Positive electrode material

Abstract

The present invention provides a positive electrode material capable of further improving load characteristics in a lithium ion secondary battery. Provided is a positive electrode material comprising: primary particles which include a lithium transition metal compound having an olivine structure; and carbon, to the surface of which the primary particles adhere, said positive electrode material including secondary particles which are obtained by aggregation of a plurality of the primary particles. In the positive electrode material, the content of carbon is more than 0.5 mass% but not more than 1.8 mass% with respect to the positive electrode material, and the lithium transition metal compound included in the positive electrode material has a crystallite size of 50-70 nm. The specific surface area of the positive electrode material is 14-45 m2 / g<sp / >.

Description

Cathode materials

[0001] The present disclosure relates to positive electrode materials.

[0002] Lithium transition metal compounds having an olivine structure are known as positive electrode active materials that can be used in lithium-ion secondary batteries. For example, Japanese Patent Application Laid-Open No. 2019-149355 proposes an electrode material having secondary particles that are aggregates of primary particles of an electrode active material and a carbonaceous coating that covers the secondary particles.

[0003] An object of one aspect of the present disclosure is to provide a positive electrode material that can further improve the load characteristics of a lithium ion secondary battery.

[0004] The first aspect is a cathode material comprising primary particles containing a lithium transition metal compound having an olivine structure, carbon attached to the surfaces of the primary particles, and secondary particles formed by aggregation of a plurality of primary particles. The cathode material has a carbon content of more than 0.5 mass% and not more than 1.8 mass% relative to the cathode material. The lithium transition metal compound constituting the cathode material has a crystallite diameter of 50 nm or more and 70 nm or less. The specific surface area of ​​the cathode material is 14 m ２ / g or more 45m ２ / g or less.

[0005] According to one aspect of the present disclosure, it is possible to provide a positive electrode material that can further improve the load characteristics of a lithium ion secondary battery.

[0006] In this specification, the term "process" refers not only to an independent process, but also to processes that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved. Furthermore, the content of each component in a composition refers to the total amount of the multiple substances present in the composition, unless otherwise specified, when multiple substances corresponding to each component are present in the composition. Furthermore, the upper and lower limits of the numerical ranges described in this specification can be arbitrarily selected and combined from the numerical values ​​exemplified as numerical ranges. Below, embodiments of the present invention are described in detail. However, the embodiments described below are examples of positive electrode materials for embodying the technical concept of the present invention, and the present invention is not limited to the positive electrode materials shown below.

[0007] The positive electrode material contains a lithium transition metal compound having an olivine structure and secondary particles formed by aggregation of a plurality of primary particles with carbon attached to the surface. The carbon content in the positive electrode material is greater than 0.5 mass% and less than or equal to 1.8 mass% of the positive electrode material. The lithium transition metal compound constituting the positive electrode material has a crystallite diameter of 50 nm or more and 70 nm or less. The specific surface area of ​​the positive electrode material is 14 m ２ / g or more 45m ２ The positive electrode material can be efficiently produced, for example, by a method for producing a positive electrode material, which will be described later.

[0008] When a positive electrode material is composed of secondary particles consisting of a plurality of primary particles containing a lithium transition metal compound having a predetermined crystallite size, and the carbon content is a predetermined amount and the specific surface area is a predetermined amount, the capacity density (e.g., 5C capacity density) under high load conditions in a lithium ion secondary battery constructed using the positive electrode material can be improved. This can be explained, for example, as follows: The larger the crystallite size (primary particle size), the greater the migration distance of lithium ions within the lithium transition metal compound, and therefore the smaller the crystallite size, the better the lithium ion conductivity. Furthermore, the larger the specific surface area, the greater the area for lithium intercalation and deintercalation, which is thought to improve output, and is particularly important under high load conditions. Furthermore, while an increase in carbon content is thought to improve electronic conductivity, an excessively high carbon content is thought to reduce lithium ion conductivity and reduce packing properties. Note that by keeping the crystallite size below a certain size and the specific surface area not exceeding a certain size, the denseness of the secondary particles is not impaired, thereby increasing discharge capacity under high load conditions while ensuring packing properties, and improving load characteristics.

[0009] A positive electrode formed using a positive electrode material has excellent packing properties in the positive electrode active material layer that constitutes the positive electrode. The packing properties of the positive electrode active material layer can be evaluated by the density of a pellet made of the positive electrode material and formed under predetermined conditions. When the pellet formation condition is 3.5 MPa, the density of the pellet made of the positive electrode material is, for example, 1.8 g / cm.３ 2.3g / cm or more ３ or less, preferably 1.9 g / cm ３ Above, 1.93g / cm ３ Above, 1.96g / cm ３ Above, 2.0g / cm ３ Above, 2.04g / cm ３ or more, or 2.05 g / cm ３ The pellet density is preferably 2.2 g / cm or more. ３ Below, 2.15g / cm ３ Below, 2.12g / cm ３ Below, 2.1g / cm ３ Below, 2.09g / cm ３ or less, or 2.08 g / cm ３ It may be the following:

[0010] The primary particles may contain a lithium transition metal compound having an olivine structure, or may consist essentially of a lithium transition metal compound having an olivine structure. Here, "substantially" means that components other than the lithium transition metal compound having an olivine structure that are inevitably contained in the primary particles are not excluded, and the content of components other than the lithium transition metal compound having an olivine structure in the primary particles is, for example, 1% by mass or less, preferably 0.5% by mass or less.

