JP2013211110A - Electrode active material and lithium ion battery, and method for producing electrode active material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: an electrode active material achieving high load characteristics, high cycle characteristics and high energy density as well as having high safety and stability, and also capable of easily detecting a discharging end state; a lithium ion battery; and a method for producing an electrode active material.SOLUTION: In an electrode active material 1 of the present invention, a surface of a particle 2 comprising LiADO(where, A represents one or two selected from the group consisting of Mn and Co, D represents one or two or more selected from the group consisting of P, Si, and S, and 0<w≤4 and 0<x≤1.5 are satisfied) is covered with a coating layer 3 comprising a composite body of a carbonaceous electron conductive substance and LiEGO(where, E represents Fe or Fe and Ni, G represents one or two or more selected from the group consisting of P, Si, and S, and 0<y≤2 and 0<z≤1.5 are satisfied), and a third region is present in a second region in the backward of a first region where the discharge potential in a discharge curve is substantially constant, while a rate of change of the discharge potential in the third region is lower than an average rate of change of the discharge potential in the second region and the discharge potential in the second region decreases.

Description

  The present invention relates to an electrode active material, a lithium ion battery, and a method for producing the electrode active material. More specifically, the present invention is a kind of a phosphate-based electrode active material having an olivine structure, which has load characteristics, cycle characteristics, and energy density. The present invention relates to an electrode active material suitable for use as an electrode material for an excellent lithium ion battery, a lithium ion battery including an electrode using the electrode active material, and a method for producing the electrode active material.

In recent years, non-aqueous electrolyte secondary batteries such as lithium ion batteries have been proposed and put into practical use as batteries that are expected to be reduced in size, weight, and capacity.
The lithium ion battery is composed of a positive electrode and a negative electrode having a property capable of reversibly inserting and removing lithium ions, and a non-aqueous electrolyte.
Lithium-ion batteries are lighter, smaller, and have higher energy than secondary batteries such as conventional lead batteries, nickel cadmium batteries, and nickel metal hydride batteries, and are portable for portable telephones, notebook personal computers, etc. Although it is used as a power source for electronic devices, it has recently been studied as a high-output power source for electric vehicles, hybrid vehicles, electric tools, and the like. High-speed charge / discharge characteristics are required for the electrode active materials of batteries used as these high-output power supplies. Application to large batteries such as smoothing of power generation load, stationary power supply, backup power supply, etc. is also being studied, and it is resource-rich and inexpensive with long-term safety and reliability. Is also considered important.

The positive electrode of the lithium ion battery is composed of an electrode material including a lithium-containing metal oxide having a property capable of reversibly removing and inserting lithium ions called a positive electrode active material, a conductive additive, and a binder. A positive electrode is formed by applying this electrode material to the surface of a metal foil called a current collector.
As the positive electrode active material of this lithium ion battery, lithium cobaltate (LiCoO ₂ ) is usually used, but in addition, lithium nickelate (LiNiO ₂ ), lithium manganate (LiMn ₂ O ₄ ), phosphoric acid Lithium (Li) compounds such as iron lithium (LiFePO ₄ ) are used.
Among these lithium compounds, lithium cobaltate and lithium nickelate have various problems such as toxicity to the human body and the environment, the amount of resources, and instability of the charged state. Further, it has been pointed out that lithium manganate is dissolved in an electrolytic solution at a high temperature.
Therefore, in recent years, a phosphate-based electrode active material having an olivine structure typified by lithium iron phosphate, which has excellent long-term safety and reliability, has attracted attention.

Since this phosphate-based electrode active material does not have sufficient electron conductivity, various measures such as finer particles and compounding with conductive materials are required to charge and discharge a large current. Many efforts have been made.
However, there is a problem in that when particles are refined or complexed using a large amount of a conductive substance, the electrode density is lowered, which in turn causes a decrease in battery density, that is, a decrease in capacity per unit volume. There is a point. Therefore, as a method for solving this problem, an organic solution is used as a carbon precursor, which is an electronically conductive material, the organic solution and electrode active material particles are mixed, dried, and the resulting dried product is non-coated. A carbon coating method has been found in which the surface of the electrode active material particles is coated with carbon by heat treatment in an oxidizing atmosphere to carbonize the organic matter.

In this carbon coating method, the surface of the electrode active material particles can be coated with the minimum amount of carbon extremely efficiently, and the conductivity can be improved without greatly reducing the electrode density. Therefore, various proposals have been made.
On the other hand, in phosphate electrode active materials such as lithium manganese phosphate (LiMnPO ₄ ) and lithium cobalt phosphate (LiCoPO ₄ ) having an olivine structure, these elements (Mn, Co) are negative with respect to carbonization of organic matter. Due to the catalytic action, it has not been easy to coat a good conductive film.
Therefore, as one of means for solving such problems, the surface of lithium manganese phosphate (LiMnPO ₄ ), which is a negative catalytic active material, is coated with lithium iron phosphate, which is an active material having a high carbonization catalytic activity ( A method of coating with LiFePO ₄ ) has been proposed (Patent Document 1). This method is an effective means for forming a conductive coating on the surface of an electrode active material having a carbonization negative catalytic action such as lithium manganese phosphate (LiMnPO ₄ ) and cobalt lithium phosphate (LiCoPO ₄ ).

