JP2015122298A - Method for manufacturing positive electrode active material for nonaqueous electrolyte secondary batteries, positive electrode active material for nonaqueous electrolyte secondary batteries, and nonaqueous electrolyte secondary battery arranged by use thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a simple and safe method for manufacturing a positive electrode active material for nonaqueous electrolyte secondary batteries which enables the achievement of a higher-level balance between thermal stability and charge/discharge capacity, and which is superior in cycle characteristics; and a nonaqueous electrolyte secondary battery arranged by use of such a positive electrode active material.SOLUTION: A method for manufacturing a positive electrode active material for nonaqueous electrolyte secondary batteries comprises: a crystallization step for producing a nickel-containing hydroxide expressed by the general formula NiCoM(OH)by adding an alkali aqueous solution to a mix aqueous solution containing at least nickel and cobalt to cause the crystallization thereof; a mixing step for mixing the resultant nickel-containing hydroxide with a lithium compound and a niobium compound into a lithium mixture; and a sintering step for sintering the lithium mixture at a temperature of 700-840°C in an oxidative atmosphere.

Description

  The present invention relates to a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, a positive electrode active material for a non-aqueous electrolyte secondary battery, and a non-aqueous electrolyte secondary battery using the same.

  In recent years, with the widespread use of portable electronic devices such as mobile phones and notebook computers, development of small and lightweight non-aqueous electrolyte secondary batteries having high energy density is strongly desired. As such a secondary battery, there is a lithium ion secondary battery. Lithium metal, lithium alloy, metal oxide, carbon, or the like is used as a negative electrode material for a lithium ion secondary battery. These materials are materials capable of removing and inserting lithium.

Research and development of such lithium ion secondary batteries is being actively conducted. Among these, a lithium ion secondary battery using a lithium transition metal composite oxide, in particular, a lithium cobalt composite oxide (LiCoO ₂ ), which is relatively easy to synthesize, as a positive electrode material is high because a high voltage of 4V is obtained. Expected as a battery having an energy density and put into practical use. In the lithium ion secondary battery using this lithium cobalt composite oxide (LiCoO ₂ ), many developments have been made so far to obtain excellent initial capacity characteristics and cycle characteristics, and various results have already been obtained. ing.

However, lithium cobalt composite oxide (LiCoO ₂ ) uses a rare and expensive cobalt compound as a raw material, which causes an increase in battery cost. For this reason, it is desired to use materials other than lithium cobalt composite oxide (LiCoO ₂ ) as the positive electrode active material.

In addition, recently, not only small secondary batteries for portable electronic devices but also expectations for applying lithium ion secondary batteries as large-sized secondary batteries for power storage and electric vehicles are increasing. . For this reason, reducing the cost of the active material and making it possible to manufacture a cheaper lithium ion secondary battery is expected to have a large ripple effect in a wide range of fields. The positive electrode active material for lithium ion secondary batteries As newly proposed materials, lithium manganese composite oxide (LiMn ₂ O ₄ ) using manganese, which is cheaper than cobalt, and lithium nickel composite oxide (LiNiO ₂ ) using nickel can be cited. it can.

Lithium-manganese composite oxide (LiMn ₂ O ₄ ) is a powerful alternative to lithium-cobalt composite oxide (LiCoO ₂ ) because it is inexpensive and has excellent thermal stability, in particular, safety with respect to ignition. Although it can be said that the theoretical capacity is only about half that of lithium cobalt composite oxide (LiCoO ₂ ), it has a drawback that it is difficult to meet the demand for higher capacity of lithium ion secondary batteries, which is increasing year by year. Further, at 45 ° C. or higher, there is a drawback that self-discharge is intense and the charge / discharge life is also reduced.

On the other hand, lithium nickel composite oxide (LiNiO ₂ ) has almost the same theoretical capacity as lithium cobalt composite oxide (LiCoO ₂ ), and shows a slightly lower battery voltage than lithium cobalt composite oxide. For this reason, decomposition | disassembly by oxidation of electrolyte solution does not become a problem, and development is performed actively from expecting higher capacity | capacitance. However, when a lithium-ion secondary battery is made using a lithium-nickel composite oxide composed solely of nickel as a positive electrode active material without replacing nickel with other elements, the cycle is higher than that of lithium-cobalt composite oxide. The characteristics are inferior. In addition, there is a disadvantage that battery performance is relatively easily lost when used or stored in a high temperature environment. Furthermore, when it is left in a high temperature environment in a fully charged state, it has a drawback that oxygen is released from a lower temperature than the cobalt-based composite oxide.

In order to solve such drawbacks, it has been studied to add niobium, which is an element more expensive than nickel, to the lithium nickel composite oxide. For example, in Patent Document 1, Li _a Ni _1-xyz Co _x M _y Nb _z O _b (however, M is used for the purpose of improving the thermal stability at the time of an internal short circuit of a lithium ion secondary battery. Is one or more elements composed of Mn, Fe and Al, 1.0 ≦ a ≦ 1.1, 0.1 ≦ x ≦ 0.3, 0 ≦ y ≦ 0.1, 0.01 ≦ z ≦ 0 0.05, 2 ≦ b ≦ 2.2), which is composed of particles having a composition composed of at least two kinds of compounds composed of lithium, nickel, cobalt, element M, niobium, and oxygen. It has a substantially spherical shell layer containing at least one compound having a niobium concentration higher than the above composition in the vicinity of or inside the surface, and the positive electrode potential is within the range of 2V to 1.5V at the first discharge. Indicates the discharge capacity of α [mAh / g], and its X-ray diffraction When the half-value width of the (003) plane of the layered crystal structure is β [deg], α and β simultaneously satisfy the conditions of 80 ≦ α ≦ 150 and 0.15 ≦ β ≦ 0.20, respectively. A positive electrode active material for a battery has been proposed.

In Patent Document 2, Li _{1 + z} Ni _1-xy Co _x Nb _y O ₂ (0.10 ≦ x ≦ 0.21, for the purpose of improving thermal stability and increasing charge / discharge capacity. 0.01 ≦ y ≦ 0.08, −0.05 ≦ z ≦ 0.10), and in the measurement by the energy dispersion method, the peak intensity of the Nb L line is represented by I _Nb and the peak intensity of the Ni L line. has been proposed to the intensity ratio _I Nb _{/ I Ni} positive electrode active material for non-aqueous electrolyte secondary battery is within 1/2 of the average value of standard deviations is the intensity ratio _I Nb _{/ I Ni} of when the _{I Ni} .

Furthermore, in Patent Document 3, the composition formula Li _x Ni _a Mn _b Co _c M1 _d M2 _e O ₂ (where M1 At least one element selected from the group consisting of Al, Ti and Mg, and M2 is at least one element selected from the group consisting of Mo, W and Nb, and 0.2 ≦ x ≦ 1.2, 0.6 ≦ a ≦ 0.8, 0.05 ≦ b ≦ 0.3, 0.05 ≦ c ≦ 0.3, 0.02 ≦ d ≦ 0.04, 0.02 ≦ e ≦ 0.06, a + b + c + d + e = 1.0)) has been proposed.

On the other hand, in Patent Document 4, Li _x Ni _(1-yz-a) Co _y Mn _z M _a O ₂ (M is Fe ₎ in order to achieve both charge / discharge capacity and safety and suppress deterioration of cycle characteristics. , V, Cr, Ti, Mg, Al, Ca, Nb and Zr are at least one element selected from the group consisting of x, y, and z, where 1.0 ≦ x ≦ 1.10. .Sub.4 ≦ y + z ≦ 0.7, 0.2 ≦ z ≦ 0.5, 0 ≦ a ≦ 0.02) A substance (A is Ti, Sn, Mg) on the surface of the lithium composite oxide , A positive electrode active material for a non-aqueous electrolyte secondary battery having a structure coated with a compound composed of at least one element selected from the group consisting of Zr, Al, Nb and Zn.

