JP3024636B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a non-aqueous electrolyte secondary battery. More specifically, the present invention relates to a lithium secondary battery or a lithium ion secondary battery, and relates to a non-aqueous electrolyte secondary battery having high capacity and improved charge / discharge characteristics, particularly improved cycle life at high temperatures, capacity storage characteristics, and self-discharge characteristics.

[0002]

2. Description of the Related Art Lithium manganate is a material that is highly expected as one of positive electrode materials for lithium ion secondary batteries. This material system was already reported in the 1950's as a subject for studying magnetic behavior (Journal of Japan).
American Chemical Society
y Vol. 78, pp3255-3260), but in 1983 Material R
seee Bulletin Vol. 18,
pp 461-472. M. Thackera
Since Y et al. reported that Li ions could be electrochemically transferred in and out, they have been studied as cathode materials for lithium secondary batteries (for example, Journal of E).
Electrochemical Society Vo
l. 136, no. 11, pp3169-3174
Or the Journal of Electroche
medical Society Vol. 138, N
o. 10, pp 2859-2864).

This lithium manganate has the chemical formula LiMn
_{It has} a spinel structure represented by ₂ O ₄ and functions as a 4V-class positive electrode material between compositions with λ-MnO ₂ . Since lithium manganate having a spinel structure has a three-dimensional host structure different from a layered structure such as that of LiCoO ₂ ,
Most of the theoretical capacity can be used, and excellent cycle characteristics are expected.

However, in practice, a lithium secondary battery using lithium manganate as a positive electrode is inevitably subjected to a capacity deterioration in which the capacity gradually decreases due to repeated charging and discharging, which is a serious problem for its practical use. Was left.

Therefore, various methods have been studied to improve the cycle characteristics of an organic electrolyte secondary battery using lithium manganate for the positive electrode. For example, characteristics improvement by improving reactivity at the time of synthesis (Japanese Patent Application Laid-Open Nos. 3-67664, 3-119656, and 3-12)
7453, JP-A-7-245106, JP-A-7-73883, etc.), and characteristic improvement by controlling the particle size (JP-A-4-198080, JP-A-5-28307, JP-A-6-295724, JP-A-7-97216, etc.), and characteristic improvement by removing impurities (JP-A-5-21063)
However, none of them has achieved satisfactory improvement in cycle characteristics.

Apart from the above, Japanese Patent Application Laid-Open No. 2-270268 discloses an attempt to improve the cycle characteristics by sufficiently increasing the composition ratio of Li with respect to the stoichiometric ratio. The synthesis of a similar excess Li-containing composite oxide is disclosed in Japanese Patent Application Laid-Open Nos.
No. 47573, JP-A-5-205744, JP-A-7-282798 and the like. The improvement of the cycle characteristics by this method can be clearly confirmed experimentally.

[0007] In addition, an effect similar to that of the Li excess composition is aimed at.
Mn spinel material LiMn_TwoO_FourAnd this
Li-Mn composite oxide Li richer than Li material_TwoM
n_TwoO _Four, LiMnO_Two, Li_TwoMnO_ThreeEtc. mixed and positive
The technique used as a pole active material is also disclosed in Japanese Patent Laid-Open No. 6-338320.
And Japanese Unexamined Patent Publication No. 7-262988.
I have. However, excessive addition of Li or another L
Cycle characteristics when mixed with i-rich compounds
Of charging / discharging capacity and charging / discharging energy
High energy density and long cycle life.
There was a problem that could not be set up. In contrast,
In Japanese Unexamined Patent Publication No. Hei 6-275276, high energy density,
High-rate charge / discharge characteristics (current during charge / discharge
Aiming at improvement of the reaction and completeness of the reaction,
Attempts to increase the product have been made,
Achieving a lifetime is difficult.

On the other hand, studies have been made to improve the characteristics by adding another element to the ternary compound of Li-Mn-O. For example, addition, doping of Co, Ni, Fe, Cr, Al or the like (Japanese Unexamined Patent Publication No. Hei 4-14195)
No. 4, JP-A-4-160758, JP-A-4-160758
160976, JP-A-4-237970,
JP-A-4-282560, JP-A-4-28966
No. 2, JP-A-5-28991, JP-A-7-1
No. 4572). The addition of these metal elements is accompanied by a reduction in charge / discharge capacity, and further measures are required to satisfy the total performance.

In the study of the addition of other elements, boron addition is expected to improve other characteristics, such as cycle characteristics and self-discharge characteristics, with almost no decrease in charge / discharge capacity.
For example, JP-A-2-253560, JP-A-3-29
No. 7058 and Japanese Unexamined Patent Publication No. Hei 9-115515 disclose that fact. In each case, manganese dioxide or lithium-manganese composite oxide is mixed with a boron compound (for example, boric acid) in a solid phase or immersed in an aqueous solution of a boron compound, and then heat-treated to synthesize a lithium-manganese-boron composite oxide. ing. Since the composite particles of the boron compound and the manganese oxide have reduced surface activity, it was expected that the reaction with the electrolytic solution would be suppressed and the storage characteristics of the capacity would be improved.

However, the mere addition of boron causes grain growth and a decrease in tap density, and does not directly lead to an increase in capacity as a battery. In addition, depending on the synthesis conditions, the capacity may be reduced in the effective potential range when combined with the carbon anode, or the reaction with the electrolytic solution may be insufficiently suppressed, which is not necessarily effective in improving the storage characteristics. Was not.

As described above, various approaches have been attempted to improve the cycle characteristics of lithium manganate.
At present, the cycle characteristics are comparable to those of the mainstream Co-based alloys, and particularly, the degradation mechanism is promoted in a high-temperature use environment. Therefore, further improvements are required to realize the cycle characteristics in a high-temperature use. In particular, considering the future expansion of application fields such as notebook computers and electric vehicles, it can be said that securing the cycle characteristics at high temperatures is becoming increasingly important.

[0012]

Although lithium manganate LiMn ₂ O ₄ as has been described above [0005] a composite oxide gather great expectations current mainstream of the positive electrode active substitute material for material LiCoO _2, a conventional LiMn ₂ A battery using O ₄ has two problems: (1) it is difficult to achieve both high energy density (high charge / discharge capacity) and high cycle life; and (2) the storage capacity is reduced by self-discharge. Was.

