JP2003086181A - Positive electrode active material and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode active material and a lithium ion secondary battery, the material being capable of achieving a secondary battery having both a high capacity characteristic and an excellent cycle characteristic when used as a positive electrode material of the battery. SOLUTION: The positive electrode active material is made of an oxide with a hexagonal system represented by the general formula: Lix Co1-y My O2 wherein M represents at least one kind of metal element having an ion radius greater than Co<3+> , the metal element having an average valence of 3, with 0.4<=x<=2.0, and 0<y<=0.2.

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 and a lithium ion secondary battery using the active material, and particularly to a lithium ion secondary battery having a good balance between battery capacity and cycle characteristics when used as a positive electrode material of a battery. The present invention relates to a positive electrode active material that can realize a secondary battery and a lithium ion secondary battery.

[0002]

2. Description of the Related Art In recent years, a relatively safe carbon-based negative electrode material has been developed, and further development of a non-aqueous electrolyte having a higher decomposition voltage has progressed, and a high-voltage non-aqueous electrolyte secondary battery has been put into practical use. . In particular, the rechargeable battery using lithium ion has a high discharge potential, is lightweight, and has a high energy density. Therefore, the demand for it as a power source for devices such as mobile phones, laptop computers, and cameras with integrated cameras is rapidly increasing. Has been expanded to.

This lithium ion secondary battery comprises a positive electrode and a negative electrode capable of reversibly occluding and releasing lithium ions,
It is composed of a non-aqueous electrolytic solution 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.

[0005]

However, in the battery using the metal oxide such as the lithium cobalt composite oxide, the discharge potential becomes higher than that in the battery using the other two composite oxides. There is a drawback that the discharge capacity in the potential range where the non-aqueous electrolyte does not decompose is reduced to about 1/2 of the theoretical capacity. Moreover, since cobalt, which is a scarce resource, is used as a material, there is a problem that the manufacturing cost becomes high.

On the other hand, in the battery using the lithium nickel composite oxide, the theoretical capacity is large and an appropriate discharge potential can be obtained, but on the other hand, the stability of the charge / discharge battery is higher than that of the battery using the lithium cobalt composite oxide. However, there was a problem that it was not put to practical use.

That is, in the charged state of the battery, deoxidation reaction from the positive electrode active material and oxidation reaction of the electrolytic solution occur, but these reactions are exothermic and the reaction rate increases as the temperature rises. The calorific value also increases sharply. Therefore, thermal runaway in which the exothermic reaction rapidly progresses at a specific temperature is likely to occur, and the safety of the battery is likely to be impaired.

On the other hand, for the purpose of improving the safety of the battery, which is easily inhibited by the above-mentioned exothermic reaction, it has been attempted to replace a part of the Ni component constituting the positive electrode active material with a metal element such as Mn, Co or Al. However, the capacity is remarkably reduced and it has not been put to practical use.

On the other hand, in the battery using the lithium-manganese composite oxide, the safety of the battery is improved by introducing the above-mentioned substitution element, but the battery capacity is remarkably reduced when charging and discharging are repeated at a high temperature of 40 ° C. or higher. There was a problem that became.
Unless these technical problems are solved, it is difficult to substitute the lithium cobalt composite oxide.

The present invention has been made to solve the above problems, and in particular, when used as a positive electrode material of a battery, it is possible to realize a secondary battery having an excellent balance of high capacity characteristics and cycle characteristics. An object is to provide a possible positive electrode active material and a lithium ion secondary battery using the active material.

[0011]

Means for Solving the Problems In response to the technical demand for high capacity, which has been rapidly increasing in recent years, the present inventors have hitherto attempted to effectively fill a battery with a lithium cobalt composite oxide. Although we have been aiming for higher capacity, there is a limit to higher capacity by merely improving the filling. Therefore, the present inventors have another highly feasible means for further improving the capacity of the lithium-cobalt composite oxide battery, that is, increase the end-of-charge voltage to increase the capacity per unit weight of the positive electrode active material. Therefore, we have been studying the means to increase the battery capacity.

However, when a charge / discharge cycle of charging and discharging a lithium cobalt composite oxide, which is widely used at present, to a high potential exceeding 4.2 V is repeated, the capacity remarkably decreases as the cycle progresses. There was found. Therefore, in order to improve the balance between the battery capacity and the cycle characteristics and simultaneously increase both characteristics, it is necessary to incorporate some improvement in the lithium cobalt composite oxide.

In order to solve the above-mentioned problems, the present inventors prepared positive electrode active materials having various compositions, and obtained the composition of the positive electrode active material, the ionic radius of the added component, and the size of the lattice spacing of the crystal. We compared and examined the effect of the on battery cycle characteristics and capacity characteristics.

