JP2000077073A - Water-based lithium-ion battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a water-based lithium-ion battery which is highly safe, low-cost, and operates stably in a neutral aqueous solution. SOLUTION: A positive electrode 1 made of a substance capable of inserting and removing lithium ions, and a chemical formula Li _x V ₃ O
_A battery was composed of a negative electrode 3 made of a compound represented by _y (x = 1 to 1.2, y = 7.9 to 8.2) and an aqueous electrolyte 5 containing lithium ions.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an aqueous lithium-ion battery that operates using an aqueous solution as an electrolyte.

[0002]

2. Description of the Related Art As a high energy density secondary battery, a lithium secondary battery is on the market. In this battery, as disclosed in Japanese Patent Application Laid-Open No. 63-121260, an oxide capable of taking lithium ions in and out of the positive electrode active material at a relatively noble potential is used, and a relatively low potential is applied to the negative electrode active material. The battery is constructed using a non-aqueous solution of a lithium salt as an electrolytic solution by using carbon or metal lithium capable of taking lithium ions in and out of the battery. Usually, in this battery, the oxide as the positive electrode active material has a potential of +3 to +4 V with respect to metallic lithium, and the carbon and the like as the negative electrode active material have a potential of +1 V or less with respect to metallic lithium. Since lithium ions are taken in and out, the battery voltage becomes 3 to 4 V. In order to operate at such a high voltage, a non-aqueous solution that is difficult to electrolyze is used as an electrolyte.

On the other hand, an aqueous battery using an aqueous solution as an electrolytic solution is disclosed in Japanese Patent Publication No. 09-508490. Here, a battery is formed using a composite oxide of lithium and manganese for the positive electrode, a composite oxide of lithium and manganese or vanadium for the negative electrode, and mainly using an alkaline aqueous electrolyte.

[0004]

The non-aqueous solvent used in general lithium batteries always has a risk of ignition or explosion, and has become a major problem in the manufacture and use of batteries. In particular, when the positive and negative electrodes of a battery are short-circuited inside or outside of the battery for some reason, the non-aqueous electrolyte causes a reaction between the non-aqueous electrolyte and the negative electrode or the positive electrode due to rapid temperature rise, and the resulting heat generation causes a further temperature rise. It has been pointed out that, in turn, it triggers other reactions and phase changes one after another, eventually leading to explosive combustion.

Further, as is apparent from the principle of the battery, if even a small amount of water is mixed in the battery, power loss due to the electrolysis reaction of water, consumption of lithium due to reaction with water, and generation of electricity due to the electrolysis occur. This causes extremely serious problems such as an increase in the internal pressure due to the gas and rupture of the battery. for that reason,
In the manufacture of a lithium ion battery, special equipment and a great deal of labor are required to completely remove water, which also contributes to raising the cost of the battery.

[0006] On the other hand, the above-mentioned problem basically does not occur in an aqueous battery using an aqueous solution as an electrolytic solution. However, there is a problem that oxide-based electrode materials generally have poor stability in neutral or acidic aqueous solutions. In particular, since the negative electrode material has a lower oxidation-reduction potential than the positive electrode material, it is susceptible to oxidation by hydrogen ions in water.
Metallic lithium anodes can be said to be the most extreme example. Therefore, in order to obtain a battery that operates stably, it is necessary to use an alkaline electrolyte.

However, in an alkaline aqueous solution, water
Is decomposed to generate oxygen.
Because it moves in a direction that is lower than that,
High noble potential when charging the positive electrode.
The charge / discharge capacity inherent in the cathode material cannot be
It is difficult to make full use of the quantity. For example, pH = 12
The oxygen generation potential of the aqueous solution is + with respect to the standard hydrogen electrode.
0.52V. On the other hand, ordinary cathode materials, such as
For example, LiCoO₂ And LiNiO₂ , LiMn ₂O₄
 Are +3.5 to +4.2 with respect to metallic lithium.
V, ie +0.5 to +1.2 with respect to the standard hydrogen electrode
Oxygen generation in aqueous solution with pH = 12 because it is charged with V
It is extremely difficult to charge without.