In Patent Document 5, excellent thermal stability, and in order to obtain a high charge-discharge _{capacity, Li 1 + z Ni 1-} x-y Co x M y O 2 ( wherein x, y, z is 0.10 ≦ x ≦ 0.21, 0.015 ≦ y ≦ 0.08, −0.05 ≦ z ≦ 0.10 is satisfied, M is from Al, Mn, Nb or Mo, which has an affinity for oxygen superior to nickel A positive electrode active material for a secondary battery for a non-aqueous electrolyte is proposed in which two kinds of M represented by at least two selected elements and having an average valence exceeding 3) are impregnated or adhered.
The proposals disclosed in Patent Documents 1 to 5 are all aimed at achieving both thermal stability and charge / discharge capacity, but if the amount of niobium added is small, the charge / discharge capacity is large, but sufficient heat is provided. Although stability could not be obtained and a large amount of niobium was added, the thermal stability was good, but there was a problem that the charge / discharge capacity could not be secured. There is also a problem that it is difficult to ensure excellent cycle characteristics.

  Recently, the demand for higher capacity for small secondary batteries such as portable electronic devices is increasing year by year, and sacrificing capacity to ensure safety is a benefit of the high capacity of lithium nickel composite oxide. You will lose. In addition, the use of lithium ion secondary batteries for large-sized secondary batteries is also prominent. In particular, there is great expectation as a power source for hybrid vehicles, electric vehicles, or stationary storage batteries for power storage. Furthermore, these batteries are also required to have a long life, and it is important to have excellent cycle characteristics. In such applications, it is a major challenge to solve the problem of lithium nickel composite oxides that are inferior in safety and to achieve both safety and higher capacity and longer life.

JP 2002-151071 A JP 2006-147500 A JP 2012-014887 A JP 2008-153017 A JP 2008-181839 A

In view of the above problems, the present invention uses a positive electrode active material for a non-aqueous electrolyte secondary battery capable of achieving both high thermal stability, charge / discharge capacity, and cycle characteristics, and the positive electrode active material. An object of the present invention is to provide a non-aqueous electrolyte secondary battery having excellent safety, high capacity, and excellent cycle characteristics.
Furthermore, this invention aims at providing the industrial manufacturing method of the said positive electrode active material for nonaqueous electrolyte secondary batteries.

  The present inventors diligently studied the addition of niobium to the lithium metal composite oxide in order to improve the thermal stability. The niobium compound having a predetermined particle size was mixed with a lithium compound and a nickel-containing hydroxide. Knowledge that the positive electrode active material for non-aqueous electrolyte secondary batteries produced by firing can be uniformly added with niobium, has good thermal stability, and has a high charge / discharge capacity As a result, the present invention has been completed.

That is, the manufacturing method of the positive electrode active material for a non-aqueous electrolyte secondary battery according to the present invention have the general formula _{Li d Ni 1-a-b} -c Co a M b Nb c O 2 ( where, M is Mn, V, At least one element selected from Mg, Ti, and Al, 0.03 ≦ a ≦ 0.35, 0 ≦ b ≦ 0.10, 0.001 ≦ c ≦ 0.05, 0.95 ≦ d ≦ 1.20) and a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery comprising a lithium transition metal composite oxide composed of particles having a polycrystalline structure, wherein at least nickel and cobalt are contained. An aqueous alkaline solution is added to the mixed aqueous solution to cause crystallization, and a general formula Ni _{1-a′-b ′} Co _{a ′} M _{b ′} (OH) ₂ (where M is selected from Mn, V, Mg, Ti, and Al) At least one element, 0.03 ≦ a ′ ≦ 0.35, 0 ≦ b ′ ≦ 0 A crystallization step for obtaining a nickel-containing hydroxide represented by formula (1), a nickel-containing hydroxide, a lithium compound, and a niobium compound having an average particle size of 0.1 to 10 μm mixed to form lithium. It includes a mixing step of obtaining a mixture and a firing step of firing the lithium mixture at 700 to 840 ° C. in an oxidizing atmosphere to obtain a lithium transition metal composite oxide.
The niobium compound is preferably niobic acid or niobium oxide.

Moreover, in the said manufacturing process, it is preferable to include the heat processing process which heat-processes a nickel containing hydroxide at the temperature of 105-800 degreeC before a mixing process.
Moreover, it is preferable after the said baking process that a lithium transition metal complex oxide is made into a slurry in the ratio of 100-2000 g / L with respect to 1L of water, and the water-washing process of washing with water is included.

The positive electrode active material for a non-aqueous electrolyte secondary battery of the present invention have the general formula _{Li d Ni 1-a-b} -c Co a M b Nb c O 2 ( where, 0.03 ≦ a ≦ 0.35,0 ≦ b ≦ 0.10, 0.001 ≦ c ≦ 0.05, 0.95 ≦ d ≦ 1.20, M is represented by at least one element selected from Mn, V, Mg, Ti and Al) A positive electrode active material for a non-aqueous electrolyte secondary battery comprising a lithium transition metal composite oxide composed of polycrystalline particles, having a porous structure and a specific surface area of 0.8 to 3.0 m ² / g and an alkali metal content other than lithium is 20 ppm by mass or less.

  The lithium transition metal complex oxide preferably has a crystallite size of 10 to 150 nm.

  The non-aqueous electrolyte secondary battery of the present invention is characterized in that the positive electrode active material for a non-aqueous electrolyte secondary battery is used as a positive electrode.

The positive electrode active material for a non-aqueous electrolyte secondary battery according to the present invention makes it possible to realize high thermal stability, charge / discharge capacity, and excellent cycle characteristics. By using the active material, high safety can be achieved. A non-aqueous electrolyte secondary battery having battery capacity and excellent cycle characteristics can be obtained. Therefore, the non-aqueous electrolyte secondary battery according to the present invention satisfies the recent demand for higher capacity and longer life for small secondary batteries such as portable electronic devices, and also for hybrid vehicles, electric vehicles, or stationary storage batteries. It is suitable for a power source used for a large-sized secondary battery.
Furthermore, the method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery is easy and suitable for production on an industrial scale, and is extremely useful industrially.

Cross section of coin battery used for battery evaluation

1. Method for producing positive electrode active material for non-aqueous electrolyte secondary battery The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery comprising the lithium transition metal composite oxide of the present invention comprises: (A) a mixed aqueous solution containing at least nickel and cobalt A crystallization step of adding an aqueous alkali solution to crystallize to obtain a nickel-containing hydroxide represented by a specific general formula; (B) mixing a niobium compound, a nickel-containing hydroxide and a lithium compound to form a lithium mixture; And (C) a firing step of firing the lithium mixture at 700 to 840 ° C. in an oxidizing atmosphere. Hereinafter, detailed description will be given for each process.

(A) crystallization step crystallization process is represented by the general formula _{Ni 1-a'-b 'Co} a' M b '(OH) 2 ( provided that at least 1 M is the Mn, V, Mg, selected from Ti and Al A nickel-containing hydroxide composed of secondary particles represented by the following: 0.03 ≦ a ′ ≦ 0.35, 0 ≦ b ′ ≦ 0.10) Is what you get.