As the cause, technical problems in battery manufacturing and compatibility with an electrolytic solution have been pointed out. However, when attention is paid to the positive electrode material itself and the influence caused by the positive electrode material, the following is considered. Can be

First, the reasons why high energy density cannot be realized include non-uniform reaction, phase separation, excessive imbalance in the composition ratio of Li and Mn, influence of impurities, insufficient tap density, and the like.

The heterogeneity of the reaction and the separation of the phases
Process depending on the process, but firing after dry mixing
In general, the mixing uniformity, starting material particle size and firing temperature
Is determined. That is, the reaction proceeds on the solid phase surface.
Therefore, mixing of the Li source and the Mn source is insufficient,
If the diameter is too coarse or the firing temperature is too high, Mn _Two
O_Three, Mn_ThreeO_Four, Li_TwoMnO_Three, LiMnO_Two, Li_TwoM
n_TwoO_Four, Li_TwoMn_FourO₉, Li_FourMn_FiveO₁₂Phase like
Generated, causing a drop in battery voltage and a drop in energy density.
Wake up.

The cause of capacity deterioration due to charge / discharge cycles is L
The average valence of Mn ions changes between trivalent and tetravalent as charge compensation as i enters and exits, and therefore Jahn-Tel
Ler distortion is generated in the crystal, and Mn is eluted from lithium manganate or the impedance is increased due to Mn elution. That is, as the cause of the capacity deterioration in which the charge / discharge capacity is reduced by repeating the charge / discharge cycle, the influence of impurities, the elution of Mn from lithium manganate, the precipitation of the eluted Mn on the negative electrode active material or the separator, and the Inactivation due to the release of the material particles, the influence of the acid generated by the contained water, and the deterioration of the electrolytic solution due to the release of oxygen from lithium manganate are considered.

Assuming that a single spinel phase is formed, the elution of Mn is caused by disproportionation of trivalent Mn in the spinel structure to tetravalent Mn and divalent Mn, so that Mn is dissolved in the electrolyte. It can be considered that Mn is easily dissolved in water, or it is eluted due to the relative shortage of Li ions, and the generation of irreversible capacity due to repeated charging and discharging and disorder of the atomic arrangement in the crystal Is promoted, and the eluted Mn ions precipitate on the negative electrode or the separator, and L
It appears to hinder i-ion migration. Lithium manganate distorts Li ions, cubic symmetry is distorted by the Jahn-Teller effect, and is accompanied by expansion and contraction of several% of the unit cell length. Therefore, by repeating the cycle, it is expected that some electrical contact failures occur and the released particles do not function as an electrode active material.

Further, it is considered that the release of oxygen from lithium manganate is facilitated along with the elution of Mn. Lithium manganate with many oxygen vacancies has a 3.3 V plateau capacity that increases with the passage of cycles, and as a result, the cycle characteristics also deteriorate. Further, it is presumed that a large amount of oxygen release affects the decomposition of the electrolytic solution, and the deterioration of the electrolytic solution may cause cycle deterioration. In order to solve this problem, improvement of the synthesis method, addition of other transition metal elements, Li excess composition, and the like have been studied.However, in order to simultaneously satisfy both high discharge capacity and high cycle life. Not reached.

Therefore, reduction of Mn elution, reduction of lattice distortion, reduction of oxygen deficiency, and the like are derived as countermeasures.

Next, the cause of the decrease in the storage capacity due to self-discharge is that if the phenomena of internal short-circuit such as insufficient alignment of the positive electrode and the negative electrode due to the manufacturing process of the battery and mixing of metal scraps from the electrodes are excluded, the storage characteristics can be improved. It is considered that the improvement of the stability of lithium manganate with respect to the electrolytic solution, that is, the suppression of elution of Mn, reaction with the electrolytic solution, release of oxygen, and the like are effective.

In particular, when used in a high-temperature environment, both of these deteriorations are promoted, which is a major obstacle to expanding the application. However, since the material system that can expect the potential to satisfy the performance required for the current high-performance secondary battery, such as high electromotive force, voltage flatness during discharge, cycle characteristics, and energy density, is limited, the charge / discharge capacity is limited. There is a need for a new spinel-structured lithium manganate that does not deteriorate and has excellent cycle characteristics and storage characteristics.

Incidentally, Japanese Patent Application Laid-Open No. H10-112318
According to the report, LiMn was used as the positive electrode active material. _TwoO_FourLithium polymer
Gangan composite oxide and LiNiO_TwoSuch as lithium nickel
It is described that a mixed oxide with a composite oxide is used.
You. According to this publication, the irreversible capacity in the first charge / discharge
Is compensated for, and a large charge / discharge capacity is obtained.
You. Japanese Patent Application Laid-Open No. 7-235291 also discloses a positive electrode active material.
LiMn as substance_TwoO_FourLithium manganese composite oxidation
LiCo_0.5Ni_0.5O_TwoIt is noted that
It is listed.

However, according to the study of the present inventors,
Simply using a mixed oxide of lithium manganese composite oxide and lithium nickel composite oxide as the positive electrode active material, the charge / discharge characteristics, especially cycle life at high temperatures, capacity storage characteristics, and self-discharge characteristics, must always be satisfied. No results were obtained.

Accordingly, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a nonaqueous electrolyte secondary battery having excellent battery characteristics, particularly, charge / discharge cycle characteristics, storage characteristics, and safety. With the goal.

[0025]

The present inventors have conducted various studies to achieve the above object, and as a result, the surface area or particle size of the lithium nickel composite oxide to be mixed has been found to be different from the charge / discharge characteristics, especially The present inventors have found that there is an extremely large effect on the cycle life at a high temperature and the improvement in capacity storage characteristics and self-discharge characteristics, and have reached the present invention.

