JP2011096641A - Manufacturing method of active material, and manufacturing method of lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method of an active material capable of improving rate characteristics of a lithium ion secondary battery. <P>SOLUTION: The manufacturing method of the active material includes a process of forming an intermediate by heating a phosphate source, a vanadium source, a water-soluble organic compound and water for 1 to 12 hours, and a process of heating the intermediate, lithium salt and water. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an active material manufacturing method and a lithium ion secondary battery manufacturing method.

LiCoO ₂ is widely used as a positive electrode active material for lithium ion secondary batteries. However, it has been pointed out that LiCoO ₂ has a high raw material cost and has a low thermal stability and a safety problem. As positive electrode active materials that overcome these problems, phosphoric acid-based positive electrode active materials such as LiFePO ₄ and LiVOPO ₄ have attracted attention. LiVOPO ₄ is known as a compound that can realize a charge / discharge voltage of 4 V class among phosphoric acid positive electrode materials (see Patent Document 1 and Non-Patent Documents 1 and 2 below).

Special table 2003-520405 gazette

Journal of The Electrochemical Society, 151 (6) A796-A800 (2004) Electrochemistry, 71 No. 12 (2003), 1108-1110

LiVOPO ₄ has a plurality of crystal structures such as triclinic crystal (α-type crystal) and orthorhombic crystal (β-type crystal), and is known to have different electrochemical characteristics depending on the crystal structure. Since the β-type crystal of LiVOPO ₄ has a linear and shorter ion conduction path (lithium ion path) than the α-type crystal, it has a characteristic of reversibly inserting and desorbing lithium ions (hereinafter, “ Reversible "). Therefore, the larger the ratio of β-type crystals contained in LiVOPO _4, tends to improve the charge and discharge capacities and rate characteristics of batteries using LiVOPO _4. Therefore, a method for producing an active material capable of obtaining a single phase of LiVOPO ₄ β-type crystal is desired.

However, in the method for producing LiVOPO ₄ described in Patent Document 1 and Non-Patent Documents 1 and 2, the conditions for obtaining β-type crystals are severe, and it is difficult for the present inventors to obtain a single phase of β-type crystals. Found.

  The present invention has been made in view of the above-described problems of the prior art, and a method for producing an active material capable of improving the rate characteristics of a lithium ion secondary battery, and a lithium ion secondary using the active material It aims at providing the manufacturing method of a battery.

  In order to achieve the above object, a method for producing an active material according to the present invention includes a step of heating a phosphoric acid source, a vanadium source, a water-soluble organic compound and water for 1 to 12 hours to form an intermediate, Heating the intermediate, the lithium salt, and water.

According to the present invention, an active material containing a LiVOPO ₄ β-type crystal can be formed. In the lithium ion secondary battery including the active material obtained by the present invention as the positive electrode active material, the rate characteristics can be improved as compared with the lithium ion secondary battery using LiVOPO ₄ formed by the conventional manufacturing method. It becomes.

  In the present invention, the water-soluble organic compound may be at least one selected from the group consisting of sucrose, glucose, methylcellulose, ethylcellulose, fructooligosaccharide and polyvinyl alcohol.

  In the present invention, the ratio [C] / [V] of the number of moles of carbon [C] contained in the water-soluble organic compound and the number of moles [V] of vanadium contained in the vanadium source is adjusted to 0.01-8. It is preferable to do. More preferably, the ratio [C] / [V] is adjusted to 0.1-4. Thereby, the effect of this invention becomes remarkable.

A method for producing a lithium ion secondary battery according to the present invention includes a current collector, an active material layer that is located on the current collector and includes an active material obtained by the method for producing an active material according to the present invention, Forming an electrode having the following. Thereby, it becomes possible to manufacture a lithium ion secondary battery having higher rate characteristics than a lithium ion secondary battery using LiVOPO ₄ formed by a conventional manufacturing method.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the active material which can improve the rate characteristic of a lithium ion secondary battery, and the manufacturing method of a lithium ion secondary battery using the said active material can be provided.

