JP2022157172A - Positive electrode material for lithium ion secondary battery, positive electrode for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Abstract

To provide a positive electrode material for a lithium ion secondary battery and a positive electrode for a lithium ion secondary battery capable of obtaining a lithium ion secondary battery with excellent cycle characteristics, and to provide a lithium ion secondary battery comprising the positive electrode for a lithium ion secondary battery.SOLUTION: For a positive electrode material for a lithium ion secondary battery, a high-energy side peak intensity in quadrupole splitting of iron ions is higher than a low-energy side peak intensity in quadrupole splitting of iron ions, in Mossbauer spectra obtained by the Mossbauer spectroscopy.SELECTED DRAWING: None

Description

The present invention relates to a positive electrode material for lithium ion secondary batteries, a positive electrode for lithium ion secondary batteries, and a lithium ion secondary battery.

In the olivine-based positive electrode active material, lithium ions in the crystal diffuse only in one-dimensional direction. There was a problem that the diffusion of lithium ions could not catch up and the overvoltage (voltage drop) increased.

Conventionally, an olivine-based positive electrode active material composed of lithium iron phosphate (LiFePO ₄ ) or lithium manganese iron phosphate (LiMn _1-x Fe _x PO ₄ ) is synthesized so that the content of trivalent Fe is extremely low. It is known that, by doing so, it is possible to increase the capacity of a lithium ion secondary battery having a positive electrode containing an olivine-based positive electrode active material. In Patent Document 1, in the Mössbauer spectrum obtained by Mössbauer spectroscopy, by specifying the isomer shift value, a small amount of trivalent Fe is intentionally dissolved in the crystal to cause defects in the crystal. This allows lithium ions to diffuse in two or three dimensions within the crystal.

Japanese Patent No. 6477948

However, in using lithium-ion secondary batteries, there has been a demand for further improvement in cycle characteristics, which is an index of durability.

The present invention has been made in view of the above circumstances, and a positive electrode material for a lithium ion secondary battery and a positive electrode for a lithium ion secondary battery that can obtain a lithium ion secondary battery having excellent cycle characteristics, and this An object of the present invention is to provide a lithium ion secondary battery having a positive electrode for a lithium ion secondary battery.

As a result of intensive studies to solve the above problems, the present inventors have found that the Mössbauer spectrum obtained by analyzing the positive electrode material for lithium ion secondary batteries by Mössbauer spectroscopy shows that the quadrupolar splitting of iron ions is high. By producing a positive electrode material for a lithium ion secondary battery so that the intensity of the peak on the energy side is greater than the intensity of the peak on the low energy side of quadrupolar splitting of iron ions, lithium ion secondary batteries with excellent cycle characteristics can be obtained. The inventors have found that a positive electrode material for lithium ion secondary batteries can be obtained from which the following batteries can be obtained, and have completed the present invention.

The present invention has the following aspects.
[1] In the Mössbauer spectrum obtained by Mössbauer spectroscopy, the intensity of the peak on the high-energy side of the quadrupolar splitting of iron ions is greater than the intensity of the peak on the low-energy side of the quadrupolar splitting of iron ions, lithium Cathode material for ion secondary batteries.
[2] The average particle diameter measured with a laser diffraction particle size distribution analyzer, comprising primary particles having a carbonaceous coating formed on the surface of the electrode active material particles and aggregated particles in which a plurality of the primary particles are aggregated. is 0.3 μm or more and 5.0 μm or less, the positive electrode material for a lithium ion secondary battery according to [1].
[3] The positive electrode material for a lithium ion secondary battery according to [2], wherein the primary particles have an olivine structure with an average particle size of 50 nm or more and 500 nm or less.
[4] The positive electrode material for a lithium ion secondary battery according to [2] or [3], wherein the carbonaceous film has a thickness of 0.5 nm or more and 10 nm or less.
[5] A positive electrode for a lithium ion secondary battery comprising an electrode current collector and a positive electrode mixture layer formed on the electrode current collector, wherein the positive electrode mixture layer comprises [1] to A positive electrode for lithium ion secondary batteries, containing the positive electrode material for lithium ion secondary batteries according to any one of [4].
[6] A lithium ion secondary battery having a positive electrode, a negative electrode and a non-aqueous electrolyte, comprising the positive electrode for a lithium ion secondary battery according to [5] as the positive electrode.

According to the positive electrode material for a lithium ion secondary battery of the present invention, a positive electrode material for a lithium ion secondary battery and a positive electrode for a lithium ion secondary battery that can obtain a lithium ion secondary battery having excellent cycle characteristics, and the lithium A lithium ion secondary battery having a positive electrode for an ion secondary battery can be provided.

FIG. 2 is a diagram showing a state in which a Mössbauer spectrum is split into two. 1 is a diagram showing a Mössbauer spectrum of the positive electrode material of Example 1. FIG. 3 is a diagram showing the Mossbauer spectrum of the positive electrode material of Comparative Example 1. FIG.

Embodiments of the positive electrode material for lithium ion secondary batteries, the positive electrode for lithium ion secondary batteries, and the lithium ion secondary battery of the present invention will be described.
It should be noted that the present embodiment is specifically described for better understanding of the gist of the invention, and does not limit the invention unless otherwise specified.

[Positive material for lithium-ion secondary batteries]
In the positive electrode material for a lithium ion secondary battery of the present embodiment, in the Mössbauer spectrum obtained by Mössbauer spectroscopy, the intensity of the peak on the high energy side of the quadrupolar splitting of iron ions is low. greater than the intensity of the peak on the energy side.

The positive electrode material for a lithium ion secondary battery of the present embodiment has the intensity of the peak on the low energy side of quadrupolar splitting of iron ions in the Mössbauer spectrum obtained by Mössbauer spectroscopy (hereinafter referred to as "peak intensity A" The ratio (peak intensity B/peak intensity A) of the intensity of the peak on the high energy side of quadrupole splitting of iron ions (hereinafter sometimes referred to as “peak intensity B”) is 1.02 It is preferably 1.04 or more, more preferably 1.04 or more. The upper limit of the ratio (peak intensity B/peak intensity A) may be 1.1 or less, or 1.08 or less.
When the ratio (peak intensity B/peak intensity A) is 1/02 or more, a decrease in capacity during charging and discharging is suppressed.

