JP2002110226A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive acqueous lithium secondary battery having a large capacity, excellent output characteristics, and excellent rate characteristic, providing very high safety, and using an acqueous solution as electrolyte by finding a positive electrode active substance capable of taking out a sufficient capacity. SOLUTION: This lithium secondary battery includes a positive electrode having a basic composition of LiMnO2 and layerlike structure and using lithium manganese composite oxide as an active substance, a negative electrode using a substance capable of absorbing and desorbing lithium as an active substance, and electrolyte composed of an acqueous solution in which lithium salt is dissolved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a method for storing and storing lithium.
More particularly, the present invention relates to a lithium secondary battery including an electrolytic solution composed of an aqueous solution.

[0002]

2. Description of the Related Art Lithium secondary batteries utilizing the insertion and extraction of lithium have a high energy density.
With the miniaturization of mobile phones, personal computers, and the like, they have become widespread in the field of communication devices and information-related devices. Also, in the field of automobiles, the development of electric vehicles is urgent due to resource and environmental issues, and lithium secondary batteries are being studied as power sources for electric vehicles.

[0003] Lithium secondary batteries currently in practical use generally include a positive electrode using a lithium transition metal composite oxide as a positive electrode active material, a negative electrode using a carbon material or the like as a negative electrode active material, and a lithium salt formed of an organic material. The secondary battery is composed of a non-aqueous electrolyte dissolved in a solvent and has a high voltage of 4V class.

[0004] However, the above-mentioned lithium secondary battery uses a non-aqueous organic solvent as an electrolytic solution, and its safety is a problem. Further, for example, when the battery is overcharged or exposed to a high-temperature environment, it may be difficult to ensure sufficient safety. In particular, a secondary battery as a power source for a vehicle or the like is large in size, uses a large amount of an organic solvent, and is expected to be used under severe conditions such as an operating temperature. Safety matters.

[0005] In addition, the presence of even a small amount of water in the battery causes various problems such as generation of gas due to the electrolysis reaction of water, consumption of lithium due to the reaction between water and lithium, and corrosion of battery constituent materials. For this reason, in the manufacture of lithium secondary batteries, a thorough dry environment is required, and special equipment for completely removing moisture and a great deal of labor are required, which raises the cost of batteries. Cause.

On the other hand, the above problem does not basically occur in an aqueous lithium secondary battery using an aqueous solution as an electrolyte. Further, in general, an aqueous solution has better conductivity than a non-aqueous solution, so that the reaction resistance of the battery is reduced and the output characteristics and the rate characteristics of the battery are improved. However, the aqueous lithium secondary battery needs to be operated in a potential range where the electrolysis reaction of water does not occur, and the voltage of the aqueous lithium secondary battery is usually
It has to be lower than that of non-aqueous system.

Therefore, in the aqueous lithium secondary battery,
In an electric potential range in which oxygen and hydrogen are not generated by the electrolysis of water, it is necessary to use an active material which can reversibly insert and extract lithium ions and is stable in an aqueous solution.

As an example of this water-based lithium secondary battery, Japanese Patent Application Laid-Open No. 9-508490 discloses L-type as a positive electrode active material.
LiMn ₂ O ₄ , VO as iMn ₂ O _4, etc.
A battery using No. ₂ or the like is disclosed. Also, Japanese Patent Application Laid-Open
-77073 discloses that LiCoO 2 is used as a positive electrode active material.
₂ , a battery using Li (Ni, Co) O ₂ , LiMn ₂ O ₄ or the like and LiV ₃ O ₈ or the like as a negative electrode active material is disclosed.

[0009]

However, when the present inventor conducted additional tests, it was found that LiCoO ₂ , a positive electrode active material,
Li (Ni, Co) O ₂ , LiMn ₂ O _4, etc. have too high a potential for reversibly occluding / desorbing lithium ions, and it is difficult to take out the capacity in a potential range where the electrolysis reaction of water does not occur. It turned out to be. That is, a positive electrode active material capable of extracting a sufficient capacity from an aqueous lithium secondary battery has not been found.

On the other hand, LiMn ₂ O _{4 as} a negative electrode active material has poor reversibility, and VO ₂ and LiV ₃ O ₈ have a relatively high potential for inserting and extracting lithium ions and are close to the positive electrode potential. There is a problem that a sufficient voltage cannot be secured.

