JP7707592B2 - Method for producing positive electrode active material for lithium secondary battery - Google Patents

Description

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

Lithium secondary batteries are widely used as small, lightweight secondary batteries with high energy density, and there is a demand for higher capacity active materials used in the positive and negative electrodes.

A lithium composite compound represented by LiMO ₂ (where M represents a metal element such as Ni, Co, or Mn) having an α-NaFeO _2- type layered structure is known as a positive electrode active material having a high charge/discharge capacity. This positive electrode active material shows a tendency for the capacity to increase as the ratio of nickel increases, in particular, and is therefore expected to be a positive electrode active material that can achieve high energy density in batteries.
There is a demand for lithium composite compounds that have high productivity and low production costs.

In the manufacturing process of positive electrode active materials, a technology has been proposed that uses a rotary kiln as a firing furnace in which the reaction to form lithium composite compounds takes place. A rotary kiln has the advantage that it does not require a special firing vessel and can easily maintain an oxidizing atmosphere inside the furnace.

For example, Patent Document 1 discloses that the furnace tube of the sintering furnace used in the rolling heat treatment process is made of ceramics, nickel, tungsten, molybdenum, titanium, or an alloy containing these metals as the main components.

Patent Document 2 also discloses that in a furnace core tube with a double structure having an inner cylindrical tube, at least one of the inner layer of the furnace core tube and the outer layer of the inner cylindrical tube is made of metallic nickel material or nickel alloy material.

International Publication No. 2017/213002 JP 2019-75253 A

However, ceramic furnace tubes make it difficult to increase the size of the equipment and achieve high productivity, and with alloy furnace tubes whose main component is metal, there is a risk that metal components will become mixed into the raw materials over long-term use.

Therefore, the present invention aims to provide a positive electrode active material for lithium secondary batteries that is high quality, highly productive, and has low production costs.

A method for producing a positive electrode active material for a lithium secondary battery according to one embodiment of the present invention includes a mixing step of mixing a lithium compound with a compound containing a metal element other than Li in the following formula (1), and a firing step of firing a precursor obtained through the mixing step to obtain a lithium composite compound represented by the following formula (1), wherein the firing step includes at least a heat treatment step of performing a heat treatment on the precursor while rolling it in a core tube of a firing furnace, and is characterized in that in the core tube, an aluminum oxide obtained by surface oxidizing pure aluminum is formed on the outermost layer of the powder-contacting part, and an alloy layer between the pure aluminum and a base material of the core tube is formed on the underlying layer.
Li _1+a M1O _2+α ...(1)
(In the formula (1), M1 is a metal element other than Li, and includes at least Ni and any one of Mn, Co, Al, Ti, Zr, Mo, Nb, W, V, Cr, Mg, Ca, Cu, Zn, and Sn , the proportion of Ni in M1 is 70 atomic % or more, and a and α are numbers that satisfy −0.1≦a≦0.2 and −0.2≦α≦0.2.)

In addition, in the manufacturing method of the positive electrode active material for lithium secondary batteries of the present invention, the sintering furnace is provided with a first air supply pipe that injects an oxidizing gas toward the inner surface side of the furnace core tube, and a second air supply pipe that flows the oxidizing gas in the axial direction of the furnace core tube, and it is preferable that the first air supply pipe has a plurality of injection ports whose opening direction is the circumferential direction of the first air supply pipe.

In addition, in the method for producing a positive electrode active material for a lithium secondary battery of the present invention, it is preferable that the legs that hold the first air supply pipe in the furnace tube are not joined to the furnace tube.

In the method for producing a positive electrode active material for a lithium secondary battery of the present invention, the heat treatment step preferably includes a step of maintaining the material at 700° C. or higher for 2 hours or longer.
In another aspect of the present invention, a method for producing a positive electrode active material for a lithium secondary battery includes a mixing step of mixing a lithium compound with a compound containing a metal element other than Li in the following formula (1),
and a calcination step of calcining the precursor obtained through the mixing step to obtain a lithium composite compound represented by the following (1),
The firing step includes at least a heat treatment step of performing a heat treatment on the precursor while rolling it in a furnace tube of a firing furnace,
the firing furnace includes a first air supply pipe that injects an oxidizing gas toward an inner peripheral surface side of the furnace core tube, and a second air supply pipe that flows the oxidizing gas in an axial direction of the furnace core tube;
The first air intake pipe has a plurality of injection ports whose opening direction is in a circumferential direction of the first air intake pipe,
In the furnace core tube, an aluminum oxide is formed on the outermost layer of the powder contact portion, and an alloy layer of aluminum and the base material of the furnace core tube is formed on the underlying layer, and the legs that hold the first air supply pipe are not joined to the furnace core tube.
It is characterized by:
Li _1+a M1O _2+α ...(1)
(In the formula (1), M1 is a metal element other than Li and contains at least Ni, the proportion of Ni in M1 is 70 atomic % or more, and a and α are numbers that satisfy −0.1≦a≦0.2 and −0.2≦α≦0.2.)

The present invention makes it possible to provide a positive electrode active material for lithium secondary batteries that is high quality, highly productive, and has low production costs.

FIG. 2 is a flow diagram of a method for producing a positive electrode active material according to an embodiment of the present invention. FIG. 1 is a diagram showing a schematic structure of a rotary kiln 1 used for producing a positive electrode active material according to an embodiment of the present invention. FIG. 1 is a diagram showing a schematic structure of a rotary kiln 1B used in the examples.

