Lithium-ion secondary battery

Abstract

To provide a lithium ion secondary battery which enables satisfactory enhancement in rate and cycle characteristics.SOLUTION: A lithium ion secondary battery comprises at least a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes at least a collector, and a positive electrode active material layer formed on the collector. In the lithium ion secondary battery, a positive electrode active material included in the positive electrode active material layer contains particles (A) represented by the following formula (a), and particles (B) represented by the following formula (b): LiNiaCobMncM1wO2 (a); and LifMngFehM2xPO4 (b). The particles (A) account for 60 to 95 pts. mass in a total weight of 100 pts. mass of the particles (A) and the particles (B). The particles (B) make an amount of water measured in a particular condition a particular proportion.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a lithium ion secondary battery having excellent rate characteristics and cycle characteristics. [Background technology]

[0002] Layered lithium composite oxides, such as layered lithium-nickel-cobalt-manganese composite oxide (NCM), have a layered crystalline structure in which lithium atomic layers and transition metal atomic layers are alternately stacked with oxygen atomic layers in between. Such layered lithium composite oxides are used as positive electrode active materials in high-power, high-capacity lithium-ion secondary batteries.

[0003] In lithium-ion secondary batteries that use such layered lithium composite oxides as the positive electrode active material, charging and discharging are carried out by the desorption and insertion of lithium ions into the layered lithium composite oxides. However, the capacity decreases as the number of charge-discharge cycles increases, and this decrease in battery capacity can become particularly significant over long-term use.

[0004] Under these circumstances, various developments have been made to realize high-performance lithium-ion secondary batteries using highly useful layered lithium composite oxides. For example, Patent Document 1 discloses a lithium secondary battery equipped with a secondary battery positive electrode containing a lithium-nickel-cobalt-manganese composite oxide with a specified number of cobalt atoms and lithium iron manganese phosphate with a specified number of manganese atoms, and attempts to realize a lithium secondary battery with excellent energy density by improving the initial coulombic efficiency of the positive electrode. Patent Document 2 discloses a lithium secondary battery in which the positive electrode contains a positive electrode active material consisting of a specific amount of an olivine compound represented by a specific formula and a lithium nickel oxide represented by a specific formula, and aims to improve charge / discharge cycle efficiency, etc. Furthermore, Patent Document 3 discloses a lithium ion secondary battery containing a layered lithium-nickel-cobalt-manganese composite oxide (NCM) and a lithium composite oxide (LMPO) having an olivine structure as positive electrode active materials, in an attempt to improve input characteristics while ensuring the safety of the lithium ion secondary battery. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-159388 [Patent Document 2] International Publication No. 2012 / 147929 [Patent Document 3] Japanese Patent Application Laid-Open No. 2016-076317 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in any of the techniques described in the literature, in order to realize a lithium-ion secondary battery that is excellent in both rate characteristics and cycle characteristics, the composition and water absorption characteristics of the positive electrode active material that forms the positive electrode have not yet been sufficiently examined, and there is room for improvement.

[0007] Therefore, an object of the present invention is to provide a lithium ion secondary battery that can sufficiently improve both the rate characteristics and the cycle characteristics. [Means for solving the problem]

[0008] Therefore, the present inventors have conducted extensive research to solve the above-mentioned problems, and have found that a lithium ion secondary battery can be obtained that is equipped with a positive electrode formed from a positive electrode active material that contains a specific amount of Li-Ni-Co-Mn oxide particles (so-called NCM particles) and the remainder being lithium manganese iron phosphate particles (so-called LMFP particles) that contain both manganese (Mn) and iron (Fe), and that is capable of achieving both high rate characteristics and excellent cycle characteristics because the LMFP particles have unique water absorption characteristics.

[0009] That is, the present invention provides a lithium ion secondary battery including at least a positive electrode, a negative electrode, and an electrolyte, The positive electrode includes at least a current collector and a positive electrode active material layer formed on the current collector, and the positive electrode active material included in the positive electrode active material layer includes particles (A) represented by the following formula (a) and particles (B) represented by the following formula (b), the particles (A) are contained in an amount of 60 parts by mass to 95 parts by mass per 100 parts by mass of the total weight of the particles (A) and the particles (B); The particles (B) are dried at 300°C for 24 hours under atmospheric pressure, and then left to stand in an environment at a temperature of 20°C and a relative humidity of 50% for x hours. In the measurement of the moisture content y detected by a Karl Fischer moisture meter until the temperature is raised to 250°C, the moisture content y is determined by taking x as 1. 1 and the amount of water when x is 24 2 The ratio (y 1 / y 2 ) is 0.3 or less. LiNi a Co b Mn c M 1 w O2 (a) (In formula (a), M 1 represents one or more elements selected from Mg, Ti, Nb, Fe, Cr, Si, Al, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi, and Ge. a, b, c, and w are within the range of 0.5≦a<1, 0 <b<0.5、0<c<0.5、0≦w≦0.3、かつ3a+3b+3c+(M1 The number satisfying (valence of Mn)×w = 3 is shown. ()) Li f Mn g Fe h M 2 x PO4···(b) (In formula (b), M 2 represents one or more elements selected from Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, and Gd. f, g, h, and x satisfy 0 < f ≤ 1.2, 0.08 ≤ g < 0.6, 0.5 < h ≤ 1.1, 0 ≤ x ≤ 0.3, and 1 / 9 ≤ g / h < 1, and f + (valence of Mn)×g + (valence of Fe)×h + (valence of M 2 represents the number satisfying ×x = 3.).) is provided.

