Positive electrode material for lithium ion secondary battery, method for producing same, positive electrode for lithium ion secondary battery, and lithium ion secondary battery

Abstract

Provided is a positive electrode material for a lithium ion secondary battery, the positive electrode material containing aggregated particles obtained by aggregating a plurality of primary particles of a positive electrode active material containing lithium iron phosphate coated with a carbonaceous coating, the positive electrode active material having a prescribed composition containing lithium iron phosphate, the rate of change in the lattice area of the b-axis-c-axis surface of the positive electrode material before and after full charge (((lattice area before charge-lattice area after full charge) / lattice area before charge) * 100) is 1.10%-1.33% (inclusive). The positive electrode material has excellent cycle characteristics and high input / output characteristics when used as a positive electrode of a lithium ion secondary battery.

Description

Technical Field

[0001] The present invention relates to a positive electrode material for a lithium ion secondary battery and a method for producing the same, a positive electrode for a lithium ion secondary battery, and a lithium ion secondary battery. Background Art

[0002] Compared with lead batteries and nickel-metal hydride batteries, lithium-ion secondary batteries have higher energy density and output density, and are used in a variety of applications, including home backup power supplies for small electronic devices such as smartphones, power tools, etc. In addition, large-capacity lithium-ion secondary batteries are being promoted for use in vehicles such as electric vehicles and for storage of renewable energy such as solar power generation and wind power generation.

[0003] Lithium-ion secondary batteries have at least a positive electrode, a negative electrode, and an electrolyte. As electrode materials constituting the positive electrode, lithium metal composite oxides having the property of being able to reversibly embed / extract lithium ions, such as lithium cobalt oxide (LiCoO2) or lithium manganese oxide (LiMn2O4) belonging to oxide-based positive electrode materials, and lithium iron phosphate (LiFePO4) belonging to olivine-based positive electrode materials, can be used. Improvements have been made from various viewpoints such as high capacity, long life, improved safety, and low cost of the battery.

[0004] The aforementioned lithium iron phosphate (LiFePO4) uses iron, which is abundant and cheap, and is therefore a material that is easy to achieve low cost. In addition, lithium iron phosphate has excellent properties that are lacking in oxide-based positive electrode materials represented by lithium cobalt oxide, such as the strong covalent bond between phosphorus and oxygen, and no oxygen is released at high temperatures, thus having excellent safety.

[0005] On the other hand, lithium iron phosphate has an olivine structure, so it has low Li ion diffusivity and electronic conductivity, and its input-output characteristics are inferior to those of oxide-based positive electrode materials. The lower the operating temperature of the battery, the more obvious the difference in characteristics. Therefore, it was previously believed that lithium iron phosphate with an olivine structure is not suitable for automotive applications such as electric vehicles and hybrid vehicles, which require high input-output characteristics even in low-temperature areas.

[0006] In order to improve the charge and discharge characteristics at low temperatures, various studies have been conducted, such as miniaturization of primary particles of lithium iron phosphate, optimization of conductive carbonaceous coatings, and coating of the surface of positive electrode active materials with Li ion conductors. However, the essential disadvantages of lithium iron phosphate have not yet been overcome.

[0007] In order to improve the cycle characteristics of lithium iron phosphate, it has been proposed to reduce cracks and fractures of active material particles caused by volume changes of crystals.

[0008] For example, Patent Document 1 proposes a composition LiFe 1-x M xThe electrode material is PO4 (M is one or more elements selected from Mg, Ca, Sr, Ba, Sc, Y, Zn, Al, Ga, In, Si and rare earth elements, 0<x<0.5).

[0009] Prior art literature

[0010] Patent Literature

[0011] Patent Document 1: Japanese Patent Application Publication No. 2001-307726 Summary of the invention

[0012] Problem that the invention aims to solve

[0013] Lithium iron phosphate has the following problems: the volume change of the crystal accompanying the charge and discharge reaction is large, and the active material particles have structural defects such as cracks and fractures, which cause the charge and discharge capacity and cycle characteristics to decrease. In this regard, according to Patent Document 1, it is believed that a long-life lithium secondary battery with large discharge capacity and excellent cycle stability can be provided.

[0014] The electrode material proposed in Patent Document 1 focuses on Li 1-y The phase change speed of the two-phase coexistence reaction of FePO4 and FePO4 is accelerated by adding elements and the volume change is reduced, thereby reducing the polarization during charge and discharge, and reducing the capacity reduction caused by charge and discharge cycles under the premise of high capacity.

[0015] However, lithium iron phosphate has problems with input-output characteristics in addition to cycle stability, and Patent Document 1 does not conduct sufficient research on this problem.

[0016] In view of the problems of the above-mentioned prior art, one aspect of the present invention is to provide a positive electrode material comprising lithium iron phosphate having excellent cycle characteristics and high input-output characteristics when used as a positive electrode of a lithium ion secondary battery, a method for manufacturing the same, a positive electrode for a lithium ion secondary battery, and a lithium ion secondary battery.

[0017] Solutions for solving problems

[0018] <1> A positive electrode material for a lithium ion secondary battery, comprising aggregated particles, wherein the aggregated particles are formed by aggregating a plurality of primary particles of a positive electrode active material covered with a carbonaceous film,

[0019] The positive electrode active material comprises lithium iron phosphate, wherein the lithium iron phosphate comprises lithium (Li), iron (Fe), magnesium (Mg) and a metal element A (A) in a mass ratio of Li:Fe:Mg:A=x:y:z:1-yz, wherein x is 0.9<x<1.1, y is 0.2<y<1.0, z is 0<z<0.1, and 0.2<y+z≤1, and the metal element A is at least one additional element selected from the group consisting of Zn, Co, Ni, Ti and V,

[0020] The change rate of the lattice area of ​​the positive electrode material on the b-axis-c-axis plane before and after full charge ([(lattice area before charge-lattice area after full charge) / lattice area before charge]×100) is 1.10% or more and 1.33% or less.

[0021] <2> The positive electrode material for a lithium-ion secondary battery according to <1>, wherein the rate of change of the lattice volume of the positive electrode material before charging and after full charging ([(lattice volume before charging - lattice volume after full charging) / volume before charging] × 100) is 4.90% or more and 6.06% or less.

[0022] <3> The positive electrode material for a lithium ion secondary battery according to <1> or <2>, wherein the positive electrode material has a crystallite diameter of 80 nm or more and 150 nm or less, and a specific surface area of ​​6.0 m 2 / g or more and 14.0m 2 / g or less.

[0023] <4> The positive electrode material for a lithium ion secondary battery according to <1> or <2>, wherein the tap density of the positive electrode material is 1.1 g / cm 3 The density of the pressed powder when the positive electrode material is compressed into a circular shape with a diameter of 20 mm under a pressure of 16 kN is 2.4 g / cm 3 above.

[0024] <5> A method for producing a positive electrode material for a lithium ion secondary battery, which is a method for producing the positive electrode material for a lithium ion secondary battery according to <1> or <2>,

[0025] The above method has the following steps:

[0026] A crystallization step, using a metal source having a valence of 2, 3 or both and a phosphoric acid source, to obtain crystallized particles through a crystallization reaction;

[0027] A water washing step, washing and drying the crystallized particles to obtain a precursor of the positive electrode active material;

[0028] a mixing step of mixing the precursor of the positive electrode active material, a lithium source and a carbon source to obtain a raw material mixture; and

[0029] a calcining step of calcining the raw material mixture in a non-oxidizing atmosphere to obtain a positive electrode material containing the positive electrode active material;

[0030] In the precursor of the positive electrode active material, a mass ratio of P to a total amount of Fe, Mg, and the metal element A (P / (Fe+Mg+A)) is 0.95 or more and 1.03 or less.

[0031] <6> The method for producing a positive electrode material for a lithium ion secondary battery according to <5>, wherein the specific surface area of ​​the precursor of the positive electrode active material is 7.0 m 2 / g or more and 12.0m 2 / g or less, and a particle size (D50) at a cumulative percentage of 50% in the cumulative particle size distribution is 0.5 μm or more and 5.0 μm or less.

[0032] <7> A positive electrode for a lithium ion secondary battery, comprising: an aluminum current collector, and a positive electrode composite material layer formed on the aluminum current collector.

[0033] The positive electrode composite material layer contains the positive electrode material for lithium ion secondary battery according to claim 1 or 2.

[0034] <8> A lithium ion secondary battery comprising at least a positive electrode, a negative electrode and an electrolyte,

[0035] The positive electrode is the positive electrode for a lithium ion secondary battery according to claim 7.

