Method for producing positive electrode active material

Abstract

To provide a method of manufacturing a cathode active material capable of contributing to improvement of battery performance.SOLUTION: A manufacturing method includes: a preparation step S10 of preparing a starting material containing at least a transition metal element; a preparation step S20 of preparing a transition metal solution by dissolving the transition metal element in the starting material into an extraction liquid; a precursor generation step S30 of crystallizing a cathode active material precursor by adding an alkaline solution to the transition metal solution; and an active material generation step S40 of generating a cathode active material by heating the cathode active material precursor together with a lithium compound. In the manufacturing method, a sulfate ion concentration of the transition metal solution in implementing the precursor generation step is made equal to or less than 0.9 mol / L.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The technology disclosed herein relates to a method for producing a positive electrode active material. Specifically, the technology disclosed herein relates to a method for producing a positive electrode active material for a lithium ion secondary battery. [Background technology]

[0002] Lithium-ion secondary batteries are widely used in various devices such as mobile terminals and vehicles. The positive electrode active material of these lithium-ion secondary batteries is, for example, a lithium transition metal composite oxide. Examples of transition metals contained in the lithium transition metal composite oxide include Ni, Co, and Mn.

[0003] Patent Document 1 discloses an example of a precursor for this type of positive electrode active material (lithium transition metal composite oxide). As described in Patent Document 1, in the production of a positive electrode active material, a transition metal sulfate (such as NiSO4, CoSO4, or MnSO4) is typically prepared as a starting material. Next, this sulfate is dissolved in water to prepare a mixed aqueous solution (transition metal solution). An alkaline solution (such as NaOH) is then added to this transition metal solution. This precipitates a compound containing the transition metal (positive electrode active material precursor). This positive electrode active material precursor is then mixed with a lithium compound and calcined. This produces a lithium transition metal composite oxide. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2018 / 123995 Summary of the Invention [Problem to be solved by the invention]

[0005] Recently, there has been an increasing demand for improved performance in lithium-ion secondary batteries. The technology disclosed herein has been made in response to such demands, and aims to provide a method for producing a positive electrode active material that can contribute to improving battery performance. [Means for solving the problem]

[0006] To address the above-mentioned problems, there is provided a method for producing a positive electrode active material (hereinafter also simply referred to as "production method") having the following configuration.

[0007] The method for producing a cathode active material disclosed herein includes the steps of: preparing a starting material containing at least a transition metal element; preparing a transition metal solution by dissolving the transition metal element in the starting material in an extract; crystallizing a cathode active material precursor by adding an alkaline solution to the transition metal solution; and heating the cathode active material precursor together with a lithium compound to produce the cathode active material. The disclosed method is characterized in that the sulfate ion concentration of the transition metal solution during the precursor production step is 0.9 mol / L or less.

[0008] As mentioned above, in the production of typical positive electrode active materials, sulfates of transition metals (such as NiSO4, CoSO4, and MnSO4) are used as starting materials. Therefore, sulfates such as sodium sulfate (Na2SO4) may be mixed into the produced positive electrode active material. As a result of the inventor's investigation, it was found that an increase in the amount of sulfates mixed into the positive electrode active material increases the initial resistance of the lithium ion secondary battery. The production method disclosed herein is based on this finding. Specifically, the production method disclosed herein involves reducing the sulfate ions (SO4 2- ) concentration to 0.9 mol / L or less. This reduces the amount of sulfate mixed into the positive electrode active material after production, which contributes to reducing the initial resistance of lithium-ion secondary batteries. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a flowchart illustrating a method for producing a positive electrode active material according to the first embodiment. [Figure 2] FIG. 2 is a flowchart illustrating a preparation process in another embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the technology disclosed herein will be described with reference to the drawings. It should be noted that matters other than those specifically mentioned in this specification that are necessary for implementing the technology disclosed herein can be understood as design matters for a person skilled in the art based on the prior art in the relevant field. The technology disclosed herein can be implemented based on the contents disclosed in this specification and common technical knowledge in the relevant field. Furthermore, the expression "A to B" indicating a range in this specification means not less than A and not more than B, and also encompasses the meanings of "preferably greater than A" and "preferably smaller than B."

