Method for manufacturing positive electrode active material for secondary battery

Abstract

This method for manufacturing a positive electrode active material for a secondary battery involves a mixing step for mixing an Li compound, an Ni compound, and a first alkaline earth metal compound to obtain a powder mixture, a firing step for firing the powder mixture to obtain an LiNi composite oxide, a water washing step for mixing the LiNi composite oxide with water to obtain slurry, and stirring the slurry to wash the LiNi composite oxide with water, a separation step for separating a water-containing cake of the LiNi composite oxide from the slurry, and a drying step for drying the water-containing cake to obtain powder of the LiNi composite oxide, wherein the method further involves an addition step for adding a second alkaline earth metal compound to the LiNi composite oxide after the water washing step, and each of the first and second alkaline earth metal compounds includes at least one selected from the group consisting of Ca compounds and Sr compounds.

Description

Method for producing positive electrode active material for secondary battery CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This disclosure claims the benefit of priority to Japanese Patent Application No. 2024-013465, filed on January 31, 2024 in the Japan Patent Office, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure relates to a method for producing a positive electrode active material for a secondary battery.

[0003] Patent Document 1 describes a compound having the general formula Li ａ Ni １－ｘ－ｙ－ｚ Co ｘ Mn ｙ Al ｚ M ｂ O ２ They propose a positive electrode material characterized by the formula "wherein 1.04≦a≦1.08, 0.04≦x≦0.08, 0.025≦y≦0.06, 0.03≦z≦0.09, 0.015≦b≦0.06, and M is Zr, at least one selected from Al, Ti, Mg, Na, Ca, Nb, Ba, Si, P, W, and Sr, and B."

[0004] Special Publication No. 2023-502088

[0005] In order to improve the performance of secondary batteries, efforts have been made to increase the capacity of positive electrode active materials. However, as the capacity of positive electrode active materials increases, their durability tends to decrease.

[0006] One aspect of the present disclosure relates to a method for producing a positive electrode active material for a secondary battery, the method comprising: (i) a mixing step of mixing a Li compound, a Ni compound, and a first alkaline earth metal compound to obtain a mixed powder; (ii) a firing step of firing the mixed powder to obtain a LiNi composite oxide; (iii) a water-washing step of mixing the LiNi composite oxide with water to obtain a slurry and stirring the slurry to wash the LiNi composite oxide with water; (iv) a separation step of separating a water-containing cake of the LiNi composite oxide from the slurry; and (v) a drying step of drying the water-containing cake to obtain the LiNi composite oxide powder, the method further comprising, after the water-washing step, a step of applying a second alkaline earth metal compound to the LiNi composite oxide, wherein the first alkaline earth metal compound and the second alkaline earth metal compound each comprise at least one compound selected from the group consisting of a Ca compound and a Sr compound.

[0007] According to the present disclosure, a highly durable positive electrode active material can be obtained. The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0008] 1 is a longitudinal cross-sectional view schematically illustrating an internal structure of a secondary battery according to an embodiment of the present disclosure.

[0009] The following describes embodiments of the present disclosure using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the expression "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less." In the following description, when lower and upper limits of numerical values ​​related to specific physical properties or conditions are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit.

[0010] The present disclosure also encompasses combinations of two or more features arbitrarily selected from the appended claims, i.e., two or more features arbitrarily selected from the appended claims can be combined unless a technical contradiction arises.

[0011] Secondary batteries include lithium ion secondary batteries that use a material that reversibly absorbs and releases at least lithium ions as a negative electrode active material, lithium secondary batteries in which lithium metal precipitates at the negative electrode during charging and dissolves during discharging, electrolyte secondary batteries that contain a liquid electrolyte (electrolytic solution), solid batteries that contain a gel electrolyte, and all-solid-state batteries that contain a solid electrolyte.

[0012] To improve the performance of secondary batteries, studies have been conducted to increase the capacity of the positive electrode active material, for example, by increasing the Ni content of the positive electrode active material. However, the durability of the positive electrode active material tends to decrease as its capacity increases. Specifically, the increase in capacity is equivalent to an increase in the amount of Li ions that are repeatedly inserted and removed from the crystalline structure of the positive electrode active material, which increases the change in the crystalline structure during charging and discharging. Particularly at full charge, the amount of Li ions that are removed from the crystalline structure increases as the capacity increases, making the crystalline structure very unstable. This makes the crystalline structure prone to collapse and capacity degradation due to side reactions, such as reactions with the electrolyte.

[0013] Therefore, it has been considered to provide a predetermined protective component for protecting the positive electrode active material on the surface layer of the primary particles or secondary particles of the positive electrode active material.

[0014] In the present disclosure, durability of a positive electrode active material is improved by providing a compound of an alkaline earth metal M to the surface layer portion of primary particles or secondary particles of the positive electrode active material. The compound of the alkaline earth metal M includes at least one compound selected from the group consisting of a Ca compound and a Sr compound.

[0015] However, if the protective component cannot be appropriately applied to the surface layer portion of the primary particles or secondary particles of the positive electrode active material, deterioration of the positive electrode active material may progress locally or the protective component may dissolve into the electrolyte, resulting in a decrease in storage characteristics.

[0016] In view of the above, one object of the method for producing a positive electrode active material for a secondary battery according to an embodiment of the present disclosure (hereinafter also referred to as "production method M") is to maximize the effect of the protective component by suitably protecting the surface layer portions of the primary particles or secondary particles of the positive electrode active material with the protective component.

