TWI869450B - Positive electrode for lithium-ion secondary battery and lithium-ion secondary battery - Google Patents

Abstract

A positive electrode for a lithium ion secondary battery having high high temperature storage stability is provided. The positive electrode for a lithium ion secondary battery is produced using specific lithium composite oxide particles (A) and specific lithium-based polyanion particles (B), wherein the median particle size of the secondary particles of the particles (A) is set smaller than the median particle size of the secondary particles of the particles (B).

Description

Positive electrode for lithium-ion secondary battery and lithium-ion secondary battery

The present invention relates to a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery having the positive electrode.

As positive active materials for lithium-ion secondary batteries, polyanion compounds with excellent thermal stability have attracted much attention. As polyanion positive active materials, lithium iron phosphate having an olivine structure, or lithium manganese iron phosphate or lithium manganese phosphate in which part or all of the iron in lithium iron phosphate is replaced by manganese has been examined.

However, polyanionic cathode active materials have the problem of insufficient electrical conductivity. Therefore, an invention for using polyanionic cathode active materials mixed with cathode active materials was proposed, wherein the cathode active material is a composite oxide containing lithium and a transition metal such as nickel, cobalt, and manganese (Patent Document 1). [Prior Art Document] [Patent Document]

[Patent Document 1] Japanese Patent Application Publication No. 2018-142420

[The problem that the invention wants to solve]

Patent document 1 states that the average particle size of the secondary particles of the "first electrode active material" (i.e., the polyanion-based positive electrode active material) is set to 0.01 to 0.30 times the average particle size of the secondary particles of the "second electrode active material" (i.e., the composite oxide of lithium and other transition metals), so that the secondary particles of the first electrode active material can enter the gaps formed by the secondary particles of the second electrode active material, thereby promoting the improvement of the electrode density. At the same time, the first electrode active material fills the gaps between the secondary particles of the second electrode active material, thereby improving the thermal conductivity of the electrode for the lithium ion secondary battery compared to the case of the second electrode active material alone.

However, when a positive electrode having a polycationic positive electrode active material is used in a lithium-ion secondary battery and exposed to high temperatures, metal elements such as manganese and iron will be dissolved from the polycationic positive electrode active material such as lithium manganese iron phosphate, and the stability of the battery at high temperatures may be impaired.

The object of the present invention is to provide a positive electrode for a lithium-ion secondary battery having high storage stability, particularly at high temperatures. [Means for Solving the Problem]

As a result of in-depth research for the above purpose, the inventors have found that in a positive electrode for a lithium ion secondary battery having a polyanion-based positive electrode active material and a lithium composite oxide-based positive electrode active material, when the median particle size of the secondary particles of the polyanion-based positive electrode active material is set larger than the median particle size of the secondary particles of the lithium composite oxide-based positive electrode active material, the discharge capacity is not easily reduced even when stored at a high temperature. The present invention includes the following contents, but is not limited thereto. [1]. A positive electrode for a lithium secondary battery, comprising lithium composite oxide particles (A) represented by the following formula (I) or (II) and lithium-based polyanion particles (B) represented by the following formula (III), LiNi _a Co _b Mn _c M ¹ _x O ₂ … (I) In formula (I), M ¹ is one or more elements selected from Mg, Ti, Nb, Fe, Cr, Si, Al, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi and Ge, a, b, c, x are numbers satisfying 0.3≦a<1, 0<b≦0.7, 0<c≦0.7, 0≦x≦0.3, and 3a+3b+ 3c+(valence of M ¹ )×x=3, LiNi _d Co _e Al _f M ² _y O ₂ …(II) In formula (II), M ² is one or more elements selected from Mg, Ti, Nb, Fe, Cr, Si, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi and Ge, d, e, f, y are numbers satisfying 0.4≦d＜1, 0＜e≦0.6, 0＜f≦0.3, 0≦y≦0.3, and 3d+3e+3f+(valence of M ² )×y=3, LiFe _m Mn _n M ³ _o PO ₄ …(III) In formula (III), M ^{M 3} is Co, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd or Gd, m, n and o are numbers satisfying 0≦m≦1, 0≦n≦1, 0≦o≦0.3, and m+n≠0, and satisfying 2m+2n+(the value of M ³ )×o=2, the median particle size of the secondary particles of the aforementioned particles (A) is smaller than the median particle size of the secondary particles of the aforementioned particles (B). [2]. A positive electrode for a lithium ion secondary battery as described in [1], wherein the median particle size of the secondary particles of the aforementioned particles (A) is less than 0.80 times the median particle size of the secondary particles of the aforementioned particles (B). [3]. A positive electrode for a lithium ion secondary battery as described in [1] or [2], wherein the powder pH of the aforementioned particle (A) is greater than the powder pH of the aforementioned particle (B). [4]. A positive electrode for a lithium ion secondary battery as described in any one of [1] to [3], wherein the aforementioned particle (A) is a compound represented by the aforementioned formula (I). [5]. A positive electrode for a lithium ion secondary battery as described in [4], wherein x in the aforementioned formula (I) is 0. [6]. A positive electrode for a lithium ion secondary battery as described in any one of [1] to [5], wherein the aforementioned particle (B) is a compound in the aforementioned formula (III) that satisfies 0＜m≦1, 0＜n≦1 and o=0. [7]. A lithium ion secondary battery having a positive electrode for a lithium ion secondary battery as described in any one of [1] to [6]. [8]. The lithium ion secondary battery as described in [7], wherein a lithium salt containing fluorine is used as an electrolyte. [Effect of the Invention]

By using the positive electrode of the present invention, it is possible to manufacture a lithium ion secondary battery whose discharge capacity is not easily reduced even when exposed to high temperature.