Effects of the Invention

[0010] According to the present invention, a highly useful lithium-ion secondary battery that exhibits excellent rate characteristics and also excellent cycle characteristics can be realized.

Modes for Carrying Out the Invention

[0011] Hereinafter, the present invention will be described in detail. The lithium-ion secondary battery of the present invention is a lithium-ion secondary battery including at least a positive electrode, a negative electrode, and an electrolyte, The positive electrode includes at least a current collector and a positive electrode active material layer formed on the current collector, The positive electrode active material contained in the positive electrode active material layer is composed of particles (A) represented by the following formula (a) and particles (B) represented by the following formula (b), The particles (A) are contained in an amount of 60 to 95 parts by mass in 100 parts by mass of the total weight of the particles (A) and the particles (B), After the particles (B) are subjected to a step of drying at 300°C for 24 hours under atmospheric pressure conditions and then left standing in an environment of 20°C and 50% relative humidity for x hours, the amount of moisture y detected until the temperature is raised to 250°C in the measurement using a Karl Fischer moisture meter. In the measurement of the amount of moisture y, when x is 11 and the water content y when x is 24 2 and the ratio (y 1 / y 2 ) shall be 0.3 or less. LiNi a Co b Mn c M 1 w O2 ··· (a) (In formula (a), M 1 represents one or more elements selected from Mg, Ti, Nb, Fe, Cr, Si, Al, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi, and Ge. a, b, c, w are numbers that satisfy 0.5 ≦ a < 1, 0 < b < 0.5, 0 < c < 0.5, 0 ≦ w ≦ 0.3, and 3a + 3b + 3c + (valence of M 1 ) × w = 3.) Li f Mn g Fe h M 2 x PO4 ··· (b) (In formula (b), M 2 represents one or more elements selected from Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, and Gd. f, g, h, and x are numbers that satisfy 0 < f ≦ 1.2, 0.08 ≦ g < 0.6, 0.5 < h ≦ 1.1, 0 ≦ x ≦ 0.3, and 1 / 9 ≦ g / h < 1, and f + (valence of Mn) × g + (valence of Fe) × h + (valence of M 2 ) × x = 3.)

[0012] That is, the lithium ion secondary battery of the present invention is a lithium ion secondary battery including at least a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode includes at least a current collector and a positive electrode active material layer formed on the current collector, and the positive electrode active material included in the positive electrode active material layer contains 60 parts by mass to 95 parts by mass of the particles (A) which are Ni-rich (high Ni content) NCM particles, and the remainder, i.e., 5 parts by mass to 40 parts by mass of the particles (A), are Fe-rich (high Fe content) LMFP particles, and the positive electrode is formed of a positive electrode active material, and the particles (B) contained in the positive electrode active material are mixed at a specific ratio (y 1 / y 2 ) values, these particles have unique water absorption properties over a temperature transition from 20°C to 250°C. In this way, by employing a positive electrode formed from a positive electrode active material consisting of two types of particles, each of a specific amount of the above-mentioned particles (A) and particles (B) having unique water absorption properties, it is believed that while particles (A) are contained, the coexisting particles (B) can effectively suppress moisture migration to particles (A) during positive electrode formation, battery manufacturing, etc. Furthermore, it is believed that it is possible to effectively exhibit the excellent electronic conductivity inherent to particles (A) while further improving stability through particles (B), thereby effectively improving both rate characteristics and cycle characteristics.

[0013] The positive electrode active material for forming the positive electrode of the lithium ion secondary battery of the present invention contains particles (A) represented by the following formula (a) in an amount of 60 to 95 parts by mass per 100 parts by mass of the total weight of particles (A) and particles (B): LiNi a Co b Mn c M 1 w O2 (a) (In formula (a), M 1 represents one or more elements selected from Mg, Ti, Nb, Fe, Cr, Si, Al, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi, and Ge. a, b, c, and w are within the range of 0.5≦a<1, 0 <b<0.5、0<c<0.5、0≦w≦0.3、かつ3a+3b+3c+(M1 The number satisfies (valence of x) × w = 3.

[0014] The particles (A) represented by the formula (a) are Ni-rich Li-Ni-Co-Mn oxide particles (NCM particles), so-called lithium composite oxide particles, which have a layered rock salt structure and are secondary particles formed by aggregation of primary particles. By containing such particles (A) in the above amount, it is possible to contribute to improving cycle performance while maintaining good rate performance.

[0015] M in formula (a) 1 represents one or more elements selected from Mg, Ti, Nb, Fe, Cr, Si, Al, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi, and Ge, and is preferably Mg or Al. In addition, a, b, c, and w in the above formula (a) are in the range of 0.5≦a<1, 0 <b<0.5、0<c<0.5、0≦w≦0.3、かつ3a+3b+3c+(M 1 a is a number that satisfies the formula (valence of each element) × w = 3. a is preferably 0.7 or more.

[0016] In the particles (A), Ni, Co, and Mn are known to contribute to the battery capacity and output characteristics. From the viewpoint of cycle characteristics, it is also known that some of these transition elements contribute to the battery capacity and output characteristics. 1 It is preferable that these metal elements M 1 It is believed that the substitution by stabilizes the crystal structure of the particles (A), so that destruction of the crystal structure can be suppressed even when charge and discharge are repeated, thereby realizing excellent cycle characteristics.

[0017] Specific examples of the NCM particles represented by the formula (a) include LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.75 Co 0.15 Mn 0.05 Mg 0.05 O2, LiNi 0.75 Co 0.15 Mn0.05 Al 0.05 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2 or LiNi 0.5 Co 0.3 Mn 0.2 O2, etc. Among them, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.6 Co 0.2 Mn 0.2 Particles made of O2 are preferred.

