JP4505695B2 - Cathode active material for non-aqueous electrolyte secondary battery and method for producing the same - Google Patents

Description

【００４１】
【表２】

【００４２】
前記、表１と２から、実施例の電池は比較例の電池と比較して、８０％以上のいずれも高い容量維持率を保ちながら、高いタップ密度を実現し、充填性が向上していることがわかる。
【００４３】
【発明の効果】
本発明になる非水系電解質二次電池用正極活物質は、非水系二次電池の正極活物質として用いることで、高いサイクル特性を維持したまま、正極としての成形性、充填密度の向上を図ることが可能であり、単位体積当たりの初期容量の大きな二次電池を提供することができるという効果がある。
【図面の簡単な説明】
【図１】 正極活物質を用いたコイン電池の概略縦断面図である。
【符号の説明】
1 正極（評価用電極）
2 セパレーター（電解液含浸）
3 リチウム金属負極
4 ガスケット
5 正極缶
6 負極缶[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a positive electrode active material for a non-aqueous electrolyte secondary battery and a method for producing the same. More specifically, the present invention improves the formability and packing density as an electrode without impairing high cycle characteristics, and is further high as a battery. The present invention relates to a positive electrode active material for a non-aqueous secondary battery that can have an initial capacity, and a method for producing the same.
[0002]
[Prior art]
In recent years, with the widespread use of portable devices such as mobile phones and laptop computers, there is an increasing demand for small and lightweight secondary batteries with high energy density. There is a non-aqueous electrolyte type lithium ion secondary battery as such, and research and development have been actively conducted and put into practical use.
[0003]
This lithium ion secondary battery includes a positive electrode using a lithium-containing composite oxide as an active material, and a material capable of inserting and extracting lithium, such as lithium, a lithium alloy, a metal oxide, or carbon, as an active material. The main components are a negative electrode to be processed and a separator or a solid electrolyte containing a non-aqueous electrolyte. Among these components, lithium cobalt composite oxide (LiCoO ₂ ), lithium nickel composite oxide (LiNiO ₂ ), lithium manganese composite oxide (LiMn ₂ O ₄ ), etc. are being studied as positive electrode active materials. is there. In particular, a battery using a lithium cobalt composite oxide as a positive electrode has been developed to obtain excellent initial capacity characteristics and cycle characteristics, and various results have been obtained and put into practical use. .
[0004]
However, since lithium cobalt complex oxide uses rare and expensive cobalt as a raw material, it is a major cause of the cost increase of the positive electrode active material and the battery. In addition, lithium nickel composite oxide using nickel, which is cheaper than cobalt, is advantageous in terms of cost and capacity, and is being developed as a promising alternative material for lithium cobalt composite oxide. Batteries that use lithium-nickel composite oxide as the positive electrode active material have the risk of decomposition, heat generation, and ignition when held at high temperatures due to the instability of the positive electrode active material in the charged state. There are still a lot of problems to do.
[0005]
On the other hand, although lithium manganese composite oxide has a slightly smaller capacity than lithium cobalt composite oxide and lithium nickel composite oxide, it is cheaper than cobalt and nickel, and uses abundant resources. Therefore, it is expected to be a next-generation positive electrode material.
[0006]
A lithium ion secondary battery is required to have a high initial discharge capacity (initial capacity) and a small capacity deterioration (cycle characteristics) due to repeated charge / discharge cycles. Furthermore, with respect to the capacity, from the viewpoint of miniaturization described at the beginning, a battery having a large discharge capacity per unit volume is required.
[0007]
However, lithium-manganese composite oxides have poor cycle characteristics when a lithium-ion secondary battery is made using a material synthesized solely with manganese as the positive electrode active material, compared to when used or stored in high-temperature environments. The battery has a drawback of easily damaging the battery performance.
[0008]
In order to solve these disadvantages, a method of replacing a part of manganese with a metal element such as chromium, nickel, cobalt, etc. has been proposed. This improves the stability of the crystal structure, and improves cycle characteristics and high temperature holding characteristics. It turned out to be improved.