Further, since the alkaline aqueous solution is highly corrosive, there is a problem in safety when the liquid leaks to the outside, and the materials of the battery exterior material and the electrode current collector are limited to those having alkali resistance. Will be.

Therefore, according to the present invention, the present invention operates stably not only in an alkaline aqueous solution but also in a neutral aqueous electrolyte.
An object of the present invention is to provide a safe and low-cost aqueous lithium-ion battery. In addition, neutral here means strictly pH
= 7.0, not 6 to 8 in terms of pH.

[0010]

The invention according to claim 1 is
A positive electrode made of a substance capable of inserting and removing lithium ions, and a chemical formula Li _x V ₃ O _y (x = 1 to 1.
2. A water-based lithium-ion battery is characterized by comprising a negative electrode made of a compound represented by the formula (2, y = 7.9 to 8.2) and an aqueous electrolyte containing lithium ions.

The compound used for the negative electrode is an oxide having a composition in the vicinity of LiV ₃ O ₈ , and does not necessarily have to be crystalline, and tetravalent vanadium partially coexists. Take. This compound can repeatedly insert and remove lithium ions with good reproducibility at a potential near -0.3 V or lower than the standard hydrogen electrode. Therefore, +0.5 to +
Combined with a normal cathode material operating at 1.2V,
A 1 to 2 V class battery that can be repeatedly charged and discharged without causing decomposition of water can be configured. Also, when a substance capable of inserting and removing lithium ions at a potential of +0.5 V or less with respect to the standard hydrogen electrode is used for the positive electrode, a battery having a voltage corresponding to that can be obtained.

As the electrolyte, lithium sulfate, lithium chloride, lithium hydroxide, lithium nitrate, lithium acetate,
Lithium borate, lithium phosphate, lithium perchlorate,
Aqueous solutions of various lithium compounds such as lithium borofluoride and lithium phosphorus fluoride can be used. These electrolytes can be used alone or in combination.

According to a second aspect of the present invention, in the battery according to the first aspect, an aqueous solution having a pH of 6 or more is used as the aqueous solution electrolyte.

FIG. 2 shows the results from the aqueous solution at various pH values.
Oxygen evolution potential, hydrogen evolution potential, chemical formula Li_xV₃O_y 
(X = 1-1.2, y = 7.9-8.2)
Lithium ion insertion / extraction potential of compound and typical cathode
LiNiO, one of the materials ₂ Lithium ion insertion and removal
The potentials are shown. As shown in FIG.
= 6 aqueous solution hydrogen generation potential with respect to the standard hydrogen electrode
−0.35 V, so that the lithium
It is almost the same as the potential at which dereaction occurs. But in general
As is known, water actually decomposes to produce hydrogen
Requires an overvoltage, and the equilibrium-derived potential
Hydrogen generation hardly occurs. Therefore, pH =
In the aqueous solution of No. 6, the above-mentioned negative electrode material stably
It is possible to cause an insertion / removal reaction. On the other hand, on the positive electrode side
Has an oxygen evolution potential of +0.88 V in an aqueous solution of pH = 6.
Therefore, considering the overvoltage, the usual cathode material
Material such as LiCoO₂And LiNiO₂, LiMn₂O
₄Can operate without oxygen evolution.

In an aqueous solution having a pH of 6 or more, the hydrogen generation potential moves in a negative direction, so that the chemical formula Li _x V ₃ O
The lithium insertion / removal reaction of the compound represented by _y (x = 1 to 1.2, y = 7.9 to 8.2) does not cause any problem, and the battery of the present invention operates stably. However, the generation of oxygen may occur depending on the selection of the positive electrode material, so that it is not necessarily preferable in terms of capacity.

