JP6485232B2 - Method for producing positive electrode active material - Google Patents

Description

  The present invention relates to a method for producing a positive electrode active material used for a positive electrode of a lithium ion secondary battery.

  One type of non-aqueous secondary battery in which a non-aqueous electrolyte mediates electrical conduction between electrodes is a lithium ion secondary battery. A lithium ion secondary battery is a secondary battery in which lithium ions are responsible for electrical conduction between electrodes in charge and discharge reactions. Compared to other secondary batteries such as nickel-hydrogen storage batteries and nickel-cadmium storage batteries, the energy density Is high and the memory effect is small. Therefore, lithium ion secondary batteries can be used for small power sources such as portable electronic devices and household electric appliances, stationary power sources such as power storage devices, uninterruptible power supply devices, power leveling devices, ships, railways, hybrid vehicles, Applications are expanding to medium and large power sources such as drive power sources for electric vehicles.

In particular, when a lithium ion secondary battery is used as a medium- or large-sized power source, it is required to increase the energy density of the battery. In order to realize a high energy density of the battery, it is necessary to increase the energy density of the positive electrode and the negative electrode, and it is required to increase the capacity of the active material used for the positive electrode and the negative electrode. As a positive electrode active material having a high charge / discharge capacity, a lithium composite compound represented by a chemical formula of LiM′O ₂ (M ′ represents an element such as Ni, Co, or Mn) having an α-NaFeO ₂ type layered structure. The powder is known. This positive electrode active material is expected to be a positive electrode active material that realizes higher energy of the battery, since the capacity tends to increase especially as the Ni ratio increases.

As such a positive electrode active material, _a powder of a lithium-containing compound represented by Li _a Ni _b M1 _c M2 _d (O) ₂ (SO ₄ ) _X and a method for producing the same are disclosed (see Patent Document 1 below). ). An object of the invention described in Patent Document 1 is to provide a lithium mixed metal oxide in which secondary particles are not destroyed in battery production (positive electrode) and do not become powder. As a solution, the difference in D10 value measured according to ASTM B 822 between the initial powder and the powder compressed at 200 MPa is 1.0 μm or less.

The manufacturing process of the lithium-containing compound powder described in Patent Document 1 includes a step of preparing a co-precipitated nickel-containing precursor having a predetermined porosity, and the nickel-containing precursor and the lithium-containing compound are mixed. And obtaining a precursor mixture. Examples of the lithium-containing compound include lithium carbonate, lithium hydroxide, lithium hydroxide monohydrate, lithium oxide, lithium nitrate, or a mixture thereof. Moreover, the resulting precursor mixture, CO ₂ free (CO ₂ content 0.5ppm or less) heated to 200 to 1000 ° C. in multiple stages to produce a powder product while using an oxygen-containing carrier gas And crushing the powder by ultrasonic waves and sieving the crushed powder.

  According to Patent Document 1, the reaction control related to the temperature holding stage of the manufacturing process described above provides a product free from agglomeration of strongly sintered secondary particles. Thereby, in electrode manufacture, it is supposed that the grinding | pulverization process in which the square particle | grains which have an angle | corner which causes destruction of a particle | grain in a material bed under a high pressure is formed can be omitted.

Special table 2010-505732

On the other hand, instead of heating the precursor mixture as in Patent Document 1, a mixture of lithium carbonate and a compound containing Ni is fired, for example, chemical formula LiM′O ₂ (M ′ is a metal element containing Ni In order to produce a lithium composite compound having a high Ni concentration and a layered structure in which the atomic ratio (Ni / M ′) of Ni in M ′ is 0.7 or more, there are the following problems. In order to industrially mass produce a lithium composite oxide with high Ni concentration, it is necessary to advance the synthesis reaction in a large amount and uniformly. However, it was found that when a mixture of lithium carbonate and a compound containing Ni is heated, a large amount of carbon dioxide gas is generated from the lithium carbonate, so that a large amount and a uniform synthesis reaction are inhibited. This is because, when carbon dioxide gas is generated, the oxygen partial pressure is lowered and the reaction of oxidizing the Ni valence from divalent to trivalent is inhibited. In particular, the present inventors have found that a lithium composite compound having a high Ni concentration has problems such as a significant decrease in capacity when Ni is not sufficiently oxidized.

  The present invention has been made in view of the above problems, and industrially mass-produces a positive electrode active material composed of a lithium composite oxide having a high Ni concentration by firing a mixture of lithium carbonate and a compound containing Ni. It aims at providing the manufacturing method of the positive electrode active material which can be manufactured.

  In order to achieve the above object, a method for producing a positive electrode active material of the present invention is a method for producing a positive electrode active material used for a positive electrode of a lithium ion secondary battery, except for lithium carbonate and Li in the following formula (1). A mixing step of mixing a compound containing each of the metal elements, a baking step of baking the mixture obtained in the mixing step in an oxidizing atmosphere to obtain a lithium composite compound represented by the following formula (1), And the firing step includes a first heat treatment step of obtaining a first precursor by heat-treating the mixture at a heat treatment temperature of 200 ° C. or more and 400 ° C. or less for 0.5 hours or more and 5 hours or less. A second heat treatment step for obtaining a second precursor by heat-treating the first precursor at a heat treatment temperature of 450 ° C. or higher and lower than 700 ° C. for 2 hours or more and 50 hours or less; and More than 700 ℃ A third heat treatment step of obtaining the lithium composite compound by heat treatment at a heat treatment temperature of 50 ° C. or less for 2 hours or more and 50 hours or less, and oxidation of the second heat treatment step and the third heat treatment step The oxygen concentration of the sexual atmosphere is 80% or more.

_{Li 1 + a Ni b Mn c} Co d M e O 2 + α ... (1)

  In the formula (1), M is at least one element selected from the group consisting of Mg, Al, Ti, Zr, Mo, and Nb, and a, b, c, d, e, and α are , −0.1 ≦ a ≦ 0.2, 0.7 ≦ b ≦ 0.9, 0 ≦ c ≦ 0.35, 0.05 ≦ d ≦ 0.30, 0 ≦ e ≦ 0.35, b + c + d + e = 1 and −0.1 ≦ α ≦ 0.1.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to industrially mass-produce the positive electrode active material which consists of Ni high concentration lithium complex oxide which has a layered structure.

The flowchart which shows embodiment of the manufacturing method of the positive electrode active material of this invention. The flowchart which shows the modification of the manufacturing method of the positive electrode active material shown to FIG. 1A. The typical fragmentary sectional view which shows one Embodiment of the lithium ion secondary battery provided with the positive electrode containing a positive electrode active material.

  Hereinafter, embodiments of the method for producing a positive electrode active material of the present invention will be described in detail.

  The manufacturing method of the positive electrode active material of this embodiment is a method for manufacturing the positive electrode active material used for the positive electrode of non-aqueous secondary batteries, such as a lithium ion secondary battery. First, the positive electrode active material manufactured by the positive electrode active material manufacturing method of this embodiment will be described in detail.

(Positive electrode active material)
The positive electrode active material produced by the production method of the present embodiment is a powder of a Ni-rich lithium composite compound having an α-NaFeO ₂ type layered structure and represented by the following formula (1).

