Positive electrode active material, positive electrode active material particles, positive electrode, lithium-ion secondary battery, and method for producing positive electrode active material particles

Abstract

The objective of the present disclosure is to provide, in a lithium-ion secondary battery, a positive electrode active material that can keep the potential difference between the potential during charging and the potential during discharging from increasing. The present disclosure relates to a positive electrode active material represented by formula (1). Formula (1): LiaMnbMc(PdSe)O4.00 (In the formula, M is one element selected from the group consisting of Fe, Co, Ni, Al, Mg, Sn, Nb, B, Cu, Cr, Mo, Ru, V, Ga, Ca, Sr, Ba, Ti and Zr, 1.00 < a ≤ 1.50, 0.40 ≤ b < 1.10, 0.00 ≤ c ≤ 0.20, 0.50 ≤ d < 1.00, and 0.00 < e ≤ 0.50.)

Description

Positive electrode active material, positive electrode active material particles, positive electrode, lithium-ion secondary battery, and method for manufacturing positive electrode active material particles

[0001] The present invention relates to a positive electrode active material, positive electrode active material particles, a positive electrode, a lithium-ion secondary battery, and a method for producing positive electrode active material particles.

[0002] Lithium-ion batteries, a type of non-aqueous electrolyte secondary battery, have a large charge and discharge capacity and are primarily used as batteries for portable electronic devices. Furthermore, their use in electric vehicles is increasing, and further performance improvements are expected.

[0003] Lithium iron phosphate (LiFePO4) is commonly used as the positive electrode active material for lithium-ion secondary batteries. However, rare metals are not always readily available due to their limited supply and high cost. Therefore, in recent years, there has been a demand for positive electrode active materials that use substances other than rare metals. One such positive electrode active material is lithium iron phosphate (LiFePO4). ４ ) is being considered. Lithium iron phosphate is inexpensive and safe because it uses fewer rare earth elements, but it has the problem of low energy density. Therefore, in lithium iron phosphate, lithium manganese phosphate (LiMnPO) is being considered in which iron is replaced with manganese. ４ Lithium manganese phosphate is also being considered as a positive electrode active material. Lithium manganese phosphate maintains the same cost and safety as lithium iron phosphate, while having a higher energy density and being less prone to charge-discharge reactions. Therefore, to suppress the problems of lithium iron phosphate and lithium manganese phosphate, lithium manganese iron phosphate, in which some of the manganese is replaced with iron, is being considered as a positive electrode active material.

[0004] For example, Patent Document 1 discloses a lithium manganese iron phosphate cathode active material in which carbon is supported on a lithium manganese iron phosphate compound represented by a specific formula.

[0005] Japanese Patent Publication No. 2016-081866

[0006] The positive electrode active material described in Patent Document 1 has some of the manganese in lithium manganese phosphate replaced with iron. Lithium-ion secondary batteries using such a positive electrode active material tend to undergo charge-discharge reactions more easily compared to lithium-ion secondary batteries using unsubstituted lithium manganese phosphate. However, it was found that the potential difference between the potential during charging and the potential during discharging is large in lithium-ion secondary batteries using the positive electrode active material described in Patent Document 1. When this potential difference is large, the amount of electricity that can be discharged is less than the amount of electricity required for charging. In other words, the amount of energy that can be released is less than the energy required for charging, and it was found that the energy efficiency tends to deteriorate.

[0007] This invention has been made in view of these circumstances, and aims to provide a positive electrode active material that can suppress the increase in the potential difference between the potential during charging and the potential during discharging in a lithium-ion secondary battery.

[0008] The inventors diligently studied to solve the above problems. As a result, they found that the positive electrode active material represented by formula (1) can solve the above problems, and thus completed the present invention.

[0009]

[0010] In equation (1), M is one selected from the group consisting of Fe, Co, Ni, Al, Mg, Sn, Nb, B, Cu, Cr, Mo, Ru, V, Ga, Ca, Sr, Ba, Ti, and Zr, where 1.00 < a ≤ 1.50, 0.40 ≤ b < 1.10, 0.00 ≤ c ≤ 0.20, 0.50 ≤ d < 1.00, and 0.00 < e ≤ 0.50.

[0011] In other words, the present invention includes the following embodiments: [1] A positive electrode active material represented by formula (1). (wherein M is one selected from the group consisting of Fe, Co, Ni, Al, Mg, Sn, Nb, B, Cu, Cr, Mo, Ru, V, Ga, Ca, Sr, Ba, Ti, and Zr, and 1.00 < a ≤ 1.50, 0.40 ≤ b < 1.10, 0.00 ≤ c ≤ 0.20, 0.50 ≤ d < 1.00, and 0.00 < e ≤ 0.50.) [2] The positive electrode active material according to [1], wherein b and c satisfy 0.50 < b + c < 1.00. [3] The positive electrode active material according to [1] or [2], wherein a, b and c satisfy 1.80 < a + b + c < 2.20. [4] The positive electrode active material according to any one of [1] to [3], wherein d and e satisfy 0.80 < d + e < 1.20. [5] A positive electrode active material represented by formula (2), as described in any of [1] to [4]. (wherein M is selected from the group consisting of Fe, Co, Ni, Al, Mg, Sn, Nb, B, Cu, Cr, Mo, Ru, V, Ga, Ca, Sr, Ba, Ti, and Zr, and 0.50 ≤ x < 1.00, 0.50 ≤ y < 1.00, 0.00 ≤ α ≤ 0.10, 0.00 ≤ β ≤ 0.20, and 0.00 ≤ γ ≤ 0.20.) [6] The positive electrode active material according to [5], wherein |x - y| ≤ 0.10. [7] Positive electrode active material particles comprising the positive electrode active material according to any one of [1] to [6]. [8] The positive electrode active material particle according to [7], wherein a carbonaceous film exists in at least a portion of the surface of the positive electrode active material particle. [9] Agglomerated positive electrode active material particle composed of aggregated positive electrode active material particles according to [7] or [8].

[10] A positive electrode comprising a positive electrode active material according to any one of [1] to [6].

[11] A lithium-ion secondary battery comprising a positive electrode comprising a positive electrode active material according to any one of [1] to [6], a negative electrode, and a separator.

[12] A method for producing positive electrode active material particles, comprising: a mixture preparation step of preparing a mixture in which raw materials comprising lithium, manganese, phosphorus, and sulfur are mixed in a first solvent comprising one or more selected from the group consisting of water, ethylene glycol, diethylene glycol, triethylene glycol, and tetraethylene glycol; and a positive electrode active material particle synthesis step of heating the mixture to obtain positive electrode active material particles comprising a positive electrode active material represented by formula (1). (In the formula, M is one selected from the group consisting of Fe, Co, Ni, Al, Mg, Sn, Nb, B, Cu, Cr, Mo, Ru, V, Ga, Ca, Sr, Ba, Ti, and Zr; 1.00 < a ≦ 1.50, 0.40 ≦ b < 1.10, 0.00 ≦ c ≦ 0.20, 0.50 ≦ d < 1.00, 0.00 < e ≦ 0.50.)

[13] The heating in the positive electrode active material particle synthesis step is performed by means including resistance heating or electromagnetic wave irradiation heating, the manufacturing method according to

[12] .

[14] The manufacturing method according to

[12] or

[13] , including a washing step of washing the positive electrode active material particles.

[15] A dispersion liquid preparation step of dispersing the positive electrode active material particles in a second solvent containing one or more selected from the group consisting of water, methanol, ethanol, propanol, butanol, ethylene glycol, diethylene glycol, triethylene glycol, and tetraethylene glycol to obtain a dispersion liquid; a carbon source addition step of adding a carbon source soluble in the second solvent to the dispersion liquid; a precipitation step of drying the dispersion liquid to which the carbon source has been added to precipitate the positive electrode active material particles; and a carbonaceous film forming step of heating the precipitated positive electrode active material particles to form a carbonaceous film in at least a partial region on the surface of the positive electrode active material particles, the manufacturing method according to any one of

[12] to

[14] .

[0012] According to the present invention, in a lithium ion secondary battery, it is possible to provide a positive electrode active material capable of suppressing an increase in the potential difference between the potential during charging and the potential during discharging.

[0013] This is an example of a schematic diagram of a lithium-ion secondary battery having coin-type cells. This is a graph showing the results of the example. This is a graph showing the results of the example. This is an image showing the results of the example. This is an image showing the results of the example. This is an image showing the results of the example. This is a graph showing the results of the example. This is an image showing the results of the example. This is an image showing the results of the example. This is an image showing the results of the example. This is a graph showing the results of the example. This is a graph showing the results of the example. This is a graph showing the results of the example. This is a graph showing the results of the example. This is a graph showing the results of the example. This is a graph showing the results of the example. This is a graph showing the results of the example. This is a graph showing the results of the example. This is a graph showing the results of the example. This is a graph showing the results of the example. This is a graph showing the results of the example. This is an image showing the results of the example. This is an image showing the results of the example. This is a table showing the results of the example. This is a graph showing the results of the example. This is a graph showing the results of the example. This is a graph showing the results of the example. This is a graph showing the results of the example. This is a graph showing the results of the example.

[0014] The embodiments of the present invention (hereinafter referred to as "these embodiments") will be described in detail below, with reference to the drawings as necessary. However, the present invention is not limited thereto, and various modifications are possible without departing from its essence. In the drawings, the same elements will be denoted by the same reference numerals, and redundant explanations will be omitted. Furthermore, unless otherwise specified, positional relationships such as up, down, left, and right will be based on the positional relationships shown in the drawings. Moreover, the dimensional ratios in the drawings are not limited to those shown.

[0015] 1. Lithium-ion secondary battery FIG. 1 is an example of a schematic diagram of a lithium-ion secondary battery having a coin-type cell according to an embodiment of the present invention. In FIG. 1, the right half shows a cross-sectional view inside the coin-type cell, and the left half shows a side view of the coin-type cell. The lithium-ion secondary battery 1 of the present embodiment (hereinafter, also simply referred to as "lithium-ion secondary battery 1") is not particularly limited, but for example, includes a negative electrode terminal 2, a negative electrode 3, a separator 4, an insulating packing 5, a positive electrode 6, and a positive electrode can 7. Alternatively, the lithium-ion secondary battery 1 is not particularly limited, but for example, includes a positive electrode containing the positive electrode active material of the present embodiment, a negative electrode, and a separator.

[0016] The shape of the lithium-ion secondary battery 1 is not particularly limited, and for example, it may be a coin type as shown in FIG. 1, or may be a cylindrical type, a laminated type, a button type, or the like.