According to a third aspect of the present invention, as the positive electrode material,
Cobalt, nickel, manganese, vanadium and nio
One or more elements selected from the group consisting of
Claim 1 or Claim 2 using a composite oxide of lithium and lithium.
Claims 2 is to constitute the aqueous lithium ion battery.
Features. These compounds have the formula Li_xV₃O _y
(X = 1-1.2, y = 7.9-8.2)
By combining with a negative electrode made of a compound,
To obtain a water-based lithium-ion battery that operates stably
Can be.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a longitudinal sectional view showing an example of an embodiment of the present invention and schematically showing a structure of an aqueous lithium ion battery. In this battery, a positive electrode having a positive electrode material 1 fixed on a current collector metal 2 and a negative electrode having a negative electrode material 3 fixed on another current collector metal 4 are separated by an aqueous electrolyte 5 containing lithium ions. It is configured. The portion of the aqueous electrolyte 5 may be an aqueous solution itself, or provided with a porous partition wall that allows only ions and a solvent to pass without passing electrons.
The partition wall may be impregnated with an aqueous electrolyte.
The exterior material 6 is for preventing the leakage and evaporation of the liquid and for maintaining the shape of the battery, and may be a closed type or provided with a communication hole with the outside so that the electrolyte can be replenished. It may be.

The positive electrode material 1 is made of a material capable of inserting and extracting lithium ions. In addition, the positive electrode material 1 has a conductivity-imparting agent for increasing conductivity, and has sufficient strength for the current collector metal 2 by maintaining the shape of the electrode. A binder for fixing at is mixed as necessary. As the substance capable of inserting and removing lithium ions, a compound capable of inserting and removing lithium ions at a potential of about 0 to +1.5 V with respect to a standard hydrogen electrode is preferably used. Specifically, a composite oxide of lithium with a transition metal such as cobalt, nickel, manganese, vanadium, or niobium, or an organic substance such as disulfide can be used. As the conductivity-imparting agent, graphite powder or the like is suitably used, and as the binder, polyvinylidene fluoride or polytetrafluoroethylene is preferably used.

As a method of mixing the substances constituting the positive electrode and fixing them on the current collector, a method of mixing and kneading the respective material powders in a mortar or the like, followed by compacting and holding the powder on the current collector, or a method of holding the solvent on the current collector. A method of kneading a material constituting the positive electrode therein to form a paste and then applying the paste on a current collector is used.

The negative electrode material 3 has a chemical formula of Li _x V ₃ O _y (x
= 1 to 1.2, y = 7.9 to 8.2), but as in the case of the positive electrode, a conductivity-imparting agent for increasing the conductivity and the shape of the electrode are maintained. A binder for fixing to the current collector metal 4 with sufficient strength is mixed as necessary. As the conductivity-imparting agent, graphite powder or the like is suitably used, and as the binder, polyvinylidene fluoride or polytetrafluoroethylene is preferably used. Also,
As a method of mixing the substances constituting the negative electrode and fixing the mixed substance on the current collector, the same method as in the case of the positive electrode can be applied.

As the current collector of the positive electrode and the negative electrode, metals such as aluminum, stainless steel, copper, and platinum, and highly conductive solid foils, plates, rods, and nets such as carbon can be used.

Examples of the aqueous electrolyte containing lithium ions include lithium sulfate, lithium chloride, lithium hydroxide, lithium nitrate, lithium acetate, lithium borate, lithium phosphate, lithium perchlorate, lithium borofluoride, and lithium phosphorus fluoride. Of various lithium compounds are used. These electrolytes can be used alone or in combination. The concentration of the electrolyte may be such that lithium ions entering and leaving both electrodes are not depleted in the vicinity of the electrodes, but generally desirably about 1 mol / l or more.

[0024]

Next, specific embodiments of the present invention will be described.