A ′ indicating the cobalt content is 0.03 ≦ a ′ ≦ 0.35, preferably 0.05 ≦ a ′ ≦ 0.35, and 0.07 ≦ a ′ ≦ 0.20. More preferably, it is more preferably 0.10 ≦ a ′ ≦ 0.20.
In order to exhibit excellent cycle characteristics, it is preferable that 0 <b ′ ≦ 0.10, which is a range in which not only Ni and Co but also M is necessarily included, and 0.01 ≦ b ′ ≦ 0. 0.07 is more preferable.

As a method for obtaining the nickel-containing hydroxide, the following production method is preferred.
For example, an alkaline solution is added to a mixed aqueous solution containing at least nickel and cobalt in the reaction vessel to obtain a reaction aqueous solution, and the reaction solution is stirred at a constant speed to control the pH of the reaction aqueous solution. The hydroxide is co-precipitated and crystallized so as to have an atomic ratio. Here, as the mixed aqueous solution, a sulfate solution, a nitrate solution, or a chloride solution can be used. Moreover, sodium hydroxide, potassium hydroxide, etc. can be used for an alkaline solution, for example.

  As for the pH region in the coprecipitation, in the absence of a complexing agent, pH = 10 to 11 is selected, and the temperature of the mixed aqueous solution is over 60 ° C. and not more than 80 ° C. In the absence of a complexing agent, crystallization at a pH exceeding 11 results in fine particles, poor filterability, and spherical particles cannot be obtained. On the other hand, if the pH is less than 10, the rate of hydroxide formation is remarkably slow, Ni remains in the filtrate, and the amount of precipitation of Ni deviates from the target composition, making it impossible to obtain a mixed hydroxide of the target ratio. End up. Therefore, by adjusting the pH to 10 to 11 and keeping the temperature of the mixed aqueous solution above 60 ° C., the phenomenon that the precipitation amount of Ni deviates from the target composition and does not cause coprecipitation is raised, the reaction temperature is increased, and the solubility of Ni is increased. It is avoided by raising. At this time, if the temperature of the mixed aqueous solution exceeds 80 ° C., the slurry concentration becomes high due to a large amount of water evaporation, the solubility of Ni decreases, and crystals such as sodium sulfate are generated in the filtrate, and the impurity concentration The problem of lowering the charge / discharge capacity of the positive electrode material, such as increase in the positive electrode material, is not preferable.

  On the other hand, when an ammonium ion supplier such as ammonia is used as a complexing agent in addition to the reaction aqueous solution, the solubility of Ni increases, so the pH range is from pH 10 to 12.5, and the temperature range is also from 50 ° C. to 80 ° C. Can be spread. In the reaction vessel, the ammonia concentration in the aqueous reaction solution is preferably maintained at a constant value within the range of 3 to 25 g / L in order not to cause the following problems. First, ammonia acts as a complexing agent, and if the ammonia concentration is less than 3 g / L, the solubility of metal ions cannot be kept constant. Since particles are not formed and gel-like nuclei are easily generated, the particle size distribution is also likely to be widened. On the other hand, when the ammonia concentration exceeds 25 g / L, the solubility of metal ions becomes too high, and the amount of metal ions remaining in the reaction aqueous solution increases, resulting in compositional deviation and the like. Further, when the ammonia concentration varies, the solubility of metal ions varies, and uniform hydroxide particles are not formed. Therefore, it is preferable to maintain a constant value. For example, the ammonia concentration is preferably maintained at a desired concentration by setting the upper and lower limits to about 5 g / L. In addition, although an ammonium ion supply body is not specifically limited, For example, ammonia, ammonium sulfate, ammonium chloride, ammonium carbonate, ammonium fluoride etc. can be used.

  After reaching a steady state, the precipitate is collected, filtered and washed with water to obtain a nickel-containing hydroxide. Alternatively, a mixed aqueous solution and an alkaline solution, and in some cases, an aqueous solution containing an ammonium ion supplier are continuously supplied to overflow from the reaction vessel, and a precipitate is collected, filtered and washed to obtain a nickel-containing hydroxide. You can also.

  The nickel-containing hydroxide after crystallization is preferably sufficiently washed with water in order to reduce the residual amount of impurities, particularly alkali metals such as sodium.

The method for blending the nickel-containing hydroxide with at least one additive element selected from M = Mn, V, Mg, Ti and Al (hereinafter also referred to as “additive element M”) is particularly limited. However, from the viewpoint of increasing the productivity of the crystallization step, an aqueous solution containing the additive element M is added to the above mixed aqueous solution containing nickel and cobalt, and a nickel-containing hydroxide (added) is added. A method of coprecipitation of (including element M) is preferred.
As an aqueous solution containing the additive element M, for example, aluminum sulfate, sodium aluminate, titanium sulfate, ammonium peroxotitanate, potassium titanium oxalate, manganese sulfate, magnesium sulfate, magnesium chloride, vanadium sulfate, ammonium vanadate, etc. are used. Can do.
The metal element M is at least one element selected from Mn, V, Mg, Ti, and Al, and may be optionally added to improve thermal stability, storage characteristics, battery characteristics, and the like. it can.

Further, in order to optimize the crystallization conditions and facilitate the control of the composition ratio, a coating step for coating M may be provided after crystallization by adding an alkaline aqueous solution to a mixed aqueous solution containing at least nickel and cobalt. Good.
The coating method is not particularly limited and a conventionally known method can be used. For example, 1) Crystallization is performed by adding an alkaline aqueous solution to a mixed aqueous solution (excluding additive element M) containing nickel and cobalt. A method of coating the nickel-containing hydroxide with the additive element M, or 2) preparing a mixed aqueous solution containing nickel, cobalt, and a part of the additive element M, and nickel-containing hydroxide (including the additive element M) And the content of M is adjusted by coating the additive element M on the coprecipitate.

(A) 'heat treatment step The mixing step is a step of mixing a niobium compound, a nickel-containing hydroxide and a lithium compound to obtain a lithium mixture, but in the production method of the present invention, before mixing with the lithium compound, For example, a heat treatment step of heat-treating the nickel-containing hydroxide can be provided, and the heat-treated nickel-containing hydroxide may be mixed with a lithium compound. Further, a compound containing the element M may be added in this step.

  By the heat treatment step, moisture contained in the nickel-containing hydroxide can be removed, and moisture remaining in the nickel-containing hydroxide up to the firing step can be reduced. By sufficiently removing moisture remaining in the nickel-containing hydroxide, the ratio of the number of atoms of metals other than lithium (Me) and the number of atoms of lithium (Li) in the positive electrode active material to be produced (Li / Me) can be prevented from varying. Note that it is only necessary to remove moisture to such an extent that Li / Me of the positive electrode active material does not vary, and thus it is not always necessary to convert all the composite hydroxides to composite oxides. However, in order to further reduce the variation in Li / Me, it is preferable to convert the composite hydroxide in the nickel-containing hydroxide to the composite oxide. In the heat treatment step, it may be heated to a temperature at which residual moisture in the nickel-containing hydroxide is removed, and is preferably set to 105 to 800 ° C. For example, residual moisture can be removed by heating the composite hydroxide to 105 ° C. or higher. In addition, if it is less than 105 degreeC, since it takes a long time to remove a residual water | moisture content, it is not industrially suitable. When the temperature exceeds 800 ° C., the particles converted into the composite oxide may sinter and aggregate. When converting the composite hydroxide to the composite oxide, it is preferable to heat at a temperature of 350 to 800 ° C.

  The atmosphere in which the heat treatment is performed is not particularly limited, and is preferably performed in an air stream that can be easily performed. Further, the heat treatment time is not particularly limited, but if it is less than 1 hour, the residual moisture in the composite hydroxide may not be sufficiently removed, so that it is preferably at least 1 hour, more preferably 5 to 15 hours. And the installation used for heat processing is not specifically limited, What is necessary is just to be able to heat a composite hydroxide in an air stream, and an electric furnace without a blower dryer and gas generation can be used conveniently.

