JP2013206562A - Cathode material for lithium-ion secondary battery, cathode for lithium-ion secondary battery, lithium-ion secondary battery, and method of producing cathode material for lithium-ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve rate characteristics in high-rate discharge of β-LiVOPO.SOLUTION: The cathode material for a lithium-ion secondary battery is characterized in that a ratio D90/D50 of a cumulative particle diameter D90 to a cumulative particle diameter D50 of β-LiVOPOis 1.10 to 6.25 inclusive.

Description

  The present invention relates to a positive electrode material for a lithium ion secondary battery, a positive electrode for a lithium ion secondary battery, a lithium ion secondary battery, and a method for producing a lithium ion secondary battery positive electrode material.

In recent years, a polyanionic positive electrode active material typified by lithium iron phosphate (LiFePO ₄ ) has been studied as a positive electrode active material excellent in crystal stability and thermal stability even at high temperatures. Non-aqueous electrolyte batteries using LiFePO ₄ as a positive electrode active material have been put into practical use for power tools, have a high discharge capacity of 160 mAh / g, and have a high rate performance due to the electron conductive carbonaceous support technology on the surface of the positive electrode active material. It is also excellent.

However, the operating potential of LiFePO ₄ is 3.42 V with respect to the Li / Li ⁺ standard, which is lower than the operating potential of the positive electrode active material used in general-purpose batteries. It is enough.

Therefore, LiVOPO ₄ has been proposed as a polyanion positive electrode active material having a higher operating potential than LiFePO ₄ .

Among them, it is known that LiVOPO ₄ is used as a positive electrode material for lithium ion secondary batteries in α type and β type.

Among these, it is known that a non-aqueous electrolyte battery using β-type β-LiVOPO ₄ as a positive electrode active material can obtain a capacity of about 100 mAh / g at a low rate discharge of C / 50. (For example, refer to Patent Document 1) Further, it is known that a high capacity of 140 mAh / g can be obtained only by a low rate discharge of C / 40 in another manufacturing method (Non-Patent Document 1).

JP 2003-68304 A

J. et al. Barker, et. al. , J. et al. Electrochem. Soc. , 151 (6) A796-800 (2004)

However, the nonaqueous electrolyte battery using β-LiVOPO ₄ as the positive electrode active material has a problem in rate characteristics in high rate discharge. (For example, see Non-Patent Document 1)

  Accordingly, the present invention provides a positive electrode material for a lithium ion secondary battery having excellent high rate discharge characteristics, a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery, and a method for producing a positive electrode material for a lithium ion secondary battery. The purpose is to do.

The positive electrode material for a lithium ion secondary battery of the present invention is characterized in that the ratio (D90 / D50) of the cumulative particle diameter D90 and the cumulative particle diameter D50 of β-LiVOPO ₄ is 1.10 or more and 6.25 or less. To do.

Β-LiVOPO ₄ in which D90 / D50 is in the range of 1.10 to 6.25 is a packing density in the electrode as compared to the case where only small particles are present due to the presence of large particles. Therefore, it is considered that the high rate discharge characteristics are improved.

The positive electrode material for a lithium ion secondary battery according to the present invention further includes a particle size A at the apex on the large particle size side and a particle size B at the apex on the small particle size side in the relative frequency distribution of the particle size distribution measurement of β-LiVOPO ₄ . The ratio (A / B) is preferably 0.97 or more and 10.71 or less.

  High rate discharge characteristics are obtained when the ratio of the particle size ratio (A / B) of the apex on the large particle size side and the small particle size side in the relative frequency distribution of the positive electrode material is in the range of 0.97 to 10.71. Further improve. When the value of A / B is larger than 10.71, it is considered that the high-rate discharge characteristics are deteriorated due to an increase in the number of large particles. When the value of A / B is smaller than 0.97, it is considered that the high-rate discharge characteristics are deteriorated due to a decrease in the packing density of the positive electrode material in the electrode.

  A method for producing a positive electrode material for a lithium ion secondary battery according to the present invention includes a precursor synthesis step of obtaining a precursor by drying a mixture containing a lithium source, a phosphate source, a vanadium source, a reducing agent, and water; It is preferable to provide a heat treatment step for heat treating the precursor obtained in the precursor synthesis step.

  In the method for producing a positive electrode material for a lithium ion secondary battery according to the present invention, it is more preferable that the heat treatment is performed at 400 ° C. to 650 ° C. in the heat treatment step.

  When the positive electrode material for a lithium ion secondary battery obtained by heat treatment at 400 ° C. to 600 ° C. is used as the positive electrode active material, it becomes easy to improve the high rate discharge characteristics of the lithium ion secondary battery.