In general, when these metal elements are added, the raw metal element compound, manganese compound, and lithium compound are sufficiently pulverized and mixed in order to improve the reactivity and to promote the reaction more uniformly. Then it is necessary to synthesize.
[0009]
However, since the lithium manganese composite oxide obtained by such a method has very fine particles in the process, the formability when forming the positive electrode is poor and the packing density as an electrode does not increase. As a result, the battery capacity per unit volume is low.
[0010]
Therefore, as a method for improving the reactivity and making the reaction proceed more uniformly, the metal element compound to be added, the manganese compound, and the lithium compound are dissolved in a solvent, mixed, spray-dried and dried, and simultaneously reacted. Although a method of proceeding has been proposed, the lithium manganese composite oxide obtained by this method has a form of secondary particles in which fine primary particles are aggregated, but the inside of the secondary particles is hollow and has a sufficient density. The strength was not obtained, and as a result, the packing density as an electrode could not be increased.
[0011]
[Problems to be solved by the invention]
As described above, in the conventional non-aqueous electrolyte secondary battery using lithium manganese composite oxide as the positive electrode active material, while maintaining high cycle characteristics, the moldability and filling density as an electrode are improved, and the battery has a high initial capacity. It has been difficult to provide
[0012]
The present invention has been made by paying attention to such problems, and the object of the present invention is to maintain the high cycle characteristics by adding elements, and to maintain the high cycle characteristics without impairing the moldability and packing density of the positive electrode. An object of the present invention is to provide a positive electrode active material for a non-aqueous electrolyte secondary battery capable of improving the initial capacity and a method for producing the same.
[0013]
[Means for Solving the Problems]
As a result of various researches conducted by the present inventors to solve the above-mentioned problems, when applying a lithium manganese composite oxide in which a part of manganese is substituted with titanium to a positive electrode active material, fine primary particles are aggregated and compared. By adding a metal element compound so that the shape of the raw material of the manganese compound with powder characteristics that composes closely packed spherical or elliptical secondary particles is not destroyed, and mixing this with a lithium compound and heat-treating By using the obtained lithium manganese composite oxide, it is possible to prevent the occurrence of the above-mentioned problems, and to form a battery having a high discharge capacity per unit volume while maintaining excellent cycle characteristics and excellent moldability and filling properties. As a result, the present invention has been completed.
[0014]
That is, the positive electrode active material for a nonaqueous electrolyte secondary battery according to the first embodiment of the present invention is lithium represented by LiMn _2-x Ti _x O ₄ (provided that 0 <x ≦ 0.33 is satisfied). The manganese composite oxide is a positive electrode active material for a non-aqueous electrolyte secondary battery, wherein the secondary particles of the lithium manganese composite oxide have a spherical shape or an elliptical spherical shape.
[0015]
In addition, the method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to the first embodiment of the present invention includes a titanium compound in which the molar ratio of manganese to titanium is 2-x: x (where x is 0 < x ≦ 0.33 is satisfied), and a manganese compound in which the shape of secondary particles added in advance is spherical or oval is mixed with a lithium compound, and the mixture is obtained by heat treatment It is.
The titanium compound is an oxide of titanium. Further, when the titanium oxide is added to the manganese compound, the titanium oxide is dispersed in a solvent, and this is converted into a manganese compound. The impregnated material is mixed with a lithium compound.
[0016]
In the production method according to the second embodiment, the heat treatment temperature of the compound is 600 ° C. or higher and lower than 950 ° C., and the heat treatment is performed for 4 hours or longer.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail. The present invention relates to an active material in which a part of manganese is substituted with titanium for improving cycle characteristics.
