JP4983356B2 - 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.

  In general, a lithium secondary battery using a non-aqueous electrolyte (non-aqueous lithium secondary battery) is composed of a lithium transition metal composite oxide as a positive electrode active material, a carbon material as a negative electrode active material, and lithium in an organic solvent as an electrolyte. It is configured using a nonaqueous electrolytic solution obtained by dissolving a salt. A non-aqueous lithium secondary battery can exhibit a high electromotive force of 3 to 4 V class in a single cell.

However, the following problems have been pointed out for non-aqueous lithium secondary batteries.
That is, a non-aqueous lithium secondary battery contains a non-aqueous electrolyte solution such as an organic solvent that has flammability and high volatility as an electrolyte solution, and must be handled with care. In particular, it is necessary to take measures against, for example, overheating due to overcharging or damage to the battery due to pressure increase and impact.
Such a problem is particularly important in applications that require large batteries and are used under harsh conditions, such as electric vehicles and hybrid vehicles.

  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. From this point of view, the non-aqueous lithium secondary battery has a problem that it is difficult to cope with mass production in the future especially for secondary batteries for electric vehicles.

  On the other hand, there is a lithium secondary battery (aqueous lithium secondary battery) using an aqueous solution as an electrolytic solution. 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.

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.

  When a neutral, ie, pH = 7 electrolyte is used, the water decomposition voltage is 2.62V for hydrogen generation potential and 3.85V for oxygen generation potential. In addition, when a strong alkaline, that is, pH = 14 electrolytic solution is used, the water decomposition voltage has a hydrogen generation potential of 2.21 V and an oxygen generation potential of 3.44 V. In water-based lithium secondary batteries, hydrogen generation may occur when the negative electrode is exposed to a potential lower than the hydrogen generation potential, and oxygen generation occurs when the positive electrode is exposed to a potential higher than the oxygen generation potential. May occur. Actually, since there is a gas generation overvoltage, it can be used to a potential slightly outside the range, but as an active material for an aqueous lithium secondary battery, a material having as much charge / discharge capacity within this range as possible is desired. ing.

So far, LiMn ₂ O ₄ , LiFePO _{4 and the} like have been proposed as positive electrode active materials for water-based lithium secondary batteries (see Patent Documents 1 to 3), and Fe-based oxides, Fe A polyanion compound and various lithium vanadium oxides have been proposed (see Patent Documents 4 to 8). Recently, an aqueous lithium secondary battery containing LiMn ₂ O ₄ as a positive electrode active material and containing a titanium-based polyanion as a negative electrode active material has been proposed (see Non-Patent Document 1).

However, the water-based lithium secondary battery having a conventional configuration has insufficient durability, and has a problem in that the capacity is very likely to decrease when charging and discharging are repeated. Specifically, since LiMn ₂ O ₄ has a Li insertion / release potential close to the decomposition potential of water, the potential could not be increased sufficiently to increase the capacity, and charge / discharge was repeated. There was a problem that the capacity was greatly reduced. In addition, LiFePO ₄ has a Li insertion / desorption potential at a potential at which water does not decompose. Therefore, the capacity decrease when charging and discharging is repeated is smaller than that of LiMn ₂ O _4, but it is practical. It was still not enough.

JP-T 9-508490 JP 2002-110221 A JP 2002-260722 A JP 2000-340256 A JP 2002-110221 A Japanese Patent Laid-Open No. 2001-102086 JP 2000-77073 A JP 2003-17057 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 an object of the present invention is to provide an aqueous lithium secondary battery excellent in durability when repeated charging and discharging are performed.

The present invention is a water based lithium secondary battery having a positive electrode, a negative electrode, and an aqueous electrolyte solution obtained by dissolving a lithium salt in water, and using Li ions as mobile ions.
The positive electrode has the general formula Li _a V _b (PO ₄ ) _c X _d (where 0 ≦ a ≦ 4, 1 ≦ b ≦ 2, 1 ≦ c ≦ 3, d is 0 or 1, X is F or OH) Containing a positive electrode active material whose main component is a vanadium phosphate compound represented by
The negative electrode contains a negative electrode active material whose main component is a substance in which insertion and desorption of Li occurs at a lower potential than the vanadium phosphate compound (claim 1). .

  The aqueous lithium secondary battery of the present invention contains the vanadium phosphate compound represented by the general formula as a main component of the positive electrode active material. Therefore, the capacity of the water-based lithium secondary battery is less likely to decrease even when charging and discharging are repeated, and can exhibit cycle durability superior to that of the prior art. Although the reason why the cycle durability is improved by the positive electrode active material has not been elucidated, the positive electrode active material containing the vanadium phosphate compound as a main component is caused by having a three-dimensional Li channel in the crystal structure. Considered as one.