[0011] First Embodiment A first embodiment of the manufacturing method disclosed herein will be described below. Note that, after describing the cathode active material to be manufactured, specific steps of the manufacturing method according to this embodiment will be described.

[0012] [Cathode active material] The manufacturing method according to this embodiment produces a lithium transition metal composite oxide for use as a positive electrode active material in a lithium ion secondary battery. This lithium transition metal composite oxide is an oxide containing lithium (Li) and a transition metal element. Examples of the transition metal element include nickel (Ni), cobalt (Co), and manganese (Mn). Specific examples of this lithium transition metal composite oxide include lithium nickel composite oxide (LN oxide), lithium cobalt composite oxide (LC oxide), lithium nickel manganese composite oxide (LNM oxide), lithium manganese cobalt composite oxide (LMC oxide), lithium nickel cobalt composite oxide (LNC oxide), and lithium nickel cobalt manganese composite oxide (LNCM oxide). Note that these lithium transition metal composite oxides may contain elements (additive elements) other than Li and the transition metal element. These additional elements include Mg, Ca, Al, Ti, V, Cr, Si, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, Sn, B, C, Si, P, S, F, Cl, Br, I, and the like.

[0013] Although the details will be described later, the manufacturing method according to this embodiment can reduce the amount of sulfates mixed into the positive electrode active material after manufacturing. Specifically, the manufacturing method according to this embodiment reduces the amount of sulfur ions (SO4 2- The content of sulfur ions (SO4) can be reduced to 1.2 wt% or less (preferably 0.9 wt% or less, more preferably 0.6 wt% or less, and particularly preferably 0.3 wt% or less). This reduces the initial resistance of the lithium ion secondary battery. 2- The "content of sulfur ions (SO4 2- ) the content of sulfur ions can be calculated based on the weight of the sulfur element.

[0014] [Method of manufacturing battery materials] Next, a method for producing a positive electrode active material according to this embodiment will be described. Fig. 1 is a flowchart illustrating a method for producing a positive electrode active material according to this embodiment.

[0015] 1, the method for producing a battery material according to this embodiment includes a preparation step S10, a preparation step S20, a precursor production step S30, and an active material production step S40. Each step will be described below using the example of producing an LNCM oxide as a positive electrode active material.

[0016] (1) Preparation process S10 In the preparation step S10, starting materials containing at least transition metal elements are prepared. As will be described in detail later, the starting materials can be appropriately selected from conventionally known materials depending on the composition of the target (cathode active material). For example, in this embodiment, an LNCM oxide containing Ni, Co, and Mn as transition metal elements is produced. In this case, a starting material containing Ni, Co, and Mn is prepared in this step. Note that one or more types of starting materials may be prepared in this step. Specifically, a single type of starting material capable of supplying all of the necessary transition metal elements may be prepared. Alternatively, multiple types of starting materials may be prepared, and the necessary transition metal elements may be supplied by combining these multiple types of starting materials. For example, in this embodiment, a starting material containing Ni and Co (NiCo source) and a starting material containing Mn (Mn source) are separately prepared. This allows the Ni, Co, and Mn necessary for producing the LNCM oxide to be supplied. Note that the NiCo source in this embodiment is prepared by calcining the cathode active material (LNCM oxide) of a used lithium-ion secondary battery. On the other hand, manganese chloride (MnCl2) is used as the Mn source.

[0017] (2) Preparation process S20 In the preparation step S20, a transition metal solution is prepared by dissolving the transition metal elements in the starting materials in an extract. This step can be performed using any conventionally known procedure, as long as it can prepare a transition metal solution containing the desired transition metal elements. In this embodiment, first, the NiCo raw material (calcined LNCM oxide) is immersed in ammonia water. This causes nickel oxide (NiO) and cobalt oxide (CoO) in the calcined LNCM oxide to leach into the ammonia water. This results in a solution containing Ni and Co (NiCo solution). On the other hand, manganese oxide (MnO) in the calcined LNCM oxide is less likely to leach into the ammonia water. Therefore, in this embodiment, manganese chloride is separately prepared as a Mn source. This manganese chloride is then dissolved in another extract (e.g., water) to prepare a solution containing Mn (Mn solution). This Mn solution is then added to the NiCo solution to prepare a transition metal solution containing Ni, Co, and Mn (NiCoMn solution).

