JP5256649B2 - Water-based lithium secondary battery - Google Patents

Description

  The present invention relates to a battery active material used as an active material of a battery such as a water based lithium secondary battery, and a water based lithium secondary battery using the battery active material.

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. Configured.

  On the other hand, there is an aqueous lithium secondary battery using an aqueous solution as an electrolytic solution. Since the water-based lithium secondary battery does not require a dry environment like a non-aqueous lithium secondary battery in the manufacturing process, the manufacturing cost can be greatly reduced. Furthermore, since aqueous electrolytes generally have higher electrical conductivity than non-aqueous electrolytes, aqueous lithium secondary batteries have the advantage of lower internal resistance than non-aqueous lithium secondary batteries.

However, on the other hand, an aqueous lithium secondary battery is required to be used in a potential range where no electrolysis reaction of water occurs, and therefore, an electromotive force is smaller than that of a non-aqueous lithium secondary battery.
When calculated from the electrolysis voltage of water, the limit of electromotive force is about 1.2V. However, in reality, overvoltage is necessary to generate gas by electrolysis, so about 2V is expected to be the limit. Is done.

  Thus, in the water based lithium secondary battery, cost reduction and reduction of internal resistance can be achieved 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.

What is important for the construction of an aqueous lithium secondary battery is that it is stable in an aqueous solution and has an activity capable of reversibly occluding and desorbing a large amount of lithium in a potential range where oxygen and hydrogen are not generated by electrolysis of water. The substance is that an active material that can exhibit a large capacity in a specific potential range is used.
Further, it is desired to use a neutral to alkaline electrolyte as the electrolyte. Li-containing oxides mainly used as an active material generally have poor stability in an acidic aqueous solution, and a large amount of H ⁺ ions in an acidic electrolyte may inhibit the rocking chair reaction of pure Li ⁺ ions. Because there is.

In the case of using a neutral, ie, pH = 7 electrolyte, the water decomposition voltage is such that the hydrogen generation potential is 2.62 V (vs. Li ⁺ / Li) and the oxygen generation potential is 3.85 V (vs. Li). ⁺ / Li). In addition, when an electrolytic solution having strong alkalinity, that is, pH = 14 is used, the water decomposition voltage has a hydrogen generation potential of 2.21 V (vs. Li ⁺ / Li) and an oxygen generation potential of 3.44 V (vs. Li ⁺ / Li).
In the water-based lithium secondary battery, as the positive electrode active material, a material that contains Li and is charged by extracting Li, that is, a material that increases the potential is preferable. On the other hand, as the negative electrode active material, a material whose potential is reduced by inserting Li is suitable.
In addition, since the potential width of the aqueous lithium secondary battery is smaller than that of the non-aqueous lithium secondary battery, both the positive electrode and the negative electrode have flat potential curves in order to increase the energy density as much as possible. It is desired to use an active material.

So far, LiMn ₂ O ₄ , LiFePO _{4 and the} like have been proposed as positive electrode active materials used in water-based lithium secondary batteries (see Patent Documents 1 to 3). These positive electrode active materials are relatively stable in an aqueous solution and can realize a relatively high capacity.

On the other hand, manganese oxides, iron oxides, iron oxide hydroxides, vanadium oxides, titanium-based polyanion compounds, and the like have been proposed as negative electrode active materials (see Patent Documents 3 to 10 and Non-Patent Document 1).
Among these, the most promising material is considered to be vanadium oxide as the negative electrode active material of the aqueous lithium secondary battery. In particular, LiV ₂ O ₄ having a spinel structure can cause stable reversible insertion / extraction of Li in a potential range where water does not decompose, and can exhibit a relatively high capacity in an aqueous electrolysis solution. it can.

However, lithium vanadium oxides such as LiV ₂ O ₄ contain vanadium (V) designated as a rare metal storage target metal. Therefore, the negative electrode active material using lithium vanadium oxide has a problem that it is difficult to cope with mass production and the manufacturing cost increases. In addition, many compounds containing vanadium as a main component are designated as poisonous or deleterious substances, which may increase the environmental load.
In addition, the charge / discharge cycle characteristics of an aqueous lithium secondary battery using a lithium vanadium oxide such as LiV ₂ O ₄ are not yet sufficient, and there is a problem that the capacity tends to decrease when charge / discharge is repeated.