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 of the positive electrode has the general formula Li _a V _b (PO ₄ ) _c X _e (where 0 ≦ a ≦ 4, 1 ≦ b ≦ 2, 1 ≦ c ≦ 3, d is 0 or 1, X is F Or a vanadium phosphate compound represented by OH) as a main component. When a, b, c, and d are out of the above ranges, it is difficult to insert and desorb Li, and the capacity may be reduced.

In the aqueous lithium secondary battery, the positive electrode is formed, for example, by mixing a conductive material and a binder with the positive electrode active material, and adding a suitable solvent as necessary to form a paste-like positive electrode mixture, If necessary, it can be compressed to increase the electrode density.
The conductive material is for ensuring the electrical conductivity of the positive electrode, and for example, a material obtained by mixing one or more carbon material powders such as carbon black, acetylene black, and graphite can be used.

The binder plays a role of connecting the active material particles and the conductive material particles. For example, a fluorine-containing resin such as polytetrafluoroethylene, polyvinylidene fluoride, and fluororubber, or heat such as polypropylene, polyethylene, and polyethylene terephthalate. A plastic resin, a polyacrylonitrile-based polymer, or the like can be used.
As a solvent for dispersing these active material, conductive material, and binder, for example, an organic solvent such as N-methyl-2-pyrrolidone can be used.

  As with the positive electrode, for example, the negative electrode is formed by mixing a negative electrode active material with a conductive material or a binder, adding a suitable solvent as necessary, and forming a paste-like negative electrode mixture, and then as necessary. It can be formed by pressing.

  As the negative electrode active material, a material that causes Li insertion and desorption at a lower potential than the vanadium phosphate compound used as 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, for example, using an electrolytic solution for evaluation such as a 6 mol / L LiNO ₃ aqueous solution, a reference electrode such as a silver-silver chloride electrode, a counter electrode such as a platinum wire (φ0.3 × 5; coiled), cyclic voltammetry I do. 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 lower 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 contains a compound having a spinel structure (claim 4).
In this case, the water based lithium secondary battery can be operated more stably and the durability can be further improved.

Further, the compound having the spinel structure is preferably a LiV ₂ O ₄ (claim 5).
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 LiV ₂ O ₄ (lithium vanadium oxide). In addition, the charge / discharge capacity can be improved.

  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.

Moreover, the said water-system lithium secondary battery has aqueous solution electrolyte solution formed by melt | dissolving lithium salt in water as electrolyte solution.
Examples of such a lithium salt include LiNO ₃ , LiOH, LiCl, and Li ₂ S. These lithium salts can be used alone or in combination of two or more.

Preferably, the lithium salt contains at least lithium nitrate (Claim 3).
In this case, the high amount of lithium nitrate can be utilized to increase the amount of Li ions present in the electrolytic solution. Therefore, the reaction resistance can be reduced, and the range of freedom of electrolyte design can be expanded.

The aqueous electrolyte solution preferably has a pH of 3 to 11. (Claim 2)
When the pH of the aqueous electrolyte is less than 3, the compound represented by the general formula Li _a V _b (PO ₄ ) _c X _d becomes unstable, and the battery capacity and charge / discharge cycle characteristics may be reduced. There is. In addition, the hydrogen generation potential becomes high, and hydrogen generation may occur on the negative electrode. On the other hand, when the pH exceeds 11, since the electrolysis potential of water, that is, the oxygen generation potential is lowered, oxygen may be easily generated at the positive electrode. More preferably, the pH of the aqueous electrolyte solution is 5 to 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.

Example 1
Next, an embodiment of the water based lithium secondary battery of the present invention will be described with reference to FIGS. This example is an example in which a water based lithium secondary battery is manufactured, and the capacity retention rate (charge / discharge cycle characteristics) when the water based lithium secondary battery is repeatedly charged and discharged a plurality of times is evaluated.

As shown in FIG. 1, 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 In this example, the positive electrode 2 contains Li ₃ V ₂ (PO ₄ ) ₃ or LiVPO ₄ F as a positive electrode active material. The negative electrode 3 contains LiV ₂ O ₄ as a negative electrode active material. The aqueous electrolyte solution is obtained by dissolving LiNO ₃ as a lithium salt in water.

  In the water based lithium secondary battery 1, a separator 4 is disposed in a CR2016 type battery case 11 together with a positive electrode 2 and a negative electrode 3 in a state of being sandwiched between them. An aqueous electrolyte solution is injected into the battery case 11. A gasket 45 is disposed at an end in the battery case 11, and the battery case 11 is sealed with a sealing plate 12.

