# Patent

## Title
Electrode material for lithium-ion secondary battery

## Abstract

An electrode material for a lithium-ion secondary battery of the present invention includes particles which are made of LiFexMn1-w-x-y-zMgyCazAwPO4 (here, A represents at least one element selected from the group consisting of Co, Ni, Zn, Al, and Ga, 0.05≦x≦0.35, 0.01≦y≦0.08, 0.0001≦z≦0.001, and 0≦w≦0.02) and have an orthorhombic crystal structure, a 0.1 CA capacity during constant-current charge in a range of 4.0 V to 4.3 V is 100 mAh/g or more, and a ratio (1 CA/0.1 CA) of a 1 CA capacity to the 0.1 CA capacity during the constant-current charge in the range of 4.0 V to 4.3 V is 0.60 or more.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
Priority is claimed on Japanese Patent Application No. 2016-065794, filed Mar. 29, 2016, the content of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to an electrode material for a lithium-ion secondary battery.
Description of Related Art
Cathode materials made of LiMnPO4 are materials which can be expected to have a higher battery reaction voltage and an energy density that is approximately 20% higher than cathode materials made of LiFePO4. Therefore, cathode materials made of LiMnPO4 are expected to be used in secondary batteries for electric vehicles.
However, lithium-ion secondary batteries including a cathode which includes the cathode material made of LiMnPO4 have the following problems. (1) The low electron conductivity of bulk LiMnPO4, (2) the low Li diffusivity of bulk LiMnPO4, and (3) the anisotropic and large volume change of LiMnPO4 crystals which is caused by battery reactions attributed to the Jahn-Teller effect of manganese ions (Mn2+). Due to these problems, in lithium-ion secondary batteries, the activation energy for intercalating and deintercalating lithium ions into and from cathodes becomes high. As a result, in the lithium-ion secondary batteries, sufficient battery characteristics cannot be obtained, and battery characteristics at a low temperature significantly degrade.
In order to improve the battery characteristics of lithium-ion secondary batteries at a low temperature, active studies are underway regarding LiFexMn1-xPO4 (0<x<1) in which some of Mn in LiMnPO4 is substituted with Fe (for example, refer to Japanese Laid-open Patent Publication No. 2013-101883). In LiFexMn1-xPO4, Fe forms a solid solution, and thus electron conductivity among particles improves more than LiMnPO4. As a result, in lithium-ion secondary batteries including a cathode which includes a cathode material made of LiFexMn1-xPO4, high-speed charge and discharge performance improves.
SUMMARY OF THE INVENTION
However, there have been no reports regarding any examples in which a cathode material capable of realizing lithium-ion secondary batteries having excellent high-speed charging characteristics can be obtained using the method described in Japanese Laid-open Patent Publication No. 2013-101883. In order to realize lithium-ion secondary batteries having excellent battery characteristics at a low temperature, it is considered that 50% or more of Mn in LiMnPO4 needs to be substituted with Fe. In a lithium-ion secondary battery for which a cathode material made of LiFexMn1-xPO4 having a large substitution amount of Fe is used (hereinafter, referred to as “lithium-ion secondary battery A”), the charge and discharge capacity increases more than in a lithium-ion secondary battery for which a cathode material made of LiMnPO4 is used (hereinafter, referred to as “lithium-ion secondary battery B”). However, in the lithium-ion secondary battery A, the proportion of a battery reaction at a high voltage which is derived from LiMnPO4 decreases, and a battery reaction derived from LiFePO4 increases. Therefore, in the lithium-ion secondary battery A, an effect of improving the energy density which is expected from LiMnPO4 included in the cathode cannot be obtained. Meanwhile, the lithium-ion secondary battery B or a lithium-ion secondary battery for which a cathode material made of LiFexMn1-xPO4 having a small substitution amount of Fe is used has the following problems due to the low electron conductivity of the bulk LiMnPO4, the low Li diffusivity of the bulk LiMnPO4, and the Jahn-Teller effect of Mn2+ which have been described above. That is, the lithium-ion secondary battery has a problem in that a favorable discharge capacity or a favorable mass energy density cannot be obtained, particularly, at a low temperature or during high-speed charge and discharge.
The present invention has been made in consideration of the above-described circumstances, and an object of the present invention is to provide an electrode material for a lithium-ion secondary battery having a high mass energy density during high-speed charge and discharge.
The present inventors and the like carried out intensive studies in order to solve the above-described problems, consequently, found that, in an electrode material for a lithium-ion secondary battery including particles which are made of LiFexMn1-w-x-y-zMgyCazAwPO4, when the 0.1 CA capacity during constant-current charge in a range of 4.0 V to 4.3 V is set to 100 mAh/g or more, and the ratio (1 CA/0.1 CA) of the 1 CA capacity to the 0.1 CA capacity during the constant-current charge in the range of 4.0 V to 4.3 V is set to 0.60 or more, the following facts happen, and completed the present invention. That is, it was found that it becomes possible to suppress the expansion and contraction of the lattice volume caused by a decrease in the crystal lattice volume of LiFexMn1-w-x-y-zMgyCazAwPO4 and a change in the valence of a transition metal element during charge and discharge and increase the Li deintercalation reaction surface, and it is possible to significantly improve high-speed charge and discharge characteristics without impairing the high material energy density of LiFeaMn1-aPO4 more than necessary.
An electrode material for a lithium-ion secondary battery of the present invention includes particles which are made of LiFexMn1-w-x-y-zMgyCazAwPO4 (here, A represents at least one element selected from the group consisting of Co, Ni, Zn, Al, and Ga, 0.05≦x≦0.35, 0.01≦y≦0.08, 0.0001≦z≦0.001, and 0≦w≦0.02) and have an orthorhombic crystal structure, in which a 0.1 CA capacity during constant-current charge in a range of 4.0 V to 4.3 V is 100 mAh/g or more, and a ratio (1 CA/0.1 CA) of a 1 CA capacity to the 0.1 CA capacity during the constant-current charge in the range of 4.0 V to 4.3 V is 0.60 or more.
According to the electrode material for a lithium-ion secondary battery of the present invention, it is possible to realize a lithium-ion secondary battery having a high mass energy density during high-speed charge and discharge.


