Method for manufacturing positive electrode active material for secondary battery

Abstract

This method for manufacturing a positive electrode active material for a secondary battery comprises a step for: firing a first mixed powder, which is obtained by mixing a Li compound and a Ni compound, to obtain a LiNi composite oxide; mixing the LiNi composite oxide with water, washing the mixture with water, separating a water-containing cake of the LiNi composite oxide from a slurry after the washing with water, and drying the water-containing cake to obtain a powder of the LiNi composite oxide; and then mixing the powder of the LiNi composite oxide with a compound of an alkaline earth metal M to obtain a second mixed powder. The compound of the alkaline earth metal M contains a Ca compound and a Sr compound. In a heat treatment step, the second mixed powder is held at a temperature of 250°C or higher but lower than 350°C for at least 2 hours.

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. 2024-013466, 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 positive electrode active material for a lithium ion secondary battery, which is made by using as a raw material a base powder containing secondary particles formed by agglomeration of primary particles of a lithium metal composite oxide containing lithium, nickel, and cobalt, wherein a coating layer containing a lithium borate salt is formed on the surface of the secondary particles, the coating layer has an average thickness of 5 to 160 nm, and the coefficient of variation (CV%) indicating the variation in thickness of the coating layer is 15% or less."

[0004] Japanese Patent Application Laid-Open No. 2021-064598

[0005] Patent Document 1 describes that it is possible to provide a "positive electrode active material for lithium ion secondary batteries that has low positive electrode resistance and a high discharge capacity retention rate when used in lithium ion secondary batteries." However, since the amount of heat generated during abnormal discharge generally increases as the positive electrode resistance decreases, it is desirable to improve the heat resistance of secondary batteries. Previous proposals have room for improvement in terms of achieving both low resistance and safety.

[0006] One aspect of the present disclosure includes: (i) a first mixing step of mixing a Li compound and a Ni compound to obtain a first mixed powder; (ii) a firing step of firing the first 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; (v) a drying step of drying the water-containing cake to obtain the LiNi composite oxide powder; and (vi) a drying step of drying the LiNi composite oxide powder. and (vii) a heat treatment step of heating the second mixed powder, wherein the compound of alkaline earth metal M contains at least one compound selected from the group consisting of a Ca compound and a Sr compound, the mass content of residual Li in the LiNi composite oxide powder after the drying step is 1000 ppm or more, and the heat treatment step involves holding the second mixed powder at a temperature of 250°C or higher but lower than 350°C for 2 hours or longer.

[0007] According to the present disclosure, it is possible to obtain a highly durable positive electrode active material capable of providing a secondary battery having low resistance and excellent heat resistance. 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] In order to improve the performance of secondary batteries, for example, studies have been conducted to increase the capacity of the positive electrode active material by increasing the Ni content of the positive electrode active material. However, the durability of the positive electrode active material tends to decrease as the capacity increases. 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] However, when using a protective component, it is necessary to minimize its side effects. For example, the protective component can cause an increase in resistance. On the other hand, even if the resistance caused by the protective component can be suppressed, the lower the positive electrode resistance, the greater the amount of heat generated during abnormal discharge. In secondary batteries, low resistance and heat resistance (or safety) are mutually exclusive. Therefore, there is a strong demand for a battery that can achieve both low resistance and heat resistance.

[0015] 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 realize a secondary battery having excellent heat resistance by using a low-resistance protective component.

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

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

[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. Element M2 may account for 80 mol% or more, or may account for 99.9 mol% or more, of the metal element M1. Among the elements M2, at least one element M3 selected from the group consisting of Co, Mn, and Al is particularly preferred. Element M3 may account for 80 mol% or more, or may account for 99.9 mol% or more, of the metal element M1.

[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 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.

[0036] (iii) Water-washing step: The water-washing step is a 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. The water-washing step may be carried out until the mass content of residual Li in the LiNi composite oxide after the subsequent drying step is 1000 ppm or more and less than 3000 ppm, preferably 1000 ppm or more and less than 2000 ppm.

[0037] As described above, it is preferable that the amount of Li compound in the first mixed powder is 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 due to decomposition of the electrolyte during charge and discharge, and a decrease in capacity due to side reactions.

[0038] In other words, the residual Li refers to the 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] However, in consideration of the production cost of the positive electrode active material, an excessively long washing step using a large amount of water is undesirable. From an industrial perspective, it is important to complete the washing step in a short time using as little water as possible.

