JP4602689B2 - Positive electrode material for lithium ion secondary battery - Google Patents

Description

  The present invention relates to a positive electrode material (positive electrode active material) for a lithium ion secondary battery.

  In recent years, the demand for non-aqueous lithium ion secondary batteries as high-energy density batteries has been rapidly increasing, and research from various viewpoints has been conducted on improving the performance thereof.

  This lithium ion secondary battery is composed of three basic elements: a positive electrode and a negative electrode, and a “separator holding electrolyte” interposed between the two electrodes. In this case, a slurry obtained by mixing and dispersing a binder and, if necessary, a plasticizer in a dispersion medium, is applied to a current collector such as a metal foil or a metal mesh.

Among these, as the positive electrode active material, cobalt-based composite oxide (Li _1-x CoO ₂ ), nickel-based composite oxide (Li _1-x NiO ₂ ), manganese-based composite oxide (Li _1-x Mn ₂ O ₄₎ ), And composite materials of lithium and transition metals have been applied, and various materials based on these have been proposed.

In addition, the lithium composite oxide as described above used as a positive electrode material for a lithium ion secondary battery is generally a compound of an element (a carbonate such as Co, Ni, Mn, etc.) that is a main component of the positive electrode material for a lithium ion secondary battery. Or an oxide) and a lithium compound (lithium carbonate or the like) are mixed at a predetermined ratio and heat-treated.
As an introduction of such a method for synthesizing a lithium composite oxide, for example, the following documents can be cited.

JP-A-1-294364: “Carbon dioxide gas (carbon dioxide) is saturated in an aqueous solution containing chlorides of Ni and Co, and an aqueous sodium bicarbonate solution is added to the solution and left to stand. Lithium composite oxidation consisting of coprecipitation of salt, washing the resulting precipitate with water, drying in argon gas at 140 ° C, mixing this with lithium carbonate and heating to react in air. Manufacturing method ". Japanese Patent Application Laid-Open No. 11-307094: “Sulfuric acid aqueous solution of each component element other than lithium and ammonium bicarbonate aqueous solution to which a small amount of ammonia was added were added to the reaction vessel little by little at the same time. While maintaining the pH in the neutral range, the crystal growth of uniform composite carbonate is performed almost concentrically, and the resulting composite carbonate and lithium hydroxide are mixed and heated and fired in an oxygen gas flow atmosphere. A process for producing a lithium composite oxide comprising

  However, the present inventors have investigated the characteristics of lithium ion secondary batteries in which various lithium composite oxides are applied to the positive electrode material, and the conventional lithium composite oxides are sufficient in terms of sinterability and composition stability. It has been found that there are aspects that cannot be satisfied, and that these lead to deterioration of battery characteristics (rate characteristics, etc.).

Therefore, the present inventors previously proposed means for stably providing a positive electrode material for a lithium ion secondary battery described below as Japanese Patent Application No. 2003-1955 (see WO2004 / 064180) .
“After putting an aqueous solution of one or more of nickel chloride, manganese chloride and cobalt chloride into the lithium carbonate suspension to precipitate the carbonate, the obtained carbonate is mixed with a lithium source and fired. Or the resulting carbonate is oxidized to form an oxide, and then mixed with a lithium source and fired. Both the Na and S contents are less than 100 ppm by mass. A-O (where A is one or more of Ni, Mn and Co) lithium ion secondary battery positive electrode materials (dope such as Mg, Al, Ti, Cr, Fe, Cu or Zr for improving characteristics) Manufacturing method that may contain metal)
According to this proposed method, it is possible to stably provide a positive electrode material for a lithium ion secondary battery that is excellent in sinterability and composition stability and also excellent in battery characteristics.

  However, as a result of continued investigations by the present inventors, a Li-A-O (wherein A is one or more of Ni, Mn, and Co) lithium ion secondary battery positive electrode materials that have been conventionally known In addition, the lithium-ion secondary battery positive electrode material obtained by the method of the previous proposal (Japanese Patent Application No. 2003-1955) often has unstable low-temperature characteristics. For example, the electrode resistance is large in a low-temperature use environment. It has been recognized that there are further points to be improved in terms of battery characteristics, such as being prone to be.

  Therefore, the object of the present invention is to provide a positive electrode material (positive electrode) for a lithium ion secondary battery that can stably exhibit excellent battery characteristics (initial capacity, initial efficiency, electrode resistance, etc.) even in a low temperature environment. Active material) was placed on stable provision.

