JP2006040572A - Positive active material for aqueous lithium secondary battery and aqueous lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a positive active material for an aqueous lithium secondary battery and the aqueous lithium secondary battery containing no expensive Co, displaying high discharge capacity, and having excellent charge discharge cycle characteristics. <P>SOLUTION: The aqueous lithium secondary battery 1 has a positive electrode 2 containing a positive active material, a negative electrode 3 containing a negative active material, and an aqueous electrolyte prepared by dissolving a lithium salt in water, and the positive active material for the aqueous lithium secondary battery is provided. The positive active material has a layer structure compound represented by general formula: Li<SB>t</SB>Ni<SB>x</SB>Mn<SB>y</SB>M<SB>z</SB>O<SB>2</SB>as the main component. Wherein, 0.9≤t≤1.2; 0.4≤x≤0.55; 0.4≤y≤0.55; 0≤z≤0.2; M is at least one element selected from Mg, Al, Fe, Ti, Ga, Cu, V, and Nb. The negative active material has a substance having lower occlusion potential and release potential than the compound represented by the general formula as the main component. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a positive electrode active material for an aqueous lithium secondary battery having an aqueous electrolyte solution obtained by dissolving a lithium salt in water as an electrolytic solution, and an aqueous lithium secondary battery using the positive electrode active material.

  Non-aqueous lithium secondary batteries that use organic solvents as the solvent for the electrolyte solution are high voltage, high energy density, and can be reduced in size and weight, so they are mainly used in personal information terminals such as personal computers and mobile phones. In practical use in the field of information communication equipment, it has been widely spread. In other fields, the development of electric vehicles has been urgently promoted due to environmental issues and resource issues, and the use of such lithium secondary batteries as power sources for electric vehicles has been studied.

Generally, a non-aqueous lithium secondary battery uses a lithium transition metal composite oxide as a positive electrode active material, a carbon material as a negative electrode active material, and an electrolyte solution in which a lithium salt is dissolved in an organic solvent as a non-aqueous electrolyte solution. It is configured.
Specifically, for example, LiCoO ₂ , LiNiO ₂ , LiMn ₂ O _{4 and the} like are used as the positive electrode active material, and these active materials have a potential of 3.5 to 4.3 V with respect to the metal Li. Used in range. Moreover, a carbon material etc. are used as a negative electrode active material, and it is used in the electric potential range of about 1-0.1V. A non-aqueous lithium secondary battery can exhibit a high electromotive force of 3 to 4 V class in a single cell by combining such a positive electrode active material and a negative electrode active material.

However, the following problems have been pointed out for non-aqueous lithium secondary batteries.
That is, since the nonaqueous lithium secondary battery contains a nonaqueous electrolytic solution such as an organic solvent as an electrolytic solution, there is always a risk of ignition or explosion. The danger is particularly high in an overcharged state or a state where the battery is subjected to a high temperature environment.
Secondary batteries are devices that store and release energy electrochemically. Therefore, when the energy stored electrochemically has been suddenly converted into thermal energy due to, for example, a short circuit between the positive electrode and the negative electrode, when there is a flammable organic solvent inside, There is a risk of fire and explosion.
Such a problem is particularly fatal in applications that require large batteries, such as electric vehicles and hybrid vehicles. Further, when used as a power source for automobiles, it is used under severe conditions in terms of operating temperature and charge / discharge cycle, and it is considered that the risk of ignition and explosion becomes higher.

By the way, since the non-aqueous lithium secondary battery does not use water as an electrolyte, it is not restricted by the electrolysis reaction of water and can exhibit an electromotive force of a high voltage of about 4V. On the other hand, if moisture is present in the battery, gas is generated with electrolysis, charge / discharge cycle characteristics are reduced due to reaction with lithium, and charge / discharge efficiency is reduced due to side reactions, There is a risk of causing various problems such as corrosion of battery constituent materials.
Therefore, in a non-aqueous lithium secondary battery, it is necessary to maintain a thorough dry environment in the manufacturing process, and special equipment and a great deal of labor are required to completely remove moisture. Therefore, there exists a problem that manufacturing cost will become high. From this point of view as well, there is a problem that it is difficult to cope with mass production in the future with a view to secondary batteries for electric vehicles.