[Best Mode for Carrying Out the Invention]

The positive electrode for the lithium ion secondary battery of the present invention is manufactured using particles (A) represented by a specific formula (i.e., a composite oxide of lithium and other transition metals) and polyanion particles (B) represented by a specific formula containing lithium, other transition metals and phosphoric acid.

<Particles (A)> The particles (A) used in the positive electrode of the present invention are lithium composite oxide particles having a layered rock salt structure represented by the following formula (I) or (II): LiNi _a Co _b Mn _c M ¹ _x O ₂ …(I) (In the formula (I), M ¹ is one or more elements selected from Mg, Ti, Nb, Fe, Cr, Si, Al, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi and Ge, and a, b, c and x are numbers satisfying 0.3≦a<1, 0<b≦0.7, 0<c≦0.7, 0≦x≦0.3, and 3a+3b+3c+(valence of M ¹ )×x=3). LiNi _d Co _e Al _f M ² _y O ₂ …(II) (in formula (II), M ² is one or more elements selected from Mg, Ti, Nb, Fe, Cr, Si, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi and Ge, d, e, f, y are numbers satisfying 0.4≦d＜1, 0＜e≦0.6, 0＜f≦0.3, 0≦y≦0.3, and 3d+3e+3f+(valence of M ² )×y=3).

In order to obtain the lithium composite oxide particles (A) represented by the above formula (I), a mixed powder containing a lithium compound, a nickel compound, a cobalt compound and a manganese compound is calcined. Specifically, first, the nickel compound, the cobalt compound and the manganese compound are dissolved in water so as to obtain an aqueous solution a in a manner to obtain a desired lithium composite oxide composition. Examples of such nickel compounds, cobalt compounds and manganese compounds include sulfates, nitrates, carbonates, acetates, oxalates, oxides, hydroxides, halides and the like of these metal elements. Specifically, examples include nickel sulfate, cobalt sulfate, manganese sulfate, nickel acetate, cobalt acetate, manganese acetate and the like, but are not limited thereto. If necessary, in order to further obtain the desired composition of the lithium composite oxide, one or more elements selected from Mg, Ti, Nb, Fe, Cr, Si, Al, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi and Ge may be mixed as the metal ( ^M1 ) element substituting for a part of the lithium composite oxide.

Next, an alkaline solution is added to the aqueous solution a to make an aqueous solution b, and the dissolved metal components are subjected to a neutralization reaction while stirring to produce a co-precipitation to generate a metal composite hydroxide. The alkaline solution used here is preferably dripped in a sufficient amount to keep the aqueous solution b at pH 10 to 14. As such an alkaline solution, an aqueous solution of, for example, sodium hydroxide, potassium hydroxide, sodium carbonate, ammonia, etc. can be used, wherein, sodium hydroxide, sodium carbonate, or a mixed solution thereof is preferably used.

The temperature of the aqueous solution b in the neutralization reaction is preferably 30° C. or higher, more preferably 30 to 60° C. The stirring time of the aqueous solution b is preferably 30 to 120 minutes, more preferably 30 to 60 minutes.

After stirring, the aqueous solution b is filtered to recover the metal complex hydroxide. The recovered metal complex hydroxide is preferably dried after being washed with water. Next, the recovered metal complex hydroxide is dry-mixed with a lithium compound in such a manner as to form the desired lithium complex oxide composition, and the obtained mixed powder is calcined in an oxygen environment. Examples of the lithium compound used here include lithium hydroxide or its hydrate, lithium peroxide, lithium nitrate, lithium carbonate, and the like. From the viewpoint of increasing the powder pH of the particles (A) described later, it is better to place more lithium hydroxide. When the metal complex hydroxide and the lithium compound are dry-mixed, a conventional dry mixer or mixing granulation device such as a ball mill or a V-type mixer can be used, and a planetary ball mill capable of self-revolution is more preferably used.

The calcination of the mixed powder is preferably carried out in two stages (pre-calcining and main-calcining). By setting the calcination to two stages, in the pre-calcination, water molecules or carbon dioxide from hydroxides or carbonates in the mixed powder are removed by thermal decomposition, and then the main calcination is carried out, so that lithium composite oxide particles (A) can be obtained efficiently. The conditions for the pre-calcination are not particularly limited, and the heating temperature is preferably 1~20℃/min from room temperature. In addition, the calcination environment is preferably an atmospheric environment or an oxygen environment. The calcination temperature is preferably 700~1000℃, and more preferably 650~750℃. Furthermore, the calcination time is preferably 3 to 20 hours, more preferably 4 to 6 hours.

The obtained preliminary calcined product is decomposed and crushed with a mortar or the like, and then mixed with an appropriate amount of a binder and granulated, and then the main calcination is performed. The calcined product obtained after the main calcination is the secondary particle of the lithium composite oxide particle (A).

The conditions for the formal calcination are not particularly limited, and the temperature can be raised to 1-20°C/minute starting from room temperature again. In addition, the calcination environment is preferably an atmospheric environment or an oxygen environment. From the perspective of controlling the average particle size of the primary particles (which are secondary particles constituting the lithium composite oxide particles (A) obtained after the formal calcination) to a desired value, the calcination temperature is preferably 700-1200°C, more preferably 700-1000°C, and even more preferably 750-900°C. The calcination time is preferably 3-20 hours, and more preferably 8-10 hours.

