Patent Application
20240322133
The present invention relates to a novel cathode active material for a lithium secondary battery, a lithium secondary battery using the cathode active material, and a method for manufacturing a cathode active material for a lithium secondary battery.
In recent years, research and development on secondary batteries that contribute to improvement of energy efficiency have been conducted. In particular, lithium secondary batteries are becoming increasingly important as power sources for electric vehicles (EV), hybrid electric vehicles (HEV), and the like.
A cathode active material has attracted attention as an important component for determining the capacity of a lithium secondary battery, and development thereof has been advanced. As a cathode active material used for a lithium secondary battery, for example, a nickel-manganese-based composite oxide has been reported (refer to Mitsuharu Tabuchi, Riki Kataoka, and Koji Yazawa, “High-Capacity Li-excess lithium nickel manganese oxide as a Co-free positive electrode material”, Materials Research Bulletin, 137 (2021) 111178), and a nickel-manganese-titanium-based composite oxide has been reported (refer to J. S. Kim, C. S. Johnson, M. M. Thackeray “Layered xLiMO2·(1-x)Li2M′O3 electrodes for lithium batteries: a study of 0.95LiMn0.5Ni0.5O2·0.05Li2TiO3”, Electrochemistry Communications 4 (2002) 205-209, JP 2022-60167 A, and JP 2008-12733 A).
By the way, in a technology related to a lithium secondary battery, an object is to provide a cathode active material that does not use expensive cobalt having a resource risk, reduces the use amount of nickel as much as possible, and has excellent electrochemical characteristics.
The technology described in Mitsuharu Tabuchi, Riki Kataoka, and Koji Yazawa, “High-Capacity Li-excess lithium nickel manganese oxide as a Co-free positive electrode material”, Materials Research Bulletin, 137 (2021) 111178 is aimed at a cobalt-free cathode material, but in an XRD pattern of LiNi0.5Mn0.5O2, any bimodal peak around 2θ=65° is tailed or not clearly separated. In addition, in 7Li pj-MATPASS NMR data, peaks due to Ni, Mn—Li, and Mn—Li in a transition metal layer having a honeycomb structure are recognized near 1350 ppm and 1500 ppm, respectively, and in a main peak, a peak due to a Mn domain close to Li in a Li layer near 700 nm and a shoulder peak due to a Ni domain close to Li in the Li layer near 533 ppm are also observed. These facts indicate that in the lithium transition metal composite oxide described in Mitsuharu Tabuchi, Riki Kataoka, and Koji Yazawa, “High-Capacity Li-excess lithium nickel manganese oxide as a Co-free positive electrode material”, Materials Research Bulletin, 137 (2021) 111178, the transition metals (Ni and Mn) are not uniformly dispersed and form phase separation or domains. This suggests that the electrochemical characteristics of the lithium transition metal composite oxide disclosed in Mitsuharu Tabuchi, Riki Kataoka, and Koji Yazawa, “High-Capacity Li-excess lithium nickel manganese oxide as a Co-free positive electrode material”, Materials Research Bulletin, 137 (2021) 111178 are low.
The technology described in J. S. Kim, C. S. Johnson, M. M. Thackeray “Layered xLiMO2·(1-x)Li2M′O3 electrodes for lithium batteries: a study of 0.95LiMn0.5Ni0.5O2·0.05Li2TiO3”, Electrochemistry Communications 4 (2002) 205-209 uses 0.95LiMn0.5Ni0.5O2·0.05Li2TiO3 into which electrochemically inert Li2TiO3 is introduced for enhancing the stability of the LiNi0.5Mn0.5O2 electrode as a cathode material. However, the XRD pattern of 0.95LiMn0.5Ni0.5O2·0.05Li2TiO3 has a problem that a bimodal peak near 2θ=650 is not clearly separated, and a transition metal (Ni, Mn, or Ti) is not uniformly dispersed.
The technology described in JP 2022-60167 A suggests Li1+x(NiyMzMn1−y−z)1−xO2 (in the formula, M represents Mg and/or Ti) as a cobalt-free cathode material, and in an XRD pattern of the lithium transition metal composite oxide described in that same literature, a peak due to a Li2MnO3 domain is observed near 20=22°, and this indicates that a transition metal is not uniformly dispersed. In addition, the technology described in JP 2008-12733 A suggests a lithium transition metal composite oxide expressed by Li1+x(Mn1−n−mNimTin)1−xO2 as a cobalt-free cathode material, but the lithium transition metal composite oxide described in the same literature includes a mixed phase of a crystal phase having a layered rock salt type structure and a crystal phase having a cubic rock salt type structure, and a chemical composition of the lithium transition metal composite oxide is not uniform.
An object of the invention is to provide a cobalt-free cathode material for a lithium secondary battery in which the amount of nickel used is reduced, and to solve a problem of deterioration in electrochemical characteristics due to non-uniform dispersion of transition metals found in the related art.
To accomplish the above-described object, the invention provides the following means.
[1] A cathode active material for a lithium secondary battery containing a lithium transition metal composite oxide as a main component,
In the cathode active material for a lithium secondary battery of the invention, titanium (Ti) is added as a constituent element to LiNi0.5Mn0.5O2 of the related art, and a part of Ni0.5Mn0.5 is replaced with Li and Ti, and thus the use amount of Ni having a resource risk can be reduced while maintaining satisfactory electrochemical characteristics. In addition, when the cathode active material satisfies the range of the chemical composition and the range of the lattice constant of the lithium transition metal composite oxide expressed by General Formula (1), the transition metal is uniformly dispersed, whereby the charge and discharge capacity of the lithium secondary battery can be improved.
