Method for producing positive electrode active material for lithium ion secondary battery

Abstract

To provide a method for industrially stably mass-manufacturing a positive electrode active material for a lithium ion secondary battery.SOLUTION: A method of manufacturing a positive electrode active material for a lithium ion secondary battery includes a step of firing under a condition in which, for a mixture of a Ni compound powder having a carbon content of 0.042% by mass or more and 0.25% by mass or less and a volume average particle size of 10 μm or more and 26 μm or less and a Li compound powder having a volume average particle size of 10 μm or more and 26 μm or less, when the Li compound powder has water of crystallization, the firing time is set to 12 hours or more, and when the Li compound powder is anhydrous, the firing time is less than 12 hours, and a step of washing the resulting LiNi composite oxide with water, and firing processing is performed under a condition in which a value obtained by dividing the ratio of the substance amount of Li to the total substance amount of transition metals other than Li in the LiNi composite oxide after the water washing treatment by the ratio of the substance amount of Li to the total substance amount of transition metals other than Li in the LiNi composite oxide before the water washing processing exceeds 0.95.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a positive electrode active material for a lithium ion secondary battery, and more particularly to a method for manufacturing a high-power positive electrode active material for a lithium ion secondary battery composed of a lithium nickel composite oxide.

Background Art

[0002] In recent years, with the rapid production expansion of small information terminals such as smartphones and tablet terminals, the demand for lithium ion secondary batteries as high-capacity secondary batteries has been rapidly increasing. Lithium ion secondary batteries are already widely used as power sources for small information terminals because of their small size and high energy density, but research and development aiming at their use as large power sources mounted on hybrid vehicles and electric vehicles is also underway.

[0003] As the positive electrode active material used in lithium ion secondary batteries, lithium transition metal composite oxides are widely used, and in particular, lithium cobalt composite oxide (LiCoO2), which is relatively easy to synthesize, has been mainly used. However, since a rare and expensive cobalt compound is used as a raw material for the lithium cobalt composite oxide, the raw material cost of the positive electrode active material increases, which is a factor in increasing the price of lithium ion secondary batteries. Reducing the raw material cost of this positive electrode active material and realizing the manufacture of more inexpensive lithium ion secondary batteries will lead to cost reduction of currently popular small information terminals and in-vehicle secondary batteries, and will also enable the mounting of lithium ion secondary batteries on future large power sources, which has great industrial significance.

[0004] Therefore, as another lithium transition metal composite oxide that can be used as a positive electrode active material for lithium-ion secondary batteries, lithium nickel composite oxide (LiNiO2) has attracted attention. Lithium nickel composite oxide has advantages such as a larger charge-discharge capacity per mass compared to the currently mainstream lithium cobalt composite oxide, the raw material nickel compound being cheaper than the cobalt compound, and being stably available. Therefore, it is expected as a next-generation positive electrode active material, and research and development are being actively continued.

[0005] However, lithium nickel composite oxide has a problem that its decomposition temperature due to the desorption of oxygen from the crystal is lower than that of lithium cobalt composite oxide. Therefore, the temperature of the synthesis reaction of the lithium compound and the nickel compound cannot be increased, and the firing time has to be lengthened in order to sufficiently proceed the reaction and obtain a well-crystallized product, resulting in inferior productivity when mass-producing industrially.

[0006] Regarding the manufacturing method of this lithium nickel composite oxide, Patent Documents 1 to 4 disclose a method of mixing a lithium compound and a nickel compound and performing heat treatment, and it has been proposed to optimize the reaction time and temperature of the synthesis reaction, and the composition of the atmosphere gas during the synthesis reaction, etc. for the purpose of improving battery characteristics. In addition, many proposals have been made regarding the synthesis method by firing of lithium transition metal composite oxides in Patent Documents 5 to 7. Furthermore, Patent Document 8 describes a method of improving the characteristics of the positive electrode active material by a water washing process.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Patent Document 8

Summary of the Invention

Problems to be Solved by the Invention

[0008] However, with the technologies of Patent Documents 1 to 8 described above, it has been difficult to industrially and stably mass-produce a cathode active material for a lithium-ion secondary battery having excellent battery performance. The present invention has been made in view of the above-described conventional situation, and an object thereof is to provide a manufacturing method capable of industrially and stably mass-producing a lithium nickel composite oxide having excellent battery performance as a cathode active material for a lithium-ion secondary battery capable of obtaining high output.

Means for Solving the Problems

[0009] As a result of proceeding with research on the synthesis of a cathode active material in order to solve the above problems, the present inventors have a firing step of firing a powder raw material having a nickel compound and a lithium compound to produce a lithium nickel composite oxide, and a water washing step of water washing the composite oxide after the firing step. In the manufacturing method of the cathode active material, by controlling the ratio of the amount of substance of lithium contained in the composite oxide before and after the water washing treatment, it has been found that a cathode active material having stable battery performance can be industrially mass-produced, and the present invention has been completed.

[0010] That is, the manufacturing method of the cathode active material for a lithium-ion secondary battery according to the present invention is a manufacturing method of a cathode active material for a lithium-ion secondary battery containing at least nickel and lithium, wherein the nickel compound powder having a carbon content of 0.042% by mass or more and 0.25% by mass or less and a volume average particle diameter MV of 10 μm or more and 26 μm or less, and a volume average particle diameter MV of 10 μm or more and 26 μm or less (excluding 10 μm)Of the anhydride The mixture with the lithium compound powder is heated at a constant heating rate from the start of heating to a maximum firing temperature of 650 °C or higher and 850 °C or lower at 2 for 3 hours or more and 3.5 hours or less maintained below and a firing step of firing under such conditions, and a water washing step of water washing the lithium nickel composite oxide powder obtained in the firing step, and the ratio of the amount of lithium to the total amount of transition metals other than lithium in the lithium nickel composite oxide powder after the water washing treatment is divided by the ratio of the amount of lithium to the total amount of transition metals other than lithium in the lithium nickel composite oxide powder before the water washing treatment, and the firing step is performed under the condition that the obtained value exceeds 0.95.

Advantages of the Invention

[0011] According to the present invention, a positive electrode active material having stable battery performance can be industrially mass-produced.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0013] Hereinafter, embodiments of a method for manufacturing a positive electrode active material for a lithium-ion secondary battery according to the present invention will be described in detail. The method for manufacturing a positive electrode active material according to this embodiment of the present invention includes a firing step of firing a mixture of nickel compound powder and lithium compound powder, and a water washing step of washing the lithium nickel composite oxide powder obtained in this firing step with water. When industrially producing lithium nickel composite oxide powder used for a positive electrode active material of a lithium-ion secondary battery, generally, after filling a ceramic firing container with a mixture of nickel compound powder and lithium compound powder, the container filled with this mixture is continuously fed into a continuous firing furnace such as a roller hearth kiln or a pusher furnace and fired at a predetermined temperature and for a predetermined time. As a result, a synthesis reaction occurs and lithium nickel composite oxide powder is generated from the mixture.