The following provides a detailed description of a positive electrode active material for lithium secondary batteries (hereinafter, sometimes simply referred to as a positive electrode active material) according to one embodiment of the present invention and a method for producing the same. Note that the following description shows specific examples of the contents of the present invention, and the present invention is not limited to these. Various modifications of the present invention are possible for those skilled in the art within the scope of the technical ideas disclosed in this specification.

<Cathode active material>
The positive electrode active material according to this embodiment is a lithium composite compound having a layered rock-salt type crystal structure (hereinafter, sometimes referred to as a layered structure) that is composed of lithium and a transition metal and belongs to the space group R-3m. This positive electrode active material is capable of reversibly absorbing and releasing lithium ions by application of a voltage, and is suitably used as a positive electrode active material for lithium secondary batteries (lithium ion secondary batteries). The lithium composite compound is also called a lithium composite oxide.

The positive electrode active material according to this embodiment has the following formula (1):
Li _1+a M1O _2+α ...(1)
(In the formula (1), M1 is a metal element other than Li and contains at least Ni, the proportion of Ni per M1 exceeds 70 atomic %, and a and α are numbers that satisfy −0.1≦a≦0.2 and −0.2≦α≦0.2.)

The positive electrode active material according to this embodiment is a positive electrode active material that can realize a high energy density and a high charge/discharge capacity by having a composition in which the ratio of nickel (Ni) per metal element (M1) other than lithium (Li) exceeds 70 atomic %. The ratio of nickel (Ni) per metal element (M1) other than lithium (Li) can be an appropriate value in the range of more than 70 atomic % and less than 100 atomic %. Since it is a positive electrode active material containing a high ratio of nickel, it is important that the oxidation reaction of oxidizing Ni ²⁺ to Ni ³⁺ is efficiently carried out.

Metal elements (M1) other than lithium (Li) may include transition metal elements, non-transition metal elements, or combinations of these, in addition to nickel. Specific examples of such metal elements (M1) include manganese (Mn), cobalt (Co), aluminum (Al), titanium (Ti), zirconium (Zr), molybdenum (Mo), niobium (Nb), tungsten (W), vanadium (V), chromium (Cr), magnesium (Mg), calcium (Ca), copper (Cu), zinc (Zn), and tin (Sn). Among these, it is preferable that aluminum (Al) and/or titanium (Ti) are included from the viewpoint of stabilizing the layered structure.

The positive electrode active material according to the present embodiment has a more preferable specific composition represented by the following formula (2):
Li _1+a Ni _b Mn _c Co _d M2 _e O _2+α ...(2)
(In the formula (2), M2 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 numbers satisfying -0.1≦a≦0.2, 0.7<b≦0.9, 0≦c<0.3, 0≦d<0.3, 0≦e≦0.25, b+c+d+e=1, and -0.2≦α≦0.2.)

<Method of manufacturing positive electrode active material>
The method for producing a positive electrode active material according to the present embodiment relates to a method for synthesizing a lithium composite compound that is a positive electrode active material used in a positive electrode of a lithium secondary battery, and that is represented by the above formula (1) and has a layered rock salt type crystal structure.

(Manufacturing method flow)
FIG. 1 is a flow diagram of a method for producing a positive electrode active material according to one embodiment of the present invention.
As shown in Fig. 1, the manufacturing method of the positive electrode active material according to the present embodiment includes a mixing step S1 and a calcination step S2. A precursor is prepared from the raw material compounds through the mixing step S1, and the precursor is calcined in the calcination step S2 to synthesize a lithium composite compound that can be a material for the positive electrode of a lithium secondary battery (lithium ion secondary battery). The manufacturing method according to the present embodiment includes at least a heat treatment step, as one step constituting the calcination step S2, in which the precursor of the lithium composite compound before calcination is rolled in a rotary kiln used as a calcination furnace and heat-treated.

In the mixing step S1, a compound containing lithium (lithium compound) is mixed with a compound containing a metal element other than Li that constitutes the positive electrode active material. Examples of compounds containing lithium include lithium carbonate and lithium hydroxide. Lithium carbonate is in stable supply, easy to obtain, and inexpensive compared to lithium acetate, lithium nitrate, lithium hydroxide, lithium chloride, lithium sulfate, etc. In addition, because it has a high melting point, it causes less damage to manufacturing equipment and is highly industrially usable and practical.

Compounds containing metal elements other than Li that make up the positive electrode active material include compounds containing nickel, compounds containing manganese, compounds containing cobalt, and compounds containing other metal elements such as M2.

Examples of compounds that contain nickel include pure nickel (included as a compound for convenience), oxides, hydroxides, carbonates, acetates, etc. Among these, it is particularly preferable to use oxides or hydroxides. Unlike the case where carbonates or acetates are used, oxides and hydroxides do not generate large amounts of carbon dioxide gas during the firing process, so lithium composite compounds with a high proportion of nickel and high purity can be stably produced.

For example, oxides, hydroxides, carbonates, acetates, etc. can be used as compounds containing manganese or cobalt. Of these, it is particularly preferable to use oxides, hydroxides, or carbonates. Furthermore, for example, carbonates, oxides, hydroxides, acetates, nitrates, etc. can be used as compounds containing other metal elements such as M2. Of these, it is particularly preferable to use carbonates, oxides, or hydroxides.

In the mixing step S1, specifically, each compound of the raw material is weighed out in a predetermined elemental composition ratio corresponding to the above formula, and each compound is crushed and mixed to prepare a powdery mixture in which each compound is mixed. From the viewpoint of uniformly mixing each compound and making the particle size uniform, it is preferable to crush each compound until the average particle size becomes less than 1 μm. As a crusher for crushing the compound, for example, a general precision crusher such as a ball mill, a jet mill, or a sand mill can be used.
The raw material compound is preferably ground by wet grinding, and from an industrial point of view, wet grinding using water as a dispersion medium is particularly preferred. The solid-liquid mixture obtained by wet grinding may be dried, for example, using a dryer. As the dryer, for example, a spray dryer, a fluidized bed dryer, an evaporator, etc. can be used.

