Method for manufacturing a positive electrode active material for lithium-ion secondary batteries, positive electrode active material for lithium-ion secondary batteries, and lithium-ion secondary batteries

Abstract

The present invention provides a method for producing a positive electrode active material that has high capacity and suppresses capacity degradation during repeated charging and discharging, as well as a positive electrode active material itself. [Solution] The method comprises a mixing step of mixing a lithium transition metal composite oxide and lithium phosphate; a milling step of applying mechanical stress to convert the lithium transition metal composite oxide having a layered crystalline structure and lithium phosphate into an amorphous or low-crystallinity NiO-like rock salt type crystalline structure; and a heat treatment step of heat treating the mixture obtained in the milling step to obtain a lithium transition metal composite oxide with a layered rock salt type crystalline structure in which lithium phosphate crystallizes and is dispersed, wherein the lithium transition metal composite oxide has the general formula Li s Ni 1-x-y-z Co x Mn y M z O 2+α Represented as such, crystallized lithium phosphate coats the surface of primary particles of lithium transition metal composite oxide with a layered rock salt crystal structure, and is dispersed inside or on the surface of secondary particles of lithium transition metal composite oxide with a layered rock salt crystal structure.

Description

Technical Field

[0001] The present invention relates to a method for producing a positive electrode active material for a lithium ion secondary battery composed of secondary particles in which a plurality of primary particles are aggregated with each other, a positive electrode active material for a lithium ion secondary battery, and a lithium ion secondary battery using the positive electrode active material as a positive electrode material. This application claims priority based on Japanese Patent Application No. 2020-167454 filed in Japan on October 2, 2020, and this application is incorporated herein by reference.

Background Art

[0002] In recent years, in order to increase the cruising range of electric vehicles (EVs), secondary batteries with high energy density are strongly desired. As such a secondary battery, there is a lithium ion secondary battery (LIB) using lithium, a lithium alloy, a metal oxide, or carbon as a negative electrode.

[0003] Among the positive electrode active materials used in the positive electrode of LIB for EVs, the material that exhibits the highest capacity is LiNi 1-x-y Co x Al y O2 (hereinafter, NCA). NCA is a positive electrode active material in which a part of Ni in lithium nickelate: LiNiO2 (hereinafter, LNO) is replaced with Co and Al. By replacing Co, the change in crystal structure during charge and discharge is suppressed, and by replacing Al, the thermal stability is improved.

[0004] For example, Patent Document 1 proposes a particulate positive electrode active material composed of lithium cobaltate (LiCoO2), which enables a large current supply.

[0005] Also, Patent Document 2 proposes positive electrode particles composed of LiNi x Co y M z O2 (where M is Al or Mn, 0 < x < 1, 0 < y < 1, x + y + z = 1), which improves the discharge capacity of the lithium ion secondary battery.

Prior Art Documents

Patent Document

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, although the positive electrode active materials such as those in Patent Documents 1 and 2 exhibit high capacity, there is a problem of significant capacity degradation due to large structural changes during repeated charge and discharge.

[0008] Therefore, in view of the above problems, an object of the present invention is to suppress the capacity degradation during repeated charge and discharge, which is a drawback of LNO, by suppressing the structural changes accompanying charge and discharge and achieving high capacity.

Means for Solving the Problems

[0009] A method for manufacturing a positive electrode active material for a lithium - ion secondary battery according to one aspect of the present invention is a method for manufacturing a positive electrode active material for a lithium - ion secondary battery composed of secondary particles in which a plurality of primary particles are aggregated with each other. The method includes a mixing step of mixing a lithium transition metal composite oxide having a crystal structure of a layered structure and lithium phosphate, applying mechanical stress to the mixture obtained in the mixing step to make the lithium transition metal composite oxide having the crystal structure of the layered structure and the lithium phosphate into an amorphous or low - crystalline NiO - like rock - salt - type crystal structure in a milling step, and subjecting the mixture having the amorphous or low - crystalline NiO - like rock - salt - type crystal structure obtained in the milling step to a heat treatment step to obtain a lithium transition metal composite oxide having a layered rock - salt - type crystal structure in which lithium phosphate is crystallized and dispersed. The lithium transition metal composite oxide has the general formula Li s Ni 1-x-y-z Co x Mn y Mz O 2+α (However, 0 ≦ x ≦ 0.35, 0 ≦ y ≦ 0.35, 0 ≦ z ≦ 0.10, 1.00 ≦ s ≦ 1.30, 0 ≦ α ≦ 0.2, and M is at least one element selected from V, Mg, Mo, Nb, Ti, W, and Al), and the crystallized lithium phosphate coats the surface of the primary particles of the lithium transition metal composite oxide having the layered rock salt type crystal structure and is dispersed inside or on the surface of the secondary particles of the lithium transition metal composite oxide having the layered rock salt type crystal structure.

[0010] In this way, it is possible to suppress the capacity degradation during repeated charge and discharge, which is a drawback of LNO, while achieving a high capacity.

[0011] At this time, in the mixing step, the lithium phosphate may be mixed with the lithium transition metal composite oxide in an amount greater than 0 and not exceeding 10 wt%.

[0012] In this way, the ratio of the lithium transition metal composite oxide and the lithium phosphate becomes optimal, and it is possible to suppress the capacity degradation during repeated charge and discharge while achieving a high capacity.

[0013] At this time, in the heat treatment step, the heat treatment may be performed at a temperature of 600 to 700 °C.

[0014] In this way, it becomes further possible for the crystallized lithium phosphate to coat the surface of the primary particles of the lithium transition metal composite oxide having the layered rock salt type crystal structure and to be dispersed inside or on the surface of the secondary particles of the lithium transition metal composite oxide having the layered rock salt type crystal structure. Therefore, it is possible to suppress the capacity degradation during repeated charge and discharge while achieving a high capacity.

[0015] At this time, in the milling step, mechanical stress may be applied by mechanical milling.

[0016] By doing so, it becomes further possible to make the lithium transition metal composite oxide having a layered crystal structure and lithium phosphate into an amorphous or low-crystalline NiO-like rock salt-type crystal structure, so that high capacity and suppression of capacity degradation during repeated charge and discharge can be achieved.

