JP2006318928A - Cathode active substance for lithium secondary battery, and nonaqueous lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium secondary battery which raises an output by an internal resistance reduction and inhibits an output deterioration, a cathode active substance used for this, and a process of manufacturing it. <P>SOLUTION: The size of a crystallite measured by a Hall method from the integration width of each diffraction peak acquired from an X-ray diffraction pattern is larger than 600 Å and less than 800 Å in the cathode active substance for the lithium secondary battery. This cathode active substance is expressed with a composition formula: Li<SB>a</SB>Mn<SB>x</SB>Ni<SB>y</SB>X<SB>z</SB>O<SB>2</SB>(X is at least either Co or Al), in a range of 1≤a≤1.2, 0≤x≤0.65, 0.35≤y≤1, and 0≤z≤0.65 and it is desirable that it is a composite oxide having a stratified rock salt structure of x+y+z=1. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a positive electrode active material for a lithium secondary battery used in a small portable information terminal, a power storage power source, an electric vehicle, or the like, and a method for manufacturing the same, and has low internal resistance and increases internal resistance even when repeatedly charged and discharged. The present invention relates to a method for producing a positive electrode active material capable of suppressing the above and providing a high output from a low temperature to a high temperature region, a positive electrode active material, and a lithium secondary battery equipped with the positive electrode active material.

In recent years, lithium secondary batteries have attracted attention not only for portable terminals but also for in-vehicle use such as hybrid vehicles and electric vehicles because of their high output and light weight.
Generally, a lithium secondary battery is configured by arranging a positive electrode, a negative electrode, and a separator in a container and filling a non-aqueous electrolyte solution with an organic solvent. The positive electrode material is obtained by applying a positive electrode active material to a current collector such as an aluminum foil. Examples of the positive electrode active material include lithium cobaltate (LiCoO ₂ ) having a layered rock salt structure, lithium nickelate (LiNiO ₂ ), lithium manganate having a spinel structure (LiMn ₂ O ₄ ), and the like. A powder made of an oxide of a transition metal is mainly used. For example, Patent Document 1 discloses the production method in detail. In general, these positive electrode active materials are synthesized by mixing and firing lithium salt powders (LiOH, Li ₂ CO ₃ etc.) and transition metal oxides (MnO ₂ , Co ₃ O ₄ , NiO etc.) powders. Has been.
Since the electrical conductivity of the positive electrode active material is 10 ⁻¹ to 10 ⁻⁶ S / cm ² , which is a low value compared to a general conductor, the current collector between aluminum and the positive electrode active material or between the active materials In order to enhance the electrical conductivity, a conductive additive such as carbon powder having better electrical conductivity than the positive electrode active material is used (see, for example, Patent Document 2). Actually, carbon powder of several to several tens of percent by weight is mixed with the positive electrode material, and further kneaded with a binder such as PVdF (polyvinylidene fluoride) or PTFE (polytetrafluoroethylene). The positive electrode is manufactured through a drying and pressing process after being kneaded into a shape and applied to a current collector foil with a thickness of about 100 μm.

In general, the battery capacity per unit volume can be estimated by the product of the capacity per weight (mAh / g) of the positive electrode active material and the electrode density (g / cm ³ ). That is, one of the factors that determine the battery capacity is to increase the capacity per unit weight of the active material itself, which largely depends on the composition and purity of the raw materials. The other is to increase the density of the electrode, which largely depends on the filling property of the positive electrode active material related to the shape, particle size, particle size distribution and the like of the particles.

Conventionally, various proposals for increasing the capacity have been made. For example, in order to improve the filling property, Patent Document 3 includes composite oxide particles including Li and at least one transition element selected from the group consisting of Co, Ni, Mn, and Fe. There is described a positive electrode active material containing 90% or more of spherical and / or elliptical particles in which D1 / D2 is in the range of 1.0 to 2.0 when the longest diameter is D1 and the shortest diameter is D2.
There are also many proposals for improving the capacity by controlling the particle size and the like. For example, Patent Document 4 discloses spinel-type lithium manganese composite oxidation of ultrafine particles in which the median particle size of primary particles is 0.01 μm or more and 0.2 μm or less, and the median particle size of secondary particles is 0.2 μm or more and 100 μm or less. Things are disclosed. Further, Patent Document 5 discloses a spinel-type lithium manganese having an average particle diameter of 5 to 20 μm, a BET specific surface area of 1.0 m ² · g ⁻¹ or less, and an average crystallite diameter determined from the Hall method of 1000 mm or more. Composite oxides are disclosed.