[2] The cathode active material for a lithium secondary battery according to [1], wherein in General Formula (1), m is in a range of 1.05≤m≤1.066, x is in a range of 0.4≤x≤0.475, y is in a range of 0.4≤y≤0.475, and z is in a range of 0.05≤z≤0.133.
When optimizing the chemical composition of the lithium transition metal composite oxide expressed by General Formula (1), dispersibility of the transition metal is further improved, and the charge and discharge capacity of the lithium secondary battery can be further increased.
[3] The cathode active material for a lithium secondary battery according to [1] or [2], wherein a ratio (Mn/Ni ratio) of the number of atoms of Mn to the number of atoms of Ni on a surface of the particles of the lithium transition metal composite oxide is more than 1.0 and less than 2.5.
When the surface of the particles of the lithium transition metal composite oxide is Mn-rich, the charge and discharge capacity of the lithium secondary battery can be increased in a case where the lithium transition metal composite oxide is used as a cathode active material of the lithium secondary battery.
[4] The cathode active material for a lithium secondary battery according to any one of [1] to [3], wherein a peak intensity ratio of 1500 ppm/600 ppm is 0.15 or less in a spectrum of the lithium transition metal composite oxide which is measured by solid lithium nuclear magnetic resonance analysis (6Li-MAS-NMR) using a magic angle sample rotation method.
When the peak intensity ratio of 1500 ppm/600 ppm in the solid lithium nuclear magnetic resonance spectrum is 0.15 or less, phase separation of the transition metal is suppressed. That is, when the transition metal is uniformly dispersed, the charge and discharge capacity of the lithium secondary battery can be increased in a case where the lithium transition metal composite oxide is used as a cathode active material of the secondary battery.
[5] The cathode active material for a lithium secondary battery according to any one of [1] to [4], wherein a peak due to lithium contained in the transition metal-lithium layer is not present at 1500 ppm in the spectrum of the lithium transition metal composite oxide which is measured by the solid lithium nuclear magnetic resonance analysis (6Li-MAS-NMR).
According to this, in a case where the lithium transition metal composite oxide is used as a cathode active material of a secondary battery, the charge and discharge capacity of the lithium secondary battery can be further increased.
[6] A lithium secondary battery including: a cathode; an anode; and an electrolyte, wherein the cathode contains the cathode active material for a lithium secondary battery according to [1].
In the lithium secondary battery of the invention, when the cathode contains a cathode active material containing the lithium transition metal composite oxide as a main component, the capacity can be increased.
[7] The lithium secondary battery according to [6], wherein the electrolyte is a solid electrolyte.
The cathode active material for a lithium secondary battery of the invention is applicable to an all-solid-state lithium secondary battery.
[8] A method for manufacturing a cathode active material for a lithium secondary battery according to [1], the method including:
A cathode active material for a lithium secondary battery satisfying the range of the chemical composition and the range of the lattice constant of the lithium transition metal composite oxide expressed by General Formula (1) can be manufactured by the method including the above-described processes.
[9] The method for manufacturing a cathode active material for a lithium secondary battery according to [8], further comprising: subsequently holding the obtained lithium transition metal composite oxide at 850° C. to 950° C. for 3 hours to 12 hours after the heat-treatming.
When the slow cooling process is further provided, the surface of the lithium transition metal composite oxide particles can be made rich in Mn.
Hereinafter, preferred embodiments of the invention will be described in detail with reference to the accompanying drawings.
A cathode active material for the lithium secondary battery of the invention contains a lithium transition metal composite oxide as a main component, and is used for a cathode of the lithium secondary battery. The phrase “containing a lithium transition metal composite oxide as a main component” represents that the content of the lithium transition metal composite oxide is 75% by mass or more, preferably 80% by mass or more, more preferably 90% by mass or more, and still more preferably 99% by mass or more. The cathode active material for a lithium secondary battery may contain components other than the main component as long as the function of the present invention is not impaired.
The cathode active material for the lithium secondary battery of the invention may contain only one kind or two or more kinds of lithium transition metal composite oxides as long as the lithium transition metal composite oxide is contained as a main component.
When the cathode active material is manufactured by using the lithium transition metal composite oxide as a main component, a composition ratio (Li:Ni:Mn:Ti) of the entirety of the lithium transition metal composite oxide is also maintained in a cathode active material that is obtained. When the cathode active material obtained by using the lithium transition metal composite oxide having such a composition as a main component is used in a secondary battery, high capacity can be realized. In addition, the composition ratio of the lithium transition metal composite oxide is adjusted so as to be the same as a composition ratio required for the cathode active material to be obtained.
The lithium transition metal composite oxide used in the invention is a layered rock salt type oxide, and is in a form of particles having a layered structure of a lithium layer (Li layer) and a transition metal-lithium layer (TM-Li layer).
In a lithium transition metal composite oxide (for example, LiNi0.5Mn0.5O2) in the related art, when Li is solid-soluted in a transition element, Ni2+ involved in an oxidation-reduction reaction is converted into Ni3+, and thus it is necessary to increase the amount of Ni used in order to improve electrochemical characteristics as a cathode active material. The invention is based on the finding that an increase in the valence of Ni ions to 3 can be suppressed by adding Ti as a constituent element to LiNi0.5Mn0.5O2 and replacing a part of Ni0.5Mn0.5 with Li and Ti. Thus, in the present invention, the amount of Ni used can be reduced while the electrochemical characteristics of the cathode active material are maintained in a satisfactory manner.