[0017] At this time, in another aspect of the present invention, there is provided a positive electrode active material for a lithium ion secondary battery composed of secondary particles in which a plurality of primary particles are aggregated with each other, the lithium transition metal composite oxide having a layered rock salt-type crystal structure, and crystallized lithium phosphate, represented by the general formula kLi3PO4-(1-k)Li s Ni 1-x-y-z Co x Mn y M z O 2+α (where 0 < k < 0.1, 0 ≤ x ≤ 0.35, 0 ≤ y ≤ 0.35, 0 ≤ z ≤ 0.10, 1.00 ≤ s ≤ 1.30, 0 ≤ α ≤ 0.2, and M is at least one element selected from V, Mg, Mo, Nb, Ti, W, and Al), and the crystallized lithium phosphate covers the surface of the primary particles of the lithium transition metal composite oxide having the layered rock salt-type crystal structure and is dispersed inside or on the surface of the secondary particles of the lithium transition metal composite oxide having the layered rock salt-type crystal structure.

[0018] By doing so, high capacity and suppression of capacity degradation during repeated charge and discharge, which is a drawback of LNO, can be achieved.

[0019] At this time, in one aspect of the present invention, the lithium ion secondary battery is characterized by including at least a positive electrode containing the above positive electrode active material for a lithium ion secondary battery.

[0020] By doing so, it is possible to obtain a lithium ion secondary battery that has high capacity and can suppress capacity degradation during repeated charge and discharge, which is a drawback of LNO.

Advantages of the Invention

[0021] According to the present invention, by suppressing structural changes associated with charging and discharging, it is possible to achieve high capacity and suppress the capacity degradation during repeated charging and discharging, which is a drawback of LNO. [Brief explanation of the drawing]

[0022] [Figure 1] Figure 1 is a schematic process diagram showing a method for manufacturing a positive electrode active material for a lithium-ion secondary battery according to one embodiment of the present invention. [Figure 2] Figure 2(A) shows the SEM image of the positive electrode active material in Comparative Example 1, Figure 2(B) shows the image of Example 1, and Figure 2(C) shows the image of the positive electrode active material in Example 2. [Figure 3] Figure 3(A) shows the mapping image of the positive electrode active material in Example 1, and Figure 3(B) shows the mapping image of the positive electrode active material in Example 2. [Figure 4] Figure 4 shows the crystal structure of the positive electrode active material in Example 1, Example 2, and Comparative Example 1, as confirmed using an XRD apparatus. [Figure 5] Figure 5 shows the charge-discharge profiles when using the positive electrode active material in Example 1, Example 2, and Comparative Example 1. [Figure 6] Figure 6 shows the charge and discharge profiles when using the positive electrode active material in Examples 1, 3, and 4. [Modes for carrying out the invention]

[0023] To solve the above problems, the present inventors diligently studied positive electrode active materials for lithium-ion secondary batteries with excellent battery characteristics and found that by mixing a powder made of lithium transition metal composite oxide having a layered crystalline structure with lithium phosphate, applying mechanical stress to the mixture, and performing heat treatment, the crystallized lithium phosphate coats the surface of the primary particles of the layered rock salt-type crystalline lithium transition metal composite oxide and disperses inside or on the surface of the secondary particles of the layered rock salt-type crystalline lithium transition metal composite oxide, thereby suppressing the capacity degradation during repeated charging and discharging, which is a drawback of LNO. Preferred embodiments of the present invention will be described below.

[0024] The embodiment described below does not unduly limit the scope of the present invention as described in the claims, and modifications are possible without departing from the spirit of the invention. Furthermore, not all of the configurations described in this embodiment are necessarily essential as solutions to the present invention. A method for producing a positive electrode active material for a lithium-ion secondary battery according to one embodiment of the present invention will be described in the following order. 1. Manufacturing process for lithium transition metal composite oxides 2. Method for producing positive electrode active material for lithium-ion secondary batteries 2-1.Mixing process 2-2. Milling Process 2-3. Heat Treatment Process 3. Positive electrode active material for lithium-ion secondary batteries 4. Lithium-ion rechargeable batteries

[0025] <1. Lithium transition metal composite oxide manufacturing process> The positive electrode active material for a lithium-ion secondary battery according to one embodiment of the present invention is manufactured from a lithium transition metal composite oxide obtained by mixing a metal composite hydroxide with a lithium salt and firing it, as shown in Figure 1. The lithium transition metal composite oxide manufacturing process S10 is further manufactured by the following steps, which are described below.

[0026] (Composite hydroxide particle manufacturing process) The composite hydroxide particle manufacturing process involves adding an aqueous solution containing an alkaline solution to a mixed aqueous solution of nickel salts such as nickel(II) sulfate, cobalt salts such as cobalt(II) sulfate, manganese salts such as manganese sulfate, and additive metal salts, thereby obtaining composite hydroxide particles as a coprecipitate. The additive metal salts can be at least one element selected from the group consisting of V, Mg, Mo, Nb, Ti, W, and Al. The concentrations of cobalt, manganese, and additive elements relative to the transition metal are appropriately determined based on the composition of the lithium transition metal composite oxide and positive electrode active material, as described later. Furthermore, from the viewpoint of stabilizing the crystal structure and ensuring safety, it is preferable that the cobalt concentration be between 10 atomic% and 35 atomic%, and the additive element concentration be between 0.1 atomic% and 10 atomic%. Depending on the composition of the lithium transition metal composite oxide and positive electrode active material, it may be necessary to add cobalt salts, manganese salts, and additive metal salts.