Mössbauer spectroscopy analyzes that γ-rays emitted from Mössbauer nuclei ( ⁵⁷ Fe, which is the excitation standard in the decay process of ⁵⁷ Co→ ⁵⁷ Fe) incorporated as radioactive isotopes (radiation sources) in solids are Energy dependence of the amount of absorption or the amount of scattering emitted after absorption when resonance is absorbed without recoil by the same kind of Mössbauer nuclei in the ground state in two solids (samples) (Mössbauer spectrum) This is done by examining

In the present embodiment, the spectrum obtained by this Mössbauer spectroscopic analysis can be approximated by a Lorentz-type theoretical linear expression, the peak half-widths for each component are all equal, the peak heights at symmetrical positions are equal, Under the assumption that the spectrum is the sum of the theoretical linear shapes, curve fitting is performed to determine the peak position and obtain the area intensity of each component. The formula shown in the following formula (1) is used as the theoretical linear formula.

In the above equation (1), f(E) is the count at the Doppler velocity E, E is the Doppler velocity (linearly proportional to the energy), B is the baseline count, I _i is the absorption intensity of the i-th peak, Γ _i is the half width of the i-th peak, x _0i is the i-th peak center, δ is the isomer shift, Δ is the quadrupole splitting, and H is the internal magnetic field.
Also, the area intensity of the spectrum is defined as the relative area ratio of each component when the sum of squares of the residuals is minimized by the method of least squares.

In the present embodiment, in the spectrum obtained by Mössbauer spectroscopy, when the Fe is paramagnetic (internal magnetic field H = 0), it is obtained as a peak split into two, and the Fe is ferromagnetic or antiferromagnetic In the case of (internal magnetic field H≠0), six peaks are obtained, and no peaks other than two or six are observed.

In addition, in the Mössbauer spectrum, the spectrum being split into two means the state as shown in FIG.

Moreover, the quadrupole splitting value in the Mössbauer spectrum is the peak interval split into two. In general, when structurally isotropic ligands with different electronegativities are coordinated, when the arrangement is structurally distorted, or when these are mixed, it splits into two. It is believed that

The positive electrode material for a lithium ion secondary battery of the present embodiment includes primary particles in which a carbonaceous film is formed on the surface of an electrode active material particle, and aggregated particles (secondary particles) in which a plurality of the primary particles are aggregated. preferably included.

In the positive electrode material for a lithium ion secondary battery of the present embodiment, the average particle size measured with a laser diffraction particle size distribution analyzer is preferably 0.3 μm or more and 5.0 μm or less, and more preferably 0.4 μm or more and 4.0 μm. It is more preferably 0.45 μm or more and 3.5 μm or less. When the average particle size is at least the lower limit, the positive electrode material paste for a lithium ion secondary battery can be applied to an aluminum current collector, and the structure of the dried positive electrode mixture layer can be made uniform, and the charge and discharge reaction can be achieved. The local overvoltage associated with this can be suppressed, and the metal elution amount can be reduced. When the average particle diameter is equal to or less than the upper limit, the positive electrode material can be densely packed, and the energy density per unit volume of the positive electrode is improved.
The average particle size here means the average particle size of the primary particles in which the carbonaceous film is formed on the surface of the electrode active material particles, and the aggregated particles in which a plurality of the primary particles are aggregated.

The average particle size of the primary particles is calculated from the average particle size of the primary particles after arbitrarily selecting 100 particles observed with a scanning electron microscope (SEM).

The primary particles preferably have an olivine structure with an average particle diameter of 50 nm or more and 500 nm or less. That is, it is preferable that the electrode active material particles that constitute the primary particles have an olivine structure, and that the primary particles have an average particle diameter of 50 nm or more and 500 nm or less.
The average particle size of the primary particles is more preferably 60 nm or more and 400 nm or less, and further preferably 80 nm or more and 250 nm or less.
Here, the reason why the average primary particle size of the primary particles containing the electrode active material particles having an olivine structure coated with the carbonaceous film is set in the above range is as follows. When the average primary particle size is less than 50 nm, the mass of carbon required increases due to the increase in the specific surface area of the primary particles, and the charge/discharge capacity decreases. Furthermore, carbon coating becomes difficult, primary particles with a sufficient coating rate cannot be obtained, and a good mass energy density cannot be obtained particularly at low temperatures or at high speed charging and discharging. On the other hand, if the average primary particle size exceeds 500 nm, it takes a long time to move lithium ions or electrons in the primary particles, thereby increasing the internal resistance and deteriorating the output characteristics.

Although the shape of the secondary particles is not particularly limited, it is preferable that the secondary particles have a spherical shape because it facilitates production of a positive electrode material composed of spherical particles, particularly spherical particles.
Here, the reason why a spherical shape is preferable is as follows. The secondary particles coated with the carbonaceous film, the binder, and the solvent can be mixed to reduce the amount of the solvent when preparing the positive electrode material paste for lithium ion secondary batteries. Furthermore, it becomes easy to apply the positive electrode material paste for a lithium ion secondary battery to an electrode current collector. Moreover, if the shape is spherical, the surface area of the secondary particles is minimized, and the amount of the binder to be added can be minimized, and the internal resistance of the resulting positive electrode can be reduced.
Furthermore, by making the shape of the secondary particles spherical, particularly spherical, the closest packing is facilitated. As a result, the filling amount of the positive electrode material for lithium ion secondary batteries per unit volume increases, and as a result, the electrode density can be increased, and the capacity of the lithium ion secondary battery can be increased, which is preferable. .

The thickness of the carbonaceous coating in the primary particles is preferably 0.5 nm or more and 10 nm or less, and more preferably 0.5 nm or more and 5.5 nm or less.
The reason for setting the thickness of the carbonaceous coating to the above range is as follows. If the thickness of the carbonaceous coating is less than 0.5 nm, the thickness of the carbonaceous coating is too thin to form a film having a desired resistance value. As a result, the conductivity is lowered, and the conductivity as a positive electrode material cannot be secured. On the other hand, when the thickness of the carbonaceous film exceeds 10 nm, the battery activity, for example, the battery capacity per unit mass of the positive electrode material decreases.