On the other hand, there is an aqueous lithium secondary battery using an aqueous solution as an electrolytic solution. This aqueous lithium secondary battery is expected to be very advantageous for the problems of the non-aqueous lithium secondary battery described above.
That is, the water-based lithium secondary battery does not basically burn because it does not contain an organic solvent in the electrolytic solution. In addition, since a dry environment is not required in the manufacturing process, manufacturing costs can be significantly reduced. Furthermore, since aqueous electrolytes generally have higher conductivity than non-aqueous electrolytes, aqueous lithium secondary batteries have the advantage of lower internal resistance than non-aqueous lithium secondary batteries.

  However, water-based lithium secondary batteries are required to be used in a potential range where no water electrolysis reaction occurs. Therefore, the electromotive force of the aqueous lithium secondary battery is smaller than that of the non-aqueous lithium secondary battery. When calculated from the electrolysis potential of water, the limit is about 1.2 V. However, in actuality, an overvoltage is necessary to generate gas by electrolysis, so about 2 V is the limit. This is because water-based lithium secondary batteries are not suitable for applications such as portable devices that emphasize high energy density, that is, light and small. It is expected to be suitable for applications such as hybrid electric vehicles and eventually household distributed power supplies.

What is important for the construction of an aqueous lithium secondary battery is that it is stable in an aqueous solution and has an activity capable of reversibly occluding and desorbing a large amount of lithium in a potential range where oxygen and hydrogen are not generated by electrolysis of water. The substance is that an active material that can exhibit a large capacity in a specific potential range is used.
Further, it is desired to use a neutral to alkaline electrolyte as the electrolyte. Oxide-based electrode materials that are mainly used as active materials generally have poor stability in acidic aqueous solutions, and a large amount of H ⁺ ions in acidic electrolytes inhibit the rocking chair reaction of pure Li ⁺ ions. It is because there is a possibility of doing.

When a neutral, ie, pH = 7 electrolyte is used, the water decomposition voltage is 2.62V for hydrogen generation potential and 3.85V for oxygen generation potential. In addition, when a strong alkaline, that is, pH = 14 electrolytic solution is used, the water decomposition voltage has a hydrogen generation potential of 2.21 V and an oxygen generation potential of 3.44 V.
Therefore, as the positive electrode active material, a material from which more Li can be pulled out to a minimum of 3.85 V (pH = 7) is desired. As the negative electrode active material, a material into which more Li can be inserted up to 2.21 V (pH = 14) is desired. Since a gas generation overvoltage exists, it can be used even at a potential outside this range. However, considering self-discharge and use at a high temperature, it is desirable to use the potential within this range as much as possible.

To date, water-based lithium secondary batteries include Li—Mn oxide, Li—Ni oxide, Li—Co oxide and the like as the positive electrode active material, and Li—Mn oxide, VO as the negative electrode active material. ₂ and those containing LiV ₃ O ₈ have been proposed (see Patent Documents 1 and 2).

  However, such active materials still have insufficient discharge capacity and stability in aqueous electrolyte solutions. Therefore, the conventional water-based lithium secondary battery has a problem that the capacity is small and the capacity is easily deteriorated by repeated charging and discharging. Therefore, the current aqueous lithium secondary battery has not yet gone through the idea stage, and has not yet been practical enough to replace the conventional non-aqueous lithium secondary battery.

  In addition, an active material containing Co, such as Li—Co oxide, can exhibit a high discharge capacity, but has a problem that manufacturing cost increases because it contains expensive Co. Therefore, when an active material containing Co is used for an aqueous lithium secondary battery, the advantage of the aqueous lithium secondary battery that it can be manufactured at low cost is impaired. Therefore, there has been a problem that an active material containing Co is not suitable for use in which application of a water-based lithium secondary battery is desired, such as an electric vehicle, a hybrid electric vehicle, and a household distributed power source.

JP-T-9-508490 JP 2000-77073 A

  The present invention was made in view of such conventional problems, and does not contain expensive Co, can exhibit a high discharge capacity, and has a positive electrode active material for an aqueous lithium secondary battery excellent in charge / discharge cycle characteristics, and The present invention intends to provide an aqueous lithium secondary battery.