[0026] Namely, the present invention is the positive electrode, and (A) a lithium-manganese composite oxide, I (B1) a specific surface area X is ^{0.3 ≦ X (m 2 / g} ) Der, LiNiO _2, Li ₂
NiO ₂ , LiNi ₂ O ₄ , Li ₂ Ni ₂ O ₄ , and LiN
i _1−x M _x O ₂ (0 <x ≦ 0.5, M is Co, M
selected from the group consisting of n, Al, Fe, Cu, and Sr
Represents one or more metal elements. Selected from the group consisting of
And a lithium-nickel composite oxide comprising at least one lithium-nickel composite oxide.

In the present invention, (A) a lithium-manganese composite oxide and (B2) a D ₅₀ particle size of 40 μm
m I der _{below, LiNiO 2, Li 2 NiO 2} , LiN
_{i 2 O 4, Li 2 Ni} 2 O 4, and _{LiNi 1-x M x O 2} (0
<X ≦ 0.5, and M is Co, Mn, Al, Fe,
At least one selected from the group consisting of Cu and Sr
Represents a metal element. However, lithium nickel nickel cobalt
・ Excluding aluminum composite oxide. Selected from the group consisting of
And a lithium-nickel composite oxide comprising at least one lithium-nickel composite oxide.

In the present invention, the weight ratio of the lithium-manganese composite oxide to the lithium-nickel composite oxide is defined as [LiMn composite oxide]: [LiNi composite oxide] =
(100-a): When represented by a, it is particularly preferable that 3 ≦ a ≦ 45.

[0029]

DETAILED DESCRIPTION OF THE INVENTION According to the study of the present inventor, the lithium-manganese composite oxide as a positive electrode active material has (B1)
Lithium having a specific surface area X of 0.3 ≦ X (m ² / g)
Or nickel composite oxide, or (B2) D ₅₀ particle size is 40
By mixing and using a lithium-nickel composite oxide having a particle size of not more than μm, (1) Mn eluted in the electrolytic solution
It is clear that the amount of ions is greatly reduced, and at the same time, (2) the change in the concentration of Li ions present in the electrolyte is reduced, and (3) the deterioration and discoloration of the electrolyte are suppressed, and the generation of acid is also suppressed. Became. At that time, it is extremely noticeable that the dependence on the specific surface area or the particle size is large.

The mechanism for obtaining such a result is as follows.
Although it is not always clear, the present inventor has argued that the reason for the large elution of Mn ions in the conventional non-aqueous electrolyte secondary battery is that hydrogen ions (H ⁺ ) are generated by the reaction of water mixed in the electrolyte with the supporting salt. It was presumed that it formed and reacted with the lithium-manganese composite oxide to cause Mn elution. Particularly, in the case of LiPF ₆ or LiBF ₄ as a supporting salt, an acid is easily generated and Mn elution is large. In the present invention, on the other hand, the above-described lithium-nickel composite oxide having a predetermined specific surface area or D ₅₀ particle size contained in the positive electrode is considered to capture the hydrogen ions. As a reaction at this time, for example, a mechanism that takes in hydrogen ions and releases Li ions instead is presumed. Further, the lithium-nickel composite oxide may have some kind of catalytic poisoning effect on the reaction between the lithium-manganese composite oxide, the electrolyte, and water.

In any case, by mixing a specific lithium / nickel composite oxide with the lithium / manganese composite oxide in the positive electrode, the generation of acid in the electrolytic solution is suppressed and lithium such as lithium manganate is used. Mn eluted from the manganese composite oxide into the electrolyte is reduced, and at the same time, desorption of oxygen from the lithium-manganese composite oxide such as lithium manganate can be similarly reduced. Accordingly, structural deterioration of the lithium-manganese composite oxide itself is suppressed, and decomposition of the electrolytic solution and a change in Li concentration are suppressed, so that an increase in battery impedance can be prevented. Therefore, both cycle characteristics and capacity storage characteristics can be improved. The invention is particularly directed to Li
Even when a supporting salt such as PF ₆ and LiBF ₄ which easily generates an acid is used, both cycle characteristics and capacity storage characteristics are excellent.

Furthermore, when a material system having a larger charge / discharge capacity than the lithium / manganese composite oxide is used as the lithium / nickel composite oxide, a higher capacity can be simultaneously achieved as a secondary effect. .

Further, in the present invention, the mixing ratio of the lithium-manganese composite oxide and the lithium-nickel composite oxide is set to be [LiMn composite oxide]: [LiNi composite oxide]
= 100-a: When 3 ≦ a when expressed as a, Mn eluted from the lithium-manganese composite oxide into the electrolyte solution can be further reduced, so that the cycle characteristics and the capacity can be reduced. The storage characteristics can be improved. It is generally known that lithium-nickel composite oxide is inferior to lithium-manganese composite oxide in safety. However, by setting a ≦ 45, lithium-manganese composite oxide is originally Thus, a highly safe nonaqueous electrolyte secondary battery can be obtained.

The lithium-manganese composite oxide used in the present invention is an oxide composed of lithium, manganese and oxygen, and lithium manganate having a spinel structure such as LiMn ₂ O ₄ , Li ₂ Mn ₂ O ₄ , and LiMnO ₂ And the like. Among these, lithium manganate having a spinel structure such as LiMn ₂ O ₄ is preferable, and the [Li] / [Mn] ratio may deviate from 0.5 as long as the spinel structure is obtained, and the [Li] / [Mn] ratio As for 0.5-
0.65, preferably 0.51 to 0.6, most preferably 0.53 to 0.58.

Similarly, as long as lithium manganate has a spinel structure, the ratio of [Li + Mn] / [O] is 0.1.
It may be shifted from 75.

The particle diameter of the lithium-manganese composite oxide is expressed in terms of weight average particle diameter in consideration of the ease of preparation and uniformity of battery reaction for a slurry suitable for preparing a positive electrode.
Usually, it is 5 to 30 μm.

Such a lithium-manganese composite oxide can be produced as follows.

Manganese (Mn) raw material and lithium (L
i) As a raw material, first, as a Li raw material, for example, a lithium compound such as lithium carbonate, lithium oxide, lithium nitrate, and lithium hydroxide can be used. As a Mn raw material, for example, electrolytic manganese dioxide (EMD), Mn ₂ O ₃ , M
Various Mn such as n ₃ O ₄ and chemical manganese dioxide (CMD)
Manganese compounds such as oxides and manganese salts such as manganese carbonate and manganese oxalate can be used. But L
Easy to secure the composition ratio of i and Mn, energy density per unit volume due to difference in bulk density, easy to secure target particle size, easy to process and handle in industrial mass synthesis, In consideration of the occurrence, cost, etc., a combination of electrolytic manganese dioxide and lithium carbonate is preferable.

