JP2008034369A - Positive electrode active material for nonaqueous electrolyte secondary battery and nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a positive electrode active material for a nonaqueous electrolyte secondary battery which is excellent in output characteristics and cycle characteristics, and provide a nonaqueous electrolyte secondary mattery provide with the above material. <P>SOLUTION: The positive electrode active material is an cathode active material for a nonaqueous electrolyte secondary battery containing Ni. Initial particles are condensed to make secondary particles. In a cross-section of the secondary particles, a total cross-sectional area of the initial particles, at least a part of which is exposed out of the secondary particles, is 40% or more of the total cross-sectional area of the initial particles composing the secondary particles. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a positive electrode active material for a non-aqueous electrolyte secondary battery. Furthermore, the present invention relates to a nonaqueous electrolyte secondary battery having a positive electrode containing a specific positive electrode active material.

In recent years, higher capacity of batteries has been demanded, and nonaqueous electrolyte secondary batteries have attracted attention. In order to further increase the capacity of the nonaqueous electrolyte secondary battery, the material of the positive electrode active material has been studied. As the positive electrode active material, a lithium composite oxide containing Co (Co-based, for example, LiCoO ₂ ) is used. From the viewpoint of cost, life and output of the nonaqueous electrolyte secondary battery, a lithium composite oxide containing Ni is used. (Ni-based), lithium composite oxide containing Mn (Mn-based), or lithium composite oxide containing both Ni and Mn (mixed system) has been studied (see, for example, Patent Documents 1 and 2).

In Patent Document 1, as the positive electrode active material, Ni and Mn have substantially the same molar ratio and the crystal particle size is 0.1 to 2 μm, and the crystal particles are aggregated to have a particle size of 2 to 20 μm. A mixture with certain secondary particles is used. By producing a non-aqueous electrolyte secondary battery using such a positive electrode active material, it achieves higher output and longer life than the case of using a conventional positive electrode active material (LiCoO ₂ ) while being inexpensive and high capacity. is doing.

In Patent Document _2, the average particle size of _{LiNi {1- (x + y)} } Co x M y O in the positive electrode active material represented by ₂ an average particle diameter of primary particles and 0.3~1μm secondary particle 5 By setting the thickness to ˜15 μm, it is possible to improve the output characteristics of the nonaqueous electrolyte secondary battery, particularly at low temperatures.
JP 2002-42813 A JP 2004-87492 A

  Patent Documents 1 and 2 describe that output characteristics can be improved, but further improvements in output characteristics are required. In general, when a Co-based material is used as the positive electrode active material, it is said that the output characteristics can be changed by changing the particle size of the secondary particles. However, whether or not the output characteristics can be changed by changing the particle size of the secondary particles in the Ni system is unknown and needs to be studied.

  Further, when a charge / discharge cycle is performed on the nonaqueous electrolyte secondary battery, the positive electrode active material repeatedly expands and contracts. Therefore, the positive electrode active material composed of secondary particles may be divided into a plurality of primary particles. Generally, a conductive agent is provided on the surface of secondary particles. As shown in Patent Documents 1 and 2, when the primary particles are too small compared to the secondary particles, the primary particles present inside the secondary particles are not exposed on the surface of the secondary particles. Therefore, the conductive agent is not applied to the surface of the primary particles present inside the secondary particles. From the above, if the secondary particles are divided into a plurality of primary particles when the primary particles are too small compared to the secondary particles, the conductivity of the positive electrode cannot be ensured. As described above, when the positive electrode active material disclosed in Patent Documents 1 and 2 is used, the life of the nonaqueous electrolyte secondary battery may be shortened.

  The present invention has been made in view of the above points, and its object is to provide a positive electrode for a non-aqueous electrolyte secondary battery that is excellent in output characteristics and cycle characteristics and can have a long life. An object is to provide an active material and a non-aqueous electrolyte secondary battery.

  In order to solve the above-mentioned conventional problems, the positive electrode active material for a nonaqueous electrolyte secondary battery of the present invention is a positive electrode active material for a nonaqueous electrolyte secondary battery containing Ni. The positive electrode active material is a secondary particle in which primary particles are aggregated, and in the cross section obtained by cutting the secondary particles, the sum of the cross-sectional areas of the primary particles at least partially exposed on the surface of the secondary particles constitutes the secondary particles. And 40% or more of the total cross-sectional area of the primary particles.

  In this configuration, many of the primary particles constituting the secondary particles are exposed on the surface of the secondary particles, and many of the primary particles can come into contact with the non-aqueous electrolyte. By increasing the amount of primary particles in contact with the non-aqueous electrolyte, the output characteristics of the non-aqueous electrolyte secondary battery can be improved.

  In addition, since most primary particles can directly exchange lithium ions with the non-aqueous electrolyte without passing between adjacent primary particles (grain boundaries), the output characteristics of the non-aqueous electrolyte secondary battery are improved. be able to.

  In general, electrical conductivity is secured on the surface of the secondary particles. In this configuration, since many of the primary particles are exposed on the surface of the secondary particles, conductivity is ensured on the surface of many of the primary particles. Therefore, even when the positive electrode active material is divided into a plurality of primary particles by the charge / discharge cycle, the secondary particles have many primary particles with ensured conductivity. Therefore, it is possible to improve cycle characteristics (ability to maintain the initial battery capacity even after repeated charge and discharge) compared to the case where the amount of primary particles present on the surface of the secondary particles is small.

  With the configuration of the present invention, a lithium composite oxide containing Ni as a positive electrode active material can be more effectively used, and a nonaqueous electrolyte secondary battery excellent in output characteristics and cycle characteristics can be provided.

  Before describing this embodiment, what the present inventors examined is shown below.

  The inventors of the present application prepared two types of Co-based and Ni-based, and examined the output characteristics. Here, in the two types of positive electrode active materials, the particle diameters of the secondary particles were the same, but the exposure ratios were different. Here, the exposure ratio means that at least a part of the cross-sectional area of all primary particles constituting the secondary particles in the cross-section obtained by cutting the secondary particles so as to pass through the approximate center of the secondary particles is the secondary particles. It is the ratio of the cross-sectional area of the primary particles exposed on the surface. Hereinafter, of the two types of positive electrode active materials, the one with a higher exposure ratio is referred to as a positive electrode active material L, and the one with a lower exposure ratio is referred to as a positive electrode active material S.

  As a result of examining the output characteristics, there was no difference in the Co system, but in the Ni system, the positive electrode active material L was superior to the positive electrode active material S. In other words, it was found that the positive electrode active material L is faster than the positive electrode active material S in the Ni system in terms of the rate at which lithium ions interact with the non-aqueous electrolyte.

  In general, the positive electrode active material is a secondary particle formed by agglomeration of primary particles, and the primary particle has no grain boundary, but the secondary particle has one or more grain boundaries. . In the primary particles in which at least a part of the primary particles constituting the secondary particles is exposed on the surface of the secondary particles, lithium ions move through the primary particles and are directly exchanged with the non-aqueous electrolyte. Therefore, if the moving speed of lithium ions in the primary particles is increased, the exchange of lithium ions can be increased. On the other hand, primary particles that are not exposed on the surface of secondary particles often cannot exchange lithium ions directly with the non-aqueous electrolyte, and lithium ions move within the primary particles and at grain boundaries. Is exchanged with the non-aqueous electrolyte.

