JP2014017154A - Positive electrode active material for lithium secondary battery and lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an FeOHSO-based positive electrode active material exhibiting excellent battery capacity.SOLUTION: There are provided a positive electrode active material for a lithium secondary battery having a composition represented by the following general formula (1), and a lithium secondary battery including a positive electrode containing the positive electrode active material. FeMOHSO(1) (In the formula (1), M represents a transition metal element other than Fe, and x represents a number satisfying 0<x<1.)

Description

  The present invention relates to a positive electrode active material for a lithium secondary battery and a lithium secondary battery including the positive electrode active material.

  In recent years, with the rapid spread of information-related equipment such as personal computers, video cameras, and mobile phones, and communication equipment, development of batteries that are used as power sources has been regarded as important. Also in the automobile industry, development of high-power and high-capacity batteries for electric vehicles and hybrid vehicles is underway. Among various batteries, lithium secondary batteries are attracting attention because of their high energy density and output.

A typical lithium secondary battery has a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and an electrolyte interposed between the positive electrode and the negative electrode.
As positive electrode active materials for lithium secondary batteries, for example, LiCoO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiCoMnO _{4 and the} like are known. Moreover, what was described in patent document 1 and nonpatent literature 1 is also mentioned. Specifically, Non-Patent Document 1 discloses a lithium ion secondary battery using FeOHSO ₄ as a positive electrode active material.

JP 2004-139947 A

Electrochemistry Communications (2009) Volume 11, 1807-1810

Conventional positive electrode active materials still have problems in terms of cost, capacity characteristics, cycle characteristics, voltage characteristics, and the like, and further research and development are required.
On the other hand, FeOHSO ₄ described in Non-Patent Document 1 is a new positive electrode active material that does not contain Li and is different from the conventional positive electrode active material. In particular, since the raw material is iron and the synthesis method is inexpensive, it is expected as a low-cost material.
However, FeOHSO ₄ has a problem that a practical capacity cannot be obtained because of low Li diffusibility and low electron conductivity.

The present invention has been accomplished in view of the above circumstances, and an object of the present invention is to provide a FeOHSO ₄ positive electrode active material exhibiting excellent battery capacity.

The positive electrode active material for a lithium secondary battery of the present invention is characterized by having a composition represented by the following general formula (1).
Fe _1-x M _x OHSO ₄ (1)
(In formula (1), M is a transition metal element other than Fe, and x is a number satisfying 0 <x <1.)
The positive electrode active material for a lithium secondary battery of the present invention having the composition of the above formula (1) has an excellent battery capacity.

The positive electrode active material for a lithium secondary battery of the present invention preferably contains at least one selected from Ni and Co as M in the general formula (1).
In the positive electrode active material of the present invention, it is preferable that, in the general formula (1), the M is Ni and 0 <x ≦ 0.1.
In the positive electrode active material of the present invention, it is preferable that in the general formula (1), the M is Co and 0 <x ≦ 0.2.

  The lithium secondary battery of the present invention is a lithium secondary battery comprising a positive electrode, a negative electrode, and an electrolyte interposed between the positive electrode and the negative electrode, wherein the positive electrode is the positive electrode for a lithium secondary battery of the present invention. It is characterized by containing an active material.

According to the present invention, it is possible to provide a FeOHSO ₄ based positive active material showing excellent battery capacity.

It is a cross-sectional schematic diagram which shows the structural example of a lithium secondary battery. It is an XRD pattern of Examples 1-7 and Comparative Example 1. It is an XRD pattern of Examples 8-12. 6 is a graph showing initial discharge capacities of Examples 1 to 12 and Comparative Example 1.

[Positive electrode active material for lithium secondary battery]
The positive electrode active material for lithium secondary batteries of the present invention (hereinafter sometimes simply referred to as a positive electrode active material) has a composition represented by the following general formula (1).
Fe _1-x M _x OHSO ₄ (1)
(In formula (1), M is a transition metal element other than Fe, and x is a number satisfying 0 <x <1.)