(B) Mixing step In the mixing step, the nickel-containing hydroxide obtained in the crystallization step or the heat-treated nickel-containing hydroxide, a lithium compound, and a niobium compound are mixed.
Conventionally, when Nb is blended into an active material, generally, niobium is added to the nickel-containing hydroxide by a coating method such as wet coprecipitation / coating or spray drying, and then calcined with a lithium compound. Has been used. However, in wet coating methods such as coprecipitation / coating and spray drying, impurities derived from solutions that dissolve Nb (for example, KOH solution, oxalic acid solution, etc.) and solutions for adjusting pH during coating (for example, sulfuric acid, hydrochloric acid, nitric acid) Etc.) and the impurities derived from it remain together with the coated Nb. In particular, an alkali metal such as potassium or sodium contained in the solution adversely affects the battery capacity and cycle characteristics, and thus needs to be sufficiently reduced.

  In the present invention, in the mixing step of mixing the nickel-containing hydroxide and the lithium compound, the niobium compound is mixed and added to the solid phase. The solid phase addition of Nb is a process with excellent productivity and low load that does not require a chemical solution or the like as compared with a method of coprecipitation and coating of Nb in a wet process. Also, when Nb is co-precipitated and coated in the wet process, it is necessary to control the pH, and in some cases, the target form and amount of Nb may not be added, which is advantageous from the viewpoint of stable quality. is there. Further, in the case of solid phase addition of Nb, although depending on the Nb compound to be added, alkali metals other than lithium (hereinafter simply referred to as alkali metals) entrained in the lithium transition metal composite oxide, particularly potassium and sodium, etc. The amount of impurities can be reduced.

  In the mixing step, the nickel-containing hydroxide, the lithium compound, and the niobium compound are mixed so that the Li / Me in the lithium mixture is 0.95 to 1.20. That is, it mixes so that Li / Me in a lithium mixture may become the same as Li / Me in the positive electrode active material of this invention. This is because Li / Me does not change before and after the firing step, and Li / Me mixed in this mixing step becomes Li / Me in the positive electrode active material.

  On the other hand, when washing with water after firing as described later, Li / Me decreases by washing with water. Therefore, when washing with water, it is preferable to mix the nickel-containing hydroxide and the lithium compound in anticipation of the decrease in Li / Me. The decrease in Li / Me varies depending on the firing conditions and washing conditions, but is about 0.05 to 0.1. The decrease can be confirmed by producing a small amount of positive electrode active material as a preliminary test. . The lithium compound is not particularly limited, but for example, lithium hydroxide, lithium nitrate, lithium carbonate, or a mixture thereof is preferable because it is easily available. In particular, lithium hydroxide is more preferably used in consideration of ease of handling and stability of quality.

  Since the niobium content does not change before and after the firing step, a niobium compound corresponding to the amount of niobium added to the positive electrode active material is added. Since the reactivity changes depending on the particle size in the solid phase addition, the particle size of the niobium compound to be added is one of the important factors. The average particle size of the niobium compound is 0.1 to 10 μm, preferably 0. 5 to 8 μm. When the average particle size is smaller than 0.1 μm, there is a problem that handling of the powder becomes very difficult. When the average particle size is larger than 10 μm, the reactivity at the time of firing is lowered and into the lithium transition metal composite oxide. This is because the diffusion of Nb is insufficient, and there is a problem that thermal stability cannot be secured. It is considered that the obtained lithium transition metal composite oxide has a porous structure by solid-phase addition of a niobium compound having the above particle size. That is, although the details are unknown, the nickel-containing hydroxide obtained by crystallization is a secondary particle formed by agglomerating primary particles, and when niobium diffuses and reacts from the surface of the secondary particles, the primary particle Since the speed is not uniform among the particles, it is assumed that the shrinkage of the primary particles is non-uniform and fine voids are generated.

As the niobium compound, niobic acid, niobium oxide, niobium nitrate, niobium pentachloride, niobium nitrate, or the like can be used. Among these, niobium hydroxide or niobium oxide is preferable from the viewpoint of easy availability and avoidance of impurities that cause deterioration in thermal stability, capacity, and cycle characteristics in the fired lithium transition metal composite oxide.
In order to obtain the niobium compound having the above average particle size range, the niobium compound as a raw material can be pulverized using various pulverizers such as a ball mill, a planetary ball mill, a jet mill, a bead mill, and a pin mill. If necessary, classification may be performed by a dry classifier or sieving.
Moreover, as a particle size of the nickel containing hydroxide used for a mixing process, about 5-20 micrometers is preferable and 10-15 micrometers is more preferable.
The average particle diameter is a value measured by a laser scattering diffraction method as a volume reference average diameter (MV).

  The lithium mixture is preferably mixed well before firing. If the mixing is not sufficient, Li / Me varies among individual particles, which may cause problems such as insufficient battery characteristics. Moreover, a general mixer can be used for mixing, for example, a shaker mixer, a Laedige mixer, a Julia mixer, a V blender, etc. can be used, and shapes, such as nickel containing hydroxide, are not destroyed. To the extent, the nickel-containing hydroxide, the lithium compound, and the niobium compound may be sufficiently mixed.

(C) Firing step In the firing step, the lithium mixture obtained in the mixing step is fired in an oxidizing atmosphere at 700 to 840 ° C, preferably 700 to 830 ° C, more preferably 700 to 800 ° C. This is a step of forming a lithium transition metal composite oxide composed of particles. When the lithium mixture is fired in the firing step, niobium in the niobium compound is diffused together with lithium in the lithium compound into the nickel-containing hydroxide, so that a lithium transition metal composite oxide is formed. If the firing temperature is less than 700 ° C., lithium and niobium are not sufficiently diffused into the nickel-containing hydroxide, so that excess lithium and unreacted particles remain, or the crystal structure becomes insufficient. This causes a problem that sufficient battery characteristics cannot be obtained. If the firing temperature exceeds 840 ° C., intense sintering may occur between the formed lithium transition metal composite oxide particles, and abnormal grain growth may occur. If abnormal grain growth occurs, the particles after firing become coarse and may not retain the particle form, and when the positive electrode active material is formed, the specific surface area decreases and the positive electrode resistance increases. There arises a problem that the battery capacity decreases.

  The firing time is preferably at least 3 hours or more, more preferably 6 to 24 hours. This is because if it is less than 3 hours, the lithium transition metal composite oxide may not be sufficiently produced. The atmosphere during firing is preferably an oxidizing atmosphere, and more preferably an oxygen concentration of 18 to 100% by volume. That is, firing is preferably performed in the air or in an oxygen stream. This is because, if the oxygen concentration is less than 18% by volume, it cannot be oxidized sufficiently and the crystallinity of the lithium transition metal composite oxide may be insufficient. Considering battery characteristics in particular, it is preferable to carry out in an oxygen stream.

  In the firing step, before firing at a temperature of 700 to 840 ° C., it is preferable to calcine at a temperature lower than the firing temperature at which the lithium compound and nickel-containing hydroxide can react. By holding the lithium mixture at such a temperature, lithium and niobium are sufficiently diffused into the nickel-containing hydroxide, and a uniform lithium transition metal composite oxide can be obtained. For example, in the case of using lithium hydroxide, it is preferable to carry out calcination by holding at a temperature of 400 to 550 ° C. which is higher than the melting point of lithium hydroxide for about 1 to 10 hours.