  Here, since the positive electrode active material L has a higher exposure rate than the positive electrode active material S, in the case of the positive electrode active material L, in many cases, the lithium ions can be non-aqueous electrolyzed as long as they move within the primary particles. Direct interaction with the liquid. On the other hand, in the positive electrode active material S, in many cases, lithium ions are exchanged with the nonaqueous electrolytic solution by repeating movement within the primary particles and movement at the grain boundaries. In other words, in the positive electrode active material L, the moving speed of lithium ions depends on the moving speed in the primary particles, but in the positive electrode active material S, (moving speed in the primary particles)> (moving speed in the grain boundary). If so, the movement within the grain boundary becomes rate-limiting (the movement speed within the primary particle) <(the movement speed within the grain boundary), and the movement within the primary particle becomes the rate-limiting.

  In addition, although the Mn system was examined, the Mn system is not excellent in conductivity even if the movement rate of lithium ions at the grain boundary is increased by increasing the exposure ratio in the Mn system. Difficult to do.

  Taking the above into consideration, the above-described experimental results for Co and Ni systems will be discussed. In the Co system, the positive electrode active material L and the positive electrode active material S did not have a large difference in output characteristics. Therefore, it is considered that the movement in the primary particles is rate limiting. On the other hand, in the Ni system, the positive electrode active material L is superior in output characteristics to the positive electrode active material S, and therefore, it is considered that the moving speed at the grain boundary is rate-limiting.

  The inventors of the present application have found for the first time that the rate-determining stage of lithium ion movement is different between Ni-based and Co-based, and in Ni-based (moving speed in primary particles)> (moving speed at grain boundaries). Taking this into account, the present application was completed. Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  In the present embodiment, the positive electrode active material is a positive electrode active material for a non-aqueous electrolyte secondary battery, includes Ni, and includes secondary particles in which primary particles are aggregated. In the positive electrode active material, in the cross section obtained by cutting the secondary particles, the sum of the cross-sectional areas of the primary particles at least partially exposed on the surface of the secondary particles is the sum of the cross-sectional areas of all the primary particles constituting the secondary particles. 40% or more. In other words, in the positive electrode active material in the present embodiment, the exposure ratio is 40% or more.

  In such a positive electrode active material, many primary particles are exposed on the surface of the secondary particles, and many primary particles are in contact with the non-aqueous electrolyte, which improves the output characteristics of the non-aqueous electrolyte secondary battery. To do.

  Specifically, in the Ni system, as described above, it is considered that (moving speed in primary particles)> (moving speed at grain boundaries). However, in the positive electrode active material according to the present embodiment, since most of the primary particles constituting the secondary particles can directly exchange lithium ions with the non-aqueous electrolyte, the lithium ions do not pass through the grain boundaries. And non-aqueous electrolyte. Therefore, the positive electrode active material according to the present embodiment can improve the output characteristics of the nonaqueous electrolyte secondary battery as compared with the conventional Ni-based material.

  In the positive electrode active material according to the present embodiment, since most of the primary particles constituting the secondary particles are exposed on the surface of the secondary particles, a conductive agent is contained in most of the primary particles constituting the secondary particles. It has been applied. Therefore, even if the secondary particles are cracked and divided into a plurality of primary particles during the charge / discharge cycle, the conductive agent is applied to the surfaces of the primary particles, so that it is possible to suppress the decrease in capacity. . Thereby, cycle characteristics can be improved.

  Here, as a method of measuring the number of primary particles in the cross-section of the secondary particles, the secondary particles are cut with FIB (Focused Ion Beam), and an image of the cut surface is obtained using a scanning electron microscope. A method of measuring the number of primary particles exposed on the surface of secondary particles in the image can be mentioned. Also, after observing a large number of secondary particles using a scanning electron microscope and confirming the average particle size, a large number of secondary particles are filled with a resin, and the cut section of the resin is polished and confirmed with a scanning electron microscope. Also, the amount of primary particles in a cross section obtained by cutting secondary particles at almost half of the position can be measured.

  Moreover, as a method for obtaining the exposure ratio, primary particles exposed on the surface of the secondary particles are confirmed according to one of the methods described above, and the total cross-sectional area of the primary particles is calculated. Moreover, the average particle diameter of secondary particles is calculated | required using a scanning electron micrograph, and the cross-sectional area of secondary particles is calculated. And the ratio of exposure can be calculated | required by calculating the sum total of the cross-sectional area of a primary particle with respect to the cross-sectional area of a secondary particle.

  In the positive electrode active material according to the present embodiment, the exposure ratio is preferably 60% or more in the cross section obtained by cutting the secondary particles.

  When the exposure ratio is 60% or more, more primary particles are exposed on the surface of the secondary particles than when the exposure ratio is 40% or more. Therefore, the output characteristics and cycle life characteristics of the nonaqueous electrolyte secondary battery can be improved as compared with the case where the exposure ratio is 40% or more.

  Further, in the positive electrode active material according to the present embodiment, the exposure ratio is 40% or more, and the average length in the major axis direction of the primary particles with respect to the average particle diameter of the secondary particles in a cross section obtained by cutting the secondary particles. It is desirable that the thickness ratio is 0.2 or more.

In such a positive electrode active material, the number of primary particles exposed on the surface of the secondary particles can be increased. In addition, since the primary particles are large in the positive electrode active material according to the present embodiment, the primary particles are small even when the secondary particles are divided into a plurality of primary particles by repeating the charge / discharge cycle as described above. Compared to the case, the rate at which the metal element elutes from the primary particles into the non-aqueous electrolyte is reduced.
Therefore, even when the secondary particles are divided into primary particles, it is possible to suppress the deterioration of the capacity of the positive electrode.

  Here, instead of measuring the average length of the primary particles in the minor axis direction, the reason for measuring the average length of the primary particles in the major axis direction is shown. If secondary particles of the same size are formed using two types of primary particles with each particle, the number of primary particles exposed on the surface of the secondary particles is larger when the secondary particles are formed using elongated primary particles. Because it can be done. The average length of primary particles in the major axis direction is determined by measuring the length of each primary particle in the major axis direction by checking the cross section of the secondary particles using a scanning electron micrograph and obtaining the average value. It is possible to estimate.

  Furthermore, in the positive electrode active material according to the present embodiment, it is desirable that the exposure ratio is 40% or more and that Ni and Mn having substantially the same molar ratio are included.

  Thus, if the positive electrode active material contains substantially the same molar ratio of Ni and Mn, Ni and Mn are mixed at an atomic level of 1: 1 as disclosed in Patent Document 1. The electronic structure of Ni and the electronic structure of Mn can interact with each other. As a result, the conductivity of the positive electrode active material is improved, the output characteristics are improved, and the cycle characteristics are improved.

  The reason why the cycle characteristics are improved is thought to be that the crystal structure of the positive electrode active material is stabilized by mixing Ni and Mn at the atomic level, so that the elution of Mn from the positive electrode active material can be suppressed. .

  The reason why the crystal structure of the positive electrode active material is stabilized when Ni and Mn are mixed at an atomic level of 1: 1 may be as follows.