[0036] Effects of the Invention

[0037] According to one embodiment of the present invention, a positive electrode material containing lithium iron phosphate having excellent cycle characteristics and high input-output characteristics when used as a positive electrode of a lithium ion secondary battery and a method for producing the same, a positive electrode for a lithium ion secondary battery, and a lithium ion secondary battery can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a schematic diagram of the cross-sectional structure of the laminated battery produced in the examples and comparative examples. DETAILED DESCRIPTION

[0039] Hereinafter, the mode for implementing the present invention will be described, but the present invention is not limited to the following embodiments, and various modifications and substitutions can be applied to the following embodiments without departing from the scope of the present invention. In addition, in the following description, the description "A to B" means "Above A and Below B".

[0040] [Positive electrode materials for lithium-ion secondary batteries]

[0041] The positive electrode material for lithium ion secondary battery of this embodiment includes aggregated particles, wherein the aggregated particles are formed by aggregating a plurality of primary particles of a positive electrode active material covered with a carbonaceous film.

[0042] The positive electrode active material comprises lithium iron phosphate, wherein the lithium iron phosphate comprises lithium (Li), iron (Fe), magnesium (Mg) and a metal element A (A) in a mass ratio of Li:Fe:Mg:A=x:y:z:1-yz, wherein x is 0.9<x<1.1, y is 0.2<y<1.0, z is 0<z<0.1, and 0.2<y+z≤1, and the metal element A is at least one additional element selected from the group consisting of Zn, Co, Ni, Ti, and V,

[0043] The change rate of the lattice area of ​​the positive electrode material on the b-axis-c-axis plane before and after full charge ([(lattice area before charge-lattice area after full charge) / lattice area before charge]×100) is 1.10% or more and 1.33% or less.

[0044] The above-mentioned change rate is calculated from the lattice constants b and c of the positive electrode material.

[0045] The positive electrode material for a lithium ion secondary battery of the present embodiment has a structure including lithium iron phosphate as a positive electrode active material and having a conductive carbonaceous coating for improving the conductivity of the positive electrode material.

[0046] Hereinafter, the positive electrode material for lithium ion secondary batteries may be abbreviated as “positive electrode material”. Also, lithium iron phosphate may be abbreviated as “LFP”.

[0047] During the charge and discharge reaction, the Li ions of LFP are embedded / de-embedded at the interface between the Li-containing layer (LiFePO4) and the Li-depleted layer (FePO4). The two-layer interface refers to the b-axis-c-axis plane. In this specification, the plane parallel to the b-axis and the c-axis is referred to as the "b-axis-c-axis plane" or "bc plane". When the difference in lattice area between the two interfaces, that is, the difference in lattice area between the Li-containing layer and the Li-depleted layer, becomes larger, it will hinder the embedding / de-embedding reaction of Li ions. The greater the difference in the aforementioned lattice area, the more hindered the embedding / de-embedding reaction of Li ions is, and the lower the charge and discharge characteristics are.

[0048] The present inventors have found that the input-output characteristics of lithium-ion secondary batteries are affected by the difference in lattice area generated on the bc plane when Li ions are inserted / released during charge and discharge reactions, and that the input-output characteristics can be improved by reducing the difference in lattice area.

[0049] Hereinafter, a lithium ion secondary battery may be simply referred to as a "battery".

[0050] The charge and discharge reaction of LFP is accompanied by a volume change of the crystal, and the volume change is affected by the size of the crystal. Therefore, when the crystal becomes larger, the stress caused by the volume change on the crystal in the form of particles becomes greater, and the crystal is prone to structural defects such as cracks and breaks during the charge and discharge reaction. Therefore, repeated charge and discharge reactions will lead to an increase in crystals that do not contribute to the charge and discharge reaction, and the cycle characteristics will decrease.

[0051] On the other hand, the difference in the lattice area is a change in the crystal structure itself, which is not greatly affected by the crystal size, but affects the insertion / extraction reaction of Li ions. Therefore, by controlling the difference in the lattice area, the input-output characteristics can be improved. Moreover, the volume change of the crystal form can also be reduced, so the cycle characteristics can be improved.

[0052] In the positive electrode material of this embodiment, the change rate of the lattice area of ​​the b-axis-c-axis plane before and after full charge calculated from the lattice constants b and c of the positive electrode material, that is, the mismatch rate of the b-axis-c-axis plane is greater than 1.10% and less than 1.33%. By making the above-mentioned mismatch rate within the above-mentioned range, the obstacles to the Li ion insertion / extraction reaction can be reduced and the input-output characteristics can be improved. When the above-mentioned mismatch rate is less than 1.10%, the inactive elements in the LFP that do not contribute to the redox reaction of charge and discharge increase, and the charge and discharge capacity may decrease.

[0053] From the above viewpoints, the rate of change of the lattice area of ​​the b-axis-c-axis plane of the positive electrode material before charging and after full charge ([(lattice area before charging - lattice area after full charge) / lattice area before charging] × 100) is preferably greater than 1.11% and less than 1.26%, more preferably greater than 1.12% and less than 1.23%, and further preferably greater than 1.13% and less than 1.22%.

[0054] Here, the lattice area before charging is s, and the lattice area after full charge is s', and the mismatch ratio can be calculated using the following mathematical formula (1). The lattice area can be calculated using the lattice constant calculated from the diffraction pattern measured by an X-ray diffraction device.

[0055] Mismatch rate (%) = [(s-s') / s] × 100 (1)

[0056] Regarding the mismatch ratio, the change in lattice size during charge and discharge can be reduced by replacing Fe in LFP with divalent and trivalent elements with a small difference in ionic radius, or elements with an intermediate ionic radius relative to Fe. In particular, the mismatch ratio reduction effect is large for inactive elements such as Mg that do not change from divalent to trivalent.

[0057] In the positive electrode material, by making the amount of Mg added in the above composition z range 0<z<0.1, thereby replacing with Fe, the mismatch rate of the bc plane can be reduced. From the viewpoint of reducing the mismatch rate and further improving the output characteristics, it is preferred that z is 0.01≤z<0.1, more preferably 0.03≤z<0.1, and further preferably 0.06≤z<0.1.

[0058] The range of 1-yz in the above composition of the additional element A added to the positive electrode material can be set to 0.2<y+z≤1, that is, 0≤1-yz<0.8. The additional element A can be added for the purpose of improving battery characteristics such as output characteristics and charge and discharge characteristics.

[0059] In addition, the positive electrode material may contain phosphoric acid (PO4) in a ratio of 1+α in terms of mass ratio. That is, the positive electrode material preferably contains lithium (Li), iron (Fe), magnesium (Mg), metal element A (A) and phosphoric acid (PO4) in a ratio of Li:Fe:Mg:A:PO4=x:y:z:1-yz:1+α in terms of mass ratio, wherein x satisfies 0.9<x<1.1, y is 0.2<y<1.0, z is 0<z<0.1, and 0.2<y+z≤1. α can be preferably set to -0.05≤α≤0.03, and more preferably to -0.01≤α≤0.03. By making the phosphoric acid in the above range, magnesium can be dissolved in the crystals of the olivine structure, making it easy to replace iron.

[0060] In the positive electrode material of this embodiment, the change rate of the lattice volume of the positive electrode material before charging and after full charging can be made 4.90% or more and 6.06% or less. By making the change rate of the lattice volume within the above range, the volume change of the crystal can be suppressed, and the crystal cracks and damage can be reduced to improve the cycle characteristics.

[0061] From the above viewpoints, the rate of change of the lattice volume of the positive electrode material before charging and after full charge ([(lattice volume before charging - lattice volume after full charge) / volume before charging] × 100) is preferably greater than 4.95% and less than 6.00%, more preferably greater than 5.00% and less than 5.60%, and further preferably greater than 5.00% and less than 5.30%.

[0062] Here, the lattice volume before charging can be set as v, the lattice area after full charge can be set as v', and the change rate of the lattice volume before charging and after full charge can be calculated using the following mathematical formula (2). The lattice volume can be calculated using the lattice constant calculated from the diffraction pattern measured by an X-ray diffraction device.

[0063] Lattice volume change rate (%) = [(v-v') / v] × 100 (2)

[0064] For the positive electrode material of the present embodiment, the crystallite diameter can be made to be more than 80nm and less than 150nm. By making the crystallite diameter in the above range, the crystallinity can be improved, and the battery characteristics such as charge and discharge characteristics and cycle characteristics can be made sufficient. The crystallite diameter can be calculated based on the peak of the X-ray diffraction pattern and using the Scherrer formula. From the above viewpoint, the crystallite diameter is more preferably set to more than 110nm and less than 134nm, and further preferably set to more than 110nm and less than 133nm.

[0065] The specific surface area of ​​the positive electrode material can be 6.0 m 2 / g or more and 14.0m 2 As a result, the electrolyte is in sufficient contact with the positive electrode material, and high charge and discharge characteristics can be achieved. Furthermore, the generation of fine particles can be suppressed, thereby increasing the packing density in the battery.

