JP4600136B2 - Aqueous electrolyte lithium secondary battery - Google Patents

Description

  The present invention relates to an aqueous electrolyte lithium secondary battery having an aqueous electrolyte obtained by dissolving a lithium salt in water as an electrolyte.

  Lithium rechargeable batteries that use the lithium absorption and desorption phenomenon are high voltage, high energy density, and can be reduced in size and weight. Practical use has progressed in the field of communication equipment, and it has become widely popular. In other fields, the use of lithium secondary batteries as a power source for electric vehicles is being studied while the development of electric vehicles is urgent due to environmental issues and resource issues.

  Currently, non-aqueous lithium secondary batteries containing a non-aqueous electrolyte solution obtained by dissolving a lithium salt in an organic solvent as an electrolyte solution are mainly used as lithium secondary batteries in practical use. As such a non-aqueous lithium secondary battery, in general, a positive electrode active material composed of a lithium transition metal composite oxide, a conductive agent composed of a conductive carbon material, and a positive electrode mixture containing a binder composed of a fluororesin, A negative electrode mixture containing a positive electrode applied to a current collector made of aluminum foil, a negative electrode active material made of a carbon material capable of inserting and extracting lithium, and a binder made of a fluororesin or a water-soluble resin, What has the negative electrode formed by apply | coating to the collector which consists of foil is used.

  However, since non-aqueous lithium secondary batteries contain non-aqueous electrolytes such as organic solvents with a low flash point as the electrolyte, there is a risk of ignition or explosion due to overcharge or short circuit. . Further, when the battery is overcharged or exposed to a high temperature environment, the electrolyte solution may decompose and generate a flammable gas. For this reason, non-aqueous lithium secondary batteries are generally equipped with devices such as PCT elements and safety valves for the purpose of ensuring safety. However, since non-aqueous lithium secondary batteries originally contain a flammable solvent, it is difficult to ensure sufficient safety. In particular, when using a non-aqueous lithium secondary battery as a power source for automobiles or the like, the battery becomes larger and the amount of the organic solvent used increases. Therefore, a lithium secondary battery with higher safety has been demanded.

  In addition, in a non-aqueous lithium secondary battery, if there is even a small amount of moisture in the battery, gas is generated due to electrolysis of water, water reacts with lithium and lithium is consumed, There is a risk of corrosion of the material constituting the battery. Therefore, in a non-aqueous lithium secondary battery, it is necessary to maintain a thorough dry environment in the manufacturing process. As a result, there is a problem that special equipment and a great amount of labor are required to completely remove moisture, resulting in an increase in manufacturing cost.

On the other hand, there is an aqueous electrolyte lithium secondary battery using an aqueous solution as an electrolyte (see Patent Document 1). This aqueous electrolyte lithium secondary battery is very advantageous for the above-described problems of the non-aqueous lithium secondary battery.
That is, since the aqueous electrolyte lithium secondary battery does not contain the organic solvent, there is almost no risk of ignition. Moreover, since a dry environment is not required at the time of manufacture, the manufacturing cost can be reduced.
In general, an aqueous electrolyte lithium secondary battery needs to be charged / discharged in a potential range where electrolysis of water does not occur. However, an aqueous solution has higher conductivity than a non-aqueous solution. Compared to the secondary battery, there is an advantage that the reaction resistance of the battery is low and the output characteristics and rate characteristics of the battery are improved.

  As in the case of the non-aqueous lithium secondary battery, the positive electrode and the negative electrode of such an aqueous electrolyte lithium secondary battery are made of an electrode mixture (positive electrode mixture or negative electrode mixture) composed of an active material, a conductive agent, and a binder. It is manufactured by applying to a current collector made of metal foil or the like and drying. The aqueous electrolyte lithium secondary battery has a short history of research, and there are few reports on binders used for positive and negative electrodes. Therefore, until now, as binders for aqueous electrolyte lithium secondary batteries, fluororesins such as polyvinylidene fluoride and tetrafluoroethylene used for the positive and negative electrodes of non-aqueous lithium secondary batteries, styrene butadiene rubber, etc. The hydrocarbon-based resin has been used (see Patent Document 1 and Patent Document 2).

However, in an aqueous electrolyte lithium secondary battery containing a binder for a non-aqueous lithium secondary battery in the positive electrode or the negative electrode, the adhesion between the electrode mixture and the current collector is insufficient, and there is no water between them. There was a possibility that swelling would occur in the electrode during charging / discharging of the battery.
Moreover, the binder used for the non-aqueous lithium secondary battery has almost no affinity with water and has a property of shrinking in an aqueous solution. Therefore, when the particles of the positive electrode active material and the negative electrode active material expand and contract with charge / discharge, the binder contracted in the aqueous electrolyte solution is easily detached from the surface of the active material particles. As a result, the bonding force between the particles of the positive electrode active material and the negative electrode active material is lost, and the particles of the positive electrode active material and the negative electrode active material may slide off from the current collector. For this reason, as charge / discharge is repeated, there is a problem in that the discharge capacity tends to decrease and charge / discharge cycle characteristics that can withstand practical use cannot be exhibited.

