JP5141356B2 - Cathode active material for non-aqueous electrolyte secondary battery, method for producing the same, and non-aqueous electrolyte secondary battery using the same - Google Patents

Description

  The present invention relates to a positive electrode active material for a non-aqueous electrolyte secondary battery, a method for producing the same, 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. A lithium ion secondary battery includes a negative electrode, a positive electrode, an electrolyte, and the like, and a material capable of desorbing and inserting lithium is used as an active material for the negative electrode and the positive electrode.

Research and development of such lithium ion secondary batteries is being actively conducted. Among these, a lithium ion secondary battery using a lithium metal composite oxide, particularly a lithium cobalt composite oxide (LiCoO ₂ ), which is relatively easy to synthesize, as a positive electrode material can obtain a high voltage of 4 V, and thus has high energy. Practical use is progressing as a battery having a high density. 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, since lithium cobalt complex oxide (LiCoO ₂ ) uses a rare and expensive cobalt compound as a raw material, it 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.

  Recently, not only small secondary batteries for portable electronic devices but also expectations for using lithium ion secondary batteries as large secondary batteries for power storage and electric vehicles are increasing. Yes. For this reason, reducing the cost of the positive electrode material and making it possible to manufacture a cheaper lithium ion secondary battery can be expected to have a large ripple effect in a wide range of fields.

Newly proposed materials as positive electrode active materials for lithium ion secondary batteries include lithium manganese composite oxide (LiMn ₂ O ₄ ) using manganese, which is cheaper than cobalt, and lithium nickel composite oxide using nickel. (LiNiO ₂ ).

Lithium-manganese composite oxide (LiMn ₂ O ₄ ) is an effective alternative to lithium cobalt composite oxide (LiCoO ₂ ) because it is inexpensive and has excellent thermal stability, especially safety with respect to ignition. However, since 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 that are increasing year by year. . Moreover, at 45 degreeC or more, it has the fault that self-discharge is intense and a charge / discharge lifetime also falls.

On the other hand, lithium nickel composite oxide (LiNiO ₂ ) has substantially 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 manufactured using a lithium nickel composite oxide composed purely of nickel as a positive electrode active material without replacing nickel with other elements, the lithium cobalt composite oxide is used as the positive electrode active material. Cycle characteristics are inferior compared to when used as a substance. In addition, there is a disadvantage that battery performance is relatively easily lost when used or stored in a high temperature environment.

In order to solve such drawbacks, for example, in Patent Document 1, Li _w Ni _x Co _y B _z O _{2 is} used as a positive electrode active material capable of maintaining good battery performance during storage and use in a high temperature environment. Lithium nickel composite oxidation to which cobalt and boron represented by (0.05 ≦ w ≦ 1.10, 0.5 ≦ x ≦ 0.995, 0.005 ≦ z ≦ 0.20, x + y + z = 1) are added Things have been proposed.

Further, in Patent Document 2, Li _x Ni _a Co _b M _c O ₂ (0.8 ≦ x ≦ 1.2, 0... Is intended to improve self-discharge characteristics and cycle characteristics of a lithium ion secondary battery. 01 ≦ a ≦ 0.99, 0.01 ≦ b ≦ 0.99, 0.01 ≦ c ≦ 0.3, 0.8 ≦ a + b + c ≦ 1.2, M is Al, V, Mn, Fe, Cu And at least one element selected from Zn) have been proposed.

  However, the lithium nickel-based composite oxides obtained by these manufacturing methods have higher charge capacity and discharge capacity than the lithium cobalt composite oxide, and improved cycle characteristics. If left below, there is a problem that oxygen is released due to decomposition of the lithium nickel composite oxide from a lower temperature than the lithium cobalt composite oxide. In addition, the nickel in the lithium-nickel composite oxide that has become unstable under high-temperature environments acts as a catalyst when in contact with the electrolyte, promoting the reaction with the released oxygen and facilitating ignition. There was a sex problem.

In order to solve such a problem, for example, in Patent Document 3, for the purpose of improving the thermal stability of the positive electrode material of a lithium ion secondary battery, Li _a M _b Ni _c Co _d O _e (M is Al , Mn, Sn, In, Fe, V, Cu, Mg, Ti, Zn, Mo, and at least one metal selected from the group consisting of 0 <a <1.3, 0.02 ≦ b ≦ 0 .5, 0.02 ≦ d / c + d ≦ 0.9, 1.8 <e <2.2, and b + c + d = 1). ing. In this case, for example, when aluminum is selected as the additive element M, it is confirmed that if the amount of substitution from nickel to aluminum is increased, the decomposition reaction of the positive electrode active material is suppressed and the thermal stability is improved. However, replacing nickel with aluminum, which is effective to ensure sufficient stability, reduces the amount of nickel that contributes to the oxidation-reduction reaction accompanying the charge / discharge reaction, so the initial capacity, which is the most important for battery performance, is large. It has the problem of being lowered. This is presumably because aluminum is stable at trivalent, and nickel is also stable at trivalent because the charges are matched, and a portion that does not contribute to the oxidation-reduction reaction is generated.

  In recent years, the demand for higher capacity for small secondary batteries used in portable electronic devices and the like has been increasing, but sacrificing capacity to ensure safety is a merit of the high capacity of lithium nickel composite oxide Will not be able to meet the demand for higher capacity. In addition, a movement to use a lithium ion secondary battery for a large-sized secondary battery is also prominent. In particular, there is a great expectation as a power source for a hybrid vehicle and an electric vehicle. Thus, when used as a power source for automobiles, it is a big problem to solve the problem of the lithium nickel composite oxide that is inferior in safety.