The lithium transition metal composite oxide used in the invention is expressed by General Formula (1): LimNixMnyTizO2 (in the formula, m is in a range of 1.0<m≤1.1, x is in a range of 0.4≤x<0.5, y is in a range of 0.3<y<0.5, and z is in a range of 0<z≤0.133).
In the lithium transition metal composite oxide used in the invention, it is more preferable that in General Formula (1), m is in a range of 1.05≤m≤1.066, x is in a range of 0.4≤x≤0.475, y is in a range of 0.4≤y≤0.475, and z is in a range of 0.05≤z≤0.133. Specifically, the lithium transition metal composite oxide used in the present invention has a chemical composition in a range of Li1.05Ni0.475Mn0.475Ti0.05O2 to Li1.066Ni0.4Mn0.4Ti0.133O2. This represents that a molar ratio of Li/Ti in the transition metal-Li layer is in a range of Li:Ti=1:1 [Li(Li0.05Ni0.475Mn0.475Ti0.05)O2] to Li:Ti=1:2 [Li(Li0.066Ni0.4Mn0.4Ti0.133)O2].
The composition of the lithium transition metal composite oxide can be analyzed by inductively coupled plasma (ICP) optical emission spectrometry.
The lithium transition metal composite oxide used in the invention is a layered compound of a rhombohedral crystal system, and has a crystal structure of a space group R-3m. With regard to the lattice constant of the lithium transition metal composite oxide, an a-axis length is preferably 2.886 Å to 2.900 Å, a c-axis length is preferably 14.29 Å to 14.35 Å, and c/a is preferably 4.940 to 4.960. When the lattice constant is within the above-described range, in the lithium transition metal composite oxide, lithium ions are likely to be diffused in primary particles, and resistance is low. The lattice constant of the crystal can be determined by a least square method by measuring an X-ray diffraction pattern of the lithium transition metal composite oxide and using each index and plane spacing thereof.
In the spectra shown in
For example, in Mitsuharu Tabuchi, Riki Kataoka, and Koji Yazawa, “High-Capacity Li-excess lithium nickel manganese oxide as a Co-free positive electrode material”, Materials Research Bulletin, 137 (2021) 111178, a 7Li pj-MATPASS NMR spectrum of LiNi0.5Mn0.5O2 shown in
The lithium transition metal composite oxide used in the invention has no peak at 1500 ppm in the 6Li-MAS-NMR spectrum after the heat treatment, or a peak intensity ratio of 1500 ppm/600 ppm is 0.15 or less, 0.1 or less, or 0.05 or less. Preferably, the lithium transition metal composite oxide used in the invention has no peak at 1500 ppm. In this specification, the absence of a peak at 1500 ppm represents that a peak other than a ghost peak such as a spinning side band does not exist at 1500 ppm. The present inventors have confirmed that there is no peak other than a ghost peak at 1500 ppm by peak separation in the 6Li-MAS-NMR spectra shown in
In the lithium transition metal composite oxide used in the invention, since the transition metal is uniformly dispersed, charge and discharge capacity can be increased when the lithium transition metal composite oxide is used in a lithium secondary battery as a cathode active material.
The 6Li-MAS-NMR spectra can be measured by a solid lithium nuclear magnetic resonance apparatus using a magic angle sample rotation method.
On the contrary, for example, Mitsuharu Tabuchi, Riki Kataoka, and Koji Yazawa, “High-Capacity Li-excess lithium nickel manganese oxide as a Co-free positive electrode material”, Materials Research Bulletin, 137 (2021) 111178 discloses an XRD pattern of LiNi0.5Mn0.5O2, and as shown in
Particles of the lithium transition metal composite oxide may be primary particles or secondary particles, but are preferably secondary particles in which a plurality of primary particles are aggregated with each other because relatively dense particles are obtained.
The particles of the lithium transition metal composite oxide have a multilayer structure including a lithium layer and a transition metal-Li layer, and a composition of an outer surface preferably has a higher content ratio of Mn in comparison to a composition of a central portion. In the lithium transition metal composite oxide used in the invention, a ratio (Mn/Ni ratio) of the number of atoms of Mn to the number of atoms of Ni on the surface of the lithium transition metal composite oxide is preferably more than 1.0 and less than 2.5, and more preferably more than 1.0 and less than 2.0. When the Mn/Ni ratio is within the above-described range, movement of lithium ions is not inhibited, and the charge and discharge capacity of the lithium secondary battery becomes high in a case of being used as a cathode active material.
A composition of a manganese-rich surface can be determined, for example, by quantitative analysis of X-ray photoelectron spectroscopy (XPS). According to the XPS, the composition of the manganese-rich surface in entire particles can be analyzed. That is, an analysis result obtained by the XPS does not indicate a local composition of the entire surface of one particle, and indicates a composition of the entire surface of the particle.
