JP2005071936A - Water-based lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a water-based lithium secondary battery having an excellent charge/discharge cycle characteristic. <P>SOLUTION: This water-based lithium secondary battery 1 has: a positive electrode 2 containing a substance allowing reversible insertion and desorption of Li as a positive electrode active material; a negative electrode 3 containing a substance wherein potential when storing and desorbing lithium is lower than the positive electrode active material as a negative electrode active material; and an aqueous electrolytic solution obtained by dissolving a lithium salt into water. In the water-based lithium secondary battery 1, the positive electrode 2 contains a metal powder as a conductive auxiliary. Preferably, the metal powder is an aluminum powder, and the pH of the aqueous electrolytic solution is 6-14. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an aqueous lithium secondary battery having an aqueous electrolyte solution obtained by dissolving a lithium salt in water as an electrolyte solution.

  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 equipment and communication equipment, it has become widely popular. In other fields, the development of electric vehicles has been urgently caused by environmental problems and resource issues, and the use of such lithium secondary batteries as power sources for electric vehicles is being studied.

As this non-aqueous lithium secondary battery, conventionally, a positive electrode containing carbon as a conductive aid has been used.
In addition, nonaqueous lithium secondary batteries using metal powders such as aluminum, tantalum, and stainless steel as conductive assistants have also been developed (see Patent Document 1 and Patent Document 2).

On the other hand, there is an aqueous lithium secondary battery using an aqueous electrolyte as an electrolyte.
Compared to non-aqueous lithium secondary batteries, this water-based lithium secondary battery is extremely difficult to burn because it does not contain an organic solvent, and it does not require a dry environment at the time of manufacturing, so that the manufacturing cost is reduced. Can do. Furthermore, in general, an aqueous electrolyte has higher conductivity than a non-aqueous electrolyte, and thus has an advantage of lower internal resistance than a non-aqueous lithium secondary battery. Therefore, the water-based lithium secondary battery is particularly expected as a power source for electric vehicles.

However, water-based lithium secondary batteries have a short history of research, and there are few reports on conductive aids used for positive electrodes.
In addition, although the water-based lithium secondary battery is expected as a power source for automobiles as described above, it has not reached a practical stage.
That is, the water-based lithium secondary battery has a problem that oxygen is generated at the positive electrode when the battery voltage increases. For this reason, the water-based lithium secondary battery has a problem that the discharge capacity tends to be reduced by repeatedly performing charge / discharge, and further improvement of charge / discharge cycle characteristics is desired.
JP-A-8-78054 Japanese Patent Laid-Open No. 11-238527

  This invention is made | formed in view of this conventional problem, Comprising: It aims at providing the water-system lithium secondary battery excellent in charging / discharging cycling characteristics.

The present invention includes a positive electrode containing a material capable of reversibly inserting and desorbing Li as a positive electrode active material, and a material having a lower potential than that of the positive electrode active material when inserting and extracting lithium as the negative electrode active material. In an aqueous lithium secondary battery having a negative electrode and an aqueous electrolyte obtained by dissolving a lithium salt in water,
The positive electrode is an aqueous lithium secondary battery characterized by containing a metal powder as a conductive auxiliary agent (claim 1).

The most notable point in the aqueous lithium secondary battery of the present invention is that the positive electrode contains a metal powder as a conductive additive.
Therefore, in the water based lithium secondary battery of the present invention, the capacity is hardly deteriorated even if charging / discharging is repeated, and the charge / discharge cycle characteristics are excellent.

Next, the reason why the aqueous lithium secondary battery of the present invention produces excellent charge / discharge cycle characteristics as described above will be described in comparison with the case where carbon is contained as a conductive additive.
In general, in an aqueous lithium secondary battery, water is used as a solvent for an electrolytic solution, so that oxygen gas is generated when the battery voltage increases. However, in practice, in order to increase the battery capacity as much as possible, and to increase the battery voltage as much as possible, the battery voltage to be used is set to be high in view of the oxygen gas generation overvoltage.

  Therefore, particularly when the electrolyte is alkaline, the environment in which the oxygen gas is relatively easily generated is generated in the positive electrode when fully charged. Therefore, when carbon or the like is used as a conductive additive, it is considered that carbon that is easily oxidized is gradually oxidized and disappears. As a result, it is considered that by repeatedly charging and discharging, the resistance inside the battery increases, and the discharge capacity tends to deteriorate.

On the other hand, in the present invention, as described above, the positive electrode contains metal powder such as aluminum, tantalum, and stainless steel as a conductive auxiliary agent. This metal powder is less oxidized than carbon and has high electron conductivity.
Therefore, in the water based lithium secondary battery of the present invention, it is possible to suppress a decrease in charge / discharge capacity when charge / discharge is repeated.