[0014] As the firing container used in the above industrial production, generally, a ceramic rectangular container having an inner size in the range of 100 mm (L) × 100 mm (W) × 20 mm (H) to 500 mm (L) × 500 mm (W) × 120 mm (H) is used, and a mixture of lithium compound powder and nickel compound powder as raw materials is filled into this container so that the height from the bottom surface is within the range of 10 to 110 mm.

[0015] In order to improve the productivity of the above firing step, it is conceivable to increase the firing throughput per unit time by increasing the conveyance speed of the firing container in the continuous firing furnace to shorten the firing treatment time, or by increasing the filling amount of the mixture in the firing container. However, if the conveyance speed is too fast, there will not be enough time for the synthesis reaction between the nickel compound powder and the lithium compound powder, and the crystal growth of the particles constituting the lithium nickel composite oxide powder will be insufficient, resulting in a problem that the battery performance deteriorates. On the other hand, if the filling amount of the mixture in the firing container is too large, the depth from the surface part to the bottom of the mixture filled in the container becomes too deep, and oxygen necessary for the reaction does not sufficiently reach the mixture at the bottom. As a result, the synthesis reaction as shown in the following formula 1 hardly proceeds well, the synthesis of the lithium nickel composite oxide becomes insufficient, and problems such as a decrease in discharge capacity may occur.

[0016] [Formula 1] NiO + LiOH + 1 / 4O2 → LiNiO2 + 1 / 2H2O

[0017] Therefore, in order to synthesize lithium nickel composite oxide powder that exhibits excellent battery performance as a positive electrode active material, it is desirable to sufficiently distribute the oxygen required for the reaction between the nickel compound powder and the lithium compound powder throughout the entire mixture, especially to the bottom of the firing container. In this regard, the inventors have found that in the reaction between the nickel compound powder and the lithium compound powder, it is not necessary to sufficiently distribute oxygen throughout the entire mixture at all temperature ranges in the firing process. In the temperature range that is important for the above reaction, specifically, when the firing temperature during the firing treatment of the mixture is 450°C or higher and 650°C or lower, if the oxygen diffusion time corresponding to the depth of the mixture in the firing container can be ensured, oxygen can be sufficiently distributed in the mixture, and even if the amount of the mixture filled in the firing container increases and the depth from the surface to the bottom of the mixture in the container becomes deeper, a powder of lithium nickel composite oxide with excellent battery performance can be obtained.

[0018] More specifically, although it depends on the type of the lithium compound, in the temperature range of 450°C or higher and 650°C or lower, the solid-phase to solid-phase reaction or the liquid-phase to solid-phase reaction between the lithium compound and the nickel compound proceeds most prominently. Therefore, by ensuring that the oxygen required for the reaction is sufficiently distributed throughout the entire mixture in this temperature range, it becomes possible to obtain a powder of lithium nickel composite oxide that has reacted sufficiently. The above-mentioned firing temperature can be measured, for example, with a thermometer provided in the furnace.

[0019] For example, when a mixture of lithium hydroxide powder and nickel composite oxide powder is prepared as a raw material and the temperature of this mixture is gradually increased, the synthesis reaction of lithium nickel composite oxide starts from around 450°C. Also, when the temperature exceeds around 460°C, which is the melting point of lithium hydroxide, lithium hydroxide reacts with the nickel composite oxide while melting. If oxygen does not sufficiently reach the bottom of the firing container in this temperature range, the unreacted molten lithium hydroxide reacts with the ceramic firing container, and the amount of lithium hydroxide reacting with the nickel composite oxide is substantially insufficient. As a result, lithium-deficient sites exist in the produced lithium nickel composite oxide, leading to a decrease in battery performance. Therefore, when the firing temperature during the firing process is 450°C or higher, which is a temperature range where lithium hydroxide melts and a sufficient reaction rate can be obtained, it is important to sufficiently supply oxygen to the above mixture to significantly advance the reaction.

[0020] On the other hand, when unreacted lithium hydroxide and nickel composite oxide still exist when the firing temperature reaches 650°C and the oxygen supply to them is insufficient, the side reaction of the following formula 2 occurs, and in the lithium nickel composite oxide crystals produced, heterogeneous phases that hinder the movement of lithium ions during the battery reaction are generated, leading to deterioration of battery performance.

[0021] [Formula 2] 8NiO + 2LiOH + 1 / 2O2 → Li2Ni8O 10 + H2O

[0022] In addition, if the firing temperature is within the range of 450°C or higher and 650°C or lower as described above, there is almost no difference in the effects between the case where the firing temperature is maintained at a predetermined value and the case where the firing temperature is changed at a constant heating rate, and similar effects can be obtained in both cases. Further, since the reaction of the above formula (1) basically requires oxygen, the atmosphere gas during firing may be air with an oxygen concentration of about 20% by volume, but oxygen-enriched air with a higher oxygen concentration is preferably used. The oxygen concentration of this oxygen-enriched air is preferably 60% by volume or higher, more preferably 80% by volume or higher, by appropriately adjusting the mixing ratio of air and oxygen, for example.

[0023] In order to ensure sufficient crystallinity in the synthesis of lithium nickel composite oxide to exhibit high battery performance, the maximum firing temperature in the firing process is preferably 650°C or higher and 850°C or lower, and the holding time at this maximum firing temperature is preferably 2 hours or longer. When the maximum firing temperature is less than 650°C, or when the maximum firing temperature is 650°C or higher but the holding time is less than 2 hours, the crystallinity of the obtained lithium nickel composite oxide may be insufficient. On the other hand, when the above maximum firing temperature exceeds 850°C, the generated lithium nickel composite oxide may start a decomposition reaction accompanied by oxygen release, and the layered structure may be disrupted, deteriorating the battery performance.

[0024] Even when the above maximum firing temperature is less than 650°C, sufficient crystallinity can be obtained in the lithium nickel composite oxide by firing over a long period of time, and it is possible to synthesize it without impairing the battery performance. However, considering industrial productivity, a firing time exceeding 24 hours is not preferable. Therefore, the time for the firing container containing the mixture to pass through the firing furnace, that is, the time from the start of heating to the maximum firing temperature and the holding at the maximum firing temperature until cooling is completed, is preferably 24 hours or less.

[0025] When lithium hydroxide containing crystal water is used as the lithium compound, if the temperature is rapidly increased, the temperature of the mixture in the firing container may become non-uniform and the synthesis reaction may not be uniform. Therefore, it is preferable to take 12 hours or more for the firing time from the start of heating to the completion of holding at the maximum firing temperature. On the other hand, by using anhydrous lithium hydroxide, the time from the start of heating to the completion of holding can be made less than 12 hours.

