JP7262419B2 - Positive electrode active material for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery - Google Patents

Description

The present invention relates to a positive electrode active material for non-aqueous electrolyte secondary batteries and non-aqueous electrolyte secondary batteries.

In recent years, with the spread of mobile electronic devices such as mobile phones and laptop computers, there is a demand for the development of small, lightweight non-aqueous electrolyte secondary batteries with high energy density. There is also a demand for the development of high-output non-aqueous electrolyte secondary batteries as batteries for electric vehicles such as hybrid vehicles. Lithium ion secondary batteries are known as non-aqueous electrolyte secondary batteries that satisfy such requirements. A lithium ion secondary battery is composed of a negative electrode, a positive electrode, an electrolytic solution, and the like, and a material capable of desorbing and intercalating lithium is used as an active material for the negative electrode and the positive electrode.

Lithium ion secondary batteries are currently being vigorously researched and developed. Among them, a lithium-ion secondary battery using a layered or spinel-type lithium metal composite oxide as a positive electrode active material can obtain a high voltage of 4V class, and is being put to practical use as a battery having a high energy density. Lithium-metal composite oxides mainly proposed so far include lithium-cobalt composite oxides (e.g., LiCoO ₂ ), which are relatively easy to synthesize, and lithium-nickel composite oxides using nickel, which is cheaper than cobalt ( LiNiO ₂ ), lithium-nickel-cobalt-manganese composite oxides (eg, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ), lithium-manganese composite oxides (eg, LiMn ₂ O ₄ ), and the like.

Batteries using lithium-cobalt composite oxide as a positive electrode active material have been developed to obtain excellent initial capacity characteristics and cycle characteristics, and various results have already been obtained. However, the lithium-cobalt composite oxide uses an expensive cobalt compound as a raw material. For this reason, lithium-cobalt composite oxides are significantly higher in unit price per capacity than nickel-metal hydride batteries, and their applications as positive electrode active materials are considerably limited. Therefore, not only for small secondary batteries for portable devices, but also for large secondary batteries for power storage and electric vehicles, it is necessary to reduce the cost of positive electrode active materials and manufacture cheaper lithium ion secondary batteries. There are high expectations for making it possible, and its realization is of great industrial significance.

Lithium-nickel composite oxides using nickel, which is cheaper than cobalt, exhibit a lower electrochemical potential than lithium-cobalt composite oxides. Since it shows a high battery voltage like the cobalt system, it is being actively developed. However, when a lithium-nickel composite oxide synthesized purely from nickel is used as a positive electrode material in a lithium-ion secondary battery, the cycle characteristics are inferior to those of cobalt-based materials, and it is difficult to use in high-temperature environments. It has a drawback that the battery performance is relatively likely to be damaged by storage. Therefore, as disclosed in Patent Document 1, for example, lithium-nickel composite oxides in which nickel is partially replaced with cobalt or aluminum are generally known.

As a general method for producing a lithium-nickel composite oxide as a positive electrode active material, a nickel composite hydroxide as a precursor is produced by a neutralization crystallization method, and this precursor is combined with a lithium compound such as lithium hydroxide. A method of mixing and firing to obtain a lithium-nickel composite oxide is known. However, unreacted lithium hydroxide remains in the lithium-nickel composite oxide synthesized by this method. Unreacted lithium hydroxide causes gelation of the positive electrode mixture paste when the positive electrode active material is kneaded into the positive electrode mixture paste. Furthermore, when the positive electrode active material is charged in a high-temperature environment, unreacted lithium hydroxide is oxidatively decomposed to cause gas generation.

Therefore, according to Patent Document 2, a method is proposed in which natural water is added to the synthesized lithium-nickel composite oxide and stirred to remove lithium hydroxide. Further, Patent Document 3 proposes a method of removing unreacted alkali contained in the lithium-nickel composite oxide after firing by washing with water. However, in these washing methods using water, not only is lithium hydroxide contained in the lithium-nickel composite oxide removed during washing, but lithium is also extracted from the lattice of the lithium-nickel composite oxide. Due to the elution of a large amount of the compound, there is a problem that lithium ion deficiency occurs in the crystals on the surface, resulting in a decrease in battery capacity and an increase in battery resistance.

Therefore, according to Patent Document 4, natural water is added to the synthesized lithium-nickel composite oxide and stirred to remove the lithium hydroxide, and then the mixture is heated at 120° C. to 550° C. in an oxygen atmosphere with an oxygen concentration of 80% by volume or more. A method of heat-treating at the following temperatures has been proposed. In the positive electrode active material obtained by this method, the lithium on the surface that is deficient during washing with water is compensated from the inside of the particles, so there is no lithium deficiency on the surface, and the positive electrode resistance of the battery can be reduced. However, a very small amount of lithium is replenished from the inside of the positive electrode active material particles to the particle surface, and there is room for improvement in solving the lithium deficiency seen from the overall positive electrode active material.

JP-A-05-242891 Japanese Patent Application Laid-Open No. 2007-273108 JP-A-1996-138669 WO2014/189108

In view of such problems, the object of the present invention is to suppress the gelation of the positive electrode mixture paste, and when used in a secondary battery, obtain a high capacity and reduce the positive electrode resistance. An object of the present invention is to provide a positive electrode active material for an electrolyte secondary battery.

In order to solve the above problems, the present inventors have made intensive research on lithium metal composite oxides used as positive electrode active materials for non-aqueous electrolyte secondary batteries and methods for producing the same. By washing the powder with an aqueous solution containing a lithium salt, it is possible to remove unreacted lithium hydroxide and impurities derived from the raw material and prevent lithium from being extracted from the lattice of the lithium-nickel composite oxide. The present invention was completed after obtaining the knowledge that there is.