On the other hand, the inventors of the present application have also found the same means uniquely, and at the same time, as a more effective means, an element having carbonization catalytic activity is mixed with an organic substance, and then a negative catalytic active material is coated and heated. A carbonization method has been proposed (Patent Document 2).
According to this method, since the carbonization catalyst active element is complexed with the negative catalytic active material via the organic substance, the diffusion of the elements can be prevented even during the heating carbonization process of the organic substance, and a smaller catalyst amount However, it has sufficient carbonization activity. Therefore, it is possible to minimize a decrease in the fraction of lithium manganese phosphate (LiMnPO ₄ ), cobalt lithium phosphate (LiCoPO ₄ ), and the like that react at a higher potential.

International Publication No. 2011/032264 JP 2011-181375 A

By the way, in the conventional method of coating the surface of lithium manganese phosphate (LiMnPO ₄ ) with lithium iron phosphate (LiFePO ₄ ), an active material composed of a negative catalytic element and a carbonization catalytic element on the surface of the particle. Since the active material is in direct contact, these elements can easily diffuse under heating conditions that decompose and carbonize organic matter, resulting in a decrease in the concentration of the carbonized active element on the surface of the active material. That's not necessarily enough. Therefore, in order to prevent this decrease in the concentration of the carbonized active element, it is necessary to increase the thickness of the carbonized active material to some extent. As a result, lithium manganese phosphate (LiMnPO ₄ ) or phosphoric acid that reacts at a higher potential. There is a problem that the fraction of cobalt lithium (LiCoPO ₄ ) or the like is lowered and a sufficient capacity cannot be exhibited.

In addition, high potential positive electrode materials having an olivine structure typified by lithium manganese phosphate (LiMnPO ₄ ) and lithium cobalt phosphate (LiCoPO ₄ ) can be expected to have high energy density. It is known that the reaction proceeds in a two-phase reaction of the reduction phase and the reaction potential is almost flat up to the end of discharge. This is advantageous for extracting high energy, but on the other hand, the voltage does not drop so much until just before the end of the discharge. Therefore, when the battery is actually used as a power source for the device, the voltage is rapidly increased at the end of the discharge. There is a risk of lowering and causing device malfunction.

On the other hand, among phosphate-based electrode active materials, actives that can be used for capacity detection without impairing the advantages of lithium manganese phosphate (LiMnPO ₄ ) and lithium cobalt phosphate (LiCoPO ₄ ), which have high safety and stability. As the substance, lithium iron phosphate (LiFePO ₄ ) is most excellent. However, this lithium iron phosphate (LiFePO ₄ ) may be used in a small amount when used for capacity detection. However, when added in a small amount, it is possible to obtain a carbonaceous conductive coating for imparting conductivity for the reasons described above. Have difficulty. Thus, if added in a large amount, a carbonaceous conductive coating can be obtained, but there is a problem that the fraction of the active material having a high voltage is lowered and the discharge capacity is lowered.

  The present invention has been made in order to solve the above-described problems, and realizes high load characteristics, high cycle characteristics, and high energy density, and has high safety and stability, and the state at the end of discharge. An object of the present invention is to provide an electrode active material, a lithium ion battery, and a method for producing the electrode active material that can be easily detected.

As a result of intensive studies to solve the above problems, the present inventors have determined that Li _w A _x DO ₄ (where A is one or two selected from the group of Mn and Co, D is P, Li _y E _z GO ₄ (provided that E represents Fe) is the surface of the particle composed of one or more selected from the group of Si and S, 0 <w ≦ 4, 0 <x ≦ 1.5. , Fe and Ni, and G is one or more selected from the group of P, Si and S, 0 <y ≦ 2, 0 <z ≦ 1.5) and carbonaceous electrons In the second region where the discharge potential after the first region where the discharge potential of the discharge curve of the electrode active material coated with the coating layer composed of the composite with the conductive material is substantially constant decreases, There is a third region in which the change rate of the discharge potential is smaller than the average change rate of the discharge potential in the region 2, and this third region is detected. Ri, the state of the discharge end found that it is possible to easily detect, and have completed the present invention.

That is, the electrode active material of the present invention is Li _w A _x DO ₄ (where A is one or two selected from the group of Mn and Co, and D is selected from the group of P, Si, and S 1 The surface of the particle consisting of seeds or two or more, 0 <w ≦ 4, 0 <x ≦ 1.5) is Li _y E _z GO ₄ (where E is one of Fe, Fe and Ni) , G is one or more selected from the group consisting of P, Si, and S, 0 <y ≦ 2, 0 <z ≦ 1.5) and a composite of a carbonaceous electron conductive material An electrode active material coated with a layer, wherein the discharge potential of the second region is reduced in a second region where the discharge potential after the first region where the discharge potential of the discharge curve is substantially constant decreases. There is a third region where the change rate of the discharge potential is smaller than the average change rate.

The capacity of the third region at 60 ° C. is preferably 1/20 or more and 1/3 or less of the maximum value of the discharge capacity.
In this case, the reaction potential at 60 ° C. in the third region is preferably 3.0 V or more and 3.8 V or less.

  The lithium ion battery of the present invention is characterized by containing the electrode active material of the present invention in a positive electrode.

The production method of the electrode active material of the present invention is Li _w A _x DO ₄ (where A is one or two selected from the group of Mn and Co, and D is selected from the group of P, Si and S) Particles composed of one or more, 0 <w ≦ 4, 0 <x ≦ 1.5), a Li source, and an E source (where E is any of Fe, Fe, and Ni) And G source (where G is one or more selected from the group of P, Si and S) and an organic compound are mixed to obtain a mixture, and then the mixture is dried to obtain a dry product. Then, the organic compound is carbonized by heat-treating the dried product in a non-oxidizing atmosphere to generate a carbonaceous electron conductive material, and on the surface of the particles composed of Li _w A _x DO ₄ , Li _y E _z GO ₄ (where, E is made Fe, Fe and Ni, from either, G is P, 1 or 2 or more selected from the group of i and S, 0 <y ≦ 2, 0 <z ≦ 1.5) and a composite of the carbonaceous electron conductive material is formed. It is characterized by that.

