Production method for positive electrode active material

Abstract

A production method for a positive electrode active material according to the present invention involves producing an Na-doped precursor that comprises an oxide that includes Na and has a median diameter of no more than 25 μm on a volume-based particle size distribution, mixing the Na-doped precursor and an Li ion source that includes Li such that the molar ratio of Li to Na is 1.05–3, and then heating the resulting mixture and replacing the Na in the Na-doped precursor with Li by ion exchange to produce a positive electrode active material.

Description

Method for producing positive electrode active material

[0001] The present invention relates to a method for producing a positive electrode active material.

[0002] Non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries have excellent properties, such as high electromotive force and high energy density, despite their small size and light weight. Utilizing these excellent properties, non-aqueous electrolyte secondary batteries are used in a wide range of applications, including small electronic devices such as mobile phones and laptop computers, and large electric drive systems such as electric vehicles and hybrid vehicles.

[0003] The positive electrode active material of the non-aqueous electrolyte secondary battery is Li 0.8 Mn 0.5 Co 0.5 Oxides of Li, such as O, may be used. This type of positive electrode active material is produced by preparing a Na-doped precursor made of an oxide containing Na, and then substituting the Na in the Na-doped precursor with Li by ion exchange.

[0004] For example, Patent Document 1 describes a method for producing a positive electrode active material, which includes a Na-doped precursor preparation step for producing a sodium-containing transition metal oxide having a P2 structure belonging to the space group P63 / mmc, and an ion exchange step for substituting at least a portion of the sodium contained in the sodium-containing transition metal oxide with lithium by an ion exchange method, in which at least lithium iodide is used as a Li ion source in the ion exchange step.

[0005] Japanese Patent Application Laid-Open No. 2021-68555

[0006] In the method for producing a positive electrode active material described in Patent Document 1, a Li ion source containing Li in an amount approximately 10 times the molar ratio of the Na content in the Na-doped precursor is used in order to reduce the amount of Na remaining in the positive electrode active material after ion exchange. Thus, the use of a Li ion source in large excess relative to the Na-doped precursor results in the problem of generating a large amount of waste material containing Li after ion exchange.

[0007] On the other hand, to reduce the amount of waste containing Li, a simple method is to reduce the amount of Li ion source used in ion exchange. However, if the amount of Li ion source used in ion exchange is reduced, Na is likely to remain in the positive electrode active material after ion exchange. Therefore, in this case, it is necessary to sufficiently perform ion exchange between Na and Li by methods such as increasing the heating temperature or lengthening the heating time during ion exchange. Therefore, simply reducing the amount of Li ion source may result in an increase in the energy consumed in ion exchange.

[0008] The present invention has been made in view of the above background, and aims to provide a method for producing a positive electrode active material that can reduce the amount of Na remaining in the positive electrode active material even when the amount of Li ion source used is reduced and the energy consumed in ion exchange is reduced.

[0009] One aspect of the present invention is a method for producing a positive electrode active material for use in a non-aqueous electrolyte secondary battery, comprising: preparing a Na-doped precursor made of an oxide containing Na (sodium), the Na-doped precursor having a median diameter of 25 μm or less in a volume-based particle size distribution; mixing the Na-doped precursor with a Li ion source containing Li (lithium) so that the molar ratio of Li to Na, Li / Na, is 1.05 or more and 3 or less; and heating the mixture obtained by the mixing to replace Na in the Na-doped precursor with Li by ion exchange, thereby producing a positive electrode active material.

[0010] In the method for producing a positive electrode active material, ion exchange is performed using a Na-doped precursor having a median diameter in a volumetric particle size distribution within the specific range. By setting the median diameter of the Na-doped precursor within the specific range, Na in the Na-doped precursor can be easily replaced with Li during ion exchange. As a result, even if the amount of Li ion source used is reduced and the energy consumed in the ion exchange is reduced, the ion exchange between Na and Li can be sufficiently performed, and the amount of Na remaining in the positive electrode active material can be reduced.

[0011] Therefore, according to the above-described aspect, it is possible to provide a method for producing a positive electrode active material that can reduce the amount of Na remaining in the positive electrode active material even when the amount of Li ion source used is reduced and the energy consumed in ion exchange is reduced.

[0012] 1 is an explanatory diagram showing X-ray diffraction patterns of active materials A1, A3, and A4 in Example 1. FIG. 2 is a development view showing the internal structure of the evaluation secondary battery in Example 1.

[0013] (Method of Manufacturing Positive Electrode Active Material) In the method of manufacturing the positive electrode active material, first, the median diameter (i.e., D 50 A Na-doped precursor having a median diameter of 25 μm or less is prepared. By setting the median diameter of the Na-doped precursor to 25 μm or less, the amount of Li ion source used in the ion exchange can be reduced, and even if energy consumption is reduced, ion exchange between Na and Li can be sufficiently performed. From the viewpoint of further enhancing this effect, the median diameter of the Na-doped precursor in the volume-based particle size distribution is preferably 18 μm or less, more preferably 14 μm or less, even more preferably 10 μm or less, and particularly preferably 6 μm or less.

[0014] The volumetric particle size distribution can be measured using a laser diffraction / scattering particle size distribution analyzer (e.g., Partica (registered trademark) LA-960V2 manufactured by Horiba, Ltd.). The particle size at which the cumulative frequency of particles with small particle sizes reaches 50% in the volumetric particle size distribution obtained using the laser diffraction / scattering particle size distribution analyzer (i.e., the cumulative 50% particle size) is defined as the median diameter of the Na-doped precursor.

[0015] If the median diameter of the Na-doped precursor is larger than 25 μm, when the amount of Li ion source used is reduced or when the energy consumed in the ion exchange is reduced, the ion exchange between Na and Li may become insufficient, and the amount of Na remaining in the positive electrode active material may increase.

[0016] The lower limit of the median diameter of the Na-doped precursor is not particularly limited from the viewpoint of obtaining the above-mentioned effects, but from the viewpoint of ease of handling during electrode assembly and battery characteristics, the median diameter of the Na-doped precursor is preferably 1 μm or more, more preferably 2 μm or more, and even more preferably 3 μm or more.