[0026] As an index for determining whether or not the synthesis reaction of the lithium nickel composite oxide from the above nickel compound and lithium compound has sufficiently proceeded, the inventors of the present invention, when the lithium nickel composite oxide powder is washed with water under predetermined conditions, the ratio of the amount of lithium to the total amount of transition metals other than lithium in the lithium nickel composite oxide powder after this water washing treatment is divided by the ratio of the amount of lithium to the total amount of transition metals other than lithium in the lithium nickel composite oxide powder before this water washing treatment (hereinafter, also referred to as the ratio of the lithium metal ratios before and after the water washing treatment). It has been found that it is effective to use the value obtained. Note that the amount of substance is a physical quantity expressed in moles in the SI unit and can be measured with an ICP emission spectroscopic analyzer or the like.

[0027] In the production of the lithium nickel composite oxide, usually, for 1 mole of the amount of substance of nickel and the added transition metal element contained in the nickel compound, the amount of substance of lithium in the lithium compound is mixed and fired so as to be in excess of 1 mole. This is because in the synthesis reaction of the lithium nickel composite oxide, if the amount of substance of lithium is less than the amount of substance of nickel and the added transition metal element, lithium deficiency occurs at the lithium site in the lithium nickel composite oxide crystal, and sufficient charge-discharge capacity cannot be ensured. At the same time, the lithium-deficient site may act as a resistance layer for the charge-discharge reaction, increasing the battery resistance of the secondary battery.

[0028] Therefore, even if a sufficient synthesis reaction of the lithium nickel composite oxide occurs, there is still an excess of lithium compound in the lithium nickel composite oxide after the synthesis reaction, and most of it exists on the surface of the lithium nickel composite oxide particles and in the vicinity thereof. Since the excess lithium compound on and near the particle surface is easily removed by the water washing treatment, the ratio of the lithium metal ratios before and after the water washing treatment is usually less than 1.

[0029] On the other hand, when the ratio of the lithium metal ratios before and after this water washing treatment is close to 1, that is, when there is almost no difference in the lithium metal ratios before and after the water washing treatment, it can be considered that there is little excess lithium present on the surface of the lithium nickel composite oxide particles after the synthesis reaction. This is presumably because most of the lithium used as the raw material is dissolved in the nickel compound particles and reacts sufficiently, and the synthesis reaction of the lithium nickel composite oxide proceeds almost stoichiometrically.

[0030] Therefore, in the method for producing a positive electrode active material composed of the lithium nickel composite oxide according to the embodiment of the present invention, the ratio of the lithium metal ratios before and after the water washing treatment is adjusted to exceed 0.95. This ratio of the lithium metal ratios is preferably 0.99 or more, more preferably 0.995 or more, in order to obtain more excellent battery characteristics. Note that as the upper limit of the ratio of the lithium metal ratios before and after the water washing treatment, although it is usually less than 1 as described above, elution of nickel and the added transition metal elements may occur when the synthesis of the lithium nickel composite oxide is insufficient, and in this case, it may be greater than 1. Therefore, in the lithium nickel composite oxide in which the synthesis reaction has been sufficiently performed, the ratio of the lithium metal ratios before and after water washing does not exceed 1.

[0031] When the ratio of the lithium metal ratios before and after this water washing treatment is 0.95 or less, it can be made to exceed 0.95 by appropriately adjusting the conditions of the previous firing treatment. For example, a nickel compound powder having a smaller volume average particle diameter MV may be employed as the raw material, or a nickel compound powder calcined at a higher temperature may be used. Alternatively, the oxygen concentration of the atmospheric gas during the firing treatment may be increased, the maximum firing temperature during the firing treatment may be increased, or the holding time of the maximum firing temperature may be increased.

[0032] In an embodiment of the method for producing a positive electrode active material of the present invention, the nickel compound powder used as a raw material is not particularly limited, but nickel hydroxide or nickel oxide is preferable from the viewpoint that it is difficult to generate side reaction products other than water during the reaction. This nickel compound powder preferably has a volume average particle diameter MV of 3 μm or more and 26 μm or less, more preferably 8 μm or more and 21 μm or less, and most preferably 10 μm or more and 16 μm or less. On the other hand, the bulk density of the nickel compound powder is preferably 0.5 g / ml or more and 2.2 g / ml or less.

[0033] When the volume average particle diameter MV of the above nickel compound powder is less than 3 μm, the particle diameter of the obtained positive electrode active material becomes too small, and a sufficient packing density cannot be obtained during electrode plate production, and the amount of the positive electrode active material per unit volume of the secondary battery is small, so the battery capacity may decrease. Conversely, when the volume average particle diameter MV of the above nickel compound powder exceeds 26 μm, the contact points between the positive electrode active material particles or between the positive electrode active material particles and the conductive assistant become too few, the resistance of the positive electrode increases, and the battery capacity may decrease.

[0034] Further, when the bulk density of the nickel compound powder is less than 0.5 g / ml, the bulk density when filling the firing container during firing becomes too small, and the filling amount per firing container is small, so the productivity may be significantly reduced. Conversely, when the above bulk density exceeds 2.2 g / ml, the mixture with the lithium compound powder is densely packed, making it difficult for oxygen to diffuse into the mixture, and the time required for firing may be extended, reducing the productivity.

[0035] The nickel compound powder described above preferably has a carbon content of 0.25% by mass or less, more preferably 0.15% by mass or less, and most preferably 0.07% by mass or less. By suppressing the carbon content in the nickel compound to 0.25% by mass or less as described above, it becomes possible to efficiently dissolve lithium in the nickel compound, and the output characteristics of the positive electrode active material can be significantly improved. Although the reason for the efficient dissolution of lithium in this way is not clear, the inventors of the present invention consider that the carbon content in the nickel compound powder is mainly due to nickel carbonate or nickel carbide, and these are presumed to be inferior in reactivity with lithium compounds compared to nickel hydroxide or nickel oxide.

[0036] In the method for producing a positive electrode active material according to an embodiment of the present invention, when nickel hydroxide is used as a raw material, it is preferable to have a pretreatment step of roasting the nickel compound powder at a roasting temperature of preferably 500°C or higher and 800°C or lower to obtain nickel oxide powder before the above-mentioned firing step. By performing the roasting treatment before the firing treatment in this way, it is also possible to reduce the carbon content of the above-mentioned nickel compound powder. The above-mentioned roasting temperature can be measured, for example, with a thermometer provided in the furnace.

[0037] The lithium compound powder used as a raw material is not particularly limited, but lithium hydroxide or lithium carbonate or a mixture thereof is preferable. Considering the reaction with the nickel compound, lithium hydroxide anhydride or lithium hydroxide monohydrate having a melting point of around 480°C is more preferable, and lithium hydroxide anhydride is particularly preferable considering productivity. This is because the reaction proceeds more uniformly when lithium hydroxide melts and undergoes a solid-liquid reaction with the nickel composite oxide.