As a result of intensive studies to improve the battery capacity of FeOHSO ₄ , the present inventor has found that a high battery capacity can be obtained by substituting a part of Fe with another transition metal element. This is presumably because the lithium ion conductivity and electronic conductivity of FeOHSO ₄ were improved by substituting a part of Fe with the transition metal element M. When the crystal structures of FeOHSO ₄ and Fe _1-x M _x OHSO ₄ were compared, it was confirmed that the crystal structure of FeOHSO ₄ was maintained in Fe _1-x M _x OHSO ₄ . It is considered that M is substituted for part of Fe in the crystal of FeOHSO ₄ and can effectively change the physical characteristics of FeOHSO ₄ .
The positive electrode active material of the present invention exhibits excellent battery capacity as described above and is inexpensive because it uses Fe as a main raw material, and greatly contributes to improving battery performance and reducing costs of lithium secondary batteries. Is.

Hereinafter, the positive electrode active material of the present invention will be described in detail.
In the general formula (1), the transition metal element M is not particularly limited as long as it is other than Fe, but normally, the one that can be divalent is preferable. In the present invention, the transition metal element includes a group 12 element in addition to a metal element present between the group 3 element and the group 11 element in the periodic table.
Specific examples of the transition metal element M include Ni, Co, Ti, V, Cr, Mn, Cu, and Zn. Among these, it is preferable to include at least one selected from Ni, Co, and Cr, and it is particularly preferable to include at least one selected from Ni and Co. Among these, Co is particularly preferable because it can dramatically improve the battery capacity. The increase in capacity due to Co substitution is thought to be due to the low electron conductivity of FeOHSO ₄ being supplemented by Co.
The transition metal element M substituted for Fe may be only one type or two or more types.

In the formula (1), x indicating the amount of substitution of Fe is a number satisfying 0 <x <1, and the preferable range varies depending on the type of the transition metal element M, but the viewpoint of stability of the crystal structure and improvement of battery characteristics Therefore, usually, 0.01 ≦ x is preferable, 0.02 ≦ x is particularly preferable, and 0.03 ≦ x is particularly preferable. Further, from the viewpoint of improving battery characteristics and generating impurities, it is usually preferable that x ≦ 0.5, particularly preferably x ≦ 0.4, and particularly preferably x ≦ 0.3.
When the transition metal element M is Ni, x ≦ 0.4 is preferable, x ≦ 0.2 is particularly preferable, x ≦ 0.1 is more preferable, and x <0.1. Most preferably it is. This is because the generation of impurities is suppressed and an excellent capacity can be obtained.
When the transition metal element M is Co, x ≦ 0.2 is preferable, and x ≦ 0.1 is particularly preferable. This is because the generation of impurities is suppressed and an excellent capacity can be obtained.

The positive electrode active material of the present invention may be in the form of particles (powder) or a thin film, but is preferably in the form of particles. This is because the degree of freedom of the electrode structure is high when it is particulate. The average particle diameter of the particulate positive electrode active material is preferably 20 nm or more, particularly 50 nm or more, more preferably 100 nm or more, and on the other hand, it is preferably 100 μm or less, particularly 10 μm or less.
The average particle size of the positive electrode active material is calculated by performing surface observation (appearance observation) of the positive electrode active material with a scanning electron microscope (FE-SEM), measuring the particle size of 50 randomly selected particles. Average value.

Next, the manufacturing method of the positive electrode active material of this invention is demonstrated. The positive electrode active material of the present invention described above can be produced, for example, as follows. That is, a positive electrode active material having a composition represented by the above formula (1) Fe _1-x M _x OHSO ₄ is obtained by heating a mixture of an iron sulfate hydrate and a transition metal element M-containing compound in an air atmosphere. A substance can be produced.

Examples of hydrates of iron sulfate include FeSO ₄ · 7H ₂ O, Fe ₂ (SO ₄ ) ₃ · nH ₂ O (n = 5 or more), and the like. For this reason, FeSO ₄ .7H ₂ O is preferable.

As the transition metal element M-containing compound, for example, sulfates, nitrates, acetates, hydroxides, oxalates, chlorides and hydrates thereof can be used. From the viewpoint of impurities after synthesis, Sulfate hydrates are preferred. Specific examples of the M-containing compound include, as Ni source, nickel sulfate hexahydrate, nickel acetate tetrahydrate, nickel nitrate, etc., and Co source as cobalt sulfate heptahydrate, cobalt acetate. Examples of the Mn source such as tetrahydrate and cobalt nitrate include manganese sulfate / monohydrate and manganese acetate / tetrahydrate, and examples of the Cr source include chromium chloride / hexahydrate. The M-containing compound as the M source may be used alone or in combination of two or more.
In the above mixture, the substitution amount of Fe by the transition metal element M can be controlled by adjusting the mixing ratio of the iron sulfate hydrate and the M-containing compound. That is, the iron sulfate hydrate and the M-containing compound may be mixed so that the molar ratio of Fe to the transition metal element M in the above mixture is (1-x): x.