As a result of further studies to achieve the above object, the present inventors have obtained the following knowledge.
A) The lithium composite oxide used as the active material for the positive electrode of a lithium ion secondary battery is said to exhibit better battery characteristics as it is finer and more homogeneous, but its average particle size and specific surface area are lithium ion secondary It has a subtle effect on the capacity and efficiency of the battery, and it can be applied to the electrode current collector by adjusting both the average particle size and specific surface area of the lithium composite oxide to be within a specific range. Is maintained to prevent a decrease in capacity and efficiency, and also contributes to suppression of deterioration of low temperature characteristics.

B) Furthermore, the pores present in the aggregate of lithium composite oxide particles constituting the positive electrode material of the lithium ion secondary battery also affect the performance of the lithium ion secondary battery. It has a great influence on the characteristics.
That is, in the above-described positive electrode material (composite oxide) for Li-A-O type lithium ion secondary batteries, primary particles tend to be bonded to form secondary particles in the production process. Voids (pores) are easily formed in the secondary particles, which are a combination of the primary particles, and pores of various sizes are dispersed in the aggregate of these particles (positive electrode material: positive electrode active material). It becomes a state to do. The present inventors have found that the size of these pores greatly affects the low temperature characteristics of the battery.

   C) According to further studies by the present inventors, the ratio of “total volume of pores from a very small diameter to the vicinity of the average diameter” and “total volume of pores having a diameter of 1 μm or less” It was found that the characteristics were significantly affected. Furthermore, even if the strict average diameter is not used as a reference, a pore having a diameter of 5 μm is used as a reference (a pore having a diameter of 5 μm or more is considered to be formed by aggregation of secondary particles). By determining the ratio of “total volume of pores having a diameter of 5 μm or less” and “total volume of pores having a diameter of 1 μm or less”, a good range of low temperature characteristics can be obtained, and this ratio is in the range of 8 to 20%. Thus, it becomes possible to provide a positive electrode material (positive electrode active material) for lithium ion secondary batteries with good low-temperature characteristics by adjusting the manufacturing conditions of the positive electrode material (lithium composite oxide particles) for lithium ion secondary batteries It was confirmed.

The present invention is based on the above findings such as “the combination of the average particle size, specific surface area, and pore size distribution is important as an indicator of a positive electrode material (positive electrode active material) for lithium ion secondary batteries having excellent low-temperature characteristics”. Based on the matter,
“Li—A—O (where A is one or more of Ni, Mn, and Co) -based composite oxide particles having an average particle diameter of 2 to 15 μm. And the ratio of the total volume of pores having a diameter of 1 μm or less to the total volume of pores having a diameter of 5 μm or less present in the composite oxide particle aggregate of 8 to 1.2 m ² / g is 8 -20% positive electrode material for lithium ion secondary batteries with excellent battery characteristics at low temperatures ”
Is to provide.

  According to the present invention, a positive electrode material (positive electrode active material) that exhibits sufficiently satisfactory initial capacity and initial efficiency when applied to a lithium ion secondary battery and also exhibits good battery characteristics even in a low temperature environment is stably provided. can do.

Now, the present invention is Li-A-O (where A is one or more of Ni, Mn and Co) based lithium ion secondary battery positive electrode materials (Mg, Al, Ti, Cr, Fe, Cu for improving characteristics). Or a composite metal particle constituting the positive electrode material has an average particle diameter of 2 to 15 μm and a specific surface area of 0.3 to Adjusted to 1.2 m ² / g.
If the average particle size or specific surface area is out of the above range, not only the low temperature characteristics deteriorate, but also the applicability to a current collector such as an aluminum foil deteriorates and the capacity and efficiency of the battery also decrease.
A positive electrode material (positive electrode active material) having an average particle diameter and a specific surface area within the above ranges can be produced, for example, by the method proposed in Japanese Patent Application No. 2003-1955 described above. In this case, the average particle size increases and the specific surface area decreases as the oxidation temperature in the step of oxidizing the composite carbonate as the production raw material is increased.

