Positive electrode active material for lithium ion secondary batteries, method for producing positive electrode active material for lithium ion secondary batteries, and lithium ion secondary battery

Abstract

A positive electrode active material for lithium ion secondary batteries contains secondary particles in which a plurality of primary particles are aggregated with each other. The positive electrode active material for lithium ion secondary batteries includes: a lithium transition metal composite oxide having a layered rock salt type structure; and crystallized lithium phosphate. In crystal structure analysis by neutron diffraction of the lithium transition metal composite oxide having the layered rock salt type structure, the ratio of elements other than lithium included in the lithium site (3b site) is 3%-8%.

Description

Positive electrode active material for lithium ion secondary battery, method for producing positive electrode active material for lithium ion secondary battery, and lithium ion secondary battery

[0001] The present invention relates to a positive electrode active material for a lithium ion secondary battery, a method for producing a positive electrode active material for a lithium ion secondary battery, and a lithium ion secondary battery.

[0002] In recent years, secondary batteries with high energy density have been strongly desired to extend the driving range of electric vehicles (hereinafter also referred to as "EVs"). As such secondary batteries, lithium-ion secondary batteries, which use lithium, lithium alloys, metal oxides, or carbon as the negative electrode, have attracted attention.

[0003] With the aim of further improving the performance of lithium ion secondary batteries, positive electrode active materials for use in lithium ion secondary batteries have been studied and proposed.

[0004] The positive electrode active material used in the positive electrode of a lithium ion secondary battery for EVs is, for example, LiNi. １－ｘ－ｙ Co ｘ Al ｙ O ２ (hereinafter also referred to as "NCA") is known. NCA is a lithium nickel oxide (LiNiO ２ (hereinafter also referred to as "LNO") is a positive electrode active material in which some of the Ni is substituted with Co and Al. The Co substitution suppresses changes in the crystal structure during charge and discharge, and the Al substitution improves thermal stability.

[0005] For example, Patent Document 1 discloses a positive electrode mixture for a non-aqueous electrolyte secondary battery containing a particulate positive electrode active material made of a transition metal composite oxide, and the transition metal composite oxide is lithium cobalt oxide (LiCoO ２ ) etc.

[0006] Patent Document 2 discloses positive electrode active material particles containing a lithium salt of a transition oxide having a layered rock salt structure. ｘ Co ｙ M ｚ O ２(wherein M is Al or Mn, 0<x<1, 0<y<1, x+y+z=1) is disclosed.

[0007] Japanese Patent Publication No. 2020-136093 Japanese Patent Publication No. 2016-110714

[0008] However, the positive electrode active materials disclosed in Patent Documents 1 and 2 have a problem in that when they are applied to a lithium ion secondary battery and repeatedly charged and discharged, the capacity decreases significantly.

[0009] In view of the problems of the above-described conventional techniques, an object of one aspect of the present invention is to provide a positive electrode active material for a lithium ion secondary battery that has excellent cycle characteristics when used in the lithium ion secondary battery.

[0010] In order to solve the above-mentioned problems, one aspect of the present invention provides a positive electrode active material for a lithium ion secondary battery, which comprises secondary particles formed by agglomeration of a plurality of primary particles, the positive electrode active material comprising a lithium transition metal composite oxide having a layered rock salt structure and crystallized lithium phosphate, wherein, in a crystal structure analysis by neutron diffraction of the lithium transition metal composite oxide having the layered rock salt structure, a proportion of elements other than lithium in the lithium sites (3b sites) is 3% or more and 8% or less.

[0011] According to one embodiment of the present invention, it is possible to provide a positive electrode active material for a lithium ion secondary battery that has excellent cycle characteristics when used in the lithium ion secondary battery.

[0012] FIG. 1 shows a flow chart of a method for producing a positive electrode active material for a lithium ion secondary battery.

[0013] Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the present invention is not limited to the following embodiments, and various modifications and substitutions can be made to the following embodiments without departing from the scope of the present invention. [Positive Electrode Active Material for Lithium-Ion Secondary Batteries] Hereinafter, a positive electrode active material for lithium-ion secondary batteries (hereinafter also referred to simply as "positive electrode active material," and a lithium-ion secondary battery is also referred to simply as "secondary battery") according to the present embodiment will be described. (1) Regarding the Positive Electrode Active Material The positive electrode active material according to the present embodiment contains a lithium transition metal composite oxide having a layered rock salt structure and crystallized lithium phosphate. The positive electrode active material according to the present embodiment may be composed only of a lithium transition metal composite oxide and crystallized lithium phosphate. However, even in this case, this does not exclude the inclusion of unavoidable impurities that are mixed in during the manufacturing process, etc. (1-1) Regarding the Particle Morphology The particle morphology of the positive electrode active material according to the present embodiment is not particularly limited, but the positive electrode active material according to the present embodiment may contain secondary particles formed by agglomeration of multiple primary particles. Therefore, the positive electrode active material of this embodiment may be composed of only secondary particles, or may contain, in addition to secondary particles, primary particles that exist independently without aggregation.

[0014] The particles of the positive electrode active material of this embodiment may contain particles of a lithium transition metal composite oxide, and the primary particles and secondary particles contained in the positive electrode active material of this embodiment may be primary particles and secondary particles of a lithium transition metal composite oxide.

[0015] The particle morphology of the positive electrode active material of this embodiment can be roughly confirmed by a scanning electron microscope (SEM) observation image (hereinafter also referred to as "SEM image") of the particle appearance, but this may lack accuracy. In the positive electrode active material of this embodiment, it is preferable to process the cross section of the particle and use the SEM image to determine the presence or absence of primary particles and secondary particles.

[0016] Specifically, for example, it is preferable to embed particles of the positive electrode active material in a resin, and then perform cross-section polishing (CP) processing or the like to make the particle cross-section observable, and then observe the SEM image to determine the presence or absence of primary particles and secondary particles. It is preferable to perform the above evaluation on 50 or more particles. There is no particular upper limit on the number of particles to be evaluated, but considering the productivity of the evaluation, it is preferable to perform the evaluation on 200 or less particles.

[0017] Furthermore, when a focused ion beam (FIB) processing device is used for cross-section processing, the particle cross-section can also be confirmed using the band contrast of an electron backscatter diffraction (hereinafter also referred to as "EBSD") method, or using an attached scanning ion microscope (SIM) observation image.

[0018] The crystallized lithium phosphate contained in the positive electrode active material of this embodiment preferably covers at least a portion of the surface of the primary particles of the lithium transition metal composite oxide having a layered rock salt structure, and further preferably is dispersed inside or on the surface of the secondary particles of the lithium transition metal composite oxide having a layered rock salt structure.

[0019] The crystallized lithium phosphate coats at least a portion of the surface of the primary particles of the lithium transition metal composite oxide and is dispersed inside or on the surface of the secondary particles of the lithium transition metal composite oxide, thereby improving the cycle characteristics when the positive electrode active material of this embodiment is applied to a secondary battery. While the reason for this is unclear, it is believed that the placement of lithium phosphate on the surface of the primary particles of the lithium transition metal composite oxide or inside or on the surface of the secondary particles can suppress the reaction between the electrolyte and the lithium transition metal composite oxide, and the lithium phosphate alleviates the expansion and contraction of the lithium transition metal composite oxide that occurs during charge and discharge. (1-2) Proportion of Elements Other Than Lithium at the Lithium Sites (3b Sites) In a neutron diffraction crystal structure analysis of the lithium transition metal composite oxide having a layered rock salt structure contained in the positive electrode active material of this embodiment, the proportion of elements other than lithium at the lithium sites (3b sites) is preferably 3% to 8%, more preferably 4% to 7%.