In the first aspect of the present invention, the general formula Li _z Ni _1-xy Co _x M _y O ₂ (where 0≦x≦0.35, 0≦y≦0.10, 0.95≦z≦ 1.10, M is at least one element selected from Mn, V, Mg, Mo, Nb, Ti and Al). A substance having a lithium hydroxide content of 0.5% by mass or less, and the ratio of Li and metals other than Li (Ni, Co and M) on the surface of the powder measured by X-ray photoelectron spectroscopy A positive electrode active material for a non-aqueous electrolyte secondary battery having a composition ratio (Li/(Ni+Co+M)) of 0.80 or more and 1.5 or less is provided.

Moreover, it is preferable that the pH of the powder in a 5% by mass suspension of the powder dispersed in water is 11.5 or less.

A second aspect of the present invention provides a non-aqueous electrolyte secondary battery containing the positive electrode active material for a non-aqueous electrolyte secondary battery in its positive electrode.

ADVANTAGE OF THE INVENTION According to the positive electrode active material of the present invention, a non-aqueous electrolyte secondary in which gelation of the positive electrode mixture paste is suppressed, and a high capacity is obtained and the positive electrode resistance is reduced when used in a secondary battery A positive electrode active material for batteries is obtained. Furthermore, the manufacturing method of the present invention can easily produce this positive electrode active material, and is particularly suitable for mass production on an industrial scale, so its industrial value is extremely high.

FIG. 1 is a flow chart showing an example of a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to this embodiment. FIG. 2 is a schematic cross-sectional view of a coin-type battery used for battery evaluation. FIG. 3 shows a Nyquist plot (top) and an equivalent circuit (bottom) obtained by the impedance measurement method.

1. Method for Producing Positive Electrode Active Material for Non-aqueous Electrolyte Secondary Battery Hereinafter, an example of an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a flow chart showing a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery (hereinafter also referred to as "positive electrode active material") according to this embodiment. In addition, the following description is an example of the manufacturing method, and does not limit the manufacturing method of the present invention.

As shown in FIG. 1, a powder made of lithium-nickel composite oxide is washed with an aqueous solution of lithium carbonate (step S1). First, a powder made of a lithium-nickel composite oxide (hereinafter also simply referred to as “powder”) is prepared as a base material. The powder has the general formula Li _z Ni _1-x-y Co _x M _y O ₂ (where 0≦x≦0.35, 0≦y≦0.10, 0.95≦z≦1.10, M is , Mn, V, Mg, Mo, Nb, Ti and Al).

The method for producing the powder is not particularly limited, and it can be produced by a known method. Powder production methods include, for example, a method of mixing a compound containing lithium and a compound containing metals other than lithium (transition metals such as nickel and cobalt, aluminum, etc.) and firing the mixture; A hydroxide containing a metal other than lithium obtained by a method of spray pyrolysis treatment of an aqueous solution containing lithium or a neutralization crystallization method, or an oxide obtained by heat treatment of the hydroxide, and a lithium compound are mixed. and firing. Among these methods, the method using a hydroxide containing a metal other than lithium obtained by the neutralization crystallization method can easily control the specific surface area of the resulting powder within a desired range. Moreover, when a hydroxide or the like is used as the raw material of the powder, and substances derived from these substances remain in the powder, the production method of the present embodiment can be preferably used.

The powder is washed with an aqueous solution containing one or more lithium salts selected from water-soluble lithium salts other than lithium hydroxide (hereinafter also referred to as "lithium salt aqueous solution"). Washing is performed, for example, by dispersing the powder in an aqueous lithium salt solution and stirring. By washing with the aqueous solution containing the lithium salt, impurities such as lithium hydroxide existing on the powder surface are removed, and at the same time lithium abstraction from the crystal lattice of the powder surface is suppressed. As a result, while suppressing gelation of the positive electrode mixture paste, it is possible to obtain a positive electrode active material that has a high capacity and excellent output characteristics by reducing the positive electrode resistance of the battery.

The aqueous solution used for washing contains, as a solute, one or more lithium salts selected from water-soluble lithium salts other than lithium hydroxide. The water-soluble lithium salt other than lithium hydroxide is not particularly limited, and known lithium salts can be used. Examples include lithium carbonate, lithium hydrogen carbonate, lithium citrate, lithium acetate, lithium oxalate, lithium tartrate, Lithium sulfate, lithium nitrate, lithium chloride, lithium bromide, lithium iodide, and the like can be used. Further, by using lithium nitrate or lithium sulfate as the water-soluble lithium salt, the positive electrode resistance (hereinafter also referred to as “reaction resistance”) can be further reduced. Moreover, from the viewpoint of reducing the amount of residual sulfate group, it is preferable to use a water-soluble lithium salt other than sulfate. Moreover, lithium salt may be used individually by 1 type, and 2 or more types may be used together. In addition, solutes other than the water-soluble lithium salt may be included as long as the effects of the invention are not impaired.

Note that when the positive electrode active material is washed with an aqueous solution containing lithium hydroxide, the lithium hydroxide remaining in the positive electrode active material is one of the causes of gelation of the positive electrode mixture paste. In addition, when this positive electrode active material is charged in a high-temperature environment, the remaining lithium hydroxide is oxidatively decomposed to become one of the factors causing gas generation.