  On the other hand, regarding the composition of the positive electrode active material, spinel lithium manganese composite oxide is advantageous in terms of cost, but it is known that the capacity is low and there is a problem in durability at high temperatures. Although the improvement is progressing in Patent Document 6 and the like, it is far from practical use. In addition, lithium cobalt composite oxide (LiCoO2) and lithium nickel composite oxide (LiNiO2) having a layered rock salt structure have better capacity and durability at high temperatures than spinel type, but there are problems with cost, safety, etc. Yes, it is still difficult to put into practical use for electric vehicles. In this respect, the Li-Mn-Ni-Co composite oxide disclosed in Patent Document 7 is well-balanced in terms of capacity and cost, and is notable.

JP-A-8-17471 JP-A-10-125323 JP 2003-17050 A JP 2002-104827 A JP 2002-226214 A JP 2001-110417 A Japanese Patent No. 3244314

  Lithium secondary batteries deteriorate when repeatedly charged and discharged. This deterioration includes capacity deterioration and output deterioration. The capacity deterioration means that the amount of electricity (Ah) that can be taken out from the battery decreases when charging / discharging is repeated at a constant charge / discharge current density. For this deterioration, attempts have been made to improve the raw material composition. On the other hand, output deterioration means that when a battery is repeatedly charged and discharged, the internal resistance of the battery increases, and when the battery is discharged at a large current density, the battery voltage (V) drops and the voltage (V) This is a phenomenon in which the output (W) that can be expressed by the product of current (A) decreases. Particularly in a hybrid vehicle using both an engine and an electric motor, rapid charge / discharge is repeatedly performed in a low to high temperature environment. In this severe environment, the degree of output deterioration determines the battery life. For this reason, countermeasures against this output deterioration are eagerly desired.

  Therefore, the present invention aims to increase the output by reducing the internal resistance, the positive electrode active material that suppresses the internal resistance and reduces the output deterioration, the manufacturing method thereof, and the output characteristics using the positive electrode active material An object of the present invention is to provide an excellent lithium secondary battery.

In the positive electrode active material, when the crystallite size obtained by the Hall method is within a predetermined range, even if high output and repeated charge / discharge are performed, an increase in internal resistance value is suppressed and output deterioration is small. I found out. That is, the positive electrode active material of the present invention is characterized in that the crystallite size measured by the Hall method from the integral width of each diffraction peak obtained from the X-ray diffraction pattern is greater than 600 mm and less than 800 mm. It is said.
It has been found that when the crystallite size obtained by the Hall method is 600 mm or less, the internal resistance value tends to increase, and even when the crystallite size is 800 mm or more, the internal resistance value tends to increase. When the size of the crystallite obtained by the Hall method is 600 mm or less, it is presumed that the diffusion rate of Li ions in the crystal at the time of charge / discharge decreases because the crystal structure has not yet been firmly formed. Further, when a crystallite value of 800 mm or more is measured, grain growth, that is, sintering is also progressing, and it is considered that the characteristics including the electrolytic solution are deteriorated. Therefore, here is a range of crystallites that can suppress an increase in internal resistance. At this time, the internal resistance can be reduced to obtain a high output, which is particularly suitable for vehicles. In this regard, the positive electrode active material disclosed in Patent Document 5 is a fine particle, but does not focus on the crystallite size. Further, in Patent Document 6, the size of the crystallite obtained by the Hall method is defined, but it is 1000 mm or more and deviates from the above range. However, both Patent Documents 5 and 6 are positive electrode materials having a spinel structure, and as described above, this has a problem in durability at high temperatures, and there is no consideration for reducing internal resistance.

Further, as described above, it is generally considered that the higher the filling property of the positive electrode active material, the higher the capacity per unit volume, the better the electrical contact state between the active materials, and the higher output. . Therefore, various studies for improving the filling property have been performed, and Patent Document 3 is an example. However, in particular, when trying to obtain a positive electrode active material with low internal resistance and low output deterioration, it is possible to contain electrolyte to some extent while maintaining electrical contact when the positive electrode active material is filled inside the battery. It is thought that the feature is necessary. From this point of view, the inventor of the present application has found that the filling property of the positive electrode active material has a filling property with little output deterioration even when the power is repeatedly output and charged repeatedly.
That is, in the positive electrode active material, the filling property represented by (tap density / true density) × 100 [%] is preferably less than 55%. Here, if the filling property is too small, the positive electrode active material tends to aggregate when applied on the positive electrode, and a large amount of binder is required, so the filling rate (%) = (tap density / true density). ) Is preferably 25% to 50%, more preferably 30% to 45%. Incidentally, the positive electrode active material of Patent Document 3 has a tap density of 2.9 g / cm ³ or more, and the true density of these positive electrode active materials is distributed to about 4 to 5 g / cm ^3, and the true density is estimated to be 5. Then, the value of filling property (%) = (tap density / true density) × 100, which is an index of the present invention, is 58% or more. As described above, it is common sense that the filling property of the conventional positive electrode active material is 55% or more, and attention has been paid exclusively to raising the filling property. Thus, the filling property of the positive electrode active material of the present invention is outside the range of conventional common sense numerical values.

