JP5050346B2 - Water-based lithium secondary battery - Google Patents

Description

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

  Lithium secondary batteries using non-aqueous electrolytes have already been put to practical use in the fields related to information and communication equipment such as personal computers and mobile phones because they can achieve high voltage and high energy density, and can be reduced in size and weight. . In addition, lithium secondary batteries are expected to expand to power sources mounted on electric vehicles and hybrid electric vehicles in order to cope with resource problems and environmental problems.

Generally, a non-aqueous lithium secondary battery is a combination of a lithium transition metal composite oxide as a positive electrode active material, a carbon material as a negative electrode active material, and a non-aqueous electrolyte obtained by dissolving a lithium salt in an organic solvent. (See Patent Documents 1 and 2).
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. In a non-aqueous lithium secondary battery, a high electromotive force of 3 to 4 V class can be exhibited 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.
In other words, non-aqueous lithium secondary batteries are flammable as electrolytes and contain non-aqueous electrolytes such as highly volatile organic solvents, so there is always a risk of ignition or explosion. ing. In particular, there is a problem in safety when, for example, overheating due to overcharging or damage to the battery due to pressure increase or impact occurs.
Such a problem is considered to be fatal particularly in an application requiring a large battery such as an electric vehicle or a hybrid vehicle. 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.

  In addition, 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. Also from such a viewpoint, the non-aqueous lithium secondary battery has a problem that it is difficult to cope with mass production in the future, particularly in view of a secondary battery for an electric vehicle.

  On the other hand, there is an aqueous lithium secondary battery using an aqueous solution as an electrolytic solution (see Patent Documents 3 to 5). Since the aqueous lithium secondary battery does not contain an organic solvent in the electrolytic solution, it basically does not burn. 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, water-based lithium secondary batteries have the advantage of lower internal resistance than non-aqueous lithium secondary batteries. However, on the other hand, water-based lithium secondary batteries are required to be used in a potential range where no electrolysis reaction of water occurs, so that the electromotive force is lower than that of non-aqueous lithium secondary batteries.

  Therefore, in the water based lithium secondary battery, safety, cost, and low internal resistance are ensured at the expense of high voltage, that is, high energy density. For this reason, water-based lithium secondary batteries are not suitable for applications such as portable devices that emphasize high energy density, that is, light and small, but are relatively cost-conscious and require electric vehicles that require large batteries. It is expected to be suitable for applications such as hybrid electric vehicles and eventually household distributed power supplies.

  To date, water-based lithium secondary batteries include positive electrodes in which a positive electrode active material such as lithium iron phosphate is adhered to a positive electrode current collector made of aluminum, stainless steel, titanium, or the like, and negative electrode active materials such as lithium vanadium oxide. A battery containing a negative electrode formed by adhering a substance to a negative electrode current collector made of aluminum, stainless steel, titanium or the like has been developed (see Patent Documents 3 to 5). In particular, since a positive electrode current collector and a negative electrode current collector made of aluminum have a large gas generation overvoltage, a battery having a large electromotive force can be formed. Therefore, aluminum has been particularly suitably used as a current collector material for the water based lithium secondary battery.

  However, the current collector made of aluminum may be dissolved in an alkaline aqueous electrolyte. For this reason, in an aqueous lithium secondary battery using a current collector made of aluminum, it is necessary to control the pH of the aqueous electrolyte so that it does not become alkaline. However, in general, in a water based lithium secondary battery, a vanadium compound such as lithium vanadium oxide is preferably used as a negative electrode active material taking advantage of its capacity and stability in an aqueous solution. In the negative electrode active material, there is a problem that vanadium is easily eluted in the aqueous electrolyte solution by charging and discharging, and as a result, the pH of the aqueous electrolyte solution tends to be alkaline. Therefore, it is difficult to control the pH of the aqueous electrolyte, and in a water-based lithium secondary battery using a current collector made of aluminum, aluminum is easily dissolved, and battery performance such as capacity and charge / discharge efficiency is likely to deteriorate. There was a problem

JP-A-8-78054 Japanese Patent Laid-Open No. 11-238527 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 an aqueous lithium secondary battery having high charge / discharge efficiency and capable of exhibiting high charge / discharge efficiency even in an alkaline aqueous electrolyte. It is something to try.