FIG. 1 is a schematic cross-sectional view of a lithium ion secondary battery according to an embodiment of the present invention.

(Method for producing active material)
Below, the manufacturing method of the active material which concerns on suitable one Embodiment of this invention is demonstrated in detail.

The method for producing an active material according to the present embodiment includes a step of forming an intermediate by heating a phosphoric acid source, a vanadium source, a water-soluble organic compound and water for 1 to 12 hours, an intermediate, a lithium salt, and Heating the water. The intermediate is a precursor of LiVOPO ₄ and is a compound that does not contain Li. Below, the process of forming said intermediate body is described as a 1st heating process. Moreover, the process of heating an intermediate body, lithium salt, and water is described as a 2nd heating process.

<First heating step>
In the first heating step, a phosphoric acid source, a vanadium source, a water-soluble organic compound and distilled water are stirred to prepare a mixed solution, and the mixed solution may be heated. Thereby, an intermediate body produces | generates in a liquid mixture. The present inventors consider that VOPO ₄ .2H ₂ O, which is a hydrate of an intermediate, is generated in the mixed solution by heating the mixed solution.

In the first heating step, the mixed solution is preferably heated to 50 to 120 ° C. That is, it is preferable to adjust the temperature of the reaction for forming the intermediate from the phosphoric acid source, vanadium source, water-soluble organic compound and distilled water within the above range. When the temperature of the mixed liquid is too low, it tends to be difficult to generate an intermediate as compared with the case where the temperature of the mixed liquid is within the above range. When the temperature of the mixed liquid is too high, the particle diameter of the intermediate is increased as compared with the case where the temperature of the mixed liquid is within the above range, and the reactivity in the second heating step tends to deteriorate. In this embodiment, these tendencies are suppressed by heating the mixed solution to the above temperature range. In the first heating step, the mixed solution is heated for 1 to 12 hours. When the heating time of the mixed liquid is outside the range of 1 to 12 hours, the discharge capacity or rate characteristics tend to deteriorate. In addition, when the heating time is long, the resulting LiVOPO ₄ crystal becomes too large, and the characteristics of the battery using this will deteriorate.

  In the first heating step, while the distilled water is heated in the above temperature range for 1 to 12 hours, a mixed solution is prepared by adding a phosphoric acid source, a vanadium source, and a water-soluble organic compound to the distilled water. Good. Again, an intermediate is formed.

As the phosphoric acid source, for example, at least one selected from the group consisting of H ₃ PO ₄ , NH ₄ H ₂ PO ₄ and (NH ₄ ) ₂ HPO ₄ can be used. Two or more phosphate sources may be used in combination. As the vanadium source, for example, either V ₂ O ₅ or NH ₄ VO ₃ can be used. Two or more vanadium sources may be used in combination.

The mixing ratio of the phosphoric acid source and the vanadium source is such that the ratio of the number of moles of phosphorus element contained in the phosphoric acid source to the number of moles of vanadium element contained in the vanadium source is the stoichiometric ratio of LiVOPO ₄ (1: 1). It may be adjusted so that Note that the blending ratio of the phosphate source and the vanadium source does not necessarily satisfy the above stoichiometric ratio.

  Examples of the water-soluble organic compound include water-soluble polymers or saccharides such as sucrose, glucose, methyl cellulose, ethyl cellulose, polyvinyl alcohol, fructooligosaccharide, sorbitol, and lactose. What is necessary is just to use what is solid at room temperature as a water-soluble organic compound. The water-soluble organic compound may be carbonized by firing, which will be described later, and may remain as an impurity in the finally obtained active material and function as a conductive aid. In addition, when a water-insoluble organic compound such as polyvinylidene fluoride or a carbon material such as graphite or acetylene black is used instead of the water-soluble organic compound, it is difficult to achieve the effects of the present invention.