In the calcination step S2, the precursor obtained through the mixing step S1 is calcined to obtain a lithium composite compound having a layered structure. The calcination step S2 includes at least a heat treatment step in which the precursor of the lithium composite compound before calcination is heat-treated while being rolled in a rotary kiln used as a calcination furnace. Here, the rotary kiln used in the calcination step S2 will be described.

(Rotary kiln/firing furnace)
FIG. 2 is a diagram showing a schematic structure of a rotary kiln used for producing a positive electrode active material for a lithium secondary battery.
As shown in FIG. 2 , the rotary kiln 1 includes a furnace tube 10 , a heater 20 , a first air supply pipe 30 , a second air supply pipe 40 , and a lifter 50 .

The furnace tube 10 has a hollow, generally cylindrical shape, and has an input section for the workpiece Ma at one end in the longitudinal direction, and a recovery section for the heat-treated material at the other end. The furnace tube 10 is installed at an incline with respect to the horizontal plane so that the input section for the workpiece Ma is located above the recovery section. The precursor of the lithium composite compound is input into the furnace tube 10 from a powder input device (not shown) installed in the input section, and is heat-treated by flowing it longitudinally inside the furnace tube 10. The inclination angle of the furnace tube 10 is not particularly limited, but is, for example, in the range of 0.5 to 3°. In this specification, the input side in the longitudinal direction of the furnace tube 10 is referred to as "upstream" and the recovery side is referred to as "downstream".

The furnace core tube 10 is connected to the power of a motor or the like (not shown) via a drive gear or roller. The furnace core tube 10 is rotated around the central axis of the cylindrical shape as the axis of rotation by the drive of such a motor or the like. Therefore, the workpiece Ma fed into the furnace core tube 10 from the feed section flows down while rolling inside the furnace core tube 10 as the furnace core tube 10 rotates, and is collected by a powder collection device (not shown) in the collection section. The rotation speed of the furnace core tube 10 is not particularly limited, but is, for example, in the range of 0.5 to 3 rpm.

The base material of the furnace core tube 10 is preferably made of a metal such as Ni, W, Mo, Ti, etc., which does not emit harmful components such as chromium, or an alloy mainly composed of these metals. In the furnace core tube, aluminum oxide is formed on the outermost surface layer of the powder contact portion, which is the portion that contacts the precursor (workpiece Ma), and an alloy layer of aluminum and the base material of the furnace core tube is formed on the underlying layer.
When aluminum oxide is formed on the outermost layer of the base material of the furnace tube 10, the lithium component contained in the precursor of the lithium composite compound is less likely to react with the furnace tube 10, so that the risk of deterioration or damage of the furnace tube 10 is reduced. The aluminum oxide on the outermost layer of the base material of the furnace tube 10 can be easily formed by baking the base material coated with aluminum in an air and oxidizing the coated aluminum. The method of coating the base material with aluminum may be, for example, thermal spraying or hot-dip plating. If the base material is coated with aluminum before the furnace tube 10 is made into a can, the base material will be exposed at the joint, so it is preferable to coat it after the can is made.

The heater 20 is installed around the body of the furnace tube 10. The heater 20 covers the heating zone 120, which is a part of the longitudinal section of the furnace tube 10 and is shown by a dashed line in FIG. 2, and can heat the heating zone 120 to a target temperature. The heater 20 also preheats the preheating zone 110, which is a section upstream of the heating zone 120 and is a predetermined distance away and shown by a dashed line in FIG. 2, to a temperature lower than the target temperature. Therefore, when a precursor of a lithium composite compound is charged into the furnace tube 10 in which the heater 20 is operating, the precursor is preheated in the preheating zone 110, and then heated to the target temperature in the heating zone 120 and heat-treated while rolling. However, the heater 20 is not particularly limited in its position or number as long as it can perform uniform heat treatment on the heating zone 120. The heaters 20 may be concentrated in one location or may be divided into multiple locations, as long as the preheating zone 110 is secured to prevent the heat treatment from progressing too rapidly.

The first air supply pipe 30 constitutes a first air supply system that supplies oxidizing gas from a gas source (not shown) to the inside of the core tube 10, and injects the oxidizing gas toward the inner surface of the core tube 10 when heat-treating the workpiece Ma. The first air supply pipe 30 is arranged along the longitudinal direction inside the core tube 10, and extends over approximately the entire length of the core tube 10 from the downstream side to the upstream side. The first air supply pipe 30 is arranged along the longitudinal direction of the core tube 10, branches off from the first air supply pipe 30, and has a plurality of injection ports 32 that open at the branched ends. Each of the injection ports 32 can inject the oxidizing gas, which is pressure-fed from a gas source (not shown), in a shower-like manner toward the inside of the core tube 10. That is, the first air supply system blows the oxidizing gas onto the precursor being heat-treated while rolling, thereby directly supplying oxygen to the precursor and efficiently promoting the oxidation reaction. In addition, the carbon dioxide gas generated from the precursor is blown up by the oxidizing gas and quickly removed from the vicinity of the precursor. In other words, the carbon dioxide gas generated from the precursor and stagnating in the furnace core tube 10 is prevented from reacting again with the precursor to generate lithium carbonate again and inhibit the generation of the lithium composite compound. The opening direction of the injection port 32 may be vertically downward (in the radial direction of the furnace core tube) or in the circumferential direction of the first air supply pipe 30, but it is preferable to open the injection port 32 in the circumferential direction of the first air supply pipe 30 because the air flow in the furnace becomes a swirling flow by injecting in the circumferential direction, which allows oxygen to be supplied to the precursor and stagnant carbon dioxide gas to be efficiently discharged.