The present invention includes a positive electrode, a negative electrode, and an aqueous electrolyte solution obtained by dissolving a lithium salt in water, and a rocking chair type aqueous lithium secondary battery using Li ions as movable ions.
The aqueous electrolyte solution has a pH in the range of 4 to 12,
The positive electrode is formed by binding a positive electrode mixture containing a positive electrode active material to a positive electrode current collector,
The positive electrode active material is mainly composed of lithium iron phosphate having an olivine structure having a basic composition of LiFePO ₄ ,
The positive electrode current collector, Ri Do nickel alloy mainly composed of nickel or nickel,
The negative electrode is formed by binding a negative electrode mixture containing a negative electrode active material to a negative electrode current collector,
The negative electrode active material is composed mainly of lithium vanadium oxide,
The negative electrode current collector is an aqueous lithium secondary battery characterized by comprising stainless steel (Claim 1).

  In the water based lithium secondary battery of the present invention, the positive electrode current collector made of nickel or a nickel alloy containing nickel as a main component has high electronic conductivity. Therefore, the water based lithium secondary battery can exhibit high charge / discharge efficiency. In addition, the positive electrode current collector can be prevented from being dissolved in the alkaline aqueous electrolyte solution. Therefore, the aqueous lithium secondary battery can be stably charged and discharged in an aqueous electrolyte solution having a wider pH range.

  By the way, in general, in a water based lithium secondary battery, it is necessary to operate in a potential range in which electrolysis of water does not occur. When calculated from the electrolysis voltage of water, the electromotive force is limited to about 1.2V. However, with regard to electrolysis with gas generation, there is a gas generation overvoltage, and in fact, gas generation does not occur even if the theoretical decomposition potential is exceeded, or there are very few states even if it occurs. . Therefore, even in the water-based lithium secondary battery, an overvoltage is actually required in order to generate gas by electrolysis, so that a battery exceeding 1.2 V can be configured.

  In the aqueous lithium secondary battery of the present invention, the positive electrode current collector made of nickel or a nickel alloy can exhibit a relatively large gas generation overvoltage. Therefore, the positive electrode can be operated at a potential higher than the theoretical electrolysis potential of water. As a result, the water based lithium secondary battery can exhibit a relatively high battery voltage without generating oxygen gas from the positive electrode current collector.

  Thus, according to the present invention, it is possible to provide an aqueous lithium secondary battery that has high charge / discharge efficiency and can exhibit high charge / discharge efficiency even in an alkaline aqueous electrolyte solution.

Next, a preferred embodiment of the present invention will be described.
The water-based lithium secondary battery includes, for example, a positive electrode and a negative electrode that occlude / release lithium, a separator that is sandwiched between them, an aqueous electrolyte that moves lithium between the positive electrode and the negative electrode, and the like as main components. Can be configured.
The positive electrode active material is mainly composed of lithium iron phosphate having an olivine structure having a basic composition of LiFePO ₄ . The above-mentioned “having a basic composition” includes not only the composition represented by the composition formula but also those obtained by substituting a part of sites such as Li and Fe in the crystal structure with other elements. Means. Furthermore, it means not only those having a stoichiometric composition but also those having a non-stoichiometric composition in which some elements such as Li are deficient.

  The positive electrode is made of nickel or a nickel alloy containing nickel as a main component, for example, a positive electrode mixture in which a conductive material and a binder are mixed with the positive electrode active material, and an appropriate solvent is added as necessary. It can be formed by applying and drying on the surface of the positive electrode current collector, and compressing to increase the electrode density as necessary.

  The conductive material is for ensuring the electrical conductivity of the positive electrode. For example, one or more of carbon powder materials such as carbon black, acetylene black, natural graphite, artificial graphite, and coke are mixed. 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 polypropylene, polyethylene, polyethylene terephthalate, or the like. A thermoplastic resin or a polyacrylonitrile-based polymer can be used. In addition, an aqueous dispersion of a cellulose-based or styrene-butadiene rubber that is an aqueous binder can also 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.

