Method for manufacturing positive-electrode active material for secondary battery

Abstract

Provided is a method for manufacturing a positive-electrode active material for a secondary battery, the method comprising a step for mixing a Li compound, a Ni compound, and a compound of an alkaline earth metal M, firing the resulting mixed powder to obtain a LiNiM composite oxide, mixing the LiNiM composite oxide with water to wash the LiNiM composite oxide, separating a water-containing cake of the LiNiM composite oxide from the resulting slurry after the washing, and drying the water-containing cake to obtain a positive-electrode active material comprising the LiNiM composite oxide, the compound of the alkaline earth metal M containing at least one selected from the group consisting of Ca compounds and Sr compounds, the firing step including first firing in which the mixed powder is retained at a temperature of 400-650°C for 30 minutes or more and second firing in which the mixed powder is retained at a temperature of 700°C or higher for one hour or more after the first firing, and the mass content of residual Li in the LiNiM composite oxide after the drying step being less than 1000 ppm.

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. 2024-013464, 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 the lithium-nickel composite oxide, there is still room for improvement. In particular, if the Ni content in the composite oxide is increased in order to increase capacity, cycle characteristics may be reduced.

[0006] One aspect of the present disclosure provides a method for producing a powder of the LiNiM composite oxide, the method comprising: (i) a mixing step of mixing a Li compound, a Ni compound, and a compound of an alkaline earth metal M to obtain a mixed powder; (ii) a firing step of firing the mixed powder to obtain a LiNiM composite oxide; (iii) a water washing step of mixing the LiNiM composite oxide with water to obtain a slurry, stirring the slurry, and washing the LiNiM composite oxide with water; (iv) a separation step of separating a water-containing cake of the LiNiM composite oxide from the slurry; and (v) a drying step of drying the water-containing cake to obtain a powder of the LiNiM composite oxide, the compound of alkaline earth metal M includes at least one selected from the group consisting of a Ca compound and a Sr compound; the firing step includes a first firing and a second firing, the first firing being a process of holding the mixed powder at a temperature of 400°C or higher and 650°C or lower for 30 minutes or longer, and the second firing being a process of holding the fired product after the first firing at a temperature of 700°C or higher for 1 hour or longer; and the mass content of residual Li in the LiNiM composite oxide after the drying step is less than 1000 ppm.

[0007] According to the present disclosure, a highly durable positive electrode active material capable of providing a secondary battery with excellent cycle characteristics 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] In order to improve the performance of secondary batteries, 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, the rate of increase in resistance with charge / discharge cycles increases, making it 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, such as increasing the amount of gas generated by decomposition of the electrolyte during charge-discharge cycles.

[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 cycle characteristics by maximizing the effect of a predetermined protective component while reducing the side effects of the protective component.

[0016] Specifically, the manufacturing method M includes a mixing step, a firing step, a water washing step, a separation step, and a drying step. Each step will be described in detail below.

[0017] (i) Mixing Step The mixing step is a step of mixing a Li compound, a Ni compound, and a compound of an alkaline earth metal M to obtain a mixed powder. The Li compound, the Ni compound, and the compound of the alkaline earth metal M may each be in the form of a powder. The step of obtaining a mixed powder may be a step of dry-mixing at least three types of powders, i.e., the Li compound, the Ni compound, and the compound of the alkaline earth metal M, in 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, a mixture of a liquid medium, a Li compound, a Ni compound, and a compound of an alkaline earth metal M 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 LiNiM composite oxide (e.g., space group R-3m) having a layered rock salt structure 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 LiNiM 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 LiNiM 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 LiNiM 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 LiNiM 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 LiNiM composite oxide.

[0024] From the viewpoint of increasing the capacity of the LiNiM 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 the volumetric particle size distribution of 2 μm or more and 20 μm or less. Such Ni compound has high reactivity with LiOH and is advantageous for improving the crystallinity of the LiNiM composite oxide.