  Thus, according to the present invention, it is possible to provide an aqueous lithium secondary battery having excellent charge / discharge cycle characteristics.

The water based lithium secondary battery of the present invention (Claim 1) contains a substance capable of reversibly inserting and removing Li as the positive electrode active material.
Examples of such a positive electrode active material include a composite oxide of lithium and a transition metal, that is, a lithium manganese composite oxide having a layered structure, a lithium manganese composite oxide having a spinel structure, a lithium manganese composite oxide having an olivine structure, and a deficient type. A lithium manganese composite oxide having a layered structure, lithium iron phosphate having an olivine structure, or the like can be used.
Note that the lithium-manganese composite oxide having the deficient layer structure is Li _2-x MnO _3-y having a basic structure of a single crystal layer structure Li ₂ MnO ₃ . This is because Mn becomes tetravalent or less due to oxygen deficiency and becomes electrochemically active and can be reversibly charged / discharged, including those in which a part of Mn is substituted with Fe, Ni, and Co.

Moreover, the said positive electrode contains a metal powder as a conductive support agent.
As such a metal powder, for example, a powder of aluminum, tantalum, stainless steel or the like can be used. These metal powders can be used alone or in combination of two or more.
Preferably, aluminum or stainless steel powder is used.
Aluminum and stainless steel have a stable oxide film layer on the outermost surface, so that they are very difficult to oxidize and have excellent electron conductivity. Therefore, when these are used as a conductive additive for the positive electrode, the charge / discharge cycle characteristics of the aqueous lithium secondary battery can be further improved.

Particularly preferably, the metal powder is an aluminum powder.
In this case, the manufacturing cost can be further reduced, and the battery can be reduced in weight.

Next, the water based lithium secondary battery contains a negative electrode active material having a lower potential than the positive electrode active material when lithium is inserted and extracted.
As such a negative electrode active material, for example, an oxide or hydroxide containing a metal such as vanadium, iron, titanium, or manganese, or a composite oxide of these metal and lithium can be used. Specifically, for example, LiV ₂ O ₄ , LiV ₃ O ₈ , VO ₂ , FeOOH, or the like can be used.

  Further, as the negative electrode active material, a carbon material capable of inserting and extracting lithium ions can be used. Examples of such a carbon material include natural or artificial graphite, mesocarbon microbeads (MCMB), a fired organic compound such as phenol resin, and a powdery material such as coke.

Moreover, the said aqueous 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 SO ₄ . These lithium salts can be used alone or in combination of two or more.

Moreover, it is preferable that pH of the said aqueous solution electrolyte is 6-14.
When the pH of the aqueous electrolytic solution is less than 6, the ionic conductivity of the aqueous electrolytic solution is lowered, so that the internal resistance of the battery may be increased.

  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 the positive electrode active material with the conductive additive and binder, and adding a suitable solvent as necessary to obtain a paste-like positive electrode mixture. And it can compress and form so that an electrode density may be raised as needed.

The binding agent plays a role of anchoring the active material particles and the conductive auxiliary particles, for example, a fluorine-containing resin such as polytetrafluoroethylene, polyvinylidene fluoride, fluororubber, or polypropylene, polyethylene, polyethylene terephthalate, etc. These thermoplastic resins can be used.
For example, an organic solvent such as N-methyl-2-pyrrolidone can be used as a solvent for dispersing these active materials, conductive assistants, and binders.

In the same way as the positive electrode, for example, the negative electrode active material is mixed with a conductive additive or a binder, and if necessary, an appropriate solvent is added to form a paste-like negative electrode mixture. It can be formed by pressing accordingly.
As the conductive additive for the negative electrode, for example, a mixture of one or more carbon material powders such as carbon black, acetylene black, and graphite can be used. Further, the same metal powder as the above-described positive electrode conductive additive can also be used.

  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, a positive electrode containing aluminum powder as a conductive additive is prepared, and its suitability as an electrode for an aqueous lithium secondary battery is examined. That is, first, a positive electrode containing aluminum powder as a conductive additive is prepared, and a non-aqueous lithium secondary battery is prepared using the positive electrode. Next, by examining the relationship between the voltage and capacity of the non-aqueous lithium secondary battery, the suitability of the aluminum powder as a conductive assistant for the aqueous lithium secondary battery is evaluated.

Specifically, first, a positive electrode active material was synthesized as follows.
That is, LiH ₂ PO ₄ as a Li source, MnCO ₃ as a Mn source, and FeC ₂ O ₄ .2H ₂ O as an Fe source were prepared, and the molar ratios of Li, P, Mn, and Fe were 1: 1 respectively. These were mixed at a ratio of 0.5: 0.5. Mixing was performed for 1 hour using an automatic mortar.

