JP2000149948A - Positive active material, lithium ion secondary battery and manufacture of its positive active material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To restrain lowering of initial capacity to the minimum, and to effectively restrain lowering of capacity accompanied to advance of charge-discharge cycle at a high temperature. SOLUTION: In this lithium ion secondary battery, a positive electrode 4 having a positive active material, and a negative electrode 6 having a negative active material are arranged to be faced each other inside a battery can via a separator 5 and a nonaqueous electrolyte 3. The positive active material comprises an active material main body comprising spinel type lithium manganese compound oxide, and a coating layer coated in at least one portion of a surface of the active material main body, and the coating layer comprises an oxide of at least one kind of element selected from iron, vanadium, tungsten, molybdenum and rhenium.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positive electrode active material, a lithium ion secondary battery and a method for producing the positive electrode active material, and more particularly to a positive electrode active material and a lithium ion secondary battery capable of greatly improving high-temperature cycle characteristics. The present invention relates to a secondary battery and a method for producing the positive electrode active material thereof.

[0002]

2. Description of the Related Art In recent years, relatively safe anode materials have been developed, and non-aqueous electrolytes having a higher decomposition voltage have been developed.
High voltage non-aqueous electrolyte secondary batteries have been put to practical use. In particular, demand for lithium-ion secondary batteries as power sources for mobile phones, notebook computers, video cameras with integrated cameras, etc. is rapidly increasing due to their excellent features of high discharge potential, light weight, and high energy density. It is expanding.

[0003] This lithium ion secondary battery comprises a positive electrode and a negative electrode capable of reversibly occluding and releasing lithium ions,
And a non-aqueous electrolyte in which a lithium salt is dissolved in a non-aqueous solvent.

Examples of the positive electrode active material of the lithium ion secondary battery include lithium cobalt composite oxides such as LiCoO ₂ , lithium nickel composite oxides such as LiNiO _2, and lithium manganese composite oxides such as LiMn ₂ O ₄ . Metal oxides are commonly used.

However, in the battery using the above-mentioned lithium-cobalt composite oxide, although the theoretical capacity is increased, the discharge potential is higher than that of the batteries using the other two composite oxides, and the non-aqueous electrolyte However, there is a drawback that the discharge capacity in the potential range where is not decomposed is reduced to about 1/2 of the theoretical capacity. In addition, there is a problem that the production cost is increased because cobalt, which is a scarce resource, is used as a material.

On the other hand, a battery using a lithium nickel composite oxide has a large theoretical capacity and an appropriate discharge potential, but on the other hand, changes in the charge / discharge potential and charge / discharge related to changes in the crystal structure that occur during the charge / discharge process. No drastic solution has been made to the decrease in charge / discharge capacity due to the collapse of the crystal structure with the progress of the cycle, and there has been a problem that the characteristic stability and reliability of the battery are insufficient.

On the other hand, a secondary battery using a lithium manganese composite oxide has a slightly lower theoretical capacity than a battery using the other two composite oxides, but has a moderately high charge / discharge potential. And that the crystal structure of the positive electrode active material can be stably maintained without changing during the charge / discharge cycle. Therefore, a battery using the lithium manganese composite oxide can be charged and discharged with a capacity close to the theoretical capacity. In addition, the lithium manganese composite oxide as the positive electrode active material has a crystal structure of the positive electrode active material that is stably maintained even in an overcharged state (λMnO ₂ ) in which lithium ions are completely exhausted from the positive electrode active material, and is thermally stable. Has been confirmed to be superior to other material systems. The starting temperature of the reaction from which oxygen is released from the overcharged material is a high temperature exceeding 400 ° C. Therefore, batteries using lithium-manganese composite oxide
It is capable of charging and discharging at a capacity close to the theoretical capacity, and has the advantage of being extremely safe, with no danger of explosion due to ignition compared to batteries using other material systems. Development is progressing.

[0008]

However, a secondary battery using the above-mentioned lithium-manganese composite oxide-based positive electrode active material has a temperature of 40 ° C. or higher as compared with batteries using other material systems. There is a problem that the decrease in capacity when charging and discharging is repeated becomes remarkable, and the energy that can be taken out decreases rapidly as the amount of current to discharge increases,
It has been found that there is a problem that the so-called rate characteristic deteriorates. Unless these problems are solved, there is no commercial value as a practical secondary battery, and some measures have to be taken.

In order to solve the above-mentioned problems, for example, as disclosed in Japanese Patent Application Laid-Open No. 7-282798, a part of Mn constituting the positive electrode active material is replaced with Li. As disclosed in Japanese Patent Application Laid-Open No. 1606969, part of Mn is replaced by Co, and
As disclosed in Japanese Patent No. 289,662, a positive electrode active material substituted with Al has been proposed as being effective for improving the normal temperature cycle characteristics. The inventor of the present application has confirmed that the use of the above-mentioned positive electrode active material can also alleviate the high-temperature cycle deterioration of the secondary battery, but has also found a problem that the initial capacity is reduced at the same time. The cause of this problem is that the valence of Mn increases due to the substitution with the metal elements such as Li, Co, and Al, and the number of trivalent Mn atoms contributing to the charge / discharge reaction relatively decreases. Conceivable.

On the other hand, as means for solving the above problems,
For example, as disclosed in US Pat. No. 5,705,291, a method of controlling the particle size of the positive electrode active material to reduce the contact area between the electrolytic solution and the active material, or forming the surface of the active material with a boron-based material Processing methods have also been proposed. However, the charge / discharge reaction is caused by L
Since this is a process of performing so-called intercalation and deintercalation for taking in and out of i-ions, the capacity characteristics are closely related to the interface area between the active material and the electrolyte. For this reason, the means described in the above USP has a fatal problem of causing a decrease in the initial capacity of the battery and a decrease in the rate characteristic.