[0018] The metal concentration of the transition metal solution is preferably 1.0 mol / L or more, more preferably 1.2 mol / L or more, even more preferably 1.4 mol / L or more, and particularly preferably 1.5 mol / L or more. This allows efficient crystallization of the positive electrode active material precursor in the precursor generation step S30. On the other hand, if the metal concentration of the transition metal solution becomes too high, the metal approaches its saturated solubility, which may result in metal precipitation due to a slight temperature change. From this perspective, the metal concentration of the transition metal solution is preferably 2.6 mol / L or less, more preferably 2.4 mol / L or less, even more preferably 2.2 mol / L or less, and particularly preferably 2 mol / L or less. The mixing ratio of the transition metal elements (Ni, Co, Mn, etc.) in the transition metal solution is appropriately adjusted depending on the composition of the positive electrode active material to be produced.

[0019] In this step, a reducing agent and a pH buffer may be added to the extraction solution. This allows for easier extraction of Ni and Co elements. Hydrogen peroxide or the like can be suitably used as the reducing agent. Ammonium sulfate or the like can be suitably used as the pH buffer. When ammonium sulfate is used as the pH buffer, it is preferable to adjust the amount of the pH buffer added so that the sulfate ion concentration in the transition metal solution is 0.9 mol / L or less, as described below.

[0020] (3) Precursor generation step S30 In the precursor generation step S30, an alkaline solution is added to the transition metal solution to crystallize a positive electrode active material precursor. This step can employ any conventionally known crystallization process without particular limitations, as long as it produces a positive electrode active material precursor. Examples of alkaline solutions include an aqueous ammonia (NH3) solution, a sodium hydroxide (NaOH) solution, and a potassium hydroxide (KOH) solution. In this embodiment, an aqueous NaOH solution is added to the transition metal solution (NiCoMn solution) to adjust the pH of the transition metal solution to a strong alkaline (pH = 11 or higher). This causes NiCoMn hydroxide to crystallize in the transition metal solution. This NiCoMn hydroxide can be collected by solid-liquid separation. The collected NiCoMn hydroxide is then washed with water or the like and dried. This results in a dried NiCoMn hydroxide (positive electrode active material precursor).

[0021] (4) Active material generation step S40 In the active material production step S40, a positive electrode active material is produced by heating a positive electrode active material precursor together with a lithium compound. In this embodiment, a positive electrode active material precursor (NiCoMn hydroxide) is mixed with a lithium compound (LiCO, LiOH, etc.). The mixing ratio here is appropriately adjusted depending on the composition of the positive electrode active material to be produced. The mixture is then fired in an oxygen atmosphere. This allows the production of a positive electrode active material (LNCM oxide) for a lithium ion secondary battery.

[0022] The maximum temperature in the calcination treatment is preferably 700°C or higher, more preferably 725°C or higher, even more preferably 750°C or higher, and particularly preferably 775°C or higher. This allows the cathode active material precursor and the lithium compound to react appropriately, thereby efficiently producing the cathode active material precursor. On the other hand, if the maximum temperature is too high, excessive crystal growth may occur, which may result in a deterioration in battery characteristics. From this perspective, the maximum temperature in the calcination treatment is preferably 900°C or lower, more preferably 875°C or lower, even more preferably 850°C or lower, and particularly preferably 825°C or lower. The calcination time is preferably 3 hours or longer, more preferably 3.5 hours or longer, even more preferably 4 hours or longer, and particularly preferably 4.5 hours or longer. This allows the cathode active material precursor and the lithium compound to react sufficiently. On the other hand, the upper limit of the calcination time is not particularly limited and may be 7 hours or shorter, 6.5 hours or shorter, 6 hours or shorter, or 5.5 hours or shorter. In this specification, the term "firing time" refers to the time during which the maximum temperature is maintained.