[0066] From the above viewpoints, the specific surface area of ​​the positive electrode material is preferably 6.8 m 2 / g or above and 13.5m 2 / g or less, more preferably 8.0m 2 / g or more and 13.0m 2 / g or less, more preferably 8.7m 2 / g or more and 12.5m 2 / g or less.

[0067] In addition, the tap density of the positive electrode material can be made to be 1.1 g / cm 3 The tap density is an indicator of the filling density in the battery and is set to 1.1 g / cm 3 The above can sufficiently increase the charge-discharge capacity per unit volume. The upper limit of the tap density is not particularly limited, and can be set to 3.0 g / cm 3 the following.

[0068] In addition, the NMP (N-methyl-2-pyrrolidone) absorption of the above-mentioned positive electrode material can be made below 40mL / 100g. The NMP absorption is an index representing the amount of pores in the positive electrode material. By making the NMP absorption below 40mL / 100g, the amount of pores can be limited and the packing density in the battery can be increased. The lower limit of the NMP absorption is not particularly limited, and from the viewpoint of good contact with the electrolyte, it can be set to more than 20mL / 100g.

[0069] From the above viewpoints, the NMP absorption is preferably 28 mL / 100 g to 40 mL / 100 g, more preferably 30 mL / 100 g to 39 mL / 100 g, and even more preferably 32 mL / 100 g to 38 mL / 100 g.

[0070] NMP may be used instead of DBP (dibutyl phthalate) and the NMP absorption may be measured in accordance with JIS K6217-4:2017.

[0071] In addition, the density of the powder compact when the positive electrode material is compressed at a pressure of 16 kN can be 2.4 g / cm 3 As a result, a high-density positive electrode can be formed, which can fully increase the charge and discharge capacity per unit volume of the battery. The density of the pressed powder is not particularly limited and can be set to 3.0 g / cm 3 The density of the powder compact is preferably 2.5 g / cm 3 above.

[0072] The method for measuring the density of the compressed powder body is not particularly limited. For example, a certain amount of sample is placed in a mold with an inner diameter of 20 mm, and a load of 16 kN is applied. The volume of the sample is calculated based on the height measurement value of the sample under the load, and the mass of the sample is divided by the volume to obtain the density of the compressed powder body.

[0073] [Method for producing positive electrode material for lithium ion secondary battery]

[0074] The method for manufacturing the positive electrode material for lithium ion secondary batteries of the present embodiment is a method for manufacturing the positive electrode material for lithium ion secondary batteries of the present embodiment, and the method comprises: a crystallization step, a water washing step, a mixing step, and a sintering step. Hereinafter, the method for manufacturing the positive electrode material for lithium ion secondary batteries of the present embodiment is sometimes referred to as "the present manufacturing method".

[0075] Hereinafter, each step of the present production method will be described, and description of a part that has already been described may be omitted.

[0076] (Crystallization process)

[0077] In the crystallization step, a metal source having a valence of 2, 3, or both and a phosphoric acid source are used to obtain crystallized particles by a crystallization reaction.

[0078] As crystallization particles, a metal phosphate composite compound can usually be obtained. In addition, the amount of the phosphate source added is controlled in the crystallization step so that the ratio of the mass of P to the total mass of iron (Fe), magnesium (Mg) and metal element (A) in the crystallization particles (P / (Fe+Mg+A)) is 0.95 or more and 1.03 or less, preferably 0.96 or more and 1.02 or less.

[0079] The raw material solution in which the metal source and the phosphoric acid source are dissolved and coexist in the form of ions is added to a reaction aqueous solution adjusted to a pH range of 7 to 10 at a liquid temperature of 25° C. to cause coprecipitation. Since the raw material solution is acidic, a base is added dropwise to the reaction aqueous solution, and the raw material solution is then added dropwise while maintaining the pH in the above range, thereby obtaining crystallized particles as a coprecipitate.

[0080] A raw material solution containing a metal source and a phosphate source is added dropwise to a reaction aqueous solution controlled to a pH of 7 to 10, preferably 7 to 9, at a liquid temperature of 25°C. This controls the ratio of metal to phosphate and allows crystallization particles of uniform composition to be obtained.

[0081] In the crystallization step, the ratio of the amount of metal to phosphorus in the crystallization particles is roughly the same as that of the raw material solution. Therefore, by controlling the composition of the raw material solution, the composition of the precursor can be controlled. The ratio of the amount of P relative to the total amount of Fe, Mg and metal element A (P / (Fe+Mg+A)) can be made the same as the composition of the target precursor. (P / (Fe+Mg+A)) can be set to 0.95 to 1.03, or can be set to 0.96 to 1.02. By setting it to such a range, the residual metal elements and phosphates in the reaction aqueous solution can be reduced, and the composition of the resulting precursor can be stabilized.

[0082] The metal source may be a water-soluble divalent or trivalent metal salt, preferably a sulfate. For example, iron sulfate, magnesium sulfate, zinc sulfate, cobalt sulfate, nickel sulfate, titanium sulfate, vanadium sulfate, etc. may be used as the metal source. In addition, an acid-soluble hydroxide may also be used.

[0083] The metal source may be used alone or in combination of two or more kinds, but it is preferred to use only one kind.

[0084] The phosphoric acid source may be any water-soluble phosphoric acid compound, and preferably orthophosphoric acid (H3PO4), diammonium phosphate (NH4H2PO4), diammonium hydrogen phosphate ((NH4)2HPO4), etc., which have a low risk of impurity contamination, can be used.

[0085] The phosphoric acid source may be used alone or in combination of two or more. However, it is preferred to use only one kind.

[0086] When preparing the raw material solution, in order to adjust the valence of the metal source in the raw material solution, an oxidant such as a hydrogen peroxide (H2O2) aqueous solution may be added to the reaction aqueous solution. The amount of the oxidant added is not particularly limited and may be set in the range of 0.3 to 3.0 times the amount of the metal source.

[0087] The oxidizing agent may be used alone or in combination of two or more.

[0088] When adjusting the pH of the reaction aqueous solution, one or more selected from the group consisting of sodium hydroxide and lithium hydroxide can be used as an alkali source.

[0089] The alkali source may be used alone or in combination of two or more.

[0090] The complexing agent may be added to the aqueous reaction solution together with the alkali source. The complexing agent is not particularly limited as long as it can form a complex by bonding with iron ions, other metal ions, etc. in aqueous solution, and an example thereof is an ammonium ion donor.

[0091] The complexing agent may be used alone or in combination of two or more.

[0092] The ammonium ion donor is not particularly limited, and for example, ammonia, ammonium sulfate, ammonium chloride, ammonium carbonate, ammonium fluoride, etc. The ammonium ion concentration in the reaction aqueous solution can be in the range of 3 g / L to 25 g / L.

[0093] The ammonium ion donor may be used alone or in combination of two or more.

[0094] Alternatively, the alkali source and the complexing agent may be mixed and added to the aqueous reaction solution in the form of a buffer solution.

[0095] In the crystallization step, a Mg source is added to the reaction aqueous solution. As a result, magnesium phosphate (Mg3(PO4)2) particles can be present on the surface of the primary particles of the obtained crystallization particles, the grain boundaries between the primary particles, or both of the aforementioned surface and the aforementioned grain boundaries. As a result, the added Mg source is not only dissolved in the crystallization particles, but also exists on either or both of the surface and grain boundaries of the primary particles, and the composition ratio can be accurately controlled.

[0096] The Mg source can be added to the reaction aqueous solution alone in the form of an aqueous solution, or it can be added to the raw material solution. In this case, magnesium sulfate, magnesium hydroxide, etc. can be used as the Mg source. In the case where Mg is contained in the phosphate source or the metal source in the form of impurities, the Mg contained in the phosphate source or the metal source can also be used. That is, the method of adding the Mg source is not limited, and it can be set to a desired amount, for example, it can be set to the z range representing the Mg addition amount in the target LFP in the above composition, that is, 0<z<0.1.

[0097] The Mg source may be used alone or in combination of two or more.

[0098] (Water washing process)

[0099] In the water washing step, the crystallized particles obtained in the crystallization step are washed with water and dried to obtain the precursor of the positive electrode active material.

[0100] Hereinafter, the precursor of the positive electrode active material may be simply referred to as “precursor”.

[0101] The water washing can be carried out by any conventional method as long as the impurities can be reduced. For example, the crystallized particles can be washed by mixing and stirring any of pure water, distilled water and ion exchange water without the risk of mixing with impurities, and then performing solid-liquid separation.

[0102] The drying is not particularly limited, and the crystallized particles may be dried in a non-oxidizing atmosphere such as a vacuum atmosphere or an inert gas atmosphere from the viewpoint of preventing oxidation of the crystallized particles.