DETAILED DESCRIPTION OF THE INVENTION
An embodiment of the electrode material for a lithium-ion secondary battery of the present invention will be described.
Meanwhile, the present embodiment is a specific description for easier understanding of the gist of the present invention and, unless particularly otherwise described, does not limit the present invention.
Electrode material for lithium-ion secondary battery An electrode material for a lithium-ion secondary battery of the present embodiment includes particles which are made of LiFexMn1-w-x-y-zMgyCazAwPO4 (here, A represents at least one element selected from the group consisting of Co, Ni, Zn, Al, and Ga, 0.05≦x≦0.35, 0.01≦y≦0.08, 0.0001≦z≦0.001, and 0≦w≦0.02) and have an orthorhombic crystal structure. In addition, in the electrode material for a lithium-ion secondary battery of the present embodiment, the 0.1 CA capacity during constant-current charge in a range of 4.0 V to 4.3 V is 100 mAh/g or more, and the ratio (1 CA/0.1 CA) of the 1 CA capacity to the 0.1 CA capacity during the constant-current charge in the range of 4.0 V to 4.3 V is 0.60 or more.
The electrode material for a lithium-ion secondary battery of the present embodiment is mainly used as a cathode material for a lithium-ion secondary battery.
In the electrode material for a lithium-ion secondary battery of the present embodiment, the surfaces of primary particles of an electrode active material made of LiFexMn1-w-x-y-zMgyCazAwPO4 are preferably coated with a carbonaceous film.
The average primary particle diameter of the primary particles of the electrode active material made of LiFexMn1-w-x-y-zMgyCazAwPO4 is preferably in a range of 40 nm to 500 nm and more preferably in a range of 70 nm to 400 nm.
Here, the reason for setting the average primary particle diameter of LiFexMn1-w-x-y-zMgyCazAwPO4 particles in the above-described range is as described below. When the average primary particle diameter of LiFexMn1-w-x-y-zMgyCazAwPO4 particles is 40 nm or more, it is possible to favorably hold crystallinity while preventing the primary particles from becoming excessively small. As a result, it is possible to obtain LiFexMn1-w-x-y-zMgyCazAwPO4 particles in which the length of a crystal lattice of the LiFexMn1-xMgyAzPO4 particle in a b axis direction is distinctively shortened while maintaining the lengths in an a axis direction and in a c axis direction being large. On the other hand, when the average primary particle diameter of the LiFexMn1-w-x-y-zMgyCazAwPO4 particles is 500 nm or less, the LiFexMn1-w-x-y-zMgyCazAwPO4 particles are sufficiently miniaturized, and consequently, extremely fine particles having favorable crystallinity can be obtained.
The thickness of the carbonaceous film is preferably in a range of 1 nm to 5 nm.
The reason for setting the thickness of the carbonaceous film in the above-described range is as described below. When the thickness of the carbonaceous film is 1 nm or more, the thickness of the carbonaceous film is sufficient, and thus it is possible to form a film having a desired resistance value. As a result, conductivity does not decrease, and it is possible to ensure conductivity suitable for electrode materials. On the other hand, when the thickness of the carbonaceous film is 5 nm or less, battery activity, for example, the battery capacity per unit mass of the electrode material does not decrease.
The average primary particle diameter of the primary particles of the electrode active material made of LiFexMn1-w-x-y-zMgyCazAwPO4 coated with the carbonaceous film is preferably in a range of 60 nm to 550 nm and more preferably in a range of 70 nm to 430 nm.
Here, the reason for setting the average primary particle diameter of the primary particles of the electrode active material made of LiFexMn1-w-x-y-zMgyCazAwPO4 coated with the carbonaceous film in the above-described range is as described below. When the average primary particle diameter is 60 nm or more, there are no cases in which an increase in the specific surface area of carbonaceous electrode active material composite particles increases the mass of required carbon, and thus the charge and discharge capacity is also not decreased. Furthermore, since carbon coating is easy, it is possible to obtain primary particles having a sufficient coating ratio, and a favorable mass energy density can be obtained particularly at a low temperature or during high-speed charge and discharge. On the other hand, when the average primary particle diameter is 550 nm or less, lithium ions or electrons smoothly migrate among the carbonaceous electrode active material composite particles, and thus the internal resistance does not increase, and the output characteristics also do not deteriorate.
The shape of the primary particle of the electrode active material made of LiFexMn1-w-x-y-zMgyCazAwPO4 coated with the carbonaceous film is not particularly limited, but is preferably a spherical shape since it is easy to generate an electrode material made of spherical particles, particularly, truly spherical particles.
Here, the reason for the shape of the primary particle of the electrode active material being preferably a spherical shape is as described below. It is possible to decrease the amount of a solvent when an electrode material paste for a lithium-ion secondary battery is prepared by mixing the primary particles of the electrode active material coated with the carbonaceous film, a binding agent, and a solvent. Furthermore, it also becomes easy to apply the electrode material paste for a lithium-ion secondary battery to a current collector. In addition, when the shape of the primary particle of the electrode active material is a spherical shape, the surface area of the primary particles of the electrode active material is minimized, furthermore, it is possible to minimize the mixing amount of the binding agent added to the electrode material paste for a lithium-ion secondary battery, and the internal resistance of the obtained electrode can be decreased.
Furthermore, when the shape of the primary particle of the electrode active material is a spherical shape, particularly, a truly spherical shape, it becomes easy to closely pack the primary particles. Therefore, the amount of the electrode material for a lithium-ion secondary battery packed per unit volume increases, and consequently, it is possible to increase the electrode density, and the capacity of the lithium-ion secondary battery can be increased, which is preferable.
The amount of carbon included in the electrode material for a lithium-ion secondary battery of the present embodiment is preferably in a range of 0.5% by mass to 3.5% by mass and more preferably in a range of 0.7% by mass to 2.5% by mass.
Here, the reason for limiting the amount of carbon included in the electrode material for a lithium-ion secondary battery of the present embodiment in the above-described range is as described below. When the amount of carbon is 0.5% by mass or more, the discharge capacity at a high charge-discharge rate is increased in a case in which a battery is formed, and it is possible to realize sufficient charge and discharge rate performance. On the other hand, when the amount of carbon included in the electrode material for a lithium-ion secondary battery is 3.5% by mass or less, the amount of carbon does not become too large, and the battery capacity of a lithium-ion secondary battery per unit mass of the primary particles of the electrode active material is not decreased.
In addition, the ratio of the carbon supporting amount to the specific surface area of the primary particles of the electrode active material ([the carbon supporting amount]/[the specific surface area of the primary particles of the electrode active material]) is preferably in a range of 0.75 to 1.15 and more preferably in a range of 0.8 to 1.1.
Here, the reason for limiting the carbon supporting amount in the electrode material for a lithium-ion secondary battery of the present embodiment in the above-described range is as described below. When the carbon supporting amount is 0.75 or more, the discharge capacity at a high charge-discharge rate is increased in a case in which a battery is formed, and it is possible to realize sufficient charge and discharge rate performance. On the other hand, when the carbon supporting amount is 1.15 or less, the amount of carbon does not become too large, and the battery capacity of a lithium-ion secondary battery per unit mass of the primary particles of the electrode active material is not decreased.
Electrode Active Material
The electrode active material is made of LiFexMn1-w-x-y-zMgyCazAwPO4 having a crystal structure preferable for Li diffusion (here, A represents at least one element selected from the group consisting of Co, Ni, Zn, Al, and Ga, 0.05≦x≦0.35, 0.01≦y≦0.08, 0.0001≦z≦0.001, and 0<w≦0.02).