[0017] The preferred range of the median diameter of the Na-doped precursor can be any combination of the upper and lower limits of the median diameter described above. For example, the preferred range of the median diameter of the Na-doped precursor in the volumetric particle size distribution may be 1 μm to 25 μm, 1 μm to 18 μm, 2 μm to 18 μm, 2 μm to 14 μm, 3 μm to 10 μm, or 3 μm to 6 μm.

[0018] The reason why the amount of Na remaining in the positive electrode active material can be reduced by setting the median diameter of the Na-doped precursor within the above-mentioned specific range is not necessarily clear, but the following reason is thought to be the case. That is, during ion exchange, Li ions generated from the Li ion source diffuse into the Na-doped precursor according to the concentration gradient and are replaced with Na ions in the Na-doped precursor. Therefore, by reducing the median diameter of the Na-doped precursor, it is thought that the migration distance of Li ions from the surface to the interior of the Na-doped precursor can be shortened. As a result, it is thought that ion exchange between Na and Li can be performed efficiently.

[0019] The composition and crystal structure of the Na-doped precursor are not particularly limited as long as they have a composition and crystal structure corresponding to the composition of the desired positive electrode active material. More specifically, the Na-doped precursor may have a composition in which all Li in the composition of the desired positive electrode active material is replaced with Na.

[0020] For example, the Na-doped precursor Na (a+b) Ni x Co y Mn z M (1-x-y-z) O (2±α)(wherein M in the composition formula represents at least one additive element selected from the group consisting of Al (aluminum), Mg (magnesium), Ti (titanium), Sn (tin), Zr (zirconium), Nb (niobium), W (tungsten), B (boron), Cr (chromium), Mo (molybdenum), and V (vanadium), and a, b, x to z, and α satisfy the following: 0.6≦a≦1.33, 0<b<0.015, 0.1≦x≦0.3, 0≦y≦0.4, 0.3≦z≦0.9, 0≦α≦0.3). The crystal structure of the main phase of the Na-doped precursor may be a P2-type layered structure that can be assigned to the space group P63 / mmc. By performing ion exchange on a Na-doped precursor having such a composition and crystal structure of the main phase, Li, a Na b Ni x Co y Mn z M (1-x-y-z) O (2±α) (wherein M in the composition formula represents at least one added element selected from the group consisting of Al, Mg, Ti, Sn, Zr, Nb, W, B, Cr, Mo, and V, and a, b, x to z, and α satisfy 0.6≦a≦1.33, 0<b<0.015, 0.1≦x≦0.3, 0≦y≦0.4, 0.3≦z≦0.9, and 0≦α≦0.3), and a positive electrode active material can be easily obtained in which the crystal structure of the main phase is an O2-type layered structure that can be assigned to the space group P63mc.

[0021] The aforementioned "main phase" refers to the crystalline phase with the highest composition ratio among the crystalline phases contained in the Na-doped precursor or the positive electrode active material. The composition ratio of the crystalline phase contained in the Na-doped precursor or the positive electrode active material can be calculated based on the X-ray diffraction pattern obtained by powder X-ray diffraction. More specifically, by performing Rietveld analysis based on the X-ray diffraction pattern obtained by powder X-ray diffraction, the type of crystalline phase in the Na-doped precursor or the positive electrode active material can be identified and the composition ratio of each crystalline phase can be calculated. In powder X-ray diffraction, for example, measurements can be performed under conditions such as a diffraction angle 2θ scanning range of 10° to 80°, a scanning speed of 2° / min, a sampling width of 0.02°, and a CuKα X-ray source.

[0022] The method for producing the Na-doped precursor can take various forms. For example, a solid-phase method can be used to produce the Na-doped precursor by utilizing solid-phase diffusion. In the solid-phase method, first, multiple types of raw materials containing metal elements from among the elements constituting the Na-doped precursor are prepared, and these raw materials are mixed in a ratio corresponding to the desired composition of the positive electrode active material. The raw materials may be metal salts or metal oxides. Furthermore, the raw materials may contain one type of metal element or two or more types of metal elements.

[0023] After preparing the mixture, the mixture is fired in an oxidizing gas atmosphere to mutually diffuse the metal atoms in the raw materials. This allows a Na-doped precursor to be obtained. The oxidizing gas can be, for example, air. The firing temperature can be appropriately set within a range of 700°C to 1100°C. The holding time during firing can be appropriately set within a range of 0.5 hours to 50 hours.

[0024] Alternatively, a method for producing a Na-doped precursor may be employed in which a co-precipitation precursor containing two or more metal elements from among the metal elements contained in the Na-doped precursor is prepared by a co-precipitation method, a mixture containing the co-precipitation precursor and a Na source is prepared, and the mixture is calcined to obtain the Na-doped precursor. By preparing a co-precipitation precursor containing two or more metal elements by a co-precipitation method, the particle size distribution of the co-precipitation precursor can be more easily adjusted to within a desired range.

[0025] In the coprecipitation method, a raw material solution containing ions of one or more metal elements constituting the Na-doped precursor is dropped into a reaction solution to precipitate a coprecipitated precursor in the reaction solution. The reaction solution can be a solution having a pH that allows the metal elements in the raw material solution to precipitate. The reaction solution is preferably a neutral or alkaline solution.

[0026] The raw material solution dropped into the reaction solution may be one type or two or more types. However, depending on the combination of compounds dissolved in the raw material solution, one of the compounds may be difficult to dissolve in the raw material solution, making it difficult to adjust the ratio of metal elements in the coprecipitated precursor to the desired range. From the viewpoint of more reliably avoiding such problems, it is preferable to prepare two or more types of raw material solutions. By preparing two or more types of raw material solutions, compounds containing metal elements can be easily dissolved in each raw material solution. As a result, the ratio of metal elements in the coprecipitated precursor can be more easily adjusted to the desired range.

[0027] When two or more types of raw material solutions are prepared, for example, one of the raw material solutions may be dropped into the other raw material solution, and then the other raw material solution may be dropped into the reaction solution. Alternatively, all of the raw material solutions may be dropped into the reaction solution simultaneously. Furthermore, in addition to the raw material solutions and the reaction solution, a pH-adjusting solution for adjusting the pH of the reaction solution may be prepared, and the pH-adjusting solution may be dropped into the reaction solution together with the raw material solutions and the mixed solution to produce a coprecipitated precursor. In this case, changes in the pH of the reaction solution due to the dropping of the raw material solutions and the mixed solution can be suppressed, making it easier to produce a coprecipitated precursor having a desired composition.