A first invention is a positive electrode active material for an aqueous lithium secondary battery having an aqueous electrolyte obtained by dissolving a lithium salt in water,
The positive electrode active material is represented by the general formula _{Li t Ni x Mn y M z} O 2 (0.9 ≦ t ≦ 1.2,0.4 ≦ x ≦ 0.55,0.4 ≦ y ≦ 0.55,0 ≦ z ≦ 0.2, M is mainly composed of a compound having a layered structure represented by one or more selected from Mg, Al, Fe, Ti, Ga, Cu, V, and Nb) The positive electrode active material for an aqueous lithium secondary battery is (Claim 1).

In the positive electrode active material for aqueous lithium secondary battery of the first invention, are mainly composed of a compound of layered structure represented by the general formula _{Li t Ni x Mn y M z} O 2. And since the compound represented by the said general formula does not contain expensive Co in the composition and contains many comparatively cheap Mn, it can be manufactured at low cost. Therefore, the present invention can also be applied to uses that require mass production, such as electric vehicles, hybrid electric vehicles, and home-use distributed power supplies.

Further, the compounds of the general formula _{Li t Ni x Mn y M z} O 2 represented by a layered structure, the layered LiMnO ₂ or spinel formed main constituent metal element other than Li are only Mn LiMn ₂ O _4, etc. Compared to the above, it has more reversible capacity within a potential range where electrolysis of water does not occur. Therefore, the positive electrode active material can exhibit a high discharge capacity when used in an aqueous lithium secondary battery. Furthermore, the capacity | capacitance deterioration when charging / discharging is performed repeatedly can be prevented, and the high discharge capacity in the initial stage of charging / discharging can be maintained. Moreover, since the said positive electrode active material is stable also in aqueous solution electrolyte solution, it is suitable as a positive electrode active material of a water-system lithium secondary battery.

  Thus, according to the first aspect of the present invention, the positive electrode active material for an aqueous lithium secondary battery and the aqueous lithium secondary battery that do not contain expensive Co, can exhibit a high discharge capacity, and are excellent in charge / discharge cycle characteristics. Can be provided.

A second invention is an aqueous lithium secondary battery having a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and an aqueous electrolyte obtained by dissolving a lithium salt in water.
The positive electrode active material is represented by the general formula _{Li t Ni x Mn y M z} O 2 (0.9 ≦ t ≦ 1.2,0.4 ≦ x ≦ 0.55,0.4 ≦ y ≦ 0.55,0 ≦ z ≦ 0.2, M is mainly composed of a compound having a layered structure represented by (one or more selected from Mg, Al, Fe, Ti, Ga, Cu, V, and Nb),
The negative electrode active material is an aqueous lithium secondary battery characterized in that a main component is a substance having a lower lithium insertion / extraction potential than the compound represented by the general formula (claim 2).

The second most noteworthy in water based lithium secondary battery of the invention, the aqueous lithium secondary battery is represented by the general formula _{Li t Ni x Mn y M z} O 2 in the positive electrode of a compound of layered structure represented The main component of the active material is.
That is, the aqueous lithium secondary battery contains the positive electrode active material of the first invention. Therefore, the water based lithium secondary battery can be manufactured at a low cost, and can exhibit a higher discharge capacity than a conventional water based lithium secondary battery. Moreover, in the said water-system lithium secondary battery, cycle deterioration by charging / discharging can be suppressed.

  As described above, according to the second aspect of the invention, it is possible to provide an aqueous lithium secondary battery that does not contain expensive Co, can exhibit a high discharge capacity, and is excellent in charge / discharge cycle characteristics.

Next, an embodiment of the present invention will be described.
In the present invention, the cathode active material is represented by the general formula _{Li t Ni x Mn y M z} O 2 (0.9 ≦ t ≦ 1.2,0.4 ≦ x ≦ 0.55,0.4 ≦ y ≦ 0 .55, 0 ≦ z ≦ 0.2, M is mainly composed of a compound having a layered structure represented by one or more selected from Mg, Al, Fe, Ti, Ga, Cu, V, and Nb) .
In the general formula _{Li t Ni x Mn y M z} O 2, t, x, and if the value of y is outside the range described above, when configuring an aqueous lithium secondary battery using the positive electrode active material The discharge capacity and charge / discharge cycle characteristics may be deteriorated.