The present invention has been made in view of the above problems, and has found a positive electrode active material capable of extracting a sufficient capacity from an aqueous lithium secondary battery using an aqueous solution as an electrolyte. It is an object of the present invention to provide a water-based lithium secondary battery having a large capacity by forming a battery by combining the above.

[0012]

The lithium secondary battery of the present invention comprises a positive electrode having a basic composition of LiMnO ₂ and a lithium manganese composite oxide having a layered structure as an active material, and a material capable of inserting and extracting lithium. It is characterized by including an anode as an active material and an electrolytic solution comprising an aqueous solution in which a lithium salt is dissolved.

The present inventor examined the charge and discharge behavior of the main active material, and found that the lithium manganese composite oxide having a basic composition of LiMnO ₂ and having a layered structure could be used in a potential range in which oxygen generation due to electrolysis of water does not occur. Has found that reversible occlusion and release of a large amount of lithium ions are possible and are suitable as a positive electrode active material in an aqueous lithium secondary battery. That is, the capacity of the lithium manganese composite oxide, which is the positive electrode active material of the lithium secondary battery of the present invention, can be sufficiently taken out within a potential range in which oxygen is not generated by electrolysis of water.

Therefore, the lithium secondary battery of the present invention is an inexpensive, highly safe, high-output, large-capacity aqueous secondary battery. In addition, as will be clarified in a later experiment, even after repeated charge and discharge, the water-based secondary battery has a small decrease in capacity and excellent cycle characteristics, particularly, good cycle characteristics at high temperatures.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the lithium secondary battery of the present invention will be described below for each component.

(1) Positive Electrode Active Material As the positive electrode active material, a lithium manganese composite oxide having a basic composition of LiMnO ₂ and having a layered structure is used. Here, the basic composition means a typical composition of the lithium-manganese composite oxide. In addition to the one represented by the above composition formula, for example, a lithium site or a manganese site is represented by C
It also includes compositions such as those partially substituted with one or more other elements such as o, Ni, and Al. The stoichiometric composition is not necessarily limited to the stoichiometric composition, for example, a non-stoichiometric composition in which cation atoms of lithium or manganese inevitably produced in the production are missing or oxygen atoms are missing. Including.

FIG. 1 shows the capacity and potential (vs. Li) of the main active material.
/ Li ⁺ ). As is clear from FIG.
LiCoO ₂ and LiMn ₂ O ₄ can hardly take up their capacity in the potential range where oxygen is not generated by the electrolysis of water, and Li (Ni, Co) O ₂ is about half of the original capacity. . In practice, a small amount of Li is dissolved in water as LiOH, and the electrolyte becomes alkaline, and the oxygen generation potential is lowered.
i, Co) O ₂ also can hardly take out its capacity. On the other hand, LiMnO ₂ , which is the positive electrode active material of the lithium secondary battery of the present invention, can reversibly occlude and release a large amount of lithium ions in a potential range where oxygen is not generated by electrolysis of water. The capacity can be sufficiently taken out. Therefore, the lithium secondary battery of the present invention using LiMnO ₂ as a positive electrode active material is a large capacity secondary battery.

The lithium manganese composite oxide having a layered structure includes a lithium manganese composite oxide having a hexagonal layered structure, that is, a so-called layered rock salt structure (a space group is represented by the following formula 1), and an orthorhombic. -Manganese composite oxide having a crystalline layered structure (space group C2 / m) and lithium-manganese composite oxide having a monoclinic zigzag layered structure (space group Pmnm), of which one is Can be used alone or as a mixture of two or more.

[0019]

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Among them, it is desirable to use a lithium manganese composite oxide having a hexagonal layered rock salt structure. The lithium manganese composite oxide having a hexagonal layered rock salt structure does not cause a transition to a spinel structure having a small capacity to be taken out even after repeated charging and discharging. The next battery is
A water-based secondary battery having a larger capacity is obtained.

The production method of the lithium manganese composite oxide having a hexagonal layered rock salt structure is not particularly limited, but according to the production method discovered by the present inventors described below, A manganese composite oxide can be easily produced.

