Method for producing positive electrode active material for secondary battery

Abstract

Provided is a method for producing a positive electrode active material for a secondary battery, the method comprising: a first mixing step for obtaining a mixed powder of a Li compound and a Ni compound; a firing step for firing the mixed powder to obtain an LiNi composite oxide; a water-washing step for washing the LiNi composite oxide with water by stirring a slurry in which the LiNi composite oxide and water are mixed; a separating step for separating a water-containing cake of the LiNi composite oxide from the slurry; a second mixing step for mixing the water-containing cake and an alkaline earth metal M compound to obtain a water-containing mixture; and a drying step for drying the water-containing mixture to obtain a powder of the LiNi composite oxide, wherein the alkaline earth metal M compound includes at least one compound selected from the group consisting of Ca compounds and Sr compounds, and a value (g / (L·min)) obtained by dividing, by a water-washing time (minutes), the amount (g / L) of the LiNi composite oxide contained per unit volume of the slurry in the water-washing step is not less than 100.

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. 2024-013468, 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 proposes a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, which is made of a lithium-nickel composite oxide powder, characterized by comprising the following steps (a) to (c): (a) roasting a nickel hydroxide and / or nickel oxyhydroxide containing nickel as a main component and at least one element selected from other transition metal elements, Group 2 elements, and Group 13 elements as a secondary component in a non-reducing atmosphere at 850°C or less to prepare a nickel oxide; (b) mixing the nickel oxide and a lithium compound so that the molar ratio of the amount of lithium in the lithium compound to the total amount of nickel, other transition metal elements, Group 2 elements, and Group 13 elements in the nickel oxide is 1.00 to 1.10, and then firing the mixture in an oxygen atmosphere at a temperature in the range of 650 to 850°C to produce a lithium-nickel composite oxide having the composition formula Li ａ Ni １－ｂ M ｂ O ２ (wherein M represents at least one element selected from transition metal elements other than Ni, Group 2 elements, and Group 13 elements, a is 1.00≦a≦1.10, and b is 0.01≦b≦0.5), and (c) a step of washing the fired powder with water to a slurry concentration of 500 to 2000 g / L for a time that satisfies B / 40<A≦B / 10 (wherein A represents the water washing time expressed in minutes, and B represents the slurry concentration expressed in g / L), followed by filtering and drying to obtain a lithium nickel composite oxide powder.

[0004] JP 2014-146441 A

[0005] Although Patent Document 1 aims to improve cycle characteristics by controlling the reaction resistance of a lithium-nickel composite oxide, there is still room for improvement. In particular, it is desired to reduce the reaction resistance when the Ni content in the composite oxide is increased in order to increase capacity.

[0006] One aspect of the present disclosure is a method for producing a LiNi composite oxide, comprising: (i) a first mixing step of mixing a Li compound and a Ni 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 method for producing a LiNi composite oxide by mixing the water-containing cake with a compound of an alkaline earth metal M to obtain a LiNi composite oxide. and (vi) a drying step of drying the aqueous mixture to obtain the LiNi composite oxide powder, wherein the alkaline earth metal M compound contains at least one compound selected from the group consisting of a Ca compound and an Sr compound, and in the water-washing step, the value (g / (L·min)) obtained by dividing the content of the LiNi composite oxide per unit volume of the slurry (g / L) by the water-washing time (min) is 100 or more.

[0007] According to the present disclosure, a positive electrode active material with low reaction resistance 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, for example, studies have been conducted to increase the capacity of a positive electrode active material by increasing the Ni content of the positive electrode active material. However, the durability of a positive electrode active material tends to decrease as its capacity increases. Specifically, a phase with low crystallinity and mainly composed of nickel oxide (hereinafter also referred to as a "Ni oxide phase") is present on the surfaces of the primary and secondary particles of a Ni-containing positive electrode active material. The Ni oxide phase is highly active in side reactions and acts as a resistive component, which increases the reaction resistance and reduces the durability of the positive electrode active material. When the durability of the positive electrode active material decreases, it becomes difficult to improve the cycle characteristics of the secondary battery.

[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, the reaction resistance of a positive electrode active material is reduced by adding a compound of an alkaline earth metal M to the surface layer of primary particles or secondary particles of the positive electrode active material. However, when using a protective component such as a compound of an alkaline earth metal M, it is necessary to limit its side effects as much as possible. For example, while the protective component has the effect of reducing the reaction resistance caused by the Ni oxide phase by reacting with the Ni oxide phase, if present in excess, the protective component itself becomes a resistance component, which may actually cause an increase in the reaction resistance.

[0015] In view of the above, one of the objectives 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 while reducing the reaction resistance.