Example 1 Lithium hydroxide and vanadium pentoxide were mixed at a ratio of 2: 3.
After melting at a temperature of 0 ° C., the mixture was quenched to obtain a lithium-vanadium composite oxide. The composition of this oxide is Li _x V
₃ O _y (x = 1 to 1.2, y = 7.9 to 8.2), and it was confirmed that the crystal structure was the same as that of LiV ₃ O ₈ . To 50 parts by weight of this oxide, 45 parts by weight of carbon black and 5 parts by weight of polytetrafluoroethylene were added, mixed in a mortar, and molded into a disk having a diameter of 13 mm under a pressure of 5,000 kg / cm ^2. Was used as a negative electrode. The lithium vanadium composite oxide contained in the negative electrode was 30 mg.

Using lithium carbonate and cobalt oxide,
LiCoO ₂ was synthesized by a well-known method, and a disk-shaped positive electrode was prepared by the same mixing ratio and method as the negative electrode. LiCoO ₂ contained in the positive electrode was 30 mg.

A 1M aqueous solution of Li ₂ SO ₄ was prepared and used as an electrolyte. The pH of this electrolyte was 6.2.
A positive electrode and a negative electrode were sandwiched between platinum nets, respectively, and immersed in the above-mentioned electrolyte to form a battery. The distance between the electrodes was 1 cm.
FIG. 3 is a graph in which a change in voltage when charging / discharging is performed at a constant current of 1 mA is plotted against the composition of the negative electrode. Before charging, the negative electrode had a more noble potential, so the initial value of the first charge graph in FIG. 3 was negative. However, a sufficient amount of lithium ions was inserted in the first charge. Thereafter, a stable charge / discharge curve was obtained in the second and subsequent times. As shown in Table 1, the discharge capacity was slightly small at the first time, but was stable after the second time, and the charge / discharge efficiency defined by the ratio of the discharge capacity to the charge capacity exceeded 70%. In addition, elution of the electrode and the like were not observed even after 10 times of charging and discharging.

[0028]

[Table 1]

Example 2 Using the lithium-vanadium composite oxide synthesized in Example 1, a negative electrode disk was produced in the same manner as in Example 1. The amount of the lithium-vanadium composite oxide contained in the negative electrode was 30 mg.

According to a well-known method, LiNi _0.81 Co
A compound having a composition of _0.19 O ₂ was synthesized, and a disk-shaped positive electrode was prepared in the same manner as in Example 1. LiNi _0.81 Co _0.19 O ₂ contained in the positive electrode was 30 mg.

The positive electrode and the negative electrode are each
Between 1M Li at pH = 6.2 ₂SO₄Soak in aqueous solution
Thus, a battery was configured. The distance between the electrodes was 1 cm. 1
The voltage change when charging and discharging with a constant current of mA
Fig. 4 shows a graph of the results and Table 1 shows the capacitance values.
Shown respectively. The behavior is similar to that of the first embodiment, but the capacity is
Is nearly four times larger as shown in Table 1. this
Indicates that the positive electrode material used in Example 2 is the same as the positive electrode material used in Example 1.
Insertion / removal of lithium ions at a lower potential than pole materials
For this reason, the voltage range of 0.1 to 1.3 V tested in the present example was used.
, More lithium ions can be inserted and removed
Probably the result. This means that the cathode material
That you can increase the capacity even further.
Is shown.

Example 3 The lithium-vanadium composite oxide synthesized in Example 1 was heated at 550 ° C. for 8 hours. Although the composition and crystal structure of the obtained oxide were the same as before firing, the peak intensity of X-ray diffraction was increased about twice and it was confirmed that the crystallinity was high.
This oxide was molded into a disk in the same manner as in Example 1, and this was used as a negative electrode. The amount of the lithium-vanadium composite oxide contained in the negative electrode was 30 mg.