(2) Negative electrode active material As the negative electrode active material, a substance capable of inserting and extracting lithium is used. As a substance capable of inserting and extracting lithium, since the potential for inserting and extracting lithium is appropriate,
It is desirable to use transition metal chalcogenides. The transition metal chalcogenide has a reversible occlusion / desorption potential of lithium ions of 2 to 2.5 V (vs. Li / Li ⁺ ). And a secondary battery having a large capacity can be constructed. Among them, it is desirable to use TiS ₂ , MoS ₂ , NbS ₂ , and VS ₂ because they are inexpensive and have a large capacity per unit weight of the active material. One of these can be used alone, or two or more can be used in combination. It should be noted that, in particular, because of the large capacity, TiS
It is desirable to use ₂ .

Each of the transition metal chalcogenides is
The production method is not particularly limited, and may be produced by a commonly used method.

(3) Configuration of Positive Electrode and Negative Electrode For both the positive electrode and the negative electrode, a mixture of a positive electrode and a negative electrode obtained by mixing a powdered active material with a conductive material and a binder is used as a metal collector. It can be formed by pressure bonding or coating and drying on the surface of the electric body.

The conductive material is used to secure the electrical conductivity of the electrode. For example, one or a mixture of two or more kinds of powdered carbon materials such as carbon black, acetylene black and graphite is used. be able to. Further, the binder plays a role of binding the active material particles and the conductive material particles, for example, a polytetrafluoroethylene, polyvinylidene fluoride, a fluorine-containing resin such as fluororubber, a thermoplastic resin such as polypropylene, polyethylene and the like. Can be used.

(3) Electrolyte The electrolyte is obtained by dissolving a lithium salt as an electrolyte in water. The lithium salt is dissociated by dissolving in water and forms lithium ions in the electrolyte. Generally, an oxide-based active material is more stably present in a neutral to alkaline aqueous solution. Further, in consideration of further activating the occlusion / desorption reaction of lithium ions, it is preferable that the electrolyte used is neutral to alkaline. Here, neutral means that the pH value is about pH 6 to 8.

For example, when a neutral electrolyte of pH = 7 is used, the hydrogen generation potential by water electrolysis is 2.62 V, the oxygen generation potential is 3.85 V (vs. Li / Li ⁺ ), and p
When an alkaline electrolyte of H = 14 is used, the hydrogen generation potential is 2.21 V and the oxygen generation potential is 3.44 V (vs. L
i / Li ⁺ ). In other words, when a neutral electrolyte is used, the potential of oxygen generation due to the electrolysis of water is high. And a larger capacity can be taken out. Therefore, when a secondary battery having a larger capacity is used, a nearly neutral electrolyte, specifically, pH = 6 to
It is desirable to use an electrolyte solution that is 10.

In general, an aqueous solution has better conductivity than a non-aqueous solution. For example, a neutral aqueous solution has a conductivity 10 times or more that of a non-aqueous solution, and an alkaline aqueous solution has a conductivity of 100 times or more of a non-aqueous solution. Has electrical conductivity. For this reason, a secondary battery using an aqueous solution for the electrolyte has a lower internal resistance, particularly a reaction resistance, than a non-aqueous secondary battery, and the internal resistance becomes lower when an alkaline electrolyte is used. It will be smaller. Therefore, in order to obtain a secondary battery having better output characteristics and rate characteristics, it is desirable to use a strongly alkaline electrolyte, specifically, an electrolyte having a pH of 10 to 12.

The lithium salt that can be used as the electrolyte is not particularly limited as long as it can be dissolved in water. However, considering the stability of the oxide as the positive electrode active material,
After dissolution, it is desirable to use a lithium salt such that the electrolyte becomes neutral to alkaline. Specifically, for example,
It is desirable to use lithium nitrate, lithium hydroxide, lithium iodide and the like. Each of these lithium salts may be used alone, or two or more of these lithium salts may be used in combination. In particular, it is desirable to use lithium nitrate in order to obtain a neutral electrolyte solution because of its high solubility and therefore good conductivity.
In order to obtain a strongly alkaline electrolyte, it is desirable to use a mixture of lithium nitrate and lithium hydroxide.

Further, the concentration of the lithium salt in the electrolyte is desirably a saturation concentration or a concentration close to the saturation concentration because the electric conductivity of the electrolyte can be increased and the internal resistance of the secondary battery can be reduced. .

