JP2006073259A - Positive electrode active material and water dissolving lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a positive electrode active material for a water dissolving lithium secondary battery having a high discharging capacity and an excellent charging and discharging cycle characteristic, and to provide the water dissolving lithium secondary battery. <P>SOLUTION: The positive electrode active material is for the water dissolving lithium secondary battery having a water dissolving electrolyte made by dissolving a lithium salt in water. The positive electrode active material is formed of particles mainly consisting of a compound of an olivine structure represented by a formula Li<SB>x</SB>Fe<SB>1-y</SB>M<SB>y</SB>PO<SB>4</SB>(wherein, 0.85≤x≤1.2, 0<y≤0.5, and M is one or more kinds of elements selected from Li, Ni, Co, Mn, Mg, Al, Ti, Ga, Cu, V, Nb, Zr, Hf and Ce.). The water dissolving lithium secondary battery 1 has a positive electrode 2 containing the positive electrode active material formed of the particles, a negative electrode 3 containing a negative electrode active material and the water dissolving electrolyte made 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. 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.

On the other hand, there is a positive electrode active material for an aqueous lithium secondary battery made of lithium iron phosphate (see Patent Document 3).
This lithium iron phosphate is superior to other positive electrode active materials as described above in terms of production cost, potential flatness, and the like as a positive electrode active material.
However, the positive electrode active material made of iron iron phosphate has a problem that the active material itself has low conductivity, and a high capacity cannot be taken out when high rate discharge is performed. Further, there is a problem that the capacity is easily lowered by repeating charge and discharge, and the charge and discharge cycle characteristics are insufficient.

JP-T-9-508490 JP 2000-77073 A JP 2002-110221 A

  The present invention has been made in view of such conventional problems, and provides a positive electrode active material for an aqueous lithium secondary battery and an aqueous lithium secondary battery that can exhibit high discharge capacity and are excellent in charge / discharge cycle characteristics. It is something to try.

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 x Fe 1-y M y} PO 4 ( where, 0.85 ≦ x ≦ 1.2,0 <y ≦ 0.5, M is Li, Ni, Co, Mn, Mg 1 or more selected from Al, Ti, Ga, Cu, V, Nb, Zr, Hf, and Ce)) (Claim 1).

The positive electrode active material of the first invention is mainly composed of a compound having an olivine structure represented by the general formula Li _x Fe _{1- y My} PO ₄ . Compared with LiFePO ₄ , Li—Mn oxide, Li—Ni oxide, Li—Co oxide, and the like, the capacity of the olivine structure compound represented by the above general formula is less likely to deteriorate due to charge and discharge.
Therefore, the positive electrode active material can further suppress a decrease in capacity caused by repeated charge and discharge compared to a conventional positive electrode active material. That is, the positive electrode active material has excellent charge / discharge cycle characteristics.

The compounds of olivine structure represented by _{Li x Fe 1-y M y} PO 4 has a relatively large number of reversible capacity in the potential range where the electrolysis of water does not occur. Therefore, the positive electrode active material can exhibit a high discharge capacity as an active material for an aqueous lithium secondary battery.
Furthermore, the positive electrode active material is stable in an aqueous electrolyte solution.
Therefore, the positive electrode active material is suitable as a positive electrode active material for an aqueous lithium secondary battery.

  In addition, the compound represented by the above general formula contains relatively inexpensive Fe as a main element. Therefore, the positive electrode active material mainly containing the compound represented by the general formula 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.

  Thus, according to the first invention, it is possible to provide a positive electrode active material for an aqueous lithium secondary battery that can exhibit a high discharge capacity and is excellent in 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 is the positive electrode active material of the first invention,
The water-based lithium secondary battery according to claim 5, wherein the negative electrode active material is mainly composed of a material having a lower lithium insertion / extraction potential than the main component of the positive electrode active material.