[0040] Furthermore, if washing is carried out at a high level until the mass content of residual Li becomes less than 1000 ppm, the LiNi composite oxide tends to deteriorate due to washing, and the resistance of the LiNi composite oxide tends to increase.

[0041] To prevent deterioration of the LiNi composite oxide due to washing with water, it may be preferable to wash the composite oxide with water to an extent that the mass content of residual Li is 1000 ppm or more, leaving a certain amount of excess Li. Even in this case, the excess Li can be inactivated in the subsequent process, and capacity reduction due to gas generation and side reactions in the secondary battery is prevented.

[0042] 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 2000 ppm or less.

[0043] The water washing step is desirably carried out in a batch manner from the viewpoint of reducing the amount of water required. Batch 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, and stirring the resulting slurry in the washing tank by dispersing the LiNi composite oxide in the water. The content of the LiNi composite oxide in the slurry (slurry concentration) may be, for example, 1000 g / L or more and 3000 g / L or less, and preferably 1500 g / L or more and 2500 g / L or less. That is, the content of the LiNi composite oxide contained in 1 L (liter) of slurry may be 1000 g to 3000 g or 1500 g to 2500 g.

[0044] In the water washing step, for example, the slurry containing the LiNi composite oxide and water is preferably stirred for less than 5 minutes, which makes it possible to control the mass content of residual Li in the LiNi composite oxide after the subsequent drying step to 1000 ppm or more (and preferably less than 3000 ppm).

[0045] (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.

[0046] The moisture content of the water-containing cake can be determined from the difference in mass of the LiNi composite oxide before and after the subsequent drying step (the difference between the mass W1 of the water-containing cake before drying and the mass W2 of the LiNi composite oxide powder after drying) (moisture content (%)=100×(W1−W2) / W1).

[0047] 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.

[0048] (v) Drying Step The drying step is a step of drying the water-containing cake to obtain a powder of LiNi composite oxide. The drying conditions are not particularly limited.

[0049] 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.

[0050] 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.

[0051] The mass content of residual Li in the LiNi composite oxide after the drying step is, for example, 1000 ppm or more, and is preferably reduced to, for example, 3000 ppm or less. Since the cleaning step is performed until the mass content of residual Li is reduced to, for example, 3000 ppm or less, the residual Li can be more easily inactivated in the subsequent step.

[0052] 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, the alkaline components containing the water-soluble residual Li are eluted into the ion-exchanged water, and the LiNi composite oxide is filtered from the aqueous solution containing the alkaline components, 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.

[0053] (vi) Second Mixing Step The second mixing step is a step of mixing the dried LiNi composite oxide powder with a compound of an alkaline earth metal M to obtain a second mixed powder. The compound of the alkaline earth metal M may be a powder. The step of obtaining the second mixed powder may be a step of dry-mixing at least two types of powder, the powder of the LiNi composite oxide and the powder of the compound of the alkaline earth metal M, using a powder mixer. A preferred example of mixing conditions includes mixing using an axial mixer manufactured by Sugiyama Heavy Industries, Ltd., with a main shaft screw blade at 200 rpm and a chopper at 1000 rpm, for a period of 3 minutes to 30 minutes.

[0054] (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.

[0055] Ca compounds include CaCO3 and Ca(OH) ２ , CaO, Ca(NO ３ ) ２ , CaSO ４ , CaH ２ , CaF ２ Among them, Ca(OH) ２, CaO, CaCO3, etc. are preferred.

[0056] Sr compounds include SrCO3 and Sr(OH) ２ , SrO, Sr(NO ３ ) ２ , SrSO ４ , SrH ２ Among them, Sr(OH) ２ , SrO, SrCO3, etc. are preferred.

[0057] 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.

[0058] In the second mixed powder, the amount of the alkaline earth metal M compound is preferably in excess relative to the LiNi composite oxide. For example, when a LiNi composite oxide having a layered rock salt structure (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 mol% or more and 4 mol% or less, and preferably 0.2 mol% or more and 1.0 mol% or less. By using such an excess amount of the alkaline earth metal M compound, 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.

[0059] The median diameter (D50) in the volume-based particle size distribution of the alkaline earth metal M compound may be, for example, 0.1 μm or more and less than 5 μm. Such alkaline earth metal M compounds have a degree of agglomeration that allows them to be disintegrated in a mixer, and are highly reactive 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, 0.1 μm or more and less than 5 μm. When controlling the particle size distribution within such a range, adjustment may be made by appropriate pulverization or classification.