When Ni and Mn are dissolved at an atomic level, the dissimilar elements close to each other cause the electronic structure to interact and change. This change in the electronic structure changes the properties of each element. For example, normally, manganese element is easily dissolved in a non-aqueous electrolyte in a non-aqueous electrolyte secondary battery. For example, when LiMn ₂ O ₄ or LiMnO ₂ or the like is used as the positive electrode active material, Mn may be eluted from the positive electrode active material and deposited on the negative electrode, resulting in a shortened battery life. However, when nickel element is present near the manganese element, it is considered that the electronic state of the manganese element changes and elution of the manganese element into the non-aqueous electrolyte is suppressed, and as a result, the crystal structure is stabilized. Such a phenomenon is also observed in, for example, a nickel metal hydride battery. A hydrogen storage alloy containing Al or Mn is used for the negative electrode of the nickel metal hydride battery. Al or Mn alone is dissolved in the alkaline electrolyte. However, in the hydrogen storage alloy, Al or Mn is dissolved in a metal element such as Ni at an atomic level, so that the dissolution rate of Al or Mn in the alkaline electrolyte can be greatly reduced.

  As mentioned above, it is preferable that the positive electrode active material contains substantially the same molar ratio of Ni and Mn.

  Furthermore, in the positive electrode active material according to the present embodiment, the exposure ratio is 40% or more, the molar ratio of Ni and Mn is substantially the same, and the crystal structure is a rhombohedral structure. When the structure is approximated as a hexagonal crystal, the length in the c-axis direction of the hexagonal crystal is 14.2 mm or more, and the combined content of Ni and Mn is 60 mol% or more of the total metal elements constituting the positive electrode active material. It is desirable that With such a positive electrode active material, output characteristics and cycle characteristics can be further improved.

  This is because the order of the crystal structure (crystallinity) of the positive electrode active material can be improved when the positive electrode active material contains 60 mol% or more of Ni and Mn at substantially the same molar ratio as metal elements. In this ordered crystal structure, if the interlayer is 14.2 mm or more, the movement speed of lithium ions in the crystal (that is, the movement speed of lithium ions in the primary particles) can be sufficiently increased. It is. And if the movement speed of the lithium ion in a crystal becomes high enough, it will be thought that the movement speed of a lithium ion is influenced by the movement speed in a grain boundary. Therefore, in such a positive electrode active material, the effect obtained by increasing the exposure ratio can be greatly expressed. In the positive electrode active material, if the total content of Ni and Mn is less than 60 mol% of the content of all metal elements, it is not preferable for the following reason. If Ni and Mn are less than 60 mol%, the abundance of elements other than Ni and Mn increases, so that the preferred electronic structure of Ni and Mn is maintained even if the molar ratio of Ni and Mn is 1: 1. It will be impossible.

  That is, even if the exposure ratio is 40% or more, if the total content of Ni and Mn is less than 60 mol% of the total metal elements, the order of the crystal structure may not be maintained. Even if the exposure ratio is 40% or more, if the interlayer in the crystal structure is less than 14.2 mm, the diffusion rate of lithium ions in the crystal layer may not be sufficiently increased. Therefore, even if the ratio of the road circumference t is increased, the movement speed of lithium ions may not be increased, and it may be difficult to improve the output characteristics.

  Further, in the positive electrode active material according to the present embodiment, the exposure ratio is 40% or more, Ni and Co are included, the crystal structure is a rhombohedral structure, and the hexagonal crystal structure is approximated as a hexagonal crystal. It is desirable that the length of the crystal in the c-axis direction is 14.13 mm or more and the Ni content is 55 mol% or more of the total metal elements. With such a positive electrode active material, output characteristics and cycle characteristics can be further improved.

  This is because the order of the crystal structure of the positive electrode active material can be improved when the positive electrode active material contains 55 mol% or more of Ni, and the interlayer is 14.13% or more in the ordered crystal structure. This is because the moving speed of lithium ions in the crystal can be sufficiently increased. And it is thought that the movement speed of the lithium ion in the positive electrode active material depends on the movement speed in the grain boundary by sufficiently increasing the diffusion speed of the lithium ion in the crystal. Therefore, in such a positive electrode active material, the effect obtained by increasing the exposure ratio can be greatly expressed.

  That is, even if the exposure ratio is 40% or more, if the Ni content is less than 55%, the order of the crystal structure of the positive electrode active material may not be maintained. Even if the exposure ratio is 40% or more, if the interlayer is less than 14.13 mm, the diffusion rate of lithium ions in the crystal may not be sufficiently increased. Therefore, even if the exposure rate is increased, the movement speed of lithium ions may not be increased, and it may be difficult to improve output characteristics.

  The positive electrode active material described above can be produced using either method described below.

  A method for synthesizing a positive electrode active material is generally a method in which a metal hydroxide is fired to form a metal oxide, and a lithium salt is added to the metal oxide and fired again. In order to synthesize a positive electrode active material having a large primary particle size, it is preferable to use a metal hydroxide having a large primary particle size. That is, the first synthesis method is a method in which a positive electrode active material is synthesized using a metal hydroxide having a large primary particle size.

  The second synthesis method is to optimize the composition of materials such as metal oxides or lithium salts, or the firing conditions such as the firing temperature or firing time. In the examples described later, the positive electrode active material according to this embodiment is synthesized using the second synthesis method.

  When a non-aqueous electrolyte secondary battery (specifically, a lithium ion secondary battery) is manufactured using any of the positive electrode active materials described above, a battery excellent in output characteristics and cycle characteristics can be provided. In addition, as a manufacturing method of a nonaqueous electrolyte secondary battery, a well-known manufacturing method can be used.

  In the present embodiment, the form of the non-aqueous electrolyte is given as an example of the electrolyte, but it goes without saying that the same effect is exhibited even with a gel electrolyte.

(Example 1)
A schematic diagram of a production apparatus for synthesizing a hydroxide (a raw material of a positive electrode active material) is shown in FIG. As raw materials, a 1 mol / L nickel sulfate aqueous solution, a 1 mol / L manganese sulfate aqueous solution, and a 1 mol / L cobalt sulfate aqueous solution were used while controlling the flow rate with pumps 4 to 8 so as to achieve the composition ratio of the composite hydroxide to be produced. Then, a 5 mol / L aqueous ammonium solution and a 5 mol / L aqueous sodium hydroxide solution were continuously supplied to the reaction vessel 1 while stirring. The pumps 4 to 6 supplied a metal element sulfate aqueous solution, the pump 7 supplied a sodium hydroxide aqueous solution, and the pump 8 supplied an ammonium aqueous solution.

  Here, the manufacturing apparatus will be described. The production apparatus is provided with pipes 9 to 11 for injecting these solutions from the pumps 4 to 8 into the reaction tank 1, and the above three kinds of metal element sulfate aqueous solutions enter the reaction tank 1. The mixture was mixed before to obtain a uniform mixed aqueous solution.

  In addition, a cylindrical tube 2 is provided in the reaction tank 1, and a stirring rod 3 is provided in the tube 2. The composite hydroxide fine particles were formed in the tube 2. Since the downward force is applied to the composite hydroxide fine particles by the stirring rod 3, the composite hydroxide fine particles collide with each other. Grown and agglomerated particles. The composite oxide particles overflowed through the outside of the tube 2 as indicated by arrows in FIG. 1 and were taken out of the manufacturing apparatus. At this time, only a small amount of overflow occurred once the tube 2 was turned around, and most of the fine particles grew around the reactor 1 many times. In addition, the temperature of the reaction tank 1 was kept at 30 ° C to 50 ° C.