In the general formula _{Li t Ni x Mn y M z} O 2, M is an optional component. M is at least one selected from Mg, Al, Fe, Ti, Ga, Cu, V, and Nb. In the case of z = 0, the above formula is represented by _{Li t Ni x Mn y O 2} .
The positive electrode active material can contain M, but even when M is not contained, excellent discharge capacity and charge / discharge cycle characteristics can be exhibited.

In the general formula _{Li t Ni x Mn y M z} O 2, z is preferably 0 <z ≦ 0.2.
In this case, in the above general formula, M is an essential component, and in this case, the positive electrode active material can exhibit more excellent discharge capacity and charge / discharge cycle characteristics.
If z exceeds 0.2, the layered structure of the compound represented by the above general paper may become unstable, or the compound represented by the above general formula may no longer have a layered structure. . Therefore, in this case, the charge / discharge characteristics of the positive electrode active material may be deteriorated.

The compound represented by the general formula _{Li t Ni x Mn y M z} O 2, the type and the M, t, x, by changing the range of y, and z, there are various things. In the aqueous lithium secondary battery, one of these can be used as the positive electrode active material for the positive electrode, but two or more can be mixed and used. Furthermore, what mixed the compound of the layered structure represented with the said general formula and a well-known positive electrode active material can also be used.

The positive electrode active material is prepared by, for example, preparing a mixed hydroxide of Ni, Mn, and M using a coprecipitation method, and firing the mixed hydroxide, thereby forming an oxide solid solution of Ni, Mn, and M. And the oxide solid solution and LiOH are fired.
In firing, since the characteristics deteriorate as grain growth proceeds, it is preferable to perform firing at the lowest temperature at which a single phase is obtained and sufficient crystallization occurs.

Further, the aqueous lithium secondary battery of the second invention, the negative electrode active material, than the compound represented by the general formula _{Li t Ni x Mn y M z} O 2, the absorption and desorption potential of lithium Mainly low substances.
Examples of such materials include lithium vanadium composite oxides such as LiV ₂ O ₄ , LiV ₃ O ₈ , Li _1.5 V ₃ O _y (7 ≦ y ≦ 8), and lithium titanium composites such as Li ₄ Ti ₅ O _12. There are oxides, iron oxides, iron hydroxides, and the like.

Moreover, the said water-system lithium secondary battery has aqueous solution electrolyte solution formed by melt | dissolving lithium salt in water as electrolyte solution.
Examples of such a lithium salt include LiNO ₃ , LiOH, LiCl, and Li ₂ S. These lithium salts can be used alone or in combination of two or more.

Next, the pH of the aqueous electrolyte solution is preferably 6 to 10. (Claim 3)
If the pH of the aqueous electrolyte solution is less than 6, the compound represented by the general formula _{Li t Ni x Mn y M z} O 2 becomes unstable, it may decrease the capacity and charge-discharge cycle characteristics of the battery is there. On the other hand, when the pH exceeds 10, the electrolysis potential of water, that is, the hydrogen generation potential and the oxygen generation potential are decreased to 2.21 V and 3.44 V, respectively. Therefore, oxygen and hydrogen may be easily generated at the positive electrode and the negative electrode.

  The above-mentioned aqueous lithium secondary battery mainly includes, for example, a positive electrode and a negative electrode that store and release lithium, a separator that is sandwiched between them, and an aqueous electrolyte that moves lithium between the positive electrode and the negative electrode. Can be configured as an element.

In the aqueous lithium secondary battery, the positive electrode is formed, for example, by mixing a conductive material and a binder with the positive electrode active material, and adding a suitable solvent as necessary to form a paste-like positive electrode mixture, If necessary, it can be compressed to increase the electrode density.
The conductive material is for ensuring the electrical conductivity of the positive electrode, and for example, a material obtained by mixing one or more carbon material powders such as carbon black, acetylene black, and graphite can be used.

The binder serves to bind the active material particles and the conductive material particles. For example, a fluorine-containing resin such as polytetrafluoroethylene, polyvinylidene fluoride, or fluororubber, or heat such as polypropylene, polyethylene, or polyethylene terephthalate. A plastic resin, a polyacrylonitrile-based polymer, or the like can be used.
As a solvent for dispersing these active material, conductive material, and binder, for example, an organic solvent such as N-methyl-2-pyrrolidone can be used.