The cathode active material for the lithium secondary battery of the invention contains the above-described lithium transition metal composite oxide as a main component. As a lithium source of the lithium transition metal composite oxide, it is possible to use a known compound such as a hydroxide such as lithium hydroxide monohydrate (LiOH·H2O), a carbonate such as lithium carbonate (Li2CO3), or an acetate such as lithium acetate (CH3COOLi) and lithium acetate dihydrate (CH3COOLi·2H2O), and there is no particular limitation. In compounds of a nickel source, a manganese source, and a titanium source of the transition metal, known oxides, hydroxides, or metal salts of nickel, manganese, and titanium can be widely used, and are not particularly limited. As the nickel compound, nickel hydroxide (Ni(OH)2), nickel (II) chloride (NiCl2), nickel (II) chloride hexahydrate (NiCl2·6H2O), and nickel (II) nitrate hexahydrate (Ni(NO3)2·6H2O) can be used, but the nickel compound is not limited thereto. As the manganese compound, manganese (II) chloride (MnCl2), manganese (II) chloride tetrahydrate (MnCl2·4H2O), manganese carbonate hexahydrate (MnCO3·6H2O), manganese (II) nitrate hexahydrate (Mn(NO3)2·6H2O) and the like can be used, but the manganese compound is not limited thereto. As the titanium compound, titanium dioxide (TiO2), titanium (IV) sulfate (Ti(SO4)2), and the like can be used, but the titanium compound is no limitation thereto. The transition metal compounds can be used alone and as a composite hydroxide (for example, nickel-manganese-titanium complex hydroxide) or the like by using a coprecipitation method or the like.
The lithium transition metal composite oxide used in the invention can be synthesized by using a known method. For example, a composite hydroxide or a composite oxide of a nickel compound and a manganese compound is prepared as an intermediate compound, the intermediate compound, a titanium compound, and a lithium compound are mixed to obtain a raw material mixture, and the raw material mixture is heat-treated (for example, fired) at a predetermined temperature for a predetermined time in a predetermined atmosphere, whereby synthesis can be performed. In addition, a composite hydroxide or a composite oxide of a nickel compound, a manganese compound, and a titanium compound is prepared as an intermediate compound, the intermediate compound and a lithium compound are mixed to obtain a raw material mixture, and the raw material mixture is heat-treated (for example, fired) at a predetermined temperature for a predetermined time in a predetermined atmosphere, whereby synthesis can also be performed.
It is preferable that the lithium transition metal composite oxide obtained in the heat treatment process (the heat-treating) is further held in a predetermined temperature range for a predetermined time by a slow cooling process. Since slow cooling conditions vary depending on treatment conditions (for example, a heat treatment atmosphere such as an oxygen atmosphere, a nitrogen atmosphere, and a vacuum atmosphere) and the like, it is preferable to appropriately adjust the slow cooling conditions. The present inventors have found that the transition metal of the lithium transition metal composite oxide is uniformly dispersed, and the Mn/Ni ratio on the particle surface can be increased by appropriately selecting the heat treatment (the heat-treating) conditions and the slow cooling conditions. Hereinafter, a method for manufacturing a cathode active material for a lithium secondary battery of the invention will be described in more detail with reference to an embodiment.
In a first process, first, a predetermined amount of titanium compound and a predetermined amount of lithium compound are added to a nickel-manganese compound as an intermediate, and the mixture is dispersed and mixed in a solvent such as ethanol. Note that, a predetermined amount of intermediate compound, a predetermined amount of titanium compound, and a predetermined amount of lithium compound may be mixed not only by wet mixing using a solvent but also by dry mixing not using a solvent. For example, in a case of synthesizing Li1.05Ni0.475Mn0.475Ti0.05O2 by using titanium dioxide (TiO2) as a titanium compound and lithium carbonate (Li2CO3) as a lithium compound, Li2CO3 is preferably weighed more than the stoichiometric ratio by 1% by mass to 5% by mass, for example, 2% by mass. Similarly, in a case of synthesizing Li1.066Ni0.4Mn0.4Ti0.133O2, Li2CO3 is preferably weighed more than the stoichiometric ratio by 1% by mass to 5% by mass, for example, 2% by mass.
The nickel-manganese compound can be synthesized by a known method. In a case where the nickel-manganese compound is a hydroxide, for example, nickel sulfate hexahydrate (NiSO4·6H2O) and manganese sulfate pentahydrate (MnSO4·5H2O) are weighed so that a molar ratio of Ni:Mn becomes 1:1, pure water is added thereto to dissolve the compounds, and an aqueous alkali solution is added dropwise to the aqueous sulfate solution to coprecipitate the compounds as a nickel-manganese composite hydroxide.
As the intermediate compound, a nickel-manganese-titanium compound may be used instead of the nickel-manganese compound. For example, a predetermined amount of lithium compound is added to the nickel-manganese-titanium compound, and the mixture is dispersed and mixed in a solvent such as ethanol. Note that, a predetermined amount of nickel-manganese-titanium compound, and a predetermined amount of lithium compound may be mixed not only by wet mixing using a solvent but also by dry mixing not using a solvent. For example, in a case of synthesizing Li1.05Ni0.475Mn0.475Ti0.05O2 by using lithium carbonate (Li2CO3) as a lithium compound, Li2CO3 is weighed preferably more than the stoichiometric ratio by 1% by mass to 5% by mass, for example, 2% by mass. Similarly, in a case of synthesizing Li1.066Ni0.4Mn0.4Ti0.133O2, Li2CO3 is preferably weighed more than the stoichiometric ratio by 1% by mass to 5% by mass, for example, 2% by mass.
The nickel-manganese-titanium compound can be synthesized using a known method. In a case where the nickel-manganese-titanium compound is a hydroxide, for example, nickel sulfate hexahydrate (NiSO4·6H2O), manganese sulfate pentahydrate (MnSO4·5H2O), and titanium sulfate (Ti(SO4)2) are weighed so that the molar ratio of Ni:Mn Ti becomes 1:1:x (x is preferably 0.05 to 0.333), pure water is added thereto to dissolve the compounds, an aqueous alkali solution is added dropwise to the aqueous sulfate solution, and the compounds can be coprecipitated as a nickel-manganese-titanium composite hydroxide.