(4) Other Components In the lithium secondary battery of the present invention, the positive electrode and the negative electrode are opposed to each other to form an electrode body. A separator is interposed between the positive electrode and the negative electrode. This separator separates the positive electrode and the negative electrode and holds the electrolytic solution, and may be a cellulosic type or the like.

The shape of the lithium secondary battery of the present invention is not particularly limited, and may be various types such as a cylindrical type, a stacked type, a coin type and the like. In any case, the above-mentioned electrode body formed in accordance with the battery shape is housed in a predetermined battery case, and the positive electrode terminal and the negative electrode terminal passing from the positive electrode current collector and the negative electrode current collector to the outside. The above steps are connected using a current collecting lead or the like, and the electrode body is impregnated with the above-mentioned electrolytic solution and sealed in a battery case, whereby a lithium secondary battery can be completed.

(5) Method for Producing Hexagonal Layered Rock Salt Structure Lithium Manganese Composite Oxide The above-described lithium manganese composite oxide having a hexagonal layered rock salt structure has been conventionally synthesized by a solid-phase reaction method. −
NaMnO ₂ is dissolved in a non-aqueous solution containing lithium ions by 3
It was common to synthesize by ion exchange at a temperature of 00 ° C. or less. However, this method is industrially disadvantageous because it has to go through a complicated two-step process of solid-phase reaction and ion exchange.

In addition, a manganese raw material and a water-soluble lithium salt are hydrothermally reacted in an aqueous solution containing an excess of an alkali metal hydroxide other than lithium, whereby lithium manganese having an orthorhombic layered rock salt structure is obtained. In some cases, a composite oxide is synthesized. However, the lithium-manganese composite oxide obtained undergoes a transition to a spinel structure during repeated charging and discharging, and when used as an active material of a non-aqueous secondary battery, the cycle characteristics of the battery become poor. It was not good.

The present inventors have repeatedly conducted experiments and found a method for easily producing a lithium manganese composite oxide having a hexagonal layered rock salt structure. The method comprises mixing a manganese dioxide serving as a manganese source and a lithium hydroxide aqueous solution serving as a lithium source at a ratio such that the molar ratio of Li / Mn is 2 or more and 10 or less to prepare a dispersed aqueous solution. It comprises a preparation step and a hydrothermal treatment step of heating and holding the aqueous dispersion at a temperature of 120 ° C. or more and 250 ° C. or less.

That is, unlike the conventional solid-phase reaction, the present production method involves a hydrothermal treatment, in which manganese dioxide is dispersed in an aqueous solution of lithium hydroxide, and the resulting aqueous dispersion is heated and maintained. This is a method for obtaining a lithium manganese composite oxide having a layered rock salt structure. By using manganese dioxide as a manganese source, it is possible to obtain a lithium manganese composite oxide having a hexagonal layered rock-salt structure without a transition to a spinel structure even when charge and discharge are repeated. In addition, the present production method is a simple and industrially advantageous production method since the target lithium composite oxide can be synthesized by one-stage hydrothermal treatment. Hereinafter, each step of the present manufacturing method will be described.

(A) Dispersion aqueous solution preparation step In the dispersion aqueous solution preparation step, manganese dioxide serving as a manganese source and lithium hydroxide aqueous solution serving as a lithium source are mixed with L
In this step, i / Mn is mixed at a molar ratio of 2 or more and 10 or less to prepare a dispersion aqueous solution.

The mixing ratio of manganese dioxide and the aqueous solution of lithium hydroxide is such that the molar ratio of Li / Mn is 2 or more and 10 or less. When the molar ratio of Li / Mn is less than 2, a mixed phase with the spinel-structured lithium manganese composite oxide is formed.
This is because the ratio of i ₂ MnO ₃ increases.

The mixing of the raw materials may be carried out by uniformly dispersing the manganese dioxide in the aqueous lithium hydroxide solution, and may be carried out according to a usual method. For example, a method of dispersing ultrasonic waves using an apparatus such as an ultrasonic homogenizer, a method of dispersing by applying a high shear force with a homogenizer or the like, and the like can be mentioned.

The manganese dioxide used in the form of powder preferably has an average particle size of 0.1 μm or more and 20 μm or less. If the average particle size is less than 0.1 μm, the resulting lithium manganese composite oxide has a small particle size, making it difficult to prepare an electrode.
This is because, if the ratio exceeds, the particle size of the lithium manganese composite oxide becomes large, which is disadvantageous for the output characteristics.