[0011] The lithium transition metal compound contained in the primary particles is a phosphate compound containing at least a first metal containing at least one element selected from the group consisting of cobalt (Co), manganese (Mn), nickel (Ni), iron (Fe), copper (Cu), and chromium (Cr), lithium (Li), phosphorus (P), and oxygen (O). In addition to the first metal, lithium, and phosphorus, the lithium transition metal compound may further contain, as necessary, a second metal containing at least one element selected from the group consisting of Group 2 elements, Group 3 elements, Group 4 elements, Group 12 elements, Group 13 elements, and Group 14 elements.

[0012] The first metal preferably contains at least iron and may further contain at least one selected from the group consisting of cobalt, manganese, nickel, copper, and chromium. The iron content in the first metal may be, for example, a ratio of the number of moles of iron to the total number of moles of the first metal of 0.7 to 1, preferably 0.8 or more, 0.9 or more, or 0.95 or more. When the iron content in the first metal is within the above range, a decrease in charge / discharge capacity tends to be suppressed in a secondary battery using the positive electrode material.

[0013] The second metal may preferably include at least one selected from the group consisting of magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), scandium (Sc), yttrium (Y), titanium (Ti), zinc (Zn), boron (B), aluminum (Al), gallium (Ga), indium (In), silicon (Si), and germanium (Ge).

[0014] The lithium transition metal compound may have the following composition, for example: The ratio of the number of moles of lithium to the number of moles of phosphorus may be greater than 0.9 and less than 1.1, and preferably 0.95 or more, 0.96 or more, or 0.98 or more, and 1.05 or less, 1.02 or less, or 1.00 or less. The ratio of the number of moles of the first metal to the number of moles of phosphorus may be greater than 0.8 and 1 or less, and preferably 0.9 or more, 0.92 or more, 0.95 or more, 0.96 or more, or 0.97 or more, and 1 or less, 0.99 or less, 0.98 or less, or 0.97 or less. Furthermore, the ratio of the number of moles of the second metal to the number of moles of phosphorus may be 0 or more and less than 1, and preferably 0 or more and 0.5 or less. Furthermore, the ratio of the total number of moles of the first metal and the second metal to the number of moles of phosphorus may be greater than 0.9 and less than 1.1, and may preferably be 0.95 or more, 0.96 or more, or 0.97 or more, and may be 1.05 or less, 1 or less, 0.99 or less, 0.98 or less, or 0.97 or less.

[0015] The lithium transition metal compound may have a composition represented by the following formula (1): Li ｘ M １ｙ M ２ ｚ P.O. ４＋α (1)

[0016] In formula (1), M １ contains at least one selected from the group consisting of Co, Mn, Ni, Fe, Cu and Cr. ２ contains at least one element selected from the group consisting of Mg, Ca, Sr, Ba, Sc, Y, Ti, Zn, B, Al, Ga, In, Si, and Ge. x, y, z, and α may satisfy the following conditions: 0.9<x<1.1, 0.8<y≦1, 0≦z<1, 0.9<y+z<1.1, −0.5≦α≦0.5, and preferably satisfy the following conditions: 0.95≦x≦1.05, 0.9≦y≦1, 0≦z≦0.5, 0.95≦y+z≦1.05, −0.3≦α≦0.5.

[0017] The average particle size (Dm) of the secondary particles contained in the positive electrode material may be, for example, 1 μm or more and 20 μm or less, preferably 2 μm or more, or 4 μm or more. The average particle size of the secondary particles may be preferably 18 μm or less, or 16 μm or less. The average particle size of the secondary particles may be a volume average particle size, and the volume average particle size of the secondary particles is determined as the particle size corresponding to 50% of the cumulative volume from the small diameter side in the volume-based cumulative particle size distribution. The volume-based cumulative particle size distribution is measured, for example, using a laser diffraction particle size distribution analyzer. When the average particle size of the secondary particles is within the above range, workability during production tends to be improved.

[0018] The crystallite diameter of the lithium transition metal compound constituting the positive electrode material may be, for example, 50 nm or more and 70 nm or less, preferably 55 nm or more, 60 nm or more, 62 nm or more, or 64 nm or more, and preferably 68 nm or less, 67 nm or less, or 66 nm or less. When the crystallite diameter of the lithium transition metal compound is within the above range, lithium ion conductivity can be increased while suppressing an increase in the carbon coating amount, and load characteristics tend to be further improved. The crystallite diameter of the lithium transition metal compound corresponds to the crystallite diameter of the crystalline phase of the lithium transition metal compound contained in the primary particles constituting the secondary particles. The crystallite diameter of the lithium transition metal compound is measured, for example, as follows. The X-ray diffraction (XRD) pattern of the sample positive electrode material is measured using an X-ray diffractometer. The crystallite diameter of the sample can be determined by fitting the XRD pattern obtained by measurement to the XRD pattern of a crystal structure model of the lithium transition metal compound available from the International Center for Diffraction Data (ICDD) or the like using the least squares method.

