JP5132293B2 - Method for producing negative electrode material for non-aqueous secondary battery - Google Patents

Description

  The present invention relates to a method for producing a negative electrode material for a non-aqueous secondary battery that has a uniform crystal structure and has a stable crystal structure even after repeated charge and discharge.

  Conventionally, as a negative electrode active material of a non-aqueous secondary battery, various carbon materials such as artificial graphite, natural graphite, and hard carbon have been used because lithium can be inserted / removed. In order to increase the capacity of the battery, the performance has been improved by improving the utilization rate of these carbon materials and the packing density per electrode volume. However, since the practical amount has approached the theoretical capacity (372 mAh / g) of graphite, and the improvement in packing density has also approached the limit, it is becoming difficult to increase the capacity of batteries using current carbon materials.

  For this reason, metal lithium and silicon alloy materials have been actively studied as negative electrode active materials, but these materials are not put into practical use because of the stress associated with the expansion and contraction of the electrodes.

On the other hand, lithium vanadium oxide has been attracting attention as a high-capacity material that has a small expansion / contraction stress of the electrode (Patent Documents 1 and 2).
JP 2003-68305 A JP2007-173096

  However, the lithium vanadium oxide obtained by the manufacturing method described in Patent Documents 1 and 2 may not have lithium contained in excess in the crystal lattice at a predetermined position in the crystal lattice, so that the lattice constant varies. However, there is a problem that the charge / discharge capacity fluctuates with the increase.

  Furthermore, since the crystal structure of the conventional lithium vanadium oxide changes greatly due to the insertion / extraction of lithium ions during charge / discharge, the capacity deterioration during the charge / discharge cycle is significant. In order to solve this problem, it is effective to strengthen the crystal structure by adding a metal element and suppress the change of the crystal structure. However, in the manufacturing methods described in Patent Documents 1 and 2, when a large amount of a metal element is added, the metal element is unevenly distributed in the crystal grains, so that the effect of adding the metal element cannot be sufficiently obtained, and the cycle characteristics are improved. Issues remain.

  Accordingly, an object of the present invention is to provide a method for producing a negative electrode material for a non-aqueous secondary battery that has a uniform crystal structure and has a stable crystal structure even after repeated charge and discharge.

  The inventors of the present invention have a crystal structure of a hexagonal lithium vanadium oxide in which lithium ions are present between layers composed of vanadium, lithium in excess relative to vanadium, other additive elements, and oxygen. When the uniformity of the element increases, each element is present at a predetermined position in the crystal lattice, so that the ratio c / a of the lattice constants a and c decreases, and accordingly, the lithium vanadium oxide is used as the negative electrode active material. It was clarified that the initial capacity of the rechargeable battery was improved. Then, when a precursor is prepared through a step of dissolving a material that becomes a lithium source or vanadium source in water, it is found that each element can be made uniform at the atomic level and the crystal structure can be made uniform. The present invention was completed based on the above.

That is, the method for producing a negative electrode material for a non-aqueous secondary battery according to the present invention is a method for producing a negative electrode material for a non-aqueous secondary battery comprising a hexagonal lithium vanadium oxide, and includes V ₂ O ₅ and Li ₂ CO ₃ is dried into a precursor of an aqueous solution of a neutralized salt obtained by dispersing in water at a ratio such that the molar ratio of lithium to vanadium is 1.15 ≦ Li / V ≦ 1.35. Preparing the precursor powder by pulverizing the precursor, mixing the precursor powder and carbon powder to prepare a mixture thereof, and mixing the mixture with the precursor Firing in an inert atmosphere at a temperature not lower than the melting point of the body and not higher than the melting point of the lithium vanadium oxide.

  According to the present invention, since the precursors are produced through a process of dissolving a material that becomes a lithium source or vanadium source in water, each element is made uniform at the atomic level, so that the crystal structure is made uniform. Can be achieved.