  In the first heating step, the ratio [C] / [V] of the number of moles of carbon [C] contained in the water-soluble organic compound and the number of moles [V] of vanadium contained in the vanadium source was adjusted to 0.01-8. It is preferable to do. More preferably, the ratio [C] / [V] is adjusted to 0.1-4. When [C] / [V] is smaller than the above range, the intermediate tends to be difficult to be covered with the water-soluble organic compound during the drying step described later. Moreover, when [C] / [V] is larger than the said range, the ratio of the organic compound with respect to an active material becomes large too much, and there exists a tendency for battery capacity to fall.

  In the first heating step, it is preferable to heat the mixed solution to produce an intermediate in the mixed solution, and then dry the mixed solution using a spray dryer or the like. By drying, the production of the intermediate further proceeds in the mixed solution, and moisture is removed from the mixed solution, whereby a residue containing the intermediate is obtained. In addition, the intermediate is covered with a water-soluble organic compound during drying. Therefore, excessive grain growth of the intermediate is suppressed. When a water-insoluble organic compound is used, the intermediate is not sufficiently covered with the water-insoluble organic compound during drying. Hereinafter, a residue obtained by drying during the first heating step is referred to as a “first residue”.

  In the first step, it is preferable to dry the mixed solution in an atmosphere of 150 to 300 ° C. When the temperature of drying is too low, compared with the case where the temperature is within the above range, drying tends to be insufficient and recovery of the first residue tends to be difficult. In this embodiment, these tendencies are suppressed by drying the mixed solution in the above temperature range. When filtration drying is employed instead of drying with a spray dryer, the water-soluble organic compound is removed from the mixed solution, and a desired intermediate cannot be obtained, making it difficult to obtain the effects of the present invention. . Instead of drying with a spray dryer, freeze drying or the like may be performed.

In this embodiment, it is preferable to fire the liquid mixture or first residue containing the above-described intermediate. By this calcination, water is removed from the mixed solution containing the intermediate or the first residue, and the intermediate becomes anhydrous. That is, the present inventors consider that VOPO ₄ · 2H ₂ O becomes VOPO ₄ . If the mixed liquid containing the intermediate or the first residue is baked to change the water-soluble organic matter to be hardly soluble, the organic matter is dissolved in water again in the second heating step. In addition, baking of the liquid mixture or residue containing an intermediate is not an essential process for obtaining an intermediate.

In this embodiment, what is necessary is just to bake the liquid mixture or 1st residue containing an intermediate body in 200-500 degreeC atmosphere. If the firing temperature is too low, water tends to remain in the intermediate and the composition of LiVOPO ₄ tends to shift. When the firing temperature is too high, a hetero phase is contained in the intermediate, so that the ratio of the β-type crystal phase in the final product LiVOPO ₄ tends to decrease, and the battery capacity tends to decrease. By setting the firing temperature in the first heating step within the above range, these tendencies can be suppressed.

  The firing atmosphere of the mixed liquid containing the intermediate or the first residue is preferably an inert atmosphere such as a nitrogen atmosphere or an argon atmosphere.

<Second heating step>
In the second heating step, first, the intermediate and lithium salt obtained in the first heating step are added to distilled water, and these are stirred to prepare a mixed solution. What is necessary is just to adjust the temperature of distilled water to about 30-80 degreeC when adjusting a liquid mixture. This promotes dissolution of the lithium salt in the mixed solution.

Examples of the lithium salt include one or more selected from the group consisting of Li ₂ CO ₃ , LiF, LiNO ₃ , LiOH, LiCl, LiBr, LiI, Li ₂ SO ₄ , Li ₃ PO _4, and CH ₃ COOLi. Can be used.

The compounding ratio of the lithium salt and the intermediate is such that the ratio of the number of moles of lithium element contained in the lithium salt, the number of moles of vanadium element contained in the intermediate, and the number of moles of phosphorus element contained in the intermediate is LiVOPO ₄ The stoichiometric ratio (1: 1: 1) may be adjusted. Note that the mixing ratio of the lithium salt and the intermediate does not necessarily satisfy the above stoichiometric ratio. For example, in order to prevent the loss of Li in the finally obtained LiVOPO ₄ , a large amount of lithium salt may be added.