The present invention has been made in order to solve the above problems, and in particular, it is possible to minimize the decrease in the initial capacity and effectively reduce the decrease in the capacity accompanying the progress of the charge / discharge cycle at a high temperature. It is an object of the present invention to provide a lithium ion secondary battery that can be suppressed to a low level and a method for producing the positive electrode active material thereof.

[0012]

Means for Solving the Problems The present inventors have earnestly studied to achieve the above object, and have obtained the following findings. That is, at least a part of the surface of the spinel-type lithium manganese composite oxide particles as the active material body is coated with an oxide of at least one element selected from iron, vanadium, tungsten, molybdenum, and rhenium. At least a part of the surface of the substance or the spinel-type lithium-manganese composite oxide particles is coated with an oxide obtained by complexing at least one element selected from iron, vanadium, tungsten, molybdenum and rhenium with an alkali metal element. When a non-aqueous secondary battery (lithium ion secondary battery) is manufactured using the positive electrode active material that has been used, the decrease in the initial capacity is small, and the decrease in the capacity accompanying the progress of the charge / discharge cycle at a high temperature is reduced. It has been found that a lithium ion battery that can be suppressed can be obtained.

Further, the average particle size of the active material body is 1 to 20 μm.
By setting the range of m, the filling property of the active material in the electrode is not impaired, and the contact with the electrolytic solution is also improved, so that the absorption and release of lithium ions through the active material surface proceeds rapidly. Therefore, it has been found that when a secondary battery is formed using the above-described positive electrode active material, a secondary battery having high capacity and capable of rapid charge and discharge can be obtained.

The present invention has been completed based on the above findings. That is, the positive electrode active material according to the present invention comprises an active material body made of a spinel-type lithium manganese composite oxide, and a coating layer applied to at least a part of the surface of the active material body, and the coating layer is , Iron, vanadium,
It is characterized by comprising an oxide of at least one element selected from tungsten, molybdenum and rhenium.

Further, in the lithium ion secondary battery according to the present invention, a positive electrode having a positive electrode active material and a negative electrode having a negative electrode active material are opposed to each other in a battery can via a separator and a non-aqueous electrolyte. Wherein the positive electrode active material comprises: an active material main body made of a spinel-type lithium manganese composite oxide; and a coating layer applied to at least a part of the surface of the active material main body. The layer is made of an oxide of at least one element selected from iron (Fe), vanadium (V), tungsten (W), molybdenum (Mo) and rhenium (Re).

Further, the coating layer is a composite of at least one element selected from iron (Fe), vanadium (V), tungsten (W), molybdenum (Mo) and rhenium (Re), and an alkali metal element. It may be composed of a converted oxide.

The average particle diameter of the active material body is 1 to 20 μm.
Is desirably within the range. The average particle diameter means a secondary particle diameter in a form in which primary particles of a single crystal are aggregated.

The positive electrode active material body used in the lithium ion secondary battery according to the present invention is made of a lithium manganese composite oxide having a spinel structure. This composite oxide is represented by the general formula LiMn ₂ O ₄ , and a part of Mn is Li,
An oxide substituted with an element such as B, Al, Ti, V, Cr, Co, Ni or the like is preferably used. Further, an oxide in which a part of oxygen is replaced by an element such as S, Se, or F, and an oxide in which an element of these anions is excessively added can be preferably used.

By using the lithium manganese oxide having the spinel type crystal structure as a base of the positive electrode active material, the lithium manganese oxide is excellent in chemical stability, has a large capacity per unit weight, and is resistant to repeated charging and discharging. A lithium ion secondary battery is obtained.

The average particle size of the base (main body) of the positive electrode active material is 0.5 to 20 μm in order to improve the dispersibility and filling property of the positive electrode active material when preparing an electrode sheet.
Is preferably in the range of 0.5 to 10 μm
Is more preferable. When the average particle diameter is less than 0.5 μm, the dispersibility and the filling property of the positive electrode active material are reduced, so that it is difficult to prepare the electrode sheet. It becomes difficult to secure rate characteristics.

At least a part of the surface of the main body of the positive electrode active material used in the present invention has iron, vanadium, tungsten,
At least one selected from molybdenum and rhenium
A coating layer consisting of an oxide of the element is applied. Alternatively, at least a part of the surface of an active material main body composed of a lithium manganese composite oxide having a spinel structure includes at least one element (M) selected from iron, vanadium, tungsten, molybdenum and rhenium and an alkali metal element. A coating layer consisting of the complexed oxide is applied. These oxides may be amorphous, but from the viewpoint of battery capacity, it is considered that they are preferably crystalline.

The coating amount of the coating layer made of the above oxide is preferably in the range of 0.1 to 18% by weight, more preferably in the range of 1 to 15% by weight, based on the weight of the active material body. Is more preferable. When the deposition amount is 0.1
If the amount is less than 10% by weight, the effect of the present invention of mainly improving the high-temperature cycle characteristics will be insufficient. On the other hand, if the amount to be applied is too large to exceed 18% by weight, the capacity will decrease. Becomes noticeable.

Further, by using an oxide in which the alkali metal element (A) is compounded with the metal element (M), the electron conductivity of the coating layer can be further increased, and the battery can have a higher capacity. . As the above-mentioned alkali metal element, lithium (Li), sodium (Na), potassium (K), rubidium (Rb) and the like are preferable.
i is preferred. This is because, for example, in the charging process in which Li is separated from the active material, Li ions in the active material are replaced with Li ions present in the active material-deposited layer, and Li ions in the deposited layer are discharged to the electrolyte side. It is because there is a reaction of going.