A method for preparing the mixture is not particularly limited, and a general mixing method can be adopted. However, from the viewpoint of uniform mixing, mixing by a ball mill, a hand mill, or the like is preferable.
The heating temperature of the mixture is not particularly limited as long as it allows dehydration of iron sulfate and generation of crystals, and is usually 150 ° C. or higher, particularly preferably 200 ° C. or higher, and 400 ° C. or lower, particularly 300 ° C. or lower. In particular, it is preferably 290 ° C. or lower, more preferably 250 ° C. or lower.
Here, the air atmosphere in which the mixture is heated means an atmosphere having an oxygen concentration of 16 vol% or more, preferably an atmosphere having an oxygen concentration of 16 to 26 vol%.
The heating time can be appropriately set, but may be about 20 to 200 hours, particularly about 170 hours or less.

The obtained positive electrode active material is preferably ground and reduced in diameter as necessary from the viewpoint of improving battery characteristics. The method for grinding the positive electrode active material is not particularly limited, and a general method can be adopted. The positive electrode active material may be subjected to a treatment such as sieving in order to make the particle size uniform or to separate a positive electrode active material having a desired particle size.
In addition, the manufacturing method of the positive electrode active material of this invention is not limited to said manufacturing method.

The positive electrode active material of this invention can be used suitably as a positive electrode active material of a lithium secondary battery. Hereinafter, the lithium secondary battery of the present invention will be described.
[Lithium secondary battery]
The lithium secondary battery of the present invention is a lithium secondary battery comprising a positive electrode, a negative electrode, and an electrolyte interposed between the positive electrode and the negative electrode, wherein the positive electrode is the positive electrode for a lithium secondary battery of the present invention. It is characterized by containing an active material.
A configuration example of a lithium secondary battery will be described with reference to FIG.
In FIG. 1, a lithium secondary battery 100 has a positive electrode 2 disposed in a positive electrode can 1. On the positive electrode 2, a negative electrode 4 is disposed via an electrolyte 3. The negative electrode 4 is filled in the negative electrode cap 5, and the negative electrode cap 5 is fitted into the positive electrode can 1 to form a laminated structure of the positive electrode 2 -electrolyte 3 -negative electrode 4. The inside of the positive electrode can 1 and the negative electrode cap 5 is kept airtight by the gasket 6.
By using the positive electrode active material of the present invention, the cost of the lithium secondary battery can be reduced and the capacity can be increased.
Hereinafter, each configuration of the lithium secondary battery will be described.

(Positive electrode)
The positive electrode contains at least the positive electrode active material of the present invention, and may contain a conductive additive, a binder, an electrolyte, and the like as necessary.
Since the positive electrode active material of the present invention has been described above, the description thereof is omitted here.
Although it will not specifically limit as long as it has desired electroconductivity as a conductive support agent, For example, a conductive carbon material is mentioned. Specific examples include acetylene black, carbon black, coke, carbon fiber, and graphite.
Examples of the binder include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), styrene butadiene rubber (SBR), and the like.
The electrolyte is not particularly limited as long as it has lithium ion conductivity. As specific electrolyte, what is mentioned in description of the electrolyte mentioned later can be used.

The content of the positive electrode active material in the positive electrode is preferably larger from the viewpoint of battery capacity, and is preferably in the range of 50 to 95% by weight, particularly 60 to 90% by weight. Further, the content of the conductive auxiliary agent is preferably smaller as long as the desired electronic conductivity can be ensured, and is preferably in the range of 3 to 50% by weight, for example. Further, the content of the binder is preferably smaller as long as the positive electrode active material and the like can be stably fixed, and is preferably in the range of 1 to 10%, for example.
The thickness of the positive electrode varies greatly depending on the configuration of the lithium secondary battery, but is preferably in the range of 10 to 100 μm, for example.

The positive electrode may be provided with a positive electrode current collector that collects current from the positive electrode.
The positive electrode current collector may have any desired electronic conductivity. Examples of the positive electrode current collector material include metal materials such as stainless steel, nickel, aluminum, iron, titanium, and copper, carbon fibers, Examples thereof include carbon materials such as carbon paper and carbon cloth, and high electron conductive ceramic materials such as titanium nitride.
Although the thickness of a positive electrode electrical power collector is not specifically limited, For example, it is preferable to exist in the range of 10-100 micrometers.
Note that a battery case described later may also have a function as a positive electrode current collector.