[0023] (5) Sulfate ion control The manufacturing method disclosed herein is characterized in that the sulfate ion concentration of the transition metal solution during the precursor generation step is set to 0.9 mol / L or less. Experiments conducted by the present inventors have confirmed that the use of a transition metal solution with such a low sulfate ion concentration can reduce the amount of sulfate contamination in the manufactured positive electrode active material. Furthermore, the use of this positive electrode active material can reduce the initial resistance of lithium-ion secondary batteries.

[0024] The manufacturing method according to the first embodiment uses starting materials that are substantially free of sulfur. This allows the sulfate ion concentration of the transition metal solution to be controlled to 0.9 mol / L or less. Specifically, in this embodiment, a used cathode active material (LNCM oxide) is used as the NiCo source. Unlike conventional Ni sources such as nickel sulfate (NiSO4) and cobalt sulfate (CoSO4), the used cathode active material does not substantially contain sulfur. Furthermore, unlike conventional Mn sources such as manganese sulfate (MnSO4), manganese chloride used as the Mn source also does not substantially contain sulfur. Using these starting materials prevents the incorporation of sulfur from the starting materials. This allows the sulfate ion concentration of the transition metal solution to be controlled to 0.9 mol / L or less. Furthermore, using this transition metal solution significantly reduces the amount of sulfate (e.g., Na2SO4) precipitated in the precursor generation step S30. As a result, the amount of sulfate mixed into the positive electrode active material after production can be reduced, and the performance (initial resistance) of the lithium ion secondary battery can be improved.

[0025] In this specification, "using starting materials that are substantially free of elemental sulfur" refers to not intentionally using compounds containing elemental sulfur as starting materials. Therefore, when trace amounts of elemental sulfur are unavoidably present in the starting materials due to the manufacturing process or other factors, "using starting materials that are substantially free of elemental sulfur" is interpreted as "using starting materials that are substantially free of elemental sulfur." ​​For example, when the content of elemental sulfur relative to the total weight (100 wt%) of the starting materials is 1 wt% or less (preferably 0.1 wt% or less, more preferably 0.01 wt% or less, even more preferably 0.001 wt% or less, and particularly preferably 0.0001 wt% or less), it can be said that "using starting materials that are substantially free of elemental sulfur" is used. Using such starting materials, a transition metal solution with a sulfate ion concentration of 0.9 mol / L or less can be easily prepared.

[0026] The sulfate ion concentration of the transition metal solution is preferably 0.8 mol / L or less, more preferably 0.7 mol / L or less, even more preferably 0.6 mol / L or less, and particularly preferably 0.5 mol / L or less. As the sulfate ion concentration of the transition metal solution decreases, the amount of sulfate salts mixed into the positive electrode active material decreases, allowing for the production of a positive electrode active material with higher performance. On the other hand, the lower limit of the sulfate ion concentration of the transition metal solution is not particularly limited and may be 0 mol / L (no sulfate ions at all).

[0027] In addition, the total amount of transition metal elements in the transition metal solution (M T ) and the amount of sulfur (M S ) and the ratio (M S / M T ) is preferably 0.5 or less, more preferably 0.3 or less, even more preferably 0.1 or less, and particularly preferably 0.01 or less. This further reduces the amount of sulfates that precipitate simultaneously with the positive electrode active material precursor (hydroxide of transition metal). As a result, the amount of sulfates mixed into the positive electrode active material after production can be more suitably reduced. On the other hand, the above M S / M T The lower limit of is not particularly limited, and is 0 or more (M S = 0 mol / L) is also acceptable.

[0028] <Other embodiments> The first embodiment of the technology disclosed herein has been described above. Note that the technology disclosed herein is not limited to the first embodiment described above, and includes other embodiments with various configuration changes. Other examples of the embodiment of the technology disclosed herein will be described below.