In LiFexMn1-w-x-y-zMgyCazAwPO4, the reason for x satisfying 0.05≦x≦0.35 is as described below. Since Fe develops a charge and discharge capacity at a voltage of near 3.5 V, a decrease in the energy density caused by formation of a solid solution is milder than that of Co or Zn, and thus a relatively large amount of Fe has been set to be available for formation of a solid solution so that an improvement of low-temperature characteristics is expected while the energy density is not excessively decreased. Furthermore, Fe is a carbonization catalyst element and, when forming a Fe solid solution, improves the coating properties of the carbonaceous film and thus enables an improvement of output characteristics or low-temperature characteristics. Therefore, the amount of a solid solution at which a sufficient carbonization catalyst action can be developed has been set as a solid solution range. Particularly, 0.05≦x≦0.25 is preferred.
The reason for setting y to satisfy 0.01≦y≦0.08 is as described below. Mg is an electrochemical inert element in a voltage range of 1.0 V to 4.3 V, has a strong effect of improving electron conductivity, Li diffusivity, and the activation energy for an intercalation and deintercalation reaction of a lithium ion (Li+), and has a strong effect of improving the energy density. However, when a large amount of Mg forms a solid solution, the charge and discharge capacity and the energy density are significantly decreased, and thus a relatively small amount of Mg at which low-temperature characteristics can be sufficiently improved while the energy density is not excessively decreased has been set to be available for formation of a solid solution.
The reason for setting z to satisfy 0.0001≦z≦0.001 is as described below.
Ca is an electrochemical inert element in a voltage range of 1.0 V to 4.3 V and is a useful element capable of producing an effect of improving low-temperature characteristics when added in a small amount. However, when a large amount of Ca forms a solid solution, the charge and discharge capacity and the energy density are significantly decreased, and, in a case in which a large amount of Ca is added, Ca is present in a crystal not as a solid solution but as an impurity, and, when dissolved as an impurity during the operation of a battery, the service life characteristics deteriorate. Therefore, a relatively small amount of Ca at which low-temperature characteristics can be sufficiently improved while the energy density is not excessively decreased has been set to be available for formation of a solid solution.
The reason for setting w to satisfy 0≦w≦0.02 is as described below. Co or Zn is an electrochemical inert element in a voltage range of 1.0 V to 4.3 V, has an effect of improving electron conductivity, Li diffusivity, and the activation energy for an intercalation and deintercalation reaction of a lithium ion (Li+), and has an effect of improving the energy density, and thus Co or Zn can be appropriately added. However, when a large amount of Co or Zn forms a solid solution, the charge and discharge capacity and the energy density are significantly decreased, and thus a relatively small amount of Mg at which low-temperature characteristics can be sufficiently improved while the energy density is not excessively decreased has been set to be available for formation of a solid solution.
LiFexMn1-w-x-y-zMgyCazAwPO4 has an orthorhombic crystal structure.
Examples of the particles of made of LiFexMn1-w-x-y-zMgyCazAwPO4 in the present embodiment include LiFe0.2988Mn0.70Mg0.01Ca0.0002PO4, LiFe0.2967Mn0.70Mg0.03Ca0.0003PO4, LiFe0.2968Mn0.70Mg0.03Ca0.0002PO4, LiFe0.2967Mn0.70Mg0.03Ca0.0003PO4, LiFe0.2947Mn0.70Mg0.05Ca0.0003PO4, LiFe0.2926Mn0.70Mg0.007Ca0.0004PO4, LiFe0.2968Mn0.70Mg0.025Co0.005Ca0.0002PO4, LiFe0.2947Mn0.70Mg0.045Co0.005Ca0.0003PO4, LiFe0.1968Mn0.80Mg0.03Ca0.0002PO4, LiFe0.2897Mn0.70Mg0.05Co0.005Ca0.0003PO4, LiFe0.2957Mn0.70Mg0.04Ca0.0003PO4, LiFe0.2888Mn0.70Mg0.02Ca0.0002PO4, LiFe0.2965Mn0.70Mg0.03Co0.003Ca0.0002PO4, and the like. Among these, LiFe0.2947Mn0.70Mg0.045Co0.005Ca0.0003PO4 is preferred since the 0.1 CA capacity during constant-current charge in a range of 4.0 V to 4.3 V is great, and the ratio (1 CA/0.1 CA) of the 1 CA capacity to the 0.1 CA capacity during the constant-current charge in the range of 4.0 V to 4.3 V becomes great.
The specific surface area of the electrode material for a lithium-ion secondary battery of the present embodiment is preferably in a range of 10 m2/g to 25 m2/g, more preferably in a range of 12 m2/g to 24 m2/g, and still more preferably in a range of 13 m2/g to 22 m2/g.
Here, the reason for limiting the specific surface area of the electrode material for a lithium-ion secondary battery of the present embodiment in the above-described range is as described below. When the specific surface area is 10 m2/g or more, migration of lithium ions or electrons among the carbonaceous electrode active material composite particles does not take time, and thus the internal resistance does not increase, and the output characteristics also do not deteriorate. On the other hand, when the specific surface area is 25 m2/g or less, there are no cases in which an increase in the specific surface area of the carbonaceous electrode active material composite particles increases the mass of required carbon, and thus the charge and discharge capacity is not decreased either.
In the electrode material for a lithium-ion secondary battery of the present embodiment, the 0.1 CA capacity during constant-current charge in a range of 4.0 V to 4.3 V is 100 mAh/g or more, and the ratio (1 CA/0.1 CA) of the 1 CA capacity to the 0.1 CA capacity during the constant-current charge in the range of 4.0 V to 4.3 V is 0.60 or more.
Meanwhile, the 0.1 CA capacity (mAh/g) of the particles made of LiFexMn1-w-x-y-zMgyCazAwPO4 during constant-current charge in a range of 4.0 V to 4.3 V is measured by performing constant-current charge at a current value of 0.1 CA until the voltage of the cathode reaches 4.3 V relative to the equilibrium voltage of Li at an environment temperature of 25° C. and extracting the data of the capacity in a range of 4.0 to 4.3 V.
Here, in the electrode material for a lithium-ion secondary battery of the present embodiment, the reason for limiting the 0.1 CA capacity during constant-current charge in a range of 4.0 V to 4.3 V in the above-described range is as described below. When the 0.1 CA capacity during constant-current charge in a range of 4.0 V to 4.3 V is less than 100 mAh/g, the mass energy density decreases during high-speed charge and discharge.
In addition, in the electrode material for a lithium-ion secondary battery of the present embodiment, the reason for limiting the ratio (1 CA/0.1 CA) of the 1 CA capacity to the 0.1 CA capacity during the constant-current charge in the range of 4.0 V to 4.3 V in the above-described range is as described below. When the ratio (1 CA/0.1 CA) of the 1 CA capacity to the 0.1 CA capacity during the constant-current charge in the range of 4.0 V to 4.3 V is less than 0.60, the mass energy density decreases during high-speed charge and discharge.
Examples of the particles of made of LiFexMn1-w-x-y-zMgyCazAwPO4 having the above-described performance (properties) include LiFe0.2988Mn0.70Mg0.01Ca0.0002PO4, LiFe0.2967Mn0.70Mg0.03Ca0.0003PO4, LiFe0.2968Mn0.70Mg0.03Ca0.0002PO4, LiFe0.2967Mn0.70Mg0.003Ca0.0003PO4, LiFe0.2947Mn0.70Mg0.05Ca0.0003PO4, LiFe0.2926Mn0.70Mg0.07Ca0.0004PO4, LiFe0.2968Mn0.70Mg0.025Co0.005Ca0.0002PO4, LiFe0.2947Mn0.70Mg0.045Co0.0005Ca0.0003PO4, LiFe0.1968Mn0.80Mg0.03Ca0.0002PO4, and the like. Among these, LiFe0.2947Mn0.70Mg0.045Co0.005Ca0.0003PO4 is preferred since the 0.1 CA capacity during constant-current charge in a range of 4.0 V to 4.3 V is great, and the ratio (1 CA/0.1 CA) of the 1 CA capacity to the 0.1 CA capacity during the constant-current charge in the range of 4.0 V to 4.3 V becomes great.
According to the electrode material for a lithium-ion secondary battery of the present embodiment, it is possible to realize lithium-ion secondary batteries having a high mass energy density during high-speed charge and discharge.
Method for Manufacturing Electrode Material for Lithium-Ion Secondary Battery
A method for manufacturing an electrode material for a lithium-ion secondary battery of the present embodiment is not particularly limited, and examples thereof include a method including a step of synthesizing LiFexMn1-w-x-y-zMgyCazAwPO4 particles under pressure by heating a raw material slurry α obtained by mixing a Li source, a Fe source, a Mn source, a Mg source, a Ca source, a P source, and an A source with a solvent including water as a main component at a temperature in a range of 150° C. to 250° C. and a step of coating the surfaces of the LiFexMn1-w-x-y-zMgyCazAwPO4 particles (primary particles) with a carbonaceous film by drying a raw material slurry β obtained by dispersing the LiFexMn1-w-x-y-zMgyCazAwPO4 particles in a water solvent including a carbon source so as to granulate the slurry and then heating the slurry at a temperature in a range of 500° C. to 860° C.