After mixing, the mixture was calcined at 250 ° C. for 12 hours in an Ar atmosphere, further mixed in an automatic mortar for 1 hour, and then baked at 750 ° C. for 24 hours in an Ar atmosphere. As a result, LiMn _0.5 Fe _0.5 PO ₄ , which is a lithium manganese composite oxide having an olivine structure, was obtained. This lithium manganese composite oxide is hereinafter referred to as iron-substituted manganese olivine as appropriate.

Next, using the iron-substituted manganese olivine produced as described above as a positive electrode active material, and further using aluminum powder as a conductive additive, a lithium secondary battery having a non-aqueous electrolyte solution was produced, and charge / discharge thereof was performed. The capacity was examined.
Specifically, first, 70 parts by weight of iron-substituted manganese olivine as a positive electrode active material, 25 parts by weight of aluminum powder as a conductive additive, and 5 parts by weight of polyethylene terephthalate as a binder are mixed to form an electrode mixture. Was made. As the aluminum powder, scale particles having a purity of 99.9% and a diameter of 5 μm were used.
Subsequently, 10 mg of the electrode mixture was formed into a 10 mm pellet, which was used as the positive electrode. This positive electrode is designated as sample E.

Next, metallic lithium was prepared as a negative electrode. Further, as an electrolytic solution, a solution having a concentration of 1M was prepared by dissolving PF ₆ in a mixed solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1: 1.
Next, the positive electrode (sample E) and the negative electrode prepared as described above were placed in a battery case for a CR2016 type coin cell via a polyethylene separator having a thickness of 25 μm.

  Further, a gasket was disposed at the end of the battery case, and an appropriate amount of the above electrolyte was injected into the battery case for impregnation. Subsequently, the battery case was sealed to produce a non-aqueous lithium secondary battery. This is referred to as a battery E.

Next, for comparison with the battery E, a positive electrode (sample C) using carbon black as a conductive additive for the positive electrode was prepared, and a non-aqueous lithium secondary battery was manufactured using the positive electrode. This is referred to as a battery C.
This battery C was produced in the same manner as the battery E except that a positive electrode (sample C) containing carbon black as a conductive additive was used.

Next, the battery E and the battery C were each charged in a constant current at a current density of 0.2 mA / cm ^{2 to} a charge upper limit voltage of 4.5 V in a constant temperature bath at a temperature of 20 ° C., and then a constant temperature bath at 20 ° C. The battery was discharged at a constant current up to a discharge lower limit voltage of 2.5 V at a current density of 0.2 mA / cm ² . At this time, the voltage and capacity of each battery were measured, and a voltage-capacity curve for Li was prepared. This is shown in FIG. In FIG. 1, the vertical axis represents the potential with respect to Li, and the horizontal axis represents the capacity.

  As can be seen from FIG. 1, in the battery E and the battery C, it was found that there was almost no difference in the capacity up to 4.2 V regarded as the oxygen gas generation potential considering the overvoltage, and the initial characteristics were almost the same. . In addition, the battery E and the battery C can exhibit a very high capacity exceeding 150 mAh / g by the oxygen gas generation potential (4.2 V), and the electrodes used in the battery E and the battery C are electrolyte solutions. As a result, it was found to be suitable for an aqueous lithium secondary battery having an aqueous electrolyte solution.

(Example 2)
In this example, two types of aqueous lithium secondary batteries (battery Ea and battery Ca) having an aqueous electrolyte solution were prepared using the two types of positive electrodes (sample E and sample C) prepared in Example 1, respectively. This is an example of evaluating the characteristics.
In this example, a CR2016 type coin cell was produced as an aqueous lithium secondary battery as shown in FIG.

As shown in FIG. 2, an aqueous lithium secondary battery 1 (battery Ea) of this example includes a positive electrode 2 containing LiMn _0.5 Fe _0.5 PO ₄ , which is a substance capable of reversibly inserting and removing Li, as a positive electrode active material. The negative electrode 3 containing LiV ₂ O ₄ as a negative electrode active material, which has a lower potential when inserting and extracting lithium, than the positive electrode active material, and LiNO ₃ as a lithium salt are dissolved in water. An aqueous electrolyte solution. The positive electrode 2 contains aluminum powder as a conductive additive.

  Further, in the water based lithium secondary battery 1 of this example, the separator 4 is disposed in a CR2016 type battery case 11 together with the positive electrode 2 and the 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 is sealed with a sealing plate 12.

Next, a method for producing the water based lithium secondary battery will be described.
First, as the positive electrode, the same sample E as in Example 1 was prepared.
As the negative electrode, first, 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 additive, and 5 parts by weight of polyethylene terephthalate as a binder were mixed to form a negative electrode. A composite material was produced, and 10 mg of this negative electrode composite material was formed into a 10 mm diameter pellet.