The lithium concentration of the lithium salt aqueous solution is not particularly limited, and can be within a water-soluble range. The lithium concentration is, for example, 0.1 g/L or more and 5.0 g/L or less. If the lithium concentration is less than 0.1 g/L, the effect of preventing extraction of lithium from within the crystal lattice of the powder surface is insufficient, and the expected effect may be difficult to obtain. On the other hand, if it exceeds 5.0 g/L, an excessive lithium compound may remain in the positive electrode active material, deteriorating battery performance.

The lithium concentration of the lithium salt aqueous solution is preferably 0.3 g/L or more and 5.0 g/L or less, more preferably 0.5 g/L or more and 3.0 g/L or less, and still more preferably 1.0 g/L or more. 2.5 g/L or less. When the lithium concentration is within the above range, the lithium content in the positive electrode active material can be easily adjusted to the desired range more efficiently.

The slurry concentration of the lithium salt aqueous solution containing the powder is not particularly limited as long as the powder is uniformly dispersed in the lithium salt aqueous solution. The slurry concentration is, for example, 100 g/L or more and 3000 g/L or less. Here, the unit of slurry concentration, g/L, means the amount of powder (g) with respect to the amount of lithium carbonate aqueous solution (L) in the slurry. When the slurry concentration is within the above range, the amount of powder contained in the slurry increases as the slurry concentration increases, and a large amount of powder can be processed. If the slurry concentration is less than 100 g/L, the effect of preventing tilium from being pulled out from the crystal lattice of the powder surface is insufficient, and the expected effect may not be obtained. On the other hand, when the slurry concentration exceeds 3000 g/L, the viscosity of the slurry becomes very high, making it difficult to stir, and lithium hydroxide may not be sufficiently removed.

The slurry concentration of the lithium salt aqueous solution containing the powder is preferably 100 g/L or more and 2500 g/L or less, more preferably 200 g/L or more and 2000 g/L or less, and still more preferably 400 g/L or more and 2000 g/L or less. When the slurry concentration is within the above range, the viscosity of the slurry is within an appropriate range, and lithium hydroxide and the like can be removed more efficiently.

Washing conditions other than those described above are not particularly limited, and can be adjusted as appropriate so that lithium hydroxide and sulfate radicals remaining in the powder are sufficiently removed and the content of lithium carbonate is within the desired range. For example, when a lithium carbonate aqueous solution containing powder is stirred, the stirring time can be about 5 minutes to 1 hour. The cleaning temperature can be, for example, about 10.degree. C. to 30.degree.

During washing, lithium in the powder is eluted into the slurry, and the atomic ratio of Li in the powder may differ before and after washing. In this case, the atomic ratio that changes due to cleaning is mainly Li, and the atomic ratio of metals other than Li before cleaning is maintained after cleaning. The atomic ratio of Li to be reduced by the cleaning is, for example, about 0.03 to 0.08. Compared to cleaning with normal water, cleaning with a lithium salt aqueous solution reduces the value of the atomic ratio of Li that decreases due to cleaning, and the decrease in Li tends to be moderated. Regarding the atomic ratio of Li after washing, the amount of decrease in the atomic ratio of Li before and after washing was confirmed by a preliminary test under the same washing conditions, and the lithium metal composite oxide powder in which the atomic ratio of Li was adjusted as the base material. can be controlled by using

Next, as shown in FIG. 1, after washing with an aqueous solution of lithium carbonate, the slurry containing the powder is filtered (step S2). A method of filtration is not particularly limited, and a known method can be used. Filtration can be performed, for example, using a commonly used filtration device such as a suction filter, filter press, or centrifuge. Filtration can reduce the amount of adhering water remaining on the surface of the powder during solid-liquid separation of the slurry. When a large amount of adhering water is present, the lithium salt dissolved in the liquid may be reprecipitated, and the amount of lithium present on the surfaces of the dried lithium-nickel composite oxide particles may be outside the expected range. It is optional whether step S2 is performed or not. If step 2S is not performed, the adhering water may be removed by, for example, allowing the slurry to stand still or centrifuging to remove the supernatant.

Next, as shown in FIG. 1, after filtration, the obtained powder is dried (step S3). The drying temperature is not particularly limited, and may be any temperature that sufficiently removes the moisture contained in the powder. The drying temperature is preferably 80° C. or higher and 350° C. or lower, for example. If the drying temperature is less than 80° C., drying of the powder after washing is slowed down, so that a lithium concentration gradient occurs between the powder surface and the inside of the powder, and the battery characteristics of the obtained positive electrode active material may deteriorate. . On the other hand, when the drying temperature exceeds 350° C., the crystal structure near the surface of the powder collapses, and the battery characteristics of the resulting positive electrode active material may deteriorate. This is because the crystal structure near the surface of the powder after washing is very close to the stoichiometric ratio, or is in a state close to a charged state due to the detachment of some lithium, and is likely to collapse. Conceivable.

The drying time is not particularly limited, and the moisture content of the powder after drying is 0.2% by mass or less, more preferably 0.1% by mass or less, and further preferably 0.05% by mass or less. is preferred. The drying time is, for example, 1 hour or more and 24 hours or less. The moisture content of the powder can be measured at a vaporization temperature of 300° C. using a Karl Fischer moisture meter.

The drying atmosphere is preferably a gas atmosphere that does not contain compound components containing carbon and sulfur, or a vacuum atmosphere. The amount of carbon and sulfur in the powder can be easily controlled by washing (step S1). At the time of drying (step S3), if the powder is further dried in an atmosphere containing carbon and sulfur compound components or in a vacuum atmosphere, the amount of carbon and sulfur in the powder will change, and the expected effect may not be obtained.