JP-T 9-508490 JP 2001-52747 A

  The present invention has been made in view of such conventional problems, and is an aqueous electrolyte lithium secondary that is excellent in adhesion between a positive electrode mixture or negative electrode mixture and a current collector and excellent in charge / discharge cycle characteristics. It is intended to provide a battery.

The present invention relates to a positive electrode formed by binding a positive electrode mixture to a positive electrode current collector, a negative electrode formed by binding a negative electrode mixture to a negative electrode current collector, and an aqueous electrolyte solution obtained by dissolving a lithium salt in water In an aqueous electrolyte lithium secondary battery having
The positive electrode mixture contains a positive electrode active material made of a lithium-containing composite oxide capable of reversibly inserting and extracting lithium, a conductive agent made of a carbon-based material having conductivity, and a binder.
The negative electrode mixture is composed of a negative electrode active material composed of a composite oxide having a lower lithium storage and desorption potential than the lithium-containing composite oxide contained in the positive electrode active material, and a conductive carbon-based material. Containing a conductive agent and a binder,
The binder in the positive electrode mixture and the negative electrode mixture is characterized by using a polymer compound that is water-soluble or water-dispersible and contains a functional group that contains an oxygen atom and forms an anion in water. In the aqueous electrolyte lithium secondary battery.

The most remarkable point in the aqueous electrolyte lithium secondary battery of the present invention is that the binder in the positive electrode mixture and the negative electrode mixture is water-soluble or water-dispersible and contains oxygen atoms and anions in water. The high molecular compound containing the functional group which forms is in the point used.
Therefore, in the aqueous electrolyte lithium secondary battery, the positive electrode mixture and the negative electrode mixture are bonded to the positive electrode current collector and the negative electrode current collector made of, for example, metal foil with excellent adhesion. be able to. Therefore, in the aqueous electrolyte lithium secondary battery, the positive electrode mixture and the negative electrode mixture can be prevented from sliding off the current collector, and a high discharge capacity can be maintained even after repeated charge and discharge. Hereinafter, this effect will be described in detail.

  That is, the positive electrode current collector and the negative electrode current collector made of, for example, metal foil are usually oxidized or hydroxylated in the aqueous electrolyte solution, and the surface is covered with oxygen atoms. In the present invention, the binder contains a functional group containing an oxygen atom and forming an anion in water. Therefore, the binder can form a chemical bond between oxygen atoms of the binder and oxygen atoms or hydroxyl groups on the surfaces of the positive electrode current collector and the negative electrode current collector. Therefore, in the aqueous electrolyte lithium secondary battery, the positive electrode mixture and the negative electrode mixture can be bound to the positive electrode current collector and the negative electrode current collector with good adhesion by the binder. Therefore, in the aqueous electrolyte lithium secondary battery, water of the aqueous electrolyte enters between the positive electrode current collector and the positive electrode mixture or between the negative electrode current collector and the positive electrode mixture. It is difficult, and it is possible to prevent the positive electrode mixture and the negative electrode mixture from separating from the current collector due to the occurrence of swelling on the positive electrode and the negative electrode.

In the aqueous electrolyte lithium secondary battery, both the positive electrode active material and the negative electrode active material are composed of a composite oxide. Therefore, the binder can be bonded to the particles of the positive electrode active material and the negative electrode active material, and the bonding force between the particles of the positive electrode active material and the particles of the negative electrode active material can be increased. .
Therefore, the binder can be bonded to the positive electrode current collector and the negative electrode current collector as described above, and can also be bonded to the positive electrode active material and the negative electrode active material. Therefore, in the positive electrode mixture, the positive electrode active material is sufficiently held on the surface of the positive electrode current collector, and also in the negative electrode mixture, the negative electrode active material is sufficiently on the surface of the negative electrode current collector. Retained. Therefore, even when the positive electrode active material particles and the negative electrode active material particles expand and contract with the insertion and extraction of lithium, the binder is not easily detached from the surface of the active material.

  The binder has water solubility or water dispersibility, but the positive electrode mixture in a state of being bound to the positive electrode current collector and the negative electrode mixture in a state of being bound to the negative electrode current collector. In, the binder no longer elutes into the aqueous electrolyte. This is because the functional group that becomes an anion in water in the binder forms a chemical bond with the positive electrode current collector, the negative electrode current collector, the positive electrode active material, and the negative electrode active material as described above.

  Therefore, in the aqueous electrolyte lithium secondary battery, the positive electrode mixture and the negative electrode mixture are not easily detached from the positive electrode current collector and the negative electrode current collector, respectively. Can be maintained.