For such two-stage calcination, it is preferred to use an electric furnace, a rotary kiln, a tubular furnace, or a pusher furnace in which the oxygen concentration in the gas atmosphere is adjusted to 20 mass % or more. In order to obtain the lithium composite oxide particles (A) represented by the above formula (II), a mixed powder containing a lithium compound, a nickel compound, a cobalt compound, and an aluminum compound is calcined. Specifically, first, the nickel compound, the cobalt compound, and the aluminum compound are dissolved in water in a manner to obtain the desired lithium composite oxide composition, thereby obtaining an aqueous solution a'. Examples of such nickel compounds, cobalt compounds, and aluminum compounds include sulfates, nitrates, carbonates, acetates, oxalates, oxides, hydroxides, and halides of these metal elements. Specifically, examples include nickel sulfate, cobalt sulfate, aluminum sulfate, nickel acetate, cobalt acetate, and aluminum acetate, but are not limited thereto. If necessary, one or more elements selected from Mg, Ti, Nb, Fe, Cr, Si, Al, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi, and Ge may be mixed as metal (M ² ) elements that replace a portion of the lithium complex oxide in order to further obtain the desired lithium complex oxide composition.

Next, an alkaline solution is added to the aqueous solution a' to make an aqueous solution b', and the dissolved metal components are subjected to a neutralization reaction while stirring to produce a co-precipitation to generate a metal composite hydroxide. The alkaline solution used here is preferably dripped in a sufficient amount to keep the aqueous solution b' at pH 10 to 14. As such an alkaline solution, an aqueous solution of, for example, sodium hydroxide, potassium hydroxide, sodium carbonate, ammonia, etc. can be used, wherein, sodium hydroxide, sodium carbonate, or a mixed solution thereof is preferably used.

The temperature of the aqueous solution b' in the neutralization reaction is preferably above 40°C, more preferably 40-60°C. In addition, the stirring time of the aqueous solution b is preferably 30-120 minutes, more preferably 30-60 minutes. From the perspective of obtaining a useful positive electrode active material by setting a high bulk density composite hydroxide, an oxidizing agent such as sodium hypochlorite or hydrogen peroxide can be further added to the aqueous solution b' after the neutralization reaction.

After stirring, the aqueous solution b' is filtered to recover the metal composite hydroxide. The recovered metal composite hydroxide is preferably calcined to obtain the metal composite oxide from the viewpoint of stabilizing the grade of the obtained lithium composite oxide and reacting uniformly and fully with lithium.

The calcination conditions for obtaining the metal composite oxide from the metal composite hydroxide are not particularly limited. For example, the metal composite oxide may be calcined in an atmospheric environment at preferably 500 to 1100° C., more preferably 60 to 900° C.

Next, the metal composite oxide obtained by the above calcination is dry-mixed with a lithium compound in such a manner as to obtain the desired lithium composite oxide composition, and the obtained mixed powder is calcined in an oxygen environment. Examples of the lithium compound used herein include lithium hydroxide or its hydrate, lithium peroxide, lithium nitrate, lithium carbonate, and the like. From the viewpoint of increasing the powder pH of the particles (A) described later, it is preferred to place more lithium hydroxide. When dry-mixing the metal composite oxide and the lithium compound, a conventional dry mixer or mixing granulation device such as a ball mill or a V-type mixer can be used. During calcination, an electric furnace, a rotary kiln, a tubular furnace, or a pusher furnace may be used, with the oxygen concentration in the gas atmosphere adjusted to 20 mass% or more.

The calcination conditions of the mixed powder are preferably 650-850° C., more preferably 700-800° C., from the viewpoint of avoiding the failure of crystal growth of the obtained lithium composite oxide to cause structural instability, and from the viewpoint of avoiding the collapse of the layered structure of the lithium composite oxide to cause difficulty in the insertion and removal of lithium ions. In addition, the calcination time is preferably 5-20 hours, more preferably 6-10 hours.

The calcination of the mixed powder is preferably carried out in two stages (preliminary calcination and main calcination). By setting the calcination to two stages, in the preliminary calcination, water molecules or carbon dioxide from hydroxides or carbonates in the mixed powder are removed by heating and decomposition, and then the main calcination is carried out, so that lithium composite oxide particles (A) can be obtained efficiently. As a condition for the preliminary calcination, it is preferred to calcine at a calcination temperature of 400~600℃ for more than 1 hour. The calcined product obtained by the preliminary calcination is decomposed and crushed using a mortar, etc., and then mixed with an appropriate amount of binder and granulated, and then supplied to the main calcination. As the conditions for the main calcination, it is preferred to calcine at a calcination temperature of 650-850°C for more than 5 hours.

The calcined product obtained by the main calcination is washed with water, filtered and dried to obtain secondary particles of the lithium composite oxide particles (A). The concentration of the slurry when the calcined product obtained by the main calcination is washed with water is preferably 200 to 4000 g/L, more preferably 500 to 2000 g/L from the viewpoint of suppressing the separation of lithium from the obtained lithium composite oxide particles (A).

The electrical conductivity of the water used for water washing is preferably less than 10 μS/cm, more preferably less than 1 μS/cm, from the viewpoint of preventing precipitation of lithium carbonate in the lithium composite oxide particles (A) due to a large amount of carbon dioxide contained in the water.

Drying is preferably performed in two stages. The first stage of drying is performed at 90°C until the water content (water content measured at a vaporization temperature of 300°C) in the lithium composite oxide secondary particles is 1 mass % or less. Thereafter, the second stage of drying is preferably performed at 120°C or higher.

The average particle size of the primary particles of the lithium composite oxide represented by the above formula (I) and formula (II) is preferably 500 nm or less, more preferably 300 nm or less. The lower limit of the average particle size of the primary particles is not particularly limited, but is preferably 50 nm or more from the viewpoint of operability.

The median particle size of the secondary particles of the lithium composite oxide particles (A) represented by the above formula (I) and formula (II) is preferably 25 μm or less, more preferably 20 μm or less. The lower limit of the median particle size of the secondary particles is not particularly limited, but from the viewpoint of operability, it is preferably 1 μm or more, more preferably 5 μm or more.