The first air supply pipe 30 is preferably provided so that the amount of oxidizing gas sprayed, the spray angle, and the oxygen concentration can be adjusted in order to efficiently supply oxygen and exhaust carbon dioxide, and to prevent the powder of the workpiece from scattering. For example, the amount of sprayed can be adjusted by adjusting the gas flow rate of the first air supply system, by providing the nozzle 32 so that it can be opened and closed, and by adjusting the number of openings of the nozzle 32. The spray angle can be adjusted by providing the first air supply pipe 30 so that it can rotate around the central axis as the rotation axis. For example, the gas may be sprayed at an angle of more than 0° and less than 45° in the forward or reverse direction relative to the rotation direction of the furnace core tube 10. The spray angle can be adjusted by moving the first air supply pipe 30 horizontally or the like inside the furnace core tube 10. For example, the first air supply pipe 30 may be sprayed while stationary at a position eccentric to the central axis of the furnace core tube 10. The oxygen concentration can be adjusted by providing an oxygen concentration detection means near the inlet or outlet of the furnace tube 10, or at any other location, and monitoring and controlling the amount of oxygen so that the detected oxygen concentration is a specified value. The blowing amount, blowing angle, and oxygen concentration can also be adjusted in an appropriate combination. Note that a carbon dioxide concentration detection means can be provided instead of or in combination with the oxygen concentration detection means, and the amount of oxygen can be monitored and controlled so that the detected carbon dioxide concentration is a specified value.

The first air supply pipe 30 is held by legs 34 inside the core tube 10. A round bar or the like can be interposed near the circumferential side of the contact area between the legs 34 and the core tube 10 to restrain the legs 34 and the core tube 10 in the circumferential direction without fastening them together. This allows the driving force of the core tube 10 to be transmitted to the first air supply pipe 30, and the first air supply pipe and the core tube 10 can rotate synchronously. This round bar or the like does not restrain the legs 34 in the axial direction of the core tube, but only in the circumferential direction, and by not joining the legs 34 to the core tube 10, the first air supply pipe 30 can be easily removed from the core tube 10, improving the workability during cleaning and maintenance of the inside of the core tube 10, and ultimately improving productivity.

The second air supply pipe 40 constitutes a second air supply system that supplies oxidizing gas from a gas source (not shown) to the inside of the core tube 10, and generates an oxidizing gas flow in the axial direction of the core tube 10 when heat-treating the workpiece Ma. The oxidizing gas may flow from the upstream side to the downstream side of the core tube 10, but it is preferable to flow it from the downstream side to the upstream side of the core tube 10. The second air supply pipe 40 is located downstream of the heating zone 120 inside the core tube 10 and opens toward the upstream side of the core tube 10. In addition, the second air supply pipe 40 is located radially outward from the first air supply pipe when viewed from the axial direction, and opens into a space radially outward from the first air supply pipe 30. The second air supply pipe 40 flows the oxidizing gas in a substantially horizontal direction radially outside the first air supply pipe 30, and the oxidizing gas is exhausted to the outside from an exhaust port (not shown) provided on the upstream side of the furnace tube 10 after passing through the heating zone 120 and the preheating zone 110. That is, the second air supply system forms an air flow of the oxidizing gas inside the furnace tube 10, so that the carbon dioxide gas generated from the precursor by the heat treatment is exhausted along with the air flow together with the oxidizing gas. If the air flow of the oxidizing gas by the second air supply system flows in the opposite direction to the direction in which the precursor flows downward, the concentration of the carbon dioxide gas becomes lower toward the downstream side of the furnace tube 10, so that the amount of carbon mixed in the workpiece Ma that finishes the heat treatment downstream can be reliably reduced. The amount and direction of the oxidizing gas supplied by the second air supply pipe 40 can also be appropriately adjusted.

The oxidizing gas supplied by the first and second air supply systems is a gas that promotes a reaction with oxygen element, and oxygen gas, oxygen-enriched air, etc. are used. The oxidizing gas supplied by the first and second air supply systems preferably has an oxygen concentration of 90% or more, more preferably an oxygen concentration of 95% or more, and preferably an oxygen concentration of 100%.

The lifter 50 is provided on the inner circumferential surface of the furnace core tube 10. The lifter 50 protrudes inward from a portion of the circumferential direction of the inner circumferential surface of the furnace core tube 10, and stirs the workpiece Ma as the furnace core tube 10 rotates. That is, the stirring by the lifter 50 causes the surface powder and the bottom powder in the precursor powder to flow while being interchanged, increasing the probability of contact with oxygen and its uniformity, and efficiently removing carbon dioxide gas generated from the precursor from the gaps between particles in the powder. Therefore, by the lifter 50 stirring the precursor under the oxidizing gas supplied by the first and second air supply systems, the supply of oxygen and the exhaust of carbon dioxide gas progress effectively, and the solid-phase reaction that produces a lithium complex compound is greatly promoted.

The lifters 50 may be provided in any suitable shape and number. For example, the lifters 50 may be provided in the shape of a blade, a ridge, a pipe, a rectangular column, or the like extending in the longitudinal direction of the core tube 10, and multiple lifters 50 may be arranged at suitable intervals around the circumference of the core tube 10. The lifters 50 may be continuous with no gaps in the longitudinal direction of the core tube 10, or may be discontinuous with gaps.