[0027] The mixed aqueous solution is made alkaline by adding an aqueous solution containing an alkaline solution. When no complexing agent is added, it is preferable to select a pH range of 10 to 11 for the mixed aqueous solution, and to set the temperature of the mixed aqueous solution between 60°C and 80°C. This range allows for an appropriate reaction rate. Furthermore, it ensures a desirable solubility of Ni and prevents particle formation due to crystallization. Crystallization above pH 11 tends to form fine particles, reducing filterability and preventing the acquisition of spherical particles. Below pH 10, the hydroxide formation rate slows significantly, Ni remains in the filtrate, and the amount of Ni precipitate deviates from the desired composition, making it difficult to obtain the desired ratio of composite hydroxide. Additionally, below 60°C, the reaction rate tends to be insufficient. Furthermore, above 80°C, the evaporation rate of water increases, leading to a higher slurry concentration, decreased Ni solubility, and the formation of crystals such as sodium sulfate in the filtrate, increasing impurity concentration and reducing the charge / discharge capacity of the cathode material.

[0028] In the process of manufacturing composite hydroxide particles, it is preferable to add an aqueous solution containing an alkaline solution to the mixed aqueous solution, as well as to add a complexing agent such as ammonia. Adding a complexing agent such as ammonia can increase the solubility of Ni. When a complexing agent is used, it is preferable to select a pH range of pH = 10 to 12.5 for the mixed aqueous solution, and to set the temperature of the mixed aqueous solution in the range of 40°C to 60°C. In the reaction vessel, the concentration of the complexing agent in the mixed aqueous solution is preferably kept constant within the range of 3 g / L to 25 g / L. If the ammonia concentration is less than 3 g / L, it is not possible to maintain a constant solubility of metal ions, so plate-shaped composite hydroxide primary particles with uniform shape and particle size are not formed, and gel-like nuclei tend to form, causing the particle size distribution to spread out. On the other hand, if the ammonia concentration exceeds 25 g / L, the solubility of metal ions becomes too high, increasing the amount of metal ions remaining in the mixed aqueous solution, which tends to cause deviations in composition. Furthermore, fluctuations in ammonia concentration cause fluctuations in the solubility of metal ions, preventing the formation of uniform composite hydroxide particles. Therefore, it is preferable to maintain a constant ammonia concentration. For example, it is preferable to maintain the ammonia concentration at a desired level with an upper and lower limit of approximately 5 g / L.

[0029] (Heating process) The heating step is a process of heating the composite hydroxide particles produced in the composite hydroxide particle manufacturing process, and is performed as needed. The heating step can remove moisture contained in the composite hydroxide particles. By performing this heating step, the amount of moisture remaining in the particles until the calcination step can be reduced. In addition, since the composite hydroxide particles can be converted into composite oxide particles, it is possible to prevent variations in the ratio of metal atoms and lithium atoms in the manufactured positive electrode active material. It is sufficient to remove moisture to the extent that there is no variation in the ratio of metal atoms and lithium atoms in the positive electrode active material, so it is not necessarily required to convert all composite hydroxide particles into composite oxide particles. In the heating step, the composite hydroxide particles only need to be heated to a temperature at which residual moisture is removed, and the heating temperature is not particularly limited, but it is preferably between 105°C and 800°C. Residual moisture can be removed by heating the composite hydroxide particles to 105°C or higher. Note that below 105°C, it tends to take a long time to remove residual moisture. When the temperature exceeds 800°C, the particles converted to composite oxides tend to sinter and aggregate. The atmosphere in which the heat treatment is performed is not particularly limited, but it is preferable to perform it in an air stream, which can be easily carried out.

[0030] (Firing process) The firing process involves firing a lithium mixture, which is a mixture of particles obtained in the heating process and lithium and / or lithium compounds, to obtain a lithium transition metal composite oxide having a layered crystalline structure. Preferably, the lithium mixture has a ratio (Li / Me) of more than 1.00 and less than 1.30 between the number of atoms of metals other than lithium in the lithium mixture (i.e., the sum of the number of atoms of nickel, cobalt, and additive metals (Me)) and the number of atoms of lithium (Li). In other words, the lithium mixture is mixed so that the Li / Me ratio in the lithium mixture is the same as the Li / Me ratio in the positive electrode active material of the present invention. This is because the Li / Me ratio does not change before and after the firing process, so the Li / Me ratio to be mixed becomes the Li / Me ratio in the positive electrode active material.

[0031] The lithium compound is not particularly limited, but for example, lithium hydroxide, lithium nitrate, or lithium carbonate, or mixtures thereof, are preferred because they are readily available. In particular, lithium hydroxide is more preferred considering its ease of handling and stability of quality.

[0032] Furthermore, it is preferable to thoroughly mix the lithium mixture before firing. Thorough mixing before firing eliminates variations in Li / Me (additive metal) between individual particles, allowing for sufficient battery characteristics to be obtained.

[0033] The firing process involves firing the above-mentioned lithium mixture to obtain lithium transition metal composite oxide particles. When the lithium mixture is fired in the firing process, lithium from the lithium-containing material diffuses into the particles obtained in the heating process, thus forming a lithium transition metal composite oxide having a layered crystalline structure. The firing of the lithium mixture is carried out at a temperature of 700°C to 850°C, and is particularly preferable at a temperature of 720°C to 820°C. If the firing temperature is below 700°C, the diffusion of lithium into the particles will not be sufficient, and excess lithium and unreacted particles will remain, resulting in a state where the crystallinity is insufficient. Furthermore, if the firing temperature exceeds 850°C, vigorous sintering will occur between the particles, and abnormal grain growth will occur. As a result, the particles after firing may become coarse and unable to maintain their particle shape (the shape of spherical secondary particles described later), and when a positive electrode active material is formed, the specific surface area tends to decrease, the resistance of the positive electrode increases, and the battery capacity tends to decrease. Furthermore, the baking time is preferably at least 3 hours, and more preferably 6 hours to 24 hours.

[0034] Furthermore, the atmosphere during firing is preferably an oxidizing atmosphere, and more preferably an atmosphere with an oxygen concentration of 18% to 100% by volume. In other words, firing is preferably carried out in air or an oxygen stream. This is because if the oxygen concentration is less than 18% by volume, the composite hydroxide particles contained in the particles cannot be sufficiently oxidized, which may result in the lithium transition metal composite oxide not having sufficient crystallinity. Especially considering the battery characteristics, firing is preferably carried out in an oxygen stream.