As a result, in the positive electrode active material composed of the lithium cobalt composite oxide, especially in the charging / discharging process,
A metal element for compensating the valence so that the valence of Co ion is always close to trivalent, or an anion element such as fluorine F is added, or a metal element M having an ionic radius larger than Co ⁺³ is part of Co. It is possible to effectively prevent the decrease of Co ⁺³ ions involved in the charge-discharge reaction when the is replaced and increase the battery capacity, and to reduce the phase transition (crystal transformation) accompanying the charge-discharge, It has been found that the cycle characteristics of the battery are significantly improved due to less structural collapse, and an excellent secondary battery satisfying both capacity and cycle characteristics 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 is represented by the general formula Li _x Co _{1- y My} O ₂ (1) (where M is at least one metal element having an ionic radius larger than Co ³⁺ , And the average valence of the metal element is trivalent, 0.4 ≦ x ≦ 2.0, 0 <y ≦ 0.
It is characterized by comprising an oxide having a hexagonal crystal structure represented by 2).

[0016] Another of the positive electrode active material according to the present invention is represented by the general formula _{Li x Co 1-y M y} O 2-y F y ...... (2) ( where, M has a large ion radius than ^{Co 3+} Two
Is a valent metal element, 0.4 ≦ x ≦ 2.0, 0 <y ≦
0.2), which is composed of an oxide having a hexagonal crystal structure.

In each of the positive electrode active materials, the lattice constants a and c at x = 0.9 of the hexagonal structure oxide are 2.
It is preferable that the ranges are 80 ≦ a ≦ 2.85 and 13.80 ≦ c ≦ 14.10.

The lithium ion secondary battery according to the present invention has the general formula Li _x Co _{1- y My} O ₂ (where M is
Is at least 1 having a larger ionic radius than Co ³⁺
Hexagonal structure oxide or general formula, which is a metal element of a species and has an average valence of 3 and is represented by 0.4 ≦ x ≦ 2.0, 0 <y ≦ 0.2) Li _x Co _{1- y}
M _y O _2-y F _y (where M is a divalent metal element having an ionic radius larger than Co ³⁺ , and 0.4 ≦ x ≦
2.0, 0 <y ≦ 0.2), a positive electrode containing a positive electrode active material made of a hexagonal structure oxide, a negative electrode arranged via the positive electrode and a separator, the positive electrode, and the separator. And a battery container accommodating the negative electrode, and a non-aqueous electrolyte solution filled in the battery container.

The above general formula showing the composition of the positive electrode active material according to the present invention

[Chemical 1] In the above, the ratio x of Li varies greatly depending on the negative electrode material.
The reason for defining ≦ x ≦ 2.0 is as follows. That is, when the x value is less than 0.4, the arrangement of metal ions is disturbed, a component having a rock salt structure is easily formed, and a desired capacity cannot be obtained. On the other hand, when the x value exceeds 2.0, the capacity will decrease.

In the composition of the positive electrode active material according to the present invention,
The metal element M added to replace a part of Co is
All of them are elements having an ionic radius larger than Co ³⁺ , and these elements are charged by compensating the valence so that the valence of Co is always 3 even when the charge / discharge reaction is repeated. It is added to prevent the decrease of Co ³⁺ ions involved in the discharge reaction and increase the capacity.

Further, since these M metal elements have a larger ionic radius than Co ³⁺ , the lattice state of the complex oxide having a hexagonal structure or the distance between Co and O atoms is expanded to control the electronic state of Co. Also, it has the effect of improving the cycle characteristics of the battery even when charged and discharged at a high voltage.

In the present invention, the detailed mechanism by which the cycle characteristics can be improved is unknown, but it is presumed as follows. That is, when charge and discharge are repeated, expansion and contraction of the active material lattice and phase transition (crystal transformation) occur, and eventually crystal collapse occurs, and the conductive auxiliary material particles originally present on the surface of the active material expand the active material lattice. It is considered that the cycle characteristics are deteriorated by the fact that the electrons flow away from the surface of the active material and the electron flow deteriorates during the repeated shrinkage and phase transition (crystal transformation).

However, in the present invention, since the metal ions that are not involved in charging / discharging and have a large ionic radius are introduced, the change in the Co—O atom distance during the charging / discharging process is small and the phase transfer (crystal transformation) occurs. It is expected that the cycle characteristics will be improved because it can be expected to decrease.

That is, in the present invention, the valence of Co is always 3
Since the metal ions are replaced so as to have a valency, Co can be charged and discharged as usual. In addition, the distance between Co and O atoms is larger than that in the system without substitution, and C
Since the change in the inter-o-O atom distance is small, the change in the electrostatic crystal field and the crystal field during the charge / discharge process is small.