JP-T 9-508490 JP 2002-260722 A JP 2002-110221 A JP 2000-340256 A JP 2000-77073 A Japanese Patent Laid-Open No. 2001-102086 JP 2002-208403 A JP 2003-17057 A JP 2005-158604 A JP 2006-66085 A H. Wang et al., “Electrochimica Acta”, UK, February 15, 2007, Vol. 52, No. 9, p. 3280-3285

  The present invention has been made in view of such conventional problems, and has a low environmental load, can be applied to an aqueous lithium secondary battery having an aqueous electrolyte, and exhibits excellent charge / discharge cycle characteristics. It is an object of the present invention to provide a battery active material that can be manufactured and a water based lithium secondary battery using the battery active material.

The first invention is mainly composed of a titanium pyrophosphate compound having a composition formula of (TiO) ₂ P ₂ O ₇ ,
As an active material for an aqueous lithium secondary battery having an aqueous electrolyte obtained by dissolving a lithium salt in water, or a non-aqueous lithium ion secondary battery having a non-aqueous electrolyte obtained by dissolving a lithium salt in an organic solvent It is used for the active material material for batteries characterized by being used (Claim 1).

The active material for a battery according to the first aspect of the invention is mainly composed of a titanium pyrophosphate compound having a composition formula of (TiO) ₂ P ₂ O ₇ . The titanium pyrophosphate compound can stably insert and desorb Li in an aqueous solution, and can perform Li insertion and desorption at a potential of about 2.2 V with respect to the Li ⁺ / Li electrode. it can. This potential of about 2.2 V (vs. Li ⁺ / Li) is lower than the hydrogen generation potential (2.62 V (vs. Li ⁺ / Li)) at pH 7 described above. Is possible.
Therefore, the battery active material can exhibit excellent stability in an aqueous solution. Therefore, the battery active material can be suitably used, for example, as a negative electrode active material of an aqueous lithium secondary battery having an aqueous electrolyte, and a relatively stable aqueous lithium secondary battery can be realized.

  When the battery active material is used as, for example, a negative electrode active material of a water-based lithium secondary battery, it is possible to suppress a decrease in capacity when charging and discharging are repeated, and to improve a capacity retention rate. The battery active material can also be applied to a lithium secondary battery having a non-aqueous electrolyte solution obtained by dissolving a lithium salt in an organic solvent.

Moreover, the said active material for batteries does not use elements, such as vanadium (V) with a large environmental load, as an essential component element in the composition. Therefore, when the battery active material is used as a battery active material, the environmental load can be reduced.
The titanium pyrophosphate compound does not contain an expensive element such as vanadium as an essential component element. Therefore, by using the battery active material, the cost of the battery can be reduced.

A second invention is a water based lithium secondary battery comprising a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and an aqueous electrolyte obtained by dissolving a lithium salt in water.
The negative electrode active material is mainly composed of a titanium pyrophosphate compound having a composition formula of (TiO) ₂ P ₂ O ₇ ,
The positive electrode active material is a water based lithium secondary battery characterized in that a main component is a material that inserts and desorbs Li at a noble potential from the titanium pyrophosphate compound.

In the water based lithium secondary battery of the second invention, the negative electrode active material is mainly composed of a titanium pyrophosphate compound having a composition formula of (TiO) ₂ P ₂ O ₇ . The titanium pyrophosphate compound can stably insert and desorb Li in an aqueous solution, and can insert and desorb Li at a very low potential of about 2.2 V with respect to the Li ⁺ / Li electrode. It can be carried out. This potential of 2.2 V is very close to the limit potential for not accompanied by hydrogen generation when the hydrogen generation overvoltage is taken into account, so that the battery voltage can be made relatively high in the aqueous lithium secondary battery. Thus, a high output battery can be realized. Moreover, the said water-system lithium secondary battery can repeat charging / discharging comparatively stably, can suppress the fall of the capacity | capacitance when charging / discharging is repeated, and can improve a capacity | capacitance maintenance factor.

Moreover, the said titanium pyrophosphate compound does not have vanadium (V) with a large environmental load in the composition as an essential component. Therefore, the environmental load can be reduced in the water based lithium secondary battery.
The titanium pyrophosphate compound does not contain an expensive element such as vanadium as an essential component element. Therefore, cost reduction of the water based lithium secondary battery can be achieved.