  Similarly to the positive electrode, the negative electrode is made of, for example, a negative electrode active material mixed with a conductive material or a binder, and an appropriate solvent is added as necessary to form a paste. It can be formed by applying and drying on the surface of a negative electrode current collector made of SUS) mesh, aluminum foil, nickel foil or the like, and then compressing to increase the electrode density as necessary.

The negative electrode is preferably formed by binding a negative electrode mixture containing a negative electrode active material to a negative electrode current collector, and the negative electrode current collector is preferably made of nickel or a nickel alloy containing nickel as a main component .
In this case, the charge / discharge efficiency of the water based lithium secondary battery can be further improved.

  In addition, as the negative electrode active material, a material in which Li insertion / desorption occurs at a lower potential than the positive electrode active material can be used. Such materials can be examined by cyclic voltammetry measurements. Specifically, for example, it can be examined as follows.

That is, first, a desired material that is a candidate for a negative electrode active material, a conductive material, and a binder are mixed to prepare a mixed powder, and then pressed onto a SUS mesh to prepare a sample electrode. Next, cyclic voltammetry is performed using an electrolytic solution for evaluation such as a saturated LiNO ₃ aqueous solution, a reference electrode such as a silver-silver chloride electrode, and a counter electrode such as platinum wire (φ0.3 × 5; coiled). The measurement can be performed at a constant scanning speed using a tripolar beaker cell. In the obtained cyclic voltammogram, a substance that is reversible at a higher potential than the positive electrode active material can be used as the negative electrode active material.

Examples of such a negative electrode active material include oxides of metals such as vanadium, iron, titanium, and manganese, hydroxides of these metals, and composite oxides of these metals and lithium. More specifically, the oxide of vanadium includes VO _{2 and the} like, and the composite oxide of vanadium and lithium includes LiV ₃ O ₈ and LiV ₂ O _{4 and the} like.

Preferably, the negative electrode active material is mainly composed of a lithium vanadium oxide having a spinel structure having LiV ₂ O ₄ as a basic composition .
In this case, the electromotive force of the aqueous lithium secondary battery can be further improved by taking advantage of the low redox potential of the lithium vanadium oxide having a basic composition of LiV ₂ O ₄ . In addition, the charge / discharge capacity can be improved.
Further, in the lithium vanadium oxide, vanadium (V) may elute into the aqueous electrolyte solution and shift the pH of the aqueous electrolyte solution to the alkaline side. However, in the aqueous lithium secondary battery, As described above, since the positive electrode current collector is stable even in the alkaline aqueous electrolyte solution, the lithium vanadium oxide can be suitably used as the negative electrode active material. That is, when the lithium vanadium oxide is used as the negative electrode active material, in addition to being able to improve electromotive force and charge / discharge capacity, excellent charge / discharge efficiency can be obtained even in an alkaline aqueous electrolyte. The effect of the present invention that it can be exhibited can be exhibited more remarkably.

  As the conductive material that can be used by mixing with the negative electrode active material, the same carbon material powder as in the case of the positive electrode can be used. As the binder, a fluorine-containing resin, a thermoplastic resin, a polyacrylonitrile-based polymer, a water-based binder, or the like can be used as in the positive electrode. As a solvent for dispersing the negative electrode active material, the conductive material, and the binder, for example, an organic solvent such as N-methyl-2-pyrrolidone can be used.

  Moreover, the separator to be narrowly attached to the positive electrode and the negative electrode is for separating the positive electrode and the negative electrode and holding the aqueous electrolyte solution. For example, a thin microporous film such as cellulose, polyethylene, or polypropylene can be used.

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 the 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.

The aqueous electrolyte solution has a pH in the range of 4-12.
When the pH of the aqueous electrolyte solution is less than 4, the lithium iron phosphate, which is the main component of the positive electrode active material, becomes unstable, and the battery capacity may be reduced. In addition, the charge / discharge cycle characteristics may be deteriorated, and the capacity may be greatly reduced due to repeated charge / discharge. On the other hand, when the pH exceeds 12, since the electrolysis potential of water, that is, the hydrogen generation potential and the oxygen generation potential are lowered, oxygen may be easily generated at the positive electrode. More preferably, the pH of the aqueous electrolyte solution is 6-10.