  Patent Document 2 discloses a method of forming a mixed aqueous solution obtained by adding a reducing agent to an organic acid salt mixture, and baking the organic acid salt precursor obtained by drying the mixed aqueous solution in an inert atmosphere. In this method as well, the precursor is prepared by dissolving the raw material in water as in the present invention. However, in the method described in Patent Document 2, the organic acid added as a reducing agent is an obstacle, and lithium is not formed during firing. Since the reaction inserted between the vanadium layers is hindered, a part of lithium stays on the particle surface, and the lattice constant ratio c / a increases.

  Moreover, although the Example of patent document 1 describes mixing the powder used as a raw material in a solid-phase state, according to the said method, a metal element is not distributed uniformly in a crystal particle, but a part Is unevenly distributed in the crystal grains, the lattice constant ratio c / a is increased.

  Compared with these prior arts, according to the manufacturing method according to the present invention, a lithium vanadium oxide having a uniform crystal structure and a small lattice constant ratio c / a can be obtained. By using it as a substance, the battery characteristics of the secondary battery can be improved.

  According to the production method of the present invention, the uniformity of the crystal structure of the lithium vanadium oxide is improved, so that excess lithium or the like is uniformly substituted with vanadium, thereby reducing the lattice constant ratio c / a, Accordingly, the battery initial capacity and cycle characteristics of a non-aqueous secondary battery using the lithium vanadium oxide as a negative electrode active material can be improved.

  Hereinafter, a method for producing a negative electrode material for a non-aqueous secondary battery according to the present invention will be described in detail.

  The production method according to the present invention includes (1) a precursor production process, (2) a pulverization process, (3) a carbon mixing process, and (4) a firing process.

(1) Precursor manufacturing process In the precursor manufacturing process, first, an aqueous solution of neutralized salt is prepared by dispersing V ₂ O ₅ and Li ₂ CO ₃ in water. Both V ₂ O ₅ and Li ₂ CO ₃ are poorly soluble in water, but simultaneously dissolve in water when dispersed in water to form a neutralized salt. Thus, by dissolving the neutralized salt composed of V ₂ O ₅ as the vanadium source and Li ₂ CO _{3 as the} lithium source in water, the respective elements can be made uniform at the atomic level. The crystal structure of the obtained lithium vanadium oxide can be made uniform, and the lattice constant ratio c / a can be reduced.

Examples of the water include pure water, deionized water, and ultrapure water. The water is about 60 to 80 ° C. so that the neutralization reaction between V ₂ O ₅ and Li ₂ CO ₃ can easily proceed. It is preferable to disperse and dissolve in a hot water bath.

V ₂ O ₅ and Li ₂ CO ₃ are dispersed and dissolved in water at such a ratio that the molar ratio of lithium to vanadium is 1.15 ≦ Li / V ≦ 1.35. When Li / V <1.15, the lithium ion insertion / desorption reaction is difficult to occur due to a small amount of lithium, and thus the obtained lithium vanadium oxide cannot be used as the negative electrode active material, while Li / V> If it is 1.35, the crystal structure of the obtained lithium vanadium oxide becomes unstable because lithium is extremely excessive.

When V ₂ O ₅ and Li ₂ CO ₃ are dispersed in water to obtain an aqueous solution of a neutralized salt, it further contains an element (hereinafter referred to as Me) contained in groups IIA to IVB of the long periodic table. The acid-soluble compound is also preferably dispersed and dissolved in water. By compounding the compound containing Me and soluble in acid, a part of vanadium is substituted with Me, and the crystal structure of the obtained lithium vanadium oxide is further stabilized. Is less likely to change, and the electron conductivity is improved. For this reason, the secondary battery using the lithium vanadium oxide as the negative electrode active material does not decrease in capacity during the charge / discharge cycle, improves cycle characteristics, and increases initial capacity.

  Examples of the Me include Mg, Ti, Al, Zr, Mo, and W. Examples of the compound containing Me and soluble in an acid include the carbonate of Me. One kind of Me may be blended, or two or more kinds may be used in combination.