Next, a preferred embodiment of the present invention will be described.
In the first aspect of the invention, the battery active material may be composed mainly of a titanium pyrophosphate compound having a basic composition of (TiO) ₂ P ₂ O ₇ . The above-mentioned “having as a basic composition” means that not only the composition represented by the composition formula but also those obtained by substituting a part of sites such as Ti in the crystal structure with other elements. To do. Furthermore, it means that not only a stoichiometric composition but also a non-stoichiometric composition in which some elements are deficient or the like is included.

  The battery active material is, for example, an aqueous lithium secondary battery having an aqueous electrolyte obtained by dissolving a lithium salt in water, or a nonaqueous lithium having a nonaqueous electrolyte obtained by dissolving a lithium salt in an organic solvent. It can be used as an active material such as an ion secondary battery.

Preferably, the active material for battery material may be used as the active material of the aqueous lithium secondary battery (claim 2).
In this case, an aqueous lithium secondary battery having a high capacity retention rate when repeatedly charging and discharging can be realized by taking advantage of the excellent stability of the battery active material in an aqueous solution.

The battery active material is preferably used as a negative electrode active material.
In this case, a battery having a relatively large output can be realized.

Next, in the second invention, the water based lithium secondary battery includes, for example, a positive electrode and a negative electrode that occlude / release lithium, a separator sandwiched between them, and moves lithium between the positive electrode and the negative electrode. An aqueous electrolyte or the like can be configured as a main component.
The negative electrode contains the negative electrode active material mainly composed of a titanium pyrophosphate compound having a basic composition of (TiO) ₂ P ₂ O ₇ . As the negative electrode active material, the battery active material of the first invention can be used.

  For the negative electrode, for example, a negative electrode mixture obtained by mixing a conductive material and a binder with the negative electrode active material and adding a suitable solvent as necessary to form a paste or a slurry, for example, stainless steel (SUS) mesh, aluminum It can form by apply | coating to the surface of the negative electrode collector which consists of foil, nickel foil, etc., drying, and compressing so that an electrode density may be increased as needed.

  Further, the conductive material is for ensuring the electrical conductivity of the negative electrode. For example, one or more carbon powder powders such as carbon black, acetylene black, natural graphite, artificial graphite, and cokes are used. 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 or water can be used.

  In the same way as in the case of the negative electrode, for example, the positive electrode mixture obtained by mixing a positive electrode active material with a conductive material or a binder and adding a suitable solvent as necessary to form a paste is made of stainless steel ( It can be formed by applying and drying on the surface of a positive electrode current collector made of SUS) mesh, aluminum foil, nickel foil or the like, and compressing to increase the electrode density as necessary.

  The positive electrode active material is mainly composed of a material that can insert and desorb Li at a potential more noble than the titanium pyrophosphate compound. Such materials can be examined by cyclic voltammetry measurements. Specifically, for example, it can be examined as follows.

That is, first, a desired substance that is a candidate for a positive 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 evaluation electrolytic solution such as a ₃ mol / LLiNO 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 shows reversible insertion and desorption of Li at a higher potential than the titanium pyrophosphate compound can be used as the positive electrode active material.

It is preferable that the positive electrode active material is mainly composed of a material capable of inserting and desorbing Li at a potential no less than 0.5 V higher than that of the titanium pyrophosphate compound.
In this case, the battery voltage of the water based lithium secondary battery can be further improved.

It is preferable that the positive electrode active material contains a compound having an olivine structure, a spinel structure, or a layered structure as a main component.
In this case, the stability of the positive electrode active material can be improved, and the charge / discharge efficiency of the aqueous lithium secondary battery can be improved.

The positive electrode active material is preferably composed mainly of a lithium iron phosphate compound having an olivine structure having a basic composition of LiFePO ₄ or a lithium-manganese composite oxide having a spinel structure having a basic composition of LiMn ₂ O ₄ ( Claim 7).
In the case where the lithium iron phosphate compound is a main component, the cycle characteristics of the water based lithium secondary battery can be further improved by taking advantage of its excellent Coulomb efficiency.
In addition, when the lithium-manganese composite oxide is the main component, the high redox potential can be utilized to increase the battery voltage of the aqueous lithium secondary battery, realizing a higher energy density. can do.
In addition, the positive electrode active material mainly composed of the lithium iron phosphate compound or the lithium-manganese composite oxide contains iron or manganese that is inexpensive and less harmful as an essential transition metal element. The manufacturing cost of the water based lithium secondary battery can be further reduced, and the safety can be further improved.