  Various composite hydroxides with different compositions are prepared by changing the flow rate of aqueous metal salt solution, aqueous ammonium solution or aqueous sodium hydroxide solution, or changing the temperature of the reaction vessel or the stirring speed of the stirring rod 3 did.

  Next, these composite hydroxides were washed with water and dried, and then fired in the air to obtain composite oxides.

  Thereafter, these composite oxides were mixed with lithium carbonate so that the number of moles of Li was equal to the number of moles of metal elements of the composite oxide, and fired at 1100 ° C. in the atmosphere. Thereafter, the fired composite oxide was mixed with lithium carbonate corresponding to volatile matter Li, and fired again at 1000 ° C. By changing the firing temperature or firing time, the number of primary particles exposed on the surface of the secondary particles was adjusted.

  In this way, positive electrode active materials 1a to 1aa containing Ni and Mn were obtained. Thereafter, the total cross-sectional area of the primary particles exposed on the surface of the secondary particles was determined. Specifically, first, a large number of secondary particles were observed using a particle size distribution meter and a scanning electron microscope, and then the average particle size of the secondary particles was confirmed using a scanning electron micrograph. Thereafter, a large number of secondary particles were embedded in the resin, and the cut section of the resin was polished and confirmed with a scanning electron microscope. As a result, in the cross section obtained by cutting the secondary particles into half, primary particles at least partially exposed on the surface of the secondary particles were confirmed. And the sum total of the cross-sectional area of the primary particle which at least one part exposed to the surface of the secondary particle was calculated | required.

  Further, the total cross-sectional area of the primary particles constituting the secondary particles was determined, and the ratio (exposure ratio) to the cross-sectional area of the primary particles in which at least a part determined above was exposed on the surface of the secondary particles was determined. Moreover, the average length in the major axis direction of the primary particles was determined using a cross-sectional photograph of the secondary particles taken using a scanning electron microscope. The average particle size of the secondary particles and the average length of the primary particles in the major axis direction were obtained by scanning electron micrographs, “Image analysis type particle size distribution measurement software MAC-View version 3.5, manufactured by Mountec Co., Ltd.” To obtain the image.

  As an example, a scanning electron micrograph of the surface of the active material 1u having a composition of Ni: Mn: Co = 33: 33: 33 is shown in FIG. Further, FIG. 3 shows a scanning electron micrograph of a cut section of the resin filled with the active material 1u. In addition, when calculating | requiring both the average particle diameter in the major axis direction of a primary particle and the primary particle in the major axis direction, it calculated | required by image-processing the photograph in a visual field wider than the visual field shown to FIG. 2 and FIG. . Further, the secondary particles are a large mass in FIG. 3, and the primary particles are small particles constituting the large mass.

  From these results, the positive electrode active material 1u was confirmed to have an exposure rate of 85%. Furthermore, it was found that the average particle diameter of the secondary particles was 11 μm, and the average length of the primary particles in the major axis direction was 4.3 μm.

  Further, X-ray diffraction measurement of the positive electrode active material 1u is performed, and from the diffraction pattern, the crystal structure of the active material is a rhombohedral structure, and the crystal structure of the positive electrode active material is approximated as a hexagonal crystal The length in the c-axis direction was confirmed.

  Similarly, for other active materials, the ratio of exposure (“surface primary particle amount (%)” in Table 1), average particle size of secondary particles (“average particle size (μm)” in Table 1) The average length of primary particles in the major axis direction (“major axis direction (μm)” in Table 1), the ratio of the average length of primary particles in the major axis direction to the average particle size of secondary particles (“ Ratio ”) and the length in the c-axis direction (“ c-axis (Å) ”in Table 1) when the crystal structure of the positive electrode active material is approximated as a hexagonal crystal. The results are shown in Table 1. In addition, it confirmed that the crystal structure of all the positive electrode active materials was a rhombohedral structure.

(Comparative Example 1)
Various composite oxides were produced in the same manner as in Example 1. These composite oxides were mixed with lithium carbonate so that the number of moles of Li was equivalent to the number of moles of metal elements of the composite oxide, and fired at 950 ° C. in the atmosphere. Then, it baked again at 750 degreeC.

  In this way, positive electrode active materials 1ab to 1aj were produced. In these positive electrode active materials, as described in Patent Document 1, the primary particles did not grow sufficiently, and the secondary particles were aggregates of very fine primary particles.

  As an example, an electron micrograph of the surface of the active material 1ad having a composition of Ni: Mn: Co = 33: 33: 33 is shown in FIG. Further, this active material 1ad is filled with a resin, and a scanning electron micrograph of a cut section of the resin is shown in FIG. The primary particles had a mean particle size of about 1 μm and were a plurality of particles constituting a large lump in FIG. The secondary particles are constituted by agglomeration of a large number of primary particles, which is a large mass in FIG. 5, and the average particle size is 10 μm.

  In the same manner as in Example 1, the ratio of exposure, average particle size of secondary particles, average length of primary particles in the major axis direction, average length of primary particles in the major axis direction relative to the average particle size of secondary particles The ratio and the length in the c-axis direction when the crystal structure of the positive electrode active material was approximated as a hexagonal crystal were determined. The results are shown in Table 1.

(Comparative Example 2)
Various composite oxides were produced in the same manner as in Example 1.

  These composite oxides were mixed with lithium carbonate so that the number of moles of Li was equivalent to the number of moles of metal elements of the composite oxide, and fired at 1000 ° C. in the atmosphere. Thereafter, the fired composite oxide was mixed with lithium carbonate corresponding to volatile matter Li, and fired again at 1000 ° C. By changing the firing temperature or firing time, the number of primary particles exposed on the surface of the secondary particles was adjusted. In this way, positive electrode active materials 1ak to 1as were produced.

  In the same manner as in Example 1, the ratio of exposure, average particle size of secondary particles, average length of primary particles in the major axis direction, average length of primary particles in the major axis direction relative to the average particle size of secondary particles The ratio and the length in the c-axis direction when the crystal structure of the positive electrode active material was approximated as a hexagonal crystal were determined. The results are shown in Table 1.

  And using the positive electrode active material (1a-1aa) manufactured in Example 1, the positive electrode active material (1ab-1aj) manufactured in Comparative Example 1, and the positive electrode active material (1ak-1as) manufactured in Comparative Example 2, Nonaqueous electrolyte secondary batteries 1A to 1AS were produced in the following manner.

  First, 100 parts by weight of the positive electrode active material powder, 2.5 parts by weight of acetylene black as a conductive agent, 4 parts by weight of polyvinylidene fluoride (PVDF) as a binder, and a dispersion medium are kneaded. Thus, a positive electrode slurry was prepared. This positive electrode slurry was applied to both sides of an aluminum foil (current collector) having a thickness of 15 μm and dried. Thereafter, this positive electrode was rolled to a uniform thickness using a flat roll press, and cut into a width of 50 mm to obtain a positive electrode. When applying the positive electrode slurry to the current collector, the positive electrode slurry was applied so that a part of the current collector was exposed.