When the alkali metal element (A) is Li and the metal element (M) is iron, the Li / Fe ratio (molar ratio) is preferably in the range of 0 to 0.5. If the ratio exceeds 0.5, magnetite constituting the coating layer becomes unstable, and the electron conductivity of the coating layer is undesirably reduced.

When the alkali metal element (A) is Li and the metal element (M) is at least one of V, W, Mo and Re other than iron, the Li / M ratio is 0 to 0 in terms of molar ratio.
A range of 10 is preferred. If this ratio exceeds 10, surplus Li not taken into the coating layer is taken into the active material to reduce Mn, and Mn ⁺³ relatively increases to cause Jahn-Teller deformation, or to the active material. Li which is not taken in also becomes an excess alkali, and when this is used to prepare a coating solution, it becomes difficult to control the viscosity of the coating solution, and it may be difficult to prepare the positive electrode.

The positive electrode active material body used in the present invention can be synthesized, for example, according to the following method.
That is, lithium hydroxide, oxide, carbonate, acetate, or nitrate, electrolytic manganese dioxide, chemical manganese dioxide, manganese oxide such as Mn ₂ O ₃ , MnOOH, manganese hydroxide, manganese carbonate, nitrate , Acetate, and if necessary, various salts of Mn and a metal that partially substitutes for oxygen (O) are mixed so as to have a predetermined composition range, and the resulting raw material mixture is mixed in the air or in an oxygen atmosphere or as required. 500-900 ° C in argon atmosphere depending on
, More preferably in the range of 680 to 750 ° C.

Here, the spinel structure is synthesized at a sintering temperature of 400 ° C. or higher. However, when the sintering temperature is lower than 500 ° C., the synthesis reaction does not sufficiently proceed, and the crystallinity of the active material is reduced. Therefore, a positive electrode active material having a favorable spinel-type crystal structure as intended in the present invention cannot be obtained. On the other hand, if the firing temperature exceeds 900 ° C., oxygen vacancies and the like are likely to appear, and it is difficult to obtain a positive electrode active material body within a predetermined composition range.

In carrying out the above synthesis operation,
In order to obtain a positive electrode active material body having an average particle size of 1 to 20 μm,
The particle size of the starting manganese compound particles is 0.5 to 2
It is important that the thickness be in the range of 0 μm. If the particle size of the manganese compound particles is too small, less than 0.5 μm, the particle size of the active material obtained under the above synthesis conditions will be less than 1 μm. It becomes difficult to fill such a fine particulate active material densely without any gap in the positive electrode layer. Therefore, it is difficult to realize a high battery capacity in a limited battery volume range.

Conversely, when coarse particles having a particle size exceeding 20 μm are used as starting materials, the particle size of the generated positive electrode active material exceeds 20 μm under the above synthesis conditions. It is easy to fill such a coarse active material into the positive electrode layer without any gaps, but the contact area between the electrolyte and the active material is reduced and the battery reaction rate is reduced. Cannot be obtained. In addition, when such a coarse manganese source is used, the reaction proceeds non-uniformly during the synthesis, so that the composition varies greatly, and only an active material having a low battery capacity can be obtained.

In the above manufacturing method, part of the manganese component is replaced with Li, B, Al, Ti, V, Cr, Co, Ni.
Alternatively, a solid solution substituted with a metal element such as described above may be used. Particularly, Li, Co, and Al are effective in improving the cycle life of the battery, while Ti, V, Cr, Co, Ni, and Al are effective in improving the electronic conductivity.

An oxide obtained by substituting a part of oxygen with an element such as S, Se, or F, or a spinel structure containing an excess of these anions may be used.

As a method for producing a positive electrode active material by applying a coating layer to at least a part of the surface of the active material main body prepared as described above, the active material main body and the treatment material are mixed at a predetermined ratio. Although a method is used, in order to perform a particularly uniform coating layer forming process, as shown below, a processing material previously dissolved in a solvent and an active material main body powder are mixed to form an active material slurry. After the preparation, a method of removing the solvent is preferred.

Specifically, a lithium hydroxide solution, an ammonia solution or a mixed solution of lithium hydroxide and ammonia is
While dissolving the oxides of V, W, Mo, and Re to prepare an adherend dissolving solution, a slurry solution in which active material body powder that has been synthesized in advance is dispersed in water is prepared. A predetermined amount of the above-mentioned adherend dissolving solution is added and dried. Thereafter, the mixture is baked at a temperature of 300 to 800 ° C. to produce a positive electrode active material.

When Fe and Li are contained as the adherend components, a mixture of iron nitrate and lithium nitrate is prepared as an aqueous solution, while a slurry prepared by dispersing a previously synthesized active material body powder in water is used. A solution is prepared, and after that, a predetermined amount of the above solution is added to the slurry solution and dried.
Thereafter, the obtained mixture is baked at 300 to 800 ° C. to produce a positive electrode active material.

Further, at the stage of synthesizing an active material body such as LiMnO ₄ , V, R
A manufacturing method in which a salt such as e, W, Mo, Fe or the like is added and baked, can also be adopted. In this production method, these metals substitute a part of Mn in LiM ₂ O ₄ with a stable composite oxide (Li) having an arbitrary Li / M ratio.
A reaction for generating a -M-O), the result of the reaction and to produce the LiMn ₂ O ₄ are competing, the composition uniformity of LiM ₂ O ₄ is impaired, there is a case where the capacity is decreased. Therefore, in order to obtain a good active material, it is necessary to carefully select the composition of the raw materials and the firing conditions.