[0103] The composition of the target positive electrode active material inherits the composition ratio of the metal element and phosphoric acid of the precursor obtained in the water washing process, so the mixing ratio of the metal source and the phosphoric acid source used in the crystallization process can be determined in a manner to achieve the target composition. Specifically, for example, it is preferred that the ratio of Fe:Mg:A:PO4=y:z:1-yz:1+β in terms of the mass ratio contains iron (Fe), magnesium (Mg), metal element A (A) and phosphoric acid (PO4), wherein y is 0.2<y<1.0, z is 0<z<0.1, and 0.2<y+z≤1.0, and the metal element A is at least one additional element selected from the group consisting of Zn, Co, Ni, Ti and V. β is preferably -0.05≤β≤0.03, and more preferably -0.04≤β≤0.02.

[0104] The aforementioned precursor can also be expressed as the general formula Fe y Mg z A 1-y-z (PO4) 1+β . Here, the general formula of the composition of the entire precursor including magnesium phosphate (Mg3(PO4)2) particles present on the surface of the primary particles of the crystallization particles, on the grain boundaries, or on both. That is, Mg may also exist in the form of (Mg3(PO4)2) particles on the surface of the primary particles of the precursor, on the grain boundaries between the primary particles, or on both the aforementioned surface and the aforementioned grain boundaries, and a part of it is solid dissolved in the precursor. y, z, β and element M in the formula have been explained, so the explanation is omitted here.

[0105] The specific surface area of ​​the aforementioned precursor can be set to 7.0 m 2 / g or more and 12.0m 2 / g or less. This can improve the reactivity of the lithium source and the precursor during sintering and further suppress excessive sintering of the primary particles of LFP. The specific surface area of ​​the precursor is preferably 8.8 m 2 / g or above and 11.5m 2 / g or less, more preferably 9.5m 2 / g or more and 11.0m2 / g or less.

[0106] In addition, for the precursor, the particle size (D50) of the cumulative percentage of 50% in the cumulative particle size distribution can be made to be 0.5 μm or more and 5.0 μm or less. Thus, the packing density of the obtained positive electrode material when the battery is made can be increased, and the contact with the electrolyte can be ensured to further improve the input-output characteristics. The D50 of the precursor is preferably 1.9 μm or more and 5.0 μm or less, and more preferably 2.5 μm or more and 4.5 μm or less.

[0107] The specific surface area and D50 of the aforementioned precursor can be controlled by the crystallization conditions. By adjusting the pH, stirring conditions, etc., a precursor having a specific surface area and D50 within the above range can be obtained.

[0108] (Mixing process)

[0109] In the mixing step, the precursor of the positive electrode active material obtained in the water washing step, the lithium source, and the carbon source are mixed to obtain a raw material mixture. A phosphorus source may be further mixed as necessary.

[0110] The precursor is mixed with a lithium source so that the ratio of the amount of lithium (Li) to the metal (Me) other than lithium in the obtained LFP, x (Li / Me), is preferably greater than 0.9 and less than 1.1, and more preferably greater than 0.95 and less than 1.05. In addition, when the ratio of the amount of metal to phosphorus (P / (Fe+Mg+A)) of the precursor is less than 1, it is preferred to mix the phosphorus source and make the ratio of metal to phosphorus reach 0.99 or more and 1.03 or less. By making the P / (Fe+Mg+A) of the positive electrode material greater than 0.99, the dissolution of metal elements that are not dissolved in the crystals of the olivine structure due to the charge and discharge reaction can be suppressed, and the durability and safety of the lithium ion secondary battery can be further improved. Therefore, it is preferred to set P / (Fe+Mg+A) to 0.99 or more and 1.03 or less in the mixing process.

[0111] Since x is almost unchanged before and after the calcination step described later, x in the raw material mixture supplied to the calcination step is substantially the same as Li / Me in the obtained LFP. Therefore, in the mixing step, it is preferred to mix so that the obtained LFP has the target x.

[0112] In the firing process, x may decrease slightly due to volatilization of lithium, etc. Therefore, it is preferred that x in the mixing process is higher than x of the target LFP by the amount of decrease in x. The amount of decrease in x is approximately constant due to the firing conditions, and can therefore be easily determined by preliminary tests, etc.

[0113] The lithium source is not particularly limited, and lithium carbonate, lithium hydroxide, lithium sulfate, etc. can be used. Lithium carbonate and lithium hydroxide free of impurities can be preferably used.

[0114] The lithium source may be used alone or in combination of two or more. However, it is preferred to use only one type.

[0115] Examples of the carbon source include: graphite such as natural graphite and artificial graphite; carbon black such as acetylene black and Ketjen black; carbon fiber, sucrose, ascorbic acid and other organic compounds that decompose to produce carbonaceous materials, and one or more of them can be used.

[0116] The carbon source may be used alone or in combination of two or more.

[0117] The amount of the carbon source to be mixed is not particularly limited as long as it is an amount that allows the LFP to be coated with the conductive carbonaceous film. The amount of carbon to be mixed is 1 to 5% by mass relative to the LFP.

[0118] The phosphorus source is not particularly limited, and ammonium dihydrogen phosphate (NH4H2PO4), diammonium hydrogen phosphate ((NH4)2HPO4), and the like can be used.

[0119] The phosphorus source may be used alone or in combination of two or more.

[0120] As a mixing means for mixing the precursor and the lithium compound in the mixing step, a general mixer can be used, for example, a swing mixer, a Loedige mixer, a Julia mixer, a V-type blender, etc. can be used.

[0121] (Firing process)

[0122] In the calcining step, the raw material mixture obtained in the mixing step is calcined in a non-oxidizing atmosphere to obtain a positive electrode material containing a positive electrode active material.

[0123] In the firing step, the firing temperature for firing the raw material mixture, that is, the maximum temperature set in the firing furnace is not particularly limited, but is preferably 600°C to 850°C, and more preferably 600°C to 750°C, for example.

[0124] By setting the sintering temperature to 600°C or above, lithium can be fully diffused into the precursor, and the crystal structure of the obtained LFP can be made particularly uniform, which can fully improve the battery characteristics when used as a positive electrode active material. In addition, since the reaction between the lithium source and the precursor can be fully carried out, the remaining lithium residue or the unreacted precursor particle residue can be further suppressed.

[0125] In this manufacturing method, magnesium phosphate particles are present on either or both of the surface and grain boundaries of the primary particles of the precursor. During firing, the magnesium phosphate particles diffuse into the precursor together with the Li in the lithium source to form an olivine structure LFP in which part of the iron and magnesium are replaced, and the mismatch rate is alleviated as described above. In this manufacturing method, the above-mentioned phosphate compound is used as a precursor, and in a state where fine magnesium phosphate particles are present on either or both of the surface and grain boundaries of the primary particles of the precursor, it is mixed and fired with the lithium source. Therefore, it can be considered that, compared with the manufacturing method in which the phosphate source and the lithium source are mixed and fired as separate raw materials with an iron source, a magnesium source, etc., magnesium and additive elements are more evenly dispersed in the crystals of the LFP, and the aforementioned mitigation effect of the mismatch rate is large.

[0126] The holding time at the firing temperature is not particularly limited, and can be set under general conditions for LFP firing, for example, 0.5 to 12 hours, preferably 2 to 6 hours.

[0127] The atmosphere during firing is not particularly limited as long as it is a non-oxidizing atmosphere, and for example, an inert atmosphere such as nitrogen or a reducing atmosphere may be used.

[0128] Here, the atmosphere during firing is an atmosphere excluding gases generated by reactions during firing, and may be, for example, an atmosphere gas supplied to a firing furnace.

[0129] In the firing step, the pre-firing can be performed at a pre-firing temperature lower than the firing temperature. The pre-firing temperature can be set to, for example, 350°C to 550°C, preferably 400°C to 500°C.

[0130] The holding time at the calcination temperature can be set to, for example, 0.5 to 12 hours, and preferably 2 to 6 hours.

[0131] After the pre-firing, the material may be cooled before being subjected to the firing step, or the temperature may be raised from the pre-firing temperature to the firing temperature to continuously perform the firing step.

[0132] The atmosphere during the preliminary firing is not particularly limited, and for example, the same atmosphere as that in the firing step may be used.

[0133] The firing furnace is not particularly limited, and the raw material mixture may be fired, for example, in an inert atmosphere or a reducing atmosphere. From the viewpoint of maintaining a uniform atmosphere in the furnace, an electric furnace without gas generation is preferred, and an intermittent or continuous furnace may be used as desired. In addition, a furnace in which the raw material mixture is filled into a firing container and fired, or a furnace in which the raw material mixture is flowed and fired may be used as desired. However, from the viewpoint of particularly improving productivity, a continuous firing furnace is preferably used in the firing process.

[0134] (Crushing process)

[0135] The powder of the positive electrode material obtained through the calcining step may sometimes aggregate or lightly sinter. In this case, the present production method may optionally include a crushing step of crushing the powder after the calcining step.