  The furnace used for firing is not particularly limited as long as the lithium mixture can be fired in the atmosphere or an oxygen stream, but an electric furnace without gas generation is preferable, and a batch type or continuous type furnace is used. Either can be used.

  In the lithium transition metal composite oxide obtained by firing, sintering between particles is suppressed, but coarse particles may be formed by weak sintering or aggregation. In such a case, it is preferable to adjust the particle size distribution by crushing to eliminate the sintering and aggregation.

(D) Water washing process The water washing process is a process of washing the lithium transition metal composite oxide with water. Moreover, it is preferable to filter and dry after washing with water.
The lithium transition metal composite oxide obtained by the firing step can be used as a positive electrode active material as it is, but by removing excess lithium on the particle surface, the surface area that can be contacted with the electrolyte is increased and charged. Since discharge capacity can be improved, it is preferable to wash with water after firing. Moreover, since the weak part formed in the particle | grain surface is fully removed, a contact with electrolyte solution can increase and charge / discharge capacity can be improved.

  Furthermore, since excess lithium causes a side reaction in the non-aqueous secondary battery and causes expansion of the battery due to gas generation, it is preferably washed with water from the viewpoint of improving safety.

  As a slurry concentration at the time of washing with water, it is preferable that the amount (g) of the lithium transition metal composite oxide with respect to 1 L of water contained in the slurry is 100 to 2000 g / L. That is, as the slurry concentration increases, the amount of powder increases. When the slurry concentration exceeds 2000 g / L, the viscosity is very high and stirring becomes difficult. However, it becomes difficult to separate the powder from the powder even when peeling occurs. On the other hand, when the slurry concentration is less than 100 g / L, the amount of lithium elution is large and the amount of lithium on the surface is small due to being too dilute. However, lithium desorption from the crystal lattice of the positive electrode active material also occurs. Not only tends to collapse, but the aqueous solution having a high pH absorbs carbon dioxide in the atmosphere and reprecipitates lithium carbonate.

The water used is not particularly limited, and pure water is preferable. By using pure water, it is possible to prevent a decrease in battery performance due to adhesion of impurities to the positive electrode active material.
It is preferable that the amount of adhering water remaining on the particle surface during solid-liquid separation of the slurry is small. When the amount of adhering water is large, lithium dissolved in the liquid is reprecipitated, and the amount of lithium existing on the surface of the lithium transition metal composite oxide particles after drying increases.

Moreover, it is preferable that a water washing process includes the process of filtering and drying after water washing.
As a filtration method, a commonly used method may be used. For example, a suction filter, a filter place, a centrifuge, or the like can be used.
The drying temperature after filtration is not particularly limited and is preferably 80 to 350 ° C. If the temperature is less than 80 ° C., the drying of the positive electrode active material after washing with water becomes slow, so that a gradient of lithium concentration occurs between the particle surface and the inside of the particle, and the battery characteristics may be deteriorated. On the other hand, near the surface of the positive electrode active material, it is expected that it is very close to the stoichiometric ratio, or slightly desorbed lithium and close to the charged state. As a result, the crystal structure close to 1 may be destroyed, and the battery characteristics may be deteriorated.

2. Positive active material for a non-aqueous electrolyte secondary battery according to a non-aqueous electrolyte positive electrode active material present invention for a secondary battery, the general formula _{Li d Ni 1-a-b} -c Co a M b Nb c O 2 ( where 0 .03 ≦ a ≦ 0.35, 0 ≦ b ≦ 0.10, 0.001 ≦ c ≦ 0.05, 0.95 ≦ d ≦ 1.20, M is selected from Mn, V, Mg, Ti and Al At least one kind of element), which is composed of a lithium transition metal composite oxide composed of particles having a polycrystalline structure, has a porous structure, and has a specific surface area of 0.8 to 3.0 m ² / g. Yes, the content of alkali metals other than lithium is 20 ppm by mass or less.

  A indicating the cobalt content is 0.03 ≦ a ≦ 0.35, preferably 0.05 ≦ a ≦ 0.35, more preferably 0.07 ≦ a ≦ 0.20, and even more preferably. Is 0.10 ≦ a ≦ 0.20. Cobalt is an additive element that contributes to the improvement of cycle characteristics. However, if the value of a is less than 0.03, sufficient cycle characteristics cannot be obtained, and the capacity retention rate also decreases. On the other hand, when the value of a exceeds 0.35, the initial discharge capacity is greatly reduced.

C indicating the amount of niobium added is 0.001 ≦ c ≦ 0.05, preferably 0.001 ≦ c ≦ 0.02. Niobium is an additive element that is thought to contribute to the suppression of the thermal decomposition reaction by deoxygenation of the lithium transition metal composite oxide and is effective in improving safety. When the value of c is less than 0.001, the amount added is too small and the safety improvement is insufficient. On the other hand, although safety | security improves according to addition amount, since there exists a tendency for crystallinity to fall, when the value of c exceeds 0.05, charge / discharge capacity will fall. In addition, the cycle characteristics are also reduced. Further, within the above range, when the value of c is 0.005 or more, the resulting positive electrode active material has better thermal stability. On the other hand, when the value of c is 0.01 or less within the above range, the charge / discharge capacity of the obtained positive electrode active material becomes larger.
The form of niobium may be either a solid solution in the lithium transition metal composite oxide or a lithium niobium composite oxide at the crystal grain boundary or particle surface of the lithium transition metal composite oxide. It is preferable that it is dissolved. Here, solid solution means a state in which a niobium compound is hardly detected by EDX measurement with a transmission electron microscope.

  Furthermore, in the case of solid solution, the ratio between the grain boundary and the Nb concentration in the grain is preferably 4 times or less, more preferably 3 times or less. The ratio between the grain boundary and the Nb concentration in the grain (grain boundary / inside grain) can be obtained from the EDX measurement result of a transmission electron microscope. By dissolving in a solid solution, the effect of suppressing the thermal decomposition reaction can be enhanced even when a small amount is added.

  The additive element M is at least one element selected from Mn, V, Mg, Ti, and Al, and can be added to improve battery characteristics such as cycle characteristics and safety. If b indicating the amount of M added exceeds 0.10, the battery characteristics are further improved, but the initial discharge capacity is greatly lowered, which is not preferable. Furthermore, since 0 <b ≦ 0.10 that always includes not only Ni and Co but also M can exhibit excellent cycle characteristics, b is preferably within this range, and 0.01 ≦ b More preferably, ≦ 0.07.

  D indicating Li / Me is 0.95 ≦ d ≦ 1.20. When the value of d is less than 0.95, the charge / discharge capacity decreases. On the other hand, the charge / discharge capacity increases as the value of d increases, but when d exceeds 1.20, the safety is lowered.

  The lithium transition composite metal oxide is characterized in that the content of alkali metals other than lithium is 20 ppm by mass or less, preferably 10 ppm by mass or less. When the alkali metal content other than lithium is 20 mass ppm or less and c indicating the amount of niobium added is 0.001 ≦ c ≦ 0.05, excellent cycle characteristics can be obtained. Even if either one of the alkali metal content other than lithium or the addition amount of niobium exceeds the above range, good cycle characteristics cannot be obtained.

The positive electrode active material is made of a lithium transition metal composite oxide composed of polycrystalline particles, and has a porous structure.
In this specification, the porous structure means that the distance between any two points on the outer edge of the gap is 0.3 μm by observing the positive electrode active material particles in an arbitrary cross section (observation surface) of the scanning electron microscope (5000 times magnification). The plurality of voids as described above are present in the cross section of the positive electrode active material particles.