  Among the Me, Mg is particularly preferable. In this case, as the compound containing Mg and soluble in acid, for example, magnesium carbonate hydroxide is used.

  The Me-containing compound that is soluble in acid is preferably dispersed and dissolved in water at a ratio such that the molar ratio of Me to vanadium is 0.01 ≦ Me / V ≦ 0.06. If Me / V <0.01, the effect of adding Me does not appear, whereas if Me / V> 0.06, the crystal structure of the obtained lithium vanadium oxide may become unstable. is there.

  In the precursor manufacturing step, the obtained aqueous solution is then dried to prepare a precursor. The drying temperature for preparing the precursor is preferably 80 to 150 ° C, more preferably 100 to 140 ° C. When the drying temperature is less than 80 ° C., it takes a long time to completely precipitate the precursor. On the other hand, when the drying temperature exceeds 150 ° C., a part of the precursor is thermally decomposed to change the composition. Therefore, in the subsequent step of mixing the precursor powder and the carbon powder, the optimal weight ratio may be deviated, and the physical properties of the lithium vanadium oxide are difficult to stabilize.

  The precursor obtained in such a precursor manufacturing process is preferably a compound having a single melting point at 500 to 700 ° C. Here, having a single melting point means not a mixture but a single compound. Therefore, when there are a plurality of melting points, the obtained precursor is a mixture of a plurality of compounds, so that the crystallinity of the lithium vanadium oxide obtained through the subsequent firing step becomes non-uniform.

(2) Pulverization step In the pulverization step, the obtained precursor is pulverized into a powder form to prepare a precursor powder. Thus, by making it into a powder form, the surface area of a precursor becomes large and a subsequent reduction reaction can be accelerated | stimulated.

(3) Carbon mixing step In the carbon mixing step, the precursor powder and the carbon powder are mixed to prepare a mixture thereof. When the precursor powder and the carbon powder are mixed, the carbon powder adheres to the surface of the precursor powder and functions as a V ₂ O ₅ reducing agent in the subsequent firing step, and the precursor melts. At this time, it functions as a minute mold, holds the precursor so that it does not flow out even if it melts, and functions so that the reduction reaction proceeds efficiently. In the firing step described below, the lithium vanadium oxide obtained without the carbon powder becomes glassy and becomes amorphous, but when the carbon powder is present, a powdered hexagonal lithium vanadium oxide is obtained.

  Examples of the carbon powder include carbon black such as acetylene black, natural graphite, and artificial graphite.

(4) Firing step In the firing step, the mixture of the precursor powder and the carbon powder is fired in an inert atmosphere at a temperature not lower than the melting point of the precursor and not higher than the melting point of the lithium vanadium oxide.

The firing temperature of the mixture is preferably 1000 to 1250 ° C. When the firing temperature is less than 1000 ° C., the reduction reaction of V ₂ O ₅ is insufficient, and some carbon powder may remain without being vaporized. On the other hand, when the firing temperature exceeds 1250 ° C., vanadium and carbon react to generate carbide VC, which adversely affects battery characteristics.

Examples of the inert gas used in the baking step include N ₂ and argon.

  According to the production method of the present invention, a powdered hexagonal lithium vanadium oxide can be obtained through the above steps. The obtained lithium vanadium oxide has a uniform crystal structure and a small lattice constant ratio c / a. For this reason, when the lithium vanadium oxide is used as a negative electrode active material, a nonaqueous secondary battery with improved initial capacity and excellent cycle characteristics can be obtained.

  The present invention will be described in more detail with reference to the following examples. However, the present invention is not limited to these examples.