[0020] The proportion of elements other than lithium occupying the lithium sites (3b sites) refers to the proportion of elements other than lithium and oxygen, for example, metal elements other than lithium, occupying the lithium sites (3b sites). The proportion of elements other than lithium occupying the lithium sites (3b sites) can be obtained by analyzing a neutron diffraction pattern measured by neutron diffraction using the Rietveld method. When analyzing using the Rietveld method, a layered rock salt structure, which is the crystal structure of a lithium transition metal composite oxide, can be used as the crystal structure.

[0021] When a secondary battery is repeatedly charged and discharged, lithium is repeatedly inserted into and extracted from the interlayer space of the lithium transition metal composite oxide. However, when a secondary battery using a conventional positive electrode active material is repeatedly charged and discharged, elements other than lithium that were located at the metal sites (3a sites) may irreversibly migrate to the tetrahedral sites (6c sites).

[0022] In contrast, in the lithium transition metal composite oxide contained in the positive electrode active material of this embodiment, the proportion of elements other than lithium in the lithium sites (3b sites) is 3% or more. It is believed that by inserting elements other than lithium into the lithium sites in advance in this way, it is possible to suppress the irreversible migration of elements other than lithium to the tetrahedral sites (6c sites) during charge and discharge. Therefore, it is believed that changes in the crystal structure of the lithium transition metal composite oxide are suppressed when charge and discharge are repeated, thereby improving cycle characteristics.

[0023] Furthermore, by setting the proportion of elements other than lithium in the lithium sites (3b sites) to 8% or less, it is possible to suppress a decrease in the perfection of the crystal of the lithium transition metal composite oxide. As a result, a sufficient amount of lithium contributing to charge and discharge is secured, reaction resistance is suppressed, and battery capacity and output can be sufficiently increased. (1-3) Proportion of Lithium Element in the Metal Sites (3a Sites) In crystal structure analysis by neutron diffraction of a lithium transition metal composite oxide having a layered rock salt structure, it is preferable that the proportion of lithium element in the metal sites (3a sites) be 3% or more and 8% or less.

[0024] The lithium element inserted into the metal site (3a site) is derived from the lithium element inserted into the lithium site (3b site) that has been replaced by an element other than lithium. In other words, the lithium element inserted into the metal site (3a site) is the lithium element that should originally have been inserted into the lithium site (3b site).

[0025] Furthermore, by setting the ratio of lithium element occupying the metal site (3a site) to 3% or more and 8% or less, it means that the substituted lithium remains within the crystal structure, and it is possible to suppress a decrease in the perfection of the crystal of the lithium transition metal composite oxide. As a result, it is possible to suppress reaction resistance and sufficiently increase battery capacity and output. (1-4) Composition of Positive Electrode Active Material The composition of the positive electrode active material of this embodiment is not particularly limited, but it is preferable that the positive electrode active material has a general formula: kLi ３ P.O. ４ -(1-k)Li ｓ Ni １－ｘ－ｙ－ｚ Co ｘ Mn ｙ Mｚ O ２＋α It is preferable that the formula be represented by the following formula:

[0026] In the general formula, k, x, y, z, s, and α preferably satisfy the relationships 0<k<0.1, 0≦x≦0.35, 0≦y≦0.35, 0≦z≦0.10, 1.00≦s<1.30, and 0≦α≦0.2. Furthermore, the element M in the general formula can be at least one element selected from V, Mg, Mo, Nb, Ti, W, and Al.

[0027] In the above general formula, Li ３ P.O. ４ corresponds to lithium phosphate, and Li ｓ Ni １－ｘ－ｙ－ｚ Co ｘ Mn ｙ M ｚ O ２＋α As is clear from the above general formula, the lithium phosphate is preferably contained in a proportion of more than 0 mol % and less than 10 mol % in terms of substance amount relative to the total of the lithium phosphate and the lithium transition metal composite oxide contained in the positive electrode active material.

[0028] In addition, lithium phosphate (Li ３ P.O. ４ ) is a lithium transition metal composite oxide (Li ｓ Ni １－ｘ－ｙ－ｚ Co ｘ Mn ｙ M ｚ O ２＋α ) is preferably 10 wt % or less, and more preferably 5 wt % or less.

[0029] The composition of the lithium transition metal composite oxide can be measured by quantitative analysis using inductively coupled plasma (ICP) emission spectrometry. (1-5) Average Particle Size of Primary Particles In the positive electrode active material of this embodiment, the average particle size of the primary particles is preferably 20 nm or more and 250 nm or less, more preferably 20 nm or more and 150 nm or less, and even more preferably 70 nm or more and 150 nm or less.

[0030] By setting the average particle diameter of the primary particles of the positive electrode active material of this embodiment to 20 nm or more, it is possible to sufficiently increase the size of the primary particles contained in the positive electrode active material of this embodiment and the crystallites constituting the primary particles. Therefore, the crystallinity of the lithium transition metal composite oxide contained in the positive electrode active material can be increased, and when the positive electrode active material of this embodiment is applied to a secondary battery, the battery capacity can be increased, resulting in a secondary battery with excellent battery capacity and cycle characteristics.

[0031] As described above, the lithium transition metal composite oxide contained in the positive electrode active material of this embodiment has an element other than Li inserted into the Li site (3b site). Insertion of an element other than Li into the Li site in this manner may reduce the diffusibility of Li during charge and discharge. In response to this, the average particle size of the primary particles of the positive electrode active material of this embodiment is set to 250 nm or less, thereby reducing the diffusion distance of Li. Therefore, the reduction in diffusibility of Li due to insertion of an element other than Li into the Li site (3b site) is suppressed, and the capacity and output characteristics can be improved when applied to a secondary battery.

[0032] The average particle size of the primary particles of the positive electrode active material is determined by selecting primary particles of the positive electrode active material that can be observed in their entirety with a scanning electron microscope (SEM), measuring their major axis lengths, and averaging them. When calculating the average particle size of the primary particles, the number of primary particles to be evaluated is not particularly limited.

[0033] For example, when the positive electrode active material contains secondary particles, it is preferable to select 10 to 20 secondary particles, select primary particles from each of the selected secondary particles, and then select a total of 20 to 30 primary particles, and measure the major axis lengths of the selected primary particles.

[0034] Furthermore, when the positive electrode active material contains single primary particles that do not constitute secondary particles, it is preferable to evaluate five or more primary particles of the positive electrode active material. There is no particular upper limit on the number of primary particles to be evaluated, but it is preferable to evaluate, for example, 20 or less.

[0035] When the positive electrode active material contains only secondary particles, the above number of secondary particles can be selected, and the above number of primary particles can be selected in total from each secondary particle. The lengths of the major axes of the selected primary particles can then be measured, and the average value can be used as the average particle size of the primary particles of the positive electrode active material.

[0036] When the positive electrode active material contains both secondary particles and single primary particles, the above-mentioned number of primary particles can be selected for each of the selected number of secondary particles and single primary particles. Then, the major axis lengths of all the selected primary particles can be measured, and the average value can be used as the average particle size of the primary particles of the positive electrode active material.