[0136] Here, crushing refers to the following operation: mechanical energy is applied to an aggregate composed of multiple secondary particles generated by sintering necking between secondary particles during firing, and the secondary particles are separated without almost destroying the secondary particles themselves, thereby dissolving the aggregate. In the crushing process, it is preferred to select the crushing conditions so that the average particle size, particle size distribution, etc. of the positive electrode material after crushing reach the desired range, and implement it. After crushing, sieving can also be performed as needed.

[0137] (About the composition of positive electrode materials)

[0138] The composition of the positive electrode material produced by the above-described production method is not particularly limited. The positive electrode material includes a positive electrode active material, the positive electrode active material includes aggregated particles, and the aggregated particles are formed by aggregating a plurality of primary particles covered with a carbonaceous film.

[0139] The composition of LFP contained in the positive electrode active material may be, for example, lithium (Li), iron (Fe), magnesium (Mg) and metal element A (A) in a mass ratio of Li:Fe:Mg:A=x:y:z:1-yz. With respect to the above x, y, and z, it is preferred that x is 0.9<x<1.1, y is 0.2<y<1.0, z is 0<z<0.1, and 0.2<y+z≤1.0. The metal element A may be at least one additional element selected from the group consisting of Zn, Co, Ni, Ti, and V. x may also be 0.95≤x≤1.05.

[0140] In addition, phosphoric acid (PO 4 ) may be contained in a ratio of 1+α in terms of the above-mentioned amount ratio, and α preferably satisfies -0.05≤α≤0.03, and preferably -0.01≤α≤0.03.

[0141] The positive electrode active material can also be expressed as the general formula Li x Fe y Mg z A 1-y-z (PO4) 1+α The x, y, α and element A in the formula have been explained, and therefore, the explanation is omitted here.

[0142] According to the present production method described above, it is possible to provide a positive electrode material containing lithium iron phosphate having high input-output characteristics when used as a positive electrode of a lithium ion secondary battery.

[0143] [Positive electrode for lithium-ion secondary battery]

[0144] The positive electrode for lithium ion secondary battery of this embodiment comprises: an aluminum current collector and a positive electrode composite material layer formed on the aluminum current collector.

[0145] The positive electrode mixture layer contains the positive electrode material for lithium ion secondary battery of the present embodiment.

[0146] The positive electrode is a sheet-like member, and is formed, for example, by applying a positive electrode composite material paste containing the above-described positive electrode material as a positive electrode active material on the surface of an aluminum foil current collector (aluminum current collector) and drying the paste.

[0147] It should be noted that the positive electrode is appropriately processed according to the battery used, for example, cutting processing to form it into an appropriate size according to the target secondary battery, and pressurizing and compressing processing such as roller pressing to increase the electrode density.

[0148] A solvent is added to the positive electrode composite material and kneaded to form a positive electrode composite material paste. The positive electrode composite material is formed by mixing the above-mentioned positive electrode material in powder form, a conductive material, and a binder.

[0149] The conductive material is added to impart appropriate conductivity to the electrode. The conductive material is not particularly limited, and examples thereof include graphite (natural graphite, artificial graphite, expanded graphite, etc.), carbon black materials such as acetylene black and Ketjen Black (registered trademark), and the like.

[0150] The conductive material may be used alone or in combination of two or more.

[0151] The binder plays a role of binding the positive electrode active material particles. The binder used in the positive electrode composite material is not particularly limited, and for example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluorine rubber, ethylene propylene diene rubber, styrene-butadiene, cellulose resin, polyacrylic acid, etc. can be used.

[0152] The binder may be used alone or in combination of two or more.

[0153] It should be noted that activated carbon or the like may also be added to the positive electrode composite material. By adding activated carbon or the like to the positive electrode composite material, the double layer capacity of the positive electrode can be increased.

[0154] The solvent is a substance that dissolves the binder and disperses the positive electrode active material, the conductive material, the activated carbon, etc. in the binder. The solvent is not particularly limited, and for example, an organic solvent such as N-methyl-2-pyrrolidone can be used.

[0155] The solvent may be used alone or in combination of two or more.

[0156] In addition, the mixing ratio of each substance in the positive electrode composite material paste is not particularly limited. For example, when the solid content of the positive electrode composite material other than the solvent is set to 100 parts by mass, the content of the positive electrode active material can be 60 parts by mass to 98 parts by mass, the content of the conductive material is 1 part by mass to 20 parts by mass, and the content of the binder is 1 part by mass to 20 parts by mass, as in the positive electrode of a general lithium ion secondary battery.

[0157] [Lithium-ion secondary battery]

[0158] In the lithium ion secondary battery of the present embodiment, the positive electrode may include the positive electrode material of the present embodiment described above. Hereinafter, a configuration example of the battery of the present embodiment will be described for each component.

[0159] The battery of this embodiment has substantially the same structure as a general lithium ion secondary battery except that a positive electrode material, more specifically, a positive electrode material obtained by the above-described method for producing a positive electrode material for a lithium ion secondary battery is used as a positive electrode active material.

[0160] Specifically, the battery of the present embodiment has the following structure: a housing, a positive electrode, a negative electrode, a non-aqueous electrolyte housed in the housing, and a separator as needed. More specifically, for example, when a non-aqueous electrolyte is used, the positive electrode and the negative electrode are stacked with a separator to form an electrode body, the obtained electrode body is immersed in the non-aqueous electrolyte, and a collector lead is used to connect the positive electrode collector of the positive electrode and the positive terminal connected to the outside, and the negative electrode collector of the negative electrode and the negative terminal connected to the outside, respectively, and sealed in the housing, thereby forming the secondary battery of the present embodiment.

[0161] It should be noted that the structure of the secondary battery of this embodiment is obviously not limited to the above example, and its outer shape may also adopt various shapes such as cylindrical shape, laminated shape, etc. The positive electrode has been described above, so the description thereof is omitted.

[0162] (negative electrode)

[0163] The negative electrode is a sheet-like member formed by applying a negative electrode composite material paste on the surface of a metal foil collector such as copper and drying it. Although the components or compatibility of the negative electrode composite material paste, the raw materials of the collector, etc. are different, the negative electrode is essentially formed by the same method as the positive electrode, and can be processed in various ways as needed, just like the positive electrode.

[0164] The negative electrode composite material paste is a material prepared by adding an appropriate solvent to a negative electrode composite material obtained by mixing a negative electrode active material and a binder to form a paste.

[0165] The negative electrode active material may be, for example: a lithium-containing material such as metallic lithium and a lithium alloy; or an occlusion material that can occlude and release lithium ions.

[0166] The storage material is not particularly limited, and for example, natural graphite, artificial graphite, phenolic resin and other organic compound sintered bodies, and powders of carbon materials such as coke can be used. When the negative electrode active material adopts the storage material, as with the positive electrode, a fluorine-containing resin such as PVDF can be used as a binder, and an organic solvent such as N-methyl-2-pyrrolidone can be used as a solvent for dispersing the negative electrode active material in the binder.

[0167] (Separator)

[0168] When a non-aqueous electrolyte is used, the separator is arranged in a manner sandwiched between the positive electrode and the negative electrode, and has the function of separating the positive electrode from the negative electrode and retaining the electrolyte. The separator can be, for example, a thin film having multiple micropores such as polyethylene or polypropylene, and is not particularly limited as long as it has the above-mentioned function.

[0169] (Non-aqueous electrolyte)

[0170] As the non-aqueous electrolyte, for example, a non-aqueous electrolyte solution can be used.

[0171] As a non-aqueous electrolyte, for example, a substance obtained by dissolving a lithium salt as a supporting salt in an organic solvent can be used. In addition, as a non-aqueous electrolyte, a substance in which a lithium salt is dissolved in an ionic liquid can also be used. It should be noted that an ionic liquid refers to a salt composed of cations and anions other than lithium ions and is liquid at room temperature (25°C).

[0172] As the organic solvent, there can be used: cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate and trifluoropropylene carbonate; chain carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate and dipropyl carbonate; ether compounds such as tetrahydrofuran, 2-methyltetrahydrofuran and dimethoxyethane; sulfur compounds such as ethyl methyl sulfone and butane sultone; phosphorus compounds such as triethyl phosphate and trioctyl phosphate, etc.

[0173] The organic solvent may be used alone or in combination of two or more kinds thereof.

[0174] As supporting salts, LiPF6, LiBF4, LiClO4, LiAsF6, LiN(CF3SO2)2, and complex salts thereof can be used. In addition, the non-aqueous electrolyte can also contain a radical scavenger, a surfactant, a flame retardant, and the like.

[0175] The supporting salt may be used alone or in combination of two or more.

[0176] In addition, a solid electrolyte may be used as a non-aqueous electrolyte. A solid electrolyte has the property of being able to withstand a high voltage. Examples of the solid electrolyte include inorganic solid electrolytes and organic solid electrolytes.