Next, the manufacturing method of the water based lithium secondary battery of this example will be described.
First, Li ₃ V ₂ (PO ₄ ) ₃ as a positive electrode active material is synthesized as follows. In the synthesis, a so-called solid phase synthesis method was performed.
Specifically, first, ammonium dihydrogen phosphate, vanadium pentoxide, and lithium carbonate were mixed in a stoichiometric ratio so as to have a composition of Li ₃ V ₂ (PO ₄ ) ₃ . The mixture was formed into pellets and fired in the atmosphere at a temperature of 300 ° C. for 4 hours. Next, the pellets were pulverized and formed again into pellets, and then fired in a hydrogen stream at a temperature of 800 ° C. for 12 hours. The fired pellets were pulverized and molded again, and then fired in a hydrogen stream at a temperature of 850 ° C. for 12 hours. Then, the obtained powder was sufficiently pulverized to obtain Li ₃ V ₂ (PO ₄ ) ₃ powder. This is a positive electrode active material.

Next, LiV ₂ O ₄ as a negative electrode active material was produced as follows.
First, lithium carbonate (Li ₂ CO ₃ ) and vanadium pentoxide (V ₂ O ₅ ) were weighed in a stoichiometric ratio such that the composition was LiV ₂ O ₄ , and these were 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. In this way, LiV ₂ O ₄ was obtained. This is defined as a negative electrode active material.

Next, an aqueous lithium secondary battery is manufactured using the positive electrode active material and the negative electrode active material.
Specifically, first, 70 parts by weight of Li ₃ V ₂ (PO ₄ ) ₃ as a positive electrode active material, 25 parts by weight of carbon black as a conductive agent, and polytetrafluoroethylene (PTFE) as a binder Was mixed to prepare a positive electrode mixture.
Further, 70 parts by weight of LiV ₂ O ₄ as a negative electrode active material, 25 parts by weight of carbon black as a conductive agent, and 5 parts by weight of polytetrafluoroethylene (PTFE) as a binder are mixed, and a negative electrode mixture Was made.

Next, as shown in FIG. 1, a battery case 11 for a CR2016 type coin cell is prepared, and a positive electrode mixture is pressure-bonded at about 0.6 ton / cm ² on a mesh previously welded to the inside of the battery case 11. 2 was formed. In the same manner as this positive electrode 2, the negative electrode mixture was pressed onto the mesh at about 0.6 ton / cm ² to form the negative electrode 3.
The positive electrode 2 and the negative electrode 3 were disposed in the battery case 11 via a polypropylene separator 4.

Next, the gasket 5 was placed in the battery case 11, and an appropriate amount of an aqueous electrolyte solution was injected into the battery case 11 and impregnated. In this example, a 6 mol / L LiNO ₃ aqueous solution (pH≈5) was used as the aqueous electrolyte solution.
Next, the sealing plate 12 was disposed in the opening of the battery case 11, and the end of the battery case 11 was caulked to seal the battery case 11, thereby producing the aqueous lithium secondary battery 1. This is referred to as a battery E1.

In this example, an aqueous lithium secondary battery (battery E2) using LiVPO ₄ F as the positive electrode active material was produced.
LiVPO ₄ F was synthesized by a two-stage process of reduction and calcination. That is, first, vanadium pentoxide and ammonium dihydrogen phosphate were sufficiently mixed in a mortar to form a pellet, and reduction treatment was performed at 750 ° C. for 4 hours in a hydrogen stream. The obtained pellets were sufficiently crushed, lithium fluoride was added and mixed in a mortar. The mixture was again formed into a pellet and fired in an Ar stream at a temperature of 750 ° C. for 1 hour to obtain LiVPO ₄ F.
Battery E2 was produced in the same manner as Battery E1, except that LiVPO ₄ F was used as the positive electrode active material.

Further, in this example, in order to clarify the excellent characteristics of the battery E1 and the battery E2, two types of aqueous lithium secondary materials each having LiFePO _{4 having} an olivine structure or LiMn ₂ O ₄ having a spinel structure as a positive electrode active material are used. Batteries (battery C1 and battery C2) were produced.

Specifically, first, two types of positive electrode active materials (LiFePO ₄ and LiMn ₂ O ₄ ) were prepared.
In preparing LiFePO _{4 having} an olivine structure, first, divalent iron oxalate, lithium carbonate, and ammonium dihydrogen phosphate were used as starting materials, and a mixing ratio of Li, Fe, and P, respectively, in a molar ratio of Li: Fe : P = 1.2: 1: 1, and carbon black is mixed with LiFePO ₄ to be synthesized, and the mixing ratio (weight ratio) is LiFePO ₄ : carbon black = 95: 5. And calcined at 650 ° C. for 24 hours in an inert gas atmosphere. In this way, LiFePO ₄ having an olivine structure was obtained.