  In addition, the above-mentioned “having a basic composition” is not limited to the composition represented by the composition formula, but a part of sites such as Li, Fe, Mn, etc. in the crystal structure is substituted with other elements. It is meant to include things. Furthermore, it means that not only a stoichiometric composition but also a non-stoichiometric composition in which some elements are deficient or the like is included.

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

  Further, 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, polyacrylonitrile, polyethylene, or polypropylene can be used. .

Moreover, the said aqueous lithium secondary battery has aqueous solution electrolyte solution formed by melt | dissolving lithium salt in water as electrolyte solution.
The aqueous electrolyte solution preferably has a pH of 3 to 11. (Claim 8)
When the pH of the aqueous electrolyte is less than 3, Li is desorbed from the positive electrode active material and / or the negative electrode active material due to the chemical action of the acidic aqueous solution, and the battery voltage cannot be maintained. There is a fear. Further, in this case, there is a possibility that the insertion / extraction of Li may be inhibited by protons in the aqueous electrolyte solution. On the other hand, when the pH exceeds 11, the oxygen generation potential decreases to about 3.63 V (Li ⁺ / Li). Therefore, there is a possibility that oxygen is easily generated at the positive electrode. More preferably, the pH of the aqueous electrolyte solution is 4-10.

Furthermore, as the lithium salt can be used, for example LiNO _3, LiOH, LiCl, and Li ₂ S and the like. These lithium salts can be used alone or in combination of two or more.

It is preferable that at least lithium nitrate is used as the lithium salt.
In this case, high charge / discharge efficiency can be obtained with little side reaction.

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 terminal via lead wires, the electrode body is impregnated with the aqueous electrolyte solution, and the battery case is sealed.

Example 1
Next, examples of the present invention will be described.
In this example, an active material for a battery containing a titanium pyrophosphate compound ((TiO) ₂ P ₂ O ₇ ) as a main component is produced, and the characteristics as an electrode are evaluated.

First, a titanium pyrophosphate compound ((TiO) ₂ P ₂ O ₇ ) was synthesized as follows.
That is, first, a solution obtained by diluting titanium tetraisopropoxide ([(CH ₃ ) ₂ CHO] ₄ Ti) with 2-propanol in 150 ml of an aqueous solution obtained by diluting phosphoric acid (H ₃ PO ₄ ) with excess water, The solution was added dropwise over about 10 minutes, and the resulting precipitate was collected by filtration. The precipitate was dried at a temperature of 90 ° C. for 24 hours and then calcined in air at a temperature of 1000 ° C. for 24 hours. As a result, an active material for a battery containing a titanium pyrophosphate compound ((TiO) ₂ P ₂ O ₇ ) as a main component was obtained.

Next, in order to investigate the characteristics of the battery active material as an electrode, a test lithium secondary battery (2016 type coin battery) was prepared.
Specifically, first, 70 parts by weight of the battery active material ((TiO) ₂ P ₂ O ₇ ) synthesized as described above, 25 parts by weight of carbon as a conductive agent, and polytetragon as a binder. 5 parts by weight of fluoroethylene (PTFE) was mixed to prepare a mixed powder. Next, a battery case for a 2016-type coin cell was prepared, and 14.3 mg of mixed powder (10 mg of active material) was pressure-bonded at about 0.6 ton / cm ² on a SUS mesh previously welded to the inside of the battery case. Test electrodes were formed.
Next, a metal lithium foil having a diameter of 15 mm was prepared as a counter electrode of the test electrode, and the test electrode and the counter electrode were arranged in a battery case for a 2016 type coin cell through a polyethylene separator. Furthermore, a gasket was disposed at the end of the battery case.

Next, LiPF ₆ is dissolved to a concentration of 1 mol / L in a mixed solvent in which ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) are mixed at a volume ratio of 3: 4: 3. Thus, an electrolytic solution was prepared. An appropriate amount of this electrolytic solution was injected into the battery case and impregnated. Thereafter, the battery case was sealed to produce a test lithium secondary battery. In the production of the lithium secondary battery of this example, all the procedures after the formation of the test electrode described above were performed in a glove box in an Ar atmosphere.