[0019] Carbon adheres to the surfaces of the primary particles that make up the secondary particles. The carbon adhesion may be, for example, physical adsorption due to van der Waals forces. The attached carbon may be in the form of particles or a film, preferably a film. The amount of carbon adhered to the primary particles can be evaluated as the carbon content in the positive electrode material. The carbon content in the positive electrode material may be, for example, greater than 0.5 mass% and less than 1.8 mass%, preferably less than 1.6 mass%, less than 1.5 mass%, or less than 1.4 mass%, relative to the total mass of the positive electrode material. The carbon content in the positive electrode material may be, for example, 0.8 mass% or more, preferably more than 0.9 mass%, more than 1.0 mass%, more than 1.1 mass%, or more than 1.2 mass%, relative to the total mass of the positive electrode material. A carbon content in the positive electrode material within the above range tends to improve load characteristics while maintaining a high pellet density. The carbon content in the positive electrode material can be measured, for example, using a total organic carbon (TOC) analyzer.

[0020] The specific surface area of ​​the positive electrode material is, for example, 14 m ２ / g or more 45m ２ / g or less, preferably 15m ２ / g or more, 17m ２ / g or more, 20m ２ / g or more, or 22m ２ The specific surface area of ​​the positive electrode material is preferably 35 m ２ / g or less, 30m ２ / g or less, 28m ２ / g or less, 26m ２ / g or less, or 24m ２ / g or less. When the specific surface area of ​​the positive electrode material is within the above range, the reaction area where lithium intercalation and deintercalation occurs is increased while suppressing an increase in the amount of carbon coating, which tends to further improve load characteristics. The specific surface area of ​​the positive electrode material may be the specific surface area measured by the BET method, and is measured by a single-point method using nitrogen gas based on the BET (Brunauer Emmett Teller) theory.

[0021] The oil absorption of the positive electrode material may be, for example, less than 50 ml / 100 g with respect to N-methyl-2-pyrrolidone (NMP), preferably 40 ml / 100 g or less, 35 ml / 100 g or less, or 34 ml / 100 g or less. The oil absorption may be, for example, 10 ml / 100 g or more, preferably 15 ml / 100 g or more, 20 ml / 100 g or more, 25 ml / 100 g or more, 28 ml / 100 g or more, or 30 ml / 100 g or more. When the oil absorption is within the above range, the secondary particles can be densified, and the pellet density tends to be improved. The oil absorption of the positive electrode material is measured in accordance with the method specified in JIS K5101-13-1.

[0022] In the log differential pore volume distribution of the positive electrode material obtained by mercury porosimetry, the pore mode diameter within the pore diameter range of 0.01 μm to 10 μm may be in the range of 0.01 μm to 0.2 μm. The pore mode diameter within the pore diameter range of 0.01 μm to 10 μm may be preferably in the range of 0.015 μm or more or 0.02 μm or more, and preferably in the range of 0.1 μm or less or 0.08 μm or less. When the pore mode diameter is within the above range, the pellet density can be increased while maintaining the conductive path of lithium ions, and the load characteristics may be further improved.

[0023] In a positive electrode material, the larger the product of the specific surface area and the crystallite size of the lithium transition metal compound, and the smaller the product of the oil absorption and the carbon content, the more likely it is that the load characteristics of the secondary battery will be improved. ２ The correlation value (hereinafter simply referred to as "correlation value") obtained by dividing the product of the oil absorption (ml / 100g) of the positive electrode material and the crystallite diameter (nm) of the lithium transition metal compound by the product of the oil absorption (ml / 100g) and the carbon content (mass%) of the positive electrode material is the capacity density (mAh / cm) under high load conditions. ３ ) The correlation value may be, for example, 20 or more, preferably 28 or more, 30 or more, or 32 or more. The correlation value may be, for example, 50 or less, 45 or less, or 40 or less.

[0024] A positive electrode for a lithium ion secondary battery includes a current collector and a positive electrode active material layer disposed on the current collector and including the above-described positive electrode material. A lithium ion secondary battery including such a positive electrode can achieve excellent charge / discharge capacity.

[0025] The density of the positive electrode active material layer is, for example, 1.6 g / cm ３ 2.8g / cm or more ３ or less, preferably 1.8 g / cm ３ 2.6g / cm or more ３ Below, 1.9g / cm ３ 2.5g / cm or more ３ or less, or 2.0 g / cm ３ 2.4g / cm or more３ The density of the positive electrode active material layer is calculated by dividing the mass of the positive electrode active material layer by the volume of the positive electrode active material layer. Here, the density of the positive electrode active material layer can be adjusted by applying pressure to an electrode composition described later on a current collector.

[0026] Examples of materials for the current collector include aluminum, nickel, and stainless steel. The positive electrode active material layer can be formed by applying an electrode composition obtained by mixing the above-mentioned positive electrode material, conductive additive, binder, and the like together with a solvent onto the current collector, followed by drying and pressure treatments. Examples of conductive additives include natural graphite, artificial graphite, and acetylene black. Examples of binders include polyvinylidene fluoride, polytetrafluoroethylene, and polyamide acrylic resin. Examples of solvents include N-methyl-2-pyrrolidone (NMP).