_{Li 1 + a Ni b Mn c} Co d M e O 2 + α ... (1)

  In the formula (1), M is at least one element selected from the group consisting of Mg, Al, Ti, Zr, Mo, and Nb, and a, b, c, d, e, and α are , −0.1 ≦ a ≦ 0.2, 0.7 ≦ b ≦ 0.9, 0 ≦ c ≦ 0.35, 0.05 ≦ d ≦ 0.30, 0 ≦ e ≦ 0.35, b + c + d + e = 1 and −0.1 ≦ α ≦ 0.1.

The positive electrode active material composed of a powder of a lithium composite compound having a layered structure of α-NaFeO ₂ represented by the above formula (1) may repeat reversible insertion and desorption of lithium ions with charge and discharge. It is a layered positive electrode active material that is possible and has low resistance. The lithium composite compound particles constituting the positive electrode active material may be primary particles in which individual particles are separated, or may be secondary particles obtained by bonding a plurality of primary particles by sintering, Primary particles or secondary particles containing a free lithium compound may be used.

  The average particle diameter of the primary particles of the positive electrode active material is preferably 0.1 μm or more and 2 μm or less, for example. By making the average particle size of the primary particles of the positive electrode active material 2 μm or less, when manufacturing a positive electrode containing the positive electrode active material, the filling of the positive electrode active material in the positive electrode is improved, and a positive electrode with high energy density is manufactured. be able to. Moreover, it is preferable that the average particle diameter of the secondary particle of a positive electrode active material is 3 micrometers or more and 50 micrometers or less, for example.

  The particles of the positive electrode active material can be converted into secondary particles by granulating the primary particles produced by the method for producing the positive electrode active material described later by dry granulation or wet granulation. As a granulation means, granulators, such as a spray dryer and a rolling fluidized bed apparatus, can be utilized, for example.

In the formula (1), a represents the stoichiometric ratio of the positive electrode active material represented by the chemical formula LiM′O ₂ , that is, the excess / deficiency of Li from Li: M ′: O = 1: 1: 2. Yes. Here, M ′ represents a metal element other than Li in the formula (1). The higher the Li content, the higher the valence of the transition metal before charging, the lower the rate of transition metal valence change during Li desorption, and the improvement of the charge / discharge cycle characteristics of the positive electrode active material. Can do. On the other hand, the higher the Li content, the lower the charge / discharge capacity of the positive electrode active material. Therefore, by setting the range of a representing the excess / deficiency of Li in the formula (1) to be −0.1 or more and 0.2 or less, the charge / discharge cycle characteristics of the positive electrode active material are improved, and charge / discharge is performed. A decrease in capacity can be suppressed.

  More preferably, the range of a representing the excess or deficiency of Li in the formula (1) can be set to -0.05 or more and 0.1 or less. When a in the formula (1) is −0.05 or more, a sufficient amount of Li to contribute to charge / discharge is ensured, and the capacity of the positive electrode active material can be increased. In addition, when a in the formula (1) is 0.1 or less, sufficient charge compensation due to change in the valence of the transition metal can be ensured, and both high capacity and high charge / discharge cycle characteristics can be achieved. .

  Moreover, if b which shows the content rate of Ni in said Formula (1) is 0.7 or more, sufficient amount of Ni will be ensured to contribute to charging / discharging in a positive electrode active material, and it will aim at high capacity | capacitance. Can do. On the other hand, when b in the formula (1) exceeds 0.9, a part of Ni is replaced with a Li site, and a sufficient amount of Li to contribute to charging / discharging cannot be secured, and charging / discharging of the positive electrode active material is not possible. The discharge capacity may be reduced. Therefore, b indicating the Ni content in the formula (1) is in the range of 0.7 to 0.9, more preferably in the range of 0.75 to 0.85. While increasing the capacity of the positive electrode active material, it is possible to suppress a decrease in charge / discharge capacity.

  Further, the addition of Mn has an effect of stably maintaining the layered structure even when Li is desorbed by charging. However, when c which shows the content rate of Mn in the said Formula (1) exceeds 0.35, the capacity | capacitance of a positive electrode active material will fall. Therefore, by setting c in the formula (1) to be in the range of 0 or more and 0.35 or less, the layered structure of the positive electrode active material can be stably maintained even when Li is inserted / desorbed by charge / discharge. And a decrease in the capacity of the positive electrode active material can be suppressed.

  Moreover, if d which shows the content rate of Co in said Formula (1) is 0.05 or more, the layered structure of a positive electrode active material can be kept stable. By keeping the layered structure stable, for example, cation mixing in which Ni is mixed into the Li site can be suppressed, so that excellent charge / discharge cycle characteristics can be obtained. On the other hand, when d in the formula (1) exceeds 0.3, the ratio of Co which is limited in supply amount and high in cost is relatively increased, which is disadvantageous in industrial production of the positive electrode active material. Therefore, d indicating the content of Co in the formula (1) is in the range of 0.05 to 0.3, more preferably in the range of 0.1 to 0.2. The charge / discharge cycle characteristics of the positive electrode active material can be improved, which is advantageous in industrial mass production of the positive electrode active material.

  Further, M in the formula (1) is composed of at least one metal element selected from the group consisting of Mg, Al, Ti, Zr, Mo, and Nb. Activity can be ensured. Moreover, by substituting the metal site of the positive electrode active material with these metal elements, the stability of the crystal structure of the positive electrode active material and the electrochemical characteristics (cycle characteristics, etc.) of the layered positive electrode active material can be improved. In addition, when e which shows the content rate of M in said Formula (1) exceeds 0.35, the capacity | capacitance of a positive electrode active material will fall. Therefore, the capacity | capacitance fall of a positive electrode active material can be suppressed by making the range of e in the said Formula (1) into 0 or more and 0.35 or less.

Moreover, (alpha) in the said Formula (1) is a range which accept | permits the layered structure compound which belongs to space group R-3m, and shows the amount of oxygen available. From the viewpoint of maintaining the α-NaFeO ₂ type layered structure of the positive electrode active material, for example, a range of −0.1 or more and 0.1 or less is preferable.

  The crystal structure of the positive electrode active material particles can be confirmed by, for example, X-ray diffraction (XRD). The average composition of the positive electrode active material particles can be confirmed by high frequency inductively coupled plasma (ICP), atomic absorption spectrometry (AAS), or the like.

The BET specific surface area of the positive electrode active material particles is preferably, for example, approximately 0.2 m ² / g or more and 2.0 m ² / g or less. By setting the BET specific surface area of the particles of the positive electrode active material to be approximately 2.0 m ² / g or less, the filling property of the positive electrode active material in the positive electrode is improved, and a positive electrode having a high energy density can be manufactured. In addition, a BET specific surface area can be measured using an automatic specific surface area measuring apparatus.

  Further, the particle breaking strength of the positive electrode active material is preferably 50 MPa or more and 100 MPa or less. As a result, the particles of the positive electrode active material are not destroyed in the process of electrode preparation, and are peeled off when the slurry containing the positive electrode active material is applied to the surface of the positive electrode current collector to form the positive electrode mixture layer. Application failure such as is suppressed. The particle breaking strength of the positive electrode active material can be measured using, for example, a micro compression tester.

(Method for producing positive electrode active material)
Next, the manufacturing method of the positive electrode active material of this embodiment which manufactures the above-mentioned positive electrode active material is demonstrated. FIG. 1A is a flowchart showing each step included in the method for producing a positive electrode active material of the present embodiment. FIG. 1B is a flowchart showing each step in the modification of the method for producing the positive electrode active material of the present embodiment shown in FIG. 1A.