[0037] By setting the average particle size of the primary particles of the positive electrode active material within the above range, when the positive electrode active material is applied to a secondary battery, it is possible to particularly increase both the battery capacity and the cycle characteristics, and to achieve both. [Method for producing a positive electrode active material for a lithium ion secondary battery] The method for producing the positive electrode active material for a lithium ion secondary battery is not particularly limited as long as it can produce a positive electrode active material having the above characteristics. Hereinafter, an example of the method for producing a positive electrode active material according to this embodiment will be described.

[0038] As described above, according to the method for producing a positive electrode active material of this embodiment, the positive electrode active material already described can be produced, and therefore, some of the matters already described will not be described again.

[0039] The method for producing a positive electrode active material of this embodiment is a method for producing a positive electrode active material for a lithium ion secondary battery including secondary particles formed by aggregation of a plurality of primary particles, and can be carried out, for example, according to flow 10 shown in Fig. 1. That is, the method for producing a positive electrode active material of this embodiment can include a mixing step (S1), a milling step (S2), and a heat treatment step (S3).

[0040] In the mixing step (S1), a mixture can be prepared by mixing a lithium transition metal composite oxide having a layered rock salt structure with lithium phosphate.

[0041] In the milling step (S2), mechanical stress is applied to the mixture obtained in the mixing step (S1), and a milled product having an amorphous or low-crystalline NiO-like rock salt structure can be obtained.

[0042] In the heat treatment step (S3), the milled product obtained in the milling step (S2) can be heat-treated at a temperature of 620°C or higher and 700°C or lower by increasing the temperature at a rate of 2°C / min or lower.

[0043] An example of the configuration of each step will be described below: (1) Mixing Step (S1) In the mixing step (S1), as described above, a lithium transition metal composite oxide and lithium phosphate can be mixed to prepare a mixture.

[0044] The lithium transition metal composite oxide subjected to the mixing step (S1) has a layered rock salt structure.

[0045] The composition of the lithium transition metal composite oxide is not particularly limited and can be selected depending on the intended positive electrode active material. The lithium transition metal composite oxide can be, for example, a lithium transition metal oxide represented by the general formula: Li ｓ´ Ni1-x'-y'-z'Co ｘ´ Mn ｙ´ M ｚ´ O ２＋β It is preferable that the formula be represented by the following formula:

[0046] In the general formula, x', y', z', s', and β preferably satisfy the following conditions: 0≦x'≦0.35, 0≦y'≦0.35, 0≦z'≦0.10, 1.00≦s'<1.30, and 0≦β≦0.2. Furthermore, the element M is preferably at least one element selected from V, Mg, Mo, Nb, Ti, W, and Al.

[0047] For lithium phosphate, Li ３ P.O. ４ It can be expressed as:

[0048] In the mixing step (S1), the method for mixing the lithium transition metal composite oxide and the lithium phosphate is not particularly limited, and they can be mixed in a mortar, for example.

[0049] In the mixing step (S1), the ratio of the lithium transition metal composite oxide and lithium phosphate to be mixed is not particularly limited, but it is preferable to mix them so that the lithium phosphate content in the resulting mixture is the target composition. Therefore, for example, in the mixing step, it is preferable to mix lithium phosphate so that the ratio of the amount of substance of the lithium phosphate and the lithium transition metal composite oxide contained in the resulting mixture is greater than 0 mol% and less than 10 mol%. (2) Milling Step In the milling step (S2), mechanical stress is applied to the mixture obtained in the mixing step (S1). By applying mechanical stress to the mixture, the lithium transition metal composite oxide having a layered rock salt structure and lithium phosphate can be milled into an amorphous or low-crystalline NiO-like rock salt structure.

[0050] Although the method of applying the mechanical stress is not particularly limited, it is preferable to use mechanical milling, which makes it easier to convert the lithium transition metal composite oxide having a layered rock-salt crystal structure and lithium phosphate into an amorphous or low-crystalline NiO-like rock-salt structure.

[0051] An amorphous or low-crystalline NiO-like rock salt structure refers to a state in which, in the XRD pattern of the milled product after the milling step, the half-width of the diffraction peak of the (200) plane observed at 44° to 45° is 0.5° or more after background processing, etc. (3) Heat Treatment Step In the heat treatment step (S3), the milled product having an amorphous or low-crystalline NiO-like rock salt structure obtained in the milling step (S2) can be heat-treated. By performing the heat treatment step, a lithium transition metal composite oxide having a layered rock salt structure in which lithium phosphate is crystallized and dispersed can be obtained. Furthermore, by combining the above-mentioned milling step and the heat treatment step, the proportion of elements other than lithium in the Li site (3b site) can be easily adjusted to a desired range.

[0052] In the heat treatment step (S3), only the lithium transition metal composite oxide takes on a layered rock salt structure, and lithium phosphate does not take on a layered rock salt structure.

[0053] By carrying out the heat treatment step (S3), the crystallized lithium phosphate can coat the surfaces of the primary particles of the lithium transition metal composite oxide having a layered rock salt structure and be dispersed inside or on the surfaces of the secondary particles of the lithium transition metal composite oxide having a layered rock salt structure. The lithium phosphate can be, for example, in the form of fine crystals. (Heat Treatment Temperature) In the heat treatment step (S3), the heat treatment is preferably carried out at a temperature (heat treatment temperature) of 620°C or higher and 700°C or lower, and more preferably at a temperature of 620°C or higher and 680°C or lower. By carrying out the heat treatment within the above temperature range, the crystallized lithium phosphate can easily coat the surfaces of the primary particles of the lithium transition metal composite oxide having a layered rock salt structure and be dispersed inside or on the surfaces of the secondary particles of the lithium transition metal composite oxide having a layered rock salt structure.

[0054] In the heat treatment step (S3), the temperature is preferably increased to the heat treatment temperature at a rate (heating rate) of 2°C / min or less, more preferably 0.1°C / min or more and 1°C / min or less.

[0055] By setting the temperature rising rate to 2° C. / min or less, the reaction within the milled material can proceed uniformly, and the crystal growth of the lithium transition metal composite oxide can be promoted.

[0056] The holding time at the heat treatment temperature is not particularly limited, but is preferably 10 hours or more, and more preferably 10 hours or more and 48 hours or less.

[0057] In the heat treatment step, for example, the temperature is raised to the heat treatment temperature, maintained for a predetermined time, and then cooled to room temperature. (Atmosphere) The heat treatment atmosphere is preferably an oxidizing atmosphere. The oxidizing atmosphere preferably has an oxygen concentration of 80% by volume or more, more preferably an oxygen concentration of 85% by volume or more. Note that the oxidizing atmosphere can also be an oxygen atmosphere, and therefore the oxygen concentration can be 100% by volume or less.

[0058] The method for producing a positive electrode active material according to this embodiment may further include any optional steps as necessary. For example, it may further include a lithium transition metal composite oxide production step of preparing a lithium transition metal composite oxide to be subjected to the mixing step described above. (4) Lithium Transition Metal Composite Oxide Production Step The lithium transition metal composite oxide production step may include, for example, a composite hydroxide production step, a heating step, and a calcination step, which are described below. Note that the lithium transition metal composite oxide production step may include only one of the above steps. (4-1) Composite Hydroxide Production Step The composite hydroxide production step is a step of obtaining a composite hydroxide as a coprecipitate in a reaction aqueous solution obtained by mixing a mixed aqueous solution containing a metal salt with an alkaline aqueous solution.