  Thus, according to the present invention, it is possible to provide an aqueous electrolyte lithium secondary battery that is excellent in the adhesion between the positive electrode mixture or the negative electrode mixture and the current collector and excellent in charge / discharge cycle characteristics.

  Next, preferred embodiments of the aqueous electrolyte lithium secondary battery of the present invention will be described for each component of the battery. Here, as described above, the aqueous electrolyte lithium secondary battery includes a positive electrode formed by binding a positive electrode mixture to a positive electrode current collector, and a negative electrode formed by binding a negative electrode mixture to a negative electrode current collector. And an aqueous electrolyte solution obtained by dissolving a lithium salt in water. The positive electrode mixture contains a positive electrode active material, a conductive agent, and a binder, and the negative electrode mixture contains a negative electrode active material, a conductive agent, and a binder.

First, the binder contained in the positive electrode mixture and the negative electrode mixture will be described.
(binder)
The binder is a polymer compound that is water-soluble or water-dispersible and contains a functional group that contains an oxygen atom and forms an anion in water. Examples of the binder include a water-soluble polymer containing the functional group and a water-dispersible polymer containing the functional group. The binder is a compound that can exhibit water solubility or water dispersibility when the functional group containing an oxygen atom of the polymer compound becomes an anion. Specifically, the binder is a polymer compound containing an organic acid such as an organic carboxylic acid or an organic sulfonic acid, or an organic acid salt such as an organic carboxylate or an organic sulfonate, such as a water-soluble acrylic resin. And one or more selected from water-dispersed styrene butadiene rubber, water-soluble fluororesin, water-soluble melamine resin, water-soluble cellulose, water-soluble polybutadiene, and the like. The binder for the positive electrode mixture and the binder for the negative electrode mixture may be the same as or different from each other.

  In addition, when a salt is used as the binder, an alkali metal salt can be used. When the lithium salt is used as the alkali metal salt in consideration of elution into the electrolytic solution, the influence on charging / discharging can be eliminated.

(Positive electrode)
The positive electrode can be formed, for example, by applying the positive electrode mixture containing the positive electrode active material, the conductive agent, and the binder to the positive electrode current collector, drying, and pressing as necessary. .
The positive electrode active material is made of a lithium-containing composite oxide capable of reversibly inserting and extracting lithium. Specifically, for example, there are lithium cobalt composite oxide, lithium nickel composite oxide, lithium manganese composite oxide, lithium iron composite phosphorus oxide, and the like.
The conductive agent is for ensuring the electrical conductivity of the electrode. For example, one or more selected from carbon-based materials such as carbon black, acetylene black, ketjen black, and graphite can be used. .

The positive electrode mixture contains 50 to 95 parts by weight of the positive electrode active material, 3 to 30 parts by weight of the conductive agent, and 2 to 20 parts by weight of the binder in 100 parts by weight of the positive electrode mixture. Is preferred.
When the content of the positive electrode active material is less than 50 parts by weight, the aqueous electrolyte lithium secondary battery may not exhibit sufficient charge / discharge capacity to withstand practical use. On the other hand, when the amount exceeds 95 parts by weight, the content of the conductive agent or the binder is insufficient, and the aqueous electrolyte lithium secondary battery has a high electrical resistance or the positive electrode mixture and the positive electrode current collector. There is a risk that the adhesion to the body may be reduced.

Moreover, when content of the said electrically conductive agent is less than 3 weight part, there exists a possibility that the electroconductivity of a positive electrode may fall and the electrical resistance of the said aqueous electrolyte lithium secondary battery may become high. On the other hand, when it exceeds 30 weight part, there exists a possibility that the battery capacity of the said aqueous electrolyte lithium secondary battery may fall.
Moreover, when content of the said binder is less than 2 weight part, there exists a possibility that the adhesive improvement effect of the said positive electrode compound material and the said positive electrode electrical power collector may not fully be exhibited. On the other hand, when it exceeds 20 weight part, there exists a possibility that the battery capacity of the said aqueous electrolyte lithium secondary battery may fall.

  When the positive electrode mixture is applied to the positive electrode current collector, for example, the binder is dissolved or dispersed in water to prepare a binder liquid, and the positive electrode active material and the conductive agent are sufficiently contained in the binder liquid. After being dispersed and further adjusted to a viscosity suitable for application with water, it can be applied to the current collector.

(Negative electrode)
The negative electrode can be formed, for example, by applying the negative electrode mixture containing the negative electrode active material, the conductive agent, and the binder to the negative electrode current collector, drying, and pressing as necessary. . The negative electrode can be the same as the positive electrode except that it contains the negative electrode active material as an active material.
The negative electrode active material is composed of a composite oxide having a lithium occlusion potential and a desorption potential lower than those of the lithium-containing composite oxide contained in the positive electrode active material. Specifically, for example, there are lithium transition metal composite oxides such as lithium vanadium composite oxide and lithium titanium composite oxide, or iron hydroxide.