[0035] A positive electrode active material for a lithium-ion secondary battery according to one embodiment of the present invention is manufactured using a lithium transition metal composite oxide obtained through the above-described firing process. This is described below.

[0036] <2. Method for manufacturing positive electrode active material for lithium-ion secondary batteries> A method for producing a positive electrode active material for a lithium-ion secondary battery according to one embodiment of the present invention is a method for producing a positive electrode active material for a lithium-ion secondary battery consisting of secondary particles formed by the aggregation of a plurality of primary particles, and comprises a mixing step S20, a milling step S30, and a heat treatment step S40. Each step will be described below.

[0037] <2-1. Mixing process> In mixing step S20, the lithium transition metal composite oxide obtained through the above firing step is used. The lithium transition metal composite oxide has a layered crystalline structure. The lithium transition metal composite oxide has the general formula Li s Ni 1-x-y-z Co x Mn y M z O 2+α (wherein 0≦x≦0.35, 0≦y≦0.35, 0≦z≦0.10, 1.00≦s≦1.30, 0≦α≦0.2, and M is at least one element selected from V, Mg, Mo, Nb, Ti, W, and Al). The amount of each added element is adjusted in the lithium transition metal composite oxide manufacturing process S10 above so that it falls within the above range.

[0038] Furthermore, in mixing step S20, the lithium transition metal composite oxide and lithium phosphate are mixed. The mixing method is not particularly limited and can be, for example, mixed in a mortar and pestle. Lithium phosphate can be, for example, Li3PO4.

[0039] Furthermore, in the mixing step S20, it is preferable to mix lithium phosphate with the lithium transition metal composite oxide in an amount greater than 0 and 10 wt% or less. More preferably, it is greater than 0 and 5 wt% or less.

[0040] <2-2. Milling Process> In milling step S30, the mixture obtained in mixing step S20 is further mixed while mechanical stress is applied. By mixing the mixture while applying mechanical stress, the lithium transition metal composite oxide having a layered crystalline structure and the lithium phosphate can be converted into an amorphous or low-crystalline NiO-like rock salt type crystalline structure.

[0041] The method for applying mechanical stress to the mixture is not particularly limited, but it is preferable to apply mechanical stress to the mixture by mechanical milling. In this way, it becomes possible to further convert the lithium transition metal composite oxide having a layered crystalline structure and lithium phosphate into an amorphous or low-crystalline NiO-like rock salt type crystalline structure.

[0042] The processing apparatus for mechanical milling is not particularly limited, but for example, ball mills, vibratory mills, turbo mills, mechanofusion mills, disc mills, and planetary ball mills can be suitably used. These ball mills are preferred because they can generate a large amount of mechanical energy. Planetary ball mills are even more preferred because the pot rotates on its own axis and also revolves around the base plate, allowing for the efficient generation of high impact energy.

[0043] The conditions for mechanical milling are set appropriately to obtain a mixture of amorphous or low-crystalline NiO-like rock salt-type crystal structures. For example, when using a general planetary ball mill, the rotation speed of the base plate is preferably 100 rpm to 2000 rpm, and more preferably 300 rpm to 1000 rpm. The total processing time for the planetary ball mill is, for example, 600 minutes or more, and may be 1000 minutes or more. However, if the continuous processing time is long, the processed material may become hot and unintended side reactions may occur, so it is preferable to take a short cooling time between mixing processes. Zirconia and alumina are preferably used as the material for the container and grinding balls used in the planetary ball mill. The diameter of the grinding balls is preferably 1 mm to 20 mm. Mechanical milling is preferably carried out in an inert gas atmosphere, such as an argon atmosphere.

[0044] In this application, amorphous refers to a solid state in which the arrangement of constituent atoms does not exhibit long-range regularity like that of a crystal structure. Low crystallinity refers to a state that lies between amorphous and crystalline, consisting of single crystal grains ("crystal grains") with different orientations, where the regularity of the crystal structure is relatively short-range (for example, less than 100 nm). Amorphousness can be determined by the absence of clear peaks in the X-ray diffraction results. Low crystallinity can be determined by the X-ray diffraction results, for example, by the presence of peaks for the rock salt crystal structure of NiO, but the diffraction lines are not clear. Furthermore, in this application, NiO-like rock salt crystal structure refers to a structure in which the rock salt crystal structure of NiO is distorted.

[0045] In milling step S30, the lithium phosphate and lithium transition metal composite oxide in the mixture obtained in mixing step S20 are finely ground, the finely ground lithium phosphate and lithium transition metal composite oxide are further mixed, and the lithium transition metal composite oxide and lithium phosphate can be mixed on an atomic level. As a result, in the heat treatment step, a lithium transition metal composite oxide with a layered rock salt-type crystalline structure in which lithium phosphate crystals and is dispersed can be obtained.

[0046] <2-3. Heat treatment process> In the heat treatment step S40, by subjecting the amorphous or low-crystalline mixture having a NiO-like rock salt crystal structure obtained in the milling step S30 to heat treatment, a lithium transition metal composite oxide having a layered rock salt crystal structure in which lithium phosphate is crystallized and dispersed can be obtained. In the heat treatment step S40, only the lithium transition metal composite oxide forms a layered rock salt crystal structure, and lithium phosphate does not form a layered structure.

[0047] Also, according to the heat treatment step S40, the crystallized lithium phosphate can coat the surface of the primary particles of the lithium transition metal composite oxide having a layered rock salt crystal structure and can be dispersed inside or on the surface of the secondary particles of the lithium transition metal composite oxide having a layered rock salt crystal structure.

[0048] The cathode active material obtained through the heat treatment step S40 is represented by the general formula kLi3PO4-(1-k)Li s Ni 1-x-y-z Co x Mn y M z O 2+α (where 〈0〈k〈0.1, 0≦x≦0.35, 0≦y≦0.35, 0≦z≦0.10, 1.00≦s≦1.30, 0≦α≦0.2, and M is at least one element selected from V, Mg, Mo, Nb, Ti, W, and Al).