As another method of deposition, a method in which an active material synthesized in advance and a treatment material (deposition material) are mixed in a solid phase, and the resulting mixture is baked to form a positive electrode active material. Can also be employed. In this case, it is desirable to perform the baking treatment at a temperature at which the treatment material dissolves. In particular, when V, Mo, or W is used as a processing material component, this deposition method is suitable. For example, when V is used as a metal to be complexed with Li, it depends on the amount of Li.
The treatment is preferably performed at 0 ° C. or higher, when using Mo, preferably at 550 ° C. or higher, and when using W, the processing is preferably performed at 700 ° C. or higher.

When the active material main body powder synthesized in advance and the processing material (adhering material) are simply mixed and used as the positive electrode active material, the adhering material is uniformly formed on the surface of the active material main body. Because it is easy to peel off without being covered and not firmly attached,
It is difficult to obtain the effects of the present invention. On the contrary,
In any of the manufacturing methods used in the present invention, the adherend becomes liquid by melting or melting. LiMn ₂
In the production method of adding and sintering V, Re, W, Mo, and Fe salts at the stage of synthesizing O ₄ , the metal salt is mixed with a lithium salt and becomes liquid at a synthesis temperature of 500 to 800 ° C. Become. By going through this liquefaction process,
The adherend can uniformly cover the surface of the active material main body with uniformity that cannot be obtained by simple mixing of solid phases. Further, in the heating / baking step, atoms on the surface of the active material and atoms constituting the adherend diffuse into each other, so that the adherend is firmly adhered to the surface of the active material body.

Therefore, when the positive electrode active material prepared by each of the above manufacturing methods is used, the adherend (coating layer) is peeled off from the surface of the active material main body even when the lattice expands and contracts in the charging and discharging process. To continue to exist without
When used as a positive electrode material of a battery, good cycle characteristics can be realized.

The lithium ion secondary battery according to the present invention
A positive electrode, which is prepared by mixing and pressing the positive electrode active material and the conductive auxiliary prepared as described above together with a binder and the like,
A negative electrode having a negative electrode active material is disposed so as to face the inside of the battery can with a separator and a non-aqueous electrolyte interposed therebetween.

Here, acetylene black, carbon black, graphite or the like is used as the conductive assistant. As the binder, for example, polyterafluoroethylene (PTFE), polyvinylidene fluoride (P
VDF), ethylene-propylene-diene copolymer (E
PDM), styrene-butadiene rubber (SBR) and the like can be used.

The positive electrode is manufactured by, for example, suspending the positive electrode active material and a binder in a suitable solvent, applying the suspension to a current collecting zone, drying and then press-pressing. Here, it is preferable to use, for example, an aluminum foil, a stainless steel foil, a nickel foil, or the like as the current collecting zone.

Examples of the negative electrode active material used in combination with the positive electrode containing the lithium manganese composite oxide as described above include, for example, a carbon material that absorbs and releases lithium ions, a material containing a chalcogen compound, and an active material made of light metal. Can be used. It is particularly preferable to use a negative electrode containing a carbon substance or a chalcogen compound that inserts and extracts lithium ions, since battery characteristics such as cycle life of a secondary battery are improved.

Examples of the carbon material that occludes and releases lithium ions include coke, carbon dioxide fiber, pyrolysis gas phase carbon material, graphite, fired resin, mesophase pitch-based carbon fiber (MCF), and mesophase spherical carbon. A fired body or the like is used. In particular, heavy oil at a temperature of 250
By using mesophase pitch-based carbon fibers and mesophase spherical carbon in the form of liquid crystal which are graphitized at 0 ° C. or higher, the electrode capacity of the battery can be increased.

In addition, the carbon material is particularly 7% by differential thermal analysis.
It has an exothermic peak at 00 ° C. or higher, more preferably 800 ° C. or higher, and the intensity ratio P ₁₀₁ between the (101) diffraction peak (P ₁₀₁ ) and the (100) diffraction peak (P ₁₀₀ ) of the graphite structure by X-ray diffraction. / P ₁₀₀ that is desired to be in the range of 0.7 to 2.2. Since a negative electrode containing a carbon material having such a diffraction peak intensity ratio can rapidly absorb and release lithium ions, a combination with a positive electrode containing the positive electrode active material that directs rapid charge and discharge is particularly effective. It is.

Further, as the chalcogen compound capable of inserting and extracting lithium ions, titanium disulfide (Ti)
S ₂ ), molybdenum disulfide (MoS ₂ ), niobium selenide (NbSe ₂ ) and the like can be used. When such a chalcogen compound is used for the negative electrode, the voltage of the secondary battery is reduced, but the capacity of the negative electrode is increased, so that the capacity of the secondary battery is improved. Further, since the diffusion rate of lithium ions in the negative electrode increases, the combination with the positive electrode active material used in the present invention is particularly effective.

Examples of the light metal used for the negative electrode include aluminum, aluminum alloy, magnesium alloy, lithium metal, lithium alloy and the like.

Further, for the negative electrode containing an active material capable of inserting and extracting lithium ions, for example, the negative electrode active material and a binder are suspended in an appropriate solvent, and this suspension is applied to a current collector and dried. It is manufactured by press bonding afterwards. Examples of the current collector include copper foil, stainless steel foil,
One formed from nickel foil or the like is used. As the binder, for example, polytetrafluoroethylene (PT
FE), polyvinylidene fluoride (PVDF), ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), and the like can be used.

The separator is formed of, for example, a synthetic resin nonwoven fabric, a polyethylene porous film, a polypropylene porous film, or the like.

As the non-aqueous electrolyte, a solution in which an electrolyte (lithium salt) is dissolved in a non-aqueous solvent is used.