A method for synthesizing the LiFexMn1-w-x-y-zMgyCazAwPO4 particles is not particularly limited and, for example, a Li source, a Fe source, a Mn source, a Mg source, a Ca source, a P source, and an A (at least one element selected from the group consisting of Co, Ni, Zn, Al, and Ga) source are injected into a solvent including water as a main component and stirred, thereby preparing the raw material slurry α including a raw material of LiFexMn1-w-x-y-zMgyCazAwPO4 particles.
The Li source, the Fe source, the Mn source, the Mg source, the Ca source, the P source, and the A source are injected into the solvent including water as a main component so that the molar ratio therebetween (Li source:Fe source:Mn source:Mg source:Ca source:P source:A source), that is, the molar ratio of Li:Fe:Mn:Mg:Ca:P:A reaches 2 to 3.5:0.05 to 0.35:0.94 to 0.55:0.01 to 0.10:0.00001 to 0.001:0.95 to 1.10:0 to 0.05, the sources are stirred and mixed together, thereby preparing the raw material slurry α.
In order to uniformly mix the Li source, the Fe source, the Mn source, the Mg source, the Ca source, the P source, and the A source together, it is preferable to respectively put the Li source, the Fe source, the Mn source, the Mg source, the Ca source, the P source, and the A source into an aqueous solution state temporarily and then mix the sources together.
The molar concentration of the Li source, the Fe source, the Mn source, the Mg source, the Ca source, the P source, and the A source in the raw material slurry α is preferably in a range of 0.8 mol/L to 3.5 mol/L since it is necessary to obtain highly pure, highly crystalline, and extremely fine LiFexMn1-w-x-y-zMgyCazAwPO4 particles.
Examples of the Li source include hydroxides such as lithium hydroxide (LiOH), inorganic lithium acid salts such as lithium carbonate (Li2CO3), lithium chloride (LiCl), lithium nitrate (LiNO3), lithium phosphate (Li3PO4), lithium hydrogen phosphate (Li2HPO4), and lithium dihydrogen phosphate (LiH2PO4), organic lithium acid salts such as lithium acetate (LiCH3COO) and lithium oxalate ((COOLi)2), and hydrates thereof. As the Li source, at least one compound selected from the above-described group is preferably used.
Meanwhile, lithium phosphate (Li3PO4) can also be used as the Li source and the P source.
As the Fe source, iron compounds such as iron (II) chloride (FeCl2), iron (II) sulfate (FeSO4), and iron (II) acetate (Fe(CH3COO)2) or hydrates thereof, trivalent iron compounds such as iron (III) nitrate (Fe(NO3)3), iron (III) chloride (FeCl3), and iron (III) citrate (FeC6H5O7), lithium iron phosphate, or the like can be used.
The Mn source is preferably a Mn salt, and examples thereof include manganese (II) chloride (MnCl2), manganese (II) sulfate (MnSO4), manganese (II) nitrate (Mn(NO3)2), manganese (II) acetate (Mn(CH3COO)2), and hydrates thereof. As the Mn source, at least one compound selected from the above-described group is preferably used.
A Mg source is preferably a Mg salt, and examples thereof include magnesium (II) chloride (MgCl2), magnesium (II) sulfate (MgSO4), magnesium (II) nitrate (Mg(NO3)2), magnesium (II) acetate (Mg(CH3COO)2), and hydrates thereof. As the Mg source, at least one compound selected from the above-described group is preferably used.
The Ca source is preferably a Ca salt, and examples thereof include calcium (II) hydroxide (Ca(OH)2), calcium (II) chloride (CaCl2), calcium (II) sulfate (CaSO4), calcium (II) nitrate (Ca(NO3)2), calcium (II) acetate (Ca(CH3COO)2), and hydrates thereof. As the Ca source, at least one compound selected from the above-described group is preferably used.
As the P source, for example, at least one compound selected from phosphoric acids such as orthophosphonic acid (H3PO4) and metaphosphoric acid (HPO3), phosphoric salts such as ammonium dihydrogen phosphate (NH4H2PO4), diammonium phosphate ((NH4)2HPO4), ammonium phosphate ((NH4)3PO4), lithium phosphate (Li3PO4), lithium hydrogen phosphate (Li2HPO4), and lithium dihydrogen phosphate (LiH2PO4), and hydrates thereof is preferably used.
A Co source is preferably a Co salt, and examples thereof include cobalt (II) chloride (CoCl2), cobalt (II) sulfate (COSO4), cobalt (II) nitrate (Co(NO3)2), cobalt (II) acetate (Co(CH3COO)2), and hydrates thereof. As the Co source, at least one compound selected from the above-described group is preferably used.
A Ni source is preferably, for example, a Ni salt, and examples thereof include nickel (II) chloride (NiCl2), nickel (II) sulfate (NiSO4), nickel (II) nitrate (Ni(NO3)2), nickel (II) acetate (Ni(CH3COO)2), and hydrates thereof. As the Ni source, at least one compound selected from a group made up of the above-described compounds is preferably used.
A Zn source is preferably a Zn salt, and examples thereof include zinc (II) chloride (ZnCl2), zinc (II) sulfate (ZnSO4), zinc (II) nitrate (Zn(NO3)2), zinc (II) acetate (Zn(CH3COO)2), and hydrates thereof. As the Zn source, at least one compound selected from a group made up of the above-described compounds is preferably used.
Examples of an Al source include aluminum compounds such as chlorides, sulfoxides, nitroxides, acetoxides, and hydroxides, and at least one compound selected from a group made up of the above-described compounds is preferably used.
Examples of a Ga source include gallium compounds such as chlorides, sulfoxides, nitroxides, acetoxides, and hydroxides, and at least one compound selected from a group made up of the above-described compounds is preferably used.
The solvent including water as a main component is any one of water and water-based solvents which include water as a main component and include an aqueous solvent such as an alcohol as necessary.
The aqueous solvent is not particularly limited as long as the solvent is capable of dissolving the Li source, the Fe source, the Mn source, the Mg source, the P source, and the A source. Examples thereof include alcohols such as methanol, ethanol, 1-propanol, 2-propanol (isopropyl alcohol: IPA), butanol, pentanol, hexanol, octanol, and diacetone alcohol, esters such as ethyl acetate, butyl acetate, ethyl lactate, propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, and γ-butyrolactone, ethers such as diethyl ether, ethylene glycol monomethyl ether (methyl cellosolve), ethylene glycolmonoethylether (ethyl cellosolve), ethylene glycol monobutyl ether (butyl cellosolve), diethylene glycol monomethyl ether, and diethylene glycol monoethyl ether, ketones such as acetone, methyl ethyl ketone (MEK), methyl isobutylketone (MIBK), acetylacetone, andcyclohexanone, amides such as dimethyl formamide, N,N-dimethylacetoacetamide, and N-methyl pyrrolidone, glycols such as ethylene glycol, diethylene glycol, and propylene glycol, and the like. These aqueous solvents may be singly used or in a mixture form of two or more aqueous solvents.
Next, the raw material slurry α is put into a pressure-resistant container, is heated at a temperature in a range of 150° C. to 250° C., preferably, in a range of 165° C. to 215° C., and is hydrothermally treated for 0.5 hours to 60 hours, thereby obtaining LiFexMn1-w-x-y-zMgyCazAwPO4 particles.
When raw material slurry reaches the temperature in a range of 150° C. to 250° C., the pressure in the pressure-resistant container reaches, for example, 0.1 MPa to 2 MPa.
In this case, it is possible to control the particle diameters of the LiFexMn1-w-x-y-zMgyCazAwPO4 particles to be a desired size by adjusting the temperature and the duration during the hydrothermal treatment.
Next, the LiFexMn1-w-x-y-zMgyCazAwPO4 particles are dispersed in the water solvent including a carbon source, thereby preparing a raw material slurry β.
Next, the raw material slurry β is dried and granulated, then, is heated at a temperature in a range of 500° C. to 860° C. for 0.5 hours to 60 hours, and the surfaces of the LiFexMn1-w-x-y-zMgyCazAwPO4 particles (primary particles) with a carbonaceous film, thereby obtaining an electrode material for a lithium-ion secondary battery of the present embodiment.
The carbon source is not particularly limited, and for example, a natural water-soluble polymer such as gelatin, casein, collagen, hyaluronic acid, albumin, or starch, a semisynthetic polymer such as methyl cellulose, ethyl cellulose, methyl hydroxypropyl cellulose, carboxymethyl cellulose, hydroxymethyl cellulose, hydroxypropyl cellulose, carboxymethyl cellulose sodium, or propylene glycol alginade, a synthetic polymer such as polyvinyl alcohol, polyvinylpyrrolidone, a carbomer (carboxyvinyl polymer), polyacrylate, or polyethylene oxide, or the like can be used.
These carbon sources may be used singly or in a mixture form of two or more carbon sources.