In the material used for the positive electrode of the present invention, the median particle size of the secondary particles of the particles (A) represented by formula (I) and formula (II) is smaller than the median particle size of the secondary particles of the particles (B) described later. Preferably, "the median particle size of the secondary particles of the particles (A) is less than 0.80 times the median particle size of the secondary particles of the particles (B)." By setting the median particle size of the secondary particles of the particles (A) to be smaller than the median particle size of the secondary particles of the particles (B), the particles (A) can efficiently fill the gaps between the particles (B), and can effectively prevent metal elements such as manganese or iron from dissolving from the particles (B) at high temperatures, thereby improving the high-temperature storage characteristics of the battery.

In this specification, the average particle size of primary particles means the average value of the particle size (length of the major axis) of dozens of particles measured by electron microscope observation such as SEM or TEM. The median particle size of secondary particles means the median particle size (D50) based on volume measured by a laser diffraction particle size distribution analyzer, which can be measured by the following method: Use ethanol as a dispersion medium, use ultrasound to disperse the particles in ethanol to prepare a measurement sample, measure the particle size distribution by a laser diffraction/scattering particle size distribution measuring device (MT3300EXII manufactured by MicrotracBEL Co., Ltd.), and calculate the median particle size (D50) based on volume.

The median particle size of the secondary particles of particles (A) and (B) tends to be almost the same even in the positive electrode. Therefore, the median particle size of the secondary particles of particles (A) and (B) in the positive electrode can also be measured by, for example, the following method: Use a scanning electron microscope JCM-6000 (manufactured by JEOL Ltd.) at a magnification of ×1000 or 3000 to observe the positive electrode, and use the [Measure Length] function of the observation software (JCM-6000) to measure. The attached EDX device can be used to identify the positive electrode particles by elemental analysis.

The powder pH of the particles (A) represented by the above formula (I) and formula (II) is preferably in the range of 10 to 14, and more preferably 10 to 13. The powder pH of the particles (A) is preferably higher than the powder pH of the particles (B) described later. By increasing the powder pH of the particles (A), it is possible to effectively prevent metal elements such as manganese or iron from dissolving from the particles (B) at high temperatures, thereby further improving the high-temperature storage characteristics of the battery. The mechanism is still unclear, but the inventors speculate as follows: When a lithium-ion secondary battery is exposed to a high temperature of more than 60°C, the trace amount of water mixed into the battery will cause the supporting electrolyte (supporting electrolyte) such as LiPF ₆ generally used in the electrolyte to hydrolyze, and there is a situation where free HF is generated. When free HF attacks the particle (B), it may cause the manganese or iron in the particle (B) to dissolve, which may impair the high temperature storage characteristics of the battery. By making the median particle size smaller than the particle (B) and allowing the particle (A) with a higher powder pH to exist in the gaps between the particles (B), the attack of free HF (acid) can be weakened, and it is believed that the dissolution of metal elements from the particle (B) can be prevented.

By placing more alkaline components in the particle (A) during the manufacturing process, the alkaline residue is retained in the particle (A), thereby increasing the powder pH of the particle (A). In this specification, the powder pH can be measured by the following procedure: Add 1 g of each particle (powder) to 50 mL of pure water, stir with a stirrer at room temperature for 5 minutes, and measure the pH of the suspension after standing for 1 minute using a pH meter.

<Particles (B)> The particles (B) used in the positive electrode of the present invention are polyanion particles having an olivine structure represented by the following formula (III), which contain at least one of lithium, iron and manganese, and phosphoric acid. LiFe _m Mn _n M ³ _o PO ₄ …(III) (In formula (III), M ³ is Co, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd or Gd, and m, n and o are numbers satisfying 0≦m≦1, 0≦n≦1, 0≦o≦0.3, and m+n≠0, and satisfying 2m+2n+(valence of M ³ )×o=2). Carbon may be supported on the surface of the particles.

The lithium-based polyanion particles (B) represented by the above formula (III) can be obtained by the following procedure: (i) mixing a lithium compound and a phosphoric acid compound, and then mixing a metal salt containing at least an iron compound and/or a manganese compound to obtain a mixed solution A; (ii) subjecting the obtained mixed solution A to a hydrothermal reaction to obtain a composite B.

When the lithium-based polyanion particles (B) carry carbon, the following steps may be further performed: (iii) the obtained composite B, carbon source and water are mixed to obtain a mixed solution C; (iv) the obtained mixed solution C is dried to obtain a granulated body D; (v) the obtained granulated body D is calcined in a reducing environment or an inert environment.

As the lithium compound that can be used in step (i), there can be mentioned lithium hydroxide (e.g., LiOH, LiOH·H ₂ O), lithium carbonate, lithium sulfate, and lithium acetate, among which lithium hydroxide is preferred. The lithium compound is preferably in the form of an aqueous solution having a concentration of 5 to 50 parts by mass (more preferably 7 to 45 parts by mass) relative to 100 parts by mass of water. Before mixing the phosphoric acid compound with the lithium compound, the aqueous solution of the lithium compound is preferably stirred for 1 to 15 minutes, more preferably for 3 to 10 minutes. In addition, the temperature of the aqueous solution is preferably 20 to 90° C., more preferably 20 to 70° C.

Examples of the phosphoric acid compound to be mixed with the lithium compound include orthophosphoric acid (H ₃ PO ₄ , phosphoric acid), metaphosphoric acid, pyrophosphoric acid, triphosphoric acid, tetraphosphoric acid, ammonium phosphate, and ammonium hydrogen phosphate. Among them, phosphoric acid is preferably used, and it is preferably used in the form of an aqueous solution with a concentration of 70 to 90 mass %. The mixture of the lithium compound and the phosphoric acid compound preferably contains 2.7 to 3.3 mol of lithium, and more preferably 2.8 to 3.1 mol of lithium, relative to 1 mol of phosphoric acid.