  ADVANTAGE OF THE INVENTION According to this invention, the positive electrode material for lithium ion secondary batteries excellent in the high-rate discharge characteristic, the positive electrode for lithium ion secondary batteries, a lithium ion secondary battery, and the manufacturing method of the positive electrode material for lithium ion secondary batteries are provided. can do.

It is a schematic cross section of a lithium ion secondary battery. It is a relative frequency distribution figure with respect to the particle size of the positive electrode material for lithium ion secondary batteries of an Example.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. In addition, this invention is not limited to the following embodiment. The constituent elements described below include those that can be easily assumed by those skilled in the art and those that are substantially the same. Furthermore, the constituent elements described below can be appropriately combined.

<Lithium ion secondary battery>
As shown in FIG. 1, a lithium ion secondary battery 100 according to the present embodiment is disposed adjacent to each other between a plate-like negative electrode 20 and a plate-like positive electrode 10 facing each other, and the negative electrode 20 and the positive electrode 10. A plate-like separator 18, an electrolyte solution (not shown) containing lithium ions, a case 50 containing these in a sealed state, and one end of the negative electrode 20 electrically And a negative electrode lead 62 whose other end protrudes outside the case, and a positive electrode whose one end is electrically connected to the positive electrode 10 and whose other end protrudes outside the case. A lead 60.

  The negative electrode 20 includes a negative electrode current collector 22 and a negative electrode active material layer 24 formed on the negative electrode current collector 22. The positive electrode 10 includes a positive electrode current collector 12 and a positive electrode active material layer 14 formed on the positive electrode current collector 12. The separator 18 is located between the negative electrode active material layer 24 and the positive electrode active material layer 14. The positive electrode active material layer 14 contains the active material according to the present embodiment.

  For other members of the lithium ion secondary battery, known materials can be used as appropriate.

<Positive electrode material for lithium ion secondary battery>
The positive electrode material for a lithium ion secondary battery according to this embodiment will be described below. The lithium ion secondary battery material according to the present embodiment is represented by the general formula β-LiVOPO ₄ , and a value represented by D90 representing 90% cumulative particle size with respect to a value represented by D50 representing 50% cumulative particle size in the particle size distribution. The value of D90 / D50, which is the ratio, is in the range of 1.10 to 6.25.

  The value of D90 / D50 is more preferably in the range of 1.75 or more and 4.27 or less.

In the relative frequency distribution of the particle size distribution measurement of β-LiVOPO _{4 according} to this embodiment, the value of the ratio (A / B) of the particle size A at the vertex on the large particle size side to the particle size B at the vertex on the small particle size side is It is preferably in the range of 0.97 or more and 10.71 or less.

In β-LiVOPO ₄ , part of V may be substituted with other elements such as Fe and Mn, or part of V may be missing.

<Method for producing positive electrode material for lithium ion secondary battery>
Next, the manufacturing method of the positive electrode material which concerns on one Embodiment of this invention is demonstrated. The manufacturing method of the positive electrode material according to the present embodiment removes moisture by applying heat from the outside to a mixture of a lithium source, a phosphoric acid source, a vanadium source, a reducing agent, and water, and becomes a precursor of the positive electrode material. A precursor mixing step for obtaining a mixture; and a heat treatment step for heat-treating the obtained precursor.

(Precursor synthesis process)
In the precursor synthesis step, first, the above-described lithium source, phosphate source, vanadium source, reducing agent and water are added to prepare a mixture (aqueous solution) in which these are dispersed. In preparing the mixture, for example, first, a mixture of a phosphoric acid source, a manganese source and water may be refluxed, and then a lithium source may be added thereto.

As the lithium source, for example, at least one selected from the group consisting of LiNO ₃ , Li ₂ CO ₃ , LiOH, LiCl, Li ₂ SO ₄ and CH ₃ COOLi can be used.

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 , (NH ₄ ) ₂ HPO ₄ and Li ₃ PO ₄ can be used.

As the vanadium source, for example, at least one selected from the group consisting of V ₂ O ₅ , VO _2, and NH ₄ VO ₃ can be used.

  As the reducing agent, for example, at least one selected from the group consisting of ascorbic acid, citric acid, tartaric acid, polyethylene glycol (PEG), polyethylene (PE), and hydrazine can be used.

  In addition, you may use together 2 or more types of lithium sources, 2 or more types of phosphoric acid sources, or 2 or more types of vanadium sources, and 2 or more types of reducing agents.