In the method for manufacturing an electrode material for a lithium-ion secondary battery of the present embodiment, when the total mass of the electrode active material and the carbon source is set to 100% by mass, the additive amount (additive rate) of the carbon source is preferably in a range of 1% by mass to 15% by mass and more preferably in a range of 2% by mass to 12% by mass.
When the additive amount of the carbon source is 1% by mass or more, mixing stability in the electrode material for a lithium-ion secondary battery can be obtained. On the other hand, when the additive amount of the carbon source is 15% by mass or less, the content of a cathode active material is relatively great, and thus battery characteristics do not degrade.
According to the method for manufacturing an electrode material for a lithium-ion secondary battery of the present embodiment, it is possible to obtain a lithium-ion secondary battery having a high mass energy density during high-speed charge and discharge.
Electrode for Lithium-Ion Secondary Battery
An electrode for a lithium-ion secondary battery of the present embodiment includes an electrode current collector and an electrode mixture layer (electrode) formed on the electrode current collector, and the electrode mixture layer includes the electrode material for a lithium-ion secondary battery of the present embodiment.
That is, the electrode for a lithium-ion secondary battery of the present embodiment is obtained by forming the electrode mixture layer formed on one main surface of the electrode current collector using the electrode material for a lithium-ion secondary battery of the present embodiment.
The electrode for a lithium-ion secondary battery of the present embodiment is mainly used as a cathode for a lithium-ion secondary battery.
A method for manufacturing an electrode for a lithium-ion secondary battery of the present embodiment is not particularly limited as long as an electrode can be formed on one main surface of an electrode current collector using the electrode material for a lithium-ion secondary battery of the present embodiment.
Examples of the method for manufacturing an electrode for a lithium-ion secondary battery of the present embodiment include the following method.
First, an electrode material paste for a lithium-ion secondary battery is prepared by mixing the electrode material for a lithium-ion secondary battery of the present embodiment, a binding agent, a conductive auxiliary agent, and a solvent.
Binding Agent
The binding agent is not particularly limited as long as the binding agent can be used in a water system. As the binding agent, for example, at least one selected from a group of polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, vinyl acetate copolymers, styrene/butadiene-based latexes, acrylic latexes, acrylonitrile/butadiene-based latexes, fluorine-based latexes, silicon-based latexes is used.
When the total mass of the electrode material for a lithium-ion secondary battery of the present embodiment, the binding agent, and the conductive auxiliary agent is set to 100% by mass, the content ratio of the binding agent to the electrode material paste for a lithium-ion secondary battery is preferably in a range of 1 part by mass to 10 parts by mass and more preferably in a range of 2 parts by mass to 6 parts by mass.
Conductive Auxiliary Agent
The conductive auxiliary agent is not particularly limited, and, for example, at least one conductive auxiliary agent selected from a group of fibrous carbon such as acetylene black, Ketjen black, furnace black, vapor grown carbon fiber (VGCF), and carbon nanotube is used.
When the total mass of the electrode material for a lithium-ion secondary battery of the present embodiment, the binding agent, and the conductive auxiliary agent is set to 100% by mass, the content ratio of the conductive auxiliary agent to the electrode material paste for a lithium-ion secondary battery is preferably in a range of 1 part by mass to 15 parts by mass and more preferably in a range of 3 parts by mass to parts by mass.
Solvent
To the electrode material paste for a lithium-ion secondary battery including the electrode material for a lithium-ion secondary battery of the present embodiment, a solvent may be appropriately added in order to facilitate coating of an article to be coated such as a current collector.
A main solvent is water, but the electrode material paste may include a water-based solvent such as an alcohol, a glycol, or an ether as long as the characteristics of the electrode material for a lithium-ion secondary battery of the present embodiment are not lost.
When the total mass of the electrode material for a lithium-ion secondary battery of the present embodiment, the binding agent, the conductive auxiliary agent, and the solvent is set to 100% by mass, the content ratio of the solvent to the electrode material paste for a lithium-ion secondary battery is preferably in a range of 80 parts by mass to 300 parts by mass and more preferably in a range of 100 parts by mass to 250 parts by mass.
When the solvent is included in the above-described range, it is possible to obtain an electrode material paste for a lithium-ion secondary battery having excellent electrode formability and excellent battery characteristics.
A method for mixing the electrode material for a lithium-ion secondary battery of the present embodiment, the binding agent, the conductive auxiliary agent, and the solvent is not particularly limited as long as it is possible to uniformly mix the above-described components. Examples thereof include methods in which a kneader such as a ball mill, a sand mill, a planetary (sun-and-planet) mixer, a paint shaker, or a homogenizer is used.
Next, the electrode material paste for a lithium-ion secondary battery is applied to one main surface of the electrode current collector so as to form a coating, and the coating is dried and then pressed under pressure, whereby it is possible to obtain an electrode for a lithium-ion secondary battery including the electrode mixture layer formed on one main surface of the electrode current collector.
Lithium-Ion Secondary Battery
A lithium-ion secondary battery of the present embodiment includes the electrode for a lithium-ion secondary battery of the present embodiment (cathode), an anode, a separator, and an electrolytic solution.
In the lithium-ion secondary battery of the present embodiment, the anode, the electrolytic solution, the separator, and the like are not particularly limited.
As the anode, for example, an anode material such as metallic Li, a carbon material, a Li alloy, or Li4Ti5O12 is used.
In addition, a solid electrolyte may be used instead of the electrolytic solution and the separator.
The electrolytic solution can be produced by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) so that the volume ratio therebetween reaches 1:1 and dissolving lithium hexafluorophosphate (LiPF6) in the obtained solvent mixture so that the concentration thereof reaches, for example, 1 mol/dm3.
As the separator, it is possible to use, for example, porous propylene.
In the lithium-ion secondary battery of the present embodiment, since the electrode for a lithium-ion secondary battery of the present embodiment is used as the cathode, and thus the lithium-ion secondary battery has a high capacity and a high energy density.
EXAMPLES
Hereinafter, the present invention will be more specifically described using examples and comparative examples, but the present invention is not limited to the following examples.
Example 1
Synthesis of Electrode Material for Lithium-Ion Secondary Battery
LiFe0.2988Mn0.70Mg0.01Ca0.0002PO4 having a specific surface area of 13.9 m2/g was synthesized in the following manner.
Li3PO4 as a Li source and a P source, an aqueous solution of FeSO4 as a Fe source, an aqueous solution of MnSO4 as a Mn source, an aqueous solution of MgSO4 as a Mg source, and an aqueous solution of Ca(OH)2 as a Ca source were mixed together so that the molar ratio therebetween reached Li:Fe:Mn:Mg:Ca:P=3:0.2988:0.70:0.01:0.0002:1, thereby preparing 280 ml of a raw material slurry α.
Next, this raw material slurry α was put into a pressure resistant vessel.
After that, hydrothermal synthesis was carried out by performing a heating reaction on the raw material slurry α at 225° C. for 3.5 hours. The pH of the slurry after the hydrothermal synthesis was 5.41.
After the reaction, the atmosphere in the pressure resistant vessel was cooled to room temperature, thereby obtaining a cake-state precipitate of a reaction product.
This precipitate was sufficiently washed with distilled water a plurality of times, and the water content ratio thereof was maintained at 40% so as to prevent the precipitate from being dried, thereby producing a cake-form substance.