[0031] (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 LiNiM composite oxide and are unevenly distributed in the surface layer of the primary or secondary particles of the LiNiM composite oxide, acting as protective components that protect the positive electrode active material made of the LiNiM composite oxide. Ca and Sr also have excellent affinity with other components of the LiNiM composite oxide and are thought to easily adhere to the surface of the primary particles. Ca and Sr present in the surface layer of the primary or secondary particles of the LiNiM composite oxide effectively act as protective components for the positive electrode active material. In other words, Ca and Sr suppress the rate of increase in resistance (suppress deterioration) of the positive electrode active material, thereby contributing to improving the cycle characteristics of the secondary battery. Furthermore, Ca and Sr exist stably inside the secondary battery and are less likely to cause side effects such as gas generation.

[0032] Ca compounds include CaCO3 and Ca(OH) ２ , CaO, Ca(NO ３ ) ２ , CaSO ４ , CaH ２ , CaF ２ Among them, Ca(OH) ２ etc. are preferred.

[0033] Sr compounds include SrCO3 and Sr(OH) ２ , SrO, Sr(NO ３ ) ２ , SrSO ４ , SrH ２ Among them, Sr(OH) ２ etc. are preferred.

[0034] 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.

[0035] In the mixed powder, the amount of the alkaline earth metal M compound may be an amount sufficient to cover the surface layer of the Ni compound to some extent. For example, when a LiNiM composite oxide (e.g., space group R-3m) having a layered rock salt structure is synthesized as the positive electrode active material, the ratio of the total number of moles of Ca atoms and Sr atoms contained in the alkaline earth metal M compound to the number of moles of all metal atoms contained in the Ni compound may be 0.1 mol% or more and 4 mol% or less, or 0.2 mol% or more and 1.0 mol% or less. By using a sufficient amount of the 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 LiNiM composite oxide can be protected by a protective component containing Ca and Sr. Furthermore, it becomes easy to sufficiently remove excess alkaline earth metal M compound by washing.

[0036] The median diameter (D50) in the volumetric particle size distribution of the alkaline earth metal M compound may be, for example, 1 μm or more and 50 μm or less. The particle size distribution may be controlled within this range by appropriate pulverization or classification. Such an alkaline earth metal M compound is highly reactive with the surface layer of the LiNiM composite oxide.

[0037] (ii) Calcination Step The calcination step is a step in which the mixed powder (powder mixture of the Li compound, Ni compound, and alkaline earth metal M compound) is calcined to obtain a LiNiM composite oxide.

[0038] The firing step includes a first firing and a second firing. That is, the firing of the mixed powder includes 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.

[0039] (First Firing) The first firing is a process in which the mixed powder is held at a temperature of 400°C to 650°C for 30 minutes or more. At a temperature of 400°C to 650°C, the alkaline earth metal M compound melts and penetrates into the secondary particles of the Ni compound. A portion of the alkaline earth metal M may form a LiNiM composite oxide. The temperature of the first firing is preferably above the melting points of both the Ca compound and the Sr compound. As a result, the surfaces of the primary particles of the Ni compound are more likely to be protected by a protective component containing Ca and Sr. Furthermore, 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, the reaction between the primary particles of the Ni compound and LiOH is more likely to be promoted in the subsequent second firing.

[0040] The temperature of the first firing is preferably a temperature at which the reaction between the Li compound and the Ni compound proceeds as little as possible. By holding the firing object containing the Ni compound at such a temperature for a fixed time of 30 minutes or more, the Li compound, the Ni compound, the Ca compound, and the Sr compound become sufficiently compatible with each other, and the reaction in the subsequent second firing tends to proceed uniformly.

[0041] (Second Firing) The second firing is a process in which the mixed powder (first fired product) after the first firing is held at a temperature of 700°C or higher for at least 1 hour. 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 LiNiM composite oxide. The second firing causes crystals of the LiNiM composite oxide to grow in the presence of the Li compound and the Ca and Sr compounds. As a result, the surface layer of the primary particles deep inside the secondary particles can be protected by the protective component containing Ca and Sr. 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 LiNiM composite oxide may be deteriorated. The second firing temperature may be 1000°C or lower, or 900°C or lower.

[0042] The atmosphere for the first firing and the second firing may be, for example, an oxidizing atmosphere.