[0018]
When the lithium manganese composite oxide is considered as a battery active material, charge and discharge are performed by lithium being desorbed and inserted as ions from the crystal structure. A pure lithium manganese composite oxide that is not substituted with a metal element or the like has a problem that the capacity deteriorates compared to the initial stage when the charge / discharge cycle is repeated. This is considered to be because when the lithium ions are repeatedly desorbed and inserted from the crystal structure, the base structure is gradually destroyed, and the place where the lithium ions should return is lost from within the crystal structure. In order to prevent this structural destruction, a method of substituting a part of manganese with another element has been proposed, and it has been reported that this method improves the charge / discharge cycle.
[0019]
Generally, substituting a part of manganese with another element reduces the amount of Mn ³⁺ that determines the capacity of the positive electrode material, so the initial capacity decreases, but in LiMn _2-x Ti _x O ₄ If the condition of 0 <x ≦ 0.33 is satisfied, it is possible to suppress the capacity drop to a practically allowable range.
[0020]
However, in general, in order to substitute with another element, it is necessary to thoroughly mix the manganese raw material compound and the substituted metal raw material compound with the lithium raw material compound during the synthesis. Solid-phase reaction using powdered solid as a reaction substance starts at the contact part between solid phases, and the reaction proceeds by the formation of reaction products at these interfaces, so the particles are fine. This is because the contact area increases and a uniform composition is obtained.
[0021]
In this way, the element-substituted lithium manganese composite oxide synthesized by the method of finely pulverizing and mixing so that the composition is as uniform as possible has improved cycle characteristics as the characteristics of the substance itself. However, from the viewpoint of the positive electrode material, since there are many fine particles, the tap density that directly affects the filling properties as an electrode is low, the moldability as an electrode is poor, and it is added as a conductive auxiliary. Since the amount of carbon and binder for improving moldability must be increased, the amount of active material contained in the unit volume of the molded positive electrode is reduced, resulting in a decrease in battery capacity. Resulting in.
[0022]
On the other hand, a method has been proposed in which both manganese raw material and metal raw material are dissolved in a solvent and mixed, and then the solvent is evaporated to achieve atomic level mixing. It becomes a hollow spherical particle, and its strength and tap density are not sufficient. In addition, a method of coprecipitation of a manganese raw material and a metal raw material at the atomic level as in the coprecipitation method is the most ideal method from the viewpoint of the uniformity of the composition, but it is difficult to control the particle size of the obtained powder. It has the problem.
[0023]
Therefore, in order to ensure that the powder has as large a tap density (packing density) as possible, it is important geometrically that the powder particles are spherical and have a particle size distribution with a certain width. Considering the powder as an actual positive electrode active material, the shape of the particles is close to a sphere, the particle size distribution is as sharp as possible, the center particle size is about several μm to several tens μm, and the moldability as an electrode In consideration, it is preferable that the amount of fine powder having a particle size of 1 μm or less is as small as possible. Manganese compounds having such powder properties are actually adjustable and are also commercially available.
[0024]
When the present inventors use a manganese compound having such powder properties as a raw material and synthesize it using a metal element addition method that maintains its powder characteristics, the resulting lithium manganese composite oxidation It has been found that the product has the same powder characteristics as the manganese raw material and can avoid the above problems.
That is, the positive electrode active material for a non-aqueous electrolyte secondary battery of the present invention is a lithium manganese composite oxide represented by LiMn _2-x Ti _x O ₄ (where 0 <x ≦ 0.33 is satisfied). The shape of secondary particles of the lithium manganese composite oxide is spherical or elliptical.
[0025]
Lithium manganese composite oxide having such powder characteristics can be obtained without the above-described titanium mixing process without undergoing a pulverization and mixing step that impairs the powder characteristics of the manganese compound whose secondary particles are spherical or elliptical. The compound can be mixed with a manganese compound by, for example, pulverizing only a titanium compound, or by dissolving only the titanium compound in a solvent and dispersing it in the manganese compound. : X (wherein x satisfies the condition of 0 <x ≦ 0.33) A manganese compound added in advance is mixed with a lithium compound, and this mixture can be obtained by heat treatment.