[0059] The mixed aqueous solution may contain, as the metal salt, a metal salt of a metal other than lithium contained in the lithium transition metal composite oxide. The mixed aqueous solution may contain, for example, a nickel salt such as nickel(II) sulfate, a cobalt salt such as cobalt(II) sulfate, a manganese salt such as manganese(II) sulfate, or a metal salt of element M. The element M used in the metal salt of element M may be at least one element selected from the group consisting of V, Mg, Mo, Nb, Ti, W, and Al.

[0060] The mixed aqueous solution can contain the metal salts in proportions that match the target composition of the lithium transition metal composite oxide.

[0061] A mixed aqueous solution containing a metal salt is supplied to a reaction tank together with an alkaline aqueous solution to form a reaction aqueous solution, and a composite hydroxide can be obtained as a coprecipitate. When a composite hydroxide contains multiple types of metals, multiple types of metal salts must be supplied to the reaction aqueous solution as a mixed aqueous solution containing metal salts. For example, a mixed aqueous solution may be prepared and supplied for each metal salt. Alternatively, one or more mixed aqueous solutions containing multiple types of metal salts may be prepared and supplied.

[0062] The alkaline aqueous solution is not particularly limited, but for example, an aqueous solution of an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide can be used.

[0063] If necessary, a complexing agent may be added to the reaction solution, such as one or more of aqueous solutions of ammonia, ammonium sulfate, ammonium chloride, etc.

[0064] The composite hydroxide can be obtained by mixing the mixed aqueous solution with an alkaline aqueous solution to form a reaction aqueous solution and controlling the pH value of the reaction aqueous solution. In this case, the pH value of the reaction aqueous solution is not particularly limited, but it is preferable to control it so that it is alkaline, for example.

[0065] When no complexing agent is added to the reaction aqueous solution, the pH value of the reaction aqueous solution is preferably controlled to be 9 or more and 11 or less. In this case, the temperature of the reaction aqueous solution is preferably set in the range of 40°C or more and 80°C or less. By setting the pH value and temperature of the reaction aqueous solution in the above ranges, the reaction rate can be set in an appropriate range. In addition, the solubility of ions of metals such as nickel becomes favorable, and the formation of fine particles due to crystallization can be prevented.

[0066] Specifically, by adjusting the pH value of the reaction aqueous solution to 11 or less, the formation of fine particles can be suppressed and spherical particles can be easily formed. Therefore, when the obtained composite hydroxide is filtered, the filterability can be improved. By adjusting the pH value of the reaction aqueous solution to 9 or more, the production rate of the composite hydroxide can be made to be suitable, and a composite hydroxide of the target composition can be easily produced.

[0067] Furthermore, by setting the temperature of the reaction aqueous solution to 40° C. or higher, the reaction rate can be adjusted to an appropriate rate. By setting the temperature of the reaction aqueous solution to 80° C. or lower, the amount of water evaporation can be suppressed, and an increase in the slurry concentration can be suppressed. As a result, a composite hydroxide of the target composition can be easily obtained.

[0068] As mentioned above, a complexing agent can be added to the reaction solution, which can increase the solubility of metals such as nickel.

[0069] When a complexing agent is used, the pH value of the reaction aqueous solution is preferably controlled to be 10 or more and 12.5 or less. In this case, the temperature of the reaction aqueous solution is preferably set in the range of 40°C or more and 60°C or less.

[0070] The concentration of the complexing agent in the reaction aqueous solution is not particularly limited, but is preferably maintained at a constant value within the range of 3 g / L to 25 g / L. By setting the concentration of the complexing agent to 3 g / L or more, fluctuations in the solubility of metal ions in the reaction aqueous solution can be suppressed. As a result, composite hydroxides with uniform shape and particle size can be formed, and the width of the particle size distribution can also be suppressed.

[0071] Furthermore, by setting the concentration of the complexing agent in the reaction aqueous solution to 25 g / L or less, the solubility of the metal ions can be adjusted to a particularly suitable range, and a composite hydroxide of the target composition can be easily obtained.

[0072] Fluctuations in the complexing agent concentration in the reaction aqueous solution result in fluctuations in the solubility of metal ions, so it is preferable to maintain the complexing agent concentration at a constant value. For example, the complexing agent concentration is preferably maintained at a desired concentration with the upper and lower limits set to approximately 5 g / L or less. As mentioned above, ammonia water or the like can be used as the complexing agent, so the complexing agent concentration can also be referred to as the ammonia concentration. (4-2) Heating Step The heating step is a step of heating the composite hydroxide produced in the composite hydroxide production step and can be performed as needed. The heating step can remove moisture contained in the composite hydroxide. This heating step can reduce moisture remaining until the calcination step. Furthermore, depending on the conditions of the heating step, the composite hydroxide can be converted into a composite oxide. By performing the heating step, variations in the number of metal atoms and the ratio of lithium atoms in the produced lithium transition metal composite oxide and the positive electrode active material can be prevented.

[0073] In the heating step, it is sufficient to remove moisture to such an extent that there is no variation in the ratio of the number of metal atoms or the number of lithium atoms in the lithium transition metal composite oxide, etc., and therefore it is not necessary to convert all of the composite hydroxide into a composite oxide. When the heating step is performed, the composite hydroxide is heated to a temperature at which the residual moisture is removed, and the heating temperature is not particularly limited, but is preferably 105°C or higher and 800°C or lower. If the composite hydroxide is heated to 105°C or higher, the residual moisture can be removed in a short time. Note that, at temperatures below 105°C, it tends to take a long time to remove the residual moisture, so heating at 105°C or higher is preferable as described above. Furthermore, by setting the heating temperature to 800°C or lower, it is possible to prevent the particles converted into the composite oxide from sintering and agglomerating.

[0074] The atmosphere in which the heat treatment is carried out is not particularly limited, and it is preferably carried out in an air stream, which is easy to carry out. (4-3) Calcination Step In the calcination step, a lithium mixture obtained by mixing the heat-treated product obtained in the heating step with one or more selected from lithium and lithium compounds can be calcined. Note that the composite hydroxide prepared in the composite hydroxide production step can also be used as is instead of the heat-treated product. A lithium transition metal composite oxide having a layered rock salt structure can be obtained by the calcination step.

[0075] The lithium mixture preferably has a ratio (Li / Me) of the number of atoms of metal elements other than lithium (Me) to the number of atoms of lithium element (Li) in the lithium mixture of 1.00 or more and less than 1.30. That is, it is preferable to mix the lithium mixture so that the Li / Me in the lithium mixture is the same as the Li / Me in the lithium transition metal composite oxide that is the product. This is because the Li / Me does not change before and after the calcination step, and therefore the Li / Me in the lithium mixture becomes equal to the Li / Me in the lithium transition metal composite oxide. The above-mentioned Me refers to the total number of atoms of metal elements other than lithium in the lithium mixture or the lithium transition metal composite oxide, and examples thereof include the total number of atoms of nickel, cobalt, manganese, and the element M.

[0076] The lithium compound used in preparing the lithium mixture is not particularly limited, but for example, one or more selected from the group of lithium compounds consisting of lithium hydroxide, lithium nitrate, and lithium carbonate can be preferably used from the viewpoint of easy availability. As the lithium compound used in preparing the lithium mixture, a mixture of two or more selected from the above group of lithium compounds can also be used.

[0077] As the lithium compound, it is more preferable to use lithium hydroxide, particularly in consideration of ease of handling and stability of quality.