[0017] Hereinafter, each component of the lithium-ion secondary battery 1 will be described in detail.

[0018] 1.1. Positive electrode The positive electrode 6 of the lithium-ion secondary battery 1 contains the positive electrode active material of the present embodiment. The positive electrode 6 preferably includes a positive electrode active material composition containing a positive electrode active material and a positive electrode current collector. The positive electrode active material composition is preferably supported on the positive electrode current collector. Note that the positive electrode can 7 is a kind of positive electrode current collector.

[0019] 1.1.1. Positive electrode active material composition The positive electrode active material composition of the present embodiment contains a positive electrode active material and may contain other additives.

[0020] 1.1.1.1. Positive electrode active material The positive electrode 6 of the lithium-ion secondary battery 1 contains a positive electrode active material represented by the formula (1). In the positive electrode active material represented by the formula (1), a part of the phosphorus in lithium manganese phosphate is replaced by sulfur. By using such a positive electrode active material, it is possible to suppress an increase in the potential difference between the potential during charging and the potential during discharging. That is, there is a tendency to achieve both an improvement in the energy efficiency of the lithium-ion secondary battery 1 and the maintenance of the battery capacity. Furthermore, by using the positive electrode active material of the present embodiment, there is a tendency to suppress a decrease in the capacity of the lithium-ion secondary battery 1.

[0021]

[0022] In equation (1), M is one selected from the group consisting of Fe, Co, Ni, Al, Mg, Sn, Nb, B, Cu, Cr, Mo, Ru, V, Ga, Ca, Sr, Ba, Ti, and Zr, where 1.00 < a ≤ 1.50, 0.40 ≤ b < 1.10, 0.00 ≤ c ≤ 0.20, 0.50 ≤ d < 1.00, and 0.00 < e ≤ 0.50.

[0023] In lithium-ion secondary batteries using lithium manganese phosphate as the positive electrode active material, charging and discharging of the battery is thought to occur through the insertion and removal of lithium. In lithium manganese phosphate, the phosphorus-oxygen bond is very strong, so the lithium-oxygen interaction that is directly involved in the insertion and removal of lithium is relatively weak, which is thought to result in a slow lithium insertion and removal reaction, very high electrical resistance, and a large potential difference during charging and discharging. On the other hand, in a positive electrode active material represented by equation (1), where some of the phosphorus is replaced by sulfur and the sulfur content is 0.00 < e ≤ 0.50, the proportion of oxygen that was strongly bonded to phosphorus decreases, and instead, the proportion of oxygen bonded to sulfur, which has a weaker bonding affinity than phosphorus, increases. This increases the proportion of oxygen that contributes to the lithium-oxygen interaction. As a result, it is thought that lithium insertion and removal occur more easily, and the potential difference during charging and discharging becomes smaller.

[0024] Furthermore, in the positive electrode active material represented by formula (1), the upper limit of the sulfur content is e ≤ 0.50, so the crystal structure of lithium manganese phosphate, in which some phosphorus is substituted with sulfur, is less likely to change. Therefore, the capacity of lithium-ion secondary batteries using the positive electrode active material represented by formula (1) is less likely to decrease.

[0025] Furthermore, lithium-ion secondary batteries using a positive electrode active material represented by equation (1), where the sulfur content is 0.00 < e ≤ 0.50, tend to have a larger capacity due to the ease with which lithium is inserted and removed.

[0026] Furthermore, in lithium manganese phosphate in which some phosphorus is substituted with sulfur, it is thought that divalent manganese ions can be oxidized to tetravalent manganese ions. In this case, the greater the amount of sulfur substitution, the greater the theoretical capacity of the battery using lithium manganese phosphate as the positive electrode active material.

[0027] Furthermore, in lithium-ion secondary batteries using lithium manganese phosphate in which some of the manganese is replaced with iron as the positive electrode active material, there was a problem in which the potential decreased in multiple stages during charging and discharging, resulting in a decrease in energy density. On the other hand, in lithium-ion secondary batteries using the positive electrode active material represented by equation (1), the multi-stage change in potential during charging and discharging is suppressed, and even if it changes in multiple stages, the potential change is suppressed, thus improving the energy density of the lithium-ion secondary battery 1.

[0028] In equation (1), it is preferable that the electrical neutrality condition is satisfied. For example, if the valence of all Mn ions is 2+, then it is preferable that a + 2b + mc + 5d + 6e - 8 = 0.00 is satisfied, where m is the valence of M. However, from the standpoint of significant figures, there may be cases where a + 2b + mc + 5d + 6e - 8 appears to be slightly off from 0. For example, even if a + 2b + mc + 5d + 6e - 8 is between -0.05 and 0.05, it can be said that the electrical neutrality condition is satisfied.

[0029] In the positive electrode active material represented by formula (1), M is preferably one or more selected from the group consisting of Fe, Co, Ni, Al, Mg, Ti, B, Cr, and V, more preferably one or more selected from the group consisting of Fe, Co, Ni, Cr, and V, and even more preferably one or more selected from the group consisting of Fe, Co, and Ni. By selecting one or more from these groups, the energy efficiency of the lithium-ion secondary battery 1 tends to improve further. In particular, by selecting one or more from the group consisting of Fe, Co, Ni, Cr, and V, the electronic conductivity of the positive electrode active material improves, the potential difference during charging and discharging of the lithium-ion secondary battery 1 becomes smaller, and the energy efficiency tends to improve further.

[0030] The positive electrode active material represented by formula (1) preferably has an olivine structure as its crystal structure. The olivine structure is a structure in which the oxygen element has a hexagonal close-packed structure, with phosphorus and sulfur elements in the center of the tetrahedron where the oxygen element is at its vertex, and lithium, manganese, and the element represented by M in the center of the octahedron where the oxygen element is at its vertex. The olivine structure is preferable because oxygen is less likely to be released even at high temperatures, and it tends to have high stability at high temperatures.

[0031] Furthermore, when the positive electrode active material represented by formula (1) has an olivine structure as its crystal structure, the (010) plane tends to be easily exposed in that crystal. Since the (010) plane is the plane corresponding to the cross-section of the lithium diffusion path within the crystal structure, exposure of the (010) plane makes it easier for lithium elements to be inserted and removed during charging and discharging, which tends to reduce the potential difference during charging and discharging and further improve energy efficiency.

[0032] The crystal structure of the positive electrode active material is not particularly limited, but can be identified, for example, using an X-ray diffractometer and the Rietveld method.

[0033] In the positive electrode active material represented by formula (1), a is preferably 1.01 ≤ a ≤ 1.45, 1.05 ≤ a ≤ 1.45, 1.10 ≤ a ≤ 1.40, and 1.15 ≤ a ≤ 1.35. Alternatively, a is preferably 1.05 ≤ a ≤ 1.35. When a is within the above range, the energy efficiency of the lithium-ion secondary battery 1 tends to improve further.

[0034] In the positive electrode active material represented by equation (1), when the lithium content is 1.00 < a, i.e., a ≠ 1.00, the stability of lithium within the crystal structure of the positive electrode active material decreases, and lithium tends to migrate more easily. As a result, insertion and removal of lithium elements during charging and discharging becomes more likely, the potential difference between charging and discharging decreases, and energy efficiency tends to improve further.

[0035] Furthermore, in lithium-ion secondary batteries using lithium manganese phosphate as the positive electrode active material, lithium contributes to the charging and discharging of the battery. Therefore, when most or all of the lithium in lithium manganese phosphate contributes to the charging and discharging of the battery, the battery capacity tends to improve when a < 1.00 in the positive electrode active material represented by equation (1). To make most or all of the lithium in lithium manganese phosphate contribute to the charging and discharging of the battery, for example, the charging voltage can be increased.

[0036] In the positive electrode active material represented by formula (1), b is preferably 0.40 ≤ b ≤ 1.05, 0.40 ≤ b ≤ 1.00, 0.50 ≤ b ≤ 0.95, and 0.60 ≤ b ≤ 0.90. Alternatively, b is preferably 0.70 ≤ b ≤ 1.00. When b is within the above range, the energy efficiency of the lithium-ion secondary battery 1 tends to improve. Also, when b is within the above range, the capacity of the lithium-ion secondary battery 1 tends to increase.

[0037] In the positive electrode active material represented by formula (1), c is preferably 0.00 ≤ c ≤ 0.19, 0.00 ≤ c ≤ 0.18, 0.00 ≤ c ≤ 0.15, 0.00 ≤ c ≤ 0.10, 0.00 ≤ c ≤ 0.05, or 0.00 ≤ c ≤ 0.01. Alternatively, c may be 0.00. When c is within the above range, the energy efficiency of the lithium-ion secondary battery 1 tends to improve further.

[0038] In the positive electrode active material represented by formula (1), d is preferably 0.50 < d < 1.00, 0.60 ≤ d ≤ 0.95, 0.65 ≤ d ≤ 0.90, and 0.70 ≤ d ≤ 0.90. When d is within the above range, the energy efficiency of the lithium-ion secondary battery 1 tends to improve further.

[0039] In the positive electrode active material represented by formula (1), e is preferably 0.00 < e < 0.50, 0.05 < e < 0.50, 0.10 < e < 0.50, 0.13 ≤ e ≤ 0.45, 0.15 ≤ e ≤ 0.45, 0.15 ≤ e ≤ 0.40, 0.15 ≤ e ≤ 0.35, and 0.20 ≤ e ≤ 0.30. Alternatively, e is preferably 0.03 < e ≤ 0.30 and 0.05 ≤ e ≤ 0.30. When e is within the above range, the energy efficiency of the lithium-ion secondary battery 1 tends to improve further.

[0040] In the positive electrode active material represented by formula (1), b and c are preferably 0.50 < b + c < 1.00 and 0.60 ≤ b + c ≤ 0.95. When b and c are within the above range, the energy efficiency of the lithium-ion secondary battery 1 tends to improve further.

[0041] In the positive electrode active material represented by formula (1), a, b, and c are preferably 1.80 < a + b + c < 2.20, 1.85 ≤ a + b + c ≤ 2.15, and 1.90 ≤ a + b + c ≤ 2.10. When a, b, and c are within the above ranges, the energy efficiency of the lithium-ion secondary battery 1 tends to improve further.

[0042] In the positive electrode active material represented by formula (1), d and e are preferably 0.80 < d + e < 1.20, 0.85 ≤ d + e ≤ 1.15, and 0.90 ≤ d + e ≤ 1.10. When d and e are within the above ranges, the energy efficiency of the lithium-ion secondary battery 1 tends to improve further.