[0038] In addition, since the synthesis reaction of the lithium nickel composite oxide is basically a solid-phase reaction, the synthesis reaction proceeds more easily when the lithium compound powder and the nickel compound powder used as raw materials are uniformly mixed. For this purpose, it is preferable that the particle size of the lithium compound powder is close to that of the nickel compound powder. As described above, the preferred particle size of the nickel compound powder is such that the volume average particle size MV is 3 μm or more and 26 μm or less. Therefore, the volume average particle size MV of the lithium compound powder is preferably 26 μm or less, more preferably 21 μm or less, and most preferably 16 μm or less. However, when the particle size of the lithium compound powder becomes 5 μm or less, the energy required for pulverization becomes too large, resulting in an increase in cost. At the same time, the bulk density of the raw material mixture becomes too small, and a large furnace volume is required during firing. Therefore, in reality, the volume average particle size MV of the lithium compound powder is preferably 5 μm or more, and more preferably 10 μm or more.

[0039] In an embodiment of the method for producing a positive electrode active material of the present invention, by subjecting the lithium nickel composite oxide powder obtained by firing to a water washing treatment, as described above, excess lithium on the particle surface and in its vicinity is removed, resulting in a positive electrode active material for a lithium ion secondary battery with high capacity and high safety. The conditions for this water washing treatment are not particularly limited, and known water washing techniques can be used. However, it is preferable to perform the water washing treatment by the following method.

[0040] That is, first, to 1 part by mass of water preferably stored in a container with a stirrer at about 10 to 15°C, preferably 0.5 to 2 parts by mass, more preferably 0.75 parts by mass of lithium nickel composite oxide powder is added and stirred preferably for about 15 to 60 minutes, more preferably for about 30 minutes, thereby sufficiently removing excess lithium on the surface and in the vicinity of the lithium nickel composite oxide particles. The slurry after this water washing treatment may be separated into solid and liquid and dried by a general method. When the addition amount of the above lithium nickel composite oxide powder exceeds 2 parts by mass, not only does the viscosity of the above slurry become too high, making stirring difficult, but also the alkali in the slurry becomes high, and the dissolution rate of the deposit is affected by the chemical equilibrium and becomes slow, or separation from the powder becomes difficult even if peeling occurs.

[0041] Conversely, when the addition amount of the above lithium nickel composite oxide powder is less than 0.5 parts by mass, since the slurry concentration is too dilute, the elution of lithium is promoted, and the desorption of lithium from the crystal lattice of the lithium nickel composite oxide particles as the positive electrode active material occurs. As a result, not only does the crystal become easily broken, but also the high-pH aqueous solution in the slurry absorbs carbon dioxide gas in the atmosphere and reprecipitates lithium carbonate.

[0042] The water used for the above water washing treatment is not particularly limited, but pure water with an electric conductivity of less than 10 μS / cm is preferred, and pure water with an electric conductivity of 1 μS / cm or less is more preferred. That is, by using pure water with an electric conductivity of less than 10 μS / cm, it is possible to prevent impurities from adhering to the lithium nickel composite oxide particles as the positive electrode active material and the battery performance from deteriorating. After the solid-liquid separation of the above slurry, it is preferable that there is little adhering water remaining on the particle surface. If there is a lot of adhering water, the lithium dissolved in the adhering water will reprecipitate and remain after the subsequent drying treatment, increasing the amount of lithium present on the surface of the lithium nickel composite oxide powder. Therefore, for the solid-liquid separation device used for the solid-liquid separation after the water washing treatment, those that can keep the moisture content of the solid content after the solid-liquid separation, such as a centrifuge and a filter press, low are preferred.

[0043] There are no particular limitations on the conditions of the drying process in the latter stage of the above solid-liquid separation. However, for the wet powder cake obtained by the above solid-liquid separation, it is preferable to use a dryer capable of controlling the atmosphere inside the chamber in a gas atmosphere or a vacuum atmosphere that does not contain carbon or sulfur compound components, and perform the drying process at a drying temperature of 80°C or higher and 550°C or lower. The reason for setting the drying temperature during drying to 80°C or higher is to quickly dry the lithium nickel composite oxide particles as the positive electrode active material after the above washing treatment and prevent the occurrence of a lithium concentration gradient between the surface and the inside of the particles.

[0044] Conversely, the reason for setting it to 550°C or lower is that it is expected that in the vicinity of the surface of the lithium nickel composite oxide particles as the positive electrode active material, the stoichiometric ratio is extremely close or slightly lithium has desorbed to be in a state close to the charged state. Therefore, at a drying temperature exceeding 550°C, it may trigger the collapse of the crystal structure of the powder close to the charged state, leading to a possible decrease in electrical characteristics. Considering productivity and thermal energy cost, the drying temperature during this drying is more preferably 120 to 350°C. The above drying temperature can be measured, for example, with a thermometer provided inside the dryer.

[0045] By the method for producing a positive electrode active material according to the above-described embodiment of the present invention, a positive electrode active material with stable quality can be mass-produced. In addition, since the ratio of the lithium metal ratio before and after the washing treatment is high, lithium loss can be suppressed, and due to the synergistic effect with the above mass productivity, it is possible to achieve cost reduction of the battery, which is an essential requirement for popularizing lithium-ion secondary batteries as power sources for electric vehicles, and its industrial value can be said to be extremely high. Note that the power source for an electric vehicle includes not only an electric vehicle driven purely by electrical energy but also a power source for a so-called hybrid vehicle that is used in combination with a combustion engine such as a gasoline engine or a diesel engine.

[0046] By the manufacturing method of the embodiment of the present invention described above, various types of lithium nickel composite oxides can be produced, and they can be applied to the industrial production of positive electrode active materials for lithium ion secondary batteries. Specifically, the composition formula is Li x Ni 1-y-z M y N z O2, a lithium nickel composite oxide can be produced. Here, M in the formula is at least one element selected from Co and Mn, and N is at least one element selected from Al, Ti, Nb, V, Mg, W, and Mo. Also, x, y, and z are preferably 0.90 ≦ x ≦ 1.10, 0.05 ≦ y ≦ 0.35, 0.005 ≦ z ≦ 0.05, and more preferably y + z ≦ 0.20.

[0047] When producing a lithium nickel composite oxide having the above composition, the nickel compound powder as its raw material is a powder of a composite oxide of nickel and other transition metals (also referred to as a nickel composite oxide) or a composite hydroxide (also referred to as a nickel composite hydroxide). These powders can be produced based on known methods. For example, in the case of a powder of a nickel composite hydroxide, it can be obtained by coprecipitating nickel, cobalt or manganese, and the additive element N.

[0048] By further oxidizing and roasting the obtained nickel composite hydroxide powder, a nickel composite oxide in which cobalt or manganese and the additive element N are dissolved in nickel oxide is obtained. It should be noted that it is also possible to produce it by a method such as pulverizing and mixing nickel oxide and oxides of other additive elements. When the lithium nickel composite oxide produced by the manufacturing method of the embodiment of the present invention described above is used as a positive electrode active material for a lithium ion secondary battery, a high-output secondary battery with stable quality is obtained.

