JP2006120367A - Positive active material 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 capable of manufacturing at low cost and having high charge discharge cycle characteristics, and to provide the aqueous electrolyte secondary battery. <P>SOLUTION: The positive active material for the aqueous lithium secondary battery having an aqueous solution electrolyte prepared by dissolving a lithium salt in water is provided. The positive active material contains a compound having spinel structure represented by general formula Li<SB>x</SB>Mn<SB>2-y</SB>M<SB>y</SB>O<SB>4</SB>(in the formula, 0.8≤x≤1.2; 0<y≤0.2; M is at least one kind selected from Li, Ni, Co, Fe, Cr, V, Ti, Cu, Zn, Ga, Mg, Al, and Nb) as the main component. The aqueous lithium secondary battery 1 has a positive electrode 2 containing the positive active material, a negative electrode 3 containing a negative active material, and the aqueous solution electrolyte prepared by dissolving the lithium salt in water. <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, but relatively important to cost and safety is the top priority. It is expected to be suitable for applications such as electric vehicles and hybrid electric vehicles in which large batteries are used, as well as home-use distributed power sources.

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, the positive electrode active material made of Li—Co oxide contains Co, which is a relatively expensive metal element, as a main component. Therefore, the manufacturing cost of the water based lithium secondary battery is increased, and the original advantages of the water based lithium secondary battery are impaired. Therefore, it has been difficult to apply to applications such as electric vehicles, hybrid electric vehicles, and home-use distributed power sources that require mass production. On the other hand, Li—Mn oxides such as LiMn ₂ O ₄ are considered to be suitable as a positive electrode active material for an aqueous lithium secondary battery because they contain inexpensive Mn as a main component.

However, an active material mainly composed of a Li-Mn oxide such as LiMn ₂ O ₄ still has insufficient stability in an aqueous electrolyte solution. Moreover, the active material which has a Li-Ni oxide as a main component was also insufficient in stability in aqueous electrolyte solution. Therefore, the conventional water-based lithium secondary battery containing such an active material has a problem that the charge / discharge capacity is insufficiently stable, and the capacity is easily deteriorated by repeated charge / discharge.

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

  The present invention has been made in view of such conventional problems, and can be manufactured at low cost, and has a positive electrode active material for an aqueous lithium secondary battery and an aqueous lithium secondary that are excellent in charge / discharge cycle characteristics. It is intended to provide a 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 has the general formula Li _x Mn _{2 -y My} O ₄ (where 0.8 ≦ x ≦ 1.2, 0 <y ≦ 0.2, M is Li, Ni, Co, Fe, Cr A positive electrode active material comprising a spinel structure compound represented by V, Ti, Cu, Zn, Ga, Mg, Al, and Nb as a main component. 1).

The positive electrode active material of the first invention is mainly composed of a compound having a spinel structure represented by the above general formula, and the compound having a spinel structure represented by the above general formula Li _x Mn _{2 -y My} O ₄ is: Relatively inexpensive Mn is the main component of the metal element. Therefore, the positive electrode active material 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.

In addition, the positive electrode active material mainly composed of a compound having a spinel structure represented by the above general formula can exhibit a relatively large charge / discharge capacity, and can be a conventional Li-Mn oxide such as LiMn ₂ O ₄ or Li -Stable in the aqueous electrolyte solution as compared with the positive electrode active material made of Ni oxide.

The reason is considered as follows.
That is, the compound of spinel structure represented by the general formula _{Li x Mn 2-y M y} O 4 , a metal element M (Li to Mn site of LiMn ₂ O ₄ having a spinel structure, Ni, Co, Fe, Cr, 1 or more selected from V, Ti, Cu, Zn, Ga, Mg, Al, and Nb). It is presumed that this substituted metal element M stabilizes the spinel structure and improves the stability in an aqueous solution.
As described above, since the positive electrode active material is stable in the aqueous electrolyte solution, the charge / discharge capacity is unlikely to decrease even if charge / discharge is repeated. That is, the positive electrode active material is excellent in charge / discharge cycle characteristics.

  As described above, according to the first invention, it is possible to provide a positive electrode active material for an aqueous lithium secondary battery that can be manufactured at low cost and has excellent charge / discharge cycle characteristics.