[0018] Furthermore, the particles (A) may form a core-shell structure having a core (interior) and a shell (surface). In this case, the core may be composed of a single phase, or may be composed of two or more phases with different compositions. In an embodiment where the core is composed of two or more phases, the core may have a structure in which multiple phases are concentrically layered, or a structure in which the composition changes gradually from the surface to the center of the core. Furthermore, the shell portion may be formed on the outside of the core portion, and may be a single phase like the core portion, or may be composed of two or more phases with different compositions.

[0019] As the particles (A) having such a core-shell structure, specifically, the (core portion)-(shell portion) is, for example, (LiNi 0.8 Co 0.1 Mn 0.1 O2)-(LiNi 0.5 Co 0.3 Mn 0.2 O2), or (LiNi 0.6 Co 0.2 Mn 0.2 O2)-(LiNi 0.5 Co 0.3 Mn 0.2 Examples include particles consisting of O2.

[0020] Furthermore, the particles (A) may be coated with a metal oxide, a metal fluoride, or a metal phosphate. By coating the NCM particles with these metal oxides, metal fluorides, or metal phosphates, the metal components (Ni, Mn, Co, Mn) from the NCM particles to the electrolyte can be easily transferred. 1 As such a coating, one or more materials selected from CeO2, SiO2, MgO, Al2O3, ZrO2, TiO2, ZnO, RuO2, SnO2, CoO, Nb2O5, CuO, V2O5, MoO3, La2O3, WO3, AlF3, NiF2, MgF2, LiF, Li3PO4, Li4P2O7, LiPO3, Li2PO3F, and LiPO2F2, or a composite thereof, can be used.

[0021] The average particle size of the primary particles of the particles (A) represented by the formula (a) is 50 nm to 500 nm, more preferably 50 nm to 300 nm, from the viewpoints of being able to suppress the amount of expansion and contraction of the primary particles accompanying the insertion and desorption of lithium ions, effectively preventing particle cracking, and handling. Furthermore, the average particle size of the particles (A), which are secondary particles formed by aggregation of the primary particles (simply referred to as "average particle size of particles (A)") is preferably 3 μm to 20 μm, more preferably 5 μm to 15 μm, from the viewpoint of obtaining a battery with excellent cycle characteristics and from the viewpoint of handling. Here, the "average particle size" of the particles (A) is the D obtained from the volume-based particle size distribution based on the laser diffraction / scattering method. 50 The values ​​are the particle diameters (median diameters) at 50% of the cumulative particle size.

[0022] The tap density of the particles (A) represented by the above formula (a) is preferably 1.5 g / cm from the viewpoint of obtaining a battery with excellent cycle characteristics and from the viewpoint of handling. 3 ~3.5g / cm 3 and more preferably 2.0 g / cm 3 ~3.0g / cm 3 is. Hereinafter, the tap density means the "tap bulk density" measured by the method specified in JIS R 1628 "Method for measuring bulk density of fine ceramic powders."

[0023] In the positive electrode active material for forming the positive electrode provided in the lithium ion secondary battery of the present invention, the content of the particles (A) is 60 to 95 parts by mass, preferably 70 to 95 parts by mass, more preferably 75 to 95 parts by mass, and even more preferably 80 to 95 parts by mass, per 100 parts by mass of the total weight of the particles (A) and (B), from the viewpoint of effectively achieving both high rate characteristics and excellent cycle characteristics.

[0024] The particles (A) can be obtained, for example, by the following production method. Specifically, the production method includes a step (Ia) of preparing slurry water a by adding a nickel compound, a cobalt compound, a manganese compound, and water, and then filtering and drying the slurry water a to obtain mixture A; and a step (IIa) of adding a lithium compound to the obtained mixture A, mixing them, and then firing them.

[0025] Examples of the nickel compound used in step (Ia) include nickel sulfate, nickel acetate, etc., which may be used alone or in combination of two or more. Among these, nickel sulfate is preferred from the viewpoint of improving battery characteristics. Examples of the cobalt compound include cobalt acetate, cobalt nitrate, and cobalt sulfate. These may be used alone or in combination of two or more. Among these, cobalt sulfate is preferred from the viewpoint of improving battery characteristics. Examples of manganese compounds include manganese acetate, manganese nitrate, and manganese sulfate. These may be used alone or in combination of two or more. Among these, manganese sulfate is preferred from the viewpoint of improving battery characteristics. In addition to these nickel compounds, cobalt compounds, and manganese compounds, metals other than these compounds (M 1 ) compounds may also be used. Examples of lithium compounds include hydroxides (for example, LiOH·H2O, LiOH), sulfates, and acetates. Of these, hydroxides are preferred.

[0026] In the step (Ia), when obtaining the slurry water a, it is preferable to adjust the pH to 8 to 13, for example, by adding aqueous ammonia dropwise.

[0027] In step (IIa), when firing, it is preferable to first perform pre-firing at 500°C to 1000°C, preferably 600°C to 900°C, for 1 to 15 hours, preferably 1 to 6 hours, and then perform main firing at 500°C to 1000°C, preferably 600°C to 900°C, for 1 to 15 hours, preferably 5 to 13 hours. Furthermore, it is preferable to crush the material after pre-firing before performing main firing.