Similarly, in the case of F system as well, since the formal charge is -1, which is half of -2 of oxygen, F is less than Co than O.
The electrostatic crystal field exerted on is small. For this reason, the energy of the electrons in Co involved in charging / discharging rises and it becomes easy to charge. Therefore, it is possible to suppress the capacity decrease due to the replacement.

Specific examples of the metal element (M) having an ionic radius larger than that of Co ³⁺ and capable of substituting a part of Co include Mg ²⁺ , Ca ²⁺ , Sc ³⁺ , Y.
³⁺ , Ti ⁴⁺ , Zr ⁴⁺ , Hf ⁴⁺ , Ga ³⁺ , In
IIa group elements such as ³⁺ and Tl ³⁺ , IIIa group elements,
Examples are IVa group elements and IIIb group elements.

The addition amount y of the metal element M is 0 <in atomic ratio.
It is specified that y ≦ 0.2. Co of metal element M
It is difficult to further widen the lattice spacing of the hexagonal structure oxide even if the substitution amount y is excessively larger than 0.2, and the amount of Co ³⁺ ions involved in the charge / discharge reaction decreases. This leads to a decrease in battery capacity. Therefore, the addition amount y of the M metal element is set to 0.2 or less,
The range of 0.01 to 0.18 is more preferable.

The above general formula (1): Li _x Co _{1- y My}
In the positive electrode active material having the composition of O ₂ , at least one metal element having a valence of 3 or the average valence thereof is 3 as the metal element (M) substituting a part of Co. A plurality of metal elements having different valences are combined and added. Particularly, as the metal element M, Sc, Y, Ga, In
Is preferred.

As described above, by adding the metal element M having an average valence of three, the valence of Co in the above composition is always made equal regardless of the addition amount y of the M metal element. It is possible.

On the other hand, the general formula (2) Li _x Co _1-y
In _{M y O 2-y F y} , in order to compensate for the valence of Co, and the addition of fluorine (F) as an anion element divalent M metal element and the same ratio y. In this way, by adding the divalent metal element M and the monovalent fluorine (F) at the same ratio y, the valence of Co in the composition of the general formula (2) is changed to that of the M metal element and the fluorine. It is possible to always make them equal regardless of the added amount. From the above viewpoint, the M metal element added to the positive electrode active material having the composition of the general formula (2) is preferably a divalent alkaline earth metal element.

In the positive electrode active material according to the present invention, Co
M metal component larger than the ionic radius of the ³⁺ ion is ^replaced by Co
Is added so as to replace a part thereof, the crystal lattice spacing and Co-O interatomic distance of the hexagonal structure composite oxide forming the positive electrode active material are widened.

Therefore, when the positive electrode active material having the composition of the general formulas (1) and (2) is in a substantially discharged state, the lattice constants a and c at the time when the atomic ratio x of Li becomes 0.9, respectively. 2.80 ≦ a ≦ 2.85 and 13.80 ≦ c ≦
It becomes the range satisfying 14.10, and the lattice constants a and c become larger than those of LiCoO _{2 in} which M metal element is not added.

The positive electrode active material of the present invention can be manufactured as follows. That is, cobalt hydroxide, cobalt oxide, cobalt acetate, cobalt nitrate and oxides, hydroxides, nitrates, acetates of the substituted metal element M, and lithium carbonate, lithium hydroxide, lithium acetate, lithium nitrate, and, if necessary, Lithium fluoride and ammonium fluoride particles are sufficiently mixed and fired at a temperature of 700 to 1000 ° C. in an air atmosphere or an oxygen atmosphere. The positive electrode active material according to the present invention can be produced by pulverizing and sieving the obtained powder and further classifying the powder if necessary.

In particular, in order to obtain a positive electrode active material for a lithium ion secondary battery having a uniform structure morphology and a uniform particle size distribution by enhancing the mixing accuracy of cobalt and the M metal element or fluorine component substituting for cobalt, Lithium salt as an active material component,
A metal salt slurry or solution is prepared by dispersing or dissolving a metal salt such as a cobalt salt or a substituted metal element M in a dispersion medium, and the slurry or solution is spray-dried to form a particulate material or treated by a coprecipitation reaction. It is important to properly control the mixed state of the component elements by obtaining a particulate precipitate. Then, by baking the obtained product, it is possible to obtain a positive electrode active material in which the mixing property of each element is improved, which is preferable.

Lithium carbonate and lithium hydroxide are particularly preferably used as the lithium salt. Further, as the cobalt salt, cobalt hydroxide can be suitably used in addition to oxides of CoO, Co ₂ O ₃ , and Co ₃ O ₄ .