The lifter 50 may be provided over the entire length of the inside of the furnace tube 10, but it is preferable that the lifter 50 is provided only in the zone (heating zone 120) of the inner circumferential surface of the furnace tube 10 that is directly heated to the target heat treatment temperature by the heater 20 during heat treatment, and not provided upstream or downstream of the heating zone 120. In the preheating zone 110 and the like upstream of the heating zone 120, carbon dioxide gas is generated significantly, and if the precursor powder is stirred in such a zone, the precursor and carbon dioxide gas react to generate lithium carbonate, which may hinder the formation reaction of the lithium composite compound. On the other hand, even if the lifter 50 is provided only in the heating zone 120, it is possible to sufficiently promote the solid-phase reaction, while by not providing the lifter 50 upstream or downstream of the heating zone 120, it is possible to prevent a situation in which the precursor fine powder that is stirred more than necessary is discharged together with the oxidizing gas flow, resulting in a decrease in the recovery rate.

The rotary kiln 1 preferably has an exhaust port on the upstream side of the core tube 10 for exhausting the atmospheric gas inside the core tube 10. If the exhaust port is provided on the side of the core tube 10 rather than on the top side, carbon dioxide gas, which has a high specific gravity, can be carried along with the oxidizing gas flow and reliably discharged from the core tube 10. More specifically, the location of the exhaust port is preferably on the inner surface on the upstream side inside the core tube 10, and more preferably in the lower half of the inner surface that is lower than the rotation axis of the core tube 10.

According to the rotary kiln 1 described above, the supply of oxygen, the exhaust of carbon dioxide, and the powdering of the precursor are continuously performed, so that the heat treatment of the precursor can be performed in a short time. In particular, the supply of oxygen is performed to the core tube 10 forming a closed space, so that the heat treatment can be performed at a low cost compared to a conveying furnace or the like in which heat treatment is performed in an open space. In addition, the first air supply system directly blows an oxidizing gas onto the precursor, so that a high concentration of oxygen can be supplied to the precursor, and the carbon dioxide gas generated from the precursor can be lifted up and reliably separated and removed from the flowing precursor. In addition, the second air supply system quickly exhausts the carbon dioxide gas lifted up inside the core tube 10 to the outside of the furnace, so that the heat-treated precursor can be prevented from coming into contact with the carbon dioxide gas. In other words, with only the first air supply system, the carbon dioxide gas generated from the precursor remains without being discharged from the core tube 10, and with only the second air supply system, the carbon dioxide gas remaining in the gaps between particles in the precursor powder is difficult to remove. By using the first and second air supply systems together, the supply of oxygen and the exhaust of carbon dioxide are efficiently and continuously circulated, resulting in a heat-treated product with fewer crystal defects and impurities.

(Firing process)
Next, the firing step S2 will be described in detail.

As shown in FIG. 1, the firing process S2 preferably includes a first heat treatment process S21 for forming a first precursor, a second heat treatment process S22 for forming a second precursor, and a third heat treatment process S23 for finishing heat treatment. The rotary kiln 1 shown in FIG. 2 may be used in any of these heat treatment processes, but is preferably used in at least one of the second heat treatment process S22 and the third heat treatment process S23, and more preferably used in the second heat treatment process S22.

[First heat treatment step S21]
In the first heat treatment step S21, the mixture obtained in the mixing step S1 is heat-treated at a heat treatment temperature of 200° C. or more and 40° C. or less for 0.5 hours or more and 5 hours or less to obtain a first precursor. The first heat treatment step S21 is performed mainly for the purpose of removing highly volatile components such as moisture that hinder the synthesis reaction of the positive electrode active material from the mixture obtained in the mixing step S1. In this step, carbon dioxide gas and the like generated by the thermal decomposition of raw materials such as lithium carbonate and the combustion of impurities are removed from the mixture together with moisture.

The first heat treatment step S21 can be carried out using an appropriate heat treatment device. Specifically, for example, a roller hearth kiln, a tunnel furnace, a pusher furnace, a rotary kiln, a batch furnace, etc. can be used. Note that if the rotary kiln 1 is not used in the second heat treatment step S22 and the third heat treatment step S23, the above-mentioned roller hearth kiln, tunnel furnace, etc. can be used.

[Second heat treatment step]
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 more and 900° C. or less for 0.1 hours or more and 50 hours or less to obtain a second precursor. The second heat treatment step S22 is performed mainly for the purpose of oxidizing nickel in the first precursor from divalent to trivalent and crystallizing a lithium composite compound having a layered structure. That is, this step is a heat treatment step in which a layered structure is formed by oxidation reaction of nickel in the first precursor using lithium carbonate (Li ₂ CO ₃ ) and an oxide of M'(M'O) as reactants. In the second heat treatment step S22, if the heat treatment temperature is less than 450° C., the reaction rate of the solid-phase reaction is slowed down, and lithium carbonate remains in excess, which may increase the amount of carbon dioxide gas generated in the third heat treatment step S23. On the other hand, if the heat treatment temperature exceeds 900° C., the grain growth of the lithium composite compound may proceed excessively in this step, and it may be difficult to obtain a high-capacity positive electrode active material. In contrast, if the heat treatment temperature is set as described above, a second precursor having few coarse crystal grains can be obtained while the solid-phase reaction proceeds overall. The reaction of lithium carbonate that proceeds in the second heat treatment step S22 is represented by the following formula (3).