[0017] Specifically, the manufacturing method M includes (i) a mixing step, (ii) a calcination step, (iii) a water-washing step, (iv) a separation step, and (v) a drying step, and further includes an application step performed after the water-washing step. In the mixing step, a Li compound, a Ni compound, and a first alkaline earth metal compound are mixed. In the application step, a second alkaline earth metal compound is applied to the LiNi composite oxide obtained in the calcination step.

[0018] The applying step may be a step independent of the water-washing step and the separation step, or may be a step proceeding simultaneously with at least a part of the water-washing step or at least a part of the separation step, or may be a step inseparably included in the water-washing step or the separation step.

[0019] The first alkaline earth metal compound may contain a Ca compound but not a Sr compound, may contain a Sr compound but not a Ca compound, or may contain both a Ca compound and a Sr compound.

[0020] The second alkaline earth metal compound may contain a Ca compound but not a Sr compound, may contain a Sr compound but not a Ca compound, or may contain both a Ca compound and a Sr compound.

[0021] However, it is preferable that one of the first alkaline earth metal compound and the second alkaline earth metal compound contains at least a Ca compound, and the other contains at least a Sr compound. In other words, it is preferable that both a Ca compound and a Sr compound are used in manufacturing method M.

[0022] Each step is described in detail below. (i) Mixing Step The mixing step is a step of mixing a Li compound, a Ni compound, and a first alkaline earth metal compound to obtain a mixed powder. The Li compound, the Ni compound, and the first alkaline earth metal compound may each be a powder. The step of obtaining a mixed powder may be a step of dry-mixing at least three types of powder, i.e., the Li compound, the Ni compound, and the first alkaline earth metal compound, using a powder mixer. The powder mixer may be equipped with a stirring mechanism that rotates a screw blade and a chopper. With such a stirring mechanism, even highly cohesive powders can be disintegrated by shear stress and mixed uniformly. A specific example is an axial mixer manufactured by Sugiyama Heavy Industries, Ltd. A preferred example of mixing conditions is mixing for 3 to 30 minutes under conditions of a main screw blade of 200 rpm and a chopper of 1000 rpm.

[0023] From the viewpoint of improving the mixing efficiency, a mixture of a liquid medium (e.g., water, alcohol, etc.), a Li compound, a Ni compound, and a first alkaline earth metal compound may be stirred while volatilizing at least a portion of the liquid medium to obtain a mixed powder.

[0024] (Li Compound) Examples of Li compounds include Li2CO3, LiOH, Li2O2, Li2O, LiNO3, LiNO2, Li2SO4, LiOH.H2O, LiH, and LiF.

[0025] In the mixed powder, the amount of Li compound is preferably in excess relative to the Ni compound. For example, when synthesizing a LiNi composite oxide having a layered rock salt structure (e.g., space group R-3m) as a positive electrode active material, the ratio of the number of moles of Li atoms contained in the Li compound to the number of moles of all metal atoms contained in the Ni compound (hereinafter also referred to as the "Li / NiM1 ratio") is preferably 1.0 or more, and more preferably 1.01 or more. By using an excess of Li compound in this way, the reactivity of the synthesis reaction is increased, and the formation of the crystalline structure of the LiNi composite oxide is likely to be promoted. From the viewpoint of limiting material loss, the Li / NiM1 ratio may be 1.1 or less, or may be 1.07 or less.

[0026] Among Li compounds, LiOH is particularly preferred. The median diameter (D50) in the volume-based particle size distribution of the LiOH powder may be, for example, 0.1 μm or more and 500 μm or less. The particle size distribution may be controlled by appropriate pulverization or classification. LiOH has high reactivity with Ni compounds and is advantageous for improving the crystallinity of the LiNi composite oxide. When using LiOH, it is preferable to heat-dry the LiOH before use. In this case, the LiOH before drying may be a hydrate. Using sufficiently dried LiOH further improves reactivity during firing. The temperature for heating and drying LiOH is preferably 100°C or more and the melting point or less. The heating time for LiOH is, for example, 1 hour or more and 10 hours or less. Heat-drying may be performed in the air or in a non-oxidizing atmosphere containing nitrogen, argon, or the like.

[0027] Here, the median diameter (D50) is the diameter at which the volume cumulative value is 50% in the volume-based particle size distribution. The volume-based particle size distribution of a powder can be determined, for example, by a particle size distribution measuring device employing a laser diffraction / scattering method.

[0028] (Ni Compound) The Ni compound may be nickel hydroxide or a Ni oxide obtained by heat-treating nickel hydroxide. Both of these are advantageous raw materials for synthesizing a high-capacity LiNi composite oxide, and are less expensive than Co, etc. However, it is preferable that the Ni compound contains a metal element M1 other than Ni. The metal element M1 has the effect of stabilizing the crystal structure of the LiNi composite oxide, for example. Co is particularly advantageous for extending the life of the battery. Furthermore, Al is advantageous for improving the thermal stability of the LiNi composite oxide.

[0029] From the viewpoint of increasing the capacity of the LiNi composite oxide, the ratio of the number of moles of Ni atoms contained in the Ni compound to the number of moles of all metal atoms contained in the Ni compound (hereinafter also referred to as "Ni / NiM1 ratio") is preferably 50 mol% or more, and may be 70 mol% or more, 80 mol% or more, or 90 mol% or more. When it is less than 80 mol%, the voltage of the battery used can be increased compared to when it is 80 mol% or more to achieve high capacity. For example, when the Ni / NiM1 ratio is less than 80 mol%, the battery voltage can be increased by 0.1 V to 0.3 V compared to when the Ni / NiM1 ratio is 80 mol% or more. On the other hand, from the viewpoint of manifesting the effect of the metal element M1, the Ni / NiM1 ratio is preferably 99 mol% or less, more preferably 98 mol% or less.