  It is preferable to mix the Li source, the E source, the G source, and the organic compound so as to form a uniform liquid phase.

According to the electrode active material of the present invention, Li _w A _x DO ₄ (where A is one or two selected from the group of Mn and Co, and D is one selected from the group of P, Si, and S) The surface of the particle consisting of seeds or two or more, 0 <w ≦ 4, 0 <x ≦ 1.5) is Li _y E _z GO ₄ (where E is one of Fe, Fe and Ni) , G is one or more selected from the group consisting of P, Si, and S, 0 <y ≦ 2, 0 <z ≦ 1.5) and a composite of a carbonaceous electron conductive material In the second region where the discharge potential after the first region where the discharge potential of the discharge curve in the electrode active material covered with the layer is substantially constant decreases, the discharge rate is determined from the average rate of change of the discharge potential in the second region. since the third area change rate is small potential exists, active product the third region is composed of the above-described Li _w a _x DO ₄ It shows a shoulder-like or step-like reaction curve that reacts at a potential lower than the reaction potential of the above. Therefore, by detecting this third region, the state at the end of discharge can be easily detected. As a result, the end point of the discharge capacity of this electrode active material can be easily estimated.

According to the lithium ion battery of the present invention, since the electrode active material of the present invention is contained in the positive electrode, the state at the end of discharge can be easily detected, and the end point of the discharge capacity can be easily estimated. Therefore, when this lithium ion battery is applied to the power source of the device, it is possible to prevent the voltage from rapidly decreasing at the end of discharge and causing malfunction of the device.
As described above, it is possible to provide a lithium ion battery having high voltage, high energy density, and high load characteristics, and excellent in long-term cycle stability and safety.

According to the method for producing an electrode active material of the present invention, Li _w A _x DO ₄ (where A is one or two selected from the group of Mn and Co, and D is selected from the group of P, Si, and S) 1 type or 2 types or more, 0 <w ≦ 4, 0 <x ≦ 1.5), a Li source, and an E source (where E is one of Fe, Fe and Ni) ), A G source (where G is one or more selected from the group of P, Si, S) and an organic compound to form a mixture, and then the mixture is dried to dry Next, the organic compound is carbonized by heat-treating the dried product in a non-oxidizing atmosphere to generate a carbonaceous electron conductive material, and the surface of the particles composed of Li _w A _x DO ₄ is formed on the surface of the particles. , _Li y _E z GO ₄ (where, E is made Fe, Fe and Ni, from either, G A coating layer comprising a composite of one or more selected from the group of P, Si, and S, 0 <y ≦ 2, 0 <z ≦ 1.5) and the carbonaceous electron conductive material. Since it is generated, it is possible to easily detect the end stage state of the discharge and to easily produce an electrode active material that can easily estimate the end point of the discharge capacity.

It is sectional drawing which shows the electrode active material of one Embodiment of this invention. It is a scanning electron microscope (SEM) image which shows the electrode active material of Example 4 of this invention. It is a figure which shows the charging / discharging curve of the lithium ion battery of Example 1 of this invention. It is a figure which shows the charging / discharging curve of the lithium ion battery of Example 3 of this invention. It is a figure which shows the charging / discharging curve of the lithium ion battery of Example 5 of this invention. It is a figure which shows the charging / discharging curve of the lithium ion battery of a comparative example. It is a figure which shows the discharge differential curve of the lithium ion battery of Example 3 of this invention.

The form for implementing the manufacturing method of the electrode active material of this invention, a lithium ion battery, and an electrode active material is demonstrated.
This embodiment is specifically described for better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified.

[Electrode active material]
FIG. 1 is a cross-sectional view showing an electrode active material according to an embodiment of the present invention. The electrode active material 1 is Li _w A _x DO ₄ (where A is one selected from the group consisting of Mn and Co). Or 2 types, D is 1 type or 2 types or more selected from the group of P, Si, and S, 0 <w ≦ 4, 0 <x ≦ 1.5) (hereinafter, Li _w A _x DO ₄₎ The surface of 2 is abbreviated as Li _y E _z GO ₄ (where E is one of Fe, Fe and Ni, and G is one selected from the group of P, Si and S) The coating layer 3 is made of a composite of two or more, 0 <y ≦ 2, 0 <z ≦ 1.5) and a carbonaceous electron conductive material.

The average particle diameter of the Li _w A _x DO ₄ particles 2 is preferably 5 nm or more and 500 nm or less, more preferably 20 nm or more and 200 nm or less.
The reason is that if the average particle size is smaller than 5 nm, the crystal structure may be destroyed due to the volume change due to charge / discharge, and if the average particle size is larger than 500 nm, electrons are supplied into the particles. This is because the amount is insufficient and the utilization efficiency is lowered.

The coating layer 3 includes Li _w A _x DO ₄ particles 2, a Li source, an E source (where E is one of Fe, Fe and Ni), a G source (where G is P, Li _y E _z GO ₄ and carbonaceous electrons generated by mixing an organic compound with one or more selected from the group of Si and S, and then heat-treating in a non-oxidizing atmosphere It is a conductive layer made of a composite with a conductive substance.