As a precursor, a mixture of a lithium compound, a titanium compound, and a nickel-manganese compound, or a mixture of a lithium compound and a nickel-manganese-titanium compound is pulverized and mixed to a preferable size, and the mixture is filled in a crucible and is heat-treated. As the crucible, an alumina sagger, an alumina crucible, a platinum crucible, a gold crucible, or the like is used. In the heat treatment of the mixture, for example, a firing furnace or a roller hearth kiln is used.
The mixture put into the sagger or the crucible is heated to reach a heat treatment temperature at a temperature-rising rate of 5° C./min to 25° C./min, and preferably 10° C./min to 25° C./min. A heat treatment atmosphere is not particularly limited, and examples thereof include an atmosphere (under an air atmosphere), an oxygen flow, and the like. The heat treatment atmosphere is preferably the oxygen flow. A heat treatment time can be appropriately set in correspondence with the heat treatment temperature. The heat treatment time represents time for holding the heat treatment temperature.
In a case where a mixture of the lithium compound (for example, Li2CO3), the titanium compound (for example, TiO2), and the nickel-manganese compound is heat-treated, the heat treatment temperature is preferably 1000° C. to 1200° C., and more preferably 1000° C. to 1100° C. The heat treatment time is preferably 1 minute to 7 hours, more preferably 2 minutes to 6 hours, and still more preferably 3 minutes to 5 hours.
In a case where the mixture of the lithium compound (for example, Li2CO3) and the nickel-manganese-titanium compound is heat-treated, the heat treatment temperature is preferably 1000° C. to 1200° C., and more preferably 1000° C. to 1100° C. The heat treatment time is preferably 1 minute to 7 hours, more preferably 2 minutes to 6 hours, and still more preferably 3 minutes to 5 hours.
In a second process, a powder obtained after the heat treatment in the first process is cooled so as to reach 900° C. at a temperature-falling rate of 5° C./min to 25° C./min, and preferably 10° C./min to 25° C./min, and then the powder is held at 900° C. for 0.5 hours to 12 hours. The atmosphere for holding the powder at 900° C. is not particularly limited, and examples thereof include the atmosphere (under an air atmosphere), a nitrogen atmosphere, and an oxygen flow.
The method for manufacturing the cathode active material for the lithium secondary battery of the invention may include a process of holding the powder at 850° C. for 12 hours after the second process. Even in this case, the powder held at 900° C. in the second process is cooled to reach 850° C. at a temperature-falling rate of 10° C./min, and then the powder is held at 850° C. for 12 hours. The atmosphere for holding the powder at 850° C. is not particularly limited, and examples thereof include the atmosphere (under an air atmosphere), a nitrogen atmosphere, and an oxygen flow.
In a third process, the powder held at 900° C. in the second process is cooled so as to reach 800° C. at a temperature-falling rate of 5° C./min to 25° C./min, and preferably 10° C./min to 25° C./min, and then the powder is held at 800° C. for 0.5 hours to 12 hours. The atmosphere for holding the powder at 800° C. is not particularly limited, and examples thereof include the atmosphere (under an air atmosphere), a nitrogen atmosphere, and an oxygen flow.
In a fourth process, the powder held at 800° C. in the third process is cooled so as to reach 750° C. at a temperature-falling rate of 5° C./min to 25° C./min, and preferably 10° C./min to 25° C./min, and then the powder is held at 750° C. for 0.5 hours to 12 hours. The atmosphere for holding the powder at 750° C. is not particularly limited, and examples thereof include the atmosphere (under an air atmosphere), a nitrogen atmosphere, and an oxygen flow.
In a fifth process, the powder held at 750° C. in the fourth process is cooled so as to reach 600° C. at a temperature-falling rate of 5° C./min to 25° C./min, and preferably 10° C./min to 25° C./min, and then the powder is held at 600° C. for 0.5 hours to 20 hours. The atmosphere for holding the powder at 600° C. is not particularly limited, and examples thereof include the atmosphere (under an air atmosphere), a nitrogen atmosphere, and an oxygen flow.
In a sixth process, the powder held at 600° C. in the fifth process is cooled so as to reach 400° C. to 500° C. at a temperature-falling rate of 10° C./min, and then the powder is held at 400° C. to 500° C. for 0.5 hours to 30 hours. The sixth process may be a process of holding the powder at 450° C. for 30 hours and then holding the powder at 400° C. for 30 hours. In addition, the sixth process may be a process of holding the powder at 500° C. for 20 hours.
The slow cooling processes from the second process to the sixth process described above can be appropriately changed or omitted as necessary. In one embodiment, the slow cooling process is held at preferably 850° C. to 950° C., preferably 3 hours to 12 hours, more preferably 4 hours to 8 hours, and most preferably 6 hours after the heat treatment. The cathode active material for the lithium secondary battery of the invention can increase the Mn/Ni ratio on the particle surface of the lithium transition metal composite oxide by appropriately selecting the cooling conditions.
The lithium secondary battery of the invention is a lithium secondary battery including a cathode, an anode, an electrolyte, and other battery elements as necessary, and the cathode contains a cathode active material containing the lithium transition metal composite oxide of the above-described embodiment as a main component.