  In order to improve safety, a method has been proposed in which the positive electrode active material is covered with a different compound to prevent direct contact between the positive electrode active material and the electrolytic solution. For example, Non-Patent Document 1 proposes that the surface of a lithium nickel composite oxide is coated with magnesium oxide to improve thermal stability. However, a secondary battery using such a lithium nickel composite oxide as a positive electrode active material has a low charge / discharge capacity, and it is difficult to say that it satisfies the problem of achieving both high capacity and safety.

In Patent Document 4, the surface of the positive electrode layer-like structure oxide of the lithium secondary battery to be coated with the lithium transition metal oxide has been proposed. In this technique, the raw material solution for surface treatment, was adjusted to 5-9 pH with an organic acid and ammonia, the later layer-like structure oxide was adjusted solution concentration to 0.1 to 2 molar concentration was added, coated and layers like structure oxide 500 to 850 ° C., by the heat treatment at 3 to 48 hours, the surface of lithium transition metal oxide to obtain a positive electrode active material having a layer-like structure oxide coated in layers Yes. However, as the lithium transition metal _{oxide, LiMn 2-X M1 X O} 4, LiCo 1-X Al X O 2, LiNi 1-X Al X O 2, LiNi 1-XY Co X Al Y O 2, LiNi 1 _-XYZ Co _X M1 _Y M2 _Z O ₂ (M1 and M2 are Al, Ni, Co, Fe, Mn, V, Cr, Cu, Ti, W, Ta, Mg or Mo, 0 ≦ X <0.5, 0 ≦ Y <0.5, 0 ≦ Z <0.5), although these are oxides capable of moving lithium ions, even if such a positive electrode active material is used, about A reduction in charge / discharge capacity as much as 8% occurs. Thus, it is hard to say that this proposal also satisfies the problem of achieving both high capacity and safety.

Further, in Patent Document 5, a positive electrode material for a non-aqueous electrolyte lithium ion secondary battery in which lithium compounds such as lithium cobaltate and lithium manganate are each independently deposited on the surface of lithium nickel oxide particles using mechanofusion. Has been proposed. However, this proposal is aimed at suppressing decomposition of the electrolytic solution, and is not aimed at achieving both high capacity and safety. Moreover, as a mode of attaching to the surface, a case of covering in a layered form and a case of scattering in a lump form have been proposed, but each aspect has advantages and disadvantages, and high capacity is also seen from this viewpoint. It is hard to say that it is both safe and safe.

As described above, no lithium metal composite oxide that has both high charge / discharge capacity, thermal stability, and safety has been found, and a nonaqueous electrolyte secondary battery that solves these problems is desired. .
JP-A-8-45509 Japanese Patent Laid-Open No. 8-213015 Japanese Patent Laid-Open No. 5-242891 JP 2002-231227 A JP-A-2005-190996 "Surface Modification of LiSr0.002Ni0.9Co0.1O2by Overcoating with a Magnesium Oxide", HJ Kweon et al., Electrochem. Solid-State Lett., Volume 3, Issue 3, pp.128-130 (March 2000)

  The present invention has been made in view of the above problems, and realizes a non-aqueous electrolyte secondary battery that has both thermal stability and safety and high charge / discharge capacity. An object of the present invention is to provide a positive electrode active material that can be used.

In the method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to the present invention, the ratio of the number of lithium atoms to the total number of metal elements other than lithium is 1.02 to 1.10. First step of mixing oxide powder with water and stirring to obtain a slurry, adding an aqueous solution containing 0.5 to 1.0 mol / dm ^{3 of} cobalt salt while stirring the resulting slurry A second step of attaching fine particles of a cobalt compound to the surface of the primary particles of the lithium metal composite oxide, and heat treatment at 300 to 700 ° C. to thereby convert the nickel compound into a lithium cobalt composite oxide. It has the 3rd process made into a thing.

Further, the lithium metal composite oxide represented by the general formula _{Li z Ni 1-xy Co x} M y O 2 ( however, M is, Mn, V, Mg, Mo, Nb, at least one selected from Ti and Al Element, 0.10 ≦ x ≦ 0.21, 0 ≦ y ≦ 0.08, 1.02 ≦ z ≦ 1.10.

  Furthermore, it is desirable to control the pH of the slurry obtained in the first step to exceed 9.0.

  Furthermore, it is desirable that the slurry obtained in the first step has a slurry concentration of 2000 to 2500 g / L.

  Furthermore, the number of cobalt atoms contained in the cobalt salt used in the second step is the sum of the number of atoms of metal elements other than Li contained in the powder comprising the lithium metal composite oxide used in the first step. On the other hand, it is desirable to set it as 0.7-0.9 atomic%.

The positive electrode active material for a non-aqueous electrolyte secondary battery of the present invention is obtained by any of the manufacturing methods of the general formula _{Li z Ni 1-xy Co x} M y O 2 ( however, M is Mn, V, At least one element selected from Mg, Mo, Nb, Ti and Al, 0.10 ≦ x ≦ 0.21, 0 ≦ y ≦ 0.08, 1.02 ≦ z ≦ 1.10. A positive electrode active material for a non-aqueous electrolyte secondary battery comprising primary particles of a lithium metal composite oxide and lithium cobalt composite oxide fine particles adhering to the surface of the primary particles, and the number of cobalt atoms contained in the fine particles Is 0.7 to 0.9 atomic% with respect to the total number of Ni, Co and M atoms contained in the primary particles.