After mixing the phosphoric acid compound, the mixture may be diluted with nitrogen to terminate the reaction in the mixture and generate a precursor of lithium-based polyanion particles (tricalium phosphate, Li ₃ PO ₄ ). The pressure during nitrogen dilution is preferably 0.1-0.2 MPa, more preferably 0.1-0.15 MPa. In addition, the temperature of the mixture after mixing the phosphoric acid compound is preferably 20-80°C, more preferably 20-60°C. The reaction time is preferably 5-60 minutes, more preferably 15-45 minutes. During nitrogen dilution, from the viewpoint of making the reaction proceed well, it is preferably stirred at a stirring speed of 200-700 rpm, more preferably 250-600 rpm.

After mixing the lithium compound and the phosphoric acid compound, a metal salt containing at least an iron compound and/or a manganese compound is added to obtain a mixed solution A. The total amount of the metal salts added is preferably 0.990 to 1.010 mol, more preferably 0.995 to 1.005 mol, relative to 1.000 mol of phosphoric acid ions. If a metal (M ³ ) salt other than an iron compound and a manganese compound is added, the total amount of the iron compound, the manganese compound and the metal (M ^{3 ) salt added is preferably 0.990 to 1.010 mol, more preferably 0.995 to 1.005 mol, relative to 1.000 mol of phosphoric acid in the obtained mixed solution A. The order of adding the iron compound, the manganese compound and the metal (M 3 )} salt is not particularly limited. Furthermore, when necessary, an antioxidant may be added together with the metal salts. As the antioxidant, sodium sulfite (Na ₂ SO ₃ ), sodium hyposulfite (Na ₂ S ₂ O ₄ ), ammonia water, etc. may be used. The amount of the antioxidant added, if excessive, will inhibit the formation of active substances, so it is preferably 0.01 to 1.00 mol, more preferably 0.03 to 0.50 mol, relative to 1.00 mol of the total metal salt.

Examples of the iron compound that can be used include iron acetate, iron nitrate, iron sulfate, etc. These compounds may be used alone or in combination of two or more. Among them, iron sulfate is preferred from the viewpoint of improving the battery characteristics of the lithium-based polyanion particles (B).

As the manganese compound that can be used, manganese acetate, manganese nitrate, manganese sulfate, etc. can be cited. These can be used alone or in combination of two or more. Among them, manganese sulfate is preferred from the viewpoint of improving the battery characteristics of the lithium-based polyanion particles (B).

The particles (B) preferably contain both iron and manganese. That is, in the above formula (iii), it is preferably 0＜m≦1 and 0＜n≦1. Lithium iron manganese phosphate (LMFP) containing both iron and manganese shows the same level of safety as lithium iron phosphate (LFP) containing iron but not manganese. Since the action potential is 0.4~0.5V higher than that of LFP, it is believed that it helps to improve the safety of the battery and increase the energy density. Therefore, when manufacturing the particles (B), it is preferred to use both iron compounds and manganese compounds as metal compounds. When both the iron compound and the manganese compound are used, the molar ratio of the manganese compound to the iron compound (manganese compound: iron compound) is preferably 99:1 to 1:99, more preferably 90:10 to 10:90.

As the metal (M ³ ) salt, sulfates, halides, organic acid salts, and hydrates of metals other than iron and manganese can be used. These can be used alone or in combination of two or more. Among them, sulfates are preferred from the viewpoint of improving battery characteristics.

Next, the mixed solution (A) obtained in step (i) is subjected to a hydrothermal reaction to obtain a composite B (step (ii)). The amount of water used in the hydrothermal reaction is preferably 10 to 50 mol, more preferably 10 to 45 mol, more preferably 10 to 30 mol, and more preferably 12.5 to 25 mol, relative to 1 mol of phosphate ions contained in the mixed solution A, from the viewpoints of the solubility of the metal salt, the ease of stirring, and the efficiency of synthesis.

The temperature during the hydrothermal reaction can be above 100°C, preferably 130-180°C. The hydrothermal reaction is preferably carried out in a pressure-resistant container. If the reaction is carried out at 130-180°C, the pressure is preferably 0.3-0.9 MPa, and if the reaction is carried out at 140-160°C, the pressure is preferably 0.3-0.6 MPa. The reaction time is preferably 0.1-48 hours, more preferably 0.2-24 hours.

The obtained composite B is a lithium-based polyanion particle (B) represented by formula (III). The polyanion particle (B) can be separated by filtering the composite B, washing with water, and drying. As the drying method, freeze drying, vacuum drying, etc. can be cited.

When lithium-based polyanion particles are used to carry carbon, a carbon source is mixed into a mixture containing complex B and water to prepare a mixed solution C (step (iii)). As the carbon source, nanocellulose or a water-soluble carbon material can be cited. Nanocellulose (hereinafter also referred to as "CNF") is a fine cellulose fiber obtained by defibrating plant fibers to reduce the fiber diameter to a nanometer size of about 1 to 100 nm. The so-called water-soluble carbon material means a carbon material that dissolves 0.4 g (calculated as carbon atom conversion amount) or more in 100 g of water at 25°C, preferably 1.0 g or more. As the water-soluble carbon material, for example, one or more selected from sugars, polyols, polyethers and organic acids can be cited. Specifically, examples include monosaccharides such as glucose, fructose, galactose, and mannose; disaccharides such as maltose, sucrose, and cellobiose; polysaccharides such as starch and dextrin; polyols or polyethers such as ethylene glycol, propylene glycol, diethylene glycol, polyethylene glycol, butylene glycol, propylene glycol, polyvinyl alcohol, and glycerol; and organic acids such as citric acid, tartaric acid, and ascorbic acid. Among them, from the viewpoint of high solubility in solvents, glucose, fructose, sucrose, and dextrin are preferred, and glucose is more preferred.