[0043] In the positive electrode active material represented by formula (1), a, b, and d are preferably 0.90 < a + b + d < 3.00, 1.50 ≤ a + b + d ≤ 2.95, 2.00 ≤ a + b + d ≤ 2.90, and 2.50 ≤ a + b + d ≤ 2.85. When a + b + d is within the above range, the energy efficiency of the lithium-ion secondary battery 1 tends to improve further.

[0044] In the positive electrode active material represented by formula (1), a / (b+c) is preferably 1.02 ≤ (a / (b+c)) ≤ 2.50, 1.05 ≤ (a / (b+c)) ≤ 2.00, and 1.10 ≤ (a / (b+c)) ≤ 1.90. When a / (b+c) is within the above range, the energy efficiency of the lithium-ion secondary battery 1 tends to improve further.

[0045] In the positive electrode active material represented by formula (1), d / e is preferably 1.10 ≤ (d / e) ≤ 50.00, 1.20 ≤ (d / e) ≤ 40.00, 1.30 ≤ (d / e) ≤ 30.00, 1.40 ≤ (d / e) ≤ 20.00, and 1.50 ≤ (d / e) ≤ 10.00. When d / e is within the above range, the energy efficiency of the lithium-ion secondary battery 1 tends to improve further.

[0046] The positive electrode 6 of the lithium-ion secondary battery 1 preferably contains a positive electrode active material represented by formula (2). This tends to further improve the energy efficiency of the lithium-ion secondary battery 1.

[0047]

[0048] In equation (2), M is one selected from the group consisting of Fe, Co, Ni, Al, Mg, Sn, Nb, B, Cu, Cr, Mo, Ru, V, Ga, Ca, Sr, Ba, Ti, and Zr, and the following conditions apply: 0.50 ≤ x < 1.00, 0.50 ≤ y < 1.00, 0.00 ≤ α ≤ 0.10, 0.00 ≤ β ≤ 0.20, and 0.00 ≤ γ ≤ 0.20.

[0049] In addition, it is preferable that the electrical neutrality condition is met in formula (2).

[0050] In the positive electrode active material represented by formula (2), the preferred type of M is the same as that in formula (1).

[0051] In the positive electrode active material represented by formula (2), x is preferably 0.50 < x < 1.00, 0.60 ≤ x ≤ 0.90, and 0.65 ≤ x ≤ 0.85. When x is within the above range, the energy efficiency of the lithium-ion secondary battery 1 tends to improve further.

[0052] In the positive electrode active material represented by formula (2), y is preferably 0.50 < y < 0.95, 0.50 < y < 0.90, 0.60 ≤ y ≤ 0.87, 0.60 ≤ y ≤ 0.85, 0.65 ≤ y ≤ 0.85, and 0.70 ≤ y ≤ 0.85. When y is within the above range, the energy efficiency of the lithium-ion secondary battery 1 tends to improve further.

[0053] In the positive electrode active material represented by formula (2), α is preferably 0.00 ≤ α ≤ 0.05, 0.00 ≤ α ≤ 0.03, and 0.00 ≤ α ≤ 0.01. Alternatively, α may be 0.00. When α is within the above range, the energy efficiency of the lithium-ion secondary battery 1 tends to improve further.

[0054] In the positive electrode active material represented by formula (2), β is preferably 0.00 ≤ β ≤ 0.15, 0.00 ≤ β ≤ 0.10, 0.00 ≤ β ≤ 0.05, and 0.00 ≤ β ≤ 0.01. Alternatively, β may be 0.00. When β is within the above range, the energy efficiency of the lithium-ion secondary battery 1 tends to improve further.

[0055] In the positive electrode active material represented by formula (2), γ is preferably 0.00 ≤ γ ≤ 0.15, 0.00 ≤ γ ≤ 0.10, 0.00 ≤ γ ≤ 0.05, and 0.00 ≤ γ ≤ 0.01. Alternatively, γ may be 0.00. When γ is within the above range, the energy efficiency of the lithium-ion secondary battery 1 tends to improve further.

[0056] In the positive electrode active material represented by formula (2), preferably α = β. Also, in the positive electrode active material represented by formula (2), preferably α = β = γ. When α, β, and γ satisfy the above formula, the energy efficiency of the lithium-ion secondary battery 1 tends to improve further.

[0057] In the positive electrode active material represented by equation (2), preferably x = y. When x and y satisfy the above equation, the energy efficiency of the lithium-ion secondary battery 1 tends to improve further.

[0058] In the positive electrode active material represented by equation (2), when α = β = γ = 0.00 and x = y, x is preferably 0.50 < x < 0.95, 0.50 < x < 0.90, 0.60 ≤ x ≤ 0.85, and 0.65 ≤ x ≤ 0.85. When x is within the above range, the energy efficiency of the lithium-ion secondary battery 1 tends to improve further.

[0059] In the positive electrode active material represented by equation (2), the absolute value of x - y, |x - y|, is preferably |x - y| ≤ 0.10, |x - y| ≤ 0.08, and |x - y| ≤ 0.05. Furthermore, when |x - y| is within the above range, α is preferably 0.00 ≤ α ≤ 0.05 and 0.00 ≤ α ≤ 0.01. Furthermore, when |x - y| is within the above range, β is preferably 0.00 ≤ β ≤ 0.05 and 0.00 ≤ β ≤ 0.01. This tends to further improve the energy efficiency of the lithium-ion secondary battery 1.

[0060] The molar ratio of each element in the positive electrode active material can be adjusted by changing the mixing ratio of each raw material in the manufacturing method of the positive electrode active material. For example, when manganese sulfate and manganese chloride are used as raw materials for the manganese element in the positive electrode active material, increasing the amount of manganese chloride used relative to manganese sulfate tends to increase the manganese element content in the positive electrode active material and decrease the sulfur content. Similarly, increasing the amount of manganese element raw materials relative to lithium element raw materials in the positive electrode active material tends to increase the manganese element content in the positive electrode active material and decrease the lithium element content.

[0061] The molar ratio of each element in the positive electrode active material is not particularly limited, but can be measured, for example, by ICP emission spectroscopy.

[0062] The manganese site occupancy rate in the positive electrode active material is preferably 0.750 to 0.950, 0.800 to 0.940, 0.850 to 0.930, and 0.900 to 0.925. This tends to improve the energy efficiency and capacity of the lithium-ion secondary battery 1. In this disclosure, the manganese site occupancy rate refers to the ratio of the number of manganese sites that actually contain manganese to the total number of manganese sites in the crystal structure.

[0063] The content of the positive electrode active material is preferably 60 to 100% by mass, 70 to 100% by mass, 80 to 100% by mass, 90 to 100% by mass, 95 to 100% by mass, or 99 to 100% by mass, relative to the total amount of the positive electrode active material composition. Furthermore, the positive electrode active material composition may consist substantially of positive electrode active material or consist solely of positive electrode active material.

[0064] In this embodiment, the positive electrode active material is preferably present at the positive electrode as positive electrode active material particles. That is, the positive electrode active material particles of this embodiment preferably contain the positive electrode active material of this embodiment. In addition to the positive electrode active material, the positive electrode active material particles may also contain substances used in the manufacturing process of the positive electrode active material and a carbonaceous film, which will be described later. The content of the positive electrode active material in the positive electrode active material particles is not particularly limited, but for example, it is 80 to 100% by mass, 90 to 100% by mass, 95 to 100% by mass, and 99 to 100% by mass. Furthermore, the positive electrode active material particles may be substantially composed of the positive electrode active material or composed entirely of the positive electrode active material.

[0065] The positive electrode active material particles may have a carbonaceous film on at least a portion of their surface, or on the entire surface. The carbonaceous film is a film containing a carbon compound, and an example of a carbon compound is elemental carbon. The presence of a carbonaceous film in the positive electrode active material particles improves the conductivity of the positive electrode active material particles, and tends to facilitate the charge and discharge reactions of lithium-ion secondary batteries. The elemental carbon content in the carbonaceous film is preferably 50 to 100% by mass, 60 to 100% by mass, or 70 to 100% by mass. Furthermore, the carbonaceous film content in the positive electrode active material particles is preferably 1.0 to 10.0% by mass, 1.5 to 8.5% by mass, or 2.0 to 7.0% by mass. When the carbonaceous film content is within the above range, tends to facilitate the charge and discharge reactions of lithium-ion secondary batteries.

[0066] The median diameter (D50) of the positive electrode active material particles is not particularly limited, but for example, it is 0.01 to 30 μm, 0.02 to 20 μm, or 0.03 to 10 μm.

[0067] The median diameter is the particle size corresponding to a 50% cumulative degree in the particle size distribution measured by laser diffraction and scattering methods. It is defined as a particle size such that the number of particles with a diameter larger than the median diameter is equal to the number of particles with a diameter smaller than the median diameter.

[0068] The median diameter is not particularly limited, but can be measured using, for example, the Microtrac MT3300EXII laser diffraction particle size distribution analyzer or the Horiba LA-960 laser diffraction / scattering particle size distribution analyzer.

[0069] In this embodiment, the positive electrode active material particles preferably aggregate to form positive electrode active material aggregate particles. That is, it is preferable that the positive electrode active material aggregate particles in this embodiment are formed by the aggregation of the positive electrode active material particles in this embodiment. For example, positive electrode active material aggregate particles are formed by the aggregation of positive electrode active material particles having a carbonaceous film in at least a portion of their surface area. The median diameter (D50) of the positive electrode active material aggregate particles is not particularly limited, but for example, it is 1.0 to 50.0 μm, 1.5 to 40.0 μm, 2.0 to 30.0 μm, 2.0 to 20.0 μm, or 2.0 to 10.0 μm.

[0070] The positive electrode of this embodiment preferably includes positive electrode active material particles containing the above-mentioned positive electrode active material, and more preferably includes positive electrode active material aggregate particles formed by the aggregation of such positive electrode active material particles. Furthermore, at least a portion of the positive electrode active material particles and / or positive electrode active material aggregate particles preferably have a carbonaceous film in at least a portion of their surface, and more preferably have a carbonaceous film over the entire surface.

[0071] 1.1.1.2. Additives The positive electrode active material composition may contain additives other than the positive electrode active material. Additives are not particularly limited, but examples include conductive additives and binders.