  Furthermore, the fine particles are desirably 50 nm or less.

  Furthermore, it is desirable that the initial discharge capacity of the non-aqueous electrolyte secondary battery obtained by using the positive electrode is 190 mAh / g or more.

  In the non-aqueous electrolyte secondary battery of the present invention, any of the positive electrode active materials for non-aqueous electrolyte secondary batteries described above is used for the positive electrode.

  A non-aqueous electrolyte secondary battery that achieves both of the two characteristics of high thermal stability and safety and high charge / discharge capacity by the method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery of the present invention. A positive electrode active material that can be realized can be provided.

  By using the positive electrode active material for a non-aqueous electrolyte secondary battery of the present invention to obtain a non-aqueous electrolyte secondary battery, while satisfying the recent demand for higher capacity for small secondary batteries such as portable electronic devices, It is possible to ensure the safety required as a power source used for large-sized secondary batteries for hybrid vehicles and electric vehicles, which is extremely useful industrially.

The inventor has studied deeply about the compatibility between charge / discharge capacity and safety, which is important when a lithium metal composite oxide is used as a positive electrode active material in a non-aqueous electrolyte secondary battery. High charge / discharge capacity is achieved by attaching fine particles of lithium cobalt oxide to the surface of primary particles of lithium metal composite oxide, optimizing the amount of cobalt contained in the fine particles and controlling the form of the fine particles. And the present invention has been completed. Hereinafter, the present invention will be described in detail.
(1) the positive electrode active material the positive electrode active material for a non-aqueous electrolyte secondary battery of the present invention have the general formula _{Li z Ni 1-xy Co x} M y O 2 ( however, M is Mn, V, Mg, Mo, Nb , At least one element selected from Ti and Al, 0.10 ≦ x ≦ 0.21, 0 ≦ y ≦ 0.08, 1.02 ≦ z ≦ 1.10) Primary particles and fine particles of lithium cobalt complex oxide adhering to the surface of the primary particles, and the number of cobalt atoms contained in the fine particles is the number of Ni, Co and M atoms contained in the primary particles. It is 0.7-0.9 atomic% with respect to the sum total. Here, the fine particles are particles having a substantially spherical shape, a particle diameter of 5 nm to 500 nm, and a uniform particle diameter.

  The charge / discharge reaction of the secondary battery proceeds by reversibly entering and exiting lithium ions in the positive electrode active material. The positive electrode active material from which lithium has been extracted by charging is unstable at high temperatures, and when heated, the active material decomposes and releases oxygen, which causes combustion of the electrolyte solution and causes an exothermic reaction. Therefore, improving the thermal stability of the positive electrode material means suppressing the decomposition reaction of the positive electrode active material from which lithium has been extracted.

  As a known method for suppressing the decomposition reaction of the positive electrode active material, a part of nickel is generally substituted with an element having strong covalent bond with oxygen such as aluminum. Certainly, if the substitution amount from nickel to aluminum is increased, the decomposition reaction of the positive electrode active material is suppressed and the thermal stability is improved, but the amount of nickel contributing to the oxidation-reduction reaction accompanying the charge / discharge reaction is reduced. Therefore, since the charge / discharge capacity is reduced, the amount of substitution with aluminum must be limited to some extent. As a result, when sufficient thermal stability is ensured, sufficient reversible capacity cannot be obtained. Conversely, in order to obtain a certain level of capacity, thermal stability must be sacrificed.

Therefore, in the present invention, the general formula _{Li z Ni 1-xy Co x} M y O 2 ( provided that at least one element M is Mn, V, Mg, Mo, Nb, selected from Ti and Al, 0. 10 ≦ x ≦ 0.21, 0 ≦ y ≦ 0.08, 1.02 ≦ z ≦ 1.10) to improve thermal stability, and lithium High charge / discharge capacity and safety are sufficiently ensured by attaching fine particles made of lithium cobalt composite oxide to the surface of particles made of metal composite oxide. That is, by reducing the direct contact between the lithium metal composite oxide particles destabilized due to the deprivation of lithium and the electrolyte, the ignition is suppressed or delayed, and at the same time, the interface resistance is increased and the heat conduction is also reduced. The effect is to reduce the temperature rise.

  Generally, when the surface of the positive electrode active material is completely covered with a different compound, the movement (intercalation) of lithium ions is greatly limited, resulting in the high capacity of the lithium nickel composite oxide. The advantage is erased. However, in the present invention, the fine particles covering the primary particles made of the lithium metal composite oxide are lithium cobalt-based oxides having high lithium ion conductivity, and hardly limit the movement of lithium ions.

  Since the lithium cobalt oxide fine particles covering the particles made of the lithium metal composite oxide have sufficient lithium in the crystal in any charge / discharge state, it appears to be apparent. It is always present as a thermally stable oxide and does not become a thermally unstable state that causes ignition.

In addition, since the lithium cobalt composite oxide is attached in the form of fine particles, the specific surface area increases, so that the contact area with the electrolytic solution is increased, and the lithium metal composite oxide particles are also present in the attached portion. Lithium ion movement is hardly hindered between the electrolyte and the electrolyte.
For the reasons described above, in the present invention, a high charge / discharge capacity can be maintained with sufficient safety.

  Here, when the surface of the positive electrode active material is coated in layers instead of as fine particles, the specific surface area decreases regardless of the coating thickness, so that the coated material has high lithium ion conductivity. Even if this is the case, the contact area with the electrolytic solution becomes small, which tends to cause a decrease in charge / discharge capacity.