The content of the complex B in the mixed solution C is preferably 10-400 mass parts, more preferably 30-210 mass parts, relative to 100 mass parts of water. The content of the carbon source in the mixed solution C is preferably 0.03-320 mass parts, more preferably 0.2-140 mass parts, further preferably 0.2-64 mass parts, further preferably 0.2-28 mass parts, relative to 100 mass parts of water.

From the viewpoint of uniformly dispersing the complex B and the carbon source, it is preferable to use a disperser to process the mixed solution C. Examples of such dispersers include a disintegrator, a pulper, a low-pressure homogenizer, a high-pressure homogenizer, a grinder, a mill, a ball mill, a jet mill, a short-shaft extruder, a double-shaft extruder, an ultrasonic mixer, and a household juicer mixer. Among them, from the viewpoint of dispersion efficiency, an ultrasonic mixer is preferred. The degree of dispersion uniformity of the mixed solution C can be quantitatively evaluated using, for example, the light transmittance of a UV/visible spectrometer or the viscosity, and can also be simply evaluated by visually confirming whether the turbidity is uniform. The treatment time using the disperser is preferably 0.5 to 6 minutes, more preferably 2 to 5 minutes.

When CNF is used as a carbon source, it is preferable to wet-classify the mixed solution C from the viewpoint of effectively removing CNF in an agglomerated state. In wet classification, a sieve or a commercially available wet classifier can be used. The mesh spacing of the sieve can be changed according to the fiber length of the CNF used, but from the viewpoint of operating efficiency, it is preferably 140-160 μm.

The mixed solution C obtained in step (iii) is dried to obtain granules D (step (iv)). The drying may be performed by spray drying, media flow drying, or the like. The median particle size of the granules D may be adjusted by adjusting the drying conditions.

Next, the granules D obtained in step (iv) are calcined in a reducing environment or an inert environment (step (v)). In this way, the carbon source in the granules D can be carbonized, and lithium-based polyanion particles (B) with carbon on the surface can be obtained. The calcination conditions are preferably 400°C or above, more preferably 400-800°C, preferably 10 minutes to 3 hours, and more preferably 0.5-1 hour, as long as they are in a reducing environment or an inert environment.

The median particle size of the secondary particles of particle (B) is larger than the median particle size of the secondary particles of particle (A). Preferably, "the median particle size of the secondary particles of particle (A) is set to be less than 0.80 times the median particle size of the secondary particles of particle (B)". By setting the median particle size of the secondary particles of particle (B) to be larger than the median particle size of the secondary particles of particle (A), particle (A) can efficiently fill the gaps between particles (B) and can effectively prevent metal elements such as manganese and iron from dissolving from particles (B) at high temperatures, thereby improving the high-temperature storage characteristics of the battery.

Here, the median particle size of the secondary particles of the particle (B) refers to the median particle size of the secondary particles of the particle (B) not carrying carbon when the particle (B) is not carrying carbon, and refers to the median particle size of the secondary particles of the particle (B) including the carried carbon when the particle (B) is carrying carbon.

The median particle size of the secondary particles of particle (B) is not limited thereto, and for example, the median particle size of the secondary particles of particle (B) can be adjusted (increased) by adjusting the amount of air delivered to the nozzle during drying such as spray drying during granulation, or by optimizing the temperature.

The powder pH of the particles (B) is preferably lower than the powder pH of the particles (A). By increasing the powder pH of the particles (A), it is possible to effectively prevent metal elements such as manganese and iron from being eluted from the particles (B) at high temperatures, thereby further improving the high-temperature storage characteristics of the battery.

<Positive electrode material> The obtained particles (A) and particles (B) are mixed in a specified ratio to obtain a positive electrode material for a lithium-ion secondary battery. The mixing method is not particularly limited, as long as a device that can uniformly mix particles (A) and particles (B) is used. Examples include ball mills, sand mills, planetary mixers, high-speed shearing machines, blade mixers, high-speed mixers, etc. The mixing ratio (mass ratio) of particles (A) and particles (B) is preferably 96:4~60:40, more preferably 96:4~70:30, and even more preferably 96:4~80:20 in terms of particles (A): particles (B).

<Positive electrode> The obtained positive electrode material can be used to manufacture a positive electrode for a lithium-ion secondary battery. The positive electrode can be obtained as follows: "Add a solvent such as N-methyl-2-pyrrolidone to the above-mentioned positive electrode material, a conductive aid such as carbon black or carbon nanotubes, and a binder (binder) such as polyvinylidene fluoride, mix them thoroughly to obtain a positive electrode slurry, apply it to a collector such as aluminum foil, compress it with a roller press, and dry it to obtain a positive electrode."

The material constituting the current collector is not particularly limited, but preferably a metal, such as aluminum, nickel, iron, stainless steel, titanium, copper, and other alloys. In addition, a coating material of nickel and aluminum, a coating material of copper and aluminum, or a coating material of a combination of these metals can be preferably used. In addition, a foil in which aluminum is coated on the surface of a metal. From the viewpoint of conductivity or operating potential, aluminum, stainless steel, and copper are preferred.

The size of the current collector is not particularly limited and may be determined according to the intended use. The thickness of the current collector is also not particularly limited and is generally about 1 to 100 μm.