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.
The aqueous lithium secondary battery includes, for example, an electrode body in which the separator is sandwiched between the positive electrode and the negative electrode in a battery case having a predetermined shape, and the positive electrode current collector and the negative electrode current collector. The body can be electrically connected to the positive electrode external terminal and the negative electrode external end via lead wires, the electrode body is impregnated with the aqueous electrolyte solution, and the battery case is sealed.

Example 1
In this example, a metal suitable as a current collector for an aqueous lithium secondary battery is examined. Specifically, a tripolar beaker cell is prepared using Ni foil, Al foil, Pt foil, Ti foil, SUS foil, and Cu foil as test electrodes, and cyclic voltammetry measurement is performed.
That is, first, Ni foil, Al foil, Pt foil, Ti foil, SUS foil, and Cu foil were prepared as test electrodes. Each test electrode has a foil shape of 20 mm × 20 mm. In addition, a saturated LiNO ₃ aqueous solution was prepared as an electrolytic solution for evaluation, a silver-silver chloride (AgCl / Ag) electrode was prepared as a reference electrode, and a platinum wire (φ0.3 × 5 mm; coiled) was prepared as a counter electrode.

Next, a test electrode and a counter electrode are arranged in a container containing an electrolyte 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. . In this manner, six types of beaker cells were produced using Ni foil, Al foil, Pt foil, Ti foil, SUS foil, or Cu foil as test electrodes.
Next, in each beaker cell, a voltage was applied between the test electrode and the counter electrode. The voltage is applied by sweeping the potential at a scan 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 set to -1.3 V to One cycle was changed in the range of 2.0 V at the maximum, 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.

In FIG. 1, the potential at which the current value (vertical axis) shifts from 0 to the positive direction (horizontal axis) is the generation potential of oxygen gas. A potential at which the current value (vertical axis) shifts from 0 to a negative direction (horizontal axis) is a hydrogen gas generation potential. As can be seen from the figure, the nickel foil has the second highest oxygen generation potential after the platinum foil and the aluminum foil, that is, the oxygen gas generation overvoltage is large. Since platinum is very expensive and aluminum is eluted in an alkaline aqueous solution, a positive electrode current collector in an aqueous lithium secondary battery having an aqueous electrolyte solution is preferably a nickel current collector. I understand. Further, the nickel foil can be used as a negative electrode current collector because it is not much different from other current collectors except for aluminum in terms of hydrogen gas generation overvoltage.
As described above, the reason why nickel has a relatively large gas generation overvoltage is not clear, but it is thought that the reason is that the surface is passivated in the atmosphere. Note that this passive film is very thin, and other active materials are passed over the current collector and break through the passive film by pressurization during the subsequent electrode molding, so there is no problem in collecting the active material. it is conceivable that.

Further, in this example, a test electrode was produced by bringing LiFePO ₄ as an active material into close contact with a current collector made of nickel foil, and a tripolar beaker cell using the test electrode was produced.
Specifically, first, LiFePO ₄ having an olivine structure was synthesized as follows.
That is, first, iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), ammonium dihydrogen phosphate ((NH ₄ ) ₂ HPO ₄ ), and lithium hydroxide (LiOH.H ₂ O) Got ready. Next, iron oxalate, ammonium dihydrogen phosphate, and lithium hydroxide were blended according to the stoichiometric ratio of the composition formula LiFePO ₄ and mixed for 30 minutes using an automatic mortar. The resulting mixture was calcined at 350 ° C. for 5 hours under an argon gas atmosphere. Thereafter, the mixture was mixed for 30 minutes in an automatic mortar, and further baked at a temperature of 650 ° C. for 6 hours under an argon atmosphere, thereby obtaining LiFePO ₄ having an olivine structure.

Next, 100 parts by weight of LiFePO ₄ as an active material, 11.8 parts by weight of acetylene black as a conductive agent, and polyvinylidene fluoride as a binder (trade name “KF polymer” manufactured by Kureha Chemical Industries) 9 parts by weight was mixed, and 60-80 parts by weight of N-methyl-2-pyrrolidone as a solvent was further added and kneaded to prepare a paste-like electrode mixture. Next, 10 mg of this electrode mixture was pressure-bonded onto a current collector made of nickel foil to prepare a test electrode. In addition, an LiNO ₃ aqueous solution having a concentration of 3 mol / L was prepared as an electrolytic solution for evaluation, a silver-silver chloride (AgCl / Ag) electrode as a reference electrode, and a platinum wire (φ0.3 × 5 mm; coiled) as a counter electrode. Prepared each.