[0028] The composition of the coprecipitated precursor may be appropriately determined depending on the desired composition of the Na-doped precursor. For example, the coprecipitated precursor may contain some or all of the metal elements contained in the Na-doped precursor. Furthermore, the coprecipitated precursor may or may not contain Na. When the coprecipitated precursor contains Na, the molar ratio of Na in the coprecipitated precursor is preferably lower than the molar ratio of Na in the desired Na-doped precursor. By lowering the molar ratio of Na in the coprecipitated precursor than the molar ratio of Na in the desired Na-doped precursor, the composition of the mixture of the coprecipitated precursor and the Na source can be more easily adjusted to the desired composition.

[0029] The co-precipitated precursor obtained by the co-precipitation method is in the form of a powder. The particle size of the co-precipitated precursor affects the particle size of the Na-doped precursor. Therefore, by adjusting the particle size distribution of the co-precipitated precursor, the median diameter of the Na-doped precursor on a volume basis can be easily adjusted to fall within the above-mentioned specific range.

[0030] The particle size of the coprecipitated precursor varies depending on reaction conditions such as the stirring speed and temperature of the reaction solution. For example, when the stirring speed of the reaction solution is low, the particle size of the coprecipitated precursor tends to be large. On the other hand, when the temperature of the reaction solution is low, the particle size of the coprecipitated precursor tends to be small. Therefore, by appropriately adjusting these reaction conditions, the median diameter of the Na-doped precursor can be easily adjusted to fall within the above-mentioned specific range.

[0031] After preparing the coprecipitated precursor by the coprecipitation method, a mixture of the coprecipitated precursor and a Na source is prepared. The Na source can be a sodium salt such as sodium carbonate or sodium halide. The blending ratio of the coprecipitated precursor and the Na source can be appropriately set depending on the desired composition of the Na-doped precursor. If necessary, a compound containing a metal element other than Na can be blended into the mixture, and the molar ratio of each metal element in the mixture can be adjusted to a ratio appropriate for the desired composition of the Na-doped precursor.

[0032] After preparing the mixture in this manner, the mixture is calcined under an oxidizing gas atmosphere to mutually diffuse the metal atoms in the mixture. This allows a Na-doped precursor to be obtained. The oxidizing gas can be, for example, air. The calcination temperature and holding time during calcination can be appropriately set depending on the composition and crystalline structure of the Na-doped precursor. For example, when preparing a Na-doped precursor having the specific composition and a P2-type layered crystalline structure of the main phase, the calcination temperature is preferably 700°C or higher and 1100°C or lower, and more preferably 800°C or higher and 1000°C or lower. The holding time during calcination can be appropriately set within a range of 0.5 hours to 50 hours.

[0033] When a powdered raw material is used, the Na-doped precursor is composed of numerous particles having a particle size derived from the raw material, and these particles are bonded together to form a mass. Therefore, by crushing the Na-doped precursor, a powdered Na-doped precursor can be obtained. The method for crushing the Na-doped precursor is not particularly limited, and various methods can be used, such as manual crushing in a mortar or mechanical crushing using a ball mill. Furthermore, after crushing the massed Na-doped precursor, the powdered Na-doped precursor may be classified as needed to adjust the particle size distribution of the Na-doped precursor.

[0034] In the manufacturing method, after preparing a Na-doped precursor, the Na-doped precursor is mixed with a Li ion source containing Li so that the molar ratio Li / Na of the amount of Li to the amount of Na is 1.05 or more and 3 or less. As the Li ion source, a lithium salt such as lithium chloride or lithium nitrate can be used.

[0035] The mixture obtained by mixing is then heated to perform ion exchange between Na in the Na-doped precursor and Li in the Li ion source. As described above, in the Na-doped precursor having a volumetric median diameter within the specific range, ion exchange between Na and Li is efficiently performed. Therefore, by using such a Na-doped precursor, ion exchange between Na and Li is sufficiently performed with a relatively small amount of Li ion source so that the Li / Na value is 1.05 or more and 3 or less, and a positive electrode active material with a small amount of residual Na can be easily obtained.

[0036] If the Li / Na ratio is less than 1.05, the ion exchange is likely to be insufficient due to a lack of Li. As a result, the amount of Na remaining in the positive electrode active material may increase. On the other hand, if the Li / Na ratio is more than 3, the amount of waste generated after the ion exchange may increase.

[0037] From the viewpoint of further reducing the amount of waste generated after ion exchange, the molar ratio Li / Na of the amount of Li to the amount of Na in the mixture is preferably 1.05 or more and 2.5 or less, more preferably 1.05 or more and 2 or less, and even more preferably 1.05 or more and 1.5 or less.

[0038] From the viewpoint of more reliably obtaining the effect of reducing the amount of waste generated after ion exchange, it is preferable to mix a Na-doped precursor having a composition containing Co and a median diameter of 14 μm or less in a volume-based particle size distribution with a Li ion source so that the molar ratio of Li to Na, Li / Na, is preferably 1.05 or more and 2.5 or less, more preferably 1.05 or more and 2 or less, and even more preferably 1.05 or more and 1.5 or less. (a+b) Ni x Co y Mn z M (1-x-y-z) O (2±α)(wherein M in the composition formula represents at least one additive element selected from the group consisting of Al, Mg, Ti, Sn, Zr, Nb, W, B, Cr, Mo, and V, and a, b, x to z, and α satisfy the following conditions: 0.6≦a≦1.33, 0<b<0.015, 0.1≦x≦0.3, 0<y≦0.4, 0.3≦z≦0.9, 0≦α≦0.3), and a Na-doped precursor having a median diameter of 14 μm or less in a volume-based particle size distribution is mixed with a Li ion source so that the molar ratio of Li to Na, Li / Na, is preferably 1.05 or more and 2.5 or less, more preferably 1.05 or more and 2 or less, and even more preferably 1.05 or more and 1.5 or less, thereby more reliably obtaining the effect of reducing the amount of waste generated after ion exchange.

[0039] The heating temperature of the mixture in the ion exchange may be any temperature at which the Li ion source melts. From the viewpoint of more easily obtaining a positive electrode active material having a desired crystalline phase, the heating temperature of the mixture is preferably 250° C. or higher and 375° C. or lower. Furthermore, by performing the ion exchange at such a relatively low temperature, it is expected that the energy consumed in the ion exchange can be further reduced.