The binder is not particularly limited. For example, in addition to polyvinylidene fluoride (PVDF), there can be cited: polyethylene, polypropylene, polyethylene terephthalate (PET), polyether nitrile, polyacrylonitrile, polyimide, polyamide, cellulose, carboxymethyl cellulose (CMC) and its salt, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene/butadiene rubber (SBR), isoprene rubber, butadiene rubber, ethylene/propylene rubber, ethylene/propylene/diene copolymer, styrene/butadiene/styrene block copolymer and its hydrogenated product, styrene/isoprene/styrene block copolymer and its hydrogenated product, etc. thermoplastic polymer, polytetrafluoroethylene (PTFE), tetrafluoroethylene/hexafluoropropylene copolymer (FEP), tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer (PFA), ethylene/tetrafluoroethylene copolymer (ETFE) , polychlorotrifluoroethylene (PCTFE), ethylene/chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF) and other fluororesins, vinylidene fluoride/hexafluoropropylene fluororubber (VDF-HFP fluororubber), vinylidene fluoride/hexafluoropropylene/tetrafluoroethylene fluororubber (VDF-HFP-TFE fluororubber), vinylidene fluoride/pentafluoropropylene fluororubber (VDF-P The adhesive may be a vinylidene fluoride rubber (VDF-PFP-TFE), a vinylidene fluoride/pentafluoropropylene/tetrafluoroethylene fluoride rubber (VDF-PFMVE-TFE), a vinylidene fluoride/perfluoromethyl vinyl ether/tetrafluoroethylene fluoride rubber (VDF-PFMVE-TFE), a vinylidene fluoride/chlorotrifluoroethylene fluoride rubber (VDF-CTFE), or other vinylidene fluoride rubbers, epoxy resins, etc. Among these, polyvinylidene fluoride is the most preferred. One of these adhesives may be used alone, or two or more may be used in combination.

The solvent used in preparing the cathode slurry is not particularly limited, and examples thereof include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, methylformamide, cyclohexane, hexane, and the like.

The composition of the positive electrode slurry is not particularly limited. For example, relative to the total solid content (components other than the solvent) of the slurry, the binder is in the range of 0.1-15 mass%, preferably 1-10 mass%, more preferably 2-8 mass%, and particularly preferably 3-8 mass%. If the viewpoint of obtaining sufficient battery capacity is considered, the positive electrode material is in the range of 70-99.5 mass%, preferably 75-99 mass%, more preferably 80-98 mass%, and particularly preferably 90-97.5 mass%. As long as it is within the above range, it is believed that sufficient bonding strength can be exhibited. The content of the conductive aid is preferably in the range of 0-10 mass%, and more preferably in the range of 3-7 mass%.

<Lithium ion secondary battery> The lithium ion secondary battery of the present invention comprises at least: the above-mentioned positive electrode, negative electrode, electrolyte, and separator. The negative electrode can be any material that can absorb and store lithium ions during charging and release lithium ions during discharge, and the material composition is not particularly limited. Examples include carbon materials such as lithium metal, graphite, or amorphous carbon. It is preferred to use a carbon material as an intercalated material that can electrochemically absorb, store, and release lithium ions.

The supporting electrolyte is dissolved in an organic solvent to obtain an electrolyte solution. The organic solvent is not particularly limited as long as it is an organic solvent used in the electrolyte solution of a common lithium ion secondary battery. For example, carbonates, hydrocarbon halides, ethers, ketones, nitriles, lactones, oxolane compounds, etc. can be used.

The type of supporting electrolyte is not particularly limited, and preferably includes: an inorganic salt selected from LiPF ₆ , LiBF ₄ , LiClO ₄ , and LiAsF ₆ , a derivative of the inorganic salt, an organic salt selected from LiSO ₃ CF ₃ , LiC(SO ₃ CF ₃ ) ₂ , LiN(SO ₃ CF ₃ ) ₂ , LiN(SO ₂ C ₂ F ₅ ) ₂ , and LiN(SO ₂ CF ₃ )(SO ₂ C ₄ F ₉ ), and at least one of the organic salts. Among them, a salt containing fluorine is preferred, and LiPF ₆ is particularly preferred, because it can significantly exhibit the effect of enhancing the high temperature stability brought by the positive electrode of the present invention.

The separator is used to electrically insulate the positive electrode and the negative electrode and to retain the electrolyte. For example, a porous synthetic resin membrane, especially a porous membrane made of polyolefin polymers (polyethylene, polypropylene).

The shape of the lithium-ion secondary battery having the above structure is not particularly limited, and may be various shapes such as coin-shaped, cylindrical, square, etc., or an indefinite shape enclosed in a laminate outer package. [Example]

The present invention is specifically described below based on examples, but the present invention is not limited to these examples. <Lithium composite oxide particles (A)> As particles (A), commercially available particles (LiNi _0.5 Mn _0.3 Co _0.2 O ₂ , secondary particle median diameter (D50): 10.5 μm, powder pH: pH 10.9) were used.

<Lithium-based polyanion particles (B1)> As particles (B1), commercially available particles (LiMn _0.7 Fe _0.3 PO ₄ containing 1.5 mass % of carbon, median particle size (D50) of secondary particles: 14.8 μm, powder pH: pH 10.4) were used.

<Lithium-based polyanion particles (B2)> As particles (B2), commercially available particles (LiMn _0.7 Fe _0.3 PO ₄ containing 1.5 mass % of carbon, median particle size (D50) of secondary particles: 8.0 μm, powder pH: pH 10.4) were used.

<Example 1> The above particles (A) and particles (B1) are mixed at a mass ratio of 80:20 to prepare the positive electrode material of Example 1.

<Comparative Example 1> The above particles (A) and particles (B2) were mixed at a mass ratio of 80:20 to prepare the positive electrode material of Comparative Example 1.