[0072] The positive electrode active material composition tends to have a reduced resistance value for the positive electrode 6 when it contains a conductive additive. The conductive additive is not particularly limited, but examples include condensed aromatic ring structures such as carbon black, acetylene black, Ketjenblack, graphite, carbon nanotubes, carbon fibers, graphene, graphene oxide, and fullerene. The conductive additive may be used alone or in combination of two or more.

[0073] The addition of a conductive additive makes it easier for electricity to pass through to the interior of the positive electrode active material particles, making it easier to maintain the discharge voltage when the lithium-ion secondary battery of this embodiment is discharged at a high current, i.e., the rate characteristics tend to improve. Furthermore, if a carbonaceous film is present on the surface of the positive electrode active material particles, the carbonaceous film may deteriorate due to repeated charging and discharging of the lithium-ion secondary battery. In this regard, the addition of a conductive additive maintains the conductive paths within the positive electrode active material particles even if the carbonaceous film deteriorates, and the capacity of the lithium-ion secondary battery is maintained even after repeated charging and discharging, i.e., the cycle characteristics of the lithium-ion secondary battery tend to improve.

[0074] Preferred conductive additives include carbon nanotubes, graphene, and graphene oxide, with carbon nanotubes being more preferred. Using these conductive additives tends to improve the rate characteristics and cycle characteristics of lithium-ion secondary batteries.

[0075] The content of the conductive additive is preferably 1 to 30% by mass, 5 to 25% by mass, and 10 to 20% by mass, relative to the total amount of the positive electrode active material composition.

[0076] The inclusion of a binder in the positive electrode active material composition tends to facilitate its bonding to the positive electrode current collector described later. The binder is not particularly limited, but examples include copolymers or homopolymers having a structure derived from vinylidene fluoride, polyvinylidene fluoride (PVDF), tetrafluoroethylene (TEF), or hexafluoropropylene (HFP). The binder may be used alone or in combination of two or more types.

[0077] The binder content is not particularly limited, but is, for example, 0.5 to 10.0% by mass and 1.0 to 7.5% by mass relative to the total amount of the positive electrode active material composition.

[0078] 1.1.2. Positive electrode current collector The positive electrode 6 preferably has a positive electrode current collector. The positive electrode current collector is not particularly limited, but examples include aluminum-based current collectors such as aluminum foil, aluminum mesh, perforated aluminum sheet, and aluminum expanded sheet; stainless steel-based current collectors such as stainless steel foil, stainless steel mesh, perforated stainless steel sheet, and stainless steel expanded sheet; nickel-based current collectors such as foamed nickel and nickel nonwoven fabric; copper-based current collectors such as copper foil, copper mesh, perforated copper sheet, and copper expanded sheet; titanium-based current collectors such as titanium foil and titanium mesh; and carbon-based current collectors such as carbon nonwoven fabric and carbon woven fabric. Among these, aluminum-based current collectors are preferred from the viewpoint of mechanical strength, conductivity, mass density, and cost.

[0079] The shape of the positive electrode current collector is not particularly limited, but may be, for example, foil-shaped, rod-shaped, ingot-shaped, or a three-dimensional base material (foamed metal, mesh, woven fabric, nonwoven fabric, expanded material, etc.). For example, the positive electrode current collector may be a positive electrode can 7, which is a metal covering the outside of the battery cell, as shown in Figure 1.

[0080] 1.2. The negative electrode 3 of the lithium-ion secondary battery 1 preferably comprises a negative electrode active material composition containing a negative electrode active material and a negative electrode current collector. The negative electrode current collector composition is preferably supported on the negative electrode current collector. The negative electrode terminal 2 is a type of negative electrode current collector.

[0081] 1.2.1. Negative electrode active material composition The negative electrode active material composition contains a negative electrode active material and may also contain other additives.

[0082] 1.2.1.1. Negative electrode active material The negative electrode active material is not particularly limited, but examples include carbon-based materials such as graphite, easily crystalline carbon, and poorly sintered carbon; metallic materials such as lithium metal, tin, copper, cobalt, or alloys containing these; and silicon-based materials such as silicon thin films. The negative electrode active material may be used alone or in combination of two or more types.

[0083] The content of the negative electrode active material is preferably 60 to 100% by mass, 70 to 100% by mass, 80 to 100% by mass, 90 to 100% by mass, 95 to 100% by mass, or 99 to 100% by mass, relative to the total amount of the negative electrode active material composition. Furthermore, the negative electrode active material composition may consist substantially of negative electrode active material or consist solely of negative electrode active material.

[0084] 1.2.1.2. Additives The negative electrode active material composition may contain additives other than the negative electrode active material. The additives are not particularly limited, but examples include conductive additives and binders.

[0085] The inclusion of a conductive additive in the negative electrode active material composition tends to reduce the resistance of the negative electrode 3. The conductive additive is not particularly limited, but examples include substances similar to those used in the positive electrode. The conductive additive may be used alone or in combination of two or more types.

[0086] The content of the conductive additive is preferably 1 to 30% by mass, 5 to 25% by mass, and 10 to 20% by mass, relative to the total amount of the negative electrode active material composition.

[0087] The inclusion of a binder in the negative electrode active material composition tends to facilitate its bonding to the negative electrode current collector described later. The binder is not particularly limited, but examples include substances similar to those used in the positive electrode binder, styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC), and mixtures of SBR and CMC. The binder may be used alone or in combination of two or more types.

[0088] The binder content is not particularly limited, but is, for example, 0.5 to 10.0% by mass and 1.0 to 7.5% by mass relative to the total amount of the negative electrode active material composition.

[0089] 1.2.2. Negative electrode current collector The negative electrode 3 preferably has a negative electrode current collector. The negative electrode current collector is not particularly limited, but for example, one similar to that of the positive electrode current collector can be used. Among these, copper-based current collectors are preferred from the viewpoint of mechanical strength, conductivity, mass density, cost, etc.

[0090] The shape of the negative electrode current collector is not particularly limited, but may be, for example, foil-shaped, rod-shaped, ingot-shaped, or a three-dimensional base material (foamed metal, mesh, woven fabric, nonwoven fabric, expanded material, etc.). For example, the negative electrode current collector may be a negative electrode terminal 2, which is metal covering the upper surface of the battery cell, as shown in Figure 1.

[0091] 1.3. Separator The lithium-ion secondary battery 1 preferably has a separator 4. The separator 4 is interposed between the positive electrode and the negative electrode, allowing the movement of ions between the positive electrode and the negative electrode, and preventing internal short circuits between the positive electrode and the negative electrode. If the lithium-ion secondary battery is a sealed type, the separator is also required to have the function of holding the electrolyte. The separator is not particularly limited, but examples include thin-walled, microporous or nonwoven membranes made of polyethylene, polypropylene, polyacrylonitrile, aramid, polyimide, cellulose, glass, etc.

[0092] 1.4. Electrolyte The lithium-ion secondary battery 1 may contain an electrolyte. The electrolyte contains an electrolyte and a solvent. Examples of electrolytes include alkali metal salts such as lithium salts, and more specifically, LiPF ６ LiBF ４ LiAsF ６ LiCF ３ SO ３ , LiI, LiClO ４ Examples include non-aqueous solvents such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dimethyl ether, γ-butyrolactone, and acetonitrile. The concentration of the electrolyte is not particularly limited, but for example, it is 0.5 mol / L to 1.7 mol / L. The electrolyte and solvent may be used individually or in combination of two or more.

[0093] The electrolyte may be used by impregnating the separator 4. Alternatively, a polymer such as polyvinylidene fluoride may be swollen with the electrolyte, and the swollen polymer may be used as the gel electrolyte.

[0094] 2. Method for producing positive electrode active material particles The method for producing positive electrode active material particles according to this embodiment includes a mixture preparation step of preparing a mixture in which raw materials containing lithium, manganese, phosphorus, and sulfur are mixed in a first solvent containing one or more selected from the group consisting of water, ethylene glycol, diethylene glycol, triethylene glycol, and tetraethylene glycol; and a positive electrode active material particle synthesis step of heating the mixture to obtain positive electrode active material particles containing a positive electrode active material represented by formula (1).

[0095] Conventional liquid-phase synthesis of positive electrode active materials such as lithium manganese phosphate is carried out by high-pressure reactions using sealed containers, which tend to result in high manufacturing costs. On the other hand, the method for producing positive electrode active material particles in this embodiment does not require pressurization and is a simple synthesis method that is inexpensive and suitable for mass production, using an open container at atmospheric pressure. In other words, the method for producing positive electrode active material particles in this embodiment tends to result in lower manufacturing costs, which is preferable.

[0096] The following describes in detail the method for manufacturing positive electrode active material particles.

[0097] 2.1. Mixture Preparation Process In the mixture preparation process, a mixture is prepared in which raw materials containing lithium, manganese, phosphorus, and sulfur are mixed in a first solvent containing one or more selected from the group consisting of water, ethylene glycol, diethylene glycol, triethylene glycol, and tetraethylene glycol.

[0098] The raw materials containing lithium, manganese, phosphorus, and sulfur preferably consist of multiple compounds. The raw materials may also contain oxygen.

[0099] The raw materials containing lithium are not particularly limited, but examples include lithium hydroxide and lithium sulfate, among which lithium hydroxide is preferred.

[0100] The raw materials containing manganese are not particularly limited, but examples include manganese(II) sulfate, manganese(II) chloride, and manganese(IV) oxide, among which manganese(II) sulfate and manganese(II) chloride are preferred.

[0101] Examples of raw materials containing phosphorus include phosphinic acid, phosphoric acid, and phosphates, with phosphoric acid being preferred among these. Furthermore, raw materials containing phosphorus as phosphoric acid or phosphate ions are preferred.

[0102] Examples of raw materials containing sulfur include sulfuric acid and sulfates such as manganese(II) sulfate, with sulfates being preferred among these. Furthermore, as raw materials containing sulfur, raw materials containing sulfur as sulfuric acid or sulfate ions are preferred.

[0103] If the positive electrode active material contains metal elements other than those mentioned above, such as Fe, then a compound containing that metal element may be used as a raw material. For example, if the positive electrode active material contains iron, then iron(II) sulfate, iron(II) oxide, etc., may be used.

[0104] The mixing ratio of each raw material may be determined, for example, according to the ratio of each element in the target positive electrode active material.

[0105] The first solvent is preferably water and / or ethylene glycol. Alternatively, the first solvent may be recovered from a previous synthesis and purified by centrifugation or the like. For example, the first solvent may be water recovered and purified from a previous synthesis, or ethylene glycol recovered and purified from a previous synthesis.

[0106] The method of mixing is not particularly limited, but one example is mixing using a magnetic stirrer.