[0028] The positive electrode active material for forming the positive electrode of the lithium ion secondary battery of the present invention is particles represented by the following formula (b), and after a process of drying at 300°C for 24 hours under atmospheric pressure conditions and then leaving the particles to stand for x hours in an environment at a temperature of 20°C and a relative humidity of 50%, the amount of moisture y detected by measurement using a Karl Fischer moisture meter until the temperature is raised to 250°C is y, where x is 1. 1 and the amount of water when x is 24 2 The ratio (y 1 / y 2 ) is 0.3 or less, and the remainder of the particles (A) is contained in the total weight of the particles (A) and the particles (B). Li f Mn g Fe h M 2 x PO4 (b) (In formula (b), M 2represents one or more elements selected from Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, and Gd. f, g, h, and x satisfy 0 < f ≤ 1.2, 0.08 ≤ g < 0.6, 0.5 < h ≤ 1.1, 0 ≤ x ≤ 0.3, and 1 / 9 ≤ g / h < 1, and f + (valence of Mn) × g + (valence of Fe) × h + (valence of M 2 represents a number that satisfies ) × x = 3.)

[0029] That is, in the positive electrode active material for forming the positive electrode included in the lithium-ion secondary battery of the present invention, in the total weight of the particles (A) and the particles (B), 5 to 40 parts by mass, which is the remainder of the particles (A), is occupied by the particles (B) which are Fe-rich LMFP particles. The lithium-ion secondary battery of the present invention includes a positive electrode formed of a positive electrode active material composed of two types of particles, the particles (A) and the particles (B).

[0030] The particles (B) are so-called lithium composite oxides (LMPO) having an olivine-type structure, which contain both manganese (Mn) and iron (Fe) as at least transition metals, and are secondary particles formed by aggregation of primary particles. And the particles (B) are Fe-rich LMFP particles as indicated by the value of "g / h" in the above formula (b), which is the molar ratio of Mn and Fe constituting such particles. This also becomes a factor, and when such particles (B) occupy all of the remainder of the particles (A), the specific water absorption characteristics of the particles (B) described later also exhibit effects and contribute to the improvement of the cycle characteristics.

[0031] For the particles (B), from the viewpoint of the average discharge voltage, f is preferably 0.6≦f≦1.2, more preferably 0.65≦f≦1.15, and even more preferably 0.7≦f≦1.1. g is preferably 0.08≦g≦0.4, and more preferably 0.08≦g≦0.2. h is preferably 0.6≦h≦1.1, and more preferably 0.8≦h≦1.1. x is preferably 0≦x≦0.2, more preferably 0≦x≦0.15, and even more preferably 0≦x≦0.1. Furthermore, g / h, which is the molar ratio of Mn to Fe constituting the particles (B), is preferably 1 / 9≦g / h≦9 / 10, more preferably 1 / 9≦g / h≦2 / 3, and even more preferably 1 / 9≦g / h≦1 / 4.

[0032] Specifically, for example, LiMn 0.1 Fe 0.9 PO4, LiMn 0.2 Fe 0.8 PO4, LiMn 0.15 Fe 0.75 Mg 0.1 PO4, LiMn 0.19 Fe 0.75 Zr 0.03 PO4, LiMn 0.3 Fe 0.7 PO4, LiMn 0.4 Fe 0.6 PO4, Li 1.2 Mn 0.27 Fe 0.63 PO4, Li 0.6 Mn 0.36 Fe 0.84 PO4, etc. Among them, LiMn 0.1 Fe 0.9 PO4 or LiMn 0.2 Fe 0.8 PO4 is preferred.

[0033] Furthermore, the particles (B) may have a core-shell structure having a core portion (interior) and a shell portion (surface portion). By using particles (B) with this core-shell structure, for example, by disposing LMFP particles with a higher Me content, which are more likely to dissolve in the electrolyte, in the core portion, and disposing LMFP particles with an even higher Fe content than the core portion in the shell portion that comes into contact with the electrolyte, it is possible to suppress deterioration in cycle performance due to the particles (B) and further improve rate performance. In this case, the core portion may be composed of a single phase, or may be composed of two or more phases with different compositions. In an embodiment where the core portion is composed of two or more phases, it may have a structure in which multiple phases are concentrically layered, or a structure in which the composition changes gradually from the surface to the center of the core portion. Furthermore, the shell portion may be formed on the outside of the core portion, and may be a single phase like the core portion, or may be composed of two or more phases with different compositions.

[0034] The average particle size of the primary particles of the particles (B) represented by the formula (b) is preferably 50 nm to 180 nm, more preferably 70 nm to 150 nm, from the viewpoints of suppressing the amount of expansion and contraction of the primary particles accompanying the insertion and desorption of lithium ions, effectively preventing particle cracking, and ease of handling. Furthermore, the average particle size of particles (B), which are secondary particles formed by aggregation of the primary particles (simply referred to as "average particle size of particles (B)"), is preferably 5 μm to 20 μm, more preferably 6 μm to 18 μm, and even more preferably 7 μm to 16 μm, from the viewpoint of obtaining a battery with excellent cycle characteristics and from the viewpoint of handling. Here, the "average particle size" of the particles (B) is the D obtained by the volume-based particle size distribution based on the laser diffraction / scattering method, similar to the above-mentioned particles (A). 50 The values ​​are the particle diameters (median diameters) at 50% of the cumulative particle size.

[0035] The BET specific surface area of ​​the particles (B) contained in the positive electrode active material is preferably 10 m 2 / g~30m 2 / g, more preferably 13m 2 / g~27m 2 / g, and more preferably 15m 2 / g~25m 2 / g. The BET specific surface area can be measured using, for example, a flow-type automatic specific surface area measuring device (FlowSorbIII2305, manufactured by Shimadzu Corporation) under conditions of a nitrogen-helium mixed gas containing 30% nitrogen.