As the dispersion medium used in the metal salt slurry, various alcohols and organic solvents such as ketone solvents can be used in addition to water. At this time, it is desirable that the metal salt is insoluble or hardly soluble in the dispersion medium.

The following method can be applied as a specific method for controlling the mixed state of the constituent elements when the positive electrode active material according to the present invention is manufactured. That is, disperse the metal salt containing the active material constituents in the dispersion medium,
A step of preparing a metal salt slurry having a 99% cumulative frequency particle size (D99) of the metal salt in the slurry of 5 μm or less; a step of spray-drying the metal salt slurry to prepare agglomerated particles; A method of firing can be applied.

As the measuring instrument of the 99% cumulative frequency particle size (D99), the above-mentioned laser light scattering type particle size distribution meter which can be used for a wet sample can be used. As described above, 99 of the metal salt particles constituting the metal salt slurry
By making the% cumulative frequency particle size (D99) finer to 5 μm or less, the diffusion of the component elements easily and rapidly progresses during firing, and a uniform active material composition can be obtained. The above D99 value is 5
When a coarse metal salt is contained so as to exceed μm, a uniform and fine mixed state of the constituent elements in the active material cannot be obtained.

Further, in order to effectively control the particle size of the metal salt particles constituting the metal salt slurry, the metal salt mixed slurry is sent to the dispersion nozzle section at a pressure of 100 MPa or more,
It is preferable to crush the metal salt by a shock wave or ultrasonic vibration due to the turbulent flow that occurs in the dispersion nozzle section. The supply pressure of the metal salt slurry to the dispersion nozzle is 100MP
When it is less than a, the crushing effect of the metal salt is insufficient and it becomes difficult to obtain a predetermined D99 value. The surface activity of the crushed metal salt particles is significantly increased, and the synthesis reaction at the time of calcination is promoted. Therefore, the crushing operation is extremely effective.

A uniform composition in the form of agglomerated particles can be obtained by spray-drying the metal salt slurry thus obtained. As the spray-drying method, a disc-type or nozzle-type spray dry in which the metal salt slurry is dropped and dispersed on a disc rotating in a drying air stream can be adopted. A desired positive electrode active material for a lithium ion secondary battery can be obtained by firing the uniform composition in the form of aggregated particles.

The lithium ion secondary battery according to the present invention is
The positive electrode active material and the conductive auxiliary agent prepared as described above were mixed with a binder and the like and applied on a current collector, and the positive electrode held by pressure molding and the negative electrode having the negative electrode active material were separated by a separator and a non-electrode. It is arranged so as to face the inside of the battery can via the water electrolyte.

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

The positive electrode is manufactured, for example, by suspending the positive electrode active material and the binder in a suitable solvent, applying the suspension to a current collector, drying the suspension, and press-pressing the suspension. Here, as the current collector, for example, aluminum foil, stainless steel foil, nickel foil, or the like is preferably used.

From the viewpoint of controlling the contact state between the active material and the electrolytic solution when the positive electrode is manufactured,
It is preferable to provide the active material in the electrode sheet with pores of appropriate amount and size. That is, when there are no pores at all, the contact ratio between the electrolytic solution and the active material is reduced and the high temperature characteristics are improved, but the battery reaction itself is less likely to occur and the rate characteristics of the battery are deteriorated.

On the other hand, when the porosity is excessively large, the contact ratio between the electrolytic solution and the active material becomes large, and the battery reaction is likely to occur, so that the battery capacity characteristics, particularly rate characteristics are improved, but the high temperature characteristics are It tends to be unsustainable.

From the above viewpoint, the porosity is set in the range of 5 to 80%, more preferably 10 to 60%. Furthermore, the range of 10 to 50% is preferable. The porosity can be adjusted by controlling the degree of dispersion of the active material layer by adjusting the molding pressure when the electrode is formed by pressure molding after applying the active material on the surface of the current collector.

The porosity can also be adjusted by properly adjusting the amount of binder added to the active material. That is, it has been confirmed that when the amount of the binder increases, the porosity decreases and the battery reaction itself hardly occurs, while when the amount of the binder decreases, the porosity increases and the high temperature characteristics of the battery cannot be maintained. The appropriate amount of binder varies depending on the type of binder, the particle size of the positive electrode active material, and the like, and it is difficult to uniformly specify the amount. The porosity and the pore distribution of the active material can be measured by, for example, a mercury injection type pore distribution measuring device (autopore).

On the other hand, as the active material of the negative electrode, for example, a carbon material which occludes / releases lithium ions, a material containing a chalcogen compound, or an active material made of a light metal can be used. In particular, the use of a negative electrode containing a carbon material or a chalcogen compound that occludes / desorbs lithium ions improves battery characteristics such as the cycle life of the secondary battery, which is particularly preferable.