[0049] There is no limitation on the method for fabricating the positive electrode of a lithium-ion secondary battery using the positive electrode active material composed of the above-mentioned lithium nickel composite oxide powder. For example, it can be fabricated by the following method. That is, first, a powdery positive electrode active material, a conductive material, and a binder are mixed, and if necessary, activated carbon is further added, and this is kneaded together with a solvent for the purpose of viscosity adjustment or the like to prepare a positive electrode composite paste. The mixing ratio of the above raw materials constituting this positive electrode composite paste is a factor that determines the performance of the lithium-ion secondary battery. When the total mass of the solid content of the positive electrode composite excluding the solvent is 100 parts by mass, similar to the positive electrode of a general lithium-ion secondary battery, it is desirable that the content of the positive electrode active material be 60 to 95 parts by mass, the content of the conductive material be 1 to 20 parts by mass, and the content of the binder be 1 to 20 parts by mass.

[0050] The obtained positive electrode composite paste is applied, for example, to the surface of a current collector made of aluminum foil, and this is dried to scatter the solvent. At that time, in order to increase the electrode density, it may be pressurized by a roll press or the like if necessary. In this way, a sheet-shaped positive electrode can be fabricated. The obtained sheet-shaped positive electrode is subjected to cutting or the like to an appropriate size according to the type and size of the target battery, and then is used in the battery assembly process.

[0051] As the above conductive agent, for example, graphite (natural graphite, artificial graphite, expanded graphite, etc.), carbon black-based materials such as acetylene black and ketjen black can be used. As the binder, it plays a role of connecting the active material particles. For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene propylene diene rubber, styrene butadiene, cellulose-based resin, polyacrylic acid, etc. can be used. Activated carbon is dispersed in these positive electrode active materials, conductive materials, and binders as needed to increase the electric double layer capacitance, and a solvent that dissolves the binder is added to the obtained positive electrode composite material. As this solvent, for example, an organic solvent such as N-methyl-2-pyrrolidone can be used. The positive electrode composite material added with the solvent is preferably kneaded with a general kneader, and thus a homogeneous positive electrode composite material paste can be produced.

[0052] For the negative electrode that serves as the counter electrode of the above positive electrode, a negative electrode composite material obtained by mixing a binder with a metal lithium, a lithium alloy, etc., or a negative electrode active material capable of intercalating and deintercalating lithium ions, adding a solvent to make it into a paste form, coating it on the surface of a metal foil current collector such as copper, drying it, and compressing it as needed to increase the electrode density is used. As the above negative electrode active material, for example, powders of organic compound fired bodies such as natural graphite, artificial graphite, and phenolic resin, and carbon materials such as coke can be used. As the negative electrode binder, similar to the positive electrode, fluorine-containing resins such as PVDF can be used, and as the solvent for dispersing these active materials and binders, organic solvents such as N-methyl-2-pyrrolidone can be used. A separator is sandwiched and arranged between the above positive electrode and negative electrode. The separator separates the positive electrode and the negative electrode and holds the electrolyte, and a thin film made of polyethylene, polypropylene, etc. and having a large number of minute pores can be used.

[0053] The non-aqueous electrolyte is obtained by dissolving a lithium salt as a supporting salt in an organic solvent. The organic solvent may be selected from cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, trifluoropropylene carbonate, etc., chain carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, dipropyl carbonate, etc., ether compounds such as tetrahydrofuran, 2-methyltetrahydrofuran, dimethoxyethane, etc., sulfur compounds such as ethyl methyl sulfone, butane sultone, etc., phosphorus compounds such as triethyl phosphate, trioctyl phosphate, etc. One kind can be used alone, or two or more kinds can be mixed and used. On the other hand, as the supporting salt, one kind selected from LiPF6, LiBF4, LiClO4, LiAsF6, LiN(CF3SO2)2, etc., or a composite salt thereof can be used. The above non-aqueous electrolyte may further contain one or more of a radical scavenger, a surfactant, a flame retardant, etc.

[0054] The lithium ion secondary battery composed of the above positive electrode, negative electrode, separator and non-aqueous electrolyte can be made into various shapes such as cylindrical and laminated types. In any case of assembly, the positive electrode and the negative electrode are laminated through the separator to form an electrode body, the obtained electrode body is impregnated with the non-aqueous electrolyte, and between the current collector on the positive electrode side and the positive electrode terminal communicating with the outside, and between the current collector on the negative electrode side and the negative electrode terminal communicating with the outside, are connected using a current collecting lead or the like, and the lithium ion secondary battery can be completed by accommodating it in a sealed state in a battery case.

[0055] The lithium ion secondary battery using the positive electrode active material of the embodiment of the present invention has a high capacity and a high output. In particular, the lithium ion secondary battery using the positive electrode active material according to the embodiment of the present invention obtained in a more preferable form, for example, when used for the positive electrode of a 2032 type coin battery, a high initial discharge capacity of 165 mAh / g or more and a low positive electrode resistance can be obtained, and it is further high in capacity and high in output. Also, it can be said that it has high thermal stability and excellent safety.

[0056] Incidentally, the above-mentioned positive electrode resistance can be measured, for example, by the following method. That is, when measuring the frequency dependence of the battery reaction by the alternating current impedance method, which is a common electrochemical evaluation method, a Nyquist diagram based on the solution resistance, the negative electrode resistance and the negative electrode capacitance, and the positive electrode resistance and the positive electrode capacitance as shown in FIG. 1 can be obtained. The battery reaction at the electrode consists of a resistance component associated with charge transfer and a capacitance component due to the electric double layer. When these are represented by an electric circuit, it becomes a parallel circuit of resistance and capacitance. As a whole battery, it is represented by an equivalent circuit in which the solution resistance and the parallel circuits of the negative electrode and the positive electrode are connected in series.

[0057] Fitting calculations can be performed on the Nyquist diagram measured using this equivalent circuit to estimate each resistance component and capacitance component. The positive electrode resistance is equal to the diameter of the semicircle on the low-frequency side of the obtained Nyquist diagram. Therefore, by performing an alternating current impedance measurement on the fabricated positive electrode and performing a fitting calculation with an equivalent circuit on the obtained Nyquist diagram, the positive electrode resistance can be estimated. Next, the present invention will be specifically described using examples, but the present invention is not limited to these examples at all.

Examples

[0058] A positive electrode active material was produced by the method shown below, and a secondary battery having a positive electrode made from the obtained positive electrode active material was assembled, and its initial discharge capacity and positive electrode resistance were measured to evaluate the battery performance. The evaluation of the above battery performance was carried out by fabricating a 2032-type coin battery 1 (hereinafter referred to as a coin-type battery) having a configuration as shown in FIG. 2. That is, the coin-type battery 1 shown in FIG. 2 is composed of a substantially cylindrical case 2 and an electrode 3 housed in the case 2.