2. Positive electrode active material for non-aqueous electrolyte secondary battery The positive electrode active material according to the present embodiment has the general formula Li _z Ni _1-xy Co _x M _y O ₂ (where 0 ≤ x ≤ 0.35, 0 ≤ y ≦0.10, 0.95≦z≦1.10, M is at least one element selected from Mn, V, Mg, Mo, Nb, Ti and Al). A positive electrode active material consisting of a lithium hydroxide content of 0.5% by mass or less, and metals other than Li and Li on the surface of the powder measured by X-ray photoelectron spectroscopy (Ni, Co and M) and the composition ratio (Li/(Ni+Co+M)) is 0.80 or more and 1.5 or less. An example of an embodiment of the positive electrode active material will be described below.

[Powder overall composition]
The positive electrode active material for non-aqueous electrolyte secondary batteries has the general formula Li _z Ni _1-xy Co _x M _y O ₂ (where 0≦x≦0.35, 0≦y≦0.10, 0.95 ≦z≦1.10, M is at least one element selected from Mn, V, Mg, Mo, Nb, Ti and Al).

In the above general formula, z represents the atomic ratio of Li when the atomic ratio of metals (Ni, Co and M) other than Li in the lithium-nickel composite oxide is 1. The range of z is 0.95≤z≤1.10. When z is within the above range, the charge/discharge capacity increases as the value of z increases. When z is less than 0.95, the reaction resistance of the positive electrode increases and the battery output may decrease. On the other hand, when z exceeds 1.10, the safety of the secondary battery may deteriorate. From the viewpoint of the balance between battery output and safety, the range of z is preferably 0.97≦z≦1.05, more preferably 0.97≦z≦1.00. As described above, when a powder made of a lithium-nickel composite oxide is used as a base material and washed, Li may be eluted from this powder. Therefore, in the case of washing, the amount of decrease in Li before and after washing is confirmed by a preliminary experiment, and the atomic ratio of Li is obtained by preparing the powder before washing so that the element ratio of Li after washing is within the above range. can be within the above range.

In the above general formula, x represents the element ratio of Co when the atomic ratio of metals (Ni, Co and M) other than Li is 1. The range of x is 0≤x≤0.35, preferably 0<x≤0.35. Good cycle characteristics can be obtained by including cobalt in the positive electrode active material. This is because by substituting cobalt for a portion of nickel in the crystal lattice, it is possible to reduce the expansion and contraction behavior of the crystal lattice due to the desorption/insertion of lithium during charging and discharging.

The range of x is preferably 0.03≦x≦0.35, more preferably 0.05≦x≦0.35, from the viewpoint of improving the cycle characteristics of the secondary battery. Also, the range of x is preferably 0.03≦x≦0.15, more preferably 0.05≦x≦0.15, from the viewpoint of the battery capacity of the secondary battery. Also, the range of x is preferably 0.07≦x≦0.25, more preferably 0.10≦x≦0.20, from the viewpoint of thermal stability.

In the above general formula, y represents the elemental ratio of M (additional element) when the atomic ratio of metals (Ni, Co and M) other than Li is 1. M is at least one element selected from Mn, V, Mg, Mo, Nb, Ti and Al. The range of y is 0≦y≦0.10, preferably 0<y≦0.10, which must include M, and more preferably 0<y≦0.05. By adding M to the positive electrode active material, the durability and safety of the secondary battery containing this positive electrode active material can be improved. On the other hand, if y exceeds 0.10, the metal element that contributes to the oxidation-reduction reaction (Redox reaction) decreases, and the battery capacity may decrease. Moreover, when M is aluminum, the safety of the positive electrode active material is further improved.

In the above general formula, the elemental ratio of nickel is 0.55 or more and 1 or less when the atomic ratio of metals (Ni, Co and M) other than Li is 1. The atomic ratio of each metal element in the lithium-nickel composite oxide can be set within the above range by adjusting the mixing ratio of the raw materials containing Li, Ni, Co and M.

[Lithium hydroxide content]
The positive electrode active material of the present embodiment has a lithium hydroxide content of 0.5% by mass or less, preferably 0.2% by mass or less. If the lithium hydroxide content in the positive electrode active material exceeds 0.5% by mass, it becomes one of the causes of gelation when the positive electrode active material is kneaded into a paste. Furthermore, when the positive electrode active material is charged in a high-temperature environment, lithium hydroxide is oxidatively decomposed to become one of the causes of gas generation. Moreover, the lower limit of the lithium hydroxide content in the positive electrode active material is not particularly limited, but is, for example, 0.01% by mass or more.

Here, the lithium hydroxide contained in the positive electrode active material includes lithium hydroxide derived from raw materials used when producing the positive electrode active material. For example, it includes an unreacted product when nickel composite hydroxide or nickel composite oxide is mixed with a lithium compound such as lithium hydroxide and fired to obtain a lithium-nickel composite oxide. The lithium hydroxide content is measured by adding pure water to the obtained positive electrode active material and stirring, and then measuring the amount of lithium (Li) eluted in this pure water by neutralization titration with 1 mol/liter of hydrochloric acid. After that, the value obtained by subtracting the amount of lithium (Li) derived from the lithium salt used in washing from the amount of eluted lithium (Li) is taken as the amount of lithium (Li) derived from lithium hydroxide, and this is converted to LiOH. It is a value obtained by Here, the amount of lithium (Li) derived from the lithium salt was determined by chemically analyzing the amount of one or more species other than Li contained in the lithium salt and converting it into the amount of the lithium salt.