[0040] From the viewpoint of sufficiently carrying out the ion exchange of Na and Li, the heating time for the ion exchange is preferably 1 hour or more, while from the viewpoint of further reducing the energy consumed in the ion exchange, the heating time for the ion exchange is preferably 4 hours or less.

[0041] The manufacturing method may include a step of washing the positive electrode active material obtained after ion exchange. Washing the positive electrode active material after ion exchange can further reduce impurities contained in the positive electrode active material, such as Na ions and anions of the Li ion source. Furthermore, the manufacturing method can easily reduce the amount of Li ion source used in ion exchange, allowing the positive electrode active material to be washed with a low content of impurities derived from the anions of the Li ion source. Therefore, the manufacturing method can further reduce the amount of water used for washing, thereby further reducing the environmental impact associated with washing.

[0042] The amount of water used for washing and the number of washings can be appropriately set depending on the productivity of the positive electrode active material and the amount of impurities contained in the positive electrode active material before washing. For example, by limiting the number of washings to one, the time required for the washing operation can be easily shortened and the productivity of the positive electrode active material can be improved. When the number of washings is one, the amount of impurities contained in the positive electrode active material after washing can be easily reduced by washing the positive electrode active material using 1.5 L to 30 L of water per 1 kg of positive electrode active material.

[0043] From the viewpoint of further reducing the amount of impurities contained in the positive electrode active material after washing, the amount of water used for washing is preferably 2.5 L or more per kg of positive electrode active material, more preferably 3.5 L or more, and even more preferably 4.5 L or more. On the other hand, from the viewpoint of further reducing the amount of water used for washing, the amount of water used for washing is preferably 20 L or less per kg of positive electrode active material, more preferably 15 L or less, even more preferably 10 L or less, and particularly preferably 5 L or less.

[0044] Furthermore, when washing is performed multiple times, the amount of water used for washing can be further reduced, thereby further reducing the environmental impact associated with washing. In this case, the amount of water used for each washing may be the same or different. From the viewpoint of workability, it is preferable that the amount of water used for each washing is the same.

[0045] When washing is performed multiple times, the amount of impurities contained in the washed positive electrode active material can be easily reduced by washing the positive electrode active material using a total of 0.8 L to 30 L of water per 1 kg of positive electrode active material. From the viewpoint of further reducing the amount of water used for washing, the total amount of water used for washing is preferably 0.8 L to 20 L, more preferably 0.8 L to 15 L, even more preferably 0.8 L to 10 L, and particularly preferably 0.8 L to 5 L, per 1 kg of positive electrode active material.

[0046] In addition, when washing is performed multiple times, washing may be performed multiple times without changing the container used for washing, or the container used for washing may be changed each time a single washing is completed. From the viewpoint of workability, it is preferable to perform washing multiple times without changing the container used for washing.

[0047] (Non-aqueous electrolyte secondary battery) The positive electrode active material obtained by the manufacturing method is configured so as to be usable in the positive electrode of a non-aqueous electrolyte secondary battery that includes a positive electrode, a negative electrode, and a non-aqueous electrolyte and uses lithium ions as a charge carrier. The non-aqueous electrolyte in a non-aqueous electrolyte secondary battery that uses the positive electrode active material may be liquid or solid. In other words, non-aqueous electrolyte secondary batteries to which the positive electrode active material can be applied include, for example, secondary batteries that use a non-aqueous electrolyte solution, such as lithium ion secondary batteries, and all-solid-state secondary batteries that use a solid electrolyte and lithium ions as a charge carrier.

[0048] The lithium ion secondary battery may include, as its main components, a positive electrode containing the positive electrode active material, a negative electrode, a separator interposed between the positive electrode and the negative electrode, a nonaqueous electrolyte, additives, and a case for accommodating these components, etc. The shape of the lithium ion secondary battery may be, for example, a coin type, a cylindrical type, a laminated type, a prismatic type, etc.

[0049] The positive electrode of a lithium ion secondary battery includes a positive electrode active material and a positive electrode current collector that holds the positive electrode active material. As the positive electrode current collector, various conductors can be used, for example, metal foils such as copper foil, aluminum foil, and nickel foil, metal meshes such as stainless steel mesh, punched metals, expanded metals, etc.

[0050] The positive electrode may contain a binder interposed between the positive electrode active material and the positive electrode current collector. Examples of the binder include fluorine-based binders such as polyvinylidene fluoride and polytetrafluoroethylene, rubber-based binders such as styrene butadiene rubber, olefin-based binders such as polypropylene and polyethylene, and cellulose-based binders such as carboxymethyl cellulose.

[0051] The positive electrode may also contain a conductive agent or conductive assistant to enhance electrical conductivity. Examples of the conductive agent include graphite, carbon black, acetylene black, and cokes.

[0052] The positive electrode can be produced, for example, by the following method. First, a paste-like positive electrode mixture containing a positive electrode active material is produced. The positive electrode mixture may contain an organic solvent for dispersing or dissolving solid components such as the positive electrode active material, as necessary. This positive electrode mixture is applied to the surface of a positive electrode current collector and then dried to form a positive electrode active material layer on the surface of the positive electrode current collector. After forming the positive electrode active material layer, the positive electrode active material layer may be pressed as necessary to increase the density of the positive electrode active material layer. A positive electrode can be obtained in this manner.

[0053] The negative electrode of a lithium ion secondary battery has a negative electrode active material and a negative electrode current collector that holds the negative electrode active material. Examples of the negative electrode active material include silicon-based active materials such as Si, Si alloys, and silicon oxide; carbon materials having a graphite structure such as graphite and hard carbon; and lithium titanate (Li4Ti5O 12 For the negative electrode current collector, the same conductors as those for the positive electrode current collector can be used.

[0054] The negative electrode may contain a binder, a conductive agent, and a conductive auxiliary agent, similar to the positive electrode. The binder, conductive agent, and conductive auxiliary agent that can be used in the negative electrode are the same as those in the positive electrode.