  The negative electrode mixture contains 50 to 95 parts by weight of the negative electrode active material, 3 to 30 parts by weight of the conductive agent, and 2 to 20 parts by weight of the binder in 100 parts by weight of the negative electrode mixture. Is preferred. About these critical significance, it is the same as that of the said positive electrode compound material.

(Electrolyte)
The aqueous electrolyte lithium secondary battery contains an aqueous electrolyte obtained by dissolving a lithium salt in water as an electrolyte. The lithium salt is dissociated by dissolving in water, and lithium ions are present in the electrolytic solution. Examples of such a lithium salt include lithium nitrate, lithium sulfate, lithium hydroxide, and lithium iodide. These lithium salts may be used alone or in combination of two or more.

The aqueous electrolyte solution preferably has a pH in a range of 6 ≦ pH ≦ 10.
In this case, the stability of the positive electrode active material in the positive electrode and the negative electrode active material in the negative electrode in an aqueous solution can be improved, and occlusion of lithium ions in the positive electrode active material and the negative electrode active material. And the elimination reaction becomes smoother.

When the pH of the aqueous electrolyte is less than 6, the positive electrode active material and / or the negative electrode active material may be dissolved and cannot be stably present in the aqueous electrolyte. On the other hand, when the pH exceeds 10, a reaction occurs between the aqueous electrolyte and the positive electrode current collector or the negative electrode current collector made of, for example, a metal foil, and the electrode may not exist stably. There is.
In addition, the lithium salt concentration in the aqueous electrolyte solution should be a saturated concentration or a concentration close to it from the viewpoint of increasing the electrical conductivity of the electrolyte solution and reducing the internal resistance of the aqueous electrolyte lithium secondary battery. Is preferred.

(Current collector)
In the aqueous electrolyte lithium secondary battery, the positive electrode has the positive electrode current collector, and the negative electrode has the negative electrode current collector. As such a current collector, a metal foil or the like that is excellent in conductivity and stable in an aqueous electrolyte can be used. Specifically, for example, there is a metal foil made of nickel, aluminum, platinum, or an alloy thereof.

Moreover, it is preferable that both of the positive electrode current collector and the negative electrode current collector are made of aluminum.
In this case, the positive electrode current collector and the negative electrode current collector react in the aqueous electrolyte solution even when the pH of the aqueous electrolyte solution is in the preferred range of 6 ≦ pH ≦ 10 as described above. And the stability of the positive electrode current collector and the negative electrode current collector in the aqueous electrolyte can be improved. In this case, the manufacturing cost of the aqueous electrolyte lithium secondary battery can be reduced, and the weight can be reduced.

(Other)
In the aqueous electrolyte lithium secondary battery, a separator can be sandwiched between the positive electrode and the negative electrode.
The said separator isolate | separates a positive electrode and a negative electrode, and hold | maintains the said aqueous electrolyte solution, A stable thing can be used in the said aqueous electrolyte solution. Specifically, what consists of thin microporous films, such as a cellulose, polyethylene, and a polypropylene, can be used, for example.

Examples of the shape of the aqueous electrolyte lithium secondary battery include a cylindrical shape, a stacked shape, a coin shape, and a square shape. As the battery case that accommodates the positive electrode, the negative electrode, the aqueous electrolyte, and the like, those corresponding to these shapes can be used.
The aqueous electrolyte lithium secondary battery includes, for example, an electrode body in which the separator is sandwiched between the positive electrode and the negative electrode in a battery case having a predetermined shape, and the positive electrode current collector and the negative electrode The current collector can be produced by electrically connecting the positive electrode external terminal and the negative electrode external terminal via lead wires, impregnating the electrode body with the aqueous electrolyte solution, and sealing the battery case.

Example 1
Next, examples of the aqueous electrolyte lithium secondary battery of the present invention will be described with reference to FIGS.
As shown in FIGS. 1 and 2, the aqueous electrolyte lithium secondary battery 1 of this example includes a positive electrode 2 formed by binding a positive electrode mixture 25 to a positive electrode current collector 21, and a negative electrode current collector 31. It has the negative electrode 3 formed by binding the material 35 and an aqueous electrolyte solution obtained by dissolving lithium salt in water.

As shown in FIG. 2, the positive electrode mixture 25 includes a positive electrode active material 251 made of a lithium-containing composite oxide capable of reversibly inserting and extracting lithium, a conductive agent 253 made of a carbon-based material having conductivity, and a binder. 255.
Further, the negative electrode mixture 35 is composed of a negative electrode active material 351 made of a composite oxide having a lower lithium storage and desorption potential than the lithium-containing composite oxide contained in the positive electrode active material 25, and a carbon-based conductive material. A conductive agent 353 made of a material and a binder 355 are contained.
As the binders 255 and 355 in the positive electrode mixture 25 and the negative electrode mixture 35, polymer compounds that are water-soluble or water-dispersible and contain functional groups that contain oxygen atoms and form anions in water are used. .