  The voids have a maximum length of preferably 50% or less, more preferably 40% or less of the particle cross-sectional major axis by cross-sectional observation with a scanning electron microscope, and are preferably present at least at the grain boundaries. By having the porous structure as described above, the particle surface that can come into contact with the electrolyte when used in the positive electrode of a battery is greatly increased, and the reduction in charge / discharge capacity due to the addition of niobium is compensated to ensure safety. Sufficient charge / discharge capacity can be obtained.

  Further, the number of voids in the cross-sectional observation is obtained for any 20 or more particles, and an index obtained by dividing the total number of voids by the total particle cross-sectional length (μm) of the particles (hereinafter, “ The number of voids ”is also preferably 0.2 to 10 / μm, and more preferably 0.5 to 3 / μm. Here, the particle cross-sectional major axis is the maximum distance between any two points on the particle outer periphery on the particle observation surface. Further, in the cross-sectional observation, particles having a particle size of 20% or less of the volume reference average diameter (MV) of the subsequent positive electrode active material are excluded from the determination of the porous structure. This is because particles having a particle size of 20% or less of the average particle size are quantitatively small in the positive electrode active material and have little influence on the charge / discharge capacity. Because it may not be appropriate. When the number of voids is 0.2 to 10 / μm, an excessive contact with the electrolytic solution is suppressed and a decrease in thermal stability is suppressed, and a high charge / discharge capacity is obtained with a sufficient contact area. Can do.

The specific surface area of the positive electrode active material is 0.9 to 3.0 m ² / g, preferably 0.9 to 2.8 m ² / g. When the specific surface area is less than 0.8 m ² / g, the particle surface that can come into contact with the electrolytic solution decreases, and sufficient charge / discharge capacity cannot be obtained. On the other hand, if the specific surface area exceeds 3.0 m ² / g, the surface of the particles coming into contact with the electrolytic solution increases so that safety is lowered.

  The crystallite diameter of the lithium transition metal oxide is preferably 10 to 180 nm, and more preferably 10 to 150 nm. When the crystallite diameter is less than 10 nm, the crystal grain boundary increases so much that the resistance of the active material increases, so that sufficient charge / discharge capacity may not be obtained. On the other hand, when the crystallite diameter exceeds 150 nm, crystal growth proceeds excessively, and cation mixing in which nickel is mixed into the lithium layer of the lithium transition metal composite oxide, which is a layered compound, occurs, and charge / discharge capacity decreases.

  The average particle diameter of the positive electrode active material is preferably 5 to 20 μm, more preferably 10 to 15 μm, as D50, which is a 50% volume integrated diameter by laser scattering measurement. When the thickness is less than 5 μm, when used for the positive electrode of the battery, the packing density may decrease, and the charge / discharge capacity per volume may not be sufficiently obtained. On the other hand, when it exceeds 20 μm, a sufficient contact area with the electrolytic solution cannot be obtained, and the charge / discharge capacity may be reduced.

3. Non-aqueous electrolyte secondary battery Embodiments of the lithium ion secondary battery according to the present invention will be described in detail for each component. The lithium ion secondary battery according to the present invention is composed of the same components as those of a general lithium ion secondary battery, such as a positive electrode, a negative electrode, and a non-aqueous electrolyte. The embodiment described below is merely an example, and the nonaqueous electrolyte secondary battery of the present invention is implemented in various modifications and improvements based on the knowledge of those skilled in the art, including the following embodiment. can do. Moreover, the use of the nonaqueous electrolyte secondary battery of the present invention is not particularly limited.

(1) Positive Electrode A positive electrode mixture forming the positive electrode and each material constituting the positive electrode mixture will be described. The powdered positive electrode active material of the present invention, a conductive material, and a binder are mixed, and if necessary, a target solvent such as activated carbon and viscosity adjustment is added, and this is kneaded to obtain a positive electrode mixture paste. Make it. The respective mixing ratios in the positive electrode mixture are also important factors that determine the performance of the lithium secondary battery.

  When the total mass of the solid content of the positive electrode mixture excluding the solvent is 100% by mass, the content of the positive electrode active material is 60 to 95% by mass as in the case of the positive electrode of a general lithium secondary battery. It is desirable that the content is 1 to 20% by mass and the content of the binder is 1 to 20% by mass.

  The obtained positive electrode mixture paste is applied to the surface of a current collector made of, for example, aluminum foil, and dried to scatter the solvent. If necessary, pressurization may be performed by a roll press or the like to increase the electrode density. In this way, a sheet-like positive electrode can be produced. The sheet-like positive electrode can be cut into an appropriate size according to the intended battery and used for battery production. However, the manufacturing method of the positive electrode is not limited to the above-described examples, and may depend on other methods.

  In producing the positive electrode, as the conductive agent, for example, graphite (natural graphite, artificial graphite, expanded graphite, etc.), carbon black materials such as acetylene black, ketjen black, and the like can be used. The binder plays a role of holding the active material particles, and for example, fluorine-containing resins such as polytetrafluoroethylene, polyvinylidene fluoride, and fluororubber, and thermoplastic resins such as polypropylene and polyethylene can be used. If necessary, a positive electrode active material, a conductive material, and activated carbon are dispersed, and a solvent that dissolves the binder is added to the positive electrode mixture. Specifically, an organic solvent such as N-methyl-2-pyrrolidone can be used as the solvent. Moreover, activated carbon can be added to the positive electrode mixture in order to increase the electric double layer capacity.

(2) Negative electrode For the negative electrode, metallic lithium, lithium alloy, or the like, and a negative electrode mixture made by mixing a binder with a negative electrode active material capable of occluding and desorbing lithium ions and adding an appropriate solvent to form a paste. In addition, it is applied to the surface of a metal foil current collector such as copper, dried, and compressed to increase the electrode density as necessary.

  As the negative electrode active material, for example, natural graphite, artificial graphite, a fired organic compound such as phenol resin, or a powdery carbon material such as coke can be used. In this case, as the negative electrode binder, a fluorine-containing resin such as polyvinylidene fluoride can be used as in the case of the positive electrode, and the active material and the solvent for dispersing the binder include N-methyl-2-pyrrolidone. Organic solvents can be used.

(3) Separator A separator is interposed between the positive electrode and the negative electrode. The separator separates the positive electrode and the negative electrode and retains the electrolyte, and a thin film such as polyethylene or polypropylene and a film having many fine holes can be used.

(4) Non-aqueous electrolyte The non-aqueous electrolyte is obtained by dissolving a lithium salt as a supporting salt in an organic solvent. Examples of the organic solvent include cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate; chain carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate; and tetrahydrofuran, 2- One kind selected from ether compounds such as methyltetrahydrofuran and dimethoxyethane, sulfur compounds such as ethylmethylsulfone and butanesultone, phosphorus compounds such as triethyl phosphate and trioctyl phosphate, etc. are used alone or in admixture of two or more. be able to.

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

(5) Shape and configuration of battery The shape of the lithium secondary battery according to the present invention composed of the positive electrode, the negative electrode, the separator, and the non-aqueous electrolyte described above is various, such as a cylindrical type and a laminated type. be able to.
In any case, the positive electrode and the negative electrode are laminated via a separator to form an electrode body, and the electrode body is impregnated with the non-aqueous electrolyte. The positive electrode current collector and the positive electrode terminal communicating with the outside, and the negative electrode current collector and the negative electrode terminal communicating with the outside are connected using a current collecting lead or the like. The battery having the above structure can be sealed in a battery case to complete the battery.