[0055] The method for producing the negative electrode is the same as that for the positive electrode. That is, first, a paste-like negative electrode mixture containing the negative electrode active material is produced. The negative electrode mixture may contain an organic solvent for dispersing or dissolving solid components such as the negative electrode active material, as necessary. This negative electrode mixture is applied to the surface of the negative electrode current collector and then dried to form a negative electrode active material layer on the surface of the negative electrode current collector. After forming the negative electrode active material layer, the negative electrode active material layer may be pressed as necessary to increase the density of the negative electrode active material layer. In this manner, a negative electrode can be obtained.

[0056] The non-aqueous electrolyte solution may contain an organic solvent and an electrolyte made of a lithium salt. Examples of the lithium salt include LiPF6. Examples of the organic solvent that may be used include ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, monofluoroethylene carbonate, and methyl-2,2,2-trifluoroethyl carbonate. These organic solvents may be used alone, or two or more of them may be used in combination.

[0057] The non-aqueous electrolyte preferably contains at least one organic solvent selected from the group consisting of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate. These organic solvents have high polarity and can dissolve a large amount of electrolyte. Therefore, by preparing a non-aqueous electrolyte using these organic solvents, the transport number of charge carriers in a lithium ion secondary battery can be easily increased.

[0058] The all-solid-state secondary battery may include, as its main components, a positive electrode containing the positive electrode active material, a negative electrode containing the negative electrode active material, a separator interposed between the positive electrode and the negative electrode and containing a solid electrolyte, and a case for accommodating these components.

[0059] The positive electrode of the all-solid-state secondary battery may be composed of, for example, a positive electrode active material, a solid electrolyte, and an additive such as a conductive agent, which is added as needed. In this case, from the viewpoint of further increasing the energy density of the secondary battery, the content of the positive electrode active material in the positive electrode is preferably 60% by mass or more and 99% by mass or less, and more preferably 70% by mass or more and 95% by mass or less.

[0060] Examples of solid electrolytes used in the positive electrode of all-solid-state secondary batteries include lithium lanthanum zirconate, LiPON, and Li １＋Ｘ Al Ｘ Ge ２－Ｘ (PO4) ３ , Li-SiO-based glass, Li-Al-S-O-based glass, and other oxide solid electrolytes, ２ S-P ２ S ５ , Li ２ S-SiS ２ , LiI-Li２ S-SiS ２ , LiI-Si ２ S-P ２ S ５ , Li ２ S-P ２ S ５ -LiI-LiBr, LiI-Li ２ S-P ２ S ５ , LiI-Li ２ S-P ２ O ５ , LiI-Li ３ P.O. ４ -P ２ S ５ , Li ２ S-P ２ S ５ -GeS ２ The content of the solid electrolyte in the positive electrode is preferably 1% by mass or more and 40% by mass or less, and more preferably 5% by mass or more and 30% by mass or less.

[0061] When a conductive agent and a conductive auxiliary agent are added to the positive electrode, the content of the conductive agent and the conductive auxiliary agent can be appropriately set depending on the electrical conductivity, ionic conductivity, and the desired energy density of the secondary battery. The additives used in the positive electrode of the all-solid-state secondary battery are the same as the additives used in the positive electrode of the lithium-ion secondary battery.

[0062] The negative electrode of the all-solid-state secondary battery may be composed of, for example, a negative electrode active material, a solid electrolyte, and an additive such as a conductive agent, which is added as needed. The negative electrode active material and additive used in the negative electrode of the all-solid-state secondary battery are the same as the negative electrode active material and additive used in the negative electrode of a lithium-ion secondary battery. The solid electrolyte used in the negative electrode is the same as the solid electrolyte used in the positive electrode described above.

[0063] The separator of an all-solid-state secondary battery contains a solid electrolyte. The solid electrolyte used in the separator is the same as the solid electrolyte used in the positive electrode described above. In addition to the solid electrolyte, the separator may contain a binder to bind the particles of the solid electrolyte together as needed. Examples of binders that can be used include fluorine-containing resins such as polyvinylidene fluoride, polytetrafluoroethylene, and fluororubber, and thermoplastic resins such as polypropylene, polyethylene, and polyethylene terephthalate.

[0064] 1. Example 1 An example of the method for producing the positive electrode active material will be described. In this example, first, a coprecipitated precursor was prepared by coprecipitation, and then a mixture of the coprecipitated precursor and a Na source was calcined to produce five types of Na-doped precursors (precursors N1 to N5) shown in Table 1. These Na-doped precursors were then mixed with a Li ion source in various ratios, and then ion exchange was performed to produce the positive electrode active materials (active materials A1 to A18) shown in Table 2. The detailed methods for producing the Na-doped precursors and positive electrode active materials are as follows.

[0065] (Method for Producing Na-Doped Precursors) [Precursors N1, N2, and N5] Manganese sulfate pentahydrate as the Mn source, nickel sulfate hexahydrate as the Ni source, and cobalt sulfate heptahydrate as the Co source were weighed out so that the molar ratios of Mn atoms, Ni atoms, and Co atoms were as shown in Table 1. These compounds were dissolved in distilled water to give a metal atom concentration of 2 mol / L to prepare a first raw material solution. Separately from the first raw material solution, sodium carbonate was dissolved in distilled water to give a concentration of 2 mol / L to prepare a second raw material solution. Furthermore, distilled water was prepared in a beaker as a reaction solution.

[0066] Next, the first raw material solution and the second raw material solution were simultaneously added dropwise to the reaction solution while stirring the reaction solution at the stirring speed shown in Table 1, and Mn, Ni, and Co were coprecipitated in the beaker to obtain a coprecipitated precursor. At this time, the dropping rates of the first raw material solution and the second raw material solution were adjusted so that the pH of the reaction solution would have the value shown in Table 1. The temperature of the reaction solution during dropping and the time required from the start to the end of dropping were as shown in Table 1. After dropping of the solutions was completed, stirring of the reaction solution was continued for another hour.

[0067] The coprecipitated precursor in the beaker was then filtered and washed with distilled water. The washed coprecipitated precursor was dried by heating at 100°C for 24 hours and then crushed in an agate mortar to obtain a powdery coprecipitated precursor.

[0068] Next, the coprecipitated precursor and sodium carbonate (as a Na source) were mixed in a molar ratio of coprecipitated precursor:sodium carbonate = 74:26. ​​This mixture was heated in air at 600°C for 6 hours and then calcined at 900°C for 1 hour to obtain a Na-doped precursor. The obtained Na-doped precursor was crushed into powder using an agate mortar.