[0049] In addition, in the heat treatment step S40, it is preferably heat-treated at a temperature of 600 to 700 °C. By doing so, the crystallized lithium phosphate can further cover the surface of the primary particles of the layered rock salt type lithium transition metal composite oxide and disperse inside or on the surface of the secondary particles of the layered rock salt type lithium transition metal composite oxide. Therefore, it is possible to suppress the capacity decrease during repeated charge and discharge while achieving a high capacity. Further, the heat treatment step S40 is preferably carried out in an oxidation atmosphere with an oxygen concentration of 80% by volume or more and 100% by volume or less, and more preferably carried out in an oxygen atmosphere. The heat treatment time is preferably 5 hours or more and 24 hours or less, and more preferably 8 hours or more and 16 hours or less.

[0050] As described above, according to the method for producing a positive electrode active material for a lithium ion secondary battery according to an embodiment of the present invention, by suppressing the structural change accompanying charge and discharge, a positive electrode active material capable of suppressing the capacity decrease during repeated charge and discharge, which is a drawback of LNO, while achieving a high capacity can be obtained.

[0051] <3. Positive Electrode Active Material for Lithium Ion Secondary Battery> The positive electrode active material for a lithium ion secondary battery according to an embodiment of the present invention is a positive electrode active material for a lithium ion secondary battery composed of secondary particles in which a plurality of primary particles aggregate with each other. Further, the positive electrode active material for a lithium ion secondary battery contains a layered rock salt type lithium transition metal composite oxide and crystallized lithium phosphate, and has a general formula kLi3PO4-(1-k)Li s Ni 1-x-y-z Co x Mn y M z O 2+α (where 0 < k < 0.1, 0 ≦ x ≦ 0.35, 0 ≦ y ≦ 0.35, 0 ≦ z ≦ 0.10, 1.00 ≦ s ≦ 1.30, 0 ≦ α ≦ 0.2, and M is at least one element selected from V, Mg, Mo, Nb, Ti, W, and Al).

[0052] In the molar ratio of the amount of substance of each element shown in the general formula above, k, which indicates the content of Li3PO4, is greater than 0 and less than 0.1, and may be between 0.01 and 0.07, or between 0.03 and 0.05. When k is 0, lithium phosphate cannot coat the surface of the primary particles of the layered rock salt type crystalline lithium transition metal composite oxide. Furthermore, lithium phosphate cannot be dispersed inside or on the surface of the secondary particles of the layered rock salt type crystalline lithium transition metal composite oxide. On the other hand, when k is 0.1 or greater, the proportion of lithium transition metal composite oxide involved in charging and discharging decreases, reducing the charge and discharge capacity, and the lithium phosphate layer on the surface becomes too thick, increasing the surface resistance.

[0053] In the above molar ratio, s, which indicates the Li content, is between 1.00 and 1.30, and may be between 1.01 and 1.20, or between 1.05 and 1.10. If the molar ratio of lithium is less than 1.00, the parts of the lithium nickel composite oxide crystal that should be occupied by lithium may be occupied by other elements, which may reduce the charge and discharge capacity. On the other hand, if the molar ratio exceeds 1.30, there will be excess lithium compounds that do not contribute to charge and discharge along with the lithium nickel composite oxide, which may increase battery resistance or reduce the charge and discharge capacity.

[0054] In the above molar ratio, x, which indicates the cobalt (Co) content, is between 0 and 0.35, and may be between 0.05 and 0.35, or between 0.1 and 0.3. When cobalt is included within the above range, the battery has high capacity and excellent cycle characteristics.

[0055] In the above molar ratio, y, which indicates the manganese (Mn) content, is between 0 and 0.35, and may also be between 0 and 0.10. When manganese is contained within the above range, the thermal stability is excellent.

[0056] In the above molar ratio, z, which indicates the content of element M, is, for example, between 0 and 0.10. M can be selected from multiple elements depending on the required properties. The positive electrode active material may also contain small amounts of elements other than Ni, Co, Mn, and element M, as long as it does not hinder the effects of the present invention.

[0057] In the above general formula, α is a coefficient that changes depending on the valence of the metal elements other than lithium contained in the lithium metal composite oxide, and the atomic ratio of lithium to the metal elements other than lithium.

[0058] The lithium transition metal composite oxide has a layered rock salt crystal structure. The crystallized lithium phosphate is characterized by coating the surface of the primary particles of the layered rock salt crystal structure lithium transition metal composite oxide and being dispersed inside or on the surface of the secondary particles of the layered rock salt crystal structure lithium transition metal composite oxide.

[0059] Lithium phosphate (Li3PO4) is preferably present in an amount of 10 mol% or less relative to lithium transition metal composite oxide (LiNiCoMnMO), more preferably between 1 mol% and 7 mol%, and even more preferably between 3 mol% and 5 mol%.

[0060] Based on the above, the positive electrode active material for lithium-ion secondary batteries according to one embodiment of the present invention can suppress structural changes associated with charging and discharging, thereby enabling high capacity and suppressing the capacity degradation during repeated charging and discharging, which is a drawback of LNO.

[0061] <4. Lithium-ion rechargeable batteries> A lithium-ion secondary battery according to one embodiment of the present invention is characterized by comprising a positive electrode containing the positive electrode active material for lithium-ion secondary batteries. Furthermore, the lithium-ion secondary battery can be composed of the same components as a general lithium-ion secondary battery, for example, including a positive electrode, a negative electrode, and a non-aqueous electrolyte. The embodiments described below are merely illustrative, and the lithium-ion secondary battery of this embodiment can be implemented in various modified and improved forms based on the embodiments described herein, according to the knowledge of those skilled in the art. Moreover, the lithium-ion secondary battery of this embodiment does not particularly limit its applications.

[0062] (a) Positive electrode Using the positive electrode active material for lithium-ion secondary batteries described above, the positive electrode of a lithium-ion secondary battery is prepared, for example, as follows. First, the powdered positive electrode active material, conductive agent, and binder are mixed, and activated carbon, a solvent for viscosity adjustment, etc., are added as needed, and the mixture is kneaded to prepare a positive electrode mixture paste. The mixing ratio of each component in the positive electrode mixture paste is, for example, when the total mass of the solid content of the positive electrode mixture excluding the solvent is 100 parts by mass, it is preferable that the positive electrode active material content be 60 to 95 parts by mass, the conductive agent content be 1 to 20 parts by mass, and the binder content be 1 to 20 parts by mass, similar to the positive electrode of a general lithium-ion secondary battery.