The coverage of the carbonaceous film on the primary particles is preferably 60% or more, more preferably 80% or more. When the coverage of the carbonaceous film is 60% or more, a sufficient covering effect of the carbonaceous film can be obtained.

In addition, the amount of carbon carried relative to the specific surface area of the positive electrode material for a lithium ion secondary battery of the present embodiment ([amount of carbon carried]/[specific surface area]) is 0.5 mg/m ² or more and 2.0 mg/m ² or less. and more preferably 0.7 mg/m ² or more and 1.6 mg/m ² or less.
Here, the reason why the amount of carbon supported relative to the specific surface area in the positive electrode material for a lithium ion secondary battery of the present embodiment is limited to the above range is as follows. If the amount of carbon supported relative to the specific surface area is less than 0.5 mg/m ² , the discharge capacity at a high charge/discharge rate becomes low when a lithium ion secondary battery is formed, making it difficult to achieve sufficient charge/discharge rate performance. Become. On the other hand, when the amount of carbon carried relative to the specific surface area exceeds 2.0 mg/m ² , the amount of carbon is too large, and the battery capacity of the lithium ion secondary battery per unit mass of primary particles is lowered more than necessary.

The specific surface area of the positive electrode material for a lithium ion secondary battery of the present embodiment is preferably 5.0 m ² /g or more and 20 m ² /g or less, and 5.5 m ² /g or more and 18 m ² /g or less. more preferably 6.0 m ² /g or more and 16 m ² /g or less.
Here, the reason why the specific surface area of the positive electrode material for lithium ion secondary batteries of the present embodiment is limited to the above range is as follows. If the specific surface area is less than 5.0 m ² /g, it takes a long time for lithium ions or electrons to move within the crystal, which increases the internal resistance and deteriorates the output characteristics. On the other hand, when the specific surface area exceeds 20 m ² /g, the mass of carbon required increases due to the increase in the specific surface area of the carbonaceous electrode active material composite particles, and the charge/discharge capacity decreases. Furthermore, carbon coating becomes difficult, primary particles with a sufficient coverage cannot be obtained, and in particular, a good mass energy density cannot be obtained at low temperatures or at high-speed charging and discharging, which is not preferable.

"Electrode active material particles"
The electrode active material _{particles having an olivine structure} are not particularly limited. M is selected from the group consisting of Mg, Ca, Co, Sr, Ba, Ti, Zn, V, B, Al, Ga, In, Si, Ge and rare earth elements It is preferable to consist of at least one type, 0.85≤x≤1.1, 0≤y≤0.85, 0≤z≤0.2).

The reason why x satisfies 0.85≦x≦1.1 in Li _x Fe _1-yz A _y M _z PO ₄ is as follows. If x is less than 0.85, the amount of lithium ions in the battery will decrease when an active material containing no lithium ions is used for the negative electrode, resulting in a decrease in battery capacity. On the other hand, if x exceeds 1.1, the olivine structure cannot be maintained and the crystal stability is lowered, which is not preferable.

The reason why y in Li _x Fe _1-yz A _y M _z PO ₄ satisfies 0≦y≦0.85 is as follows. When y exceeds 0.85, the ratio of Fe becomes too small, which lowers the lithium ion diffusion speed and electron conduction speed in the crystal, and deteriorates the input/output characteristics, which is not preferable.

The reason why z satisfies 0≦z≦0.2 in Li _x Fe _1-yz A _y M _z PO ₄ is as follows. When z exceeds 0.2, the ratio of electrochemically inactive metals increases, which unfavorably lowers the battery capacity per unit mass of the positive electrode material.

Li _x Fe _1-yz A _y M _z PO ₄ in the present embodiment preferably has y=0 and z=0. That is, in the positive electrode material for a lithium ion secondary battery of this embodiment, the electrode active material particles are preferably made of _LiFePO4 .
When the electrode active material particles are made of LiFePO ₄ , the lithium ion diffusion speed and electron conduction speed in the crystal are improved, and the input/output characteristics are improved.

"Carbon coating"
A carbonaceous film is a pyrolytic carbonaceous film obtained by carbonizing an organic compound as a raw material. The carbon source that is the raw material for the carbonaceous film is preferably derived from an organic compound having a carbon purity of 42.00% or more and 60.00% or less.

As a method for calculating the "purity of carbon" of the carbon source that is the raw material of the carbonaceous film in the positive electrode material for lithium ion secondary batteries of the present embodiment, when using a plurality of types of organic compounds, the blending amount of each organic compound ( %) and the known carbon purity (%), the amount of carbon (% by mass) in the amount of each organic compound is calculated and summed, and the total amount of the organic compound (% by mass) and the total amount of carbon ( % by mass), a method of calculating according to the following formula (2) is used.
Purity of carbon (%) = total carbon content (% by mass)/total compounding amount (% by mass) x 100 (2)

According to the positive electrode material for a lithium ion secondary battery of the present embodiment, in the Mössbauer spectrum obtained by Mössbauer spectroscopy, the intensity of the peak on the high energy side of the quadrupolar splitting of iron ions is the quadrupolar splitting of iron ions. , a lithium ion secondary battery with excellent cycle characteristics can be obtained.

[Method for producing positive electrode material for lithium ion secondary battery]
The method for producing the positive electrode material for _{a lithium ion secondary battery} of the present embodiment is not particularly limited. A raw material slurry α obtained by mixing an Fe source, an A source, an M source, and a P source with a solvent containing water as a main component is heated to a temperature in the range of 100° C. or higher and 300° C. or lower. synthesizing Li _x Fe 1-yz A _{yM z} PO ₄ particles under reduced pressure; dispersing the Li _x Fe _{1- yz} A _{yM z} PO ₄ particles in a water solvent containing a carbon source After drying and granulating the raw material slurry β, the surface of the Li _x Fe _1-yz A _y M _z PO ₄ particles is heated to a temperature in the range of 500° C. or higher and 1000° C. or lower. and coating with a carbonaceous coating.

In the Mössbauer spectrum obtained by the Mössbauer spectroscopy of the positive electrode material for a lithium ion secondary battery of the present embodiment, the intensity of the peak on the high energy side of the quadrupolar splitting of iron ions is lower than that of the quadrupolar splitting of iron ions. The method for adjusting the intensity of the peak on the energy side to be greater than the intensity is not particularly limited. The target particles can be formed by using the precursor particles as a material and performing hydrothermal synthesis again to promote grain growth.