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 has the general formula Li _x Mn _{2 -y My} O ₄ (where 0.8 ≦ x ≦ 1.2, 0 <y ≦ 0.2, M is Li, Ni, Co, Fe, Cr 1 or more selected from V, Ti, Cu, Zn, Ga, Mg, Al, and Nb) as a main component,
The negative electrode active material is a water based lithium secondary battery characterized in that a main component of the positive electrode active material is a material having a lower lithium occlusion / desorption potential than that of the main component of the positive electrode active material.

In the water based lithium secondary battery of the second invention, the general formula Li _x Mn _{2 -y My} O ₄ (where 0.8 ≦ x ≦ 1.2, 0 <y ≦ 0.2, M is Li 1 or more selected from Ni, Co, Fe, Cr, V, Ti, Cu, Zn, Ga, Mg, Al, and Nb) That is, it contains the positive electrode active material of the first invention.
Therefore, taking advantage of the characteristics of the positive electrode active material, the water-based lithium secondary battery can be produced at low cost, and the charge / discharge capacity is not easily lowered even after repeated charging, and the charge / discharge cycle characteristics are excellent. It will be.

Next, an embodiment of the present invention will be described.
In the present invention, the positive electrode active material contains as a main component a compound having a spinel structure represented by the general formula Li _x Mn _{2 -y My} O ₄ . The compound represented by the general formula _{Li x Mn 2-y M y} O 4 , a part of Mn of LiMn ₂ O ₄ having a spinel structure, the metal element M (Li, Ni, Co, Fe, Cr, V , Ti, Cu, Zn, Ga, Mg, Al, and Nb).
In the general formula _{Li x Mn 2-y M y} O 4, x, y is in the range of 0.8 ≦ x ≦ 1.2,0 <y ≦ 0.2.
In the above general formula, when the values of x and y are out of the above ranges, the discharge capacity and charge / discharge cycle characteristics may be deteriorated when an aqueous lithium secondary battery is configured using the positive electrode active material. There is.

Further, as the compound represented by the above general formula Li _x Mn _{2 -y My} O ₄ , various compounds exist by changing the kind of M and the ranges of x and y. The positive electrode active material can have one of these as a main component, but can also have two or more as a main component. Furthermore, as the positive electrode active material, a mixture of a spinel structure compound represented by the above general formula and a known positive electrode active material may be used.

  The positive electrode active material is prepared by a solid phase synthesis method or the like, for example, a raw material compound such as a compound containing Li such as lithium hydroxide, a compound containing Mn such as manganese dioxide, and a compound containing a desired metal element M. It can be prepared by mixing at a blending ratio so as to obtain a desired composition in the compound represented by the above general formula and heating (baking) the mixture.

Next, the aqueous lithium secondary battery according to the second invention has 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 mainly composed of a compound having a spinel structure represented by the general formula Li _x Mn _{2 -y My} O ₄ .

The negative electrode active material is mainly composed of a material having a lower lithium occlusion / desorption potential than the main component of the positive electrode active material, that is, the compound represented by the general formula Li _x Mn _{2 -y My} O _4. Ingredients.
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.
Preferably, the main component is a lithium vanadium composite oxide.
In this case, the energy density of the water based lithium secondary battery can be further improved.

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 4)
If the pH of the aqueous electrolyte solution is less than 6, the compound of spinel structure represented by the general formula _{Li x Mn 2-y M y} O 4 becomes unstable, lowering the capacity and charge-discharge cycle characteristics of the battery There is a risk. 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.

Next, examples of the present invention will be described.
In this example, a positive electrode active material made of a compound having a spinel structure is manufactured, and an aqueous lithium secondary battery is manufactured using the positive electrode active material.
The positive electrode active material of this example is mainly composed of a compound having a spinel structure represented by LiMn _1.8 M _0.2 O ₄ (M is Mg, Ni, or Co).

As shown in FIG. 1, the aqueous lithium secondary battery 1 of this example includes a positive electrode 2 containing the positive electrode active material, a negative electrode 3 containing a negative electrode active material, and an aqueous solution obtained by dissolving a lithium salt in water. An electrolyte solution. The negative electrode active material is mainly composed of a material (LiV ₂ O ₄ ) having a lower lithium occlusion / desorption potential than the main component (LiMn _1.8 Mg _0.2 O ₄ ) of the positive electrode active material. The aqueous electrolyte solution is obtained by dissolving LiNO ₃ as a lithium salt in water.

  In the aqueous lithium secondary battery 1 of this example, 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.