  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 One cycle was changed in the range of 1.3 V at the maximum, and this cycle was performed by performing a total of 3 cycles. The current value at this time was monitored. The result is shown in FIG.

As is known from FIG. 2, a test electrode in which an active material made of LiFePO ₄ having an olivine structure is in close contact with a current collector made of nickel has a reversible amount of Li in a potential range of 0.3 V to 1.0 V. Insertion and detachment can occur. Further, as shown in FIG. 1 described above, the nickel current collector hardly generates oxygen gas up to about 1.4V. Therefore, it can be seen that a positive electrode in which a positive electrode active material made of LiFePO ₄ is adhered to a current collector made of nickel is suitable for an aqueous lithium secondary battery.

(Example 2)
In this example, a water based lithium secondary battery using a Ni foil as a positive electrode current collector is produced and its charge / discharge efficiency is evaluated. In this example, evaluation is performed using a simple battery cell having a polyethylene bag as a battery case as a water based lithium secondary battery.
As shown in FIG. 3, the aqueous lithium secondary battery 1 of this example is a rocking chair type battery using Li ions as movable ions, and an aqueous solution formed by dissolving a positive electrode 2, a negative electrode 3, and a lithium salt in water. An electrolyte solution. The pH of the aqueous electrolyte is in the range of 4-12. The positive electrode 2 is formed by binding a positive electrode mixture containing a lithium iron phosphate (LiFePO ₄ ) positive electrode active material having an olivine structure to a positive electrode current collector made of nickel.
The negative electrode 3 is formed by binding a negative electrode active material mainly composed of spinel lithium vanadium oxide (LiV ₂ O ₄ ) to a negative electrode current collector.

  Further, as shown in the figure, the aqueous lithium secondary battery 1 of this example has a positive electrode 2 and a negative electrode 3 placed in a battery case 5 made of a polyethylene bag with a separator 4 sandwiched therebetween. This is a simple battery cell. In FIG. 3, the positive electrode 2, the negative electrode 3, and the separator 4 are illustrated as being shifted from each other for the convenience of drawing, but in actuality, in order to prevent the positive electrode 2 and the negative electrode 3 from contacting each other. The separator 4 is inserted between the two so as to completely separate them.

Next, the 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 were produced, and a positive electrode and a negative electrode were produced using these.
As the positive electrode active material, olivine-structured lithium iron phosphate (LiFePO ₄ ) was produced in the same manner as in Example 1. This LiFePO ₄ (positive electrode active material), acetylene black (conductive material), polyvinylidene fluoride (binder), and N-methyl-2-pyrrolidone (solvent) are blended in the same proportions as in Example 1. A paste-like positive electrode mixture was prepared.
Next, 10 mg of the positive electrode mixture was applied to one side of a 20 μm-thick positive electrode current collector made of nickel foil and dried. After that, by densifying with a roll press, cutting, and stripping the tab part, as shown in FIG. 3, the shape that the tab part 25 protrudes to the application part of the positive electrode mixture with a width of 20 mm and a length of 20 mm as shown in FIG. A positive electrode 2 was prepared. In addition, the adhesion amount of the positive electrode active material was about 7.0 mg / cm ² per side.

Next, a spinel structure lithium vanadium oxide (LiV ₂ O ₄ ) was synthesized as a negative electrode active material as follows.
That is, first, lithium carbonate (Li ₂ CO ₃ ) and vanadium pentoxide (V ₂ O ₅ ) were blended in a stoichiometric ratio so as to be LiV ₂ O ₄ and mixed in an automatic mortar for 20 minutes. To 100 parts by weight of the mixture, 2 parts by weight of carbon powder ketjen black (TB-5500, manufactured by Tokai Carbon) was added and further mixed for 20 minutes in an automatic mortar. Next, the obtained mixture was baked at a temperature of 750 ° C. for 24 hours in an argon stream and quenched to obtain LiV ₂ O ₄ .