[0078] It is preferable that the lithium mixture be thoroughly mixed before firing in the firing step, since thorough mixing before firing eliminates variations in Li / Me between individual particles, resulting in particularly high battery performance.

[0079] In the calcination step, the lithium mixture is calcined to obtain a lithium transition metal composite oxide. When the lithium mixture is calcined in the calcination step, lithium in the lithium-containing substance is diffused into the composite hydroxide obtained in the composite hydroxide production step or the heating step, and a lithium transition metal composite oxide having a layered rock salt structure is formed.

[0080] The lithium mixture is preferably fired at 600°C or higher and 850°C or lower, more preferably 650°C or higher and 820°C or lower. By setting the firing temperature to 600°C or higher, lithium can be sufficiently diffused into the composite hydroxide, etc., and the crystallinity of the resulting lithium transition metal composite oxide can be improved. Furthermore, by setting the firing temperature to 850°C or lower, sintering between particles of the resulting lithium transition metal composite oxide can be suppressed, and abnormal grain growth can be suppressed. Therefore, coarsening of the particles of the lithium transition metal composite oxide obtained after firing can be suppressed.

[0081] The baking time is not particularly limited, but is preferably at least 3 hours or more, and more preferably 6 hours or more and 24 hours or less.

[0082] The atmosphere during firing is preferably an oxidizing atmosphere, and more preferably an atmosphere with an oxygen concentration of 18% by volume or more and 100% by volume or less. The firing process is preferably carried out in air or an oxygen stream. By setting the oxygen concentration in the firing atmosphere to 18% by volume or more, even if a composite hydroxide or the like is contained in the particles, it can be sufficiently oxidized, thereby improving the crystallinity of the lithium transition metal composite oxide. In particular, considering the battery characteristics, it is preferable to carry out the firing in an oxygen stream. [Lithium Ion Secondary Battery] The lithium ion secondary battery of this embodiment includes at least a positive electrode, a negative electrode, and a non-aqueous electrolyte, and the positive electrode can contain the above-mentioned positive electrode active material for lithium ion secondary batteries.

[0083] Below, one example of the configuration of the secondary battery of this embodiment will be described for each component. The secondary battery of this embodiment includes, for example, a positive electrode, a negative electrode, and a non-aqueous electrolyte, and is composed of the same components as a general lithium-ion secondary battery. Note that the embodiment described below is merely an example, and the lithium-ion secondary battery of this embodiment can be implemented in various forms, including the following embodiment, with various modifications and improvements based on the knowledge of those skilled in the art. Furthermore, the use of the secondary battery is not particularly limited. (Positive Electrode) The positive electrode of the secondary battery of this embodiment can include the positive electrode active material described above.

[0084] An example of a method for manufacturing a positive electrode is described below. First, the above-described positive electrode active material (powder), a conductive agent, and a binder are mixed to prepare a positive electrode mixture, to which activated carbon and a solvent for viscosity adjustment and the like are added as needed, and the mixture is kneaded to prepare a positive electrode mixture paste.

[0085] The mixing ratio of each material in the positive electrode mixture is a factor that determines the performance of the lithium-ion secondary battery, and can be adjusted depending on the application. The mixing ratio of the materials can be the same as that of the positive electrodes of known lithium-ion secondary batteries. For example, when the total mass of the solid content of the positive electrode mixture excluding the solvent is taken as 100 mass%, the positive electrode active material can be contained in proportions of 60 mass% to 95 mass%, the conductive agent can be contained in proportions of 1 mass% to 20 mass%, and the binder can be contained in proportions of 1 mass% to 20 mass%.

[0086] The resulting positive electrode mixture paste is applied to the surface of, for example, an aluminum foil current collector, and dried to remove the solvent, producing a sheet-like positive electrode. If necessary, pressure can be applied using a roll press or the like to increase the electrode density. The sheet-like positive electrode thus obtained can be cut to an appropriate size depending on the desired battery and used to produce the battery.

[0087] As the conductive agent, for example, graphite (natural graphite, artificial graphite, expanded graphite, etc.) and carbon black materials such as acetylene black and Ketjen Black (registered trademark) can be used.

[0088] The binder serves to bind the positive electrode active material particles together, and may be one or more selected from, for example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene propylene diene rubber, styrene butadiene, cellulose-based resin, polyacrylic acid, and the like.

[0089] If necessary, a solvent for dispersing the positive electrode active material, conductive agent, etc. and dissolving the binder can be added to the positive electrode mixture. Specifically, an organic solvent such as N-methyl-2-pyrrolidone can be used as the solvent. Activated carbon can also be added to the positive electrode mixture to increase the electric double layer capacity.

[0090] The method for producing the positive electrode is not limited to the above-mentioned exemplary method, and other methods may also be used. For example, the positive electrode can be produced by press-molding the positive electrode mixture and then drying it under a vacuum atmosphere. (Negative Electrode) Metallic lithium, a lithium alloy, or the like can be used for the negative electrode. Alternatively, the negative electrode may be produced by mixing a binder with a negative electrode active material capable of absorbing and desorbing lithium ions, adding an appropriate solvent to form a paste of the negative electrode mixture, applying it to the surface of a metal foil current collector such as copper, drying it, and compressing it to increase the electrode density as needed.

[0091] Examples of the negative electrode active material include natural graphite, artificial graphite, and sintered organic compounds such as phenolic resins, as well as powdered carbonaceous materials such as coke. In this case, a fluorine-containing resin such as PVDF can be used as the negative electrode binder, as in the positive electrode. An organic solvent such as N-methyl-2-pyrrolidone can be used as the solvent for dispersing these active materials and binders. (Separator) A separator can be sandwiched between the positive electrode and negative electrode as needed. The separator separates the positive electrode and negative electrode and retains the electrolyte. Known separators can be used, such as thin membranes of polyethylene or polypropylene with numerous micropores. (Non-aqueous Electrolyte) The non-aqueous electrolyte can be, for example, a non-aqueous electrolytic solution.

[0092] The non-aqueous electrolyte may be, for example, a solution of a lithium salt as a supporting salt dissolved in an organic solvent. Alternatively, the non-aqueous electrolyte may be a solution of a lithium salt dissolved in an ionic liquid. The ionic liquid is a salt that is liquid even at room temperature and is composed of cations and anions other than lithium ions.

[0093] The organic solvent may be selected from the group consisting of cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate; chain 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 butane sultone; and phosphorus compounds such as triethyl phosphate and trioctyl phosphate. One of these organic solvents may be used alone, or two or more of these may be used in combination.

[0094] The supporting salt is LiPF ６ , LiBF ４ , LiClO ４ , LiAsF ６ , LiN(CF ３ SO ２ ) ２and complex salts thereof. The non-aqueous electrolyte solution may further contain a radical scavenger, a surfactant, a flame retardant, and the like.

[0095] Alternatively, the non-aqueous electrolyte may be a solid electrolyte, which has the property of being able to withstand high voltages. Examples of the solid electrolyte include inorganic solid electrolytes and organic solid electrolytes.

[0096] Examples of inorganic solid electrolytes include oxide-based solid electrolytes and sulfide-based solid electrolytes.

