Positive electrode materials for lithium-ion secondary batteries and lithium-ion secondary batteries

Abstract

A positive electrode material for lithium-ion secondary batteries comprises an aggregate composition of positive electrode active material particles, wherein the positive electrode active material particles are in the general formula (1) (Li x A y D z The cathode material is formed by forming a carbonaceous coating on the surface of lithium metal phosphate particles (e.g., Co, Mn, Ni, Fe, etc.). The aforementioned aggregate composition comprises: a first aggregate with a grain diameter of 200 nm or more and 2000 nm or less, and a second aggregate with a grain diameter of 50 nm or less. This cathode material exhibits high Li-ion conductivity, excellent loading characteristics, and can suppress the decrease in capacity retention associated with the number of cycles during battery charge-discharge cycles.

Description

Technical Field

[0001] This invention relates to positive electrode materials for lithium-ion secondary batteries and lithium-ion secondary batteries. Background Technology

[0002] Compared to lead-acid and nickel-metal hydride batteries, lithium-ion rechargeable batteries have higher energy and power densities, and are used in various applications such as backup power for small electronic devices like smartphones, and power tools. Furthermore, the practical application of high-capacity lithium-ion rechargeable batteries is progressing, including in-vehicle use in electric vehicles, and storage for renewable energy sources such as solar and wind power generation.

[0003] Lithium-ion secondary batteries typically consist of a positive electrode, a negative electrode, an electrolyte, and a separator.

[0004] As positive electrode active materials, lithium metal oxides such as lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), and lithium iron phosphate (LiFePO4), which have the property of reversibly inserting / extracting lithium ions, were used to improve the battery from various perspectives, such as increasing battery capacity, extending battery life, improving safety, and reducing costs.

[0005] The aforementioned positive electrode active material, lithium iron phosphate (LiFePO4), uses abundant and inexpensive iron, making it a material that is easy to reduce costs. In addition, due to the strong covalent bond between phosphorus and oxygen, lithium iron phosphate does not release oxygen at high temperatures, thus possessing excellent properties not found in oxide-based positive electrode materials such as lithium cobalt oxide, such as outstanding safety.

[0006] On the other hand, lithium iron phosphate (LFP) exhibits inferior input / output characteristics compared to oxide-based cathode materials due to the low diffusion and electronic conductivity of Li ions. This poor performance becomes more pronounced at lower battery operating temperatures, thus making LFP unsuitable for automotive applications such as hybrid vehicles that require high input / output characteristics in low-temperature regions.

[0007] LiMPO4 (where M is a divalent metal element), represented by lithium iron phosphate, has an olivine structure. Due to the low diffusion and electronic conductivity of Li ions, the charge-discharge characteristics of LiMPO4 can be improved by miniaturizing the primary particles and coating the surface of each primary particle with a carbon film.

[0008] In addition, the specific surface area of ​​LiMPO4 after primary particle micronization is large, so a large amount of binder is required for the electrode composite slurry. Therefore, the primary particles coated with carbonaceous film are usually granulated to form secondary particles, thereby improving the workability of the powder and the properties of the electrode composite slurry.

[0009] The miniaturization of primary particles includes: methods for controlling nucleation and grain growth conditions during LiMPO4 synthesis, and methods for mechanically crushing coarse primary particles. The former is mainly used in wet synthesis methods, while the latter is used in solid-phase synthesis methods.

[0010] For example, Patent Document 1 discloses a method for manufacturing a lithium-ion battery cathode material composed of a LiMPO4 and carbon composite. The method is characterized by using LiMPO4 powder (M is a divalent metal element) with a particle size of less than 1 μm as raw material, adding a mixed carbon source to it to obtain a mixture, sintering the obtained mixture under vacuum or inactive atmosphere by heating, and crushing the block of the obtained sintered body.

[0011] In addition, Patent Document 2 discloses a positive electrode active material for lithium-ion batteries, characterized in that it is a positive electrode active material for lithium-ion batteries containing lithium iron phosphate microparticles. The positive electrode active material does not form secondary aggregates but exists in the form of single particles. The average particle size of the aforementioned lithium iron phosphate microparticles is 5~50nm, and more than 90% of the total number of microparticles have a particle size in the range of 3~70nm.

[0012] In addition, in order to improve battery characteristics, an electrode using a mixture of different types of positive electrode active materials was studied.

[0013] For example, Patent Document 3 discloses an electrode for a secondary battery, which has a positive electrode mixture layer containing at least an olivine-based positive electrode active material and a layered oxide-based positive electrode active material as positive electrode active materials. The total weight fraction of the aforementioned positive electrode active materials in the aforementioned positive electrode mixture layer is 80% by weight or more and 99% by weight or less, the weight fraction of the aforementioned olivine-based positive electrode active material is 10% by weight or more and 65% by weight or less, the weight fraction of the aforementioned layered oxide-based positive electrode active material is 30% by weight or more and 80% by weight or less, and the aforementioned positive electrode mixture layer contains graphene.

[0014] Existing technical documents

[0015] Patent documents

[0016] Patent Document 1: Japanese Patent Application Publication No. 2009-81002

[0017] Patent Document 2: Japanese Patent Application Publication No. 2008-159495

[0018] Patent Document 3: International Publication No. 2020 / 066909 Summary of the Invention

[0019] The problem the invention aims to solve

[0020] As in Patent Document 1 or Patent Document 2, the conductivity of Li ions can be improved by miniaturizing the primary particles of the positive electrode active material, but the following problems exist: the filling capacity of the positive electrode active material in the electrode film is impaired, making it difficult to increase the electrode film density, thereby reducing the energy density of the battery; by miniaturizing the primary particles of the positive electrode active material, the contact area between the aluminum foil, which serves as the current collector, and the positive electrode film is reduced, thereby impairing the adhesion of the electrode film.

[0021] Furthermore, it is known that in battery reactions, Fe ions dissolve from the surface of LiFePO4 primary particles into the electrolyte through a redox reaction on the particle surface. It is also known that if these dissolved Fe ions undergo reduction and precipitation on the carbon anode surface, they will hinder the movement of lithium ions incorporated into the anode, resulting in a decrease in discharge capacity with increasing charge-discharge cycles. It is generally believed that by refining the primary particles of LiFePO4, the contact area with the electrolyte increases, thereby increasing the proportion of Fe ions dissolved per unit weight of LiFePO4.

[0022] Furthermore, in Patent Document 3, by mixing olivine-based positive electrode active materials with high safety and layered oxide-based positive electrode active materials with high energy density, a balance between safety and high energy density can be achieved. However, although energy density and capacity retention were studied, improvements in loading characteristics were not investigated.

[0023] The present invention was made in view of this actual situation, and its object is to provide a positive electrode material for lithium-ion secondary batteries and a lithium-ion secondary battery using the positive electrode material, wherein the positive electrode material for lithium-ion secondary batteries has high Li-ion conductivity, excellent load characteristics, and can suppress the decrease in capacity retention rate associated with the number of cycles during charge-discharge cycles of the battery.

[0024] Solution for solving the problem

[0025] In order to solve the above-mentioned problem, the inventors conducted in-depth research and found that the problem can be solved by the following invention.

[0026] [1] A positive electrode material for lithium-ion secondary batteries, comprising an aggregate composition of positive electrode active material particles, wherein the positive electrode active material particles are formed by forming a carbonaceous coating on the surface of lithium metal phosphate particles as shown in the following general formula (1).

[0027] The aforementioned aggregate composition comprises:

[0028] The aforementioned positive electrode active material particles are a first aggregate with a grain diameter of 200 nm or more and 2000 nm or less, and a second aggregate with a grain diameter of 50 nm or less.

[0029] Li x Ay D z PO4 (1)

[0030] Wherein, A is at least one selected from the group consisting of Co, Mn, Ni, Fe, Cu, and Cr; D is at least one selected from the group consisting of Mg, Ca, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, Sc, and Y; 0.9 <x<1.1、0<y≤1、0≤z<1、0.9<y+z<1.1。

[0031] [2] According to the positive electrode material for lithium-ion secondary batteries described in [1], the average particle size of the first aggregate and the average particle size of the second aggregate are each independently 0.5 μm or more and 50 μm or less.

[0032] [3] According to the positive electrode material for lithium-ion secondary batteries described in [1] or [2], the density of the electrode film obtained by coating the electrode slurry onto aluminum foil and drying it is 1.1 g / cm³. 3 Above and 2.0 g / cm 3 The electrode slurry described below is obtained by mixing the aforementioned positive electrode material, carbon particles and binder in an N-methyl-2-pyrrolidone solvent.

[0033] [4] The positive electrode material for lithium-ion secondary batteries according to any one of [1] to [3], wherein, for a pouch cell whose structure is to use an electrode containing the aforementioned positive electrode material as a positive electrode, an electrode containing a carbon-based negative electrode material as a negative electrode, a separator is disposed between the aforementioned positive electrode and the aforementioned negative electrode, and the interior is filled with electrolyte, after performing a repeating unit consisting of 1C full charge and 1C full discharge 500 times at 60°C, the amount of metal contained in the aforementioned negative electrode obtained by disassembling the aforementioned pouch cell is less than 1,000 ppm by mass of the lithium metal phosphate particles of general formula (1) contained in the first aggregate and the aforementioned second aggregate in the aforementioned positive electrode.

[0034] [5] A lithium-ion secondary battery having at least a positive electrode, a negative electrode, and an electrolyte.

[0035] The aforementioned positive electrode includes any one of [1] to [4] for use as a positive electrode material in lithium-ion secondary batteries.

[0036] The effects of the invention

[0037] According to the present invention, a positive electrode material for a lithium-ion secondary battery and a lithium-ion secondary battery using the positive electrode material can be provided. The positive electrode material for the lithium-ion secondary battery has high Li-ion conductivity, excellent load characteristics, and can suppress the decrease in capacity retention rate associated with the number of cycles during charge-discharge cycles of the battery. Detailed Implementation

[0038] The positive electrode material for lithium-ion secondary batteries and the lithium-ion secondary battery of the present invention will be described. It should be noted that this embodiment is specifically described to better understand the spirit of the invention, and unless otherwise specified, the invention is not limited.

[0039] It should be noted that, unless otherwise specified, the numerical range “lower limit to upper limit” recorded in this instruction manual refers to the value above the lower limit and below the upper limit.

[0040] Furthermore, the upper and lower limits of the numerical ranges described in this specification can be combined arbitrarily. For example, if "A~B" and "C~D" are described as numerical ranges, the numerical ranges of "A~D" and "C~B" are also included within the scope of this disclosure. Similarly, for example, if "A and below B" and "C and below D" are described as numerical ranges, the numerical ranges of "A and below D" and "C and below B" are also included within the scope of this disclosure.