100 parts by weight of this LiV ₂ O ₄ (negative electrode active material), 11.8 parts by weight of acetylene black (conductive material), 5.9 parts by weight of polyvinylidene fluoride (binder), and N-methyl-2-pyrrolidone (Solvent) 60-80 parts by weight were blended to prepare a paste-like negative electrode mixture. Next, 10 mg of a negative electrode mixture was applied to one side of a 20 μm-thick negative electrode current collector made of SUS340 and dried. After that, as in the case of the positive electrode described above, by densifying with a roll press, cutting, and stripping the tab part, as shown in FIG. 3, the negative electrode composite material having a width of 20 mm and a length of 20 mm is applied to the application part. A negative electrode 3 having a shape such that the tab portion 35 protruded was produced. In addition, the adhesion amount of the negative electrode active material was about 7.0 mg / cm ² per side.

  Next, as shown in FIG. 3, the positive electrode 2 and the negative electrode 3 produced as described above were opposed to each other through a cellulose-based separator 4 and placed in a battery case 5 made of a polyethylene bag. Further, a lithium nitrate aqueous solution (pH = 7.0) having a concentration of 3 mol / L as an aqueous electrolyte solution is added to such an extent that the separator 4 is completely impregnated. I stopped. In this way, an aqueous lithium secondary battery 1 was produced. This is referred to as a battery E1.

In this example, a water based lithium secondary battery in which the negative electrode current collector of the battery E1 was changed to a current collector made of Ni foil was produced. This is designated as a reference battery X.
The reference battery X is a battery produced in the same manner as the battery E1 except that the negative electrode current collector was changed to a 20 μm thick Ni foil.

  In this example, an aqueous lithium secondary battery containing an aqueous electrolyte solution having a pH of 12 was prepared as the electrolyte solution. This is designated as battery E3. The aqueous electrolyte solution of the battery E3 was prepared by adding LiOH to the saturated lithium nitrate aqueous solution of pH 7 used as the electrolytic solution in the battery E1 and adjusting the pH to 12. The battery E3 is a battery produced in the same manner as the battery E1 except that an electrolyte solution having a pH of 12 was used as the aqueous electrolyte solution.

Further, in the present embodiment, as a ratio較用to prepare six types of aqueous lithium secondary battery using a positive electrode current collector made of a metal other than Ni foil (battery C1~ cell C6).
Battery C1 and battery C2 are water based lithium secondary batteries manufactured using aluminum foil as a positive electrode current collector.
The battery C1 is a battery produced in the same manner as the battery E1 except that an aluminum foil having a thickness of 20 μm was used as the positive electrode current collector.
The battery C2 is the same as the battery E1 except that an aluminum foil having a thickness of 20 μm is used as the positive electrode current collector and an aqueous electrolyte solution having a pH of 12 similar to the battery E3 is used as the aqueous electrolyte solution. The battery produced as described above.

Next, the battery C3 and the battery C4 are water based lithium secondary batteries manufactured using SUS340 foil as a positive electrode current collector.
The battery C3 is a battery produced in the same manner as the battery E1 except that a SUS340 foil having a thickness of 20 μm was used as the positive electrode current collector.
The battery C4 is the same as the battery E1 except that a SUS340 foil having a thickness of 20 μm is used as the positive electrode current collector and an aqueous electrolyte solution having a pH of 12 similar to the battery E3 is used as the aqueous electrolyte solution. The battery produced as described above.

Next, the battery C5 and the battery C6 are water based lithium secondary batteries manufactured using a Ti foil as a positive electrode current collector.
The battery C5 is a battery produced in the same manner as the battery E1, except that a 10 μm thick Ti foil was used as the positive electrode current collector.
The battery C6 is the same as the battery E1 except that a 10 μm thick Ti foil is used as the positive electrode current collector and an aqueous electrolyte solution having a pH of 12 similar to the battery E3 is used as the aqueous electrolyte solution. The battery produced as described above.