[0043] (iii) Water-washing step: The water-washing step is a step of mixing the LiNiM composite oxide with water to obtain a slurry, and stirring the slurry to wash the LiNiM composite oxide with water. The water-washing step is carried out until the mass content of residual Li in the LiNiM composite oxide after the subsequent drying step becomes less than 1000 ppm, or even 810 ppm or less.

[0044] As mentioned above, it is preferable that the amount of Li compound in the mixed powder is in excess of the amount of Ni compound. Furthermore, the amount of alkaline earth metal M compound in the mixed powder should be sufficient to cover the surface layer of the Ni compound to some extent. Therefore, it is necessary to wash the powder with water appropriately to remove unreacted Li, Ca, or Sr to a certain extent. Residual Li and residual Ca or residual Sr can cause gas generation and side reactions in the secondary battery, resulting in a decrease in capacity.

[0045] In other words, the residual Li refers to the Li element that is not incorporated into the crystal structure of the LiNiM composite oxide and remains as a Li compound other than the LiNiM composite oxide. The residual Li is present in the LiNiM composite oxide in the form of, for example, lithium carbonate, lithium oxide, or lithium hydroxide.

[0046] Residual Ca refers to Ca element remaining on the surface layer of the LiNiM composite oxide without stably adhering as a protective component, and residual Sr refers to Sr element remaining on the surface layer of the LiNiM composite oxide without stably adhering as a protective component.

[0047] Here, when the LiNiM composite oxide is washed with water until the mass content of residual Li in the LiNiM composite oxide after the subsequent drying step is less than 1000 ppm, the excess Li, Ca, or Sr remaining in the LiNiM composite oxide is sufficiently removed. Therefore, it is believed that the protective components containing Ca and Sr are present in the LiNiM composite oxide in roughly the right amount. Therefore, during charge / discharge cycles, an increase in the amount of gas generated due to decomposition of the electrolyte caused by the protective components is suppressed. As a result, a secondary battery with excellent cycle characteristics can be obtained.

[0048] 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.

[0049] 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 LiNiM 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 LiNiM composite oxide in the water. The content of the LiNiM composite oxide in the slurry (slurry concentration) may be, for example, 200 g / L or more and 2000 g / L or less, or 500 g / L or more and 1500 g / L or less. That is, the content of the LiNiM composite oxide contained in 1 L (liter) of slurry may be 200 g to 2000 g or 500 g to 1500 g.

[0050] In the water washing step, for example, it is preferable to stir the slurry containing the LiNiM composite oxide and water for 5 minutes or more, and even 15 minutes or more. This makes it possible to reduce the mass content of residual Li in the LiNiM composite oxide after the subsequent drying step to less than 1000 ppm. In this case, excess residual Ca or residual Sr in the LiNiM composite oxide is also removed to a sufficiently low level.

[0051] (iv) Separation Step The separation step is a step of separating a hydrous cake of the LiNiM composite oxide from the slurry. The hydrous cake of the LiNiM composite oxide refers to LiNiM composite oxide from which most of the water has been removed but which still contains sufficient moisture and is in a solid state rather than a powder state. The moisture content of the hydrous cake of the LiNiM composite oxide may be, for example, 3% by mass or more and 10% by mass or less.

[0052] The moisture content of the wet cake can be determined from the difference in mass of the LiNiM composite oxide before and after the subsequent drying step (the difference between the mass W1 of the wet cake before drying and the mass W2 of the LiNiM composite oxide powder after drying) (moisture content (%)=100×(W1−W2) / W1).

[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) Drying Step The drying step is a step of drying the water-containing cake to obtain a positive electrode active material made of a LiNiM composite oxide. The drying conditions are not particularly limited.

[0055] In the drying step, the LiNiM 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 LiNiM composite oxide after the drying step is in the form of powder. In other words, in the drying step, the LiNiM composite oxide may be dried until it changes from a water-containing cake state into a powder state having fluidity.