[0030] As the metal element M1 that can be contained in the Ni compound, for example, at least one element M2 selected from the group consisting of Co, Mn, Al, W, Nb, Zr, Zn, Ti, Mg, V, Si, Mo, and Cr is preferred. Among the elements M2, at least one element M3 selected from the group consisting of Co, Mn, and Al is particularly preferred.

[0031] When nickel hydroxide is used as the Ni compound, the metal element M1 may be contained in the nickel hydroxide. Specifically, when nickel hydroxide, which is a raw material for the Ni compound, is produced in an alkaline aqueous solution, a hydroxide of the metal element M1 may be produced together with the nickel hydroxide, and the nickel hydroxide and the hydroxide of the metal element M1 may be coprecipitated. For example, an alkali may be added to an aqueous solution containing a nickel salt and a salt of the metal element M1 to coprecipitate a composite hydroxide. Nickel sulfate or the like may be used as the nickel salt. A sulfate or the like may be used as the salt of the metal element M1. Sodium hydroxide or the like may be used as the alkali.

[0032] The obtained hydroxide is then heat-treated to obtain a Ni oxide containing the metal element M1. In the heat treatment of the hydroxide, for example, the hydroxide may be heated to 300°C or higher and 800°C or lower. When the hydroxide is converted into the oxide by a dehydration reaction, some of the Ni sites in the crystal lattice of the nickel oxide are substituted with the metal element M1, and a solid solution in which the metal element M1 is incorporated into the Ni oxide may be formed. The heating time of the hydroxide is, for example, 30 minutes or longer and 10 hours or shorter. The heat treatment atmosphere may be a non-oxidizing atmosphere containing nitrogen or the like, or an oxidizing atmosphere containing oxygen. The oxidizing atmosphere may be air, or an atmosphere with an oxygen concentration of 20% or higher.

[0033] According to the above method, the metal element M1 can be uniformly dispersed in the Ni compound.

[0034] Alternatively, nickel hydroxide and the hydroxide of the metal element M1 may be prepared separately, and the mixture obtained by mixing these may be heat-treated to obtain a Ni oxide containing the metal element M1, which may then be used as the Ni compound.

[0035] The median diameter (D50) in the volumetric particle size distribution of the Ni compound powder may be, for example, 2.0 μm or more and 20.0 μm or less. In order to control the particle size distribution to such a value, the Ni compound powder may be subjected to pulverization or classification as appropriate. Such Ni compounds have high reactivity with Li compounds and are advantageous for improving the crystallinity of the LiNi composite oxide.

[0036] (First alkaline earth metal compound) The first alkaline earth metal compound includes at least one of a Ca compound and a Sr compound. Ca and Sr are difficult to incorporate into the crystal structure of the LiNi composite oxide, and are unevenly distributed in the surface layer of the primary or secondary particles of the LiNi composite oxide, acting as protective components that suppress deterioration of the LiNi composite oxide. Ca and Sr suppress deterioration of the LiNi composite oxide, thereby improving the durability of the secondary battery. Ca and Sr also have excellent affinity with other components of the LiNi composite oxide, and are thought to easily adhere to the surface of the primary particles.

[0037] The first alkaline earth metal compound may contain only one of a Ca compound and a Sr compound, in which case the protective component containing Ca or Sr can be more uniformly applied to the surface layer portion of the primary particles or secondary particles of the LiNi composite oxide.

[0038] Ca compounds include CaCO3 and Ca(OH) ２ , CaO, Ca(NO ３ ) ２ , CaSO ４ , CaH ２ , CaF ２ Among these, CaO, CaCO3, etc. are preferred.

[0039] Sr compounds include SrCO3 and Sr(OH) ２ , SrO, Sr(NO ３ ) ２ , SrSO ４ , SrH ２ Among these, SrO, SrCO3, etc. are preferred.

[0040] From the viewpoint of enhancing the effects of Ca and Sr, the ratio of the total number of moles of Ca atoms and Sr atoms contained in the alkaline earth metal compound to the number of moles of all metal atoms contained in the first alkaline earth metal compound is preferably 80 mol% or more, or may be 85 mol% or more, 90 mol% or more, 95 mol% or more, or may be 100 mol%.

[0041] The amount of the first alkaline earth metal compound may be any amount sufficient to cover the surface layer of the Ni compound to some extent. For example, when a LiNiM composite oxide (e.g., space group R-3m) having a layered rock salt structure is synthesized as the positive electrode active material, the ratio of the total number of moles of Ca atoms and / or Sr atoms contained in the first alkaline earth metal compound to the number of moles of all metal atoms contained in the Ni compound may be 0.1 mol% or more and 4 mol% or less, or 0.2 mol% or more and 1.0 mol% or less. This allows a large proportion of the surface layer portion of the primary particles or secondary particles of the LiNi composite oxide to be protected by a protective component containing Ca and / or Sr.

[0042] The median diameter (D50) in the volumetric particle size distribution of the first alkaline earth metal compound may be, for example, 0.5 μm or more and 50 μm or less. Such a first alkaline earth metal compound has high reactivity with the Ni compound in the subsequent firing step. The median diameter (D50) in the volumetric particle size distribution of each of the Ca compound and the Sr compound may be, for example, 0.5 μm or more and 50 μm or less. In order to control the particle size distribution to such a value, the first alkaline earth metal compound may be crushed or classified as appropriate.

[0043] (ii) Calcination Step The calcination step is a step of calcining the mixed powder (powder mixture of the Li compound, Ni compound, and first alkaline earth metal compound) to obtain a LiNi composite oxide.

[0044] The firing step may include a first firing and a second firing. That is, the firing of the first mixed powder may include at least a two-stage firing process. The firing process may include three or more stages, but a two-stage firing process is preferred from the viewpoint of improving productivity.