[0036] The tap density of the particles (B) contained in the positive electrode active material is preferably 0.8 g / cm from the viewpoint of ensuring the expression of excellent battery characteristics. 3 ~1.5g / cm 3 and more preferably 0.9 g / cm 3 ~1.4g / cm 3 and more preferably 1.0 g / cm 3 ~1.3g / cm 3 is. The tap density means the "tap bulk density" measured by the method specified in JIS R 1628 "Method for measuring bulk density of fine ceramic powders."

[0037] The particles (B) have unique water absorption characteristics during the temperature transition from 20°C to 250°C. Specifically, after drying at 300°C for 24 hours under atmospheric pressure, the particles are allowed to stand for x hours in an environment at a temperature of 20°C and a relative humidity of 50%. In the measurement of the amount of moisture y detected by a Karl Fischer moisture meter until the temperature is raised to 250°C, this is considered to be an absolutely dry state, and when x is set to 1, the amount of moisture y is 1 and the amount of water when x is 24 2 The ratio (y 1 / y 2 ) is 0.3 or less. 1 / y 2) can be regarded as the water absorption rate from 1 hour to 24 hours based on the amount of water absorbed during a temperature change from 20°C to 250°C. Since the difference between the amount of water absorbed after 1 hour at 20°C and after 24 hours is small, particles (B) have the property of having a slow water absorption rate and being resistant to water absorption. Because particles (B) have such unique water absorption properties, even when particles (A) rich in Ni are contained, the migration of water from particles (B) to particles (A) is slowed down during the formation of the positive electrode or during the manufacture of the battery, and unnecessary side reactions (such as the production of Li2CO3) are effectively prevented, thereby effectively suppressing deterioration of rate characteristics and cycle characteristics.

[0038] In addition, the above moisture content y 1 and y 2 After heating up to 250°C, the temperature is kept constant for 20 minutes before measurement.

[0039] Water content in particle (B) y 1 In order to fully exert the effect of the unique water absorption properties, the ratio (y 1 / y 2 ) is 0.3 or less, preferably 0.28 or less, more preferably 0.26 or less, and even more preferably 0.24 or less, from the viewpoint of fully exerting the effect of the unique water absorption characteristics.

[0040] In the positive electrode active material for forming the positive electrode provided in the lithium ion secondary battery of the present invention, the specific content of the particles (B) that make up the remainder of the particles (A) is 5 to 45 parts by mass, preferably 5 to 40 parts by mass, more preferably 5 to 30 parts by mass, and even more preferably 5 to 20 parts by mass, per 100 parts by mass of the total weight of the particles (A) and (B), from the viewpoint of effectively achieving both high rate characteristics and excellent cycle characteristics by exerting the effect of the unique water absorption characteristics.

[0041] From the viewpoint of effectively achieving both high rate characteristics and excellent cycle characteristics, particles (B) are preferably particles having cellulose nanofiber-derived carbon and / or water-soluble carbon material-derived carbon supported on their surfaces. Cellulose nanofibers are the skeletal components that make up approximately 50% of all plant cell walls. They are lightweight, high-strength fibers that can be obtained by defibrating the plant fibers that make up these cell walls to nano-size. The fiber diameter of such cellulose nanofibers is 1 nm to 1000 nm, and they also have good dispersibility in water. Furthermore, the cellulose molecular chains that make up cellulose nanofibers form a periodic structure of carbon.

[0042] The water-soluble carbon material, like cellulose nanofibers, is carbonized to form carbon, and when this is supported on the surface of the particles (B), it is possible to ensure both high rate characteristics and excellent cycle characteristics. Examples of such water-soluble carbonaceous materials include one or more selected from sugars, polyols, polyethers, and organic acids. More specifically, examples include monosaccharides such as glucose, fructose, galactose, and mannose; disaccharides such as maltose, sucrose, and cellobiose; polysaccharides such as starch and dextrin; polyols and polyethers such as ethylene glycol, propylene glycol, diethylene glycol, polyethylene glycol, butanediol, propanediol, polyvinyl alcohol, and glycerin; and organic acids such as citric acid, tartaric acid, and ascorbic acid. Among these, from the viewpoint of increasing solubility and dispersibility in a solvent and effectively functioning as a carbonaceous material, glucose, fructose, sucrose, and dextrin are preferred, and glucose is more preferred.

[0043] The cellulose nanofiber-derived carbon and the water-soluble carbon material-derived carbon may support only the cellulose nanofiber-derived carbon, only the water-soluble carbon material-derived carbon, or both the cellulose nanofiber-derived carbon and the water-soluble carbon material-derived carbon.

[0044] When the surface of the particles (B) is supported with carbon derived from cellulose nanofibers and / or carbon derived from a water-soluble carbon material, the total of the atomic equivalent amount of carbon derived from the cellulose nanofibers and the atomic equivalent amount of carbon derived from the water-soluble carbon material, i.e., the total amount of carbon derived from the cellulose nanofibers and the amount of carbon derived from the water-soluble carbon material, is preferably 0.7 to 5.0 parts by mass, more preferably 0.9 to 4.0 parts by mass, and even more preferably 1.0 to 3.0 parts by mass, per 100 parts by mass of the particles (B).

[0045] In addition, when particles (B) have carbon derived from cellulose nanofibers and / or carbon derived from a water-soluble carbon material supported on their surfaces, the content of particles (B) in the positive electrode active material also includes the amount of carbon supported. Furthermore, the atomic equivalent amount (supported amount) of carbon derived from cellulose nanofibers present in particle (B) and the atomic equivalent amount (supported amount) of carbon derived from water-soluble carbon materials are values ​​determined by measurements using a carbon / sulfur analyzer.