[0029] (1) Type of starting material In the first embodiment, a used positive electrode active material (LNCM oxide) is used as the NiCo source. In the first embodiment, manganese chloride (MnCl2) is used as the Mn source. However, the manufacturing method disclosed herein is not limited to a specific starting material. For example, nickel oxide, nickel carbonate, nickel acetate, nickel chloride, nickel hydroxide, nickel nitrate, etc. can be used as the starting material (Ni source) that supplies Ni. Cobalt oxide, cobalt carbonate, cobalt acetate, cobalt chloride, cobalt hydroxide, cobalt nitrate, etc. can be used as the starting material (Co source). Manganese chloride, manganese oxide, manganese carbonate, manganese acetate, manganese chloride, manganese hydroxide, manganese nitrate, etc. can be used as the starting material (Mn source) that supplies Mn. These starting materials also substantially do not contain sulfur, so a transition metal solution with a sulfate ion concentration of 0.9 mol / L or less can be easily prepared.

[0030] Furthermore, the technology disclosed herein does not prohibit the use of sulfates (nickel sulfate, cobalt sulfate, manganese sulfate, etc.) as starting materials. In other words, sulfates may be used as part of the starting materials as long as the sulfate ion concentration of the transition metal solution can be controlled to 0.9 mol / L or less. For example, using a NiCo source (such as a used cathode active material) that is substantially free of sulfur can prevent the inclusion of sulfur elements derived from the supply of Ni and Co. Furthermore, if the cathode active material to be manufactured is a low-Mn LNCM material, the amount of Mn source added will be smaller than that of the NiCo source. In such cases, even if a sulfate (manganese sulfate) is used as the Mn source, the sulfate ion concentration of the transition metal solution can be controlled to 0.9 mol / L or less.

[0031] (2) Selection of starting materials In the above-described embodiment, starting materials with known compositions are used, and the sulfate ion concentration of the transition metal solution is controlled to 0.9 mol / L or less. However, the manufacturing method disclosed herein can also use materials with unknown compositions (such as mined minerals or recycled materials of unknown origin) as starting materials. In this case, as shown in FIG. 2, it is recommended to carry out a measurement step S12 and a raw material selection step S14 in the preparation step S10. This will be explained in detail below.

[0032] First, in the measurement step S12, the molar concentration of sulfur element in the starting material is measured. In this step, any conventionally known elemental analysis technique can be used without particular limitations. Examples of such elemental analysis techniques include ICP analysis, SEM-EDS analysis, and XRF analysis. Next, in the raw material selection step S14, starting materials whose molar concentration of sulfur element is below a predetermined threshold are selected as starting materials to be provided to the preparation step S20. This allows the sulfate ion concentration in the transition metal solution to be sufficiently reduced even when starting materials whose detailed composition is unknown are used. Note that the threshold value set in the raw material selection step S14 can be appropriately set from the perspective of suppressing the sulfate ion concentration in the transition metal solution to 0.9 mol / L or less.

[0033] The measurement step S12 and the raw material selection step S14 shown in FIG. 2 may be performed when starting materials with known compositions are used. This allows the amount of elemental sulfur present in the starting materials as an impurity to be determined, thereby more effectively suppressing the contamination of elemental sulfur from the starting materials. Furthermore, the measurement step S12 and the raw material selection step S14 described above do not need to be performed on all starting materials used. For example, the measurement step S12 and the raw material selection step S14 may be performed when the type or lot of the starting materials is changed, or when the conditions of other processes are changed. This allows the production of a positive electrode active material with a low amount of sulfate contamination without significantly reducing production efficiency.

[0034] (3) Type of extract In the first embodiment, ammonia water is used as the extracting liquid for extracting Ni and Co from the NiCo raw material. Furthermore, water is used as the extracting liquid for extracting Mn from the Mn raw material. However, the extracting liquid used in the preparation step S20 can be selected from conventionally known extracting liquids without particular limitations as long as it can extract the target transition metal element from the starting material. Other examples of this extracting liquid include organic acids such as citric acid, ascorbic acid, oxalic acid, and acetic acid, and inorganic acids such as nitric acid, hydrochloric acid, phosphoric acid, and sulfuric acid. However, sulfuric acid can cause sulfur elements to be mixed into the transition metal solution. Therefore, when using sulfuric acid as the extracting liquid, it is necessary to appropriately adjust the sulfuric acid concentration and the type of starting material so that the sulfate ion concentration of the transition metal solution is 0.9 mol / L or less.