  The manufacturing method of the positive electrode active material of the present embodiment includes a mixing step S1 in which lithium carbonate and a compound containing a metal element other than Li in the formula (1) are mixed, and the mixture obtained in the mixing step S1 is oxidized. Firing step S2 for obtaining a lithium composite compound represented by the above formula (1) by firing in a neutral atmosphere.

  In the mixing step S1, as a starting material of the positive electrode active material, in addition to lithium carbonate, a compound containing a metal element other than Li in the formula (1), for example, a Ni-containing compound, a Mn-containing compound, a Co-containing compound, M A containing compound or the like can be used. The M-containing compound is a compound containing at least one metal element selected from the group consisting of Mg, Al, Ti, Zr, Mo, and Nb.

  In the mixing step S1, the raw material powder is prepared by mixing the starting materials weighed at a ratio that gives a predetermined elemental composition corresponding to the formula (1). In the method for producing a positive electrode active material of this embodiment, lithium carbonate is used as a starting material containing Li. Lithium carbonate is excellent in industrial applicability and practicality as compared with other Li-containing compounds such as lithium acetate, lithium nitrate, lithium hydroxide, lithium chloride, and lithium sulfate.

  As the Ni-containing compound, Mn-containing compound, and Co-containing compound as a starting material for the positive electrode active material, for example, oxides, hydroxides, carbonates, sulfates, acetates, and the like can be used. Preferably, the product, hydroxide or carbonate is used. In addition, as the M-containing compound, for example, acetate, nitrate, carbonate, sulfate, oxide, or hydroxide can be used, and in particular, carbonate, oxide, or hydroxide is used. Is preferred.

  In the mixing step S1, the starting material is preferably pulverized and mixed by, for example, a pulverizer. Thereby, a uniformly mixed powdery solid mixture can be prepared. As a pulverizer for pulverizing the starting material compound, a general precision pulverizer such as a ball mill, a jet mill, or a sand mill can be used. The starting material is preferably pulverized wet. From an industrial viewpoint, the solvent used for wet grinding is preferably water. The particle size of the pulverized powder of the starting material pulverized in the mixing step S1 is an index representing the degree of mixing of the starting material.

  The particle size (cumulative distribution) of a practical starting material that can be used industrially, that is, the particle size (cumulative distribution) when measured on a volume basis is 1 μm or more as the average particle size and 10 μm or more as the maximum particle size D100. is there. In this case, the particle size of the pulverized powder of the starting material when measured on a volume basis is preferably such that D50 is 0.3 μm or less and D100 is 1.0 μm or less. Thus, by making D50 0.3 μm or less, the starting materials can be sufficiently pulverized and uniformly mixed. Further, by setting D100 to 1.0 μm or less, it is possible to promote uniform composition and crystallization in the subsequent firing step S2. The volume-based particle size distribution can be measured with a laser diffraction particle size distribution meter. By carrying out the wet grinding of the starting material, the preferred particle size distribution can be easily reached.

  In the mixing step S1, the solid-liquid mixture obtained by pulverizing the starting material in a wet manner can be dried by, for example, a dryer. As the dryer, for example, a spray dryer, a fluidized bed dryer, an evaporator or the like can be used. In particular, it is preferable to dry the solid-liquid mixture with a spray dryer to obtain a mixed powder that is granulated and dried so that D50 is 10 μm or more and 30 μm or less. By making D50 of mixed powder 10 micrometers or more, D50 of positive electrode active material can also be 10 micrometers or more, and when forming a positive electrode active material into an electrode, it can prevent that a positive electrode layer peels from current collection foil. Further, by setting D50 of the mixed powder to 30 μm or less, it is possible to make the filling of the positive electrode active material in the thickness direction in the electrode uniform and prevent the conductive path from being reduced. In order to satisfy such a preferable range of D50 of the mixed powder, a rotary disk type spray dryer is suitable.

  The bulk specific gravity of the mixed powder obtained by the mixing step S1 is preferably 0.6 g / cc or more and 0.8 g / cc or less. By setting the bulk specific gravity of the mixed powder to 0.6 g / cc or more, it becomes a mixed powder with less fine powder, and it is possible to prevent the positive electrode layer from being peeled off from the current collector foil at the time of electrode formation. Further, by setting the bulk specific gravity of the mixed powder to 0.8 g / cc or less, a particle gap is ensured, and when the mixed powder is filled in the container and heated in the firing step S2, the oxidizing atmosphere gas flows into the mixed powder. It becomes easy to let.

  In the firing step S2, the mixture obtained in the mixing step S1 is fired in an oxidizing atmosphere to produce a positive electrode active material that is a powder of the lithium composite compound represented by the formula (1). The firing step S2 of the present embodiment includes a first heat treatment step S21, a second heat treatment step S22, and a third heat treatment step S23.

  In the first heat treatment step S21, the first precursor is obtained by heat-treating the mixture obtained in the mixing step S1 at a heat treatment temperature of 200 ° C. or higher and 400 ° C. or lower for 0.5 hours or more and 5 hours or less. . The first heat treatment step S21 is performed mainly for the purpose of removing vaporized components that hinder the synthesis reaction of the positive electrode active material from the mixture obtained in the mixing step S1. That is, the first heat treatment step S21 is a heat treatment step for removing vaporized components in the mixture.

  In the first heat treatment step S21, gas is generated by vaporization, combustion, volatilization, and the like of vaporized components contained in the mixture to be heat treated, such as moisture, impurities, and volatile components accompanying thermal decomposition. When the mixture contains a carbonate such as lithium carbonate, carbon dioxide gas is generated with the thermal decomposition of the carbonate.

  In the first heat treatment step S21, if the heat treatment temperature is less than 200 ° C., the combustion reaction of impurities and the thermal decomposition reaction of the starting material may be insufficient. In the first heat treatment step S21, if the heat treatment temperature exceeds 400 ° C., a layered structure of the lithium composite compound may be formed in an atmosphere containing a gas generated from the mixture by the heat treatment. Therefore, in the first heat treatment step S21, the mixture is heat treated at a heat treatment temperature of 200 ° C. or more and 400 ° C. or less, whereby the first precursor from which the vaporized components are sufficiently removed and the layered structure is not yet formed is obtained. Can be obtained.

  In the first heat treatment step S21, if the heat treatment temperature is 250 ° C. or higher and 400 ° C. or lower, more preferably 250 ° C. or higher and 380 ° C. or lower, the vaporizing component removal effect and the formation of the layered structure are suppressed. The effect can be further improved. Further, the heat treatment time can be appropriately changed according to, for example, the heat treatment temperature, the degree of removal of vaporized components, the degree of suppression of formation of the layered structure, and the like.

  In the first heat treatment step S21, for the purpose of exhausting the gas generated from the mixture, it is preferable to perform the heat treatment under an atmosphere gas flow or under a pump exhaust. The flow rate per minute of the atmospheric gas or the displacement per minute by the pump is preferably larger than the volume of gas generated from the mixture. The volume of the gas generated from the mixture can be calculated based on, for example, the ratio of the mass of the starting material and the vaporized component contained in the mixture.