[0063] The resulting positive electrode mixture paste is applied to the surface of a current collector, for example, made of aluminum foil, and dried to allow the solvent to evaporate. If necessary, pressure may be applied using a roll press or the like to increase the electrode density. In this way, a sheet-shaped positive electrode can be produced. The sheet-shaped positive electrode can be cut to an appropriate size depending on the intended battery and used to manufacture the battery. However, the method for producing the positive electrode is not limited to the example given, and other methods may be used.

[0064] For the positive electrode conductive material, for example, graphite (natural graphite, artificial graphite, expanded graphite, etc.) or carbon black-based materials such as acetylene black and Ketjenblack (registered trademark) can be used.

[0065] The binder plays a role in holding the active material particles together, and examples of binders that can be used include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene propylene diene rubber, styrene butadiene, cellulose resins, and polyacrylic acid.

[0066] Furthermore, if necessary, a solvent is added to the positive electrode mixture to disperse the positive electrode active material, conductive agent, and activated carbon, and to dissolve the binder. Specifically, organic solvents such as N-methyl-2-pyrrolidone can be used as the solvent. In addition, activated carbon may be added to the positive electrode mixture to increase the electrical double layer capacity.

[0067] (b) Negative electrode For the negative electrode, a negative electrode mixture is used, which is made by mixing a binder with a negative electrode active material such as metallic lithium or lithium alloy, or a negative electrode active material capable of intercalating and deintercalating lithium ions, and adding a suitable solvent to form a paste. This negative electrode mixture is then applied to the surface of a metal foil current collector such as copper, dried, and compressed as needed to increase the electrode density.

[0068] As the negative electrode active material, for example, natural graphite, artificial graphite, calcined organic compounds such as phenolic resin, and powdered carbon materials such as coke can be used. In this case, as with the positive electrode, a fluororesin such as PVDF can be used as the negative electrode binder, and an organic solvent such as N-methyl-2-pyrrolidone can be used as the solvent for dispersing these active materials and binders.

[0069] (c) Separator A separator is placed between the positive and negative electrodes. The separator separates the positive and negative electrodes and holds the electrolyte. For example, a thin film made of polyethylene, polypropylene, or similar material with numerous tiny pores can be used.

[0070] (d) Non-aqueous electrolyte As a non-aqueous electrolyte, a non-aqueous electrolyte solution can be used. For example, a non-aqueous electrolyte solution may be one in which a lithium salt as a supporting salt is dissolved in an organic solvent. Alternatively, a non-aqueous electrolyte solution may be one in which a lithium salt is dissolved in an ionic liquid. An ionic liquid is a salt composed of cations and anions other than lithium ions, and which is liquid at room temperature.

[0071] As the organic solvent, one of the following may be used alone or in combination: cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate; linear carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate; ether compounds such as tetrahydrofuran, 2-methyltetrahydrofuran, and dimethoxyethane; sulfur compounds such as ethyl methyl sulfone and butanesultone; and phosphorus compounds such as triethyl phosphate and trioctyl phosphate.

[0072] As supporting salts, LiPF6, LiBF4, LiClO4, LiAsF6, LiN(CF3SO2)2, and their composite salts can be used. Furthermore, the non-aqueous electrolyte may contain radical scavengers, surfactants, and flame retardants.

[0073] Furthermore, solid electrolytes may be used as non-aqueous electrolytes. Solid electrolytes have the property of being able to withstand high voltages. Examples of solid electrolytes include inorganic solid electrolytes and organic solid electrolytes.

[0074] As inorganic solid electrolytes, oxide-based solid electrolytes, sulfide-based solid electrolytes, and the like are used.

[0075] The oxide-based solid electrolyte is not particularly limited and can be used as long as it contains oxygen (O) and has lithium-ion conductivity and electronic insulation properties. Examples of oxide-based solid electrolytes include lithium phosphate (Li3PO4) and Li3PO4N. XLiBO2N X , LiNbO3, LiTaO3, Li2SiO3, Li4SiO4-Li3PO4, Li4SiO4-Li3VO4, Li2O-B2O3-P2O5, Li2O-SiO2, Li2O-B2O3-ZnO, Li 1+X Al X Ti 2-X (PO4)3(0≦X≦1), Li 1+X Al X Ge 2-X (PO4)3(0≦X≦1), LiTi2(PO4)3, Li 3X La 2 / 3-X TiO3(0≦X≦2 / 3), Li5La3Ta2O 12 Li7La3Zr2O 12 Li6BaLa2Ta2O 12 Li 3.6 Si 0.6 P 0.4 Examples include O4.

[0076] The sulfide-based solid electrolyte is not particularly limited and can be used as long as it contains sulfur (S) and has lithium-ion conductivity and electronic insulation properties. Examples of sulfide-based solid electrolytes include Li2S-P2S5, Li2S-SiS2, LiI-Li2S-SiS2, LiI-Li2S-P2S5, LiI-Li2S-B2S3, Li3PO4-Li2S-Si2S, Li3PO4-Li2S-SiS2, LiPO4-Li2S-SiS, LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, and the like.

[0077] Furthermore, other inorganic solid electrolytes may be used besides those mentioned above; for example, Li3N, LiI, Li3N-LiI-LiOH, etc., may be used.

[0078] The organic solid electrolyte is not particularly limited as long as it is a polymer compound exhibiting ionic conductivity; for example, polyethylene oxide, polypropylene oxide, or copolymers thereof can be used. The organic solid electrolyte may also contain a supporting salt (lithium salt).