[0035] (4) Separation of elemental sulfur In addition, in each of the above-described embodiments, the sulfate ion concentration of the transition metal solution is controlled to 0.9 mol / L or less by suppressing the incorporation of sulfur elements derived from the transition metal solution materials (starting materials, extract). However, in the technology disclosed herein, it is sufficient to control the sulfate ion concentration of the transition metal solution to 0.9 mol / L or less when performing the precursor generation step S30. That is, after preparing a transition metal solution with a sulfate ion concentration exceeding 0.9 mol / L in the preparation step S20, sulfate ions may be removed from the transition metal solution. Even when adopting such a configuration, the sulfate ion concentration of the transition metal solution can be controlled to 0.9 mol / L or less, thereby producing a positive electrode active material with a low amount of sulfate contamination. A specific method for removing sulfate ions from the transition metal solution is to add calcium hydroxide. This allows sulfate ions to be removed as calcium sulfate.

[0036] (5) Type of positive electrode active material In the above-described embodiments, a lithium-nickel-cobalt-manganese composite oxide is produced as the positive electrode active material. However, the technology disclosed herein is not limited by the type of positive electrode active material to be produced. That is, the production method disclosed herein can also produce other lithium transition metal composite oxides (such as LN oxide, LC oxide, LNM oxide, LMC oxide, and LNC oxide).

[0037] [Test example] Test examples relating to the technology disclosed herein will be described below. Note that the contents of the test examples described below are not intended to limit the technology disclosed herein.

[0038] 1. Sample Preparation In this test, three types of positive electrode active materials were prepared using different manufacturing procedures. The manufacturing procedures for each example are explained below.

[0039] (1) Example 1 In Example 1, transition metal sulfates were prepared as starting materials. Specifically, nickel sulfate hexahydrate (NiSO4·6H2O) was prepared as the Ni source, cobalt sulfate heptahydrate (CoSO4·7H2O) was prepared as the Co source, and manganese sulfate pentahydrate (MnSO4·5H2O) was prepared as the Mn source.

[0040] Next, a transition metal solution was prepared by dissolving the aforementioned transition metal sulfates in water. The amounts of the starting materials added were adjusted so that the transition metal element concentration in the solution was 1.5 mol / L. The mixing ratio of the Ni source, Co source, and Mn source was adjusted to Ni:Co:Mn=8:1:1.

[0041] Next, a reaction vessel containing 0.5 wt% NH3 water was prepared. The temperature of the reaction vessel was maintained at 40°C, and while stirring at 600 rpm, a transition metal solution, 28 wt% NH3 water, and a 30 wt% NaOH aqueous solution were continuously added dropwise. The transition metal solution and the NaOH aqueous solution were added dropwise for 6 hours while adjusting the amount of NaOH aqueous solution added so that the pH in the reaction vessel was maintained at 11.4. The solid matter precipitated in the reaction vessel was then recovered by solid-liquid separation. The recovered solid matter was then washed twice with water and dried. The NiCoMn hydroxide obtained in this manner was used as a positive electrode active material precursor.

[0042] Next, a mixed material was prepared by mixing the positive electrode active material precursor with lithium hydroxide (LiOH). The mixing ratio of each material was adjusted so that the molar ratio of lithium to transition metal elements was 1.06:1. The mixed material was then subjected to a calcination treatment. The calcination conditions were a maximum temperature of 800°C, a calcination time of 5 hours, and an oxygen atmosphere. This produced LNCM oxide (positive electrode active material).