As a step prior to mixing the starting materials, it is preferable that the lithium material and the manganese material are pulverized, if necessary, to have a suitable particle size. The particle size of the Mn raw material is
Usually, it is 3 to 70 μm, preferably 5 to 30 μm. The particle size of the Li source is usually 10 μm or less, preferably 5 μm or less.
μm or less, most preferably 3 μm or less.

Since the reaction of forming the lithium-manganese composite oxide proceeds on the surface of the solid phase, if the mixing of the Li source and the Mn source is insufficient or the particle size is too coarse, the desired composition and In some cases, a lithium-manganese composite oxide having a structure cannot be obtained. For example, when producing lithium manganate having a spinel structure, if the mixing of the Li source and the Mn source is insufficient or the particle size is too coarse,
₂ O ₃ , Mn ₃ O ₄ , Li ₂ MnO ₃ , Li ₂ Mn ₄ O ₉ , Li ₄
A phase such as Mn ₅ O ₁₂ may be generated, and the battery voltage or the energy density may be lower than that of lithium manganate having a spinel structure. Therefore, in order to obtain a lithium-manganese composite oxide having a desired composition and structure, it is necessary to use the above particle size in order to increase the contact area between the lithium raw material and the manganese raw material in order to increase the uniformity of the reaction. Is preferred. Therefore, particle size control and granulation of the mixed powder may be performed. Further, by controlling the particle size of the raw material, a lithium-manganese composite oxide having a target particle size can be easily obtained.

Next, the respective raw materials are taken so that the molar ratio of Li / Mn matches the target composition ratio of the lithium-manganese composite oxide, mixed sufficiently, and fired in an oxygen atmosphere. Oxygen may be pure oxygen, or may be a mixed gas with an inert gas such as nitrogen or argon. The oxygen partial pressure at this time is about 50 to 760 torr.

The firing temperature is usually from 400 to 1000 ° C., but is appropriately selected so as to obtain a desired phase. For example, if the calcination temperature is too high to produce lithium manganate having a spinel structure, undesired phases such as Mn ₂ O ₃ and Li ₂ MnO ₃ are generated and mixed, and the battery voltage and energy density are not sufficient. If the firing temperature is too low, oxygen may become relatively excessive or the powder density may be low, which is also not preferable for realizing high capacity. Therefore, in order to produce lithium manganate having a spinel structure, the calcination temperature is preferably 600 to
900 ° C., most preferably 700-850 ° C.

The firing time can be appropriately adjusted, but is usually 6 to 100 hours, preferably 12 to 48 hours. Although the cooling rate can be adjusted as appropriate, it is preferable not to rapidly cool during the final baking treatment, and it is preferable to set the cooling rate to, for example, about 100 ° C./h or less.

The powder of the lithium-manganese composite oxide thus obtained is further classified, if necessary, to make the particle size uniform, and mixed with the lithium-nickel composite oxide described below to form a positive electrode. Used as an active material.

Next, the lithium / nickel composite oxide used in the present invention will be described. The lithium-nickel composite oxide is an oxide composed of lithium, nickel and oxygen, and is composed of LiNiO ₂ , Li ₂ NiO ₂ , LiNi ₂
O ₄ , Li ₂ Ni ₂ O ₄ , and those obtained by doping some of these oxides with other elements for stabilization, high capacity, and improved safety can be used. As an example of an oxide partially doped with another element, for example, an oxide obtained by doping another element with respect to LiNiO ₂ is LiNi _1-x M _x O ₂ (0 <x ≦ 0.
5 ) Where M is a doped metal element,
It represents one or more metal elements selected from the group consisting of Co, Mn, Al, Fe, Cu, and Sr. M may be two or more types of doped metal elements, and the sum of the composition ratios of the doped metal elements may be x.

Among them, LiNiO ₂ and LiNi
_1-x Co _x O ₂ (in this case, x is usually 0.1 to 0.4) is preferred.

In the present invention, the lithium / nickel composite
Li / Ni ratio of the composite oxide (LiNi _1-xM_xO_TwoIn the case of
Li / [Ni + M] ratio is slightly different from the stoichiometric ratio
The lithium-nickel composite acid of the present invention may be shifted.
The compound includes such a case.

In the present invention, by using such a lithium-nickel composite oxide having a specific surface area X of 0.3 or more, it is possible to effectively prevent the deterioration of the lithium-manganese composite oxide or the electrolytic solution. Becomes possible. Further, the specific surface area is usually 5.0 or less, and it is preferable to use a specific surface area of 3.0 or less, since a slurry which can be easily handled and easily applied to the electrode when the positive electrode is manufactured is obtained.

[0049] Further, as described above lithium-nickel composite oxide of the present invention may also be used as D ₅₀ particle size is 40μm or less, by a 40μm or less D ₅₀ particle size,
It is possible to effectively prevent the deterioration of the lithium-manganese composite oxide or the electrolytic solution. Further, D ₅₀ particle size is usually 1μm or more, so easy to handle easily electrode coating slurries can be performed to obtain when producing the positive electrode especially used more than 3μm preferred.

Here, the specific surface area indicates a surface area (m ² / g) per unit weight of powder, and is measured by a gas adsorption method in the present invention.

[0051] Further, the D ₅₀ particle size represents a particle diameter corresponding to the weight accumulated value 50%, is measured by laser light scattering measurement method.

Such a lithium / nickel composite oxide can be produced as follows. First, as a lithium raw material, for example, lithium carbonate,
A lithium compound such as lithium oxide, lithium nitrate, and lithium hydroxide can be used. Nickel (N
i) Nickel hydroxide, nickel oxide, nickel nitrate or the like can be used as a raw material.

It is preferable that both the lithium raw material and the nickel raw material are pulverized, if necessary, to have an appropriate particle size. In particular, in order to obtain a predetermined specific surface area, or D ₅₀ particle size, is preferably used by classifying the particle size of the nickel material.