(B) Hydrothermal Treatment Step The hydrothermal treatment step is a step of heating and maintaining the aqueous dispersion prepared in the aqueous dispersion preparation step at a temperature of from 120 ° C. to 250 ° C. If the temperature is lower than 120 ° C., the reaction does not proceed. Conversely, if the temperature exceeds 250 ° C., the cost for withstanding pressure increases.

The heating and holding time may be any time as long as the reaction can be completed completely, and is usually about 72 hours. The hydrothermal treatment is performed, for example, by putting the aqueous solution obtained in the aqueous dispersion solution preparing step into an autoclave lined with Teflon, holding the aqueous solution at a predetermined temperature for a predetermined time, and then cooling with water, or gradually cooling in the container. By doing so, the temperature can be lowered to around room temperature and then taken out.

The produced lithium manganese composite oxide is taken out of the aqueous solution by filtration, washed with water,
Dry to make a powder. In addition, filtration, washing with water,
The method of drying is not particularly limited. For example, specifically, the solution is filtered using a suction filter or the like, washed with water by ultrasonic waves or the like, further filtered in the same manner, and then dried in a drying furnace or the like.
It may be dried at a temperature of 20 ° C. for about 60 to 180 minutes.

The lithium manganese composite oxide having a hexagonal layered rock salt structure synthesized by the present manufacturing method does not change to a spinel structure even after repeated charge and discharge, and a secondary battery using this as an active material is In addition, the battery has a small decrease in capacity even after repeated charging and discharging, that is, a battery having very good cycle characteristics.

The use of the lithium manganese composite oxide having a hexagonal layered rock salt structure synthesized by the present production method is not limited to the positive electrode active material of an aqueous lithium secondary battery. It can also be used as an active material of a non-aqueous secondary battery using an aqueous electrolyte.

At present, as a positive electrode active material of a non-aqueous lithium secondary battery, LiCoO ₂ is used as the positive electrode active material because it is easy to synthesize and relatively easy to handle and has excellent charge / discharge cycle characteristics. Rechargeable batteries used for the mainstream have become mainstream. However, cobalt is scarce as a resource, and secondary batteries using LiCoO ₂ as the positive electrode active material are difficult to cope with future mass production and enlargement of electric vehicle power supplies, and are extremely expensive in terms of price. It has to be something. Therefore, the lithium manganese composite oxide, which is abundant as a resource and contains inexpensive manganese as a constituent element, is expected as a useful positive electrode active material instead of cobalt. Therefore, the present production method is a method capable of easily producing a lithium manganese composite oxide having a hexagonal layered rock salt structure, which is a useful active material.

(6) The embodiment of the lithium secondary battery of the present invention has been described above. However, the above-described embodiment is merely an embodiment, and the lithium secondary battery of the present invention includes the above-described embodiment. Various modifications and improvements can be made based on the knowledge of those skilled in the art.

[0048]

EXAMPLES Based on the above embodiment, various aqueous lithium secondary batteries having different positive electrode active materials and the like were produced, and each battery was evaluated. Below, the prepared lithium secondary battery,
The evaluation of the initial capacity and the evaluation of the cycle characteristics will be described.

<Prepared Lithium Secondary Battery> (1) Lithium Secondary Battery of Example 1 (a) Synthesis of Lithium-Manganese Composite Oxide with Layered Rock Salt Structure of Hexagonal System 6.29 g of LiOH.H ₂ O in 80 ml Was dissolved in water to prepare a LiOH aqueous solution. 2.61 g of MnO ₂ was added to this LiOH aqueous solution (Li / Mn becomes 5 in molar ratio), and ultrasonic dispersion was performed for 30 minutes to prepare a dispersion aqueous solution. Next, the aqueous dispersion was placed in an autoclave and reacted at a temperature of 200 ° C. for 7 days. After the reaction, the autoclave was cooled, the precipitate in the vessel was filtered, washed with water,
After drying at 20 ° C., a lithium manganese composite oxide having a hexagonal layered rock salt structure was obtained.