  Moreover, you may perform 1st heat processing process S21 under the pressure reduction below atmospheric pressure. Further, since the first heat treatment step S21 is not mainly intended for the oxidation reaction, the oxidizing atmosphere of the first heat treatment step S21 may be air. By using air as the oxidizing atmosphere in the first heat treatment step S21, the configuration of the heat treatment apparatus can be simplified, the supply of the atmosphere can be facilitated, the productivity of the positive electrode active material can be improved, and the manufacturing cost can be reduced. . Further, the atmosphere of the heat treatment in the first heat treatment step S21 is not limited to the oxidizing atmosphere, and may be a non-oxidizing atmosphere such as an inert gas.

  In the firing step S2, the second heat treatment step S22 is performed after the completion of the first heat treatment step S21. As shown in FIG. 1A, the first gas replacement step of replacing the oxidizing atmosphere during the step of the first heat treatment step S21. S24 may be included. Further, as shown in FIG. 1B, a first gas replacement step S24 may be performed after the first heat treatment step S21 is completed. In the first gas replacement step S24, the oxidizing atmosphere used in the first heat treatment step S21 can be exhausted and a new oxidizing atmosphere can be introduced to perform the second heat treatment. By performing the gas replacement step S24 as described above, it is possible to prevent the gas generated from the mixture of the starting materials in the heat treatment of the first heat treatment step S21 from affecting the second heat treatment step S22.

  Further, in the first gas replacement step S24, the first precursor may be once taken out from the heat treatment apparatus and again put into the heat treatment apparatus. In this case, when taking out the first precursor from the heat treatment apparatus, the oxidizing atmosphere used in the first heat treatment step S21 is exhausted, and a new oxidizing atmosphere is introduced into the same or another heat treatment apparatus together with the first precursor. The second heat treatment step S22 may be performed.

  In addition, when evacuation is performed during or after the first heat treatment step S21, the first heat treatment step S21, the first gas replacement step S24, and the second heat treatment step S22 may be performed continuously. In this case, in the first gas replacement step S24, the oxidizing atmosphere may be continuously replaced in the same heat treatment apparatus without taking out the first precursor from the heat treatment apparatus.

In the second heat treatment step S22, the first precursor obtained in the first heat treatment step S21 is heat-treated at a heat treatment temperature of 450 ° C. or higher and lower than 700 ° C. for 2 hours or more and 50 hours or less to obtain a second heat treatment step S22. A precursor is obtained. The second heat treatment step S22 is carried out mainly for the purpose of oxidizing Ni in the first precursor from divalent to trivalent and synthesizing a compound having a layered structure represented by the composition formula LiM′O _2. Is called. That is, the second heat treatment step S22 is a heat treatment step for performing the Ni oxidation reaction in the first precursor and the formation of the layered structure.

In order to develop a high capacity in a positive electrode active material having a high Ni concentration in which the range of b indicating the Ni content in the formula (1) is not less than 0.7 and not more than 0.9, Ni is used in the firing step S2. Must be oxidized from divalent to trivalent. Divalent Ni easily occupies the Li position in the layered structure LiM′O ₂ , and causes a decrease in the capacity of the positive electrode active material. Therefore, in the firing step S2, the mixture is fired in an oxidizing atmosphere to change the valence of Ni from divalent to trivalent.

In order to synthesize a compound having a layered structure represented by the composition formula LiM′O ₂ , it is necessary for the first precursor to react with oxygen in the atmosphere. The reaction for synthesizing LiNiO ₂ from lithium oxide and nickel oxide contained in the first precursor is represented by the following formula (2).

Li ₂ O + 2NiO + (1/2) O ₂ → 2LiNiO ₂ (2)

  In order to promote the Ni oxidation reaction and the reaction of the above formula (2), the atmosphere of the heat treatment in the second heat treatment step S22 is an oxidizing atmosphere containing oxygen, and the oxygen concentration is preferably 80% or more. The concentration is more preferably 90% or more, the oxygen concentration is more preferably 95% or more, and the oxygen concentration is still more preferably 100%. Further, in order to sequentially advance the Ni oxidation reaction and the reaction of the above formula (2), it is preferable to supply oxygen continuously during the heat treatment in the second heat treatment step S22, and the heat treatment is performed in an oxidizing atmosphere gas stream. Preferably it is done.

  In the second heat treatment step S22, a gas such as carbon dioxide gas is generated from the first precursor during the heat treatment by heat-treating the first precursor from which the vaporized components in the mixture of the starting materials have been removed in the first heat treatment step S21. It is possible to suppress the decrease in oxygen concentration in the oxidizing atmosphere. Therefore, the Ni precursor reaction of the first precursor in the second heat treatment step S22 proceeds smoothly, and the second precursor in which the formation reaction of the lithium composite compound has progressed uniformly can be obtained. Furthermore, the residue derived from the starting material can be sufficiently reduced.

  When the heat treatment temperature in the second heat treatment step S22 is less than 450 ° C., the formation reaction of the layered structure is remarkably slow when the first precursor is heat treated to form the second precursor having the layered structure. Become. On the other hand, when the heat treatment temperature of the second heat treatment step S22 is 700 ° C. or higher, when the first precursor is heat treated to form the second precursor having a layered structure, the grain growth progresses during the formation and oxygen and The reaction of becomes inadequate.

  Therefore, when the heat treatment temperature in the second heat treatment step S22 is set to 450 ° C. or more and less than 700 ° C., the formation reaction of the layered structure is performed when the first precursor is heat treated to form the second precursor having the layered structure. It is possible to suppress the growth of crystal grains and to prevent the reaction with oxygen from becoming insufficient. Note that the effect of suppressing the growth of crystal grains can be further improved by setting the heat treatment temperature in the second heat treatment step S22 to 450 ° C. or more and 660 ° C. or less.

  Further, in order to sufficiently react the first precursor with oxygen in the temperature range of the heat treatment in the second heat treatment step S22, the heat treatment time can be set to 2 hours or more and 100 hours or less. From the viewpoint of improving productivity, the heat treatment time in the second heat treatment step S22 is preferably 2 hours or more and 50 hours or less, and more preferably 2 hours or more and 15 hours or less.

  In the firing step S2, the third heat treatment step S23 is performed after the end of the second heat treatment step S22. As shown in FIG. 1A, the second gas replacement step of replacing the oxidizing atmosphere during the step of the second heat treatment step S22. S25 may be included. Further, as shown in FIG. 1B, a second gas replacement step S25 may be performed after the end of the second heat treatment step S22. In the second gas replacement step S25, it is possible to discharge the oxidizing atmosphere used in the second heat treatment step S22 and introduce a new oxidizing atmosphere to perform the third heat treatment. Thereby, it is possible to prevent the gas generated from the first precursor during the heat treatment in the second heat treatment step S22 from affecting the third heat treatment step S23.

  In the second gas replacement step S25, the second precursor may be once taken out from the heat treatment apparatus and again put into the heat treatment apparatus. In this case, when the second precursor is taken out from the heat treatment apparatus, the oxidizing atmosphere used in the second heat treatment step S22 is exhausted, and a new oxidizing atmosphere is introduced into the same or another heat treatment apparatus together with the second precursor. The third heat treatment step S23 may be performed. In addition, since the vaporized component of the starting material mixture is removed in the first heat treatment step, the second heat treatment step S22 and the second heat treatment step S22 are performed without performing the second gas replacement step S25 and without removing the second precursor from the heat treatment apparatus. The three heat treatment step S23 may be performed continuously.

