabricated by inserting the positive electrode, the negative electrode, and the non-aqueous electrolyte into an outer package of an aluminum laminate in an atmosphere of argon gas at normal temperature and normal pressure.

  As shown in FIGS. 1 and 2, the lithium secondary battery has a specific structure in which a positive electrode 1 and a negative electrode 2 are opposed to each other with a separator 3 therebetween. Thus, a power generation element is configured. The positive electrode 1 and the negative electrode 2 are connected to a positive electrode tab 4 made of aluminum metal and a negative electrode tab 5 made of nickel metal, respectively, so that they can be charged and discharged as a secondary battery. The power generation element composed of the positive electrode 1, the negative electrode 2, and the separator 3 is disposed in a storage space of an aluminum laminate exterior body 6 that includes a closed portion 7 whose peripheral edges are heat-sealed.

[First embodiment]
Example 1
As Example 1, the lithium secondary battery shown in the best mode for carrying out the invention was used.
The battery thus produced is hereinafter referred to as the present invention battery A1.

(Example 2)
A lithium secondary battery was produced in the same manner as in Example 1 except that cerium oxide having an average particle diameter of 14 nm and a BET specific surface area of 60 m ² / g was used as the oxide added to the positive electrode instead of yttrium oxide powder. did.
The battery thus produced is hereinafter referred to as the present invention battery A2.

(Example 3)
A lithium secondary battery was produced in the same manner as in Example 1 except that silicon oxide having an average particle diameter of 25 nm and a BET specific surface area of 110 m ² / g was used as the oxide added to the positive electrode instead of yttrium oxide powder. did.
The battery thus produced is hereinafter referred to as the present invention battery A3.

Example 4
A lithium secondary battery was produced in the same manner as in Example 1 except that tin oxide having an average particle diameter of 21 nm and a BET specific surface area of 45 m ² / g was used in place of yttrium oxide powder as the oxide added to the positive electrode. did.
The battery thus produced is hereinafter referred to as the present invention battery A4.

(Comparative example)
A lithium secondary battery was produced in the same manner as in Example 1 except that the yttrium oxide powder was not added to the positive electrode.
The battery thus produced is hereinafter referred to as comparative battery X.

(Experiment)
About the said invention battery A1-A4 and the comparison battery X, since charging / discharging was performed on the following conditions and the charging / discharging efficiency of the 1st cycle was calculated | required, the result is shown in Table 1. In addition, in Table 1, it represents with the index when charging / discharging efficiency of this invention battery A1 is set to 100.
-Charging conditions Conditions for charging to a battery voltage of 4.2 V at a current value of 2.8 mA, and subsequently charging until the current value reaches 0.7 mA while holding the voltage at 4.2 V. The temperature is 25 ° C.

-Discharging conditions Conditions for discharging to a battery voltage of 2.75 V at a current value of 2.8 mA.
Under the above charge / discharge conditions, one cycle charge / discharge was performed, the charge capacity and discharge capacity of the first cycle were determined, and the first cycle charge / discharge efficiency was calculated from the following formula (1).
First cycle charge / discharge efficiency (%)
= (Discharge capacity at the first cycle / Charge capacity at the first cycle) × 100 (1)

  As is apparent from Table 1, the batteries A1 to A4 of the present invention in which an oxide is added in the positive electrode exhibit excellent charge / discharge characteristics as compared with the comparative battery X in which no oxide is added in the positive electrode. I understand that. This is because, in a battery in which an oxide is added in the positive electrode, the liquid retention of the oxide is high, and the liquid retention of the positive electrode is improved. Even when the surface area is increased, it is considered that a large amount of the electrolytic solution in the battery system is held only by the negative electrode, and the charge / discharge reaction is smoothly performed.

[Second Embodiment]
Example 1
As Example 1, the battery A1 of the present invention shown in Example 1 of the first example was used.

(Examples 2 to 4)
A lithium secondary battery was produced in the same manner as in Example 1 except that the ratio of yttrium oxide to the positive electrode active material was 0.5% by mass, 2.1% by mass, and 5.3% by mass, respectively.
The batteries thus produced are hereinafter referred to as invention batteries B1, B2, and B3, respectively.

(Experiment)
About the said invention battery A1, B1-B3, since charging / discharging was performed on the conditions similar to the experiment of the said 1st Example and the charging / discharging efficiency of the 1st cycle was calculated | required, the result is shown in Table 2. In addition, in Table 2, it represents with the index | exponent when the charging / discharging efficiency of this invention battery A1 is set to 100. FIG.

  As is apparent from Table 2, the present invention batteries A1 and B2 in which the ratio of yttrium oxide to the positive electrode active material is 1 to 5% by mass are the present invention batteries B1 in which the ratio of yttrium oxide to the positive electrode active material is less than 1% by mass. In addition, it is recognized that the ratio of yttrium oxide to the positive electrode active material exhibits charge / discharge characteristics superior to the battery B3 of the present invention exceeding 5 mass%.

  This is because, when the ratio of yttrium oxide to the positive electrode active material is less than 1% by mass, the amount of yttrium oxide is too small, so that the liquid retention of the positive electrode becomes insufficient. On the other hand, when the ratio exceeds 5% by mass, Since the relative amount of the positive electrode binder with respect to the total amount of the substance and the oxide is reduced, the adhesion of the positive electrode active material to the positive electrode active material particles and the positive electrode current collector is reduced, and the charge / discharge characteristics are considered to be reduced. It is done.

From these facts, it can be seen that the ratio of yttrium oxide to the positive electrode active material is preferably 1 to 5% by mass. This has been confirmed not only when yttrium oxide is used but also when oxides such as cerium oxide, silicon oxide, and tin oxide are used.
[Third embodiment]
Example 1
As Example 1, the battery A1 of the present invention shown in Example 1 of the first example was used.