Next, the test electrode of the test lithium secondary battery produced as described above was kept at a constant current up to a predetermined lower limit voltage at a current density of 0.2 mA / cm ² in a constant temperature bath at a temperature of 20 ° C. Reduced and oxidized in a constant temperature bath at a temperature of 20 ° C. with a constant current at a current density of 0.2 mA / cm ^{2 to a} predetermined upper limit voltage. The lower limit voltage was 1.0 V and the upper limit voltage was 3.0 V.
Next, the test electrode of the lithium secondary battery was further reduced with a constant current at a current density of 0.2 mA / cm ^{2 to a} predetermined lower limit voltage, and a voltage-capacity curve at the time of the second reduction was prepared. Further, after the second reduction, oxidation was performed again at a constant current up to a predetermined upper limit voltage at a current density of 0.2 mA / cm ² , and a voltage-capacity curve at this time was prepared. The result is shown in FIG.

That is, FIG. 1 shows the relationship between the potential with respect to Li and the oxidation / reduction capacity for the active material (TiO) ₂ P ₂ O ₇ .
In FIG. 1, the horizontal axis represents capacity (oxidation capacity or reduction capacity), and the vertical axis represents potential (vs. Li / Li ⁺ ) with respect to Li. Further, in FIG. 1, a curve that rises to the right indicates the relationship between the potential and the capacity during oxidation, and a curve that descends to the right indicates the relationship between the potential and the capacity during reduction. The reduction capacity is obtained by dividing the value obtained by multiplying the reduction current value by the time required for reduction (hr) by the weight (g) of the active material (TiO) ₂ P ₂ O ₇ in the battery. It can be calculated. Similarly, the oxidation capacity is obtained by dividing the value obtained by multiplying the oxidation current value by the time (hr) required for oxidation by the weight (g) of the active material (TiO) ₂ P ₂ O ₇ in the battery. Can be calculated.

As can be seen from FIG. 1, the Coulomb efficiency is slightly worse, but (TiO) ₂ P ₂ O ₇ causes reversible Li insertion / extraction in the non-aqueous electrolyte. The behavior of the potential is divided into several regions. The two-phase coexistence region that has only one oxidation-reduction potential, the composition changes continuously with the insertion and removal of Li, and the potential is also Along with this, there were mixed areas and the like. Generally there are three ^{regions, 2.6V (vs.Li + /Li),2.2V(vs.Li + /} Li), and relatively flat potential 1.6V (vs.Li ⁺ / Li) I confirmed that I have.

By the way, in general, in an aqueous lithium secondary battery having an aqueous electrolyte, when an electrolyte with pH = 7 is used, the hydrogen generation potential is 2.62 V (vs. Li ⁺ / Li), and pH = 14. When the electrolytic solution is used, the hydrogen generation potential is 2.21 V (vs. Li ⁺ / Li). That is, hydrogen is generated below this hydrogen generation potential, and Li insertion / extraction does not occur in the aqueous solution. Actually, since there is a gas generation overvoltage, considering the possibility of Li insertion / desorption up to a potential slightly lower than the hydrogen generation potential, from the result of FIG. 1, the above (TiO) ₂ P _It can be seen that ₂ O ₇ has a capacity of about 150 mAh / g in the potential range as the negative electrode active material of the water based lithium secondary battery.

(Example 2)
In this example, an aqueous lithium secondary battery is produced using (TiO) ₂ P ₂ O ₇ produced in Example 1 as a negative electrode active material, and its charge / discharge cycle characteristics are evaluated.
As shown in FIG. 2, the aqueous lithium secondary battery 1 of this example includes a positive electrode 2 containing a positive electrode active material, a negative electrode 3 containing a negative electrode active material, and an aqueous electrolyte obtained by dissolving a lithium salt in water. Have As the negative electrode active material, the battery active material ((TiO) ₂ P ₂ O ₇ ) prepared in Example 1 is used.
The positive electrode 2 contains LiFePO ₄ as a positive electrode active material. Furthermore, water based lithium secondary battery 1, as the aqueous electrolyte solution contains a LiNO ₃ solution of concentration 6 mol / L.