[0079] (e) Battery shape and configuration A lithium-ion secondary battery according to one embodiment of the present invention is composed of, for example, a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte as described above. Furthermore, the shape of the lithium-ion secondary battery is not particularly limited and can be various, such as cylindrical or stacked. Regardless of the shape adopted, the positive electrode and the negative electrode are stacked with a separator in between to form an electrode body, the resulting electrode body is impregnated with a non-aqueous electrolyte, the positive electrode current collector and the positive electrode terminal that is open to the outside, and the negative electrode current collector and the negative electrode terminal that is open to the outside are connected using current collector leads, etc., and the battery is sealed in a battery case to complete the lithium-ion secondary battery.

[0080] A lithium-ion secondary battery according to one embodiment of the present invention, by having a positive electrode composed of the above-mentioned positive electrode active material, can suppress structural changes associated with charging and discharging, thereby achieving high capacity and suppressing the capacity degradation during repeated charging and discharging, which is a drawback of LNO. [Examples]

[0081] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited in any way by these examples.

[0082] (Example 1) In Example 1, 0.97 g of lithium nickelate and 0.03 g of lithium phosphate were weighed using an electronic balance in a glove box under an argon atmosphere, thoroughly mixed in an agate mortar (mixing step), and then sealed in a 40 ml zirconia container together with 59 g of φ5 mm zirconia balls.

[0083] Then, a total of 80 mechanical milling processes were performed using a Fritsch planetary ball mill (P-7), each involving processing at 600 rpm for 15 minutes followed by a 3-minute pause (milling process).

[0084] Subsequently, the zirconia container was opened in an argon-filled glove box, the zirconia balls were removed, and the mixture obtained by mechanical milling was extracted. The crystal structure of the obtained mixture was confirmed using an XRD device (Bruker, product name: D2 PHASER) and confirmed to be amorphous.

[0085] Furthermore, the mixture obtained by mechanical milling was heat-treated in a calcination furnace under an oxygen atmosphere at 650°C for 12 hours (heat treatment step) to obtain the positive electrode active material according to the present invention.

[0086] The obtained cathode active material was observed using SEM-EDS (manufactured by JEOL Ltd., product name: JCM-6000), and elemental mapping images were obtained. The SEM observation results are shown in Figure 2, and the EDX elemental mapping results are shown in Figure 3.

[0087] Furthermore, to confirm the crystal structure of the obtained cathode active material, an XRD profile was obtained using an XRD instrument (Bruker, product name: D2 PHASER). The results are shown in Figure 4.

[0088] The obtained positive electrode active material was weighed to 0.170 g in a glove box under an argon atmosphere, and placed in an ointment container with 0.010 g of acetylene black, 0.250 g of PVDF solution (Kureha 8% solution), and 70 μl of NMP, and mixed with a stirrer (Thinky AR-100).

[0089] The obtained slurry was coated onto aluminum foil, vacuum-dried at room temperature for 2 hours, and then vacuum-dried at 120°C for 2 hours to obtain electrodes.

[0090] The obtained electrodes were punched out to a diameter of φ10 mm and their weight was measured. A coin cell was then assembled using these electrodes as the positive electrode, metallic lithium as the negative electrode, a porous PE film as the separator, and 1M LiPF6 / EC(3)+DMC(7) as the electrolyte. Subsequently, a current of 30 mA / g was passed through the resulting coin cell, and charging and discharging were performed in the range of 2.5V-4.5V. The charge and discharge profiles were then confirmed, and the results are shown in Figure 5.

[0091] Furthermore, Figure 6 shows the charge-discharge profile of the positive electrode active material manufactured at a heat treatment temperature of 650°C during the heat treatment process. Note that the charge-discharge cycle was performed 20 times.

[0092] (Example 2) In Example 2, 0.95 g of lithium nickelate and 0.05 g of lithium phosphate were used. Otherwise, the positive electrode active material was synthesized in the same manner as in Example 1, a coin cell was assembled, and charging and discharging were performed.

[0093] (Example 3) In Example 3, the heat treatment temperature was set to 600°C. Otherwise, the positive electrode active material was synthesized and a coin cell was assembled and charged / discharged in the same manner as in Example 1. The charge / discharge profile at a heat treatment temperature of 600°C is shown in Figure 6.

[0094] (Example 4) In Example 4, the heat treatment temperature was set to 700°C. Otherwise, the positive electrode active material was synthesized and a coin cell was assembled and charged / discharged in the same manner as in Example 1. The charge / discharge profile at a heat treatment temperature of 700°C is shown in Figure 6.

[0095] (Comparative Example 1) In Comparative Example 1, lithium phosphate was not mixed in the mixing process, nor was the milling process performed. Otherwise, the positive electrode active material was synthesized in the same manner as in Example 1, a coin cell was assembled, and charging and discharging were performed.

[0096] (evaluation) The positive electrode active materials in Example 1, Example 2, and Comparative Example 1 were observed using a scanning electron microscope (SEM). The SEM images are shown in Figure 2. As shown in Figure 2, the positive electrode active materials in Example 1 (Figure 2(B)) and Example 2 (Figure 2(C)) were lithium transition metal composite oxides with a layered rock salt-type crystalline structure that were nanoparticle-formed by mechanical stress such as mechanical milling. Therefore, even after the heat treatment process, they maintained a small particle size of 1 μm to 5 μm. On the other hand, the positive electrode active material in Comparative Example 1 (Figure 2(A)), in which lithium phosphate was not mixed in the mixing process and the milling process was not performed, had a particle size larger than 8 μm.

[0097] Furthermore, the elemental distribution of the positive electrode active material in Example 1 and Example 2 was confirmed. The elemental distribution is shown in Figures 3(A) and 3(B). As shown in Figure 3, in the positive electrode active material of Example 1 (Figure 3(A)) and Example 2 (Figure 3(B)), the obtained mapping images confirmed that phosphorus was dispersed on the submicron order.

[0098] Furthermore, to confirm the crystal structure of the positive electrode active material in Example 1, Example 2, and Comparative Example 1, an XRD apparatus was used to confirm the crystal structure. The XRD results are shown in Figure 4. As shown in Figure 4, the obtained profiles confirmed that the main phase of the positive electrode active material in Example 1 and Example 2 has a layered rock salt type crystal structure. Also, as shown in Figure 4, since a slight diffraction peak of lithium phosphate was detected in the positive electrode active material in Example 1 and Example 2, it was confirmed that crystalline lithium phosphate is dispersed on the submicron order.