This cake-form substance was dried in a vacuum at 70° C. for two hours, raw material slurry β obtained by dispersing 2% by mass of polyvinyl alcohol adjusted in advance to be 10% by mass relative to 95% by mass of the obtained powder (particles) in a water solvent was dried and granulated, and then a thermal treatment was carried out at 735° C. for 7.5 hours. Therefore, the surfaces of the particles were coated with a carbonaceous film, and an electrode material for a lithium-ion secondary battery of Example 1 was obtained.
Example 2
Synthesis of Electrode Material for Lithium-Ion Secondary Battery
LiFe0.2967Mn0.70Mg0.003Ca0.0003PO4 having a specific surface area of 14.0 m2/g was synthesized in the following manner.
Li3PO4 as a Li source and a P source, an aqueous solution of FeSO4 as a Fe source, an aqueous solution of MnSO4 as a Mn source, an aqueous solution of MgSO4 as a Mg source, and an aqueous solution of Ca(OH)2 as a Ca source were mixed together so that the molar ratio therebetween reached Li:Fe:Mn:Mg:Ca:P=3:0.2967:0.70:0.03:0.0003:1, thereby preparing 250 ml of a raw material slurry α.
Next, this raw material slurry α was put into a pressure resistant vessel.
After that, hydrothermal synthesis was carried out by performing a heating reaction on the raw material slurry α at 225° C. for 3.5 hours. The pH of the slurry after the hydrothermal synthesis was 5.37.
Hereinafter, an electrode material for a lithium-ion secondary battery of Example 2 was obtained in the same manner as in Example 1.
Example 3
Synthesis of Electrode Material for Lithium-Ion Secondary Battery
LiFe0.2968Mn0.70Mg0.003Ca0.0002PO4 having a specific surface area of 19.5 m2/g was synthesized in the following manner.
Li3PO4 as a Li source and a P source, an aqueous solution of FeSO4 as a Fe source, an aqueous solution of MnSO4 as a Mn source, an aqueous solution of MgSO4 as a Mg source, and an aqueous solution of Ca(OH)2 as a Ca source were mixed together so that the molar ratio therebetween reached Li:Fe:Mn:Mg:Ca:P=3:0.2968:0.70:0.03:0.0002:1, and, furthermore, an aqueous solution of LiOH was added as a pH adjuster to the produced LiFe0.2968Mn0.70Mg0.03Ca0.0002PO4 so that the amount of LiOH added reached 1 mol %, thereby preparing 280 ml of a raw material slurry α.
Next, this raw material slurry α was put into a pressure resistant vessel.
After that, hydrothermal synthesis was carried out by performing a heating reaction on the raw material slurry α at 225° C. for 3.5 hours. The pH of the slurry after the hydrothermal synthesis was 5.81.
Hereinafter, an electrode material for a lithium-ion secondary battery of Example 3 was obtained in the same manner as in Example 1.
Example 4
Synthesis of Electrode Material for Lithium-Ion Secondary Battery
LiFe0.2967Mn0.70Mg0.003Ca0.0003PO4 having a specific surface area of 24.6 m2/g was synthesized in the following manner.
Li3PO4 as a Li source and a P source, an aqueous solution of FeSO4 as a Fe source, an aqueous solution of MnSO4 as a Mn source, an aqueous solution of MgSO4 as a Mg source, and an aqueous solution of Ca(OH)2 as a Ca source were mixed together so that the molar ratio therebetween reached Li:Fe:Mn:Mg:Ca:P=3:0.2967:0.70:0.03:0.0003:1, and, furthermore, an aqueous solution of LiOH was added as a pH adjuster to the produced LiFe0.2967Mn0.70Mg0.03Ca0.0003PO4 so that the amount of LiOH added reached 2 mol %, thereby preparing 280 ml of a raw material slurry α.
Next, this raw material slurry α was put into a pressure resistant vessel.
After that, hydrothermal synthesis was carried out by performing a heating reaction on the raw material slurry α at 225° C. for 3.5 hours. The pH of the slurry after the hydrothermal synthesis was 6.11.
Hereinafter, an electrode material for a lithium-ion secondary battery of Example 4 was obtained in the same manner as in Example 1.
Example 5
Synthesis of Electrode Material for Lithium-Ion Secondary Battery
LiFe0.2947Mn0.70Mg0.05Ca0.0003PO4 having a specific surface area of 13.9 m2/g was synthesized in the following manner.
Li3PO4 as a Li source and a P source, an aqueous solution of FeSO4 as a Fe source, an aqueous solution of MnSO4 as a Mn source, an aqueous solution of MgSO4 as a Mg source, and an aqueous solution of Ca(OH)2 as a Ca source were mixed together so that the molar ratio therebetween reached Li:Fe:Mn:Mg:Ca:P=3:0.2947:0.70:0.05:0.0003:1, thereby preparing 250 ml of a raw material slurry α.
Next, this raw material slurry α was put into a pressure resistant vessel.
After that, hydrothermal synthesis was carried out by performing a heating reaction on the raw material slurry α at 225° C. for 3.5 hours. The pH of the slurry after the hydrothermal synthesis was 5.52.
Hereinafter, an electrode material for a lithium-ion secondary battery of Example 5 was obtained in the same manner as in Example 1.
Example 6
Synthesis of Electrode Material for Lithium-Ion Secondary Battery
LiFe0.2926Mn0.70Mg0.007Ca0.0004PO4 having a specific surface area of 14.0 m2/g was synthesized in the following manner.
Li3PO4 as a Li source and a P source, an aqueous solution of FeSO4 as a Fe source, an aqueous solution of MnSO4 as a Mn source, an aqueous solution of MgSO4 as a Mg source, and an aqueous solution of Ca(OH)2 as a Ca source were mixed together so that the molar ratio therebetween reached Li:Fe:Mn:Mg:Ca:P=3:0.2926:0.70:0.07:0.0004:1, thereby preparing 250 ml of a raw material slurry α.
Next, this raw material slurry α was put into a pressure resistant vessel.
After that, hydrothermal synthesis was carried out by performing a heating reaction on the raw material slurry α at 225° C. for 3.5 hours. The pH of the slurry after the hydrothermal synthesis was 5.39.
Hereinafter, an electrode material for a lithium-ion secondary battery of Example 6 was obtained in the same manner as in Example 1.
Example 7
Synthesis of Electrode Material for Lithium-Ion Secondary Battery
LiFe0.2968Mn0.70Mg0.025Co0.005Ca0.0002PO4 having a specific surface area of 15.3 m2/g was synthesized in the following manner.
Li3PO4 as a Li source and a P source, an aqueous solution of FeSO4 as a Fe source, an aqueous solution of MnSO4 as a Mn source, an aqueous solution of MgSO4 as a Mg source, an aqueous solution of Ca(OH)2 as a Ca source, and an aqueous solution of CoSO4 as a Co source were mixed together so that the molar ratio therebetween reached Li:Fe:Mn:Mg:Co:Ca:P=3:0.2968:0.70:0.025:0.005:0.0002:1, thereby preparing 280 ml of a raw material slurry α.
Next, this raw material slurry α was put into a pressure resistant vessel.
After that, hydrothermal synthesis was carried out by performing a heating reaction on the raw material slurry α at 225° C. for 3.5 hours. The pH of the slurry after the hydrothermal synthesis was 5.46.
Hereinafter, an electrode material for a lithium-ion secondary battery of Example 7 was obtained in the same manner as in Example 1.
Example 8
Synthesis of Electrode Material for Lithium-Ion Secondary Battery
LiFe0.2947Mn0.70Mg0.045Co0.005Ca0.0003PO4 having a specific surface area of 13.4 m2/g was synthesized in the following manner.
Li3PO4 as a Li source and a P source, an aqueous solution of FeSO4 as a Fe source, an aqueous solution of MnSO4 as a Mn source, an aqueous solution of MgSO4 as a Mg source, an aqueous solution of Ca(OH)2 as a Ca source, and an aqueous solution of CoSO4 as a Co source were mixed together so that the molar ratio therebetween reached Li:Fe:Mn:Mg:Co:Ca:P=3:0.2947:0.70:0.045:0.005:0.0003:1, thereby preparing 280 ml of a raw material slurry α.
Next, this raw material slurry α was put into a pressure resistant vessel.
After that, hydrothermal synthesis was carried out by performing a heating reaction on the raw material slurry α at 225° C. for 3.5 hours. The pH of the slurry after the hydrothermal synthesis was 5.35.
Hereinafter, an electrode material for a lithium-ion secondary battery of Example 8 was obtained in the same manner as in Example 1.
Example 9
Synthesis of Electrode Material for Lithium-Ion Secondary Battery
LiFe0.1968Mn0.80Mg0.03Ca0.0002PO4 having a specific surface area of 14.1 m2/g was synthesized in the following manner.
Li3PO4 as a Li source and a P source, an aqueous solution of FeSO4 as a Fe source, an aqueous solution of MnSO4 as a Mn source, an aqueous solution of MgSO4 as a Mg source, and an aqueous solution of Ca(OH)2 as a Ca source were mixed together so that the molar ratio therebetween reached Li:Fe:Mn:Mg:Ca:P=3:0.1968:0.80:0.03:0.0002:1, thereby preparing 250 ml of a raw material slurry α.
Next, this raw material slurry α was put into a pressure resistant vessel.
After that, hydrothermal synthesis was carried out by performing a heating reaction on the raw material slurry α at 225° C. for 3.5 hours. The pH of the slurry after the hydrothermal synthesis was 5.48.
Hereinafter, an electrode material for a lithium-ion secondary battery of Example 9 was obtained in the same manner as in Example 1.
Comparative Example 1