The most notable point in the aqueous lithium secondary battery of the second invention is that the aqueous lithium secondary battery is the positive electrode active material of the first invention, that is, the general formula Li _x Fe _{1- y My} PO _4. (However, 0.85 ≦ x ≦ 1.2, 0 <y ≦ 0.5, M is Li, Ni, Co, Mn, Mg, Al, Ti, Ga, Cu, V, Nb, Zr, Hf, and The positive electrode active material mainly contains a compound having an olivine structure represented by (one or more selected from Ce).
Therefore, the water based lithium secondary battery can take advantage of the characteristics of the positive electrode active material of the first invention and can further suppress the capacity deterioration due to charge / discharge compared to the conventional water based lithium secondary battery. . The water based lithium secondary battery can exhibit a high discharge capacity. Furthermore, it can be manufactured at low cost.

  Thus, according to the said 2nd invention, while being able to exhibit high discharge capacity, the water-system lithium secondary battery excellent in the charge / discharge cycle characteristic can be provided.

Next, an embodiment of the present invention will be described.
In the present invention, the positive electrode active material is mainly composed of a compound having an olivine structure represented by the general formula Li _x Fe _{1 -y My} PO ₄ . The compound represented by the general formula _{Li x Fe 1-y M y} PO 4, a part of Fe of lithium iron phosphate having an olivine structure (LiFePO ₄₎ has a metal element M (M is Li, Ni, Co , Mn, Mg, Al, Ti, Ga, Cu, V, Nb, Zr, Hf, and Ce).
In the general formula _{Li x Fe 1-y M y} PO 4, x, y is in the range of 0.85 ≦ x ≦ 1.2,0 <y ≦ 0.5.
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.

The compound represented by the general formula _{Li x Fe 1-y M y} PO 4, the type and the M, by changing the range of x and y, there are various things. 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 compound having an olivine structure represented by the above general formula and a known positive electrode active material can be used.

  The positive electrode active material is obtained by, for example, a solid-phase synthesis method or the like from a raw material compound such as a compound containing Li, a compound containing Fe, a compound containing a desired metal element M, and a compound containing P It can produce by mixing with the compounding ratio which becomes a desired composition in the compound represented by a formula, and heating (baking) a mixture.

Next, it is preferable that the particles of the positive electrode active material have a coating layer made of a conductive carbon material on at least a part of the surface thereof (Claim 2).
In this case, the conductivity of the positive electrode active material can be improved. Therefore, in this case, the capacity can be taken out from the positive electrode active material at a higher current density.

The coating layer can be formed by mixing and baking the positive electrode active material or a raw material compound of a positive electrode active material that becomes a positive electrode active material after firing, and the conductive carbon material.
Examples of the conductive carbon material include ketjen black, acetylene black, carbon fiber, and activated carbon.
Further, the coating layer made of the conductive carbon material, for example, mixes an organic material containing carbon with the positive electrode active material or a raw material compound thereof, and burns the organic material to generate conductive carbon. At the same time, the carbon can be formed by covering the particles of the positive electrode active material.

The coating layer is preferably formed at a ratio of 20 parts by weight or less with respect to 100 parts by weight of the main component of the positive electrode active material.
When the coating layer exceeds 20 parts by weight, when the electrode is formed using the positive electrode active material, the active material density in the electrode may decrease, and the energy density per unit volume of the battery may decrease. is there.

Next, the compound represented by the general formula _{Li x Fe 1-y M y} PO 4, M is Li, Ni, Co, Mn, Mg, Al, Ti, Ga, Cu, V, Nb, Zr, Hf And at least one selected from Ce.
Preferably, M in the compound represented by the general formula _{Li x Fe 1-y M y} PO 4 may be be a Mn (claim 4).
In this case, the general formula is represented by _{Li x Fe 1-y Mn y} PO 4 ( where, 0.85 ≦ x ≦ 1.2,0 <y ≦ 0.5). In this case, the charge / discharge cycle characteristics of the positive electrode active material can be improved. Although this reason is not certain, it is guessed that Mn stabilizes the olivine structure of the compound represented by the above general formula.