_Li2CO3 + _2M'O + _0.5O2 → _2LiM'O2 + _CO2 ...(3)

It is more preferable that the heat treatment temperature in the second heat treatment step S22 is 600°C or higher. If it is 600°C or higher, the reaction efficiency of the above formula (3) is further improved. If it is 700°C or higher, the reaction efficiency is further improved, and this is even more preferable. In addition, it is more preferable that the heat treatment temperature in the second heat treatment step S22 is 800°C or lower. If it is 800°C or lower, the crystal grains are less likely to become coarse.

It is more preferable that the heat treatment time in the second heat treatment step S22 is 0.1 hours or more and 5 hours or less. If the heat treatment time is 5 hours or less, the time required to manufacture the positive electrode active material is shortened, and productivity can be improved.

In order to develop a high capacity in a positive electrode active material having a nickel ratio of more than 70 atomic %, it is particularly important to sufficiently oxidize the valence of nickel from divalent to trivalent. This is because divalent nickel is easily substituted for lithium sites in _LiM'O2 having a layered structure, which causes a decrease in the capacity of the positive electrode active material. Therefore, in the second heat treatment step S22, it is preferable to heat treat the first precursor in an oxidizing atmosphere in which oxygen is sufficiently supplied, and to reliably change the valence of nickel from divalent to trivalent. In addition, the carbon dioxide gas generated in the above formula (3) inhibits the progress of the reaction of formula (3) and causes a decrease in the capacity of the positive electrode active material. Therefore, in the second heat treatment step S22, it is preferable to perform heat treatment under an air flow in which carbon dioxide gas is unlikely to stagnate.

Specifically, the second heat treatment step S22 is preferably performed in an oxidizing atmosphere with an oxygen concentration of 90% or more, more preferably an oxidizing atmosphere with an oxygen concentration of 95% or more, and even more preferably an oxidizing atmosphere with an oxygen concentration of 100%. In addition, the second heat treatment step S2 is preferably performed under an oxidizing gas flow. When the heat treatment is performed under an oxidizing gas flow with a high oxygen concentration, nickel can be reliably oxidized and the carbon dioxide gas generated in the above formula (3) can be reliably eliminated.

In the second heat treatment step S22, it is preferable to perform the heat treatment while rolling the first precursor. By performing the heat treatment while rolling the first precursor, the probability of contact between the powdered first precursor and oxygen can be increased, and nickel and the like can be sufficiently oxidized. In addition, by rolling the powdered first precursor, the generated carbon dioxide gas is less likely to remain in the gaps between the particles, and the carbon dioxide gas can be efficiently removed to promote the solid-phase reaction.

When the second heat treatment step S22 is performed using a rotary kiln 1 having the configuration shown in FIG. 2, the first precursor is charged into the core tube 10 adjusted to an oxidizing atmosphere, and the first and second air supply systems and heater 20 are operated to rotate the core tube 10 at a predetermined rotation speed. That is, the first precursor that rolls and flows down from the upstream side to the downstream side in the core tube 10 of the rotary kiln 1 adjusted to an oxygen atmosphere with an oxygen concentration of 90% or more is sprayed with oxidizing gas by the first air supply system, and carbon dioxide gas generated from the first precursor is exhausted by the air flow of oxidizing gas by the second air supply system, while heat treatment is performed at a predetermined heat treatment temperature and heat treatment time. It is preferable that the carbon dioxide gas generated from the first precursor is exhausted from the axial direction of the core tube 10 through an exhaust port provided on the side of the upstream side in the core tube 10. In addition, it is preferable to perform heat treatment by adjusting at least one of the amount of oxidizing gas blown by the first gas supply system, the blowing angle, and the oxygen concentration according to the input amount of the first precursor, the heat treatment temperature, the oxygen concentration of the atmosphere, the rotation speed of the furnace tube 10, etc. However, the main purpose of the second heat treatment process S22 is to prevent the large amount of carbon dioxide gas generated from the first precursor from becoming a reaction inhibitor, as described above. It is preferable to exhaust as much carbon dioxide gas as possible in this second heat treatment process S22 and efficiently exhaust it from the inside of the furnace tube 10 in order to proceed with a series of processes. For this reason, the second heat treatment process S22 is a process in which the second gas supply system, which exhausts carbon dioxide gas, is very important. Therefore, in the second heat treatment process S22, it is preferable to adjust at least the amount of oxidizing gas blown by the second gas supply pipe 40, the blowing pressure, etc., and it is more preferable to adjust both the second gas supply system and the first gas supply system.

[Third heat treatment step]
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 more and 900° C. or less to obtain a lithium composite compound having a layered structure. The third heat treatment step S23 is mainly performed for the purpose of sufficiently oxidizing nickel in the second precursor from divalent to trivalent and growing crystal grains of the lithium composite compound having a layered structure. That is, this step is a heat treatment step in which an oxidation reaction of nickel in the second precursor and grain growth of crystal grains of the lithium composite compound are performed.

In the third heat treatment step S23, the second precursor may be left stationary for heat treatment, or may be rolled while being heat treated. By performing heat treatment while rolling the second precursor, the probability of contact between the powdered second precursor and oxygen can be increased, and nickel, etc. can be sufficiently oxidized. In addition, rolling the powdered second precursor has the advantage of more uniformly sintering the lithium composite compound.