Further, LiMn ₂ O ₄ having a spinel structure was synthesized by a so-called solid phase method. That is, lithium hydroxide (LiOH) and manganese dioxide (MnO ₂ ) are first mixed so that the mixing ratio of Li and Mn is L: Mn = 1.05: 2.0 in terms of molar ratio. It mixed for 24 hours by the ball mill in the ethanol solvent. Next, the mixture was sufficiently dried, and ball mill mixing was performed for 12 hours in a dry manner. The obtained mixed powder was fired at a temperature of 800 ° C. for 12 hours. Thus, LiMn ₂ O ₄ having a spinel structure was obtained.

Next, using these two types of positive electrode active materials, two types of aqueous lithium secondary batteries were produced in the same manner as the battery E1. Let these be the battery C1 and the battery C2.
The battery C1 was produced in the same manner as the battery E1 except that olivine-structured LiFePO ₄ was used as the positive electrode active material.
The battery C2 was produced in the same manner as the battery E1 except that LiMn ₂ O ₄ having a spinel structure was used as the positive electrode active material.

Next, charge / discharge cycle characteristics were examined for the four types of water-based lithium secondary batteries (battery E1, battery E2, battery C1, and battery C2) produced as described above (charge / discharge cycle test).
In the charge / discharge cycle test, each battery was charged at a constant current with a current density of 0.1 mA / cm ² under a temperature of 60 ° C. and then discharged at a constant current with a current density of 0.1 mA / cm ^2. Discharging was performed as one cycle, and this cycle was repeated 20 times. At this time, the end-of-charge voltage is 1.4 V for the battery E1 using Li ₃ V ₂ (PO ₄ ) ₃ as the positive electrode active material, 1.7 V for the battery E2 using LiVPO ₄ F, and LiFePO ₄ The battery C1 using the battery was 1.2V, and the battery C2 using LiMn ₂ O ₄ was 1.7V. The end-of-discharge voltage was 0.5 V for all batteries.
In each charge / discharge cycle, after charging to the end-of-charge voltage and after discharging to the end-of-discharge voltage, a charge stop time and a discharge stop time were provided for 10 minutes each. Then, for each cycle, the discharge capacity of each battery (battery E1, battery E2, battery C1, and battery C2) was measured, and the capacity retention rate was calculated.

  The discharge capacity was obtained by measuring the discharge current value (mA) for each cycle and multiplying this discharge current value by the time (hr) required for discharge. Further, the capacity maintenance rate for each cycle is based on the formula “capacity maintenance rate at the nth cycle (%) = discharge capacity at the Nth cycle (mAh) / discharge capacity at the first cycle (mAh) × 100”. Calculated. The result is shown in FIG. In FIG. 2, the horizontal axis indicates the number of cycles (times), and the vertical axis indicates the capacity retention rate (%).

As is known from FIG. 2, water-based lithium secondary batteries (battery E1 and battery E2) using a vanadium phosphate compound such as Li ₃ V ₂ (PO ₄ ) ₃ or LiVPO ₄ F as a positive electrode active material are conventional positive electrode active materials. Compared to batteries (battery C1 and battery C2) using the material (LiFePO ₄ or LiMn ₂ O ₄ ), the capacity retention rate was extremely excellent. This is considered because the positive electrode active material in the battery E1 and the battery E2 can be charged / discharged stably in an aqueous solution.

  As described above, the vanadium phosphate compound used in the battery E1 and the battery E2 is suitable for an aqueous lithium secondary battery because it can stably perform Li insertion / extraction in an aqueous solution as compared with other materials. . Such an aqueous lithium secondary battery using a vanadium phosphate compound as a positive electrode active material has high safety, can be manufactured at low cost, and can exhibit stable battery characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS Explanatory drawing which shows the structure of the water-system lithium secondary battery concerning Example 1. FIG. A line showing the relationship between the number of charge / discharge cycles and the capacity retention rate for four types of aqueous lithium secondary batteries (battery E1, battery E2, battery C1, and battery C2) containing different positive electrode active materials according to Example 1. Figure.

Explanation of symbols

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

Claims (5)

In an 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 mobile ions,
The positive electrode has the general formula Li _a V _b (PO ₄ ) _c X _d (where 0 ≦ a ≦ 4, 1 ≦ b ≦ 2, 1 ≦ c ≦ 3, d is 0 or 1, X is F or OH) Containing a positive electrode active material whose main component is a vanadium phosphate compound represented by
The water-based lithium secondary battery, wherein the negative electrode contains a negative electrode active material whose main component is a substance in which insertion and desorption of Li occurs at a lower potential than the vanadium phosphate compound.   The aqueous lithium secondary battery according to claim 1, wherein the aqueous electrolyte solution has a pH of 3 to 11.   3. The aqueous lithium secondary battery according to claim 1 or 2, wherein the lithium salt contains at least lithium nitrate.   4. The water based lithium secondary battery according to claim 1, wherein the negative electrode active material contains a compound having a spinel structure. 5. 5. The aqueous lithium secondary battery according to claim 4, wherein the compound having the spinel structure is LiV ₂ O ₄ .

Images