[0107] 2.2. Positive electrode active material particle synthesis process In the positive electrode active material particle synthesis process, the mixture obtained in the mixture preparation process is heated to obtain positive electrode active material particles containing the positive electrode active material represented by formula (1).

[0108]

[0109] In equation (1), M is one selected from the group consisting of Fe, Co, Ni, Al, Mg, Sn, Nb, B, Cu, Cr, Mo, Ru, V, Ga, Ca, Sr, Ba, Ti, and Zr, where 1.00 < a ≤ 1.50, 0.40 ≤ b < 1.10, 0.00 ≤ c ≤ 0.20, 0.50 ≤ d < 1.00, and 0.00 < e ≤ 0.50.

[0110] When heating a mixture, it is preferable to mix the mixture. While there are no particular limitations on the method of heating the mixture while mixing, one example is to use a hot stirrer.

[0111] Furthermore, methods for heating a mixture include resistance heating and electromagnetic wave irradiation heating. Electromagnetic wave irradiation heating methods include radiant heating using infrared lamps, microwave heating using microwaves, and induction heating, which heats a mixture by applying an alternating magnetic field and generating Joule loss through electromagnetic induction of a conductor.

[0112] Pressurization is not required in the positive electrode active material particle synthesis process. For example, the process may be carried out at atmospheric pressure (1 atmosphere).

[0113] The heating temperature is preferably 120 to 300°C, 130 to 280°C, 140 to 260°C, or 150 to 250°C.

[0114] The heating time is preferably 10 to 900 minutes, 50 to 800 minutes, 100 to 750 minutes, or 150 to 700 minutes.

[0115] 2.3. Particle Separation Step The method for producing positive electrode active material particles of this embodiment may include a particle separation step for separating the positive electrode active material particles obtained in the positive electrode active material particle synthesis step.

[0116] The method for separating the positive electrode active material particles is not particularly limited, but for example, one method is to place the dispersion liquid in which the positive electrode active material particles obtained in the positive electrode active material particle synthesis process is dispersed in a centrifuge and perform centrifugation, and collect the resulting precipitate by solid-liquid separation. Alternatively, the solvent obtained by centrifugation may be reused as the first solvent described above.

[0117] 2.4. Cleaning Step The method for producing positive electrode active material particles of this embodiment may include a cleaning step for cleaning the positive electrode active material particles obtained in the positive electrode active material particle synthesis step.

[0118] The method for cleaning the positive electrode active material particles is not particularly limited, but examples include dispersing the positive electrode active material particles in a solvent such as water, methanol, ethanol, propanol, butanol, ethylene glycol, diethylene glycol, triethylene glycol, or tetraethylene glycol, and then ultrasonically cleaning them. Alternatively, the positive electrode active material particles may be dispersed in a solvent such as water or alcohol, and then subjected to solid-liquid separation.

[0119] 2.5. Dispersion Preparation Step The method for producing positive electrode active material particles in this embodiment may include a dispersion preparation step in which positive electrode active material particles are dispersed in a second solvent containing one or more selected from the group consisting of water, methanol, ethanol, propanol, butanol, ethylene glycol, diethylene glycol, triethylene glycol, and tetraethylene glycol to obtain a dispersion.

[0120] The second solvent is preferably water, ethanol, and / or ethylene glycol.

[0121] The dispersion method is not particularly limited, but examples include ultrasonic dispersion or bead mill dispersion in a second solvent to which positive electrode active material particles are added.

[0122] 2.6. Carbon Source Addition Step The method for producing positive electrode active material particles of this embodiment may include a carbon source addition step in which a carbon source soluble in the second solvent is added to the dispersion obtained in the dispersion preparation step. Alternatively, in the carbon source addition step, a carbon source insoluble in the second solvent may be added to combine a carbon source soluble in the second solvent with an insoluble carbon source.

[0123] The carbon source soluble in the second solvent is not particularly limited as long as it is soluble in the second solvent. For example, when the second solvent is water, examples of carbon sources soluble in the second solvent include polyvinyl alcohol; carboxymethylcellulose; glucose such as D(+)-glucose; sucrose, fructose, cellobiose, etc. In this embodiment, a substance soluble in the second solvent means a substance in which no turbidity or precipitate can be visually observed when 1 g of the substance is mixed with 100 g of the second solvent at 25°C. Furthermore, the solubility of the carbon source soluble in the second solvent in the second solvent at 25°C is preferably 5 g / 100 mL or more, and 10 g / 100 mL or more. Alternatively, the solubility of a carbon source soluble in the second solvent in the second solvent at 25°C is not particularly limited, but for example, it may be 10 g / 100 mL or more, 20 g / 100 mL or more, 30 g / 100 mL or more, 40 g / 100 mL or more, 50 g / 100 mL or more, 60 g / 100 mL or more, or 70 g / 100 mL or more.

[0124] The melting point of the carbon source soluble in the second solvent is preferably 100 to 500°C, 125 to 450°C, 150 to 400°C, 175 to 350°C, or 200 to 300°C. This tends to facilitate the formation of a carbonaceous film of uniform thickness in the carbonaceous film formation process described later.

[0125] The carbon source insoluble in the second solvent is not particularly limited as long as it is insoluble in the second solvent. For example, when the second solvent is water, the carbon sources insoluble in the second solvent include those listed above as conductive additives, with graphene, graphene oxide, and carbon nanotubes being preferred, and carbon nanotubes being more preferred. In this embodiment, a substance insoluble in the second solvent means a substance in which turbidity or precipitation can be visually confirmed when 1 g of the substance is mixed with 100 g of the second solvent at 25°C. Furthermore, the solubility of the carbon source insoluble in the second solvent in the second solvent at 25°C is preferably 500 mg / 100 mL or less, 100 mg / 100 mL or less, 50 mg / 100 mL or less, 10 mg / 100 mL or less, 5 mg / 100 mL or less, 1 mg / 100 mL or less, and 0.5 mg / 100 mL or less.

[0126] The carbon source insoluble in the second solvent is added as an example for the purpose of electrically connecting the positive electrode active material particles. In other words, the carbon source insoluble in the second solvent is not added for the purpose of forming a carbonaceous film on the surface of the positive electrode active material particles. From this viewpoint, the melting point of the carbon source insoluble in the second solvent is not particularly limited as long as it does not dissolve in the carbonaceous film formation step described later, but for example it may be 1000°C to 5000°C or 1500°C to 4000°C.

[0127] 2.7. Precipitation Step The method for producing positive electrode active material particles in this embodiment may include a precipitation step in which the dispersion liquid to which the carbon source has been added, obtained in the carbon source addition step, is dried to precipitate positive electrode active material particles.

[0128] The carbon source added in the carbon source addition step adheres to the surface of the positive electrode active material particles precipitated in the precipitation step. The drying method is not particularly limited, but examples include heating drying such as microwave heating, resistance heating, and dielectric heating; spray drying; and freeze drying. When heating drying is performed, the heating temperature is preferably 50 to 250°C. The drying time is not particularly limited, but for example, it is 5 to 300 minutes.

[0129] 2.8. Carbonaceous Film Formation Step The method for producing positive electrode active material particles of this embodiment may include a carbonaceous film formation step in which the positive electrode active material particles precipitated in the precipitation step are heated to form a carbonaceous film on at least a portion of the surface of the positive electrode active material particles.

[0130] By heating the positive electrode active material particles precipitated in the deposition process, the carbon source attached to the surface of the positive electrode active material particles carbonizes, and a carbonaceous film is formed on at least a portion of the surface of the positive electrode active material particles, or on the entire surface. In this case, the carbonaceous film contains elemental carbon. Note that carbon sources that are insoluble in the second solvent do not constitute a carbonaceous film on the surface of the positive electrode active material particles even after the carbonaceous film formation process, and the shape of the carbon source is maintained. As a result, in the aggregated positive electrode active material particles obtained by aggregating positive electrode active material particles having a carbonaceous film, the carbon source electrically connects the positive electrode active material particles having a carbonaceous film to each other, improving conductivity and making it easier for the charge and discharge reactions of lithium-ion secondary batteries to occur.

[0131] The heating method is not particularly limited, but examples include resistance heating and electromagnetic wave irradiation heating. Examples of electromagnetic wave irradiation heating include radiant heating using infrared lamps, microwave heating using microwaves, and induction heating which heats by Joule loss generated by electromagnetic induction of a conductor when an alternating magnetic field is applied.

[0132] The heating temperature is preferably 350 to 800°C, 400 to 700°C, or 450 to 600°C.

[0133] The heating time is preferably 3 to 9 hours, 4 to 8 hours, or 5 to 7 hours.

[0134] The present invention will be described more specifically below using examples and comparative examples. The present invention is not limited in any way by the following examples. Unless otherwise specified, the experiments in the examples were conducted at room temperature (25°C) and 1 atmosphere.

[0135] 1. Synthesis of Cathode Active Material Particles 1.1. Production of Cathode Active Material Particles [Example 1] 240 mL of a mixed solvent of pure water and ethylene glycol (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was mixed with 0.12 mol of lithium hydroxide monohydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), 0.04 mol of manganese(II) sulfate pentahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and 0.04 mol of phosphoric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity 85%) to obtain a mixture. This mixture was transferred to a 500 mL glass flask and placed on a hot stirrer. A glass condenser and thermometer were attached to the flask. The mixture was then heated to 180°C while stirring. After that, it was heated for 180 minutes and maintained at 180°C. After the reaction was complete, the mixture was removed from the flask and centrifuged. The precipitate obtained by centrifugation was collected by solid-liquid separation. The collected precipitate was washed to obtain the positive electrode active material particles of Example 1. The hot stirrer used resistance heating, and no pressurization was applied during heating.

[0136] The precipitate was washed as follows: The collected precipitate was dispersed in a mixed solution containing ethanol and pure water, and then centrifuged to obtain the precipitate. This washed the precipitate. Unless otherwise specified, the precipitate was washed in the same manner as in Example 1 in the Examples and Comparative Examples.

[0137] [Example 2] The cathode active material particles of Example 2 were obtained in the same manner as in Example 1, except that the time for maintaining 180°C was changed from 180 minutes to 10 hours.

[0138] [Example 3] The same mixture as in Example 1 was transferred to a 500 mL glass flask. The flask was then placed inside a cavity-type microwave reactor (SMW-17 (product name) manufactured by Shikoku Keisoku Kogyo Co., Ltd.). A glass condenser and a thermometer were then attached to the flask. Next, the mixture was heated to 180°C by irradiating it with 2.45 GHz microwaves while stirring with a stirrer. The temperature was then maintained at 180°C for 60 minutes. After that, the cathode active material particles of Example 3 were obtained in the same manner as in Example 1.