It is preferable to carry out the third heat treatment step S23 after the second heat treatment step S22 is completed, by completely exhausting the atmospheric gas used in the second heat treatment step S22 and introducing a new atmospheric gas. In addition, when both the second heat treatment step S22 and the third heat treatment step S23 are carried out using a rotary kiln 1 having the configuration shown in FIG. 2, the second heat treatment step S22 may be carried out using a single rotary kiln 1, and then the third heat treatment step S23 may be carried out using the same rotary kiln 1, or the second heat treatment step S22 and the third heat treatment step S23 may be carried out sequentially using multiple rotary kilns 1, or the second heat treatment step S22 and the third heat treatment step S23 may be carried out continuously at one time in a single rotary kiln 1.

The present invention will be described in detail below with reference to examples, but the technical scope of the present invention is not limited to these examples.

Example 1
As starting materials for the positive electrode active material, lithium carbonate, nickel hydroxide, cobalt carbonate, and manganese carbonate were prepared. These starting materials were weighed so that the atomic ratio of Li:Ni:Co:Mn was 1.04:0.80:0.15:0.05, and a mixing step S1 was carried out. Specifically, ion-exchanged water was added and mixed so that the total weight of the starting materials was 20 mass%, and pulverization and mixing were carried out using a bead mill. The obtained solid-liquid mixture was dried using a spray dryer to obtain a raw material mixed powder.

Next, the obtained raw material mixed powder was filled into an alumina firing container and heat-treated in a roller hearth kiln in an air atmosphere at 360° C. for 1 hour (first heat treatment step S21) to obtain a first precursor. This heat treatment not only removed the moisture absorbed by the raw material mixed powder, but also thermally decomposed the nickel hydroxide and partially decomposed each carbonate, thereby removing a certain amount of carbon dioxide gas (CO ₂ ).

Next, the obtained first precursor was put into the rotary kiln 1 shown in Fig. 2, and in the rotating furnace core tube 10, while air was being supplied through the first air supply pipe 30 and the second air supply pipe 40, heat treatment was performed at 650 ° C for 0.9 hours, and then heat treatment was performed at 700 ° C for 3.5 hours. That is, the second heat treatment step S22 was performed to obtain the second precursor. At this time, in the rotary kiln 1, the furnace core tube 10 had a total tube length L1 = 3500 mm, a tube inner diameter D1 = 214 mm, and a volume V1 = 0.126 m ³ , the first air supply pipe 30 had a total tube length L2 = 3500 mm, a tube outer diameter D2 = 120 mm, and a volume V2 = 0.04 m ³ , V2/V1 = 0.32 (32%), and D2/D1 = 0.56. The inner layer (inner shell) of the furnace core tube 10 was made of metallic nickel material, the outer layer (outer shell) was made of stainless steel material, the inner layer of the first air supply pipe 30 was made of stainless steel material, and the outer layer was made of metallic nickel material. Then, pure aluminum was sprayed onto the inner layer of the furnace core tube 10 and the outer layer of the first air supply pipe 30, and then the resultant was air-baked in argon gas, and oxygen was then introduced to oxidize the surface, thereby forming an aluminum oxide on the outermost layer of the powder-contacting portion of the heat-treated portion Ma, and forming an alloy layer of aluminum and the base material nickel on the underlying layer.

Next, this second precursor was put into the rotary kiln 1B shown in FIG. 3 and heat-treated at 840° C. for 0.7 hours (third heat treatment step S23) to obtain a lithium composite compound (positive electrode active material) having a composition of Li _1.0 Ni _0.8 0Co _0.15 Mn _0.05 O _2. At this time, an alumina furnace tube 10B was used in the rotary kiln 1B. Then, the amount of unreacted lithium carbonate and lithium hydroxide remaining in the obtained positive electrode active material and the specific surface area of the positive electrode active material were measured. The measurement results are shown in Table 1.

Next, a lithium secondary battery was prepared by the following procedure using the obtained positive electrode active material as a positive electrode material. First, the positive electrode active material, the binder, and the conductive material were mixed to prepare a positive electrode mixture slurry. The prepared positive electrode mixture slurry was applied to an aluminum foil having a thickness of 20 μm, which is a positive electrode current collector, and dried at 120 ° C., and then compression molded by a press so that the electrode density was 2.0 g / cm ³ , and this was punched into a disk shape with a diameter of 15 mm to prepare a positive electrode. In addition, a negative electrode was prepared using metallic lithium as a negative electrode material. Then, a lithium secondary battery was prepared using the prepared positive electrode and negative electrode and a nonaqueous electrolyte. As the nonaqueous electrolyte, a solution in which LiPF6 was dissolved in a solvent in which ethylene carbonate and dimethyl carbonate were mixed so that the volume ratio was 3:7, and the concentration was 1.0 mol / L was used.

Next, the initial discharge capacity of the lithium secondary battery was measured using the following procedure. First, the battery was charged at a constant current and voltage of 0.2 CA until the end-of-charge voltage reached 4.3 V. Then, the battery was discharged at a constant current of 0.2 CA until the end-of-charge voltage reached 2.5 V, and the discharge capacity was calculated from the discharge current at that time. The results are shown in Table 1.

[Measurement of core tube weight loss rate]
The first precursor obtained in the same manner was used to perform the firing in the second heat treatment step S22 shown in Example 1, and then the weight of the furnace tube was measured. A process including temperature increase and decrease was repeated as one cycle, and the weight of the furnace tube was measured after each cycle. That is, the weight of the furnace tube after the cycle was divided by the weight of the furnace tube before use to obtain the weight reduction rate (amount of thinning), and durability was evaluated. The results after 300 cycles and 500 cycles are shown in Table 1.