Synthesis of Electrode Material for Lithium-Ion Secondary Battery
LiFe0.2998Mn0.70Ca0.0002PO4 having a specific surface area of 13.2 m2/g was synthesized in the following manner.
Li3PO4 as a Li source and a P source, an aqueous solution of FeSO4 as a Fe source, an aqueous solution of MnSO4 as a Mn source, and an aqueous solution of Ca(OH)2 as a Ca source were mixed together so that the molar ratio therebetween reached Li:Fe:Mn:Ca:P=3:0.2998:0.70:0.0002:1, thereby preparing 280 ml of a raw material slurry α.
Next, this raw material slurry α was put into a pressure resistant vessel.
After that, hydrothermal synthesis was carried out by performing a heating reaction on the raw material slurry α at 225° C. for 3.5 hours. The pH of the slurry after the hydrothermal synthesis was 5.44.
Hereinafter, an electrode material for a lithium-ion secondary battery of Comparative Example 1 was obtained in the same manner as in Example 1.
Comparative Example 2
Synthesis of Electrode Material for Lithium-Ion Secondary Battery
LiFe0.2996Mn0.70Ca0.0004PO4 having a specific surface area of 24.1 m2/g was synthesized in the following manner.
Li3PO4 as a Li source and a P source, an aqueous solution of FeSO4 as a Fe source, an aqueous solution of MnSO4 as a Mn source, and an aqueous solution of Ca(OH)2 as a Ca source were mixed together so that the molar ratio therebetween reached Li:Fe:Mn:Ca:P=3:0.2996:0.70:0.0004:1, and, furthermore, an aqueous solution of LiOH was added as a pH adjuster to the produced LiFe0.2996Mn0.70Ca0.0004PO4 so that the amount of LiOH added reached 2 mol %, thereby preparing 280 ml of a raw material slurry α.
Next, this raw material slurry α was put into a pressure resistant vessel.
After that, hydrothermal synthesis was carried out by performing a heating reaction on the raw material slurry α at 225° C. for 3.5 hours. The pH of the slurry after the hydrothermal synthesis was 6.14.
Hereinafter, an electrode material for a lithium-ion secondary battery of Comparative Example 2 was obtained in the same manner as in Example 1.
Comparative Example 3
Synthesis of Electrode Material for Lithium-Ion Secondary Battery
LiFe0.2969Mn0.70Mg0.003Ca0.0001PO4 having a specific surface area of 9.6 m2/g was synthesized in the following manner.
Li3PO4 as a Li source and a P source, an aqueous solution of FeSO4 as a Fe source, an aqueous solution of MnSO4 as a Mn source, an aqueous solution of MgSO4 as a Mg source, and an aqueous solution of Ca(OH)2 as a Ca source were mixed together so that the molar ratio therebetween reached Li:Fe:Mn:Mg:Ca:P=3:0.2969:0.70:0.03:0.0001:1, and, furthermore, an aqueous solution of LiOH was added as a pH adjuster to the produced LiFe0.2969Mn0.70Mg0.003Ca0.0001PO4 so that the amount of LiOH added reached 2 mol %, thereby preparing 280 ml of a raw material slurry α.
Next, this raw material slurry α was put into a pressure resistant vessel.
After that, hydrothermal synthesis was carried out by performing a heating reaction on the raw material slurry α at 225° C. for 3.5 hours. The pH of the slurry after the hydrothermal synthesis was 4.54.
Hereinafter, an electrode material for a lithium-ion secondary battery of Comparative Example 3 was obtained in the same manner as in Example 1.
Comparative Example 4
Synthesis of Electrode Material for Lithium-Ion Secondary Battery
LiFe0.2967Mn0.70Mg0.03Ca0.0003PO4 having a specific surface area of 27.3 m2/g was synthesized in the following manner.
Li3PO4 as a Li source and a P source, an aqueous solution of FeSO4 as a Fe source, an aqueous solution of MnSO4 as a Mn source, an aqueous solution of MgSO4 as a Mg source, and an aqueous solution of Ca(OH)2 as a Ca source were mixed together so that the molar ratio therebetween reached Li:Fe:Mn:Mg:Ca:P=3:0.2967:0.70:0.03:0.0003:1, and, furthermore, an aqueous solution of LiOH was added as a pH adjuster to the produced LiFe0.2967Mn0.70Mg0.03Ca0.0003PO4 so that the amount of LiOH added reached 3 mol %, thereby preparing 280 ml of a raw material slurry α.
Next, this raw material slurry α was put into a pressure resistant vessel.
After that, hydrothermal synthesis was carried out by performing a heating reaction on the raw material slurry α at 225° C. for 3.5 hours. The pH of the slurry after the hydrothermal synthesis was 6.76.
Hereinafter, an electrode material for a lithium-ion secondary battery of Comparative Example 4 was obtained in the same manner as in Example 1.
Comparative Example 5
Synthesis of Electrode Material for Lithium-Ion Secondary Battery
LiFe0.2908Mn0.70Mg0.09Ca0.0002PO4 having a specific surface area of 13.7 m2/g was synthesized in the following manner.
Li3PO4 as a Li source and a P source, an aqueous solution of FeSO4 as a Fe source, an aqueous solution of MnSO4 as a Mn source, an aqueous solution of MgSO4 as a Mg source, and an aqueous solution of Ca(OH)2 as a Ca source were mixed together so that the molar ratio therebetween reached Li:Fe:Mn:Mg:Ca:P=3:0.2908:0.70:0.09:0.0002:1, thereby preparing 280 ml of a raw material slurry α.
Next, this raw material slurry α was put into a pressure resistant vessel.
After that, hydrothermal synthesis was carried out by performing a heating reaction on the raw material slurry α at 225° C. for 3.5 hours. The pH of the slurry after the hydrothermal synthesis was 5.47.
Hereinafter, an electrode material for a lithium-ion secondary battery of Comparative Example 5 was obtained in the same manner as in Example 1.
Evaluation of Electrode Materials for Lithium-Ion Secondary Battery
(1) Amount of Carbon
The amount of carbon in the electrode material for a lithium-ion secondary battery of each of the examples and the comparative examples was measured using a carbon analyzer (trade name: EMIA-220V, manufactured by Horiba Ltd.). The results are shown in Table 1.
(2) Specific Surface Area
The specific surface area of the electrode material for a lithium-ion secondary battery of each of the examples and the comparative examples was measured using a specific surface area meter (trade name: BELSORP-mini, manufactured by MicrotracBEL Corp.) by means of a BET method in which nitrogen (N2) adsorption was used. The results are shown in Table 1.
Production of Lithium-Ion Secondary Battery
The electrode material for a lithium-ion secondary battery of each of the examples and the comparative examples, polyvinylidene fluoride (PVdF) as a binding agent, and acetylene black (AB) as a conductive auxiliary agent were added to N-methyl-2-pyrrolidone (NMP) which was a solvent so that the mass ratio therebetween in a paste reached electrode material:AB:PVdF=90:5:5, and the components were mixed together, thereby preparing an electrode material paste for a lithium-ion secondary battery.
Next, the electrode material paste for a lithium-ion secondary battery was applied to a surface of a 30 μm-thick aluminum foil (current collector) so as to form a coating, and the coating was dried, thereby forming an electrode mixture layer on the surface of the aluminum foil. After that, the electrode mixture layer was pressed under a predetermined pressure so as to obtain a predetermined density, thereby producing a cathode for a lithium-ion secondary battery of each of the examples and the comparative examples.
Next, a circular plate having a diameter of 16 mm was produced from the cathode for a lithium-ion secondary battery using a shaping machine by means of punching, was dried in a vacuum, and then a lithium-ion secondary battery of each of the examples and the comparative examples was produced using a stainless steel (SUS) 2016 coil cell in a dried argon atmosphere.
Metallic lithium was used as an anode, a porous polypropylene film was used as a separator, and a LiPF6 solution (1M) was used as an electrolytic solution. As the LiPF6 solution, a solution obtained by mixing ethylene carbonate and ethyl methyl carbonate so that the volume ratio therebetween reached 1:1 was used.
Evaluation of Lithium-Ion Secondary Batteries
(1) Battery Characteristics
The battery characteristics of the lithium-ion secondary batteries were evaluated. At an environmental temperature of 25° C., constant-current charge was performed at a current value of 0.1 CA until the voltage of the cathode reached 4.3 V relative to the equilibrium voltage of Li. Subsequently, after one-minute retention, constant-current discharge was performed at a current value of 0.1 CA until the voltage of the cathode reached 2.0 V relative to the equilibrium voltage of Li. After that, constant-current charge was performed at a current value of 1 CA until the voltage of the cathode reached 4.3 V relative to the equilibrium voltage of Li. For the charges at 0.1 CA and 1 CA, the data of the capacity in a range of 4.0 to 4.3 V was extracted, and the capacities at 1 CA and 0.1 CA were compared with each other, thereby evaluating the high-speed charge and discharge of Mn. The evaluation results are shown in Table 1.