[0041] 1. Positive electrode material for lithium-ion secondary batteries

[0042] The positive electrode material for lithium-ion secondary batteries in this embodiment contains a positive electrode material comprising an aggregate composition of positive electrode active material particles, wherein the positive electrode active material particles are formed by forming a carbonaceous coating on the surface of lithium metal phosphate particles as shown in general formula (1), and the aggregate composition comprises a first aggregate and a second aggregate.

[0043] Hereinafter, the positive electrode material for lithium-ion secondary batteries will sometimes be abbreviated as "positive electrode material".

[0044] In addition, lithium metal phosphate represented by general formula (1) is sometimes simply referred to as "lithium metal phosphate". It should be noted that the "metal" in "lithium metal phosphate" refers to "A" in general formula (1). y D z ".

[0045] [First Aggregate]

[0046] The first aggregate is an aggregate of positive electrode active material particles formed by forming a carbonaceous coating on the surface of lithium metal phosphate particles as shown in general formula (1), wherein the grain diameter of the positive electrode active material particles is 200 nm or more and 2000 nm or less. That is, the positive electrode active material particles, which are primary particles with a grain diameter of 200 nm or more and 2000 nm or less, aggregate to form secondary particles as aggregates of positive electrode active material particles.

[0047] It is generally known that hydrogen fluoride, produced during the reaction of fluorine-containing electrolytes with water, causes metals to dissolve from the positive electrode active material. Additionally, it is believed that in olivine-type phosphate compounds, during battery charging and discharging, electrochemical reactions occur through a two-phase reaction between a Li-rich phase and a Li-depleted phase. At the phase boundary between the Li-rich and Li-depleted phases, the lattice strain becomes extremely large, and therefore, the positive electrode active material at this boundary dissolves due to a redox reaction with hydrogen fluoride, resulting in metal dissolution.

[0048] Hereinafter, the boundary between the rich Li phase and the poor Li phase will sometimes be abbreviated as "phase boundary".

[0049] If the grain diameter of the positive electrode active material particles in the first aggregate is 200 nm or larger, the grain diameter is sufficiently large, thus mitigating the lattice strain at the phase boundaries. Furthermore, the area of ​​the phase boundaries is significantly reduced relative to the volume of the positive electrode active material, thereby decreasing the amount of metal dissolution to a negligible level. Additionally, it reduces porosity within the positive electrode film, thereby increasing the energy density when used in batteries.

[0050] On the other hand, by setting the grain diameter of the positive electrode active material particles in the first aggregate to less than 2000 nm, in the case of coexistence of Li-rich phase and Li-poor phase during the charging and discharging stage, cracking caused by crystal strain at the outer edge of the grain in a direction perpendicular to the crystal surface is suppressed, thereby improving the battery life characteristics.

[0051] From the viewpoint of improving the energy density of the battery and suppressing metal dissolution, the grain diameter of the positive electrode active material particles in the first aggregate is preferably 200 nm or more and 1800 nm or less, more preferably 200 nm or more and 1700 nm or less.

[0052] When the lithium metal phosphate is LiFePO4, from the viewpoint of improving capacity retention, the grain diameter of the positive electrode active material particles in the first aggregate is more preferably 500 nm or more and 1700 nm or less, and even more preferably 600 nm or more and 1700 nm or less.

[0053] In lithium metal phosphate, LiFe 0.3 Mn 0.7 In the case of PO4, from the viewpoint of improving capacity retention, the grain diameter of the positive active material particles in the first aggregate is more preferably 230 nm or more and 800 nm or less, and even more preferably 250 nm or more and 600 nm or less.

[0054] [Second aggregate]

[0055] The second aggregate is an aggregate of positive electrode active material particles formed by forming a carbonaceous coating on the surface of lithium metal phosphate particles as shown in general formula (1), wherein the grain diameter of the positive electrode active material particles is less than 50 nm. That is, positive electrode active material particles, which are primary particles with a grain diameter of less than 50 nm, aggregate to form secondary particles that are aggregates of positive electrode active material particles.

[0056] If the grain diameter of the positive electrode active material particles in the second aggregate is less than 50 nm, then due to the high surface energy, the phase separation of the Li-rich and Li-depleted phases within a single grain, as described above, will not occur. During the charge and discharge reactions of the battery, it becomes a solid solution phase in which the Li-rich and Li-depleted phases coexist within a single grain. As a result, when the battery reaction stops, Li crossover occurs between the grains, resulting in a mixed morphology of primary particles of the Li-rich and Li-depleted phases. Therefore, phase boundaries are difficult to form within the grains, and the amount of metal dissolution is reduced to a negligible level. In addition, a large contact field with the electrolyte can be ensured, resulting in high Li-ion conductivity.

[0057] Hereinafter, the charging and discharging reaction of a battery will sometimes be abbreviated as "battery reaction".

[0058] From the viewpoint of high Li ion conductivity and suppression of metal dissolution, the grain diameter of the positive electrode active material particles in the second aggregate is preferably 48 nm or less, and more preferably 45 nm or less.

[0059] There is no particular limitation on the lower limit of the grain diameter of the positive electrode active material particles in the second aggregate. From the viewpoint of improving crystallinity to improve battery characteristics, it can be preferably set to 10 nm or more, and more preferably 15 nm or more.

[0060] As described above, in cathode materials containing olivine-type phosphate compounds, by using different types of aggregates with grain diameters of 50 nm or less and 200 nm or more but less than 2000 nm for the cathode active material particles, metal dissolution accompanying the battery reaction can be suppressed. For example, in a cycle test of a pouch cell with a cathode and a cathode as described later, the amount of metal contained in the cathode can be kept below 1,000 ppm by mass relative to the total mass of the cathode active material particles in the cathode material contained in the cathode, i.e., the total mass of the cathode active material particles contained in the first aggregate and the second aggregate, thereby suppressing battery degradation accompanied by metal dissolution from the cathode active material.

[0061] Furthermore, the amount of additives used when forming an insulating organic polymer coating on the negative electrode surface, such as vinylene carbide, which is added to suppress the reduction and precipitation of metals dissolved in the electrolyte inside the battery to the negative electrode, can be kept low. As a result, battery durability can be improved, and Li ion conductivity can be increased to enhance load characteristics.

[0062] Here, the amount of metal dissolved from the positive electrode material during the battery reaction can be evaluated, for example, by a pouch cell configured as follows: an electrode containing the positive electrode material of this embodiment is used as the positive electrode, an electrode containing a carbon-based negative electrode material is used as the negative electrode, a separator is disposed between the aforementioned positive electrode and the aforementioned negative electrode, and the interior is filled with electrolyte.

[0063] The positive electrode is obtained by coating an electrode slurry onto an aluminum foil, drying it, and then pressing it under uniaxial pressure of 600 kgf. The electrode slurry is obtained by mixing the positive electrode material of this embodiment, carbon particles, and a binder in an N-methyl-2-pyrrolidone solvent. After subjecting the manufactured pouch cell to 500 cycles of full charging and full discharging at 1C at 60°C, the pouch cell is disassembled to remove the negative electrode. The amount of metal (Ma) contained in the removed negative electrode is quantitatively analyzed using, for example, high-frequency inductively coupled plasma (ICP) emission spectroscopy. Furthermore, by calculating the total mass (Mc) of lithium metal phosphate particles contained in the first and second aggregates of the positive electrode, and determining the ratio of the metal amount (Ma) to the total mass (Mc) (Ma / Mc), the amount of metal leaching can be evaluated.

[0064] It should be noted that the total mass (Mc) of lithium metal phosphate particles contained in the first and second aggregates in the positive electrode is usually unchanged before and after the manufacturing of the pouch cell.

[0065] In the aforementioned "total mass (Mc) of lithium metal phosphate particles contained in the first and second aggregates in the positive electrode," the first and second aggregates, as described above, refer to aggregates of positive electrode active material particles formed by a carbonaceous coating on the surface of lithium metal phosphate particles. On the other hand, the total mass (Mc) of lithium metal phosphate particles refers to the total mass of the lithium metal phosphate particles themselves without a carbonaceous coating. Since the carbonaceous coating in the electrode is difficult to analyze, the mass of the lithium metal phosphate particles after deducting the analyzable carbonaceous coating is used in the calculation of Mc. More specifically, for example, in the case where the chemical composition of lithium metal phosphate is LiFePO4, the sum of the masses of the main constituent elements Li, Fe, P, and O, and the masses of impurity elements, is Mc.

[0066] If metal ions dissolved from the positive electrode are reduced and deposited at the negative electrode, the insulating properties of the deposited film, commonly known as the solid electrolyte interface (SEI), composed of electrolyte decomposition products containing Li-containing compounds, are compromised, and a new SEI is formed on the surface of the negative electrode particles. As the amount of SEI deposited increases, the Li in the electrolyte... + The number of ions decreases. On the other hand, the Li in the electrolyte... + In the case of a reduced number of ions, PF6 becomes a counterion. - Ions remain in the electrolyte. Therefore, in order to maintain electroneutrality, Li... + Ions are released from the positive electrode active material particles into the electrolyte to compensate for this, resulting in a reduction in the total amount of Li in the positive electrode active material particles during charging and discharging. This leads to a decrease in battery capacity (battery degradation). Therefore, under repeated charge-discharge tests under the same conditions, the higher the amount of metal dissolved in the positive electrode material, the greater the degree of battery degradation. Thus, the impact on battery degradation is determined by comparing the amount of metal contained in the negative electrode with the total mass of lithium metal phosphate particles in the positive electrode.

[0067] The electrodes for lithium-ion secondary batteries in this embodiment, as described above, all contain a first aggregate and a second aggregate in which metal dissolution during the battery reaction is suppressed.

[0068] The positive electrode active material particles of the first aggregate have a grain diameter of 200 nm to 2000 nm, which suppresses the formation of cracks in the positive electrode active material particles and the decrease in capacity retention during charge-discharge cycles. Furthermore, it can improve the energy density when used in batteries. On the other hand, the positive electrode active material particles of the second aggregate have a grain diameter of 50 nm or less, exhibiting high Li-ion conductivity and excellent loading characteristics.

[0069] Therefore, the electrode for lithium-ion secondary batteries of this embodiment, by containing an aggregate composition comprising a first aggregate and a second aggregate, can exhibit mutually excellent battery characteristics, has high Li-ion conductivity, excellent load characteristics, and can suppress the decrease in capacity retention rate associated with the number of cycles during charge-discharge cycles of the battery.

[0070] Hereinafter, we will describe the common items in the first aggregate and the second aggregate contained in the positive electrode material for lithium-ion secondary batteries. Hereinafter, the "first aggregate and the second aggregate" will sometimes be referred to collectively as the "aggregate of this embodiment".