[Li/(Ni + Co + M) on powder surface]
In the positive electrode active material of the present embodiment, the composition ratio (Li/(Ni + Co + M)) of Li and metals other than Li (Ni, Co and M) on the surface of the powder measured by X-ray photoelectron spectroscopy is 0.80. 1.5 or less, preferably 0.80 or more and 1.45 or less, more preferably 0.93 or more and 1.45 or less, still more preferably 0.95 or more and 1.45 or less, particularly preferably 1.00 or more 1.45 or less. When Li/(Ni + Co + M) on the powder surface is less than 0.80, lithium ion deficiency occurs on the particle surface, so when used for the positive electrode of a secondary battery, the conduction path of lithium ions is hindered and the discharge capacity decreases. This is one of the factors that increase reaction resistance. Since the reaction resistance is reduced, the voltage lost in the battery is reduced, and the voltage actually applied to the load side is relatively increased, resulting in high output. On the other hand, when Li/(Ni+Co+M) on the powder surface exceeds 1.5, excessive lithium compounds such as lithium hydroxide are present on the powder surface, which is one of the factors causing gelation of the positive electrode mixture paste. Furthermore, when a positive electrode active material having an excessive lithium compound present on its surface is charged in a high-temperature environment, the lithium compound may decompose to generate gas, which may deteriorate the battery characteristics. In addition, if there is a lithium compound that does not contribute to charge/discharge, an extra amount of negative electrode material corresponding to the irreversible capacity of the positive electrode active material is used when constructing the battery. As a result, the capacity per weight and per volume of the battery as a whole may be reduced, and the excess lithium accumulated in the negative electrode as irreversible capacity may pose a safety problem.

The composition ratio (Li/(Ni+Co+M)) of Li and metals other than Li on the surface of the powder can be measured by X-ray photoelectron spectroscopy, as described in detail in Examples below. Further, the powder surface is measured by an X-ray photoelectron spectroscopy (XPS) device (manufactured by ULVAC-Phi, Inc., Versa ProbeII) from the surface of the positive electrode active material to a depth of several nm to about 10 nm toward the center. means the area of

[Powder pH]
The positive electrode active material of the present embodiment has a powder pH of 11.5 or less in a 5% by mass suspension solution in which the powder is dispersed in water. If the pH exceeds 11.5, the positive electrode mixture paste may gel when the positive electrode active material is kneaded into the paste. Although the lower limit of the powder pH is not particularly limited, it is, for example, preferably 10.5 or higher, more preferably 11.0 or higher.

[Average particle diameter]
Although the average particle size of the positive electrode active material of the present embodiment is not particularly limited, for example, when it is 3 μm or more and 25 μm or less, the battery capacity per volume of the positive electrode active material can be increased, and safety is high. A secondary battery with good cycle characteristics can be obtained. The average particle size is a value measured by a laser diffraction particle size distribution analyzer.

[Specific surface area]
Although the specific surface area of the positive electrode active material of the present embodiment is not particularly limited, for example, when it is 1.0 m ² /g or more and 7.0 m ² /g or less, the particle surface that can be in contact with the electrolytic solution is sufficient. If the specific surface area is less than 1.0 m ² /g, the surface area of the particles that can come into contact with the electrolytic solution is reduced, and sufficient charge/discharge capacity may not be obtained. On the other hand, if the specific surface area exceeds 7.0 m ² /g, the surface of the particles that come into contact with the electrolytic solution may become too large, resulting in a decrease in safety. The specific surface area is a value measured by a specific surface area measuring device using the BET method based on the nitrogen gas adsorption method.

The positive electrode active material of the present embodiment can be easily mass-produced on an industrial scale by using the above-described method for producing a positive electrode active material.

3. Non-Aqueous Electrolyte Secondary Battery A non-aqueous electrolyte secondary battery according to the present embodiment includes the positive electrode active material described above in the positive electrode. The non-aqueous electrolyte secondary battery of the present embodiment can be composed of a positive electrode, a negative electrode, a separator, and a non-aqueous electrolytic solution, like general non-aqueous electrolyte secondary batteries. In the following, embodiments of the non-aqueous electrolyte secondary battery will be described in detail with respect to each component and the shape and configuration of the battery.

[Positive electrode]
The positive electrode compound material forming the positive electrode and each material constituting the same will be described. The powdery positive electrode active material of the present invention, a conductive material, and a binder are mixed, and if necessary, activated carbon and a solvent for viscosity adjustment are added, and the mixture is kneaded to form a positive electrode mixture paste. make. The mixing ratio of each material in the positive electrode mixture is also an important factor that determines the performance of the lithium secondary battery.

The mixing ratio of each material in the positive electrode mixture is not particularly limited, but similar to the positive electrode of a general lithium secondary battery, the total solid content of the positive electrode mixture excluding the solvent is 100% by mass. It is desirable to contain 60% by mass or more and 95% by mass or less of the positive electrode active material, 1% by mass or more and 20% by mass or less of the conductive material, and 1% by mass or more and 20% by mass or less of the binder.

The obtained positive electrode mixture paste is applied, for example, to the surface of a current collector made of aluminum foil, and dried to scatter (evaporate) the solvent. If necessary, pressure may be applied by a roll press or the like in order to increase the electrode density. Thus, a sheet-like positive electrode can be produced. The sheet-like positive electrode can be cut into an appropriate size depending on the intended battery, and used for manufacturing the battery. However, the method for producing the positive electrode is not limited to the above examples, and other methods may be used.