The method for producing the positive electrode is not particularly limited. For example, it can be formed using a positive electrode material in which a positive electrode active material is mixed with other materials such as a conductive additive and a binder. Specifically, the above-mentioned material constituting the positive electrode and a solvent are kneaded, and the obtained positive electrode material is applied on the base material and / or rolled on the base material, and if necessary, The positive electrode can be manufactured by performing a drying process, a pressurizing process, a heating process, or the like.
The solvent used for producing the positive electrode is not particularly limited as long as it has volatility, and can be appropriately selected. Specific examples include ethanol, acetone, N, N-dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP) and the like.
The method for applying the positive electrode material is not particularly limited, and general methods such as a doctor blade method, an ink jet method, and a spray method can be used. The method for rolling the positive electrode material is not particularly limited, and for example, a roll press or the like can be used.

(Negative electrode)
The negative electrode contains at least a negative electrode active material.
Examples of the negative electrode active material include a metal active material and a carbon active material. Examples of the metal active material include metals Li, In, Al, Si, and Sn. On the other hand, examples of the carbon active material include mesocarbon microbeads (MCMB), highly oriented graphite (HOPG), hard carbon, and soft carbon. The shape of the negative electrode active material may be, for example, a film shape or a particle shape. In the case of a film-like negative electrode active material, the negative electrode active material itself can usually be used as the negative electrode. The average particle diameter of the particulate negative electrode active material is, for example, preferably 10 nm to 10 μm, and more preferably 30 nm to 1 μm.

The negative electrode may contain at least one of a conductive additive, a binder, and an electrolyte as necessary.
In this case, the content of the negative electrode active material contained in the negative electrode is preferably larger from the viewpoint of capacity, for example, in the range of 50 to 98% by weight, particularly 60 to 98% by weight, especially 80 to 95% by weight. It is preferable that Further, the content of the conductive auxiliary agent is preferably less as long as the desired electronic conductivity can be ensured, for example, 0.5 to 35% by weight, particularly preferably in the range of 0.5 to 10% by weight. . In addition, the content of the binder component is preferably smaller as long as the negative electrode active material and the like can be stably fixed, and is, for example, in the range of 0.5 to 10% by weight, particularly 0.5 to 2% by weight. preferable.
In addition, since it is the same as that used for the positive electrode mentioned above about the conductive support agent, binder, and electrolyte which are used for a negative electrode, description here is abbreviate | omitted.

The thickness of the negative electrode varies greatly depending on the configuration of the lithium secondary battery, but is preferably in the range of 10 to 100 μm, for example.
The negative electrode may be provided with a negative electrode current collector that collects current from the negative electrode. The material of the negative electrode current collector is not particularly limited as long as it has conductivity, and examples thereof include copper, stainless steel, nickel, and aluminum. Examples of the shape of the negative electrode current collector include a foil shape, a plate shape, and a mesh shape. Further, the battery case may have a function as a negative electrode current collector.

  The manufacturing method of a negative electrode is not specifically limited. For example, a foil-like negative electrode active material and a negative electrode current collector can be overlaid and pressurized to laminate the negative electrode and the negative electrode current collector. Alternatively, a negative electrode material containing a negative electrode active material and a binder is prepared, and the material is applied or rolled onto a base material (for example, a negative electrode current collector), and dried to obtain a negative electrode and a negative electrode current collector. Can be laminated.

(Electrolytes)
The electrolyte is interposed between the positive electrode and the negative electrode, and is responsible for lithium ion conduction between these electrodes. The form of the electrolyte is not particularly limited, and examples thereof include an electrolytic solution, a gel electrolyte, and a solid electrolyte.