LiNi _0.81 Co synthesized in Example 2
_0.19 O ₂ was molded into a disk in the same manner as in Example 1 to form a positive electrode. LiNi contained in this positive electrode
_{0. 81} Co _0.19 O ₂ was 30 mg.

The positive electrode and the negative electrode were each formed by a platinum net.
Between 1M Li at pH = 6.0 ₂SO₄Soak in aqueous solution
Thus, a battery was configured. The distance between the electrodes was 1 cm.

FIG. 5 is a graph showing the change in voltage when charging / discharging was performed at a constant current of 1 mA with respect to the composition of the negative electrode. The average voltage of charging and discharging is higher than in the case of Example 2, but the capacity is smaller. This is considered to be related to the crystallinity of lithium vanadium oxide.

Example 4 45 parts by weight of the lithium-vanadium composite oxide synthesized in Example 1 and carbon black 4
5 parts by weight, 10% of polyvinylidene fluoride
100 parts by weight of the N-methyl-2-pyrrolidone solution was added and kneaded. To this was added 100 parts by weight of N-methyl-2-pyrrolidone, and the mixture was further kneaded and coated on a stainless steel foil so that the thickness after drying was 0.1 mm, and this was used as a negative electrode. The area to which the negative electrode material was applied was 4 cm ² , and the lithium vanadium oxide contained therein was 15 mg. Similarly, LiNi _0.81 synthesized in Example 2
A mixture of 45 parts by weight of Co _0.19 O _{2 and} 45 parts by weight of carbon black was mixed with 10% N-polyvinylidene fluoride.
100 parts by weight of a methyl-2-pyrrolidone solution is added and kneaded, and 100 parts by weight of N-methyl-2-pyrrolidone is added and further kneaded so that the thickness after drying is 0.1 mm on a stainless steel foil. This was used as a positive electrode.
The area to which the positive electrode material was applied was 3.8 cm ² , and the lithium vanadium oxide contained therein was 15 mg.

The positive electrode and the negative electrode were prepared at pH = 6.2.
Was immersed in a 1 M aqueous solution of Li ₂ SO ₄ to form a battery.
The distance between the electrodes was 1 cm.

Table 1 shows the capacitance values when charging and discharging were performed at a constant current of 1 mA.

Example 5 A positive electrode and a negative electrode were prepared using the same materials and methods as in Example 2, and the same charge and discharge were performed in a 1M aqueous lithium chloride solution having a pH of 6.0. The battery behavior was almost the same as in Example 3. Table 1 shows the capacitance values.

Example 6 A positive electrode and a negative electrode were prepared using the same materials and methods as in Example 2, and the same charge / discharge was performed in a 5% aqueous solution of lithium borofluoride having a pH of 6.1. The battery behavior was almost the same as in Example 3. Table 1 shows capacitance values
Shown in

Example 7 A positive electrode and a negative electrode were prepared using the same materials and techniques as in Example 2, and were repeatedly charged and discharged in a 1M aqueous solution of lithium sulfate. FIG. 6 shows a graph in which the change in the discharge capacity is plotted against the number of repetitions of charge and discharge. The discharge capacity increases by about 20% during the first 10 cycles and then decreases, but maintains about 90% of the initial value even after 30 repetitions of charge / discharge. FIG. 6 also shows the charge / discharge efficiency defined by the ratio of the discharge capacity to the charge capacity, which is an extremely high value of 90% or more except for the first few cycles.

Comparative Example 1 LiMn ₂ O ₄ was synthesized from lithium carbonate and manganese dioxide by a well-known method, and molded into a disk in the same manner as in Example 1 to produce a negative electrode. LiMn ₂ O ₄ contained in the negative electrode was 30 mg. Also, LiNi _0.81 C synthesized in Example 2
o _0.19 O ₂ was molded into a disk in the same manner as in Example 1 to form a positive electrode. LiN contained in this positive electrode
i _0.81 Co _0.19 O ₂ was 30 mg.