Next, the composition of the positive electrode active material of the present invention is represented by the composition formula Li _a Mn _x Ni _y X _z O ₂ (X = Co, at least one of Al), and 1 ≦ a ≦ 1.2, 0 ≦ x ≦ The composite oxide has a layered rock salt structure in the range of 0.65, 0.35 ≦ y ≦ 1, 0 ≦ z ≦ 0.65 and x + y + z = 1. This composition forms a spinel structure or a layered structure depending on the mixing ratio of Mn, Ni, and Co, the firing atmosphere, and the firing temperature. As the positive electrode active material of the present invention, it was confirmed that the composition of the multi-component composite oxide having the layered rock salt structure as described above was effective. Then, the inventors of the present application further studied as follows. Increasing the amount of Mn in this composition is advantageous in terms of cost, but a spinel phase tends to occur, causing a problem in capacity and durability at high temperatures. Further, when the amount of Ni is increased, there is a problem of safety (overcharge, nail penetration, bursting at the time of collapse, ignition, etc.). As for Co, a large content is disadvantageous in terms of cost. In addition, a composition that can be synthesized in the atmosphere is desirable for low cost. In some cases, a part of Co can be replaced with Al in consideration of capacity, safety, and cost. However, if a large amount of Al is substituted, the safety increases and the cost is advantageous, but the capacity tends to decrease. In view of the above, in the present invention, the following composition is more desirable because only the layered rock salt structure is obtained even when fired in the air, and the capacity, safety and cost are balanced. That is, it is represented by the composition formula Li _a Mn _x Ni _y X _z O ₂ (X: at least one of Co or Al), 1 ≦ a ≦ 1.2, 0.2 ≦ x ≦ 0.5, 0.35 ≦ y ≦ 0.8, 0 ≦ z It is a complex oxide with a layered rock salt structure in the range of ≦ 0.45 and x + y + z = 1. More desirably, the composite oxide has a layered rock salt structure in the range of 1 ≦ a ≦ 1.2, 0.2 ≦ x ≦ 0.5, 0.35 ≦ y ≦ 0.7, 0.1 ≦ z ≦ 0.45 and x + y + z = 1.

Next, in the positive electrode active material and the method for producing the same of the present invention, the raw material is a lithium salt powder (LiOH, Li ₂ CO _3, etc.), a transition metal oxide (MnO ₂ , Co ₃ O ₄ , NiO etc.) powder, water The slurry raw material in which aluminum oxide or aluminum oxide is mixed and fired to synthesize the positive electrode active material and the mixing step are important. In the case of a multi-component composition used in the present invention, it is necessary to make the composition in a mixed state uniform. Otherwise, the composition varies depending on the location, and a portion where the spinel phase is generated and the capacity is reduced is generated, and the performance of the positive electrode active material as a whole is deteriorated.
In other words, the positive electrode active material of the present invention is pulverized so that at least a lithium salt and a transition metal oxide, cobalt oxide and / or aluminum hydroxide, aluminum oxide, etc. are dispersed in a solvent, and the median particle diameter D50 is 1 μm or less. The mixed slurry is used as a raw material. As a result of the study by the present inventors, it has been found that when D50 is larger than 1 μm, a tendency to adversely affect the characteristics starts to appear.

  Next, in the method for producing a positive electrode active material for a non-aqueous lithium secondary battery of the present invention, at least a lithium salt and a transition metal oxide, cobalt oxide and / or aluminum hydroxide, aluminum oxide, etc. are dispersed in a solvent, and a median is obtained. After grinding and mixing so that the particle size D50 is 1 μm or less, this raw material slurry is dried, granulated and granulated, and then fired at a temperature of 800 ° C. to 1100 ° C. in air, nitrogen atmosphere or oxygen atmosphere And then pulverizing, followed by heat treatment at a temperature of 400 ° C. to 900 ° C. in air, nitrogen atmosphere or oxygen atmosphere.