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, a main component compound of olivine structure represented by the general formula _{Li x Fe 1-y Mn y} PO 4.

Further, the negative electrode active material, than the compound represented by the general formula _{Li x Fe 1-y Mn y} PO 4, absorption and desorption potential of lithium as a main component low material.
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 6)
When the pH of the aqueous electrolyte is less than 6, the olivine structure compound represented by the above general formula Li _x Fe _{1- y My} PO ₄ becomes unstable, and the battery capacity and charge / discharge cycle characteristics decrease. 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.

Example 1
Next, examples of the present invention will be described.
In this example, while producing the said positive electrode active material, the characteristic as a positive electrode active material for water-system lithium secondary batteries is evaluated.
The positive electrode active material of this example is composed of particles mainly composed of a compound having an olivine structure represented by LiFe _0.85 Mn _0.15 PO ₄ . The positive electrode active material particles have a coating layer made of a conductive carbon material on at least a part of the surface thereof.

Hereinafter, the manufacturing method of the positive electrode active material of this example is demonstrated. In this example, a positive electrode active material is produced by a solid phase synthesis method.
That is, first, as raw materials for the positive electrode active material, iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), lithium carbonate (Li ₂ CO ₃ ), manganese carbonate (MnCO ₃ ), and diphosphate Ammonium hydrogen (NH ₄ H ₂ PO ₄ ) was weighed in a stoichiometric ratio such that Li: Fe: Mn: PO ₄ was 1.05: 0.85: 0.15: 1 Were mixed into a pellet.

The pellets were calcined in a heating furnace at a temperature of 350 ° C. in a nitrogen atmosphere to produce substitutional lithium phosphate (LiFe _0.85 Mn _0.15 PO ₄ ). Next, the substituted lithium phosphate is taken out from the heating furnace and pulverized, and 10 parts by weight of the conductive carbon material ketjen black is added to 100 parts by weight of the pulverized product (substituted lithium phosphate). Mixed.
The obtained mixture was again solidified into a pellet and fired at a temperature of 750 ° C. for 24 hours in a nitrogen atmosphere. Thereafter, the mixture was sufficiently crushed to prepare a positive electrode active material composed of particles of LiFe _0.85 Mn _0.15 PO ₄ having an olivine structure. This is designated as Sample E1.

An example of the particles of the positive electrode active material obtained in this example is shown in FIG.
As shown in FIG. 1, in the positive electrode active material 20 of this example, a coating layer 22 made of ketjen black, which is a conductive carbon material, is formed on at least a part of the surface of the particles 21 of LiFe _0.85 Mn _0.15 PO _4. It was.

Further, in this example, in order to clarify the excellent characteristics of the sample E1, a positive electrode active material made of LiNi _0.8 Co _0.2 O ₂ having a layered rock salt structure was prepared for comparison. This is designated as Sample C1.
Sample C1 was prepared by synthesizing from a metal salt of Li, Ni, and Co by a liquid phase method.

Next, the characteristics as the positive electrode active material of the water based lithium secondary battery are evaluated for the sample E1 and the sample C1. Specifically, cyclic voltammogram measurement was performed using two kinds of test electrodes each containing the sample E1 and the sample C1.
Specifically, first, 70 parts by weight of sample E1, 25 parts by weight of carbon as a conductive agent, and 5 parts by weight of polytetrafluoroethylene (PTFE) as a binder were mixed to prepare a mixed powder. Next, 10 mg of this mixed powder was pressure-bonded onto a SUS mesh at about 0.6 ton / cm ² to prepare a test electrode.
In addition, a 3M LiNO ₃ aqueous solution was prepared as an electrolyte for evaluation, a silver-silver chloride electrode as a reference electrode, and a platinum wire (φ0.3 × 5; coil shape) as a counter electrode.