[0071] [composition]

[0072] In this embodiment, the first aggregate and the second aggregate contained in the electrode for a lithium-ion secondary battery are both aggregates of positive electrode active material particles formed by forming a carbonaceous coating on the surface of lithium metal phosphate particles as shown in the following general formula (1).

[0073] Li x A y D z PO4 (1)

[0074] Wherein, A is at least one selected from the group consisting of Co, Mn, Ni, Fe, Cu, and Cr; D is at least one selected from the group consisting of Mg, Ca, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, Sc, and Y; 0.9 <x<1.1、0<y≤1、0≤z<1、0.9<y+z<1.1。

[0075] There are no particular limitations on the composition of lithium metal phosphate particles as long as they have the above-mentioned structure, but they are preferably composed of transition metal lithium phosphate compounds with an olivine structure.

[0076] In general formula (1), A is preferably Co, Mn, Ni, and Fe, and more preferably Co, Mn, and Fe. Additionally, D is preferably Mg, Ca, Sr, Ba, Ti, Zn, and Al. By making the lithium metal phosphate particles into olivine-type phosphate compounds containing these elements, a cathode composite material layer capable of achieving higher discharge potential and higher safety can be fabricated. Furthermore, these materials are preferred due to their abundant resources.

[0077] From the perspective of high discharge capacity and high energy density, lithium metal phosphate particles can be configured as olivine-type phosphate compounds as shown in the following general formula (2).

[0078] Li x2 Fe y2 Mn 1-y2-z2 M z2 PO4 (2)

[0079] [Where M is selected from at least one of Mg, Ca, Co, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, Sc, and Y, 0.9] <x2<1.1、0.05≤y2≤1.0、0≤z2≤0.14。]

[0080] The first and second aggregates can be formed independently using lithium metal phosphate particles with different compositions, or they can be formed using lithium metal phosphate particles with the same composition.

[0081] There are no particular limitations on the mixing of the first aggregate and the second aggregate. For example, the mass ratio (a1:a2) of the first aggregate (a1) to the second aggregate (a2) can be 1:9 to 9:1. In other words, a1 / a2 can be 1 / 9 to 9 / 1.

[0082] When the lithium metal phosphate particles are LiFePO4, from the viewpoint of improving capacity retention, the mass ratio (a1:a2) of the first aggregate (a1) to the second aggregate (a2) is preferably 1:9 to 5:5 (a1 / a2 is 1 / 9 to 5 / 5), and from the viewpoint of simultaneously improving capacity retention and discharge capacity ratio, it is more preferably 1:9 to 4:6 (a1 / a2 is 1 / 9 to 4 / 6).

[0083] LiFe phosphate metal particles are LiFe 0.3 Mn 0.7 In the case of PO4, from the viewpoint of improving capacity retention, the mass ratio (a1:a2) of the first aggregate (a1) to the second aggregate (a2) is preferably 2:8 to 8:2 (a1 / a2 is 2 / 8 to 8 / 2), and more preferably 3:7 to 7:3 (a1 / a2 is 3 / 7 to 7 / 3).

[0084] [Particle structure]

[0085] The cathode material in this embodiment is composed of secondary particles, which are aggregates of the cathode active material particles that serve as the primary particles described above. The cathode material in this embodiment may also contain independent primary particles.

[0086] The shape of the primary particles of the positive electrode active material is not particularly limited, but spherical shape is preferred, and more preferably round. By making the primary particles of the positive electrode active material spherical, the amount of solvent used in preparing the positive electrode material paste can be reduced, and the positive electrode material paste can be easily coated onto the current collector. It should be noted that the positive electrode material paste can be prepared, for example, by mixing the positive electrode material of this embodiment, carbon particles, binder resin (binder), and solvent.

[0087] The average particle size of the aggregates can be 0.5 μm or more and 50 μm or less in both the first and second aggregates. By making the average particle size of the aggregates 0.5 μm or more, preferably 1.0 μm or more, and more preferably 1.5 μm or more, when preparing a cathode material paste by mixing the cathode material, conductive additive, binder, and solvent, a large amount of conductive additive and binder is required. This increases the battery capacity per unit mass of the cathode composite layer in the cathode of the lithium-ion secondary battery. On the other hand, by making the average particle size of the aggregates 50 μm or less, preferably 25 μm or less, and more preferably 15 μm or less, the dispersibility and uniformity of the conductive additive and binder in the aforementioned cathode composite layer can be improved. This results in a higher discharge capacity during high-speed charge and discharge of the lithium-ion secondary battery.

[0088] When the lithium metal phosphate particles are LiFePO4, from the viewpoint of improving capacity retention, the average particle size of the first aggregate is more preferably 5 μm or more and 15 μm or less, and even more preferably 9 μm or more and 15 μm or less; from the viewpoint of improving capacity retention, the average particle size of the second aggregate is more preferably 3 μm or more and 13 μm or less, and even more preferably 5 μm or more and 10 μm or less.

[0089] LiFe phosphate metal particles are LiFe 0.3 Mn 0.7 In the case of PO4, from the viewpoint of improving capacity retention, the average particle size of both the first aggregate and the second aggregate is more preferably 5 μm or more and 15 μm or less, and even more preferably 7 μm or more and 13 μm or less.

[0090] The average particle size of the aggregates can be determined by observing the cross-section of the particles using a scanning electron microscope (SEM). For example, the cathode material of this embodiment is embedded in resin or the like, processed in a way that allows observation of the cross-section of the cathode material, and then the cross-section of the cathode material is observed using an SEM. For more than 100 aggregates randomly selected from this cross-section observation, the maximum and minimum diameters are measured, and the average of the maximum and minimum diameters is taken as the particle size of each aggregate. By averaging the particle sizes obtained from the measured aggregates, the average particle size can be determined.

[0091] [Carbon coating]

[0092] The positive electrode active material particles constituting the positive electrode material of this embodiment have a carbonaceous coating formed on the surface of lithium metal phosphate particles as shown in general formula (1).

[0093] Carbon is generally conductive, therefore carbon coatings, carbon particles, and various carbon materials are conductive.

[0094] For the carbon coating, a thickness of 1.0 nm or more, preferably 1.1 nm or more, is desirable. If the thickness of the carbon coating is 1.0 nm or more, the overall resistance to electron movement within the carbon coating can be suppressed. This suppresses the increase in the internal resistance of the lithium-ion secondary battery and prevents voltage drop at high charge / discharge rates. Conversely, a thickness of 10.0 nm or less, preferably 7.0 nm or less, is desirable. If the thickness of the carbon coating is 10.0 nm or less, the formation of steric hindrance hindering lithium-ion diffusion within the carbon coating can be suppressed, thereby reducing the resistance to lithium-ion movement. This also suppresses the increase in the battery's internal resistance and prevents voltage drop at high charge / discharge rates.

[0095] When the lithium metal phosphate particles are LiFePO4, from the viewpoint of improving capacity retention, the thickness of the carbon coating is more preferably 1.1 nm or more and 6.0 nm or less, and even more preferably 1.1 nm or more and 5.5 nm or less.

[0096] LiFe phosphate metal particles are LiFe 0.3 Mn 0.7 In the case of PO4, from the viewpoint of improving capacity retention, the thickness of the carbon coating is more preferably 1.5 nm or more and 5.0 nm or less, and even more preferably 1.7 nm or more and 4.0 nm or less.

[0097] The coating rate of the carbonaceous film relative to the lithium metal phosphate particles is not particularly limited, but it is preferably 60% or more, and more preferably 80% or more. By achieving a coating rate of 60% or more, the coating effect of the carbonaceous film can be fully obtained.

[0098] The thickness of the aforementioned carbonaceous coating can be determined as follows: a thin film sample formed by cross-section processing of the cathode material is prepared, and the thickness of the carbonaceous coating on the surface of multiple primary particles is measured using a transmission electron microscope (TEM) and averaged to obtain the thickness. Furthermore, the coverage rate of the carbonaceous coating can be determined as follows: particles are observed using a transmission electron microscope (TEM), energy dispersive X-ray microanalyzer (EDX), etc., the proportion covering the particle surface is calculated, and the average value is used to determine the coverage rate.

[0099] For the cathode material of this embodiment, the carbon content can be 0.7% by mass or more and 3.0% by mass or less.

[0100] By ensuring the carbon content of the cathode material is 0.7% by mass or more, a sufficient number of contact points between the carbon coatings can be ensured to form a sufficient conductive path. This improves electron conduction speed and maintains battery capacity even with faster charge and discharge rates in lithium-ion batteries. Furthermore, by keeping the aforementioned carbon content below 3.0% by mass, the distance that lithium ions travel within the carbon coating during the charge and discharge reaction (battery reaction) can be shortened, suppressing the decrease in lithium ion migration speed and maintaining battery capacity.

[0101] Based on the above viewpoint, the carbon content of the cathode material is more preferably 0.8% by mass or more and 3.0% by mass or less, and even more preferably 0.9% by mass or more and 2.9% by mass or less.

[0102] Here, the carbon content can be quantitatively analyzed using a carbon analyzer and used as an expression for the carbon content in the cathode material.

[0103] When the lithium metal phosphate particles are LiFePO4, from the viewpoint of improving capacity retention, the carbon content in the cathode material is more preferably 1.5% by mass or more and 2.9% by mass or less, and more preferably 2.0% by mass or more and 2.9% by mass or less.

[0104] LiFe phosphate metal particles are LiFe 0.3 Mn 0.7 In the case of PO4, from the viewpoint of improving capacity retention, the carbon content in the cathode material is more preferably 1.0% by mass or more and 2.7% by mass or less, and more preferably 1.0% by mass or more and 2.1% by mass or less.

[0105] [Specific surface area]

[0106] For the cathode material in this embodiment, the specific surface area can be 5m². 2 / g or more and 35m 2 / g or less.

[0107] By making the specific surface area of ​​the positive electrode material 5m² 2 By increasing the particle size of the primary particles constituting the cathode material to over 35 m² / g, the time required for lithium ions and electrons to move is shortened, increasing the capacity during high-current and low-temperature operation. Furthermore, by achieving a specific surface area of ​​35 m² / g for the cathode material... 2 Below / g, it can suppress the viscosity increase of the cathode material paste.

[0108] Based on the above viewpoints, a specific surface area of ​​7m² is more preferable. 2 / g or more and 34m 2 / g or less, more preferably 9m 2 / g or more and 33.5m 2 / g or less.

[0109] When the lithium metal phosphate particles are LiFePO4, from the viewpoint of improving capacity retention, the specific surface area of ​​the cathode material is further preferably 10 m². 2 / g or more and 33.5m 2 / g or less, more preferably 11m 2 / g or more and 33.5m 2 / g or less.