  When the raw material slurry is dried and fired, there are usually a method in which the slurry is put into a filter cloth and pressed, dehydrated, dried and fired, a method of drying and firing using a slurry dryer, etc. When the mixed raw material dried by these methods is fired, sintering proceeds considerably before the reaction is sufficiently completed, and it takes time to disintegrate in a subsequent process, and it becomes difficult to control the particle size distribution. Further, if the firing temperature is lowered to avoid the progress of sintering, there is a concern that an unreacted portion remains. For this reason, in this invention, it has the process of granulating a raw material slurry into a granular form, using a spray dryer etc., and baking. Due to this process, sintering can be performed in a short time without progressing the sintering, and crushing in the subsequent process can be completed in a short time. Moreover, it is easy to control the particle size distribution. As a result of examination in Examples, the firing temperature is desirably 800 ° C to 1100 ° C. Moreover, it is desirable to heat-treat after crushing. When heat treatment is performed, battery characteristics are stabilized. This is considered to have an effect of removing strain generated in the positive electrode active material during crushing. The heat treatment temperature is preferably 400 ° C to 900 ° C.

  Moreover, although it is the said crushing process, it is not preferable to carry out with a strong crushing force. In addition to giving a strong damage to the crystal structure itself of the positive electrode active material, there is a possibility that the change over time in the particle size during the pulverization is steep and pulverization is excessive, which makes management difficult in the process. Therefore, it is desirable to use a ball in which the surface of the ball used for grinding is coated with an organic material such as a resin. That is, the present invention is characterized in that the crushing is performed using a ball coated with an organic material such as a resin as a medium. The material to be coated is preferably nylon or the like. When this method is used, crushing does not proceed excessively, and damage to the crystal structure of the positive electrode active material can be minimized. Further, it is also preferable in that the strain can be removed in a short time by the heat treatment in the next step, and the contamination entering from the ball at the time of crushing can be burned off in the heat treatment step because it is an organic substance.

  From the above, in the positive electrode active material for a lithium secondary battery and the method for producing the same of the present invention, at least a lithium salt and a transition metal oxide, cobalt oxide and / or aluminum hydroxide, aluminum oxide, etc. are dispersed in a solvent. The mixture is pulverized and mixed so that the median particle diameter D50 is 1 μm or less, and the raw slurry is dried, granulated and granulated, and then at a temperature of 800 ° C. to 1100 ° C. in air, nitrogen atmosphere or oxygen atmosphere Calcination and pulverization, followed by heat treatment at a temperature of 400 ° C. to 900 ° C. in air, nitrogen atmosphere or oxygen atmosphere, and classification to obtain a positive electrode active material. Production parameters (composition, particle size of mixed raw material slurry, firing temperature, crushing condition, heat treatment temperature) so that the filling property and crystallite size of the positive electrode active material produced by this process fall within the scope of the present invention. Etc.).

  According to the present invention, a positive electrode active material manufacturing method and a positive electrode active material with high output and low output deterioration due to internal resistance reduction, and a lithium secondary material excellent in output characteristics using this positive electrode active material, particularly suitable for vehicles. A battery can be provided.

Hereinafter, embodiments of the present invention will be described based on examples. First, the parameter measuring method and means in the present invention will be described below.
First, a method for measuring the filling property of the positive electrode active material will be described. About 200 g of the positive electrode active material vacuum-dried at 120 ° C. for about 8 hours was filled into a cylindrical sample holder by free fall, and tapped 180 times at a rate of once per second. The tap density (weight / apparent volume) was calculated from the apparent volume and weight of the positive electrode active material after tapping. For the measurement in the present invention, a powder tester (type: PT-D) manufactured by Hosokawa Micron Corporation was used. The true density of the positive electrode active material was calculated from the lattice constant obtained from X-ray diffraction and the weighed composition. When calculation is difficult, you may measure using the particle density measuring device by a commercially available pycnometer method. From the tap density and the true density obtained from the above, filling property = (tap density / true density) × 100 was obtained by calculation.