[0097] The oxide-based solid electrolyte is not particularly limited, and for example, an oxide-based solid electrolyte containing oxygen (O) and having lithium ion conductivity and electronic insulation can be suitably used. For example, lithium phosphate (Li ３ P.O. ４ ), Li ３ P.O. ４ N Ｘ , LiBO ２ N Ｘ , LiNbO ３ , LiTaO ３ , Li ２ SiO ３ , Li ４ SiO ４ -Li ３ P.O. ４ , Li ４ SiO ４ -Li ３ VO ４ , Li ２ Alumni ２ O ３ -P ２ O ５ , Li ２ O—SiO ２ , Li ２ Alumni ２ O ３ -ZnO, Li １＋Ｘ Al Ｘ Ti ２－Ｘ (P.O. ４ ) ３ (0≦X≦1), Li １＋Ｘ Al Ｘ Ge ２－Ｘ (P.O. ４ ) ３(0≦X≦1), LiTi ２ (P.O. ４ ) ３ , Li ３Ｘ La ２／３－Ｘ TiO ３ (0≦X≦2 / 3), Li ５ La ３ Ta ２ O １２ , Li ７ La ３ Zr ２ O １２ , Li ６ BaLa ２ Ta ２ O １２ , Li ３．６ Si ０．６ P ０．４ O ４ One or more selected from the above can be used.

[0098] The sulfide-based solid electrolyte is not particularly limited, and for example, a sulfide-based solid electrolyte containing sulfur (S) and having lithium ion conductivity and electronic insulation can be suitably used. ２ S-P ２ S ５ , Li ２ S-SiS ２ , LiI-Li ２ S-SiS ２ , LiI-Li ２ S-P ２ S ５ , LiI-Li ２ S-B ２ S ３ , Li ３ P.O. ４ -Li ２ S-Si ２ S., Li. ３ P.O. ４ -Li ２ S-SiS ２ , LiPO ４ -Li ２ S-SiS, LiI-Li ２ S-P ２ O ５ , LiI-Li ３ P.O. ４ -P ２ S ５One or more selected from the above can be used.

[0099] In addition, inorganic solid electrolytes other than those mentioned above may be used. For example, Li ３ N, LiI, Li ３ N-LiI-LiOH or the like may also be used.

[0100] The organic solid electrolyte is not particularly limited as long as it is a polymer compound exhibiting ion conductivity, and examples thereof include polyethylene oxide, polypropylene oxide, and copolymers thereof. The organic solid electrolyte may also contain a supporting salt (lithium salt). (Shape and Configuration of Lithium-Ion Secondary Battery) The lithium-ion secondary battery of the present embodiment described above can be formed into various shapes, such as a cylindrical shape or a laminated shape. Regardless of the shape, if the secondary battery of the present embodiment uses a nonaqueous electrolyte solution as the nonaqueous electrolyte, the positive electrode and the negative electrode are laminated with a separator interposed therebetween to form an electrode assembly, the resulting electrode assembly is impregnated with a nonaqueous electrolyte solution, and the positive electrode current collector and the positive electrode terminal connected to the outside, and the negative electrode current collector and the negative electrode terminal connected to the outside are connected using current collecting leads or the like, and the battery can be sealed in a battery case.

[0101] As described above, the secondary battery of this embodiment is not limited to a battery using a non-aqueous electrolytic solution as the non-aqueous electrolyte, and may be, for example, a secondary battery using a solid non-aqueous electrolyte, i.e., an all-solid-state battery. When an all-solid-state battery is used, the components other than the positive electrode active material may be changed as necessary.

[0102] The secondary battery of this embodiment can be used for various purposes. Since the secondary battery of this embodiment can be a high-capacity, high-output secondary battery, it is suitable, for example, as a power source for small portable electronic devices (such as notebook personal computers and mobile phone terminals) that always require high capacity, and is also suitable as a power source for electric vehicles that require high output.

[0103] Furthermore, since the secondary battery of this embodiment can be made smaller and have higher output, it is suitable as a power source for electric vehicles, which are subject to space restrictions. The secondary battery of this embodiment can be used not only as a power source for electric vehicles that are driven purely by electrical energy, but also as a power source for so-called hybrid vehicles that use the secondary battery in combination with a combustion engine such as a gasoline engine or a diesel engine.

[0104] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0105] First, the evaluation methods for the positive electrode active materials and secondary batteries obtained in the following examples and comparative examples will be described. (Evaluation of Positive Electrode Active Materials) The obtained positive electrode active materials were evaluated as follows.

[0106] (a) Evaluation of Composition The composition of the positive electrode active material was analyzed by ICP atomic emission spectrometry. The evaluation results are shown in the "Composition" column of Table 1. The proportion of lithium phosphate shown in the composition column means the mass proportion relative to the lithium transition metal composite oxide.

[0107] (b) The proportion of elements other than lithium in the lithium site (3b site), and the proportion of lithium in the metal site (3a site) The positive electrode active material prepared in the following examples and comparative examples was filled into a vanadium tube of about 2 g and then completely sealed. Then, neutron diffraction measurements were performed in double frame mode at BL20 (Ibaraki Prefecture Materials Structure Analysis System, iMATERIA) at the Japan Proton Accelerator Research Complex (J-PARC) under conditions that yielded an intensity of 10,000 cps or more.

[0108] The neutron diffraction pattern obtained from the rear detector bank was subjected to Rietveld analysis using the analysis software Z-Code. At this time, the space group was set to R-3m, and lithium and contaminating metal elements (metal elements other than lithium in the lithium site) were set as chemical species at the 3b site, and metal elements other than lithium and contaminating lithium were set as chemical species at the 3a site. The metal elements other than lithium refer to nickel, cobalt, manganese, and element M, and were selected and set according to the composition of each example. The metal element other than lithium was nickel in Examples 1 to 4 and Comparative Examples 1 and 2. The metal elements other than lithium were nickel, cobalt, and aluminum in Example 5, and nickel, manganese, and cobalt in Example 6.

[0109] After refining the scale factor, lattice constant, and global parameters, the site occupancy and atomic coordinates were refined. The atomic displacement parameters for lithium were 0.9 to 1.0 Å. ２ , nickel, cobalt, manganese, and element M: 0.3 to 0.5 Å ２ , oxygen: 0.6 to 0.8 Å ２ The site occupancy rate and atomic coordinates were refined, and the reliability parameter S value was considered to have converged when it was 3.0 or less. The site occupancy rate of metal elements other than lithium at the obtained lithium site (3b site) was calculated, and the results are shown in the column "Ratio of elements other than Li at the Li site (3b site)" in Table 1. The site occupancy rate of lithium elements at the obtained metal site (3a site) was also calculated, and the results are shown in the column "Ratio of Li element at the metal site (3a site)" in Table 1.

[0110] Powder neutron diffraction measurements can also be performed using a High Resolution Powder Diffractometer (HRPD) installed in the Japan Atomic Energy Agency's JRR-3 reactor or a Super HRPD at J-PARC.

[0111] (c) Average particle size of primary particles In the SEM observation image of the lithium transition metal composite oxide particles, 15 secondary particles that could be observed in their entirety by image analysis were selected, and primary particles were selected from each of the selected secondary particles. In this case, a total of 30 primary particles were selected. The major axis lengths of the selected primary particles were then measured, and the average value was taken as the average primary particle size.

[0112] The evaluation results are shown in the column "Average particle size of primary particles" in Table 1. (Fabrication and evaluation of lithium ion secondary batteries) (a) Fabrication of coin-type batteries Coin-type batteries were fabricated using the positive electrode active materials fabricated in the following examples and comparative examples.