  In addition, when adhered to the surface of the primary particles in the form of a lump, the specific surface area is reduced at that portion, and the contact area with the electrolytic solution is reduced correspondingly, thereby easily reducing the charge / discharge capacity. In addition, the lithium metal composite oxide is in an unstable state at the portion in direct contact with the electrolytic solution as in the prior art, and the effect of improving safety is reduced.

  In the present invention, since the fine particles composed of the lithium cobalt composite oxide are adhered to the primary particles composed of the lithium metal composite oxide, the above-described effects are exerted on the lithium metal composite oxide destabilized in the charged state. And can provide high safety. Moreover, since lithium ion conduction can be effectively maintained with a high specific surface area, a reduction in charge / discharge capacity can be minimized.

  The number of cobalt atoms contained in the fine particles comprising the lithium-cobalt composite oxide is 0.7 to 0.9 atomic% with respect to the total number of Ni, Co and M atoms contained in the lithium metal composite oxide particles. By doing, since the coating layer is sufficiently thin and the movement of lithium ions is not restricted, both high charge / discharge capacity and safety can be achieved.

  On the other hand, if it is less than 0.7 atomic%, the effect of reducing the contact between the lithium metal composite oxide and the electrolyte is not sufficient. On the other hand, if it exceeds 0.9 atomic%, the fine particles comprising the lithium cobalt composite oxide tend to aggregate, which is not preferable.

  The fine particles made of a lithium nickel composite oxide preferably have a diameter of 50 nm or less. Thereby, also in the part to which the lithium cobalt complex oxide is adhered, the contact area between the fine particles and the electrolytic solution is increased, and lithium ions can be sufficiently moved.

  Adhesion to the primary particles made of lithium metal composite oxide may have a part that is not attached, but uniform thinness is preferable because the movement of lithium ions is not restricted. Since the contact area of a thing and electrolyte solution becomes large, it is not preferable.

  In addition, if the diameter of the fine particle made of lithium cobalt oxide is larger than 50 nm, the thickness of the fine particle adhesion portion made of lithium cobalt oxide increases, which may adversely affect the movement of lithium ions. Is not preferable. In addition, it is not preferable that the diameter of the fine particles made of lithium cobalt oxide is less than 5 nm because the aggregation of the fine particles easily proceeds and adversely affects the adhesion uniformity and lithium ion conductivity.

  The surface property of such a lithium metal composite oxide can be determined by observing with a field emission scanning electron microscope (FE-SEM, manufactured by HITACHI, S-4700), and the non-aqueous electrolyte secondary battery of the present invention. As for the positive electrode active material for use, it has been confirmed that fine particles of lithium cobalt-based oxide having a diameter of 5 nm to 50 nm are thinly and uniformly attached on the surface of particles made of a lithium metal composite oxide.

The effect of attaching fine particles of lithium cobalt complex oxide to the surface of the lithium metal complex oxide is, for example, lithium cobalt complex oxide, lithium manganese complex oxide, lithium nickel cobalt manganese complex oxide, The present invention can be applied not only to the positive electrode active material described in the present invention but also to a commonly used positive electrode active material for a lithium secondary battery.
(2) Method for Producing Positive Electrode Active Material The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to the present invention is to obtain a slurry by mixing and stirring a powder comprising a lithium metal composite oxide with water. A second step of adding fine particles of a cobalt compound to the surface of primary particles of a lithium metal composite oxide by adding an aqueous solution containing a nickel salt while stirring the resulting slurry; and It has the 3rd process which makes the said cobalt compound the lithium cobalt type complex oxide fine particles by heat-processing at 300-700 degreeC.
(First step)
First, a powder made of a lithium metal composite oxide having a ratio of the number of lithium atoms to the total number of metal elements other than lithium of 1.02 or more and 1.10 or less is mixed with water and stirred. To obtain a slurry. The lithium metal composite oxide may contain an element other than a metal element that can replace the metal element, and the number of atoms of the element other than the metal element is added to the total number of atoms of the metal element. Further, the lithium metal composite oxide, from the viewpoint of high capacity and thermal stability, the general formula _{Li z Ni 1-xy Co x} M y O 2 ( however, M is, Mn, V, Mg, Mo , Nb, Ti And at least one element selected from Al, 0.10 ≦ x ≦ 0.21, 0 ≦ y ≦ 0.08, 1.02 ≦ z ≦ 1.10. It is preferable.

  The ratio of the number of lithium atoms in the lithium metal composite oxide to the total number of atoms of the metal elements other than lithium is 1.02 or more and 1.10 or less. If the atomic ratio is less than 1.02, the lithium that elutes when slurryed will dissolve more than the amount necessary to maintain the charge / discharge capacity, leading to a decrease in capacity, and the lithium dissolved into the slurry. The amount is insufficient, the reaction with an aqueous solution in which a cobalt salt added later is dissolved becomes insufficient, and it becomes difficult to sufficiently attach the fine particles of the cobalt compound. Even if it exceeds 1.10, the crystallinity of the lithium metal composite oxide is lowered and the thermal stability is not sufficient.

  In order to obtain a high charge / discharge capacity, it is preferable to use, as the water for obtaining the slurry, general pure water that does not contain impurities harmful to the positive electrode active material.