  In the third heat treatment step S23, the second precursor obtained in the second heat treatment step S22 is heat-treated at a heat treatment temperature of 700 ° C. or higher and 850 ° C. or lower to obtain a positive electrode active material made of a lithium composite compound. In the third heat treatment step S23, the Ni oxidation reaction in which Ni in the second precursor is oxidized from divalent to trivalent is sufficiently advanced, and the positive electrode active material made of a lithium composite compound obtained by the heat treatment exhibits electrode performance. Therefore, the main purpose is to grow crystal grains. That is, the third heat treatment step S23 is a heat treatment step for performing Ni oxidation reaction and crystal grain growth in the second precursor.

  In order to sufficiently advance the Ni oxidation reaction, the atmosphere of the heat treatment in the third heat treatment step S23 is an oxidizing atmosphere containing oxygen, and the oxygen concentration is preferably 80% or more, and the oxygen concentration is 90% or more. More preferably, the oxygen concentration is more preferably 95% or more, and even more preferably 100%.

  Note that if the heat treatment temperature in the third heat treatment step S23 is less than 700 ° C., the crystallization of the second precursor becomes insufficient, and if it exceeds 850 ° C., the layered structure of the second precursor is decomposed to form divalent Ni. The capacity | capacitance of the positive electrode active material produced | generated and obtained will fall. Therefore, by setting the heat treatment temperature in the third heat treatment step S23 to 700 ° C. or more and 850 ° C. or less, the grain growth of the second precursor is promoted and the decomposition of the layered structure is suppressed. Capacity can be improved. In addition, by setting the heat treatment temperature in the third heat treatment step S23 to 700 ° C. or higher and 840 ° C. or lower, the effect of promoting grain growth and the effect of suppressing the decomposition of the layered structure can be further improved.

  In the third heat treatment step S23, if the oxygen partial pressure is low, heat is required to promote the Ni oxidation reaction. Therefore, when the oxygen supply to the second precursor is insufficient, it is necessary to increase the heat treatment temperature. When the heat treatment temperature is raised, decomposition of the layered structure is unavoidable, and good electrode characteristics cannot be obtained in the obtained positive electrode active material. Therefore, in order to sufficiently supply oxygen to the second precursor, the heat treatment time in the third heat treatment step S23 can be set to 2 hours or more and 100 hours or less. From the viewpoint of improving the productivity of the positive electrode active material, the heat treatment time in the third heat treatment step S23 is preferably 2 hours or more and 50 hours or less, and more preferably 2 hours or more and 15 hours or less.

  As described above, in the method for manufacturing the positive electrode active material of the present embodiment, carbon dioxide gas is sufficiently supplied from the mixture in the first heat treatment step S21 of the firing step S2 in which the mixture obtained in the mixing step S1 is fired in an oxidizing atmosphere. The first precursor in which the generation of carbon dioxide gas by heating is suppressed can be obtained. And in 2nd heat treatment process S22 of baking process S2, the fall of the oxygen partial pressure of oxidizing atmosphere is suppressed by suppressing generation | occurrence | production of the carbon dioxide gas from a 1st precursor, and Ni oxidation of a 1st precursor is suppressed. The reaction is promoted and proceeds in a large amount and uniformly to obtain the second precursor. Furthermore, also in the third heat treatment step S23 of the firing step S2, the generation of carbon dioxide gas from the second precursor is suppressed, so that the reduction of the oxygen partial pressure in the oxidizing atmosphere is suppressed, and the second precursor Ni As the oxidation reaction proceeds in a large amount and uniformly, the growth of crystal grains proceeds. Therefore, it is possible to obtain a positive electrode active material having a high capacity and an excellent capacity retention ratio in which divalent Ni remaining in the Ni-rich lithium composite compound having a layered structure is reduced.

  The effect of the positive electrode active material manufacturing method of the present embodiment becomes significant when the weight of the positive electrode active material to be manufactured is large, for example, several hundred g or more. If the weight of the active material to be produced is several grams, there is little influence from the gas generated from the starting material in the firing step S2, but if the positive electrode active material is mass-produced on an industrial scale, the starting material is used in the firing step S2. This is because the volume of the generated gas increases and the oxygen partial pressure in the oxidizing atmosphere in the heat treatment process tends to decrease.

  If the first heat treatment step S21 is omitted in the firing step S2, the oxygen partial pressure is reduced in the second heat treatment step S22 and the third thermal heat treatment step S23. As a result, it is necessary to perform a heat treatment at a higher temperature in order to sufficiently advance the formation reaction of the layered structure accompanied by the oxidation of Ni, which exceeds the preferable temperature range. Further, if the second heat treatment step S22 is omitted, it is not preferable because the grain growth proceeds while the Ni oxidation reaction is insufficient. Further, if the third heat treatment step S23 is omitted, appropriate electrode characteristics cannot be obtained.

(Positive electrode and lithium ion secondary battery)
Hereinafter, the positive electrode for a non-aqueous secondary battery using the positive electrode active material manufactured by the above-described method for manufacturing a positive electrode active material, and the configuration of a non-aqueous secondary battery including the same will be described. FIG. 2 is a schematic partial cross-sectional view of the positive electrode 111 of the present embodiment and the non-aqueous secondary battery 100 including the positive electrode 111.

  The non-aqueous secondary battery 100 of the present embodiment is, for example, a cylindrical lithium ion secondary battery, and is housed in a bottomed cylindrical battery can 101 that contains a non-aqueous electrolyte, and the battery can 101. A wound electrode group 110 and a disk-shaped battery lid 102 that seals the upper opening of the battery can 101 are provided. The battery can 101 and the battery lid 102 are made of, for example, a metal material such as stainless steel or aluminum, and the battery lid 102 is fixed to the battery can 101 by caulking or the like via a sealing material 106 made of an insulating resin material. Thus, the battery cans 101 are sealed by the battery lid 102 and are electrically insulated from each other. The shape of the non-aqueous secondary battery 100 is not limited to a cylindrical shape, and other arbitrary shapes such as a square shape, a button shape, and a laminate sheet shape can be adopted.

  The wound electrode group 110 is manufactured by winding a long strip-like positive electrode 111 and a negative electrode 112 facing each other with a long strip-like separator 113 around a winding center axis. In the wound electrode group 110, the positive electrode current collector 111 a is electrically connected to the battery lid 102 via the positive electrode lead piece 103, and the negative electrode current collector 112 a is electrically connected to the bottom of the battery can 101 via the negative electrode lead piece 104. Connected. An insulating plate 105 is disposed between the wound electrode group 110 and the battery lid 102 and between the wound electrode group 110 and the bottom of the battery can 101 to prevent a short circuit. The positive electrode lead piece 103 and the negative electrode lead piece 104 are members for current extraction made of the same material as the positive electrode current collector 111a and the negative electrode current collector 112a, respectively, and the positive electrode current collector 111a and the negative electrode current collector, respectively. 112a is joined by spot welding or ultrasonic pressure welding.

  The positive electrode 111 of this embodiment includes a positive electrode current collector 111a and a positive electrode mixture layer 111b formed on the surface of the positive electrode current collector 111a. As the positive electrode current collector 111a, for example, a metal foil such as aluminum or an aluminum alloy, an expanded metal, a punching metal, or the like can be used. The metal foil can have a thickness of, for example, about 15 μm to 25 μm. The positive electrode mixture layer 111b includes a positive electrode active material manufactured by the above-described method for manufacturing a positive electrode active material. The positive electrode mixture layer 111b may include a conductive material, a binder, and the like.