Next, a charge / discharge test was performed on each of the batteries (battery E1 , reference battery X, battery E3, and batteries C1 to C6) manufactured as described above.
In the charge / discharge test, each battery was charged to a charge upper limit voltage of 1.4 V with a constant current of a charge current density of 0.5 mA / cm ² under a temperature of 20 ° C., and the charge capacity at this time was determined. . After charging, after resting for 1 minute, the battery was discharged at a constant current of a discharge current density of 0.5 mA / cm ² to a discharge lower limit voltage of 0.1 V, and the discharge capacity at this time was determined. The charging capacity (mAh / g) is obtained by dividing the value obtained by multiplying the charging current value (mA) by the time (hr) required for charging by the weight (g) of the positive electrode active material LiFePO ₄ in the battery. This can be calculated. Similarly, the discharge capacity (mAh / g) is obtained by multiplying the discharge current value (mA) by the time (hr) required for discharge by the weight (g) of the positive electrode active material LiFePO ₄ in the battery. It can be calculated by dividing. Table 1 shows the charge capacity and discharge capacity of each battery.
Moreover, charge / discharge efficiency was computed from the result of the charge capacity and discharge capacity of each battery based on the formula of charge / discharge efficiency (%) = discharge capacity (mAh / g) / charge capacity (mAh / g) × 100. The results are shown in Table 1.

As is known from Table 1, the battery E1 , the reference battery X, and the battery E3 produced using Ni foil as the positive electrode current collector are batteries C1 to C6 that use a metal other than Ni as the positive electrode current collector. It can be seen that the charge / discharge efficiency is superior. Moreover, when the reference battery X and the battery E1 are compared, it can be seen that the charge / discharge efficiency of the reference battery X is slightly higher than that of the battery E1. This is presumably because the reference battery X uses a nickel foil excellent in electronic conductivity not only in the positive electrode current collector but also in the negative electrode current collector. Further, in the battery E3, since an alkaline (pH = 12) aqueous electrolyte solution is used, the oxygen generation overvoltage is lowered and the efficiency is slightly lowered. However, the battery E3 showed higher charge / discharge efficiency than the battery C2, the battery C4, and the battery C6 that contained an alkaline aqueous electrolyte and used other positive electrode current collectors made of Al, SUS340, and Ti. .

  Further, it can be seen that the battery C1 using the aluminum foil as the positive electrode current collector can obtain the highest charge capacity due to the high oxygen generation overvoltage of the positive electrode current collector made of the aluminum foil. However, when the pH is shifted to alkaline, the capacity and charge / discharge efficiency are greatly reduced (see battery C2 in Table 1). On the other hand, when nickel is used for the positive electrode current collector, even when an alkaline aqueous electrolyte solution is used (battery E3), the decrease in capacity is small and high charge / discharge efficiency can be exhibited.

( Reference example )
In this example, an 18650 type cylindrical battery having a wound electrode is manufactured.
As shown in FIG. 4, the aqueous lithium secondary battery 6 of this example has a cylindrical shape, and includes a positive electrode 61, a negative electrode 62, a separator 63, a gasket 64, a battery case 7, and the like. The battery case 7 is a 18650-type cylindrical battery case, and includes a cap 71 and an outer can 72. In the battery case 7, a sheet-like positive electrode 61 and a negative electrode 62 are arranged in a state of being wound together with a separator 63 sandwiched therebetween, and a wound electrode is formed.

Further, a gasket 64 is disposed inside the cap 71 of the battery case 7, and an aqueous electrolyte is injected into the battery case 7.
A positive electrode current collecting lead 611 and a negative electrode current collecting lead 621 are provided on the positive electrode 61 and the negative electrode 62 by welding, respectively. The positive electrode current collector lead 611 is connected to the positive electrode current collector tab 612 disposed on the cap 71 side by welding. Further, the negative electrode current collecting lead 621 is connected to the negative electrode current collecting tab 622 disposed at the bottom of the outer can 72 by welding.
Further, a saturated lithium nitrate aqueous solution (pH≈7) is used as the aqueous electrolyte solution, and the aqueous electrolyte solution is injected into the battery case 7.