  Next, 100 parts by weight of graphite as a negative electrode active material, 6 parts by weight of PVDF as a binder, and a dispersion medium were kneaded to prepare a negative electrode slurry. This negative electrode slurry was applied to both sides of a copper foil (current collector) having a thickness of 10 μm, dried and rolled, and then cut into a width of 52 mm to obtain a negative electrode. When applying the negative electrode slurry to the current collector, the negative electrode slurry was applied so that a part of the current collector was exposed.

Subsequently, a current collecting lead is welded to each of the positive electrode and the negative electrode where the current collector is exposed, a polyethylene separator having a thickness of 27 μm is sandwiched between the positive and negative electrodes, and wound in a spiral shape. An electrode group was constructed. The electrode group was accommodated in a stainless steel bottomed cylindrical battery can, and a non-aqueous electrolyte was poured into the battery can to prepare non-aqueous electrolyte secondary batteries 1A to 1AS. Here, the thickness of the battery can was 25 μm, the outer diameter of the battery can was 18 μm, and the height of the battery can was 65 mm. As the non-aqueous electrolyte, a solution obtained by dissolving 1M LiPF ₆ in a solvent in which EC (ethylene carbonate) and EMC (ethyl methyl carbonate) were mixed at a volume ratio of 1: 3 was used.

  These nonaqueous electrolyte secondary batteries 1A to 1AS were aged in an atmosphere of 450 ° C. for 7 days in a state of being charged with a current of 0.4 A until the voltage reached 4.1 V. Thereafter, the battery was discharged in an atmosphere of 25 ° C. until the voltage reached 3.0 V, and then charged at a constant current with a current of 0.4 A until the voltage reached 4.2 V. Thereafter, constant voltage charging was performed at 4.2 V, and the charging was terminated when the current reached 0.2 A. After standing for 30 minutes, discharging was performed until the voltage reached 3 V at a current of 0.4 A. The battery capacities of the nonaqueous electrolyte secondary batteries 1A to 1AS were all 1.2 Ah.

  After that, constant current charging was performed until the voltage reached 4.2V at a current of 0.4A in a 25 ° C atmosphere, and then constant voltage charging was performed at a voltage of 4.2V, and charging was performed when the current reached 0.2A. Ended. Thereafter, it was left for 2 hours in an atmosphere of 0 ° C. and discharged at a current of 3 A for 10 seconds. And the difference (voltage difference) between the open circuit voltage measured before discharging for 10 seconds with a current of 3 A and the open circuit voltage measured after discharging for 10 seconds with a current of 3 A was confirmed.

  The voltage difference includes not only the voltage difference generated due to the positive electrode active material but also the voltage difference generated due to the battery member resistance. It can be made very small by doing. In order to clarify the voltage difference caused by the positive electrode active material, the true voltage difference was obtained by subtracting the voltage difference caused by the member resistance from the voltage difference. The member resistance was the AC resistance at 1 kHz of each battery. In addition, since the reciprocal of this true voltage difference is proportional to the output of the nonaqueous electrolyte secondary battery, this reciprocal was used as an output index. Table 1 shows the output value of the non-aqueous electrolyte secondary battery 1AD as 100.

  Next, the following charge / discharge cycle was performed to examine cycle characteristics. Specifically, in a 45 ° C. atmosphere, constant current charging is performed at a current of 1 A until the voltage reaches 4.2 V, then constant voltage charging is performed at 4.2 V, and charging is performed when the current reaches 0.2 A. After being left for 30 minutes, the battery was discharged at a current of 1 A until the voltage reached 3V. Table 1 shows the number of cycles when the battery capacity was less than 60% of the initial battery capacity.

  Hereinafter, the results of the nonaqueous electrolyte secondary batteries 1A to 1AA of Example 1 are compared with the results of the nonaqueous electrolyte secondary batteries 1AB to 1AS of Comparative Examples 1 and 2, and the experimental results are considered.

  As shown in Table 1, the nonaqueous electrolyte secondary batteries 1A to 1AA had higher exposure ratios and excellent output characteristics and cycle characteristics than the nonaqueous electrolyte secondary batteries 1AB to 1AS of the comparative example.

  That is, when the exposure ratio is less than 40%, it is difficult to improve the output characteristics and the cycle life characteristics regardless of the composition of the positive electrode active material. On the other hand, when the exposure ratio was 40% or more, the output characteristics and the cycle characteristics could be improved.

  If the exposure ratio is 40% or more, the output characteristics are improved because many primary particles are exposed on the surface of the secondary particles, so that lithium ions can be exchanged without passing through the grain boundaries between the primary particles. This is because it becomes possible. When the exposure ratio was 40% or more, the effect obtained by increasing the exposure ratio could be greatly expressed.

  If the exposure ratio is 40% or more, the reason why the cycle characteristics are improved is as follows. The first reason is that if the exposure rate is 40% or more, many primary particles are exposed on the surface of the secondary particles, and conductivity is ensured on the surface of the secondary particles. This is because even when the secondary particles are cracked as a result of expansion and contraction of the positive electrode active material, the conductivity of many primary particles can be ensured. The second reason is that in the positive electrode active material of Example 1, the primary particles are larger than the positive electrode active materials of Comparative Examples 1 and 2, so that the specific surface area is small. As a result, the secondary particles are a plurality of primary particles. This is because the rate at which a metal such as Mn elutes on the surface of the primary particles can be reduced even when the particles are divided. The third reason is that since the output characteristics can be improved, the temperature rise of the battery in the charge / discharge cycle can be suppressed, and as a result, the rate of elution of metal elements such as Mn can be reduced. .

  Note that the reason why the positive electrode active materials are compared with each other is that the output characteristics and cycle characteristics differ depending on the composition of the positive electrode active materials.

  The results of the nonaqueous electrolyte secondary batteries 1A to 1AA of Example 1 will be further discussed.

  In the nonaqueous electrolyte secondary batteries 1J to 1AA using the positive electrode active materials 1j to 1aa, the output characteristics and the cycle characteristics are further improved compared to the nonaqueous electrolyte secondary batteries 1A to 1I using the positive electrode active materials 1a to 1i. We were able to. In the positive electrode active materials 1j to 1aa, since the exposure ratio was 60% or more, the above-described effect could be greatly expressed as compared with the case where the exposure ratio was 40% or more and less than 60%.

  In addition, when the exposure ratio is 60% or more and less than 80% (positive electrode active materials 1j to 1r) and when the exposure ratio is 80% or more (positive electrode active materials 1s to 1aa), output characteristics and cycle There was no significant difference in characteristics. The reason for this is that when the exposure ratio is 80% or more, the primary particles are larger, so the specific surface area becomes smaller, and as a result, the effect obtained by increasing the exposure ratio is offset. thinking. Specifically, if the exposure ratio is less than 40%, the grain boundaries increase, and the movement speed of lithium ions between the active material and the nonaqueous electrolytic solution decreases. On the other hand, if the exposure ratio exceeds 80%, the specific surface area becomes small, so that the contact area with the non-aqueous electrolyte is lowered, and the reaction rate is lowered. Therefore, the exposure ratio may be 40% or more, preferably 40% or more and 100% or less, preferably 60% or more and 100% or less, and most preferably 60% or more and 80% or less. The exposure ratio of 100% means that all the primary particles constituting the secondary particles are exposed on the surface of the secondary particles.