[0016] Specifically, the manufacturing method M comprises (i) a first mixing step, (ii) a calcination step, (iii) a water washing step, (iv) a separation step, (v) a second mixing step, and (vi) a drying step. Each step will be described in detail below.

[0017] (i) First Mixing Step The first mixing step is a step of mixing a Li compound and a Ni compound to obtain a mixed powder. The Li compound and the Ni compound may each be powder. The step of obtaining a mixed powder may be a step of dry-mixing at least two powders, the Li compound and the Ni 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.

[0018] From the viewpoint of increasing the mixing efficiency, the mixture of the liquid medium, the Li compound, and the Ni compound may be stirred while volatilizing at least a part of the liquid medium to obtain a mixed powder.

[0019] (Li Compound) Examples of Li compounds include Li2CO3, LiOH, Li2O2, Li2O, LiNO3, LiNO2, Li2SO4, LiOH.H2O, LiH, and LiF.

[0020] 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, 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.

[0021] 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.

[0022] 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.

[0023] (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.

[0024] 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 80 mol% or more, or may be 85 mol% or more, 90 mol% or more, or may be 95 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, and more preferably 98 mol% or less.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] According to the above method, the metal element M1 can be uniformly dispersed in the Ni compound.

[0029] 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.

[0030] The Ni compound powder may have a median diameter (D50) in a volumetric particle size distribution of, for example, 2 μm or more and 20 μm or less. Such Ni compounds have high reactivity with Li compounds and are advantageous in improving the crystallinity of the LiNi composite oxide.

[0031] (ii) Firing Step The firing step is a step of firing the first mixed powder (powder mixture of a Li compound and a Ni compound) to obtain a LiNi composite oxide.

[0032] 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.

[0033] (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 melts and penetrates into the secondary particles of the Ni compound. Some of the Li compound may form a LiNi composite oxide. For example, if 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.

[0034] The temperature of the first baking 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 compound and the Li compound at such a temperature for a certain period of 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 baking tends to proceed uniformly.

[0035] (Second Firing) The second firing is a process in which the first 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 an environment rich in Li compounds. 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.

[0036] (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.

[0037] As mentioned above, in the first mixing step, the amount of Li compound is preferably in excess of the amount of Ni compound. Therefore, it is necessary to perform appropriate water washing 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.

[0038] 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.

[0039] 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.

[0040] The area where the Ni oxide phase is formed on the surface of the positive electrode active material tends to increase or decrease depending on the water washing time and the slurry concentration. The "slurry concentration" is defined as the content (g / L) of the LiNi composite oxide contained per unit volume of the slurry prepared in the water washing step.

[0041] The slurry concentration may be, for example, 1000 g / L to 3000 g / L, or 1500 g / L to 2500 g / L, In other words, the content of the LiNi composite oxide contained in 1 L (liter) of the slurry may be 1000 g to 3000 g, or 1500 g to 2500 g.

[0042] Hereinafter, the value (g / (L·min)) obtained by dividing the slurry concentration by the water washing time (min) will also be referred to as "water washing degree."

[0043] Washability (g / (L·min)) = slurry concentration (g / L) / washing time (min)

[0044] The degree of water washing is an index of the degree of water washing. The water washing time (minutes) may be counted as the stirring time of the slurry obtained by mixing the LiNi composite oxide with water.

[0045] In the water washing step, the stirring conditions do not substantially affect the degree of water washing as long as the slurry concentration is sufficiently low. Therefore, the mass content of residual Li depends on the "degree of water washing" determined by the slurry concentration and washing time. However, stirring is preferably performed under conditions in which 80 mass % or more (preferably 100%) of the LiNi composite oxide in the slurry is constantly flowing without settling.

[0046] By controlling the degree of water washing to 100 or more, the generation of a Ni oxide phase on the surface of the positive electrode active material is suppressed, and the attachment sites of the protective component to be attached in the subsequent process can be limited. As a result, the attachment of the protective component to the positive electrode active material can be optimized, and an increase in reaction resistance due to an excess of the protective component can be suppressed.

[0047] The degree of water washing may be 120 or more, or 200 or more. However, in consideration of the production cost of the positive electrode active material, a washing step that is too short and uses a small amount of water is undesirable from the viewpoint of minimizing the influence of residual Li. Therefore, the upper limit of the degree of water washing is preferably set to, for example, 400 or less.

[0048] In the water washing step, the washing time, i.e., the time for stirring the slurry containing the LiNi composite oxide and water, is preferably set to, for example, 10 minutes or less, more preferably 1 minute or less, which can prevent excessive adhesion of the protective component to the surface of the positive electrode active material in the subsequent step.