TABLE 1









Syn-

Specific
Amount
Mn part charge



Example

thet-

surface
of C
capacity
1 C/0.1 C


Comparative
Composition
ic pH
pH
area
[% by
[mAh/g]
capacity















Example
[% by mass]
[−]
adjuster
[m2/g]
mass]
0.1 C
1 C
ratio


















Example 1
LiFe0.2988Mn0.70Mg0.01Ca0.0002PO4
5.41
None
13.9 
0.93
100.1
60.6
0.605


Example 2
LiFe0.2967Mn0.70Mg0.03Ca0.0003PO4
5.37
None
14.0
0.84
102.4
65.8
0.643


Example 3
LiFe0.2968Mn0.70Mg0.03Ca0.0002PO4
5.81
LiOH 
19.5
1.26
100.9
68.9
0.682





1 mol %







Example 4
LiFe0.2967Mn0.70Mg0.03Ca0.0003PO4
6.11
LiOH 
24.6
1.26
103.2
75.5
0.732





2 mol %







Example 5
LiFe0.2947Mn0.70Mg0.05Ca0.0003PO4
5.52
None
13.9
0.92
105.9
72.8
0.688


Example 6
LiFe0.2926Mn0.70Mg0.07Ca0.0004PO4
5.39
None
14.0
0.93
100.7
61.4
0.610


Example 7
LiFe0.2968Mn0.70Mg0.025Co0.005Ca0.0002PO4
5.46
None
15.3
1.08
105.6
72.2
0.684


Example 8
LiFe0.2947Mn0.70Mg0.045Co0.005Ca0.0003PO4
5.35
None
13.4
0.93
107.7
82.2
0.764


Example 9
LiFe0.1968Mn0.80Mg0.03Ca0.0002PO4
5.48
None
14.1
1.12
113.2
70.3
0.621


Comparative 
LiFe0.2998Mn0.70Ca0.0002PO4
5.44
None
13.2
0.97
88.9
46.3
0.521


Example 1










Comparative 
LiFe0.2996Mn0.70Ca0.0004PO4
6.14
LiOH 
24.1
1.10
96.3
57.4
0.596


Example 2


2 mol %







Comparative 
LiFe0.2969Mn0.70Mg0.03Ca0.0001PO4
4.54
None
9.6
0.92
100.5
51.3
0.510


Example 3










Comparative 
LiFe0.2967Mn0.70Mg0.03Ca0.0003PO4
6.76
LiOH 
27.3
1.26
104.2
56.7
0.544


Example 4


3 mol %







Comparative 
LiFe0.2908Mn0.70Mg0.09Ca0.0002PO4
5.47
None
13.7
1.02
96.6
59.9
0.620


Example 5









From the results of Table 1, it was found that, in the electrode materials for a lithium-ion secondary battery of Examples 1 to 9, the specific surface areas were in a range of 13.9 m2/g to 24.6 m2/g, the 0.1 CA capacity during constant-current charge in a range of 4.0 V to 4.3 V was 100.1 mAh/g or more, and the ratio (1 CA/0.1 CA) of the 1 CA capacity to the 0.1 CA capacity during the constant-current charge in the range of 4.0 V to 4.3 V was 0.605 or more, and thus the 1 CA capacity during constant-current charge in a range of 4.0 V to 4.3 V was 60.6 mAh/g or more.
In contrast, it was found that, in the electrode material for a lithium-ion secondary battery of Comparative Example 1, the 0.1 CA capacity during constant-current charge in a range of 4.0 V to 4.3 V was 88.9 mAh/g, and the ratio (1 CA/0.1 CA) of the 1 CA capacity to the 0.1 CA capacity during the constant-current charge in the range of 4.0 V to 4.3 V was 0.521, and thus the 1 CA capacity during constant-current charge in a range of 4.0 V to 4.3 V was 46.3 mAh/g.
It was found that, in the electrode material for a lithium-ion secondary battery of Comparative Example 2, the 0.1 CA capacity during constant-current charge in a range of 4.0 V to 4.3 V was 96.3 mAh/g, and the ratio (1 CA/0.1 CA) of the 1 CA capacity to the 0.1 CA capacity during the constant-current charge in the range of 4.0 V to 4.3 V was 0.596, and thus the 1 CA capacity during constant-current charge in a range of 4.0 V to 4.3 V was 57.4 mAh/g.
It was found that, in the electrode material for a lithium-ion secondary battery of Comparative Example 3, the specific surface area was 9.6 m2/g, and the ratio (1 CA/0.1 CA) of the 1 CA capacity to the 0.1 CA capacity during the constant-current charge in the range of 4.0 V to 4.3 V was 0.510, and thus the 1 CA capacity during constant-current charge in a range of 4.0 V to 4.3 V was 51.3 mAh/g.
It was found that, in the electrode material for a lithium-ion secondary battery of Comparative Example 4, the specific surface area was 27.3 m2/g, and the ratio (1 CA/0.1 CA) of the 1 CA capacity to the 0.1 CA capacity during the constant-current charge in the range of 4.0 V to 4.3 V was 0.544, and thus the 1 CA capacity during constant-current charge in a range of 4.0 V to 4.3 V was 56.7 mAh/g.
It was found that, in the electrode material for a lithium-ion secondary battery of Comparative Example 5, the 0.1 CA capacity during the constant-current charge in the range of 4.0 V to 4.3 V was 96.6 mAh/g, and thus the 1 CA capacity during constant-current charge in a range of 4.0 V to 4.3 V was 59.9 mAh/g.
Since the electrode material for a lithium-ion secondary battery of the present invention includes particles which are made of LiFexMn1-w-x-y-zMgyCazAwPO4 (here, A represents at least one element selected from the group consisting of Co, Ni, Zn, Al, and Ga, 0.05≦x≦0.35, 0.01≦y≦0.08, 0.0001≦z≦0.001, and 0≦w≦0.02) and have an orthorhombic crystal structure, the 0.1 CA capacity during constant-current charge in a range of 4.0 V to 4.3 V is 100 mAh/g or more, and the ratio (1 CA/0.1 CA) of the 1 CA capacity to the 0.1 CA capacity during the constant-current charge in the range of 4.0 V to 4.3 V is 0.60 or more, lithium-ion secondary batteries including electrodes for a lithium-ion secondary battery produced using this electrode material for a lithium-ion secondary battery have a high charge and discharge capacity and a high energy density during high-speed charge and discharge, and thus it is possible to apply the present invention to next-generation secondary batteries from which a higher voltage, a higher energy density, higher load characteristics, and higher-speed charge and discharge characteristics are expected, and in the case of next-generation secondary batteries, the effects are extremely strong.

## Claims

1. An electrode material for a lithium-ion secondary battery comprising:
particles which are made of LiFexMn1-w-x-y-zMgyCazAwPO4 (here, A represents at least one element selected from the group consisting of Co, Ni, Zn, Al, and Ga, 0.05≦x≦0.35, 0.01≦y≦0.08, 0.0001≦z≦0.001, and 0≦w≦0.02) and have an orthorhombic crystal structure,
wherein a 0.1 CA capacity during constant-current charge in a range of 4.0 V to 4.3 V is 100 mAh/g or more, and a ratio (1 CA/0.1 CA) of a 1 CA capacity to the 0.1 CA capacity during the constant-current charge in the range of 4.0 V to 4.3 V is 0.60 or more.

2. The electrode material for a lithium-ion secondary battery according to claim 1, wherein a specific surface area of the particles is in a range of 10 m2/g to 25 m2/g.