[0177] Examples of the inorganic solid electrolyte include oxide-based solid electrolytes and sulfide-based solid electrolytes.

[0178] The oxide-based solid electrolyte is not particularly limited, and for example, a substance containing oxygen (O) and having lithium ion conductivity and electronic insulation can be preferably used. Specifically, examples of the oxide-based solid electrolyte include lithium phosphate (Li3PO4), 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 、Li6BaLa2Ta2O 12 , Li 3.6 Si 0.6 P 0.4 O4, etc. As the oxide-based solid electrolyte, one or more selected from these can be used.

[0179] As sulfide-based solid electrolytes, there are no particular limitations, and for example, substances containing sulfur (S) and having lithium ion conductivity and electronic insulation can be preferably used. Specifically, as sulfide-based solid electrolytes, for example, there can be mentioned: 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, LiI-Li3PO4-P2S5, etc. The sulfide-based solid electrolyte can use one or more selected from these.

[0180] It should be noted that as the inorganic solid electrolyte, substances other than those listed above may be used, for example, Li 3 N, LiI, Li 3 N—LiI—LiOH, etc. may be used.

[0181] The organic solid electrolyte is not particularly limited as long as it is a polymer compound showing ion conductivity, and examples thereof include polyethylene oxide, polypropylene oxide, and copolymers thereof. The organic solid electrolyte may also contain a supporting salt (lithium salt).

[0182] Only one kind of organic solid electrolyte may be used, or two or more kinds of organic solid electrolyte may be used.

[0183] (Shape and structure of secondary battery)

[0184] As described above, the battery of the present embodiment can be made into various shapes such as cylindrical and stacked. Regardless of the shape, in the case where the secondary battery of the present embodiment uses a non-aqueous electrolyte as a non-aqueous electrolyte, the positive electrode and the negative electrode are stacked across a separator to form an electrode body. The following structure can be made: the obtained electrode body is immersed in a non-aqueous electrolyte, and a collector lead is used to connect the positive electrode collector and the positive terminal connected to the outside, and the negative electrode collector and the negative terminal connected to the outside, and sealed in a battery case.

[0185] It should be noted that the battery of the present embodiment is not limited to the mode of using non-aqueous electrolyte as non-aqueous electrolyte, for example, a secondary battery using solid non-aqueous electrolyte, i.e., an all-solid-state battery, can also be made. In the case of making an all-solid-state battery, the composition other than the positive electrode material can be appropriately changed as needed.

[0186] As mentioned above, the battery of the present embodiment uses the positive electrode material of the present embodiment as the material of the positive electrode, so the battery capacity and input-output characteristics are excellent. Therefore, the battery of the present embodiment can be preferably used as a rechargeable battery for clean energy vehicles such as portable information terminals such as portable phones, smart phones, tablet computers, notebook personal computers, portable music players, digital cameras, medical equipment, HEV (Hybrid Electric Vehicle), EV (Electric Vehicle), PHEV (Plug-in Hybrid Electric Vehicle), etc.

[0187] Example

[0188] Hereinafter, the present embodiment will be described in more detail with reference to examples. However, the present embodiment is not limited to the following examples.

[0189] 1. About the evaluation method

[0190] In the following Examples and Comparative Examples, evaluation was performed by the following methods.

[0191] (1) Composition analysis: The composition of the precursor of the positive electrode active material and the positive electrode material was measured by ICP emission spectrometry.

[0192] (2) Lattice area and lattice volume of the bc plane of the positive electrode material: A composite material layer of the active material is peeled off from the positive electrode of a lithium-ion secondary battery (evaluation battery) produced using the method described below, and the layer is measured using an X-ray diffraction device. Based on the diffraction pattern of the resulting composite material layer, the lattice constant and lattice volume before charging are calculated.

[0193] Next, at room temperature (25°C), the evaluation battery prepared by the method described below is fully charged with an upper limit voltage of 3.7V and a constant current of 0.1C, and then the evaluation battery is disassembled and the positive electrode is removed. The removed positive electrode is thoroughly cleaned with diethyl carbonate (DEC), dried under reduced pressure at room temperature for 12 hours, and then vacuum dried at 50°C for 12 hours. The composite material layer of the active material is peeled off from the dried positive electrode and measured using an X-ray diffraction device. The lattice constant after charging is obtained based on the diffraction pattern of the obtained composite material layer, and the lattice area and lattice volume are calculated.

[0194] (3) Crystallite diameter of positive electrode material: A diffraction pattern was measured using an X-ray diffraction apparatus under the following measurement conditions.

[0195] Radiation source: Cu-Kα

[0196] Step size: 0.01° / step

[0197] Scan speed: 3 seconds / step

[0198] In the measured diffraction pattern, the crystallite diameter was calculated from the following formulae (i) and (ii) using the half value width (B) of the peak in the range of 2θ of 28.8 to 30.8°.

[0199] Crystallite diameter (nm) = {0.9 × 1.5418 × 0.1} / {β × COS (〔29.78 / 2〕 × π / 2)} (i)

[0200] β=(Bb) (ii)

[0201] Wherein B is the half-value width of the peak in the range of 2θ of 28.8 to 30.8° in the diffraction pattern measured for the positive electrode active material particles, and b is the half-value width of the reference sample Si (2θ=47.3°).

[0202] (4) Specific surface area: The specific surface area of ​​the precursor and the positive electrode material was determined by the BET single-point method based on nitrogen adsorption.

[0203] (5) Average particle size of the precursor: The particle size at which the cumulative percentage on a volume basis is 50% (D50) is determined using a laser diffraction scattering particle size distribution measuring apparatus.

[0204] (6) Tap density: The tap density of the positive electrode material was measured by shaking a sample container containing the positive electrode material 500 times using a tapping machine (KRS-409, manufactured by Kuramochi Scientific Instruments Mfg. Co., Ltd.).

[0205] (7) NMP absorption amount of positive electrode material: measured using NMP in accordance with JIS K 6217-4:2017.

[0206] (8) Density of pressed powder of positive electrode material:

[0207] 3g of sample is placed in a mold with an inner diameter of 20mm, and the mold is vibrated several times to make the sample surface flat. A preload of 10kN is applied to the mold three times and then released, and then a load of 16kN is further applied to the mold. Based on the height measurement values ​​of the sample in the loaded state and the unloaded state, the volume of the sample is calculated, and the compression density when pressurized and depressurized is obtained respectively.

[0208] (9) Production of evaluation batteries and evaluation of battery characteristics

[0209] As a lithium ion secondary battery for evaluation, a laminated secondary battery 10 (see Figure 1 ), and the battery characteristics (load characteristics, DC resistance, and cycle characteristics) were evaluated. The method for manufacturing the laminated secondary battery 10 and the method for evaluating the battery characteristics are as follows.

[0210] (Production of positive electrode)

[0211] The positive electrode material, polyvinylidene fluoride (PVdF) as a binder, and acetylene black (AB) as a conductive aid were mixed so that the mass ratio reached positive electrode material: PVdF: AB = 90:5:5 to obtain a mixture. Then, N-methyl-2-pyrrolidone (NMP) as a solvent was added to the mixture to give fluidity, thereby preparing a slurry.

[0212] Next, the slurry was coated on an aluminum (Al) foil (current collector) having a thickness of 30 μm and vacuum dried at 120° C. for 12 hours. Subsequently, the slurry was cut into strips having a coating width of 35 mm and repeatedly pressed twice with a roller press at a roller gap of 5 μm and a roller feed speed of 0.5 m / min to produce the positive electrode 11 of each embodiment and comparative example.

[0213] (Manufacturing of lithium-ion secondary batteries)

[0214] like Figure 1 As shown, the laminated secondary battery (evaluation battery) 10 has the following structure: a laminate of a positive electrode 11, a separator 12, and a negative electrode 13 is immersed in an electrolyte and sealed by a laminate (aluminum laminate film) 14. It should be noted that a positive electrode tab 15 is connected to the positive electrode 11, and a negative electrode tab 16 is connected to the negative electrode 13, and the positive electrode tab 15 and the negative electrode tab 16 are exposed outside the laminate 14.

[0215] A negative electrode composite material paste is applied on a copper foil to prepare a negative electrode 13. The paste is a mixture of natural graphite powder with an average particle size of about 20 μm and polyvinylidene fluoride. The obtained positive electrode 11 and negative electrode 13 are cut into a predetermined size, and the positive electrode tab 15 is welded to the positive electrode 11, and the negative electrode tab 16 is welded to the negative electrode 13. The positive electrode 11 and the negative electrode 13 are arranged in a laminate 14 via a separator 12 formed of a porous polypropylene film.

[0216] Next, in a glove box with an Ar gas atmosphere and a dew point of -80°C, ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a ratio of EC:DEC = 50:50 (volume %) to obtain a mixed solution. LiPF6 was added to the mixed solution and dissolved to a concentration of 1 mol / dm 3 Then, 2 mass % of vinylene carbonate (VC) was added to obtain an electrolyte solution. The obtained electrolyte solution was injected into the laminate 14 and sealed to produce the laminate type secondary battery 10.