Furthermore, in the present invention, the ratio of the total volume of pores having a diameter of 1 μm or less to the total volume of pores having a diameter of 5 μm or less present in the aggregate of composite oxide particles constituting the positive electrode material is regulated to 8 to 20%. The
If this ratio is less than 8%, it is difficult to maintain a sufficient electrolyte solution, resulting in deterioration of battery characteristics. On the other hand, if the ratio exceeds 20%, the electrical contact of the composite oxide particles is insufficient, and it becomes difficult to ensure the conductivity between the composite oxide particles, and the material which impairs the activity of the particle surface such as a binder. Acts to reduce the electrochemical reaction surface, so that good low-temperature characteristics (such as electrode resistance at low temperatures) cannot be stably exhibited. Of course, even if the ratio is in the range of 8 to 20%, if the average particle size or specific surface area of the composite oxide particles is out of the specified range, the adverse effect on the low temperature characteristics is large.
A lithium ion secondary battery positive electrode material in which the ratio of “total volume of pores having a diameter of 1 μm or less” to “total volume of pores having a diameter of 5 μm or less” is 8 to 20% is disclosed in, for example, Japanese Patent Application No. 2003-1955 In the proposed method, it can be produced by measures such as setting a higher oxidation temperature in the step of oxidizing the composite carbonate as a production raw material.

In addition, the positive electrode material (positive electrode active material) for lithium ion secondary batteries according to the present invention can be more specifically manufactured, for example, as follows.
First, a lithium carbonate suspension is prepared. The lithium carbonate concentration of the liquid to be produced is suitably about 20 to 600 g / l.
Subsequently, an aqueous solution of Ni, Mn, Co chloride having a desired composition is added or dropped into the prepared lithium carbonate suspension. At this time, a chloride aqueous solution of a different element such as Al, Si, Mg, Ca, Ti, Cr (a dopant element known as a battery characteristic improving element) may be added. Depending on the carbonate to be obtained, a single aqueous solution of Ni chloride, Mn chloride and Co chloride may be used.
The charging rate of the aqueous chloride solution is preferably adjusted so that the total addition amount is added in 10 minutes to 20 hours.
The liquid temperature may be room temperature, but it may be heated. In addition, it is desirable to stir the lithium carbonate suspension at a stirring speed of 50 to 400 rpm when the chloride aqueous solution is added. The stirring speed is determined according to the tank to be used.
A carbonate having a desired particle diameter can be obtained depending on the charging speed and stirring speed.

The carbonate thus obtained is oxidized into an oxide by oxidation treatment (firing in an oxidizing atmosphere such as the atmosphere), mixed with a lithium source (lithium carbonate, etc.), and heat treated (firing) to form lithium. A composite oxide is obtained and used as a positive electrode material (active material) for a lithium secondary battery. The oxidation treatment is preferably carried out under the condition of maintaining at a high temperature of 1000 to 1100 ° C. for 1 to 10 hours.
The oxidation treatment may be performed after the Li-containing composite metal carbonate is dried, or may be performed without drying.
The oxidation treatment can be carried out in a continuous furnace or other furnaces in addition to a normal stationary furnace.

By the way, Japanese Patent Laid-Open No. 11-135119 discloses that “primary particles having a size of 2 μm or less are aggregated, and a pore radius of 30 μm (0.003 μm) or less among 200 μm (0.02 μm) or less pores in the positive electrode active material. Li-A-O (provided that the total volume of voids having a pore radius of 30 mm or less and 0.002 cm ³ / g or less) A is one or more of Ni, Mn, and Co) Active material for non-aqueous secondary batteries consisting of complex oxides ”
This relates to an active material for a battery that can suppress oxidative decomposition of an organic solvent during high-temperature storage in a charged state and exhibits good discharge characteristics even after storage. Thus, the present invention is different from the present invention aiming at imparting particularly good low temperature characteristics. Therefore, in the above invention, the pores defined are those having a very small radius of 30 mm (0.003 μm) or less. The effect of the present invention cannot be obtained with such a definition.
The invention will now be illustrated by examples.

First, 0.4 liter of a lithium carbonate suspension (lithium carbonate concentration: 420 g / l) in which lithium carbonate was suspended in water was prepared.
Next, while stirring this lithium carbonate suspension (room temperature) at 300 rpm, Ni: Mn: Co chloride aqueous solution (Ni, Mn, Co chloride having a composition of Ni: Mn: Co of 1: 1: 1 was added thereto. The total concentration of the product was 2.9 mol / l and a room temperature aqueous solution) was added at 0.6 liter at an addition rate of 0.3 L / hr.
This treatment resulted in the formation of fine precipitates in the solution. The precipitates were filtered and separated, and then further examined by washing with water and drying. As a result, NiMnCo-based composite carbonates (Ni : Mn: Co = 1: 1: 1).