  As with the positive electrode, for example, the negative electrode active material is mixed with a conductive material or a binder, and if necessary, an appropriate solvent is added to form a paste-like negative electrode mixture. And press to form.

  In addition, the separator to be narrowly attached to the positive electrode and the negative electrode separates the positive electrode and the negative electrode and holds the electrolytic solution. For example, a thin microporous film such as cellulose, polyethylene, or polypropylene can be used.

  Examples of the shape of the water based lithium secondary battery include a coin shape, a cylindrical shape, and a square shape. As the battery case that accommodates the positive electrode, the negative electrode, the separator, the aqueous electrolyte, and the like, those corresponding to these shapes can be used.

Example 1
Next, an embodiment of the present invention will be described with reference to FIGS.
In this example, an aqueous lithium secondary battery is produced using a positive electrode active material and the positive electrode active material, and the discharge capacity and charge / discharge cycle characteristics of the aqueous lithium secondary battery are evaluated.

In this example, the positive electrode active material is mainly composed of a compound having a layered structure represented by a compound having a layered structure represented by LiNi _1/2 Mn _1/2 O ₂ .
As shown in FIG. 1, the aqueous lithium secondary battery 1 of this example includes a positive electrode 2 containing a positive electrode active material, a negative electrode 3 containing a negative electrode active material, and an aqueous electrolyte obtained by dissolving a lithium salt in water. Have As described above, the positive electrode active material is mainly composed of a compound having a layered structure represented by LiNi _1/2 Mn _1/2 O ₂ . The negative electrode active material is mainly composed of a material having a lower lithium insertion / desorption potential than LiNi _1/2 Mn _1/2 O ₂ , that is, Li _1.5 V ₃ O _y (7 ≦ y ≦ 8). The aqueous electrolyte solution is obtained by dissolving LiNO ₃ as a lithium salt in water.

  In the water based lithium secondary battery 1, a separator 4 is disposed in a CR2016 type battery case 11 together with a positive electrode 2 and a negative electrode 3 in a state of being sandwiched between them. An aqueous electrolyte solution is injected into the battery case 11. A gasket 5 is disposed at an end in the battery case 11, and the battery case 11 is sealed with a sealing plate 12.

Next, a manufacturing method of the water based lithium secondary battery of this example will be described.
First, a compound having a layered structure represented by LiNi _1/2 Mn _1/2 O ₂ is synthesized as a positive electrode active material as described below.
In the synthesis of LiNi _1/2 Mn _1/2 O ₂ , production of mixed hydroxides of Ni and Mn using a so-called coprecipitation method, production of Ni and Mn oxide solid solutions by firing, and the oxide solid solutions It was synthesized through a three-stage reaction of baking with LiOH.

Specifically, first, nickel nitrate (II) .hexahydrate (Ni (NO ₃ ) ₂ .6H ₂ O), manganese acetate (II) .anhydrous (C ₄ H ₆ MnO ₄ ), LiNi _An aqueous solution was prepared by dissolving in water according to a stoichiometric ratio such that the composition was _1/2 Mn _1/2 O ₂ . At this time, the concentration of each element (Ni and Mn) in the aqueous solution was adjusted to 1 mol / L.
Next, 10% aqueous ammonia was added dropwise until the pH of the aqueous solution reached 8, to obtain a mixed hydroxide of Ni and Mn. The resulting mixed hydroxide was thoroughly washed and dried.

Next, this mixed hydroxide was calcined at a temperature of 400 ° C. in a nitrogen stream. Thereby, an oxide solid solution of Ni and Mn was obtained.
Subsequently, lithium hydroxide monohydrate (LiOH.H ₂ O) was dissolved in water according to the stoichiometric ratio of the composition LiNi _1/2 Mn _1/2 O ₂ , and the oxidation obtained above The solid solution was added and mixed. Mixing was performed for 12 hours using a ball mill. After drying, the mixture was heated in air at a temperature of 150 ° C. for 1 hour, and then calcined at a temperature of 1000 ° C. for 12 hours. In this way, a compound having a layered structure represented by LiNi _1/2 Mn _1/2 O ₂ was produced. This is a positive electrode active material.