The positive electrode and the negative electrode were each
Between 1M Li at pH = 6.3 ₂SO₄Soak in aqueous solution
Thus, a battery was configured. The distance between the electrodes was 1 cm.

FIG. 7 is a graph showing a change in voltage with respect to the charging / discharging time when charging / discharging is repeatedly performed at a constant current of 0.5 mA.

As shown in FIG. 7, the first charging
Then, the voltage does not rise from around 0.8V. this
Means that the current supplied for charging is
The elution of the negative electrode was actually observed.
Was. In this case, even if charging current flows, lithium
Since no on-insertion has occurred, the discharge capacity is almost
Not observed. Although not shown in the figure, the second and third times
Even when charging and discharging the eyes, the situation is almost
They were the same. In the third charge, the voltage is increased to 1.3V.
But this is mainly due to the LiNi
_0.81Co_0.19O ₂Of Lithium Ion from Water
Is thought to be approaching the limit. Negative electrode corrosion still
And no discharge capacity is shown. 5 times
In the eyes, there is no lithium ion that can be desorbed from the positive electrode
And almost no charge capacity or discharge capacity
You.

LiMn ₂ O ₄ is said to be stable as a negative electrode in an alkaline aqueous solution. However, when used as a negative electrode in a neutral aqueous solution as described above, corrosion and elution are severe, and a lithium ion battery that operates stably cannot be obtained.

[0047]

The aqueous lithium ion battery according to the present invention uses an aqueous solution as an electrolyte, so there is no danger of ignition or explosion, and it can be manufactured in air without using special equipment such as a dry box. Since a drying process is not required, a low-cost battery can be manufactured. Further, since the device operates in a neutral solution, it is not necessary to use a highly corrosive alkali, and the capacity can be increased.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view schematically showing a structure of an aqueous lithium ion battery, showing an example of an embodiment of the present invention.

FIG. 2 is an explanatory view showing a relationship between an oxygen generation potential and a hydrogen generation potential from an aqueous solution at various pH values, and a lithium ion insertion / extraction potential of a lithium metal oxide.

FIG. 3 is a graph showing a voltage change during charging and discharging in Example 1 of the present invention with respect to a negative electrode composition.

FIG. 4 is a graph showing a voltage change during charging and discharging in Example 2 of the present invention with respect to a negative electrode composition.

FIG. 5 is a graph showing a voltage change at the time of charging and discharging in Example 3 of the present invention with respect to a negative electrode composition.

FIG. 6 is a graph showing changes in discharge capacity and charge / discharge efficiency with respect to the number of repetitions of charge / discharge in Example 7 of the present invention.

FIG. 7 is a graph showing a change in voltage during charging and discharging in Comparative Example 1 with respect to charging and discharging time.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Positive electrode material 2 Positive electrode current collector metal 3 Negative electrode material 4 Negative electrode current collector metal 5 Aqueous electrolyte 6 Exterior material

Continued on the front page F term (reference) 5H003 AA02 AA08 AA10 BB05 BB12 BD03 BD06 5H014 AA02 EE10 HH01 HH08 5H029 AJ03 AJ12 AJ14 AK03 AL03 DJ09 HJ01 HJ10

Claims (3)

[Claims] 1. A positive electrode comprising a substance capable of inserting and removing lithium ions, and a chemical formula Li _x V ₃ O _y (x
= 1-1.2, y = 7.9-8.2) An aqueous lithium-ion battery comprising a negative electrode comprising a compound represented by the following formula: and an aqueous electrolyte containing lithium ions. 2. The aqueous lithium ion battery according to claim 1, wherein the pH of the aqueous electrolyte is 6 or more. 3. The method according to claim 1, wherein the positive electrode is selected from the group consisting of cobalt, nickel, manganese, vanadium and niobium.
3. The aqueous lithium-ion battery according to claim 1, comprising a composite oxide of one or more kinds of elements and lithium.