[0217] (Evaluation of lithium-ion secondary batteries)

[0218] The obtained laminated secondary battery 10 was evaluated by the following method.

[0219] (Load characteristics (discharge capacity ratio))

[0220] For the charge and discharge test of the laminated secondary battery, the charge and discharge were repeated three times at room temperature (25°C) with a cut-off voltage of 2.5V to 3.7V and a constant current of 0.1C. Then, at an ambient temperature of 0°C, the lithium ion secondary battery was charged and discharged with a cut-off voltage of 2.5V to 3.7V and a constant current of 0.1C, and the discharge capacity was measured. Furthermore, at an ambient temperature of 0°C, the battery was charged with a cut-off voltage of 2.5V to 3.7V and 0.1C, and discharged at 3C, and the discharge capacity was measured.

[0221] The ratio of the measured 3 C discharge capacity to the 0.1 C discharge capacity was used as the load characteristic and calculated by the following formula (A).

[0222] Load characteristics (%) = (3C discharge capacity / 0.1C discharge capacity) × 100 (A)

[0223] (Direct Current Resistance (DCR))

[0224] At room temperature (25°C), the charge depth (SOC) of the laminated battery 10 was adjusted to 50% at a charge rate of 0.1C, and the charge side and the discharge side were alternately energized at 1C, 3C, 5C and 10C rates for 10 seconds each at an ambient temperature of 0°C. The current value after 10 seconds at each rate was plotted on the horizontal axis, and the voltage value was plotted on the vertical axis. The slope of the approximate straight line obtained based on the least squares method was used as "charge side = input DCR" and "discharge side = output DCR". A 10-minute stop time was set when changing the power supply direction and the power supply current at each current.

[0225] (Cycle Characteristics (Capacity Retention Rate))

[0226] At an ambient temperature of 60°C, charge and discharge were repeated 300 times at a cutoff voltage of 2.5V to 3.7V and a constant current of a charge and discharge rate of 1C. The ratio of the 300th discharge capacity to the first discharge capacity was calculated as the cycle characteristic using the following formula (B).

[0227] Cycle characteristics (%) = (300th discharge capacity / 1st discharge capacity) × 100 (B)

[0228] [Example 1]

[0229] (Preparation of Precursor of Positive Electrode Active Material)

[0230] 279 g of FeSO4·7H2O as an Fe source was dissolved in pure water. 130 g of H3PO4 (75 mass % aqueous solution) as a P source and 3.08 g of Mg(OH)2 were mixed and dissolved therein. Next, 170 g of H2O2 (30 W / V % aqueous solution) as an oxidant and pure water for concentration adjustment were added to the mixed aqueous solution to prepare a raw material solution. In addition, sodium hydroxide (30 mass % aqueous solution) and ammonia water (28 mass % aqueous solution) were mixed at a molar ratio of 1:1 to prepare a buffer solution.

[0231] Next, pure water was added to a reaction container equipped with a stirrer and a pH sensor, and the buffer solution and the raw material solution were added while stirring so that the pH of the reaction aqueous solution in the container reached a range of 8.0 to 8.5 at 25° C. After all the prepared raw material solutions were added, stirring was continued for 30 minutes to allow the reaction to proceed completely.

[0232] After the reaction was completed, the reaction aqueous solution was filtered and washed with distilled water several times to obtain filter cake-like crystal particles, which were vacuum dried at 50° C. for 24 hours to obtain a precursor of the positive electrode active material (FePO4).

[0233] The obtained precursor was observed with a scanning electron microscope, and it was confirmed that it was composed of secondary particles formed by aggregation of primary particles of FePO4, and fine Mg3(PO4)2 particles of 10nm to 300nm existed on the particle surface of the primary particles.

[0234] The ratio of P to the total amount of Fe, Mg and metal element A (P / (Fe+Mg+A)) is 0.98. In addition, the specific surface area is 9.7 m 2 / g, and D50 was 2.3 μm. Table 1 shows the evaluation results of the obtained precursor.

[0235] (Production and evaluation of positive electrode materials)

[0236] The obtained precursor is mixed with lithium hydroxide (LiOH) as a Li source so that the ratio of the total amount of Li to the total amount of metal elements other than Li reaches Li / (Fe+Mg)=1.02. Furthermore, diammonium phosphate (NH4H2PO4) is mixed in the mixed powder of the precursor and the Li source so that the ratio of the total amount of P to Fe, Mg and metal element A (P / (Fe+Mg+A)) reaches 1.00. Furthermore, sucrose powder with a carbon content of 2.5% by mass relative to the synthesized positive electrode active material is mixed to obtain a raw material mixture.

[0237] Next, the obtained raw material mixture was heat treated at 700° C. for 5 hours in a nitrogen atmosphere to synthesize a positive electrode active material and load carbon on the surface of the positive electrode active material particles, thereby producing a positive electrode material for a lithium ion secondary battery of Example 1.

[0238] X-ray diffraction analysis confirmed that the obtained positive electrode material produced LFP as a positive electrode active material. In addition, scanning electron microscope observation confirmed that the obtained positive electrode material was composed of a positive electrode active material including secondary particles formed by aggregation of primary particles of LFP.

[0239] The composition of the obtained positive electrode material was confirmed to be Li:Fe:Mg:PO4=1.02:0.95:0.05:1 in terms of mass ratio. Table 2 shows the obtained positive electrode material and the evaluation results of the lithium ion secondary battery using the positive electrode material.

[0240] [Example 2]

[0241] The positive electrode material for lithium ion secondary battery of Example 2 was prepared and evaluated in the same manner as in Example 1, except that 6.12 g of Mg(OH) 2 and 129 g of H 3 PO 4 (75% aqueous solution) were mixed when preparing the precursor of the positive electrode active material.

[0242] The obtained precursor was observed with a scanning electron microscope, and it was confirmed that it was composed of secondary particles formed by aggregation of primary particles of FePO4, and fine Mg3(PO4)2 particles of 10nm to 300nm existed on the particle surface of the primary particles.

[0243] The ratio of P to the total amount of Fe, Mg and metal element A (P / (Fe+Mg+A)) is 0.96. In addition, the specific surface area is 10.6 m 2 / g, and D50 was 4.1 μm. Table 1 shows the evaluation results of the obtained precursor.

[0244] X-ray diffraction analysis confirmed that the obtained positive electrode material produced LFP as a positive electrode active material. In addition, scanning electron microscope observation confirmed that the obtained positive electrode material was composed of a positive electrode active material including secondary particles formed by aggregation of primary particles of LFP.

[0245] The composition of the obtained positive electrode material was confirmed to be Li:Fe:Mg:PO4=1.02:0.905:0.095:1 in terms of mass ratio. The evaluation results of the obtained precursors are summarized in Table 1, and the evaluation results of the obtained positive electrode materials and lithium ion secondary batteries using the same are summarized in Table 2.

[0246] [Example 3]

[0247] A positive electrode material for a lithium ion secondary battery of Example 3 was prepared and evaluated in the same manner as in Example 1 except that the amount of Mg(OH) 2 mixed when preparing the precursor of the positive electrode active material was changed to 1.19 g.

[0248] The obtained precursor was observed with a scanning electron microscope, and it was confirmed that it was composed of secondary particles formed by aggregation of primary particles of FePO4, and fine Mg3(PO4)2 particles of 10nm to 300nm existed on the particle surface of the primary particles.

[0249] The ratio of P to the total amount of Fe, Mg and metal element A (P / (Fe+Mg+A)) is 0.99. In addition, the specific surface area is 8.4 m 2 / g, and D50 was 1.2 μm. Table 1 shows the evaluation results of the obtained precursor.

[0250] X-ray diffraction analysis confirmed that the obtained positive electrode material produced LFP as a positive electrode active material. In addition, scanning electron microscope observation confirmed that the obtained positive electrode material was composed of a positive electrode active material including secondary particles formed by aggregation of primary particles of LFP.

[0251] The composition of the obtained positive electrode material was confirmed to be Li:Fe:Mg:PO4=1.02:0.98:0.02:1 in terms of mass ratio. Table 2 shows the obtained positive electrode material and the evaluation results of the lithium ion secondary battery using the positive electrode material.

[0252] [Example 4]

[0253] A positive electrode material for a lithium ion secondary battery of Example 4 was prepared and evaluated in the same manner as in Example 1 except that the precursor of the positive electrode material was mixed with lithium hydroxide (LiOH) as a Li source so that the mass ratio of Li, Fe and Mg became Li / (Fe+Mg)=1.00.