Thereafter, the Li / Ni ratio is adjusted to the desired composition ratio of the lithium-nickel composite oxide, mixed well, and fired in the same manner as in the production of the lithium-manganese composite oxide. The firing temperature is about 500 to 900 ° C. The calcined obtained lithium-nickel composite oxide, preferably to obtain the desired specific surface area, or D ₅₀ particle size lithium-nickel composite oxide, and further classified.

For the positive electrode used in the nonaqueous electrolyte secondary battery of the present invention, a mixture of such a lithium-manganese composite oxide and a lithium-nickel composite oxide is used as a positive electrode active material.

In the present invention, in addition to such a mixture of the lithium-manganese composite oxide and the lithium-nickel composite oxide, the positive electrode active material such as LiCoO ₂ is generally known as the positive electrode active material. May be used in combination. For safety and the like, a commonly used additive such as Li ₂ CO ₃ may be further added.

The method for producing the positive electrode is not particularly limited. For example, for example, a powder of lithium-manganese composite oxide and a powder of lithium-nickel composite oxide are mixed with a binder together with, for example, a conductivity-imparting agent and a binder. After mixing (slurry method) with a suitable dispersing medium that can be dissolved, the mixture is applied to a current collector such as an aluminum foil, and then the solvent is dried and then compressed by a press or the like to form a film.

The conductivity-imparting agent is not particularly limited, and those usually used such as carbon black, acetylene black, natural graphite, artificial graphite and carbon fiber can be used. As the binder, a commonly used binder such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) can be used.

On the other hand, as the negative electrode active material, a carbon material such as lithium, lithium alloy, graphite or amorphous carbon capable of inserting and extracting lithium is used.

The separator is not particularly limited.
Glass fibers, porous synthetic resin films and the like can be used. For example, a polypropylene or polyethylene porous film is suitable in terms of a thin film, large area, film strength and film resistance.

As the solvent for the non-aqueous electrolyte, those which are commonly used may be used. For example, carbonates, chlorinated hydrocarbons, ethers, ketones, nitriles and the like can be used. Preferably, ethylene carbonate (EC), propylene carbonate (P
C), at least one of γ-butyrolactone (GBL) and the like, and diethyl carbonate (DE) as a low-viscosity solvent.
C), at least one selected from dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), esters and the like, and a mixture thereof is used. EC + DEC, P
C + DMC or PC + EMC is preferred.

As a supporting salt, LiClO ₄ , LiI, L
iPF ₆ , LiAlCl ₄ , LiBF ₄ , CF ₃ SO ₃ Li
For example, at least one type is used. In the present invention, the use of a supporting salt which easily generates an acid can suppress the acid in the electrolytic solution, and is particularly preferable when LiPF ₆ or LiBF ₄ is used, since the most effective effect can be obtained. The concentration of the supporting salt is, for example, 0.8 to 1.5M.

As the configuration of the battery, a prismatic type, a paper type,
Various shapes such as a laminated type, a cylindrical type, and a coin type can be adopted. In addition, components include a current collector, an insulating plate, and the like, but these are not particularly limited, and may be selected according to the above shape.

[0064]

EXAMPLES Hereinafter, the present invention will be further described with reference to Examples, but the present invention is not limited thereto. The specific surface area was measured by Quanta Chrome's Quant.
using aSorb, D ₅₀ particle size, Micro Tra
It measured using FRA made from c company.

[Evaluation Test Example 1] In the synthesis of lithium manganate, lithium carbonate (Li ₂ CO ₃ ) and electrolytic manganese dioxide (EMD) were used as starting materials.

Prior to the mixing of the starting materials, Li ₂ CO ₃ was pulverized and EMD was classified for the purpose of improving the reactivity and obtaining lithium manganate having the target particle size. When lithium manganate is used as a positive electrode active material of a battery, a weight average particle size of 5 to 30 μm is preferable in consideration of ensuring reaction uniformity, ease of slurry preparation, safety, and the like. 5 to 30 μm, which is the same as the target particle size of lithium oxide.

[0067] On the other hand, since the Li ₂ CO ₃ in order to ensure homogeneous reaction the following particle size desired 5 [mu] m, D ₅₀ particle size 1.
Pulverization was performed so as to be 4 μm.

The EMD and Li ₂ CO ₃ thus adjusted to a predetermined particle size were converted to [Li] / [Mn] = 1.05 / 2.
Were mixed so that

This mixed powder was dried under an oxygen flow atmosphere at 80
Baking at 0 ° C. Next, fine particles having a particle size of 1 μm or less in the obtained lithium manganate particles were removed by an air classifier. At this time, the specific surface area of the obtained lithium manganate was about 0.9 m ² / g.

The tap density was 2.17 g / cc, the true density was 4.09 g / cc, the D50 particle size was 17.2 μm,
The powder had a lattice constant of 8.227 °.

On the other hand, LiNiO ₂ having a specific surface area of 1.7 m ² / g was prepared as a lithium-nickel composite oxide.

The lithium manganate prepared as described above and LiNiO ₂ were mixed at the ratio shown in Table 1, and 5 g of the mixed powder was mixed with propylene carbonate (PC) containing LiPF ₆ (concentration: 1 M) and dimethyl carbonate (DMC). Of a mixed solvent (50:50 (% by volume)) was placed in a closed container.

These closed containers were heated to 80 ° C.
Left for days. Thereafter, the electrolytic solution was extracted, and the Mn ion concentration in the electrolytic solution was analyzed by ICP. Table 1 shows the results.
Shown in

[0074]

[Table 1]

(In Table 1, a is the same as the above, ie, lithium-manganese composite oxide: lithium-nickel composite oxide when [lithium-nickel composite oxide] is represented by (100-a): a.) From this result, the higher the mixing ratio of LiNiO ₂ , the smaller the amount of Mn eluted in the electrolytic solution. That is, it is expected that the stability of the positive electrode active material will increase even when the battery is used in a high-temperature environment. In particular, when the content of LiNiO ₂ is less than 3%, although the effect of suppressing the dissolution of Mn can be seen at least by the addition, it is preferable to add 3% or more to obtain a satisfactory effect. More preferably, it is 10% or more.