Examples of the carbon material that absorbs and releases lithium ions include coke, carbonic acid fiber, pyrolytic gas phase carbon material, graphite, resin fired body, mesophase pitch carbon fiber (MCF) or mesophase spherical carbon. A fired body or the like is used. In particular, heavy oil is heated to a temperature of 250
The electrode capacity of the battery can be increased by using liquid crystalline mesophase pitch carbon fibers or mesophase spherical carbon graphitized at 0 ° C. or higher.

Further, the carbonaceous material is 7
It has an exothermic peak at 00 ° C. or higher, and more preferably at 800 ° C. or higher, and has a graphite structure of (10
1) and diffraction peak _{(P 101)} (100) intensity ratio _{P 10} 1 _{/ P 100} of a diffraction peak _{(P 100)} is 0.7
It is desirable to be within the range of 2.2. Since a negative electrode containing a carbon material having such a diffraction peak intensity ratio is capable of rapidly absorbing and releasing lithium ions, it is particularly effective to combine it with a positive electrode containing the positive electrode active material directed to rapid charge / discharge. Is.

Further, as the chalcogen compound which absorbs and releases the lithium ion, titanium disulfide (Ti
S _2), molybdenum disulfide _(MoS 2), can be used niobium selenide (NbSe _2), and the like. When such a chalcogen compound is used for the negative electrode, the voltage of the secondary battery is lowered but the capacity of the negative electrode is increased, so that the capacity of the secondary battery is improved. Furthermore, since the diffusion rate of lithium ions in the negative electrode is increased, 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 the active material that absorbs and releases lithium ions, for example, the negative electrode active material and the binder are suspended in a suitable solvent, and the suspension is applied to a current collector and dried. It is manufactured by press-pressing later. Examples of the current collector include copper foil, stainless steel foil,
The 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 made of, for example, a synthetic resin non-woven fabric, a polyethylene porous film, a polypropylene porous film, or the like.

As the non-aqueous electrolytic solution, a solution prepared by dissolving an electrolyte (lithium salt) 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),
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), sulfolane (S)
Examples thereof include sulfides such as L) and dimethyl sulfoxide (DMSO).

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

Further, the lithium ion is occluded in the negative electrode.
When using a negative electrode active material containing a carbon substance to be released, from the viewpoint of improving the cycle life of a secondary battery including a negative electrode, EC and PC and γ-BL, EC and PC and MEC, and 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 particularly preferable to use a mixed solvent consisting of and DEC.

As the electrolyte, for example, lithium perchlorate
(LiClO_Four), Lithium hexafluorophosphate (LiPF
₆), Lithium borofluoride (LiBF_Four), Arsenic hexafluoride
Lithium (LiAsF₆), Trifluorometasulf
Lithium sulfonate (LiCF_ThreeSO_Three), Bistrifluor
Lithium methylsulfonylimide [LiN (CF _Three
SO_Two)_Two] Lithium salts such as In particular, Li
PF₆, LiBF_Four, LiN (CF_ThreeSO_Two)_TwoUsing
This is desirable because it improves conductivity and safety.

The amount of these electrolytes dissolved in the non-aqueous solvent is preferably set in the range of 0.1 to 3.0 mol / l. If the concentration of the electrolyte solution is higher than 3.0 mol / l, the reaction between the positive electrode active material and the electrolyte solution becomes active in a high temperature range, and the safety of the battery decreases, which is not preferable.

Further, it is possible to enhance the safety of the battery by using a gel electrolyte in which an electrolyte component is dispersed in a gel polymer matrix, instead of the liquid non-aqueous electrolyte solution. Examples of the polymer material (polymer) that constitutes the matrix of the gel electrolyte include polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyethylene oxide (PEO), polyvinyl chloride (PVC), and polyacrylate (PMM).
At least one polymer selected from A) can be used. As the electrolyte, the above-mentioned electrolyte can be mentioned.

When the above gel electrolyte is used, there is no great change in battery characteristics, but a polymer material is used.
Has the function of binding the electrolyte components to each other in addition to the function of holding the electrolyte components, so that liquid leakage does not occur as compared with the liquid non-aqueous electrolyte solution. Further, since it is not a flammable material, there is no risk of explosion or ignition even if an abnormal charging operation is performed, and there is a possibility that the safety of the battery will be improved. Further, since the volume can be reduced as compared with the non-aqueous electrolyte, there is room for downsizing the battery.