Examples of the non-aqueous solvent include cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC), dimethyl carbonate (DMC), and the like.
Chain carbonates such as methyl ethyl carbonate (MEC) and diethyl carbonate (DEC), chain ethers such as dimethoxyethane (DME), diethoxyethane (DEE) and ethoxymethoxyethane, tetrahydrofuran (THF), 2-methyltetrahydrofuran ( 2-M
eTHF) and other cyclic ethers and crown ethers, γ-
Fatty acid esters such as butyrolactone (γ-BL), nitrogen compounds such as acetonitrile (AN), and sulfolane (S
L) and sulfides such as dimethylsulfoxide (DMSO).

The above non-aqueous solvents may be used alone or as a mixture of two or more. In particular, E
DMC, MEC, DEC, DME, DEE, TH with at least one selected from C, PC, and γ-BL, and at least one selected from EC, PC, and γ-BL
It is desirable to use a mixed solvent comprising at least one selected from F, 2-MeTHF and AN.

Further, the above-mentioned lithium ions are stored in the negative electrode.
When a negative electrode active material containing a carbon material to be released is used, from the viewpoint of improving the cycle life of a secondary battery including the negative electrode, EC and PC and γ-BL, EC and PC and MEC, EC
And PC and DEC, EC and PC and DEE, EC and AN, E
C and MEC, PC and DMC, PC and DEC, or EC
It is preferable to use a mixed solvent consisting of and DEC.

As the electrolyte, for example, lithium perchlorate (LiClO ₄ ), lithium hexafluorophosphate (LiPF
₆ ), lithium borofluoride (LiBF ₄ ), lithium arsenic hexafluoride (LiAsF ₆ ), lithium trifluorometasulfonate (LiCF ₃ SO ₃ ), lithium bistrifluoromethylsulfonylimide [LiN (CF ₃
SO ₂ ) ₂ ]. In particular, Li
It is desirable to use PF ₆ , LiBF ₄ , or LiN (CF ₃ SO ₂ ) ₂ because conductivity and safety are improved. The amount of these electrolytes dissolved in the non-aqueous solvent is desirably set in the range of 0.1 to 3.0 mol / l.

According to the lithium ion secondary battery having the above structure, the positive electrode active material having the coating layer made of an oxide of at least one element selected from the group consisting of Fe, V, W, Mo and Re formed on the surface of the active material. Since the substance is used, direct contact between the electrolyte and the positive electrode active material is prevented, elution of Mn is effectively prevented, and the high-temperature cycle characteristics of the battery are significantly improved. Further, since the coating layer does not hinder the diffusion of Li ions and has good electron conductivity, a high capacity can be maintained even when the coating layer is formed. Therefore, even when an active material that is fine particles and has a large contact area with the electrolytic solution is used, a secondary battery having high initial capacity and rate characteristics and excellent high-temperature cycle characteristics can be obtained.

The coating layer was formed of an oxide in which an alkali metal element such as Li and a metal element such as V, W, Mo, and Re were compounded, and the Li / M ratio was set to about 0 to 10. In such a case, it is considered that the structure of the coating layer is basically a βNaV ₂ O ₅ structure or a ReO ₃ structure. For example, the ReO ₃ structure has a structure in which units of M surrounded by an octahedron by oxygen are simply arranged in a cubic structure, and
A site for accommodating ions and a path for diffusing Li ions are provided. Furthermore, as a result of these M oxides interacting with oxygen greatly, the coating layer has a wide d-band and is excellent in electron conductivity. Therefore, the reaction speed of the battery can be further increased, and the capacity of the battery can be increased.

On the other hand, when the coating layer is formed of an oxide of Fe, part of the iron oxide is magnetite having excellent electron conductivity, so that the effect of preventing the dissolution of Mn and the electron conductivity are obtained. An improvement effect is obtained.

Since the coating layer has good electron conductivity, the amount of the conductive auxiliary agent in the positive electrode sheet can be reduced, and the active material filling amount in the battery cell can be relatively increased. Thus, the battery capacity can be further increased.

[0058]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described more specifically with reference to the following examples. The present invention is not limited to the following examples,
The present invention can be implemented with appropriate modifications without departing from the scope defined by the gist of the present invention and the elements described in the claims.

Examples 1 to 26 and Comparative Examples 1 to 7 Lithium ion secondary batteries according to Examples and Comparative Examples were prepared according to the following procedures, and the characteristics thereof were compared and evaluated.

[Preparation of Positive Electrode] LiOH.H ₂ O powder and M
nO ₂ (electrolytic Mn) powder and Li: Mn = 1.1:
A predetermined amount was prepared and mixed so as to have an atomic ratio of 1.9 to prepare a raw material mixture, and the obtained raw material mixture was calcined at a temperature of 750 ° C. for 15 hours in an oxygen atmosphere to obtain Li _1.1. A positive electrode active material base powder having a composition of Mn _1.9 O ₄ was produced.

(For Example 1) Lithium nitrate and iron nitrate were blended so that the Li / Fe ratio became the value shown in Table 1, and dissolved in pure water to prepare a coating material solution. The active material body (Li _1.1 Mn _1.9 O ₄ ) was added to the adherend solution so as to have a prescribed adherence amount shown in Table 1, and mixed well.
The obtained mixture was dried and heat-treated at 500 ° C. for 3 hours to produce a positive electrode active material for Example 1.

(For Examples 2 to 17) The oxides of the adherend elements shown in Table 1 and lithium hydroxide (sodium hydroxide in Example 6) were used, and the adherend composition (A / M) was as shown in Table 1. And dissolved in pure water to prepare respective adherend solutions. The active material body (Li _1.1 Mn) is contained in each adherend solution.
_1.9 O ₄ ) was added and mixed well so as to obtain a predetermined coating amount shown in Table 1. The resulting mixture is dried and 500
By performing a heat treatment at 3 ° C. for 3 hours, positive electrode active materials for Examples 2 to 17 were produced.