[0113] Specifically, 0.170 g of the positive electrode active material obtained in the following Examples and Comparative Examples was weighed out in a glove box under an argon atmosphere. The weighed positive electrode active material was then placed in a container together with 0.010 g of acetylene black, 0.250 g of polyvinylidene fluoride (PVDF) solution (8% solution manufactured by Kureha), and 70 μL of N-methyl-2-pyrrolidone (NMP), and mixed using a stirrer (AR-100 manufactured by Thinky).

[0114] The slurry obtained by stirring was coated on an aluminum foil, and the coated aluminum foil was vacuum dried at room temperature for 2 hours, and then vacuum dried at 120° C. for 2 hours to obtain an electrode.

[0115] The obtained electrode was punched out to a diameter of 10 mm, and the weight was measured, and this was used as a positive electrode.

[0116] Metallic lithium was used as the negative electrode, and a porous polyethylene (PE) film was used as the separator. The positive electrode, separator, and negative electrode were stacked in that order. The stack of the positive electrode, separator, and negative electrode was then impregnated with an electrolyte and sealed between the positive electrode can and the negative electrode can. A gasket was placed between the positive electrode can and the negative electrode can to restrict relative movement and secure the two components in a non-contact state, i.e., to maintain an electrically insulated state.

[0117] The electrolyte is 1M LiPF ６ A 3:7 mixture (volume ratio) of ethylene carbonate (EC) and diethyl carbonate (DEC) with ethylene carbonate (EC) as a supporting electrolyte was used.

[0118] The coin-type battery was fabricated in a dry room controlled at a dew point of -60°C.

[0119] (b) Evaluation of Discharge Capacity and Cycle Characteristics The capacity retention rate, which indicates cycle characteristics, was evaluated by measuring the capacity retention rate after 100 charge / discharge cycles for a coin-type battery.

[0120] Specifically, a current of 30 mA / g was applied to the coin battery, and the battery was charged to a cutoff voltage of 4.5 V and then discharged to a cutoff voltage of 2.5 V. This cycle was repeated 100 times. The capacity retention rate, which is the ratio of the discharge capacity at the 100th cycle after conditioning to the discharge capacity at the first cycle, was calculated and evaluated.

[0121] The discharge capacity at the first cycle is shown in the "Discharge Capacity" column of Table 1, and the capacity retention rate is shown in the "Cycle Characteristics" column of Table 1. (Production Conditions of Positive Electrode Active Material) [Example 1] (1) Lithium Transition Metal Composite Oxide Production Process Nickel hydroxide obtained by a known method and lithium hydroxide monohydrate were mixed to prepare a lithium mixture.

[0122] Nickel hydroxide and lithium hydroxide monohydrate were thoroughly mixed using a shaker mixer (TURBULA Type T2C, manufactured by WAB) to prepare a lithium mixture.

[0123] The raw materials were weighed and mixed so that the ratio of the number of lithium atoms (Li) to the number of nickel atoms (Ni) in the resulting lithium mixture (Li / Ni) would be 1.00.

[0124] This lithium mixture was fired in an oxygen atmosphere (firing step).

[0125] In the firing step, the temperature was raised to a firing temperature of 650° C. and held at the firing temperature of 650° C. for 12 hours. After being held at the firing temperature, the material was cooled to room temperature.

[0126] The XRD pattern of the lithium transition metal composite oxide obtained after firing was measured using a powder X-ray diffractometer (manufactured by Bruker, product name: D2 PHASER), and it was confirmed that it had a layered rock salt structure. (2) Mixing Step In the mixing step, 0.97 g of the lithium transition metal composite oxide having the layered rock salt structure and 0.03 g of lithium phosphate were weighed on an electronic balance in a glove box under an argon atmosphere and thoroughly mixed in an agate mortar to prepare a mixture. (3) Milling Step In the milling step, the mixture obtained in the mixing step was sealed in a 40 mL zirconia container together with 59 g of φ5 mm zirconia balls in the glove box.

[0127] Then, using a Fritsch planetary ball mill (P-7), mechanical milling was performed 96 times in total, with the milling being performed at 600 rpm for 15 minutes and then stopped for 3 minutes.

[0128] Thereafter, in a glove box under an argon atmosphere, the zirconia container was opened, the zirconia balls were removed, and the milled product obtained by mechanical milling was taken out.

[0129] The milled product was analyzed using a powder X-ray diffractometer (D2 Phaser manufactured by Bruker) with CuKα radiation at a tube voltage of 30 V and a tube current of 10 mA, and the XRD pattern was measured in the 2θ range of 10° to 100°. As a result, the half-width of the (200) diffraction peak observed at 44° to 45° was 0.5° or more, and a broad diffraction peak derived from a NiO-like rock salt structure was observed. This confirmed the presence of an amorphous or low-crystalline NiO-like rock salt structure. (4) Heat Treatment Step The milled product obtained in the milling step was heated in an oxygen atmosphere in a calciner to a heat treatment temperature of 650°C at a heating rate of 1°C / min, and then maintained at the heat treatment temperature for 12 hours to obtain a positive electrode active material.

[0130] The obtained positive electrode active material was embedded in a thermosetting resin and then polished using a cross-section polisher (SM-09010, manufactured by JEOL Ltd.). The cross sections of 100 particles were observed using a Schottky field emission scanning electron microscope (Ultra 55, manufactured by Carl Zeiss). As a result, it was confirmed that the positive electrode active material was composed of secondary particles formed by agglomeration of multiple primary particles.

[0131] In the following Examples 2 to 6, the positive electrode active materials were evaluated using the same procedure, and it was confirmed that the obtained positive electrode active materials were composed of secondary particles formed by aggregation of a plurality of primary particles.

[0132] To confirm the crystalline structure of the obtained positive electrode active material, the XRD pattern was measured using a powder X-ray diffractometer. As a result, it was confirmed that the lithium transition metal composite oxide had a layered rock salt structure. Furthermore, since the diffraction peak of lithium phosphate was faintly observed, it was confirmed that the crystallized lithium phosphate was dispersed on the submicron order.

[0133] The positive electrode active materials were evaluated in the same manner in Examples 2 to 6 below, and it was confirmed that the lithium transition metal composite oxide had a layered rock salt structure. Furthermore, in Examples 2 to 6 below, the diffraction peaks of lithium phosphate were also faintly observed, confirming that the crystallized lithium phosphate was dispersed on the submicron order.

[0134] The obtained positive electrode active material was observed using SEM-EDS (trade name: JCM-6000, manufactured by JEOL Ltd.) to obtain an element mapping image. As a result, it was confirmed that phosphorus was dispersed on the submicron order, and from the phosphorus distribution, it was confirmed that lithium phosphate coated at least a portion of the surface of the primary particles of the lithium transition metal composite oxide and was dispersed inside or on the surface of the secondary particles of the lithium transition metal composite oxide.

[0135] The positive electrode active material was also observed by SEM-EDS using the same procedure in the following Examples 2 to 6. As a result, it was confirmed that phosphorus was dispersed on the submicron order, and from the phosphorus distribution, lithium phosphate coated at least a portion of the surface of the primary particles of the lithium transition metal composite oxide and was dispersed inside or on the surface of the secondary particles of the lithium transition metal composite oxide.

[0136] Other evaluation results are shown in Table 1.