(B) Preparation of Lithium Secondary Battery A lithium secondary battery was prepared using the above-mentioned lithium manganese composite oxide as a positive electrode active material. In the positive electrode, first, 70 parts by weight of a lithium manganese composite oxide as an active material were mixed with 25 parts by weight of carbon as a conductive material and 5 parts by weight of polytetrafluoroethylene as a binder. Next, this mixed powder was welded to the inside of a coin cell in advance.
It was crimped on a US mesh at about 0.6 ton / cm ² to form a positive electrode.

For the negative electrode to be opposed, TiS ₂ was used as an active material. As with the positive electrode, first, TiS ₂ 7 as an active material
To 0 parts by weight, 25 parts by weight of carbon as a conductive material and 5 parts by weight of polytetrafluoroethylene as a binder were mixed. Next, about 0.6 t of this mixed powder was placed on a SUS mesh welded in advance to the inside of the coin cell.
It was crimped at on / cm ² to form a negative electrode.

The separator used was a cellulosic separator. In addition, LiNO ₃ as a lithium salt was dissolved in water to form a 5M LiNO ₃ aqueous solution, which was used as an electrolyte. The pH of the electrolyte was set to about 7. The positive electrode and the negative electrode were opposed to each other with a separator interposed therebetween, and an appropriate amount of the electrolytic solution was injected and impregnated, and then housed in a 2032-type coin-type battery case, whereby an aqueous lithium secondary battery was manufactured. The aqueous lithium secondary battery produced in this manner was used as the secondary battery of Example 1.

(2) Lithium secondary battery of Example 2 In the secondary battery of Example 1, 5 M of Li
An aqueous solution adjusted to pH 12 by adding an appropriate amount of LiOH to an aqueous solution of NO ₃ was used as an electrolyte. Except for that, an aqueous secondary battery was manufactured in the same manner as in Example 1, and a lithium secondary battery of Example 2 was obtained.

(3) Lithium Secondary Battery of Example 3 (a) Synthesis of Monoclinic Zigzag Layered Structure Lithium Manganese Composite Oxide 2.52 g of LiOH.H ₂ O was dissolved in 80 ml of water, An aqueous LiOH solution was prepared. To this aqueous LiOH solution, 2.37 g of Mn ₂ O ₃ was added instead of MnO ₂ (Li / Mn becomes 2 in molar ratio), and ultrasonic dispersion was performed for 30 minutes to prepare a dispersion aqueous solution. Next, the aqueous dispersion was placed in an autoclave and reacted at a temperature of 200 ° C. for one day. After the reaction, the autoclave was cooled, and the precipitate in the vessel was filtered, washed with water, and dried at 120 ° C. to obtain a monoclinic zigzag layered lithium manganese composite oxide.

(B) Preparation of Lithium Secondary Battery A lithium secondary battery was prepared using the above-mentioned lithium manganese composite oxide as a positive electrode active material. The manufacturing method was the same as in Example 1 except that the positive electrode active material was different. Then, the produced aqueous secondary battery was used as the lithium secondary battery of Example 3.

(4) Lithium Secondary Battery of Comparative Example 1 In the secondary battery of Example 1, the electrolyte was changed to a non-aqueous electrolyte, and ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 3: 7. A lithium secondary battery of Comparative Example 1 was manufactured in the same manner as in Example 1, except that a mixed solvent of LiPF ₆ at a concentration of 1 M was used as an electrolytic solution.

(5) Lithium Secondary Battery of Comparative Example 2 (a) Synthesis of Layered Rock Salt Structure LiNi _0.8 Co _0.15 Al _0.05 O _{2 It} was synthesized by a so-called solid phase reaction method. LiOH
H ₂ O, Ni (OH) ₂ , Co ₃ O ₄ , and Al (OH) ₃ are each 1: 1.
8 :: 0.15: 0.05. Then, this mixture is placed in an oxygen stream at a temperature of 900 ° C. for 24 hours.
After calcination for a period of time, cooling and crushing, powdered layered rock salt structure LiNi _0.8 Co _0.15 Al _0.05 O ₂ was obtained.

(B) Production of Lithium Secondary Battery A lithium secondary battery was produced using the above-mentioned LiNi _0.8 Co _0.15 Al _0.05 O ₂ as a positive electrode active material. The manufacturing method was the same as in Example 1 except that the positive electrode active material was different. The produced aqueous secondary battery was used as a lithium secondary battery of Comparative Example 2.