The slurry concentration is preferably 2000 to 2500 g / L. When the slurry concentration is less than 2000 g / L, the volume of the slurry increases, and when considering the amount of drainage and the amount that can be produced at one time, the efficiency is low and the cost tends to be disadvantageous. On the other hand, if the concentration exceeds 2500 g / L, the viscosity of the slurry becomes too high, and it becomes difficult to stir uniformly.
(Second step)
Next, an aqueous solution in which a cobalt salt is dissolved is added while stirring the obtained slurry. The cobalt salt is not particularly limited, but it is preferable to use a cobalt salt that is readily soluble in water, such as cobalt sulfate, cobalt nitrate, cobalt chloride, cobalt acetate, cobalt citrate, and cobalt oxalate. It is preferred from the uniformity of crystallization that the cobalt salt is completely dissolved in water and added while stirring the slurry.

Here, the cobalt concentration in the aqueous solution containing the cobalt salt needs to be 0.5 to 1.0 mol / dm ³ . When the concentration is higher than 1.0 mol / dm ^3, fine particles made of a cobalt compound that adhere to the surface of primary particles made of a lithium metal composite oxide aggregate. On the other hand, if the concentration is less than 0.5 mol / dm ³ , the concentration is low, so that a large amount of aqueous solution is required, and the pH of the slurry is easily changed.

  The rate at which the aqueous solution containing the cobalt salt is added is not particularly limited, but can be arbitrarily determined so as to crystallize on the surface of the primary particles made of the lithium metal composite oxide in consideration of the slurry concentration.

  Cobalt present as ions in the aqueous solution in which the added cobalt salt is dissolved crystallizes as a cobalt compound mainly composed of hydroxide, and then collides with primary particles composed of lithium metal composite oxide in the slurry. To adhere to the surface or by crystallization on the primary particle surface. At that time, since almost all of the cobalt contained in the aqueous solution is crystallized and adheres to the surface of the primary particles, the aqueous solution preferably contains an amount corresponding to the desired amount of cobalt contained in the fine particles. That is, the cobalt contained in the aqueous solution is preferably 0.7 to 0.9 atomic% with respect to the total number of atoms of the metal elements other than lithium contained in the primary particles.

  Moreover, when adding the aqueous solution containing cobalt salt to a slurry, it is preferable to control so that pH of a slurry exceeds 9.0. When the pH of the slurry is 9.0 or less, a cobalt compound due to a neutralization reaction is difficult to be generated, and a large amount of lithium is desorbed from the lithium nickel composite oxide constituting the primary particles during the process. It causes a significant decrease in capacity. Furthermore, the influence of pH is very large on the shape of the cobalt compound covering the surface. When the pH is 9.0 or less, the cobalt compound tends to be coarse particles, tends to be a layered coating after firing, and the contact area with the electrolytic solution decreases, leading to a decrease in charge / discharge capacity. In addition, since the compound is difficult to be precipitated by neutralization at a low pH, the coating tends to be incomplete, and the safety improvement is insufficient.

  On the other hand, in the present invention, by adjusting the pH so that it always exceeds 9.0, nucleation of a large amount of cobalt compound is caused, and furthermore, by controlling grain growth, uniform cobalt compound fine particles Has also gained adhesion.

  The pH of the slurry is not particularly limited, but it is sufficient if it can be maintained at a level exceeding 9.0 and not exceeding 13.5.

    In the production method according to the present invention, a powder composed of a lithium metal composite oxide and water are mixed to form a slurry. By the pH adjustment described above, the pH of the slurry is raised so that it exceeds 9.0. However, although a small amount of lithium ions in the powder composed of the lithium metal composite oxide is eluted in the slurry and raises the pH of the slurry, the pH is raised by dissolving lithium hydroxide in the slurry. It is preferable to exceed zero.

Lithium in the slurry crystallizes on the surface of primary particles made of a lithium metal composite oxide together with cobalt in the cobalt salt, and a lithium cobalt-based oxide is formed by subsequent heat treatment. Therefore, the lithium hydroxide added in advance is sufficient in an amount commensurate with the amount of the lithium cobalt composite oxide produced by the subsequent heat treatment. Moreover, when lithium hydroxide is added excessively, excess lithium increases and battery characteristics deteriorate. Moreover, the amount of wasted lithium is increased, which is not preferable from the viewpoint of cost.
(Third step)
After filtration and drying, the cobalt compound containing lithium present on the surface of the lithium metal composite oxide is converted into fine particles made of lithium cobalt composite oxide by performing heat treatment at a temperature of 300 to 700 ° C. Thus, a positive electrode active material for a non-aqueous electrolyte secondary battery comprising primary particles of lithium metal composite oxide and lithium cobalt composite oxide fine particles adhering to the primary particle surface is obtained. The lithium cobalt composite oxide may contain a metal other than lithium in the lithium metal composite oxide that is the primary particle.

  The slurry may be filtered by any of the commonly used methods, such as suction filtration, filter press, and centrifugal separation. The drying method is not particularly limited, but it is preferably dried in an inert atmosphere or vacuum, particularly vacuum drying, in order to prevent deterioration of electrical characteristics when used as a positive electrode active material for a non-aqueous electrolyte secondary battery. . For example, it is preferable to vacuum dry at 90 to 210 ° C., preferably 150 ° C. or more for 14 hours or more.

  Moreover, heat processing temperature shall be 300-700 degreeC. If it is less than 300 degreeC, conversion to lithium cobalt type complex oxide is not enough, and if it exceeds 700 degreeC, the primary particle which consists of lithium metal complex oxide will raise | generate sintering or aggregation. The atmosphere at the time of the heat treatment is preferably an oxidizing atmosphere such as an air atmosphere or an oxygen atmosphere in order to convert the lithium cobalt complex oxide. The heat treatment time is not particularly limited, but may be a time that can be sufficiently converted into the lithium cobalt composite oxide in consideration of the amount of the lithium metal composite oxide and the heat treatment temperature.