The coating layer 3 preferably contains 30% by mass or more and 99% by mass or less, more preferably 50% by mass or more and 95% by mass, in terms of carbon, when the carbonaceous electronic conductive material is converted to carbon. It is as follows.
The carbonaceous electronic conductive material can impart desired electronic conductivity to the electrode active material 1 by containing the carbonaceous material in a carbon conversion value of 30 mass% or more and 99 mass% or less.
Also, Li _w A _x DO ₄ particles 2, Li source, by appropriately changing the content of the composition ratio and each of the E sources and G source, it is possible to easily impart the desired remaining capacity detection.

The thickness of the coating layer 3 is preferably 0.1 nm or more and 25 nm or less, more preferably 2 nm or more and 10 nm or less.
The reason is that if the thickness is less than 0.1 nm, the electronic conductivity of the coating layer 3 itself is insufficient, and as a result, the electronic conductivity as the electrode active material 1 is greatly reduced. This is because if the thickness is greater than 25 nm, the ratio of the high-voltage active material in the electrode active material 1 is reduced, and the active material is hardly used effectively.

Thus, taking into consideration the average particle diameter of Li _w A _x DO ₄ particles 2 and the thickness of the coating layer 3, the average particle diameter of the electrode active material 1 is 5 nm to 550 nm, preferably 20 nm to 300 nm. Become.
Since this electrode active material 1 has a sharp average particle diameter range and excellent monodispersibility, when this electrode active material 1 is used as a positive electrode of a lithium ion battery, the electrical characteristics of this positive electrode are extremely high. It becomes uniform and the variation in characteristics becomes extremely small. Therefore, the obtained lithium ion battery has high voltage, high energy density, high load characteristics, and excellent long-term cycle stability and safety.

[Method for producing electrode active material]
In the method for producing an electrode active material of the present embodiment, Li _w A _x DO ₄ particles, a Li source, an E source, a G source, and an organic compound are mixed to form a mixture, and then the mixture is dried. The dried product is then heat-treated in a non-oxidizing atmosphere to carbonize the organic compound to produce a carbonaceous electron conductive material. On the surface of the Li _w A _x DO ₄ particles, In this method, a coating layer made of a composite of Li _y E _z GO ₄ and a carbonaceous electron conductive material is generated.

_{Incidentally,} Li _w A x DO ₄ particles, Li source, the A source and D sources, these molar ratio (Li source: A source: D source) w: x: 1 and the main component of water so The precursor solution of Li _w A _x DO ₄ is stirred to give a precursor solution of Li _w A _x DO ₄ , and this precursor solution is put in a pressure resistant container, and at a high temperature and high pressure, for example, 120 ° C. or higher and 250 ° C. or lower, 0.2 MPa or higher Can be obtained by hydrothermal treatment for 1 hour or more and 24 hours or less.

Examples of the Li source used in the method for producing the electrode active material include lithium hydroxide (LiOH), lithium carbonate (Li ₂ CO ₃ ), lithium chloride (LiCl), and lithium phosphate (Li ₃ PO ₄ ). One or more selected from the group consisting of lithium inorganic acid salts, lithium organic acid salts such as lithium acetate (LiCH ₃ COO) and lithium oxalate ((COOLi) ₂ ), and hydrates thereof are preferably used. It is done. In particular, a raw material that forms a uniform solution phase with an E source, a G source, and an organic compound, such as lithium chloride and lithium acetate, is preferable.

As the E source, a compound containing any of Fe, Fe and Ni, for example, iron chloride (II) (FeCl ₂ ), iron sulfate (II) (FeSO ₄ ), iron acetate (II) (Fe (CH ₃ COO) ₂ ) and the like, or hydrates thereof, or these iron compounds or hydrates thereof, nickel chloride (II) (NiCl ₂ ), nickel sulfate (II) (NiSO ₄ ), nickel acetate ( II) Nickel compounds such as (Ni (CH ₃ COO) ₂ ) or mixtures thereof with hydrates are preferably used.

Moreover, since the reduction effect by an organic compound can be expected, the iron compounds include iron nitrate (III) (Fe (NO ₃ ) ₃ ), iron chloride (III) (FeCl ₃ ), iron citrate (III) (FeC ₆ Trivalent iron compounds such as H ₅ O ₇ ) are also preferably used.
In particular, iron (II) chloride (FeCl ₂ ), iron (II) acetate (Fe (CH ₃ COO) ₂ ), iron (II) sulfate (FeSO ₄ ), iron nitrate (III) (Fe (NO ₃ ) ₃ ) , Iron (III) citrate (FeC ₆ H ₅ O ₇ ) and the like are preferable because they form a uniform solution phase with the Li source, the G source and the organic compound.

Examples of the G source include phosphoric acid such as orthophosphoric acid (H ₃ PO ₄ ) and metaphosphoric acid (HPO ₃ ), ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ), and diammonium hydrogen phosphate ((NH ₄ ) _2. HPO ₄ ), ammonium phosphate ((NH ₄ ) ₃ PO ₄ ), and phosphate sources such as hydrates thereof, silicon oxide (SiO ₂ ), silicon tetramethoxide (Si (OCH ₃ ) ₄ ), etc. One or more selected from the group of Si sources such as silicon alkoxide, S sources such as diammonium sulfate ((NH ₄ ) ₂ SO ₄ ) and sulfuric acid (H ₂ SO ₄ ) are preferably used.
In particular, orthophosphoric acid, sulfuric acid and the like are preferable because they form a uniform solution phase with the Li source, the E source and the organic compound.