[0099] Next, the discharge capacity retention rate with respect to the number of charge-discharge cycles was checked. The results are shown in Figure 5. As shown in Figure 5, the positive electrode active materials in Example 1 (LiNiO2-3%Li3PO4) and Example 2 (LiNiO2-5%Li3PO4) showed little decrease in discharge capacity when cycled repeatedly, and had excellent cycle characteristics. On the other hand, the positive electrode active material in Comparative Example 1 showed a large decrease in discharge capacity when cycled repeatedly, and its cycle characteristics were inferior to those of the examples. Note that "LiNiO2-3%Li3PO4" in the examples and in Figures 2 to 6 indicates that the weight ratio of lithium phosphate to the total weight of lithium phosphate and lithium transition metal composite oxide is 3% by weight, and "LiNiO2-5%Li3PO4" indicates that the weight ratio of lithium phosphate to the total weight of lithium phosphate and lithium transition metal composite oxide is 5% by weight.

[0100] Furthermore, the charge-discharge profiles of positive electrode active materials manufactured at heat treatment temperatures of 600°C to 700°C were examined. The results are shown in Figure 6. In Figure 6, the plots rising from the bottom left to the top right represent the capacity and voltage during charging, and the plots rising from the bottom right to the top left represent the capacity and voltage during discharging. As shown in Figure 6, the decrease in discharge capacity was small even after repeated charging and discharging, indicating excellent cycle characteristics. In addition, the positive electrode active material processed at 650°C in Example 1 exhibited the best cycle characteristics.

[0101] Based on the above, the present invention makes it possible to suppress structural changes associated with charging and discharging, thereby achieving high capacity and suppressing the capacity reduction during repeated charging and discharging, which is a drawback of LNO.

[0102] Although each embodiment and example of the present invention has been described in detail above, it will be readily apparent to those skilled in the art that many modifications are possible without substantially departing from the novel aspects and effects of the present invention. Therefore, all such modifications are considered to fall within the scope of the present invention.

[0103] For example, any term that appears at least once in the specification or drawings alongside a broader or synonymous term may be replaced with that different term anywhere in the specification or drawings. Furthermore, the method for producing the positive electrode active material for lithium-ion secondary batteries, the positive electrode active material for lithium-ion secondary batteries, and the configuration and operation of the lithium-ion secondary battery are not limited to those described in the embodiments and examples of the present invention, and various modifications are possible. [Explanation of Symbols]

[0104] S10 Lithium transition metal composite oxide manufacturing process, S20 Mixing process, S30 Milling process, S40 Heat treatment process

Claims

1. A method for producing a positive electrode active material for a lithium-ion secondary battery, comprising secondary particles formed by the aggregation of multiple primary particles, A mixing step of mixing a lithium transition metal composite oxide having a layered crystalline structure with lithium phosphate, A milling step is performed in which mechanical stress is applied to the mixture obtained in the mixing step to convert the lithium transition metal composite oxide having the layered crystalline structure and the lithium phosphate into an amorphous or low-crystalline NiO-like rock salt type crystalline structure. The process includes a heat treatment step in which the mixture of amorphous or low-crystallinity NiO-like rock salt-type crystalline structures obtained in the milling step is heat-treated to obtain a lithium transition metal composite oxide with a layered rock salt-type crystalline structure in which lithium phosphate crystals are crystallized and dispersed. The lithium transition metal composite oxide has the general formula Li ｓ Ni １－ｘ－ｙ－ｚ Co ｘ Mn ｙ M ｚ O ２＋α (where 0 ≤ x ≤ 0.35, 0 ≤ y ≤ 0.35, 0 ≤ z ≤ 0.10, 1.00 ≤ s ≤ 1.30, 0 ≤ α ≤ 0.2, and M is represented by at least one element selected from V, Mg, Mo, Nb, Ti, W, and Al), A method for producing a positive electrode active material for a lithium-ion secondary battery, characterized in that the crystallized lithium phosphate coats the surface of the primary particles of the layered rock salt-type crystalline lithium transition metal composite oxide and is dispersed inside or on the surface of the secondary particles of the layered rock salt-type crystalline lithium transition metal composite oxide.

2. The method for producing a positive electrode active material for a lithium-ion secondary battery according to claim 1, characterized in that, in the mixing step, lithium phosphate is mixed with the lithium transition metal composite oxide in an amount greater than 0 and 10 wt% or less.

3. The method for producing a positive electrode active material for a lithium-ion secondary battery according to claim 1 or 2, characterized in that the heat treatment step is performed at a temperature of 600 to 700°C.

4. The method for producing a positive electrode active material for a lithium-ion secondary battery according to any one of claims 1 to 3, characterized in that the milling step involves applying mechanical stress by mechanical milling.

5. A positive electrode active material for lithium-ion secondary batteries, comprising secondary particles formed by the aggregation of multiple primary particles, A lithium transition metal composite oxide having a layered rock salt crystal structure and crystallized lithium phosphate, and having the general formula kLi ３ PO ４ -(1 - k)Li ｓ Ni １－ｘ－ｙ－ｚ Co ｘ Mn ｙ M ｚ O ２＋α (where 0 < k < 0.1, 0 ≤ x ≤ 0.35, 0 ≤ y ≤ 0.35, 0 ≤ z ≤ 0.10, 1.00 ≤ s ≤ 1.30, 0 ≤ α ≤ 0.2, M is at least one element selected from V, Mg, Mo, Nb, Ti, W, and Al) and represented by The positive electrode active material for a lithium-ion secondary battery is characterized in that the crystallized lithium phosphate coats the surface of the primary particles of the layered rock salt-type crystalline lithium transition metal composite oxide and is dispersed inside or on the surface of the secondary particles of the layered rock salt-type crystalline lithium transition metal composite oxide.

6. A lithium-ion secondary battery characterized by comprising a positive electrode containing at least the positive electrode active material for lithium-ion secondary batteries described in claim 5.

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