In the second heating step, it is preferable to obtain a second residue by drying a mixed solution containing the intermediate, lithium salt and distilled water with a spray dryer or the like. Thus, it is possible to suppress deviation in composition of LiVOPO _4, can suppress a decrease in battery capacity. In the second heating step, it is preferable to dry the mixed solution containing the intermediate, lithium salt and distilled water in an atmosphere of 150 to 300 ° C. When the drying temperature is too low, drying tends to be insufficient and recovery of the second residue tends to be difficult as compared with the case where the temperature is within the above range. These tendencies can be suppressed by drying the mixed solution in the above temperature range. In the case of adopting filtered and dried in place of the drying with a spray drier, lithium element from the mixed liquid will be removed, it is difficult to obtain a LiVOPO _4.

In the second heating step, the second residue is fired. Or in a 2nd heating process, the liquid mixture containing an intermediate body, lithium salt, and distilled water is baked. By this firing, an active material containing a LiVOPO ₄ β-type crystal can be formed.

In the second heating step, it is preferable to fire the mixed solution or second residue containing the intermediate, lithium salt and distilled water in an atmosphere of 400 to 700 ° C. If the firing temperature is too low, the degree of crystal growth of LiVOPO ₄ tends to be small, and the degree of improvement in its capacity density tends to be small. If the firing temperature is too high, crystal growth of LiVOPO ₄ proceeds excessively, the particle size of LiVOPO ₄ increases, and the Li diffusibility in the crystal tends to decrease. For this reason, the degree of improvement in the rate characteristics of the battery using LiVOPO ₄ obtained tends to be small. By setting the firing temperature in the second heating step within the above range, these tendencies can be suppressed.

  The firing time in the second heating step may be 3 to 20 hours. The firing atmosphere may be a nitrogen atmosphere, an argon atmosphere, or an air atmosphere.

Since the LiVOPO ₄ obtained by the method for producing an active material according to the present embodiment is a single-phase β-type crystal that is excellent in reversibility of lithium ions, the present inventors have found that the rate characteristics of a battery using this improve. Think. In other words, in the method for producing an active material according to the present embodiment, it is considered that a β-type crystal of LiVOPO ₄ can be obtained with a higher yield than the conventional production method. In the case where the intermediate, water-soluble organic compound, lithium salt and water are heated after forming the intermediate by heating the phosphoric acid source, vanadium source and water without using the water-soluble organic compound. This makes it difficult to produce β-type crystals and makes it difficult to obtain the effects of the present invention.

(Method for producing lithium ion secondary battery)
A positive electrode active material layer 14 containing an active material and a conductive additive obtained by the manufacturing method according to the present embodiment is formed on the positive electrode current collector 12 (see FIG. 1). In this way, the positive electrode 10 including the positive electrode current collector 12 and the positive electrode active material layer 14 formed on the positive electrode current collector 12 is manufactured. Further, a negative electrode material layer 24 containing a negative electrode active material such as graphite is formed on the negative electrode current collector 22. In this way, the negative electrode 20 including the negative electrode current collector 22 and the negative electrode active material layer 24 formed on the negative electrode current collector 22 is produced.

  Next, the negative electrode lead 60 and the positive electrode lead 62 are electrically connected to the negative electrode 20 and the positive electrode 10, respectively. Thereafter, the separator 18 is arranged in contact with the negative electrode 20 and the positive electrode 10 to form the power generation element 30. At this time, the surface of the negative electrode 20 on the negative electrode active material layer side and the surface of the positive electrode 10 on the positive electrode active material layer side are arranged in contact with the separator 18.

  Next, the power generation element 30 is inserted into the battery case 50, and further an electrolyte solution is injected. Subsequently, the lithium ion secondary battery 100 is completed by sealing the opening of the battery case 50 in a state where the leading ends of the negative electrode lead 60 and the positive electrode lead 62 are arranged outside the battery case.

  As mentioned above, although one suitable embodiment of the manufacturing method of the active material and lithium ion secondary battery which concerns on this invention was described in detail, this invention is not limited to the said embodiment.