[0139] [Example 4] The positive electrode active material particles of Example 4 were obtained in the same manner as in Example 1, except that the amount of lithium hydroxide monohydrate was changed from 0.12 mol to 0.10 mol.

[0140] [Example 5] The positive electrode active material particles of Example 5 were obtained in the same manner as in Example 1, except that 0.032 mol of manganese(II) sulfate pentahydrate and 0.008 mol of iron(II) sulfate heptahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were used instead of 0.04 mol of manganese(II) sulfate pentahydrate.

[0141] [Comparative Example 1] The positive electrode active material particles of Comparative Example 1 were obtained in the same manner as in Example 1, except that 0.04 mol of manganese(II) chloride tetrahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used instead of 0.04 mol of manganese(II) sulfate pentahydrate.

[0142] [Example 6] In Example 1, the cathode active material particles were dispersed in pure water to obtain a dispersion. Polyvinyl alcohol (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., degree of saponification: 78-82 mol%) was added as a first carbon source at a concentration of 3% by mass relative to 100% by mass of the solid content of the dispersion. D(+)-glucose (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added as a second carbon source at a concentration of 20% by mass relative to 100% by mass of the solid content of the dispersion. The dispersion was then ultrasonically treated. Next, the ultrasonically treated dispersion was dried using a spray dryer (manufactured by Nippon Buch Co., Ltd., B-290 (product name)) with the drying inlet temperature set to 200°C to obtain a powder. The obtained powder was then heated in a gas displacement furnace under an argon atmosphere (an inert gas) at 500°C for 6 hours to obtain heat-treated cathode active material particles. As will be described later, these heat-treated cathode active material particles had a carbonaceous film on their surface.

[0143] [Example 7] Positive electrode active material particles were obtained by heat treatment in the same manner as in Example 6, except that the positive electrode active material particles obtained in Example 2 were used.

[0144] [Example 8] Positive electrode active material particles were obtained by heat treatment in the same manner as in Example 6, except that the positive electrode active material particles obtained in Example 3 were used.

[0145] [Example 9] Positive electrode active material particles were obtained by heat treatment in the same manner as in Example 6, except that the positive electrode active material particles obtained in Example 4 were used.

[0146] [Example 10] Positive electrode active material particles were obtained by heat treatment in the same manner as in Example 6, except that the positive electrode active material particles obtained in Example 5 were used.

[0147] [Example 16] Positive electrode active material particles were obtained in the same manner as in Example 2, except that 0.004 mol of manganese(II) sulfate pentahydrate and 0.036 mol of manganese(II) chloride tetrahydrate were used instead of 0.04 mol of manganese(II) sulfate pentahydrate. Then, the positive electrode active material particles were subjected to the same treatment as in Example 6 to obtain the heat-treated positive electrode active material particles of Example 16.

[0148] [Example 17] Positive electrode active material particles were obtained in the same manner as in Example 2, except that 0.008 mol of manganese(II) sulfate pentahydrate and 0.032 mol of manganese(II) chloride tetrahydrate were used instead of 0.04 mol of manganese(II) sulfate pentahydrate. Then, the positive electrode active material particles were subjected to the same treatment as in Example 6 to obtain the heat-treated positive electrode active material particles of Example 17.

[0149] [Example 18] Positive electrode active material particles were obtained in the same manner as in Example 2, except that 0.012 mol of manganese(II) sulfate pentahydrate and 0.028 mol of manganese(II) chloride tetrahydrate were used instead of 0.04 mol of manganese(II) sulfate pentahydrate. Then, the positive electrode active material particles were subjected to the same treatment as in Example 6 to obtain the heat-treated positive electrode active material particles of Example 18.

[0150] [Example 19] Positive electrode active material particles were obtained in the same manner as in Example 2, except that 0.012 mol of manganese(II) sulfate pentahydrate and 0.028 mol of manganese(II) chloride tetrahydrate were used instead of 0.04 mol of manganese(II) sulfate pentahydrate.

[0151] To the dispersion obtained by dispersing the cathode active material particles in pure water, 20% by mass of cellobiose (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and 4.5% by mass of carboxymethylcellulose were added as a second solvent, which is a carbon source soluble in pure water, relative to 100% by mass of the solid content of the dispersion. Furthermore, 3% by mass of single-walled carbon nanotubes (CNTs) (manufactured by OCSiA, TUBALL) were added as a carbon source insoluble in pure water, relative to 100% by mass of the solid content of the dispersion. Subsequently, the dispersion was ultrasonically treated. Next, the ultrasonically treated dispersion was dried using a spray dryer (manufactured by Nippon Buch Co., Ltd., B-290 (product name)) with the drying inlet temperature set to 200°C to obtain a powder. Next, the obtained powder was heated in a gas displacement furnace under an argon atmosphere, which is an inert gas, at 500°C for 6 hours to obtain the heat-treated cathode active material particles of Example 19. In the heat-treated positive electrode active material particles of Example 19, the carbon content was 7.2% by mass relative to the total amount of positive electrode active material particles. The carbon content measured here originates from the carbonaceous film and carbon nanotubes.

[0152] [Example 20] Heat-treated cathode active material particles of Example 20 were obtained in the same manner as the heat-treated cathode active material particles of Example 19, except that graphene oxide (GO) (manufactured by Graphenea) was added at a concentration of 3% by mass relative to 100% by mass of the solid content of the dispersion, instead of carbon nanotubes as a carbon source insoluble in pure water. In the heat-treated cathode active material particles of Example 20, the carbon content was 6.0% by mass relative to the total amount of cathode active material particles. The carbon content measured here is derived from the carbonaceous film and graphene oxide.

[0153] [Example 21] Positive electrode active material particles were obtained in the same manner as in Example 2, except that 0.02 mol of manganese(II) sulfate pentahydrate and 0.02 mol of manganese(II) chloride tetrahydrate were used instead of 0.04 mol of manganese(II) sulfate pentahydrate. Then, the positive electrode active material particles were subjected to the same treatment as in Example 6 to obtain the heat-treated positive electrode active material particles of Example 21.

[0154] Incidentally, the content of the carbonaceous film on the surface of the positive electrode active material particles in Examples 6 to 10, 16 to 18, and 21 was 3 to 6% by mass with respect to the total amount of the positive electrode active material particles.

[0155] [Comparative Example 2] Positive electrode active material particles obtained by heat treatment in the same manner as in Example 6 were obtained, except that the positive electrode active material particles obtained in Comparative Example 1 were used.

[0156] [Comparative Example 4] Positive electrode active material particles were obtained in the same manner as in Example 2, except that lithium hydroxide monohydrate was changed from 0.12 mol to 0.08 mol. Then, the same treatment as in Example 6 was performed on the positive electrode active material particles to obtain heat-treated positive electrode active material particles of Comparative Example 4.

[0157] Incidentally, the positive electrode active material particles in each example formed positive electrode active material agglomerated particles by aggregating.

[0158] 1.2. XRD Analysis XRD analysis was performed on the positive electrode active material particles obtained in Examples 1 to 5, 16 to 18, 21 and Comparative Examples 1 and 4. Specifically, using an X-ray diffractometer (manufactured by Rigaku, MiniFlex600C (product name)), the measurement conditions were set as target Cu Kα, tube voltage 40 kV, tube current 15 mA, scanning range 10 to 70° (2θ), step width 0.02°, and scan speed 10° / min step, and charts of the positive electrode active material particles obtained in each example were obtained. The crystal phases having the interplanar spacings corresponding to each peak in the chart were associated to identify the crystal phases and confirm the presence or absence of different phases. The lattice constant and the interatomic bond distance were calculated using the Rietveld method.

[0159] The charts obtained as a result of the XRD analysis are shown in FIGS. 2 and 8. The peak positions in the charts obtained from the positive electrode active material particles of Examples 1 to 5, 16 to 18, 21, and Comparative Example 4 shown in FIGS. 2 and 8 are all olivine-type lithium manganese phosphate (LiMnPO ４The diffraction peak positions matched those calculated using the crystal structure data of (ICSD#97763), indicating the absence of a different phase. In other words, it was found that the positive electrode active material particles of Examples 1-5, 16-18, 21, and Comparative Example 4 adopt an olivine-type crystal structure. The P-O bond distance calculated by Rietveld analysis was consistent with that of the olivine-type LiMnPO ４ The P-O bond distance was significantly shorter compared to that in [the other case]. Therefore, it was found that some of the sites where P is located in the crystal structure are substituted with S. Although the position of the diffraction peak in XRD analysis is shifted due to the substitution of some of the sites where P is located with S, the shift is very slight. Therefore, the diffraction peak positions in the XRD analysis of the positive electrode active material particles of Examples 1-5, 16-18, 21, and Comparative Example 4 were compared with olivine-type lithium manganese phosphate (LiMnPO4). ４ By comparing the diffraction peak positions calculated using the crystal structure data of the ) with these positions, the presence or absence of a different phase can be determined.

[0160] Furthermore, Figure 11 shows the manganese site occupancy rate in the positive electrode active material particles for each example, calculated from the XRD analysis results. The numbers in parentheses are the standard deviations. If the manganese site occupancy rate is less than 1, it is presumed that the manganese sites are substituted with lithium or that lattice defects exist.

[0161] Furthermore, the peak positions in the chart obtained from the positive electrode active material particles of Comparative Example 1 shown in Figure 2 are all listed in the standard database for olivine-type LiMnPO ４ The peak position was consistent with that of the olivine-type LiMnPO4. ４ The P-O bond distance is almost identical to that in the crystal structure, indicating that the sites where P is located in the crystal structure are not substituted with elements other than P.

[0162] 1.3. ICP Emission Spectroscopy Analysis The positive electrode active material particles obtained in Example 6 were dissolved in acid to obtain a sample solution. Then, ICP emission spectroscopy analysis was performed using this sample solution. As a result, the positive electrode active material particles of Example 6 were found to be Li １．２１ Mn ０．７９(P ０．７５ S ０．２４ ) O ４．００ It was found to have the following composition. This composition satisfies the electrical neutrality condition. In addition, since carbon is not detected in ICP emission spectroscopy, the carbonaceous film attached to the surface of the positive electrode active material particles in Example 6 is not reflected in the above composition. It was found that the positive electrode active material particles obtained in Examples 7 and 8 also have a similar composition.