Although the method for synthesizing the Li _x Fe _1-yz A _y M _z PO ₄ particles is not particularly limited, for example, Li source, Fe source, A (at least one selected from the group consisting of Mn, Co and Ni) source, M (at least one selected from the group consisting of Mg, Ca, Co, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge and rare earth elements) source and P source, water and stirred to prepare a raw material slurry α containing a precursor of Li _x Fe _1-yz A _y M _z PO ₄ .

These Li source, Fe source, A source, M source and P source are combined in their molar ratio (Li source: Fe source: A source: M source: P source), that is, Li: Fe: A: M: P It is added to a solvent containing water as a main component so that the molar ratio is 0.85-5:0.1-2:0-2:0-2:1-2, and stirred and mixed to form a raw material slurry α. Prepare.
Considering that these Li source, Fe source, A source, M source and P source are uniformly mixed, each of the Li source, Fe source, A source, M source and P source was once made into an aqueous solution. After that, it is preferable to mix.
The molar concentrations of the Li source, the Fe source, the A source, the M source and the P source in this raw material slurry α are highly pure, highly crystalline and extremely fine Li _x Fe _1-yz A _y M _z . Since it is necessary to obtain PO4 particles, it is preferably 0.1 mol/L or more and ₃ mol/L or less.

Li sources include, for example, hydroxides such as lithium hydroxide (LiOH), lithium carbonate (Li ₂ CO ₃ ), lithium chloride (LiCl), lithium nitrate (LiNO ₃ ), lithium phosphate (Li ₃ PO ₄ ). , lithium inorganic acid salts such as dilithium hydrogen phosphate (Li ₂ HPO ₄ ), lithium dihydrogen phosphate (LiH ₂ PO ₄ ), lithium acetate (LiCH ₃ COO), lithium oxalate ((COOLi) ₂ ), etc. Examples include organic acid salts and hydrates thereof. At least one selected from these groups is preferably used as the Li source.
Lithium phosphate (Li ₃ PO ₄ ) can also be used as a Li source and a P source.

Examples of the Fe source include iron such as iron (II) chloride (FeCl ₂ ), iron (II) sulfate (FeSO ₄ ), and iron (II) acetate (Fe(CH ₃ COO) ₂ ) as divalent Fe compounds. A compound or a hydrate thereof is used, and trivalent Fe compounds include lithium iron phosphate (III) (FePO ₄ ), iron nitrate (III) (Fe(NO ₃ ) ₃ ), iron chloride (III) (FeCl ₃ ), an iron compound such as iron (III) citrate (FeC ₆ H ₅ O ₇ ), or a hydrate thereof. Only the divalent Fe compound may be used as the Fe source, only the trivalent Fe compound may be used as the Fe source, or both the divalent Fe compound and the trivalent Fe compound may be used as the Fe source. If both a divalent Fe compound and a trivalent Fe compound are used as Fe sources, trivalent Fe can easily be dissolved in the crystal, which is preferable.

Mn sources are preferably Mn salts such as manganese (II) chloride (MnCl ₂ ), manganese (II) sulfate (MnSO ₄ ), manganese (II) nitrate (Mn(NO ₃ ) ₂ ), manganese (II) acetate )(Mn(CH ₃ COO) ₂ ) and hydrates thereof. At least one selected from these groups is preferably used as the Mn source.

Co sources are preferably Co salts such as cobalt (II) chloride (CoCl ₂ ), cobalt (II) sulfate (CoSO ₄ ), cobalt (II) nitrate (Co(NO ₃ ) ₂ ), cobalt (II) acetate )(Co(CH ₃ COO) ₂ ) and hydrates thereof. At least one selected from these groups is preferably used as the Co source.

Ni sources are preferably Ni salts such as nickel (II) chloride (NiCl ₂ ), nickel (II) sulfate (NiSO ₄ ), nickel (II) nitrate (Ni(NO ₃ ) ₂ ), nickel (II) acetate )(Ni(CH ₃ COO) ₂ ) and hydrates thereof. At least one selected from these groups is preferably used as the Ni source.

Mg sources are preferably Mg salts such as magnesium (II) chloride (MgCl ₂ ), magnesium (II) sulfate (MgSO ₄ ), magnesium (II) nitrate (Mg(NO ₃ ) ₂ ), magnesium (II) acetate ) (Mg(CH ₃ COO) ₂ ) and hydrates thereof. At least one selected from these groups is preferably used as the Mg source.

Ca sources are preferably Ca salts such as calcium chloride (II) (CaCl ₂ ), calcium sulfate (II) (CaSO ₄ ), calcium nitrate (II) (Ca(NO ₃ ) ₂ ), calcium acetate (II) ) (Ca(CH ₃ COO) ₂ ) and hydrates thereof, and at least one selected from the group consisting of these is preferably used.

Co sources are preferably Co salts such as cobalt (II) chloride (CoCl ₂ ), cobalt (II) sulfate (CoSO ₄ ), cobalt (II) nitrate (Co(NO ₃ ) ₂ ), cobalt (II) acetate )(Co(CH ₃ COO) ₂ ) and hydrates thereof. At least one selected from these groups is preferably used as the Co source.

Sr sources are preferably Sr salts such as strontium carbonate (SrCo ₃ ), strontium sulfate (SrSO ₄ ), strontium hydroxide (Sr(OH) ₂ ), and at least one selected from the group consisting of these. Seeds are preferably used.

As a Ba source, Ba salts are preferred, for example, barium (II) chloride (BaCl ₂ ), barium (II) sulfate (BaSO ₄ ), barium (II) nitrate (Ba(NO ₃ ) ₂ ), barium (II) acetate )(Ba(CH ₃ COO) ₂ ) and hydrates thereof, and at least one selected from the group consisting of these is preferably used.

Ti sources are preferably Ti salts, such as titanium chloride (TiCl ₄ , TiCl ₃ , TiCl ₂ ), titanium oxide (TiO), and hydrates thereof, which are selected from the group consisting of these. At least one is preferably used.