Example 1
A mixed aqueous solution of nickel sulfate and cobalt sulfate, aqueous sodium aluminate solution, 25 mass% sodium hydroxide solution, 25 mass so that the molar ratio of nickel: cobalt: aluminum is 81.5: 15.0: 3.5. % Ammonia water was simultaneously added to the reaction vessel, the pH was maintained at 11.8 based on the liquid temperature of 25 ° C., the reaction temperature was maintained at 50 ° C., and the ammonia concentration was maintained at 10 g / L. The resulting nickel cobalt aluminum composite hydroxide was formed. After the inside of the reaction vessel is stabilized, a hydroxide slurry is recovered from the overflow port, filtered, washed with water and dried to be nickel cobalt aluminum composite hydroxide (nickel-containing hydroxide: Ni _0.815 Co _0.150 Al _{0 0.035} (OH) ₂ ) was obtained. (Crystallization process)

Next, the average particle diameter of 0.6 μm using the nickel cobalt aluminum composite hydroxide (nickel-containing hydroxide), commercially available lithium hydroxide, and a nano grinding mill (Nano Grinding Mill manufactured by Sanrex Industries Co., Ltd.) Niobic acid (Nb ₂ O ₅ .xH ₂ O) powder pulverized so that the Li / Me is 1.10, and then the nickel-containing hydroxide is maintained. The lithium mixture was obtained by sufficiently mixing with strength using a shaker mixer device (TURBULA Type T2C manufactured by Willy et Bacofen (WAB)). The target niobium addition amount c ′ was set to 0.01. (Mixing process)

The lithium mixture was inserted into a magnesia firing vessel, and heated up to 500 ° C. at a heating rate of 2.77 ° C./min in an oxygen stream at a flow rate of 6 L / min. Hold for 3 hours. Then, after heating up to 780 degreeC with the same temperature increase rate and hold | maintaining for 12 hours, it furnace-cooled to room temperature, and obtained lithium transition metal complex oxide. (Baking process)
The obtained lithium transition metal composite oxide was mixed with pure water so that the slurry concentration was 1500 g / L to prepare a slurry, washed with water for 30 minutes using a stirrer, and then filtered. After filtration, it was held at 210 ° C. for 14 hours using a vacuum dryer and cooled to room temperature to obtain a positive electrode active material. (Washing process)

[Evaluation of positive electrode active material]
Crystal calculated from Scerler's formula using the composition of the positive electrode active material obtained by quantitative analysis by ICP emission spectrometry, the 2θ of the (003) plane and the half-value width in the diffraction pattern obtained by XRD measurement of the positive electrode active material The following table shows the child diameter, the specific surface area measured by the BET method, the volume-based average diameter (MV) measured by the laser scattering diffraction method, and the number of voids, which are indices related to the porosity obtained by observing the particle cross section with a scanning electron microscope. It is shown in 1.
Moreover, the amount of alkali metals other than lithium was measured by atomic absorption spectrometry and is shown in Table 1 together.
When the cross section of the obtained positive electrode active material was observed with a transmission electron microscope, no heterogeneous phase was observed, and by EDX analysis, niobium was uniformly distributed in the positive electrode active material particles, and the grain boundaries and grains were It was confirmed that the Nb concentration ratio was 3 times or less.

[Evaluation of initial discharge capacity]
The initial capacity evaluation of the obtained positive electrode active material was performed as follows. 70% by mass of the active material powder was mixed with 20% by mass of acetylene black and 10% by mass of PTFE. Lithium metal was used as the negative electrode, and an equivalent mixed solution of ethylene carbonate (EC) and diethyl carbonate (DEC) (made by Toyama Pharmaceutical Co., Ltd.) using 1M LiClO ₄ as a supporting salt was used as the electrolyte. A 2032 type coin battery as shown in FIG. 2 was fabricated in an Ar atmosphere glove box whose dew point was controlled at −80 ° C.

The prepared battery is left for about 24 hours, and after the open circuit voltage OCV (open circuit voltage) is stabilized, the current density with respect to the positive electrode is set to 0.5 mA / cm ² and charged to a cutoff voltage of 4.3 V to obtain an initial charge capacity. The capacity when the battery was discharged to a cutoff voltage of 3.0 V after a one hour rest was defined as the initial discharge capacity.

[Evaluation of cycle characteristics]
The cycle characteristics were evaluated as follows. For each battery, at a temperature of 25 ° C., charge to CC at a rate of 1C up to 4.4V (charge voltage required) and pause for 10 minutes, then CC discharge to 3.0V at the same rate and pause for 10 minutes The charging / discharging cycle was repeated 200 cycles. The discharge capacities at the first cycle and the 200th cycle were measured, and the percentage of the 2C discharge capacity at the 200th cycle to the 2C discharge capacity at the first cycle was determined as a capacity retention rate (%).

[Evaluation of safety of positive electrode]
The safety evaluation of the positive electrode is performed by CCCV charging (constant current-constant voltage charging. First, charging is operated at a constant current) to a cutoff voltage of 4.5V using a 2032 type coin battery manufactured by the same method as described above. Then, the battery was disassembled with care so as not to short-circuit, and the positive electrode was taken out. 3.0 mg of this electrode was measured, 1.3 mg of the electrolyte was added, sealed in an aluminum measurement container, and a temperature increase rate of 10 ° C./degree using a differential scanning calorimeter (DSC) PTC-10A (Rigaku). The heat generation behavior was measured from room temperature to 300 ° C. in min.
Table 1 shows the evaluation results of the initial capacity, cycle characteristics, and safety of the positive electrode active material.

(Example 2)
A positive electrode active material was obtained and evaluated in the same manner as in Example 1 except that the average particle size of niobic acid was 8 μm. The evaluation results of the obtained positive electrode active material are shown in Table 1, respectively.

(Example 3)
A positive electrode active material was obtained and evaluated in the same manner as in Example 1 except that the niobium compound was niobium oxide and the average particle size of the niobium compound was 1 μm. The evaluation results of the obtained positive electrode active material are shown in Table 1, respectively.

Example 4
A positive electrode active material was obtained and evaluated in the same manner as in Example 1 except that the target niobium addition amount c ′ was 0.05. The evaluation results of the obtained positive electrode active material are shown in Table 1, respectively.
(Example 5)
A positive electrode active material was obtained and evaluated in the same manner as in Example 1 except that the target niobium addition amount c ′ was 0.005. The evaluation results of the obtained positive electrode active material are shown in Table 1, respectively.
(Example 6)
A positive electrode active material was obtained and evaluated in the same manner as in Example 1 except that the target niobium addition amount c ′ was 0.001. The evaluation results of the obtained positive electrode active material are shown in Table 1, respectively.

(Example 7)
A positive electrode active material was obtained and evaluated in the same manner as in Example 1 except that the firing temperature was 700 ° C. The evaluation results of the obtained positive electrode active material are shown in Table 1, respectively.

(Example 8)
A positive electrode active material was obtained and evaluated in the same manner as in Example 1 except that the firing temperature was 830 ° C. The evaluation results of the obtained positive electrode active material are shown in Table 1, respectively.

Example 9
A nickel cobalt aluminum composite hydroxide was heat treated at 700 ° C. for 6 hours to obtain a composite oxide, and then a positive electrode active material was obtained and evaluated in the same manner as in Example 1 except that it was mixed with lithium hydroxide and niobic acid. The evaluation results of the obtained positive electrode active material are shown in Table 1, respectively.