  In the nonaqueous electrolyte secondary batteries 1A, 1J, and 1S in which the total content of Ni and Mn is less than 60 mol% among the nonaqueous electrolyte secondary batteries 1A to 1AA, the cycle characteristics are increased by increasing the exposure ratio. Improved, but the output characteristics did not improve.

  The reason for this is considered to be that in the positive electrode active materials 1a, 1j and 1s, the length in the c-axis direction when the crystal structure of the positive electrode active material is approximated as a hexagonal crystal is less than 14.2 mm.

  That is, the following reasons can be considered. In the positive electrode active materials 1a, 1j, 1s, 1ab, and 1ak, the total content of Ni and Mn is less than 60 mol% of the total metal elements constituting the positive electrode active material, so that the crystal layer of the positive electrode active material is ordered Therefore, it was difficult to grow, and the length in the c-axis direction when the crystal structure of the positive electrode active material was approximated as a hexagonal crystal was less than 14.2 mm without sufficiently developing. Therefore, the interlayer in the crystal structure of the positive electrode active material becomes narrow, and the diffusion rate of lithium ions in the primary particles cannot be sufficiently increased. As a result, the diffusion rate in the primary particles becomes the rate-determining rate of movement of lithium ions, and the effect obtained by increasing the exposure rate is small.

As a particularly significant comparison, the results for LiCoO ₂ mainly composed of Co are shown in Table 2 (batteries 2AA, 2AJ, 2AS, 2AT and 2AU), but the output characteristics and cycle characteristics are increased even when the exposure ratio is increased. There was no significant effect.

  Further, among the non-aqueous electrolyte secondary batteries 1A to 1AA, in the non-aqueous electrolyte secondary batteries 1H, 1I, 1Q, 1R, 1Z, 1AA where Ni and Mn are not substantially in the same molar ratio, the exposure ratio is increased. However, in the nonaqueous electrolyte secondary batteries 1B to 1E, 1K to 1N, and 1T to 1W, in which Ni and Mn are substantially in the same molar ratio and the total content of Ni and Mn is 60 mol% or more In comparison, the degree of improvement in output characteristics and cycle characteristics was small.

  The reason is considered to be related to the following reasons. The first reason is that when Ni and Mn are not present in the same molar ratio, the crystal layer of the positive electrode active material cannot be grown in an orderly manner. Therefore, even if the interlayer of the crystal layer is 14.2 mm or more, lithium ions in the crystal This is because the movement speed of is slow. Second, when Ni and Mn are not present in the same molar ratio, the electronic structure of Ni and the electronic structure of Mn are not changed by the interaction, so that the conductivity of the positive electrode active material is improved and the crystal of the positive electrode active material is crystallized. This is because it is difficult to improve the stability of the structure. For these two reasons, in the nonaqueous electrolyte secondary batteries 1H, 1I, 1Q, 1R, 1Z and 1AA, the effects obtained by increasing the exposure ratio (improvement of output characteristics and cycle characteristics), and the positive electrode active The effect obtained by stabilizing the crystal structure of the substance (suppression of elution of Mn from the positive electrode active material) could not be expressed greatly.

  In addition, non-aqueous electrolyte secondary batteries 1F, 1G, 1O, 1P using positive electrode active materials 1f, 1g, 1o, 1p, 1x, and 1y in which the molar ratio of Ni and Mn is shifted by 10% from the same molar ratio, In 1X and 1Y, both the output characteristics and the cycle characteristics were not significantly different from the nonaqueous electrolyte secondary batteries 1E, 1N, and 1W using the positive electrode active materials 1e, 1n, and 1w having the same molar ratio. From this, “the molar ratio of Ni and Mn is the same” means that not only the molar ratio of Ni and Mn is exactly the same, but also the difference between the molar ratio of Ni and the molar ratio of Mn. The case where it is within 10% is also included.

  As described above, the positive electrode active material contains Ni and Mn in the same molar ratio, the crystal structure is a rhombohedral structure, and the length of the hexagonal crystal in the c-axis direction is approximated when the crystal structure is approximated as a hexagonal crystal. When the thickness is 14.2% or more and the total content of Ni and Mn is 60 mol% or more of all metal elements constituting the positive electrode active material, the output characteristics and the cycle characteristics can be most improved.

  Further, from Table 1, when the ratio of the average length in the major axis direction of the primary particles to the average particle size of the secondary particles is 0.20 or more, the output characteristics and the cycle characteristics are improved. Here, the larger the ratio of the average length of the primary particles in the major axis direction to the average particle size of the secondary particles, the larger the number of primary particles exposed on the surface of the secondary particles.

  Moreover, if the ratio of the average length of the primary particles in the major axis direction to the average particle size of the secondary particles is 1 or less, the shape of the secondary particles can be prevented from becoming distorted. As a result, when preparing a positive electrode active material paste, applying a positive electrode active material paste to a current collector, and applying a positive electrode active material paste to a current collector and then rolling to produce a positive electrode This is advantageous. Therefore, the ratio of the average length of the primary particles in the major axis direction to the average particle size of the secondary particles is preferably 1 or less.

  Furthermore, it was found from Table 1 that the ratio of the average length in the major axis direction of the primary particles to the average particle size of the secondary particles is more preferably 0.3 or more. That is, this ratio is preferably 0.2 or more, more preferably 0.2 or more and 1 or less, and further preferably 0.3 or more and 1 or less.

(Example 2)
A hydroxide (raw material of a positive electrode active material) containing Ni as a main component was produced using the reaction tank 1 of FIG. As the raw material, a 1 mol / L nickel sulfate aqueous solution, a 1 mol / L cobalt sulfate aqueous solution, and a 1 mol / L aluminum sulfate aqueous solution ( In some cases, it may be a 1 mol / L manganese sulfate aqueous solution instead of a 1 mol / L aluminum sulfate aqueous solution), a 5 mol / L ammonium aqueous solution, and a 5 mol / L sodium hydroxide aqueous solution while stirring them in the reaction vessel 1. Continuously fed. Thereafter, various composite hydroxides having different compositions were produced in the same manner as in Example 1.

  Next, each of these composite hydroxides was washed with water and dried, and then fired in the air to obtain a composite oxide. In this way, various composite oxides having different element ratios of Ni, Co, and Al and secondary particle sizes in the composite oxide were prepared.

  These composite oxides were mixed with lithium carbonate so that the number of moles of Li was equivalent to the number of moles of metal elements of the composite oxide, and fired at 1100 ° C. in the atmosphere. Thereafter, the fired composite oxide was mixed with lithium carbonate corresponding to volatile matter Li, and fired again at 1000 ° C. By changing the firing temperature and firing time, the amount of primary particles exposed on the surface of the secondary particles was controlled.

  In this way, positive electrode active materials 2a to 2x were obtained. Thereafter, the total cross-sectional area of the primary particles exposed on the surface of the secondary particles was determined. Specifically, first, a large number of secondary particles were observed using a particle size distribution meter and a scanning electron microscope, and then the average particle size of the secondary particles was confirmed using a scanning electron micrograph. Thereafter, a large number of secondary particles were embedded in the resin, and the cut section of the resin was polished and confirmed with a scanning electron microscope. As a result, in the cross section obtained by cutting the secondary particles into half, primary particles at least partially exposed on the surface of the secondary particles were confirmed. And the sum total of the cross-sectional area of the primary particle which at least one part exposed to the surface of the secondary particle was calculated | required.