[0163] Furthermore, ICP emission spectroscopy analysis was similarly performed on the positive electrode active material particles obtained in Examples 9-10, 16-18, 21, and Comparative Examples 2 and 4. The results are as follows. These compositions satisfy the electrical neutrality conditions. Example 9: Li １．０４ Mn １．００ (P ０．８７ S ０．１０ ) O ４．００ Example 10: Li １．２７ Mn ０．７１ Fe ０．１８ (P ０．８３ S ０．１４ ) O ４．００ Example 16: Li １．０１ Mn １．０２ (P ０．９５ S ０．０３ ) O ４．００ Example 17: Li １．０７ Mn １．００ (P ０．９１ S ０．０６ ) O ４．００ Example 18: Li １．１１ Mn ０．９８ (P ０．９０ S ０．０７ ) O ４．００ Example 21: Li １．１５ Mn ０．９３ (P ０．８５ S ０．１２ ) O ４．００ Comparative example 2: Li ０．９６ Mn １．０５ (P ０．９９ ) O ４．００ Comparative example 4: Li ０．９４ Mn １．０６ (P ０．８９ S ０．０８ ) O ４．００

[0164] 1.4. Neutron Diffraction Analysis Neutron diffraction analysis was performed on the positive electrode active material particles obtained in Example 6. Specifically, a neutron diffraction analyzer (J-PARC BL19, Engineering Materials Diffractometer "TAKUMI") was used to obtain charts of the positive electrode active material particles obtained in Example 6 in the range of lattice plane spacing from 3.3 to 0.4. The crystal phases with corresponding lattice plane spacings were matched to each peak in the chart to identify the crystal phases and confirm the presence or absence of different phases. The lattice constant, interatomic bond distance, and occupancy rate of each element were calculated using the Rietveld method.

[0165] Figure 3 shows the chart obtained from the neutron beam analysis. In the chart obtained from the positive electrode active material particles of Example 6 shown in Figure 3, each peak position corresponds to the olivine-type lithium manganese phosphate (LiMnPO) registered in the Inorganic Crystal Structure Database (ICSD). ４ The diffraction peak positions were consistent with those calculated using the crystal structure data of (ICSD#97763), indicating the absence of a different phase. In other words, it was found that the positive electrode active material particles of Example 6 adopt an olivine-type crystal structure. Furthermore, Rietveld analysis revealed that some of the sites where P is located in the crystal structure are substituted with S. In addition, the ratio of P to S determined from the occupancy rate was consistent with the results of ICP emission spectroscopy.

[0166] 1.5. Electron Microscope Observation The positive electrode active material particles of Example 6 were observed using an electron microscope. Specifically, a scanning electron microscope (Hitachi SU9000 (product name)) was used to observe the particle size and shape. The acceleration voltage during observation was 10 kV. The observation results are shown in Figures 4A and 4B.

[0167] Next, a thin film sample was prepared by cross-sectionally processing the positive electrode active material particles of Example 6 using a focused ion beam processing apparatus. An image of the sample was obtained using a scanning transmission electron microscope (JEOL Ltd., JEM-ARM200F NEOARM (product name)) and the thickness of the carbonaceous film was evaluated. The image is shown in Figure 5A. Subsequently, elemental analysis of the cross-section was performed by energy-dispersive X-ray spectroscopy (EDX). The acceleration voltage during EDX observation was set to 60 kV. The results of the EDX analysis performed along the line indicated by the black line in Figure 5A are shown in Figure 5B. Furthermore, Figure 6A shows the results of high-resolution observation of a cross-section of the positive electrode active material particles of Example 6, different from the cross-section shown in the image in Figure 5A, using the scanning transmission electron microscope at an acceleration voltage of 200 kV.

[0168] The electron microscope image in Figure 4A shows that the obtained positive electrode active material particles have a spindle-like shape. Furthermore, the electron microscope image in Figure 4B shows that the spindle-shaped positive electrode active material particles aggregate to form positive electrode active material aggregate particles. In addition, the image in Figure 5A shows that a film approximately 3 nm thick is formed over the entire surface of the particles.

[0169] The graph in Figure 5B shows that the abundance of carbon increases around 0–10 nm and 40–50 nm. This indicates that a carbonaceous film is formed on the surface of the positive electrode active material particles. Furthermore, the elemental analysis results show that sulfur, oxygen, manganese, and phosphorus are present inside the positive electrode active material particles.

[0170] Furthermore, Fast Fourier Transform (FFT) analysis was performed on the region enclosed by the white frame in the high-resolution image of the particle cross-section shown in Figure 6A. The results are shown in Figure 6B. In addition, when the orientation of the incident electron beam is set to [-1 0 0], the olivine-type LiMnPO ４ The electron diffraction pattern was calculated. The calculation results are shown in Figure 6C. Figures 6B and 6C show good agreement, and in the image shown in Figure 6A, the olivine type LiMnPO ４ It was found that a carbonaceous film is formed on the (010) surface.

[0171] Furthermore, it was found that the positive electrode active material particles in Examples 7-10, 16-18, 21, and Comparative Example 4 were similar to those of the positive electrode active material particles in Example 6.

[0172] Next, electron microscope observations were performed on the positive electrode active material particles of Examples 19 and 20. Specifically, a scanning electron microscope (Hitachi SU9000 (product name)) was used. The acceleration voltage during observation was 10 kV. The observation results are shown in Figures 9 and 10. In Figures 9 and 10, the image on the left shows the observation results of aggregated positive electrode active material particles, and the image on the right shows the results of a magnified observation of a part of the surface of the aggregated positive electrode active material particles.

[0173] As shown in Figure 9, carbon nanotubes were in contact with multiple positive electrode active material particles within the positive electrode active material aggregate particles of Example 19. Also, as shown in Figure 10, a sheet of graphene oxide was in contact with and covered multiple positive electrode active material particles within the positive electrode active material aggregate particles of Example 20.

[0174] 1.6. Particle Size Measurement The median diameter of the aggregated positive electrode active material particles of Example 18 was measured using a laser diffraction / scattering particle size distribution analyzer (Horiba, Ltd., LA-960 (product name)). Specifically, the aggregated positive electrode active material particles of Example 18 were dispersed in ethanol by ultrasonic stirring for 2 minutes. After that, the median diameter was measured using the above measuring device. The median diameter of the aggregated positive electrode active material particles of Example 18 was 4.134 μm.

[0175] 2. Evaluation 2.1. Charge / Discharge Characteristics [Example 11] A suspension of the positive electrode active material composition was prepared by mixing and stirring the positive electrode active material particles of Example 6 with a conductive additive (acetylene black, Denka Black (product name), manufactured by Denka Co., Ltd.): polyvinylidene fluoride = 85:10:5 (mass ratio), and adding N-methylpyrrolidone. This suspension was coated onto aluminum foil (thickness 0.02 mm) using a doctor blade and dried to support the positive electrode active material composition on the aluminum foil. The aluminum foil was then pressed to obtain a positive electrode sheet. Next, the positive electrode sheet was punched out into a circle with a diameter of 16 mm to prepare a test positive electrode. The positive electrode obtained in this way was assembled with a negative electrode consisting of metallic lithium (metallic lithium foil with a diameter of 20 mm and a thickness of 0.2 mm), a separator (a porous polypropylene sheet with a diameter of 24 mm and a thickness of 0.025 mm, Cellguard #2400 (product name), manufactured by Cellguard Co., Ltd.), and an electrolyte in a stainless steel container (manufactured by Hosen Co., Ltd.) to fabricate a coin-type lithium-ion secondary battery for evaluating charge-discharge characteristics. This coin-type lithium-ion secondary battery was designated as the lithium-ion secondary battery of Example 11. As the electrolyte, a mixed solvent of ethylene carbonate and diethyl carbonate in a 1:1 (volume ratio) was used, and LiPF4 ６ A solution containing 1 mol / L was used.

[0176] Except for using the positive electrode active material particles of Examples 7 to 10 and Comparative Example 2, the coin-type lithium-ion secondary batteries of Examples 12 to 15 and Comparative Example 3 were manufactured in the same manner as the manufacturing method of the coin-type lithium-ion secondary battery of Example 11. Table 1 shows the positive electrode active material particles used in the lithium-ion secondary batteries of Examples 11 to 15 and Comparative Example 3, and the figure numbers representing the measurement results of the charge and discharge characteristics of these lithium-ion secondary batteries.

[0177] Except for using the positive electrode active material particles of Examples 16, 18-20 and Comparative Example 4, the coin-type lithium-ion secondary batteries of Examples 22-25 and Comparative Example 5 were manufactured in the same manner as the manufacturing method of the coin-type lithium-ion secondary battery of Example 11. Table 2 shows the positive electrode active material particles used in the lithium-ion secondary batteries of Examples 22-25 and Comparative Example 5, and the figure numbers representing the measurement results of the charge and discharge characteristics of these lithium-ion secondary batteries.

[0178] The coin-type lithium-ion secondary batteries of Examples 11-15, 22-25, and Comparative Examples 3 and 5 were charged under a constant current of 0.1C until the inter-electrode voltage reached 4.3V or 4.5V. Subsequently, they were discharged under a constant current of 0.1C until the inter-electrode voltage reached 2.0V. The charge-discharge curves were measured at this time. The measurement results for the lithium-ion secondary batteries of Examples 11-15, 22-25, and Comparative Examples 3 and 5 are shown in Figures 7A-K, respectively.

[0179]

[0180]

[0181] For the lithium-ion secondary batteries of Examples 11-15 and 22-25, it was confirmed that the potential difference between charge and discharge was small in the region where the voltage during charge and discharge was approximately constant (for example, the region of 20-100 mAh / g in Examples 11-14, the region of 40-80 mAh / g in Example 15, the region of 10-60 mAh / g in Example 22, the region of 20-120 mAh / g in Examples 23-24, and the region of 20-100 mAh / g in Example 25). Furthermore, although the lithium-ion secondary battery of Example 15 showed multi-stage potential changes during charge and discharge due to the partial substitution of Mn with Fe, the degree of these potential changes was suppressed. The lithium-ion secondary battery made using the positive electrode active material particles of Example 17 and the lithium-ion secondary battery made using the positive electrode active material particles of Example 21 showed similar charge and discharge characteristics to the lithium-ion secondary battery of Example 23.