In the lithium secondary battery of the invention, a known battery element of the lithium secondary battery can be employed as it is except that the cathode contains a cathode active material containing the above-described lithium transition metal composite oxide as a main component. The lithium secondary battery of the invention may have any of a coin type, a button type, a cylindrical type, a square type, and a laminate type. In addition, the lithium secondary battery of the invention is applicable to a wide range of applications such as mobile devices including mobile phones and notebook computers, and in-vehicle applications.
Hereinafter, as an embodiment of the lithium secondary battery of the invention, a lithium secondary battery (coin-type lithium secondary battery) using an electrolytic solution will be described. Each battery element described below can be similarly applied to an all-solid-state lithium secondary battery not using an electrolytic solution.
As illustrated in
In the lithium secondary battery of this embodiment, a cathode mixture is prepared by blending a conductive agent, a binder, and the like with the cathode active material for the lithium secondary battery of this embodiment as necessary, and a cathode can be manufactured by pressing the cathode mixture to a current collector (not shown). As the current collector, a stainless steel mesh, an aluminum foil, or the like can be preferably used. As the conductive agent, acetylene black, ketjen black, or the like can be preferably used. As the binder, tetrafluoroethylene, polyvinylidene fluoride, or the like can be preferably used.
Blending of the cathode active material, the conductive agent, and the binder in the cathode mixture is not particularly limited. The content of the conductive agent in the cathode mixture is preferably 1% by mass to 15% by mass, and more preferably 0.1% by mass to 5% by mass. The content of the conductive agent in the cathode mixture is preferably 0.1% by mass to 10% by mass, and more preferably 0.1% by mass to 5% by mass. It is preferable to blend the cathode active material, the conductive agent, and the binder such that the remainder (a portion other than the cathode active material and the conductive agent) in the cathode mixture becomes the cathode active material.
In the lithium secondary battery of this embodiment, as a counter electrode with respect to the cathode, a known electrode, for example, a metal-based material such as metallic lithium and a lithium alloy, a carbon-based material such as graphite and mesocarbon microbeads (MCMB), and a silicon-based material such as Si, a Si alloy, and silicon oxide, which functions as an anode active material and is capable of intercalating and deintercalating lithium, can be employed.
Known battery elements can be employed as the separator, the battery container, and the like.
As the electrolyte, a known electrolytic solution, a known solid electrolyte, or the like can be employed. As the electrolytic solution, for example, a solution obtained by dissolving an electrolyte such as lithium perchlorate or lithium hexafluorophosphate in a solvent such as ethylene carbonate (EC), dimethyl carbonate (DMC), propylene carbonate (PC), or diethyl carbonate (DEC) can be used.
In addition, the all-solid-state lithium secondary battery can have a similar structure as in a known all-solid-state lithium secondary battery except that a cathode active material containing the above-described lithium transition metal composite oxide as a main component is used.
In the case of the all-solid-state lithium secondary battery, as the electrolyte, for example, solid electrolytes such as a polymer-based solid electrolyte such as a polyethylene oxide-based polymer compound and a polymer compound including at least one or more of a polyorganosiloxane chain and a polyoxyalkylene chain, a sulfide-based solid electrolyte, and an oxide-based solid electrolyte can be used.
For the cathode of the all-solid-state lithium secondary battery, for example, a cathode mixture containing a solid electrolyte in addition to the cathode active material, the conductive agent, and the binder can be carried to a cathode current collector such as aluminum, nickel, and stainless steel.
In the lithium secondary battery of this embodiment, since the cathode contains a cathode active material containing the above-described lithium transition metal composite oxide as a main component, high capacity is realized.
Next, examples of the invention will be described, but the invention is not limited to these examples.
Li2CO3 (manufactured by Kojundo Chemical Lab. Co., Ltd.), and Ni0.5Mn0.5(OH)2 and TiO2 (manufactured by Kojundo Chemical Lab. Co., Ltd.) were weighed so that a ratio of Li:Ni:Mn:Ti becomes 1.05:0.45:0.45:0.05 in terms of a molar ratio, and in consideration of Li evaporation, Li2CO3 was weighed to be rich by 2% by mass based on the stoichiometric ratio. The total mass of Li2CO3 (manufactured by Kojundo Chemical Lab. Co., Ltd.), and Ni0.5Mn0.5(OH)2 and TiO2 (manufactured by Kojundo Chemical Lab. Co., Ltd.) was set to 2.1 g. The materials were dispersed and mixed in ethanol by a mortar. Thereafter, the resulting mixture was filled in a JIS standard platinum crucible. The mixture filled in the platinum crucible in the air was heated at a temperature-rising rate of 15° C./min and fired at 1050° C. for 5 minutes by using a firing furnace to obtain a lithium-nickel-manganese-titanium oxide of Example 1.
A chemical composition of an obtained sample was analyzed by an ICP optical emission spectrometer (trade name: Agilent 5110 VDV, manufactured by Agilent Technologies), and the results are shown in Table 1. As shown in Table 1, it was confirmed that Li:Ni:Mn:Ti=1.05:0.44:0.46:0.05.
In addition, an X-ray diffraction pattern of the obtained sample was measured with a powder X-ray diffractometer (trade name: SmartLab, manufactured by Rigaku Corporation), and a lattice constant was determined by the least square method using each index and plane spacing. When a space group of the obtained sample was set to R-3m, and the lattice constant was determined, a was 2.88758 (7) Å, c was 14.3078 (5) Å, and c/a was 4.9549. The powder X-ray diffraction pattern is shown in
In addition, the results obtained by analyzing the composition of the surface layer of the obtained sample by quantitative analysis with an X-ray photoelectron spectroscopy (XPS) analyzer (trade name: K-Alpha+, manufactured by Thermo Fisher Scientific) are shown in Table 1. As shown in Table 1, a Mn/Ni ratio in the surface layer was 1.20.