[0043] (2) Example 2 In Example 2, first, a used cathode active material (LNCM oxide, Ni:Co:Mn = 8:1:1) was calcined (calcination temperature: 650 °C, calcination time: 5 hours, calcination atmosphere: oxygen atmosphere). The calcined cathode active material was then used as a NiCo source. The cathode active material was immersed in an NH3 solution to leach out the NiCo contained in the cathode active material. The NH3 solution used was a mixed solution of 28 wt% NH3, 0.5 mol / L (NH4)2SO4, and 0.5 mol / L HO2. The NH3 immersion treatment involved immersion in the NH3 solution maintained at 80 °C for 8 hours while stirring at 500 rpm. The solid content was then removed by solid-liquid separation to obtain a solution containing Ni and Co (NiCo solution). Next, manganese sulfate pentahydrate (MnSO4·5H2O) was added to this NiCo solution so that the total concentration of the transition metal elements was 1.5 mol / L. This resulted in a transition metal solution containing Ni, Co, and Mn. In Example 2, a positive electrode active material precursor was crystallized from the transition metal solution using the same procedure as in Example 1 above, and then an LNCM oxide (positive electrode active material) was produced using the positive electrode active material precursor.

[0044] (3) Example 3 In Example 3, calcination and NH3 leaching were performed under the same conditions as in Example 2, except that a LiNi composite oxide was used as the used positive electrode active material. This resulted in the extraction of only Ni from the used positive electrode active material. Next, cobalt sulfate heptahydrate (CoSO4·7H2O) and manganese sulfate pentahydrate (MnSO4·5H2O) were added to this Ni solution to prepare a transition metal solution containing Ni, Co, and Mn. Then, in Example 3, LNCM oxide (positive electrode active material) was produced according to the same procedure as in Example 1.

[0045] 2.Evaluation Test (1) Measurement of sulfur contamination During the preparation of Examples 1 to 3, a portion of the transition metal solution was collected and the amount of elemental sulfur was measured using ICP analysis. The sulfate ion concentration (mol / L) of the transition metal solution was calculated based on the measurement results. In this test, the amount of sulfate in the LNCM oxide after preparation was also measured. To measure the amount of sulfate, a solution was prepared by dissolving the LNCM oxide in nitric acid. This solution was then subjected to ICP analysis to measure the amount of elemental sulfur. The sulfate ion concentration in the positive electrode active material was calculated based on the measured amount of elemental sulfur. The results are shown in Table 1.

[0046] (2) Fabrication of lithium-ion secondary batteries Next, lithium ion secondary batteries were fabricated using the positive electrode active materials of Examples 1 to 3. First, a positive electrode active material, a conductive material (graphite), a binder (polyvinylidene fluoride powder), and N-methyl-2-pyrrolidone (NMP) were mixed to prepare a positive electrode composite slurry. The mixing ratio of each raw material was set to 1 part by mass of the conductive material and 0.9 parts by mass of the binder relative to 100 parts by mass of the positive electrode active material. Next, the positive electrode composite slurry was applied to both sides of a positive electrode current collector (aluminum foil) and dried. The dried coating was then rolled with a rolling roller. In this way, a positive electrode having a positive electrode active material layer formed on both sides of the positive electrode current collector was fabricated.

[0047] Next, a negative electrode was fabricated by forming a negative electrode active material layer on both sides of a negative electrode current collector (copper foil). The negative electrode active material layer was a mixture of a negative electrode active material (graphite), a binder (styrene butadiene rubber), and a thickener (carboxymethyl cellulose). The mixing ratio of the negative electrode active material, binder, and thickener was set to 98:1:1. A porous film with a three-layer structure of PP / PE / PP was prepared as a separator. A wound electrode assembly was then fabricated using the positive electrode, negative electrode, and separator. Electrode terminals were then attached to this wound electrode assembly, which was then housed in an aluminum battery case. A nonaqueous electrolyte was then poured into the battery case, and a lithium-ion secondary battery for evaluation was fabricated. In this test, the non-aqueous electrolyte used was a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 3:4:3, to which 1.0 mol / L of LiPF6 was added.