[0027] Lithium-ion secondary battery The lithium-ion secondary battery includes the above-described lithium-ion secondary battery positive electrode. The lithium-ion secondary battery is configured to include, in addition to the lithium-ion secondary battery positive electrode, a lithium-ion secondary battery negative electrode, a non-aqueous electrolyte, a separator, etc. For the lithium-ion secondary battery negative electrode, non-aqueous electrolyte, separator, etc., those for lithium-ion secondary batteries described in, for example, JP 2002-075367 A, JP 2011-146390 A, JP 2006-12433 A (the disclosures of which are incorporated herein by reference in their entirety) can be used as appropriate.

[0028] 2. Method for Producing Positive Electrode Material A method for producing a positive electrode material may include: a preparation step of preparing a raw material mixture containing a first metal source containing at least one selected from the group consisting of cobalt, manganese, nickel, iron, copper, and chromium, a lithium source, a carbon source, and a liquid medium, wherein at least one of the first metal source and the lithium source contains a phosphate; a granulation step of granulating the raw material mixture to obtain a precursor having a volume average particle size of 5 μm to 30 μm; and a heat treatment step of heat-treating the precursor at a temperature in the range of 500° C. to 700° C. to obtain a heat-treated product. The heat-treated product obtained in the heat treatment step may contain a positive electrode material.

[0029] In the preparation step, a raw material mixture containing a first metal source, a lithium source, a carbon source, and a liquid medium is prepared. The first metal source may include a metal compound containing a first metal atom, which includes at least one selected from the group consisting of cobalt, manganese, nickel, iron, copper, and chromium, or a simple substance of the first metal atom. Examples of the metal compound include phosphates, nitrates, carbonates, oxides, and the like, and may include at least phosphate. The first metal source includes at least an iron compound, preferably iron phosphate (e.g., Fe ３ (P.O. ４ ) ２ ) and may further contain a metal compound containing at least one selected from the group consisting of cobalt, manganese, nickel, copper, and chromium. When the iron contained in the first metal source is divalent iron, carbonization of the carbon source tends to occur before crystal growth of the lithium transition metal compound, which tends to increase the specific surface area of ​​the resulting positive electrode material and increase the discharge capacity under high load conditions. The ratio of the number of moles of iron contained in the first metal source to the total number of moles of the first metal atoms contained in the first metal source may be, for example, 0.7 or more and 1 or less, preferably 0.8 or more, 0.9 or more, or 0.95 or more.

[0030] The content of the first metal source contained in the raw material mixture may be, for example, greater than 0.8 and not more than 1.8, and preferably not less than 0.9 and not more than 1.6, as the ratio of the number of moles of the first metal atoms to the total number of moles of phosphorus contained in the raw material mixture.

[0031] The lithium source may include a lithium compound, etc. Examples of the lithium compound include lithium phosphate, lithium carbonate, and lithium hydroxide. The lithium source preferably contains at least lithium phosphate (e.g., Li ３ P.O. ４The content of the lithium source contained in the raw material mixture may be, for example, greater than 0.9 and less than 1.1, and preferably 0.95 or more and 1.05 or less, as a ratio of the number of moles of lithium contained in the lithium source to the total number of moles of phosphorus contained in the raw material mixture. The content of the lithium source contained in the raw material mixture may be, for example, 1 or more and 1.1 or less, as a ratio of the number of moles of lithium contained in the lithium source to the number of moles of the first metal atoms contained in the first metal source. The content may be preferably 1.01 or more, or 1.02 or more, and may be preferably 1.07 or less, or 1.05 or less.

[0032] The carbon source may be elemental carbon or a carbon compound capable of generating carbon by heat treatment. Examples of carbon compounds that can be contained in the carbon source include dextrin, sucrose, starch, etc., and the carbon source may contain at least one selected from the group consisting of these. From the viewpoint of the carbonization rate, the carbon source preferably contains dextrin.

[0033] The content of the carbon source contained in the raw material mixture may be, for example, 15 mass% or more and 30 mass% or less, preferably 16 mass% or more, 18 mass% or more, 19 mass% or more, or 20 mass% or more, and preferably 25 mass% or less, 24 mass% or less, or 23 mass% or less, relative to the total mass of the first metal atoms contained in the raw material mixture.

[0034] The liquid medium may contain at least water, and may further contain a water-soluble organic solvent such as alcohol or acetone in addition to water. The raw material mixture may be configured as a fluid slurry. The concentration of the first metal source contained in the raw material mixture may be, for example, 3% by mass or more and 15% by mass or less, preferably 4% by mass or more and 10% by mass or less, in terms of the concentration of the first metal atoms.

[0035] The raw material mixture may further contain, as necessary, a second metal source containing a second metal atom containing at least one element selected from the group consisting of Group 2 elements, Group 3 elements, Group 4 elements, Group 12 elements, Group 13 elements, and Group 14 elements. The second metal source may contain a metal compound containing the second metal atom, a simple substance of the second metal atom, or the like. Examples of the metal compound include phosphates, oxides, carbonates, halides, and the like, and may contain at least a phosphate.

[0036] The second metal atom may preferably include at least one selected from the group consisting of magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), scandium (Sc), yttrium (Y), titanium (Ti), zinc (Zn), boron (B), aluminum (Al), gallium (Ga), indium (In), silicon (Si), and germanium (Ge).