  The organic compound is not particularly limited as long as it is an organic compound that generates carbon by heat treatment in a non-oxidizing atmosphere. For example, higher monohydric alcohols such as hexanol and octanol, allyl alcohol, propinol ( (Propargyl alcohol), unsaturated monohydric alcohols such as terpineol, sugars such as glucose, sucrose, and lactose, polyvinyl alcohol (PVA), and the like. In particular, glucose, sucrose, polyvinyl alcohol (PVA) and the like are preferable because they form a uniform solution phase with a Li source, an E source, a G source and an organic compound.

These Li source, E source, G source and organic compound may be used in combination to form a uniform solution phase, and there are no particular limitations on individual materials.
Moreover, in order to form a uniform solution phase, you may add pH adjusters, such as an acid and an alkali.
As such a pH adjuster, one or more selected from the group of inorganic acids such as hydrochloric acid, sulfuric acid and nitric acid, and organic acids such as formic acid, acetic acid, citric acid, lactic acid and ascorbic acid are preferably used. Used. In particular, an organic acid is preferable because no residue other than carbon is produced after thermal decomposition.

These Li source, E source, the concentration of the organic compounds in the mixture obtained by mixing G source and an organic compound (slurry) is not particularly limited, the surface of the Li _w A _x DO ₄ particles, Li _In order to uniformly form a coating layer composed of a composite of _y E _z GO ₄ and a carbonaceous electron conductive material, the content is preferably 1% by mass or more and 25% by mass or less.

  The solvent for dissolving the organic compound is not particularly limited as long as it dissolves the organic compound. For example, water, methanol, ethanol, 1-propanol, 2-propanol (isopropyl alcohol: IPA), butanol , Alcohols such as pentanol, hexanol, octanol, diacetone alcohol, ethyl acetate, butyl acetate, ethyl lactate, propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, esters such as γ-butyrolactone, diethyl ether, ethylene Glycol monomethyl ether (methyl cellosolve), ethylene glycol monoethyl ether (ethyl cellosolve), ethylene glycol monobutyl ether (butyl cellosolve), die Glycol monomethyl ether, ethers such as diethylene glycol monoethyl ether.

  Also, acetone, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), ketones such as acetylacetone, cyclohexanone, amides such as dimethylformamide, N, N-dimethylacetoacetamide, N-methylpyrrolidone, ethylene glycol, diethylene glycol, propylene Examples thereof include glycols such as glycol. These may be used alone or in admixture of two or more. However, safety and price, dissolution of Li source, E source and G source and organic compounds are dissolved. Water is preferred because of its ease.

In this production method, a mixture obtained by mixing Li _w A _x DO ₄ particles, a Li source, an E source, a G source, an organic compound, and, if necessary, a solvent is heated to 50 ° C. to 200 ° C. in a dryer. The dried product is dried for 1 to 48 hours, and then the dried product is treated with a non-oxidizing atmosphere, for example, an inert atmosphere such as nitrogen (N ₂ ) gas or hydrogen (H ₂ ) gas in an amount of 2 to 5% by volume. An organic compound is carbonized by heat-treating in a reducing atmosphere such as nitrogen (N ₂ ) gas containing carbon to generate a carbonaceous electron conductive material, and Li _y E _z is formed on the surface of Li _w A _x DO ₄ particles. A coating layer made of a composite of GO ₄ and a carbonaceous electron conductive material is generated.
Also, Li _w A _x DO ₄ particles, Li source, E source, G source, an organic compound, it is also possible to use a spray drier for drying the mixture obtained by mixing a solvent, if necessary. When dried using a spray dryer, it can be expected that the positive electrode fillability and productivity can be improved by obtaining a spherical electrode active material.

As the heat treatment condition, by generating an electronic conductive material of the carbonaceous and organic compound is carbonized, Li _w A _x DO ₄ the surface of the particles, lithium manganese phosphate (LiMnPO ₄₎ or cobalt phosphate lithium ( LiCoPO ₄₎ and Li _y E _z GO ₄ at a low potential than the active material of the center shows an electrochemical reaction, such as in the range of temperature and time that the coating layer is produced consisting of a complex with electron-conductive material of the carbonaceous For example, the heat treatment temperature is preferably 500 ° C. or more and 1000 ° C. or less, and the heat treatment time is preferably 1 hour or more and 24 hours or less depending on the temperature during the heat treatment.

As described above, the surface of the Li _w A _x DO ₄ particles 2 is covered with the coating layer 3 made of the composite of Li _y E _z GO ₄ and the carbonaceous electron conductive material, and the average particle diameter is 5 nm or more and 550 nm. Hereinafter, the electrode active material 1 having a thickness of preferably 20 nm or more and 300 nm or less can be easily produced.

[Lithium ion battery]
The lithium ion battery of this embodiment contains the electrode active material of this embodiment in the positive electrode.
In order to produce the positive electrode of this embodiment, the electrode active material, a binder composed of a binder resin, and a solvent are mixed to prepare an electrode forming paint or an electrode forming paste. At this time, a conductive aid such as carbon black may be added as necessary.

As the binder, that is, the binder resin, for example, polytetrafluoroethylene (PTFE) resin, polyvinylidene fluoride (PVdF) resin, fluororubber, and the like are preferably used.
The mixing ratio of the electrode active material and the binder resin is not particularly limited. For example, the binder resin is 1 part by mass or more and 30 parts by mass or less, preferably 3 parts by mass or more and 100 parts by mass of the electrode active material. 20 parts by mass or less.
As the solvent used for the electrode forming paint or electrode forming paste, a solvent similar to the solvent for dissolving the organic compound described above is suitable, and description of the solvent is omitted here.