Furthermore, before the heat treatment is performed, it is preferable to crush with a sieve having an opening of 100 to 200 μm, preferably 100 μm. By crushing, aggregation of the lithium metal composite oxide can be prevented during the heat treatment.
(3) Non-aqueous electrolyte secondary battery The non-aqueous electrolyte secondary battery of the present invention includes a positive electrode, a negative electrode, a non-aqueous electrolyte solution, and the like, and includes the same components as those of a general non-aqueous electrolyte secondary battery. The embodiment described below is merely an example, and the nonaqueous electrolyte secondary battery of the present invention can be variously modified based on the knowledge of those skilled in the art based on the embodiment described in the present specification. It can be implemented in an improved form. Moreover, the use of the nonaqueous electrolyte secondary battery of the present invention is not particularly limited.
(A) Positive electrode Using the positive electrode active material for a non-aqueous electrolyte secondary battery obtained as described above, for example, a positive electrode of a non-aqueous electrolyte secondary battery is produced as follows.

  First, a powdered positive electrode active material, a conductive material, and a binder are mixed, and, if necessary, a target solvent such as activated carbon and viscosity adjustment is added and kneaded to prepare a positive electrode mixture paste. Each mixing ratio in the positive electrode mixture paste is also an important factor for determining the performance of the non-aqueous electrolyte secondary battery. When the total mass of the solid content of the positive electrode mixture excluding the solvent is 100 parts by mass, the content of the positive electrode active material is 60 to 95 parts by mass in the same manner as the positive electrode of a general non-aqueous electrolyte secondary battery, and the conductive material It is desirable to set the content of 1 to 20 parts by mass and the content of the binder to 1 to 20 parts by mass.

  The obtained positive electrode mixture paste is applied to the surface of a current collector made of, for example, an aluminum foil and dried to disperse 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 or the like according to the target battery and used for battery production. However, the method for manufacturing the positive electrode is not limited to the above-described examples, and other methods may be used.

  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 anchoring the active material particles. For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluorine rubber, ethylene propylene diene rubber, styrene butadiene, cellulosic resin, polyacrylic. An acid or the like 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. Activated carbon can be added to the positive electrode mixture in order to increase the electric double layer capacity.
(B) Negative electrode A negative electrode mixture in which a negative electrode active material capable of occluding and desorbing lithium ions is mixed with a binder and an appropriate solvent is added to the negative electrode. 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 PVDF can be used as in the case of the positive electrode, and as a solvent for dispersing these active materials and the binder, N-methyl-2-pyrrolidone or the like can be used. Organic solvents can be used.
(C) 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 minute holes can be used.
(D) 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.
(E) Battery shape and configuration The shape of the non-aqueous electrolyte secondary battery of the present invention composed of the positive electrode, negative electrode, separator and non-aqueous electrolyte described above can be various, such as a cylindrical type and a laminated type. Can be.

In any case, the positive electrode and the negative electrode are laminated via a separator to form an electrode body, and the obtained electrode body is impregnated with a non-aqueous electrolyte and communicated with the positive electrode current collector and the outside. The positive electrode terminal 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 and sealed in a battery case to complete a non-aqueous electrolyte secondary battery. .
(F) Characteristics The secondary battery using the positive electrode active material of the present invention has an initial discharge capacity of 190 mAh / g or more. In addition, in differential scanning calorimetry (DSC, manufactured by BRUKER, DSC3100SA), compared with a positive electrode active material that is not covered with fine particles of a lithium cobalt-based composite oxide, the heat generation start temperature is significantly increased and the heat generation amount is reduced. Therefore, it can be said that it is excellent in safety.

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 is a perspective view and a cross-sectional view showing a coin battery used for battery evaluation.
Example 1
200 g of lithium metal composite oxide powder represented by Li _1.030 Ni _0.82 Co _0.15 Al _0.03 O ₂ was added to 80 mL of pure water and stirred to form a slurry at a slurry concentration of 2500 g / L.
Next, cobalt sulfate (manufactured by Wako Pure Chemical Industries, cobalt sulfate (II) seven water) is used so that cobalt is 0.8 atomic% with respect to the total number of nickel, cobalt and aluminum atoms in the lithium metal composite oxide. (Japanese product) was weighed, and pure water was added thereto so that the cobalt concentration was 0.5 mol / dm ³ to prepare an aqueous solution. Thereafter, the aqueous solution was added dropwise over 1 hour while stirring the obtained slurry. After completion of dropping, the slurry was suction filtered, and the obtained powder was vacuum-dried at 150 ° C. for 14 hours, and further crushed through a sieve having an opening of 100 μm.

  20 g of the obtained powder was weighed and placed in a 10 cm × 5 cm × 5 cm alumina firing container, and a temperature increase rate of 5 ° C./min in a 100% oxygen stream at a flow rate of 1.5 L / min using a tubular electric furnace. The temperature was raised to 500 ° C., heat treated for 2 hours, and then cooled to room temperature. Finally, it was passed through a sieve having an opening of 50 μm and pulverized again to obtain a positive electrode active material which is a lithium metal composite oxide powder on which fine particles made of lithium cobalt composite oxide adhere to the surface.

  The initial capacity evaluation of the obtained positive electrode active material was performed as follows.