Subsequently, a test electrode and a counter electrode are arranged in a container containing an electrolytic solution so that they face each other, and a reference electrode is arranged between the test electrode and the counter electrode to produce a tripolar beaker cell. did.
Next, a voltage was applied between the test electrode and the counter electrode. The voltage is applied by sweeping the potential at a scanning speed (potential sweep speed) of 2 mV / sec (potential difference from the reference electrode) by cyclic voltammetry, and the potential of the test electrode (potential difference from the reference electrode) is -0.4V to The cycle changed in the range of 1.2 V was set to 1 cycle, and this cycle was carried out by performing a total of 3 cycles. The current value at this time was monitored. The result is shown in FIG.

For sample C1, a test electrode was prepared in the same manner as sample E1 to produce a three-electrode beaker cell, and a cyclic voltammogram was measured under the same conditions as for sample E1. The result is shown in FIG.
In FIG. 2, the cyclic voltammogram when the sample E1 is used as a test electrode is indicated by a solid line, and the cyclic voltammogram when the sample C1 is used as a test electrode is indicated by a dotted line. In FIG. 2, the horizontal axis represents the potential difference (V) between the test electrode and the reference electrode, and the vertical axis represents the current value (mA).

  As can be seen from FIG. 2, in the test electrode using the sample C1, Li elimination occurs in the aqueous electrolyte, but even if the potential is lowered thereafter, almost no current flows due to the insertion reaction. . Further, the peak of the elimination reaction depends on the decomposition potential of water, and it can be seen that the sample C1 cannot take out a sufficient capacity as the positive electrode active material of the aqueous lithium secondary battery.

On the other hand, the sample E1 has only one oxidation-reduction potential, and the bottom of the peak was within the decomposition potential of water. Moreover, in sample E1, it can confirm that the reversible insertion and removal of Li are performed efficiently.
Therefore, it turns out that the sample E1 is suitable as a positive electrode active material of an aqueous lithium secondary battery.

(Example 2)
This example is an example in which a water-based lithium secondary battery is produced using the sample E1 produced in Example 1 as a positive electrode active material, and its charge / discharge cycle characteristics are evaluated.
As shown in FIG. 3, 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 solution obtained by dissolving a lithium salt in water. Have The positive electrode active material is composed of a compound having an olivine structure represented by LiFe _0.85 Mn _0.15 PO _4, that is, the sample E1 prepared in Example 1. Further, the negative electrode active material is mainly composed of a material having a lower lithium absorption / desorption potential than that of the main component of the positive electrode active material (LiFe _0.85 Mn _0.15 PO ₄ ), that is, LiV ₂ O ₄ . 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 positive electrode active material and a negative electrode active material are prepared.
As a positive electrode active material, the sample E1 produced in Example 1 was prepared.

As the negative electrode active material was prepared LiV ₂ O _4.
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 raw material mixture was prepared by mixing for 120 minutes in an automatic mortar. Subsequently, this mixture was press-molded and fired in a hydrogen stream at 700 ° C. for 3 hours to prepare a mixture of V ₂ O ₃ and Li ₂ VO ₂ . This mixture was mixed in an automatic mortar for 20 minutes, and then baked at a temperature of 750 ° C. for 24 hours in an oxygen-containing gas composed of carbon dioxide and oxygen, with the oxygen partial pressure adjusted to 1.0 × 10 ⁻³ atm. LiV ₂ O ₄ was produced.

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. 3, a battery case 11 for a CR2016 type coin cell is prepared, and a positive electrode mixture is crimped onto a mesh previously welded to the inside of the battery case 11 at about 0.6 ton / cm ^2. 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.

In this example, in order to clarify the excellent characteristics of the battery E1, two types of aqueous lithium secondary batteries each having LiNi _0.8 Co _0.2 O ₂ having a layered rock salt structure or LiFePO ₄ having an olivine structure are used as the positive electrode active material. Secondary batteries (battery C1 and battery C2) were produced.

Specifically, first, two types of positive electrode active materials (LiNi _0.8 Co _0.2 O ₂ , LiFePO ₄ ) were prepared.
The sample C1 prepared in Example 1 was prepared as LiNi _0.8 Co _0.2 O ₂ having a layered rock salt structure.