Ｈａｌｌ法によると、正極活物質の回折ピ−クのθとその積分幅βには以下の関係が成り立つ。

ここでεは結晶子のサイズ、λはCuKα線の波長、ηは正極活物質の歪に相当する。Ｘ軸にsinθ、Ｙ軸にβcosθを取り、得られた正極活物質の回折ピ−クのθとその積分幅βをＸ軸とＹ軸にプロットしてゆき、最小二乗法を用いて近似直線を引く。Ｙ軸との交点から結晶子サイズεを求める。図２に測定結果の一例を示す。因みに図２の結晶子サイズは850Åを示している。このＨａｌｌ法から求まる結晶子の大きさは、実際の電子顕微鏡観察から得られる結晶子のサイズとはかならずしも一致しない場合がある。しかし、電子顕微鏡に比べ測定方法が簡便であり、本発明では正極活物質の性能と相関を見出したので、評価方法としてＸ線回折によるＨａｌｌ法を使用した。 An evaluation method for measuring the size of the crystallite is shown. In the present invention, an Rigaku X-ray diffractometer (RINT2500) was used. A positive electrode active material was filled in a sample cell, and X-ray diffraction was measured by a reflective diffractometer method using a monochromatic CuKα ray having a wavelength of 1.5406 mm as a radiation source. The measurement range was 2θ with a range of 10 to 70 degrees. The angle step during measurement was 0.006 deg and the scanning speed was 1 deg / min. Subtracting the diffraction intensity by K [alpha ₂ line from the obtained X-ray diffraction curves, each diffraction peak - to determine the integral breadth beta _M of click. β _M includes a component β derived from the positive electrode active material and a component βs derived from the measuring device. The integral width βs derived from the measuring device is obtained in advance. For the calculation of βs, the integral width of the diffraction peak obtained from the high-purity silicon powder for X-ray standard was used. Next, the integral width β derived from the positive electrode active material was determined using the following equation.

According to the Hall method, the following relationship holds between the diffraction peak θ of the positive electrode active material and its integral width β.

Here, ε corresponds to the crystallite size, λ corresponds to the wavelength of the CuKα ray, and η corresponds to the strain of the positive electrode active material. Taking sinθ on the X-axis and βcosθ on the Y-axis, plotting the diffraction peak θ and the integral width β of the obtained positive electrode active material on the X-axis and Y-axis, and using the least-squares method pull. The crystallite size ε is obtained from the intersection with the Y axis. FIG. 2 shows an example of the measurement result. Incidentally, the crystallite size in FIG. The crystallite size obtained from the Hall method may not always match the crystallite size obtained from actual electron microscope observation. However, since the measurement method is simpler than that of an electron microscope and the present invention has found a correlation with the performance of the positive electrode active material, the Hall method based on X-ray diffraction was used as an evaluation method.

  Next, the battery characteristic evaluation method of a positive electrode material is shown. A positive electrode material, a conductive additive (carbon powder), and a binder (8 wt% PVdF / NMP) were kneaded in an agate bowl at a weight ratio of 85: 10: 5 to obtain a slurry-like composite material. The obtained mixture was applied to a thickness of about 200 μm on a current collector (Al foil) having a thickness of 20 μm with a stainless steel spatula. The applied composite material was pre-dried at 80 ° C. for 2 hours, then cut into predetermined dimensions (width 10 mm, length approximately 50 mm) and pressed with a mold at a pressure of 1.5 t / cm 2 for 2 min. Finally, vacuum drying was performed at 120 ° C. for 2 hours to obtain a test positive electrode. At this time, the thickness of the applied positive electrode material is about 100 μm. The size of the coated part is 10 mm × 10 mm.

A simple battery was prepared by the following procedure. The positive electrode was transferred into an Ar atmosphere glove box kept at a dew point of −60 ° C. or lower, infiltrated with an electrolyte (EC: DMC = 1: 2, electrolyte 1M-LiPF ₆ ), and then a separator (25 μm thick porous) Polyethylene film), a 1 mm-thick metallic lithium counter electrode with a sufficiently thin oxide film, and a reference electrode, stacked together, sandwiched between coin-shaped stainless steel plates, and sealed in a glass bottle with a terminal to obtain a simple battery. Allow the cell to remain electrochemically balanced for several hours, and then connect each terminal (test electrode, counter electrode, reference electrode) to a charge / discharge measuring device (TOSCAT-3100 manufactured by Toyo System) to perform measurement. It was. The current density with respect to the electrode area at the time of charge was 0.5 mA / cm ² . When the potential of the positive electrode became 4.3 V vs. Li reference electrode, the charging was completed, and the initial voltage was measured when the current density during discharging was changed to 0.5, 3.0, 6.0 mA / cm ² . The discharge current density was taken on the horizontal axis and the battery voltage was taken on the vertical axis, and the internal resistance of the battery was calculated from the slope. In the measurement of the rate of increase in internal resistance after leaving at high temperature, the battery that has been measured at room temperature is first charged again, and then the battery is disassembled in a glove box and the positive electrode is taken out. Next, 7 g of the electrolytic solution is weighed and sealed in a sealed container. The sealed container is left in a hot water bath maintained at 50 ° C. After leaving it for 10 days, the positive electrode was taken out in the globe box and incorporated in a simple battery, and the internal resistance was measured in the same manner as described above. The value obtained by dividing the resistance change before and after leaving by the resistance before leaving and multiplying by 100 was taken as the rate of increase in internal resistance (%).