[0069] The Na-doped precursors (precursors N1, N2, and N5) obtained in this manner were all composed of a crystalline phase having a P2-type layer structure. The median diameters in the volume-based particle size distributions of precursors N1, N2, and N5 were as shown in Table 1. The volume-based particle size distributions of precursors N1, N2, and N5 were measured using a laser diffraction / scattering particle size distribution analyzer ("Partica (registered trademark) LA-960V2" manufactured by Horiba, Ltd.).

[0070] [Precursors N3 and N4] The preparation method for precursors N3 and N4 was the same as that for precursors N1, N2, and N5, except that cobalt sulfate heptahydrate was not dissolved in the first raw material solution. Precursors N3 and N4 were both composed of a crystalline phase having a P2-type layered structure. The median diameters in the volume-based particle size distributions of precursors N3 and N4 were as shown in Table 1.

[0071] (Method of Preparing Positive Electrode Active Material) A Li ion source was prepared by mixing lithium nitrate and lithium chloride in a molar ratio of lithium nitrate:lithium chloride = 88:12. This Li ion source was mixed with a Na-doped precursor so that the molar ratio of Li to Na, Li / Na, was the value shown in Table 2 to prepare a mixture. The mixture was then heated in the air at the heating temperature and holding time shown in Table 2 to perform ion exchange between Na and Li. After the ion exchange was completed, the positive electrode active material was washed with distilled water, dried, and then crushed using an agate mortar to obtain the positive electrode active materials (active materials A1 to A18) shown in Table 2.

[0072] The composition and crystal structure of the positive electrode active material obtained as described above were identified by the following method.

[0073] (Composition of Positive Electrode Active Material) The molar ratio of each metal element in the positive electrode active material was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The compositions of active materials A1 to A18, assuming an oxygen molar ratio of 2, are shown in the "Composition of Positive Electrode Active Material" column in Table 2.

[0074] (Identification of Crystal Structure) The crystalline phases contained in active materials A1 to A18 were identified by powder X-ray diffraction. A "SmartLab (registered trademark)" manufactured by Rigaku Corporation was used as the X-ray diffractometer. The characteristic X-rays irradiated were CuKα rays, the X-ray tube voltage was 40 kV, and the irradiation current was 50 mA. As an example of the X-ray diffraction pattern of a positive electrode active material, the X-ray diffraction patterns of active materials A1, A3, and A4 are shown in FIG. 1. The vertical axis of FIG. 1 represents the diffraction intensity (relative intensity), and the horizontal axis represents the diffraction angle 2θ (unit: °).

[0075] Based on the obtained X-ray diffraction patterns, the crystalline phase contained in each positive electrode active material was identified, and it was confirmed that active materials A1 to A18 were all composed of a crystalline phase having an O2-type layered structure. Furthermore, as shown in FIG. 1 , active material A4, which had a Li / Na ratio smaller than the specific range and used an insufficient amount of Li ion source in the ion exchange, exhibited a reduced intensity of the diffraction peak appearing near a diffraction angle of 37° compared to active material A3, which used a sufficient amount of Li ion source. This is thought to be due to the relatively large amount of Na remaining in the positive electrode active material, causing some distortion in the O2-type layered structure. Although not shown in the figure, it was also confirmed that active materials A8 and A12, which, like active material A4, used an insufficient amount of Li ion source, also exhibited a reduced intensity of the diffraction peak appearing near a diffraction angle of 37° compared to active materials using a sufficient amount of Li ion source.

[0076] (Discharge Capacity Measurement) Next, the discharge capacity of the positive electrodes containing the active materials A1 to A18 was measured by the following method.

[0077] [Configuration and Manufacturing Method of Evaluation Secondary Battery] The configuration and manufacturing method of the evaluation secondary battery 1 used to measure the discharge capacity will be described with reference to Fig. 2. The secondary battery 1 is a CR2032 type coin battery including a positive electrode 2, a counter electrode 3, a separator 4, and a non-aqueous electrolyte solution 5.

[0078] More specifically, the secondary battery 1 has a case 11 that is a cylindrical case with a bottom and a relatively small height, and a top lid 12 that closes the opening of the case 11. A space is formed between the case 11 and the top lid 12. The top lid 12 is joined to the case 11 by crimping.

[0079] A positive electrode 2, a counter electrode 3, a separator 4, and a non-aqueous electrolyte 5 are accommodated in the space between the case 11 and the top lid 12. A rubber packing 15 is disposed between the positive electrode 2 and the separator 4. A spacer 13 and a washer 14 are provided between the top lid 12 and the positive electrode 2. The spacer 13 is disposed so as to abut against the positive electrode 2. The washer 14 is disposed between the spacer 13 and the top lid 12. Specifically, the spacer 13 in this example is a disk-shaped stainless steel plate.

[0080] The positive electrode 2 has a positive electrode current collector 21 and a positive electrode active material layer 22 provided on the positive electrode current collector 21, the positive electrode active material layer 22 containing any one of the active materials A1 to A18 shown in Table 2. The positive electrode current collector 21 in this example is specifically a disk-shaped aluminum foil. The diameter of the aluminum foil is 16 mm. The counter electrode 3 in this example is specifically a disk-shaped lithium foil. The diameter of the lithium foil is 19 mm.

[0081] The separator 4 is made of polypropylene and is interposed between the positive electrode 2 and the counter electrode 3. A nonaqueous electrolyte 5 is filled at least between the positive electrode 2 and the counter electrode 3 in the secondary battery 1. The nonaqueous electrolyte 5 in this example is prepared by dissolving LiPF6 at a concentration of 1 mol / L in a mixed solvent obtained by mixing equal volumes of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate.

[0082] Next, a method for manufacturing the secondary battery 1 will be described. First, polyvinylidene fluoride as a binder was dissolved in N-methyl-2-pyrrolidone to prepare a binder solution with a concentration of 12% by mass. 1.0 g of a positive electrode active material and 0.1178 g of carbon black as a conductive additive were added to 0.49 g of this binder solution and kneaded to obtain a positive electrode mixture. This positive electrode mixture was applied to one side of a positive electrode current collector 21 and then dried to form a positive electrode active material layer 22. Thereafter, the positive electrode active material layer 22 was pressed together with the positive electrode current collector 21 using a roll press, thereby adjusting the density of the positive electrode active material layer 22 to 2.4 to 2.8 g / cm. ３ was increased to

[0083] The pressed positive electrode active material layer 22 was punched together with the positive electrode current collector 21 into a disk shape having a diameter of 16 mm to obtain a positive electrode 2. A counter electrode 3 was also prepared by punching a lithium foil into a disk shape having a diameter of 19 mm.