  The active material obtained by the production method according to the present invention can also be used as an electrode material for electrochemical devices other than lithium ion secondary batteries. Examples of such electrochemical elements include secondary batteries other than lithium ion secondary batteries such as metal lithium secondary batteries, and electrochemical capacitors such as lithium capacitors. These electrochemical elements can be used for power sources such as self-propelled micromachines and IC cards, and distributed power sources arranged on or in a printed circuit board.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example.

Example 1
<First heating step>
A mixed solution was prepared by adding 4.68 g of NH ₄ VO ₃ as a vanadium source, 5.28 g of (NH ₄ ) ₂ HPO ₄ as a phosphoric acid source, and 1 g of sucrose to 200 ml of ion-exchanged water. . [C] / [V] in the mixed solution was adjusted to 0.90. The mixture was heated at 80 ° C. for 4 hours. The mixed solution after heating was dried at about 200 ° C. with a spray dryer to obtain a first residue. This first residue was calcined in argon at 450 ° C. for 4 hours to obtain an intermediate. In firing the first residue, the argon atmosphere was heated to 450 ° C. over 1 hour.

<Second heating step>
100 ml of ion-exchanged water and 2.96 g of Li ₂ CO ₃ as a lithium salt were added to the intermediate, and the mixture was stirred at 25 ° C. to prepare a mixed solution. Next, the liquid mixture containing the intermediate, Li ₂ CO ₃ and ion-exchanged water was dried at about 200 ° C. with a spray dryer to obtain a powdery second residue. The second residue was calcined in an argon atmosphere at 450 ° C. for 4 hours. In firing the second residue, the argon atmosphere was heated to 450 ° C. over 4 hours. This obtained the active material of Example 1. From the results of Rietveld analysis based on powder X-ray diffraction (XRD), it was confirmed that the active material of Example 1 was a single phase of LiVOPO ₄ β-type crystal.

(Examples 2 to 41, Comparative Examples 1 to 7)
In Examples 2-41 and Comparative Examples 5-7, the compounds shown in Tables 1 to 3 were used as water-soluble organic compounds. In Comparative Examples 1 and 2, a carbon material shown in Table 1 was used instead of the water-soluble organic compound. In Comparative Example 3, not the water-soluble organic compound but the water-insoluble organic compound shown in Table 1 was used. In Comparative Example 4, none of the water-soluble organic compound, the water-insoluble organic compound, and the carbon material was used.

  In Examples 2-41 and Comparative Examples 5-7, the input amount of the water-soluble organic compound into 100 ml of ion-exchanged water was the value shown in Tables 1-3. In Comparative Examples 1 and 2, the input amount of the carbon material into 100 ml of ion-exchanged water was the value shown in Table 1. In Comparative Example 3, the amount of the water-insoluble organic compound added to 100 ml of ion-exchanged water was the value shown in Table 1. In Examples 2-41 and Comparative Examples 5-7, [C] / [V] was adjusted to the values shown in Tables 1-3. In addition, [C] / [V] in Comparative Examples 1 and 2 shown in Table 1 is a ratio of the number [C] of carbon contained in the carbon material and the number [V] of vanadium contained in the vanadium source. [C] / [V] of Comparative Example 3 shown in Table 1 is the ratio of the number of carbons [C] contained in the water-insoluble organic compound to the number [V] of vanadiums contained in the vanadium source.

  The heating time of the liquid mixture in the 1st heating process of Examples 2-41 and Comparative Examples 1-3, 5-7 was the value shown to Tables 1-3. The heating time of Example 10 is the shortest among all examples, and the heating time of Example 11 is the longest among all examples. The heating time of Comparative Example 6 is the shortest among all examples and all comparative examples, and the heating time of Comparative Example 5 is the longest among all examples and all comparative examples.

  Except for the above, the active materials of Examples 2 to 41 and Comparative Examples 1 to 7 were obtained in the same manner as in Example 1.