(For Example 18) Iron nitrate was dissolved in pure water to prepare a coating material solution. The active material body (Li _1.1 Mn _1.9 O ₄ ) was added to the adherend solution so that the adhered amount was 1 wt%, and the mixture was thoroughly mixed. The obtained mixture was dried and heat-treated at 500 ° C. for 3 hours to produce a positive electrode active material for Example 18.

(Examples 19 to 22) The oxides of the adherend elements shown in Table 1 were dissolved in aqueous ammonia to prepare adherend solutions. The above-mentioned active material body (L
i _1.1 Mn _1.9 O ₄ ) was added so as to have a predetermined coating amount shown in Table 1 and mixed well. The obtained mixture was dried and heat-treated at 500 ° C. for 3 hours to obtain Example 1.
The positive electrode active materials for 9 to 22 were produced.

(For Example 23) Powdered vanadium oxide and lithium hydroxide were mixed in a predetermined amount so that the Li / V ratio was 1/1 to prepare an adherend mixture. Next, the mixture of the adherends was added to the active material body (Li _1.1 Mn _1.9 O ₄ ) so that the amount of the adherend was 1% by weight, and the obtained mixture was heat-treated at 500 ° C. for 3 hours. Then, by cooling after the adhered material was melted, a positive electrode active material for Example 23 was produced.

(For Example 24) A predetermined amount of powdery molybdenum oxide and lithium hydroxide were mixed so that the Li / Mo ratio became 1/1, to prepare a mixture of adherends. Next, this mixture of the adherends was added to the active material body (Li _1.1 Mn _1.9 O ₄ ) so that the adhered amount was 1% by weight, and the obtained mixture was heat-treated at 600 ° C. for 3 hours. Then, by cooling after the adhered material was melted, a positive electrode active material for Example 24 was produced.

(For Example 25) A predetermined amount of powdered tungsten oxide and lithium hydroxide were mixed so that the Li / W ratio became 1/1 to prepare an adherend mixture. Next, this mixture of adherends is added to the active material body (Li _1.1 Mn _1.9 O ₄ ) so that the adhered amount becomes 1% by weight, and the obtained mixture is subjected to 850 in a pure oxygen atmosphere. By heat treatment at ℃ for 3 hours and cooling after the adhered material has melted.
A positive electrode active material for Example 25 was produced. (For Example 26) LiOH · H ₂ O, LiF and electrolytic manganese MnO ₂ were mixed with a molar ratio LiOH · H ₂ O: LiF: MnO ₂ = such that the anionic element (F) became excessive.
It was prepared at a ratio of 0.96: 0.22: 1.82 and mixed well. Next, the obtained mixture was calcined at a temperature of 750 ° C. for 12 hours to prepare an active material body having a composition of Li _1.18 Mn _1.82 O _3.77 F _0.23 . Next, 1 wt% of W oxide was applied to the surface of the active material main body under the same conditions as in Example 2 to produce a positive electrode active material for Example 26.

(For Comparative Example 1) The active material (Li _1.1 Mn _1.9 O ₄ ) obtained in Example 1 was prepared so that the Li / B ratio was 1/1 and the amount of coating was 1% by weight. Predetermined amounts of LiOH.H ₂ O and B ₂ O ₃ were added so that the mixture was heat-treated at 600 ° C. to prepare a positive electrode active material for Comparative Example 1.

(For Comparative Example 2) A positive electrode active material for Comparative Example 2 was used without forming a coating layer on the surface of the active material (Li _1.1 Mn _1.9 O ₄ ) obtained in Example 1.

(For Comparative Example 3) LiOH.H ₂ O powder and M
The nO ₂ (electrolytic manganese) powder and the cobalt oxide powder were prepared so as to have an atomic ratio of Li: Mn: Co = 1: 1.7: 0.3, and were sufficiently mixed. The mixture was calcined at 750 ° C. under oxygen reflux to prepare a parent active material (LiMn _1.7 Co _0.3 O ₄ ). The positive electrode active material for Comparative Example 3 was used without forming a coating layer on the surface of the active material.

(For Comparative Example 4) The active material (Li _1.1 Mn _1.9 O ₄ ) obtained in Example 1 was added with Li / A as an adherend.
Predetermined amounts of lithium nitrate and manganese nitrate are added so that the ratio of l + Mn is 1/2 and the amount of coating is 2% by weight. The mixture was calcined at 550 ° C. under reflux to prepare a positive electrode active material for Comparative Example 4.

(For Comparative Example 5) 1% by weight of V ₂ O ₅ as an adherend was added to the active material (Li _1.1 Mn _1.9 O ₄ ) obtained in Example 1, and ball mill mixing was carried out for 15 hours. Thus, a positive electrode active material of Comparative Example 5 was prepared.

(For Comparative Example 6) 2% by weight of MoO ₃ as an adherend was added to the active material (Li _1.1 Mn _1.9 O ₄ ) obtained in Example 1, and ball mill mixing was performed for 24 hours. The positive electrode active material of Comparative Example 6 was prepared.

(For Comparative Example 7) Li ₂ CO ₃ powder and MoO
₂ powder was mixed so that Li / Mo was 2/1, and the resulting mixture was heated at 630 ° C. in an oxygen stream to remove Li ₂ M.
oO ₄ was synthesized. By heating the obtained Li ₂ MoO ₄ to 700 ° C. in hydrogen gas, L as an adherend is
i ₂ MoO ₃ was prepared. Next, 10 parts by weight of the adherend (Li ₂ MoO ₃ ) is blended with 70 parts by weight of the active material (Li _1.1 Mn _1.9 O ₄ ) obtained in Example 1, and ball mill mixing is performed for 15 hours. Thus, a positive electrode active material for Comparative Example 7 was prepared.