Examples of the electrolyte solution include a non-aqueous electrolyte solution containing a lithium salt and a non-aqueous solvent.
Examples of lithium salts include inorganic lithium salts such as LiPF ₆ , LiBF ₄ , LiClO _4, and LiAsF ₆ ; and LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , An organic lithium salt such as LiC (CF ₃ SO ₂ ) ₃ can be used.
Examples of the non-aqueous solvent include ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), butylene carbonate, γ-butyrolactone, sulfolane, acetonitrile, Examples include 1,2-dimethoxymethane, 1,3-dimethoxypropane, diethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, and mixtures thereof. Moreover, low volatile liquids, such as an ionic liquid, can also be used as a non-aqueous solvent.
The concentration of the lithium salt in the non-aqueous electrolyte is preferably in the range of 0.5 mol / L to 3 mol / L, for example.
The electrolytic solution can be disposed between the positive electrode and the negative electrode in a state in which the separator is impregnated. Examples of the separator include a porous film such as polyethylene, polypropylene, cellulose, and polyvinylidene fluoride; a nonwoven fabric such as a resin nonwoven fabric and a glass fiber nonwoven fabric; and a laminate thereof.

  Examples of the gel electrolyte include those obtained by gelling the non-aqueous electrolyte with a polymer. Specifically, gelation can be performed by adding a polymer such as polyethylene oxide (PEO), polyacrylonitrile (PAN), or polymethyl methacrylate (PMMA) to the nonaqueous electrolytic solution.

Examples of the solid electrolyte include an oxide solid electrolyte and a sulfide solid electrolyte.
As the oxide solid electrolyte, Li _{a Xb} Y _{cP d} O _e (X is at least one selected from the group consisting of B, Al, Ga, In, C, Si, Ge, Sn, Sb and Se). Y is at least one selected from the group consisting of Ti, Zr, Ge, In, Ga, Sn and Al, and a to e are 0.5 <a <5.0, 0 ≦ b <2. .98, 0.5 ≦ c <3.0, 0.02 <d ≦ 3.0, 2.0 <b + d <4.0, 3.0 <e ≦ 12.0. NASICON type oxide; perovskite type oxide such as Li _x La _1-x TiO ₃ ; Li ₄ XO ₄ -Li ₃ YO ₄ (X is at least one selected from Si, Ge, and Ti, and Y is And at least one selected from P, As and V) and Li ₃ DO ₃ -Li ₃ LISON type oxide such as YO ₄ (D is B, Y is at least one selected from P, As and V); Li—La—Zr—O type oxide such as Li ₇ La ₃ Zr ₂ O ₁₂ Garnet-type oxides and the like.
Examples of the sulfide solid electrolyte include a Li ₂ S—P ₂ S ₅ compound, a Li ₂ S—SiS ₂ compound, and a Li ₂ S—GeS ₂ compound.
The solid electrolyte may be amorphous or crystalline. A crystalline solid electrolyte can be obtained, for example, by firing an amorphous solid electrolyte.

  The thickness of the electrolyte between the positive electrode and the negative electrode varies greatly depending on the configuration of the lithium secondary battery, but is preferably in the range of 1 to 100 μm, for example.

(Other)
The lithium secondary battery of the present invention may have other constituent members other than the positive electrode, the electrolyte, and the negative electrode as described above.
A lithium secondary battery usually has a battery case that houses a positive electrode, a negative electrode, and an electrolyte. The shape of the battery case is not particularly limited, and specific examples include a coin type, a flat plate type, a cylindrical type, and a laminate type.
In the case where the lithium secondary battery takes a structure (for example, a laminated structure or a wound structure) in which a laminated body in which a positive electrode, an electrolyte, and a negative electrode are arranged in this order is repeatedly stacked, from the viewpoint of safety, It is preferable to have a separator between the positive electrode and negative electrode which belong to different laminated bodies. Examples of such a separator include porous films such as polyethylene and polypropylene; and nonwoven fabrics such as a resin nonwoven fabric and a glass fiber nonwoven fabric.

[Synthesis of positive electrode active material]
Example 1
Iron sulfate heptahydrate (FeSO ₄ · 7H ₂ O, manufactured by Nacalai Tesque) and nickel sulfate hexahydrate (NiSO ₄ · 6H ₂ O, manufactured by Nacalai Tesque) in a molar ratio of 97: 3 The mixture was mixed in a mortar and fired at 280 ° C. for 168 hours in an air atmosphere to obtain a powder of Fe _0.97 M _0.03 OHSO ₄ .

(Example 2)
Fe _0.95 Ni _{0 .5} in the same manner as in Example 1 except that iron sulfate heptahydrate and nickel sulfate hexahydrate were mixed at a molar ratio of 95: 5 _. A powder of ₀₅ OHSO ₄ was obtained.