[0037] The content of the second metal source contained in the raw material mixture may be, for example, a ratio of the number of moles of the second metal atoms to the total number of moles of phosphorus contained in the raw material mixture of 0 or more and less than 1, and preferably 0 or more and 0.5 or less. Furthermore, the ratio of the total number of moles of the first metal atoms and the second metal atoms to the total number of moles of phosphorus contained in the raw material mixture may be more than 0.9 and less than 1.1, and preferably 0.95 or more and 1.05 or less.

[0038] The raw material mixture may further contain a phosphate compound as needed. Examples of the phosphate compound include ammonium phosphate and phosphoric acid. Furthermore, ammonium dihydrogen phosphate may be used as the ammonium phosphate. The content of the phosphate compound contained in the raw material mixture may be, for example, 0 mol% or more and 3 mol% or less (0 mol% or more and 0.03 mol% or less) in terms of the ratio of the number of moles to the total number of moles of the first metal atoms contained in the raw material mixture, and may preferably be 0.5 mol% or more and 2.5 mol% or less. Also preferably, it may be 1.0 mol% or more, or 1.5 mol% or more, and may be 2 mol% or less, or 1.8 mol% or less. When the amount of the phosphate compound contained in the raw material mixture is within the above range, a positive electrode material with improved crystallinity tends to be obtained.

[0039] The raw material mixture may contain a pH adjuster as needed. Examples of pH adjusters include citric acid, sulfuric acid, and ammonium carbonate. The content of the pH adjuster in the raw material mixture may be appropriately adjusted so that the raw material mixture has a desired pH.

[0040] The raw material mixture can be prepared by pulverizing a composition containing a first metal source, a lithium source, a carbon source, a liquid medium, and, if necessary, a second metal source, a phosphate compound, a pH adjuster, etc. The pulverization can be carried out using, for example, a ball mill, a vibration mill, a roll mill, a mortar and pestle mill, etc. The raw material mixture obtained by the pulverization may be prepared as a slurry having fluidity.

[0041] The pulverization treatment can be carried out so that the volume average particle size of the raw material mixture is 0.05 μm or more and 1 μm or less, preferably 0.1 μm or more and 0.5 μm or less. The solid content concentration of the raw material mixture may be, for example, 5 mass % or more and 50 mass % or less, preferably 10 mass % or more and 30 mass % or less. The volume average particle size of the raw material mixture is measured using a laser diffraction particle size distribution analyzer.

[0042] In the granulation step, at least a portion of the liquid medium contained in the prepared raw material mixture is removed to obtain a dried precursor. The volume average particle size of the precursor may be, for example, 5 μm or more and 30 μm or less, and preferably 7 μm or more and 25 μm or less. Methods for drying the raw material mixture include spray drying and fluidized bed drying, with spray drying being preferred. Here, the volume average particle size of the precursor is measured using a laser diffraction particle size distribution analyzer.

[0043] In the heat treatment step, the precursor is heat-treated to obtain a heat-treated product. The heat treatment temperature may be, for example, in the range of 500°C to 700°C, preferably in the range of 600°C to 650°C. The heat treatment step may include raising the temperature to a predetermined heat treatment temperature, maintaining the heat treatment temperature, and lowering the temperature from the heat treatment temperature. The rate of temperature rise from room temperature to the heat treatment temperature may be, for example, 2.5°C / min to 5°C / min, preferably 3.0°C / min or more or 3.3°C / min or more, and preferably 4.5°C / min or less or 4.2°C / min or less. The heat treatment time for maintaining the heat treatment temperature may be, for example, 0.1 hours to 15 hours, preferably 0.2 hours to 0.3 hours, or 0.4 hours or more, and preferably 12 hours or less, 8 hours or less, or 5 hours or less. The rate of temperature decrease from the heat treatment temperature to room temperature may be, for example, 1° C. / min or more and 600° C. / min or less.

[0044] The atmosphere in the heat treatment step may be an inert gas atmosphere containing, for example, a rare gas such as nitrogen or argon. The inert gas atmosphere may have an inert gas content of, for example, 90% by volume or more, preferably 95% by volume or more, or 98% by volume or more. The heat treatment may also be carried out under a flow of inert gas.

[0045] The pressure in the atmosphere in the heat treatment step may be atmospheric pressure, or may be under pressurized or reduced pressure conditions. The pressurized condition may be, for example, a gauge pressure greater than 0 MPa and less than 0.1 MPa, preferably greater than 0 MPa and less than 0.05 MPa. The reduced pressure condition may be, for example, a gauge pressure greater than -0.1 MPa and less than 0 MPa, preferably greater than -0.05 MPa and less than 0 MPa.

[0046] The heat treatment of the precursor can be carried out using, for example, a box-type atmospheric furnace, a tubular furnace, a carbon rotary kiln, etc. The heat treatment of the precursor can be carried out, for example, by filling the precursor into a crucible, boat, etc. made of aluminum oxide. In addition to aluminum oxide materials, carbon materials such as graphite, boron nitride (BN), molybdenum materials, etc. can also be used.

[0047] The heat-treated product obtained in the heat treatment step may be subjected to treatments such as pulverization, dispersion, washing, filtration, classification, etc., and may be subjected to at least pulverization and classification.