In the above embodiment, when the metal element (M) contained in the coating layer is Mo, W, Re, (Li)
was calculated deposited amount as _{x O) x / 2 · MO} 3. When the metal element is V, the deposition amount is calculated as (Li ₂ O) _{x / 2} · V ₂ O ₅ , and when the metal element (M) is Fe, (Li ₂ O) _{x The} amount of deposition was calculated as _{/ 2} · Fe ₃ O ₄ .

The respective positive electrode active material powders for the above Examples and Comparative Examples, acetylene black as a conductive additive, and Teflon (registered trademark) powder as a binder were in a weight ratio of 80:
Each was mixed at a ratio of 17: 3 to form a positive electrode mixture. Next, each positive electrode mixture was attached to a current collector (stainless steel) to prepare positive electrodes each having a size of 10 mm × 0.5 mm.

[Preparation of Negative Electrode] A negative electrode was prepared by integrally attaching a lithium metal foil to a current collector band made of stainless steel.

[Preparation of Reference Electrode] A lithium metal foil was integrally attached to a stainless steel current collector,
A 10 mm square reference electrode was prepared.

[Preparation of Non-Aqueous Electrolyte] A predetermined amount of LiClO ₄ as an electrolyte was dissolved in a mixed solvent composed of propylene carbonate and dimethoxyethane so that the concentration thereof became 1 mol / l, and a non-aqueous electrolyte for each battery was prepared. A water electrolyte was prepared.

[Preparation of Battery for Positive Electrode Evaluation] Battery members such as the positive electrode, the negative electrode, the reference electrode, and the sufficiently dried non-aqueous electrolyte solution prepared as described above were placed in an argon atmosphere, and these battery members were removed. The batteries for evaluation of the lithium ion secondary batteries according to Examples and Comparative Examples each having a beaker-type glass cell as shown in FIG. 1 were respectively assembled.

As shown in FIG. 1, each evaluation battery 1 includes a glass cell 2 as a battery container, and a predetermined amount of the non-aqueous electrolyte 3 is accommodated in the glass cell 2. The positive electrode 4 and the negative electrode 6 housed in the bag-shaped separator 5 are laminated with the separator 5 interposed therebetween, and this laminated body is immersed in the non-aqueous electrolyte 3 in the glass cell 2. ing. The two holding plates 7 sandwich and fix the laminate between them. The reference electrode 8 housed in the bag-shaped separator 5 is immersed in the non-aqueous electrolyte 3 in the glass cell 2.

One end of each of the three electrode wirings 9 extends to the outside through the upper surface of the glass cell 2, while the other end is connected to the positive electrode 4, the negative electrode 6, and the reference electrode 8. Connected to each other. Such a glass cell 2 is subjected to a sealing treatment so that the air does not enter inside during the charge / discharge test.

[Battery Evaluation] (4 V Class Charge / Discharge Test) The reference electrode and the positive electrode were tested at a current value of 1 mA for the lithium ion secondary battery evaluation batteries according to the examples and comparative examples prepared as described above. Was charged until the voltage reached 4.3 V, and the current was stopped for 30 minutes. Then 1mA
A cycle test is performed in a 55 ° C. oven in which a charge / discharge cycle in which discharge is performed at a current value of 3 V until the voltage between the reference electrode and the positive electrode reaches 3 V and the current is stopped for 30 minutes is repeated 30 times (cycle). did.

The initial capacity and the capacity after 30 cycles after repeated charging and discharging of 1 mA were measured, and the results shown in Table 1 below were obtained.

[0085]

[Table 1]

As is clear from the results shown in Table 1 above,
Lithium ion secondary batteries according to the respective examples using a positive electrode active material having a coating layer composed of an oxide of a predetermined metal element (M) and an alkali metal element (A) are comparative examples in which other components are applied. Comparative Examples 1 and 4 and Comparative Examples 2 and 3 in which a coating layer was not formed
It was found that the capacity was larger than those of the batteries of Comparative Examples 5, 6, and 7 using the positive electrode active material formed by simply mixing the active material body and the adherend. Further, even after the charge / discharge cycle proceeded, the decrease in the capacity was small, and it was confirmed that excellent battery characteristics were exhibited.

Examples 7 to 7 in which the composition of the coating layer was the same and the amount of the coating layer was varied from 0.1 to 18%.
From the results shown in FIG. 11, it was found that even when the deposition amount was increased, the decrease in the initial capacity was small, the cycle retention at 55 ° C. was significantly increased, and excellent high-temperature cycle characteristics were obtained.

Further, from the results shown in Examples 12 to 17, even when the element ratio (A / M ratio) of the oxide constituting the coating layer was changed, no significant difference appeared in the initial capacity. It was also confirmed that the high-temperature cycle retention greatly changed depending on the amount of deposition.

Here, the positive electrode active material used in the present invention is manufactured by applying a coating layer made of a predetermined oxide on the surface of the active material body, and in the step of applying this coating layer, the material to be applied is treated with a solution. Alternatively, the bonding strength between the deposition layer and the surface of the active material main body is greatly increased by applying the coating in a molten state, or by applying a heating operation during or after the deposition. In this regard, as a conventional example, a positive electrode active material in which an adherend component and an active material body are simply bonded using a binder or a conventional positive electrode active material in which an adherend material and an active material body are simply mixed and adhered is obtained. A special effect that cannot be obtained is obtained in the present invention.