Examples will be described below.
(Examples 1 and 2)
The positive electrode active material of the present invention was manufactured along the manufacturing process shown in FIG. First, lithium carbonate (Li ₂ CO ₃ ), manganese dioxide (MnO ₂ ), cobalt oxide (Co ₃ O ₄ ), and nickel oxide (NiO) powders were used as raw materials, and the composition of the positive electrode active material was Li _1.08 Mn _0.33 Ni Weighed to _0.36 Co _0.31 O ₂ . For the subsequent pulverization and mixing, media mainly composed of zirconia (ZrO ₂ ) were used. Raw material powder, pure water, and media were put into a ball mill pot and mixed in a wet manner. The median particle size D50 of the raw material powder is about 10 μm at the initial stage, and a raw material slurry was prepared in which the median particle size D50 of the raw material powder was changed in the range of 10 to 0.5 μm (4 steps of 10, 5, 1, 0.5 μm). . Here, the raw material slurry of Example 1 has a median particle size of 1 μm, and the raw material slurry of Example 2 has a median particle size of 0.5 μm. The time required for the median particle size D50 of the raw material powder to be 0.5 μm was about 24 hours. After mixing, an appropriate amount of an aqueous solution in which PVA (polyvinyl alcohol) is dissolved is added and further mixed for 1 hour. The slurry is then removed from the pot and transferred to a storage tank. Granulate with a spray dryer and dry to produce granules with a diameter of 10-100 μm. The granules are then fired in the atmosphere at 1000 ° C. for 4 hours. After firing, pulverization with a ball mill is performed until a particle size distribution with a maximum particle size of 20 μm or less is obtained using media coated with a resin coat or the like. Next, heat treatment was performed in the atmosphere at 600 ° C. for 4 hours, and classification was performed with a vibration sieve having an aperture of 63 μm to obtain a positive electrode active material. Using this positive electrode active material, the filling property and the internal resistance and the rate of increase in internal resistance due to a simple battery were measured.

(Comparative Example 1)
A positive electrode active material produced by the same manufacturing method as in Examples 1 and 2 above, but using a raw material slurry having a median particle diameter D50 of 10 μm in the raw material powder at the stage of the raw material slurry.

(Comparative Example 2)
A positive electrode active material produced by the same production method as in Examples 1 and 2 above, but using a raw material slurry having a median particle diameter D50 of 5 μm of the raw material powder at the stage of the raw material slurry.

(Comparative Examples 3-6)
A raw material was weighed in a predetermined amount so as to have the same composition as in Examples 1 and 2, and the raw material powder median particle diameter D50 was changed in the range of 10 to 0.5 μm (4 steps of 10, 5, 1, 0.5 μm). The slurry was prepared and this was made into Comparative Examples 3-6. Next, these slurries were dried with a slurry drier. Then, it is fired at 1000 ° C. for 4 hours in the atmosphere. In this comparative example, as in the above example, since it is not granulated before firing, the mixed raw material becomes a cookie-like sintered body after firing. Next, crushing after firing is performed. However, since the sintering progresses and is hard, the media coated with the resin coat of the present invention cannot be crushed. Therefore, ball milling using a zirconia ball having higher crushing ability was performed. Next, heat treatment was performed in the atmosphere at 600 ° C. for 4 hours, and classification was performed with a vibration sieve having an aperture of 63 μm to obtain a positive electrode active material. Using this positive electrode active material, fillability, internal resistance, and internal resistance increase rate were measured.

(Comparative Example 7)
A raw material slurry was prepared in which a predetermined amount of raw materials were weighed so as to have the same composition as in Examples 1 and 2, and the median particle diameter D50 of the raw material powder was 0.5 μm. Next, the slurry was granulated with a spray dryer and dried to prepare granules having a diameter of 10 to 100 μm. Then, it is fired at 1000 ° C. for 4 hours in the atmosphere. Next, ball milling using zirconia balls was performed. Thereafter, heat treatment was performed in the atmosphere at 600 ° C. for 4 hours, and classification was performed with a vibration sieve having an aperture of 63 μm to obtain a positive electrode active material. Using this positive electrode active material, fillability, internal resistance, and internal resistance increase rate were measured.
The obtained evaluation results are shown in Table 1.