[0059] Case 2 consists of a positive electrode can 2a that is hollow and has an opening at one end, and a negative electrode can 2b that is hollow and has an opening at one end, and is arranged inside the positive electrode can 2a with this opening facing the opening of the above positive electrode can 2a. By arranging the negative electrode can 2b and the positive electrode can 2a with their openings facing each other in this way, a space for accommodating the electrode 3 is formed by the negative electrode can 2b and the positive electrode can 2a. The electrode 3 consists of a positive electrode 3a, a separator 4, and a negative electrode 3b, and by laminating them in this order so as to line up from the positive electrode can 2a side, the positive electrode 3a abuts against the inner surface of the positive electrode can 2a via the current collector 5, and the negative electrode 3b can be accommodated in the case 2 in a state of abutting against the inner surface of the negative electrode can 2b via the current collector 5. Note that the current collector 5 is also interposed between the positive electrode 3a and the separator 3c.

[0060] A gasket 2c is provided between both edge portions of the positive electrode can 2a and the negative electrode can 2b, and by means of this gasket 2c, the positive electrode can 2a and the negative electrode can 2b can be fixed so as not to move relative to each other while maintaining a non-contact state. This gasket 2c also has a function of sealing the gap between the positive electrode can 2a and the negative electrode can 2b and hermetically and liquid-tightly blocking between the inside of the case 2 and the outside.

[0061] The above coin-type battery 1 for evaluation was manufactured by the following method. That is, first, 52.5 mg of a positive electrode active material, 15 mg of acetylene black, and 7.5 mg of polytetrafluoroethylene resin (PTFE) were mixed while being ground in a mortar, and the obtained mixture was press-molded at a pressure of 100 MPa into a diameter of 11 mm and a thickness of 100 μm to produce the positive electrode 3a. The produced positive electrode 3a was dried in a vacuum dryer at 120 °C for 12 hours and then used.

[0062] For the negative electrode 3b, a negative electrode sheet in which a mixture of graphite powder having a volume average particle diameter MV of 20 μm and polyvinylidene fluoride was applied to a copper foil was punched into a disk shape with a diameter of 14 mm. For the separator 4, a polyethylene porous membrane with a film thickness of 25 μm was used. As the electrolytic solution, an equal amount mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (manufactured by Toyama Chemical Co., Ltd.) with 1 M LiClO4 as a supporting electrolyte was used.

[0063] Using these positive electrode 3a, negative electrode 3b, separator 4 and electrolyte, the coin-type battery 1 described above was assembled in a glove box with an Ar atmosphere in which the dew point was controlled to -80°C. The initial discharge capacity and the positive electrode resistance of the fabricated coin-type battery 1 were measured by the following method to evaluate the battery performance.

[0064] The initial discharge capacity was measured after leaving the coin-type battery 1 at room temperature for 24 hours after fabrication until the open circuit voltage OCV (Open Circuit Voltage) was stabilized, and then charging it to a cut-off voltage of 4.3V with a current density with respect to the positive electrode of 0.1mA / cm 2 and taking the capacity when discharging it to a cut-off voltage of 3.0V after a 1-hour rest as the initial discharge capacity.

[0065] Also, when the coin-type battery 1 was charged at a charging potential of 4.1V and measured by the AC impedance method using a frequency response analyzer and a potentiostat (manufactured by Solartron, 1255B), a Nyquist plot as shown in Fig. 1 was obtained. Since this Nyquist plot is represented as the sum of the characteristic curves showing the solution resistance, the negative electrode resistance and its capacitance, and the positive electrode resistance and its capacitance, fitting calculations were performed using an equivalent circuit based on this Nyquist plot to calculate the value of the positive electrode resistance. Next, the lithium nickel composite oxide of the example of the present invention used for the above positive electrode active material will be specifically described.

[0066] (Example 1) As the raw material nickel compound powder, a powder of nickel composite hydroxide prepared by a known crystallization method was used. That is, into a reaction tank storing a 2 mol / L aqueous sodium sulfate solution, a 2 mol / L mixed aqueous solution of nickel sulfate and cobalt sulfate prepared such that the molar ratio of the amounts of substance of nickel and cobalt is 88:9, and a 1 mol / L aqueous sodium aluminate solution were continuously dropped while stirring at an input amount such that the molar ratio of the amounts of substance of nickel, cobalt, and aluminum is 88:9:3. In parallel, 28% aqueous ammonia was added so that the ammonia concentration in the solution in the reaction tank becomes 5 g / L, and further, a 40% aqueous sodium hydroxide solution was added so that the pH in the reaction tank becomes 11.5 - 12.5. At this time, the reaction was advanced while finely adjusting the pH in the reaction tank with the addition amount of the above 40% aqueous sodium hydroxide solution so as to obtain the particle size of the target nickel composite hydroxide.

[0067] The slurry containing the produced nickel composite hydroxide was overflowed from the reaction tank and recovered, and this was filtered using a Buchner funnel to obtain a cake. 1 L of a 40% aqueous sodium hydroxide solution and 19 L of deionized water were added per 1 kg of the obtained cake, and after stirring for 30 minutes, it was filtered using a Buchner funnel to obtain a cake. 20 L of deionized water was added per 1 kg of the obtained cake, and after stirring for 30 minutes, the washing process of filtering with a Buchner funnel was further repeated twice to obtain a nickel composite hydroxide cake. The obtained nickel composite hydroxide cake was charged into a stationary hot air dryer and dried at a drying temperature of 110°C for 24 hours to obtain a powdery nickel composite hydroxide. The obtained nickel composite hydroxide powder had a volume average particle size MV of 11.8 μm and a carbon grade of 0.25 mass%.

[0068] As the lithium compound powder, which is the other raw material, lithium hydroxide was used. This lithium hydroxide was obtained by dehydrating lithium hydroxide monohydrate (LiOH·H₂O) through vacuum drying to form anhydrous lithium hydroxide, and then pulverizing it with a jet mill until the volume average particle size MV reached 14.1 μm. The volume average particle size MV of the above powder was measured using a particle size distribution measuring device based on the laser diffraction scattering method, and the carbon content of the nickel composite hydroxide powder was measured using the high-frequency combustion-infrared absorption method.

[0069] Next, the above anhydrous lithium hydroxide powder and nickel composite hydroxide powder were weighed so that the molar ratio of the amount of substance of lithium to transition metals other than lithium was 1.020:1.000, and they were thoroughly mixed. The obtained mixture was filled into a ceramic firing container with an inner size of 280 mm (L) × 280 mm (W) × 90 mm (H), and this was loaded into a roller hearth kiln, which is a continuous firing furnace. In an atmosphere gas with an oxygen concentration of 80% by volume, the firing temperature measured with a furnace thermometer was raised from 600 °C to 765 °C at a constant heating rate over about 100 minutes, and then fired at a temperature pattern of holding at the maximum firing temperature of 765 °C for 210 minutes to synthesize lithium nickel composite oxide.