[0254] X-ray diffraction analysis confirmed that the obtained positive electrode material produced LFP as a positive electrode active material. In addition, scanning electron microscope observation confirmed that the obtained positive electrode material was composed of a positive electrode active material including secondary particles formed by aggregation of primary particles of LFP.

[0255] The composition of the obtained positive electrode material was confirmed to be Li:Fe:Mg+PO4=1:0.95:0.05:1 in terms of mass ratio. The evaluation results of the precursors are summarized in Table 1, and the evaluation results of the obtained positive electrode materials and lithium ion secondary batteries using the positive electrode materials are summarized in Table 2.

[0256] [Example 5]

[0257] A positive electrode material for a lithium ion secondary battery of Example 5 was prepared and evaluated in the same manner as in Example 1 except that the precursor of the positive electrode material was mixed with lithium hydroxide (LiOH) as a Li source so that the mass ratio of Li, Fe and Mg became Li / (Fe+Mg)=0.98.

[0258] X-ray diffraction analysis confirmed that the obtained positive electrode material produced LFP as a positive electrode active material. In addition, scanning electron microscope observation confirmed that the obtained positive electrode material was composed of a positive electrode active material including secondary particles formed by aggregation of primary particles of LFP.

[0259] The composition of the obtained positive electrode material was confirmed to be Li:Fe:Mg+PO4=0.98:0.95:0.05:1 in terms of mass ratio. The evaluation results of the precursors are summarized in Table 1. The evaluation results of the obtained positive electrode materials and lithium ion secondary batteries using the positive electrode materials are summarized in Table 1.

[0260] [Comparative Example 1]

[0261] When making the precursor of the positive electrode active material, Mg(OH)2 was not mixed but 131g of H3PO4 (75% aqueous solution) was mixed, and when mixing the precursor and the Li source, ammonium dihydrogen phosphate was not mixed. Except for this, the positive electrode material for the lithium ion secondary battery of Comparative Example 1 was made and evaluated in the same manner as Example 1.

[0262] The obtained precursor was observed with a scanning electron microscope, and it was confirmed that it was composed of secondary particles in which primary particles of FePO 4 were aggregated.

[0263] The ratio of P to the total amount of Fe, Mg and metal element A (P / (Fe+Mg+A)) is 1.04. In addition, the specific surface area is 10.4 m 2 / g, and D50 was 1.8 μm. Table 1 shows the evaluation results of the obtained precursor.

[0264] X-ray diffraction analysis confirmed that the obtained positive electrode material produced LFP as a positive electrode active material. In addition, observation of the obtained positive electrode material using a scanning electron microscope revealed that the positive electrode material was composed of a positive electrode active material including secondary particles formed by aggregation of primary particles of LFP.

[0265] The composition of the obtained positive electrode material was confirmed to be Li:Fe:PO4=1.02:1:1.04 in terms of mass ratio. The evaluation results of the obtained precursors are summarized in Table 1, and the evaluation results of the obtained positive electrode materials and lithium ion secondary batteries using the same are summarized in Table 2.

[0266] [Comparative Example 2]

[0267] A positive electrode material for a lithium ion secondary battery of Comparative Example 2 was prepared and evaluated in the same manner as in Example 1, except that 8.36 g of Mg(OH) 2 and 128 g of H 3 PO 4 (75% aqueous solution) were mixed when preparing the precursor of the positive electrode active material.

[0268] The obtained precursor was observed with a scanning electron microscope, and it was confirmed that it was composed of secondary particles formed by aggregation of primary particles of FePO4, and fine Mg3(PO4)2 particles of 10nm to 300nm existed on the particle surface of the primary particles.

[0269] The ratio of P to the total amount of Fe, Mg, and metal element A (P / (Fe+Mg+A)) is 0.94. In addition, the specific surface area is 8.7 m 2 / g, and D50 was 2.7 μm. Table 1 shows the evaluation results of the obtained precursor.

[0270] X-ray diffraction analysis confirmed that the obtained positive electrode material produced LFP as a positive electrode active material. In addition, scanning electron microscope observation confirmed that the obtained positive electrode material was composed of a positive electrode active material including secondary particles formed by aggregation of primary particles of LFP.

[0271] The composition of the obtained positive electrode material was confirmed to be Li:Fe:Mg+PO4=1.02:0.875:0.125:1 in terms of mass ratio. The evaluation results of the obtained precursors are summarized in Table 1, and the evaluation results of the obtained positive electrode materials and lithium ion secondary batteries using the same are summarized in Table 2.

[0272] [Table 1]

[0273] Table 1

[0274]

[0275] [Table 2]

[0276]

[0277] As shown in Table 1, it was confirmed that the load characteristics, DC resistance and cycle characteristics of Examples 1 to 5 were better than those of Comparative Examples 1 and 2. It is believed that since Mg substitution was not performed in Comparative Example 1, the lattice strain and volume change accompanying the Li ion insertion / extraction reaction were large, and the load characteristics and input-output characteristics were reduced. It is believed that since the amount of Mg substitution in Comparative Example 2 was too much, the resistance of each reaction increased, and the load characteristics and input-output characteristics were reduced.

[0278] As described above, it was confirmed that the positive electrode material and the production method of the present embodiment can provide a positive electrode material containing lithium iron phosphate having high input-output characteristics and cycle characteristics when used as a positive electrode of a lithium ion secondary battery.

[0279] Description of Reference Numerals

[0280] 10 Laminated secondary battery

[0281] 11 Positive electrode

[0282] 12 Dividers

[0283] 13 Negative electrode

[0284] 14 Laminate

[0285] 15. Positive electrode tab

[0286] 16 Negative electrode tab

Claims

1. A positive electrode material for a lithium ion secondary battery, comprising aggregated particles, wherein the aggregated particles are formed by aggregating a plurality of primary particles of a positive electrode active material covered with a carbonaceous film, The positive electrode active material comprises lithium iron phosphate, which contains lithium (Li), iron (Fe), magnesium (Mg) and metal element A (A) in a mass ratio of Li:Fe:Mg:A=x:y:z:1-yz, wherein: x is 0.9<x<1.1, y is 0.2<y<1.0, z is 0<z<0.1, and 0.2<y+z≤1, and the metal element A is at least one additional element selected from the group consisting of Zn, Co, Ni, Ti and V, The change rate of the lattice area of ​​the positive electrode material on the b-axis-c-axis plane before and after full charge, i.e., [(lattice area before charge-lattice area after full charge) / lattice area before charge]×100 is greater than 1.10% and less than 1.33%.

2. The positive electrode material for lithium ion secondary battery according to claim 1, wherein The change rate of the lattice volume of the positive electrode material before charging and after full charging, that is, [(lattice volume before charging-lattice volume after full charging) / volume before charging]×100 is 4.90% or more and 6.06% or less.

3. The positive electrode material for lithium ion secondary battery according to claim 1 or 2, wherein The positive electrode material has a crystallite diameter of 80 nm or more and 150 nm or less, and a specific surface area of ​​6.0 m 2 / g or more and 14.0m 2 / g or less.

4. The positive electrode material for lithium ion secondary battery according to claim 1 or 2, wherein The tap density of the positive electrode material is 1.1 g / cm 3 The density of the pressed powder when the positive electrode material is compressed into a circle with a diameter of 20 mm under a pressure of 16 kN is 2.4 g / cm 3 above.

5. A method for producing a positive electrode material for a lithium ion secondary battery, which is a method for producing the positive electrode material for a lithium ion secondary battery according to claim 1 or 2, The method comprises the following steps: A crystallization step, using a metal source having a valence of 2, 3 or both and a phosphoric acid source, to obtain crystallized particles through a crystallization reaction; A water washing step, washing and drying the crystallized particles to obtain a precursor of the positive electrode active material; a mixing step of mixing the precursor of the positive electrode active material, a lithium source and a carbon source to obtain a raw material mixture; and a calcining step of calcining the raw material mixture in a non-oxidizing atmosphere to obtain a positive electrode material containing the positive electrode active material; In the precursor of the positive electrode active material, a ratio of P to the total amount of Fe, Mg and the metal element A, that is, P / (Fe+Mg+A), is 0.95 or more and 1.03 or less.

6. The method for producing a positive electrode material for a lithium ion secondary battery according to claim 5, wherein: The specific surface area of ​​the precursor of the positive electrode active material is 7.0 m 2 / g or more and 12.0m 2 / g or less, and a particle size D50 of 50% of the cumulative percentage in the cumulative particle size distribution is 0.5 μm or more and 5.0 μm or less.

7. A positive electrode for a lithium ion secondary battery, comprising: an aluminum current collector, and a positive electrode composite material layer formed on the aluminum current collector, The positive electrode composite material layer contains the positive electrode material for a lithium ion secondary battery according to claim 1 or 2.

8. A lithium ion secondary battery comprising at least a positive electrode, a negative electrode and an electrolyte, The positive electrode is the positive electrode for a lithium ion secondary battery according to claim 7.

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