(Comparative Example 1)
A positive electrode active material was obtained and evaluated in the same manner as in Example 1 except that the average particle size of niobic acid was 15 μm. When the obtained positive electrode active material was observed with a scanning electron microscope, an unreacted niobium compound was confirmed. Therefore, the positive electrode active material was added to a 100 g / L aqueous potassium hydroxide solution and stirred at 80 ° C. for 10 minutes. The reaction niobium compound was dissolved and filtered to remove the niobium compound, and then the composition of the positive electrode active material was analyzed in the same manner as in Example 1. The Nb content was below the lower limit of analysis. The evaluation results of the obtained positive electrode active material are shown in Table 1, respectively.

(Comparative Example 2)
A positive electrode active material was obtained and evaluated in the same manner as in Example 1 except that the target niobium addition amount c ′ was changed to 0.07. The evaluation results of the obtained positive electrode active material are shown in Table 1, respectively.

(Comparative Example 3)
A mixed aqueous solution of nickel sulfate and cobalt sulfate, aqueous sodium aluminate solution, 25 mass% sodium hydroxide solution, 25 mass so that the molar ratio of nickel: cobalt: aluminum is 81.5: 15.0: 3.5. % Ammonia water was added to the reaction vessel at the same time, no niobium compound was added during the mixing step, and the firing temperature was set to 740 ° C. In the same manner as in Example 1, a positive electrode active material was obtained and evaluated. did. The evaluation results of the obtained positive electrode active material are shown in Table 1, respectively.

(Comparative Example 4)
A positive electrode active material was obtained and evaluated in the same manner as in Example 1 except that the firing temperature was 850 ° C. The evaluation results of the obtained positive electrode active material are shown in Table 1, respectively.

(Comparative Example 5)
Niobium salt solution (30 g / L) prepared by dissolving niobic acid (Nb ₂ O ₅ xH ₂ O) in caustic potash in a slurry obtained by mixing nickel cobalt aluminum composite hydroxide obtained in the crystallization process with pure water. ) Is added dropwise with sulfuric acid while adjusting the pH to 10.0, thereby preparing an Nb-coated nickel cobalt aluminum composite hydroxide (hereinafter also referred to as “Nb-coated nickel hydroxide”), and a mixing step In Example 1, a positive electrode active material was obtained and evaluated in the same manner as in Example 1 except that the niobium compound was not mixed and the Nb-coated nickel hydroxide (Nb content c ′ was 0.01) was used. The evaluation results of the obtained positive electrode active material are shown in Table 1, respectively.

(Evaluation)
As shown in Table 1, in Examples 1 to 9 of the present invention, the initial discharge capacity of the obtained positive electrode active material generally exceeds 185 mAh / g, indicating that the material can be used as the positive electrode active material. The capacity retention rate is also 85% or more, which indicates that it has excellent cycle characteristics. In Example 4, since the amount of niobium added is large, the cycle characteristics are slightly lower than those in the other examples.
Moreover, the maximum exothermic peak height by DSC measurement is 4.0 cal / sec / g or less, and the calorific value is greatly suppressed as compared with the conventional positive electrode active material to which niobium of Comparative Example 3 is not added. I understand.
In Comparative Example 1 in which the average particle size of the niobium compound was 15 μm, the reactivity of the niobium compound was low, so that the amount of unreacted niobium compound increased and Nb was not contained in the positive electrode active material. For this reason, the maximum exothermic peak height was very high at 7.1 cal / sec / g, and the thermal stability was not good.
In Comparative Example 2, since the amount of niobium added was as high as 0.07, the initial discharge capacity was greatly reduced to 182.6 mAh / g. The cycle characteristics are also low.
Comparative Example 3 is a conventional positive electrode active material to which niobium is not added. Although the initial discharge capacity is high and the cycle characteristics are excellent, the maximum exothermic peak height is very high as 7.0 cal / sec / g. The thermal stability was not good.
In Comparative Example 4, since the mixture is baked at a high temperature, cation mixing in which nickel is mixed into the lithium layer of the lithium transition metal composite oxide, which is a layered compound, occurs and the specific surface area is small, so the initial discharge capacity is greatly reduced. did. Moreover, the maximum exothermic peak height is high.
In Comparative Example 5, since niobium was added by the coating method, the content of alkali metal as an impurity was high, so that the cycle characteristics were lower than in the examples.

  In order to take advantage of the non-aqueous electrolyte secondary battery of the present invention that has high initial capacity and excellent cycle characteristics while being excellent in safety, small portable electronic devices that always require high capacity and long life It is suitable for use as a power source for equipment. In addition, in power sources and stationary storage batteries for electric vehicles, it is indispensable to secure safety by increasing the size of the battery and to install an expensive protection circuit to ensure higher safety. Since the lithium ion secondary battery of the present invention has excellent safety, not only is it easy to ensure safety, but also it is possible to simplify expensive protection circuits and reduce costs. It is suitable as a power source for electric vehicles and a stationary storage battery. The power source for electric vehicles can be used not only for electric vehicles driven purely by electric energy, but also for power sources for so-called hybrid vehicles used in combination with combustion engines such as gasoline engines and diesel engines.

1 Lithium metal negative electrode 2 Separator (electrolyte impregnation)
3 Positive electrode (Evaluation electrode)
4 Gasket 5 Negative electrode can 6 Positive electrode can 7 Current collector

Claims (7)

Formula _{Li d Ni 1-a-b} -c Co a M b Nb c O 2 ( where, M is at least one element selected Mn, V, Mg, Ti and Al, 0.03 ≦ a ≦ 0.35, 0 ≦ b ≦ 0.10, 0.001 ≦ c ≦ 0.05, 0.95 ≦ d ≦ 1.20), and lithium composed of particles having a polycrystalline structure A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery comprising a transition metal composite oxide,
An aqueous alkaline solution is added to a mixed aqueous solution containing at least nickel and cobalt for crystallization, and the general formula Ni _{1-a′-b ′} Co _{a ′} M _{b ′} (OH) ₂ (where M is Mn, V, Mg, Ti And a crystallization step for obtaining a nickel-containing hydroxide represented by 0.03 ≦ a ′ ≦ 0.35 and 0 ≦ b ′ ≦ 0.10. ,
A mixing step of mixing the obtained nickel-containing hydroxide, a lithium compound, and a niobium compound having an average particle size of 0.1 to 10 μm to obtain a lithium mixture; and firing the lithium mixture at 700 to 840 ° C. in an oxidizing atmosphere. A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, comprising a firing step of obtaining a lithium transition metal composite oxide.   The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the niobium compound is niobic acid or niobium oxide.   3. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, further comprising a heat treatment step of heat-treating the nickel-containing hydroxide at a temperature of 105 to 800 ° C. before the mixing step. Production method.   The lithium transition metal composite oxide is made into a slurry at a rate of 100 to 2000 g / L with respect to 1 L of water after the firing step, and includes a water washing step of washing with water. The manufacturing method of the positive electrode active material for non-aqueous electrolyte secondary batteries. Formula _{Li d Ni 1-a-b} -c Co a M b Nb c O 2 ( where, 0.03 ≦ a ≦ 0.35,0 ≦ b ≦ 0.10,0.001 ≦ c ≦ 0.05 0.95 ≦ d ≦ 1.20, wherein M is at least one element selected from Mn, V, Mg, Ti and Al), and is a lithium transition metal composite oxide composed of polycrystalline particles A positive electrode active material for a non-aqueous electrolyte secondary battery comprising:
A positive electrode for a non-aqueous electrolyte secondary battery having a porous structure, a specific surface area of 0.9 to 3.0 m ² / g, and an alkali metal content other than lithium of 20 mass ppm or less Active material.   6. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 5, wherein the lithium transition metal composite oxide has a crystallite diameter of 10 to 180 nm.   A non-aqueous electrolyte secondary battery comprising the positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 5 or 6 as a positive electrode.

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