Hereinafter, a method for producing the positive electrode active material (LiMn _1.8 Mg _0.2 O ₄ ) of this example will be described. In this example, a positive electrode active material is produced by a solid phase synthesis method.
That is, first, lithium hydroxide (LiOH), manganese dioxide (MnO ₂ ), and magnesium carbonate (MgCO ₃ ) are used as raw materials for the positive electrode active material, and Li: Mn: Mg is 1.05: 1.9: 0.0. The sample was weighed at a stoichiometric ratio of 1 and ball milled using an ethanol solvent for 24 hours. Subsequently, the obtained mixed powder was sufficiently dried and dry ball mill mixing was performed for 12 hours to obtain a starting material.
The obtained starting material was baked at a temperature of 800 ° C. for 12 hours to prepare a positive electrode active material made of LiMn _1.8 Mg _0.2 O ₄ having a spinel structure. This is designated as Sample E1.

In this example, two types of positive electrode active materials mainly composed of LiMn _1.8 Ni _0.2 O ₄ and LiMn _1.8 Co _0.2 O ₄ were also produced.
The positive electrode active material mainly composed of LiMn _1.8 Ni _0.2 O ₄ is the same as the sample E1 except that nickel nitrate hexahydrate (NiNO ₃ .6H ₂ O) is used instead of magnesium carbonate. Produced. This is designated as Sample E2.
The positive electrode active material mainly composed of LiMn _1.8 Co _0.2 O ₄ is the same as the sample E1 except that cobalt chloride hexahydrate (CoCl ₂ .6H ₂ O) is used instead of magnesium carbonate. Produced. This is designated as Sample E3.

In this example, for comparison with the samples E1 to E3, a positive electrode active material (sample C1) made of spinel-structured LiMn ₂ O ₄ and a positive-electrode active material made of layered rock salt structure LiNi _0.8 Co _0.2 O _2. A material (sample C1) was prepared.
Sample C1 was prepared in the same manner as Sample E1 except that magnesium carbonate was not used.
Sample C2 was synthesized from a metal salt of Li, Ni, and Co by a liquid phase method.

Next, LiV ₂ O ₄ was produced as a negative electrode active material.
Specifically, first, lithium carbonate (Li ₂ CO ₃ ) and vanadium pentoxide (V ₂ O ₅ ) are prepared, and these are in accordance with the stoichiometric ratio so as to become the target substance LiV ₂ O _4. The mixture was obtained by mixing for 20 minutes in an automatic mortar. Thereafter, 2 parts by weight of ketjen black (TB-5500 manufactured by Tokai Carbon Co., Ltd.), which is a carbon powder, was added to 100 parts by weight of the mixture, and further mixed for 20 minutes in an automatic mortar. Next, this mixture was fired at 750 ° C. for 24 hours in an argon stream, and then rapidly cooled to produce LiV ₂ O ₄ .

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 sample E1 as a positive electrode active material, 25 parts by weight of carbon black as a conductive agent, and 5 parts by weight of polytetrafluoroethylene (PTFE) as a binder are mixed. A positive electrode mixture was produced.
Further, 70 parts by weight of LiV ₂ O ₄ as a negative electrode active material, 25 parts by weight of carbon black as a conductive agent, and 5 parts by weight of polytetrafluoroethylene (PTFE) as a binder are mixed, and a negative electrode mixture Was made.

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, as the positive electrode active material, the sample E2, the sample E3, the sample C1, or the sample C2 prepared in advance as described above is used, and the other four types of batteries (batteries) are the same as the battery E1. E2, battery E3, battery C1, battery C2) were produced.
That is, the battery E2 was manufactured in the same manner as the battery E1 except that a positive electrode active material (sample E2) mainly composed of spinel-structured LiMn _1.8 Ni _0.2 O ₄ was used.
The battery E3 was manufactured in the same manner as the battery E1 except that a positive electrode active material (sample E3) mainly composed of spinel-structured LiMn _1.8 Co _0.2 O ₄ was used.
The battery C1 was manufactured in the same manner as the battery E1 except that a positive electrode active material (sample C1) mainly composed of spinel-structured LiMn ₂ O ₄ was used.
The battery C2 was prepared in the same manner as the battery E1 except that a positive electrode active material (sample C2) mainly composed of LiNi _0.8 Co _0.2 O ₂ having a layered rock salt structure was used.