  In the water based lithium secondary battery 1, a separator 4 is disposed in a 2016 type battery case 11 together with a positive electrode 2 and a negative electrode 3 in a state of being sandwiched between them. An aqueous electrolyte solution is injected into the battery case 11. A gasket 45 is disposed at an end in the battery case 11, and the battery case 11 is sealed with a sealing plate 12.

Next, a manufacturing method of the water based lithium secondary battery 1 of this example will be described.
First, LiFePO ₄ as a positive electrode active material is synthesized as follows.
That is, first, lithium carbonate, iron oxalate having a valence of iron, and ammonium dihydrogen phosphate are mixed so that the molar ratio of Li: Fe: P is 1.2: 1: 1, respectively. Mixed. Next, the obtained mixed powder was formed into a pellet and fired at a temperature of 650 ° C. for 24 hours in an argon gas atmosphere. In this way, LiFePO ₄ was obtained. In the synthesis of LiFePO _4, the molar ratio of Li and Fe in LiFePO ₄ obtained 1: whereas a 1, the molar ratio of the above as Li and Fe is 1.2: 1 and In this way, the Li source and the Fe source are mixed. This is because Li was blended in consideration that Li is easily released into the atmosphere at a high temperature.
Further, as the negative electrode active material, the same battery active material ((TiO) ₂ P ₂ O ₇ ) as in Example 1 was produced.

Next, an aqueous lithium secondary battery is produced using the positive electrode active material and negative electrode active material produced as described above.
Specifically, first, 70 parts by weight of the negative electrode active material, 25 parts by weight of carbon black as a conductive agent, and 5 parts by weight of polyethylene terephthalate as a binder were mixed to prepare a negative electrode mixture. This negative electrode mixture 14.3 mg (active material amount 10 mg) was pressure-bonded at about 0.6 ton / cm ² on a SUS mesh previously welded to the inside of the coin cell to form the negative electrode 3 (see FIG. 2).
Further, 70 parts by weight of LiFePO ₄ as a positive electrode active material, 25 parts by weight of carbon black as a conductive agent, and 5 parts by weight of polyethylene terephthalate as a binder were mixed to prepare a positive electrode mixture. 14.4 mg of this positive electrode mixture (active material amount 10 mg) was pressure-bonded at about 0.6 ton / cm ² on a SUS mesh previously welded to the inside of the coin cell to form the positive electrode 2 (see FIG. 2).

Next, as shown in FIG. 2, a battery case 11 for a 2016-type coin cell was prepared, and the positive electrode 2 and the negative electrode 3 were arranged in the battery case 11. At this time, a polyethylene separator 4 was disposed between the positive electrode 2 and the negative electrode 3.
Next, a gasket 45 was placed in the battery case 11, and an appropriate amount of an aqueous electrolyte solution was injected into the battery case 11 and impregnated. In this example, a 6 mol / L LiNO ₃ aqueous solution (pH≈4.5) was used as the aqueous electrolyte solution.
Next, the sealing plate 12 is disposed in the opening of the battery case 11, and the battery case 11 is sealed by caulking the ends of the battery case 11 and the sealing plate 12, thereby producing the aqueous lithium secondary battery 1. did. This is referred to as a battery E.

In this example, for comparison with the battery E, an aqueous lithium secondary battery (battery C) containing LiV ₂ O ₄ as a negative electrode active material was produced. Battery C was produced in the same manner as Battery E, except that LiV ₂ O ₄ was used as the negative electrode active material.
LiV ₂ O ₄ was synthesized as follows.
That is, first, lithium carbonate (Li ₂ CO ₃ ) and vanadium pentoxide (V ₂ O ₅ ) were weighed according to the stoichiometric ratio of the composition formula (LiV ₂ O ₄ ) and mixed in an automatic mortar for 20 minutes. Thereafter, 2 parts by weight of carbon black (TB-5500 manufactured by Tokai Carbon Co., Ltd.) was added to 100 parts by weight of the mixture, and further mixed for 20 minutes in an automatic mortar. The mixture was baked in an argon stream at a temperature of 750 ° C. for 24 hours and then rapidly cooled to obtain LiV ₂ O ₄ .
Using the obtained LiV ₂ O ₄ , a water-based lithium secondary battery was produced in the same manner as the battery E except for the above. This is battery C.