[0110] LiFe phosphate metal particles are LiFe 0.3 Mn 0.7 In the case of PO4, from the viewpoint of improving capacity retention, the specific surface area of ​​the cathode material is further preferably 12 m². 2 / g or more and 30m 2 / g or less, more preferably 16m 2 / g or more and 26m 2 / g or less.

[0111] [Tap density]

[0112] For the cathode material in this embodiment, the tap density can be 1.0 g / cm³. 3 Above and 2.0 g / cm 3 the following.

[0113] By making the tap density of the positive electrode material 1.0 g / cm³ 3 In this way, the contact area between the positive electrode active material and the electrolyte will not increase excessively, thus inhibiting the dissolution of metal from the positive electrode active material. Furthermore, by setting the tap density of the positive electrode material to 2.0 g / cm³... 3 As the contact area between the positive electrode active material and the electrolyte increases, the insertion / extraction of lithium ions into the positive electrode active material becomes easier, thus increasing the capacity.

[0114] Based on the above viewpoints, the tap density of the cathode material is more preferably 1.1 g / cm³. 3 Above and 1.9g / cm 3 The following is a further preferred value of 1.2 g / cm³. 3 Above and 1.8g / cm 3 the following.

[0115] When the lithium metal phosphate particles are LiFePO4, from the viewpoint of improving capacity retention, the tap density of the cathode material is further preferably 1.2 g / cm³. 3 Above and 1.7g / cm 3 the following.

[0116] LiFe phosphate metal particles are LiFe 0.3 Mn0.7 In the case of PO4, from the viewpoint of improving capacity retention, the tap density of the cathode material is further preferably 1.2 g / cm³. 3 Above and 1.6g / cm 3 The following is a further preferred value: 1.3 g / cm³ 3 Above and 1.5g / cm 3 the following.

[0117] [Electrode film density]

[0118] The positive electrode material for the lithium-ion secondary battery in this embodiment can achieve an electrode film density of 1.1 g / cm³. 3 Above and 2.0 g / cm 3 The following is an example of achieving an electrode film density of 1.1 g / cm³. 3 The above methods achieve a high energy density in lithium-ion secondary batteries, reaching 2.0 g / cm³. 3 The following steps can ensure sufficient contact with the electrolyte, thereby further improving the load characteristics.

[0119] When the lithium metal phosphate particles are LiFePO4, from the viewpoint of improving capacity retention, the electrode film density is preferably 1.2 g / cm³. 3 Above and 1.9g / cm 3 The preferred value is 1.2 g / cm³. 3 Above and 1.8g / cm 3 the following.

[0120] LiFe phosphate metal particles are LiFe 0.3 Mn 0.7 In the case of PO4, from the viewpoint of improving capacity retention, the electrode film density is preferably 1.1 g / cm³. 3 Above and 1.8g / cm 3 The preferred value is 1.2 g / cm³. 3 Above and 1.7g / cm 3 the following.

[0121] Here, the electrode film density can be determined by coating an electrode slurry onto an aluminum foil and drying it, and then measuring the density of the resulting electrode film. The electrode slurry is obtained by mixing the positive electrode material, carbon particles, and binder of this embodiment in an N-methyl-2-pyrrolidone (NMP) solvent. The electrode film density can be calculated as follows: the mass (Em), area, and thickness of the electrode film are measured, and the mass (Em) is divided by the volume (Ev) of the electrode film obtained from its area and thickness (Em / Ev).

[0122] 2. Manufacturing method of positive electrode material for lithium-ion secondary batteries

[0123] The method for manufacturing the positive electrode material for lithium-ion secondary batteries in this embodiment is not particularly limited, and may include, for example, the following steps: step (A), obtaining lithium metal phosphate particles; step (B), adding an organic compound to the lithium metal phosphate particles obtained in step (A) to prepare a mixture; step (C), placing the mixture into a firing sagger for firing; and step (D), mixing the positive electrode material obtained in step (C). The method for manufacturing the positive electrode material for lithium-ion secondary batteries in this embodiment may further include, arbitrarily, a crushing step to adjust the average particle size of the aggregates (secondary particles), etc.

[0124] [Process (A)]

[0125] In step (A), there is no particular limitation on the method for manufacturing the above-mentioned lithium metal phosphate particles. For example, the lithium metal phosphate particles shown in the following general formula (1) can be obtained by hydrothermal synthesis. The shape of the obtained lithium metal phosphate particles can be, for example, granular.

[0126] Li x A y D z PO4 (1)

[0127] Wherein, A is at least one selected from the group consisting of Co, Mn, Ni, Fe, Cu, and Cr; D is at least one selected from the group consisting of Mg, Ca, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, Sc, and Y; 0.9 <x<1.1、0<y≤1、0≤z<1、0.9<y+z<1.1。

[0128] In the hydrothermal synthesis method, for example, a slurry-like mixture is obtained by mixing a Li source, an A source, a P source, water, and a D source as required, which are components of general formula (1), and the resulting slurry-like mixture is subjected to hydrothermal synthesis. According to the hydrothermal synthesis, lithium metal phosphate particles are generated as a precipitate in water. The obtained precipitate can be a precursor of lithium metal phosphate particles. In this case, the target lithium metal phosphate particles can be obtained by calcining the precursor of lithium metal phosphate particles.

[0129] In hydrothermal synthesis, a pressure-resistant sealed container is preferred. By pressurizing and heating in the pressure-resistant sealed container, lithium metal phosphate particles or their precursors are synthesized by hydrothermal reaction. By selecting the conditions, a first aggregate or a second aggregate can be obtained.

[0130] As reaction conditions for the hydrothermal synthesis of the first aggregate, the heating temperature is preferably 170°C or higher and 200°C or lower, more preferably 175°C or higher and 195°C or lower. On the other hand, as reaction conditions for the hydrothermal synthesis of the second aggregate, the heating temperature is preferably 120°C or higher and 170°C or lower, more preferably 150°C or higher and 165°C or lower.

[0131] Furthermore, when obtaining the first aggregate, the reaction time is preferably 2 hours or more and 96 hours or less, more preferably 3 hours or more and 72 hours or less, and even more preferably 4 hours or more and 48 hours or less. On the other hand, when obtaining the second aggregate, the reaction time is preferably 2 hours or more and 48 hours or less, more preferably 3 hours or more and 24 hours or less, and even more preferably 4 hours or more and 12 hours or less.

[0132] Furthermore, the pressure during the reaction is preferably 0.1 MPa or more and 22 MPa or less, more preferably 0.1 MPa or more and 17 MPa or less.

[0133] By setting the reaction conditions within the aforementioned range, positively active material particles constituting the first aggregate or the second aggregate can be obtained.

[0134] The molar ratio (Li:A:D:P) of the Li source, A source, D source and P source is preferably 2.5~4.0:0~1.0:0~1.0:0.9~1.15, more preferably 2.8~3.5:0~1.0:0~1.0:0.95~1.1.

[0135] Here, as the Li source, for example, preferably is at least one selected from the group consisting of hydroxides such as lithium hydroxide (LiOH); lithium inorganic acid salts such as lithium carbonate (Li2CO3), lithium chloride (LiCl), lithium nitrate (LiNO3), lithium phosphate (Li3PO4), dilithium hydrogen phosphate (Li2HPO4), and lithium dihydrogen phosphate (LiH2PO4); lithium organic acid salts such as lithium acetate (LiCH3COO) and lithium oxalate ((COOLi)2); and their hydrates.

[0136] It should be noted that lithium phosphate (Li3PO4) can also be used as a Li source and a P source.

[0137] Examples of sources of A include chlorides, carboxylates, and sulfates containing at least one of the following groups: Co, Mn, Ni, Fe, Cu, and Cr. For example, Li... x A y D zWhen A in PO4 is Fe, iron compounds or their hydrates, such as ferric chloride (II) (FeCl2), ferric sulfate (II) (FeSO4), ferric acetate (II) (Fe(CH3COO)2), ferric nitrate (III) (Fe(NO3)3), ferric chloride (III) (FeCl3), ferric citrate (III) (FeC6H5O7), metallic iron, and lithium iron phosphate can be used as sources of Fe.

[0138] Examples of D sources include chlorides, carboxylates, and sulfates containing at least one of the following groups: Mg, Ca, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, Sc, and Y. For example, Li... x A y D z When D in PO4 is Ca, the sources of Ca include calcium hydroxide (II) (Ca(OH)2), calcium chloride (II) (CaCl2), calcium sulfate (II) (CaSO4), calcium nitrate (II) (Ca(NO3)2), calcium acetate (II) (Ca(CH3COO)2), and their hydrates.

[0139] Examples of phosphoric acid (P) sources include phosphoric acid (H3PO4), ammonium dihydrogen phosphate (NH4H2PO4), and diammonium hydrogen phosphate ((NH4)2HPO4). Among these, at least one selected from the group consisting of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate is preferred as the P source.

[0140] [Process (B)]

[0141] In step (B), an organic compound is added to the lithium metal phosphate particles obtained in step (A) to prepare a mixture.

[0142] First, an organic compound is added to the aforementioned lithium metal phosphate particles, and then a solvent is added to slurry them.

[0143] Regarding the amount of organic compound mixed with lithium metal phosphate particles, when the total mass of the organic compound is converted into carbon elements, it is preferably 0.15 parts by mass and 15 parts by mass and less than 100 parts by mass of lithium metal phosphate particles, more preferably 0.45 parts by mass and 4.5 parts by mass and less than 100 parts by mass of lithium metal phosphate particles.

[0144] If the amount of organic compound in the formulation is 0.15 parts by mass or more per 100 parts by mass of lithium metal phosphate particles, the carbonaceous coating produced by the thermal decomposition of the organic compound can achieve a coating rate of over 80% on the surface of the lithium metal phosphate particles. This improves the high-input and cycle characteristics of lithium-ion batteries.

[0145] On the other hand, by keeping the amount of organic compound mixed with lithium metal phosphate particles at 15 parts by mass or less per 100 parts by mass, the reduction in the lithium-ion battery capacity due to the relatively lower mixing ratio of lithium metal phosphate particles can be suppressed. Furthermore, by keeping the amount of organic compound mixed with lithium metal phosphate particles at 15 parts by mass or less, the decrease in the bulk density of the cathode material due to excessive loading of the carbon coating on the lithium metal phosphate particles can be suppressed. It should be noted that by suppressing the decrease in the bulk density of the cathode material, the decrease in electrode film density can be suppressed, thereby suppressing the reduction in the capacity of the lithium-ion battery per unit volume.