[0060] (vii) Heat Treatment Step The heat treatment step includes a process of holding the second mixed powder at a temperature of 250°C or higher but lower than 350°C for 2 hours or longer. This enhances the protective effect of the LiNi composite oxide and further reduces the reaction resistance in the normal temperature range. On the other hand, even if the reaction resistance is reduced, the heat resistance during abnormal heat generation is not impaired.

[0061] If the temperature in the heat treatment step is less than 250° C., the effect of reducing the reaction resistance is hardly obtained. On the other hand, if the temperature in the heat treatment step is 350° C. or higher, the crystal structure of the surface layer of the LiNi composite oxide is easily damaged, which tends to reduce the discharge capacity and increase the reaction resistance.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] Examples of the material for the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0066] Examples of the binder include polytetrafluoroethylene and polyvinylidene fluoride.

[0067] Examples of the conductive agent include graphite such as natural graphite and artificial graphite, and carbon blacks such as acetylene black.

[0068] 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.

[0069] Examples of the material for the negative electrode current collector include copper, copper alloy, nickel alloy, nickel alloy, and stainless steel.

[0070] As the binder, in addition to the materials exemplified for the positive electrode, styrene butadiene rubber or the like can be used.

[0071] As the thickener, for example, carboxymethyl cellulose (CMC) and its modified products (including salts such as Na salt) can be used.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] (Additional Note) The above description discloses the following techniques. (Technology 1) A method for producing a powder mixture of Li and Ni compounds, comprising: a first mixing step of mixing a Li compound and a Ni compound to obtain a first mixed powder; a firing step of firing the first 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 drying step of drying the hydrous cake to obtain a powder of the LiNi composite oxide; a second mixing step of mixing the powder of the LiNi composite oxide with a compound of an alkaline earth metal M to obtain a second mixed powder; and a heat treatment step of heating the second mixed 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 the mass content of residual Li in the powder of the LiNi composite oxide after the drying step is 1000 ppm or more, The method for producing a positive electrode active material for a secondary battery according to Technique 1, wherein the second mixed powder is maintained at a temperature of 250°C or higher and lower than 350°C for two hours or longer in the heat treatment step. (Technology 2) The method for producing a positive electrode active material for a secondary battery according to Technique 1, wherein the compound of alkaline earth metal M has a median diameter (D50) of 0.1 μm or higher and lower than 5 μm in a volume-based particle size distribution. (Technology 3) The method for producing a positive electrode active material for a secondary battery according to Technique 1 or 2, wherein, in the second mixed powder, a 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 higher and 4 mol% or lower. (Technology 4) The method for producing a positive electrode active material for a secondary battery according to any one of Techniques 1 to 3, 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 higher. (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 the amount of the Li compound in the first mixed powder is in excess of the amount of the Ni compound.(Technology 6) The method for producing a positive electrode active material for a secondary battery according to any one of Techniques 1 to 5, 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.

[0080] 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.

[0081] Example 1 (1) First Mixing Step A first mixed powder was obtained by dry-mixing LiOH, which is a Li compound, and Ni oxide containing a metal element M1 (Co, Al).

[0082] LiOH was prepared by heating and drying lithium hydroxide monohydrate (median diameter (D50) 1 μm) at 150° C. for 1 hour.

[0083] 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 Al ０．０５ 0 (median diameter (D50) 10 μm).

[0084] In the first 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.

[0085] (2) Firing Step The mixed powder packed in the sagger was fired at 750° C. for 5 hours in an oxidizing atmosphere to obtain a fired body (LiNi composite oxide).

[0086] (3) Water-washing step: The fired product (LiNi composite oxide) was mixed with water in a washing tank to obtain a slurry, which was stirred for 10 minutes, and the LiNi composite oxide was washed with water. The content of the LiNi composite oxide in the slurry (slurry concentration) was 2000 g / L.

[0087] (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.

[0088] (5) Drying step: The water-containing cake was dried in air at 200°C for 3 hours to obtain a LiNi composite oxide (LiNi ０．９０ Co ０．０５ Al 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 1377 ppm.