Example 1
In a 70 ° C. water bath, 40.3 g of V ₂ O ₅ (purity 99.9%) was dispersed in 300 g of pure water. Thereafter, 20.0 g of Li ₂ CO ₃ (purity 99.0%) was added and stirred for 15 minutes to prepare an aqueous solution of neutralized salt. This was dried at 130 ° C. for 10 hours, and the resulting dried product was pulverized to prepare a precursor powder. Further, 47.5 g of the precursor powder and 2.5 g of Denka Black (registered trademark, manufactured by Denki Kagaku Kogyo) were mixed in an automatic mortar for 1 hour, and then calcined in a nitrogen stream at 1200 ° C. for 3 hours to obtain lithium vanadium oxide Li. _1.1 V _0.9 O ₂ was obtained. FIG. 1 shows the result of differential thermal analysis (DTA) performed on the precursor. As shown in FIG. 1, a single melting point was confirmed in the vicinity of 600.degree.

(Comparative Example 1)
Lithium vanadium oxide was prepared by mixing 36.8 g of V ₂ O ₄ (purity 99.9%) and 20.0 g of Li ₂ CO ₃ in an automatic mortar for 1 hour and then firing at 1200 ° C. for 3 hours in a nitrogen stream. Li _1.1 V _0.9 O ₂ was obtained.

(Comparative Example 2)
40.3 g of V ₂ O ₅ (purity 99.9%), 20.0 g of Li ₂ CO _{3 and} 118.1 g of oxalic acid dihydrate were dissolved in 300 g of pure water to prepare a mixed aqueous solution. This was dried at 120 ° C. to obtain a mixed oxalate salt as a precursor. Furthermore, the obtained precursor was calcined at 1200 ° C. for 3 hours in a nitrogen stream to obtain lithium vanadium oxide Li _1.1 V _0.9 O ₂ .

(Example 2)
In a 70 ° C. water bath, 39.0 g of V ₂ O ₅ (purity 99.9%) is dispersed in 300 g of pure water. Thereafter, 20.0 g of Li ₂ CO ₃ (purity 99.0%) and 1.4 g of 4MgCO ₃ .Mg (OH) ₂ .5H ₂ O (42.8% as MgO) were added and stirred for 15 minutes. An aqueous solution of neutralized salt was prepared. This was dried at 130 ° C. for 10 hours, and the resulting dried product was pulverized to prepare a precursor powder. Further, 47.5 g of the precursor powder and 2.5 g of Denka Black (registered trademark, manufactured by Denki Kagaku Kogyo) were mixed in an automatic mortar for 1 hour, and then calcined in a nitrogen stream at 1200 ° C. for 3 hours to obtain lithium vanadium oxide Li. _1.1 V _0.86 Mg _0.04 O ₂ was obtained.

(Comparative Example 3)
V ₂ O ₄ (purity _{99.9%) 35.2g, Li 2 CO} 3 20.0g, was mixed 1 hour in an automatic mortar _{4MgCO 3 · Mg (OH) 2} · 5H 2 O 1.8g, nitrogen The lithium vanadium oxide Li _1.1 V _0.86 Mg _0.04 O ₂ was obtained by firing at 1200 ° C. for 3 hours in an air stream.

(Comparative Example 4)
V ₂ O ₅ (purity 99.9%) 39.0 g, Li ₂ CO ₃ 20.0 g, 4MgCO ₃ · Mg (OH) ₂ · 5H ₂ O 1.4 g, oxalic acid dihydrate 118.1 g Dissolved in 300 g of water to prepare a mixed aqueous solution. This was dried at 120 ° C. to obtain a mixed oxalate salt as a precursor. Furthermore, the obtained precursor was calcined at 1200 ° C. for 3 hours in a nitrogen stream to obtain lithium vanadium oxide Li _1.1 V _0.86 Mg _0.04 O ₂ .