[0146] The organic compounds used in the preparation of the mixture are not particularly limited as long as they can form a carbonaceous coating on the surface of lithium metal phosphate particles. Examples include polyvinyl alcohol (PVA), polyvinylpyrrolidone, cellulose, starch, gelatin, carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, polyacrylic acid, polystyrene sulfonic acid, polyacrylamide, polyvinyl acetate, phenol, phenolic resin, glucose, fructose, galactose, mannose, maltose, sucrose, lactose, glycogen, pectin, alginic acid, glucomannan, chitin, hyaluronic acid, chondroitin, agarose, polyether, and polyols. Examples of polyols include polyethylene glycol, polypropylene glycol, polyglycerol, and glycerol. Only one type can be used, or two or more can be used in combination.

[0147] Here, by using low-molecular-weight organic compounds such as sucrose and lactose as the organic compounds, a carbonaceous coating can be easily formed on the primary particle surface of lithium metal phosphate particles without any omissions. However, on the other hand, there is a tendency for the degree of carbonization of the carbonaceous coating obtained through thermal decomposition to be lower, making it difficult to form a carbonaceous coating that can achieve sufficient resistance reduction. In addition, by using such low-molecular-weight organic compounds, the amount of micropores in the carbonaceous coating increases, and the overall micropore ratio increases.

[0148] On the other hand, by using high-molecular-weight organic compounds such as polyvinyl alcohol and polyvinylpyrrolidone, and organic compounds with benzene ring structures such as phenolic resins, there is a tendency for the carbonization degree of the carbonaceous coating obtained through thermal decomposition to increase, which can achieve sufficient resistance reduction. However, there is a tendency to make it difficult to form a carbonaceous coating on the surface of the primary particles of the cathode material without omission, and there are problems such as difficulty in achieving sufficient resistance reduction of the lithium-ion battery cathode material. In addition, by using such high-molecular-weight organic compounds and organic compounds with benzene ring structures, the amount of micropores in the carbonaceous coating is reduced, and the overall micropore ratio is reduced.

[0149] Therefore, it is preferable to use a suitable mixture of low-molecular-weight organic compounds and high-molecular-weight organic compounds, or organic compounds with benzene ring structures.

[0150] In particular, low-molecular-weight organic compounds, when used in powder form, are easier to mix with lithium metal phosphate particles, resulting in positive electrode active material particles with a carbonaceous coating formed on the primary surface of the lithium metal phosphate particles without any omissions. Therefore, this is preferred. Furthermore, unlike high-molecular-weight organic compounds, low-molecular-weight organic compounds are easily soluble in solution, eliminating the need for prior dissolution processes. This reduces processing steps and lowers the cost of dissolution operations.

[0151] When adding solvent to lithium metal phosphate particles, the solid content of the solvent containing lithium metal phosphate particles is preferably adjusted to 10-60% by mass, more preferably 15-55% by mass, and even more preferably 25-50% by mass. By keeping the solid content within the above range, the tap density of the obtained lithium-ion battery cathode material can be within the above range.

[0152] Examples of solvents mentioned above include: water; alcohols such as methanol, ethanol, 1-propanol, 2-propanol (isopropanol: IPA), butanol, pentanol, hexanol, octanol, and diacetone alcohol; esters such as ethyl acetate, butyl acetate, ethyl lactate, propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, and γ-butyrolactone; ethers such as diethyl ether, ethylene glycol monomethyl ether (methyl cellosolve), ethylene glycol monoethyl ether (ethyl cellosolve), ethylene glycol monobutyl ether (butyl cellosolve), diethylene glycol monomethyl ether, and diethylene glycol monoethyl ether; ketones such as acetone, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), acetylacetone, and cyclohexanone; amides such as dimethylformamide, N,N-dimethylacetylacetamide, and N-methylpyrrolidone; and diols such as ethylene glycol, diethylene glycol, and propylene glycol. One or more of these solvents may be used. Water is the preferred solvent among these.

[0153] It should be noted that a dispersant may also be added as needed.

[0154] As for the method of dispersing lithium metal phosphate particles and organic compounds in a solvent, there is no particular limitation as long as the lithium metal phosphate particles are uniformly dispersed and the organic compounds are dissolved or dispersed. Examples of apparatus for such dispersion include planetary ball mills, vibratory ball mills, bead mills, paint mixers, and grinding mills—medium-stirring dispersion devices that use high-speed stirring of the medium particles. Particularly when obtaining a second cathode material, pulverization using a planetary ball mill, vibratory ball mill, or bead mill is preferred. The pulverization time is not particularly limited; for example, it can be set to 24 hours or more but less than 360 hours.

[0155] A mixture can be prepared by slurrying lithium metal phosphate particles and organic compounds, followed by drying and granulation. The drying and granulation method is not particularly limited; for example, spray pyrolysis can be used, where the slurry is sprayed into an atmosphere at a temperature above 110°C and below 200°C for drying, resulting in granulated mixtures.

[0156] In this spray pyrolysis method, in order to quickly dry and generate approximately spherical granules, the droplet size during spraying is preferably 0.01 μm or more and 100 μm or less.

[0157] [Process (C)]

[0158] In step (C), the mixture obtained in step (B) is placed into a firing vessel, such as a firing sagger, for firing.

[0159] As a firing sagger, a firing sagger made of a material with excellent thermal conductivity, such as carbon, is suitable for use.

[0160] The firing temperature is preferably above 600°C and below 790°C, more preferably above 640°C and below 770°C. By selecting the conditions, a first cathode material or a second cathode material can be obtained.

[0161] The firing temperature for obtaining the first aggregate is preferably set to 690°C or higher and 790°C or lower, more preferably 700°C or higher and 770°C or lower. On the other hand, the firing temperature for obtaining the second aggregate is preferably set to 600°C or higher and 680°C or lower, more preferably 630°C or higher and 670°C or lower.

[0162] If the firing temperature is above 630°C, the decomposition and reaction of the organic compounds proceed fully, allowing for complete carbonization. As a result, a low-resistivity carbonaceous coating can be formed on the obtained positive electrode active material particles. Conversely, if the firing temperature is below 790°C, the grains of the lithium-ion battery positive electrode material do not grow further, maintaining a sufficiently high specific surface area. Consequently, when forming a lithium-ion battery, the discharge capacity at high charge / discharge rates increases, achieving optimal charge / discharge rate performance.

[0163] The firing time is as long as the organic compound is fully carbonized, and there are no special restrictions, such as more than 0.1 hours and less than 100 hours.

[0164] The firing atmosphere is preferably an inactive atmosphere composed of inactive gases such as nitrogen (N2) and argon (Ar) or a reducing atmosphere containing reducing gases such as hydrogen (H2). If it is desired to further suppress the oxidation of the mixture, a reducing atmosphere is more preferred.

[0165] During the calcination process (C), organic compounds decompose and react to generate carbon. This carbon then adheres to the surface of the lithium metal phosphate particles, forming a carbonaceous coating. Thus, the surface of the lithium metal phosphate particles is covered by this carbonaceous coating, becoming the positive electrode active material particles.

[0166] After firing in step (C), the resulting fired product is sometimes observed to have some agglomeration. Therefore, after firing, a crushing step can also be included to crush the resulting fired product.

[0167] Here, crushing refers to the following operation: applying mechanical energy to a fired product composed of multiple secondary particles, which is formed during firing due to sintering and necking between secondary particles, to separate the secondary particles with minimal damage to the particles themselves, thereby breaking down the fired product. In the crushing process, for example, a pin mill, hammer mill, or pulverizer can be used to crush the product to a degree that does not damage the secondary particles. After the crushing process, sieving or other methods can be performed as needed to adjust the particle size distribution.

[0168] [Process (D)]

[0169] In step (D), the first aggregate obtained in the calcination of step (C) is mixed with the second aggregate. The mixing ratio of the first aggregate to the second aggregate is not particularly limited and can be selected according to the required characteristics of the cathode material. For example, the mass ratio (a1:a2) of the first aggregate (a1) to the second aggregate (a2) can be set to 1:9 to 9:1. In other words, a1 / a2 can be 1 / 9 to 9 / 1.

[0170] When the lithium metal phosphate particles are LiFePO4, from the viewpoint of improving capacity retention, the mass ratio (a1:a2) of the first aggregate (a1) to the second aggregate (a2) is preferably 1:9 to 5:5 (a1 / a2 is 1 / 9 to 5 / 5), and from the viewpoint of simultaneously improving capacity retention and discharge capacity ratio, it is more preferably 1:9 to 4:6 (a1 / a2 is 1 / 9 to 4 / 6).

[0171] LiFe phosphate metal particles are LiFe 0.3 Mn 0.7 In the case of PO4, from the viewpoint of improving capacity retention, the mass ratio (a1:a2) of the first aggregate (a1) to the second aggregate (a2) is preferably 2:8 to 8:2 (a1 / a2 is 2 / 8 to 8 / 2), and more preferably 3:7 to 7:3 (a1 / a2 is 3 / 7 to 7 / 3).

[0172] 3. Lithium-ion secondary batteries

[0173] The lithium-ion secondary battery of this embodiment has a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode contains the lithium-ion battery positive electrode material of this embodiment.

[0174] Hereinafter, lithium-ion secondary batteries will sometimes be abbreviated as "battery".

[0175] [positive electrode]

[0176] In the fabrication of the positive electrode, a positive electrode material for a lithium-ion secondary battery of this embodiment, containing an aggregate composition comprising a first aggregate and a second aggregate, is mixed with a binder formed from a binder resin and a solvent to prepare a positive electrode composite paste. Conductive additives such as carbon black, acetylene black, graphite, Ketjen black, natural graphite, and artificial graphite can be added as needed.

[0177] As an adhesive, i.e., an adhesive resin, suitable materials include, for example, polytetrafluoroethylene (PTFE) resin, polyvinylidene fluoride (PVdF) resin, fluororubber, etc.

[0178] The mixing ratio of the cathode material compound to the binder resin is not particularly limited. For example, relative to 100 parts by weight of the cathode material, the binder resin is set to 1 to 30 parts by weight, preferably 3 to 20 parts by weight.

[0179] The solvent used in the positive electrode composite paste can be selected appropriately based on the properties of the binder resin.

[0180] Examples include: water; alcohols such as methanol, ethanol, 1-propanol, 2-propanol (isopropanol: IPA), butanol, pentanol, hexanol, octanol, and diacetone alcohol; esters such as ethyl acetate, butyl acetate, ethyl lactate, propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, and γ-butyrolactone; ethers such as diethyl ether, ethylene glycol monomethyl ether (methyl cellosolve), ethylene glycol monoethyl ether (ethyl cellosolve), ethylene glycol monobutyl ether (butyl cellosolve), diethylene glycol monomethyl ether, and diethylene glycol monoethyl ether; ketones such as acetone, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), acetylacetone, and cyclohexanone; amides such as dimethylformamide, N,N-dimethylacetylacetamide, and N-methylpyrrolidone; and diols such as ethylene glycol, diethylene glycol, and propylene glycol. Only one of these can be used, or two or more can be used in combination.