[0049] The water washing step may be carried out so that the mass content of residual Li in the LiNi composite oxide after the subsequent drying step is 1000 ppm or more, or 1200 ppm or more, thereby significantly suppressing an increase in reaction resistance due to the generation of a Ni oxide phase when a protective component is imparted in the subsequent second mixing step and drying step.

[0050] On the other hand, if the washing with water is insufficient, an excessive amount of unreacted Li may remain in the LiNi composite oxide after the washing step. Such residual Li may cause gas generation and side reactions in the secondary battery, resulting in a decrease in capacity. Therefore, the mass content of residual Li in the LiNi composite oxide after the drying step is preferably 3000 ppm or less, and more preferably 2100 ppm or less.

[0051] (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 LiNi composite oxide that has been mostly dehydrated but still contains sufficient moisture and is in a solid state rather than a powder state. The moisture content of the hydrous cake of LiNi composite oxide may be, for example, 3% by mass or more and 10% by mass or less.

[0052] The moisture content of the hydrous cake can be determined from the difference in mass of the LiNi composite oxide before and after drying (the difference between the mass W1 of the hydrous cake before drying and the mass W2 of the LiNi composite oxide powder after drying) (moisture content (%) = 100 × (W1 - W2) / W1). The hydrous cake can be dried by holding it in air at a temperature of 180°C to 220°C for 1 hour.

[0053] 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.

[0054] (v) Second Mixing Step The second mixing step is a step of mixing the hydrous cake with a compound of an alkaline earth metal M to obtain a hydrous mixture. The compound of alkaline earth metal M to be mixed with the hydrous cake may be in powder form. Alternatively, an aqueous solution in which the compound of alkaline earth metal M is dissolved in water in advance may be mixed with the hydrous cake of LiNi composite oxide. The step of obtaining a hydrous mixture may be a step of mixing the hydrous cake of LiNi composite oxide with the aqueous solution of the compound of alkaline earth metal M using a kneader, mixer, or the like. One preferred example of mixing conditions is mixing using an axial mixer manufactured by Sugiyama Heavy Industries Co., 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.

[0055] The mixed powder may be obtained by stirring a mixture of a liquid medium (for example, water or an alcohol such as ethanol), a water-containing cake, and a powder of a compound of an alkaline earth metal M while volatilizing at least a portion of the liquid medium. In this case, the amount of the liquid medium is preferably 10 parts by mass or less per 100 parts by mass of the mixture of the water-containing cake of the LiNi composite oxide and the powder of the compound of an alkaline earth metal M.

[0056] (Alkaline Earth Metal M Compound) The alkaline earth metal M compound includes at least one selected from the group consisting of a Ca compound and a Sr compound. Ca and Sr are not easily incorporated 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. For example, Ca and Sr suppress the rate of increase in resistance of the LiNi composite oxide (suppress deterioration), thereby contributing to improved 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. Furthermore, Ca and Sr exist stably inside the secondary battery, making them less likely to cause side effects such as gas generation.

[0057] Ca compounds include CaCO3 and Ca(OH) ２ , CaO, Ca(NO ３ ) ２ , CaSO ４ , CaH２ , CaF ２ Among these, CaO, CaCO3, etc. are preferred.

[0058] Sr compounds include SrCO3 and Sr(OH) ２ , SrO, Sr(NO ３ ) ２ , SrSO ４ , SrH ２ Among these, SrO, SrCO3, etc. are preferred.

[0059] 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 compound of the alkaline earth metal M to the number of moles of all metal atoms contained in the compound of the alkaline earth metal M is preferably 80 mol % or more, or may be 85 mol % or more, 90 mol % or more, or may be 95 mol % or more.

[0060] In the aqueous mixture, the amount of the alkaline earth metal M compound may be an amount sufficient to cover the surface layer of the LiNi composite oxide to some extent. For example, when a layered rock salt structure LiNi composite oxide (e.g., space group R-3m) is synthesized as the positive electrode active material, the ratio of the total number of moles of Ca atoms and Sr atoms contained in the alkaline earth metal M compound to the number of moles of all metal atoms other than Li contained in the LiNi composite oxide may be 0.1 to 4.0 mol%, or may be 0.2 to 1.0 mol%. By using a sufficient amount of alkaline earth metal M compound in this way, a large proportion of the surface layer portion of the primary particles or secondary particles of the LiNi composite oxide can be protected by a protective component containing Ca and Sr. Therefore, gas generation and capacity reduction due to side reactions in the secondary battery caused by residual Li are suppressed.