According to the positive electrode active material having the above structure and the lithium ion secondary battery using the active material, Co ³⁺
C is a metal element with a predetermined valence that has a larger ionic radius than ions.
Since it has a composition in which a part of o is substituted or a composition containing fluorine, it is possible to obtain excellent cycle characteristics with a small decrease in capacity even when charging and discharging are repeated at a high voltage, and
As a result of the valence of Co ³⁺ being compensated by the above-mentioned substituted metal element or fluorine component, reduction of Co ³⁺ ions involved in the charge / discharge reaction can be prevented, and thus a secondary battery having both high capacity characteristics and excellent cycle characteristics. Can be realized.

[0064]

BEST MODE FOR CARRYING OUT THE INVENTION Next, the 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 appropriately modified and implemented within a range not departing from the scope defined by the gist of the present invention and the elements described in the claims.

According to the following procedure, lithium ion secondary batteries for positive electrode evaluation according to each example and comparative example as shown in FIG. 1 were prepared, and their characteristics were comparatively evaluated.

[Preparation of Positive Electrode Active Material] Examples 1 to 10 and Comparative Examples 1 and 2 Cobalt oxide, each substituted metal salt, lithium fluoride and Li so that the composition of the positive electrode active material is as shown in the left column of Table 1. ₂ CO ₃
And were dispersed in pure water to prepare a metal salt slurry having a solid content concentration of 15% by mass. 100 parts of this metal salt slurry
The metal salt was crushed by supplying it to a wet crusher (Nano maker: manufactured by Nanomizer Co., Ltd.) in a state of being pressurized to MPa, and the particle size thereof was adjusted.

Here, the wet pulverizer is provided with a pair of jet nozzles facing each other, the metal salt slurry is jetted from the jet nozzles at a predetermined pressure at an extremely high speed, and the impact due to the collision of the metal salt slurry jetted oppositely. It is a device that crushes metal salts by force. Then, the 50% cumulative frequency particle size (D
50 value) and 99% cumulative frequency particle size (D99 value)
When measured by Microtrack II of EEDS & NORTHRUP, the D50 value was 1.0 to 1.2 μm and the D99 value was 5.0 to 5.2 μm.

Each of the pulverized metal salt slurries is spray-dried by a spray dryer to prepare agglomerated particles,
The obtained aggregated particles are further baked at a temperature of 850 ° C. for 10 hours, and then cooled at a temperature decrease rate of 1 ° C./min to prepare positive electrode active materials for lithium-ion secondary batteries according to Examples and Comparative Examples. did.

[Preparation of Positive Electrode] Each of the above positive electrode active material powders, acetylene black as a conductive aid, and Teflon (registered trademark) powder as a binder were mixed in a weight ratio of 80: 1.
The mixture was mixed at a ratio of 7: 3 to obtain a positive electrode mixture. Next, each positive electrode mixture was attached to a current collector (stainless steel) to prepare a positive electrode of 10 mm square × 0.5 mm. Subsequent battery preparation work was carried out in a glove box filled with argon gas.

[Preparation of Negative Electrode] Lithium metal foil was integrally attached to a stainless steel current collector to prepare negative electrodes.

[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 Solution] LiPF ₆ as an electrolyte was added to a mixed solvent of ethylene carbonate and methyl ethyl carbonate at a concentration of 1 mol.
A predetermined amount was dissolved so that the amount became 1 / l, and nonaqueous electrolyte solutions for batteries according to Examples 1 to 8 and Comparative Examples 1 and 2 were prepared.

On the other hand, as a gel electrolyte for Examples 9 to 10, 88 mol% of an electrolyte having the above composition and 12 mol% of polyacrylonitrile were mixed at 120 ° C. to prepare a polyvinylidene fluoride (PVdF) gel electrolyte. .

[Preparation of Battery for Evaluating Positive Electrode] Battery members such as the positive electrode, the negative electrode, the reference electrode, and the non-aqueous electrolyte solution prepared as described above were placed in an argon atmosphere, and the battery members shown in FIG. Batteries for evaluation of lithium-ion secondary batteries according to the respective examples and comparative examples equipped with the beaker type glass cells as shown were respectively assembled.

As shown in FIG. 1, each evaluation battery 1 is provided with a glass cell 2 as a battery container, and 20 ml of the non-aqueous electrolyte 3 is contained in the glass cell 2. The positive electrode 4 and the negative electrode 6 housed in the bag-shaped separator 5 are stacked with the separator 5 interposed therebetween, and this stacked body is the nonaqueous electrolytic solution or gel electrolyte 3 in the glass cell 2. It is dipped or filled in. The two holding plates 7 sandwich and fix the laminated body therebetween. The reference electrode 8 housed in the bag-shaped separator 5 is immersed or filled in the non-aqueous electrolyte solution or the gel electrolyte 3 in the glass cell 2.