[0181] Next, the positive electrode composite paste is coated onto one side of the aluminum foil and then dried to obtain an aluminum foil with a coating film formed on one side of the mixture of the positive electrode material and the binder resin.

[0182] Next, the coating is pressed and dried to create an electrode (positive electrode) with a positive composite material layer on one side of the aluminum foil.

[0183] In this way, it is possible to produce a positive electrode for lithium-ion secondary batteries that has high input characteristics and excellent cycle performance.

[0184] [negative electrode]

[0185] The negative electrode is a sheet-like component formed by coating a negative electrode composite paste onto the surface of a current collector such as copper foil and drying it. Although the composition of the negative electrode composite paste, its formulation, and the raw materials of the current collector may differ, the negative electrode is essentially formed using the same method as the aforementioned positive electrode and undergoes various treatments as required, just like the positive electrode.

[0186] Negative electrode composite paste is a paste-like substance made by adding appropriate solvents to a negative electrode composite material that contains negative electrode active material and binder.

[0187] The negative electrode active material can be, for example, lithium metal, lithium alloys or other lithium-containing materials, or storage materials that enable lithium ion absorption and deintercalation.

[0188] The absorbent material is not particularly limited; for example, sintered organic compounds such as natural graphite, artificial graphite, and phenolic resin, as well as powdered carbonaceous materials such as coke, can be used. When this absorbent material is used as the negative electrode active material, similarly to the positive electrode, fluorinated resins such as PVDF can be used as the binder, and organic solvents such as N-methyl-2-pyrrolidone can be used as the solvent to disperse the negative electrode active material in the binder.

[0189] [Separator]

[0190] The separator is sandwiched between the positive and negative electrodes when using a non-aqueous electrolyte, and functions to separate the positive and negative electrodes and retain the electrolyte. The separator can be, for example, a thin membrane such as polyethylene or polypropylene with multiple micropores; there are no particular limitations as long as it has the aforementioned functions.

[0191] [Non-aqueous electrolytes]

[0192] As a non-aqueous electrolyte, a non-aqueous electrolyte solution can be used, for example.

[0193] As a non-aqueous electrolyte, for example, a solution of lithium salt as the supporting salt dissolved in an organic solvent can be used. Alternatively, a solution containing lithium salt dissolved in an ionic liquid can be used as a non-aqueous electrolyte. It should be noted that an ionic liquid is a salt composed of cations and anions other than lithium ions, which is liquid at room temperature (25°C).

[0194] As organic solvents, cyclic carbonates such as ethylene carbonate, propylene carbonate, butyl carbonate, and trifluoropropylene carbonate; chain carbonates such as diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, and dipropyl carbonate; ether compounds such as tetrahydrofuran, 2-methyltetrahydrofuran, and dimethoxyethane; sulfur compounds such as ethyl methyl sulfone and butyryl lactone; and phosphorus compounds such as triethyl phosphate and trioctyl phosphate can be used.

[0195] Organic solvents can be used alone, or two or more can be used in combination.

[0196] As supporting salts, LiPF6, LiBF4, LiClO4, LiAsF6, LiN(CF3SO2)2, and their complex salts can be used. Furthermore, the non-aqueous electrolyte can contain free radical scavengers, surfactants, and flame retardants.

[0197] You can use only one type of salt, or you can use a mixture of two or more.

[0198] In addition, solid electrolytes can be used as non-aqueous electrolytes. Solid electrolytes have the property of being able to withstand high voltages. Inorganic solid electrolytes and organic solid electrolytes can be cited as examples of solid electrolytes.

[0199] Examples of inorganic solid electrolytes include oxide-based solid electrolytes and sulfide-based solid electrolytes.

[0200] There are no particular limitations on the type of oxide-based solid electrolyte. For example, oxide-based solid electrolytes containing oxygen (O) and possessing lithium-ion conductivity and electronic insulation properties can be used. Specifically, examples of oxide-based solid electrolytes include lithium phosphate (Li3PO4) and Li3PO4N. X LiBO2N X , LiNbO3, LiTaO3, Li2SiO3, Li4SiO4-Li3PO4, Li4SiO4-Li3VO4, Li2O-B2O3-P2O5, Li2O-SiO2, Li2O-B2O3-ZnO, Li 1+X Al X Ti 2-X (PO4)3 (0≤X≤1), Li 1+X Al X Ge 2-X (PO4)3 (0≤X≤1), LiTi2(PO4)3, Li 3X La 2 / 3-X TiO3 (0≤X≤2 / 3), Li5La3Ta2O 12 Li7La3Zr2O 12 Li6BaLa2Ta2O12 Li 3.6 Si 0.6 P 0.4 O4, etc. Oxide-based solid electrolytes can use more than one of them.

[0201] There are no particular limitations on the type of sulfide-based solid electrolyte. For example, sulfide-based solid electrolytes containing sulfur (S) and possessing lithium-ion conductivity and electronic insulation properties can be used. Specifically, examples of sulfide-based solid electrolytes include Li₂S-P₂S₅, Li₂S-SiS₂, LiI-Li₂S-SiS₂, LiI-Li₂S-P₂S₅, LiI-Li₂S-B₂S₃, Li₃PO₄-Li₂S-Si₂S, Li₃PO₄-Li₂S-SiS₂, LiPO₄-Li₂S-SiS, LiI-Li₂S-P₂O₅, and LiI-Li₃PO₄-P₂S₅. Sulfide-based solid electrolytes selected from one or more of these types can be used.

[0202] It should be noted that, as an inorganic solid electrolyte, substances other than those mentioned above can be used, such as Li3N, LiI, Li3N-LiI-LiOH, etc.

[0203] As an organic solid electrolyte, there are no particular limitations as long as it is a polymeric compound exhibiting ion conductivity; for example, polyethylene oxide, polypropylene oxide, and their copolymers can be used. Furthermore, organic solid electrolytes can contain supporting salts (lithium salts).

[0204] Organic solid electrolytes can be used in single or multiple ways.

[0205] [Shape and structure of secondary batteries]

[0206] As described above, the battery of this embodiment can be manufactured in various shapes, such as cylindrical or stacked. Regardless of the shape, in the case of using a non-aqueous electrolyte as the non-aqueous electrolyte in the secondary battery of this embodiment, the electrode body is formed by stacking the positive and negative electrodes with a separator in between. The structure can be made such that the non-aqueous electrolyte permeates the resulting electrode body, and current-collecting leads are used to connect the positive current collector to the positive terminal leading to the outside, and the negative current collector to the negative terminal leading to the outside, and the battery is sealed within the battery casing.

[0207] It should be noted that the battery in this embodiment is not limited to using a non-aqueous electrolyte as the non-aqueous electrolyte form. For example, it can also be made into a secondary battery using a solid non-aqueous electrolyte, i.e., an all-solid-state battery. When making an all-solid-state battery, the structure other than the positive electrode material can be appropriately modified as needed.

[0208] As described above, the battery of this embodiment uses the positive electrode material of this embodiment as the positive electrode material, thus exhibiting excellent battery capacity and input / output characteristics. Therefore, the battery of this embodiment is suitable for use as a rechargeable battery in portable information terminals such as mobile phones, smartphones, tablet computers, and laptop computers; portable music players; digital cameras; medical devices; and clean energy vehicles such as HEVs (Hybrid Electric Vehicles), EVs (Electric Vehicles), and PHEVs (Plug-in Hybrid Electric Vehicles).

[0209] Example

[0210] The present invention will now be described in detail by way of examples and comparative examples. It should be noted that the present invention is not limited to the methods described in the examples.

[0211] Manufacturing of lithium-ion battery cathode materials

[0212] (Example 1)

[0213] The olivine-type compound LiFePO4 is prepared as follows.

[0214] Using Li3PO4 as the Li source and P source, and FeSO4 aqueous solution as the Fe source, they were mixed in a molar ratio of Li:Fe:P = 3:1:1 to prepare 2.2 L of raw material slurry A1.

[0215] Next, the raw material slurry A1, placed in a pressure-resistant container, was heated at 190°C for 48 hours to carry out hydrothermal synthesis. After the reaction, the atmosphere inside the pressure-resistant container was cooled to room temperature, yielding a precipitate of the reaction product in filter cake form.

[0216] The precipitate was thoroughly washed multiple times with distilled water and kept at a moisture content of 40% without drying to form a filter cake.

[0217] The filter cake was vacuum dried at 70°C for 2 hours. Relative to 95% by mass of the obtained LiFePO4 particles, 4.5% by mass of an aqueous solution of polyacrylic acid as the first carbon source and 0.5% by mass of a water-dispersible phenolic resin as the second carbon source were dispersed in an aqueous solvent to obtain raw material slurry α1.

[0218] After the raw material slurry α1 was dried and granulated, it was calcined at 735°C for 2 hours in a nitrogen (N2) atmosphere using a rotary kiln manufactured by Chugai Ro Co., Ltd. This yielded the first aggregates whose particle surfaces were coated with a carbonaceous film.

[0219] Then, the raw material slurry A1, placed in a pressure-resistant container, was heated at 160°C for 4 hours to carry out hydrothermal synthesis. After the reaction, the atmosphere inside the pressure-resistant container was cooled to room temperature to obtain a precipitate of the reaction product in the form of a filter cake.

[0220] The precipitate was thoroughly washed multiple times with distilled water and kept at a moisture content of 40% without drying to form a filter cake.

[0221] The filter cake was vacuum dried at 70°C for 2 hours. Relative to 95% by mass of the obtained LiFePO4 particles, 4.5% by mass of an aqueous solution of polyacrylic acid as the first carbon source and 0.5% by mass of a water-dispersible phenolic resin as the second carbon source were dispersed in an aqueous solvent. After adjusting the solid content concentration to 20% by mass, the mixture was pulverized for 280 hours using a bead mill with a bead diameter of 0.1 mm to obtain the raw material slurry α2.

[0222] After drying and granulating the raw material slurry α2, it was calcined at 650°C for 2 hours in a nitrogen (N2) atmosphere using a rotary kiln manufactured by Chugai Ro Co., Ltd. This yielded a second aggregate with a carbonaceous coating covering the surface of the particles.

[0223] The first and second aggregates were weighed in a mixing ratio of 2:8 [mass of the first aggregate (a1): mass of the second aggregate (a2) = 2:8] and mixed using a mixer to obtain the cathode material mixture of Example 1.

[0224] (Example 2)

[0225] Weighing was performed so that the mixing ratio (a1:a2) of the first lithium-ion battery cathode material and the second lithium-ion battery cathode material obtained in Example 1 was 9:1 (mass of the first aggregate: mass of the second aggregate = 9:1). Otherwise, the cathode material mixture of Example 2 was obtained in the same manner as in Example 1.