As a Zn source, Zn salts are preferred, such as zinc (II) chloride (ZnCl ₂ ), zinc (II) sulfate (ZnSO ₄ ), zinc (II) nitrate (Zn(NO ₃ ) ₂ ), zinc (II) acetate )(Zn(CH ₃ COO) ₂ ) and hydrates thereof. At least one selected from these groups is preferably used as the Zn source.

Examples of the B source include boron compounds such as chlorides, sulfates, nitrates, acetates, hydroxides and oxides, and at least one selected from the group consisting of these is preferably used.

Examples of the Al source include aluminum compounds such as chlorides, sulfates, nitrates, acetates, and hydroxides, and at least one selected from the group consisting of these is preferably used.

Examples of Ga sources include gallium compounds such as chlorides, sulfates, nitrates, acetates and hydroxides, and at least one selected from the group consisting of these is preferably used.

Examples of the In source include indium compounds such as chlorides, sulfates, nitrates, acetates and hydroxides, and at least one selected from the group consisting of these is preferably used.

Examples of Si sources include sodium silicate, potassium silicate, silicon tetrachloride (SiCl ₄ ), silicates, organosilicon compounds, etc. At least one selected from the group consisting of these is preferably used. be done.

Ge sources include, for example, germanium compounds such as chlorides, sulfates, nitrates, acetates, hydroxides and oxides, and at least one selected from the group consisting of these is preferably used.

Examples of rare earth element sources include chlorides, sulfates, nitrates of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. , acetates, hydroxides, oxides, etc., and at least one selected from the group consisting of these is preferably used.

Examples of P sources include phosphoric acids such as orthophosphoric acid (H ₃ PO ₄ ) and metaphosphoric acid (HPO ₃ ), ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ), diammonium hydrogen phosphate ((NH ₄ ) ₂ HPO ₄ ), ammonium phosphate ((NH ₄ ) ₃ PO ₄ ), lithium phosphate (Li ₃ PO ₄ ), dilithium hydrogen phosphate (Li ₂ HPO ₄ ), lithium dihydrogen phosphate (LiH ₂ PO ₄ ), etc., and at least one selected from these hydrates are preferably used.

The solvent containing water as the main component is either water alone or an aqueous solvent containing water as the main component and optionally containing an aqueous solvent such as alcohol.
The aqueous solvent is not particularly limited as long as it can dissolve the Li source, Fe source, A source, M source and P source. For example, alcohols such as methanol, ethanol, 1-propanol, 2-propanol (isopropyl alcohol: IPA), butanol, pentanol, hexanol, octanol, diacetone alcohol, ethyl acetate, butyl acetate, ethyl lactate, propylene glycol monomethyl ether Esters such as acetate, propylene glycol monoethyl ether acetate, γ-butyrolactone, diethyl ether, ethylene glycol monomethyl ether (methyl cellosolve), ethylene glycol monoethyl ether (ethyl cellosolve), ethylene glycol monobutyl ether (butyl cellosolve), Ethers such as diethylene glycol monomethyl ether and diethylene glycol monoethyl ether, acetone, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), acetylacetone, ketones such as cyclohexanone, dimethylformamide, N,N-dimethylacetoacetamide, N-methylpyrrolidone and amides such as ethylene glycol, diethylene glycol and propylene glycol. These aqueous solvents may be used singly or in combination of two or more.

Next, this raw material slurry α is placed in a pressure vessel, heated to a temperature in the range of 100° C. or higher and 300° C. or lower, preferably 100° C. or higher and 250° C. or lower, and hydrothermally treated for 1 hour or more and 72 hours or less, Li _x Fe _1-yz A _y M _z PO ₄ particles are obtained.
In this case, the particle size of the Li _x Fe _1-yz A _y M _z PO ₄ particles can be controlled to a desired size by adjusting the temperature and time during the hydrothermal treatment.

Next, the Li _x Fe _1-yz A _y M _z PO ₄ particles are dispersed in an aqueous solvent containing a carbon source to prepare a raw material slurry β.
Next, this raw material slurry β is dried and granulated, and then heated at a temperature in the range of 500° C. or higher and 1000° C. or lower, preferably 500° C. or higher and 800° C. or lower for 1 hour or longer and 100 hours or shorter. , Li _x Fe _1-yz A _y M _z PO ₄ particles are coated with a carbonaceous film to obtain a positive electrode material for a lithium ion secondary battery of the present embodiment. Here, heating at a temperature of less than 500° C. is not preferable because carbonization of the carbonaceous film is insufficient and the electrical conductivity is remarkably lowered. On the other hand, heating at a temperature exceeding 1000° C. is not preferable because some of the lithium volatilizes and the battery capacity decreases.

"carbon source"
The carbon source is not particularly limited as long as it is an organic compound capable of forming a carbonaceous film on the surface of the electrode active material particles.
As the organic compound, a compound having solubility in water or dispersibility in water is preferable.
For example, salicylic acid, catechol, hydroquinone, resorcinol, pyrogallol, phloroglucinol, hexahydroxybenzene, benzoic acid, phthalic acid, terephthalic acid, phenylalanine, water-dispersed phenol resin, sugars such as sucrose, glucose, lactose, malic acid, Carboxylic acids such as citric acid, unsaturated monohydric alcohols such as allyl alcohol and propargyl alcohol, ascorbic acid, polyvinyl alcohol, etc., may be mentioned, and one type or two or more types may be mixed to make the carbon purity 42.00% or more. can be used.

In the method for producing a positive electrode material for a lithium ion secondary battery of the present embodiment, the amount (addition rate) of the carbon source added is 0.5 when the total mass of the electrode active material particles and the carbon source is 100% by mass. It is preferably 1% by mass or more and 15% by mass or less, more preferably 1% by mass or more and 10% by mass or less.

If the amount of the carbon source added is less than 0.5% by mass, the mixing stability in the positive electrode material for lithium ion secondary batteries is lowered, which is not preferable. On the other hand, when the amount of the carbon source added exceeds 15% by mass, the content of the positive electrode active material becomes relatively small, and the battery characteristics deteriorate, which is not preferable.

In addition, when using a plurality of types of organic compounds as the carbon source, the blending amount of each organic compound is adjusted as described above so that the purity of carbon in the organic compound is 42.00% or more and 60.00% or less. to adjust.