70% by mass of the positive electrode active material powder was mixed with 20% by mass of acetylene black (manufactured by Denki Kogyo Co., Ltd.) and 10% by mass of PTFE (manufactured by Daikin Kogyo Co., Ltd.). A pellet was prepared 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) (made by Toyama Pharmaceutical Co., Ltd.) using 1M LiClO ₄ as a supporting salt was used as the electrolyte. Using these, a 2032 type coin battery as shown in a perspective view and a cross-sectional view in FIG. 1 was produced in a glove box in an Ar atmosphere in which the dew point was controlled at −80 ° C.

  The produced battery was left for about 24 hours, and after the open circuit voltage (OCV) was stabilized, the current density with respect to the positive electrode was set to 0.5 mA / cm 2 and charged to a cutoff voltage of 4.3 V. The initial charge capacity was defined as the capacity when discharged to a cutoff voltage of 3.0 V after 1 hour of rest.

  Evaluation of the safety of the positive electrode was performed as follows using a 2032 type coin battery manufactured by the same method as described above.

  First, CCCV charging up to a cutoff voltage of 4.5V (constant current-constant voltage charging. Charging method using a two-phase charging process in which charging starts at a constant current and then ends at a constant voltage) Then, it was disassembled with care not to short-circuit, and the positive electrode was taken out. 3.0 mg of the obtained positive electrode was weighed, 1.3 mg of the electrolyte was added and sealed in an aluminum measuring container, and the temperature rising rate was 10 ° C./min using a differential scanning calorimeter (DSC, manufactured by BRUCKER, DSC3100SA). The exothermic rate from room temperature to 400 ° C. was measured and the exothermic start temperature and the exothermic peak intensity were measured. The heat generation start temperature is 255 ° C. The exothermic peak intensity was 3 cal / sec · g, and the relative ratio was calculated with the exothermic peak intensity obtained in Comparative Example 1 described later as 100.

  Table 1 shows the initial discharge capacity, the heat generation start temperature, and the heat generation peak intensity obtained as a relative ratio obtained as described above. Table 1 also shows the ratio (L / Me ratio) between the number of lithium atoms and the total number of metal elements other than lithium, and the aqueous solution cobalt concentration.

FIG. 2 shows a surface fine region photograph of the positive electrode active material obtained using FE-SEM (manufactured by HITACHI, S-4700).
(Example 2)
A positive electrode active material was obtained in the same manner as in Example 1 except that a lithium metal composite oxide represented by Li _1.025 Ni _0.82 Co _0.15 Al _0.03 O ₂ was used, and the same evaluation as in Example 1 was performed. It was. The evaluation results are shown in Table 1.

FIG. 3 shows a surface fine region photograph of the positive electrode active material obtained using FE-SEM (manufactured by HITACHI, S-4700).
(Example 3)
A positive electrode active material was obtained in the same manner as in Example 1 except that a lithium metal composite oxide represented by Li _1.020 Ni _0.82 Co _0.15 Al _0.03 O ₂ was used, and the same evaluation as in Example 1 was performed. It was. The evaluation results are shown in Table 1. The evaluation results are shown in Table 1.
Example 4
Example 1 except that a lithium metal composite oxide represented by Li _1.025 Ni _0.82 Co _0.15 Al _0.03 O ₂ was used and the cobalt concentration of the aqueous solution was adjusted to 1.0 mol / dm ^3. Similarly, the positive electrode active material was obtained and evaluated in the same manner as in Example 1. The evaluation results are shown in Table 1. The evaluation results are shown in Table 1.
(Comparative Example 1)
A positive electrode active material was obtained in the same manner as in Example 1 except that a lithium metal composite oxide represented by Li _1.015 Ni _0.82 Co _0.15 Al _0.03 O ₂ was used, and the same evaluation as in Example 1 was performed. It was. The evaluation results are shown in Table 1. The evaluation results are shown in Table 1.

FIG. 4 shows a surface fine region photograph of the positive electrode active material obtained using FE-SEM (manufactured by HITACHI, S-4700).
(Comparative Example 2)
Example 1 except that a lithium metal composite oxide represented by Li _1.025 Ni _0.82 Co _0.15 Al _0.03 O ₂ was used and the cobalt concentration of the aqueous solution was adjusted to 0.3 mol / dm ^3. Similarly, the positive electrode active material was obtained and evaluated in the same manner as in Example 1. The evaluation results are shown in Table 1. The evaluation results are shown in Table 1.
(Comparative Example 3)
Example 1 except that a lithium metal composite oxide represented by Li _1.025 Ni _0.82 Co _0.15 Al _0.03 O ₂ was used and the cobalt concentration of the aqueous solution was adjusted to 1.3 mol / dm ^3. Similarly, the positive electrode active material was obtained and evaluated in the same manner as in Example 1. The evaluation results are shown in Table 1. The evaluation results are shown in Table 1.

[Evaluation]
From FIG. 2 and FIG. 3 which are FE-SEM photographs of actual examples, in the portions adhering to the surface of the primary particles made of lithium metal composite oxide having a diameter of about 0.3 to 0.5 μm, the diameter is 10 to 20 nm, respectively. It can be seen that the fine particles of the lithium cobalt complex oxide are relatively uniformly attached. On the other hand, in FIG. 4 which is the FE-SEM photograph of Comparative Example 1, it can be seen that the aggregation of the fine particles is greatly advanced. That is, it can be seen that the Li / Me ratio needs to be 1.02 or more in order to uniformly deposit the lithium cobalt based composite oxide as fine particles on the surface of the lithium metal composite oxide.