[0226] (Example 3)

[0227] The raw material slurry A1 used in hydrothermal synthesis was heated in a pressure vessel at 190°C for 20 hours to obtain the first lithium-ion battery cathode material. The mixture of the first aggregate and the second aggregate was weighed in such a way that the mixing ratio (a1:a2) was 5:5 (mass of the first aggregate:mass of the second aggregate = 5:5). Otherwise, the cathode material mixture of Example 3 was obtained in the same manner as in Example 1.

[0228] (Example 4)

[0229] The heating conditions of the raw material slurry A1 during hydrothermal synthesis in a pressure vessel were set to 190°C for 10 hours to obtain the first lithium-ion battery cathode material. Otherwise, the cathode material mixture of Example 4 was obtained in the same manner as in Example 3.

[0230] (Example 5)

[0231] The raw material slurry A1 used in hydrothermal synthesis was heated in a pressure vessel at 190°C for 4 hours to obtain the first aggregate. The grinding time of the bead mill was set to 48 hours to obtain the second aggregate. Otherwise, the cathode material mixture of Example 5 was obtained in the same manner as in Example 3.

[0232] (Example 6)

[0233] The raw material slurry A1 used in hydrothermal synthesis was heated in a pressure vessel at 180°C for 8 hours to obtain the first aggregate. Otherwise, the cathode material mixture of Example 6 was obtained in the same manner as in Example 5.

[0234] (Example 7)

[0235] The raw material slurry A1 used in hydrothermal synthesis was heated in a pressure vessel at 180°C for 5 hours to obtain a first aggregate. The first aggregate and the second aggregate were weighed in such a way that the mixing ratio (a1:a2) of the first aggregate and the second aggregate was 9:1 (mass of the first aggregate: mass of the second aggregate = 9:1). Otherwise, the cathode material mixture of Example 7 was obtained in the same manner as in Example 5.

[0236] (Example 8)

[0237] Weighing was performed so that the mixing ratio (a1:a2) of the first aggregate and the second aggregate obtained in Example 7 was 2:8 (mass of the first aggregate: mass of the second aggregate = 2:8). Otherwise, the cathode material mixture of Example 8 was obtained in the same manner as in Example 7.

[0238] (Example 9)

[0239] Using Li3PO4 as the Li and P source, FeSO4 aqueous solution as the Fe source, and MnSO4 aqueous solution as the Mn source, they were mixed in a molar ratio of Li:Fe:Mn:P = 3:0.3:0.7:1 to prepare 2.2 L of raw material slurry B1.

[0240] Next, the raw material slurry B1, placed in a pressure-resistant container, was heated at 190°C for 6 hours to carry out hydrothermal synthesis. After the reaction, the atmosphere inside the pressure-resistant container was cooled to room temperature, yielding a precipitate of the reaction product in filter cake form.

[0241] The precipitate was thoroughly washed multiple times with distilled water and kept at a moisture content of 40% without drying to form a filter cake.

[0242] The filter cake was vacuum dried at 70°C for 2 hours, relative to the obtained LiFe 0.3 Mn 0.7 95% by mass of PO4 particles were dispersed in an aqueous solvent with 4.5% by mass of an aqueous solution of polyacrylic acid as the first carbon source (based on solids content) and 0.5% by mass of a water-dispersible phenolic resin as the second carbon source (based on solids content) to obtain raw material slurry β1.

[0243] After drying and granulating the raw material slurry β1, it was calcined at 735°C for 2 hours in a nitrogen (N2) atmosphere using a rotary kiln manufactured by Chugai Ro Co., Ltd. This yielded the first aggregates whose particle surfaces were coated with a carbonaceous film.

[0244] Then, the raw material slurry B1, placed in a pressure-resistant container, was heated at 160°C for 4 hours to carry out hydrothermal synthesis. After the reaction, the atmosphere inside the pressure-resistant container was cooled to room temperature to obtain a precipitate of the reaction product in the form of a filter cake.

[0245] The precipitate was thoroughly washed multiple times with distilled water and kept at a moisture content of 40% without drying to form a filter cake.

[0246] The filter cake was vacuum dried at 70°C for 2 hours, relative to the obtained LiFe 0.3 Mn 0.7 95% by mass of PO4 particles were dispersed in an aqueous solvent with 4.5% by mass of polyacrylic acid aqueous solution as the first carbon source and 0.5% by mass of water-dispersible phenolic resin as the second carbon source. After adjusting the solid content concentration to 20 wt%, the mixture was pulverized for 280 hours using a bead mill with a bead diameter of 0.1 mm to obtain raw material slurry β2.

[0247] After drying and granulating the raw material slurry β2, it was calcined at 650°C for 2 hours in a nitrogen (N2) atmosphere using a rotary kiln manufactured by Chugai Ro Co., Ltd. This yielded a second aggregate with a carbonaceous coating covering the surface of the particles.

[0248] The first and second aggregates were weighed in a mixing ratio (a1:a2) of 5:5 (mass of the first aggregate:mass of the second aggregate = 5:5), and mixed using a mixer to obtain the cathode material mixture of Example 9.

[0249] (Example 10)

[0250] The raw material slurry B1 used in hydrothermal synthesis was heated in a pressure vessel at 180°C for 6 hours to obtain the first aggregate. Otherwise, the cathode material mixture of Example 10 was obtained in the same manner as in Example 9.

[0251] (Comparative Example 1)

[0252] The olivine-type compound LiFePO4 was prepared as follows. Li3PO4 was used as the Li and P source, and an aqueous solution of FeSO4 was used as the Fe source. They were mixed in a molar ratio of Li:Fe:P = 3:1:1 to prepare 2.2 L of raw material slurry A1.

[0253] Next, the raw material slurry A1, placed in a pressure-resistant container, was heated at 180°C for 1 hour to carry out hydrothermal synthesis. After the reaction, the atmosphere inside the pressure-resistant container was cooled to room temperature, yielding a precipitate of the reaction product in filter cake form.

[0254] The precipitate was thoroughly washed multiple times with distilled water and kept at a moisture content of 40% without drying to form a filter cake.

[0255] The filter cake was vacuum dried at 70°C for 2 hours. For 95% by mass of the obtained LiFePO4 particles, 12.5% ​​by mass of glucose aqueous solution as the first carbon source and 2.5% by mass of hydroxyethyl cellulose as the second carbon source were dispersed in an aqueous solvent to obtain raw material slurry γ1.

[0256] After drying and granulating the raw material slurry γ1, it was calcined at 735°C for 2 hours in a nitrogen (N2) atmosphere using a rotary kiln manufactured by Chugai Ro Co., Ltd. Thus, a lithium-ion battery cathode material of Comparative Example 1, in which the surface of the particles was coated with a carbonaceous film, was obtained.

[0257] (Comparative Example 2)

[0258] The olivine-type compound LiFePO4 was prepared as follows. Li3PO4 was used as the Li and P source, and an aqueous solution of FeSO4 was used as the Fe source. They were mixed in a molar ratio of Li:Fe:P = 3:1:1 to prepare 2.2 L of raw material slurry A1.

[0259] Next, the raw material slurry A1, placed in a pressure-resistant container, was heated at 170°C for 1 hour to carry out hydrothermal synthesis. After the reaction, the atmosphere inside the pressure-resistant container was cooled to room temperature to obtain a precipitate of the reaction product in filter cake form.

[0260] The precipitate was thoroughly washed multiple times with distilled water and kept at a moisture content of 40% without drying to form a filter cake.

[0261] The filter cake was vacuum dried at 70°C for 2 hours. For 95% by mass of the obtained LiFePO4 particles, 2.5% by mass of glucose aqueous solution as the first carbon source and 0.5% by mass of hydroxyethyl cellulose as the second carbon source were dispersed in an aqueous solvent to obtain raw material slurry δ1.

[0262] After drying and granulating the raw material slurry δ1, it was calcined at 735°C for 2 hours in a nitrogen (N2) atmosphere using a rotary kiln manufactured by Chugai Ro Co., Ltd. Thus, a lithium-ion battery cathode material of Comparative Example 2, in which the surface of the particles was coated with a carbonaceous film, was obtained.

[0263] (Comparative Example 3)

[0264] The olivine-type compound LiFePO4 was prepared as follows. Li3PO4 was used as the Li and P source, and an aqueous solution of FeSO4 was used as the Fe source. They were mixed in a molar ratio of Li:Fe:P = 3:1:1 to prepare 2.2 L of raw material slurry A1.

[0265] Next, the raw material slurry A1, placed in a pressure-resistant container, was heated at 180°C for 1 hour to carry out hydrothermal synthesis. After the reaction, the atmosphere inside the pressure-resistant container was cooled to room temperature, yielding a precipitate of the reaction product in filter cake form.

[0266] The precipitate was thoroughly washed multiple times with distilled water and kept at a moisture content of 40% without drying to form a filter cake.

[0267] The filter cake was vacuum dried at 70°C for 2 hours. For 95% by mass of the obtained LiFePO4 particles, 12.5% ​​by mass of glucose aqueous solution as the first carbon source and 2.5% by mass of hydroxyethyl cellulose as the second carbon source were dispersed in an aqueous solvent to obtain raw material slurry γ1.

[0268] After drying and granulating the raw material slurry γ1, it was calcined at 735°C for 2 hours in a nitrogen (N2) atmosphere using a rotary kiln manufactured by Chugai Ro Co., Ltd. This yielded the first aggregates whose particle surfaces were coated with a carbonaceous film.

[0269] Then, the raw material slurry A1, placed in a pressure-resistant container, was heated at 170°C for 1 hour to carry out hydrothermal synthesis. After the reaction, the atmosphere inside the pressure-resistant container was cooled to room temperature to obtain a precipitate of the reaction product in the form of a filter cake.

[0270] The precipitate was thoroughly washed multiple times with distilled water and kept at a moisture content of 40% without drying to form a filter cake.

[0271] The filter cake was vacuum dried at 70°C for 2 hours. For 95% by mass of the obtained LiFePO4 particles, 2.5% by mass of glucose aqueous solution as the first carbon source and 0.5% by mass of hydroxyethyl cellulose as the second carbon source were dispersed in an aqueous solvent to obtain raw material slurry δ1.

[0272] After drying and granulating the raw material slurry δ1, it was calcined at 735°C for 2 hours in a nitrogen (N2) atmosphere using a rotary kiln manufactured by Chugai Ro Co., Ltd. This yielded a second aggregate with a carbonaceous coating covering the surface of the particles.

[0273] The first and second aggregates were weighed in a mixing ratio (a1:a2) of 5:5 (mass of the first aggregate: mass of the second aggregate 5:5), and mixed using a mixer to obtain the cathode material mixture of Example 9.