[0045] (First Firing) The first firing is, for example, a process in which the mixed powder is held at a temperature of 400°C or higher and 650°C or lower for 30 minutes or longer. At a temperature of 400°C or higher and 650°C or lower, the Li compound and the first alkaline earth metal compound melt and penetrate into the secondary particles of the Ni compound. A portion of the first alkaline earth metal compound may form a LiNi composite oxide. For example, when the Li compound contains LiOH, the LiOH melts and easily penetrates into the secondary particles of the Ni compound, facilitating the reaction. As a result, in the subsequent second firing, the reaction between the primary particles of the Ni compound and LiOH is more likely to be promoted.

[0046] The temperature of the first firing is preferably a temperature at which the reaction between the Li compound and the Ni compound proceeds as little as possible. By maintaining the Ni oxide and the Li compound at such a temperature for a fixed time of 30 minutes or more, the Li compound and the Ni compound become sufficiently compatible with each other, and the reaction in the subsequent second firing tends to proceed uniformly.

[0047] (Second Firing) The second firing is a process in which the mixed powder (first fired product) after the first firing is held at a temperature of 700°C or higher for 1 hour or more. That is, the second firing is performed at a temperature at which the reaction between the Li compound and the Ni compound proceeds to produce a LiNi composite oxide. The second firing causes crystals of the LiNi composite oxide to grow in the presence of the Li compound. The second firing temperature may be 730°C or higher, or 750°C or higher. However, if the second firing temperature is too high, the crystal structure of the LiNi composite oxide may be deteriorated. The second firing temperature may be 1000°C or lower, or 900°C or lower. The atmosphere for the first firing and the second firing may be, for example, an oxidizing atmosphere.

[0048] (iii) Water-washing Step The water-washing step is a step in which the LiNi composite oxide is mixed with water to obtain a slurry, and the slurry is stirred to wash the LiNi composite oxide with water.

[0049] As mentioned above, it is preferable that the amount of Li compound is in excess of the amount of Ni compound in the mixing step. Therefore, it is necessary to wash the mixture with water appropriately to remove unreacted Li to a certain extent. Residual Li causes gas generation and side reactions in the secondary battery, resulting in a decrease in capacity.

[0050] Residual Li refers to Li element that is not incorporated into the crystal structure of the LiNi composite oxide and remains as a Li compound other than the LiNi composite oxide. The residual Li is present in the LiNi composite oxide in the form of, for example, lithium hydroxide, lithium carbonate, or lithium oxide.

[0051] The water washing step is preferably carried out in a batch manner from the viewpoint of reducing the amount of water required. The batchwise water washing is carried out, for example, by charging a predetermined amount of LiNi composite oxide and a predetermined amount of water into a predetermined washing tank, dispersing the LiNi composite oxide in water, and stirring the resulting slurry in the washing tank.

[0052] The water-washing step may include a step of applying a second alkaline earth metal compound to the LiNi composite oxide. In this case, the application step may be performed by dissolving the second alkaline earth metal compound in the slurry used in the water-washing step. For example, an aqueous solution containing the second alkaline earth metal compound dissolved in water may be prepared, the aqueous solution may be mixed with the LiNi composite oxide to prepare a slurry, and the LiNi composite oxide may be stirred to wash the LiNi composite oxide with water. Alternatively, a slurry may be prepared by mixing water with the LiNi composite oxide, and a previously prepared aqueous solution containing the second alkaline earth metal compound dissolved in water may be appropriately added to the slurry during the stirring of the slurry to wash the LiNi composite oxide with water.

[0053] The concentration of the second alkaline earth metal compound in the slurry may be controlled so that the ratio of the total number of moles of Ca atoms and / or Sr atoms contained in the second alkaline earth metal compound to the number of moles of metal atoms other than Li contained in the LiNi composite oxide after the subsequent drying step is 0.1 mol % to 4 mol %, or 0.2 mol % to 1 mol %, thereby making it possible to effectively protect areas of the surface layer portions of the primary particles or secondary particles of the LiNi composite oxide that are insufficiently protected by the protective component containing Ca and / or Sr.

[0054] (iv) Separation Step The separation step is a step of separating a hydrous cake of LiNi composite oxide from the slurry. The hydrous cake of LiNi composite oxide refers to a LiNi composite oxide from which most of the water has been removed but which still contains sufficient moisture.

[0055] The separation step may be carried out by any method, and industrially advantageous methods include those using a vacuum dehydrator, a centrifugal concentration dehydrator, a multi-disk dehydrator, a filter press, a belt press, a screw press, a rotary drum screen, a belt screen, a vibrating screen, a multi-disk wave filter, etc.

[0056] The imparting step may be a step of mixing the second alkaline earth metal compound with the water-containing cake obtained in the separation step to obtain a water-containing mixture. Alternatively, the second alkaline earth metal compound may be imparted to the LiNi composite oxide in the water-washing step by the method described above, and the second alkaline earth metal compound may be mixed with the water-containing cake.

[0057] The second alkaline earth metal compound to be mixed with the hydrous cake may be in the form of a powder. Alternatively, an aqueous solution in which the second alkaline earth metal compound is dissolved in water may be mixed with the hydrous cake of the LiNi composite oxide. In this case, the aqueous solution in which the second alkaline earth metal compound is dissolved may be sprayed onto the hydrous cake.

[0058] The step of obtaining the hydrous mixture may be a step of mixing a hydrous cake of the LiNi composite oxide with an alkaline earth metal compound (or an aqueous solution thereof) using a kneader, a mixer, etc. One preferred example of mixing conditions is mixing using an axial mixer manufactured by Sugiyama Heavy Industries, Ltd., with a main screw blade speed of 200 rpm and a chopper speed of 1000 rpm, for a period of 3 minutes to 30 minutes.