<Comparative Example 2> Only the above-mentioned particles (B2) were used to prepare the positive electrode material of Comparative Example 2. <Manufacture of lithium-ion secondary battery> A layer comprising 97.5 mass% of each positive electrode material of Example 1 and Comparative Examples 1 and 2, 0.5 mass% of carbon nanotubes, and 2 mass% of polyvinylidene fluoride was coated on both sides of a 16μm thick aluminum foil in a single-sided coating amount of 18mg/cm2 for Example 1 and Comparative Example ¹ and 21mg/cm2 for Comparative Example ² , and then pressed and vacuum dried to obtain a double-sided coated positive electrode of size 30mm×40mm, one sheet each.

A layer containing 95 mass % of graphite, 2.5 mass % of styrene butadiene rubber, and 2.5 mass % of carboxymethyl cellulose was coated on both sides of an 8 μm thick copper foil at a single-sided coating weight of 9.8 mg/cm ² , and then pressed and vacuum dried to obtain two double-sided coated negative electrodes with a size of 32 mm × 42 mm.

The obtained positive electrode, negative electrode, and two polypropylene separators with a thickness of 28 μm and a size of 35 mm × 45 mm were laminated in the order of negative electrode-separator-positive electrode-separator-negative electrode, inserted into a laminated outer casing, injected with 0.6 mL of ethyl carbonate + diethyl carbonate solution (3:7) containing ₁ M LiPF6, and sealed to produce a lithium ion secondary battery.

<60℃ Storage Test> (1) The capacity of each lithium-ion secondary battery obtained at 25℃ and 13mA was confirmed under the following conditions. The discharge capacity at this time was set to (1): Charge 13mA-4.2V/CCCV (6.5h) Charge stop 10 minutes Discharge 13mA-2.5V/CC Discharge stop 10 minutes (2) Next, store at 60℃, 7 days, SOC 100%. The conditions are as follows: 25℃ charge 13mA-4.2V/CCCV (6.5h) 25℃ charge stop 10 minutes 60℃, 7-day storage 25℃ discharge 13mA-2.5V/CC 25℃ discharge stop 10 minutes (3) Next, similar to (1), confirm the capacity at 25℃, 13mA under the following conditions. The discharge capacity at this time is set to (3): Charge 13mA-4.2V/CCCV(6.5h) Charge stop 10 minutes Discharge 13mA-2.5V/CC Discharge stop 10 minutes (4) The discharge capacity maintenance rate after high temperature (60°C) maintenance is calculated by the following formula: Discharge capacity maintenance rate (%) = (discharge capacity of (3)/discharge capacity of (1)) × 100.

The results of the discharge capacity maintenance rate of each lithium ion secondary battery are as follows: Battery using the positive electrode material of Example 1: 95% Battery using the positive electrode material of Comparative Example 1: 85% Battery using the positive electrode material of Comparative Example 2: 80% It can be seen that the lithium ion secondary battery using the positive electrode of the present invention is less likely to have a lower discharge capacity than the lithium ion secondary battery of the comparative example even after being kept at a high temperature for a certain period of time. By using the positive electrode of the present invention, a lithium ion secondary battery with higher stability at high temperatures can be formed.

Claims (7)

A positive electrode for a lithium ion secondary battery comprises a lithium composite oxide particle (A) represented by the following formula (I) or (II) and a lithium-based polyanion particle (B) represented by the following formula (III): LiNi _a Co _b Mn _c M ¹ _x O ₂ … (I) In the formula (I), M ¹ is one or more elements selected from Mg, Ti, Nb, Fe, Cr, Si, Al, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi and Ge, a, b, c and x are numbers satisfying 0.3≦a<1, 0<b≦0.7, 0<c≦0.7, 0≦x≦0.3, and 3a+3b+3c+(valence of M ¹ )×x=3, LiNi _d Co _a Al _f M ² _y O ₂ …(II) In formula (II), M ² is one or more elements selected from Mg, Ti, Nb, Fe, Cr, Si, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi and Ge, d, e, f, y are numbers satisfying 0.4≦d<1, 0<e≦0.6, 0<f≦0.3, 0≦y≦0.3, and 3d+3e+3f+(valence of M ² )×y=3, LiFe _m Mn _n M ³ _o PO ₄ …(III) In formula (III), M ^{M 3} is Co, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd or Gd, m, n and o are numbers satisfying 0≦m≦1, 0≦n≦1, 0≦o≦0.3, and m+n≠0, and satisfying 2m+2n+(the valence of M ³ )×o=2, the mixing ratio (mass ratio) of the aforementioned particles (A) and the aforementioned particles (B) is 96:4~60:40 in terms of particles (A): particles (B), the median particle size of the secondary particles of the aforementioned particles (A) is smaller than the median particle size of the secondary particles of the aforementioned particles (B), and the powder pH of the aforementioned particles (A) is larger than the powder pH of the aforementioned particles (B). In the positive electrode for lithium-ion secondary battery of claim 1, the median particle size of the secondary particles of the aforementioned particles (A) is less than 0.80 times the median particle size of the secondary particles of the aforementioned particles (B). A positive electrode for a lithium ion secondary battery as claimed in claim 1 or 2, wherein the aforementioned particle (A) is a compound represented by the aforementioned formula (I). As in claim 3, the positive electrode for lithium-ion secondary battery, wherein x in the aforementioned formula (I) is 0. A positive electrode for a lithium ion secondary battery as claimed in claim 1 or 2, wherein the aforementioned particle (B) is a compound satisfying 0<m≦1, 0<n≦1 and o=0 in the aforementioned formula (III). A lithium ion secondary battery having a positive electrode for a lithium ion secondary battery as described in any one of claims 1 to 5. As in claim 6, the lithium ion secondary battery has a lithium salt containing fluorine as an electrolyte.