[Positive electrode for lithium ion secondary battery]
The positive electrode for a lithium ion secondary battery of the present embodiment includes an electrode current collector and a positive electrode mixture layer (positive electrode) formed on the electrode current collector, and the positive electrode mixture layer is contains a positive electrode material for a lithium ion secondary battery.
That is, the positive electrode for a lithium ion secondary battery of the present embodiment is formed by forming a positive electrode mixture layer on one main surface of an electrode current collector using the positive electrode material for a lithium ion secondary battery of the present embodiment. is.

The method for producing the positive electrode for a lithium ion secondary battery of the present embodiment is particularly a method that can form a positive electrode on one main surface of an electrode current collector using the positive electrode material for a lithium ion secondary battery of the present embodiment. Not limited. Examples of the method for producing the positive electrode for a lithium ion secondary battery of the present embodiment include the following methods.
First, a positive electrode material paste for lithium ion secondary batteries is prepared by mixing the positive electrode material for lithium ion secondary batteries of the present embodiment, a binder, a conductive agent, and a solvent.

"Binder"
As the binding agent, that is, binder resin, for example, polytetrafluoroethylene (PTFE) resin, polyvinylidene fluoride (PVdF) resin, fluororubber, and the like are preferably used.

The content of the binder in the positive electrode material paste for lithium ion secondary batteries is 100% by mass when the total mass of the positive electrode material for lithium ion secondary batteries, the binder, and the conductive aid according to the present embodiment is , preferably 1% by mass or more and 10% by mass or less, more preferably 2% by mass or more and 6% by mass or less.

"Conductivity aid"
The conductive agent is not particularly limited, but at least selected from the group of fibrous carbon such as acetylene black, ketjen black, furnace black, vapor grown carbon fiber (VGCF), carbon nanotube, etc. One type is used.

The content of the conductive agent in the positive electrode material paste for lithium ion secondary batteries is 100% by mass when the total mass of the positive electrode material for lithium ion secondary batteries, the binder, and the conductive agent according to the present embodiment is 100% by mass. , preferably 1% by mass or more and 15% by mass or less, more preferably 3% by mass or more and 10% by mass or less.

"solvent"
In the positive electrode material paste for lithium ion secondary batteries containing the positive electrode material for lithium ion secondary batteries according to the present embodiment, a solvent is added as appropriate in order to facilitate coating on an object to be coated such as an electrode current collector. may
The solvent used in the electrode-forming paint or electrode-forming paste may be appropriately selected according to the properties of the binder resin.
Examples of solvents include alcohols such as water, methanol, ethanol, 1-propanol, 2-propanol (isopropyl alcohol: IPA), butanol, pentanol, hexanol, octanol, and diacetone alcohol, ethyl acetate, butyl acetate, and lactic acid. Esters such as ethyl, propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, γ-butyrolactone, diethyl ether, ethylene glycol monomethyl ether (methyl cellosolve), ethylene glycol monoethyl ether (ethyl cellosolve), ethylene glycol Ethers such as monobutyl ether (butyl cellosolve), diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, acetone, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), acetylacetone, ketones such as cyclohexanone, dimethylformamide, N,N-dimethylacetate Amides such as acetamide and N-methylpyrrolidone, glycols such as ethylene glycol, diethylene glycol and propylene glycol can be used. These may be used alone or in combination of two or more.

The content of the solvent in the positive electrode material paste for lithium ion secondary batteries is 60 parts by mass when the total mass of the positive electrode material for lithium ion secondary batteries, the binder and the solvent according to the present embodiment is 100 parts by mass. It is preferably at least 400 parts by mass and more preferably at least 80 parts by mass and not more than 300 parts by mass.
By containing the solvent in the above range, it is possible to obtain a positive electrode material paste for a lithium ion secondary battery that is excellent in electrode formability and battery characteristics.

The method for mixing the positive electrode material for a lithium ion secondary battery of the present embodiment, the binder, the conductive aid, and the solvent is not particularly limited as long as it is a method capable of uniformly mixing these components. For example, a method using a kneader such as a ball mill, sand mill, planetary mixer, paint shaker, homogenizer or the like can be mentioned.

Next, the positive electrode material paste for a lithium ion secondary battery is applied to one main surface of the electrode current collector to form a coating film, the coating film is dried, and then pressure-bonded to form an electrode current collector. A positive electrode for a lithium ion secondary battery having a positive electrode material mixture layer formed on one main surface can be obtained.

According to the positive electrode for lithium ion secondary batteries of the present embodiment, since the positive electrode material for lithium ion secondary batteries of the present embodiment is contained, a lithium ion secondary battery having excellent cycle characteristics can be obtained.

[Lithium ion secondary battery]
The lithium ion secondary battery of this embodiment includes the positive electrode for lithium ion secondary battery of this embodiment, a negative electrode, a separator, and an electrolytic solution.

In the lithium-ion secondary battery of the present embodiment, the negative electrode, electrolytic solution, separator and the like are not particularly limited.
As the negative electrode, for example, negative electrode materials such as metal Li, carbon materials, Li alloys, and Li ₄ Ti ₅ O ₁₂ can be used.
Also, a solid electrolyte may be used instead of the electrolytic solution and the separator.

The electrolytic solution is, for example, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) mixed in a volume ratio of 1:1, and lithium hexafluorophosphate ( _LiPF6 ) to a concentration of 1 mol/dm ³ , for example.
Porous propylene, for example, can be used as the separator.

Since the lithium ion secondary battery of the present embodiment includes the positive electrode for a lithium ion secondary battery of the present embodiment, it has excellent cycle characteristics.

EXAMPLES The present invention will be specifically described below with reference to examples and comparative examples. In addition, this invention is not limited to the form as described in an Example.