  As shown in FIG. 1, the aqueous electrolyte lithium secondary battery 1 of this example has a cylindrical shape, and includes a positive electrode 2, a negative electrode 3, a separator 4, a gasket 5, a battery case 6, and the like. The battery case 6 is a 18650 type cylindrical battery case, and includes a cap 61 and an outer can 62. In the battery case 6, a sheet-like positive electrode 2 and a negative electrode 3 are arranged in a state of being wound together with a separator 4 sandwiched therebetween, and the aqueous electrolyte lithium secondary battery 1 is a wound electrode. have.

Further, the gasket 5 is disposed inside the cap 61 of the battery case 6, and a water-based electrolyte is injected into the battery case 6.
The positive electrode 2 and the negative electrode 3 are respectively provided with a positive electrode current collecting lead 28 and a negative electrode current collecting lead 38 by welding. The positive electrode current collecting lead 28 is connected by welding to a positive electrode current collecting tab 285 arranged on the cap 61 side. Further, the negative electrode current collecting lead 38 is connected by welding to a negative electrode current collecting tab 385 disposed on the bottom of the outer can 62.
Further, a saturated concentration of lithium nitrate aqueous solution (pH = 7) is used as the aqueous electrolyte, and the aqueous electrolyte is injected into the battery case 6.

Next, the manufacturing method of the aqueous electrolyte lithium secondary battery of this example will be described.
First, an olivine-structured lithium iron composite phosphorus oxide represented by a composition formula LiFePO ₄ was prepared as a positive electrode active material, and acetylene black was prepared as a conductive agent. As binders, sodium carboxymethylcellulose containing carboxyl groups and water-dispersed styrene butadiene rubber containing carboxyl groups were prepared.

Next, sodium carboxymethyl cellulose is dissolved in water to prepare a 5 wt% aqueous carboxymethyl cellulose solution (binder liquid 1), and styrene butadiene rubber is dispersed in water to give a water solution of 49 wt% styrene butadiene rubber in an emulsion state. A dispersion (binder liquid 2) was prepared.
Next, in the binder liquid 1, the said positive electrode active material and the electrically conductive agent were disperse | distributed and mixed, and also the binder liquid 2 was added and mixed, and the positive electrode compound material was obtained. This positive electrode mixture is composed of 85 parts by weight of lithium iron composite phosphate (LiFePO ₄ ) as a positive electrode active material, 10 parts by weight of acetylene black as a conductive agent, 2 parts by weight of carboxymethyl cellulose as a binder, and water-dispersed styrene butadiene rubber 3 Parts by weight.

  Next, an aluminum foil having a thickness of 20 μm was prepared as a positive electrode current collector. Water was evaporated by applying a positive electrode mixture on both sides of the positive electrode current collector and drying it. Furthermore, it pressed with the linear pressure of about 1 ton / cm, and produced the sheet-like positive electrode.

Next, a lithium vanadium composite oxide having a spinel structure represented by a composition formula LiV ₂ O ₄ was prepared as a negative electrode active material, and acetylene black was prepared as a conductive agent. Further, sodium carboxymethyl cellulose containing a carboxyl group and a water-dispersed styrene butadiene rubber containing a carboxyl group were prepared as binders.

Next, in the same manner as in the case of the positive electrode, the binder liquid 1 and the binder liquid 2 are prepared, and the negative electrode active material and the conductive agent are dispersed and mixed in the binder liquid 1, and the binder liquid 2 is further added and mixed. A negative electrode mixture was obtained. This negative electrode mixture is composed of 85 parts by weight of lithium vanadium composite oxide (LiV ₂ O ₄ ) as a negative electrode active material, 10 parts by weight of acetylene black as a conductive agent, 2 parts by weight of carboxymethyl cellulose as a binder, and water-dispersed styrene butadiene rubber. 3 parts by weight.
Next, as in the case of the positive electrode, the negative electrode mixture was applied to both sides of a negative electrode current collector made of aluminum foil having a thickness of 20 μm, dried, and pressed at a linear pressure of about 1 ton / cm. Was made.

Next, as shown in FIG. 1, a positive electrode current collecting lead 28 and a negative electrode current collecting lead 38 were welded to the sheet-like positive electrode 2 and negative electrode 3 obtained as described above, respectively. Next, the positive electrode 2 and the negative electrode 3 were wound in a state where a cellulose separator 4 was sandwiched between them, to produce a spiral wound electrode.
Subsequently, the wound electrode was inserted into an 18650 type cylindrical battery case 6 including an outer can 62 and a cap 61. At this time, the positive electrode current collecting lead 28 is connected to the positive electrode current collecting tab 285 disposed on the cap 61 side of the battery case 6 by welding, and the negative electrode current collecting lead 385 is disposed on the negative electrode current collecting tab 385 disposed on the bottom of the outer can 6. 38 were connected by welding.