(6) Lithium Secondary Battery of Comparative Example 3 In the secondary battery of Example 1, the positive electrode active material used in the lithium secondary battery of Comparative Example 2 was a layered rock salt structure LiNi.
_0.8 Co _0.15 Al _0.05 O ₂ was changed, and an appropriate amount of LiOH was added to a 5 M LiNO ₃ aqueous solution as an electrolytic solution.
An aqueous secondary battery was produced in the same manner as in Example 1 except that an aqueous solution whose pH was adjusted to 12 was used as an electrolytic solution, and a lithium secondary battery of Comparative Example 3 was obtained.

(7) Lithium Secondary Battery of Comparative Example 4 In the secondary battery of Example 1, the negative electrode active material was changed to V ₂
An aqueous secondary battery was produced in the same manner as in Example 1 except that O ₅ was used, and a lithium secondary battery of Comparative Example 4 was obtained.

<Evaluation of Initial Capacity> Each of the lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 4 was charged and discharged under the following two conditions, and the initial capacity was measured. The first charge / discharge condition is as follows: at a temperature of 20 ° C., a current density of 1.0 mA / c.
charge to a charge upper limit voltage of 1.5 V with a constant current of m ² ,
Next, discharge was performed at a constant current of 0.5 mA / cm ^{2 to} a discharge lower limit voltage of 0.05 V. Also, the second
Were performed in the same manner as in the first condition, except that the charging upper limit voltage was 1.8 V. Table 1 shows the initial capacity of each lithium secondary battery.

[0062]

[Table 1]

In Table 1, "cannot be charged" means that the charging end voltage did not reach even after 9 hours from the start of charging. As is clear from Table 1, the secondary batteries of Examples 1 to 3 all have large capacities. In particular, it can be seen that the secondary battery of Example 1 using the lithium manganese composite oxide having a hexagonal layered rock salt structure as the positive electrode active material has a large capacity. This is because the lithium manganese composite oxide as the positive electrode active material could occlude and desorb lithium ions in a potential range in which oxygen generation due to electrolysis of water does not occur. On the other hand, the lithium secondary batteries of Comparative Examples 2 to 4 were not charged. This is due to the generation of oxygen or hydrogen by the electrolysis of water, and the occlusion and storage of lithium ions.
It is considered that the desorption could not be performed.

FIG. 2 shows a change in potential during charging of the lithium secondary batteries of Comparative Examples 2 and 3. In FIG. 2, the same positive electrode and negative electrode as those of Comparative Examples 2 and 3 were used, and ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 3: 7.
The potential change of a non-aqueous secondary battery using a non-aqueous electrolyte in which LiPF ₆ was dissolved at a concentration of 1 M in a mixed solvent
It is shown by a solid line for reference.

FIG. 2 shows that the potential of the lithium secondary battery using the non-aqueous electrolyte rapidly rises after the start of charging, whereas the secondary batteries of Comparative Examples 2 and 3 have a constant potential. Once reached, the potential does not rise any further. In addition, the secondary battery of Comparative Example 3 using the electrolyte of pH 12 has a lower potential than the secondary battery of Comparative Example 2 using the electrolyte of pH 7. In other words, the higher the pH value, the lower the oxygen generation potential at the positive electrode. Therefore, it is considered that there is a high possibility that oxygen is generated at the positive electrode during charging of the secondary batteries of Comparative Examples 2 and 3.

Further, the capacity of the secondary battery of Example 1 is considerably larger than that of the secondary battery of Comparative Example 1. This is because the aqueous solution has higher conductivity than the non-aqueous solution, so the secondary battery of Example 1 using the aqueous solution for the electrolytic solution is:
This is probably because the internal resistance was smaller than that of the secondary battery of Comparative Example 1 using the non-aqueous solution. In addition, in order to evaluate the internal resistance of each secondary battery of an Example and a comparative example, the impedance of each secondary battery was measured. As a measuring method, an AC resistance of 1 KHz was measured by a four-terminal method. Table 2 shows the measurement results.

[0067]

[Table 2]

As is clear from Table 2, the non-aqueous electrolyte used in the secondary battery of Comparative Example 1 has a higher resistance than the aqueous electrolyte used in the other secondary batteries. The resistance value is about four times that of the secondary battery. Therefore, it is understood that the lithium secondary battery of the present invention using the aqueous solution as the electrolytic solution has a large capacity, a small internal resistance, and excellent output characteristics and rate characteristics.