[Production of positive electrode material for lithium ion secondary battery]
[Example 1]
LiOH was used as the Li source, NH ₄ H ₂ PO ₄ was used as the P source, and FeSO ₄ .7H ₂ O was used as the Fe source. to prepare a uniform slurry mixture of 200 mL.
Then, this mixture was placed in a pressure-resistant sealed container with a capacity of 500 mL, and hydrothermal synthesis was performed at 170° C. for 2 hours. After the reaction, the reaction product was cooled to room temperature (25° C.) to obtain a precipitated cake-like reaction product. The reaction product was thoroughly washed several times with distilled water and kept at 30% moisture content so as not to dry out to obtain a precursor.
Furthermore, 200 mL of a slurry mixture was prepared again in the same manner as above. 2 g of the precursor was added to the slurry mixture, placed in a 500 mL pressure-resistant sealed container, and the second hydrothermal synthesis was performed at 170° C. for 12 hours. After the reaction, the reaction product was cooled to room temperature (25° C.) to obtain a precipitated cake-like reaction product. The reaction product was thoroughly washed several times with distilled water, and the moisture content was kept at 30% so as not to dry out to form a cake-like substance. A small amount of the cake-like material was sampled and vacuum-dried at 70° C. for 2 hours, and the resulting powder was measured by X-ray diffraction, which confirmed the formation of single-phase LiFePO ₄ .
20 g of the resulting cake-like LiFePO ₄ (electrode active material) and 0.73 g of polyvinyl alcohol as a carbon source were mixed with water so that the total amount was 100 g. A slurry (mixture) having a dispersed particle diameter (d50) of 100 nm was obtained.
Then, using a spray dryer, the mixture was dried and granulated at a temperature at which the drying outlet temperature was 60° C. to obtain a granulated powder.
The obtained granulated powder was heat-treated at 700° C. for 1 hour using a rotary kiln in a nitrogen atmosphere to obtain a carbonaceous-coated granule (hereinafter referred to as “carbonaceous-coated granule”). Obtained.
The obtained carbonaceous-coated granules were pulverized using a jet mill apparatus (trade name: SJ-100, manufactured by Nisshin Engineering Co., Ltd.) at a supply rate of 180 g/hour to obtain a carbonaceous-coated electrode. A positive electrode material containing an active material (hereinafter referred to as "carbonaceous-coated electrode active material") was obtained.

[Example 2]
A positive electrode material containing a carbonaceous-coated electrode active material was obtained in the same manner as in Example 1, except that the amount of the precursor added during the second hydrothermal synthesis was changed to 1 g.

[Example 3]
A positive electrode material containing a carbonaceous-coated electrode active material was obtained in the same manner as in Example 1, except that the amount of the precursor added during the second hydrothermal synthesis was changed to 4 g.

[Comparative Example 1]
LiOH was used as the Li source, NH ₄ H ₂ PO ₄ was used as the P source, and FeSO ₄ .7H ₂ O was used as the Fe source. to prepare a uniform slurry mixture of 200 mL.
Next, this mixture was placed in a pressure-resistant sealed container with a capacity of 500 mL, and hydrothermal synthesis was performed at 170° C. for 12 hours. After the reaction, the reaction product was cooled to room temperature (25° C.) to obtain a precipitated cake-like reaction product. The reaction product was thoroughly washed several times with distilled water, and the moisture content was kept at 30% so as not to dry out to form a cake-like substance. A small amount of the cake-like material was sampled and vacuum-dried at 70° C. for 2 hours, and the resulting powder was measured by X-ray diffraction, which confirmed the formation of single-phase LiFePO ₄ .
20 g of the resulting cake-like LiFePO ₄ (electrode active material) and 0.73 g of polyvinyl alcohol as a carbon source were mixed with water so that the total amount was 100 g. A slurry (mixture) having a dispersed particle diameter (d50) of 100 nm was obtained.
Then, using a spray dryer, the mixture was dried and granulated at a temperature at which the drying outlet temperature was 60° C. to obtain a granulated powder.
The obtained granulated powder was heat-treated at 800° C. for 1 hour using a rotary kiln in a nitrogen atmosphere to obtain carbonaceous-coated granules.
The obtained carbonaceous-coated granules were pulverized using a jet mill apparatus (trade name: SJ-100, manufactured by Nisshin Engineering Co., Ltd.) at a supply rate of 180 g/hour to obtain a carbonaceous-coated electrode active material. A positive electrode material containing

[Comparative Example 2]
A positive electrode material containing a carbonaceous-coated electrode active material was obtained in the same manner as in Comparative Example 1, except that the heat treatment temperature was set to 700° C. and the pulverization by the jet mill was not performed.

[Comparative Example 3]
A positive electrode material containing a carbonaceous-coated electrode active material was obtained in the same manner as in Comparative Example 2, except that the amount of the carbon source added was 2.5 g.

"evaluation"
The positive electrode materials obtained in Examples 1 to 3 and Comparative Examples 1 to 3 were evaluated as follows. Table 1 shows the results.

[Evaluation of positive electrode material for lithium ion secondary battery]
(1) Particle size distribution of positive electrode material The positive electrode material is dispersed in water, and the particle size distribution of the positive electrode material contained in the dispersion is measured using a particle size distribution meter (trade name: LA-920, manufactured by Horiba, Ltd.), using JIS Z8825. It was measured by a method according to "Particle Size Analysis - Laser Diffraction/Scattering Method".

(2) Measurement of average particle size of primary particles The average particle size of the primary particles containing the electrode active material particles and the carbonaceous coating formed on the surface of the electrode active material particles is observed with a scanning electron microscope (SEM). It was obtained by averaging the particle diameters of 200 or more primary particles measured by .

(3) Measurement of thickness of carbonaceous coating The thickness of the carbonaceous coating was obtained by averaging the thickness of each coated carbon layer in 30 fields of view by observation with a transmission electron microscope (TEM).

(4) Mössbauer Spectral Measurement The positive electrode materials of Examples 1 to 3 and Comparative Examples 1 to 3 were subjected to Mössbauer spectroscopic analysis by Mössbauer spectroscopy. Mössbauer spectra were measured by transmission method. Details are shown below.
Measurement method: uniform acceleration mode, room temperature, under normal pressure Radiation source: ⁵⁷ Co/Rh matrix, 1.85 [GBq]
Method of velocity axis calibration: Of the six magnetic splitting peaks of the spectrum of the pure iron foil at room temperature, the center positions of the four inner peaks were defined as X2, X3, X4, and X5 [channel], and were obtained by the following formula. .
X0[channel]=(X2+X3+X4+X5)/4
Γ[channel]=20.422/{0.0835(X5−X2)+0.8385(X4−X3)}
Assuming that the spectrum obtained by this Mössbauer spectroscopic analysis can be approximated by a Lorentzian theoretical linear equation, fitting was performed using numerical calculation software, and the intensity of each peak was calculated. The intensity ratio was calculated as peak intensity on the high energy side/peak intensity on the low energy side.
2 shows the Mössbauer spectrum of the positive electrode material of Example 1. FIG. 3 shows the Mössbauer spectrum of the positive electrode material of Comparative Example 1. FIG.