[0056] By the above process, secondary particles formed by aggregation of primary particles of the LiNiM composite oxide can be obtained. The median diameter (D50) of the volume-based particle size distribution of the secondary particles of the LiNiM composite oxide is, for example, 2 μm or more and 20 μm or less.

[0057] The mass content of residual Li in the LiNiM composite oxide after the drying process is reduced to less than 1000 ppm. Furthermore, as a result of performing the cleaning process until the mass content of residual Li is reduced to less than 1000 ppm, residual Ca or residual Sr is also sufficiently removed. Therefore, when the LiNiM composite oxide after the drying process is used as a positive electrode active material, the side effects of Ca and Sr are significantly limited, while the effects of the protective components containing Ca and Sr are significantly manifested. In other words, a positive electrode active material having high capacity and excellent durability can be obtained.

[0058] The mass content of residual Li in the LiNiM composite oxide after the drying step can be determined by neutralization titration. For example, 1.0 g of a sample of the LiNiM composite oxide after the drying step is shaken and dispersed in 30 mL of ion-exchanged water at 25°C, the alkaline components containing water-soluble residual Li are eluted into the ion-exchanged water, and the LiNiM composite oxide is filtered from the aqueous solution containing the strong alkaline components, and the filtrate obtained is titrated. ２ The 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 LiNiM composite oxide is calculated by converting the mass of the LiNiM composite oxide into the mass of Li.

[0059] An example of a secondary battery using a positive electrode active material made of a LiNiM composite oxide obtained by the above-described manufacturing method will now be described.

[0060] The secondary battery includes a positive electrode containing a positive electrode active material made of at least a LiNiM composite oxide, a negative electrode, an electrolyte, and a separator interposed between the positive electrode and the negative electrode.

[0061] 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.

[0062] Examples of the material for the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0063] Examples of the binder include polytetrafluoroethylene and polyvinylidene fluoride.

[0064] Examples of the conductive agent include graphite such as natural graphite and artificial graphite, and carbon blacks such as acetylene black.

[0065] 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.

[0066] Examples of the material for the negative electrode current collector include copper, copper alloy, nickel alloy, nickel alloy, and stainless steel.

[0067] As the binder, in addition to the materials exemplified for the positive electrode, styrene butadiene rubber or the like can be used.

[0068] As the thickener, for example, carboxymethyl cellulose (CMC) and its modified products (including salts such as Na salt) can be used.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] (Additional Notes) The above description discloses the following technology: (Technology 1) A cathode active material comprising: a mixing step of mixing a Li compound, a Ni compound, and a compound of an alkaline earth metal M to obtain a mixed powder; a firing step of firing the mixed powder to obtain a LiNiM composite oxide; a water washing step of mixing the LiNiM composite oxide with water to obtain a slurry and stirring the slurry to wash the LiNiM composite oxide with water; a separation step of separating a water-containing cake of the LiNiM composite oxide from the slurry; and a drying step of drying the water-containing cake to obtain a cathode active material made of the LiNiM composite oxide, wherein the compound of the alkaline earth metal M includes at least one selected from the group consisting of a Ca compound and a Sr compound; the firing step includes a first firing and a second firing, and the first firing is a process of holding the mixed powder at a temperature of 400°C or higher and 650°C or lower for 30 minutes or longer, The method for producing a positive electrode active material for a secondary battery according to Technique 1, wherein the second firing is a process of holding the fired product after the first firing at a temperature of 700°C or higher for one hour or more, and the mass content of residual Li in the LiNiM composite oxide after the drying step is less than 1000 ppm. (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 1 μm or more and 50 μm or less 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 the water-washing step includes stirring the slurry for five minutes or more. (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 in the 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 contained in the Ni compound is 0.1 mol % or more and 4 mol % 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 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 6) The method for producing a positive electrode active material for a secondary battery according to any one of Technologies 1 to 5, wherein the amount of the Li compound in the mixed powder is in excess of the amount of the Ni compound.(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 mixed powder, the amount of the compound of the alkaline earth metal M is an amount sufficient to cover a surface layer of 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.

[0077] 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.