[Measurement of crystal structure]
From the results of Rietveld analysis based on powder X-ray diffraction (XRD), it was confirmed that each of the active materials of Examples 2 to 41 and Comparative Examples 1 to 7 contained a β-type crystal phase of LiVOPO ₄ .

[Production of evaluation cell]
A mixture of the active material of Example 1, polyvinylidene fluoride (PVDF) as a binder, and acetylene black was dispersed in N-methyl-2-pyrrolidone (NMP) as a solvent to prepare a slurry. The slurry was prepared so that the weight ratio of the active material, acetylene black, and PVDF was 90: 5: 5 in the slurry. This slurry was applied onto an aluminum foil as a current collector, dried, and then rolled to obtain an electrode (positive electrode) on which an active material layer containing the active material of Example 1 was formed.

Next, the obtained electrode and the Li foil as the counter electrode were laminated with a separator made of a polyethylene microporous film interposed therebetween to obtain a laminate (element body). This laminate was put in an aluminum laminator pack, and 1M LiPF ₆ solution was injected as an electrolyte into the aluminum laminate pack, followed by vacuum sealing to produce an evaluation cell of Example 1.

  In the same manner as in Example 1, evaluation cells were produced using each of the active materials of Examples 2 to 41 and Comparative Examples 1 to 7 alone.

[Measurement of discharge capacity]
Using the evaluation cell of Example 1, the discharge capacity (unit: mAh / unit) when the discharge rate is 0.1 C (current value at which discharge is completed in 10 hours when constant current discharge is performed at 25 ° C.) g) was measured. In addition, using the evaluation cell of Example 1, the discharge capacity (unit: unit) when the discharge rate is 10 C (current value at which discharge is completed in 0.1 hour when constant current discharge is performed at 25 ° C.). mAh / g) was measured.

  In the same manner as in Example 1, the discharge capacities of the evaluation cells of Examples 2 to 41 and Comparative Examples 1 to 7 were measured.

[Evaluation of rate characteristics]
The rate characteristics (unit:%) of Example 1 were determined. The rate characteristic is the ratio of the discharge capacity at 10 C when the discharge capacity at 0.1 C is 100%. The results are shown in Table 1.

  In the same manner as in Example 1, the rate characteristics of the evaluation cells in Examples 2 to 41 and Comparative Examples 1 to 7 were obtained. The results are shown in Tables 1-3.

  The rate characteristic is preferably 30% or more, and more preferably 40% or more.

  It was confirmed that the rate characteristics of each evaluation cell of Examples 1-41 in which the mixed liquid containing the water-soluble organic compound was heated for 1 to 12 hours was 30% or more.

  It was confirmed that the rate characteristics of the evaluation cells of Comparative Examples 1 to 7 were less than 30%.

  DESCRIPTION OF SYMBOLS 10 ... Positive electrode, 20 ... Negative electrode, 12 ... Positive electrode collector, 14 ... Positive electrode active material layer, 18 ... Separator, 22 ... Negative electrode collector, 24 ... Negative electrode Active material layer, 30 ... power generation element, 50 ... case, 60, 62 ... lead, 100 ... lithium ion secondary battery.

Claims (5)

A step of heating a phosphoric acid source, a vanadium source, a water-soluble organic compound and water for 1 to 12 hours to form an intermediate;
Heating the intermediate, lithium salt and water,
A method for producing an active material. The water-soluble organic compound is at least one selected from the group consisting of sucrose, glucose, methylcellulose, ethylcellulose, fructooligosaccharide and polyvinyl alcohol.
The manufacturing method of the active material of Claim 1. The ratio [C] / [V] of the number of moles of carbon [C] contained in the water-soluble organic compound and the number of moles [V] of vanadium contained in the vanadium source is adjusted to 0.01-8.
The manufacturing method of the active material of Claim 1 or 2. Adjusting the ratio [C] / [V] to 0.1-4,
The manufacturing method of the active material of Claim 3. The electrode which has a collector and the active material layer which is located on the said collector and contains the active material obtained by the manufacturing method of the active material as described in any one of Claims 1-4 is formed. Comprising the steps of:
A method for producing a lithium ion secondary battery.

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