[0059] (Second alkaline earth metal compound) The second alkaline earth metal compound contains at least one of a Ca compound and a Sr compound. However, if the first alkaline earth metal compound does not contain a Ca compound, the second alkaline earth metal compound preferably contains a Ca compound. If the first alkaline earth metal compound does not contain a Sr compound, the second alkaline earth metal compound preferably contains a Sr compound. The second alkaline earth metal compound may contain both a Ca compound and a Sr compound.

[0060] The Ca compound and Sr compound may be selected arbitrarily from the compounds exemplified as the first alkaline earth metal compound, for example.

[0061] From the viewpoint of enhancing the effect of Ca or Sr, the ratio of the total number of moles of Ca atoms and Sr atoms contained in the second alkaline earth metal compound to the number of moles of all metal atoms contained in the second alkaline earth metal compound is preferably 80 mol% or more, or may be 85 mol% or more, 90 mol% or more, or may be 95 mol% or more.

[0062] When the second alkaline earth metal compound is applied to the water-containing cake in a powder state, the median diameter (D50) in the volumetric particle size distribution of the second alkaline earth metal compound may be, for example, 0.5 μm or more and less than 50 μm. Such a second alkaline earth metal compound has high reactivity with the surface layer portion of the LiNi composite oxide in the subsequent drying step. The Ca compound and the Sr compound used as the second alkaline earth metal compound may each have a median diameter (D50) in the volumetric particle size distribution of, for example, 0.5 μm or more and less than 50 μm. In order to control the particle size distribution to such an extent, the second alkaline earth metal compound may be crushed or classified as appropriate.

[0063] It is preferable to limit the amount of the second alkaline earth metal compound used so that the ratio of the total number of moles of Ca atoms and Sr atoms to the number of moles of all metal atoms other than Li contained in the LiNi composite oxide after the subsequent drying step is 0.1 mol % or more and 4 mol % or less, which makes it possible to maximize the effect of the protective component.

[0064] (v) Drying Step The drying step is a step of drying the water-containing cake or the water-containing mixture to obtain a powder of the LiNi composite oxide to which Ca and Sr have been added. The drying conditions are not particularly limited.

[0065] In the drying step, the LiNi composite oxide may be held at a temperature of, for example, 100°C or higher and 250°C or lower for 30 minutes or longer. The LiNi composite oxide after the drying step is in the form of powder. In other words, in the drying step, it is sufficient to dry the LiNi composite oxide until it assumes a powder state with flowability.

[0066] By the above process, secondary particles formed by aggregation of primary particles of the LiNi composite oxide can be obtained. The median diameter (D50) of the secondary particles of the LiNi composite oxide in the volume-based particle size distribution is, for example, 2 μm or more and 20 μm or less.

[0067] (vi) Heat Treatment Step The dried LiNi composite oxide powder may be further heat treated. The heat treatment is an optional process. For example, the LiNi composite oxide powder may be held at a temperature of 350°C or higher and 550°C or lower. This enhances the protective effect of the LiNi composite oxide, further improving its storage characteristics.

[0068] An example of a secondary battery using a positive electrode active material made of a LiNi composite oxide obtained by the above-described manufacturing method will now be described.

[0069] The secondary battery includes a positive electrode containing a positive electrode active material made of at least a LiNi composite oxide, a negative electrode, an electrolyte, and a separator interposed between the positive electrode and the negative electrode.

[0070] The positive electrode includes, for example, a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector. The positive electrode mixture layer may be formed on one surface or both surfaces of the positive electrode current collector. The positive electrode mixture contains a positive electrode active material as an essential component and may contain a binder, a conductive agent, etc. as optional components.

[0071] Examples of the material for the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0072] Examples of the binder include polytetrafluoroethylene and polyvinylidene fluoride.

[0073] Examples of the conductive agent include graphite such as natural graphite and artificial graphite, and carbon blacks such as acetylene black.

[0074] The negative electrode includes, for example, a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector. The negative electrode mixture layer may be formed on one surface or both surfaces of the negative electrode current collector. The negative electrode mixture contains a negative electrode active material as an essential component and may contain a binder, a thickener, etc. as optional components.

[0075] Examples of the material for the negative electrode current collector include copper, copper alloy, nickel alloy, nickel alloy, and stainless steel.

[0076] As the binder, in addition to the materials exemplified for the positive electrode, styrene butadiene rubber or the like can be used.

[0077] As the thickener, for example, carboxymethyl cellulose (CMC) and its modified products (including salts such as Na salt) can be used.

[0078] Examples of the negative electrode active material include carbon materials, silicon, silicon compounds, tin, tin compounds, aluminum, zinc, etc. Examples of the carbon material include graphite (natural graphite, artificial graphite, etc.), amorphous carbon, etc.

[0079] The electrolyte may be a liquid electrolyte in which a solute such as a lithium salt is dissolved in a solvent. The solvent may be a non-aqueous solvent or water. Alternatively, the electrolyte may be a solid electrolyte.

[0080] The electrolyte contains, for example, a non-aqueous solvent and a lithium salt that dissolves in the non-aqueous solvent. Examples of the non-aqueous solvent include cyclic carbonate esters, chain carbonate esters, cyclic carboxylic acid esters, and chain carboxylic acid esters. The lithium salt includes LiPF ６ etc. are used.

[0081] The separator has high ion permeability and adequate mechanical strength and insulation. The separator can be made of a microporous thin film, woven fabric, nonwoven fabric, or the like. Polyolefins such as polypropylene and polyethylene are preferred as the separator material.