[0048] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[0049] In the following examples and comparative examples, the ratio of the number of moles of lithium to the number of moles of phosphorus and the ratio of the number of moles of iron to the number of moles of phosphorus were measured using an inductively coupled plasma atomic emission spectrometer (ICP-AES; manufactured by PerkinElmer). The carbon content was measured using a total organic carbon meter (TOC meter; manufactured by Shimadzu Corporation, ON-LINE TOC-V). ＣＳＨ ) was measured. The volume average particle size was measured using a laser diffraction particle size distribution analyzer (SALD-3100, manufactured by Shimadzu Corporation). The specific surface area by the BET method was measured by a single-point method using nitrogen gas. The crystallite size was measured using X-ray diffraction. Specifically, an X-ray diffraction spectrum (tube current 45 mA, tube voltage 200 kV) was measured using CuKα radiation, and the diffraction peak obtained near 2θ = 32 degrees (attributed to the (031) plane in the space group Pmnb) was fitted by the least squares method using a PseudoVoigt function to calculate the values ​​of θ and β. From the diffraction peak attributable to the (031) plane determined by X-ray diffraction, the crystallinity was calculated using the following formula (2):

[0050] D=K'λ / (βcosθ) (2)

[0051] In the above formula, D represents crystallinity (Å), λ represents the wavelength of the X-ray source (1.54 Å in the case of CuKα), β represents the integral width (radian), θ represents the diffraction angle (degree), and K' is measured using sintered Si for optical system adjustment (manufactured by Rigaku Denki Co., Ltd.), and the value at which the crystallinity D due to the (022) plane is 1000 Å when using the above formula (2) is used. The value obtained by multiplying the obtained crystallinity D (Å) by 10 is the crystallite diameter (nm). The oil absorption relative to NMP was measured by dropping NMP while mixing to form a slurry, and the amount of NMP was measured. The pore mode diameter was also measured using a POREMASTER-60 manufactured by Anton Paar (formerly known as Quantachrome).

[0052] Example 1 Iron phosphate (Fe ３ (P.O. ４ ) ２ ) in pure water to prepare a slurry having an iron atom concentration of 8.02 mass %, and 1496.3 g of lithium phosphate (Li ３ P.O. ４ 86.3 g of dextrin, 4.2 g of ammonium dihydrogen phosphate, 3.0 g of citric acid, and 1,233 g of pure water were placed in a ball mill and mixed for 40 hours using zirconia balls to finely grind the mixture. Then, 177.6 g of a 15% by mass dextrin solution was added, and the mixture was further ground for 3 hours.

[0053] The ratio of the number of moles of lithium atoms contained in the lithium phosphate to the number of moles of iron atoms contained in the raw material mixture (Li / Fe) was 1.04, and the ratio of the number of moles of ammonium dihydrogen phosphate to the number of moles of iron atoms contained in the raw material mixture (PO ４ The ratio of the mass of dextrin to the mass of iron atoms contained in the raw material mixture (C / Fe) was 22 mass%, and the ratio of the mass of citric acid to the mass of iron atoms contained in the raw material mixture (C / Fe) was 2.5 mass%.

[0054] The pulverized raw material mixture was spray-dried to obtain a precursor with an average particle size of 7 μm to 8 μm. Scanning electron microscope (SEM) observation revealed that the particle size of the primary particles constituting the precursor was several tens of nanometers. 50 g of the obtained precursor was packed into an alumina crucible measuring 90 mm in length and width and 50 mm in height, and heat-treated at 650°C for 11 hours in a nitrogen gas atmosphere to obtain the heat-treated product of Example 1. During the heat treatment, nitrogen gas was flowed horizontally near the top edge of the crucible at a rate of 10 L / min.

[0055] The phase of the heat-treated product was identified using an X-ray diffractometer. As a result of analysis using CuKα rays (wavelength: λ=1.54 nm) as X-rays, the composition was found to be LiFePO ４ In the following examples and comparative examples, the heat-treated product was confirmed to have a composition of LiFePO ４ An olivine-type lithium transition metal compound represented by the formula:

[0056] The heat-treated product obtained in Example 1 had a ratio of the number of moles of lithium to the number of moles of phosphorus (Li / P) of 0.99, a ratio of the number of moles of iron to the number of moles of phosphorus (Fe / P) of 0.97, a carbon content (C) of 1.2 mass%, a volume average particle size (Dm) of 7.6 μm, and a specific surface area (BET) measured by the BET method of 22 m ２ The positive electrode material had a specific surface area of ​​3.5 μm / g, an oil absorption capacity of 31 ml / 100 g in NMP, and a crystallite diameter of 65.5 nm for the olivine-type lithium transition metal compound. The correlation value obtained by dividing the product of the specific surface area of ​​the positive electrode material and the crystallite diameter of the lithium transition metal compound by the product of the oil absorption capacity of the positive electrode material and the carbon content of the positive electrode material was 39. Furthermore, the pore mode diameter within the pore diameter range of 0.01 μm to 10 μm was 0.025 μm.

[0057] Example 2 A heat-treated product of Example 2 was prepared in the same manner as in Example 1, except that the amount of the dextrin solution used in Example 1 was changed to 224.0 g.

[0058] The heat-treated product obtained in Example 2 had a ratio of the number of moles of lithium to the number of moles of phosphorus of 0.99, a ratio of the number of moles of iron to the number of moles of phosphorus of 0.97, a carbon content of 1.8 mass%, a volume average particle size of 6.9 μm, and a specific surface area measured by the BET method of 35 m ２ The olivine-type lithium transition metal compound had an oil absorption of 39 ml / 100 g and an NMP oil absorption of 39 ml / 100 g. The crystallite diameter of the olivine-type lithium transition metal compound was 59.8 nm.