[0084] Next, the counter electrode 3, separator 4, packing 15, positive electrode 2, spacer 13, and washer 14 were sequentially stacked in the case 11, and the nonaqueous electrolyte 5 was poured into the case 11. Thereafter, the top lid 12 was placed on the opening of the case 11, and the space between the case 11 and the top lid 12 was sealed by crimping. In this way, the secondary battery 1 was obtained.

[0085] [Discharge Capacity Measurement] A charge / discharge device (Bio-Logic's "BCS-815") was used to measure the discharge capacity. First, the secondary battery was charged in constant current-constant voltage mode at a temperature of 25°C. The current density in the constant current mode was 1 / 10 C, and when the voltage of the secondary battery reached 4.8 V, it was switched to constant voltage mode. In the constant voltage mode, the charging voltage was 4.8 V, and charging was continued until the current value reached 0.01 mA. After the current value in the constant voltage mode reached 0.01 mA, charging and discharging were paused for 10 minutes to stabilize the potential of the secondary battery. Thereafter, the secondary battery was discharged to 2.0 V at a constant current density of either 1 / 10 C or 1 C, and the discharge curve at this time was obtained.

[0086] Based on the discharge curves obtained by the above method, the discharge capacity at each current density was calculated, and these values ​​are listed in the "Discharge Capacity" column of Table 2. Note that "C," the unit of current density during charge and discharge, refers to the current density at which the charge rate or discharge rate theoretically reaches 100% in one hour. In other words, the secondary battery can theoretically be fully discharged by discharging at a current density of 1 C for one hour. In this example, the current density corresponding to 1 C is specifically 250 mA / g.

[0087]

[0088]

[0089] As shown in Table 2, active materials A2 and A3 were prepared by ion-exchanging precursor N1, whose volume-based particle size distribution had a median diameter within the specified range. Furthermore, in preparing active materials A2 and A3, ion-exchange was performed using a Li ion source in an amount such that the Li / Na ratio was within the specified range. Therefore, in these active materials, ion-exchange was sufficiently performed, similar to active material A1, which was ion-exchanged using a large excess of Li ion source, and the amount of residual Na in the active material was small. Furthermore, the discharge capacities of active materials A2 and A3 were comparable to those of active material A1, demonstrating good discharge capacities.

[0090] In contrast, the amount of Li ion source used in the ion exchange in active material A4 was too small, resulting in insufficient ion exchange and a higher amount of Na than in active materials A1 to A3. Furthermore, the discharge capacity of active material A4 was lower than that of active materials A1 to A3.

[0091] Comparing active materials A5 to A8 using precursor N2 and active materials A9 to A12 using precursor N3, it can be seen that, similar to active materials A1 to A4, by using a Na-doped precursor with a median diameter within the specified range, sufficient ion exchange can be achieved even when the amount of Li ion source used is reduced. Furthermore, active materials A6 and A7 obtained in this manner had discharge capacities comparable to those of active material A5, which was ion-exchanged using a large excess of Li ion source. Similarly, active materials A10 and A11 had discharge capacities comparable to those of active material A9.

[0092] In contrast, the amount of Li ion source used in the ion exchange for active material A8 was too small, resulting in insufficient ion exchange. As a result, the amount of Na remaining in active material A8 was greater than that of active materials A5 to A7, which were prepared using the same Na-doped precursor. Furthermore, the discharge capacity of active material A8 was lower than that of active materials A5 to A7. Similarly, the amount of Na remaining in active material A12 was greater than that of active materials A9 to A11, which were prepared using the same Na-doped precursor. Furthermore, the discharge capacity of active material A12 was lower than that of active materials A9 to A11.

[0093] Furthermore, a comparison of active materials A13 and A14 also shows that the use of a Na-doped precursor having a median diameter within the above-mentioned range ensures sufficient ion exchange even in active material A14, which uses a reduced amount of Li ion source. Furthermore, active material A14 has a discharge capacity comparable to that of active material A13, which was ion-exchanged using a large excess of Li ion source.

[0094] Active materials A16 to A18 were prepared using a Na-doped precursor having a median diameter within the specified range, with the amount of Li ion source used in the ion exchange within the specified range, and with a shortened retention time during the ion exchange. On the other hand, active materials A16 to A18 exhibited residual Na amounts and discharge capacities comparable to those of active material A15, which was subjected to ion exchange using a large excess of Li ion source under conditions with a longer retention time than active materials A16 to A18. Therefore, by comparing active materials A15 to A18, it can be seen that by using a Na-doped precursor having a median diameter within the specified range and with the amount of Li ion source used in the ion exchange within the specified range, sufficient ion exchange is possible even when the retention time during ion exchange is shortened and energy consumption is reduced.

[0095] 2. Example 2 This example describes the measurement of the amount of remaining impurities when a positive electrode active material was washed by various methods. In this example, first, precursor N1 was prepared by the same method as in Example 1. Next, a Li ion source was prepared by mixing lithium nitrate and lithium chloride at a molar ratio of lithium nitrate:lithium chloride = 88:12. This Li ion source and precursor N1 were mixed so that the molar ratio of Li to Na, Li / Na, was the value shown in Table 3 to prepare a mixture. Then, the mixture was heated in the air at 280°C for 4 hours to perform ion exchange between Na and Li, thereby obtaining a positive electrode active material.

[0096] Next, 5 g of the positive electrode active material thus obtained was washed with water. The number of washings and the total amount of water used for washing were as shown in Table 3. The column "Total amount of water per kg of positive electrode active material" in Table 3 shows the total amount of water used for washing converted into the amount per kg of positive electrode active material (unit: L).

[0097] After washing, the positive electrode active material was dried and then crushed using an agate mortar to obtain active materials B1 to B20 shown in Table 3. The amount of Na contained in the active material and the amount of Cl derived from the anions of the Li ion source were measured by inductively coupled plasma atomic emission spectrometry. Table 3 shows the amount of Na and the amount of Cl contained in the active material. The amount of Na and the amount of Cl shown in Table 3 are the atomic ratios relative to all elements in the active material.