  Next, the battery case 6 was impregnated with a saturated lithium nitrate aqueous solution (pH = 7) as an aqueous electrolyte. The gasket 5 was disposed inside the cap 61, and the cap 61 was disposed in the opening of the outer can 62. Subsequently, the battery case 6 was hermetically sealed by caulking the cap 61, whereby the aqueous electrolyte lithium secondary battery 1 was produced. This is referred to as a battery E1.

  In the aqueous electrolyte lithium secondary battery 1 (battery E1) of this example, as shown in FIG. 2, the binders 255 and 355 contained in the positive electrode mixture 25 and the negative electrode mixture 35 are water-soluble or water-dispersible. And a polymer compound containing an oxygen atom and a functional group that forms an anion in water. Therefore, the adhesion between the positive electrode mixture 25 and the positive electrode current collector 21 and the negative electrode mixture 35 and the negative electrode current collector 31 are excellent. Therefore, in the aqueous electrolyte lithium secondary battery 1, the positive electrode mixture 25 and the negative electrode mixture 35 can be prevented from sliding off from the positive electrode current collector 21 and the negative electrode current collector 31, respectively. The battery 1 can maintain a high discharge capacity even after repeated charging and discharging.

That is, the positive electrode current collector 21 and the negative electrode current collector 31 made of aluminum foil are usually oxidized or hydroxylated, and the surfaces are covered with oxygen atoms. The positive electrode active material 251 and the negative electrode active material 351 are oxides and contain oxygen atoms. Furthermore, the binders 255 and 355 contain a functional group (carboxyl group) that contains an oxygen atom and forms an anion in water.
Therefore, as in this example, when the positive electrode active material 251 and the negative electrode active material 351 are dispersed in the aqueous solution and aqueous dispersion of the binders 255 and 355 to produce the positive electrode mixture 25 and the negative electrode mixture 35, the positive electrode mixture 25 and In the negative electrode mixture 35, the functional groups (carboxyl groups) of the binders 255 and 355 can react with oxygen atoms in the positive electrode active material 251 and the negative electrode active material 351 to form a chemical bond. Furthermore, when the positive electrode mixture 25 and the negative electrode mixture 35 are applied to the positive electrode current collector 21 and the negative electrode current collector 31 and dried, oxygen atoms and a binder 255 on the surfaces of the positive electrode current collector 21 and the negative electrode current collector 31 are obtained. And 355 functional groups can react to form a chemical bond.

  Thus, in the aqueous electrolyte lithium secondary battery 1 of this example, the binder 255 can be bonded to both the positive electrode active material 251 and the positive electrode current collector 21 by chemical bonding. Further, the binder 355 can be bonded to both the negative electrode active material 351 and the negative electrode current collector 31 by a chemical bond.

Moreover, since the binders 255 and 355 have affinity with water, they hardly shrink even in the aqueous electrolyte solution.
Further, in the aqueous electrolyte lithium secondary battery 1 of this example, as described above, the binder 255 forms a chemical bond with the positive electrode active material 251 and the positive electrode current collector 21 by using a functional group that can be an anion. The binder 355 forms a chemical bond with the negative electrode active material 351 and the negative electrode current collector 31 by using a functional group that can be an anion. Therefore, in the aqueous electrolyte lithium secondary battery 1, the binders 255 and 355 are no longer dissolved in the aqueous electrolyte.

Therefore, in the aqueous electrolyte lithium secondary battery 1 of this example, the positive electrode current collector 21 and the positive electrode mixture 25 and the negative electrode current collector 31 and the negative electrode mixture 35 are excellent in adhesion, and the positive electrode current collector 21 It is difficult for water of the aqueous electrolyte solution to enter between the positive electrode mixture 25 and between the negative electrode current collector 31 and the negative electrode mixture 35, and the positive electrode 2 and the negative electrode 3 are swelled to generate the positive electrode mixture 25 and the negative electrode. The composite material 35 can be prevented from being detached from the positive electrode current collector 21 and the negative electrode current collector 31, respectively.
Further, in the aqueous electrolyte lithium secondary battery 1 of the present example, the binders 255 and 355 can firmly bond the active material particles 251 and 351, respectively, and the active material particles 251 and 351 can be charged and discharged. As the material expands and contracts, the binders 255 and 355 can maintain the bond between the active material particles 251 and 351, respectively. Therefore, even when charging and discharging are repeated, the particles of the positive electrode active material 251 and the negative electrode active material 351 are hardly detached. Therefore, the aqueous electrolyte lithium secondary battery of this example is excellent in charge / discharge cycle characteristics because the battery capacity does not easily decrease even when charge / discharge is repeated.