Li2CO3 (manufactured by Kojundo Chemical Lab. Co., Ltd.), and Ni0.5Mn0.5(OH)2 and TiO2 (manufactured by Kojundo Chemical Lab. Co., Ltd.) were weighed so that a ratio of Li:Ni:Mn:Ti becomes 1.05:0.45:0.45:0.05 in terms of a molar ratio, and in consideration of Li evaporation, Li2CO3 was weighed to be rich by 2% by mass based on the stoichiometric ratio. The total mass of Li2CO3 (manufactured by Kojundo Chemical Lab. Co., Ltd.), and Ni0.5Mn0.5(OH)2 and TiO2 (manufactured by Kojundo Chemical Lab. Co., Ltd.) was set to 2.1 g. The materials were dispersed and mixed in ethanol by a mortar. Thereafter, the resulting mixture was filled in a JIS standard platinum crucible. The mixture filled in the platinum crucible in the air was heated at a temperature-rising rate of 15° C./min and fired at 1050° C. for 5 minutes by using a firing furnace to obtain a lithium-nickel-manganese-titanium oxide of Example 1.
The powder after the heat treatment was cooled so as to reach 850° C. at a temperature-falling rate of 10° C./min, and then the powder was held at 850° C. for 6 hours in the air.
Thereafter, a lithium-nickel-manganese-titanium oxide of Example 2 was obtained by leaving the powder as is until the temperature of the powder reached room temperature.
The chemical composition of the obtained sample was analyzed in a similar manner as in Example 1. The results are shown in Table 1. As shown in Table 1, it was confirmed that Li:Ni:Mn:Ti was 1.09:0.44:0.42:0.05. In the chemical composition, Mn is reduced as compared with Example 1 because Mn moves to the surface layer of the sample obtained by the heat treatment and further reaches the platinum crucible. The reason why Li is increased as compared with Example 1 is because calculation was performed on the assumption that the phase sum of the chemical composition was set to 2 and thus the decrease in Mn was allocated to Li. The decrease in Mn depends on the amount of a precursor filled in the crucible.
In addition, the space group of the obtained sample was set to R-3m, and it was confirmed that the phase is a single phase in which all peaks can be indexed. When lattice constants were further determined, a was 2.88675 (7) Å, c was 14.3067 (8) Å, and c/a was 4.9560. The powder X-ray diffraction pattern is shown in
In addition, the results obtained by analyzing the composition of the surface layer of the obtained sample are shown in Table 1. As shown in Table 1, the Mn/Ni ratio in the surface layer was 1.47.
The lithium-nickel-manganese-titanium oxide of Example 1 or the lithium-nickel-manganese-titanium oxide of Example 2 as a cathode active material, acetylene black (AB) as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder were blended in a weight ratio of 8:1:1 by using N-methyl-2 pyrrolidone (NMP) as a solvent to prepare slurry. Thereafter, aluminum foil having a thickness of 15 μm was coated with the slurry and was dried to prepare a cathode having a diameter of 14ϕ. A coating area density was set to 4.5 mg/cm2, and a volume density was set to 2.3 g/cm3. With respect to the cathode, a lithium metal having a thickness of 200 μm and a diameter of 16ϕ was used as a counter electrode, and a polyethylene microporous film having a thickness of 20 μm and a diameter of 18ϕ was used as a separator. An electrolytic solution was set to a 1.2 mol/L solution obtained by dissolving lithium hexafluorophosphate (LiPF6) in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio: 3:4:3), and a lithium secondary battery (2032 coin type cell) having the structure illustrated in
With regard to the prepared lithium secondary battery, a charge and discharge test was conducted at a constant current at a rate of 0.05 C, a current density of 12.5 mA/g, and a cutoff potential of 4.7 V to 2.5 V under a temperature condition of 55° C. to evaluate charge and discharge characteristics. The charge and discharge test was initiated from charging.
As illustrated in
Further, the obtained samples of Examples 1 and 2 were analyzed by Li-MAS-NMR (trade name: AVANCE 300, manufactured by Bruker Corporation). The results are shown in
Li2CO3 (manufactured by Kojundo Chemical Lab. Co., Ltd.), and Ni0.5Mn0.5(OH)2 and TiO2 (manufactured by Kojundo Chemical Lab. Co., Ltd.) were weighed so that a ratio of Li:Ni:Mn:Ti becomes 1.066:0.40:0.40:0.133 in terms of a molar ratio, and in consideration of Li evaporation, Li2CO3 was weighed to be rich by 2% by mass based on the stoichiometric ratio. The total mass of Li2CO3 (manufactured by Kojundo Chemical Lab. Co., Ltd.), and Ni0.5Mn0.5(OH)2 and TiO2 (manufactured by Kojundo Chemical Lab. Co., Ltd.) was set to 2.1 g. The materials were dispersed and mixed in ethanol by a mortar. Thereafter, the resulting mixture was filled in a JIS standard platinum crucible. The mixture filled in the platinum crucible in the air was heated at a temperature-rising rate of 15° C./min and fired at 1100° C. for 5 minutes by using a firing furnace to obtain a lithium-nickel-manganese-titanium oxide of Example 3. An electron micrograph of the obtained lithium-nickel-manganese-titanium oxide is shown in
Results obtained by analyzing the chemical composition of the obtained sample in a similar manner as in Example 1 are shown in Table 2. As shown in Table 2, it was confirmed that Li:Ni:Mn:Ti was 1.09:0.40:0.38:0.13. In the chemical composition, Mn is reduced as compared with a charged composition because Mn moves to the surface layer of the sample obtained by the heat treatment and further reaches the platinum crucible. The reason why Li is increased as compared with Example 1 is because calculation was performed on the assumption that the phase sum of the chemical composition was set to 2 and thus the decrease in Mn was allocated to Li. The decrease in Mn depends on the amount of a precursor filled in the crucible.