[0082] An example of the structure of a secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and an electrolyte are housed in an outer casing. Alternatively, instead of a wound electrode group, other types of electrode groups may be used, such as a stacked electrode group formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween. The secondary battery may be in any shape, such as a cylindrical shape, a prismatic shape, a coin shape, a button shape, or a laminate shape.

[0083] The structure of the secondary battery will be described below with reference to Fig. 1. Fig. 1 is a longitudinal cross-sectional view of a cylindrical nonaqueous secondary battery 10 that is an example of this embodiment. However, the present disclosure is not limited to the following configuration.

[0084] The secondary battery 10 includes an electrode group 18, an electrolyte (not shown), and a cylindrical battery can 22 with a bottom that accommodates these. A sealing body 11 is crimped to the opening of the battery can 22 via a gasket 21, thereby sealing the battery. The sealing body 11 includes a valve body 12, a metal plate 13, and an annular insulating member 14 interposed between the valve body 12 and the metal plate 13. The valve body 12 and the metal plate 13 are connected to each other at their respective centers. A positive electrode lead 15a extending from a positive electrode plate 15 is connected to the metal plate 13. Thus, the valve body 12 functions as an external terminal for the positive electrode. A negative electrode lead 16a extending from a negative electrode plate 16 is connected to the inner bottom surface of the battery can 22. An annular groove 22a is formed near the open end of the battery can 22. A first insulating plate 23 is disposed between one end face of the electrode group 18 and the annular groove portion 22a. A second insulating plate 24 is disposed between the other end face of the electrode group 18 and the bottom of the battery can 22. The electrode group 18 is formed by winding a positive electrode plate 15 and a negative electrode plate 16 with a separator 17 interposed therebetween.

[0085] (Additional Notes) The above description discloses the following technology: (Technology 1) A method for producing a positive electrode active material for a secondary battery, comprising: a mixing step of mixing a Li compound, a Ni compound, and a first alkaline earth metal compound to obtain a mixed powder, a firing step of firing the mixed powder to obtain a LiNi composite oxide, a water-washing step of mixing the LiNi composite oxide with water to obtain a slurry and stirring the slurry to wash the LiNi composite oxide with water, a separation step of separating a water-containing cake of the LiNi composite oxide from the slurry, and a drying step of drying the water-containing cake to obtain the LiNi composite oxide powder, further comprising, after the water-washing step, an application step of applying a second alkaline earth metal compound to the LiNi composite oxide, wherein each of the first alkaline earth metal compound and the second alkaline earth metal compound comprises at least one compound selected from the group consisting of a Ca compound and a Sr compound. (Technology 2) The method for producing a positive electrode active material for a secondary battery according to Technology 1, wherein one of the first alkaline earth metal compound and the second alkaline earth metal compound contains at least a Ca compound, and the other contains at least a Sr compound. (Technology 3) The method for producing a positive electrode active material for a secondary battery according to Technology 1 or 2, wherein the applying step includes adding the second alkaline earth metal compound to the slurry in the water washing step. (Technology 4) The method for producing a positive electrode active material for a secondary battery according to any one of Technology 1 to 3, wherein the median diameter (D50) in a volume-based particle size distribution of the first alkaline earth metal compound is 0.5 μm or more and 50 μm or less. (Technology 5) The method for producing a positive electrode active material for a secondary battery according to any one of Technology 1 to 4, wherein the applying step includes mixing the second alkaline earth metal compound with the water-containing cake obtained in the separating step. (Technology 6) The method for producing a positive electrode active material for a secondary battery according to Technology 5, wherein the second alkaline earth metal compound mixed with the water-containing cake has a median diameter (D50) in a volume-based particle size distribution of 0.5 μm or more and 50 μm or less.(Technology 7) The method for producing a positive electrode active material for a secondary battery according to any one of Techniques 1 to 6, wherein a ratio of the total number of moles of Ca atoms and Sr atoms to the number of moles of all metal atoms other than Li contained in the LiNi composite oxide after the drying step is 0.1 mol % or more and 4 mol % or less. (Technology 8) The method for producing a positive electrode active material for a secondary battery according to any one of Techniques 1 to 7, wherein a ratio of the number of moles of Ni atoms contained in the Ni compound to the number of moles of all metal atoms contained in the Ni compound is 50 mol % or more. (Technology 9) The method for producing a positive electrode active material for a secondary battery according to any one of Techniques 1 to 8, wherein the amount of the Li compound in the mixed powder is in excess of the amount of the Ni compound. (Technology 10) The method for producing a positive electrode active material for a secondary battery according to any one of Techniques 1 to 9, wherein the Ni compound contains a metal element M1 other than Ni, and the metal element M1 contains at least one selected from the group consisting of Co, Mn, Al, W, Nb, Zr, Zn, Ti, Mg, V, Si, Mo, and Cr.

[0086] Hereinafter, the present disclosure will be specifically described based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0087] Example 1 (1) Mixing Step A Li compound, LiOH, a Ni oxide containing a metal element M1 (Co, Mn), and a Ca compound (Ca(OH) ２ ) were dry mixed to obtain a mixed powder.

[0088] LiOH was prepared by heating and drying lithium hydroxide monohydrate (median diameter (D50) 1.0 μm) at 150° C. for 1 hour.

[0089] The Ni oxide containing the metal element M1 was prepared by heating Ni hydroxide containing the metal element M1 produced by the coprecipitation method in air at 700°C for 2 hours. ０．９０ Co ０．０５ Mn ０．０５ 0 (median diameter (D50) 10 μm).

[0090] Ca compound (Ca(OH) ２ The median diameter (D50) of the particles is 10 μm.