[0046] The particles (B) can be obtained, for example, by the following production method. Specifically, the particles (B) can be obtained by the following steps (Ib) to (IVb): (Ib) A step of adding a metal compound including a lithium compound, a manganese compound, and an iron compound, a phosphate compound, carbon derived from cellulose nanofibers and / or carbon derived from a water-soluble carbon material, and water to obtain a slurry water b, and then subjecting the slurry to a hydrothermal reaction to obtain a composite B. (IIb) A step of adding the obtained composite B and water to obtain slurry water c. (IIIb) A step of subjecting the obtained slurry water c to spray drying to obtain granules Z (IVb) A step of firing the obtained granules Z The manufacturing method includes the steps of:

[0047] The above-mentioned step (Ib) is a step in which metal compounds including lithium compounds, manganese compounds, and iron compounds, phosphate compounds, carbon derived from cellulose nanofibers and / or carbon derived from a water-soluble carbon material, and water are added to obtain slurry water b, which is then subjected to a hydrothermal reaction to obtain composite B.

[0048] As the lithium compound, the same compounds as those used for the particles (A) can be used, but hydroxides are preferred. As the manganese compound, the same ones as those used for the particles (A) can be used. Examples of iron compounds include iron acetate, iron nitrate, and iron sulfate. These may be used alone or in combination of two or more. Among these, iron sulfate is preferred from the viewpoint of improving battery characteristics. The metal compounds include at least the manganese compounds and iron compounds, and further include metals other than the manganese compounds and iron compounds (M 2 ) compounds may also be used. Examples of the phosphoric acid compound include orthophosphoric acid (H3PO4, phosphoric acid), metaphosphoric acid, pyrophosphoric acid, triphosphoric acid, tetraphosphoric acid, ammonium phosphate, ammonium hydrogen phosphate, etc. Among these, it is preferable to use phosphoric acid.

[0049] More specifically, step (Ib) includes a step (ib-1) of mixing a phosphate compound with a slurry water b' containing a lithium compound to obtain a composite B'; A step (ib-2) of subjecting the obtained composite B' and a slurry water b containing metal compounds including at least a manganese compound and an iron compound to a hydrothermal reaction to obtain a composite B. It is preferable to have:

[0050] The slurry water b' after mixing with the phosphoric acid compound preferably contains 2.0 to 4.0 moles, more preferably 2.0 to 3.1 moles, of lithium per mole of phosphoric acid, and the lithium compound and phosphoric acid compound may be used in such amounts. More specifically, the slurry water b' after mixing with the phosphoric acid compound preferably contains 2.7 to 3.3 moles, more preferably 2.8 to 3.1 moles, of lithium per mole of phosphoric acid.

[0051] Nitrogen is purged into the slurry water b' after mixing with the phosphoric acid compound to complete the reaction in the slurry water, obtaining a composite B', which is a precursor of particle (B), as a slurry. Purging nitrogen allows the reaction to proceed in a state where the dissolved oxygen concentration in the slurry water b' is reduced, and the dissolved oxygen concentration of the resulting slurry water containing composite B' is also effectively reduced, thereby suppressing the oxidation of the metal compound added in the next step. In the slurry water b' containing composite B', the precursor of particle (B) exists as fine dispersed particles. Composite B' is obtained as a composite of trilithium phosphate (Li3PO4) and carbon derived from cellulose nanofibers and / or carbon derived from a water-soluble carbon material.

[0052] Next, in step (ib-2), composite B' obtained in step (ib-1) and slurry water b containing metal compounds including at least a manganese compound and an iron compound are subjected to a hydrothermal reaction to obtain composite B. The total amount of these metal compounds added is preferably 0.99 mol to 1.01 mol, and more preferably 0.995 mol to 1.005 mol, per mol of phosphate ions contained in the slurry water A.

[0053] The amount of water used in the hydrothermal reaction is preferably 10 mol to 50 mol, and more preferably 12.5 mol to 45 mol, per mol of phosphate ions contained in the slurry water b, from the viewpoints of the solubility of the metal compound, ease of stirring, synthesis efficiency, etc.

[0054] The hydrothermal reaction may be carried out at a temperature of 100°C or higher, preferably 130°C to 200°C. The hydrothermal reaction is preferably carried out in a pressure-resistant vessel, and when the reaction is carried out at 130°C to 200°C, the pressure is preferably 0.3MPa to 1.6MPa, and when the reaction is carried out at 140°C to 160°C, the pressure is preferably 0.3MPa to 0.6MPa. The hydrothermal reaction time is preferably 0.1 hours to 48 hours, more preferably 0.2 hours to 24 hours. The obtained complex B is isolated by filtration, washing with water, and drying, which can be performed by freeze-drying or vacuum drying.

[0055] The step (Ib) may be a step of adding trilithium phosphate particles instead of adding a lithium compound and a phosphate compound. Specifically, the step (Ib) may be a step of adding trilithium phosphate particles, a metal compound including a manganese compound and an iron compound, carbon derived from cellulose nanofibers and / or carbon derived from a water-soluble carbon material, and water to obtain slurry water b, which is then subjected to a hydrothermal reaction to obtain composite B. The manganese compound, iron compound, and carbon derived from cellulose nanofibers and / or carbon derived from water-soluble carbon materials that can be used here are the same as those described above, and other conditions may also be selected appropriately as described above.