[0137] The composition of the obtained positive electrode active material was LiNiO in mass proportion as shown in Table 1. ２ -3% Li ３ P.O. ４ In terms of the ratio of the amount of substance, it is 0.975LiNiO ２ -0.025Li ３ P.O. ４ This becomes:

[0138] Furthermore, the above-described coin-type batteries were fabricated using the obtained positive electrode active materials and evaluated. The evaluation results are shown in Table 1. [Examples 2 and 3] Positive electrode active materials and coin-type batteries were fabricated and evaluated under the same conditions as in Example 1, except that the heat treatment temperature in the heat treatment step was changed.

[0139] The heat treatment temperature was 665°C in Example 2 and 635°C in Example 3.

[0140] The evaluation results are shown in Table 1. [Example 4] In the mixing step, 0.92 g of the lithium transition metal composite oxide prepared in Example 1 and 0.08 g of lithium phosphate were weighed on an electronic balance and thoroughly mixed in an agate mortar to prepare a mixture, which was then subjected to the milling step and the heat treatment step. A positive electrode active material and a coin-type battery were produced and evaluated under the same conditions as in Example 1 except for the above points.

[0141] The composition of the obtained positive electrode active material was LiNiO in mass proportion as shown in Table 1. ２ -9% Li ３ P.O. ４ In terms of the ratio of the amount of substance, it is 0.929LiNiO ２ -0.071Li ３ P.O. ４ This becomes:

[0142] The evaluation results are shown in Table 1. [Example 5] The lithium transition metal composite oxide production process was not carried out, and LiNi was used as a lithium transition metal composite oxide having a layered rock salt structure. ０．８５ Co ０．１０ Al ０．０５ O ２ The mixture was subjected to a mixing step. 0.97 g of the lithium transition metal composite oxide and 0.03 g of lithium phosphate were weighed using an electronic balance and thoroughly mixed in an agate mortar to prepare a mixture, which was then subjected to a milling step and a heat treatment step. A positive electrode active material and a coin-type battery were produced and evaluated under the same conditions as in Example 1, except for the above points.

[0143] The composition of the obtained positive electrode active material was LiNi in mass proportion as shown in Table 1. ０．８５ Co ０．１０ Al ０．０５ O ２ -3% Li ３ P.O. ４ In terms of the ratio of the amount of substance, it is 0.976LiNi ０．８５ Co ０．１０ Al ０．０５ O ２ -0.024Li ３ P.O. ４ Example 6 The lithium transition metal composite oxide production process was not performed, and LiNi was used as a lithium transition metal composite oxide having a layered rock salt structure. ０．８ Mn ０．１ Co ０．１ O ２ The mixture was subjected to a mixing step. 0.97 g of the lithium transition metal composite oxide and 0.03 g of lithium phosphate were weighed using an electronic balance and thoroughly mixed in an agate mortar to prepare a mixture, which was then subjected to a milling step and a heat treatment step. A positive electrode active material and a coin-type battery were produced and evaluated under the same conditions as in Example 1, except for the above points.

[0144] The composition of the obtained positive electrode active material was LiNi in mass proportion as shown in Table 1. ０．８ Mn ０．１ Co ０．１ O ２ -3% Li ３ P.O. ４ In terms of the ratio of the amount of substance, it is 0.975LiNi ０．８ Mn０．１ Co ０．１ O ２ -0.025Li ３ P.O. ４ Comparative Example 1 In Comparative Example 1, lithium phosphate was not mixed in the mixing step, and the milling step was not performed. That is, the lithium transition metal composite oxide obtained in the lithium transition metal composite oxide production step was directly subjected to the heat treatment step. A positive electrode active material and a coin-type battery were produced and evaluated under the same conditions as in Example 1, except for the above points. The evaluation results are shown in Table 1. Comparative Example 2 A positive electrode active material and a coin-type battery were produced and evaluated under the same conditions as in Example 1, except for the fact that the heat treatment temperature in the heat treatment step was changed to 610°C.

[0145] The evaluation results are shown in Table 1.

[0146] (Evaluation Results) As shown in Table 1, in Examples 1 to 6, positive electrode active materials were obtained which contained secondary particles formed by agglomeration of a plurality of primary particles, a lithium transition metal composite oxide having a layered rock salt structure, and crystallized lithium phosphate, and in which the proportion of elements other than lithium in the lithium sites (3b sites) of the lithium transition metal composite oxide was 3% or more and 8% or less.

[0147] It was also confirmed that the positive electrode active materials obtained in Examples 1 to 6 had a higher capacity retention rate when applied to a lithium ion secondary battery, compared to the positive electrode active materials obtained in Comparative Examples 1 and 2, which did not satisfy the above requirements. That is, it was confirmed that positive electrode active materials excellent in cycle characteristics were obtained in Examples 1 to 6.

[0148] Furthermore, it was confirmed that the positive electrode active materials obtained in Examples 1 to 6 also had higher discharge capacities when applied to lithium ion secondary batteries compared to the positive electrode active material obtained in Comparative Example 2.

[0149] This application claims priority based on Japanese Patent Application No. 2023-123254, filed with the Japan Patent Office on July 28, 2023, the entire contents of which are incorporated herein by reference.

[0150] S1 Mixing process S2 Mixing process S3 Heat treatment process

Claims

1. A positive electrode active material for a lithium ion secondary battery comprising secondary particles in which a plurality of primary particles are aggregated together, the positive electrode active material for a lithium ion secondary battery comprising a lithium transition metal composite oxide having a layered rock salt structure and crystallized lithium phosphate, wherein in a crystal structure analysis of the lithium transition metal composite oxide having the layered rock salt structure by neutron diffraction, the proportion of elements other than lithium in the lithium sites (3b sites) is 3% or more and 8% or less.

2. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the average particle size of the primary particles is 20 nm or more and 250 nm or less.

3. A positive electrode active material for a lithium ion secondary battery according to claim 1 or 2, wherein in a crystal structure analysis by neutron diffraction of the lithium transition metal composite oxide having the layered rock salt structure, the proportion of lithium elements in the metal sites (3a sites) is 3% or more and 8% or less.

4. The positive electrode active material for a lithium ion secondary battery is represented by the general formula: kLi ３ P.O. ４ -(1-k)Li ｓ Ni １－ｘ－ｙ－ｚ Co ｘ Mn ｙ M ｚ O ２＋α 3. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein k is at least one element selected from the group consisting of V, Mg, Mo, Nb, Ti, W, and Al, and 0≦x≦0.35, 0≦y≦0.35, 0≦z≦0.10, 1.00≦s<1.30, and 0≦α≦0.

2.

5. A positive electrode active material for a lithium ion secondary battery according to claim 1 or 2, wherein the crystallized lithium phosphate covers at least a portion of the surface of a primary particle of the lithium transition metal composite oxide having a layered rock salt structure, and is dispersed inside or on the surface of a secondary particle of the lithium transition metal composite oxide having a layered rock salt structure.

6. A method for producing a positive electrode active material for a lithium ion secondary battery comprising secondary particles in which a plurality of primary particles are aggregated together, the method comprising: a mixing step of mixing a lithium transition metal composite oxide having a layered rock salt structure with lithium phosphate to prepare a mixture; a milling step of applying mechanical stress to the mixture obtained in the mixing step to form a milled product having an amorphous or low-crystalline NiO-like rock salt structure; and a heat treatment step of heating the milled product obtained in the milling step at a rate of 2°C / min or less and heat treating it at a temperature of 620°C to 700°C.

7. A lithium ion secondary battery comprising at least a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode contains the positive electrode active material for lithium ion secondary batteries according to claim 1 or 2.

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