[Evaluation Test Example 2] The sealed container prepared in Evaluation Test Example 1 was similarly heated to 80 ° C and left for 20 days.
Thereafter, the electrolytic solution was extracted, and the Li ion concentration in the electrolytic solution was analyzed by atomic absorption. Table 2 shows the results.

[0077]

[Table 2]

Propylene carbonate (PC) containing LiPF ₆ (concentration 1M) and dimethyl carbonate (DM)
Considering that the Li concentration in the electrolytic solution of the mixed solvent (C) (50:50 (vol%)) is about 6400 ppm,
When the LiNiO ₂ mixture ratio is 3% or more, it can be said that the decrease in the Li concentration in the electrolytic solution can be suppressed. Assuming that the Mn concentration is 1/3 or less when lithium / nickel composite oxide is not mixed, a is preferably 3 or more also from the viewpoint of suppressing the decrease in the Li concentration in the electrolytic solution.

[Evaluation Test Example 3] Lithium manganate synthesized in the same manner as in Evaluation Test Example 1 was used as the lithium-manganese composite oxide, and the specific surface area of the lithium-nickel composite oxide was 3.0 m ² / g. , 2.36m ² /
g, 1.50m ² /g,0.71m ² /g,0.49m ²
/G,0.30m ² /g,0.25m ² / g of the seven types of L
iNi _0.8 Co _{0. 2} O ₂ powder were prepared.

Next, lithium manganate and LiNi _0.8 Co _0.2 O ₂ having various specific surface areas are mixed at a predetermined weight mixing ratio (a
= 0, 3, 5, 10, 15, 20, 30, 35), and 5 g of the mixed powder and LiPF ₆ were mixed in the same manner as in Evaluation Test Example 1.
(Concentration 1M) mixed solvent of propylene carbonate (PC) and dimethyl carbonate (DMC) (50: 5
0 (% by volume) of the electrolytic solution was placed in a closed container.

These closed containers were heated to 80 ° C.
Left for days. Thereafter, the electrolytic solution was extracted, and the Mn ion concentration in the electrolytic solution was analyzed by ICP. Figure 1 shows the results.
Shown in It can be seen that the greater the specific surface area, the higher the effect of suppressing Mn elution.

From the results of Evaluation Test Example 3, it was found that when the specific surface area of the lithium-nickel composite oxide was less than 0.3 m ² / g, the effect of suppressing the dissolution of Mn was too small. M when oxide is not mixed
It does not become less than 1/3 of n concentration. Therefore, the specific surface area is 0.
It can be seen that the effect is not recognized unless it is 3 m ² / g or more.

[Evaluation Test Example 4] Lithium manganate synthesized in the same manner as in Evaluation Test Example 1 was used as the lithium-manganese composite oxide, and as the lithium-nickel composite oxide, D ₅₀ was 2 μm, 3 μm, 15 μm, 26 μm,
Six kinds of LiNi _0.8 Co _0.2 O _{2 of} 40 μm and 45 μm
Powder was prepared.

Next, lithium manganate and LiNi _0.8 Co _0.2 O ₂ having various specific surface areas are mixed at a predetermined weight mixing ratio (a
= 0, 3, 5, 10, 15, 20, 30, 35), and 5 g of the mixed powder and LiPF ₆ were mixed in the same manner as in Evaluation Test Example 1.
(Concentration 1M) mixed solvent of propylene carbonate (PC) and dimethyl carbonate (DMC) (50: 5
0 (% by volume) of the electrolytic solution was placed in a closed container.

These closed containers were heated to 80 ° C.
Left for days. After that, the electrolyte is extracted, and the
The Mn ion concentration was analyzed by ICP. Figure 2 shows the result.
Shown in The smaller the particle size, the more the effect of suppressing Mn elution
It turns out that it is high. Also, D ₅₀Particle size larger than 40 μm
The mixing ratio α of the lithium nickel composite oxide is set to a> 45.
Even with the elution amount of Mn mixed with lithium / nickel composite oxide
If not, the concentration does not become 1/3 or less of the Mn concentration. Therefore
D₅₀If the particle size is not less than 40 μm, the effect is recognized.
I can't.

[Evaluation Test Example 5] Lithium manganate synthesized in the same manner as in Evaluation Test Example 1 was used as the lithium-manganese composite oxide, and the specific surface area of the lithium-nickel composite oxide was 4.5 m ² / g. 3.2m ² /
^{g, 3.0m 2 /g,1.50m 2 /g,0.30m 2} /
g of five types of LiNi _0.8 Co _0.2 O ₂ powder were prepared.
Lithium manganate, LiNi _0.8 Co _0.2 O ₂ and carbon black as a conductivity-imparting agent are dry-mixed, added to N-methyl-2-pyrrolidone (NMP) in which PVDF as a binder is dissolved, and kneaded to obtain a uniform mixture. The slurry was dispersed to prepare a battery slurry. At this time, lithium manganate: LiNi _0.8 Co _0.2 O ₂ : conductivity imparting agent: PVD
The mixing ratio of F: NMP = 30: 10: 5: 5: 50 (% by weight) (a = 25).

After measurement with a Brookfield viscometer, the slurry was uniformly applied on a 25 μm-thick aluminum metal foil, and NMP was evaporated to obtain a positive electrode sheet. Table 3 shows the specific surface area, the slurry and the state of application.

[0088]

[Table 3]

According to Table 3, when the specific surface area is larger than 3.0 m ² / g, gelation occurs and electrode coating becomes difficult.
It is understood that the specific surface area is preferably 3.0 m ² / g or less.

[Evaluation Test Example 6]
As the nickel-nickel composite oxide, D₅₀Is 2 μm,
3 μm, 15 μm, 26 μm, 40 μm, 45 μm L
iNi_0.8Co_0.2O_TwoEvaluation test example except that powder was prepared
In the same manner as in No. 5, a positive electrode sheet was obtained. In Table 4, D ₅₀Particle size
This shows the state of slurry and application.