Next, this electrode-forming paint or electrode-forming paste is applied to one side of the metal foil, and then dried to form a coating film comprising a mixture of the above electrode material and binder resin on one side. Get a metal foil.
Next, this coating film is pressure-bonded and dried to produce a current collector (electrode) having a positive electrode layer on one surface of the metal foil.
By using the current collector (electrode) as a positive electrode, a lithium ion battery can be obtained.

In this lithium ion battery, during the second region where the discharge potential after the first region in which the discharge potential of the discharge curve is substantially constant decreases, the discharge potential of the second region is determined based on the average change rate of the discharge potential in the second region. There is a third region (hereinafter referred to as a shoulder portion) having a small change rate.
Here, the capacity of the shoulder portion at 60 ° C. is preferably 1/20 or more and 1/3 or less of the maximum value of the discharge capacity.
The reason why the capacity at 60 ° C. of the shoulder portion is limited to the above range is that this range is a shoulder shape or a step shape that reacts at a potential lower than the reaction potential of the active material made of Li _w A _x DO ₄ . This is because the reaction curve can be sufficiently detected and the remaining capacity remaining after the detection can be sufficiently secured. As a result, it is possible to avoid the serious problem of causing a malfunction due to a rapid voltage drop at the end of discharge in the device, while eliminating the possibility of greatly damaging the capacity of the high potential portion.

  In this lithium ion battery, by detecting the discharge potential at 60 ° C. of the shoulder portion and confirming that the detected value is in the above range, the state at the end of discharge can be easily detected. As a result, the end point of the discharge capacity of this electrode active material can be easily estimated.

In this lithium ion battery, the reaction potential at 60 ° C. of the shoulder portion is preferably 3.0 V or more and 3.8 V or less.
The reason why the reaction potential of this shoulder portion at 60 ° C. is limited to the above range is that this range has a distinctly different potential from the high potential portion, so that the detection is easy and the energy of the remaining capacity portion is sufficient. This is because it can be secured at a high level.

As described above, according to the electrode active material 1 of the present embodiment, the surface of the Li _w A _x DO ₄ particle 2 is made of a composite of Li _y E _z GO ₄ and a carbonaceous electron conductive material. The discharge potential of the second region is covered by the coating layer 3 and the second region where the discharge potential after the first region where the discharge potential of the discharge curve in the electrode active material 1 is substantially constant decreases. Since there is a third region where the change rate of the discharge potential is smaller than the average change rate, the shoulder region or step shape in which the third region reacts at a potential lower than the reaction potential of the Li _w A _x DO ₄ particles 2. Therefore, by detecting this shoulder portion, it is possible to easily detect the end stage of the discharge, and as a result, to easily estimate the end point of the discharge capacity of the electrode active material 1. Can do.

According to the lithium ion battery of the present embodiment, since the electrode active material of the present embodiment is contained in the positive electrode, the state at the end of discharge can be easily detected, and the end point of the discharge capacity can be easily estimated. it can. Therefore, when this lithium ion battery is applied to the power source of the device, it is possible to prevent the voltage from rapidly decreasing at the end of discharge and causing malfunction of the device.
As described above, it is possible to provide a lithium ion battery having high voltage, high energy density, and high load characteristics, and excellent in long-term cycle stability and safety.

According to the method for producing an electrode active material of the present embodiment, Li _w A _x DO ₄ particles, a Li source, an E source, a G source, and an organic compound are mixed to form a mixture, and then this mixture is converted into a mixture. The dried product is dried to obtain a carbonaceous electron conductive material by carbonizing the organic compound by heat-treating the dried product in a non-oxidizing atmosphere, and particles of Li _w A _x DO ₄ are produced. Since a coating layer made of a composite of Li _y E _z GO ₄ and a carbonaceous electron conductive material is generated on the surface, the end state of the discharge can be easily detected, and the end point of the discharge capacity can be easily determined An electrode active material that can be estimated can be easily produced.

  EXAMPLES Hereinafter, although this invention is demonstrated concretely by Examples 1-5 and a comparative example, this invention is not limited by these Examples.

(Synthesis of LiMnPO ₄ particles)
LiMnPO ₄ common to Examples 1 to 4 and Comparative Example was prepared as follows.
Li ₃ PO ₄ was used as the Li source and P source, MnSO ₄ .5H ₂ O was used as the Mn source, and these were dissolved in pure water so that the molar ratio was Li: Mn: P = 3: 1: 1. 200 mL of the precursor solution was prepared.

Subsequently, this precursor solution was put into a pressure vessel and hydrothermal synthesis was performed at 170 ° C. for 24 hours. After this reaction, the reaction mixture was cooled to room temperature to obtain a precipitated cake-like reaction product.
Next, the precipitate was washed with distilled water 5 times to wash away impurities, and then kept at a water content of 30% so as not to be dried, to obtain cake-like LiMnPO ₄ .
A small amount of sample was taken from this cake-like LiMnPO ₄ and vacuum-dried at 70 ° C. for 2 hours, and the powder obtained was identified by X-ray diffraction. As a result, single-phase LiMnPO ₄ was produced. It was confirmed that

(Synthesis of LiCoPO ₄ particles)
LiCoPO ₄ of Example 5 was produced as follows.
Li ₃ PO ₄ was used as the Li source and P source, CoSO ₄ .7H ₂ O was used as the Co source, and these were dissolved in pure water so that the molar ratio was Li: Co: P = 3: 1: 1. 200 mL of the precursor solution was prepared.