The following comparative test was conducted on the bonding strength between the above-mentioned coating layer and the active material body. That is, the positive electrode active material 10 prepared in Example 20 and Comparative Example 6
g was suspended in 100 ml of ethanol (EtHO) and irradiated with ultrasonic waves for 10 minutes. After removing the supernatant of the suspension, 100 ml of ethanol was added again, and ultrasonic waves were irradiated for 10 minutes. After removing the supernatant again, it was separated by filtration and dried. Then, the surface composition (Mo / Mn atomic ratio) of each dried sample before and after washing with ethanol was measured by the XPS method, and the results shown in Table 2 below were obtained.

[0091]

[Table 2]

As is clear from the results shown in Table 2 above,
In the positive electrode active material of Comparative Example 6 in which the coating layer was formed by simply mixing the Mo oxide and the active material body, the surface of the active material body was not completely covered with the Mo oxide, and the active material was slightly cleaned by ultrasonic cleaning. It can be confirmed that a large amount of Mo oxide has been peeled off from the surface of the substance main body. In Table 1 above,
The reason why the cycle retention rate of the battery according to Comparative Example 6 is low is that M
o The surface coating by the oxide is not perfect, and the adherend mixture that is easily peeled off due to washing etc. easily peels off from the active material surface as the active material lattice expands and contracts during the battery charging and discharging process.・ It can be guessed that this is due to falling off.

On the other hand, in the battery according to Example 20 in which the same Mo oxide was applied to the surface of the active material through a heating operation, the positive electrode active material was used, and the Mo oxide as the material to be applied was not used. Since the bonding strength with the material surface is extremely high, it can be confirmed that the coating layer does not peel even after the charge / discharge cycle of the battery, and that excellent life characteristics and cycle maintenance ratio can be obtained.

[0094]

As described above, according to the method for producing the lithium ion secondary battery and the cathode active material thereof according to the present invention, at least one element selected from the group consisting of Fe, V, W, Mo and Re is used. Since a positive electrode active material in which a coating layer made of an oxide is formed on the surface of the active material is used, direct contact between the electrolyte and the positive electrode active material is prevented, elution of Mn is effectively prevented, and the battery , The high-temperature cycle characteristics are greatly improved. Further, since the coating layer does not hinder the diffusion of Li ions and has good electron conductivity, a high capacity can be maintained even when the coating layer is formed.
Therefore, even when an active material that is fine particles and has a large contact area with the electrolytic solution is used, a secondary battery having high initial capacity and rate characteristics and excellent high-temperature cycle characteristics can be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a structure of a lithium ion secondary battery for evaluating a positive electrode.

[Explanation of symbols]

 1 Evaluation (lithium ion secondary) battery 2 Glass cell (battery container) 3 Non-aqueous electrolyte 4 Positive electrode 5 Separator 6 Negative electrode 7 Holding plate 8 Reference electrode 9 Electrode wiring

 ──────────────────────────────────────────────────の Continued on the front page F term (reference) 5H003 AA04 BA01 BA03 BB05 BC01 BC05 BC06 BD02 BD04 5H014 AA01 AA02 BB01 BB06 CC01 CC07 EE05 EE10 HH00 HH01 5H029 AJ05 AK03 AL06 AL12 AM03 AM04 AM05 CJ02 CJ08 DJJ17 HJ05 HJ13

Claims (10)

[Claims] 1. An active material body made of a spinel-type lithium manganese composite oxide, and a coating layer applied to at least a part of the surface of the active material body, wherein the coating layer is made of iron, vanadium, and tungsten. A positive electrode active material comprising an oxide of at least one element selected from the group consisting of, molybdenum and rhenium. 2. The coating layer according to claim 1, wherein the coating layer is made of an oxide compounded with at least one element selected from iron, vanadium, tungsten, molybdenum and rhenium and an alkali metal element. The positive electrode active material described in the above. 3. The positive electrode active material according to claim 1, wherein an average particle size of the active material body is in a range of 1 to 20 μm. 4. The coating amount of the coating layer is 0.1 to 18% by weight based on the weight of the active material body.
Or the positive electrode active material according to 2. 5. A lithium ion secondary battery in which a positive electrode having a positive electrode active material and a negative electrode having a negative electrode active material are opposed to each other in a battery can with a separator and a non-aqueous electrolyte interposed therebetween. The material comprises an active material body made of a spinel-type lithium manganese composite oxide, and a coating layer applied to at least a part of the surface of the active material body,
The lithium ion secondary battery, wherein the coating layer is made of an oxide of at least one element selected from iron, vanadium, tungsten, molybdenum and rhenium. 6. The coating layer according to claim 5, wherein the coating layer is made of an oxide compounded with at least one element selected from iron, vanadium, tungsten, molybdenum and rhenium and an alkali metal element. The lithium ion secondary battery according to the above. 7. The lithium ion secondary battery according to claim 5, wherein the average particle size of the active material body is in a range of 1 to 20 μm. 8. The method according to claim 5, wherein the coating amount of the coating layer is 0.1 to 18% by weight based on the weight of the active material body.
Or the lithium ion secondary battery according to 6. 9. An active material body powder comprising a spinel-type lithium manganese composite oxide is prepared, and a compound containing at least one element selected from iron, vanadium, tungsten, molybdenum, rhenium and an alkali metal element is dissolved in a solvent. A method for producing a positive electrode active material, comprising preparing a solution by dissolving the same in the solution, adding the active material main body powder to the solution, mixing, drying, and heat-treating the obtained mixture. 10. An active material body powder comprising a spinel-type lithium manganese composite oxide is prepared, and iron, vanadium,
A compound containing at least one element selected from the group consisting of tungsten, molybdenum, rhenium and an alkali metal element is mixed with the active material body powder, and the resulting mixture is heated and cooled after the compound has passed through a molten state A method for producing a positive electrode active material.