(Example 2)
A lithium secondary battery was produced in the same manner as in Example 1 except that yttrium oxide having an average particle diameter of 2.5 μm and a BET specific surface area of 22 m ² / g was used.
The batteries thus produced are hereinafter referred to as the present invention battery C1.

(Experiment)
About the said invention battery A1 and C1, since charging / discharging was performed on the conditions similar to the experiment of the said 1st Example and charging / discharging efficiency of the 1st cycle was calculated | required, the result is shown in Table 3. In addition, in Table 3, it represents with the index | exponent when the charging / discharging efficiency of this invention battery A1 is set to 100. FIG.

  As is apparent from Table 3, the present invention battery A1 using yttrium oxide having an average particle diameter of 33 nm exhibits charge / discharge characteristics superior to those of the present invention battery C1 using yttrium oxide having an average particle diameter of 2.5 μm. Is recognized. This is considered to be because the liquid retaining property of the electrolytic solution is lowered when the average particle diameter of the yttrium oxide powder is too large as in the battery C1 of the present invention.

Although not shown in Table 3 above, it was confirmed that when the average particle diameter of yttrium oxide powder or the like exceeds 1 μm, the liquid retention of the electrolytic solution is lowered. On the other hand, when the average particle diameter of yttrium oxide powder or the like is less than 10 nm, it was confirmed that the handling becomes difficult. Therefore, it can be seen that the average particle diameter of the yttrium oxide powder or the like is preferably 10 nm or more and 1 μm or less.
In addition, regarding the BET specific surface area of the yttrium oxide powder and the like, it was also confirmed that when the BET specific surface area is less than 5 m ² / g, the liquid retention of the electrolytic solution is lowered. Therefore, it can also be seen that the BET specific surface area of yttrium oxide powder or the like is preferably 5 m ² / g or more.

  The present invention can be applied not only to a driving power source of a mobile information terminal such as a mobile phone, a notebook computer, and a PDA, but also to a large battery such as an in-vehicle power source of an electric vehicle or a hybrid vehicle.

It is a front view of the battery which concerns on the best form of this invention. It is AA arrow sectional drawing of FIG.

Explanation of symbols

1: Positive electrode 2: Negative electrode 3: Separator

Claims (16)

In a lithium secondary battery having a negative electrode containing silicon as a negative electrode active material, a positive electrode containing a lithium transition metal composite oxide as a positive electrode active material, and a non-aqueous electrolyte,
In the positive electrode, a liquid retaining powder for holding the nonaqueous electrolyte is added. The liquid retaining powder has an average particle diameter of 10 nm to 1 μm and a BET specific surface area of 5 m ² / g or more. A lithium secondary battery that is regulated. The lithium secondary battery according to claim 1, wherein the liquid retaining powder does not have a lithium storage capacity. The lithium secondary battery according to claim 1, wherein the liquid retaining powder is uniformly dispersed in the positive electrode. The lithium secondary battery according to claim 1, wherein at least one oxide selected from the group consisting of yttrium oxide, cerium oxide, silicon oxide, and tin oxide is used as the liquid retaining powder. The lithium secondary battery according to claim 1, wherein an addition ratio of the liquid retaining powder to the lithium transition metal composite oxide is regulated to 1% by mass or more and 5% by mass or less. The lithium secondary battery according to claim 1, wherein carbon dioxide is dissolved in the non-aqueous electrolyte. The positive electrode is a positive electrode current collector in which a positive electrode mixture layer containing a positive electrode active material containing the lithium transition metal composite oxide, the liquid retaining powder, a positive electrode binder, and a positive electrode conductive agent is formed of a conductive metal foil. The lithium secondary battery according to claim 1, which has a structure arranged on the body. The negative electrode has a structure in which a negative electrode mixture layer containing a negative electrode active material containing silicon and / or a silicon alloy and a negative electrode binder is disposed on the surface of a negative electrode current collector made of a conductive metal foil. The lithium secondary battery according to Items 1 to 7. The lithium secondary battery according to claim 8, wherein the negative electrode is disposed by sintering the negative electrode mixture layer on a surface of the negative electrode current collector. In a lithium secondary battery having a negative electrode containing silicon as a negative electrode active material, a positive electrode containing a lithium transition metal composite oxide as a positive electrode active material, and a non-aqueous electrolyte,
A lithium secondary battery, wherein at least one oxide powder selected from the group consisting of yttrium oxide, cerium oxide, silicon oxide, and tin oxide is added to the positive electrode. The lithium secondary battery according to claim 10, wherein the oxide powder is uniformly dispersed in the positive electrode. The lithium secondary battery according to claim 10 or 11, wherein an addition ratio of the oxide powder to the lithium transition metal composite oxide is regulated to 1% by mass or more and 5% by mass or less. The lithium secondary battery according to claim 10, wherein carbon dioxide is dissolved in the non-aqueous electrolyte. The positive electrode is a positive electrode current collector in which a positive electrode mixture layer including a positive electrode active material including the lithium transition metal composite oxide, the oxide powder, a positive electrode binder, and a positive electrode conductive agent is formed of a conductive metal foil. The lithium secondary battery according to claim 10, which has a structure disposed on top. The negative electrode has a structure in which a negative electrode mixture layer containing a negative electrode active material containing silicon and / or a silicon alloy and a negative electrode binder is disposed on the surface of a negative electrode current collector made of a conductive metal foil. Item 15. A lithium secondary battery according to item 10-14. The lithium secondary battery according to claim 15, wherein the negative electrode is disposed by sintering the negative electrode mixture layer on a surface of the negative electrode current collector.