  Thereafter, the total cross-sectional area of the primary particles constituting the secondary particles was determined, and the ratio of the cross-sectional area of the primary particles at least a part of which was exposed on the surface of the secondary particles (the ratio of exposure) was determined. . Further, the length of the primary particles in the major axis direction was calculated using a cross-sectional photograph of a scanning electron microscope. The average particle size of the secondary particles and the average length of the primary particles in the major axis direction were calculated by subjecting a cross-sectional photograph of a scanning electron microscope to image processing.

  Moreover, the X-ray diffraction measurement of this positive electrode active material was performed. From the diffraction pattern, it was confirmed that the crystal structure of the positive electrode active material was a rhombohedral structure, and the length of the hexagonal crystal in the c-axis direction was approximated when the crystal structure was approximated as a hexagonal crystal. Table 2 shows the composition and physical properties of the positive electrode active material.

(Comparative Example 3)
A composite oxide was produced in the same manner as in Example 2. These composite oxides were mixed with lithium carbonate so that the number of moles of Li was equivalent to the number of moles of metal elements of the composite oxide, and fired at 950 ° C. in the atmosphere. Then, it baked again at 750 degreeC.

  In this way, positive electrode active materials 2aa to 2ai were produced. In these positive electrode active materials, the primary particles were not sufficiently grown as described in Patent Document 1, and the secondary particles were aggregates of very fine primary particles.

  In the same manner as in Example 2, the exposure ratio, the average particle size of the secondary particles, the average length in the major axis direction of the primary particles, the average length in the major axis direction of the primary particles relative to the average particle size of the secondary particles, And the length of the c-axis direction of the hexagonal crystal when the crystal structure of the positive electrode active material was approximated as a hexagonal crystal was determined. The results are shown in Table 2.

(Comparative Example 4)
A composite oxide was produced in the same manner as in Example 2.

  These composite oxides were mixed with lithium carbonate so that the number of moles of Li was equal to the number of moles of metal elements of the composite oxide, and fired in the atmosphere. Thereafter, the fired composite oxide was mixed with lithium carbonate corresponding to the volatile matter Li, and fired again. By changing the firing temperature and firing time, the amount of primary particles exposed on the surface of the secondary particles was adjusted. In this way, positive electrode active materials 2aj to 2ar were prepared.

  Thereafter, in the same manner as in Example 2, the exposure ratio, the average particle size of the secondary particles, the average length in the major axis direction of the primary particles, the average length in the major axis direction of the primary particles relative to the average particle size of the secondary particles In addition, the length in the c-axis direction of the hexagonal crystal when the crystal structure of the positive electrode active material was approximated as a hexagonal crystal was obtained. The results are shown in Table 2.

(Comparative Example 5)
A Co oxide was prepared in the same manner as in Example 2 except that only a cobalt sulfate aqueous solution was used as the metal element sulfate solution. This Co oxide was mixed with lithium carbonate so that the number of moles of Li was equal to the number of moles of the metal element, and fired in the air. Thereafter, the fired oxide was mixed with lithium carbonate corresponding to the volatile component Li, and fired again. By changing the firing temperature and firing time, the amount of primary particles exposed on the surface of the secondary particles was adjusted. In this way, positive electrode active materials 2as, 2at and 2au were produced.

  In the same manner as in Example 2, the exposure ratio, the average particle size of the secondary particles, the average length in the major axis direction of the primary particles, the average length in the major axis direction of the primary particles relative to the average particle size of the secondary particles, And the length of the c-axis direction of the hexagonal crystal when the crystal structure of the positive electrode active material was approximated as a hexagonal crystal was determined. The results are shown in Table 2.

  Using these positive electrode active materials 2a to 2x and 2aa to 2au, non-aqueous electrolyte secondary batteries 2A to 2X and 2AA to 2AU were produced in the same manner as in Example 1.

  Next, in the same manner as in Example 1, output characteristics and cycle characteristics were measured for the nonaqueous electrolyte secondary batteries 2A to 2X and 2AA to 2AU. The results are shown in Table 2. In Table 2, the output (%) is shown with the output value of the nonaqueous electrolyte secondary battery 2AF being 100.

  As shown in Table 2, the non-aqueous electrolyte secondary batteries 2A to 2X manufactured using the positive electrode active material of Example 2 (positive electrode active materials 2a to 2x) are the same as those of Comparative Examples 3 to 5 (positive electrode active material). It was found that both the output characteristics and the cycle characteristics were improved as compared to the nonaqueous electrolyte secondary batteries 2AA to 2AU manufactured using the substances 2aa to 2au). The reason is the same as in Example 1.

  In the following, the results of the nonaqueous electrolyte secondary batteries 2A to 2X of Example 2 are compared with the results of the nonaqueous electrolyte secondary batteries 2AS, 2AT, and 2AU of Comparative Example 5, and the experimental results are considered.

  In the nonaqueous electrolyte secondary batteries 2AS, 2AT, and 2AU, even when the exposure ratio was increased, neither the cycle characteristics nor the output characteristics showed an improvement trend. The reason why the cycle characteristics were not improved even when the ratio of exposure was increased is that the positive electrode active material does not contain Ni, and therefore it depends on the charge / discharge cycle compared to the case where the positive electrode active material contains Ni. This is considered to be because the generation of cracks in the positive electrode active material can be suppressed.

  The reason why the output characteristics are not improved even when the exposure ratio is increased is considered as follows. That is, if Ni is not contained in the positive electrode active material, the length of the crystal structure of the positive electrode active material in the c-axis direction is not sufficiently developed, so that the interlayer in the crystal structure is narrow. The diffusion rate could not be increased sufficiently. Therefore, the diffusion rate of lithium ions in the primary particles becomes the rate-determining rate of diffusion of lithium ions, and as a result, the effect obtained by increasing the exposure ratio cannot be expressed greatly.

  The positive electrode active materials 2a, 2b, 2i, 2j, 2p, and 2r of the nonaqueous electrolyte secondary batteries 2A, 2B, 2I, 2J, 2Q, and 2R of Example 2 each have a Ni content of 55 in the positive electrode active material. The length in the c-axis direction of the hexagonal crystal when the crystal structure of the positive electrode active material was approximated as a hexagonal crystal was less than 14.13 mm. In such nonaqueous electrolyte secondary batteries 2A, 2B, 2I, 2J, 2Q, and 2R, output characteristics and cycle can be obtained even when the exposure ratio is increased as compared with other nonaqueous electrolyte secondary batteries of Example 2. The degree of improvement in characteristics was small.

  The reason why the improvement of the output characteristics is small in the non-aqueous electrolyte secondary batteries 2A, 2B, 2I, 2J, 2Q and 2R is considered as follows. That is, since the Ni content in the positive electrode active material is low, an ordered crystal structure cannot be grown, and the length of the crystal structure of the positive electrode active material in the c-axis direction cannot be sufficiently developed. Therefore, it is difficult to sufficiently increase the diffusion rate of lithium ions in the primary particles. For this reason, the diffusion rate in the primary particles becomes the rate-determining factor for the movement of lithium ions, so that the effect obtained by increasing the exposure rate could not be expressed greatly.