(Example 3)
Fe _0.9 Ni _{0 .5} in the same manner as in Example 1 except that iron sulfate heptahydrate and nickel sulfate hexahydrate were mixed at a molar ratio of 90:10 _. A powder of ₁ OHSO ₄ was obtained.

Example 4
Fe _0.8 Ni _{0. 6} was conducted in the same manner as in Example 1 except that iron sulfate heptahydrate and nickel sulfate hexaxahydrate were mixed at a molar ratio of 80:20 _. A powder of ₂ OHSO ₄ was obtained.

(Example 5)
Fe _0.7 Ni _{0. 5} was prepared in the same manner as in Example 1 except that iron sulfate heptahydrate and nickel sulfate hexahexahydrate were mixed at a molar ratio of 70:30 _. A powder of ₃ OHSO ₄ was obtained.

(Example 6)
Fe _0.6 Ni _{0 .6} in the same manner as in Example 1 except that iron sulfate heptahydrate and nickel sulfate hexahydrate were mixed at a molar ratio of 60:40 _. A powder of ₄ OHSO ₄ was obtained.

(Example 7)
Fe _0.5 Ni _{0 .5} in the same manner as in Example 1 except that iron sulfate heptahydrate and nickel sulfate hexahydrate were mixed at a molar ratio of 50:50 _. A powder of ₅ OHSO ₄ was obtained.

(Example 8)
Instead of nickel sulfate hexahydrate, cobalt sulfate heptahydrate (CoSO ₄ · 7H ₂ O, manufactured by Nacalai Tesque) is used, and iron sulfate heptahydrate and cobalt sulfate heptahydrate are mixed. A powder of Fe _0.97 Co _0.03 OHSO ₄ was obtained in the same manner as in Example 1 except that mixing was performed so that the molar ratio was 97: 3.

Example 9
Fe _0.95 Co _{0 .5} in the same manner as in Example 8, except that iron sulfate heptahydrate and cobalt sulfate heptahydrate were mixed at a molar ratio of 95: 5 _. A powder of ₀₅ OHSO ₄ was obtained.

(Example 10)
Fe _0.97 Co _{0 .6} in the same manner as in Example 8 except that iron sulfate heptahydrate and cobalt sulfate heptahydrate were mixed at a molar ratio of 90:10 _. A powder of ₁ OHSO ₄ was obtained.

(Example 11)
Fe _0.8 Co ₀ .6 in the same manner as in Example 8 except that iron sulfate heptahydrate and cobalt sulfate heptahydrate were mixed at a molar ratio of 80:20 _. A powder of ₂ OHSO ₄ was obtained.

(Example 12)
Fe _0.6 Co _{0 .6} in the same manner as in Example 8 except that iron sulfate heptahydrate and cobalt sulfate heptahydrate were mixed at a molar ratio of 60:40 _. A powder of ₄ OHSO ₄ was obtained.

(Comparative Example 1)
Iron sulfate heptahydrate was fired in the same manner as in Example 1 to obtain a FeOHSO ₄ powder.

[Crystal structure analysis]
After pulverizing the active material powders of Examples 1 to 12 and Comparative Example 1 obtained above with a mortar and passing through a 100 μm mesh, the crystal structure analysis was performed by the X-ray diffraction method (XRD method) under the following conditions. Went. The results are shown in FIGS.
<XRD conditions>
・ Measurement device: Rigaku Ultima4 one-dimensional detector used ・ Scanning range: 10 ° -80 °
・ Scanning speed: 10 ° / min
・ Total number of times: 3 times

As shown in FIG. 2, when comparing the XRD pattern of Example 1 and Comparative Example _{1~7, Fe 1-x Ni x} OHSO 4 of Examples 1 to 7 in (0.03 ≦ x ≦ 0.5) It was confirmed that the structure of the main phase FeOHSO ₄ was maintained. That is, it was confirmed that part of Fe in FeOHSO ₄ can be replaced with Ni. However, in Examples 5 to 7 where x ≦ 0.3 or more, since NiSO ₄ is generated as an impurity, the solid solution limit can be said to be about x = 0.2.
In addition, as shown in FIG. 3, in Fe _1-x Co _x OHSO ₄ (0.03 ≦ x ≦ 0.4) of Examples 8 to 12, no major impurities were observed, and the structure of FeOHSO ₄ Was confirmed to remain stable. That is, it was confirmed that a part of Fe of FeOHSO ₄ can be replaced by solid solution with Co.