In addition, the space group of the obtained sample was set to R-3m, and it was confirmed that the phase is a single phase in which all peaks can be indexed. When lattice constants were further determined, a was 2.89293 (12) Å, c was 14.3323 (7) Å, and c/a was 4.9543. The powder X-ray diffraction pattern is shown in
In addition, the results obtained by analyzing the composition of the surface layer of the obtained sample are shown in Table 2. As shown in Table 2, the Mn/Ni ratio in the surface layer was 1.06.
Li2CO3 (manufactured by Kojundo Chemical Lab. Co., Ltd.), and Ni0.5Mn0.5(OH)2 and TiO2 (manufactured by Kojundo Chemical Lab. Co., Ltd.) were weighed so that a ratio of Li:Ni:Mn:Ti becomes 1.066:0.40:0.40:0.133 in terms of a molar ratio, and in consideration of Li evaporation, Li2CO3 was weighed to be rich by 2% by mass based on the stoichiometric ratio. The total mass of Li2CO3 (manufactured by Kojundo Chemical Lab. Co., Ltd.), and Ni0.5Mn0.5(OH)2 and TiO2 (manufactured by Kojundo Chemical Lab. Co., Ltd.) was set to 2.1 g. The materials were dispersed and mixed in ethanol by a mortar. Thereafter, the resulting mixture was filled in a JIS standard platinum crucible. The mixture filled in the platinum crucible in the air was heated at a temperature-rising rate of 15° C./min and fired at 1100° C. for 5 minutes by using a firing furnace to obtain a lithium-nickel-manganese-titanium oxide of Example 3.
The powder after the heat treatment was cooled so as to reach 900° C. at a temperature-falling rate of 10° C./min, and then the powder was held at 900° C. for 6 hours or shorter in the air.
Thereafter, a lithium-nickel-manganese-titanium oxide of Example 4 was obtained by leaving the powder as is until the temperature of the powder reached room temperature. An electron micrograph of the obtained lithium-nickel-manganese-titanium oxide is shown in
Results obtained by analyzing the chemical composition of the obtained sample in a similar manner as in Example 3 are shown in Table 2. As shown in Table 2, it was confirmed that Li:Ni:Mn:Ti was 1.09:0.40:0.38:0.13. In the chemical composition, Mn is reduced as compared with a charged composition because Mn moves to the surface layer of the sample obtained by the heat treatment and further reaches the platinum crucible. The reason why Li is increased as compared with Example 1 is because calculation was performed on the assumption that the phase sum of the chemical composition was set to 2 and thus the decrease in Mn was allocated to Li. The decrease in Mn depends on the amount of a precursor filled in the crucible.
In addition, the space group of the obtained sample was set to R-3m in a similar manner as in Example 3, and it was confirmed that the phase is a single phase in which all peaks can be indexed. When lattice constants were further determined, a was 2.89424 (8) Å, c was 14.3366 (6) Å, and c/a was 4.9535. An X-ray diffraction pattern is shown in
In addition, results obtained by analyzing the composition of the surface layer of the obtained sample in a similar manner as in Example 3 are shown in Table 2. As shown in Table 2, the Mn/Ni ratio in the surface layer was 1.08.
The lithium-nickel-manganese-titanium oxide of Example 3 or the lithium-nickel-manganese-titanium oxide of Example 4 as a cathode active material, acetylene black (AB) as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder were blended in a weight ratio of 8:1:1 by using N-methyl-2 pyrrolidone (NMP) as a solvent to prepare slurry. Thereafter, aluminum foil having a thickness of 15 μm was coated with the slurry and was dried to prepare a cathode having a diameter of 14ϕ. A coating area density was set to 4.5 mg/cm2, and a volume density was set to 2.3 g/cm3. With respect to the cathode, a lithium metal having a thickness of 200 μm and a diameter of 16ϕ was used as a counter electrode, and a polyethylene microporous film having a thickness of 20 μm and a diameter of 18ϕ was used as a separator. An electrolytic solution was set to a 1.2 mol/L solution obtained by dissolving lithium hexafluorophosphate (LiPF6) in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio: 3:4:3), and a lithium secondary battery (2032 coin type cell) having the structure illustrated in
With regard to the prepared lithium secondary battery, a charge and discharge test was conducted at a constant current at a rate of 0.05 C, a current density of 12.5 mA/g, and a cutoff potential of 4.7 V to 2.5 V under a temperature condition of 55° C. to evaluate charge and discharge characteristics. The charge and discharge test was initiated from charging.
As illustrated in
Further, the obtained samples were analyzed by Li-MAS-NMR (trade name: AVANCE 300, manufactured by Bruker Corporation). The results are shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 2023-044683 | Mar 2023 | JP | national |