Moreover, LiFePO _{4 having} an olivine structure was produced as follows.
That is, first, iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), lithium carbonate (Li ₂ CO ₃ ), and ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ) are mixed with Li: Fe. : Weighed so that PO ₄ was 1.05: 1: 1, and mixed well in a mortar to harden into pellets. The obtained pellets were calcined in a heating furnace at a temperature of 350 ° C. in a nitrogen atmosphere to obtain a calcined product. Next, the calcined product was taken out from the heating furnace and pulverized, and 10 parts by weight of the coated carbon material ketjen black was mixed with the calcined product, and sufficiently mixed in a mortar. The mixture was again solidified into pellets and baked in a nitrogen atmosphere at a temperature of 750 ° C. for 24 hours, and then sufficiently crushed to obtain LiFePO ₄ having an olivine structure.

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 contains LiNi _0.8 Co _0.2 O ₂ having a layered rock salt structure as a positive electrode active material.
The battery C2 is the same as the battery E1 except that it contains olivine-structured LiFePO ₄ as the 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 30 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. 4, 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 LiFe _0.85 Mn _0.15 PO ₄ as a positive electrode active material is referred to as a battery E1, a battery configured using LiNi _0.8 Co _0.2 O ₂ is represented as a battery C1, and configured using LiFePO _4. The resulting battery was designated as 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 very excellent initial discharge capacity of 78 mA / g or more. The battery E1 had a very good initial discharge capacity compared to the battery C1, and had a sufficient initial discharge capacity that could withstand practical use although it did not reach the battery C2.

  In addition, as is known from FIG. 4, the battery E1 has a slightly lower initial discharge capacity than the battery C2, but maintains a high discharge capacity of about 57 mAh / g even after 30 cycles of charge and discharge. It can be seen that the cycle characteristics are excellent. On the other hand, in the battery C2, although the initial discharge capacity is high, the discharge capacity is reduced to a low value of less than 50 mAh / g after 30 cycles, and it is understood 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 30 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 30 cycles of charge and discharge, and could not withstand practical use.

Thus, it can be seen that the battery E1 containing LiFe _0.85 Mn _0.15 PO ₄ as the positive electrode active material can exhibit a sufficiently high discharge capacity practically and has excellent charge / discharge cycle characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory diagram showing an example of a cross-sectional structure of positive electrode active material particles according to Example 1; The diagram which shows the cyclic voltammogram of two types of positive electrode active materials (sample E1 and sample C1) concerning Example 1. FIG. FIG. 3 is an explanatory diagram showing a configuration of an aqueous lithium secondary battery according to Example 2. The diagram which shows the charging / discharging cycle characteristic of three types of water-system lithium secondary batteries (battery E1, battery C1, and battery C2) concerning Example 2. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Water-system lithium secondary battery 2 Positive electrode 20 Positive electrode active material 3 Negative electrode

Claims (6)

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 x Fe 1-y M y} PO 4 ( where, 0.85 ≦ x ≦ 1.2,0 <y ≦ 0.5, M is Li, Ni, Co, Mn, Mg 1 or more selected from Al, Ti, Ga, Cu, V, Nb, Zr, Hf, and Ce)) .   2. The positive electrode active material according to claim 1, wherein the particles of the positive electrode active material have a coating layer made of a conductive carbon material on at least a part of the surface thereof.   3. The positive electrode active material according to claim 2, wherein the coating layer is formed at a ratio of 20 parts by weight or less with respect to 100 parts by weight of the main component of the positive electrode active material. _{4. The} positive electrode active material according to claim 1, wherein M in the compound represented by the general formula Li _x Fe _{1 -y My} PO ₄ is Mn. 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 the positive electrode active material according to any one of claims 1 to 4,
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.   6. The aqueous lithium secondary battery according to claim 5, wherein the aqueous electrolyte solution has a pH of 6 to 10.

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