  The precursor synthesis step may be mixed at normal temperature, or may be mixed at a temperature equal to or higher than normal temperature using an oil bath or the like.

It is desirable to stir the aqueous solution as the raw material mixture for 10 to 40 hours. By stirring, the particle size of β-LiVOPO ₄ produced by segregation of dissolved components from the aqueous solution can be reduced. Moreover, by stirring within 40 hours, it can suppress that the decomposition | disassembly of the vanadium source which is a raw material advances, can prevent the viscosity increase of aqueous solution, and can stir easily.

(Heat treatment process)
In the heat treatment step, it is assumed that the precursor obtained in the precursor synthesis step is in an amorphous state, so that β-LiVOPO ₄ can be synthesized by heat-treating the precursor in the atmosphere.

The temperature of the heat treatment is preferably 400 ° C to 600 ° C, and more preferably 500 ° C to 550 ° C. When the heat treatment temperature is too low, the β-LiVOPO ₄ phase is not formed and the precursor partially maintains the amorphous structure, and the charge / discharge characteristics of the active material tend to be lowered. If the heat treatment temperature is too high, the β-LiVOPO ₄ phase decomposes or undergoes a phase change, and the target phase cannot be obtained. By setting the temperature of the heat treatment within the above range, it can be stably obtained.

(Particle size distribution measurement)
The particle size distribution measurement can be performed using an existing apparatus such as a microtrack.

  The particle size corresponding to 50% by weight in a particle size cumulative curve plotted with the weight percentage (%) passing through a sieve of a certain particle size on the vertical axis and the particle size on the horizontal axis of the logarithmic scale is D50, weight percentage 90 % Is D90.

  The ratio of D50 and D90 calculated as described above, that is, D90 / D50 is in the range of 1.10 to 6.25, and preferably in the range of 1.75 to 4.17.

  When the particle size distribution measurement result is expressed as a relative frequency distribution, if two peaks of relative frequency distribution are seen, the particle size at the peak of the peak on the large particle size side is A, and the particle at the peak of the peak on the small particle size side Let B be the diameter.

  For the vertices defined as A and B, the particle diameter may be in any range, and A and B are distinguished by distinguishing between the particle diameters of the two vertices.

  As a result of the particle size distribution measurement, when two relative frequency distributions are not found in the relative frequency distribution diagram, the value of A / B is not defined.

  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
0.2 mol of H ₃ PO ₄ and 180 ml of distilled water were put in a container and stirred at room temperature. Further, 0.1 mol of V ₂ O ₅ was added and stirring was continued. Thereafter, hydrazine was added and stirring was continued. Thereafter, 0.1 mol of LiOH.H ₂ O was added. The obtained mixed solution was allowed to stand at room temperature without stirring for 80 h.

  The raw material mixed solution was dried at 80 ° C. to obtain a precursor.

The obtained precursor was heat-treated at 550 ° C. for 4 hours in an air atmosphere to obtain β-LiVOPO ₄ .

The obtained β-LiVOPO ₄ and carbon black were mixed at a ratio of 84 parts by mass to 8 parts by mass for 3 minutes to obtain a β-LiVOPO ₄ / C mixed powder.

<Fabrication of half cell>
92 parts by mass of the positive electrode material after heat treatment and 8 parts by mass of PVDF (polyvinylidene fluoride) were added to NMP (N-methyl-2-pyrrolidone) to prepare a positive electrode paint. The ratio of Li ₃ V ₂ (PO ₄ ) ₃ , carbon black and PVDF, which is a solid content in the positive electrode paint, is adjusted to β-LiVOPO ₄ : carbon black: PVDF = 84 parts by mass: 8 parts by mass: 8 parts by mass. did.

The positive electrode coating material was applied to an aluminum foil having a thickness of 20 μm. The applied positive electrode coating material was dried and then rolled to obtain a positive electrode. Next, the lithium foil was cut into a predetermined size and attached to a copper foil (thickness: 15 μm) to obtain a negative electrode. The positive electrode and the negative electrode were laminated with a separator made of a polyethylene microporous film interposed therebetween to obtain a laminate (element body). An aluminum foil (width 4 mm, length 40 mm, thickness 80 μm) and nickel foil (width 4 mm, length 40 mm, thickness 80 μm) were ultrasonically welded as leads to the positive electrode and the negative electrode, respectively. The lead was wrapped with heat-bonded polypropylene (PP) previously grafted with maleic anhydride. This is to improve the sealing performance between the lead and the case. The case of the lithium ion secondary battery was made of an aluminum laminate material, and the configuration thereof was prepared as PET (12) / Al (40) / PP (50). PET is polyethylene terephthalate and PP is polypropylene. The value in parentheses represents the thickness of each layer (unit: μm). At this time, bags were made so that PP was inside. The upper laminate was placed in a case, and 1 M LiPF ₆ / EC + DEC (30:70 volume ratio), which is an electrolytic solution, was injected into the case.