[0059] Example 3 A heat-treated product of Example 3 was prepared in the same manner as in Example 1, except that the amount of the dextrin solution used in Example 1 was changed to 184.0 g.

[0060] The heat-treated product obtained in Example 3 had a ratio of the number of moles of lithium to the number of moles of phosphorus of 0.99, a ratio of the number of moles of iron to the number of moles of phosphorus of 0.97, a carbon content of 1.4 mass%, a volume average particle size of 7.6 μm, and a specific surface area measured by the BET method of 24 m ２ The olivine-type lithium transition metal compound had an oil absorption of 33 ml / 100 g and an NMP oil absorption of 33 ml / 100 g. The crystallite diameter of the olivine-type lithium transition metal compound was 64.8 nm.

[0061] Example 4 A heat-treated product of Example 4 was prepared in the same manner as in Example 1, except that the amount of the dextrin solution used in Example 1 was changed to 152.0 g.

[0062] The heat-treated product obtained in Example 4 had a ratio of the number of moles of lithium to the number of moles of phosphorus of 1.00, a ratio of the number of moles of iron to the number of moles of phosphorus of 0.98, a carbon content of 1.1 mass%, a volume average particle size of 6.9 μm, and a specific surface area measured by the BET method of 15 m ２ The olivine-type lithium transition metal compound had an oil absorption of 30 ml / 100 g and an NMP oil absorption of 30 ml / 100 g. The crystallite diameter of the olivine-type lithium transition metal compound was 68.4 nm.

[0063] Comparative Example 1 A heat-treated product of Comparative Example 1 was prepared in the same manner as in Example 2, except that the amount of ammonium dihydrogen phosphate was changed to 3.6 g.

[0064] The heat-treated product obtained in Comparative Example 1 had a ratio of the number of moles of lithium to the number of moles of phosphorus of 1.00, a ratio of the number of moles of iron to the number of moles of phosphorus of 0.98, a carbon content of 1.9 mass%, a volume average particle size of 6.7 μm, and a specific surface area measured by the BET method of 31 m ２ The olivine-type lithium transition metal compound had an oil absorption of 39 ml / 100 g and an NMP oil absorption of 39 ml / 100 g. The crystallite diameter of the olivine-type lithium transition metal compound was 49.0 nm.

[0065] Comparative Example 2 A heat-treated product of Comparative Example 23 was prepared in the same manner as in Example 1, except that ammonium dihydrogen phosphate was not added and the amount of the dextrin solution was changed to 136.0 g.

[0066] The heat-treated product obtained in Comparative Example 2 had a ratio of the number of moles of lithium to the number of moles of phosphorus of 1.01, a ratio of the number of moles of iron to the number of moles of phosphorus of 0.99, a carbon content of 0.5 mass%, and a specific surface area measured by the BET method of 17 m ２ The olivine-type lithium transition metal compound had an oil absorption of 41 ml / 100 g and an NMP oil absorption of 41 ml / 100 g. The crystallite diameter of the olivine-type lithium transition metal compound was 51.9 nm.

[0067] Comparative Example 3 A heat-treated product of Comparative Example 3 was prepared in the same manner as in Example 1, except that the amount of ammonium dihydrogen phosphate was changed to 4.7 g, the amount of dextrin solution was changed to 240.0 g, and the temperature for the heat treatment of the precursor was changed to 700°C.

[0068] The heat-treated product obtained in Comparative Example 3 had a ratio of the number of moles of lithium to the number of moles of phosphorus of 1.00, a ratio of the number of moles of iron to the number of moles of phosphorus of 0.98, a carbon content of 1.5 mass%, and a specific surface area measured by the BET method of 23 m ２ The olivine-type lithium transition metal compound had an oil absorption of 39 ml / 100 g and an NMP oil absorption of 39 ml / 100 g. The crystallite diameter of the olivine-type lithium transition metal compound was 79.0 nm.

[0069] Comparative Example 4 A heat-treated product of Comparative Example 42 was prepared in the same manner as in Example 1, except that the amount of ammonium dihydrogen phosphate was changed to 4.7 g and the amount of the dextrin solution was changed to 240.0 g.

[0070] The heat-treated product obtained in Comparative Example 4 had a ratio of the number of moles of lithium to the number of moles of phosphorus of 0.99, a ratio of the number of moles of iron to the number of moles of phosphorus of 0.99, a carbon content of 1.9 mass%, and a specific surface area measured by the BET method of 46 m ２ The olivine-type lithium transition metal compound had an oil absorption of 50 ml / 100 g and an NMP oil absorption of 50 ml / 100 g. The crystallite diameter of the olivine-type lithium transition metal compound was 61.3 nm.

[0071] Comparative Example 5 A heat-treated product of Comparative Example 5 was produced in the same manner as in Example 4, except that the space containing the alumina crucible was conditioned with a nitrogen gas atmosphere before the heat treatment of the precursor, and the nitrogen gas was not flowed at 10 L / min during the heat treatment.