[0078] Example 1 (1) Mixing Step A Li compound, LiOH, a Ni oxide containing a metal element M1 (Co, Mn), and a Ca compound (Ca(OH) ２ ) and Sr compounds (Sr(OH) ２ ) were dry mixed to obtain a mixed powder.

[0079] LiOH was prepared by heating and drying lithium hydroxide monohydrate (median diameter (D50) 1.0 μm) at 150° C. for 1 hour.

[0080] 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).

[0081] Ca compound (Ca(OH) ２ ) and Sr compounds (Sr(OH) ２ ) have median diameters (D50) of 10 μm and 5 μm, respectively.

[0082] In the mixed powder, the ratio of the total number of moles of Ca atoms and Sr atoms to the number of moles of all metal atoms contained in the Ni oxide was 0.4 mol %.

[0083] 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.

[0084] (2) Firing Step (First Firing) The mixed powder packed in the sagger was fired at 550° C. for 3 hours in an oxidizing atmosphere to obtain a first fired body.

[0085] (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 (LiNiM composite oxide).

[0086] (3) Water-washing step: The second fired product (LiNiM composite oxide) was mixed with water in a washing tank to obtain a slurry, which was then stirred for a predetermined time to wash the LiNiM composite oxide. The content of LiNiM composite oxide in the slurry (slurry concentration) was 1000 g / L. Note that because the slurry concentration was sufficiently low, conditions such as the stirring speed did not affect the degree of washing.

[0087] (4) Separation Step After the water washing step, a water-containing cake of LiNiM composite oxide was separated from the slurry using a filter press.

[0088] (5) Drying step The water-containing cake is dried to obtain a LiNiM composite oxide (LiNi ０．９０ Co ０．０５ Mn ０．０５ O ２ The mass content of residual Li in the LiNiM composite oxide after the drying step was determined by the method described above and was found to be 780 ppm. Note that conditions such as the stirring speed do not affect the degree of washing, so the mass content of residual Li depends on the slurry concentration and washing time.

[0089] Examples 2 to 5, Comparative Examples 1 to 3 Positive electrode active materials were synthesized in the same manner as in Example 1, except that the production conditions were changed as shown in Table 1.

[0090] In Table 1, A1 to A5 are the positive electrode active materials of Examples 1 to 5, respectively, and B1 to B3 are the positive electrode active materials of Comparative Examples 1 to 3, respectively. ０．９０ Co ０．０５ Mn ０．０５Ni instead of O ０．９０ Co ０．０５ Al ０．０５ O ２ In A1 to A5 and B1 to B3, the mass content of residual Li was changed by changing the stirring conditions and water washing conditions when preparing the mixed powder. In B1, no Ca compound or Sr compound was mixed into the mixed powder. In B3, a one-stage firing process was used, in which firing was performed at 750°C from the beginning, without performing the two-stage firing process.

[0091]

[0092] (Preparation of Secondary Battery) [Preparation of Positive Electrode] Positive electrodes were prepared using positive electrode active materials A1 to A5 and B1 to B3 in the following manner. Specifically, N-methyl-2-pyrrolidone (NMP) was added to a positive electrode mixture containing the positive electrode active material, acetylene black, and polyvinylidene fluoride (PVDF) in a mass ratio of 95:2.5:2.5, and the mixture was stirred to prepare a positive electrode slurry. The positive electrode slurry was applied to the surface of aluminum foil, which was a positive electrode current collector, and the coating was dried to form an unrolled layer on both sides of the aluminum foil. Next, the unrolled layer was rolled to obtain a positive electrode active material having a density of 3.6 g / cm. ３ The total thickness of the positive electrode after rolling was 160 μm.

[0093] [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.

[0094] [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.

[0095] [Secondary Battery Fabrication] An electrode group was fabricated by attaching tabs to each electrode and spirally winding the positive and negative electrodes with a separator (14 μm thick) interposed between them so that the tabs were positioned at the outermost periphery. The separator had a 10 μm-thick microporous polyethylene substrate layer and a 4 μm-thick heat-resistant layer laminated on one side (the positive electrode side) of the substrate layer. The heat-resistant layer contained 20% by mass of inorganic oxide filler (alumina) and the remainder was composed of aromatic polyamide (aramid). The electrode group was inserted into an exterior case made of aluminum laminate film and vacuum-dried at 105°C for 2 hours. After that, an electrolyte solution was injected, and the opening of the exterior case was sealed to obtain a secondary battery. Hereinafter, batteries using positive electrode active materials A1 to A5 and B1 to B3 will be referred to as secondary batteries A1 to A5 and B1 to B3, respectively.