  The reason why the cycle characteristics were not improved so much in the nonaqueous electrolyte secondary batteries 2A, 2B, 2I, 2J, 2Q and 2R is that the Ni content is lower than the Co content in the positive electrode active material. This is because the progress of cracks in the positive electrode active material can be suppressed even when the charge / discharge cycle is performed.

  From the above, the positive electrode active material contains Ni and Co, the crystal structure is a rhombohedral structure, and the length of the hexagonal crystal in the c-axis direction when the crystal structure is approximated as a hexagonal crystal is 14. The output characteristics and the cycle characteristics were most improved when the content was 13% or more and Ni contained 55 mol% or more as a metal element.

  Next, among the nonaqueous electrolyte secondary batteries 2C to 2H, 2K to 2P, and 2S to 2X, the experimental results of the nonaqueous electrolyte secondary batteries 2H, 2P, and 2X and other nonaqueous electrolyte secondary batteries were compared. . Here, the nonaqueous electrolyte secondary batteries 2C to 2H, 2K to 2P, and 2S to 2X each have a positive electrode active material containing Ni and Co, and the crystal structure of the positive electrode active material is a rhombohedral structure. When the crystal structure is approximated as a hexagonal crystal, the length from the c-axis direction to the crystal structure is 14.13 mm or more, and the content of Ni in the positive electrode active material is 55% of all metal elements constituting the positive electrode active material. It was more than mol%. In addition, the nonaqueous electrolyte secondary batteries 2H, 2P, and 2X had positive electrode active materials (positive electrode active materials 2h, 2p, and 2x) containing Mn, respectively.

  In the non-aqueous electrolyte secondary batteries 2H, 2P, and 2X using the positive electrode active materials 2h, 2p, and 2x containing Mn, the cycle characteristics used the positive electrode active material that does not contain Mn even if the ratio of exposure was increased. It turned out that it is inferior compared with a nonaqueous electrolyte secondary battery. The reason for this is that since Ni and Mn do not exist at substantially the same molar ratio, the crystal structure of the positive electrode active material cannot be stabilized at the atomic level, and as a result, Mn is eluted from the positive electrode active material. I think this is because.

  As shown in Table 2, when the ratio of the average length of the primary particles in the major axis direction to the average particle size of the secondary particles was 0.2 or more, the output characteristics and the cycle characteristics were improved. The reason is as described in the first embodiment.

  As described above, the nonaqueous electrolyte secondary battery according to Example 2 is excellent in output characteristics and cycle characteristics.

  The average length of the primary particles in the major axis direction is desirably 2 μm or more. When the average length in the major axis direction of the primary particles is short, the ratio of the average length in the major axis direction of the primary particles to the average particle size of the secondary particles satisfies 0.2 or more and 1 or less. The average particle size of the particles is also reduced. As the primary particles and secondary particles become smaller, the specific surface area of the secondary particles increases, so the area where the positive electrode active material comes into contact with the non-aqueous electrolyte solution becomes very large, and the elution rate of metal elements on the surface of the secondary particles increases. As a result, the life characteristics of the nonaqueous electrolyte secondary battery are deteriorated. Therefore, the average length of primary particles in the major axis direction is desirably 2 μm or more. Furthermore, in order to obtain the effect of suppressing the secondary particles from becoming smaller, the average length in the major axis direction is desirably 3 μm or more.

  In Examples 1 and 2, the positive electrode active material was obtained by performing the firing step of mixing and firing the composite oxide and Li ions twice. When a positive electrode active material is manufactured using this method, primary particles can be enlarged as compared with a manufacturing method in which the firing step is performed once as shown in Comparative Example 1.

  In addition, you may manufacture a positive electrode active material using manufacturing methods other than the manufacturing method shown in the said Example 1 and 2. For example, when preparing a composite hydroxide, a low-concentration metal salt aqueous solution, a sodium hydroxide aqueous solution and an ammonia aqueous solution are used, and an aqueous solution having a low temperature and a residence time (the same amount as the capacity of the reaction tank 1 is charged). By increasing the time, the primary particles of the composite hydroxide can be increased. If such a composite hydroxide is used, the structure of the positive electrode active material of the present application can be obtained even by using a conventional method of introducing Li in one step as in Comparative Example 3.

  In addition, when composite carbonate is used as a raw material without using a composite hydroxide as a raw material, the structure of the positive electrode active material of the present application can be obtained even by using a conventional method in which Li is introduced in one step. It is. This is because relatively large primary particles can be produced when a lithium composite oxide is produced using carbonate as a raw material.

  The positive electrode active material for a non-aqueous electrolyte secondary battery according to the present invention and the non-aqueous electrolyte secondary battery using the same have excellent output characteristics and a large charge / discharge cycle life. It is effective for applications such as power tool power supplies, electric vehicle power supplies, hybrid electric vehicle power supplies, and household power supplies.

It is the schematic of the reaction tank which synthesize | combines the composite hydroxide which is a raw material of a positive electrode active material. It is the scanning electron micrograph which image | photographed the surface of the positive electrode active material concerning this invention. It is the scanning electron micrograph which image | photographed the cross section of the positive electrode active material concerning this invention. It is the scanning electron micrograph which image | photographed the surface of the conventional positive electrode active material. It is the scanning electron micrograph which image | photographed the cross section of the conventional positive electrode active material.

Explanation of symbols

1 reaction tank 2 tube 3 stirring rod 4-8 pump 9-11 tube

Claims (7)

A positive electrode active material for a non-aqueous electrolyte secondary battery containing Ni,
Secondary particles in which primary particles are aggregated,
In the cross-section obtained by cutting the secondary particles, the total cross-sectional area of the primary particles, at least a part of which is exposed on the surface of the secondary particles, is 40% of the total cross-sectional area of the primary particles constituting the secondary particles. The positive electrode active material for a non-aqueous electrolyte secondary battery as described above.   In the cross section obtained by cutting the secondary particles, the total cross-sectional area of the primary particles, at least a part of which is exposed on the surface of the secondary particles, is 60% of the total cross-sectional area of the primary particles constituting the secondary particles. The positive electrode active material for a nonaqueous electrolyte secondary battery according to claim 1, which is as described above.   2. The nonaqueous electrolyte according to claim 1, wherein, in a cross section obtained by cutting the secondary particles, a ratio of an average length in a major axis direction of the primary particles to an average particle size of the secondary particles is 0.2 or more. Positive electrode active material for secondary battery.   The positive electrode active material for a nonaqueous electrolyte secondary battery according to claim 1, comprising substantially the same molar ratio of Ni and Mn. The crystal structure is a rhombohedral structure,
When the crystal structure is approximated as a hexagonal crystal, the length of the hexagonal crystal in the c-axis direction is 14.2 mm or more,
The positive electrode active material for a nonaqueous electrolyte secondary battery according to claim 4, wherein the total content of Ni and Mn is 60 mol% or more of the content of all metal elements. Ni and Co are included,
The crystal structure is a rhombohedral structure,
When the crystal structure is approximated as a hexagonal crystal, the length of the hexagonal crystal in the c-axis direction is 14.13 mm or more,
The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the content of Ni is 55 mol% or more of the content of all metal elements.   The nonaqueous electrolyte secondary battery using the positive electrode active material as described in any one of Claim 1 to 6.