[0070] Next, 0.75 parts by mass of the above lithium nickel composite oxide was added to 1 part by mass of pure water at 15 °C with an electric conductivity in the range of 1 to 10 μS / cm stored in a container equipped with a stirrer to form a slurry. After stirring this slurry for 30 minutes, the solid content was recovered by filtration, and this was loaded into a vacuum dryer with a chamber internal pressure of 0.1 kPa or less and dried at an internal temperature of 110 °C over 12 hours. In this way, the lithium nickel composite oxide as the positive electrode active material of Sample 1 was obtained.

[0071] The lithium metal ratio (the value obtained by dividing the amount of lithium by the amount of transition metal elements other than lithium) of the obtained lithium nickel composite oxide of Sample 1 before the water washing treatment was 1.030, and the lithium metal ratio of the lithium nickel composite oxide after the water washing treatment was 0.993. Therefore, the ratio of the lithium metal ratios before and after the water washing treatment was 0.964, which was greater than 0.95. From this result, it is considered that the synthesis reaction proceeded almost stoichiometrically during firing to produce a lithium nickel composite oxide with little lithium loss.

[0072] As a result of evaluating the battery characteristics of Coin Cell 1 having a positive electrode prepared by using the obtained lithium nickel composite oxide of Sample 1 as a positive electrode active material, the initial discharge capacity was 203.8 mAh / g. The positive electrode resistance is represented as a relative value with the smallest positive electrode resistance among all the samples prepared including Examples 2 to 3 described later being 100, and it was 115.

[0073] Furthermore, the lithium nickel composite oxides of Samples 2 to 5 were synthesized in the same manner as in the case of Sample 1 above, except that the particle size after pulverization of anhydrous lithium hydroxide was changed to 15.6 μm, 18.4 μm, 20.9 μm, and 25.3 μm instead of 14.1 μm, and the ratios of the lithium metal ratios before and after the water washing treatment of them were determined. As a result, the ratios of the lithium metal ratios before and after the water washing treatment of Samples 2 to 5 were 0.964, 0.961, 0.958, and 0.951, respectively.

[0074] Furthermore, coin cells were fabricated in the same manner as Sample 1 and their battery characteristics were evaluated. For Sample 2, the initial discharge capacity was 200.9 mAh / g and the relative value of the positive electrode resistance was 140. For Sample 3, the initial discharge capacity was 202.8 mAh / g and the relative value of the positive electrode resistance was 154. For Sample 4, the initial discharge capacity was 200.3 mAh / g and the relative value of the positive electrode resistance was 205. For Sample 5, the initial discharge capacity was 201.4 mAh / g and the relative value of the positive electrode resistance was 258.

[0075] (Example 2) The nickel composite hydroxide prepared in the same manner as in Example 1 was calcined in an air atmosphere at a calcination temperature of 400 °C for 5 hours using a small muffle furnace to produce a nickel composite oxide. The obtained nickel composite oxide had a volume average particle size MV of 11.8 μm and a carbon content of 0.21% by mass. For anhydrous lithium hydroxide, the one with the same average particle size MV of 25.3 μm as that of Sample 5 was used. Thereafter, the lithium nickel composite oxide of Sample 6 was synthesized in the same manner as in Example 1, and the battery characteristics were evaluated in the same manner as in Example 1. As a result, the ratio of the lithium metal ratio before and after the water washing treatment was 0.950, the initial discharge capacity was 201.5 mAh / g, and the relative value of the positive electrode resistance was 266. Furthermore, lithium nickel composite oxides of Samples 7 to 10 were synthesized in the same manner as in the case of Sample 6 above, except that the calcination temperature during calcination using a small muffle furnace was changed to 500 °C, 650 °C, 700 °C, and 750 °C instead of 400 °C.

[0076] As a result, in Sample 7, the nickel composite oxide had a volume average particle size MV of 11.5 μm and a carbon content of 0.15% by mass. The ratio of the lithium metal ratio before and after the water washing treatment was 0.958, the initial discharge capacity was 208.6 mAh / g, and the relative value of the positive electrode resistance was 222. In Sample 8, the nickel composite oxide had a volume average particle size MV of 11.6 μm and a carbon content of 0.066% by mass. The ratio of the lithium metal ratio before and after the water washing treatment was 0.963, the initial discharge capacity was 207.5 mAh / g, and the relative value of the positive electrode resistance was 169. In Sample 9, the nickel composite oxide had a volume average particle size MV of 11.5 μm and a carbon content of 0.042% by mass. The ratio of the lithium metal ratio before and after the water washing treatment was 0.964, the initial discharge capacity was 209.0 mAh / g, and the relative value of the positive electrode resistance was 166. In Sample 10, the nickel composite oxide had a volume average particle size MV of 11.5 μm and a carbon content of 0.029% by mass. The ratio of the lithium metal ratio before and after the water washing treatment was 0.965, the initial discharge capacity was 207.8 mAh / g, and the relative value of the positive electrode resistance was 154.

[0077] (Example 3) The nickel composite hydroxide prepared in the same manner as in Example 1 was calcined in an air atmosphere using a small muffle furnace at a calcination temperature of 650 °C for 5 hours to produce a nickel composite oxide. The obtained nickel composite oxide had a volume average particle size MV of 11.6 μm and a carbon content of 0.066 mass%. For anhydrous lithium hydroxide, the one with the same average particle size MV of 18.4 μm as in Sample 3 was used.

[0078] These were mixed under the same conditions as in Example 1, and further charged into a ceramic firing container in the same manner as in Example 1 and fired in a roller hearth kiln. However, firing was carried out under the condition of holding at a maximum firing temperature of 550 °C for 330 minutes in an atmospheric gas with an oxygen concentration of 80 vol%. In this way, the lithium nickel composite oxide of Sample 11 was synthesized, and the battery characteristics were evaluated in the same manner as in Example 1. As a result, the ratio of the lithium metal ratio before and after the water washing treatment was 0.941, the initial discharge capacity was 197.6 mAh / g, and the relative value of the positive electrode resistance was 386.

[0079] In a roller hearth kiln, in an atmospheric gas with an oxygen concentration of 80 vol%, the firing temperature was raised from 600 °C to 650 °C at a constant heating rate over about 100 minutes, and then the lithium nickel composite oxide was synthesized under the firing condition of holding at a maximum firing temperature of 650 °C for 210 minutes. Thereafter, the lithium nickel composite oxide of Sample 12 was prepared in the same manner as in Example 1, and the battery characteristics were evaluated. As a result, the ratio of the lithium metal ratio before and after the water washing treatment was 0.970. Also, the initial discharge capacity was 208.7 mAh / g, and the relative value of the positive electrode resistance was 109.

[0080] In a roller hearth kiln, in an atmospheric gas with an oxygen concentration of 80 vol%, the lithium nickel composite oxide of Sample 13 was synthesized in the same manner as in the case of Sample 12 above, except that the firing temperature was raised from 600 °C to 750 °C at a constant heating rate over about 100 minutes, and then fired under the condition of holding at a maximum firing temperature of 750 °C for 210 minutes, and the battery characteristics were evaluated. As a result, the ratio of the lithium metal ratio before and after the water washing treatment was 0.971, the initial discharge capacity was 209.8 mAh / g, and the relative value of the positive electrode resistance was 100.