In producing the water-based lithium secondary battery 6 of this example, first, a paste-like positive electrode mixture and negative electrode mixture were produced in the same manner as the battery E1 of Example 2. That is, the positive electrode mixture contains manganese spinel (LiFePO ₄ ) as the positive electrode active material, and the negative electrode mixture contains LiV ₂ O ₄ as the negative electrode active material. The positive electrode mixture and the negative electrode mixture were applied to both surfaces of a positive electrode current collector and a negative electrode current collector made of a nickel foil having a thickness of 20 μm, and dried. Thereafter, the positive electrode current collector is made dense with a roll press and coated with the positive electrode mixture, cut into a shape with a width of 54 mm and a length of 500 mm, and the negative electrode current collector coated with the negative electrode mixture with a width of 56 mm, The sheet was cut into a shape having a length of 520 mm, and a sheet-like positive electrode 61 and a negative electrode 62 were produced.

Thereafter, a part of the positive electrode mixture was scraped off from the end of the coating portion where the positive electrode mixture was applied in the positive electrode 61 so that the final length of the coated portion of the positive electrode mixture was 450 mm. Similarly, for the negative electrode, the length of the coating portion of the final negative electrode mixture in the negative electrode was set to 500 mm.
In addition, the adhesion amount of the positive electrode active material and the negative electrode active material was about 7 mg / cm ² for each side.

  Next, the positive electrode current collector lead 611 and the negative electrode current collector lead 621 were welded to the sheet-like positive electrode 61 and negative electrode 62 obtained as described above, respectively. Next, the positive electrode 61 and the negative electrode 62 were wound in a state where a polyethylene separator 63 having a width of 58 mm and a thickness of 25 μm was sandwiched between them, to produce a spiral wound electrode.

  Subsequently, the wound electrode was inserted into an 18650-type cylindrical battery case 7 including an outer can 72 and a cap 71. At this time, the positive electrode current collecting lead 611 is connected to the positive electrode current collecting tab 612 disposed on the cap 71 side of the battery case 7 by welding, and the negative electrode current collecting lead is disposed on the negative electrode current collecting tab 622 disposed on the bottom of the outer can 72. 621 was connected by welding.

  Next, the battery case 7 was impregnated with a saturated lithium nitrate aqueous solution (pH≈7) as an aqueous electrolyte solution. The gasket 64 was disposed inside the cap 71 and the cap 71 was disposed in the opening of the outer can 72. Subsequently, the battery case 7 was hermetically sealed by caulking the cap 71, and the aqueous lithium secondary battery 6 was produced.

In the aqueous lithium secondary battery 6 of this example, as in the battery E1 , the reference battery X, and the battery E3 of Example 2, a positive electrode active material made of LiFePO ₄ was adhered to a positive electrode current collector made of nickel foil. It has a positive electrode. Therefore, the aqueous lithium secondary battery 6 of this example has high charge / discharge efficiency, and can exhibit high charge / discharge efficiency even in an alkaline aqueous electrolyte solution.

The diagram which shows the cyclic voltammogram in the aqueous solution electrolyte of the collector which consists of various metals concerning Example 1. FIG. According to the second embodiment, the diagram showing a cyclic voltammogram of an electrode made in close contact with the active material composed of LiFePO ₄ in a current collector made of nickel foil. Explanatory drawing which shows the structure of the water based lithium secondary battery (simple battery cell) concerning Example 2. FIG. Explanatory drawing which shows the structure of the water based lithium secondary battery (18650 type cylindrical battery) concerning a reference example .

Explanation of symbols

1 (6) Aqueous lithium secondary battery 2 (61) Positive electrode 3 (62) Negative electrode 4 (63) Separator

Claims (1)

In a rocking chair type aqueous lithium secondary battery having a positive electrode, a negative electrode, and an aqueous electrolyte obtained by dissolving a lithium salt in water, and using Li ions as movable ions,
The aqueous electrolyte solution has a pH in the range of 4 to 12,
The positive electrode is formed by binding a positive electrode mixture containing a positive electrode active material to a positive electrode current collector,
The positive electrode active material is mainly composed of lithium iron phosphate having an olivine structure having a basic composition of LiFePO ₄ ,
The positive electrode current collector, Ri Do nickel alloy mainly composed of nickel or nickel,
The negative electrode is formed by binding a negative electrode mixture containing a negative electrode active material to a negative electrode current collector,
The negative electrode active material is composed mainly of lithium vanadium oxide,
An aqueous lithium secondary battery , wherein the negative electrode current collector is made of stainless steel .

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