Next, Li _1.5 V ₃ O _y (7 ≦ y ≦ 8) as a negative electrode active material was produced as follows.
First, 1.688 g of lithium carbonate (Li ₂ CO ₃ ) and 8.31 g of vanadium pentoxide (V ₂ O ₅ ) were prepared, and 10 g of these mixtures were mixed in an automatic mortar for 2 hours. Next, the mixture was heated to 680 ° C. at a temperature increase rate of 4.33 ° C./min in an argon stream and held for 12 hours. Thereafter, it was cooled at a rate of 4.33 ° C./min. In this way, Li _1.5 V ₃ O _y (7 ≦ y ≦ 8) was produced. This is defined as a negative electrode active material.

Next, an aqueous lithium secondary battery is manufactured using the positive electrode active material and the negative electrode active material.
Specifically, first, 70 parts by weight of LiNi _1/2 Mn _1/2 O ₂ as a positive electrode active material, 25 parts by weight of carbon black as a conductive agent, and polytetrafluoroethylene (PTFE as a binder) ) Was mixed to prepare a positive electrode mixture.
Further, Li _1.5 V ₃ O _y (7 ≦ y ≦ 8) as a negative electrode active material is 70 parts by weight, carbon black as a conductive agent is 25 parts by weight, and polytetrafluoroethylene (PTFE) as a binder is used. 5 parts by weight was mixed to prepare a negative electrode mixture.

Next, as shown in FIG. 1, a battery case 11 for a CR2016 type coin cell is prepared, and a positive electrode mixture is pressure-bonded at about 0.6 ton / cm ² on a mesh previously welded to the inside of the battery case 11. 2 was formed. In the same manner as this positive electrode 2, the negative electrode mixture was pressed onto the mesh at about 0.6 ton / cm ² to form the negative electrode 3.
The positive electrode 2 and the negative electrode 3 were disposed in the battery case 11 via a cellulose separator 4 having a thickness of 25 μm.

Next, the gasket 5 was placed in the battery case 11, and an appropriate amount of an aqueous electrolyte solution was injected into the battery case 11 and impregnated. In this example, a 3M LiNO ₃ aqueous solution (pH≈7) was used as the aqueous electrolyte.
Next, the sealing plate 12 was disposed in the opening of the battery case 11, and the end of the battery case 11 was caulked to seal the battery case 11, thereby producing the aqueous lithium secondary battery 1. This is referred to as a battery E1.

Further, in this example, in order to clarify the excellent characteristics of the battery E1, two types of aqueous lithium secondary batteries each having LiMnO _{2 having} a layered structure or LiNi _0.8 Co _0.2 O ₂ having a layered rock salt structure as a positive electrode active material are used. Secondary batteries (battery C1 and battery C2) were produced.

Specifically, first, two types of positive electrode active materials (LiMnO ₂ and LiNi _0.8 Co _0.2 O ₂ ) were prepared.
In producing LiMnO _{2 having} a layered structure, 6.29 g of lithium hydroxide monohydrate was first dissolved in 80 ml of ion exchange water, and 2.61 g of manganese dioxide was further added, followed by ultrasonic dispersion for 30 minutes. The molar ratio of lithium to manganese was 5: 1. The mixture was reacted in an autoclave lined with a fluororesin under hydrothermal conditions at 200 ° C. for 7 days. The precipitate in the container was filtered, then washed with water, and dried at 120 ° C. to obtain LiMnO ₂ having a layered structure (space group R3-m).
The layered rock salt structure LiNi _0.8 Co _0.2 O ₂ was prepared by synthesizing from a metal salt of Li, Ni, and Co by a liquid phase method.

Next, using these two types of positive electrode active materials, two types of aqueous lithium secondary batteries were produced in the same manner as the battery E1. Let these be the battery C1 and the battery C2.
The battery C1 is the same as the battery E1 except that it includes LiMnO ₂ having a layered structure as a positive electrode active material.
The battery C2 is the same as the battery E1 except that it includes LiNi _0.8 Co _0.2 O ₂ having a layered rock salt structure as a positive electrode active material.

Next, charge / discharge cycle characteristics were examined for the three types of aqueous lithium secondary batteries (battery E1, battery C1, and battery C2) produced as described above (charge / discharge cycle test).
In the charge / discharge cycle test, each battery was charged to a battery voltage of 1.2 V at a constant current of a current density of 0.5 mA / cm ² under a temperature of 60 ° C., and then a current density of 0.5 mA / cm ² . Charging / discharging at a constant current up to a battery voltage of 0.1 V was taken as one cycle, and this cycle was repeated 10 times. In each charging / discharging cycle, after charging to 1.2V and discharging to 0.1V, a charging suspension time and a discharging suspension time were provided for 1 minute each. And the discharge capacity of each battery (battery E1, battery C1, battery C2) was measured for every cycle.