[Production of lithium ion secondary battery]
Using the positive electrode materials of Examples 1 to 3 and Comparative Examples 1 to 3, lithium ion secondary batteries were produced.
N-methyl-2-pyrrolidinone (NMP) as a solvent, a positive electrode material, acetylene black (AB) as a conductive agent, and polyvinylidene fluoride (PVdF) as a binder are added to the mass ratio in the paste. Then, positive electrode material:AB:PVdF=90:5:5 and mixed to prepare a positive electrode material paste.
Next, this positive electrode material paste is applied to the surface of an aluminum foil (electrode current collector) having a thickness of 30 μm to form a coating film, the coating film is dried, and the coating film is crimped so that it has a predetermined density. Then, a positive electrode mixture layer was formed on the surface of the aluminum foil to obtain a positive electrode plate having the aluminum foil and the positive electrode mixture layer.
Thereafter, the positive electrode plate was punched out into a plate shape having a tab margin around a square with a positive electrode area of 9 cm ² using a molding machine. Furthermore, a test electrode (positive electrode) was produced by welding a tab to the tab margin.

Next, natural graphite as a negative electrode active material, styrene-butadiene latex (SBR) as a binder, and carboxymethyl cellulose (CMC) as a viscosity modifier are added to pure water as a solvent in a mass ratio of the paste. Graphite:SBR:CMC=98:1:1 was added and these were mixed to prepare a negative electrode material paste (for negative electrode).
The prepared negative electrode material paste (for negative electrode) is applied to the surface of a copper foil (current collector) having a thickness of 10 μm to form a coating film, the coating film is dried, and a negative electrode mixture layer is formed on the copper foil surface. Thus, a negative electrode plate having a copper foil and a negative electrode mixture layer was obtained.
Thereafter, the negative electrode plate was punched out into a plate shape having a tab margin around a square with a negative electrode area of 9 cm ² using a molding machine. Further, a tab was welded to the tab margin to prepare a negative electrode.

The prepared positive electrode and negative electrode are opposed to each other via a 25 μm thick separator made of porous polypropylene, and a 1 mol/L lithium hexafluorophosphate (LiPF ₆ ) solution as a non-aqueous electrolyte (non-aqueous electrolyte solution). After being immersed in 500 mL, it was sealed with a laminate film to produce a lithium ion secondary battery. As the LiPF ₆ solution, a mixture of ethylene carbonate and diethyl carbonate in a volume ratio of 1:1 was used.

[Evaluation of Lithium Ion Secondary Battery]
(1) Measurement of capacity retention rate At an environmental temperature of 25°C, the discharge capacity was measured by constant current charge/discharge with a charge current of 2C and a discharge current of 2C, and the measured value was taken as the initial discharge capacity.
After that, set the environmental temperature to 45° C., set the charging current to 2 C and the discharging current to 2 C, and perform constant current charging and discharging 600 times. Then, set the environmental temperature to 25° C. again, charge current to 2 C, discharge current was set to 2 C, the discharge capacity was measured by constant current charging and discharging, and the discharging capacity after the cycle was obtained.
The capacity retention rate was calculated according to the cycle test according to the following formula (3).
Cycle test capacity retention rate = discharge capacity after cycle / initial discharge capacity (3)

From the results shown in Table 1, in the lithium ion secondary batteries using the positive electrode materials for lithium ion secondary batteries of Examples 1 to 3, the intensity of the peak on the high energy side of quadrupolar splitting of iron ions is Since the intensity is higher than the peak intensity on the low energy side of quadrupole splitting of ions, the cycle test capacity retention rate is high and the cycle characteristics are excellent.
On the other hand, in the lithium ion secondary batteries using the positive electrode materials for lithium ion secondary batteries of Comparative Examples 1 to 3, the intensity of the peak on the high energy side of the quadrupolar splitting of iron ions and the quadrupolar splitting of iron ions Since the intensity of the peak on the low energy side is equal to that of , the cycle test capacity retention rate is low and the cycle characteristics are poor. In addition, the lithium ion secondary battery using the positive electrode material for lithium ion secondary batteries of Comparative Example 3 and the lithium ion secondary battery using the positive electrode material for lithium ion secondary batteries of Example 2 are different from each other. Although the ratio is the same, the lithium ion secondary battery using the positive electrode material for lithium ion secondary batteries of Comparative Example 3 has a low first cycle capacity. The performance is inferior to that of the lithium ion secondary battery used.

The positive electrode material for lithium ion secondary batteries of the present invention is useful as a positive electrode for lithium ion secondary batteries.

Claims (6)

In the Mössbauer spectrum obtained by Mössbauer spectroscopy,
A positive electrode material for a lithium ion secondary battery, wherein the intensity of the quadrupolar splitting of iron ions on the high energy side is higher than the intensity of the quadrupolar splitting of iron ions on the low energy side. including primary particles in which a carbonaceous coating is formed on the surface of the electrode active material particles, and aggregated particles in which a plurality of the primary particles are aggregated,
2. The positive electrode material for a lithium ion secondary battery according to claim 1, having an average particle size of 0.3 [mu]m or more and 5.0 [mu]m or less as measured by a laser diffraction particle size distribution analyzer. 3. The positive electrode material for a lithium ion secondary battery according to claim 2, wherein said primary particles have an olivine structure with an average particle size of 50 nm or more and 500 nm or less. 4. The positive electrode material for a lithium ion secondary battery according to claim 2, wherein said carbonaceous film has a thickness of 0.5 nm or more and 10 nm or less. A positive electrode for a lithium ion secondary battery comprising an electrode current collector and a positive electrode mixture layer formed on the electrode current collector,
A positive electrode for a lithium ion secondary battery, wherein the positive electrode mixture layer contains the positive electrode material for a lithium ion secondary battery according to any one of claims 1 to 4. A lithium ion secondary battery having a positive electrode, a negative electrode and a non-aqueous electrolyte,
A lithium ion secondary battery comprising the positive electrode for a lithium ion secondary battery according to claim 5 as a positive electrode.

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