Note that LiV ₂ O ₄ has a charge / discharge potential with respect to Li / Li ^{+ in the} vicinity of 2.4 V, that is, within a potential range in which hydrogen generation does not occur due to electrolysis of water. Therefore, this LiV ₂ O ₄ can reversibly absorb and desorb a large amount of lithium ions in a potential range where hydrogen is not generated, and is suitable as a negative electrode active material for an aqueous lithium secondary battery. is there.

As an aqueous electrolyte, a LiOH aqueous solution (pH≈14) having a concentration of 1 M was prepared by dissolving LiOH in water.
Next, as shown in FIG. 2, a polyethylene separator 4 having a thickness of 25 μm was sandwiched between the positive electrode 2 and the negative electrode 3, and this was placed in a CR2016 type battery case 11.

  Further, the gasket 5 was disposed at the end in the battery case 11, and an appropriate amount of the above aqueous electrolyte solution was injected into the battery case 11 and impregnated. Subsequently, the sealing plate 12 was disposed, and the end portion of the battery case 11 was caulked, whereby the battery case 11 was sealed to produce the aqueous lithium secondary battery 1. This is designated as battery Ea.

Further, in this example, in order to clarify the excellent characteristics of the battery Ea, a positive electrode containing carbon black as a conductive additive, that is, the sample C prepared in Example 1 was used as a positive electrode for comparison. An aqueous lithium secondary battery was prepared. This is referred to as a battery Ca.
This battery Ca is produced in the same manner as the battery Ea except that a positive electrode (sample C) containing carbon black as a conductive additive is used.

Next, a charge / discharge cycle test was performed on the battery Ea and the battery Ca.
In the charge / discharge cycle test, each battery was charged to a battery voltage of 1.4 V at a constant current of 0.5 mA / cm ² under a temperature of 60 ° C., and then the current density of 0.5 mA / cm ^2. Charging / discharging to a battery voltage of 0.1 V at a constant current of 1 cycle was performed, and this cycle was repeated 50 times. In each charge / discharge cycle, after charging to 1.4 V and after discharging to 0.1 V, a charging pause time and a discharge pause time were provided for 1 minute each. And the discharge capacity of each battery (battery Ea and battery Ca) 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 ( Calculated by dividing by g).
The result is shown in FIG.
In FIG. 3, the horizontal axis indicates the number of cycles (times), and the vertical axis indicates the discharge capacity (mAh / g).
The initial discharge capacities of the batteries Ea and Ca are shown in Table 1 below.

  As is known from Table 1, both the battery Ea and the battery Ca showed a high initial discharge capacity exceeding 60 mAh / g. Further, as is known from Table 1, the battery Ea and the battery Ca showed the initial discharge capacity of almost the same size.

Next, as is known from FIG. 3, the battery Ea maintains a discharge capacity of about 90% of the initial discharge capacity even after 50 cycles of charge and discharge, and has a high discharge capacity after the cycle test. Was maintained.
On the other hand, in the battery Ca, after 50 cycles of charging and discharging, the discharge capacity was reduced to about 78% of the initial discharge capacity, and the discharge capacity after the cycle test was reduced to about 50 mAh / g.

From the results of this example, it can be seen that the charge / discharge cycle characteristics of the water-based lithium secondary battery can be improved by using aluminum powder, that is, powder that is not easily oxidized and has high electron conductivity, as the positive electrode conductive additive.
In this example, an aqueous lithium secondary battery was fabricated using aluminum powder as the metal powder of the conductive additive, and its characteristics were evaluated. However, instead of aluminum powder, powder such as stainless steel or titanium was used. Similar results can be obtained with the

FIG. 3 is an explanatory diagram showing the relationship between the capacity and potential of each battery according to Example 1; Sectional drawing which shows the structure of the water based lithium secondary battery concerning Example 2. FIG. Explanatory drawing which shows the result of the charging / discharging cycle test of each battery concerning Example 2. FIG.

Explanation of symbols

1 Water-based lithium secondary battery 2 Positive electrode 3 Negative electrode 4 Separator

Claims (3)

A positive electrode containing a material capable of reversibly inserting and desorbing Li as a positive electrode active material; a negative electrode containing a material having a lower potential than the positive electrode active material as a negative electrode active material when inserting and extracting lithium; In an aqueous lithium secondary battery having an aqueous electrolyte solution obtained by dissolving a lithium salt in water,
The positive electrode contains a metal powder as a conductive additive, and is a water based lithium secondary battery.   2. The aqueous lithium secondary battery according to claim 1, wherein the metal powder is an aluminum powder.   3. The aqueous lithium secondary battery according to claim 1, wherein the aqueous electrolyte solution has a pH of 6 to 14. 4.

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