Next, a charge / discharge cycle test was performed on the two types of water-based lithium secondary batteries, the battery E and the battery C.
In the charge / discharge cycle test, each battery (battery E and battery C) was charged to a battery voltage of 1.4 V at a constant current of a current density of 0.1 mA / cm ² at a temperature of 20 ° C. Charging / discharging to discharge the battery voltage to 0.6 V with a constant current of density 0.1 mA / cm ^{2 was} defined as one cycle, and this cycle was repeated 20 times. The initial discharge capacity and the discharge capacity at the 20th cycle were measured. The discharge capacity was calculated by dividing the value 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 in the battery. FIG. 3 shows an initial discharge curve for the battery E1. Further, the capacity retention rate after 20 cycles was calculated from the initial discharge capacity and the 20th cycle discharge capacity. The results are shown in Table 1.

As can be seen from FIG. 3, it can be seen that (TiO) ₂ P ₂ O ₇ can operate as a negative electrode active material of an aqueous lithium secondary battery.
Further, as is known from Table 1, an aqueous lithium secondary battery (battery E) using (TiO) ₂ P ₂ O ₇ as a negative electrode active material is an aqueous lithium secondary battery using LiV ₂ O ₄ as a negative electrode active material. It can be seen that the capacity retention rate of the battery (battery C is improved and excellent charge / discharge cycle characteristics are exhibited.
Therefore, the active material mainly composed of (TiO) ₂ P ₂ O ₇ can be particularly preferably used as the negative electrode active material of the water based lithium secondary battery, and the water based lithium secondary battery using such a negative electrode active material is Excellent charge / discharge cycle characteristics can be exhibited. An active material containing (TiO) ₂ P ₂ O ₇ as a main component has a low environmental load and does not contain an expensive metal element, and thus can be manufactured at low cost.

According to Example 1, graph showing the relationship between the potential and the capacity of the battery active material _{(TiO) 2 P 2 O 7} . FIG. 3 is an explanatory diagram showing a configuration of an aqueous lithium secondary battery according to Example 2. The diagram which shows the relationship between the capacity | capacitance at the time of the first discharge of the water-system lithium secondary battery (battery E) concerning Example 2, and a voltage.

Explanation of symbols

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

Claims (9)

The main component is a titanium pyrophosphate compound having a composition formula of (TiO) ₂ P ₂ O ₇ ,
As an active material for an aqueous lithium secondary battery having an aqueous electrolyte obtained by dissolving a lithium salt in water, or a non-aqueous lithium ion secondary battery having a non-aqueous electrolyte obtained by dissolving a lithium salt in an organic solvent An active material for a battery, characterized by being used . According to claim 1, said active material for battery material, active material for a battery, characterized in that used as the active material of the aqueous lithium secondary battery.   3. The battery active material according to claim 1, wherein the battery active material is used as a negative electrode active material. In an aqueous lithium secondary battery containing a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and an aqueous electrolyte obtained by dissolving a lithium salt in water,
The negative electrode active material is mainly composed of a titanium pyrophosphate compound having a composition formula of (TiO) ₂ P ₂ O ₇ ,
The water-based lithium secondary battery, wherein the positive electrode active material contains as a main component a substance that inserts and desorbs Li at a potential more noble than the titanium pyrophosphate compound.   5. The aqueous lithium secondary material according to claim 4, wherein the positive electrode active material is mainly composed of a material capable of inserting and desorbing Li at a potential of 0.5 V or higher than the titanium pyrophosphate compound. battery.   6. The aqueous lithium secondary battery according to claim 4, wherein the positive electrode active material contains a compound having an olivine structure, a spinel structure, or a layered structure as a main component. 7. The positive electrode active material according to claim 4, wherein the positive electrode active material is an iron phosphate lithium compound having an olivine structure having a basic composition of LiFePO ₄ or lithium-manganese having a spinel structure having a basic composition of LiMn ₂ O _4. An aqueous lithium secondary battery comprising a composite oxide as a main component.   The aqueous lithium secondary battery according to any one of claims 4 to 7, wherein the aqueous electrolyte solution has a pH of 3 to 11.   The aqueous lithium secondary battery according to any one of claims 4 to 8, wherein at least lithium nitrate is used as the lithium salt.

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