[0081] In a roller hearth kiln, a lithium nickel composite oxide of Sample 14 was synthesized in the same manner as in the case of Sample 12 except that the firing temperature was increased from 600 °C to 850 °C at a constant heating rate over about 100 minutes in an atmosphere gas with an oxygen concentration of 80% by volume, and then fired under the condition of holding at the maximum firing temperature of 850 °C for 210 minutes, and the battery characteristics were evaluated. As a result, the ratio of the lithium metal ratio before and after the water washing treatment was 0.952, the initial discharge capacity was 199.9 mAh / g, and the relative value of the positive electrode resistance was 254.

[0082] In a roller hearth kiln, a lithium nickel composite oxide of Sample 15 was synthesized in the same manner as in the case of Sample 12 except that the firing temperature was increased from 600 °C to 765 °C at a constant heating rate over about 100 minutes in an atmosphere gas with an oxygen concentration of 50% by volume, and then fired under the condition of holding at the maximum firing temperature of 765 °C for 210 minutes, and the battery characteristics were evaluated. As a result, the ratio of the lithium metal ratio before and after the water washing treatment was 0.938, the initial discharge capacity was 196.9 mAh / g, and the relative value of the positive electrode resistance was 400.

[0083] In a roller hearth kiln, a lithium nickel composite oxide of Sample 16 was synthesized in the same manner as in the case of Sample 12 except that the firing temperature was increased from 600 °C to 765 °C at a constant heating rate over about 100 minutes in an atmosphere gas with an oxygen concentration of 60% by volume, and then fired under the condition of holding at the maximum firing temperature of 765 °C for 210 minutes, and the battery characteristics were evaluated. As a result, the ratio of the lithium metal ratio before and after the water washing treatment was 0.954, the initial discharge capacity was 201.8 mAh / g, and the relative value of the positive electrode resistance was 251.

[0084] In a roller hearth kiln, a lithium nickel composite oxide of Sample 17 was synthesized in the same manner as in the case of Sample 12 except that the firing temperature was increased from 600 °C to 765 °C at a constant heating rate over about 100 minutes in an atmosphere gas with an oxygen concentration of 70% by volume, and then fired under the condition of holding at the maximum firing temperature of 765 °C for 210 minutes, and the battery characteristics were evaluated. As a result, the ratio of the lithium metal ratio before and after the water washing treatment was 0.957, the initial discharge capacity was 202.6 mAh / g, and the relative value of the positive electrode resistance was 226.

[0085] In a roller hearth kiln, in an atmosphere gas with an oxygen concentration of 80% by volume, the firing temperature was raised from 600 °C to 765 °C at a constant heating rate over about 100 minutes, and then the lithium nickel composite oxide of Sample 18 was synthesized in the same manner as in the case of Sample 12 except that it was fired under the condition of holding at the maximum firing temperature of 765 °C for 100 minutes, and the battery characteristics were evaluated. As a result, the ratio of the lithium metal ratio before and after the water washing treatment was 0.946, the initial discharge capacity was 198.5 mAh / g, and the relative value of the positive electrode resistance was 311.

[0086] In a roller hearth kiln, in an atmosphere gas with an oxygen concentration of 80% by volume, the firing temperature was raised from 600 °C to 765 °C at a constant heating rate over about 100 minutes, and then the lithium nickel composite oxide of Sample 19 was synthesized in the same manner as in the case of Sample 12 except that it was fired under the condition of holding at the maximum firing temperature of 765 °C for 150 minutes, and the battery characteristics were evaluated. As a result, the ratio of the lithium metal ratio before and after the water washing treatment was 0.953, the initial discharge capacity was 199.7 mAh / g, and the relative value of the positive electrode resistance was 262.

[0087]

Table 1

[0088] As is clear from the results in Table 1 above and FIGS. 3 to 4, there is a correlation between the ratio of the lithium metal ratio before and after the water washing treatment, the initial discharge capacity, and the positive electrode resistance. It can be seen that if the ratio of the lithium metal ratio before and after the water washing treatment exceeds 0.95, a positive electrode active material with a large discharge capacity and a small positive electrode resistance can be stably obtained.

Explanation of Signs

[0089] 1 Coin-type battery 2 Case 2a Positive electrode can 2b Negative electrode can 2c Gasket 3 Electrode 3a Positive electrode 3b Negative electrode 4 Separator 5 Current collector

Claims

1. A method for producing a positive electrode active material for a lithium-ion secondary battery, comprising at least nickel and lithium, the method comprising: heating a mixture of a nickel compound powder having a carbon content of 0.042% by mass or more and 0.25% by mass or less and a volume average particle diameter MV of 10 μm or more and 26 μm or less and an anhydrous lithium compound powder having a volume average particle diameter MV of 10 μm or more and 26 μm or less (excluding 10 μm) at a constant heating rate from the start of heating and performing a firing process under conditions of holding at a maximum firing temperature of 650°C or more and 850°C or less for 2 hours or more and 3.5 hours or less; and a water washing process of water washing the lithium nickel composite oxide powder obtained in the firing process, wherein the ratio of the amount of lithium to the total amount of transition metals other than lithium in the lithium nickel composite oxide powder after the water washing process is divided by the ratio of the amount of lithium to the total amount of transition metals other than lithium in the lithium nickel composite oxide powder before the water washing process, and the firing process is performed under conditions where the obtained value exceeds 0.

95. A method for producing a positive electrode active material for a lithium-ion secondary battery, characterized in that.

2. The method for producing a positive electrode active material for a lithium-ion secondary battery according to claim 1, wherein the nickel compound powder is a powder of nickel oxide or nickel hydroxide.

3. The method for producing a positive electrode active material for a lithium-ion secondary battery according to claim 1 or 2, wherein the lithium compound powder is lithium hydroxide powder.

4. The method for producing a positive electrode active material for a lithium-ion secondary battery according to any one of claims 1 to 3, wherein the oxygen concentration of the atmosphere gas when firing the mixture of the nickel compound powder and the lithium compound powder is 60% by volume or more.

5. The method for producing a positive electrode active material for a lithium-ion secondary battery according to any one of claims 1 to 4, wherein the nickel compound powder contains a powder of a composite oxide or composite hydroxide of nickel and other transition metals.

6. The method for producing a positive electrode active material for a lithium-ion secondary battery according to any one of claims 1 to 5, further comprising a pretreatment step of roasting the nickel compound powder at a roasting temperature of 500°C or more and 800°C or less to obtain a nickel oxide powder before the firing step.

7. The lithium nickel composite oxide has a general formula Li ｘ Ni １－ｙ－ｚ M ｙ N ｚ O ２ (wherein M is at least one element selected from Co and Mn, N is at least one element selected from Al, Ti, Nb, V, Mg, W, and Mo, x is 0.90 to 1.10, y is 0.05 to 0.35, and Z is 0.005 to 0.05), and a method for producing a positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 6.

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