(Example 2)
Next, this example is different from the aqueous electrolyte lithium secondary battery (battery E1) produced in Example 1 in the seven types of aqueous electrolyte lithium secondary batteries (types of binder in the positive electrode mixture and negative electrode mixture) ( This is an example in which a battery E2 and batteries C1 to C6) are produced and charge / discharge cycle characteristics are evaluated.

The battery E2 was produced in the same manner as the battery E1 except that a water-soluble acrylic resin containing a carboxylic acid group was used as a binder.
Specifically, first, a water-soluble acrylic resin is dissolved in water to prepare a 5 wt% water-soluble acrylic resin aqueous solution (binder liquid 3). The same positive electrode active material and conductive material as in Example 1 are added to this binder liquid 3. The positive electrode mixture was prepared by dispersing the agent. This positive electrode mixture contains 85 parts by weight of lithium iron composite phosphate (LiFePO ₄ ) as a positive electrode active material, 10 parts by weight of acetylene black as a conductive agent, and 5 parts by weight of a water-soluble acrylic resin as a binder.
Next, in the same manner as in Example 1, the positive electrode mixture was applied to the positive electrode current collector, dried, and pressed to prepare a positive electrode for the battery E2. The negative electrode was also produced in the same manner as in Example 1 except that the binder liquid 3 was used.
Next, a wound electrode was produced using the positive electrode and the negative electrode in the same manner as in Example 1, and further, as in Example 1, a 18650 type cylindrical aqueous electrolyte lithium secondary battery (battery E2) was prepared. Produced.

The battery C1 was prepared in the same manner as the battery E1 in Example 1 except that polyvinylidene fluoride that was water-insoluble and did not contain oxygen-containing functional groups was used as the positive electrode side binder. That is, the battery C1 was prepared using polyvinylidene fluoride as the positive electrode binder, and the same sodium carboxymethyl cellulose and water-dispersed styrene butadiene rubber as the battery E1 as the negative electrode binder. The positive electrode mixture of the positive electrode of the battery C1 contains 85 parts by weight of a positive electrode active material (LiFePO ₄ ), 10 parts by weight of a conductive agent (acetylene black), and 5 parts by weight of a binder (polyvinylidene fluoride). Was used. Others are the same as the battery E1.

The battery C2 was prepared in the same manner as the battery E1 of Example 1 except that polyacrylonitrile containing a water-insoluble oxygen-containing functional group was used as the positive electrode side binder. That is, the battery C2 was prepared using polyacrylonitrile as the binder for the positive electrode and sodium carboxymethylcellulose and water-dispersed styrene butadiene rubber similar to those for the battery E1 as the binder for the negative electrode. Further, as the positive electrode mixture of the positive electrode of the battery C2, a material containing 85 parts by weight of a positive electrode active material (LiFePO ₄ ), 10 parts by weight of a conductive agent (acetylene black) and 5 parts by weight of a binder (polyacrylonitrile) is used. It was. Others are the same as the battery E1.

The battery C3 was produced in the same manner as the battery E1 of Example 1 except that polyvinylidene fluoride which was insoluble in water and did not contain a functional group containing oxygen was used as the binder on the negative electrode side. That is, the battery C1 was prepared using polyvinylidene fluoride as the negative electrode binder, and the same carboxymethyl cellulose sodium and water-dispersed styrene butadiene rubber as the battery E1 as the positive electrode binder. The negative electrode mixture of the negative electrode of the battery C3 contains 85 parts by weight of a negative electrode active material (LiV ₂ O ₄ ), 10 parts by weight of a conductive agent (acetylene black), and 5 parts by weight of a binder (polyvinylidene fluoride). A thing was used. Others are the same as the battery E1.

The battery C4 was prepared in the same manner as the battery E1 of Example 1 except that polyacrylonitrile containing a functional group containing oxygen and containing water was used as the binder on the negative electrode side. That is, the battery C4 was prepared using polyacrylonitrile as the binder for the negative electrode, and sodium carboxymethyl cellulose and water-dispersed styrene butadiene rubber similar to those for the battery E1 as the positive electrode binder. The negative electrode mixture for the negative electrode of battery C4 contains 85 parts by weight of a negative electrode active material (LiV ₂ O ₄ ), 10 parts by weight of a conductive agent (acetylene black), and 5 parts by weight of a binder (polyacrylonitrile). Was used. Others are the same as the battery E1.

The battery C5 was produced in the same manner as the battery E1 of Example 1 except that polyvinylidene fluoride that was insoluble in water and did not contain a functional group containing oxygen was used as the binder on the positive electrode side and the negative electrode side. As the positive electrode mixture of the positive electrode of the battery C5, a material containing 85 parts by weight of a positive electrode active material (LiFePO ₄ ), 10 parts by weight of a conductive agent (acetylene black) and 5 parts by weight of a binder (polyvinylidene fluoride) was used. . As the negative electrode material of the negative electrode, use a negative electrode active material (LiV ₂ O ₄₎ 85 parts by weight, the conductive agent and (acetylene black) 10 parts by weight, a binder those containing (polyvinylidene fluoride) 5 parts by weight It was. Others are the same as the battery E1.