In manufacturing the positive electrode, 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 conductive material.

In addition, the binder plays a role of binding the active material particles. Thermoplastic resins such as resins, polyacrylic acid, polypropylene, and polyethylene can be used.

Further, if necessary, the positive electrode active material, the conductive material, and the activated carbon may be dispersed, and a solvent that dissolves the binder may be added to the positive electrode mixture. As the solvent to be added, for example, an organic solvent such as N-methyl-2-pyrrolidone can be used. In addition, activated carbon may be added to the positive electrode mixture in order to increase the electric double layer capacity.

[Negative electrode]
For the negative electrode, metal lithium, a lithium alloy, etc., or a negative electrode active material capable of intercalating and deintercalating lithium ions, mixed with a binder, added with an appropriate solvent and made into a paste, was made into a negative electrode compound material such as copper. is coated on the surface of the metal foil current collector, dried, and compressed to increase the electrode density as necessary.

As the negative electrode active material, for example, natural graphite, artificial graphite, baked organic compounds such as phenol resin, and powdery carbon substances such as coke can be used. In this case, as the negative electrode binder, similarly to the positive electrode, fluorine-containing resin such as polyvinylidene fluoride can be used. Organic solvents can be used.

[Separator]
A separator is interposed between the positive electrode and the negative electrode. The separator separates the positive electrode from the negative electrode and holds the electrolyte, and may be a thin film of polyethylene, polypropylene, or the like, having a large number of fine holes.

[Non-aqueous electrolyte]
The non-aqueous electrolytic solution is obtained by dissolving a lithium salt as a supporting salt in an organic solvent. Examples of organic solvents include cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate; chain carbonates such as diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate, and dipropyl carbonate; One selected from ether compounds such as methyltetrahydrofuran and dimethoxyethane, sulfur compounds such as ethylmethylsulfone and butane sultone, and phosphorus compounds such as triethyl phosphate and trioctyl phosphate, and the like, or a mixture of two or more thereof is used. be able to.

As the supporting salt, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiN(CF ₃ SO ₂ ) ₂ and the like, and composite salts thereof can be used. Furthermore, the nonaqueous electrolytic solution may contain a radical scavenger, a surfactant, a flame retardant, and the like.

[Battery Shape and Configuration]
The shape of the lithium secondary battery according to this embodiment can be various shapes such as a cylindrical shape and a laminated shape. Regardless of which shape is used, the positive electrode and the negative electrode are laminated via a separator to form an electrode assembly, and the electrode assembly is impregnated with the non-aqueous electrolyte. A current collecting lead or the like is used to connect between the positive electrode current collector and the positive electrode terminal connected to the outside, and between the negative electrode current collector and the negative electrode terminal connected to the outside. A battery can be completed by sealing the above structure in a battery case.

EXAMPLES The present invention will be described below with reference to examples and comparative examples, but the present invention is not limited to the following examples. The examples and comparative examples were evaluated based on the measurement results using the following apparatus and method.

[Whole particle composition]
After dissolving the lithium-nickel composite oxide powder used as the base material in nitric acid, the composition ratio of each component was measured by an ICP emission spectrometer (ICPS-8100, manufactured by Shimadzu Corporation). Moreover, the obtained positive electrode active material was measured by the same method as described above.

[Particle surface composition]
The positive electrode active material thus obtained was measured using an X-ray photoelectron spectrometer (Versa ProbeII, manufactured by ULVAC-Phi, Inc.). At this time, monochromatic Al-Kα rays (1486.7 eV) were used as the X-ray source, the tilt angle was 45°, the pass energy was 187.85 eV, and the degree of vacuum was 10 ^-7 Pa. .

[Lithium hydroxide content]
100 ml of ultrapure water was added to 10 g of the obtained positive electrode active material powder, stirred for 5 minutes, filtered, and the filtrate was titrated with 1 mol/liter hydrochloric acid to measure up to the second neutralization point. The amount of alkali content neutralized with hydrochloric acid was taken as the total amount of lithium hydroxide (LiOH) and the amount of lithium (Li) derived from the lithium salt used for washing. Then, as shown in the following formula, the amount obtained by subtracting the amount of Li derived from the lithium salt used for washing from the amount of alkali content neutralized by neutralization titration is the amount of Li derived from lithium hydroxide (LiOH). bottom. The amount of lithium (Li) derived from the lithium salt used for washing was calculated from the lithium salt content determined by the following method.

(Amount of Li derived from lithium hydroxide) = (amount of alkali content neutralized with hydrochloric acid) - (amount of Li derived from lithium salt used for washing) (formula)

The lithium hydroxide content was obtained by converting the amount of Li derived from lithium hydroxide (LiOH) calculated by the above formula into the amount of LiOH.

(lithium carbonate, lithium citrate and/or lithium acetate)
These lithium salt contents are determined by measuring the total carbon element (C) content with a carbon sulfur analyzer (CS-600 manufactured by LECO), and converting the measured total carbon element amount into each lithium salt. I asked for it.
(lithium sulfate)
The lithium sulfate content was obtained by measuring the elemental sulfur (S) content by ICP emission analysis and converting the measured elemental sulfur (S) content into lithium sulfate.
(lithium nitrate)
The lithium nitrate content was measured by stirring the positive electrode active material powder in ultrapure water to elute lithium nitrate, filtering the filtrate, and measuring the nitrate group content of the filtrate by ion chromatography. It was determined by converting the nitrate content into lithium nitrate.