[0098]

[0099] As shown in Table 3, the amounts of Na and Cl in active materials B1 to B14 and active materials B16 to B18 were similar to those in active materials B19 and B20. Therefore, from these results, it can be seen that the amount of water used to wash the active material can be easily reduced by reducing the amount of Li ion source used in the production process of the positive electrode active material.

[0100] Although the above has described aspects of the method for producing the positive electrode active material based on the examples, the specific aspects of the method for producing the positive electrode active material according to the present invention are not limited to the aspects of the examples, and the configuration can be changed as appropriate within the scope that does not impair the spirit of the present invention.

[0101] For example, the method for producing the positive electrode active material can take the following aspects [1] to [7].

[0102] [1] A method for producing a positive electrode active material for use in a non-aqueous electrolyte secondary battery, comprising: preparing a Na-doped precursor made of an oxide containing Na and having a median diameter of 25 μm or less in a volume-based particle size distribution; mixing the Na-doped precursor with a Li ion source containing Li so that the molar ratio of Li to Na, Li / Na, is 1.05 or more and 3 or less; and heating the mixture obtained by the mixing to replace Na in the Na-doped precursor with Li by ion exchange, thereby producing a positive electrode active material.

[0103] [2] The positive electrode active material is Li a Na b Ni x Co y Mn z M (1-x-y-z) O(2±α) (wherein M in the composition formula represents at least one additive element selected from the group consisting of Al, Mg, Ti, Sn, Zr, Nb, W, B, Cr, Mo, and V, and a, b, x to z, and α satisfy 0.6≦a≦1.33, 0<b<0.015, 0.1≦x≦0.3, 0≦y≦0.4, 0.3≦z≦0.9, and 0≦α≦0.3), and a crystal structure of a main phase of the cathode active material is an O2-type layered structure that can be assigned to space group P63mc.

[0104] [3] The Na-doped precursor is Na (a+b) Ni x Co y Mn z M (1-x-y-z) O (2±α) (wherein M in the composition formula represents at least one additive element selected from the group consisting of Al, Mg, Ti, Sn, Zr, Nb, W, B, Cr, Mo, and V, and a, b, x to z, and α satisfy the following conditions: 0.6≦a≦1.33, 0<b<0.015, 0.1≦x≦0.3, 0≦y≦0.4, 0.3≦z≦0.9, 0≦α≦0.3), and the crystal structure of a main phase of the Na-doped precursor is a P2-type layered structure that can be assigned to the space group P63 / mmc.

[0105] [4] The method for producing a positive electrode active material according to any one of [1] to [3], wherein in the preparation of the Na-doped precursor, a coprecipitated precursor containing two or more metal elements selected from the metal elements contained in the Na-doped precursor is prepared by a coprecipitation method, a mixture containing the coprecipitated precursor and a Na source containing Na is then prepared, and the mixture is calcined to obtain the Na-doped precursor. [5] The method for producing a positive electrode active material according to any one of [1] to [4], wherein in the ion exchange, the mixture is heated at a temperature of 250°C or higher and 375°C or lower for one hour or longer.

[0106] [6] The method for producing a cathode active material according to any one of [1] to [5], wherein the cathode active material obtained by the ion exchange is washed once using 1.5 L to 30 L of water per 1 kg of the cathode active material. [7] The method for producing a cathode active material according to any one of [1] to [5], wherein the cathode active material obtained by the ion exchange is washed multiple times using a total of 0.8 L to 30 L of water per 1 kg of the cathode active material.

Claims

1. A method for producing a positive electrode active material for use in a non-aqueous electrolyte secondary battery, comprising: preparing a Na-doped precursor made of an oxide containing Na and having a median diameter of 25 μm or less in a volumetric particle size distribution; mixing the Na-doped precursor with a Li ion source containing Li so that the molar ratio of Li to Na, Li / Na, is 1.05 or more and 3 or less; and heating the mixture obtained by the mixing to replace Na in the Na-doped precursor with Li by ion exchange, thereby producing a positive electrode active material.

2. The positive electrode active material is Li a Na b Ni x Co y Mn z M (1-x-y-z) O (2±α) (wherein M in the composition formula represents at least one added element selected from the group consisting of Al, Mg, Ti, Sn, Zr, Nb, W, B, Cr, Mo, and V, and a, b, x to z, and α satisfy 0.6≦a≦1.33, 0<b<0.015, 0.1≦x≦0.3, 0≦y≦0.4, 0.3≦z≦0.9, and 0≦α≦0.3), and a crystal structure of a main phase of the cathode active material is an O2-type layered structure that can be assigned to space group P63mc.

3. The Na-doped precursor is Na (a+b) Ni x Co y Mn z M (1-x-y-z) O (2±α) (wherein M in the composition formula represents at least one additive element selected from the group consisting of Al, Mg, Ti, Sn, Zr, Nb, W, B, Cr, Mo, and V, and a, b, x to z, and α satisfy the following conditions: 0.6≦a≦1.33, 0<b<0.015, 0.1≦x≦0.3, 0≦y≦0.4, 0.3≦z≦0.9, 0≦α≦0.3), and the crystal structure of a main phase of the Na-doped precursor is a P2-type layered structure that can be assigned to the space group P63 / mmc.

4. The method for producing a positive electrode active material according to claim 3, wherein in preparing the Na-doped precursor, a coprecipitated precursor containing two or more metal elements selected from the metal elements contained in the Na-doped precursor is prepared by a coprecipitation method, a mixture containing the coprecipitated precursor and a Na source containing Na is then prepared, and the Na-doped precursor is obtained by calcining the mixture.

5. The method for producing a positive electrode active material according to any one of claims 1 to 4, wherein the mixture is heated at a temperature of 250°C or higher and 375°C or lower for one hour or longer in the ion exchange.

6. The method for producing a positive electrode active material according to any one of claims 1 to 4, wherein the positive electrode active material obtained by the ion exchange is washed once using 1.5 L to 30 L of water per kg of the positive electrode active material.

7. The method for producing a positive electrode active material according to any one of claims 1 to 4, wherein the positive electrode active material obtained by the ion exchange is washed multiple times using a total of 0.8 L to 30 L of water per kg of the positive electrode active material.

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