[0056] The above step (IIb) is a step of adding water to the composite B obtained in step (Ib) to obtain a slurry water c. The solid content concentration of the slurry water c is preferably 5% by mass to 60% by mass, more preferably 10% by mass to 55% by mass, and even more preferably 15% by mass to 50% by mass.

[0057] The above step (IIIb) is a step of subjecting the slurry water c obtained in step (IIb) to spray drying to obtain granules Z. In the spray drying, the operating conditions may be appropriately set depending on the device used, such as a micro-mist dryer equipped with a four-fluid nozzle.

[0058] The step (IVb) is a step of calcining the granules Z obtained in the step (IIIb). The calcination conditions in the step (IVb) are preferably a reducing atmosphere or an inert atmosphere, the calcination temperature is preferably 500°C to 1000°C, more preferably 550°C to 900°C, and the calcination time is preferably 0.5 hours to 12 hours, more preferably 1 hour to 6 hours.

[0059] To obtain the positive electrode active material for forming the positive electrode of the lithium ion secondary battery of the present invention, the particles (A) and particles (B) are adjusted to the above-mentioned amounts and then mixed by a conventional method.

[0060] To form a positive electrode using such a positive electrode active material, the positive electrode active material is mixed with acetylene black, ketjen black, polyvinylidene fluoride, N-methyl-2-pyrrolidone, or the like to prepare a positive electrode slurry, which is then applied to a current collector and press-molded. In this case, the unique water absorption properties of particles (B) effectively suppress moisture transfer to particles (A), thereby effectively preventing unwanted side reactions.

[0061] The lithium ion secondary battery of the present invention comprises, as essential components, the above-mentioned positive electrode, a negative electrode and an electrolyte, or the above-mentioned positive electrode and a negative electrode. The negative electrode is not particularly limited in terms of material composition, and any known material composition can be used as long as it can absorb lithium ions during charging and release them during discharging. For example, lithium metal, graphite, silicon-based materials (Si, SiOx), lithium titanate, or carbon materials such as amorphous carbon can be used. It is preferable to use an electrode formed of an intercalating material capable of electrochemically absorbing and releasing lithium ions, particularly a carbon material. Furthermore, two or more of the above negative electrode materials may be used in combination, such as a combination of graphite and silicon-based materials.

[0062] The electrolyte is not particularly limited in type, but is preferably an electrolytic solution in which a supporting salt is dissolved in an organic solvent. The organic solvent is not particularly limited as long as it is an organic solvent typically used in electrolytic solutions for lithium ion secondary batteries, and examples thereof include carbonates, halogenated hydrocarbons, ethers, ketones, nitriles, lactones, and oxolane compounds.

[0063] The supporting salt is not particularly limited in type, but is preferably at least one of inorganic salts selected from LiPF6, LiBF4, LiClO4, and LiAsF6, derivatives of these inorganic salts, organic salts selected from LiSO3CF3, LiC(SO3CF3)2, LiN(SO3CF3)2, LiN(SO2C2F5)2, and LiN(SO2CF3)(SO2C4F9), and derivatives of these organic salts.

[0064] In a more preferred embodiment, the lithium-ion secondary battery of the present invention includes a separator as a constituent element. The separator serves to electrically insulate the positive and negative electrodes and to retain the electrolyte. For example, a porous synthetic resin film, particularly a porous film of a polyolefin polymer (polyethylene, polypropylene), may be used.

[0065] The shape of the lithium ion secondary battery of the present invention is not particularly limited, and may be various shapes such as coin-shaped, cylindrical, or rectangular, or may be an irregular shape enclosed in a laminate exterior. [Example]

[0066] The present invention will be specifically described below based on examples, but the present invention is not limited to these examples.

[0067] <Lithium composite oxide particles (A-1)> As particles (A-1), commercially available particles (LiNi 0.5 Mn 0.3 Co 0.2 O2: average particle size 12 μm) was used.

[0068] <Lithium composite oxide particles (A-2)> As particles (A-2), commercially available particles (LiNi 0.8 Mn 0.1 Co 0.1 O2: average particle size 13 μm) was used.

[0069] <Lithium-based polyanion particles (B-1)> As particles (B-1), commercially available particles (LiMn 0.3 Fe 0.7 PO4: average particle size 14 μm, carbon atom equivalent 0.9% was used.

[0070] <Lithium-based polyanion particles (B-2)> As particles (B-2), commercially available particles (LiMn 0.3 Fe 0.7 PO4: average particle size 14 μm, carbon atom equivalent 0.7% was used.

[0071] <Lithium-based polyanion particles (B-3)> As particles (B-3), commercially available particles (LiMn 0.1 Fe 0.9 PO4: average particle size 15 μm, carbon atom equivalent amount 1.0% was used.

[0072] <Lithium-based polyanion particles (B-4)> As particles (B-4), commercially available particles (LiMn 0.7 Fe 0.3 PO4: average particle size 14 μm, carbon atom equivalent 1.0% was used.

[0073] <Lithium-based polyanion particles (B-5)> As particles (B-5), commercially available particles (LiMn 0.3 Fe 0.7 PO4: average particle size 15 μm, carbon atom equivalent amount 0.5% was used.

[0074] <Amount of carbon atoms (supported amount)> The amount of carbon atoms (supported amount) of the obtained particles was measured using a carbon / sulfur analyzer (EMIA-220V2, manufactured by Horiba, Ltd.).

[0075] <<Measurement of the average particle size of particles (secondary particles)>> The particle size distribution of the particles was measured using a laser diffraction device (Microtrac MT3000II, manufactured by MicrotracBEL). The measurement conditions were particle transmittance: transmission, particle shape: non-spherical, particle refractive index: 1.52. Ethanol was used as the solvent, and the solvent refractive index: 1.36.