Subsequently, this precursor solution was put into a pressure vessel and hydrothermal synthesis was performed at 170 ° C. for 24 hours. After this reaction, the reaction mixture was cooled to room temperature to obtain a precipitated cake-like reaction product.
Next, the precipitate was washed with distilled water 5 times to wash away impurities, and then kept at a moisture content of 30% so as not to be dried, to obtain cake-like LiCoPO ₄ .
A small amount of sample was taken from the cake-like LiCoPO ₄ and vacuum-dried at 70 ° C. for 2 hours. The powder obtained was identified by X-ray diffraction. As a result, single-phase LiCoPO ₄ was produced. It was confirmed that

Example 1
Polyvinyl alcohol 10% aqueous solution as an organic compound so as to be 5 parts by mass in terms of solid content, LiCH ₃ COO as a Li source, Fe (CH ₃ COO) ₂ as a Fe source, and H ₃ PO ₄ as a phosphoric acid source Each mass was adjusted so as to be 5 parts by mass in terms of LiFePO ₄ , poured into pure water, dissolved by stirring, and a transparent and uniform solution was obtained. Into this solution, 95 parts by mass of LiMnPO ₄ was added, and the mixture was stirred and suspended. The resulting slurry was dried at 100 ° C. for 10 hours using a drier. Heat treatment was performed for a time, and the electrode active material of Example 1 was obtained.

(Example 2)
An electrode active material of Example 2 was obtained in the same manner as Example 1 except that FeSO ₄ was used instead of Fe (CH ₃ COO) ₂ as the Fe source.

(Example 3)
As an organic compound, a 10% aqueous solution of polyvinyl alcohol so that the solid content is 5 parts by mass, LiCH ₃ COO as a Li source, iron (III) citrate (FeC ₆ H ₅ O ₇ ) and phosphoric acid as a Fe source As a source, each mass was adjusted so that H ₃ PO ₄ was converted to LiFePO ₄ to 8 parts by mass, and each mass was adjusted and poured into pure water, and dissolved by stirring to obtain a transparent and uniform solution. Into this solution, 92 parts by mass of LiMnPO ₄ was added, stirred and suspended, and the resulting slurry was dried at 100 ° C. for 10 hours using a drier. Heat treatment was performed for a time, and the electrode active material of Example 3 was obtained.

Example 4
The electrode active material of Example 4 was obtained in the same manner as in Example 3 except that the slurry was dried at 120 ° C. using a spray dryer instead of drying at 100 ° C. for 10 hours using a dryer. It was. A scanning electron microscope (SEM) image of the electrode active material of Example 4 is shown in FIG.

(Example 5)
An electrode active material of Example 5 was obtained in the same manner as Example 3 except that LiCoPO ₄ was used instead of LiMnPO ₄ .

(Comparative example)
A solution containing an organic compound, a Li source, an Fe source and a phosphate source was obtained in the same manner as in Example 1 except that LiOH was used as the Li source and (NH ₄ ) H ₂ PO ₄ was used as the phosphate source. It was. A precipitate was formed in this solution.
Furthermore, 95 parts by mass of LiMnPO ₄ was added to this solution, and stirring, drying and heat treatment were performed in the same manner as in Example 1 to obtain a comparative electrode active material.

“Production of lithium-ion batteries”
The positive electrodes of Examples 1 to 5 and Comparative Examples were prepared.
Here, each electrode active material obtained in each of Examples 1 to 5 and Comparative Example, acetylene black (AB) as a conductive additive, polyvinylidene fluoride (PVdF) as a binder, and N-methyl-2-pyrrolidinone as a solvent Using (NMP), these were mixed to prepare pastes of Examples 1 to 5 and Comparative Examples. The mass ratio in the paste, LiMnPO ₄ or LiCoPO ₄ : AB: PVdF, was 85: 10: 5.
Next, these pastes were applied onto an aluminum (Al) foil having a thickness of 30 μm and dried. Then, it compacted with the pressure of 40 Mpa, and set it as the positive electrode.

Next, this positive electrode was punched out into a disk shape having an area of 2 cm ² using a molding machine, vacuum dried, and then subjected to Example 1 using a 2032 coin type cell made of stainless steel (SUS) in a dry Ar atmosphere. Lithium ion batteries of ˜5 and Comparative Example were prepared. Metal Li was used for the negative electrode, a porous polypropylene film was used for the separator, and a 1M LiPF ₆ solution was used for the electrolyte solution. As a solvent for this LiPF ₆ solution, a solvent having a ratio of ethylene carbonate to diethyl carbonate of 1: 1 was used.

"Battery characteristics test"
The battery characteristics test of each of the lithium ion batteries of Examples 1 to 5 and Comparative Example was performed until the potential of the test electrode reached a predetermined charging voltage with respect to the Li equilibrium potential at an environmental temperature of 60 ° C. and a charging current of 0.1 CA. The battery was charged and rested for 1 minute, and then discharged to 2.0 V with a discharge current of 0.1 CA.
The charging voltage was 4.5 V for the Mn type lithium ion batteries of Examples 1 to 4 and the comparative example, and 4.9 V for the Co type lithium ion battery of Example 5.

In Example 2, FeSO ₄ was used in place of Fe (CH ₃ COO) ₂ as the Fe source, but the discharge curve was almost the same as that of the lithium ion battery of Example 1.
Moreover, in Example 4, it dried using the spray dryer instead of the dryer, but the discharge curve was the same as that of the lithium ion battery of Example 3. In Example 4, a spherical electrode active material was obtained by using a spray dryer, the positive electrode filling property was improved, and the productivity was also improved.