  From the above results, when the particle size of the raw material slurry is 1 μm or less, there is a tendency for the internal resistance to decrease. This is thought to be due to an increase in composition uniformity. In addition, even in the case of Comparative Examples 1 and 2 having a filling property of 55% or more even when the product is manufactured by the manufacturing process of the present invention, the internal resistance and the rate of increase in internal resistance are relatively large, which satisfies the present invention. I can't. In addition, the reason why the samples prepared in Comparative Examples 3 to 6 have high filling property is that granulation is not performed before firing, so that the positive electrode active material becomes a cookie-like hard sintered body after firing. That is, it is considered that it is difficult to crush, and even after crushing, the sintered grains are mixed and the particle size distribution is wide. However, in the case of Comparative Example 7, which was granulated and fired and crushed with a zirconia ball having a strong crushing ability, the filling property was 25%, which made it difficult to apply on the electrode. FIG. 3 shows the relationship between the fillability and the rate of increase in internal resistance. The filling property can be controlled by adjusting the crushing time of the crushing process using the resin coat ball of the present invention, further reducing the filling property and improving the characteristics including the electrolyte, and improving the internal resistance. It seems possible to reduce the rate of increase. In applications that require high output, such as hybrid cars, the internal resistance is desirably 35Ω or less and the rate of increase in internal resistance is desirably 20% or less in the simple cell evaluation. From this, the positive electrode active material of this invention is suitable as a lithium secondary battery for vehicles.

Next, the composition of the positive electrode active material was examined. Examples and comparative examples are shown below.
(Examples 3 to 7)
The following powders are lithium carbonate (Li ₂ CO ₃ ), manganese dioxide (MnO ₂ ), cobalt oxide (Co ₃ O ₄ ), nickel oxide (NiO), and aluminum hydroxide (Al (OH) ₃ ) It measured so that it might become in the composition range of the positive electrode active material of this invention. Subsequent manufacturing methods and steps were the same as in Examples 1 and 2. However, the median particle diameter D50 of the raw material slurry was 0.5 μm.

(Comparative Examples 8-11)
As raw materials, lithium carbonate (Li ₂ CO ₃ ), manganese dioxide (MnO ₂ ), cobalt oxide (Co ₃ O ₄ ), and nickel oxide (NiO) powders are outside the composition range of the positive electrode active material of the present invention shown below. Weighed so that Subsequent manufacturing methods and steps were the same as in Examples 1 and 2. However, the median particle diameter D50 of the raw material slurry was 0.5 μm.
The initial capacity was evaluated using the positive electrode active materials of the above Examples and Comparative Examples. The composition produced in Examples 3-7 and Comparative Examples 8-11 is shown in FIG. FIG. 4 shows a Mn—Ni— (Co, Al) ternary phase diagram, in which the inside of the double line is the composition range of the present invention and the shaded portion is a more desirable composition range. Also, ● represents an example, and ▲ represents a comparative example. These evaluation results are shown in Table 2. The crystal structure was determined from the X-ray diffraction pattern.

From the above results, only the layered rock salt structure is stably obtained in the composition regions of the present invention in Examples 3 to 7. In Examples 5 and 6, a part of Co was replaced with Al, but a stable layered rock salt structure was obtained although the capacity was slightly reduced. As described above, Al is a substituted species that can reduce the amount of expensive Co used and thus can provide cost merit. In each case, the capacity is 130 mAh / g or more, the internal resistance is 35Ω or less, and the resistance increase rate is 20% or less. Moreover, the spinel structure is produced | generated with the layered rock salt structure except this composition area | region of Comparative Examples 8-11. Increasing the content of Mn increases the cost merit, but as in Comparative Examples 10 and 11, an unknown X-ray diffraction pattern that seems to be an unreacted material starts to be observed, and the capacity is extremely large. It is falling. The generation of the spinel phase not only lowers the capacity, but also has poor internal resistance and rate of resistance increase compared to the examples. There is also concern about the high temperature durability. In this evaluation, the capacity is preferably 130 mAh / g or more for an electric vehicle.
A stable layered structure can be obtained even in a region where the amount of Co is large in a composition region other than the present invention, and there may be obtained performance that satisfies the capacity, internal resistance, resistance increase rate, and high-temperature durability. This is an area that cannot be put to practical use.