[0091]

[Table 4]

Table 4 shows that when the D ₅₀ particle size is smaller than 3 μm, gelation occurs and electrode coating becomes difficult, so that the D ₅₀ particle size is preferably 3 μm or more.

[Evaluation Test Example 7] Lithium manganate synthesized in the same manner as in Evaluation Test Example 1 was used as the lithium-manganese composite oxide, and the lithium-nickel composite oxide had a specific surface area of 1.7 m ² / g. LiNi _0.8 Co _0.2
2320 coin cells were made using O ₂ .

That is, the positive electrode is made of lithium manganate: LiN
i _0.8 Co _0.2 O ₂ : conductivity imparting agent: PTFE = 72:
The mixture kneaded at a mixing ratio (a = 10) of 8:10:10 (% by weight) was rolled to a thickness of 0.5 mm, and was rolled to a diameter of 12 m.
m. Here, carbon black was used as the conductivity-imparting agent. The negative electrode is φ14mm, thickness 1.5
mm of metal Li was used, and a porous PP film having a thickness of 25 μm was used as a separator. The electrolyte is LiClO ₄ (concentration 1)
M) and a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (50:50 (% by volume)).

At the same time, for comparison, the positive electrode was made of lithium manganate: conductivity imparting agent: PTFE = 80: 10: 10
(Weight%), except that LiNi _0.8 Co _0.2 O ₂ was not contained, and a negative electrode, a separator, and an electrolytic solution were similarly used to produce a 2320 coin cell.

Using these coin cells, a charge / discharge cycle test was performed. The cycle is 0.5m for both charging and discharging
A / cm ² constant current, charge / discharge voltage range is 3.0 to 3.0
The test was performed at 4.5 V vs. Li. The evaluation temperature is 10 ° C
To 60 ° C in steps of 10 ° C.

# 50 / # 1 (discharge capacity at 50th cycle with respect to discharge capacity at 1st cycle) based on cycle evaluation temperature of coin cells containing LiNi _0.8 Co _0.2 O ₂ (Example) and not containing (Comparative Example) Table 5 shows the residual capacity ratio (%). The coin cell according to the present invention has a higher capacity remaining ratio even when the cycle temperature is increased.

[0098]

[Table 5]

[Evaluation Test Example 8] Lithium manganate synthesized in the same manner as in Evaluation Test Example 1 was used as the lithium / manganese composite oxide, and the lithium / nickel composite oxide had a specific surface area of 1.7 m ² / g. LiNi _0.8 Co _0.2
An 18650 cylindrical cell was prototyped using O ₂ .

That is, first, lithium manganate, LiNi
_0.8 Co _0.2 O ₂ and carbon black as a conductivity-imparting agent were dry-mixed, and uniformly dispersed in N-methyl-2-pyrrolidone (NMP) in which PVDF as a binder was dissolved to prepare a slurry. Thick the slurry to a thickness of 25
After coating on a μm aluminum metal foil, NMP was evaporated to form a positive electrode sheet. The solid content ratio in the positive electrode was lithium manganate: LiNi _0.8 Co _0.2 O ₂ : conductivity imparting agent: PVDF = 72: 8: 10: 10 (% by weight). At this time, a = 10.

On the other hand, the negative electrode sheet was made of carbon: PVDF =
NMP was mixed so as to have a ratio of 90:10 (% by weight).
And applied to a copper foil having a thickness of 20 μm.

The electrode sheets of the positive electrode and the negative electrode produced as described above were wound up through a 25 μm-thick polyethylene porous membrane separator to obtain a cylindrical battery.

The electrolytic solution uses 1M LiPF ₆ as a supporting salt,
The solvent used was a mixed solvent of propylene carbonate (PC) and diethyl carbonate (DEC) (50:50 (vol%)).

At the same time, for comparison, LiNi was contained in the positive electrode.
_An 18650 cylindrical cell was fabricated in the same manner except that _0.8 Co _0.2 O ₂ was not included, and the solid content ratio was changed to lithium manganate: conductivity imparting agent: PVDF = 80: 10: 10 (% by weight).

A charge / discharge cycle test at 55 ° C. was performed using these cylindrical cells. Charging was performed up to 4.2 V at 500 mA, and discharging was performed up to 3.0 V at 1000 mA. FIG. 3 shows a comparison of the cycle characteristics of the discharge capacity at 55 ° C. of the cylindrical cell when LiNi _0.8 Co _0.2 O ₂ is contained (Example) and when it is not contained (Comparative Example). It can be seen that the capacity of the cylindrical cell according to the embodiment of the present invention is less deteriorated even when the charge / discharge cycle is repeated.

Further, a charge / discharge cycle test at 55 ° C. was performed for 10 times using the cylindrical cells of the above Examples and Comparative Examples.
After 0 cycles, the impedance of each cylindrical cell was measured by the AC impedance method. The comparison is shown in FIG. It can be seen that the embodiment according to the present invention has smaller DC equivalent resistance and interface resistance.

[Evaluation Test Example 9] Lithium manganate synthesized in the same manner as in Evaluation Test Example 1 was used as the lithium-manganese composite oxide, and the specific surface area of the lithium-nickel composite oxide was 1.7 m ² / g. LiNi _0.8 Co _0.2
An 18650 cylindrical cell was prototyped using O ₂ .

The production method of the 18650 cylindrical cell was performed in the same manner as in Evaluation Test Example 8.

In this evaluation test example, the solid content weight ratio in the positive electrode was lithium manganate: LiNi _0.8 Co
_0.2 O ₂ : conductivity imparting agent: PVDF = 80-x: x: 1
The test was performed with x (% by weight) at 0:10 as shown in Table 6. Table 6 also shows a (= x · 100/80, synonymous with the above-mentioned a).

Using the cylindrical cell thus produced, 5
A volume storage test at 5 ° C. was performed.

The battery was charged at a constant current of 500 mA to 4.2 V, and then charged at a constant voltage of 4.2 V for 2 hours. Thereafter, the discharge capacities of a case where the battery was discharged without leaving a standing time at room temperature and a case where the battery was discharged after being left at room temperature for 28 days were measured. The capacity measurement was performed under a room temperature environment at 500 mA and a cutoff potential of 3.0 V.