Next, charge / discharge cycle characteristics were examined for the five types of water-based lithium secondary batteries (battery E1 to battery E3, battery C1, and battery C2) manufactured 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 to a battery voltage of 0.1 V was defined as one cycle, and this cycle was repeated 20 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 E3, battery C1, and 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 a spinel-structured LiMn _1.8 Mg _0.2 O ₄ as a positive electrode active material is referred to as a battery E1, and a battery configured using a spinel-structured LiMn _1.8 Ni _0.2 O ₄ is referred to as a battery E2. A battery configured using LiMn _1.8 Co _0.2 O ₄ having a structure is represented as a battery E3, a battery configured using LiMn ₂ O ₄ having a spinel structure is represented as a battery C1, and LiNi _0.8 Co _0.2 O ₂ having a layered rock salt structure is used. The battery configured as described above was represented as battery C2.

  In addition, Table 1 below shows the initial charge capacity and initial discharge capacity of the batteries E1 to E3, the battery C1, and the battery C2. 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 batteries E1 to E3 can exhibit a very excellent initial discharge capacity of 86 mA / g or more. The battery E1 had a very good initial discharge capacity compared to the battery C2, and had a sufficient initial discharge capacity that could withstand practical use although it did not reach the battery C1.

  In addition, as is known from FIG. 2, the batteries E1 to E3 maintain a high discharge capacity of about 64 mAh / g even after 20 cycles of charge / discharge, although the initial discharge capacity is slightly inferior to that of the battery C1. It can be seen that the charge / discharge cycle characteristics are excellent. On the other hand, in the battery C1, although the initial discharge capacity is high, after 20 cycles, the discharge capacity is reduced to a low value of less than 60 mAh / g, indicating that the charge / discharge cycle characteristics are insufficient. Further, in the battery C2, the change between the initial discharge capacity and the discharge capacity after 20 cycles is small, but both of them are very low. That is, the battery C2 could only exhibit a very low discharge capacity of less than 35 mAh / g at the maximum in 20 cycles of charge and discharge, and could not withstand practical use.

When the positive electrode active materials (sample E1 to sample E3) used in the batteries E1 to E3 and the positive electrode active material (sample C1) used in the battery C1 are compared, the samples E1 to E3 are the sample C1 (LiMn ₂ O ₄ ) It has a composition in which a part of Mn is substituted with a substitution element Mg, Ni, or Co. Therefore, as is known from the results of FIG. 2 described above, the water-based lithium secondary batteries (battery E1 to battery E3) using the positive electrode active materials (sample E1 to sample E3) in which a part of Mn is substituted with a substitution element are obtained. Compared with a water-based lithium secondary battery (battery C1) using a non-substituted positive electrode active material (sample C1), the stability of the discharge capacity when the charge / discharge cycle is repeated, that is, the charge / discharge cycle characteristics are improved. (See FIG. 2).

  In the batteries E1 to E3, the positive electrode active materials (samples E1 to E3) contain Mn, which is relatively inexpensive, as a main component of the metal element. These positive electrode active materials contain Mg, Ni, and Co in addition to Mn, but these elements are very minute amounts of 0.2 mol with respect to 1.8 mol of Mn. Therefore, the positive electrode active materials (sample E1 to sample E3) can be synthesized relatively inexpensively, and the manufacturing cost of the batteries E1 to E3 can be reduced.

  Thus, it can be seen that the batteries E1 to E3 can be manufactured at low cost and have excellent charge / discharge cycle characteristics.

Explanatory drawing which shows the structure of the water-system lithium secondary battery concerning an Example. The diagram which shows the charging / discharging cycle characteristic of five types of water based lithium secondary batteries (battery E1-battery E3, battery C1, and battery C2) concerning an Example.

Explanation of symbols

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

Claims (4)

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 has the general formula Li _x Mn _{2 -y My} O ₄ (where 0.8 ≦ x ≦ 1.2, 0 <y ≦ 0.2, M is Li, Ni, Co, Fe, Cr , V, Ti, Cu, Zn, Ga, Mg, Al, and Nb), and a spinel structure compound represented by the main component. 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 has the general formula Li _x Mn _{2 -y My} O ₄ (where 0.8 ≦ x ≦ 1.2, 0 <y ≦ 0.2, M is Li, Ni, Co, Fe, Cr 1 or more selected from V, Ti, Cu, Zn, Ga, Mg, Al, and Nb) as a main component,
The water-based lithium secondary battery, wherein the negative electrode active material is mainly composed of a material having a lower lithium occlusion / desorption potential than the main component of the positive electrode active material.   3. The aqueous lithium secondary battery according to claim 2, wherein the negative electrode active material contains a lithium vanadium composite oxide as a main component.   4. 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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