[0091] In the mixed powder, the ratio of the number of moles of Ca atoms to the number of moles of all metal atoms contained in the Ni oxide was 0.001 (0.1 mol %).

[0092] In the mixed powder, the ratio of the number of moles of Li atoms contained in LiOH to the number of moles of all metal atoms contained in Ni oxide (Li / NiM1 ratio) was 1.03.

[0093] (2) Firing Step (First Firing) The mixed powder filled in the sagger was fired at 600° C. for 3 hours in an oxidizing atmosphere to form a compact, thereby obtaining a first fired body.

[0094] (Second Firing) Subsequently, the first fired body was fired in an oxidizing atmosphere at 750° C. for 5 hours to obtain a second fired body (LiNi composite oxide).

[0095] (3) Water washing step: The second fired product (LiNi composite oxide) was mixed with water in a washing tank to obtain a slurry, and the slurry was stirred to wash the LiNiM composite oxide. The water used for the slurry contained a Sr compound (Sr(OH) ２ ) was dissolved.

[0096] Sr compound (Sr(OH) ２ The amount of Sr) is an amount that makes the ratio of the number of moles of Sr atoms to the number of moles of all metal atoms other than Li contained in the LiNi composite oxide in the slurry 0.001 (0.1 mol %).

[0097] (4) Separation Step After the water washing step, a water-containing cake of LiNi composite oxide was separated from the slurry using a filter press.

[0098] (5) Drying step: The aqueous mixture was dried in a vacuum at 200°C for 3 hours to obtain a LiNiM composite oxide containing Ca and Sr (LiNi ０．９０ Co ０．０５ Mn ０．０５ O ２ ) powder was obtained.

[0099] Examples 2 to 10, Comparative Examples 1 to 3 Positive electrode active materials were synthesized in the same manner as in Example 1, except that the production conditions were changed as shown in Table 1. In Table 1, alkaline earth compounds are indicated by "M."

[0100] In Table 1, A1 to A10 are the positive electrode active materials of Examples 1 to 10, respectively, and B1 to B3 are the positive electrode active materials of Comparative Examples 1 to 3, respectively.

[0101] In A2 and A3, the amount of Ca compound contained in the mixed powder in the mixing step and the amount of Sr compound dissolved in the slurry in the water washing step were changed.

[0102] In A4 and A8, Ni is used as Ni oxide. ０．９０ Co ０．０５ Mn ０．０５ Ni instead of O ０．９０ Co ０．０５ Al ０．０５ O ２ was used.

[0103] In A5 to A8, the same process as A1 to A4 was carried out, except that an Sr compound was used instead of a Ca compound in the mixing step and a Ca compound was used instead of a Sr compound in the water washing step.

[0104] In B1, neither a Ca compound nor a Sr compound was used.

[0105] In B2, the mixed powder contained the same amount of Sr compound in terms of moles as the Ca compound in the mixing step, but the Sr compound was not dissolved in the slurry in the water washing step.

[0106] In B3, a Ca compound was dissolved in the slurry in the water washing step in an amount equal in moles to that of the Sr compound, whereas no Ca compound was mixed into the mixed powder in the mixing step.

[0107] In A9, the same process as A1 was carried out, except that the amount of Ca compound contained in the mixed powder in the mixing step and the amount of Sr compound dissolved in the slurry in the water washing step were changed to excess amounts.

[0108] In A10, the same process as A5 was carried out, except that the amount of Sr compound contained in the mixed powder in the mixing step and the amount of Ca compound dissolved in the slurry in the water washing step were changed to excess amounts.

[0109]

[0110] (Preparation of Secondary Battery) [Preparation of Positive Electrode] Positive electrodes were prepared using positive electrode active materials A1 to A10 and B1 to B3 in the following manner. Specifically, N-methyl-2-pyrrolidone (NMP) was added to a positive electrode mixture containing the positive electrode active material, acetylene black, and polyvinylidene fluoride (PVDF) in a mass ratio of 95:2.5:2.5, and the mixture was stirred to prepare a positive electrode slurry. The positive electrode slurry was applied to the surface of aluminum foil, which was a positive electrode current collector, and the coating was dried to form an unrolled layer on both sides of the aluminum foil. Next, the unrolled layer was rolled to obtain a positive electrode active material having a density of 3.6 g / cm. ３ The total thickness of the positive electrode after rolling was 160 μm.

[0111] [Fabrication of Negative Electrode] A negative electrode mixture containing graphite, sodium carboxymethyl cellulose (CMC-Na), and styrene-butadiene rubber (SBR) as negative electrode active materials in a mass ratio of 96:2:2 was added with water and stirred to prepare a negative electrode slurry. Next, the negative electrode slurry was applied to the surface of copper foil as a negative electrode current collector, and the coating was dried and then rolled to form a negative electrode active material layer on both sides of the copper foil. The density of the negative electrode active material in the negative electrode active material layer was 1.6 g / cm ３ The total thickness of the negative electrode was 170 μm.

[0112] [Preparation of Non-Aqueous Electrolyte] An electrolyte solution was prepared by dissolving LiPF6 at a concentration of 1.0 mol / L in a mixed solvent containing ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7.

[0113] [Secondary Battery Fabrication] An electrode group was fabricated by attaching tabs to each electrode and spirally winding the positive and negative electrodes with a separator (14 μm thick) interposed between them so that the tabs were positioned at the outermost periphery. The separator had a 10 μm-thick microporous polyethylene substrate layer and a 4 μm-thick heat-resistant layer laminated on one side (the positive electrode side) of the substrate layer. The heat-resistant layer contained 20% by mass of inorganic oxide filler (alumina) and the remainder was composed of aromatic polyamide (aramid). The electrode group was inserted into an exterior case made of aluminum laminate film and vacuum-dried at 105°C for 2 hours. After that, an electrolyte solution was injected, and the opening of the exterior case was sealed to obtain a secondary battery. Hereinafter, batteries using positive electrode active materials A1 to A10 and B1 to B3 will be referred to as secondary batteries A1 to A10 and B1 to B3, respectively.