[Powder pH]
5.0 g of the obtained positive electrode active material powder was dispersed in 100 ml of distilled water to prepare a 5% by mass suspension, and the pH value of the suspension was measured after stirring at 25° C. and room temperature for 30 minutes.

[Determination of Gelation of Paste]
For 20 g of the obtained positive electrode active material, put 2.2 g of PVDF (manufactured by Kureha Chemical Industry Co., Ltd., model number KF polymer # 1100) and 9.6 ml of NMP (manufactured by Kanto Chemical Industry) in a container, and kneader (Nippon Seiki Seisakusho, product The mixture was sufficiently mixed for 10 minutes with a non-bubbling kneader (model number NBK-1) at a rotational speed of 2000 rpm to prepare a paste. The obtained paste was transferred to a glass bottle, sealed, stored in a dry box at a temperature of 25° C. and a dew point of −40° C., and the fluidity of the paste was observed after being left for 24 hours. After standing for 24 hours, the case where the fluidity of the paste did not change was evaluated as ⊚, the case where the fluidity of the paste was changed but the fluidity was changed was evaluated as ◯, and the case where gelation occurred was evaluated as ×.

[Evaluation of battery characteristics]
(1) Production of Coin Battery for Evaluation To 70% by mass of the positive electrode active material thus obtained, 20% by mass of acetylene black and 10% by mass of PTFE were mixed, and 150 mg of the mixture was taken out to prepare a pellet, which was used as a positive electrode. Lithium metal was used as the negative electrode, and an equivalent mixed solution of ethylene carbonate (EC) and diethyl carbonate (DEC) with 1 M LiClO ₄ as the supporting electrolyte (manufactured by Toyama Pharmaceutical Co., Ltd.) was used as the electrolyte, and the dew point was −80 ° C. A 2032-type coin-type battery as shown in FIG. The 2032-type evaluation coin-type battery BA includes a lithium metal negative electrode 1 as a negative electrode, a separator 2 impregnated with an electrolytic solution, a positive electrode 3, a gasket 4, a negative electrode can 5, a positive electrode can 6, and a current collector. 7.

(2) Discharge capacity The prepared coin-type battery BA is left for about 24 hours, and after the open circuit voltage OCV (open circuit voltage) is stabilized, the current density for the positive electrode is set to 0.5 mA / cm and charged to a cutoff voltage of 4.3 V. The charge capacity was evaluated as the discharge capacity, and the capacity when the battery was discharged to a cutoff voltage of 3.0 V after resting for 1 hour was evaluated as the discharge capacity.

(3) Positive Electrode Resistance The prepared coin-type battery BA was charged at a charging potential of 4.1 V, and measured by the AC impedance method using a frequency response analyzer and a potentiogalvanostat (manufactured by Solartron, 1255B). The upper part of FIG. 2 shows the Nyquist plots obtained. Since this Nyquist plot is expressed as the sum of characteristic curves showing solution resistance, negative electrode resistance and its capacity, and positive electrode resistance and its capacity, fitting calculation is performed using the equivalent circuit shown in the lower part of FIG. 2 based on this Nyquist plot. was performed to calculate the value of the positive electrode resistance. Positive electrode resistance was evaluated as a relative value with Example 1 as 100.

(Example 1)
Lithium nickel represented by Li _1.03 Ni _0.88 Co _0.09 Al _0.03 O ₂ obtained by a known technique of mixing and firing an oxide powder containing nickel as a main component and lithium hydroxide A sintered powder of composite oxide was obtained. This powder was used as the base material. This powder had an average particle size of 12.0 μm and a specific surface area of 1.2 m ² /g. The average particle size was measured using a laser diffraction particle size distribution meter (Microtrac, manufactured by Nikkiso Co., Ltd.), and the specific surface area was measured using a specific surface area measuring device (Kantasorb QS-10, manufactured by Yuasa Ionics Co., Ltd.), using nitrogen gas adsorption. It was evaluated using the BET method by.

A lithium carbonate aqueous solution prepared so that the amount of lithium was 1.5 g/L was added to the lithium nickel composite oxide powder (base material) to adjust the slurry concentration to 750 g/L. After stirring and washing this slurry for 30 minutes, the powder taken out by filtration is dried in a vacuum atmosphere while being held at a temperature of 210° C. for 14 hours to obtain a positive electrode active material composed of a lithium-nickel composite oxide. rice field. The atomic ratio z of Li was 0.992 when the obtained positive electrode active material was measured by an ICP emission spectrometer.

(Example 2)
In Example 2, a positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the concentration of the lithium carbonate aqueous solution was adjusted so that the amount of lithium was 0.3 g/L.
(Example 3)
In Example 3, a positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the concentration of the lithium carbonate aqueous solution was adjusted so that the amount of lithium was 0.7 g/L.
(Example 4)
In Example 4, a positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the concentration of the lithium carbonate aqueous solution was adjusted so that the amount of lithium was 1.0 g/L.
(Example 5)
In Example 5, a positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the concentration of the lithium carbonate aqueous solution was adjusted so that the amount of lithium was 2.5 g/L.
(Example 6)
In Example 6, a positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the concentration of the lithium carbonate aqueous solution was adjusted so that the amount of lithium was 3.0 g/L.

(Example 7)
In Example 7, a positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the concentration of the slurry was set to 100 g/L.
(Example 8)
In Example 8, a positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the concentration of the slurry was changed to 375 g/L.
(Example 9)
In Example 9, a positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the concentration of the slurry was changed to 1500 g/L.
(Example 10)
In Example 10, a positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the concentration of the slurry was adjusted to 3000 g/L.