[Battery performance evaluation]
A lithium secondary battery was produced using the active material powders of Examples 1 to 12 and Comparative Example 1 obtained above, and a charge / discharge test was performed.
(Production of lithium secondary battery)
A positive electrode was produced as follows. That is, first, each positive electrode active material that was ground with a mortar and passed through a 100 μm mesh was mixed with carbon particles (Ketjen Black, ECP-600JD, manufactured by KB international) using a ball mill (100 rpm). The obtained mixture was mixed with acetylene black (manufactured by Denki Kagaku Kogyo, HS-100), PVdF (manufactured by Kureha, KF polymer # 9305), and N-methyl-2-pyrrolidone (solvent, manufactured by Wako), and a positive electrode slurry. Was prepared. In the positive electrode slurry, the ratio (wt%) of the positive electrode active material, carbon particles, acetylene black, and PVdF was 56: 14: 20: 10.
The positive electrode slurry was applied onto an aluminum foil (current collector) by a doctor blade method, dried at 80 ° C., and then rolled with a press to obtain a positive electrode having a basis weight of 3 mg / cm ² and a thickness of 15 μm.

On the other hand, metallic lithium was prepared as a negative electrode.
In addition, a solution in which ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) are mixed at 3: 3: 4 (volume ratio) is mixed with lithium salt (LiPF ₆ ) at a concentration of 1 mol / L. In this way, a non-aqueous electrolyte was prepared.
As a separator, a laminated porous film (manufactured by Ube Industries) of a polypropylene porous membrane and a polyethylene porous membrane was prepared.

  A SUS 2032 type coin cell container was prepared, and a positive electrode, a separator, and a negative electrode were laminated and accommodated in this order. A separator was impregnated with a non-aqueous electrolyte to produce a coin-type lithium secondary battery.

(Charge / discharge test)
The coin-type lithium secondary battery produced above was subjected to a charge / discharge test as follows. That is, first, constant current charging was performed at a lower limit of 2.0 V at 0.1 C with respect to an actual capacity of 150 mAh / g (theoretical capacity: 155 mAh / g). Then, constant current discharge was performed to 4.0V at 0.1 C, and the initial discharge capacity (Li desorption capacity) was obtained. The results are shown in FIG.
As shown in FIG. 4, when Comparative Example 1 and Examples 1 to 7 into which Ni was introduced were compared, the specific capacity was most effective when the Ni substitution amount was 10 atm% or less (X ≦ 0.1). Was confirmed to increase. From the result of the above XRD measurement, when it exceeds 10 atm%, it is considered that a sufficient capacity increasing effect cannot be obtained because an impurity phase is generated.
Further, as shown in FIG. 4, when comparing Comparative Example 1 and Examples 8 to 12 in which Co is introduced, the capacity is effectively reduced in the range where the Co substitution amount is 20 atm% or less (x ≦ 0.2). It was confirmed that it could be increased. Further, in comparison with Examples 1 to 7 in which Ni is introduced and Examples 8 to 12 in which Co is introduced, when replaced with Co, the capacity can be dramatically increased, and a capacity close to the theoretical capacity can be obtained. I understand that. When the amount of Co is 40 atm% (x = 0.4), the capacity is decreased, which is considered to be due to the generation of impurities.

DESCRIPTION OF SYMBOLS 1 ... Positive electrode can 2 ... Positive electrode 3 ... Electrolyte 4 ... Negative electrode 5 ... Negative electrode cap 6 ... Gasket 100 ... Lithium secondary battery

Claims (5)

A positive electrode active material for a lithium secondary battery, characterized by having a composition represented by the following general formula (1).
Fe _1-x M _x OHSO ₄ (1)
(In formula (1), M is a transition metal element other than Fe, and x is a number satisfying 0 <x <1.)   The positive electrode active material for a lithium secondary battery according to claim 1, wherein, in the general formula (1), the M includes at least one selected from Ni and Co.   The positive electrode active material for a lithium secondary battery according to claim 1 or 2, wherein, in the general formula (1), the M is Ni and 0 <x ≦ 0.1.   The positive electrode active material for a lithium secondary battery according to claim 1, wherein, in the general formula (1), the M is Co and 0 <x ≦ 0.2. A lithium secondary battery comprising a positive electrode, a negative electrode, and an electrolyte interposed between the positive electrode and the negative electrode,
A lithium secondary battery, wherein the positive electrode includes the positive electrode active material for a lithium secondary battery according to any one of claims 1 to 4.

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