  From Table 1, it can be seen that in Examples 1 to 4 of the present invention, the DSC exotherm start temperature was as high as 255 ° C. or higher, and the peak relative intensity was also decreased as compared with Comparative Example 1. That is, fine particles made of lithium cobalt composite oxide are sufficiently adhered to the surface of primary particles made of lithium metal composite oxide, so that the interfacial resistance is increased, heat conduction is suppressed to some extent, and lithium is lost. It is considered that the direct contact between the destabilized lithium metal composite oxide particles and the electrolytic solution was suppressed, and the reaction with oxygen desorbed from the positive electrode active material became relatively slow.

  Moreover, in Examples 1-4 of this invention, an initial stage discharge capacity is also 194 mAh / g or more, and it turns out that the positive electrode active material which made thermal stability and safety and high charge / discharge capacity compatible is obtained.

  On the other hand, in Comparative Examples 2 and 3, the heat generation start temperature by DSC is lower than that of the Example, and the heat generation peak relative intensity is not much reduced as compared with Comparative Example 1. This is because the fine particles of the lithium cobalt composite oxide are aggregated, so that the effect of the lithium cobalt composite oxide fine particles is reduced and the primary particle surface of the lithium metal composite oxide can be reliably covered. It is possible that contact with the electrolyte has increased.

From the above, in order to obtain a positive electrode active material in which fine particles made of lithium nickel composite oxide are adhered to the surface of the lithium metal composite oxide primary particles and both thermal stability and safety are compatible with high charge / discharge capacity, The ratio of Li and metal elements other than Li in the lithium metal composite oxide, that is, the Li / Me ratio is set to 1.02, and the cobalt concentration in the aqueous solution is set to 0.5 to 1.0 mol / dm ³ . It turns out that it is appropriate to do.

  The non-aqueous electrolyte secondary battery of the present invention, which is excellent in safety and has a high charge / discharge capacity, is a small portable electronic device (such as a notebook personal computer or a mobile phone terminal) that always requires a high capacity. ) Is suitable for the power source.

  Further, in a power source for an electric vehicle, it is indispensable to ensure safety by increasing the size of the battery and an expensive protection circuit for ensuring higher safety. On the other hand, the non-aqueous electrolyte secondary battery of the present invention has excellent safety, so that not only is it easy to ensure safety, but also an expensive protection circuit is simplified to reduce the cost. Therefore, it is also suitable as a power source for electric vehicles. The present invention can be used not only as a power source for an electric vehicle driven purely by electric energy but also as a power source for a so-called hybrid vehicle used in combination with a combustion engine such as a gasoline engine or a diesel engine.

It is the perspective view and sectional drawing which show the coin battery used for the initial stage capacity | capacitance evaluation of a positive electrode active material. It is a photograph of the surface fine area | region of the positive electrode active material for non-aqueous electrolyte secondary batteries obtained in Example 1 of this invention. It is a photograph of the surface fine area | region of the positive electrode active material for non-aqueous electrolyte secondary batteries obtained in Example 2 of this invention. It is a photograph of the surface fine area | region of the positive electrode active material for nonaqueous electrolyte secondary batteries obtained by the comparative example 1 of this invention.

Explanation of symbols

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 (9)

A slurry is obtained by mixing and stirring a powder of a lithium metal composite oxide having a ratio of the number of lithium atoms and the total number of metal elements other than lithium of 1.02 to 1.10 with water. In one step, while stirring the resulting slurry, an aqueous solution containing 0.5 to 1.0 mol / dm ^{3 of} a cobalt salt is added to the surface of the primary particles made of the lithium metal composite oxide from the cobalt compound. A non-aqueous electrolyte secondary battery comprising: a second step of adhering fine particles to be formed; and a third step of heat treating at 300 to 700 ° C. to convert the cobalt compound into a lithium-cobalt composite oxide. For producing a positive electrode active material for use. The lithium metal composite oxide represented by the general formula _{Li z Ni 1-xy Co x} M y O 2 ( however, M is at least one element Mn, V, Mg, Mo, Nb, selected from Ti and Al, The nonaqueous electrolyte secondary battery according to claim 1, wherein: 0.10 ≦ x ≦ 0.21, 0 ≦ y ≦ 0.08, 1.02 ≦ z ≦ 1.10. For producing a positive electrode active material for use.   The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the pH of the slurry obtained in the first step is controlled to exceed 9.0.   The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the slurry obtained in the first step has a slurry concentration of 2000 to 2500 g / L.   The number of cobalt atoms contained in the cobalt salt used in the second step is based on the total number of atoms of metal elements other than Li contained in the powder composed of the lithium metal composite oxide used in the first step. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the content is 0.7 to 0.9 atomic%. Obtained by the production method according to any one of claims 2 to 5, the general formula _{Li z Ni 1-xy Co x} M y O 2 ( however, M is, Mn, V, Mg, Mo , Nb, Ti and Al Primary particles of a lithium metal composite oxide represented by at least one element selected from: 0.10 ≦ x ≦ 0.21, 0 ≦ y ≦ 0.08, 1.02 ≦ z ≦ 1.10. And a positive electrode active material for a non-aqueous electrolyte secondary battery comprising fine particles of lithium cobalt composite oxide adhering to the surface of the primary particles, and the number of cobalt atoms contained in the fine particles is included in the primary particles A positive electrode active material for a non-aqueous electrolyte secondary battery, characterized in that it is 0.7 to 0.9 atomic% with respect to the total number of atoms of Ni, Co, and M.   The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 6, wherein the fine particles are 50 nm or less.   The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 6 or 7, wherein an initial discharge capacity of the non-aqueous electrolyte secondary battery obtained by using the positive electrode is 190 mAh / g or more.   A non-aqueous electrolyte secondary battery, wherein the positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 6 is used for a positive electrode.