[0072] The heat-treated product obtained in Comparative Example 5 had a ratio of the number of moles of lithium to the number of moles of phosphorus of 1.00, a ratio of the number of moles of iron to the number of moles of phosphorus of 0.97, a carbon content of 1.3 mass%, a volume average particle size of 7.3 μm, and a specific surface area measured by the BET method of 13 m ２ The olivine-type lithium transition metal compound had an oil absorption of 43 ml / 100 g and an NMP oil absorption of 43 ml / 100 g. The crystallite diameter of the olivine-type lithium transition metal compound was 69.3 nm.

[0073] Evaluation Pellet Density The pellet density was evaluated using the heat-treated products produced in the Examples and Comparative Examples. 2.0000 g of the olivine-type lithium transition metal compound heat-treated product was weighed and filled into a 20 mm mold, compressed at 3.5 MPa, and the reduction in height was measured. The pellet density was calculated by measuring the weight per volume. The measurement results are shown in Table 1.

[0074]

[0075] The discharge capacity of the positive electrode active materials of the Examples and Comparative Examples was evaluated as follows.

[0076] Assembly of Evaluation Battery Preparation of Positive Electrode A positive electrode mixture slurry was prepared by dispersing 87.5 parts by mass of the positive electrode active material, 2.5 parts by mass of acetylene black, and 10 parts by mass of polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP). The obtained positive electrode mixture slurry was applied to an aluminum foil current collector, dried, and then compression-molded using a roll press. The positive electrode was then cut to a predetermined size to prepare a positive electrode.

[0077] A negative electrode slurry was prepared by dissolving 97.5 parts by mass of artificial graphite, 1.5 parts by mass of carboxymethyl cellulose (CMC), and 1.0 part by mass of SBR (styrene butadiene rubber) in pure water. The resulting negative electrode slurry was applied to a copper foil current collector, dried, and then compression-molded using a roll press. The negative electrode was then cut to a predetermined size.

[0078] After attaching lead electrodes to the current collectors of the positive and negative electrodes, a separator was placed between the positive and negative electrodes, and the resultant was housed in a bag-shaped laminate pack. This was then vacuum dried at 65°C to remove moisture adsorbed to each component. An electrolyte solution was then injected into the laminate pack under an argon atmosphere, and the pack was sealed to prepare a battery for evaluation. The electrolyte solution was prepared by mixing ethylene carbonate (EC) and methyl ethyl carbonate (MEC) in a volume ratio of 3:7, and dissolving lithium hexafluorophosphate (LiPF ６ ) was dissolved to a concentration of 1 mol / L. The evaluation battery thus obtained was placed in a thermostatic chamber at 25° C. and aged with a weak current, and then evaluated as follows.

[0079] Discharge Capacity Using the prepared evaluation battery, constant voltage / constant current charging (cutoff current 0.005 C) was performed at a charge voltage of 3.65 V and a charge current of 0.1 C, followed by constant current discharging at a discharge end voltage of 2.0 V and a discharge current of 5 C, to measure the discharge capacity (mAh / g), and the 5 C capacity density was calculated using the measured pellet density value.

[0080]

[0081] It can be seen that the evaluation battery using the positive electrode material of the example has a high 5C capacity density and improved load characteristics.

[0082] The disclosure of Japanese Patent Application No. 2021-181867 (filing date: November 8, 2021) is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards described herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. a positive electrode material including primary particles containing a lithium transition metal compound having an olivine structure and carbon attached to surfaces of the primary particles, the secondary particles being formed by aggregation of a plurality of the primary particles; the carbon content is greater than 0.5% by mass and less than or equal to 1.8% by mass of the positive electrode material; The lithium transition metal compound has a crystallite size of 50 nm or more and 70 nm or less, Specific surface area is 14m ２ / g or more 45m ２ / g or less.

2. 2. The positive electrode material according to claim 1, which has an oil absorption of less than 50 ml / 100 g with respect to N-methyl-2-pyrrolidone.

3. the lithium transition metal compound includes a first metal including at least one selected from the group consisting of cobalt, manganese, nickel, iron, copper, and chromium, lithium, and phosphorus; a phosphate compound which may contain a second metal which contains at least one selected from the group consisting of Group 2 to Group 4 elements and Group 12 to Group 14 elements; 3. The positive electrode material according to claim 1, having a composition in which a ratio of the number of moles of lithium to the number of moles of phosphorus is greater than 0.9 and less than 1.1, a ratio of the number of moles of the first metal to the number of moles of phosphorus is greater than 0.8 and less than 1, a ratio of the number of moles of the second metal to the number of moles of phosphorus is greater than 0 and less than 1, and a ratio of the total number of moles of the first metal and the second metal to the number of moles of phosphorus is greater than 0.9 and less than 1.

1.

4. The positive electrode material according to claim 1 or 2, wherein the lithium transition metal compound has a composition represented by the following formula: Li ｘ M １ ｙ M ２ ｚ PO ４＋α (In the formula, M １ contains at least one selected from the group consisting of Co, Mn, Ni, Fe, Cu and Cr. ２ contains at least one element selected from the group consisting of Mg, Ca, Sr, Ba, Sc, Y, Ti, Zn, B, Al, Ga, In, Si, and Ge, and x, y, z, and α satisfy the following conditions: 0.9<x<1.1, 0.8<y≦1, 0≦z<1, 0.9<y+z<1.1, −0.5≦α≦0.5.