Table 6 shows the storage capacity (shown as 4 W capacity) of the prototyped cylindrical cell after being left for 28 days and the ratio of the storage capacity to the capacity (shown as 0 W capacity) when the battery was discharged without the storage period. Is shown. LiNi _0.8 Co _0.2 O
_In contrast to the case where ₂ was not added (when x = 0), the case where added was added, the storage stability of the volume was high even after standing for 28 days. Also,
The capacity of the cylindrical cell also increased due to the mixing effect of the high capacity lithium-nickel composite oxide.

[0113]

[Table 6]

[Evaluation Test Example 10] A safety test was performed using the cylindrical cell prepared in Evaluation Test Example 9. Table 7 shows the results. When lithium manganate is used as the main positive electrode active material, the safety is higher than that of Co-based, and the difference in safety under more severe conditions is emphasized. The test was employed.

In the round bar crush test, one round of the battery was
/ 2 crushed. The nail insertion test is a test for forcibly causing an internal short circuit by inserting a nail into a battery, and a 4 mm nail was used. Details are UL-164
Performed according to 2.

In the round bar crushing test, a small amount of steam was observed when x was 40 or more, and ignition occurred when x was 52 or more. On the other hand, in the nailing test, smoking was observed when x exceeded 36, and ignition occurred at 48 or more. As the proportion of the lithium-nickel composite oxide increases, it becomes more difficult to ensure safety. Therefore, from the viewpoint of safety, x is 36 or less and a ≦ 45.

[0117]

[Table 7]

Summarizing the results of the above evaluation test examples, the lithium-nickel composite oxide to be mixed has a specific surface area X from the viewpoint of Mn elution and the applicability and printability of the slurry.
Is most preferably 0.3 ≦ X ≦ 3.0 (m ² / g).

[0119] Further, lithium-nickel composite oxide is mixed, the coating of the aspects and slurries of Mn elution is most suitable that the viewpoint than D ₅₀ particle size of the printing property is 3μm or 40μm or less.

Further, the ratio of the lithium-manganese composite oxide to the lithium-nickel composite oxide is determined from the viewpoint of elution of Mn and safety from the viewpoint of [LiMn composite oxide]:
When [LiNi composite oxide] = (100-a): a, 3 ≦ a ≦ 45 is preferable.

[0121]

According to the present invention, the elution of Mn from the lithium-manganese composite oxide, which is the active material used in the nonaqueous electrolyte secondary battery, and the change in the Li concentration in the electrolyte are suppressed.
It is possible to provide a nonaqueous electrolyte secondary battery in which the charge and discharge cycle, particularly the charge and discharge life at a high temperature, is greatly improved.
In addition, the nonaqueous electrolyte secondary battery of the present invention has improved capacity storage characteristics. Further, according to the present invention, it is possible to provide a non-aqueous electrolyte secondary battery excellent in safety.

[Brief description of the drawings]

FIG. 1 is a graph showing the results of measuring the Mn concentration in an electrolyte solution immersed in an electrolyte solution at 80 ° C. for 20 days while changing the mixing ratio of lithium / nickel composite oxide and the specific surface area.

FIG. 2 is a graph showing the results obtained by changing the Mn concentration in the electrolyte solution, the mixing ratio of the lithium-nickel composite oxide, and D ₅₀ when immersed in the electrolyte solution at 80 ° C. for 20 days.

FIG. 3 is a diagram showing cycle characteristics of discharge capacity at 55 ° C. of the present invention and a conventional cylindrical cell.

FIG. 4 is a diagram showing the impedance of the present invention and a conventional cylindrical cell.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-10-112318 (JP, A) JP-A-9-180718 (JP, A) JP-A-10-92430 (JP, A) JP-A-6-180318 60887 (JP, A) JP-A-8-217452 (JP, A) JP-A-8-138670 (JP, A) JP-A-11-3698 (JP, A) JP-A-11-54120 (JP, A) JP-A-10-199508 (JP, A) JP-A-5-13107 (JP, A) JP-A-8-171935 (JP, A) JP-A-8-50895 (JP, A) JP-A-6-349493 (JP, A) JP-A-9-55211 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 4/58 H01M 4/02 H01M 10/40

Claims (6)

(57) [Claims] 1. A positive electrode comprising: (A) a lithium-manganese composite oxide; (B1) a specific surface area X satisfying 0.3 ≦ X (m ² / g), and LiNiO ₂ , Li ₂ NiO ₂ , LiNi ₂ O ₄ , Li ₂
Ni ₂ O ₄ and LiNi _1-x M _x O ₂ (0 <x ≦ 0.5
And M represents one or more metal elements selected from the group consisting of Co, Mn, Al, Fe, Cu, and Sr. A non-aqueous electrolyte secondary battery comprising at least one lithium-nickel composite oxide selected from the group consisting of: 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the specific surface area X of the lithium-nickel composite oxide further satisfies X ≦ 3.0 (m ² / g). 3. A positive electrode comprising: (A) a lithium-manganese composite oxide; (B2) LiNiO ₂ , Li ₂ NiO ₂ , LiNi ₂ O _{4 having} a D ₅₀ particle size of 40 μm or less and 3 μm or more . Li ₂
Ni ₂ O ₄ and LiNi _1-x M _x O ₂ (0 <x ≦ 0.5
And M represents one or more metal elements selected from the group consisting of Co, Mn, Al, Fe, Cu, and Sr. However, excluding lithium-nickel-cobalt-aluminum composite oxide. A non-aqueous electrolyte secondary battery comprising at least one lithium-nickel composite oxide selected from the group consisting of: 4. The lithium-manganese composite oxide and a lithium-manganese composite oxide
[LiMn]
Complex oxide]: [LiNi complex oxide] = (100−
a): when represented by a, 3 ≦ a ≦ 45
The non-aqueous electrolyte secondary according to any one of claims 1 to 3.
battery. 5. The lithium-manganese composite oxide,
5. A lithium manganate having a spinel structure.
The non-aqueous electrolyte secondary battery according to any one of the above. 6. The support salt in the electrolyte is LiPF ₆ or
The non-aqueous solution according to claim 1, which is LiBF _4.
Electrolyte secondary battery.