Next, the results of study on the effect of heat treatment are shown.
(Examples 8 and 9 and Comparative Examples 12 to 14)
A positive electrode active material was produced by the same raw materials, composition, production method and steps as in Examples 1 and 2. However, the median particle diameter D50 of the raw material slurry was 0.5 μm. And the temperature in the heat treatment process was changed. The internal resistance and the rate of increase in internal resistance in this example and the comparative example were measured.
The results are shown in Table 3.

  From the above results, it can be seen that the internal resistance and the rate of increase in internal resistance can be kept low by performing heat treatment. As described above, it is presumed that the intracrystalline strain of the positive electrode active material received in the crushing process can be relaxed. It is inferred that when the heat treatment temperature was 1000 ° C., the sintering started to progress and the grain shape changed. In addition, it is surmised that the internal resistance and the rate of increase in internal resistance deteriorated because the intracrystalline strain could not be relaxed well by the heat treatment at 300 ° C. As described above, in applications that require high output such as hybrid cars, the internal resistance is desirably 35Ω or less and the rate of increase in internal resistance is desirably 20% or less in the simple cell evaluation. Accordingly, it was found that the heat treatment temperature is preferably 400 to 900 ° C.

Next, the examination result about a crystallite is shown.
(Examples 10 to 13 and Comparative Examples 15 and 16)
A positive electrode active material was produced by the same raw materials, composition, production method and steps as in Examples 1 and 2. However, the median particle diameter D50 of the raw material slurry was 0.5 μm. And the calcination temperature was changed. The crystallite size, internal resistance, and internal resistance increase rate in this example and comparative example were measured.
The results are shown in Table 4.

  From the above results, it can be seen that the crystallite size can be controlled by the firing temperature. As the firing temperature increases, the diffusibility of atoms increases and the crystallite size grows, but the sintering proceeds in the same manner, and there is a high possibility that grains will be mixed after crushing. For this reason, it is thought that the filling property of the positive electrode active material is also increased, the characteristics including the electrolytic solution are deteriorated, and the internal resistance and the internal resistance increase rate are considered to be high values. Further, when fired at 700 ° C., a crystallite size of 400 mm or less is measured. In other words, there is a high possibility that the crystal structure has not yet been firmly formed, and it is presumed that it adversely affects the diffusion rate of Li ions during charge and discharge. FIG. 5 shows the relationship between the crystallite size and the rate of increase in internal resistance. In this evaluation, the rate of increase in internal resistance is preferably 20% or less. Accordingly, it was found that the size of the crystallite is preferably in the range of 400 to 850 mm.

It is a flowchart which shows the process of the manufacturing method by this invention. It is a figure showing the measurement of the crystallite by Hall method. It is a characteristic diagram which shows the relationship between a filling property and an internal resistance raise rate. It is a Mn-Ni- (Co, Al) ternary phase diagram showing the composition of the complex oxide described in the examples and comparative examples of the present invention. It is a characteristic diagram which shows the relationship between the crystallite size by Hall method, and an internal resistance raise rate.

Claims (5)

In the positive electrode active material, the crystallite size measured by the Hall method from the integral width of each diffraction peak obtained from the X-ray diffraction pattern is greater than 600 mm and less than 800 mm. Positive electrode active material for secondary battery. The positive electrode active material is represented by a composition formula Li _a Mn _x Ni _y X _z O ₂ (X = Co, at least one of Al), and 1 ≦ a ≦ 1.2, 0 ≦ x ≦ 0.65, 0.35 ≦ y ≦ 1 4. The positive electrode for a lithium secondary battery according to claim 1, wherein the positive electrode is a composite oxide having a layered rock salt structure in a range of 0 ≦ z ≦ 0.65 and x + y + z = 1. Active material. The positive electrode active material is represented by a composition formula Li _a Mn _x Ni _y X _z O ₂ (X: at least one of Co or Al), and 1 ≦ a ≦ 1.2, 0.2 ≦ x ≦ 0.5, 0.35 ≦ y ≦ 0.8 5. The positive electrode active material for a lithium secondary battery according to claim 4, wherein the positive electrode active material is a composite oxide having a layered rock salt structure in a range of 0 ≦ z ≦ 0.45 and x + y + z = 1. Disperse at least lithium salt and transition metal oxide, cobalt oxide and / or aluminum hydroxide, aluminum oxide, etc. used to synthesize the positive electrode active material in a solvent, and pulverize so that the median particle diameter D50 is 1 μm or less. 6. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the mixed slurry is used as a raw material. A nonaqueous lithium secondary battery comprising the positive electrode active material according to claim 1.

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