The battery C6 was produced in the same manner as the battery E1 of Example 1 except that polyacrylonitrile containing a functional group containing oxygen was used as the binder on the positive electrode side and the negative electrode side. As the positive electrode mixture of the positive electrode of the battery C6, a material containing 85 parts by weight of a positive electrode active material (LiFePO ₄ ), 10 parts by weight of a conductive agent (acetylene black) and 5 parts by weight of a binder (polyacrylonitrile) was used. As the negative electrode material of the negative electrode was used and the negative electrode active material (LiV ₂ O ₄₎ 85 parts by weight, the conductive agent and (acetylene black) 10 parts by weight, those containing 5 parts by weight of the binder (polyacrylonitrile) . Others are the same as the battery E1.

Next, a charge / discharge cycle test was performed on the battery E1, the battery E2, and the batteries C1 to C6, and the discharge capacity retention rates at the 100th cycle were compared.
The charge / discharge cycle test is performed for each battery (battery E1, battery E2, and battery C1 to battery C6) up to a battery voltage of 1.4 V at a constant current of 0.5 mA / cm ² at a temperature of 60 ° C. Charging and discharging was then carried out by repeating the cycle for 100 cycles, with one cycle consisting of charging and discharging at a constant current of 0.5 mA / cm ^{2 to} a battery voltage of 0.1 V. In each charge / discharge cycle, after charging to 1.4V and after discharging to 0.1V, a charging pause time and a discharge pause time were provided for 10 minutes each. And about each battery, the discharge capacity of 1st cycle (first time) and 100th cycle was measured.

The discharge capacity is determined by measuring the discharge current value (mA) at the first cycle or the 100th cycle, and multiplying this discharge current value by the time (hr) required for discharge to obtain the positive electrode active capacity in the battery. Calculated by dividing by weight (g) of material.
The results are shown in Table 1.
Further, the capacity retention rate was calculated from the following equation from the discharge capacity at the first time and the 100th cycle.
Capacity maintenance ratio (%) = (discharge capacity at 100th cycle / initial discharge capacity) × 100

As known from Table 1, there was almost no difference in the initial discharge capacity in any of the aqueous electrolyte lithium secondary batteries of the batteries E1 and E2 and the batteries C1 to C6.
However, the battery E1 and the battery E2 are superior in the discharge capacity maintenance rate compared to the batteries C1 to C6. Moreover, in the battery E1 and the battery E2, no swelling or the like occurred in the positive electrode or the negative electrode, and the electrode did not peel off.
Therefore, the battery E1 and the battery E2 using a water-soluble polymer or a water-dispersible polymer containing a group that forms an anion in water as a binder include a positive electrode mixture, a negative electrode mixture, and a current collector. It can be seen that the adhesion of the battery is excellent and the charge / discharge cycle characteristics are excellent.

BRIEF DESCRIPTION OF THE DRAWINGS Explanatory drawing which shows the structure of the aqueous electrolyte lithium secondary battery concerning Example 1. FIG. BRIEF DESCRIPTION OF THE DRAWINGS Explanatory drawing which shows the structure of the positive electrode (negative electrode) concerning Example 1. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Water-system electrolyte lithium secondary battery 2 Positive electrode 21 Positive electrode collector 25 Positive electrode compound 251 Positive electrode active material 253 (353) Conductive agent 255 (355) Binder 3 Negative electrode 31 Negative electrode collector 35 Negative electrode compound 351 Negative electrode active material

Claims (3)

An aqueous system having a positive electrode formed by binding a positive electrode mixture to a positive electrode current collector, a negative electrode formed by binding a negative electrode mixture to a negative electrode current collector, and an aqueous electrolyte solution obtained by dissolving a lithium salt in water In the electrolyte lithium secondary battery,
The positive electrode mixture contains a positive electrode active material made of a lithium-containing composite oxide capable of reversibly inserting and extracting lithium, a conductive agent made of a carbon-based material having conductivity, and a binder.
The negative electrode mixture is composed of a negative electrode active material composed of a composite oxide having a lower lithium storage and desorption potential than the lithium-containing composite oxide contained in the positive electrode active material, and a conductive carbon-based material. Containing a conductive agent and a binder,
The binder in the positive electrode mixture and the negative electrode mixture is characterized by using a polymer compound that is water-soluble or water-dispersible and contains a functional group that contains an oxygen atom and forms an anion in water. Aqueous electrolyte lithium secondary battery.   The aqueous lithium secondary battery according to claim 1, wherein the aqueous electrolyte has a pH in a range of 6 ≦ pH ≦ 10.   3. The aqueous electrolyte lithium secondary battery according to claim 1, wherein both of the positive electrode current collector and the negative electrode current collector are made of aluminum.

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