[0182] In the case of the lithium-ion secondary battery of Comparative Example 3, the capacity decreased further when it was charged until the inter-electrode voltage reached 4.3V. Furthermore, when it was charged until the inter-electrode voltage reached 4.5V and then discharged until the inter-electrode voltage reached 2.0V, the capacity was also small, and a large potential difference was observed during charging and discharging. This is presumed to be because Li is difficult to detach from the positive electrode active material particles in Comparative Example 4.

[0183] Regarding the lithium-ion secondary battery of Comparative Example 5, it was found that the region in which the voltage during charging and discharging remains nearly constant is narrow, at 10 to 30 mAh / g, and the capacity is also small. This is presumed to be because Li is less likely to detach from the positive electrode active material particles of Comparative Example 4.

[0184] In measuring the charge-discharge curves, the voltage is measured at specific time intervals. The graphs in Figures 7A-K and 12A-B plot the measurement results. In the graphs in Figure 7F and others, the dotted lines represent the plotted measurement results. The solid lines in the graphs in Figures 7A-K appear as solid lines because the measurement time intervals are small; these solid lines are actually collections of plotted measurement results. The applied current and weight remain constant throughout the measurement of the charge-discharge curves.

[0185] 2.2 Rate Characteristics A rate characteristic test was conducted on the batteries of Examples 24 and 23 to confirm the relationship between voltage and capacity when discharged at various current values. Specifically, for the battery of Example 24, charging was first performed under a constant current condition of 0.1C until the inter-electrode voltage reached 4.3V.

[0186] Next, under constant current conditions of 0.1C, the discharge was performed until the inter-electrode voltage reached 2.0V, and the discharge curve was measured.

[0187] After discharging under constant current conditions at 0.1C, the battery was charged under the same conditions as described above, i.e., under constant current conditions at 0.1C, until the inter-electrode voltage reached 4.3V. Then, under constant current conditions at 0.2C, the battery was discharged until the inter-electrode voltage reached 2.0V, and the discharge curve was measured.

[0188] After discharging under constant current conditions at 0.2C, charging was performed under the same conditions as described above. Then, under constant current conditions at 0.5C, the battery was discharged until the inter-electrode voltage reached 2.0V, and the discharge curve was measured.

[0189] After discharging under constant current conditions of 0.5C, charging was performed under the same conditions as described above. Then, under constant current conditions of 1C, the battery was discharged until the inter-electrode voltage reached 2.0V, and the discharge curve was measured.

[0190] After discharging under constant current conditions at 1C, charging was performed under the same conditions as described above. Then, under constant current conditions at 2C, the battery was discharged until the inter-electrode voltage reached 2.0V, and the discharge curve was measured.

[0191] After discharging under constant current conditions at 2C, charging was performed under the same conditions as described above. Then, under constant current conditions at 3C, the battery was discharged until the inter-electrode voltage reached 2.0V, and the discharge curve was measured. The discharge curve obtained in this way is shown in Figure 12A.

[0192] A rate characteristic test was performed on the battery of Example 23 using the same procedure as the rate characteristic test for the battery of Example 24. The obtained discharge curve is shown in Figure 12B.

[0193] As can be seen from the comparison of Figures 12A and 12B, the battery of Example 24, which has CNTs, was found to maintain its discharge voltage even when discharged at a high current, meaning it has excellent rate characteristics. Furthermore, in the battery of Example 25, an improvement in rate characteristics was also observed with the addition of GO.

[0194] 2.3. Cycle Characteristics A cycle characteristics test was conducted on the batteries of Examples 24 and 23 to confirm the relationship between capacity and the number of cycles when charge-discharge cycles were repeated at various current values.

[0195] Specifically, the battery of Example 24 was charged under a constant current condition of 0.1C until the inter-electrode voltage reached 4.3V. Then, under a constant current condition of 0.1C, it was discharged until the inter-electrode voltage reached 2.0V. This charge-discharge cycle was repeated five times, and the capacity for each cycle was calculated from the resulting charge-discharge curves.

[0196] After five charge-discharge cycles under a constant current of 0.1C, the battery of Example 24 was charged under a constant current of 0.1C until the inter-electrode voltage reached 4.3V. Then, it was discharged under a constant current of 0.2C until the inter-electrode voltage reached 2.0V. This charge-discharge cycle was repeated five times, and the capacity for each cycle was calculated from the resulting charge-discharge curves.

[0197] After five charge-discharge cycles under a constant current of 0.2C, the battery of Example 24 was charged under a constant current of 0.1C until the inter-electrode voltage reached 4.3V. Then, it was discharged under a constant current of 0.5C until the inter-electrode voltage reached 2.0V. This charge-discharge cycle was repeated five times, and the capacity for each cycle was calculated from the resulting charge-discharge curves.

[0198] After five charge-discharge cycles under a constant current of 0.5C, the battery of Example 24 was charged under a constant current of 0.1C until the inter-electrode voltage reached 4.3V. Then, it was discharged under a constant current of 1C until the inter-electrode voltage reached 2.0V. This charge-discharge cycle was repeated five times, and the capacity for each cycle was calculated from the resulting charge-discharge curves.

[0199] After five charge-discharge cycles under a constant current of 1C, the battery of Example 24 was charged under a constant current of 0.1C until the inter-electrode voltage reached 4.3V. Then, it was discharged under a constant current of 2C until the inter-electrode voltage reached 2.0V. This charge-discharge cycle was repeated five times, and the capacity for each cycle was calculated from the resulting charge-discharge curves.

[0200] After five charge-discharge cycles under a constant current of 2C, the battery of Example 24 was charged under a constant current of 0.1C until the inter-electrode voltage reached 4.3V. Then, it was discharged under a constant current of 3C until the inter-electrode voltage reached 2.0V. This charge-discharge cycle was repeated five times, and the capacity for each cycle was calculated from the resulting charge-discharge curves.

[0201] After five charge-discharge cycles under a constant current of 3C, the battery of Example 24 was charged under a constant current of 0.1C until the inter-electrode voltage reached 4.3V. Then, under a constant current of 0.1C, it was discharged until the inter-electrode voltage reached 2.0V. This charge-discharge cycle was repeated 10 times, and the capacity for each cycle was calculated from the resulting charge-discharge curves. The test results obtained in this way are shown in Figure 13A.

[0202] A rate characteristic test was performed on the battery of Example 23 using the same procedure as the cycle characteristic test for the battery of Example 24. The obtained test results are shown in Figure 13B.

[0203] As can be seen from the comparison of Figures 13A and 13B, the battery of Example 24, which has CNTs, was found to maintain its capacity even when repeatedly charging and discharging at various current values, meaning it has excellent cycle characteristics. Furthermore, in the battery of Example 25, improvement in cycle characteristics was also observed with the addition of GO.

[0204] 1...Lithium-ion secondary battery, 2...Negative electrode terminal, 3...Negative electrode, 4...Separator, 5...Insulating packing, 6...Positive electrode, 7...Positive electrode casing.

Claims

1. The positive electrode active material represented by formula (1). (In the formula, M is one element selected from the group consisting of Fe, Co, Ni, Al, Mg, Sn, Nb, B, Cu, Cr, Mo, Ru, V, Ga, Ca, Sr, Ba, Ti, and Zr, where 1.00 < a ≤ 1.50, 0.40 ≤ b < 1.10, 0.00 ≤ c ≤ 0.20, 0.50 ≤ d < 1.00, and 0.00 < e ≤ 0.50.) 2. The positive electrode active material according to claim 1, wherein b and c satisfy 0.50 < b + c < 1.

00.

3. The positive electrode active material according to claim 1, wherein a, b, and c satisfy 1.80 < a + b + c < 2.

20.

4. The positive electrode active material according to claim 1, wherein d and e satisfy 0.80 < d + e < 1.

20.

5. The positive electrode active material according to claim 1, represented by formula (2). (In the formula, M is one selected from the group consisting of Fe, Co, Ni, Al, Mg, Sn, Nb, B, Cu, Cr, Mo, Ru, V, Ga, Ca, Sr, Ba, Ti, and Zr, 0.50 ≦ x < 1.00, 0.50 ≦ y < 1.00, 0.00 ≦ α ≦ 0.10, 0.00 ≦ β ≦ 0.20, 0.00 ≦ γ ≦ 0.20.) 6. The positive electrode active material according to claim 5, wherein |x - y| ≤ 0.

10.

7. Positive electrode active material particles comprising the positive electrode active material described in any one of claims 1 to 6.

8. The positive electrode active material particle according to claim 7, wherein a carbonaceous film is present on at least a portion of the surface of the positive electrode active material particle.

9. Aggregated positive electrode active material particles, wherein the positive electrode active material particles described in claim 8 are aggregated.

10. A positive electrode comprising the positive electrode active material described in any one of claims 1 to 6.

11. A lithium-ion secondary battery comprising: a positive electrode containing the positive electrode active material described in any one of claims 1 to 6; a negative electrode; and a separator.

12. A method for producing positive electrode active material particles, comprising: a mixture preparation step of preparing a mixture in which raw materials containing lithium, manganese, phosphorus, and sulfur are mixed in a first solvent containing one or more selected from the group consisting of water, ethylene glycol, diethylene glycol, triethylene glycol, and tetraethylene glycol; and a positive electrode active material particle synthesis step of heating the mixture to obtain positive electrode active material particles containing a positive electrode active material represented by formula (1). (In the formula, M is one element selected from the group consisting of Fe, Co, Ni, Al, Mg, Sn, Nb, B, Cu, Cr, Mo, Ru, V, Ga, Ca, Sr, Ba, Ti, and Zr, where 1.00 < a ≤ 1.50, 0.40 ≤ b < 1.10, 0.00 ≤ c ≤ 0.20, 0.50 ≤ d < 1.00, and 0.00 < e ≤ 0.50.) 13. The manufacturing method according to claim 12, wherein the heating in the positive electrode active material particle synthesis step is carried out by means including resistance heating or electromagnetic wave irradiation heating.

14. The manufacturing method according to claim 12, comprising a washing step of washing the positive electrode active material particles.

15. A manufacturing method according to claim 12, comprising: a dispersion preparation step of dispersing the positive electrode active material particles in a second solvent containing one or more selected from the group consisting of water, methanol, ethanol, propanol, butanol, ethylene glycol, diethylene glycol, triethylene glycol, and tetraethylene glycol to obtain a dispersion; a carbon source addition step of adding a carbon source soluble in the second solvent to the dispersion; a precipitation step of drying the dispersion to which the carbon source has been added to precipitate the positive electrode active material particles; and a carbonaceous film formation step of heating the precipitated positive electrode active material particles to form a carbonaceous film on at least a portion of the surface of the positive electrode active material particles.

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