The battery characteristics of the lithium vanadium oxides obtained in the examples and comparative examples were evaluated as follows. The obtained lithium vanadium oxide was classified so as to be a powder having a maximum particle size of 75 μm or less. Denka black (registered trademark, manufactured by Denki Kagaku Kogyo Co., Ltd.) 6 wt% and polyvinylidene fluoride 4 wt% were added to 90 wt% of the powder, and N-methylpyrrolidone was further added as a solvent to form a slurry. This was applied to a 15 μm copper foil so as to have a concentration of 10 mg / cm ² , dried at 130 ° C., punched into a disk having a diameter of 13 mm, and pressed to a predetermined thickness to produce a negative electrode. Using this negative electrode, a coin cell using metallic lithium as a positive electrode was produced, and the battery characteristics were evaluated. In the coin cell, a 20 μm polyethylene porous membrane was used as a separator, and ethylene carbonate / diethyl carbonate = 3/7 LiPF ₆ 1.2M was used as an electrolyte.

  The prepared coin cell is charged with a constant current (0.5 C) -constant voltage (4.2 V), and then subjected to 30 cycles of 0.5 C discharge to a discharge starting voltage of 2.75 V. The discharge capacity after the end of 30 cycles was measured.

  In addition, as an evaluation of the crystal structure of the lithium vanadium oxide obtained in Examples and Comparative Examples, a high power X-ray diffractometer was used, using a Cu target, a voltage of 50 kV, a current of 300 mA, and a step width of 0.02 °. The lattice constant was calculated using the results measured at a scan speed of 1 ° per minute. These results are shown in Table 1. In addition, the discharge capacity of the 1st cycle in Table 1 shows a ratio when the value of the comparative example 1 is set to 100%.

  When Example 1 and Comparative Examples 1 and 2 and Example 2 and Comparative Examples 3 and 4 are compared, as shown in Table 1, all of the examples have a smaller lattice constant ratio c / a. It was found that the crystal structure was highly uniform. This coincided with the fact that all of the examples had a larger initial capacity. Further, in all of the examples, the cycle characteristics were also better. For this reason, it is considered that the example according to the present invention is less likely to disturb the crystal structure during the charge / discharge cycle.

3 is a DTA chart showing the melting point of the precursor of Example 1. FIG.

Claims (7)

A method for producing a negative electrode material for a non-aqueous secondary battery comprising a hexagonal lithium vanadium oxide,
An aqueous solution of a neutralized salt obtained by dispersing V ₂ O ₅ and Li ₂ CO ₃ in water at a ratio such that the molar ratio of lithium to vanadium is 1.15 ≦ Li / V ≦ 1.35. A step of preparing a precursor by drying,
Pulverizing the precursor to prepare a precursor powder;
Mixing the precursor powder and carbon powder to prepare a mixture thereof;
And baking the mixture at a temperature not lower than the melting point of the precursor and not higher than the melting point of the lithium vanadium oxide in an inert atmosphere, and a method for producing a negative electrode material for a non-aqueous secondary battery. When V ₂ O ₅ and Li ₂ CO ₃ are dispersed and dissolved in water, they further contain at least one element Me selected from the group consisting of elements included in groups IIA to IVB of the long periodic table The non-aqueous two-component compound according to claim 1, wherein a compound soluble in sialic acid is dissolved and dissolved in water at a ratio such that the molar ratio of Me to vanadium is 0.01 ≦ Me / V ≦ 0.06. A method for producing a negative electrode material for a secondary battery.   The method for producing a negative electrode material for a non-aqueous secondary battery according to claim 2, wherein the Me is at least one element selected from the group consisting of Mg, Ti, Al, Zr, Mo, and W.   The method for producing a negative electrode material for a non-aqueous secondary battery according to claim 2 or 3, wherein the compound containing Me and soluble in acid is magnesium carbonate hydroxide.   The method for producing a negative electrode material for a non-aqueous secondary battery according to claim 1, wherein a drying temperature for preparing the precursor is 80 to 150 ° C.   The method for producing a negative electrode material for a non-aqueous secondary battery according to claim 1, wherein the firing temperature of the mixture is 1000 to 1250 ° C.   The method for producing a negative electrode material for a non-aqueous secondary battery according to claim 1, wherein the precursor has a single melting point at 500 to 700 ° C. 7.