  The negative electrode 112 includes a negative electrode current collector 112a and a negative electrode mixture layer 112b formed on the surface of the negative electrode current collector 112a. As the negative electrode current collector 112a, metal foil such as copper or copper alloy, nickel or nickel alloy, expanded metal, punching metal, or the like can be used. The metal foil can have a thickness of, for example, about 7 μm or more and 10 μm or less. The negative electrode mixture layer 112b contains a negative electrode active material used in a general lithium ion secondary battery. The negative electrode mixture layer 112b may include a conductive material, a binder, and the like.

  As the negative electrode active material, for example, one or more of a carbon material, a metal material, a metal oxide material, and the like can be used. As the carbon material, graphites such as natural graphite and artificial graphite, carbides such as coke and pitch, amorphous carbon, carbon fiber, and the like can be used. Further, as the metal material, lithium, silicon, tin, aluminum, indium, gallium, magnesium and alloys thereof, and as the metal oxide material, a metal oxide containing tin, silicon, lithium, titanium, or the like is used. it can.

  As the separator 113, for example, a polyolefin resin such as polyethylene, polypropylene, or a polyethylene-polypropylene copolymer, a microporous film such as a polyamide resin or an aramid resin, a nonwoven fabric, or the like can be used.

  The positive electrode 111 and the negative electrode 112 can be manufactured through, for example, a mixture adjustment process, a mixture coating process, and a molding process. In the mixture adjustment step, for example, using a stirring means such as a planetary mixer, a disper mixer, and a rotation / revolution mixer, the positive electrode active material or the negative electrode active material is stirred and mixed with a solution containing a conductive material and a binder, for example. Homogenize to prepare a mixture slurry.

  As the conductive material, a conductive material used in a general lithium ion secondary battery can be used. Specifically, for example, carbon particles such as graphite powder, acetylene black, furnace black, thermal black, and channel black, carbon fibers, and the like can be used as the conductive material. For example, the conductive material can be used in an amount of about 3% by mass to 10% by mass with respect to the total mass of the mixture.

  As the binder, a binder used in a general lithium ion secondary battery can be used. Specifically, for example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene, polyhexafluoropropylene, styrene-butadiene rubber, carboxymethylcellulose, polyacrylonitrile, modified polyacrylonitrile, and the like can be used as the binder. As the binder, for example, an amount of about 2% by mass to 10% by mass with respect to the mass of the entire mixture can be used. The mixing ratio of the negative electrode active material and the binder is preferably 95: 5 by weight, for example.

  As the solvent of the solution, N-methylpyrrolidone, water, N, N-dimethylformamide, N, N-dimethylacetamide, methanol, ethanol, propanol, isopropanol, ethylene glycol, diethylene glycol, glycerin depending on the type of binder. , Dimethyl sulfoxide, tetrahydrofuran and the like.

  In the mixture coating step, first, the mixture slurry containing the positive electrode active material and the negative electrode active material adjusted in the mixture adjustment step are applied to, for example, a bar coater, a doctor blade, a roll transfer machine, or the like. By the means, it apply | coats on the surface of the positive electrode collector 111a and the negative electrode collector 112a, respectively. Next, the positive electrode current collector 111a and the negative electrode current collector 112a coated with the mixture slurry are each heat-treated to volatilize or evaporate the solvent of the solution contained in the mixture slurry, thereby removing the positive electrode current collector. A positive electrode mixture layer 111b and a negative electrode mixture layer 112b are formed on the surfaces of 111a and the negative electrode current collector 112a, respectively.

  In the molding step, first, the positive electrode mixture layer 111b formed on the surface of the positive electrode current collector 111a and the negative electrode mixture layer 112b formed on the surface of the negative electrode current collector 112a are subjected to, for example, a roll press or the like. Each is pressure-molded using a pressure means. Thereby, the positive electrode mixture layer 111b can be made to have a thickness of, for example, about 100 μm to 300 μm, and the negative electrode mixture layer 112b can be made to have a thickness of, for example, about 20 μm to 150 μm. Thereafter, the positive electrode current collector 111a and the positive electrode material mixture layer 111b, and the negative electrode current collector material 112a and the negative electrode material mixture layer 112b are each cut into long strips, whereby the positive electrode 111 and the negative electrode 112 can be manufactured. .

  The positive electrode 111 and the negative electrode 112 manufactured as described above are wound around the winding central axis in a state of being opposed to each other with the separator 113 interposed therebetween, so that a wound electrode group 110 is formed. In the wound electrode group 110, the negative electrode current collector 112a is connected to the bottom of the battery can 101 via the negative electrode lead piece 104, and the positive electrode current collector 111a is connected to the battery lid 102 via the positive electrode lead piece 103 for insulation. A short circuit with the battery can 101 and the battery lid 102 is prevented by the plate 105 and the like, and the battery can 101 is accommodated. Thereafter, a non-aqueous secondary battery 100 is manufactured by injecting a non-aqueous electrolyte into the battery can 101, fixing the battery lid 102 to the battery can 101 via the sealing material 106, and sealing the battery can 101. Can do.

Examples of the electrolytic solution injected into the battery can 101 include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate (VC), methyl acetate (MA), and ethyl methyl. In a solvent such as carbonate (EMC) or methylpropyl carbonate (MPC), lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium perchlorate (LiClO ₄ ) or the like is used as an electrolyte. It is desirable to use a dissolved one. The concentration of the electrolyte is preferably 0.7M or more and 1.5M or less. In addition, a compound having a carboxylic acid anhydride group, a compound having a sulfur element such as propane sultone, or a compound having boron may be mixed in these electrolytic solutions. The purpose of adding these compounds is to suppress the reductive decomposition of the electrolyte solution on the negative electrode surface, prevent the reduction and precipitation of metal elements such as manganese eluted from the positive electrode on the negative electrode, improve the ionic conductivity of the electrolyte solution, and make the electrolyte flame retardant It may be selected appropriately according to the purpose.

  The non-aqueous secondary battery 100 having the above configuration uses the battery lid 102 as a positive electrode external terminal and the bottom of the battery can 101 as a negative electrode external terminal, and accumulates power supplied from the outside in the wound electrode group 110, and The electric power accumulated in the rotating electrode group 110 can be supplied to an external device or the like. As described above, the non-aqueous secondary battery 100 according to the present embodiment includes, for example, small power sources such as portable electronic devices and household electric devices, stationary power sources such as uninterruptible power sources and power leveling devices, ships, railways, and hybrids. It can be used as a drive power source for automobiles, electric cars and the like.

  Hereinafter, the Example based on the manufacturing method of the positive electrode active material of this invention and the comparative example different from the manufacturing method of the positive electrode active material of this invention are demonstrated.

Example 1
First, lithium carbonate, nickel hydroxide, cobalt carbonate, and manganese carbonate were prepared as starting materials for the positive electrode active material. Next, each starting material is weighed so that the atomic ratio of Li: Ni: Co: Mn is 1.04: 0.80: 0.10: 0.10, pulverized with a pulverizer, and wet mixed Thus, a slurry was prepared, and the resulting slurry (mixture) was dried with a spray dryer (mixing step). Thereafter, the dried mixture was fired to obtain a fired powder (firing step).