[0026]
That is, by using a manganese compound added in advance so that the molar ratio of manganese to titanium is 2-x: x in the range of 0 <x ≦ 0.33, it is suppressed to a practically acceptable capacity reduction. If this condition is not met, the initial capacity will decrease.
[0027]
Examples of the lithium compound used in the present invention include lithium carbonate, lithium hydroxide, lithium hydroxide monohydrate, lithium nitrate, and lithium peroxide.
In addition, the manganese compound has powder characteristics such as manganese oxide, manganese hydroxide, manganese chloride, manganese carbonate, manganese nitrate, manganese sulfate, manganese acetate, etc., with secondary particles having a spherical or elliptical shape. If there is, it can be suitably used.
[0028]
Further, finely pulverized powder that can sufficiently undergo solid phase reaction with a titanium compound, or any powder that can be dissolved in a solvent and uniformly dispersed, adhered, and reacted with a manganese compound can be used. Among these, titanium oxide can be dispersed in a solvent (for example, water) and can be easily added to a manganese compound. This oxide is dispersed in a solvent, and a manganese compound is charged into the manganese compound. The resulting mixture is uniformly dispersed and added to the manganese compound, mixed with the lithium compound, and heat-treated. Thus, a lithium manganese composite oxide can be obtained without impairing the powder characteristics.
[0029]
In addition, by setting the heat treatment temperature to 600 ° C or more and less than 950 ° C, the metal element M can be completely dissolved without causing a heterogeneous phase such as a compound of the additive metal element M, and the crystal structure has high integrity. Can be realized. Preferably, a higher initial capacity can be realized by setting the heat treatment temperature to 700 ° C. or higher and 850 ° C. or lower.
[0030]
Note that if the heat treatment temperature is less than 600 ° C., the crystallinity deteriorates because the reaction is insufficient, while if it exceeds 950 ° C., the crystal structure changes from cubic to tetragonal, which is not preferable. Further, the heat treatment is preferably performed for 4 hours or longer, and heat treatment for less than 4 hours causes a decrease in crystallinity and appearance of a different phase.
[0031]
When a positive electrode active material in which a part of manganese according to the present invention is substituted with titanium and the shape of secondary particles is spherical or elliptical spherical is used, forming as an electrode while maintaining high cycle characteristics by substitution of metal elements And a high initial capacity as a battery. Hereinafter, an embodiment according to the present invention will be described in detail with reference to the preferred drawings.
[0032]
【Example】
(Example)
Commercially available lithium hydroxide monohydrate, spherical manganese dioxide, and titanium oxide were prepared in order to synthesize a positive electrode active material in which a part of manganese was substituted with titanium. Spherical manganese dioxide and titanium oxide are weighed so that the molar ratio of manganese to titanium is (1) 1.83: 0.17, (2) 1.89: 0.11, (3) 1.94: 0.06, and then the titanium oxide is completely dispersed. Titanium oxide was dispersed in a quantity of pure water.
[0033]
Thereafter, spherical manganese dioxide was put into the solution and stirred while heating to volatilize water, thereby obtaining a dry powder. This dry powder and lithium hydroxide monohydrate were weighed so that the molar ratio of lithium to manganese + titanium was 1: 2, and mixed well enough to maintain the shape of spherical secondary particles. .
[0034]
The mixed powder was calcined at 475 ° C. for 2 hours in an oxygen stream, then calcined at 800 ° C. for 20 hours, and cooled to room temperature.
[0035]
When the obtained fired product was analyzed by powder X-ray diffraction using Cu Ka line, only a desired positive electrode active material having a spinel structure could be confirmed in a single phase. Moreover, when the lattice constant was obtained from the Rietveld analysis of the powder X-ray diffraction pattern, the lattice constant decreased linearly as the amount of titanium added to the samples (1) to (3) increased. The solid solution of titanium was confirmed. Moreover, the tap density of the obtained positive electrode active material was measured. Table 1 shows the lattice constant and tap density.