[0048] (3) Evaluation of output characteristics The evaluation battery was charged to half of its initial capacity at a constant current of 0.5 It in a 25°C environment. After stopping charging and leaving it for 15 minutes, the battery was charged at a constant current of 0.1 It for 10 seconds, and the voltage was measured. Next, the battery was discharged for 10 seconds equivalent to the charge capacity, and then the current value was changed and the voltage was measured when the battery was charged for 10 seconds. The battery was then discharged for 10 seconds equivalent to the charge capacity. While measuring the voltage, the above-mentioned charge / discharge cycle was repeated at current values ​​ranging from 0.1 It to 2 It. The initial resistance was then measured based on the measured voltage and current values. The results are shown in Table 1. In Table 1, the percentages are shown relative to the initial resistance of Example 1, which is set to 100%.

[0049] [Table 1]

[0050] As shown in Table 1, it was confirmed that the sulfate ion concentration of the transition metal solution was reduced by using LNCM oxide as part of the starting material. It was also found that controlling the sulfate ion concentration of the transition metal solution to 0.9 mol / L or less reduced the amount of sulfate contamination in the positive electrode active material. Furthermore, as shown in Examples 2 and 3, it was confirmed that a positive electrode active material with a low amount of sulfate contamination could reduce the initial resistance of a lithium-ion secondary battery by more than 5%.

[0051] The technology disclosed herein has been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and variations of the specific examples exemplified above. In other words, the technology disclosed herein encompasses the aspects described in items 1 to 8 below.

[0052] <Item 1> a preparation step of preparing a starting material containing at least a transition metal element; a preparation step of preparing a transition metal solution by dissolving the transition metal element in the starting material in an extract; a precursor production step of crystallizing a positive electrode active material precursor by adding an alkaline solution to the transition metal solution; an active material production step of producing a positive electrode active material by heating the positive electrode active material precursor together with a lithium compound; Including, A method for producing a positive electrode active material, wherein the sulfate ion concentration of the transition metal solution when the precursor producing step is carried out is set to 0.9 mol / L or less.

[0053] <Item 2> 2. The method for producing a positive electrode active material according to item 1, wherein the starting material contains at least Ni and / or Co.

[0054] <Item 3> 3. The method for producing a positive electrode active material according to item 1 or 2, wherein the starting material is a positive electrode active material of a used lithium ion secondary battery.

[0055] <Item 4> The preparation step includes: a measuring step of measuring the molar concentration of elemental sulfur in the starting material; a raw material selection step of selecting a starting material having a molar concentration of sulfur element below a predetermined threshold as a starting material to be provided to the preparation step; 4. The method for producing a positive electrode active material according to any one of items 1 to 3, comprising:

[0056] <Item 5> 5. The method for producing a positive electrode active material according to any one of items 1 to 4, wherein the extracting liquid is at least one selected from the group consisting of ammonia water, citric acid, ascorbic acid, oxalic acid, acetic acid, nitric acid, hydrochloric acid, phosphoric acid, and sulfuric acid.

Claims

1. a preparation step of preparing a starting material containing at least a transition metal element; a preparation step of preparing a transition metal solution by dissolving the transition metal element in the starting material in an extract; a precursor production step of crystallizing a positive electrode active material precursor by adding an alkaline solution to the transition metal solution; an active material production step of producing a positive electrode active material by heating the positive electrode active material precursor together with a lithium compound; Including, the starting material includes a positive electrode active material of a used lithium ion secondary battery containing at least Ni and / or Co, The content of elemental sulfur relative to the total weight of the starting materials is 1 wt % or less, The extract contains at least sulfuric acid or a sulfate salt, The method for producing a positive electrode active material, wherein the sulfate ion concentration of the transition metal solution during the precursor production step is set to 0.9 mol / L or less.

2. The preparation step includes: a measuring step of measuring the molar concentration of elemental sulfur in the starting material; a raw material selection step of selecting a starting material having a molar concentration of sulfur element below a predetermined threshold as a starting material to be provided to the preparation step; The method for producing a positive electrode active material according to claim 1 , comprising:

3. The method for producing a positive electrode active material according to claim 1 , wherein the extracting solution further contains at least one selected from the group consisting of ammonia water, citric acid, ascorbic acid, oxalic acid, acetic acid, nitric acid, hydrochloric acid, and phosphoric acid.

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