Further, one end of each of the three electrode wirings 9 penetrates the upper surface of the glass cell 2 and is led out to the outside, while the other ends thereof are the positive electrode 4, the negative electrode 6 and the reference electrode 8. Respectively connected to. Such a glass cell 2 is sealed so that the atmosphere does not enter inside during the charge / discharge test.

[Battery Evaluation] (Initial Capacity) Regarding the batteries for evaluation of the lithium ion secondary batteries according to the respective examples and comparative examples prepared as described above, a reference electrode and a positive electrode were formed at a current value of 0.1 C. Charging was performed until the potential difference between the electrodes reached 4.6 V, and then constant voltage charging was continued for 4 hours, and then the current was stopped for 30 minutes. Next, constant current discharge was performed at a current value of 0.1 C until the potential difference between the positive electrode and the reference electrode reached 3 V, and the amount of electricity discharged during this period was measured.

The composition of the positive electrode active material was Li (CoM) (OF) ₂ to Li _0.9 (C) during the initial charging of the batteries for evaluation of the lithium ion secondary batteries according to the examples and comparative examples.
oM) (OF) ₂ until constant capacity charging is performed, the cell is disassembled, a part of the positive electrode active material is taken out, and X
The lattice constants a and c of the complex oxide having a hexagonal crystal structure were measured using a line diffraction method (XRD method).

Further, using the positive electrode active materials prepared as described above as the battery using the non-aqueous electrolyte, the lithium ion secondary batteries 10 having the structure shown in FIG. Each was produced. Note that graphitized MCF (mesophase pitch carbon fiber) was used as the negative electrode active material constituting the secondary battery. PVDF was used as the binder for the positive electrode and the negative electrode. Further, while aluminum foil was used as the positive electrode current collector, copper foil was used as the negative electrode current collector.

That is, each lithium ion secondary battery 10
In, a bottomed cylindrical battery container 1 made of stainless steel
An insulator 18 is arranged at the bottom of No. 4. Electrode group 15
Are stored in the battery container 14. The electrode group 1
5 is formed in a structure in which a band-shaped material in which a positive electrode 12, a separator 13 and a negative electrode 11 are laminated in this order is spirally wound so that the negative electrode 11 is located outside. The separator 13 is formed of, for example, a non-woven fabric or a polypropylene porous film.

Non-aqueous electrolytes (Examples 1 to 8 and Comparative Examples 1 and 2) are contained in the battery container 14. The insulating sealing plate 19 having an opening at the center is used for the battery container 14
The insulating sealing plate 19 is fixed to the battery container 14 in a liquid-tight manner by being caulked inside the upper opening. The positive electrode terminal 20 is fitted in the center of the insulating sealing plate 19. One end of the positive electrode lead 17 is connected to the positive electrode 12, and the other end is connected to the positive electrode terminal 2.
0 respectively. The negative electrode 11 is connected to a battery container 14, which is a negative electrode terminal, via a negative electrode lead (not shown).

The lithium-ion secondary batteries according to the respective examples and comparative examples prepared as described above were charged at a constant voltage with a potential difference of 4.6 V at a current value of 0.1 C,
Further, the ratio of the discharge capacity to the initial discharge capacity in the first cycle after repeating 30 times the charge / discharge cycle of discharging at a current value of 0.1 C until the potential difference became 3 V was measured as the cycle maintenance rate.

The results of each measurement are shown in Table 1 below.

[0084]

[Table 1]

As is clear from the results shown in Table 1 above,
In the batteries according to each example using the positive electrode active material in which the content of the M metal element substituting a part of Co and the content of fluorine (F) were defined in an appropriate range, the initial capacity was 143-148.
High mAh / g and good cycle retention rate,
It was found that it has both excellent capacity characteristics and cycle characteristics.

On the other hand, in Comparative Example 1 using conventional LiCoO ₂ containing no M metal element and F component, the capacity is high, but the effect of preventing phase transition and the valence compensation effect for Co are not obtained. Therefore, it was reconfirmed that the cycle characteristics were deteriorated. Further, in the battery of Comparative Example 2 in which the contents of the M metal component and the F component contained in the active material were out of the ranges specified by the present invention, both the capacity and the cycle retention rate were lowered, and the battery characteristics were not sufficient. I was able to confirm that.

[0087]

As described above, according to the positive electrode active material and the lithium ion secondary battery using the active material according to the present invention, Co is a metal element having a predetermined ^valence larger than that of Co ³⁺ ions and having a predetermined valence. Since it has a partially substituted composition or a composition containing fluorine, excellent cycle characteristics can be obtained with little decrease in capacity even when charging and discharging are repeated at high voltage, and by the above substituted metal element and fluorine component. As a result of compensating for the valence of Co ³⁺ , it is possible to prevent the decrease of Co ³⁺ ions involved in the charge / discharge reaction, and thus it is possible to realize a secondary battery having both high capacity characteristics and excellent cycle characteristics. .