<Evaluation of Cycle Characteristics> Next, the lithium secondary battery of Example 1 was subjected to a charge / discharge cycle test in order to evaluate the cycle characteristics. The charge / discharge cycle test was performed at two temperatures: room temperature, 20 ° C .; The conditions were as follows: charging was performed at a constant current of 1.0 mA / cm ^{2 to} a charge end voltage of 1.5 V, and then the current density was 0.5 mA.
A charge / discharge cycle of discharging at a constant current of / cm ^{2 to} a discharge end voltage of 0.05 V was defined as one cycle, and this cycle was repeated 100 cycles. The pause between charging and discharging was 1 minute. Similarly, a charge / discharge cycle test with a charge end voltage of 1.8 V was also performed.

The discharge capacity in each cycle was measured, and the results are shown in the graphs of FIG. 3 and FIG. FIG.
FIG. 4 shows the discharge capacity in each cycle under the temperature of 100 ° C.
Is a graph showing the discharge capacity in each cycle at 60 ° C. when the charge end voltage is 1.5 V. FIG. 4 also shows the discharge capacity of the lithium secondary battery of Comparative Example 1, which is a non-aqueous secondary battery, which was similarly subjected to a cycle charge / discharge test at 60 ° C.

As is apparent from FIG. 3, regardless of the end-of-charge voltage, the capacity is slightly reduced by the initial 10 charge / discharge cycles, but the capacity thereafter is small and the cycle characteristics are good. Further, from FIG. 4, the lithium secondary battery of Example 1 was obtained at a high temperature of 60 ° C.
It can be seen that the capacity decrease due to repeated charging and discharging is small, and the cycle characteristics are very good. On the other hand, in the lithium secondary battery of Comparative Example 1, the capacity was significantly reduced. This is considered to be because the lithium secondary battery of Comparative Example 1 uses a non-aqueous organic solvent for the electrolytic solution, and thus has a large internal resistance and is easily affected by an increase in resistance due to charge and discharge.

Accordingly, it was confirmed that the lithium secondary battery of the present invention was a secondary battery having a small capacity reduction and good cycle characteristics even after repeated charging and discharging. Also,
In particular, it was confirmed that the secondary battery had good cycle characteristics at high temperatures.

[0073]

The lithium secondary battery of the present invention is an aqueous lithium secondary battery using an aqueous solution as an electrolytic solution. By using a suitable positive electrode active material, it is inexpensive and has very high safety. And it becomes a high output and large capacity secondary battery. In addition, even if charge and discharge are repeated, a decrease in capacity is small, and a secondary battery having excellent cycle characteristics, particularly at high temperatures, is obtained.

[Brief description of the drawings]

FIG. 1 shows the relationship between the capacity of each active material and the potential (vs. Li ^/ Li ⁺ ).

FIG. 2 shows charging curves of the lithium secondary batteries of Comparative Examples 2 and 3.

FIG. 3 shows the discharge capacity of each cycle of the lithium secondary battery of Example 1 at 20 ° C.

FIG. 4 shows the discharge capacity in each cycle at 60 ° C. of the lithium secondary batteries of Example 1 and Comparative Example 1.

Continued on front page F-term (reference) 4G048 AA04 AB02 AC06 AD06 AE05 5H029 AJ02 AJ03 AJ05 AJ12 AK03 AL04 AM00 AM07 DJ17 HJ13 5H050 AA02 AA07 AA08 AA15 BA08 CA09 CB05 FA19 HA02 HA13

Claims (4)

[Claims] 1. A positive electrode using a lithium manganese composite oxide having a basic composition of LiMnO ₂ and having a layered structure as an active material, a negative electrode using a material capable of inserting and extracting lithium as an active material, and an aqueous solution in which a lithium salt is dissolved And a lithium secondary battery comprising: 2. The lithium secondary battery according to claim 1, wherein the lithium manganese composite oxide having a layered structure is a lithium manganese composite oxide having a hexagonal layered rock salt structure. 3. The lithium secondary battery according to claim 1, wherein the substance capable of inserting and extracting lithium is a transition metal chalcogenide. 4. The transition metal chalcogenide is TiS
₂ , at least one selected from MoS ₂ , NbS ₂ and VS ₂
The lithium secondary battery according to claim 3, which is at least one kind.