  FIG. 3 shows the discharge curve of the lithium ion battery of Example 1 at an environmental temperature of 60 ° C., FIG. 4 shows the discharge curve of the lithium ion battery of Example 3, and FIG. 5 shows the discharge curve of the lithium ion battery of Example 5. FIG. 6 shows a discharge curve of the lithium ion battery of the comparative example. 3-5, the arrow shows the position of the shoulder portion.

With respect to the discharge curve of the lithium ion battery of Example 3, a differential curve obtained by differentiating the capacity with voltage was obtained. The results are shown in FIG.
The shoulder portion starting voltage (shoulder voltage) obtained from the maximum value of the differential curve was 3.70 V, and the discharge capacity (capacity before shoulder) to the shoulder starting point was 140 mAh / g. On the other hand, the discharge capacity at 2.00 V is 155 mAh / g, and the ratio (shoulder capacity ratio) of the capacity after the shoulder portion (capacity at 60 ° C. in the third region) to the total capacity (maximum discharge capacity). Was (155-140) /155=0.097.
The same evaluation was performed for the lithium ion batteries of Examples 1, 2, 4, 5 and Comparative Examples. These evaluation results are shown in Table 1.

According to Table 1, in Examples 1-5, the shoulder voltage 3.5~3.7V from LiFePO ₄ contained in the coating layer 3 made of a complex with electron-conductive material LiFePO ₄ and the carbonaceous A shoulder capacity ratio of 5% or more was recognized. This is because if capacity detection is performed with a shoulder voltage, a warning can be issued when the capacity remains at 5% or more, and there is sufficient time margin to prevent malfunction of the device due to sudden voltage drop in advance. It turns out that it is obtained.

On the other hand, in the comparative example, no shoulder voltage is observed in the charge / discharge curve, and the shoulder capacity ratio obtained from the differential curve is as small as 1.4%, which is the same as that in the carbonaceous coating layer as compared with Example 1. In spite of the presence of LiFePO _4, a sufficient time margin for preventing malfunction of the device due to a sudden voltage drop cannot be obtained in advance, and there is a concern that the malfunction of the device due to a sudden voltage drop may occur. It became the result.
Furthermore, according to this example, the carbonaceous coating layer capable of imparting good conductivity to LiMnPO ₄ even when LiFePO ₄ is less than 10% by mass with respect to LiMnPO ₄ . As a result, the capacity to react at a high voltage characteristic of LiMnPO ₄ could be sufficiently maintained.

In Examples 1 to 5, in order to reflect the behavior of the electrode active material itself in the data, metallic lithium was used for the negative electrode, but instead of metallic lithium, carbon materials such as natural graphite, artificial graphite, and coke, lithium An anode material such as an alloy or Li ₄ Ti ₅ O ₁₂ may be used.
Moreover, although acetylene black was used as a conductive support agent, carbon materials such as carbon black, graphite, ketjen black, natural graphite, and artificial graphite may be used.

Further, a LiPF ₆ solution in the electrolyte solution, the ratio of ethylene carbonate and diethyl carbonate as the solvent for this LiPF ₆ solution 1: 1 of what has been used, respectively, LiBF ₄ solution and LiClO ₄ in place of LiPF ₆ solution A solution may be used, and propylene carbonate or diethyl carbonate may be used instead of ethylene carbonate.
Moreover, you may use a solid electrolyte instead of electrolyte solution and a separator.

1 the electrode active material 2 Li _w A _x DO ₄ particles 3 covering layer

Claims (6)

Li _w A _x DO ₄ (where A is one or two selected from the group of Mn and Co, D is one or more selected from the group of P, Si and S, and 0 <w ≦ 4, 0 <x ≦ 1.5), Li _y E _z GO ₄ (where E is one of Fe, Fe and Ni, G is a group of P, Si and S) An electrode active material coated with a coating layer composed of a composite of one or more selected from the group consisting of 0 <y ≦ 2, 0 <z ≦ 1.5) and a carbonaceous electron conductive material. There,
In the second region where the discharge potential after the first region in which the discharge potential of the discharge curve is substantially constant decreases, the third change rate of the discharge potential is smaller than the average change rate of the discharge potential in the second region. An electrode active material characterized by the presence of a region.   2. The electrode active material according to claim 1, wherein the capacity of the third region at 60 ° C. is not less than 1/20 and not more than 1/3 of the maximum value of the discharge capacity.   The electrode active material according to claim 2, wherein a reaction potential at 60 ° C. of the third region is 3.0 V or more and 3.8 V or less.   A lithium ion battery comprising the electrode active material according to any one of claims 1 to 3 in a positive electrode. Li _w A _x DO ₄ (where A is one or two selected from the group of Mn and Co, D is one or more selected from the group of P, Si and S, and 0 <w ≦ 4, 0 <x ≦ 1.5), a Li source, an E source (where E is one of Fe, Fe and Ni), a G source (where G is P, 1 type or 2 or more types selected from the group of Si and S) and an organic compound are mixed to form a mixture, and then the mixture is dried to obtain a dry product. the organic compounds are carbonized to generate electron-conducting substance carbonaceous by heat treatment in the _Li w _a to the surface of the x DO of _{four particles,} Li _y E z GO ₄ (where, E is, It consists of any one of Fe, Fe and Ni, and G is one kind selected from the group of P, Si and S 2 or more, 0 <y ≦ 2, 0 <z ≦ 1.5) and a coating layer made of a composite of the carbonaceous electron conductive material is produced. .   6. The method for producing an electrode active material according to claim 5, wherein the Li source, the E source, the G source, and the organic compound are mixed so as to form a uniform liquid phase.