[0274] [The fabrication of lithium-ion secondary batteries]

[0275] The positive electrode material mixture obtained in the examples and comparative examples, acetylene black (AB) as a conductive additive, and polyvinylidene fluoride (PVdF) as a binder were mixed in N-methyl-2-pyrrolidone (NMP) at a mass ratio of electrode material:AB:PVdF = 90:5:5 to prepare a positive electrode material paste. The obtained paste was coated on an aluminum foil with a thickness of 30 μm and dried to obtain a dried electrode film. The density of the obtained dried electrode film was measured. Then, the dried electrode film was pressed together to a specified density to form an electrode plate.

[0276] The obtained electrode plate was punched into a 3×3cm shape. 2 (Coated surface) + plate-shaped pre-reserved electrode tabs, weld the electrode tabs, and make the test electrode.

[0277] On the other hand, a coated electrode coated with natural graphite is also used as the counter electrode. A porous polypropylene membrane is used as the separator. Furthermore, a 1 mol / L lithium hexafluorophosphate (LiPF6) solution is used as the non-aqueous electrolyte solution. It should be noted that the solvent used in this LiPF6 solution is a solvent obtained by mixing ethylene carbonate and diethyl carbonate in a 1:1 volume ratio and adding 2% ethylene carbonate as an additive.

[0278] Using the test electrode, counter electrode, and non-aqueous electrolyte prepared as described above, laminated battery cells were fabricated as examples and comparative examples.

[0279] Evaluation of lithium-ion battery cathode materials

[0280] The physical properties of the lithium-ion battery cathode materials obtained in the examples and comparative examples, as well as the components contained in these lithium-ion battery cathode materials, were evaluated. The evaluation methods are described below. The results are shown in Table 1.

[0281] (1) Carbon content

[0282] Carbon content (c) was determined using a carbon analyzer (manufactured by Horiba Manufacturing Co., Ltd., model: EMIA-220V).

[0283] (2) Specific surface area

[0284] The specific surface area was determined by the BET method based on nitrogen (N2) adsorption using a specific surface area meter (manufactured by BEL, Inc., trade name: BELSORP-mini).

[0285] (3) Tap density

[0286] The tap density was determined according to the method in JIS R 1628:1997, "Determination of Bulk Density of Fine Ceramic Powders". A specified mass of sample was collected from the positive electrode material for lithium-ion secondary batteries and placed into a 10 mL glass graduated cylinder. The sample was vibrated together with the graduated cylinder, and the volume of the sample was measured when the volume no longer changed. The mass of the sample was divided by the volume of the sample, and the value obtained was taken as the tap density of the lithium-ion battery positive electrode material.

[0287] (4) Carbon coating thickness (average)

[0288] Using a focused ion beam processing observation device (Hitachi High-Technologies Corporation, trade name: FB2100), thin film samples with cross-sectional processing of secondary particles of lithium-ion battery cathode material were prepared. Using a field emission transmission electron microscope (Hitachi High-Technologies Corporation, trade name: HF2000), the average value of the carbon coating thickness (average value) was obtained by measuring 10 points at each of 10 primary particles based on the captured images.

[0289] (5) Grain diameter

[0290] The grain diameter was determined by the half-width (FWHM) of the (211) plane, which was measured by powder X-ray diffraction using CuKα rays, and calculated using the Scherer formula.

[0291] (6) Average particle size

[0292] The cathode material blend was embedded in resin and processed in a manner that allowed for cross-sectional observation. The cross-section of the cathode material blend was then observed using a scanning electron microscope (SEM). For 100 aggregates randomly selected from this cross-sectional observation, the maximum and minimum diameters were measured, and the average of the maximum and minimum diameters was taken as the particle size of each aggregate. Furthermore, the average particle size of the aggregates was calculated by averaging the measured particle sizes.

[0293] [Evaluation of Lithium-ion Batteries]

[0294] The obtained lithium-ion batteries were evaluated using the following methods. The results are shown in Tables 1 and 2.

[0295] (1) Load characteristics (discharge capacity ratio)

[0296] At room temperature (25°C), the lithium-ion battery was repeatedly charged and discharged three times under a constant current (charging for 10 hours, discharging for 10 hours) at a cutoff voltage of 2.5V~3.7V and a charge / discharge rate of 0.1C. The discharge capacity of the third test was taken as the discharge capacity at 0.1C. Then, at room temperature (25°C), the battery was charged at a cutoff voltage of 2.5V~3.7V and a charge rate of 0.2C (charging for 5 hours) and discharged at 3C (discharging for 20 minutes), and the discharge capacity was measured.

[0297] The ratio of the 3C discharge capacity to the 0.1C discharge capacity is taken as the load characteristic and calculated according to the following formula (2).

[0298] Discharge capacity ratio (%) = (3C discharge capacity / 0.1C discharge capacity) × 100… (2)

[0299] (2) DC resistance (DCR)

[0300] For lithium-ion batteries, at an ambient temperature of 25°C, charge at a current of 0.1C for 5 hours, and adjust the depth of charge (SOC) to 50%. For batteries adjusted to 50% SOC, set the ambient temperature to 0°C and measure the DCR. Perform the following cycles sequentially: Cycle 1: "1C charge for 10 seconds → rest for 10 minutes → 1C discharge for 10 seconds → rest for 10 minutes"; Cycle 2: "3C charge for 10 seconds → rest for 10 minutes → 3C discharge for 10 seconds → rest for 10 minutes"; Cycle 3: "5C charge for 10 seconds → rest for 10 minutes → 5C discharge for 10 seconds → rest for 10 minutes"; Cycle 4: "10C charge for 10 seconds → rest for 10 minutes → 10C discharge for 10 seconds → rest for 10 minutes". Measure the voltage 10 seconds after each charge and discharge cycle. An approximate straight line is plotted with each current value on the horizontal axis and the voltage after 10 seconds on the vertical axis. The slope of the approximate straight line is used as the DC resistance during charging (input DCR) and the DC resistance during discharging (output DCR).

[0301] (3) Cyclic characteristics

[0302] For the aforementioned laminated battery cell, the process was repeated 500 times at an ambient temperature of 60°C, a cutoff voltage of 2.5V~3.7V, and a charge / discharge rate of 1C with constant current (charging for 1 hour followed by discharging for 1 hour). The laminated battery cell was then brought to a fully discharged state under a constant current discharge of 0.1C (hereinafter referred to as the "battery cell after the test").

[0303] The capacity retention rate is calculated by the ratio of the 0.1C discharge capacity after repeated tests to the initial 0.1C discharge capacity according to the following formula (3).

[0304] Capacity retention rate (%)

[0305] = (0.1C discharge capacity after repeated tests / initial 0.1C discharge capacity) × 100… (3)

[0306] (4) Metal leaching amount

[0307] The battery cells used in the cycle performance evaluation were disassembled after testing, and the negative electrode layer was separated and the acid was dissolved. The transition metal content [metal content (Ma)] was quantified using ICP emission spectroscopy. The ratio of this to the total mass (Mc) of lithium metal phosphate particles contained in the first and second aggregates of the positive electrode in the laminated battery cells used in the cycle performance evaluation (Ma / Mc) was taken as the metal dissolution amount.

[0308] [Table 1]

[0309]

[0310] [Table 2]

[0311]

[0312] As shown in Tables 1 and 2, in lithium-ion batteries using a cathode material comprising the first and second aggregates of the embodiments, where the first aggregate is composed of positive electrode active material particles with a grain diameter of 200 nm or more and 2000 nm or less, and the second aggregate is composed of positive electrode active material particles with a grain diameter of 50 nm or less, the metal dissolution after 500 cycles is suppressed to below 1,000 ppm by mass, resulting in a capacity retention rate of over 80% after 500 cycles. Furthermore, by using the cathode material of the embodiments, the electrode film density in the dry state can be achieved to be 1.1 g / cm³. 3 The above enables the production of high-energy-density lithium-ion batteries.

[0313] On the other hand, in lithium-ion secondary batteries using a cathode material from a comparative example with a grain diameter greater than 50 nm and less than 200 nm, the metal leaching after 500 cycles was greater than 1,000 ppm by mass. As a result, the capacity retention after 500 cycles was less than 80%, and the electrode film density in the dry state was less than 1.1 g / cm³. 3 It is impossible to obtain lithium-ion secondary batteries with excellent cycle characteristics and high energy density.

[0314] Based on the above, it has been confirmed that the lithium-ion secondary battery obtained from the cathode material of this embodiment has excellent cycle characteristics and high energy density.

Claims

1. A positive electrode material for lithium-ion secondary batteries, comprising an aggregate composition of positive electrode active material particles, wherein the positive electrode active material particles are formed by forming a carbonaceous coating on the surface of lithium metal phosphate particles represented by the following general formula (1). The aggregate composition comprises: The positive electrode active material particles consist of a first aggregate with a grain diameter of 200 nm or more and 2000 nm or less, and a second aggregate with a grain diameter of 50 nm or less. Li x A y D z PO4 (1) in, A is at least one selected from the group consisting of Co, Mn, Ni, Fe, Cu, and Cr; D is at least one selected from the group consisting of Mg, Ca, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, Sc, and Y; 0.9 <x<1.1、0<y≤1、0≤z<1、0.9<y+z<1.1。 2. The positive electrode material for lithium-ion secondary batteries according to claim 1, wherein, The average particle size of the first aggregate and the average particle size of the second aggregate are each independently greater than 0.5 μm and less than 50 μm.

3. The positive electrode material for lithium-ion secondary batteries according to claim 1 or 2, wherein, The electrode film obtained by coating the electrode paste onto aluminum foil and drying it has a density of 1.1 g / cm³. 3 Above and 2.0 g / cm 3 The electrode slurry described below is obtained by mixing the positive electrode material, carbon particles and binder in an N-methyl-2-pyrrolidone solvent.

4. The positive electrode material for lithium-ion secondary batteries according to claim 1 or 2, wherein, For a pouch cell whose structure is to use an electrode containing the positive electrode material as the positive electrode, an electrode containing a carbon-based negative electrode material as the negative electrode, a separator disposed between the positive electrode and the negative electrode, and whose interior is filled with electrolyte, after performing a repetitive cycle consisting of 1C full charge and 1C full discharge at 60°C for 500 times, the amount of metal contained in the negative electrode obtained by disassembling the pouch cell is less than 1,000 ppm by mass of the sum of the mass of lithium metal phosphate particles of general formula (1) contained in the first aggregate and the second aggregate in the positive electrode.

5. A lithium-ion secondary battery, comprising at least a positive electrode, a negative electrode, and an electrolyte. The positive electrode comprises the positive electrode material for lithium-ion secondary batteries as described in claim 1 or 2.

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