[Brief description of drawings]

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

FIG. 2 is a half cross-sectional view showing a structural example of a lithium ion secondary battery according to the present invention.

[Explanation of symbols]

1 Evaluation (lithium ion secondary) battery 2 glass cells (battery container) 3 Non-aqueous electrolyte, gel electrolyte 4 positive electrode 5 separator 6 Negative electrode 7 Presser plate 8 reference electrode 9 electrode wiring 10 Lithium-ion secondary battery 11 Negative electrode 12 Positive electrode 13 separator 14 Battery container 15 electrode group 16 insulating paper 17 Positive electrode lead 18 Insulator 19 Insulation sealing plate 20 Positive terminal

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Shota Endo             8th Shinsugita Town, Isogo Ward, Yokohama City, Kanagawa Prefecture             Ceremony company Toshiba Yokohama office (72) Inventor Kazuki Amamiya             8th Shinsugita Town, Isogo Ward, Yokohama City, Kanagawa Prefecture             Ceremony company Toshiba Yokohama office (72) Inventor Hiromasa Tanaka             8th Shinsugita Town, Isogo Ward, Yokohama City, Kanagawa Prefecture             Ceremony company Toshiba Yokohama office (72) Inventor Ryo Sakai             8th Shinsugita Town, Isogo Ward, Yokohama City, Kanagawa Prefecture             Ceremony company Toshiba Yokohama office (72) Inventor Yasuhiro Shirakawa             8th Shinsugita Town, Isogo Ward, Yokohama City, Kanagawa Prefecture             Ceremony company Toshiba Yokohama office (72) Inventor Kyomasa Oya             8 East Shinsugita-cho, Isogo-ku, Yokohama-shi, Kanagawa             Shiba Electronics Engineering Co., Ltd. F term (reference) 5H029 AJ03 AJ05 AK03 AL04 AL06                       AL12 AM03 AM04 AM05 AM07                       BJ02 BJ14 DJ17 HJ02 HJ04                 5H050 AA07 AA08 BA17 CA08 CB05                       CB07 CB12 FA05 FA19 HA02                       HA04

Claims (7)

[Claims] 1. A general formula _{Li x Co 1-y M y} O 2 ( where, M is at least one metallic element having a larger ionic radius than ^{Co 3+,} and the average valence of the metal element trivalent in And a hexagonal structure oxide represented by 0.4 ≦ x ≦ 2.0, 0 <y ≦ 0.2). 2. The general formula Li_xCo_1-yM_yO_2-yF
_y(However, M is Co ³⁺Has a larger ionic radius
It is a divalent metal element, and 0.4 ≦ x ≦ 2.0, 0 <y ≦
0.2) Hexagonal structure oxide
A positive electrode active material characterized by: 3. The lattice constants a and c at x = 0.9 of the hexagonal structure oxide are 2.80 ≦ a ≦ 2.85,1.
3.80 ≦ c ≦ 14.10, The positive electrode active material according to claim 1 or 2, wherein Wherein the general formula _{Li x Co 1-y M y} O 2 ( where, M is at least one metallic element having a larger ionic radius than ^{Co 3+,} and the average valence of the metal element trivalent in And a positive electrode containing a positive electrode active material composed of a hexagonal structure oxide represented by 0.4 ≦ x ≦ 2.0 and 0 <y ≦ 0.2), and the positive electrode and the positive electrode. A lithium ion secondary battery comprising a negative electrode, a battery container accommodating the positive electrode, the separator and the negative electrode, and an electrolyte filled in the battery container. 5. The general formula Li_xCo_1-yM_yO_2-yF
_y(However, M is Co ³⁺Has a larger ionic radius
It is a divalent metal element, and 0.4 ≦ x ≦ 2.0, 0 <y ≦
0.2) Positive electrode composed of hexagonal structure oxide
Through the positive electrode containing the active material, the positive electrode and the separator
And the arranged negative electrode, the positive electrode, the separator and
A battery container for housing the negative electrode and filling the battery container
A lithium electrolyte,
On secondary battery. 6. The lattice constants a and c of the oxide of the hexagonal structure at x = 0.9 are 2.80 ≦ a ≦ 2.85,1.
3.80 ≦ c ≦ 14.10, The lithium ion secondary battery according to claim 4 or 5, characterized in that 7. The lithium ion secondary battery according to claim 4, wherein the electrolyte is at least one of an organic non-aqueous electrolyte solution and a gel electrolyte.