  Specifically, a bead mill was used as a pulverizer, wet mixing was performed using water as a solvent, and operation was continued until the particle size was stabilized. When the particle size of the obtained slurry was measured with a laser diffraction particle size distribution analyzer, D50 = 0.13 μm and D100 = 0.26 μm. The slurry was dried using a rotary disk type spray dryer to obtain a dried mixed powder having D50 = 17 μm and a bulk specific gravity of 0.74 g / cc.

  Next, 1 kg of the mixture (mixed powder) obtained in the mixing step is filled into an alumina container having a length of 300 mm, a width of 300 mm, and a height of 100 mm, and then in an air atmosphere at a heat treatment temperature of 350 ° C. for 1 hour by a continuous transfer furnace. Then, heat treatment was performed (first heat treatment step). In the first heat treatment step, water vapor accompanying thermal decomposition of nickel hydroxide and carbon dioxide accompanying thermal decomposition of cobalt carbonate and manganese carbonate are generated. Next, the obtained powder (first precursor) was heat-treated for 10 hours at a heat treatment temperature of 600 ° C. in an oxygen stream in a continuous transfer furnace in which the atmosphere in the furnace was replaced with an atmosphere having an oxygen concentration of 90% or more. (Second heat treatment step). In the second heat treatment step, cobalt carbonate and manganese carbonate that could not be reacted in the first heat treatment step are thermally decomposed to generate carbon dioxide. Moreover, since it reacts with each oxide of nickel, cobalt, and manganese after thermal decomposition to form a precursor of a lithium composite oxide, lithium carbonate decomposes and releases carbon dioxide. Furthermore, the obtained powder (second precursor) was subjected to heat treatment for 10 hours at a heat treatment temperature of 800 ° C. in an oxygen stream by a continuous transfer furnace in which the atmosphere in the furnace was replaced with an atmosphere having an oxygen concentration of 90% or more. A fired powder (lithium composite compound) was obtained (third heat treatment step). In the third heat treatment step, the reaction of the above formula (2) proceeds due to oxidation of nickel, so that lithium carbonate as a reaction residue is decomposed into lithium oxide and carbon dioxide, and carbon dioxide is generated. In order to synthesize the lithium composite oxide, carbon dioxide generated in the second and third heat treatment steps is quickly exhausted, and sufficient oxygen that promotes the oxidation reaction is important.

The obtained fired powder was classified to a mesh size of 53 μm or less to obtain a positive electrode active material. As a result of analyzing the element ratio of the positive electrode active material by ICP, Li: Ni: Mn: Co was 1.02: 0.80: 0.10: 0.10. When X-ray diffraction measurement was performed, a diffraction pattern corresponding to the α-NaFeO ₂ type layered structure was obtained, and the lattice constants were a = 0.287 nm and c = 1.42 nm. The specific surface area was 0.37 m ² / g.

(Example 2)
A positive electrode active material was produced in the same manner as in Example 1 except that the temperature of the first heat treatment step that was 350 ° C. in Example 1 was reduced to 250 ° C.

(Example 3)
A positive electrode active material was produced in the same manner as in Example 1 except that the temperature of the second heat treatment step that was 600 ° C. in Example 1 was increased to 650 ° C.

Example 4
A positive electrode active material was produced in the same manner as in Example 1 except that the temperature of the second heat treatment step that was 600 ° C. in Example 1 was reduced to 550 ° C.

(Example 5)
A positive electrode active material was produced in the same manner as in Example 1 except that the temperature of the second heat treatment step that was 600 ° C. in Example 1 was reduced to 500 ° C.

(Comparative Example 1)
In the firing step, a positive electrode active material was synthesized in the same manner as in Example 1 except that the first heat treatment step was omitted, and X-ray diffraction measurement and specific surface area were measured. The obtained lattice constants were a = 0.287 nm and c = 1.41 nm. The specific surface area was 0.40 m ² / g.

(Comparative Example 2)
In the firing step, a positive electrode active material was synthesized in the same manner as in Example 1 except that the first heat treatment step and the second heat treatment step were omitted, and X-ray diffraction measurement and specific surface area were measured. The obtained lattice constants were a = 0.287 nm and c = 1.42 nm. The specific surface area was 0.38 m ² / g.

(Comparative Example 3)
A positive electrode active material was produced in the same manner as in Example 1 except that the temperature of the first heat treatment step that was 350 ° C. in Example 1 was reduced to 150 ° C.

(Comparative Example 4)
A positive electrode active material was produced in the same manner as in Example 1 except that the temperature of the second heat treatment step that was 600 ° C. in Example 1 was increased to 700 ° C.

(Comparative Example 5)
A positive electrode active material was produced in the same manner as in Example 1 except that the temperature of the second heat treatment step that was 600 ° C. in Example 1 was reduced to 400 ° C.

(Comparative Example 6)
A positive electrode active material was produced in the same manner as in Example 1 except that both the second heat treatment and the third heat treatment performed in an oxidizing atmosphere having an oxygen concentration of 90% or more in Example 1 were performed in an air atmosphere.

(Production of lithium ion secondary battery)
Using the positive electrode active materials produced in Example 1 to Example 5 and Comparative Example 1 to Comparative Example 6, the lithium ion of Examples 1 to 5 and Comparative Example 1 to Comparative Example 6 was respectively obtained by the following procedure. A secondary battery was produced.

First, a positive electrode active material, a binder, and a conductive material were mixed to prepare a positive electrode mixture slurry. Then, the prepared positive electrode mixture slurry is applied to a 20 μm thick aluminum foil as a positive electrode current collector, dried at 120 ° C., and then compressed by a press so that the electrode density becomes 2.0 g / cm ^3. It was molded and punched into a disk shape with a diameter of 15 mm to produce a positive electrode. Moreover, the negative electrode was manufactured using metallic lithium as a negative electrode active material.

Next, a lithium ion secondary battery was manufactured using the manufactured positive electrode, negative electrode, and non-aqueous electrolyte. As the non-aqueous electrolyte, a solution in which LiPF ₆ is dissolved in a solvent in which ethylene carbonate and dimethyl carbonate are mixed so that the volume ratio is 3: 7 so that the final concentration is 1.0 mol / L is used. It was.

  Next, for each of the lithium ion secondary batteries of Examples 1 to 5 and Comparative Examples 1 to 6, a charge / discharge test was performed to measure the initial discharge capacity. Charging was performed at a constant current and a constant voltage up to a charging end voltage of 4.4 V with a charging current of 0.2 CA, and discharging was performed at a constant current up to a discharge end voltage of 2.5 V with a discharging current of 0.2 CA. Thereafter, charging and discharging were repeated for 50 cycles with a charge and discharge current of 1.0 CA, a charge end voltage of 4.4 V, and a discharge end voltage of 2.5 V. The percentage of the value obtained by dividing the discharge capacity measured at the 50th cycle by the discharge capacity measured at the 1st cycle was calculated and defined as the capacity maintenance rate. The results are shown in Table 1.

  From the above results, in the lithium ion secondary battery of Example 1 in which the positive electrode active material manufactured through the firing process including the first heat treatment, the second heat treatment, and the third heat treatment was used for the positive electrode, the 0.2 C discharge capacity 198 Ah / kg, 1C initial discharge was 180 Ah / kg, and the capacity retention rate was 81%. The first heat treatment step is performed at a temperature of 250 ° C. or more and 400 ° C. or less, the second heat treatment step is performed at a temperature of 450 ° C. or more and less than 700 ° C., and the third heat treatment step is performed at 700 ° C. or more and 840 ° C. or less. Similarly, good results were obtained with the lithium ion secondary batteries of Example 2 to Example 5.