[0114] (Evaluation) [Initial Capacity] In a 25°C environment, the secondary battery was subjected to constant current charging at a current of 0.5 C until the voltage reached 4.2 V, and then constant voltage charging at a voltage of 4.2 V until the current reached 0.05 C. After a 10-minute break, the secondary battery was subjected to constant current discharging at a current of 0.7 C until the voltage reached 2.5 V. The discharge capacity (Ci) at this time was determined as the initial capacity.

[0115] For batteries A1, B4, A5, and B5, the relative values ​​of the initial capacities of battery B4 and battery A5 in Example 5 when the initial capacity of battery A1 in Example 1 is taken as 100 are shown in Table 1. The larger the relative value, the better the performance.

[0116] [Capacity Retention Rate After Storage] The battery was charged to determine the discharge capacity Ci, and then stored in a charged state at 50° C. for 90 days, after which the discharge capacity (Cc) was determined. The capacity retention rate was calculated as the ratio (%) of the discharge capacity Cc after storage to the initial discharge capacity Ci, which was taken as 100%.

[0117] The capacity retention rate of each battery is shown in Table 1 as a relative value when the capacity retention rate of battery B1 of Comparative Example 1 is set to 100. The larger the relative value, the better the battery.

[0118] As shown in Table 1, the batteries A1 to A10 of the examples, which used positive electrode active materials to which alkaline earth metal M had been added in the mixing and water-washing steps, all exhibited higher capacity retention rates than the batteries B1 to B3 of the comparative examples. The batteries A9 to A10 of the examples had low initial capacities due to the addition of excessive amounts of Ca compound and Sr compound in the mixing and water-washing steps, but had excellent capacity retention rates after storage.

[0119] The positive electrode active material obtained by the manufacturing method according to the present disclosure is suitable for use in secondary batteries that require high capacity and high reliability. Secondary batteries are suitable for use as main power sources and power storage devices for, for example, portable electronic devices, electric vehicles, hybrid vehicles, and the like. While the present invention has been described with respect to presently preferred embodiments, such disclosure should not be construed as limiting. Various modifications and variations will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. Accordingly, the appended claims should be construed to embrace all modifications and variations that do not depart from the true spirit and scope of the present invention.

[0120] 10: Secondary battery, 11: Sealing body, 12: Valve body, 13: Metal plate, 14: Insulating member, 15: Positive electrode plate, 15a: Positive electrode lead, 16: Negative electrode plate, 16a: Negative electrode lead, 17: Separator, 18: Electrode group, 21: Gasket, 22: Battery can, 22a: Groove portion, 23: First insulating plate, 24: Second insulating plate

Claims

1. A method for producing a positive electrode active material for a secondary battery, comprising: a mixing step of mixing a Li compound, a Ni compound, and a first alkaline earth metal compound to obtain a mixed powder; a firing step of firing the mixed powder to obtain a LiNi composite oxide; a water-washing step of mixing the LiNi composite oxide with water to obtain a slurry and stirring the slurry to wash the LiNi composite oxide with water; a separation step of separating a hydrous cake of the LiNi composite oxide from the slurry; and a drying step of drying the hydrous cake to obtain the LiNi composite oxide powder, wherein the method further comprises, after the water-washing step, a step of applying a second alkaline earth metal compound to the LiNi composite oxide, wherein the first alkaline earth metal compound and the second alkaline earth metal compound each comprise at least one compound selected from the group consisting of a Ca compound and a Sr compound.

2. The method for producing a positive electrode active material for a secondary battery according to claim 1, wherein one of the first alkaline earth metal compound and the second alkaline earth metal compound contains at least a Ca compound, and the other contains at least a Sr compound.

3. The method for producing a positive electrode active material for a secondary battery according to claim 1, wherein the applying step includes adding the second alkaline earth metal compound to the slurry in the water washing step.

4. The method for producing a positive electrode active material for a secondary battery according to claim 1, wherein the first alkaline earth metal compound has a median diameter (D50) in a volume-based particle size distribution of 0.5 μm or more and 50 μm or less.

5. The method for producing a positive electrode active material for a secondary battery according to claim 1, wherein the applying step includes mixing the second alkaline earth metal compound with the water-containing cake obtained in the separating step.

6. The method for producing a positive electrode active material for a secondary battery according to claim 1, wherein the second alkaline earth metal compound has a median diameter (D50) in a volume-based particle size distribution of 0.5 μm or more and 50 μm or less.

7. The method for producing a positive electrode active material for a secondary battery according to claim 1, wherein the ratio of the total number of moles of Ca atoms and Sr atoms to the number of moles of all metal atoms other than Li contained in the LiNi composite oxide powder after the drying step is 0.1 mol % or more and 4 mol % or less.

8. The method for producing a positive electrode active material for a secondary battery according to claim 1, wherein the ratio of the number of moles of Ni atoms contained in the Ni compound to the number of moles of all metal atoms contained in the Ni compound is 50 mol % or more.

9. The method for producing a positive electrode active material for a secondary battery according to claim 1, wherein the Ni compound contains a metal element M1 other than Ni, and the metal element M1 contains at least one element selected from the group consisting of Co, Mn, Al, W, Nb, Zr, Zn, Ti, Mg, V, Si, Mo, and Cr.

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