Comparative Example 1
The first precursor obtained in the same manner was charged into the rotary kiln 1 shown in FIG. 2 and subjected to a heat treatment at 650° C. for 3.5 hours, i.e., a second heat treatment step S22, to obtain a second precursor.
Next, this second precursor was put into a rotary kiln 1B and subjected to heat treatment (third heat treatment step S23) at 840 ° C for 0.7 hours to obtain a lithium composite compound (positive electrode active material) having a composition of Li _1.0 Ni _0.80 Co _0.15 Mn _0.05 O _2. Then, the amount of unreacted lithium carbonate and lithium hydroxide remaining in the obtained positive electrode active material and the specific surface area of the positive electrode active material were measured. Furthermore, a lithium secondary battery was prepared and the discharge capacity was obtained. The results are shown in Table 1.
The difference between Example 1 and Comparative Example 1 is the temperature and time of the second heat treatment step S22.

(Comparative Example 2)
Similarly, the first precursor obtained was put into a rotary kiln 1' shown in FIG. 2, and heat-treated at 650 ° C for 0.9 hours, and then heat-treated at 700 ° C for 3.5 hours. At this time, the rotary kiln 1' had the same dimensions as in the embodiment, the inner layer of the furnace core tube 10 was made of metallic nickel material, the outer layer was made of stainless steel material, the inner layer of the first air supply pipe 30 was made of stainless steel material, the outer layer was made of metallic nickel material, and the powder contact part of the heat treatment part Ma was occupied by nickel material. Similarly, the process including the temperature increase and the temperature decrease was repeated as one cycle, and the weight reduction rate of the furnace core tube was measured. The results after 300 cycles and 500 cycles are shown in Table 1.

In Example 1 and Comparative Example 1, the second heat treatment step S22 in Example 1 includes a step (stage) of maintaining at 700° C. or higher for 2 hours or more, which shows that the amount of residual unreacted lithium carbonate is small and the solid-phase reaction proceeds.
In addition, in Example 1, by making the powder-contacting portion of the furnace tube occupied by aluminum oxide, the lithium component contained in the precursor of the lithium composite compound is less likely to react with the furnace tube, reducing the amount of thinning of the furnace tube and improving durability. As a result, the life of the furnace tube is extended, which contributes to lower production costs.

S1 Mixing process
S2 Firing process
S21 First heat treatment step
S22 Second heat treatment step
S23 Third heat treatment step
1, 1B Rotary kiln (firing furnace)
10, 10B Furnace tube
20, 20B Heater
30, 30B 1st air supply pipe
32 injection port 34 leg
40, 40B second air intake pipe
50, 50B Lifter
110, 110B Preheating zone
120, 120B Heating zone
Ma Processing object

Claims (5)

A mixing step of mixing a lithium compound with a compound containing a metal element other than Li in the following formula (1);
and a calcination step of calcining the precursor obtained through the mixing step to obtain a lithium composite compound represented by the following (1),
The firing step includes at least a heat treatment step of performing a heat treatment on the precursor while rolling it in a furnace tube of a firing furnace,
a first layer of the furnace tube that is in contact with the powder and is formed of aluminum oxide obtained by surface oxidizing pure aluminum , and an underlying layer of the first layer is an alloy layer of the pure aluminum and a base material of the furnace tube.
Li _1+a M1O _2+α ...(1)
(In the formula (1), M1 is a metal element other than Li, and includes at least Ni and any one of Mn, Co, Al, Ti, Zr, Mo, Nb, W, V, Cr, Mg, Ca, Cu, Zn, and Sn , the proportion of Ni in M1 is 70 atomic % or more, and a and α are numbers that satisfy −0.1≦a≦0.2 and −0.2≦α≦0.2.) the firing furnace includes a first air supply pipe that injects an oxidizing gas toward an inner peripheral surface side of the furnace core tube, and a second air supply pipe that flows the oxidizing gas in an axial direction of the furnace core tube;
2. The method for producing a positive electrode active material for a lithium secondary battery according to claim 1, wherein the first air supply pipe has a plurality of injection ports whose opening direction is in a circumferential direction of the first air supply pipe . The method for producing a positive electrode active material for a lithium secondary battery according to claim 2, characterized in that the legs that hold the first air supply pipe in the furnace tube are not joined to the furnace tube. 4. The method for producing a positive electrode active material for a lithium secondary battery according to claim 1, further comprising the step of maintaining the material at 700° C. or higher for 2 hours or more in the heat treatment step. A mixing step of mixing a lithium compound with a compound containing a metal element other than Li in the following formula (1);
and a calcination step of calcining the precursor obtained through the mixing step to obtain a lithium composite compound represented by the following formula (1),
The firing step includes at least a heat treatment step of performing a heat treatment on the precursor while rolling it in a furnace tube of a firing furnace,
the firing furnace includes a first air supply pipe that injects an oxidizing gas toward an inner peripheral surface side of the furnace core tube, and a second air supply pipe that flows the oxidizing gas in an axial direction of the furnace core tube;
The first air intake pipe has a plurality of injection ports whose opening direction is in a circumferential direction of the first air intake pipe,
In the furnace core tube, an aluminum oxide is formed on the outermost layer of the powder contact portion, and an alloy layer of aluminum and the base material of the furnace core tube is formed on the underlying layer, and the legs that hold the first air supply pipe are not joined to the furnace core tube.
The present invention relates to a method for producing a positive electrode active material for a lithium secondary battery.
Li _1+a M1O _2+α ...(1)
(In the formula (1), M1 is a metal element other than Li and contains at least Ni, the proportion of Ni in M1 is 70 atomic % or more, and a and α are numbers that satisfy −0.1≦a≦0.2 and −0.2≦α≦0.2.)