[0089] (6) Second mixing step: Mixing the powder of LiNi composite oxide and the Ca compound (Ca(OH) ２ ) and Sr compounds (Sr(OH) ２ ) were dry mixed to obtain a second mixed powder.

[0090] Ca compound (Ca(OH) ２ ) and Sr compounds (Sr(OH) ２ ) have median diameters (D50) of 0.17 μm and 0.24 μm, respectively.

[0091] In the second mixed powder, 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 are 2 mol % and 1 mol %, respectively.

[0092] (7) Heat Treatment Step The second mixed powder was subjected to heat treatment by being held in air at 300° C. for 3 hours.

[0093] Examples 2 and 3, Comparative Examples 1 to 7 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.

[0094] In Table 1, A1 to A3 are the positive electrode active materials of Examples 1 to 3, respectively, and B1 to B7 are the positive electrode active materials of Comparative Examples 1 to 7, respectively.

[0095] In A3, B1, B3, B6, and B7, Ni is used as Ni oxide. ０．９０ Co ０．０５ Al ０．０５ Ni instead of O ０．９０ Co ０．０５ Mn ０．０５ O ２ was used.

[0096] In A1 to A3 and B1 to B8, the mass content of residual Li was changed by changing the stirring conditions, firing conditions, and water washing conditions when preparing the first mixed powder.

[0097] In A1 to A3 and B1 to B8, the median diameter (D50) of the Ca compound and Sr compound in the second mixed powder, the ratio 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, and the heat treatment conditions were changed.

[0098] In B1, the Ca compound and the Sr compound were not mixed into the second mixed powder, and the heat treatment step was not performed.

[0099] In B2, only the heat treatment step was carried out without mixing the Ca compound and the Sr compound into the second mixed powder.

[0100] In B3, the Ca compound and the Sr compound were mixed into the second mixed powder, but the heat treatment step was not performed.

[0101]

[0102] (Preparation of Secondary Battery) [Preparation of Positive Electrode] Positive electrodes were prepared using positive electrode active materials A1 to A3 and B1 to B7 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.

[0103] [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 / cm３ The total thickness of the negative electrode was 170 μm.

[0104] [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.

[0105] [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 A3 and B1 to B7 will be referred to as secondary batteries A1 to A3 and B1 to B7, respectively.

[0106] (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%.

[0107] 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.

[0108] [Heat resistance (safety)] (a) In a 25°C environment, each 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, the battery was charged at a constant voltage of 4.2 V until the current reached 0.05 C. (b) The battery charged in (a) was placed in a 130°C environment, and the surface temperature of the battery was measured in Kelvin (K). The reciprocal of the highest temperature reached on the battery surface within 2 hours after being placed in the 130°C environment was calculated as the heat resistance.

[0109] Table 2 shows the relative values ​​of the initial resistance of each battery when the initial resistance of secondary battery B1 of Comparative Example 1 is set to 100, and the relative values ​​of the reciprocal of the surface temperature of each battery when the reciprocal of the surface temperature of secondary battery B1 of Comparative Example 1 is set to 100. The smaller the relative value, the better the initial resistance, and the larger the relative value, the better the heat resistance.

[0110]

[0111] As shown in Table 2, the batteries A1 to A3 of the examples, which used a positive electrode active material obtained by maintaining the second mixed powder containing a LiNi composite oxide having a residual Li mass content of 1000 ppm or more and a compound of an alkaline earth metal M within a predetermined temperature range for a predetermined time, all exhibited lower initial resistance and higher heat resistance than the batteries B1 to B7 of the comparative examples.

[0112] 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.

[0113] 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 powder mixture comprising: a first mixing step of mixing a Li compound and a Ni compound to obtain a first mixed powder; a firing step of firing the first 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 drying step of drying the hydrous cake to obtain a powder of the LiNi composite oxide; a second mixing step of mixing the powder of the LiNi composite oxide with a compound of an alkaline earth metal M to obtain a second mixed powder; and a heat treatment step of heating the second mixed 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 the mass content of residual Li in the powder of the LiNi composite oxide after the drying step is 1000 ppm or more, In the heat treatment step, the second mixed powder is maintained at a temperature of 250° C. or higher and lower than 350° C. for 2 hours or longer.

2. 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 0.1 μm or more and less than 5 μm.

3. A method for producing a positive electrode active material for a secondary battery as described in claim 1, wherein in the second mixed powder, 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.

4. 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.

5. 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.

6. 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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