[0096] (Evaluation) [Initial Capacity] In a 25°C environment, the secondary battery was subjected to constant current charging at a current of 0.5 C until the voltage reached 4.2 V, and then constant voltage charging at a voltage of 4.2 V until the current reached 0.05 C. After a 10-minute break, the secondary battery was subjected to constant current discharging at a current of 0.7 C until the voltage reached 2.5 V. The discharge capacity (Ci) at this time was determined as the initial capacity.

[0097] [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%.

[0098] 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.

[0099] [Capacity Retention Rate (Durability)] The cycle of charge, rest, and discharge used to determine the discharge capacity Ci was counted as one cycle, and the discharge capacity (Cc) was calculated after 300 cycles. The capacity retention rate was calculated as the ratio (%) of the discharge capacity Cc to the initial discharge capacity Ci, which was taken as 100%.

[0100] [Rate of Resistance Increase] After 300 cycles of charge, rest, and discharge for determining the discharge capacity Ci, the DCIR(Rc) was calculated in the same manner as above. The ratio (%) of the difference (ΔR) between the initial DCIR(Ri) and DCIR(Rc) to the initial DCIR(Ri) was calculated as the rate of resistance increase.

[0101] Table 2 shows the relative values ​​of the capacity retention rate and the resistance increase rate of each battery, where the capacity retention rate and the resistance increase rate of secondary battery B1 of Comparative Example 1 are set to 100. The larger the capacity retention rate, the better the value, and the smaller the resistance increase rate, the better the value.

[0102]

[0103] As shown in Table 2, the batteries A1 to A5 of the examples, which used positive electrode active materials containing a protective component containing Ca and Sr and which were subjected to a first firing in which the mixed powder was held at a temperature of 400°C to 650°C for 30 minutes or more and a second firing in which the fired product after the first firing was held at a temperature of 700°C or higher for 1 hour or more, and in which the mass content of residual Li in the LiNiM composite oxide after the drying step was controlled to less than 1000 ppm, all exhibited higher capacity retention rates (durability) and lower resistance increase rates than the batteries B1 to B3 of the comparative examples.

[0104] 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.

[0105] 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 powdered LiNiM composite oxide, comprising: a mixing step of mixing a Li compound, a Ni compound, and a compound of an alkaline earth metal M to obtain a mixed powder; a firing step of firing the mixed powder to obtain a LiNiM composite oxide; a water washing step of mixing the LiNiM composite oxide with water to obtain a slurry and stirring the slurry to wash the LiNiM composite oxide; a separation step of separating a hydrous cake of the LiNiM composite oxide from the slurry; and a drying step of drying the hydrous cake to obtain a powdered LiNiM composite oxide, wherein the compound of the alkaline earth metal M comprises at least one compound selected from the group consisting of a Ca compound and a Sr compound; the firing step comprises a first firing and a second firing, and the first firing is a process of holding the mixed powder at a temperature of 400°C or higher and 650°C or lower for 30 minutes or longer; the second firing is a process of holding the fired product after the first firing at a temperature of 700°C or higher for one hour or longer, and the mass content of residual Li in the LiNiM composite oxide after the drying step is less than 1000 ppm.

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 1 μm or more and 50 μm or less.

3. The method for producing a positive electrode active material for a secondary battery according to claim 1, wherein the water washing step includes stirring the slurry for 5 minutes or more.

4. The method for producing a positive electrode active material for a secondary battery according to claim 1, wherein in the mixed powder, 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 contained in the Ni compound is 0.1 mol % or more and 4 mol % or less.

5. 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.

6. 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 mixed powder is in excess of the amount of the Ni compound.

7. 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.

Images