[0061] The median diameter (D50) in the volume-based particle size distribution of the alkaline earth metal M compound may be, for example, 1 μm or more and 50 μm or less. Such an alkaline earth metal M compound has high reactivity with the surface layer portion of the LiNi composite oxide in the subsequent heat treatment step. The median diameter (D50) in the volume-based particle size distribution of each of the Ca compound and the Sr compound may be, for example, 1 μm or more and 50 μm or less. The particle size distribution may be controlled by appropriate pulverization or classification.

[0062] (vi) Drying Step The drying step is a step of drying the aqueous mixture to obtain a powder of a LiNi composite oxide to which Ca and Sr have been added. The drying conditions are not particularly limited.

[0063] 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, the LiNi composite oxide may be dried until it changes from a water-containing cake state to a powder state having fluidity.

[0064] 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.

[0065] The mass content of residual Li in the LiNi composite oxide after the drying step is, for example, 1000 ppm or more, and preferably, for example, 1200 ppm or more.

[0066] The mass content of residual Li in the LiNi composite oxide after the drying step can be determined, for example, by neutralization titration. For example, 1.0 g of a sample of the LiNi composite oxide after the drying step is shaken and dispersed in 30 mL of ion-exchanged water at 25°C, a strong alkaline component containing water-soluble residual Li is eluted into the ion-exchanged water, and the LiNi composite oxide is filtered from the aqueous solution containing the strong alkaline component, and the filtrate obtained is titrated. ２The solution is titrated to the second end point with hydrochloric acid having a concentration of 1 mol / L under atmospheric pressure. The amount of hydrochloric acid required for the titration to the first end point and the amount of hydrochloric acid required for the titration to the second end point are used to calculate the alkali components as the mass of LiOH and Li ２ CO ３ The mass content of Li in 1.0 g of the LiNi composite oxide is calculated by converting the mass of Li into the mass of LiNi.

[0067] (vii) 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 maintained at a temperature of 350°C or higher and 550°C or lower. Such a heat treatment step enhances the protective effect of the LiNi composite oxide, further reduces reaction resistance, and improves the capacity retention rate during charge / discharge cycles.

[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 Note) The above description discloses the following technology: (Technology 1) A method for producing a powder of the LiNi composite oxide, comprising: a first mixing step of mixing a Li compound and a Ni 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; a second mixing step of mixing the water-containing cake with a compound of an alkaline earth metal M to obtain a water-containing mixture; and a drying step of drying the water-containing mixture to obtain a powder of the LiNi composite oxide, wherein 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, A method for producing a positive electrode active material for a secondary battery, wherein in the water-washing step, a value (g / (L·min)) obtained by dividing the content of LiNi composite oxide per unit volume of the slurry (g / L) by the water-washing time (min) is 100 or more. (Technology 2) A method for producing a positive electrode active material for a secondary battery according to Technology 1, wherein a mass content of residual Li in the LiNi composite oxide powder after the drying step is 1000 ppm or more. (Technology 3) A method for producing a positive electrode active material for a secondary battery according to Technology 1 or 2, wherein a median diameter (D50) in a volume-based particle size distribution of the compound of alkaline earth metal M is 1 μm or more and 50 μm or less. (Technology 4) A method for producing a positive electrode active material for a secondary battery according to any one of Technology 1 to 3, wherein a water content of the water-containing cake is 3 mass% or more and 10 mass% or less. (Technology 5) The method for producing a positive electrode active material for a secondary battery according to any one of Technologies 1 to 4, wherein in the aqueous mixture, a ratio of the total number of moles of Ca atoms and Sr atoms contained in the compound of the alkaline earth metal M to the number of moles of all metal atoms other than Li contained in the LiNi composite oxide is 0.1 mol % or more and 4 mol % or less. (Technology 6) The method for producing a positive electrode active material for a secondary battery according to any one of Technologies 1 to 5, 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 80 mol % or more.(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 in the first mixed powder, an amount of the Li compound is in excess relative to the Ni compound. (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 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) First Mixing Step A mixed powder was obtained by dry-mixing LiOH, which is a Li compound, and Ni oxide containing a metal element M1 (Co, Mn).

[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] 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.

[0091] (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 obtain a first fired body.

[0092] (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).

[0093] (3) Water-washing step: The second fired product (LiNi composite oxide) was mixed with water in a washing tank to obtain a slurry, which was stirred for 10 minutes to wash the LiNi composite oxide with water. The slurry concentration was 2000 g / L, and the water washability was 200 (g / (L·min)).

[0094] (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. The water content of the water-containing cake was 5% by mass.

[0095] (5) Second mixing step: Mixing a water-containing cake of LiNi composite oxide with a Ca compound (Ca(OH) ２ ) and Sr compounds (Sr(OH) ２ ) was mixed to obtain a water-containing mixture.