Next, the obtained composite carbonate was dried and divided into four, and subjected to an oxidation treatment in air at each oxidation temperature shown in Table 1 for 3 hours, and each of Ni: Mn: Co was 1: 1: 1. A fine-grained composite oxide having a composition was obtained.
These composite oxides and lithium carbonate were mixed and heat-treated in air at each heat treatment temperature shown in Table 1. In mixing, the molar ratio of Li / metal was 1.10.

When the powder of each compound obtained by the above heat treatment was measured by powder X-ray diffraction, it was confirmed that it was a layered lithium composite oxide having a chemical composition of Li _x Ni _0.33 Mn _0.33 Co _0.33 O ₂ (x = 1.10). It was.
The powder characteristics of these lithium composite oxide powders are also shown in Table 1.
The average particle size of the lithium composite oxide powder is a laser diffraction method, the specific surface area is a BET method using nitrogen, and the pore size distribution (pores having a diameter of 1 μm or less with respect to the total volume of pores having a diameter of 5 μm or less) The ratio of the total volume of each was measured by the mercury intrusion method.

  For reference, the distribution of pores in each sample is shown in FIG.

Subsequently, the characteristic investigation of the lithium ion secondary battery which used each obtained lithium complex oxide as the positive electrode active material was conducted.
In this investigation, NMP (N-methylpyrrolidone) was used as a solvent, and the obtained lithium composite oxide (active material) 85 wt%, acetylene black 8 wt%, and PVDF (polyvinylidene fluoride) 7 wt% were mixed. After producing the slurry, this was applied to an aluminum foil, dried, and then press-molded to obtain a “positive electrode sample for evaluating a lithium ion secondary battery”.

Evaluation of initial capacity and initial efficiency is a 2032 type coin cell type evaluation lithium ion secondary battery using the above positive electrode samples as the positive electrode and using lithium foil as the counter electrode, and 1 mol of LiPF ₆ is used as the electrolyte in the EC. This was carried out using a solution in a solvent having a ratio of (ethylene carbonate) / DMC (dimethyl carbonate) of 1: 1.
The charging conditions were 4.3V CCCV (constant current constant voltage mode), and the discharging conditions were 3.0V CC (constant current mode).
For the evaluation of the low temperature characteristics, 1 mol of LiPF ₆ was added as an electrolytic solution to PC (propylene carbonate), EC (ethylene carbonate), EMC (ethyl methyl carbonate) (1: 1: A battery using graphite as a negative electrode was prepared using the material dissolved in 3), and the electrode resistance of the positive electrode at −20 ° C. was measured by an impedance analyzer.
These evaluation results are shown in Table 2.

From the results shown in Tables 1 and 2, the following can be confirmed.
That is, a composite oxide (average ratio, specific surface area, and pore size distribution (ratio of the total volume of pores having a diameter of 1 μm or less to the total volume of pores having a diameter of 5 μm or less) within the specified range of the present invention ( Example 1) shows excellent initial capacity, initial efficiency, and low temperature characteristics when used as a positive electrode material (positive electrode active material) for a lithium ion secondary battery, whereas the average particle diameter and specific surface area of the composite oxide. Alternatively, when the pore size distribution is outside the specified range of the present invention (Comparative Example 1, Comparative Example 2 and Comparative Example 3), the initial capacity, initial efficiency, and low temperature characteristics are inferior.

  In the above-mentioned “Example”, only examples relating to lithium composite oxides containing both Ni, Mn and Co are shown, but when lithium composite oxides containing Ni, Mn or Co alone are targeted, Ni, For lithium composite oxides containing two types of Mn and Co, and lithium composite oxides containing Mg, Al, Ti, Cr, Fe, Cu, Zr or other dopant metals It has also been confirmed that excellent results can be obtained as well.

It is the graph which compared the distribution condition of the pore regarding the sample which concerns on an Example and a comparative example.

Claims (1)

Li-A-O (where A is one or more of Ni, Mn, and Co) based composite oxide particles, and the average particle diameter of the composite oxide particles aggregate is 2 to 15 μm, The ratio of the total volume of pores having a diameter of 1 μm or less to the total volume of pores having a specific surface area of 0.3 to 1.2 m ² / g and existing in the composite oxide particle diameter of 5 μm or less is 8 to 20 %, A positive electrode material for a lithium ion secondary battery excellent in battery characteristics at low temperature.

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