[0036]
Using the obtained active material, a battery was produced as follows, and the battery characteristics according to charge / discharge capacity were measured. 90 wt% of the active material powder was mixed with 5 wt% of acetylene black and 5 wt% of PVDF (polyvinylidene fluoride), and NMP (n-methylpyrrolidone) was added to make a paste. This was applied to a 20 μm thick aluminum foil so that the weight of the active material after drying was 0.05 g / cm ² , vacuum-dried at 120 ° C., and punched into a disk shape having a diameter of 1 cm to obtain a positive electrode.
[0037]
As shown in FIG. 1, the equivalent amounts of ethylene carbonate (EC) and dimethyl carbonate (DMC) using lithium metal as the positive electrode 1 and negative electrode 3 obtained and 1M LiClO ₄ as the supporting salt in the electrolyte solution. A mixed solution was used. The separator 2 made of polyethylene was impregnated with an electrolytic solution and assembled into a 2032 type coin battery in a glove box in an Ar atmosphere in which the dew point was controlled at −80 ° C. In FIG. 1, 4 is a gasket, 5 is a positive electrode can, and 6 is a negative electrode can.
[0038]
The battery assembled in this way is left for about 24 hours after assembly, and after the open circuit voltage (OCV) has stabilized, the current density with respect to the positive electrode is 0.5 mA / cm ² and the charge / discharge test is performed at a cutoff voltage of 4.3 to 3.0 V. Went. Table 2 shows the obtained discharge capacity (initial capacity) at the first cycle, the discharge capacity at the 50th cycle, and the ratio of the discharge capacity at the 50th cycle to the initial capacity (capacity maintenance ratio).
[0039]
(Comparative example)
In order to synthesize a pure positive electrode active material in which a part of manganese is not replaced with an element, commercially available lithium hydroxide monohydrate, spherical manganese dioxide was weighed so that the molar ratio of lithium to manganese was 1: 2. Except for the above, a positive electrode active material was synthesized in the same manner as in the Examples, and a lithium coin secondary battery was further produced. The results obtained are shown in Tables 1 and 2.
[0040]
[Table 1]

[0041]
[Table 2]

[0042]
From Tables 1 and 2, the battery of the example achieves a high tap density while maintaining a high capacity retention rate of 80% or more compared to the battery of the comparative example, and the filling property is improved. I understand that.
[0043]
【The invention's effect】
The positive electrode active material for a non-aqueous electrolyte secondary battery according to the present invention is used as a positive electrode active material for a non-aqueous secondary battery, thereby improving the formability and packing density as a positive electrode while maintaining high cycle characteristics. Therefore, it is possible to provide a secondary battery having a large initial capacity per unit volume.
[Brief description of the drawings]
FIG. 1 is a schematic longitudinal sectional view of a coin battery using a positive electrode active material.
[Explanation of symbols]
1 Positive electrode (Evaluation electrode)
2 Separator (electrolyte impregnation)
3 Lithium metal negative electrode
4 Gasket
5 Positive electrode can
6 Negative electrode can

Claims (2)

A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery comprising a lithium manganese composite oxide represented by the general formula LiMn _2-x Ti _x O ₄ (0 <x ≦ 0.33),
The oxide of titanium is dispersed in a solvent, manganese and titanium molar ratio is 2-x: x (x is 0 <x ≦ 0.33) as a shape of the secondary particles is spherical or oval A spherical manganese compound is added to the solution in which the titanium oxide is dispersed, the manganese oxide is impregnated in the manganese compound, and then dried to obtain a dry powder. A non-aqueous electrolyte characterized by mixing with a lithium compound at such a strength that the shape of the secondary particles is maintained, and heat-treating the mixture to obtain a lithium manganese composite oxide in which the shape of the secondary particles is spherical or elliptical A method for producing a positive electrode active material for a secondary battery.   2. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the heat treatment of the mixture is performed at 600 ° C. or higher and lower than 950 ° C. for 4 hours or longer.

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