[0096] Ca compound (Ca(OH) ２ ) and Sr compounds (Sr(OH) ２ ) have median diameters (D50) of 10 μm and 5 μm, respectively.

[0097] In the aqueous mixture, the ratios of the number of moles of Ca atoms and Sr atoms to the number of moles of all metal atoms contained in the Ni oxide were 0.3 mol % and 0.1 mol %, respectively.

[0098] (6) Drying step: The aqueous mixture was dried in air at 200°C for 3 hours to obtain a LiNi composite oxide containing Ca and Sr (LiNi ０．９０ Co ０．０５ Mn ０．０５ O ２ The mass content of residual Li in the LiNi composite oxide powder after the drying step was determined by the method described above and was found to be 1218 ppm.

[0099] Examples 2 to 4, Comparative Examples 1 to 5 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.

[0100] In Table 1, A1 to A4 are the positive electrode active materials of Examples 1 to 4, respectively, and B1 to B5 are the positive electrode active materials of Comparative Examples 1 to 5, respectively.

[0101] In A4, Ni is used as Ni oxide. ０．９０ Co０．０５ Mn ０．０５ Ni instead of O ０．９０ Co ０．０５ Al ０．０５ O ２ was used.

[0102] In A1 to A4 and B1 to B5, the stirring conditions, firing conditions, and degree of water washing when preparing the mixed powder in the first mixing step were changed to change the mass content of residual Li in the LiNi composite oxide after the drying step.

[0103] In B1 to B4, no Ca compound and no Sr compound were mixed into the water-containing mixture.

[0104]

[0105] (Preparation of Secondary Battery) [Preparation of Positive Electrode] Positive electrodes were prepared using positive electrode active materials A1 to A4 and B1 to B5 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.

[0106] [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.

[0107] [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.

[0108] [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 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 A4 and B1 to B5 will be referred to as secondary batteries A1 to A4 and B1 to B5, respectively.

[0109] (Evaluation) [Initial Resistance] In a temperature environment of 25°C, the battery was charged at a constant current of 0.3 C until the voltage reached 4.2 V, and then at a constant voltage of 4.2 V until the current reached 0.05 C. Next, the battery was discharged at a constant current of 0.3 C for 100 minutes to bring the state of charge (SOC) to 50%.

[0110] The voltage value was measured when a battery with an SOC of 50% was discharged for 10 seconds at current values ​​of 0 A, 0.1 A, 0.5 A, and 1.0 A. The relationship between the discharge current value and the voltage value after 10 seconds was approximated to a straight line by the least squares method, and the initial DCIR (Ri) was calculated from the absolute value of the slope.

[0111] The relative initial resistance of each battery, where the initial resistance of secondary battery B1 of Comparative Example 1 is taken as 100, is shown in Table 1. The smaller the relative value, the better the initial resistance.

[0112] As shown in Table 1, the batteries A1 to A4 of the examples, which used positive electrode active materials prepared by drying a water-containing mixture containing a LiNi composite oxide having a water washing degree of 100 or more and a compound of an alkaline earth metal M, all exhibited lower initial resistances than the batteries B1 to B5 of the comparative examples.

[0113] 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.

[0114] 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 first mixing step of mixing a Li compound and a Ni 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; a second mixing step of mixing the hydrous cake with a compound of an alkaline earth metal M to obtain a hydrous mixture; and a drying step of drying the hydrous mixture to obtain the LiNi composite oxide powder, wherein 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, and wherein in the water-washing step, the value (g / (L·min)) obtained by dividing the content of the LiNi composite oxide per unit volume of the slurry (g / L) by the water-washing time (min) is 100 or more.

2. The method for producing a positive electrode active material for a secondary battery according to claim 1, wherein the mass content of residual Li in the LiNi composite oxide powder after the drying step is 1000 ppm or more.

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

4. The method for producing a positive electrode active material for a secondary battery according to claim 1, wherein the water content of the water-containing cake is 3% by mass or more and 10% by mass or less.

5. The method for producing a positive electrode active material for a secondary battery according to claim 1, wherein in the aqueous mixture, the ratio of the total number of moles of Ca atoms and Sr atoms contained in the compound of alkaline earth metal M to the number of moles of all metal atoms other than Li contained in the LiNi composite oxide is 0.1 mol % or more and 4 mol % or less.

6. 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 80 mol % or more.

7. The method for producing a positive electrode active material for a secondary battery according to claim 1, wherein the amount of the Li compound in the first mixed powder is in excess of the amount of the Ni compound.

8. 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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