(Example 11)
In Example 11, a positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the aqueous lithium carbonate solution was changed to an aqueous lithium citrate solution.
(Example 12)
In Example 12, a positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the aqueous lithium carbonate solution was changed to an aqueous lithium acetate solution.
(Example 13)
In Example 13, a positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the lithium carbonate aqueous solution was changed to a lithium nitrate aqueous solution.
(Example 14)
In Example 14, a positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the lithium carbonate aqueous solution was changed to a lithium sulfate aqueous solution.

(Comparative example 1)
In Comparative Example 1, a positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the step of washing with an aqueous lithium carbonate solution was not performed.
(Comparative example 2)
In Comparative Example 2, a positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that pure water was used instead of the lithium carbonate aqueous solution.
(Comparative Example 3)
In Comparative Example 3, a positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that pure water was used instead of the lithium carbonate aqueous solution and the slurry concentration was 375 g/L.
(Comparative Example 4)
In Comparative Example 4, a positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that pure water was used instead of the lithium carbonate aqueous solution and the slurry concentration was 3000 g/L.
(Comparative Example 5)
In Comparative Example 5, a positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that an aqueous lithium hydroxide solution was used instead of the aqueous lithium carbonate solution.

(evaluation)
Table 1 shows the production conditions and evaluation results of the positive electrode active materials obtained in Examples and Comparative Examples.
As is clear from Table 1, the positive electrode active materials obtained in Examples 1 to 14 have a lithium hydroxide content of 0.5% by mass or less, and the powder surface of the obtained positive electrode active material Li / (Ni + Co + M ) is 0.80 or more and 1.5 or less. In addition, the obtained positive electrode active material was found to be useful as a positive electrode active material because gelling during paste kneading was suppressed, the discharge capacity was high, and the positive electrode resistance was low.

On the other hand, in Comparative Example 1, since the step of washing with an aqueous lithium salt solution was not performed, the lithium hydroxide content was as high as 1.06 and the powder surface Li/(Ni+Co+M) was as high as 6.28. In addition, gelation during paste kneading was observed in the obtained positive electrode active material, and furthermore, the discharge capacity was low, and the battery performance was inferior to that of the examples.

In Comparative Examples 2 and 3, since the cleaning was performed using pure water, Li/(Ni+Co+M), which represents the composition ratio of the powder surface, is as low as 0.80 or less. In addition, the obtained positive electrode active material has a low discharge capacity and a high positive electrode resistance, and is inferior in battery performance compared to the examples.

In Comparative Example 4, since the slurry was washed with pure water at a slurry concentration of 3000 g/L, the lithium hydroxide content was as high as 0.51, and Li/(Ni + Co + M), which represents the composition ratio of the powder surface, was as high as 2.01. . In addition, gelation during paste kneading was observed in the obtained positive electrode active material, and the battery performance was inferior to that of the examples.

In Comparative Example 5, since the lithium hydroxide aqueous solution was used for washing, the lithium hydroxide content was as high as 0.78, and the Li/(Ni+Co+M) representing the composition ratio of the powder surface was as high as 4.69. In addition, gelation was observed during paste kneading in the obtained positive electrode active material, and furthermore, the discharge capacity was lowered, and the battery performance was inferior to that of the examples.

From the above results, the positive electrode active material obtained by using the manufacturing method of the present embodiment can suppress the gelation of the positive electrode mixture paste when used as the positive electrode material of the battery, and furthermore, can reduce the positive electrode resistance of the battery. reduced, high capacity and excellent output characteristics. Moreover, it can be seen that the positive electrode active material of the present embodiment is useful as a positive electrode active material for non-aqueous electrolyte secondary batteries.

A non-aqueous electrolyte secondary battery containing the positive electrode active material obtained by the present invention in the positive electrode can be suitably used as a power source for small portable electronic devices (laptop personal computers, mobile phone terminals, etc.) that are always required to have a high capacity. It can also be suitably used for electric vehicle batteries that require high output.

In addition, the non-aqueous electrolyte secondary battery according to the present invention has excellent safety and can be made smaller and higher in output. be able to. The non-aqueous electrolyte secondary battery according to the present invention can be used not only as a power source for electric vehicles driven purely by electric energy, but also as a power source for so-called hybrid vehicles used in combination with a combustion engine such as a gasoline engine or a diesel engine. can be used.

BA... Coin type battery for evaluation 1... Lithium metal negative electrode 2... Separator (impregnated with electrolytic solution)
3 ... positive electrode (electrode for evaluation)
4 Gasket 5 Negative electrode can 6 Positive electrode can 7 Current collector

Claims (3)

general formula Li _z Ni _1-x-y Co _x M _y O ₂ (where 0≦x≦0.35, 0≦y≦0.10, 0.95≦z≦1.10, M is Mn, At least one element selected from V, Mg, Mo, Nb, Ti and Al) for a positive electrode active material for a non-aqueous electrolyte secondary battery comprising a lithium-nickel composite oxide powder represented by V, Mg, Mo, Nb, Ti and Al, containing lithium hydroxide The amount is 0.5% by mass or less, and the composition ratio (Li/(Ni + Co + M)) of Li and metals other than Li (Ni, Co and M) on the surface of the powder measured by X-ray photoelectron spectroscopy is 0.80 or more and 1.5 or less, a positive electrode active material for non-aqueous electrolyte secondary batteries. 2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the powder has a pH of 11.5 or less in a 5% by mass suspension of the powder in water. A non-aqueous electrolyte secondary battery comprising the positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 or 2 in a positive electrode.

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