The discharge capacity is determined by measuring the discharge current value (mA) for each cycle, and multiplying this discharge current value by the time (hr) required for discharge to obtain the weight of the positive electrode active material in the battery (g ).
The result is shown in FIG.

In FIG. 2, the horizontal axis indicates the number of cycles (times), and the vertical axis indicates the discharge capacity (mAh / g). In the figure, a battery configured using LiNi _1/2 Mn _1/2 O ₂ as a positive electrode active material is referred to as a battery E1, a battery configured using LiMnO ₂ is referred to as a battery C1, and LiNi _0.8 Co _0.2 O _2. A battery constituted by using was represented as a battery C2.
The initial charge capacity and initial discharge capacity of the battery E1, the battery C1, and the battery C2 are shown in Table 1 below. The initial charge capacity and the initial discharge capacity indicate the first charge capacity and discharge capacity of each battery. Further, the charge capacity is obtained by measuring the charge current value (mA) and multiplying the charge current value by the time (hr) required for charging, as the weight (g) of the positive electrode active material in the battery. It was calculated by dividing.

As is known from Table 1, it can be seen that the battery E1 can exhibit a large initial discharge capacity although the initial charge capacity is smaller than those of the battery C1 and the battery C2.
In addition, as is known from FIG. 2, the battery C2 exhibited a unique characteristic that the discharge capacity increased until the third cycle and then decreased, but all of the battery E, the battery C1, and the battery C2 are substantially the same. The rate of decrease in discharge capacity was shown. However, the battery E has a remarkably high initial discharge capacity compared to the batteries C1 and C2, and maintains this high discharge capacity even after repeating the charge / discharge cycle, and the discharge is higher than that of the batteries C1 and C2. It turns out that capacity can be demonstrated.

  Thus, it turns out that the battery E1 of this example can exhibit a high capacity | capacitance compared with the conventional water-system lithium secondary battery, and can exhibit the outstanding charging / discharging cycling characteristics. Further, the battery E1 does not contain expensive Co in the composition of the positive electrode active material, and can be manufactured at a low cost.

BRIEF DESCRIPTION OF THE DRAWINGS Explanatory drawing which shows the structure of the water-system lithium secondary battery concerning Example 1. FIG. The diagram which shows the charging / discharging cycle characteristic of three types of water based lithium secondary batteries (battery E1, battery C1, battery C2) concerning Example 1. FIG.

Explanation of symbols

1 Water-based lithium secondary battery 2 Positive electrode 3 Negative electrode

Claims (3)

A positive electrode active material for an aqueous lithium secondary battery having an aqueous electrolyte obtained by dissolving a lithium salt in water,
The positive electrode active material is represented by the general formula _{Li t Ni x Mn y M z} O 2 (0.9 ≦ t ≦ 1.2,0.4 ≦ x ≦ 0.55,0.4 ≦ y ≦ 0.55,0 ≦ z ≦ 0.2, M is mainly composed of a compound having a layered structure represented by one or more selected from Mg, Al, Fe, Ti, Ga, Cu, V, and Nb) A positive electrode active material for an aqueous lithium secondary battery. In an aqueous lithium secondary battery having a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and an aqueous electrolyte obtained by dissolving a lithium salt in water,
The positive electrode active material is represented by the general formula _{Li t Ni x Mn y M z} O 2 (0.9 ≦ t ≦ 1.2,0.4 ≦ x ≦ 0.55,0.4 ≦ y ≦ 0.55,0 ≦ z ≦ 0.2, M is mainly composed of a compound having a layered structure represented by (one or more selected from Mg, Al, Fe, Ti, Ga, Cu, V, and Nb),
The water-based lithium secondary battery, wherein the negative electrode active material contains a substance having a lower lithium insertion / extraction potential than the compound represented by the general formula.   The aqueous lithium secondary battery according to claim 2, wherein the aqueous electrolyte solution has a pH of 6 to 10.

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