  On the other hand, in the lithium ion secondary batteries of Comparative Example 1 and Comparative Example 2 in which the positive electrode active material in which the first heat treatment and the first heat treatment and the second heat treatment are omitted in the firing step is used for the positive electrode, The numerical value was lower than the result of the lithium ion secondary battery. Further, Comparative Example 3 in which the temperature of the first heat treatment step was lowered to 150 ° C., Comparative Example 4 in which the temperature of the second heat treatment step was raised to 700 ° C., and comparison in which the temperature of the second heat treatment step was lowered to 400 ° C. Also in the lithium ion secondary battery of Example 5, the numerical value was lower than each Example. In addition, in the lithium ion secondary battery of Comparative Example 6 in which the first heat treatment to the third heat treatment were all performed in an air atmosphere, the discharge capacity was significantly reduced. Thereby, it has confirmed that the positive electrode active material excellent in the capacity | capacitance and the capacity | capacitance maintenance factor was obtained with the manufacturing method of the positive electrode active material of Example 1- Example 5.

(Example 6)
Next, a positive electrode active material was produced in the same manner as in Example 1 except that the wet mixing time in the mixing step was reduced to 50% of Example 1, and a lithium ion secondary battery of Example 6 was produced. In addition, when the particle size of the pulverized powder of the starting material contained in the slurry after wet mixing before dry granulation in the mixing step was measured with a laser diffraction particle size distribution meter, D50 = 0.18 μm and D100 = 0.45 μm. there were.

(Example 7)
Next, a positive electrode active material was produced in the same manner as in Example 1 except that the wet mixing time was reduced to 38%, and a lithium ion secondary battery of Example 7 was produced. The particle size of the pulverized powder of the starting material contained in the slurry after wet mixing measured in the same manner as in Example 6 was D50 = 0.27 μm and D100 = 1.3 μm.

(Example 8)
Next, a positive electrode active material was produced in the same manner as in Example 1 except that the wet mixing time was reduced to 25%, and a lithium ion secondary battery of Example 8 was produced. In addition, the particle size of the pulverized powder of the starting material contained in the slurry after wet mixing measured in the same manner as in Example 6 was D50 = 0.36 μm and D100 = 5.1 μm.

  Next, for each of the lithium ion secondary batteries of Example 6, Example 7, and Example 8, a charge / discharge test was performed under the same conditions as the lithium ion secondary battery of Example 1, and the initial discharge capacity was measured. The capacity maintenance rate was calculated. Table 2 shows a comparison between the results of the lithium ion secondary batteries of Examples 1 and 6 and the results of the lithium ion secondary batteries of Examples 7 and 8.

  The lithium ion secondary batteries of Example 1 and Example 6 exhibited a relatively high discharge capacity and capacity retention rate. On the other hand, in the lithium ion secondary batteries of Example 7 and Example 8, the discharge capacity is high as in the lithium ion secondary batteries of Example 1 and Example 6, but the capacity maintenance rate after 50 cycles decreases. And deterioration due to the charge / discharge cycle was observed. That is, when the mixing time in the mixing step is short and the D50 and D100 of the mixed powder increase, the capacity after the cycle decreases.

  From the above, it has been found that in order to obtain a positive electrode active material having a high capacity and an excellent capacity retention ratio as in Example 1 and Example 6, it is necessary to sufficiently mix the starting materials. In the mixing step, the particle size of the pulverized powder of the starting material before dry granulation when measured on a volume basis is preferably such that D50 is less than 0.27 μm, D100 is less than 1.3 μm, and D50 is 0.00. It was found that 2 μm or less and D100 of 1.0 μm or less are more preferable.

  The embodiment of the present invention has been described in detail with reference to the drawings, but the specific configuration is not limited to this embodiment, and there are design changes and the like without departing from the gist of the present invention. They are also included in the present invention.

100 lithium ion secondary battery, 111 positive electrode, S1 mixing step, S2 firing step, S21 first heat treatment step, S22 second heat treatment step, S23 third heat treatment step, S24 first gas replacement step, S25 second gas replacement step

Claims (8)

A method for producing a positive electrode active material used for a positive electrode of a lithium ion secondary battery,
A mixing step of mixing lithium carbonate and a compound containing a metal element other than Li in the following formula (1);
Firing the mixture obtained in the mixing step in an oxidizing atmosphere to obtain a lithium composite compound represented by the following formula (1),
The firing step includes
A first heat treatment step of obtaining a first precursor by heat-treating the mixture at a heat treatment temperature of 200 ° C. or more and 400 ° C. or less for 0.5 hours or more and 5 hours or less;
A second heat treatment step of obtaining a second precursor by heat-treating the first precursor at a heat treatment temperature of 450 ° C. or more and less than 700 ° C. for 2 hours or more and 50 hours or less;
A third heat treatment step of obtaining the lithium composite compound by heat-treating the second precursor at a heat treatment temperature of 700 ° C. or more and 850 ° C. or less for 2 hours or more and 50 hours or less;
And the oxygen concentration of the oxidizing atmosphere in the second heat treatment step and the third heat treatment step is 80% or more.
_{Li 1 + a Ni b Mn c} Co d M e O 2 + α ... (1)
In the formula (1), M is at least one element selected from the group consisting of Mg, Al, Ti, Zr, Mo, and Nb, and a, b, c, d, e, and α are , −0.1 ≦ a ≦ 0.2, 0.7 ≦ b ≦ 0.9, 0 ≦ c ≦ 0.35, 0.05 ≦ d ≦ 0.30, 0 ≦ e ≦ 0.35, b + c + d + e = 1 and −0.1 ≦ α ≦ 0.1. The heat treatment temperature of the first heat treatment step is 250 ° C. or more and 400 ° C. or less,
The heat treatment temperature of the second heat treatment step is 450 ° C. or more and 660 ° C. or less,
2. The method for producing a positive electrode active material according to claim 1, wherein the heat treatment temperature in the third heat treatment step is 700 ° C. or higher and 840 ° C. or lower. The first heat treatment step is a step of obtaining the first precursor from which a vaporized component contained in the mixture is removed,
The second heat treatment step is a step of obtaining the second precursor by advancing a Ni oxidation reaction that oxidizes divalent Ni contained in the first precursor to trivalent Ni.
2. The positive electrode active material according to claim 1, wherein the third heat treatment step is a step of obtaining the lithium composite compound by advancing the Ni oxidation reaction and crystal grain growth of the second precursor. Production method.   2. The method for producing a positive electrode active material according to claim 1, further comprising a first gas replacement step of replacing the oxidizing atmosphere during or after the first heat treatment step.   5. The positive electrode according to claim 4, wherein the first gas replacement step is performed after the first heat treatment step, and the heat treatment is performed by introducing a new oxidizing atmosphere in the second heat treatment step. A method for producing an active material.   2. The method for producing a positive electrode active material according to claim 1, further comprising a second gas replacement step of replacing the oxidizing atmosphere during or after the second heat treatment step.   The positive electrode according to claim 6, wherein the second gas replacement step is performed after the second heat treatment step, and the heat treatment is performed by introducing a new oxidizing atmosphere in the third heat treatment step. A method for producing an active material. The method for producing a positive electrode active material according to claim 1, wherein the oxidizing atmosphere in the first heat treatment step is air.

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