<Measurement of discharge capacity>
Using the half cell of Example 1, rate characteristics were measured. Specifically, the discharge capacity (unit: mAh / g) was measured when the discharge rate was changed from 0.1 C (current value at which discharge was completed in 10 hours when constant current discharge was performed) to 10 C. The results are shown in Table 1. The discharge capacity shown in Table 1 is the discharge capacity per gram of active material. In the measurement, charging and discharging were performed at 0.1 C to 10 C with a theoretical capacity of β-LiVOPO ₄ as a positive electrode active material being 159 mAh / g. The upper limit charging voltage was 4.3 V (VS. Li ^/ Li ⁺ ), and the lower limit discharging voltage was 2.8 V (VS. Li ^/ Li ⁺ ). The charging was performed until the positive electrode voltage reached the upper limit charging voltage and the charging current was attenuated to 1 / 20C. The measurement temperature was 25 ° C.

<Measurement of particle size distribution>
The β-LiVOPO ₄ powder obtained by the heat treatment step was dispersed by performing ultrasonic treatment for 3 minutes in a mixed solution of 90 parts by weight and 10 parts by weight of hexametaphosphoric acid, and the particle size distribution of the particles was measured. From the particle size accumulation curve, D50 was determined for the particle size corresponding to 50% by weight, D90 for the weight corresponding to 90% by weight, and the ratio D90 / D50 was calculated to be 4.17. Also, in the relative frequency distribution, two peaks were confirmed, and the ratio (A / B) of the particle size A at the apex on the large particle size side to the particle size B at the apex on the small particle size side was 0.97. there were. A relative frequency distribution diagram is shown in FIG.

(Example 2)
The active material and half cell of Example 2 were produced in the same manner as in Example 1 except that the aqueous solution mixed with the raw materials was stirred for 40 hours.
The particle size distribution was measured in the same manner as in Example 1. From the particle size accumulation curve, the particle size corresponding to 50% by weight was D50, the weight corresponding to 90% was D90, and the ratio D90 / D50 was calculated to be 3.20. Further, in the relative frequency distribution, two peaks are confirmed, and the ratio (A / B) of the particle size A at the vertex on the large particle size side to the particle size B at the vertex on the small particle size side is 10.71. there were. A relative frequency distribution diagram is shown in FIG.

(Example 3)
The active material and half cell of Example 2 were produced in the same manner as in Example 1 except that the aqueous solution mixed with the raw materials was stirred for 24 hours.
The particle size distribution was measured in the same manner as in Example 1. From the particle size accumulation curve, D50 was determined for the particle size corresponding to 50% by weight, D90 for the weight corresponding to 90% by weight, and the ratio D90 / D50 was calculated to be 1.75. Also, in the relative frequency distribution, two peaks are confirmed, and the value of the ratio (A / B) of the particle size A at the vertex on the large particle size side to the particle size B at the vertex on the small particle size side is 5.40. there were. A relative frequency distribution diagram is shown in FIG.

Example 4
The active material and half cell of Example 2 were produced in the same manner as in Example 1 except that the aqueous solution mixed with the raw materials was stirred for 12 hours.
When the particle size distribution was measured in the same manner as in Example 1, the ratio D90 / D50 = 6.25. The A / B value was 12.83.

(Example 5)
The active material and half cell of Example 2 were produced in the same manner as in Example 1 except that the aqueous solution mixed with the raw materials was stirred for 6 hours.
When the particle size distribution was measured in the same manner as in Example 1, the ratio D90 / D50 = 5.10 was obtained. Moreover, the value of A / B was 3.91.

(Example 6)
The active material and half cell of Example 2 were produced in the same manner as in Example 1 except that the aqueous solution mixed with the raw materials was stirred for 48 hours.
When the particle size distribution was measured in the same manner as in Example 1, the ratio D90 / D50 = 1.10 was obtained. Moreover, the value of A / B was 0.47.

(Example 7)
In the precursor synthesis step, after all the lithium source, phosphoric acid source, and vanadium source as raw materials were mixed, instead of standing, the mixture solution was stirred at a stirring temperature of 45 ° C. for 80 hours, as in Example 1. The active material and half cell of Example 7 were produced by the method described above.
When the particle size distribution was measured in the same manner as in Example 1, the ratio D90 / D50 = 2.29 was obtained. The A / B value was 4.81.