[0076] The water absorption properties of the particles A-1 to A-2 and particles B-1 to B-5 were further evaluated according to the following method. The results are shown in Table 1, along with the physical properties of each particle.

[0077] Evaluation of water absorption characteristics (water content y 1 and moisture content y 2 Measurement of Each of the particles was dried at 300°C under atmospheric pressure for 24 hours to obtain an absolute dry state. Then, the particles were allowed to stand for one day in an environment at 20°C and 50% relative humidity to absorb moisture from the atmosphere until equilibrium was reached. The temperature was then raised to 250°C and maintained for 20 minutes. The amount of volatilized moisture was measured using a Karl Fischer moisture meter (MKC-610, manufactured by Kyoto Electronics Manufacturing Co., Ltd.), with the starting point being when the temperature reached 250°C and the ending point being when the constant temperature at 250°C was reached.

[0078] [Table 1]

[0079] [Examples 1 to 8, Comparative Examples 1 to 3] According to the formulation shown in Table 2, the particles were mixed using a planetary mixer (PLM-2, manufactured by Inoue Seisakusho Co., Ltd.) to obtain a positive electrode active material. Next, the resulting positive electrode active material was used as a positive electrode material to fabricate a positive electrode for a lithium-ion secondary battery. Specifically, the resulting positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 90:5:5, and N-methyl-2-pyrrolidone was added to the mixture and thoroughly kneaded to prepare a positive electrode slurry. The positive electrode slurry was applied to a current collector made of 20 μm thick aluminum foil using a coating machine and vacuum dried at 80 °C for 12 hours. The resulting mixture was then punched into a φ14 mm disk and pressed for 2 minutes at 16 MPa using a hand press to form a positive electrode.

[0080] Next, a coin-type secondary battery was constructed using the above positive electrode. A lithium foil punched to a diameter of 15 mm was used as the negative electrode. The electrolyte was a 1 mol / L solution of LiPF6 in a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 3:7. A polymer porous film was used as the separator. These battery components were assembled and housed in an atmosphere with a dew point of -50°C or lower using standard methods to obtain a coin-type secondary battery (CR-2032).

[0081] The discharge capacities of the obtained coin-type secondary batteries were measured at 0.2 C (34 mA / g) and 5 C (850 mA / g) in an environment at an ambient temperature of 30°C using a charge / discharge capacity measuring device (HJ-1001SD8, manufactured by Hokuto Denko Corporation), and the capacity ratio (%) was calculated using the following formula (x). The cut-off voltage during charging was 4.25 V, and the cut-off voltage during discharging was 2.0 V. Capacity ratio = {(Discharge capacity at 5 C) / (Discharge capacity at 0.2 C)} × 100 (x)

[0082] Furthermore, a 1000-cycle charge-discharge test was conducted at a temperature of 45°C and a charge-discharge rate of 1C (170mA / g), and the discharge capacity was measured to determine the capacity retention rate (%) using the following formula (y): The cut-off voltage during charging was 4.25V, and the cut-off voltage during discharging was 2.0V. Cycle characteristics = {(Discharge capacity after 1000 cycles) / (Discharge capacity after 1 cycle)}×100 (y) The weight of the active material contained in the electrode (mg / cm 2 ) and density (g / cm 3 The results are shown in Table 3.

[0083] [Table 2]

[0084] [Table 3]

Claims

1. A lithium ion secondary battery including at least a positive electrode, a negative electrode, and an electrolyte, the positive electrode includes at least a current collector and a positive electrode active material layer formed on the current collector, The positive electrode active material contained in the positive electrode active material layer includes particles (A) represented by the following formula (a) and particles (B) represented by the following formula (b), the particles (A) are contained in an amount of 60 parts by mass to 95 parts by mass per 100 parts by mass of the total weight of the particles (A) and the particles (B); The particles (B) are dried at 300°C for 24 hours under atmospheric pressure, and then left to stand for x hours in an environment at a temperature of 20°C and a relative humidity of 50%. In the measurement of the moisture content y detected by a Karl Fischer moisture meter until the temperature is raised to 250°C, the moisture content y is determined by taking x as 1. 1 and the amount of water when x is 24 2 The ratio (y 1 / y 2 ) is 0.22 to 0.29, a lithium ion secondary battery L)) a Co b Mn c M 1 w O 2 ・・・(a) (In formula (a), M 1 represents one or more elements selected from Mg, Ti, Nb, Fe, Cr, Si, Al, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi, and Ge. a, b, c, and w satisfy the following conditions: 0.5≦a<1, 0<b<0.5, 0<c<0.5, 0≦w≦0.3, and 3a+3b+3c+(M 1 (valence of x) × w = 3. Li f Mn g Fe h M 2 x 2O 4 ・・・(b) (In formula (b), M 2 represents one or more elements selected from Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, and Gd. f, g, h, and x satisfy 0<f≦1.2, 0.08≦g<0.6, 0.5<h≦1.1, 0≦x≦0.3, and 1 / 9≦g / h<1, and the formula f+(valence of Mn)×g+(valence of Fe)×h+(valence of Mn) 2 (valence of x) × x = 3.

2. The moisture content y in the particles (B) contained in the positive electrode active material 1 The lithium ion secondary battery according to claim 1, wherein the total amount of ZnO is 2000 ppm or less.

3. 3. The lithium ion secondary battery according to claim 1, wherein the particles (B) contained in the positive electrode active material have carbon derived from cellulose nanofibers and / or carbon derived from a water-soluble carbon material supported on their surfaces.

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