(Example 8)
In the precursor synthesis step, after all the lithium source, phosphoric acid source, and vanadium source as raw materials were mixed, instead of standing, the mixture solution was stirred at 60 ° C. for 80 hours, and the same as in Example 1. The active material and half cell of Example 8 were produced by the method described above.
When the particle size distribution was measured in the same manner as in Example 1, the ratio D90 / D50 = 2.85 was obtained. The A / B value was 3.37.

Example 9
In the precursor synthesizing step, after all the lithium source, phosphoric acid source, and vanadium source as raw materials were mixed, instead of standing, the mixture solution was stirred at 80 ° C. for 80 hours, and the same as in Example 1. The active material and half cell of Example 9 were produced by the method described above.
When the particle size distribution was measured by the same method as in Example 1, it was found that the ratio D90 / D50 = 3.62. The A / B value was 9.61.

(Comparative Example 1)
The active material and half cell of Example 2 were produced in the same manner as in Example 1 except that the aqueous solution mixed with the raw materials was stirred for 24 hours and the heat treatment temperature was 650 ° C.
Moreover, when the particle size distribution was measured by the same method as in Example 1, it was D90 / D50 = 8.50. Moreover, the value of A / B was 15.68.

(Comparative Example 2)
The active material and half cell of Example 2 were produced in the same manner as in Example 1 except that the aqueous solution mixed with the raw materials was stirred for 40 hours and the heat treatment temperature was 650 ° C.
Moreover, when the particle size distribution was measured in the same manner as in Example 1, it was D90 / D50 = 10.00. The A / B value was 23.05.

In the same manner as in Example 1, discharge capacity values per 1 g of 0.1 C active material of Example 2 and Comparative Example 1 were obtained. The results are shown in Table 1. As can be seen from Example 1 to Example 9, the positive electrode material in which the value of D90 / D50 falls within the range of 1.10 or more and 6.25 or less showed a good discharge capacity of around 140 mAh / g at 0.1C. On the other hand, the positive electrode material outside this range had a discharge capacity of around 120 mAh / g. Moreover, the same tendency was seen also in the charge / discharge measurement in 1C performed as a comparison of the high rate discharge characteristic. Further, as can be seen from Example 1 to Example 9, the ratio between the particle size A at the apex on the large particle size side and the particle size B at the apex on the small particle size side obtained from the relative frequency distribution diagram of the particle size distribution ( Also from the value of A / B), the positive electrode material in the range of 0.97 to 10.71 showed high high-rate discharge characteristics. Further, as can be seen from Example 4, the positive electrode material whose (A / B) value is out of the range of 0.97 to 10.71 also has a D90 / D50 range of 1.10 to 6.25. Although no deterioration of the high rate discharge characteristics was observed when entering, positive electrode materials in which both values were out of the specified range as shown in Comparative Examples 1 and 2 showed a decrease in high rate discharge characteristics.

Claims (7)

A positive electrode material for a lithium ion secondary battery, wherein the ratio (D90 / D50) of the cumulative particle diameter D90 and the cumulative particle diameter D50 of β-LiVOPO ₄ is 1.10 or more and 6.25 or less. The ratio (A / B) of the particle size A at the apex on the large particle size side to the particle size B at the apex on the small particle size side in the relative frequency distribution of the particle size distribution measurement of β-LiVOPO ₄ is 0.97 or more and 10 The positive electrode material for a lithium ion secondary battery according to claim 1, wherein the positive electrode material is .71 or less.   The positive electrode for lithium ion secondary batteries using the positive electrode material for lithium ion secondary batteries of Claim 1 or 2.   The lithium ion secondary battery using the positive electrode for lithium ion secondary batteries of Claim 3.   Lithium ion secondary comprising a precursor synthesis step for obtaining a precursor by drying a mixture containing a lithium source, a phosphate source, a vanadium source, a reducing agent and water, and a heat treatment step for heat-treating the obtained precursor A method for producing a positive electrode material for a battery.   The lithium ion secondary battery positive electrode material according to claim 5, wherein in the precursor synthesis step, a mixture containing a lithium source, a phosphate source, a vanadium source, a reducing agent, and water is left standing or stirred. Production method.   The method for producing a positive electrode material for a lithium ion secondary battery according to claim 5 or 6, wherein in the heat treatment step, the mixture is heat treated at 400 ° C to 650 ° C.

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